Characterization of groundwater based on δ²H, δ¹⁸O and Cl⁻ concentration beneath the Osaka Plain, Southwest Japan

抄録

The origins of groundwater in the aquifers beneath the Osaka Plain, Japan, were investigated based on the spatial distribution and relationship of δ²H, δ¹⁸O and Cl⁻ concentration in groundwater and river water. Groundwaters beneath the Osaka Plain have unique geochemical characteristics in each aquifer, which can be categorized into three types: shallow (AI), middle (AII) and deep (AIII). Groundwater in AI aquifers originated from surface water, including local precipitation, seawater and riverwater. AII aquifers are abundantly recharged with freshwater, while seawater also infiltrated through the Uemachi Fault. In the AIII aquifers, two types of saline groundwater in addition to freshwater were present beneath the whole plain: fossil seawater and deep-seated groundwater similar to Arima-type brine. The Arima-type brine is known to exhibit distinct oxygen isotope shifts; i.e., the brine is enriched in ¹⁸O and ²H compared with the local meteoric water, shifting the water isotope signature away from the LMWL. The fossil seawater type groundwater was not considerably altered, whereas the deep-seated type groundwater had experienced high-temperature alteration. The both saline groundwaters were diluted by freshwater with lower δ²H and δ¹⁸O than present local meteoric waters. Such freshwaters have lower δ¹⁸O in AIII aquifers than in AII aquifers, suggesting ultrafiltration, which causes the O-isotopic fractionation when passing through thick sedimentary formations. This study revealed the geochemical characteristics of the groundwaters, which originated from different sources and recharged into different aquifers, in a whole sedimentary basin.