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In 2011, a curious marine scientist captured a series of photos of the ocean floor that left him disturbed. Using a sea drone, he documented dozens of corroding industrial barrels, scattered 12 miles off the coast of Los Angeles. Tests later showed that the sediment inside contained exceptionally high concentrations of DDT, a pesticide banned in the 1970s, and other chemical waste.
How serious his discovery was for marine life — and for the humans who consume that marine life — depended in part on whether he had captured the bulk of this eerie chemical graveyard or just a tiny piece.
A decade later, that scientist, David Valentine, a professor of biology and earth science at the University of California, Santa Barbara, has an answer. This week, a group of scientists shared the results of an extensive mission focused on mapping the area. They counted more than 25,000 barrels that they believe may contain DDT-laced industrial waste.
Astonished by the number, the scientists operating the sonar devices used to detect the barrels began running tests to make sure they were not malfunctioning, according to Eric Terrill, the leader of the expedition and director of the Marine Physical Laboratory at the Scripps Institution of Oceanography.
In an interview, Dr. Terrill compared the search to space exploration. In areas where they had expected to find, say, a single moon, the sonar images hinted at something more in the vein of the Milky Way.
“It was hard to wrap my head around the density of targets,” he said.
The findings, which were presented to California’s congressional delegation at a briefing on Monday, may help explain the extraordinarily high rate of cancer in adult sea lions in the area, Dr. Valentine, who served as a consultant on the recent mission, said. The latest images also suggest that a ticking time bomb lurks 3,000 feet below the surface.
Some of the barrels may have been languishing for as long as 70 years, Dr. Valentine estimated. But because the 3 foot by 2 foot industrial drums are now disintegrating, it is possible that the waste is more of a threat now than when the barrels were dumped there in the 1940s, ’50s and ’60s.
“As these drums potentially lose their containment function, the materials will make their way into the environment and food web,” Dr. Terrill said.
This should not affect people swimming or surfing in the area, Dr. Valentine said, because DDT does not dissolve in water. But it may have already entered the food chain, working its way into fish and other marine life, he said.
In a statement on Monday, Senator Dianne Feinstein, who organized the congressional briefing, called the barrels “one of the biggest environmental threats on the West Coast.”
“The expedition’s findings confirm fears that a large number of barrels containing DDT-laced industrial waste were dumped off the coast of California and are now impacting marine life and potentially public health,” she said.
Precisely how much DDT the barrels contain is not yet clear. It is also possible that they contain a different type of petrochemical waste, Dr. Terrill said.
The recent mission involved 31 scientists and engineers from the Scripps Institution of Oceanography, the National Oceanic and Atmospheric Administration and their partners. Last month, as they mapped 36,000 acres of steep seafloor between Catalina Island and Los Angeles — an area bigger than the city of San Francisco, as The Los Angeles Times noted — the researchers were trying to determine how many barrels lay beneath the water. The mission was inspired by a scientific paper Dr. Valentine published in 2019 and an L.A. Times investigation published in October that expanded on the paper.
The thousands of barrels now littering the ocean floor — the scientists believe that they still have captured only the tip of the iceberg — are remnants of a time before DDT carried the menacing connotations it does now.
Dichloro-diphenyl-trichloroethane was first synthesized in 1874. In 1939, Paul Hermann Müller figured out that it could kill insects, a discovery that earned him a Nobel Prize in 1948. Attitudes toward this useful tool for agriculture and fighting malaria shifted drastically after the publication of Rachel Carson’s groundbreaking 1962 environmental best seller, “Silent Spring.”
Ms. Carson warned that overused pesticides like DDT washed into waterways and moved along the food chain, threatening delicate ecosystems for birds, fish and, ultimately, humans.
The book “made a powerful case for the idea that if humankind poisoned nature, nature would in turn poison humankind,” Eliza Griswold wrote in a 2012 New York Times Magazine article about how Carson’s book ignited the environmental movement.
In 1972, DDT was banned in the United States. The fact that the seafloor off the coast of California contains remnants from DDT’s heyday has long been known. What startled Dr. Terrill, Dr. Valentine and other scientists was the density and location of the barrels, which were discovered outside previously documented dumping sites. The discovery may also help explain phenomena that other scientists have been investigating.
“The uniquely high body burden of DDT in top predators feeding in Southern California waters has been known for some time,” said Lihini Aluwihare, a Scripps chemical oceanographer who was not part of the mission, in a statement. In 2015, Dr. Aluwihare published a study that found high concentrations of DDT in the blubber of bottlenose dolphins.
“The extent of the dumping ground helps to explain some of these previous observations,” she said.
Senator Feinstein said she planned to ask the Justice Department to find out which companies dumped the barrels and to hold them accountable. Her office declined to elaborate on which companies would be investigated. Montrose Chemical Corporation, at one time the world’s largest manufacturer of DDT, was repeatedly named in the briefing. In 1990, the Justice Department filed a lawsuit against the company for discharging DDT into California’s waters. Montrose agreed to settlements worth millions of dollars.
Dr. Valentine said the team could not yet recommend a course of action for mitigating the risks presented by the barrels. Studying the barrels’ contents and toxicity is a next step, according to Christopher Reddy, a senior scientist at the Woods Hole Oceanographic Institution.
“That will allow others to estimate the current and future impacts on humans and marine life,” he said, to then determine the safest way to limit the dangers posed by “these chemical time capsules.”
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