Estuarine Water Quality Response Modeling for Tidal Flooding Susceptibility

This study investigates estuarine water quality response to coastal flooding in the Mason Creek in Norfolk, Virginia. The Mason Creek basin connects to Willoughby Bay which eventually reaches the Atlantic Ocean, serving as a primary drainage basin for both point and nonpoint source loadings from the Norfolk Naval Base and surrounding residential communities, and commercial businesses.  Due to the increase in tidal flooding frequency and accelerated climate change, loading characteristics from contributing land had been gradually changed over recent years, becoming more susceptible to water quality degradation and eutrophication episodes. The current regulatory dissolved oxygen requirement is 3.2 mg/L.

To characterize the estuarine water quality response, in situ samplings were conducted for key water quality parameters of turbidity, salinity, and dissolved oxygen over tidal cycles from multiple locations. Subsequently, flux bulk dispersion coefficients were estimated to develop a steady-state response matrix (SSRM) loading flux framework consists of four equal volume segments representative of the Mason Creek basin. Results from SSRM were then used to formulate a two-dimensional, laterally averaged, longitudinal, hydrodynamic, and water quality model by using CEQUAL-W2 to further evaluate various loading and loading reduction scenarios and corresponding estuarine water quality responses with an emphasis on dissolved oxygen (DO) level as the key evaluatory variable.  One of the scenarios included the use of aerators to improve DO during months with low DO levels, and the result indicated DO level improvement beyond regulatory dissolved oxygen requirement in three out of the four segments in the Mason Creek basin.