Home Abstract Introduction Hydrogeology of S. Florida - Floridan Aquifer System   >  Upper Floridan Aquifer   -  Middle Confining Unit   -  Lower Floridan Aquifer - Hydrogeology at Alligator Alley Site Ground-Water Movement Subsurface Storage Summary and Conclusions References PDF Version

Hydrogeology of Southern Florida: Floridan Aquifer System

Upper Floridan Aquifer

The Upper Floridan aquifer in southern Florida chiefly consists of permeable zones in the Tampa, Suwannee, and Ocala Limestones and in the upper part of the Avon Park Formation. On the basis of aquifer tests and a regional flow model, the transmissivity is estimated to range from 10,000 to 60,000 feet squared per day (ft²/d) (Bush and Johnston, in press). The contained ground water is brackish. The salinity of the ground water generally increases with increasing depth and with distance downgradient and southward from central Florida. Ground-water temperatures also generally increase downgradient and southward from the recharge area in central Florida. However, temperatures along the southeastern coast are lowest (about 70.0 °F) owing to heat transfer to the Atlantic Ocean (Straits of Florida) (Sproul, 1977, p. 75) and (or) to heat transfer to cooler saltwater in the Lower Floridan aquifer (Kohout, 1965). Temperature and salinity anomalies that are related to upwelling ground water from the Lower Floridan aquifer are discussed later in this report.

Figure 4. (above) Potentiometric surface of peninsular Florida in May 1980 and the area of potable ground water, Upper Floridan aquifer (revised from Johnston and others, 1981, and Healy, 1982). [larger version] Figure 5. (above) Seismic reflection profile B-B' in the western Straits of Florida showing submarine karst on the Miami Terrace. (Line of section shown in fig. 2.) [larger version]

Water movement is chiefly lateral through highly permeable zones of dissolution at or near the top of each formation. Ground-water movement in May 1980 was generally southward from the area of highest head near Polk City in central Florida to the Gulf of Mexico and to the Atlantic Ocean (fig. 4). The area of highest freshwater head is herein referred to as the "Polk City high." Prior to development (late 1800's or early 1900's), the head in south Florida probably was 5 to 10 ft higher than at present. As water use increased and wells were drilled in the area north of Lake Okeechobee, water levels were lowered and a saddle formed in the potentiometric surface, as shown by the close spacing of the 40- to 70-ft contours toward the center of the peninsula. Hydraulic gradients in southern Florida were reduced, resulting in a decrease of natural discharge by submarine springs along the southeastern coast and the movement of seawater inland to a new position of equilibrium.

The concave shape of the contours on the 1980 potentiometric surface map along the southeastern coast indicates convergence of flow toward the submerged karst on the Miami Terrace between Fort Lauderdale and Miami. Ground-water discharge in this area is also suggested by computer flow modeling as described by Bush and Johnston (in press). The rugged topography of the submarine terrace was formed by the collapse of solution features (sinkholes) in the underlying limestone. A seismic reflection profile (fig. 5) across the Miami Terrace shows the pinnacles and troughs associated with the submerged karst and the northward-prograding sediments of Miocene through Pleistocene age unconformably overlying the Suwannee Limestone. Currents and perhaps upwelling freshwater from submarine springs are probably responsible for the lack of sediment on the terrace and terrace slope. Malloy and Hurley (1970) reported that rock samples from dredge hauls on the Miami Terrace by the University of Miami's Institute of Marine Science (now the Rosenstiel School of Marine and Atmospheric Sciences) indicated that the ocean floor is composed of fossiliferous phosphatic limestone that contains numerous foraminifers, chiefly Miogypsina sp., Giobigerina sp., miliolids, and rotalids. Rock samples from equivalent depths in artesian wells at Miami and Fort Lauderdale contain the same fossil assemblages and are identified as the Suwannee Limestone of Oligocene age.

Sinkholes on the Miami Terrace are both filled and unfilled. Those that are unfilled probably are active submarine springs. The large filled sinkhole in the center of the seismic profile (fig. 5) is about 2 mi wide and may be related to the collapse in the highly cavernous dolostone (the Boulder Zone) in the lower part of the Oldsmar Formation. The Boulder Zone is discussed by Miller (1986, p. B65-B66). Sinkholes generally are present throughout Florida and are prominent in central Florida as chains of sinkhole lakes. The sinkholes are chiefly in Tertiary limestones along joints or fractures that trend generally northwestward to southeastward (and to a lesser extent southwestward to northeastward). Sinkholes in southern Florida are virtually obscured because they are filled by deposits that are Miocene or younger in age. Their presence, however, is often indicated by drilling and by local salinity or temperature anomalies.