United States
Environmental Protection
Agency
Nitrogen and Phosphorus Pollution in the Mississippi River Basin:
Findings of the Wadeable Streams Assessment
Introduction
Beginning as a tiny outlet stream in
northern Minnesota at Lake Itasca,
the Mississippi River is the largest
river in the United States and drains
all or part of 31 states and more than
1,245,000 square miles of the country's
heartland. The Mississippi River has
played an important role in influencing
America's economic and cultural
development, and its watershed (i.e.,
the land area it drains) is considered
the most productive agricultural land
in the country. Today, however, the
watershed of the Mississippi River
suffers from the effects of excess
inputs of nitrogen and phosphorus,
often referred to as "nutrient
pollution." Many studies have linked
this pollution to the contamination
of drinking water, surface water,
and ground water throughout the
Mississippi River watershed and to the
growth of the hypoxic (oxygen-poor)
zone in the northern Gulf of Mexico.
*
Sixty percent of North American birds use the
Mississippi River as their migratory flyway.
(Photo: U.S. Fish and Wildlife Service)
The Wadeable Streams Assessment
In 2005, the U.S. Environmental Protection Agency (EPA) began a new program to
assess the condition of the nation's water resources. Partnering with the states,
tribes and other agencies, EPA launched the National Aquatic Resource Surveys
(NARS). The goals of the surveys are to quantify the health of the nation's waters
based on a set of physiochemical and biological indicators, and to identify the
relative importance of key stressors, including nitrogen and phosphorus. Each
survey uses standardized field and lab methods and is designed to produce
unbiased estimates of the condition of the whole water resource being studied (i.e.,
rivers and streams, lakes and reservoirs, wetlands, or coastal waters).
The Wadeable Streams Assessment (WSA), released in 2006, was the first such
survey of the condition of the nation's small streams. The assessment shows that 30
percent of streams across the country have high levels of nitrogen or phosphorus
and that these streams are twice as likely to have poor biological condition as those
with lower levels of nutrients. ("Biological condition" is a comprehensive indicator
of the health of a water body). In addition, the findings of the assessment illustrate
that while the problem is widespread, sites with the highest concentrations of
nitrogen occur in the Upper Mississippi sub-basin (Figure 1 and Table 1).
Figure 1. Total nitrogen concentrations (Source: WSA)
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Findings for the Mississippi River Basin
EPA developed "thresholds" to compare
nitrogen and phosphorus levels for the
WSA. These thresholds are regionally
specific and based on nutrient
concentrations found in a collection
of reference sites within that region. A
reference site is a water body that is
"least disturbed," meaning that it is as
close to natural conditions as can be
found in that region.
Nationally, assessment results show
that 32 percent of streams have high
levels of nitrogen and 31 percent of
streams have high levels of phosphorus.
By comparison, in the Mississippi River
Basin, 39 percent of streams have high
levels of nitrogen and 32 percent of
streams have high levels of phosphorus.
Findings for nitrogen and phosphorus
in the Mississippi River Basin are
presented below (Table 1), but a few key
messages are:
* In general, high levels of nitrogen
affect more streams than high
phosphorus levels, except in the
Upper Missouri, the Arkansas and
Lower Mississippi sub-basins
• In the Upper Mississippi sub-basin,
where 61 percent of the land area is
used for agricultural production, 50
percent of streams have high levels
of nitrogen (Figure 2).
• The Ohio sub-basin has the highest
percentage of streams affected by
nutrients in the Mississippi River
basin (55 percent of stream miles
affected by nitrogen, and 43 percent
affected by phosphorus).
The results of the assessment provide
insight into the problem of nitrogen and
phosphorus pollution in the Mississippi
River Basin. Future river and stream
surveys conducted by EPA and its
partners will continue to provide crucial,
consistent and comparable information
to allow water quality managers and
others to track the health of these
important water resources.
Table I Percent of wade able streams
with high levels of nutrients
(Source: WSA)
Overall Mississippi
Basin
Total U.S. (lower 48)
Total MS River Basin
Upper Mississippi
Ohio
Tennessee*
Upper Missouri
Lower Missouri
Arkansas
Lower Mississippi*
Phosphorus
30.9%
32.5%
23.4%
43.2%
18.1%
22.4%
27.7%
41.2%
38.6%
Nitrogen
31.8%
39.5%
50.4%
54.6%
36.3%
18.6%
34.9%
25.9%
1.6%
* Small sample sizes in these sub-basins result in
lower statistical significance
Upper Missouri
j Arkansas-Wiite-Red
3 LtMer Mississippi
I Tennessee
r"\
• High
• Medium
Low
Figure 2. Percent of streams with high, medium and low levels of nitrogen and
percent of land area in agricultural use (Source: WSA)
&EPA
United States
Environmental Protection
Agency
For more information on the Wadeable Streams Assessment and the National Aquatic
Resource Surveys, visit www.epa.gov/aquaticsurveys. Additional information on nitrogen and
phosphorus pollution in the Mississippi River Basin can be found at www.epa.gov/msbasin
and www.epa.gov/nutrientpollution.
US EPA Office of Water Monitoring Branch (4503T), 1200 Penn. Ave., NW, Washington, DC 20460
December 2011, EPA 841-F-11-004
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