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Enhanced in Situ Reductive Dechlorination for Groundwater

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Enhanced in Situ Reductive Dechlorination for Groundwater Enhanced In Situ Reductive Dechlorination for Groundwater On this page: Schematic Introduction Other Technology Names Description Development Status Applicability Cost Duration Implementability Considerations Resources Schematic Enhanced In Situ Bioremediation Introduction https://frtr.gov/matrix/Enhanced-In-Situ-Reductive-Dechlorinated-for-Groundwater/ 1/18 8/3/2020 Technology Screening Matrix | Federal Remediation Technologies Roundtable Enhanced in situ reductive dechlorination (ERD) is the process of modifying chemical, physical, and biological conditions in the aquifer to stimulate the microbial degradation of contaminants under anaerobic conditions to harmless end products (e.g., carbon dioxide [CO2 ] or ethene). It is used to remediate volatile organic compounds (CVOCs) (e.g., trichloroethylene [TCE]) and certain pesticides and other chlorinated organic contaminants. ERD commonly is used to treat dissolved phase plumes downgradient of source areas, but also can be designed to treat CVOC source areas including those that contain dense non- aqueous phase liquid (DNAPL). Other Technology Names Biostimulation Bioaugmentation Enhanced anaerobic bioremediation Enhanced Natural Attenuation Halorespiration Dehalorespiration Description ERD is the process through which chlorine atoms attached to an organic compound are sequentially removed under anoxic (no oxygen) conditions. Application of ERD is comprised of biostimulation and sometimes bioaugmentation to modify existing geochemical and biological conditions in an aquifer to facilitate degradation of contaminants (AFCEC, NAVFAC, ESTCP, 2004). Biostimulation refers to the introduction of an electron donor (carbon source) into the aquifer for stimulating microbial growth. The carbon source is used as food by native microbes, which in turn, produce hydrogen through fermentation reactions. This process depletes the aquifer of dissolved oxygen (DO) and other electron accepters including nitrate, sulfate, and ferric iron, which lowers the oxidation-reduction potential (ORP), thereby creating the conditions for reductive dechlorination to occur (EPA, CLU-IN, 2016; NAVFAC, 2007a). Bioaugmentation refers to the introduction of microbes, which may be required at some sites to accelerate the acclimation and biodegradation process if the existing microbial population is not adequate at the onset of treatment (ITRC, 2002; ITRC, 2008b). At many sites, microbes capable of dechlorinating cis-1,2- https://frtr.gov/matrix/Enhanced-In-Situ-Reductive-Dechlorinated-for-Groundwater/ 2/18 8/3/2020 Technology Screening Matrix | Federal Remediation Technologies Roundtable dichloroethene (cis-1,2-DCE) and vinyl chloride (VC) are not present in suicient quantities resulting in a condition referred to as "cis-1,2-DCE and VC stall" (NAVFAC, 2007b). In those cases, bioaugmentation can be used in combination with biostimulation to achieve the necessary conditions to overcome stall and promote complete reductive dechlorination (NAVFAC, 2013). Under the right conditions, ERD has been proven successful as a remedial strategy to treat chlorinated solvent source areas, including those that contain low to moderate levels of DNAPL mass (ITRC, 2007; ITRC, 2008a). However, ERD is more commonly used to treat dissolved phase plumes downgradient of the source area, or to create reactive barriers to prevent further migration of a plume. Reaction Mechanisms ERD can occur through several reaction (degradation) mechanisms including: Direct Anaerobic Reductive Dechlorination is a biological process where microbes receive energy and grow through sequential reductive dechlorination. In this reaction, chlorinated compounds serve as the terminal electron acceptor, and hydrogen (sometimes acetate as well) serves as the electron donor. Hydrogen is produced by fermenting populations that utilize the added organic substrate. Anaerobic reductive dechlorination can also be referred to as halorespiration or dehalorespiration. A common anaerobic dechlorination pathway is the degradation of tetrachloroethene (PCE) to TCE to cis-1,2-DCE and small amounts of trans-DCE to VC to ethene (Lorenz and Loler, 2016). The chlorinated ethene molecule is the electron acceptor in this reaction. Although fermentation products such as acetate may serve as an electron donor, hydrogen is recognized as the predominant electron donor for anaerobic dechlorination of CVOCs. Following similar degradation pathways, chloroethanes and chloromethanes may also be sequentially reduced by anaerobic dechlorination. For chloroethanes, 1,1,1- trichloroethane (TCA) is reduced to 1,1-dichloroethane (DCA) then chloroethane (CA) and finally to ethane. For chloromethanes, carbon