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Radon in groundwater
Analysis of causes and development of a prediction methodology
Skeppström K.PhD. Student
Dept. of Land and Water Resources Engineering, KTH
Layout of presentation
Radon (focus of Rn in groundwater)
Objective of project / Phases involved
Methodology
Results & Discussions
Radon• Radioactive
• Colourless, odourless, noble gas
• Exists as 3 main isotopes:
•222Rn (uranium decay series, 238U),Half-life ( T1/2) = 3.8 days
•220Rn (Thorium decay series, 232Th), T1/2 = 56 seconds
•219Rn (Actinium decay series, 235U), T1/2 = 4 seconds
• Cancer risk
• 500 cases of lung cancer/year in Sweden; smokers have a higher risk.
• Risk of developing of other cancers ?
214Po
210Bi
210Pb
206Pb (stable)
238U (parent)
234Th
234Pa
234U
230Th
226Ra
222Rn
218Po
214Pb
214Bi
Uranium decay series
210Po
+
+
Regulatory limits(Sweden)
Radon in water Radon in air
Radon > 1000 Bq/l
Gränsvärde för otjänligt
Radon: 400 Bq/m3
Riktvärde för radon i befintliga bostäder
Radon > 100 Bq/l
Gränsvärde för tjänligt med anmärkning
Radon: 200 Bq/m3
Gränsvärde för radon i nya bostäder
Radon problems in water
Surface water Groundwater
Dug wells
(soil/sand aquifer)
Drilled wells
(Hard rocks)
How radon in water is a problem?
1000 Bq/l
in water100 Bq/m3 in air
Dish washing 95 %
Shower 60 – 70 %
Bath 30 – 50 %
Washing machine 90 – 95 %
Tap water 10 – 45 %
WC 30 %
Radon in water-
Water extracted from drilled wells (fracture
water)
Radon emanated in mineral grain
escape in the pore space
Pore space filled with water- Radon dissolves in the water
Transport mechanisms
• Diffusion
• Convection
Prerequisites
Presence of parent elements, 238U or 226Ra
Recoil Theory
How is it a problem ?
Dosimetry
• 1000 Bq/l is dangerous
Precipitation of 238U 234U, 230Th, 226Ra from water to surface of
fractureLeaching of 238U and 234U
Emanation of 222RnContent of 238U in the rock:
10ppm
Concentration of 222Rn in groundwater: 5 milj Bq/m3
Concentration of 222Rn in Bedrock: 0.33Bq/m3
rocks
222 Rn
0 k m 2 0 k m 4 0 k m
Radon content in wells in the county of Stockholm
N
Rn conc. (Bq/L)
0 to 100 100 to 500 500 to 1000 1000 to 64000
0
100
500
1000
Rn
(B
q/L
)
0 k m 2 0 k m 4 0 k m
Radon risk areascalculated usingkriging.
N
(W hite areas havetoo few wells)
(Knutsson & Olofsson, 2002)
Any deduction?
Granite types of rocks with high
uranium concentration
High radon concentration in
water
not always the case
Hypothesis of project
The hypothesis stipulates that the occurrence of radon from groundwater is governed by a number of well-defined factors ranging from:
• Geological (bedrock, soil, tectonic structures, flow pattern and surrounding environment)
• Chemical (oxidation reaction, other processes in water)
• Topographical (difference in elevation and slope that determine flow pattern and renewal tendency and frequency)
• Technical (withdrawal system & frequency which determine circulation as well as ventilation possibilities.
Purpose of research
Map processes and factors influencing radon content in groundwater
Develop a prediction model, based on statistics, that can be used to determine areas at risk.
Phases of the project
Phase 1Using GIS and multivariate analysis of
data to assess factors affecting radon
concentration – REGIONAL LEVEL
Phase 2Detailed study at Ljusterö to determinespatial & temporal variation of radon concentrations due to a range of factors.LOCAL SCALE
Phase 3 Development of risk prediction model
Phase 11. Data collection from:
Swedish National Land Survey (elevation and landuse data)
Swedish Geological Survey, SGU (soil & bedrock geology, fractures, radiometric)
Municipalities (data about wells and radon content)
2. Data transformation and extraction using ArcGIS and its spatial analyst function
3. Statistical analyses including multivariate analysis of data.
Factors considered
• Elevation
• Soil geology
• Bedrock
• Fracture zone
• Landuse
• Uranium content
Variables Derived factors
• Altitude difference
• Predominant soil, bedrock, landuse within a certain vicinity e.g. 200 m
• Slope of the terrain
Geographical Information System (GIS)
• GIS is a computer system for managing spatial data.
