Simulation of seawater quality around Wailingding Island for coral protection

Abstract

A hydrodynamic and water quality model for seawater around Wailingding Island was configured and validated. The model was further used to predict future scenarios in terms of increasement in sewage discharge and upgradation in sewage treatment, and the corresponding impacts on coral growth were assessed. Water quality parameters of interest included dissolved oxygen (DO), biochemical oxygen demand (BOD), NH4+-N and NO3−-N. The validation results showed that the correlation coefficients for hydrodynamics and water quality were about 0.9 and 0.7, respectively; and the sewage discharge optimization should mainly control dissolved inorganic nitrogen (DIN) to support coral growth. The prediction and assessment indicated that when there was no sewage discharge, the DIN concentration was approximately 0.312 mg/L, which couldn't meet eutrophication required by coral, reducing the DIN concentration economically and effectively to decrease the impacts on coral was the goal of sewage discharge optimization. When sewage discharge rose to 1500 t/d, the DIN concentration of the area with coral distributed increased by 10.3%. Considering that local sewage treatment cost was expensive and seawater exchange capacity was strong, as well as to maximize the benefits, BOD and DIN emission control standards were proposed: 218 mg/L and 42 mg/L, respectively. These results were helpful for sewage discharge optimization and coral protection at Wailingding Island.