Scientists-turned-detectives look to crack the case of the missing DDT

A mystery in the ocean off the Palos Verdes Peninsula is drawing out the detective instincts of scientists. Call it the case of the missing DDT. Over the decades, tons of the pesticide flowed through Los Angeles County’s sewers and settled into the world’s largest underwater toxic hotspot. But the last round of tests has indicated that the contamination has all but vanished.

Now researchers are working to understand why.

The lead detective is the Environmental Protection Agency, and for a while the EPA was pretty certain it knew the story. After the Montrose Chemical Company stopped sending toxic waste into the ocean, federal studies estimated 110 tons of DDT were buried in sediment 200 feet below the surface. The EPA even declared the Palos Verdes Shelf a Superfund site. 

Then, says EPA’s regional administrator Jared Blumenfeld, researchers took more samples in 2009. “What we thought would be there, and had been there before…was not there," he said.

What they found suggests there are only 20 tons of DDT off Palos Verdes, a 90 percent drop from what they found before. The findings froze cleanup plans. They forced an audit of lab work. And now scientists are retracing their steps, sampling sediment over 17 square miles of the Palos Verdes Shelf. Again.

EPA has been consulting with the Ocean Monitoring and Research Group from the Los Angeles County Sanitation Districts. Bill Power is with the sanitation districts. A few weeks back on the county’s boat, Power showed off a metal box. 

“We always call it our pipe organ because there’s three rows of three tubes in there and then inside of those metal tubes we slide another metal tube that has the core,” Power said.

Researchers shot those tubes into the silty sea floor of the Palos Verdes Shelf. Over a few weeks, they have taken about 80 new core samples from dozens of locations.

Technical advisors and other area scientists have wondered aloud whether the data from five years ago may have been off. Power doesn’t think so.

Neither does L.A. County Sanitation’s supervising environmental scientist, Joe Gully. Gully suggests the new core samples will simply reinforce the mystery of the missing DDT and underscore the central questions of the riddle:

Multiple detectives, multiple suspects

As with any good mystery, investigators are looking at multiple suspects.

Weather and ocean currents are one. Sediment transport and mixing may just have moved the DDT into deeper parts of the ocean. Another strong suspect is  microorganisms that may be chomping away at the DDT and breaking it down faster.

But those theories do nothing to explain another puzzling finding: why another group of industrial compounds called PCBs are almost completely absent from the 2009 core samples. Those test showed levels of PCBs also fell 90 percent. Mark Gold is a longtime technical advisor for researchers studying contamination in the Palos Verdes Shelf. 

“No one can come up with a scenario for the PCBs disappearing, because the PCBs are not nearly as biodegradable as the DDT,” Gold says. “It just doesn’t make sense.”

To complicate matters, tissue samples from fish swimming and feeding near the hotspot so far aren’t showing big changes, like the sediment tests have. Guang-Yu Wang, a staff scientist for the Santa Monica Bay Restoration Commission, says you’d expect a big drop in sediment pollution would mean less contamination in fish.

“But we have not seen that in fish tissue concentration,” Wang says. “It’s a big mystery. I can only guess that we will still see contaminated fish for many years to come.”

And that's what Wang and other scientists say matters most, particularly since there have been warnings against catching and eating local fish for more than 30 years.

A few years ago, EPA began moving forward with plans to clean up the toxic sediment off Palos Verdes, but those plans have now been put on hold because of the missing DDT.

The price tag for the cleanup is tens of millions of dollars, and EPA’s Blumefeld says federal officials won’t decide to do it without evidence it’s necessary.

“So the question is, from our perspective, what should a remedy look like?” he says. 

Some environmental advocates warn the clock is ticking: They say the longer it takes to decide on what to do, the more likely people will start to forget the warnings against eating local fish – especially if questions linger about the disappearing DDT.