Development of Water-Management System and Impact on the Hydrology of Southeastern Florida : Water-Table Fluctuations of the Surficial Aquifer System

The surficial aquifer system generally is considered to contain water under unconfined or water-table conditions. Discharge occurs by seepage to canals, the Atlantic Ocean, and evapotranspiration. Canal seepage has a major effect on water levels due to the highly transmissive nature of the surficial aquifer system in southeastern Florida. Canals display both gaining or losing flow characteristics depending on whether ground water is discharged from the surficial aquifer system to canals or surface water in canals seeps into the aquifer system (fig. 37). Ground-water levels are highest near the impounded water-conservation areas and lowest near the coast; consequently, the hydraulic gradient in southeastern Florida is seaward and discharge to the Atlantic Ocean occurs near the coast, except under extreme drought conditions in Miami-Dade County (Schroeder and others, 1958; Sherwood and Klein, 1963).

Figure 37. Canals of southern Florida exhibiting both (A) gaining and (B) losing characteristics depending on relative stage level. Modified from Klein and others (1975). [larger image]

For this study, a comparative analysis of average water-table conditions was used to evaluate temporal and spatial changes in the flow system resulting from drainage of the former wetland areas by a surface-water conveyance system. “Average-condition” maps were prepared to illustrate the evolution of the water-management system and its impact on the configuration of the water table in southeastern Florida. The major features of the flow system in the surficial aquifer system, illustrated in a series of potentiometric surface maps showing average conditions between 1940-44, 1970-74, and 1990-94 (figs. 38-40), were prepared using ground-water level and stage data from the files of the USGS, SFWMD, and the LWDD. In southeastern Florida, water levels are highest in September or October (end of wet season) and lowest during April or May (end of dry season). Instead of portraying synoptic conditions, a 5-year interval of time was used to dampen or smooth the effect of unusually wet or dry months.

Figure 38. Average water levels in the surficial aquifer system in southeastern Florida during (A) April 1940-44 and (B) October 1940-44. Derived from data and files of the U.S. Geological Survey, South Florida Water Management District, and Lake Worth Drainage District. [larger image]

Figure 39. Average water levels in the surficial aquifer system in southeastern Florida during (A) April 1970-74 and (B) October 1970-74. Derived from data and files of the U.S. Geological Survey, South Florida Water Management District, and Lake Worth Drainage District. [larger image]

Figure 40. Average water levels in the surficial aquifer system in southeastern Florida during (A) April 1990-94 and (B) October 1990-94. Derived from data and files of the U.S. Geological Survey, South Florida Water Management District, and Lake Worth Drainage District. [larger image]

Predevelopment Water-Table Conditions and Ground-Water Flow

Figure 41. (A) City of Miami water well in 1899(?) at the Miami River headwaters, and (B) unidentified man withdrawing water from a freshwater spring in Biscayne Bay in 1890. Photographs courtesy of Historical Museum of South Florida, Ralph Middleton Monroe Collection. [larger image]

Few data are available to evaluate the configuration of the predevelopment water table at the beginning of the 20th century, and only limited data document conditions prior to 1939. The water table probably formed a subtle reflection of Atlantic Coastal Ridge topography, with stage levels in the adjoining Everglades of sufficient height to discharge surface water through the intervening transverse glades. In Miami-Dade County, Everglades wetland areas were reported to lie within 3 mi of the coastal community of Coconut Grove (fig. 13) in 1887 (Shaler, 1890), with a reported surface-water stage of 10 ft above NGVD 1929 (Parker and others, 1955, p. 580). In Palm Beach County, historic surface-water outflows from marsh areas exited along the Loxahatchee-Hungryland Sloughs or from an outlet of the Hillsboro Marsh (Land and others, 1973). Fresh ground water discharged along the Atlantic Coastal Ridge shoreline and offshore as submarine springs. Springs reportedly discharged nearshore as freshwater boils in the shallow waters of Biscayne Bay; freshwater discharge in some areas occurred in sufficient quantities to permit mariners to collect fresh drinking water directly from the bay in a boat (fig. 41B). Discharging freshwater springs were labeled on the U.S. Coast and Geodetic Survey (1896) map of Biscayne Bay (Parker and others, 1955, Kohout and Kolipinski, 1967).

