Barrel of DDT found off the coast of Santa Catalina Island in California. (Image credit: David Valentine, UC Santa Barbara/RV Jason)

A survey recently mapped over 27,000 barrels of industrial waste and DDT.

The sea bottom near southern California has been hiding a very dirty secret: decades of discarded chemicals in thousands of barrels. And the toxic debris field is even bigger than anyone expected, containing at least 27,000 drums of DDT and industrial waste, scientists recently discovered.

High concentrations of DDT (dichlorodiphenyltrichloroethane, an insecticide that was widely used for pest control during the 1940s and 1950s) were previously detected in ocean sediments between the Los Angeles coast and Catalina Island, in 2011 and 2013. At the time, scientists who searched the seafloor in the area identified 60 barrels (possibly containing DDT or other waste) and found DDT contamination in sediments, but the full extent of the area’s contamination was unknown.

Now, a research expedition presents a clearer picture of the deep-sea dump site. Their findings reveal a stretch of ocean bottom studded with at least 27,000 industrial waste barrels — and possibly as many as 100,000, researchers with Scripps Institution of Oceanography at the University of California said in a statement.

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From March 10 to March 24, a team of 31 experts onboard the Scripps research vehicle Sally Ride created high-resolution acoustic maps of the seafloor at the San Pedro Basin, covering 36,000 acres (146 square kilometers) from 12 miles (19 kilometers) off the coast of southern California to 8 miles (13 km) from Catalina Island.

Two underwater autonomous vehicles (AUVs) named REMUS 6000 and Bluefin swam through depths up to 3,000 feet (900 meters) below sea level, using sonar to pinpoint the locations of the barrels.

These containers were quite small — less than 3 feet (1 m) tall — and those that were buried looked even smaller in the sonar scans, expedition member Sophia Merrifield, a Scripps oceanographer and data scientist, said at a virtual news conference on April 27. The researchers therefore had to develop algorithms that would automate the process for identifying and counting such tiny objects, Merrifield explained.

“We needed to be able to pump hundreds of gigs [gigabytes] through an algorithm that would detect these very small, very bright targets,” she said.

Images of the 60 sunken barrels spotted in 2011 and 2013 helped the scientists calibrate their algorithms. The result categorized not only an object’s location but also its size and brightness, “so that we can do further pattern analysis and classification of the types of targets,” Merrifield said.

From the AUV scans and data analysis, the expedition scientists discovered that more than 90% of the survey area contained some debris, Eric Terrill, chief scientist of the expedition and director of the Marine Physical Laboratory at Scripps, said at the news conference. Researchers found 100,000 pieces of human-made debris and identified the subset that were likely barrels holding DDT and other types of industrial waste, Terrill said.

The seafloor survey covered 36,000 acres in the San Pedro Basin. The known dumpsite is roughly 12 miles offshore Palos Verdes, and eight miles from Santa Catalina Island. (Image credit: Scripps Institution of Oceanography at UC San Diego)

“Irreversible damage”