Evaluating the restoration of Egypt’s Mediterranean Manzala Lagoon: a multi-index assessment of water quality and heavy metals

Lake Manzala, the largest coastal lagoon along Egypt’s Mediterranean delta, represents an ecosystem of exceptional ecological and socioeconomic significance, yet it has endured decades of severe environmental degradation. This study presents a comprehensive post‑restoration assessment of its water quality during 2021–2022 to evaluate the effectiveness of ongoing dredging and wastewater‑treatment initiatives. Twelve monitoring stations were systematically surveyed seasonally across the lake’s four sectors, and a wide array of physicochemical variables, nutrient salts, and heavy‑metal concentrations were determined. Integrated indices, including the Canadian Water Quality Index (CWQI), Aquatic Toxicity Index (ATI), Trophic State Index (TSI), Pollution Index (PI), and Heavy-Metal Pollution Index (HPI), were applied, alongside multivariate statistical analysis (Principal Component Analysis, PCA) to interpret spatial patterns and dominant controlling factors. Results revealed a persistent north–south gradient of deterioration, with the southern and southeastern sites receiving heavy loads of organic and inorganic pollutants from the Bahr El‑Baqar, Hadous, and Serw drains. The lake remains markedly hypereutrophic; mean BOD5 and COD values exceeded international thresholds, and high concentrations of Pb, Cd, Ni, and Cr indicate serious ecological risk. CWQI values (39–64) classified most areas as poor to marginal, while HPI values (> 180) indicate heavy‑metal pollution stress. PCA distinguished two principal drivers: (1) anthropogenic organic and metal loading, explaining about 52% of the total variance, and (2) natural oxygenation and self‑purification processes with a minor influence. Despite improved circulation from dredging and the commissioning of the Bahr El-Baqar wastewater treatment plant, overall chemical restoration remains incomplete. Long-term recovery requires catchment-wide nutrient control, sediment remediation, and continuous monitoring to restore the ecological balance and sustain the environmental and economic value of this vital wetland.