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