The Everglades National Park
FIU IDH 4007

Groundwater in South Florida

Camilo Ponton
IDH 4007
Prof.Machonis
Spring 2003

 

   Groundwater is one of our most important and widely available resources, yet people’s perceptions of the subsurface environments where it comes from are often unclear and incorrect. Observations on the land surface give the impression that Earth is solid. Because of this observation many people believe that groundwater occurs only in underground rivers. In reality, most of the subsurface environment is not solid at all. It includes tiny pore spaces between grains of soil and sediment, and narrow joints and fractures in bedrock. Together, these spaces add up to an immense volume. It is in these small openings that groundwater collects and moves.

   Considering all of the Earth’s water, only about 1% occurs underground. Nevertheless, this small percentage stored in the rocks and sediments beneath earth surface, is a large quantity. When the oceans are excluded and only sources of freshwater are considered, the significance of groundwater becomes more apparent. The largest volume occurs as glacial ice (roughly 85%). Second in rank is groundwater with 14% of the total. However when ice is excluded and only liquid water is considered, more than 94% of all freshwater is groundwater.

   Without question groundwater represents the largest reservoir of freshwater that is readily available to humans. Its value in terms of economics and human well-being is incalculable (Tarbuck, 125).
When rain falls, some of the water runs off, some evaporates, and the remainder soaks into the ground. This last path is the primary source of underground water. The amount of water that takes each of these paths, however, varies greatly both in time and space. Influential factors include steepness of slope, nature of surface material, intensity of rainfall, and type and amount of vegetation. Heavy rains falling on steep slopes underlain by impervious materials will obviously result in a high percentage of the water running off. Conversely, if rain falls steadily and gently upon more gradual slopes composed of materials that are easily penetrated by the water, a much larger percentage of water soaks into the ground (Hydrogeology, Fall 2002).

   It is important to be acquainted with the following terms in order to have a better understanding of groundwater flow.

   Aquifer: Rock or sediment that is saturated with water and sufficiently permeable to transmit economic quantities of water    to wells and springs.

   Confined Aquifer: An aquifer that is overlain by other rock layers or confining unit.

   Unconfined Aquifer: Aquifer with no confining units overlaying it, thus having direct contact with the surface. Also termed    water table aquifer.

   Porosity: The ratio of the volume of void spaces in a rock or sediment to the total volume of the rock or sediment.

   Permeability: The ability of a material to allow the passage of a liquid, such as water through rocks. A measure of the    water bearing capacity of subsurface rock. With respect to water movement, it is not just the total magnitude of porosity    that is important, but also the size of the voids and the extent to which they are interconnected.

   Water table: Surface that divides the point between saturated and unsaturated pore space in the bedrock. Below the       water table all the pore space is saturated with water. In this way the water table is the top of the water surface in the    saturated part of the aquifer (Fetter,28)

    The quantity of groundwater that can be stored depends on the porosity of the material. Voids most often are spaces between sedimentary particles, but also common are joints, faults, and cavities formed by the dissolving of soluble rock such as limestone.

   Variations in porosity can be great. Sediment is commonly quite porous, and open spaces may occupy 10 to 50 percent of the sediment's total volume. Pore space depends on the size and shape of the grains, how they are packed together, the degree of sorting, and in sedimentary rocks, the amount of cementing material. For example, clay may have a porosity as high as 50 percent, while some gravels may have only 20 percent voids.

   Porosity alone cannot measure a material's capacity to yield groundwater. Rock or sediment may be very porous but, still not allow water to move through it. The pores must be connected to allow water flow, and they must be large enough for water to go through, the permeability of a material is also very important.
Groundwater flows through small, interconnected openings. The smaller the pore spaces, the slower the water moves. This idea is expressed in the table below. Groundwater is divided into two categories: that portion which will drain under the influence of gravity (called specific yield) and that part which is retained on particle and rock surfaces and in tiny openings (called specific retention). Specific yield indicates how much water is actually available for use, while specific retention indicates how much water remains bound in the material. For example, clay's ability to store water is great owed to its high porosity, but its pore spaces are so small that water is unable to move through it. Thus, clay's porosity is high but because its permeability is poor, clay has a very low specific yield.

