Coastal Plain aquifer system, the Floridan aquifer system, or both. A new method for dividing the Upper and Lower Floridan aquifers was proposed and a new term, "composite unit", was introduced to refer to regionally extensive litho-stratigraphic units of rock, previously classified as one of eight "Middle Confining Units" by Miller (1986), which have been found to be neither confining nor permeable across their entire extents. Three regionally mapable litho-stratigraphic units are used to consistently divide the Upper and Lower Floridan aquifer in the revised framework: the Bucatunna Clay Confining Unit, the Middle Avon Park Composite Unit, and the Lisbon-Avon Park Composite Unit. The Upper and Lower Floridan aquifers behave as a single hydrogeologic unit in areas where these composite units are leaky. Lower Floridan aquifer The Lower Floridan aquifer is generally less permeable than the Upper Floridan aquifer and the water produced can be highly mineralized and/or saline; however, the Lower Floridan aquifer is relatively fresh water to the base of the system in central Florida and in updip areas of central and southern Georgia and Alabama. A new basal permeable zone is mapped throughout the Florida peninsula, and slightly into southeastern Georgia, that incorporates the previously established Boulder Zone and Fernandina permeable zone; this more extensive unit is called the Oldsmar permeable zone. The Oldsmar permeable zone appears to have higher permeability, far greater than the cavernous areas of the Boulder and Fernandina permeable zones, and contains freshwater in the central peninsula area. This newly delineated areally extensive basal unit containing freshwater may influence the movement of freshwater water through the deepest part of the aquifer system toward the discharge areas. The Oldsmar permeable zone, which is part of the Lower Floridan aquifer, is of interest because it may be an important alternative source of water where it is confined (and isolated) beneath the Upper Floridan aquifer and may be important to the offshore movement of groundwater in areas previously unknown. General hydraulic characteristics The carbonate rocks that compose the Floridan aquifer system have highly variable hydraulic properties, including porosity and permeability. Transmissivity within the aquifer system has been reported over a range of more than six orders of magnitude, from less than 8 ft2/d () to greater than 9,000,000 ft2/d (), with the majority of values ranging from 10,000 to 100,000 ft2/d (). Where the aquifer is unconfined or thinly confined, infiltrating water dissolves the rock and