What Happened to Toxins in L.A. Seafloor Superfund Site?

Scientists are investigating a mystery off the coast of Los Angeles: the process by which 200,000 pounds of toxic material at a badly contaminated seafloor site seem to have dispersed.

Decades ago, massive amounts of DDT and PCBs were dumped off L.A. County's Palos Verdes Peninsula. The area was declared a Superfund site.

But recently, according to the Los Angeles Times, "samples taken from the sediment suggest more than 100 metric tons of the banned pesticide DDT and industrial compounds known as polychlorinated biphenyls, or PCBs, have vanished from one of the country's most hazardous sites, almost a 90 percent drop in just five years."

As a result, the U.S. Environmental Protection Agency has suspended cleanup efforts and ordered new tests over the next year.

Scientists have several theories, the Times says:

The chemicals could be escaping into the water. Or they may have been covered by clean sediment deposited by the Palos Verdes Peninsula's landslide-prone coastline.

EPA officials are nearly certain some of the polluted material is being swept off the shelf into waters more than 3,000 feet deep. They also say the compounds are shedding chlorine to become less toxic.

"We know it is breaking down, we just don't know the mechanism," said Judy Huang, project manager for the Palos Verdes Shelf Superfund site.

One scientist told the Times that the seafloor is just one indicator of the toxic chemicals' presence in the environment.

"The most important information is whether fish are still contaminated," said Guang-yu Wang, senior scientist at the Santa Monica Bay Restoration Commission. "For now, all the data are showing they are still unsafe to eat."

Here's the story from the Associated Press:

LOS ANGELES — The Environmental Protection Agency has suspended planned cleanup efforts at a California site where industrial waste on the ocean floor appears to be declining naturally.

Scientists are at a loss to explain the rapid drop of toxic chemical levels across the 17-square-mile site about 200 feet below the ocean surface and two miles off the Los Angeles County coast, the Los Angeles Times reported Sunday.

Samples taken from sediment suggest more than 100 metric tons of the banned pesticide DDT and industrial compounds known as PCBs have dropped by nearly 90 percent in just five years.

In response to the discovery, the EPA halted its cleanup plan and ordered a new round of tests to be completed during the next year. Researchers began collecting samples from the seafloor last month.

"The precipitous drop needs to be explained,'' EPA Regional Administrator Jared Blumenfeld told the newspaper. "The question we're answering is: Is the DDT still there?''

Millions of pounds of the pesticide were dumped into county sewers from the late 1940s to the early 1970s. Wastewater, also contaminated with PCBs from industrial sites, emptied into the ocean and settled on the ocean floor. The area was declared an EPA Superfund site in 1996.

A legal battle over the pollution ended in 2000 with Montrose Chemical Corp, other manufacturers and the county sanitation agency paying $140 million in settlements.

Scientists have several theories about what's happening to the toxic chemicals.. The chemicals could be escaping into the water or covered by clean sediment deposited by the Palos Verdes Peninsula's landslide-prone coastline.

EPA officials are nearly certain some of the polluted material is being swept off the shelf into waters more than 3,000 feet deep. They also say the compounds are shedding chlorine to become less toxic.

"We know it is breaking down, we just don't know the mechanism,'' said Judy Huang, project manager for the Palos Verdes Shelf Superfund site.

Once the results of the latest tests are available, likely by the end of 2014, the agency will decide whether to proceed with the cleanup or make other plans.