Formatted Title
Optimizing Bioremediation with Low pH Tolerant Dhc
Background/Objectives
Tetrachloroethene (PCE) and trichloroethene (TCE) can be reductively dechlorinated to cis-dichloroethene (cDCE) under anaerobic conditions by a variety of microbes. Continued dechlorination of cDCE to VC and subsequently to ethene is conducted uniquely by Dehalococcoides (Dhc) bacteria containing the appropriate vinyl chloride reductive dehalogenase genes. For sites that are lacking native Dhc with the appropriate functional genes, commercial Dhc cultures are often injected into the groundwater to complete the cDCE and VC dechlorination to ethene step.
Bioremediation of chlorinated ethenes and many other chlorinated compounds is optimal at neutral pH (i.e., 6.8-7.5). pH values below 6.0 is problematic for bioremediation of chlorinated ethenes with the cDCE and VC to ethene step being the most affected with either slow dechlorination rates or even complete stalling. Given that both reductive dechlorination and fermentation of commonly used electron donors are acid generating processes, enhanced bioremediation has the potential to decrease pH into the inhibitory range, even if prior to bioremediation amendment addition the pH was acceptable.
Approach/Activities
Modifying aquifer pH using buffering agents such as sodium bicarbonate and various commercial formulations has become increasingly common. Aquifer pH modification has shown varying degrees of success and is generally considered challenging depending on application method, site geology and geochemistry. In certain cases, especially where pH is near or slightly below 6.0, the use of bioaugmentation cultures acclimated to lower pH has the potential to reduce the need for aquifer neutralization. Increasing evidence indicates that complete dechlorination to ethene is possible below pH 6.0 with pH tolerant bioaugmentation cultures. These cultures can be used to minimize or avoid the need for aquifer pH neutralization.
Results/Lessons Learned
The development and field use of low pH acclimated cultures will be discussed and case studies presented. At one Site in Florida, with the pH ranging between 5.5 and 6.0, PCE, TCE and cDCE were completely dechlorinated to VC and ethene within 6 months of bioaugmentation with a low pH tolerant bioaugmentation culture. At another Site, where the pH was as low as 5.5, TCE (up to 100,000 ug/L) was dechlorinated to ethene after bioaugmentation with a low pH tolerant bioaugmentation culture. The benefits of reducing or eliminating the need for aquifer neutralization will be discussed along with lessons learned.