Helium isotopes and 36Cl in saline groundwater from the Osaka Basin, Southwest Japan: Concurrent change in isotopic ratio during groundwater flow

Isotopic composition and concentration of solute and dissolved gases in groundwater a useful indicator for identifying groundwater flow path and estimating groundwater age. Helium and chlorine isotopes are of special importance for a very old groundwater. Helium-4 concentrations in groundwaters often increase with water residence time because of the accumulation of radiogenic He. Combining radiogenic He concentration with 3He/4He ratio, He isotopes has a potential to elucidate both origin and age of groundwater. Chlorine-36 is a radioactive nuclide, which decays with a half-life of 301,000 years and is thus applicable to dating very old groundwater. Another possible application of this isotope is a method using subsurface produced 36Cl to investigate the origin and evolution of saline water. Morikawa et al. (2008) investigated the helium isotopes in deep groundwater from the Osaka Basin in which unusual saline water containing upper mantle-like helium welled out. The Osaka basin is a tectonic subsidence basin, which consists of 500-2,000m thick Late Pliocene to Pleistocene sediments. The granitic basement rocks have been broken into two major blocks by the thrust fault trending north to south. Observed 3He/4He variation in deep groundwater was clearly related with the distribution of active faults and geological structure. Highest value which was identical to the upper mantle one was found out near the two major tectonic lines, Median Tectonic Line and Arima-Takatsuki Tectonic Line, which are located on the north border and near the south border of the Osaka Basin, respectively. The 3He/4He ratios decrease with increasing distance from these tectonic lines. The lowest 3He/4He ratio was observed for the groundwater in the middle-east part of the basin. It has been proposed a model that observed spatial distribution of 3He/4He ratio reflects the movement of fluids through the fault and following dissolution of radiogenic 4He during groundwater flow (Morikawa et al., 2008). The amount of accumulated 4He corresponds to the age for hundreds of thousands of years. In this study, we investigated the distribution of 36Cl/Cl ratio to examine this model. Most of the observed 36Cl/Cl ratios were less than 1 x 10-14. Especially low ratios (<2x10-15) were observed for the samples from the vicinity of the tectonic lines, whereas relatively high 36Cl/Cl ratio was observed in the middle-east part of the basin. Considering that a secular equilibrium value of the aquifer would be around 10-14, increasing 36Cl/Cl ratio toward the middle part of the basin indicates the direction of deep groundwater flow. It is consistent with a tendency inferred from 3He/4He results.