Fluxes and budgets of anthropogenic metals in the Santa Monica and San Pedro Basins off Los Angeles: review and reassessment

Sediment cores collected during 1990–1991 from the Santa Monica and San Pedro Basins were dated by 210Pb and analyzed for a suite of metals to update the record of metal contamination. Downcore profiles of Cr, Cu, Zn, Ag, Cd, Sn, Sb, and Pb in the eastern slope and throughout the basin floor are characterized by a steady increase since the turn of the century to a subsurface maximum dated around 1970 and then followed by a decrease toward recent years. This trend is consistent with the history of metal emissions from sewage outfalls. The decreases from the subsurface maxima toward the sediment-water interface reflect improvements in wastewater treatment and source control practices. The distribution of anthropogenic Ba indicates that it was contributed by offshore dumping, primarily in the north San Pedro Basin. Budgets of anthropogenic metals in the deep basin account for about 12% of the Cr, 3% of the Zn, 8–9% of the Pb, and 5% of the Cu, Ag and Cd discharged from sewage outfalls. From residence times and transport pathways of water masses in the SM-SP Basin, it can be inferred that most sewage metals in the dissolved form and on fine particles are exported further offshore.