Caractérisation hydrogéophysique des cavités karstiques du Causse d’El Hajeb-Ifrane (Maroc) et impact sur la vulnérabilité des ressources en eau: Apport de la géologie structurale, de la géomatique et de la géophysique

Tabular Middle Atlas (TMA) of Morocco holds the most important water aquifer which provides water for the people living in the cities of El Hajeb, Meknes, Fez and nearby cities. This aquifer is located in Liassic dolomite and limestone and constitutes an important karst environment. However, this water resource is vulnerable due to anthropogenic activities and natural phenomenon combined with the ongoing variation in climate system and increasing population growth, and hence the problem gets more alarming. It is facing more problems - such water pollution, water table level fluctuation, sinkholes collapse, increase water turbidity and water spring drying up and hence to the need for monitoring and establishing a protection plan. In order to contribute to the recent initiative of the protection of karstic environment of water springs in TMA of Morocco, the present research work intends to characterize different karst landforms in Jurassic carbonate rocks of TMA in the Causse of El Hajeb which are the origins of pollution identified in water samples and to estimate the level of vulnerability across this region. By adopting a multidisciplinary methodology approach integrating remote sensing, applied geophysics and on-ground-based lithology and structural measurements. Using satellite imagery (Copernicus Sentine2, Landsat, Terra-ASTER, ASTER- GDEM), and GIS tools, we delineate the extent of carbonate rocks which hold water reservoir and areas with intense human activities in the aim of enhancing the understanding of karst environment system in order to establish a roadmap for evaluation of water vulnerability. The results obtained remotely were validated by in-situ data obtained during intense fieldworks conducted in TMA. In addition, we conducted Geophysical Surveys using electrical resistivity tomography, Electrical Vertical Sounding and Self Potential Geophysical methods. PaPRIKa method was used in order to estimate the level of Vulnerability of the Causse of El Hajeb. Satellite images and geophysical data were processed and analyzed in the aim of getting information on soil parent materials, on characteristics (lithology, fractures, and layer thickness) and on the functioning of epikarst zone. On these carbonate rocks fractures mapped on satellite viiiimages using automatic and manual extraction revealed two main faults systems in NE-SW and NW-SE involved in surface water infiltration and groundwater drainage respectively, this has been validated by on ground fracture measurements in different location of karst aquifers. The location of different karst landforms (sinkholes, avens, water spring, poljes and lakes) were also identified and some of them are believed to the origin of chemical and turbidity revealed geochemical analysis, on water springs (Ain Ribaa, Ain Bittit, Ain Maarouf...). The results showed that a big number of karst landforms in this area are located on NE-SW fault system affecting mainly dolomite and limestone of Lower and Middle Liassic, while NW-SE fault system was identified as pathway which drains water from TMA to the basin of Saiss located in North of El Hajeb. Landcover mapping revealed that in the Causse of El Hajeb 54 % carbonate rocks, 15 % soil in basalts and clays, 30 % vegetative cover (forest, farming areas and other vegetation), 0.9 % artificial construction and water surface for the rest. Geophysical surveys conducted along some sinkholes and boreholes data provided geological information on the reservoir (in Liassic dolomite and limestone) and its protection (Quaternary clays and basalts). They allowed us to identify surface extension of these sinkholes as well as their continuity in the subsurface. The results of vulnerability study show that the part of Liasic aquifer belonging to the watershed of the Tizguit River and the region of the Ribaa and Bittit springs are vulnerable due to the karstic forms and geological formations which outcrop in this area. Areas of high to very high vulnerability spread in the center of the basin and on the border between the Causse d'El Hajeb. Medium to very high vulnerability regions were also identified. These results show that it is necessary to establish a protection plan for sinkholes identified in vulnerable areas.