Home Abstract >Introduction - Purpose and Scope - Previous Investigations - Acknowledgments Hydrogeology of S. Florida Ground-Water Movement Subsurface Storage Summary and Conclusions References PDF Version

Introduction

In October 1978, the U.S. Geological Survey began a 4-year (yr) study of the Floridan aquifer system (formerly called the Tertiary limestone aquifer) of the Southeastern United States, as part of a national ground-water resources investigative program called Regional Aquifer-System Analysis (RASA). The objectives of the Florida RASA project were to describe the hydrogeology, geochemistry, and flow of ground water in the aquifer system (Johnston, 1978). Variations in water quality, hydraulic head, and water temperature within the carbonate rocks that make up the Floridan aquifer system in southern Florida suggest that the flow system is complex. Movement of fresh and brackish ground water through the upper part of the Floridan aquifer system has been documented chiefly on the basis of measured head and hydraulic gradients. Movement of saltwater in the lower part, however, has been the subject of speculation because definitive head data are lacking.

Kohout (1965) and Kohout and others (1977) hypothesized inland flow of seawater through the lower part of the Floridan aquifer system on the basis of a temperature anomaly (reversed geothermal gradient) in several deep wells in southern Florida. The driving force, Kohout concluded, was geothermal heat which produced a convection flow cell wherein cold seawater was heated as it flowed inland through the lower part, called the Boulder Zone, then moved upward through vertical paths (such as ancient sinkholes) in the overlying confining units and mixed with the seaward-flowing freshwater in the upper part (fig. 1). Opposing views were expressed by Vernon (1970), who suggested that the temperature anomaly was due only to heat conduction, and Sproul (1977), who concluded that existing data were insufficient to support either hypothesis.

Figure 1. Idealized hydrogeologic cross section through peninsular Florida showing concept of cyclic flow of seawater induced by geothermal heating (proposed by Kohout, 1965). (Location of site 11 shown in fig. 2.) [larger version]

A major part of the RASA investigation in southern Florida involved drilling a test well, G-2296, also referred to as the Alligator Alley test well (fig. 2, site 10). About 44 miles (mi) west of Fort Lauderdale, the well was drilled to a depth of 2,811 feet (ft) during 1980-81 to obtain hydrogeologic and hydrochemical data, including selected isotopic analyses, within the Floridan aquifer system. Data from the Alligator Alley test well indicated that carbon-14 might be useful in determining the direction and rate of ground-water movement in the lower saltwater part of the aquifer system - information that would be useful in assessing the long-term effects of deep-well injection. Earlier studies by Osmond and others (1968) suggested that uranium isotopes also could be used to assess ground-water movement. Supplementary data were obtained from deep wells that were drilled by local utilities at Fort Lauderdale, Miami, and Stuart for injection of treated wastewater into the Boulder Zone - a cavernous, highly transmissive dolostone that underlies southeastern Florida at a depth of about 3,000 ft.

Injection of liquid wastes into the saline water of the Boulder Zone as a pollution-control measure was started in 1943 with the injection of brine at an oil field a few miles west of Naples. Subsequently, the practice expanded rapidly, and numerous high-capacity, municipally operated, wastewater injection wells are now in use along the southeastern coast of Florida (Vernon, 1970; Vecchioli and others, 1979; Meyer, 1984). Determination of the direction of ground-water movement in the lower part of the Floridan aquifer system was, therefore, a very necessary and important part of the RASA project in southern Florida.

This report is one of nine chapters in U.S. Geological Survey Professional Paper 1403 that describe various aspects of the geology, hydrology, and geochemistry of the Floridan aquifer system. Those chapters most related to this report (chapter G) include the summary (chapter A) by Johnston and Bush (1988) and chapters on the hydrogeologic framework (chapter B) by Miller (1986), regional ground-water hydraulics (chapter C) by Bush and Johnston (in press), and ground-water geochemistry (chapter I) by Sprinkle (in press).