Development of Water-Management System and Impact on the Hydrology of Southeastern Florida : Assessment of Saltwater Intrusion

Under predevelopment conditions, the freshwater-saltwater interface in southeastern Florida extended close to the coastline, and freshwater was discharging from springs on the floor of Biscayne Bay, where the aquifer extends and is hydraulically connected to the Atlantic Ocean (Parker and others, 1955). The natural balance between freshwater and saltwater was first altered following the emplacement of canal drainage works accompanied by the oxidation and compaction of peat soils that functioned as a sponge, helping to maintain higher ground-water levels in the underlying aquifer. Designed to control deadly catastrophic flooding in the late 1920s, completion of the protective Hoover Dike at Lake Okeechobee in 1937 signaled the demise and subsidence of a large natural peat storage reservoir, funneling surface water into narrow channels that debouched surface water at the coastline. Natural overland flow processes served to supply water to the surficial aquifer system; however, once severed, saltwater intrusion issues accelerated. Although ground-water movement extends into the shallow surface marine environment, as recently documented in southern Biscayne Bay between Coconut Grove and Homestead (Langevin, 2001), only a limited part of the present-day submarine ground-water discharge to Biscayne Bay is fresh—most is recirculated seawater (Kohout and Kolipinski, 1967, Langevin, 2001).

Saltwater intrusion has been an issue of concern in southeastern Florida since the early 1930s, with its most severe effect evident during the 1940s. Following a lengthy drought and prolonged period of uncontrolled drainage, the water supply in Miami-Dade County experienced its greatest threat by saltwater contamination in May-June 1945, also recording its lowest water levels (Leach and others, 1972). Uncontrolled drainage of the Everglades, peat and muck soil loss due to oxidation-compaction processes, associated lowering of the water table within the peat mantle and underlying surficial aquifer system, and coastal well-field withdrawals have collectively contributed to the landward advance of saltwater in the surficial aquifer system as exemplified along the coastal reach of the Miami Canal. Saltwater intrusion concerns of the 1930s and 1940s centered mostly on contamination of the Miami-Hialeah Well Field (fig. 18) and other private wells for agricultural supply with little regard to the impact on the local ecology. In Broward County, saltwater intrusion became an issue during the drought of 1945-46 (Parker and others, 1955, p. 677) because of tidal canal seepage near the Fort Lauderdale (Dixie) Well Field (fig. 18).

Figure 50. Line depicting the landward limit of the saltwater interface in southeastern Florida in 1996. Modified from Broward County Department of Natural Resource Protection (1994), Sonenshein (1997), Fitterman and Deszcz-Pan (1999), and Hittle (1999). [larger image]

The mechanics and character of saltwater intrusion in the surficial aquifer system in southeastern Florida have been evaluated extensively in previous studies. Some of the researchers include Parker (1945), Parker and others (1955), Klein (1957), Kohout (1960; 1964), Cooper (1959), Leach and others (1972), Hughes (1979), Klein and Waller (1985), Koszalka (1995), Sonenshein and Koszalka (1996), Merritt (1996b), Sonenshein (1997), and Langevin (2001). The position of the present-day saltwater interface (fig. 50) is a result of three principal mechanisms: (1) lateral landward movement of seawater in the surficial aquifer system from the Atlantic Ocean, (2) seepage from tidal canals containing saline water, and (3) upconing of relict seawater in response to well-field withdrawals.

In coastal areas under natural conditions, saltwater is present only near the shoreline and is balanced by a relatively higher onshore freshwater table having relatively high levels. Under equilibrium conditions, a slightly higher column of freshwater is necessary because freshwater is less dense than saltwater. Assuming static ground-water flow conditions of the Ghyben-Herzberg principle, there is approximately a 40-ft column of freshwater below sea level for each foot of fresh-water above sea level. The lowering of Everglades surface-water stage levels by gravity drainage in combination with intensive municipal withdrawals for water supply has helped to create an imbalance between freshwater and saltwater levels, contributing to the landward movement of saltwater.

Recognizing that the position of the saltwater front in the Biscayne aquifer during the 1950s was dynamically stable seaward of a position predicated by the Ghyben-Herzberg principle, Kohout (1964) concluded that dynamic equilibrium assumptions of static saltwater were incorrect. He determined that freshwater discharged coastward and upward beneath Biscayne Bay on the basis of fluid pressure and chloride concentration field observations, a mappable zone of diffusion, and flow-net analyses. Saltwater flows back in a landward cycle from the seabed floor into the zone of diffusion, then upward and back to the sea floor (figs. 51 and 52). Tidal fluctuations, freshwater head change, and head loss within the landward cycle of saltwater movement help to minimize intrusion in a coastal aquifer (Cooper, 1959).

Figure 51. Cross sections through the Cutler Ridge area showing (A) lines of equal chloride concentration, and (B) results from a flow-net analysis, September 18, 1958. From Kohout (1964). [larger image]

Figure 52. Cross section through the Silver Bluff area showing the zone of diffusion. From Kohout (1960). [larger image]

Saltwater encroachment in the surficial aquifer system in northern and central Miami-Dade County and southern Broward County (figs. 53-56) has been well documented by several researchers, including Parker and others (1955), Kohout (1960), Leach and Grantham (1966), Leach and others (1972), Hughes (1979), Merritt (1996b), and Sonenshein (1997). Uncontrolled drainage from the principal conveyance canals between 1904 and the early 1940s lowered ground-water levels nearly 6 ft in the Everglades area, providing considerable opportunity for increased lateral intrusion into the surficial aquifer system (fig. 53). Large withdrawals from the Hialeah-Miami Springs Well Field (fig. 50) further contributed to canal seepage and the intrusion of tidal seawater into the Miami Canal. Seepage of saltwater from coastal tidal canals further contributed to saltwater intrusion problems.

