Dewatering DOE – Managing and Monitoring a Contaminant Plume Near an Excavation Dewatering Operation

CDM Smith developed a 3D groundwater flow and mass transport model using MODFLOW/MT3D to simulate various dewatering scenarios during the planning and implementation stages of a trichloroethylene (TCE) source area excavation and treatment action. The work was conducted at a Department of Energy (DOE) site in Ohio undergoing deactivation and decommissioning (D&D) activities. The model was used to determine the required groundwater extraction rates and simulate the dewatering activities in real time to monitor the location of the groundwater divide between the dewatering area and an adjacent plume. The dewatering operation was not allowed to accelerate the migration of the adjacent TCE plume, which was being managed by withdrawals near the plume core. CDM Smith used the model to simulate groundwater levels, head contours, and flow-direction vectors to complete flow field assessments during the planning and dewatering stages. Using the model throughout the dewatering process allowed for model verification under several different groundwater extraction rates. In addition, monitoring wells located in the vicinity of the groundwater divide were regularly gaged, and these data were used to compare the observed groundwater divide to the simulated groundwater divide. The location of the groundwater divide was monitored to assess the potential for the nearby plume to be affected by the dewatering activities in the excavation area. After dewatering and excavation activities were completed, the groundwater model was used to simulate sulfate transport (a byproduct of the source area stabilization) to a nearby surface water body to confirm that elevated sulfate levels would not adversely affect the water body. GIS and 3D visualizations were used to assist in the design, development, and presentation of the dewatering system and estimates.