210Pb DEPOSITION RATES IN A TROPICAL …...v Contents Chapter 1: Introduction 1 1.1 Overview 1 1.2 Alligator Rivers Region 6 1.3 Project objectives 9 Chapter 2: Literature Review:

QUEENSLAND UNIVERSITY OF TECHNOLOGY SCHOOL OF PHYSICAL AND CHEMICAL SCIENCES

MEASUREMENT OF 222Rn EXHALATION RATES AND

210Pb DEPOSITION RATES IN A TROPICAL

ENVIRONMENT

Submitted by Cameron Lawrence (B. App. Sc., M. App. Sc.) to the School of Physical and Chemical Sciences, Queensland University of Technology, in partial fulfilment of therequirements of the degree of Doctor of Philosophy.

March 2005

i

Key Words Radon, exhalation, emission, Lead-210, deposition, excess, redistribution, budget,

Kakadu, Ranger, uranium, mining, radionuclides, isotopes, soil moisture, radium,

activity concentration, land application, soil erosion, atmospheric transport,

geomorphic landscapes, tropics, Alligator Rivers Region, environmental

radioactivity, Jabiru, atmospheric dispersion, soil profile, diurnal, seasonality, wet

season, dry season, precipitation scavenging, aerosol transport, aerosol removal,

Hadley circulation, water inundation

ii

Acknowledgements I owe great thanks to my supervisor Dr. Riaz Akber for all his support and effort

during the course of this project, his assistance and direction has been invaluable.

Many thanks also go to my external supervisor and the other support staff of the

Enrad group at eriss, Dr. Paul Martin, Dr. Andreas Bolhöfer, Mr. Bruce Ryan, Mrs.

Therese Fox and Mr. Peter Medley, for their countless hours of assistance, sample

analysis and data retrieval. I also owe many thanks to the remainder of the eriss

team, especially the Jabiru Field Station, for their support during my time in Jabiru.

Eriss provided my accommodation and all work facilities for the 20 months of my

stay at Jabiru and for that I am gratefully appreciative.

Major parts of this project would not have been possible without the

assistance of ERA personnel, specifically Mr. Ian Marshman, for arranging access to

the Ranger sampling locations.

Special thanks go to my father, Eoin Lawrence, for all his support during my

studies over the years. His support has fantastic providing me with sound advice in

all the major decisions I’ve had to make. I only hope that I can continue to live up to

his expectations as I enter the next phase of this life.

Most of all I am pleased to have the support and love of my partner Saski

who has a direct understanding of the personal commitments required to complete

this work. Her support over the last year in all things has been phenomenal and I look

forward to providing her the same support in all matters in her life.

iii

Abstract This thesis provides the measurements of 222Rn exhalation rates, 210Pb deposition

rates and excess 210Pb inventories for locations in and around Ranger Uranium Mine

and Jabiru located within Kakadu National Park, Australia. Radon-222 is part of the

natural 238U series decay chain and the only gas to be found in the series under

normal conditions. Part of the natural redistribution of 222Rn in the environment is a

portion exhales from the ground and disperses into the atmosphere. Here it decays

via a series of short-lived progeny, that attach themselves to aerosol particles, to the

long lived isotope 210Pb (T1/2 = 22.3 y). Attached and unattached 210Pb is removed

from the atmosphere through wet and dry deposition and deposited on the surface of

the earth, the fraction deposited on soils is gradually transported through the soil and

can create a depth profile of 210Pb. Here it decays to the stable isotope 206Pb

completing the 238U series.

Measurements of 222Rn exhalation rates and 210Pb deposition rates were

performed over complete seasonal cycles, August 2002 – July 2003 and May 2003 –

May 2004 respectively. The area is categorised as wet and dry tropics and it

experiences two distinct seasonal patterns, a dry season (May-October) with little or

no precipitation events and a wet season (December-March) with almost daily

precipitation and monsoonal troughs. November and April are regarded as

transitional months. As the natural processes of 222Rn exhalation and 210Pb deposition

are heavily influenced by soil moisture and precipitation respectively, seasonal

variations in the exhalation and deposition rates were expected. It was observed that 222Rn exhalation rates decreased throughout the wet season when the increase in soil

moisture retarded exhalation. Lead-210 deposition peaked throughout the wet season

as precipitation is the major scavenging process of this isotope from the atmosphere.

Radon-222 is influenced by other parameters such as 226Ra activity

concentration and distribution, soil porosity and grain size. With the removal of the

influence of soil moisture during the dry season it was possible to examine the effect

of these other variables in a more comprehensive manner. This resulted in

categorisation of geomorphic landscapes from which the 222Rn exhalation rate to 226Ra activity concentration ratios were similar during the dry season. These results

can be extended to estimate dry season 222Rn exhalation rates from tropical locations

from a measurement of 226Ra activity concentration.

iv

Through modelling the 210Pb budget on local and regional scales it was

observed that there is a net loss of 210Pb from the region, the majority of which

occurs during the dry season. This has been attributed to the fact that 210Pb attached

to aerosols is transported great distance with the prevailing trade winds created by a

Hadley Circulation cell predominant during the dry season (winter) months. By

including the influence of factors such as water inundation and natural 210Pb

redistribution in the soil wet season budgeting of 210Pb on local and regional scales

gave very good results.

v

Contents

Chapter 1: Introduction 1 1.1 Overview 1 1.2 Alligator Rivers Region 6 1.3 Project objectives 9

Chapter 2: Literature Review: Previous research in relation to radon emanation, migration, exhalation and 210Pb deposition 10 2.1 Overview 10 2.2 Radon emanation 10

2.2.1 Introduction 10 2.2.2 Radon emanation and radium distribution 12 2.2.3 Radon emanation and soil moisture 14 2.2.4 Radon emanation, soil porosity and grain size 17 2.2.5 Radon emanation, pore size and number 18 2.2.6 Radon emanation and soil temperature 20 2.2.7 Variations in emanation coefficients for radon isotopes 21

2.3 Radon migration, exhalation and soil gas concentration 22 2.3.1 Introduction 22 2.3.2 Radon exhalation measurements techniques 24 2.3.3 Radon exhalation surveys 24 2.3.4 Radon migration, exhalation, soil gas concentration and soil

moisture 29 2.3.5 Radon exhalation, soil gas concentration and atmospheric

pressure 31 2.3.6 Radon exhalation, soil gas concentration and temperature 32 2.3.7 Radon exhalation, soil gas concentration and wind speed 33 2.3.8 Radon diffusion theory 33 2.3.9 Radon exhalation temporal variations 36 2.3.10 Radon migration, exhalation and soil gas concentration

summary 38 2.4 Pb-210 deposition 39

2.4.1 Introduction 39 2.4.2 Pb-210 depositional rate studies 41 2.4.3 Pb-210 soil studies 45 2.4.4 Pb-210 deposition and geographical location 49 2.4.5 Pb-210 atmospheric concentration studies 50 2.4.6 Pb-210 summary 51

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2.5 Chapter summary 52

Chapter 3: Project location, site selection and measurement schedules 55 3.1 Overview 55 3.2 Exhalation from open ground – Investigation of

physical parameters [226Ra activity concentration, distribution in grains, grain size and porosity] 55 3.2.1 Ranger operations 55 3.2.2 Ranger site selection 60 3.2.3 Ranger measurement schedule 63

3.3 Seasonal and diurnal radon exhalation [moisture, pressure and temperature] 66 3.3.1 Site selection 66 3.3.2 Seasonal site measurement schedule 70 3.3.3 Diurnal measurement schedule 71

3.4 Excess 210Pb soil sampling 72 3.5 Pb-210 deposition sampling 76

Chapter 4: Methodology 77 4.1 Overview 77 4.2 Available techniques for radon exhalation

measurements 78 4.3 Radon exhalation measurement with charcoal

canisters 79 4.3.1 Charcoal canister counting system, calibration & efficiency 82

4.4 Radon emanometers 83 4.4.1 Emanometer calibration 87 4.4.2 Associated emanometer measurements 88

4.5 Soil moisture readings 89 4.6 Soil activity concentration measurements 91

4.6.1 Geofizika GS-512 portable gamma detector 91 4.6.2 Determination of 226Ra from gamma dose rates 93 4.6.3 Soil sampling and preparation 94 4.6.4 Excess 210Pb analysis of soil samples 97

4.7 Pb-210 deposition measurement 97 4.8 HPGe gamma spectroscopic system 99

4.8.1 Calibration of spectroscopy system for project samples 102

Chapter 5: Radon sources 107

vii

5.1 Overview 107 5.2 Rn-222 exhalation rate and 226Ra activity 107 5.3 Diurnal measurements of radon exhalation 128 5.4 Seasonal measurements of radon exhalation 135 5.5 Chapter summary 143

Chapter 6: Lead-210 deposition and excess 145 6.1 Overview 145 6.2 The 210Pb story 145 6.3 Pb-210 deposition 147

6.3.1 Seasonal 210Pb results 147 6.3.2 Annual depositional rate, average values and residency time 153 6.3.3 Pb-210 deposition summary 156

6.4 Pb-210 excess in soil samples 156 6.4.1 Pb-210 inventories 156 6.4.2 Penetration half depth 161 6.4.3 Excess 210Pb summary 163

6.5 Magela Land Application Area 164 6.5.1 Introduction 164 6.5.2 Uranium-238, 226Ra and 210Pb depth profile inventories 164 6.5.3 Experimental plot inventories 168 6.5.4 Radium-226 and 210Pb distribution 170 6.5.5 Magela Land Application Area summary 172

6.6 Chapter summary 173

Chapter 7: Lead-210 budget 175 7.1 Introduction 175 7.2 Hadley circulation 175 7.3 Local area 210Pb budget 176

7.3.1 Fate of Ranger 222Rn 177 7.3.2 Determination of 222Rn exhalation rates from 210Pb deposition

and excess 210Pb inventories 177 7.3.3 Determination of excess 210Pb inventories from 210Pb deposition 179 7.3.4 Determination of 210Pb deposition and inventories from 222Rn

exhalation rates 182 7.3.5 Local area 210Pb budget summary 183

7.4 Regional 210Pb budget 184 7.4.1 Kakadu dry season 222Rn emission 184 7.4.2 Kakadu wet season 210Pb budget 188

7.5 Chapter Summary 191

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Chapter 8: Conclusions and future directions 193 8.1 Project outcomes 193 8.2 Future directions 197 8.3 Conclusions 198

ix

List of Figures Figure 1.1: The uranium and actinium natural decay series with radon isotopes highlighted ....................................................................................................3 Figure 1.2: Global population weighted average of human exposure to natural sources of radiation, total 2.4 mSv.y-1 (UNSCEAR 2000) .......................4 Figure 1.3: Alligator Rivers Region, Northern Territory, Australia, curtesy Supervising Scientists Division .................................................................................7 Figure 2.1: Fate of 222Rn nucleus just after 226Ra-222Rn transmutation, R is the recoil range of 222Rn nucleus in solid material. A: Recoil and embedding in same grain. B: Recoil, ejection from grain and stopping in interstitial space C: Recoil, ejection from grain, crossing air gap and embedding in neighbouring grain. D: Recoil, ejection from grain and embedding in neighbouring grain. E: Recoil and stopping in water in the interstitial space. .12 Figure 2.2: On right scanning electron micrograph of monazite (top) and zircon (bottom). On left thorium distribution in the same grain (Holdsworth and Akber 2004) ........................................................................................................14 Figure 2.3: Emanation coefficient as function of increasing water content for sample of till sieved into various grain sizes. (Adapted from Markkanen and Arvela (1992)) ............................................................................................................16 Figure 2.4: The ratio of the saturated emanation coefficient to dry emanation coefficient for increasing moisture (Emission ratio). (Adapted from Sun and Furbish (1995)) ..........................................................................................................16 Figure 2.5: Emanation coefficient for increasing grain size and differences between radium distribution for natural samples. Surface Ra has thickness equal to the recoil range (40 nm). (Adapted from Greeman and Rose (1995))18 Figure 2.6: Radon emanation, migration and exhalation ...................................23 Figure 2.7: Typical 210Pb soil profiles for various soil uses (adapted from Walling et al. (2003)).................................................................................................48 Figure 3.1: Ranger Uranium Mine, numbers indicate approximate sampling locations used for this project ..................................................................................57 Figure 3.2: Flow chart of Ranger processing (ERA 2005) ..................................58 Figure 3.3: Original Magela Land Application Area (MLAA)...............................64 Figure 3.4: Map of region displaying seasonal sites............................................69 Figure 3.5: Dry season (April-October) wind rose for Jabiru East (data courtesy of Australian Bureau of Meteorology) [26 years averaged] ................73 Figure 3.6: Map of Jabiru and Ranger, numbers indicate approximate locations of selected sites for soil samples ...........................................................74 Figure 4.1: Charcoal canister ..................................................................................80 Figure 4.2: Radon emanometer ..............................................................................84 Figure 4.3: Cutter used to accurately place emanometer saucer......................85 Figure 4.4: Schematic of radon/thoron emanometer ...........................................86 Figure 4.5: Set up for emanometer calibration .....................................................88 Figure 4.6: Default calibration curve for Diviner 2000 soil moisture probe ......90 Figure 4.7: Geofizika Brno NaI(Tl) GS-512 gamma spectrometer in use at Rangers waste rock dump .......................................................................................93 Figure 4.8: Base of discs used for soil samples ...................................................96 Figure 4.9: 210Pb deposition collector deployed at Oenpelli ...............................98 Figure 4.10: eriss detector room, Darwin (Photograph by Bruce Ryan).........100

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Figure 4.11: Calibration curves and equations for pressed disc soil samples, energy unit is keV ....................................................................................................104 Figure 4.12: Efficiency calibration for resin samples .........................................105 Figure 5.1: Plot of 222Rn exhalation rates vs. 226Ra activity concentrations for all sampling sites .....................................................................................................122 Figure 5.2: Ratio of 222Rn exhalation rate to 226Ra activity concentration (RE-R) for locations during dry conditions ........................................................................124 Figure 5.3: Plot of 222Rn exhalation rate vs. 226Ra activity concentration for all sites categorised by geomorphic groups .............................................................127 Figure 5.4: Diurnal variations of atmospheric pressure observed at Jabiru East (Data courtesy of Australian Bureau of Meteorology) ..............................129 Figure 5.5: Diurnal variations in atmospheric and soil temperatures at Jabiru East (Data courtesy of Australian Bureau of Meteorology) ..............................129 Figure 5.6: Normalised 222Rn exhalation rate for all sites vs. time of day of measurement ...........................................................................................................131 Figure 5.7: Normalised 220Rn exhalation rate for all sites vs. time of day of measurement ...........................................................................................................131 Figure 5.8: 222Rn exhalation rate vs. soil temperature.......................................132 Figure 5.9: 220Rn exhalation rate vs. soil temperature.......................................132 Figure 5.10: 222Rn exhalation rate vs. change in atmospheric pressure ........133 Figure 5.11: 220Rn exhalation rate vs. change in atmospheric pressure ........133 Figure 5.12: Seasonal variations of 222Rn exhalation rates and cumulative rainfall ........................................................................................................................136 Figure 5.13: Seasonal variations of 222Rn exhalation rates and cumulative rainfall continued .....................................................................................................137 Figure 5.14: Averaged 222Rn exhalation rates and atmospheric concentrations for sampling periods at Mudginberri ..........................................139 Figure 5.15: 2002-2003 wet season moisture profiles for Jabiru East ...........141 Figure 5.16: 2003 dry season soil moisture profiles for Jabiru East ...............141 Figure 5.17: Mirray soil moisture profiles, all readings ......................................143 Figure 6.1: Jabiru East 210Pb deposition and cumulative rainfall.....................148 Figure 6.2: Oenpelli 210Pb deposition and cumulative rainfall ..........................149 Figure 6.3: Relationship between excess 210Pb and 40K...................................160 Figure 6.4: Relative cumulative excess 210Pb versus depth for soil scrapes.162 Figure 6.5: Relative cumulative excess 210Pb versus depth for soil cores .....162 Figure 6.6: Inventory depth profile for 2cm sectioned cores from irrigated TM1 and non-irrigated TM2 ...................................................................................165 Figure 6.7: Inventory depth profile for 5cm sectioned cores, irrigated (core 2) and averaged non-irrigated cores.........................................................................165 Figure 6.8: U-238 inventory depth profile for irrigated core TM1.....................166 Figure 6.9: U-238 inventory depth profile for irrigated core 1 and core 2.......166 Figure 6.10: Inventory depth profile for scrape 1 from experimental plot.......169 Figure 6.11: Inventory depth profile for scrape 2 from the experimental plot 169 Figure 6.12: Activity concentration depth profile for core collected by J. Storm 1994 (fine grains, <2mm only)...............................................................................170 Figure 6.13: Distribution of 226Ra in soil fractions for 0-10cm ..........................171 Figure 6.14: Distribution of 210Pb in soil fractions for 0-10 cm .........................172 Figure 7.1: Global Hadley circulation model (curtesy Australian Bureau of Meteorology) ............................................................................................................176 Figure 7.2: Natural redistribution of 210Pb............................................................181

xi

Figure 7.3: Radium-226 versus 238U to 10 cm to demonstrate 226Ra/238U disequilibrium ...........................................................................................................181 Figure 7.4: Geomorphic landscapes of the Kakadu region (Adapted from Lowry and Knox (2002)) .........................................................................................186 Figure 7.5: Wetland area of Kakadu (Santos-Gonzalez et al. 2002) ..............189

xii

List of Tables Table 2-1: Worldwide reported 222Rn exhalation rates ........................................26 Table 3-1: Classification of ore grades at Ranger................................................56 Table 3-2: Number, name and position of sites selected at Ranger, refer to Figure 3.1....................................................................................................................60 Table 3-3: Measurement dates for Ranger Mine sites ........................................64 Table 3-4: Number of Ranger measurements ......................................................65 Table 3-5: Seasonal measurement sites, names and locations ........................68 Table 3-6: Sites and dates of diurnal measurements ..........................................72 Table 3-7: Sites for excess 210Pb soil samples .....................................................75 Table 3-8: Soil collection dates and samples taken.............................................75 Table 4-1: Emanometer calibration check results ................................................88 Table 4-2: Standards for Pressed Disc Geometry .............................................103 Table 4-3: Standards for resin...............................................................................103 Table 4-4: Corrections factors applied to activity concentrations of soil samples .....................................................................................................................106 Table 5-1: Dry season values of 222Rn exhalation rates from all measurement sites ...........................................................................................................................108 Table 5-2: 226Ra activity concentrations and RE-R ratio .....................................110 Table 5-3: Analysis result of geomorphic clusters .............................................126 Table 5-4: Correlation coefficients for diurnal measurements .........................134 Table 6-1: Results from Jabiru East 210Pb deposition collector .......................149 Table 6-2: Results from Oenpelli 210Pb deposition collector.............................150 Table 6-3: Seasonal and annual 210Pb depositional rates and rainfall .........154 Table 6-4: Total inventories of excess 210Pb.......................................................158 Table 7-1: Measured and estimated seasonal and annual 222Rn exhalation rates for Jabiru East ................................................................................................179 Table 7-2: Estimated and observed excess 210Pb inventories .........................180 Table 7-3: Estimated 210Pb deposition rate and excess 210Pb inventory ........183 Table 7-4: Dry season 222Rn emission from Kakadu National Park ................187 Table 7-5: Wet season kappa for various geomorphic landscapes ................188 Table 7-6: Wet season 222Rn emission from Kakadu National Park ...............190 Table 7-7: Estimated Kakadu region 222Rn exhalation rate, wet season 210Pb deposition rate and total excess 210Pb inventory................................................191

xiii

Statement of Original Authorship The work contained in this thesis has not been previously submitted for a degree or diploma at any other tertiary educational institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made. Signed __________________________

Date_______________________________

1

1

Introduction

1.1 Overview Discovery of 222Rn gas emanating from samples of 226Ra is attributed to German

physicist Friedrich Ernst Dorn in 1900 who was performing work in relation to the

recent discovery of the three types of radioactivity by Ernest Rutherford in 1899

(Romer 1964). Radon-222 is present in nature as the only gas found in the natural 238U radioactive decay series, Figure 1-1; it is the direct progeny of 226Ra, has a half-

life of 3.82 days, is colourless, inert, the major radon isotope, a noble gas, which is

the heaviest gaseous element. Radon-222 is soluble in water (bulk solubility =

1.95*10-4 at standard temperature and pressure) and highly soluble in organic

solvents (Chemical Rubber Company 2004; UNSCEAR 1982). Two other isotopes

of radon exist in nature and are members of the 232Th and 235U natural radioactive

decay series, they are 220Rn (thoron) and 219Rn (actinon) respectively, Figure 1.1. The

half-lives of 220Rn (55.6 s) and 219Rn (3.96 s) are much shorter than that of 222Rn and

as a result they are not as useful for environmental studies although exposure to 220Rn and its progeny is considered for radiological dose assessment in certain

industries.

The radon isotopes, specifically 222Rn, are reported as contributing the largest

component of human exposure to natural radiation (UNSCEAR 2000), Figure 1-2.

Trace quantities of 238U, 232Th, 235U and their progeny are found in all natural rocks

and soils. As a result radon isotopes are emitted in some quantity from every natural

and a number of man-made surfaces. Exposure via inhalation of the radon isotopes

and their progeny has been associated with an increase in the risk of lung cancer

(Smith 1988; Jacobi 1988; Lubin and Boice 1997). Concentration measurements of 222Rn and its progeny for the determination of radiation doses to occupationally

exposed individuals and members of the public living in proximity to supervised

radiation areas are standard practice. The International Commission on Radiological

Protection (ICRP 1991) recommendations for limits to ionising radiation from man-

made sources are 20 mSv.yr-1, effective dose, for occupationally exposed workers

2

and 1 mSv.yr-1 for members of the public; this does not include medical exposure as

a patient or natural background.

Occupational exposure to 222Rn is common in mining and milling industries,

predominantly in the uranium mining industry. Sand mining of 232Th and 238U rich

sands for minerals like zircon, rutile, monazite and ilmenite also increases exposure

to 220Rn and 222Rn. Other than mining and milling a number of areas are subject to

accumulation of 222Rn and 220Rn thus increasing exposure of people working or

living in such environments, these include poorly ventilated natural caves, basements

and cellars. Accumulation of 222Rn in poorly ventilated houses, that enters the living

area from crawl spaces or basements, has been identified as a health risk in a number

of countries around the world (UNSCEAR 2000).

Eventually 222Rn decays through a series of short-lived progeny, with

half-lives ranging from 104 μs to 26.8 minutes, until it reaches the “long-lived”

isotope 210Pb with a half-life of 22.3 years. A portion of the natural radon isotopes

are emitted into the atmosphere from the rock and soil surfaces around the globe in a

process known as exhalation. The depth from which exhalation occurs depends on a

large number of factors including the isotope half-life, soil porosity, soil moisture,

soil temperature, air pressure, air temperature and wind speed. Having a longer half-

life, 222Rn is exhaled from greater depths than 220Rn that in turn is exhaled from

greater depths than 219Rn. In normal soils 222Rn is exhaled from the top few metres, 220Rn from the top few decimetres and 219Rn from the top few centimetres.

Radon-222 entering the atmosphere mixes as a gas and is subject to atmospheric

transport. The progeny of 222Rn are electrically charged, having been stripped of

electrons during the decay process, a majority of them attach to aerosols existing in

the atmosphere.

3

Figure 1.1: The uranium and actinium natural decay series with radon isotopes highlighted

Pb-206 Stable

Po-214 104μs

Bi-214 19.9m

Pb-210 22.3y

Po-218 3.10m

Rn-222 3.82d

Ra-226 1600y

Th-230 7.54*104y

U-234 2.46*106y

Pa-234m 1.17m

Th-234 24.1d

U-238 4.47*109y

Uranium Decay Series

α α

α

α

α

β

β

β

β

Pb-214 26.8m

α

α

β

Po-210 138d

Bi-210 5.01d

β

α

Actinium Decay Series

Pb-207 Stable

Po-211 0.516s

Bi-211 2.14m

Tl-207 4.77m

Pb-211 36.1m

Po-215 1.78ms

Rn-219 3.96s

Ra-223 11.4d

Th-227 18.7d

Ac-227 21.8y

Fr-223 21.8m

Pa-231 3.28*104y

Th-231 25.5h

U-235 7.04*108y

α

α

α

α

α

α

α

α

α

β

β

β

β

β

β

98.6%

1.4%

0.3%

99.7%

4

Thorium Decay Series

Figure 1-1 continued: The thorium natural decay series with thoron highlighted

TerrestialCosmicIngestionInhalation

Figure 1.2: Global population weighted average of human exposure to natural sources of radiation, total 2.4 mSv.y-1 (UNSCEAR 2000)

α

β α

Rn-220 55.6s

Ra-224 3.66d

Th-228 1.91y

Ac-228 6.15h

Ra-228 5.75y

Th-232 1.41*1010y

β

50% 17%

21%

12%

α

α

α

α

Pb-207 Stable

Po-211 0.516s

Bi-211 2.14m

Tl-207 4.77m

Pb-212 10.6h

Po-216 0.145s

β

β

β

64.1%

35.9%

α

5

Aerosols are removed from the atmosphere by two processes; the first is dry

deposition and the second is precipitation scavenging. The latter of these removes the

larger amount of aerosols from the atmosphere over a shorter period of time.

Removal of radioactive particles in the atmosphere therefore comes through their wet

and dry deposition. The deposition of attached and unattached 222Rn progeny occurs

on all surfaces over the globe. The portion that is deposited on land is either washed

into the water systems or is transported, mixed and covered by more new soil

creating a depth profile of deposition history. Profiling excess 210Pb and examining

its behaviour in soils is a major component of this project.

The natural phenomenon of radon exhalation, its atmospheric mixing and the

deposition of its progeny back onto the surface of the globe have numerous

applications. Measurements of radon exhalation rates from the ground and radon and

radon progeny concentrations in the atmosphere have been used for the following

applications:

− For indirect measurement of radium isotopes;

− Ground fault line identification and earthquake prediction;

− Radiological dose assessment;

− Tracers for determination of atmospheric transport;

− Investigation of the aerosol attachment process;

− For studies of the prevailing conditions that control the exhalation.

Measurement of 210Pb in soils has the potential for use in soil redistribution

studies. The man-made fission product 137Cs is commonly used for these types of

studies (Zapata 2003) but cessation of above ground nuclear testing means there is no

replenishment of 137Cs into the atmosphere. The natural exhalation of 222Rn into the

atmosphere replenishes 210Pb almost continuously. Also there was a greater amount

of 137Cs released into the atmosphere of the northern hemisphere than the southern

during the above ground nuclear tests. This makes using 137Cs in the southern

hemisphere even more limited. Another surface deposited natural radionuclide, 7Be,

can be used but its short half-life of 55 days limits potential applications. As the half-

lives of 210Pb and 137Cs are comparable, 22.3 years and 30.1 years respectively, 210Pb

can be used for the same environmental applications as 137Cs along with providing

new applications, these include;

6

− Use in soil sedimentation studies for agriculture and land care

management;

− Transport and bioturbation of soil particles;

− Soil erosion studies;

− Indicator of deposition history;

− Indicator of 222Rn progeny transport in the atmosphere.

An aim of this project is to provide potential for development of further

applications using 210Pb.

1.2 Alligator Rivers Region This project’s main aim is to budget the 210Pb within the Alligator Rivers Region of

tropical northern Australia. This project was developed due to the current lack of

knowledge and understanding of the behaviour of 222Rn and 210Pb within tropical

regions. Budgeting involves identification of the sources and sinks of 210Pb within a

region and producing a model of the exhalation and deposition process that matches

observed values at selected locations. The area selected was the Alligator Rivers

Region of the Northern Territory, Australia shown in Figure 1-3. This region

encompasses the world heritage listed Kakadu National Park and includes the

operational Ranger Uranium Mine (RUM), owned by Energy Resources of Australia

(ERA). The region also hosts a number of former uranium mine sites and

undisturbed uranium deposits. This area was selected as it lies within the tropics, is

representative of a complete geomorphic system, and the Ranger Uranium Mine acts

as a localised 222Rn source. At Jabiru East there is a government field station

laboratory (eriss) with which the project was performed in collaboration with. They

provided full support during the time spent in the region.

7

Figure 1.3: Alligator Rivers Region, Northern Territory, Australia, curtesy Supervising Scientists Division

8

The region is classified as wet and dry tropics with those two distinct seasons

in the year. The wet season is usually from November to March and the dry season is

from May to September. The months of April and October are transitional periods

between these seasons. The local Aboriginal people have categorised eight seasons in

the area corresponding to temperature and humidity changes that occur throughout

various times of the year. The effects that this contrast in weather conditions has on

the exhalation and deposition rates of 222Rn and 210Pb have received very little

attention over the years. Hence a primary reason for selection of a wet-dry tropical

location was to study the influence of season on the exhalation and deposition rates

of these radionuclides. Average annual rainfall for the region is 1485.3 mm per

annum, occurring mostly in the wet season and the average annual pan evaporation

rates are 2591.5 mm per annum (ABM 2005). The prevailing winds for the region

are easterly to south-easterly during the dry season and northerly to north-westerly

during the wet season (ABM 2004).

In the Alligator Rivers Region the wet season deposits large amounts of rain

over a period of 4-5 months. While several rain-producing systems have been

identified in northern Australia they all fall into the two main categories of

convective or cyclonic systems. A convective rain event is intense but short-lived

(minutes to hours) and covers relatively small areas (1-100 km2). The contrast is the

cyclonic rain events and monsoon troughs created as low-pressure systems passing

across the area. Cyclonic and monsoon rains last for a few days to weeks and cover

much larger areas.

Jabiru is the only township located within Kakadu National Park while

Oenpelli is an Aboriginal township located in Arnhem Land in the Alligator Rivers

Region. A number of tourist accommodation resorts, ranger stations and Aboriginal

settlements are also located within the region. Jabiru lies about 12 km west of the

Ranger mine, some 260 km east of Darwin. It has a population of about 1200 people

that includes mining personnel, eriss personnel, local Aboriginals, park rangers,

resort staff, people employed in associated community facilities and families of all

the above. Jabiru is a closed township and most people leave after their term of

employment has ceased. Gunbalunya, also known as Oenpelli, is located 60 km

north-north-east of the Ranger mine; it is an Aboriginal township 15 km inside

Arnhem Land from Cahill’s Crossing with a population of approximately 750, the

9

population increases slightly in the wet season as a number of people move from the

surrounding region to the town for shelter and facilities.

Tourist accommodation centres are located within Kakadu National Park at

Cooinda, South Alligator River, Jabiru and Ubirr. Each centre varies in the number

of people that they can accommodate and the peak tourist influx is during the dry

season from May to September. On average some 200,000 people visit Kakadu every

year and spend an average of 4 days in the region. Aboriginal settlements are located

throughout the Alligator Rivers Region and vary in population throughout the year

depending on the season, hunting opportunities and traditional ceremonies.

1.3 Project objectives The aim of this project is to investigate 222Rn exhalation, 210Pb deposition and excess 210Pb inventories within the wet and dry tropical region of Kakadu National Park,

Northern Territory, Australia. The specific objectives for the project are outlined as

follows:

− Investigate the principal contributing meteorological, geographical &

geological factors that affect the exhalation of 222Rn and deposition of 210Pb;

− Measure the seasonal variations of 222Rn exhalation rates at several sites;

− Measure the seasonal variations of 210Pb depositional rate in both wet and

dry deposition;

− Study the transport of 210Pb through the surface layers of the soil through

the measurement of radionuclides in soil samples;

− Model the 210Pb budget in the Kakadu region.

This project was performed in collaboration between eriss and QUT and a

work plan was established at an early stage of the project. The details of site

selection and schedule of measurements are found in Chapter 3 while information

regarding all equipment and methodology used throughout the project is covered in

Chapter 4.

10

2

Literature Review: Previous research in relation to radon emanation, migration, exhalation and 210Pb deposition

2.1 Overview The aim of this chapter is to provide details on the type of work previously reported

in this field and establishes that the work presented in this project is unique. Since its

discovery radon emanation, migration, exhalation and the deposition of its progeny

has been studied extensively. A comprehensive review on the process of radon

emanation and migration through soils and rocks has been performed previously

(Tanner 1964; Tanner 1980). These reports are referred to in relation for work

performed up to 1980 while this review mainly covers research performed after that

date.

Studies of 210Pb deposition date back to the early 1960’s and a compilation of

global results are maintained and updated by the Laboratory of Glaciology and

Geophysics of the Environment, France (Preiss et al. 1998). They provide results for

the majority of 210Pb deposition studies performed over the last 40years along with a

list of all relevant publications.

2.2 Radon emanation 2.2.1 Introduction Emanation of radon atoms is defined as their ejection from their source material

sometime after the radioactive decay of the parental radium isotope. The process of

emanation for the isotopes 220Rn and 219Rn is the same as that for 222Rn, with

recoiling ranges and diffusion lengths differing due to different alpha energies and

half-lives. Studies and models of radon emanation have been performed by a number

of researchers with some work concentrating on physical factors shown to affect its

emanation such as soil moisture and porosity; pore size and number; radium

concentration and distribution; grain size and shape; atmospheric and soil

temperature as well as atmospheric pressure (Markkanen and Arvela 1992; Baixeras

et al. 2001; Morawska and Phillips 1992; Morawska and Jefferies 1994; Mosley et al.

1996; Schumann and Gundersen 1996; Semkow 1990; Fleischer 1987; Menetrez et

al. 1996; Amin et al. 1995; Gan et al. 1986; Greeman and Rose 1995; Sun and

11

Furbish 1995; Maraziotis 1996; Iskander et al. 2004; Grasty 1997; Strong and Levins

1982; Goh et al. 1991; Barton and Ziemer 1986; Misdaq et al. 1998; Holub and

Brady 1981; Nazaroff and Nero 1988; Megumi and Mamuro 1974; Landa 1987a).

A large number of studies have been performed that simply report

measurement techniques and radon emanation rates for various materials including

uranium ores, tailings, construction materials, rocks, soils and minerals (Rogers and

Nielson 1991; Ferry et al. 2002a; Martino and Sabbarese 1997; Bossew 2003; Ho

and Weng 1981; Quindos et al. 1994; Singh and Ghuman 1988; Howard et al. 1995;

McCorkell 1986; Funtua et al. 1997; Holdsworth and Akber 2004; McCorkell et al.

1981; Barillon et al. 1991; Beckman and Balek 2002; Klein et al. 1995; Chao et al.

1997; Chao and Tung 1999; Burke et al. 2003; Bigu and Hallman 1993; Landa

1987b; Zahorowski et al. 1994; Sonter et al. 2002; IAEA 1992). This review focuses

on radon emanation from natural materials but the process of emanation is similar for

all materials.

Radon is created after the radioactive disintegration of its parent, an isotope

of radium. Under normal conditions radium, and all the isotopes in the decay chain

before it, are in a solid phase. While some of these isotopes are slightly soluble in

water in general radium will be produced at the site of the original uranium or

thorium atom. If this is deep within a medium then radon will be produced within

that medium and remain trapped. For radium close to or on the surface of a medium

there is the chance that some of the radon produced might escape into the interstitial

space with the momentum of recoil from radioactive decay. The fraction of radon

atoms that escape from a medium into the interstitial space is known as the

“emanating power” or “emanation coefficient”, this is dimensionless being the

fraction of emanating radon atoms to total radon atoms. For soils Tanner (1964,

1980) describes that upon the creation of a radon atom one of the following three

things are likely to occur as shown in Figure 2.1;

− It will travel a short distance within the grain and become embedded

within the grain (A);

− It can travel across the interstitial space between grains and become

embedded in another grain (C & D); or

− It is released into the interstitial space where diffusion and transport

mechanisms migrate the radon (B & E).

12

Figure 2.1: Fate of 222Rn nucleus just after 226Ra-222Rn transmutation, R is the recoil range of 222Rn nucleus in solid material. A: Recoil and embedding in same grain. B: Recoil, ejection from grain and stopping in interstitial space C: Recoil, ejection from grain, crossing air gap and embedding in neighbouring grain. D: Recoil, ejection from grain and embedding in neighbouring grain. E: Recoil and stopping in water in the interstitial space.

The recoil range of a 222Rn atom after the disintegration of 226Ra are given as

30-50 nm for solids, 95 nm in water and 64000 nm in air (Tanner 1980; Greeman and

Rose 1995). The diffusion coefficient of radon through solid materials is of the order

of 10-20 cm2.s-1, so considering 222Rn with a 3.8 day half-life any produced deep

within a soil grain are most likely to disintegrate close to its position at the end of

recoil after 226Ra transmutation. This means that 222Rn will only emanate from a thin

layer (30-50 nm) on a soil grains surface as recoil after 226Ra transmutation is the

dominant emanation mechanism.

2.2.2 Radon emanation and radium distribution The distribution of radium in soil grains are described as the key factor that

affects radon emanation (Greeman and Rose 1995; Semkow 1990; Schumann and

Gundersen 1996; Morawska and Jefferies 1994; Morawska and Phillips 1992;

Holdsworth and Akber 2004; Nazaroff and Nero 1988; Landa 1987b; Landa 1987a).

R

B

CD

E

A

13

Original models of radon emanation assumed that radium distribution was

homogeneous throughout the grain and that these grains were also spherical

(Morawska and Phillips 1992). This does not represent the common situation where

uranium and radium mobilization result in a higher proportion of radium deposited

on the surface of soil grains (Morawska and Phillips 1992; Schumann and Gundersen

1996; Greeman and Rose 1995). For materials with a homogenous distribution of

radium it is expected that the emanation coefficients would be low compared to those

with a surface distribution. Materials exhibiting emanation coefficients higher than

expected for a homogenous distribution supports the conclusion that radium in those

materials are most likely close to surface distributed. In a study on the emanation of 220Rn from samples of monazite and zircon Holdsworth and Akber (2004) provided

an example of the variation in 222Rn emanation rates from different 226Ra

distributions. As may be seen in Figure 2.2, thorium (and hence 228Ra) distribution of

the two types of grains are different, monazite has a uniform distribution while

zircon has specks of thorium mainly on smaller sized grains. The resulting

experimental emanation coefficients for the two minerals was reported as

(9.0±2.6)*10-4 and (4.1±1.9)*10-2 for monazite and zircon respectively.

Fractures and fissures on the surface of a grain, referred to as pores, from

previous radioactive decays, chemical or weathering effects, effectively increase the

surface area of the grain and can increase its emanation coefficient by up to a factor

of two (Schumann and Gundersen 1996; Semkow 1990; Morawska and Phillips

1992). These pores provide more surface area and radium trapped within the soil

grain may now be in close proximity to the interstitial space created by these pores.

These pores will have a greater effect on the emanation coefficient for material with

a homogenous radium distribution compared with those with a surface distribution.

Further details of the effect of these pores are covered in section 2.2.5.

14

Figure 2.2: On right scanning electron micrograph of monazite (top) and zircon (bottom). On left thorium distribution in the same grain (Holdsworth and Akber 2004)

While radium distribution is certainly the key factor affecting the radon

emanation coefficient of materials other factors such soil moisture, porosity,

temperature, grain size and atmospheric pressure are all known to influence it. The

combined effect of all these variables explains large variations in the emanation

coefficients observed for various materials.

2.2.3 Radon emanation and soil moisture After radium distribution the soil moisture content has been described as the

next most important factor affecting the radon emanation coefficient. Water filling

the pores of grains and the interstitial space between grains shortens the range of

radon atoms exiting the grain. This will stop radon atoms from embedding

themselves into neighbouring grains or crossing pores to embed in the same grain.

Since radon is soluble, water may also aid in the release of radon trapped in pores on

15

the surface of a grain caused by atoms embedding into the grain (B and D in Figure

2.1) (Fleischer 1987; Tanner 1980; Morawska and Phillips 1992; Semkow 1990).

The effect of moisture content on radon emanation coefficients has been

reported extensively (Tanner 1964; Tanner 1980; Menetrez et al. 1996; Greeman and

Rose 1995; Markkanen and Arvela 1992; Fleischer 1987; Grasty 1997; Sun and

Furbish 1995; Megumi and Mamuro 1974; Mosley et al. 1996; Strong and Levins

1982; Goh et al. 1991; Barton and Ziemer 1986; Semkow 1990; Nazaroff and Nero

1988). All work on the subject has shown that small amounts of moisture increase

the radon emanation coefficient but continual increase in moisture eventually reduces

the measured emanation coefficient (Schumann and Gundersen 1996; Menetrez et al.

1996; Fleischer 1987; Markkanen and Arvela 1992; Megumi and Mamuro 1974;

Barton and Ziemer 1986). This effect is shown in Figure 2.3 and Figure 2.4 taken

from two of the studies on the topic. The amount of moisture that produces the

maximum level of emanation is dependent on the type of soil, grain size and its

porosity.

