<p>The scarcity of clean water in semi-arid regions is a pressing global issue, leading to increase reliance on reservoirs for drinking water supply and irrigation. While dams help address water shortages, their water quality remains vulnerable to contamination from both natural and anthropogenic pressures. This study presents an integrated evaluation of the surface water quality in the Hassan II dam (eastern Morocco), including, for the first time, sampling from the central reservoir to ensure a comprehensive special assessment. The results show that average concentrations of Trace Metal Elements (TMEs), including As (1.5 µg·L⁻¹), Cd (2.1 µg·L⁻¹), Cu (2.2 µg·L⁻¹), Pb (5.2 µg·L⁻¹), and Zn (3.4 µg·L⁻¹), remain below international guideline limits. Irrigation Water Quality Indices indicates generally suitable conditions, with a Sodium Adsorption Ratio (SAR) values ranging from 0.64 to 1.76, corresponding to excellent to good irrigation quality. In contrast, the Water Quality Index (WQI) ranges between 100 and 593, classifying the water as unsuitable for drinking according to Moroccan regulations. Heavy Metal Pollution Index values (HPI &lt; 100) categorize the reservoir as low-polluted overall. Multivariate statistical analyses, including Principal Component Analysis (PCA) and Cluster Analysis (CA), identify both watershed lithology and anthropogenic activities as responsible for water chemistry. This process-based interpretation provides deeper insight into the mechanisms governing water quality variations. Overall, the findings reveal a contrasted water quality status, with acceptable conditions for irrigation but significant limitations for drinking purposes. The study provides a robust baseline dataset and supports informed decision-making for monitoring and sustainable management of reservoir systems in semi-arid environments.</p>