Water chemistry in a number of mountainous streams of east Asia

To determine the processes and factors responsible for the chemical composition of mountainous stream waters, correlations among solute concentrations in streams were examined in 13 forested watersheds from Japan, Jiulianshan (southern China) and Chiang Mai (northern Thailand), which had different soil parent materials and climatic conditions. The Na+ concentration in stream water was significantly correlated with both Cl− and Si concentrations in the regions near the sea (Kagawa and Shibecha), but was correlated with the Si concentration in the regions far from the sea (Mt. Hiei, Takayama, Jiulianshan and Chiang Mai). In some watersheds in Kagawa, the Si concentration was correlated with Cl−, probably reflecting the relatively dry climate conditions. In Kagawa and Mt. Hiei, SO42− concentrations in stream water were inversely correlated with NO3− concentrations, suggesting that nitrification inhibited SO42− discharge, and denitrification and root NO3− uptake enhanced it. The alkalinity was mainly carried by HCO3− and organic acid anions. According to the theory of ion exchange equilibrium, high alkalinity was closely related to divalent cations (Ca2+ and Mg2+) more than to Na+. In the streams where concentrations of SO42− and divalent cations were low (Chiang Mai), alkalinity was closely related to Na+ reflecting production by weathering.