CHAPTER I INTRODUCTION

1.0 GENERAL

The word radiation was used until about 1900 to describe electromagnetic waves.

Around turn of the century, electrons, x-rays, and natural radioactivity were discovered and

were also included under the umbrella of the term radiation. In the 1920's, De Broglie

developed his theory of the duality of matter, which was soon afterward proved, correct by

electron diffraction experiments and distinction between particles and waves cased to be

important. Today, radiation refers to the whole electromagnetic spectrum as well as to all the

atomic and subatomic particles that have been discovered.

One of the many ways in which different types of radiation are grouped together is in

terms of ionizing and non-ionizing nature. The word ionizing refers to the ability of the radiation

to ionize a gas through which it passes.

Non-ionizing radiation is electro-magnetic radiation with wavelength about 1.0 nm [or]

longer. The part of the electromagnetic spectrum includes radio waves, microwaves, visible

light [=770 nm to 390 nm], and ultraviolet [ =390 nm to 1 nm].

Ionizing radiation includes the rest of the electromagnetic spectrum like, x-rays [=1.0-

0.005 nm] and gamma rays. It also includes all atomic and subatomic particles, such as

electrons, positrons, protons, alphas, neutrons, heavy ions and mesons. Radiation released into

the environment may cause harm to humans. In order to assess the effects of naturally existing

radiation on people, the measurement of radioactivity levels in the environment becomes

imperative.

1.1 SOURCES OF RADIATION

Radioactivity and ionizing radiation associated with it have existed in earth long before

life emerged. Indeed, they were present in space before earth itself appeared. Radioactive

materials become part of earth at its very formation. Hence, mankind has all along evolved in

an environment of natural background radiation.

The exposure to ionizing radiation can be classified into two groups. (i) External

exposure, resulting from source external to the body and (ii) Internal exposure, resulting from

radioactivity residual in the body. The cosmic radiation and radiation arising out of earth’s crust

and the building material used for construction of houses and buildings give raise to the

external exposure. The radioactivity naturally present in the body as well that, which enters it

through inhalation and ingestion and is retained in the body contributes to internal exposure.

These two comes under the natural background radiation. The natural background radiation

has two components: One is originating from extraterrestrial source such as cosmic rays, the

other having a terrestrial origin such as radioactive nuclides that exist in the earth’s crust. Total

contribution from the natural sources to the Indian population works out to 2.3 mSv/y as

against the global value of 2.4 mSv/y (UNSCEAR-2008). However, in some localities wide

variations, the natural radiation level may increase slightly due to scientific, industrial and

technological activities.

EXTRATERRESTRIAL SOURCES

The extraterrestrial high energy particles which originate from cosmos and which are

known as primary cosmic rays impinge continuously on earth’s atmosphere. These particles

consist of 87% proton, 11% alpha particles and 1% nuclei of atomic number between 4 and 26

and about 1% of electron of very high energy. The interaction of the primary cosmic radiation

with the atmospheric nuclei of nitrogen, oxygen and argon many other radionuclide 3H, 4Be, 14C

and 22Na are other important cosmogenic radionuclide to which mankind is exposed. The dose

rates in the air due to cosmic rays vary little with latitude but vary significantly with altitude,

doubling approximately every once in 1500m. Global average annual effective dose

contribution from cosmogenic radionuclides through internal exposure is estimated to be about

0.015mSv/y (UNSCEAR-2008).

In general, individual annual effective doses from cosmic ray radiation around the world

ranges between 0.26 to 2.00 mSv/y with a mean value of 0.380 mSv/y. Average effective dose

from cosmic ray radiation in India is estimated to be about 0.355 mSv/y (Mishra et al:1971).

Since the dose rate from cosmic radiation increases with altitudes, in high altitude locations like

Gulmarg in India, the annual effective dose to residents is about 830 μSv/y (Benville et al 1987).

