Abstract. Air sparging in an aquifer below a less permeable horizontal layer is modeled using a two-phase flow approach. Supported by numerical simulations, we show that a steady state situation is reached. For an analysis of the steady state, we distinguish three different flow regimes, which occur between the well screen and the unsaturated zone. Just below the interface that separates the high and the low permeable layers a regime with almost hydrostatic capillary pressures develops. We use this observation to derive an ordinary differential equation for the pressure at the interface, which leads to an approximation of the air flow pattern just below and within the low permeable layer. The approximation provides an estimate for the radius of influence as a function of the physical parameters. The agreement between the analytical approximation and the numerical steady state results is almost perfect when heterogeneity is increased. With a few modifications the analysis applies also to a dense non-aqueous phase liquid (DNAPL) spill above a less permeable layer. Comparison with an illustrative numerical simulation shows that the analytical approximation provides a good estimate of the radial spreading of the DNAPL flow on top of and within the low permeable layer. we model air sparging below a less permeable horizontal layer with large lateral extension. We assume that the different layers have similar structure but different mean pore size, that is, the similar media assumption [Leverett, 1941;Miller, 1980]. We aim at investigating the air flow through layered soils, in particular the quantitative effect of the degree of heterogeneity and the position of the interface that separates the layers on the resulting radius of influence.In section 2 we present the transient model: the basic equations with their saturation dependent relative permeability and capillary pressure functions and the geometry of the domain including the two different layers. To accommodate analysis of the steady state situation, we reformulate the problem in dimensionless form and identify the governing dimensionless numbers. Thus we present similar equations with different parameters for the two subdomains, which are linked by continuity of capillary pressure and of the vertical air velocity component at the interface.In section 3 we analyze the steady state situation that occurs when air flow from the injection well to the vadose zone has been established. Emphasis is given to the region just below the interface, where air spreads mainly horizontally. We assume that flow in this region is ruled by vertical equilibrium, despite a small vertical air velocity component across the interface. An ordinary differential equation for the capillary pressure at the interface governs the radial extension of air below the interface.In section 4 we present the results of numerical simulations that are based on the transient model, which show that indeed a steady state situation is approached. In terms of capillary pressure the numerical solutions are co...