RISK OF NITRATE IN GROUNDWATERS OF THE UNITED-STATES - A NATIONAL PERSPECTIVE

Nitrate contamination of groundwater occurs in predictable patterns, b ased on findings of the U.S. Geological Survey's (USGS) National Water Quality Assessment (NAWQA) Program. The NAWQA Program was begun in 19 91 to describe the quality of the Nation's water resources, using nati onally consistent methods. Variables affecting nitrate concentration i n groundwater were grouped as ''input'' factors (population density an d the amount of nitrogen contributed by fertilizer, manure, and atmosp heric sources) and ''aquifer vulnerability'' factors (soil drainage ch aracteristic and the ratio of woodland acres to cropland acres in agri cultural areas) and compiled in a national map that shows patterns of risk for nitrate contamination of groundwater. Areas with high nitroge n input, well-drained soils, and low woodland to cropland ratio have t he highest potential for contamination of shallow groundwater by nitra te. Groundwater nitrate data collected through 1992 from wells less th an 100 ft deep generally verified the risk patterns shown on the natio nal map. Median nitrate concentration was 0.2 mg/L in wells representi ng the low-risk group, and the maximum contaminant level (MCL) was exc eeded in 3% of the wells. In contrast, median nitrate concentration wa s 4.8 mg/L in wells representing the high-risk group, and the MCL was exceeded in 25% of the wells.