Home Biscayne aquifer Hydrologic system Water quality Ground-water withdrawal Vulnerability of Biscayne aquifer Future problems of potable supplies Other drinking water sources Summary & References PDF Version

The hydrologic system of southeast Florida is significantly controlled and managed by the SFWMD. The District maintains and operates a system of canals, levees, pump stations, control structures in canals, and large water-conservation areas for flood control, water conservation, and salinity control. The levees impound freshwater in Lake Okeechobee and three water conservation areas (figs. 3 and 7). The levees also prevent overland sheet flow from the Everglades eastward through the agricultural and urban coastal areas.

Water releases from Conservation Area 3 (fig. 3) are made to the south during the year to sustain plants and animals in the Everglades National Park. During much of the dry season, water levels near the coast ate maintained by direct seepage from the water-conservation areas. During prolonged drought, the canals transfer water from Lake Okeechobee through the conservation areas to points of need along the coast. Lake water is shared by local municipalities, Everglades National Park, and agricultural interests.

The hydrology of southeast Florida was affected significantly before 1945 when the prime objectives were land drainage and reclamation. Water levels were lowered an estimated 5-6 ft in parts of southeast Florida as a result of the virtual uncontrolled drainage (Leach and others, 1972, p. 96). An adverse effect was saltwater intrusion into some municipal wells of the water-supply system of Miami.

The period after 1946 was one of flood protection, water control and conservation, and water management. Control structures were placed in all major drainage canals and water-conservation areas were established in the Interior. The control structure’s were opened to discharge flood water during rainy seasons, and were closed to prevent overdrainage of ground water, thereby slowing or halting saltwater intrusion. Water stored in the conservation areas helped to sustain high water levels near the coast.

Saltwater intrusion is an ever-present threat. The water-control and management practices have stabilized the inland movement of saltwater into the aquifer in most areas. Some inland migration still occurs, primarily in south Dade County.

The major problems facing water-management agencies are: (1) Satisfying the water demands of an increasing population while making sufficient water available for irrigation and maintaining aquatic environments; and (2) Upgrading and maintaining water quality in the Biscayne aquifer, the water conservation areas, and in the canals and lakes.

The regional flow of ground water in southeast Florida is seaward. Locally, however, the direction of flow may be influenced by drainage canals or well fields. Water levels are highest in the water-conservation areas and lowest along the coast, along uncontrolled reaches of canals, and in the centers of large municipal well fields. During rainy seasons control structures in canals are opened in order to discharge surplus water to prevent flooding in urban and agricultural areas. Opening the controls lowers the level in the canals, thereby, permitting more ground water to move to the canals and then seaward. Rainy season high-water levels of June 1968, one of the highest of record in southeast Florida, are shown in figure 7.

Figure 7. -- High ground-water levels, June 1968 (Leach and others, 1972, figure 32). [larger image]	Figure 8. -- Low ground-water levels, May 1962 (Leach and others, 1972, figure 30). [larger image]

At the beginning of the dry season control structures are closed and normally remain closed until the next rainy season. Figure 8 shows water levels near the end of the dry season of 1961-62, a near record low in southeast Florida. At that time the canals were picking up ground water in inland areas and transporting it to the coastal areas where canal levels were being maintained to help retard saltwater intrusion by infiltration from canals.

All the fresh ground water used in Dade and Broward Counties is from the Biscayne aquifer. Large capacity municipal wells commonly contain 40 to 60 ft of large diameter casing, 6 to 42 in., and open hole below the casing to 75 or 100 ft in Dade County, and deeper in Broward County. Yields are from 500 to more than 7,000 gal/min (gallons per minute); water-level drawdowns are small. Specific capacities from tests of several days' duration are from about 1,000 gal/min/ft of drawdown in Dade County to 200 gal/min/ft in north Broward and southeast Palm Beach Counties.

In the agricultural areas of south and interior Dade County, irrigation wells are usually rotary drilled to depths of 25 to 35 ft. Casing is not required because the aquifer is solely limestone. Hundreds of these wells are drilled at spacings as small as 300 ft. A large capacity irrigation pump mounted on a truck is moved from well to well and each is pumped for short intervals at rates of 500 to 1,000 gpm.