It needs to be noted that the reduction in emanation reported by most

literature and displayed in Figure 2.3 and Figure 2.4 is actually an artefact of the

measurement technique. Measurement of radon in these studies requires it to be in its

gaseous phase and not in a dissolved state. The emanation coefficient is the

proportion of radon atoms liberated into the interstitial space regardless of the

medium filling that space. So in essence any increase in moisture increases the

emanation coefficient till it reaches a steady state and the reductions experimentally

observed are a measurement limitation. Moisture ensures that there is less radon

available in the gas phase and this will have a direct impact on the radon

concentration and migration. There is general agreement on the topic that it only

requires a small amount of moisture filling the interstitial space to increase

emanation. The reduction in emanation coefficient seen with higher moisture

contents is attributed to a reduction in the diffusion of radon trapped in the water.

16

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30

Water Content (%)

Em

anat

ion

Coe

ffici

ent (

%)

0.074mm 0.125-0.25mm1-2mm 2-4mm2-4mm

Figure 2.3: Emanation coefficient as function of increasing water content for sample of till sieved into various grain sizes. (Adapted from Markkanen and Arvela (1992))

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100

Moisture Saturation (%)

Em

issi

on R

atio

Rn-222

Figure 2.4: The ratio of the saturated emanation coefficient to dry emanation coefficient for increasing moisture (Emission ratio). (Adapted from Sun and Furbish (1995))

17

2.2.4 Radon emanation, soil porosity and grain size Both soil porosity and grain size are known to affect radon emanation

(Misdaq et al. 1998; Markkanen and Arvela 1992; Amin et al. 1995; Maraziotis

1996; Baixeras et al. 2001; Landa 1987b; Schumann and Gundersen 1996; Tanner

1980; Morawska and Phillips 1992; Barton and Ziemer 1986; Semkow 1990;

Greeman and Rose 1995; Sun and Furbish 1995; Nazaroff and Nero 1988). Porosity

is defined as the ratio of empty space volume to solid material volume. It should be

noted that grain size and interstitial space are interrelated as grain size increases so

does interstitial space, however this does not necessarily increase porosity as the

increases may balance out. Smaller sized grains may also have larger than expected

values of porosity especially if the grains are filled with pores providing more

interstitial space. Porosity also depends upon the amount of compaction of the

material. An increase in interstitial space, hence porosity, means that radon has a

greater chance of stopping in the interstitial space after ejection from a grain. It has

been shown in models and experiments that the radon emanation coefficient is

directly proportional to porosity (Sun and Furbish 1995; Misdaq et al. 1998;

Maraziotis 1996).

While increasing grain size increases the interstitial space, proportionally less

radon will escape from large grains due to the smaller surface area to volume ratio.

Hence larger emanation coefficients are observed for smaller grain sizes. Published

results on the topic from both modelling and experiments shows that the radon

emanation coefficient is inversely proportional to the grain size (Morawska and

Phillips 1992; Semkow 1990; Amin et al. 1995; Markkanen and Arvela 1992;

Greeman and Rose 1995; Maraziotis 1996; Barton and Ziemer 1986; Tanner 1964;

Baixeras et al. 2001). In their models Maraziotis (1996) made the assumption that

radium was distributed homogenously while Semkow (1990) looked at surface

distribution. Morawska and Philips (1992) modelled both aspects while Greeman and

Rose (1995) calculated that surface distributed radium provided higher emanation

coefficients compared with same sized grains with a homogenous distribution, shown

in Figure 2.5.

18

0

20

40

60

80

100

0.01 0.1 1 10 100

Particle Diameter (mm)

Em

anat

ion

Coe

ffici

ent (

%)

Surface Ra Homogeneous Ra

Figure 2.5: Emanation coefficient for increasing grain size and differences between radium distribution for natural samples. Surface Ra has thickness equal to the recoil range (40 nm). (Adapted from Greeman and Rose (1995))

All work on the topic agrees that regardless of radium distribution an inverse

relationship between radon emanation and grain size exists. The recoil range of 222Rn

in solid materials is approximately 40 nm, which may be smaller than the thickness

of the 226Ra surface distribution of up to a few micrometres. Perhaps it is because of

this reason that authors have observed decreasing emanation coefficients with

increasing grain sizes even for situations where 226Ra is distributed more on the grain

surface.

2.2.5 Radon emanation, pore size and number To date there has been no published quantitative experimental work

examining the effect of granular pores on radon emanation. This is likely due to

difficulties in quantifying the size and number of pores of various types of material.

Three publications have studied the effect through various forms of modelling and

examined pore size, number and type of radium distribution (Morawska and Phillips

1992; Semkow 1990; Maraziotis 1996).

Granular pores increase the amount of surface area on a grain providing radon

with more opportunity to escape thus increasing the emanation coefficient (Semkow

1990; Morawska and Phillips 1992). The model produced by Semkow (1990)

examined the effect that the number and distribution of pores had upon emanation.

This was performed through a comprehensive analysis of various pore sizes and their

19

distribution on a grain. The results showed that high emanation coefficients were

proportional to high pore densities as pores provide more open space for radon to

stop in the interstitial space (Semkow 1990).

Radium distribution is related to the effect that surface pores have on radon

emanation. If radium is homogenously distributed throughout a grain, then granular

pores will provide access that allows radon to emanate. If the radium is surface

distributed then pores have minimal effect on the emanation coefficient unless the

radium is surface distributed within the pores as well (Morawska and Phillips 1992;

Semkow 1990).

Morawska and Philips (1992) produced four models examining variations in

radium distribution and the effect of inner particle porosity (grain with cracks and

fissures). The first two models examined variations in the radium distribution with no

inner porosity while the latter two examined the same two types of radium

distributions but this time in the presence of inner particle porosity. This study also

examined the effect of grain size. Results from the work agree well with those

published by Semkow (1990) in that particles with higher inner porosity (pores) had

higher emanation coefficients. This was seen to be consistent over a range of grain

sizes.

Maraziotis (1996) produced a model based upon homogeneous distribution of

radium within a grain using cylindrical pores. They were particularly interested in the

relationship between particle radius and the size of pores on the grains. Their results

show that as the pore radius increases the emanation coefficient decreases for

constant particle radius. However as the distribution of the pores increases, thus

increases the porosity of a grain, the emanation coefficient also increases which is in

agreement to the work performed by both Semkow (1990) and Morawska and

Phillips (1992).

The real situation may be entirely different to the results of the models above

since it has been shown that radium distribution is generally on the surface, that

pores on a grain may vary in shape and size and the distribution of pores may vary

from one grain to another even in the same material. It is clear that for a

comprehensive understanding of emanation from samples that measurements of the

grains average size, shape, radium distribution, pore size and pore distribution should

be performed (Morawska and Phillips 1992).

20

Radon atoms that embed themselves into neighbouring grain produce recoil

pore up to 30-50 nm depth and 1 nm in diameter. The energy involved in the recoil is

enough to melt the solid material in the pore. It was originally thought that the radon

atom would be trapped in this pore, as the thermal conductivity coefficient for

cooling was several orders of magnitude greater than the diffusion coefficient.

However Tanner (1980) continues by mentioning that experiments show that there is

a brief period of time where the solid material is vaporized and the thermal

conductivity coefficient is comparable with the diffusion coefficient so it is possible

that the radon will escape back into the interstitial space.

2.2.6 Radon emanation and soil temperature Radon emanation from soils is proportional to temperature (Stranden et al.

1984; Gan et al. 1986; Markkanen and Arvela 1992; Iskander et al. 2004; Goh et al.

1991). This occurs because the physical sorption of gases onto solids is temperature

dependent. As temperature increases gases desorb from solid materials and become

freely available. This effect has been clearly demonstrated for radon absorption on

charcoal.

Markkanen and Arvela (1992) noted that the variation in emanation coefficient due

to temperature was much greater in dry samples than samples with moisture in them.

Desorption of radon as a result of temperature increase will release it into the

interstitial space, as it is soluble in water the measured increase in emanation from

moist samples is less than that of dry samples although the actual increase would be

the same. It has been noted (Stranden et al. 1984) that the effect of temperature is not

as large as the effect of moisture on the emanation coefficient. That is, for the range

of ambient conditions, moisture in the interstitial space increases emanation greater

than desorption by an increase in temperature. The most recent of these studies

(Iskander et al. 2004) provides a function for the determination of emanation

coefficient across temperature range (-20 oC to 40 oC). A result of the work

performed by Iskander et al. (2004) is a function usable for both dry and wet soils,

provided in Equation 2-1.

E = 0.21T + 14.8 (R2 = 0.98) Equation 2-1

21

Where E – Emanation coefficient (%) T – Temperature (oC) 2.2.7 Variations in emanation coefficients for radon isotopes

Most of the work mentioned in Sections 2.2.2-2.2.6 are concerned with 222Rn

although a small amount includes 220Rn (Howard 1987; Howard et al. 1995; Martino

and Sabbarese 1997; Sun and Furbish 1995; Greeman and Rose 1995; Megumi and

Mamuro 1974). No work has been performed for the emanation of 219Rn due to

measurement difficulties given the isotope’s short half-life (3.96 s). Other than recoil

and diffusion ranges the process of radon emanation should be similar for the three

natural radon isotopes. Studies that have examined both 220Rn and 222Rn emanation

found that 220Rn emanation coefficients are approximately 10% less than those of 222Rn (Megumi and Mamuro 1974; Greeman and Rose 1995; Sun and Furbish 1995).

We know that 220Rn atoms have more kinetic energy as a result of the decay

process compared to 222Rn atoms. As a result 222Rn atoms have a larger range in all

media. Tanner (1964, 1980) reports that the range in air of the various radon isotopes

is 64 μm for 222Rn, 83 μm for 220Rn and 92 μm for 219Rn. The chances of a 220Rn or 219Rn atom embedding into a neighbouring grain after being expelled from its

original grain are much greater than a 222Rn atom. However greater range means that

there should be a greater contribution from deeper within the grain and that the two

effects should balance out. This would be the case for homogeneously distributed

radium but as it has been pointed out previously that radium is generally surface

distributed in natural materials. This is one explanation why 220Rn emanation

coefficients are smaller than those for 222Rn in soils; similarly 219Rn emanation rates

would be even smaller. The situation would be further complicated if there are

differences in the soil grain distribution of 224Ra and 223Ra compared to 226Ra but this

has not been studied.

The model by Sun and Furbish (1995) predicted that an increase in porosity

increases 222Rn emanation greater than 220Rn emanation but moisture would have a

similar affect on both. Increased porosity provides more surface area for emanation

but the increased range of 220Rn atoms mean they are more likely to cross a pore and

embed into the other side. However the reduction in recoil range due to the presence

of water is enough to stop both isotopes and increase emanation similarly. They also

made a comparison of their theoretical results to the existing experimental results and

found a good correlation between them.

22

Overall the large amount of studies performed examining radon emanation

rates can be grouped; soils, rocks and man made materials. Typically soils have 222Rn

emanation coefficients between 0.1-0.3 while rocks are generally less, 0.01-0.1, as a

result of their denser packed structure and low porosity, man made materials vary

greatly depending upon the characteristics of the material.

2.3 Radon migration, exhalation and soil gas concentration 2.3.1 Introduction

The fraction of radon atoms that reach the interstitial space become subject to

diffusion and transport mechanisms at work within the soil. The type of mechanism

that predominates depends on the type of soil, its moisture content and the

underlying geological conditions. Transport is a pressure driven flow of soil gases

that can be described by Darcy’s law while diffusion is the flow of soil gases due to a

concentration gradient and is described by Fick’s law.

Radon close to the soil surface boundary will diffuse into the atmosphere.

This process is known as exhalation. The depth from which radon is removed from

the soil into the atmosphere depends on the type of soil, its moisture content, the

isotopes half-life and underlying geology. For 222Rn the depth is generally about 1-2

m in unsaturated soils, deeper for sands and shorter for saturated and compacted soils

(Tanner 1964; Holdsworth and Akber 2004). Diffusion depths for 220Rn and 219Rn

are greatly reduced due to their shorter half-lives.

The process of radon emanation, migration and exhalation is sketched in

Figure 2.6. For unsaturated, undisturbed rocks and soils, diffusion is the dominant

mechanism for radon migration as once released into the interstitial space it follows

the concentration gradient and moves towards the surface (Tanner 1964). Later

Tanner (1980) describes that in unsaturated, fractured rocks and disturbed soils,

transport can become the dominant mechanism for radon migration but it is

dependent on the geological conditions such as the amount of fracturing or

disturbance. In saturated rocks and soils, transport is the dominant mechanism for

radon migration as radon is dissolved in the water and moves with it.

23

Figure 2.6: Radon emanation, migration and exhalation

There have been a large number of studies published involving measurements

of radon exhalation and its soil gas concentration; they can be divided into the

following main categories:

− Equipment for exhalation measurements;

− Exhalation rate surveys; and

− Studies examining the influencing physical factors and temporal

variations etc.

The following physical factors are shown to influence the exhalation of radon

into the atmosphere:

− Soil moisture;

− Atmospheric pressure;

− Soil temperature;

− Atmospheric temperature; and

− Wind speed.

Radon-222 migration and the physical factors that influence it can be studied

comprehensively by simultaneously taking 222Rn soil gas concentration

measurements along with measurements of the physical factors mentioned

previously. A number of studies have reported this type of work and are mentioned

in section 2.3.4.

3: Rn exhalation Soil surface

Interstitial space

Soil grain

2: R

n m

igra

tion

1: Rn emanation

24

2.3.2 Radon exhalation measurements techniques There is a large amount of published literature devoted to various types of radon

exhalation measurement techniques (Harley 1992; Escobar et al. 1999; Keller and

Schutz 1988; Martino et al. 1998; Kearney and Krueger 1987; Davey 1995; Abu-

Jarad 1989; Barillon et al. 1993; Bigu and Elliot 1994; Bland and Norlander 1988;

Dave and Lim 1982; Kirichenko 1970; Lehmann et al. 2003; Ielsch et al. 2002;

Nikezic and Urosevic 1997; Gan et al. 1983; Countess 1976; Akber et al. 2002; Bigu

1984; Abdelrazek 1984; Savvides et al. 1985; Zahorowski and Whittlestone 1996;

Labed et al. 1994; Balcazar et al. 1999; Lawrence 2001; NCRP 1988; Khan et al.

1980; Akber et al. 1980; IAEA 1992). Techniques are firstly classified as either

passive or active. Passive systems have no electrical components and rely upon the

natural properties of radon and/or its progeny. Active systems do have electrical

components and are based upon scintillation chambers coupled to photomultiplier

tubes or silicon surface barrier detectors.

Radon-220 is known to contribute significantly in measurements of radon

exhalation and a variety of methods exist to either distinguish or eliminate 220Rn

contribution. Any contribution of 219Rn is considered to be negligible since most

measurement techniques are designed in such a way that 219Rn will decay before the

measurement takes place.

It is not the aim of this review to provide a detailed analysis on the techniques

that are available for radon exhalation measurements. Details of the techniques used

for the measurements performed in this project are found in Chapter 4.

2.3.3 Radon exhalation surveys The exhalation rates of 222Rn and 220Rn from various sources such as soils,

mineral sands, rocks, construction materials, uranium tailings and uranium ores have

been the subject of numerous studies (Todd and Akber 1996; Mason et al. 1982; Hart

and Levins 1986; Schery et al. 1989; Morris and Fraley 1989; Hutter 1996; Nielson

et al. 1996; Whittlestone et al. 1996; Denagbe 2000; Jha et al. 2000; El-Dine et al.

2001; Sengupta et al. 2001; Sroor et al. 2001; Shweikani et al. 1995; Kerrrigan and

O'Connor 1990; El-Amri et al. 2003; Sharma et al. 2003; Kumar et al. 2003;

Evangelista and Pereira 2002; Al-Jarallah 2001; Somashekarappa et al. 1996;

Whittlestone et al. 1998; Abu-Jarad and Fremlin 1982; Cosma et al. 1996; Cheng and

Porritt 1981; Labed et al. 1996; Singh et al. 1999; Tufail et al. 2000; Oufni and

25

Misdaq 2001; Ramola and Choubey 2002; Oufni 2003; Koarashi et al. 2000; Ferry et

al. 2002b; Jovanovic 2001; Andjelov and Branjnik 1996; Carrera et al. 1997; Ielsch

et al. 2001; Schery and Petschek 1983a; Fleischer et al. 1980; Kvasnicka 1990; Todd

1998; Bollhöfer et al. 2003; Wilkening et al. 1974; Auty and Preez 1994; Schery and

Whittlestone 1986; Megumi and Mamuro 1974; Sonter et al. 2002; Lenzen and

McKenzie 1999).

This project is primarily interested in 222Rn exhalation from natural soils and

uranium ore. A global average value for 222Rn exhalation rates from natural soils is

given (UNSCEAR 2000) as 16 mBq.m-2.s-1 taken from work performed by

Wilkening et al. (1974). This survey covered a range of sites throughout New

Mexico, Texas, Hawaii and Alaska but the average is weighted towards the readings

obtained from New Mexico, as this was the largest sample set. Results from a

number of 222Rn exhalation rate surveys, including Wilkening et al, (1974), and

others performed globally are shown in Table 2-1. The large variation in the values

reported indicate that while the UNSCEAR (2000) average value is a reasonable

estimate of global 222Rn exhalation rates it cannot account for all geomorphic types

and is not valid for all seasons. True 222Rn or 220Rn exhalation rates from an area can

only be obtained through direct measurement.

26

Table 2-1: Worldwide reported 222Rn exhalation rates

Reference Location

No.+ Technique* Average Rn-222 Rate#

(mBq.m-2.s-1)

Reference Location No.+ Technique* Average Rn-222 Rate#

(mBq.m-2.s-1)

Wilkening et al. (1974)

New Mexico

180 Acc, Flow, Vert

24±12 Whittlestone et al. (1998)

Tasmania, Australia

Not reported

Eman 31


Texas

9 Acc 10±4.3 Whittlestone et al. (1998)

Tasmania, Australia

Not reported

Eman 15


Hawaii

29 Acc 11±18 Nielson (1996)

Florida, Undeveloped

882 CC 15


Alaska

18 Acc, Flow 6.9±1.8 Whittlestone et al. (1996)

Hawaii

34 Eman 2.6±4.8

UNSCEAR (2000)

All Above

236 All Above 16±13 Nielson (1996)

Florida, Developed

112 CC 48

Somashekarappa et al. (1996)

Kiaga, India

12 Acc 31±19 Auty and duPreez (1994)

Jabiluka Eastern Decline

Not reported

CC 25±5

Sengupta et al. (2001)

Bihar, India

15 SSNTD 1.4±1.1 Auty and duPreez (1994)

Jabiluka Mine Valley

Not reported

CC 46±31

27

*Techniques: Acc-Accumulator, Flow-Flow through scintillometer, Vert-Estimation from vertical profile, Eman-Emanometer, SSNTD-Solid state nuclear track detector, NTD-Nuclear track detector, CC-Charcoal canisters +Number of measurements #Average 222Rn exhalation rate with reported error

Reference Location


(mBq.m-2.s-1)

Reference Location


(mBq.m-2.s-1)

Todd et al. (1998) Jabiru, Australia

22 Eman 64±25 Evangelista et al. (2002)

King George Is.

41 NTD 1.6±1.5

Ielsch et al. (2001)

France 89 Eman 45±22 Schery et al. (1989)

Australia

105 Eman, Acc 22±4.4

28

There is only a handful of literature that reports 222Rn exhalation rates from

ambient Australian soils (Todd 1998; Schery et al. 1989; Schery and Whittlestone

1986; Whittlestone et al. 1998; Lenzen and McKenzie 1999; Auty and Preez 1994).

Of these, the report by Schery and Whittlestone (1986) details 222Rn and 220Rn

exhalation rates from a number of locations throughout Australia and is the most

comprehensive Australian study of 222Rn exhalation to date. Their report provided

Australian averages of 222Rn exhalation rates and was used to estimate Australian

exposure to natural radiation by UNSCEAR (UNSCEAR 1993; UNSCEAR 2000).

The work performed by Todd (1998) was performed in the Alligator Rivers Region

in the Northern Territory, Australia. This work reported a range of 222Rn and 220Rn

exhalation rates from a number of locations around the Ranger uranium mine. The

work carried out by Schery and Whittlestone (1986) also contained readings from

around the Ranger mine and Jabiru region. However both of these studies have only

provided a snap shot of 222Rn exhalation rates from the area. Todd (1998) has

provided diurnal readings but mentioned that further work was required to

understand the parameters affecting radon exhalation rates from a tropical

environment. Auty and duPreez (1994) performed a survey of 222Rn exhalation rates

from North Ranger (Jabiluka). The uranium deposit here is between 20-200 m deep

and mostly covered with sandstone, as 222Rn diffusion lengths are only a few metres

the work can be considered to be on ambient soils. They reported 222Rn exhalation

rates of 46±31 mBq.m-2.s-1 and 25±5 mBq.m-2.s-1 for the mine valley and eastern

decline, respectively. These studies are important as they were performed in the same

region where this project was based. Whittlestone et al. (1998) examined 222Rn

exhalation rates from a number of sites in Tasmania and Victoria they reported

winter and summer values to observe seasonal variations of 222Rn exhalation rates.

More details of temporal variations are covered in section 2.3.9.

A number of studies of 222Rn exhalation rates have been performed from

rehabilitated uranium mines, undisturbed ore-bodies, operational uranium mines and

tailing and ore samples from Australia (Mason et al. 1982; Kvasnicka and Auty

1994; Davy et al. 1978; Kvasnicka 1990; Bollhöfer et al. 2003; O'Brien and

Whittlestone 1981; Hart 1986; Strong and Levins 1982; Akber et al. 2002; Harris and

Chandler 1992; Davey 1994; Sonter 1987; Martin et al. 2002). The report by Mason

et al. (1982) for the 222Rn exhalation from Ranger mine only performed

measurements from the waste rock dump and tailings dam wall. Readings were also

29

performed at Nabarlek and Rum Jungle. A value of +9649-32 mBq.m-2.s-1/%U3O8 was

reported for waste rock material at the three sites. This value corresponds to +0.910.47-0.31 mBq.m-2.s-1(222Rn)/Bq.kg-1(226Ra). Kvasnicka and Auty (1994) were able

to perform measurements on the ground above ore body 3 before mining commenced

but the sample set of three charcoal cups provides poor statistical analysis. They

obtained average pre mining 222Rn exhalation rates of 4.1 Bq.m-2.s-1 and 2.5

Bq.m-2.s-1 for ore bodies 1 and 3 respectively obtained from calculations derived

from experimental results. This report provides the only pre mining 222Rn exhalation

rates for Ranger ore body 3. Previously Kvasnicka (1990) used the results reported

by Mason et al. (1982) to model the exhalation rates from Ranger for an atmospheric

dispersion model. The other studies listed were performed at Nabarlek, Northern

Territory (Bollhöfer et al. 2003; Martin et al. 2002), Koongarra, Northern Territory

(Davy et al. 1978), Lake way, Western Australia (O'Brien and Whittlestone 1981),

Olympic Dam, South Australia (Akber et al. 2002; Davey 1994; Harris and Chandler

1992; Sonter 1987) or were laboratory based using Australian samples (Hart 1986;

Strong and Levins 1982)

Measurement of the 222Rn exhalation rates from various sources at Ranger

uranium mine is important for the modelling of the 210Pb budget within the region.

To date most reports of 222Rn produced by the Office of the Supervising Scientist

have concentrated on the measurement of 222Rn and 222Rn progeny concentrations

within the Jabiru and Jabiru East region as a means of determining mining related

doses received by members of the public in the region (Akber et al. 1991; Akber et

al. 1994b; Akber and Pfitzner 1992; Akber et al. 1994a; Whittlestone 1992; Akber

and Pfitzner 1994; Kvasnicka 1990; Akber et al. 1993; Peterson et al. 1993)

2.3.4 Radon migration, exhalation, soil gas concentration and soil moisture

Similar to radon emanation the most important factor affecting radon

migration and exhalation is soil moisture. Numerous authors have examined the

effects of soil moisture or precipitation on 222Rn exhalation rates and soil gas

concentrations (Tanner 1980; Tidjani 1988; Kraner et al. 1964; Grasty 1993;

Washington and Rose 1990; Shweikani et al. 1995; Hutter 1996; Tanner 1964;

Thomas et al. 1992; Luetzelschwab et al. 1989; Asher-Bolinder et al. 1990;

Whittlestone et al. 1996; Jha et al. 2000; Schery et al. 1989; Ferry et al. 2001; Graaf

30

et al. 1992; Megumi and Mamuro 1974; Stranden et al. 1984; Owczarski et al. 1990;

Koarashi et al. 2000; Ferry et al. 2002b; Hart 1986; Nazaroff and Nero 1988; Todd

and Akber 1996).

Laboratory and field studies show that small amounts of soil moisture

increase 222Rn exhalation, while further increasing the moisture content, reduces it

(Owczarski et al. 1990; Todd and Akber 1996; Megumi and Mamuro 1974; Stranden

et al. 1984; Schery et al. 1989). Peak 222Rn exhalation rates were noted to coincide

with same saturation levels that produce peak 222Rn emanation (Owczarski et al.

1990; Stranden et al. 1984). Stranden (1984) provided an explanation that the

addition of small amounts of water increases emanation. Some of the additional 222Rn atoms that stopped in the pore space will escape from the water and diffuse to

the soil surface. It may be likely that the exhalation to emanation ratio may be

smaller at these saturation levels than for dry soils but this has not been investigated.

Addition of more water to the soil traps 222Rn within the soil and reduces the

exhalation rate. Todd and Akber (1996) study of a monazite sample showed that

three days after simulated rainfall 220Rn exhalation rates increased. The increase

lasted for four days and exhalation was 20% higher than the recorded initial dry

exhalation rate. This behaviour was likely due to the sample surface layers drying out

following the simulated rain event and the moisture content of the sample reaching a

level that increases exhalation as mentioned above.

The ability for soil to retain moisture primarily depends upon the soil

porosity. Soil porosity plays the dominating role in the length of time that 222Rn and 220Rn exhalation rates remain reduced at a site after precipitation. The sooner water

penetrates deeper into the ground or is evaporated from the soil the faster the

recovery to normal radon exhalation rates

The remainder of the studies performed on the topic of radon exhalation and

moisture are based upon field work and most of them are more concerned with

precipitation rather than the soil moisture content (Washington and Rose 1990;

Thomas et al. 1992; Luetzelschwab et al. 1989; Jha et al. 2000; Ferry et al. 2001;

Grasty 1994; Kraner et al. 1964; Shweikani et al. 1995; Koarashi et al. 2000; Ferry et

al. 2002b). The work of these authors has shown that 222Rn and 220Rn exhalation

rates decrease directly after moderate to heavy rain events while soil gas

concentrations increase. The rain events examined in these cases increase soil

moisture levels to values that retard exhalation. It has been shown that regions with

31

low precipitation have high exhalation rates and regions with high precipitation have

low exhalation rates (Wilkening et al. 1974; Washington and Rose 1990; Tidjani

1988; Kraner et al. 1964; Grasty 1993). Soil depth profiles of 222Rn concentration

after precipitation have shown that there is a delay in the increase of 222Rn

concentration at large depths due to the time it takes for moisture to penetrate

through the soil transporting 222Rn with it (Kraner et al. 1964). The depth to which

moisture penetrates the soil has a direct influence on the 222Rn exhalation rate and

concentration at that site (Kraner et al. 1964).

2.3.5 Radon exhalation, soil gas concentration and atmospheric pressure

After soil moisture, atmospheric pressure has been described as being the

most important meteorological condition affecting 222Rn and 220Rn exhalation and

soil gas concentration (Tanner 1980; Tanner 1964). Studies have shown that

increasing atmospheric pressure decreases exhalation rates and increases soil gas

concentrations while the opposite is observed for a decrease in atmospheric pressure

(Wilkening et al. 1974; Fleischer et al. 1980; Edwards and Bates 1980; Schery and

Gaeddert 1982; Schery et al. 1982; Clements and Wilkening 1974; Janssens et al.

1988; Chen et al. 1995; Owczarski et al. 1990; Jha et al. 2000; Koarashi et al. 2000;

Kraner et al. 1964; Thomas et al. 1992). It is noted that these effects are observed in

short periods of time during pressure changes and that after pressure changes pass

exhalation rates return to previous levels.

Kraner et al. (1964) concluded that changes in the depth profile of 222Rn

concentration levels, due to changes in atmospheric pressure, correlated well with

what was theoretically expected from the diffusion of gases from soil media.

Changes in atmospheric pressure will create pressure induced fluctuations of air in

the soil. An increase in pressure will push air down into the soil retarding exhalation

and increasing soil gas concentration. A decrease in pressure draws air out of the

ground increasing exhalation and reducing soil gas concentration. It was noted that it

is important for detection equipment to allow for changes in atmospheric pressure at

the point of measurement.

Clements and Wilkening (1974) designed a simple analytical model to

examine the effect of atmospheric pressure on 222Rn exhalation and compared it with

reported laboratory and field studies. Their model combined the effects of diffusion

32

and pressure induced transport and they concluded that diffusion remained the

predominant migration mechanism. The results obtained from their model agreed

with those reported by Kraner et al. (1964). They stated that the magnitude of change

in 222Rn exhalation was dependent on the magnitude and rate of the pressure change.

That is pressure changes over short periods of time vary the radon exhalation rate

more than similar changes over longer periods. This is because quick pressure

changes influence soil gases more than slower changes that may have little or no

influence on soil gases. The work performed since these early studies serves to

justify the results obtained in them.

Some studies have reported a poor correlation for 222Rn and 220Rn exhalation

rates with atmospheric pressure changes (Tidjani 1988; Schery et al. 1989; Graaf et

al. 1992). They all reported that observed pressure changes were small and that other

variables may be dominant.

2.3.6 Radon exhalation, soil gas concentration and temperature It has been mentioned previously in Section 2.2.6 that 222Rn emanation

increases with temperature. It has been documented and reported that a good

correlation between soil temperature and the 222Rn soil gas concentration also exists

(Washington and Rose 1990; Hutter 1996). Washington and Rose (1990) reported

that changes in soil temperature have less effect on 222Rn concentrations in dry soils

than they do for moist soils. This is due to desorption of radon from solids into the

interstitial space, mentioned in Section 2.2.6. If the interstitial space is filled with air

radon is freely available to diffuse to the surface however if it is filled with moisture

radon will remain dissolved in water.

Natural fluctuations in ambient temperature do not have much of an influence

on 222Rn and 220Rn exhalation as soil moisture and atmospheric pressure. Their is

agreement in studies performed that the increase in exhalation due to temperature is

minor if any (Jha et al. 2000; Stranden et al. 1984; Schery and Petschek 1983a;

Schery et al. 1989; Tidjani 1988; Hutter 1996; Morris and Fraley 1989). In a number

of these studies, where regression analysis has been used, the influence of

temperature on exhalation has been reported with low correlation coefficients. Since

temperature affects the physical properties known to influence radon exhalation and

the diffusion coefficient the net effect that temperature has on radon soil gas

concentrations and exhalation rates is likely due to these other dominant parameters

being affected.

33

2.3.7 Radon exhalation, soil gas concentration and wind speed Wind speed has been reported as theoretically having an effect on 222Rn

exhalation and soil gas concentration (Kraner et al. 1964; Wilkening et al. 1974).

Kraner et al. (1964) reported that increased wind speed would deplete 222Rn from the

upper layers of the ground, increase the concentration gradient and increase the

exhalation rate. Direct measurements for the effects of wind speed are difficult since

common measurement techniques involve covering the sampling area reducing wind

speed to negligible velocities.

Only one recent study has examined the influence of wind speed on

exhalation rates (Jha et al. 2000). They reported that the wind speed variations from

the region studied are not strong enough to cause any considerable change in the

exhalation rate. Also they theorised that even extreme wind speeds would only

produce minor variations in exhalation rate through depletion of soil gas from the

upper layers of the ground.

2.3.8 Radon diffusion theory Fick’s law of diffusion of gases through porous media covers radon migration

through unsaturated, undisturbed rocks and soils (Nazaroff and Nero 1988). It is

shown here as Equation 2-2.

02

2

=+−ε

λρλD

REDC

dxCd

Equation 2-2

Where

C: interstitial concentration of radon isotope (Bq.m-3)

x: soil depth measured from the soil-air interface (m)

λ: radon isotope disintegration constant (s-1)

D: diffusion coefficient of radon isotope (m2.s-1)

E: radon isotope’s emanation coefficient for material

R: parent radium isotope’s activity concentration of the material (Bq.kg-1)

ρ: bulk density of material (kg.m-3)

ε: porosity of the material

The resultant radon exhalation rate at the soil-air interface can then be determined by

⎟⎠⎞

⎜⎝⎛−=

dxdCDJ ε

Equation 2-3

34

where

dxdC − is determined as the solution from Equation 2-2, using the appropriate

boundary conditions Equation 2-3 becomes

⎟⎠⎞

⎜⎝⎛×=

LHLREJ tanhexp λρ

Equation 2-4

where

H: sample thickness (m)

L: diffusion length (m)

with

λDL = Equation 2-5

Radium activity concentration and distribution in the soil primarily affects

radon emanation and has been covered in Section 2.2.2. For 222Rn exhalation, from

ambient undisturbed soils, correlation with 226Ra activity concentration has been

reported as relatively weak due to the other parameters that play a role (Ielsch et al.

2001; Oufni 2003; Sharma et al. 2003; Sroor et al. 2001). Yet those authors who

have studied 222Rn exhalation rates from both uranium bearing minerals and ambient

soils have reported differences in 222Rn exhalation rates of orders of magnitude

(Todd 1998; Schery et al. 1989). Migration of radon is negligible to the radon soil

gas concentration at a given point and it has been shown that the relationship with

radium activity concentration is stronger than that seen between radon exhalation and

radium activity concentration (Luetzelschwab et al. 1989; Asher-Bolinder et al.

1990; Grant et al. 2001; Choubey et al. 1999)

The diffusion length of 222Rn in undisturbed soils is approximately 1-2 m as

previously mentioned in Section 2.3.1. As such areas where soil cover is shallow or

areas where soil is porous, the underlying geological rock formations may have a

direct influence on the 222Rn exhalation rates observed. This is especially the case if

the 226Ra concentration of the rock formations differs greatly from the overlying soil.

A number of studies have been performed that examine underlying geology and

uranium concentration variations and their relationship to the 222Rn exhalation rate

and soil gas concentration (Choubey et al. 1999; El-Dine et al. 2001; Ielsch et al.

2001; Ramola et al. 1988; Sengupta et al. 2001; Varley and Flowers 1992; Akber et

al. 1980; Khan et al. 1980). These types of studies include the determination of

35

underlying uranium mineralisation and detection of ore bodies through enhanced

exhalation rates. Tanner (1980) reported that in fractured rocks and disturbed soils

transport mechanisms become the dominant method for 222Rn and 220Rn migration

meaning that Fick’s law presented above in Equation 2-2 no longer holds for these

conditions.

Ielsch et al. (2001) examined the variations in 222Rn exhalation rates and atmospheric

concentrations at the soil atmospheric boundary over two transects from coastal to

inland France. Variations in geology and uranium concentration over the two

transects were recorded and compared with the 222Rn measurements. A number of

uranium rich granites were located along these transects and it was found that while

increased 222Rn exhalation rates were positively correlated with increased uranium

concentration it was not the only contributing factor. Other factors such as fractures,

variations in depth of soil cover over granite formations and soil type were also

important. Similar studies to this have been performed in India and England and

report that uranium concentration is an important, but not the only influencing factor

for increased 222Rn exhalation (Varley and Flowers 1992; Sengupta et al. 2001;

Choubey et al. 1999).

Studies performed over fault lines and fractured rocks are in agreement with

the conclusions drawn by Tanner (1980) in that transport dominates and high 222Rn

exhalation rates are observed from these areas. This leads into a whole area of study

where 222Rn exhalation measurements have been used for the determination of fault

lines, as an earthquake precursor and to examine the underlying geological

conditions. This topic is beyond the scope of this project and has not been considered

in this review.

Soils with large porosity have more interstitial space and have greater 222Rn

and 220Rn exhalation rates (Shweikani et al. 1995). The increase in exhalation is due

to two factors. Firstly larger interstitial space will increase emanation with a

reduction in the number of 222Rn and 220Rn atoms that embed into nearby grains.

Secondly the larger space in the soil provides an easier diffusion path to the soil

surface. A result of this is that 222Rn and 220Rn will exhale from greater depths in

these types of soils. Shweikani et al. (1995) noted, using a constant moisture value of

14%, there was a non-linear behaviour in the effect of porosity on the 222Rn

exhalation rate compared to the linear relationship expected from Fick’s law,

Equation 2-2. They argued that the effect of moisture created the variation in

36

emanation as having the same moisture content and varying porosity would vary the

actual amount of moisture layer on each grain. This was in agreement with

theoretical emanation modelling reported by other authors (Tanner 1964; Semkow

1990; Maraziotis 1996; Markkanen and Arvela 1992).

A model by Owczarski et al. (1990) demonstrated the effect that a variation in

soil porosity has upon 222Rn exhalation. Even though this work was primarily about

variations in 222Rn exhalation due to changes in soil moisture and atmospheric

pressure, their use of five different soil types with different porosity provides a good

display of the effect that porosity has on 222Rn exhalation. A variation in exhalation

rates between less porous materials (clay) and more porous materials (sand) was

evident in their study.

2.3.9 Radon exhalation temporal variations Precipitation, soil moisture, atmospheric pressure and temperature vary

temporally. Systematic variations may occur with the time of day and between

seasons. A number of studies have been performed that measure 222Rn and 220Rn

exhalation rates and soil gas concentrations over time to observe changes resulting

from variations of these parameters (Jha et al. 2000; Wilkening et al. 1974;

Whittlestone et al. 1998; Segovia et al. 1987; Thomas et al. 1992; Ferry et al. 2002b;

Ferry et al. 2001; Washington and Rose 1990; Hutter 1996; Torri et al. 1988; Tidjani

1988; Winkler et al. 2001). Most of these studies however focus on measurement of 222Rn concentrations in soil gas and only three of them are on 222Rn exhalation rates

(Whittlestone et al. 1998; Wilkening et al. 1974; Jha et al. 2000). Even though there

is typically an inverse relationship between the two the lack of seasonal exhalation

studies leaves a gap in the body of knowledge.

Only one study (Jha et al. 2000) has been performed within a tropical region,

Jaduguda, India, which lies just south of the Tropic of Cancer. The study was

performed within a uranium mineralised zone and reports good correlations of 222Rn

exhalation rates with soil temperature and atmospheric concentration values. They

observed only minor fluctuations in the overall seasonal results and conclude that the

composite influence of the meteorological parameters masked the effect of individual

parameters. Wilkening et al. (1974) only performed 222Rn exhalation measurements

from winter through to spring at a site in Rio Grande valley, Socorro, USA. This was

not a long enough time scale to observe any major seasonal variations in the radon

exhalation rate. Finally, Whittlestone et al. (1998) performed comparison

37

measurements for different seasons. The study was based on data measured from

sites in Victoria and Tasmania. They reported reduced 222Rn and 220Rn exhalation

rates in winter. This was related to the fact that the rainfall rates were higher during

winter. Winter minima of exhalation rates in US and European studies have been

related to snow cover and frozen ground which restricts exhalation from the ground

(Washington and Rose 1990; Winkler and Rosner 2000).

Temporal variations in soil gas concentrations have been observed at many

locations (Wilkening et al. 1974; Washington and Rose 1990; Hutter 1996; Winkler

et al. 2001; Torri et al. 1988; Tidjani 1988). Of particular interest is the study by

Tidjani (1988) that has been performed in the African tropical country of Senegal.

The author did not find seasonal variations in 222Rn soil gas concentrations at the

onset of the wet season. They commented this was contrary to what they expected

given the reported retardation of 222Rn with soil moisture. Again Wilkening et al.

(1974) measurements over a few months aren’t enough to justify any seasonal

variations although they did report variations up to 30% that correlated well with

barometric pressure changes, these measurements took place in New Mexico.

Washington and Rose (1990) observed winter minima and summer maxima of 222Rn

soil gas concentrations from three years of data collected from sites in Pennsylvania

and New Jersey. This was in contrast to what had been reported by other authors

from different locations, such as those performed in New Jersey and Germany

(Winkler et al. 2001; Hutter 1996), but it was in agreement with results obtained

from an Italian study (Torri et al. 1988). Washington and Rose (1990) concluded that

winter minima and summer maxima were due to the seasonal variations in the

rainfall, soil moisture, atmospheric pressure and temperature. Another study of 222Rn

soil gas concentrations from New Jersey (Hutter 1996) reported spring to summer

lows and autumn to winter highs and agree well with exhalation studies. A study of

the seasonal 222Rn soil gas concentration in Germany (Winkler et al. 2001) also

reported spring to summer lows and autumn to winter highs. Peaks observed in 222Rn

soil gas concentrations over the summer months correlated well with rainfall patterns

that saturate the soil and prevent 222Rn migration.

