IMPLICATIONS FOR BIOAUGMENTATION AT A RABITT PROTOCOL SITE REVISITED

摘要

Recently there has been a significant promotion in the use of bioaugmentation at sites considered to be recalcitrant in the degradation of dichloroethylene (DCE) with emphasis on Dehalococcoides ethenogenes as the limiting factor. It appears that qualitative detection of Dehalococcoides is only a start; more needs to be done with regard to quantification of organisms as well as their specific activity. In essence, for many sites that test negative for the dechlorinators in question, simply supplying “more time and more electrons” is all that is required. At the former Alameda Naval Air Station (NAS) in Alameda, CA, there was an opportunity to re-apply electron donor, in the form of HRC?, at a site treated almost two years previously with organic acids by liquid injection and extraction as part of the Strategic Environmental Research and Development Program (SERDP) funded Reductive Anaerobic Biological In-Situ Treatment Technology (RABITT) protocol. From this effort, some peripheral but seminal observations on the value of bioaugmentation have emerged. Groundwater collected from four wells in the RABITT test cell was tested for the presence of specialized dehalogenating microbes: Dehalobacter, Dehalococcoides, and Desulfuromonas. Two of the wells are located within the RABITT test cell; one well is located downgradient, and one is upgradient of the test cell. Dehalococcoides is recognized as potentially the most essential for complete biodegradation to ethene. All of these bacteria, however, are known chloroethene dechlorinators. The microbial test results indicate the presence of two dehalogenating microbial populations (Dehalococcoides and Desulfuromonas) in the RABITT test cell and downgradient wells, and inconclusive evidence of microbial populations in the upgradient (background) well. At the same time, DCE is elevated at the site after there was a historical period of degradation during the original RABITT test program. A revised interpretation of the data begins with a definition of three basic stages of microbial activity: I) Present, but suboptimal for detection by current genetic probe technology II) Present and detectable, but suboptimal for reductive dechlorination III) Present and detectable and able to facilitate reductive dechlorination Baseline microbial studies indicated the presence of native dehalogenating bacteria in the RABITT test cell, and inconclusive in wells outside the treatment zone. These bacteria were likely stimulated by the original electron donor (butyric acid).