TERRESTRIAL SOURCE

Terrestrial source of radiation are primordial radionuclides, produced during the birth of

the universe. The half-lives of these radionuclides are sufficiently long as to be active even

today. Secondary radionuclides are derived from the radioactive decay of primordial

radionuclides. These radionuclides are components of the three radioactive series. These are

uranium 238U series, actinium 235U series and thorium series 232Th.

Estimated modified source including mining of heavy metals, coal fired power plants,

mining of phosphate rocks and its use as fertilizers, production of natural gas, gas mantles and

luminescent dial and air travel contribution to the background radiation to the Indian

population works out to be 1.2 x 10-3mSv/y; atmospheric weapon tests contributes about 0.045

mSv/y, medical exposure contributes about 0.048 mSv/y and exposure due to nuclear power

production contributes about 5.0x10-5mSv/y to the background radiation (Ramachandran:

2011).

1.2 NATURE OF PRIMORDIAL AND FALLOUT RADIONUCLIDES

238Useries, 232Th series and 40K are the primordial radionuclides and have half-lives

sufficiently high as long as to be active even today. Secondary radionuclides are derived from

the radioactive decay of primordial radionuclides. These radionuclides are components of the

three radioactive series. These are 238U series, 235U series and 232Th series.

Among, the singly occurring radionuclides 40K are most important one. It has a half-life

of 1.3 x 109 years and decays by beta emission to 40Ca followed by K-capture to an excited state

of 40Ar, which returns to the ground state, by gamma ray emission. The abundance of 40K is

0.0118% and its specific activity is about 29600 Bq/Kg [Eisenbudet al1997]. Since potassium is

widely distributed in the environmental matrix, 40K constitutes a major source of environmental

radiation. However, potassium is an essential element which is under close homeostatic control

in the body. The average mass concentration for an adult male is about 2g of potassium per kg

of body weight. The isotopic ratio of 40k is 1.18x10-4 and the average activity mass

concentration of 40k in the body is about 60Bq/kg. The highest annual absorbed dose [270Gy]

is received in red bone marrow and the lowest in the thyroid [100Gy]. It delivers an annual

effective dose equivalent to about 180Sv (UNSCEAR 2008).

The families of radionuclides belonging to 238U series, 235U series and 232Th series

account for much of the background radiation level in the environment. Among these three

series, the 235U series is less important since it makes up only 0.73% by weight of natural

uranium compared to 99.27% for 238U, and the activity ratio 235U/238U is less than 0.05. In

addition, 223Ra and subsequent members of the series are relatively short lived and do not

appear in the environment in significant concentration (UNSCEAR-2008).

238U is the head of a series of 14 principal nuclides. This series can be divided into five

sub-series in which the activity of the precursor controls to a large degree the activities of the

decay products. Thus: 238U234U; 230Th; 226Ra: 226Ra214Po: and 210Pb210Po. These sub-series

deliver an annual dose of 51-Sv, 71-Sv, 71-Sv, 850-Sv and 12-Sv respectively due to

intake by inhalation and ingestion of nuclides into the body (UNSCEAR 2008). 238U has a half-live

of 4.47 x 109 years and decays to 234Th by alpha emission. 238U is the most abundant [99.27%]

isotope of natural uranium and is found in all rocks and soil with varied concentration. Further,

because of its presence in the soil and phosphate-based fertilizers, 238U finds its way into food

and human tissue. The daily intake of uranium from all dietary sources is about 0.011-0.018 Bq

[5 Bq annually] (Eisenbud et al 1997).

