Hydrogeological Characterization and Groundwater Resources Assessment of the Chtouka Coastal Aquifer (SOUTH-WEST, Morocco)

Abstract

The coastal aquifer of Chtouka, with an area of 1250 km2, is located south of Agadir city in the Massa basin. The Chtouka aquifer is made up of a complex of heterogeneous formations mainly represented by alluvium and Plio-Quaternary lacustrine limestones which rest on the marls and sandy marls of the Miocene, which constitutes the substratum of this aquifer. The groundwater of the Chtouka aquifer is mainly used for agricultural and drinking water supply of Biougra, Ait Amira centers, and the rural communes of the plain. The average water withdrawals from the groundwater during the period of 2000-2015 are estimated at 85 Mm3/year, of which 90% corresponds to withdrawals by agricultural pumping. These withdrawals generated 58 Mm3/year which has led to a depletion of the groundwater reserves. The aquifer reservoir is mainly supplied by the infiltration of rainwater and partly by the irrigation water percolation by gravity which covers only 2% of the irrigation perimeter with a surface area of 17260 ha. The analysis of piezometric records, carried out for the series of years 1968-2020, showed a general decrease of groundwater level with drawdown values between 3 and 35 m. The sectors most affected by this decrease are located in the northern half where values reach 35 m in the northwest, 9 m in the north around Biougra Ait Amira, and around 14 m in the south. Further east, the recorded cumulative drawdown values oscillate around 8 m. These drawdowns are lower in the coastal zone to the west and hardly exceed 3 m. The obvious drop in the northern sectors of the study area is due to the reduction in groundwater recharge following the succession of dry years, but mainly due to the intensive exploitation of groundwater by agricultural pumping. In the northwest, the drop in the piezometric level is moderated by the impoundment of the modern irrigated perimeter of the Massa large hydraulics, served from the Youssef Ben Tachefine dam water reservoir. To improve our knowledge and our relevant understanding of this aquifer’s functioning, a hydrogeological study was carried out to identify and assess these water resources, in terms of updated piezometric and new balance. Indeed, the aquifer was greatly impacted by the last successive droughts which have hit the region since 2014. This updated assessment of these groundwater resources will contribute to better management of the Chtouka aquifer and will assist the manager in making the right decision regarding water resources planning.