FGL - Ground water on remote coral atoll islands in tropical ocearns

"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."
https://www.sciencedirect.com/science/article/pii/S0022169420301013