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Underground Barrier:
New Technique to Stop Spread of Contaminants from Hazardous Waste Sites

April 17, 1995

By Jeffery Kahn, [email protected]

BERKELEY -- A new technique that could make it possible to contain and prevent the spread of underground hazardous waste has been field tested by Lawrence Berkeley Laboratory researchers.

Collaborating with the Bechtel Corp., LBL demonstrated the feasibility of the concept during January at a site with complex subsurface conditions similar to that at Hanford, Washington. The test involved only a small area but the results were very encouraging. 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 LBL 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 barrier that surrounds the contaminated site.

LBL earth scientists Karsten Pruess and George Moridis liken their technique to the creation of an underground isolation chamber. The researchers stress that each site is unique and cleanup strategies always will vary. However, they believe their approach is a vitally-needed supplement to today's standard cleanup method.

"Up until now," said 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. In some cases, the groundwater treatment times required for remediation can stretch over 50 years or more. Over time, water in the ground can cause the contamination to spread. Depending on what is nearby, water supplies, rivers, residential areas, and human health can be further jeopardized by these delays.

To develop its underground isolation chamber approach, LBL is using every tool in its research belt. The lab is a world leader in developing technology to analyze, model, and see the underground landscape. This expertise has been tapped to provide insight into what might work in ground that can vary from sandy to a mix of clay, gravel, and boulders.

As the project has moved from the lab to the field, LBL project chemists John Apps and Peter Persoff have 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.

The Bechtel Corp. handled the operational aspects of the recent field test. Moridis and Pruess say they chose the site -- a Los Banos, California sand and gravel quarry -- because the underground there is such a jumble, and a severe test for any gel barrier fluid.

"The problem is heterogeneity," explained Pruess. "Natural environments have an intrinsic variability that you cannot anticipate or control. Companies like Bechtel that inject grout into the ground -- to strengthen dams and foundations, to keep water out of construction sites -- run into this all the time. The stories are legend. At one site, a grouting contractor injected over 1,000 sacks worth of cement slurry down a hole, and every bit of it completely disappeared. Twenty feet away, they were unable to inject more than a few dozen sacks."

For the recent field test, LBL injected two fluids, a colloidal silica and a polysiloxane fluid. 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 a containment 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, these barriers have other potential applications. These include the lining and capping of landfills, the stabilization of slide-prone slopes, as well as the prevention of soil liquefaction in earthquake-prone areas.

LBL researchers say the next step in their development efforts is a larger scale test to show the ability to contain the area around an underground storage tank. The ongoing effort is sponsored by the Department of Energy's Office of Technology development (headed by Clyde Frank) under the In Situ Remediation Integrated Program (led by Jef Walker and Mary Peterson).

LBL is a national laboratory that conducts unclassified scientific research for the U.S. Department of Energy. It is located in Berkeley, California, and is managed by the University of California.