GEOLOGY AND GROUND-WATER RESOURCES 

OF THE LAWRENCEVILLE AREA, GEORGIA

By Melinda J. Chapman, Thomas J. Crawford, and W. Todd Tharpe

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 98-4233

Prepared in cooperation with the

CITY OF LAWRENCEVILLE, GEORGIA

Atlanta, Georgia

1999
For additional information write to:

District Chief

U.S. Geological Survey

3039 Amwiler Road, Suite 130

Peachtree Business Center

Atlanta, GA 30360-2824

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Federal Center

Denver, CO 80225
Contents iii

CONTENTS

Abstract.......................................................................................................................................................................................1

Introduction ...............................................................................................................................................................................2

Background.......................................................................................................................................................................2

Purpose and scope ............................................................................................................................................................4

Description of the study area ............................................................................................................................................4

Methods of investigation ..................................................................................................................................................5

Previous studies ................................................................................................................................................................5

Well and surface-water station numbering systems..........................................................................................................7

Acknowledgments ............................................................................................................................................................7

Regional geology .......................................................................................................................................................................7

Geologic mapping—interpretations and limitations.........................................................................................................7

Lithologic units ................................................................................................................................................................8

Amphibolite ...........................................................................................................................................................8

Biotite gneiss .........................................................................................................................................................8

Button schist ...........................................................................................................................................................9

Granite gneiss ........................................................................................................................................................9

Magnetite quartzite.................................................................................................................................................9

Quartzite and aluminous schist...............................................................................................................................9

Diabase dikes........................................................................................................................................................10

Hydrogeologic sections .................................................................................................................................................10

Ground-water resources ...........................................................................................................................................................12

Well inventory and data compilation .............................................................................................................................12

City of Lawrenceville historical well records ......................................................................................................12

Well yields ............................................................................................................................................................12

Relation of well yield to lithology .................................................................................................................................13

Amphibolite .........................................................................................................................................................13

Biotite gneiss .......................................................................................................................................................16

Button schist ........................................................................................................................................................16

Granite gneiss .......................................................................................................................................................17

Quartzite and aluminous schist ............................................................................................................................17

Interpretation of borehole geophysical logs ..................................................................................................................17

Rhodes Jordan Wellfield.......................................................................................................................................20

Maltbie Street well................................................................................................................................................21

Pike Street well.....................................................................................................................................................21

Gwinnett County Airport well..............................................................................................................................28

Analyses of ground-water levels.....................................................................................................................................28

Response of local ground-water levels to pumpage .............................................................................................29

Areal effects of ground-water withdrawals .........................................................................................................34

Ground-water quality......................................................................................................................................................34

Summary and conclusions .......................................................................................................................................................45

References cited .......................................................................................................................................................................45
iv Contents

ILLUSTRATIONS

[Plates are in pocket in back of report]

Plate 1. Lithologic map showing the distribution of major rock types, fault contacts, and well locations 

in the Lawrenceville area, Georgia

2. Hydrogeologic sections showing general dip of lithologic units inferred from surface 

geologic mapping and subsurface lithologic and fracture data collected from wells 

in the Lawrenceville area, Georgia

Figure 1. Map showing location of (A) the study area in Gwinnett County and physiographic 

provinces in Georgia, and (B) the Rhodes Jordan Wellfield and observation wells 

in Lawrenceville, Georgia........................................................................................................................ 3

2. Subsurface lithologic characteristics and fractures penetrated by wells 14FF08, 14FF26, 

14FF27, and 14FF42.............................................................................................................................. 18

3. Fracture identification and relative comparison of fractures in Rhodes Jordan Wellfield 

well 14FF16 using borehole geophysical logs....................................................................................... 22

4. Fracture identification and relative comparison of fractures in Maltbie Street well 14FF08 

using borehole geophysical logs ............................................................................................................ 24

5. Fracture identification and relative comparison of fractures in Pike Street well 14FF27 

using borehole geophysical logs ............................................................................................................ 26

6. Fracture identification and relative comparison of fractures in Gwinnett County Airport 

well 14FF42 using borehole geophysical logs....................................................................................... 30

Figures 7-10. Hydrographs showing: 

 7. Continuous ground-water levels at the Rhodes Jordan Wellfield during 

June–September 1995 ............................................................................................................................ 32

 8. An extended 18-day pumping period and ground-water-level recovery at the Rhodes 

Jordan Wellfield during September-October 1996................................................................................. 33

 9. Influence of pumping from the bedrock aquifer on ground-water levels in the Rhodes Jordan 

Wellfield, October 1996, in (A) regolith well 14FF43, and (B) bedrock observation 

well 14FF16 ........................................................................................................................................... 35

10. Influence of pumping at the Rhodes Jordan Wellfield on ground-water levels in 

bedrock observation wells, May 1996–January 1997............................................................................ 36

Figure 11. Trilinear diagram of water-quality analyses of ground-water samples and City Lake sample 

collected during October–November 1995 ............................................................................................ 40

12. Trilinear diagram of water-quality analyses of ground-water samples and City Lake sample 

collected during August 1996 ............................................................................................................... 41
Contents v

TABLES

Table 1. Records of wells inventoried in the Lawrenceville area, Georgia..................................................................... 14 

2. Physical properties and concentrations of inorganic constituents in ground-water samples 

collected from the Rhodes Jordan Wellfield during October-November 1995 .......................................... 38

3. Physical properties and concentrations of inorganic constituents in ground-water samples collected 

from the Rhodes Jordan Wellfield and outlying bedrock observation wells during August 1996 ............ 39

4. Physical properties and concentrations of inorganic constituents in water samples collected from 

regolith wells and City Lake at the Rhodes Jordan Wellfield during August 1996 ................................... 42

5. Volatile organic compounds included in laboratory analyses .......................................................................... 43

6. Concentrations of volatile organic compounds detected in ground-water samples collected 

from the Rhodes Jordan Wellfield during October-November 1995 .......................................................... 44

7. Concentrations of volatile organic compounds detected in ground-water samples collected from the 

Rhodes Jordan Wellfield and outlying bedrock observation wells during August 1996 ........................... 44

CONVERSION FACTORS AND VERTICAL DATUM

Conversion Factors

Multiply by to obtain

Length

inch 0.254 centimeter

foot (ft) 0.3048 meter 

mile (mi) 1.609 kilometer 

Volumetric Rate

gallon per day (gal/d) 3.785 x 10-3 cubic meter per day 

million gallons per day (Mgal/d) 3.785 x 10-3 million cubic meters per day 

gallon per minute (gal/min) 6.309 x 10-5 cubic meter per second

 gallon per minute (gal/min) 2.228 x 10-3 cubic foot per second

Area

square mile (mi2) 2.59 square kilometer

acre (ac) 4,047 meter squared 

Concentration

microgram per liter (μg/L) 1 part per billion

Temperature

Temperature in degrees Fahrenheit (° F) can be converted to degrees Celsius (° C) as follows:

 ° C = (° F -32)/1.8 

Temperature in degrees Celsius (° C) can be converted to degrees Fahrenheit (° F) as follows:

 ° F = 1.8 (° C) +32

Vertical Datum

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic 

datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called 

Sea Level Datum of 1929. 
Abstract 1

GEOLOGY AND GROUND-WATER RESOURCES OF 

THE LAWRENCEVILLE AREA, GEORGIA

By Melinda J. Chapman, Thomas J. Crawford, and W. Todd Tharpe

ABSTRACT

The population of the Atlanta Metropolitan area continues 

to grow at a rapid pace and the demand for water supplies 

steadily increases. Exploration for ground-water resources, 

as a supplement to surface-water supplies, is being 

undertaken by many city and county governments. The 

application of effective investigative methods to 

characterization of the complex igneous and metamorphic 

fractured bedrock aquifers of the Piedmont physiographic 

province is essential to the success of these ground-water 

exploration programs. The U.S. Geological Survey, in 

cooperation with the City of Lawrenceville, Ga., began a 

study in December 1994 to apply various investigative 

techniques for field characterization of fractured 

crystalline-bedrock aquifers near Lawrenceville.

Five major lithologic units were mapped in the 

Lawrenceville, Ga., area as part of an ongoing study of 

ground-water resources—amphibolite, biotite gneiss, 

button schist, granite gneiss, and quartzite/aluminous 

schist. These units generally are thin in outcrop width, 

have low angles of dip (nearly 0 to 20 degrees, dip 

reversals occur over short distances), and exhibit some 

shearing characteristics. The most productive unit for 

ground-water resources, on the basis of subsurface data 

collected through 1997, is the amphibolite. Historically, 

two wells drilled into this unit are recognized as having 

possibly the highest yields in the Piedmont region of 

northern Georgia. The City of Lawrenceville refurbished 

one well at the Rhodes Jordan Wellfield in 1990, and has 

pumped this well at an average rate of about 230 gallons 

per minute since 1995. 

In general, the composition of water collected from the 

bedrock wells, regolith wells, and City Lake is similar; 

calcium and bicarbonate are the dominant cation and anion, 

respectively. Water from the regolith wells and the lake 

have lower concentrations of major ions than does water 

from the bedrock wells. Many of the ground-water samples 

collected from the Rhodes Jordan Wellfield during 

October–November 1995, and from the wellfield and three 

additional observation well sites during August 1996, 

contain volatile organic compounds. Volatile organic 

compounds were detected in ground-water samples 

collected from several bedrock and regolith wells located in 

urban areas. Trace concentrations of tetrachloroethylene, 

trichloroethylene, 1,1-dichloroethane, 

trichlorofluoromethane, 1,1,1-trichloroethane, and 

cis-1,2-dichloroethene were detected. Methyl-tert-butylether (MTBE)—a compound used to increase the octane 

level in gasoline—was detected at concentrations above 

expected urban background levels in bedrock wells in the 

Rhodes Jordan Wellfield. Concentrations of MTBE ranged 

from 0.6 to 12 micrograms per liter in October–November 

1995, and from 0.6 to 26 micrograms per liter in 

August 1996. 

Continuous ground-water-level data suggest that the 

fractured crystalline-bedrock aquifer (amphibolite unit) at 

the Rhodes Jordan Wellfield, generally is dewatered to a 

depth near a productive fracture during the regular 

pumping cycle of 18 hours per day, 5 days on and 2 days 

off per week. However, when the stress on the aquifer is 

increased by extending the pumping period up to as much 

as 18 days, or by pumping longer that 18 hours per day, the 

aquifer exhibits an unusual condition of recovery. Areal 

effects of pumping have been observed at distances of as 

much as one mile, extending across surface-water 

drainage divides.
2 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

INTRODUCTION

Availability of water for public supply, industrial use, and 

landscape irrigation is a major factor in the economy and 

quality of life in large urban areas of the southeastern 

United States. Water resources in fractured crystallinebedrock aquifers are becoming more important in the 

Atlanta Metropolitan area of northern Georgia as the need 

for water supplies escalates because of increased 

development and rapid population growth. The population 

increased from about 1.684 million in 1970, to nearly 2.834 

million in 1990; an increase of more than 68 percent (U.S. 

Bureau of the Census, 1991). The water supply for the 

metropolitan area primarily is derived from surface water. 

Most of the population in the urban area depends on water 

from the Chattahoochee River for drinking water supply; 

however, the actual percentage of water used from this 

source largely is unknown (Marella and others, 1993). The 

largest impoundment is Lake Sidney Lanier, located 

upstream and northeast of the Atlanta Metropolitan area. 

Surface-water supplies require substantial storage capacity 

in impoundments to meet minimum supply requirements 

during periods of peak demand. Impoundments are 

extremely expensive to develop and are susceptible to 

drought. Ground-water resources have the reputation of 

being difficult to obtain in the Piedmont region because of 

the complex hydrogeology of the fractured igneous and 

metamorphic rocks. Although the quantity of ground water 

available from a well drilled in these fractured crystalline 

rocks may not be comparable to that of a reservoir, 

generally a few 100,000 gallons per day (gal/d), wells can 

serve as a primary resource in rural communities, and as 

supplemental resources in many suburban communities.

The City of Lawrenceville, Ga., a northeastern suburb of 

Atlanta (fig. 1), includes several of the fastest growing 

industrial and residential areas in the metropolitan area. The 

average demand for water ranges from a low of about 1.5 

million gallons per day (Mgal/d) in the winter, to a high of 

about 2.9 Mgal/d in the summer (E&C Consulting 

Engineers, Inc., 1995). A small percentage of the city’s 

public water supply—about 10 percent—currently (1998) is 

obtained from ground-water sources, and projects have been 

initiated to expand ground-water development. The city is 

now investigating the possibility of using ground-water 

supplies to provide a primary water source to serve an 

estimated population of about 20,000 (Mr. Mike Bowie, 

City of Lawrenceville, oral commun., 1998). Well yields in 

this area, as well as in all igneous/metamorphic-rock 

terranes, are extremely variable and can range from zero to 

several hundred gallons per minute (gal/min). Thus, there is 

imminent need to improve methods of ground-water 

resource evaluation in these complex igneous/metamorphicrock hydrogeologic settings. 

The fractured crystalline-bedrock aquifers in this area of the 

Georgia Piedmont are comprised of metamorphic rocks, 

including schists, gneisses, and amphibolites, as well as 

igneous granitic rocks. These rocks have been exposed to 

intense heat and pressure during metamorphism, as well as 

structural deformation, and subsequent chemical and 

physical weathering. These forces have resulted in an 

extremely heterogeneous aquifer system characterized by a 

broad range of physical properties. Characterization of these 

fractured crystalline-bedrock aquifers often is difficult at 

any scale of investigation. Predictions of the availability of 

ground water in the Piedmont physiographic province are 

difficult because of the lack of detailed knowledge of the 

geology and its relation to the storage and flow of ground 

water. Conceptual advances have been made in relating the 

occurrence of ground water to geologic structures, rock 

type, and topographic setting; and the understanding and 

prediction of local hydrogeologic conditions has improved. 

Background

The fractured crystalline-bedrock aquifers of the 

Lawrenceville area have proved to be a dependable source 

of ground water for more than 80 years. Four wells were 

drilled and used for municipal supply in Lawrenceville prior 

to initiation of the city’s purchase of surface water from 

Gwinnett County in the mid-1970’s. The first production 

well was drilled in 1912 by cable-tool methods at the 

Rhodes Jordan Park (fig.1). Original records indicate that 

the well had an estimated yield of as much as 400 gal/min. 

This well was used for municipal supply for several 

decades. The city reportedly pumped 200,000 gal/d from 

the well in 1942 (USGS site visit January 12, 1943, 

written commun.). 

Three additional wells (fig. 1) were drilled in the 1940’s. 

An additional production well was drilled in 1945 near the 

1912 well (14FF10) at Rhodes Jordan Park, in what has 

become known as the Rhodes Jordan Wellfield. A second 

well was drilled in 1947 at the Maltbie Street site. A third 

well was drilled at the Pike Street site in the 1940’s (Mr. 

Don Martin, City of Lawrenceville, oral commun., 1996). 

This historical well was located about 200 ft east of current 

well 14FF27. Drilling records are not available for the 

historical Pike Street well. The wells at the Rhodes Jordan 

Park and Maltbie Street site had high sustained yields. The 

Pike Street well yield was considered unreliable (Mr. Don 

Martin, City of Lawrenceville, oral commun., 1996). 

Subsequent to the shutdown of the wells in the 1970’s, the 

city refurbished the two wells at the Rhodes Jordan Park in 

1990 (Special Environmental Services, 1991). The Maltbie 

Street well was revisited in 1995. The Pike Street well was 

apparently paved over during construction activities for an
Introduction 3

Chattahoochee River

Chattahoochee River

Fall Line

GEORGIA

COASTAL PLAIN

CUMBERLAND 

PLATEAU

BLUE RIDGE

PIEDMONT

Atlanta

VALLEY 

AND 

RIDGE

Lawrenceville

Lawrenceville

Dacula

Gwinnett

County

STUDY AREA

0 10 MILES

0 10 KILOMETERS

02208118

City Lake

N Yellow River basin

14FF43

US Highway 29

RHODES JORDAN

WELLFIELD

Shoal Creek

14FF08 

Maltbie

Street Site) 1060

1040

1040

1020

1020

1020

1020

1040

1060

1000

980

1000

1000

1040

1060

1080

1060

1040

1040

1020

1020

1100

1080

1060

1040

1100

1120

DRAINAGE-BASIN

BOUNDARY

14FF17

14FF36

14FF10

14FF18

14FF16

14FF26 14FF37

14FF38 14FF39

14FF27

14FF27

14FF09 (Pike Street Site)

Figure 1. Location of (A) the study area in Gwinnett County and physiographic provinces in Georgia, and 

(B) the Rhodes Jordan Wellfield and observation wells in Lawrenceville, Georgia.

Gwinnett County Airport

Well 14FF42

2 miles

A.

B.

Alcovy River basin

02208118

Base modified from U.S. Geological Survey

Lawrenceville, Georgia; 1:24,000, 1992 

Well and 

 number

Stage recorder 

 and number

0 125 250 METERS

EXPLANATION

0 500 1,000 FEET
4 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

office park development; a new test well was drilled near 

the original well location in 1995. However, aquifer-test 

data from the 1995 Pike Street well suggest that the well 

does not yield enough ground water for development into a 

production well. 

In December 1994, the U.S. Geological Survey (USGS) 

began a study in cooperation with the City of Lawrenceville, 

to determine hydrogeologic characteristics of fractured 

crystalline-bedrock aquifers (fig. 1). The objectives of the 

overall Lawrenceville study are to (1) evaluate the regional 

hydrogeologic setting of the Lawrenceville, Ga., study area 

by correlating subsurface and surface lithology; (2) 

delineate and characterize subsurface secondary fractures 

that control aquifer permeability; and (3) monitor the 

response of the bedrock ground-water system to pumpage 

by characterizing areal influence of ground-water pumping 

from the fractured bedrock aquifer system. Two scales of 

investigation are being conducted as part of the 

Lawrenceville study—regional and local.