Of note is that there has been little work performed on seasonal variations in 222Rn and 220Rn exhalation rate from open ground and the main focus has been on

soil gas concentration measurements. This is perhaps because soil gas concentration

is more related to 222Rn entry into homes. It is most likely that seasonal variations in

38

exhalation rates are also as broad as those observed for soil gas concentration

measurements and may be site specific.

Revision of the above studies was important for this work as the region this

work was performed in experiences distinct wet and dry seasons. From theory and

the studies previously published it was expected that 222Rn exhalation rates in the

Kakadu region should decrease throughout the wet season.

2.3.10 Radon migration, exhalation and soil gas concentration summary

There is wide and varied literature dealing with the subject of radon migration,

soil gas concentration and exhalation. Some of this work focuses on measurement

techniques but a large portion examines various soil and meteorological parameters

known to influence exhalation, migration and soil gas concentrations. There is an

agreement in all work that soil moisture plays the most dominant role in affecting

radon exhalation because of its solubility and thus exhalation is retarded after

precipitation. Moisture has a capping effect and traps radon in the soil before

sweeping it through the soil as the moisture passes downwards. The capping effect

dominates and radon soil gas concentrations increase after precipitation. The other

parameters that have been noted to influence radon are atmospheric pressure; soil

and atmospheric temperature; wind speed; radium activity concentration and

distribution; soil grain size, porosity, compaction and underlying geological

formations. The effect of most of these has been examined through either laboratory

or field measurements. It appears that the magnitude of influence of these parameters

varies from site to site and they are also seen to combine and mask out the effects of

other parameters. It has been noted that there is a lack of knowledge about the

seasonal dependency of radon exhalation that needs to be further investigated. There

is also a lack of knowledge about the composite behaviour of the influencing

parameter that effect radon exhalation.

A number of studies examined include measurements of both 222Rn and 220Rn.

This is a result of improved detection equipment that provides a means of

distinguishing between the contributions of the two isotopes for simultaneous

measurements. A number of the studies mentioned above have provided both 222Rn

and 220Rn exhalation rates from their work (Schery and Petschek 1983b; Bigu and

Elliot 1994; Zahorowski and Whittlestone 1996; Schery et al. 1989; Whittlestone et

39

al. 1996; Schery and Petschek 1983a; Whittlestone et al. 1998; Megumi and Mamuro

1974; Todd and Akber 1996). The same geological, meteorological and seasonal

factors that affect 222Rn exhalation rates also affect 220Rn exhalation rates in a similar

manner with a variation in the magnitude of the effect only. It can only be assumed

that they also affect 219Rn exhalation rates but measurement of this isotope in all but

laboratory conditions is difficult.

2.4 Pb-210 deposition 2.4.1 Introduction

Radon-222 atoms exhaled from the ground spread into the atmosphere where they

remain until they decay. Most of radons direct progeny, 218Po, is electrically charged

(~88%), a result of the decay process, and a large proportion (~90%) become

attached to aerosol particles (Pagelkopf and Porstendorfer 2003). The processes

involved in attachment has been the subject of many studies (Bandi and Phillips

1988; Bouland and Chouard 1992; Cheng et al. 2000; Hopke 1996; Morawska and

Jamriska 1997; Planinic et al. 1997; Porstendorfer et al. 2000; Tokonami 2000;

Winkler et al. 1998; El-Hussien and Ahmed 1995). It is reported that the attached

fraction attaches to aerosol particles with an average size distribution in the range

0.07-1 μm (Winkler et al. 1998; Morawska and Jamriska 1996; Tu et al. 1994;

Porstendorfer et al. 2000) while the unattached fraction forms clusters with trace

gases and vapours in air with a size spectrum between 0.5-3 nm (Pagelkopf and

Porstendorfer 2003). The half-lives of the direct progeny of 222Rn are such that

within 4 hours more than 99% have decayed into 210Pb. Meanwhile both attached and

unattached fractions are subject to atmospheric transport mechanisms that occur at

the place of their formation (Winkler et al. 1998; Koch et al. 1996). It is reported that

precipitation is the dominant scavenger of the attached fraction (Preiss and Genthon

1997; Patterson and Lockhart 1964), while dry deposition through plate-out and fall-

out removes the unattached fraction (Schery et al. 1992; Lupu and Cuculeanu 1999).

Precipitation scavenging can occur as a result of in cloud rainout of particles within

the rain clouds, or below cloud washout of dust and particles in the atmosphere.

Contrary results have been reported for the mean residency time of 210Pb in

the atmosphere, it has been reported as being between a few days or up to a hundred

days. Residency times of 5-9 days are typically reported for temperate regions (Beks

et al. 1998; Pourchet et al. 2000; Koch et al. 1996; Moore et al. 1977). Even with the

40

broad range of reported residency times they are all shorter than the half-life of 210Pb,

22.3 years. After deposition on the ground 210Pb penetrates into the soil over time to

a depth that depends on soil characteristics such as grain size, porosity, moisture

content and attachment properties.

The natural process of 222Rn progeny attachment, 210Pb deposition and soil

transport has a wide variety of applications including the following:

− For the determination of aerosol movement and residency times in the

lower atmosphere (Preiss et al. 1996; Koch et al. 1996; Winkler et al.

1998; Martin 2003; Lupu and Cuculeanu 1999);

− The examination of radon progeny deposition on indoors surfaces for

health studies (Lively and Ney 1987; Leung et al. 2000; Morawska and

Jamriska 1996; Nikezic and Yu 1999);

− Using existing data to develop models of global sources and deposition of 210Pb (Preiss and Genthon 1997; Preiss et al. 1996);

− For studies of the prevailing meteorological conditions controlling 210Pb

atmospheric concentration and deposition over seasonal changes or

individual events (Rangarajan et al. 1986; El-Hussien et al. 2001; Winkler

and Rosner 2000; Bonnyman et al. 1972; Rosner 1988; Branford et al.

1998; Kim et al. 2000; Beks et al. 1998; Martin 2003; Melieres et al.

2003; Zahorowski et al. 2004; Todd et al. 1989; Baskaran 1995; Baskaran

et al. 1993; Hussain et al. 1990; Turekian et al. 1983);

− To investigate the history of 210Pb deposition and its soil transport through

analysis of soil profiles (Pourchet et al. 2000; Nozaki et al. 1978; Moore

and Poet 1976; Matthews and Potipin 1985; Thomson et al. 2002);

− Investigating the effect of soil surface and geomorphic changes on the 210Pb atmospheric concentration and deposition (Branford et al. 1998;

Patterson and Lockhart 1964; Lupu and Cuculeanu 1999);

− Applied to the measurement of erosion and sedimentation deposition in

water systems (Imboden and Stiller 1982; Zapata 2003; Li et al. 2003;

Walling et al. 2003; Heijnis 1999; Matthai et al. 1998; Bonniwell et al.

1999; Heijnis et al. 1987; Wallbrink and Murray 1993; Belyaev et al.

2004; Ugur et al. 2004; Zhang et al. 2003; Matisoff et al. 2002; Milton et

41

al. 2001; Whiting et al. 2001; Walling and He 1999; Wallbrink et al.

1999; Ivanovich and Harmon 1992; Ivanovich and Harmon 1982).

This study focuses on the deposition of 210Pb.

2.4.2 Pb-210 depositional rate studies Radon-222 exhaled into the atmosphere from land is the only source of 210Pb

to the atmosphere. Currently a global database exists containing the results of

numerous studies of 210Pb atmospheric concentrations, depositional rates and lake

sedimentation rates (Preiss et al. 2003). For depositional rates this database currently

holds results from more than 100 sites over all continents and is the most extensive

collection of 210Pb depositional rate values available to date. While a compiled list of

this work is too large to add to this chapter it is noted that worldwide 210Pb

depositional rates range from 2 Bq.m-2.y-1 (Argentina Islands) to 465 Bq.m-2.y-1

(Kitami, Japan) with an average value of 124 Bq.m-2.y-1. Of note is that values

reported for the Northern Hemisphere are larger than those for the Southern

Hemisphere due to larger landmass, hence 210Pb source, found in the Northern

Hemisphere.

Some of the major contributors to this database have performed

measurements of 210Pb deposition for many years investigating seasonal variations

and related them meteorological patterns (Baskaran 1995; Winkler and Rosner 2000;

Melieres et al. 2003; Su et al. 2003; Rosner 1988; Baskaran et al. 1993; Turekian et

al. 1983; Todd et al. 1989; Beks et al. 1998; Bonnyman et al. 1972; Kim et al. 2000).

Others have investigated deposition over shorter periods of time or even through

individual rain events (Martin 2003; Caillet et al. 2001; Pettersson and Koperski

1991; Fujitaka et al. 1992). It is best to understand the effects occurring within

individual rain events before broadening the concepts to seasonal studies.

It has been mentioned that rainfall is the dominant scavenger of 210Pb from

the atmosphere. This occurs through two methods, “in-cloud rainout” and “below-

cloud washout” (Martin 2003; Caillet et al. 2001). In-cloud rainout refers to the

removal of 210Pb attached to aerosols that themselves are attached to water molecules

within clouds. Below-cloud washout refers to the removal of attached and unattached 210Pb within the atmosphere collected on rain as it descends. Fujitaka et al. (1992)

showed that near surface 210Pb atmospheric concentrations returned to normal levels

3hours after precipitation events that caused significant washout. That study was

performed in the Chiba region, Japan (35oN) but it was noted that the region receives

42

monsoonal rain patterns. Caillet et al. (2001) performed measurements of 210Pb

deposition from individual rain events at a study site in Switzerland. They noted the

heavier rainfall produced more washout compared to light rain events and reported a

mean reload time of 1-2 days for 210Pb.

A number of measurements from the east coast of the United States have been

made around Chesapeake Bay (Todd et al. 1989; Kim et al. 2000). A report

published by Todd et al. (1989) examined the atmospheric characteristics of 210Pb

along the South Virginia Coast from 1982-1985. Both wet and dry depositional rates

were measured and it was found that wet deposition provided more than 90% of the

total deposition. Their investigation into seasonal variations found spring maxima

and autumn minima for the 210Pb depositional rates. These were attributed to

precipitation events and ocean air mixing respectively. The results were in agreement

with other reports from coastal regions of the United States. Kim et al. (2000)

performed a more recent study of the atmospheric factors that affect the 210Pb

depositional rate at Stillpond in the north of Chesapeake Bay. The study measured 210Pb deposition for one year. They also reported a strong correlation between the

depositional rate and precipitation and a spring maximum and autumn minimum.

They noted that at this location a period of at least two weeks between weak

precipitation events was required to obtain similar depositional rates again.

Martin (2003) analysed uranium and thorium series radionuclides deposited

in rainwater from a number of tropical storms between 1985-1989 at Jabiru and

Jabiru East in the Northern Territory, Australia. Samples were collected from a total

of 16 storms, 9 at Jabiru East and the 7 in the township of Jabiru. This work was of

interest as the area of study was the same as the one used for this project. He noted

that most of the 210Pb depositional rate measured in these storms was sourced from

in-cloud rainout while a ‘substantial fraction’ of the measured activity concentrations

of 238U, 234U, 230Th and 226Ra was due to below-cloud washout of dust transported

from the nearby Ranger mine. He also determined 210Pb residency times and reported

widely ranging values from 0-70 days. It was noted that residency times were smaller

in the mid to late wet season, a result of the constant removal of 210Pb from the

atmosphere due to frequent rain events. For six of the storms he collected sequential

samples and noted that for three of them there was an initial large deposition of 210Pb

that decreased rapidly throughout the remainder of the storm. It was reported as a

result of below-cloud washout of 210Pb created from 222Rn decay in the atmosphere

43

rather than washout of 210Pb attached to suspended dust. He noted that sequences of

light rain tended to have larger 210Pb concentrations due to partial washout. This

finding was in agreement with work previously published (Turekian et al. 1983).

In contrast to the work performed by Martin (2003) Pettersson and Koperski

(1991) measured the dry deposition of 238U series radionuclides on passive vinyl

collectors as a function of distance from Ranger mine. They derived an estimate of

the dry deposition of each long-lived 238U series radionuclide at Jabiru East to be

about 10 Bq.m-2.y-1. From their work a value of 27 Bq.m-2.y-1 for 210Pb can be

determined.

The global database includes Australian data of 210Pb depositional

measurements performed by Bonnyman et al. (1972) covering six years of sampling.

This is the only report available for annual 210Pb depositional rates from Australia

including the tropical northern Australia. The report observed a correlation between 210Pb depositional rates and precipitation events. It was also reported that very high

depositional rates were recorded in hot, dry inland areas after rare rain events

occurred. Lead-210 depositional rates followed the same patterns as those of the

fission product Cs-137 and they also followed a seasonal trend that was related to the

rainfall pattern. This report includes results from the tropical locations of Darwin and

Townsville where the 210Pb deposition rates obtained were 95 Bq.m-2.y-1 and 38

Bq.m-2.y-1 respectively.

The group responsible for maintaining the global database noted the lack of

results for 210Pb depositional rates from tropical locations (Preiss et al. 1998). They

recently reported results from a study site in French Guiana, South America,

covering two years of continuous monitoring (Melieres et al. 2003). The site is

tropical, experiencing monsoonal, wet season summers and dry season winters with

an average annual rainfall of 3010mm most of which occurs in the wet season,

January-June. They reported a mean annual 210Pb deposition rate of 163±75

Bq.m-2.y-1 and also observed that deposition was proportional to rainfall but only for

rainfall events less than 15 cm over 15 days. This linearity was masked by strong

fluctuations when the rainfall increased to higher values. No attempt was made to

determine seasonal trends for the data but the information displayed shows

depositional peaks occurring during the monsoonal months for both years of study.

A long term 210Pb depositional rate measurement project was performed in

the Netherlands (Beks et al. 1998). Results from four locations, Groningen, Texel, de

44

Bilt and Bithoven have been reported with the latter two sites being the work of

another author. Sampling at these sites was performed from 1987-1994, 1991-1996,

1991 and 1987-1991 respectively. Annual 210Pb depositional rates for the sites are

determined as 71±20 Bq.m-2.y-1, 82±33 Bq.m-2.y-1, 56 Bq.m-2.y-1 and 71±38

Bq.m-2.y-1 respectively, where the errors are determined as the standard deviation of

annual data. The authors reported large random daily variations related to

precipitation events. Overall, lower depositional rates are likely to be a result of land

and sea air mixing. Summer depositional rates were higher than winter and related to

storms and precipitation events that are more common in the summer months. The

large annual variability was reported as being controlled by the number of heavy rain

events and thunderstorms each year.

Studies of 210Pb depositional rate in Germany have been performed from

1981-1999 the results of which have been reported in two articles (Rosner 1988;

Winkler and Rosner 2000). In agreement with other work, they reported a strong

correlation between precipitation and the amount of 210Pb deposition. Seasonal

fluctuations of 210Pb depositional rate observed were summer maximums and winter

minimums related to precipitation events that are more common in summer. Also the

ground is capped with ice throughout the winter months that will reduce 222Rn

exhalation as a 210Pb source. The site used in this study is 10 km north of Munich and

classified as a continental location with a prevalent westerly wind carrying moist

Atlantic air. The resultant averaged annual 210Pb depositional rate was 180±42

Bq.m-2.y-1 and was in general agreement with other research performed in Europe.

Results of a continuous monitoring study from Taiwan, 1996-2001, have also

reported seasonality in 210Pb depositional rates with winter peaks and summer

minimums (Su et al. 2003). Winter peaks were related to the southward movement of

the Mongolian High when the northeast monsoon brings 210Pb enriched air masses

towards Taiwan. During summer the southwest monsoon introduces 210Pb depleted

air masses from the South China Sea and western Pacific resulting in reduced 210Pb

deposition. The switch of monsoon patterns largely explains the annual cycle of 210Pb

deposition. While they did not report annual depositional rates they did observe the

typical correlation of deposition with precipitation events. They also related

increased deposition with dust storms that come from continental Asia laden with 210Pb.

45

Researchers in the United States have a number of stations monitoring 210Pb

depositional rate over many years. Baskaran et al. (Baskaran 1995; Baskaran et al.

1993) have provided reports on the seasonal variability of 210Pb depositional rate

from Galveston and College stations in Texas from 1990-1993. In the first report

(Baskaran et al. 1993) they did not observe any obvious seasonal fluctuation in the 210Pb depositional rate. However they did report that precipitation was the controlling

factor for the amount of deposition. They also examined the amount of deposition

after individual precipitation events to measure the contribution of each event to the

total deposition. In a second paper Baskaran (1995) reported that a small number of

precipitation events (4-6%) contributed a large amount (20-30%) of the total annual 210Pb deposition. This is related to the fact the most washout of 210Pb occurs within

the first half hour of precipitation. Rain events lasting for long durations will

generally deposit less 210Pb as they continue (Martin 2003). Investigation for

seasonal fluctuations continued, since other North American stations had reported

spring-summer highs and autumn-winter lows of 210Pb depositional rate. These

further investigations resulted in Baskaran (1995) reporting the same seasonal

fluctuations from their stations but on a much smaller scale and also heavily

dependent on precipitation events. Annual 210Pb deposition rates were 170 Bq.m-2.y-1

and 130 Bq.m-2.y-1 for Galveston and College, respectively.

An earlier report by Turekian et al. (1983) measured the 210Pb depositional

rate at New Haven in Connecticut and Bermuda. From a model, they expected

depositional rates at Bermuda to be quite large as it is downwind from a continental

source. Observations showed that the Bermuda depositional rate was 70% of that

predicted by the model due to a seasonal high pressure system that deflects

continental air. For New Haven the 210Pb deposition rate was reasonably correlated

precipitation with but this was not the case at Bermuda. Overall the reported

depositional rates are 202 Bq.m-2.y-1 for New Haven and 115 Bq.m-2.y-1 for

Bermuda.

2.4.3 Pb-210 soil studies Lead-210 settles upon the surface of the earth through dry and wet deposition

and has a strong affinity for soil and sediment particles (He and Walling 1997).

Initially 210Pb will attach to soil particles close to the surface of the soil reducing in

concentration with respect to depth. It should be noted that 210Pb found in the surface

soil can be a result of deposition or decay within the ground from 222Rn. Given that

46

the ground component of 210Pb can be determined from an assumption that it is in

equilibrium with 226Ra the excess, deposited, 210Pb can be determined. Measurements

of soil profiles indicate that excess 210Pb is redistributed vertically in the surface

layers of the soil. He and Walling (1997) explain this that redistribution reflects the

physio-chemical and biological processes that operates within the soil including

diffusion, convection and bioturbation. Ion exchange is the most dominant

absorption method but this is reversible and excess 210Pb can be replaced by other

ions leaving it free to re-enter the soil. Lead-210 released from one location may be

re-absorbed at another and transported downward with soil moisture, while free in

the soil it may also become subject to molecular diffusion. Bioturbation associated

with vertical mixing by soil fauna also represents an important mechanism for

redistribution of excess 210Pb in the soil. With a mean lifetime of 32 years, excess 210Pb should travel to depths of 10-20 cm in normal soils.

It is possible to use measurements of 210Pb in soils, snow or ice as a means of

determining an integrated average of previous depositional history for a region. Such

measurements can also be used for studies of erosion in surface soils and

determination of sedimentation rates in water systems. Measurement techniques,

applications and various studies of 210Pb in soils and ice cores have been reported by

a number of researchers (Matthews and Potipin 1985; Moore and Poet 1976; Nozaki

et al. 1978; Zapata 2003; Walling et al. 2003; Li et al. 2003; Branford et al. 1998;

Pourchet et al. 2003; Schulz et al. 2003; Kim et al. 1997; Roos et al. 1994; Graustein

and Turekian 1986; Wallbrink and Murray 1993; Thomson et al. 2002; Huh and Su

2004; Heijnis 1999; Heijnis et al. 1987; Imboden and Stiller 1982; Matthai et al.

1998; Bonniwell et al. 1999; Belyaev et al. 2004; Ugur et al. 2004; Zhang et al. 2003;

Walling and He 1999; Milton et al. 2001; Matisoff et al. 2002; Whiting et al. 2001;

Ivanovich and Harmon 1992; Ivanovich and Harmon 1982).

The use of 210Pb for erosion and sedimentation studies has become the most

common use for this radionuclide. Until recently, focus lay on 137Cs but this fission

product has only been released into the atmosphere during above ground atomic

explosions or by an accidental release, such as Chernobyl. As above ground nuclear

testing ceased in the early 1960’s there is no longer an input source of 137Cs into the

top layers of the soil. Due to redistribution and radioactive decay there is a constant

reduction in the 137Cs inventory. Compound this with the fact that equatorial regions

and the southern hemisphere received less deposition of 137Cs, 210Pb is a suitable

47

alternative (Walling et al. 2003). Pb-210 is constantly replenished due to continuous

constant release of 222Rn from the surface soils across the world. The soil interactions

between the two radionuclides are very similar (He and Walling 1997) so researchers

of soil erosion will likely turn to 210Pb for future studies as the 137Cs profile

disappears. For short-term erosion studies the short-lived radionuclide 7Be has been

successfully used, but with a half-life of 55 days its use is limited to these short-term

studies and studies are best coupled with measurements that include other

radionuclides (Zapata 2003).

Moore and Poet (1976) noted that the profile of 210Pb will be different for

disturbed soils compared to undisturbed soils and that from analysis of the 210Pb

profile a date for the soil disturbance could be determined. They were also able to

use their results to determine an estimate of the annual 210Pb depositional rate for the

area of study in Colorado. Nozaki et al. (1978), Roos et al. (1994) and more recently

Zapata (2003) have also reported that analysis of the atmospheric component of 210Pb

in soil profiles can be used to provide the historical depositional rate for about the

last 100years. A similar method was used by Pourchet et al. (2003) to determine past 210Pb depositional rates over Antarctica through a measurement of snow and ice core

samples. Measurement of 210Pb from salt marshes has also shown it to be useful as a

geochronological tool (Kim et al. 1997; Thomson et al. 2002).

In Zambia samples were taken from undisturbed and disturbed sites showing

the clear distinction in 210Pb profiles as a result of erosion and farming (Walling et al.

2003). Of notice in this work was the ability to accurately determine tillage depths of

soils from an analysis of depth profile. The example of soil profiles in Figure 2.7 is

from their work. For this work erosion or sedimentation was determined from

analysis of the total excess 210Pb inventory compared with an averaged standard

undisturbed profile. Where the total inventory was less than the standard, it was

concluded that erosion has occurred and where it was greater, sedimentation has

occurred.

48

b) Communal cultivation soil profile with erosion

0 10 20 30 40 50 60 70 80

0-2

4-6

8-10

12-14

16-18

20-22

24-26

28-30

Dep

th (c

m)

Unsupported 210Pb content (Bqkg-1)

Total inventory = 1505 Bqm2

Tillage Depth

c) Commercial cultivation soil profile with deposition

0 10 20 30 40 50 60 70 80

0-2

4-6

8-10

12-14

16-18

20-22

24-26

28-30

Dep

th (c

m)


Tillage Depth


d) Bush grazing soil profile with erosion

0 10 20 30 40 50 60 70 80

0-2

4-6

8-10

12-14

16-18

20-22

24-26

28-30

Dep

th (c

m)



a) Undisturbed soil profile

0 10 20 30 40 50 60 70 80

0-2

4-6

8-10

12-14

16-18

20-22

24-26

28-30

Dep

th (c

m)



Figure 2.7: Typical 210Pb soil profiles for various soil uses (adapted from Walling et al. (2003))

In Australia Wallbrink and Murray (1993) examined the potential of 210Pb for

erosion studies by setting up an experimental erosion field and generating artificial

erosion processes. In conjunction with measurements of 7Be and 137Cs they found

that they were able to distinguish various erosion processes based up on

measurements of radioactivity in the sedimentation. They found that sediments

moved from surface soils had high concentrations of 7Be and 210Pb while those

derived from gully erosion had much lower concentrations. Similarly Li et al. (2003)

used 137Cs and the 210Pb/137Cs ratio to determine the main sources of erosion in the

hills of western China. They were able to determine that gully erosion and not

surface soil was the dominant erosion process contributing to sedimentation in the

Yellow River. However Huh and Su (2004) noted that 210Pb deposition rates vary

spatially and temporally making it more difficult to determine whether varied values

are a result of erosion or natural processes. They noted that erosion studies are

possible if variations in 210Pb depositional rates are known along with their

relationship with precipitation events of the study region.

In the past radiochemical techniques were used to determine 210Pb in water

and soils. Reports the like of Matthews and Potipin (1985) detail the techniques

49

involved in sample preparation for the extraction of 210Pb fallout from soils for alpha

beta counting. More commonly today though, due to the improved technology of

low-level germanium gamma detectors, soil samples are analysed through the

detection of the low energy gamma ray (46 keV) that is emitted during 210Pb decay.

This has reduced the analytical time required to process samples although there are

some issue involved with self-absorption of the low-energy gamma ray. Matthews et

al. (1985) continued on to determine that the maximum penetration depth for 210Pb in

normal soils was 25 cm and that migration distance and time is different to 137Cs.

Primarily 210Pb is used for sedimentation studies in fluvial systems to

examine the build up of sedimentation through analysis of the radionuclide

concentrations (Zapata 2003). There is a large body of work on this topic but it is

considered to be beyond the scope of this review.

2.4.4 Pb-210 deposition and geographical location It has been observed in a number of studies that geographical location also

has an influence on 210Pb deposition. In some cases it may be a result of the

prevailing winds carrying 210Pb rich or poor air from other locations, in other areas it

might be due to the interaction between topography that affects the rainfall patterns,

such as the effect that mountainous regions have on inducing or preventing rainfall.

A number of studies report that coastal locations have less 210Pb deposition as a

result of mixing 210Pb poor air from the ocean and 210Pb rich air from the land (Beks

et al. 1998; Bonnyman et al. 1972; El-Hussien et al. 2001; Kim et al. 2000; Patterson

and Lockhart 1964; Winkler and Rosner 2000; Todd et al. 1989; Baskaran et al.

1993; Hussain et al. 1990; Preiss et al. 1996; Turekian et al. 1983).

As has been previously mentioned, Turekian et al. (1983) specifically

investigated the depositional rates of 210Pb from an east coast site on continental

United Sates and the island of Bermuda. The results obtained for the coastal sites

agreed well with observations from similar sites in the United States. However they

expected Bermuda to have a high depositional rate as it is downwind from a

continental source of 210Pb. The values obtained from Bermuda did not agree with

their expectation and further analysis showed that the effect of ocean air mixing was

underestimated and that the prevailing weather conditions kept the 210Pb depositional

rate at Bermuda at lower levels compared to mainland sites.

It should be noted that the prevailing meteorological conditions of an area

dominate more than the effect of localised geography even though the two may be

50

linked. This has been reported by Patterson and Lockhart (1964) who showed that

the meteorological conditions prevailing in their area of study were much more

significant to 210Pb deposition than the geographical location. This study looked at

the depositional rates of 210Pb along the 80th meridian in an attempt to observe

latitudinal variation. The results showed up to an order of magnitude difference in

the depositional rates between the various stations leading them to the conclusion

that meteorological conditions prevail.

Localised variations in geography and topography have been examined for

their effect on the depositional rate of 210Pb (Lupu and Cuculeanu 1999; Branford et

al. 1998). Lupu and Cuculeanu (1999) reported that the vegetation cover over an area

had a dramatic effect on the vertical profile of the concentration of radon progeny. It

was concluded that this would have a major effect on the 210Pb depositional rate at

these locations. Branford et al. (Branford et al. 1998) measured 210Pb depositional

rates from the coast to inland sites in Scotland, over an area with three distinct

mountain peaks. The results showed that deposition was greatest on the ocean-facing

face of the first mountain and deposition increased with increasing altitude for this

mountain. A weaker, similar observation was made for the second mountain but not

for the third. They concluded that the deposition was related to precipitation events

and that the results for the ocean facing mountain strongly showed the effect of its

influence to induce rainfall that reduces with increasing distance from the coast. In

this case a strong correlation was made between the localised geography and 210Pb

deposition but this was due to the influence that mountains have on inducing

precipitation events.

2.4.5 Pb-210 atmospheric concentration studies There is of course a relationship between 210Pb depositional rates and

atmospheric concentrations. This relationship is a result of 210Pb below-cloud

washout during rain events that reduces 210Pb atmospheric concentrations. A number

of the studies mentioned have also investigated 210Pb atmospheric concentrations,

performed correlations with depositional rates and investigated seasonal fluctuations

(Rangarajan et al. 1986; El-Hussien et al. 2001; Winkler and Rosner 2000;

Bonnyman et al. 1972; Rosner 1988; Beks et al. 1998; Todd et al. 1989; Baskaran

1995; Baskaran et al. 1993). Some of these studies have also examined 7Be and other

radionuclide depositional rates as it can be performed during analysis of 210Pb.

51

Until the study by Melieres et al. (2003) the majority of work on 210Pb

atmospheric concentrations within tropical regions was from India (Rangarajan et al.

1986). Rangarajan et al. (1986) reported averaged annual readings of 210Pb

concentrations from their various collection sites across India over many years of

study. They showed a distinctive decrease in 210Pb atmospheric concentrations during

the monsoonal period. They also reported that this was a result of dominant sea

breezes and below-cloud washout. They observed clearly though that in the short

term 210Pb concentrations varied seasonally with dry season peaks and wet season

troughs at all of the study locations. These results differ from those reported by

Melieres et al. (2003) but noted that differences in the prevalent winds can explain

this. Guiana experiences ocean winds for most of the year and occasionally Saharan

plumes carried all the way to South America creating occasional 210Pb peaks. In

India, especially Bombay, dry season winds come from inland sources carrying more 210Pb while the wet season winds come from the ocean with a lower 210Pb

concentration.

An article published by El-Hussein et al. (2001) examined the seasonal

variation of 210Pb concentration in the surface air at El-Minia, Egypt. They reported

summer minimums and winter maximums of the concentration of 210Pb in the surface

air. With low rainfall in this region the variation was attributed to an inversion effect

of the surface air layers during the different seasons. They also observed daily

variations that they attributed to atmospheric mixing. The results reported were in

agreement with patterns observed elsewhere around the world.

The other studies all show that 210Pb deposition increases with rainfall while

atmospheric concentrations decrease due to below-cloud washout. This will reach a

point where the replenishment of 210Pb from exhaling 222Rn into the atmosphere is

retarded due to capping effect by moisture in the soil so further rainfall will result in

reduced deposition rates and further reduced atmospheric concentrations. It has also

been noted that locations with high 210Pb atmospheric concentrations initially have

large depositional rates. It is evident that below-cloud washout of airborne 210Pb is a

significant contributor to the total 210Pb depositional rate.

2.4.6 Pb-210 summary The previous section provided a detailed description of the current state of

knowledge of 210Pb deposition and its application to soil studies. It has touched on

the topics of geographical depositional studies and its use for geochronology plus

52

broadly examining the relationship between 210Pb depositional rates and atmospheric

concentrations. Perhaps the most important information to be drawn from this section

of the review is the relationship between 210Pb deposition and precipitation,

especially that a small number of rain events can contribute a large amount of the

annual deposition rate.

Of note is the lack of 210Pb depositional rate measurements from tropical

regions, especially from Australia. While Bonnyman et al. (1974) performed a very

good study the results focused upon urban locations. Given the application of 210Pb

for erosion studies a complete knowledge of depositional rates at specific locations is

important for holistic study.

Even with the existence of a global database for 210Pb atmospheric

concentrations and depositions there has been little attempt to correlate these with 222Rn exhalation rates on regional or global scales.

2.5 Chapter summary This chapter has reported a summary of the current literature available on radon

emanation, migration, exhalation and the resultant deposition of 210Pb. The summary

shows the lack of current research on these topics within Australia compared to the

number of studies that are being performed internationally. Australia is a major

source of 222Rn, and thus 210Pb, in the southern hemisphere and the only major source

within Oceania. The report by Bonnyman (1972) is the only long-term study of 210Pb

depositional rates from Australia and has provided the values reported by Preiss et al.

(2003) for their global database of 210Pb concentrations and depositional rates from

around the world. There have been more recent reports of radon exhalation rates

from a number of ambient Australian soils. Schery et al. (1989) provided Australian

averages of 222Rn and 220Rn exhalation rates for the international UNSCEAR reports

(UNSCEAR 1993; UNSCEAR 2000) while Todd (1998) examined the rate of 222Rn

and 220Rn in the vicinity of the Ranger uranium mine in the Northern Territory of

Australia. These are the two most recent works on radon exhalation within Australia

and neither examined seasonal fluctuations or provided correlation between radon

exhalation rates and geological or meteorological conditions. Only Whittlestone et al.

(1998) started to address these issues but that work was also only a snapshot of the

overall picture.

53

Other than studies performed on operational or rehabilitated mine sites there

is a current lack of 222Rn exhalation studies being performed within Australia. While

a number of stations are monitoring 222Rn and 222Rn progeny atmospheric

concentrations no correlations have been made between atmospheric concentrations

and exhalation rates or 210Pb depositional rates. While a number of studies have been

performed on radon exhalation from abandoned and rehabilitated uranium mines and

natural ore bodies within the Alligator Rivers Region, to date no substantial survey

has been performed to determine the 222Rn source term from the Ranger uranium

mine. The reports produced for the Ranger mine source have been incomplete or

relied partially on model-based estimates for 222Rn exhalation rates from the various

identified sources at the site. Since the mine is a localised source of 222Rn within the

region measurements of the source term will be helpful in modelling the 210Pb budget

for the region, and for estimates of dose to people from inhalation of 222Rn progeny

sourced from the mine site.

The work covering the process of radon emanation and its migration is very

comprehensive. The models produced in more recent years (Semkow 1990;

Maraziotis 1996) provides ideas to solve some of the remaining questions of radon

emanation.

The Office of the Supervising Scientist (OSS), the Australian Atomic Energy

Commission (AAEC), Australian Radiation Protection and Nuclear Safety Agency

(ARPANSA) and the Northern Territory Department of Mines and Energy have all

carried out measurements of 222Rn exhalation rates and surface air concentrations

from various locations within the Alligator Rivers Region of the Northern Territory.

Analysis of this data and continued measurements can be used to examine the

specific meteorological, geographical and geological conditions that are involved in

the process of 222Rn exhalation and 210Pb deposition. Research and reports from

tropical areas in the Southern Hemisphere are lacking information on the physical

factors that affect 222Rn exhalation its transport and transport of its progeny. Most

reports have been made for the important assessment of radiological dose received

by members of the public living in the vicinity of operational or abandoned mines.

The location of this project, in the wet and dry tropics, is of intrinsic interest

for 222Rn exhalation and 210Pb deposition rate measurements. Periods of several

months without rainfall are extremely useful to observe the effect of influencing

parameters, such as 226Ra activity concentration, soil porosity, grain size,

54

atmospheric pressure and temperature, on 222Rn exhalation. Also, throughout the dry

season, all 210Pb deposition is dry deposition. The contrast then occurs for the months

of the wet season that gives extreme values of precipitation and soil moisture

expected to retard 222Rn exhalation and increase 210Pb deposition. The effect of early

or late wet season rains are also of interest to observe the influence these events have

on 222Rn exhalation and 210Pb deposition. The study is therefore of intrinsic interest

as it will provide results more decisive than those that would be obtained in a

temperate region.

55

3

Project location, site selection and measurement schedules

3.1 Overview The aim of this chapter is to introduce the reader in greater detail to the region that

the project was performed in, the sampling sites selected within and the measurement

schedules used. The tropical region of the Alligator Rivers, which encompasses

Kakadu National Park in the Northern Territory of Australia, was selected. It is this

region that eriss monitors for any environmental impact of the Ranger Uranium

project and performs a large range of research projects over many scientific fields. A

brief description of the region has been provided in Chapter 1.

Each of the project objectives outlined in Chapter 1 have had measurement

sites and schedules determined. First was identification of sites that should enable

investigating the effect of various parameters influencing 222Rn exhalation; secondly

the selection of sites to perform measurements of 222Rn exhalation rate over one

seasonal cycle; thirdly a selection of sites for the collection of soil scrapes and cores

to measure excess 210Pb following deposition; and finally the selection of sites to use

for the collection of 210Pb wet and dry deposition. These sites, sampling locations,

relevant maps, measurement schedules and reasons for selection are covered in the

following sections.

3.2 Exhalation from open ground – Investigation of physical parameters [226Ra activity concentration, distribution in grains, grain size and porosity]

3.2.1 Ranger operations The Ranger ore bodies were discovered during an airborne radiometric survey in

1969 and confirmed by drilling in the mid 1970’s. After Government approval,

mining at Ranger Uranium Mine commenced on ore body 1 in May 1980. Ore body

1 was mined out in 1994 and the remaining pit is currently used as the primary

tailings dam. Government approval for mining ore body 3 was given in 1996 and

mining commenced in July 1997; this ore body was still being mined at the time of

writing of this thesis in January 2005. Operations for processing commenced in 1981

56

and continue today. Between 1981 and 1997 the processing plant produced

approximately 3000 tonnes of U3O8 per annum, improvements completed in 1997

upgraded this to 5000 tones per annum. The total expected U3O8 reserve from both

Ranger ore bodies are estimated at 122,000 tonnes (Kendall 1990).

Mining at Ranger is performed by the open cut method due to the shallow

depth of the ore bodies (ore body 1, 5-70 m; ore body 3, 5-200 m). A large amount

of waste rock has been removed from each of the pits and used to create the original

tailings dam and the retention pond walls. Ore body 1 had an average uranium

concentration of 0.33% while ore body 3 averages 0.27%. Blasting is used to

dislodge rock from the pit walls; it is then collected using a heavy mechanical

excavator and loaded into open tray dump trucks. At the top of the pit ore trucks stop

under a radioactivity discriminator array consisting of four large sodium iodide

(NaI(Tl)) detectors. Here the load is graded before being dumped onto the

appropriate stockpile relating to its grade. Uranium concentrations used for

classification of ore grades at Ranger are listed in Table 3-1. Grade 1 ore is referred

to as waste rock; its low uranium concentration makes it uneconomical for milling. A

lay out of the Ranger uranium mine operation is provided as Figure 3.1.

Table 3-1: Classification of ore grades at Ranger

Ore Grade Number U3O8 (%)

1 0-0.02

2 0.02-0.08

3 0.08-0.12

4 0.12-0.2

5 0.2-0.35

6 0.35-0.5

7 >0.5

57

Figure 3.1: Ranger Uranium Mine, numbers indicate approximate sampling locations used for this project A flow chart of the processing operation performed at Ranger is provided as

Figure 3.2. Ore is processed on site to produce U3O8 that is shipped to international

customers. Processing starts by crushing and grinding the ore to fine particulate sizes

using rock crushers and rod mills. Ore is fed into the primary crusher with an average

uranium concentration of 0.26%. This average input value is achieved by feeding the

crusher from stockpiles of varying grades, based on calculation of the required

amounts. A secondary discriminator array, similar to the one located at the top of the

pit #3 is located at the entrance of the primary crusher and used to confirm the grade

of each load.

58

Figure 3.2: Flow chart of Ranger processing (ERA 2005)

59

After passing through primary, secondary and tertiary crushers the fine ore is

stored in a silo before being ground further in a series of rod mills. The finely ground

output from the rod mills is added to sulphuric acid in a leach tank where the

uranium leaches into a solution over 24 hours. Leaching removes approximately 90%

of the uranium from the ore. Sulphuric acid is produced on site in an acid plant from

stockpiles of sulphur. The uranium solution is separated from the solid waste in a

circuit of wash tanks. The solid waste, or tailings, are neutralized with lime and

pumped to the primary tailings dam. Kerosene and ammonia are added to the

uranium solution in the clarifier and filter tanks. This removes the uranium from the

acidic solution into a kerosene solution. More ammonia is added in the precipitation

tanks and uranium is precipitated from the solution as ammonium diuranate, more

commonly referred to as yellowcake. Final traces of liquid are removed in a

centrifuge and through heating the yellowcake at temperatures of 800 oC in a

calciner; this removes the final traces of ammonia. The final product is U3O8,

uranium oxide, which is a dark green powder.

A large amount of wastewater is produced during milling, and management

of wastewater is an important issue for Ranger. From information available at the

onset of their operations the company expected lower rainfall and higher evaporation

rates for the region than proved to be the case. It proved to be an ongoing operational

problem for Ranger. The wet season of 1982-1983 had 20% lower rainfall than

average and was classified as a drought year. This resulted in Ranger importing

water into the site. By 1985 Ranger was carrying more water than the system could

handle and plans were put in place to reduce the excess water load. A number of

proposals were made including pumping water directly into Magela Creek.

Eventually permission was given to irrigate excess water onto a land filter on the

Ranger side of Magela now known as the Magela land application area. Irrigation

commenced in 1985 and continues to the present day but only during years where the

water loads are in excess. Water irrigated onto this region comes from retention pond

2 and contains trace quantities of uranium series radioactive elements including 226Ra, the direct parent of 222Rn.

