Metal exposure profiles at metal-contaminated sites in rivers across Japan

Understanding realistic exposure profiles of mixtures is crucial for effectively predicting the ecological risks and effects of metal mixtures in natural environments on a large scale (e.g., at a country level). In this study, we aimed to compile available measurement data for metals relevant to ecological risk assessment in rivers across Japan, identify metals of particular concern based on their relative ecological risks, and derive realistic exposure profiles of these metals based on the compiled data. We focused on six metals of concern (Ni, Cu, Zn, Pb, Cd, and Al) selected by comparing available measurement data and 10% inhibition concentrations for the daphnid Ceriodaphnia dubia. We then compiled measurement data on these metal concentrations, ensuring sufficiently low detection/quantification limits relevant to ecological risk assessments, from a total of 531 riverine sites. At 194 metal-contaminated sites, concentrations generally increased in the following order: Cd (median: 0.013 μg/L), Pb (0.072 μg/L) < Ni (0.45 μg/L) < Cu (1.2 μg/L) < Zn (9.1 μg/L), Al (22 μg/L). Using hierarchical cluster analysis, we classified the metal-contaminated sites into three groups (Group 1: 56 sites; Group 2: 104 sites; and Group 3: 34 sites). Group 1 and Group 3 were characterized by higher concentrations of Cd and Ni, respectively, compared to Group 2. Further compilation and accumulation of measurement data, particularly in small rivers (e.g., tributaries of major rivers), are required to more accurately assess contamination levels and ecological risks from metals in rivers nationwide.