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Nuclear wastes from Hanford's processing for separation of plutonium are stored in massive, buried, single‐shell tanks in 18 tank farms. These so‐called tank wastes were initially thermally hot because of radioactive decay, and many exhibited extreme chemical character in terms of pH, salinity, and radionuclide concentration. At present, 67 of the 149 single shell tanks are suspected to have released over 1.9 million L of tank waste to the vadose zone, with most leak events occurring between 1950 and 1975. Boreholes have been placed through the largest vadose zone plumes to define the extent of contaminant migration and to develop conceptual models of processes governing the transformation, retardation, and overall transport of tank waste residuals. Laboratory studies with sediments so collected have shown that ion exchange, precipitation and dissolution, and surface complexation reactions have occurred between the tank wastes and subsurface sediments, moderating their chemical character and retarding the migration of select contaminants. Processes suspected to facilitate the far‐field migration of immobile radionuclides including stable aqueous complex formation and mobile colloids were found to be potentially operative but unlikely to occur in the field, with the exception of cyanide‐facilitated migration of 60Co. Certain fission product oxyanions (Mo, Ru, Se, Tc) and nitrates are the most mobile of tank waste constituents because their adsorption is suppressed by large concentrations of waste anions, the vadose zone clay fraction is negative in surface charge, and, unlike Cr, their reduced forms are unstable in oxidizing environments. Reaction/process‐based transport modeling is beginning to be used for predictions of future contaminant mobility and plume evolution.

Some of the defense wastes at the Hanford Site in southeastern Washington State are stored in the vadose zone. It is possible that natural recharge could mobilize and transport the contaminants in these wastes to the groundwater, as well as influence groundwater velocities and directions. The objective of this study was to estimate the areal distribution of natural recharge for use as a boundary condition for a groundwater flow and transport model. A geographic information system (GIS) was used to identify all possible combinations of soil type and vegetation and assign to each an appropriate estimate of recharge. The strategy was to assign estimates based on field data and supplement with simulation results only when necessary. The estimated rates varied from 0.7 to 127.1 mm/yr. The order of preference for assigning estimates was lysimetry > water content measurements > tracers > modeling, based on qualitative estimates of the relative error of each method as applied. The GIS software was used to estimate the annual recharge volume attributable to specific soil‐vegetation combinations. The total annual recharge volume was 8.47 × 109 L for the 765 km2 portion of the site containing the major waste storage areas. This volume is from 2 to 10 times higher than estimates of runoff and groundwater flow from adjacent higher elevations and is equivalent to facility discharges in 1992. The recharge map showed the impact of a 1984 fire on increasing recharge; it also illustrated the higher recharge rates associated with disturbed soils in the waste storage areas.

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Large quantities of radioactive and chemical wastes resulting from Pu production for nuclear weapons are located in the vadose zone at the USDOE's Hanford Site, north of Richland, WA. The vadose zone here is characterized by often highly stratified glacial‐fluvial sediments that give rise to complex subsurface‐flow paths that contribute to uncertainty of contaminant fate and transport. Research efforts have focused on answering questions of contaminant transport from the viewpoint of geologic, biologic, geochemical, and hydrologic controls. This special section highlights key research topics concerning vadose zone problems at the Hanford Site. Research indicates that some of the contaminant species (137Cs, 60Co, 90Sr) are retained by Hanford sediments as a result of geochemical reactions, rendering them effectively immobile except under extremely saline or acidic conditions, while other species (99Tc, 129I, 3H) are typically mobile and have moved deep into the vadose zone and subsequently into groundwater. In addition, large quantities of organics, including carbon tetrachloride, have moved in complex ways as both vapor and liquid in the subsurface. Observed transport of mobile species is linked to liquid discharges and to elevated recharge rates that occur primarily at waste sites where land surfaces are void of vegetation and where winter rains have subsequently penetrated the subsurface wastes. A series of papers in this issue documents progress to date in understanding transport rates at Hanford, why anisotropy strongly affects the distribution of subsurface contaminants, why organic contaminants are difficult to find in the deep vadose zone, and what the impacts of hypersaline fluids are on waste form degradation and subsequent transport.