Description of Study Area:
Hydrogeology

Miami-Dade, Broward, and Palm Beach County are underlain by deposits of Holocene to Tertiary age, which comprise the surficial aquifer system, intermediate confining unit, and Floridan aquifer system. Holocene deposits, including inland freshwater peat and marl, coastal marsh peat, and beach deposits, form a thin veneer overlying the surficial aquifer system - the principal source of water for the tri-county area. The intermediate confining unit underlies the surficial aquifer system and separates it from the deeper Floridan aquifer system (fig. 6).

Figure 6. Geologic formations, aquifers, and confining units of the surficial aquifer system in central Miami-Dade County. From Fish and Stewart (1991). [larger version]

Large-scale deposition of freshwater peat deposits began about 5,700 to 4,800 years before present, infilling an elongated bedrock trough (fig. 7) that separates the Atlantic Coastal Ridge and Big Cypress Swamp (Gleason and others, 1984). Classified on the basis of plant components (Davis, 1943; Cohen and Spackman, 1984; Jones, 1948), Holocene peat deposits were formed in freshwater marsh areas in which the hydroperiod exceeded 270 days per year (Duever and others, 1994). Lengthy hydroperiods maintain anaerobic conditions that are needed to ensure accumulation of plant vegetation and minimize dry season oxidation, consolidation, and compaction of organic sediments.

Figure 7. (A) Configuration of bedrock surface underlying peat deposits and (B) distribution of peat deposits in southeastern Florida. Modified from Parker and others (1955) and Gleason and others (1984). [larger version]

Prior to development of the modern Everglades Agricultural Area, peat deposits ranged from 6 to 17 ft thick (Stephens and Johnson, 1951). The stage level of surface water overlying these peat deposits ranged from 18 to 24 ft near Lake Okeechobee. Peat deposits thinned where they extend southward and were reported to have been 2.5 ft thick in the southern part of the Everglades (Gleason and others, 1984). In the northernmost part of the Everglades, a calcareous marl sequence separates the peat deposit from underlying limestone bedrock; freshwater marl is considered indicative of a shorter hydroperiod that cannot support peat accumulation (Gleason and others, 1984). Drainage works in agricultural areas designed to control flooding south of the lake have contributed greatly to oxidation- and compaction-driven subsidence of peat deposits (fig. 8). Subsidence ranged from 3 to 9 ft in the Everglades Agricultural Area and was as much as 3 ft in nonirrigated areas to the south and southeast (Ingebritsen and others, 1999).

Figure 8. Everglades drainage conveyance features that contributed to a gradual loss of peat and peat soils through subsidence, compaction, and oxidation. Modified from Stephens and Johnson (1951) and Ingebritsen and others (1999). [larger image]

Hydroperiods characterized by lengthy flooding and anaerobic conditions supported the accumulation of dead plants into deposits of peat (Cohen and Spackman, 1984). Predevelopment peat deposits within the location of the modern Everglades Agricultural Area ranged from 6 to17 ft thick (Stephens and Johnson, 1951) with land-surface altitudes of 18 to 24 ft near Lake Okeechobee (South Florida Water Management District, 2003). The average thickness of peat deposits thinned to 2.5 ft in the southern Everglades (Gleason and others, 1984). Near Fort Lauderdale along the North New River Canal in eastern Broward County (fig. 7), the subsidence of peat deposits lowered land-surface altitude as much as 5 ft between 1915 and the early 1950s (Stephens, 1984).

The surficial aquifer system of southeastern Florida is wedge shaped, thickening eastward toward the Atlantic Ocean. The system ranges from less than 360 ft thick in western Broward and north-central Palm Beach Counties to greater than 400 ft thick in coastal areas of northeastern Broward and southeastern Palm Beach Counties (fig. 9). In the Miami-Dade County area, the surficial aquifer system ranges from 140 to 240 ft thick.