     Table: Selected Values of Porosity, Specific Yield, and Specific Retention

Material
Porosity
Specific Yield
Specific Rentention
Soil
55
40
15
Clay
50
2
48
Sand
25
22
3
Gravel
20
19
1
Limestone
20
18
2
Sandstone
11
6
5
Granite
0.1
0.09
0.01
Basalt
11
8
3


Values in Percent by Volume
Source: U.S. Geological Survey Water Supply, Paper 2220, 1987

Principal Aquifers in Florida:

   The Floridan Aquifer is the biggest aquifer and is imperative for water supply to much of the state of Florida. It underlies Florida and parts of Georgia, Alabama, as well as South Carolina. It gets its recharge from northern and central Florida, where it is unconfined. It discharges along the coastline, to streams and in many springs in northern and central Florida. It is composed of mainly of mudstones. The upper unit of the Floridan Aquifer has moderate to high transmissivity (5x104 ft2/day in confined areas and 106 ft2/day in unconfined areas, has good water quality and is the major supply of potable water in North and Central Florida. The lower unit has extremely high transmissivity, but the water quality is saline and is used for disposal of treated sewage water (Deep Well Injection). In South Florida, the Floridan Aquifer is brackish and is not useful for water supply.

    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. The aquifer extends beneath the Biscayne Bay (from whence it was named) and the Atlantic Ocean.
It is one of the most prolific aquifers in the world and it is and is composed of very porous limestone units (large pores created by secondary dissolution). The Biscayne Aquifer is unconfined and shallow; the water table varies from a few feet deep to as much as 150 feet near east coast. The aquifer is very susceptible to pollution due to its great permeability, unconfined nature, near surface location and its existence close to polluting urban areas (many landfills, leaking gasoline tanks, airports, and industry). Transmissivity of the Biscayne Aquifer is 3x105 ft2/day to 2x106 ft2/day (Aich, 17).

Aggressive Pumping:

   Excessive water pumping from the Biscayne aquifer has caused water table to lower. The aquifer could be recharged only through pervious surfaces. That is why it is very important to preserve the Everglades. As population grows, not only more water is being pumped out every day, but also new buildings, malls, and roads, (impervious surfaces) cover the porous limestone, obstructing the recharge of the aquifer.

   Florida’s rain pattern in the long term consists of years with surplus of fresh water and years of scarce rainwater. The surplus water is disposed to the ocean, to prevent flooding. This surplus rainwater endangers the estuarine environments.
On the contrary, during the dry years, the water table lowers down risking the productivity of the aquifer due to salt-water intrusions, and generating high risk of sinkhole formation.

The Aquifer Storage and Recovery Project:

   Aquifer Storage and Recovery is an important technology that is proposed to provide water to the Everglades and South Florida's people and farms in the coming decades. In addition, ASR will help manage water levels in Lake Okeechobee, and reduce damaging discharges to coastal estuaries. Called "ASR", it is a process through which excess water is pumped deep underground for storage in a confined aquifer and recovered when needed.
ASR is a technology that has been in use for more than 30 years in the United States and since 1983 in Florida. As it is proposed in the Comprehensive Everglades Restoration Plan, this technology would be used to deliver water to the population as well as the environment.

   The Everglades needs water to be restored. One goal of the Comprehensive Everglades Restoration Plan is to capture 1.7 billion gallons of water currently sent to sea each year during periods of heavy rainfall in order to avoid flooding. Much of this water will be captured and stored underground in ASR wells for use later by natural ecosystems, people and farms or in either surface or in ground storage areas (ASR Brochure, Jan. 2002).

   Excess rainwater will be captured in a reservoir or lake, pumped from the area, treated, and injected approximately 1,000 feet underground for storage in a confined, porous aquifer. The injected fresh water forms a bubble within the aquifer's heavier, brackish water. The fresh water can be retrieved during dry periods.

   “The role of ASR in the Comprehensive Everglades Restoration Plan is extremely significant, as the storage capacity of ASR provides benefits which are not otherwise possible through surface water storage. ASR allows us to store large volumes of water when rainfall is plentiful, and retrieve it later as needed. Unlike surface reservoirs, little water is lost to evaporation and large land acquisition costs are avoided. ASR is a technology that has been identified to provide water to the Everglades and also to the urban population and farms, and to help restore Lake Okeechobee and the Caloosahatchee and St. Lucie estuaries” (ASR Brochure, Nov. 2002).

   ASR involves pumping freshwater which has been treated to drinking water standards, approximately 1,000 feet underground where it is stored in a confined aquifer and can be recovered later. The pumped freshwater displaces the brackish water of the Upper Floridan Aquifer, resulting in an underground reservoir of freshwater on a relatively small scale. The Comprehensive Everglades Restoration Plan (CERP) proposes to use up to 333 ASR wells to store as much as 1.6 billion gallons of freshwater per day to ensure water for the Everglades, improve conditions in Lake Okeechobee and prevent damaging releases of freshwater to coastal estuaries. Some water also would be available to support agriculture and to protect urban wells located near the coast from saltwater intrusion (ASR, Nov. 2002).

   ASR pilot projects will be located around Lake Okeechobee, adjacent to the Hillsboro Canal south of the Arthur R. Marshall Loxahatchee National Wildlife Refuge, and near the Caloosahatchee River. The number of five milliongallon a day (mgd) ASR wells varies by project, but each site will include pumping and water treatment facilities, monitoring wells and equipment needed for operational testing.

   Design studies for the Pilot Projects are currently in progress. When these studies are complete, a Draft Pilot Project Design Report (PPDR) and Draft Environmental Impact Statement (EIS) will be prepared and distributed for public and agency comment in accordance with National Environmental Policy Act (NEPA) guidelines. After the Final PPDR/EIS has been approved, construction plans and specifications will be prepared and the ASR Pilot Project constructed.

   Following construction, operational tests will be conducted for two years under the guidance of the Pilot Projects. The Lake Okeechobee and Hillsboro ASR Pilot Projects were authorized in the Water Resources Development Act (WRDA) of 1999; the Caloosahatchee (C 43) River ASR Pilot Project was authorized in the Water Resources Development Act (WRDA) of 2000.

   Lake Okeechobee ASR Pilot Project: This project will locate ASR wells at three sites around Lake Okeechobee. In this way, a geographic understanding of ASR system performance around Lake Okeechobee can be established. One site is expected to have a cluster of three ASR wells to demonstrate how multiple wells perform; the other sites will have one well each. Operational testing and data collection, which follow construction, are to be completed in the fall of 2009.

   Hillsboro ASR Pilot Project: This project is located in southern Palm Beach County just south of the Arthur R. Marshall Loxahatchee National Wildlife Refuge and north of the Hillsboro Canal on land owned by the South Florida Water Management District. The project consists of a cluster of three ASR wells. Operational testing and data collection, which follow construction, are to be completed in the summer of 2009.

   Caloosahatchee (C 43) River ASR Pilot Project: This project is located just west of LaBelle, near the Caloosahatchee River on land owned by the South Florida Water Management District in Hendry County. One ASR well is planned. Construction and testing are to be completed in the spring of 2010 (US Army Corps of Engineers, ASR Regional Study 2002).

   The scope of the ASR Regional Study was established by a multi agency team and was based on recommendations of the South Florida Ecosystem Restoration Working Group and the National Academy of Science. The study goes well beyond what can be learned from the Pilot Projects to address the potential effects of large scale ASR on water levels, on water quality within the aquifer, on surface waters, such as Lake Okeechobee, and on plants and animals. The study also will develop a regional computer model of the Floridan Aquifer system to be used in evaluating impacts in specific areas.


WORKS CITED

Aquifer Storage and Recovery for the Comprehensive Everglades Restoration
Plan. Informative Brochure, January 2002.

Aquifer Storage and Recovery Regional Study and Pilot Project. Informative
Brochure, November 2002.

Fetter, Allan. Introduction to Hydrogeology. Prentice-Hall, 2001.

Tarbuck J., and Lutgens M.K. Earth, An Introduction to Physical Geology.
Prentice-Hall, 1999.

US Army Corps of Engineers. ASR Regional Study, Lake Okeechobee ASR
Pilot Project, and Hillsboro ASR Pilot Project. Glen B. Landers 2002.


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