Influence of Watershed Topography on the Chemistry of Stream Water in a Mountainous Area

摘要

The relation among the mean slope angle of a watershed, the stream water quality, and the chemical composition of the plagioclase component of granite was examined for 17 watersheds located in the southern part of the Northern Japan Alps in central Japan. The 17 watersheds were mainly composed of the granite. The ionic composition of the stream water can be classified into two patterns: the Ca–HCO₃-type and the Ca·Na–HCO₃-type. In all the watersheds, Na+ and Ca2+ occur in the groundwater that flows into the stream mainly due to the kaolinization of plagioclase. As compared to the watersheds with Ca–HCO₃-type stream water, the Na+/Ca2+ ratio for stream water in watersheds with Ca·Na–HCO₃-type stream water was closer to that for plagioclase in granite. This implies that the groundwater flowing in the Ca·Na–HCO₃-type watersheds is deeper than that in the Ca–HCO₃-type watersheds. The mean slope angle of Ca·Na–HCO₃-type watersheds was less acute as compared to that of Ca–HCO₃-type watersheds. The mean slope angle of the watersheds is negatively correlated with the Na+/Ca2+ ratio for stream water (r = −0.78, P ≤ 0.001). It is suggested that the groundwater quality at greater depths, which is comparable with the Na/Ca ratio of plagioclase, contributes more to the stream water quality in the gently sloping watersheds than in the steep watersheds. This study clarified that there is a close relation between the mean slope angle of watersheds and the depth of groundwater discharged to the stream. Moreover, it is believed that the relationship between the mean slope angle of watersheds and the Na+/Ca2+ ratio for stream water is an effective and simple index for understanding the relative depth of groundwater.