This project explores 1,4-dioxane, which was widely used as a stabilizer in chlorinated solvents. It has emerged as an important groundwater contaminant throughout the United States. Typical of many ethers, 1,4-dioxane is miscible in water, and has both a low Henry’s Law constant and a low organic carbon partitioning coefficient, making it difficult to treat by typical remediation approaches, such as airstripping and carbon adsorption. Moreover, because of its use as a solvent stabilizer, 1,4-dioxane is frequently co-mingled with chlorinated volatile organic compounds (CVOCs) in aquifers, yet it is not treated by the most common bioremediation approach for these contaminants, which entails adding carbon substrates to aquifers to stimulate reductive dechlorination of CVOCs. The key objective of this project is to demonstrate that a novel multiple primary substrate (MPS) cometabolic biosparging technology can meet the DoD’s needs for reliable, flexible, and cost effective treatment of groundwater with comingled 1,4-dioxane and CVOCs. The ability of the technology to achieve regulatory levels of 1,4-dioxane and CVOCs in the groundwater, and the cost of the technology will be evaluated.

Schematic of the hybrid cometabolic treatment system in this demonstration that utilized groundwater recirculation plus oxygen sparging. Primary substrate gases used to promote aerobic cometabolism of target chemicals were isobutane and methane.

In this demonstration project, which is based on recent data from SERDP Project ER-2303, Evaluation of Branched Hydrocarbons as Stimulants forIn Situ Cometabolic Biodegradation of 1,4-Dioxane and Its Associated Co-Contaminants, multiple gaseous substrates will be utilized to stimulate the aerobic degradation of 1,4-dioxane and CVOCs by distinct groups of widely distributed bacteria. One group of bacteria that grows on isobutane can biodegrade 1,4-dioxane to ≤1 μg/L. This activity is consistently observed in both commercially sourced and newly isolated isobutane-oxidizing bacteria, as well as in aquifer microcosms. Isobutane-oxidizers can also biodegrade a range of CVOCs including 1,1,1-trichloroethane, and vinyl chloride, but are unreactive toward TCE. In the treatment system to be used in the project, isobutane will be used to specifically promote biodegradation of 1,4-dioxane and some CVOCs. One or more other primary gaseous substrates such as methane, propane, or isobutylene will be used to stimulate bacteria capable of rapidly degrading CVOCs such as TCE that are not effectively biodegraded by isobutane-oxidizing bacteria. This approach represents a significant advance for traditional single gas biosparging technology in that it utilizes a new understanding of microbial communities and monooxygenase enzyme stimulation and inhibition to specifically target DoD sites with mixed 1,4-dioxane and CVOCs.

Co-mingled plumes containing 1,4-dioxane and CVOCs are a primary concern and challenge for the DoD. There are currently no cost-effective options for the treatment of groundwater plumes containing this mixture of contaminants. The goal of this project is to demonstrate effective joint treatment of these contaminants using a novel MPS biosparging approach. The approach has the potential to be a cost effective, reliable, and flexible technology for addressing many DoD sites with co-mingled 1,4-dioxane and CVOCs, and potentially other important COCs such as NDMA, and 1,2,3-trichloropropane. A second important advantage of the aerobic MPS approach to DoD is that the geochemistry of an aerobic aquifer is largely preserved during treatment via this method, with none of the long-term secondary groundwater impacts that commonly occur with anaerobic CVOC treatment, such as H₂S and CH₄ accumulation, acidification, and mobilization of numerous reduced metals, such as Fe, Mn, and As. In addition, for CVOCs, accumulation of VC or other toxic metabolites is not anticipated based on known metabolic pathways. This approach is expected to represent a significant advance in treatment of mixed CVOC and 1,4-dioxane plumes prevalent at many DoD sites. (Anticipated Project Completion - 2023)