Conceptual Site Model Development and Confirmation of a Bedrock Stratigraphic Preferential Pathway

A remedial investigation and feasibility study conducted for the United States Army Corps of Engineers at the former Glenburn Ground-to-Air Transmitter Station evaluated trichloroethylene in groundwater and private water supply wells. Bedrock geology data collected using surface geophysics, borehole geophysics of existing wells, outcrop mapping and previous geologic reports were combined with water quality data into a Conceptual Site Model used to develop a bedrock investigation. A relatively low cost approach using a water well air-hammer drill rig combined with borehole geophysics, packer tests, and discrete-interval well completions was used to define the nature and extent of contamination, and to correlate preferential pathways between the bedrock wells. The investigation documented a long narrow plume of trichloroethylene in the bedrock aquifer, the nature and extent of which was controlled by bedrock stratigraphy controlled preferential pathways. Pumping test results confirmed the elongated nature of the stratigraphic preferential pathways and documented hydrogeologic connections between specific vertical intervals in multiple bedrock wells. Subsequent years of water quality monitoring documented stable or declining concentrations in the groundwater. Natural attenuation, institutional controls, point-of-use water treatment and long-term monitoring were chosen as the primary remedial action elements in the Decision Document.