Georgia Water Science Center

USGS Scientific Investigations Report 2006-5070

Geohydrology of the Lower Apalachicola–Chattahoochee–Flint River Basin, Southwestern Georgia, Northwestern Florida, and Southeastern Alabama

This report is available online in pdf format (8 MB): USGS SIR 2006-5070 (Opens the PDF file in a new window. )

Lynn J. Torak and Jaime A. Painter

U.S. Geological Survey Scientific Investigations Report 2006-5070, 80 pages (Published 2006)


The lower Apalachicola–Chattahoochee–Flint (ACF) River Basin contains about 4,600 square miles of karstic and fluvial plains and nearly 100,000 cubic miles of predominantly karst limestone connected hydraulically to the principal rivers and lakes in the Coastal Plain of southwestern Georgia, northwestern Florida, and southwestern Alabama. Sediments of late-middle Eocene to Holocene in hydraulic connection with lakes, streams, and land surface comprise the surficial aquifer system, upper semiconfining unit, Upper Floridan aquifer, and lower semiconfining unit and contribute to the exchange of ground water and surface water in the stream-lake-aquifer flow system. Karst processes, hydraulic properties, and stratigraphic relations limit ground-water and surface-water interaction to the following hydrologic units of the stream-lake-aquifer flow system: the surficial aquifer system, upper semiconfining unit, Upper Floridan aquifer, and lower confining unit. Geologic units corresponding to these hydrologic units are, in ascending order: Lisbon Formation; Clinchfield Sand; Ocala, Marianna, Suwannee, and Tampa Limestones; Hawthorn Group; undifferentiated overburden (residuum); and terrace and undifferentiated (surficial) deposits. Similarities in hydraulic properties and direct or indirect interaction with surface water allow grouping sediments within these geologic units into the aforementioned hydrologic units, which transcend time-stratigraphic classifications and define the geohydrologic framework for the lower ACF River Basin. The low water-transmitting properties of the lower confining unit, principally the Lisbon Formation, allow it to act as a nearly impermeable base to the stream-lake-aquifer flow system.

Hydraulic connection of the surficial aquifer system with surface water and the Upper Floridan aquifer is direct where sandy deposits overlie the limestone, or indirect where fluvial deposits overlie clayey limestone residuum. The water level in perched zones within the surficial aquifer system fluctuates independently of water-level changes in the underlying aquifer, adjacent streams, or lakes. Where the surficial aquifer system is connected with surface water and the Upper Floridan aquifer, water-table fluctuations parallel those in adjacent streams or the underlying aquifer.

The upper semiconfining unit ranges in thickness from a few feet in the northwestern part of the lower ACF River Basin near the outcrop areas of the Upper Floridan aquifer to about 400 feet to the south and east of Lake Seminole in the Tifton Upland and Tallahassee Hills. In some areas in Florida, several hundred feet of unconsolidated clay and sand or low-permeability sediments fill paleosinks and inhibit the exchange of water between land surface and the Upper Floridan aquifer. Sand and clay content of the upper semiconfining unit controls the vertical hydraulic conductivity and influences the rate of vertical leakage of ground water across the top of the Upper Floridan aquifer. Variations in the hydrologic and geologic settings of the lower ACF River Basin have created distinct local patterns of ground-water-level fluctuations that allowed the establishment of 14 geohydrologic zones for identifying local variations in the saturated proportion of total semiconfining-unit thickness basinwide. Differences in the saturated proportion of total thickness influences potential recharge to the Upper Floridan aquifer.

Ground-water levels in the upper semiconfining unit respond to infiltration of precipitation, dry climatic conditions, evapotranspiration, changes in surface-water level, and ground-water withdrawal and discharge (springflow). Drought conditions and heavy rainfall disrupt seasonal patterns of ground-water-level fluctuations by creating either unusually low or high levels, respectively, although water-level data do not indicate long-term declines.

The Upper Floridan aquifer is the principal water-bearing hydrologic unit of the stream-lake-aquifer flow system in the lower ACF River Basin. The diversity and complexity of thickness, lithology, and hydraulic properties within and among the geologic units constituting the Upper Floridan aquifer create equally diverse hydrologic characteristics depending on location in the basin. Hydraulic properties of aquifer transmissivity, hydraulic conductivity, storage coefficient, and specific yield have been enhanced in the Upper Floridan aquifer by limestone dissolution, resulting in increased ground-water flow to wells through interconnected systems of solution openings, fractures, and joints in the limestone, and increased storage capacity of the aquifer and its hydraulic connection with surface water. Stream-aquifer interaction varies in the basin according to the proximity of the aquifer to surface water and degree of hydraulic connection, or separation, of the aquifer from surface water by other hydrologic units. Seasonal ground-water-level response to natural and human-made stresses affects ground-water discharge to streams and reflects the local heterogeneity of hydraulic properties in the aquifer. Increased agricultural pumpage since the mid-1970s and drought conditions from the early to mid-1980s and from 1998 to 2002 caused noticeable declining trends in groundwater level along the Flint River and eastward across the Solution Escarpment into adjacent river basins.




Purpose and Scope

Previous Studies

Well and Climatological-Station Identification and Surface-Water Station

Numbering Systems

Study Area


Physiography and Drainage


Geologic Setting

Hydrologic Setting


Hydrologic Characteristics

Surficial Aquifer System

Upper Semiconfining Unit


Vertical Hydraulic Conductivity and Leakage Potential

Ground-Water-Level Fluctuations

Geohydrologic Zones and Potential Recharge to the Upper Floridan Aquifer

Upper Floridan Aquifer


Hydraulic Properties and the Effect of Limestone Dissolution on Ground-Water Flow

Transmissivity and Hydraulic Conductivity

Storage Coefficient and Specific Yield

Seasonal and Long-Term Ground-Water-Level Fluctuations

Climatic Effects

Effects of Ground-Water Withdrawal

Surface-Water Influence

Lower Confining Unit


References Cited

Appendix A. Geologic and Hydrologic Data for Selected Wells in the Lower Apalachicola–Chattahoochee–Flint River Basin

Appendix B. Stream and Well Hydrographs and Rainfall Graphs Accessed Interactively from Figure 2



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