Evaluating dynamic mechanisms and formation process of freshwater lenses on reclaimed atoll islands in the South China Sea

Abstract

Groundwater on remote coral atoll islands in tropical oceans occurs in the form of freshwater groundwater lenses (FGLs) that serve as an important water resource for local inhabitants and ecosystems. Continuous recharge of freshwater from rainfall can flush out salt water and eventually create a FGL beneath the natural or reclaimed island. However, the process whereby a FGL grows in reclaimed islands, and the dynamic mechanisms and formation process are unclear. This study used numerical modeling for a reclaimed island in the South China Sea to evaluate the process of mixing between freshwater and saltwater, dynamic mechanisms, and the formation process of FGLs influenced by tidal action. Our results revealed that the long-term average flow lines of FGLs were superimposed by short-term tidal fluctuations beneath the atoll island. The tidal signals move rapidly inland from the lower aquifer and are then propagated upwards to drive the water level in the Holocene sediments. This causes an oscillation of the FGLs and increased mixing between salt and fresh water. The formation of the FGLs can be divided into three stages: preparatory, formation, and pseudo steady-state. Yongshu Island (a reclaimed island) was used as a case study. The island’s preparatory phase lasted approximately 2.5 years, and will take 20 years to form a stable FGL (with a thickness of about 15 m). Recharge rates, and the nature of the contact between the Holocene and the underlying Pleistocene aquifers, determine the shape of the FGLs beneath the island. Abstraction during the formative period of the FGLs will increase the time required to reach a stable state. The results obtained enhance the understanding of the formation of FGLs, and can provide a reference for the management of freshwater resources on atoll islands.