RADIONUCLIDE TRANSPORT ABOVE A NEAR-SURFACE WATER-TABLE .2. VERTICAL-DISTRIBUTION OF GAMMA-ACTIVITIES WITHIN SOIL PROFILES IN RELATION TO WHEAT ROOTING DENSITY AND SOIL-TO-PLANT TRANSFERS

Radiochemical results are described from a replicated field lysimeter experiment to investigate the upward migration of radionuclides in veg etated soils above near-surface contaminated groundwater. Water tables were maintained at depths of 35 cm (shallow) and 65 cm (deep), below which a cocktail of radionuclides including Cs-137, Cd-109, Co-60, and Na-22 was Introduced. Results of the vertical migration and plant upt ake are presented from the first 2 yr of an ongoing experiment with wi nter wheat (Triticum aestivum L.). As expected, Cs-137, (CO)-C-60, and Cd-109 proved to be highly sorbed to the soil solid phase in the regi ons of the water table and exhibited a lesser degree of up-profile tra nsport than Na-22 in shallow lysimeters. However, in deep lysimeters, Cs-137 and Cs-60 appeared to be preferentially accumulated at the soil surface; a biological translocation mechanism is postulated to accoun t for this observation. It was clear from a statistical analysis of ve rtical radionuclide distributions that, in physicochemical terms, Na-2 2 was the most mobile of the radionuclides studied. The distribution o f this radionuclide within the soil profiles showed a significant shif t between the two growth seasons examined, whereas the distributions o f the other radionuclides remained static between 1990 and 1991. Each radionuclide showed a distinct and consistent distribution pattern thr oughout the crop tissues, although the differences in tissue specific activities for individual radionuclides were found to be nonsignifican t with the exception of Na-22. Soil-to-plant transfer factors (TFw) we re calculated using mean soil specific activities weighted according t o the fractional abundance of roots and measured soil specific activit ies in discrete soil layers. The TFw values were significantly lower i n 1991 than in 1990 in both shallow and deep lysimeters; it is postula ted that this was due to a much greater flux of water through the crop in 1990, which was characterized by high evapotranspiration rates and a substantial negative water budget. In both years, TFw values were, in general, significantly higher for deep lysimeters than for shallow lysimeters. This is thought to provide evidence of enhanced radionucli de absorption by the relatively small fraction of roots in the vicinit y of the deeper water table.