The hydrogeological study of the Rharb coastal basin, located in the semi-arid northwest region of Morocco, focuses on its two aquifers: the Plio-Quaternary aquifer characterized by high-quality water with salt concentrations ranging from 0.4 to 2 g/L, and the Upper Quaternary aquifer, with lower water quality (2 to 6 g/L). The deep aquifer is overexploited for agricultural purposes. This overexploitation has led to declining piezometric levels and the worsening of the oceanic intrusion phenomenon. The study aims to develop a numerical model for a period of 15 years, from 1992/93 to 2006/07 for monitoring groundwater quantity and quality, considering recharge, exploitation, and basin characteristics. A hydrodynamic model based on storage coefficient calibration identifies overexploitation for irrigation, increasing from 93 Mm³ in 1993 to 170 Mm³ in 2007, as the primary driver of declining water levels. A hydrodispersive model highlights higher salt concentrations in the shallow aquifer (up to 6 g/L), high nitrate concentrations due to human activity, and pinpoints areas of seawater intrusion approximately 500 m from the shoreline. Although the deeper aquifer remains relatively preserved, negative hydraulic balances from −15.4 Mm³ in 1993 to −36.6 Mm³ in 2007 indicate an impending critical period.

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.