Executive Summary >Introduction Study Area & Objective Methods Results Summary & Recommendations Acknowledgements & References Statement of Work Appendix A Appendix B PDF Version

The Northwest Miami-Dade County Freshwater Lake Plan Area (commonly referred to as the "Lake Belt Area") is vital to the future planning and development of South Florida (Fig. 1). This 230-km² (89 mi²) area is located within one of the most environmentally sensitive parts of the state. It also provides half of the limestone mining resources used in the state every year. The majority of lands located within the Lake Belt Area are wetlands that were once part of the historical Everglades watershed and were part of the Shark River Slough headwaters. Historically, Shark River Slough was a deep-water slough that collected flows from the eastern portion of the Everglades, including the western side of the Atlantic coastal ridge, and moved that water to the southwest (Fig. 1).

Figure 1. Location of the Lake Belt Study Area adjacent to the Everglades National Park, Miami-Dade County, and Broward County in south Florida. The study area includes a portion of the Shark River Slough waterway to the west and Water Conservation Areas (WCA) to the northwest. [larger image]

In 1850, the federal government passed the Swamp and Overflowed Lands Act. Which granted the State of Florida the right to drain and develop the Everglades. By the 1930s, public works projects had successfully created 645 km (400 mi) of canals in the Everglades. At the eastern edge of the Everglades, a levee (East Coast Protective Levee, ECPL) was constructed to stop water from flowing east (Fig. 2). As a drought protection measure, shallow impoundments (Water Conservation Areas, WCA) were created in the Everglades to the west of the study area (Fig. 2). These impoundments are interconnected with a network of canals that traverses the study area and have gates (control structures) that control the flow of water through the area. This system of canals and impoundments provides a mechanism that helps speed water flow through the Everglades. Water stored in these areas provided a dry season water supply for the lower coast of Florida.

After completion of the ECPL and the adjacent WCA, lands east of the levee were cut off from surface water sheet flow and ground-water levels were lowered to provide flood protection. In addition, northeast Shark River Slough has received significantly reduced flow and the flora and fauna of the greater Everglades ecosystem, as a whole, has been impacted.

One of the consequences of the drainage has been an increase in ground-water flows from the WCA and Everglades National Park (ENP) to the urban drainage networks, to ultimately discharge into the ocean. One of the fundamental prerequisites for restoring the Everglades ecosystem is restoring the hydrology of the area. Hydrologic restoration efforts to date have focused on restoring more natural hydropatterns by implementing rainfall-driven water deliveries, improving water conveyance throughout the system, increasing storage capacity, and controlling the amount of water that is lost from the WCAs and Everglades National Park. Preventing water loss from the ecosystem through seepage is an integral component of restoration.

During the last several years, the greater Everglades ecosystem has been the focus of much attention. It has been a difficult task of balancing the needs of the mining industry and other land uses in the Lake Belt Area with the needs of the Everglades ecosystem. Hydropattern restoration is central to the recovery of several threatened and endangered species in the Everglades because of declining populations due to the disruption of their normal breeding patterns that resulted from disrupted water flow through the ecosystem. The recovery of these species will require the restoration of hydropatterns in the WCAs and ENP.

Figure 2. The location of water management canals and East Coast Protective Levee (ECPL) within the Lake Belt study area that provide flood protection by discharging storm runoff and Everglades seepage to Biscayne Bay. These canals also collect and discharge large volumes of seepage from the Everglades. (Modified from Northwest Dade County Freshwater Lake Belt Plan, 1997.) [larger image]

Water management is key to balancing the needs of Everglades Restoration, flood protection, and economics of the adjacent communities. Figure 2 shows the water-management canals within the study area. One of the by-products of flood protection has been the urbanization of significant areas of former Everglades located just east of the WCAs (Fig. 1, 3). Palm Beach and Broward Counties have been extensively urbanized to the edge of the Everglades in some areas. The pattern in north-central Miami-Dade County is different because the mining industry in north-central Miami-Dade County purchased large tracts of land during the 1960s and 1970s and since the mid 1970s the Dade County Comprehensive Plan has designated this area for open land uses and prohibited urban development (Fig. 3).

