USGS

National Water-Quality Assessment Program

Nutrients in the Nation's Waters--Too Much of a Good Thing?

U.S. Geological Survey Circular 1136
WHAT PARTS OF THE NATION ARE MOST AFFECTED?

Nitrate concentrations in ground water were highest in the Northeast, Great Plains, and along the West Coast. The agricultural land in these regions is used for intensive row-crop farming, soils are generally well drained, and much of the land is underlain by unconsolidated material, such as sand and gravel, through which water and nitrate can move quickly. These areas receive some of the highest rates of irrigation and fertilizer application in the Nation. Use of inorganic fertilizer is particularly heavy in the Great Plains and West Coast States. Application of manure on croplands is extensive in the Northeast. California has the largest amount of irrigated cropland in the Nation (about 17 percent of the national total), and Nebraska has the second-largest amount. The combination of permeable soils, high rates of fertilizer application, and irrigation provides a large source of nitrate and a high potential for nitrate movement down to the water table.

Nitrate concentrations were generally higher in ground water than in streams. The primary exception was in the Midwest, where poorly drained soils restrict downward water movement and artificial drainage provides a quick path for nutrient-rich runoff to reach streams.

Nitrate concentrations generally are low in both ground water and streams in agricultural areas throughout the Southeast. Poor soil drainage, an abundance of soil organic carbon, warm temperatures, and high rainfall combine to produce conditions that restrict nitrate formation in soils and shallow ground water of this region. Forested buffer strips between and within agricultural fields also are common and might retain nutrients.

Concentrations also are low in ground water and streams of the arid Southwest. The sources of nitrate are limited in this region. Only a small percentage of the region is cultivated; most agricultural areas are used for rangeland and pasture.

The results of local investigation are of general value to many districts, and a knowledge of one locality must be derived from an examination of many other locations.---John Wesley Powell (second Director of the U.S. Geological Survey), 1886

(10K GIF)

fig13

Median nitrate concentrations in streams and shallow ground water in agricultural areas of the 20 NAWQA study units.

The MEDIAN is the middle value of a group of data; half of the data are lower than the median, and half are higher. In this report, we use the median to represent values generally expected for a group of nutrient concentrations.
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