The Associated Press wrote last week that the BLM said the deaths of 71 wild horses on a military site in Nevada was not caused by elevated nitrate levels in water sources.

Testing of a watering hole showed that the nitrate levels came from a combination of animal waste, natural soil nitrogen and concentration from evaporation, Kirsten Cannon of the Bureau of Land Management told the A.P.

The animals died in July 2007 of nitrate toxicity at the Tonopah Test Range, a proving ground for ballistics and bombing experiments. The area is home to 250 wild horses.

"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 BLM. 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 told the AP, "I think it’s important that a number of educated eyes look at this thing, not to discount the conclusions but to figure out what happened. I'm a little anxious about just leaving a toxic pond out there — a multitude of animals are sensitive to high nitrates."

The study found nitrate levels of more than 3,000 parts per million, about 30 times more than acceptable levels. It recommends filling in the pond or monitoring the water levels to determine whether the horses should be allowed to get to the water.

"We just have to be heads-up about it," Lamm said. "You don't leave toxic stuff lying around in your backyard."