226Ra, 222Rn and 210Pb (which head the three sub series of 238Useries) and 210Po which

belongs to 210Pb sub series are other important radionuclides in 238U series. 226Ra has a half-life

of 1620 years and becomes 222Rn through alpha decay. Chemical properties of radium are

similar to that of calcium and therefore enter the human body through food chain easily and

gets concentrated in bones. More than 70% of the radium in the body is contained in box, the

remaining fraction being distributed rather uniformly in soft tissues. The average annual dietary

intake of 226Ra in areas of normal radiation background is 15Bq (UNSCEAR-2008). The 222Rn is a

radioactivity gas with half-life of 3.8 days. The radon gas exhaled from the earth surface into

the vertical convention and turbulence. 222Rn and its daughter products enter the human body

mainly through inhalation. The average annual intake in normal background area is about

2,00000Bq through in halation and 300Bq through ingestion (UNSCEAR 2008).

210Pb occurs in nature in partial radioactivity equilibrium with its immediate daughters

210Bi and 210Po. The complete radioactive equilibrium of 210Pb→210Bi→210Po in the biosphere is

usually not reached due to the difference in the influence of biological, metrological, chemical

and other factors on each of these nuclides (Jaworowski-1967). 210Pb has a half-life of 22 years

and decays to 210Bi by beta emission. 219Bi is also a beta emitter with a half-life of 5 days and

decays to 210Po.

The 210Po has a half-life of 138 days and is an alpha emitter [5.3Mev]. Consumption of

food is usually the most important route by which 210Pb and 210Po enter the human body. The

absorbed dose from the 210Pb sub series depend mainly on the highly energetic alpha particles

of 210Po, as the contribution from the beta emissions of 210Po and 210Bi amounts to just about

10% of the total absorbed dose (UNSCEAR-2008).

The natural alpha emitter, Polonium-210 of the 238U series, is of radiological interest for

a number of reasons but mainly because of its large contribution to the natural radiation dose

received by many species (cherry and Shannon 1974) have placed 210Po in very hazardous

radiation materials group and further evidence from the literature indicates that 210Po is

accumulated strongly by organism and transferred via food chain. The main source of 210Po

entering into the environmental is the exhalation of 222Rn from the ground and its subsequent

decay in the atmosphere, resulting in 210Po deposition on earth’s surface primarily scavenged

by rainout processes (Abe and Abe, 1980) may be transferred to humans via diet, there is a

distinct need for investigation of the accumulation of this nuclide by aquatic organisms. A

system is scanty in literature. Moreover, this kind of study is inadequate with respect to

different habitats.

Another major terrestrial source of natural background radiation is thorium series. 238Th

is the head of a series of 11 radionuclides and can be divided into three sub series: 232Th itself;

228 Ra 224Ra; and 220Rn 208Pb. The sub series deliver an annual dose of 3Sv, 13Sv and

160SV respectively due to intake by inhalation and ingestion (UNSCEAR2008). 232Th has a half-

life of 1.4x1010 years and decay to 228Ra by alpha emission. Although 232Th is not as widely

redistributed as uranium in rocks and soils. Certain rocks such as ingenious are found to certain

232Th four times that of 238U. However, since the specific activity of 232Th is 4.07 Bq/Kg

compared to 12.21 Bq/Kg of 238U, the radioactivity of the two nuclides is more [or] less the

same (Eisenbud 1997). Wren et al -1985 have reported the body content of 232Th of about

80mBq, of which 60% is in the Skelton.

228Ra which heads the sub series of 232Th series is similar to 222Ra in toxic behavior. 228 Ra

has a half-life of 5.76 years and decays to 228Ac by beta emission. 228Ra is much more available

to plants and animals than 232Th. Therefore the activity concentration of 228Ra in humans are

mostly due to the dietary intake of 228Ra itself and not due to the decay of 232Th. The annual

activity intake arising from in halation is estimated to be 0.001Bq, while that from ingestion of

food is considerably larger, about 15Bq in area of normal radiation background. The estimated

average activity mass concentrations in bone and tissues in humans are 50mBq and 4mBq

respectively in areas of normal background radiation. Another nuclide of some interest in this

series is 220Rn which has also known has thoron. It has a half-life of 55sec.

