Computer simulation of DDT distribution in Palos Verdes shelf sediments

Prior to the imposition of effective source-control measures in 1970, large quantities of DDT were discharged to the Los Angeles County municipal sewer system and, subsequently, to the Pacific Ocean. A fraction of this material accumulated among sediments of the Palos Verdes shelf. While the bulk of the DDT lies 10-40 cm below the sediment surface, its fate may be affected by future wastewater-treatment decisions at the Joint Water Pollution Control Plant (JWPCP), Los Angeles County's 385-mgd treatment plant. To assess the impact of impending JWPCP secondary treatment requirements on shelf sediment quality (processes that potentially affect the distribution of chemical tracers among those sediments, including background sedimentation rate), contributions from effluent-related solids, sediment mixing, and diffusive transport were incorporated in mathematical models. Model projections of surface-sediment quality at the most heavily contaminated sites are sensitive to projected effluent solids concentrations, and thus treatmentlevel decisions at JWPCP. Full secondary treatment projections of surface-sediment DDT concentration are 75% higher than those corresponding to continuation of present treatment practice.