Abstract. To provide insight into the mechanism of bubble migration in in situ air sparging in a coarse porous medium, the rise of air bubbles through a granular porous medium is studied through video photography enhanced with image analysis. The porous medium is simulated by a fully saturated cylindrical glass column filled with 4-mm glass beads in random packing order. The experimental procedure for introducing and visualizing a single air bubble is discussed, and the average rise velocity as a function of the emerging bubble radius is directly measured. Compressed air is injected into the base of the column through the use of an electronically controlled solenoid valve. The resulting bubble motion is recorded by camcorder, and still frames are captured and enhanced from the videotape with an image analyzer. The vertical rise of a bubble through the porous medium displays a linear dependence on time. This is the first study to quantify the terminal velocity of an air bubble rising in a stationary porous medium. Comparison is made to experimental data on air bubbles in water and Soltrol obtained using the same video photography and image analysis techniques. Although a porous medium is sometimes represented as a bundle of capillary tubes, an air bubble rising in a granular porous medium is shown to exhibit significantly different behavior from a bubble rising in either a capillary tube or a tube with a diameter comparable to the equivalent diameter of the bubble. The effect of a bubble chain, a series of single bubbles separated vertically from each other by a few centimeters, on air bubble migration in a granular porous medium is also discussed. IntroductionIn situ air sparging (IAS) is an innovative technology applied to remediate groundwater contaminated by volatile organic compounds (VOCs). Air sparging is the direct injection of compressed air into the formation beneath the water table. As the air rises through the saturated aquifer, the volatile compounds dissolved in the water diffuse into the air and are then volatile contaminants from water to air phase, thus affecting the efficiency of an air-sparging operation. To provide insight into the mechanism of discrete bubble migration in air sparging, we have developed a technique for studying the motion of an air bubble rising through a column of granular porous media. This is the first work to directly measure the terminal velocity of a bubble in a stationary porous medium. 2.Literature ReviewThe behavior of air bubbles in a porous medium has been the subject of a wide variety of treatments. A qualitative study of airflow patterns through a water-saturated porous medium was conducted by Ji et al. [1993]. Laboratory visualization ex-1131

A remediation technique involving surfactant foam (SF) was investigated by using a micromodel with a view toward improved contaminant displacement. A residual trichloroethylene (TCE) saturation was emplaced in an otherwise water-saturated micromodel. Air flow was introduced to a surfactant solution flow (Sodium C14 - 16 olefin sulfonate 2%(w)) commonly utilized in surfactant flushing to obtain the SF. Conventional remediation strategies, surfactant flushing, water flooding, and gas flooding were also employed for comparison to the SF flooding. The removal efficiency was quantified by directly measuring the remaining TCE blob area with an image analyzer. The micromodel visualization showed that the SF is capable of displacing a higher amount of TCE as compared to surfactant solution alone. The SF flooding removed 99% of the residual TCE, while surfactant flushing removed 41% with 25 pore volumes of the same surfactant solution. Based on micromodel visualization, the resulting enhanced removal of TCE blobs due to direct and indirect displacements, increased solubility and blob snap-off were investigated. The micromodel was also oriented vertically, and no vertical movement of the TCE was observed during SF flooding. Thus, the SF flooding achieved high TCE removal efficiency without lowering the interfacial tension to an ultralow value and causing vertical migration, which has been of particular concern in TCE removal.

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