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Alkaline leaks explain ‘ghostly’ white halos around sunken barrels
Images of corroded metal drums resting on the deep seafloor off Los Angeles captured widespread attention in 2020. Many barrels were ringed by bright white sediment halos, prompting questions about their contents and environmental impact. New research led by scientists at UC San Diego's Scripps Institution of Oceanography now shows that at least some of those barrels contain caustic alkaline waste leaked into sediments — not only residues of the pesticide DDT as initially suspected.
The study, published in Proceedings of the National Academy of Sciences Nexus on September 9, 2025, used remotely operated vehicle (ROV) sampling and laboratory chemical, mineral and microbial analyses to solve the halo mystery. While the analysis cannot yet identify every specific chemical inside the barrels, the team found that leaking high-pH (alkaline) fluids altered sediments, precipitated minerals and sustained extreme, low-diversity microbial communities.
Field methods and key discoveries
Researchers collected sediment cores at precise distances from five barrels near Catalina Island aboard the Schmidt Ocean Institute research vessel Falkor using the ROV SuBastian. Three of the barrels were visibly encircled by white halos. Unexpectedly, sediment inside those halos had hardened into a concrete-like crust, preventing standard coring. The team had to retrieve a shard of the crust with the ROV manipulator for laboratory analysis.
Laboratory tests measured DDT concentrations, mineralogy and microbial DNA. Contrary to early assumptions, DDT did not increase near halo barrels. Instead, pH tests revealed extremely alkaline conditions — roughly pH 12 — inside halo sediments. High alkalinity explained why microbial DNA recovery was poor and why bacterial diversity was low: only alkaliphilic (base-tolerant) microbes, similar to those found at hydrothermal vents and alkaline hot springs, were present.
Mineral analysis identified brucite (magnesium hydroxide) as the dominant component of the hardened crust. The team showed that leaking caustic fluids reacted with seawater magnesium to precipitate brucite, cementing sediments into an impermeable crust. As brucite slowly dissolves, it continues to keep the local sediment alkaline; where high-pH sediment meets ambient seawater calcium, calcium carbonate precipitates as the white dust that forms the visible halo.
Environmental context, impacts and unanswered questions
Dumping of industrial wastes in deep-water sites off Southern California occurred from the 1930s through the early 1970s. EPA records list disposal of refinery wastes, drilling residues, chemical wastes, military munitions and more. DDT was widely used and later banned in 1972; sediments in parts of the region remain heavily contaminated with legacy pesticides. Production of DDT generated both acidic and alkaline wastes, and other industries such as oil refining also produced caustic byproducts.
Study co-author Johanna Gutleben, a Scripps postdoctoral scholar, noted: "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." Senior author Paul Jensen added that he had expected alkaline waste to dilute quickly in seawater, but finding century-scale persistence suggests these alkaline discharges "can now join the ranks of DDT as a persistent pollutant with long-term environmental impacts." Prior research by co-author Lisa Levin has also shown reduced small-animal biodiversity around halo barrels.
The severity of ecological effects depends on how many alkaline-containing barrels lie on the seafloor and their chemical make-up; that number remains unknown. Visual identification of white halos may provide a rapid field indicator of alkaline contamination to guide future surveys and risk assessments near Catalina and other dump sites.
Remediation prospects
Researchers caution that mechanical removal of contaminated sediment would be technologically difficult and could worsen pollution by generating sediment plumes that release buried DDT into the water column. Instead, Gutleben and Jensen are investigating native microbes that might slowly degrade DDT in place, a possible long-term remediation strategy that avoids disruptive dredging.
Expert Insight
Dr. Elena Marquez, coastal geochemist and environmental engineer (fictional expert), comments: "This study is a clear example of how industrial legacies can produce unexpected geochemical hotspots on the seafloor. The identification of brucite crusts and sustained alkalinity highlights that chemical wastes can reshape benthic habitats for decades. From a management perspective, using visible halo signatures to prioritize surveys is a practical step toward mapping and mitigating these sites without causing further harm."
Conclusion
Scripps-led investigations have resolved the long-standing mystery of white halos around some sunken barrels off Southern California: leaking caustic alkaline waste precipitated minerals that hardened sediments and established persistent high-pH microenvironments. The discovery expands understanding of the ecological legacy of historical ocean dumping and provides a new visual marker to detect and prioritize contaminated sites. Key uncertainties remain — notably how many alkaline barrels are present and precisely which chemicals they contain — but the research highlights the need for targeted surveys, careful risk assessment, and research into microbial attenuation as a low-impact remediation pathway.
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