Regional evaluation of the hydrogeologic framework, hydraulic properties, and chemical characteristics of the intermediate aquifer system underlying southern west-central Florida

Three major aquifer systems-the surficial aquifer system, the intermediate aquifer system, and the Floridan aquifer system-are recognized in the approximately 5,100-square-mile southern west-central Florida study area. The principal source of freshwater for all uses is ground water supplied from the three aquifer systems. Ground water from the intermediate aquifer system is considered only moderately abundant compared to the Upper Floridan aquifer, but it is an important source of water where the Upper Floridan aquifer contains water too mineralized for most uses. In the study area, the potential ground-water resources of the intermediate aquifer system were evaluated by regionally assessing the vertical and lateral distribution of hydrogeologic, hydraulic, and chemical characteristics.

Although the intermediate aquifer system is considered a single entity, it is composed of multiple water-bearing zones separated by confining units. Deposition of a complex assemblage of carbonate and siliciclastic sediments during the late Oligocene to early Pliocene time resulted in discontinuities that are reflected in transitional and abrupt contacts between facies. Discontinuous facies produce water-bearing zones that may be locally well-connected or culminate abruptly. Changes in the depositional environment created the multilayered intermediate aquifer system that contains as many as three zones of enhanced water-bearing capacity. The water-bearing zones consist of indurated limestone and dolostone and in some places unindurated sand, gravel, and shell beds, and these zones are designated, in descending order, as Zone 1, Zone 2, and Zone 3. Zone 1 is thinnest (<80 feet thick) and is limited to <20 percent (southern part) of the study area. Zone 2, the only regionally extensive zone, is characterized by moderately low permeability. Zone 3 is found in about 50 percent of the study area, has the highest transmissivities, and generally is in good hydraulic connection with the underlying Upper Floridan aquifer. In parts of the study area, particularly in southwestern Hillsborough County and southeastern De Soto and Charlotte Counties, Zone 3 likely is contiguous with and part of the Upper Floridan aquifer.

Transmissivity of the intermediate aquifer system ranges over five orders of magnitude from about 1 to more than 40,000 feet squared per day (ft2/d), but rarely exceeds 10,000 ft2/d. The overall transmissivity of the intermediate aquifer system is substantially lower (2 to 3 orders of magnitude) than the underlying Upper Floridan aquifer. Transmissivity varies vertically among the zones within the intermediate aquifer system; Zone 2 has the lowest median transmissivity (700 ft2/d), Zone 1 has a moderate median transmissivity (2,250 ft2/d), and Zone 3 has the highest median transmissivity (3,400 ft2/d). Additionally, the transmissivity varies geographically (from site to site) within a zone. Specifically, a region of relatively low transmissivity (<100 ft2/d) throughout the vertical extent of the intermediate aquifer system is present in the central part of the study area. This low transmissivity region is encompassed by a larger region of moderately low transmissivity (<1,000 ft2/d) that covers a large part of the study area.

Clay beds and fine-grained carbonates form the confining units between the water-bearing zones and are characterized by low leakance. Leakance through the intermediate aquifer system confining units ranges over 4 orders of magnitude from 4.2x10-7 to 6.0x10-3 feet per day per foot [(ft/d)/ft]. Despite the large range, the geometric mean and median leakances of individual confining units are within the same order of magnitude, 10-5 (ft/d)/ft, which is 2 orders of magnitude less than the median leakance of the semiconfining unit within the Upper Floridan aquifer.

Major ion concentrations in water from the intermediate aquifer system, and throughout the ground-water flow system, generally increase with depth. T