Field characterization of a trichloroethene (TCE) source area in fractured mudstones produced a detailed understanding of the geology, contaminant distribution in fractures and the rock matrix, and hydraulic and transport properties. Groundwater flow and chemical transport modeling that synthesized the field characterization information proved critical for designing bioremediation of the source area. The planned bioremediation involved injecting emulsified vegetable oil and bacteria to enhance the naturally occurring biodegradation of TCE. The flow and transport modeling showed that injection will spread amendments widely over a zone of lower-permeability fractures, with long residence times expected because of small velocities after injection and sorption of emulsified vegetable oil onto solids. Amendments transported out of this zone will be diluted by groundwater flux from other areas, limiting bioremediation effectiveness downgradient. At nearby pumping wells, further dilution is expected to make bioremediation effects undetectable in the pumped water. The results emphasize that in fracture-dominated flow regimes, the extent of injected amendments cannot be conceptualized using simple homogeneous models of groundwater flow commonly adopted to design injections in unconsolidated porous media (e.g., radial diverging or dipole flow regimes). Instead, it is important to synthesize site characterization information using a groundwater flow model that includes discrete features representing high- and low-permeability fractures. This type of model accounts for the highly heterogeneous hydraulic conductivity and groundwater fluxes in fractured-rock aquifers, and facilitates designing injection strategies that target specific volumes of the aquifer and maximize the distribution of amendments over these volumes.
The multichannel analysis of surface waves ͑MASW͒ seismic method was used to delineate a fault zone and gently dipping sedimentary bedrock at a site overlain by several meters of regolith. Seismic data were collected rapidly and inexpensively using a towed 30-channel land streamer and a rubberband-accelerated weight-drop seismic source. Data processed using the MASW method imaged the subsurface to a depth of about 20 m and allowed detection of the overburden, gross bedding features, and fault zone. The fault zone was characterized by a lower shear-wave velocity ͑V s ͒ than the competent bedrock, consistent with a large-scale fault, secondary fractures, and in-situ weathering. The MASW 2D V s section was further interpreted to identify dipping beds consistent with local geologic mapping. Mapping of shallowfault zones and dipping sedimentary rock substantially extends the applications of the MASW method.
The hydrogeologic framework of fractured sedimentary bedrock at the former Naval Air Warfare Center (NAWC),1 Trenton, New Jersey, a trichloroethylene (TCE)‐contaminated site in the Newark Basin, is developed using an understanding of the geologic history of the strata, gamma‐ray logs, and rock cores. NAWC is the newest field research site established as part of the U.S. Geological Survey Toxic Substances Hydrology Program, Department of Defense (DoD) Strategic Environmental Research and Development Program, and DoD Environmental Security Technology Certification Program to investigate contaminant remediation in fractured rock. Sedimentary bedrock at the NAWC research site comprises the Skunk Hollow, Byram, and Ewing Creek Members of the Lockatong Formation and Raven Rock Member of the Stockton Formation. Muds of the Lockatong Formation that were deposited in Van Houten cycles during the Triassic have lithified to form the bedrock that is typical of much of the Newark Basin. Four lithotypes formed from the sediments include black, carbon‐rich laminated mudstone, dark‐gray laminated mudstone, light‐gray massive mudstone, and red massive mudstone. Diagenesis, tectonic compression, off‐loading, and weathering have altered the rocks to give some strata greater hydraulic conductivity than other strata. Each stratum in the Lockatong Formation is 0.3 to 8 m thick, strikes N65°E, and dips 25° to 70°NW. The black, carbon‐rich laminated mudstone tends to fracture easily, has a relatively high hydraulic conductivity and is associated with high natural gamma‐ray count rates. The dark‐gray laminated mudstone is less fractured and has a lower hydraulic conductivity than the black carbon‐rich laminated mudstone. The light‐gray and the red massive mudstones are highly indurated and tend to have the least fractures and a low hydraulic conductivity. The differences in gamma‐ray count rates for different mudstones allow gamma‐ray logs to be used to correlate and delineate the lithostratigraphy from multiple wells. Gamma‐ray logs and rock cores were correlated to develop a 13‐layer gamma‐ray stratigraphy and 41‐layer lithostratigraphy throughout the fractured sedimentary rock research site. Detailed hydrogeologic framework shows that black carbon‐rich laminated mudstones are the most hydraulically conductive. Water‐quality and aquifer‐test data indicate that groundwater flow is greatest and TCE contamination is highest in the black, carbon‐ and clay‐rich laminated mudstones. Large‐scale groundwater flow at the NAWC research site can be modeled as highly anisotropic with the highest component of permeability occurring along bedding planes.
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