Carbon-14 age and chemical evolution of Ca(HCO3) 2-type groundwater of age less than 8,000 years in a confined sandy and muddy Pleistocene aquifer, Japan

Machida, Isao; Suzuki, Yohey; Takeuchi, Mio

Carbon-14 age and chemical evolution of Ca(HCO3) 2-type groundwater of age less than 8,000 years in a confined sandy and muddy Pleistocene aquifer, Japan | Datação por carbono-14 e evolução química de águas subterrâneas do tipo Ca(HCO3) 2-com menos de 8,000 anos num aquífero arenoso e lodoso confinado plistocénico, Japão Âge au Carbone14 et évolution chimique d’eaux souterraines de type Ca(HCO3) 2 de moins de 8000 ans d’âge dans un aquifère sablo-argileux captif du Pléistocène au Japon Edad carbono-14 y evolución química de agua subterránea tipo Ca(HCO 3) 2-de edad menor que 8000 años en un acuífero confinado arenoso y fangoso del Pleistoceno, Japón

2013

Machida, Isao | Suzuki, Yohey | Takeuchi, Mio

The Pleistocene Kimitsu aquifer was selected for examination of the relationship between groundwater age and chemical evolution of Ca(HCO₃)₂-type groundwater. For the most part, the aquifer is confined and composed mainly of quartz and feldspar with a small amount of calcite. The groundwater ages calculated by ¹⁴C were adjusted by using a carbon mass-balance method and corrected for effects of ¹⁴C diffusion. Groundwater ages in the Kimitsu aquifer vary from modern (upgradient) to approximately 2,400 years at 4.4 km from the edge of the recharge area. The ¹⁴C age was verified by groundwater velocity calculated from the hydraulic gradient and hydraulic conductivity. The confined groundwater evolved to Ca(HCO₃)₂-type around 50 years after recharge and this has been maintained for more than 8,300 years due to low chemical reactivity, derived from equilibrium with calcite, kaolinite and Ca-montmorillonite. In addition, high pH prevents the dissolution of Fe and Mn. Consequently, the rate of increase in electrical conductivity ranges from 10 to 30 μS/cm per 1,000 years. On the other hand, leakage from the deep region, which is recognized from high Cl– levels, causes remarkable increases in CH₄ and HCO₃ – concentrations, resulting in an apparent sulfidic zone at 500-m depth in most downgradient regions.

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