A CE-QUAL-W2 Model of Neuse Estuary for Total Maximum Daily Load Development

In support of a total maximum daily load ~TMDL! analysis, hydrodynamic and water quality conditions in the Neuse River Estuary were simulated for a 43 month period beginning in June 1996 using the two-dimensional, laterally averaged model CE-QUAL- W2. Several modifications to the model were made for this application including the further development of the existing sediment diagenesis submodel and an empirically derived light extinction prediction. The simulation period included two extreme nitrogen loading events and three summers with chlorophyll a concentrations that exceeded the water quality standard of 40 mg/L. A multiobjective, statistically based procedure was used to calibrate the model to observed chlorophyll a concentrations. The model was then used to predict changes in chlorophyll a concentrations and water quality standard exceedances in estuary regions for a variety of total nitrogen load reductions. The model predicted that a load reduction of approximately 5% was needed to lower the frequency of water quality standard violations below 10%. The process-based modeling approach was found to be a valuable tool for a TMDL analysis, with certain reservations related to its relatively high-data needs and its inability to fully address prediction uncertainty.

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