The last global assessment of the contamination of Lake Geneva surface sediments (metals and nutrients) was carried out in 1988. A new survey was needed to assess the current state of surface sediments with respect to metallic and organic micropollutants. The present study is based on the analysis of i) sedimentological parameters and trace metals at 80 sites; ii) phosphorus and its chemical forms on 50 sites; iii) 54 substances distributed in 12 groups of organic micropollutants (PAHs, PCBs, pesticides, etc.) on 31 sites; and iv) 24 substances poorly documented for Lake Geneva (dioxins, PFOS, brominated flame retardants, ...) on 5 sites for an exploratory study. In addition, ecotoxicological tests based on the growth and mortality of ostracod Heterocypris incongruens were carried out at 30 sites to compare the results with the micropollutant levels.
Most measured substances on the 80 and 31 sites were detected. For trace metals, concentrations generally have values two to three times higher than the natural background, except for mercury that is up to 20 times more concentrated. Metal concentrations are similar to those measured in 1988, with the exception of mercury, which shows a marked overall decrease in concentrations. For organic micropollutants, their distribution follows two main trends: an increase with the remoteness of the Rhone River mouth (DDT, PCB, etc.), or a quite homogeneous distribution over the whole lake but with some higher values close to the lakeshore (e.g. PBDE, Endosulfan).
The median concentrations of the main groups of organic substances are ΣPAH 781 μg/kg ; Σ7PCBi 8.78 μg/kg; ΣDDT 2.87 μg/kg; ΣHCH 0.86 μg/kg; Σ7PBDEi 0.12 μg/kg.
The exploratory study at 5 sites revealed the presence of dioxins and furans, hexabromocyclododecane and perfluorinated compounds.
The site of Vidy Bay off Lausanne remains the most contaminated site of the lake, both for metals and most organic substances, with: Hg 2.44 μg/kg; Cd 1.47 μg/kg; ΣPAH 3214 μg/kg; Σ7 PCBi 79.1 μg/kg; Σ7 PBDEi 2.7 μg/kg; ΣDDT 9.0 μg/kg; ΣHCH 0.83 μg/kg.
The ecotoxicological tests have classified the sites into three categories, with 13 sites considered to be of good sediment quality, 10 middle quality sites and 7 poor quality sites.
A first approach to the potential toxicity of sediments to benthic organisms based on chemical data indicates that sediments are not expected to induce significant toxicity except at the Vidy Bay site. The results of the ostracod tests reveal a different situation, the significance of which remains to be explored.