Scientists Finally Know What's Inside Mysterious 'Halo' Barrels Sunken Off Los Angeles Coast

The toxic barrels that captured public attention in 2020 appear to have “halos” around them — and researchers have finally figured out why. (Image courtesy of the Schmidt Ocean Institute.)

Thousands of barrels of industrial waste have covered the ocean floor off the coast of Los Angeles for decades, but scientists still don't fully understand what chemicals the dump is releasing into the environment.

Now research has shown that some of the chemicals leaking from the barrel cemetery were highly alkaline, the opposite of acids, and they were still concentrated enough to kill most life living nearby.

According to the U.S. Environmental Protection Agency, between the 1930s and the early 1970s, radioactive waste, refinery waste, chemical waste, oil drilling waste and military explosives were dumped in 14 deep-sea dumps off the coast of Southern California.

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The massive underground dump came to public attention in 2020, when an LA Times article reported that deep-sea robotic surveys had found dozens of barrels scattered across the seafloor. Then, in 2021 and 2023, follow-up surveys by the Scripps Institution of Oceanography in California found nearly 27,000 barrel-like objects and more than 100,000 pieces of general trash on the seafloor. Some suspected that the barrels, many of which were surrounded by whitish halos in the sediment, contained the now-banned pesticide DDT, as the area is heavily contaminated with it.

However, to this day, the total number of barrels on the seabed and what they contain remain unknown.

Now, Scripps Institute microbiologist Johanna Gutleben and her colleagues have released the results of an analysis of sediment samples taken near five barrels using a remotely operated vehicle in 2021. They found that DDT contamination levels did not increase the closer they got to the barrels, so they say the barrels were free of the chemical.

White halos were found around three of the barrels tested, and all samples taken near these barrels had extremely high pH levels (around 12) and very few microbes living there, leading the team to argue that the barrels contained caustic alkaline waste that can damage organic matter and leach high concentrations of potentially toxic metals.

The team's research was published Tuesday (September 9) in the journal PNAS Nexus.

“Up until now we have mainly been looking for DDT. No one has thought about alkaline waste before, and we may have to start looking for other substances as well,” Gutleben said.

Analysis of the samples did not reveal what specific chemicals were in the barrels, but it should be noted that DDT production, like oil refining, produces alkaline waste.

“One of the major byproducts of DDT production was acid, and they didn't put it in the barrels,” Gutleben said. “It makes you wonder: What was worse than the acidic waste of DDT that it deserved to be put in the barrels?”

Because the researchers found very limited amounts of microbial DNA near the barrels, they say the alkaline waste likely transformed parts of the seafloor into extreme conditions where most life cannot survive. However, they did find traces of some specialized bacteria — species from families adapted to alkaline environments, such as deep-sea hydrothermal vents and alkaline hot springs.

The team also figured out how these strange halos form. When alkaline waste flows out of the barrels, it reacts with magnesium in the water to form a mineral form of magnesium hydroxide called brucite, creating a crust similar to concrete. The brucite then slowly dissolves, maintaining a high pH in the sediment and causing reactions in the surrounding seawater. This results in the formation of calcium carbonate, which settles as a white dust around the barrels.