CONTAMINANT TRANSPORT AND REMEDIATION IN AN EXTENSIONAL FAULT ZONE

The convergence of commingled groundwater volatile organic plumes into a network of steeply-dipping en echelon extensional faults and bedding-parallel fractures created the need to better understand the geologic structures and contaminant transport in this bedrock aquifer. The study area is in the Passaic Formation of the Newark Basin of New Jersey and consists of interbedded shales, siltstones, mudstones and sandstones. The fault segments, with up to 800 feet of offset, are buried under the terminal moraine and previously unmapped. The recently named Farmhaven Brook Fault zone, discovered during a groundwater remedial investigation, is subparallel to other major intrabasinal fault systems and will be included on the upcoming Perth Amboy Quadrangle NJGS geologic map. We used several approaches to better characterize the overall fault zone as well as local fracture networks, including data from outcrops, borehole televiewer, caliper, natural gamma and flow meters, bedrock topography, fluorescent tracers, straddle-packer testing, aquifer hydraulic testing, seismic refraction velocity cross-sections and 3D-visualization.

Understanding bedrock architecture on both site-wide and local scales was vital to protecting receptors and developing effective remedial designs. Vertical hydraulic gradients over the 5,000-ft. long commingled VOC plume are generally downward with the exception of the steeply-dipping en echelon breaks where strong upward gradients and even flowing artesian conditions prevail. VOC-impacted bedrock groundwater is transported up in the fault into the shallow overburden aquifer, creating a contaminated outlier of shallow groundwater leading to vapor intrusion investigations, one residential mitigation and surface water issues.

We performed fluorescent tracer-dye tests in bedrock at two injection wells, designated for the application of bioremediation amendments, to evaluate transport rates and migration pathways in bedrock. Data collected during water extraction, dye injections, and dye migration was synthesized with previously identified site characteristics to develop a high-resolution map of the source area fracture network. This data was used to finalize the design and execute amendment injections targeting a chlorinated solvent-impacted bedrock aquifer.