page_title: Floridan aquifer
section_title: Hydrology and geology
hierarchical_section_title: Floridan aquifer / Hydrology and geology
caption_attribution_description: English: Idealized geologic diagram showing the confining layer that separates the Upper Floridan and surficial aquifers and plays an important role in determining water quality in the Upper Floridan aquifer.
context_page_description: The Floridan aquifer system, composed of the Upper and Lower Floridan aquifers, is a sequence of Paleogene carbonate rock which spans an area of about 100,000 square miles in the southeastern United States. It underlies the entire state of Florida and parts of Alabama, Georgia, Mississippi, and South Carolina.
  The Floridan aquifer system is one of the world's most productive aquifers and supplies drinking water for nearly 10 million people. According to the United States Geological Survey, total withdrawals from the Floridan aquifer system in 2000 were ranked 5th highest of all principal aquifers in the nation at 3,640 million gallons per day. Of the total, 49% was used for irrigation, 33% was used for public water supply, 14% was used for industrial purposes, and 4% was used for domestic self-supply.
  The Floridan aquifer system is the primary source of drinking water for most cities in central and northern Florida as well as eastern and southern Georgia, including Brunswick, Savannah, and Valdosta.
context_section_description: The carbonate rocks that form the Floridan aquifer system are of late Paleocene to early Oligocene age and are overlain by low-permeability clays of Miocene age (upper confining unit) and surficial sands of Pliocene and Holocene age (surficial aquifer system). In west-central Florida, north Florida, and along the updip margin of the system, the limestone crops out and the aquifer system is unconfined. Where low-permeability clays of the upper confining unit are present and substantial, the system is confined and groundwater is contained under pressure. The upper confining unit is particularly thick in Coastal Georgia and South Florida; downward leakage of water through the upper confining unit in these areas is minimal and the Floridan aquifer system is thickly confined. Low permeability limestone rocks of Paleocene age (e.g. Cedar Keys Formation) form the base of the Floridan aquifer system. The Floridan aquifer system ranges in thickness from less than 100 feet (30 m) in updip areas where the rocks pinch out to more than 3,700 feet (1,100 m) in southwestern Florida. Recharge, flow, and natural discharge in the Floridan aquifer system are largely controlled by the degree of confinement provided by upper confining units, the interaction of streams and rivers with the aquifer in its unconfined areas, and the interaction between fresh and saline water along the coastlines.
  Where the Floridan aquifer system is at or near land surface (areas shaded brown in image above), clays are thin or absent and dissolution of the limestone is intensified and many springs and sinkholes are apparent. Transmissivity of the aquifer in karstified areas such as these is much higher owing to the development of large, well-connected conduits within the rock (see image at right). Springs form where the water pressure is great enough for the groundwater to flow out on the land surface. More than 700 springs have been mapped in Florida. Wakulla Springs in Wakulla County is one of a number of major outflows of the aquifer with a flow rate of 200–300 million US gallons (0.76–1.14 million cubic metres; 610–920 acre feet) of water per day. A record peak flow from the spring on April 11, 1973, was measured at 14,324 US gallons (54.22 m³) per second – equal to 1.24 billion US gal (4.68 million m³; 3,800 acre⋅ft) per day.
  The Floridan aquifer system is composed of two main aquifers: the Upper Floridan aquifer and the Lower Floridan aquifer. These aquifers are separated by sediments that range from low-permeability clays in the panhandle (Bucatunna Clay) and low-permeability dolomites and gypsiferous anhydrite in west-central Florida to permeable limestones along the east coast of Florida and elsewhere. Where these intervening sediments and rock are permeable, the Upper and Lower Floridan aquifers behave as a single unit. Conversely, where the intervening sediments are less permeable, there is less hydraulic connection between the Upper and Lower Floridan aquifers.
caption: Idealized geologic diagram showing the confining layer that separates the Upper Floridan and surficial aquifers and plays an important role in determining water quality in the Upper Floridan aquifer (from Berndt and others, 2015).[18]
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