SELECT AND OPTIMIZE ENGINEERING DESIGN USING A GROUNDWATER MODEL AT THE ENVIRONMENTAL MANAGEMENT WASTE MANAGEMENT FACILITY (EMWMF), OAK RIDGE, TENNESSEE

A three-dimensional groundwater flow and transport model was developed for use on the Environmental Management Waste Management Facility (EMWMF) at DOE’s Oak Ridge Reservation (ORR) to select and evaluate engineering alternatives for lowering the groundwater level under the EMWMF and to help the performance analyses and monitoring activities. This sitespecific flow model was developed from an area regional groundwater flow model and the previous site-specific model used for initial disposal cell conceptual design. The groundwater flow model was developed by (1) revising model input to reflect the local geology based on field reconnaissance and information gained during site development and construction of Phase I of the EMWMF, (2) incorporating the current site topography and engineering features, (3) adding properties to represent the as-built EMWMF, and (4) utilizing latest groundwater monitoring data in and around the EMWMF. The refined model consists of 1.3 million cells with a grid spacing of 10 × 10 ft (3 m x 3 m) and finer vertical layers (11 layers) to represent more precisely the construction/engineering design features and other site-specific conditions. The model was used to formulate and evaluate various engineering alternatives for lowering the water table beneath the EMWMF in terms of hydraulic effectiveness and impact on EMWMF performance and waste acceptance criteria (WAC). Fate-transport simulation was used to evaluate the long-term EMWMF performance by predicting the contaminant movement and the future contaminant plume migration. Based on the detailed alternative performance analysis and other qualitative criteria, the undercell drain alternative was selected. The undercell drain was successfully constructed in early 2004. Following the completion of the undercell drain at the EMWMF, the model was used to conduct the transient modeling analysis on the groundwater level change in response to the installation of the undercell drain. The model was also used as a tool for developing a groundwater-monitoring plan to identify monitoring locations and evaluating the design basis for future cell expansion. The current monitoring data suggest that the implemented alternative has the desired effect on the groundwater levels at the site as predicted by the model.