Scientific Investigations Report 2006–5157
U.S. Geological Survey Scientific Investigations Report 2006–5157
By Connor J. Haugh
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Arnold Air Force Base (AAFB) occupies about 40,000 acres in the eastern part of the Highland Rim physiographic region in Coffee and Franklin Counties, Tennessee. The area is characterized by fractured carbonate rock terrane that complicates evaluation of ground-water flow. Numerous site-specific ground-water contamination investigations have been conducted at designated Solid Waste Management Units (SWMUs) at AAFB. Several synthetic volatile organic compounds (VOCs), primarily chlorinated solvents, have been identified in the ground water at AAFB. Two ground-water contaminant plumes that originate at AAFB, the “SWMU 8 plume” and the “northwest plume,” have been shown to extend to regional discharge points outside the AAFB boundary. In 2002, the U.S. Geological Survey (USGS), in cooperation with the U.S. Air Force, AAFB, began an investigation to further refine the understanding of the regional ground-water system in the AAFB area and to update the previous computer ground-water flow model incorporating new data and information collected since 1992.
The updated ground-water flow model incorporates revised structure maps of the top-of-rock surface and the top of the Chattanooga Shale and the preferential regional flow paths identified by investigations conducted since 1992. The preferential regional flow paths play an important role in ground-water movement and contaminant transport in the AAFB area. The model is calibrated to steady-state conditions defined by detailed water-level and streamflow data collected in 2002. Particle-tracking simulations were used with the model to simulate ground-water flow paths and travel times from selected sites at AAFB. The flow paths indicated by the particle-tracking simulations agree reasonably well with maps of interpreted contaminant plumes.
Currently (2005), ground-water withdrawal wells are operating at SWMU 1&2, SWMU 5, SWMU 8, and SWMU 10, and dewatering occurs at many facilities at the Main Test Area (MTA). Particle-tracking results show that no particles leave these SWMUs while the ground-water withdrawal wells are pumping. Three particle-tracking simulations were run to analyze the effects of dewatering facilities on flow paths at the MTA. These simulations indicate that the dewatering facilities have a substantial effect on flow paths from the MTA and are effective in containing most of the ground water in this area.
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Contact Information: Connor J. Haugh