60

3.2.2 Ranger site selection Radon-222 is released into the atmosphere during some stages of the uranium

mining and milling process. Searching previous literature of work performed at

Ranger has identified the major 222Rn sources as Pit #1, ore body 3 (now Pit #3), the

ore stockpiles, waste rock dump, tailings dam, land application area and the milling

plant (Akber et al. 1993; Kvasnicka 1990). All of these locations are shown in Figure

3.1; the numbered locations indicate sites where 222Rn exhalation measurements were

performed during this project. They are identified in Table 3-2 that also provides the

site name and Global Positioning System (GPS) coordinates, given in the WGS84

Universal Transverse Mercator (UTM) coordinate system. The UTM coordinate

system was used for all positioning during the course of this project.

Both pits have been identified as 222Rn sources since it is exhaled from the

uranium bearing rocks in the walls and floors. In pit #3 222Rn is likely to be emitted

in larger quantities during the blasting process. Atmospheric inversion and calm

conditions keep 222Rn in the pit, so most will decay within the pit. Occasionally,

conditions arise that transport 222Rn out of the pit; this has been observed through

continuous monitoring of 222Rn close to the edge of the pit. During the wet season

both pits fill with large amounts of water and exhalation of 222Rn is retarded due to

the saturation of rocks and soil with water.

Table 3-2: Number, name and position of sites selected at Ranger, refer to Figure 3.1

GPS Site Number Site Name

53L UTM

1 Pit #1 0273430 8596120

2 Pit #3 0273703 8597772

3 Grade 2 Ore Stock Pile 0273135 8596547

4 Waste Rock Dump 0273010 8597072

5 Laterite Stockpile 0273330 8596804

6 Magela Land Application

Area

0275015 8597449

6 Experimental Plot (In MLAA) 0275058 8597406

7 Grade 7 Ore Stock Pile 0274186 8596781

61

All stockpiles at Ranger exhale 222Rn and since they consist of uranium

bearing material the exhalation rates from the stockpiles were expected to be a major

contributing source. Three ore stockpiles of differing grades and a laterite stockpile

were selected for measurement of 222Rn exhalation rates. The three ore stockpiles

selected were the waste rock dump, grade 2 and grade 7 stockpiles. It was expected

that a correlation between uranium concentration and 222Rn exhalation rate would be

observed.

Waste rock makes up large surface areas in the operational areas of the

Ranger mine. It was used in construction of the tailings dam and retention pond

walls. It is an important material on site as it will be used to create the final

landforms and as a surface covering for both pits. Sections of the waste rock dump

have been ploughed and vegetated as part of the rehabilitation process while other

sections are compacted flat by heavy vehicles. A compacted section and a

rehabilitated section, located next to each other, were selected for 222Rn exhalation

measurements. The aim was to compare results from both sections with each other

and other measured stockpiles.

Laterite is a fine-grained soil created through weathering of rocks. Laterite

soil is found in tropical regions or in regions that were once tropical; it has been

leached of soluble minerals but still contains large concentrations of iron oxides and

iron hydroxides. It is generally reddish in colour due to the presence of these iron

compounds. Large amounts of laterite soil were removed from above the ore bodies

at Ranger. The laterite contains elevated concentrations of uranium and it is theorized

that the uranium deposit at Ranger has leached through this laterite layer. During the

early stages of the Ranger project laterite was processed for its uranium but this

became uneconomical when global uranium prices declined.

The ore crushing process releases 222Rn that might have otherwise been

trapped within the rock fragment. As ore passes through the crushing and grinding

phases it is broken into smaller and smaller grain sizes. Smaller grains have greater 222Rn emanation rates due to their larger surface area to volume ratio. Crushing also

creates large amounts of dust, and so 222Rn emanating from airborne dust particles

passes straight into the atmosphere. With these processes at work the mill has

previously been identified as an important 222Rn emission source from the Ranger

site (Akber et al. 1993; Kvasnicka 1990). Unfortunately determining the mill source

term would have presented several difficulties including measurements problems due

62

to the various engineering control systems in place and associated occupational

health and safety concerns with the operations required to obtain samples. It was

therefore decided that determining the mill source term was out of the scope of this

project.

Radium-226 is one of the waste products released into the primary tailings

dam in the form of wet slurry. Tails slurry is saturated with water so little 222Rn

exhalation is expected from the water saturated tailings.

As previously mentioned, Ranger releases some of its excess retention pond 2

water through irrigation onto a land filter called the Magela Land Application Area

(MLAA). This area, just to the south of Magela creek, encompasses 33ha of

Eucalyptus woodland that has been irrigated with retention pond 2 wastewater since

1985. Wastewater from retention pond 2 contains trace amounts of 226Ra and 238U

along with other uranium series radionuclides, salts and sulphates. Deposition of

water onto this site makes the land application area a unique site for 222Rn exhalation

studies. The water is deposited using irrigation sprinklers attached to pipes laid

across the area. Surface deposition results in a higher 226Ra concentration in the

surface 5-10 cm of soil at this site compared with natural locations. Irrigated and

non-irrigated sections along with a test experimental plot all within the land

application area boundary were selected for measurements. The irrigated section

selected for study had had not been irrigated in a number of years due to broken

pipes in the region. Assessment of future 222Rn exhalation rates from the Magela

Land Application Area was obtained from analysis of the results from this section.

A small section of land within the boundary of the Magela land application

area was used as a test plot to study the effects of surface deposition of salts,

sulphates and radionuclides. The plot, known as the Experimental Plot, was set up

for a collaboration project between the Office of the Supervising Scientist and the

CSIRO (Willett et al. 1993). Over the course of nine months starting in late August

1988 the plot was irrigated with synthetic water designed to match the quality of the

retention pond 2 water irrigated onto the land application area. The project’s aim was

to estimate long-term effects and observe movement of deposited salts, sulphates and

radionuclides. The experimental plot became the focus of intensive study for three

years after its irrigation. Results of these studies were presented at a symposium and

published in the symposiums proceedings (Akber 1991).

63

3.2.3 Ranger measurement schedule Measurements performed at the stockpiles, pits and waste rock dump were limited

due to access restrictions at the Ranger mine site. They were performed on weekends

when work at the mine ceased for maintenance, they were performed during the dry

seasons of 2002 and 2003. Access to the waste rock dumpsites was easier as it was

not used for dumping. Two methods of measuring 222Rn exhalation rates were

employed and complete details of equipment can be found in Chapter 4. Along with 222Rn exhalation rates gamma doses and 226Ra activity concentrations were also

recorded where possible. Generally, accessibility determined the number and

duration of measurements performed.

Ease of accessibility to the Magela Land Application Area meant that a more

intensive survey could be performed there. Three sections of the land application

area were selected; the first was the abandoned irrigated section mentioned

previously, while the other two were non-irrigated sections representing two different

soil types shown in Figure 3.3. Over 90 charcoal canisters for 222Rn exhalation

measurements were deployed across these three sections for periods of 3-5 days.

Gamma doses, 226Ra activity concentrations and 222Rn exhalation measurements,

using an emanometer, were measured at 11 sites. Soil cores were also taken from 9

of these sites to obtain radionuclide depth profiles.

The position of the Experiment Plot may be seen on the bottom right hand

side of Figure 3.3. Work there commenced with collection of two soil scrapes

followed later by gamma dose rate, 222Rn exhalation rate and soil radionuclide

concentration measurements. Twenty-five charcoal canisters were deployed across

this site for 222Rn exhalation measurements while gamma doses and 226Ra activity

concentrations were measured at each location where a charcoal canister was placed.

Unanalysed core samples from a previous study on the area were also prepared for

gamma counting (Storm and Martin 1995).

The dates of all measurements performed at Ranger are given in Table 3-3.

Type and number of measurements performed are seen in Table 3-4.

64

Figure 3.3: Original Magela Land Application Area (MLAA) Table 3-3: Measurement dates for Ranger Mine sites Site Measurement Dates

Grade 2 Ore Stockpile (OSP2) 08/09/2002

Grade 7 Ore Stockpile (OSP7) 07/07/2002

Laterite Stockpile 05/08/2002

Waste Rock Dump (WRD) 09/07/2002-10/07/2002

Pit #1 03/10/2003

Pit #3 12/10/2003

Magela Land Application Area (MLAA) 06/2002-07/2002

Experimental Plot 20/07/2003-29/07/2003

65

Table 3-4: Number of Ranger measurements Site

Area

Emanometer Charcoal Canisters

Soil Samples

Sodium Iodide

Gamma Dose Rate

OSP 2

Pad

6

9

1

16

15

Rim - 10 1 - 10

OSP 7

Pad

4

5

1

-

5

Rim 8 5 2 - 3

Laterite

Pad

5

15

-

19

20

Rim 4 9 - - 13

Push Zone 1 6 - - 7

WRD

Rehabilitated

10

11

1

9

21

Pad 10 10 2 9 20

Pit #1

Bench

6

28

3

-

27

Wall - 2 - - -

Pit #3

Pad

4

21

1

-

21

Rubble 2 7 1 - 7

Rock - 2 - - 2

LAA

Irrigated

8

33

8

8

8

Non-irrigated 11 60 9 11 11

Experimental

Plot

- 24 4 24 24

66

3.3 Seasonal and diurnal radon exhalation [moisture, pressure and temperature]

3.3.1 Site selection Eight sites were selected for seasonal 222Rn exhalation rate measurements. These

sites were selected on the basis of the following criteria:

− Accessibility;

− Proximity to laboratory;

− Availability of other relevant data for the site;

− Range of natural and disturbed sites;

− Acceptance for use of the site from Aboriginal Traditional Owners;

− Safety.

Accessibility was an essential criterion as during the wet season the rivers

and creeks swell making some areas inaccessible. It was extremely important to

ensure that most sites were accessible all year round. Proximity to the Jabiru field

station was also important; as most equipment used was fragile, so long journeys

over unsealed roads were undesirable. One site, Mirray, 40 km from the laboratory

was selected as a representative background site for the Kakadu region far enough

away from any possible mine related influence. Secondly, Mirray is on the side of a

hill and therefore to some extent could provide information of seasonal 222Rn

exhalation rate variability from sloped locations.

There are three locations where eriss monitors airborne dust and radon

progeny concentrations; (i) Mudginberri radon station; (ii) Jabiru East; and (iii)

Jabiru Town (Water Tower). Dust concentrations are measured for a period of one

week every month while radon progeny concentrations are measured over a day

every month. Radon gas monitors constructed by the Australian Nuclear Science and

Technology Organization (ANSTO) are used at Mudginberri and Jabiru East to

continuously monitor radon concentrations. With the availability of this additional

data and as they meet all requirements listed, these three sites were included for

seasonal 222Rn exhalation measurements. It was desirable that human disturbed sites

and natural sites be studied to determine if disturbed sites have variations in 222Rn

exhalation rates when compared with natural sites. For mining influenced seasonal

sites the waste rock dump and irrigated section of the Magela Land Application Area

were used. The remaining two sites are relatively undisturbed, the first being close to

67

the edge of the Magela Creek at the northern end of the Jabiru East aerodrome.

While Magela Creek site didn’t have all year round accessibility it does have a

naturally high 222Rn exhalation rate as previously identified (Todd et al. 1998). The

final site was on the non-irrigated section of the Magela Land Application Area.

The eight sites selected are listed in Table 3-5 providing a reference

number, site name and the Global Positioning System (GPS) location of each of

them. The map displayed in Figure 3.4 shows the position of these sites around the

Jabiru and Ranger project region.

Mudginberri radon station lies 10 km from Ranger mine and 2 km from an

Aboriginal campsite at Mudginberri billabong. A large area around the site was

previously used as a stockyard for an abattoir based where the Mudginberri campsite

is now located. A result of its use as a stockyard is that the ground at the site is firmly

compacted. It has been cleared of the typical Eucalyptus woodland that dominates

the region. The monitoring station here is used to monitor airborne dust, radon and

radon progeny concentrations. The results of this monitoring are used to calculate

effective doses for people living in the Mudginberri campsite. The station has full

year accessibility, a secure compound, an automated weather station and a permanent

radon gas monitor. The site was attractive for use because it met all the criteria.

Mudginberri radon station is the first of three sites used by eriss for the airborne dust,

radon and radon progeny measurements. This site is representative of a disturbed

location.

The eriss Jabiru field station is located at Jabiru East approximately 3 km

from the operational pit at Ranger. The site was once the location of the township of

Jabiru East that was demolished in 1990. Common Eucalyptus trees found in the

region were planted for rehabilitation. At the rear of the eriss compound is a smaller

compound used for meteorological measurements, it was within this compound that

seasonal 222Rn exhalation measurements were carried out. Nearby in the eriss

compound measurements of airborne dust, radon and radon progeny concentrations

are carried out. This site is accessible all year, secure, within 300 m of the weather

station based at Jabiru East aerodrome and has a permanent radon gas monitor. This

site is representative of a disturbed but rehabilitated location.

68

Table 3-5: Seasonal measurement sites, names and locations GPS Site Number Site Name

53L UTM

1 Mudginberri 0267389 8607610

2 Mirray 0251463 8576966

3 Jabiru East 0271339 8599366

4 Jabiru Water Tower 0264330 8598104

5 Waste Rock Dump 0272326 8596788

6 Irrigated Land Application Area 0274960 8597404

7 Non-irrigated Land Application Area 0275057 8597390

8 Magela Creek 0272441 8600260

The third site used for monitoring airborne dust and radon progeny is the

water tower located in the township of Jabiru some 15 km from the mine. This site is

accessible all year round, has a secure compound and access to additional data that

can be used in conjunction with 222Rn exhalation measurements. This site, similar to

large areas of Jabiru Town, is covered with compacted soil excavated from what is

now Jabiru Lake. The idea was to ensure that sections of town were above the

flooding levels of nearby creeks. This site also represents a disturbed region.

Mirray is an eroded hillock some 35 km south south west of the Ranger mine.

There is a lookout at the top of the hillock joined by an 800 m walking trail to the car

park at the base that is accessible all year. An ambient site 30 m from the car park on

the side of the walking trail near the base of the hill was selected for our seasonal 222Rn measurements. This undisturbed site was selected to be representative of

ambient 222Rn exhalation rates from natural sites of the region. The site is on sloped

ground so analysis of the data, especially from the wet season, was expected to

provide information about the importance that soil moisture drainage may have on 222Rn exhalation rates.

69

Figure 3.4: Map of region displaying seasonal sites

Waste rock covers large surface areas of the mine site and has been identified

previously as one of the main 222Rn sources from Ranger. The measurements

performed on the mine at the sites covered in Section 3.2 only provide 222Rn

exhalation rates for dry season conditions. A site at the rear of the waste rock dump

was selected to perform seasonal 222Rn exhalation rates measurements. This site,

located near the old tailings dam is accessible all year. Previous work performed at

Ranger has measured 222Rn exhalation rates from the waste rock dump but never

examined it over a seasonal cycle. Results obtained from these measurements were

70

expected to provide insight into the seasonality of 222Rn exhalation rates from ore

stockpiles structures. At the end of operations Ranger will be rehabilitated and large

areas layered with a covering of waste rock. Studying the seasonality of 222Rn

exhalation from this site now provides an idea of expected 222Rn exhalation rates

from the rehabilitated site in future.

The Magela land application area is accessible all year and close to the eriss

field station. Irrigated and non-irrigated sites were selected within zone 6A and the

type II soil non-irrigated sections respectively, shown in Figure 3.3. In zone 6A a site

beside the access track to Georgetown Billabong near water sample bore #32 was

selected. From the type II soil non-irrigated section a site close to the access track to

water sample bore #31 was selected. The irrigated site is a natural site that only

differs from the surrounding woodland because of its previous irrigation with mine

processed water. Both sites represent the common Eucalyptus woodland and are both

kept free from fires during the dry season. These sites are within 200 m of each other

and both located on type II soil. Comparison of seasonal 222Rn exhalation rates for

these sites will enable investigating any variations as a result of the surface

deposition of 226Ra on the irrigated section. Measurements on the irrigated section

will also provide insight into expected 222Rn exhalation rates from the remainder of

the Magela Land Application Area after mining operations cease.

The final site used for seasonal measurements was at the edge of Magela

Creek 700 m down a track at the northern end of the Jabiru East aerodrome. Todd et

al. (1998) measurements at this site identified it as having a high 222Rn exhalation

rate. It was easily identifiable from an old car body lying nearby. Proximity to

Magela creek however meant the site was covered with water during the wet season

once Magela Creek swelled. Radon-222 exhalation rates during these months are

assumed to be zero. The site is representative of natural open ground with alluvial

sandy soil.

3.3.2 Seasonal site measurement schedule The seasonal sites were measured for 222Rn exhalation rates every month for a year.

Readings commenced in August 2002 and ceased in July 2003. The only exception

was the Magela creek site where readings commenced in September 2002, also with

the swelling of the creek this site was not measured over the period January-March

2003. The two measurement techniques used were charcoal canisters and

emanometers; both are described in detail in Chapter 4. Five charcoal cups were

71

deployed for up to several days each month and when equipment was available three

measurements were taken over one day each month using the emanometers. The

emanometers were not available between October-December 2002 as they were

being calibrated. When emanometer measurements were performed soil temperature,

atmospheric temperature, humidity and emanometer collection chamber temperature

were also recorded.

As soil moisture affects 222Rn exhalation, soil moisture depth profiles were

measured once a month at four of the seasonal sites. Compact ground, at

Mudginberri, waste rock dump and Jabiru water tower plus the likelihood of its

submersion at Magela creek meant that the required PVC tubing for the soil moisture

probe was install at other sites. The sites Mirray, Jabiru East and both Magela Land

Application Area sites were suitable for the required PVC tubing to be installed for

the soil moisture probe. Originally it was planned to install the tubing to a depth of 1

m but practicality meant that some tubes only reached half that depth. Soil moisture

readings were performed when emanometer measurements were made or during

charcoal canister plantation periods.

3.3.3 Diurnal measurement schedule Regional data shows that atmospheric concentrations of 222Rn and its progeny vary

diurnally with concentrations peaking between 7-10am (Whittlestone 1992; Akber

and Pfitzner 1992; Akber et al. 1994a; Akber et al. 1994b). Few studies, local or

internationally, have attempted to observe diurnal variations in 222Rn exhalation

rates. Todd et al. (1998) performed two partial diurnal measurements and one

complete diurnal measurement from two sites in the Jabiru region. Martin et al.

(2002) also reported one diurnal measurement for a site at Nabarlek; this was

indicative work leading to this project.

Atmospheric temperature, humidity, soil temperature and the emanometer

collection chamber temperature were measured over the course of the diurnal

measurements.

72

Table 3-6: Sites and dates of diurnal measurements

Site Measurement Date

Mudginberri 2-3 April 2003

Mirray 20-21 March 2003

Jabiru water tower 15-16 March 2003

Mudginberri 1-2 September 2003

Jabiru water tower 2-3 August 2003

Magela creek 15-16 August 2003

Jabiru East 20-21 August 2003

Diurnal measurements were performed at the end of the wet season 2003 and

again during the dry season 2003. This was aimed to investigate if any diurnal

variations were associated with the soil moisture levels. The sites and dates that these

measurements took place are listed in Table 3-6. As diurnal measurements required

either constantly travelling to or staying at a site for 24 hours time and manpower did

not allow for all sites to be measured during both seasons. Measurements were made

with an emanometer and readings were taken every hour and a half during the wet

season. The dry season regime changed with readings taken every two hours from

start till midnight and then at 3 am and 6 am. Since access to the mine site over 24

hours was not possible to arrange therefore neither the waste rock dump nor Magela

Land Application Area was included in the diurnal studies.

3.4 Excess 210Pb soil sampling In the Jabiru region during the dry season the wind blows predominantly from an

easterly to south easterly direction as shown on a dry season wind rose, Figure 3.5,

which was created from data obtained from the Australian Bureau of Meteorology

weather station at Jabiru East. It was expected that maximum 222Rn exhalation from

Ranger would occur during the dry season. Expecting some correlation between 222Rn exhalation and 210Pb deposition rates a number of samples were collected from

the region with the main focus being on samples downwind, to the west of Ranger. In

total 8 sites were selected for soil sample collection for analysis of excess 210Pb.

Locations are shown in Figure 3.6 while numbers, names and corresponding GPS

positions are provided in Table 3-7. Dates of sample collection are provided in Table

3-8.

73

Sites for this section were selected mainly on the basis of accessibility,

direction and distance from the mine. A site near Georgetown billabong on the east

side of the mine is opposite to the prevailing wind direction during the dry season.

Jabiru East lies within the second most dominant dry season wind sector while the

site on the side of the Arnhem highway lies between both dominant dry season wind

sectors. Sampling occurred at various times throughout the project when time and

facilities permitted. Soil excess 210Pb results should also aid in determining transport

range and residency time of 210Pb in a tropical atmosphere.

Figure 3.5: Dry season (April-October) wind rose for Jabiru East (data courtesy of Australian Bureau of Meteorology) [26 years averaged]

74

Figure 3.6: Map of Jabiru and Ranger, numbers indicate approximate locations of selected sites for soil samples

75

Table 3-7: Sites for excess 210Pb soil samples

GPS Site Number Site Name

53L UTM

1 West of original Tailings Dam 0271501 8596426

2 Eastern Side Barallil Creek 0267244 8596826

3 Western Side Barallil Creek 0266130 8598056

4 Jabiru Township 0265336 8597684

5 Georgetown Billabong 0275587 8597120

6 Jabiru East 0271301 8599844

7 Side of Arnhem Highway 0268579 8599616

8 South of original Tailings Dam 0272171 8595464

9 West of Retention Pond 1 0272136 8598050

10 Magela Land Application Area (including

Experimental Plot)

~0274925

~8597583

Table 3-8: Soil collection dates and samples taken

Site Name Date Collected Sample West of Old Tailings Dam 20/3/04 Core

Eastern Side Barallil Creek 31/1/04 Core & Scrape

Western Side Barallil Creek 30/8/03 Core

Jabiru Township 30/8/03 Core

Georgetown Billabong 28/3/04 Core & Scrape

Jabiru East 21/2/02 &

27/8/03

Core & Scrape

Side of Arnhem Highway 16/9/03 Core & Scrape

South original Tailings Dam 10/10/03 Core & Scrape

West of Retention Pond 1 26/9/03 Core

Non-Irrigated Magela Land Application Area 22/2/02, 19/7/02

& 26/7/02

Cores

Irrigated Magela Land Application Area

(including Experimental Plot)

20/2/02-1/8/03 Cores & Scrapes

76

3.5 Pb-210 deposition sampling Only two deposition collectors were constructed so their deployment locations were

carefully considered. A previous report examined the levels of various radionuclides,

including 210Pb, in rainwater collected from Jabiru East and Jabiru Town (Martin

2003). Keeping in mind the factors such as previous information on deposition

values, security, accessibility and a weather station less than 300 m away at the

Jabiru East aerodrome, Jabiru East was selected as one of the sites for a deposition

collector. Jabiru East was also used for seasonal 222Rn exhalation measurements and

soil sampling. Use of additional data collected from this site may allow for the 210Pb

deposition measurements to be more holistic.

Since the first collector was deployed close to Ranger mine it was decided

that the second collector should be installed further away to measure ambient 210Pb

deposition for the region away from any possible influence of the mine. Close

proximity to a weather station and security for the equipment were other

considerations. The second deployment location selected was another Aboriginal

community station, Gunbalunya, previously known as Oenpelli missionary station.

Gunbalunya is approximately 50 km north north west of Jabiru inside Arnhem Land.

With approval of traditional owners the collector was placed in the yard of an

employee from the local school. This site is adjacent to a floodplain on the northern

edge of the township. Rainfall measurement records in Gunbalunya date back to

1910. In 1957 the weather station was upgraded and in addition to rainfall data now

provides information about temperature, humidity and wind speed and direction.

Gunbalunya is considered to be far enough away from any mining site that 210Pb

depositional rates here should be representative for the ambient values in the region.

Sampling using these collectors was performed on a monthly basis over a period of

one year starting in May 2003.

77

4

Methodology

4.1 Overview The aim of this chapter is to detail the methodology and analytical techniques used

throughout the project. Some equipment, such as the charcoal canisters and radon

emanometers were used across various sections. Methods of operation and sampling

processes are all described here.

Two types of 222Rn exhalation measurement devices were used; (i) passive

charcoal canisters; and (ii) an active system designed by ANSTO known as an

emanometer. Soil moisture was recorded using a DIVINER soil moisture probe at

four seasonal sites. Soil, emanometer and atmospheric temperature values were

recorded at all sites when the emanometer was used. A portable thallium doped

sodium iodide (NaI(Tl)) detector, Geofizika GS-512, was used to measure equivalent

uranium and thorium activity concentrations and percent potassium at all seasonal

sites and the majority of sites at Ranger mine where 222Rn exhalation rates were

measured. Gamma dose rates were recorded at most sites with a MINI environmental

meter type 6-80. Wet and dry deposition collectors for 210Pb deposition were

designed with an ion exchange resin column attached to the base. Soil samples were

taken with soil corers and scrapers then prepared for gamma spectroscopy analysis.

The Australian Bureau of Meteorology supplied meteorological data from their local

weather stations.

A work plan was created to break up the project into specific tasks as follows:

− Establishment of dry season 222Rn exhalation rate from various sources

located on the Ranger uranium mine;

− Measurement of seasonal variations in 222Rn exhalation rate at several

locations around the Jabiru/Ranger region;

− Measurement of 222Rn diurnal exhalation rate variations from several

sites;

− Measurement of excess 210Pb from soil samples taken from the

Jabiru/Ranger region;

78

− Measurement of 210Pb fallout in wet and dry deposition for one seasonal

cycle from two sites for study of 222Rn transport.

4.2 Available techniques for radon exhalation measurements A variety of techniques are available for 222Rn exhalation rate measurements.

Techniques are classified as being either passive or active; active systems are those

requiring electrical power and passive are those that do not. Active systems have the

advantage of being able to perform prompt measurements of 222Rn exhalation while

passive systems have to be left at the measurement location for periods of time

ranging from a few days to a few months. The following passive devices have been

extensively used in the past:

− Charcoal canisters;

− Nuclear track detectors;

− Electret ion chamber.

Active systems generally have air pumps to move air from a collection

chamber into a counting chamber. Most active systems utilize scintillation chambers

to determine the activity of the air sampled, with a few using silicon surface barrier

detectors.

It was noted by Rutherford that 222Rn was absorbed onto activated charcoal.

In the mid 1970’s Countess (Countess 1976) described a way in which canisters

filled with activated charcoal could be used to measure 222Rn exhalation and

atmospheric concentration by gamma spectroscopy analysis. Heating activated

charcoal will desorb 222Rn and its progeny providing a reusable material for 222Rn

measurement. Numerous researchers have used this technique over the years as an

easy method to perform 222Rn measurements. For exhalation measurements canisters

are deployed upturned over the sample surface, sealed and left for an exposure period

between 3-7 days. Upon collection canisters are sealed to trap in 222Rn, and then left

for at least four hours to allow for the establishment of secular equilibrium between 222Rn and its short-lived progeny prior to gamma spectroscopic analysis. The amount

of 222Rn trapped was determined through measurement of gamma rays emitted

following the decay of the 222Rn progeny, 214Bi and/or 214Pb. The amount of 222Rn

the canister was exposed to was determined from an integration of recorded gamma

counts over exposure time then simple calculation provides the 222Rn exhalation rate.

79

This was the technique selected for this project and is discussed in further detail in

section 4.3.

Nuclear track detectors are a film-like material that is damaged by alpha

particles incident upon them. Etching the film after exposure to alpha particles

widens the damage tracks enough so they can be counted manually under a

microscope or automatically using electronic equipment. A number of alpha sensitive

materials are used as track detectors, some of the more common ones are cellulose

nitrate (Kodak’s LR115 and Russian DNC), polycarbonate (Bayer’s Makrofol E),

and allyl digycol carbonate (Chinese developed CR-39). Placing a strip of the

detector in the bottom of a canister, upturning and sealing it over a sample is a means

of measuring 222Rn exhalation rates. A barrier is employed to remove any

contribution from 220Rn. When 222Rn enters the canister alpha particles from the

decay of its progeny leave tracks on the film that can be etched and counted. Radon-

222 exposure can be determined from an integration of track counts over exposure

time and exhalation determined through simple calculation.

The final passive technique examined here is the electret ion chamber. This

device, developed by Rad Elec Inc., consists of a chamber with an electret disc and a 220Rn barrier. Electret discs are a dielectric material that holds a quasi-permanent

charge and they are charged prior to deployment. The negative charge on the electret

disc draws alpha particles towards it as they are positively charged. The alpha

particles deposit energy onto the disc and it loses some of its charge. The final charge

on the electret is read with a capacitance meter. If the chamber is placed over the

surface of a sample 222Rn exhalation can be determined as a function of exposure

time and the difference between initial and final charges.

4.3 Radon exhalation measurement with charcoal canisters Since Countess (1976) developed the technique for 222Rn measurements using

charcoal canisters it has been improved upon and has proven to be a reliable method

for the measurement of 222Rn exhalation rates and atmospheric concentration. For

this project charcoal canisters were designed and are shown in Figure 4.1. One

hundred of these were made and each canister was filled with approximately 25

grams of activated charcoal. Exposure times recommended are 3-5 days but shorter

times can be used at sites with elevated 222Rn exhalation rates.

80

Figure 4.1: Charcoal canister

Some authors (Samuelsson 1987; Aldenkamp et al. 1992) have reported that

closed cans, such as charcoal canisters, lying over the ground will underestimate the

free radon exhalation rate to the atmosphere. Measurement conditions in this study

are such that this effect will be small. In a number of situations, charcoal canister and

emanometer measurements were performed simultaneously at the same site and the

readings obtained by both methods statistically overlap with each other.

Charcoal canisters are prepared by heating them in an oven at 110oC for at

least 24 hours prior to deployment. Heating desorbs any 222Rn and its progeny

currently in the charcoal. Directly prior to deployment canisters are removed from

the oven and sealed. They are taken to the sampling site where the lids are removed

and the canister upturned, pressed firmly into the ground and sealed with soil if

required. At the end of exposure they are collected, resealed with lids and tape then

returned to the laboratory for measurement using a Geofizika GS-256 gamma

spectrometer.

By placing and sealing a canister over the surface of a material all 222Rn

exhaling from that surface will enter the canister and be absorbed by the activated

charcoal. Previous work (Bollhöfer et al. 2003; Spehr and Johnston 1983) show that 222Rn exhalation rates can be calculated using the following equation:

( )( )ec

d

tt

tc

eea

etRJλλ

λ

ε

λ−− −−

=1.1..

... )(2

Equation 4-1

81

where

J: is the average 222Rn exhalation rate (Bq.m-2.s-1)

R: is the net count rate after background subtraction (s-1)

tc: is the counting time (s)

λ: is the 222Rn decay constant (s-1)

td: is the delay time between collection and counting (s)

a: is the surface area covered by a canister (m2)

ε: is the counting efficiency of the detector system (s-1.Bq-1)

te: is the exposure time (s)

The following assumptions are made with Equation 4-1:

− There is negligible 222Rn activity on the charcoal at the start of exposure;

− The supply of 222Rn is constant over the exposure period;

− There is no saturation of 222Rn in charcoal over exposure time;

− As the canister is sealed temperature and humidity effects can be ignored;

− Back diffusion of 222Rn from the canister into the ground is unlikely due to

the high concentration gradient.

Derivation of Equation 4-1 starts with a charcoal canister exposed to a source

of 222Rn for time te. During exposure 222Rn activity will grow towards a steady state

value, AE, and 222Rn activity, Ae, at the end of exposure is given by:

( )etEe eAA λ−−= 1 Equation 4-2

If t is any time after exposure, 222Rn activity at time t will be: t

et eAA λ−= Equation 4-3

If the canister is counted for time tc after a delay of time td, average 222Rn activity

will be:

[ ]cdcddcd

c

cd

d

t

c

te

ttt

c

ett

t

t

c

ett

tt

cave e

teAee

tAdte

tAdtA

tA λ

λλλλ

λλ−

−+−−+−

+

−=⎥⎦

⎤⎢⎣

⎡ −=== ∫∫ 11 )(

Equation 4-4 Therefore:

( )( ) ( )( )ec

d

ec

d

tt

tc

tt

tave

E

ee

etR

ee

etAA−−−− −−

=−−

=11

...

11

...

ε

λλ λλ

Equation 4-5

All symbols have the same meaning given for Equation 4-1.

82

When canisters are used as emanometers the 222Rn exhalation rate is given by:

aAJ Eλ

= Equation 4-6

Providing us with Equation 4-1.

This is the same calculation method used by eriss for their work at the

rehabilitated uranium mine Nabarlek (Bollhöfer et al. 2003) and was originally

published by Spehr and Johnston (1983).

4.3.1 Charcoal canister counting system, calibration & efficiency Charcoal canisters were counted on a portable gamma ray spectrometer, Geofizika

GS-256. This device consists of a NaI(Tl) detector coupled to a photo-multiplier tube

connected to a control unit. The detectors NaI(Tl) crystal is 75mm in diameter and

height, coupled to the photo-multiplier tube all housed in a water resistant aluminium

cylinder. The control unit is a microprocessor system with a liquid crystal display

and is able to provide a gamma spectrum over 256 channels. The unit can be

controlled by an external computer, on which the spectra can also be recorded. The

system’s energy range is 12 keV to 3 MeV, with an energy resolution of

approximately 12 keV per channel. Normally a 137Cs source is housed at the end of

the detector and used as a reference source by adjusting the gain to bring the 137Cs

662 keV peak to channel 55. For the purpose of charcoal canister measurements the 137Cs source was removed since it interferes with the 222Rn progeny, 214Bi gamma

energy at 609 keV. The system has automatic dead time correction so the live time is

extended till the effective count time reaches the required value. The detector was

housed in a lead castle to reduce background counts from external sources.

Originally this was in an environmental radioactivity counting laboratory but moved

to another laboratory in August 2002 when the original laboratory was

decommissioned and relocated. Further information regarding the operation of the

GS-256 is available in the user manual (Geofizika 1990).

Throughout the project most canisters were counted for 600 seconds but a

few were counted for 300 seconds. A 600 second counting time was used for good

counting statistics and easy measurement of many canisters over a couple of days. At

the end of each measurement spectra were downloaded from the GS-256 and saved

onto a computer in a file format that could be accessed by a spreadsheet program.

The file contains the complete spectrum, counting time and sum of counts over four

set regions of interest. The GS-256 can store data from the four regions of interest or

83

the entire spectra but its memory can only hold sixty-two complete spectra. The

GS-256 had recently been used for counting charcoal canisters from 222Rn exhalation

measurements and regions of interest were preset on the energy peaks of the 222Rn

progeny isotopes; 214Pb at 242 keV, 295 keV and 353 keV; and 214Bi at 609keV. To

cover each of these peaks completely the regions of interest were set over channels

18-22 (216-264 keV), 23-27 (276-324 keV), 28-31 (336-372 keV) and 47-54

(564-648 keV). After correction for background the net counts under these peaks is

used as R in Equation 4-1.

Analysis of various spectra observed a drift in peak positions, caused by drift

in amplification, especially for the 214Bi peak at 609 keV. It was decided that one

broad region of interest should be used to determine the net counts, R, in Equation

4-1. A region of interest covering channels 18-60 (216-719 keV) was used as it

covers all energy peaks and allows for some drift in higher energy peaks. To

determine the counting efficiency of the system a charcoal canister, with 25g of

activated charcoal spiked with 327.0 Bq of 226Ra, was used (Bollhöfer et al. 2003). A

detection efficiency of (6.96±0.3)% was obtained.

Background counts were performed using two different methods. Originally

all deployed canisters were measured before deployment and values averaged to

obtain a background for that set of readings. Later during the project it was decided

that background counts could be made from a random selection of canisters not

deployed in the field, these were averaged and used as background for that set of

readings.

4.4 Radon emanometers Two radon emanometers, designed and constructed by ANSTO, are in possession of

eriss, one of these systems is shown in Figure 4.2. They are active radon sampling

systems with the ability to distinguish between the isotopes 222Rn and 220Rn. They

sample air from a large sampling saucer that is placed over a surface and are able to

provide radon exhalation rates within an hour.

84

Figure 4.2: Radon emanometer

The system consists of an enclosed box, to house the detection equipment, a

large metal sampling saucer that is placed over the sampling surface, a computer for

system control and tubing to couple the sampling saucer to the detector. To obtain a

good seal between the sampling saucer and ground a cutter, shown in Figure 4.3, is

used to indent the soil where sampling will take place.

85

Figure 4.3: Cutter used to accurately place emanometer saucer

The detector unit contains two scintillation cells coupled to separate

photomultiplier tubes. Air in the sampling saucer is mixed using a small electrical

fan at the top of the saucer. Sampled air pumped into the detector unit is filtered to

remove 222Rn progeny before entering the first scintillation chamber. In this chamber

total alpha counts are recorded which represents the contribution of 222Rn and 220Rn.

The air then passes through a delay line and reaches the second scintillation chamber

6 minutes after exiting the first. With a half-life of 55.6 seconds a 6 minute delay is

adequate time for the 220Rn concentration to reduce to approximately 2% of the

concentration in the first chamber. In the second scintillation chamber the alpha

count is measured again but this time the primary contribution is from 222Rn. The air

is then pumped back into the sampling saucer. A schematic of the emanometer is

shown in Figure 4.4.

A laptop computer with the control program installed controls various

functions of the detector system. The interface controls sampling, high voltage

settings, discriminator voltage settings and displays current sampling information.

Complete analysis consists of six 6 minute counting periods where the final

measurement draws ambient air into the first chamber while the remaining sampled

air is counted in the second chamber. When the control program is active and the

system is turned on it samples ambient air that is used as a background reading.

86

Figure 4.4: Schematic of radon/thoron emanometer

Data analysis requires a second program supplied with the emanometers. The

average of the background measurements are used to correct the readings obtained

from sampling. The analysis program outputs 222Rn and 220Rn exhalation rates once

supplied with the background and sampling measurement details from the data file

recorded during sampling. The following assumptions are made in analysis:

− There are no errors due to leaks of 222Rn and 220Rn from sealing, edge

effects or back diffusion;

− Counting error can be estimated as the square root of counts.

The equation for 222Rn exhalation rate determination that the emanometer uses has

been previously reported but is also supplied (Zahorowski and Whittlestone 1996):

( )( ) ( )( )[ ]∑=

=++ −−−=

5

121,2,11,11,2

k

kkkkkR bCbCDJ ββ

Equation 4-7

Where

( ) 11,2,11,2,1

−++ −= kkkkD αββα

87

C1,k: Counts in cell 1 at increment k

C2,k±1: Counts in cell 2 at increment k+1

β2,k±1, β1,k, α1,k & α2,k±1: Constants (where efficiency of cells is accounted for)

b1 & b2: Background counts for cell 1 and cell 2

4.4.1 Emanometer calibration The emanometers are labelled with their model and serial numbers RTE 2 001 and

RTE 2 002 but for this project they were referred to as RTE1 and RTE2. Twice

during the project the emanometers were freighted to QUT for calibration. The

method used for calibration was the same as that previously reported for the

equipment (Todd and Akber 1996).

Calibration requires the use of certified 222Rn and 220Rn sources that were

available at QUT. Radon is pumped from the source into the sampling saucer

through the line that returns air from the detector to the sampling saucer. This set up

is shown in Figure 4.5. This input source of 222Rn or 220Rn is representative of a

constant exhalation rate. Problems were encountered obtaining consistent 220Rn

values from RTE2 so no results of this isotope have been reported from this

equipment.

Calibration checks were performed in April-May 2002 and October-

December 2002. Results from the calibration checks are displayed in Table 4-1.

Exhalation results reported in later chapters from emanometer readings have been

efficiency corrected. The certified source was not variable so it could not be

determined if the discrepancy occurs over a range of 222Rn exhalation rates.

88

Figure 4.5: Set up for emanometer calibration Table 4-1: Emanometer calibration check results Emanometer Source 222Rn

exhalation (mBq.m-2.s-1)

220Rn exhalation

(mBq.m-2.s-1)

222Rn efficiency

(%)

220Rn efficiency

(%)

RTE1 222Rn +

220Rn 127±10 1068±152 72.9±5.8 49.0±6.9

RTE2 222Rn + 220Rn

128± - 74.0±2.2 -

4.4.2 Associated emanometer measurements When the emanometers were used a number of associated parameters were recorded.

It is known that soil and meteorological variables influence 222Rn exhalation.

Variables such as soil temperature, atmospheric temperature, rainfall, pressure,

humidity, wind speed, wind direction and emanometer temperature may vary

throughout the course of a series of measurements. These variables are important to

record, as they will likely influence any diurnal fluctuations of 222Rn exhalation. Soil

temperature was measured with a Taylor 9878 soil temperature probe while

atmospheric temperature, humidity and emanometer temperature was measured with

a Vaisala HM34 humidity and temperature meter.

The other meteorological parameters, atmospheric pressure, rainfall, wind

speed and wind direction were obtained from either Bureau of Meteorology or eriss

weather stations. A Bureau weather station is located at Jabiru East aerodrome and

89

individuals at Jabiru Town Council and Cooinda record rainfall. An eriss weather

station located at Mudginberri only records wind speed and direction.

