New Technique Stops Spread of Hazardous Waste

A new technique developed by Berkeley Lab researchers could make it possible to contain and prevent the spread of underground hazardous waste.

Collaborating with the Bechtel Corp., Berkeley Lab demonstrated the feasibility of the concept in January 1995 at a site with complex subsurface conditions similar to those at Hanford, Washington. If the development effort continues to be successful, the technique could substantially reduce both the financial and environmental costs associated with the country's thousands of hazardous waste sites.

To immobilize waste, the Berkeley Lab team drills a string of wells around and beneath the perimeter of the area that is to be contained. Then, they inject a fluid into the wells which is able to permeate the ground before later gelling and forming an impermeable, solid barrier that surrounds the contaminated site.

Earth scientists Karsten Pruess and George Moridis liken their technique to the creation of an underground isolation chamber.

"Up until now," says Moridis, "the country has been fighting a losing battle. Huge areas can be contaminated by just a few gallons of hazardous fluids, and once they get into the ground, contaminants are very difficult to strip from the soil. Unfortunately, the state of the art of cleaning up these sites is the same as it was 30 years ago-we dig the soil out and truck it to a hazardous waste site."

The costs and limitations of this approach have severely handicapped the nation's cleanup efforts. Thousands of contaminated sites have been identified but few have been cleaned up. Typically, years elapse before cleanup begins at a site.

As they moved their technique from the lab to the field, Berkeley Lab project chemists John Apps and Peter Persoff worked with chemical companies including Dow Corning, DuPont, and Philadelphia Quartz Corp. to refine the performance of the gel barrier fluids. Fluids chosen for testing are environmentally benign, so much so that they have been given a categorical exclusion from EPA and NEPA regulations.

For the recent field test, LBL injected two fluids, a colloidal silica and a polysiloxane fluid into the unsaturated soil zone above the water table. Several days later researchers excavated the grout plumes, slicing the earth away to reveal how well they had managed to penetrate and saturate the uneven ground.

The colloidal silica performed satisfactorily but the polysiloxane exceeded expectations. Bechtel specialists who have been injecting materials into the ground for two decades say the material did something they had never seen before.

"Not only did we see a uniform, almost symmetrical plume," reports Moridis,"but the polysiloxane displayed a remarkable ability to penetrate small and large pores in the relatively clay soil matrix."

When injected into the earth, polysiloxane has a viscosity similar to water. A catalyst causes it to turn into a strong, rubber-like polymer, and controls how quickly this occurs. Soil chemistry, which can cause a compound to solidify prematurely, does not significantly affect polysiloxane. The product, a silicon-chain polymer, was developed by Dow Corning for Berkeley Lab.

Precisely how long such barriers would last in the earth -- a relatively short time or a millennium -- must still be determined, although the life expectancy of similar products is 30 to 50 years. During remediation, even a barrier with a lifetime of months can be useful, helping to contain or redirect groundwater flows.

Besides their use at hazardous waste sites,such barriers have other potential applications. These include the lining and capping of landfills, the stabilization of slide-prone slopes, and the prevention of soil liquefaction in earthquake-prone areas.