Carbon-14 study of groundwater in the sedimentary rocks at the Tono study site, central Japan

Isotopic investigations using 14C of groundwater were carried out to understand the hydraulic conditions in the sedimentary rocks at the Tono study site, central Japan. 14C activities of groundwater observed range from 2 to 32 % Modern Carbon (pMC). Measured 14C activities of groundwater are corrected by the isotopic mass balance model based on 14C activities, δ13C values, concentrations of the dissolved inorganic carbon (DIC) and δ13C values of non-active carbon dissolved into the groundwater from carbonate minerals and organics. Assuming that the groundwater reservoir is comparable to the piston flow situation, the relative 14C ages of groundwaters were calculated from the corrected 14C activity. The relative 14C age suggests that the groundwater infiltration from the upper part of the sedimentary rocks to the lower part takes several thousands of years, or that the groundwater in the lower part of sedimentary rocks is derived from long distance flow from the surface through the unconformity between the sedimentary rocks and basement granite. The flow rate calculated by relative 14C ages shows similar values to those estimated by computer simulation using hydraulic pressure and conductivity data. Hydraulic conditions at the Tono study site inferred from 14C activity agree with those suggested from hydrogeological analyses. Isotopic approaches using 14C activity can be applied as geochemical evaluation for interpretations from the hydraulic study.