CHEMICAL COMPOPITION OF THE DRAINAGE FROM ASO WELDED TUFF : ON THE ONO RIVER

Abstract

One of the characteristics of river waters in Kyushu island is high content of SiO2. The author has investigated the variation of the chemical composition of the river waters with the difference of geological feature. The results are as follows. In the Ono river system, the SiO2, content of the drainage from Aso welded tuff area is 53.6 ppm (43.4% to the salinity), in tributaries which start from other igneous rocks and flow through Aso tuff it is 42.0 ppm (39.8% to the salinity), and in tributaries which start from argillaceous sedimentary rocks and flow through Aso welded tuff it is 24.8 ppm (26.6% to the salinity). In the Banjo river system, the whole of which basin is covered with argillaceous sedimentary rocks, the Si02 content is 12.9 ppm (25.7% to the sallinity). This fact indicates that the SiO2, content in the water of the rivers varies with the geological feature of their reaches. The SiO2, content in the drainage of Aso welded tuff is in direct proportion to HCO3 content. Therefore the dissolution of rocks is cuased by the water charged with carbon dioxide. In Aso welded tuff area the Na content in river waters is out of proportion to Cl content. On the other hand, the quantity of (Na + K)- Cl is in direct proportion to SiO2, content. Accordingly, almost all the quantity of alkali metals in river waters comes from resolution of rocks at Aso welded tuff area. It is concluded that the alkali quantity which comes from rocks is 82% to the total quantity of alkali metals in river waters of Aso welded tuff, and argillaceous sedimentary rock area that percentage is 40.