Investigation of matrix fluids in fractured aquifers through various gas analyse

We investigated the matrix fluids in a fractured aquifer in French Brittany (Ploemeur, national observatory site). We used four wells close to each others, two of them being pumped for several hours. The four wells were monitored during the whole experiment for chemical composition (major and trace elements) and dissolved gases content: CFC, SF6, Rn, noble gases and He isotopes. After an equilibration phase, the chemical and gas concentrations evolve during the relaxation phase following pumping. The production of matrix fluids is evidenced through the following observations: * CFC concentrations decrease, indicating waters with longer residence time; * He, Rn and SF6 concentrations increase with time, indicating a potential production mechanism. SF6 and Rn production have indeed been measured in this aquifer. However, it is interesting to notice that these three elements do not have the same reactivity. they do not have similar production mechanisms and diffusion coefficients; * Some chemical elements such as silica increase indicating silicate alteration processes; * The redox conditions also vary with time during the relaxation phase, indicating more reduced conditions with U concentration increases; * Ar increases also indicate either an Ar production or a recharge temperature of 5°C. This experiment shows that fluids with various compositions in fractured aquifers are related to various degree of micro- and macro-porosity, and may be associated to various pumping conditions.