For several years now, one question has held the key to understanding just how much we should worry about the hundreds of tons of DDT that had been dumped off the coast of Los Angeles:
How, exactly, has this decades-old pesticide — a toxic chemical spread across the seafloor 3,000 feet underwater — continued to reenter the food web?
Now, in a highly anticipated study, researchers have identified tiny zooplankton and mid-to-deep-water fish as potential links between the contaminated sediment and the greater ecosystem.
For the first time, chemical analyses confirmed that these deep-sea organisms are contaminated by numerous DDT-related compounds that match similar chemical patterns found on the seafloor and animals higher up on the food chain.
“This DDT pollution happened several decades ago, there’s no new source, it’s been banned ... but this old source is still polluting the deep-ocean biota, which is really alarming,” said Eunha Hoh, whose lab at San Diego State’s School of Public Health led the study’s chemical analysis. “We’re not talking about zooplankton collected in 1960 — we’re talking about zooplankton collected now, in the deep ocean, that is still polluted with DDT.”
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Hoh’s team had already found significant amounts of DDT-related chemicals in present-day dolphins and coastal-feeding condors (and a recent study by another team even connected an aggressive cancer in sea lions to DDT). But even though DDT has clearly been accumulating at the top of the food chain, how the DDT reached these animals has been somewhat of a mystery. Key questions remain on whether it has been coming from more shallow sources (such as the Palos Verdes Shelf Superfund site, where DDT had been discharged for years via the sewer system), or from the deep-sea sediment itself.
“It really [hits home] this concept that nothing is untouched,” said Lihini Aluwihare, a chemical oceanographer whose lab at UC San Diego’s Scripps Institution of Oceanography helped piece together the many multi-disciplinary aspects to the study. “Establishing the current distribution of DDT contamination in deep-sea food webs lays the groundwork for thinking about whether those contaminants are also moving up through deep-ocean food webs into species that might be consumed by people.”
The study, published Monday in Environmental Science & Technology Letters, is one of many research efforts sparked by a 2020 Los Angeles Times report that detailed the little-known history of ocean dumping off the Southern California coast — and how the nation’s largest manufacturer of DDT had for years disposed of its waste at sea.
One team of scientists, in an attempt to map and scan the seafloor for DDT-related waste, discovered instead a multitude of discarded military explosives from the World War II era. Another team unearthed records showing that barrels of radioactive waste had also been dumped at sea.
And during an urgent investigation into old and forgotten records, the U.S. Environmental Protection Agency discovered that from the 1930s to the early 1970s, 13 other areas off the Southern California coast had also been approved for all manner of dumping — including the disposal of various refinery byproducts and 3 million metric tons of petroleum waste.
Barrels of DDT waste — along with other chemicals — were likely poured directly into the ocean near Catalina Island, according to federal regulators.
As for the DDT, which is short for dichlorodiphenyltrichloroethane, scientists have so far confirmed that much of what’s still sitting on the seafloor remains in its most potent form and is buried barely 6 centimeters deep — raising concerns about just how easily it could remobilize and spread by reentering the food web.
In a world dominated with concerns over microplastics and “forever chemicals,” DDT persists as an unresolved problem — long after the pesticide was banned in 1972 following Rachel Carson’s book “Silent Spring.”
DDT was banned 50 years ago, but its toxic legacy continues to affect the California marine ecosystem and threaten various animal species.
With this latest study, researchers sought to demonstrate how the chemical is still likely making its way up from the deep seafloor by coming into contact with zooplankton, which get eaten by deep-sea fish, which then swim around and get eaten by midwater fish and marine mammals higher and higher up the food chain.
Hoh joined forces with Aluwihare’s lab at Scripps, where a microbiology team also provided sediment analysis and a deep-sea biologist helped determine which organisms to sample — and where across Southern California to collect them.
In addition to zooplankton, which are a window into the base of the food chain, one particular type of fish, myctophids, proved to be key.
Also known as lanternfish, myctophids are tiny, unassuming fish that travel remarkable distances from the deep ocean all the way to the surface. (One of the most abundant and widespread fish in the world, myctophids make up roughly 65% of all deep-sea biomass on Earth.) The researchers methodically ground up each fish sample, extracted the lipid (DDT tends to be stored in fat), and assessed the contamination with an unprecedented level of scrutiny.
The findings have been sobering: Wherever they looked, they found DDT. Even the “control” samples they tried to collect — as a way to compare what a normal fish sample farther away from the known dumping area might look like — ended up riddled with DDT.
“This is one of the missing pieces that we’ve been waiting to see,” said David Valentine, who has been leading the broader research community on this issue since his team at UC Santa Barbara first shed light on the startling amounts of DDT still spread across the seafloor. “We know there’s a ton of stuff down there ... but seeing these compounds in deep-dwelling organisms really points to a link.”
Valentine, who was not involved in the study, noted a number of interesting new clues.
One key to tracking the legacy of DDT through the marine ecosystem is identifying and then comparing the patterns of every chemical that appear in various animals — a technique called “non-targeted analysis.” That can help fingerprint where all the DDT is coming from, and how it’s moving and accumulating at different levels of the food chain.
Monitoring programs typically use a targeted approach — searching for only four to eight specific DDT compounds. But by using non-targeted methods, scientists in this new study were able to identify an entire suite of DDT-related chemicals, including a particularly suspicious compound, TCPM, that poses unknown threats to the ecosystem. These currently unmonitored chemicals were also present in the blubber of dolphin carcasses that had washed ashore, as well as in the sediment collected near the known dumping area.
“This gives us a much more realistic view of what the potential ecological and human health impacts can be,” said Mark Gold, an environmental scientist at the Natural Resources Defense Council. The study, he says, lays bare how the traditional approach to testing and monitoring for only a few DDT compounds “grossly underestimates the concentrations of DDT in the sediment and in organisms.”
Gold, who was not affiliated with the study but has spent more than 30 years pushing for DDT cleanup along the coast, said much more work needs to be done on all fronts to truly reckon with the chemical’s legacy in Southern California. In addition to the DDT spread across the deep sea and the Palos Verdes Shelf, the mouth of the Dominguez Channel has also been identified for decades as a hot spot.
The road ahead is long. Twenty-four members of Congress, led by U.S. Sen. Alex Padilla (D-Calif.) and Rep. Salud Carbajal (D-Santa Barbara), recently urged the Biden administration to dedicate long-term funding to both studying and remediating the issue. Officials at the EPA, meanwhile, have been considering their next steps in collaboration with a number of state and federal agencies.
Academic research groups, including the ones in San Diego and the one led by Valentine at UC Santa Barbara, are also continuing to seek answers. Chief among them are determining the boundary of the dump site, mapping the spread of the pollution and tracking its migration across the food web.
For environmental chemists Margaret Stack, the first author of the latest study, and fellow author Raymmah Garcia, a doctoral candidate at Scripps, seeing once-popular pesticides such as DDT continue to move so pervasively through the ecosystem makes them wonder about all the other chemicals still being used today without question — chemicals that might also come back to haunt us many decades from now.
“I often find myself feeling frustrated when looking at this data and then seeing that we’re still using chemicals without testing them, without understanding their impacts,” said Stack, a research specialist at San Diego State’s School of Public Health. “It feels like we’re not doing anything differently.”
“How many more times,” she said, “are we going to go through the same story?”
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