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

摘要

Isotopic investigations using C-14 Of groundwater were carried out to understand the hydraulic conditions in the sedimentary rocks at the Tone study site, central Japan. C-14 activities of groundwater observed range from 2 to 32% Modern Carbon (pMC). Measured C-14 activities of groundwater are corrected by the isotopic mass balance model based on C-14 activities, delta C-13 values, concentrations of the dissolved inorganic carbon (DIC) and delta C-13 values of nonactive carbon dissolved into the groundwater from carbonate minerals and organics. Assuming that the groundwater reservoir is comparable to the piston flow situation, the relative C-14 ages of groundwaters were calculated from the corrected C-14 activity. The relative C-14 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 C-14 ages shows similar values to those estimated by computer simulation using hydraulic pressure and conductivity data. Hydraulic conditions at the Tone study site inferred from C-14 activity agree with those suggested from hydrogeological analyses. Isotopic approaches using C-14 activity can be applied as geochemical evaluation for interpretations from the hydraulic study. (C) 2001 Elsevier Science Ltd. All rights reserved. [References: 38]