NUMERICAL MODELING OF HYDROLOGY ON ATOLL ISLANDS

The modeling of atoll island hydrology is of critical scientific and socio-economic importance: The fresh-water lenses are the major sources of water supply in many Pacific and Indian Ocean atoll islands and the lens distribution determines the diagenetic environments of porosity modification in carbonate platforms. The thickness of the lens is typically estimated based on the width and average hydraulic conductivity of the island and the rate of meteoric water recharge by using the Dupuit-Ghyben-Herzberg (DGH) methods. In this study, complex 2- and 3-dimensional hydrological models are developed and solved through the TOUGH2 multiphase flow simulator to determine the effects of various controls on the size of the freshwater lens and the structure of the mixing zone in atoll island groundwater systems. Our simulations show a systematic reduction in freshwater lens thickness and an increase in the width of the mixing zone with the introduction of permeability heterogeneities. The effects of tidal fluctuations are modeled both directly and indirectly via time dependent sea level positions and hydrodynamic dispersion coefficients, respectively. Modifications and enhancements to the DGH model are developed to account statistically for the primary effects of these controls as well as the 3D effect of island geometries.