Faults of Florida

Mary M Stewart

Applied Hydrology

Floridian Aquifer system

 

Hydrologic Setting

There are five principal aquifers or aquifer systems in Florida. An aquifer system consists of two or more aquifers that have a hydraulic connection. The shallowest of these systems is the undifferentiated Surficial Aquifer System (SAS). The SAS consists of the sand and gravel aquifer and the Biscayne aquifer. The deepest aquifer system is the Floridan aquifer. Only a small part of this system is exposed at land surface. The Floridan Aquifer System consists of a thick sequence of carbonate rocks of Tertiary Age. In between the Floridan Aquifer System and the Surficial Aquifer System is the Intermediate Aquifer System.

In most places, the Floridan Aquifer System can be divided into the Upper Floridan and the Lower Floridan aquifers, separated by a less permeable middle confining unit. The Floridan Aquifer underlies about 100,000 square miles and underlies a large part of the coastal plain of Georgia, smaller areas of coastal Alabama and South Carolina, and All of Florida. Many cities in Florida rely on the Floridan aquifer for potable water, including Tallahassee, Jacksonville, and Saint Petersburg. Refer to Figure 1 for a map of the Floridan Aquifer System.



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Figure 1: Map of the Floridan Aquifer System. Courtesy USGS

 

Regional Climate and Influence


Figure 2: Climate

The Florida climate is tropical, meaning it is warm and moist. The average temperatures are mild from the upper 90’s in the summer to lower 70’s in the winter throughout most of Florida.

Rainfall varies in annual amounts, seasonal distribution and location. Distribution of rainfall across the state of Florida varies as much as 100 inches in the panhandle to less than 40 inches during a calendar year. Seasonal variations are the dry season and the wets season. The dry season is from October through November, the wet season being from November through to next October. The wettest periods occur in late winter or early spring and throughout the summer.

During the wet season, afternoon rain showers occur daily. For a typical afternoon shower, peak rainfall rates are 0.7 to 1.8 inches per hour. During the dry season, it may be weeks or months without any rain.

The result of such extended dry periods followed by sudden, intense rainfall is rapid runoff and little infiltration. Such long, periods of time extends between rain events so that the soil is so dry that the soils actually repels water, thereby decreasing infiltration further. In this situation, infiltration is driven strictly by gravity. Conversely, it is not uncommon for the amount and rate of rainfall to exceed the rate of infiltration from the surficial aquifer into the intermediate and Floridan aquifers such that the surficial aquifer becomes saturated and cannot accept any more rainfall. It could be days or even weeks before the surficial aquifer can accept more water. Thus much of the rain that falls onto Florida is lost through runoff.

Mechanisms of Recharge


Florida is characterized by high rainfall and low topographic relief. Highly permeable rock or soil is present at or near land surface throughout much of Florida.

According to Miller, the source of all freshwater in Florida is precipitation. Most of this precipitation falls directly onto land, however, some of it is from rainfall in Alabama or Georgia that flows southward into Florida in either streams or groundwater inflows. Most of the precipitated water moves along short paths then discharges as base-flow into streams. Some of the water leaks downward across the leaky clayey confining beds to recharges intermediate, where it can leak down to the Floridan. This infiltration is only possible where the potentiometric surface of the aquifer is less than the depth of the water table in the surficial aquifer.

The amount of water available as recharge to the Floridan aquifer is that part of rainfall after losses to runoff and evapo-transpiration that infiltrates to the water table and continues to move downward. Recharge also occurs from infiltration of rainfall where the limestone of the Floridan is at or near land surface and sinkholes have breached the upper confining unit.

In areas where the elevation of the water table of the surficial aquifer is higher then the potentiometric surface of the Floridan aquifer, the water moves from the surficial aquifer downward through the upper confining unit to the Floridan aquifer. In areas where the Upper Floridan potentiometric surface is at a higher elevation than the water table, the direction of leakage is upward from the Floridan to the surficial aquifer resulting springs and flowing artesian wells.

