Observations and modelling of seawater intrusion for a small limestone island aquifer

Seawater intrusion in coastal aquifers is a widespread and serious problem in many parts of the world. Groundwater degradation due to seawater intrusion is often accentuated in small island aquifers due to overpumping or limited replenishing amounts of precipitation. Seawater intrusion and groundwater dynamics in a small limestone aquifer on Yoron Island, Japan, were investigated. Observations in the field were compared with results of a numerical model based on the sharp interface approach coupled with a tank model in order to take into account groundwater recharge and evapotranspiration. The field observations indicated that the sinusoidal fluctuations of groundwater level near the coastline occur in response to diurnal tides. The tidal fluctuations are damped, however, at a distance of 1 km from the coastline and the seasonal variation of groundwater level is small. The transition zone between freshwater and seawater in the inland coral limestone layer is narrow relative to the aquifer thickness, and the estimated interface by the Ghyben-Herzberg relation does not always represent the location of the transition zone for unsteady conditions. Suitable site-specific parameters used in the tank model, as well as effective porosity and hydraulic conductivity of the groundwater aquifer, are discussed, and observed and modelled groundwater level fluctuations are compared for the study site. It is shown that calculated groundwater levels and location of seawater intrusion by the coupled numerical model are in relatively good agreement with observed results.