Secrets of Los Angeles’ White Halo Barrels Revealed

What was so corrosive that it was encapsulated in steel and sunk on the bottom of the ocean for all time? Along the coast of Los Angeles, thousands of rusting drums have lain in darkness for over a half century, their fillings a mystery. Recently, scientists made an amazing discovery: numerous barrels are leaking extremely alkaline waste, producing inhospitable deep-sea ecosystems where nearly nothing can live.

From the 1930s through the early 1970s, the deep waters off Southern California served as a disposal site for industrial waste. The U.S. Environmental Protection Agency has identified 14 offshore locations that accepted such things as refinery sludge and byproducts of oil drilling as well as chemical waste, radioactive contents, and military ordnance. Barges took the trash out to sea, where steel drums occasionally perforated to guarantee sinking were dumped into the ocean. Historical accounts are patchy, but over 300,000 barrels are estimated to have been dumped in the San Pedro Basin.

Public pressure increased in 2020 when deep-sea robot explorations yielded photos of rusting barrels littering the seafloor, some ringed with ghostly white haloes in the sediment. These “haloes” were first suspected to indicate DDT contamination since sediments within the area are heavily contaminated with the outlawed pesticide. But in 2021, when scientists at Scripps Institution of Oceanography took samples of sediments around five drums with the remotely operated vehicle SuBastian, they detected no rise in DDT levels close to the barrels.

Instead, three halo-beset barrels whispered a different tale. The pH of the sediment was approximately 12 similar to household bleach and microbial activity was virtually non-existent. “To date we have been primarily searching for DDT. No one was concerned with alkaline waste prior to this and we will possibly need to begin searching for other substances as well,” indicated Johanna Gutleben, lead author of the study.

The origin of such alkalinity is still unclear, but both DDT production and oil refining have a history of creating strongly basic waste streams. “One of the primary waste streams from DDT manufacturing was acid and they didn’t put that into barrels,” Gutleben said. “It makes you question: What was worse than DDT acid waste to warrant being placed into barrels?”

The team found in its analysis that when alkaline waste seeps into seawater, it reacts with dissolved magnesium to create brucite, a mineral form of magnesium hydroxide. This brucite cements overlying sediment into a concrete-like crust, rendering sampling physically challenging. As the brucite gradually dissolves, it preserves the high pH of the sediment and causes additional chemical reactions that precipitate calcium carbonate, which settles as a pale white dust the halo.

These conditions simulate the chemistry of alkaline hot springs and natural hydrothermal vents, favoring extremophile microbes specialized in high pH. DNA sequencing showed low-diversity communities with a predominance of alkaliphilic bacteria, some of which were related to species that live in deep aquifers and hyperalkaline springs. Macrofaunal diversity was also lower in halo zones, as found in previous ecological surveys.

What alarms researchers is the persistence of the contamination. “It’s shocking that 50-plus years later you’re still seeing these effects,” said Paul Jensen, senior author of the study. Alkaline waste, like DDT, now qualifies as a persistent pollutant one that resists dilution and continues to alter benthic ecosystems decades after disposal.

About one-third of visually detected barrels show halos, although the actual ratio is unknown. The Scripps researchers propose halo detection as a quick, nondestructive tool for charting the extent of contaminated alkaline waste. The research also highlights the utility of state-of-the-art ROV technology in exploring dangerous sites, making accurate sediment coring, mineral sampling, and high-resolution imaging possible at depths close to 900 meters in low-oxygen environments.

The results, reported in PNAS Nexus, add depth to the portrait of Southern California’s industrial dump legacy. They expose not only the chemical and mineral changes caused by alkaline waste but also its ability to construct artificial “extremophile hotspots” on the seafloor an unintended, long-term experiment in ocean geochemistry and microbial ecology.