• Purpose of GIS• Organisation• Visualisation• Spatial Query• Combination• Analysis• Prediction
What is my objective?
For each well, relevant spatial patterns need to be extracted from the factor maps
GISSoftware: ArcMap
Spatial analyst functionGeostatistical software
To generate continuous
surfaces with a spatial
resolution of 50 m
+
Derive factors
Data obtained in
different formats, e.g
ASCII, point vector
Ultra editsoftware
Methodology using GIS
Topography
Geology
Radiom etric
Landuse
R a ster fo rm a t
P ix e l s ize : 5 0 m x 5 0 m
C o n tin u o u s su rface
Factors
. ....
.. .
.... wells Wells X Y Rn Factor 1
Data preparation Data extraction Database
Statistical methods• Which method?• Relate radon concentration with a large number of
variables• Variables are both qualitative and quantitative in
nature• Non-normal distribution of many variables• Use of covariance and correlations ? Careful with
the interpretations• Not much information about association between variables • Non-linear associations can exist• Very sensitive to ‘ wild observations- outliers ’
Statistical Analyses
Use of multivariate analysis of data– Each observational unit is characterised by several
variables.– It enables us to consider changes in several
properties simultaneously– Non normality of data (non parametrical tests)
Statistical Methods1. Analysis of variance2. Principal Component Analysis (PCA)
PCA method
• Eigenvectors of a variance-covariance matrix
• Linear combinations of these variables
• Its general objectives:• Data reduction (A small amount of k components
account for much of the variability of the data)
• Interpretation (may reveals relationships that were not previously suspected)
Descriptive Statistics
Statistic parameters
Number of wells 4439Minimum radon concentration (Bq/l) 4.0 Maximum radon concentration (Bq/l) 63560Mean radon concentration (Bq/l) 492Median value 230 Variance 1505978 Standard deviation 1227
Summary of results
High radon concentration in drilled wells is related to:
– Low altitude
– Granite rocks
– Close distance to fracture
– When overlying geology is lera/silt
– Infrequent use of wells (summer houses)
– An overview of the terrain in the surrounding of the wells (flat or hilly) is also of interest in connection to groundwater flow tendencies and speed of flow.
Risk Variable MethodData collectionData collection
Statistical analysesStatistical analyses Expert assessmentExpert assessment
Selection of significant variables
Selection of significant variables
Determination of risk values
Determination of risk values
Determination of uncertainty valuesDetermination of uncertainty values
Suming up risk and uncertainty
values
Suming up risk and uncertainty
values
Final Risk EvaluationFinal Risk Evaluation
Preparation Phase(Expert system)
Operational phase(User Interface)
Define study areaDefine study area
Risk Variable Modelling (RVM)
V1 x R1 + V2 x R2 + V3 x R3 + ……….+ Vn x Rn = FRV
FRV = Final risk value
• Where Vi= a risk value for a specific variable (-2 to +2)
Ri = the rating of the variable (1 to 3)
Field studies at Ljusterö
Why Ljusterö?
• Number of wells = 198• 141 wells exceeding 500 Bq/l (71%)
• 96 wells exceeding 1000 Bq/l (48%)
• Radon concentration• Mean = 1942 Bq/l
• Minimum = 50 Bq/l
• Maximum = 63560 Bq/l
What was done?
To choose 3-4 study areas on Ljusterö, exhibiting drastic fluctuations in the radon concentration and to perfom detailed study
at these locations
Detailed study• Analysis of geology (bedrock type, fracture zones,
tectonic zones and fracture filling minerals, soil type and soil depth)
• Altitude and other terrain considerations• Analysis of technical factors (wells technical design,
hauling system, spatial temporal extraction patterns of wells)
• Radiometric measurements of radiation (from soil around wells as well as measurements of radiation in wells and in tap water)
• Chemical analyses in water samples (U, Ra, Rn, fluoride and other water components)