Stable carbon isotope monitoring of in situ bioaugmentation for enhanced reductive dechlorination of halogenated hydrocarbons

Injection of electron donor to stimulate reductive dechlorination of trichloroethene (TCE) has been demonstrated to be an effective strategy for remediation of contaminated groundwater. At a number of sites, however, complete reductive dechlorination of TCE to ethene is not attained because the appropriate microbial community is not present. Addition of Dehalococcoides spp. to groundwater to achieve complete reductive dechlorination of TCE is being tested at Naval Weapons Station Seal Beach, CA. To help assess the effectiveness of this process, the stable carbon isotope compositions of TCE and its byproducts, cis-dichloroethene (cDCE), vinyl chloride (VC) and ethene are being measured during the experiment. Two different methods of bioremediation are being tested. In the “active” cell groundwater is continuously pumped from downgradient wells and re-injected into two upgradient wells. Electron donor (1-3% Na-lactate) has been added to the injection line either weekly or monthly. In the “passive” cell, no circulation of groundwater is done, but electron donor is added to three injection wells monthly. When reducing conditions were reached in the groundwater (late 2008), the bioaugmentation culture was added to both experimental cells with the electron donor. In the active cell, addition of electron donor prior to introduction of the bioaugmentation culture stimulated significant increases in the concentrations of cDCE, but only trace VC and ethene. In the passive cell, production of cDCE was observed, but at lower levels. The δ13C values of TCE ranged from -20‰ to 28‰ (averaging -24‰). The δ13C values of cDCE were generally 1-2‰ per mil lower than those of the TCE, representing fractionation during the biological conversion from TCE to cDCE. Following bioaugmentation, significant production of VC has been observed in the active cell, with corresponding increases in δ13C values of TCE and cDCE. In several wells, the δ13C values of the cDCE have become higher than that of the TCE, indicating efficient conversion of cDCE to VC. To date, only low levels of ethene have been observed in the active cell and this is reflected by the δ13C values of the VC, which are approaching the initial δ13C values of TCE. In the passive cell, dechlorination to VC is observed but less widespread than in the active cell. Where VC is observed, however, there is also significant production of ethene. The δ13C values of both cDCE and VC in these wells have been shifted to very high values (to as high as 1‰ for cDCE and -4‰ for VC) indicating significant levels of complete reductive dechlorination are occurring in this cell. The variations in the carbon isotope compositions of TCE and byproducts clearly indicate that bioaugmentation has led to significantly enhanced levels of reductive dechlorination at this site.