Remediation of chlorinated solvent DNAPL sites often meets with mixed results. This can be attributed to the diametrically opposed nature of the impacts, where the disparate dissolved phase plume is more manageable than the localized, high-concentration source area. A wide range of technologies are available for downgradient plume management, but the relative mass of contaminants in a DNAPL source area generally requires treatment for such technologies to be effective over the long term. In many cases, the characteristics of DNAPL source zones (e.g., depth, soil heterogeneity, structural limitations) limit the available options. This article describes the successful full-scale implementation of in situ chemical reduction (ISCR) enhanced bioremediation of a TCE DNAPL source zone. In this demonstration, concentrations of TCE were rapidly reduced to below the maximum contaminant level (MCL) in less than six months following implementation. The results described herein suggest that ISCR-enhanced bioremediation is a viable remedial alternative for chlorinated solvent source zones.
TCE concentrations in samples collected from the site monitoring wells and direct-push sampling ranged as high as 259,000 and 592,000 μg/L, respectively. These concentrations suggested a high probability of the presence of TCE DNAPL, likely associated with a release in the early 1980s. The project team identified an achievable RAO of reducing TCE concentrations to below the 1 percent of TCE solubility threshold (i.e., 11,000 μg/L). Five months following completion of a PRB and supplemental treatment zones composed of EHC and KB-1, the RAO was met in all 23 of the monitoring points. Furthermore, the US EPA MCL for TCE (5 μg/L) was achieved in 9 of the 23 mointoring points. The TCE percent mass reduction ranged as high as 99.1 percent in the injection zone, and 98.6 percent downgradient of the injection zone. Other data confirmed that the injected amendments are advecting downgradient and treating potential DNAPL mass located in inaccessible areas under the adjacent building. Removal of TCE and its degradation products is occurring by both abiotic and microbially mediated pathways. A weight-of-evidence approach to data assessment confirms that ISCR is a potentially effective remedial technique for chlorinated-solvent DNAPL source zones in relatively deep, heterogeneous aquifers.