PARTICLE-SIZE HETEROGENEITY AND SUBSURFACE STRATIFICATION IN AIR SPARGING: LABORATORY EXPERIMENTS-FIELD IMPLICATIONS

Air-sparging laboratory experiments were performed to assess the effects of particle-size heterogeneity and sediment stratification on air-flow geometry and area of impact. Results were interpreted in terms of the three main air-flow geometries described in the literature: chamber flow, channelized flow, and pervasive/bubbly flow. Compared to sparging experiments in monostratigraphic layers, air sparging of coarsening-upward sequences can increase the extent of the sediment column impacted by air, due to changes in air-flow geometry and/or sparge angle at strata transition boundaries. Air sparging of fining-upward sequences can also increase the extent of sediment impacted in an overlying unit in which channelized flow occurs by generating multiple air-source points. Laboratory simulations of the air-sparging injection interval, representative grain size, and stratified saturated zone at a field site were also performed. These experiments indicated the occurrence of chamber flow at the site, and revealed the effects of pulsed air sparging. The effect is to fill in areas of the sedimentary column between air-flow chambers that otherwise would not be affected by air during continuous sparging. This phenomenon, observed in the laboratory, is a likely explanation for remediation performance at many fine-grained sites.