Effects of soil heterogeneity on airflow patterns and hydrocarbon removal during in situ air sparging

This paper presents the results of a laboratory investigation performed to study the effect of soil heterogeneity on the removal of benzene from six d ifferent homogeneous and heterogeneous soil profiles using in situ air spar ging. Air injected in homogeneous coarse sand profiles traveled in channels within a parabolic zone. Within fine gravel, injected air traveled in bubb les and was confined to a smaller zone of influence than within the sand pr ofiles. 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 inclusio ns were less than a factor of 10, air freely entered into the fewer permeab ility soil regions. However, when differences were greater than a factor of 10, the injected air followed a path of least resistance and avoided regio ns 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 t he result of locations outside of the zone of influence or airflow diversio n due to permeability differences between adjacent soil layers/lenses, did not experience rapid contaminant removal and required rate-limiting diffusi on for contaminant removal. Overall, the importance of determining the degr ee of soil heterogeneity at a given site with a detailed site investigation prior to implementing air sparging is demonstrated.