U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5273

Prepared in cooperation with (in alphabetical order) 

the Arkansas Natural Resources Commission, Arkansas Oil and Gas Commission, 

Duke University, Faulkner County, Shirley Community Development Corporation, and 

the University of Arkansas at Fayetteville, and 

the U.S. Geological Survey Groundwater Resources Program 

Shallow Groundwater Quality and Geochemistry in the 

Fayetteville Shale Gas-Production Area, North-Central 

Arkansas, 2011
Cover:

Left, Drilling rig and equipment used in the Fayetteville Shale gas-production area, north-central

Arkansas. 

Right, Pond with synthetic liner used to store water at shale gas-production facility in the Fayetteville

Shale area, north-central Arkansas.

Bottom, Freshwater pond and distribution lines for source water used in drilling and hydrofracturing in

the Fayetteville Shale gas-production area, north-central Arkansas. 

All photographs by Timothy M. Kresse, U.S. Geological Survey.
Shallow Groundwater Quality and 

Geochemistry in the Fayetteville Shale 

Gas-Production Area, North-Central 

Arkansas, 2011

By Timothy M. Kresse, Nathaniel R. Warner, Phillip D. Hays, Adrian Down, 

Avner Vengosh, Robert B. Jackson

Prepared in cooperation with (in alphabetical order) 

the Arkansas Natural Resources Commission, Arkansas Oil and Gas Commission, 

Duke University, Faulkner County, Shirley Community Development Corporation, 

and the University of Arkansas at Fayetteville, and 

the U.S. Geological Survey Groundwater Resources Program

Scientific Investigations Report 2012–5273

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Kresse, T.M., Warner, N.R., Hays, P.D., Down, A., Vengosh, A., and Jackson, R.B., 2012, Shallow groundwater quality and 

geochemistry in the Fayetteville Shale gas-production area, north-central Arkansas, 2011: U.S. Geological Survey 

Scientific Investigations Report 2012–5273, 31 p.
iii

Acknowledgments

The authors would like to thank numerous persons for their assistance during all phases 

of the study. Several people were instrumental and deserve special recognition in the early 

reconnaissance phase involving the petitioning of homeowners interested in participating in the 

study and collecting all pertinent information including home addresses and well information. 

Tom Kimmons of the Shirley Community Development Corporation gathered information 

related to homeowners with domestic wells in Van Buren County, and also assisted in locating 

the homes and interacting with the homeowners during sample collection in Van Buren County. 

Faulkner County Judge Preston Scroggin, Randy Higgins, and Randy Ingram were instrumental 

in spearheading efforts related to similar activities in Faulkner County. All homeowners are 

acknowledged for their participation in the study, their kindness extended to the sampling 

teams, and their sharing of information related to land-use and gas-production activities in each 

of the counties.

The authors additionally would like to thank Jay Hansen, Senior Petroleum Geologist with 

the Arkansas Oil and Gas Commission, for information related to the chemistry of flowback 

and produced waters associated with gas production in the Fayetteville Shale. Larry Bengal, 

Director of the Arkansas Oil and Gas Commission, deserves a special thanks for sharing 

his knowledge and insight into every aspect of gas-production in Arkansas. Jon Karr, Duke 

University Environmental Isotope Laboratory assisted in the isotopic measurements of water 

and gases. Finally, Roger Miller, Arkansas Department of Environmental Quality, for providing 

data on organic analysis of samples from domestic wells in the Fayetteville Shale production 

area. The U.S. Geological Survey’s Groundwater Resources Program provided supplementary 

funding for completion of this report.

v

Contents

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

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

Purpose and Scope .............................................................................................................................2

Description of Study Area ..................................................................................................................2

Overview of Hydrogeologic Setting ..................................................................................................4

Water Sample Collection and Analysis Methods ....................................................................................6

Shallow Groundwater Quality and Geochemistry....................................................................................7

Historical Groundwater Quality and Geochemistry .......................................................................8

Current Study Groundwater Quality and Geochemistry ................................................................9

Groundwater Quality and Spatial and Temporal Comparative Analysis ............................9

Groundwater Geochemistry and Geochemical Evolution ..................................................11

Rock-Water Interaction ..................................................................................................13

Reduction-Oxidation Processes ....................................................................................19

Methane and Carbon Isotopes ......................................................................................20

Trace Metals .....................................................................................................................24

Limitations Associated with Data Interpretation ...................................................................................26

Implications of Study Results ...................................................................................................................27

Summary .......................................................................................................................................................27

References Cited .........................................................................................................................................29

