Summary appraisals of the nation's ground-water resources– South Atlantic-Gulf region

Abstract

Precipitation in the 270,000-square-mile South Atlantic-Gulf Region ranges from 44 to 80 inches, and the average runoff is about 15 inches. The ground-water discharge that forms the base flow of streams is conservatively estimated to be about 78,000 million gallons per day the equivalent of about 6 inches of precipitation. On this basis, the regional sustained ground-water supply is about 286,000 gallons per day per square mile. Projected water use through 2020 indicates that about 10 percent of the supply will meet the region's requirement for ground water. Enormous quantities of ground water are available in the South Atlantic-Gulf States in the extensive aquifers that underlie the Coastal Plain province. Large supplies of ground water can be developed also in places in the Cumberland Plateau and the Valley and Ridge provinces. In the Piedmont and Blue Ridge provinces ground water is available only in modest quantities. The principal Coastal Plain aquifers consist largely of deltaic sand and gravel deposits of Cretaceous to Quaternary age; however, a notable exception is the highly permeable Tertiary limestone aquifer, which underlies parts of four States. Ground water in the Blue Ridge and the Valley and Ridge provinces is available from openings along fissures and joints in consolidated rocks and from the unconsolidated sand and gravel in the residuum and alluvium that overlie the bedrock. In parts of these areas faulting and folding has created favorable conditions for the development of moderate supplies of ground water. In the Piedmont province modest quantities of water are available from joints and fractures. Solution cavities in massive limestone units in some areas underlying the Cumberland Plateau section yield very large quantities of ground water. Most of the major Coastal Plain aquifers are recharged where they are exposed and water moves downdip to the south. The general direction of ground-water movement in the Coastal Plain aquifers is seaward. There is, however, movement of water vertically and laterally that affects pressure and quality in every aquifer. Ground water of good chemical and physical quality is available generally except in a small area in western Alabama and eastern Mississippi and in some small coastal areas. Freshwater extends to depths of slightly more than 3,000 feet in some areas. In many areas, water can be produced that does not require any treatment for general use. The more common objectionable constituents or properties are excessive iron, fluoride, hardness, color, and acidity. Contamination or pollution of ground water is not a major regional problem, but it is significant in local areas. Many of these local problems of contamination were the result of earlier improper disposal of oil-field and industrial waste. Saltwater intrusion either directly from the sea or from estuaries has been reported in every State in the region, and the potential for the proliferation of this problem is very large. The relationship of ground water and surface water is significant at many places in the region where large ground-water withdrawals affect swamps, lakes, and the flow of streams and springs. There is a potential in some areas for using ground water during dry periods for low-flow augmentation of stream supplies. In other areas it is feasible to recharge ground-water reservoirs artificially by means of injection wells, pits, and water spreading. Although large water-level declines caused by withdrawals have occurred locally, water-level changes have been small or modest when considered on a regional basis owing to the abundant rainfall and favorable recharge conditions. The saline-water resources of the region are known to be extremely large. In some areas where the availability of freshwater is limited, saline ground water is available for desalinization to supplement freshwater supplies. Also, the saline aquifers are potential storage sites for injected freshwater. Ground-water problems generally are not severe. Critical situations are restricted to areas where large quantities of ground water are being withdrawn or where aquifers are contaminated by oil-field or industrial waste. Large withdrawals in coastal areas have caused some saltwater intrusion. In other localities, highly mineralized water may have migrated along fault zones to freshwater aquifers. Many of the present problems can be resolved or ameliorated by redistributing withdrawals or developing alternative water sources.