Relative contributions of natural and waste-derived organics to the subsurface transport of radionuclides

Our laboratory is studying the role of organic compounds in the subsurface transport of radionuclides at shallow-land burial sites of low-level nuclear waste, including a commercial site at Maxey Flats, Kentucky, and an aqueous waste disposal site. At the Maxey Flats site, several radionuclides, notably Pu and 60Co, appear to exist as anionic, organic complexes. Waste-derived organics, particularly chelating agents such as EDTA, HEDTA and associated degradation products (e.g., ED3A), are abundant in aqueous waste leachates and appear to account for the complexation. EDTA, and probably other waste-derived chelating agents as well, are chelated to the Pu and 60Co in the leachates, potentially mobilizing these radionuclides. In contrast, at the low-level aqueous waste disposal site, naturally-occurring organics, ranging from low molecular weight (MW) acids to high MW humic acids, account for the bulk of the groundwater's organic content. Certain radionuclides, notably 60Co, 103Ru and 125Sb, are mobile as anionic complexes. These radionuclides are clearly associated with higher MW organics, presumably humic and fulvic acids with nominal MW's > 1000. It is clear, therefore, that naturally-occurring organics may play an important role in radionuclide transport, particularly at nuclear waste burial sites containing little in the way of waste-derived organics