The hydrogeologic characteristics of the surficial, intermediate, and Floridan aquifer systems determine areas and rates of recharge to the Floridan aquifer. The water table of the surficial aquifer is the upper surface of the zone of saturation. The intermediate confining unit, where present, retards the movement of water between the surficial and Floridan aquifers and confines the Floridan aquifer under artesian conditions. Vertical movement, or leakage, of water through the confining unit is dependent upon its thickness and hydraulic conductivity.

Surface Water Bodies

The distribution of water bodies is determined by the age and depth of the aquifer formations, the elevation of the potentiometric surface in the Floridan aquifer, and the depth of the water table of the surficial aquifer. Where the potentiometric surface is higher than the water table, the water is discharged into springs and wetlands. Where the potentiometric surface is lower, water infiltrates and recharges the aquifers. Where the potentiometric surface and the water table are of equal elevation, lakes form.

There are more than 7,700 lakes in Florida. Refer to Figure 3 for a distribution of lakes in Florida. Most abundant amount of lakes for a given area in the world is in the panhandle. Many of the lakes in central and western peninsular Florida were formed by sinkholes that are the result of dissolution of part of the limestone bedrock. Where the Floridan aquifer discharges into sinkholes, springs form. Sinkholes allow rapid infiltration into or discharge from the Floridan aquifer. There are 27 springs or groups of springs in Florida. The source of water for these springs is the Floridan Aquifer. They discharge at an average rate of 100 cubic feet per second (cfs). The locations of springs are primarily determined by environmental factors of geology and hydrology, such as a down-cutting river or local karst. Refer to Figure 4 for a map of the distribution of springs.


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Figure 3: Distribution of Lakes.

 

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Figure 4: Distribution of springs.

South Florida is dominated by wetlands. The wetlands occur as a result of sheet-flow from precipitation or from discharge of the aquifer. The largest wetland system in the world is the Everglades in south Florida, extending over 1.5 million acres in size. Refer to Figure 5 for the distribution of wetlands.


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Figure 5: Distribtuion of wetlands.

Hydrologic and Geologic Properties

The Floridan aquifer consists of a series of carbonate rocks of Tertiary age. The most permeably being the Ocala Limestone and the upper part of the Avon Park Formation of Eocene Age. The Suwannee Limestone can be an important source of water, but it is less extensive than the Eocene age formations. In some places, where it is permeable enough, the Tampa Limestone is considered to be part of the Floridan aquifer. The Tampa Limestone is Miocene age. Also included where permeable enough are the lower part of the Avon Park formation, Oldsmar formation (Eocene age), the upper part of the Cedar Keys formation (Paleocene age), and the lower part of the Hawthorn formation. Refer to Figure 6 for a table of the formations and ages.

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Figure 6: Ages of formations.

The upper Floridan is highly permeable and consists of the Suwannee and Ocala Limestones, and the upper part of the Avon Park formation. The middle confining unit may be divided into seven discrete units that vary in depth and rock type. The Lower Floridan aquifer is less studied since it tends to be salty.

Two very important zones to note are the Fernandina permeable zone in northern Florida and southeastern Georgia and the Boulder Zone. Flow in the Fernandina permeable zone is upwards into the Upper Floridan aquifer. The Boulder Zone was named because it is difficult to drill into, no actual boulders exist in this zone. Despite the hardness of these rocks, they are cavernous and, therefore extremely permeable. This zone is used primarily for receiving treated wastes and sewage.

The top and bottom of the Floridan aquifer does not coincide with the same formations throughout its extent. The Floridan aquifer thickens to the southeast so that the main source of recharge is located where the Floridan aquifer outcrops in northern Florida. Refer to figure 7 for a map of the thickness of the aquifer. The Floridan aquifer is about 250 feet thick in south-central Georgia and 3,000 feet thick in southeastern Florida. At its northern extent, the formations are connected and water flows between to form the Upper Floridan. As the Floridan extends south, a middle confining unit develops and splits the Floridan into an Upper and Lower Floridan aquifer.

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Figure 7: Thickness of the Floridan aquifer.

