National Water-Quality Assessment Program
Nonpoint and Point Sources of Nitrogen In Major Watersheds
of the United States
By Larry J. Puckett
[Electronic version of Water-Resources Investigations Report
94-4001]
Estimates of nonpoint and point sources of nitrogen were made for 107
watersheds located in the U.S. Geological Survey's National Water-Quality
Assessment Program study units throughout the conterminous United States.
The proportions of nitrogen originating from fertilizer, manure, atmospheric
deposition, sewage, and industrial sources were found to vary with
climate, hydrologic conditions, land use, population, and physiography.
Fertilizer sources of nitrogen are proportionally
greater in agricultural areas of the West and the Midwest than in other
parts of the Nation. Animal manure contributes large proportions of
nitrogen in the South and parts of the Northeast. Atmospheric
deposition of nitrogen is generally greatest in areas of greatest
precipitation, such as the Northeast. Point sources (sewage and industrial)
generally are predominant in watersheds near cities, where they may account
for large proportions of the nitrogen in streams. The transport of nitrogen
in streams increases as amounts of precipitation and runoff increase and is
greatest in the Northeastern United States. Because no single nonpoint
nitrogen source is dominant everywhere, approaches to control nitrogen must
vary throughout the Nation. Watershed-based approaches to understanding
nonpoint and point sources of contamination, as used by the National
Water-Quality Assessment Program, will aid water-quality and environmental
managers to devise methods to reduce nitrogen pollution.
Congress has charged the U.S. Geological Survey's National Water-Quality
Assessment (NAWQA) Program with conducting an assessment of the quality of
the Nation's water resources. The goals of the NAWQA Program are to describe
the status and trends in the quality of a large representative part of the
Nation's streams and ground-water resources and to provide a sound,
scientific understanding of the primary natural and human factors that
affect the quality of these resources (Leahy and Wilber, 1991). Since
its passage in 1972, the Federal Water Pollution Control Act (commonly
referred to as the Clean Water Act) has focused on efforts to reduce
discharges of pollutants from sewage-treatment plants and other point
sources. The ultimate goal of the Clean Water Act is to restore and
maintain the chemical, physical, and biological integrity of the Nation's
waters. However, in 1990, approximately 37 percent of the U.S. river
miles that were tested still did not fully support the uses designated
to them by the States (U.S. Environmental Protection Agency, 1992).
Until recently, little was known about the magnitudes of the various
nonpoint and point sources of nitrogen. This knowledge gap has made
it difficult to develop a national strategy for nonpoint-source pollution
prevention and control.
Many of the water-pollution-control measures instituted since 1972 have
focused on reducing discharges from point sources. However, one recognized
limitation of the Clean Water Act is its lack of controls on nonpoint-source
pollution (Knopman and Smith, 1993). These sources are called
"nonpoint" because they contribute pollutants to the receiving
rivers and streams at numerous and widespread locations, rather than at a
single discharge point. Commercial fertilizer and animal manure are two
important nonpoint sources of nitrogen. Both are applied to agricultural
land throughout the United States. Atmospheric deposition is another
important nonpoint source of nitrogen but receives much less recognition
than other sources.
Eutrophication of surface waters is the major environmental effect of
excessive nutrient inputs. The abundant growth of algae and aquatic
plants that may develop in nutrient-rich waters is often unsightly and
can restrict recreational uses of lakes and slow-moving rivers and streams.
When this vegetation decomposes, it consumes large quantities of oxygen,
which leads to fish kills, foul odors and tastes, and increased
water-treatment costs.
On an annual basis, about 11.5 million tons of nitrogen are applied as
commercial fertilizer for agricultural purposes throughout the United
States (figure 1 [14K GIF]).
Between 1945 and 1985, commercial nitrogen fertilizer
use increased twentyfold in the United States, from about 594,000 to almost
11.5 million tons per year (Alexander and Smith, 1990). A portion of the
fertilizer that is applied to fields returns to the atmosphere as ammonia
gas, and most of the rest is either taken up by plants or converted to
nitrate in the soil. Consequently, most of the dissolved nitrogen that
enters streams from runoff of agricultural fertilizer occurs as nitrate.
Nitrate is a very mobile form of nitrogen---it is not readily retained by the
soil and is highly soluble in water. Because of the mobility of nitrate,
farmers may apply it in greater quantities than crops require. Also, given
its high solubility, nitrate may be washed into adjacent streams by rain, or
it may leach into the ground-water system.
Each year the 7 billion farm animals in this country produce millions of tons
of manure that contains an estimated 6.5 million tons of nitrogen
(figure 2 [15K GIF]).
