Conceptual Site Model of PCE Migration in the Subsurface
Following the release of 1,200 gallons of virgin tetrachloroethene (PCE) from an AST onto a paved courtyard area and the detection of PCE in bedrock groundwater several years later, Cox-Colvin was contracted, in 1995, to develop and verify a conceptual site model (CSM) of PCE migration in the subsurface. Development and verification of the CSM was the initial step of Interim Stabilization Measures (ISMs) implemented through the RCRA Corrective Action program at this permitted midwestern manufacturing facility. The CSM postulated that following the release, PCE entered the subsurface through cracks in the asphalt surface, through cracks between the surrounding buildings and pavement, and through a nearby industrial sewer line catch basin. Once in the subsurface, the CSM assumed the migration of the dense non-aqueous phase liquid (NAPL) laterally within building footers and utility backfill and vertically through the underlying glacial lacustrine deposits until reaching a low permeability glacial till.
To refine and verify the CSM, a limited field investigation was conducted to evaluate the hydraulic connection between hydrogeologic units, to verify that the majority of the PCE mass was present in the glacial lucstrine deposits and the foundation footers, and to evaluate the possible presence of a NAPL within the soil column. The objectives were met through a continuous rate pumping test, detailed soil sampling, and geotechnical testing. DNAPL in soil was identified through real time use of ultraviolet light and a photoionization detector. The investigation confirmed that the majority of the PCE mass, including NAPL, was contained within the glacial lacustrine unit and backfill material surrounding building footers and utilities; that vertical migration was impeded by the underling gray till; and that inadequately constructed wells installed by a previous consultant served as the primary vertical migration pathway to bedrock groundwater. Mapping of the slope of the till surface suggested the future migration direction on the till surface at depth.