Evaluating physicochemical and organic pollution dynamics using WQI and OPI in the Sidi Chahed reservoir and its tributaries (Oued Mikkès and Oued Mellah), Morocco

This study evaluates the spatial and seasonal dynamics of physicochemical parameters and water quality in the Sidi Chahed reservoir and its tributaries over a complete hydrological year (December 2022-November 2023). Monthly sampling was performed at six stations representing tributary inputs, the confluence, and different reservoir depths. Standard laboratory analyses and in situ measurements were complemented by statistical approaches, including ANOVA, correlation analysis, and principal component analysis. Two indices—the Water Quality Index (WQI) and Organic Pollution Index (OPI)—were applied to assess overall water quality and organic pollution levels. Results revealed pronounced seasonal variability in temperature, conductivity, turbidity, nutrients, and organic matter, driven by climatic fluctuations and upstream anthropogenic pressures. WQI values (17.3-47.4) classified the water as excellent to good, while OPI values (2.67—1.33) indicated moderate organic pollution, particularly in tributaries during spring and autumn. Reservoir stations showed improved conditions in summer due to dilution and stratification effects. Overall, the system remains functional but requires strengthened monitoring and better management of wastewater and agricultural inputs.