The South Florida Ecosystem Restoration Program is an intergovernmental effort, involving a number of agencies, to reestablish and maintain the ecosystem of south Florida. One element of the restoration effort is the development of a firm scientific basis for resource decision making. The U.S. Geological Survey (USGS), one of the agencies, provides scientific information as part of the South Florida Ecosystem Restoration Program. The USGS began their own program, called the South Florida Ecosystem Program, in fiscal year 1995 for the purpose of gathering hydrologic, cartographic, and geologic data that relate to the mainland of south Florida, Florida Bay, and the Florida Keys and Reef ecosystems.

Over the years, the construction of canals and levees has altered the natural hydrologic conditions of the Everglades. The canals and levees were constructed to convey water, prevent flooding, and store water in conservation areas for future use. The U.S. Army Corps of Engineers (COE) in Jacksonville, Fla., is planning to construct gated spillways and culverts to allow for the restoration of natural sheetflow conditions to Everglades National Park (ENP). These proposed changes may further affect the hydrologic conditions of ENP and other parts of the ecosystem, thus leading to the following questions:

• Is ground water flowing to Biscayne Bay a significant component of the water budget in south Florida?
• Would the quantity of ground water flowing to Biscayne Bay be greatly affected by changes in the operation of gates and control structures in canals?
• How much change in ground-water discharges to Biscayne Bay has occurred due to modifications to the hydrologic system?

Federal, State, and local agencies need to know the effect that changes in the operation of gates and control structures in canals will have on the water-budget components of the natural water systems in south Florida, including ground-water flow to Biscayne Bay.

The objectives of this project are to define the geohydrologic characteristics of the surficial aquifer system in parts of Biscayne Bay and apply numerical modeling techniques to estimate ground-water discharges to the bay. An assessment will also be made describing the potential effects of some water-management operations in canal and water-conservation areas on the rates of ground-water discharges to Biscayne Bay.

Quantification of ground water flowing to Biscayne Bay is needed as input to two interagency projects: the South Florida Ecosystem Restoration Program and the Biscayne Bay Feasibility Study. The principal objective of the Biscayne Bay Feasibility Study is to investigate ongoing construction/dredging projects and propose solutions to alleviate adverse factors that affect the bay and to aid in the development of guidelines for future management of the natural resources of Biscayne Bay. The Biscayne Bay Feasibility Study includes the implementation of a surface-water circulation model which will be developed by the Waterway Experimental Station of the COE. Quantification of ground-water discharges to Biscayne Bay is needed as input to the bay water circulation model.

Several surveys during the late 19th and early 20th centuries describe the occurrence of large quantities of ground-water flow to Biscayne Bay by way of underground channels or conduits (Parker and others 1955). Some of the most notable quotations from these surveys include the following:

• "Nearly all the rainwater appears to find its way by underground channels to the sea, where we can note its emergence in great springs" (Shaler, 1890, p.146).
• "Great springs of constant flow emerging in large numbers [are present] along the shore" (Griswold, 1896, p.53).
• "All along this shore, Coconut Grove [in the vicinity of Coral Gables], there are places where the freshwater comes up through the rocks under the saltwater with quite a head. It no doubt comes from the Everglades by subterranean passages" (Willoughby, 1898, p.65).
• "One of these springs was estimated to be flowing at a rate of 100 gallons per minute in the Coconut Grove area [in the vicinity of Coral Gables]" (Fuller, 1904, p.266).
• "The abundant spring water off the shore, while organically pure, and beautifully clear and tasteless, was somewhat hard" (Munroe, 1980, p.118 and 218).
• Sewell (1933) indicated that fresh ground water was obtainable in wells from 50 to 60 feet deep at the mouth of the Miami River.

Figure 1 -- A flowing well in Biscayne Bay. Photograph from Kohout (1967).

The construction of the drainage and flood-control network in southeastern Florida began during the early 20th century for the purpose of managing the water resources of the area. This drainage canal network affected the hydrologic pattern in southeastern Florida by replacing sheetflow with canal flow, thereby significantly reducing the altitude of the water table and diminishing ground-water flow to Biscayne Bay. This led to the inland movement of the saltwater interface as described by Parker and others (1955, p.589). Kohout (1960) indicated that there was still ground water discharging to the bottom of Biscayne Bay, but no quantification of the amount of ground-water discharges to the bay was made at the time. A well discharging silty ground water to the surface of Biscayne Bay (Kohout, 1967) is shown in figure 1. In 1967, Kohout estimated discharges to the bay in the Cutler Ridge area by assuming Darcian flow and considering the tidal cycle; he estimated that 210 cubic feet per square foot of flow section area was discharged during a 12.5-hour tidal cycle.

Data compiled during this project will include the collection of water-level data, core samples, and water-quality data (especially chloride and specific conductance data) from observation wells and test holes. The wells and test holes will be drilled and installed offshore and nearshore in an attempt to define the freshwater-saltwater mixing zone and the nature, location, and quantity of ground-water flow to Biscayne Bay. Core samples will be collected and analyzed to determine the geologic and hydrogeologic characteristics of the subsurface material underlying the bay and near the coast. Gamma and electromagnetic induction resistivity logs will be run at selected wells to aid in the determination of the geohydrologic horizons and the definition of salinity profiles. A model analysis will be performed to further define the saltwater-freshwater mixing zone and to estimate ground-water flow to Biscayne Bay. A preliminary ground-water flow model and a refined model will be used for the analysis. Both models are described in detail in the next sections of this paper. Some historical and seasonal simulations (including near predevelopment conditions) will be performed. Additionally, an attempt will be made with the model to estimate the location of the saltwater-freshwater zone. These data will be made available to various Federal, State, and local agencies.

