In order to understand the significance of stratum stratification characteristics during the formation of underground freshwater lens in the coral reef islands

the particle size distribution of the samples drilled at various depths of a coral reef island in the South China Sea is analyzed. The zone and stratification of the island are determined according to the particle size characteristics in the horizontal and vertical directions

respectively. The area of each zone is determined based on the range covered by each drilling

and the thickness of each stratum is determined by the thickness of the samples with similar particle size in each drill hole. The permeability of the strata is controlled by the particle size distribution of different zones and strata of the loose layer above the reef limestone

while the vertical and horizontal permeabilities control the difficulty of the underground freshwater lens formation

the total thickness of the freshwater lens

and the thickness of the transition zone. The numerical model is established by considering the total area of the island and the number of strata. Finally

the numerical results of groundwater concentration characteristics at different times

horizontal positions and depths are obtained.