For the 15 metals for which data were available from both sources (Ni, Cu, Zn, Pb, Cd, Al, Mn, Fe, Co, Rb, Sr, Ba, Ti, V, and Cr), we calculated the ratio of the dissolved concentration of each metal to the corresponding IC10 (hereafter, toxic unit, TU) as well as the sum of the TUs at each sampling site. By calculating the contributions of each metal to the sum of the TUs at each sampling site and ranking the metals based on the 95th percentiles of these contributions, the metals in descending order were Ni (0.94), Zn (0.64), Cd (0.61), Al (0.31), and Cu (0.14) at sites where the sum of the TUs ≥ 1 (i.e., metal-contaminated sites) (Fig. ). The 95th percentiles of the contributions for Cu, Fe, and Co were similar (0.14, 0.13, and 0.09, respectively). Among these three metals, Cu was included in the following analysis because its IC10 of 12 μg/L was somewhat higher than the U.S. EPA hardness-adjusted water quality criterion (e.g., 6.6 μg/L at a water hardness of 70 mg/L) and because ecological risks associated with Cu have been a concern in Japan . Lead was included in the following analysis because the IC10 for Pb reported by was 67 μg/L, which is more than an order of magnitude higher than the U.S. EPA hardness-adjusted water quality criterion of 1.7 μg/L at a water hardness of 70 mg/L. It should be noted that the IC10 of 0.37 μg/L for Ni is two orders of magnitude lower than the U.S. EPA water quality criterion of 38 μg/L at a water hardness of 70 mg/L. Alghouth the direct use of IC10 values requires some caution, IC10 values are likely acceptable for screening metals of ecological concern. The following data collection and analysis were conducted for a total of six metals (Ni, Cu, Zn, Pb, Cd, and Al).