Preliminary Model

A preliminary ground-water flow model will be constructed using the SWIP code (INTERA Environmental Consultants, Inc., 1979) as modified by Merritt (1994). The model solves ground-water flow equations accounting for fluid density and viscosity dependence on temporal changes of pressure, temperature, and solute transport. Density changes are important because the differences in density between the fresh ground water and the saline bay water affect flow and water levels. Data from an existing ground-water flow model (Merritt, 1995), which includes most of Dade County from Tamiami Trail south (fig. 2), will be transferred to the new proposed model grid using a geographic information system (GIS).

The southern Dade County model by Merritt (1995) is a comprehensive model that subdivides the surficial aquifer in four layers (one layer is used to represent overland flow) and considers the effects of rainfall, evapotranspiration, canal water levels, canal control structures, areal and vertical changes in permeability, aquifer thickness, production wells, and tidal boundaries. All of the parameters from the southern Dade County model by Merritt (1995) to be used in the southwestern part of the preliminary model will also be generated for the northern and northeastern parts of the model grid using GIS technology. Hydrogeologic data from the southern Dade County model will be extrapolated to the area of the grid that covers Biscayne Bay. For purposes of this preliminary model, the boundary conditions used will be constant head or pressure, whichever is determined to be appropriate for the steady-state simulation. Water-level data from wells, canals, and tidal gage sites will be used to represent the boundaries. This preliminary model will be used to select the locations for drilling test holes and observation wells and provide an initial estimate of ground-water flow to Biscayne Bay. These data will be made available to the Waterway Experimental Station (the COE research group that will be performing simulations of surface-water circulation in Biscayne Bay) and to the Jacksonville office of the COE and to the South Florida Water Management District for their effort on the restoration of ENP.

Figure 2 -- Southern Dade County model area by Merritt (1995) and proposed Biscayne Bay model area. (click on image for a larger version.)

Refined Model

• A comprehensive report is planned. This report will include the numerical model analysis, focusing on an evaluation of the effects of management activities on ground-water flow to Biscayne Bay.
• Fact Sheets are planned on an annual basis to summarize the progress of the project.

Tentative dates and the planned project activities are summarized below:

04/96 - 06/96: Conduct well and data inventory. Apply preliminary model analysis to design location of wells. Select drilling sites and obtain permits.
07/96 - 10/96: Conduct nearshore corehole drilling and well construction.
08/96 - 12/96: Conduct offshore corehole drilling and well construction.
12/96: Begin refined model construction and calibration.
04/97: Apply model to estimate ground-water discharges to Biscayne Bay under different scenarios.
07/97: Begin writing of comprehensive report.
10/97: Continue application of the model and start model sensitivity analysis.
10/98: Complete comprehensive report.

Fuller, M.L., 1904, Contributions to the hydrology of eastern United States: U.S. Geological Survey Water-Supply Paper 102.

Griswold, L.S., 1896, Notes on the geology of southern Florida: Harvard Coll. Mus. Comp. Zoology Bulletin, v. 28, no. 2.

INTERA Environmental Consultants, Inc., 1979, Revision of the documentation for a model for calculating effects of liquid waste disposal in deep saline aquifers: U.S. Geological Survey Water-Resources Investigations Report 79-96, 73p.

Kohout, F.A., 1960, Cyclic flow of salt water in the Biscayne aquifer of southeastern Florida: Journal of Geophysical Research, v. 65, no. 7, p. 2133-2141.

----- 1967, Relation of seawater and landward flow of ground water to the salinity of Biscayne Bay: M.S. Thesis, The University of Miami, Coral Gables, 98p.

Merritt, M.L., 1994, A rewetting approximation for a simulator of flow in a surficial aquifer overlain by seasonally inundated wetlands: Ground Water, v. 32, no. 2, p.286-292.

----- 1995, Simulation of the water-table altitude in the Biscayne aquifer, southern Dade County, Florida, water years 1945-89: U.S. Geological Survey Open-File Report 95-337, 88 p.

Munroe, R.M., 1930, The commodore's story: Ives Washburn Press.

Parker, G.G., Ferguson, G.E., Love, S.K., and others, 1955, Water resources of southeastern Florida: U.S. Geological Survey Water-Supply Paper 1255, 965p.

Sewell, John, 1933, John Sewell's memoirs and history of Miami: Miami's water supply and its source: Miami, Florida, Franklin Press, v. 1, chap. 6.

Shaler, N.S., 1890, The topography of Florida: Harvard Coll. Mus. Comp. Zoology Bulletin, v. 16, no. 7.

Willoughby, H.L., 1898, Across the Everglades: J.B. Lippincott Company.

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For more information contact:

Florida District Chief
Email: dc_fl@usgs.gov
U.S. Geological Survey
9100 N.W. 36th Street
Suite 107
Miami, FL 33178
(305) 526-2895

Related information:

SOFIA Project: Ground-Water Discharge to Biscayne Bay