Ground-water resources of the Cahaba River basin in Alabama - Subarea 7 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-Chattahooochee-Flint and Alabama-Coosa-Tallapoosa 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 availablity in the Cahaba River basin in Alabama, Subarea 7 of the Apalachicola-Chattahoochee-Flint and Alabama-Coosa-Tallapoosa River basins, and to estimate the possible effects of increased ground-water use within the basin. Subarea 7 encompasses about 1,030 square miles in north-central Alabama. Subarea 7 encompasses parts of the Piedmont, Valley and Ridge, and Coastal Plain physiographic provinces. The Piedmont Province is 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 can behave as a porous-media aquifer. The Valley and Ridge Province is underlain by fracture- and solution-conduit aquifer systems, similar in some ways to those in the Piedmont Province. Fracture-conduit aquifers predominante in the well-consolidated sandstones and shales of Paleozoic age; solution-conduit aquifers dedominate 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 Cahaba 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 atuomated 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 763 cubic feet per second at Centreville, Ala., where the Cahaba River exits Subarea 7 into Subarea 8. Mean-annual baseflow represented about 48 percent of total mean-annual stream discharge for the period of record. Stream discharge for selected sites on the Cahaba River and its tributaries were compiled for the years 1941, 1954, and 1986, during which sustained droughts occurred throughout most of the Apalachicola-Chattahoochee-Flint and Alabama-Coosa-Tallapoosa River basin area. Stream discharges were assumed to be sustained entirely by baseflow during the latter periods of these droughts. Estimated baseflow near the end of these droughts averaged about 21 percent of the estimated mean-annual baseflow in Subarea 7 (ranged from about 16 to 25 percent for individual drought years). The potential exists for the development of ground-water resources on a regional scale throughout Subarea 7. Estimated ground-water use in 1990 was about 2 percent of the estimated mean-annual baseflow, and 9.7 percent of the average drought baseflow near the end of the droughts of 1941, 1954, and 1986. Because ground- water use in Subarea 7 represents a relatively minor percentage of ground- water recharge, even a large increase in ground-water use in Subarea 7 is likely to have little effect on ground-water and surface-water occurrernce in Alabama. Indications of long-term ground-water dec