Remediation of a Fractured Rock Aquifer Containing Trichloroethylene Dense Nonaqueous Phase Liquid

Extensive chlorinated solvent contamination, including trichloroethylene (TCE) dense nonaqueous phase liquid (DNAPL), was found in ground water at a Navy Installation Restoration Program site on a former Navy property in Pennsylvania. Ground water at the site is encountered in the Triassic age Stockton Formation, which consists of gently dipping, alternating sandstone and mudstone units. Ground-water flow is primarily through fractures in the coarser grained units, while the finer grained mudstones generally act as semi-confining units. An aggressive pump-and-treat remedy was selected for the site, with the goal of source area containment and mass removal. A municipal water supply well in operation approximately 2,000 feet downgradient from the site has been impacted by site-related and non site-related contamination. An air stripping unit on the municipal well is used to treat the water prior to public consumption. The remedial approach included the installation of extraction wells in the DNAPL zone. Extensive real-time hydraulic and water-quality testing and analysis and DNAPL screening procedures were employed through an iterative process during the installation of extraction wells to address the inherent uncertainties of fracture flow and minimize the potential for DNAPL migration. Measures were also taken to safeguard the municipal well against the possibility of contamination being released during the remedy implementation. A Technical Impracticability (TI) evaluation was also performed, and a TI Waiver subsequently was granted by EPA for the site to waive applicable or relevant and appropriate requirements (ARARs) in the DNAPL zone. Currently (2007), 12 extraction wells are in operation at the site. Contaminant concentration decreases since the startup of the extraction system are two to three orders of magnitude in both the source and downgradient areas. The project is an example of how an aggressive pumpand-treat approach can be successfully employed at a fractured rock DNAPL site in an area of high ground-water vulnerability.