Radionuclide Transport above a Near‐Surface Water Table: II. Vertical Distribution of Gamma Activities within Soil Profiles in Relation to Wheat Rooting Density and Soil‐to‐Plant Transfers

Abstract

AbstractRadiochemical results are described from a replicated field lysimeter experiment to investigate the upward migration of radionuclides in vegetated 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 137Cs, 109Cd, 60Co, and 22Na was introduced. Results of the vertical migration and plant uptake are presented from the first 2 yr of an ongoing experiment with winter wheat (Triticum aestivum L.). As expected, 137Cs, 60Co, and 109Cd proved to be highly sorbed to the soil solid phase in the regions of the water table and exhibited a lesser degree of up‐profile transport than 22Na in shallow lysimeters. However, in deep lysimeters, 137Cs and 60Cs appeared to be preferentially accumulated at the soil surface; a biological translocation mechanism is postulated to account for this observation. It was clear from a statistical analysis of vertical radionuclide distributions that, in physicochemical terms, 22Na was the most mobile of the radionuclides studied. The distribution of this radionuclide within the soil profiles showed a significant shift between the two growth seasons examined, whereas the distributions of the other radionuclides remained static between 1990 and 1991. Each radionuclide showed a distinct and consistent distribution pattern throughout the crop tissues, although the differences in tissue specific activities for individual radionuclides were found to be nonsignificant with the exception of 22Na. Soil‐to‐plant transfer factors (TFw) were calculated using mean soil specific activities weighted according to the fractional abundance of roots and measured soil specific activities in discrete soil layers. The TFw values were significantly lower in 1991 than in 1990 in both shallow and deep lysimeters; it is postulated that this was due to a much greater fiux 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 radionuclide absorption by the relatively small fraction of roots in the vicinity of the deeper water table.