Appendixes (available online only)

1. Results of spring and fall 2012 Standard Reference Sample (SRS) inter-laboratory 

comparison study

2. Historical groundwater-quality analyses extracted from the U.S. Geological 

Survey National Water Information System database for Cleburne, Conway, 

Faulkner, Independence, Van Buren, and White Counties, Arkansas

3. Water-quality, isotope, and methane data collected from domestic wells in 

Van Buren and Faulkner Counties, Arkansas, July 2011 through November 2011

Figures

1. Map showing location of the Fayetteville Shale study area sampling sites and 

gas-production wells ..................................................................................................................3

2. Map showing geology of the Fayetteville Shale study area with generalized 

geologic cross section ................................................................................................................5

3. Graph showing relation between dissolved solids concentrations and specific 

conductance values for historical data (1951–83) in the Fayetteville Shale gasproduction area, north-central Arkansas ................................................................................9

4. Graph showing relation between total cation and anion milliequivalent 

concentrations for historical data (1951–83) in the Fayetteville Shale gasproduction area, north-central Arkansas ..............................................................................10

5. Boxplots for selected constituents for A, historical data and B, data from 127 

wells in the Fayetteville Shale gas-production area, north-central Arkansas ................12

6. Conceptual model of geochemical evolution of groundwater with increased 

rock-water interaction represented by increasing dissolved solids 

concentrations ...........................................................................................................................13
vi

7. Graph showing relation between dissolved solids concentrations and pH 

values from data collected in the Fayetteville Shale gas-production area, northcentral Arkansas ........................................................................................................................14

8. Graph showing relation between dissolved solids concentrations and 

A, bicarbonate, B, sulfate, C, chloride, D, calcium, E, magnesium, and F, sodium 

concentrations from data collected in the Fayetteville Shale gas-production 

area, north-central Arkansas ..................................................................................................15

9. Graph showing relation between sodium/chloride ratios and A, calcium plus 

magnesium/bicarbonate ratios constructed using milliequivalent concentrations 

and B, dissolved solids concentrations from data collected in the Fayetteville 

Shale gas-production area, north-central Arkansas ...........................................................16

10. Trilinear diagram showing the percentage distribution of major ions from data 

collected in the Fayetteville Shale gas-production area, north-central Arkansas ........17

11. Graph showing relation between dissolved solids concentrations and pH values 

with a depiction of the evolution of chemical water types from data collected in 

the Fayetteville Shale gas-production area, north-central Arkansas ..............................18

12. Graph showing relation between dissolved solids concentration and chloride 

as percent of total anions in milliequivalents per liter from data collected in the 

Fayetteville Shale gas-production area, north-central Arkansas .....................................18

13. Graph showing relation between nitrate and A, iron and B, manganese 

concentrations from data collected in the Fayetteville Shale gas-production 

area, north-central Arkansas ..................................................................................................21

14. Graph showing relation between nitrate concentrations and pH values from data 

collected in the Fayetteville Shale gas-production area, north-central Arkansas ........22

15. Graph showing relation between dissolved solids concentrations and A, iron 

and B, arsenic concentrations from data collected in the Fayetteville Shale 

gas-production area, north-central Arkansas ......................................................................23

16. Graph showing relation between dissolved solids and methane concentrations 

from data collected in the Fayetteville Shale gas-production area, northcentral Arkansas ........................................................................................................................24

17. Graph showing relation between dissolved solids and boron concentrations 

from data collected in the Fayetteville Shale gas-production area, north-central 

Arkansas .....................................................................................................................................25

18. Graph showing relation between dissolved solids and strontium concentrations 

from data collected in the Fayetteville Shale production area, north-central 

Arkansas .....................................................................................................................................25

Tables

1. Descriptive statistics for selected water-quality constituents from historical 

data collected from 1951 through 1983 from 43 wells in the Fayetteville Shale 

gas-production area, north-central Arkansas ........................................................................8

2. Descriptive statistics for selected water-quality constituents from data 

collected in 2011 from 127 wells in Van Buren and Faulkner Counties, Arkansas .........11
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29)

Altitude, as used in this report, refers to distance above the vertical datum.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Shallow Groundwater Quality and Geochemistry in the 

Fayetteville Shale Gas-Production Area, North-Central 

Arkansas, 2011

By Timothy M. Kresse1, Nathaniel R. Warner2, Phillip D. Hays1, Adrian Down2, Avner Vengosh2, and 

Robert B. Jackson2

Abstract

The Mississippian Fayetteville Shale serves as an 

unconventional gas reservoir across north-central Arkansas, 

ranging in thickness from approximately 50 to 550 feet and 

varying in depth from approximately 1,500 to 6,500 feet 

below the ground surface. Primary permeability in the 

Fayetteville Shale is severely limited, and successful 

extraction of the gas reservoir is the result of advances 

in horizontal drilling techniques and hydraulic fracturing 

to enhance and develop secondary fracture porosity and 

permeability. Drilling and production of gas wells began in 

2004, with a steady increase in production thereafter. As of 

April 2012, approximately 4,000 producing wells had been 

completed in the Fayetteville Shale.

