STABLE CARBON ISOTOPE INVESTIGATION OF BIOLOGICALLY ENHANCED DISSOLUTION OF TETRACHLOROETHENE NEAR THE DNAPL SOURCE ZONE
Compositional and isotopic results provide evidence for reductive dechlorination near the source zone at PCE concentrations greater than saturation. The isotopic composition of the parent compound (PCE) in a system with pure phase present remains largely unchanged (30.0 +/- 0.5 permil) due to the rate of dissolution of isotopically non-fractionated DNAPL (controlled by groundwater flow rates) outweighing the production of isotopically fractionated PCE due to biodegradation. Hence close to the source zone, confirmation of reductive dechlorination is based primarily on the appearance of the less chlorinated breakdown products and isotopic signatures for those products consistent with biodegradation. The isotope values of trichloroethene (TCE) ranged from -28.8 to 25.0 permil, indicative of dechlorination of TCE to of 1,2-dichloroethene (cDCE). The isotope values of cDCE ranged from -30.8 to 22.8 permil while the isotope values of vinyl chloride (VC) ranged from -57.2 to 34.4 permil. In all cases cDCE and VC were more depleted in 13C than the parent product from which they were derived. Over time, all of the breakdown products of reductive dechlorination of PCE showed isotopic enrichments trends consistent with reductive dechlorination through to the final non-chlorinated product, ethene. Enhanced dissolution due to biodegradation of PCE was demonstrated in this study based on differences in total mass flux between the control model aquifer and bioaugmented model aquifer.