Thousands of small diameter (2-inch) wells are used throughout the year for irrigation of residential lawns and shrubs. These wells, about 20 to 50 ft deep, are normally pumped at rates of 25 to 40 gpm. In areas near the coast or adjacent to tidal canals no fresh ground water is available so residences use municipal water for lawn irrigation. Shallow wells of small diameter are also used for domestic supplies in areas not serviced by municipal systems.

Figure 9. -- Response of water levels in well G-86 to heavy rainfall, April 1942 (Parker and others, 1955, p. 216). [larger image]

The Biscayne aquifer is recharged principally by rainfall. The average annual rainfall in the lower cast coast area varies areally from 58 to 64 in; the annual extremes experienced are 29 in and 106 in (Leach and others, 1972, p. 9-10). The rainy season, June - October, contributes about 70 percent of the total. During this period heavy rains are associated with tropical disturbances and frequent short, local downpours. Light to moderate rainfall during the dry season is associated with cold fronts moving southward through Florida.

The oolitic limestone and sand that form the upper surface of the aquifer readily absorb rainfall and move it rapidly to the water table. The rapid response of the water table to rainfall in the Miami area is indicated in figure 9. Infiltration of rainfall is retarded but not prevented in interior parts of Dade and Broward Counties where thin marl deposits cover the surface, and along the shallow elongate depressions that dissect the urban area. Other sources of recharge to the aquifer are: (1) Connate ground water of inferior quality (Parker and others, 1955, fig. 221) along the upper reaches of the Miami, the North New River, and the Hillsboro Canals in Broward and Palm Beach Counties (northwest of the limits of the Biscayne aquifer) that is transferred eastward during dry seasons; (2) Water from Lake Okeechobee released by the SFWMD into the Miami Canal during the later weeks of the dry seasons to replenish the Miami area; and (3) Effluent from septic tanks, certain sewage treatment plant and disposal ponds scattered throughout the urban area.

Figure 10. -- Long-term hydrographs of selected wells, 1940-70 (Klein and others, 1975, p. 65-69); locations figure 7. [larger image]

Parker and others (1955) and Meyer (1971) estimated that 20 in of the approximately 60 in of annual rainfall in Dade County is lost directly by evaporation, about 20 in is lost by evapotranspiration after infiltration, 16 to 18 in is discharged by canals and by coastal seepage,and the remainder is utilized by man. Sherwood and others (1973. p. 49) indicated comparable values for Broward County. Thus, nearly 50 percent of the rainfall that infiltrates the Biscayne aquifer is discharged to the ocean, a reflection of the high degree of connection between the aquifer and the canal system.

Figure 10 (see fig. 8 for well locations) shows long-term water-level fluctuations in observation wells in the east edge of the Everglades in Dade County (Well G-596), in south Dade County (G-613), in north Dade County (S-18), and in inland Broward County (G-616). The decline of the minimum and average water levels and the increase in the magnitude of fluctuation in wells G-596 and G-613 are the result of improved drainage in south Dade County by the digging of Canals 1, 102 and 103 (fig. 7) after 1960. The response to drainage was similar in the vicinity of well G-616 in north-central Broward County. In contrast, minimum and average levels have increased in well S-18 in north Dade County, and the range in fluctuation has decreased.

One of the most important factors in flood control and water management in southeast Florida is the hydraulic connection between the Biscayne aquifer and the canals that dissect the aquifer (fig. 11). This inter-connection brings about benefits and problems. The benefits are (1) Flood prevention by the rapid removal of excess water to the ocean through operation of control structures in canals; and (2) the movement of ground water from the interior to the coastal areas where it can infiltrate the aquifer and maintain high water levels to retard saltwater intrusion. Problems related to good aquifer-canal inter-connection are: (1) The movement of saltwater into the aquifer along the coast and tidal canals during times of low water; and (2) the threat of pollutants entering the aquifer from the land surface or from canals, and moving long distances. The degree of connection can be affected by the amount of sediment that accumulates along the bottom of the canals. Thus, the rate at which a canal can recharge or discharge water to or from an aquifer may change over the years because of the accumulation or removal of channel-bottom sediments.

Data relating the response of the ground-water levels to changes in canal levels (Pitt, 1976, p. 5) indicate the connection between the aquifer and the canals ranges from excellent in the permeable limestone areas of south Dade County, to poor in north parts of Broward County where the permeability of the sand is comparatively low. The influence that canals have on fluctuations of ground-water levels after rain storms in south Dade County is shown by the short-period hydrographs in figure 12 (Klein and others. 1975, p. 65); the rate of recession of ground-water levels after storms was increased markedly after canals were constructed.

