Date of Award
Master of Science (MS)
Environmental Engineering and Science
Freedman, David L
Lee , Cindy
Finneran , Kevin T
Chlorinated ethenes are among the most prevalent groundwater contaminants found at hazardous waste sites throughout the United States. Two such sites are the Statesville FCX OU3 Superfund Site in North Carolina and the C-area at the Savannah River Site (SRS) in Aiken, SC. Both sites are contaminated with two of the more common chlorinated ethenes; tetrachloroethene (PCE) and trichloroethene (TCE). Anaerobic biological reductive dechlorination is an increasingly common treatment option. There are two main concerns as to the feasibility of this approach. The first is if the microbes are present that are needed for carrying out complete reductive dechlorination of PCE and TCE, via sequential reduction to cis-1,2-dichloroethene (cDCE) and vinyl chloride (VC) to ethene (or ethane). The presence of Dehalococcoides is essential; since this is the only genera known that can chlororespire cDCE and VC to ethene. Dehalococcoides are also capable of chlororespiring PCE and TCE, although numerous other genera can accomplish these steps metabolically. Given the high level of toxicity of VC relative to the other chlorinated ethenes (it is a known human carcinogen), the last dechlorination step is typically considered the most critical. The second concern regarding the feasibility of bioremediation by reductive dechlorination is if the geochemistry of the site is favorable. Geochemical conditions include the redox level, availability of competing electron acceptors, supply of fermentable electron donor, and pH. The latter condition was a focus of this thesis. The prevailing groundwater pH at both sites is below 6, which is generally considered too low to achieve complete reductive dechlorination.
The specific objectives of this thesis were:
1) To evaluate the feasibility of using anaerobic bioremediation to treat groundwater contaminated with PCE at the Statesville FCX OU3 Superfund Site, based on a microcosm study with groundwater and soil from the site; and
2) To develop a bioaugmentation culture for anaerobic reductive dechlorination of chlorinated ethenes that is capable of rapid activity at pH levels of 6.0 and below.
Based on the results of the research performed in this study, the following conclusions were reached regarding the microcosm study of the NC hazardous waste site:
* It is highly unlikely that biostimulation alone will lead to complete reductive dechlorination of PCE to ethene at the Statesville site, even with the groundwater pH adjusted to near neutral. The amount of electron donor added in this study was considerably higher than the demand, both for competing electron acceptors and reductive dechlorination. Field monitoring data indicate that reductive dechlorination to cDCE is occurring in some locations. This suggests that some areas of the site have a higher population of microbes capable of reducing PCE to cDCE compared to the area from which soil and groundwater were obtained for the microcosm study. Nevertheless, complete dechlorination to ethene requires a significant population of Dehalococcoides, which were (presumptively) absent in the microcosm study and appear to either be absent or present in extremely low numbers in the field (based on a lack of significant levels or VC or ethene accumulation).
* Bioaugmentation can be an effective strategy for achieving complete dechlorination of PCE to ethene if the pH of the groundwater is adjusted to near neutral conditions. Each of the electron donors tested (lactate, EOS¨ and HRC-A¨) supported bioaugmentation.
* The fact that a 0.1% (v/v) dose of bioaugmentation culture was effective indicates that growth of the culture occurs in the pH-adjusted groundwater. This is an important finding from the perspective of achieving activity in the field in areas that receive a low dose of culture.
* Adjusting the pH of the groundwater and soil was more challenging than expected under laboratory conditions. Although a relatively low dose of alkali was needed to raise the pH from below 5 to approximately 6, a much larger amount of base was needed to increase the pH to near neutral. If bioaugmentation is to be implemented in the field, it would be desirable to use a culture that can tolerate a pH close to 6. Alternatively, an even better approach would be to develop a culture that can tolerate the in situ pH, or at least one that is effective in a pH range of 5.0-5.5. No such commercial cultures are currently available. However, the results of this thesis suggest it should be feasible to develop such a culture.
* Bioaugmentation was effective in the treatment that was not pH adjusted, although apparently not until the pH increased sufficiently to allow for growth of the chlororespiring microbes. The increase in pH was likely attributable to addition of lactate as the electron donor. The results indicate that the bioaugmentation culture remained viable for a number of months at low pH, before the pH eventually became favorable for growth.
Based on the results of the research performed in this study, the following conclusions were reached regarding the development of an enrichment culture capable of growth at pH levels of 6 or lower:
* Using inoculum from a wetland part of the NC hazardous waste site, enrichment cultures were developed that are capable of reductively dechlorinating PCE to ethene at pH levels of approximately 5.9 to 6.1. At pH levels of approximately 5.5, reductive dechlorination of PCE continued; however, cDCE became the dominant daughter product.
* Using inoculum from the Twin Lakes wetland area at SRS, microcosms were developed that reductively dechlorinated cDCE and VC to ethene at a pH of approximately 5.6, with activity continuing when the pH was as low as 5.4. Although additional work is needed to achieve a sediment-free enrichment culture, these preliminary results support the prospect that complete reductive dechlorination of chlorinated ethenes occurs at reasonable rates in low pH environments.
Hickey, Michael, "Evaluation of Tetrachloroethene Dechlorination Under Low pH Conditions in Microcosms and Enrichment Cultures" (2010). All Theses. 1041.