Carbon tetrachloride flow and transport in the subsurface of the 216-Z-9 trench at the Hanford Site

As a result of past practices, up to 580 m(3) carbon tetrachloride (CT) was discharged to waste sites at the 200 West Area of the USDOE's Hanford Site near Richland, WA. Three-dimensional modeling was conducted to enhance the current conceptual model of CT distribution beneath the major disposal site (216-Z-9). The simulations, using the STOMP code, focused on migration of dense nonaqueous phase liquid (DNAPL) consisting of CT and codisposed organics under scenarios with differing sediment properties, sediment distribution, waste properties, and waste disposal history. Simulation results support a conceptual model for CT distribution where CT in the DNAPL phase migrated primarily in a vertical direction below the disposal site and where some CT DNAPL likely migrated across the water table into the regional aquifer. Results also show that the lower permeability Cold Creek unit retained more CT DNAPL within the vadose zone than other hydrologic units during the infiltration and redistribution process. Due to the relatively high vapor pressure of the CT, the resulting vapor plumes are extensive and influenced by density-driven advection. Any continued migration of CT from the vadose zone to the groundwater is likely through interaction of vapor phase CT with the groundwater and not through continued DNAPL migration. Additional simulations assessed the impacts of soil vapor extraction (SVE) as a remediation method. These simulations showed rapid CT removal associated with the assumed local equilibrium of CT between the phases. Additional efforts are needed to enhance the understanding of rate-limited volatilization to improve simulation of the SVE process and to provide a basis for refining the design and operation of SVE systems.