Because of the lag between the July 2007 horse deaths and the sample collection effort in February when the watering hole was filled with about six feet of water from rain and snow melt, the team couldn't eliminate the possibility that nitrate-laced de-icing fluids from operations at a nearby, military airfield contributed to the horses' deaths.

"No samples collected for this study contained detectable amounts of glycol de-icers or typical organic de-icer additives," the team reported in its conclusions.

"However, because of the time elapsed between the wild horse deaths in July 2007 and sampling by DRI in February 2008, the negative results for these analytes does not definitely affirm that glycol-based de-icers were not present in the main lake depression at the time of the wild horse deaths," the report reads.

The $80,630 study was conducted for the Bureau of Land Management and paid for by the BLM, the Air Force and the Department of Energy -- agencies that have a stake in the restricted Tonopah Test Range, where weapons logistics tests are conducted inside the sprawling Nellis Air Force Range, about 30 miles southeast of Tonopah.

Kevin Dye, a former Air Force technical sergeant who worked at the Tonopah range in the 1990s, estimates that 21,000 pounds per year of urea, a de-icing compound high in nitrate, was used on the airfield's runway from 1982 to 1996.

Dye said the fluids routinely streamed into the desert unchecked within a mile or two of where the horses died last year and not far from where 61 died in 1988 from drinking water tainted with nitrogen compounds from ammonia.

Dye reacted on Thursday to Hershey's comments, pointing to sampling data that showed high levels of other contaminants such as lead, arsenic, barium, uranium and aluminum, which is a catalyst sometimes used to enhance the blasting power of ammonium nitrate.

Aluminum in the horses' watering hole, described in the study as the "main lake depression," was more than 100 times in excess of the Environmental Protection Agency's secondary drinking water standard for humans.

Although the aluminum standard is not enforceable because aluminum is more of a taste or smell factor rather than a health risk, the data still indicate very high concentrations of aluminum.

"The extremely high aluminum levels were not explained. There are too many unanswered questions that they should have addressed but didn't," Dye said.

Lead levels were three times greater than the EPA's safe drinking water standard, even though the pond is not a drinking water source for humans.

Likewise, arsenic was three times the standard; barium was 4.8 times the standard; and uranium levels were 4.4 times more than the standard allows for people.

Hershey acknowledged that the main lake depression had at least something to do with explosive materials. It was dug 20 years ago by the Department of Energy.

"They called it a gun pit," he said.

Nitrate levels in preliminary water tests taken from the pond shortly after the dead horses were found were 66 times in excess of the safe drinking water standard for humans and 30 times acceptable levels for livestock.

Nevertheless, Hershey said, "We didn't find any evidence of nitrates from explosives like TNT."

That finding was derived from so-called "isotopic tests," which are based on the ratio of naturally occurring nitrogen isotopes. The signature for nitrogen in man-made urea, for example, is 24 times less than the measurement for nitrogen in manure.

As such, Hershey's team concluded the nitrate contamination most likely came from natural sources that were concentrated by evaporation of the pond's water in the heat of summer.

Sen. Harry Reid, D-Nev., who asked the Air Force and Interior Departments to conduct the study in August, a month after the 71 carcasses were found, intends to further examine the Desert Research Institute study.

Reid and his staff "are taking a close look at this report and will continue to monitor the situation," his spokesman, Jon Summers, said in a statement Thursday.

Contact reporter Keith Rogers at krogers@ reviewjournal.com or 702-383-0308.