ISOTOPE TRACKING OF FLUID INFILTRATION THROUGH THE VADOSE ZONE AT HANFORD, WASHINGTON

During the Cold War era, the Hanford site was used for the production of nuclear weapons fuel. The significant amounts of high-level radioactive waste generated there are currently stored in tanks at the site, some of which are known to be leaking. Also, lower-level waste was discharged directly to the ground. In order to understand the movement of these contaminants through the vadose zone, we have been measuring the isotopic compositions of pore water extracted from vadose zone core samples and groundwater from Hanford.

The vadose zone at Hanford consists of glacial sediments with interbedded caliche layers. Water contents here range from 2 to 8% (by weight) with perched water zones developed above some of the less permeable sediments and caliches. The oxygen and hydrogen isotope compositions of vadose zone pore water in a core drilled at a relatively undisturbed site are significantly evaporated (d18O values shifted by 2-4‰). Exceptions include samples from a perched water layer and from the saturated zone, both of which have not been evaporated. This indicates that they are primarily derived from lateral fluid flow, not vertical infiltration. Analyses of the tritium content and chemical composition of the perched water indicates that it is partially derived from site activities (the core was drilled ~50 m from an infiltration pond). In addition, a series of infiltration tests were conducted a separate site. In May/June of 2000, 5 aliquots of ~4000 L of water were released to the subsurface to simulate a leaking tank. D2O and Br- were added to the third of these aliquots. After 3 weeks, the tracers had spread out laterally through unsaturated sediments above a low-permeability layer. The highest measured concentrations were >50% of the initial (despite the tracer solution representing only 20% of the total water injected). After 2.5 months, the tracer solution had spread somewhat further (maximum concentrations had dropped to ~30% of the initial). After 9 months, there was little difference in the D2O concentrations, indicating no further movement of the tracers. These studies indicate that infiltration of fluids through the vadose zone is limited unless enhanced by excess fluid.