Estimation of relative groundwater age in the granite at the Tono research site, central Japan

Isotopic studies have been conducted to develop a method to estimate subsurface hydraulic conditions in the granite at the Tono research site, central Japan. The groundwaters were classified into three groups according to residence time, based on the 3H concentration: (1) modern water recharged from the surface within the last 40 years, (2) a mixture between modern water and relatively old groundwaters, and (3) relatively old groundwater which is devoid of 3H. An attempt was made to date the 3H-free groundwater using 14C. The relative groundwater age estimated by simple model, which assumes no addition of dead carbon ranges from 6500 to 14,000 years B.P. The calcite precipitations on the fracture surface are also classified into three groups, based on carbon isotope compositions: (1) precipitation from groundwater within the last 50,000 years, (2) precipitation from the solution with different δ13C values from groundwater in the past more than 50,000 years, and (3) mixture of new calcite partly precipitated within the last 50,000 years and relatively old calcite. These data suggest the possibility that the conductive (open) fractures connected to the surface can be estimated by 14C determination in the calcite on fracture surfaces.