TECHNOLOGICALLY ENHANCED NATURAL RADIATION

The modern scientific and technology practices contribute, through slightly to the

prevailing natural background radiation level in the environment. Phosphate industry and coal

fired electric power stations are the major contributors. By burning coal the activities of these

naturally occurring radionuclides are distributed from underground to biosphere. The world

annual production of coal was about 3.1x1012kg in 1985 (UNSCEAR, 1987). Coal is used mostly

commonly for industrial purposes power generation and space heating. The average activity

concentrations in coal are 50Bq/kg of 40k and 20Bq/kg each of 238U and 232Th.

Mining and processing phosphate ores distribute 238U and its decay products among

varies products by products and wastes of the phosphates industry. Industrial effluence, the

use of phosphate fertilizers in agriculture, the use of it’s by products in the building industry are

the possible source of enhanced levels of natural radiation. In 1982 the estimated world

production of phosphate rock was about 130millions tones. The consumption of phosphate

fertilizers was about 30millions tones. The typical concentration of 238U in sedimentary

phosphate is 1500Bq/kg [UNSCEAR-2008]. Building materials contain higher concentration of

226Ra may enhance the indoor exposures to radon and its decay products. Consumer products

such as radio luminous products, electronic and electrical devices, smoke detectors and

ceramics glassware, alloys etc containing uranium or thorium also contributes to the

technologically enhanced natural radiation level [UNSCEAR-2008].

SOURCE OF ARTIFICIAL RADIONUCLIDE

The use of radioisotopes in medicine, nuclear weapon test and nuclear power reactors

are the major sources of artificial radionuclides among more than 200 different radionuclides

produced in these artificial source, the long lived radionuclides 90Sr [28.8years] and 137Cs

[30.2years] which are produced in nuclear weapon test and reactors contribute significantly to

the back ground radiation level in the environment. 90Sr is chemically similar to calcium and

therefore enters the human body following a path similar to that of calcium. On the other hand,

137Cs is chemically similar to potassium and enters the human body following the path of

potassium. However, 137Cs is tightly bound by soil and thus the uptake by plant from soil is

relatively less compared to that of 90Sr. however the direct contamination is possible. In fact

external radiation associated with past atmospheric nuclear weapon test has been attributed to

Cs.

Although several hundred radionuclides are produced by nuclear explosions, only the

radionuclides 131I, 90Sr, 137Cs and 14C contribute significantly to human exposure. The dose from

131I are delivered in a matter of weeks , those from 90Sr and 137Cs are delivered for few decades

while from 14C will be delivered over thousands of year. The collective dose due to all

atmospheric nuclear explosions was estimated to be 3x10-7mSv.

The subject matter of the thesis has been only the primordial radionuclides and fallout

radionuclides and their literature survey an given below :

1.3 LITERATURE SURVEY

Scientific observations so far shows that major part of the internal dose that people

receive from natural sources is derived from terrestrial sources from the decay of Uranium-238

series and to a lesser extent, from the decay of Thorium-232 series and Potassium-40. A large

number of studies have been carried out by various group of investigators in different parts of

the world to measure radiation level radionuclide distribution and transportation in the

environment.

The global average of 238U series, 232Th series, and 40K in soil is 35, 35, and 400 Bq/kg,

respectively (UNSCEAR-2008). In the Indian context, these values are 31, 63, and 394 Bq/ kg,

respectively (Kamath et al 1996).The world’s average value of radium equivalent activity is 370

Bq/kg reported by organization of Economic and control department (OECD). The absorbed

dose rate estimated from soil for Indian sub-continent is about 69 nGy/h (Kamath et al 1996)

and the world average is 51 nGy/h [UNSCEAR-2008]. Extensive studies shows that an average

outdoor terrestrial gamma dose rate of 60 nGy/h in the world ranging from 10 to 200 nGy/h

(Taskin et al, 2009). The world average value of outdoor annual effective dose equivalents for

soil samples is 70μsv (Orgun et al 2007)

1.4 RADIOACTIVITY IN SOIL AND ITS DISTRIBUTION

The radioactivity of soil is that of the rock from which it is derived diminished by the

leaching action of moving water, diluted by increased porosity and by added water and

inorganic matter and augmented by sorption and precipitation of radionuclides from incoming

water. Soil may have been produced from the weathered top layer of still intact bedrock below,

transported laterally from the same rock unit, or transported from an entirely different rock

unit or type some distance away. Although most soil probably is derived from transported

material, it is also probable that it is material from the same rock unit or type. Water is the

dominant transport medium, but other means of transport are wind and human activities.

