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

The performance of in situ air sparging is controlled by the distribution o f air pathways in the subsurface, which is in turn controlled by the struct ure 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 press ures of the pores. At the field scale, pore size distributions are usually correlated with heterogeneous structures (e.g. layers) within the medium, w hich control the macroscopic distribution of the air. The processes that pr oduce an observed air distribution at a particular site are complicated, an d are potentially well suited to modeling with multiphase flow models. Rece nt numerical modeling of heterogeneous media appears to successfully repres ent 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 st udied, as well as to capture the relevant aspects of the discrete air/water distribution (i.e., represent air channels at the centimeter or smaller sc ale). This will, in general, require a level of site data that is not avail able and numerical models that require many millions of computational eleme nts. As a consequence, at least for the foreseeable future, numerical model ing of the air sparging process will continue to play a vital role as a con ceptual tool with Limited predictive capability at sites. (C) 2000 Elsevier Science B.V. All rights reserved.