Origin and residence time of groundwater in the Tadla basin (Morocco) using multiple isotopic and geochemical tools

Abstract

Summary Groundwater resources in the Tadla basin stem from surface water recharge and different groundwater inflows, forming a multilayered aquifer system, which hosts one of Morocco’s most important groundwater reservoirs. The hydrodynamic infrastructure; i.e. the relationship between all regional aquifers, recharge, and the residence time of waters poses a serious challenge for current water management and future exploitation in aiming for a long-term sustainable utilization. A combined hydrogeologic and isotopic investigation using hydrochemical and isotopic tracers such as 18 O, 2 H, 3 H, 13 C and 14 C was carried out in order to determine the sources of water recharge to the aquifers, the groundwater flow system, and the residence time of these waters. More than one hundred point measurements throughout the study area in varying wells, boreholes, springs and rivers were investigated. Chemical compositions of the groundwater indicate an important influence by the host carbonate rocks from each of the Tadla aquifers. Stable isotope results indicate the existence of two groups of groundwater corresponding to the unconfined aquifer in the north and the confined aquifer in the south. The δ 18 O, δ 2 H, 3 H, and 14 C data indicate that the High Atlas Mountains in the south and east of the basin, which are characterized by high rainfall and low δ 18 O and δ 2 H values, are currently the major source of recharge for the Tadla aquifers. A significant recharge zone lies in the northern part of the basin where all the aquifers outcrop. The confined zones show depleted 18 O values, corresponding to the signature of recharge water from the Atlas Mountains. Moreover, all isotope data demonstrated clearly that the Tassaout springs, which are located in the southwest of the basin and were previously interpreted as representing natural outlet of the deep aquifers, are comprised of young waters with depleted 18 O and 2 H signatures, suggesting a high altitude recharge from the Atlas Mountain. In contrast, the unconfined parts of the aquifers show higher values of δ 18 O, indicating an evaporation phenomenon, which occurs during infiltration or recharge from irrigation. The mixing process of old and recent waters is confirmed by 14 C and 3 H. The isotopic data also indicates probable interaction and flow between the different aquifers. The isotopic and hydrochemical data is therefore essential in confirming possible mixing relationships between aquifers whereas traditional hydraulic data is not capable to provide a quantitative assessment of these relationships. The data generated in this study will certainly encourage the revision and improvement of the current hydrological water resources model for the Tadla basin. The results provide a framework for development of a comprehensive management plan in which water exploitation shifts towards areas with modern recharge and where young and high quality groundwater is found.