Three-dimensional structure and sources of groundwater masses beneath the Osaka Plain, Southwest Japan

Abstract

Study region: Quaternary tectonic basin beneath the Osaka Plain, Southwest Japan Study focus: Groundwaters of different sources were categorized by the cluster analysis, using six components; Cl–, alkalinity, Na+ and Ca2+, δ18O and δ2H, and the following characterization by the relationships between δ18O and δ2H and among Cl–, B and Li concentrations, and Li isotopic ratios. Then, three-dimensional mapping was applied to visualize the distribution of the categorized groundwater masses in the whole basin. New hydrological insights for the region: The sources of groundwater was categorized into two freshwater and three saline water end-members; 1) modern local meteoric water in the aquifers to depths of 300 m beneath the plain and surrounding mountains, 2) fossil freshwater that is more depleted in δ18O and δ2H and enriched in alkalinity than the other, 3) modern seawater recharged since the last submergence appearing in mostly to depths of 300 m beneath the plain, 4) fossil seawater probably recharged before the last submergence remaining in the base of the basin, and 5) deep-seated Arima-type brine, appearing along perimeter active faults at the northern and southern boundaries of the plain. The method used here successfully visualize the different groundwater masses and will be applicable to evaluate the sustainability of groundwater resources in the similar hydrogeological settings such as coastal plains of Asian countries.

Keywords

• Oxygen and hydrogen isotopes
• Lithium isotopes
• Saline groundwater
• Active faults
• Tectonic basin