Date of this Version
Nitrogen, particularly in the form of nitrate, is the most common contaminant in aquifer systems (Freeze and Cherry, 1979). Hallberg (1989) points to agriculture as the most substantial anthropogenic source of nitrate, and Keeney (1986) suggests that this is caused by the intensive and extensive land-use activities associated with crops and animal production. The discussion of the occurrence of nitrogen in groundwater beneath agricultural systems is presented by examining the factors influencing aquifer vulnerability to nitrogen contamination, and by characterizing the geographic distribution of groundwater contamination by nitrogen. Factors that influence aquifer vulnerability are presented in the context of exposure to nitrogen sources from general agricultural systems and hydrologic conditions that facilitate transfer of those sources to groundwater. This analysis focuses on the occurrence of nitrate in the United States because data are readily available on many variables needed for such an analysis. Data from the US Geological Survey National Water Quality Assessment Program (NAWQA, Gilliom et al., 1995); the Census of Agriculture; the National Resources Inventory; and the State Soils Geographic Database [STATSGO (United States Department of Agriculture, 1994)] provide an unique opportunity to directly relate nitrogen in groundwater to agricultural systems at a national or continental scale. Results of international research and monitoring are introduced to compare the occurrence of nitrogen in similar agricultural and hydrologic systems supported by literature and data available from the United Nations Food and Agriculture Organization (FAO).