Nutrient Control Actions for Improving Water Quality in the Mississippi River Basin and Northern Gulf of Mexico (2009) / Chapter Skim
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2 Nutrient Inputs and Water Quality Effects
2 Nutrient Inputs and Water Quality Effects
Pages 13-26
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From page 13... ...
, the Clean Water Act has achieved many successes in helping address point source effluent into the Mississippi River. Today, the more challenging water quality problems across the river basin and in the northern Gulf of Mexico derive from inputs of nonpoint source pollutants, especially nutrients.1 Nonpoint source pollutants derive from a variety of unconfined and unchanneled sources of water pollution, such as runoff flowing across agricultural lands, forests, and urban lawns, streets, and other paved areas.
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From page 14... ...
500 km FIGURE 1. Mississippi River basin, major tributaries, land uses, and typical summertime extent of northern Gulf of Mexico hypoxia (in red)
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From page 15... ...
. The transport and delivery of nutrients across the Mississippi River basin have been affected substantially by changes in land use and related agricultural practices, including the installation of subsurface drainage systems, as well as nutrient inputs from fertilizers and manure (Baker et al., 2008)
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From page 16... ...
This degradation is of particular concern across the Mississippi River basin because of the predominance of annual rowcrop agriculture. Factors contributing to this region's high productivity and high acreages in row-crop agriculture include naturally rich soil, adequate annual precipitation, relatively flat to gently rolling terrain, and a hydrologicallymodified landscape from which excess water drains rapidly and easily.
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From page 17... ...
These maps also show that large percentages of total nutrient yields derive from a relatively small number of watersheds across the river basin. SOURCE: Reprinted, with permission, from Alexander et al.
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From page 18... ...
If dissolved oxygen falls below about 2 milligrams per liter, that portion of the water body is said to be hypoxic and sometimes is referred to as a "dead zone." The hypoxia zone is a seasonal but perennial feature of the coastal waters downstream from the Mississippi River discharge into the gulf and is most prevalent from late spring through late summer. Although hypoxia is mainly a bottom-water condition, oxygen-depleted waters often extend upward into the lower one-half to two-thirds of the water column.
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From page 19... ...
Values are milligrams/liter (mg/l) of dissolved oxygen.
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From page 20... ...
For example, there is only limited regulatory authority that the Clean Water Act grants to the federal government to regulate loadings from nonpoint sources of water pollutants. Many economic factors also will affect future nutrient loadings across the basin and discharges into the gulf, further complicating nutrient control measures.
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From page 21... ...
. If a 9-year period of trend data at a minimum is necessary to recognize whether changes in land use practices, or changes in fertilizer applications, or other nutrient management practices can affect water quality, it may require decades for nutrient control actions in the Mississippi River basin to be reflected in changes in the areal extent of northern Gulf of Mexico hypoxia.
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From page 22... ...
This large concentration of the sources of nitrogen loadings in a relatively small number of hydrologic units/watersheds is important information for any program designed to reduce nutrient loadings to the Mississippi River and the northern Gulf of Mexico. Indeed, this information may identify opportunities for substantially reducing the areal extent of northern Gulf of Mexico hypoxia.
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From page 23... ...
. © by the American Chemical Society.
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From page 24... ...
Finding/recommendation 1: Realizing progress toward reducing the areal extent of northern Gulf of Mexico hypoxia will require an acknowledgement that there will be a considerable time lag -- roughly a decade, at a minimum -- between nutrient reduction actions across the river basin and ecological and water quality responses downstream in the gulf. Finding/recommendation 2: Purposeful targeting of nutrient control efforts toward areas of higher nutrient loadings will be essential to realize the greatest initial reductions in nutrient loadings.
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From page 25... ...
NUTRIENT INPUTS AND WATER QUALITY EFFECTS 25 point source dischargers to monitor nutrient concentrations -- nitrogen and phosphorus -- in effluent at their discharge point as a condition of their National Pollutant Discharge Elimination System (NPDES) permits.
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