Chemistry of surface water at a volcanic summit area, Norikura, Central Japan: Multivariate statistical approach

Abstract

Many hydrochemical studies on chemical formation of shallow ground water have been reported as results of water–rock interaction, and contamination of paleo-brine or human activities, whereas the preliminary formation of precipitation source in the recharged region has not been established yet. The purpose of this research work is to clarify the geochemical process of water formation from a water source unpolluted by seawater or human activity. Norikura volcano, located in western part of central Japan provided a suitable source for this research purpose, and hence chemical compositions of water samples from the summit and the mountainside area of Norikura volcano were determined. Most samples in the summit area showed very low electrical conductivity, and lower than 12 μS/cm . On the basis of the chemical compositions, principal component analysis (PCA) and factor analysis (FA), such as kinds of multivariate statistical techniques were used to extract geochemical factors affecting hydrochemical process. As a result, three factors were extracted. The first factor showed high loading on K +, Ca 2+, SO 4 2− and SiO 2, and this factor was interpreted due to influence of the chemical interaction between acidic precipitated water and rocks. The second factor showed high loading on Na + and Cl −, and it was assumed to be an influence of seawater salt. The third factor showed loading on NO 3 −, and it was interpreted to be caused by biochemical effect of vegetation. The proportionate contributions of these factors to the evolution of water chemical composition were found to be 45%, 20% and 10% for factors 1, 2 and 3, respectively. The same exploration at the mountainside of Norikura volcano revealed that the chemical variances of the non-geothermal water samples were highly influenced by water–rock interactions. The silicate dissolution showed 45% contribution for all chemical variances, while the adsorption of Ca 2+ and Mg 2+ by precipitation or ion exchange showed 20% contribution. The seawater salt influence or biochemical effect was statistically negligible in this area. The clear differentiation of geochemical process on water formation was found between the summit area and the mountainside area.