The Natural Georgia Series: The Flint River

Design by Lenz Design, Decatur, Georgia.

The Underground Waters of the Flint River Basin

By David W. Hicks and Stephen P. Opsahl

Groundwater is available most everywhere in the Flint River Basin. However, the aquifer yield varies greatly from location to location, and it can be difficult to obtain a usable supply of water. Both are dependent on the water storing and transmitting characteristics of the rocks or sediment present beneath the area. Typically the yield of a well is significantly higher in the Coastal Plain than in the Piedmont; however, some wells in the Piedmont can yield as much as several hundred gallons per minute on a sustained basis. In the lower Flint River Basin, most wells used for agricultural irrigation and for municipal supply typically yield more than 1,000 gallons per minute, and yields of more than 4,400 gallons per minute have been measured from large-diameter wells southwest of Albany.

Historically, most communities in the northern part of the state relied on groundwater as a drinking water source. The city of Lawrenceville was one of the first communities to develop its groundwater supplies. In 1912 the city drilled its first well that produced more than 400 gallons per minute. The city of Atlanta maintained numerous wells as a part of its water distribution system for many years. Today, many communities in the metropolitan Atlanta area are conducting groundwater exploration to supplement their dwindling surface-water supplies. As surface-water supplies become more scarce in this region, groundwater exploration could increase.

The abundance of groundwater supplies in the Albany area of Georgia has long been recognized. Albany became known as the "Artesian City" due to the work of Col. John P. Fort who successfully completed the first flowing artesian well in Georgia in 1881 at a location about 10 miles west of Albany. In the 120 years since Col. Fort's discovery, groundwater use has increased beyond anyone's wildest dreams. During the typically dry early summer months, groundwater use may be several billion gallons per day in the middle and lower Flint River Basin. This high rate of use has taken its toll on the environment. In some parts of the Flint River Basin, groundwater levels have declined markedly as a direct result of water use. In the area around Albany and to the northwest, artesian wells no longer flow. In fact, since the 1940s water levels in the deeper aquifers have declined more than 150 feet as a result of water use. In other areas, groundwater pumping has been associated with declines in streamflow and spring flows.

In the Piedmont physiographic province of the upper Flint River Basin the availability of water in crystalline-rock aquifers is related directly to the complexity of the geologic terrains. Crystalline-rock aquifers are present primarily along foliation (zones where there are mineralogical differences between layers), fractures, joints, geologic contacts between formations and rock types, veins, pegmatites, and other geologic features that have been enhanced by differential weathering. Differential weathering may be one of the most important factors affecting the ability of crystalline-rock aquifers to function. This type of weathering occurs as adjacent rock units composed of different minerals weather at variable rates because of the mineralogical differences. As the easily weathered minerals dissolve, voids and solution openings develop. These secondary openings enhance the rocks' ability to store and transmit groundwater. Groundwater in crystalline-rock settings is present in irregularly distributed, highly localized, and discontinuous water-bearing zones. Unlike aquifers in the Coastal Plain, Piedmont aquifers are not extensive and have relatively limited water-transmitting capability.

Coastal Plain geology is markedly different from that of the Piedmont. About 150 million years ago, during the Cretaceous Period, the seas advanced upon the Georgia landscape. The advances and retreats of the seas persisted on the Georgia Coastal Plain for nearly 130 million years, and for the most part, the seas halted at the imaginary line now referred to as the fall line. This imaginary line separates the Piedmont physiographic province from the Coastal Plain and extends from Columbus, through Macon, to Augusta.

As a result of the many advances and retreats of the ancient seas, many layers of sediment were deposited. Some of the layers are limestone. Limestone is much like cement in that it has very few pores. Because the limestone is hard and brittle it is easily fractured. As water circulates through the fractures it dissolves the calcium carbonate that solidifies the limestone. Over time the fractures become enlarged as the calcium carbonate is dissolved. The solution-enlarged openings enhance the natural ability of the limestone to store and transmit groundwater.

Other layers consist of sand and gravel and contain relatively large void spaces between the individual grains, called pores. Some of the layers consist of silt and clay that are very fine grained and do not contain large void spaces. The relative percentage of the void spaces compared to the individual grains is referred to as the "porosity" of the sediment layer. When the individual pores are interconnected, the sediment is said to be permeable because it will allow the flow of water between the pores. The "porosity" and "permeability" are measures of the sediments' ability to store and transmit water. Sediment that exhibits both of these characteristics forms the Coastal Plain aquifers. Sediments that do not exhibit permeability form the confining layers that separate the stratified aquifers. Both the storage and transmitting capability of the aquifers varies vertically and areally.

Because the ancient seas came upon the Georgia landscape from the southeast, they were shallower to the northwest and deeper to the southeast. As a result, the sedimentary layers that contain the Coastal Plain aquifers are thinner towards Columbus, and progressively thicken toward the Georgia coast.

Underground water is derived directly or indirectly from rainfall. Water falling on the surface of the earth is taken up by evaporation, is consumed (transpired) by plants, is carried off by surface drainage, or infiltrates into the soil. Piedmont crystalline-rock aquifers rely on the porosity of the weathered rock, called saprolite, that overlie the crystalline rocks for water storage. The fractures within the crystalline rocks contain very limited storage space for groundwater. The Coastal Plain aquifers are recharged almost entirely by the rain that falls in the areas where the aquifer is near land surface. As each aquifer becomes deeper in the geologic profile toward the southeast, it no longer receives the benefit of local rainfall and recharge.

The Georgia Coastal Plain sediments contain as many as eight separated, major aquifers, five of which are found within the Cretaceous-age sediments. From oldest to youngest, the Cretaceous aquifers are the Tuscaloosa, Eutaw, Blufftown, Cusseta Sand, and Providence. The remaining three aquifers, from oldest to youngest, are the Clayton, Claiborne, and Floridan. The Flint River and many other area streams have eroded into each of the Cretaceous aquifers between the fall line and Montezuma. From Montezuma south to the northern part of Lake Blackshear, the streams cut into the upper part of the Claiborne aquifer. The Clayton aquifer, which lies vertically between the Providence and the Claiborne, is not present near the Flint River. In the river reach between Lake Blackshear and Lake Seminole, many streams in the lower Flint River Basin dynamically interact with the Upper Floridan aquifer. In the areas where the Coastal Plain aquifers interact with area streams, groundwater may be discharged into the stream, or streamflow may be lost to the aquifer. The direction of flow is dependent on the relative hydraulic head (pressure) relation where the stream and the aquifer meet.

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