HYDROGEOLOGY OF THE GULF TROUGH- APALACHICOLA EMBAYMENT AREA, GEORGIA Madeleine F. Kellam Lee L. Gorday Georgia Department of Natural Resources Environmental Protection Division Georgia Geologic Survey Cover Photo: Windmill on a domestic well in Tarrytown, Montgomery County, Georgia. HYDROGEOLOGY OF THE GULF TROUGHAPALACHICOLA EMBAYMENT AREA, GEORGIA Madeleine F. Kellam Lee L. Gorday Georgia Department of Natural Resources Lonice C. Barrett, Commissioner Environmental Protection Division Harold F. Reheis, Assistant Director Georgia Geologic Survey William H. McLemore, State Geologist Atlanta 1990 BULLETIN 94 M. Wallace Holcombe IN MEMORIAM Wallace Holcombe, the Geologic Survey's Senior Enviromnental Technician for the last ten years, passed away on October 8, 1990. Wallace was always prepared to go the extra mile to see that , the needs of the Geologic Survey staff were met. His diligence in maintaining the geological equipment and field vehicles in proper working condition allowed the staff to maintain their field and laboratory schedules. Wallace's contributions to the success of this report and numerous others over the last ten years will be greatly missed. TABLE OF CONTENTS Page Abstract .......................................................................................................................................... 1 Introduction .................................................................................................................................... 1 General ..................................................................................................................................... 1 Purpose .................................................................................................................................... 2 Scope ........................................................................................................................................ 2 Sources of data ......................................................................................................................... 2 Methods of investigation............................................................................................................ 3 Acknowledgements.......................................................................................................................... 4 Description of study area .............................................. ......... ......... ................................................ 4 Location .................................................................................................................................... 4 Demography and population ............................... .... ......... ................................ ....... .................. 4 Physiography and drainage ....................................................................................................... 7 Climate ..................................................................................................................................... 7 Previous investigations ....................................................... ........................... .................................. 7 Geologic framework ................................................................. ;....................................................... 11 Hydrologic framework .................... :................................................................................................ 11 General ..................................................................................................................................... 11 Surficial aquifer ........................................................................................................................ 12 Floridan aquifer system ............................................................................................................. 12 Claibome aquifer ...................................................................................................................... 15 Clayton aquifer ......................................................................................................................... 17 Cretaceous aquifers .................................................................................................................. 17 Potentiometric trends and water use ............................................................................................... 19 General ..................................................................................................................................... 19 Potentiometric surface of the upper Floridan aquifer ................................................................. 19 Potentiometric trends ................................................................................................................ 21 Water use ................................................................................................................................. 24 Well construction ...................................................................................................................... 28 Ground-water availability ................................................................................................................ 28 General ..................................................................................................................................... 28 Well characteristics and aquifer properties ................................................................................ 28 Specific capacity ..................................................................................................................30 Specific capacity indices .......................................................................................................30 Transmissivity ...................................................................................................................... 30 Hydraulic conductivity .........................................................................................................31 Recharge ................................................................................................................................... 31 Ground-water flow .................................................................................................................... 31 Analysis of ground-water availability ......................................................................................... 31 Lithologic factors and availability of ground water ............................................................... 32 Ground-water flow factors and availability of ground water ................................................. 32 Ground-water quality ...................................................................................................................... 33 General .....................................................................................................................................33 Ground water-quality in the Gulf Trough and Apalachicola Embayment .................................... 33 Sulfate in ground water ........................................................................................................ 34 Distribution of sulfate .................................................................................................... 34 Source of sulfate ............................................................................................................34 Barium in ground water ....................................................................................................... 34 Distribution of barium ...................................................................................................34 Source ofbarium ........................................................................................................... 35 Natural radioactivity in ground water ................................................................................... 35 Distribution of radioactivity ............................................................................................ 35 Source of radioactivity .................................................................................................... 36 v TABLE OF CONTENTS (Continued) Page Summary ........................................................................................................................................ 37 Recommendations ........................................................................................................................... 38 References ...................................................................................................................................... 39 Appendix A. Depth to the top and bottom ofthe Floridan aquifer system .........................................43 Appendix B. Wells used in hydraulic parameter and inorganic chemistry plates .............................. 55 Appendix C. Wells used in barium and gross alpha plates ............................................................... 