Groundwater flow system in the Kamikita Plain, Japan, inferred from geochemical tracers including ³⁶Cl
Dr Yuki Tosaki1, Dr Noritoshi Morikawa1, Dr Kohei Kazahaya1, Mr Hitoshi Tsukamoto1, Mr Tsutomu Sato1, Dr Hiroshi Takahashi1, Mr Masaaki Takahashi1, Dr Akihiko Inamura1
1Geological Survey of Japan, AIST, Tsukuba, Japan
For intermediate depth disposal of radioactive waste, evaluation of the impacts of long-term phenomena is important including glacial-interglacial sea-level changes (up to about 120-130 m). Because the movement of coastlines results in a shift of the discharge area of a regional groundwater flow system, sea-level changes can considerably affect groundwater flow regimes especially in coastal areas. To study its influence on groundwater flow, we attempt to extract palaeohydrological information from groundwater in a coastal sedimentary basin (the Kamikita Plain, NE Japan), using chemical and isotopic tracers including ³⁶Cl.
The Kamikita Plain is characterized by marine terraces developed widely throughout the plain. Positions of former shorelines corresponding to MIS 1, 5e, 7, and 9 have been reconstructed from the terraces and tephra layers. The difference in the elevations of these terraces indicate that the plain has uplifted with an average rate of 0.1–0.2 mm/yr at least in the late Quaternary period. The gradient of continental shelf is rather gentle in the offshore area of the southern part of the Kamikita Plain. It suggests that the horizontal extension of groundwater flow system during the interglacial-glacial transition is relatively large for the study area.
Based on water chemical composition, groundwater samples obtained from existing boreholes were divided into (1) Ca-HCO₃ type shallow fresh groundwater (>-400 m a.s.l.; Pliocene-Pleistocene aquifers), (2) Na-Cl type deep saline/brackish groundwater (-700 to -1,200 m a.s.l.; Miocene aquifers), and (3) Na-HCO₃ type deep fresh groundwater (-400 to -1,300 m a.s.l.; Miocene aquifers). Stable oxygen and hydrogen isotope ratios are distributed between the local meteoric water line and the value of seawater, indicating the mixing between meteoric water and seawater. Since ³H is detected in about half of the Ca-HCO₃ type shallow fresh groundwater, they were recharged by relatively recent precipitation. The δD values of the Na-HCO₃ type deep fresh groundwater are ~10‰ lower than those of recent precipitation, suggesting that they were recharged in a colder period than the present. The ³⁶Cl/Cl ratios of the Na-Cl type deep saline/brackish groundwaters are comparable to the secular equilibrium value of the aquifer, except for coastal locations. This would indicate that old trapped seawater partly remains in Miocene aquifers of the plain.
The above results show that meteoric groundwater currently flows at a depth shallower than -400 m. During the glacial period, meteoric water reached to a depth of around -1,000 m due to sea level drop. However, old seawater trapped in deep Miocene aquifers are remaining in inland locations. It suggests that the meteoric flushing during the glacial period is rather limited in a sedimentary aquifer system as compared to a fractured rock aquifer system.
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.
Dr. Yuki Tosaki is a senior researcher at the Geological Survey of Japan, AIST. His area of research involves groundwater dating using isotope tracers and its applications. He focuses on long-term changes in coastal groundwater flow systems, which is associated with safety issues for the disposal of radioactive waste.