Freshwater lens

A body of fresh groundwater that floats above denser seawater within a permeable coastal island. It forms when rainfall infiltrates the island and recharges its groundwater.

Ghyben–Herzberg relation: A hydrostatic relation estimating the depth of a freshwater–saltwater interface from freshwater head above sea level. It gives a first estimate of how far the lens extends below sea level.
Atoll: A ring-shaped coral reef, island, or chain of islands surrounding a lagoon. Atolls commonly rely on freshwater lenses stored within narrow, low-lying islands.
Aquifer: A permeable geological formation that stores and transmits groundwater. A permeable island aquifer provides the material in which the lens is held.
Bikini Atoll: An atoll in the Marshall Islands, part of the Ralik Chain in the western Pacific. Its groundwater studies document freshwater-lens behavior in a low-lying atoll setting.
Groundwater recharge: The entry of water into a saturated groundwater system, commonly through infiltration from the land surface. Rainfall recharge replenishes the freshwater that sustains the lens.
Groundwater pumping: The extraction of groundwater from an aquifer using wells or pumps. Pumping lowers freshwater head and may draw saline water into the lens.
Porosity: The fraction of a material’s volume occupied by pores or open spaces. Pore spaces store the lens’s water within island sediments or rock.
Kiritimati: A coral atoll in the Republic of Kiribati, also known as Christmas Island. Its large, irregular island provides a contrasting setting for lens shape and freshwater supply.
Freshwater–saltwater interface: The transition zone separating fresh groundwater from saline groundwater. This boundary marks the lens’s lower edge, though mixing makes it a zone rather than a sharp line.
Saltwater upconing: The upward movement of saline groundwater beneath a pumping well in a coastal aquifer. A well can pull the lens’s saline base upward toward its intake.

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