Estimating Dissolved Oxygen Depletion from Anthropogenic and Riverine Loading Using a Three-Dimensional Water Quality Model

An application of the three-dimensional water quality model EFDC (Environmental Fluid Dynamics Code) was developed for the Lower Cape Fear River Estuary, North Carolina, to investigate the effects of various organic matter (OM) and ammonia load reduction scenarios on the dissolved oxygen (DO) concentrations within the estuary. Hydrodynamic model calibration consisted primarily of varying the width and bottom roughness of shallow model cells located adjacent to the river and estuary channels. The twenty-one state variable water quality model available in EFDC included multiple dissolved and particulate organic carbon constituents, as well as organic and inorganic nutrients, DO, and three phytoplankton constituents. To better characterize the decomposition rates riverine and wastewater inputs, both labile and refractory dissolved OM constituents were used. Over the three-year time period (2002-2005) for which the freshwater and point source loadings were developed, approximately 10% of the OM loading and 50% of the ammonia loading to the estuary came from the twenty wastewater point sources that discharged directly into the estuary. A number of scenario tests were conducted to investigate the system's sensitivity to loading changes. Additional water quality model runs were conducted to investigate how changes in model kinetic parameters might affect the system's sensitivity to loading changes. For instance, the impact of turning off the organic matter and ammonia loadings from all wastewater treatment plants was investigated for various sediment oxygen demand (SOD) rates. In general, it was found that even though DO concentrations were significantly affected by changes in SOD values, the sensitivity of the system to OM load changes was not significantly affected by changing parameter values. It was also found that for this system, relatively large changes in both OM loadings and SOD were needed to eliminate water quality violations. A 50% reduction in both OM loading and SOD raised the 10th percentile DO concentration during the summer season to 5.2 mg/L. Since anthropogenic loads only accounted for about 10% of OM loads into the estuary, even completely eliminating these sources was found to be insufficient to eliminate water quality violations. Large changes (approximately 70%) in both OM loading and SOD were needed to completely eliminate water quality standard violations in this estuary.