36Cl ages of deep saline groundwater in coastal sedimentary areas in Japan

Abstract:

Climate-induced sea-level fluctuations can have impacts on groundwater flow regimes, especially in coastal areas. A sea level decline leads to a seaward movement of the discharge area of regional groundwater flow system, accompanying increased hydraulic heads in aquifers that enhances deeper groundwater flow. It also brings drastic changes in shorelines and associated topography of coastal areas, which potentially affect groundwater flow regimes. Therefore, an assessment of the influence of sea-level change on groundwater system is especially important.

In this study, we focus on several sedimentary basins in Japan, including the Ishikari Plain, the Tsugaru Plain, and the Kanto Plain. Saline groundwater samples were collected from deep boreholes located over the plains, and analyzed for ³⁶Cl/Cl ratios. Several rock samples taken from each area were measured for whole-rock chemical compositions to determine the secular equilibrium ³⁶Cl/Cl ratios in deep aquifers. The obtained ³⁶Cl ages are generally greater than 100 kyr, and tend to become older in inland areas. These age distribution patterns for each area are compared with the locations of past shorelines. The results may show some factors, including topography and geology, which may control the influence of sea-level change on groundwater systems in coastal sedimentary areas.

Acknowledgement: Main part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of the Nuclear Regulation Authority, Japan.