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A core sample of limestone from the aquifer shows the holes and cracks in the rock that allow water to move underground.
Florida, at times, seems as much a liquid state as it does terra firma. Slosh through its marshes and wetlands, and you’ll probably agree. We live in a land literally shaped by water, a peninsula boasting more than 1,197 miles of coastline and laced with 50,000 miles of rivers and streams, 7,000 lakes and more than 700 known springs.
The vast majority of the water that comes from our collective taps — water we use for drinking, washing, bathing and irrigating our landscapes — comes not from visible flows but from sources deep beneath the ground.
This is a story about Florida’s aquifer system, an underground labyrinth harboring billions of gallons of sparkling water, with a mysterious network of chasms and porous rock and vast confining layers of clay. The aquifer has inspired enduring legends (think of Ponce de Leon’s futile search for Florida’s Fountain of Youth) and it has helped shaped the culture of the Sunshine State. The water ebbing and flowing beneath our feet represents one of the most prolific aquifer systems in the world.
Understanding its complexities is critical to the mission of the St. Johns River Water Management District as it makes daily decisions about water use for its 18-county region.
Florida has five main aquifer systems that serve most of the state’s needs. The Floridan aquifer system is the principal source of freshwater in the St. Johns District, and for the state. The Floridan underlies most of Florida and extends out to parts of Georgia, South Carolina and beneath the Atlantic Ocean and the Gulf of Mexico.
“The Floridan is about 100,000 square miles in area and can be thousands of feet thick,” says Jeff Davis, a District hydrologist who is helping to map the aquifer system. “That’s a potential for a huge volume of water.”
The Floridan is mainly composed of limestone and dolomite. Limestone acts as a sponge, absorbing and holding water. The texture, however, doesn’t feel like a sponge at all. It is very hard and full of voids and crevices that allow water to move freely through it.
How did the aquifer come to be, and what exactly is the composition of limestone? Rewind the clock 55 million years or so, and Florida would have resembled the Bahamas, Davis says.
“Millions of years ago, Florida was composed of mud flats, corals, shells and algae,” Davis says. “Over time, shells and the skeletons of dead sea life accumulated and hardened into the rocks that comprise the Floridan.”
As the centuries crawled by, the seas rose and fell, exposing the peninsula. The Appalachian Mountains to the north eroded and brought sand and clay to create areas that confined water, called “confining units.” Slightly acidic rainwater dissolved the limestone, creating large cavities that, today, are underwater caves and caverns. This ongoing dissolution has created what scientists call a karst, which is characterized by a topography that includes sinkholes and subterranean limestone caverns, carved by groundwater.
“The Floridan isn’t a static system,” Davis says. “The ability of the rocks to store or transmit water changes in response to changes in the aquifer’s water chemistry and even forces that fracture the rocks. Sea level changes of hundreds of feet have created varying conditions. At times, fresh, slightly acidic water flowed through the system and dissolved the limestone, thereby increasing porosity and permeability. Other times, highly mineralized water flowed through and filled void spaces by depositing carbonate cement. There is a constant process of deposition and dissolution.”
Among their work, scientists monitor water quality, taking measurements of the spring discharge at Salt Springs.
Davis and Don Boniol are District hydrologists who update hydrogeologic models — maps of the Floridan aquifer system, if you will. These maps provide input for groundwater models, which are essential to the District in performing water supply assessments and evaluating water use permit applications. In simplest terms, District staff can make educated decisions on issuing permits for consumptive uses if they have accurate models to inform them about available water below the ground.
Using high-tech equipment and boring holes deep into the ground, District geologists are mapping the Floridan and recording information about rock type, flow zones and water quality so they can judge the depth and elevation of the aquifers and confining units. The log data standardizes information from one test well to the next. Amassing and assembling the information is like constructing a 3-D jigsaw puzzle image of the Floridan, Davis says.
“There have been many reports and studies done on the aquifer,” Davis says. “But it is time to update the mapping, with new information from other agencies and sources, into a single mapping framework.”
Knowing where potable water is most plentiful is key to the District’s water supply planning process. Water in the Floridan may seem limitless, but overpumping can have immediate and long-term impacts on groundwater resources.
The Floridan is under constant pressure, the result of an endless cycle of recharge and discharge. Recharge occurs when water seeps from the land’s surface down through the layers of earth into an aquifer.
Florida generally receives 50 to 55 inches of rainfall each year, but much of it — about 37 inches — evaporates or runs off the land into surface waters before it has a chance to soak into the ground. That leaves about 13 inches annually to recharge the aquifer, in limited areas.
Pumping too much water from the aquifer can draw salt water up from deeper zones below the aquifer or laterally from the ocean. Overpumping can also result in a pressure drop in the aquifer, causing spring flows to diminish and wetlands to dry out.
To the uninitiated, the District’s research and mapping of the Floridan aquifer system may seem esoteric and somewhat intimidating. However, improving our understanding of this critical water source is necessary to ensuring Florida will continue to flourish.