Anaerobic reductive dechlorination processes are typically inhibited at pH less than about 5.5, making anaerobic bioremediation largely ineffective in acidic aquifers. The primary objective of this project is to demonstrate and validate aerobic treatment of chlorinated volatile organic compounds (cVOCs) in an acidic aquifer, such as those that characterize the coastal plain aquifer systems running from Long Island NY, south to North Carolina, and then west across the panhandle of Florida to southern Texas. A large number of military sites are present over these aquifer systems. An acidic aquifer with a dilute cVOC plume will be targeted as a demonstration site, since dilute plumes remain a significant challenge for the Department of Defense (DoD). A sustainable, solar-powered treatment system will be utilized to provide necessary remediation amendments with maximum flexibility and minimal operation and maintenance requirements.

Recent experiments conducted under SERDP Project ER-2531 reveal that methanotrophs as well as bacteria capable of aerobic growth on ethane and ethene are present in many acidic aquifers and are capable of cometabolically biodegrading a range of common cVOCs to non-toxic products. During this ESTCP demonstration, the project team will utilize an off-the-grid system to supply the amendments necessary to stimulate cometabolic organisms to degrade cVOCs in an acidic aquifer. The amendments include a cometabolic substrate (methane, ethane or ethene), oxygen, and inorganic nutrients. The system will be capable of operating in a biosparging-only mode (supplying gaseous amendments to biosparge wells via compressed air); a recirculation mode (recirculating groundwater with a solar-powered pump and adding amendments to the flowing water stream); or in a hybrid mode, in which both approaches are utilized simultaneously. This flexibility makes it possible for the system to be applied in many different configurations and geological settings, and in remote areas without a power supply. The project team believes this approach has the potential to be the first cost-effective and sustainable technology for addressing plumes of cVOCs in acidic aquifers at DoD sites.

Groundwater Sampling at a Low pH Site in New Jersey

cVOCs such at trichloroethene still represent one of the largest remediation challenges and costs at commercial and military sites in the United States. Potential clean-up costs are in the billions of dollars. One significant challenge remaining for remediating these contaminants and protecting downgradient receptors is the treatment of plumes present in acidic aquifers. The key bacteria that promote the complete dehalogenation of chlorinated ethenes to ethene (e.g., Dehalococcoides spp.) are not active under acidic conditions; thus, parent cVOCs either do not degrade, or degrade partially, resulting in production of terminal daughter products such as cis-dichloroethene or vinyl chloride (a known carcinogen with a federal maximum contaminant level of 2 µg/L). Thus, monitored natural attenuation is rarely a viable option at such sites. Remedial costs are particularly high at sites where contamination is extensive, but concentrations are low, as there are few in situ technologies to meaningfully enhance plume attenuation rates under these conditions. This project will demonstrate an effective, sustainable biological remediation option for enhancing remediation of plumes in acidic aquifers without generation of toxic daughter products. The approach also will not require traditional grid power. Rather, gas pressure and solar energy will be used to distribute amendments and for system control, making this approach applicable even in remote locations. This remedial technology is anticipated to have broad applicability for the military and commercial entities. (Anticipated Project Completion - 2025)

Hwangbo, M., Y. Shao, P.B. Hatzinger, and K. Chu. 2023. Acidophilic Methanotrophs: Occurrence, Diversity, and Possible Bioremediation Applications. Environmental Microbiology Reports, 1–17. doi.org/10.1111/1758- 2229.13156.