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By Sharon E. Kroening
Originally published as USGS Fact Sheet 121-98, December
1998
IntroductionConcerns about nutrientsNutrient sources within the Upper Mississippi River BasinImplications to Water QualityReferencesThe amount of nutrients contained
in fertilizer, livestock manure, municipal wastewater, atmospheric deposition,
and legume residues were quantified in each of the major drainage basins
within the Upper Mississippi River Basin study unit (fig.
1) as part of the U.S. Geological Survey's National Water-Quality Assessment
Program. These sources of nutrients may potentially affect surface- and
ground-water quality, so knowledge about the relative importance of each
source may assist in the management of surface and ground waters within
the study unit. The relative importance of each nutrient source was expected
to vary among each of the four drainage basins due to differences in land
use across the study unit.
[go to fig. 1] Fertilizer and livestock manure were potentially large sources of nitrogen and phosphorus in each of the four drainage basins. However, nitrogen in legume residues was a more important source in the Upper Mississippi, St. Croix, and Lower Mississippi River Basins because hay comprised a larger part of the total acreage of crops grown in these basins. Atmospheric deposition comprised a larger percentage of the nitrogen sources in the St. Croix River Basin compared to the other three drainage basins probably because amounts of the other sources are relatively low. Nitrogen and phosphorus yields in streams were greatest in the Lower Mississippi River Basin and the Minnesota River Basin, where amounts of nonpoint sources of these constituents also were the greatest per square mile. IntroductionIn 1994, the U.S. Geological
Survey began studies in the Upper Mississippi River Basin as part of the
National Water-Quality Assessment Program. The purpose of the program is
to assess the quality of a large, representative part of the Nation's water
resources.
There are four major drainage
basins in the study unit--Upper Mississippi, Lower Mississippi, St. Croix,
and Minnesota River Basins. In this paper, the Upper Mississippi River
Basin (UMRB) is defined as the entire drainage of the Mississippi River
upstream of Anoka, Minnesota, and the Lower Mississippi River Basin (LMRB)
is defined as the drainage of the Mississippi River from Anoka, Minnesota
to the outlet of Lake Pepin (excluding the drainage areas of the Minnesota
and St. Croix Rivers).
Land use varies among these
four drainage basins. The Minnesota River Basin (MRB) consists primarily
of agricultural land. Soybeans and corn are the principal crops grown,
and pigs are the dominant type of livestock raised in this basin. Data
obtained from the state agricultural censuses (Iowa State University, 1994;
Minnesota Agricultural Statistics Service, 1995; North Dakota Agricultural
Statistics Service, 1994; South Dakota Agricultural Statistics Service,
1994) showed the MRB produced approximately 75 percent of the soybeans
and 65 percent of the corn and pigs raised in the study unit in 1993.
Land use in the UMRB is primarily a mixture of agriculture and forest. The major crops grown in this drainage basin are hay and corn. The predominant livestock raised in the UMRB are dairy cows, beef cows, and pigs. Data obtained from the state agricultural censuses (Minnesota Agricultural Statistics Service, 1995; Wisconsin Agricultural Statistics Service, 1994) showed over half of the hay grown in the study unit in 1993 was from the UMRB. About half of the milk cows, 45 percent of the beef cows, and about 20 percent of the pigs raised in the study unit in 1993 also were from the UMRB. The LMRB consists primarily of agricultural land and also includes the TCMA. The major crops grown in this drainage basin are corn, soybeans, and hay. The major livestock raised are pigs and cattle. Data obtained from the state agricultural censuses (Minnesota Agricultural Statistics Service, 1995; Wisconsin Agricultural Statistics Service, 1994) showed that about 10 percent of the corn, soybeans, hay, pigs, beef cows, and milk cows raised in the study unit in 1993 were from the LMRB. The St. Croix River Basin (SCRB) consists predominantly of forested and agricultural land. The major crops produced in this drainage basin are hay and corn. The major livestock raised in the SCRB are cattle. Data obtained from the state agricultural censuses (Minnesota Agricultural Statistics Service, 1995; Wisconsin Agricultural Statistics Service, 1994) showed about 20 percent of the hay and less than 5 percent of the corn produced in the study unit in 1993 was from the SCRB. In 1993, about 15 percent of the milk cows and beef cows in the study unit also were raised in the SCRB. back to topback to U.S. Geological Survey in Minnesota homepage Concerns About NutrientsNutrients are essential for
plant and animal growth. However, elevated concentrations of nutrients
in slower-moving reaches of streams and in lakes can result in eutrophication.
Eutrophication is the excessive growth of aquatic plants and algae. This
excessive growth can result in the water smelling and tasting foul, which
may ultimately increase water treatment costs if the water is used for
drinking. In addition, when these plants and algae die dissolved oxygen
may be depleted in the water, which can ultimately result in fish kills.
Excessive applications of nutrients to the land surface also could result
in the leaching of more soluble forms, such as nitrate and ammonium, to
ground water. Nitrate concentrations greater than 10 milligrams per liter
(mg/L) in drinking water can cause a potentially-fatal condition called
methemoglobinemia (blue-baby syndrome) in humans. Un-ionized ammonia in
water can be toxic to fish and other aquatic life.
