IMPROVING SUBSURFACE GEOLOGIC MAPPING OF THE MIGRATION OF CCL₄ BENEATH THE HANFORD NUCLEAR WASTE SITE

Carbon tetrachloride was used at Hanford's Plutonium Reclamation Facility to recover plutonium from waste streams generated during the production of special nuclear materials for the United State's nuclear arsenal. An estimated 1,000 metric tons of the material was discharged into the ground in the central portion of Washington State's Hanford Site from 1955 to 1973; however, a significant fraction of the original discharged material cannot be detected or accounted for. Extensive modeling and transport research is providing new insights into migration of CCl₄ beneath the Hanford disposal facilities. Modeling contaminant transport at the Hanford Site is complicated by spatial variations in its subsurface geology. A combination of fluvial deposits, calcrete, loess, and younger silts, sands, and gravels deposited by numerous catastrophic ice age floods compose the site's aquifer and thick vadose zone. Past erosional and depositional events result in uncertainty as to the location of contacts between sedimentary units, and contaminant movement is strongly influenced by variations in sediment texture and composition. While the site is well-studied, a more detailed, lithofacies-based geologic model is needed to improve predictions of future contaminant migration. We show how the geologic model is being improved by the integration of our interpretation of geophysical logs and geologists' drilling records with laboratory measurements quantifying carbon tetrachloride concentrations along the primary groundwater flowpath.