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Hydrogeology and the Distribution and Origin of Salinity in the Floridan Aquifer System, Southeastern Florida
Water Resource Investigations Report 94-4010

By Ronald S. Reese

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Introduction
Hydrogeology of the Floridan Aquifer System
Collection and Analysis of Salinity Data
Evaluation of Formation Salinity
Distribution of Salinity in the Floridan Aquifer System
Origin of Salinity
Summary and Conclusions
References Cited
Appendix: Inventory of Wells
PDF Version

Abstract

The Floridan aquifer system in southeastern Florida consists of the Upper Floridan aquifer, the middle confining unit, and the Lower Floridan aquifer and ranges from Paleocene to Oligocene in age. The top of the rocks of Eocene age in the study area approximates the top of the Upper Floridan aquifer and coincides with an important flow zone.

An upper zone of brackish water and a lower zone of water with salinity similar to that of seawater are present in the Floridan aquifer system. The brackish-water zone is defined as that in which water has a dissolved-solids concentration of less than 10,000 milligrams per liter (chloride concentration less than about 5,240 milligrams per liter), and water in the saline-water zone has a dissolved solids concentration of about 35,000 milligrams per liter (about 18,900 milligrams per liter chloride concentration). The brackish-water and saline-water zones are separated by a transitional zone, typically 100 feet thick, in which salinity increases abruptly with depth. The base of the brackish-water zone lies within the Upper Floridan aquifer along the coast but extends into the middle confining unit inland. The brackish-water zone is as much as 1,200 feet thick inland, whereas the Upper Floridan aquifer is typically 500 to 600 feet thick. Chloride concentrations range from about 850 to 5,640 milligrams per liter and from about 1,410 to 3,330 milligrams per liter in the upper and lower intervals of the brackish-water zone, respectively.

The base of the brackish-water zone and the top of the saline-water zone were approximately determined mostly through the use of resistivity borehole geophysical logs. Changes in lithology or permeability do not usually control the position of the boundary between the brackish-water and saline-water zones.

Calculations of the depth of a brackish-water/saline-water interface using the Ghyben-Herzberg relation show good agreement between calculated and actual positions of the interface, indicating equilibrium between the zones.

Several areas of high salinity (chloride concentrations greater than 3,000 milligrams per liter) are present in the upper interval of the brackish-water zone near the coast, and in one of these areas in northeastern Broward County, salinity decreases with depth from the upper to lower interval. Available data indicate that in areas of high salinities, the Upper Floridan aquifer has relatively high transmissivity. The high salinities could be a result of seawater preferentially encroaching into zones of higher permeability in the Upper Floridan aquifer during Pleistocene high stands of sea level and incomplete flushing of the seawater by the present-day flow system.

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Conversion Factors, Vertical Datum, Abbreviations, and Acronyms
Multiply By To Obtain
inch (in.) 25.4 millimeter
foot (ft.) 0.3048 meter
mile (mi.) 1.609 kilometer
foot per day (ft/d) 0.000353 centimeters per second
foot squared per day (ft2/d) 0.9290 meter squared per day
gallon per minute (gal/min) 0.00006309 cubic meter per second
pound per square inch per foot
(lb/in2/ft)
22.6214 kilopascal per meter
gram per cubic centimeter
(g/cm3)
62.43 pound per cubic foot

 

Sea Level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) - a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level datum of 1929.

 

Equations for temperature conversion between degrees Celsius (°C) and degrees Fahrenheit (°F)
°C = 5/9 x (°F - 32)
°F = (9/5 °C) + 32

 

Abbreviated water-quality units
mg/L = milligrams per liter
µS/cm = microsiemens per centimeter

 

Acronyms
API American Petroleum Institute (standard units for natural gamma-ray radioactivity measured in boreholes)
FDEP Florida Department of Environmental Protection
GWSI Ground-Water Site Inventory System (a U.S. Geological Survey computerized well data storage and retrieval system)
QWDATA U.S. Geological Survey Water-Quality Data storage and retrieval computer system
RASA Regional Aquifer System Analysis
USGS U.S. Geological Survey



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