Development of Three-Dimensional Hydrodynamic and Water Quality Models to Support Total Maximum Daily Load Decision Process for the Neuse River Estuary, North Carolina

Abstract

The Neuse River Estuary was included on the North Carolina Department of Water Quality 303(d) list for nutrients, and was scheduled for total maximum daily load (TMDL) development by spring of 2001. The water quality target of the TMDL was determined by the state to be the chlorophyll-a concentrations in the estuary. EPA Region 4 partnered with the State of North Carolina to provide technical assistance and guidance for nutrient TMDL development in the Neuse River Estuary. The goal was the development of hydrodynamic and water quality models that are sufficient to simulate the complex circulation and water quality kinetics within the system, including salinity and temperature stratification, wind driven seiching, dissolved oxygen stratification, and longitudinal and lateral variations in nutrient and chlorophyll-a concentrations. A three-dimensional, hydrodynamic, and water quality model was developed in the estuary from Maw Point at the Pamlico Sound boundary, to upstream at Streets Ferry Bridge above New Bern, North Carolina. The complex three-dimensional hydrodynamics of the Neuse estuary were modeled using the Environmental Fluid Dynamics Code (EFDC). EFDC was applied with water surface elevation forcing at the downstream boundary, freshwater inflows at the upstream boundaries, and wind over the water surface of the modeled domain. Water surface elevation, flows, currents, salinity, and temperature were simulated using EFDC. The U.S. EPA Water Quality Analysis Simulation Program (WASP6) was applied for the water quality portion of the model. The eutrophication model of WASP was used to simulate the complex nutrient transport and cycling in the estuary. The purpose of the water quality model is to predict a response in chlorophyll-a and dissolved oxygen concentrations as a function of nutrient loadings and transport throughout the Neuse River Estuary. The model was used to evaluate various loading scenarios and the impact on water quality within the “use support” areas within the 303(d) listed segments. The hydrodynamic and water quality models were calibrated for 1998 and confirmed for 1999 and 2000. A comparison of the model simulations with the extensive dataset shows that the models are accurately simulating the longitudinal/seasonal distribution of the hydrodynamics, mass transport, and water quality. The water quality model was used to evaluate TMDL scenarios.