The Floridan aquifer is varies throughout. Except for a few exposures in sinkholes in northern Florida and Southern Georgia, the Floridan is confined. In and near outcrop areas, the Upper Floridan aquifer is semiconfined. In many places, especially where the intermediate aquifer exists, the Floridan is leaky, varying in how much leakage occurs based on the amount of clay in the middle confining layer. Also, in southern Florida, flowing artesian wells are common. Springs, where the Floridan discharges, are common throughout the state of Florida. Reported values for leakance of the Floridan in Southwest Florida ranges from 1X10-6 gpd/ft3 to 234 gpd/ft3.

The permeability of the formations vary with age. Older formations and thinner formations have higher permeability whereas the younger formations and thicker formations have lower permeability. This is because the older limestones have been exposed to dissolution for a longer time and, thus, have more dissolutions, cavities and sinkholes. In northern peninsular Florida, the Paleocene and lowermost Eocene rocks contain sand and are much less permeable than the carbonate rocks of the Floridan. Due to the contrast in permeability, these sandy strata form the base of the Floridan aquifer system in this area. In south-central Georgia and northern peninsular Florida, evaporite minerals have filled the pore spaces in upper Eocene rocks, and these low-permeability beds comprise the Floridan in this area.

On average, the transmissivity values for the Floridan range from less than 10,000 square feet per day (sfd) to more than 1 million (sfd). In most areas the transmissivity is values for the Upper Floridan aquifer is around 250,000 (sfd) where it is unconfined or the upper confining unit is less than 100 ft thick. In the western panhandle the Floridan is thinner and the transmissivity values are less than 10,000 (sfd). In southwest Florida, the transmissivity values are reportedly range from 3 gallons per day per foot (gpd/ft) in the fine-grained sands of the Ocala limestone in northern Sarasota County to 70,000,000 gpd/ft in the thick, cavernous zones at the northern boundary of Hardee County.

The specific yield is negligible in the confined conditions of the Floridan aquifer. In Southwest Florida, storativity range from 1X10-5 to 4X10-1 for the Upper Floridan and 3X10-6 to 2X10-3 for the intermediate aquifer. The variation of these values is due to the variation of lithology and aquifer matrix.


Most of the Floridan is suitable for potable water use. The groundwater increases in mineral content and total dissolved solids (TDS) the longer travels through the aquifer so that deeper the aquifer, the more mineralized the water. In some places, the Floridan aquifer contains gypsum and/or anhydrite that contribute large amounts of calcium sulfate to the water. In other places, the Floridan contains saltwater where it connects to the coast. The Lower Floridan is commonly brackish or salty with a high TDS. In south Florida, the Upper Floridan also becomes unusable as a potable water supply due to high TDS.

In Florida, the unsaturated zone is very small and often non-existent. The unsaturated zone decreases further south. In the areas of the Biscayne aquifer and near the coast, it is not uncommon for the water table to be at land surface for most of the year. In north Florida, the unsaturated zone is more significant, and has a greater economic impact. North Florida is riddled with karst terrain and the aquifer is cavernous. When the water level of the Floridan Aquifer drops, the caverns become empty. Without the water providing hydrostatic pressure, the caverns then collapse into a sinkhole.

Aquifer Management

There are five water management districts (WMD) in Florida. The WMDs administer flood protection programs, and perform technical investigations into water resources. They are also responsible managing drought preparedness programs; land management, acquisition, and protection in relation to sensitive lands; and the ‘Save Our Rivers’ program. They manage consumptive use, aquifer recharge, well construction, and surface water management.

Where the Floridan contains fresh water, it is the main source of potable water. Many cities in Florida depend on the Floridan for potable water, including Jacksonville, Gainesville, Tallahassee, Orlando, Clearwater, Tampa, and Saint Petersburg.

It is also a source of water hundreds of thousands of people using smaller community supply systems and private wells. Total Withdrawals from the Floridan aquifer in 1985 was more than 2..5 billion gallons per day. These rates have increased somewhat. Despite this amount of pumping, water levels in the Floridan have decreased only slightly, except around public water supply wells where the cone of depression extends a mile or more deep.