63 Appendix D. Permeabtlity test results .............................................................................................. 71 ILLUSTRATIONS Figure 1. Location of study area .. ..... ... .... .. ... .... .. ... ... .. .. .. ... ... ... ....... .. ..... .. .. ... ... .. .. .. ...... ... .. .. ........ ... ... ... ... .. 5 2. Approximate location of the Gulf Trough and Apalachicola Embayment..................................... 6 3. Physiographic districts of the study area .. .. .. .. .. .. .. .. .. .. ...... .. .. .. .. .. .. ...... .. .... .. .. .. ...... .. .. .. .. .. .. .. .. .. .. . 8 4. Geographic extent of the surficial aquifer in the study area ....................................................... 13 5. Geographic extent of the Floridan aquifer system in Georgia ...................................................... 14 6. Approximate geographic extent of the Claiborne aquifer in the study area ................................. 16 7. Approximate geographic extent of the Clayton aquifer in the study area .................................... 18 8. Water level in the Upper Floridan aquifer, May 1985 ................................................................. 20 9. Water levels in Floridan aquifer wells, Ben Hill and Mitchell Counties ....................................... 22 10. Water levels in Floridan aquifer wells, Worth and Toombs Counties ........................................... 23 11. Water-level change, Floridan aquifer system, 1969-1978 ........................................................... 25 12. Typical well construction, Floridan aquifer system ..................................................................... 29 13. Areas at greatest risk for elevated levels of natural radioactivity in ground water .......................40 TABLES 1. Number of acres irrigated, 1974 and 1984 ................................................................................. 26 2. Water use in the study area, by county, in million gallons per day ............................................. 27 PLATES (Plates in separate envelope) 1. Stratigraphic units in the GulfTrough-Apalachicola Embayment area and their relationship to the Floridan aquifer system 2. Thickness of sediments overlying the Floridan aquifer system in the Gulf Trough-Apalachicola Embayment area 3. Geology and configuration of the top of the Floridan aquifer system in the GulfTrough-Apalachicola Embayment area 4. Geology and configuration of the base of the Floridan aquifer system in the GulfTrough-Apalachicola Embayment area 5. Thickness of the Floridan aquifer system in the GulfTrough-Apalachicola Embayment area 6. Locations of wells used in mapping aquifer properties and water quality 7. Specific capacity values for wells tapping the Floridan aquifer system in the GulfTrough-Apalachicola Embayment area 8. Specific capacity indices for wells tapping the Floridan aquifer system in the GulfTrough-Apalachicola Embayment area 9. Estimated transmissivity values for wells tapping the Floridan aquifer system in the Gulf Trough- Apalachicola Embayment Area vi PLATES (Continued) (Plates in separate envelope) 10. Estimated hydraulic conductivity values for wells tapping the Floridan aquifer system in the Gulf Trough-Apalachicola Embayment area 11. Major ion geochemistry of ground water from the Floridan aquifer system in the Gulf Trough- Trough-Apalachicola Embayment area 12. Total dissolved solids in ground water from the Floridan aquifer system in the Gulf Trough- Apalachicola Emabayment area 13. Distribution of sulfate in ground water from the Floridan aquifer system in the Gulf Trough- Apalachicola Embayment area 14. Distribution of barium in ground water from the Floridan aquifer system in the Gulf Trough- Apalachicola Embayment area 15. Distribution of gross alpha activity in ground water from the Floridan aquifer system in the Gulf Trough-Apalachicola Embayment area Vll HYDROGEOLOGY OF THE GULF TROUGH -APALACHICOLA EMBAYMENT AREA, GEORGIA Madeleine F. Kellam Lee L. Gorday ABSTRACT The geologic make-up and hydrologic properties of the Floridan aquifer system in the study area are controlled by the presence of the Gulf Trough-Apalachicola Embayment. The Floridan aquifer system, within the trough-embayment, is composed ofdense, deep-water limestones; and it is thickly overlain by Miocene and younger sediments. Throughout the GulfTrough, and in most of the Apalachicola Embayment, the permeability of the aquiferislowerthanthat typical ofthe Floridan system outside the trough-embayment. This is due to a combination offactors, including the low primarypermeabilityofthe deep-waterlimestones ofthe trough-embayment; limited development of secondary permeability due to thick overburden; and, possibly, a lack of joints or fractures to enhance movement of ground water. However, certain areas withintheApalachicola Embayment and along its south flank are exceptions to this trend. The contact between the Miocene and Oligocene sediments in these areas is a zone of enhanced permeability. The quality ofwater from the Floridan aquifer system is reduced in certain parts ofthe study area. Inthe Colquitt-Thomas-Grady Counties area, sluggish ground-water flow through the lower parts of the aquifer has inhibited the dissolution of gypsum and the removal of sulfate from the aquifer, causing high sulfate levels. The source of elevated levels of barium in ground water from Ben Hill County is not understood. High concentrations of natural radioactivity (mainly as Radium-226) occuringround waterfromtheWheelerMontgomery and Tift-Berrien Counties areas. The ultimate source is Uranium-238, probably derived from weathered crystalline rocks of the Piedmont. The uranium was incorporated in the crystal structure ofthe abundant phosphate minerals of the Miocene confining sediments. Oxidizing recharge waters flowing through these sedi- ments dissolved uranium and transported it into the aquifer. The uranium was deposited on the aquifer matrix in areas where reducing conditions were encountered. The pyrite content ofthe troughembayment limestones provided such reducing conditions. asdid decaying organicmattertrapped in paleo-sinkholes within the top ofthe Oligocene strata. Radioactive decay now contributes Radium-226 to ground water in these areas. INTRODUCTION GENERAL This investigation is the culmination of a multi-year study by the Georgia Geologic Survey of the geology and ground-water hydrology of the GulfTrough andApalachicola Embayment, which are part of a subsurface paleo-marine channel system in the Georgia Coastal Plain. The Floridan aquifer system, the most widely used aquifer in the Coastal Plain, is affected bythe presence ofthe Apalachicola Embayment, and by its northeastward extension, the Gulf Trough. Both the quality and the availability of ground water in and near the trough-embayment are reduced. In view of the continuing water needs of municipal, agricultural, and industrial users of this aquifer, a comprehensive study of the geology and hydrogeology of this area was conducted in order to explain the causes of reduced well yields and water quality. This study of the Gulf Trough-Apalachicola Embayment includes three reports. A data report (McFadden and others, 1986) presents lithologic logs and a table surrnnarizing stratigraphic data on all wells used in the ensuing geologic and hydrologic investigations. The locations of these wells are displayed on a 1:500,000 scale base map. A geologic report (Huddlestun and others, in preparation) presents the stratigraphic frame- 1 work for the hydrologic investigation of the Gulf Trough and Apalachicola Embayment. The geologic report contains lithologic and faunal descriptions of stratigraphic units, isopach and structure-contour maps, and discussions of the nature, origin, and geologic history of the troughembayment. The present report, which was completed in 1988, describes the aquifers in the Gulf Trough-Apalachicola Embayment area, presents data on the availability and quality of ground waterfrom the Floridan aquifer system, and makes recommendations for the future development of ground-water resources in the area. PURPOSE The Floridan aquifer system is the most widely used aquifer in Georgia. Potentiometric maps of the Floridan system in Georgia consistently show an anomalous steepening of the potentiometric surface trending northeastward across the Coastal Plain, from Grady County in the southwest, to Bulloch County in the northeast. This anomaly roughly parallels the trend of the Gulf Trough-Apalachicola Embayment. The Floridan aquifer system in the vicinity of this anomaly exhibits poor well yields and locally reduced water quality, including abnormally high concentrations of barium, sulfate, and natural radioactivity. The present study was designed primarily to examine the hydrogeology of the Floridan aquifer system in the Gulf TroughApalachicola Embayment area. The goal of this study was to assess the principal controls on the occurrence, availability, and quality of ground water from the Floridan