If not properly handled and disposed of, this manure can add to the nitrogen
in streams. Where farm animals are allowed to roam freely, large amounts of
nitrogen are distributed over the landscape and represent a true nonpoint
source of nutrients. However, where animals are confined to feedlots,
barns, or sheds, they become more of a point-source nitrogen problem. In
these situations, large quantities of manure commonly are concentrated in
one location, and the nutrients that leach to ground and surface waters from
storage areas may pose a water-quality problem.
The nitrogen in atmospheric deposition originates primarily from the
combustion of fossil fuels, such as coal and oil
(figure 3 [10K GIF]). Atmospheric
deposition may be in a wet form as rain, snow, hail, fog, and freezing
rain or in a dry form as particulates, gases, and droplets. The largest
sources are point sources---coal- and oil-burning electric utilities and
large industries that together account for about 53 percent of nitrogen
emissions. However, automobiles, trucks, buses, and other forms of
transportation account for approximately 38 percent of nitrogen emissions.
Atmospheric inputs, in particular, have been largely ignored within the
context of nonpoint-source pollution because they do not fit the traditional
definition of a nonpoint source. For example, releases of nitrogen into the
air from point sources, such as the com-bustion processes of powerplants and
industries, are called nonpoint sources of water pollution when that nitrogen
reaches water bodies through precipitation. More than 3.2 million tons of
nitrogen are deposited in the United States each year from the atmosphere
(figure 4 [10K GIF]).
This means that about 54 percent of the nitrogen emitted from
fossil-fuel-burning plants, vehicles, and other sources in the United States
(Sisterson, 1990) is deposited on U.S. watersheds.
Point sources of nitrogen consist primarily of a variety of large and small
industries and publicly and privately owned wastewater-treatment plants.
They are distinguished from nonpoint sources in that they discharge directly
into streams at a discrete point. The most recent nationwide estimates of
nonpoint- and point-sources of pollution were compiled in 1984 (Gianessi and
Peskin, 1984). These estimates indicated that during the period 1978 through
1981, point sources discharged approximately 1.3 million tons of nitrogen per
year compared to 21.4 million tons from nonpoint sources. Industrial sources
and municipal sewage-treatment plants accounted for about 26 and 74 percent,
respectively, of the point-source total. However, point sources represented
only 5.7 percent of the total nitrogen added to the environment, whereas
agricultural nonpoint sources accounted for 93.5 percent.
The contribution of nonpoint sources to the total nitrogen added to major
watersheds of the United States varies nationally from nearly zero in some
predominantly urban watersheds to as much as 100 percent in agricultural
and other rural watersheds (figure 5 [18K GIF]).
In spite of this variability, some
broad generalizations can be made. In the Western United States, where
agriculture is intensive, commercial fertilizers are the dominant source of
nitrogen. Atmospheric deposition is the second most important source,
particularly in those western watersheds devoted to forestry or in remote
headwater areas. In watersheds of the Central and the Southeastern United
States, commercial fertilizers are the dominant nitrogen source, again due
to intensive agricultural. Animal manure is an important secondary source
in the Central and the Southeastern watersheds as a result of cattle, hog,
poultry, and other livestock production. In the Northeastern United States,
where agriculture is less intensive, atmospheric deposition is the dominant
source of nitrogen in most watersheds, and animal manure is the second most
important source.
Areas of the country with the greatest rainfall and atmospheric pollution
also have the greatest amounts of nitrogen deposited from the atmosphere
(figure 4 [10K GIF]).
For example, in the Northeastern United States, atmospheric
deposition of nitrogen in rain, snow, and other forms accounts for about
one-third of the total nitrogen inputs to watersheds. The amount of nitrogen
carried by streams also is generally greatest where runoff is greatest.
Runoff is greatest in the Northeastern United States as are the amounts of
nitrogen transported in streams (figure 6 [17K GIF]).
The proportion of in-stream nitrogen accounted for by point sources is also
variable in the NAWQA Program watersheds
(figure 7 [16K GIF]). Streams near large cities
such as Denver in the South Platte River Basin, Colorado, Dallas-Fort Worth
in the Trinity River Basin, Texas, and Atlanta in the
Appalachicola-Chattahoochee-Flint River Basin, Georgia
(figure 7 [16K GIF]) receive
relatively large proportions (up to 77 percent) of their nitrogen from
point sources, such as sewage-treatment plants. However, point-source
inputs of nitrogen account for less than 10 percent of the
in-stream nitrogen in more than 50 percent of the watersheds studied, and
less than one-half of the nitrogen in 90 percent of the watersheds studied.
This means that in more than 90 percent of those watersheds, nonpoint sources
of nitrogen account for more than one-half of the nitrogen in streams.
