Dynamics of Irregular Freshwater Lenses Evolution in Thin Aquifers of Reclaimed Circular Islands

Abstract Fresh groundwater lenses (FGLs) are important freshwater resources for offshore reclaimed islands. However, previous studies usually neglected the dynamic evolution process of FGLs, which can last for decades in thin reclaimed islands. This study employed numerical simulation to analyze the morphology and volumetric evolution of FGLs in a saltwater‐filled reclaimed island. We found that the FGL could develop into an irregular pattern in its decades‐long evolution process, with the radius‐to‐thickness ratio of the island aquifer playing a critical role in shaping FGL morphology. When the ratio is below 29.7, the FGL fully develops into a regular lens‐like shape, characterized by a thick center and thinner edges. Conversely, high ratio leads to an irregular, basin‐like FGL formation. The periphery of the irregular FGL initially contacts the aquifer base, leaving significant residual saltwater at the island's center. At this point, the growth rate of the freshwater volume is maximized. An increase in hydraulic conductivity delays the appearance of irregular FGL formation and accelerates the removal of residual saltwater. When longitudinal dispersivity exceeds 5 m, freshwater firstly forms at the periphery of the island; otherwise, it predominantly forms at the center. Furthermore, increased recharge rates notably accelerate freshwater formation and increase the proportion of freshwater volume. These findings provide new insights into the hydrodynamic mechanisms driving irregular FGL evolution and offer technical support for optimizing fresh groundwater resource utilization on reclaimed islands.