Stratification d'un aquifère continental. Les sources jumelles, froides et thermo-minéralisées de Camou (massif des Arbailles, Pyrénées-Atlantiques)

The Camou springs (Arbailles Massif, French Western Pyrenees) display an unusual close association of a typically cold karstic spring that drains the Urgonian western limb of the Arbailles, and a thermo-mineralized spring (33.5 °C; salinity 17.7 g/L). The latter gains its mineralization at the contact of Triassic evaporites mainly through a deep loop in the Apanice syncline. The fast upflow of this deep water is made at the cross of large active lines (the North-Pyrenean thrust and the Arbailles-Roncal fault). Speleo-diving in the nearby Maddalen Cave allowed reaching the phreatic passage at the origin of the cold spring, which however also crosses the thermal body in the third sump (S3). 6 pressure-temperature dataloggers were settled in both water bodies during 6 months. The dataloggers located downstream on either side of the thermohaline show at the beginning of flood first a lift of the thermal body, then an invasion of the whole phreatic passage by the cold floodwater, controlled by head changes in the karst aquifer. From observation of these mechanisms, we deduce a hydrodynamic model with a warm plume rising into the cold aquifer, without significant mixing. This does not actually require independent watertight routes, both water bodies can coexist including in the same conduit. This model implies the existence of unknown secondary passages close to the spring, which allow during low stage an independent draining of each water body toward separate outlets. Such type of stratified aquifer linked to density differences are common in coastal karsts (Florida, French Calanques…), in the evaporite karsts (Schlotten of the Harz in Germany, Kungur karst in Ural…), but remain poorly identified in continental areas. The Camou twin springs pouring out in the same porch are an outstanding example, together with the Mescla spring in French Alpes-Maritimes.