Where Floridan aquifer is salty or has a high TDS, it has been used as a repository for treated sewage and industrial wastes. This occurs along the southeastern coast of Florida. Near Orlando, drainage wells are used to divert runoff into the Floridan Aquifer.

Also, in southern Florida, the salty water is used for cooling at power plants and is converted to fresh water by desalinization plants. In 1985, 17 million gallons per day of saltwater was utilized for desalinization, mixing with potable water, or cooling systems.

The water supply in the Floridan is threatened by increasing population and demands that results in over-pumping, contamination through injection wells, and contamination through sinkholes. In the past, people thought ‘out of site out of mind.’ They thought if they junk and trash disappeared, then it wouldn’t be a problem, so they disposed of stuff into sinkholes and springs. Springs are fed by the Floridan aquifer where the potentiometric surface is higher than the water table. So, this disposal, in effect, was into the Floridan aquifer. The result is that the sinkholes have become clogged and water no longer flows. This lack of flow has caused fresh spring waters to become stagnant with algae growth. It has also caused springs to stop flowing.

In general, the Floridan aquifer has withstood the massive amounts of pumping imposed on it and water levels have not declined dramatically. However, locally, pumping rates have caused salt water intrusion from the coast and upward migration of lower waters with higher TDS.

The aquifer is contaminated with salt water and, locally, may be contaminated from discharges depending on the nature and amount of the spill and where it was located. In north Florida, the Floridan outcrops and is most vulnerable to contamination by mankind. Further south, deep injection wells inject pre-treated waste and sewage water into the aquifer. The long-term effects of injection wells are not known. Much debate has pursued on this topic. It is the fear that these wastes will migrate upward under leaky conditions. It is not a problem to public water supply systems that treat and monitor the water they supply. However, private and domestic well owners are at risk because they do not have the advanced treatment and testing standards of public water suppliers.

Many agencies are responsible for the cleanup and protection of groundwater, the agency responsible varies by location. In some areas, the counties are responsible for protection and management. The WMDs are also responsible to some extent. In still other areas, the health department manages contamination monitoring and cleanup. Increasing up the ladder of responsibility, the local agencies and WMDs report to the Florida department of Environmental Protection. Next up the ladder is the federal Environmental Protection Agency.

Conclusion

The Floridan Aquifer is an important source of potable water in most of Florida. The thickness and depth of the aquifer increases southward. the water quality also decreases with depth, becoming salty and increases in mineral content. Also, local over-pumping has caused salt-water intrusion. There are five water management districts in Florida that are responsible for controlling withdrawal, groundwater usage, stream management, and researching water quality and sources. The Florida Department of Environmental Protection and Environmental Protection Agency are responsible for cleanup and prevention of contamination.

References


Miller, James A., "Hydrogeology of Florida." Randazzo, Anthony F., and Douglas S. Jones, eds. The Geology of Florida. Gainesville, FL: University Press of Florida, 1997.

Publication of Archival Library and Museum Material. 3 Apr. 2008. State University System of Florida. 23 Apr. 2008 <http://palmm.fcla.edu/>.

Rosenau, Jack C. Springs of Florida. Tallahassee, Florida: Bureau of Geology, Florida Department of Natural Resources, 1977. Publication of Archival Library and Museum Materials. State University System of Florida. 30 Apr. 2008 <http://fulltext10.fcla.edu/cgi/t/text/text-idx?&c=feol&idno=uf00000232&format=text>.

Technical Services Department Resource Regulation Of The Southwest Florida Water Management District. "Aquifer Characteristics within the Southwest Florida Water Management District." <i></i> Fourth Edition (2006): 0-30. ? Southwest Florida Water Management District. 30 Apr. 2008 <http://www.swfwmd.state.fl.us/permits/wellconstruction/aquifer_characteristics.pdf>.

U.S. Department of the Interior. U.S. Geological Survey. Scientific Investigations Report. Feb. 2008.  S.C. Water Science Center. 23 Apr. 2008 <http://www.usgs.gov/>.

United States Department of Commerce. National Oceanic and Atmospheric Administration. NOAA. Apr. 2008. 30 Apr. 2008 <http://www.noaa.gov/index.html>.