IC 30 GEORGIA STATE DIVISION OF CONSERVATION DEPARTMENT OF MINES,MINING AND GEOLOGY GARLAND PEYTON, Director THE GEOLOGICAL SURVEY Information Circular 30 GEOLOGY AND GROUND-WATER RESOURCES OF CRYSTALLINE ROCKS DAWSON COUNTY, GEORGIA By Charles W. Sever U.S. Geological Survey Prepared in cooperation with the U.S. Geological Survey ATLANTA 1964 IC 30 GEORGIA STATE DIVISION OF CONSERVATION DEPARTMENT OF MINES,MINING AND GEOLOGY GARLAND PEYTON, Director THE GEOLOGICAL SURVEY Information Circular 30 GEOLOGY AND GROUND-WATER RESOURCES OF CRYSTALLINE ROCKS DAWSON COUNTY, GEORGIA By Charles W. Sever U.S. Geological Survey Prepared in cooperation with the U.S. Geological Survey ATLANTA 1964 CONTENTS Page Abstract ------------------------------------------------------------------------------------------------ _________ ____________________ ___________ __ ________________ ______ 4 Introduction _____________________ _____________________ ___________________________________________________________________________ __ ____ ______ _________________________ 4 Purpose, scope, and methods of investigation ----------------------------------------------------- ____________________________________ 4 Location and extent of area --------------------------------------------------------------------------------- __ ____ ______________________________ 5 Well-, spring-, and outcrop-numbering system _____ ------------------------------------------- ____________________________________ 5 Previous investigations ______________----------------------------------________________________________ _______ __ __________________________________ 5 AcknowIedgment s ________________________ ______________ _________________________ ----------------------------- __________ _______________________________ 5 Geography ------------------------------------------------------------------------------------------------------------------------------------------------------------- 5 Surface features -------------------------------------------------------------------------------------------- ________ __ ___ ------------------------------- 5 Climate ----------------------------------------------------------------------------------------------------------------________ ____________________________________ 6 Agricultural and industrial development __________________________________________________ _________________________ ____________ _________ 7 The geologic formations and their water-bearing characteristics ----------------------------------------------------------------- 7 Zone A ___________________________________________________________________________________________________________________________________________________________ 7 Mica schist _____________________________________________________________________________________________________________________________________________ 8 Description of the rocks -------------------------------------------------------------------------------------------------------------- 8 Water-bearing characteristics ------------------------------------------------------------------------------------------------- 8 Sillimanite gneiss and schist ----------------------------------------------------------------------------------------------------------_____ 10 Description of the rocks ------------------------------------------------------------------------------------------------------------- 10 Water-bearing characteristics -----------------------------------------------------_______ ______________________________________ 10 Amphibolite ----------------------------------------------------------------------------------------------------------- _______________________________ 10 Description of tl~ e rocks ------------------------------------------------- ___________ _________ ______________________________________ 10 Water-bearing characteristics ________------------------------------------------------------------------------------------------- 10 Quartzite _______________________________________________________________________________________________________________________________________________ 10 Description of the rocks ---------------------------------------------------- ___________ _______ ___________________________________ 10 Water-bearing characteristics ____________ _ _____________________________ _____ ___________________________ 10 Zone B 11 Description of the rocks ---------------------------------------------------------------------- ______________ _ 11 Water-bearing characteristics _ __ ___ ______________ _________ ______ _______ ______________________________ 11 Zone C 12 Biotite Schist ___________________________________ ------------------------------------------------- ____ ____ _________________ 12 Description of the rocks _______ ----------------------------------------- __________________ Water-bearing characteristics _______________ __ _______________________ ______ _____ ----------- ---- 12 12 Amphibolite -------------------------------------------------------------------------------------------- ______ ___ _____________ 12 Description of the rocks -------------------------------------------------------------- ______ _________ _____________ 12 Water-bearing characteristics ___________________________________________________ _______ _________________ __________________ 13 Metagraywacke __________ __---------------------------------------------------------------------------------- _____ ___ __________________ 13 Description of the rocks ---------------------------------------------------------------------- ______________________________________ 13 Water-bearing characteristics ____________________ _____________________________ ______________________ _____________________ _____ 13 Great Smoky Group ---------------------------------------------------------------------------------------------- _ ___ ______________________________ 13 Zone D ------------------------------------------------------------------------------------------------------------- ______________________________________ 14 Description of the rocks ---------------------------------------------------------------------------- ________________________________ 14 Water-bearing characteristics ------------------------------------------------------------- _________ ____ __________________ ____ 14 Zone E -------------------------------------------------------------------------------------------------------------- ------------------------------------ 14 Description of the rocks ------------------------------------------------------------------------------------------------------------- 14 Water-bearing characteristics --------------------------------------------------------------------------------------------------- 14 Zone F ---------------------------------------------------------------------------------------------------------------------------------------------------- 15 Description of the rocks -------------------------------------------------------------------------------------- ______________________ 15 Water-bearing characteristics ___________________________ ---------------------------------------------------------------------- 15 2 CONTENTS Page Alluvium -------------------------------------------------------------------------------------------------------------------______________________________________ 16 Description of the rocks __________ ---------------------------- ____________ --------------------------------------- __________________ 16 Water-bearing characteristics ___________________________ ----------------------- _______ -------------------------------------- 16 Structure ---------------------------------------------------------------------------------------------------------- ----- ------- ------------------------ --- 16 Weathering ---------------------------------------------------------------------------------------------------------------- ------------------ _______________________ 17 Water Resources -------------------------------------------------------------------------------------------------------------------------------------------------- 17 The hydroIogic cycle ------------------------------------------------------------------------------------------------------------------------____________ 17 Surface water ----------------------------------------_____-------------------------------------------------------------____________________________________ 17 Streams ________________________________________________----------------__________------------------------______________________________________________ 17 Lakes _____________________________________________________________________-------___---------------------------------_______________________________ 19 Ground Water -----------------------------------------------------------------------------------------------------------____________________________ 19 Recharge -------------------------------------------------------------------------------------------------- _____ ___ ________ ___ _ 19 Movement ----------------------------------------------------------------------------------------------------------- _______________________________ 19 Field observations _____________________----------------------------------------------------------------- _________ __________ _____ ________ 19 Weathering as an indicator ----------------------------------------------------------------------------------------------- __ _ 20 Pumping test --------------------------------------------------------------------------------- ___ ____ ___________ __________________ 21 Laboratory studies ---------------------------------------------------------------------------------_____________________________ 21 Variation in streamflow due to ground-water runoff ____________________________________________________ 21 Rate of m ovement -------------------------------------------------------------------------- _______ ___________ _______________________ 22 Storage _____________________________________________________ __________________________________________________________ ___________________________________ 22 Discharge ----------------------------------------------------------------------------------------------------------------------------------------------- 23 Chemical quality -------------------------------------------- ____-------------------------------------------- ________________________________ _____ 23 Iron _________________________________________________________________________________________________________________________________________________ 25 Calciurn ------------------------------------------------------------------------------------------------ _______ ------------------------------------- 25 Magnesium -------------------------------------------------------------------------------------------------------------------------------------- 26 Sodium and potassium __________________________ ------------------------------------------------------------------------------------- 26 Carbonate and bicarbonate _________________ ---------------------------------------- ----------------------------------------------- 26 Sulfate ----------------------------------_______________----------------------------------------------- _______--------------------------------- _____ 26 ChIoride ___________________________________________________________________________________________________________________________________________ 26 Nitrate _____________________-------------------------------------------------------------------------------------------____________________ _______ 26 pH ------------------------------------------------------------------------------------------------------------------- __________________________ 27 Carbon dioxide --------------------------------------------------------------------------------------------------- ___ ____________ 27 Hardness ----------------------------------------------------------------------------------------------------------------------------------- 28 Fluoride ______________________------------------------------------------------------------------------------ __________________ _____ 28 Ternperature ____________________________________----------------------------------------------------------- ______ _____________________________ 2 8 Well yield -----------------------------------------------------------------------------------------------______________________-------------------------- 28 Selecting a well site --------------------------------------------------------------------------------------------------------------------------------------------- 29 DeveIopment --------------------------------------------------------------------------------------------------------------------------------------------------------- 31 References --------------------------------------------------------------------------------------------------------------_____________________________------------------ 31 3 TABLES Page Table 1. Dug wells in zone A __ _ __ __ ___ ________________________________ _ 8 2. Analyses of data on drilled wells _______________________________________ 9 3. Dug wells in migmatite of zone B (analysed with respect to topography) ____________________________________________ 11 4. Dug wells in zone C ____________________________________________________________ 12 5. Dug wells in zones E and F. (Analysed with respect to topography) ---------------------------------------------------------------- 15 6. Thickness of alluvial deposits, Dawson County ____________ _ ___ _ 16 7. Chemical analyses of ground water, Dawson County ____________ __ ____________ ____________ 24 8. Recommended limits of water quality for domestic use_________ ________ ____ 25 9. Iron content of ground water, Dawson County ____________________________________________________________ 25 10. Combined sodium and potassium content of ground water, Dawson County _____________________________________________________________________________ ---------------- ------- ----- 25 11. Carbon dioxide in ground water, Dawson County ___________________________ _________________ 26 12. Yield of drilled wells and springs ________________________________________________________________ _______ ___________________ 29 13. Information to be considered in selecting sites for dug wells _______________________________________________ 30 14. Topographic control on the water table __________________________________________________ _____________________________ 30 15. Topography versus blasting of dug wells ------------------------------------------ ___ __________ __ _____ _________ 30 16. Topographic influences that should be considered in selecting the site for a drilled well ---------------------------------------------------------------------------- _____ _____________________ 30 17. Yield per foot versus depth of well ---------------------------------------------------------- ___________________________________ 30 ILLUSTRATIONS Page Figure 1. Map of Georgia showing Dawson County _____________________________________________________________________________ 5 2. Map showing geology, and location of wells, springs and outcrops, Dawson County, Georgia __ ______________________________________________________ In Pocket 3. Physical geographic divisions of Dawson County ---------------------------------------------------------- 6 4. The effects of structure and topography on the yield of wells in quartzite _ _______ _______ ______ ______ _ __ __ _________________ __________ ___________________________ __ __ __ 11 5. Jointing and bedding in the metagraywacke of zone C _____________ ----------------------------------------------- 13 6. Geologic structural map of Dawson County, Georgia ________________________________________________ In Pocket 7. Slickensides along fault at location 470-1630-A ________________________ ----------------------------------------------- 17 8. Overturned recumbent fold in muscovite schist of zone F at location 530-1600-A __ _______________ __ _____ ___ _ __________________________________ -------- ----- 18 9. Seeps show paths of water movement in saprolite. Road cut is near Doraville, DeKalb County. (Seep. 20 for explanation.) _____________ 20 10. Weathering along joints in biotite gneiss _____________________________ _ 21 11. Relationship between direction of streamflow and the number of planar openings transected per unit length of stream ____________________________________________ 21 12. Relative ground-water runoff plotted in the direction of the major axis of stream basins at GNL, Dawson County ____ ----------------------------------------------- 22 13. Storage and availability of ground water in interstitial pores _----------------------------------------------- 23 14. Ground-water temperatures in degrees Fahrenheit, Dawson County (October 1959) --------------------------------------------------------- ----------------------------------------------- 27 15. Seasonal changes in the temperature of ground water _____________ ----------------------------------------------- 28 4 GEOLOGY AND GROUND-WATER RESOURCES OF CRYSTALLINE ROCKS, DAWSON COUNTY, GEORGIA by Charles W. Sever ABSTRACT Dawson County, in the central part of northern Georgia 58 miles north of Atlanta, has a mean annual precipitation of 52 inches, an average annual temperature of 58F., and an average growing season of about 195 days. Rocks in Dawson County are grouped in five distinct geologic divisions; they are designated zones A, B, and C, Great Smoky Group, and alluvium. The age and regional stratigraphic position of zones A, B, and C are uncertain. A thick schist sequence makes up the rocks of zone A and is divisible into five mappable lithologic units. From the oldest to the youngest, the units are mica schist and interbedded sillimanitemica schist; mica schist and interbedded amphibolite; garnet-mica schist; quartzite and garnet amphibolite; and mica schist. The rocks of zone B lie along a fault between the rocks of zone A and zone C and consist of mixed igneous and metamorphic rocks referred to as migmatite. The rocks of zone C lie along the northeastsouthwest belt and consist of three units -biotite schist, amphibolite, and metagraywacke. The rocks of the Great Smoky Group are metasedimentary and are divided into zones D, E, and F. Alluvial deposits composed of poorly sorted silt, sand, and gravel occur along the major streams. The average yield per foot of drilled wells, based on the total footage, was 0.08 gpm (gallons per minute) in mica schist, 0.23 gpm in quartzite, 0.06 gpm in migmatite, 0.09 gpm in amphibolite, 0.04 gpm in metagraywacke, 0.13 gpm in zone F, and 0.08 gpm in zone E. Also, the average yield per foot of drilled wells was 0.08 gpm on hilltops, 0.10 gpm on slopes, and 0.15 gpm in valleys and draws. The yield per foot of drilled wells decreased with well depth. It averaged 0.24 gpm between the land surface and 100 feet, 0.09 gpm for 300 feet, 0.03 gpm for 400 feet, and 0.02 gpm for wells deeper than 400 feet. Studies indicate that the greatest volume of ground water moves along cleavage, schistosity, and (or) bedding planes, which in Dawson County generally are parallel except on the nose of folds. Ground water from the metamorphic rock of Dawson County is soft and of good chemical quality, but carbonic acid dissolved in the water makes it corrosive. Water from alluvial deposits is soft and contains a small amount of acid. The pH of ground water ranged from 4.9 to 6.8. The carbon dioxide content ranged from 5 to 135 ppm (parts per million). Water from 39 percent of the wells and springs contained more than 0.3 ppm iron. Water from 95 percent of the wells and springs had a ratio of alkalinity to carbon dioxide of less than 3:1, and 92 per cent had a ratio of less than 1.1. The lowest ratio was 1 :8. The seasonal changes in ground-water temperature in wells where the water table was more than 40 feet below the land surface averaged about 1.5F., but in wells where the water table was less than 15 feet below the land surface it averaged about 15.2F. The seasonal change in the temperature of ground water at springs was about 2F. The temperature of the shallow ground water decreased the higher the land-surface altitude. Also, both Lake Sidney Lanier and geologic structure affect the temperature of ground water. Topography, lithology, and strike of the rocks are the three most important geologic factors to consider when selecting a well site. INTRODUCTION Purpose, Scope, and Methods of Investigation In October 1958, the U.S. Geological Survey in cooperation with the Georgia Department of Mines, Mining, and Geology, began an investigation of the geology and ground-water resources of Dawson County, Ga., as part of the statewide study of ground-water resources. The purpose of the investigation was to determine the occurrence and chemical quality of ground water and the relation of yields of wells and springs to geology and topography. A study was made of ground-water development in the county. Field work was started in November 1958 and completed in February 1960. It consisted of inventorying wells and springs ; gathering information from well drillers; estimating the discharge of water at wells, springs, and road cuts; mapping geologically and measuring a network of observation wells to determine seasonal water-level fluctuations and changes in ground-water temperature. Water-level measurements were made in wells at about one-third of the rural residences in Dawson County. The discharges of the springs were estimated. Chemical analyses were made by the U.S. Geological Survey Laboratory, Ocala, Fla. Determination of water pH as a part of the study of the distribution of carbon dioxide was made in the field. Geophysical data, furnished by the Geophysics Branch, U.S. Geological Survey, 5 and aerial photographs were used as aids in geologic mapping. Location and Extent of Area Dawson County includes 213 square miles in north-central Georgia (fig. 1). It is bordered on the east by Hall and Lumpkin Counties, on the 85" 84" 83" 35" .