Figure 11. -- Hydraulic connection between a canal and an aquifer (Klein and others, 1975, p. 38). [larger image]	Figure 12. -- Response of ground-water levels in well S-182 to rainfall and canal drainage (after Klein and others, 1975, p. 65). [larger image]

Figure 13. -- Record high water levels in south Dade County, September 1960 (Klein and others, 1975, p. 63). [larger image]

The effectiveness of the canal system in removing ground water to prevent flooding in south Dade County is shown by comparing figures 7 and 13. Figure 13 shows the record high levels in south Dade County as a result of extremely heavy rainfall associated with two tropical disturbances in September 1960. Peak water levels exceeded 10.5 ft above sea level and much of south Dade County was flooded. In contrast, figure 7 shows ground-water levels after the comparable heavy rainfall in May - June 1968 when the canal-drainage system in south Dade County was operational. Drainage by the canals prevented the formation of the high, elongate ground-water mound shown in figure 13; rather, isolated mounds of lower elevation were formed in the intercanal areas.

The degree of effectiveness of the hydraulic connection is also indicated by the rate at which canal water can infiltrate downward and laterally into the aquifer during dry seasons when wafer levels in canals are higher than adjacent ground-water levels in the coastal area. Principal areas of concern are the canal reaches in the vicinity of the major municipal well fields. The canal-bottom sediments tend to retard but do not prevent the infiltration of canal water into the aquifer in these areas. Investigations by Meyer (1972) in the Miami Springs-Hialeah well-field area adjacent to the Miami Canal, showed that infiltration from the Miami Canal comprised 52 percent of the total water pumped in 1970 and 55 percent in 1971. Infiltration is also retarded by beds of fine sand below the oolitic limestone.

The response of ground-water levels to control-structure operations is shown in figures 14 and 15. The water-table contour pattern for July 21, 1959 shows that when the control was open ground water was moving toward all reaches of Canal C-2 and then to Biscayne Bay. The pattern for May 24, 1962 shows that when the control structure had been closed for several months, the water level in the canal was higher than adjacent ground-water levels and water was infiltrating the aquifer along all reaches, particularly to the north, toward the Alexander Orr well field. The hydrographs of C-2 Canal and wells within the basin (fig. 15) show the rapid response of ground-water levels to changes in canal levels, further evidence of the good connection between the aquifer and the canal.

Figure 14. -- Water levels in Canal 2 area, July 21, 1959 (Sherwood and Leach, 1962) and May 24, 1962 (Sherwood and Klein, 1963). [click on images above for larger version]

Figure 15. -- Response of ground-water levels to changes in water levels in Canal 2, July 17-26, 1959 (Sherwood and Leach, 1962, p. 15); well locations in figure 14. [larger image]

Saltwater intrusion affects the entire coastal zone of the Biscayne aquifer. Saltwater extends inland from the coast and along tidal streams and canals. It moves inland and upward in response to low ground-water levels, and seaward and downward in response to high ground-water levels.

The sequential maps in figure 16 show that most of the saltwater intrusion in the Miami area took place before 1946 when canal flow was virtually uncontrolled and ground-water levels were greatly lowered. Intrusion was halted during subsequent years as a result of installation of control structures in canals by the local and State water-control and water-management agencies. A readvance of saltwater occurred during the prolonged dry season of 1970-71 in the Miami and south Dade County areas as shown in figure 17.

The inland advance of saltwater in the Miami area was due to the fact that control structures in the Miami, Tamiami, and Coral Gables Canals were placed too far upstream for the effective control of saltwater movement (Parker and others, 1955, p. 705). The advance in the Fort Lauderdale area, similarly, was the result of placement of structures too far inland in the North New River and South New River Canals. The particularly wide zone of salty ground water in southeast Dade County was the result of the maze of canals and mosquito-control ditches which lowered water levels and permitted saltwater intrusion (Parker and others. 1955, p. 711).

Figure 16. -- Areas affected by saltwater intrusion 1904-1971 (Parker and others, 1955, Fig. 169). [click on images above for larger version]	Figure 17. -- Southeast Florida showing inland extent of saltwater intrusion, 1975. [larger image]