LOS ANGELES – A DDT deposit in the ocean off Los Angeles County has rapidly shrunk, shocking experts and casting doubt on the need to mount a controversial $60-million Superfund cleanup, according to new data.

For decades, government officials and scientists have estimated that 110 tons of the banned pesticide – the world's largest deposit of DDT – have been sprawled on the ocean floor, where it was discharged by a now-defunct Los Angeles company.

But now only about 14 tons remain, according to the latest testing by the U.S. Environmental Protection Agency.

What has happened to the DDT off Palos Verdes Peninsula is, for now, a scientific mystery. Scientists have no explanation for how almost 90 percent might have vanished in a mere five-year period – between tests in 2004 and 2009 – after decades of a slow, gradual decline.

“It’s a dramatic decrease,” said EPA site manager Judy Huang. “It’s a lot smaller. We don’t fully understand why.”

Mark Gold, associate director of the UCLA Institute of the Environment and Sustainability, said he was in “absolute shock.”

“There was no foreshadowing of this,” said Gold, who has served on the site’s technical review committee since the 1990s. “There’s an order of magnitude less DDT today than what was there five years ago. It doesn’t make sense to me that this degree of a change would have occurred within the last five years. It’s very difficult to assess where it went.”

One theory is that it's a statistical fluke, that when researchers retrieved the new sediment samples from the ocean floor, they somehow missed all the hot spots of DDT. Another is that something has accelerated the pace at which the pesticide has been dechlorinating – perhaps more microbes are suddenly gobbling more of it up and breaking it down into byproducts containing less chlorine. Another possibility is that it has diffused into the water and spread throughout the ocean, although there is no evidence of that and no known impetus for such a rapid change.

“It may be that something is changing out there,” said Robert Eganhouse, a research chemist with the U.S. Geological Survey who began studying the site in the 1970s. “You’re not having any new DDT added so over time the concentrations are going to go down. The $64,000 question is: What is it that controls it?”

A toxic legacy for fish, birds and people

Between 1947 and 1971, pesticide manufacturer Montrose Chemical Corp. discharged millions of pounds of DDT into Los Angeles County sewers, which empty two miles offshore in an area called the Palos Verdes Shelf. The depositcovers about 17 square miles of the ocean floor.

DDT was banned in the United States in 1972 because it was accumulating in the environment, causing the near-extinction of bald eagles, pelicans and other birds.

Off Los Angeles, the DDT and other banned substances called polychlorinated biphenyls, or PCBs, have been seeping into the food web, leaving fish, birds and marine mammals with high levels of contamination. Anglers are warned to not eat white croaker and other bottom-dwelling fish caught offshore from Santa Monica to Seal Beach because of the risk of cancer and neurological effects from high PCB levels. And until 2007 bald eagles on Santa Catalina Island, 22 miles off Los Angeles, had been unable to reproduce because DDT thinned their eggs.

Declaring in 2000 that the site poses “an unacceptable risk to human health and the environment,” the EPA has been planning to cover part of the Palos Verdes Shelf with a thick cap of clean sand at an estimated cost of $60 million.

But now there may be little need to build that controversial cap, which is unprecedented in deep ocean waters. The deposit not only has shrunk in size, but its peak concentrations, or hot spots, of DDT and PCBs have been dramatically reduced, according to the new EPA data and maps. The contaminant levels already are what the EPA had hoped to achieve with the cap.

“We’ve either achieved [our goal] without the cap, or close to it,” Huang said. “We have a lot better understanding now based on the 2009 data,” she said. But, “to me, this whole project is like a giant jigsaw puzzle.”

The 100 samples from 59 spots on the ocean floor were collected back in 2009. But then the EPA hired outside experts to review the unexpected findings, which were then confirmed last fall. The data, obtained by Environmental Health News, have not been made public until today.

There have been no major storms, earthquakes or landslides that could explain such a huge, rapid change.

“We’ve been seeing declines in fish tissues and surface sediments for the past 20 years. But this [new finding] was more than what we expected. It appears that the change has accelerated,” said Joseph Gully, supervising environmental scientist at the Los Angeles County Sanitation Districts, which operates the sewage outfall that discharged Montrose’s DDT into the ocean.

“DDTs don’t break down that quickly. I don’t see this as the bacteria all of a sudden eating up all this stuff. It’s a bit of a mystery,” added Steve Bay, head of toxicology at the Southern California Coastal Water Research Project, a government-funded institute that studies pollution off the coast of Southern California.

Christopher Sherwood, a USGS geological oceanographer who has studied the site's sediment since 1991, suspects that the new data may be a "statistical fluke" because there is nothing he can think of that would make DDT transform so quickly.

"If I were a betting man, I'd say that next time we look we'll see a higher inventory of DDE than we did in 2009," he said. "My personal hypothesis is that they just missed the places with the highest contamination." He acknowledged that such a huge error–missing 90 percent of the deposit–would be "really bad statistical luck."

Essentially, what really is happening on that deep ocean floor remains a mystery. "We're all just grasping at straws," Sherwood said.

A long, twisted saga

The vanishing act is the latest twist in the convoluted saga of the Palos Verdes Shelf, which is considered the most studied piece of ocean floor on Earth. Every day, hundreds of millions of gallons of the county’s treated sewage are discharged there.

The EPA declared the Palos Verdes Shelf a Superfund site in 1996. Two miles offshore in 200-foot deep ocean waters, it is one of the most unusual and challenging sites on the nation's list of the worst dumpsites.

