Interpreting Groundwater Chemistry and Estimating Residence Time of Groundwater in the Mt. Fuji Area Based on a Dissolution Kinetics-Fluid Flow Model and Chlorofluorocarbons (CFCs) Concentration

Abstract

Relationships of concentrations of elements (Ca, Mg, Si) in groundwater from the Mt. Fuji area are obtained using analytical data on groundwater. Relationships of Ca, Mg, and H4SiO4 concentrations can be explained in terms of the dissolution of basaltic glass and precipitations of secondary minerals (halloysite, allophane, montmorillonite, and α-cristobalite). The relationship between residence time and H4SiO4 concentration of groundwater is obtained based on dissolution kinetics-piston fluid flow and a perfectly mixing fluid flow model. Using this relationship, the dissolution rate constant experimentally determined in an open system (not in a closed system) (k = 10−10.7 molSi/m2 s) and analytical data on H4SiO4 in groundwater, the residence time of groundwater (Kakitagawa site, southeastern foot and Fuji Yoshida site, northern foot) is estimated to be ca. 20 years for A/M (A: surface area of rock (m2), M: mass of water (kg)) = 20 based on a dissolution kinetics-fluid flow model. This estimated residence time is roughly consistent with other methods (He isotope tritium, chlorofluorocarbons (CFCs) ages (10-40 years), indicating that the dissolution kinetics-fluid flow model using dissolved silica concentration is a potentially useful method for estimating the residence time of groundwater in a young volcanic region. It is also reported that CFCs ages are ca. 30-40 years, which are the first data obtained for the Mt. Fuji area. This suggests that CFCs data are very useful for providing the residence time of groundwater in an area at a high elevation.