THREE-DIMENSIONAL MODELING OF ANTHROPOGENIC IMPACTS TO DISSOLVED OXYGEN IN A STRATIFIED ESTUARINE ENVIRONMENT: SAVANNAH RIVER ESTUARY

Two concurrent programs to determine anthropogenic impacts on the dissolved oxygen (DO) environment in the Lower Savannah River Estuary are in process. The first is in support of an Environmental Impact Statement (EIS) for a proposed harbor expansion project where the present channel would be deepened, and the second is in support of a TMDL for DO. The overlapping goals for the two programs allowed for the development of a single comprehensive field and modeling program to evaluate the variety of potential anthropogenic impacts on the DO environment.The Lower Savannah River Estuary is a complex, highly dynamic, partially-mixed estuary, with high annual river flow and a large tide range that experiences varying degrees of stratification. Urban and industrial portions of the City of Savannah and a large port facility are located on the river in close proximity to the Savannah Wildlife Refuge. Anthropogenic impacts to the river that affect DO primarily include a deep dredged channel and a series of wastewater outfalls containing significant loads of biochemical oxygen demand (BOD) and ammonia. Continuous monitoring data identified a longitudinal DO concentration sag in the harbor area showing frequent and prolonged hypoxic conditions. The dynamic DO environment showed vertical variations up to 4 mg/l and tidal variations (not diurnal) up to 3 mg/l based on proximity to the affected harbor area. Physical aspects of the system clearly influence the DO environment and are important in distinguishing between natural and anthropogenic sources and predicting impacts.To accurately simulate this dynamic and complex environment, a curvilinear coordinate, boundary-fitted, hydrodynamic, and water quality model system was utilized. The 3-D hydrodynamic model simulated water surface elevation, currents, salinity, and temperature for lower river system. The model proved capable of reproducing observed transient physical phenomena including stratification and collapse and is in good agreement with observed values. The 3-D water quality model, based on EPA-WASP5 kinetics and the same curvilinear coordinate system, used the same model grid, and directly imported the hydrodynamic predictions. The water quality model was applied to simulate the full DO balance in the estuary, including BOD, the nitrogen and phosphorus kinetics and sediment flux interactions. The models were calibrated for a 3 month period in the summer of 1999. Comparisons of model predictions to observations were made and statistics developed. The calibration simulation was re-run with the anthropogenic loads removed and an analysis made for the model predicted difference in DO in the river as a result. Preliminary results indicate that improvements in DO concentration in the water column would be on the order of less than 0.1 (mg/l) in the Savannah Harbor area. Reasons for the small change include the unaltered SOD load and the relatively large influence of the marsh loads on the Lower River.