Development of Transient Hydrodynamic and Hydrodispersive Models in Semi-Arid Environments

The article presents a hydrogeological study through numerical modeling of the coastal basin of Rharb, located in the northwest of Morocco, a region characterized by a semi-arid environment. This basin consists of two aquifers of Plioquaternary and Upper Quaternary age, separated by a lower Quaternary aquitard. The Plioquaternary aquifer hosts abundant and good-quality groundwater, while the Upper Quaternary aquifer hosts a mediocre quality groundwater with negligible hydrogeological interest. In this semi-arid context, for health reasons, a significant portion of the population exploits the second, deeper aquifer. Unfortunately, this is not the case for the majority of the population, who cannot access the second aquifer due to financial constraints (exploitation costs). Overexploitation of the Plioquaternary aquifer by agricultural pumping causes water stress, leading to a decline in piezometric levels and seawater intrusion, causing a degradation of the local hydrosystem. The main objective of the study is to develop a decision support tool, in the form of a Multicodes numerical environment, to understand the circulation and storage mode of groundwater, aimed at maintaining this resource and preserving the environment, considering both quantitative and qualitative constraints. The creation of this digital environment detailing the intrinsic characteristics of the basin leads to monitoring the quantity and quality of both aquifers in response to external influences such as recharge, exploitation and pollution. The development of a transient hydrodynamic model over a 15 years period confirms that overexploitation of the deep aquifer is the main cause of the decline in groundwater level. Subsequently, the development of a hydrodispersive model indicates higher salt concentrations in the shallow aquifer than in the deeper aquifer, and locates the seawater. Despite the qualitative constraints, the quality of the deep aquifer is relatively preserved thanks to the clayey aquitard acting as a barrier against the infiltration of agricultural pollutants in the central basin, and the renewal of these waters in the recharge zones at the peripheries. However, deficit hydraulic balances signal the beginning of a relatively critical period.