Quantifying Mobilization of Chlorinated Ethene Compounds Following Bioaugmentation in a Fractured Mudstone

Remediation technologies are often applied in fractured rock; however, it is challenging to evaluate their effectiveness with sparse monitoring that is typical in these heterogeneous geologic environments. Monitoring geochemistry usually entails boreholes that target individual or closely spaced permeable fractures. However, the primary porosity of the rock (or rock matrix) is likely to retain the majority of contaminant mass due to diffusion from fractures. Monitoring changes in geochemistry in the rock matrix is not economical because of the high cost of collecting rock cores.

This investigation demonstrates that integrating site characterization with groundwater modeling and the strategic location of monitoring boreholes can be used to evaluate the effectiveness of remediation conducted in fractured rock. Bioaugmentation of chlorinated ethene (CE) compounds was conducted in the mudstone underlying the former Naval Air Warfare Center, West Trenton, NJ. Groundwater modeling was used to design locations for monitoring, and the location and injection volume of bioaugmentation amendments. Groundwater fluxes from the model were coupled with CE concentrations from monitoring boreholes to formulate a CE mass balance of the region targeted in the bioaugmentation. Differences in CE fluxes into and out of the rock volume identified the total CE mobilized from diffusion, desorption, and NAPL dissolution under pre- and post-injection conditions. The mobilized CE mass was compared with the initial CE mass in the rock matrix estimated from analyses of rock core conducted prior to the bioaugmentation.

The CE mass mobilized per year prior to the bioaugmentation was small relative to the total CE mass in the rock matrix, indicating that hundreds of years would be needed for current pumping and natural attenuation to achieve remedial objectives. The post-injection CE mobilization rate increased by an order of magnitude, but multiple remediation applications would be needed over decades to reduce CE concentrations to acceptable limits.