Abstract: The hydrological properties of calcareous sand in South China Sea are very important for the study of the formation of underground freshwater lens on islands. The permeability, specific water capacity,and specific yield of calcareous sands were studied by laboratory test first; and combined with the two-dimensional numerical model, their influence on the formation of freshwater lens of islands was analyzed. The result shows that the calcareous sand in South China Sea is poorly gradated and unevenly distributed, and the permeability coefficient is usually between 0.023 and 110 m/d, greatly changed in the study area. The porosity and specific yield are mainly between 0.40-0.55 and 0.012-0.310, respectively, which reflects that the calcareous sand has the characteristics of good water capacity but poor water supply compared with the continental sand with the same particle size range. The influence of hydrological properties of calcareous sand on the formation of freshwater lens is mainly reflected in the velocity, thickness, resource reserve,and formation time. Among them, the better the permeability of calcareous sand is, the faster the groundwater flow rate is, which strengthens the mixing effect between brackish and fresh water, resulting in a thinner freshwater lens and less reserves. Specific yield mainly affects the resources and reserves of freshwater lens in calcareous sand, but has little influence on the thickness and shape of freshwater lens.

Key words: South China Sea, calcareous sands, hydrological properties, freshwater lens, numerical simulation