In Van Buren and Faulkner Counties, 127 domestic 

water wells were sampled and analyzed for major ions and 

trace metals, with a subset of the samples analyzed for 

methane and carbon isotopes to describe general water 

quality and geochemistry and to investigate the potential 

effects of gas-production activities on shallow groundwater 

in the study area. Water-quality analyses from this study 

were compared to historical (pregas development) shallow 

groundwater quality collected in the gas-production area. 

An additional comparison was made using analyses from 

this study of groundwater quality in similar geologic and 

topographic areas for well sites less than and greater than 

2 miles from active gas-production wells.

Chloride concentrations for the 127 groundwater 

samples collected for this study ranged from approximately 

1.0 milligram per liter (mg/L) to 70 mg/L, with a median 

concentration of 3.7 mg/L, as compared to maximum and 

median concentrations for the historical data of 378 mg/L 

and 20 mg/L, respectively. Statistical analysis of the data sets 

revealed statistically larger chloride concentrations (p-value 

<0.001) in the historical data compared to data collected 

for this study. Chloride serves as an important indicator 

parameter based on its conservative transport characteristics 

and relatively elevated concentrations in production waters 

associated with gas extraction activities. Major ions and trace 

metals additionally had lower concentrations in data gathered 

for this study than in the historical analyses. Additionally, no 

statistical difference existed between chloride concentrations 

from water-quality data collected for this study from 94 wells 

located less than 2 miles from a gas-production well and 

33 wells located 2 miles or more from a gas-production well; a 

Wilcoxon rank-sum test showed a p-value of 0.71. 

Major ion chemistry was investigated to understand 

the effects of geochemical and reduction-oxidation (redox) 

processes on the shallow groundwater in the study area along 

a continuum of increased rock-water interaction represented 

by increases in dissolved solids concentration. Groundwater 

in sandstone formations is represented by a low dissolved 

solids concentration (less than 30 mg/L) and slightly acidic 

water type. Shallow shale aquifers were represented by 

dissolved solids concentrations ranging upward to 686 mg/L, 

and water types evolving from a dominantly mixedbicarbonate and calcium-bicarbonate to a strongly sodiumbicarbonate water type. 

Methane concentration and carbon isotopic composition 

were analyzed in 51 of the 127 samples collected for 

this study. Methane occurred above a detection limit of 

0.0002 mg/L in 32 of the 51 samples, with concentrations 

ranging upward to 28.5 mg/L. Seven samples had methane 

concentrations greater than or equal to 0.5 mg/L. The carbon 

isotopic composition of these higher concentration samples, 

including the highest concentration of 28.5 mg/L, shows the 

methane was likely biogenic in origin with carbon isotope 

ratio values ranging from -57.6 to -74.7 per mil. Methane 

concentrations increased with increases in dissolved solids 

concentrations, indicating more strongly reducing conditions 

with increasing rock-water interaction in the aquifer. As such, 

groundwater-quality data collected for this study indicate 

that groundwater chemistry in the shallow aquifer system in 

the study area is a result of natural processes, beginning with 

recharge of dilute atmospheric precipitation and evolution 

1

U.S. Geological Survey, Little Rock, Ark. 

2

Division of Earth and Ocean Sciences, Nicholas School of the 

Environment, Duke University, Durham, N.C.
2 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

of observed groundwater chemistry through rock-water 

interaction and redox processes. 

Introduction

Gas production in the Fayetteville Shale area of northcentral Arkansas has provided important economic benefits for 

the State and the individual counties (University of Arkansas, 

2008); however, local citizens and citizen groups have voiced 

numerous concerns regarding potential public health and 

environmental effects associated with gas production. Issues 

important to citizens include potential effects to surface-water 

and groundwater quality, noise, traffic, road damage, and air 

pollution. These concerns parallel those voiced in other newly 

developed shale-gas production areas throughout the country, 

where oil and gas development is moving into communities 

where residents have no prior history and experience 

with these activities and the potential effects to their local 

resources, infrastructure, health, and culture (New York Times, 

variously dated).