Figure 9. Base and extent of the surficial aquifer system in southeastern Florida. Modified from Fish (1988), Shine and others (1989), and Fish and Stewart (1991). [larger image]

The surficial aquifer system (Fish, 1988; Fish and Stewart, 1991) comprises a sequence of highly permeable limestone, quartz sand, shell, and terrigeneous mudstone of Pliocene to Holocene age. The sand content of the surficial aquifer system is high to the north and east; limestone and sandstone are more prominent to the south and west. Lithologic units that compose the surficial aquifer system are thin and lens like, and the complete stratigraphic sequence is not present at any one place. Some rock units interfinger, whereas other units are considered lateral equivalents of one another.

The surficial aquifer system has been divided into separate aquifers and semiconfining (leaky) units of quartz sand, terrigeneous mudstone, and limestone (Fish, 1988; Fish and Stewart, 1991). The Fort Thompson Formation, Anastasia Formation, and Key Largo Limestone yield the most water and constitute the prolific Biscayne aquifer (fig. 6). Defined as a sole-source aquifer (U.S. Environmental Protection Agency, 1979), the karstic Biscayne aquifer comprises highly permeable limestone and less-permeable sandstone and sand of Pleistocene and late Pliocene age. The Biscayne aquifer generally is considered to extend northward from southeastern Monroe County and southernmost Miami-Dade County into southern Palm Beach County. The Miami Limestone, Pamlico Sand, Fort Thompson Formation, Anastasia Formation, and Key Largo Limestone make up the Biscayne aquifer in Miami-Dade and Broward Counties. In southern Palm Beach County, the Anastasia and Fort Thompson Formations compose the Biscayne aquifer and are considered equivalent to the “cavity-riddled” zone (Fischer, 1980) or “zone of secondary permeability” (Swayze and Miller, 1984). The Biscayne aquifer does not extend into central and northern Palm Beach County; however, a moderately to highly transmissive limestone sequence forms its lateral hydrogeologic equivalent and has been defined as the non-Biscayne production zone (Shine and others, 1989).

In western Miami-Dade County, the eogenetic Biscayne aquifer is a triple porosity system characterized by: (1) a matrix of interparticle porosity and separate vug porosity; (2) touching-vug porosity that forms preferred, stratiform passageways; and less common, (3) conduit porosity formed by thin solution pipes, bedding-plane vugs, and cavernous vugs (Cunningham and others, 2003; 2006, in press). Karst features that include sinkholes and caves have been documented in Miami-Dade County along the Atlantic Coastal Ridge extending southward from south Miami to Everglades National Park (Cressler, 1993). Parker and others (1955, p. 269) have described cavernous permeability and solution holes within the Miami Limestone and Fort Thompson Formation that create turbulent flow conditions in some wells. Paleotopographic relief on karstic subaerial exposure surfaces (paleokarst) are well documented in the Lake Belt area of north-central Miami-Dade County (Cunningham and others, 2004). Although karst features have not been documented in Broward or Palm Beach Counties, the “cavity-riddled” zone (Fischer, 1980) is likely associated with karst dissolution.

Figure 10. Distribution of transmissivity within the surficial aquifer system in southeastern Florida. Modified from Fish (1988), Shine and others (1989), and Fish and Stewart (1991). [larger image]

Permeability of the surficial aquifer system is highest in areas where the Biscayne aquifer has been subject to extensive dissolution. Transmissivity of limestone-rich areas is greater than 1,600,000 ft²/d, but decreases to about 54,000 ft²/d where the aquifer mostly consists of sand (fig. 10). Because of high permeability and large withdrawals, the Biscayne aquifer is subject to contamination by saltwater intrusion. Yields of 1,000 to more than 7,000 gal/min are reported for some wells.

The intermediate confining unit separates the surficial aquifer system from the more deeply buried Floridan aquifer system—a 1,000-ft-thick sequence of poorly permeable carbonate rock, clay, clay-rich sand, and sand that form the Hawthorn Group (Miller, 1997, fig. 6.24). Permeable limestone and lesser dolomitic rocks compose the Floridan aquifer system, which mostly contains brackish to saline water in southeastern Florida. The Floridan aquifer system is not hydraulically connected to the Biscayne aquifer. Use of the Floridan aquifer system in Miami-Dade, Broward, and Palm Beach County is limited to aquifer storage and recovery (ASR), reverse osmosis, and wastewater injection within the deeply buried Boulder Zone.