Characterization of a Large Hexavalent Chromium and PFAS Plume in Fractured Bedrock

Hexavalent chromium and per- and polyfluoroalkyl substances (PFAS) were released to fractured bedrock by a chromium plating facility. Sampling for these materials in monitoring and residential water wells determined that the extent of groundwater impacts was approximately one square mile extending to a depth of over 400 feet. The results of investigations conducted at the site indicate that it is likely that a significant portion of the area was not impacted by the release but can instead be attributed to background conditions.

The investigations focused on collecting several lines of evidence to distinguish between site impacts and background conditions. Multiple rounds of groundwater samples were collected and analyzed for metals, PFAS and other organic compounds. To evaluate groundwater migration pathways site investigations included conducting surficial and down-hole geophysical investigations, packer testing and pumping tests to evaluate the hydraulic characteristics and fracture interconnectivity of the bedrock.

Lines of evidence used to distinguish between areas impacted by the release and background areas include: characterization of the fractured bedrock groundwater flow, hexavalent to total chromium ratios, PFAS fingerprinting, changes in groundwater physical conditions, groundwater anionic and cationic composition, and characterization of the composition of the rock matrix.