Monitoring Of An In-Situ Air Sparging Experiment Using Electrical Resistivity Tomography

Abstract

Electrical resistivity tomography (ERT) was successfully used for monitoring the air distribution in an in-situ air sparging (IAS) experiment at the Oregon Graduate Institute of Science and Technology. The experiment evaluated the removal of residual gasoline from the source zone of a simulated gasoline spill in a tank filled with water-saturated sands The percent difference in resistivity provided a quantitative view of air saturation in the sand tank. The initial background resistivity image suggests the tank filled with water-saturated sands is geoelectrically non-homogenous though the sands in tank are homogeneous in the sense of grain sizes. The background resistivity images also showed the evidence of trapped residual gasoline around the water table. The in-situ air sparging (IAS) in three different modes was carried out. The IAS at a flow rate of 20 cubic feet per minute (cfm) resulted in a larger radius of influence than the IAS at 5 cfm. At the same air flow rate, the IAS in the pulse mode created a larger affected zone than the IAS in the continuous mode. Two weeks was not long enough for the trapped air to escape from the tank. Unlike the continuous IAS operation, the pulse operation produced a more complex air distribution in the tank. Skewed airflow at both 5 cfm and 20 cfm flow rates indicated that there are preferential pathways in the tank. Introduction An in-situ air sparging (IAS) experiment was conducted at the Oregon Graduate Institute (OGI). The experiment evaluated the removal of residual gasoline from the source zone of a simulated gasoline spill in a tank filled with water-saturated sands. The petroleum hydrocarbon contaminants in groundwater can be effectively removed using insitu air sparging (IAS). The removal of MTBE contained in the gasoline from source zones containing residual gasoline may be effective using IAS. In order for cleanup to be completely effective, the injected air needs to reach almost every portion of the source zone since most of the less-soluble residual gasoline needs to come into contact with the sparging air. The IAS was carried out at three modes: a flow rate of 5 cubic feet per minute (cfm), a flow rate of 20 cfm, and an on-and-off pulse operation at a flow rate of 20 cfm and with a cycle of 2 hours on and 2 hours off. Electrical resistivity tomography (ERT) has been proven to be an effective tool for IAS monitoring (LaBrecque et al., 1995; LaBrecque et al., 1996; Schima et al., 1996). To find the effective IAS flow volume, ERT was used for monitoring the air distribution in this IAS experiment.