Response of Freshwater Lenses to Precipitation and Tides

Coral islands are home to unique underground freshwater bodies generally known as freshwater lenses. There are differences in the development, formation, and properties of steady-state freshwater lenses among different coral islands because of the influence of tides, precipitation, island sizes, and hydrogeological conditions. This study investigated the response pattern of the groundwater system of coral islands to tides and precipitation based on field observation. Moreover, numerical simulation was performed to explore the effect mechanism of precipitation and tides on the formation of the freshwater lens. Field observation data and simulation boundary data were processed at three time scales to analyze the effect of the time scale on the simulation results. The groundwater chloride concentration (converted from monitored conductivity) of coral islands fluctuates periodically. In particular, tides mainly affect the periodicity of the fluctuation, whereas precipitation mainly affects the peak concentration. Monthly data are suitable for revealing the overall trend of groundwater properties, while hourly data are suitable for revealing the periodicity. During the formation and development of the freshwater lens, precipitation mainly affects the groundwater chloride concentration, whereas tides mainly affect the groundwater hydraulic head. The stabilization time point and chloride concentration of the freshwater lens are mainly affected by precipitation factors. The larger the minimum time scale of the boundary condition, the greater the simulation error. Time scales have a greater effect on the error of the steady-state chloride concentration of the freshwater lens than on the errors of the stabilization time point and steady-state hydraulic head.