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Gas leak clean up work hailed

Spectrum Volume 17 Issue 20 - February 16, 1995

For decades, gasoline service stations have used underground storage sites to hold petroleum for their customers. Similarly, chemical companies have used storage tanks as their repositories. There are thousands of these active underground storage sites in the country.

In recent years, the Environmental Protection Agency (EPA) has identified some of these facilities as a source of soil and water pollution. In some cases, the pollution is an emitted vapor that contaminates the soil and water. New technologies, investigated by Virginia Tech civil engineering faculty member Mark Widdowson, are being hailed as successful treatment processes in a localized experiment. Widdowson now hopes to employ this technology on a wider basis.

Widdowson's work began several years ago while he was a faculty member at the University of South Carolina. He received a grant from the state's Hazardous Waste Management Research Fund to demonstrate remediation technologies at an underground storage tank in the Piedmont region of South Carolina.

At this particular site, a leak had occurred at an underground gasoline storage tank. The pure gas had polluted the soil and filtered down to the water table.

One of Widdowson's removal methods, labeled soil-vapor extraction, calls for the installation of wells above the water table. Using a sophisticated type of vacuum, the vapor is pulled up through the well. The vapor can subsequently be treated.

Part of the consideration in installing the wells is the type of soil the engineers need to drill through. The Piedmont region, which extends into Virginia, is mostly clay in nature and has a lower permeability than the adjacent coastal plain, comprised of sandy soil.

Widdowson, who specializes in hydrosystems engineering, determined that from a fluid-mechanics point of view, the technology called for a significant number of wells, closely spaced. Once this layout was decided on, the increase in the number of wells led to the next question: would the technology be economically feasible?

Since soil-vapor extraction allows removal of the contaminated mass without the full-scale removal of the soil and water, the methodology is considered cost-effective. Previous expensive technologies called for the extraction of the contaminated soil and water, the treatment of the material, and the subsequent replacement of it.

A second technology Widdowson is researching is called air sparging. He describes it as being "similar in nature to vapor extraction. Wells are placed below the water table, air is injected, and as the air bubbles up through the aquifer, it strips out the contamination in the ground water. It also helps put oxygen back in the ground, which the bacteria can use to help destroy the contaminant."

The soil-vapor extraction process primarily addresses soil contamination, while air sparging targets polluted ground water.

Widdowson hopes to be able to apply his process to work now being done in Virginia to clean up contaminated sites near underground storage tanks.