Testing of a watering hole showed that the nitrate levels came from a combination of animal waste, natural soil nitrogen and concentration from evaporation, said Kirsten Cannon of the Bureau of Land Management.
The animals died in July 2007 of nitrate toxicity at the Tonopah Test Range, a proving ground for ballistics and bombing experiments. The area, within the U.S. Air Force's Nellis test and training range about 210 miles northwest of Las Vegas, was home to about 250 wild horses.
Some workers at the test range have said they suspect de-icing chemicals used to keep runways clear at the high desert location were responsible for the death of the horses, but the study discounts the possibility.
"It appears unlikely that human influence, such as contamination from urea or glycol-based deicing fluids, played a significant role in the high nitrogen concentrations," the Desert Research Institute said in its report for the land management agency.
The research institute said in its report that while human contamination was unlikely, it "cannot be definitively discounted" because deicers and other organic compounds quickly degrade over time. The institute collected 22 samples in February 2008, including seven of water and 15 of sediment.
Willis Lamm of the Alliance of Wild Horse Advocates said Monday that whether or not humans caused the high nitrate levels, it was important for officials to find out why the levels were high to prevent more animals from dying in the future.
"I think its important that a number of educated eyes look at this thing, not to discount the conclusions but to figure out what happened," Lamm said. "I'm a little anxious about just leaving a toxic pond out there - a multitude of animals are sensitive to high nitrates."
The study of the pond found nitrate levels of more than 3,000 parts per million, about 30 times more than acceptable levels. Cannon said the U.S. Environmental Protection Agency standard for nitrate levels in human drinking water is less than 10 parts per million, while acceptable levels for livestock are below 100 parts per million.
Cannon said the area was fenced off last year to keep horses away from the water. She said agency and Air Force officials will decide on a plan to manage the pond.
The study recommends either filling it in or monitoring the water levels to determine whether the horses should be allowed to get to the water.
The study said more horse deaths could result if wild horses were allowed to drink the water during times of drought when the pond's shrinking size concentrates the chemicals in it.
"We just have to be heads-up about it," Lamm said. "You don't leave toxic stuff lying around in your backyard."
