In hydrology a lens is a convex layer of fresh groundwater that floats on top of denser saltwater. It arises when rainwater seeps down through a soil surface and then gathers over a layer of seawater at or down to about five feet below sealevel. Freshwater lenses are often found on small coral or limestone islands and atolls, where wells dug into them may be the only natural source of potable water. 
Ghyben-Herzberg lenses are a simple model of the process.
- Ryan T. Bailey, John W. Jenson, Arne E. Olsen (04-2008). "An Atoll Freshwater Lens Algebraic Model for Groundwater Management in the Caroline slands". University of Guam, Water and Environmental Research Institute of the Western Pacific. Retrieved 2010-11-25.
- "Freshwater lens". United Nations Educational, Scientific and Cultural Organization. Retrieved 2010-11-25.
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