Georgia Geologic Survey Information Circular 63
The Greater Atlanta Region encompasses about 6,000 square miles in the Piedmont physiographic province of west-central Georgia. Municipal and industrial water supplies in the area are derived mainly from surface water taken from rivers, streams, and impoundments. Large withdrawals now and predicted for the future are causing concern about surface-water sources being able to meet the rising demands. This study was conducted to assess the availability of ground water in the crystalline rocks of the area, and to devise methods for locating sites for high-yielding wells that could provide alternative sources of supply.
The Greater Atlanta Region is roughly divided in half by the Chattahoochee River, which follows a comparatively straight southwesterly course for nearly 110 miles across the area. Streams in the north half of the area, including the Chattahoochee River basin, mainly have rectangular and trellis drainage styles and clearly show the influence of geologic control. The topography and drainage are closely related to bedrock permeability and conventional methods for locating high-yielding well sites apply to most of the area. In contrast, the south half of the area has a superimposed dendritic drainage style in which streams developed more or less independently of the underlying geology. There, the topography and drainage are poorly related to bedrock permeability; many high-yielding wells occupy ridge crests, steep slopes, and bare-rock areas normally considered to be sites of low yield potential.
To better understand the occurrence of ground water in the area, detailed geologic studies were made of 1,051 high-yielding well sites. The results showed that large well yields are available only where aquifers have localized increases in permeability. This occurs mainly in association with certain structural and stratigraphic features, including: (1) contact zones between rocks of contrasting character and also within multilayered rock units, (2) fault zones, (3) stress relief fractures, (4) zones of fracture concentration, (5) small-scale geologic structures that localize drainage development, (6) folds that produce concentrated jointing, and (7) shear zones. Methods for selecting high-yielding well sites using these structural and stratigraphic features are outlined in the report.
Borehole geophysical techniques were used to study the nature of water-bearing openings. Sonic televiewer logs revealed that in several wells the water-bearing openings consist of horizontal or nearly horizontal fractures 1 to 8 inches in vertical dimension. The fractures were observed in granitic gneiss, biotite gneiss, gneiss interlayered with schist, and in quartz-mica schist. The writers believe the openings are stress relief fractures formed by the upward expansion of the rock column in response to erosional unloading. Core drilling at two well sites confirmed the horizontal nature of the fractures and showed no indication of lateral movement that would associate the openings with faulting.
Wells that derive water from horizontal fractures characteristically remain essentially dry during drilling until they penetrate one or two high-yielding fractures. The fractures are at or near the bottom of the wells. The high-yielding fractures are at or near the bottom of wells because: (1) the large yields were in excess of the desired quantity and, therefore, drilling ceased, or (2) in deep wells yielding 50 to 100 gal/min or more, the large volume of water from the fracture(s) "drowned out" the pneumatic hammers in the drill bits, effectively preventing deeper drilling. Twenty-five wells in the report area are known to derive water from bottom-hole fractures, all of which are believed to be horizontal stress relief fractures. Other wells in the area are reported to derive water from bottom-hole fractures, which also are believed to be stress relief fractures. These wells occupy a variety of topographic settings, including broad valleys, ridge crests, steep slopes, and bare-rock areas, indicating that stress relief fractures are present beneath uplands and lowlands alike.
Wells deriving water from stress relief fractures have much greater average depths than wells reported from other crystalline rock areas. Many of the wells are 400 to 550 feet or more deep and derive water from a single fracture at the bottom of the hole. In one area, 62 percent of the wells that supply 50 gallons per minute or more are from 400 to more than 600 feet deep. The chance of obtaining large well yields from stress relief fractures is significantly increased by drilling to about 620 feet.
In general, moderate quantities of ground water presently are available in the report area. Most of the 1,165 high-yielding wells that were inventoried during this study supply from 40 to more than 200 gallons per minute. The distribution of these wells with respect to topography and geology indicates that most were located for the convenience of the users and that the large yields resulted mainly from chance, rather than from thoughtful site selection. By employing the site selection methods outlined in this report, it should be possible to develop large supplemental ground-water supplies in most of the area from comparatively few wells.
Coweta, Fayette, Henry, and Clayton Counties in the south part of the area that include the communities of Newnan, Shenandoah, Peachtree City, and Fayetteville are expected to grow rapidly during the next 25 years. Because of unfavorable quality conditions in the Chattahoochee River, these communities and surrounding areas are being forced to turn to small, marginal streams as water-supply sources. These streams are vulnerable to pollution from nonpoint sources and are seriously affected by prolonged drought. For these reasons, the southern Atlanta area is one that ~an benefit greatly from supplemental groundwater supplies. At present, all of Coweta County outside the city of Newnan uses ground water exclusively, and much of the four-county area soon may require ground water for supplemental or primary sources of supply. Large quantities of ground water are available in the four counties, as indicated by the presence of 168 wells that supply 40 to more than 200 gallons per minute.
Contrary to popular belief, many wells in the Greater Atlanta Region are highly dependable and have records of sustaining large yields for many years. Sixty-six mainly industrial and municipal wells have been in use for periods of 12 to more than 30 years without experiencing declining yields.
Well water in the area generally is of good chemical quality and is suitable for drinking and most other uses. Concentrations of dissolved constituents are fairly consistent throughout the area, and except for iron, rarely exceed drinking water standards.
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