Using Resistivity Tomography To Monitor Air Sparging

Air sparging is a relatively new technique for remediation of<br>ground water contaminated with petroleum hydrocarbons. In this<br>technique, air is injected below the water table. The air is usually<br>removed by vacuum extraction in the vadose zone. Remediation<br>occurs through a combination of contaminant partitioning into the<br>vapor and enhanced biodegradation.<br>Although remedial efficiency is strongly a function of the<br>pattern of air flow, the distribution of the injected air in space and<br>time is poorly understood. Cross-borehole resistivity surveys were<br>carried out at an air sparging site to address these unknowns. The<br>site is a former service station in Florence, Oregon. Resistivity<br>measurements were made using six 40 ft deep wells, one of which<br>was the air sparge (air injection) well. Data were collected over a two<br>week period during and after several sparge events. Tomographic<br>resistivity reconstructions were generated between wells using an<br>algorithm that assumes axially symmetric structures. The movement<br>of air throughout time is clearly defined by high resistivity regions.