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.