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 in updip areas where the rocks pinch out to more than 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 of water per day. A record peak flow from the spring on April 11, 1973, was measured at per secondequal to 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. Upper Floridan aquifer The Upper Floridan aquifer is the main source of water withdrawn from the Floridan aquifer system due to high yields and proximity to land surface. Groundwater in the Upper Floridan is fresh in most