--~-t--- \ DAWSON COUNTY ''\Rom~ I 34"-t-'- + - - - , - + - - - - ; o c - - - 1 - three and four digit number represents a Mercator-projection coordinate measured in thousands of feet. The designation 500-1600-1 represents a well or spring within the square bounded on the east by the 500,000-foot and on the south by the 1,600,000-foot coordinates. The number after the second dash indicates that the well or spring was the first to be located in that square. The designation 500-1600-A represents an outcrop within the same square as above. Well records and other basic data not included in this report are on file at the district office, U.S. Geological Survey, Ground Water Branch, 19 Hunter Street, S.W., Atlanta, Georgia. Previous Investigations 82" -L "'"" ---84"____ --~~'e 83 l t;i 50 Q 50 MILE'S 82 Figure 1:-- Mop of Georgia showing Dawson County. north by Gilmer County, on the west by Pickens and Cherokee Counties, and on the south by Forsyth County. Long 8400'E. and lat 3430'N. intersect in southeastern Dawson County. Dawsonville, the county seat, is 58 miles north of Atlanta and is accessible by U.S. Highway 19. Other highways in the county are Georgia Highways 9E, 52, 53, 136, 226, 183, and 318. The 1960 census listed 3,590 people in the county. Dawsonville is the only incorporated town and has a population of about 200. Well-, Spring-, and Outcrop-numbering System To facilitate the location of wells, springs, outcrops, and other features, a numbering system based on the transverse Mercator projection by the U.S. Coast and Geodetic Survey is used in this report. (See fig. 2.) In previous Georgia reports, wells and springs were numbered either serially within each quadrangle or county, or numbered by geographic coordinates based on longitude and latitude grids. In this report each location is given a three and a four digit number separated by a dash, which represents the east-west and northsouth coordinates, respectively, of the square in which the well, spring, or outcrop is located. Each General information concerning the geology of Dawson County is included in a report by Crickmay (1952). Bayley (1928) and LaForge and Phalen (1913) mapped the western and northern parts of the county, respectively, as Carolina Gneiss (of former usage). Furcron and Teague (1945) mapped the kyanite deposits of the county and divided the Carolina Gneiss into two units, the Oglethorpe Formation and the Amicalola Gneiss. Detailed geologic and hydrologic studies were begun at the GNL (Georgia Nuclear Laboratory) site in 1956 by Stewart, Callahan, and others and were continued to 1962 (written communication). Related studies have been made in nearby areas by Hurst (1955 and 1956), Mundorff (1950), and Herrick and LeGrand (1949). Additional references relating to the geology and ground-water resources of the area are given as citations and references in this report. Acknowledgments The assistance of well owners who provided data on their wells is greatly appreciated. Special acknowledgment is due the Gainesville Well Drilling Co., City Ice Co., Murphy Well Drilling Co. Oasis Well Drillers, Inc., Virginia Supply and Well Co., All Purpose Boring Co., and Mr. Gordon Graham for their cooperation in furnishing records of wells. Mr. Glynn Wallace, Dawson County Sheriff, is especially thanked for furnishing office space to the author. The Geophysics Branch, U.S. Geological Survey, furnished aerial radioactivity and geomagnetic data, which aided in delineating lithologic units and geologic structure. The author acknowledges the interest and assistance of the staff of the Georgia Department of Mines, Mining, and Geology. GEOGRAPHY Surface Features Dawson County lies within the Blue Ridge and Piedmont physiographic provinces (Fenneman, 1938). (See fig. 3.) The Blue Ridge is a series of prominent round - crested mountains in the northwestern part of the county, and the Piedmont is a series of prominent hills near the base of the mountains but changes to flat-topped undulating hills toward the south. Scarps of 500 to 700 feet separate the two provinces. 6 N t 5 level below land- surface datum (feet) I ="i' - - - - il: 1Well has adeauate yield 1Users reported water quality as shown 'Mud accumul~tes in well -~ lWell caves 0 .~, ..c .~, ~ 1.5~~ z" ~~I~~ 8 Cl,)o(&O ~ ... z 0 >".,'- =): = 0 .:c.: ., 0 0 ;J e,:) . 0 0 ll. .=:~=c.: I ;J \ = z0 l>-".', .:c.: :J, z 0 =il: >".,-' .:0cc.: ;J 1Well required dynamiting i=l: z 0 >."-, ' .:0c.: 1:5 =.... 0J"CC C11bi!Q)bll~ .... +l ~ ..... U) Ql ~or ill "..?::C~ A. Wells in zone A analyzed with respect to lithology Quartzite 1 36 0101 00100100010 30 Amphibolite 10 29 15-50 3 5 2 8 1 1 2 1 7 3 0 7 2 0 8 0 Sillimanite gneiss 21 36 7-51 2 16 3 12 4 5 8 4 9 8 4 9 9 3 9 0-30 and schist Mica schist 35 29 4-56 4 29 2 15 5 15 10 10 15 14 4 17 10 5 20 0-18 All rocks 67 30 4-56 9 51 7 36 10 21 21 15 31 27 8 33 21 9 37 0-30 Hilltop Slope Valley and draw All locations B. Wells in mica schist analyzed with respect to topography 17 34 18-56 2 15 0 2 25 10 7 6 4 9 2 6 3 13 26 16-35 2 9 2 8 0 5 3 4 6 4 1 8 5 5 16 4-27 0 5 0 5 0 0 0 0 5 1 1 3 2 35 29 4-56 4 29 2 15 25 15 10 10 15 14 4 17 10 4 10 1 7 0 3 5 20 12-39 0-? 0 0-39 Hilltop Slope Valley and draw All locations C. Wells in sillimanite gneiss and schist analyzed with respect to topography 10 42 31-51 1 8 1 7 1 2 4 1 5 4 1 5 5 0 5 0 10 29 19-43 1 7 2 5 "2 3 4 2 4 3 3 4 4 2 4 0-30 17 7 0100 10010100010 4 21 36 7-51 2 16 3 12 4 5 8 4 9 8 4 9 9 3 9 0-30 1lnformation reported by well owners or tenants. 2Hard water. 9 Table 2.-Analyses of data on dTilled wells Average depth Number of wells (feet) Reported yield ( gpm) Range Average Per foot of well ......0. ~ Quartzite "~>..-. Sillimanite gneiss and schist "E..:.: 0" Mica schist <""0 ~ All wells 1 150 3 323 16 194 20 213 ---------------------------------------- ---------- < rzl 0 N s.s5bn:".~a~c: Hilltop Slope """g_'_mg <""0" Valley and dlaw 8 12 None 234 171 ------- 0- 99 ." -.o.a-";' ~~ ."..0- 0~ .C..l) ~ o..=~ 100-199 200-299 <"""~"c. 300-399 "0 400+ Ncne 11 139 5 215 2 300 2 478 35 5-25 0-80 0-80 0-20 4-80 4-30 0-80 6-14 6 35 0.23 15 .05 16 .08 17 .08 ---------- 15 .06 19 .11 ------- 15 .11 21 .10 10 .03 6 .01 .s.2 Hilltop ..,..= c .5 f~ Slope "5""g_' m~ Valley and draw ~o- p:j <"" All wells z ~ 0- 99 0 N .o. a-";' ~"'~"'~~ 100-199 200-299 ~ o..=~ < """~"c. 300-399 "0 400+ 3 248 4-13 9 .04 4 127 7-17 13 .10 3 163 7-20 14 .09 10 174 4-20 11 .06 2 71 7-17 12 .17 7 154 4-20 11 .07 0 0 1 423 13 13 .03 0 Metagraywacke ... ~ .......,c. c >- Amphibolite u ~ z "E..:.: 0" Biotite schist ."- Zone E p.. p "E..:.: <0uu u 0 ~ Zone F All wells 0p:; c.!) .s -~ ..,..= c Hilltop >< ~ 0 ~ U1 .s ~-z Slope 8"""g'_~m " surface datum (feet) .5 .a:!;i ~ .~ Q ..", z=8 -:S """0' ."."" < " > .s ..,..., I .="'" 1!::"0 Well has1 adequate yield = ~ z Q ~ >"- Q =-'I =;:J Users 1 reported water quality as shown "0 Q Q (.;) . Q Q ~ = ~ Q =-'I =;:J Mud' accumulates in well = ~ z Q ~ >"- = Q -'I =;:J Well caves1 Wells required dynamiting1 = ~ ~ = z Q ~ >"- Q =-'I =;:J z 0 ~ >"- '2=Q ;:J ,... ~ ......= ~ .... IV"'CC "="..""~ ~...E0:C.. Hilltop 5 50 34-58 2 3 0 5 0 0 5 0 0 4 1 0 4 1 0 0-12 Slope 5 35 16-50 1 4 0 4 0 1 4 0 1 3 1 1 4 0 1 0 Valley and draw 2 7 7-7 0 2 0 2 0 0 2 0 0 0 2 0 2 0 0 0 All locations 1}4 40 7-58 3 9 2 11 0 3 11 0 3 7 4 3 10 1 3 0-12 1Inforin8.tion reported by well owners or tenants. 2The topography was not recorded at two well locations. 12 draws yield twice as much per foot as those on hilltops. The depth to water below the land surface in dug wells averages 7 feet in valleys and draws, 35 feet on slopes, and 50 feet on hilltops (table 3). About 25 percent of the wells are inadequate, which probably is due to insufficient well depth. Dug wells in valleys and draws, and some on hilltops and slopes, require casing because of caving. About 20 percent of the hilltop wells reportedly required blasting. The water is generally of good quality. Two chemical analyses of water from migmatite are given in table 7. ZONE C The rocks mapped as zone C in this report were mapped by Crickmay (Cook and others, 1939) as Ashland Mica Schist. The name Ashland Mica Schist is not retained in this report and zone C is divided into three lithologic units ; biotite schist, amphibolite, and metagraywacke. BIOTITE SCHIST Description of the rocks The biotite schist unit consists of interbedded biotite schist, biotite gneiss, garnet-biotite schist, garnet-biotite gneiss, and thin discontinuous layers of amphibolite. The rocks show evidence of shearing and the occasional development of mylonite. Fair exposures of this rock were observed along the Etowah River at the GNL site. The rocks generally are coarsely crystalline, strongly foliated, and moderately jointed. Foliation and cleavage are generally parallel. The rocks have been intruded by granite near zone B. Water-bearing characteristics No drilled wells have penetrated this rock, but at least six wells are dug in the saprolite. The dug wells averaged 36 feet in depth to the water table (table 4-A), and their yields are adequate for domestic and farm uses. Some dug wells were reported to cave. Blasting was required in others to reach saturated rock. The water is reported to be soft but corrosive. AMPHIBOLITE Description of the rocks Overlying the biotite schist is 500 to 800 feet of dark-green to black amphibolite and thin interbedded lenses of pale orange-colored schist. The amphibolite is composed mainly of quartz, plagioclase, and hornblende, but minor amounts of magnetite, mica, and epidote occur in a few places. This rock is well jointed and at least three sets of joints were recognized in most outcrops. Secondary manganese oxide coats most of the joint surfaces. This amphibolite is a prominent ridge former and can be traced easily. A dark yellowish-orange to dusty-red soil develops on the amphibolite, and in most road cuts it has a cellular appearance. Table 4.-Dug wells -in zone C (Numbers represent wells reported unless otherwise shown) Location Metagraywacke Amphibolite Biotite schist All rock types Water level """~ .=";8. below land- surface datum (feet) .5 1Well bas adequate yield 1Users reported water qual'ty as shown I 1 Mud accumulates in well 1Well caves 1Well required dynamiting -!.l; .s ~ .0."...,. 8 z " ""~""" ".~..' <";. .5 = ~ = ~ = ~ = ~ = = ~ .,..cu"'CC ~=~~'-t""=l" z 0 >""-' I .:>=0=>: 'tl 0 0 " 0 = ... 0 .>: 0 i = ~ ;:;J z 0 >""-' 0 .>=: :>= z0 >""-' 0 .>=: :>= z 0 >""-' :.>>==0: blltl.bcal.i..!,.V..,- t: .... r.nQ.l rr 0 CIS G.J ~o-.-.o-~ A_ Wells in zone C analyzed with respect to lithology 29 41 3-64 3 11 15 13 0 16 12 1 16 13 0 16 2 2 25 4-12 4 28 20-33 0 2 2 2 0 2 1 0 3 0 1 3 1 0 3 0 G 36 25-60 0 4 2 4 1 1 2 0 4 2 1 3 1 1 4 0-4 39 39 3-64 3 17 19 19 1 19 15 1 23 15 2 22 4 3 32 0-12 B. Wells in metagraywacke analyzed with respect to topography Hilltop 16 45 28-64 1 7 8 7 0 9 7 0 9 7 0 9 0 21 15 4 Slope 8 35 20-44 2 3 3 5 0 3 4 1 3 5 0 3 0 1 7 12 Valley and draw 5 12 3-30 0 1 4 1 0 4 1 0 4 1 2 2 2 0 3 0 All locations 29 41 3-64 3 11 15 13 0 16 12 1 16 13 2 14 2 2 25 4-12 1lnformation reported by well owners or tenants. 'This well reportedly failed in 1954 and contained only 1.2 feet of water on 11-05-57. It needs to be deepened. 