GROUNDWATER LENS DYNAMICS OF ATOLL ISLANDS

A variable-density groundwater model is used to analyze the effects of various controls on the size of the freshwater lens, the structure of the transition zone, and the propagation of tidal fluctuations in a two-layer atoll island groundwater system. Modeling results indicate that mixing of fresh water and saltwater occurs primarily as a result of oscillating vertical flow due to tidal fluctuations and depends to a lesser extent on transverse dispersion along the dominantly horizontal recharge-discharge path of flow. The controls on the amount of mixing are: (1) the accumulated vertical distance, which increases with tidal range and is restricted by vertical permeabilities, and (2) vertical longitudinal dispersion. Comparison of cross-sectional simulations of atoll islands using nontidal and tidal models shows that the nontidal model must use artificially high values of transverse dispersivity to compensate for the lack of tidally driven, vertical mixing processes. Although the tidal model has high computational requirements, it can be used to calibrate vertical permeabilities and is best suited for problems dealing with groundwater resource evaluations, hydrologic events, and hydrologic processes. The limitations of the nontidal model are that it cannot be used for calibration of vertical permeabilities and will not realistically simulate those cases in which transition zones are thick or recharge low.