Researchers Identify Caustic Alkali in 'Halo' Barrels Off Southern California

Scientists have identified a highly caustic alkaline substance leaking from thousands of submerged barrels off the Southern California coast, resolving a mystery about the white "halos" that surround the containers on the seafloor.

Researchers using the remotely operated vehicle SuBastian collected sediment from around barrels discovered in the San Pedro Basin in 2021 and found the sediment around the haloed containers to be extremely alkaline, with pH values near 12. Analysis reported in PNAS Nexus shows the alkalinity, not the pesticide DDT, is producing the concrete-like crusts and white rings that give the barrels their name.

The barrels were first noted during surveys of deep-water dump sites off Southern California. Between the 1930s and the early 1970s, federal records and historical accounts indicate that 14 designated deep-water sites were used to discard a wide range of industrial and military materials. The Environmental Protection Agency has described the historical disposals as including refinery wastes, filter cakes, oil-drilling residues, chemical wastes, refuse, military explosives and radioactive materials.

Because the San Pedro Basin and nearby areas are known to be contaminated with DDT — a pesticide manufactured in large quantities in the region in the mid-20th century — scientists initially feared the barrels contained acidic, DDT-laden sludge. That hypothesis was tested by measuring pH and chemical concentrations at set distances from haloed barrels. Instead of acidic signatures expected from DDT production wastes, the team found highly alkaline sediment and a hard, cemented crust.

Lead author Johanna Gutleben, a researcher at UC San Diego’s Scripps Institution of Oceanography, said the composition undermines the DDT-origin hypothesis. "One of the main waste streams from DDT production was acid and they didn’t put that into barrels," Gutleben said. "It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?"

Laboratory analysis identified the primary mineral in the hardened crust as brucite, a magnesium hydroxide. Researchers concluded that an alkaline waste leaking from the containers reacted with magnesium in seawater and porewater to precipitate brucite, cementing sediment grains into a concrete-like layer. As brucite slowly dissolves, it maintains high pH in the surrounding sediment. The alkaline plume then reacts with seawater to form calcium carbonate, creating the dusty white halo visible on the seafloor.

The chemical reactions appear to have long-lasting ecological consequences. Inside the halo rings, microbial communities are dominated by alkaliphilic bacteria typically associated with hydrothermal vent environments, while overall biodiversity within the rings is markedly lower than in surrounding sediments. Co-author Paul Jensen, a marine microbiologist at Scripps, said the persistence of the effect is striking. "It’s shocking that 50-plus years later you’re still seeing these effects," he said. "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."

Although the surrounding seafloor contains elevated levels of DDT that reflect the region’s industrial history, the study reports that DDT concentrations did not vary with distance from the halo barrels, suggesting the pesticide contamination is unrelated to the halo-forming leaks. Historical accounts have implicated manufacturers such as the Montrose Chemical Corporation in extensive DDT discharges to local waters during the mid-20th century, but the new findings indicate multiple waste types were placed in the deep-water sites over several decades.

The discovery reframes how scientists and regulators should approach mapping and assessing legacy pollution at the historic dump sites. "DDT was not the only thing that was dumped in this part of the ocean and we have only a very fragmented idea of what else was dumped there," Gutleben said. "We only find what we are looking for and up to this point we have mostly been looking for DDT. Nobody was thinking about alkaline waste before this and we may have to start looking for other things as well."

The authors say the results could help improve efforts to inventory and prioritize monitoring of seabed contamination at the deep-water disposal sites. Determining the number and distribution of haloed barrels will be necessary to quantify the full environmental footprint of these long-buried wastes and to guide any future remediation or management decisions. The study underscores that materials deposited decades ago can produce persistent and unexpected chemical environments on the seafloor.

The research team collected samples with the ROV SuBastian and performed laboratory pH measurements and mineral analyses to reach their conclusions. The findings are published in PNAS Nexus and add new detail to an evolving picture of how historical ocean dumping has altered deep-sea habitats off Southern California.