[A-HW23] Isotope Hydrology 2021

[AHW23-P02] Geochemical characteristics and origins of saline groundwater in the southern Boso Peninsula, Chiba, Japan

*Masaya Yasuhara¹, Akihiko Inamura², Ryotaro Mabuchi³, Seongwon Lee¹, Noritoshi Morikawa² (1.Rissho University, 2.Geological Survey of Japan, AIST, 3.Out-Sourcing Technology Co., Ltd. )

Keywords:deep-seated saline water, saline water ascent, river water, mixing, chloride ion, isotope

Ascent of deep-seated saline water toward the ground surface and its mixing with local shallow groundwater and river water has long been recognized to take place in and around the shallow well, Kanamari Kobo Ido (Tateyama City) and Yunotani (Futtsu City), both in the southern Boso Peninsula, Chiba, Japan. The saline water in this region is characterized by its high chloride ion (Cl^-) concentration of about 5,100-5,800 mg/L and 1,000-1300 mg/L for the Kanamari Kobo Ido and Yunotani, respectively. The oxygen and hydrogen isotopes, Li/Cl ratio, and ³He/⁴He ratio support an assumption that saline water is of see water origin which was subjected to sediment-water solute exchange to a large extent. On the other hand, we attempted to quantify the amount of Cl^- brought to the ground surface from the depth by the ascent of deep-seated saline water. As the result, its Cl^- amount was estimated at about 120 kg/day in November 2019 for the Kanamari Kobo Ido and its vicinity. Taking into account the Cl^- concentration of Kanamari Kobo Ido groundwater, the volume of deep-seated saline water mixed with river water and shallow groundwater in the area was estimated to be 24 m³/day.