A mechanism that permits the separation and concentration of particles in natural systems is proposed. According to the mechanism, the particles in an aquifer are moved by an alternating flow of ground water. The retardation factor oscillates in synchronism with the alternating water flow. The retardation factor or the flow depend on the position in the aquifer (the particle velocity is u=v/R, where v is the water velocity and R is the retardation factor). The alternating flow is induced by tides, geysers or periodic rains. The alternating water flow produces temperature and water composition oscillations in the aquifer. Moreover, the alternating flow is caused by pressure oscillations: therefore, the retardation factor (which depends on pressure, temperature and water composition) oscillates. The dependence of the retardation factor on position is due to the inhomogeneity of the aquifer medium or to the position dependence of temperature, composition or pressure. Different particles may be concentrated at different places in the aquifer or in different aquifers. The particle concentration and the time necessary for concentration are estimated. The particles may be atoms, molecules, colloids or microorganisms. Such a mechanism may have played a role in chemical evolution.