Coupling aggressive mass removal with microbial reductive dechlorination for remediation of DNAPL source zones: a review and assessment

Abstract

The infiltration of dense non-aqueous-phase liquids (DNAPLs) into the saturated subsurface typically produces a highly contaminated zone that serves as a long-term source of dissolved-phase groundwater contamination. Applications of aggressive physical-chemical technologies to such source zones may remove > 90% of the contaminant mass under favorable conditions. The remaining contaminant mass, however, can create a rebounding of aqueous-phase concentrations within the treated zone. Stimulation of microbial reductive dechlorination within the source zone after aggressive mass removal has recently been proposed as a promising staged-treatment remediation technology for transforming the remaining contaminant mass. This article reviews available laboratory and field evidence that supports the development of a treatment strategy that combines aggressive source-zone removal technologies with subsequent promotion of sustained microbial reductive dechlorination. Physical-chemical source-zone treatment technologies compatible with posttreatment stimulation of microbial activity are identified, and studies examining the requirements and controls (i.e., limits) of reductive dechlorination of chlorinated ethenes are investigated. Illustrative calculations are presented to explore the potential effects of source-zone management alternatives. Results suggest that, for the favorable conditions assumed in these calculations (i.e., statistical homogeneity of aquifer properties, known source-zone DNAPL distribution, and successful bioenhancement in the source zone), source longevity may be reduced by as much as an order of magnitude when physical-chemical source-zone treatment is coupled with reductive dechlorination.

Figure 1. Representative photograph from laboratory-scale (63.5 cm length × 38 cm height × 1.4 cm thickness) infiltration and entrapment PCE-DNAPL (dyed red with 10⁻⁴ M Oil Red-O for visualization).

Figure 2. Representation of air sparging with soil vapor extraction in a shallow, relatively homogeneous, unconfined aquifer with a well-defined DNAPL source zone. Arrows represent tortuous air channels into which contaminants partition and are subsequently recovered through soil vapor extraction wells.

Figure 3. Representation of ISCO in a shallow, relatively homogeneous, unconfined aquifer with a well-defined DNAPL source zone. Contaminant destruction occurs in situ as depicted by the representative chemical reaction. Alternatively, implementation of ISCO technologies may use a point-to-point flood similar to that shown in Figure 4.

Figure 4. Representation of subsurface flushing technologies in a shallow, relatively homogeneous, unconfined aquifer with a well-defined DNAPL source zone (generalized to include steam, co-solvent, and surfactant). Insets represent DNAPL recovery mechanisms (top, mobilized bank of free product collecting DNAPL ganglia; bottom, reduction in entrapped DNAPL mass through solubilization).

Figure 5. Depiction of DNAPL source-zone conceptual models used in the example calculations: (A) IGP ratio, (B) HGP ratio, (C) LGP ratio, and (D) ZGP ratio. All control volumes are the same size and contain equal amounts of PCE-DNAPL.

Figure 6. Percent DNAPL mass remaining as a function of time for (A) SEAR followed by bioenhancement in all four scenarios and (B) three alternative remediation strategies in LGP scenario.