Toxic legacy: Decades-old DDT found in deep sea fish

In the mid-20th century, the ocean off the Los Angeles coast became a disposal site for significant quantities of the pesticide Dichlorodiphenyltrichloroethane (DDT), due to the activities of the largest DDT manufacturer in the nation. 

This chemical, notorious for its persistent toxicity, continues to impact the marine environment of L.A. more than 50 years later. Although legal at the time, the full scale of this pollution at an offshore dump site near Catalina Island, 15 miles offshore, only came to broader public attention in 2020, raising substantial concern among scientists and the public alike.

DDT chemicals in deep-sea fish

Recent findings from researchers at UC San Diego’s Scripps Institution of Oceanography and San Diego State University (SDSU) have shown that deep-sea fish and sediments near the Catalina Island dump site are still laden with DDT-related chemicals, indicating that DDT-related chemicals discarded into the ocean decades ago continue to influence marine food webs.

Since the rediscovery of this dump site near Catalina Island, scientists have endeavored to determine the current scope and severity of the contamination. It remains a critical concern whether these old chemicals, now settled on the ocean floor, are confined to the seabed or are circulating through marine ecosystems, potentially endangering wildlife and human health.

DDT contamination in marine food webs

“These are deep-sea organisms that don’t spend much time at the surface and they are contaminated with these DDT-related chemicals,” said co-author Lihini Aluwihare, a professor of ocean chemistry at Scripps. 

“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.”

Montrose Chemical Corporation

From 1948 to at least 1961, barges hired by Montrose Chemical Corporation would journey from the Port of Los Angeles towards Catalina, discharging waste containing sulfuric acid and up to 2% pure DDT directly into the Pacific. 

This practice, which was legal until 1972, largely went unnoticed, overshadowed by another disposal method used by Montrose: discharging a more dilute acidic slurry that also contained DDT into the ocean off Palos Verdes through L.A. County sewers. 

The Palos Verdes Shelf, where an estimated 100 tons of DDT accumulated in sediments, was declared an underwater Superfund Site by the Environmental Protection Agency in 1996, and in 2000, Montrose was ordered to pay $140 million for environmental restoration. 

Research has linked DDT pollution in this area to health issues in local sea lions, dolphins, bottom-feeding fish, and even coastal California condors, likely affected by consuming contaminated marine animals.

Toxic legacy of offshore dumping 

The rediscovery of Montrose’s offshore dumping near Catalina, now known as Dumpsite 2, was made by UC Santa Barbara researcher David Valentine using an undersea robot. This discovery gained significant public exposure in 2020 through a series of investigative reports by the Los Angeles Times, highlighting the toxic legacy of offshore dumping in the region.

Valentine and Scripps researchers have since mapped extensive DDT contamination across an area of the seafloor larger than San Francisco. The ongoing challenge is to determine whether this pollution remains localized or if it is migrating through the marine environment in ways that could harm aquatic life or humans.

Extent of the DDT contamination 

In 2021, Aluwihare, SDSU’s Eunha Hoh, and their team began addressing two crucial questions: Are the DDT-related chemicals on the seafloor near Dumpsite 2 entering the food chain? Can they develop a unique chemical fingerprint for the contamination from Dumpsite 2 to distinguish it from pollution emanating from the Palos Verdes Shelf?

Sediment and deep-sea animal samples collected near Dumpsite 2 revealed the presence of an extensive array of DDT-related chemicals, previously identified in the blubber of dolphins off Southern California’s coast. This broader spectrum of DDT compounds, known as DDT+, is critical for understanding the full extent of contamination.

DDT-related compounds in fish

Analysis of these samples showed that 215 fish across three species near Dumpsite 2 contained 10 DDT-related compounds, all present in the sediment samples. 

Two fish species were collected from depths between 546 meters (1,791 feet) and 784 meters (2,572 feet), and a third species ranged from these depths up to the surface. Fish from shallower waters had lower contaminant levels and lacked two of the DDT-related compounds found in deeper-dwelling fish.

How were the fish exposed to DDT?

“None of these fish species are known to feed in the sediment of the seafloor. There must be another mechanism that is exposing them to these contaminants,” said co-author Anela Choy, a biological oceanographer at Scripps. 

“One possibility is that physical or biological processes are resuspending sediments around Dumpsite 2 and allowing these contaminants to enter deeper water food webs.”

While it’s not yet possible to definitively exclude the Palos Verdes Superfund Site as a contamination source, the study’s findings – including the specific contaminants identified and their distribution across different species and depths – suggest that pollution from the seafloor could be infiltrating the marine food chain.

“This is evidence that DDT compounds are making their way into the deep ocean food web,” said lead author Margaret Stack, an environmental chemist at SDSU. “That is cause for concern because it’s not a big leap for it to end up in marine mammals or even humans.”

This “will help us figure out what to do as far as mitigation and what not to do in terms of offshore development that could make this problem worse by stirring up these contaminants,” Hoh added.

Further research is needed

Aluwihare highlighted the need for further research to identify the source of the DDT contaminants found in the deep-sea fish and to determine whether similar contamination affects larger, open-ocean fish species consumed by people.

As the investigation continues, the researchers are analyzing samples from fish species targeted by recreational anglers and commercial fisheries, including basses and sanddabs. Comparing these findings with sediment samples from the Palos Verdes Shelf and Dumpsite 2 may help pinpoint the source of the toxins.

“We are still seeing this DDT contamination in deep-sea organisms and ocean sediments more than 50 years after they were dumped there. I’m not sure if that company expected the consequences of their pollution to last this long, but they have,” Hoh concluded.

The study is published in the journal Environmental Science & Technology Letters.

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