How did the fresh water found on Yongshu Island come from?

The fresh water in Yongshu Island is formed by the "fresh water lens". "Freshwater lens" is mostly formed in coral reef islands, which is a renewable and limited underground freshwater resource of coral reefs. Above it is shallow groundwater level, which is not deeply buried, and the bottom is connected with seawater through pore caves of coral limestone, forming a relatively thin transition zone. Because of the different densities of salty and fresh water, groundwater floats on salty water, with thin edges and thick middle, just like a lens, forming a unique underground fresh water resource on the island.

Yongshu Island used to be Yongshu Reef, a submarine uplift structure created by corals and other reef-building organisms in a long geological period. It is an annular coral island in Nansha Islands, China. There was no fresh water before because of the special geological structure of coral reefs. The lower part of coral island reef is Tertiary or Pleistocene karst limestone, with extremely developed pores/caves, strong permeability, easy circulation of seawater, and no freshwater lens can be formed; The upper limestone diagenesis is late, the holes and caves are undeveloped, the permeability is poor, and the seawater is not easy to penetrate, but the rainwater is easy to stay, which can form a freshwater lens.

The amount of water in the freshwater lens changes with the change of precipitation and pumping loss. When precipitation occurs, the lens body is replenished, the thickness increases and the reserves increase. When pumping water loses water, the thickness decreases and the reserves decrease.

Freshwater lenses are mostly formed on islands with a width of more than 270 meters. Since July 20 13, China has reclaimed land on Yongshu Reef on a large scale. By June of 20 15, the area reached 2.803 square kilometers, which has met the formation width of freshwater lenses. Therefore, there is no fresh water underground in Yongshu Island earlier. Fresh water was detected here only because China reclaimed the sea to build an island.