Transfer of Radiocesium from Soil-to-Plant by Field Experiment

MATERIALS AND METHODS

Location of the study area: The geographical coordinates of the AERE campus are 23.55^ON latitude and 90.11^OE 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 25^OC 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 (K_D): Measurement of the distribution coefficient (K_D) 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 K_D 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 K_D 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 K_D 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 K_D, the equilibrium condition between soil and liquid phase was attained after 3 h. At equilibrium, the K_D was found to be 320 ml/gm. The K_D 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.