Metal contamination along the French Mediterranean coast: From background to current status

The French Mediterranean coast has a long history of anthropogenic metal contamination, yet current contamination levels remain largely unquantified. This study aims to provide a comprehensive overview of metal accumulation along the coastline by establishing natural background levels, identifying spatial contamination patterns, and assessing associated ecological risks. Major elements (Al, Fe) and trace metals (Mn, Pb, Zn, Cd, Cu) were measured in surface sediments from 74 sites and in three sediment cores using ICP-MS, while mercury (Hg) was analyzed using an AMA instrument. Natural background concentrations were determined based on the depth layers of three sediment cores that predate the year 1917. This timeframe has been recognized as the pre-industrial period by applying multivariate change point analysis. Approximately 60 % of sediment samples exhibited a Pollution Load Index (PLI) above 1, indicating significant anthropogenic enrichment across the coast. The Mean Effects Range Median Quotient (m-ERM-Q) exceeded 1 only at Cortiou and Toulon, suggesting a localized ecological risk. At Cortiou, Cd (2.85 mg/kg) and Cu (212.0 mg/kg) were between Effect Range-Low (ERL) and Effect Range-Median (ERM) thresholds, while Pb (264.0 mg/kg), Hg (6.73 mg/kg), and Zn (654.0 mg/kg) were above ERM, due to wastewater discharge. In Toulon Bay, Pb (79.34 mg/kg) and Cu (50,00 mg/kg) were between the ERL and ERM, while Hg (5.57 mg/kg) exceeded the ERM, linked with long-term naval and industrial activities. These findings demonstrate pervasive metal enrichment and ecological risks driven by different sources and highlight the urgent need for targeted mitigation strategies.