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Water on Earth - Explaining the Wonders of the Biscayne Aquifer

By Dr. Anteneh Abiy, Hydrogeologist for The Everglades Foundation

Excerpt from The Everglades Foundation Science Insider, Summer 2023, Volume 8

Invisible Water

From medieval qanats and hand-dug wells to modern-day water supply systems, humankind has been dependent on underground freshwater for eons. A hidden resource, groundwater plays a vital role in the well-being and survival of life on our planet, supplying freshwater in many places of the world. Of that small fraction of freshwater on Earth not locked in glaciers and ice caps, over 99% is buried underground in the voids and cracks of the Earth.


In South Florida, five million residents, and countless tourists use over three billion gallons of water every day—for drinking and bathing, watering our lawns, growing our crops, and servicing our industries. Buried in the pores of underground limestone, our groundwater drives our economy, thanks to the Biscayne Aquifer. All aquifers have one thing in common: They don’t produce water, they just hold it.


The Birth of the Biscayne Aquifer

The State of Florida rests on a larger mass of rocks that rise steeply from the ocean floor like a plateau. Called the Florida platform, this basement land mass is found 0.5 to three miles below the surface, mainly underwater. Made of both metamorphic and igneous rocks, for 200 million years, its geology changed episodically. Cycles of ice ages and warmer periods over geological scales of time exposed, then buried, the Florida platform with sedimentary rock, containing alternating layers of carbonate and silicate sediments.


The combination of warm and shallow marine water provided ideal conditions for depositing carbonate sediments. When sea levels receded during the last ice age, the sedimentary rocks formed through this process were exposed to rainfall and dissolution allowing them to hold large volumes of freshwater, with the silicate layers of clay and fine sand sediments serving as a watertight boundary at the base of the aquifer. Like layers of cake and frosting, these alternating layers of rocks are grouped into three major aquifer systems.

The deepest and thickest landmass covering the bottom basement rocks is the Floridan Aquifer System. Water in this aquifer is salty in South Florida and fresh in Central and Northern Florida. A thick mass of sand and clay called the Intermediate Aquifer separates the predominantly salty Floridan Aquifer from a freshwater aquifer above, the Surficial Aquifer System. Covering the southern and parts of northeastern Florida, this surficial aquifer was likely exposed to acid dissolution from rainfall. Consequently, it has a wide range of carbonate dissolution features that can store water, like sinkholes, caves, and other voids.


Getting its name from Biscayne Bay, the Biscayne Aquifer is part of this Surficial Aquifer System serving southern Palm Beach, Broward, Miami-Dade and Monroe Counties. This land mass just under our feet is comprised of highly porous limestone. With an average thickness of only 80 feet, the Biscayne Aquifer supplies freshwater to millions of Floridians for two reasons:

  1. Its large, interconnected voids store large volumes of water.

  2. It receives water directly from the surface. When a drop of water hits the ground, it infiltrates quickly to move around the underground limestone. This means water flows freely directly into our water supply wells.

Sit and Recharge

The Biscayne Aquifer is recharged predominantly by the Everglades. South Florida is blessed with an abundance of rainfall, 50 to 60 inches annually. When this water from the Everglades percolates underground, the large pores in the limestone serve as an “underground river” allowing it to travel south and southeast to the coastal regions. This subsurface flow from the Everglades to the aquifer is its main source of recharge. However, Everglades water is also transported into urban areas through canals. These extremely leaky canals recharge the aquifer, albeit at a much lower rate than the Everglades.


The urban areas of South Florida receive rain too, but since they are highly developed and paved, the water is quickly drained out to the coast to avoid flooding. Consequently, rainfall in urban areas does not recharge the aquifer much.


Therefore, the hundreds of water supply wells in Miami-Dade, Broward, Palm Beach and Monroe Counties, serving millions of residents, gain water from the Everglades. The water flowing into faucets, showers and lawns was recently a raindrop over South Florida that splashed into the Everglades, seeped into the limestone pore space beneath our feet, and then was pumped out, treated, and distributed. Your water comes from the Biscayne Aquifer, whose survival depends on the Everglades.

Is There Enough Water

With more than three billion gallons of water being used every day, the pressure on

the Biscayne Aquifer’s water supply is mounting. With increasing urbanization, climbing

populations, and influxes of tourists, the demand for freshwater will remain high in South Florida. In addition, sea level rise and saltwater intrusion are compromising the aquifer’s freshwater availability.


This means that groundwater is the essence of the water supply system in the

region, one highly dependent on the Everglades. Ensuring high water levels in the

Everglades is a priority to maintain a supply of freshwater in the future. By sending

more freshwater through the Everglades, instead of dumping it to tide, Everglades

restoration will help guarantee a sustainable and increasing source of aquifer

recharge.

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