Groundwater Chemistry in Granitic Rcok Area Interpreted in Terms of a Coupled Fluid Flow-Dissolution and Precipitation Kinetics Model

The chemical composition of shallow groundwater in the granitic rock area (Tsukuba-Kaba area, central Japan ; Kamaishi mine, northern Japan) was determined. Thermochemical calculations on the concentrations of Si, Na, K, Ca, H+, and HCO3- in shallow groundwater (Tsukuba-Kaba area, central Japan) in equilibrium with silicate minerals in granitic rocks indicate that the equilibrium between shallow groundwater and silicate minerals (feldspars, kaolinite, amorphous silica) is not attained. Therefore, a coupled fluid flow-dissolution kinetics model was applied to interpret the chemical composition of shallow groundwater. Assuming reasonable values of residence time, solubilities (equilibrium concentration), and dissolution rate of Na-Ca feldspar, the ratio of the surface area of rock to the mass of water (A/M) for shallow groundwater was estimated to be 0.15～8.7. The width of a fracture in a granitic rock is estimated to be 0.2mm～2cm, considering estimated A/M and uncertainty of parameter values. The estimated value of the width is much wider than that obtained by pore sizes experimentally determined using the mercury intrusion method for a hand-specimen scale granitic sample, which is n × 10-1μm. It is suggested that the interaction of groundwater and country rocks takes place dominantly on the surface of a fracture with a width of ca. 2 mm and the chemical composition of groundwater is controlled by such interaction and fluid flow in the fracture. Silica concentration of deep groundwater in the granitic rock area (Kamaishi, northern Japan) is in equilibrium with SiO2 mineral (chalcedony). Based on a coupled fluid flow-dissolution-precipitation kinetics model the relationship between residence time of deep groundwater and A/M was derived, and the residence time for reasonable values of A/M (1) is estimated to be more than 40 years