4.5 Soil moisture readings It is been shown in Chapter 2 that soil moisture is regarded as the most important

factor influencing 222Rn exhalation rates. At four of the seasonal sites PVC access

tubes were installed to allow access for a Diviner 2000 soil moisture probe. Initially

it was planned for most seasonal sites to have these access tubes installed but

difficulties experienced with installation meant that only four sites could be

measured. Even then, hardness of the ground at time of installation meant that not all

of these sites could have the access tube installed to the preferred depth of 1 m. The

sites selected were the irrigated Magela Land Application Area, non-irrigated Magela

Land Application Area, Mirray and Jabiru East.

A Diviner 2000 soil moisture probe was used to perform the soil moisture

measurements. While a number of systems were investigated the Diviner 2000 was

the most versatile. It consists of a data logger unit with display and interface along

with the probe that can scan to a depth of 1 m and provides results in 10 cm intervals.

The sensor head of the probe transmits a high frequency pulse (>100 MHz), it

receives the reflected signal from the surrounding soil. Moisture changes the

dielectric constant of soil thus the capacitance of soil increases with an increase in

moisture. Frequency of response is inversely proportional to capacitance so the

frequency of the reflected signal is related to the amount of moisture in the soil. The

logger records the raw frequency of response but analysis is performed from a ratio

to ensure data quality between units. This scaled frequency (SF) is used to determine

the volumetric water content of the soil. Scaled frequency is determined from the raw

counts via Equation 4-8.

)/()( WASA FFFFSF −−= Equation 4-8 Where

FA: frequency response in air (Hz)

FW: frequency response in water (Hz)

FS: raw counts or frequency obtained from the soil (Hz)

The values of FA and FW are obtained through a simple calibration procedure

for the unit. This calibration is performed with the unit set to calibration mode and

the probe placed into PVC housing, readings are then performed with the PVC

housing in air (FA), and then immersed in water (FW). These values provide the

90

boundary conditions, air (0% moisture) and water (100% moisture). From them the

scaled frequency is obtained from a scan and a preset calibration curve of moisture

content versus scaled frequency is used to obtain the volumetric moisture content.

This default calibration curve and equation are shown in Figure 4.6 and has been

derived from data collected from sand, sandy loam and organic potting mix.

A depth profile is created for each site showing the measurement depth of the

site and all readings performed there. The logger/display unit can provide

information such as the scaled frequency and moisture content in a simple graphic

format. The logger can also be downloaded to a computer and imported into

spreadsheet programs for further analysis. Further information regarding the Diviner

2000 can be found in the user manual (Technologies 2000).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60 80 100

Volumetric Soil Water Content(%)

Sca

led

Freq

uenc

y (S

F)

SF=0.27(VWC0.33)

Figure 4.6: Default calibration curve for Diviner 2000 soil moisture probe

91

4.6 Soil activity concentration measurements The activity concentration of material in the ground at all sites was performed by one

of the following three methods.

− Gamma spectroscopic analysis using portable field NaI(Tl) detector

(GS-512);

− Calculation from recorded gamma dose rates;

− Gamma spectroscopic analysis from collected sample; core, scrape or

surface.

Method of analysis was determined based upon site parameters, and the type

of information required. Measurement of 210Pb in soils was only possible by

laboratory gamma spectroscopic analysis of collected samples. Cores and scrapes

collected from the selected sites were processed for this section. Where 226Ra activity

concentration was required a portable NaI(Tl) gamma detector (Geofizika Brno GS-

512) was used. However the GS-512 was not suitable for all locations due to

problems recording measurements at locations with very high 238U equivalent

activity concentrations, such as a large number of sites found at Ranger. In these

cases the gamma dose rates recorded at the sites were used to calculate the 238U

activity concentration and hence 226Ra activity concentration. Some surface soil

samples were collected by hand for gamma spectroscopic analysis to compare with

values obtained through calculation from gamma dose rates.

4.6.1 Geofizika GS-512 portable gamma detector

Similar to the previously described GS-256 unit the GS-512 has a number of

different features. Manufactured by Geofizika Brno it contains a 3” diameter, 3”

thick thallium doped sodium iodide (NaI(Tl)) crystal coupled to a photomultiplier

tube housed in a water resistant aluminium casing and a data/display unit. It is

specifically designed for field use and records the gamma spectrum over 512

channels. A photograph of the system is shown in Figure 4.7. This unit is able to

directly analyse measured spectra to provide prompt values of equivalent uranium

and equivalent thorium activity concentrations and percent potassium. For field

measurement the detector is mounted on a tripod frame keeping the crystal at a

height of 1m above ground surface. The system is calibrated on test pads with known 238U, 232Th and 40K activity concentrations, prior to use the system had been

calibrated in 1998. Stability is checked regularly by eriss at a test site. Like the GS-

92

256 this system also houses a 137Cs source and uses the 662 keV peak as a reference

peak for spectrum stabilisation..

Determination of uranium activity concentration is performed by

measurement of the 214Bi 1764 keV gamma peak. Analysis then assumes that

equilibrium exists between 214Bi, 226Ra and 238U. It is because of this indirect

measurement that results are expressed as equivalent uranium. Similarly 232Th is

determined from its progeny 208Tl. Percent potassium is obtained directly from the 40K 1461 keV gamma peak after taking into account the natural 40K abundance of

potassium. Results are expressed in ppm eU, ppm eTh and %K. Measurement is

performed over set regions of interest programmed into the detector. With the 137Cs

source the detector establishes the position of these regions of interest setting the 137Cs 662 keV gamma peak onto reference channel 110. Radionuclide activity

concentrations are determined through matrix multiplication of count rates and

calibration constants. As 214Bi is a progeny of 226Ra, the equivalent uranium activity

concentration represents a measure of 226Ra rather than 238U. Further information of

the method of operation of this system can be found in the user manual (Geofizika

1998).

Given that the GS-512 measures 214Bi for its activity concentration

calculations, measurements obtained at the irrigated Magela Land Application Area

are more representative of the 226Ra activity concentration than 238U because excess 226Ra was deposited at the site during irrigation. It was realised that the 137Cs source

in the detector was not active enough to set the reference channel while

measurements were performed at some Ranger ore stockpiles or within pit #3. As a

result the GS-512 would only measure for 9 seconds before shutting itself down.

While a number of these spectra were analysed the peak drift was substantial and not

consistent so this data was of no use. As a result one of the other two methods of

determining the radionuclide concentrations was used at these sites.

93

Figure 4.7: Geofizika Brno NaI(Tl) GS-512 gamma spectrometer in use at Rangers waste rock dump 4.6.2 Determination of 226Ra from gamma dose rates

The gamma dose at any location is a combination of gamma rays emitted from the

natural decay series, 40K and cosmic radiation. UNSCEAR (2000) provides

conversion coefficients that can be used to estimate the above ground gamma dose

rate at any location if concentrations of these radionuclides are known. Alternatively,

at locations where the uranium concentration is high an approximate value of the

concentration can be obtained from the gamma dose rate using the following

relationship previously reported (Akber et al. 2004b):

94

CU ≈ 0.16 (HTotal – HCosmic) Equation 4-9 Where

HTotal: Gamma dose rate (nSvhr-1) in air

HCosmic: Gamma dose due to cosmic radiation (nSvhr-1)

CU: Uranium concentration (ppm)

For locations where the GS-512 was unable to obtain complete spectra

because it could not locate the reference source peak, Equation 4-9 was used to

estimate 238U equivalent concentration. It was then assumed that 238U is in

equilibrium with all solid progeny and so an estimate of 226Ra activity concentration

could be obtained. It is noted that this holds only for locations where 238U

concentrations are high. This method was used to determine the majority of 226Ra

activity concentrations for ore stockpile grade 7, ore stockpile grade 2, pit #1, pit #3

and laterite rim and push zones. A value of 66 nSv.hr-1 was used as the cosmic

component of gamma dose. This value was obtained for the region in a previous

study (Marten 1992).

4.6.3 Soil sampling and preparation Soil samples were collected from a number of sites where further analysis was

required other than results supplied by the GS-512 or gamma dose rates. In particular

analysis of excess 210Pb could only be performed from collection of soil cores and

scrapes sectioned by depth, fractionated and analysed by gamma spectroscopy. It

also allowed for comparison of results obtained using the GS-512 and calculation

from gamma dose rates. Samples were collected using one of the three following

methods:

− Hand collection;

− Soil cores;

− Soil scrapes.

Hand collection was the means of obtaining samples from locations where

readings could not be performed with the GS-512 or gamma meter. They were

collected from a number of locations at the waste rock dump, ore stockpile grade 7,

ore stockpile grade 2 and the laterite stockpile. Three samples from the general area

of each location were taken and the results averaged to obtain 226Ra activity

concentrations for sites where no other reading was possible. Samples were collected

95

by hand and placed into a sample bag, labelled and returned to the laboratory to be

dried and prepared for gamma spectroscopy.

Soil cores and scrapes were used to determine excess 210Pb from the sites

listed (Section 3.4). A number of cores were also collected from the Magela land

application area and experimental plot to determine the depth profile of irrigated

radionuclides. Collected cores were sectioned with the smallest practical sectioning

length being about 1 cm. Generally, most cores were sectioned every 5 cm, cores

collected at the irrigated Magela Land Application Area were sectioned every 2 cm

for the first 10 cm and then every 5 cm thereafter. The corer breaks in half for

preparation and sample removal. A locking pin keeps the corer together during

sample collection. Preparation requires a clear plastic sheet lined on the inside of the

corer for the sample to sit in, the plastic makes removal of the sample in one piece an

easy task. The sample is collected by forcibly inserting the corer into the ground with

aid of a sand hammer. The corer is then removed, compaction and friction with the

plastic lining generally keeps the core in place. In cases when the corer is firmly

stuck a levering device can be used to remove it.

The soil corer only covers a small surface area (19.6 cm2) but will collect

samples to depths of 30+ cm. In order to collect samples from a greater surface area a

soil scraper was used. Scrapes were taken to a depth of 10 cm in sections, 0-1 cm,

1-2.5 cm, 2.5-5 cm and 5-10 cm. Scrapes were only collected at sites that also had

cores collected. Scrapes provide a better analysis of radionuclides in the top layers of

soil, especially excess 210Pb, as scrapes cover a larger surface area with smaller

sections. Samples are collected by inserting the surface frame onto the ground,

securing it with a tent peg then digging a small trench at the collection end of the

frame and lined with a plastic sheet to allow for easy collection of the samples. This

trench need only be as deep as the current sampling section and can be enlarged as

needed. The handle is then connected to the scraper at the desired first section depth

and the scraper drawn across frame. This draws the soil onto the plastic lining in the

trench that is collected and placed into an appropriately labelled bag. Once the first

scrape is collected the handle can be set to the next depth and the process is repeated

until the desired total depth is reached.

96

Figure 4.8: Base of discs used for soil samples

Sampled cores and scrapes were returned to the laboratory where the cores

required sectioning. All sections were then weighed before being placed in an oven

set at 80oC to dry for at least 24 hours. Upon removal from the oven the samples

were weighed again so moisture content could be determined. Then the sections were

fractionated using a 2 mm sieve, each fraction was weighed so the amount of pebbles

and fine grains was known. Each fractionated section was crushed to powder using

either a Labtechnics or Rocklabs grinding mill. The sample was then pressed into a

disc, similar to that shown in Figure 4.8, using a manual hydraulic press. Samples

were spooned onto the disc, pressed for a minute at 7000 psi, removed, topped up

and the process repeated until the sample was flush with the top of the disc. Once

full, a greased O-ring was put in place and the lid screwed on to seal the sample.

Establishment of equilibrium of 222Rn progeny with 226Ra is achieved by sealing the

sample, after 20 days samples were analysed on an HPGe gamma spectroscopy

system.

Hand grab samples collected from Ranger were not fractionated and were

crushed using a separate mill to avoid contamination. They were pressed and sealed

as described previously in this section.

97

4.6.4 Excess 210Pb analysis of soil samples Excess 210Pb in surface soils is a result of its atmospheric deposition on the surface

followed by transport, mixing and covering in the soil creating a soil profile. Excess 210Pb is determined with two steps; (i) measuring 226Ra activity concentration and

using that as the equilibrium component of 210Pb activity concentration; (ii)

subtraction of the equilibrium component of 210Pb from measured 210Pb, as displayed

in Equation 4-10. This is valid under the assumption that contribution of 210Pb

created from 222Rn decay within the soil from 222Rn diffused from a greater depth

and leaching of 226Ra from the surface to a deeper position in the soil are both

negligible.

Excess 210Pb (Bq.kg-1) = Measured 210Pb (Bq.kg-1) – Equilibrium 210Pb (Bq.kg-1)

Equation 4-10 4.7 Pb-210 deposition measurement Collection of wet and dry 210Pb deposition was performed using a large deposition

collection system. An ion exchange resin column was connected to the base of the

collector. The ion exchange resin extracts metallic ions that pass through it. A

photograph of the system is shown in Figure 4.9, this shows the large stainless steel

collection tray attached via a tube to the resin column underneath. The collection tray

has a diameter of 85 cm covering a total surface area of 0.57 m2. During the dry

season only a tube attaches the resin column to the collection tray but during the wet

season the tube is replaced by a bucket to store water since the resin retards flow.

Two of these were designed and constructed in Brisbane and then forwarded onto

Jabiru for deployment at Jabiru East and Oenpelli.

This system is designed to collect total 210Pb deposition that is washout,

rainout and fallout. For the first four months a filter was placed above the first resin

column to remove large particulate matter, it was noted that the filter caused

blockages and its use was ceased before the onset of the wet season. The ion

exchange resin used was Amberlite IR 120 (H) designed to remove ionic metals

passing through it, thus removing atmospheric 210Pb passing through the column. The

resin itself must be kept moist in order to retain its properties so a U-tube attached to

the bottom of the column filled with water and hung up keeps the resin moist.

98

Figure 4.9: 210Pb deposition collector deployed at Oenpelli

A monthly sample collection and resin replacement routine was decided

upon. Sample collection required filter removal, for the months it was there, then

rinsing the collector with distilled water to remove deposited 210Pb on the collector

and pass it through the column. Once the rinse was complete the resin was collected

and replaced with new prepared resin. The system is then connected back together

with a new filter in place and the U-tube filled with water to cover the resin.

Collection routine during the wet season changed slightly, in some cases the column

was blocked with matter stopping flow and the collector bucket was partially full.

Detaching the U-tube allowed water to flow through but very slowly. It was easier to

remove the resin column, unblock it, collect the water and then pass it back through

99

the unblocked column. The system has four resin chambers but only two chambers

were used in each collector for faster flow rates.

Resin required chemical preparation before use to flush out any metallic ions

in it and the resin was reusable by performing the same chemical flushing process.

This involved rinsing 30 ml of resin with 200 ml of 6M HCl followed by 200 ml of

distilled water, this was performed using a 50 ml glass column with a bottom valve.

Flushing it with acid removes metals trapped in the resin and distilled water rinses

out any remaining acid so the resin will again hold metals. After preparation it could

be stored for prolonged periods providing it remained covered with demineralised

water.

Collected filters and resins were sent to Darwin for analysis by gamma

spectroscopy for the 210Pb 46 keV gamma peak. The filters were placed in the same

size disc used for soil samples and a similar efficiency calibration as the soil samples

was used. The true efficiency of the filters was never determined as they were

abandoned during the project. Analysis of the filter results and observation of dry

season 210Pb deposition rates showed that the activity on the filters was negligible.

The resin was placed in a similar but larger disc. The results for the resin were

corrected for system efficiency that was determined for the resin. When filters were

used the 210Pb was determined as a combination of the filter and resin, but it is noted

that the value determined for the filter is incorrect, as the efficiency was never

determined, but negligible.

4.8 HPGe gamma spectroscopic system Set up and operation of the eriss High Purity Germanium (HPGe) gamma

spectroscopy system is covered in an internal report (Marten 1992). Information

provided in this section summarises that report to show the operation and

maintenance of the system ensuring high quality results are obtained in this project

using gamma spectroscopy analysis. The original report should be referenced for

precise details and further information of the systems.

A number of HPGe detectors are available at the eriss facility originally in

Jabiru but now based in Darwin. Samples were analysed on detector N, G or O with

the majority analysed on O to avoid discrepancies from using multiple detectors.

Detectors N and O are EG&G Ortec ADCAM 100 systems and detector G is a

Canberra 7229N, all are mounted in lead shielding constructed from 100mm low-

100

level lead. Shielding minimizes background radiation so the lower limit of detection

is improved. The room housing the detectors has been designed to minimize

background radiation by using low activity concentration materials in construction.

The detectors and their associated electronics are kept at a temperature of 21 oC to

avoid electrical noise caused by temperature variations. A photograph of the eriss

detector room at the new Darwin laboratory is shown in Figure 4.10.

To obtain the best energy resolution, detectors must be cooled to reduce

thermally induced leakage currents and electrical noise. To achieve this, detectors are

thermally coupled to a 30 L liquid nitrogen Dewar by a copper-cooling rod

surrounded by a cryosorption material such as charcoal. Keeping a detector cool is

very important, as warming up a detector with high voltage still applied may damage

or even destroy it. Dewars are filled with liquid nitrogen weekly.

Figure 4.10: eriss detector room, Darwin (Photograph by Bruce Ryan)

101

HPGe gamma detectors operate on the principle gamma rays passing through

the detector crystal will interact with it via one of three processes, photoelectric

effect, Compton scattering or pair production. Any interaction deposits energy and

generates electrons creating electron-hole pairs in the germanium crystal. The

quantity of charge produced is directly proportional to the energy of the incident

photon. The high voltage applied to a detector ensures complete collection of the

charge, although the exact high voltage varies between makes and models and it is

generally of the order of a few thousand volts. Detectors are fitted with charge

sensitive preamplifiers acting as an interface between a pulse processor and the

analysis electronics. The preamplifier integrates and amplifies the ionised charge

produced by incident interacting radiation. From the preamplifier the signal passes to

an amplifier to increase signal strength from a few millivolts to a few volts. The

amplifier amplifies and shapes the signal before the next stage. Signal to noise ratio

can be improved at the amplifier stage.

Finally a Multi Channel Analyser (MCA) receives the signal from the

amplifier converting it into a number using an analogue to digital converter (ADC).

The number produced represents a channel in the spectra and corresponds to the

energy of the incident photon. Energy windows can be varied to view high or low

energy areas of the gamma spectrum. An electronic gate controls the ADC so some

signals may be missed while one is processed and this results in dead time. A counter

registers the amount of pulses received by the ADC during processing and is

representative of the dead time so the result can be internally corrected by extending

the live counting time. Results of counts versus channel number can be displayed on

a computer using a program named MAESTRO. This is the user interface providing

a visual means to examine spectra.

At completion of sample analysis a test source is placed on top of the sample

and counted for 600 seconds. This source contains 50 kBq and 30 kBq of 238U and 210Pb, respectively, and is used to ensure that no peak shifts occurred during analysis.

The test source also provides data for attenuation correction of the sample. A

program named GPEAK is used to extract gamma peak intensities and uncertainties

from analysed spectra. Results from use of the test source and background analysis

allow GPEAK to output peak intensities and standard deviations corrected for dead

time.

102

Determination of activity concentrations of radionuclides from samples is

performed from the output of GPEAK by a program called GOUT. This program

requires the sample weight to provide activity concentrations (Bq.kg-1) and was the

final analysis step used to obtain results reported throughout this thesis.

Maintenance of the gamma spectroscopy system, background counts,

standards analysis, filling liquid nitrogen Dewars and checking the electronics is

performed on a regular basis. Members of eriss staff perform these tasks according to

strict quality assurance requirements ensuring the system runs correctly and results

are accurate and precise. Currently a report is in preparation covering details of the

new Darwin laboratory, associated set up and calibration of the detector system.

4.8.1 Calibration of spectroscopy system for project samples The sample preparation technique of pressing crushed samples into discs was

originally developed at the Australian Institute of Marine Science and is a new

method employed by eriss. Prior to this soil samples were cast into resins of various

geometries for gamma spectroscopic analysis. The press disc method is quicker than

casting with an average of four medium sized sample discs being pressed per hour

compared with casting of four samples a day. Being a new technique for the

laboratory it was required to create calibration standards so the detectors could be

calibrated for this geometry. A total of twelve standards were prepared using 238U, 232Th and 226Ra of known activity. A small amount of active sample was mixed with

low-level sand used by eriss as a background standard. They were mixed, dried and

pressed into the sample discs using the same method described in Section 4.6.3 for

soil samples.

To create a mix of activities across the samples two batches for each radio

nuclide was prepared and two standards created from each batch. For uranium a

BL-5 mixture containing 7% U3O8, in the form of a powder was directly added to the

ground sand. The thorium standard was prepared from thorium nitrate salt that was

ground with a mortar and pestle and placed in the oven to dry at 80 oC overnight.

Thorium nitrate salt is hygroscopic so it was quickly weighed and then mixed with

the sand to make the standards. These standards were pressed as quickly as possible

to avoid absorption of more water. Radium was in the form of liquid that was added

to the sand, and then dried in an oven at 80 oC overnight before making the

standards. Weights, activities and standard identifications of these standards are

shown in Table 4-2 and the obtained calibration curves are shown in Figure 4.11.

103

Table 4-2: Standards for Pressed Disc Geometry Sample ID Standard Type Weight (g) Activity (Bq)

ZQ0001 14.09 242.97 ZQ0002 16.04 276.68 ZQ0003 15.46 266.06 ZQ0004

238U

15.62 268.81 ZQ0005 15.46 135.68 ZQ0006 14.95 131.14 ZQ0007 14.23 162.96 ZQ0008

232Th

15.11 173.06 ZQ0009 14.52 203.94 ZQ0010 15.57 218.71 ZQ0011 14.88 342.81 ZQ0012

226Ra

15.06 347.00 Table 4-3: Standards for resin

Sample ID Standard Type Resin Volume (ml) Activity (Bq)

ZR018 51.7 34.7 ZR019 41.7 33.6 ZR029 26.5 34.6 ZR021 25.3 44.5 ZR022 34.9 42.9 ZR023 30.6 48.8 ZR024

210Pb

15.8 42.3

Similarly the resin used to collect 210Pb deposition also required standards

and background samples to determine detector efficiency for this geometry. At the

start of the deposition collection regime no specific volume of resin was used so it

was necessary to perform efficiency calibration across a range of volumes. Several

standards and one background were prepared using varying amounts of resin to

determine the detector efficiency over a range of resin volumes. The details of the

standards prepared for the resin are shown in Table 4-3.

104

1

2

3

ln e

ffici

ency

1

2

3

ln e

ffici

ency

1

2

3

ln e

ffici

ency

3 4 5 6 7ln Energy

det N

det G

det O

Figure 4.11: Calibration curves and equations for pressed disc soil samples, energy unit is keV

For ln Energy<5.5 ln eff=-10.94+6.01x-0.67x2

For ln Energy>5.5 ln eff=12.40-2.64x+0.13x2





105

5.0

7.0

9.0

11.0

13.0

15.0

17.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0Resin Volume (ml)

Effi

cien

cy %

Figure 4.12: Efficiency calibration for resin samples

The analysis of these samples provided an efficiency plot shown in Figure

4.12. There is a clear rise in efficiency above 40 ml of resins. A result from these

tests was that a resin volume of 25 ml was used for the remainder of the deposition

collection regime to avoid variations in counting efficiency. The average detector

efficiency determined for this volume is (11.14±0.37)%. Filter efficiencies were

assumed to be the same as the soil samples, it is noted that the filters were

insignificant in terms of their contribution to the total 210Pb.

The low energy 46 keV gamma emitted by 210Pb is easily self-absorbed even

through small samples and it is necessary to correct for this self-absorption that is

related to the samples density (Cutshall et al. 1983). Cutshall et al. (1983) provided a

means of correcting for this absorption through determination of a correction factor

that can be applied to the results. The correction factor is provided by the following

equation:

1)/()/ln(

−=

ITIT

OA

Equation 4-11

106

Where

A: sample photon emission rate

O: attenuated sample output

T: attenuated beam intensity

I: unattenuated beam intensity

The correction factor A/O is multiplied to the 210Pb activity concentrations

provided by GPEAK. T is the peak area counts for each sample and for this work I is

the test source peak area counts attenuated through the calibration standards, that is

the self-absorption corrections are corrected for the calibration standards. After

analysis of the self-absorption corrections for more than 50% three categories for

different corrections were distinguished. Samples were divided into sand, <2mm

fraction and >2mm fraction. The final average correction factors applied to the 210Pb

activity concentrations for each detector are shown in Table 4-4.

Table 4-4: Corrections factors applied to activity concentrations of soil samples Sample\Detector G N O

Sand 1.01±0.01 0.98±0.06 1.01±0.02

<2mm Fraction 1.03±0.04 0.95±0.01 1.03±0.02 >2mm Fraction 1.19±0.04 1.10±0.03 1.28±0.07

107

5

Radon sources

5.1 Overview This chapter presents the findings and analysis of 222Rn exhalation and associated

measurements carried out during the course of this project.

The sites covered represent a large variability in the factors that are known to

influence 222Rn exhalation. Grain size, porosity, 226Ra concentration and its

distribution within the grain and soil moisture content varied dramatically. Grain size

ranged from large rocks to fine grain while the porosity ranged from loose sands to

compact soils. Radium-226 concentration varied from ambient values to values

typical for mine-grade uranium bearing rocks and its distribution was expected to

vary from homogeneous to mainly on the surface of the grain. Moisture content of

the ground changed due to seasonal variations from dry to wet.

5.2 Rn-222 exhalation rate and 226Ra activity Radon-222 exhalation rate values given in Table 5-1 are representative of dry

conditions. Charcoal cups and emanometers were used to take the readings

(Procedural details are given in sections 4.3 and 4.4). Mine sites readings were taken

during the dry season of 2002 and 2003; seasonal sites readings are the averages for

the dry periods, August-October 2002 and May-July 2003. The location and

description of the sampling sites has been supplied in Chapter 3.

Three different methods were used to measure 226Ra activity concentration,

insitu gamma spectroscopy using a NaI(Tl) system (Section 4.6.1), HPGe gamma

spectroscopy of prepared soil samples (Section 4.6.2), and estimates through external

equivalent gamma dose rate measurements at 1m above ground (Akber et al. 2004b;

UNSCEAR 2000). Values of 226Ra activity concentrations for the corresponding sites

listed in Table 5-1 are provided in Table 5-2. The value RE-R in Table 5-2 is the ratio

of 222Rn exhalation rate (mBqm-2s-1) to 226Ra activity concentrations (Bqkg-1).

108

Table 5-1: Dry season values of 222Rn exhalation rates from all measurement sites Site Methods

* Number of

Measurements 222Rn Exhalation Rate (mBq.m-2.s-1)

Area Arithmetic Mean+

Geometric Mean^

Laterite Stockpile

Pad

C+E

20

(5.2±0.6)*103

4.4*103(1.1)

Push C+E 7 (8.1±1.5)*104 6.6*104(1.4)

Rim C+E 13 (3.8±0.5)*104 3.3*104(1.2)

Ore Stock Pile 2

Pad

C+E

15

(1.0±0.2)*104

7.2*103(1.3)

Rim C 10 (7.3±2.2)*103 4.3*103(1.5)

Waste Rock Dump

Pad

C+E

20

(5.3±1.0)*102

4.2*102(1.2)

Rehabilitated C+E 21 (9.4±1.0)*102 8.3*102(1.1)

Overburden E 4 (9.7±1.7)*102 9.2*102(1.2)

Ore Stock Pile 7

Pad

C+E

9

(3.1±0.7)*103

2.6*103(1.3)

Rim E 8 (9.5±3.5)*102 5.3*102(1.6)

Pile C 5 (1.7±0.7)*103 1.3*103(1.4)

Land Application

Area

Non-Irrigated Type II

Soil

C+E

30

70±3

69(1.0)

Non-Irrigated Type III

Soil

C+E 30 70±4 67(1.0)

Irrigated C+E 34 (1.1±0.1)*102 1.1*102(1.1)

* C – Charcoal Cups, E – Emanometers + Arithmetic mean with arithmetic standard error ^ Geometric mean with geometric standard error

109

Table 5-1: Continued

Site Methods

* Number of

Measurements 222Rn Exhalation Rate (mBq.m-2.s-1)

Area Arithmetic Mean+

Geometric Mean^

Mine Pit #1

General

C+ E

33

(5.0±0.5)*102

4.1*102(1.1)

Wall C 3 (3.0±0.5)*102 3.0*102(1.2)

Mine Pit #3

Rocks

C

2

(1.0±1.0)*103

2.3*102(8.8)

Pad C+E 25 (2.5±0.6)*103 1.2*103(1.3)

Rubble C+E 9 (1.7±0.7)*103 9.1*102(1.5)

Mudginberri C+E 44 35±2 33(1.1)

Mirray C+E 45 39±2 36(1.1)

Jabiru East C+E 45 43±2 40(1.1)

Jabiru Water Tower C+E 46 18±1 16(1.1)

Magela Creek C+E 31 (2.1±0.2)*102 1.8*102(1.1)

Waste Rock Dump

Seasonal Site

C+E

45

(2.5±0.2)*102

2.0*102(1.1)

Land Application

Area Non-Irrigated

Seasonal Site

C+E

39

43±5

36(1.1)

Irrigated Seasonal Site C+E 37 68±4 63(1.1)

Experimental Plot

Irrigated

C

19

(1.4±0.1)*102

1.3*102(1.1)

Non-Irrigated C 5 43±1 43(1.1)

* C – Charcoal Cups, E – Emanometers + Arithmetic mean with arithmetic standard error ^ Geometric mean with geometric standard error

110

Table 5-2: 226Ra activity concentrations and RE-R ratio

Site

Area Methods * Number of

Measurements 226Ra

Concentration (Bq.kg-1)+

RE-R+

Laterite Stockpile

Pad

A+B

22

(1.15±0.05)*104

0.40±0.04

Push C 7 (1.98±0.07)*104 4.06±0.72

Rim C 13 (1.74±0.07)*104 0.60±0.02

Ore Stock Pile 2

Pad

A+B

18

(5.15±0.43)*103

1.86±0.30

Rim C 10 (5.79±0.47)*103 1.58±0.59

Waste Rock Dump

Pad

A, B+C

19

(1.22±0.10)*103

0.47±0.09

Rehabilitated A+B 20 (1.30±0.10)*103 0.77±0.09

Overburden A 4 578±373 2.80±1.28

Ore Stock Pile 7

Pad

B+C

6

(2.77±0.23)*104

0.10±0.02

Rim B+C 4 (4.37±0.44)*104 0.02±0.01

Pile# B 1 (3.22±0.50)*104 0.04±0.01

Land Application

Area

Non-Irrigated Type

II Soil

A

5

74±6

1.21±0.07

Non-Irrigated Type

III Soil

A 5 73±9 0.85±0.07

Irrigated A 8 147±6 0.67±0.04

* A – Insitu gamma spectroscopy GS-512 NaI(Tl) Detector, B – Gamma spectroscopic from soil sample, C – 226Ra Activity estimated from external gamma dose rate + Arithmetic mean and arithmetic standard error # Counting error from gamma spectroscopic analysis supplied

111

Table 5-2: Continued

Site Area

Methods * Number of Measurements

226Ra Concentration

(Bq.kg-1)+

RE-R+

Mine Pit #1

General

B+C

30

(1.27±0.08)*103

0.41±0.04

Wall - 0 - -

Mine Pit #3

Rocks

-

0

-

-

Pad B+C 22 (9.93±2.22)*103 0.38±0.12

Rubble B+C 8 (8.50±1.31)*103 0.23±0.11

Mudginberri A 3 182±2 0.19±0.01

Mirray A 3 47±1 0.82±0.04

Jabiru East A 3 52±1 0.83±0.05

Jabiru Water Tower A 3 46±1 0.39±0.02

Magela Creek A 3 43±4 4.94±0.5

Waste Rock Dump

Seasonal Site

A

3

478±18

0.52±0.05

Land Application

Area

Non-Irrigated

Seasonal Site

A

9

58±1

0.75±0.08

Irrigated Seasonal

Site

A 6 144±1 0.67±0.04

Experimental Plot

Irrigated

A

19

356±5

0.40±0.01

Non-Irrigated A 5 125±3 0.35±0.01

* A – Insitu gamma spectroscopy GS-512 NaI(Tl) Detector, B – Gamma spectroscopic from soil sample, C – 226Ra Activity estimated from external gamma dose rate + Arithmetic mean and arithmetic standard error

112

Both the arithmetic and geometric means are provided for the 222Rn

exhalation rates in Table 5-1 along with the standard error for each (geometric

standard error provided in parenthesis). Radon-222 is a random variable that depends

upon a number of other variables that may also behave randomly in nature, such as

radioactive disintegration of 226Ra to produce 222Rn, the direction of recoil of 222Rn

ion, the local moisture conditions, atomic diffusion etc. A number of studies

including the present one have demonstrated that 222Rn exhalation rate distribution is

better represented by a log-normal distribution rather than a normal distribution

(Akber et al. 2004b; Akber et al. 2002; Holdsworth and Akber 2004). This behaviour

indicates that the random variables involved in 222Rn exhalation interact in a

multiplicative rather than an additive manner.

A total of 626 222Rn exhalation readings were performed in dry season

conditions covering 16 different sites both on and off the Ranger mineral lease. Of

the total, 317 were performed on identified 222Rn sources directly from the Ranger

mine; the remaining 309 at sites that cover natural and man influenced areas. Along

with the exhalation measurements a total of 235 measurements of 226Ra

concentrations were obtained through various methods described in Chapter 4. In

150 cases 226Ra activity concentration was obtained through insitu measurements

using a NaI(Tl) field detector. In order to cross-reference some of these results 15

samples were collected by hand and prepared for analysis on the HPGe system. In

cases where insitu gamma spectroscopy results were not available 226Ra activity

concentration was derived from an approximation using the gamma dose rate. This

approximation is only valid for uranium bearing material above 0.02% U3O8. This

approximation has been justified in (Akber et al. 2004b) using a mathematical

relationship described by UNSCEAR (2000). Wherever it was possible 226Ra activity

concentration measurements were taken at the same spot where 222Rn exhalation

measurements were performed. The value of RE-R in Table 5-2 is the ratio of the 222Rn exhalation rate to 226Ra activity concentration (mBq.m-2.s-1/Bq.kg-1). In Table

5-2 it is the average of individual calculations performed for such pairs of

measurements.

At Ranger, stockpile formation starts by paddock dumping in a designated

area to build up a base. Once the base is firm and high enough an access ramp is

created to reach the top of the pile, which is then compacted and flattened using

graders and other heavy vehicles. From this point the stockpile is a tipping head for

113

loads where the ore is dumped either over or near the edge of the pile to be bulldozed

over the edge. Paddock dumping on the pad is also performed in order to grow the

stockpile vertically and the process is repeated. The tip head is changed regularly to

ensure equal spreading of the material onto the pile. The top of the pile that has been

compacted and flattened for the heavy vehicle access is known as the ‘pad’. Around

the edge of each stockpile is a raised head of uncompacted material that has been left

after dumping, this area is known for the sake of this project as the ‘rim’. The rim is

less compacted and generally filled with non-pulverised material. At the periphery of

the pad before the rim there is sometimes a zone where bulldozers manipulate the

material, this area is referred to as the ‘push zone’. A push zone has less compaction

than the pad but is still made from a similar material. As described through the

literature review in Chapter 3, soil grain size and soil porosity can affect 222Rn

exhalation. For this reason measurements in these different areas have been

distinguished from one another.

The results across the laterite stockpile show how variations in porosity can

influence 222Rn exhalation to a great extent. Laterite is a fine-grained soil created

from the weathering of rocks. It has been leached of soluble minerals but still

contains large concentrations of iron oxides and iron hydroxides. Laterite at Ranger

is a reddish fine-grained uranium bearing material. As such the only variations

between the pad, push and rim zones were the degree of soil compaction and hence

the soil porosity. As may be seen in Table 5-2 226Ra activity concentrations for these

three areas are comparable but the corresponding 222Rn exhalation rates differ

greatly. For the pad zone a value of 4.4*103(1.1) mBq.m-2.s-1 is obtained while the

less compacted push and rim zones have rates of 6.6*104(1.4) mBq.m-2.s-1 and

3.3*104(1.2) mBq.m-2.s-1 respectively. These are also the largest 222Rn exhalation

rates recorded on the mine site and indicate that soil porosity is an important

contributing factor to the 222Rn exhalation from this site.

Emanometers were used to measure 222Rn exhalation from a pile of

overburden material that had been laid a few weeks earlier. This overburden material

was also made up of fine grains and appeared similar to a laterite push zone area.

Four readings of the 222Rn exhalation rate averaged at 9.2*102(1.2) mBq.m-2.s-1. The 226Ra activity concentration of this soil is 578±373Bq.kg-1 and by comparison the 222Rn exhalation behaviour appears to be similar to that of the push zone of the

laterite.

114

The ore stockpiles are mostly made from the schist rock of the upper mine

sequence. They have a wide range of rock sizes from boulders to fine grains. The rim

zones from the ore stockpiles mainly contain larger rocks; the pad zones contain

similar sized rocks but they are compacted with smaller sized gravel in between. The

average 222Rn exhalation rate from a grade 7 ore stockpile pad is 2.6*103(1.3)

mBq.m-2.s-1 while for the rim and pile areas they are 5.3*102(1.6) mBq.m-2.s-1 and

1.3*103(1.4) mBq.m-2.s-1, respectively. Similarly for the grade 2 ore stockpile the pad

area has a higher 222Rn exhalation rate than that for the rim, the values being

7.2*103(1.3) mBq.m-2.s-1 and 4.3*103(1.5) mBq.m-2.s-1, respectively. Rims at these

stockpiles contain larger sized rocks 10-20 cm in size while the pad area is

compacted and contains a mix of large rocks with small rocks and pulverised gravel

down to fine grains. The compaction on the pad results in reduced porosity that will

reduce the overall 222Rn exhalation due to constrictions and tortuous path lengths. On

the other hand the smaller sized material will provide more surface area for 222Rn to

emanate from it. It appears that, in case of Ranger stockpiles the net result of these

two competing effects is an increase in the exhalation rate from the pad area.

Of particular interest in the case of the ore stockpiles was the variation seen

in 222Rn exhalation rate between the grade 7 and grade 2 stockpiles. The stockpiles

are graded with regard to the uranium concentration (Table 3-1). Since the grade 7

stockpile has a higher 226Ra concentration than the grade 2 stockpile we expect that

the 222Rn exhalation rate would be higher there considering that both stockpiles

consist of similar sized material. A possible explanation may lie in the differences in

the stockpile heights. The grade 7 stockpile was about 3-4 metres in height above the

surface of the natural soil while the grade 2 stockpile was much larger being closer to

25 metres. The waste rock dump pad area was about 10 metres high. The ratio RE-R

for these three sites increases with increasing stockpile height. If diffusion is the

dominant mechanism of transport of 222Rn from the subsurface then the exhalation

rate may be given by the relationship (Holdsworth and Akber 2004):

⎟⎠⎞

⎜⎝⎛×=

LHREJ tanhexp λρ

Equation 5-1

115

Where

J: 222Rn exhalation rate (mBq.m-2.s-1)

Eexp: 222Rn emanation coefficient

ρ: bulk density of soil (kg.m-3)

R: 226Ra activity concentration (Bq.kg-1)

λ: 222Rn decay constant (s-1)

L: diffusion length (L = λ/D )

D: diffusion coefficient of 222Rn through the material (m2.s-1)

H: height of stockpile (m)

This suggests that RE-R for different stockpiles of similar geomorphologic conditions

should be proportional to tanh (H/L). Using approximations of the stockpile heights

for the three sites calculations were carried out to estimate the 222Rn diffusion lengths

through the stockpiles. The results obtained tend to suggest that for stockpile type

structures 222Rn diffusion length is likely to be a few tens of metres in magnitude.

This estimate is substantially different from that commonly reported in the literature

for dry soils (1-2 m) (Graaf et al. 1992) or for sandy type materials (1.9-2.5 m)

(Holdsworth and Akber 2004). Other compounding factors not investigated may

include differences in uranium distribution for rocks of different ore grades or

potential mixing of different grade material between stockpiles.

Waste rock is the lowest grade ore at Ranger; it is made up of upper mine

sequence schist material. Waste rock is an important material in terms of the post

rehabilitation situation at Ranger as it is likely to be the covering surface material for

the pits and original tailings dam. Three areas of waste rock were selected for study;

a pad area similar to those found on other stockpiles; a rehabilitated area tilled and

planted with sparse vegetation maximum 3 metres in height; and the seasonal site

that was tilled but contained little vegetation. The pad and rehabilitated sections of

the waste rock dump have comparable 226Ra activity concentrations, (1.22±0.10)*103

Bq.kg-1 and (1.30±0.10)*103 Bq.kg-1 respectively, but differing values of 222Rn

exhalation rates, 4.2*102(1.2) mBq.m-2.s-1 and 8.3*102(1.1) mBq.m-2.s-1 respectively.