system in the study area. Specifically, the following aspects were to be addressed: 1) the cause of the potentiometric anomaly; 2) ground-water occurrence and movement in the Upper Floridan aquifer; and; 3) water quality, particularly the mechanisms which produce the abnormally high barium, sulfate, and natural radioactivity levels which appear to be associated with the Gulf Trough-Apalachicola Embayment. SCOPE Prior studies of the ground-water hydrology of the Gulf Trough area were hampered by an incomplete understanding of its complex geology. This study used data from approximately 500 wells to define the stratigraphy ofthe GulfTroughApalachicola Embayment area. The interpretation of the geology of the trough-embayment area which has emerged from this study allows a more comprehensive view of the hydrogeology of the trough-embayment area than had been possible previously. The hydrogeology phase of this study was designed (1) to describe the lithology and extent of aquifers in the vicinity of the troughembayment; (2) to produce new data on the hydraulic characteristics of the Floridan aquifer system in the Gulf Trough and Apalachicola Embayment; (3) to discuss the hydraulic characteristics ofthe Floridan aquifer systemin the area; and (4) to examine the possible causes of the reduced quality and availability of ground water from the Upper Floridan aquifer in and near the trough-embayment. SOURCES OF DATA Data for the hydrogeologic study were gatheredfrom a variety ofsources, both published and unpublished. Published sources oflithologic data and stratigraphic data include collections of well logs by Herrick (1961) and Applin and Applin (1964). Asumma:ry ofpetroleum exploration wells in Georgia (Swanson and Gernazian, 1979) provided stratigraphic and well location data. In addition to well logs from these sources, a number of additional wells were examined and five wells were cored and examined specifically for this project. These were all wells for which the Georgia Geologic Survey (GGS) retains cutting or core samples in its cuttings library or wells drilled (cored) specifically for this project. All are assigned a sequential registration number, known as a GGS number, and are available for inspection at the Georgia Geologic Survey in Atlanta. The stratigraphic data obtained from all the above sources have been published in Georgia Geologic Survey Information Circular 56 (McFadden and others, 1986). Sources of unpublished geologic data include the files of the GGS in Atlanta and those of the U. S. Geological Survey (USGS), Water Resources Division office in Doraville, Georgia. These files include lithologic logs prepared by staffofthe GGS and USGS and a small number of logs prepared by the staffs of petroleum exploration companies. Hydrologic data for this study were obtained primarily from the files ofthe GGS, the USGS, ~nd the Water Resources Management Branch ofthe Georgia Environmental ProtectionDivision. These data include well-construction details, production-test data, and water-quality analyses. Published water-quality data were also included in 2 this study (Grantham and Stokes, 1976). Permeability tests were conducted on cores collected during the project, and the results are summarized in Appendix D. The potentiometric maps of the Floridan aquifer used in this report were produced by the Water Resources Division of the USGS (Clarke and others, 1979; Bush and others, 1987). METHODS OF INVESTIGATION The wells chosen for inclusion in the study were those for which the most complete information was available in the form of lithologic logs or cuttings, well construction data, and verifiable locations. These locations were field checked wherever possible. Geophysical and paleontological logs were also available in some case. In addition, five wells were cored and geophysically logged, specifically, for this study. The definition ofthe Floridan aquifer system in the study area was reexamined on the basis of the revised stratigraphic interpretation ofthe Gulf Trough-Apalachicola Embayment area. Using lithologic and geophysical logs ofall wells from the stratigraphic data base, the top and base of the Floridan aquifer system in the study area were determined and its thickness was mapped. The depth to the top ofthe Upper Floridan aquifer was also calculated and mapped. Appendix A lists the wells used in mapping the aquifer and includes land surface elevations, depth to top of the aquifer, and elevations of its top and base. More complete information on these wells is presented by McFadden and others (1986). Specific-capacity maps were constructed using data from well production tests. These tests variedgreatlyin duration, rangingfrom an hour or less to several days. Construction ofthe wells also varied widely in such details as diameter, depth, and length of open borehole. Specific-capacity indices were obtained by dividing the specificcapacity values by the length of open borehole of the well; thus, normalizing, in part, the varying well construction. Maps were constructed to show the range and distribution of specific-capacity indices. Appendix B summarizes construction data for these wells. Time-drawdown data, needed to calculate transmissivity (T). storativity (S). and hydraulic conductivity, are vexy limited for the study area. However, an estimate of transmissivity can be obtained from the specific capacity (Q /s). Lohman (1979, p. 52) noted a relationship between specific capacity and transmissivity of confined aquifers which can be expressed as: T = 2.3CQ/s ) 41t w . log (2.25Tt/r2~ S\ where pumping rate (Q). drawdown (s). and duration of test (t) are measured during the well production test, and rw is the diameter of the well.This relationship was used to estimate the transmissivity of the aquifer at wells for which time-drawdown data is not available. Pertinent data on these wells is summarized in Appendix B. Storage cooefficients (S) for the Floridan aquifer system in the study area were, also, not generally available. AS value typical for confined aquifers (0.0001) was used in the transmissivity estimations. The effect of changes in the S on the estimated transmissivity value is relatively small; an order of magnitude change in the S value, typically, produces only an ll per cent change in the estimated transmissivity. Transmissivity estimates derived using this method agreed well with values obtained using time-drawdown data. Tests ofvertical hydraulic conductivity were conducted on 140 core samples from the five core holes drilled for this study and from two U. S. Gypsum cores. Test samples were selected at intervals ofapproximately 25 feet Sampling intervals varied, however, where sample gaps occurred or where the core was fragmented. The sides of core sampleswere sealed bywrapping themtightly with impermeable tape. Polyvinylchloride caps were fitted on the ends of the samples and sealed with silicone sealer. The samples were oriented vertically and saturated with water. Water was introduced from the bottom of the core sample to minimize the possiblity of trapping air in the sample. Core samples varied greatly in permeabilty, making it neccessary to employ both constant head and falling head tests. Samples with relatively high vertical hydraulic conductivity values were tested using the constant head method. In this method, a constant head gradient was established across the sample while measuring the rate of flow. A constant head was maintained by the use of an overflow tube on the supply side of the sample. The flow rate was measured by noting the time required to fill a known volume. Head gradients used ranged up to approximately 2 feet. Measurements were made at three or more different gradients for each sample. For each measurement, the flow rate was plotted against the head gradient. If the values plotted on a line that extended through the origin, then laminar 3 flow conditions through the sample were assumed. A non-linear plot of values was assumed to indicate turbulent flow through the sample. These samples were retested at lower head gradients. In several instances, scatter of the head gradient versus flow rate plot could not be attributed to turbulent flow. Measurements repeated several hours later showed a linear plot, suggesting that the saturation of the sample may have caused swelling of clay minerals. A longer interval between saturation and testing eliminated this problem. When flow through the core sample was too small to be measured readily by timing, the filling of a known volume, falling head tests were used. Fallinghead testsmeasure the rate atwhich water enters the sample. Following saturation, a tube of known cross-sectional area, open to the sample, was filled with water. The rate at which the water level in the tube dropped was measured. The initial head gradient (the difference in height between the water level in the supply tube and the discharge point) ranged up to 5.5 feet. The head values were plotted against time to insure that there were no sudden changes in the rate of