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, radionuclides might not be homogeneously distributed in soils due to heterogeneous water flow and solute transport. One important cause of heterogeneous transport is preferential flow. The aim of this study was to investigate the spatial distribution of radionuclides in the soil in relation to preferential flow paths and to assess the possible consequences for their transfer from soil to plants. We identified the preferential flow paths in a forest soil by staining them with a blue dye, and we compared radionuclide activity in samples from the stained preferential flow paths with those from the unstained soil matrix. The activities of the atmospherically deposited radionuclides 137Cs, 210Pb, 239,240Pu, 238Pu, and 241Am were enriched in the preferential flow paths by a factor of up to 3.5. Despite their different depositional histories, the distribution of the radionuclides between preferential flow paths and matrix was similar. Our findings indicate increased transport of radionuclides through the preferential flow paths, representing 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 into soil-to-plant transfer models. Taking the correlated radionuclide and root distribution between the two flow regions into account provides a more physical and biological basis for the calculation of plant activities with transfer models than using the homogeneously mixed bulk soil activities as input parameters.