Decrease of pH of River and Lake Water in Mountainous Region in Central Japan and Its Relation to Acid Rain

Abstract

Annual changes of the pH of river and lake water in the mountainous region in central Japan were investigated by using the water quality monitoring data in Nagano prefecture from 1972 to 1989. By this analysis, the annual trend of the decrease of the pH of river and lake water was confirmed in the upper stream area of Saigawa, Himekawa and Tenryugawa (Matsukawa), where the bedrocks of their watersheds consist of acidic rocks such as the granitic rocks and rhyolite. However, such a decrease of pH of river water was not obvious in the upper stream area of Chikumagawa, Nakatsugawa, Tenryugawa (Koshibugawa) and Fujigawa, where their bedrocks consist of neutral rocks and basic rocks, such as andesite and basalt, and sedimentary rocks. The decrease of pH of rievr water was also not obvious in the middle stream area, where the influence of neighboring anthropogenic pollution was not negligible.