Researchers at UC San Diego's Scripps Institution of Oceanography have finally solved the mystery of why barrels dumped on the seafloor off Los Angeles are surrounded by mysterious halos.
In a new study published in Proceedings of the National Academy of Sciences Nexus, Scripps scientists have revealed that these eerie halos are caused by caustic alkaline waste leaching out of the barrels.
“The world's oceans have been widely used as disposal sites for industrial waste,” says the paper. “This was particularly true off the coast of southern California where waste streams associated with the oil and gas industry, the military, and the manufacturing of chemicals including DDT, were legally discharged into the sewer or dumped directly into the ocean from the 1930s to the early 1970s.”
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The legacy of DTT
DDT – or Dichlorodiphenyltrichloroethane – is a pesticide that was banned in the US in the 1970s and in the UK in the 1980s because of its negative impact on the environment and its potential to harm human health. Despite this, it has lingered in the environment and still threatens marine animals today.
Some people had been concerned that these barrels might contain DDT but the researchers weren’t able to confirm exactly which chemicals were inside the barrels. Alkaline waste is produced during the DDT manufacturing process but it is also a product of other industries, such as oil refineries.
Although they did find DDT in the sediment, it didn’t appear to come from the barrels. “The concentration of DDT and its breakdown products were highly elevated relative to control sites yet did not vary with distance from the barrels, suggesting that they were not associated with the contamination,” says the study. It’s likely that the toxic chemical was discharged directly into the ocean.
“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,” says the study’s lead author Johanna Gutleben, a Scripps postdoctoral scholar. “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.”
Impact on the ocean floor
Around the barrels, the seabed had turned as hard as concrete and had a pH of around 12 – that’s the same as household bleach. These areas had also created their own ecosystems akin to hydrothermal vents with bacteria that have adapted to survive in strongly alkaline environments.
And what about the halos? The researchers discovered how they formed too. The hard, concrete-like substance around the barrels was an alkaline substance called brucite that formed when the alkaline waste seeped out of the barrels and reacted with magnesium in the water. As this dissolves into the seawater around it, it creates calcium carbonate which settles around the area like a white halo.
Removing the contaminated sediments from the seafloor isn’t a viable option. It would be logistically hard to do and the removal process might cause further harm to the environment. “The highest concentrations of DDT are buried around 4 or 5 centimetres below the surface – so it's kind of contained,” says Jensen. “If you tried to suction that up you would create a huge sediment plume and stir that contamination into the water column.”
Although the barrels were dumped decades ago, the experts are concerned about the ongoing impact of these chemicals on the marine ecosystem. According to the paper, they may “have unforeseen, long-term consequences.”
“It's shocking that 50-plus years later you're still seeing these effects,” says senior author Paul Jensen, emeritus marine microbiologist at Scripps. “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 localised impact on microbes.”
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Top image: a discarded barrel on the seafloor off the coast of Los Angeles. The image was taken during a survey in July 2021 by remotely operated vehicle SuBastian. Credit: Schmidt Ocean Institute.