DISTRIBUTION OF RADIOACTIVITY

The geologic map, which usually shows the type and extent of the uppermost bedrock,

must be used with caution in estimating the inherent terrestrial radioactivity of a locality. The

soil layer about 0.25m thick furnishes the external radiation from the ground and can mask

bedrock of substantially greater or lesser radioactivity. Within moderated distance of

mountains with exposed rocks of abnormal radioactivity, the out washed erosion products from

the mountain leads to a covering that may be more radioactive than the bedrock.

Biochemical process modifies the inherent radioactivity of the soil in several ways. The

development of root system stabilizes the soil as its water content increases. Humic acid

accelerate decomposition of the rock material, resulting in smaller grain sizes and greater water

content, greater porosity and less permeability for the soil. The decomposition of organic

matter tends to change the lower soil from oxidation to a reducing environment, reducing

uranium from its mobile hexa-valent state to its immobile tetravalent state and decomposing

the hydrous iron oxides that entrap radium and other elements present in minute

concentration. The acid condition in some soil should also minimize retention of radionuclides

taken up by calcium carbonate. The overall effect of soil development is to reduce the average

level of external radiation and to reduce the range of concentration of the radionuclides

comparison with source rocks.

1.5 TRANSFER OF RADIONUCLIDES TO PLANTS

Plant take up the radionuclides in soil water, but are selective in doing so. Tracer has

indicated that movement into the plant root system and to the leaf or other terminal site may

require only day time. A detail of this report is given in the following section.

TRANSFER OR UPTAKE OF RADIONUCLIDES

Uptake of a long-lived radionuclide by plants depends to a considerable degree on

whether it remains within the root zone and the extent to which it is chemically available

[Availability] for transport to root endings [Migration], mechanism of plant uptake and

translocation to the edible portion.

AVAILABILITY OF RADIONUCLIDES IN SOIL

The availability of radionuclides for biological uptake is of vital importance for their

transfer through the food chain to human beings. The availability of radionuclides in the soil for

the plant uptake is determined through short and long-term assessments. Since it is mainly the

radionuclides present in the soil, which are thought to be available for plant uptake, the study

on the processes that are controlling the flow of radionuclides into and away from soil solution

is done by short and long-term assessments.

SHORT-TERM ASSESSMENT

Many methods of short-term assessment of radionuclides have been tried. The most

obvious method is to grow plant in radioactivity-induced soil and measure the amounts of

nuclides taken up by plants. This has been applied in many investigations. However, the

method has the following drawbacks.

1. Study of bioavailability of radionuclides through short-term assessment is a time

consuming process depending on the growth period of plants.

2. It is impossible to determine the specific conditions or seasons during which the uptake

of radionuclides from soil to plant is high or low.

3. There are quiet large differences between plants with regard to uptake of radionuclides,

so it is very difficult to arrive at general conclusions.

4. Plant uptake of radionuclide cations is mainly from soil water. It would be ideal for

short-term studies to analyze the contents of the soil water. However, this is very

difficult, since a significant proportion of soil water is closely associated with the soil

matrix and will not be extracted even by high-speed centrifuge force. This small volume

of soil water available under field moisture conditions also, makes detection of

radionuclides difficult.

5. The short-term assessment gives the information only for readily available radionuclides

and not for the radionuclides produced during disintegration process, which are

essential for long term predictions of food- chain transfer.

