Georgia Water Science Center

USGS Open-File Report 95-321

GROUND-WATER RESOURCES OF THE LOWER APALACHICOLA-CHATTAHOOCHEE-FLINT RIVER BASIN IN PARTS OF ALABAMA, FLORIDA, AND GEORGIA—SUBAREA 4 OF THE APALACHICOLA-CHATTAHOOCHEE-FLINT AND ALABAMA-COOSA-TALLAPOOSA RIVER BASINS

This report is available online in pdf format (39 MB): USGS OFR 95-321 (Opens the PDF file in a new window. )

Lynn J. Torak and Robin John McDowell

U.S. Geological Survey Open-File Report 95-321, 145 pages (Published 1996)

ABSTRACT

The study area is underlain by Coastal Plain sediments of pre-Cretaceous to Quaternary age consisting of alternating units of sand, clay, sandstone, dolomite, and limestone that gradually thicken and dip gently to the southeast. The Upper Floridan aquifer is composed of an offlapping sequence of clastic and carbonate sediments consisting of the Clinchfield Sand, the Ocala, Suwannee, and Tampa Limestones, and the Marianna Formation. The Intermediate system consists of the Intracoastal, Chipola, and Jackson Bluff Formations, is limited in areal extent to the southern part of the basin in Florida, and constitutes an aquifer of low yield. The aquifer-stream-reservoir (flow) system is defined by surface water in hydraulic connection with aquifers and semiconfining units.

Simulation of the flow system by using the U.S. Geological Survey’s MODular Finite-Element model (MODFE) of two-dimensional ground-water flow indicated that ground-water availability in Alabama is affected most by changes to lateral and vertical boundary conditions to the Upper Floridan aquifer that might occur in that state, and is affected minimally by changes to ground- and surface-water levels in Georgia. Incomplete hydrologic information precludes definitive assessment of ground- water-resource potential, overpumpage, and potential for additional development; however, simulated-increased pumpage at more than 3 times the October 1986 rates caused drying of the Upper Floridan aquifer in parts of Miller and Lee Counties, Ga. Evaluation of ground-water-development potential in the virtually untapped Intermediate system has questionable reliability due to the lack of data.

Increased hypothetical pumpage over October 1986 rates for the Upper Floridan aquifer, located almost entirely in Georgia, indicated reduction in ground-water discharge to streams that reduced flow in the Apalachicola River and to the Bay, especially during droughts. Water budgets prepared from simulation results indicate that discharge to streams and recharge by horizontal and vertical flow are principal hydro-logic mechanisms for moving water into, out of, or through aquifers. The Intermediate system contributes less than 2 percent of the total simulated ground-water discharge to streams; thus, it does not represent an important source of water for the Apalachicola River and Bay.


CONTENTS

Abstract

Introduction

Purpose and scope

Area of study and physiography

Ground-water use

Methods of investigation

Previous studies

Well- and surface-water-station numbering system

Acknowledgments

Hydrogeology

Geologic setting

Hydrologic setting

Hydrologic characteristics

Overlying semiconfining units

Intermediate system

Underlying semiconfining Unit

Upper Floridan aquifer

Lower confining unit

Ground water levels

Seasonal fluctuations

Long term effects of drought conditions and pumpage

Effects of surface water features

Ground-water quality

Surface water

Drainage

Streamflow

Dams and navigational improvements

Evaluation of ground-water resources

Conceptualization of the flow system

Mathematical model

Governing equation

Boundary and initial conditions

Numerical model

Simulation approach

Steady-state analysis

Limitations

Advantages

Transient analysis

Finite element mesh

Boundary conditions

Regional ground water flow

Flow across streambeds

Vertical leakage

Springflow

Hydraulic property zones

Distribution of ground-water withdrawal

Calibration to October 1986 conditions

Ground-water-level residuals

Computed stream-aquifer flows

Simulated potentiometric surfaces

Directions of ground-water movement

Surface-water influence on ground-water flow

Water-budget analysis

Zero-pumpage conditions

Upper Floridan model

Intermediate model

Effects of pumpage and boundary conditions on flow system

Stream aquifer flow decline

Changes in boundary flow

Ground-water-level change

Upper Floridan aquifer

Intermediate system

Accuracy of results

Transient response of flow system to pumpage changes

Potential for changes to water quality

Ground-water-development potential

Conclusions

References cited

Appendix A

 


REPORT AVAILABILITY

This report is available online in pdf format (39 MB): USGS OFR 95-321 (Opens the PDF file in a new window. )
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