Structure and workings of a montane karstic hydrosystem in a semi-arid climate: The case of the Beni Mellal Atlas (Morocco)

The northern slope of the Beni Mellal Atlas is a complex area where two large units (Atlas and the Tadla Plain) meet by means of thrust sheets of diverse composition. A lot of springs originating from the relief and boreholes of the plain have enabled the identification of consequent groundwater reserves, hardly known until now. This study reveals the characterisation of Atlasic aquifers (Atlas and piemont) with karstic behaviour, the demonstration of their interactions, and also their relationship with the Tadla Plain aquifers. The application of structural, hydrodynamic, physicochemical and isotopic methods have enabled the characterisation of the Beni Mellal Atlas Aquifer and the specification of the hydrodynamic connections between different aquifers in a semi-arid zone. The geological data (lithology and structure) have deline ated a 365 km(2) Atlasic intake area. The structural analysis shows a dense fracturing of the Liassic massif (limestones and dolomites). This Liassic unit is overthrusting the northern series at the Dir level (piemont), bringing together permeable facies in some places, thus easing the hydraulic connections or re-infiltration of the waters of the Atlasic springs. The hydrodynamic and hydrochemical studies (recessions, sorted discharges, cross correlation analysis, frequency distribution analysis of carbonated tracers, discriminant factor analysis-AFD) demonstrate that the system has a very inertial behaviour, which acts as a shallow karst during high water periods. The hydrochemical characterisation confirms the presence of units displayed by the structural analysis. The hypothesis put into the fore by other methods is confirmed by the isotope study: the aquifers of the plain are recharged by the same area as the Atlasic springs within a very wide system with several inputs. (C) 1997 Elsevier Science Limited.