DDT and other chemicals left by decades of pesticide manufacturing may be spreading from the offshore sediments of the Palos Verdes Shelf (PVS), a University of Southern California Sea Grant study suggests.
The level of DDT detectable in PVS sediments has declined steadily since production of the insecticide was banned, and marine scientists have believed the decline was primarily due to the substance's chemical decay over time.
To the contrary, the Sea Grant study indicates the decline may be largely attributable to processes that continue to pull the substance out of contaminated sediments -- even deeply buried sediments -- and distribute it in water currents to areas outside the zone where it was first deposited by sewage outflow between 1950 and 1971. The findings appear in the February issue of the journal Environmental Science and Technology, published today (Feb. 1).
Sea Grant researcher Eddy [cq] Y. Zeng, Ph.D., measured the quantities of DDT and PCBs (polychlorinated biphenyls; another sediment contaminant) in the water samples collected at various distances from the ocean floor at numerous points along the PVS. He collected the samples by pumping large volumes of water (1,100 to 2,300 liters) through glass-fiber filters and resin-packed columns that collect both particulate- bound and dissolved contaminants.
Dr. Zeng found that DDT and PCBs are widely distributed in the PVS water column and appear to be leaching from the sediments. DDT was detected in all of the samples, and PCBs were detected in all but one. The highest concentrations of contaminants were consistently found at sites with the highest sediment concentrations.
Concentrations of DDT and PCBs in the water decreased
exponentially with increasing distance from the ocean floor.
Utilizing equations that describe the tendency of a compound to
go from being particulate-bound (adsorbed onto a particle) to
being dissolved in water, Zeng found t
Contact: Phyllis Grifman
University of Southern California