DIGGING IT: Postdoc Dennis Smithenry and Luthy survey the Hunter's Point site as the Aquamog tills in carbon. Linda A. Cicero

dick luthy's idea can be likened to burnt toast.

As he has poked around contaminated U.S. harbors for several decades, the former Navy deep-sea diver has found plenty of cancer-causing polychlorinated biphenyls, or PCBs. “The chemicals tend to accumulate in the fatty tissues of clams,” says Luthy, a professor of civil and environmental engineering. “They don’t really kill clams or worms, but they move up the food web so that eventually you have organisms at the top of the chain with unhealthy levels of PCBs or DDT.”

In the sites he’s visited, Luthy also has turned up hunks of black carbonaceous matter. Charred wood near San Francisco. Coal in the waters bordering Milwaukee. Over decades, many of the PCBs migrate to the carbon particles. The charred wood and coal in harbor sediments absorb the chemicals and remove them from the underwater biota buffet.

Which brings us to burnt toast. Luthy recently gave a talk about his work at Bio-X, Stanford’s interdisciplinary initiative in the biosciences. The physicians in the audience “related immediately to the idea,” he says. “They’ve known for a long time that an antidote for certain kinds of poisoning is to give activated carbon. You sorb up [the poison] with the carbon and it passes through the gut and is eliminated. And if you don’t have activated carbon, in a pinch you can use burnt toast.”

In early 2000, it occurred to Luthy that maybe clams would like some extra burnt toast, figuratively speaking. “What if we added a material like activated carbon to sediments, to soak up the contaminants?” Luthy thought, after consulting with water quality control boards and with representatives of the Environmental Protection Agency about how to clean up Superfund sites. “Maybe that would be a way to handle the sediment contamination problem.”

Luthy is testing his concept in South Basin, a 100-acre bay near southeast San Francisco. The sediments are filled with PCBs from a nearby landfill, and the Navy is charged with the cleanup.

At first, Luthy says, “I was thinking we could just get out there with shovels and mix the carbon in.” But there were all those acres of tidal flats. “Some of our sponsors, like the Department of Energy, said, ‘How does that scale up? How do you go from a little garden plot to something big?’”

Luthy, who helped develop underwater pickup trucks during his Navy years, brought in a big gun last summer—an industrial-strength rototiller mounted on a barge. Operated by Aquatic Environments of Alamo, Calif., the Aquamog can float in at high tide and sit on the sediment at low tide. “What we’re doing is adding activated carbon to the upper layer of sediment—about the top foot,” Luthy says. “Contamination can go down a lot deeper, but you don’t have to treat everything—just the biologically active layer where clams, worms and shrimp-like things burrow down and live.”

KNEE DEEP: Luthy, PhD student Pamela McLeod, MS ’02, and Army researcher Rod Millward harvest clams that have spent a month in the muck. Dennis Smithenry

He and several postdocs are testing the effect of the tilled-in carbon on clams.
They place Macoma nasuta clams in cages and bury them in the treated sediment for a month, then collect the tiny bivalves and test for PCBs in their tissues.

“We’ve done a whole suite of tests in the lab to look at physical, chemical and biological measurements, and the tests show that it actually does work,” Luthy says. “You can see a reduction of up to maybe 90 percent in a short-term test of one to six months.”

Previous Superfund cleanups of marine contaminants have involved dredging or capping. But, Luthy says, there always are residual contaminants after an area is dredged, and piling several feet of sand on top of sediment can wipe out habitats and turn bays into beaches.

“Instead, we are trying to change the chemistry of the sediment,” Luthy says. “If we can control the exposure of PCBs to the organisms that live in the sediment, then we’ll stop the transfer of PCBs up the food chain—and that’s what it’s all about.” Burnt toast for everyone.

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