Abstract: In this research, soil-to-plant Transfer Factor (TF) for radiocesium has been measured by field experiment. Soil and grassy plant grown in a contaminated land containing radiocesium in the Atomic Energy Research Establishment (AERE) campus, Savar, Dhaka were collected to investigate the transfer of radiocesium in the soil-plant system. The activities of radiocesium in soil and grassy plant were measured using High Purity Germanium (HPGe) detector coupled with associated accessories. Using the measured activities, TF values were found within the range of 2.7H10•2 to 4.7H10•2, which are reasonably comparable with the values found in the literature. Soil characteristics were also investigated to assist the measured TF values for the corresponding soil. This data set might be useful while calculating radiological human dose via the ingestion pathway in the tropical environment.
INTRODUCTION
After accidental release of radionuclides in the environment, they are deposited on ground and consequently accumulated in the plant through uptake process. Among deposited radionuclides, radiocesium (137Cs) is a dominant fission product, which has high relative mobility in the soil-plant system, long-term bioavailability, high radiotoxicity and is long-lived (Rosen and Eriksson, 1998). The plant uptake of deposited 137Cs from soil, commonly expressed as soil-to-plant Transfer Factor (TF) is widely used while calculating radiological human dose via the ingestion pathway. Soil to plant transfer factor; an important parameter needs to be measured adequately to predict a reliable dose to the population through ingestion pathway (IAEA, 2001). There are many studies regarding the TF values for 137Cs performed both for tropical and temperate environments (Skarlou, 1996; IAEA, 1994; Nisbet and Woodman, 2000; Othaman and Yassine, 1995; Bunzl and Kracke, 1987; Jalil et al., 2002). However, a few data performed by pot experiment (Jalil et al., 2002) is available in Bangladesh especially in the vicinity of AERE campus where major nuclear facilities (3 MW TRIGA Mark-II research reactor, a radioactive waste disposal facility likely to be established and a central radioactive waste processing and storage facility) in the country have been established. In the Atomic Energy Research Establishment (AERE) campus, there is a contaminated land containing 137Cs. In the present work, TF value of 137Cs for grassy plants grown in the contaminated land has been investigated. Generally TFs of 137Cs vary often by more than four orders of magnitude depending upon soil type, pH, solid/liquid distribution coefficient, exchangeable K+, organic matter content, plant species and other environmental conditions (Absalom et al., 2001). The low solid-liquid distribution coefficient (KD) due to the low clay content and high NH4+ concentration in the soil solution enhance TF value of 137Cs. Moreover, the usual low K+ level in the solid phase and in the soil solution and high organic matter content also enhance root uptake. In the present work, soil characteristics have also been investigated to assist the measured TF values for the corresponding soil. The results obtained in the present work might be useful while calculating radiological human dose via the ingestion pathway in the tropical environment valid for many Asian countries.
MATERIALS AND METHODS
Location of the study area: The geographical coordinates of the AERE campus are 23.55ON latitude and 90.11OE longitude. It stands on a topographic high land, which is surrounded by an alluvial flood plain whose average elevation is about 10.70 m above mean sea level. AERE campus and its adjoining areas have a tropical humid climate with high summer temperature, excessive air humidity, heavy rainfall and cool dry winter. The mean annual temperature is about 25OC and the average annual rainfall is about 2.1 m. The southwest monsoon continuing from March to October causes rainfall in the area. Heavy precipitation occurs during the period June-July.
Soil characteristics: A wide range of physical and chemical properties of soil such as pH, exchangeable K+, organic matter content and clay content is generally responsible for enhancing TF values of 137Cs to plant, both directly and indirectly. Since the uptake of 137Cs to plant is based on soil characteristics, a data set of soil type based on Marshal textural classification was investigated. Apart from this, some available data such as exchangeable K+, organic matter content and clay content for the same soil described elsewhere (Rabbani et al., 1996; Frissel, 2001) were shown in Table 1. The data set described in Table 1 is the average value of the corresponding soil property.
Transfer factor (TF): The term soil-to-plant TF measures the transfer of radionuclides from soil to plant when uptake by plant root is the only process affecting the transfer. From the observed activity concentrations of the radionuclide in the plant and in the corresponding soil, the TF values were calculated according to the following equation.
| (1) |
The TF is expressed in this experiment in fresh mass basis because fresh mass is required in the radiological assessment. Grassy plant grown at contaminated land containing 137Cs in the AERE campus were selected to measure TF values under field condition.
