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A 2,811-foot deep test well, designated G-2296, was drilled during 1980 in The Everglades of south Florida along Alligator Alley (Interstate 75), as part of the Floridan Regional Aquifer-System Analysis (RASA) project. A 16-inch diameter steel casing was installed and grouted with cement from land surface to a depth of 895 feet, below which a nominal 8-inch diameter exploratory hole was drilled to a total depth of 2,811 feet. A 2-inch diameter steel monitor tube with perforations from 811 to 816 feet was grouted with cement in the outer annulus. Hydraulic packers were used to isolate selected zones for water sampling and measurement of water levels. The Alligator Alley test well penetrated the surficial and intermediate aquifer systems and terminated in the Floridan aquifer system. The surficial aquifer system is about 180 feet thick and is composed chiefly of sandy limestone of the Tamiami Formation of Pliocene age. The intermediate aquifer system is composed of three artesian limestone aquifers and related confining beds and occurs between 180 and 770 feet in Miocene deposits. The top of the Floridan aquifer system in the Alligator Alley test well is considered to coincide with the top of the Suwannee Limestone of Oligocene age at 770 feet on the basis of hydraulic head and water chemistry data. In U.S. Geological Survey Professional Paper 1403-B, published in 1986, Miller, in describing the regional hydrogeologic framework of the Floridan, placed the top of the Floridan at about 950 feet at this test well on the basis of apparent porosity changes within the Suwannee Limestone. This discrepancy of placing the top of the Floridan aquifer system at two different depths at this test well site is due to the difference in the criteria defined for the regional framework and for the area studies. For area studies, more refinements are needed than for a regional study. About 67 percent of the total thickness of the Floridan aquifer system was penetrated by the well. The formations that comprise the Floridan aquifer system in the well (from shallowest to deepest) include the Suwannee and Ocala Limestones and the Avon Park and Oldsmar Formations. The Floridan aquifer system is composed of many water-bearing zones that are chiefly horizontal solution zones in the limestone. The dominant water-bearing zones occur at about 1,030 feet near the top of the Ocala Limestone of late Eocene age and at about 2,560 feet in the Oldsmar Formation of early Eocene age. The 1,030-foot water-bearing zone (zone 3) contains brackish artesian water (chloride concentration of about 800 milligrams per liter), and the 2,560-foot water-bearing zone (zone 13) contains salty artesian water similar in composition to seawater (chloride concentration of about 19,000 milligrams per liter). The head in the 1,030-foot zone is about 59 feet above sea level (at prevailing density), whereas that in the 2,560-foot zone is about 7 feet above sea level (at prevailing density). However, a possibility exists that the head in the 1,030-foot zone is several feet higher than the measured value because of loss of head due to internal circulation when the well is shut-in. Pressure gradients in the well indicate a saltwater flow system and a freshwater flow system. The theoretical saltwater-freshwater interface, based on buoyancy and the pressure gradients, would occur at 1,918 feet. However, the salinity distribution shows a thick mixing zone between 1,600 and 2,200 feet that suggests a complex relation between the two flow systems. A reverse geothermal gradient is indicated by the coldest water (24.5 °C) occurring at 2,811 feet (the bottom of the well). Radiocarbon activities measured in dissolved inorganic carbon species (unadjusted for fractionation and reactions during transit in the aquifer) indicate that the saltwater in the 2,560-foot zone (zone 13) in the lower part of the aquifer system is younger than brackish water in the upper part, and that brackish water in the 1,030-foot zone (zone 3) in the upper part of the aquifer system is relatively younger than that in the overlying and underlying zones. Comparison of the carbon-14 activity of the saltwater from the deep zone (zone 13) with that from wells of comparable depth at Fort Lauderdale and Miami, east of the test well, indicates that the carbon-14 activity of the saltwater increases (is younger) toward Fort Lauderdale and the Atlantic Ocean. The carbon-14 activity gradient indicates that seawater flows inland through the 2,560-foot zone. The potentiometric surface for the Upper Floridan aquifer indicates that brackish water moves seaward in the 1,030-foot zone. INTRODUCTION Purpose and Scope In October 1978, the U.S. Geological Survey began a 4-year study of the Floridan aquifer system of the southeastern United States, as part of a national investigative program called RASA (Regional Aquifer-System Analysis). The objectives of the Floridan RASA project were to describe the hydrogeology, geochemistry, and flow in the Floridan aquifer system (Johnston, 1978). Data for the Floridan aquifer system in south Florida are sparse because the system contains saline water and is of little use for supply, it occurs at relatively great depth (generally more than 500 feet), and adequate potable supplies are, for the most part, available from surficial aquifers. Variations in water quality, head, and temperature are known to occur within the limestones that comprise the aquifer system (Kohout, 1965; Meyer, 1971; Meyer, 1974), thereby suggesting a complex flow regimen. To obtain reliable data and fill in the gap between the eastern and western coasts of south Florida, an exploratory test well was drilled in the central Everglades, south of Lake Okeechobee (fig. 1). After assessing the distribution of existing data (Smith and others, 1982), a drilling site was selected in west Broward County, about 5 miles east of the Broward-Collier County line and 0.4 mile north of mile post 54 on Alligator Alley (Everglades Turnpike, Interstate 75, State Road 84) (fig. 1). The site is on right-of-way of the South Florida Water Management District

Summary of well construction, testing, and preliminary findings from the Alligator Alley test well, Broward County, Florida