decline. Samples of low permeability, typically, required several days to saturate. Those samples which did not saturate after four days or more could not be tested and are assumed to have a vertical hydraulic conductivity of0.001 ft/day or less. This appears to be a conservative figure, as several samples which did saturate had vertical hydraulic conductivity values lower than 0.001 ft/day. Hydraulic conductivity values were calculated using readily available equations derived from Darcy's Law (Freeze and Cherry, 1979, p. 335-336). Vertical hydraulic conductivity data are presented in Appendix D. Water-quality maps were produced using a combination of published and unpublished data. Sequential identification numbers were assigned to the wells and municipal water systems used in the maps, keyed to appendices which list sources of data. ACKNOWLEDGEMENTS The authors would like to thank the many people whose assistance helped make this report possible. Sue Rodenbeck and Susan Mosteller collected and checked much ofthe data on which this study was based. Stephen McFadden partici- pated in the early portion of this study and also provided information on research into the occurrence of radionuclides in ground water. John Fernstrom, now retired from the Georgia Environmental Protection Division's Ground-water Program, contributed data onthe occurrence ofnatural radioactivity in the GulfTrough/Apalachicola Embayment area. Wendell Pope, of Abner Pope and Sons Well Drilling, supplied well data and cooperatedwith geophysical logging ofwells in the vicinity of Alapaha. Grady Thompson, of Bishop Pump and Well Service, provided well data and general observations on the nature of the Gulf Trough/Apalachicola Embayment. He also as-_ sisted in repairing a damaged inflatable packer. James Miller and Woody Hicks ofthe U.S. Geological Survey reviewed the manuscript and made many helpful comments. DESCRIPTION OF STUDY AREA LOCATION The 27-county study area is located in the Coastal Plain Province of Georgia. The Coastal Plain covers approximately 60 percent of the state's total area and contains Georgia's major aquifers. The study area extends northeastward from the southwestern comer of the state to the Savannah River, an area of approximately 11,550 square miles (Figure 1). The study area takes in the Apalachicola Embayment and Gulf Trough (Figure 2), as well as adjacent portions of the Coastal Plain. The Apalachicola Embayment occupies the southwestern end of the study area, and the GulfTrough occupies the central portion. DEMOGRAPHY AND POPULATION The total estimated population of the study area was 492,900 in 1985 (Bachtel, 1987). with a populationdensity ofapproximately 43 people per square mile. The population is primarily rural, producing agricultural and forest products as the main economic activities. A number ofsmall cities are located in the study area, eight ofwhich have populations in excess of 10,000. These eight cities are: Bainbridge, Douglas, Fitzgerald, Moultrie, Statesboro, Thomasville, Tifton, and Vidalia. Only Moultrie (15,508) and Thomasville (18,352) contain populations greater than 15,000 (Bachtel, 4 0 20 40 Ml I II I I 1---4 1----1 0 20 40 KM N t Figure 1. Location of the study area. 5 0 20 40 Ml E3 E3 I 1-1 H 0 20 40 KM N t Figure 2. Approximate location of the Gulf Trough and Apalachicola Embayment. (After Huddlestun and others, in prep.) 6 1987). PHYSIOGRAPHY AND DRAINAGE Portions offive physiographic districts make up the study area: the Tifton Upland, Vidalia Upland, Dougherty Plain, Bacon Terraces, and Barrier Island Sequences Districts (Clark and Zisa, 1976). TheTiftonandVidalia Uplands comprise the majority of the study area (Figure 3). These two districts are topographically high areas, ranging in elevation from about 500 feet above sea level in the north and northeast to about 100 feet above sea level in the southeast. The regional slope is southeastward, towards the Atlantic coast. Drainage in these areas is well developed and dendritic. The westem edge ofthe study area, including portions of Decatur, Grady, Mitchell, and Worth Counties, lies in the Dougherty Plain, a gentlyrolling karstic lowland. The regional slope of this district is southwestward, towards the Gulf of Mexico. Maximum elevations of about 300 feet above sea level occur in the northeast and a minimum elevation of about 77 feet above sea level in the southwest, at Lake Seminole. The Dougherty Plain contains few surface streams but many sinkhole lakes and marshes. Portions of Appling, Atkinson, Bacon, Ben Hill, Coffee, Irwin, and Jeff Davis Counties lie in the Bacon Terraces District, an area of subtly dissected terraces, paralleling the present coastline. Terrace levels range in elevation from about 330 to about 160 feet above sea level. Drainage is southeast-trending, dendritic, and extended. The easternmost end of the study area, ineluding Effingham County and portions ofBulloch and Screven Counties, lies in the Barrier Island Sequence District, an area of abandoned shorelines parallel to the present coast. This area exhibits slight to moderate dissection, with marshes occupying poorly drained lowlands. The study area is crossed by several of the state's major rivers. The Flint River forms the western boundary of the study area and flows through Decatur County in the extreme southwestern part ofthe study area. The Ocmulgee and Oconee Rivers join in the vicinity of Jeff Davis, Montgomery, and Toombs Counties to form the Altamaha River. The Savannah River forms the eastern boundary of the study area. CLIMATE The climate ofthe study area is influenced in the west by the Gulf of Mexico and in the east by the Atlantic Ocean. Winters are generally mild, and summers are hot and humid. The mean annual temperature for the period ofrecord, 1951 to 1980, was 65.7 F at the Tifton experiment station of the National Oceanic and Atmospheric Administration (NOAA). Mean annual precipitation for the same period and location was 46.61 inches. March and July are, usually, the wettest months oftheyearinthe studyarea: while October and November are the driest. Evapotranspiration rates are highest in the spring and summer. PREVIOUS INVESTIGATIONS The Floridan aquifer system has been known by a number ofnames, among them, the principal artesian aquifer, the Tertiary limestone aquifer, the Floridan aquifer, and most recently and formally, the Floridan aquifer system. The large geographic extent of the Floridan aquifer system in Georgia has, until recently, limited the detail of most investigations. Early reports on the hydrogeology of the Floridan in Georgia were of a reconnaissance nature, due to the incompletely understood geology of the Coastal Plain. More detailed work, on a single county or smaller scale, has added to the general understanding of the hydrogeology of the aquifer; and as more detail emerges from these small studies, a larger regional picture of the Floridan aquifer system is being developed. The influence of the Gulf TroughApalachicola Embayment on the Floridan aquifer system in Georgia has only recently been studied. One ofthe earliest reconnaissance studies of ground water in Georgia was that of McCallie (1908), who described wells and springs throughout Georgia and included many driller's logs and water-quality analyses in his descriptions. He identified the upper Eocene limestone, which he called the Vicksburg-Jackson limestone, as the major water-bearing unit of the Coastal Plain. Stephenson and Veatch (1915) related ground water to stratigraphy, and summarized the geology and water resources of the Coastal Plain by county, including information on well construction, well yields, subsurface geology, and water quality. Meinzer (1923). in his summary ofgroundwater occurrence in the United States, identified the Eocene and Oligocene formations of Georgia as important water-bearing strata. In view of the rapid rate at which groundwater resource development was occurring in the Southeast, Warren (1944) published results of investigations of limestone aquifers of the Coastal 7 EXPLANATION [I!J] Barrier Island Sequence District l:i\\J Bacon Terraces District tf=j Dougherty Plain District ({;WJi1 Vidalia Upland District ~ Tifton Upland District /l{(~~~~~~~ 33~,..(.;A:;=x.=::.;::~:" -33o kSCREVENiif.:', tt;jitlt~~{ )' "''j;?Niffi/fi;,;~,YHc\IH~~ ~;,,, ~fir.)I ..............._..,~,.,.,,.,,...:.:.,... Illj ,t,C;;}-'r.i,i.;-i:;.i~;S':{5~1:'~''~-'.'~''i'g'Cl{'A~'"~Ni'lD'l>LlM''E":'RU',Hf.'~:,<~:',W:".?~;,'i.',~.',m'.,t".,u'.'~,.r:.,.. - ,,,, -w ""''lf~~~~\tli~R@1'1_ ~I].~ ) ~"r'NI.I l 11' G ~ H-41.1, ' :~TAT 00 ....;;.........:.::":L'":;:':",,"J.!..<. -'"' DAVIS'...;f;::i-':"-A:P:PoL,I;N:G,'.:~:l .82o PELHAM ESCARPMENT<, ,..t ,c:.c:: ;: F ~-MIT f_ c-_-_-_:-_-_-._:: 31-t-: f=t_--D_E-C_-AT_-UR 1- /- COLQUITT COOK - 31 '.:::::::-:-::;::f-. ;!.!.~$....