REEVALUATION OF THE PERMEABLE MATERIAL AT THE BASE OF THE UNCONFINED AQUIFER NEAR LERF, HANFORD SITE, WASHINGTON

A groundwater evaluation in the vicinity of the Liquid Effluent Retention Facility (LERF) in the 200 East Area, Hanford, Washington, provided an interesting opportunity to reevaluate a conceptual site model. An aquifer is a formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs. However, the implementation of this definition can at times be challenging given certain beliefs and tenets held about a particular hydrogeologic framework. At the LERF, a RCRA-compliant groundwater monitoring network was established with one upgradient and three downgradient wells and operated successfully for many years. Declining groundwater levels caused two of the downgradient wells to go dry; consequently, the groundwater monitoring network was out of compliance. The U.S. Department of Energy, in collaboration with the Washington State Department of Ecology and the U.S. Environmental Protection Agency, developed a series of groundwater evaluation questions. In working through these questions, hydrogeologic data in the vicinity of LERF was reevaluated in light of the declining water table, including data developed during the Basalt Waste Isolation Project related to the uppermost nature and extent of fractured basalt flow top in the vicinity of a structural high near LERF. Consequently, a plan was developed to drill two additional wells. Data collected during the installation and subsequent monitoring of these two wells was evaluated in conjunction with historical hydrogeologic and analytical data from the site to support refinement of the site conceptual model. The evaluation suggest that the aquifer in the vicinity of LERF includes two geologic formations containing both fractured and porous media and that a compliant RCRA groundwater monitoring network can be reestablished at LERF.