As a result of their widespread use as degreasers, solvents, and dry-cleaning agents, chlorinated solvents are among the most prevalent of groundwater contaminants found at Department of Defense (DoD) sites. These compounds commonly exist as dense non-aqueous phase liquids (DNAPL) that serve as long-term sources of contamination as they slowly solubilize into moving groundwater. Laboratory studies have shown that the addition of hydrogen as an electron donor is effective in stimulating the biological reductive dechlorination of chlorinated solvents. The challenge in transferring this technology to the field is to distribute and mix the hydrogen effectively with the contaminants in-situ. One promising method to mix hydrogen in contaminated groundwater is low-volume pulsed hydrogen biosparging (LVPB-H2). This technology has been evaluated partially by the project team in several laboratory studies and in one Air Force pilot field test.
The objective of this project was to further develop LVPB-H2 by determining the effective zone of influence and persistence of the hydrogen pulse needed to achieve rapid reductive dechlorination. The efficacy of hydrogen addition for reductive dechlorination of a simulated chlorinated solvent dissolved plume and source area was evaluated.
LVPB-H2 introduces a small volume of gas to stimulate in-situ biological reductive dechlorination while minimizing hydrogen breakthrough to the surface. The hydrogen gas is injected in short pulses to improve the contaminant mass removal rates as the result of increased groundwater mixing. Experiments were conducted using a 5400-gallon Experiment Controlled Release System, developed by the DoD's Advanced Applied Technology Demonstration Facility for testing emerging remediation technologies. Use of this apparatus allowed the project team to perform controlled studies of hydrogen breakthrough, distribution, persistence, and the resulting dechlorination processes. Besides vapor and water sampling during the experiments, Time Domain Reflectometry was used to visualize distriubtion and dissolution of the sparged hydrogen gas.
For the simulated dissolved plume experiment, 82 percent of the total tetrachloroethene (PCE) entering the tank was removed and 78 percent was biotransformed using one sparge well. Two percent of the added PCE was lost through volatilization. Methanogenesis, sulfate reduction, and high hydrogen gas saturations did not prevent reductive dechlorination. The maximum measured dissolved hydrogen delivery radius was 7 feet although it is expected to be greater at actual field sites. The hydrogen persistence decreased from 2-4 days to less than 1 day as the experiment progressed. The DNAPL experiment demonstrated that reductive dechlorination to cis-dichloroethene (cis-DCE) was possible within 3 feet of a PCE DNAPL source, with trichloroethene (TCE) being the predominant dechlorination product. Enhanced DNAPL dissolution was not observed due to the low rates of biodegradation that were sustained. This project was completed in FY 2002.
Hydrogen biosparging represents an innovative, cost-effective technology for the management of chlorinated solvent plumes because of hydrogen's low cost and its ability to promote rapid dechlorination. LVPB-H2 may be implemented in various configurations for active plume remediation, passive barriers to plume migration, or remediation of source zones.