Air distribution during in situ air sparging: an overview of mathematical modeling.

The performance of in situ air sparging is controlled by the distribution of air pathways in the subsurface, which is in turn controlled by the structure of the medium to be sparged. The specific pathways that the air follows are determined, at the grain scale, by the distribution of air entry pressures of the pores. At the field scale, pore size distributions are usually correlated with heterogeneous structures (e.g. layers) within the medium, which control the macroscopic distribution of the air. The processes that produce an observed air distribution at a particular site are complicated, and are potentially well suited to modeling with multiphase flow models. Recent numerical modeling of heterogeneous media appears to successfully represent expected distributions of air; however, current models do not provide a tool to predict sparging performance. For this to be the case, the models need to represent the detailed structure of the medium at the site to be studied, as well as to capture the relevant aspects of the discrete air/water distribution (i.e., represent air channels at the centimeter or smaller scale). This will, in general, require a level of site data that is not available and numerical models that require many millions of computational elements. As a consequence, at least for the foreseeable future, numerical modeling of the air sparging process will continue to play a vital role as a conceptual tool with limited predictive capability at sites.