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Underground Water

What is groundwater?

Subsurface water contained in the interconnected pores below the water-table of an aquifer is termed 'Groundwater'. Groundwater occurs when water flows through the impermeable surface barriers that lets water to flow through the unsaturated zone and collect in the spaces made by interconnecting pores. The impermeable layer can be of bedrock or that of one rock. Once the water flows down to the saturated zone, it flows from the high water-table point to a lower point, this flow of water occurs due to percolation. Water percolating within the saturated zone may, eventually, flow to streams, rivers or other surface waterbodies.

 

What is groundwater recharge?

Groundwater recharge is the process by which aquifers are replenished with water from the surface. A number of factors influence the rate of recharge including the soil type, plant cover, slope, rainfall intensity, and the presence and depth of confining layers and aquifers.  Most of Florida's groundwater recharge occurs in the summer months when precipitation is highest. Recharge also occurs with locally heavy rainstorms during the rest of the year. Groundwater typically discharges into a lake or river, maintaining its level or flow in dry seasons.

Which aquifers are in Florida?

Florida's aquifers yield large quantities of groundwater, which provide drinking water to at least 90% of the state's population. The five principal aquifers system in the state are:

  • Floridian aquifer

  • Biscayne aquifer

  • Sand and Gravel aquifer

  • Shallow aquifer

  • Highly mineralized aquifer

The Floridan aquifer is one of the most productive aquifers in the world in terms of total water yield.. The thick limestone of the Floridan aquifer underlie all of the state and extend into parts of Alabama, Georgia, and South Carolina.  Most of the Floridan aquifer lies beneath several hundred feet of sediments. For much of the state, it is the principal source of water supply.  It is generally not usable in the region south of Lake Okeechobee due to its high salt content.  There, as well as in  the extreme western part of the state and along much of both coasts, other aquifers are used to supply fresh water.

The Biscayne aquifer underlies an area of about 4,000 square miles and is the principal source of water for all of Dade and Broward Counties and the southeastern part of Palm Beach County in southern Florida.  Major population centers that depend on the Biscayne aquifer for water supply include Boca Raton, Pompano Beach, Fort Lauderdale, Hollywood, Hialeah, Miami, Miami Beach, and Homestead. The Florida Keys also are supplied primarily by water from the Biscayne aquifer that is transported from the mainland by pipeline. Throughout much of the mapped area, the top of the aquifer is at or near the land surface.   Because the Biscayne aquifer is highly permeable and lies at shallow depths everywhere, it is readily susceptible to contamination.

The sand and gravel aquifer is the primary water supply in far western part of the state.  As its name indicates, the sand and gravel aquifer consists largely of inter-bedded layers of sand and gravel. The thickness of the aquifer generally increases to the west and southwest from its thin outcrop along the Walton-Washington County line, and in southeastern Escambia County it is 400 to 500 feet thick.

A shallow, non-artesian aquifer is present over much of the state, but in most areas it is not an important source of  water because a better supply is available from other aquifers.  In south Florida, the shallow aquifer is the major source of groundwater in Martin, Palm Beach, Hendry, Lee, Collier, Indian River, St. Lucie, Glades and Charlotte counties.  It is also an important source of water in some Atlantic and Gulf coastal areas where the artesian ground water is highly mineralized. The composition and hydrologic characteristics of the shallow aquifer differ from place to place. The thickness of the shallow aquifer system is typically less than 50 feet, but  is as much as 400 feet in Indian River and St. Lucie Counties; 250 feet in Martin and Palm Beach Counties; and 150 feet in eastern St. Johns County.

Can aquifers dry up completely?

Groundwater resources are depleted or "mined" when pumping from an aquifer is not matched by recharge. This can happen in two ways: (1) by over pumping or (2) by decreasing recharge; due to drought for example. The drying up of an aquifer should not be confused with the failure of individual wells in that aquifer, which happens much more frequently. Well failures can have different causes. A well may be too shallow, so that a temporary decline in water levels lowers the water table below the bottom of the well. Alternatively, the screen at the pipe opening at the bottom of the well may become plugged by mineral and/or bacterial deposits.

