AN INNOVATIVE BOREHOLE APPARATUS FOR CHARACTERIZING DEPTH-DEPENDENT MICROBIAL ACTIVITY AND GROUND-WATER CHEMISTRY IN FRACTURED ROCKS AT THE NAVAL AIR WARFARE CENTER, WEST TRENTON, NJ

Fractured rock aquifers offer challenges in collecting water samples that characterize ground-water chemistry and microbial activity. Samples collected from boreholes intersecting multiple fractures may not accurately represent in situ geochemical and microbial conditions. The hydraulic conductivity of fractures can vary over orders of magnitude, which when coupled with complex fracture connectivity, can lead to spatially heterogeneous microbial and geochemical conditions. In fractured rock aquifers contaminated by volatile organic compounds (VOCs), the intricacy of the geochemistry and microbial activity will depend on the fractures that have been active in transporting the VOCs.

A retrievable multiple-packer apparatus was installed in a 52-meter-deep borehole in dipping fractured mudstones of the Triassic Lockatong Formation that underlie the former Naval Air Warfare Center, West Trenton, NJ. The site is the focal point of U.S. Geological Survey field research on the fate of dense non-aqueous phase liquids in fractured rock aquifers. At this site, trichloroethene, which was accidentally introduced into the ground water, has been partially transformed to cis-1,2-dichloroethene, vinyl chloride, and ethene by natural microbial reductive dechlorination. The borehole apparatus was designed to characterize water chemistry and microbial activity with depth using retrievable sampling devices installed at multiple locations in the borehole. Detailed geophysical and hydraulic characterization were conducted to position 5 packers that isolated 6 borehole intervals. Water samples could be collected at land surface from 4 of the 6 intervals; 2 intervals could not be accessed from land surface. In all 6 borehole intervals, in situ sampling devices were installed, including (1) crushed core acting as a microcosm for microbial growth, (2) diffusion bag samplers for analyses of VOCs and ionic species, and (3) gas-filled syringes to sample for hydrogen to characterize the redox conditions of indigenous microbial communities. The borehole apparatus was installed for approximately 1 year. Water samples were collected from borehole intervals intermittently, and the apparatus was removed to retrieve the in situ sampling devices. The water samples and in situ sampling devices are currently being analyzed.