LONG-TERM ASSESSMENT

To obtain information for radionuclide produced during the disintegration process,

various chemical extraction methods are being applied along with the gamma ray

spectrometry. The same was recommended by UNSCEAR for long-term assessment, which has

been used in the present investigation and is briefly discussed in the second chapter.

FACTORS AFFECTING AVAILABILITY OF RADIONUCLIDES

Generally the factors that affect the migration are also important for bioavailability and

this includes the type of radionuclides, form of deposition and soil type. The bioavailability will

be reduced with time, although the extent to which this occurs varies between soil types.

1.6 MIGRATION

Migration of radionuclides is either upward or downward movement of radionuclides in

soil. Study of migration is of important for several reasons. The downward migration is of

important during the analysis of fall out radioactivity. Vertical movement of radionuclides from

bottom to top layer of soil increases the uptake of radionuclides to plant and hence the upward

migration of radionuclide is also important. Also, the external dose from the radionuclides in

the soil decreases when they are down in the soil profile and if the nuclide migration is below

the root zone, the plant uptake will becomes smaller, thereby decreasing the internal dose

through food. Possible contamination of ground water and surface water is yet another reason

to study migration.

EXTERNAL RADIATION DOSE

External radiation dose from the soil is only due to the gamma emitting radionuclides,

such as primordial radionuclides and natural fall out radionuclides. The main factors that

reduce the dose rate from the soil are radioactive decay and vertical migration of radionuclides.

INTERNAL RADIATION DOSE

Human beings are exposed to internal radiation dose either from ingestion or inhalation

of radionuclides. Ingestion of radionuclides through food chain can be primary or secondary.

Primary ingestion occurs when human consumes plant materials containing radionuclides;

secondary is the consumption of animal products contaminated by fodder or soil ingestion.

MECHANISM OF MIGRATION

Mechanism of migration is the process in soil, which cause radionuclides to be

transported within the soil profile. This mechanism is the subject of some debate, but the

following suggestion is acceptable. Transport of radionuclides in the soil can occur either in

solute or in colloidal form. Generally it has been supposed that solute transport dominates, and

most modeling efforts have been directed towards that end. Both U and Th have very slow

migration whereas K and Po have a high migration rate due to it’s the chemical property

[Sheppard et al, 1998]. The dominant species of U and Th in soil are, specifically UO22+, Th4+

cations, strongly absorbed by soils, since soil have net negative charge. The retention of these

radionuclides is created in fine texture soils. Since, U and Th are quiet immobile in soils, any

mechanism that increases their mobility becomes important in plant uptake of these

radionuclides. Such mechanisms include the formation of the organic complexes and

association with colloids. Different experiments show that organic complexes and colloids can

increase the mobility of U and Th as well as their plant uptake [Sheppard et al, 1998].

Moreover the constant deposition of 210Po in soil and water could lead to higher

radioactivity concentration in soil. The uptake and migration of 210Po is highly dynamic than the

other natural alpha emitter. The accumulation of these radionuclides on the soil enters into

plant through soil water.

1.7 MECHANISM OF PLANT UPTAKE

The long-term pathway of radionuclides to human being is through plant uptake and food

ingestion. For this reason, study of plant uptake of radionuclides is important. Plants may

uptake radionuclides through the leaves or through the roots. Here root uptake is more

important because primordial radionuclides are largely available in the soil. The plant roots take

up U and Th in the same way as the chemical analogues P. 40K is in the form of K+ and the plants

tend to take up K+ more easily than P. But in the case natural fallout radionuclide 210Po is closely

related with atmospheric moisture and dust particles. Certain species like moos and orchid

depends on atmospheric moisture and dust particles for their nutrition and the plants absorb

the result of atmospheric content of 210Po. Not only that most of the plant species having the

strong capacity of ion exchange and are able to hold, in addition to nutrients, the nuclides that

are transport through rainwater and moisture [Hasanem, 1972].

