Missouri River Planning Recognizing and Incorporating Sediment Management (2011) / Chapter Skim
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6 Water Quality and Missouri River Sediment Management
6 Water Quality and Missouri River Sediment Management
Pages 103-123
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From page 103... ...
The statement of task to this committee also reflected concerns that sediment introduction, with associated phosphorus, may have detrimental effects on water quality within the river and as far away as the northern Gulf of Mexico. As described through this report, high concentrations of sediment were a natural feature of the preregulation river and important to its native species, and also important to land-building processes in parts of the Mississippi River delta.
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From page 104... ...
In considering the full range of implications of the Corps of Engineers habitat projects along the Missouri River, it is therefore important to understand not only provisions of the Endangered Species Act, but also provisions of the Clean Water Act -- especially setting water quality standards for sediment and phosphorus concentrations. This chapter responds to two questions in this report's statement of task: • What is the significance of the Missouri River sediments to the gulf of Mexico Hypoxia problem?
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From page 105... ...
The zone of hypoxia, and nutrient loadings and water quality across the Mississippi River basin, have been evaluated by many scientists and research teams. Box 6-1 summarizes several recent studies, reports, and initiatives addressing Mississippi River and northern Gulf of Mexico water quality.
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From page 106... ...
. Among several reasons why the northern Gulf hypoxic zone has proven to be a stubborn water quality remediation challenge is that it is affected by factors other than Atchafalaya-Mississippi river nutrient discharges, and that the areal extent varies from year to year.
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From page 107... ...
A team of USGS scientists has been employing a spatially referenced regression on watershed attributes (SPARROW) water quality model to determine spatial patterns in nutrient yields across the Mississippi River basin (Alexander et al., 2008)
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From page 108... ...
. Total phosphorus loads did not change significantly during this period.
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From page 109... ...
4. Reducing the size of the hypoxic zone and improving water quality in the Mississippi River basin will require considerable reductions in nitrogen and phosphorus loads.
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From page 110... ...
Actual values would almost assuredly be less than this estimated, upper-end range. Potential Sediment Bypass Around Gavins Point Dam It also is possible to estimate the potential total phosphorus load to the Gulf resulting from moving sediment around Gavins Point Dam -- an aggressive, perhaps unlikely -- sediment management measure (and described
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From page 111... ...
CURRENT AVERAGE LOAD TO GULF FROM MISSISSIPPI RIVER BASIN: 154,000 CURRENT LOAD FROM MISSOURI RIVER: 25,536-30,400 (17-20 percent of total load to Gulf) ESTIMATED UPPER BOUND LOADS FROM CORPS SWH PROJECTS: 10,700-19,400 (6-12 percent of total load to Gulf)
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From page 112... ...
presents results from a biophysical model that relates the areal extent of Gulf hypoxia to AprilJune total phosphorus loads. The modeled response curve from this study (Figure 6-4)
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From page 113... ...
For purposes of comparison, 40,150 metric tons/year figure is 2.1-3.7 times larger than the upper bound estimated range of 10,658-19,418 metric tons per year of phosphorus estimated from the Corps SWH projects, and 11-21 times the estimated range from moving sediment past Gavins Point Dam. Thus, even the upper-bound estimates of additional total phosphorus
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From page 114... ...
. Given the multiple causes of the year-to-year variation in the area of hypoxia in the northern Gulf of Mexico, it is not appropriate to relate discharges from select sites of relatively small nutrient loadings across the river basin with changes in the areal extent of the hypoxic zone in any given year.
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From page 115... ...
That being said, the Corps of Engineers Missouri River restoration projects, and any additional future projects, deliver additional nutrients to the river and Gulf at a time that federal and state agencies, and a variety of nongovernmental organizations, are seeking ways to reduce nutrient loadings across the Mississippi River basin. WATER QUALITY CRITERIA FOR SEDIMENT AND NUTRIENTS This report does not intend to suggest that load increases of any size or in any location can be ignored in permitting for the discharges of sediment and nutrients into waterbodies.
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From page 116... ...
The EPA required replacing Florida's narrative sediment and nutrient criteria with numeric criteria and expected such numeric criteria to be protective of designated uses of the waterbody itself as well as downstream waters. Clearly, the EPA effort to define numeric water quality criteria for the Missouri River is part of an ongoing national agency effort to replace narrative with numeric criteria that protects local and downstream waters.
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From page 117... ...
The three evaluations above offer different approaches to setting nutrient criteria for water quality. Using these multiple lines of evidence, the next step is to define numeric total nitrogen (TN)
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From page 118... ...
Water Quality and the Historic Missouri: A Reference Condition As discussed in previous chapters, the preregulation Missouri River carried a substantial sediment load. And that load, as well as the nutrients (especially phosphorus)
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From page 119... ...
the years -- may be essential to attaining designated uses that support native species. Nutrients Associated with Sediments as Water Quality impairments As previously discussed, phosphorus is a nutrient closely correlated with sediment.
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From page 120... ...
This value is considered too low given the historic range of suspended sediments, and the likely range of particulate phosphorus in undeveloped watersheds presented above, but this difference does illustrate the uncertainty in understanding the background phosphorus load in the system. The actions of the Missouri Clean Water Commission highlight the need for closer integration of the nutrient criteria development process and water quality management decision making.
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From page 121... ...
The MRRIC also could help mediate disagreements among federal and state agencies on proposed water quality criteria. Sediment Releases and Water Quality Compliance The development of narrative or numeric criteria considers historical nutrient and sediment factors in setting limits on sediment and phosphorus discharges to the mainstem river and as a basis for regulating such discharges.
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From page 122... ...
A comparison of potential phosphorus loads from Corps SWH projects, with load increments required to produce measureable changes in the areal extent of Gulf hypoxia, shows that these projects will not significantly change the extent of the hypoxic area in the Gulf of Mexico. Additional comparisons of other alternatives for reintroducing sediment to the river -- namely, bypassing sediment around Gavins Point Dam -- yield a similar conclusion that they will not significantly change the areal extent of the hypoxic zone.
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From page 123... ...
This monitoring should be conducted to ensure that sediment or other pollutants discharged to the river comply with applicable water quality criteria.
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