Field condition: In 1988, an experiment >Uptake of 137Cs by Commonly Consumed Fresh Water Fishes= was carried out within the AERE campus (Annual Report, BAEC, 1989). Upon completion of the experiment, liquid radioactive wastes originating from this experiment was stored in 200 l drums in an isolated interim storage room within the campus.
| Table 1: | Soil characteristics in the AERE campus |
In 2000, an area (~20H30 m2) adjacent to the storage room was found contaminated with 137Cs liquid wastes, confirmed by an investigation. The contaminated area is mostly covered by grassy plant (mainly grass) especially during summer and rainy seasons. In the present research work, an investigation on the transfer of 137Cs from contaminated soil to grassy plant grown in this area was carried out under field condition. Different locations were selected for the collection of soil (at a depth of 0-20 cm) and grassy plant samples. Roots were separated from the grassy part of the plants. Collected soils and grassy plants from each location were transported to the laboratory for processing before measurements. These samples were air-dried and grounded with pestle and motor. Special precaution was taken to avoid cross contamination. In the representative plant and soil samples, 137Cs activity was measured directly with the HPGe detector in Marinelli beaker geometry (height .16.5 cm and diameter = 14.5 cm). The mean 137Cs concentration in soil was observed to be about 30"7 Bq kgG1 with a maximum of 64 Bq kgG1. A typical spectrum of 137Cs having centroid position at 662 keV for both soil and plant is shown in Fig. 1.
Distribution coefficient (KD): Measurement of the distribution coefficient (KD) can be performed by the Batch Method with any radionuclide on any soil material or rock. In most cases, the soil material or rock is continually agitated to facilitate mixing and contact. At specified times, to approach equilibrium conditions, the solid and solution are separated. The KD can be determined as the ratio of concentration of a radionuclide absorbed to solid phase to the concentration of that radionuclide left in liquid phase. The distribution coefficient represents the partition of the solute in the soil matrix and soil water, assuming that equilibrium conditions exist between the soil and solution phases.
| Fig. 1: | Gamma Spectra of 137Cs for Soil and grassy plants |
The KD used to express the exchange of radionuclides between the dissolved and sediment sorbed phases for a given radionuclide for steady state conditions at equilibrium can be expressed as,
| (2) |
The KD for the collected soils was investigated using Batch Method after attaining equilibrium between solid and liquid phase.
RESULTS AND DISCUSSION
The granular compositions of the soil are shown in Table 1. The soil is composed of 11.20% sand, 43.30% silt and 46.50% clay, which lie in silty clay soil based on Marshal textural classification. The chemical properties are also shown in Table 1. The gamma spectra of 137Cs for soil and grassy plants are shown in Fig. 1. It can be seen in this Figure that the peak at 662 keV corresponds to 137Cs. The soil concentration is firmly higher than the concentration in plant. The inset as in Fig. 1 gives the concentration of grassy plant on an expanded scale.
It can be seen in this Fig. 2 that the TF values were 3.10H10G2, 3.20H10G2, 3.50H10G2, 2.50H10G2, 2.90H10G2, 4.20H10G2, 3.5H10G2, 2.8H10G2 and 3.70H10-2 for points 1, 2, 3, 4, 5, 6, 7, 8 and 9, respectively within the contaminated area. The average TF value was found to be 0.0324"0.004. The measured values are found to be comparable with the values described elsewhere (Jalil et al., 2002; Shang and Leung, 2002). The highest TF value was found to be 4.2H10G2 at point 6 while the lowest TF value was found to be 2.5H10G2 at point 4.
| Fig. 2: | TF values for 137Cs for the grassy plants |
| Table 2: | Transfer factors of 137Cs published by other Organization (Jalil et al., 2002; Shang and Leung, 2002) |
The variations lie within the ranges compared to the values described in Table 2 which indicate a less considerable variation among different points in the contaminated area. This attributes to a slight variation while calculating radiological dose to the population considering ingestion pathway if uses this measured value. The TF values for the plants could be applied in order to predict more reliable dose assessment via the ingestion pathway.
According to the kinetic study for KD, the equilibrium condition between soil and liquid phase was attained after 3 h. At equilibrium, the KD was found to be 320 ml/gm. The KD value might be useful for dose calculation considering the ground water pathway.
CONCLUSIONS
Soil to plant TF values for the grassy plant grown in the contaminated land of AERE campus have been investigated. Based on Marshal textural classification, it is found that the selected soil is silty clay. The TF values of the grassy plants at different positions were found to be reasonably comparable with the value found in the literature. It has been observed that the variation of measured TF values is acceptable at different positions.
This may attribute to a very small variation of dose considering a single TF value of a plant while calculating radiological dose to the population via ingestion pathway. The measured soil characteristics might provide a data set to assist the measured TF values. This data set might be useful while calculating radiological human dose via the ingestion pathway in the tropical environment.
ACKNOWLEDGMENTS
The authors are thankful to technical staffs of Radiation Monitoring and Waste Management Laboratory, Institute of Nuclear Science and Technology, Atomic Energy Research Establishment, Savar, Dhaka, Bangladesh for their cooperation during experiments.