The western part of the Salt River Valley area (WSRV) is a large alluvial basin in central Arizona. The WSRV includes the city of Phoenix and much of the surrounding metropolitan area, which had a population of 2.1 million residents in 1990. In addition to being a major population center, large parts of the valley have been intensively farmed since the early part of the century. Both agricultural and non-agricultural activities have affected the quality of ground water in the basin. Ground water in several areas of the valley exceeds the 10 mg/L (as N) maximum contaminant level set by the U.S. Environmental Protection Agency (USEPA). Although in some areas high nitrate concentrations have a natural source, in other areas nitrates are the result of one or more human activities.