Evaluating the Effect of Subsurface Structures on Freshwater Lenses in Coral Reefs

Freshwater groundwater lenses (FGLs) serve as an important water resource for remote coral atoll islands, and different hydrogeological data of the islands determine the growth and decay of the FGLs. This study conducted a three-dimensional (3D) numerical model using hydrogeological data from a reclaimed island to evaluate the effects of different locations and sizes of subsurface structures on the formation and macroscale stability of the FGLs. The results revealed that freshwater formation begins earlier than the base model when the subsurface structures are present. Constructing underground structures in Pleistocene aquifers leads to a significant increase in freshwater volume with the enlargement of the structure’s size. When these structures are built within Holocene aquifers, as the burial site moves from the edge towards the center of the area, the maximum freshwater volume decreases slightly, with the largest difference being approximately 4 × 10⁵ m³. When the structure constructed in the Holocene aquifer and its central axis does not coincide with the center of the simulated area, the maximum thickness of the FGLs will be shifted from the center of the simulated area to 20–25 m on the side of the structure. When constructed in the Pleistocene aquifer, the thickness of the FGLs increases. This study provides methodology for planning and management of reclaimed lands for water sustainability in the future in coastal areas.