A remedial investigation was conducted on a chlorinated volatile organic compound (CVOC) groundwater plume in the Portland Arkose, located in the Harford basin of central Connecticut.  Primary contaminants of concern are trichloroethylene (TCE), tetrachloroethylene (PCE), and their degradation products. The site is underlain by the Portland Arkose, an early Jurassic reddish-brown sandstone and siltstone. Top of bedrock ranges from 4 to 25 feet below ground surface. The top 20 feet of bedrock is heavily weathered with significant horizontal to subhorizontal fractures. This is underlain by more component bedrock with lower fracture density. Groundwater at the site ranges from 0.5 to 16.9 feet below the ground surface. A targeted remedial approach was required to remediate the groundwater plume in multiple fracture zones.

CVOCs were historically released at the ground surface and migrated through the thin overburden and downward into the fractured shallow bedrock resulting in elevated concentrations in the shallow, heavily fractured zone as well as migrating into areas of lower hydraulic conductivity. Elevated concentrations of CVOCs have persisted due to limited natural dilution and complex flow paths.

Subsurface characterization included downhole geophysical logging at three bedrock boreholes where logs included caliper, acoustic and optical televiewer, natural gamma, fluid temperature, fluid conductivity, single-point resistance, and stressed & ambient heat-pulse measurements.  Pre-remedial groundwater geochemistry and CVOC samples were obtained from the three bedrock boreholes at multiple depths ranging from 14 to 69 feet using passive diffusion bag (PDB) samplers, targeting high transmissive zones identified by geophysical data.

Geophysical results suggest the contaminant migration pathway is dominated by shallowly dipping fractures ranging from 3–5 degrees up to 81–86 degrees in competent bedrock, however the overlying fractured weathered bedrock allows for vertical (downward and upward) and horizontal migration of contaminants.  Water-bearing fractures contributing to groundwater flow and contaminant migration are found within this shallow dip range. These findings were integrated into a conceptual site model and 3D plume model of high-density baseline sampling data, which guided strategic placement of injection wells for in-situ remediation via microbe-enhanced reductive dechlorination, targeting the most transmissive zones. This approach maximized areal distribution of the remedial amendment into impacted groundwater. Post-injection geochemical and analytical sampling of the groundwater showed initial reductions in CVOC concentrations, particularly in areas with well-connected fracture networks.