The sandy microdune systems of the Sahel are important for biomass production, in that they trap and store water. We have studied the movement of water over and in a dune and the chemistry of the water to understand this aspect of the systems. We experimented with simulated rain using a field sprinkling infiltrometer. We applied demineralized water with a chemical composition similar to that of the natural rain on a 1-m² plot. The plot was delimited by a metallic two-level setting: the first enabled us to collect surface runoff, while the second measured subsurface flow. Water samples were taken at 5- to 10-minute intervals throughout each simulation for chemical analysis (alkalinity, SO4^2–, F^–, NO^3–, Ca²⁺, Mg²⁺, Na⁺, K⁺ and Si). Mass balances, combined with a simple mixture model involving one tracer (chloride) and two reservoirs (old and new waters), were calculated. The equilibrating pressures of the CO₂ (pCO₂) and the saturation index with respect to specified minerals (e.g. calcite, fluorite, silicates) were also calculated by the AQUA ion-pair model. The solute concentrations decrease in surface runoff as well as in subsurface water, except for F^– and Si in the subsurface. The pCO₂ decreased to a pressure less than the atmospheric pressure. The difference between measured concentrations and concentrations computed with the mixing model highlighted interactions between the soil and water. The dissolution of calcite which consumes CO₂, and the cation exchanges, dominated, whereas the dissolutions of fluorite, silicates and gypsum appear secondary. Reactive mineral stocks were quickly exhausted, especially in the surface flow.