Researchers identify caustic alkaline waste in 'halo' barrels off Southern California

Scientists have determined that thousands of submerged “halo” barrels discovered off Southern California contain a caustic alkaline waste that is leaching into the seafloor and producing distinctive white rings, not the pesticide DDT as many feared.

Researchers from the Scripps Institution of Oceanography and collaborators reported in PNAS Nexus that sediment sampled around the barrels was highly alkaline, with pH values near 12, and chemically altered into a concrete-like crust made primarily of the mineral brucite. The finding indicates that the halos are formed when alkaline waste reacts with seawater chemistry, not by acidic sludge associated with DDT production.

The team used the remotely operated vehicle SuBastian to collect sediment and crust samples from a set distance around barrels in the San Pedro Basin, near Los Angeles, where the containers were first observed in 2021. Investigators expected acidic signatures if the barrels contained DDT-related waste, because one major waste stream from DDT manufacture was acidic; instead they found extreme alkalinity. "One of the main waste streams from DDT production was acid and they didn’t put that into barrels," lead author Dr. Johanna Gutleben said. "It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?"

Laboratory analysis showed the hardened crust to be largely brucite — a magnesium hydroxide mineral. The researchers concluded that an alkaline substance leaking from the barrels reacted with magnesium in seawater to precipitate brucite, which cemented the sediment into a stiff, concrete-like matrix. As brucite slowly dissolves, it maintains locally elevated pH and drives further reactions that convert surrounding sediment to calcium carbonate, producing the white, dusty halos visible in remotely captured images.

Although the wider seafloor around the dump sites is contaminated with persistent pollutants including DDT, the study found that DDT concentrations did not vary with distance from the halo barrels, suggesting the halos are unrelated to that pesticide. Southern California was a major site of DDT manufacture; records indicate the Montrose Chemical Corporation in Torrance produced large quantities of DDT and reportedly may have dumped acidic sludge containing the chemical into federal waters between 1947 and 1982. More broadly, the Environmental Protection Agency has documented that 14 deep‑water sites off Southern California were used for legal dumping from the 1930s through the early 1970s, receiving refinery wastes, filter cakes, oil drilling wastes, chemical wastes, refuse, military explosives and even some radioactive material.

The barrels’ alkaline leakage has transformed local microbial communities in a manner the researchers compared to the environmental effects of deep-sea hydrothermal vents. Within halo rings, only specialized extremophile bacteria typically associated with high-pH environments survive; overall biodiversity inside the halos is markedly lower than in surrounding sediments. "It’s shocking that 50-plus years later you’re still seeing these effects," said co-author Dr. Paul Jensen, a marine microbiologist at Scripps. The team said the persistent, localized change in seafloor chemistry demonstrates long-term ecological consequences from historic dumping.

The discovery was enabled by direct sampling of the crust, which in situ appears impermeable; the ROV had to chip away fragments and bring them to the surface for pH measurements and mineralogical analysis. Those measurements were crucial to distinguishing an alkaline waste source from the acidic sludge long associated in public discussion with the region’s DDT legacy.

The authors noted that historical records of ocean disposal are fragmentary and biased toward concerns already under investigation. "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. The research team cautioned that mapping and assessing the full extent of pollution at historic dump sites will require looking for a wider variety of chemical signatures, including alkaline wastes that previously received less attention.

Regulatory agencies and researchers will need to reconcile the new chemical evidence with historical disposal records to quantify environmental impact and guide future monitoring. The study underscores how legacy waste, legally deposited decades ago, can continue to alter ocean chemistry and benthic ecosystems long after dumping ceased.