GEOCHEMISTRY IN HANFORD'S VADOSE ZONE: RECENT SCIENTIFIC FINDINGS AND KEY UNCERTAINTIES

The vadose zone at Hanford is an unsaturated region between 10 and 60 m in depth that lies between surface soil and groundwater. The vadose zone sediments are of glacio-fluvial origin and physical properties can exhibit extreme vertical heterogeneity at the 0.25 m scale and above. The geochemical regime is mildly calcareous. The sediments are relatively unweathered and contain minor detrital phyllosilicates and few secondary metal oxides. The Hanford vadose zone has received large inventories of inorganic (Na, NO3, Cr), radioactive (137Cs, 90Sr, 60Co, 99Tc, U, Pu) and organic (e.g., CCl4) contaminants as a result of past practices disposal (e.g., cribs, retention basins, and disposal trenches) and the leakage of single shell (SS)-high level waste (HLW) tanks. Most of these contaminants are sorbed to vadose sediments by different mechanisms, and some are migrating to and entering groundwater. Here we focus on the migration behavior of HLW, dealing with several case studies from Hanford's tank farms (S-SX; B-BX-BY; T-TX-TY) that are currently under regulatory mandated characterization. Contaminant distributions will be shown beneath SS tanks that have leaked waste solutions of markedly different composition and overall sediment reactivity (e.g., REDOX boiling waste, Sr recovery waste, and high U metal waste). The development of conceptual models for the geochemical interaction/retardation of select contaminants [e.g., 137Cs, 90Sr, U, and Cr(VI)] within these plumes will be highlighted using information from laboratory sorption/desorption studies, and results of mineralogic residence/chemical speciation measurements by different methods that include X-ray microscopy and spectroscopy. Important adsorption and precipitation/dissolution processes will be identified and insights will be provided on fruitful and needed areas of fundamental research to resolve critical issues of contaminant migration.