In the Shibukuro and Tama River system, Akita, where river water is acidified by inflow of acidic hot spring water, mixing process of confluent river water is explained by changes in the isotope ratios of Sr and S as well as the chemical composition of dissolved components. Based on the relationships between Sr and S isotopic ratios and their reciprocal concentrations, the mixing ratio of waste water from a neutralization plant of acidic hot spring water with river water of Shibukuro River was estimated as 1 to 1, and at the confluence of Shibukuro and Tama Rivers the mixing ratio of these two was estimated to be 3 to 7 or 4 to 6. These mixing ratios were mostly consistent with those obtained from the concentrations of dissolved chemical components. These consistent mixing ratios suggest that the chemical composition of river water was mainly controlled by the mixing of the waste water and the tributary river waters, and the influence of precipitation of insoluble salts was negligible. The contribution of the waste water was about 10% at Tose located downstream from the confluence of Shibukuro and Tama Rivers.
