TMDL Development for the Savannah Harbor Estuary: Problem, Approach, and Tools

The Savannah Harbor Dissolved Oxygen TMDL is the most controversial TMDL performed by EPA Region 4 and has had an extensive track record of public involvement along with Federal and State agencies in a technical review role. The Savannah Harbor Estuary has numerous anthropogenic, hydrological, and meteorological factors that influence the dissolved oxygen concentrations. The dominating anthropogenic factors are the discharge of wastewaters from large industrial and wastewater facilities; withdrawals of freshwater by power plants for urban supplies; and pollution by nonpoint source runoff. In addition to anthropogenic sources, there is an extensive marsh system that exports natural sources of BOD from vegetation decay along with a large BOD source in the Atlantic Ocean. The hydrological regime of the estuary is complicated by spring tides on the order of 10–12 feet in range and freshwater flow distribution between interconnected parts of the estuary: the Savannah River, the Front River, the Middle River, the Little Back River, the Back River, the South Channel, and the offshore portions in the Atlantic Ocean, as well as numerous meandering channels and marshes.The Savannah Harbor Estuary is bathymetrically complex, with shallow 3–6 meter parts of the harbor, cut by a 14-meter ship channel that follows through the Front River up to Port Wentworth. Water, salinity, and temperature balance are forming under the influence of tidal intrusions of the ocean waters and freshwater flow from upstream parts of Savannah River.The hydrodynamic and water quality models were put under extensive scrutiny by the Federal and State agencies that would be using the model for NPDES permits, a dissolved oxygen TMDL, harbor deepening impacts, and a dissolved oxygen ecosystem restoration study. Previous modeling efforts of the Savannah Harbor were determined to be lacking defensibility on the handling of the vertical mixing numerical scheme, boundary conditions, and resolving the upstream portions of the river. The Federal and State agencies wanted one management tool for the harbor that could be used on the various studies. Therefore, the grid resolution, the boundaries, inputs, and outputs had to be approved by a working technical group through the process.The Environmental Fluid Dynamics Code (EFDC) and the US EPA Water Quality Analysis Simulation Program (the latest version-WASP 7.0) was applied to the harbor and calibrated to a 1999 dataset and validated to a 1997 dataset. The WASP model was used for the nutrient enrichment, eutrophication, and dissolved oxygen depletion dynamics. The water quality model allows for simulating a response in dissolved oxygen, nutrients, and chlorophyll-a concentrations as a function of various loading and transport scenarios. The model has a user-friendly interface that eases the problem of input files creation and model output analyzing. The hydrodynamics are critically important for simulation of transport of water quality constituents. The WASP 7.0 has the option of adopting the information from the output of the EFDC model. The EFDC modeling package has been well tested for simulating three-dimensional flow and transport in complex environments including rivers, estuaries, and in the offshore shelf. The EFDC model has a highly accurate transport scheme that provides a capability to resolve sharp gradient problems that are typical for salinity regimes in shallow estuaries.The model domain is located near Savannah, Georgia and is on the border between Georgia and South Carolina. The domain extends upstream on the Savannah River to river mile 61.0 near Clyo, Georgia. The downstream end of the model extends approximately 17 miles offshore from Oysterbed Island to cover the navigational channel of Savannah Harbor.