Estimation of weathering in mountainous river basins observed from dissolved inorganic constituents in river water

Abstract

In our research, the dissolved inorganic components and alkalinity in mountainous river water were measured and rock weathering in the basin was evaluated. The survey area are upper reaches of Asai River in Shiraiwa Forest Park, Hirakawa City, having four small drainage basins. Observation was started from May 2018, and covers summer season, snow season, and snow melting season. Rock weathering by water containing atmospheric carbon dioxide dissolves alkali and alkaline earth elements and silicate from base rock. The counter anion is hydrogen carbonate in the river water of neutral pH, and alkalinity and silicate concentrations are increased by the weathering. The results of river water measurement showed the ratio of seasonal change of alkalinity and silicate concentrations has linear relationship. The slope was almost between 0.5 and 0.7 for all sites in upper reaches. When river water flow was unusually high because of heavy rain and snow melting, the linearity was deteriorated. It suggest that the river water alkalinity and silicate concentration change was regulated by local weathering of feldspar including Orthoclase and Plagioclase, considering the ratio of alkalinity to silicate in the river water.