They're scattered all around the United States, more than 1,200 of them, waiting for cleanup. Some are old military bases or abandoned factories. Others are gas stations with leaky underground tanks. And they're only the beginning of a long, arduous task.
Over the next 30 years, the US may have to clean up as many as 350,000 Superfund sites at a cost of up to $250 billion, according to the US Environmental Protection Agency.
How will taxpayers pay for that?
One solution is to find cheaper cleanup technologies. One of the most promising innovations right now involves microscopic iron particles. At least four teams of researchers are using these "nanoparticles" to attack some of the most vexing underground pollutants, including chromium-6, the groundwater pollutant made famous in the movie "Erin Brockovich."
If these nanotechnologies prove successful, they could reduce cleanup costs at selected Superfund sites by 75 percent, researchers suggest, perhaps saving billions of dollars.
"Using iron nanoparticles is one of the hottest new technologies to emerge in recent years," says Paul Tratnyek, an environmental chemist at Oregon Health & Science University and one of several researchers working on the technology.
Iron's contaminant-removal power arises from the fact that it rusts. "When it does so in the presence of groundwater contaminants, it can convert them into less toxic or nontoxic materials," says Dr. Wei-xian Zhang, an engineering professor at Lehigh University in Bethlehem, Pa.
The nanoparticles are particularly useful because of their size - a single human hair is 500 to 5,000 times as wide. At that scale, they can move through microscopic flow channels in soil and rock, reaching and destroying groundwater pollutants that larger particles cannot.
"Developing new technologies capable of locating and effectively treating areas contaminated with subsurface pollutants is difficult," says Greg Lowry, an engineering professor at Carnegie Mellon University in Pittsburgh. "This is because it is often difficult to locate the exact site of contamination because records are poor for many old waste sites and the primary contamination sources, such as storage tanks, were removed many years ago.
"Some of these contaminants move deep underground, and predicting their movement is difficult. So there are few reliable technologies to treat these sites," Dr. Lowry adds.
Working with researchers at Pacific Northwest National Laboratory and the University of Minnesota, Dr. Tratnyek has found that iron nanoparticles can effectively destroy carbon tetrachloride, a toxic organic chemical once widely used in dry cleaning and in degreasers. Its widespread production and use have contaminated soil and groundwater at many sites.
But the ultra-small size of iron nanoparticles alone is not enough to neutralize carbon tetrachloride; the chemistry is crucial. For example, when Tratnyek's team compared two leading types of particles, only one converted carbon tetrachloride to a mixture of relatively harmless product consisting of iron oxide with a magnetite shell high in sulfur. "This type of nanoparticle is now commercially available," says Tratnyek. (The other type of particle, coated with oxidized boron, created chloroform, a toxic and persistent contaminant.)
Other researchers are using nanoparticles containing palladium to convert toxic organic chemicals into harmless products. The palladium enables the iron to react with organic contaminants much faster than other types of iron nanoparticles, says Dr. Zhang.
When air-conditioner manufacturer Trane tested these nanoparticles at its Trenton, N.J., plant in 2001, they reduced the toxic organic solvent trichloroethylene, or TCE, in nearby well water by about 96 percent after 12 hours. More recently, field tests have been performed at several sites including a GlaxoSmithKline pharmaceutical facility in North Carolina.
TCE, widely used to remove grease from metal parts, is found in some 60 percent of Superfund sites, according to Lowry. His research team has designed a different iron nanoparticle that also can reach underground pockets of TCE. It uses reactive iron, which quickly breaks down solvents such as TCE into harmless byproducts. This reactive iron is coated with two layers of polymers. The outer "water-loving" one allows the nanoparticles to travel through an aquifer. When it reaches TCE, an inner "water-hating" polymer layer enables the nanoparticles to contact the water-insoluble TCE.
To remove chromium-6 - the "Brockovich" contaminant - from groundwater and industrial wastewater, two researchers are assembling an iron nanoparticle inside ferritin, an animal blood protein. When removed from the ferritin, this nanoparticle can convert toxic metals into a form that makes them much easier to remove from water, according to chemist Daniel Strongin and co-workers at Temple University in Philadelphia and Montana State University at Bozeman. For example, they use the iron nanoparticles plus visible light or sunlight to convert chromium-6 to insoluble chromium-3 (trivalent chromium), which can then be filtered out.
Using iron nanoparticles to clean groundwater can provide significant savings. For example, a $20 million cleanup project might cost $5 million using nanoparticles, a savings of 75 percent, says Zhang. Iron nanoparticle prices have dropped from $15 to $23 per pound in 1995 to $9 to $11 per pound today, thereby reducing their site treatment costs.