DDE in Sediments of the Palos Verdes Shelf, California: In Situ Transformation Rates and Geochemical Fate

From 1947 to 1971 the world’s largest manufacturer of DDT discharged process wastes into the sewers of Los Angeles County. Roughly 870–1450 t of DDT were released to the ocean off Palos Verdes, CA, a portion of which (∼100 t) resides in sediments on the continental shelf and slope. The most abundant DDT compound in the sediments, p,p′-DDE, is degrading by reductive dechlorination, but the rate of transformation and factors controlling it are not well understood. In order to estimate in situ transformation rates and predict the long-term fate of p,p′-DDE, box cores were collected in 1992 and 2003 from a single location on the Palos Verdes Shelf and analyzed for 8 DDT compounds and 84 polychlorinated biphenyl (PCB) congeners. The PCBs show no evidence of dechlorination, and inventories did not change between 1992 and 2003. By contrast, the inventory of p,p′-DDE decreased by 43%, whereas that of p,p′-DDMU, the putative reductive dechlorination product, increased by 34%. The first-order transformation rate for p,p′-DDE at the study site is 0.051 ± 0.006 yr⁻¹. A multistep reaction model suggests that inventories of p,p′-DDE and p,p′-DDMU will continue to decline, whereas that of p,p′-DDNU will reach a maximum around 2014.