In 2020, haunting images of corroded metal barrels in the deep ocean off Los Angeles leapt into the public consciousness . Initially linked to the toxic pesticide DDT, some barrels were encircled by ghostly halos in the sediment. It was unclear whether the barrels contained DDT waste , leaving the barrels' contents and the eerie halos unexplained.
Now, new research from UC San Diego's Scripps Institution of Oceanography reveals that the barrels with halos contained caustic alkaline waste, which created the halos as it leaked out. Though the study's findings can't identify which specific chemicals were present in the barrels, DDT manufacturing did produce alkaline as well as acidic waste. Other major industries in the region such as oil refining also generated significant alkaline waste.
"One of the main waste streams from DDT production was acid and they didn't put that into barrels," said Johanna Gutleben, a Scripps postdoctoral scholar and the study's first author. "It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?"
The study also found that the caustic waste from these barrels transformed portions of the seafloor into extreme environments mirroring natural hydrothermal vents — complete with specialized bacteria that thrive where most life cannot survive. The study authors said the severity and extent of this alkaline waste's impacts on the marine environment depend on how many of these barrels are sitting on the seafloor and the specific chemicals they contained.
Despite these unknowns, Paul Jensen, emeritus marine microbiologist at Scripps and senior author of the study, said that he would have expected the alkaline waste to quickly dissipate in seawater. Instead, it has persisted for more than half a century, suggesting this alkaline waste "can now join the ranks of DDT as a persistent pollutant with long-term environmental impacts."
The study, published today in the Proceedings of the National Academy of Sciences Nexus and supported by NOAA and the University of Southern California's Sea Grant program, continues Scripps' leadership role in unspooling the toxic legacy of once-legal ocean dumping off the coast of Southern California. The findings also provide a way of visually identifying barrels that formerly contained this caustic alkaline waste.
"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," said Gutleben. "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."
From the 1930s until the early 1970s, 14 deep-water dump sites off the coast of Southern California received "refinery wastes, filter cakes and oil drilling wastes, chemical wastes, refuse and garbage, military explosives and radioactive wastes," according to the EPA . A pair of Scripps-led seafloor surveys in 2021 and 2023 identified thousands of objects , including hundreds of discarded military munitions . The number of barrels on the seafloor remains unknown. Sediments in the area are heavily contaminated with the pesticide DDT , a chemical banned in 1972 now known to harm humans and wildlife. Scant records from this time period suggest DDT waste was largely pumped directly into the ocean .
Gutleben said she and her co-authors didn't initially set out to solve the halo mystery. In 2021, aboard the Schmidt Ocean Institute's Research Vessel Falkor, she and other researchers collected sediment samples to better understand the contamination near Catalina. Using the remotely operated vehicle (ROV) SuBastian, the team collected sediment samples at precise distances from five barrels, three of which had white halos.
The barrels featuring white halos presented an unexpected challenge: Inside the white halos the sea floor suddenly became like concrete, preventing the researchers from collecting samples with their coring devices. Using the ROV's robotic arm, the researchers collected a piece of the hardened sediment from one of the halo barrels.
The team analyzed the sediment samples and the hardened piece of halo barrel crust for DDT concentrations, mineral content and microbial DNA. The sediment samples showed that DDT contamination did not increase closer to the barrels, deepening the mystery of what they contained.
During the analysis, Gutleben struggled to extract microbial DNA from the samples taken through the halos. After some unsuccessful troubleshooting in the lab, Gutleben tested one of these samples' pH. She was shocked to find that the sample's pH was extremely high — around 12. All the samples from near the barrels with halos turned out to be similarly alkaline. (An alkaline mixture is also known as a base, meaning it has a pH higher than 7 — as opposed to an acid which has a pH less than 7).
This explained the limited amount of microbial DNA she and her colleagues had been able to extract from the halo samples. The samples turned out to have low bacterial diversity compared to other surrounding sediments and the bacteria came from families adapted to alkaline environments, like deep-sea hydrothermal vents and alkaline hot springs.
Analysis of the hard crust showed that it was mostly made of a mineral called brucite. When the alkaline waste leaked from the barrels, it reacted with magnesium in the seawater to create brucite, which cemented the sediment into a concrete-like crust. The brucite is also slowly dissolving, which maintains the high pH in the sediment around the barrels, and creates a place only few extremophilic microbes can survive. Where this high pH meets the surrounding seawater, it forms calcium carbonate that deposits as a white dust, creating the halos.
"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."
Prior research led by Lisa Levin , study co-author and emeritus biological oceanographer at Scripps, showed that small animal biodiversity around the barrels with halos was also reduced. Jensen said that roughly a third of the barrels that have been visually observed had halos, but it's unclear if this ratio holds true for the entire area and it remains unknown just how many barrels are sitting on the seafloor.
The researchers suggest using white halos as indicators of alkaline waste could help rapidly assess the extent of alkaline waste contamination near Catalina. Next, Gutleben and Jensen said they are experimenting with DDT contaminated sediments collected from the dump site to search for microbes capable of breaking down DDT.
The slow microbial breakdown the researchers are now studying may be the only feasible hope for eliminating the DDT dumped decades ago. Jensen said that trying to physically remove the contaminated sediments would, in addition to being a huge logistical challenge, likely do more harm than good.
"The highest concentrations of DDT are buried around 4 or 5 centimeters below the surface — so it's kind of contained," said Jensen. "If you tried to suction that up you would create a huge sediment plume and stir that contamination into the water column."
In addition to Gutleben, Jensen and Levin, Sheila Podell, Douglas Sweeney and Carlos Neira of Scripps Oceanography co-authored the study, alongside Kira Mizell of the U.S. Geological Survey.