Figure 53. Saltwater intrusion (shown in red) near the Miami Canal in Miami-Dade County, 1904-43. [larger image]

Figure 54. Saltwater intrusion (shown in red) near the Miami Canal in Miami-Dade County, 1946-62. [larger image]

Figure 55. Saltwater intrusion (shown in red) near the Miami Canal in Miami-Dade County, 1969-95. [larger image]

Figure 56. Saltwater intrusion in southern Broward and northern Miami-Dade Counties in 1945, 1969, and 1993. Modified from Merritt (1996b). [larger image]

During the 1940s, saltwater intrusion was mitigated mostly through use of temporary sheet-pile dam control structures. Salinity control structures in Miami-Dade County were constructed by 1946 to prevent overdrainage and increase water levels in the coastal part of the surficial aquifer system, and also to allow the timely release of water during floods. Ultimately, these measures helped reverse saltwater intrusion in central Miami-Dade County. The construction of surface-water control structures and improved water-management practices have considerably reduced the area of saltwater contamination from its maximum extent during the 1950s and early 1960s (fig. 54). During the 1970s, Tamiami Canal salinity coastal structures were replaced with a new structure located 3 mi downstream, further reducing the intrusion of tidal canal water south and west of the Hialeah-Miami Springs Well Field (figs. 50 and 55) according to Klein and Ratzlaff (1989). During the 1980s and 1990s, chloride concentrations continued to decline as indicated in monitoring wells near the well field (fig. 55) because of a reduction in well withdrawals (table 1) as reported by Sonenshein and Koszalka (1996).

The temporal saltwater encroachment near Miami Canal, Miami-Dade County, has been well illustrated (figs. 53-55). Sparse well data with a suitable chloride sampling period and wide distribution, however, limit preparation of a similar temporal map series over a larger multicounty area. Minor divergence between the mapped position of the 1984 interface (Klein and Waller, 1985) and the 1995 interface (Sonenshein, 1997) is attributable to interpretive differences resulting from limited well control data and not caused by a change in long-term hydrologic conditions.

The saltwater interface extended no more than 1 mi from the coastline in most of Broward County prior to 1948, with the greatest intrusion occurring along the North New River Canal (Vorhis, 1948). Movement of the interface in Broward County is partly attributed to Central and Southern Florida Flood Control Project drainage canal construction during the 1950s and 1960s. Enhanced drainage helped to lower the ground-water table, and convert marginal wetland areas to suitable land for development. The saltwater interface lies well inland along the North and South New River Canals in east-central Broward County (fig. 56) where control structures have been constructed several miles inland to provide access for boating.

Landward movement of the interface in southeastern Broward and northeastern Miami-Dade County is evident in figure 56 (Merritt, 1996b). Saltwater intrusion forced closure of the Sunny Isles and East Side Well Fields in 1972 and 1977 (Merritt, 1996b). Additionally, tidal canal seepage and coastal saltwater intrusion have increased chloride concentrations near the Dania Cutoff, South New River, and Hollywood Canals (Merritt, 1996b, p. 25), affecting supply wells at the Dania, Broward 3A, and Hollywood Well Fields.

Water managers in southeastern Broward County, cognizant of low topography and its effect on saltwater encroachment, have closely monitored wells located east of the Hallandale Well Field since 1970. The westward progression of the saltwater interface (fig. 57) seems to maintain a similar physical structure to the “diffuse” coastal transition zones mapped near Cutler Ridge and Silver Bluff in Miami-Dade County (figs. 51 and 52). At the Hallandale Well Field (fig. 57), the 1,000-mg/L chloride concentration line is nearly horizontal at depths of about 75 and 160 ft below land surface but vertical at greater depths, indicating vertical and horizontal concentration gradients, respectively. Merritt (1989) suggested that these chloride concentration gradients are indicative of preferential high-permeability flow zones.

Figure 57. Movement of the saltwater interface near the Hallandale Well Field in southern Broward County, 1976-99. Derived from data and files of the U.S. Geological Survey. [larger image]

Coastal well field withdrawals, elevated ground-water levels, and the effect of the canal drainage system are principal factors influencing the movement of the saltwater interface in Palm Beach County. For example, the LWDD maintains ground-water levels as high as 15 ft above NGVD 1929 using a system of pumps, control structures, and equalizing canals (fig. 50). Accordingly, the influence of saltwater intrusion is substantially less in this area; however, saltwater intrusion has occurred in other areas of Palm Beach County near coastal well fields (fig. 50). The interface is reported to underlie the Highland Beach, Delray Beach, and Boynton Beach Well Fields and the coastal well fields of Boca Raton (Hittle, 1999). In response to increased salinity in coastal wells, the City of Boca Raton reduced withdrawals in those wells and has increased production from supply wells in the inland well fields. Saltwater intrusion also has been reported near the Tequesta Well Field in northeastern Palm Beach County (Scott and others, 1977), where the position of the interface probably reflects the effect of Loxahatchee River drainage lowering water levels in the aquifer (Hittle, 1999).