The effect of a step change in seabed depth on spreading discharged brine effluents from a two-outfall system

Discharged brine effluents from coastal desalination plants through marine outfall systems contain some reactive chemicals that may be derived from corrosion product, toxic antifoulants, antiscalants and other chemicals used in desalinating seawater. These effluents are subject to significant loss due to temporal decay that varies with water depth as the results of consumption by bacteria or radioactive decay, heat loss or evaporation through the surface, and break up or dissolution by turbulence. A far-field model using a two-dimensional decay-advection-diffusion equation with two point sources in a simple step seabed profile is presented to study the variability of decay with water depth. Analytical solutions are illustrated graphically by plotting contours of concentration to simulate the spreading of discharged brine effluent plumes from two outfalls in coastal waters. The maximum value of the compounded concentration at the shoreline is formulated and used as a measure for assessing the quality standards of coastal waters. It is found that, for coastal effluent discharges, the modern engineering practice which installs a two-port diffuser at the end of the outfall pipeline does produce low potential environmental impact.