Feeding microbes to get rid of nitrates.

Many sources of drinking water, polluted by fertilizers and other contaminants, contain high concentrations of nitrate, which studies have shown to cause cancer in humans. No simple method exists for removing nitrates, and groundwater rich in this contaminant has little use except on crops.

Now, researchers at the Department of Agriculture's Agricultural Research Service (ARS) in Fort Collins, Colo., say they may have a technique for reducing nitrate concentrations by supplying corn or soybean oil to microbes living in aquifers.

To survive, the microorganisms require carbon and either nitrate or oxygen to oxidize the carbon. Adding oil to nitrate-rich water provides a source of extra carbon that enables the microbes to make use of the abundant nitrogen, report William J. Hunter and Ronald F. Follett in the July Agricultural Research. Microbes convert most of their nitrate supply into harmless nitrogen gas.

Follett and Hunter tested their idea for groundwater cleanup by injecting oil into glass tubes containing sand and water from a polluted aquifer. The researchers then pumped the water slowly through the tubes. The water had 14 to 19 milligrams of nitrate-nitrogen (the nitrogen present in nitrates) per liter, which exceeds the safe concentration of 10 milligrams per liter, says Follett.

The oil became embedded in the sand, where it and the microbes formed an organic filter in the tube. Within 1 to 2 days of the oil infusion, the microbes began removing nitrates from the water that passed through the tube. As long as they had enough oil, the microbes kept nitrate concentrations to almost zero for the year-long experiment.

One gram of oil enabled the microbes to remove 260 milligrams of nitrate-nitrogen from 26 liters of water, the ARS team calculates.

An oil-based approach to removing nitrogen could have pitfalls, the scientists acknowledge. The microbe-oil filter could plug up pores in aquifers; make water taste or smell bad or be totally undrinkable; or pollute water with nitrites, a dangerous by-product of denitrification, they note.

The researchers hope to conduct field tests, in which they will either inject a mixture of oil and water near the base of a well or force oil down a well, says Hunter. They may also devise an above-ground water filter that uses oil.

"Overall, I think it's an intriguing concept," says Ralph S. Baker of ENSR Consulting, Engineering, and Remediation in Acton, Mass. However, "there's a lot of work to do to make something like this effective," he warns, citing particularly the concerns about clogging and nitrite production.

Battelle Memorial Institute, a research group in Richland, Wash., has a patent on a similar oil-based system for removing nitrates from water. A Battelle scientist, formerly with ARS, came up with the idea for the technology, but the institute has not formally tested it, says Glendon W. Gee of Battelle. The institute is looking for a commercial partner to help develop the system.

Other researchers have used bacteria to remove heavy metals and organic chemicals from damp soil. One group grows bacteria on blankets of coconut hull fibers that it then lays on contaminated areas (SN: 3/4/95, p.138).