Dynamics and fates of trace metals chronically input in a Mediterranean coastal zone impacted by a large urban area

Quantification and characterization of chronic inputs of trace metals and organic carbon in a coastal Mediterranean area (the city of Marseille) during the dry season was carried out. The 625 km2 watershed includes two small coastal rivers whose waters are mixed with treated wastewater (TWW) just before their outlet into the sea. Dissolved and particulate Cu, Pb, Cd, Zn, Co, Ni and organic carbon concentrations in the rivers were comparable to those in other Mediterranean coastal areas, whereas at the outlet, 2- to 18-fold higher concentrations reflected the impact of the TWW. A non-conservative behavior observed for most of the studied metals in the mixing zone was validated by a remobilization experiment performed in the laboratory. The results showed that sorption/desorption processes could occur with slow kinetics with respect to the mixing time in the plume, indicating non-equilibrium in the dissolved/particulate metal distribution. Thus, a sample filtration immediately after sampling is strictly required.

► Evaluation of the chronic inputs from a large agglomeration to the coastal zone. ► Evidencing the influence of WWTP inputs vs. rivers to the elements discharge at sea. ► Comprehension of organic carbon and metals behavior and fate in the salinity gradient. ► Fast sorption processes control metal dissolved/particulate fractionation. ► Correct dissolved metal assessment requires on-board sample filtration.