Dilution and Neutralization Effects of the Tributary Uguigawa River on the Ohtakigawa River Near Mount Ontake in Central Japan

摘要

The water quality of the Ohtakigawa River is affected by Mount Ontake volcanic activity. The Makio Dam in the Ohtakigawa is one of the supply water sources for Aichi and Gifu Prefectures. For improvement of water quality management at Makio Dam, this study is aimed at understanding the effects of the Uguigawa River on the water quality in the Ohtakigawa. Samples were separated into dissolved and suspended matters. This study focuses on eight elements (Al, Ca, Fe, K, Mg, Na, S, and Si). The Uguigawa shows pH neutrality (pH 6.7) and low concentrations (ex. S: 0.33 mgL~(-1)). In the Ohtakigawa after the Uguigawa, the water is diluted. As a result, pH is neutralized and element concentrations are lowered (pH 5.4 to 6.3, S: 14.6 to 7.43 mgL~(-1)). In particular, the concentrations of Al and Fe are significantly decreased compared with other elements, because Al and Fe precipitate on the river bottom likely owing to the neutralization effect. In the neutralization process, dissolved Al and Fe become insoluble forms Al(OH)_3 and Fe(OH)_3. It is found that dilution and neutralization effects of the Uguigawa play important roles in the water quality of the Ohtakigawa.