Electrical Geophysical Performance Monitoring of Amendment Enhanced Bioremediation

Abstract

Enhanced bioremediation is increasingly used to accelerate cleanup of recalcitrant compounds and has been identified as the preferred remedial treatment at a substantial number of contaminated DoD (Department of Defense) sites. However, blind introduction of amendments can lead to suboptimal or ineffective remediation and it is generally unknown how the introduced amendments are distributed throughout the treatment zone. Consequently, performance monitoring tools are needed which provide operational staff and regulators timely, actionable information on the spatiotemporal behavior of amendments. As amendments and degradation products often have electrical properties which differ from ambient soil properties time lapse electrical geophysical monitoring has the potential to be such a tool. We discuss the field implementation and initial results of a Hydrogeophysical Performance Monitoring System (HPMS) which provides performance monitoring data on an enhanced bioremediation effort which is currently occurring at the Brandywine site in Brandywine, Maryland. Introduction Bioremediation Due to the potential for substantial cost savings there is a continuing interest in the use of in situ remedial approaches. These include both the use of permeable reactive barriers, monitored or enhanced natural attenuation and contaminant sequestration approaches. One of the most appealing approaches for organics is the use of biological processes which break down the contaminants in benign components. This process can be enhanced or facilitated by the injection of amendments and/or microbes. A range of different amendments exists, all of which serve as nutrients for the microbial communities. Amendments include both water soluble amendments (such as lactate, ethanol and molasses) as well as slow release compounds (such as vegetable oil, Hydrogen Release Compound ®, a proprietary mixture, and mulch) (table 1). In general, it is not uncommon to inject thousands of gallons of amendment mixture per injection point. One of the primary challenges for a remediation based approach is to ensure the adequate distribution of amendment in the subsurface. While analysis of aqueous samples can provide accurate information on amendment distribution the cost of sampling and subsequent laboratory analysis is substantial. In addition, sampling effort only provide limited spatial and temporal information on amendment distribution.