In watersheds that are largely agricultural, such as the Red River of the
North in North Dakota and Minnesota and the Palouse River in Washington,
nitrogen from commercial fertilizers accounts for 84 and 87 percent,
respectively, of the total nitrogen added to the watersheds. Similarly,
in watersheds where animals are raised in large quantities, such as the
Susquehanna River in Pennsylvania and the White River in Arkansas, nitrogen
from animal manure accounts for 54 and 56 percent, respectively, of the
total nitrogen added to the watersheds.
Because dominant nitrogen sources vary among watersheds, it would be
difficult to implement a single management strategy for nitrogen reduction
that would be effective throughout the Nation. The proportion of nutrients
deposited in the watershed that eventually find their way into streams
also is variable. In most of the watersheds examined, only a small
percentage of the nitrogen deposited in the watershed immediately reaches
and is measured in the stream---the rest is removed with harvested crops,
incorporated into woody vegetation, returned to the atmosphere by biological
and chemical processes, or transported to the ground-water system.
If water enters the ground-water system, then it may take a long time
before the water is discharged to a stream. Therefore, the effects of
current land-use practices may continue to show up in streams years or
even decades from now.
Proportions of nonpoint and point sources of nitrogen vary from watershed
to watershed throughout the United States, not necessarily
following political boundaries. These proportions vary as a function of
land use, population, hydrologic
conditions, climate, and physiography. This variability suggests that in
addition to efforts to reduce nitrogen releases from all sources to the
extent feasible, pollution-prevention plans need to be developed on an
individual watershed basis. In this way, the importance of sources, as
well as differences in climate, soils, and water-management practices, can
be taken into consideration. Developing a more complete understanding of
which sources have the greatest effects on water chemistry in watersheds,
and the timing of those effects, is essential to providing effective
prevention and control programs.
Point sources are commonly a major source of nitrogen to streams near large
urban areas. In many areas of the United States waste-treatment plants
and other urban point sources still release large amounts of nitrogen to
downstream river reaches. Localized effects of these point sources can
exceed those of nonpoint sources scattered throughout the watershed.
Therefore, concentrating pollution-prevention efforts entirely on nonpoint
sources may not remedy water-quality problems near major cities.
Atmospheric deposition of nitrogen can be a major source of nitrogen that
is not addressed by water-quality legislation. Because most of the
sources of atmospheric deposition are point sources, this form of pollution
is currently controlled by reducing nitrogen oxide emissions. Commonly these
point sources are located outside of the political boundaries of watersheds
that receive this atmospherically deposited nitrogen and, therefore, may not
be controlled through State and local government regulations. Recent
amendments to the Clean Air Act (1990) have mandated a 2-million-ton
reduction in nitrogen oxide emissions (approximately 10 percent) below 1980
levels by 2000. However, after 2000, nitrogen oxide emissions are projected
to increase again, which means that atmospheric deposition of nitrogen will
remain a factor that needs to be considered in nitrogen-management plans.
Eutrophication of large rivers, lakes, reservoirs, estuaries, and shallow
marine environments is the most immediate environmental consequence of
nitrogen pollution in surface waters. Estuarine and shallow marine
environments, such as the Gulf of Mexico, Puget Sound, Chesapeake Bay,
Albemarle and Pamlico Sounds, Long Island Sound, and others, are considered
to be sensitive to large inputs of nitrogen. Environmental planners for the
Chesapeake Bay have found it necessary to control the cumulative amount of
nitrogen from all watersheds that contribute to the bay as part of their
management plan (Fisher and Oppenheimer, 1991). Water-quality managers in
other areas of the Nation will probably find that a similar approach is
required in their watersheds. For example, nitrogen discharged to the Gulf
of Mexico from the Mississippi River may originate as fertilizer, animal
manure, or atmospheric deposition in Minnesota, Arkansas, or Ohio.
Quantitative information is meager or unavailable for several potentially
important sources of nitrogen. Leachate from septic systems, urban
runoff, combined sewage overflow, and contaminated ground water are several
potentially important nitrogen sources that are difficult to quantify, either
because they are poorly understood or because little data on them exists.
It is important to determine the amount of nitrogen contributed by these
undocumented sources in watersheds to target those that contribute most to
nitrogen-pollution problems. For example, it might be of little value to
mandate reductions in commercial fertilizer application rates if combined
sewage overflows contribute a proportionally greater amount of the total
nitrogen to a stream.
Data needed to develop pollution-prevention plans are either often not
being collected or not readily available. A significant limitation in
determining the importance of nonpoint and point sources of nitrogen or
other contaminants is the general lack of coordination between point-source-
effluent monitoring and water-quality monitoring pro-grams (Zogorski and
others, 1990). For example, most wastewater-treatment plants are required to
monitor their effluent for ammonia nitrogen, but not for nitrate or organic
nitrogen. Therefore, the total amount of nitrogen that enters critical
estuaries and water bodies must be estimated. Source estimates could be
improved by mandating that chemical-constituent data required by
effluent-monitoring programs be consistent with those required by
stream-water-quality monitoring programs.