~ I 83 -~ 8140 Study Area 0 20 40 60 Kilometers o 20 40 Miles After Clark and Zisa, 1976. Figure 3. Physiographic districts of the study area. Plain. Warren noted that the principal artesian aquifers in the Southeast are limestones of Eocene and Oligocene age, which crop out in a belt extending northeastward from the southwestern corner of Georgia, roughly paralleling the Fall Line. Recharge areas were identified in the area of outcrop, and also in the Valdosta area. where lime sinks allow direct access to the aquifer. Warren included the Oligocene Suwannee Limestone and the Eocene Ocala Limestone in the principal artesian aquifer, and identified the impeiVious clays and marls of the Miocene Hawthorne Formation as the upper confining unit. Transmissivity values for the principal artesian aquifer were calculated to range from 100,000 to 1,000,000 square feet per day (ft2 / d). Herrick and Wait (1956) characterized the availability and quality of ground water from Coastal Plain strata, including the middle Eocene to lowerMiocene principal artesian aquifer. They cited dolomitization as the cause of reduced utility ofthe principal artesian aquifer in the vicinity of Grady, Thomas. and Colquitt Counties, and mentioned the presence of a "subsurface structural trough" in this area as the cause ofdecreased permeability ofthe aquifer and increased hardness ofthe ground water. In a 1960 study, Wait described ground-water quality in the Ocala Limestone, characterizing the water as moderately hard to hard, slightly alkaline, and of calcium bicarbonate type. He noted that in some areas of the Tifton Upland, water from the Ocala Limestone contained elevated levels of sulfate. The principal artesian aquifer was redefined to include limestones ofMiocene age. Wait presented water-quality data for southwestern Georgia, and discussed the relationship of structural features and sinkholes to water-quality trends in the area. In a study that has formed the basis for much ofthe subsequentgeologic and hydrogeologic work in the Coastal Plain, Herrick (1961) published a collection of lithologic logs ofwells in the Coastal Plain of Georgia. He described well cuttings from 354 wells, noting possible water-bearing units in each. Among the reports detailing ground-water resources of single counties in the study area was that of Owen (1963) on the geology and groundwater resources of Mitchell County. Mitchell County was divided into three hydrologic zones. the Dougherty Plain, the solution escarpment. and the Tifton Upland. Mitchell County lies, for the most part, in the Dougherty Plain. In this karstic lowland, the Ocala Limestone is at the land surface, overlain by a blanket of residuum. Water in the Ocala Limestone is under unconfined con- ditions, except where the overburden of clayey residuum is sufficiently thick to confine it. Wells drilled in the Dougherty Plain often tap the middle Eocene Tallahatta Formation, in addition to the Ocala. Few wells have been drilled in the solution escarpment. probably due to the greater thickness of overburden in this area. In the Tifton Upland, the Ocala Limestone is deeply buried by thick Miocene and Oligocene deposits, which Owen interpreted as evidence of downwarping in the extreme southeastern part of the county. The term principal artesian aquifer was not used in this study because the Oligocene and Eocene limestone are unconfined overmost ofMitchell County. Callahan (1964) attempted to quantify the amount of water available from Coastal Plain aquifers, including the principal artesian aquifer. The principal artesian aquifer was redefined as an aquifer system made up of a number of interconnected water-bearing strata. Aquifer geometry. flow wnes, water quality, and recharge were considered. An estimated maximum "safe yield" of the principal artesian aquifer system was calculated, and the probable effects of maximum pumpage on water levels in the aquifer and in streams were assessed. Using potentiometric maps of the principal artesian aquifer, Callahan noted an apparent decrease in transmissivity in the position now identified as the Gulf TroughApalachicola Embayment. Two northeast-trending faults, offsetting or constricting the permeable wnes ofthe aquifer, were postulated as the cause of this anomaly. Sever (1966). in another small-scale study, surveyed the ground water and geology of Thomas County. He identified the Eocene Ocala Limestone and Oligocene Suwannee Limestone as the principal aquifer in the county. excluding the middle Eocene Lisbon Formation due to its highly mineralized waters. Sever noted the wide range of weli yields in Thomas County, from 60 gallons per minute (gal/min) in the northeast. to 3,000 gal/ min in the southeast; and he postulated a fault, the Ochlockonee fault, to explain the steep gradient of the potentiometric surface of the principal artesian aquifer. In addition, certain water-quality anomalies were identified, including elevated levels of sodium chloride and sulfate. In a paper on the Tertiary limestone aquifers of the southeastern states. Stringfield (1966) identified the Ocala Limestone and the Suwannee Limestone as the major water-bearing units in Georgia, and described the hydraulic properties, recharge, and water quality of these units. The steepening of the hydraulic gradient across the 9 Georgia Coastal Plain was discussed and anumber of possible explanations were given. A written communicationfrom H.E. LeGrand Un Stringfield, 1966, p. 123) attributed the anomaly to the proximity of the recharge area. LeGrand's theory was that there was upward leakage from the principal artesian aquifer to the overlying Miocene rocks, which diminished as the water moved under the confiningHawthome Formation, and as the aquifer became thicker and more permeable. Stringfield (p.132) attributed the potentiometric anomaly to "recharge and discharge relationships and...changes in the permeability and thickness of the limestone." Sever and Herrick (1967) discussed the origin of poor-quality ground water, high in sulfate, iron, flouride, and total dissolved solids, derived from wells in the Grady County area. They concluded that this water, formerly thought to be from the Ocala Limestone, was probably being obtained from a limestone of early Oligocene age, never before described in Georgia. Sever (1969) reported the results of aquifer tests and water-quality analyses at the cities of Alapaha, Coolidge, Fitzgerald, and Thomasville. Transmissivityvalues calculated for wells tapping the Oligocene and Eocene limestones ranged from 16,000-ft2I d at Fitzgerald to as large as 2, 700,000 ft2I d at Thomasville. The extremely high trans- missivityvalue obtainedfrom theThomasville test was attributed to solution of the limestone along structurally-producedjoints and fractures. In addition to calculations of transmissivity, storage coefficients were derived where possible, and the effects of future pumpage were estimated. Zimmerman (1977) explained variations in the transmissivity ofthe principal artesian aquifer in Colquitt County as the result offacies changes across a paleogeographic feature which he identified with the Suwannee Strait. Reduced transmissivity was attributed to deposition offine-grained clastic sediments within the strait, contrasted with deposition of more permeable carbonates outside the feature. The Ochlockonee fault of Sever (1966) was used to explain such waterquality anomalies as high concentrations of dissolved solids, especially sulfate. Krause (1979), in a study ofthe geohydrology of Brooks, Lowndes, and westem Echols Counties, described the principal artesian aquifer in Brooks County as containing rocks ofthe Claibome Group, the Ocala Limestone, Suwannee Limestone, and limestone of the lower Hawthome Formation. These limestones are jointed, enhancing solution of the limestone and allowing conduit flow of ground water. Recharge takes place through the area's many sinkholes and permeable lake bottoms and the bed of the Withlacoochee River. Gelbaum (1978), in a paper on the geology and ground water of the Gulf Trough, extended the trough northeastward into Screven and Effingham Counties on the basis of potentiomet..: ric maps of the principal artesian aquifer. She discussed possible causes oflow yields from wells in the vicinity ofthe trough, suggesting that many wells may not penetrate the aquifervery deeply, or at all, due to the greater depth to the top of the aquifer in the Gulf Trough. Another possibility mentioned was a thinning of Oligocene strata in the trough, making the aquifer thinner overall. Faulting parallel to the trough and facies changes across the trough were also considered as possible causes oflowwellyields. In laterwork on the Gulf Trough, Gelbaum and Howell (1982) used specific-capacity indices to characterize groundwaterflow across different areas ofthe trough. The potentiometric anomaly which marks the trough was described as the result of a combination of structural and depositional factors. They attributed the reduced transmissivity of the aquifer in the trough to facies changes resulting in denser limestones deposited in the trough, with downfaulted blocks locally forming ground-water flow barriers. Bush (1982) simulated the predevelopment flow in the Tertiary