Field observations and modeling of dispersion processes around ocean outfalls

Abstract

Buoyant plumes discharged from ocean outfalls are important for dispersing municipal and industrial wastewater into the coastal marine environment. Ocean outfalls are designed to produce high levels of initial dilution to minimize environmental impacts. However, very few field studies of operating ocean outfalls have been reported in the literature. In recent years comprehensive field studies have been conducted around large ocean outfall systems. One set of experiments focused on the outfall off Whites Point which is operated by the Sanitation Districts of Los Angeles County. Historically, it has been an important point source of pollutants in the Southern California Bight. The authors' results show that dispersion and mixing levels of the outfall plume depend strongly on local currents and density stratification. They developed a technique for detecting the plume under general ocean conditions and use this to examine plume structure under a variety of conditions. They estimate dilution levels, heights of plume rise, turbulent mixing rates, and the effects of time-varying currents on plume structure. More recent work has focused on modeling plume dynamics based on measurements of currents and stratification over seasonal time scales. The model has been applied to a produced water plume near Carpinteria, CA. Produced water is a pollutant associated with oil drilling activity and is discharged through ocean outfalls. A goal of this study is to predict the exposure of test populations of marine invertebrates to the produced water plume. Other recent research has examined the effects of the internal tide on the wastewater plume of the Sand Island outfall near Honolulu.