Longitudinal dissolved-oxygen profiles of the Ashley River for various hydrologic and point-source loading conditions were determined using results from water-quality simulations by the Branched Lagrangian Transport Model. The study area included the Ashley River from S.C. Highway 165 at Bacon Bridge to S.C. Highway 17 near the confluence with the Charleston Harbor. Hydraulic data for the Branched Lagrangian Transport Model were simulated using the U.S. Geological Survey BRANCH one-dimensional unsteady-flow model. Data used to apply and calibrate the BRANCH model included timeseries of water-level data at three locations and measured tidal-cycle streamflows at four locations. Data used to apply and calibrate the Branched Lagrangian Transport Model included timeseries of salinity concentrations at three locations, high- and low-slack tide longitudinal salinity profiles from six sampling locations, nutrient and biochemical oxygen demand concentrations collected over a tidal cycle during two sampling surveys for six locations, nutrient and biochemical oxygen demand concentrations collected over five slack tides over two and three days during two sampling surveys for three locations, and continuous water temperature data and dissolved oxygen concentrations at three locations.
A sensitivity analysis of the simulated dissolved-oxygen concentrations to model coefficients and data inputs indicated the simulated dissolved-oxygen concentrations were most sensitive to equilibrium temperatures due to the effect of temperature on reaction rate kinetics. Of the model coefficients, the simulated dissolved-oxygen concentrations were most sensitive to sediment oxygen demand.
Scenario simulations were used to evaluate four point-source loading conditions to the system by comparing simulated dissolved-oxygen concentrations with a condition where there is no point-source discharge into the system (no-load condition). The September 1992 loading condition decreased the one-day dissolved-oxygen concentration of September 25, 1992, by 29.0 percent or less as compared to a no-load condition. Setting all the point-source loadings to advanced secondary treatment (10 milligrams per liter of ammonia-nitrogen (mg/L) and 20 mg/L of 5-day biochemical oxygen demand) decreased the total ultimate oxygen demand loading to the system by 28 percent and decreased the one-day mean dissolved-oxygen concentrations from the no-load condition by 29.9 percent or less.
Setting all the point-source loadings to advanced treatment (2 mg/L of ammonia-nitrogen and 10 mg/L of 5-day biochemical oxygen demand) decreased the total ultimate oxygen demand loading to the system by 78 percent and decreased the one-day mean dissolved-oxygen concentrations from the no-load condition by 8.1 percent or less. Setting all the point-source loadings to reclaimed-use treatment (0.5 mg/L of ammonia-nitrogen and 5 mg/L of 5-day biochemical oxygen demand) decreased the total ultimate oxygen demand loading to the system by 91 percent and decreased the one-day mean dissolved-oxygen concentrations from the no-load condition by 5.2 percent or less.
--- 1998 By Paul A. Conrads