Analysis of mass transfer processes during advective air movement in contaminated soil

Air sparging (AS) and soil vapor extraction (SVE) are remediation techniques that are particularly suitable for the removal of Volatile Organic Contaminants (VOC) from contaminated soil and ground water. However, the design and operation of these techniques are mainly subjective due to the lack of design tools suitable for field scale applications. The goal of this research is to develop and evaluate a numerical model of the mass transfer processes during advective air movement in soil to aid in the design and operation of AS/SVE systems. The model consists of two separate modules; steady state unsaturated air flow module, and transient multiphase contaminant transport module that incorporates non-equilibrium first order kinetics mass transfer. The solution in space was obtained using the Galerkin\u27s finite elements formulation, while the solution in time was obtained using the central finite difference scheme. The model was used to analyze the first order mass transfer processes and to study the effect of air permeability field heterogeneity on the contaminant removal. It was found that the removal rate is proportional to the air-water (stripping) and NAPL-air (volatilization) first order coefficients. The analysis also shows that the equilibrium assumption was valid for the sorption/de-sorption mass transfer process. The results showed that contaminant removal depends on the air flow distribution, which in turn is dependent on the heterogeneity of the air flow field. The model was used to analyze two case studies, which involve the removal of Benzene, Toluene, Ethyl Benzene and Xylenes (BTEX). For both case studies, all model input were pre-determined or compiled using literature data. In one of the case studies more emphasis was put on studying the effect of pulse and flow domain heterogeneity on the contaminant removal time. Pulse sparging operation was approximated assuming uniform contaminant redistribution after each sparging cycle. The close comparison between the simulated and observed BTEX concentration in the aqueous phase indicated that the uniform redistribution of the contaminant after each sparging cycle is a valid assumption wherever the soils and site conditions are favorable for air sparging operation