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| Ground-water conditions in southern Florida |
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| Ground-water conditions in southern Florida |
This report has been reformatted for presentation on the World Wide Web. The official text of WRIR 01-4275 (6.3 MB download) is available in PDF format. The Adobe PDF Reader program is available, at no cost, from Adobe.
Although southern Florida generally receives abundant annual rainfall, retention of this resource is low and a large percentage is discharged to the ocean through natural and anthropogenic surface-water drainage systems. This is particularly true when extreme rainfall events occur. Much of this water is discharged to the ocean and does not provide recharge to aquifers. Additionally, some of the water that recharges the aquifers is lost to evapotranspiration. Water that is retained in the aquifers becomes a source of water supply for municipal, domestic, and agricultural purposes.
In Broward, Miami-Dade, Hendry, Martin, and St. Lucie Counties, ground water is the sole source of municipal water supply. Ground water provides 83 to 93 percent of the municipal water supply for the remaining counties in the study area (Marella, 1999). In 1998, about 68 percent of the municipal water extracted in the study area was withdrawn from the Biscayne aquifer in Miami-Dade and Broward Counties (R.L. Marella, U.S. Geological Survey, written commun., 1998). These withdrawals supported a population of 3,551,204 in Miami-Dade and Broward Counties as well as 85,646 people in Monroe County (Bureau of Economic and Business Research, 1998). This population represents about 65 percent of the population in southern Florida at that time.
Water usage is directly related to population (Toomey and Woehlcke, 1979). Between 1980 and 1998, the population in the United States increased by about 19 percent (U.S. Bureau of the Census, 1996; U.S. Census Bureau, 1999), whereas the population in the study area increased by more than 30 percent. A comparison of water-use data compiled by R.L. Marella (U.S. Geological Survey, written commun., 1998) and population estimates (Bureau of Economic and Business Research, 1998) illustrates this relation in figure 3 (91K). Between 1980 and 1998, the population in Miami-Dade and Broward Counties increased by 34 percent, which corresponds to an increase in ground-water withdrawals of 26 percent (fig. 3A(91K)). Relative increases in population and water use are even greater in the southwestern and northeastern parts of the study area (fig. 3B-C(91K)). Between 1980 and 1998, the population increased in these areas by 109 and 82 percent, respectively. This growth corresponded to increases in ground-water withdrawals of 99 and 91 percent, respectively. Population in the study area is increasing at a rate that is well above the national growth rate.
Florida typically receives about 54 in. of rainfall annually (Southeast Regional Climate Center, 2001). In southern Florida, eastern Miami-Dade and Broward Counties generally receive about 60 to 64 in. of rain annually, whereas southwestern Florida receives about 52 to 56 in. of rain annually (Winsberg, 1996). The wet season generally lasts from June to October, and the dry season lasts from November to May. About 70 percent of the annual rainfall occurs during the wet season. Florida is susceptible to large differences in annual precipitation caused by the effects of El Niño and La Niña. November to March precipitation in El Niño years can be about 30 percent higher than normal. In years affected by La Niña, precipitation can be 10 to 30 percent lower than normal from autumn to spring (The Florida Consortium, 2001). Tropical storms and hurricanes that produce tremendous amounts of rainfall over very short periods can also contribute to large differences in annual precipitation.
Waller (1985) cites four types of droughts:
The first three definitions can be described in readily quantifiable terms. The fourth, however, is more difficult to quantify, but most closely describes the concerns of water managers in southern Florida. Water managers commonly refer to this fourth type of drought as a "water shortage." The primary concern of water managers is to provide sufficient water supply to the public, while at the same time minimizing any detrimental effects to the water supply or the environment. It is difficult to quantify a water shortage because the long-term effects to the water-supply system, or the potential for such effects, may not be precisely known. In the case of ground water, the potential for adverse effects to the aquifer depends largely on the characteristics of the aquifer. These characteristics commonly vary throughout the aquifer, and are generally known from aquifer tests conducted at a limited number of locations.
Sustained droughts occurred: during various time periods. These periods include: 1943-46 (Parker and others, 1955); 1949-57 (Waller, 1985; Bridges and others, 1991, p. 231-238); 1960-63 (Waller, 1985; Bridges and others, 1991, p. 231-238); 1970-77 (Benson and Gardner 1974; Waller, 1985; Bridges and others, 1991, p. 231-238); 1980-82 (Waller, 1985; Bridges and others, 1991, p. 231-238); 1985 (South Florida Water Management District, 1985); and 1989-90 (Trimble and others, 1990).
Influxes of water to, and withdrawals from, the aquifers also tend to vary from location to location. Complex mathematical models are used to approximate the potential effects of water shortages on aquifers. Quantifying the severity of a drought is a combination of evaluating model results with all available data and using considerable professional judgment.
The rates of ground-water withdrawal from southern Florida aquifers required to support increasing population demands have often exceeded rates of aquifer recharge. As a result, cones of depression have developed in the potentiometric surface of most aquifers near many of the public-supply well fields. During droughts, the growth potential of these cones of depression increases because recharge is reduced and withdrawals often increase. It is during droughts that the balance between withdrawal rates and recharge rates is most critical.
Because the Biscayne aquifer is highly transmissive, the water-management system in southeastern Florida can be, and has been, operated in a manner to mitigate the detrimental effects of increased ground-water usage in this aquifer. Cones of depression have formed around the major well fields in the Biscayne aquifer, but these cones of depression are limited to spatial extent and depth (Sonenshein and Koszalka, 1996; A.C. Lietz, U.S. Geological Survey, written commun., 2001), when compared to those that have formed in the mid-Hawthorn and sandstone aquifers.
