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The Development of a Hydrodynamic and Water Quality Model to Support TMDL Determinations and Water Quality Management of a Stratified Shallow Estuary: Mobile Bay, Alabama

Mobile Bay is located in the southwestern portion of Alabama and is currently on the State of Alabama’s §303(d) list of impaired waters. Large portions of the estuary are listed for organic enrichment, low dissolved oxygen, and fecal coliform. Mobile Bay is a shallow, stratified estuary in which the water quality is controlled by freshwater inflow from the Mobile River, wind, tidal variation and anthropogenic loadings. Because of the complexities of transport and the interaction with water quality, simplistic modeling approaches could not be used to develop a TMDL or be used as management tools for the waterbody. Several components of the EPA TMDL Modeling Toolbox are being used to develop the TMDL and to be used in the future by the State of Alabama as a management tool for water quality issues in the bay. The primary components of the Toolbox that is being used are: the Water Resources Database, Environmental Fluid Dynamics Code, and the Water Quality Analysis Simulation Program. There are several other ancillary tools within the Toolbox that will aid in the application of models. Introduction Total Maximum Daily Load (TMDL) development for the Mobile Bay estuary requires the application of sophisticated modeling tools to link changes in loadings to the system through management strategies to improvement of water quality. Typically, models are applied to waterbodies for the express purpose of determining the TMDL. In this application the there are two goals: (1) determine the TMDL and (2) develop a management tool that can be used by the Alabama Department of Environmental Management (ADEM) in the future to manage water quality in the bay. Mobile is located on the northern coast of the Gulf of Mexico. Geomorphologically, it is a combination of the drowned river valley and bar-built estuary. Considering its shallow depths, Mobile Bay is bathymetrically complex, with deep holes in the northeastern part and a manmade island in the northwest area. There are two openings in the lower part of the Bay: to the Gulf of Mexico and Mississippi Sound. A ship channel, 12-meters deep, cuts through the Main Pass to the Port of Mobile. Mobile Bay’s water quality is highly influenced by its natural geographic location, weather patterns of the watershed, and human uses. The Mobile River system delivers 95% of the total freshwater input to the bay. The average discharge of the system is about 1,840 m/s, but during winter-spring rainfalls it can exceed 14,000 m/s and during summer low-flow season decrease to 240 m/s. The Mobile Bay drainage system is nation’s sixth largest in area and fourth largest in discharging volume. As a result, urban and agricultural development in the Bay’s surrounding areas and in areas far outside the coastal region impact Mobile Bay’s water quality characteristics. Hypoxic and anoxic conditions are common in Mobile Bay and are generally prevalent during the summer months. These frequently stressed water quality conditions marked by stratification with low dissolved oxygen. Specific for the bay environmental problem is the ‘Jubilee’ (local name) phenomenon. It is the east shoreward movement of dense concentrations of fish and invertebrates that has been observed in Mobile Bay since at least 1867, before any significant man-made environmental impact had been registered. Presumably this phenomenon associates with the formation and shoreward movement of a low oxygen zone. This persistent pattern of hypoxia occurs when winds blowing from the mainland drive surface waters from shore, causing deeper, poorly oxygenated water to move into the shallows. Mobile Bay waters are monitored for pathogens and often are subject to closings, advisories, or warnings during situations when excess fresh waters entered Mobile Bay through Mobile River or due to sanitary sewer collection system failures. The Alabama §303(d) list of impaired waters includes parts of Mobile Bay impaired by organic enrichment/low dissolved oxygen and pathogens (Figure 1). TMDLs will be developed for all of the listed segments. Mobile Bay (Pathogens) Mobile Bay (OE/DO) Bon Secour Bay (Pathogens) Gulf of Mexico (Mercury) Portersville Bay (Pathogens) Mississippi Sound (Pathogens) Bayou la Batre (OE/DO and Pathogens) Fish River (Pathogens) Intracoastal Waterway (OE/DO) Intracoastal Waterway (OE/DO) MOBILE COUNTY