Implication of Guigo and L'Hajeb Causses in the renewal and circulations of Saïs basin groundwaters (Middle-Atlas Causses, Morocco).

International audienceIn a context of overexploitation of the karst system of the Middle Atlas Causses feeding the Saïs basin and the current climatic variations, the study of circulations and of renewal of waters of this system in the Fes-Meknes area becomes essential for the population, in order to maintain a sufficient quality of waters with a good management.By coupling hydrochemical and isotopic analysis methods (delta-D, delta O-18, Radon-222), saturation indices obtained from PHREEQC code and principal component analysis (PCA) of ten springs and three wells, a first conceptual model of groundwater flows of this karst system was obtained . These waters are mainly renewed by the rainfall of L'Hajeb Causse and secondarly by the rainfall of Guigo Causse containing several springs. Hydrochemistry and saturation indexes allowed us to highlight two types of waters: a main contribution of Liasic origin and two low contributions of Triassic origin at the southern extremities (SE, SW) of the basin. We pointed out the existence of five local recharge zones of different altitudes (900 to 1500 m asl.) including the two main mixing zones to the south (SE, SW). Radon-222 showed areas of rapid exchange (upwelling time less than two weeks) between waters of Liasic aquifer and the ones of Triassic origin with high radon activity.The use of PCA on hydrochemical data, allowed us to refine the kind of waters, their transit times and highlighted the existence of several mixing zones between the Triassic aquitard and the Liasic aquifers in more or less faulted structures for the two causses. Our results allow us to obtain a first conceptual model of groundwater circulations between the two causses and the Saïs basin.Previous campaigns of electrical resistivity tomography coupled with electromagnetic measurements (EM34) revealed lateral and vertical variations of electrical conductivity changing with the depth along the North-South axis, and a preferential drain perpendicularly to the causses (EGU 2009). We can note that groundwater flows are conditioned by two major fracture networks, NE-SW and NW-SE directions (Bentayeb & Leclerc 1977, Amraoui 2005). A karstic complex water circulation is especially developed locally at the border of the two hydrogeological units (Qarqori 2015)