Heterogeneity and potential aquatic toxicity of hydrogen peroxide concentrations in selected rivers across Japan.

Hydrogen peroxide (H2O2) is a reactive oxygen species formed in natural water. It is reportedly toxic to aquatic organisms with a predicted no-effect concentration (PNEC) of about 380 nM. In this study, a countrywide investigation of H2O2 concentrations in selected rivers across Japan was conducted to identify rivers that pose toxicity concerns. Twelve rivers with a total catchment area of 13,646 km2 were selected from different prefectures. Spatial and temporal variation studies showed that the H2O2 concentrations (avg. 320 nM, n = 111) varied by two orders of magnitude (range 21-2929 nM) across the rivers. The Yamato River in Osaka and Nara prefectures and the Kokubu River in Chiba Prefecture had the highest concentrations at 276-669 nM and 236-2929 nM, respectively. >75% of the data from the two rivers were either close to or exceeded the PNEC. Most of the results for the other rivers were less than the PNEC. There was a clear seasonal variation in the H2O2 concentrations, with the highest values obtained in summer because of high solar irradiation. The H2O2 concentration had the highest positive correlation (r = 0.61, p < 0.01, n = 111) with the product of dissolved organic carbon and solar radiation intensity, which suggests that these two factors in combination are important in determining the H2O2 concentrations in river water. It was also observed that bigger rivers had lower H2O2 concentration and vice-versa. This shows that the size of a river may influence its H2O2 concentration. This study is the first countrywide survey of H2O2 concentrations in different rivers and evaluation of their relationship with the PNEC. The data provide insight on the factors influencing the concentrations of H2O2 in river water.