In the experimental catchment area of the Fontaine de Vaucluse, a long term monitoring has been completed. It enables the study of the system functioning with variable hydrological conditions. Coupling flow rates and Mg2+ concentrations permits highlighting the particular contribution of water with great Mg2+ concentration during low water periods. It appears that these high Mg2+ concentrations are linked not only to the low flow rate itself, but also to flow rate of previous flood periods. Thus, the average residence time of discharged water during low flow period, which is usually linked to the severity of the low water level, seems to be influenced by previous flood period too. Great floods and related high pressures could lead to circulation of hardly mobilized water with very high residence time.