The nitrogen ion, NO3-, is derived from nitric acid and is an important source of nitrogen in fertilizers. Nitrate pollution of drinking water, shallow wells being particularly vulnerable, is of concern because infants are especially sensitive. A nitrate drinking water standard has been set under the Safe Drinking Water Act. An Environmental Protection Agency national survey of drinking water wells conducted from 1988 to 1990 indicated that 2.4% of rural domestic wells contained nitrate at or above the 10 mg/L standard. Higher rates of contamination have been found in areas of high vulnerability; for example, surveys along the upper Des Moines river indicate that 20 to 30% of wells exceed the standard.