Building The Right Barrier Stops Spread Of Deadly Pollutants

This permeable reactive barrier technology, in use for less than a decade, promises to be a highly effective pollution control tool. But environmental engineers at The Johns Hopkins University have found that cleanup crews may be making crucial miscalculations in designing these pollution neutralizing walls. To correct this problem, researchers William A. Arnold and A. Lynn Roberts have prepared a new mathematical model that should lead to safer and more cost-effective barriers. They presented their findings Sunday, March 26, at the national meeting of the American Chemical Society in San Francisco.

Pollution cleanup barriers must be designed properly to keep cancer-causing chemicals away from drinking water supplies. Each project can cost more than $1 million, most of which pays for the materials. "If you build these barriers too thin, you won't get complete treatment of the contaminated water," Arnold explained. "But if you build them too thick, then you're using two or three times more iron than you need, and you're greatly increasing the cost of the cleanup. This also limits the number of sites where you can use this technology because it's difficult to place thick barriers in front of pollutants located at large depths."

While preparing his doctoral thesis at Johns Hopkins, Arnold studied the chemical reactions that take place when certain contaminants collide with iron filings. Roberts, an associate professor in the Johns Hopkins Department of Geography and Environmental Engineering, collaborated on the research as Arnold's thesis advisor.

The pair focused on chlorinated ethylenes, suspected cancer-causing chemicals commonly found in dry cleaning solvents and de-greasing solutions. Through improper disposal at military and industrial sites, these deadly chemicals have seeped into underground water supplies and begun to migrate. Robert Gillham of the University of Waterloo in Canada developed the permeable reactive barrier technology about nine years ago as a way to de-toxify such plumes before they contaminate nearby drinking water supplies. Generally, the process calls for digging a trench to the level of the contaminated water. Workers then pour in tons of iron filings to form a permeable wall, usually one to six feet thick, that should remove toxins as the water moves through it. This method is now being used in full-scale and pilot projects at more than two dozen groundwater contamination sites in the United States and other nations.

To study the effectiveness of this process, Arnold and Roberts set up lab experiments at Johns Hopkins that mimicked the chemical reactions taking place in the field, but at an accelerated rate. They discovered that chlorinated ethylene molecules must compete for access to a limited number of sites on the iron surfaces where reactions can take place. "So at higher concentrations of pollution, there is more of this competition, and the reaction occurs more slowly," said Arnold, who, after receiving his Ph.D. at Johns Hopkins, became an assistant professor of civil engineering at the University of Minnesota. "At lower concentrations, the reaction takes place more quickly."

He and Roberts developed a new mathematical model that incorporates such variations. "We found that it's crucial that you know the exact concentration of the pollution, and you must use the correct mathematical model when you calculate how wide to build these barriers," Arnold said. "Our new model requires a lot more experimental data, which means you have to do a lot more preliminary testing in the lab. But if we can design these barriers more accurately, they will operate more effectively and more economically."