Naturally occuring nitrates killed 71 wild horses on Nellis

By Mary Manning

Monday, July 21, 2008 | 2:50 p.m.

The nitrates that caused the deaths of 71 wild horses last summer north of Las Vegas came from natural causes, the Bureau of Land Management said today.

The horses were found around a shallow pond of water on the Air Force's Nellis Test and Training Range in June.

According to a study conducted by the Desert Research Institute, nitrate concentrations soared because of evaporation and because of animal waste and natural nitrogen deposited in soils from the. The study concluded high nitrates led to the horses' deaths.

In July 2007 initial test results found high levels of nitrates in samples taken from water in the shallow pond and from blood serum and eye fluid taken from dead horses. The results indicated nitrate levels more than 3,000 parts per million.

The U.S. Environmental Protection Agency standard for human drinking water is less than 10 parts per million.

Livestock can tolerate higher levels, but problems occur when levels exceed 400 parts per million and acceptable levels should be below 100 parts per million, the Environmental Protection Agency.

The area where the dead horses were found is called Cactus Flat. The depression where the nitrate-laced water was found was fenced last year and remains fenced so horses cannot drink out of that source, the Bureau of Land Management said.

The Bureau of Land Management and the Air Force must decide how to manage the area to prevent future wild horse deaths.