Published laboratory studies conducted at Rice University have demonstrated that rapid dechlorination activity can occur in the immediate vicinity of pure chlorinated solvent dense non-aqueous phase liquid (DNAPL), cause dramatic changes in the mass transfer and partitioning characteristics of the DNAPL, and result in rapid DNAPL dissolution. Source zone bioremediation harnesses the natural metabolism of dehalorespiring organisms, capable of thriving at high concentrations of chlorinated solvent contamination, to modify the dissolution characteristics of DNAPLs. If source zones could be effectively treated using low-cost bioremediation technology, significant reduction in remediation life-cycle costs could be achieved at Department of Defense sites. Carefully controlled, near-field scale demonstrations are required to validate this benefit. A controlled test of source zone bioaugmentation at a near-field scale using tetrachloroethene (PCE) DNAPL was successfully completed in the Experimental Controlled Release System (ECRS) at Rice University. Use of this system for assessing the potential for DNAPL source zone bioremediation provides a means for avoiding many of the difficulties inherent in field-scale work (adequate estimation of the mass and composition of DNAPL initially present, an inability to operate a parallel independent control study, and the high costs generally associated with experimental work at this scale).
Having demonstrated the ability to construct, bioaugment, and monitor DNAPL source zones in a controlled release system, a quantitative demonstration of DNAPL source zone bioremediation was conducted in a cost-effective manner with a known initial DNAPL mass and composition and a parallel independent control. The objectives of this work were to (1) provide a basis for critical analysis of the extant field data from ongoing tests to determine if source zone longevity is being biologically impacted at these sites, (2) allow for the characterization of microbial ecology in the DNAPL source zone and downgradient using molecular techniques for tracking and enumerating critical populations, (3) determine whether PCR-based analysis targeting phylogenetic or catabolic biomarkers could be a reliable and cost-effective tool to estimate dechlorination rates, and (4) develop a basis for cost and effectiveness considerations at field scale.
In the first phase of this project (Phase I), two 11.7 m3 ECRS, packed with sandy model aquifer material and amended with PCE DNAPL, were operated in parallel with identical flow regimes and electron donor amendments. Hydrogen Releasing Compound® (HRC) and later dissolved lactate served as electron donors to promote dechlorination. One ECRS was bioaugmented with an anaerobic dechlorinating consortium directly into the source zone, and the other served as a control (biostimulated only) to determine the benefits of bioaugmentation. The presence of halorespiring bacteria in the aquifer matrix prior to bioaugmentation, shown by nested PCR with phylogenetic primers, suggests that dechlorinating catabolic potential may be somewhat widespread. PCR analyses demonstrated that the bacteria present in the culture used for bioaugmentation in the ECRS prevailed for almost a year. Unfortunately, even with Dehalococcoides present, complete dechlorination to ethene was achieved at minimum (<1μM). Results demonstrated that the low concentration of ethene produced in this first phase was not due to washout of the dechlorinating organisms. It was also demonstrated that as long as the electron acceptor was not limiting, there was greater energy flow to the dechlorinating populations than to the methanogens.
Phase II compared the fate of a mixed DNAPL source zone under a natural attenuation scenario (no treatment, natural rates of dissolution) with a most probable engineering approach that included biostimulation and bioaugmentation. The same experimental ECRS tanks used in Phase I were emptied and repacked with uncontaminated sandy soil. HRC® was continuously added in the influent as a pre-hydrolized (diluted) mixture consisting of 50:50 v/v HRC®: deionized water. The effluent concentration of ethene measured in the biostimulated and bioaugmented tank (approximately 4μM) was 4-fold higher than Phase I. This suggests a more complete dechlorination activity that was most likely the result of the slower groundwater seepage velocity used in this experiment (0.4 m/d) compared to the Phase I experiment (1.6 m/d). Cumulative mass balance calculations showed that the total mass removed at the end of the experiment in the biostimulated and bioaugmented tank was near 47% of the total mass of PCE added to the tank. Of this removal, 26% was removed by dissolution (as measured by the mass of PCE in the effluent) and 21% by dechlorination to lesser chlorinated products, mainly trichloroethene and cis-dichloroethene. In the control tank, 34% of the PCE added to the tank was removed, with 31% being removed by dissolution and 3% by dechlorination. The benefit of biostimulation and bioaugmentation was observed with higher (7-fold) dechlorination activity compared to the control tank.
Overall, the results obtained corroborate that source zone reductive dechlorination of PCE is possible at near-field scale and that a system bioaugmented with a competent halorespiring consortium can enhance DNAPL dissolution and dechlorination processes at significantly greater rates than in a system that is biostimulated only. The results also demonstrate that the bioaugmentation of the ECRS system was successful and that the organisms present in the inoculum became the dominant organisms in the ECRS. Unfortunately, even with Dehalococcoides present, complete dechlorination to ethene was not achieved and analysis of the groundwater and sand from the ECRS systems with molecular biology techniques did not give any insight into why dechlorination was incomplete. Results demonstrate that the lack of ethene production was not due to washout of the dechlorinating organisms. It was also demonstrated that there was no correlation between cell numbers and activity of methanogens and dechlorinators and that as long as the electron acceptor was not limiting, there was greater energy flow to the dechlorinating populations than to the methanogens. (Project Completed - 2009)