Abstract
In the western part of Aso-valley, the unpermeable layer is located about 200 m below the ground which is supposed to decline from Aso-valley to the northern slope of Aso-somma. Considering that much SO42- is contained in the groundwater and rivers in the western part of Aso-valley, it is possible that much SO42-will be also found in some springs in the northern slope of Aso-somma. Since the water qualities of a river represent those of springs in the watershed, the authors investigated the water qualities of the upper reach of Tsuetate-river, running in the northern slope of Aso-somma, by taking into account the confluence of the branches. The results and some discussions were presented in this study.
