Just a little bit below the top 6 feet of ground – an area ecologists have studied extensively – is a large layer of naturally occurring nitrate. Nobody knew it was there. Nobody knows why it has stayed in the ground, instead of being taken up by plants as a key nutrient.
“We were drilling at the Nevada Test Site looking for a different chemical, and there was this very strange (instrument registration) peak,” said Michelle Walvoord, who was a graduate student at New Mexico Tech when she made the discovery. “I was very surprised when we learned there was a large inventory of nitrate there,” said Walvoord, now a hydrologist with the U.S. Geological Survey. “I thought for sure somebody else must have noticed it before, but we did a lot of background checking, and nobody had.”
The discovery is significant because nitrate is also a common groundwater pollutant. As she studied the phenomenon, Walvoord learned conventional irrigation methods in agriculture – not more advanced drip irrigation systems – push the layer of naturally occurring nitrate deeper into the ground until it mixes into the water table. The U.S. Geological Survey studied three desert areas in Nevada to see how the nitrate behaved. In an area that had been irrigated for about 100 years, the nitrate had dropped all the way into the water table 100 meters beneath the ground.