1.8 TRANSFER CO-EFFICIENT

Environmental assessment of uptake of radionuclides, mostly use food- chain models to

determine the dose to man from radionuclide release to the biosphere. Traditional food chain

models require a plant/soil substrate transfer co-efficient, referred to as a concentration ratio

[CR.]. Concentration ratio or transfer coefficient is generally accepted and widely used in the

environmental transport models and describes the amounts of radionuclide expected to enter a

plant from soil substrate [Martinez etaI 1996].

The C.R. approach defines the concentration of radionuclides in plant and soil substrates

are linearly related and that the line defining the relationship goes through the origin. Hence,

under normal conditions, the C.R. for an element is a constant [Sheppard and Sheppard 1985].

Canon, in 1952, supported the linear relationship existing for radionuclides between plant and

soil concentration.

However, the uptake of radionuclides, both essential and non-essential for growth of

plants, differs and may follow the non-linear relationship also. Morishima et al, in 1976, has

reported a non-linear relationship between plant and soil concentration of radionuclides. Wirth

etal, in 1985, has also stated the C.R. of 40K, as an essential element for plants, was not linearly

related to the soil concentration.

Mordberg et al, in 1976, obtained C.R. values by fitting linear and hyperbolic function to

the data obtained using different soil concentration of isotopes in U-Ra series. They have

showed that the CR was constant over their observed soil concentrations. Cannon [1952] also

suggested a linear relationship existing for uranium between plant and soil concentration.

However Morishima et al, 1976 reported a non-linear relationship for uranium between

vegetable and soil concentration. Lopatkina et al 1970 has suggested that the linearity

assumption may hold only for primitive plant forms. Shepard and Shepard 1985study the linear

assumption for uranium in plants and substrate at two ore bodies on the Precambrian shield.

They formed that the relation between CR and substrate concentration observed in native

plants tend to be hyperbolic with a tendency for CR values to be larger at lower substrate

concentration. It appears; therefore that uranium may behave as the essential element at

lower substrate concentrations. Many data suggest that plant concentration are unrelated to

soil concentration, and when a line is fit towards the diagram the intercept is usually greater

than zero [Simon and Ibrahim 1987]. Shepard and Evenden 1988 reported log normal

distributions of concentration ratio. They proposed that the linearity assumption although

contrary to what is usually expected from Nutrition elements, may be valid for limited range of

soil concentration are for elements not physiologically regulated by plants. Many experiment

designed to measure CR values have used only a few different soil concentrations and thus

cannot statistically assist the linearity assumption. Simon and Ibrahim 1987 proposed that

curvilinear reflecting a saturation type mechanism in plants is more appropriate.

It is seen from the figure 1.1, the plant readily takes-up elements essential for plant

growth when soil substrate concentrations are low, whereas plant uptake of non-essential

element is generally constant in this substrate concentration range [Timperley et al 1970]. At

high soil substrate concentration, plant uptake of essential and non-essential elements can

either be constant [non-toxicity] or can decrease, leading to toxicity or death. Toxicity begins at

a toxicity threshold and hence uptake, begins to be impaired [Sheppard et al, 1985]. Lethal

substrate concentrations of an essential or non-essential element may not exist, and in that

case the CR may be constant for several orders of magnitude.

------ ( 1 )

The C.R. concept is CR= Cp/Cs = constant, and it is probably useful for non –essential

elements at non-toxic substrate levels. The lake of correlation between plant and substrate

concentration for non-essential elements could be explained if the element is mimicking an

essential element [Shepard and Shepard 1985]. This fact would result in a non-linear

relationship between the CR and substrate concentration at low substrate concentration. This

relationship could be modeled as where Cp is the concentration in plant and

CS is the concentration in the substrate. This results in bC

a

C

CCR

ss

p . If the first

relationship does not have a zero intercept this would be that CR is independent on the

substrate concentration.

