Discharging mechanism of springs around the fault in the southeastern coast of Yonaguni Island

Abstract

Chemical and isotopic compositions of spring water distributed along the southeastern coast of Yonaguni Island, which is the westernmost part of Japan, have been determined. The chemical and isotopic compositions of these springs suggest that they contain a few percent of seawater. The chemical and isotopic compositions of the springs indicate that they are affected by summer rainfall. On the other hand, the sulfur hexafluoride concentration shows the atmospheric equilibrium concentration with groundwater in 2016, which indicates a residence time of about three years. However, all of the springs were supersaturated with respect to calcium carbonate, suggesting that the effects of seawater were immediate and may have been due to the immediate influence of seawater, and thus may have been discharged before re-equilibrium was reached.