Ground-water resources of the Upper Chattahoochee River basin in Georgia and Subarea 1 of the Apalachicola-Chattahoochee-Flint and Alabama-Coosa-Tallapoosa river basins

Abstract

Drought conditions in the 1980’s focused attention on the multiple uses of the surfaceand ground-water resources in the Apalachicola-Chattahoochee-Flint (ACF) and Alabama-Coosa-Tallapoosa (ACT) River basins in Georgia, Alabama, and Florida. State and Federal agencies also have proposed projects that would require additional water resources and revise operating practices within the river basins. The existing and proposed water projects create conflicting demands for water by the States and emphasize the problem of water-resource allocation. This study was initiated to describe ground-water availability in the upper Chattahoochee River basin of Georgia, Subarea 1 of the ACF and ACT River basins, and to estimate the possible effects of increased ground-water use within the basin. Subarea 1 encompasses about 2,430 square miles in the Piedmont and Blue Ridge physiographic provinces of north-central Georgia. Subarea 1 includes about 28 percent of the total 8,740 square-mile area of the Chattahoochee 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 approximates steadystate conditions and discharges chiefly to major drains such as the Chattahoochee River. Ground-water discharge to major drains originates from all flow regimes. Mean-annual ground-water discharge to streams (baseflow) is considered to approximate 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 1 was estimated to be 2,570 cubic feet per second. Mean-annual baseflow represented about 69 percent of total mean-annual stream discharge for the period of record. 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 droughts (1941 and 1954) averaged about 19.3 percent of the estimated mean-annual baseflow in Subarea 1. The potential exists for the development of ground-water resources on a regional scale throughout Subarea 1. Estimated ground-water use in 1990 was about 1.1 percent of the estimated mean-annual baseflow; and 5.8 percent of the average drought flow. Because ground-water use in Subarea 1 represents a relatively minor percentage of groundwater recharge, even a large increase in ground-water use in Subarea 1 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 1 are insufficient to draw conclusions.