Purpose and Scope

This report addresses results of the regional scale of an 

ongoing study that has focused on the identification of 

major lithologic units at the land surface, applications of 

borehole geophysical logs to lithologic identification and 

fracture characterization in the subsurface, and the response 

of the aquifer system to stress from pumping. Regional 

findings in the study area are presented to (1) describe 

lithologic units and the geologic setting; (2) characterize the 

hydrogeologic setting where subsurface data are available; 

and (3) evaluate ground-water availability and quality. An 

overall assessment of the availability of ground water is 

discussed in relation to geologic factors within each 

lithologic unit, such as characteristic fracturing of specific 

rock types. Applications of borehole geophysical logs 

include fracture delineation and characterization, as well as 

lithologic identification. Ground-water levels were 

monitored to evaluate the response of the aquifer 

to pumping. 

Field work conducted as part of the regional study has 

included geologic mapping, the compilation of wellinventory data, collection of borehole geophysical logs, 

ground-water-level monitoring, collection of geologic core, 

well drilling, and ground-water-quality sampling. Regional 

surface lithologic units are described as a basis of 

correlation of current and future subsurface data. The 

surface expression of geologic contacts of lithologic units 

are presented at the 1:24,000 scale. From data compiled 

through 1997, subsurface lithologic and fracture data are 

available for nine bedrock wells at four sites—the Rhodes 

Jordan Wellfield (two production wells (one in use) and four 

observation wells), the Maltbie Street well, the Pike Street 

well, and the Gwinnett County Airport well. The use of 

borehole geophysical logs in fracture delineation and 

qualitative comparison is presented and discussed for each 

of the four sites. 

Description of the Study Area

The study area encompasses about 44 square miles (mi2) 

and includes the City of Lawrenceville in Gwinnett County, 

Ga., and is located about 20 mi northeast of Atlanta (fig. 1). 

The northeastern quarter of the Luxomni and the northern 

half of the Lawrenceville, 7 1/2-minute topographic 

quadrangles (plate 1) are included in the study area. 

Lawrenceville is located in the Piedmont physiographic 

province, Winder Slope District of the Southern Piedmont 

Section (Clark and Zisa, 1976). Topography is gently 

rolling, with altitudes ranging from about 780 feet (ft) to 

1,170 ft above mean sea level. The Winder Slope District is 

dissected by the headwater tributaries of major streams 

draining to the Atlantic Ocean (Clark and Zisa, 1976). The 

study area contains part of the headwaters for the Yellow 

River (west) and the Alcovy River (east). This area was 

selected because the geologic and hydrogeologic settings are 

representative of the fractured, crystalline metamorphic and 

igneous rocks of the Piedmont and Blue Ridge 

physiographic provinces in Georgia, and includes a rapidly 

growing urban area in need of increased water supply. 

The climate of the study area generally is mild—having 

warm, humid summers, and temperate winters. The 1996 

average annual temperature for the National Oceanic and 

Atmospheric Administration (NOAA) station at Winder, 

Ga., (station WINDER 1 SSE), located about 16 mi from 

Lawrenceville, was about 60 degrees Fahrenheit. Annual 

precipitation at Winder was about 52 inches during 1996 

(National Oceanic and Atmospheric Administration, 1996). 

Four well sites in the study area have been characterized at a 

local scale to better assess specific geologic controls on 

ground-water availability. The Rhodes Jordan Wellfield 

(RJWF, fig. 1) includes two bedrock production wells, four 

bedrock observation wells, and three regolith wells (1997). 

The wellfield encompasses an area of about 3 acres. A 

25-acre recreational lake is near the wellfield in the Rhodes 

Jordan Park. Three additional single-well sites—Maltbie 

Street (about 0.9 mi N80W from RJWF), Pike Street (about 

1.0 mi S80W from RJWF), and the Gwinnett County 

Airport well (about 1.8 mi N55E)—were studied as part of 

the regional investigation (fig. 1). 
Introduction 5

Methods of Investigation

The investigation of ground-water resources in the 

Lawrenceville area included geologic mapping; compilation 

of well information; and collection and analysis of borehole 

geophysical logs, well cores, ground-water-level data, and 

ground-water-quality samples. All data (geology, well 

characteristics, topography, hydrography, and roads) were 

digitized and incorporated into a spatially oriented 

database for geographic information system applications. 

All well-construction information is stored in the USGS 

Ground-Water Site Inventory (GWSI) database for 

subsequent retrieval. 

Geologic mapping included the identification of regional 

lithologies and structural measurements, as well as the 

notation of textural characteristics and degree of 

development of foliation/compositional layering. The 

distribution of rock types and lithologic contacts were 

mapped in detail across the study area. General observations 

regarding potential structural and textural characteristics 

that may enhance ground-water availability also were noted 

in selected areas. 

Compilation and assimilation and compilation of well data 

included construction and yield data obtained from the 

USGS GWSI database. Additional well information was 

compiled during field reconnaissance. Local water-well 

drillers, consultants, and well owners provided additional 

information. Well-inventory information included well 

depth, casing depth, construction, and yield, when available. 

Well-yield data were related to lithologic, structural, and 

weathering characteristics of geologic units. Water-bearing 

potential of geologic units was evaluated by correlating rock 

type and structural features with characteristics of 

production zones in wells. 

Borehole geophysical logs were used to determine 

characteristics of water-yielding zones. Fractures 

intersecting the borehole in bedrock wells were identified 

using an integrated suite of borehole geophysical logs—

including caliper, focused resistivity, long- and shortnormal resistivity, gamma, acoustic televiewer and velocity, 

spontaneous potential, fluid temperature and resistivity, 

deviation, heat-pulse flowmeter, video camera logs, and 

single-borehole radar surveys. As part of the local studies, 

subsequent to fracture zone identification, the orientation of 

fractures within each zone was interpreted from acoustic 

televiewer logs and directional borehole radar surveys 

(Chapman and Lane, 1996). 

Drill core was collected from boreholes at the Rhodes 

Jordan Wellfield (well 14FF26) to a depth of about 380 ft 

and the Gwinnett County Airport (well 14FF42) to a depth 

of about 600 ft. The coreholes remain open and currently 

(1998) are used as observation wells. A second observation 

well (14FF39) was later drilled, using air-rotary methods, 

near well 14FF26 to assess yield. It was completed to a 

depth of about 180 ft. 

Ground-water-quality samples were collected from eight 

bedrock wells, and three regolith wells to characterize 

general ground-water quality (major ionic constituents) and 

to evaluate possible volatile organic compounds contamination of ground water from urban activities. A sample also 

was collected from City Lake for a comparison of surfacewater chemistry with chemical data from ground-water 

samples. Two sets of samples were collected—(1) October–

November 1995, during a pumping shutdown; and (2) 

August 1996, during a period of sustained pumping. Water 

samples from the bedrock wells were collected using a 

submersible Redi-Flow 2 Grundfos pump. The sampling 

pump was first lowered below casing depth to remove 

stagnant casing water. Then, the pump was lowered to a 

fracture depth for sample collection. Water from bedrock 

wells was pumped until all field parameters (specific 

conductance, pH, temperature, and dissolved oxygen) 

stabilized. Regolith wells first were purged using a bailer, 

and then sampled by attaching a bottom-emptying device to 

the bailer.

Previous Studies

A complete presentation of hydrogeologic investigations in 

the Piedmont physiographic province of northern Georgia is 

beyond the scope of this report. However, Chapman and 

Peck (1997) listed and discussed several studies that have 

evaluated ground-water resources in the fractured crystalline-bedrock aquifers of the Atlanta Metropolitan area. 

Some of the more recent concepts of factors affecting 

ground-water availability in the fractured crystalline rocks 

of the Atlanta Metropolitan area were presented by Cressler 

and others (1983). The availability, quality, and quantity of 

ground water in fractured crystalline rocks and methods 

used for locating well sites were discussed. Cressler and 

others (1983) stated that wells could serve as alternative or 

supplemental sources of water supply. Results from that 

study indicated the highest well yields in the Atlanta area 

are associated with wells tapping contact zones between 

rocks of contrasting lithology, fault zones, stress-relief 

(horizontal) fractures, drainage features controlled by local 

structural characteristics, concentrated jointing within 

folded rocks, and shear zones. Topographic drainage 

features may or may not be related to underlying waterbearing features in the rocks. From data gathered using 

borehole geophysical logs of wells, Cressler and others 
6 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

(1983) determined that the size, spacing, and 

interconnection of water-bearing openings differ greatly 

from one rock type to another. Cressler and others (1983) 

determined that the range in well yield within an identified 

water-bearing unit is highly variable, and high-yielding 

wells are present in each unit. Local features in the rocks 

were recognized as generally controlling well yield. They 

also noted that water from wells open to mafic rock types 

contained higher concentrations of iron, magnesium, 

manganese, dissolved solids, and possibly chloride, than 

water from wells open to granitic rock in the Atlanta 

Metropolitan area. The pH of water samples collected from 

wells completed in mafic rocks also was relatively high 

compared with samples collected from wells completed in 

granitic rocks. 

Previous regional geologic mapping, which included the 

Lawrenceville and Luxomni quadrangles, was compiled by 

McConnell and Abrams (1984). The data sources for the 

Luxomni quadrangle were Atkins and Higgins (1980) and a 

modification of the 1:24,000-scale reconnaissance map by 

Atkins and Morris (1982). The Lawrenceville quadrangle 

was modified from the 1:24,000-scale reconnaissance map 

by Dooley (unpublished). 

Higgins and others (1988) published a study of the structure, 

stratigraphy, tectonostratigraphy, and evolution of the 

southernmost part of the Appalachian orogen. Major 

lithologic units in the vicinity of the Lawrenceville and 

Luxomni topographic 1:24,000 quadrangles were defined, 

named, and assigned to proposed thrust sheets; primarily the 

Zebulon thrust sheet and the Sandy Springs thrust sheet. 

Both of those thrust sheets are considered to be closely 

associated with a large outcrop area of Silurian and 

Devonian granites. Concepts and interpretations presented 

by Higgins and others (1988) are being revised and 

reinterpreted through more detailed mapping, such as that 

presented in this report. 

As part of the refurbishment of the Rhodes Jordan 

production wells in the early 1990’s (wells 14FF10 and 

14FF16, fig. 1), a hydrogeologic investigation was 

conducted by Special Environmental Services, Inc., (1991) 

and Radzieta (1993). The investigation was conducted in the 

vicinity of the Rhodes Jordan Wellfield and consisted of 

limited geologic structural mapping, well rehabilitation, and 

observation well drilling, a watershed analysis, assessment 

of potential sources of contamination, and aquifer testing. 

Four observation wells were drilled near the production 

wells, two tapping the deeper bedrock aquifer and two 

shallow wells tapping the regolith. One bedrock well was 

located along the principal geologic strike (N60E) from the 

production well, and the second was located perpendicular 

to strike (N30W). Both of the bedrock wells tap biotitehornblende gneisses and amphibolites, and have estimated 

yields of 150 gal/min along strike, and 100 gal/min 

perpendicular to strike. Using estimates of recharge to the 

aquifer through precipitation and infiltration applied to the 

watershed, the maximum yield available to a production 

well was estimated to be about 310 gal/min. The sustainable 

yield was estimated to be 200 to 250 gal/min. 

In 1995, E&C Consulting Engineers, Inc., presented a plan 

for the development of ground-water resources in the 

Lawrenceville area. Phase I of their investigation included 

property as far as 1 mi outside the city limits, and consisted 

of the selection of favorable zones suitable for the development of ground-water resources. Zones were selected on the 

basis of interpretations from lineament analyses, geologic 

mapping, identification of soil types, characterization of 

watershed morphology, topography, areas of potential 

ground-water recharge, and identification of potential 

contaminant sources to the ground-water system. Phase II is 

planned to include the selection of test-well drilling sites 

using surface geophysical surveys. Six geologic units were 

identified: quartzite/schist, button schist, biotite gneiss/

hornblende gneiss/amphibolite, schistose gneiss, hornblende 

gneiss/amphibolite, and the Lithonia Gneiss (E&C 

Consulting Engineers, Inc., written commun., 1995). 

Evidence of thrust faulting, including abrupt terminations of 

rock units, shear fabrics, and sheared limbs of folds near 

stratigraphic boundaries, were noted. Foliation/

compositional layering was noted as having orientations 

northeast to north-south, and dipping less than 30 degrees 

(E&C Consulting Engineers, Inc., written commun., 1995). 

The characterization of subsurface fractures at the Rhodes 

Jordan Wellfield was discussed in Chapman and others 

(1997) and in Chapman and Lane (1996). Fractures were 

delineated in four bedrock wells using borehole geophysical 

logs, in correlation with drilling and geologic logs where 

available. The most useful traditional geophysical logs were 

the caliper, focused resistivity, and acoustic televiewer. 

Additional comparison of fractures was made using heatpulse flowmeter logs to determine vertical flow within the 

boreholes. More recent technology used to characterize 

subsurface fractures included directional borehole radar 

surveys, which propagate high-frequency electromagnetic 

signals into the rock (amphibolite) at distances of more than 

100 ft from the well. Orientations (strike and dip) of 

fractures intersecting the wells were determined from 

acoustic televiewer logs and directional borehole radar 

surveys. These fracture orientations were then compared 

with interpretations of fracture strike from surface 

azimuthal resistivity surveys and surface geologic 

structural mapping. 
Regional geology 7

In general, several productive (transmissive) fractures strike 

in the N10-60W direction. An additional dominant fracture 

set has strikes ranging from N80-90E and N80-90W. 

Secondary, or less transmissive fractures have strikes of N-S 

to N20E and N50-70E.

Well and Surface-Water Station 

Numbering Systems

Wells in Georgia are numbered using a system based on 

USGS topographic maps. Each 7 1/2-minute topographic 

quadrangle map in Georgia has been assigned a number and 

letter designation beginning at the southwest corner of the 

State. Numbers increase sequentially eastward through 39—

letters advance northward through “Z,” then double-letter 

designations “AA” through “PP” are used. The letters “I,” 

“O,” “II,” and “OO” are not used. Wells and springs 

inventoried in each quadrangle are numbered sequentially 

beginning with “1.” Thus, the second well inventoried in 

the Lawrenceville quadrangle (designated 14FF) is 

designated 14FF02.

Wells in the USGS GWSI database are assigned a 15-digit 

identification number based on the latitude and longitude 

grid system. The first six digits denote the degrees, 

minutes, and seconds of latitude. The next seven digits the 

degrees, minutes, and seconds of longitude. The last two 

digits (assigned sequentially) identify wells within a onesecond grid. 

The USGS established a standard identification numbering 

system for all surface-water stations in 1950. Stations are 

numbered according to downstream order. Stations on a 

tributary entering upstream of a main-stream station are 

numbered before, and listed before that station. No 

distinction is made between continuous-record and partialrecord stations. Each station has a unique number that 

includes a two-digit Part number (02 refers to natural 

drainage into the Eastern Gulf of Mexico) and 6 to 12-digit 

downstream order number within the part number. Gaps are 

left in the series of numbers to allow for new stations that 

may be established; hence, the numbers are not consecutive. 

All records for a drainage basin, encompassing more than 

one state, can easily be correlated by Part number and 

arranged in downstream order. 

Acknowledgments

The authors would like to thank Mr. Bartow Jenkins, Mayor, 

City of Lawrenceville; and Mr. Don Martin, Director of 

Public Works, City of Lawrenceville, for their support in 

initiating and continuing this project. Special thanks go to 

Mr. Roy Wilson, City of Lawrenceville Water Department 

Superintendent; and Mr. Mike Bowie, City of 

Lawrenceville, Water Treatment Plant Operator, for their 

enthusiastic support of field work conducted during this 

project. Appreciation also is extended to Mr. Juan Ruiz, 

E&C Consulting Engineers, Inc., for his support and 

cooperation throughout this study. Many thanks go to Mr. 

Andrew Clark, Kemron Environmental Services, Inc., for 

his cooperation in allowing the monitoring of ground-water 

levels in wells installed by Kemron near the Rhodes Jordan 

Wellfield. Appreciation also is extended to Mr. Mark 

Hubner, Middle Georgia Water Systems, for his support and 

exchange of information.

REGIONAL GEOLOGY

The Piedmont physiographic province of northern Georgia 

is composed of metamorphic and igneous rocks extending to 

an unknown depth below land surface. Ground-water 

exploration generally is conducted at depths of less than 

1,000 ft, and in most cases, to a maximum depth of about 

600 ft. In addition to the exposure of extreme heat and 

pressure from metamorphism, these rocks have been 

extensively folded and faulted. Primary porosity is virtually 

nonexistent. The igneous intrusions, although coarsergrained, also most often have extremely low primary 

porosity. Ground water in the fractured crystalline bedrock 

is stored in secondary porosity fractures. The term 

“fractures” includes openings along foliation planes, joints, 

stress-relief (erosional unloading) fractures, and brittle 

fractures related to faulting.

Geologic Mapping—Interpretations 

and Limitations

Geologic mapping in the study area included identifying 

rock types, grouping of rock types into lithologic units, 

delineating lithologic unit contacts, and selected structural 

measurements. Mapping was conducted using 1:24,000-

scale topographic base maps. The study area includes parts 

of two 7 1/2-minute quadrangles in the Piedmont 

physiographic province of Georgia (fig. 1); the 

Lawrenceville and the Luxomni quadrangles. The geologic 

map presented in plate 1 of this report is a lithologic map; 

no stratigraphic order is given or implied, and no ages of 

rock units are given. 

