Researchers agree that nitrate concentrations in unpolluted groundwater seldom exceed the National Public Health Service 10 ppm NO 3 -N standard. Recent United States Geological Survey data show that almost every state has areas where nitrate levels exceed the standard. However, a recent Environmental Protection Agency study found that only 2. 7 percent of rural wells exceeded the standard. Several recent studies in the Great Plains and Midwest have found localized areas where nitrate concentrations in groundwater have been increasing. Sources of Nitrate in Groundwater Cultivated soil usually contains between 1,500 and 4,000 pounds of organically bound nitrogen per acre.
Plants are unable to use organic nitrogen directly.
The forms of organic nitrogen fertilizers used in agriculture include animal manures, human wastes, composts, sewage sludge, legume crops and green manure crops. Plants do not necessarily use all the nitrate in chemical fertilizers or all the nitrate produced when organic matter decomposes. When the nitrogen supply is greater than the amount plants use, nitrate can accumulate in the soil and be lost from the system. In today's agriculture, with greater nitrogen inputs for higher crop yields, efficiencies of nitrogen use may be lower and the potential for losses may increase. In some natural ecosystems, nitrogen is almost always in short supply, and little is lost from the system.