LAS VEGAS -- A study released by the Bureau of Land Management shows that high levels of nitrates present in water in a depression near Cactus Flat were not from industrial sources.

The high nitrates were from a combination of evaporative concentration and animal waste and natural soil nitrogen from air deposition.

Last June, 71 wild horses on the Nevada Wild Horse Range, which is within the U.S. Air Force's Nellis Test and Training Range, died from nitrate poisoning at the site of the depression.

The study conducted by Desert Research Institute determined that the two sources of high nitrates that caused the deaths of the horses were:

* An increase in the concentration of naturally occurring nitrate due to high rates of evaporation of the water, and

* The chemical conversion of nitrogen to nitrate that occurs through a process called nitrification in natural materials, including animal waste and natural soil nitrogen produced by aerial deposition.

In July 2007, initial test results found high levels of nitrates in samples taken from water from the depression as well as in the deceased horses' blood serum and ocular fluid.

These indicated nitrate levels of more than 3,000 parts per million (ppm).

The U.S. Environmental Protection Agency standard for drinking water for human consumption is less than 10 ppm. Livestock can tolerate higher levels than humans but problems are known to occur when levels exceed 400 ppm, and acceptable levels should be below 100 ppm.

DRI was contracted to determine the source of nitrates through water, sediment and soil samples.

The Cactus Flat depression remains fenced since last year so horses do not drink out of the area. The study recommends a few management options, particularly to fill in the depression, or control access to it, and monitor water levels and chemistry.

A joint decision on future management of the depression will be made by BLM and Air Force.

The full report is available online at http://www.blm.gov/nv/st/en/fo/lvfo/blm_programs/wild_horse_and_burro/_2008_nitrate_test.html.