Over the next 50 years, significant change will come to the Lake Belt Area through major public and private investments. Mining interests will excavate limestone (Fig. 3) in accordance with federal, state and local permits, creating the largest network of freshwater lakes in south Florida. Miami-Dade County will implement wellfield protection regulations to protect the public water supply. The South Florida Water Management District (SFWMD) will acquire and restore wetlands necessary to mitigate for wetland losses that occur due to mining activities. The SFWMD and US Army Corps of Engineers (USACE) will implement the Comprehensive Everglades Restoration Plan (CERP, Fig. 4), including several major components that may be located within the Lake Belt Area. These future investments together compose an enormous opportunity to accomplish a number of public benefits related to Everglades restoration, water supply protection, public recreation, and the supply of building materials critical to the Florida economy.

Successful implementation of CERP will require a thorough understanding of the geology and hydrogeology of the Everglades, and specifically the Lake Belt Area. The primary concern to the SFWMD is surface- and ground-water flow, hydrology, and geology of the upper 61 m (200 ft) represented predominantly in this area by the Miami Limestone, Fort Thompson Formation and Tamiami Formation.

Figure 3. Map describing land uses for the Lake Belt Study Area. (Modified from Northwest Dade County Freshwater Lake Belt Plan, 1997.) [larger image]

Figure 4. Explanation of proposed Comprehensive Everglades Restoration Plan (CERP) Components within the Lake Belt Study Area. Note location of Krome Avenue and Tamiami Trail (Hwy 41) as the study area of Nemeth and others (2000). (Modified from Northwest Dade County Freshwater Lake Belt Plan, 1997.) [larger image]

Previous Studies

Florida has a long history of geologic investigation, for a thorough up-dated summary the reader is referred to Randazzo and Jones (1997). Herein is a much limited referencing to the most relevant geologic and hydrogeologic studies. There have been numerous studies of the Quaternary rocks of southern Florida and the Florida Keys (e.g. Ginsburg, 1956; Stanley, 1966; Hoffmeister and Multer, 1968; Enos and Perkins, 1977; Harrison and Coniglio, 1985; Lidz and others, 1991; Shinn and others, 1989; Ludwig and others, 1996). Pre-Quaternary geology of southern Florida has been less studied until recently (e.g. Johnson, 1986; Warzeski and others, 1996). These studies refer primarily to regional geologic framework, lithologic, and stratigraphic topics. The following are more specific studies to the Lake Belt Area.

USACE - 1953 Pump Test Study in Central and Southern Florida

The USACE - Jacksonville District conducted pumping studies in south Florida in the early 1950s to examine the 'underseepage quantities ' and geologic sections along the conservation areas and protective levee system in what is now called the Lake Belt Area of the eastern Everglades (USACE Serial No. 20). The report states that pumping tests demonstrate that there is good correlation between aquifer lithology and thickness, and underseepage quantities. High quantities of underseepage were found at locations where the foundation was hard, solution-riddled limestone. The geologic cross sections and descriptions were excellent and have been included in this report where appropriate.

Seepage Study of the East Everglades

Nemeth and others (2000) developed a coupled ground- and surface-water model (MODBRANCH) to estimate ground-water flow. This discussion relies heavily on the Nemeth and others (2000) report and the reader is referred to that report for details. The relative importance of Nemeth and others (2000) is due to its location in the central portion of this report's study area. Nemeth and others (2000) identified seepage characteristics were for L-31N, a canal and levee system that separates the East Everglades from urban areas (Fig. 2, 3). The seepage study site is 110 km² (43 mi²) and is located in the central portion of the Lake Belt Area study near the Tamiami Trail and Krome Avenue in Miami-Dade County, Florida (Fig. 4).

This site has been extensively monitored for ground-water stage, surface water stage and flow, and rainfall due to a municipal wellfield located within 2 km (1.2 mi) of the Everglades and concerns over the impacts of this wellfield on seepage rates.

The hydrogeology and some aquifer characteristics of the Nemeth and others (2000) study area are well defined based on previous studies by Causaras (1987) and Fish and Stewart (1991). There are two semiconfining layers of low-permeability limestone in the study area. The shallower semiconfining layer is about 0.6 m (2 ft) thick and is located at the top of the Fort Thompson Formation, just below the Miami Limestone. The deeper semiconfining layer averages about 1.5 m (5 ft) thick and has nearly the same slope as the upper surface of the Tamiami Formation (Causaras, 1987). Regional water-table maps indicate that ground-water flows from west to east beneath Levee 31N (Fish and Stewart, 1991).

