The Floridan Aquifer System (FAS) is an underground reservoir that serves as a primary source of freshwater for the southeastern United States. It is one of the world’s most prolific aquifers, providing water for agriculture, industry, and millions of people. This system consists of a thick sequence of carbonate rocks that store and transmit groundwater across a vast region.
Geographical Footprint of the Aquifer
The horizontal extent of the Floridan Aquifer System covers an estimated area of about 100,000 square miles. This footprint underlies the entire state of Florida, stretching north into parts of Alabama, Georgia, and South Carolina. The boundaries are defined by the extent of the permeable carbonate rock layers that hold the water.
The aquifer’s presence across this multi-state region is not uniform, as the depth to the water-bearing rock changes significantly. In central and northern Florida and parts of southern Georgia, the aquifer is unconfined, meaning its upper surface is close to the land surface. This proximity allows for easy access to the water and direct interaction with the surface environment.
In other regions, particularly southern and coastal portions, the aquifer becomes deeply confined beneath thick, less-permeable layers of rock and sediment. This confinement means the water is under pressure and protected from surface contamination, but it requires deeper drilling for access. The varying surface expression of the system is a primary factor in how water is managed and withdrawn across the Southeast.
Vertical Extent and Internal Structure
The Floridan Aquifer System is defined by its significant vertical depth as well as its horizontal spread. The thickness of the water-bearing rock layers varies dramatically, ranging from less than 100 feet where the rock pinches out to more than 3,700 feet in the deep subsurface of southwestern Florida. This vertical profile contains the system’s entire volume of stored groundwater.
The system is separated into two main components: the Upper Floridan Aquifer (UFA) and the Lower Floridan Aquifer (LFA). These aquifers are separated by a middle confining unit, which is a layer of rock with much lower permeability. This unit acts as a partial barrier, slowing the vertical movement of water between the upper and lower sections.
The Upper Floridan Aquifer is the primary source of freshwater used across the region, as it is shallower and more permeable. The Lower Floridan Aquifer is much deeper and tends to contain water that is more mineralized or brackish, particularly in coastal and southern areas. While the deeper layer is less frequently tapped for drinking water, it remains a significant part of the aquifer’s overall structure.
Water Volume and Supply Capacity
The volume of water stored within the Floridan Aquifer System represents one of the largest reservoirs of fresh groundwater in the world. The Upper Floridan Aquifer alone is estimated to contain a volume of water that is approximately one-fifth the total amount of water found in all of the Great Lakes combined. This comparison helps illustrate the magnitude of the resource.
This storage capacity translates into a high-producing supply for the region, supporting the drinking water needs of nearly 10 million people. In addition to public supply, the aquifer is heavily utilized for agricultural irrigation, which accounts for nearly half of the total water withdrawn. Industrial operations also rely on the aquifer, making it a foundation for the regional economy.
Withdrawal rates from the FAS are substantial, with total usage often measured in billions of gallons per day. For example, total withdrawals in 2000 were approximately 3.64 billion gallons per day, ranking it among the highest-producing aquifers in the United States. Monitoring these withdrawal rates is necessary to ensure the long-term sustainability of the resource for both human and environmental needs.
Geological Composition and Recharge Mechanisms
The physical structure of the Floridan Aquifer System is defined by its unique geological composition, characteristic of a karst aquifer. The system is predominantly composed of thick layers of limestone and dolomite, which are carbonate rocks formed from marine sediments. These rocks contain countless pores and fractures that allow them to store and transmit large quantities of water.
The process of recharging this water reserve relies on rainfall that filters down through the ground. In areas where the aquifer is unconfined and close to the surface, rainwater can percolate directly into the limestone. This direct infiltration is the most efficient method of recharge for the system.
Geological features like sinkholes and fractures in the limestone bedrock also play an important part in recharge. These openings act as conduits, funneling surface water and rainfall directly into the underground system. Conversely, the aquifer discharges its water through natural outlets, most notably the more than 700 springs scattered across Florida, which represent the visible overflow of the groundwater system.