On the surface, Canal Creek looks like a postcard Chesapeake Bay tributary, with red-winged blackbirds swooping over the tidal marsh lining its banks.
But the creek flows through Aberdeen Proving Ground in Harford County, where for decades the Army produced and tested chemical weapons, and dumped its wastes indiscriminately. The creek's bottom and the marsh muck are riddled with toxic chemicals like PCBs, pesticides like DDT and metals including lead and mercury. Fish and turtles in the creek are tainted as well.
Now, researchers are working in the creek to test a new use for a familiar substance, activated carbon, to see if it can "lock up" the contaminants left in the sediments by long-gone practices. Scientists and the military hope a pelletlike form of the material, which is commonly used in water filters, may one day help rid the Chesapeake and other polluted water bodies of troublesome toxic hot spots that pose a threat to fish, wildlife and people.
It is one potential approach the Army is studying to help with many of its contaminated sites. It may be years yet before the Pentagon settles on how to proceed on all of the hot spots.
Ben Amos and Ken Cerreto, senior scientists with Exponent, a California-based consulting firm, slogged through knee-deep muck in the marsh one steamy day last week to check on the SediMite pellets they had spread at selected points along Canal Creek in frozen December. They shoved clear plastic tubes into the water and mud, capped them and tugged them out, filled with a foot or more of dark gunk.
"We think we see some changes in the color here that say there's still some carbon here," said Amos, holding one tube up. A good sign, he explained, that the pellets persisted through winter and spring.
Amos then drove the samples to the University of Maryland, Baltimore County for a scientific analysis of how the pellets are working. There, Upal Ghosh, associate professor of civil and environmental engineering and SediMite's co-developer, will test to see whether contaminants in the muck have been reduced from levels before the field trial began. He'll also assess whether worms exposed to the mud are picking up as many toxic chemicals.
Activated carbon is widely used to treat drinking water and to keep aquariums free of impurities. Although it can bind itself to certain chemicals and effectively remove them from water, getting contaminants out of sediments is far more challenging. Carbon in its raw form is a powdery substance that would easily wash away if put in open, flowing water, Ghosh explained.
So, in collaboration with Charles Menzie, a principal scientist with Exponent, Ghosh came up with a way to form pellets made of activated carbon mixed with clay and sand. The pellets sink to the bottom and work their way into the sediments — where the contaminants are — before eventually breaking up.
After a series of promising lab tests, the collaborators have been trying the pellets out in real-world conditions, including two contaminated sites in the bay, both military installations. The other's at Fort Eustis near Newport News, Va.
Aberdeen Proving Ground was selected for a field trial in part because, with its history of chemical weapons production and of testing explosives, the 72,000-acre base has multiple contaminants in sediments. The 13,000-acre Edgewood area bordering the bay and the Bush and Gunpowder rivers has so many hot spots, the entire peninsula is classified as a federal Superfund toxic waste cleanup priority.
SediMite is being tested in two spots along Canal Creek. One is the tidal marsh, which has elevated levels of PCBs, or polychlorinated biphenyls, at one time a widely used electrical insulating material that's since been shown to cause cancer in lab animals, as well as other serious health effects. At the other site further downstream, mercury, a potent neurotoxin, lurks in the sediments.
Fish and turtles from Canal Creek had elevated levels of both contaminants in them, according to a 2008 public health assessment of the proving ground by the federal Agency for Toxic Substances and Disease Registry.
Fish get toxic chemicals in their bodies by eating worms and clams that have lived and fed in the contaminated sediments. There's a risk to humans as well from eating fish and wildlife that's been in the area. Even when contaminant levels are relatively low in the wild, they tend to increase as more of the tainted food is consumed.
In lower Canal Creek, red-winged blackbirds and other marsh-frequenting birds are at risk of picking up mercury from the bugs they feed on, said Cindy Gilmour, a senior scientist with the Smithsonian Environmental Research Center in Edgewater, who is handling that portion of the field trial.
Lab tests have shown that the activated carbon in the SediMite pellets binds with the mercury, she said, and keeps it from getting into the water saturating the sediments.