It was the focal point of one of the largest court casesin U.S. history that has sought natural resource damages. The case, filed by the U.S. Justice Department in 1990, led to an acrimonious trial in federal court in 2000. About $140 million in settlement money was paid by Montrose, several other manufacturers and the county sanitation agency.

Ironically, at the time, Montrose company representatives had argued in court that the deposit would degrade naturally so there was no need for costly cleanup. But their key expert was thrown out by the judge when the government provided evidence that he was not a biodegradation expert and may have manipulated some data.

Montrose “was right in saying it was dechlorinating. There was no dispute,” Eganhouse said. “But their rates were not realistic. Data didn’t exist at that time to make an accurate prediction.”

Eganhouse's previous analysis of one location on the shelf shows that the breakdown product, called DDMU, is now more prevalent out there than the original DDT/DDE. Virtually nothing is known about the toxicity of that substance or whether it has potential effects on wildlife or human health.

“The general idea exists that the fewer chlorines, the less toxic a substance is, but nothing can replace actual toxicological studies,” Eganhouse said.

Sherwood analyzed about 30 years of DDT data from many locations on the Palos Verdes Shelf and said the rate of decline in the bulk of the sediment was only 1 to 2 percent per year, far slower than anything that could explain the reported loss of 96 tons in five years.

He is particularly suspicious about the accuracy of the new EPA data because it showed most of the PCBs had vanished too, and PCBs do not break down like DDT does.

There also was nothing in Eganhouse's analysis, albeit at only one location, to suggest that the pace of dechlorination has been accelerating. “An analysis of data between 1992 and 2003 and a 2009 core showed the rates haven’t changed out there,” he said.

The upshot, he said, is that “we cannot presently say for sure if the rates have changed in the past or if they will change in the future.”

Eganhouse is now analyzing more locations on the shelf that could provide key answers within six months about what’s happening out there now.

Evolution of microbes?

Bay said perhaps there has been an “evolution or adaptation of the microbes” that break down DDT. Maybe something “revved them up” since 2004, after a decades-long lag, he said.

James Tiedje of Michigan State University said these types of chlorine-gobbling microbes “can have logarithmic growth.” The growth of their colonies starts out slowly, then reaches a point of exponential growth that transforms the entire environment.

“Some can grow up over night. There’s a number of examples where microbes in nature grow up and increase their numbers substantially and increase their rate of dechlorination,” said Tiedje, Distinguished Professor of microbiology and director of the university’s Center for Microbial Ecology.

Changes in carbon, for one, can cause an explosion of growth because it provides more food. “If more carbon is breaking down, especially when anaerobic, the microbes reach fatty acids and hydrogen and that’s what the dechlorinators like,” Tiedje said.

There has been a major change in the conditions of that part of the ocean floor. Los Angeles County switched to full secondary sewage treatment in late 2002, which greatly reduced the volume of organic substances released into the ocean and increased the oxygen in the system. Tiedje, however, thought that might have the opposite effect, slowing the breakdown of the chlorinated chemicals.

Another possibility – one that alarms some experts–is that the chemicals diffused into the water so they are no longer on the ocean floor.

Gully of the county sanitation agency said that scenario is concerning because it means more fish in a wider area could be contaminated directly through the water, not just through the food web. Any cleanup would be virtually impossible. Water testing, however, has not detected any increase, he said.

Fish and birds are the real indicators

When the area’s bottom-dwelling fish were last tested in 2004, they remained highly contaminated, “and that’s the part people shouldn’t lose sight of," said Gold, who was executive director of the environmental group Heal the Bay before joining UCLA last year.

“Despite the dramatic drop in the sediment, we have not seen similar, commensurate drops in the fish. This recent result should lead to a much greater, more focused look at fish concentrations,” Gold said.

Tiedje said the fish–and consequently the eagles, seabirds and the rest of the food web–may remain contaminated for a long time, since fish are “good scavengers.”

“You have to have these sediments really cleaned up for it to have an effect on the fish,” he said.

Huang said the EPA now plans to do more sediment testing this fall to see what’s happened since 2009.

“We should go out and take another look because of the difference between 2009 and 2004,” she said. One lingering question: If it was 14 tons in 2009, what is it now? “We know it’s declining. It is getting smaller. How much smaller I do not know,” she said.

According to its 2000 plan, the EPA’s goal for capping the site was to “immediately bring” average DDTconcentrations to 78 parts per million, down from the 150 ppm found in 2004. But the average is now calculated at 58 ppm. For PCBs, the goal was to reduce it to 7 ppm and the new data show it is now only 0.23 ppm.

"We’ve had concerns about capping it from day one,” Gully said. “The risks associated with putting a cap down are not insignificant. If it’s not done well, it can make a bad situation way worse.”

Nevertheless, at EPA, “capping is definitely still under consideration,” Huang said. But she added that other options now must be considered since it is so much smaller. One idea is to find ways to enrich the microbes so they work even faster.

More paralysis by analysis?

Gold worries that the shrinking deposit will lead to more paralysis at EPA. “The inaction of the last decade in a weird way is being rewarded,” he said. “But from my perspective, this project shouldn’t end.”

Sherwood said all eyes will be on the next round of sampling to see if it confirms the huge drop in contamination.

"If it does, there'd be reason to celebrate," he said. "But I'm not popping open the champagne yet."

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.