Field measurements of chloride profiles in the water table region of a deep aquifer are interpreted in terms of haline convection at the microscale level. Under natural flow conditions the flow field is characterized by a small Peclet number (between 6.25×10−2 and 6.25×10−3) so that molecular diffusion dominates over mechanical dispersion. It is suggested that haline convection is a major transport and mixing mechanism at the water table region of granular aquifers under natural flow conditions wherever the intervention of man changed the chemical composition of natural replenishment. In the studied region it is estimated that the critical density difference which overcomes viscous drag forces is 0.23–0.28 kg/m3. This value agrees with the magnitude of the critical free convection parameter (modified Rayleigh number) of earlier models.