A hypothetical relationship for the C.R. for essential and non-essential element over a

range of soil substrate concentration is shown in the Figure 1.1.

Tracy el at 1983 studied the uptake of Radium, 210Pb and U from soil to garden produce.

They observed a clear trend of increasing concentration in soil in plant with increase in

concentration in soil substrate. In order to express this trend quantitatively the linear

regression analysis was carried out for the logarithmic of the mean vegetable concentration

verses the logarithmic of the soil concentration. This involves fitting of function of the form

b

sp aCC or1b

saCCR , where a and b are parameters to be determined.

A value of b equal to 1.00 would imply a linear relationship. They found that their result

were not inconsistent with the linearity assumption at least for a concentration up to 37 Bq/g in

soil. They use of concentration ratio to predict plant concentration from low substrate

concentration for environmental assessment purpose should be investigated further for

Radium nuclides known to mimic essential elements and should be measured under natural

conditions.

TRANSLOCATION WITHIN PLANTS

The radionuclides taken up by plants are translocated in order to reach the plant edible

parts or food crops. Because of the adsorption of U and Th on cell wall materials, the mobility of

U and Th to the edible parts of the plants are restricted. As a result, the radionuclide

concentrations are higher in tissues, found in lower part of the plant. For this reason root crops

usually have higher transfer coefficient values than the other parts for U and Th. However, for

40K, the concentration ratio depends only upon the nutrient requirement of the plant [Mengel

et al, 1979].

1.9 SCOPE AND AIMS OF STUDY

The above literature survey shows that extensive studies have been carries out on

natural background radiation and distribution of radionuclides in the environmental of different

regions throughout the world. These studies indicate that natural background radiation level is

not the same for different regions. Fluctuations have been observed from region to region. The

distribution activity of important radionuclides 238U series and 232Th series, 40k in the natural

samples of soil sand vegetation and other food items has also been reported to be fluctuation

in environs of different regions. This fluctuation is found to be significantly high in sample of

high background radiation areas. The survey also shows that the intake of radionuclides

estimated and reported for the population of different environs is different. It is also clear from

the above survey that the studies on variation of radionuclides concentration with depth and

with grain size of soil are quite sparse. In view of all these, this investigation was initiated with

one aim objective is that to study the distribution, transportation and uptake behavior in plats

of natural and fallout radionuclides in soil.

Natural radiation level varies only within relatively narrow limits in most of the places on

the earth. However, a wide deviation from the normal radiation level has been observed in

some localities due to abnormally high soil concentrations of radioactive minerals. The

radionuclides studied in the soil are 238U series, 232Th series,40K and 210Po, because in long term,

these three radionuclides are the most important and also the source of the other

radionuclides. These radionuclides enter into plants from soil and thereby to human body

through ingestion. Hence, the present investigation focuses on the study of radionuclides

distribution in soil and transfer or uptake of radionuclides from soil to plants.

The objective of the present study is to measure the concentration of primordial

nuclides in soils and its dose to the environment. Especially concerning the vertical migration

and transfer to plant system in Longwood forest located in Western Ghats region.

The main aim of this work is to contribute a bit more and study about how the natural

radionuclides are been incorporated in the plant from its substrate concentration. Although

these species are not directly involved in the human food chain, information on the

concentration level and the transfer of radium nuclides from contaminated soil will provide

important data on the transfer mechanism in the case of those species more directly involved in

the human diet.

The specific aims are classified as

To assess and try to understand the behavior of primordial radionuclides present in

forest soils and to measure the radiation in the local environment of long wood forest of

western Ghats region.

To understand and study their distribution with in the soil and depth in the forest

ecosystem.

To investigate Soil to plant transfer of primordial and fall out radionuclides in forest

Soils.

To understand the distribution of theses radionuclides in the different parts and

different types of the plants species.

To identify the plants which shows higher affinity to these radionuclides and apply the

CR model for the same plants

To attain possible conclusion in plant uptake of radionuclides.