tetrachloride (CT) can be reduced to chloroform (CF) then methylene chloride (MC) to chloromethane (CM) and finally to methane (Stroo, Lesson et al., 2013; ITRC, 2002). Cometabolic Anaerobic Reductive Dechlorination is a process where chlorinated compounds are reduced by non-specific enzymes or co-factors that are generated for metabolism by another compound (i.e., the primary substrate) in an anaerobic environment. Cometabolism does not produce energy for or benefit the microbe producing the non-specific enzyme or co- factor. To sustain the cometabolic process, suicient primary substrate is required to support the microbe producing the non-specific enzyme or co- https://frtr.gov/matrix/Enhanced-In-Situ-Reductive-Dechlorinated-for-Groundwater/ 3/18 8/3/2020 Technology Screening Matrix | Federal Remediation Technologies Roundtable factor (Stroo and Ward, 2010; ITRC, 2008b). Reliance on this indirect strategy can lead to the accumulation of toxic end products such as cis-dichloroethene (cDCE) and vinyl chloride (VC) as cometabolism has slower reaction rates than direct reductive dechlorination (ITRC, 2002). Abiotic Reductive Dechlorination is a chemical degradation reaction where a chlorinated hydrocarbon is degraded (i.e., reduced) by reactive compounds, such as metal sulfides. By definition, the reaction is not microbially mediated. Adding organic substrate to an aquifer lowers the redox environment and can produce ferrous iron and hydrogen sulfide through biologically-mediated iron and sulfate reduction, respectively. It is the reactive compounds that abiotically reduce the chlorinated solvents (Stroo, Lesson et al., 2013; Stroo and Ward, 2010). More information is provided in the In Situ Biogeochemical Transformation Processes profile. Distinguishing between the various pathways at sites is diicult and oen not possible given all the variables in the subsurface. As such, all three pathways are recognized to concurrently contribute to enhanced anaerobic degradation. Common Amendments All of the above processes generally require the introduction of an electron donor to produce the necessary conditions for biodegradation to occur (AFCEC, NAVFAC et al., 2010; Borden, 2006). These electron donors/carbon sources can be separated into three principal groups based on their physical characteristics including: Aqueous (soluble): Aqueous compounds are highly soluble and can be more readily distributed across large areas by groundwater flow/advection, assuming reasonable flow velocity. However, they also are readily bioavailable, and, therefore, are consumed in a relatively short time. Examples of soluble substrates include lactate and fatty acids, butyrate, methanol, ethanol, benzoate, molasses, and high fructose corn syrup. Slow-Release: Slow release compounds tend to have low solubility limits and greater viscosities than their aqueous counterparts, making them more diicult to emplace in the aquifer. However, because they are less soluble (and less bioavailable), they persist much longer. This slow-release aspect allows these substrates to serve as long-lasting sources of electron donor and minimize the number of substrate injections performed at a site. Their properties enables the substrate to remain relatively immobile in the subsurface. Viscous fluid substrates, including emulsified and neat vegetable oils and hydrogen release compounds, are included in this category (ESTCP, 2009). https://frtr.gov/matrix/Enhanced-In-Situ-Reductive-Dechlorinated-for-Groundwater/ 4/18 8/3/2020 Technology Screening Matrix | Federal Remediation Technologies Roundtable Solid Substrates: Typical solid phase substrates, consisting of materials such as mulch, compost and chitin, are used either as a one-time amendment or in biobarrier construction. Degradation of the substrate by microbial processes provides a number of breakdown products, including metabolic and humic acids, which act as secondary fermentable substrates. At many sites, the aquifer is bioaugmented with commercially available microbial consortia consisting of one or more of Dehalococcoides (DHC), Dehalobacter, sulfate reducers, methanogens, and fermentative microbes, which can degrade chlorinated ethene, chlorinated ethane, and mixed plumes. It has been found that the microbial population must also contain necessary functional genes to facilitate the ERD process. For example, DHC must contain the function gene vinyl chloride reductase (vcrA) to degrade VC or a "stall" condition will occur (Stroo, Lesson et al., 2013; Lorenz and Loler, 2016). Microbial cultures should be added only aer the necessary redox conditions have been achieved in the aquifer. DCE and VC require stronger anaerobic conditions (i.e., low redox conditions) than TCE for the ERD process to proceed to completion (e.g. to ethene) or a "stall" condition may occur (NAVFAC, 2007b). The cost
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