Environmental Geochemistry and Health, 1991, Vol 13(3), 139
Abstract: Soil-to-plant transfer of radionuclides: lysimeter-based studies at Imperial College P. Wadey, Q. Hu, M.J. Minski and G. Shaw* imperial College Reactor Centre, Silwood Park, Ascot, Berks, SL5 7TE, England
Introduction
Contamination of soils by mdionuclides can occur either as a result of atmospheric deposition or migration from contaminated ground waters. The former pathway may result from accidental or operational releases from nuclear plant, while the latter is of significance when considering the potential contamination of the biosphere from buried sources, such as nuclear waste repositories. Since 1984 a series of lysimeter-based studies has been carried out on these topics at Imperial College's field station at Silwood Park. These were initially aimed at providing data on the soil-to-plant transfer of radionuclides for the validation of numerical models predicting the dose to man following accidental atmospheric releases. Over the last three years, however, these have been extended under the auspices of a major Nirex-funded programme to examine the up-profile migration of radionuclides in simulated contaminated water tables at different depths below the soil surface. Studies of Soil Surface Contamination
In the case of an atmospheric release of radioactivity during the crop growth season the terrestrial foodchain pathway is, in most instances, the dominant exposure route (Nair, 1984). Deposition of fallout following such a release results in the contamination of the surface layers of the soil, the overall immobility of elements such as caesium ensuring that the contaminated layer remains in the upper region of the soil column. Uptake of radioactive contaminants by crops is dependent on many soil and crop parameters but is usually quantified for the purposes of radiological assessment modelling by means of the soil-to-plant transfer factor (TF), defined as plant specific activity (Bq kg-1) TF =
(1)
soil specific activity (Bq kg-1) TF values are normally calculated using total soil specific activities determined for the 'rooting zone', normally assumed to occupy the top 15-20 cm of the soil profile. Models such as FOODWEB, however, identify that * To whom correspondence should be addressed.
physico-chemical reactions within the soil may render radionuclides only partially available for crop uptake (Nair, 1984). Using a sequential leaching scheme adapted from that of Tessier et al. (1979) we have determined that the degree of chemical 'lability' of a radionuclide within a soil is strongly correlated with its availability as determined by the TF (Table 1). TF values shown within Table 1 are typically < 1 and suggest that the process of soil-to-plant transfer results in an attenuation of radioactivity. Table 1 Mean chemical labilities and TF values for five fission and activation products supplied as a surface application to lysimeter-contained soils ('zero' values represent specific activities below detection limits).
134Cs t37Cs
106gu
Radionuclide
54Mn 6~
Mean LT Mean TF
12.70 3.18 0.36 0.16 'zero' 1.7e-1 1.4e-2 2.2e-3 0.3e-3 'zero'
* LT is defined as the ratio of ion-exchangeable - total soil radionuclide specific activity.
Studies of Soil Contamination via Groundwaters
In 1988 eight of the lysimeters previously used for the study of crop uptake of mdionuclides following surface applications of radioactivity were converted to allow the establishment and automatic maintenance of artificial water tables at 35 and 65 cm below the soil surface. A cocktail of 13- and y-emitting radionuelides (22Na, 137Cs, 1~ 6~ 99Tc and 36C1) was introduced into this system in 1990 via the 'ground water' in order to simulate the leaching of radionuclides from buried nuclear waste repositories. An experimental programme is presently under way to determine the degree of up-profile migration of radioactivity with time together with the concurrent development of a soil-plant-water model of radionuclide transfer from a saturated sub-surface soil horizon (Karavokyfis et al., 1991). Preliminary results suggest that overall uptake of radionuclides by wheat growing in these lysimeters is significantly enhanced compared to previous studies involving surface contaminations and the potential consequences of these findings are being assessed.
140
Acknowledgements The authors thank CEGB (now Nuclear Electric) and UK Nirex Ltd for financial support. References Karavokytis, I., Buder, A.P. and Wheatcr, H.S. 1991. The development and validation of a coupled soil-plant-wamr model
(SPWO. Nircx safety series repo~ NSS/R225, UK Nirex Ltd. (in press) 138 pp. Nail S. 1984. Models for the evaluation of ingestion doses from the consm'nption of terrestrial foods follow2mgan atmospheric radioactive release. CEGB Report No. RD/B/5200/N84. Tessier, A.P., Campbell, G.C. and Bisson, M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. AnalyticalChemistry, 51(7), 844-851.
