In the future, quality management of ground waters will depend on better knowledge of natural mixing phenomena in porous media. The paper is a contribution to the basic hydrodynamics of interfacial instability in stratified flow in porous media. Although flows in porous media generally involve extremely low Reynolds numbers (the so-called Darcy flows), the existence of several mechanisms which could induce instability has been recognized for many years. In particular, most of these effects can appear in the instabilities associated with an interface between two fluids in a porous medium (Soils).