Source areas of chlorinated volatile organic compounds (VOCs) in groundwater create and perpetuate dilute groundwater plumes that subsequently pose risks to downgradient receptors for decades or centuries. Although bioremediation has been applied to treat many contaminant source areas over the past two decades, the overall success of this treatment and factors that differentiate successful from unsuccessful treatment applications have not been thoroughly assessed. The primary project objective of this project was to assess the long-term effectiveness of past biological treatment approaches for remediating source areas of chlorinated ethene-contaminated aquifers.
To meet the project objective, a large database was developed from 15 historical VOC sites that included: 1) site location; 2) VOC concentrations over time; 3) hydrogeology; 4) geochemistry; 5) water chemistry; 6) abundances of relevant microbial biomarkers where available; 7) treatment approach; and 8) other relevant site data. Statistical analyses were then performed to identify factors that may promote or prevent successful application of different bioremediation strategies. Secondly, sampling was conducted at five sites to obtain a current snapshot of the effectiveness of past treatment approaches using traditional measures (e.g., contaminant concentrations) and new tools, including analysis of key dehalogenating organisms/genes via molecular biological tools (MBTs); passive flux meters (PFM) to assess current contaminant flux; and compound specific isotope analysis (CSIA) to quantify whether degradation is ongoing. The historical and current data were used to draw conclusions about the long-term effectiveness of VOC bioremediation at the five sites.
Analysis of the database indicated that a number of factors correlated positively or negatively with overall concentration reduction of VOCs at contaminated sites including, minimum sulfate after treatment (-), sulfate depletion (+), initial oxidation-reduction potential (ORP) (-), maximum iron (+), presence of fine grained materials (-), initial sulfate (-), initial chlorinated ethene concentrations (+), minimum ORP (-), and presence of dense non-aqueous phase liquid 1/0 (+) in decreasing order. Factors associated with accumulation of cis-1,2-dichloroethene (cis-DCE) as a daughter product of trichloroethene or tetrachloroethene dechlorination (i.e., cis-DCE stall) included minimum sulfate after treatment (+), initial iron (-), initial sulfate (+), aquifer heterogeneity (-), and initial ORP (+) in decreasing order. Other factors of importance included low initial pH (-) and previous remedial applications (+). Among the five sites where current assessments or remedial effectiveness were completed, all showed evidence of ongoing biodegradation a few to several years after amendments were applied. PFMs, MBTs and CSIA provided valuable assessments of whether necessary organisms/genes (MBTs) were present at site locations, whether degradation was ongoing (CSIA), and the overall affect of treatment on contaminant flux (PFMs). The cost for conducting the aforementioned analyses (PFMs, MBTs, CSIA) at a typical site are provided in the Final Report.
The database analysis provides insight on factors contributing to bioremediation success and has no implementation issues. The primary end-users of the new site assessment technologies (MBT, PFM, CSIA) are expected to be Department of Defense (DoD) site managers and their contractors, consultants and engineers. The general concerns of these end users are likely to include the following: (1) technology availability and cost; (2) appropriate application of the technology at DoD sites; and (3) interpretation of CSIA, MBT and PFM data. These technologies are commercially available, easily implemented, and various resources are available to assist with data interpretation as described in the Final Report.
Haluska, A.A. 2018. Performance Assessment of Bioremediation and Bioaugmentation of Chloroethene DNAPL Source Zones, Ph.D. Dissertation, University of Florida.