Aquifer depletion is a serious problem in Florida and the United States. Coastal communities in the state are often affected as our increasing population demands more water. As more fresh water is pumped and its supply is depleted, salt water from the ocean is drawn into the wells resulting in contamination. Also, the depletion of deeper aquifers may be permanent where the weight of overlying sediments causes the aquifer to compress as the water is pumped out. The aquifer would therefore never again be fully recharged even if pumping ceased, because its capacity to store water has been reduced. This occurs in areas of high water demand such as large municipalities and agricultural operations that require heavy irrigation.

How does groundwater become contaminated?

Sometimes groundwater contamination occurs naturally, but serious contamination is usually the result of human activities on the land surface. Each human activity has a particular impact on groundwater. Some agricultural activities add nitrate nitrogen and pesticides to groundwater. Residential areas with septic systems usually add nitrate nitrogen, bacteria, viruses, and synthetic organics used in household cleaning products and septic tank cleaners.  Industrial activities tend to add organic chemicals and metals, though in widely varying amounts. Gasoline storage areas (including service stations) may have leaks and spills of petroleum products. Roadways contribute petroleum pollutants leaked from vehicles and metals from exhaust fumes.  The most concentrated impact comes from older sanitary landfills, whose leachate may contain many different chemicals at relatively high concentrations.

Except where contaminated water is injected directly into an aquifer, essentially all groundwater pollutants enter the aquifer through recharge water from the land surface. Sinkholes are a special case as they can provide a direct connection to the aquifer and allow contaminants in surface runoff to move straight to the groundwater.

Since groundwater moves slowly, many years may pass before a pollutant released on the land surface above the aquifer is detected in water taken from the aquifer some distance away. Unfortunately, this means that contamination is often widespread before being detected. Even if release of the contaminant is stopped, it may take many years for an aquifer to purify itself naturally.

What causes springs?

Springs are created when groundwater naturally flows to the surface. Groundwater discharged from a spring may have traveled many miles before reaching the surface. Usually, spring water discharges occur or increase when rain has recharged the groundwater system in upland areas. Springs occur throughout the state, but are most abundant in north central Florida. These are also the largest springs which may discharge several million gallons per day into local rivers.

What are sinkholes and how do they occur?

Sinkholes are closed depressions in the land surface formed by dissolution of near-surface rocks or by the collapse of the roofs of underground channels and caverns. Sinkholes are a natural, common geologic feature in places underlain by soluble rocks such as the limestone and dolomite that form the Floridan aquifer system.  Under natural conditions, sinkholes form slowly and expand gradually.  However, activities such as dredging, constructing reservoirs, diverting surface water, and pumping groundwater can accelerate the rate of sinkhole expansions, resulting in the abrupt formation of collapse-type sinkholes, some of which are spectacular.

What is the relationship between groundwater and surface water?

Groundwater and surface water (along with precipitation and Evapotranspiration) are interdependent parts of the hydrologic cycle. Unlike groundwater, surface water can be seen all around: lakes, ponds, rivers, streams and wetlands.

Rivers usually start as small streams and get larger as they flow downstream.  The water they gain is often groundwater. Such a steam is called a gaining stream. It is also possible for streams to lose water to the ground at some points,  In these cases, aquifers are replenished by water from the losing stream.  A stream which flows near the surface of an aquifer will lose water to the aquifer if the water surface in the stream is higher than the water table of the aquifer.  A stream will gain water if the water surface of the stream is lower than the water table in the adjacent land.  (See full-size image for illustration of gaining and losing streams.)

Sinkholes also play an important role in connecting groundwater and surface water. Lakes commonly occupy the depressions created by sinkhole collapse. Some streams lose their entire flow at low-flow stages to the upper Floridan aquifer through swallow holes in the stream bed.  Springs and wells bring the water from the aquifer up to the surface.

Click here to see full-size image of gaining and losing streams.

Is groundwater safer to consume than surface water?

Groundwater is generally safer than surface water for drinking because of the filtration and natural purification processes which take place in the ground. These processes however, become ineffective owing to sewage, fertilizers, and toxic chemicals, which may seep into the groundwater supply.

Household, commercial, and industrial wastes that end up in dumps, waste lagoons, or septic systems can pollute groundwater. Acid rain also threatens to recharge aquifers with contaminated water.

Generally, groundwater is not as easily contaminated as surface water, but once it is contaminated, it is much more difficult to clean up because of its relative inaccessibility.

 
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Last Revised on: 11/20/2007