The phenomenon of hydrodynamic dispersion occurs in problems of underground mixing of waters of different quality. In these problems, any identifiable solute may serve as a tracer whose concentration distribution indicates the mixing. A review is given of the microscopic and macroscopic factors of the medium and the liquid, which affect the mixing phenomenon. The paper presents a generalized macroscopic dispersion theory based on the hydrodynamics of the microflow through a porous medium model and on statistical averaging procedures. The macroscopic parameters appearing in the average transport equations and their interrelations are analyzed. The problem of determining the tracer concentration distribution, under field conditions encountered in the practice, is stated mathematically in the form of a set of equations and boundary and initial conditions.