Over 90% of impaired streams in the coastal plain of Georgia are listed for violating established dissolved oxygen (DO) standards. Violations normally occur during summer months when temperatures are high and flow low or nonexistant. In accordance with Section 303(d) of the Clean Water Act, Georgia must develop Total Maximum Daily Loads (TMDLs) for waters not meeting established water quality standards. However, recent research indicates that low levels of DO may occur naturally in southeastern, low gradient, blackwater streams and that current standards may be inappropriate for these ecosystems. The Georgia Department of Natural Resources – Environmental Protection Division has chosen to use the Georgia DoSag mathematical model for developing DO TMDLs in streams and rivers.
The Georgia DoSag model
The Georgia DoSag model is used routinely by the Georgia EPD to simulate DO dynamics in rivers for the National Pollution Discharge Elimination System (NPDES). In these applications, the model is used primarily to determine the effect of biochemical oxygen demand (BOD) on stream DO. DoSag uses a modified version of the Streeter-Phelps equation (Streeter and Phelps, 1925) to calculate DO stepwise through a stream network. It is a steady-state, one-dimensional, advection-dispersion, mass-transport, deterministic model (figure 1).
Because the Streeter-Phelps equation fails under low flow or no flow conditions (flow velocity is in the denominator), DoSag can not be used under the flow conditions during which we are most likely to observe extremely low DO concentrations. DoSag was developed by Dr. Roy Burke of the Georgia EPD in the late 1980’s specifically for use during NPDES permitting by the agency. There is no peer reviewed documentation describing and evaluating the model currently available. Because DoSag has not been extensively applied in coastal plain streams and rivers, the objective of this research was to determine the applicability of DoSag in this ecoregion.
This work involved the parameter estimation, calibration, sensitivity analysis, and validation of the DoSag model in the Little River Experimental Watershed (LREW) (figure 2) in south-central Georgia, USA, which is generally accepted to be representative of coastal plain watersheds in the southeastern USA.
The model was successfully calibrated and then validated for several dates indicating that it can be applied in the coastal plain (figure 3).
Sensitivity analysis indicated that sediment oxygen demand (SOD) is the most critical parameter of the model. This finding resulted in extensive work on measuring SOD in coastal plain watersheds which is described on other pages.
Anna Cathey, Ph.D. student, Agricultural & Biological Engineering Dept, University of Florida
George Vellidis, , Professor; Biological and Agricultural Engineering, University of Georgia
Matt Smith, Research Leader, Animal Manure and By-Products Laboratory, USDA-ARS, Beltsville, Maryland
Richard Lowrance, Ecologist, Southeast Watershed Research Laboratory, USDA-ARS, Tifton, Georgia
Roy Burke, Principle Environmental Engineer, Georgia Department of Natural Resources – Environmental Protection Division, Atlanta, Georgia