Geologic mapping of parts of the Lawrenceville and 

Luxomni quadrangles was conducted intermittently during 

1991–95. Reconnaissance mapping was conducted in the 

Lawrenceville quadrangle during 1991–92 as part of the 

Athens, Ga., 30-minute x 60-minute project of the USGS 

National Geologic Mapping Program. The northern part of 

the Lawrenceville quadrangle and the northeastern part of 
8 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

the Luxomni quadrangle were mapped during December 

1994 to July 1995. The lithologic map of the Lawrenceville 

quadrangle is more accurate than the lithologic map of the 

Luxomni quadrangle because, by 1995, when part of the 

Luxomni quadrangle was mapped, the area was essentially 

suburban and exposures were extremely scarce. Therefore, 

some units were distinguished almost entirely on the basis 

of associated soil colors. For example, the biotite gneiss 

(bg) generally weathers to an orange-colored soil, whereas 

the granite gneiss (gg) generally weathers to a tan or yellowcolored soil. In residential subdivisions, which cover more 

than 75 percent of the area in the Luxomni quadrangle, 

several factors had to be considered in separating the biotite 

gneiss (bg) from the granite gneiss (gg): (1) differences in 

color of soil resulting from differences in mineral 

composition of the units; (2) variations in soil color 

because of different degrees of weathering; (3) movement of 

soils during subdivision construction; (4) movement of soils 

during landscaping. Therefore, these units and their contacts 

are less constrained in such residential areas than in those 

areas where surburbanization is less encompassing, as in 

most of the Lawrenceville quadrangle. In areas having 

landscaped residential subdivisions, more reliable 

mapping data commonly are restricted to small ungrassed 

spots, some stretches of small creeks, and sparse outcrops 

and roadcuts. 

Geologic mapping in the southeastern United States, an area 

of deep weathering and abundant vegetation, requires that 

all lithologic contacts be “projected.” Contacts are 

occasionally observed at precise points, along traverses, or 

while attempting to “walk the contact.” More often, contacts 

can be interpreted within a few feet or a few 10’s of feet—

based on many and varied criteria; thus, they are 

“projected.” In some places, such as on broad floodplains or 

colluvium-covered slopes, projection accuracy may be off 

by a few 100’s of feet. 

Other difficulties in mapping and interpreting the geology of 

the study area are a result of the low-angle dips of most 

structural features and of most map units. Dips of less than 

10 degrees are very difficult to measure; foliation/

schistosity, compositional layering/bedding, axial planes, 

and linear features typically exhibit dips of less than 10 

degrees. The low-angle dips of mappable rock units result in 

contacts that “V” upstream. The lithologic patterns shown in 

plate 1 reflect those patterns that result from folding.

Lithologic Units

The lithologic map of the study area (plate 1) shows seven 

mappable units; five of these seven units are shown on 

section figures (plate 2). The units shown are, in no 

stratigraphic or structural order: (1) amphibolite (a); (2) 

biotite gneiss (bg); (3) button schist (bs); (4) granite gneiss 

(gg); (5) magnetite quartzite (mq); (6) quartzite and 

aluminous schist (qs); and (7) diabase dikes (d). The 

magnetite quartzite unit and diabase dikes are considered 

minor units. 

A major revision of the stratigraphic nomenclature in the 

Atlanta, Athens, and Cartersville 30-minute by 60-minute 

topographic quadrangles is currently (1998) underway 

(Thomas J. Crawford, State University of West Georgia, 

written commun., 1998). To correlate the lithologic units of 

this report with published named units would be misleading 

to the reader.

Amphibolite

The amphibolite unit (shown as “a” on plates 1 and 2) 

consists of a fine- to medium-grained, dark-green to 

greenish-black, massive to thinly laminated, hornblendeplagioclase and plagioclase-hornblende amphibolite, and 

hornblende gneiss. These rocks generally contain epidote 

and garnet, and also may be locally chloritic. Thinly 

laminated, medium-grained, magnetite quartzite (mq) units 

about 1 to 20 ft thick, are common in and characteristic of, 

the amphibolite unit. This unit contains insignificant 

amounts (generally less than one percent) of fine- to 

medium-grained, generally amphibole-bearing granofels. 

These rocks, alter to an ocher color when partially 

weathered. The final weathering product of the amphibolite 

unit is a very characteristic dark-red clayey soil.

In the Lawrenceville area, highly productive yields (greater 

than 200 gal/min sustained yield) are obtainable from wells 

tapping the amphibolite unit. This unit typically is well 

jointed, but most joints appear to be tight at land surface. 

Differential weathering can occur parallel to the thin, but 

well-developed, compositional layering. Productive fracture 

zones appear to be the result of differential weathering at the 

intersection of low-angle foliation (compositional layering) 

and steeply-dipping joint planes. 

Biotite Gneiss

The biotite gneiss unit (shown as “bg” on plates 1 and 2) 

consists of a gray to grayish-brown to dark-gray, mediumto coarse-grained, biotite-rich gneiss. This biotite gneiss 

typically is schistose in texture and often is pegmatitic. 

Regionally, the unit generally contains rare lenses and pods 

of hornblende-plagioclase amphibolite; locally, however, 

these lenses may be fairly common. Also, the biotite gneiss 

locally contains small red garnets. The biotite gneiss unit 

characteristically contains small pods and lenses of altered 

ultramafic rocks; typically, soapstones and serpentinites; 
Regional geology 9

originally, these inclusions probably were pyroxenites, 

dunites, and peridotites. Some of the ultramafic rocks have 

original crystal textures indicative of pyroxenites, but the 

crystals of pyroxene have been completely serpentinized 

and/or uralitized, and many rocks have been altered to 

soapstones. The biotite gneiss unit is intensely deformed, as 

can be observed in most outcrops. Outcrops are rare, even 

on steep hillsides of tributary streams, because the rock 

weathers deeply. However, unweathered rock may be 

exposed along large streams. The biotite gneiss unit 

characteristically weathers to a uniform, slightly 

micaceous, dark-red saprolite and clayey dark-red soil. 

Vermiculitic mica also is characteristic of soils formed in 

the unit. Outcrops are rare, even on steep hillsides of 

tributary streams. 

The biotite gneiss unit generally is considered to be a 

potentially productive water-bearing unit. The waterbearing characteristics of the unit are directly related to 

lithologic composition and differential weathering of the 

constituent lithologies. The amphibolite layers common 

in the biotite gneiss are bounded by intraformational 

contacts that weather differently than the surrounding 

gneiss. Within the biotite gneiss, compositional layering 

results in differential weathering that enhances permeability.

The generally high potassium-feldspar content results in 

deep weathering with subsequent potential ground-waterstorage capability. 

Button Schist

The button schist unit (shown as “bs” on plates 1 and 2) is a 

dark-gray to brownish-gray garnet schist with lesser 

amounts of interlayered biotite gneiss and scarce amphibolite. The schist and the associated biotite gneiss typically 

have a sheared texture. The schist locally contains scattered 

kyanite or staurolite, indicating that retrogressive metamorphism probably did not accompany the shearing. In the 

Luxomni quadrangle, feldspathic quartzites and 

manganiferous-schist zones are present within the button 

schist unit. The button schist unit is named for its weathering characteristics that resemble micaceous “buttons.” 

The button schist unit generally is resistant to weathering, 

and is considered to have relatively low production 

potential as a water-bearing unit. Despite the sheared 

texture, the button schist is poorly jointed and has tightlyspaced folia-tion planes. This unit does not outcrop as large 

areas, such as the amphibolite, biotite gneiss, and granite 

gneiss units.

Granite Gneiss

The granite gneiss unit (shown as “gg” on plates 1 and 2) is 

a complex of granite and granite gneiss. The lithology 

typically is a light gray to whitish-gray, medium-grained, 

micaceous gneiss, having well-defined gneissic layering; 

and typically is (but not everywhere), contorted (generally 

on a scale of 0.1 to 2 inches). Garnet segregations in lenses 

as large as 7 ft by 7 ft locally, are present. The most 

common rock mapped is the granite gneiss unit, which is a 

poorly foliated metagranite. Concentrations of biotite may 

foliate the rock. Because of the small content of biotite, in 

most exposures this foliation is recognizable and 

measurable only where the rock is slightly weathered. 

Pavement outcrops, where a large surface area of rock is 

exposed in flat-lying topographic areas, are characteristic of 

this unit, but are rare in some areas. Where deeply 

weathered, the unit forms light-whitish-yellow, sandy soils. 

Scattered xenoliths are mainly amphibolite. This rock is 

extensively quarried in some areas for aggregate, curbstone, 

and monument stone.

Generally, the granite gneiss is considered a poor waterbearing unit because of its massive character, uniform 

weathering, and general lack of structural features or 

discontinuities needed to create open fractures. Where the 

granite gneiss is unsheared and poorly foliated, it typically 

is poorly jointed, and where present, the joints generally 

are tight. 

Magnetite Quartzite

The magnetite quartzite (shown as “mq” on plate 1) is 

composed of finely laminated (less than 0.4 inch), mediumgrained, magnetite quartzite. This unit occurs in layers 

about 1 to 20 ft thick, and is scattered within the 

amphibolite unit (a). The magnetite quartzite typically has 

thin (about 0.4–1.6 inches) quartz-magnetite layers, with 

subhedral (typical) to euhedral (only locally) magnetite 

crystals as much as 0.4 inches in size; but commonly about 

0.04 inches. The quartz-magnetite layers alternate with 

about 10–20 inches thick quartz layers that may contain a 

small percentage of magnetite. The magnetite crystals are 

shiny where the rock has been recently broken, but 

generally are oxidized to gray or black in color. Magnetite 

clumps that generally disrupt the layering are locally as 

large as 8 inches, but are typically about 0.4 inches. 

The magnetite quartzite unit is too thin to be considered a 

major water-bearing unit in the study. However, the 

presence of the unit creates an opportunity for differential 

weathering, and could enhance ground-water movement 

along the contact with its enclosing rocks.

Quartzite and Aluminous Schist 

The quartzite and aluminous schist unit (shown as “qs” on 

plate 1) consists of a white to yellowish, sugary to vitreous, 

slightly graphitic to non-graphitic quartzite with accessory 

muscovite, garnet (generally flattened and elongate) and 
10 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

aluminosilicate minerals. This unit typically is present in 

layers about 1 to 4 ft thick, interlayered with feldspathic 

quartzite and garnetiferous quartzite schist. The quartzite is 

typically in contact with, and in most areas not mappable as 

a separate unit from, the aluminous schist. This quartzite 

and schist unit forms low topographic ridges that are 100 

to 200 ft above intervening valleys, weathering to a quartzrich saprolite/soil. The aluminous schist part of the unit 

typically is a tan- to yellow-weathering, sheared or buttontextured, quartzose, garnet schist, and generally contains 

kyanite or staurolite. 

A characteristic of the quartzite and aluminous schist unit in 

the study area is that the rocks are of high metamorphic 

grade and are partly granitized/migmatized; although these 

features are secondary, they are ubiquitous and serve as one 

of the identifying features of the quartzite and aluminous 

schist unit. Dikes and sills of “sweat-out” pegmatites 

pervade the schists, gneisses, and quartzites of the quartzite 

and aluminous schist unit (qs), and small bodies of 

granitoid are common. Quartzites in this unit are granular, 

thoroughly recrystallized, and typically contain garnet and 

aluminosilicate minerals (kyanite, staurolite, or sillimanite). 

The quartzite and aluminous schist unit generally is 

considered to be a potentially productive water-bearing 

unit. Quartzite in the unit is commonly well jointed, having 

spacings of 4 to 12 inches as noted in outcrops. The 

quartzite also contains open foliation planes. Schists in the 

unit are well jointed, but joints are not as closely spaced as 

in the quartzite. This situation allows water to accumulate at 

the base of the quartzite and discharge as springs where 

gently dipping isoclines intersect stream valleys. However, 

because this rock unit is relatively thin and generally 

underlies high topographic areas, subsurface fractures in the 

quartzite may not receive adequate recharge for large 

ground-water yield and sustained production. 

Diabase Dikes

Diabase dikes (shown as “d” on plate 1) consist of fine-to 

medium-grained, dark-gray to black, augite diabase, that 

locally contains small amounts of olivine, hypersthene, 

hornblende, magnetite, and pyrite. Dikes generally are 

about 16 to 66 ft wide. The diabase weathers to a dark-red 

clayey soil containing spheroidal boulders with 

unweathered rock inside an armoring, ocherous rind. 

Diabase dikes cross the study area in two major northwesttrending swarms containing hundreds of en-echelon dikes. 

Each swarm consists of closely spaced, thin, relatively short 

(from a few feet to about 0.3 mi), discontinuous, offset enechelon dikes. The diabase dikes are mostly mapped on the 

basis of float and are therefore difficult to locate and 

accurately delineate. An early report about diabase dikes of 

the Georgia Piedmont by Lester and Allen (1950) shows the 

general distribution and pervasive northwest trend of these 

dikes. A later report by Watts and Noltimier (1974) 

discusses the significance of paleomagnetism in these 

Mesozoic dikes. 

The diabase dikes typically are well jointed, but are too thin 

to be considered a water-bearing unit. However, the 

presence of the dikes increases the probability for 

differential weathering, which could enhance ground-water 

movement along the contact of the dike with the adjacent 

country rock.

Hydrogeologic Sections

The structural geology of the study area is quite different 

from that generally presented in published studies of the 

southeast part of the Piedmont physiographic province. In 

the Lawrenceville area, areal distribution of lithologic units, 

measured planar and linear structural features, and 

subsurface lithologic data from wells all indicate the same 

two general relations of mappable rock units: 

• rock units are inclined at very low to moderate 

angles, and dip direction varies over short 

distances; and 

• relative structural position of lithologic units varies 

over short distances. 

To illustrate these relations, hydrogeologic sections (plate 

2) were constructed along alignments where sufficient 

subsurface information was available to allow reasonable 

projection of surface data. Subsurface lithologic units are 

projected into the sections as preliminary interpretations 

between well pairs, and between wells and surficial lithologic contacts. As data collected for this study is expanded, 

interpretations of the subsurface hydrogeologic setting may 

differ from those in the sections shown on plate 2. 

The vertical scale of the hydrogeologic sections (plate 2) is 

the same as the horizontal scale (i.e., no vertical 

exaggeration). This allows observation of “to-scale” 

relations among topography, horizontal distance, and 

subsurface features. A short distance (0.6 to 0.9 mi) was 

selected for the sections so that the subsurface lithologic 

and fracture data can be distinguished from interpretive 

surface lithologic contacts without difficulty.

All contacts on the hydrogeologic sections (plate 2) are 

shown as thrust faults, although not shown on plate 1. This 

interpretation is based on geologic mapping throughout the 

study area, combined with subsurface (corehole and well) 

data. The relative positions of the lithologic units at land 

surface, combined with structural data and apparent 

thicknesses, and the relative positions of the lithologic units 

and thicknesses in wells, suggest thrust faults as being an 
Regional geology 11

applicable model for the contact relations between 

lithologic units. A general discussion of thrusting in the 

southernmost Appalachians in Georgia and Alabama is 

presented in Higgins and others (1988). Where subsurface 

data are available, interpretation of lithologic units is 

shown. Fractures tapped by each well or corehole are shown 

on the hydrogeologic sections, along with corresponding 

well yields, if any. Subsurface fracture dip angles were 

interpreted from directional borehole radar surveys and 

digital acoustic televiewer logs, and were projected into the 

section plane. Within the plane of each section, depiction of 

the direction and magnitude of dip of the lithologic units 

was constrained by available surface and subsurface data; 

projection between control points was based on relations 

observed during surface geologic mapping. 

Hydrogeologic sections were constructed along three 

alignments (shown on plate 1):

A–A'—from the York Casket Company well (13FF12) 

to the Pike Street well (14FF27) along a N25W 

alignment; 

B–B'—from the Maltbie Street well (14FF08) to the 

Pike Street well along a N10E alignment; and 

C–C'—through the Rhodes Jordan corehole/well 

(14FF26), along a north-south alignment. 

The alignments of these three hydrogeologic sections are 

roughly perpendicular to outcrop trend, and were selected to 

incorporate as much subsurface data as possible.

Section A–A' (plate 2) is the westernmost section, extending 

from the Pike Street well (A',14FF27), N25W to the York 

Casket company well (A,13FF12). An apparent antiformal 

structural crest is inferred to be present a short distance 

northwest of Redland Creek and the railroad. A known 

water-producing unit in the Lawrenceville area, the 

amphibolite, apparently is thicker toward the northwest. The 

interpreted subsurface contact between the biotite gneiss 

unit and the amphibolite unit at the York production well 

(13FF12) were projected from a nearby York monitoring 

well (MW-3D; Atlanta Environmental Management, 1995) 

located about 170 ft west-southwest of well 13FF12. This 

projected contact is a minimal depth, shown at the point of 

total depth of the well, which taps the biotite gneiss unit, 

(actual depth of the contact is unknown). The original airlift yield of well 13FF12 was about 254 gal/min. This 

hydrogeologic section shows increasing thickness the 

amphibolite near the crest of the antiform at Redland 

Creek; compared with 80 ft at the Pike Street well on the 

south flank of the antiform. The variations in thickness 

probably are tectonic—the result of faulting and/or 

intraformational folding. Thickness of the amphibolite unit 

in well 13FF12 is unknown. The variations in thickness 

probably are tectonic—the result of faulting and/or 

intraformational folding. 

