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Florida Science Center

SIR 2006-5013


Prepared in cooperation with
Southwest Florida Water Management District


2006


Regional Evaluation of the Hydrogeologic Framework, Hydraulic Properties, and Chemical Characteristics of the Intermediate Aquifer System Underlying Southern West-Central Florida

Lari A. Knochenmus



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CONTENTS

Abstract
Introduction
      Purpose and Scope
      Description of the Study Area
Hydrologic Data Sources
      Acknowledgments
      Definitions and Terms
Geologic Framework
      Depositional History
      Stratigraphy
      Hydrogeologic Framework
      Evolution of Hydrogeologic Unit Nomenclature
      Properties of the Aquifers Overlying and Underlying the Intermediate Aquifer System
Intermediate Aquifer System
      Permeable Units
            Zone 1
            Zone 2
            Zone 3
      Confining Units
            Clay Beds
            Leakance Across Confining Units
      Hydraulic Head Differences
Regional Evaluation of the Intermediate Aquifer System
Summary
Selected References
List of ROMP Reports

Appendix 1A-B. (A) Major ion data and (B) Pie charts showing major ion composition of water samples from wells at selected Regional Observation and Monitoring-well Program (ROMP) sites, 2001
Appendix 2A-B. Water levels in and differences among zones and adjacent aquifers at selected Regional Observation and Monitoring-well Program (ROMP) sites, (A) April 2001 and (B) October 2001


Abstract

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. The chemical composition of water in the intermediate aquifer system is more varied than the adjacent aquifers. This variation is related to the different types of minerals forming the aquifer matrix, and complex mixing with waters from adjacent aquifers. The dominant water type throughout the intermediate aquifer system is mixed-cation bicarbonate, typically with nearly equal concentrations of calcium and magnesium ions. At the southern extent of the study area, sodium-chloride type water is present throughout the intermediate aquifer system. Calcium-magnesium sulfate type water is present along the coastal margin from Tampa Bay to southern Sarasota County in Zone 3 and in two Zone 2 wells in west-central Sarasota County.

Differences in hydraulic head generally are greater between adjacent zones within the intermediate aquifer system than between the intermediate aquifer system and adjacent aquifers. Heads differences between Zone 1 and the surficial aquifer system range from -1 (downward) to 3 (upward) feet, and heads are nearly the same (<1 foot) between Zone 3 and the Upper Floridan aquifer in much of the study area. In contrast, head differences across the upper and lower groups of confining units of the intermediate aquifer system range from -48 to 14 feet and from -119 to 24 feet, respectively. These head differences among the zones indicate some degree of hydraulic separation between zones within the intermediate aquifer system.


Suggested Citation:

Knochenmus, L.A., 2006, Regional Evaluation of the Hydrogeologic Framework, Hydraulic Properties, and Chemical Characteristics of the Intermediate Aquifer System Underlying Southern West-Central Florida: U.S. Geological Survey Scientific Investigations Report 2006-5013, 52 p.

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U.S. Geological Survey
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lari@usgs.gov



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