25,000 barrels of undissolved pesticides on the seabed

Around the mid-20th century, a series of containers of DDT pesticide were dumped into the sea and have not yet decomposed, posing a great risk to the environment .

DDT is a synthetic high molecular organic substance, containing chlorine, in the form of a white powder, with a characteristic odor, insoluble in water.

DDT is highly durable and toxic , and takes a long time to decompose in the natural environment.

A barrel of DDT at the bottom of the ocean, near the shore of Catalina Island, California. (Photo: AP).

Decades ago, industrial companies in southern California used the coast as a dumping ground for toxic chemicals, including the pesticide DDT, for years. In a new study, scientists found that DDT is still present in high concentrations on the ocean floor and has not broken down, the Guardian reported on March 24.

"We still see original DDT on the seafloor from 50, 60, 70 years ago. That shows it doesn't break down the way we thought it would," David Valentine, a scientist at the University of California Santa Barbara, said on March 23. The contaminated area on the seafloor is even larger than the city of San Francisco.

DDT was once widely used in the United States as an agricultural pesticide and sprayed in large quantities on beaches to kill mosquitoes. It has been linked to cancer, other human diseases, and even mass animal deaths.

In the 1970s, DDT was banned in the United States due to its effects on wildlife and potential risks to humans. Research has also linked exposure to the chemical to breast cancer and reproductive problems. Near the central California coast, which was also a DDT dumping ground, a 20-year study found exposure to the pollutant was linked to higher rates of cancer and herpes infections in sea lions.

Southern California was the center of DDT production in the United States. The Montrose Chemical Corporation in Torrance, S.C., produced large quantities of DDT from the end of World War II until 1982, when Congress banned the practice. During that time, up to 2,000 barrels of acid waste containing DDT were dumped into the ocean each month. Workers sometimes poked holes in the barrels to make them sink faster.

In 2021, a team of experts from the Scripps Institution of Oceanography at the University of California San Diego and the National Oceanic and Atmospheric Administration (NOAA) conducted a two-week survey using seafloor robots, sonar images, and other data. The survey found more than 25,000 barrels of waste. Scientists also found more than 100,000 man-made objects across the entire survey area.

According to the latest analysis by scientists, the densest layer of DDT is only about 6 centimeters deep in the seabed sediment . "Trawls, cables, can bring it back to the surface. Feeding animals, like a whale, can go down and dig up the seabed, and things can get stirred up," Valentine said.