HYDROSTRATIGRAPHIC CONTROLS ON CONTAMINANT TRANSPORT AND RETENTION IN FRACTURED MUDSTONES OF THE LOCKATONG FORMATION OF THE NEWARK BASIN

Trichloroethene (TCE), a dense non-aqueous-phase liquid, was used in industrial processes at the former Naval Air Warfare Center (NAWC), West Trenton, N.J. from the mid-1950’s to the mid-1990’s, and high concentrations have been detected in groundwater samples at the NAWC. Below the weathered zone, groundwater flow and transport of TCE and related chlorinated volatile organic compounds (CVOCs) takes 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 that underlie the site.

CVOCs were measured in 294 rock core samples from 5 coreholes by methanol extraction. CVOC’s concentrations were above detection limits in nearly all rock-core samples (51 out of 53) from the weathered zone, above 14.9-23 meters depth, depending on location. In contrast, concentrations in core samples from deeper unweathered strata were below detection in most samples (173 out of 241). However, high CVOC concentrations in deeper unweathered strata were present in isolated core samples from highly fractured dark carbon-rich mudstones and fractured dark-gray laminated mudstones, and in adjacent core samples from massive unfractured strata. These results suggest that CVOC diffusion from deeper fractures only penetrated a few centimeters into the unweathered rock matrix, possibly retarded by sorption on organic carbon in the darker mudstones. Overall, most of the contaminant mass in the aquifer is not in the aqueous phase in water flowing in permeable fractures (secondary porosity), but is retained in the rock matrix (primary porosity) or sorbed to the aquifer mudstones. Integrating the hydrogeologic framework and the hydraulic and water-quality characterizations, including the rock core results, suggests that CVOC concentrations in groundwater flowing in permeable fractures will remain elevated for many years due, in part, to gradual back-diffusion and desorption from the mudstones. Remediation below the weathered zone may potentially only be required near permeable fractures, due to limited diffusion into massive mudstone strata.