Toxic contamination of Southern California’s coast lives on decades later: Study

Toxic pesticides dumped off Southern California's coast decades ago are staying put — deep in adjacent ocean sediments and in the fish that reside in these habitats, a new study has found.

Current levels of dichlorodiphenyltrichloroethane (DDT), an agricultural insecticide banned in 1972, remain highest in sediments and fish located closest to the old discharge sites, according to the study, published Monday in the Proceedings of the National Academy of Sciences.

DDT contamination remains persistent in the Palos Verdes region about 15 miles off the coast of Catalina Island, according to the previous research.

From about 1948 to 1961, barges contracted by DDT-producer Montrose Chemical Corporation released waste that held up to 2 percent pure DDT directly into the Pacific Ocean in this area, past studies have shown.

An estimated 100 tons of the chemical ended up in the Palos Verdes sediments offshore. Ultimately, the Environmental Protection Agency declared the area a hazardous waste site in 1996, and four years later, a judge ordered Montrose to pay $140 million in damages.

Research has since confirmed contamination and health issues in local sea lions, dolphins, bottom-feeding fish and coastal California condors. As such, the ways in which the compounds move through the food chain have been under increasing scrutiny.

Monday's study was able to reveal robust relationships between DDT concentrations in fish and their precise habitat, diet and location — information the authors said they hope will lead to more accurate fish consumption advisories.

“I was surprised by how strong the relationship was,” lead author Lillian McGill, a postdoctoral researcher at the University of California San Diego’s Scripps Institution of Oceanography, said in a statement.

That relationship, McGill continued, was “strong enough to reasonably predict DDT concentrations in a fish based on where it was caught and its diet and habitat.”

To investigate these connections, the scientists compiled information from nine datasets, which included 60 fish species in the area from 1998 to 2021.

After analyzing chemical concentrations in both ocean sediments and fish, the researchers used statistical models to unravel the relationships between DDT distribution and location, diet and habitat of the species.

In encouraging news, the researchers said they found that DDT contamination in fish has declined over time and that most fish caught in the region were safe for consumption.

“The jury is still out, but I think one of the main reasons for the decrease is that the contaminated sediment is slowly being buried by new sediment,” said co-author Brice Semmens, a Scripps marine biologist, in a statement.

“This could be why we are seeing less in the food web over time,” Semmens added.

Nonetheless, the scientists did identify some notable exceptions, including bottom-dwellers like halibut that were caught near highly contaminated areas, such as Palos Verdes.

Overall, the researchers observed that the highest DDT concentrations were found near these dump sites, even though they have been out of commission for half a century.

These findings, the authors stressed, attest to the ability of certain chemical pollutants to persist in the ocean over time. Yet their relatively localized presence and predictability could help both reduce exposure and inform future assessments of other affected species, according to the researchers.

“If DDT contamination is moving through the ocean in predictable ways, then we can mitigate our exposure while still making use of the ocean,” Semmens said.