Until 1972, the Pacific waters of Southern California served as a dumping ground for hazardous and industrial wastes. More than 50 years later, corroded metal barrels still litter the seafloor off the coast of Los Angeles, and scientists are only beginning to understand the consequences of casually tossing them into the ocean.
Images of the barrels first surfaced in 2020, with some of them encircled by mysterious white halos on the seafloor. Experts initially linked the barrels to DDT—a toxic pesticide that Montrose Chemical Corporation regularly dumped nearby—but an EPA investigation could not confirm that hunch. Now, researchers have discovered that the halo-encircled barrels actually contained caustic alkaline waste that transformed parts of the seafloor into extreme environments.
“We only find what we are looking for and up to this point we have mostly been looking for DDT,” Johannah Gutleben, a postdoctoral scholar at UC San Diego’s Scripps Institution of Oceanography and first author of the new study, said in a statement. “Nobody was thinking about alkaline waste before this and we may have to start looking for other things as well.”
The findings, published Tuesday in the journal PNAS Nexus, show that supposedly contained alkaline waste can cause pollution 50 years down the line, leading to unforeseen consequences for benthic communities, according to the researchers.
A mystery solved by accident
When Gutleben and her colleagues set sail aboard the Schmidt Ocean Institute’s “Falkor” Research Vessel in 2021, their goal wasn’t to solve the mystery of the halos, UC San Diego Today reports. The team set out to measure contamination levels near Santa Catalina Island, using a remotely operated vehicle (ROV) called SuBastian to collect sediment cores near five deep-sea waste barrels.
Three of those five barrels did have halos, however. Inside those white circles, the sediment was as hard as concrete, preventing the researchers from extracting samples with their coring devices. Instead, they used the ROV’s robot arm to snatch a piece of the hardened sediment from one of the barrels.
Analysis of the sediment samples showed that DDT levels did not increase closer to the barrels, suggesting they are not the source of this pollutant. Samples taken near the barrels with halos showed extremely alkaline pH levels, and the hard crust proved to be made of a mineral called brucite.
This led the researchers to believe that alkaline waste leaked from the barrels and reacted with magnesium in the seawater to create brucite, which cemented the seafloor. As the brucite slowly dissolves, this keeps sediment pH levels elevated around the barrels. When seawater reacts with the alkaline sediment, circular, dusty white calcium carbonate deposits—or halos—take shape.
A perfect home for extremophiles
Only highly specialized microbes can survive in such alkaline conditions. This explains why Gutleben and her colleagues struggled to extract microbial DNA from the sediment samples taken through the halos. The few species they did detect were extremophiles adapted to alkaline environments, such as deep-sea hydrothermal vents or alkaline hot springs.
“This adds to our understanding of the consequences of the dumping of these barrels,” senior author Paul Jensen, emeritus marine microbiologist at Scripps, said in the statement. “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.”