13 Water-bearing characte ristics Only two wells have bee n drilled in to the amphibolite . One we ll is 80 feet deep, but its yield is unknow n. The other well is 74 feet deep and has a yield of about 7 gpm. The average yield per foot of well is about 0.09 gpm (table 2). Four dug wells were inventoried. They averaged 28 feet in depth. The y ields of dug wells are adequate fo r most domestic and farm uses. Well owners r eported t hat blasting was not necessary to get water fro m t he amphiboli te and th at they had no problems wit h mud accu mulating in wells. The water is reported to be soft. Two chemical ana lyses of \\'ater fro m t he amphiboli te are g iven in table 7. METAGRAYWACKE Description of the rocks The rocks mapped as metagraywacke generally ar e metamorphosed sed iments. Lithologic varia- Hydrothermal quartz veins are comm on and are usually subconcordant with t he fo liation. A good expos ure of metag ra ywacke is at location 5301570-A; graded beds occur at t his location. Generally the quartz-biotite sc hist forms prominent ridges, and t he more feldspathic gneisses form lowlands. Shearing formed openings in which gold and pyrite were deposited. Eye-shaped pyrite-bearing qua rtz-feldspar pods 1 to 6 inches across are present in many places; t hese can be seen at location 530-1 570-A . Copper is present spor ad icall y . The amount and type of joint ing vary connsiderably with li t hology (fig. 5) . Gneisses are more j ointed than schists . Water-bearing characteristics The metagray wacke yields only small amounts of water to drilled wells . The average yield is about 0.04 gpm per foot of drilled well. Drilled F ig ure 5.-Join ti ng and bedding in t he metagraywacke of zone C. t ions along the strike are common. The individual beds co nsi st of garnet-biotite schist, garnet-biotite gneiss, biotite gneiss, and quartz-biotite sch ist. Thin cliscontinuous bod ies of amphibolite and epid ote-h ornblende gne iss also are presen t throughout t he metagraywacke. Rocks of this un it are s im ilar in mineralogy to the biotite gneiss but are well bedded in contrast to t he massive appearance of the biotite gnei ss. Thickness of t he beds ranges from a f raction of an inch to several feet . Bed thickness and composition var y along the strike. The lowermost 20 to 40 feet of this uni t is a biotite sc hist wh ich contains conglom- erat ic pebbles of qua rtz; the individual beds are 2 to 8 inches thick and are graded. Bedd ing in the metagrayvvacke is sh ow n in figure 5. wells in ventoried aver aged 400 feet in dept h (table 2). Data on dug wells in t he metagraywacke were analyzed with respect to topographic location of t he well (table 4-B). The average depth to the water table below t he land surface on hilltops is 45 feet, on slopes it is 35 feet, and in valleys and draws it is onl y 12 feet . Th e probabili ty of having to blast in order to reach saturated rock is greatest on hilltops . Valley and draw wells frequently cave unless cased. GREAT SMOKY GROUP The name Great Smoky co nglomerate was used by Keith (1904, p. 6) for a thi ck sequence of interbedded conglomerate, graywacke, q uartzite, 14 schist, and slate in North Carolina. The Great Smoky conglomerate was renamed the Great Smoky Formation by LaForge and Phalen (1913). Detailed mapping by Hurst (1955) showed the sequence of rocks to be divisib~e. into four. distin~t lithologic units, each of suffi~Ient stratigraphic importance to warrant formatwnal rank. There- fore Hurst reclassified the Great Smoky Forma- tion'as the Great Smoky Group and divided it into the Copperhill, Hughes Gap, _Hothouse, and Dean Formations. Based upon lithology the Great Smoky Group in Dawson County is divided into three zones- zoneD, zone E, and zone F. These zones are lithologically similar and possibly equi- valent respectively to the Copperhill, Hughes Gap, and Hothouse Formations of Hurst (1955). Criteria for the determination of the age of the Great Smoky Group were not observed in Dawson County, but the Great Smoky Group is considered to be Precambrian. In the extreme northern part of the county, lithologic changes along the strike, similarity in lithology between the upper part of zone F and zone that Dc~nanbde uncertain described structure, create a only by mapping complex in great detail. For this reason the northern part of Daw- son County was mapped as Great Smoky Group undifferentiated. Within this undifferentiated area kyanite schist similar to that within zone E was observed as well as conglomerate, meta- graywacke, and quartzite that could be either zone D or zone F. In the remainder of the county the lithologic zones were mapped. ZONED Description of the rocks About half of zone D is metagraywacke, and the remainder consists of biotite schist, biotite conglomerate, irregularly banded biotite gneiss, quartzite, metasiltstone, kyanite-~ill!manite-mica schist and meta-arkose. Pseudodwnte occurs as eye-shaped pods in some areas. A thick coarse- grained amphibolite occurs within this Z?ne. T~e amphibolite is mineralogically and stratigraphic- ally similar to the Newtown sill described by Hurst (1955, p. 59) near Ducktown, Tenn. The best ex- posure of the amphibolite is at location 470-1630- lBo~abtuitona more easily 470-1630-C. accessible outcrop Graded bedding is occurs at well pre- served and is abundant in some areas. The con- glomerate and schist occur as thick, massive non- persistent beds. The metasiltstone is thinly bedded; beds range from one-twentieth to one- half inch in thickness. The metagraywacke usu- ally occurs as alternating beds of biotite schist and biotite gneiss. The best exposures of zone D are along Georgia Highway 136 near the Dawson- Pickens County line. Water-bearing characteristics Springs are the only developed source of water in zone D in Dawson County. Four springs were inventoried; their yields ranged from 5 to 12 gpm. The water is reported to be soft and of good chemical quality. Because of the rugged topography, hillside springs are numerous and springs probably will continue to be the principal source of water from this zone. ZONE E Description of the rocks Most of zone E consists of amphibolite, amphibole-biotite schist, biotite-amphibole gneiss, garnet-biotite-amphibole gneiss, quartzite, kyanitebiotite schist and gneiss, sillimanite - kyanite schist, and quartz-garnet-muscovite schist. The abundance of garnet, kyanite, sillimanite, dark-red biotite, and amphibole indicate a high iron and aluminum content in these rocks. The sillimanite occurs as thin, barely discernible crystals in the schist beds of the central part of the county. The sillimanite crystals increase in size and abundance northward, and in the vicinity of Faucetts Lake the coarse fibrous sillimanite called fibrolite occurs sporadically. Sillimanite was not observed in these rocks south of Georgia Highway 53. A characteristic of zone E in the central and southwestern part of the county is the interlocking to radiating clusters of kyanite which megascopically resemble conglomeratic pebbles of milky quartz. In the schistose beds kyanite occurs as elliptically-shaped porphyroblasts about half an inch in diameter, but the amount of kyanite decreases toward the northwest, where it appears to have been partially replaced by sillimanite. Schistosity generally is wavy owing to the porphyroblastic growth of crystals of kyanite, sillimanite, and garnet. Weathered out garnet-mica-quartz buttons about 1 inch in diameter and 0.3-inch thick are common; buttons more than 2 inches in diameter and 0.5-inch thick are scarce. The rocks of this zone are well jointed but do not weather deeply. A shallow-dark-red soil develops, which contains abundant pebbles of garnet and kyanite. Water-bearing characteristics Drilled wells in zone E average 146 feet in depth and 13 gpm in yield; this is about 0.09 gpm per foot of well (table 2). Dug wells average 37 feet to the water table (table 5). Wells on hilltops average 49 feet to the water table, wells on slopes 37 feet, and wells in valleys or draws 14 feet. Of the owners interviewed who had knowledge of the construction of their wells, 6 of 28 reported inadequate yields, 6 of 37 reported corrosive water, 4 of 31 reported mud accumulation, 2 of 31 reported occasional caving, and 8 of 12 reported that their wells had to be blasted to obtain water. One well was reported blasted at least 53 feet before reaching saturated rock. Wells that fail during dry weather, contain corrosive waters, accumulate mud, or require blasting to reach saturated rock are more numerous on hilltops than on slopes or in valleys and draws. Caving wells are most numerous in valleys and draws. Water in zone E generally is corrosive and high in iron. Four chemical analyses of water from this zone are given in table 7. 15 Table 5.-Dug wells in zones E and F (analysed with respect to topography) (Numbers represent wells reported unless otherwise shown) - - - - - ------------ - Topographic location "tl Water level ~ ="~tl .<: y below landsurface I datum 1Well has adequate 1Users reported water qual'ty 1Mud accumulates ~ (feet) I yield as shown in well 1Well caves 1Well required dynamiting .!2 0: )1: ...<: I :} 0... "'lt"llo s'".,Q z ~ .."'"..." ~ < > .s c )1: c c ~ ~ =)1: c c )1: .... <1.1"0 ~-s . '"c llo ~"tl z 0 ~ >'"- ..cc0:.: l"tl I o :::> 1<3 .. 0 0 P.. ..cc0:.: :::> ~ z0 >'"- ..c0:c.: :::> z 0 ~ >'"- c 0 "::c":>' z0 ~ >'"- c 0 :":c:">' ~ Gmbc:.o ./-.llo..be...- uli.It. =".>: .~. -==..~"tt ,.. o..O~ I:- .. (L)Q ,-< ..c~ ~ ~ " -"tl .! ~ ~;~ ...0"tl .. ....c. c. ... .,=~ 0 .... Q " .S:"ii~~ ,0...., s~ 0,...., 0,...0., Ground water from,the alluvial deposits is of good chemical quality. It is soft and noncorrosive. The water is well suited for domestic and industrial use. However, some wells immediately adjacent to streams may yield water which is muddy after rains and which contains coliform bacteria or other objectionable matter. An example is well 560-1570-21, which is 14 feet deep and is about 10 feet from a polluted stream. Mundorff (1950, p. 13), after studying alluvial deposits in North Carolina, found that wells 100 to 200 feet from polluted streams yielded water of good quality. The deposits of sand and gravel that fill many stream valleys are potential sources of large quantities of water. Information concerning the flood-plain deposits of Dawson County are listed in table 6. Based on the above data, the known yields of wells in alluvial sand and gravel in other similar areas, and the large yield reported from a dug well (570-1570-20) in alluvium in Dawson County, wells and infiltration galleries having yields of several hundred gpm can be developed in alluvial deposits along the Etowah River in Dawson County. Such wells and galleries would be from 20 to 30 feet deep. STRUCTURE The rocks of zone A are folded and generally overturned toward the northwest (fig. 6) Fold axes trend N.45E. to N.75E. Zone B is a fault zone, and slickensides are numerous. The hanging wall (zone A) moved northwest over the footwall (zone C). The rocks of zone C are folded into tight overturned isoclinal folds whose axial planes strike N.40E. to N.60E. and dip about 60 SE. This zone is bounded on both sides by faults. Neither of the faults was observed, but several criteria indicate their occurrence. A fault southeast of zone C was first mapped by Furcron and Teague (1945). Criteria observed by the author are: (1) the migmatite of zone B is tabular and has been injected into adjacent rocks, (2) radioactive and magnetic anomalies terminate abruptly, (3) slickensides are numerous in zone B, and (4) the topography changes abruptly along the southeast edge of zone C. This change was recognized by LaForge (1925) who used it to separate two of his physiographic divisions, the Fairburn and the Gainesville platforms. A fault northwest of zone C was first mapped in Dawson County by Stose and Ljungstedt (1932). Criteria used by the author to establish this fault are: (1) zone F of the Great Smoky Group is thinned, and all younger beds are absent, (2) radioactive and magnetic anomalies terminate abruptly, (3) slickensides and shear zones occur locally, and (4) the topography changes abruptly along the northwest edge of zone C. This change was recognized by LaForge (1925) who used it to separate two more of his physiographic divisions, the Dahlonega and Atlanta Plateaus. Most of the rocks of the Great Smoky Group occupy the upright east limb and the southern nose of a plunging anticline overturned toward the west. The overturned beds of the west limb are exposed along Georgia Highway 136 west of the county. The structural trends of the Great Smoky Group closely follow the configuration of the belt of Murphy Marble to the west, as shown on the Geologic Map of Georgia (Cook and others, 1939). Superimposed on this anticline are nu- 17 merous minor folds whose axial trends are generally east-west. The rocks of t he Great Smoky Group a re broken by numerous high-angle fa ults (fig . 