Spatio-Temporal Assessment of Surface Water Quality Using the Water Quality Index: Insights from the Marchica Lagoon, Morocco

Abstract The lagoon system in northeastern Morocco constitutes the largest regional paralic hydrological system between the Mediterranean Sea and the African continent. Given its importance, this lagoon is one of five Sites of Biological and Ecological Interest (SIBE) and has been classified as a protected ecosystem site under the Ramsar Convention. Assessing spatial and seasonal variations in surface water quality is essential to preserving the ecological integrity of this lagoon, which is often subjected to increasing anthropogenic pressures and suffers from eutrophication due to discharges from wastewater treatment plants, agricultural sites and various inputs transported by the watercourses draining its catchment area. This study assesses the water quality of the Nador (Marchica) lagoon using the Water Quality Index (WQI) applied to 16 monitoring stations grouped into four sectors: Oued Caballo, Oued Selouane, Kariat Arkmane, and Beni Enssar, across all four seasons. Water quality index calculations were performed in accordance with to the standards for Moroccan drinking water (MaP) and irrigation water (MaiR). The results indicate pronounced spatial heterogeneity in water quality. Under the Moroccan drinking-water guideline, the Caballo and Oued Selouane wadis exhibit very high WQI values (> 100), indicating very poor water quality and unsuitability for human consumption, these sites also exhibit elevated WQI values (75–110), indicating that treatment is required before irrigation. In contrast, Kariat Akremane located closer to the lagoon inlet and farther from major pollution sources shows substantially lower WQI values, indicating comparatively better water quality. Beni Enssar presents intermediate conditions, likely influenced by urban runoff but partly moderated by marine water exchange. Overall, the observed degradation is mainly attributed to untreated domestic wastewater, industrial discharges, and intensifying agricultural activities. These findings underscore the need for integrated watershed management and continuous monitoring to limit local algal proliferation and support the long-term ecological sustainability of the Nador Lagoon.