Transport and diagenesis of trace metals and organic matter in Palos Verdes shelf sediments affected by a wastewater outfall

Particles from the Whites Point/JWPCP outfalls operated by the Los Angeles County Sanitation District (LACSD) have been discharged onto the Pales Verd es (PV) shelf, Southern California, since the late 1930s. Since the early 1 950s, they have made a significant contribution to the sedimentary deposits on the shelf. In order to study the transport and diagenesis of organic ca rbon (OC) and associated trace metals, replicate sediment cores were collec ted during 1996 and 1997 at four different sites at the similar to 60 m iso bath on the PV shelf, and analyzed far OC, Ag, Al, Cd, Cr, Cu, Mn, Ni, Pb, and Zn. We conclude from these results that a significant fraction of OC an d associated heavy metals were transported laterally on silt particles from shallower environments. Cross-shelf transport of sediments caused multiple peaks in measured profiles of OC and trace metals at site 6C, 2 km away fr om the outfall. The same mechanism is likely to contribute to a concentrati on decrease that is smaller than that expected from decreases from the Whit es Point outfall emissions. Based on Pb/OC ratios in sediments, deposited i n 1971, and comparisons to the outfall from the same year, we estimate that 50 +/- 10% of the OC deposited in the early 1970s, now buried at 30-50 cm depth, had oxidized since that time, implying a half-life of about 26 years for the outfall-OC, as an upper limit. The average OC oxidation rate at pe ak depth (about 2 mg C cm(-2) year(-1)) is, however, only about 10% of the present-day OC accumulation rate (20 mg C cm(-2) year(-1)), which itself is adding not much more than 1% per year to the post-1950s OC inventory (simi lar to 1500 mg cm(-2)). We furthermore estimate that the OC inventory in PV shelf sediments in 1971 was equivalent to about 35% of that emitted by the outfall. OC and trace metal inventories did not decrease in the period 198 1 to 1997, contrary to those of other contaminants such as DDTs and PCBs. ( C) 2001 Elsevier Science B.V. All rights reserved.