Effects of Soil Heterogeneity on Airflow Patterns and Hydrocarbon Removal during In Situ Air Sparging

Abstract

This paper presents the results of a laboratory investigation performed to study the effect of soil heterogeneity on the removal of benzene from six different homogeneous and heterogeneous soil profiles using in situ air sparging. Air injected in homogeneous coarse sand profiles traveled in channels within a parabolic zone. Within fine gravel, injected air traveled in bubbles and was confined to a smaller zone of influence than within the sand profiles. Heterogeneous soil profiles were subjected to airflow patterns that were combinations of patterns observed in homogeneous soil profiles. When hydraulic conductivity differences between adjacent soil layers or inclusions were less than a factor of 10, air freely entered into the lower permeability soil regions. However, when differences were greater than a factor of 10, the injected air followed a path of least resistance and avoided regions of lower permeability. Regions that were subjected to high airflow were remediated rapidly as a result of efficient vapor-phase partitioning due to volatilization. Regions that were not subjected to high airflow, whether the result of locations outside of the zone of influence or airflow diversion due to permeability differences between adjacent soil layers/lenses, did not experience rapid contaminant removal and required rate-limiting diffusion for contaminant removal. Overall, the importance of determining the degree of soil heterogeneity at a given site with a detailed site investigation prior to implementing air sparging is demonstrated.