Spatial and temporal distribution of contaminated, effluent-affected sediment on the Palos Verdes margin, southern California

A sedimentary deposit on the continental margin near the Palos Verdes Peninsula, California is comprised of sewage effluent and geologic materials and is contaminated with metals, pesticides (including DDT and associated compounds), and PCBs. The deposit was mapped with subbottom acoustic profilers, and sediment cores were analyzed for geochemical and physical properties to determine the volume of the deposit and the distribution and mass of contaminants. Mapping showed that the deposit ranges up to 60-cm thick, has a total volume exceeding 9 million m3, and covers over 40 km2. Virtually the entire effluent-affected deposit is contaminated with DDT and PCBs. Nearly half of the area of the deposit lies on the continental slope, but 70–75% of the volume of the deposit and total mass of DDT reside on the continental shelf. Analysis of data collected biennially since 1981 by the Sanitation Districts of Los Angeles County show that the mass of DDT has apparently decreased at some stations but has remained essentially constant at others. Temporal changes in mass per unit area of DDT are not statistically significant (at the 90% confidence level) at the most contaminated locations over a 16-yr period. The results of this mapping effort were used as a basis for modeling efforts described elsewhere in this issue.