The Floridan Aquifer system is considered one of the world's most productive, underlying southern Alabama, Georgia, South Carolina and all of Florida. That's about 100,000 square miles of permeable, 250 to 3,000 feet thick subsurface rock serving as a main source of potable water to cities such as Savannah, Brunswick, Jacksonville, Tallahassee, Orlando, St. Petersburg as well as smaller communities.

The Floridan Aquifer is a confined aquifer divided into upper and lower units comprising a number of formations of calcareous sedimentary rock that are Tertiary in age. Though the permeability of the aquifer varies by location and formation, the aquifer is described as being ten times more permeable than its confining layers. The main geologic units making up the Upper Floridan aquifer are the Ocala, Suwannee, and Tampa Limestones as well as the upper portion of the Avon Park Formation while the Lower Floridan aquifer includes the lower portion of the Avon Park Formation, the Oldsmar Limestone and the upper part of the Cedar Keys Formation. The Boulder Zone is the base unit of the Lower Floridan aquifer underlying thirteen counties in southern Florida. This zone is highly permeable because it is made up of fractured and highly dissolved dolomite creating large caverns and groundwater flow ducts. There is a middle confining unit that in theory divides the upper and lower aquifers and prevents the flow of groundwater between the two, but it consists of different materials from clay to fine-grained limestones located at various depths.

While the Floridan Aquifer provides almost all the potable water for the northern part of Florida, the aquifer contains nonpotable water south of Lake Okeechobee. The water here is mixed with encroaching salt water making it unsuitable for human use. The Boulder Zone is used for storage of treated sewage. The overlying limestone and dolomite is somewhat impermeable, and, it is believed to be confining enough to prevent the upward flow of the waste into the upper portion of the aquifer.

Source of information and a map of the aquifer can be found here: