Formatted Title
Integrating qPCR Methods for Design Optimization of Biogeochemical Treatment of DNAPL
Background/Objectives
Chlorinated solvents including trichloroethene (TCE), trichloroethane (TCA), and their degradation products have impacted fractured bedrock (metamorphic and igneous rock types) at a Superfund site in the northeastern USA. In situ treatment is required by the USEPA to address residual source dense, nonaqueous phase liquid (DNAPL) in the fractured bedrock. The proposed injections will leverage the synergistic effects of enhancing biotic and abiotic degradation of dissolved chlorinated compounds via biological anaerobic reductive dechlorination (ARD) and biologically mediated abiotic degradation (BMAD). However, a thorough biogeochemical characterization of the treatment zone was required for the planned injections to be successful. A robust microbial community is vital for sustained degradation of chlorinated solvents. Therefore, quantification of the relevant, diverse microbial species and genes capable of inducing ARD and BMAD was an integral part of the pre-design investigation. The overarching objective of the microbial component of the investigation was to develop a quantifiable and systematic approach to characterize the biogeochemical nature of the Site. Advanced molecular-based tools (MBTs) were used to obtain a snapshot of the abundance and distribution of the native microbiome; elucidate the type and quantity of supplements for injection; and inform the ideal adaptive management approach for future performance monitoring protocols.
Approach/Activities
The pre-design investigation included an assessment of select biogeochemical parameters indicative of or affecting microbiological pathways known to induce chlorinated solvent degradation. To determine the genetic potential for biotic and abiotic degradation, groundwater samples were collected and analyzed using two complementary quantitative polymerase chain reaction (qPCR) MBTs that would investigate each degradation pathway: QuantArray®-Chlor and QuantArray®-BGC. In addition, samples were also analyzed for Site constituents of concern, degradation products, terminal electron acceptors, general water quality parameters, essential microbial nutrients, and anaerobically fermentable substrates.
Results/Lessons Learned
The project findings indicate that MBTs, combined with a full geochemical analysis and well-informed interpretation is a valuable tool for optimizing in situ remedy design via ARD-BMAD. QuantArray®-Chlor results indicate that the source area is hospitable to Dehalococcoides and Dehalobacter, key dechlorinating organisms for ARD, with the highest concentrations in the proposed injection zone. QuantArray®-BGC results show that the populations of iron-reducing and sulfate-reducing bacteria can likely support ongoing and future enhanced BMAD. The results were also used to inform reagent amount, injection locations, and will be used to determine the frequency of future treatment. Overall, these microbial testing results foster confidence that the optimum design will be selected for the in situ component of the remedy. The results demonstrate the importance of combining these QuantArray® methods for optimizing the design of synergistic biotic and abiotic treatment approaches to enhance DNAPL dissolution.