New technique removes and destroys underground contaminants
An environmental technology developed by Lawrence Livermore National Laboratory is cleaning a heavily contaminated industrial site in Visalia, California, in a few years, not centuries, as would be required using conventional methods. Called dynamic underground stripping, this new technique heats soil and groundwater to both remove underground contaminants and destroy them in place.
Southern California Edison put this technology to work at its former Visalia pole yard, a four-acre site, used from the 1920s to 1980 to treat power poles with preservatives such as creosote and "penta" (pentachlorophenol), dissolved in diesel fuel. Over the years, residues leaked into the ground and contaminated the groundwater.
These wood preservatives are particularly difficult to clean because they are more dense than water and tend to sink through the soil down through the water table. Until now, there had been no practical way to clean this type of effluent.
In its first nine months of use at Visalia, dynamic underground stripping removed, or destroyed in place, an amount of contaminants that would have required more than 1,000 years with traditional pump-and-treat cleanup, the kind used at the site since 1975.
With pump-and-treat, essentially pumping up polluted water and cleaning it at the surface with state-of-the-art filtration, about 275 pounds of contaminants were removed during one nine-month period. During a similar amount of time, dynamic underground stripping removed some 540,000 pounds of contaminants nearly 2,000 times more.
Since the clean-up started in June 1997, engineers have removed a total of 902,000 pounds of contaminants at the Visalia site.
It is believed by some experts that the new technology could be used to efficiently and economically destroy the pollutants found at about one-fourth of the nation's 1,300 Superfund sites.
Dynamic underground stripping acts in two ways. One is by using steam heat and pressure to drive contaminants to extraction wells, where they are easily removed from the soil and water. The other is by allowing heat and forced air to chemically break down many contaminants in place, converting them into harmless compounds.
Another vital component of dynamic underground stripping is the imaging technology called electrical resistance tomography. This Lawrence Livermore innovation was refined by SteamTech Environmental Services of Bakersfield, California, during work at the Visalia site.
The technology uses sensors to measure electric resistance and temperatures underground to create computer-based, three-dimensional images that show the location of the contaminants and their temperature measurements. Those images allow specialists on the surface to monitor and control the heating process in real time.
Environmental scientists estimate that the new cleanup technique could complete the pole yard cleansing within two years. Total cost for the cleanup, including follow-up monitoring, is estimated at less than (US) $20 million. Pump-and-treat was costing more than $1 million a year and would have subjected Southern California Edison to a liability of up to $75 million.
Working with University of California, Berkeley researchers, Lawrence Livermore developed the steamcleaning technology and cleaned a 7,600-gallon gasoline leak at its main facility in Livermore before Edison contracted to apply the technology on a full-scale basis at the Visalia site.
In 1996, the gasoline site was deemed "closed", no longer requiring clean-up by the Environmental Protection Agency, the state Regional Water Quality Control Board, and the state Department of Toxic Substances Control.
Lawrence Livermore is also conducting further research on the oxidation process that destroys pollutants in place.