Assessing Seasonal Pollution Sources, Metal Pollution and Water Quality Indices in the Qholora Estuary, South Africa

Estuaries along South Africa’s coastline are increasingly subjected to anthropogenic pressures that disrupt their biogeochemical function and increase the risk of contamination. This study presents the first seasonal assessment of heavy metal contamination and water quality indices in the Qholora Estuary, Eastern Cape Province. Surface water samples collected during wet and dry seasons were analysed for physicochemical properties and heavy metals (As, Cd, Cu, Fe, Hg, and Pb). Multiple pollution metrics (Pollution Index (PI), Nemerow Pollution Index (NPI), Heavy Metal Evaluation Index (HEI), Heavy Metal Pollution Index (HPI)), ecological risk indices ((Ecological Risk Index (ERI), and Potential Ecological Risk Index (PERI)), and the Water Quality Index (WQI) were applied and supported by Principal Component and Cluster Analyses to identify dominant pollutant, contamination sources and seasonal hydro-geochemical controls. Results reveal strong seasonal contrasts: wet-season conditions showed elevated ionic concentrations and enhanced mobilisation of Cu, Pb, Cd, Hg, and Fe due to storm-driven runoff and sediment resuspension, while dry-season patterns reflected evapo-concentration, prolonged residence times, and pH-mediated metal partitioning. Across indices, heavy metal contamination remained low in the dry season but increased significantly in the wet season, especially for Hg, which posed moderate to considerable ecological risk at most sites, indicating emerging ecological pressure under high-flow conditions. These findings highlight a generally low risk under average conditions but a pronounced seasonally vulnerable estuarine system, underscoring the need for intensified monitoring during periods of increased runoff. The study establishes an important baseline for regional water resource management.