Mysterious white halos emerging around sunken barrels of chemical waste on the seafloor off California's coast have been found to contain traces of an alkaline substance, providing a tantalizing clue to their origins.
Thousands of containers with unknown contents were dumped into the Pacific off the coast of Los Angeles, near Catalina, in the 20th century. In the past decade, researchers manning remote underwater robots have repeatedly come across their corroding remains.
A recent sonar survey detected around 27,000 of the barrels scattered across the San Pedro basin – a mere fraction of the estimated half-million that may have been dumped by the DDT industry, particularly the Montrose Chemical Company.
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Testing has repeatedly shown the noxious insecticide is still present in the seafloor in this area, but the barrels have been ruled out as its source.
"Acid waste containing DDT was stored in large above-ground storage tanks, transported to the Port of Los Angeles in tanker trucks, pumped to Cal Salvage's barges that were later towed to Disposal Site #2, and dumped into the ocean," the EPA reported in 2021.
The steel barrels, they said, more likely contained other chemical substances.
New research from a team led by Johanna Gutleben, a marine biologist at the University of California San Diego's Scripps Institution of Oceanography, concurs: while DDT and its byproducts are comparatively abundant in the San Pedro basin waters near the Catalina dumping site, they aren't more concentrated closer to the barrels, as may be expected if the barrels were the DDT's source.
But the strange white 'halos' and concretions that encircle many of these barrels can provide a clue as to their contents.
Sediment samples collected from these rings provided further evidence that their contents were not acid sludge: in fact, it's quite the opposite.
"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. We only find what we are looking for, and up to this point we have mostly been looking for DDT," Gutleben explains.
Samples of the sediment surrounding three halo-ringed barrels – some of it so solid that the researchers had to switch from their usual core sampling devices to instead deploy a robotic arm, just to chip away a piece – were brought back to the lab for analysis.
This is when Gutleben realized the samples were extremely alkaline, with a pH level so high that the only microbes to inhabit the sediment were those usually found at hydrothermal vents and alkaline hot springs.
"One of the main waste streams from DDT production was acid, and they didn't put that into barrels," Gutleben says. "It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?"
The solid material surrounding the barrels consists mostly of a mineral named brucite. Whatever the waste is, alkaline compounds appear to be reacting with the magnesium content in the surrounding seawater, forming a solid, concrete-like material.
As the brucite dissolves, it continues to elevate the pH of the surrounding sediment, forming ghostly halos of calcium carbonate in the process.
"This adds to our understanding of the consequences of the dumping of these barrels," says Scripps marine biologist Paul 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."
The research was published in PNAS Nexus.