- U.S. Geological Survey
Environment Canada Climate Information Branch
Nonpoint and Point Sources of Nitrogen
Excerpted from: Puckett, L.J., Nonpoint and point sources of nitrogen in major watersheds of the
United States: U.S. Geological Survey, National Water-Quality Assessment Program pamphlet.
Why Be concerned about nitrogen?
Nitrogen is a public health concern when concentrations exceed 10 micrograms per liter, however,
this is more common in ground water than in surface water. Other potential effects of nitrogen
pollution include ammonia toxicity to aquatic life and oxygen depletion when unoxidized forms
of nitrogen are dischrarged into streams. Eutrophication of surface waters is te 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 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, amd increased water-treatment costs.
What are the major sources of nitrogen?
The major nonpoint sources of nitrogen are fertilizer, animal manure, and atmospheric deposition.
Point sources consist primarily of a variety of large and small industries and publically and
privately owned wastewater-treatment plants. They are distinguished from nonpoint sources in
that they discharge directly into streams at a discrete point. On an annual basis, about 11.5 million
tons of nitrogen are applied as commercial fertilizer for agricultural purposes throughout the
United States. 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. More than 3.2 million tons of
nitrogen are deposited in the United States each year from the atmosphere. During the period
1978 through 1981, point sources discharged approximately 835,000 tons of nitrogen per year.
Point sources are often a major source of nitrogen pollution to streams near large urban areas.
Do the proportions of nonpoint and point sources of nitrogen vary throughout the United States?
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.
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.
What environmental problems are caused by nitrogen pollution?
Eutrophication of large rivers, lakes, reservoirs, estuaries, and shallow marine environemnts 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 sensitive to large inputs of nitrogen.
Nonpoint and Point Sources of Nitrogen
In Major Watersheds of the United States
Return to the U.S. Geological Survey National Water Conditions
Water Quality Tables