Technetium Attenuation in the Vadose Zone: Role of Mineral Interactions

High-level waste (HLW) has leaked into the vadose zone from buried single-shell tanks at the Hanford Site. Contaminant plumes containing radionuclides are slowly migrating toward the groundwater table. The accepted model of contaminant migration places technetium (Tc) at the leading edge of the plume due to the high mobility of the anionic species, TcO{sub 4}{sup -}, in its oxidized state. However, recent drilling and sampling programs at the B-BX-BY and S-SX Hanford Site tank farms have revealed that the Tc distribution within the contaminant plume is more complex than anticipated and some Tc may be retained in the vadose zone as insoluble precipitates. This research project investigates possible mechanisms by which Tc associated with HLW leakages can be attenuated in the vadose zone. It also will provide fundamental information on surface mediated reduction/precipitation reactions of Tc on Fe{sup II}-containing mineral surfaces and the stability of Tc precipitates under conditions in the va dose zone at the Hanford Site. This information will provide much needed data for the development of models of Tc mobility in the vadose zone following closure of the tank farms. With reliable models, a major cost reduction in remediation efforts may be achieved by selecting sites that present the greatest environmental threat and designing remediation methods with the greatest efficacy