Despite years of research and technology development, chlorinated volatile organic compounds (CVOCs) remain the primary contaminants of concern for the Department of Defense (DoD). One of the greatest challenges remaining for remediating these contaminants at DoD sites and protecting downgradient receptors is the treatment and/or control of deep, large, dilute plumes. A disproportionate share of resources and attention often goes to a relatively few such sites because of the costs and technical difficulties involved in treating large volumes of groundwater dispersed over large areas. The recirculation-based aerobic cometabolic biodegradation (ACB) technology has been considered an effective tool in treating multiple CVOCs and 1,4-dixoane (1,4-D) concurrently. However, because of perceived high operation and maintenance costs due to bioclogging in the vicinity of substrate injection wells as well as potential site access constraints for construction of conveyance pipeline for large-scale groundwater recirculation, this technology has been underused. The overall objective of this project is to demonstrate that recirculation-based ACB can be cost-effective for treating deep, large, dilute plumes. Key specific objectives include: (1) evaluating the benefit of infrequent, longduration, alternate substrate pulses together with a novel temporary microbial substrate utilization inhibitor, acetylene, for bioclogging control; (2) assessing the technical, implementation, and cost benefits of using angled wells to eliminate the need for recirculation piping; and (3) verify the cost competitiveness of the technology in comparison with the conventional pump-and-treat approach for large dilute plume management.
This project will use an angled injection/extraction well pair to perform groundwater recirculation and to intercept a deep, dilute plume. The use of angled wells helps create large-scale recirculation without the need for long piping to convey recirculated groundwater between the injection/extraction well pair. A customized mixture of short-chain hydrocarbons developed through laboratory microcosm studies and oxygen and/or hydrogen peroxide will be pulsed to recirculated groundwater to stimulate in situ ACB activity to treat multiple contaminants concurrently. A temporary inhibitor, acetylene, coupled with infrequent, long-duration, and pulsed substrate injection will be used to sustain the degradation activity and, at the same time, significantly reduce the potential of bioclogging in the vicinity of the injection well. The project will use a Venturi injector to reduce the aboveground footprint of the amendments addition system because of its high efficiency in gaseous substrate solubilization and ability to accommodate a high flow rate.
The project results are expected to demonstrate the cost-effectiveness of the improved ACB technology for bioremediation of deep, large, dilute plumes. Commingled plumes containing 1,4-D and CVOCs are a primary concern and challenge for the DoD; this technology has the potential to treat multiple contaminants concurrently to very low levels and meet site-specific cleanup goals, reducing the need for expensive treatment upgrades to the pump-and-treat approach (e.g., advanced oxidation needed for 1,4-D treatment). The technology will have a small footprint by using angled wells to avoid the need for conveyance piping for recirculated groundwater, which may be difficult or costly to install where access is limited. An updated technical guidance and accompanying screening design software, developed based on the engineering principles verified during this project, will also help site managers and/or remediation practitioners assess whether their sites can benefit from this technology. (Anticipated Project Completion - 2024)