Section B–B' (plate 2) is east of section A–A' extending 

from the Pike Street well (B', 14FF27) N10E through the 

Maltbie Street well (14FF08), and beyond to the 

amphibolite-biotite gneiss contact. Along this alignment a 

large variation in thickness of the amphibolite unit is 

apparent -80 ft at the Pike St. well and greater than 350 ft at 

the Maltbie Street well. The Maltbie Street well is 

completed in the amphibolite to a total depth of about 

350 ft. The total thickness of the amphibolite is 

indeterminate at this site. From the interpretation of average 

dip direction at land surface to the northeast, an apparent 

antiformal structural also may be present along this 

alignment, as in section A–A'. As discussed in section A–A', 

the variation in thickness of section B–B' is interpreted as 

being the result of tectonic processes.

Section C–C' (plate 2) is the easternmost section, extending 

from the granite gneiss-biotite gneiss contact south of the 

Rhodes Jordan wellfield (near C'), north through the 

wellfield corehole/well to the amphibolite-biotite gneiss 

contact, north of the railroad (C). The wellfield corehole 

began in amphibolite and tapped the amphibolite/biotite 

gneiss contact at a depth of about 292 ft. There are no 

definitive data for thickness of lithologic units north of the 

Rhodes Jordan Wellfield. From the interpretation of average 

dip direction at land surface to the north, an apparent 

antiformal structure also may be present along this 

alignment, as in sections A–A' and B–B'.

Because of the lack of detailed subsurface structural data, 

the fault contacts are represented as unmodified lines in the 

sections. Surface mapping shows large variations in strike 

orientations, and dip angle and orientation, over short 

distances within a single lithologic unit. This may be 

primarily intraformational, or it may also apply to the 

interpreted fault contacts. Surface exposures of contacts are 

not sufficient for this degree of resolution. 

These three hydrogeologic sections indicate a broad 

antiform trending east-west. The crest of this antiform is 

coincident with a topographic low underlain by amphibolite, the major water-bearing unit in this area. 

Present-day surface drainage is directed onto a broad and 

long area of amphibolite unit outcrop. Well-developed 

foliation that parallels compositional layering, dips at low 

angles, and intersects several sets of closely spaced, 

generally steeply dipping joints, and numerous random 

fractures, creates a setting that is particularly conducive to 

ground-water movement and potentially highyielding wells. 
12 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

GROUND-WATER RESOURCES

Many factors must be considered to adequately assess 

ground-water resources in a fractured crystalline-rock 

hydrogeologic setting, such as the Piedmont physiographic 

province of northern Georgia. The hydrogeologic 

framework of Piedmont crystalline-bedrock aquifers in the 

Southeastern United States typically includes a weathered 

regolith zone overlying unweathered bedrock. Thickness of 

the regolith ranges from less than 1 ft in granitic rocks to 

more than 100 ft in a weathered gneiss. The regolith may 

include soil, alluvium, colluvium, and saprolite (weathered 

bedrock that retains original structural characteristics). The 

regolith generally has a porosity ranging from about 20 to 

30 percent (Heath, 1984); but it can have low permeability 

if substantial clay is present. In some rock types, a 

transition zone may be as present between the regolith and 

the bedrock. The transition zone may be highly permeable, 

it consists of slightly weathered “blocks” of bedrock. The 

bedrock has been exposed to intense heat and pressure, and 

generally has a primary porosity of less than two percent. 

Bedrock permeability is controlled by secondary fracturing. 

Fractures include features such as joints and open foliation 

planes. Some rocks may respond differently to stresses 

such as faulting, folding, and metamorphism, and develop a 

stronger foliation and numerous joint planes that can 

enhance differential weathering and ground-water 

movement. Rocks also respond to “unloading” from 

weathering at the land surface, producing stress-relief 

fractures. These processes enhance and greatly control 

ground-water movement. Determination of the 

characteristics of the fracture pattern in an area can advance 

the understanding of permeability characteristics of a 

crystalline-bedrock aquifer.

Well Inventory and Data Compilation

The collection of well data included the compilation of 

records from the U.S. Geological Survey Ground-Water 

Site Inventory (GWSI) database, local drillers, consultants, 

and well owners. Data were field verified where possible. 

Twenty-nine bedrock (drilled) wells and eight regolith 

(bored) wells were inventoried in the study area (table 1). 

Yields range from about 3 to 471 gal/min. Total depths of 

bedrock wells range from about 105 to 605 ft. Depths of 

regolith wells range from about 15 to 40 ft.

City of Lawrenceville Historical Well Records

As mentioned previously, the City of Lawrenceville first 

drilled a production well in 1912 (well 14FF10; fig. 1, table 

1) using cable-tool methods. This well, located at the 

current Rhodes Jordan Wellfield, was refurbished in 1990 

(Special Environmental Services, 1991). During the postWorld War II era, three additional wells were drilled; a 

second well at the Rhodes Jordan location (well 14FF16), a 

well on Maltbie Street (well 14FF08), and a well on Pike 

Street (well 14FF09). Drilling records are not available for 

the historical Pike Street well (14FF09). 

The City of Lawrenceville historically is recognized as 

having two of the highest yielding wells in the Piedmont. 

These wells (14FF10 and 14FF16, plate 1; fig. 1) were 

located at the Rhodes Jordan Wellfield. Herrick and 

LeGrand (1949) stated that the City of Lawrenceville, 

population 2,223, derives its municipal water supply from 

two “deep” bedrock wells. These two wells had the highest 

reported yield, at that time, in the Georgia Piedmont. Water 

from the wells was treated by chlorination only. The first 

well, drilled in 1912, (well 14FF10; fig. 1; table 1), had an 

initial reported yield of about 470 gal/min, which had 

declined to less than 200 gal/min by 1947. A second 

production well, drilled in 1945 (well 14FF16; fig. 1; table 

1), had a reported yield of about 400 gal/min, (USGS 

unpublished data, December, 1945). Historical records also 

indicate that this well was pumped at 471 gal/min for 6 

hours, and had a pumping water level of about 110 ft. Both 

wells were reported to have penetrated “hornblendic rocks” 

(amphibolite), dipping to the southeast at 35 degrees 

(Herrick and LeGrand, 1949). Additional unpublished data 

from a USGS site visit in January 1948 also indicate that 

the Maltbie Street well (14FF08; fig.1; table 1; drilled in 

1947) was initially tested at a rate of 365 gal/min for 

20 hours, producing 18 ft of drawdown. The large groundwater production in the Lawrenceville area was linked to 

fault-block overthrusting, which was described to be at a 

low angle from the southeast (Herrick and LeGrand, 1949). 

Subjacent underlying rock beds were crushed and turned 

concave upward (dragged), which resulted in a zone of 

large and numerous openings in the rocks. Wells located to 

the northwest or southeast of the “belt of hornblendic 

rocks” were not expected to yield significant ground water. 

The quality of production-well water was said to be good 

overall, but mineralized (Herrick and LeGrand, 1949).

Well Yields

Well yields for the study area, based on available data 

included in this report, range from about 3 to more than 470 

gal/min (table 1). The largest yield estimates, generally 

more than 400 gal/min, are from historical pumping 

information for the City of Lawrenceville municipal wells. 

Wells completed prior to about 1950 (Mr. Johnny Robinson, 

Robinson Well Company, oral commun., 1995) were drilled 

using cable-tool methods, the records from which do not 

provide an initial yield estimate. Yields from wells drilled 
Ground-water resources 13

using a cable tool were estimated from aquifer tests or longterm pumping records. Later well drilling was accomplished 

using air-rotary methods, whereby cumulative air-lift yield 

estimates are recorded during drilling activities. 

Relation of Well Yield to Lithology

Subsurface data collected from wells can be placed in a 

regional hydrogeologic framework by evaluating well 

characteristics in terms of mapped lithologic units. The 

lithologic units shown in plate 1—amphibolite (a), biotite 

gneiss (bg), and button schist (bs), granitic gneiss (gg), and 

quartzite and aluminous schist (qs)—are relatively thin; a 

few hundred's of feet to a few thousand's of feet in outcrop 

width. Geologic contacts and internal compositional 

differences within lithologic units in much of the study area 

are undulatory and inclined at low angles. The relative 

vertical position of the rock units varies areally, due to 

intrusive relations, recumbent folding, and (or) 

overthrusting. Because the rocks have complex intrusive 

and (or) structural relations, wells that begin in one rock 

unit at land surface may be drilled into, and produce water 

from, different lithologies below the surficial unit. 

Additionally, the “thinness” of these lithologic units may 

have resulted in different responses to tectonic stress and 

weathering than the same lithologies in a thicker, less 

complex geologic setting. However, these thin lithologic 

units may extend over large areas because of an undulatory, 

nearly horizontal, structural attitude combined with low 

topographic relief.

Comparison of well yields with lithologic units mapped at 

land surface, on a regional scale such as that of the study 

area, is difficult. Many factors must be considered before 

data can be accurately evaluated. Often, no subsurface 

lithologic information is available for domestic wells, 

which generally comprise the majority of the wells 

inventoried in the study area. Also, reported well yields can 

be inaccurate. Generally, domestic wells do not have yields 

as large as wells drilled for municipal or industrial 

purposes; however, a small yield, such as 5 gal/min, is 

sufficient for a single household. Consequently, domestic 

wells generally are shallower and do not encounter as many 

water-bearing zones as the deeper municipal or industrial 

wells; however, some domestic wells can have high initial 

air-lift yields of 100 gal/min or more.

An overall comparison of the well yield data shows that the 

amphibolite has notably higher yields than the other four 

major lithologic units (table 1); however, most exploration 

for public supply has been focused on areas where the 

amphibolite is near land surface. Historically, the municipal 

wells at the Rhodes Jordan Wellfield and Maltbie Street, 

drilled in areas of amphibolite outcrops, are two of the most 

productive sites in the Piedmont physiographic province of 

Georgia. From limited data available at four well sites (data 

collected through 1997), subsurface fractures appear to be 

more numerous in the amphibolite compared with other 

rock types. 

Examples of wells that have encountered extremely low 

yields are wells 14FF31 and 14FF35 (plate 1; table 1); these 

two wells were drilled into granitic gneiss. These wells were 

drilled to depths of about 600 ft, which is the typical total 

depth of exploration in the Piedmont, and yield only 3 and 4 

gal/min, respectively; three “dry” wells were drilled on the 

same property prior to drilling well 14FF35. 

Although the number of wells inventoried in the various 

units influences the comparison of well-yield data with 

lithologic units, a general comparison of potential waterbearing characteristics of each mappable unit is discussed in 

the following section. Subsurface well data are discussed 

where available.

Amphibolite

From the well-inventory-data compilation, yields for ten 

wells drilled in the amphibolite outcrop area range from 

about 10 to 471 gal/min (table 1) (although some wells may 

tap other lithologic units at depth); six wells have no 

associated yield information. Historically, this rock unit has 

been referred to as “hornblendic rocks” by geologists, and 

as “black granite,” by drillers. Five of the ten wells are 

located at the Rhodes Jordan Wellfield. Production zones in 

the known high-yielding municipal wells in the 

Lawrenceville area are located in the amphibolite unit. At 

the Rhodes Jordan Wellfield, five wells yield from about 

100 to more than 400 gal/min, and the city well located at 

Maltbie Street (14FF08, fig. 1) was reported to yield more 

than 400 gal/min (table 1). Of the three domestic wells 

having yield data, the range is from about 25 to 60 gal/min. 

At a depth of about 423 ft in the Pike Street well (14FF27), 

a large fracture zone (initial yield of 50 gal/min) was 

encountered in the amphibolite. Overall aquifer-testing 

yield was less than the initial report of 100 gal/min 

(Mark Hubner, Middle Georgia Water Systems, oral 

commun., 1996).

Surface exposures of the amphibolite typically are highly 

jointed, as are other amphibolite units throughout the 

Piedmont physiographic province of Georgia, and strongly 

foliated (compositional layering). Strike and dip of the 

foliation is variable, even at the scale of a few acres, and 

joint set orientations are diverse. Core drilling was 

conducted at the Rhodes Jordan Wellfield to derive 

mineralogical, textural, and structural characteristics of the
14 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia Table 1. Records of wells inventoried in the Lawrenceville area, Georgia

[Well type: B, bored or augered; D, drilled; Use of water: H, domestic; U, unused; d, destroyed; N, industrial; P, public supply; S, livesto

—, data not available. Geologic unit: bg, biotite gneiss; bs, button schist; a, amphibolite; gg, granite gneiss; qs, quartzite schist. NOTE: 

estimated from 1:24,000-scale topographic maps]

Well 

number Owner Latitude Longitude

Land-surface 

altitude 

(feet)

Well 

depth 

(feet)

Casing 

depth 

(feet)

Casing 

diameter 

(inches)

Well 

type

Use of 

water

Date 

constructe

13FF05 David Brannon 33°55'35" 84°00'11" 1,030 325 30 6 D

H October 197

13FF06 James A. Daily 33°59'38" 84°02'48" 1,045 234 43 6 D

H April 1963 

13FF07 Elmore F. Stewart 33°59'33" 84°01'31" 1,020 128 89 6 D

H October 196

13FF08 E.A. Barton 33°56'03" 84°01'13" 940 143 11 6 D

H August 197

13FF12 York Casket Company 33°57'54" 84°00'02" 1,045 265 54 6 D

N August 197

14FF05 David Manchester 33°55'01" 83°59'55" 940 105 39 6 D

H April 1977 

14FF06 Dacula, Georgia 33°59'20" 83°53'47" 1,050 375 23 8 D d 1947 14FF08 City of Lawrenceville 33°57'39" 83°59'40" 1/1,018.60 348 28 8 D

U 1947 

14FF09 City of Lawrenceville 3/33°57'22" 3/83°59'43" 3/1,050 — 

—

— D d 4/1940’s

14FF10 City of Lawrenceville 33°57'34" 83°58'44" 5/993.29 1/177 20 8 D P 1912 14FF11 Gerald Hanson 33°56'39" 83°57'16" 1,040 170 27 6 D 

H November 19

14FF12 Dr. Charles Brand 33°56'43" 83°56'14" 1,030 265 49 6 D 

H April 1971 

14FF13 Oscar M. Dunnagan 33°57'35" 83°56'35" 1,070 200 25 6 D

H June 1960 

14FF14 James Banner 33°57'51" 83°54'46" 1,010 295 52 6 D

H September 19

14FF16 City of Lawrenceville 33°57'35" 83°58'44" 5/994.64 302 7/6, 26 7/12,8 D P 1945 14FF17 City of Lawrenceville 33°57'32" 83°58'47" 5/991.66 212 22 6 D

U November 19

14FF18 City of Lawrenceville 33°57'32" 83°58'43" 5/999.36 162 24 6 D

U November 19

14FF19 Jim Dunnagan 33°57'40" 83°56'49" 1,060 162 19 6 D

H 1958 

14FF20 Dunnagan 33°57'33" 83°57'01" 1,040 130 13 6 D 

H

— 

14FF21 Jim Dunnagan 33°57'44" 83°56'37" 1,060 200 13 6 D S — 14FF26 City of Lawrenceville 33°57'33" 83°58'45" 5/993.38 380 33 4 D

U March 199

14FF27 City of Lawrenceville 33°57'20" 83°59'43" 1/1,050.71 600 7/59, 91 7/10, 6

D

U March 199
 Ground-water resources 15 1/Surveyed by E&C Consulting Engineers, Inc. 2/Historical reported yield. Aquifer-test data collected in December 1995 indicate yield of 350 gallons per minute. 3/Approximate location and altitude—well abandoned. 4/Approximate date, oral communication, Don Martin, City of Lawrenceville, Ga., 1996. 5/Surveyed by U.S. Geological Survey. 6/Original depth of 386 feet, currently blocked at depth indicated. 7/Double-cased well. First number in casing depth corresponds with first number in casing diameter. 8/Initial air-lift test yield. Subsequent aquifer-test data do not support this yield. 9/Surveyed by Gwinnett County, Ga. 14FF29 Macon Ogletree 33°56'32" 83°57'15" 1,040 200 50 6 D U 1966 

14FF30 J.D. Bowen 33°59'16" 83°53'17" 1,130 110 — — D H 1958 14FF31 Erwin Jones 33°56'44" 83°56'05" 870 605 35 6 D H April 1989 14FF32 Charles Vernable 33°59'24" 83°56'46" 1,080 40 40 — B H — 14FF33 Hubert Hickman 33°58'20" 83°59'34" 1,090 36 36 30 B H 1940 14FF34 Mike Crow 33°57'40" 83°59'16" 1,060 217 68 6 D U 1937 14FF35 Roe Chatham 33°58'07" 83°53'28" 1,085 605 10 6 D H May 1988 14FF36 City of Lawrenceville 33°57'34" 83°58'46" 1/991.67 16 6 2 B U July 1995 14FF37 City of Lawrenceville 33°57'32" 83°58'43" 1/999.36 15 5 2 B U July 1995 14FF38 City of Lawrenceville 33°57'34" 83°58'45" 1/992.95 25 15 2 B U July 1995 14FF39 City of Lawrenceville 33°57'33" 83°58'45" 993 180 36 6 D U October 19914FF40 Gwinnett County Airport 33°58'32" 83°58'34" 1,060 17 — 30 B d — 14FF41 Gwinnett County Airport 33°58'30" 83°58'31" 1,065 27 — 30 B d — 14FF42 Gwinnett County Airport 33°58'39" 83°57'23" 9/1,029.29 599 35 8 D U May 1996 14FF43 Kemron 33°57'25" 83°58'45" 990 21 17 2 B U August 199

Table 1. Records of wells inventoried in the Lawrenceville area, Georgia (Continued) [Well type: B, bored or augered; D, drilled; Use of water: H, domestic; U, unused; d, destroyed; N, industrial; P, public supply; S, livestoc

—, data not available. Geologic unit: bg, biotite gneiss; bs, button schist; a, amphibolite; gg, granite gneiss; qs, quartzite schist. NOTE: 

estimated from 1:24,000-scale topographic maps] Well number Owner Latitude Longitude Land-surface altitude (feet) Well depth (feet) Casing depth (feet) Casing diameter (inches) Well type Use of water Date constructe
16 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

amphibolite at depth. About 292 ft of amphibolite was cored 

at well 14F26 (fig. 2). Compared with other amphibolite 

units in the Piedmont physiographic province (fig. 1), this 

unit appears to have a highly sheared foliation (stronger 

compositional layering), as well as significant 

concentrations of biotite and pyrite. Open-space 

mineralization, consisting primarily of zeolites (stilbite) and 

calcite, also was noted in the core samples. 