Explanation for this difference is the porosity of the ground at each site. The

rehabilitated area has been tilled and had trees planted there while the pad area was

compacted with heavy vehicles regularly driving over the area. Vegetation found on

the rehabilitated area is well developed and will have extensive root structures. Roots

116

break up the ground and maintain moisture thus increasing the soil porosity and soil

moisture at this location. The effect of this increase in porosity and moisture is

observed with a difference in 222Rn exhalation rates between the two sites. The

effects that vegetation can have upon the 222Rn exhalation rates has been previously

reported (Morris and Fraley 1989; Morris and Fraley 1994).

Radium-226 activity concentration at the seasonal waste rock site was

recorded as 478±18 Bq.kg-1, almost one third of the other two waste rock sites. The

observed dry season 222Rn exhalation rate from this location was 2.0*102(1.1)

mBq.m-2.s-1. Ground at the seasonal site had been tilled for planting but the

vegetation here was very sparse with a maximum height of 1.5 m. RE-R values for the

three waste rock sites are 0.47±0.09, 0.52±0.05 and 0.77±0.09 (mBq.m-2.s-1/Bq.kg-1)

for the pad, seasonal and rehabilitated sites, respectively. The pad and seasonal sites

are similar with the only difference being tilling and light vegetation at the seasonal

site, this may have resulted in the slightly higher RE-R value. The rehabilitated site is

more vegetated and tilled, given all other parameters are similar this is the only

explanation for the higher RE-R value obtained here. As waste rock will be the

covering surface material for large sections of the project area after rehabilitation the

effects of vegetation on 222Rn exhalation rates should be taken into consideration.

Ranger’s waste rock dump has been subject to previous 222Rn exhalation rate

surveys. One performed by Mason et al. (Mason et al. 1982) derived an exhalation

rate per percent of ore grade of +9649-32 mBq.m-2.s-1%-1 of U3O8 for waste rock and

ore at Ranger, Nabarlek and Rum Jungle. When using this value with known grade

uranium concentrations we obtain similar exhalation rates to those observed from ore

stockpiles and pits during the course of this project, with the exception of the grade 7

stockpile. The average value reported by Mason et al. of 49 mBq.m-2.s-1%-1 of U3O8

was used by Kvasnicka (Kvasnicka 1990) to derive the 222Rn source term for Ranger

from the stockpiles, pit #1 and ore body 3. These values were then used for input to

an aerial dispersion model. That work determined the emission rates (Bq.s-1) given

the area for each source then averaged the exhalation rate over 500 m x 500 m grids.

Results presented in this project show that broad approaches cannot

accurately determine 222Rn exhalation rates from uranium mines without knowledge

of contributing variables. The more holistic approach made in this project should

enable better determination of Ranger’s 222Rn source term.

117

Pit #1 has been mined out and is now used as the primary tailings dam. A

bench, above the wet season water level was used for the 222Rn exhalation rate

survey as being representative of the 222Rn exhalation from this pit. Grasses and

other vegetation had taken hold on the bench and were present in most areas covered

in the survey. A section of the retaining wall at the rear of this bench was also

measured as a representation of the 222Rn exhalation rate from pit walls. The average

readings for the bench and pit wall are 4.1*102(1.1) mBq.m-2.s-1 and 3.0*102(1.2)

mBq.m-2.s-1 respectively. These levels of 222Rn exhalation rate compare with values

obtained at the waste rock pad location. Radium-226 activity concentration is

(1.27±0.08)*103 Bq.kg-1 on the bench that is also similar to the value measured at the

waste rock pad and rehabilitated sites. The ground structure on the bench is quite

different from that observed at the waste rock dump. The base of the bench is solid

rock covered in a layer of smaller rocks and fine grains. While the exact depth of

cover is unknown on the bench it is likely to be approximately 1 m. While 222Rn

from the waste rock dump, with similar 226Ra activity concentration and 222Rn

exhalation rates, maybe diffused from much deeper, the smaller grains observed on

the pit #1 bench may emanate more 222Rn resulting in an exhalation rate comparable

to that at the waste rock dump pad.

Measurements were also performed in the operational pit #3. The survey here

performed readings from a pad area compacted by heavy machinery, a pile of rubble

similar to rim zones on stockpiles as well as two measurements directly on rocks.

Results obtained give 222Rn exhalation rates of 1.2*103(1.3) mBq.m-2.s-1,

9.1*102(1.5) mBq.m-2.s-1 and 2.3*102(8.8) mBq.m-2.s-1 respectively. Radium-226

analysis could only be performed through laboratory gamma spectrographic analysis

of collected samples or derived from the gamma dose rate. It was expected that ore

grades and therefore 226Ra activity concentrations would vary over small distances in

the pit. Of particular interest are the results recorded for 222Rn exhalation rates of the

two rock samples. Independently they recorded values of 26±7 mBq.m-2.s-1 and

(2.0±0.1)*103 mBq.m-2.s-1 indicative of the vast difference that can be observed

between two similar samples. Radium-226 activity concentrations for these samples

were derived from gamma dose rates and do not accurately reflect the true values of

the rocks. Whether they were vastly differing in 226Ra activity concentrations or

distribution is not known.

118

Results of measurements performed on the operational mine site show large

variations in individual readings taken from stockpiles and pit #3 when compared to

the other sites. Some values recorded were unexpectedly low or high but there is no

doubt that these variations are real. Localised factors at these sites such as variations

in radioactivity content of various rocks and differing path lengths due to the

underlying material are the likely cause of these observed variations.

With ease of access to the site a thorough 222Rn exhalation survey was

performed across a large section of the Magela land application area in July and

August 2002. In total ninety-three sites over irrigated and non-irrigated sections of

the area were measured. The majority of these measurements were with charcoal

cups but at twelve sites additional measurements and sampling was performed

including emanometers, soil core, gamma dose rates and NaI(Tl). Sixty of the sites

are on non-irrigated areas and the remaining thirty-three from an irrigated region,

zone 6A. Non-irrigated sites are further divided between two soil types dominant to

the region known as type II and type III soil (Chartres et al. 1988). Results from the

non-irrigated sections show natural exhalation from both soil types are comparable

with values of 69(1.1) mBq.m-2.s-1 and 67(1.1) mBq.m-2.s-1 recorded for type II and

type III soils respectively. Radium-226 activity concentrations for these two

locations are also similar with values of 74±6 Bq.kg-1 and 73±9 Bq.kg-1 for type II

and type III soils respectively. These values are equivalent to ambient values

reported previously from the region (Todd et al. 1998) of 64±25 mBq.m-2.s-1.

It is known that the contribution to 222Rn exhalation is greater from the

surface layers of the soil decreasing with depth as shown by the tangential hyperbolic

relationship of Equation 5-1. Radium-226 deposited by surface irrigation onto the

Magela Land Application Area is absorbed into the surface layers of the soils

providing a very strong vertical gradient of 226Ra at this location. Additional

measurements were carried out by collection of soil cores from the region and are

reported in Chapter 6 and show the vertical gradient of 226Ra, this has also been

shown by other authors from samples collected from the land application area (Akber

and Marten 1991). Radium-226 activity concentration measurements over the area

gave a mean and standard error of 147±6 Bq.kg-1. These measurements were

performed using the GS-512 that provides an average value of 226Ra based upon

measurement of the 214Bi gamma ray. Due to surface irrigation of 226Ra the irrigated

area has elevated 222Rn exhalation rate of 1.1*102(1.1) mBq.m-2.s-1.

119

The mean dry season 222Rn exhalation rates from the non-irrigated and

irrigated seasonal sites are 36(1.1) mBq.m-2.s-1 and 63(1.1) mBq.m-2.s-1 respectively.

While these values are lower than the mean values reported for non-irrigated and

irrigated sections they still lie within the range observed. Radium-226 activity

concentration at the non-irrigated seasonal site is 58±1 Bq.kg-1 that is less than the

average value for type II non-irrigated soil. The non-irrigated seasonal site was also

at the edge of a small clearing with little vegetation and compact ground. The

combination of low 226Ra activity concentration and soil compaction results in lower 222Rn exhalation from this site when compared to the remainder of type II non-

irrigated soil area. Radium-226 activity concentration at the irrigated seasonal site,

144±1 Bq.kg-1 is comparable with the average value reported across the irrigated

section. The reduced 222Rn exhalation rate observed at this site must be the result of

other influencing factors. Soil at this site appeared more compact than the

surrounding area and it was close to a groundwater sampling bore. Specific locations

selected for deployment of charcoal cups around this site might not have received a

homogenous spread of irrigated water due to blockage from vegetation. Radon-222

exhalation rates of this level were observed across the irrigated site during the survey

but they represent the lower level of observed rates.

A detailed 222Rn exhalation rate survey was performed at the experimental

land application plot in July 2003. The survey included deployment of twenty-four

charcoal cups for measurement of 222Rn exhalation rates, collection of soil samples,

measurement of gamma dose rates and 226Ra activity concentration. Average 226Ra

activity concentration at the site is 356±5 Bq.kg-1, over twice the value recorded from

the irrigated section of the Magela land application area, while the mean 222Rn

exhalation rate is 1.3*102(1.1) mBq.m-2.s-1. The 222Rn exhalation rate here

statistically overlaps with values observed at the irrigated land application area.

Differences in 226Ra activity concentrations between the sites are not reflected in a

difference in 222Rn exhalation rate. The experimental plot was specifically selected as

it was devoid of large amounts of vegetation while the irrigated land application area

is representative of the common eucalyptus woodland of the region. We have

previously shown the influence that vegetation has upon 222Rn exhalation rates at the

waste rock dumps sites the effect is shown again here in the differences between the

experimental plot and irrigated land application area.

120

Effort was made to select a non-irrigated area around the experimental plot

but results of 226Ra activity concentration measurements show some water must have

settled around the edges of the site during irrigation. Similar to the irrigated section

of the experimental plot increased 226Ra activity concentration has not resulted in

increased 222Rn exhalation rate, measured as 43(1.1) mBq.m-2.s-1. This value is

comparable with non-irrigated sections of the land application area. Hence increases

in 226Ra activity concentration through land application of water have not resulted in

proportional increases in 222Rn exhalation rates.

The 222Rn exhalation rate observed at Mudginberri was greater than expected

with the mean dry season exhalation rate being 33(1.1) mBq.m-2.s-1, a 226Ra

concentration of 182±2 Bq.kg-1 was recorded that was high for natural soils of the

region. This site was once used as a holding pen for an abattoir located nearby at the

Mudginberri campsite. The area is bare of trees but speargrass (Sorghum intrans)

grows abundantly during the wet season. The site that measurements were taken was

within and around a radon station compound devoid of vegetation other than small

grasses. Ground at the compound is very compacted to an extent that at times during

the dry season it was not possible even to insert the soil temperature probe. This

compaction is probably a result of the area being devoid of vegetation and its

previous use as a cattle holding pen, when cattle and vehicles were regularly driven

over the area. While measured 226Ra concentration here is higher for an ambient site,

low porosity keeps 222Rn trapped in the soil. The result is a higher exhalation rate

compared with other natural sites of the region but not as high as more porous sites

observed with similar 226Ra activity concentrations.

Jabiru’s water tower is built on soil excavated from the location of Jabiru

Lake. In fact earth excavated from the lake was used over large areas of Jabiru to

build it up above the surrounding floodplain. It is typical that ground built up in this

fashion is compacted with heavy machinery resulting in solidly compacted ground at

the site. The site is in an open paddock devoid of vegetation other than grass.

Radium-226 activity concentration here is 46±1 Bq.kg-1, similar to other natural soils

in the region. The compaction and lack of vegetation has a noticeable effect upon the

rate of 222Rn exhalation here with the recorded dry season value of 16(1.1)

mBq.m-2.s-1.

Mirray is a natural undisturbed location representative of the region. Dry

season levels of 222Rn exhalation rates here have a mean value of 36(1.1) mBq.m-2.s-1

121

comparable to sites like the non-irrigated land application and the value for common

woodland in the region reported by Todd et al (1998). Radium-226 activity

concentration here is 47±1 Bq.kg-1, also a typical level comparable with that of the

natural soils in the region. Vegetation consisted of varieties of trees, shrubs and

grasses; the soil at the site was relatively porous.

Jabiru East is a rehabilitated area once the location of a township that

serviced Ranger mine. The town was demolished in 1990 and the site rehabilitated

through clearing and planting of local vegetation species, soil at the measurement site

is relatively porous. In the decade since rehabilitation the area has recovered well but

is noticeably different from surrounding woodland as it has younger vegetation. The

mean dry season 222Rn exhalation at this site is 40(1.1) mBq.m-2.s-1 with the 226Ra

concentration being 52±1 Bq.kg-1. The 222Rn exhalation rate here is comparable with

other natural sites measured.

The site alongside Magela creek consists of porous sandy soil. Todd et al.

(1998) previously measured 222Rn exhalation rates here and recorded high readings.

During this work it was observed that 226Ra activity concentration here is similar to

other natural sites with a value of 43±4 Bq.kg-1 but the mean 222Rn exhalation rate is

much greater being 1.8*102(1.1) mBq.m-2.s-1. The soil here is alluvial with no rocks

and a sandy consistency with high porosity. High 222Rn exhalation rate observed here

is due to high soil porosity and small grain size. Vegetation consisted of grasses and

a few small trees.

Careful selection of the sampling sites enabled us to select sites that covered a

broad range of 226Ra activity concentrations over four orders of magnitude. This has

in turn provided a large range of 222Rn exhalation rates also over four orders of

magnitude. Plotting the results of 222Rn exhalation rates against 226Ra activity

concentrations in Figure 5.1 shows that a weak positive correlation between the

variables exists.

122

y = (0.452 ± 0.384)xR2 = 0.176

10

100

1000

10000

100000

10 100 1000 10000 100000226Ra Activity Concentration (Bq.kg-1)

222 R

n Ex

hala

tion

Rat

e (m

Bq.

m-2

.s-1

)

Figure 5.1: Plot of 222Rn exhalation rates vs. 226Ra activity concentrations for all sampling sites

This data is presented in a different format in Figure 5.2 where a plot of the

ratio, 222Rn exhalation rate/226Ra activity concentration (RE-R) for all the dry season

results is shown organised in ascending order of RE-R. From this figure we observe

the broad trends discussed referring to comparisons or variations in soil parameters

from various sites. Sites have been categorised into one of four geomorphic clusters,

non-compacted boulders, barren, vegetated and non-compacted fine grains. Despite a

number of orders of magnitude variation in 226Ra activity concentration and 222Rn

exhalation rates sites belonging to similar geomorphic structures have clustered

together. The greatest RE-R values are for sites of non-compacted fine-grained

material such as Magela, laterite push zone and over burden.

A result of identifying these clusters enables a reanalysis of the correlation

between 222Rn exhalation rates and 226Ra activity concentration similar to that

performed in Figure 5.1. Locations such as the grade 7 ore stockpile pad, grade 2 ore

stockpile rim and grade 2 ore stockpile pad, as displayed in Figure 5.2, are outliers

from their clusters due to the variation in 222Rn diffusion length discussed previously.

With these outliers removed a clearer pattern and better relationships are established

123

for three of the four geomorphic categories. The results of reanalysis are displayed in

Figure 5.3. The results from Figure 5.3 are summarised in Table 5-3 with the outliers

removed.

124

Figure 5.2: Ratio of 222Rn exhalation rate to 226Ra activity concentration (RE-R) for locations during dry conditions

0.01

0.10

1.00

10.00

OSP 7 Rim

OSP7 Pile

OSP7 Pad

Pit3 ro

ckMudginberr

iPit 3

Rubble

Expplot N

onPit3

pad JWT

Expplot Ir

rLate

rite p

adPit 1

genera

lWRD Pad

LAA Irr Sea

sonal

WRD Seaso

nalLate

rite r

imLAA Irr

LAA Non Sea

sonal

WRD Reh

abMirr

ayJa

biru Eas

tLAA N

IRR III

LAANIR II

OSP2 Rim

OSP2 Pad

Overb

urden

Laterit

e push

zone

Magela

Location

222 R

n Fl

ux (m

Bq.

m-2

.s-1

)/226 R

a C

once

ntra

tion

(Bq.

kg-1

)

Vegetated Non-compacted bouldersNon-compacted fine grainsBarren

125

For barren sites such as Jabiru water tower, all stockpile pads, Mudginberri,

pit bases and the experimental plot an average RE-R of 0.49±0.14 mBq.m-2.s-1/Bq.kg-1

is obtained. With the exception of the experimental plot they are all disturbed sites

where compaction of the soil has taken place as a result of human influence. In most

cases this compaction has resulted in a decrease in the 222Rn exhalation rate

compared with natural undisturbed sites. The grade 2 and grade 7 ore stockpile pads

are clear outliers and their removal from the analysis provides a lower, but

statistically better RE-R of 0.36±0.04 mBq.m-2.s-1/Bq.kg-1.

Vegetated woodland and rehabilitated sites such as Jabiru East, rehabilitated

waste rock, Magela land application area and Mirray have an RE-R of 0.82±0.06

mBq.m-2.s-1/Bq.kg-1. This group includes all common woodland sites investigated

and represents the ratio for relatively porous vegetated soils of the region.

The greatest RE-R values are obtained from non-compacted fine grains such as

Magela, laterite push zone and overburden locations. The average RE-R for these

regions is 3.1±1.0 mBq.m-2.s-1/Bq.kg-1 indicating that the porosity of the soil at these

locations plays a dominant role in controlling the 222Rn exhalation rates.

The cluster of non-compacted boulders has the lowest RE-R value of

0.12±0.05 mBq.m-2.s-1/Bq.kg-1. This covered the stockpile rims, piles and rocks

studied in pit #3. This result is expected, as it is known 222Rn has difficulty

emanating from within enclosed crystalline lattices found in rocks. Again the grade 2

ore stockpile rim provides an outlier in this set and is likely the result of 222Rn

diffusion from greater depths as mentioned previously, this was removed from

calculation of the average.

The results from the regression analysis of the four geomorphic clusters are

provided in Table 5-3. An important outcome of this work is the 222Rn exhalation

rate to 226Ra activity concentration relationship derived as a result of this regression

analysis. This relationship kappa (κ), in Table 5-3, is an RE-R derived from regression

analysis that can be applied to similar geomorphic structures at dry tropical locations.

All geomorphic clusters, with the exception of non-compacted boulders returned

positive correlations with good determination coefficients. Due to large air gaps and

random 222Rn diffusion paths within non-compacted boulder structures the typical

positive relationship between 222Rn exhalation rate and 226Ra activity concentration

does not hold. The results presented in Table 5-3 will be used in Chapter 7 to

determine the dry season 222Rn source term for the Jabiru and Kakadu regions.

126

Table 5-3: Analysis result of geomorphic clusters Geomorphic

group

Average RE-R

(mBq.m-2.s-1/Bq.kg-1)

κ (mBq.m-2.s-1/Bq.kg-1) R2

Barren 0.36±0.04 0.27±0.05 0.82

Vegetated 0.82±0.06 0.61±0.03 0.98

Non-compacted

fine grains

3.1±1.0 2.70±0.42 0.93

Non-compacted

boulders

0.12±0.05 - -

127

Figure 5.3: Plot of 222Rn exhalation rate vs. 226Ra activity concentration for all sites categorised by geomorphic groups

Barren

ΦRn-222 = (0.27 ± 0.05)CRa-226

R2 = 0.82p < 0.01

10

100

1000

10000


222 R

n Ex

hala

tion

Rat

e (m

Bq.

m-2

.s-1

)

Non-compacted fine grains

ΦRn-222 = (2.70 ± 0.42)CRa-226

R2 = 0.93p < 0.01

10

100

1000

10000

100000


222 R

n Ex

hala

tion

Rat

e (m

Bq.

m-2

.s-1

)Vegetated

Φ Rn- 222 = (0.61 ± 0.03)CRa- 226

R 2 = 0.98p < 0.01

10

100

1000

10 100 1000 10000226Ra Activity Concentration (Bq.kg-1)

222 R

n Ex

hala

tion

Rat

e (m

Bq.

m-2.s

-1)

Non-compacted Boulders

100

1000

10000

1000 10000 100000226Ra Activity Concentration (Bq.kg-1)

222 R

n Ex

hala

tion

Rat

e (m

Bq.

m-2

.s-1

)

128

5.3 Diurnal measurements of radon exhalation Diurnal measurements of 222Rn and 220Rn exhalation rates were recorded seven times

from five of the seasonal sites. Towards the end of the wet season 2002 readings

were performed at Mudginberri, Jabiru Water Tower and Mirray sites. Later that year

during the dry season measurements were performed at Mudginberri, Jabiru Water

Tower, Jabiru East and Magela Creek. The aim was to observe any possible

correlation of 222Rn or 220Rn exhalation rates with meteorological parameters. On the

basis of information from the reported literature parameters of primary interest were

soil temperature and atmospheric pressure.

Literature shows that radon emanation is directly proportional to soil

temperature (oC) and that the radon exhalation rate is directly proportion to the

square root of the soil temperature (oC). The effect of temperature variations on

diffusion is limited to the top few decimetres of soil. Temperature should therefore

influence 220Rn exhalation more than 222Rn exhalation as 220Rn is exhaled only from

the top few decimetres of soil. Other studies have shown 222Rn exhalation is

inversely proportional to atmospheric pressure. The inverse relationship between 222Rn exhalation rate and atmospheric pressure has been used previously to explain

observed diurnal variations in 222Rn exhalation rates. These potentially influencing

parameters have been discussed in detail in Chapter 2 of the thesis.

In the Alligator Rivers Region the atmospheric pressure, atmospheric

temperature and soil temperature vary diurnally. To demonstrate this, measurements

for a selected period of time are shown in Figure 5.4 and Figure 5.5. Atmospheric

pressure records low values in the mid afternoon and high values in the morning.

Temperature variations are as expected with minimums recorded before dawn and

maximums in the mid afternoon. In the region, on a diurnal basis, typical

atmospheric temperature and pressure variations are 12-14 oC and 5-7 hPa

respectively.

It was noted that variations in the collection chamber temperature measured

during these tests followed the same pattern as air temperature variations. As

variations in air temperature have not been reported to influence radon exhalation no

analysis using this variable has been performed.

129

1008

1010

1012

1014

1016

20/08/030:00

20/08/036:00

20/08/0312:00

20/08/0318:00

21/08/030:00

21/08/036:00

21/08/0312:00

21/08/0318:00

DateTime

Atm

osph

eric

Pre

ssur

e (h

Pa)

Figure 5.4: Diurnal variations of atmospheric pressure observed at Jabiru East (Data courtesy of Australian Bureau of Meteorology)

10

20

30

40

20/08/030:00

20/08/036:00

20/08/0312:00

20/08/0318:00

21/08/030:00

21/08/036:00

21/08/0312:00

21/08/0318:00

DateTime

Tem

pera

ture

(o C)

Site Air Temp. Site Soil Temp. Weather Station Air Temp.

Figure 5.5: Diurnal variations in atmospheric and soil temperatures at Jabiru East (Data courtesy of Australian Bureau of Meteorology)

130

Combining the information supplied in Figure 5.4 and Figure 5.5 with

previous studies it is expected that peak 222Rn exhalation should occur during the mid

or late afternoon when atmospheric pressure is dropping and temperature is at a

maximum. For the region of this study a small set of advantageous diurnal

measurements was reported (Todd et al. 1998). They reported poor correlation

coefficients (R-values) between 222Rn, 220Rn exhalation rates and soil temperature,

-0.536 and 0.368 respectively, and atmospheric pressure, 0.428 and -0.482

respectively. Variation between positive and negative correlations is further display

of the poor relationship between the variables

In Figure 5.6 and Figure 5.7 the combination of the seven diurnal

measurements has been reported. For each site the average daily 222Rn and 220Rn

exhalation rates have been used to normalise the data. The observations suggest that

for both radon isotopes, diurnal trends, if any, are masked by the random variation of

the signal. Standard deviations of all the measurements are 0.46 and 0.15 for 222Rn

and 220Rn respectively. This displays the general scatter in the results with no distinct

pattern.

To further investigate any variation with the influencing parameters that

change diurnally, the data are plotted as a function of soil temperature in Figure 5.8

and Figure 5.9 and the change in exhalation rate against the change in atmospheric

pressure in Figure 5.10 and Figure 5.11. Change in atmospheric pressure and change

in exhalation rates are obtained through subtracting the previous readings from the

current one. No trends appear in the data suggesting that we are observing systematic

variations with these parameters. To confirm this regression analysis was performed

on the data displayed in Figure 5.6 to Figure 5.11 similar to that performed by Todd

et al. (1998), the results returned poor correlations varying between positive and

negative relationships for the variables. These statistical results are displayed in

Table 5-4

Diurnal measurements were performed during the wet season and again

during the dry season to see if diurnal variations were dependent upon the moisture

content of the soil. Separate analysis of these results indicated no diurnal variations

or relationship with soil temperature and atmospheric pressure during either season.

131

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

6:00 10:00 14:00 18:00 22:00 2:00 6:00 10:00 14:00Time of Day

Nor

mal

ised

222 R

n E

xhal

atio

n R

ate

Jabiru Wet Mirray Wet Mudginberri WetJabiru Dry Magela Dry Jabiru East DryMudginberri Dry

Figure 5.6: Normalised 222Rn exhalation rate for all sites vs. time of day of measurement

0.4

0.6

0.8

1

1.2

1.4

1.6

6:00 10:00 14:00 18:00 22:00 2:00 6:00 10:00 14:00Time of Day

Nor

mal

ised

220 R

n E

xhal

atio

n R

ate

Jabiru Wet Mirray Wet Mudginberri Wet Jabiru DryMagela Dry Jabiru East Dry Mudginberri Dry

Figure 5.7: Normalised 220Rn exhalation rate for all sites vs. time of day of measurement

132

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

25 27 29 31 33 35 37 39

Soil Temperature (oC)

Nor

mal

ised

222 R

n E

xhal

atio

n R

ate

Jabiru Wet Mirray WetMudginberri Wet Jabiru DryMagela Dry Jabiru East DryMudginberri Dry

Figure 5.8: 222Rn exhalation rate vs. soil temperature

0.4

0.6

0.8

1

1.2

1.4

1.6

25 27 29 31 33 35 37 39Soil Temperature (oC)

Nor

mal

ised

220 R

n E

xhal

atio

n R

ate

Jabiru Wet Mirray Wet Mudginberri WetJabiru Dry Magela Dry Jabiru East DryMudginberri Dry

Figure 5.9: 220Rn exhalation rate vs. soil temperature

133

-80

-60

-40

-20

0

20

40

60

80

100

-3 -2 -1 0 1 2 3Change in Atmospheric Pressure (hPa)

Cha

nge

in 22

2 Rn

Exh

alat

ion

Rat

e (m

Bq.

m-2

.s-1

)


Figure 5.10: 222Rn exhalation rate vs. change in atmospheric pressure

-1000

-800

-600

-400

-200

0

200

400

600

800

-3 -2 -1 0 1 2 3

Change in Atmospheric Pressure (hPa)

Cha

nge

in 22

0 Rn

Exh

alat

ion

Rat

e (m

Bq.

m-2

.s-1

)


Figure 5.11: 220Rn exhalation rate vs. change in atmospheric pressure

134

Table 5-4: Correlation coefficients for diurnal measurements Radon-222 Exhalation Rate correlation against

Site Atmospheric

Temperature

Humidity Soil

Temperature

Pressure

Change

Mudginberri Dry - 0.03 0.27 -0.08

Jabiru East Dry -0.16 0.11 -0.20 0.64

Magela Dry -0.22 0.23 0.15 -0.01

Jabiru Dry 0.38 -0.20 0.49 0.03

Mudginberri Wet -0.56 0.59 -0.56 0.11

Mirray Wet -0.49 0.64 -0.59 0.27

Jabiru Wet -0.12 -0.12 -0.04 0.42

An important finding of this experiment relates to the input to models used

for radiological impact assessment of Ranger. These results show that daytime

measurements of 222Rn exhalation rates from Ranger, as were performed in Section

5.2, are valid for dose assessment calculations. The methodology used for the

charcoal cups is also justified. If diurnal variations exist then charcoal cups should be

collected at the same time of day that they were deployed so complete diurnal cycles

are measured. As no diurnal cycles in 222Rn exist in the region the results obtained

from charcoal cup measurements deployed and collected at different times of day are

valid.

135

5.4 Seasonal measurements of radon exhalation Expectations from theory and previous studies indicated that wet season rainfall

should retard 222Rn exhalation rates. Sites selected for seasonal measurements have

been described previously in Chapter 3. Charcoal cups and emanometers were used

to measure 222Rn exhalation rates. The equipment used, the measurement procedures

and schedules have been described previously in Chapters 3 and 4.

Many authors have reported the effects of precipitation and soil moisture on 222Rn emanation and exhalation rates as covered in detail in Chapter 2. It is widely

accepted that when water penetrates soil it fills interspatial spaces and increases 222Rn emanation as recoiling atoms are slowed more quickly in water than air. Above

certain soil moisture content 222Rn exhalation rates decrease because 222Rn is soluble

and becomes trapped in wet soils. However in the large collection of work

investigating the effects of moisture on 222Rn emanation and exhalation little work

has been performed for a systematic coverage over a seasonal cycle, particularly

within tropical regions. At most sites it was observed that 222Rn exhalation rates

decreased throughout the wet season. Radon-222 exhalation rates were recorded over

a 4 to 10 day period on a monthly basis between August 2002 and July 2003 for eight

sites. Results of this work are graphically shown in Figure 5.12 and Figure 5.13.

Cumulative rainfall recorded from the nearest meteorological station at Jabiru East,

Jabiru Town or Cooinda is also displayed.

136

Magela Land Application Area Irrigated Zone

0

200

400

600

800

1000

1200

1400

Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul

2002-2003

Rai

nfal

l (m

m)

0

20

40

60

80

100

120

140

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)

Cumulative Rainfall Rn-222 Exhalation

Jabiru East

0

200

400

600

800

1000

1200

1400

Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul2002-2003

Rai

nfal

l (m

m)

0

20

40

60

80

100

120

140

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)


Magela Creek

0

200

400

600

800

1000

1200

1400


2002-2003

Ran

ifall

(mm

)

0

60

120

180

240

300

360

420

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)

Cumlative Rainfall Rn-222 ExhalationMagela Land Application Area Non-Irrigated

0

200

400

600

800

1000

1200

1400


2002-2003

Rai

nfal

l (m

m)

0

20

40

60

80

100

120

140

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)


Figure 5.12: Seasonal variations of 222Rn exhalation rates and cumulative rainfall

137

Waste Rock Dump

0

200

400

600

800

1000

1200

1400


Rai

nfal

l (m

m)

0

80

160

240

320

400

480

560

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)

Cumlative Rainfall Rn-222 Exhalation

Jabiru Water Tower

0

200

400

600

800

1000

1200

1400

1600


Rai

nfal

l (m

m)

0

4

8

12

16

20

24

28

32

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)

Cumlative Rainfall Rn-222 Exhalation

Mirray

0

200

400

600

800

1000

1200


Rai

nfal

l (m

m)

0

30

60

90

120

150

180

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)

Cumulative Rain Rn-222 Exhalation

Mudginberri

0

200

400

600

800

1000

1200

1400


Rai

nfal

l (m

m)

0

20

40

60

80

100

120

140

Rad

on E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)


Figure 5.13: Seasonal variations of 222Rn exhalation rates and cumulative rainfall continued

138

Elevated 222Rn exhalation rates were observed during January 2003 at Jabiru

East, irrigated Magela land application area, Mirray and Mudginberri. Conditions at

these sites were similar to other sites so explanation of this outlier comes from

analysis of precipitation events and its influence on 222Rn emanation. The day prior

to charcoal cup deployment recorded 124 mm of rain over 24 hours till 9 am, this

was the largest rain event that wet season. During charcoal cups exposure only light

rain events occurred. Precipitation occurring before cup deployment has increased 222Rn emanation. Also 222Rn exhaled from layers underneath became trapped in the

water. It is possible that evaporation of water during charcoal cup exposure has

released trapped 222Rn and resulted in the observed increased 222Rn exhalation rate. A

similar effect has previously been observed and reported (Todd and Akber 1996). In

that study 220Rn exhalation rates, three days after addition of water to a monazite

sample increased by 20% compared to dry exhalation rates. As 222Rn has a much

longer half-life compared to 220Rn it can remain trapped in water for many days.

There are large errors associated with the January measurements that are an effect of

the large variations in individual measurements. It indicates that transport of 222Rn in

water, rather than normal diffusion, is dominant. Movement of 222Rn with water

through soil and evaporation through capillary channels has resulted in some cups

recording higher 222Rn exhalation rates.

Elevated levels were observed at Mirray during February and March along

with the January peak. This site is on sloping ground and most water deposited their

runs off quickly. This may be an indication that enough water is retained at the site to

increase emanation followed by an increase in exhalation due to evaporation

explained previously. There is a slight increase in 222Rn exhalation at the non-

irrigated Magela land application site in January and February but it is not as great as

that observed at other sites. This site is similar to the irrigated Magela land

application area with more soil compaction and without as much vegetation. The

slight increase observed there may be a result of minimal water absorption,

placement of charcoal canisters away from major evaporative sites or a combination

of these.

139

0

2

4

6

8

10

12

14

16

18

20

9-12Aug

9-12Sep

7-9Oct

8-11Nov

6-9Dec

8-10Jan

4-7Feb

4-10Mar

4-10Apr

3-8May

3-10Jun

3-8Jul

Deployment Date 2002-2003

222 R

n A

tmos

pher

ic C

once

ntra

tion

(Bq.

m-3

)

0

20

40

60

80

100

120

140

160

222 R

n E

xhal

atio

n R

ate

(mB

q.m

-2.s

-1)

Rn-222 Atmospheric Concentration Rn-222 Exhalation

Figure 5.14: Averaged 222Rn exhalation rates and atmospheric concentrations for sampling periods at Mudginberri

Monitoring of 222Rn atmospheric concentrations has been conducted in the

region for many years. Data collected from this monitoring program shows seasonal

variations of 222Rn concentrations with a low throughout the wet season (Akber et al.

1993). Lower 222Rn exhalation rates in the wet season would be expected to be a

major contributor to low 222Rn atmospheric concentrations.

The Mudginberri sampling site is adjacent to an atmospheric radon

monitoring station maintained by eriss. Data from this station was used to obtain

average 222Rn concentrations for the days when the exhalation measurements were

performed. The aim is to see any direct correlation between the local 222Rn

exhalation rate and atmospheric concentration. The general trend is a decrease in

both 222Rn atmospheric concentrations and exhalation rates during the wet season as

shown in Figure 5.14, however correlation between the two variables is not clear.

Particularly the exhalation peak in January 2003 does correspond to an increase in

atmospheric concentration. It is known that a number of meteorological factors that

change on a daily basis, such as inversion layers and wind speed, substantially

change 222Rn atmospheric concentration in the area. Hence, these meteorological

140

factors override the day-to-day variations that 222Rn exhalation rates have on

atmospheric 222Rn concentrations.

Peak 222Rn exhalation rates were not observed at the waste rock dump, Jabiru

water tower or non-irrigated Magela land application area during January 2003. The

waste rock dump is very porous with a good capacity to absorb large amounts of

water. Water deposited onto the waste rock dump will quickly travel to depth. Jabiru

water tower however is compact with low capacity to absorb water and the majority

of water deposited at this site runs off. Therefore there is little absorption and so no

increase in emanation or exhalation as a result. Mudginberri has also been noted as

being compact but compaction here is not as great as at the Jabiru water tower, this is

confirmed by considering formation methods. Mudginberri was cleared and used as a

cattle holding pen while Jabiru water tower was constructed on excavated soil

compacted with heavy machinery. Therefore it is likely that larger amounts of water

are absorbed at Mudginberri increasing emanation and releasing 222Rn during

evaporation.

Effects of soil moisture on 222Rn exhalation are complex and controlled by

more than one implication of a moisture profile. Previous studies (Hart 1986; Todd

and Akber 1996) have demonstrated the effect that increasing moisture content have

on 222Rn and 220Rn exhalation rates. Those works examined total moisture content of

a sample. Field observations in this study show that a diversity of moisture content

exists in the soil depths that change throughout a wet season. Observations at Jabiru

East are displayed in Figure 5.15. These measurements are discrete, however it is

expected that the moisture changes due to the intensity, duration and intervals of rain

events. In turn this will have an effect on 222Rn exhalation rates and during a wet

season 222Rn exhalation rates from localised areas may vary greatly over short

periods of time. As the dry season progresses, soil moisture levels drop rapidly

compared with wet season values and remain relatively constant (Figure 5.16).

Wet season variations in soil moisture profiles similar to that seen in Figure

5.15 were observed from all four sites selected for measurement. Some differences in 222Rn exhalation rates observed throughout the wet season can be explained through

examination of soil moisture profiles. After precipitation, moisture penetrates the

ground and seeps downward. The rate and depth of penetration depends on factors

such as amount of deposited water, current soil moisture and soil porosity. For

normal soils, recent rain will increase soil moisture in top layers and this will have a

141

0.00

5.00

10.00

15.00

10 20 30 40 50 60 70 80 90

Depth (cm)

% M

oist

ure

by V

olum

e

October November January February March

Figure 5.15: 2002-2003 wet season moisture profiles for Jabiru East

0.00

1.00

2.00

3.00

4.00

10 20 30 40 50 60 70 80 90

Depth (cm)

% M

oist

ure

by V

olum

e

May June July

Figure 5.16: 2003 dry season soil moisture profiles for Jabiru East

142

capping effect on 222Rn exhaled from depth. If a long enough time passes between

precipitation events, moisture penetrates deeper and 222Rn exhalation will occur from

the top layers of soil but will be capped at a deeper level.

Soil moisture measurements at Jabiru East for the month of January 2003

were performed on the 10th. This was three days after the heavy rain event

previously mentioned and soil moisture was less than 5% at all depths. Water

deposited has either runoff, evaporated or penetrated to depths beyond measurement

or a combination of these. Considering the compact nature of the ground at the site

runoff is the most likely explanation. Similar soil moisture values were observed at

three of the four measurement sites where no January peak in 222Rn exhalation was

observed, the exception being Mirray where a peak was observed. At Mirray 30-50

cm readings of soil moisture were high.

All soil moisture readings from Mirray are displayed in Figure 5.17. The soil

moisture reading for January indicate that either water absorbed uphill is travelling

through the soil at depth or water deposited has penetrated quickly to depth, the latter

being more plausible given the porous nature of soil at the site. Compared to

November’s results where 222Rn exhalation is low due to capping in the soils surface

layers January’s soil moisture profile is almost opposite. February and March

moisture levels are low compared with other wet season levels and correspond with

elevated 222Rn exhalation rates. January’s peak in 222Rn exhalation is still likely to be

a result of increased emanation followed by release during evaporation. February and

March results are due to increased emanation with increased soil moisture but the

moisture levels are not high enough to retard exhalation.

The soil moisture access tube at non-irrigated Magela land application area

went to a depth of 50 cm. Unexpected elevated levels of soil moisture were recorded

there at the 50 cm depth throughout the dry season. This site is only 20 m from land

that is irrigated almost daily over the course of the dry season. The observed

elevation in soil moisture may be indicative of water movement from the irrigated

area.

143

0

2

4

6

8

10

12

14

10 20 30 40 50

Depth (cm)

% M

oist

ure

by V

olum

e

Aug-02 Sep-02 Nov-02 Jan-03 Feb-03 Mar-03 Apr-03 May-03 Jun-03 Jul-03

Figure 5.17: Mirray soil moisture profiles, all readings 5.5 Chapter summary Investigations of the parameters known to influence 222Rn exhalation have been

performed within a tropical region and are presented. It has been shown that while 226Ra activity concentration is an important contributing factor of 222Rn exhalation, it

can be dominated by soil variables such as porosity and grain size. Dry season results

obtained from waste rock dumps, ore stockpiles, overburden, land application area

(irrigated and non-irrigated) and five ambient sites were used to determine our

conclusions, as the influence of soil moisture could be neglected. Diffusion length of 222Rn through rocky stockpile formations may be of the order of a few tens of metres

that differ greatly from standard soils. Compaction of the ground and its resultant

reduction of soil porosity decrease 222Rn exhalation rates. Vegetation with

established root structures lead to higher 222Rn exhalation rates.

Differences in 222Rn exhalation rates for sites based upon their

geomorphologic structure were displayed with the 222Rn exhalation rate to 226Ra

activity concentration ratio (RE-R). It was observed that sites of similar geomorphic

structure had comparable RE-R values and that four broad groups could be

formulated. Regression analysis of results from the four geomorphic groups provided

144

linear relationships with very good correlations for three of the four groups; (i)

vegetated; (ii) barren; and (iii) non-compacted fine grain sites. No relationship was

determined for non-compacted boulders due to large variations in 222Rn diffusion

paths. These results are used in Chapter 7 to determine the 222Rn source term for the

Kakadu region.