6). Slickensides are common along t h e fau lts, as at locat ion 470-1630-A (fig. 7). 'T'he dip of bedding is usually less than 40 degrees except near fau lts where it is erratic, and overturned beds are common. The fault between zone C and the Great Smoky Group caused steepening of dips and overt urning of beds in the Great Smoky Group for several thousand feet northwest of t he fault. Figure 7.-Slickensides along fau lt at location 470-1630-A. Figure 8 shows s uch an overturned fold in muscovite schist in the lower part of t he Great Smoky Group, zone F. Schistosity and axial-plane cleavage are parallel to bedding except on t he nose of fo lds (fig. 8b). WEATHERING The weathering of rocks may be divided into two principal categories, disintegration and decomposition. Weathering is caused by ph ysical changes, the most important of which is rock expansion due to unloading, and chemical changes that are induced by ground water percolating downward through joints, schistosity planes, and other openings in t he rock (Reiche, 1950 ) . In the first stages of weathering certain mineral grains or gr oups of grains are broken apart by t he expa nsion due to h ydration of micaceo us minerals. Rock durin g t hese first stages of weathering is referred to as disintegrated rock. The feldspars are particularly s usceptible to breakage because of t heir cleavage and solubility . Onl y minor chemical weatherin g occ urs at f irst, but, as part icle size diminishes and m ore surface area is exposed, the chemical alteration of many of the minerals proceeds until clay min erals are formed. Altera t ion of t he original minerals to clay is accompanied by h ydration a nd expansion so t hat small cracks and joints formed in the first stages ten d to become closed. T he end product of t his weathering is decomposed rock. All gradation s occur between decomposed rock and fresh rock . In general the disintegrated rock is far more permeable than the decomposed rock and is the principal zone tapped by wells. WATER RESOURCES The Hydrologic Cycle Precipitation is the source of all water in the county . Water vapor condenses and fa lls as snow or r a in on the land surface where it either seeps into t he soil, evaporates, or runs into lakes and streams. Part of t he water that enters the soil returns to the atmosphere by transpiration and evaporation, and part is absorbed by various minerals. The remainder of the water seeps downward to the saturated zone. After water reaches the saturated zone, t he top of whi ch is called the water table, it may percolate through interstit ial pores in the saprolite or along joints or other openings within hard rock. Ground water slowly moves downgradient to places where t he water table is close to or inter sects the land surface, where it is discharged either by sprin gs and swamps or by evaporation and transpiration. It may seep directly from the saturated zone to stream beds. SURFACE WATER Streams Two major river systems, the Coosa and the Chattahoochee, drain DawHon County. Drainage 18 a. Overall view of nose of folc!. b. Closeup of nose of fo ld showing departure of axia l plane cleavage f rom bedding . Figure 8.--0vertumed recumbent fo ld in muscovite schist of zone F at location 530-1600-A. on the northwestem slopes of the Highland is northwestward into the Coosawattee River in Gilmer Co unty. Drainage on the southeastern slopes of the Highland is southward in t he Amicalola Creek valley into the Etowah River. Shoal Creek, another tributary to the Etowah River, and the Etowah River, drain t he central part of the county . The Etowah and Coosawattee Ri ve rs are part of t he Coosa River system. Drainage in t he 19 southeast corner of the county is into Lake Sidney Lanier on the Chattahoochee River system. These waters eventually reach the Gulf of Mexico. The geographic arrangement of the streams is a result of geologic structure, differences in rock hardness, and the physiographic history of the region. Streams in the county have relatively high rates of runoff per unit area owing to the hilly terrane. The average flow of the Etowah River at the gaging station near Dawsonville, 1 mile upstream from Georgia Highway 53, was 239 cfs (cubic feet per second) for the 19-year period between 1940 and 1959 (U.S. Geological Survey, 1960, p. 287). The average flow of Amicalola Creek at the gaging station near Dawsonville, at Georgia Highway 53, was 215 cfs for a 12-year period between 1939 and 1952 (U.S. Geological Survey, 1955, p. 226). The flow of Shoal Creek during October 1956 above its confluence with the Etowah River was about 30 cfs (Stewart and others, 1961, written communication). Water from several small streams throughout the county is used by farmers to water chickens and livestock. About 150,000 gallons of water from the Etowah River are filtered and chlorinated daily for use at GNL. Other uses of streams in the area are for recreation and for cooling in the production of illicit white liquor called "moonshine." Lakes About 12 miles south of the Dawson-Pickens County line, the Chestatee and Chattahoochee Rivers are ponded by Buford Dam to form Lake Sidney Lanier. The lake floods about 2,600 acres of land in Dawson County at a lake elevation of 1,070 feet above mean sea level. Water surrounds an additional 200 acres of islands. The lake is used for the generation of electric power and is a major recreational area. In 1954 the county contained 20 small artificial lakes which ranged in size from about 1 to 45 acres. One of the most popular is Lake Amicalola, a 7-acre lake in Amicalola Falls State Park. In addition, 12 small flood- retarding dams, under construction in 1960, will create lakes from 6 to 25 acres in area which also will be used for recreation. Lakes help to raise the water table beneath adjacent hill slopes by reducing the ground-water gradient. This creates additional storage of ground water and helps reduce the number of well failures adjacent to the lakes during dry seasons. GROUND WATER Ground water is the water in the rocks below the water table. It is water that issues from, or may be pumped from, springs or wells. In the use of any spring or well, a continuing supply of ground water is as important as an adequate yield. All ground water supplied must be transported through openings in rock, but openings and kinds of rocks differ greatly. The character of both the openings and the rock are determined by the physical and chemical properties of the rock and geological processes. To gain knowledge about the water-bearing character of the rocks, geologic studies are necessary. Recharge The principal influences on recharge are precipitation, plantlife cover, season of the year, soil type, dip of planar features, and topography. Of these precipitation is obviously the most important for without it recharge would not be possible. Precipitation, plant cover, and the season of the year are interrelated in their influences. During the growing season, infiltrated rainfall is used by plants or is evaporated directly from the soil because of high summer temperatures, and apparently little rainfall recharges the rocks except after exceptionally heavy or prolonged rains. Each large elm, oak, or similar tree can transpire as much as 500 gpd (gallons per day) or about as much as 10 people or 10,000 chickens will use in a day. Frequently during the growing season a wooded area will transpire all of a single rainfall without allowing recharge to the groundwater reservoir. In contrast, during the winter when most trees are dormant, even light rains recharge aquifers. Lower temperatures during the winter result in lower evaporation rates. Also, most rains during winter are slow and steady and have little runoff in contrast to the short hard summer thundershowers which inundate the streams. Snows also recharge aquifers during winter. Thus, the amount of annual recharge to the ground-water reservoir is controlled much more by precipitation during the winter, which averages about 17 inches, than by precipitation during the remainder of the year. Rate of recharge also is controlled by the porosity and permeability of the soil. Sandy soils are more permeable, and water moves more rapidly into and through them than it does in clay soils. In Dawson County most soils are formed in place from the underlying rocks and vary in texture and composition with variations in the rocks ; thus recharge varies from one rock unit to another. The dip of bedding, cleavage, and schistosity affect recharge. Where these planar openings are horizontal, they aid lateral movement to discharging springs but inhibit the vertical movement of water and retard recharge. Where the planes dip at high angles, water moves readily down them to the water table. Topography influences recharge because runoff is more rapid on steep slopes than on flat lands, and surface water has less time to infiltrate and recharge an aquifer. Thus, the greatest recharge occurs during the winter and in areas of low relief, sandy soil, steeply dipping planar openings, and dormant plant life. MOVEMENT Field observations Generally the movement of water downward through the soil is vertical (section a, fig. 9). 20 Figure 9.