A three dimensional pollutant transport model is presented. The code is based on a particle tracking technique : the random-walk method, which is based on the analogy between the advection-dispersion equation and the Fokker-Planck equation. The velocity field is calculated by a mixed hybrid finite element method. To avoid particle accumulation in regions with dispersion contrasts, a new efficient method is developed to allow conservation of the fluxes between high and low dispersive regions. Comparisons are made on a layered aquifer example between this method and other algorithms commonly used and show the efficiency of the new method. To validate the code, a 3D experiment made on a laboratory model have been simulated. It represents a heterogeneous aquifer of about six meter length, one meter width and one meter depth. The porous medium is made from three different sorts of sand. Sodium chloride is used as tracer. Comparisons between simulated and measured values show very satisfactory results.