The hydrochemistry of surface waters in andesitic volcanic area, Norikura volcano, central Japan.

Many hydrochemical studies have been reported on surface waters and a major contributor of the water chemistry is considered to be rock dissolution. However, the extent of rock dissolution effect on surface water chemistry has still not been fully understood. The purpose of this research is to clarify the effect to give better understanding on the nature of surface water chemistry. Natural surface water samples were collected from a typical andesitic volcanic area, and major chemicals were investigated in terms of water-rock interaction. The analytical results were subjected to multivariate statistical techniques to understand the geographical distribution and to extract geochemical potential factors affecting the chemical concentration of waters. High correlation between the major chemicals and the altitude of sampling points was found among non-geothermal waters. The mole ratios of summation Sigmacations/Si were significantly high in meteoric waters (2.5), low in the surface waters from the summit area of the volcano (0.4), and the median in the surface waters at the mountainside region (1-2) and in geothermal waters (1-2). The basement rocks around the observation areas showed a relatively low ratio (0.4). On the basis of the water chemical compositions, these chemical variation were quantitatively interpreted by the incongruent dissolution reactions for primary minerals in the basement rocks followed by the formation of secondary minerals. The variations of Na+/(Ca2+ + Na+) ratio took positive correlation with [Si] for the non-geothermal waters, and high linear correlation was observed between log(a(Ca2+)/a2(H+)) and log(a(Na+)/a(H+)). These relations were interpreted by an ion-exchange reaction. The factor analysis (FA) gave two major factors to describe the chemical behaviors mentioned above. The contribution proportionate for each factor was calculated as; 65% was the incongruent rock dissolution, and 16% was the ion-exchange reaction.