The U.S. Geological Survey implemented the NAWQA Program in 1991 to provide
water-quality information that will be useful to policymakers and managers
at the national, State, and local levels. To meet this goal, the NAWQA
Program is currently assessing the status and trends in the quality of a
large representative part of the Nation's streams and ground waters.
Liaison committees of interested national, State, and local managers and
scientists meet regularly with the NAWQA Program staff to ensure that program
findings are relevant to the needs of water-resources managers, provide
review of products, provide assistance in field activities, and identify
management implications of findings. The NAWQA Program also is designed to
provide a sound, scientific understanding of the natural and human factors
that affect water quality (Leahy and Wilber, 1991). In addition to
determining the sources and assessing the effects of nitrogen and other
nutrients on water quality, the NAWQA Program investigators are evaluating
the effects of pesticides and volatile organic compounds in water and
collecting aquatic biological data as indicators of overall water-quality
conditions.
The NAWQA Program consists of studies of 60 major river and aquifer systems,
which are referred to as "study units"
(figure 8 [26K GIF]). As a group, the
60 study units account for about one-half of the land area of the
conterminous United States and 60 to 70 percent of its water use and
population served by public water supply. The NAWQA Program is using a
watershed-based approach to assess the quality of the Nation's ground and
surface waters. The NAWQA Program results have already influenced State
legislation and management plans for pesticides and organic compounds in
Kansas, Washington, and elsewhere.
The water-quality and much of the nutrient source data in this report
were retrieved from electronic data bases, verified, and analyzed by
the following U.S. Geological Survey personnel: Scott K. Anderholm,
Clyde E. Asbury, Richard W. Bell, Hugh E. Bevans, Joel D. Blomquist,
Bernadine A. Bonn, Gary R. Buell, Gregory M. Clark, James C. Ebbert,
Suzanne R. Femmer, Gary T. Fisher, Douglas A. Freehafer, Elizabeth A. Frick,
Karen E. Greene, Robert A. Hainly, Charles R. Kratzer, David W. Litke,
Jeffrey D. Martin, Gerard McMahon, Patrick J. Phillips, David C. Reutter,
Dale M. Robertson, John K. Stamer, Lan H. Tornes, Peter C. Van Metre,
Michael D. Woodside, and Marc J. Zimmerman. Four of the illustrations were
compiled and formatted from electronic data bases by Kerie J. Hitt.
- Alexander, R.B., and Smith, R.A., 1990, County-level estimates
of nitrogen and phosphorus fertilizer use in the United States, 1945 to
1985: U.S. Geological Survey Open-File Report 90-130, 12 p.
- Fisher, D.C., and Oppenheimer, M., 1991, Atmospheric nitrogen
deposition and the Chesapeake Bay Estuary: Ambio, v. 20, p. 102-108.
- Gianessi, L.P., and Peskin, H.M., 1984, An overview of the RFF
Environmental Data Inventory---Methods and preliminary results: Resources
for the Future, Washington, D.C., 111 p.
- Kohout, E.J., Miller, D.J., Nieves, D.S., Saricks, C.L., and
Stodolsky, F., 1990, Month and state current emission trends for
NOx, SOx, and VOC---Methodology and results:
Argonne National Laboratory.
- Knopman, D.S., and Smith, R.A., 1993, Twenty years of the Clean
Water Act---Has U.S. water quality improved?: Environment, v. 35, no. 1,
p. 16-41.
- Leahy, P.P., and Wilber, W.G., 1991, National Water-Quality
Assessment Program: U.S. Geological Survey Open-File Report 91-54, 2 p.
- Sisterson, D.L., 1990, Detailed SOx-S and
NOx-N mass budgets for the United States and Canada,
in Acidic deposition---State of Science and Technology, Appendix
8A: National Acid Precipitation Assessment Program, Washington, D.C., p.
8A-1--8A-10.
- U.S. Environmental Protection Agency, 1992, National Water
Quality Inventory---1990 report to Congress: U.S. Environmental Protection
Agency,
p. 5.
- Zogorski, J.S., Blanchard, S.F., Romack, R.D., and Fitzpatrick,
F.A., 1990, Availability and suitability of municipal wastewater information
for use in a National Water-Quality Assessment---A case study of the Upper
Illinois River Basin in Illinois, Indiana, and Wisconsin: U.S. Geological
Survey Open-File Report 90-375, 68 p.
