Review papersTerrestrial freshwater lenses: Unexplored subterranean oases

- Terrestrial freshwater lenses (TFLs) are usually high-value freshwater resources.
- Recent discoveries suggest a higher TFL prevalence than previously thought.
- TFLs provide habitat refugia and water supply in often inhospitable conditions.
- The TFL typology presented here identifies more than six distinct types.
- This study is the first step towards improving global awareness of TFLs.

Freshwater lenses are lenticular bodies of fresh (TDS < 1000 mg/L) groundwater that develop above more saline groundwater within the same host aquifer due in part to buoyancy. In contrast to the widely studied situation of freshwater lenses in coastal aquifers, the formation, location and persistence of freshwater lenses in terrestrial settings are poorly understood. This is despite inland aquifers commonly containing saline groundwater, particularly in arid and semi-arid climates, and the local occurrences of freshwater being critical for ecosystems and human endeavour. We identify and classify known terrestrial freshwater lenses (TFLs) using four formation categories, namely topography, geology, groundwater-surface water interaction and recharge mechanisms. The resulting typology highlights the importance of buoyancy in the formation of TFLs in otherwise unlikely situations, implying that TFLs may be more prevalent than previously thought. TFLs represent some of the most vulnerable and precious freshwater resources on Earth that require considerably more research into mechanisms of formation and threats to their existence.