limestone aquifer. The model revealed that the majority of flow in the aquifer prior to development occurred in the unconfined and thinly confined portions of the aquifer. In these areas, high recharge and discharge produced an active shallowflow zone and a less active deeperzone. Transmissivityvalues for unconfined and shallowly confined areas commonly exceeded 1,000,000 square feet per day (ft21d). The thickly confined areas ofthe aquifer had lower transmissivities, due to the retarded discharge and sluggish ground-water flow. The geology and configuration of the top of the Floridan aquifer system was mapped by Miller (1986). He followed Gelbaum in attributing the reducedtransmissivityofthe aquiferinthe trough to faulting, stating that extensive graben faulting along the trend of the trough could have dropped low-permeability Miocene clastics into contact with permeable limestones of the aquifer, effectively damming ground-water flow across the trough. In 1986, Miller restated this conclusion in the context of a Regional Aquifer System Analysis (RASA). Although this work is the most complete and comprehensive report on the geology of the Floridan aquifer system to date, it employed rela- 10 tively few data specific to the Gulf TroughApalachicola Embayment area. An unpublished M. S. thesis by Korosy (1984) delineated ground-water flow patterns in the Ochlockonee River area of northwest Florida and southwest Georgia. Korosy used uranium isotope distributions to identify recharge areas and areas where the development ofsecondary permeability in the limestones of the Floridan aquifer is inhibited by retarded ground-water flow and thick overburden. GEOLOGIC FRAMEWORK The study area is located in the Coastal Plain geologic province. The Coastal Plain is underlain by a seaward thickeningwedge ofsediments ranging in age from late Cretaceous to Holocene, resting unconformably on a basement complex of Piedmont crystalline rocks; Triassic grabens filled with red-bed sediments and volcanic rocks; and Paleozoic sedimentary rocks. Coastal Plain sedimentruy units generally dip to the southeast and exhibit an outcrop pattern that strikes northeast to southwest. The oldest outcropping sedimentary units of the Coastal Plain are exposed along the Fall Line in southwest Georgia, and the youngest crop out along the coast. The Apalachicola Embayment along with the GulfTrough, its narrow northeastward extension, is a linear, subsurface depression continuous with the Gulf of Mexico (Figure 2). The troughembayment exhibits as much as 600 feet ofburied relief and varies in width from 35 miles in the extreme southwestern corner of the state to approximately 6 miles at its narrowest in Jeff Davis County. The feature cannot be traced east of central Bulloch County. The Gulf Trough-Apalachicola Embayment area is distinguished by radical changes in the geometry and lithology of stratigraphic units in the study area (Plate 1). The presence of the Gulf Trough is first apparent in Claibornian-age sediments, which show a facies boundary between clastic and carbonate sedimentation which approximatesthe position ofthe trough-embayment. Claibornian sediments are also anomalously thin in the vicinity ofthe feature. The Upper Eocene in the trough-embayment is represented by a dense, fine-grained, relatively deep-water limestone that is thinner than the adjacent Upper Eocene Ocala Limestone. The boundary between Eocene and Oligocene sediments is difficult to distinguish in well cuttings from the trough-embayment, due to their lithologic similarity. The Lower Oligocene Ochlockonee Formation ofthe trough-embayment is a dense, fine-grained limestone. There is no typical Suwannee Limestone in the troughembayment. The Okapilco Member of the Suwannee, a coarser-grained, more variable, coralline limestone, occupies its stratigraphic position. In general, the Oligocene section in the trough-embayment is thicker than normal and contains a deeper-water faunal assemblage. Lower Miocene sediments in the trough-embayment are also unusually thick, particularly the Parachucla Formation of Aquitanian age. The Apalachicola Embayment and Gulf Trough are interpreted to have been produced by the Suwannee Currentwhich flowed from the Gulf of Mexico to the Atlantic Ocean and inhibited sedimentation in the trough-embayment duiing the middle and upper Eocene (Huddlestun and others, in preparation). Falling sea level during the Early Oligocene probably initiated the cessation of the current. The filling of the troughembayment began, continuing into the lower Miocene. Plate 1 shows generalized stratigraphic columns for representatiye parts of the study area. The stratigraphy and geologic history of the Gulf Trough and Apalachicola Embayment are complex, and this report addresses only those aspects pertinent to a discussion of the hydrogeology of the area. For a more thorough treatment of the geology of the Gulf Trough-Apalachicola Embayment area, the reader is referred to Huddlestun and others (in preparation). HYDROLOGIC FRAMEWORK GENERAL Aquifers are rock units which store significant quantities ofwater and transmit that waterto wells which tap the rock units. The amount of water considered to be significant varies with the water needs and water availability of an area. A confined, or artesian, aquifer is one which is overlain by a layer of relatively impermeable material. Pressure inthe aquifer exceeds atmospheric pressure, causing water to rise above the level of the aquifer in tightly cased wells tapping the aquifer. The imaginary surface, coinciding with the level to which water from the aquifer will rise in artesian wells is called the potentiometric smface. An unconfined aquifer is one which contains water in contact with the atmosphere by way of the open spaces in the permeable material. The 11 upper smface of water in an unconfined aquifer is called thewater table. For this reason, unconfined aquifers are also called water-table aquifers. Water in unconfined aquifers is at abnosphertc pressure at the water table. The configuration of the water table is usually a subdued replica of the land smface. The Coastal Plain province of Georgia contains the State's major aquifers. Of these, the Upper Floridan aquifer, a confined ~arbonate aquifer of Middle Eocene to Oligocene age, is the most widely used. In much ofthe study area, the Floridan is overlain by sediments of Miocene and younger age, which are sufficiently permeable to form an unconfined surficial aquifer. The surficial aquifer in the study area locally yields quantities of water sufficient for domestic supply. Other major aquifers which extend into the study area include the Clayton, Claiborne, and Providence aquifers and the Cretaceous aquifer system. All are confined aquifers. Because these aquifers are more deeply burted than the Flortdan aquifer system, they are used extensively only in areas where the Floridan system is absent or yields insufficient water. Increased costs associated with the drilling ofdeep wells make use ofthe Floridan aquifer system the most practical in areas where it yields sufficient water. SURFICIAL AQUIFER Miller (1986) noted that the Floridan aquifer systeminmostplaces is overlainbyan unconfined surficial aquifer (Figure 4). In the study area, the surficial aquifer is composed of sediments of Miocene to Holocene age, which vary greatly in thickness and permeability. In areas where the Upper Floridan aquifer or its upper confining unit crop out, such as the Dougherty Plain (Figure 4) no surficial aquifer is present. The surficial aquifer in the majority of the study area is made up primarily of unconsolidated clastic sediments ofthe Miocene Hawthorne Group. Although the Hawthorne Group sediments are characteristically high in clay content and act as the upper confining unit for much ofthe Flortdan aquifer, they vary greatly in lithology over the study area. Beds of coarser matertallocally yield supplies of water adequate for domestic supply. The Hawthorne Group also varies greatly in thickness in the study area, due to the presence of the Gulf Trough-Apalachicola Embayment. Deposition ofthe Hawthorne Group sediments completed the majority of infilling of the trough, primarily through deposition of the limestones of the lower Miocene Parachucla Formation. Areal variations in lithology produce widely different well yields over the study area, even in wells drtlled to the same depths, or in the same formations. Although the Parachucla Formation contains considerable amounts of dense dolomite, it may locally yield adequate amounts of water for domestic supply, particularly in areas where it contains beds of coquinoid limestone. Seasonal variations in water levels and well yields may also be extreme, as water-table aquifers respond quickly to fluctuations in precipitation. The surficial aquifer is used primarilyfor domestic supply because of its small and variable well yields. Locally, the surficial aquifer may yield adequate waterfor other purposes, but indrought years water supplies from this aquifer