13:45 〜 14:00
[AHW19-01] Long-term changes in deep groundwater flow in the Kamikita Plain as inferred from chemical and isotopic tracers
★Invited Papers
キーワード：groundwater flow system、sedimentary rock area、coastal area、saline groundwater、isotope、Kamikita Plain
Long-term sea-level changes induced by Quaternary climate change, which can reach up to 120–130 m, may affect groundwater flow regimes, particularly in coastal areas. As sea levels fall from present to glacial levels, the discharge areas of regional groundwater flow systems would shift seawards, potentially increasing deep groundwater flow that was previously stagnant. Assessing the impact of such a long-term phenomenon is crucial for the safety assessment of radioactive waste disposal. This study aims to extract palaeohydrological information from groundwater in the the Kamikita Plain, NE Japan, using chemical and isotopic tracers. The evolution of groundwater in the Kamikita Plain, including the origin of water and salinity, is interpreted in the context of interglacial–glacial transgression–regression cycles.
Groundwater samples were collected from existing boreholes in 2008–2009 and 2016–2018 and analyzed for their chemistry and isotopes. The groundwater samples were categorized into three groups based on their composition: (1) shallow groundwater of Ca-HCO3 type; (2) deep fresh groundwater of Na-Cl to Na-HCO3 type (Cl− <200 mg/L; sampled at approximately 500–1,200 m depth); and (3) deep saline/brackish groundwater of Na-Cl type (Cl− >200 mg/L; sampled at approximately 600–1,200 m depth). The δD values of the deep freshwaters are about 10‰ lower than those of recent precipitation, indicating recharge during a colder period than the present. The 36Cl/Cl ratios of the saline component in the deep saline/brackish waters were found to be comparable to the secular equilibrium value of the aquifer. These data suggest that old saline water, presumably seawater trapped in the deep Miocene aquifer, is still extensively present inland. In contrast, younger saline/brackish waters with low 36Cl/Cl are found mainly along the coast, particularly in the deeper aquifer.
The occurrence of low δD freshwaters at depths of about −1,000 m a.s.l. inland, such as in the northern plain, and the distribution of relatively young saline/brackish waters in coastal areas both suggest that meteoric flushing of older saline waters can occur during glacial periods, but only to a limited extent and area. The greater depth of meteoric water circulation during the interglacial-glacial transition may have been influenced by the increased distance between recharge and discharge areas, as well as the topography of high mountainous recharge areas.
Acknowledgement: The main part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (NRA), Japan.
Groundwater samples were collected from existing boreholes in 2008–2009 and 2016–2018 and analyzed for their chemistry and isotopes. The groundwater samples were categorized into three groups based on their composition: (1) shallow groundwater of Ca-HCO3 type; (2) deep fresh groundwater of Na-Cl to Na-HCO3 type (Cl− <200 mg/L; sampled at approximately 500–1,200 m depth); and (3) deep saline/brackish groundwater of Na-Cl type (Cl− >200 mg/L; sampled at approximately 600–1,200 m depth). The δD values of the deep freshwaters are about 10‰ lower than those of recent precipitation, indicating recharge during a colder period than the present. The 36Cl/Cl ratios of the saline component in the deep saline/brackish waters were found to be comparable to the secular equilibrium value of the aquifer. These data suggest that old saline water, presumably seawater trapped in the deep Miocene aquifer, is still extensively present inland. In contrast, younger saline/brackish waters with low 36Cl/Cl are found mainly along the coast, particularly in the deeper aquifer.
The occurrence of low δD freshwaters at depths of about −1,000 m a.s.l. inland, such as in the northern plain, and the distribution of relatively young saline/brackish waters in coastal areas both suggest that meteoric flushing of older saline waters can occur during glacial periods, but only to a limited extent and area. The greater depth of meteoric water circulation during the interglacial-glacial transition may have been influenced by the increased distance between recharge and discharge areas, as well as the topography of high mountainous recharge areas.
Acknowledgement: The main part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (NRA), Japan.