Dams act as both sinks and potential secondary sources of sediments and trace metal elements (TMEs), generating environmental risks for ecosystems and human uses. The Hassan II Dam, a key drinking water reservoir in the High Moulouya (Eastern Morocco), illustrates this dual role as it secures water supply while receiving inputs from upstream mining residues, soils, and tributaries. This study evaluates the geochemical properties and TMEs dynamics of dam sediments in response to concerns about their behavior and potential contamination.

Twenty-seven sediment samples were analyzed for granulometry, mineralogy, physico-chemical parameters, and TMEs (Zn, Pb, As, Cu). Granulometry was determined by laser granulometer. Mineralogy and major elements were analyzed by X-ray diffraction (XRD) and X-ray fluorescence (XRF), respectively. Physico-chemical parameters included pH, electrical conductivity (EC), organic matter (OM), and carbonate content (CaCO₃), quantified by loss on ignition (LOI). TMEs were measured using inductively coupled plasma mass spectrometry (ICP-MS). Contamination levels were assessed through enrichment factor (EF), geo-accumulation index (Igeo), and pollution load index (PLI). Correlation matrices were used to explore relationships between TMEs and sediment properties.

Results show sediments dominated by fine particles (≈80% silt and clay) and silicate, carbonate minerals, under alkaline conditions (pH 8–9) with CaCO₃ contents of 18–19%. Average TMEs concentrations were 43.9 mg/kg for As, 56.3 mg/kg for Cu, 37.2 mg/kg for Pb, and 152.6 mg/kg for Zn, pointing to polymetallic contamination. Indices confirm moderate to strong enrichment, suggesting ecological toxicity risks. Zn and Cu display strong correlations and affinities with Fe/Mn oxides and alumino-silicates, indicating vulnerability to redox-driven release during anoxic phases. Their antagonism with carbonates reflects competition for adsorption sites. In contrast, Pb shows distinct behavior, suggesting different behavior or legacy sources.

These findings underscore the importance of understanding TMEs' behavior in dam sediments to predict their fate and ecological risks. Within reservoir sinks, sediments act as hotspots of contamination, with heavy inflows triggering resuspension. Coupled risks include ecological toxicity to organisms and long-term threats to water supply and storage capacity. Addressing these issues requires further source tracing, particularly for Pb and risk-aware management strategies that jointly address sediment influx and TMEs mobility.