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Stanford Report, March 8, 2000

Removing toxic dirt could worsen the environment


How do you clean up dirt? Scientists have grappled with this dilemma for years as they've tried to get rid of environmental contaminants. But now a team of scientists suggests that some pollutants may be so tightly bound to pieces of sediment that they can't harm us. The research could lead to simpler and safer ways to clean up the environment.

"Maybe we should pay attention to the stuff coming off and not worry about the stuff that's bound," says Richard Luthy, the Silas H. Palmer Professor in Stanford's Department of Civil and Environmental Engineering. Luthy is part of a team that recently won the Cleanup Project of the Year Award from the Strategic Environmental Research and Development Program, which is administered by the U.S. Department of Defense in partnership with the Department of Energy, the Environmental Protection Agency and numerous other federal and non-federal organizations. The team found that the cleanup process itself can stir up toxins and expose people to hazards that would normally stay buried.

"Many people don't realize that some contaminants are so bound up that the best thing you can do is leave them alone," says research team member Richard Zare, the Marguerite Blake Wilbur Professor in Natural Science at Stanford.

The group came to its conclusions after looking at contaminants called polycyclic aromatic hydrocarbons (PAHs) in a Milwaukee harbor. A major class of organic contaminants, PAHs likely accumulate in the harbor from nearby coal-processing operations. To maintain navigability, workers must dredge the harbor and dump the resulting sediment in a disposal facility. It doesn't take long for such a facility to brim with contaminated sediment.

To study this contamination problem, the group has been performing so-called "particle scale observations." Luthy provides this analogy: "Imagine you had a mixture of salt and pepper, and with a pair of tweezers you pull out a piece of pepper and say, 'Well, I wonder what's on this?'"

That's what Luthy's group has done with samples of harbor sediment. Seb Gillette, a graduate student working on the project, demonstrates this by putting tiny grains on a platter and placing them in a chamber that detects contaminants via spectroscopy. He analyzes a spot measuring 40 microns in diameter -- about half that of a human hair. As the Milwaukee harbor spans 44 acres, that's quite a detailed analysis.

"Without this kind of in-depth investigation, it's difficult to think ahead and come up with effective strategies for cleaning up the land," argues Zare. Many people view land contamination on a large scale, so trying to clean up an entire 44-acre site would be expensive. Detailed analysis can help identify trouble spots that need remediation.

So the team set out to take a close look at how contaminants are bound to sediment particles. They put sediment samples in water and watched how easily the toxins washed off. Some washed off quickly, but most stayed bound to the particles. The scientists found that this happened because there were two different types of particles. It was as if some were sticky and others were slick.

Moreover, it turns out that the contaminants stick to the outside -- but not the inside -- of sticky particles. When remediators think about cleaning up contaminated sites, "we can try to sequester PAHs in the sediment by adding more of these [sticky] particles," says Upal Ghosh, an engineering research associate working with Luthy. In other words, maybe sticky particles could be dumped in to sop up the noxious toxins released by contaminated slick particles.

Alternatively, microbes can be enlisted to eat up and degrade PAH contaminants. Then the remaining sediment may be used for landscaping. A disposal facility would then become more like a transfer facility. Sediment would come in, be treated, and then be shipped out. This potential cleanup strategy has generated great excitement among scientists and environmentalists.

The group is now gearing up to apply this research to the San Francisco Bay. The plan is to put together a team of people to study the area's contamination problems at scales ranging from the subparticle level to the level of whole organisms.

To assess the situation, the group will start by looking at clams living in the bay. Clams eat by filtering particles from seawater, and compounds such as PAHs accumulate in their tissues. That makes them good sentinels to warn of pollution problems.

"We would like to move on to other contaminants as well," adds Ghosh. The Stanford group hopes to work with researchers from the U.S. Geologic Survey in Menlo Park, the University of California-Santa Cruz's Environmental Sciences Group and Stanford's Hopkins Marine Station to look at other long-lived, hazardous organic compounds. SR