-Seeps show paths of water movement in saprolite. Road cut is near Doraville, DeKalb County. (See p. 20 for explanation.) Small quantities of water were observed to move laterally (not shown in fig. 9) through the sub- soil immediately above decomposed rock, indicating a smaller porosity and (or) permeability of the underlying saprolite. ering as an indicator, the following observations and conclusions were made. Water movement is not uniformly distributed but tends to be concentrated in a few openings, such as the one shown in figure 10. Upon entering decomposed rock, movement of the water is influenced by relic rock textures and structural features, particularly bedding and schistosity. Note the abrupt change in direction of water movement at the top of decomposed rock (below point b, fig. 9). Some water moved along the joints (c and d, fig. 9), but most of the water moved parallel to the foliation through pores created by weathering (e, fig. 9). All the quartz veins examined in the saprolite were well fractured and more permeable than the surrounding material, as is the one discharging water at point f in figure 9. Water that moves downward through the saprolite comes in contact with a tremendous decrease in porosity as it enters hard rock. Stewart and others (1961, written communication) found the average porosity of saprolite to be along sheeting or exfoliation joints, high-angle joints, bedding planes, and schistosity planes and through interstitial pores in the saprolite. Most water appears to move through interstitial pores in the saprolite immediately above hard rock. Weathering as an indicator To a degree the extent of weathering along each planar opening is indicative of its role as a conduit for water circulation. Using weath- Although this opening appears to be wide in the photograph, it is thinner than a pencil lead. A zone of weathered rock has developed on each side of the plane, and, after exposure, is rapidly eroding away. Weathering along a few steeply dipping planes, as shown in figure 10, probably extends to great depth and accounts for the occasional well that taps large supplies of water below 500 feet. However, the number of joints about 46 percent and the average porosity of fresh rock to be about 4.5 percent at the GNL. The ratio of porosities is about 10:1. Inasmuch as all the water moving in the saprolite cannot flow into the few openings available in hard rock, most of it accumulates immediately above the hard rock. Accumulation of the water increases the hydraulic gradient, causing the water to be shunted laterally within the saprolite and amount of weathering along most joints decrease rapidly with depth. The extensive joints are the most highly weathered ones. Decomposition is greatest along the uppermost sheeted zones and less great along the most prominent high-angle joints, indicating that these transport water. Where high-angle joints transect the alternating schist and gneiss beds of metagraywacke, decomposition is least adjacent to the gneiss beds, intermediate adjacent to the schist beds, ancl greatest adjacent to the 21 Figure 10.-Weathering along joints in biotite gneiss. bedding planes - indicating that more vvater moves along the bedding planes. The amount of weathering varies considerably with lithology. This is why the different litholog ic units have different water yield. (See ta- ble 12) For example, as the feldspar content in schist increases, the weathered zone along joints, and along bedding and schistosity planes increases perceptibly in width . along planes oriented parallel to the strike of the rocks, then streams flowing normal to the strike should transect a greater number of these planes and should receive a greater volume of groundwater runoff per unit length of stream than streams flowing in other directions (fig. 11). Pumping test A pumping test was made at GNL (Stewart and others, 1961, written communication) in sa- prolite of the rr.igmatite of zone B. The cone of depression was elliptical with its major axis parallel to the direction of strike of the rocks. 1 The cone represent s a dewatered area, and it is obvious from its shape that much more water moved toward the pumping well parallel to the strike of th e rocks than normal to it. Laboratory studies Two samples of saprolite were collected at the same location (Stewart and others, 1961, written communication) to determine the differences in permeability parallel to and normal to the strike of the rocks. The permeability parallel to the strike was about 0.2 gpd per square foot, but the permeability normal to the strike was only about 0.009 gpd per square foot. Variation in streamflow due to ground-water runoff If the principal movement of ground water IS 1S chi stus ity, axia l- pla ne c lean1ge, and bed di ng a re ge nera ll y pan1 ll el in the a rea, and H is not kn own w h ich o f t hese actually contro l t he direct io n of water mo ve ment. F o r co n v~ n i e n c e of di sc uss io n thei r or ie n tat i on is g iv C> n a~ the st dke and d i p of th e toc k s. Figure 11.-Re laf ionshi p befween di re c f ion of sfreomf low ond fhe number of p lanar openings fransecfed per unif l engf h of stream . A study of the increase in streamflow due to the ground-water runoff made at GNL by R. F . Carter (Stewart and others, 1961, written communication) showed a variation in ground-water runoff with direction of streamflow. Groundwater runoff is plotted against strike and dip of the rocks in figure 12. The length of line reptesents runoff in cubic feet per second per square 22 mile of drainage area. The greatest groundwater runoff is to streams flowing normal to the strike of the rocks. Also, 59 percent of the streams are on dip slopes while only 9 percent are on scarp slopes ; 32 percent are on strike slopes. Rate of movement The rate of flow of ground water as determined by Stewart and others (1961, written communication) varied between 0.007 and 0.14 foot per day in hard rock, between 0.15 and 0.40 foot per day in weathered migmatite, and between 0.59 foot and 1.58 feet per day in weathered metagraywacke. Storage Storage capacity is an essential aspect of an aquifer. To understand where and how water is stored, the nature and distribution of openings must be studied. In crystalline rocks water is stored in the interstices and along various 16 -so -.90 -1.00 -1.10 Length of line represents ground-water runoff in cubic feet per second per square mile -1.20 -1.30 -1.40 After R.F. Corter in Stewart and others 1961 written communication / Measurement sites 8 Figure 12.- Relative ground-water runoff plotted in the direction of the major ax1s of stream basins at GN L, Dawson County. 23 planar openings. Ground water also is stored in alluvial deposits. The porosity of a rock - a unit measure of its storage capacity - is the ratio of the aggregate volume of interstices in the rocks to its total volume expressed as a percentage (Meinzer, 1923, p. 19). Figure 13-A shows the difference in interstitial storage space in fresh rock and weathered rock. Completely saturated, a volume of saprolite 1 acre in area and 10 feet thick would contain :g 2 0 c: ~ 0 o I ., "0 ' 0 iii poroslt)':. 4.5 percent) fresh rock {average 5 10 15 20 Thickness of saturated material, in feet A. Water stored in interstitial pores 'c": E 3,-----,-----.--------,,------~ 0 0"' "c:' 2 E s 2 ~ 0 0 specific yield: 0 percent) 0 5 10 15 20 Thickness of saturated material, in feet 8. Amount of stored interstitial water available to wells Figure 13,-- Storage and availability of ground water in interstitial pores. about 1,500,000 gallons of water, whereas an equal volume of fresh rock would contain only about 150,000 gallons. This is a ratio of 10:1. But this is not as significant a difference as the specific yield. Specific yield is an expression of the amount of stored water that is available to wells and is defined as the ratio of the volume of water which a rock, after being saturated, will yield by gravity to its own volume, expressed as a percentage (Meinzer, 1923, p. 28). Figure 13-B shows the difference in specific yield of weathered rock and fresh rock. Completely saturated, a volume of saprolite 1 acre in area and 10 feet thick would contain about 850,000 gallons of water available to wells, or about 57 percent of the water stored. None of the water stored in interstitial pores in fresh rock is available to wells. The pores are so minute that capillary forces prohibit movement of the water. Because of the many difficulties involved, direct measurements have not been made to determine the porosity due to jointing and other open breaks within fresh rock. LeGrand and Mundorff (1952, p. 10), and numerous other investigators believe the porosity to be less than 1 percent. Ellis (1906, p. 21) believes the porosity to be less than onehalf of one percent. Observations in quarries and cuts, as in figure 11, suggest that the porosity of fresh rock due to jointing alone is less than 1 percent. Thus, practically all ground water available to wells is stored in weathered rock. Wells obtain some water from breaks in hard rock and from weathered rock adjacent to the breaks. As weathering generally decreases rapidly with depth porosity decreases correspondingly. The thickness of the saprolite, and hence the amount of water stored, varies with topography and with rock type. No storage data are available for alluvial deposits in Dawson County, but on the basis of the known geology it is concluded that a large quantity of water is stored in the alluvium. Discharge Ground-water discharge is divided into two categories -- natural and artificial. The principal natural discharge is by springs. Artificial discharge is restricted to wells and deep road cuts. Most springs discharge from the saprolite immediately above hard rock, but the yield of these springs is usually less than 1 gpm. Springs that flow from joint, bedding, or foliation planes are less numerous but usually yield 1 to 5 gpm. Many of the larger springs are associated with faults and occur at intervals along fault traces. Yields of these springs are generally more than 5 gpm. The largest spring yields about 90 gpm and is the town supply for Dawsonville. The total use of ground water in Dawson County, based on an estimated per capita use of 50 gpd per person and 5 gpd per 100 chickens, is about 220,000 gpd. No estimate of the discharge from road cuts has been attempted, but it is not much. Chemical Quality Natural water is not chemically pure, as it contains dissolved gasses and impurities from the air and many minerals dissolved from the rocks with which it comes in contact. Chemical analyses of ground water in Dawson County were made to determine the amounts of dissolved constituents in the water. The amounts of silica, iron, calcium, magnesium, sodium, potassium, bicarbonate, sulfate, chloride, nitrate, and fluoride in the water were determined and are expressed in parts per million. The pH, hardness, 24 Owner Location Table 7.-Chemical analyses of ground water, Dawson County (Analyses by the U. S. Geological Survey except as indicated.) -----~- ~ -~ ~---~---- Parts per million Aquifer 0 iii a 0 = .;! -";; ..!:; u .as CX1 M cr,j cv:) M 00'> ~ C'l coI r( 0 ~ 0 ,....; cr.) cr.) ~ H I ~ H ~ H 0'> .... I ~ 0 H 0 0 ....., rn :B 'r-n' ."::' ."::' 0 0 N N 0 0 A A mended limit of iron for domestic use (table 8) is 0.3 ppm (parts per million). Water containing more than 0.3 ppm will stain fabrics, utensils, and fixtures, and 0.5 ppm is detectable by taste. Water having a high iron content favors the growth of the organism Crenothrix. This organism forms reddish-brown deposits in water pipes, partly or completely clogging them. The iron content in waters varies with lithology (table 9) Calcium Calcium carbonate forms a soft scale and calcium sulfate forms a hard scale in boilers and 26 cooking utensils. Calcium also is a soap consumer. Water in some amphibolite of zone A is reported by well owners to contain enough calcium carbonate to form a scale in domestic cooking utensils. Magnesium Magnesium is generally present in small quantities in water that contains calcium. Magnesium carbonate, which is precipitated by heating, forms a soft friable scale. Calcium and magnesium sulfate when precipitated together form a dense porcelainlike scale. The highest magnesium content analyzed was 25 ppm. This is less than the recommended limit of 125 ppm shown in table 8. Sodium and potassium Water containing more than 5 ppm sodium and (or) potassium may cause foaming in boilers. More than 300 ppm sodium salts in water causes a saline taste. The sodium and potassium content of ground water varies between lithologies (table 10). More than 50 percent of the water from zones B and C contained more than 5 ppm sodium and potassium combined, but none of the water in zone A and only 25 percent of the water in the Great Smoky Group contained more than 5 ppm. The average content of all water analyzed is only 7.9 ppm. Carbonate and bicarbonate Calcium and magnesium bicarbonate are the most abundant dissolved mineral matter in ground water but have comparatively little effect on the utility of water unless present in very large amounts. Carbonate is not present in the ground water of Dawson County. Sulfate Most sulfate is derived from oxidation of metallic sulfides, sulfur-bearing organic compounds, or fertilizers containing sulfate. Sulfate is purgative and causes a bitter taste in water if present in excess of 250 ppm. Sulfate in excess of 100 ppm causes hard scale in boilers if calcium and magnesium cations are present. Ground water in Dawson County contains little sulfate. Chloride Sodium chloride is a characteristic constituent of sewage, and any appreciable pollution of water by sewage is accompanied by a measurable increase in chloride. Small quantities of chloride also are dissolved from some rock materials. Chloride gives a salty taste to water if present in quantities greater than 250 ppm. Ground water in Dawson County is low in chloride. Nitrate Fertilizers contribute to the nitrate content in water supplies and some nitrogen is dissolved Table 11.