Chapter 3 - Nitrate in Ground Water in the United States

Abstract

Many activities of modern society, including agricultural land use, contribute nitrate contamination to ground water. Natural background areas commonly show <2 mg/L NO3-N in shallow ground water; agricultural areas often exhibit > 10 mg/L, at least seasonally. Expanded use of nitrogen fertilizers has profitably increased agricultural production, but it has also had an adverse impact on water quality in many areas. In responsive ground-water settings, nitrate concentrations have increased concurrent with the increased use of N-fertilizers. Wells and ground water exhibiting nitrate contamination have been noted in every state in the USA. The major areas exhibiting problems with nitrate contamination of ground water are: 1. areas of widespread grain production (particularly corn), marked by intensive row-cropping and heavy fertilization; 2. areas with locally intensive animal feeding and handling operations; and, 3. areas of irrigation and fertilization of vegetable and specialty crops, particularly shallow-rooted vegetable crops on sandy soils. Ground water contributes to the nitrate contamination of surface waters through base-flow to streams and lakes. Nitrate loads have increased in many streams, particularly from the midwestern grain-belt eastward, since 1974, in spite of reductions in point-source loads. These increases were highly correlated with many measures of agricultural activity. Nitrate concentrations in ground water typically show significant variability in many dimensions: with depth, spatially, and over time; and these variations are often inter-related. Recognition of this variability is important to understand the extent of the problem and to design programs to mitigate it. At present, nitrate contamination is only evident in shallow portions of the ground-water flow system. This is, in part, a function of time; relative to the dimensions of the ground-water system. In many areas, particularly in deeper portions of the ground-water system the impact of recent nitrate leaching may not be noted for several decades.