often prove to be unreliable. Even in areas where it is not used to supply water to wells, the surficial aquifer is important as a source of recharge to the Upper Floridan aquifer. FLORIDAN AQUIFER SYSTEM The Floridan aquifer system is a thick sequence of permeable carbonate rocks, ranging in age from Paleocene to Miocene, which are in some degree of hydraulic connection. The Floridan aquifer system blankets all ofFlorida, most ofthe Coastal Plain of Georgia (Figure 5), and adjacent portions of South Carolina and Alabama. The Floridan aquifer system consists of a single permeable zone in its updip regions, confined by less permeable sediments. Downdip, the aquifer system contains two permeable zones, the Upper and Lower Flortdan aquifers, separated by one of a number of local confining units designated by Miller (1986) as middle confining units I-VIII. The Floridan aquifer system is represented onlybythe Upper Floridan aquifer throughout most of the study area. Although Miller (1986) and Bush (1982) mapped an intra-aquifer low permeability zone in the area southeast of the Apalachicola Embayment, this study did not divide the aquifer system into upper and lower permeable zones. The Floridan aquifer system is separated from the surficial aquifer by a relatively impermeable confining unit. This upper confining unit varies considerably in age, lithology, thickness, and permeability. The interbedded clays and sands of the Miocene Hawthorne Group form the confining unit for the Floridan aquifer system over most of the Coastal Plain of Georgia. Locally, however, the Miocene section also contains dense dolomites or other carbonate layers which form a portion ofthe confining unit. The Suwannee Limestone, a major component of the Upper Flortdan aquifer, locally 12 0 20 40 Ml I I E""3 I 1---1 1---1 0 20 40 KM N t Figure 4. Geographic extent of the surficial aquifer in the study area. Shaded area indicates aquifer. (After Miller, 198 6.) 13 0 20 40 Ml E3 E3 I t=-1 H 0 20 40 KM N t Figure 5. Geographic extent of the Floridan aquifer system. Shaded area indicates aquifer. (After Miller, 1986.) 14 forms a part of the confining layer. The stratigraphic relationship between the Miocene and the Oligocene sediments in the study area is complex, and for this reason, the top and thickness ofthe upperconfiningunit ofthe Floridan system have not been mapped. Plate 2 shows the thickness of all sediments overlying the Floridan aquifer system, including both the surficial aqui- fer and the upper confining unit of the Floridan system. The confining Miocene sediments have been removed by erosion in parts ofthe study area. The Pelham Escarpment, an erosional feature of the Coastal Plain, divides the Dougherty Plain in southwestern Georgia from the Tifton and Vidalia Uplands ofthe central Coastal Plain. West of this scarp, erosion has removed most of the Miocene sediments and all but a fringe of Oligocene sediments. The Upper Eocene Ocala Limestone. a major part of the Floridan aquifer system, is exposed at the surface. overlain only by clayey residuum. Locally, this residuum is sufficiently thick and impermeable to produce confined or semi-confined conditions in the Floridan. The upper confining unit has been breached by the Withlacoochee River and by numerous sinkholes in Brooks County. and in the Lowndes and westem Echols Counties to the east. Recharge to the Floridan aquifer system takes place in this area from the river and through sinkholes and porous lakebeds (Krause, 1979). The boundaries of the Floridan aquifer system cross both rock- and time-stratigraphic boundaries. Within the study area, the Upper Floridan aquifer is primarily Middle Eocene to Oligocene in age. Any ofthese units maybe oflow permeabilitylocally and maybe excluded from the aquifer as a result. The massive limestone of the lowermost Miocene may form a part ofthe aquifer in a few small areas. Plates 3 through 5 illustrate the geology and geometry of the Floridan aquifer system in the study area. The geology and configuration of the top of the aquifer is shown in Plate 3. as determined by examination of well cuttings, cores and geophysical logs and by permeability testing of core samples. The top of the aquifer in the study area conforms most closely to the top ofOligocene sediments. Exceptions include areas such as the Dougherty Plain, where the Oligocene sediments have been removed by erosion. and the southcentral partoftheApalachicola Embayment, where the lowermost Miocene limestones may be sufficiently permeable to form a portion ofthe aquifer. The regional strike of the top of the Floridan aquifer system is southwest to northeast, but the direction of dip varies due to the presence of the Gulf Trough-Apalachicola Embayment. The top of the aquifer also varies in degree ofdip, from a rate of 3.5 feet per mile southeast of the Apalachicola Embayment to 62 feet per mile on the south flank ofthe GulfTrough. The subsurface reliefofthe top of the aquifer in the trough-embayment averages 350 feet, with a maximum of approximately 500 feet in the vicinity of southwestern Tift County. Plate 4 shows the geology and configuration of the base of the Floridan aquifer system. This boundary is the base of the lowermost permeable limestone, as determined on the basis of well cuttings, cores. electric logs, and permeability tests. The lower confining unit varies in age and lithology, from the impermeable limestones ofthe upperEocene intheThomas-Brooks-southeastern Colquitt-southern Cook Counties area, to indurated sands ofthe middle Eocene southeast ofthe Gulf Trough. Plate 5 shows the thickness of the Floridan aquifer system in the study area. At its thickest, near the central portion of the study area, the aquiferis 800-900feet thick.The aquiferis thinnest in Screven and northern Effingham Counties where the Oligocene and upper Eocene limestones grade into clastic sediments. The aquifer is also thin in the Apalachicola Embayment and southwestern GulfTrough, where impermeable limestones make up the lower portion of the Oligocene section. The Gulf Trough-Apalachicola Embayment is an area of rapid and complex facies changes. The trough-embayment contains limestones of a deeper-water origin than those beyond the flanks of the feature (Huddlestun and others. in preparation). Thus, theflanksofthetrough-embayment represent areas ofrapid facies change. Additional facies changes mark the transition from the Apalachicola Embayment to the GulfTrough, and also the northeastern termination of the Gulf Trough. The relationship of the Floridan aquifer system to the stratigraphic units in the study area is shown on Plate 1, using a series ofstratigraphic columns keyed to various parts of the study area. Complete descriptions ofthese stratigraphic units can be found in Huddlestun and others (in preparation). CLAIBORNE AQUIFER The Claibome aquifer extends into the westem part of the study area (Figure 6), and it underlies the Floridan systemin Mitchell, northern Worth, and extreme northwestern Colquitt Counties (McFadden and Perriello, 1983). The 15 0 20 40 Ml I I E3 I 1---1 1---1 0 20 40 KM N t Figure 6. Approximate geographic extent of the Claiborne aquifer in the study area. Shaded area indicates aquifer. (After Arora, 1984.) 16 Claibome aquifer is composed of Middle Eocene sands of the Tallahatta Formation in its updip portion, and in some areas it includes clastic sediments ofthe lower part ofthe overlying Lisbon Formation and the underlying Hatchetigbee Formation. The Claibome is confined above by the clay-rich upper part of the Lisbon Formation. In the downdip region, where the Claibome aquifer enters the study area, the distinctions between the Claibome aquifer and the overlying Floridan aquifer system become less distinct due to facies changes in both aquifers. Although the Upper Eocene to Middle Eocene section is carbonate, the uppermost Claibome sediments consist of relatively impermeable glauconitic limestones which serve to confine the Claibome aquifer. Recharge to the Claibome aquifer is through its outcrop area in the northwestem part of the Coastal Plain and possibly through downward leakage from the Floridan aquifer system. Outside the study area, in the vicinity ofAlbany, declining head in the Claibome aquifer may be causing such leakage (McFadden and Perriello, 1983). Potentiometric declines in the Albany area, and throughout the area occupied by the Claibome aquifer, suggest that it is not a good candidate for extensive development in the study area. CLAYTON AQUIFER The Lower Paleocene Clayton aquifer extends intothe study area inwestemMitchell and northem Worth Counties (Figure 7). This aquifer underlies the Claibome aquifer and is separated from it by a confining unit which consists of the Nanafalia Formation and the clay-rich upper Clayton Formation (McFadden and Perriello, 1983). The Clayton aquiferismade up ofpermeable limestone of the middle unit of the Clayton Formation. It locally includes permeable sands ofthe upper and lower parts ofthe Clayton Formation. The Clayton aquifer is