Fate of heavy metals in the water-scarce Nile Delta lagoons: a post-Aswan Dam perspective

Deltas around the world are increasingly threatened by metal pollution, caused by human activity, endangering the health of vital ecosystems. This study presents a comprehensive review of the evolution of heavy metals in the arid Nile Delta, aiming to establish a model for the metals' source-sink dynamics and their underlying drivers. Based on an overall assessment of heavy metal distribution across 639 sites interpreted by numerous previous studies, we synthesized individual datasets into an integrated delta-scale model, to reassess the source-sink pollution dynamics. Using pre-Aswan Dam data (prior to 1964) as a reference point, we defined major heavy metals (Pb, Cu, Zn, Cd, Cr, Mn, Sr, Ni) in deltaic surface sediments as being of anthropogenic origin, through statistical analyses. We found a latitudinal (south-to-north) increasing trend in anthropogenic metal concentrations, which were progressively transported from the delta plain to coastal lagoons over the past half-century. Integrated pollution indices revealed that metal concentrations in the lagoons were 3-4 times higher than those in various delta sectors, i.e. river branches, canals, drains, soils and coastal beaches, indicating a cumulative process-consequence following dam construction. The increasing enrichment factor (EF) values of lagoonal metals are closely correlated with proxies of social development, including wastewater discharge, fertilizer application, GDP, population, etc., among which the water stress level (WSL) plays a determinant role in raising EF value. In-depth study revealed a non-linear relationship between EF and WSL, indicating that lagoon sediments will lose their capacity to absorb anthropogenic metals by the end of 2060, implying an irreversible metal contamination crisis in the water-scarce Nile Delta. Our study calls for an urgent policy reform and implementation to improve coastal management.