An assessment of streams by the Minnesota Pollution Control Agency (1992) indicated nutrient compounds were a frequent cause of river reaches in the study unit not supporting their designated uses, which includes swimming, boating, and the protection and fish and wildlife. Total phosphorus concentrations in the Mississippi River and some of its tributaries in the study unit have exceeded the recommended criterion of 0.1 mg/L as phosphorus set by the U.S. Environmental Protection Agency (1986) to control eutrophication (Kroening and Andrews, 1997). Nitrate concentrations in streams draining the southern and southeastern part of the study unit have, at times (primarily during the spring and summer), exceeded the Maximum Contaminant Level (MCL) of 10 mg/L for drinking water set by the U.S. Environmental Protection Agency (1986) for the protection of human health (Payne, 1994; Kroening and Andrews, 1997). Nitrate is listed by the Minnesota Pollution Control Agency (1992) as a frequent contaminant in ground water. Within the study unit, concentrations in ground water have at times exceeded the MCL of 10 mg/L as nitrogen (Kroening and Andrews, 1997). back to topback to U.S. Geological Survey in Minnesota homepage Nutrient Sources Within the Upper Mississippi River BasinFertilizer, livestock manure,
municipal wastewater, atmospheric deposition, and legume residues are sources
of nitrogen and/or phosphorus which may affect surface- and ground-water
quality. Nutrients may reach surface waters from these sources by direct
discharge, runoff, or atmospheric deposition. Nutrients may reach ground
water from these sources by leaching through the soil. It is recognized
that some of the nutrients contained in livestock manure may have originated
as fertilizer, atmospheric deposition, or as part of legumes. However,
the amount of nutrients contained in livestock manure was quantified in
this analysis because manure may affect surface- and ground-water quality.
Legume residues are not traditionally regarded as a potential source of
nitrogen to surface or ground water, but part of the nitrogen contained
in legumes is fixed from the atmosphere and can become available to crops
in subsequent years as a result of mineralization to more plant-available
forms (University of Wisconsin-Extension, 1994). The amounts of nitrogen
and phosphorus contained in each of the above mentioned sources were calculated
using data from 1991, 1992, or 1993 for the MRB, SCRB, UMRB, and the study
unit as described by Kroening and Andrews (1997). For the LMRB, the amount
of nitrogen and phosphorus contained in each source was obtained by difference.
Approximately 760,000 tons of nitrogen and 132,000 tons of phosphorus from fertilizer, livestock manure, municipal wastewater, atmospheric deposition, and legume residues have the potential to affect water quality in the study unit each year (fig. 2). Most of the nitrogen and phosphorus from these sources in the study unit are from the MRB and the UMRB (fig. 2). However, these are the largest of the four drainage basins. Greater amounts of nutrients per unit area may increase the likelihood of transport to surface or ground water. On a per-square-mile basis, most of the nitrogen and phosphorus from the five sources quantified in this analysis (table 1) was to the MRB and LMRB. Fertilizer and livestock manure were the predominant sources of nitrogen and phosphorus to each of the drainage basins (figs. 3 and 4). These sources comprised from 57 to 83 percent of the nitrogen sources and from 91 to 99 percent of the phosphorus sources in each of the four drainage basins. The relative importance of the other sources of nitrogen in the study unit varied among each of the drainage basins. Nitrogen in legume residues was a more important source in the UMRB, SCRB, and LMRB relative to the MRB because hay comprises a larger part of the total acreage of crops grown these drainage basins. Atmospheric deposition is a more important source of nitrogen in the SCRB compared to most of the other sources probably because inputs from fertilizer, manure, and legume residues in this basin are relatively low. Municipal wastewater discharges comprise a larger nitrogen and phosphorus source in the LMRB relative to the other drainage basins because of direct discharges to the Mississippi River from the TCMA. [go to fig. 3] [go to fig. 4] There were greater amounts of fertilizer and livestock manure applications per square mile in the MRB and LMRB relative to the UMRB and SCRB (table 2). However, the amounts of fertilizer and livestock manure applied in each drainage basin is not uniform. For example, in the UMRB more fertilizer and manure is probably applied in the southern and western parts where agri-cultural land use is more concentrated (fig. 1). More nitrogen is available per square mile from legume residues in the LMRB (table 2) because almost half of the crops grown in this basin in 1993 were hay and soybeans. [go to table 2] back to topback to U.S. Geological Survey in Minnesota homepage Implications to Water QualityThis analysis illustrates
the importance of nonpoint sources of nitrogen and phosphorus, especially
fertilizer and livestock manure, to the major drainage basins of the study
unit. Nitrogen and phosphorus yields in streams were greatest in the MRB
and LMRB, where amounts of nonpoint sources of these constituents also
were the greatest per square mile (table 2). Nitrogen and phosphorus from
nonpoint sources generally are transported to streams by runoff. However,
point sources, which generally discharge directly to streams in the study
unit (Kroening and Andrews, 1997), may be important local sources of nutrients
in the study unit.
References
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