LAS VEGAS (AP) — Federal officials say they believe nitrate contamination killed 71 wild horses last month along a dry lake bed near a desert airfield, but they're uncertain of source of the pollution.

In an Aug. 10 statement from Bureau of Land Management cited "nitrate toxicity" as the most likely cause of death for the horses found about a mile south of the Tonopah Test Range.

"High levels of nitrates were found in some water samples taken from a pond the horses used for drinking on a dry lake bed," the statement said. The levels were at least 66 times the safe drinking standards for humans and 30 times acceptable levels for livestock.

The Tonopah range, a high desert airfield 210 miles northwest of Las Vegas, has been a destination for commuter aircraft that shuttle workers from Las Vegas to remote sites where tests of stealth aircraft and other high-tech aircraft have been conducted.

A Nellis Air Force Base spokesman, Capt. Justin McVay, said civil engineers who are probing the horse deaths along with the BLM don't know the nitrate's source.

"Until the investigation is complete, I don't think they'll speculate on what the root cause is," he told the Las Vegas Review-Journal in Wednesday editions.

One former worker at the test site told the newspaper he believes the contamination came from illegal dumping of nitrate compounds used in the de-icing of planes and a nearby runway.

"I know what the root cause was," said former Air Force tech sergeant Kevin Dye. "It was runway de-icing fluid because that's what we used to put on the runways up there, ammonium nitrate."

Dye said dumping of ammonium nitrate, ammonium phosphate and other chemicals was routine when he worked there from 1990 to 1998.

"This time of year is when they clean out the de-icing tanker trunks. They just pull them up to the fence and wash them out and let it run off into the desert," Dye said. He noted that there is "no water reclamation system up there."

He reported the environmental compliance issue to the Environmental Protection Agency in 1996 but the EPA took no action, the newspaper reported.

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Dante Pistone, a spokesman for the Nevada Division of Environmental Protection, said the division's staff hasn't received any information from Nellis and the BLM, the lead agencies in the investigation.

Airports that use de-icing fluids must apply to the division's Bureau of Water Pollution Control for an industrial storm-water permit.

"Before that permit is issued, they would have to submit ... a plan detailing how they plan to handle the de-icing fluids to keep them out of the waters of the state," he wrote in an e-mail Tuesday.

He confirmed that the Nellis Air Force Base has a permit covering de-icing fluids.

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