After defying cleanup for decades, toxins may yield to new approach
Thirty years after they were banned because of their persistence and toxicity, polychlorinated biphenyls, or PCBs, are still fouling places like the San Francisco Bay Estuary and New York’s Grasse and Hudson Rivers. Getting rid of them, through dredging or burying them, is proving to be near impossible in some places. Stanford researchers think the answer may be to fight harmful chemicals with environmentally friendly chemicals.
“We’re seeing encouraging results in the field by using activated carbon to ‘sorb’ the PCBs, rendering them much less harmful,” says Dick Luthy, a professor of civil and environmental engineering, a department he also chairs. He and his students have been working in the field since 2005 to test whether carbon can be mixed into contaminated sediment to make PCBs unavailable to the organisms that live there. “Three years ago, no one knew how they were going to do this,” says Luthy. “But we’ve now demonstrated this is all feasible to do.”
The stakes are quite high. Because of the favorable industrial use properties of PCBs, such as non-flammability and electrical insulation, U.S. companies manufactured more than 1.5 billion pounds of the oily, waxy toxins before they were banned in 1977, the EPA reports. But PCBs have caused cancer, and adversely affected the immune system, reproductive system, nervous system, and endocrine system in animals in which they have been tested. There is also clear evidence that PCBs can spread through the food web, meaning that they are passed on from smaller creatures (e.g. worms and fish) to the larger ones that eat them (e.g. people).
Can’t remove ‘em? Absorb ‘em.
The most obvious way to remediate PCB-contaminated sediments is to remove the sediment, but dredging is not only very expensive, but also can backfire, Luthy says. “Removing the top layers of sediment deposited after the PCB ban often exposes older layers that are more contaminated because they were the top layer when PCBs were still being manufactured.”
Burying the contaminated sediment under a new layer of sand or gravel also doesn’t always work well either, Luthy says. Such caps may be subject to erosion and may destroy productive habitats or fill productive wetlands.
What Luthy and his students have been doing instead is leaving the PCBs right where they are, but trapping them in a way that renders them unavailable and therefore benign for aquatic organisms. To do this, they use “activated carbon,” which is made by grinding and heating up coal and then infusing it with steam or carbon dioxide. The result is a coarse powder of granules, somewhat like ground coffee, each with lots of very small channels where PCBs can permeate and bond with the carbon.
When Luthy and his students treat a site, they apply about a one inch-thick layer and mix it into the top foot of sediment, which is the region where bottom-dwelling organisms live. The carbon concentration turns out to be about 3 percent by weight.
The technique has worked well both in lab and field testing. Two years ago Luthy’s then doctoral student Pam McLeod, now a postdoctoral researcher, led a lab-based effort to test the activated carbon idea with sediment from the Grasse River, where the industrial giant Alcoa has been trying to remediate PCB contamination. She mixed various amounts of carbon into some of the Grasse River sediment. She then put live clams in the sediment samples and let them live there for one month. Clams feed on nutrients in the sediment so they are great test organisms for evaluating PCB uptake.
In McLeod’s experiment, the clams living in the sediment with the highest concentration of carbon took in 95 percent less PCBs than clams who lived in untreated sediment. The higher the concentration of carbon, the greater the reduction was of PCB uptake in the clams.
Other lab experiments produced similar results, so last year Luthy’s team got the chance to do field testing in the San Francisco Bay along the muddy shores of Hunters Point, where a former naval shipyard has left a PCB concentration of 1 to10 parts per million. The team mixed carbon into test plots in the sediment at low tide on Jan. 25, 2006.
They went back to the site six months later, in July, to place clams there and conduct other tests. Subsequent analyses found that the test plots gave up 33 to 66 percent less PCBs than untreated plots depending on type of measurement. The reduction in the clams was substantial, although less than in lab tests. A likely reason, Luthy says, is because with only small test plots of sediment holding activated carbon, the overlying water in the field site was still rich in PCBs. Clams may have taken in some PCBs from the water. Hypothetically, treating a much wider area of sediment will result in greater reductions in PCB uptake as less of the toxins are available to seep into the overlying water.
Still, a complicating factor may have been the unusually high heat this past July in San Francisco. Could it have affected the clams? To gather more data, Luthy and his students went back last month, almost exactly a year after the experiment began, to gather sediment for lab tests with clams in temperature controlled rooms and also to augment the field testing by dipping in thin strips of polyethylene plastic, which will absorb PCBs much like clams would. Periodically over the next weeks and months the researchers will return to the site to pull out more of the strips to compare the amount of PCB absorption they experience.
Future directions
With good results so far, Luthy’s group is working with Alcoa and the Navy to perform broader scale field tests of the method in the Grasse River and more comprehensive sampling at Hunters Point.
Luthy’s group has also received a new grant to study whether ecological activity rebounds in sites treated with activated carbon. Will creatures return to and thrive in cleaned-up sediment?
“It’s a new area for me and it’s a new area for our discipline, to see how with some intervention like this the sediment-dwelling community changes,” Luthy says. “We anticipate some beneficial changes if we make the sediment a healthier environment.”
Those changes could undo decades of toxic threat to everyone from clams to human beings.
