Disappearing DDT?
Two miles off Palos Verdes Peninsula, the nation’s largest deposit of the powerful pesticide DDT lies spread across the ocean floor. As the chemical has lingered there for decades, poisoning eagles, fish and other marine life, scientists have puzzled over a pivotal question:
Is nature doing its own cleanup? Or must humans intervene?
In a scientific finding that could have profound importance for hundreds of environmental cleanups, researchers reported this month that DDE--the residual of the banned pesticide DDT--disappears naturally as it breaks down into a less hazardous substance.
The Michigan State University scientists--using sediments from the Palos Verdes shelf--conducted laboratory tests showing that DDE is quickly gobbled up by bacteria found in ocean sediments. The microbes replace a chlorine atom with a hydrogen one--a transformation that reduces the danger to ocean life.
Yet, even with this promising news, critical questions remain regarding what the laboratory experiment means about DDE in the real world--especially the 130 tons polluting 27 square miles of the ocean floor off Palos Verdes.
The new research, published recently in the journal Science, adds a new dimension to a high-stakes conflict over whether the deposit should be cleaned up or left alone.
For a quarter-century ending in 1971, Montrose Chemical Co. manufactured DDT, one of the most widely used pesticides in history, at a plant near Torrance. During that time, the factory discharged millions of pounds of the chemical into Los Angeles County’s sewage pipeline, which empties onto the Palos Verdes shelf.
DDT was banned in the United States 25 years ago after it wiped out vast numbers of birds, including bald eagles, peregrine falcons and pelicans.
But the legacy of the chemical that remains off Palos Verdes still poses a daunting environmental challenge, and lawsuits over it make up one of the largest natural resource cases in U.S history.
The federal government and the state of California are seeking hundreds of millions of dollars in damages from Montrose and six other companies, Los Angeles County and more than 150 cities that discharged into the county sewage system.
In addition, the U.S. Environmental Protection Agency has named the Palos Verdes shelf a Superfund site, and expects to propose a cleanup solution this summer. The EPA’s likely remedy is to cap part of the ocean floor with a thick barrier of sand, an unprecedented effort that could cost as much as $300 million.
Montrose Chemical Co. has long argued that a cleanup is unnecessary because DDE--the site’s most prevalent pollutant--is vanishing on its own.
But until now, most scientists believed that it was a stubborn, stable compound that didn’t break down in oxygen-free places like ocean sediments.
The discovery that DDE is transformed by natural microbes “is news to everybody,” said Fred Schauffler, the EPA’s project manager for the Palos Verdes Superfund site. “This experiment demonstrates that this degradation pathway really can occur.”
But Schauffler said the compound probably degraded much more quickly in the laboratory than it does in nature.
“Although Mother Nature may be doing something out there to take care of the DDE, it is not at all clear that it is happening at a pace quick enough to satisfy our concerns,” Schauffler said.
“I certainly don’t see this [DDE discovery] as a compelling reason for not taking any action out there,” he said.
Federal officials argue that if the pesticide off Palos Verdes was breaking down in a matter of months, as it did in the laboratory, the 30-year-old deposit would be long gone. Instead, a massive volume remains, and they suspect that it will take many more decades to naturally degrade.
As long as the chemical lies on the surface of the ocean floor, Southern California’s marine life will suffer the effects of the poison.
Fish, such as white croaker, are highly contaminated around Palos Verdes, and birds and marine mammals that eat the fish accumulate the pesticide in their tissues. Santa Catalina Island’s bald eagles still cannot produce chicks because so much DDT is passed on to their eggs. For humans, eating bottom-dwelling fish from the Palos Verdes area poses a high cancer risk.
The microbe experiment was conducted by James Tiedje and John Quensen at Michigan State University’s Center for Microbial Ecology. In their laboratory, Tiedje and Quensen added DDE marked with radioactive tracers.
In 32 weeks, microbes in the sediment transformed half the DDE into a compound with less chlorine called DDMU. DDMU does not build up in animal or human tissue as readily.
Although the research was funded by Montrose--which has high stakes in the outcome--the two scientists are nationally renowned for their work with chlorinated compounds.
Michael Aitken, an associate professor of environmental engineering at the University of North Carolina at Chapel Hill, called them “very credible scientists who did a very straightforward, well-designed experiment.”
“This proves pretty definitively that DDE can be degraded by the anaerobic microorganisms in these [Palos Verdes] sediments,” Aitken said.
But, Aitken added, the rate of the transformation found in the laboratory “cannot be extrapolated” to the ocean because the test, in several ways, does not replicate natural conditions.
A major difference between the lab and nature is that DDE was added to the water for the tests instead of being bound up in the sediment for years. That probably makes it much easier for the microbial “bugs” to consume it, Aitken said.
The lab tests were also conducted in room-temperature water--about 35 degrees Fahrenheit warmer than the ocean bottom. Microbes thrive in warmer water.
To address such uncertainties, the Michigan team has mounted new experiments that more closely replicate the temperature and other conditions of the ocean floor. Results are expected this year.
*
In the meantime, Tiedje and Quensen say the government should reevaluate its decisions on cleaning up dump sites to factor in the natural breakdown of DDE. More than 3,000 U.S. lakes, rivers and ocean areas are contaminated with DDE.
“I don’t want to portray that the rate [of degradation] we see in the laboratory would be the rate we see in the field,” Tiedje said. But he said, the experiment “shows there are organisms there and a transformation occurs, and that’s important.”
EPA officials, however, remain skeptical that letting Mother Nature take over would protect the ocean environment off Palos Verdes.
“If it were only that easy,” Schauffler said, “that would be great. But there’s still a significant amount of DDT off Palos Verdes in the surface sediments. We would hate to walk away from that.”
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
The DDT Legacy
Even thought DDT has been banned for more than 25 years, residue of the potent presticide, which breaks down into a less toxic form called DDE, remains sprawled across the ocean floor of the Palos Verdes Peninsula. From 1947 through 1971, the manufacturer, based near Torrance, discharged millions of pounds of the pesticide into the ocean via Los Angeles County’s sewage pipeline.
Bacteria on the ocean bottom transofrm DDT into DDE, which then changes into a less toxic compound known as DDMU.
