Expanding Freshwater Lenses in Circular Islands Using Cut-Off Walls: A Three-Dimensional Model

Freshwater lenses are vital for ecosystems and water supply on small islands. Cut-off walls in aquifers can enhance these lenses, but previous studies relied on two-dimensional (2D) models, overlooking spatial complexity. This study uses a three-dimensional (3D) model of a circular island to evaluate how wall distance, length, and depth affect freshwater lens dynamics. Walls improve storage by blocking lateral saltwater intrusion and directing flow toward the lens core, reducing mixing and expanding freshwater. Compared to 2D models, the 3D model shows a thinner lens and narrower transition zone. Unlike 2D results, no critical wall depth exists beyond which storage stops increasing. Instead, the growth rate always slows with deeper walls. Longer, deeper, and coast-proximal walls significantly boost storage, with volume increases up to 20 times (e.g., relative length = 1, depth = 27.5 m, distance = 0 m). Sensitivity analysis indicates depth and length are more influential than distance. These findings aid in optimizing wall designs, though practical constraints like cost and constructibility must be considered for sustainable groundwater management.