Isotopic Tool and Problematic of Recharge in Semi-Arid Area: The Case of the Coast Zone of Essaouira Basin (Morocco)

Abstract

The Essaouira coastal zone is one of the Moroccan semi-arid areas with annual rainfall not exceeding 300 mm/year. This area contains several main aquifers such as: The Plio-quaternary aquifer consisting of sandstones, the Turonian aquifer composed of chalk-dolomitic rocks, the Barremian-Aptian aquifer content in calcareous clay formations, fossil limestone and sandstone and the Hauterivian aquifer housed formations of marl clay alternating with hard siliceous limestone and calcareous marl. The study had framework on the most important aquifers of the Eassouira coast zone, while having as principal objectives the dating of underground waters of the region and the sampling of main water points that supply drinkable water to the city as well as to the rural agglomerations. Different water samples were collected from drillings, sources and wells belonging to the Plio-quaternary, Turonian, Barremian-Aptian and Hauterivian aquifers of the studied region. Their electric conductivity, pH and temperature were measured in situ and water samples were collected for ¹⁸O, ²H and ³H determinations. A meteoric local line was determined and compared to the world meteoric line.The apparent radiocarbon ages of the studied aquifers were evaluated. The recharge to the main aquifers of the studied region was investigated. It has been shown by this study that the recharge rate to the deep Turonian aquifer is very low. This may cause a shortage in water supply to the Essaouira city and its surrounding region. Among the concerned aquifers, the Turonian provides more than 50 % of drinkable water to Essaouira city and rural villages and must therefore be subject to a rational and durable exploitation of the groundwater resources.

Keywords: Essaouira basin, semi-arid, ¹⁸O, ²H, ³H and ¹⁴C isotopes, aquifer recharge