-Carbon dioxide in ground U'ater, DaU'son County Sample Date No. collected Source Location number Field pH ..".E.. ... "k"o!-o~ ..:l j=~- -38 E iQ:d: ~~ 0 ~ SQ)- .u;c.S:~!S~eo. N"C~ .... o O'"' 0 M ;.:. u0 ooo ~Xu Aquifer 1 01-19-60 drilled 500-1600-43 5.6 59.5 8 32 1:4 Zone E 2 01-19-60 spring 500-1600-11 5.7 57.0 11 35 1:3 Do 3 01-19-60 dug 500-1600-23 6.4 56.0 37 21 2:1 Migmatite 4 01-19-60 do 500-1600-39 6.0 58.0 8 13 1:2 Zone E 5 01-19-60 do 500-1600-49 5.3 57.0 6 48 1:8 Do 6 01-19-60 drilled 500-1600-50 5.7 59.0 15 48 1:3 Do 7 01-19-60 dug 500-1570-15 6.3 58.0 49 39 1:1 Do 8 01-20-60 do 530-1570-20 6.8 20 5 4:1 Alluvium 9 01-20-60 drilled 530-1570-14 6.1 43 54 1:1 Amphibole-mica schist 10 01-20-60 dug 530-1570-24 5.5 18 91 1:5 Metagraywacke 11 01-20-60 spring 530-1570-23 5.4 11 70 1:6 Do 12 01-20-60 drilled 500-1600-33 6.1 52.0 23 29 1:1 Zone E 13 01-20-60 spring 500-1600-32 5.6 56.0 12 48 1:4 Do 14 01-20-60 drilled 500-1630-13 6.2 55.0 56 56 1:1 Do 15 01-20-60 spring 500-1630-29 5.4 57.0 10 63 1:6 Great Smoky Group undifferentiated 16 01-20-60 dug 500-1630-34 6.0 51.0 63 100 1:1 Do 17 01-20-60 do 530-1600-24 6.1 56.0 11 14 1:1 Zone F 18 01-21-60 bored 560-1570-25 6.0 44.0 30 48 1:2 Sillimanite-amphibole-mica schist 19 01-21-60 drilled 560-1570-23 5.7 54.5 17 54 1:3 Amphibole-biotite gneiss 20 01-21-60 do 560-1570-11 6.2 54.5 30 30 1:1 Quartzite 21 01-21-60 dug 560-1570-3 5.8 59.5 17 43 1:3 Amphibole-biotite gneiss 22 01-21-60 do 560-1570-5 5.8 57.0 42 106 1:3 Quartzite 23 01-21-60 do 560-1570-49 5.6 57.0 24 96 1:4 Amphibolite 24 01-21-60 do 560-1570-91 5.6 56.0 8 32 1:4 Mica schist 25 01-21-60 spring 560-1570-92 5.3 58.0 17 135 1:8 Migmatite 1Chemical analyses by Roger Landrum, Georgia Geological Survey, January 25, 1960. co, content computed as follows: ppm co, = 1.589 x 10 [H+l x ppm alkalinity as HCOa. 27 from rocks, but most of the nitrate in water is considered to be the oxidation product of nitrogenous organic material (usually sewage). The presence of abnormal quantities of nitrate may indicate poor sanitary conditions. The recommended limit is 44 ppm (table 8). Two dug wells were found to contain more than 44 ppm; both are polluted because of inadequate protection, and both wells are abandoned. Poor construction accounts for most of the pollution of well water. pH The pH is the negative logarithm of the hydrogen-ion concentration in water and is an expression of the acidity or alkalinity. A low pH is evidence of a high concentration of hydrogen ions, or acidity, and a high pH is evidence of a low concentration of hydrogen ions, or alkalinity. Neutral water has a pH of 7.0. Ground water in Dawson County is decidedly acid, and the pH of some water is as low as 4.9. A portable meter was used to determine the pH at 25 sites (table 11) immediately after collection of water samples; the determinations are considered to be nearer the natural pH than those in table 14 (laboratory determinations). The pH ranged from 5.3 to 6.8 in the field and averaged about 5.9. Water in alluvium has the highest pH, and water in metagraywacke has the lowest average pH of all waters examined - indicating that lithology influences the pH of ground water. Carbon dioxide The principal cause of corrosiveness of ground water in Dawson County is carbon dioxide. Carbon dioxide is absorbed from the atmosphere, from humus in the upper soil layer, and from soil air. The carbon dioxide content in soil air is 10 to 100 times greater than in the atmo- N 1 --dI (.)' L&JI wo~I, ~~ u; Figure 14.- Ground-water temperatures in degrees (October 1959) 28 Contours based on 39 control points Fahrenheit, Dawson County sphere (Hem, J. D., 1960, oral communication). Thus, percolating waters have access to and absorb large amounts of carbon dioxide. The carbon dioxide content of ground water in Dawson County is shown in table 11. If the ratio of alkalinity (bicarbonate) to carbon dioxide is less than 3:1, the water is corrosive. Of the 25 water samples tested, 23 were decidedly corrosive, one was slightly corrosive (No. 3, table 11), and one was noncorrosive (No 8, table 11). The noncorrosive water was from the alluvial deposit adjacent to the Etowah River. Corrosive waters cause reddish - brown or rust - colored stains on linen and porcelain fixtures where iron or steel plumbing is used and blue or blue-green stains where copper or brass plumbing is used. Corrosive water attacks and gradually destroys plumbing, appliances, pumps, and the like. The corrosiveness of water can be neutralized by treatment with various commercial products. Water in dug wells and springs can be neutralized with crushed limestone or marble. Hardness Hardness is the property of water attributable to the presence of alkaline earths. Hardness is caused almost entirely by calcium and magnesium. Other constituents, such as iron, aluminum, strontium, barium, zinc, or free acid also cause hardness. Hardness that can be removed by boiling is referred to as carbonate hardness, and the remainder, if any, as noncarbonate hardness. Hardness causes scale in boilers, destroys soap to form soap curds, and destroys dyestuffs. Water with a hardness of 0 to 60 ppm is classified "soft"; 61 to 120 ppm, "moderately hard"; 121 to 180 ppm, "hard"; and more than 181 ppm, "very hard." The hardness of water in Dawson County averages 28 ppm and ranges between 2 and 89 ppm (table 7.) Of the 19 analyses in table 7, 16 may be classified as soft and 3 as moderately hard. Fluoride Fluoride in excessive concentrations is undesirable in water used for drinking because it may cause spotting of the tooth enamel and may effect skeletal bone structure. Fluoride is a natural constituent in much of the ground water in the study area and is dissolved from numerous complex fluoride-bearing minerals found in the rocks ; the most important of these minerals are apatite, hornblende and mica. Fluoride content in water from the study area ranged from 0.0 to 0.2 ppm. The recommended maximum limit for fluoride content in water in this area is 1.2 ppm, according to the U.S. Public Health Service. (See table 8.) Temperature The temperature of ground water approximates the mean annual air temperature; but season of the year, and, locally, Lake Sidney Lanier also affect the ground-water temperature in Dawson County. The altitude of the land surface in Dawson County ranges from about 1,000 feet in the southern part to 3,200 feet in the northern part. Ground-water temperature decreases with increasing altitude (fig. 14). This decrease is caused by a decrease of the mean annual air temperature and more snowmelt at higher altitudes. The temperature of ground water decreases adjacent to Lake Sidney Lanier. The reason for the decrease is not understood, but the temperature 500-1630-18 DEPTH- 9 FEET 64 62 a; ~ 60 ~ .c ~ 58 """::''';. 56 ""0' .S 54 ~ ::J ~~ 52 '~ ~ 50L-~L-~--~--_l___L___L__J ___~--i_~ ~ 62,_-,,--,--~---.---.---,--,---,---,---, 530-1600-24 60 DEP~TH~-"35~FDEE~TEP=T=H5-0=0B_ 1=5O70=_FIOE=ET==============~ 500-1600-23 58 SPRING Figure 15:--Seasonal changes in the temperature of ground water. of the water in Lake Lanier at a depth of 135 feet was about 47 F during October 1959 (Mr. William T. Alley, U.S. Army Corp of Engineers, oral communication, October, 1960). The ground-water temperature decreases 1 degree along a northeast-trending belt passing through Dawsonville. This belt roughly corresponds to zone C and is attributed to increased altitude of land surface within the zone. The temperature of water in 22 wells and springs were measured periodically between July 1959 and April 1960. The seasonal ground-watertemperature fluctuation is far greater in shallow wells than in deeper ones (fig. 15). This was consistent in all the wells measured. Changes in temperature of ground water lagged from 40 to 60 days behind changes in temperature of the air. The average seasonal temperature change was 15.2F at shallow depths (less than 15 feet to the water table), 1.5F in deeper wells (more than 40 feet to the water table), and 1.9F in springs. During the summer water in shallow wells is warmer than that in deeper wells whereas during the winter water in shallow wells is considerably colder than that in deep wells. Well Yield The term yield, as used in this report, designates the maximum rate at which water can 29 Location Hilltop Slope Valley and draw Zone A Quartzite Mica schist Amphibolite Sillimanite-biotite gneiss and schist Amphibole orthogneiss Zone B (migmatite) Zone C Biotite schist Amphibolite Metagraywacke Great Smoky Group Zone F Zone E ZoneD Alluvium All locations Table 12.