Data collected from wells drilled into the amphibolite in the 

study area suggest that the amphibolite is more fractured at 

depth than are other lithologic units. These fractures are the 

secondary porosity and permeability that transmit ground 

water. Tectonic stresses, and later erosional unloading, 

probably created the closely spaced fractures in the 

amphibolite. The apparent increased presence of fracturing 

in the amphibolite may be related to the location of that 

unit at the crest of the apparent antiformal structure 

(indicated from the hydrogeologic sections shown in plate 

2). Stress related to the folding may have enhanced 

fracturing potential of the amphibolite unit. In addition, 

well-developed compositional layering has created zones of 

weakness, which were vulnerable to both tectonic stress and 

weathering, which also enhanced ground-water movement. 

With this compositional layering and a fracture network, 

subsequent unloading by erosion of broad valleys, could 

have resulted in “stress release” and opening of the fracture 

system. The “stress release” discussed here is not a release 

of inherent stress associated with deep-seated igneous 

intrusions. Rather, it is more comparable to the upward 

expansion of the rock as a result of the removal of overlying 

rocks in broad valleys (elastic rebound). Large rock 

fragments noted during air-rotary drilling and from core 

samples often have fractures partially filled with minerals. 

Zeolites (stilbite) are the most abundant mineral, although 

epidote and iron sulfide minerals also are common. The 

presence of openings partly filled with mineral growth 

indicates that some of the stress-relief fractures have been 

open for considerable time.

Biotite Gneiss

From the well-inventory-data compilation, the range of 

yields for wells drilled into the biotite gneiss outcrop area 

(the well may tap other lithologic units at depth), is from 

about 5 to 254 gal/min (table 1). Of these seven wells 

having well yield data, six are domestic wells having yields 

that range from about 5 to 50 gal/min (table 1). A much 

higher yield of 254 gal/min was reported for well 13FF12, 

which was drilled for industrial purposes (plate 1). The 

corehole drilled at the Rhodes Jordan Wellfield (well 

14FF26) penetrates the biotite gneiss at a depth of about 

292 ft (fig. 2). The corehole continued in the biotite gneiss 

unit to a depth of about 320 ft, where the button schist was 

penetrated. The biotite gneiss unit was again penetrated at a 

depth of about 355 ft and continued through the total depth 

of the corehole to about 380 ft. A significant fracture zone 

(potential production zone (PPZ)) was noted near the 

contact between the amphibolite unit and biotite gneiss unit 

at about 300 ft. The Pike Street well (14FF27) also 

penetrates the biotite gneiss unit from about 239–365 ft 

below land surface and 445–585 ft (fig. 2). A large fracture 

zone was noted during the drilling of this well at about 

335–339 ft below land surface had an estimated yield of 

50 gal/min. The biotite gneiss unit was penetrated at depths 

of about 100–255, 275–280 ft, 352–361 ft, and 382–600 in 

the Gwinnett County Airport corehole (well 14FF42; fig. 

2). In this well, the biotite gneiss unit is mostly unfractured 

and mixed with layers of amphibolite.

Larger well yields are most probable where the biotite 

gneiss is strongly foliated and well jointed. The biotite 

gneiss generally is deeply weathered, due to the higher 

feldspar composition; therefore, the regolith could provide 

storage of shallow ground water that recharges the deeper 

fracture zones. Minor rock types within the biotite gneiss 

unit, such as pegmatites and quartz veins, respond 

differently to stress and weathering, and could also 

significantly enhance water-bearing potential. Quartz veins 

respond to stress in a “brittle” manner, and often are highly 

fractured or granulated. Pegmatites are coarser-grained than 

the biotite gneiss and consequently weather differently, 

providing pathways for ground-water movement. The 

biotite gneiss generally is well-foliated (compositional 

layering) and can have well-developed joints.

Button Schist

From the well-inventory-data compilation, the range of 

yields for a total of three domestic wells drilled into the 

button schist outcrop area (the well may tap other lithologic 

units at depth), is from about 23 to 100 gal/min (table 1). 

The button schist is interpreted as having resulted from 

shearing and recrystallization of the biotite gneiss. These 

two rock units have similar mineralogy, containing 

muscovite, biotite, feldspar, and quartz, but with an increase 

in muscovite over biotite in the schist. Although the button 

texture is related to intense shearing of the schist, the 

foliation planes generally are not open at the surface or at 

depth. In outcrop, the button schist does not weather deeply 

and does not contain prominent or regular jointing. 

Sections of the button schist were retrieved from the two 

coreholes (wells 14FF26 and 14FF42, plate 1; figs. 1 and 2) 

drilled in the study area. A section of the button schist was 

retrieved from about 320–355 ft below land surface from 

the Rhodes Jordan Wellfield corehole (well 14FF26, fig. 2). 

The button schist also was penetrated in the Gwinnett County 
Ground-water resources 17

Airport corehole (well 14FF42), in an alternating sequence 

with the biotite gneiss unit, at depths of about 255–275 ft, 

280–352 ft, and 361–382 ft (fig. 2). Shear foliation in the 

button schist is strong and no fractures were observed in 

core samples.

Granite Gneiss

From the well-inventory-data compilation, the range of 

yields for a total of four wells having well yield data drilled 

into the granite gneiss outcrop area (the well may tap other 

lithologic units at depth), is from about 3 to 20 gal/min 

(table 1). This unit is known for “dry” wells, and may not 

yield enough water for domestic use. One property owner 

(well 14FF35, table 1) had to drill four wells, three of which 

were considered “dry.” An historical municipal well 

(14FF06, plate 1) owned by the City of Dacula yielded 20 

gal/min. The Pike Street well (14FF27; fig. 1) penetrates the 

granite gneiss from land surface to a depth of about 178 ft 

(fig. 2). The estimate air-lift yield of this well was initially 

100 gal/min; however, subsequent aquifer testing indicates a 

much lower overall yield. A sand collapse zone, yielding 

about 42 gal/min, was encountered in the Pike Street well 

(14FF27) within the granite gneiss, below apparent 

competent bedrock (original surface casing to 59 ft), from 

about 62–86 ft below land surface. This zone was sealed 

prior to well completion. 

The physical characteristics of the granite gneiss vary 

throughout the study area, but it generally is granitic in 

texture and composition and is either massive or has only a 

weak foliation. Jointing is scarce and irregular in this 

lithologic unit. Generally, the granite gneiss is relatively 

uniform in composition, but depth of weathering is quite 

variable. In some areas, pavement outcrops are prominent, 

whereas in other areas, the granite gneiss is weathered to 

depths of several ten's of feet. In general, this lithologic unit 

is considered to have a little overall potential for high-yield 

wells. Massive granitic rocks can, however, yield significant 

quantities of water to wells if stress-relief fractures are 

present at depth. Because stress-relief fractures are nearly 

horizontal, there is little surface evidence for their presence. 

Quartzite and Aluminous Schist

Yield information is not available for the only well 

(14FF30; plate 1) drilled in a quartzite and aluminous schist 

outcrop area (the well may tap other lithologic units at 

depth). In the subsurface, the quartzite and aluminous schist 

unit has only been identified in one well in the study 

area—the Pike Street well (14FF27, fig. 2). In this well, the 

unit occurs at a depth of about 178–239 ft below land surface. A fracture was noted near the contact with the overlying granitic gneiss unit at a depth of about 178–179 ft. 

Yield from this fracture was estimated to be about 8 to 

10 gal/min. 

Differential weathering is well developed in the quartzite 

and aluminous schist unit, as the felsic schist is much more 

susceptible to weathering than the siliceous quartzite. The 

schist generally is well foliated, but has fewer joints than the 

highly fractured quartzite, which most likely responded 

brittlely to tectonic stress and unloading during the most 

recent weathering cycle. The thickness of the quartzite is 

highly variable throughout the study area. Because of its 

resistance to weathering, the quartzite often underlies higher 

topographic areas and has little recharge potential. Although 

its physical characteristics suggest that the quartzite would 

be a potentially productive aquifer, its general thinness and 

typical high topographic position are not considered to be 

favorable hydrogeologic conditions for adequate groundwater yields in the study area.

Interpretation of Borehole Geophysical Logs

Subsurface information and lithology and fracture data, are 

available at four sites in the study area: (1) Rhodes Jordan 

Park Wellfield; (2) Maltbie Street; (3) Pike Street; and (4) 

the Gwinnett County Airport. Lithologic variations between 

these sites are shown in figure 2. In some areas, gamma logs 

can be used to distinguish rock types and lithologic units. 

Geologic core was collected from wells at the Rhodes 

Jordan Park (14FF26) and Gwinnett County Airport 

(14FF42), and cuttings were collected from the Pike Street 

well (14FF27). The lithology in the Maltbie Street well 

(14FF08) was interpreted from geophysical logs. 

Of these four sites, the larger yielding wells, Rhodes Jordan 

Wellfield (14FF10, -16, -17, -18, -39) and Maltbie Street 

(14FF08), primarily penetrate the amphibolite unit (fig. 1). 

Production at the Rhodes Jordan Wellfield averages 230 gal/

min. The Maltbie Street well (14FF08) has been tested at 

350 gal/min. Originally projected to yield 100 gal/min, the 

Pike Street well (14FF27) penetrates five different lithologic

units; however, subsequent aquifer testing suggests a much 

lower overall yield. The Gwinnett County Airport well 

(14FF42) penetrates three different lithologic units, and has 

an estimated yield of about 10 gal/min. 

In resistive crystalline rocks, such as those in the Piedmont 

physiographic province of northern Georgia, fractures can 

be identified as distinctive anomalies on borehole 

geophysical logs. Typically, fractures are recognized most 

readily as an increase in borehole diameter on the caliper 

log, and a decrease in resistivity on electric logs (saturated 

fractures). Fractures intersecting the borehole in bedrock 

wells were identified using an integrated suite of borehole 

geophysical logs, including caliper, focused resistivity,
18 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

B

B

M

M

M

M

Biotite schist

14FF08 14FF26

1,050

1,000

950

900

850

800

750

700

650

600

550

500

450

400

Feet

DATUM IS SEA LEVEL

NOT TO HORIZONTAL SCALE

Amphibolite/

 Muscovite schist

Amphibolite/

 Biotite gneiss

Interlayered Button 

 schist/Biotite gneiss/

 Amphibolite

Amphibolite/

 Button schist

Quartzite

Granite gneiss 

Quartzite and

 aluminous schist

Amphibolite 

Button schist

Biotite gneiss

Fracture zone—

 Length tapped 

 by borehole

Lithologic unit

Amphibolite

Biotite gneiss

Button schist

Granitic gneiss

Schist/Quartzite

Muscovite schist

Rock type

Potential pro-

 duction zone

Biotite gneiss/

 Button schist

Figure 2. Subsurface lithologic characteristics and fractures tapped by wells 14FF08, 14FF26, 

14FF27, and 14FF42. Fracture zones interpreted from well 14FF26 and 14FF42.

Rhodes Jordan Wellfield

sustained yield 230 gal/min

(Yield data from 14FF39) 

Aquifer test yield

350 gallons per minute (gal/min)

Production zone

determined from

aquifer test

PPZ

PPZ

PPZ

PPZ

PPZ

PPZ

75 gal/min

12 gal/min

75 gal/min
Ground-water resources 19

B

B

B

B B

B

B

B

B

B

B

B B

B

M

M

M

M

M

M

M

M

0 250 500 750

Gamma, in APIU

(American Petroleum Institute Units)

14FF27 14FF42

1,050

1,000

950

900

850

800

750

700

650

600

550

500

450

400

Feet

Initial total yield

100 gal/min

42 gal/min

Total estimated yield 10 gal/min

4 gal/min

4 gal/min

50 gal/min

8-10 gal/min

50 gal/min

25 gal/min
20 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

long- and short-normal resistivity, natural gamma, acoustic 

televiewer and velocity, spontaneous potential, fluid 

temperature and resistivity, deviation, heat-pulse flowmeter, 

video camera logs, and single-borehole radar surveys. 

Subsequent to fracture zone identification, the orientation of 

each zone was interpreted from acoustic televiewer logs and 

directional borehole radar surveys. 

Rhodes Jordan Wellfield

The Rhodes Jordan Park Wellfield (fig. 1) was the first 

local-scale study site in Lawrenceville. The wellfield 

includes a bedrock production well, five bedrock 

observation wells, and three regolith observation wells (data 

collected through 1997). Bedrock observation wells are 

located at distances of 10, 100, and 250 ft from the 

production well. The aquifer is a sheared and highly jointed 

amphibolite that is overlain by about 25 ft of regolith. A 

production well (14FF10, table 1) penetrating fracture zones 

in this aquifer supplies ground water to the City of 

Lawrenceville, at an average rate of about 230 gal/min. Four 

bedrock observation wells near the production well yield 

more than 100 gal/min each (the fifth well 14FF26 is a 

corehole and has no yield estimate). All of the bedrock 

observation wells respond within 10 minutes when the 

production well is pumped. Various borehole geophysical 

techniques and surface resistivity methods were used to 

characterize the fracture system. The geophysical 

interpretations were compared with surface geologic 

structural data recorded near the study site (Chapman and 

Lane, 1996; Chapman and others, 1997). 

Subsurface lithologic data from the Rhodes Jordan Wellfield 

were obtained from historical drillers' logs of wells 14FF10 

and 14FF16, geologists’ logs of wells 14FF17 and 14FF18, 

and the interpretation of borehole geophysical logs and 

geologic core samples collected as part of this study. The 

aquifer has been described as biotite hornblende gneiss/

amphibolite. Historical drillers’ logs of well 14FF16 

describe the rock as black, gray, and blue granite. The 

corehole drilled at the Rhodes Jordan Wellfield (fig. 2; plate 

2; well 14FF26) provided essential geologic and 

hydrogeologic information regarding the mineralogical and 

textural characteristics of the amphibolite aquifer, as well as 

characteristics of water-bearing zones within the 

amphibolite. Well 14FF10, the current production well, has 

an estimated total depth (from historical drilling records) of 

about 386 ft; this well currently is blocked at about 177 ft 

(Mr. Jim Breakey, Atlanta Drilling and Exploration, Inc., 

oral commun.,1998). Based on this information, the 

corehole (well 14FF26; figs. 1 and 2; table 1) was drilled to 

a total depth of about 380 ft (fig. 2). As expected, the 

dominant lithologic unit from land surface to about 292 ft 

was the amphibolite. Below that depth, the biotite gneiss 

unit was penetrated to a depth of about 320 ft. The button 

schist unit was penetrated from about 320–355 ft. The 

remainder of the borehole was identified as the biotite 

gneiss unit from about 355–380 ft. 

Well 14FF39 (fig.1; table 1) was later drilled near the 

corehole (well 14FF26), by air-rotary methods, to a depth of 

about 180 ft to determine which fractures noted in the 

corehole would produce significant ground-water yield. The 

total estimated air-lift yield for well 14FF39 was more than 

150 gal/min. Total depth of well 14FF39 was only about 

180 ft due to high hydrostatic borehole pressure from 

productive fractures at about 120–125 ft and 170–185 ft. 

The maximum air-lift yield estimate from air-rotary drilling 

rigs is about 150 gal/min from a 6-inch diameter borehole 

(Mr. Johnny Robinson, Robinson Well Company, oral 

commun., 1995). 

Interpretations of natural gamma logs indicate an overall 

recognizable lower baseline response for the amphibolite 

unit than for the other major lithologic units. The gamma 

log, a passive tool that measures total gamma radiation in a 

borehole within a selected energy range, generally reflects 

the presence of gamma-emitting radioisotopes such as 

potassium-40 and daughter products of the uranium and 

thorium-decay series (Keys, 1990). The gamma log of well 

14FF26 (fig. 2) indicates lower gamma counts (in American 

Petroleum Institute units, APIU) within the amphibolite 

(0–292 ft below land surface), compared to the button schist 

and biotite gneiss at depth. The button schist and biotite 

gneiss units both contain much larger percentages of 

potassium feldspar compared to the amphibolite, which 

primarily contains calcium plagioclase feldspar. Many 

fracture zones within the amphibolite have corresponding 

gamma peaks (fig. 2). Secondary minerals in fracture zones 

may include clays (resulting from chemical breakdown of 

the crystalline rock), resulting in higher gamma readings, as 

compared with the amphibolite. 

