Forensic scientists solved the mystery of what killed a large portion the the wild mustang herd in southern Nevada using a technique called stable isotope ratio mass spectrometry.

In July 2007, 71 horses of the 250-head herd of wild horses were found dead near Main Lake depression in southern Nevada. Testing performed by the California Animal Health and Food Safety Laboratory system found high levels of nitrate (NO_3-) and nitrite (NO_2-). The horses were negative for botulin, anatoxin-a, microcystins, and testing for organic compounds was also negative. The lake itself–which the herd commonly used–had high nitrate levels. Other ions such as chloride and sulfate were also markedly high.

After catching wind of the fallen horses, a handful of public groups speculated that the horses had suffered a repeat of the 1988 dumping of nitrate-containing deicing fluids which resulted in the accidental poisoning of 61 wild horses near Cactus Flat, Nev.

Nitrate is formed naturally via biological transformation of ammonia (NH⁴⁺) in soil, sediments and water. It is also synthetically produced by humans in large quantities used in such products as fertilizers and homemade explosives