Résumé
This study aims to determine the pattern of contamination of water and lagoon sediments and monitor their spatial dispersion through the coastal urban areas. A sedimentological, mineralogical and chemical investigations of the surface marine sediments of the lagoon and the surrounding nearshores were conducted in 2019 and 2020. The determination of the chemical quality of the water has also been studied. Grain size results revealed a relevant muddy distribution (< 0.063 mm) in the lagoon and the port sites. Mineralogical identification provided by XRD analysis showed that the most common components are quartz and calcite elements. Major and trace metals quantification by ICP-MS highlighted that surface water contained systematically arsenic at concentrations ranging from 1.5 to 1.9 times the environmental quality standard (EQS). Most of the water samples also showed high concentrations of Zn. A very high concentration of copper (12 µg/L, i.e., 7 times the EQS) was measured in the water column in the northeast zone of the lagoon. Very high concentrations of copper (402 ppm) and lead (245 ppm) were measured in the sediments collected in the harbor entrance channel. Overall, the analysis of the sediments of the low valley of Medjerda River and the nearshore of Ghar El Melah lagoon reveals a critical chemical enrichment of traces element (As, Cu, Ni, Pb and Zn) mainly trapped in muddy terrestrial sediment probably enriched by urban waste.