Predicting Organic Accumulation in Sediments Near Marine Outfalls

Abstract

The impacts of wastewater discharges in coastal waters are largely exhibited in sediment composition changes. Processes controlling the accumulation of organic material in sediments near submerged sewage outfalls are described. A simplified model is then formulated for predicting particle deposition and organic accumulation in surface sediments. The model is based on mathematical descriptions of coastal transport, particle dynamics, and organic carbon cycles and includes a second-order kinetic description for the coagulation and settling of sewage particles and natural organic materials. Sample calculations are presented demonstrating the importance of coagulation and settling behavior and tidal motion in determining the pronounced changes in deposition and sediment composition near outfalls. Model applications are also presented for the Orange County and Los Angeles County outfalls using predetermined modeling coefficients. Results compare quite well to field observations for both outfalls demonstrating the capabilities of the model in predicting deposition and accumulation of organic material in surface sediments near sewage outfalls in deep coastal waters.