How Scientists Discovered a Massive Toxic Dump Site Off the LA Coast

A long-forgotten chemical dumping ground off the coast of Los Angeles raises questions about the damage caused by deepwater waste sites.
​Image: David Valentine, UC Santa Barbara / ROV Jason
Image: David Valentine, UC Santa Barbara / ROV Jason
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A massive industrial dump site 10 miles off the coast of Los Angeles has been leaking the harmful chemical DDT into the ocean for decades, according to scientists who discovered the whereabouts of the underwater landfill.

Few people were even aware of the dumping ground until researchers led by David Valentine, a professor of microbiology and geochemistry at the University of California Santa Barbara, used submersibles to locate the 3,000-foot-deep site between Long Beach and Catalina Island.


Valentine was first tipped off to the potential existence of the dumping ground when he dug up a 1985 report by a team of scientists at the California Regional Water Quality Control Board that described the waste disposal practices of chemical companies from the 1940s to 1970s. 

“The report had been shelved and completely forgotten about,” Valentine said in an email. “I came across a reference to the report as a citation in another report from the era that I picked up.” 

“I tracked down the original report and was aghast at what I read,” he added. “They estimated up to half-a-million barrels of DDT waste were discarded to the deep ocean.” 

The discovery inspired Valentine and his colleagues to search for this long-overlooked dumping ground. He finally got the opportunity to follow up on his suspicions while leading oceanographic cruises in the area in 2011 and 2013. His team sent autonomous vehicles to look for the site in spare moments between their other research objectives.

The submersibles revealed barrels of toxic waste scattered across the seafloor, some of which appeared to be slashed open, possibly to help them sink. Valentine and his colleagues described the dumping practice as “inherently sloppy, with the contents readily breaching containment and leading to regional scale contamination,” in a 2019 study that summarized their findings.


The insecticide dichlorodiphenyltrichloroethane (DDT) was widely used in the 20th century, but was banned in the United States in 1972 due to its immense negative effects on public health and the environment. Humans exposed to high doses of DDT, usually through food sources, may experience vomiting, diarrhea, and seizures in humans. The chemical is also considered a probable carcinogen by the Environmental Protection Agency, and has adverse effects on wildlife in contaminated ecosystems.

Despite all these risks, DDT-laced barrels were legally discarded in deepwater sites in the mid-20th century by a contractor acting on behalf of Montrose Chemical Corporation of California, Valentine said.

“During this era, dumping of industrial waste was permitted,” he explained, noting that the sites have coordinates that span a radius of three nautical miles. “We focused our efforts on one of these sites. We don't have a good estimate for what's actually there. We observed barrels in our cameras, and more using acoustic imaging, but the estimates from the 1980s report put the number [of barrels] in the [hundreds of thousands].”

Montrose was once the largest producer of DDT in the United States, but the company was forced to stop manufacturing the chemical in the wake of the national ban. The company’s dumping practices were known to some extent, as Montrose was ultimately sued over a shallower DDT waste site called the Palos Verdes Shelf, which was only two miles off the coast of Los Angeles County, according to CBS News.


Scientists have tentatively linked the contamination at the Palos Verdes Shelf to elevated levels of DDT in ocean animals that live near the waste site, which could explain why California sea lions are suffering from an unusually high rate of cancer.  

“About 25% of adults, subadults, have cancer and that is an extremely alarming number,” Cara Field, medical director at the Marine Mammal Center, told CBS Los Angeles. “Given the very severely high rate and how abnormal it is, it is really important that we understand what is driving this disease in these animals.”

It’s still unclear just how much the deepwater sites have been leaking toxic chemicals, which makes it challenging to determine how these dumping grounds are affecting surrounding ecosystems. Valentine said it’s “certainly possible” that harmful contamination from the relatively unexplored waste sites may be damaging ocean life that comes into contact with them. Just weeks ago, a new expedition deployed remote-operated robots to continue mapping the extent of the DDT waste sites. 

“DDT is known to cause various problems,” he noted, “but we don't yet have a good handle on how much DDT waste is actually present or the extent to which it is being reintroduced to animals.”