Aquifers, Springs, and Sinkholes

What is an aquifer?  Two-thirds of the freshwater on earth is found underground.  This “groundwater” is held in a naturally occurring reservoir called an aquifer.  An aquifer is made up of the layers of limestone and sandshell under the earth’s crust.  Water is held here in the tiny spaces between soil particles or cracks in the bedrock, similar to a sponge.  The holes in the rock allow the water to freely flow through it.  Aquifers are a source for well and spring water.  The water held there also forms the “water table”.  There are three types of aquifers in Florida:  deep, intermediate, and shallow.  Northeast Florida gets its groundwater from a deep aquifer, the Floridan.    The Floridan aquifer is the largest aquifer in the southeastern United States and stretches 82,000 square miles beneath Florida and parts of Alabama, Georgia and South Carolina.  This aquifer averages 1,000 feet thick.  It is the principal water supply for much of the state of Florida.

What is groundwater?  Groundwater is simply the supply of fresh water found beneath the earth’s surface, usually stored in aquifers.  Groundwater only moves if there is sufficient pressure to force it through the porous layers of the aquifer.  Groundwater moves from areas of high pressure to areas of low pressure.  It may be accessed by wells or as it naturally discharges from springs.  Some discharging water has just entered the aquifer, while some has been stored there for thousands of years. The water in the Floridan aquifer has been there from 50 – 26,000 years.

Groundwater can be accessed for human use through wells and pumps.   When an area of the aquifer is under lots of pressure, an artesian well can easily draw water.  The water naturally flows up this well to escape these high subterranean pressures.   However, if the aquifer is not under great pressure, there is no reason for the water to move up a well on its own.  Instead, a mechanical pump must be utilized to bring this water to the surface.  The water in the aquifer is referred to as the water table.

Can an aquifer dry up completely?  Under natural conditions, there is a balance between the amount of water entering the aquifer and the amount leaving it. Groundwater is replenished through a process called groundwater recharge - rain seeps through the soil and into the layers of limestone and sandshell.  On average, Florida receives 50 inches of rain each year. Only 13 inches actually reach the aquifer, the other 37 inches evaporate or become runoff.  The balance in the aquifer may be naturally disrupted when a draught causes lower-than-normal amounts of water to enter the aquifer.  The use of wells has begun to deplete groundwater resources, disrupting the natural balance.  When the aquifer is overpumped, or the rate of pumping is not matched by recharge, the aquifer is depleted.   Because of the increased demand on the groundwater supply, aquifer depletion has become a serious problem in Florida and the United States in general. Some alternate water sources include using reclaimed water and removing the salt from brackish and salt water.

How does groundwater become contaminated?  Some groundwater contamination occurs naturally. Saltwater intrusion occurs when too much fresh water is pumped out of the aquifer and saltwater from the ocean is drawn in to replace it.  Serious contamination usually occurs because of human activities.  This contamination includes the runoff of agricultural pesticides, synthetic organics from household cleaning products, and petroleum and metals from roadways.  Leakage of bacteria from septic systems, as well as petroleum from gasoline storage tanks, also creates a serious hazard to groundwater purity.  Most of these pollutants enter the aquifer through “recharge water” from the surface. Since groundwater moves slowly, it may take many years for a pollutant to be detected.

What causes springs?  A spring is an area where groundwater from an aquifer naturally flows to the earth’s surface. Springs are fed through cracks in the underlying rock of the aquifer allowing for the free flow of water to the surface.

Florida has over 320 springs, which discharge almost 8 billion gallons of water each day.  Most of Florida’s springs are located in the northern half of the state and are connected to the Floridan aquifer. The quality of spring water is generally representative of the quality of the nearby groundwater system.  Most spring water has excellent quality.  The temperature of spring water ranges from 66 F to 97 F.  The temperature of spring water in north Florida averages about 70 F.

What causes sinkholes?  Sinkholes occur when soil on the surface collapses into an underground cavity in the underground limestone and sandshell layers. These cavities occur when rainwater percolates through the outer layer of sediments and reaches the limestone, gradually disssolving the rock and weakening the structural integrity of the underlying layers. This weakening process is often triggered by changes in water levels in the aquifer.

References:

“The Florida Water Resource Primer”, http://muck.soils.ufl.edu/primer/h2ohome.html
Florida’s Underground Water Supply, St. John’s River Water Management Distirct, November 1998 

Provided by the Pelotes Island Nature Preserve
http://pelotes.jea.com