Georgia Water Science Center
This report is available online in pdf format (39 MB): USGS OFR 95-321 (
)
Lynn J. Torak and Robin John McDowell
U.S. Geological Survey Open-File Report 95-321, 145 pages (Published 1996)
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.
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
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