Old Metal Scraps Offer New Way of Cleaning Polluted Groundwater
PORTLAND, Ore. — For most business owners, a mountain of scrap metal is nothing more than a big, unsightly nuisance. But to environmental scientists and engineers at Oregon Health & Science University, piles of old metal actually hold the key to a new technology that''s beginning to make a difference in cleaning up the nation's contaminated groundwater.
Take, for example, Central Point, Ore., a mid-sized community five miles northwest of Medford, where chlorinated solvents in the soil were threatening local water wells. A growing plume of perchloroethylene (PCE) and trichloroethylene (TCE) - chemicals known to cause cancer in humans was slowly moving through the soil from an old industrial facility, posing a potential hazard to nearby wells and local surface water. Because the bulk of the contamination was under the building, engineers knew they had to find a new approach.
The new approach -- a wall of sand-sized grains of iron metal buried in the ground to intercept and dissolve contaminants -- is one that is starting to take hold in Oregon and nationwide. Given the proliferation of contaminated sites in Oregon alone, the iron wall technology, as it's known, is becoming an effective alternative nationally to standard forms of groundwater treatment.
Researchers at OHSU's West Campus tackling chlorinated solvents and explosives
Chlorinated solvents are regularly used in dry cleaning and degreasing, and were commonly used 15 years ago by the electronics industry. These compounds bind tightly to the soil and can contaminate groundwater even in small concentrations. As groundwater flows, a plume of contaminated water develops, increasing the risk of human exposure.
One way to clean up the underground is to intercept the contaminated plume and degrade the pollutants, which is where the iron wall technology comes in. The iron barrier -- usually made of iron filings and sometimes sand -- are placed directly in the path of a plume of contaminated groundwater. The resulting chemical reactions readily dissolve the pollutants.
The iron wall technology was first developed around 1990 at the University of Waterloo in Ontario. Scientists there stumbled upon the approach while reviewing earlier work and, although they recognized they might have a new approach to groundwater remediation, they initially had little understanding of how iron actually broke down the chlorinated solvents.
Paul Tratnyek, Ph.D., associate professor of chemistry in the OGI School of Science & Engineering at OHSU, began studying the science behind the iron wall technology in 1992 and was the first chemist to explain how the iron breaks down pollutants.
"When the contaminated water interacts with the metal, a chemical reaction similar to rusting occurs that decomposes the chlorinated solvent," said Tratnyek. A resulting byproduct is chloride which is found in table salt as well as carbon dioxide, methane and dissolved iron. All of these byproducts are harmless in groundwater.
Next up: Full-scale field tests with iron wall technology
In addition to examining in the lab how contaminants move in the water, Tratnyek has teamed with colleague Richard Johnson, Ph.D., who directs the center for groundwater research at the OGI School of Science & Engineering, to conduct large-scale field experiments on the Hillsboro, Ore.-based campus.
Using an artificial aquifer the world's largest measuring 70' x 30 and 15' deep, Tratnyek and Johnson are effectively simulating chlorinated solvent spills and cleanup using the iron wall technology. The researchers can simulate 50 years of groundwater contamination in one week. By pushing water through the iron wall at a set rate, they can observe the pressure and the chemical composition of what comes out of the aquifer.
"We're trying to see how long the iron wall lasts, which will help us determine its cost-effectiveness," said Tratnyek. If we have to dig up and replace all the iron walls after five years, they're probably not cost-effective. But if they last 30 years, they're probably a good deal. So we're trying to figure out how much contaminated water we can run through the iron wall before it croaks.
Johnson and Tratnyek are currently testing the iron wall chemistry at Oregon's Umatilla Army Base, where some groundwater is contaminated with the explosive compounds TNT and RDX. Johnson is in the final stages of selecting another Department of Defense military site for a full-scale field test of the technology on munitions.
Although it is effective on explosives, textile dyes, and arsenic, the iron wall approach does have its limitations, cautioned Tratnyek. "We don't expect it, as currently conceived, to be able to clean up gasoline, for example," he said. Scientists also are studying how effective the iron wall can be in cleaning up contaminants at depths greater than 75 feet.
First site of iron wall technology in Oregon working well.
The iron wall in Central Point -- the first and only one in Oregon -- was installed during a four-month period, beginning in late 1997, and seems to be working well, said Dave Graham, principal hydrogeologist for Environmental Management Services in Medford, the company that tackled the Central Point site. The iron wall goes right in the ground, and no one even knows it's there.
The standard pump-and-treat remediation approach was considered but abandoned because it would have required continual on-site monitoring. The iron wall isn't the cheapest technology, Graham said, but if you figure in the long-term maintenance and monitoring costs, it becomes a pretty viable technology. Coupled with other groundwater remediation techniques, the iron wall is definitely a technology of the future, said Graham.