Evolution of the chemical and isotopic composition of groundwater under a semi-arid climate; the case of the Cenomano-Turonian aquifer within the Essaouira basin (Morocco)

This study evaluated how the chemical and isotopic composition of the groundwater in the Cenomano-Turonian aquifer of the Essaouira basin from Morocco has changed in response to low annual rainfall associated with climate change during the period 1995–2018. The study of the chemical composition of groundwater shows a Cl–Ca–Mg type facies which was maintained during the observation period (1995–2018). The electrical conductivity (EC) and the Cl⁻ content of groundwater increased during the study period in response to low rainfall with a mean values range from 1214 (1995) to 2482 µs/cm (2018) for EC and from 11.36 (1995) to 14 meq/L (2018) for Cl⁻. Stable isotope measurements indicate that the Cenomano-Turonian aquifer is recharged by rainfall of Atlantic origin. Levels of ¹⁸O during the period 2007 and 2016 were generally stable with slight fluctuations (≈ 1‰) that may be due to analytical errors, suggesting that climate change has no effect on the isotopic signature of groundwater in the study area. Tritium levels in groundwater decreased between 2007 and 2016, possibly due to low recharge rates associated with low annual rainfall during this period.