Wurstner et al. (1995) summarize previous groundwater flow and contaminant transport model development and applications at the Hanford Site. Recently, a two-layer model of the unconfined aquifer, based on the VAM-3D code, was developed and applied (Law et al. 1996). This model was applied to simulate migration of tritium, nitrate, iodine-129, trichloroethylene, chloroform, uranium, and carbon tetrachloride plumes in the aquifer to provide input for developing the Hanford Site-wide Groundwater Remediation Strategy (Chiaramonte et al. 1996). These flow and transport simulations were conducted for a time period of 200 years and predicted that the groundwater flow system required 100 to 125 years to reach a steady-state condition. At steady state, the flow direction is predominantly from west to east. As the heads decrease and the water table is predicted to be within the Ringold Formation, the groundwater flow velocities were predicted to decline significantly, slowing the rate of migration of the various contaminant plumes. The Chiaramonte et al. (1996) model predicted that tritium discharged to the SALDS will not migrate beyond the Central Plateau, which is the area that encompasses the 200-West and 200-East Areas. The tritium plume to the east of the 200 Area is predicted to continue migrating toward the Columbia River and to reduce in size because of radioactive decay.
Other flow and transport models of the unconfined aquifer have been developed and applied. Connelly et al. (1992a, b) developed models of the 200-East and 200-West Areas to support Resource Conservation and Recovery Act (RCRA) and Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) investigations.
Golder Associates, Inc. constructed a site-wide model that was applied to support the Washington State Discharge WAC 173-216 permit application for the SALDS. This model was applied with the assumption that the source concentrations remained constant over the entire simulation period (205 years), rather than 20 years as assumed for the current model. This assumption resulted in prediction of a large tritium plume extending north of Gable Butte.
Future efforts in groundwater modeling will be focused on integrating the
multiple databases and models into a single set of conceptual and numerical
models for the Hanford Site, to ensure consistency in how data are interpreted
and how models are applied at Hanford.