Contaminant dispersal on the Palos Verdes continental margin: I. Sediments and biota near a major California wastewater discharge

Discharge of treated wastewaters, over five decades, to the Palos Verdes shelf has produced a sediment mound that contains historically discharged contaminants such as DDT and PCBs. Since 1971, emissions of effluent solids and contaminants have decreased dramatically. Surface sediment quality has improved. Benthic and epibenthic organisms have concomitantly increased in range and diversity. However, the partly buried reservoir of historically discharged contaminants continues to be available to the food chain. Moreover, invertebrates and fish disturb this reservoir via locomotion, feeding, respiration and habitat formation. Vertical distribution studies at four Palos Verdes sites have shown that a variety of organisms live in sediments to depths of at least 35 cm. They penetrate throughout the zone of greatest contamination. Annelids dominate, and mollusks are usually second most common. Approximately 60–80% of the biomass and 95–98% of the organisms occupy the top 10 cm. However, important bioturbators also live in deeper sediments. The faunal composition and behavior have been parameterized in a manner that supports numerical modeling. The Palos Verdes sediment contaminant reservoir is being disturbed by bioadvection, biodiffusion, and physical processes. Sediment-bound contaminants are being biodiffused up from the subsurface to upper sediments, where they periodically undergo resuspension and redistribution.