Numerical Groundwater Flow Modeling of a Northern Appalachian Coal Basin Bedrock Valley Exhibiting Stress-Relief Fractures

Permitting of fly ash impoundments requires a solid understanding of the groundwater system, both the physical properties governing flow and the natural variability of the groundwater geochemistry. At the site in this case study, sandstones of the Monongahela, Conemaugh, and Allegheny formations form the major water-bearing aquifers with thick shale sequences forming intervening aquitards. Borehole geophysics conducted at the site indicates that fracture flow plays a dominant role in the movement of groundwater. The fly ash ponds in this study are located in a high-relief, upland setting and stress-relief fracturing is expected to play an important role near the valley sides. Groundwater monitoring conducted at the site during the last 28 years, including stable isotope analysis, has provided considerable data on the flow system. As part of a recent permit application to increase the capacity of the existing fly ash pond, we developed a 20-square-mile, 10-layer, steady-state groundwater flow model using MODFLOW-NWT, a stand-alone version of MODFLOW 2005. Pre-processing of the model, including development of the geological conceptual model, was conducted in GIS to increase the efficiency of the modeling process. The challenge at the site is to distinguish constituents naturally present in groundwater and those potentially due to leaching from the fly ash reservoirs. Impacts from extensive coal mining and petroleum exploration over the last 100 years further complicate the interpretation of groundwater geochemistry. Particle tracks were generated from the numeric modeling simulations and the results helped to verify the groundwater flow-paths as indicated by groundwater contours and geochemical data. Ultimately the model was a key component in the permit, allowing an understanding of future conditions and guiding development of future monitoring plans.