Delineation of Trichloroethene and Related Contaminants in Weathered and Unweathered Sedimentary Rock, NAWC, New Jersey

The U.S. Geological Survey, Toxic Substances Hydrology and National Research Programs, in cooperation with the U.S. Navy and the Strategic Environmental Research and Development Program, is conducting multidisciplinary research on the distribution and fate of chlorinated volatile organic compounds (CVOCs) in weathered and unweathered fractured sedimentary rocks that underlie the former Naval Air Warfare Center (NAWC), West Trenton, New Jersey. Trichloroethene (TCE), a dense non-aqueous-phase liquid, was used in industrial processes at NAWC from the mid-1950s to the mid-1990s, and high concentrations have been detected in groundwater samples. Below the weathered zone, groundwater flow and transport of CVOCs take place locally along bedding-plane fractures and thin fissile or laminated strata in gently-dipping mudstones of the Triassic-age Lockatong Formation of the Newark Basin. Concentrations of CVOCs in most rock-core samples from the shallow weathered zone exceed limits of detection. In contrast, concentrations in core samples from deeper unweathered strata are generally below detection. However, high CVOC concentrations are present in isolated unweathered-rock samples from highly fractured dark carbon-rich mudstones and fractured dark-gray laminated mudstones. Consistent results from five coreholes, along with hydraulic and water-quality characterization, suggest a conceptual model for the migration of CVOCs from or near the land surface into the aquifer. Prior to pump-and-treat remediation (P&T), groundwater flow was generally horizontal in the high-permeability weathered zone, and the bulk of contaminant mass was shallow. CVOCs diffused into the weathered and fractured rock for decades. After P&T began in 1995, primarily using bedrock wells open below the weathered zone, CVOCs were pulled downward from the weathered zone into sub-cropping, dipping high-permeability bedding plane fractures. The data indicate diffusion of TCE and other CVOCs from deeper fractures only penetrated a few centimeters into the unweathered rock matrix, possibly due to sorption of CVOCs on the organic material in the dark mudstones.