Super typhoon induced high silica export from Arakawa River, Japan.

Dissolved silicate (DSi) and particulate silica (PSi) concentrations were measured at Arakawa River and at sewage treatment plants (STP) during October 2018 to October 2019. These included flooding observations after super Typhoon Hagibis. At ordinary water levels, the STP effluents were found to be the largest source of DSi in the river. Although DSi concentrations during the flooding events (165 μmol L-1) decreased by about 25% compared to that of ordinary water level (221 μmol L-1), PSi was more than sixteen times higher value (301 μmol L-1) compared to that of ordinary water level (18 μmol L-1). Loading amounts of DSi and PSi (± 1 standard error) were 1.5 × 108 (± 0.1 × 108) and 0.15 × 108 (± 0.02 × 108) mol year-1, respectively, excluding the data of Typhoon Hagibis. Loading amounts during flooding events of DSi and PSi were 1.2 × 108 (± 0.1 × 108) and 2.4 × 108 (± 0.4 × 108) mol 15 days-1, respectively. Although the silica loading at ordinary water level was mainly derived from DSi, the silica loading during flooding events was extremely large due to both high level of DSi and PSi; moreover, it was higher than the annual loading amount.