Several diurnal measurements were performed to observe possible variations

of 222Rn or 220Rn exhalation rates with changes in soil temperature or atmospheric

pressure. Diurnal variations of soil temperature and atmospheric pressure occur but

have no observable effect, if any, on either 222Rn or 220Rn exhalation rates.

Distinct wet and dry seasons enabled observation of the effect of precipitation

on 222Rn exhalation. In general, 222Rn exhalation rate reduces during the wet season.

Site to site and day to day behaviours could be complex, even localised variations

could occur due to uneven soil moisture profiles and possible evaporation as the

ground heats up after a precipitation event.

Large variations between individual wet season measurements result in large

uncertainties in site averages and such variations indicate that transport in water is

dominant over diffusion. Porous sites and very compacted sites displayed expected

seasonal trends with reduced 222Rn exhalation throughout the wet season.

Soil moisture was measured with a probe that provides a moisture depth

profile at 10 cm intervals. Soil moisture profiles at all sites varied throughout the wet

season due to water transport and strengths, times and duration of precipitation

events. This explains the large variations of 222Rn exhalation rates observed

throughout the wet season. Radon-222 exhalation can be expected to change over

short periods of time throughout the wet season as soil moisture profiles vary.

145

6

Lead-210 deposition and excess

6.1 Overview This chapter provides results of the seasonal 210Pb depositional rate measurements

and excess 210Pb inventories of the soil samples collected from the region. Distinct

seasonality in depositional rates is observed at Jabiru East and Oenpelli with the

majority of deposition occurring in the wet season. Jabiru East had a larger annual

depositional rate than Oenpelli, which may be a result of its proximity to Ranger

mine. Dry season deposition rates are low and indicative of long 210Pb atmospheric

residency times, the order of a few months.

Excess 210Pb inventories from 14 soil cores collected across the region are

included. These results have been compared with an excess 210Pb inventory derived

from the deposition rate measurements. Penetration half depth of 210Pb for the region

was determined to be 2.5±0.7 cm, which compares well with values reported for

Queensland, Australia (Akber et al. 2004a). Discrepancies in excess 210Pb inventories

observed with some samples have been investigated and explained.

One unique site, the Magela Land Application Area, was studied in detail for

retention of spray irrigated radionuclides found in Ranger wastewater. The soil’s

retention of 226Ra and 210Pb was within the top few centimetres with a distribution on

the fine grain fraction. Uranium-238 retention differed with results showing

horizontal and vertical transport more than expected at the onset of irrigation

program.

6.2 The 210Pb story After entering the atmosphere 222Rn spreads vertically and horizontally and

eventually decays, via its short-lived progeny, into 210Pb. With a half-life of 3.82

days for 222Rn, and a reported average residency time of between 5-9 days for 210Pb

in the atmosphere (Beks et al. 1998; Pourchet et al. 2000; Koch et al. 1996; Moore et

al. 1977), deposition of 210Pb occur at large distances from its point of origin.

Previous studies have shown that the vertical distribution of 222Rn and its progeny

decreases rapidly with increasing altitude (Cuculeanu and Lupu 1996; Moore et al.

1977). Both model and experimental studies of the horizontal distribution of 222Rn

146

from Ranger have shown that mine related concentrations decrease to negligible

levels within a few kilometres of the source (Urban et al. 1992; Davey 1994). This

means the majority of 222Rn concentration at environmental sites is mostly due to the

local contribution.

As 222Rn is soluble in water, the exhalation rate from water bodies is

extremely low. This causes some complexity to the budgeting estimates for the

Kakadu region, as there are large floodplains, creeks and billabongs that expand with

wet season rains and shrink throughout the dry season. The size to which they

expand and shrink is dependent on the rainfall of the wet season. Generally

floodplains fill and are submerged throughout the wet season but the size of creeks

and billabongs can vary dramatically with short-term changes in rainfall patterns.

As 222Rn moves through the atmosphere it eventually decays and its transport

characteristics change to that of a solid. The majority of progeny rapidly attach

themselves to aerosol particles in the atmosphere and become subject to the same

transport and deposition patterns as the aerosol particles they are attached to. This

has been detailed in Chapter 2.

Typically, surface soil is finer and more compact than the soil below it and as

a result it has greater moisture retention and a smaller diffusion coefficient that

causes torturous 222Rn path lengths. Radon-222 migrating through soil that reaches

this layer can become trapped beneath it and will decay there. This is part of the

natural process that redistributes 210Pb in the environment and results in a higher 210Pb concentration relative to 226Ra in the surface layers of soil. This process

combined with deposition contributes to excess 210Pb found in surface soils. For this

work excess 210Pb resultant from 222Rn being trapped in the surface soil is considered

to be negligible.

Another consideration is the high solubility of 226Ra in water. Some 226Ra

normally found in surface soils will leach down after precipitation events. The

amount of 226Ra leached out of the surface soil depends primarily on its distribution

in the soil grain as surface distributed 226Ra is more accessible to water than

homogenously distributed 226Ra. Lead-210 is much less mobile so 226Ra leaching

results in excess 210Pb with respect to 226Ra in surface soils. This effect has been

investigated in this chapter.

147

6.3 Pb-210 deposition 6.3.1 Seasonal 210Pb results

As described in Chapter 3 Jabiru East and Oenpelli were selected for 210Pb

deposition measurement over one seasonal cycle. Oenpelli lays approximately 50 km

north north west of the Ranger operation and Jabiru East lays 3 km west of the

operational pit #3. Lead-210 was collected in an ion exchange resin column attached

to the base of a wet and dry deposition collector as described in Chapter 4. The

results obtained from these measurements are shown in Figure 6.1 and Figure 6.2 and

tabulated in Table 6-1 and

148

Table 6-2. During the wet season the East Alligator River becomes impassable so a

light aircraft was chartered to collect the Oenpelli samples. Due to weather

conditions and flight availability the monthly collection routine was disrupted.

Collection periods were between five and six weeks for Oenpelli and it was decided

that Jabiru East sample collections should match those of Oenpelli. Due to

operational problems the Jabiru East collector was overlooked during April 2004 and

the sample was not collected until May 2005.

Figure 6.1: Jabiru East 210Pb deposition and cumulative rainfall

0

200

400

600

800

1000

1200

1400

1600

1800

2000

May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun

Month (2003-2004)

Rai

nfal

l (m

m)

0

50

100

150

200

250

300

350

400

210 Pb

Dep

ositi

on F

lux

(mB

q.m

-2.d

ay-1

)

Rainfall Pb-210 Deposition

149

Figure 6.2: Oenpelli 210Pb deposition and cumulative rainfall

Table 6-1: Results from Jabiru East 210Pb deposition collector Sampling dates Rainfall (mm) Average 210Pb deposition

(mBq.m-2.day-1)

1/5/03-31/5/03 5.4 32.4

1/6/03-30/6/03 0.2 0

1/7/03-31/7/03 0 16.1

1/8/03-31/8/03 0 24.1

1/9/03-30/9/03 0.8 23.3

1/10/03-31/10/03 31.2 369.2

1/11/03-2/12/03 105 260.8

3/12/03-9/1/04 482.8 242.8

10/1/04-6/2/04 458.2 174.0

7/2/04-12/3/04 538.6 165.1

13/3/04-18/5/04 149.8 89.7

19/5/04-23/6/04 53 34.7

0

200

400

600

800

1000

1200

1400

1600

1800

2000

May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun

Month (2003-2004)

Rai

nfal

l (m

m)

0

20

40

60

80

100

120

140

160

180

210 Pb

Dep

ositi

on (m

Bq.

m-2

.day

-1)

Rainfall Pb-210 Deposition

150

Table 6-2: Results from Oenpelli 210Pb deposition collector Sampling dates Rainfall (mm) Average 210Pb deposition

(mBq.m-2.day-1)

1/5/03-31/5/03 1.2 2.9

1/6/03-30/6/03 0 15.8

1/7/03-31/7/03 0 2.0

1/8/03-31/8/03 0 0.8

1/9/03-30/9/03 5.4 10.2

1/10/03-31/10/03 0 5.3

1/11/03-3/12/03 79.5 58.7

4/12/03-9/1/04 572.2 (64.0 calculated)

10/1/04-13/2/04 549.4 61.5

14/2/04-13/3/04 308.2 57.9

14/3/04-19/4/04 242.8 114.7

20/4/04-17/5/04 4 49.3

18/5/04-23/6/04 124.4 47.2

There were problems with sample collection at Oenpelli for October and

December 2003, a result of the resin column being blocked with organic matter. This

does not represent a large problem for October as meteorological data shows that

there were no rain events during that month. An average value obtained from all

other dry season months has been used to represent the October deposition. Loss of

the December sample is problematic and in order to obtain data for modelling

deposition is obtained from normalising January’s deposition against rainfall and

calculating December’s deposition from this. January was used over instead of all

wet season months, as the rainfall amounts for December and January are similar.

This problem was overcome during later collection when a bucket was used to

collect any spilt water. After blockages were removed from the column, excess water

was placed back into the top of the collector and allowed to pass through the resin

before the sample was collected.

The results displayed in Figure 6.1 and Figure 6.2 show distinct seasonality

for 210Pb depositional rates at Jabiru East and Oenpelli. As expected 210Pb

depositional rates are greater during the wet season. In some cases dry season

151

deposition rates are lower than the detection limits. This is evidence that precipitation

scavenging of 210Pb is the dominant removal mechanism of 210Pb from the

atmosphere.

At Jabiru East maximum 210Pb deposition is observed at the onset of the wet

season and it decays exponentially throughout the wet season. At the start of the wet

season 210Pb atmospheric concentrations are at a peak and the rain scavenges this.

Exponential decrease is a result of a similar reduction in the local 210Pb source term.

In Chapter 5 the waste rock dump seasonal site had a delay in retardation of 222Rn

exhalation rate at the onset of the wet season. This was because stockpile porosity

meant that early wet season rains penetrated the stockpile with minimal effect on 222Rn exhalation. Providing this occurs for all stockpiles then there is an exponential

decrease in 222Rn source term from Ranger as the wet season progresses. This may be

reflected in the 210Pb deposition rates observed at Jabiru East as a component of this

deposition is a result of the Ranger operation.

It may also be possible as 222Rn exhalation rate measurements were discrete

that the local area source term follows a similar pattern to the stockpiles. This would

most likely occur in porous soils and can also explain the wet season exponential

decrease in 210Pb deposition rates at Jabiru East. As the 210Pb source term reduces,

atmospheric concentrations and 210Pb deposition reduce in a similar pattern. Lead-

210 deposition rates in May 2003, April and May 2004 are higher than dry season

averages and are a result of light rain events that occurred during these months.

Similar results were observed at Oenpelli when rain events occurred during dry

season months.

Oenpelli displays a relatively steady state 210Pb depositional rate throughout

the wet season with the exception of a peak in April 2004. The local 210Pb term is

likely to suffer a similar decrease but precipitation events occur at frequencies that

maintain deposition rates at steady state values. The peak in April 2004 is a result of

reduced precipitation events and a likely increase in 222Rn exhalation rates

throughout this month. Normally convective storms occur in the late afternoon or

early evening every few days throughout March and April. The time between

precipitation events allows for an increase in 222Rn exhalation rates as the soil dries

and the moisture aids emanation, this in turn results in higher 210Pb atmospheric

concentration and deposition rates. Readings for April 2004 at Jabiru East can not be

used for comparison as they are averaged over two months.

152

The overall low 210Pb deposition rates at Oenpelli may be attributed to a

regional lack of 210Pb during the wet season. Oenpelli is surrounded by floodplains

and inaccessible other than air throughout the wet season, the township is an island

that rises out of the surrounding floodplain. The area will likely have very low 210Pb

atmospheric concentrations during the wet season resulting in low 210Pb deposition.

At Oenpelli there is also relatively increased 210Pb deposition in May and

June 2004 compared with normal dry season values. This is due to uncharacteristic

rain events occurring during these months. In May there were four rain events totally

4 mm, while June displayed heavier rains, 6 events totally 124 mm. The 210Pb

deposition rates are similar for these months being evidence that light rain events are

as efficient in scavenging 210Pb from the atmosphere as heavier events, this

observation supports previous studies (Baskaran 1995; Baskaran et al. 1993) and the

observations at Jabiru East. As 210Pb deposition rates for these months are similar to

averaged wet season values for Oenpelli it is speculated that the increase in surface

area due to reduction in inundated floodplains results in greater 222Rn emission in the

area and increases 210Pb atmospheric concentrations. The scavenging events during

these months, although only a few, have higher 210Pb atmospheric concentrations

available to extract. Therefore a few rain events result in averaged 210Pb deposition

rates similar to wet season values.

The results obtained here only provide 210Pb deposition rate monthly

averages. Previous work performed in the area provides information on 210Pb

deposition rates from individual rain events, including sequential sampling for six

rainstorms (Martin 2003). Three rainstorms sequentially sampled displayed large

amounts of 210Pb washout in the first few millimetres while the other storms

sequentially sampled did not. The former supports the conclusion discussed for

Oenpelli deposition rates in May and June 2004 that light rains can scavenge a lot of 210Pb, most likely true if the majority of 210Pb deposits in the first few millimetres of

rain. Of note from the study by Martin (2003) are the results obtained from light

rainfall at Jabiru East on the 20/9/85 with high 210Pb deposition, (1.7±0.1 kBq.l-1).

September rain, while not uncommon, is unseasonable and usually represents the

first rain for many months. A conclusion drawn from this study and Martin (2003) is

that the large deposition rates observed during October 2003 at Jabiru East are a

result of scavenging the large amount of available 210Pb. The first few rain events are

153

likely responsible for the majority of the 210Pb deposition observed during this

month.

6.3.2 Annual depositional rate, average values and residency time

Values for annual 210Pb depositional rates as well as wet and dry season depositional

rate averages are listed in Table 6-3. It has been reported that 210Pb deposition rates

can vary annually by a factor of three (Bonnyman and Molina-Ramos 1974; Beks et

al. 1998; Baskaran 1995). This project has only observed results from one seasonal

cycle and whether these results are indicative of an average year cannot be

determined.

From the global database an average annual southern hemisphere 210Pb

depositional rate is determined as 53±4 Bq.m-2.y-1 (Preiss et al. 2003). The observed

annual 210Pb depositional rate at Jabiru East of 49±14 Bq.m-2.y-1 is within the bound

of the southern hemisphere average. Oenpelli’s annual deposition rate of 15±4

Bq.m-2.y-1 is low but still comparable with the southern hemisphere average and

other values reported in the database. Annual 210Pb deposition at Jabiru East also

compares well and is within bounds of the Australian depositional rates reported by

Bonnyman et al. (1972) from which an average 56±4 Bq.m-2.y-1 is derived. Annual 210Pb at Jabiru East also compare with Darwin where an average 210Pb deposition rate

of 90±9 Bq.m-2.y-1 was reported (Bonnyman et al. 1972). Over the six-year study

period annual 210Pb deposition at Darwin varied between 60 Bq.m-2.y-1 to 195

Bq.m-2.y-1. The 2003-2004 wet season was higher than average with a total of 1765

mm of rainfall and 126 rainy days compared to the annual average of 1485.3 mm and

104 rainy days. While 210Pb is scavenged by precipitation the additional inundation

in the region reduces 222Rn exhalation rates and the local source of 210Pb. This may

explain the lower value of 210Pb deposition rates observed over the course of this

study.

For both locations approximately 90% of the annual 210Pb deposition occurs

during the wet season. The duration and intensity of the wet season will be the major

influencing factor in annual variations of 210Pb deposition in this region.

154

Table 6-3: Seasonal and annual 210Pb depositional rates and rainfall

Jabiru East+ Oenpelli# Rate\Location

Pb-210

deposition

Rainfall

(mm)

Pb-210

deposition

Rainfall

(mm)

Wet Season

(mBq.m-2.day-1)

217±43 1765.4 65±10 1880.5

Dry Season

(mBq.m-2.day-1)

22±5 59.4 6±3 6.6

Annual Rate

(Bq.m-2.y-1)

49±14 1824.8 15±4 1887.1

+Jabiru East wet season 1/10/03-18/5/04 #Oenpelli wet season 1/11/03-23/6/04 (included early dry season rains)

The low 210Pb deposition rates observed at Oenpelli has been attributed to the

fact that Oenpelli is surrounded by floodplains during the wet season. Most

deposition occurs from 222Rn exhaled during the wet season and at this site there is a

lack of a local source term as billabongs swell and floodplains are inundated with

water.

A previous study in the region used sticky vinyl aligned horizontally and

vertically to measure dry deposition rates of a number of uranium series

radionuclides in the vicinity of Ranger (Pettersson and Koperski 1991). That work

produced a model for determination of dry deposition rates at various distances from

Ranger. Using the model for Jabiru East results in very large dry depositional rates

(~500 Bq.m-2.y-1) leading to the conclusion that they may have encountered problems

with the horizontal measurement technique. From his thesis, Pettersson provides an

average dry deposition of 10 Bq.m-2.y-1 for all long-lived uranium series

radionuclides at Jabiru East (Pettersson 1990) The 210Pb dry deposition rate at Jabiru

East was determined to be 27 Bq.m-2.y-1 which was reported as being likely to be the

upper limit (Pettersson and Koperski 1991). From the work presented here a 210Pb

dry deposition rate of 8.0±1.8 Bq.m-2.y-1 and 2.2±1.1 Bq.m-2.y-1 are obtained for

Jabiru East and Oenpelli respectively, the value for Jabiru East is comparable to that

determined by Pettersson and Koperski (1991)

155

From the results presented in Figure 6.1 and Figure 6.2 the low 210Pb dry

deposition rates indicate that dry season 210Pb atmospheric residency times are higher

than values reported at other locations. Martin (2003) reported residency times

between 0-70 days averaging 18 days. He continued by determining early and late

wet season residency times with averages of 36 days and 8 days respectively. From

Pettersson and Koperski (1991) a dry season 210Pb atmospheric residency time of 140

days is derived. Both these results imply that 210Pb does not settle easily during the

dry season and that it remains suspended for many months or settles and resuspends.

Pettersson and Koperski (1991) also derived 210Pb depositional velocities and

showed that beyond 2 km from Ranger’s operational pit depositional velocities of 210Pb reduce to 0.25 cm.s-1, at Jabiru East 210Pb depositional velocity is 0.28 cm.s-1.

Some dry deposition observed at Jabiru East is dust transported from Ranger but the

techniques used in this study could not distinguish this from 210Pb originating from

exhaled 222Rn decay in the atmosphere.

Differences observed between the 210Pb deposition rates at Jabiru East and

Oenpelli can be attributed to a number of factors. Firstly the proximity of Jabiru East

to the Ranger operation means that a proportion of the 210Pb deposition there is a

result of the mine operation. Oenpelli is closer to the coast than Jabiru so there is

ocean air mixing, especially during the wet season when dominant winds come from

the northwest. This explanation raises the question however why the reported value

of 210Pb deposition at Darwin, of 90±9 Bq.m-2.yr-1 (Bonnyman et al. 1972), is higher

than Jabiru East or Oenpelli. A more plausible explanation is that during the wet

season the 210Pb source term for Jabiru East and Oenpelli is reduced as creeks,

billabongs, floodplains and the soil fill with water retarding 222Rn exhalation.

Throughout the wet season Oenpelli is only accessible by air as the East Alligator

River swells, becoming impassable, the township itself is surrounded by floodplains.

Jabiru East is further inland and there is less inundation of water in this region

compared to Oenpelli. The ratio of dry to wet season 210Pb depositional rates for the

two locations is approximately the same indicating that 222Rn and its progeny is

evenly mixed over the region.

Low annual 210Pb deposition rates in the region are most likely attributed to

prevailing meteorological conditions. Complete budgeting of 210Pb and explanation

of the dry season loss shall be examined in further detail in Chapter 7.

156

6.3.3 Pb-210 deposition summary The distinctive seasonality of 210Pb deposition is a result of the precipitation that

occurs during the wet season. The exponential decrease in deposition rates

throughout the wet season at Jabiru East is likely caused by a gradual reduction in the

localised 210Pb source A similar reduction in source term is expected at Oenpelli but

frequency of precipitation events retains 210Pb deposition rates at steady state values.

Unseasonal rain events that occurred in May and June 2004 at Oenpelli kept 210Pb

deposition rates at wet season values due to increased 210Pb atmospheric

concentrations.

Analysis shows that the dry season 210Pb atmospheric residency time is of the

order of a few months. Compared to globally reported 210Pb deposition rates,

Oenpelli is on the lower bound and Jabiru East is comparable. The 210Pb deposition

rate at Jabiru East compares well to Australian and Darwin averages obtained from

previous research. Air mixing, retarded wet season 222Rn exhalation rates, long 210Pb

atmospheric residency times causing low dry deposition rates explain the low annual 210Pb deposition rates. Oenpelli’s location being surrounded by floodplains, results in

low wet season 210Pb atmospheric concentrations and deposition rates.

6.4 Pb-210 excess in soil samples This section details the results from analysis of excess 210Pb from the soil cores and

scrapes collected in the Jabiru region. The sampling locations have been specifically

dealt with previously in Chapter 3 and the sampling techniques are covered in

Chapter 4. All samples were measured on HPGe detectors and analysed using the

GPEAK program.

6.4.1 Pb-210 inventories Results of excess 210Pb inventories from the sampling sites provided in Chapter 3 are

provided in Table 6-4. This table provides excess 210Pb over 226Ra and 238U

inventories to a depth of 10cm for all ambient sites. The exception is the non-

irrigated Magela Land Application Area, where the inventory of excess 210Pb over 238U is not presented due to increased levels of 238U observed in the upper layers of

soil that is speculated to be a result of horizontal transport of 238U from the irrigated

zones.

From the results presented in Table 6-4 a discrepancy is observed at the site

west of the original tailings dam. This site is 200 m from the Ranger ore stockpiles

157

and has high levels of 238U, 226Ra and 210Pb in the surface layers of soil indicative of

dust transport from the stockpiles. This dust deposition masks the excess 210Pb

deposited as a result of 222Rn decay in the atmosphere. This site has not been

included in determination of the regional average excess 210Pb inventory. To support

this conclusion it is noted that the site to the south of the original tailings dam, which

is also close to the ore stockpiles but not within any dominant wind sector, has 238U, 226Ra and 210Pb values similar to ambient sites. With the exception of the four sites;

(i) western side of Barallil Creek; (ii) eastern side of Barallil Creek; (iii) west side of

original tailings dam; and (iv) Georgetown billabong the remaining results closely

match.

158

Table 6-4: Total inventories of excess 210Pb

Site Excess 210Pb over 226Ra

(kBq.m-2) to 10 cm depth

Excess 210Pb over 238U

(kBq.m-2) to 10 cm depth

West of Old Tailings Dam -0.5±1.0 -7.4±1.3

Eastern Side Barallil

Creek

0.88±0.30 0.47±0.65

Western Side Barallil

Creek

7.9±0.9 3.2±1.8

Jabiru Township 1.8±0.3 1.8±0.9

Georgetown Billabong 4.5±0.6 1.0±1.1

Jabiru East 3.4±0.5 2.3±0.8

Side of Arnhem Highway 2.4±0.4 1.8±0.9

South original Tailings

Dam

2.4±0.5 1.1±0.8

West of Retention Pond 1 2.3±1.0 2.9±2.0

Non-Irrigated Magela

Land Application Area

Core 10

4.1±0.5

-

Core 13 2.0±0.4 -

Core TM2 1.9±0.8 -

Average (minus West

Tailings Dam

3.0±0.6 1.8±0.3

Average above (minus

Eastern Barallil Creek &

Georgetown)

2.5±0.3

2.0±0.3

159

The variations from average observed at the western side of Barallil Creek,

eastern side of Barallil Creek and Georgetown billabong is explained due to the clay

content of the soil. Potassium and its natural isotope 40K is an indicator of clay

content of soil (Ivanovich and Harmon 1992). Clay has good absorption properties

for fine particles and this influences 222Rn exhalation and excess 210Pb. Clay will trap 222Rn exhaled below it creating an of excess 210Pb and also deposited excess 210Pb

moving through the soil will become trapped in clay soils. Georgetown billabong and

the western side of Barallil Creek have high 40K inventories explaining the excess 210Pb inventories observed. The eastern side of Barallil Creek has a low 40K

inventory indicating it is a sandy soil. It is speculated at this site the retention of fine

particles is poor explaining the low excess 210Pb inventory here. A demonstration of

the relationship between excess 210Pb and 40K is shown in Figure 6.3 where a positive

correlation is observed.

The average excess 210Pb inventories for the region, over 226Ra and 238U, are

3.0±0.6 kBq.m-2 and 1.8±0.3 kBq.m-2 respectively. The western side of Barallil

Creek, eastern side of Barallil Creek and Georgetown billabong sites are noted as

being outliers due to their clay content. However an alternative explanation for the

excess 210Pb inventories differing at these sites may be the potential influence of

fluvial processes. These three sites are all located near natural water bodies and

fluvial movements may result in erosion or deposition of fine particles providing

unreliable excess 210Pb inventories. Removal of these outliers changes the 210Pb

inventories to 2.5±0.3 kBq.m-2 and 2.0±0.3 kBq.m-2 over 226Ra and 238U respectively

Differences between sites and the difference between the excess 210Pb over 226Ra

compared to excess 210Pb over 238U can be explained due to other influencing factors

that will be examined in Chapter 7.

160

R2 = 0.59

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 2040K (kBq.m-2)

Exce

ss 21

0 Pb

over

226 R

a (k

Bq.

m-2

)

Eastern Side Barallil Creek Western Side Barallil CreekJabiru Township Georgetown BillabongJabiru East Side of Arnhem HighwaySouth of original Tailings Dam West of Retention Pond 1Core 10 Core 13

Figure 6.3: Relationship between excess 210Pb and 40K

No relationship between excess 210Pb inventory and distance from the mine

was observed, indicating that 222Rn exhaled from the mine is readily mixed in the

atmosphere and distributed evenly across the region. As no samples were collected

from locations further away from Ranger than Jabiru the data presented cannot be

used to determine if any relationship exists between excess 210Pb inventories and

distance from Ranger on a larger scale.

No relationship between excess 210Pb inventory and the dominant dry season

wind direction was observed. This project focused on sites that are downwind of

Ranger during the dry season as these sites were readily accessible. Georgetown

billabong, the non-irrigated Magela Land Application Area and south of the original

tailings dam were all outside of the dominant dry season wind sectors from Ranger

with Georgetown billabong and the Magela Land Application Area in the dominant

wet season wind sector. None of these sites show large deviations from regional

average excess 210Pb inventory with the exception of Georgetown billabong that has

been discussed.

While there are no previously reported values of excess 210Pb inventories for

this region a comparison with work performed in Queensland, Australia and the

161

global database can be made (Akber et al. 2004a; Preiss et al. 2003; Pfitzner et al.

1993). Akber et al. (2004) presented an average excess 210Pb inventory of 2.4±0.5

kBq.m-2 for eight sites studied over Southeast Queensland while Pfitzner et al. (2004)

reported an excess 210Pb inventory of 1.5±0.2 kBq.m-2 for sites around Townsville,

North Queensland. Results reported here are in good agreement with both these

studies. The global database contains results of 155 measurements from five

continents, excess 210Pb inventories in soils range from 0.228±0.007 kBq.m-2

measured in Portugal to 14±0.04 kBq.m-2 recorded in Japan. The average global

excess 210Pb inventory determined from the database is 3.7±0.2 kBq.m-2, also in

close agreement with the results presented here.

6.4.2 Penetration half depth It is expected for undisturbed soils that the majority of excess 210Pb will lay within

the top layers of the soil. Lead-210 will redistribute in soils due to diffusion,

convection and bioturbation creating a depth profile. Depth profiles for the relative

cumulative excess 210Pb inventory for the scrapes are presented in Figure 6.4 and for

the cores in Figure 6.5. The samples from Jabiru East, eastern side of Barallil Creek,

side of Arnhem highway and west of retention pond 1 all had large uncertainties

associated with them and were excluded from analysis. This was due to a reduction

in 210Pb inventories compared to 226Ra resulting in increased errors and reduced

cumulative excess 210Pb inventories. This may have been due to the small cross

sectional area of the corer and the collection of an unrepresentative portion of the

larger than 2mm fraction. Georgetown billabong was also removed from analysis

because of the large amounts of excess 210Pb in this sample. Due to cluttering of the

graphs, as the depth points are the same for most samples in Figure 6.4 and Figure

6.5, error bars have only been displayed for one data series. They are representative

of the errors associated with the other data series.

162

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 2 4 6 8 10 12Depth (cm)

Rel

ativ

e C

umul

ativ

e Ex

cess

210 P

b In

vent

ory

Eastern Side Barallil Creek Georgetown BillabongJabiru East Side of Arnhem HighwaySouth of original Tailings Dam

Figure 6.4: Relative cumulative excess 210Pb versus depth for soil scrapes

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 5 10 15 20Depth (cm)

Rel

ativ

e C

umul

ativ

e Ex

cess

210 P

b In

vent

ory

Western Side Barallil Creek Jabiru TownshipSouth of original Tailings Dam Core 10Core 13 TM2 Core

Figure 6.5: Relative cumulative excess 210Pb versus depth for soil cores

163

The trendlines displayed in Figure 6.4 and Figure 6.5 are derived from a

penetration half depth equation. The penetration half depth value was determined

using the data-fit program Datafit v8.0, by Oakdale Engineering, from all the

samples displayed. Penetration half depth is the depth at which the inventory is half

the total inventory and has been previously reported for 7Be studies (Wallbrink and

Murray 1996). It is provided by a solution of Equation 6-1 for τ:

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛ ×

−=τ

deII TE)2ln(1

Equation 6-1

Where d: is the depth (cm) IE: is the relative cumulative excess 210Pb inventory at depth d IT: is the total relative cumulative excess 210Pb inventory for the sample τ : is the penetration half depth (cm).

From these samples a regional penetration half depth of 2.5±0.7 cm has been

derived. There are currently no reported values for penetration half depth of excess 210Pb. However Akber et al. (2004) presented a value for excess 210Pb penetration

half depth in Southeast Queensland of 3.6±0.3 cm at the SPERA 2004 conference.

The results presented here are comparable with this value and differences are likely

attributable to variations in soil types and methods of 210Pb redistribution in the soils.

The regional penetration half depth of 2.5±0.7 cm means that 210Pb can be used for

soil redistribution studies in this region.

6.4.3 Excess 210Pb summary With the removal of outliers due to the high clay content or fluvial processes the

average excess 210Pb inventory is 2.5±0.3 kBq.m-2 and 2.0±0.3 kBq.m-2 over 226Ra

and 238U respectively. The relationship between excess 210Pb and 40K, representative

of clay content, justifies the removal of the outliers. The site to the west of the

original tailings dam displayed high deposition of dust particles from the nearby

stockpiles making excess 210Pb measurements here undeterminable. Other than the

dust deposition at this one site there was no clear relationship between excess 210Pb

inventories and distance from the mine site indicating that 222Rn transported from the

mine is evenly mixed over the sampled area. The excess 210Pb inventories observed

in this study compared well with results reported in Queensland, Australia and

globally. Using a previously established calculation the penetration half depth for

excess 210Pb in the region was determined as 2.5±0.7 cm, comparable to a value of

3.6±0.3 cm reported for Southeast Queensland.

164

6.5 Magela Land Application Area 6.5.1 Introduction

It has been previously mentioned that excess water from retention pond 2 at Ranger

is irrigated onto an area of land between the processing plant and Magela Creek,

commonly known as the Magela Land Application Area. A section of this site,

known as the experimental plot, was selected in 1988 for a study of this process. An

irrigated section of the application area and the experimental plot were selected for

an extensive 222Rn exhalation rate survey, the results of which have been presented in

Chapter 5. A number of soil samples were taken from both areas to investigate the

distribution of the 226Ra deposited and its influence on the 222Rn exhalation rate. It

was shown in Chapter 5, that for both locations the effect of surface deposition has

had little influence upon the 222Rn exhalation rate. While the 222Rn exhalation rates

are enhanced at these locations the 222Rn exhalation rate to 226Ra activity

concentration ratios are similar to ambient sites of the same geomorphic structure.

6.5.2 Uranium-238, 226Ra and 210Pb depth profile inventories Measurements of 226Ra activity concentration, using insitu gamma

spectrometry at the irrigated Magela Land Application Area and the experimental

plot was presented in Table 5.2. It was mentioned that these values were not

representative for the area due to assumptions that the GS-512 makes in its

calculations. Soil samples collected from the area provide a much better analysis of

the 226Ra activity concentration, depth profile and distribution in the soil. The 226Ra

and 210Pb inventory depth profiles for these samples are displayed in Figure 6.6 and

Figure 6.7 while the 238U inventory depth profiles are displayed in Figure 6.8 and

Figure 6.9. Comparisons to non-irrigated areas have been included for 226Ra and 210Pb.

165

0.0

0.5

1.0

1.5

2.0

0-5 5-10 10-15 15-20Section (cm)

Inve

ntor

y ((

kBq.

m-2

).cm

-1)

Ra-226 Core2 (Irrigated MLAA)Pb-210 Core2 (Irrigated MLAA)

0.0

0.5

1.0

1.5

2.0

0-5 5-10 10-15 15-20Section (cm)

Inve

ntor

y ((k

Bq.

m-2).c

m-1)

Ra-226 Average (Non-irrigated MLAA)

Pb-210 Average (Non-irrigated MLAA)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0-2 2-4 4-6 6-8 8-10 10-15 15-20Section (cm)

Inve

ntor

y ((

kBq.

m-2

).cm

-1)

Ra-226 TM2 (Non-irrigated MLAA)Pb-210 TM2 (Non-irrigated MLAA)

Figure 6.6: Inventory depth profile for 2cm sectioned cores from irrigated TM1 and non-irrigated TM2

Figure 6.7: Inventory depth profile for 5cm sectioned cores, irrigated (core 2) and averaged non-irrigated cores

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0-2 2-4 4-6 6-8 8-10 10-15 15-20Section (cm)

Inve

ntor

y ((k

Bq.

m-2

).cm

-1)

Ra-226 TM (Irrigated MLAA)1Pb-210 TM1 (Irrigated MLAA)

166

0

10

20

30

40

50

60

70

80

0-2 2-4 4-6 6-8 8-10 10-15 15-20

Section (cm)

238 U

Inve

ntor

y ((

kBq.

m-2

).cm

-1)

U-238 Core TM1 (Irrigated MLAA)

Figure 6.8: U-238 inventory depth profile for irrigated core TM1

0

10

20

30

40

50

60

70

80

90

100

0-5 5-10 10-15 15-20Section (cm)

238 U

Inve

ntor

y ((

kBq.

m-2).c

m-1

)

U-238 Core1 (Irrigated MLAA) U-238 Core2 (Irrigated MLAA)

Figure 6.9: U-238 inventory depth profile for irrigated core 1 and core 2

167

Surface irrigation is evident in Figure 6.6 and Figure 6.7 in the 226Ra and 210Pb inventories compared with the non-irrigated counterparts. Deposited 226Ra and 210Pb is held in the top few centimetres of the soil, with Figure 6.6 indicating that it is

held within the top 2 cm of the soil. As the irrigated water contains 226Ra greatly in

excess over 210Pb a substantial fraction of the 210Pb seen in the top 2 cm is from

ingrowth from 226Ra in the soil. This explains why the 210Pb is elevated in the surface

soil but is not as high as 226Ra.

The distribution of irrigated water across the area is not even, as seen by the

large variations in radionuclide inventories in the top layers of the soil for the

irrigated samples. This is due to the type of impulse sprinklers used for water

application and obstruction of spray patterns by trees and shrubs (Akber and Marten

1991). While an exponential decrease is observed in Figure 6.7 and the inventories

are higher than the non-irrigated area to a depth of 15 cm, below the first section

inventories become comparable with other ambient sites. These results agree with

findings from an earlier study into spray irrigation application of radionuclides at the

area (Willett and Bond 1991). Willett and Bond (1991) noted that for all soil types 226Ra and 210Pb absorption should be within the top few centimetres of soil and that

they would not easily remobilise after initial bonding. They also realised that

remobilisation would most likely occur with 238U as seen from the depth profiles of

Figure 6.8 and Figure 6.9. Willett and Bond (1991) showed that 238U absorption was

lowest on soil classified as Unit III followed by Unit I, with Unit II soils retaining the

greatest amount in the upper layers. Soil classification on the Magela Land

Application Area was part of an earlier study (Chartres et al. 1988).

Core 1 was taken from Unit II soil and core 2 was collected from Unit III soil.

Core TM1 was collected from an area close to the boundary of these two soil types.

While no accurate determination can be made for 238U penetration due to the small

sample set it is estimated as being 6-8 cm for core TM1, 15-20 cm for core 1 and 5-

10 cm for core 2. Below these levels the 238U inventory is comparable to that of other

ambient sites in the area.

Willett and Bond (1991) estimated that the top 50 cm of soil should retain

applied radionuclides for up to 100 seasons of application. Using an assumption of

1800mm.yr-1 of irrigation Willett and Bond (1991) also estimated that there should

be no downward movement of 238U beyond 4 cm until the surface soils are brought to

saturation after approximately 22 years of irrigation. From 1986-2000 there has been

168

an average of 964±221 mm.yr-1 of irrigated water applied to this area above normal

rainfall averaging 1485.3 mm.yr-1. The results indicate that 238U mobilization has

occurred beyond the estimated value. The retention of 226Ra and 210Pb in the top 2 cm

demonstrates that the vertical movement of 238U is not primarily due to physical

movement of the soil particles. It is also noted that two of the three non-irrigated

sites had high 238U inventories in the surface layers of the soil indicating there has

been horizontal mobilisation of 238U from the irrigated sections. The

6.5.3 Experimental plot inventories The experimental plot was the site of previous study for absorption of radionuclides

and movement of ions (Bond and Willett 1991; Akber and Marten 1991). After the

original intensive sampling program this area has been left relatively undisturbed

other than the decommissioning of a nearby shed and a gamma dose rate survey

(Storm and Martin 1995). During the course of this project the experimental plot was

selected for an extensive 222Rn exhalation rate survey and two scrapes were collected

from undisturbed locations on the plot. A soil core from previous work collected by

J. Storm in 1994 was unprocessed and available at eriss, this core was prepared and

analysed during the course of this project. The inventory depth profiles for 238U, 226Ra and 210Pb of the scrapes and the activity concentration depth profile of the core

are displayed in Figure 6.10, Figure 6.11 and Figure 6.12 respectively.

From the depth profiles of Figure 6.10 and Figure 6.11 it is seen that peak

radionuclide activity occurs within the first two sections down to 2.5 cm. These

figures show that there is an increase in activity compared to ambient sites down to

the extent of the scrapes 5-10 cm. From the analysis of the core it is noted that

increased activity is extends to the 6.7-11.4 cm section beyond which activities level

out to ambient values. It is noted that 226Ra and 210Pb inventories for the experimental

plot are more than an order of magnitude greater than those observed from the

irrigated Magela Land Application Area while 238U inventories are comparable. This

is because the concentrations of 210Pb and 226Ra applied to the experimental plot were

one and two orders of magnitude greater, respectively, and 238U concentrations were

twice as great compared to retention pond 2 water, (Akber and Marten 1991). The

observed inventories presented in Figure 6.10 and Figure 6.11 is consistent with

loads applied to the experimental plot and reported by Akber and Marten (1991).

169

0

10

20

30

40

50

60

70

80

0-1 1-2.5 2.5-5 5-10Section (cm)

Inve

ntor

y ((

kBq.

m-2

).cm

-1)

U-238 Ra-226 Pb-210

Figure 6.10: Inventory depth profile for scrape 1 from experimental plot

0

10

20

30

40

50

60

70

0-1 1-2.5 2.5-5 5-10Section (cm)

Inve

ntor

y ((

kBq.

m-2

).cm

-1)

U-238 Ra-226 Pb-210

Figure 6.11: Inventory depth profile for scrape 2 from the experimental plot

170

10

100

1000

10000

100000

0-1.9 1.9-6.7 6.7-11.4 11.4-16.2 16.2-20.9 20.9-25.7 25.7-30.4 30.4-35.2 35.2-38.9

Section (cm)

Act

ivity

Con

cent

ratio

n (B

q.kg

-1)

U-238 Ra-226 Pb-210

Figure 6.12: Activity concentration depth profile for core collected by J. Storm 1994 (fine grains, <2mm only)

The increase in 226Ra and 210Pb inventories observed to 10 cm at the

experimental plot is a result of the saturation of absorption sites resulting in a

downward movement of 226Ra and 210Pb. No more bonding in the upper layers of the

soil occurs and these radionuclides, transported with soil moisture, penetrate deeper

until they are absorbed. The 238U depth profile at the experimental plot is comparable

with that observed at other irrigated sections.