Even though population and ground-water usage in southwestern Florida are substantially less than in Broward and Miami-Dade Counties, the effects of ground-water withdrawals are much more evident. The confined and semiconfined aquifers in southwestern Florida are substantially less transmissive than the Biscayne aquifer. As a result, these aquifers in southwestern Florida, which have shown the largest declines in water levels, respond differently to stress than an unconfined aquifer, and large cones of depression have formed in many of the aquifers in this region. During 1974-98, water levels in parts of the mid-Hawthorn and sandstone aquifers, as well as the lower Hawthorn producing zone declined by about 1 ft/yr on average (Prinos and Overton, 2000).
As cones of depression from major municipal well fields increase in size, they may intersect with areas of influence of neighboring water-supply wells, thus causing the water levels in these wells to fall below the pump intakes. This problem has been reported in southwestern Florida a number of times during recent dry periods. Once the dry period ends, however, water levels in the aquifer can recover sufficiently to allow the affected wells to resume operation. If the cones of depression continue to grow, however, these periods of lost pumpage in neighboring wells may become prolonged.
If water levels are lowered sufficiently, aquifer compaction and land subsidence could occur. Water in the pore spaces of rocks and sediments helps to support the weight of the overlying materials. If this support is lost because of decreased water levels, it is possible for the materials comprising the aquifer to permanently compact or collapse. In this case, even if water levels recovered to higher levels, the loss of pore space in the compacted materials would prevent the aquifer from holding as much water as in previous instances.
In some cases, sinkholes may form as water levels are lowered. Several large and deep sinkholes are present in southern Florida (Parker and Cooke, 1944); however, catastrophic sinkhole formation in southern Florida is not generally considered to be a significant factor (Sinclair and Stewart, 1985; Spencer and Lane, 1995). The potential for sinkhole formation, land subsidence, and aquifer compaction is related to properties of the materials forming the aquifer and the diagenetic and geologic history of these materials.
Saltwater contamination has been observed in all of the principal water-supply aquifers of southern Florida. In many cases, this contamination has been caused by lowered freshwater head in aquifers near the coast, which in turn, has resulted in lateral intrusion of seawater (Merritt, 1996; Sonenshein and Koszalka, 1996; Schmerge, 2001). Another major source of saltwater contamination is cross-aquifer contamination (Fitzpatrick, 1986; Schmerge, 2001). Cross-aquifer contamination has been caused by wells that are open to multiple aquifers or have casings that have been corroded or broken. In some cases, poor natural confinement may have allowed cross-aquifer contamination. In some areas, contamination has been attributed to upconing of saltwater from the lower parts of the aquifers (McCoy, 1962).
In many cases, lateral saltwater intrusion was caused by the lowering of the water table in a large area through the use of drainage canals or other features, such as boat basins (Klein, 1954; Schroeder and others, 1958; Klein and Waller, 1985). Initially many canals did not have salinity control structures. As a result, saline-water intruded directly into the canals or intruded where the freshwater head around the canals decreased. After salinity control structures were added, the rates of landward intrusion of saltwater were reduced; however, this issue remains a concern because rates of ground-water withdrawal from coastal aquifers in southern Florida are increasing.
Examples of each source of contamination have been documented in southwestern Florida. The water-table aquifer (west coast) was contaminated by the lateral intrusion of saltwater from the Gordon River, which killed several rows of litchi trees in the Caribbean Botanical Gardens near Naples (McCoy, 1962). Wedderburn and others (1982) documented an area of contamination in the water-table aquifer (west coast) of Lehigh Acres that may have been caused by either upconing of saltwater from lower parts of the aquifer, or by contamination from leaking wells drilled into deeper aquifers. In the lower Tamiami aquifer, both lateral saltwater contamination from the Gulf of Mexico and cross-aquifer contamination through leaking wells have occurred (Schmerge, 2001). Declines in water levels in the mid-Hawthorn aquifer allowed downward movement of saltwater from the surficial aquifer system and upward movement of saltwater from the Floridan aquifer system to contaminate parts of the mid-Hawthorn aquifer (Fitzpatrick, 1986).
In the Biscayne aquifer, lateral intrusion of saltwater occurred in both Miami-Dade and Broward Counties. In southeastern Broward County, the saltwater front moved inland as much as 0.5 mi between 1945 and 1993 (Merritt, 1996). Koszalka (1995), in his examination of saltwater encroachment in eastern Broward County, showed that chloride concentration increased in monitoring wells east of the major well fields between 1980 and 1990.
In Miami-Dade County, the use of poorly regulated drainage canals caused 1 to 3 mi of saltwater encroachment along the coast, and also caused saltwater contamination 6 mi inland along the Miami Canal from 1904 to 1953 (Parker and others, 1955; Schroeder and others, 1958). Parker and others (1955) indicated that much saltwater encroachment occurred during a major drought between 1943 and 1946. This drought caused record low water levels in 1945. During a 27-month period that overlapped 1943-44, the interface moved inland by about 2,000 ft.
Improved control of the water-management system in Miami-Dade County has helped to mitigate saltwater encroachment. Between 1953 and 1995, the saltwater front in much of Miami-Dade County remained in about the same location. Some additional encroachment occurred between 1970 and 1971 (Klein and Waller, 1985) and between 1984 and 1990 in south-central and southeastern Miami-Dade County (Sonenshein, 1997). However, the amount of landward movement of the interface during these periods was minor relative to that which occurred prior to 1953.
In the surficial aquifer system in southern Martin and Palm Beach Counties, saltwater underlies several of the major well fields (Hittle, 1999). This creates a situation where upconing of the saltwater interface could occur under certain circumstances. Near the Hobe Sound Well Field, lateral movement of the saltwater interface is occurring in a sandy limestone production zone. Saltwater has intruded to within about 500 ft of a production well in that area (Hittle, 1999).
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