Abstract: Soil-to-plant transfer of radionuclides: lysimeter-based studies at Imperial College P. Wadey, Q. Hu, M.J. Minski and G. Shaw* imperial College Reactor Centre, Silwood Park, Ascot, Berks, SL5 7TE, England
Introduction
Contamination of soils by mdionuclides can occur either as a result of atmospheric deposition or migration from contaminated ground waters. The former pathway may result from accidental or operational releases from nuclear plant, while the latter is of significance when considering the potential contamination of the biosphere from buried sources, such as nuclear waste repositories. Since 1984 a series of lysimeter-based studies has been carried out on these topics at Imperial College's field station at Silwood Park. These were initially aimed at providing data on the soil-to-plant transfer of radionuclides for the validation of numerical models predicting the dose to man following accidental atmospheric releases. Over the last three years, however, these have been extended under the auspices of a major Nirex-funded programme to examine the up-profile migration of radionuclides in simulated contaminated water tables at different depths below the soil surface. Studies of Soil Surface Contamination
In the case of an atmospheric release of radioactivity during the crop growth season the terrestrial foodchain pathway is, in most instances, the dominant exposure route (Nair, 1984). Deposition of fallout following such a release results in the contamination of the surface layers of the soil, the overall immobility of elements such as caesium ensuring that the contaminated layer remains in the upper region of the soil column. Uptake of radioactive contaminants by crops is dependent on many soil and crop parameters but is usually quantified for the purposes of radiological assessment modelling by means of the soil-to-plant transfer factor (TF), defined as plant specific activity (Bq kg-1) TF =
(1)
soil specific activity (Bq kg-1) TF values are normally calculated using total soil specific activities determined for the 'rooting zone', normally assumed to occupy the top 15-20 cm of the soil profile. Models such as FOODWEB, however, identify that * To whom correspondence should be addressed.
physico-chemical reactions within the soil may render radionuclides only partially available for crop uptake (Nair, 1984). Using a sequential leaching scheme adapted from that of Tessier et al. (1979) we have determined that the degree of chemical 'lability' of a radionuclide within a soil is strongly correlated with its availability as determined by the TF (Table 1). TF values shown within Table 1 are typically < 1 and suggest that the process of soil-to-plant transfer results in an attenuation of radioactivity. Table 1 Mean chemical labilities and TF values for five fission and activation products supplied as a surface application to lysimeter-contained soils ('zero' values represent specific activities below detection limits).
134Cs t37Cs
106gu
Radionuclide
54Mn 6~
Mean LT Mean TF
12.70 3.18 0.36 0.16 'zero' 1.7e-1 1.4e-2 2.2e-3 0.3e-3 'zero'
* LT is defined as the ratio of ion-exchangeable - total soil radionuclide specific activity.
Studies of Soil Contamination via Groundwaters
In 1988 eight of the lysimeters previously used for the study of crop uptake of mdionuclides following surface applications of radioactivity were converted to allow the establishment and automatic maintenance of artificial water tables at 35 and 65 cm below the soil surface. A cocktail of 13- and y-emitting radionuelides (22Na, 137Cs, 1~ 6~ 99Tc and 36C1) was introduced into this system in 1990 via the 'ground water' in order to simulate the leaching of radionuclides from buried nuclear waste repositories. An experimental programme is presently under way to determine the degree of up-profile migration of radioactivity with time together with the concurrent development of a soil-plant-water model of radionuclide transfer from a saturated sub-surface soil horizon (Karavokyfis et al., 1991). Preliminary results suggest that overall uptake of radionuclides by wheat growing in these lysimeters is significantly enhanced compared to previous studies involving surface contaminations and the potential consequences of these findings are being assessed.
140
Acknowledgements The authors thank CEGB (now Nuclear Electric) and UK Nirex Ltd for financial support. References Karavokytis, I., Buder, A.P. and Wheatcr, H.S. 1991. The development and validation of a coupled soil-plant-wamr model
(SPWO. Nircx safety series repo~ NSS/R225, UK Nirex Ltd. (in press) 138 pp. Nail S. 1984. Models for the evaluation of ingestion doses from the consm'nption of terrestrial foods follow2mgan atmospheric radioactive release. CEGB Report No. RD/B/5200/N84. Tessier, A.P., Campbell, G.C. and Bisson, M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. AnalyticalChemistry, 51(7), 844-851.
