Impact of preferential flow on radionuclide distribution in soil

Migration of radionuclides in soils and their transfer to edible plants are usually estimated using volume-averaged bulk concentrations. However, radi onuclides might not be homogeneously distributed in soils due to heterogene ous water flow and solute transport. One important cause of heterogeneous t ransport is preferential flow. The aim of this study was to investigate the spatial distribution of radionuclides in the soil in relation to preferent ial flow paths and to assess the possible consequences for their transfer f rom soil to plants. We identified the preferential flow paths in a forest s oil by staining them with a blue dye, and we compared radionuclide activity in samples from the stained preferential flow paths with those from the un stained soil matrix. The activities of the atmospherically deposited radion uclides Cs-137, Pb-210, Pu-239,Pu-240, Pu-238, and Am-241 were enriched in the preferential flow paths by a factor of up to 3.5. Despite their differe nt depositional histories, the distribution of the radionuclides between pr eferential flow paths and matrix was similar. Our findings indicate increas ed transport of radionuclides through the preferential flow paths, represen ting a possible risk of groundwater contamination. Furthermore, enrichment of radionuclides in the preferential flow paths might influence the uptake by plants. The heterogeneous radionuclide distribution in the soil and the more intense rooting in the preferential flow paths can be incorporated int o soil-to-plant transfer models. Taking the correlated radionuclide and roo t distribution between the two flow regions into account provides a more ph ysical and biological basis for the calculation of plant activities with tr a nsf er models than using the homogeneously mixed bulk soil activities as input parameters.