The combination of the quality index, isotopic, and GIS techniques to assess water resources in a semi-arid context (Essaouira watershed in Morocco)

Abstract

In arid and semi-arid zones, water resources remain unknown in terms of quality and quantity. The current paper aims to evaluate the water quality for drinking and irrigation purposes within the Essaouira basin and to define the origin and recharge areas of groundwater. 122 samples from the Plio-Quaternary, Cenomano-Turonian, and Hauterivian aquifers, as well as surface water, were taken in July 2020 are the subject of hydrogeochemical analyses. Based on the results of these analyses, almost all elements are within the allowable limit fixed by WHO except Cl− and SO42− in a few points, which require treatment before human consumption. The indices SAR, %Na, KR, MR, CR, RSC, PI, and EC, show that the water of the study area is adequate for agricultural purposes. Therefore, the waters of Essaouira basin meet the standards according to the indices used except for EC where almost 50% of the points have poor quality and for CR the waters require a non-corrosive pipe. The spatial distribution of WQI shows a slight improvement of the quality even though the prolonged drought in the region. This is explained by the impact of the Covid 19 pandemic, which has caused a total shutdown of tourist activity. Tracing the waters using stable isotopes 18O and 2H show that the recharge of these waters is ensured by precipitation of Atlantic origin without significant evaporation and the recharge altitude varies between 400 and 1400 m. The mixing ratios deduced from the mass balance of stable isotopes shows that surface water contributes between 32.16 and 70.42% to the Cenomano-Turonian aquifer recharge. However, the results of this study could provide valuable information on the water quality for drinking and agricultural purposes.