Our goal is to use isotopic measurements to understand how contaminants are introduced to and stored in the vadose zone, and what processes control migration from the vadose zone to groundwater and then to surface water. We have been using the Hanford Site in south-central Washington as our field laboratory, and our investigations are often stimulated by observations made as part of the groundwater monitoring program and vadose zone characterization activities. Understanding the transport of contaminants at Hanford is difficult due to the presence of multiple potential sources within small areas, the long history of activities, the range of disposal methods, and the continuing evolution of the hydrological system. Observations often do not conform to simple models, and cannot be adequately understood with standard characterization approaches, even though the characterization activities are quite extensive. One of our objectives is to test the value of adding isotopic techniques to the characterization program, which has the immediate potential benefit of addressing specific remediation issues, but more importantly, it allows us to study fundamental processes at the scale and in the medium where they need to be understood. Here we focus on two recent studies at the waste management area (WMA) T-TX-TY, which relate to the sources and transport histories of vadose zone and groundwater contamination and contaminant fluid-sediment interaction. The WMA-T and WMA-TX-TY tank farms are located within the 200 West Area in the central portion of the Hanford Site (Fig. 2). They present a complicated picture of mixed groundwater plumes of nitrate, {sup 99}Tc, Cr{sup 6+}, carbon tetrachloride, etc. and multiple potential vadose zone sources such as tank leaks and disposal cribs (Fig. 3). To access potential vadose zone sources, we analyzed samples from cores C3832 near tank TX-104 and from C4104 near tank T-106. Tank T-106 was involved in a major event in 1973 in which 435,000 L of high activity waste leaked to the vadose zone over a seven-week period. Other nearby tanks (T-103 and T-101) are also suspected of having leaked or overfilled. Pore water from these cores was analyzed for U and Sr isotopic compositions. Increasing {sup 99}Tc concentration in monitoring well 299-W11-39 (to 27,000 pCi/L in 2005) near the northeast corner of the WMA-T area prompted the emplacement of a series of new wells, 299-W11-25B, W11-45 (down gradient), and W11-47 (Fig. 3), during which depth discrete samples were collected below the groundwater surface. The depth profile from W11-25B revealed high {sup 99}Tc concentrations peaking at 182,000 pCi/L at {approx}10 m below the water table (Dresel et al. 2006). We obtained aliquots for isotopic analysis of groundwater samples produced by purge-and-pump sampling during the drilling of W11-25B, -45 and -47. In addition we have analyzed groundwater samples from monitoring wells in the vicinity of WMA T-TX-TY.
