NUMERICAL SIMULATION OF UPCONING AND FRESHWATER LENS BREAKAGE IN A CORAL ISLAND

A freshwater lens is reproducible but limited underground freshwater resources on a coral island. The bottom of the lens connects seawater. When freshwater is pumped from a well, the drawdown will bring on raise of seawater to form an upconing. If the pumping rate is too high, the freshwater lens will be broken by the upconing and freshwater reserve will greatly decrease. In order to research into the forming process of the upconing and the decrement of the freshwater reserve due to the rupture of the lens, the surface of a lens on YongXin Island was accounted with a numerical model deduced by the mass conservation, Darcy law and Ghyhen-Herzberg approximation. Then the development process of the upconing was simulated. The contours of small lenses and the decrement of the freshwater reserve were computed after a great freshwater lens being broken. The results showed: the error between the calculation value and measure value of the lens contour was between 3% and 15%. The height and incidence of the upconing would increase with the pumping rate and time. When the rate was under a critical value, the upconing would reach a stable state. Otherwise, the freshwater lens would be broken by the upconing. If the freshwater lens on YongXin Island was broken into two small lenses, the freshwater reserve would decrease by 26.8%. These results can be applied to establishing exploitation project of the freshwater lens on coral islands so as to realize safe, scientific, and sustainable exploitation and use of underground freshwater resources of coral islands.