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Title: Supplemental water quality analysis St. Johns Bayou and New Madrid Floodway
Authors: United States. Army. Corps of Engineers. Memphis District.
Ashby, Steven L.
Ruiz, Carlos E.
Deliman, Patrick N.
Keywords: Big Oak Tree State Park
Water quality
New Madrid Floodway
St. Johns Bayou
Publisher: Environmental Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: ERDC/EL SR ; 00-7.
Description: Special Report
Abstract: A supplemental assessment of water quality data was conducted to describe potential impacts on water quality in the St. Johns Bayou and New Madrid Floodway as a result of the proposed flood control project. Existing water quality data from Federal and state resource agencies and literature-based information on land use effects on water quality were compiled for evaluation. Results of this evaluation were used to describe water quality conditions and, in conjunction with land cover and hydrology information, the relative transport/retention of selected materials associated with various hydrologic events based on selected surface water elevations. Specific issues addressed included evaluation of (1.) the effects of hydrologic changes on water quality for both the area impacted by the proposed project and in relationship to the overall water quality of the Mississippi River, (2.) the effects on water quality associated with potential changes in pesticide use, and 3) the effects of proposed groundwater supplement on Big Oak Tree State Park. Water quality in the area reflects conditions typical for basins where agriculture is the dominant land use. In general, nutrient concentrations (with the exception of phosphorus) were not excessively high except during periods of elevated flow, and basin concentrations were not substantially different than observations for the Mississippi River. Sediment concentrations were generally lower than concentrations in the Mississippi River and increase with runoff as expected. With the exception of a few occasional high concentrations of nitrates, groundwater quality is acceptable although phosphorus concentrations in groundwater were generally higher than in surface waters. Point sources were the most notable sources of extremes (high nutrients or low dissolved oxygen concentrations). Potential changes in pesticide usage and impacts on water quality were evaluated with an assessment of potential changes in cropping practices, a literature review of herbicide transport research, and an assessment of pesticide data compiled in the data retrievals. In general, pesticide concentrations were relatively low in surface and subsurface waters, and water supply concentrations were below water quality criteria for drinking water. The impact of pesticides, atrazine in particular, on public groundwater resources is expected to be minimal. Water quality with the project alternative should be similar to conditions that exist during periods of no flooding. Material processing should be similar between these two scenarios as well. The basin most likely retains or removes material from headwaters and floodwaters, and this process is maximized during low-water periods and is comparable with the alternative project. Impacts to the water quality of the Mississippi River with the proposed or alternative project in place are not expected to be discernible, due to the overwhelming volume of water in the Mississippi River relative to floodwater volume in the project area. Mass balance estimates indicated that impacts to material loads of the Mississippi River are less than 0.1% for moderate flows with the project. Potential impacts to Big Oak Tree State Park with the project are likely to be associated with a decreased supply of sediments and the associated sustainability of the site. Historical alteration in the flow regime associated with the development of agriculture in the area during the 20th century has been suggested as a major mechanism contributing to the decline at the park. The use of groundwater to restore a flooding regime more conducive to the sustainability of the park is suggested, and the impacts of reduction in material supply (e.g. sediments) can be lessened with the use of surface water when available.
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