IMPORTANCE OF HYDROGEOLOGIC CHARACTERIZATION TO SUCCESSFUL BIOAUGMENTATION OF CONTAMINATED FRACTURED SEDIMENTARY ROCKS

The U.S. Geological Survey, in cooperation with the U.S. Navy, is conducting an in situ bioaugmentation experiment in dipping fractured sedimentary rocks underlying the former Naval Air Warfare Center (NAWC), West Trenton, NJ. The fractured mudstones are contaminated with trichloroethene (TCE) and its degradation products cis-1,2-dichloroethene and vinyl chloride. The bioaugmentation experiment was initiated in October 2008 by injecting a consortium of bacteria that degrade TCE and an electron donor consisting of edible oil substrate and lactate. These amendments augment and stimulate the growth of the subsurface bacteria population, which in turn enhances contaminant degradation rates. Prior to injection of the amendments, a multidisciplinary hydrogeologic investigation was conducted to characterize the rock structure and the heterogeneity of flow and contaminant transport properties, which are critical factors in the design of remediation strategies such as bioaugmentation. This hydrogeologic characterization focused on a region between the injection well and a pumping well located 40 m away. Characterization to a depth of 40 m included (1) developing a geologic framework of individual dipping mudstone beds identified using rock core stratigraphy and geophysical logs, (2) borehole flow logging to locate permeable bedding-plane fractures, (3) single- and cross-hole hydraulic testing to estimate flow properties and likely flow paths between the injection and pumping wells, and (4) tracer testing to estimate transport properties along these flow paths. This systematic characterization, supplemented by transport modeling for hypothesis testing, enabled prediction of the primary mudstone beds into which the bioaugmentation amendments would most likely migrate, and of arrival times to packer-isolated borehole monitoring intervals. Post-bioaugmentation monitoring has shown substantial reductions in TCE concentrations at locations predicted to be affected by the bioaugmentation, as well as changes in geochemistry indicative of stimulated microbial activity. These results affirm the importance of hydrogeologic characterization for guiding the design, implementation, and monitoring of bioaugmentation in fractured sedimentary-rock aquifers.