Peat and muck deposits were an extremely important predevelopment component of Everglades surface- and ground-water hydrology. These deposits functioned as a storage reservoir to a water column that extended upward from the underlying aquifer, helping to maintain a higher water table and to prolong the hydroperiod (U.S. Department of the Interior, 1969; Stephens, 1984). Assuming peat deposits were fully saturated during predevelopment conditions, the combined height of the water column contributed largely to coastal ground-water hydrology, thereby restricting movement of a coastal saltwater interface. The SFWMD’s Natural Systems Model, a two-dimensional coupled surface- and ground-water flow model designed to simulate predevelopment response of the Everglades system (MacVicar and others, 1984), provides some insight to possible predevelopment ground-water flow conditions. Simulated hydraulic gradients were greatest near the Atlantic Coastal Ridge with ground water flowing to the coast and to the west (Fennema and others, 1994).

Average 1940-44 Water-Table Conditions

The April and October 1940-44 maps (fig. 38) show conditions in the surficial aquifer system before the extensive, modern-day drainage and conveyance system was developed as part of the Central and Southern Florida Flood Control Project of the 1950s and 1960s. The five major canals (St. Lucie, West Palm Beach, Hillsboro, North New River, and Miami Canals) that extend from Lake Okeechobee to the southeastern coast and the Tamiami Canal were the largest existing man-made hydrologic features. Smaller and less extensive canal systems also were present in Miami-Dade and Palm Beach Counties. An extensive levee system, gated canal structures along the coast to prevent saltwater intrusion, and impoundment structures had not yet been built.

Everglades drainage efforts remained uncontrolled through the early 1940s, causing a lowering of water levels in the surficial aquifer system underlying the Atlantic Coastal Ridge, in wetland areas adjoining the ridge, and along major drainage canals. The uncontrolled drainage contributed to movement of saline water into canals and the surficial aquifer system. The lowest water levels in the surficial aquifer system, recorded in 1945, were attributed to the combined effect of long-term uncontrolled canal drainage and an extended drought (Schroeder and others, 1958, fig. 14)

South of the Tamiami Canal in Miami-Dade County, ground water discharged to the east and southeast; an eastward hydraulic gradient characterized the water table between the Tamiami and Miami Canals. The highest reported water levels between 1939 and 1946 were on September 23, 1940, along the Atlantic Coastal Ridge between the Tamiami and Miami Canals (Parker and others, 1955, p. 211). The water table formed a large mound with ground-water movement toward both the Everglades and the coast.

A cone of depression adjoining the Miami Canal surrounded the City of Miami Well Field during the early 1940s. Now referred to as the Hialeah-Miami Springs Well Field (fig. 18, Miami-Hialeah), its primary source of recharge was the Miami Canal. The LWDD in Palm Beach County generally maintained higher water levels than those in Broward and Miami-Dade Counties (Parker and others, 1955, p. 173). Sparse water-level control points limited analysis of conditions during the 1940-44 time frame north of the LWDD.

Average 1970-74 Water-Table Conditions

The 1970-74 water-table maps (fig. 39) represent average hydrologic conditions in the surficial aquifer system following completion of the Central and Southern Florida Flood Control Project. Engineered features that affected the altitude and configuration of the water table in the surficial aquifer system during this period include coastal gated canal structures, extensive levees and surface-water impoundments (water-conservation areas), pump systems, improved secondary canals, and the construction/expansion of several large well fields. By the 1970s, hydraulic gradients were greatest near landward conservation areas (Fennema and others, 1994), whereas the lower ground-water levels and shallower coastal hydraulic gradients increased potential for saltwater intrusion (Leach and others, 1972).