confined below by clay layers in the lower Clayton Formation and upper Providence Sand. Recharge is by leakage from other aquifers and by infiltration in the area of outcrop. This aquifer has a relatively small outcrop area; thus recharge to it is limited (McFadden and Perriello, 1983). The Clayton aquifer has been extensively developed inthe area northwest oftheApalachicola Embayment. Large ground-water withdrawals, combined with the limited recharge to this aquifer, have resulted in dramatic head declines in the Clayton aquifer. Although a small portion of this aquifer extends into the study area, its future development potential is low. Because this aquifer underlies the more productive Floridan aquifer system, no wells tapping the Clayton exclusively are known in the study area. CRETACEOUS AQUIFERS The interbedded sands and clays ofthe Cretaceous stratigraphic units of the Coastal Plain form a number of aquifers and intervening confining units throughout the area. Pollard and Vorhis (1980) identified seven such Cretaceous aquifers in the Coastal Plain and designated them aquifers A1 through A,. These aquifers are rarely tapped in the study area, due to the ease of obtainingwaterfrom the shallowerFloridan aquifer system. Aquifer A extends into Screven County, in 1 the northeastem portion of the study area. In 1976, 1.5 million gallons of water were pumped from this aquifer for industrial use in Screven County (Pollard and Vorhis, i980). The Providence aquifer of southwestern Georgia, also called Aquifer~. is unconformably overlain by the Clayton Formation. It is composed of the the upper sand member of the Upper Cretaceous Providence Sand. Lithology of the aquifer is variable, ranging from a sand in the updip region to a coquina in the downdip region. The aquifer underlies a portion ofMitchell County, and the northern part ofWorth County. Recharge to the Providence aquifer is through its area of outcrop. Discharge is to streams and also to the Clayton aquifer, through upward leakage. The declining head in the Clayton aquifer has increased the potential for such upward leakage. Pollard and Vorhis (1980) also identified an aquifer, which they designated~ composed primarily of the Cretaceous Cusseta Sand. Where aquifer ~ underlies the study area, it is not separated from the Providence aquifer (aquifer~) by a confining unit. Hence, Pollard and Vorhis called this aquifer ~CA. also called the Providence-Cusseta aquifer. No wells are known in the study area which tap this aquifer exclusively. The greatest development potential for the Cretaceous aquifers is north of the study area, in their updip regions, where they are closer to the surface and contain a greater percentage of sand. Due to the availability ofwater from the shallower Floridan aquifer system, the Cretaceous aquifers are rarely tapped in the study area. Northeast of the Gulf Trough, aquifer A1 is used quite extensively, and the Providence aquifer is used in the vicinity ofAlbany and Americus. Potential for use of the Providence aquifer also exists in northern 17 0 20 40 Ml I I E3 I r:::l 1---f 0 20 40 KM N ,.,.---.../'' t Figure 7. Approximate geographic extent of the Clayton aquifer in the study area. Shaded area indicates aquifer. -40 Q) -.... Q) as ~ -45 Ben Hill County 72 73 74 75 76 11 18 19 80 81 82 83 84 85 86 87 calendar year b) -35 Mitchell County -40 Q) --Q) c . Q5 > -45 Q) -.... Q) as ~ -so -55 18 79 80 81 82 83 84 8 5 86 87 calendar year Figure 9. Water levels in Floridan aquifer wells. a>. Trees Inc., northern Ben Hill County. b). Wright well, eastern Mitchell County. 22 a) -190 - ~ CD -Q) -195 c:: (i5 > Q) -'- Q) ttS -200 ~ Worth County 72 73 7 4 7 5 76 77 78 79 80 81 82 83 8 4 85 86 87 cal en dar year b) Toombs County -155 Q) --Q) c:: -160 -(i5 > CD -'- -165 (!) ttS 3: -170 74 7 5 76 77 78 79 80 81 82 83 84 8 5 86 87 calendar year Figure 10. Water levels in Floridan aquifer wells. a>. City of Sylvester, Worth County. b). City of Vidalia, Toombs County. 23 in water levels over large areas. Development of aquifers as water supplies produces changes in recharge and discharge relationships. Declines in water levels commonly result from large withdrawals of ground water. In the Floridan aquifer system, water levels over much of the Coastal Plain have declined (Figure 11). No decline is seen at the northwest extent of the aquifer near the outcrop area, andinthe Brooks-Lowndes-ThomasCook Counties area, where the aquifer is recharged. The largest water-level declines appear at Savannah and in Wayne and Long Counties in the vicinity of Jesup and Doctortown. These declines are the result of large industrial withdrawals to supply the paper industries in these cities. Regional water-level declines resulting from these large withdrawals do not extend westward of the Gulf Trough, suggesting that the low transmissivity of the aquifer within the trough prevents the pumpage-produced pressure changes from extending further westward. A comparison of the May 1980 (Krause and Hayes, 1981) and May 1985 (Bush and others, 1987) potentiometricmaps ofthe upperpermeable zone ofthe Floridan aquifer system (now called the upper Floridan aquifer) in Georgia shows that major water-level decline occurred in only one portion of the study area. The southwestern portion of the state, in and adjacent to the Dougherty Plain, showed a water-level decline of 10 to 30 feet. This decline was brought about by a combination of local drought conditions and resulting increased pumpage during this time. WATER USE Totalreported ground-water use inthe study area in 1985 was approximately 179.4 million gallons per day (Mgal/d) (Turlington and others, 1987). Most ofthis waterwas withdrawn from the Floridan aquifer system. Municipalitieswere once the mainconsumers ofgroundwaterinthe Coastal Plain and still rely almost exclusively on wells to provide adequate water to meet public-supply needs. Agricultural withdrawals, however, account for an increasing percentage ofground-water use and in most counties have surpassed municipal use. Locally, industrial withdrawals form a growing segment of total ground-water use. Ground water is used for thermoelectric power generation in two counties of the study area. The largest ground-waterwithdrawals inthe study area are for agricultural purposes, including both irrigation and livestock use. Recent decades have seen phenomenal growth in the number of acres of irrigated farmland (Table 1, after Bachtel, 1987). With the advent in the seventies of sophisticated irrigation systems supplied by water wells, ground-water withdrawals have played an increasingly large role in crop irrigation. The largest agriculturalwithdrawals in 1985, an average of 32.94 million gallons per day (Mgal/d), were in Decatur County (Turlington and others, 1987). Four other counties reported agricultural withdrawals in excess of 5 Mgal/d: Mitchell, with 11.29 Mgal/d; Colquitt, with 7.54 Mgal/d; Tift, with 6.58 Mgal/d; and Screven, with 5.10 Mgal/ d. Some ofthe withdrawals reported from Decatur and Mitchell Counties, which borderthe Dougherty Plain, mayhave been obtainedfrom the Clayton or Claiborne aquifers. Total reported agricultural use in the study area in 1985 was 106.08 Mgal/ d. These figures are average daily-use estimates which do not take into account the highly seasonal nature of irrigation withdrawals. The city ofThomasville is the largest municipal user of ground water in the study area, withdrawing 4.51 million gallons per day for public supply purposes (Turlington and others, 1987). Three other cities in the study area reported withdrawals in excess of 3.00 Mgal/d. They were: Adel, with 3.71; Douglas, with 3.11; and Moultrie, with 3.08 Mgal/d. Total reported ground-water withdrawal for public supply in the study area for 1985 was 45.45 Mgal/d. Self-supplied domestic and commercial withdrawals locally form a large segment of county-wide ground-water use. Estimates ofground-water use inthis categoryinclude all household water users not supplied by public water systems, as well as commercial users such as restaurants, hotels, stores and other businesses. These amounts also include withdrawals by military and recreational facilities, schools, hospitals, prisons, and other institutions (Turlington and other, 1987). Total withdrawals for these and other categories are presented in Table 2. Industrial users locally account for significant ground-water withdrawals. Colquitt and Thomas Counties contain two of the largest population centers in the study area, and industrial withdrawals are correspondingly high. Significant withdrawals for industrial use in 1985 were reported for Colquitt County, 1.30 Mgal/d and Thomas County, 1.28 Mgal/d. BothJe:IIDavis and Screven Counties have established textile industries which withdraw large quantities of ground water, 1.68 Mgal/d inJeffDavis, and 1.36 Mgal/d in Screven County. Total industrial and mining use in 1985 was 9. 78 Mgal/d. Thermoelectric power generation, a separate category ofwater 24 EXPLANATION - - 8 - Line of equal water-level change.