By compiling measurement data on metal concentrations relevant to ecological risk assessments, we were able to derive realistic compositions and concentration levels of six metals (Ni, Cu, Zn, Pb, Cd, and Al) at metal-contaminated sites in rivers across Japan (see Table for the percentiles of metal concentrations in individual groups). This information would be useful in choosing levels of metal concentrations relevant to those found in Japanese rivers for metal mixture toxicity tests, enabling a better assessment of their actual ecological risks. However, it should be noted that the two field studies included in our dataset focused on the impacts of specific metals such as Ni or Cu, Zn, Cd, and Pb . In addition, have measured metal concentrations in the mainstreams of major rivers across Japan but did not make measurements in tributary streams. Because levels of contamination in small streams and rivers are more likely to be high because of their limited capacity for dilution , further compilation and accumulation of measurement data are therefore required to more accurately assess contamination levels and ecological risks from metals in rivers across Japan. To this end, it is essential to encourage the open accessibility of measurement data, along with their geographical coordinates, in easy-to-use formats in publications such as peer-reviewed papers, particularly those written in Japanese. Furthermore, the bioavailability of metals was not considered when selecting the metal-contaminated sites in this study. While important water quality variables such as Ca, Mg, and dissolved organic carbon (DOC) were collected as complementary data (Table ), DOC was not measured by . Because the influence of DOC on bioavailability can be significant for some metals , compiling measurement data on such water quality variables in addition to metals, as well as developing predictive models when measurements are unavailable, will be essential for more accurate metal risk assessments.