Editing Diego Garcia (section)

===Fresh water supply===
Diego Garcia is the above-water rim of a coral atoll composed of [[Holocene]] coral rubble and sand to the depth of about {{convert|36|m|ft|abbr=on}}, overlaying [[Pleistocene]] limestone deposited at the then-sea level on top of a [[seamount]] rising about {{convert|1800|m|ft|abbr=on}} from the floor of the Indian Ocean. The Holocene sediments are porous and completely saturated with sea water. Any rain falling on the above-water rim quickly percolates through the surface sand and encounters the salt water underneath. Diego Garcia is of sufficient width to minimise tidal fluctuations in the [[aquifer]], and the rainfall (in excess of 102.5&nbsp;inches/260&nbsp;cm per year on average)<ref>[[#NRMP|Natural Resources Management Plan (2005)]], paragraph 2.5.2.</ref> is sufficient in amount and periodicity for the fresh water to form a series of convex, freshwater, [[Ghyben-Herzberg lenses]] floating on the heavier salt water in the saturated sediments.<ref name="bare_url">{{cite web |url=http://geography.about.com/library/misc/ucghyben.htm |title=Salt Water vs. Fresh Water – Ghyben-Herzberg Lens |publisher=Geography.about.com |date=9 April 2012 |access-date=21 June 2012 |archive-date=5 December 2010 |archive-url=https://web.archive.org/web/20101205001839/http://geography.about.com/library/misc/ucghyben.htm |url-status=dead }}</ref><ref name="Urish">[[#Urish|Urish (1974)]], p. 27.</ref>

The horizontal structure of each lens is influenced by variations in the type and porosity of the subsurface deposits, which on Diego Garcia are minor. At depth, the lens is globular; near the surface, it generally conforms to the shape of the island.<ref>[[#Urish|Urish (1974)]], p. 28.</ref> When a Ghyben-Herzberg lens is fully formed, its floating nature will push a [[Hydraulic head|freshwater head]] above [[mean sea level]], and if the island is wide enough, the depth of the lens below mean sea level will be 40 times the height of the water table above sea level. On Diego Garcia, this equates to a maximum depth of 20 m. However, the actual size and depth of each lens is dependent on the width and shape of the island at that point, the permeability of the aquifer, and the equilibrium between recharging rainfall and losses to evaporation to the atmosphere, transpiration by plants, tidal advection, and human use.

In the plantation period, shallow wells, supplemented by rainwater collected in [[cistern]]s, provided sufficient water for the pastoral lifestyle of the small population. On Diego Garcia today, the military base uses over 100 shallow "horizontal" wells to produce over 560,000 L per day from the "Cantonment" lens on the northwest arm of the island—sufficient water for western-style usage for a population of 3,500. This 3.7&nbsp;km<sup>2</sup> lens holds an estimated 19 million m<sup>3</sup> of fresh water and has an average daily recharge from rainfall over 10,000 m<sup>3</sup>, of which 40% remains in the lens and 60% is lost through [[evapotranspiration]].<ref>Hunt, Charles D. "Hydrogeology of Diego Garcia". In: [[#Vacher|Vacher & Quinn (1997)]], pp. 909–929. {{doi|10.1016/S0070-4571(04)80054-2}}.</ref>

Extracting fresh water from a lens for human consumption requires careful calculation of the sustainable yield of the lens by season because each lens is susceptible to corruption by [[saltwater intrusion]] caused by overuse or drought. In addition, overwash by tsunamis and tropical storms has corrupted lenses in the Maldives and several Pacific islands. Vertical wells can cause salt upcoming into the lens, and overextraction will reduce freshwater pressure resulting in lateral intrusion by seawater. Because the porosity of the surface soil results in virtually zero runoff, lenses are easily polluted by fecal waste, burials, and chemical spills. Corruption of a lens can take years to "flush out" and reform, depending on the ratio of recharge to losses.<ref name="bare_url" />

A few natural depressions on the atoll rim capture the abundant rainfall to form areas of freshwater wetlands.<ref name="Stoddart_127_142">D. R. Stoddart (1971): "Land vegetation of Diego Garcia". In: [[#Stoddart|Stoddart & Taylor (1971)]], pp. 127–142.</ref> Two are of significance to island wildlife and to recharge their respective freshwater lenses. One of these is centred on the northwest point of the atoll; another is found near the Point Marianne Cemetery on the southeast end of the airfield. Other, smaller freshwater wetlands are found along the east side of the runway, and in the vicinity of the receiver antenna field on the northwest arm of the atoll.<ref>[[#NRMP|Natural Resources Management Plan (2005)]], paragraph 3.3.2.1.</ref>

Also, several man-made freshwater ponds resulted from excavations made during construction of the airfield and road on the western half of the atoll rim. These fill from rainfall and from extending into the Ghyben-Herzberg lenses found on this island.<ref>Stephen W. Surface and Edward F.C. Lau, "Fresh Water Supply System Developed on Diego Garcia", The Naval Civil Engineer, Winter 1985</ref>