Numerical Study of Physical and Biogeochemical Processes Controlling Dissolved Oxygen in an Urbanized Subtropical Estuary: Vitória Island Estuarine System, Brazil

Subtropical estuaries such as the Vitória Island Estuarine System (VIES), located on the central coast of Brazil, are under strong anthropic pressure. Poor water quality is one of the impacts usually caused by the discharge of untreated effluents into the estuary. The purpose of this study is to understand the stress to which the VIES water quality is subjected, evaluating the dissolved oxygen (DO) balance, as well as the physical and biogeochemical processes that govern it. The computational fluid dynamics technique was used. The continuity and Reynolds-averaged Navier–Stokes equations are vertically integrated and solved numerically by the finite difference method in an orthogonal curvilinear grid. A similar procedure is performed for the advection–diffusion-reaction equation for scalar parameters such as temperature, salinity, oxygen, nutrients, and others. The results of the numerical simulations show that the main DO reduction process is mineralization. Reaeration and ocean water contributed positively to the increase in DO. As the movement of water is governed mainly by the astronomical tide, the critical moments for water quality occur in the quadrature, with less dispersion of the constituents due to lower velocities and lower dilution. In the regions of the main channel, near the Itanguá River and the Costa Channel, the largest degradations of water quality are found. Point sources had less spatial influence, with changes in DO observed only near the discharges.