“It makes you wonder: What was worse than DDT acid waste to deserve being put into barrels?” asks Johanna Gutleben, looking out from the deck of an expedition vessel. Her question cuts to the heart of a mystery growing on the Pacific seafloor for decades.
From the 1930s to the early 1970s, barges carried industrial sludge from refineries, chemical plants, and military installations in Southern California to 14 deepwater dump sites. Steel drums perforated occasionally to hasten their sinking were rolled overboard, contributing to a toxic repository of radioactive waste, oil-drilling waste, explosives, and chemical byproducts. For years, the common assumption had been that scores of the barrels contained DDT residue, a pesticide banned in 1972 but still lingering in appalling concentrations in sediments nearby.
That narrative changed when sediment cores next to five barrels were pulled up in deep-sea surveys in 2021, carried out using the Schmidt Ocean Institute’s remotely operated vehicle (ROV) SuBastian. Three of them were encircled by ethereal white halos on the seafloor “halos” that scientists had been stumped about since they first appeared in ROV footage. Lab analysis revealed something unexpected: Sediments within the halos were not heavier with DDT than elsewhere. Rather, they were highly alkaline, with pH readings of around 12 about the same as household bleach and virtually devoid of microbial DNA.
The chemistry pointed to caustic alkaline waste, a byproduct of such processes as refining oils and manufacturing DDT. When waste leaked into seawater, it mixed with dissolved magnesium to form brucite, a magnesium hydroxide mineral that bound the sediment together as a crusty cement. As slowly dissolving brucite, it maintained surrounding pore water at high pH. At the boundary where such a high-pH region was in contact with normal seawater, calcium carbonate was precipitated out and deposited as the white dust that forms the halos.
These conditions have persisted for more than half a century. “It’s shocking that 50-plus years later you’re still seeing these effects,” said Paul Jensen, emeritus marine microbiologist at Scripps Institution of Oceanography. The persistence challenges the assumption that alkaline waste would quickly neutralize in seawater, placing it alongside DDT as a long-term, persistent pollutant.
The biological impact is spectacular. Sediments in the halo zone hosted few bacterial families extremophiles that can survive in hyperalkaline environments such as deep-sea vents and serpentinizing springs. Taxa like Serpentinicella alkaliphila, capable of growth at pH 10.1, and cousins of Alkaliphilus transvaalensis, tolerating pH 12.5, were discovered through gene sequencing. Macrofaunal richness was also reduced, showing that these alkaline hotspots have recharted benthic communities into bounded, extreme environments.
The appearance of the visual halo signature also offers a valuable method of contamination mapping. An estimated one-third of barrels observed to date display halos, Jensen estimates, even though the true ratio is not known because tens of thousands of sonar-discovered debris targets exist. The presence of halos as a surrogate for the alkaline waste would accelerate assessment of the overall dump site without having to sample all barrels.
Technically, the discovery underscores the significance of premium ROV technology in offshore pollution forensics. SuBastian‘s manipulator arms pulled out cemented brucite crusts that resisted conventional coring, and high-definition imaging recorded halo morphology for subsequent spatial analysis. Such precision sampling and visual mapping were instrumental to correlating mineral growth with chemical leakage.
The finding also reframes the broader contamination picture. While sediments across the San Pedro Basin remain heavily laced with DDT and its breakdown products, the alkaline waste barrels represent a parallel hazard one that alters geochemistry and biology without adding to the pesticide load. In effect, two distinct legacies of mid-20th-century dumping now overlap: organic pollutants that bioaccumulate through food webs, and inorganic caustics that engineer microhabitats hostile to most marine life.
For policy-makers and environmentalists, the message is sobering. The alkaline waste has not diluted and spread widely in over five decades, confounding expectations of oceanic buffering. Its mineral by-products find their way into the physical structure of the seafloor, and its chemical legacy continues to suppress biodiversity in localised regions. As Gutleben explained, We only find what we are looking for… Nobody was thinking about alkaline waste before this and we may have to start looking for other things as well.
