Ground-water resources of the Coosa River basin in Georgia and Alabama; Subarea 6 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 surface- and 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 Coosa River basin of Georgia and Alabama, Subarea 6 of the ACF and ACT River basins, and estimate the possible effects of increased ground-water use within the basin. Subarea 6 encompasses about 10,060 square miles in Georgia and Alabama, totaling all but about 100 mi2 of the total area of the Coosa River basin; the remainder of the basin is in Tennessee. Subarea 6 encompasses parts of the Piedmont, Blue Ridge, Cumberland Plateau, Valley and Ridge, and Coastal Plain physiographic provinces. The major rivers of the subarea are the Oostanaula, Etowah, and Coosa. The Etowah and Oostanaula join in Floyd County, Ga., to form the Coosa River. The Coosa River flows southwestward and joins with the Tallapoosa River near Wetumpka, Ala., to form the Alabama River. The Piedmont and Blue Ridge Provinces are underlain by a two-component aquifer system that is composed of a fractured, crystalline-rock aquifer characterized by little or no primary porosity or permeability; and the overlying regolith, which generally behaves as a porous-media aquifer. The Valley and Ridge and Cumberland Plateau Provinces are underlain by fracture- and solution-conduit aquifer systems, similar in some ways to those in the Piedmont and Blue Ridge Provinces. Fracture-conduit aquifers predominate in the well-consolidated sandstones and shales of Paleozoic age; solution-conduit aquifers predominate in the carbonate rocks of Paleozoic age. The Coastal Plain is underlain by southward-dipping, poorly consolidated deposits of sand, gravel, and clay of fluvial and marine origin. 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 steady-state conditions and discharges chiefly to major drains such as the Coosa River, and in upstream areas, to the Etowah and Oostanaula Rivers. 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 in Georgia was estimated to be about 4,000 cubic feet per second (ft3/s) (from the headwaters to the Georgia-Alabama State Line), 5,360 ft3/s in Alabama, and 9,960 ft3/s for all of Subarea 6 (at the Subarea 7-Subarea 8 boundary). Mean annual baseflow represented about 60 percent of total mean-annual stream discharge for the period of record. Stream discharge for selected sites on the Coosa River and its tributaries were compiled for the years 1941, 1954, and 1986, during which sustained droughts occurred throughout most of the ACF-ACT area. Stream discharges were assumed to be sustained entirely by baseflow during the latter periods of these droughts. Estimated baseflow near the end of the individual drought years ranged from about 11 to 27 percent of the estimated mean-annual baseflow in Subarea 6. The potential exists for the development of ground-water resources on a regional scale throughout Su