6.5.4 Radium-226 and 210Pb distribution Previous work has reported that when irrigated the radionuclides 238U, 226Ra and 210Pb absorb to the fine grain fraction of soil (Akber and Marten 1991). This is

expected as fine grains have better absorption properties due to their larger surface

area to volume ratios compared with larger grains. This makes areas with coarse

grains unsuitable locations for the surface deposition radionuclides. To demonstrate

this Figure 6.13 displays the 226Ra <2mm inventory plotted against the 226Ra >2mm

inventory for irrigated and non-irrigated sampling sites.

171

1.0

10.0

100.0

1000.0

1.0 10.0 100.0226Ra >2mm fraction (kBq.m-2)

226 R

a <2

mm

frac

tion

(kB

q.m

-2)

Irrigated Soil Non-irrigated Soil

Figure 6.13: Distribution of 226Ra in soil fractions for 0-10cm

The continuous line shown in Figure 6.13 is the trendline for the non-irrigated

samples which was the same as a line of equality indicating that 226Ra distribution in

normal soils is even across the <2mm and >2mm fractions while all irrigated sites lie

on the upper side of the line of equality. For the irrigated sites both fractions have

higher 226Ra inventories compared with the non-irrigated sites. This is due to

absorption of 226Ra onto the larger fraction but also shows that dry sieving does not

remove the entire finer fraction from the larger grains. The two outliers seen in the

upper right are the experimental plot scrapes where deposited 226Ra concentrations

were much higher. The results agree with those previously reported, that 226Ra

deposited from spray irrigation adheres to the finer grains of soil.

Lead-210 also absorbs on finer grains and this is an attractive feature for a

radionuclide to be used in soil transport studies, as finer grains are subject to erosion.

The distribution plot for 210Pb is shown in Figure 6.14 where it is noted that it is

primarily attached to the finer grains at most sites, the continuous line shown in this

figure is a line of equality. This effect is observed at irrigated and non-irrigated sites

due to the atmospheric deposition of excess 210Pb.

172

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0210Pb >2mm Inventory (kBq.m-2)

210 P

b <2

mm

Inve

ntor

y (k

Bq.

m-2

)

Irrigated Non-irrigated

Figure 6.14: Distribution of 210Pb in soil fractions for 0-10 cm 6.5.5 Magela Land Application Area summary

The irrigation of retention pond 2 water has occurred here since 1986 and extended

to surrounding areas. It was introduced to provide Ranger a means of dealing with

excess water loads. From the analysis presented it can be seen that the irrigation of

water has an impact by increasing the 238U, 226Ra and 210Pb inventories compared to

ambient soils. For 226Ra and 210Pb the increase is within the top few centimetres of

the soil due to the soils capacity to absorb these radionuclides. Absorption of 238U

was known to be different and original studies expected that 238U would penetrate

deeper which has been shown in this analysis.

Increased inventories and greater depth penetration of 226Ra and 210Pb at the

experimental plot is a result of the high concentrations used at this site. This resulted

in saturation of the upper soil layers and these radionuclides penetrated deeper into

the soil before being absorbed. In agreement with other reports it was shown the

absorption of 226Ra and 210Pb is primarily on fine grains. For 226Ra this was evident

only at irrigated sites but for 210Pb it was evident at all locations due to the

atmospheric deposition of excess 210Pb.

173

6.6 Chapter summary The results presented in this chapter are essential for determination of the regional 210Pb budget performed in Chapter 7. Measurements of annual 210Pb deposition rates

and excess 210Pb in the soils around the Jabiru region also provides a picture of the

natural cycle of the 238U series from 222Rn exhalation to 210Pb deposition and its

transport through the soil.

It has been shown that 210Pb deposition rates measured between May 2003

and May 2004 for Jabiru East and Oenpelli are low compared to globally reported

values. Partially this is due to low dry season 210Pb deposition when 222Rn exhalation

is at a maximum resulting in a net loss of 210Pb from the region. At the onset of the

wet season at Jabiru East initially high deposition rates are observed that

exponentially decrease as rain slowly retards the local 210Pb source. Oenpelli displays

a more steady state 210Pb depositional rate over the wet season with the exception of

the April 2004 peak. This is due to a balance between the source term and

scavenging events that result in an even distribution. Overall Oenpelli’s low annual

deposition rate is due to dilution of 210Pb from air mixing and a lack of 222Rn

exhalation during the wet season as the area is surrounded by inundated floodplains.

From a previous study by Pettersson and Koperski (1991) and observations

dry season 210Pb atmospheric residency times are determined to be of the order of a

few months (140 days). This agrees with the upper value of residency times reported

by Martin (2003) of 70 days determined for an early wet season rain event.

Excess 210Pb inventory was measurable at most sampled sites. At some

locations high clay content, fluvial processes or dust deposition from Ranger

stockpiles resulted in high and undeterminable values respectively. Average excess 210Pb inventories for the region agree well with values reported for Queensland,

Australia and globally. The penetration half depth was found to be 2.5±0.7 cm which

agrees well with a result reported for Southeast Queensland.

The Magela Land Application Area was subject to an intensive 222Rn

exhalation rate survey and soil collection to analyse radionuclide deposition resultant

from spray irrigation. It was observed that 226Ra and 210Pb was retained within the

top few centimetres of the soil while 238U penetrated deeper. Absorption

predominantly occurs on the finer grain soil fraction for 226Ra at the irrigated region

and for 210Pb at all locations. Horizontal transport of 238U from the irrigated section

174

to a non-irrigated section is evident and vertical transport of 238U is greater than

originally expected.

175

7

Lead-210 budget

7.1 Introduction Radon-222 exhalation gives birth to 210Pb that attaches to aerosols. Its behaviour in

the environment through transport and deposition provides an insight into the

behaviour of the aerosols it is attached to. The objective here is to extend the

knowledge of 222Rn exhalation, 210Pb deposition and excess 210Pb inventories to

examine this behaviour. The aim of this chapter is to make comparisons between

estimated and observed values of 222Rn exhalation rates, 210Pb deposition rates and

excess 210Pb inventories and also to examine the relationship between them.

Explanation for the annual loss of 210Pb from the region is also provided.

Budgeting 210Pb is performed firstly on local scale of the Jabiru region then on

a broader scale over Kakadu. In order to perform this some justifiable assumptions

have been made and reasoning behind these is provided. However given the half-life

of 210Pb, long atmospheric residency time determined from the previous chapter and

large annual variations in meteorological conditions in the region it must be noted

that inventory calculations determined from deposition measurements over 1 year are

less useful.

7.2 Hadley circulation It is speculated on the basis of the results that the reason for the net loss of 210Pb during the dry season is a result of it being transported away from the region by

the dominant dry season winds. These winds are created by an atmospheric

circulation pattern known as the Hadley circulation pattern. Explanation of this

circulation pattern is presented here and a model is shown in Figure 7.1.

This circulation pattern was proposed by a British physicist George Hadley

(1685-1768) in 1735 to explain global air circulation patterns. Hot, moist air from

equatorial regions rises into the tropopause losing moisture as it rises thus creating

convection rain events. The now dry air is cycled towards the pole but descends at

the mid-latitudes where the global desert bands lies creating high pressure cells. The

descending air then moves back towards the equator, which is at low pressure.

Rotation of the earth creates the Coriolis effect that turns the southerly wind into

176

south easterly and easterly wind. Hadley circulation is further complicated by the

tilting of the earth on its axis and by landmasses.

7.3 Local area 210Pb budget A large number of measurements were performed around the Jabiru area. These

include; 222Rn exhalation rates from ambient sites of Jabiru East, Jabiru water tower

and the non-irrigated Magela Land Application Area; 210Pb deposition rates from

Jabiru East; and excess 210Pb inventory measurements from 11 ambient sites. For this

local area it is possible to perform a budget of 210Pb. Lead-210 depositional rate

measurements were performed over one seasonal cycle from May 2003 to May 2004

at Oenpelli however no additional data, such as measurements of 222Rn exhalation

rates or excess 210Pb inventories were taken so it is not possible to perform 210Pb

budgeting for the Oenpelli region.

It has been reported that 210Pb deposition rates can vary annually by a factor

of three (Bonnyman and Molina-Ramos 1974; Beks et al. 1998; Baskaran 1995).

This complicates budgeting estimates and comparisons, as a long-term 210Pb

depositional analysis for the region is not available.

Figure 7.1: Global Hadley circulation model (curtesy Australian Bureau of Meteorology)

177

7.3.1 Fate of Ranger 222Rn Measured 222Rn and 222Rn progeny mine related contributions at Jabiru East

are 14% and 10% respectively and at Jabiru 5% and 15% respectively (Whittlestone

1992). The results for Jabiru East compare well with another measurement study that

reported the mine related contribution of 222Rn atmospheric concentrations there to

be 20% (Akber and Pfitzner 1994). Model based estimates for Jabiru East and Jabiru

reported annual mine contributions of 222Rn atmospheric concentration are 17% and

14% respectively (Akber et al. 1993). This compares well with an earlier model that

estimated the mine related component of exposure to 222Rn progeny at Jabiru to be

approximately 16% of the annual average (Kvasnicka 1990).

The information provided shows that despite being a strong point 222Rn

source term, 222Rn from Ranger is readily dispersed in the local region and makes a

small contribution to the local 222Rn atmospheric concentrations, 210Pb deposition

and excess 210Pb inventories. The incremental additional 210Pb contribution to the

local area is likely to be within the experimental uncertainties reported through this

thesis.

7.3.2 Determination of 222Rn exhalation rates from 210Pb deposition and excess 210Pb inventories

Assuming that 222Rn atomic exhalation to the environment is equal to the 210Pb

atomic deposition in the Jabiru area, the data presented in Table 6-1 and Table 6-2

can be used to estimate 222Rn exhalation rates. Measured and estimated seasonal and

annual 222Rn exhalation rates for Jabiru East are shown in. Measured seasonal 222Rn

exhalation rates in Table 7-1 for Jabiru East exclude the transitional months of

October and April. The 222Rn exhalation rates for these months are non-indicative of

the seasonal value due to variations in soil moisture and the influence it has on 222Rn

exhalation. However the transitional months October and April are included in

determination of the wet season 222Rn exhalation rates from the 210Pb depositional

measurements as the scavenging events that occur in these months cannot be ignored.

Measured 222Rn exhalation rates are higher than those predicted on the basis

of 210Pb deposition. The discrepancy, of a factor of two during the wet season,

increases to a factor of twenty in the dry season. On an annual basis it results in a

factor of seven higher for the measured exhalation rates over the estimated value.

Lack of scavenging events throughout the dry season results in atmospheric

178

residency times of the order of months and it is speculated that locally produced 210Pb transports large distances away from the region resulting in a net loss of 210Pb.

The annual 222Rn exhalation rate estimated from the 210Pb inventories agrees

well with the annual value derived from the 210Pb deposition rates but is slightly

higher. This is explained due to the natural redistribution of 210Pb in the soil that will

be discussed and also as the 2003-2004 wet season was above average, with 1760

mm recorded for a year to the start of May 2004 compared with the annual average

of 1485.3 mm. There were 126 rainy days in the year up to the start of May 2004

compared to an annual average of 104. It is speculated that this increased

precipitation over this wet season has reduced the regional 222Rn exhalation and 210Pb

source and resulted in a lower estimate of 222Rn exhalation rates compared to that

derived from excess 210Pb inventories.

An explanation for the regional net loss during the dry season comes from 210Pb attached to aerosols being transported from the region. During the dry season

regional surface winds are easterly to south easterly, known as trade winds, created

by the Hadley circulation cell and Coriolis effect Kakadu is located between 12-14oS

and receives the full influence of the easterly to south easterly trade winds. Lead-210

attached to aerosols from the Kakadu region is transported by this wind to the ocean,

where it is likely scavenged by more frequent precipitation events and mixed with 210Pb deficient air. This air is lifted into the upper atmosphere where it is further

diluted before being swept back towards the surface.

The wet season value estimated for 222Rn exhalation rates from the 210Pb

deposition rate at Jabiru East is comparable and within the statistical bounds of the

observed value. The uncertainties can be attributed a number of phenomena. The

measurements were performed over different wet seasons so variations in

precipitation and soil moisture and the effect that this has upon 222Rn exhalation rates

explains some uncertainty. Also 210Pb is deposited evenly across the region while 222Rn only exhales from land not inundated with water. Uncertainty also lies with the

fact that 222Rn exhalation rate measurements were discrete, between one and several

days a month, while the 210Pb deposition values are determined as an average of a

continuous monthly measurement. There was also a small number of 222Rn

exhalation measurement sites compared with the size of the region.

179

Table 7-1: Measured and estimated seasonal and annual 222Rn exhalation rates for Jabiru East

222Rn exhalation rate (mBq.m-2.s-1)

Measured (Jul

02-Jun 03)

Estimated from 210Pb deposition

(May 03-May 04)

Estimated from

excess 210Pb

inventory

Wet season

average

23612 * 5.9±0.4 -

Dry season average 483038 * 1.8±0.1 -

Annual average 322327 3.7±0.4 5.3±0.6

* Excludes October and April as transitional months

7.3.3 Determination of excess 210Pb inventories from 210Pb deposition

From the observed 210Pb deposition rates estimates of expected excess 210Pb

inventories can be made. The excess component of 210Pb in the surface soil is

obtained by averaging deposition over the mean lifetime (τ) of 32 years. Using the

annual deposition rate reported in Table 6-1 to represent averaged annual loadings of 210Pb to the surface soils for Jabiru East and Oenpelli measured and estimated

inventories are shown in Table 7-2. For Jabiru East and the Jabiru region the values

observed compare well with the estimations.

Differences can be attributed to the physical factors affecting 226Ra, 222Rn and 210Pb transport through the soil. A model of the natural process of 210Pb redistribution

is shown in Figure 7.2. It has been previously mentioned that 226Ra can be leached

down or removed with water infiltration. To demonstrate this effect occurring in the

region a plot of 226Ra against 238U is provided in Figure 7.3 where the continuous line

represents a state of equilibrium. The majority of values lie below this line of

equilibrium. Removal of 226Ra is insufficient to explain the total excess 210Pb

inventory observed but it in part explains why the excess 210Pb over 226Ra inventory

derived from the soil samples is higher than those predicted from 210Pb deposition.

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Table 7-2: Estimated and observed excess 210Pb inventories

Jabiru East Jabiru Region Oenpelli

Measured Inventory

over 226Ra (kBq.m-2)

3.4±0.5 2.5±0.3 -

Measured Inventory

over 238U (kBq.m-2)

2.3±0.8 2.0±0.3 -

Estimated Inventory

(kBq.m-2)

1.6±0.5 - 0.48±0.13

Radium-226 loss can account for the difference between the excess 210Pb

inventory over 226Ra and that over 238U. Another contributing factor lies with the fact

that not all 222Rn diffusing through the soil exhales into the atmosphere. Some

becomes trapped in the upper layers of the soil and in the water above it where it

decays. This is most likely to be predominant during the wet season when

precipitation has a capping effect upon 222Rn exhalation. More compact surface soil

than the soil underneath can lead to a similar effect. This results in more excess 210Pb

in the surface layers of the soil than can be attributed to atmospheric deposition and

this is also shown in Table 7-2.

Two outliers were removed for the purpose of Figure 7.3. The sites lay

between the irrigated section and the Magela Creek boundary fence. The data points

lay far to the right of the plot and the high 238U observed in the surface layers is

indicative of horizontal transport of 238U from the irrigated section of the Magela

Land Application Area.

181

Figure 7.2: Natural redistribution of 210Pb

0.0

2.0

4.0

6.0

8.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0238U Inventory (kBq.m-2)

226 R

a In

vent

roy

(kBq

.m-2

)

Eastern Side Barallil Creek Western Side Barallil Creek Jabiru Township

Georgetown Billabong Jabiru East Side of Arnhem Highway

South of original Tailings Dam West of Retention Pond 1 Core 10 (Non-Irrigated MLAA)

Figure 7.3: Radium-226 versus 238U to 10 cm to demonstrate 226Ra/238U disequilibrium

182

7.3.4 Determination of 210Pb deposition and inventories from 222Rn exhalation rates

With the net loss of 210Pb from the region estimation of annual 210Pb deposition rates

from the 222Rn exhalation rate is difficult. It is possible to provide an estimation of

the wet season 210Pb deposition rates from the observed 222Rn exhalation rates. This

estimation is based upon wet season 222Rn exhalation between November and March.

The January 222Rn exhalation anomaly, if existing for a site, has been removed from

the analysis. To determine the contribution of dry season deposition to the excess 210Pb inventory, given the dry season loss of 222Rn, the measured dry season

deposition is used. For determination of the excess 210Pb inventory wet season

deposition rates extend from October to March. Contribution of the transitional

month of April is considered to be comparable to the dry season. The results for

estimated wet season 210Pb deposition rate and total excess 210Pb inventories are

provided in Table 7-3

Approximately 28% of Kakadu is inundated with water during the wet season

(Santos-Gonzalez et al. 2002). Water inundation is greater near the coast at the

mouth of the rivers where the floodplain coverage is greater. For the Jabiru region an

approximation of 20% inundation is used to determine the values in the final column

of Table 7-3. Using this approximation a more accurate determination of 210Pb

deposition rates and excess 210Pb inventories from 222Rn exhalation rates is obtained.

While excess 210Pb inventories are similar to those observed they are higher

because of the assumptions made and do not take into account the observed removal

of 226Ra or capping of 222Rn that both result in additional excess 210Pb in the surface

soil. Other uncertainties are again attributed to the fact that measurements were

performed over different wet seasons and 222Rn exhalation rate measurements are not

truly indicative of the real 222Rn exhalation rates, as they were discrete and limited in

area.

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Table 7-3: Estimated 210Pb deposition rate and excess 210Pb inventory

Jabiru East Jabiru region

Jabiru region (inc. inundation)#

Estimated* 487±167 390±83 312±66 Wet season 210Pb deposition rate (mBq.m-2.day-1) Measured+ 212±14 - -

Estimated from exhalation*

3.0±0.2 2.4±0.2 1.9±0.2

Estimated from deposition+

1.6±0.5 - -

210Pb inventory (kBq.m-2)

Measured 3.4±0.5 2.5±0.3 2.5±0.3

* November 2002 to March 2003 + October 2003 to March 2004 # 20% inundation assumed 7.3.5 Local area 210Pb budget summary

The results presented in this section show there is a good comparison between 222Rn

exhalation rates, 210Pb deposition rates and excess 210Pb inventories for the wet

season on a local scale. The fact that 222Rn exhalation measurements were performed

in the wet season previous to the 210Pb deposition measurements explains some of the

discrepancies between the results. At the onset of the project it was planned that 222Rn exhalation rate and 210Pb deposition rate measurements would be performed

simultaneously. A delay in the construction of the 210Pb deposition collectors resulted

in this not being possible.

Local water inundation is a possible explanation for why estimated 222Rn

exhalation rates are lower and estimated 210Pb deposition rates and excess 210Pb

inventories are higher than the observed values. Some mixing with 210Pb deficient air

is also likely to occur during the wet season with predominant winds coming from

the ocean across the floodplains. Discrete measurements of 222Rn are also not truly

indicative of the monthly average when compared to averaged continuous

measurements of 210Pb deposition. Small areas of measurement compared to the

relatively large area that the model is applied to will also create discrepancies.

184

A variation between excess 210Pb inventories obtained from measured soil

samples and those derived from depositional measurements occurs because of two

phenomena. First water acts as a cap on 222Rn exhalation trapping it in the upper

layers of the soil where it decays. Secondly 222Ra can be removed the soil with water

infiltration. These phenomena create additional excess 210Pb, more than can be

accounted for by deposition.

Overall it is observed that there is a net loss of 210Pb from the region and it

has been speculated that this is due to a much broader effect caused by the trade

winds created by a Hadley Circulation cell. Removed 210Pb is likely deposited over

the equatorial regions to the north west of Kakadu. From the data presented it is

estimated that for Jabiru East there is a net loss of 210Pb of 350±75 Bq.m-2.yr-1 while

49±14 Bq.m-2.yr-1 was deposited from May 2003 to April 2004. Approximately 78%

of this loss occurs during the dry season while 90% of the deposition occurs during

the wet season.

7.4 Regional 210Pb budget Using the results presented in this thesis with other information available for the

region it is possible to make estimates of Kakadu’s 222Rn exhalation rates. The value

κ presented in Chapter 5, for determination of 222Rn exhalation rates from 226Ra

activity concentrations, is used to estimate dry season 222Rn exhalation rates across

Kakadu.

Deriving a wet season κ from a limited dataset the wet season 222Rn

exhalation rate for Kakadu can also be derived. This is used to estimate the 210Pb

depositional rate and excess 210Pb inventory for Kakadu as a whole.

7.4.1 Kakadu dry season 222Rn emission Kakadu National Park covers 19,800 km2 and has been classified into its geomorphic

landscapes in a previous study (Lowry and Knox 2002). A map adapted from Lowry

and Knox (2002) is shown as Figure 7.4. Chapter 5 distinguished four geomorphic

landscapes and a value (κ) for three of them that could be used to estimate 222Rn

exhalation rates from 226Ra activity concentrations. Ambient values of 226Ra activity

concentrations from measurements performed in this project are combined with

ambient values provided from other previous work performed in the region and are

used as an estimate for total averaged ambient 226Ra activity concentrations (Todd et

185

al. 1998). The following assumptions have also been made to determine the dry

season exhalation rates that are presented in Table 7-4:

− Eroded Koolpinyah, Koolpinyah and dissected foothill surfaces are

considered to be vegetated;

− Coastal and alluvial floodplains are considered to be fine grained

material;

− Plateau is modelled as 50% the value of non-compacted boulders, this

accounts for the solid matrix;

− The 226Ra activity concentration of the plateau is the average of the 226Ra

activity concentration from all sites (the surface soils in the region

originate from erosion of the plateau over millions of years);

− The amount of water coverage in the dry season is considered negligible

(perhaps up to 10% but within the bounds of uncertainties);

− The area of barren sites is considered to be negligible.

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Figure 7.4: Geomorphic landscapes of the Kakadu region (Adapted from Lowry and Knox (2002))

Koolpinyah Surface – 17%

Coastal Floodplains – 11%

Alluvial Floodplains – 7%

Dissected Foothills – 7%

Plateau – 17%

Eroded Koolpinyah Surface– 41%

Kakadu Boundary

12oS

13oS

14oS

132oE 133oE

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Table 7-4: Dry season 222Rn emission from Kakadu National Park

Geomorphic landscape Area (km2) Dry Season 222Rn

Emission (Bq.s-1)

Koolpinyah Surface 3,366 (1.2±0.2)*108

Dissected Foothills 1,386 (4.7±0.6)*107

Eroded Koolpinyah Surface 8,118 (2.8±0.4)*108

Alluvial Floodplains 1,386 (1.1±0.2)*108

Coastal Floodplains 2,178 (1.8±0.4)*108

Plateau 3,366 (9.9±5.0)*106

Kakadu Total 19,800 (7.4±0.6)*108

Hadley Circulation cells, previously discussed, create easterly and south

easterly trade winds during the winter months, dry season, through Kakadu. It was

observed that the dry season 210Pb deposition rate at Jabiru East was twenty times

lower than could be accounted for from the source term. The dry season 222Rn source

from Kakadu is assumed to suffer the same fate that is observed on the local scale in

the Jabiru region. It is speculated that this will be similar at all tropical locations that

experience dry seasons with dominant winds created by Hadley Circulation.

It has been shown in section 7.2.1 that Ranger operations represented

approximately (0.8±0.1)% of the 222Rn emission from the Kakadu region. From the

estimate provided in Table 7-4 again using and the averaged value of 7.04 MBq.s-1

for Ranger’s dry season 222Rn emission it is redetermined that Ranger’s 222Rn

emission represents (0.95±0.08)% of the total emission from Kakadu. The two

estimates are comparable and discrepancies attributed to the assumptions made in

determination of the 222Rn emission for Kakadu.

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7.4.2 Kakadu wet season 210Pb budget From a smaller dataset covering only several locations a wet season κ is determined

for the geomorphic landscapes shown in Table 7-5. The small data set meant that

regression analysis was not possible and the value κ presented is the average of the

readings for each geomorphic landscape. An additional geomorphic landscape is

included in Table 7-5 as observations at the seasonal site of Mirray hillock noted it

had a different wet season 222Rn exhalation rate compared to other vegetated areas.

This is likely a result of water transport downhill from the site reducing the influence

of moisture on 222Rn exhalation rates.

Water inundation during the wet season varies the 222Rn exhalation rate as

observed in the seasonal results of Chapter 5. Currently the exact amount of wetlands

within Kakadu is under debate because of the imagery tools used. To date the best

estimate is that approximately 28% of the region is either swamp or land subject to

inundation (Santos-Gonzalez et al. 2002). This compares well to the 18% classified

as coastal or alluvial floodplains in Figure 7.4 and the additional 10% is attributed to

the addition of swamps in the latter report.

Table 7-5: Wet season kappa for various geomorphic landscapes

Geomorphic Landscape Average (κ) (mBq.m-2.s-1/Bq.kg-1)

Barren 0.13±0.09

Vegetated 0.20±0.11

Foothills 0.57±0.20

189

Figure 7.5: Wetland area of Kakadu (Santos-Gonzalez et al. 2002)

halla

This figure is not available online. Please consult the hardcopy thesis available from the QUT Library

190

The wet season 222Rn emission from Kakadu is determined using the

information in Table 7-5, a wet season inundation of 28%, and the following

assumptions:

− Floodplains have no 222Rn exhalation during the wet season;

− Eroded Koolpinyah and Koolpinyah surfaces are considered to be

vegetated;

− The additional 10% water inundation is over eroded Koolpinyah and

Koolpinyah surfaces;

− The area of barren sites is considered to be negligible;

− Plateau is modelled as 25% the value of dry season non-compacted

boulders, this accounts for the solid matrix and decrease in 222Rn

exhalation due to water coverage;

− The 226Ra activity concentration of the plateau is the average of the 226Ra

activity concentration from all sites (the surface soils in the region

originate from erosion of the plateau over millions of years).

The wet season 222Rn emission source term from Kakadu is presented in

Table 7-6.

Table 7-6: Wet season 222Rn emission from Kakadu National Park

Geomorphic landscape Area (km2) Wet Season 222Rn

Emission (Bq.s-1)

Koolpinyah Surface 3,366 (3.5±0.8)*107

Dissected Foothills 1,386 (3.7±1.4)*107

Eroded Koolpinyah

Surface

8,118 (8.4±2.0)*107

Alluvial Floodplains 1,386 0

Coastal Floodplains 2,178 0

Plateau 3,366 (5.0±2.5)*106

Kakadu Total 19,800 (1.6±0.3)*108

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Table 7-7: Estimated Kakadu region 222Rn exhalation rate, wet season 210Pb deposition rate and total excess 210Pb inventory

Estimated average wet

season 222Rn exhalation

rate (mBq.m-2.s-1)

Estimated wet season 210Pb deposition rate

(mBq.m-2.day-1)

Estimated total excess 210Pb inventory (kBq.m-2)

8.1±1.3 334±53 2.1±0.2

From the results presented in Table 7-6 this it was possible to determine the

Kakadu regional wet season 210Pb deposition rate and excess 210Pb inventory using a

similar approach as used in section 7.2.4, these results are presented in Table 7-7.The

results presented in Table 7-7 agree well with the estimates determined for the Jabiru

region presented previously in this chapter and the measured results presented in

Chapter 5 and Chapter 6.

7.5 Chapter Summary This chapter focused on the relationship between 222Rn exhalation, 210Pb deposition

and excess 210Pb inventories. Using the data collected throughout the project it is

observed that there is a net loss of 210Pb from the region. The majority of this loss

occurs during the dry season. The lack of scavenging events and trade winds created

by a Hadley Circulation cell transports 210Pb attached to aerosols away from the

region where it is likely deposited over the ocean and land masses to the north west

of the Northern Territory.

The results of Chapter 5 and Chapter 6 allowed for determination of estimates

of 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories to be

compared with the observed values. While there is substantial loss of 210Pb, the

majority during the dry season, 90% of 210Pb deposition occurs during the wet

season. When 210Pb redistribution, regional water inundation, measurement times and

methodology are considered the estimations compare very well to the measured

values. There is a loss of 350±75 Bq.m-2.yr-1 of 210Pb from the Jabiru region, 78% of

which occurs in the dry season.

For Kakadu a dry season 222Rn emission of (7.4±0.6)*108 Bq.s-1 is derived

from estimations using justifiable assumptions. This shows that the Ranger source

term, although a strong point source is negligible on a broader scale. Kakadu’s 222Rn

emission during the wet season is reduced to (1.6±0.3)*106 Bq.s-1 when water

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inundation and reduction in 222Rn exhalation rates due to soil moisture are accounted

for. Regional estimated 210Pb deposition rates for the wet season and total excess 210Pb inventories for Kakadu compare well with the values measured in the Jabiru

region.

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8

Conclusions and future directions

8.1 Project outcomes From a study of the literature soil moisture is the most dominant variable that

influences 222Rn exhalation rates. The location of the project made it possible to

observe the influence of other variables, such as 226Ra activity concentrations, grain

size and soil porosity for various geomorphic landscapes, by performing dry season

measurements. These measurements showed that similar geomorphic landscapes

have similar 222Rn exhalation rate to 226Ra activity concentration ratios (RE-R).

Regression analysis on the dataset of dry season measurements was performed to

obtain a value (κ) for three of the four geomorphic landscapes identified. These

coefficients (κ) can be applied to determine dry season 222Rn exhalation rates for

tropical locations from a measurement of the 226Ra activity concentration. In general

compacted ground had decreased 222Rn exhalation rates while root structures,

associated with vegetation, creates more porous soil resulting in higher exhalation

rates.

Several diurnal measurements of 222Rn and 220Rn exhalation rates were

performed, some towards the end of the wet season and others during the dry season.

Even though there are diurnal variations in soil temperature and atmospheric

pressure, which both influence 222Rn and 220Rn exhalation rates and there are diurnal

variations in 222Rn atmospheric concentrations no diurnal variations in the exhalation

rates were observed. Diurnal variations, if any, are most likely masked by normal

fluctuations in exhalation of these isotopes. Results presented here confirmed and

agree with a previous study in the region that also reported no diurnal variations.

Distinctive seasonality of 222Rn exhalation rates was observed at the eight

seasonal sites. With the onset of the wet season increased soil moisture retarded 222Rn exhalation rates. Variations in the magnitude of retardation between sites was

due to the porosity of the local ground where more porous sites allow moisture to be

drawn away so after the first few weeks of rain 222Rn exhalation was not greatly

affected. Localised variations in 222Rn exhalation at a site occur during the wet

season due to uneven soil moisture distributions. A peak in 222Rn exhalation rates

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observed at some sites during January 2003 is most likely due to an uneven moisture

profiles and evaporation that results in a release of trapped 222Rn. While the 222Rn

exhalation rate measurements were discrete they are representative of the seasonal

variations for the region. Variations in soil moisture were observed through

measurement of soil moisture profiles. Dry season soil moisture profiles varied

minimally as small amounts of moisture were removed but large variations

throughout the wet season were a result of the intensity, intervals and duration of

precipitation events. It is expected there are large variations in 222Rn exhalation rates

over short periods of time due to these variations in soil moisture.

Distinct seasonality in 210Pb deposition rates is observed with 90% of the

deposition occurring during the wet season. Jabiru East displayed an exponential

decrease in the 210Pb deposition rates throughout the wet season while Oenpelli

maintained a relative steady state value. Observations at Jabiru East are explained

that there is a gradual reduction in the localised source term throughout the wet

season. At Oenpelli the effect on the 210Pb source term is likely to differ but

scavenging events are occurring at frequencies that result in a steady level of 210Pb

deposition. The lower wet season 210Pb deposition rates at Oenpelli compared to

Jabiru East are because of its location, surrounded by water inundated floodplains

that significantly reduce the local 210Pb source term.

Compared to globally reported values Jabiru East and Oenpelli have low

annual 210Pb depositional rates. In part this is due to the net loss of 210Pb from the

region that occurs during the dry season. Lead-210 attached to aerosols is drawn

away with the winter easterly to south easterly trade winds that result from a Hadley

Circulation cell. Lead-210 is transported from the region towards equatorial zones

where it is most likely deposited over the ocean and landmasses to the north west of

the Kakadu region. However the 210Pb deposition rates at both sites are within the

bounds of globally reported values, particularly southern hemisphere values. Jabiru

East is within bounds of the Australian and southern hemisphere averages and

compares well with values previously reported for Darwin.

Excess 210Pb inventories were measured in most of the soil samples collected

at 14 sites around Jabiru and Ranger. With the exception a few samples that returned

anomalous values due to dust deposition from Ranger stockpiles or high clay content

in the soils, excess 210Pb inventories over the region were relatively even. This

indicates that 222Rn and its progeny is dispersed evenly and well mixed over the

195

region within a few kilometres of a strong point source. Also as the majority of 210Pb

deposition occurs during the wet season the retardation of 222Rn exhalation reduces

the contribution from Ranger and makes it difficult to distinguish from the ambient

signal. As such there was no clear relationship between excess 210Pb inventories and

distance from the Ranger operation for the sampling locations.

Vertical transport of 210Pb in the soil was observed through analysis to

determine the penetration half depth of 2.5±0.7 cm. This was similar to a value

reported for Southeast Queensland.

Using a disused section of the Magela Land Application Area an intensive

study of 222Rn exhalation, 226Ra, 210Pb and 238U deposition was performed. From

previous studies it was expected that retention of 226Ra and 210Pb would be within the

top few centimetres of the soil and 238U penetration should be deeper. Radon-222

exhalation rates over the area, while enhanced, reflected the expectation that 226Ra is

absorbed in the top few centimetres. Analysis of soil samples collected in the area

confirmed this. Absorption of irrigated 226Ra over at the Magela Land Application

Area was predominantly on the finer grain material as was absorption of the naturally

deposited excess 210Pb at all locations. Adherence to finer grains for surface

deposited radionuclides is due to better ionic bonding and larger surface area to

volume ratios of small grains compared to larger grains. Uranium-238 transport at

the site differed from what was estimated at the onset of the irrigation program.

Analysis of non-irrigated samples close to the irrigated areas indicates there has been

horizontal transport of 238U from the irrigated area.

By combining the data presented in Chapter 5 and Chapter 6 the relationship

between 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories is

observed. Lead-210 budget was first performed on a local scale for the Jabiru region,

where wet season estimations agreed well with the measured values when factors

such as 210Pb redistribution, water inundation, measurement times and methodology

was considered. Dry season estimates of 222Rn exhalation rates were twenty times

lower than the observed values due to the net loss of 210Pb from the region.

The defined geomorphic landscapes of Kakadu were categorised to match the

geomorphic landscapes identified in this project. Hills were added as an additional

geomorphic landscape for the wet season. Using the coefficient (κ) it was possible to

obtain wet and dry season estimates of the 222Rn emission source term for the

Kakadu region of (7.4±0.6)*108 Bq.s-1 and (1.6±0.3)*106 Bq.s-1 respectively. From

196

the wet season emission it was possible to estimate the average 210Pb deposition rate

and excess 210Pb inventory for Kakadu, these estimations agreed very well with the

values observed on a local scale around Jabiru.

The lack of 210Pb scavenging events during the dry season results in long

atmospheric residency times calculated from previous work to be of the order of a

few months (140 days). This results in a net loss of 210Pb from the region that is a

result of it being transported to equatorial regions with the south east trade winds

caused by a Hadley Circulation cell. This loss has made complete 210Pb budgeting for

the region beyond the scope of this project. The quantity of the loss observed,

350±75 Bq.m-2.yr-1, was an unexpected result.

The objectives outlined in Chapter 1 have been met and a number of

additional objectives were achieved. Primarily the major project outcomes were:

− Investigating the principal contributing meteorological, geographical &

geological factors that affect the exhalation of 222Rn and deposition of 210Pb;

− Measurement of the seasonal variations in 222Rn exhalation rates from

several sites;

− Measurement of the seasonal variations in 210Pb deposition rates in wet

and dry deposition;

− Study the transport of 210Pb through the surface layers of the soil through

the measurement of radionuclides in soil samples;

− Modelling the 210Pb budget in the Kakadu region.

Additionally the following outcomes were also achieved:

− Measurement of the dry season 222Rn exhalation rates from numerous

geomorphic landscapes at Ranger Uranium Mine;

− Derivation of a value (κ) to determine 222Rn exhalation rates from

measured 226Ra activity concentration for four geomorphic landscapes

applicable to dry tropical locations;

− Measurement of diurnal variations of 222Rn and 220Rn exhalation rates for

the region;

− Determination of the dry season removal mechanism of 210Pb from the

region;

197

− Investigation of 238U, 226Ra and 210Pb retention at the Magela Land

Application Area;

− Modelling of the 210Pb budget on a local scale and determination of the

net loss of 210Pb.

8.2 Future directions This study has drawn to a close but has given rise to a number of potential future

research projects.

No relationship between ore grade and 222Rn exhalation rates for the

measurements performed at the Waste Rock Dump, grade 2 and grade 7 ore

stockpiles was observed. On the basis of diffusion theory it was stated that diffusion

lengths of 222Rn in stockpile structures differed substantially from normal ground and

are of the order of tens of metres compared to 1-2 m in normal soils. Only three ore

stockpiles at Ranger were measured and other than observation and personal

communications no direct measurement of the stockpile heights were recorded. For

rehabilitation purposes and the determination of radiological impact future studies

should investigate this matter further to determine whether 222Rn diffusion lengths

are greater in stockpiles structure or whether the results presented here are due to

other variables.

While it was possible to determine the 222Rn exhalation rate to 226Ra activity

concentration ratios (RE-R) for four geomorphic structures results for non-compacted

boulders were derived from a small dataset and are relatively inconclusive. The lack

of information for 222Rn exhalation rates and 226Ra activity concentrations for the

plateau region meant that some justifiable assumptions were required to determine

the 210Pb budget for Kakadu. Future studies should add to the dataset for better

determination of RE-R ratios and κ values as well as expanding on the classification

of geomorphic landscapes.

Measurements of soil moisture during this project were discrete, performed

once a month when 222Rn exhalation measurements were performed. Comprehensive

analysis of a single site throughout the wet season with frequent measurement of 222Rn exhalation rates and soil moisture would provide a clearer view of the effect of

soil moisture. Furthermore experimental laboratory procedures could be developed to

investigate the speculated evaporative effect that is believed to have given rise to the 222Rn exhalation anomaly in January 2003.

198

Averaged monthly 210Pb deposition rates provide a clear picture for seasonal

measurements but lack the determination of individual rain event contributions. A

previous study investigated a number of precipitation events and provided good

analysis of the 210Pb contribution of these events. A more holistic approach would be

to perform measurements of 210Pb deposition more frequently over a wet season.

Either a daily or weekly measurement schedule throughout a wet season would allow

for better determination of the individual contribution of precipitation events.

Observations should include the intensity, duration and type of precipitation events

for complete analysis. If performed in conjunction with a strict 222Rn exhalation rate

program for a site as previously discussed a much clearer determination of the wet

season 210Pb budget could be obtained.

The net loss of 210Pb from the region will most likely be observed on a

broader scale. If it were possible to perform measurements of 210Pb deposition rates

from sites such as Melville or Tiwi Island or as far as East Timor and Indonesia an

answer to the 210Pb loss might be obtained.

Soil measurement analysis of samples taken from the Magela Land

Application Area non-irrigated area returned elevated levels of 238U in the upper

levels of the soil. This indicates that there has been some horizontal transport of 238U

from the irrigated sections. Transport of 238U at the Magela Land Application Area

has been noted by eriss who have marked it as a priority research project. When

complete the results of this project will provide a more detailed analysis of the

transport of 238U from the irrigated sections than what has been provided here.

8.3 Conclusions At the onset of this project it was noted that there was a lack of information

regarding 222Rn exhalation and 210Pb deposition from tropical locations. The results

of this project have increased the body of knowledge for the natural cycle of 222Rn

exhalation, 210Pb deposition and its transport through the soil. This information was

obtained from the tropical location of Kakadu National Park, Australia. The dry

season allowed for the influence that soil moisture has on 222Rn exhalation to be

removed as a variable so a better estimate of the other influencing factors could be

determined

Only a small number of studies have attempted to combine and analyse 222Rn

exhalation rates and 210Pb deposition rates at a location. Analysis has found there is a

199

net loss of 210Pb from the region and confirmed that precipitation is the major

scavenging process of 210Pb from the atmosphere. With determination of the

atmospheric residency times this has further increased knowledge of the behaviour of 210Pb, and the aerosols it attaches to, in the atmosphere.

The measured excess 210Pb inventory in the surface soils and a penetration

half depth of 2.5±0.7 cm is good for use as a tracer in erosion studies providing a

good baseline can be established. The project has generated sufficient information

for future studies to be developed from the findings.

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210Pb DEPOSITION RATES IN A TROPICAL …...v Contents Chapter 1: Introduction 1 1.1 Overview 1 1.2 Alligator Rivers Region 6 1.3 Project objectives 9 Chapter 2: Literature Review: