Supply of silicate and alkalinity into river water by rock weathering and their seasonal variations

Abstract

Hirosaki city is surrounded by mountainous areas with different geological settings. One is Towada-Hakkoda mountain area with volcanic eruption product and the other is Shirakami mountain area with old sedimentary rock with oceanic origin. The mountainous river water showed different concentration of silicate, calcium, and magnesium, which mainly come from rock weathering. These rock weathering components showed seasonal increase in summer and minimum concentration in snow melt season. Rivers from Towada-Hakkoda mountain showed high alkalinity and silicate concentrations than rivers from Shirakami mountain. The correlation of alkalinity and silicate was significant. By assuming simple mixing model of rock weathering components of andesite and carbonate, the overall dissolved components in the mountainous rivers are well explained. Silicate in these river are utilized for rice growth during summer growing season in the rice paddy located in the middle river area.