DISTINGUISHING ANTHROPOGENIC URANIUM IN A HIGH BACKGROUND ENVIRONMENT

The Rocky Flats Environmental Technology Site (Site) near Golden Colo. is underlain by the Rocky Flats Alluvium, derived from glacial outwash that may, in part, be sourced from the Ralston Buttes Uranium District, southwest of the Site. The Site contains ground water with detectable concentrations of dissolved U in some site background sampling wells. Both enriched and depleted U were processed at various times at the Site and some material was disposed of in the environment. A major challenge for the Site has been to determine whether the uranium detected in ground water was due to natural or man-made processes. High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP/MS) methods were used to investigate uranium (U) isotopic ratios in ground water for the Site. Historic Alpha Spectroscopy results for isotopic U were not accurate enough to determine natural ratios and concentrations seemed to be linked to lithologic units. Ground water pathways interact with finer grained sediments, which may contribute natural U to the water. Most ground water becomes surface water before leaving the site and must meet an ambient standard of 10 pCi/l. The Solar Ponds area of the site has concentrations up to 850 pCi/l but the full impact of contamination is blurred by natural concentrations in the stream alluvium. With impending cleanup deadlines in 2006 there is no time to waste in disputing the background data. The Site will use the HR-ICP/MS data to resolve issues with respect to U contamination at the Solar Ponds and elsewhere on Site. A collaborative sampling plan has been used to distinguish background uranium from contamination at all areas of the site exceeding the surface water standard of 10 pCi/l U in ground water. The plan evaluates the sample results against the natural abundances of U235 and U238 and the presence of U236, an activation product present in anthropogenic U. Those results have shown two additional areas where buried depleted U has impacted ground water. Additional evaluation of samples with low levels of U236 is being investigated with Thermal Ionization Mass Spectrometry (TIMS) to help define the extent of impacted ground and surface waters. An alternative background protocol is being developed from these results.