|Successful bioremediation recognized in
Microorganisms polish off the job
The Nebraska Department of Environmental Quality recently presented 1998 environmental excellence awards to six companies and organizations remediating the Hastings, Nebraska Superfund site. One recipient was the U.S. Army of Corps of Engineers, which has been working since 1987 to clean up the former Blaine Naval Ammunition Depot east of Hastings. Pilot-testing with methane-enhanced biodegradation, a technology pioneered by the Department of Energy, shows promise to deliver the final blow to a tenacious contaminant plume.
Built in 194243 on a site comprising nearly 50,000 acres, the Blaine NAD operated for 15 years. At one point during World War II, it was producing 40 percent of all U.S. Navy munitions. After ceasing operations in 1957, the plant took nine years to decommission. The largest of seven subsites in the Hastings Superfund site, much of the NAD has been put to other uses, including a community college, an industrial park, and an animal research center operated by the Department of Agriculture. But contaminated ground-water plumes covering six square miles still require remediation.
The Kansas City district of USACE has used a variety of advanced in situ techniques to avoid the traditional pump-and-treat method, which is expensive and depletes the ground water as well. One plume of particular concern involves trichloroethylene, a potentially carcinogenic cleaning solvent, which was found in concentrations as high as 16,000 parts per billion. The plume extended below the water table but not past a confining clay layer into the lower aquifer.
Soil vapor extraction was initiated first to reduce TCE contamination in the unsaturated zone, which is sandy silt interrupted by less-permeable lenses. A month later USACE installed a 200-foot-long horizontal well 125 feet below the surface to pump air into the saturated zone just above the confining clay layer. The rising air bubbles stripped TCE from the ground water, to be drawn off by the SVE wells in the soil above the water table. This "air sparging" technique, patented and developed at DOEs Savannah River Site, pulled TCE levels down to a few hundred parts per billion or less, but eventually they plateaued.
Enter the microorganisms
USACE is now pilot-testing bioremediation to "polish" the remaining TCE from the ground water, using a process developed and demonstrated in the early 1990s at the Savannah River Site, funded in part by the forerunner of the Office of Science and Technology. The technology won a 1995 R&D 100 award for the Savannah River Technology Center.
At the Nebraska site, the same horizontal well as that used for air sparging is being used to inject natural gas and triethyl phosphate into the ground water. Populations of indigenous microorganisms rise sharply in the suddenly nutrient-rich environment. The microorganisms produce a series of enzymes to metabolize the methane in the injected natural gas. One of the enzymes, methane monooxygenase, also fortuitously breaks TCE down into TCE-epoxide. Thats why the process is sometimes referred to as "cometabolism." The TCE-epoxide rapidly breaks down further into carbon dioxide and chloride salts.
The SRTC research found that the best operating campaign combined continuous injection of nitrogen and phosphorus nutrients with pulsed injections of methane. The process can be monitored by various means, including measuring the microbe populations and tracing the fate of the methane. But the ultimate proof is the decrease in the contaminant levels. Results are still preliminary at the NAD pilot test, but at some monitoring wells TCE has dropped to 5 parts per billion (the maximum concentration level allowed by regulation) or even nondetectable levels, according to Mirek Towster, USACE technical manager.
Gaseous Nutrient Injection for In Situ Bioremediation, patented by DOE, has been licensed to more than 30 companies, says Terry Hazen, one of the principal investigators, now at Lawrence Berkeley National Laboratory. As with the USACE project in Nebraska, many projects ostensibly "demonstrating" the technology are actually cleaning up sites. For instance, a recent feasibility study at SRSs D Area oil seepage basin remediated methylene chloride, vinyl chloride, dichloroethylene, trichloroethylene, and tetrachloroethylene in the unsaturated and saturated zones. The process was so successful the site applied for a no-action record of decision.
In addition to bioremediation, USACE is testing two in-well stripping technologies, including No-VOCs (see Initiatives, February 1997). A feasibility study will soon determine which of the technologies are appropriate for further use at the Blaine NAD.