As a first step in aquifer characterization at the Rhodes 

Jordan Wellfield study site, depths to fracture zones were 

determined for each bedrock observation well. The 

identification of fracture zones within the amphibolite in 

well 14FF16 (plate 1; fig. 1; table 1) is shown in figure 3 

(p. 22–23). This well was drilled in 1945, and while the 

driller's log does not list fracture zones specifically, zones at 

which the “old” well became “muddy” were noted at 

42–44 ft, 140–170 ft, and 195 ft. Fractures are most readily 

identified from the caliper log, which shows borehole 

diameter enlargement, and electrical logs (focused 

resistivity). During drilling activities, borehole diameter 

generally is increased near fracture zones, as the bit is able 

to remove more material. In saturated, resistive crystalline 

rocks, fractures are identified as low resistivity anomalies 

(negative inflections). Images of fractures are produced 
Ground-water resources 21

from acoustic televiewer logs. Based on interpretations of 

these logs, major fractures were identified in well 14FF16 at 

depths of about 43, 77–87, 92, 134, 169, 175, and 255 ft 

(fig. 3). During sustained pumping of production well 

14FF10, the uppermost fractures at 43, 77–87, and 92 ft are 

regularly dewatered. However, drawdown rates are 

recognizably reduced near the 134 ft fracture, and the 134, 

169, and 175 ft fractures have been interpreted as 

production zones. The deeper 255 ft fracture appears to be 

of a lesser magnitude, a contributing rather than a 

production zone fracture, from geophysical interpretations. 

A relative comparison of fractures also was made using 

fluid conductivity, temperature, and heat-pulse flowmeter 

logs. Inflections or slope variations may be observed in 

fluid resistivity and temperature logs (fig. 3), as ground 

water moving into the borehole from a particular fracture 

zone may have different dissolved-solids concentrations or 

temperatures. Variations in the dissolved-solids concentrations are indicated by variations in the fluid resistivity logs. 

In well 14FF16, inflections were noted on both the fluid 

resistivity (increasing resistivity, lower dissolved solids) 

and temperature (increasing) logs near the fracture at 175 ft, 

and in the temperature log (increasing) near fracture at 255 

ft. The heat-pulse flowmeter log (fig. 3) measures vertical 

flow rates within the borehole. A positive rate, or upward 

flow, was measured throughout well 14FF16, most likely 

because of the close proximity to production well 14FF10, 

located about 10 ft away. During the flowmeter logging of 

well 14FF16, nearby well 14FF10 was being pumped at a 

rate of about 230 gal/min, with the pump column set at 

about 160–170 ft. The highest flow rates of about 10 to 11 

gal/min were measured near the 77–87 and 92 ft fracture 

zones. Turbulent flow was evident near the 134 ft fracture, 

suggesting direct connection with the production well at that 

depth. The fracture zones at 169 and 175 ft also were 

measured at about 7 gal/min. Overall, much less water was 

being contributed by the deepest fracture at 255 ft (fig. 3). 

Maltbie Street Well

The Maltbie Street well (14FF08, plate 1; figs. 1 and 2; 

table 1) was drilled using cable-tool methods in 1947, to a 

total depth of about 350 ft. No drilling records or geologic 

information is available from historical records. However, 

using borehole geophysical logs, the rock type and 

characteristics of fracture zones were determined (figs. 2 

and 4). From the low gamma readings (generally less than 

100 APIU, fig. 2), the lithology of well 14FF08 was 

interpreted to be entirely that of the amphibolite unit. The 

rock type was later visually confirmed from the video log. 

Fracture delineation was accomplished using data primarily 

from the caliper and focused resistivity logs (fig. 4), and 

from acoustic televiewer logs. One fracture zone penetrated 

by the Maltbie Street well intersects more than 10 ft of the 

borehole. The most significant fracture zones identified 

from borehole geophysical logs were delineated at depths of 

48–54 ft, 101.5–115 ft, 185–191 ft, 205.5–210.5 ft, 

222–227 ft, and 231–233 ft (figs. 2 and 4). Additional 

smaller, or contributing fracture zones were identified at 

141.5–145 ft, 319.5–320.5 ft, 324–328 ft, and 343–346 ft. 

This well has very large (more than 15 inches in diameter) 

caliper peaks; the historical cable-tool drilling method 

apparently “cleans out” fracture zones better than modern 

air-rotary methods. Inflections were noted on the 

temperature and fluid resistivity logs near the 185–191 ft, 

and 233 ft fracture zones. An inflection also was noted on 

the fluid resistivity log near the 48–55 ft and 100–116 ft 

fracture zones. Inflections on the temperature and fluid 

resistivity logs identify fractures where water is moving into 

or out of the borehole.

A constant-discharge aquifer test was conducted using 

this well, in December 1995. The Maltbie Street well was 

pumped at a rate of 350 gal/min for a total of 96 hours, 

and had 62 ft of drawdown (U.S. Geological Survey, 

unpublished data, 1995). The pump was lowered to a depth 

near the 101.5–115 ft fracture zone, which is labeled as a 

production zone (PZ) in figure 4. Additionally, the deeper 

fractures at 185–191 ft, 205.5–210.5 ft, 222–227 ft, 

and 231–233 ft are interpreted as potential production 

zones (PPZ, fig. 4). The Maltbie Street well currently 

(1998) is used as an observation well. This well may be 

put into production within the next few years (Mr. Juan 

Ruiz, E&C Consulting Engineers, Inc., oral 

commun., 1997).

Pike Street Well

A test well (14FF27- Pike Street well; plate 1; fig. 1; table 

1) was drilled by the City of Lawrenceville in March 1995 

near an abandoned, former city supply well (14FF09) 

drilled in the 1940’s. The 1995 Pike Street well was drilled 

to a depth of about 600 ft. Geologic logs indicate that well 

14FF27 penetrates five different lithologic units (figs. 2 and 

5), including granite gneiss, quartzite and aluminous schist, 

biotite gneiss, amphibolite, and button schist. The biotite 

gneiss unit is repeated below the amphibolite unit (fig. 2). 

Surface casing in well 14FF27 is telescoped (table 1) 

because of a sand-collapse zone below the outer casing 

(10 inches, 0–59 ft), from about 62–86 ft, within apparent 

competent rock in the granite gneiss unit. This sand zone 

was cased off (6-inch casing, 0–91 ft) prior to the advancement of the borehole and produced about 42 gal/min. The 

open borehole below the telescoped casing produced about 

100 gal/min from the initial air-lift test. Subsequent aquifer 

testing suggests that the yield of well 14FF27 is 

significantly less than 100 gal/min. 
22 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

Amphibolite

Fracture zone—

 Length tapped 

 by borehole

Potential

production

zone

Lithologic unit

Amphibolite

Rock type

EXPLANATION

PPZ

PPZ

Production

zone—Determined from

sustained yield 

of 230 gallons

per minute

of nearby 

well 14FF10

Dewatered

during

sustained

pumping

PPZ

Caliper,

in inches

6 8 10 12 14 16

Bit size change

0

50

100

150

200

250

300

DEPTH BELOW LAND SURFACE, IN FEET

14FF16

Figure 3. Fracture identification and relative comparison of fractures in Rhodes Jordan Wellfield 

well 14FF16 using borehole geophysical logs.
Ground-water resources 23

Focused resistivity,

in ohm-meters

Fluid resistivity,

in ohm-meters

Temperature, in

degrees Celsius

Heat-pulse flow meter, 

in gallons per minute

0 500 750 250 1,000 1,250 1,500 1,750 35 40 45 50 17 17.5 18 0 2 4 6 8 10 12

Turbulent flow
24 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

0

50

100

150

200

250

300

350

14FF08

Caliper,

in inches

96-hour constant-rate aquifer test

yield of 350 gallons per minute

DEPTH BELOW LAND SURFACE, IN FEET

PZ

PPZ

PPZ

PPZ

PPZ

5.5 8 10.5 13 15.5 18

Figure 4. Fracture identification and relative comparison of fractures in Maltbie Street well 14FF08 using 

borehole geophysical logs.

Amphibolite

Potential

production

zone

Amphibolite

EXPLANATION

PPZ

Production

zone

PZ

Fracture zone—

 Length tapped 

 by borehole

Lithologic unit

Rock type
Ground-water resources 25

Fluid resistivity,

in ohm-m

Gamma, in APIU

(American Petroleum Institute Units)

Temperature, in

degrees Celsius

Focused resistivity,

in ohm-m

0 500 750 250 1,000 1,250 1,500 1,750 0 50 100 200 150 250 350 300 15.5 16.5 16 17.5 17 18 35 40 45 50 55
26 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

5 5.5 7 7.5 6 6.5

0

50

100

150

200

250

300

350

400

450

500

550

600

DEPTH BELOW LAND SURFACE, IN FEET

Caliper,

in inches

Original air-lift yield of 100 gallons per minute

(gal/min); aquifer test suggests lower yield

Well 14FF27 Total

estimated

yield, in

gal/min

4 gal/min

42 gal/min

4 gal/min

8–10 gal/min

50 gal/min

50 gal/min

25 gal/min

Granite gneiss

Granitic gneiss

Biotite gneiss

Schist/Quartzite

Muscovite schist

Biotite schist

Amphibolite

Biotite gneiss

Amphibolite

Button schist

Quartzite and

 aluminous schist

Amphibolite/

 Muscovite schist

Amphibolite/

 Button schist

Quartzite

Figure 5. Fracture identification and relative comparison of fractures in Pike Street well 14FF27 using 

borehole geophysical logs.

EXPLANATION

B

B

B

B B

B

B

B

B

B

B B

B B

B

M

M M

M

M

M

M M

M

M

Fracture zone—

 Length tapped 

 by borehole

Lithologic unit

Rock type
Ground-water resources 27

50 175 300 425 550 675 800 16 17 18 19 20 30 40 60 70 80 90 50 100 110

Fluid resistivity,

in ohm-m

Temperature, in

degrees Celsius

Gamma, in APIU

(American Petroleum Institute Units)

0 250 750 500 1,000 1,200 1,500 1,750

Focused resistivity,

in ohm-m
28 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

Four water-bearing fracture zones were observed during 

initial drilling of the Pike Street well (14FF27). Two fracture zones having the largest estimated yield of 50 gal/min 

each were noted. The first 50 gal/min fracture zone is within 

a minor amphibolite rock interpreted as part of the biotite 

gneiss unit (331–349 ft). The second 50 gal/min fracture 

zone is within the amphibolite unit (423–430 ft) (plate 2; 

fig. 2; fig. 5). Two additional water-bearing zones were 

noted near lithologic contacts and/or quartz-rich zones. A 

yield of about 8–10 gal/min was noted near the granite 

gneiss contact with the quartzite and aluminous schist unit 

(178–179 ft). A yield of about 25 gal/min was noted within 

a quartz-rich zone of amphibolite and biotite schist 

(534–536 ft) in the biotite gneiss unit (fig. 5). 

From interpretations of borehole geophysical logs, fracture 

zones were delineated in the Pike Street well (14FF27) at 

depths of about 101–105 ft, 115–116 ft, 164–170 ft, 

178–179 ft, 293–296 ft, 331–349 ft, 363-398 ft, 423–430 ft, 

534–536 ft, and 571–572 ft (figs. 2 and 5). Good correlation 

of water-bearing zones and indicated fractured zones was 

observed from the caliper log and focused resistivity log of 

the Pike Street well (14FF27; figs. 2 and 5). The caliper log 

indicates an extensive fracture zone from about 330 to 420 

ft. The focused resistivity log extends the zone of low 

resistivity, interpreted as fracture concentrations, from about 

270–450 ft. Images of the fractures also were interpreted 

from the digital acoustic televiewer logs and video logs. 

Temperature and fluid resistivity inflections were noted near 

the 330–420 ft fracture zone. 

Distinguishing lithologic units using the gamma logs is 

somewhat difficult in the Pike Street well. However, 

recognizably higher readings of more than 350 APIU from 

about 95–130 ft are associated with the granite gneiss (figs. 

2 and 5).

Gwinnett County Airport Well

A corehole was drilled at the Gwinnett County Airport (well 

14FF42; plate 1; fig. 2; table 1) in 1996 to expand the hydrogeologic database. This well was drilled to a total depth of 

about 600 ft; penetrating the amphibolite unit from land 

surface to about 100 ft. Below the amphibolite unit, a mixed 

sequence of the biotite gneiss unit and the button schist unit 

was noted from 100 to 600 ft (figs. 2 and 6). The well has a 

small estimated total yield of about 10 gal/min. The few 

fractures noted during drilling, and from the caliper and 

focused resistivity logs were from 40–41 ft, 66–69 ft, 76–79 

ft and 81–83 ft in the amphibolite unit, and from about 361–

363 ft in the biotite gneiss unit (fig. 6. p. 30–31). The temperature log indicates borehole flow near the 76–79 ft and 

81–83 ft fractures. Additional fractures identified from core 

samples are shown in figure 6 (p. 30-31). Generally, the 

biotite gneiss and button schist units are nearly impermeable. The gamma log of well 14FF42 (figs. 2 and 6) shows 

much higher overall readings within the coarser, muscoviterich button schist unit, and characteristically low readings 

within the amphibolite unit. The biotite gneiss unit had a 

comparable gamma baseline to the amphibolite unit. The 

biotite gneiss unit in this area is mixed, having significant 

layers of minor amphibolite (fig. 6, p. 30–31).

Analyses of Ground-Water Levels

A major objective of this study is to evaluate the effects of 

ground-water withdrawals on the bedrock aquifer in 

Lawrenceville. Continuous water-level data, recorded in 

wells at 30-minute intervals, have been collected from 

March 1995 through the present (June 1998). Analyses of 

continuous ground-water-level data suggest that the 

response of the aquifer (drawdown and recovery) is directly 

related to the volume removed and the overall stress on the 

ground-water system.

Tharpe and others (1997) discussed the effects of groundwater withdrawals at the Rhodes Jordan Wellfield on both 

local and areal bedrock water levels. Typical pumping 

cycles and the resulting drawdown and recovery in bedrock 

observation wells in the immediate vicinity of the production well, and two bedrock wells located at distances of 0.9 

and 1.0 mi, were discussed. The usual pumping cycle at the 

wellfield is five days on and two days off. Drawdown in 

bedrock wells near the production well typically exceeds 

70 ft during the first 5-day pumping cycle. The effects on 

nearby regolith ground-water levels also were analyzed during a one-month period. However, during the summer, when 

demand is high, the pumping period has been extended to as 

many as 62 days. When the well is allowed to recover for 

the usual 2-day weekend period, a net recovery is evident in 

bedrock ground-water levels when pumping resumes. 

Tharpe and others (1997) also discussed the areal effects of 

pumping from the Rhodes Jordan Wellfield. Two bedrock 

observation wells located 0.9 mi N80W (Maltbie St. well 

14FF08, plate 1, fig. 1) and 1.0 mi S80W (Pike St. well 

14FF27) from the production well, were monitored. Weekly 

pumping cycles are evident in records of ground-water 

levels in both wells. The Maltbie Street well (14FF08) had 

the greater drawdown. Additional significance was noted, in 

that these two bedrock observation wells are located in a 

different drainage basin than the Rhodes Jordan Wellfield 

production well. The Rhodes Jordan Wellfield is located in 

the Alcovy River basin, the Maltbie Street observation well 

(14FF08) is located in the Yellow River basin, and the Pike 

Street observation well (14FF27) is located near the divide 

between the two basins.
Ground-water resources 29

Response of Local Ground-Water Levels 

to Pumpage

As part of this study, continuous ground-water-level data 

were collected from the Rhodes Jordan Wellfield since 

March 1995, to observe the response of the ground-water 

system to pumpage. In general, ground-water levels are 

measured by pressure transducers and recorded at 

30-minute intervals. As other wells have been drilled or 

refurbished in the Lawrenceville area, the network of 

observation wells has expanded to include three additional 

sites; the Pike Street site in November 1995, Maltbie 

Street site in December 1995, and the Gwinnett County 

Airport site in June 1996. The effects of pumpage, 

both during net drawdown and net recovery conditions, 

have been documented across surface-water 

drainage divides.

Although a permit for withdrawal of up to 300,000 gal/d 

was issued for the Rhodes Jordan production well (14FF10) 

in 1993 (Mr. Juan Ruiz, E&C Consulting Engineers, Inc., 

oral commun., 1996), ground-water withdrawals at the 

Rhodes Jordan Wellfield only were periodic until the 

summer of 1995. If the daily pumping and recovery 

periods, as well as the pumping rate remain fairly constant, 

the bedrock aquifer is dewatered to a depth near a 

productive fracture, and the rate of drawdown gradually 

decreases. From geophysical log interpretations (not 

presented graphically in this report) and ground-water-level 

observations, the primary water-producing fractures are at 

depths of 128–130.5 ft and 172.5–178 ft in production well 

14FF10. During mid-July through mid-September 1995, the 

production well was pumped for 62 consecutive days. 

Pumping duration averaged about 15 hours during this 

period, and the rate of withdrawal was about 245 gal/min. 

Data from observation well 14FF17 (fig. 7) show the 

response of ground-water levels in the bedrock aquifer 

during the pumping period that began on July 19, 1995, and 

ended on September 18, 1995. This extended pumping 

period yielded about 16.4 million gallons (Mgal) of ground 

water, and total drawdown was about 108 ft. Ground-water 

levels recovered to prepumping levels after about 11 days 

(Tharpe and others, 1997). 

Various hydrologic conditions have been observed since 

pumping resumed at the Rhodes Jordan Wellfield on 

February 1, 1996, continuing through 1997. The bedrock 

ground-water system reached a full recovery after the 

shutdown of the production well 14FF10 on September 18, 

1995, and maintained that level for several months. 

Pumping resumed in early February at a rate of about 230 

gal/min. The primary pumping schedule used by the City of 

Lawrenceville, is 5 days on and 2 days off (weekend), 

resulting in weekly withdrawals ranging from about 1.2 to 

1.5 Mgal (E&C Consulting Engineers, Inc., written 

commun., 1996). The early drawdown during the first 

week (5 days) of pumping was about 75 ft; weekend 

recovery ranged from about 15 to 30 ft. As pumping 

continues, weekly and daily drawdown decrease, and 

ground-water levels approach equilibrium with the 

pumping rate. Although “steady-state” conditions may 

never actually occur, weekly net drawdown in the bedrock 

aquifer may be only 1 to 2 ft. In general, at the 230 gal/min 

pumping rate, the lowest ground-water levels observed 

have been about 135 ft below land surface at the Rhodes 

Jordan Wellfield. 

