areas, though locally may be brackish or saline, particularly in coastal areas with saltwater intrusion problems, and in South Florida. The Upper Floridan aquifer includes the uppermost or shallowest permeable zones in the Floridan aquifer system. In the northern half of the study area, this aquifer behaves as a single hydrogeologic unit and is undifferentiated. In the southern half of the study area, including most of central and southern Florida, the Upper Floridan aquifer is thick and can be differentiated into three distinct zones, namely the uppermost permeable zone, the Ocala Lower-Permeability Zone, and the Avon Park Permeable Zone. The base of the Upper Floridan aquifer is marked by two composite units (see below) and one confining unit in the middle part of the Floridan aquifer system: the Lisbon-Avon Park Composite Unit or the Middle Avon Park Composite Unit, and the Bucatunna Clay Confining Unit. In updip areas, the base of the Upper Floridan is either coincident with the top of the confining units above the Claiborne, Lisbon, or Gordon aquifers, or it lies above any clay bed that marks the boundary between mostly carbonate and mostly clastic units or previously mapped numbered MCUs of Miller (1986). If one or more evaporite units are present, such as middle confining unit MCUIII near Valdosta in south-central Georgia (Miller, 1986) or MCUII in southwestern Florida (Miller, 1986), the base of the Upper Floridan aquifer is coincident with the top of the evaporite unit. In regions where no distinct lower permeability unit is known to be present, the base of the Upper Floridan is extrapolated along a horizon that allows for a stratigraphic grouping of permeable rock into the upper or lower parts of the aquifer system. In southeastern Alabama, northern Florida, Georgia, and South Carolina, the stratigraphic units are grouped into the Lisbon-Avon Park Composite Unit. In peninsular Florida, this horizon is coincident with one or more evaporite-bearing or non-evaporite-bearing units of the Middle Avon Park Composite Unit. In the panhandle of Florida and southwest Alabama, the base is coincident with the top of the Bucatunna Clay Confining Unit. Middle Confining and Composite Units The hydrogeologic framework of the Floridan aquifer system was revised by the U.S. Geological Survey in 2015. The extent of the system was revised to include some of the updip clastic facies which grade laterally into the Lower Floridan aquifer and have been previously included in the Southeastern