Contaminant Transport in a Fault Zone Aquifer

Clusters of domestic and irrigation wells along the trace of the Farmhaven Brook Fault Zone attest to the highly permeable nature of the fault zone aquifer. The convergence of commingled groundwater plumes into the systematic network of steeply-dipping en echelon extensional fractures and bedding-parallel fractures aligned subparallel to the fault zone created the need to better understand the structures and contaminant transport in this fractured aquifer. The bedrock in this New Jersey study area is the Passaic Formation of the Newark Basin consisting of interbedded shales, siltstones, mudstones, and sandstones. We used several approaches to better characterize fracture networks, including data from outcrops; borehole televiewer, caliper, natural gamma and flow metering logging, bedrock topography, tracers, aquifer hydraulic tests, seismic refraction velocity cross-sections and 3D-visualization.

Two north-south trending, steeply-dipping en echelon faults, lying approximately 1000 feet apart, are the focus of this presentation. Vertical gradients over the 4000-foot VOC plume are generally downward with the exception of these steeply-dipping en echelon breaks where strong upward gradients and even flowing artesian conditions prevail. VOC-impacted bedrock groundwater is transported up into the shallow overburden aquifer over one of these breaks, creating a contaminated island leading to vapor intrusion investigations, one residential mitigation and surface water issues. The upwelling of “clean” bedrock groundwater on other fault segments and temporal variations in vertical head in bedrock control the position of the commingled overburden plumes. Two seismic refraction lines were run across one of the en echelon breaks and a portion of the fault ramp and then modeled to better understand the fracture networks and buried fluvial deposits. The analysis of spatial and temporal geochemistry trends were also used to create the conceptual fracture flow model. The installation of additional overburden and bedrock wells, aquifer testing, and downhole geophysics in early 2013 will supplement our research.