Identification of modern meteoric water, glacial meteoric water, and fossil seawater in a deep borehole at the coastal area of Horonobe, north Japan, using groundwater dating and paleoclimatic proxy methods

Abstract

Groundwater flow in deep areas of coastal regions has attracted significant attention for geological radioactive waste disposal or geological CO2 storage. To investigate groundwater flow in coastal areas, groundwater dating methods with the potential to assess slow groundwater flow were used at depths of up to 1200 m at Hamasato, Horonobe, northern Japan. Multiple groundwater dating methods were applied using pore water in drilling core and pumping groundwater. This study shows that modern meteoric water, glacial meteoric water, and fossil seawater are distributed in sand and silt layers at 300–360 and 90–300 m in depth and in silt and mud layers below 800 m in depth, respectively. Modern meteoric water infiltrated after and glacial meteoric water infiltrated during the latest glacial period were identified using δD and δ18O values, estimated recharge temperature from noble gas concentrations, and 14C concentrations. The estimated recharge temperatures of modern and glacial meteoric waters were 8.1 ± 2.0 and 2.1 ± 2.0 °C, respectively, whereas the δD and δ18O values were −68 ± 3‰ and −10.3 ± 0.2‰, −81 ± 2‰ and −11.4 ± 0.3‰, respectively. The 14C age of modern meteoric water was 10–16 ka. However, the 14C age of glacial meteoric water could not be estimated because it was highly diluted by 14C-free CO2 from methanogenesis and marine carbonate dissolution. Fossil seawater was identified using δD, δ18O, 36Cl, and 4He. δD and δ18O values underwent water-rock interaction and approached inland fossil seawater with increasing depth. 36Cl/Cl values did not reach secular equilibrium in situ in the Yuchi Formation due to its relatively short geological age and consolidation-induced porosity changes. However, the Yuchi Formation seawater is considered extremely old because its 36Cl/Cl ratio is higher than that of modern seawater as a result of 36Cl production, which also explains the observed 36Cl/Cl increases with depth toward the secular equilibrium ratio (36Cl/Clse). 4He concentrations were in the order of 10−6 ccSTP/gw, corresponding to an in situ accumulation age of 1–2 Ma, neglecting the external flux. The contribution of external flux could be negligible for the short geological age and rapid sedimentation rates. 4He ages were equivalent to the geological age of the Yuchi Formation. Core investigations can interpolate groundwater investigations because cores, particularly those from low-hydraulic conductivity layers, can be collected continuously. In addition, stable and radioactive isotopes and noble gases can help identify modern meteoric water, glacial meteoric water, and fossil seawater, assisting in understanding the groundwater flow velocity.