Why is fresh water scarce on most Tuamotu atolls?

Fresh water is scarce on most atolls in the Tuamotu Archipelago primarily because these islands are low-lying and flat, consisting of narrow sandbanks resting on coral reefs.12

Unlike high volcanic islands that can support streams or rivers, atolls lack the elevation and landmass to sustain surface water sources. The limited fresh water available on these islands is typically stored in a thin, lens-shaped layer that permeates the coral structure, resting above the denser salt water beneath the surface. Because of this restricted supply, residents must rely on wells and rainwater storage tanks to conserve water for their needs.2