Hydrogeochemical processes and long-term effects of sea-level rise in an uplifted atoll island of Minami-Daito, Japan

Study region: An uplifted atoll of Minami-Daito Island, Japan. Study focus: Major ions and stable isotopes (δ²H and δ¹⁸O) of groundwater at fifteen observation wells, surface water at eight representative lakes and one seawater site were measured to unravel the dominant processes controlling the chemistry of water, its spatial distribution and to identify the salinization mechanism caused by long-term sea-level rise. New hydrological insights for the region: Rainfall is the main source for groundwater and lake water. Evaporation affects both the ion concentration and the stable isotopes of the lake water. Geochemical modeling suggests that freshwater-seawater mixing is the main process increasing concentrations of Na⁺, Cl⁻, Mg²⁺, and SO₄²⁻, whereas dissolution of calcite and dolomite increases concentrations of Ca²⁺, Mg²⁺, and HCO₃⁻ in groundwater. Fresh groundwater and lake water (i.e., Cl⁻ < 500 mg/L) are largely distributed along a SW-NE direction, but they have been reduced since the 1970s. Sea-level rise causes an increase in the salinity of lake water by flowing through fractures being connected from lakes to the northern coast, then spreading to other lakes through the artificial channels built in the years.