FEATURE
Identifying the Major Sources of Nutrient Water Pollution A national watershed-based analysis connects nonpoint and point sources of nitrogen and phosphorus w i t h regional land use and other factors. LARRY J.
S
ince the passage of the Clean Water Act (CWA) in 1972, more than $540 billion has been spent on water pollution controls. In spite of this massive commitment of funds, in 1992 approximately 44% of U.S. river miles tested still did not fully support the uses designated by the states (i). One wellrecognized problem is the lack of controls in the CWA for nonpoint-source pollution (2). Preventing or controlling pollution from nonpoint sources has been stymied, in part, because little was known about the relative magnitudes of nonpoint and point sources of nutrients at the national level. Only recently has it been possible to estimate the major sources of nutrients entering watersheds and relate these inputs and resulting stream loads to land use patterns and regional settings. Analysis of these new data reveals that nutrient inputs into watersheds vary according to land use practices, that atmospheric nitrogen may be a more important source of nutrient contamination than previously believed, and that in some localities point sources are still the major water quality problem. These findings underscore the need for individualized watershed management plans for preventing and controlling water pollution in the United States. The CWA has traditionally focused on reducing discharges of pollutants, particularly nitrogen and phosphorus, to surface waters from sewage treatment plants and other point sources. Because of the Act's point-source focus, nearly 90% of the monies allocated to water pollution prevention have been targeted at point sources (1). Congress has been considering revisions to the Act since 1993, but it is unclear at this time how nonpoint-source pollution will be addressed. In spite of Congressional uncertainty, many state and federal water quality programs have begun to tackle nonpoint sources by focusing on watershedbased management plans. However, to develop ef-
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fective plans, it is necessary to identify and quantify the dominant sources of nonpoint-source pollution at the watershed scale, and then eliminate, to the greatest extent possible, those sources contributing most to water quality problems. Using information from various databases, such as the U.S. Geological Survey's National Water Quality Assessment program, Census of Agriculture, and the National Trends Network, it is now possible to estimate the relative magnitudes of the major nonpoint and point sources of nutrients. The estimates allow three key points to be addressed: the proportion of nonpoint and point sources of nutrients in U.S. watersheds, the continuing importance of point sources of nutrients, and the relative importance of atmospheric inputs as a nonpoint source of nitrogen.
Identifying the major sources Among the various sources of water quality impairment, agricultural practices are ranked as the most important factor for rivers and lakes and third in importance for estuaries (J). Going one step further and delineating agricultural practices with respect to specific pollutants, nutrient contamination is identified as the second most important contribution to river and lake pollution and first in importance for estuaries. Commercial fertilizer is the primary agricultural nonpoint source of nitrogen and phosphorus. Between 1945 and 1993, the use of nitrogen in commercial fertilizers in the United States increased 20fold, rising from about 0.5 million to nearly 10.3 million metric tons per year. Phosphorus use has increased as well, from about 0.5 million to nearly 1.8 million metric tons per year (Figure 1). Much of the fertilizer is applied in the upper Midwest "corn belt" states (Figure 2). Studies have estimated that farmers may apply 24 to 38% more fertilizers than crops require because of uncertainties 0013-936X/95/0929-408AS09.00/0 © 1995 American Chemical Society
Agricultural practices are the most important factor in water quality impairment for rivers and lakes (above), but in large urban centers point sources remain a major nutrient source (below).
associated with weather and soil nutrient status (3,4). Animal manure is another agricultural pollution source. Each year in the United States the manure from 7.5 billion farm animals produces an estimated 5.9 million and 1.8 million metric tons of nitrogen and phosphorus, respectively. The upper Midwest has the greatest manure nutrient input (Figure 3). Where farm animals are allowed to graze freely, die manure is distributed over the landscape and represents a nonpoint source of nutrients. However, where animals are confined to feedlots, barns, or sheds, they may become more of a point source nutrient problem. If the nutrient content of applied ma-
nure is not accounted for when computing crop requirements, the excess may move into surface and ground waters. Atmospheric inputs have been largely ignored as nonpoint-source pollution because they originate primarily at point sources. For example, releases of nitrogen oxides into the air from point sources become nonpoint sources of water pollution when that nitrogen reaches water bodies as rainfall. More than 2.9 million metric tons of nitrogen are deposited in the United States each year from the atmosphere (5), primarily in the northeastern states (Figure 4). About 53% of these nitrogen emissions come from coal- and VOL. 29, NO. 9, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY • 4 0 9 A
FIGURE 1
Changes in fertilizer nitrogen and phosphorus use U.S. data show that from 1945 to 1993 nitrogen use jumped 20-fold and phosphorus use tripled.
TABLE 1
Watershed classification by dominant land use Values under the land use classifications are the percentage of each category in the watershed, except for population density, which is persons per square kilometer. Dominant land use classification
Crop and pasture
Agriculture Urban Forest Mixed
> 40 < 40 < 10 38 < 40 > 50