As the demand for water supply in the City of 

Lawrenceville increased, particularly during the summer 

months, the pumping cycle was extended to as many as 

62 days. When the pumping period was extended on a 

daily basis by the number of hours per day, or by 

several days (assuming the pumping rate remains 

constant), a unique hydrologic condition of net recovery 

was observed. 

The unusual hydrologic condition of net recovery in the 

bedrock aquifer was observed after a 12-day pumping cycle 

in September 1996 (fig. 8). Well 14FF10 was pumped at an 

average rate of about 0.26 Mgal/d, and allowed to recover 

for only a few hours each day. Total volume pumped during 

this 12-day cycle was about 3.1 Mgal. At the conclusion of 

the 12-day pumping cycle, the well was shut down to 

recover for the usual 2-day weekend period. As the 5-day, 

weekly pumping cycle resumed, instead of the normal loss 

or decline of hydraulic head in the bedrock aquifer, the net 

effect was an increase, or gain, in bedrock ground-water 

levels of about 6 ft over a period of about 3 weeks (fig. 8). 

The weekly pumping volume did not decrease, but rather 

increased from 1.26 to 1.38 Mgal during the observed 

“recovery” period (Tharpe and others, 1997). This apparent 

recovery condition has been observed several times during 

this study. 

The cause of the net recovery observed during pumping is 

indiscernible at this time. The areal influence of pumping 

may extend across a hydraulic boundary in the groundwater system, or the ground-water system may be 

exhibiting a pressure response, resulting in a more regional 

rate of recovery that is greater than pumping withdrawals at 

the Rhodes Jordan Wellfield. Or, the recovery may reflect 

the presence of steeper hydraulic gradients, from the overstressed period, resulting in larger induced ground-water 

flow to the wellfield, that is apparently unaffected by the 

general 5-day pumping cycle. The influence of significant 

rainfall on bedrock ground-water levels is uncertain at this 

time. However, this unusual recovery response has been 

observed during relatively dry periods in the spring, 

summer, and fall. 
30 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

5.5 6 6.5 7 14FF42

Caliper, in inches

DEPTH BELOW LAND SURFACE, IN FEET

Total estimated yield

10 gallons per minute

0

50

100

150

200

250

300

350

400

450

500

550

600

Amphibolite

Biotite gneiss

Button schist

Fracture

Amphibolite

Biotite gneiss

Interlayered Button

 schist/Biotite gneiss/

 Amphibolite

Button schist

Amphibolite/

 Biotite gneiss

Figure 6. Fracture identification and relative comparison of fractures in Gwinnett County Airport 

well 14FF42 using borehole geophysical logs.

Bit size change

Fracture zone—

 Length tapped 

 by borehole

Lithologic unit

Rock type

EXPLANATION
Ground-water resources 31

0 500 250 750 1,000 1,250 1,500 1,750 0 50 150 100 200 250 350 300 16 17 18 19

Focused resistivity,

in ohm-m

Gamma, in APIU

(American Petroleum Institute Units)

Temperature, in

degrees Celsius
32 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia WATER LEVEL, IN FEET BELOW LAND SURFACE

Well 14FF17

August

1995

June July

 0

50

100

150

29 7 6 13 20 27 3 4 10 17 24 31

September October

14 21 28

Figure 7. Continuous ground-water levels at the Rhodes Jordan Wellfield 

during June 29–October 4, 1995. (Data recorded in 30-minute intervals.)
Ground-water resources 33

60

70

80

90

100

110

120

130

WATER LEVEL, IN FEET BELOW LAND SURFACE

3 7 11 12 15 15 19 23 27 31 4 8

September October

Well 14FF17

3.12 million gallons

(Mgal)

Pumping/

recovery cycle

1.26

Mgal

1.32

Mgal

1.38

Mgal

1.48

Mgal

1996

Figure 8. An extended 18-day pumping period and ground-water-level

recovery at the Rhodes Jordan Wellfield during September 3–October 15, 

1996. (Data recorded in 30-minute intervals.)
34 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

Continuous ground-water-level data also indicate that 

ground-water withdrawals in the Rhodes Jordan Wellfield 

affect water levels in the overlying regolith near the 

production well. Weekly pumping cycles are evident in the 

hydrograph of regolith observation well 14FF43 (figs. 1, 2, 

and 9a), located about 950 ft southwest of bedrock 

production well 14FF10 (figs. 1, 2, and 9b, data from nearby 

observation well 14FF16)). The time lag for the response of 

water levels in well 14FF43 to the weekly cycles of the 

production well averaged about 8 hours during October 

1996. During these 5-day pumping cycles, regolith groundwater levels in well 14FF43 declined a maximum of about 

0.11 ft; whereas the water level in nearby bedrock well 

14FF16 (fig. 9b), located about 15 ft from the pumped well, 

declined as much as 35 ft per weekly cycle (Tharpe and 

others, 1997).

Areal Effects of Ground-Water Withdrawals

Continuous ground-water-level data indicate that pumping 

at the Rhodes Jordan Wellfield directly affects water levels 

in the Maltbie Street well (14FF08, located about 0.9 mi 

N80W of the wellfield), and the Pike Street well (14FF27, 

located about 1 mi S80W of the wellfield) (figs. 1 and 10). 

The Maltbie Street well is in a separate surface-water 

drainage basin (Yellow River basin), compared with the 

Rhodes Jordan Wellfield, which is in the Alcovy River 

basin. The Pike Street well is located near the drainage 

divide between these two watersheds. 

The areal effects of pumping are evident in the similarity 

between hydrographs from the Rhodes Jordan Wellfield, 

Maltbie Street, and Pike Street wells (shown in figs.10a, b, 

and c, respectively). In general, the magnitude of drawdown 

per weekly pumping cycle is greater in the Maltbie Street 

well (0.75 to 3.10 ft) than in the Pike Street well (0.34 to 

0.82 ft). This indicates that the hydraulic connection 

between pumped well 14FF10 and observation well 14FF08 

is greater than between wells 14FF10 and 14FF27. These 

observations suggest that ground-water recharge areas may 

not be limited to watershed basins, and suggest the presence 

of connecting fracture systems between the wells at a 

distance of at least 1 mi (Tharpe and others, 1997).

Ground-Water Quality

As part of the evaluation of the geochemical setting and 

potential impact of urban activities on ground-water quality 

in the fractured crystalline-bedrock aquifer near 

Lawrenceville, ground-water samples were collected during 

October–November 1995 from the Rhodes Jordan Wellfield 

(table 2), and from the wellfield and outlying bedrock 

observation wells during August 1996 (table 3). A sample of 

surface water was collected from City Lake at the Rhodes 

Jordan Wellfield during both sampling events. The October–

November 1995 samples are representative of a recovering 

ground-water system. After periodic pumping in the spring, 

and constant pumping from mid-July through midSeptember 1995, the Rhodes Jordan production well 

(14FF10) was shutdown about 6 weeks prior to sampling. 

The August 1996 samples are representative of a stressed 

ground-water system. Well 14FF10 had been pumped, 

generally, on a 5-day-per-week schedule, since February 

1996. Samples were analyzed for major ions and volatile 

organic compounds. 

In general, the composition of water collected from the 

bedrock wells, regolith wells, and the lake is similar; 

calcium and bicarbonate are the dominant cation and anion, 

respectively (figs. 11 and 12). Two exceptions are samples 

collected from wells 14FF16 and 14FF42, which have 

sulfate as the dominant anion (tables 2 and 3). Sulfate 

concentrations were as high as 300 milligrams per liter 

(mg/L) in well 14FF16 in October 1995. A high 

concentration of iron, 44 mg/L, also was noted in the 

October 1995 sample from well 14FF16. Mixing problems 

were evident, as field parameters did not stabilize in well 

14FF16 during that sampling event. Iron and sulfate 

concentrations have been observed to fluctuate during 

pumping in well 14FF10. These fluctuations most likely are 

related to variable oxidation/reduction conditions in the 

wells, during drawdown (dewatering of fracture zones) and 

subsequent recovery of ground-water levels during pump 

shutdown. The oxidation of minerals such as pyrite (FeS) in 

the rock and fractures in wells occurs during dewatering, 

breaking down pyrite through the oxidation of iron and 

dissolution of sulfate ions. The subsequent introduction of 

reduced bedrock ground water during recovery may return 

iron to solution (Hem, 1989). 

Ground water collected from the regolith wells generally 

has lower concentrations of major ions (most likely from the 

lack of direct contact with crystalline bedrock), and has 

lower values of pH (most likely from the infiltration of 

acidic precipitation), than water from the bedrock wells. 

Analyses of ground-water samples collected during the two 

sampling events indicate that calcium concentrations range 

from about 9.8 to 60 mg/L in bedrock wells, from about 

2.4 to 15 mg/L in regolith wells, and is about 8.9 and 

8.6 mg/L in the lake samples (tables 2 and 4), respectively. 

Measurements of field alkalinities (bicarbonate 

concentrations) range from 45 to 103 mg/L in the bedrock 

wells, and from about 17 to 91 mg/L in the regolith wells. 

Water samples collected from the lake had an alkalinity 

value of 30 mg/L in August 1996. Specific-conductance 

field measurements range from 110 to 685 microSiemens 

per centimeter (μS/cm) for the bedrock wells, from 
Ground-water resources 35

(A) Well 14FF43

11.2

11.3

11.4

11.5

11.6

11.7

80

90

100

110

120

130

30 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 30

September October 

(B) Well 14FF16

Blank where

data missing

WATER LEVEL, IN FEET BELOW LAND SURFACE

1996

Figure 9. Influence of pumping from the bedrock aquifer on ground-water 

levels in the Rhodes Jordan Wellfield, September 30–October 30, 1996,

in (A) regolith well 14FF43, and (B) bedrock observation well 14FF16

(data recorded in 30-minute intervals).
36 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

(A) Rhodes Jordan Wellfield

 Wells 14FF16, 14FF17, and 14FF18

40

60

80

100

120

140

160

May June July Aug Sept Oct Nov Dec Jan

1996 1997

(B) Maltbie Street

 Well 14FF08

WATER LEVEL, IN FEET BELOW LAND SURFACE

14

16

18

20

22

Blank where data

are missing

Blank

where data

are missing

Figure 10. Influence of pumping at the Rhodes Jordan Wellfield on groundwater levels in bedrock observation wells, May 1, 1996 to January 20, 1997.
Ground-water resources 37

WATER LEVEL, IN FEET BELOW LAND SURFACE

(C) Pike Street

 Well 14FF27

May June July Aug Sept Oct Nov Dec Jan

1996 1997

12.0

12.5

13.0

13.5

14.0

14.5

15.0

Blank where data

are missing

Figure 10. Influence of pumping at the Rhodes Jordan Wellfield on 

ground-water levels in bedrock observation wells, May 1, 1996 to 

January 20, 1997—continued.
38 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

Table 2. Physical properties and concentrations of inorganic constituents in ground-water samples collected from the Rhodes 

Jordan Wellfield during October–November 1995

[mm, millimeter; mg/L, milligram per liter; μg/L, microgram per liter; μS/cm, microSiemens per centimeter at 25 degrees Celsius; CaCO3, 

calcium carbonate; <, less than; nd, none detected; na, not analyzed; —, not applicable]

Physical property or 

constituent, 

in units

Reporting 

limit, 

in units

Sampling site and date

Bedrock wells Regolith wells Surface 

water

14FF16 

10/31/95

14FF17 

10/30/95

14FF18 

10/30/95

14FF39 

10/31/95

14FF36 

11/06/95

14FF37 

11/07/95

City Lake 

11/06/95

Physical properties

Specific conductance (field), in 

μS/cm 

1.0 685 215 227 243 80 na 85

Specific conductance 

(laboratory), in μS/cm

1.0 651 230 244 243 78 174 83

pH (field), in standard units — 6.3 7.1 6.9 6.6 5.4 na 6.4 

pH (laboratory), in standard 

units 

— 6.0 6.8 6.8 6.7 5.9 5.9 7 

Water temperature (field), in 

degrees Celsius

— 17.9 17.3 18 17 na na 13 

Dissolved oxygen (field), 

in mg/L

— 0.04 0.03 0.04 0.03 0.04 na 5.1 

Air pressure (field), in mm of 

mercury 

— na 746 746 na na na na 

 Major dissolved constituents

Hardness as CaCO3, in mg/L 1.0 253 95 88 104 24 65 29 

Dissolved solids, in mg/L 1.0 505 153 157 151 52 119 47

Calcium, dissolved, in mg/L 0.02 60 26 23 25 3.5 13 8.9 

Magnesium, dissolved, in mg/L 0.004 25 7.3 7.5 10 3.7 8.0 1.6 

Sodium, dissolved, in mg/L 0.06 10 7.0 9.9 5.0 3.3 7.5 1.9 

Potassium, dissolved, in mg/L 0.1 4.8 3.3 3.5 2.5 1.2 1.0 3.5 

Alkalinity (field), in mg/L — 45 96 80 103 na na na 

Alkalinity (laboratory), in mg/L 1.0 na 100 74 105 na 66 25

Sulfate, dissolved, in mg/L 0.1 300 10 30 13 1.3 13 6.8 

Chloride, dissolved, in mg/L 0.1 6.7 4.9 8.2 3.9 4.0 3.3 1.8 

Fluoride, dissolved, in mg/L 0.1 0.2 0.4 0.7 0.2 0.2 0.2 0.2 

Bromide, dissolved, in mg/L 0.01 0.06 0.04 0.05 0.02 0.14 0.06 0.04

Silica, dissolved as Silicon 

oxide, in mg/L 

0.05 46 33 28 27 14 31 6.1 

Nitrite-Nitrate, dissolved as 

Nitrogen, in mg/L 

0.05 nd nd nd nd 1.2 0.32 0.25 

Iron, dissolved, in μg/L 10 44,000 760 1,800 970 17 <10 160

Manganese, dissolved, in μg/L 3.0 850 120 260 150 910 580 30 

Carbon dioxide, dissolved, in 

mg/L

0.1 12.8 na na na na na na
Ground-water resources 39

Table 3. Physical properties and concentrations of inorganic constituents in ground-water samples collected from the 

Rhodes Jordan Wellfield and outlying bedrock observation wells during August 1996

[mm, millimeter; mg/L, milligram per liter; μg/L, microgram per liter; μS/cm, microSiemens per centimeter at 25 degrees Celsius; 

CaCO3, calcium carbonate; nd, none detected; na, not analyzed; —, not applicable]

Physical properties or 

constituent, 

in units

Reporting 

limit

Bedrock well and sampling date

14FF08 

08/13/96

14FF10 

08/15/96

14FF16 

08/15/06

14FF17 

08/15/96

14FF18 

08/15/96

14FF27 

08/14/96

14FF39 

08/15/96

14FF42 

08/13/96

Physical properties

Specific conductance (field), 

in μS/cm

1.0 174 218 295 219 225 110 260 301

Specific conductance (laboratory), 

in μS/cm

1.0 209 238 320 238 239 122 282 335

pH (field), in standard units — 6.3 7.0 6.8 7.4 6.9 6.2 6.6 7.3

pH (laboratory), in standard units — 7.0 7.1 6.8 7.2 6.9 6.6 6.8 7.2

Water temperature (field), in 

degrees Celsius

— 17.1 18.0 19.2 19.0 19.3 18.1 18.5 18.6

Dissolved oxygen (field), in mg/L — 0.04 0.07 4.52 0.83 0.05 4.08 3.15 0.06

Air Pressure (field), in mm 

of mercury

— 734 740 740 740 740 738 740 734

Major constituents

Hardness as CaCO3, in mg/L 1.0 78 90 129 91 85 41 120 140

Dissolved solids, in mg/L 1.0 138 155 212 159 153 93 178 242

Calcium, dissolved, in mg/L 0.02 18 24 32 25 22.0 9.8 27 42

Magnesium, dissolved, in mg/L 0.004 8.0 7.4 12 7.0 7.2 3.9 13 9.6

Sodium, dissolved, in mg/L 0.06 7.4 8.8 6.6 11 9.4 7.9 5.8 5.7

Potassium, dissolved, in mg/L 0.1 3.8 3.4 3.4 3.3 3.4 2.5 2.7 2.9

Alkalinity (field), in mg/L 1.0 75 83 49 89 76 48 96 53

Alkalinity (laboratory), in mg/L 1.0 75 80 54 94 75 37 96 51

Sulfate, dissolved, in mg/L 0.1 17 24 91 13 28 4.2 39 110

Chloride, dissolved, in mg/L 0.1 5.9 6.7 3.8 7.1 7.5 7.7 3.9 1.4

Fluoride, dissolved, in mg/L 0.1 0.1 0.7 0.3 0.9 0.8 nd 0.2 nd

Bromide, in mg/L 0.01 0.03 0.07 0.03 0.06 0.05 0.04 0.02 0.06

Silica, dissolved, in mg/L 0.05 31 31 30 34 28 27 28 38

Nitrite-Nitrate, dissolved, in mg/L 0.05 0.06 nd 0.05 0.08 nd 1.7 0.1 0.06

Barium, total, in μg/L 100 nd na na na na na na na

Cobalt, total, in μg/L 0.1 2.0 na na na na na na na

Copper, total, in μg/L 0.1 nd na na na na na na na

Iron, dissolved, in μg/L 10 960 1,000 18 500 1,600 360 330 1,500

Lead, total, μg/L 1.0 nd na na na na na na na

Manganese, dissolved, in μg/L 3.0 94 170 190 110 270 17 160 110

Mercury, in mg/L 0.1 nd na na na na na na na

Nickel, total, in μg/L 0.1 5.0 na na na na na na na

Zinc, total, in μg/L 10 nd na na na na na na na

Cyanide, total, in mg/L 0.01 nd na na na na na na na

Carbon, organic, as C, dissolved, 

in mg/L

0.1 nd 0.2 na 0.2 0.2 nd 0.6 0.1

Carbon dioxide, dissolved, in mg/L 0.1 71.1 14.5 14.8 na na na na na
40 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

