This article presents the results of a laboratory investigation of the mechanisms of surfactant-enhanced air sparging (SEAS) in different media. Two kinds of media (medium sand and gravel) were used in one-dimensional column experiments, designed to determine (1) the functional relationship between the air saturation and surface tension of water during SEAS, and (2) the contaminant removal mechanisms in different air travel modes (channels and bubbles) under different surface tension values. The results demonstrated that when air traveled in the form of channels, a decrease in surface tension accordingly reduced capillary pressure in porous media. Air saturation therefore increased, thereby considerably improving contaminant removal. The variations in removal efficiency under different surface tension values coincide with the trend of air saturation change. When air traveled in the form of bubbles, the SEAS-induced air saturation in the column was directly affected by foam stability and foamability, rather than by the surface tension of water. Surfactant addition improved only the contaminant removal rate, but the decrease in lingering concentration was insignificant. The results of this study can serve as theoretical bases for SEAS application in contaminated sites.