Using Turbulence Model Results to Quantify Oxygen Reaeration in an Estuary Dissolved Oxygen Model

An alternate means of quantifying oxygen reaeration was investigated in a model of dissolved oxygen in an estuary. The three-dimensional hydrodynamic and water quality model EFDC was used to simulate dissolved oxygen (DO) conditions in North Carolina's Lower Cape Fear River Estuary. A review of DO monitoring data showed that the upper portion of the water column was frequently undersaturated with respect to dissolved oxygen even though hypoxia was not usually observed in the bottom waters. In the impaired area of the estuary, surface reaeration was expected to be a significant source of DO to the water column. Even though reaeration has been shown to be dependent on the local energy dissipation rate near the water surface, water quality models typically use macroscale measurements of wind and water velocity to establish the reaeration rate coefficient. In this study we investigated the use of results from the turbulence closure submodel of a hydrodynamic model to quantify the local energy dissipation rate and, in turn, the dissolved oxygen mass transfer coefficient. Significant differences were seen in the statistical distribution of reaeration rates. The existing formulation showed a log-normal distribution, with a relatively small number of high reaeration rates. The new formulation showed a higher abundance of relatively high reaeration rates, and these high rates were seen more often during the summer period when DO was lowest in the estuary. Although the differences in predicted DO between the two methods were not dramatic, minimum DO in the estuary was as much as 1.0 mg/L higher using the new formulation. Based upon these results, it does seem that further investigation of the method for quantifying reaeration rate in estuarine water quality models is justified.