Formatted Title
Evaluation of Enhanced In Situ Bioremediation of Chlorinated Ethenes in Groundwater Using Molecular Tools
Background/Objectives
Background/Objectives. The purpose of the in situ bioremediation (ISB) system was to promote biodegradation of chlorinated ethenes in groundwater via biostimulation alone or in combination with bioaugmentation in the source area. The groundwater has displayed elevated concentrations (> 1 milligram per liter) of total chlorinated volatile organic compounds (cVOCs) including: tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-1,2-DCE), and 1,1-dichloroethene. Groundwater at the site occurs at a depth range of 26 to 30 feet below ground surface (bgs).
The site is underlain by basal glacial till composed primarily of silt, gravel, and cobbles, with relatively little sand and clay. It is dense and compacted, with generally very low permeability typical of basal till. A conspicuous cobble zone is present over most of the site at depths of approximately 15 to 30 feet bgs. A relatively thin but significant layer of silty sand and gravel was identified at a depth between 40 to 49 feet in the general vicinity of the source area. This thin zone provided an important control for limiting the vertical spread of contamination and was targeted by permanent injection wells for amendment distribution.
Approach/Activities
Approach/Activities. A series of injections was performed in four different locations at the site showing elevated cVOC concentrations. Injection wells were used to inject carbon sources during the initial biostimulation event and the Dehalococcoides microbial consortium during the second event. The injection amendments included: biostimulation with quick release carbon substrate and an emulsified oil substrate, EOS Pro, to sustain the microbial population for an extended time period and support reductive dechlorination. A monitoring program was developed to evaluate effectiveness of the treatment system including field parameters, cVOCs, geochemical parameters, and key microbial populations and genes. Due to vinyl chloride (VC) levels increasing in some downgradient monitoring wells, bioaugmentation with Dehalococcoides was performed in third quarter of 2023.
Results/Lessons Learned
Results/Lessons Learned: Less than one year after injecting the carbon substrates, a one to three order of magnitude increase in total organic carbon (TOC) in downgradient monitoring wells was observed corresponding to a decrease in oxidation reduction potential (ORP) to as low as -101 millivolts. As the EOS was degraded, a significant increase in acetone and 2-butanone was observed, which was probably produced during anaerobic fermentation processes. About 19 months post ISB injection, there was up to a 98% reduction in PCE, a 99.7% reduction in TCE and a 99.9% reduction in cis-1,2-DCE in downgradient wells. Bioaugmentation was conducted in the area around MW-6D, which promoted a significant increase in Dehalococcoides, and genes associated with the biodegradation of VC. These significant changes in microbial characteristics corresponded to a decrease in VC.
The groundwater geochemistry indicated that competing electron acceptors were sufficiently reduced to develop conditions conducive to reductive dechlorination. This was further supported by significant decreases in concentrations of cVOCs in downgradient wells post-biostimulation and bioaugmentation. This presentation will discuss the varying hydrogeologic, geochemical and microbiological conditions of site groundwater and how these challenges were addressed to maximize biodegradation of chlorinated ethenes in groundwater and how microbial parameters provided key insight concerning how to enhance complete reductive dechlorination.