Two models to analyze freshwater lenses in coral atolls are presented first; a freshwatersaltwater interface model and a dispersion model. In the freshwatersaltwater interface model, especially when the horizontal hydraulic conductivity varies exponentially with the horizontal coordinate, the analytical solution can be obtained. In the dispersion model, the horizontal and vertical dispersion coefficients are treated as a function of the hydraulic conductivity respectively, determined with experimental and field data. The estimated values of natural replenishment in the dispersion model were approximately 63∼78% of those in the freshwater-saltwater interface model at the New Zealand Airfield district of Christmas Island in the Republic of Kiribati.
Secondly groundwater environment in Gogo Isla n d in Japan is investigated through simulations of seawater intrusion into the confined aquifer at the Yura district and the phreatic aquifer at the Kitaura district. The ratios of the horizontal dispersion coefficient to hydraulic conductivity obtained at both districts and from other experimental and field data are proportional to the thickness of the aquifer, and the proportional constant is as large as 10^-3. A hydrologic cycle was simulated at the Kitaura district, and the outflow amount of groundwater into the sea was approximately 9% of annual precipitation in Matsuyama City in 1985. Key words: small island, coral atoll, freshwater lens, freshwater-saltwater interface model, dispersion model