100

80

60

40

20

40

0

20

0

0

20

40

60

80

100 0

20

40

80

60

100

0

0

0

20

20

20

40

40

40

60

60

60

80

80

100

100

80

100

100

1

80

80

100

60

60

40

40

20

20

0

0

80

100

60

40

20

0

100

80

60 MAGNESIUM

SULFATE PLUS CHLORIDE

CARBONATE PLUS BICARBONATE

CALCIUM PLUS MAGNESIUM

SODIUM PLUS POTASSIUM

SULFATE

6

2

4 7

5 3

5

3

2

4

1 7

6

2 6

4 7

5

3

1

CALCIUM CHLORIDE

PERCENT

PERCENT

PERCENT

1 14FF16 Bedrock well 10-31-95

2 14FF17 Bedrock well 10-30-95

3 14FF18 Bedrock well 10-30-95

4 14FF39 Bedrock well 10-31-95

5 City Lake Surface water 11-06-95

6 14FF36 Regolith well 11-06-95

7 14FF37 Regolith well 11-07-95

Number Well ID Sampling-site type Date of sample

EXPLANATION

Figure 11. Trilinear diagram of water-quality analysis from ground-water and City Lake

water samples collected during October-November 1995.
Ground-water resources 41

100

80

60

40

20

40

0

20

0

0

20

40

60

80

100 0

20

40

80

60

100

0

0

0

20

20

20

40

40

40

60

60

60

80

80

100

100

80

100

100

80

80

100

60

60

40

40

20

20

0

0

80

100

60

40

20

0

100

80

60 MAGNESIUM

SULFATE PLUS CHLORIDE

CARBONATE PLUS BICARBONATE

CALCIUM PLUS MAGNESIUM

SODIUM PLUS POTASSIUM

SULFATE

CALCIUM CHLORIDE

PERCENT

PERCENT

PERCENT

1 14FF16 Bedrock well 08-15-96

2 14FF17 Bedrock well 08-15-96

3 14FF18 Bedrock well 08-15-96

4 14FF39 Bedrock well 08-15-96

5 City Lake Surface water 08-16-96

6 14FF36 Regolith well 08-16-96

7 14FF37 Regolith well 08-16-96

8 14FF38 Regolith well 08-16-96

9 14FF08 Bedrock well 08-13-96

10 14FF27 Bedrock well 08-14-96

11 14FF42 Bedrock well 08-13-96

12 14FF10 Bedrock well 08-15-96

4 3

9 12

5

2 7

8

1

11

8 6

2 10

5

7

9

12

3

4

1

11

11

5 2 10

6

7

8

4

1 9

12 3

10

6

Number Well ID Sampling-site type Date of sample 

EXPLANATION

Figure 12. Trilinear diagram of water-quality analysis from ground-water and City Lake

water samples collected during August 1996.
42 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

Table 4. Physical properties and concentrations of inorganic constituents in water samples 

collected from regolith wells and City Lake at the Rhodes Jordan Wellfield during August 1996

[mm, millimeter; mg/L, milligram per liter; μg/L, microgram per liter; μS/cm, microSiemens per 

centimeter at 25 degrees Celsius; CaCO3, calcium carbonate; <, less than; nd, none detected; 

na, not analyzed; —, not applicable]

Physical property 

or constituent, 

in units

Reporting 

limit

Sampling site and date

Regolith well Surface 

water

14FF36 

08/16/96

14FF37 

08/16/96

14FF38 

08/16/96

City Lake

08/16/96

Physical properties

Specific conductance (field), in μS/cm 1.0 59 147 180 87 

Specific conductance (laboratory), in μS/cm 1.0 56 151 188 85

pH (field), in standard units — 5.2 6.0 6.5 7.8

pH (laboratory), in standard units — 6.3 6.3 6.6 6.9 

Water temperature (field), in degrees Celsius — na na na na 

Dissolved oxygen (field), in mg/L — na na na na 

Air Pressure (field), in mm of mercury — na na na na 

Major constituents

Hardness, as CaCO3, in mg/L 1.0 16 50 76 30 

Dissolved solids, in mg/L 1.0 44 105 137 46

Calcium, dissolved, in mg/L 0.02 2.4 9.8 15 8.6 

Magnesium, dissolved, in mg/L 0.004 2.4 6.2 9.3 2.1 

Sodium, dissolved, in mg/L 0.06 3.3 7.9 6.1 2.7 

Potassium, dissolved, in mg/L 0.1 1.1 0.8 0.8 2.5 

Alkalinity (field), in mg/L 1.0 17 89 91 30

Alkalinity (laboratory), in mg/L 1.0 15 55 88 31

Sulfate, dissolved, in mg/L 0.1 1.0 13 2.8 6.0 

Chloride, dissolved, in mg/L 0.1 3.9 3.0 3.5 2.3 

Fluoride, dissolved, in mg/L 0.1 nd nd 0.2 0.1 

Bromide, in mg/L 0.01 0.2 0.04 0.06 nd

Silica, dissolve, in mg/L 0.05 15 29 44 3.1 

Nitrite-Nitrate, dissolved as Nitrogen, mg/L 0.05 1.3 0.49 0.26 0.05 

Iron, dissolved, in μg/l 10 <10 <10 190 16

Manganese, dissolved, in μg/L 3.0 51 170 840 <3.0 

Carbon, organic as C, dissolved, in mg/L 0.1 0.7 1.0 3.6 2.8 

Cyanide, total, in mg/L 0.01 na na na na
Ground-water resources 43

59 to 180 μS/cm for the regolith wells, and are 85 and 87 

μS/cm from the lake samples, respectively. Values of field 

pH range from about 6.2 to 7.4 for the bedrock wells, from 

about 5.2 to 6.5 for the regolith wells, and about 6.4 and 

7.8, from the lake, respectively (tables 2, 3, and 4). 

Results of the analyses for volatile organic compounds 

indicate the presence of nonregulated organic compounds in 

ground-water samples collected from the Rhodes Jordan 

Wellfield, and low-level concentrations of regulated 

compounds in samples collected from the Maltbie Street 

well (14FF08) and the Pike Street well (14FF27) (plate 1; 

fig. 1). A complete list of volatile organic compounds 

included in the analyses is presented in table 5. The primary 

nonregulated organic compound detected at the Rhodes 

Jordan Wellfield is methy-tert-butyl-ether (MTBE) (tables 6 

and 7). MTBE was detected in all of the bedrock 

observation wells at the Rhodes Jordan Wellfield in 

October-November 1995, and in several of the bedrock 

wells in August 1996, except observation well 14FF17. 

This compound also was detected at a concentration of less 

than 1 microgram per liter (μg/L) at the Gwinnett County 

Airport well (14FF42) (table 7). Concentrations of MTBE 

detected in wells at the Rhodes Jordan Wellfield range from 

about 0.6 to 12 μg/L in samples collected during October–

November 1995 (table 6), and about 0.6 to 26 μg/L in 

samples collected during August 1996 (table 7). In quarterly 

samples of the Rhodes Jordan production well (14FF10) 

collected by Kemron Environmental Services, Inc., during 

September–October 1996, and January, March, and June 

1997, concentrations of MTBE were somewhat consistent, 

ranging from about 21 to 25 μg/L (Kemron Environmental 

Services, Inc.,1997a–d).

Other organic compounds detected in the Lawrenceville 

wells include trace concentrations (less than 1 μg/L) at the 

Rhodes Jordan Wellfield of: tetrachloroethylene (PCE) in 

bedrock wells 14FF10 and 14FF16; and regolith wells 

14FF37 and 14FF38 in August 1996 (table 6); 1,1-

dichloroethane, cis-1,2-dichloroethene, and trichloroethylene 

(TCE) in well 14FF17, October–November 1995 (table 5); 

trichlorofluoromethane, and 1,1,1-trichloroethane in well 

14FF37, October–November 1995 (table 6) and 1,1-

dichlorethylene and 1,1,1-trichloroethane in August 1996 

(table 7). Organic compounds detected in samples collected 

from the Maltbie Street well (14FF08) in August 1996 

include 1.3 μg/L PCE, and less than 1 μg/L TCE, 

chloroform, and cis-1,2-dichloroethene (table 7). The 

sample collected from the Pike Street well (14FF27) in 

August 1996 contained 2.7 μg/L PCE and less than 1 mg/L 

chloroform (table 7). 

Table 5. Volatile organic compounds included in laboratory analyses

Dibromomethane

Dichlorobromo-methane

Carbon tetrachloride

1,2-Dichloroethane

Bromoform

Chlorodibromo-methane

Chloroform

Toluene

Benzene 

Chlorobenzene

Chloroethane

Ethylbenzene

Methyl bromide

Methyl chloride

Methylene chloride

Tetrachloroethylene (PCE)

Trichloroflouromethane

1,1-Dichloroethane

1,1-Dichloroethylene

1,1,1-Trichloroethane

1,1,2-trichloroethane

0-Dichlorobenzene

1,2-Dichloropropane

1,2-trans-dichloroethene

1,2-trans-dichlorobenzene

1,3-Dichlorobenzene

1,4-Dichlorobenzene

2-Chloroethylvinylether

Dichlorodifluoromethane

Napthalene

Trans-1,3-Dichloropropene

Cis-1,3-Dichloropropene

Trichloroethylene (TCE)

Hexachlorobutadiene

Vinyl chloride

Cis-1,2-Dichloroethene

Styrene

1,1-Dichloropropene

2,2-Dichloropropane 

1,3-Dichloropropane

1,2,4-Trimethylbenzene

Isopropyl benzene

N-Propylbenzene

1,3,5-Trimethylbenzene

0-Chlorotoluene

P-Chlorotoluene

Bromochloromethane

N-Butylbenzene

Sec-butylbenzene

Tert-butylbenzene

P-Isopropyltoluene

1,2,3-Trichloropropane

1,1,1,2-Tetrachloroethane

1,2,3-Trichlorobenzene

1,2-Dibromoethane

Methyl-tert-butyl-ether (MTBE)

Xylene

Bromobenzene

Dibromochloropropane

Trichloro-trifluoroethane
44 Geology and Ground-Water Resources of the Lawrenceville Area, Georgia

Table 6. Concentrations of volatile organic compounds detected in ground-water samples collected from the Rhodes Jordan 

Wellfield during October–November 1995

[Concentrations are in micrograms per liter (μg/L); nd, none detected; reporting limit is 0.2 μg/L for all compounds; <0.2, trace 

concentration detected]

Chemical compound, in units

Sampling site and date

Bedrock well Regolith well Surface 

water

14FF16 

10/31/95

14FF17 

10/30/95

14FF18 

10/30/95

 14FF39 

10/31/95

14FF36 

11/06/95

14FF37 

11/05/95

City Lake 

11/06/95

Trichloroflouromethane, in μg/L nd nd nd nd nd <0.2 nd

1,1-Dichloroethane, in μg/L nd <0.2 nd nd nd nd nd

1,1,1-Trichloroethane, in μg/L nd nd nd nd nd <0.2 nd

Trichloroethylene (TCE), in μg/L nd <0.2 nd nd nd nd nd

Cis-1,2-Dichloroethene, in μg/L nd <0.2 nd nd nd nd nd

Methyl-tert-buty-ether (MTBE), in μg/L 9.2 0.6 12.0 0.7 nd nd nd

Table 7. Concentrations of volatile organic compounds detected in ground-water samples collected from the Rhodes Jordan 

Wellfield and outlying bedrock wells during August 1996

[Concentrations are in micrograms per liter (μg/L); nd, none detected; reporting limit is 0.2 μg/L for all compounds; <0.2, trace concentration 

detected]

Chemical 

compound 

in units

Sampling site and date

Bedrock well Regolith well Surface 

water

14FF08 

08/15/96 

14FF10 

08/15/96 

14FF16 

08/13/96 

14FF17 

08/13/96 

14FF18 

08/15/96 

14FF27 

08/15/96 

14FF39 

08/15/96 

14FF42 

08/14/96

14FF36 

08/16/96 

14FF37 

08/16/96 

14FF38 

08/16/96

City Lake 

08/16/96

Chloroform, 

in μg/L

0.5 nd nd nd nd 0.2 nd nd nd nd nd nd

Tetrachloroethylene 

(PCE), in 

μg/L 

1.3 0.6 0.3 nd nd 2.7 nd nd nd <0.2 1.3 nd

1,1-Dichloroethylene, 

in μg/L 

nd nd nd nd nd nd nd nd nd <0.2 nd nd

1,1,1-

Trichloroethane, in 

μg/L 

nd nd nd nd nd nd nd nd nd 0.3 nd nd

Trichloroethylene 

(TCE), in 

μg/L

0.5 nd nd nd nd nd nd nd nd nd 0.2 nd

Cis-1,2-

Dichloroet

hene, in 

μg/L

0.6 nd nd nd nd nd nd nd nd nd nd nd

Methyl-tertbutyl-ether 

(MTBE), 

in μg/L

nd 19 0.6 nd 26 nd 5.7 <0.2 nd nd nd nd
Summary and Conclusions 45

SUMMARY AND CONCLUSIONS

The exploration for ground-water resources is being 

undertaken by many local municipalities in the Atlanta 

Metropolitan area because of the continued rapid pace of 

population growth, and the corresponding increased 

demand for water supplies. The igneous and metamorphic 

fractured bedrock aquifers in this area of the Piedmont 

physiographic province are highly complex and often 

difficult to characterize. The application of effective 

methods of characterization is essential to the success of 

these ground-water exploration programs. 

As part of an ongoing study conducted by the U.S. 

Geological Survey, in cooperation with the City of 

Lawrenceville, five major lithologic units were mapped in 

the Lawrenceville area at a scale of 1:24,000, as the basis of 

the regional hydrogeologic framework needed to apply field 

investigative techniques to characterize fractured 

crystalline-bedrock aquifers. These lithologic units are: 

granite gneiss, quartzite and aluminous schist, biotite 

gneiss, amphibolite, and button schist. These units generally 

are thin, a few hundred to a few thousand feet in outcrop 

width, and have low angles of dip (nearly 0 to 20 degrees). 

In many places, dip reversals occur over short distances 

inferring the presence of structural folding of the rocks. All 

of the units exhibit some shearing characteristics. 

Of the five lithologic units mapped in the study area, the 

amphibolite unit has the most potential for the production of 

ground-water resources. From limited data available at four 

well sites (data collected through 1997), subsurface 

fractures appear to be more numerous in the amphibolite 

compared with other rock types. Historically, two wells at 

the Rhodes Jordan Wellfield and a well at the Maltbie Street 

site, were known to have possibly the highest sustained 

yields in the Piedmont physiographic province of northern 

Georgia. Sustained production at the Rhodes Jordan 

Wellfield during 1995–1997 averaged 230 to 250 gallons per 

minute. The Maltbie Street well was tested at 350 gallons 

per minute in 1995. 

The apparent increased presence of fracturing in the 

amphibolite may be related to the location of that unit at the 

crest of the antiformal structure. Stress related to the folding 

may have enhanced fracturing potential of the amphibolite 

unit. Well-developed compositional layering also has 

created zones of weakness, vulnerable to both tectonic 

stress and weathering. With this compositional layering and 

a fracture network, subsequent unloading by erosion of 

broad valleys, could have resulted in “stress release” and 

opening of the fracture system. This envisioned “stress 

release” is comparable to the upward expansion of the rock 

as a result of the removal of overlying rocks in broad valleys

(elastic rebound).

The composition of water collected from the bedrock wells, 

regolith wells, and City Lake is similar; calcium and 

bicarbonate are the dominant cation and anion, respectively. 

Concentrations of major ions generally are lower in water 

from the regolith wells and the lake than in water from the 

bedrock wells. High concentrations of iron (44 milligrams 

per liter) and sulfate (300 milligrams per liter) also were 

detected in samples from the bedrock wells. Volatile organic 

compounds were detected in most wells from water samples 

collected during October–November 1995 and August 1996. 

Trace concentrations, generally less than 1 microgram per 

liter, of tetrachloroethylene, trichloroethylene, 

1,1-dichloroethane, 1,1-dichloroethylene, 

trichlorofluoromethane, 1,1,1-trichloroethane, 

cis-1,2-dichloroethene, and chloroform were detected from 

bedrock wells located in urban areas. Elevated 

concentrations of one compound, methyl-tert-butyl-ether, 

commonly known as MTBE, were detected in wells at the 

Rhodes Jordan Wellfield. Detected concentrations of MTBE

range from 0.6 to 12 micrograms per liter in October–

November 1995, and from 0.6 to 26 micrograms per liter in 

August 1996. MTBE also was detected at a concentration of 

less than one microgram per liter in a sample from the 

Gwinnett County Airport well. 

Analyses of continuous ground-water-level data suggest 

that the response of the aquifer (drawdown and recovery) is 

directly related to the volume of water removed and the 

overall stress on the ground-water system. If the daily 

pumping and recovery periods, as well as the pumping rate 

remain fairly constant, the aquifer is dewatered to a depth 

near a productive fracture, and the rate of drawdown 

gradually decreases. However, if the pumping period is 

extended on a daily basis (increased pumping hours), or by 

several days (assuming the pumping rate remains constant), 

a unique hydrologic condition of net recovery has been 

observed. The effects of pumpage, both during net 

drawdown and net recovery conditions, have been 

documented across surface-water drainage divides.

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