The highest water levels in Miami-Dade County typically occurred in the water-conservation areas. South of the Tamiami Canal, the hydraulic gradient was predominately to the south and southeast, whereas north of the Tamiami Canal, the hydraulic gradient was eastward. In Broward County, water was diverted from major canals by free flow or was pumped into local canals to maintain water levels in the surficial aquifer system, and to artificially recharge local well fields by canal seepage. Water levels were highest in Broward County near the water-conservation areas and also in the northeastern part of the county (fig. 39). In the latter area, surface water was pumped from the Hillsboro Canal into secondary canals, creating an artificial mound (Fish, 1988, p. 68). A steep hydraulic gradient separated this mound from the northwestern edge of the Prospect Well Field (fig. 19).

In Palm Beach County, water levels were highest in Water Conservation Area 1, the LWDD, and the northwestern part of the county (figs. 35 and 39). Gated canals and lateral canals maintained 16-ft-high water levels in the LWDD. These multipurpose canals were used to drain excess surface water during the wet season, maintain water levels in the surficial aquifer system, and minimize saltwater intrusion caused by coastal well withdrawals.

Closed depressions in the water-table surface evident in the 1970-74 maps (fig. 39) and also the 1990-94 maps (fig. 40), discussed next, are attributable to large water withdrawals from municipal well fields in Miami-Dade and Broward Counties. These cones of depression are associated with withdrawals from the Miami-Hialeah, Alexander Orr, Southwest, Northwest, Dixie, Prospect, and Pompano Beach Well Fields (figs. 19 and 20). Withdrawals from other smaller well fields, including those in Broward and Palm Beach Counties, have resulted in similar but smaller hydrologic features not evident on the water-table maps of this scale.

Average 1990-94 Water-Table Conditions

By 1990-94, a water-management system had evolved to include hydraulic control structures and a system of levees, impoundments, and gated conveyance canals that collectively affected the altitude and configuration of the water table. The 1990-94 water-table maps (fig. 40) illustrate average conditions during a dry-season month (April) and wet-season month (October). The overall configuration of the water table in the surficial aquifer system is similar to that seen for average conditions during 1970-74 (fig. 39). For example, the highest water levels are maintained in the water-conservation areas, and the movement of ground water is east and southeast toward the coast. In Miami-Dade County, withdrawals from the Northwest Well Field (fig. 20), constructed in the early 1980s, helped to lower water levels. Well-field withdrawals are in evidence by the occurrence of a cone of depression not previously seen. The Northwest Well Field was built as an alternative municipal water supply following contamination concerns associated with Hialeah-Miami Springs supply wells. Withdrawals were curtailed in the Hialeah-Miami Springs Well Field from 1984 to 1992 (Sonenshein and Koszalka, 1996), but reinitiated following the completion of a new treatment facility. Withdrawals from the Northwest Well Field have been subsequently reduced.

The configuration of the potentiometric surface in Broward County in 1990-94 (fig. 40) is largely the same as in 1970-74 (fig. 39). Prior to 1985, agricultural users in the Deerfield Beach area relied on water pumped from the Hillsboro Canal to supply irrigation canals, also helping to recharge the surficial aquifer system. In northwestern Palm Beach County, water levels exceeding 20 ft were maintained. Stage levels in Water Conservation Area 1 and the LWDD canals (fig. 35) were both greater than 16 ft during the wet and dry seasons. An increase in municipal and domestic withdrawals, combined with regulatory change in canal stage, have cumulatively affected the altitude and configuration of the water table in Palm Beach County (Miller, 1988, p. 190). The City of Boca Raton withdraws water from the Hillsboro Canal to maintain coastal ground-water levels, prevent saltwater intrusion, and recharge the surficial aquifer system in the vicinity of the city’s western well field (McKenzie, 1995).