Pasturing cattle above groundwater contaminated with nitrates doesn’t add to the problem, provided the ground isn’t fertilized.
That’s according to Lloyd Owens, a soil scientist at USDA’s ARS North Appalachian Experimental Watershed Laboratory in Coshocton, OH. He found that it doesn't make any difference in groundwater nitrate levels whether cattle are on the pasture or not. But he says pastures with high nitrate levels can't be fertilized for at least a few years, until the levels drop sufficiently.
EPA guidelines for drinking water stipulate 10 parts per million (ppm) nitrate-nitrogen as the maximum allowable safe level for drinking water. Owens studied problem pastures with groundwater nitrate-nitrogen levels of 13 to 26 ppm, caused by heavy experimental fertilization for 11 years before the study. He stopped fertilizing for a seven-year study to see if that would bring nitrate levels down to safe levels. For comparison, he let cattle graze on two pastures, and fenced them out and made hay from two other pastures.
In the groundwater underneath three pastures, the nitrate-nitrogen levels dropped below 10 ppm within three years; after five years, the levels below all four pastures fell to 2 to 4 ppm.
Because of soil conditions, some fields are more prone to high nitrate levels. Fertilizing every year can eventually turn them into problem fields, says Owens.
Withholding fertilizer caused only a slight decrease in grass growth, he reports.