-Yield of drilled wells and springs Wells Drilled wells Reported yield (gpm) Range Average Per foot of well Number of springs A. Analysed with respect to topography 26 0-40 16 0.08 0 21 4-80 16 .10 1 25 3 7-40 21 .15 18 B. Analysed with respect to lithology 20 0-80 17 .08 7 1 35 .23 0 16 0-80 16 .08 4 0 0 3 5-25 15 .05 2 0 10 4-20 11 4 2-30 12 0 2 3-11 7 2 2-30 16 16 3-40 18 5 11-40 30 11 3-27 13 0 0 50 0-80 16 1 .06 9 .05 14 2 .09 0 .04 12 .11 41 .13 15 .09 19 4 0 .09 71 Springs Yield (gpm) Range Average 1-25 6 1-90 15 1-5 2 1-4 2 1-5 3 2 2 2-15 7 1-4 3 2-2 2 1-4 3 1-90 14 1-90 18 3-20 10 5-12 9 1-90 10 1The topographic location of 28 springs was not recorded. be withdrawn from an aquifer by wells and is expressed in gallons per minute. Yield is influenced by porosity, permeability, jointing, storage, recharge, and other geologic and hydrologic factors. The topography in Dawson County is a re- sult of differences in resistance of the rocks to erosion. The resistance of a rock to erosion is affected by its chemical and physical properties. The most resistant rocks form ridges, and the least resistant form valleys. Studies were made of the relationship between lithology, topography, and yield of wells. Wells in valleys and draws have larger yields than wells on hilltops and slopes (table 12-A). For drilled wells the yield per foot of well in valleys and draws is almost twice that for wells on hilltops. Also, the average yield of springs in valleys is 2.5 times that of springs on slopes. Because dug wells seldom are tested, little is known about their yield, but the variation in yield should correspond to that of springs and drilled wells. Yield also depends upon lithology. The variation in yield between lithologies is shown in ta- ble 12-B. The various lithologic units can be compared or rated best on the basis of yield per foot of drilled well. For example, in the quartzite of zone A the average yield is 0.23 gpm per foot of well, whereas in the metagraywacke of zone C the average yield is only 0.04 gpm per foot of well. Also, the average yield of springs in zone F is nine times as great as that of those in the mica schist and amphibolite of zone A and in the biotite schist of zone C. SELECTING A WELL SITE If the influences of topography, lithology, and the direction of ground-water movement were considered when selecting a well site, the cost would be less and the yield would be greater. Other factors influence the selection of a well site, but topography and lithology always should be considered. Table 13 summarizes the influences of topography on depth to the water table, necessity for blasting to reach saturated rock, need for well casing, yield of well, and the quality of water from dug wells. Although dug wells in 30 Table 13.-lnformation to be considered in selecting sites for dug wells --------------- Percent of wells inventoriedt Topographic location Average depth to water (feet) Blasting required Casing required Yield inadequate Water of poo,r quality - - - - - - - - - - -~--- -- - - - Hilltop 39 41 7 24 21 Slope 32 30 15 15 7 Valley 12 25 50 15 10 and draw 1Based on information reported by well owners. Table 14.-Topographic cont.' ol on the water table Depth to water below land surface (feet) Topo- Number graphic of below Location wells 0-10 11-20 21-30 31-40 41-50 51-60 60 (Percent of wells inventoried) Hilltop 95 0 5 10 30 33 15 7 Slope 65 0 17 27 32 14 8 2 Valley 24 55 33 4 8 0 0 0 and draw Table 15.-Topography versus blasting of aug wells Topographic location Number Number Number Average Range in of wells of wells of wells footage footage inventoried blasted blasted blasted blasted Hilltop 27 11 41 27 4-53 Slope 23 7 30 13 6-30 Valley 12 and draw 3 25 4 2-6 Table 16.-Topogmphie influences that should be considered in selecting the site for a drilled well -------------- ---- Topography Average yield per foot of well (gpm) Average well depth (feet) Average amourct of casing required (feet) for dynamiting to reach water. These two factors are analyzed more completely in tables 14 and 15. Note that the depth to water below the land surface in 55 percent of the hilltop wells is more than 40 feet, whereas in 55 percent of the valley and draw wells it is less than 10 feet. Also as much as 53 feet in hilltop wells needed t~ be dynamited, whereas the most reported for valley or draw wells was 6 feet. Table 16 summarizes the influences of topography on the construction and average yield of drilled wells. Drilled wells in valleys and draws have a much higher average yield per foot of wells, are shallower, and require less casing than those on either slopes or hilltops. Part of the influence of lithology is incorporated in the discussion under topography because, other things being equal, rocks most resistant to erosion compose the hills, and the rocks least resistant underlie valleys and draws. Yield of the various rocks is summarized in table 12 and discussed in the chapter on well yield. Dug wells in alluvium in Dawson County indicate that the alluvial deposits are very permeable and probably will yield large amounts of water to wells and infiltration galleries. Aquifer properties of the alluvium are discussed in the chapter on geology. Recharge and discharge areas affect the yield of wells and should be considered in selecting a well site. If a pumping well induces recharge from a nearby stream or lake, the yield will be greater than that of a well not so favorably located, other factors being equal. A well near a spring, another pumping well, or a road cut reaching below the water table usually has a lower yield than one more favorably located. The fact that ground-water movement in the area is principally parallel to the strike of the rocks is important in selecting a well site near another well. Wells several hundred feet apart along the strike could cause immediate interference and a decline in the yield of both wells, whereas those a short distance apart normal to the strike could be pumped for a long period of time without causing interference. The general strike in the southeastern part of Dawson County is northeast, and in the northwestern part of the county it is north. The yield per foot of drilled well decreases with well depth (table 17). Yield per foot aver- Hilltop 0.08 202 73 Table 17.-Yield per foot versus depth of well Slope .10 177 67 Depth of well below Yield (gpm) Valley and draw .15 164 62 land surface Number Average of depth Per fcot (feet) wells (feet) Range Average of well valleys and draws may require casing to prevent caving, they are shallower, require less blasting, have higher average yields, usually produce water of better quality, and usually cost less to construct than wells at other locations. Probably the most important topographic influences on the cost of dug wells are depth to water and the need 0- 99 100-199 200-299 300-399 400+ 31 6 76 3-43 18 0.24 28 148 3-30 13 .09 8 218 0-80 19 .09 3 300 6-14 10 .03 5 436 2-30 11 .02 aged 0.24 gpm between the land surface and 100 feet, 0.09 gpm to 200 feet, 0.09 gpm to 300 feet, 0.03 gpm to 400 feet, and 0.02 gpm below 400 feet. DEVELOPMENT About 33 percent of the wells were inventoried. Type of development was distributed as follows: 64 percent dug wells, 20 percent springs, 15 percent drilled wells, and 1 percent bored wells. The type of development varied with topography and prosperity. Figure 2 shows the concentration of spring utilization in the northern and western areas of high relief and a higher than average percentage of drilled wells near Lake Lanier. The percentage of new homes having drilled wells is considerably higher than the county average. Of the inventoried wells constructed between 1949 and 1959, 45 percent are drilled wells. Many owners and tenants prefer drilled wells, and the trend toward drilled wells will probably continue as long as prosperity permits. The water supply for the town of Dawsonvine comes from a spring which flows about 90 gpm. Only 30 to 50 percent of the flow is used. The ground-water resources of Dawson County have not been fully developed as shown by the large volume of ground water which annually flows from undeveloped springs. REFERENCES Adams, G. F., 1926, Geology of Alabama, crystalline rocks: Alabama Geol. Survey Spec. Report 14, map, p. 25-40. Bayley, W. S., 1928, Geology of the Tate quadrangle Georgia: Georgia Geol. Survey Bull. 43, 167 p. ' Cooke, C. W. Crickmay, G. W., Butts, Charles, Hayes, C. W., Keith, Arthur, McCallie, S. W., 1939, Geologic map of Georgia: Georgia Geol. Survey map. Crickmay, W. S., 1952, Geology of the crystalline rocks of Georgia: Georgia Geol. Survey Bull. 58, 52 p. Ellis, E. E., 1906, Occurrence of water in crystalline rocks: U. S. Geol. Survey Water-Supply Paper 160, p. 19-28. Fenneman, N. M., 1938, Physiography of eastern United States: New York, McGraw-Hill, 7 pl., 691 p. Furcron, A. S., and Teague, K. H., 1945, Mica-bearing pegmatites of Georgia: Georgia Geol. Survey Bull. 48, 191 p. Herrick, S. M., and LeGrand, H. E., 1949, Geology and ground-water resources of the Atlanta area, Georgia: Georgia Geol. Survey Bull. 55, 124 p. Hurst, V. J., 1955, Stratigraphy, structure, and mineral resources of the Mineral Bluff quadrangle, Georgia: Georgia Geol. Survey Bull. 63, 137 p. ---1956, Geologic map of Kennesaw Mountain-Sweat Mountain area, Cobb County, Georgia: Georgia Geol. Survey map. Keith, Arthur, 1903, Description of the Cranberry quadrangle (North Carolina-Tennessee): U. S. Geol. Survey Geol. Atlas, folio SO. ---1904, Description of the Asheville quadrangle (North Carolina-Tennessee): U. S. Geol. Survey Atlas folio 116. LaForge, Lawrence, 1925, Physical geography of Georgia: Georgia Geol. Survey Bull. 42, 165 p. LaForge, Lawrence, and Phalen, W. C., 1913, Description of the Ellijay quadrangle (Georgia): U. S. Geol. Survey Geol. Atlas, folio 187. LeGrand, H. E., and Mundorff, M. J., 1952, Geology and ground water in the Charlotte area, North Carolina: North Carolina Dept. Conserv. Devel., Div. Mineral Res. Bull. 63, 88 p. Meinzer, 0. E., 1923, Outline of ground-water hydrology with definitions: U. S. Geol. Survey Water-Supply Paper 494, 71 p. Mundorff, M. J., 1950, Flood-plain deposits of North Carolina Piedmont and' mountain streams as a possible source of ground-water supply, Preliminary report: North Carolina Dept. Conserv. Devel., Div. Mineral Res. Bull. no. 59, 20 p. Reiche, Parry, 1950, A survey of weathering processes and products: University of New Mexico Publ. in Geol., no. 3, 95 p. Stose, G. W., and Ljungstedt, 0. A., 1932, Geologic map of the United States: U. S. Geol. Survey map. U. S. Dept. of Agriculture, 1956, Dawson County farm statistics 1900-1955. U. S. Geological Survey, 1955, Surface Water Supply of the United States 1952: U. S. Geol. Survey WaterSupply Paper 1234, part 2-B. U. S. Geological Survey, 1960, Surface Water Supply of the United States 1959: U. S. Geol. Survey WaterSupply Paper 1624, part 2-B. U. S. Public Health Service, 1962, Drinking Water Standards: Public Health Service Pub. 956. 32 N L ME R I -I 1I \ 2 un f I \ \ ' () \ \ 0 c:. 2 \ L_ __ >- 1- z ::J 0 ) (j) z w z v'~~/ \ I I I 'I ( E X p L AN A T 0 N - Du- - - - - - Fault Long-dashed where approxi mate ly located; short-dashed where inferred ; dotted where concea l ed . U, up thrown side; D, downthrown si de Anticline Showing tra ce of axial plane Overturned ant icline Showing tra ce of axial plan e t Syncline Showing trace of axi a I plane Plunge of minor anticline Plunge of minor overturned anticline Plunge of m1nor synclin e Strike and dip of beds Str ike and dip of beds where the upper bed cannot be distinguished Strike and dip of ove rturned beds Appare nt dip Dot marks paint of observat io n Strike and d ip of beds and plunge of slicken sides Vv- General ized str ike and di p of undulatin g beds X..o Strike and dip of beds where the upper bed can be distinguished by graded bedding Bearing ond plunge of l ineat ion . Po int of observat ion at ba se of arrow Strike and d i p of fol i at i on and rake of l i neation Str ike and dip of beds and plunge of lineation Strike and dip of paral lel beds and fol iation Str i ke and d i p of paralle l beds and foliation whe re the upper beds cannot be d i stinguished Strike and dip of joint Strike and d ip of vertical joint 3$~10 '1' S4 oo oo" >- 1z - ::J 0 (.) w :>::: 0 a:: w I (.) Bose mop compiled from mop of Oowson County, GeorQiO Deportment ol Public Health, 1957. F 0 Rsy T H c 0 u NT y I 4 MILES 3 J s4"oo' oo" -.... Geolo gy by Cho rle s W. S ever, 1959 Figure 6-. Geologic structural map of Dawson County, Georgia.