Hydrogeology of the Surficial Aquifer System

The surficial aquifer system underlies central Miami-Dade County to a depth of about 55 m (180 ft) below sea level (Fig. 5). The unconfined Biscayne aquifer in the upper part of the surficial aquifer system consists of the Pamlico Sand, Miami Limestone, Anastasia Formation, Key Largo Limestone, and the Fort Thompson Formation all of Pleistocene age as well as contiguous, highly permeable beds of the Tamiami Formation of Pliocene and Miocene ages. Fish and Stewart (1991) use permeability as a means to define the basal contact of the Biscayne aquifer beneath Miami-Dade and Broward Counties. The contact is where the Fort Thompson Formation, Anastasia Formation, or Key Largo Limestone grade laterally into less-permeable facies. If there are contiguous, highly permeable limestone or calcareous sandstone beds of the Tamiami Formation, the lower boundary is the transition from these beds to subjacent sands or clayey sands. Where the contiguous beds of the Tamiami Formation do not have sufficiently high permeability, the Fort Thompson Formation, Anastasia Formation, or Key Largo Limestone is the base of the Biscayne aquifer. In general, the basal contact extends from about 13 to 27 m (44 to 84 ft) below sea level in the southwestern and northeastern corners of the Lake Belt study area (Fig. 5). Below the Biscayne aquifer are less permeable limestone, sand, and sandstone of the Tamiami Formation.

The hydraulic conductivity is estimated to be 29,000 ft/d (feet per day) in the Biscayne aquifer and 470 ft/d in the Tamiami Formation below the aquifer (Fish and Stewart, 1991).

The Biscayne aquifer is recharged by rainfall in upland areas. This recharge infiltrates directly to the aquifer or by surface water that seeps downward through wetland sediments to the aquifer. In 1953, Levees 31N, 30 (to the north) and 31W (south of the study area) were constructed to store excess water during the wet season and transfer the excess water to areas of need during the dry season (Fish and Stewart, 1991).

Figure 5. Hydrogeologic section showing formations, aquifers, and confining units of the surficial aquifer system in northcentral Miami-Dade County, (modified from Fish and Stewart, 1991). Section extends along Tamiami Canal (C-4 Canal). See Figure 2 for location. [larger image]

South Florida Stratigraphic Studies

Stratigraphic investigations of Miocene-Pliocene siliciclastics of southern Florida prior to the 1990s focused on lithostratigraphy (Peck and others, 1979; Wedderburn and others, 1982; Peacock, 1983; Missimer, 1984; Knapp and others, 1986; Scott, 1988; Smith and Adams, 1988; Missimer 1992). More recently, stratigraphic framework studies of the Miocene-Pliocene of southern Florida include Evans and Hines (1991), Warzeski and others (1996), Missimer (1997), Cunningham and others (1998), Guertin and others (1999), Missimer (1999), and Guertin and others (2000). In a regional stratigraphic study of southern Florida Reese and Cunningham (1999), and Cunningham and others (2001) provide excellent reviews of literature and stratigraphic discussion.

Reese and Cunningham (1999) have described the lithostratigraphy, geology and hydrogeology of south Florida including the study area (Fig. 6). Following on the earlier report, Cunningham and others (2001) integrated lithologic and paleontologic data from 89 test coreholes and cuttings from 18 test wells to map the lithostratigraphic boundaries, and developed facies associations and sequence stratigraphy of southern Florida. In their report, they use established chronologies integrated with new biostratigraphic data to describe depositional sequences of the proposed Long Key Formation (Cunningham and others, 1997) within the Peace River Formation (Hawthorn Group) and Ochopee Limestone and Unnamed Sand Members of the Tamiami Formation. Cunningham and Aviantara (2001) used ground-penetrating radar, digital optical borehole images, and cores to characterize the Biscayne aquifer. Their findings indicated that conduit-flow pathways within the Fort Thompson Formation are produced by well-connected, solution-enlarged pore space. These solution-enlarged pore spaces vary as a result of depositional textures, diagenesis in a meteoric-water system, and vertical position within the stacked lithofacies that combine to form each upward-shallowing cycle. Each depositional facies has unique solution features that are characteristic of the unit and can facilitate accurate assessment of the depositional and diagenetic facies. This would indicate that even though the rock-fabric facies within the Fort Thompson Formation stratigraphic cycles is moderately variable it is characteristically conformable within much of the Lake Belt study area.

Figure 6. Lithostratigraphic units recognized in south Florida by Reese and Cunningham (1999). Chart shows generalized geology, and relationship with hydrogeologic units as modified from Olson (1964), Hunter (1968), Miller (1990), Missimer (1990), and Weedman and others (1990). [larger image]

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