The in situ steam enhanced extraction process, called thermally enhanced recovery (Praxis Environmental Services Inc. 1994), removes volatile and semivolatile organic compounds from an area of contaminated soil or groundwa-ter without excavation. The process operates through the use of wells constructed in the contaminated soil. High-quality steam is added to the soil through some wells, called injection wells. Other wells, known as extraction wells, operate under vacuum to remove liquid and vapor contaminants and water from the soil. Injecting steam into the ground raises the temperature of the soil and causes the most volatile compounds to vaporize. In addition, pressure gradient is formed between the injection and extraction wells which drives the flow of steam and vaporized contaminants towards the extraction wells (U.S. EPA 1993a). Raising the temperature of the soil matrix also assists in removing less volatile compounds by increasing their in situ vapor pressure. After the entire soil mass being treated has reached the steam temperature, as determined by soil-temperature monitors, and steam breakthrough occurs at the extraction wells, the flow of steam continues only intermittently with a constant vacuum applied to the extraction wells. The vacuum extraction removes much of the remaining contamination. As the soil in the high permeability region cools, the steam remaining in the low permeability region evaporates the contaminants.
The technology is cost-effective for large and deep areas of contamination where technologies requiring excavation are difficult or impossible. The process can be applied in sections to treat an area of any size and depth. If the site, however, contains a high concentration (>200 ppm) of heavier-than-water organics, a possibility exists that these compounds might be mobilized downward into groundwater. In addition, treatment of shallow (<10 feet) contaminated areas is less cost-effective than deeper areas compared to other technologies.
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