Enhanced in situ bioremediation of chlorinated ethenes: from in situ microcosms to full-scale application

Abstract

Concentrations greater than 20 mg/L of chlorinated volatile organic compounds (cVOCs) including tetrachloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (cDCE) have been present in site groundwater for more than four decades. To promote a faster clean-up time, an in situ bioremediation approach was evaluated using In-Situ Microcosms® (ISMs) followed by a full-scale in situ bioremediation approach. The ISM study evaluated slow-release versus quick-release carbon substrates with and without bioaugmentation using the chlorinated ethene degrading culture, SDC-9™. After a three-month incubation period, the ISMs were retrieved. The ISMs amended with a carbon source with or without bioaugmentation displayed greater than a 93% reduction in TCE, which corresponded to an increase in cDCE in all the ISMs. The Dehalococcoides population and gene abundances associated with chlorinated ethene biodegradation (tceA, bvcA, vcrA) increased three orders of magnitude in the bioaugmented ISMs over the natural attenuation ISM and carbon source only amended ISMs. Additionally, the SDC-9™ and AquaBupH®, an enhanced emulsified oil substrate (EOS®) and buffer, amended ISM unit showed the highest level of vinyl chloride and similar level of ethene to the SDC-9™ and EHC®, a controlled-release, organo-iron substrate, ISM. However, the AquaBupH and SDC-9™ ISM displayed the highest level of acetate, demonstrating active fermentation processes. The results of the ISM study indicated that a combined approach of biostimulation along with bioaugmentation effectively promoted conditions conducive to reductive dechlorination. The full-scale in situ bioremediation system coupled biostimulation with bioaugmentation using SDC-9™ to successfully reduce chlorinated ethenes in groundwater. The slow-release carbon substrate, EOS-100® served as a sustained carbon source along with CoBupH (buffering agent similar to AquaBupH), facilitating the production of hydrogen, through volatile fatty acid fermentation. This led to the reduction of chlorinated ethenes over the subsequent three years, showcasing minimal rebound in contaminant levels. Two rounds of bioaugmentation notably increased the Dehalococcoides population, accelerating the biodegradation processes, which is setting up the site for monitored natural attenuation. This study shows that using ISMs to guide full-scale bioremediation design resulted in an effective cVOC biodegradation system that led to quicker and more sustainable clean-up.