Floridan

The Floridan aquifer system supplies drinking water to nearly all of Florida, southern Georgia, and parts of Alabama and South Carolina — about ten million people. It is one of the most productive aquifers in the world. It is also being pumped faster than it recharges in every major coastal city it serves, and the consequence is two failure modes happening at once: the land surface above the aquifer is sinking, and saltwater from the Atlantic and the Gulf is moving inland to occupy the space the fresh water used to fill.

What it is, geologically

The Floridan is a stack of Eocene-to-Oligocene limestones — the Avon Park, Ocala, and Suwannee formations, primarily — that lies beneath most of Florida and extends north into Georgia, eastern Alabama, and southeastern South Carolina. Carbonate rock dissolves where slightly acidic water moves through it, and the result over millions of years is karst: a labyrinth of dissolved channels, cavities, and conduits. Karst aquifers are extraordinarily productive — wells can pull thousands of gallons per minute from a single hole — but the same property that makes them productive makes them extraordinarily vulnerable.

Water that falls on the surface above karst can reach deep wells in days, not centuries. There is no slow filtration through dense rock the way there is in the Denver Basin. A spilled drum of solvent at the surface can be in someone's drinking water by the end of the week. The trade-off for high productivity is low residence time and low natural protection.

Saltwater intrusion: the slow front

Most of the Floridan's outcrops sit near the coast. When fresh groundwater is pumped faster than it can recharge, the pressure that holds back ocean water along the boundary drops, and saltwater moves inland. Once it's in, it doesn't leave; the only ways to retreat the saline front are to stop pumping or to inject fresh water back into the aquifer.

Specific frontiers as of the 2020s:

• Brunswick, Georgia — saltwater has moved approximately 8 miles inland over the last 50 years, contaminating municipal wells and forcing relocation deeper into the aquifer.
• Hilton Head, South Carolina — coastal Floridan wells across the island have been retired or are at active risk.
• Tampa Bay region — the city of Tampa runs a well-retirement program, paying private well owners to abandon their wells in favor of municipal supply, partly to protect the aquifer from saltwater pull.
• Florida Panhandle — deep wells in coastal counties are turning brackish; some have already exceeded the EPA secondary standard for chloride.
• Atlantic coast (Jacksonville south to Cape Canaveral) — slower-moving fronts but ongoing.

If you're on a private well within ~10 miles of the coast and you've noticed your water tasting saltier or your water heater corroding faster, it's not your imagination. Test for chloride and sodium, not just bacteria.

Subsidence and sinkholes: the karst surprise

The flip side of pumping a karst aquifer is the surface above it. Karst is structurally a network of cavities supported by the residual rock between them. When the water that fills those cavities is removed, the unsupported cavity roof can collapse — a sinkhole.

Pasco, Hernando, and Hillsborough counties in west-central Florida are the most sinkhole-prone area in the United States. Insurance markets there require sinkhole coverage as a separate rider. Most "spontaneous" sinkholes correlate with periods of accelerated groundwater pumping — drought years, irrigation peaks, or new municipal well-field expansions. The rock didn't change; the water that held it up went away.

Beyond visible sinkholes, regional subsidence — gradual sinking of the ground surface over wide areas — has been measured by InSAR satellite radar across central and northern Florida and southern Georgia at rates of inches per decade. It does not look catastrophic, but it cracks foundations, tilts buildings, and reverses the gradients on stormwater drainage.

Water quality

The Floridan's water is generally hard (limestone dissolution gives high calcium carbonate) and frequently has elevated iron and hydrogen sulfide ("rotten egg" smell) in the deeper, more reducing parts of the aquifer. Quality varies dramatically with depth and proximity to recharge.

• Bacteria — karst conduits transmit surface contamination fast. Floridan wells should be tested for coliform and E. coli annually, more often after heavy rain. See the bacteria page.
• Nitrates — Florida's intensive citrus, vegetable, and dairy operations push nitrate into the upper Floridan in many counties. Suwannee River basin and central Florida's "fern belt" have documented elevated nitrate. See nitrates.
• Hardness — universally high. Most homes have a softener.
• Iron and manganese — common in deeper wells, especially in the Lower Floridan. See iron and manganese.
• Saltwater intrusion markers — sodium, chloride, sulfate. Test these specifically if you're coastal.
• PFAS — Florida has multiple military installations and airports with documented AFFF firefighting-foam contamination plumes affecting the Floridan: Tyndall AFB, NAS Pensacola, NAS Jacksonville, MacDill AFB. See PFAS.

PFAS risk modeling

The USGS 2024 national PFAS-in-groundwater predictive model estimates a 27.8% mean probability of detecting any PFAS compound at typical private-well drinking-water depth across the Floridan. Roughly 14.1% of the aquifer's area falls in the model's >50% probability band — land where a freshly drilled domestic well is predicted more likely than not to show some detectable PFAS. That is above the national CONUS aquifer median of 19%, and places the Floridan in the upper half of CONUS Principal Aquifers.

This is a 1 km-resolution statistical prediction, not a measurement of any specific well. What PFAS is and how it gets into groundwater → · How to test a private well for PFAS → · Full state-and-aquifer breakdown with methodology →