Offshore DDT Deposit Spreading, Research 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 that DDT and PCBs are readily transported from sediment into the water column, even in the absence of any physical disturbance.

The scientist's model also predicts that ocean currents may be transporting significant quantities of DDT and PCBs from the PVS to adjacent estuaries and bays.

The widespread distribution of DDT in sediments of the Santa Monica and San Pedro basins supports that prediction, and the Sea Grant scientist believes that migration from PVS sediment is the only mechanism that could account for so much DDT there.

According to Zeng's calculations, none of the alternatives -- continued outputs from the sewer discharge pipes that originally caused the contamination, aerial deposition or land-derived runoff -- could account for such quantities of DDT.

The Environmental Protection Agency has designated the PVS accumulation of DDT and PCBs as a Superfund site.

Zeng and Richard Teh-Lung Ku, Ph.D., a professor of earth sciences in the USC College of Letters, Arts and Sciences, are now looking into the chemical fate of DDT and PCBs in offshore sediments and seawater.

The research was funded by the University of Southern California Sea Grant program, a partnership between USC and the National Oceanic and Atmospheric Administration.