U.S. Geological Survey Open-File Report 96-492, 48 p.
Subarea 2 encompasses about 4,100 square miles (mi2), which includes about 2,250 mi2 of the Chattahoochee River basin and 1,850 mi2 of the Flint River basin in the Piedmont physiographic province of west-central Georgia and eastern Alabama. Subarea 2 includes about 26 percent of the total 8,740 square-mile area of the Chattahoochee River basin and about 22 percent of the total 8,460 square-mile area of the Flint River basin. The study area is underlain by a two-component aquifer system composed of a fractured, crystalline-rock aquifer characterized by little or no primary porosity or permeability; and the overlying weathered regolith (saprolite), which generally behaves as a porous-media aquifer. In some areas, a transition zone lies between the regolith and unweathered crystalline bedrock.
The conceptual model described for this study qualitatively subdivides the ground-water flow system into local (shallow), intermediate, and regional (deep) flow regimes. Ground-water discharge to tributaries mainly is from local and intermediate flow regimes and varies seasonally. The regional flow regime probably represents steady-state conditions and discharges chiefly to major drains such as the Chattahoochee and Flint Rivers. Ground-water discharge to major drains originates from all flow regimes. Mean-annual ground-water discharge to streams (baseflow) is considered to represent the long-term, average recharge to ground water. The mean-annual baseflow was estimated using an automated hydrograph-separation method, and represents discharge from the local, intermediate, and regional flow regimes of the ground-water flow system. Mean-annual baseflow exiting Subarea 2 was estimated to be 5,800 cubic feet per second. Mean-annual baseflow represented about 68 percent of total mean-annual stream discharge in the Chattahoochee River basin and 49 percent in the Flint River basin at the Subarea 2-Subarea 3 boundary.
Stream discharge for selected sites on the Chattahoochee River and its tributaries were compiled for the years 1941, 1954, and 1986, during which sustained severe droughts occurred throughout most of the ACF-ACT area. Stream discharge was assumed to be sustained entirely by baseflow during the latter periods of these droughts. Estimated baseflow (unregulated) near the end of the individual drought years averaged about 13 percent of the estimated mean-annual baseflow in the Chattahoochee River basin and 8.7 percent in the Flint River basin in Subarea 2.
The potential exists for the development of ground-water resources on a regional scale throughout Subarea 2. Estimated ground-water use in 1990 was about 1.2 percent of the estimated mean-annual baseflow in Georgia and 1.7 percent in Alabama; and 11.4 and 14.5 percent of the average drought flow in Georgia and Alabama, respectively. Because ground-water use in Subarea 2 represents a relatively minor percentage of ground-water recharge, even a large increase in ground-water use in Subarea 2 in one State is likely to have little effect on ground-water and surface-water occurrence in the other. Indications of long-term ground-water level declines were not observed; however, the number and distribution of observation wells for which long-term water-level measurements are available in Subarea 2 are insufficient to draw conclusions.
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