Barrels of industrial waste dumped into the Pacific Ocean decades ago are still affecting the deep seafloor off the Los Angeles coast, a new study has found.

The discarded barrels, which have for years been encircled by mysterious white halos, have likely been leaking a caustic alkaline waste product into the marine environment, according to the study, published on Tuesday in the Proceedings of the National Academy of Sciences Nexus.

Scientists had initially linked the halos, observed in images captured in 2020, to the toxic pesticide DDT, whose manufacturing processes unleashed copious amounts of acidic waste into the ocean from the 1930s through the 1970s.

Although DDT manufacturing did produce alkaline byproducts in addition to the acidic waste, other major industries such as oil refining generated significant alkaline residuals as well, the study authors noted.

“One of the main waste streams from DDT production was acid and they didn’t put that into barrels,” lead author Johanna Gutleben, a postdoctoral scholar at the University of California San Diego’s Scripps Institution of Oceanography, said in a statement.

“It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?” Gutleben asked.

The researchers found that the caustic, alkaline waste emanating from these barrels was so powerful that it transformed parts of the seafloor into extreme environments reminiscent of natural hydrothermal vents. These vents contain specialized bacteria that thrive in conditions where most life cannot survive, the authors noted.

The severity and extent of this mystery waste’s impacts, however, depend on just how many of the barrels are planted on the seafloor and the specific chemicals they contain, the researchers explained.

Yet senior author Paul Jensen, an emeritus marine microbiologist at Scripps, said in a statement that he would have expected the alkaline waste to dissipate quickly in seawater.

Instead, he observed, the contaminants have persisted for more than half a century — indicating that this waste “can now join the ranks of DDT as a persistent pollutant with long-term environmental impacts.”

While the number of barrels remains unknown, the researchers deployed a remotely operated vehicle to collect sediment cores adjacent to five of the containers. They then analyzed the sediment samples and hardened pieces of halo barrel crust, to assess DDT levels, mineral content and microbial DNA.

As for DDT, the scientists found that concentrations of the chemical and its byproducts were highly elevated relative to control sites but did not change with distance from the barrels — leading the researchers to conclude that the DDT was not linked to this contamination.

On the other hand, they discovered that the pH levels of the microbial DNA were remarkably high, indicating that the halos contained waste that was very alkaline, also known as a base.

These sediments, the researchers observed, had low bacterial diversity compared to other surrounding sediments, and the bacteria came from families habituated to alkaline environments, such as deep-sea hydrothermal vents or alkaline hot springs.

Although the researchers were not able to identify the specific chemicals present inside the barrels, they stressed the importance in now using white halos to help track alkaline waste — and thereby determine the extent of the contamination in this region.

The authors also said that they are now experimenting with DDT contaminated sediments from the dump sites to pinpoint microbes capable of breaking down the toxic pesticide.

“This adds to our understanding of the consequences of the dumping of these barrels,” said Jensen. “It’s shocking that 50-plus years later you’re still seeing these effects. We can’t quantify the environmental impact without knowing how many of these barrels with white halos are out there, but it’s clearly having a localized impact on microbes.”