Development of fresh groundwater lens in coastal reclaimed islands

Groundwater serves as a significant freshwater supply for coastal regions. In many coastal areas, extensive islands have been created by filling the sea. These reclaimed islands not only provide valuable space for urban development, but also can serve as valuable aquifers. Recharge of fresh water from rainfall can flush out salt water and eventually create a freshwater lens beneath the reclaimed island. However, how a fresh groundwater lens grows in reclaimed islands and the factors that control its growth are unclear. Here we use a series of three-dimensional density-dependent flow and transport models to investigate the process of fresh-salt water mixing and quantify the potential maximum volume of fresh groundwater that can occur beneath reclaimed islands. Our results show that the recharge rate controls the growth rate and the initialization of the fresh groundwater lens, whereas the permeability of the underlying fill materials and bedrock determines the shape of freshwater lens beneath the island but has a minor influence on freshwater volume. Increasing porosity enlarges the total water volume but diminishes the proportion of fresh water as the larger volume of original salt water in the aquifer requires more recharge to flush and dilute; longitudinal dispersivity promotes the fresh-salt water circulation in a certain range but limits the maximum fresh volume storage, and vertical transverse dispersivity facilitates extending mixing areas to enlarge the freshwater lens. Heterogeneity in the hydraulic conductivity field increases the variability of groundwater flow, restricts the average speed of fresh-salt water circulation, and results in a decrease in the growth rate and maximum volume of the freshwater lens. Our study has large implications for planning and management of reclaimed lands for water sustainability in the future in coastal areas.