Importance of Hydrogeologic Characterization to Effective 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 mudstones underlying the former Naval Air Warfare Center (NAWC), West Trenton, NJ. The rocks 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. Prior to injection of the amendments, a multidisciplinary hydrogeologic investigation was conducted to characterize the rock structure and the highly heterogeneous distribution of groundwater flow and contaminant transport properties, which are critical factors in the design of remediation strategies such as bioaugmentation.

 The characterization focused on a region between the injection well and a pumping well 40 m away. Characterization to a depth of 40 m included (1) developing a three-dimensional geologic framework that delineates individual dipping mudstone beds identified using rock core stratigraphy and geophysical logs, (2) single- and cross-hole hydraulic testing of multiple fracture zones to estimate flow properties and flow paths between the injection and pumping wells, and (3) cross-hole and push-pull tracer testing to estimate transport properties along these paths. Transport modeling also was conducted to simulate the cross-hole tracer test and to guide the bioaugmentation injection strategy. This systematic characterization and modeling enabled prediction of the primary mudstone beds into which the bioaugmentation amendments would likely migrate, and of arrival times at packer-isolated borehole monitoring intervals. Post-bioaugmentation monitoring has shown substantial reductions in TCE concentrations at borehole intervals 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.