Crosswell Seismic Imaging Of An In-Situ Air Stripping Waste Remediation Process

The restoration of environmentally contaminated sites at DOE facilities has become a major<br>effort in the past several years. The variety of wastes involved and the differing characteristics have<br>driven the development of new restoration and monitoring technologies. One of the new remediation<br>technologies is being demonstrated at the Savannah River Site near Aiken, South Carolina. In<br>conjunction with this demonstration, a new technology for site characterization and monitoring of the<br>remediation process has been applied by Sandia National Laboratories.<br>The remediation technology being demonstrated involves the in-situ air stripping of<br>contaminated soils and groundwater. The geology at the site consists of horizontally-layered<br>unconsolidated sands, clayey sands, sandy clays, and clays. A leaking process sewer line contaminated<br>the area under study with chlorinated volatile organic compounds (CVOC’s), primarily<br>trichloroethylene (TCE) and tetrachloroethylene (PCE). To remove the contaminants, two horizontal<br>wells were drilled: a vapor extraction well at depths from 9 m to 26 m (AMH-2), and an injection well<br>at depths from 36 m to 56 m (AMH-1). Figure 1 shows the projection of these horizontal wells on the<br>surface. Air was injected into the lower well beneath the water table depth of 42 m, and a vacuum<br>was applied to the upper well to extract the injected air. As the air passed through the subsurface, the<br>CVOC’s were dissolved into the gas phase and brought out the extraction well.<br>Little was known about the distribution of the air injected into the earth. This air distribution<br>has a direct effect on which areas are remediated and also on the flow regimes set up during the air<br>injection process. A method for imaging the air distribution was developed at Sandia using crosshole<br>seismic data and based on the changes in seismic velocities as a result of changes in saturation due to<br>the injected air.