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St. Louis Bay is a vital waterbody in the Mississippi Gulf Coast Region with designated uses of shellfish harvesting and primary contact recreation. Several waterbody segments in the St. Louis Bay watershed are on the Mississippi 1998 Section 303(d) List of Waterbodies as impaired due to pathogens, which are indicated by the presence of fecal coliform bacteria. A comprehensive hydrologic, hydrodynamic, and water quality modeling system was developed by the Civil Engineering Department at Mississippi State University to support the development of TMDLs for the St. Louis Bay estuary and other 303(d) listed waterbody segments within the watershed.

The modeling system includes the Nonpoint Source Model (NPSM) and the Environmental Fluid Dynamics Code (EFDC) and considers both point source and nonpoint source loadings. The Mississippi Department of Environmental Quality (MDEQ) and the Environmental Protection Agency (EPA) have conducted intensive field data acquisition projects to provide additional data to facilitate better understanding of this complex estuarine system and to provide data for model calibration and validation. The modeling system predicted a reduction in the total fecal coliform load of approximately 27 percent was necessary for St. Louis Bay to meet shellfish harvesting water quality standards. Because over 99 percent of the allocated load is contributed by nonpoint sources, those sources were the focus for reduction. The waters of St. Louis Bay are in various stages of restriction for shellfish harvesting, and one of the goals of the St. Louis Bay fecal coliform TMDL is to improve water quality to allow for upward reclassification of the waters to once again allow shellfish harvesting where appropriate. Additional stakeholder input is needed to develop an appropriate implementation plan for this watershed. However, the proactive efforts of several programs and organizations that already focus conservation activities in the St. Louis Bay watershed provide the opportunity for this to continue as a TMDL success story.