ENVIRONMENTAL CONDITIONS OF DDT DECHLORINATION IN SEDIMENTS OF THE PALOS VERDES SHELF, CALIFORNIA

Sediments on the Palos Verdes Shelf (PVS), CA are contaminated with DDX (all forms of DDT), including traces of the parent compound p,p’-DDT (1-chloro-4-[2,2,2-trichloro-1-(4-chlorophenyl)ethyl]benzene and dechlorination products, principally p,p’-DDE (1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethyl]benzene). The DDX in the sediments originated from 1947-1971 discharge of wastewater from a chemical manufacturing plant through a submarine outfall into waters of PVS. The DDX can be directly taken up by sediment infauna or remobilized by resuspension into the overlying water where contact with marine organisms and bioaccumulation may occur. Birds of prey are vulnerable to the DDX contamination as they feed on marine organisms and may experience egg shell thinning. Part of the PVS is a USEPA Superfund site being considered for remediation. Microbially-mediated reductive dechlorination of DDX occurs within the sediments; butthe major DDT compound, in both field and laboratorythe biogeochemical factors controlling dechlorination rates remain uncertain.

Natural attenuation studies are focused on understanding the biogeochemical factors that affect in situ rates of DDX dechlorination. We examined pore water redox chemical indicators (dissolved iron and sulfur species, gases, trace metals) and distributions of DDX compounds in pore water of contaminated shelf sediments and in sediments from three sites on PVS as a part of a larger remediation study. Laboratory microcosm studies indicate that rates of dechlorination of DDT, DDX are impacted by the nature of the biogeochemical environment, being (1) inhibited in the sulfate reduction zone in the presence of sulfide, and (2) enhanced by methanogenic substrates (e.g. H₂). All sites had significant iron reduction zones near the surface, with the site farthest from the outfall having higher relative rates of dechlorination, and the most extensive iron reduction zone. This site also had the deepest sulfate reduction zone, and lowest concentrations of sulfide and H₂, supporting the conclusions of the laboratory microcosm studies. Sediment DDX data and pore water redox chemistry results support the conclusion that natural attenuation of DDX by reductive dechlorination is occurring in these sediments in both iron and sulfate reduction redox zones.