Physics of atoll groundwater flow

Large-scale steady-state groundwater flow in atoll carbonate platforms resu lts from temperature and salinity- induced density gradients. Atolls are bu ilt on top of a basaltic substrate that provides geothermal heating from be neath. Moreover; they are immersed in the tropical ocean where temperature decreases rapidly with depth. Groundwater circulation in these platforms ha s long been associated with the geothermal heat flux because it is capable of generating inward and upward flow of oceanic oceanic origin water by buo yancy effects. This study shows that hydraulic circulation occurs even in t he absence of a geothermal flux because the combination of the cold subsurf ace ocean waters with the warm surface conditions is sufficient to maintain a convection cell within the carbonate platform. Using a one-dimensional a nalytical model, validated by more sophisticated two-dimensional simulation s, we can investigate the interaction between these two driving forces. The flow rate inside the platform is, in fact, a function of the ratio of the geothermal flux to the temperature gradient in the ocean. It increases with the geothermal flux but decreases with the oceanic temperature gradient. T his one-dimensional model also shows that Inking salinity effects on densit y into account increases the flow rates transiting through the platform by a third.