One of the most common concerns is related to perceived 

degradation of groundwater quality in domestic wells. 

Appropriate groundwater-quality data to address groundwater 

concerns are lacking, and up through the summer of 2010, 

no water-quality sampling had been performed to confirm 

or deny anecdotal evidence of groundwater contamination 

in Arkansas from gas-production activities. The purpose of 

this study by the U.S. Geological Survey, in cooperation with 

the Arkansas Natural Resources Commission, Arkansas Oil 

and Gas Commission, Duke University, Faulkner County, 

Shirley Community Development Corporation, the University 

of Arkansas at Fayetteville, and the U.S. Geological Survey 

Groundwater Resources Program, was to sample groundwater 

from domestic wells, document groundwater quality and 

geochemistry, and investigate potential effects to groundwater 

quality from gas-production activities in Faulkner and Van 

Buren Counties, Arkansas.

The Mississippian Fayetteville Shale serves as an 

unconventional gas reservoir across north-central Arkansas. 

The thickness of the Fayetteville Shale ranges from 

approximately 50 to 550 feet and varies in depth from 

approximately 1,500 feet to 6,500 feet below the ground 

surface (Southwestern Energy, 2012). Primary permeability 

in the Fayetteville Shale is very low, as is typical of dense 

shales, and economic extraction of gas has been made possible 

by advances in horizontal drilling and hydraulic fracturing of 

the formation, which are techniques that enhance secondary 

fracture porosity and permeability and maximize connectivity 

and gas yield to the production well (King, 2012). These 

technological advancements have made possible hydrocarbon 

production from formations that previously were not 

economically viable drilling targets and have expanded oil and 

gas industry operations into areas where such operations did 

not exist. Shale gas development in Arkansas began in 2004, 

and production has increased steadily thereafter. As of April 

2012, approximately 4,000 producing gas wells had been 

completed in the Fayetteville Shale (Arkansas Oil and Gas 

Commission, 2012).

Gas-well drilling and completion activities in the 

Fayetteville Shale have the potential to affect water quality in 

shallow aquifers through the loss of fluids used in every step 

of gas production, including drilling, hydraulic fracturing, and 

storage and handling of flowback water. Transport pathways 

for surface-derived contaminants include potential leakage 

from earthen pits used to store drilling, hydraulic fracturing, 

and flowback fluids, leakage from pipes, and losses from pond 

overflows and spills during transport (King, 2012). Water 

contamination can possibly be associated with hydraulic 

fracturing through changes in the permeability of the shale 

gas formation and overlying geological units because of the 

hydraulic fracturing process, migration of hydraulic fracturing 

fluids through existing vertical fractures that connect to the 

shallow aquifer (Warner and others, 2012), and, more likely, 

upward migration of gas and pressurized fluids through poorly 

cased and grouted sections of the shale-gas well bore (Atlantic 

Council, 2011; King, 2012). Hydraulic fracturing in a properly 

cased and cemented wellbore has been cited as the lowest risk 

for shallow groundwater contamination in the entire welldevelopment process (King, 2012). The potential for migration 

of fracturing fluids can increase where the target formation is 

shallower, thus reducing the separation distance between the 

gas-production zone and the shallow groundwater, or where 

increased hydraulic connectivity exists through deep faulting 

zones (Warner and others, 2012).

Purpose and Scope

The purpose of this report is to describe general water 

quality and geochemistry of shallow groundwater in the 

Fayetteville Shale gas-production area in north-central 

Arkansas and evaluate the potential effects, if any, from 

activities related to shale-gas drilling and production. 

Groundwater wells were sampled during 2011 in two counties 

of north-central Arkansas: Van Buren County and Faulkner 

County. This report presents and assesses field and inorganic 

water-quality data, including major ions, trace metals, methane 

gas, and selected isotopes. Any constituents, notably organic 

compounds, that might have been added to fracturing fluids 

for gas drilling were not analyzed for this study.

Description of Study Area

The study area is located in north-central Arkansas in 

Van Buren and Faulkner Counties; four of the sampling 

locations were slightly outside of these two counties but 

near to the county lines (fig. 1). The Fayetteville Shale 

gas-production area is approximately 2,500 square miles, 

and wells sampled for this study were distributed across an 

approximate area of 850 square miles or about one-third of the 

total gas-production area.
Introduction 3

Figure 1. Location of the Fayetteville Shale study area sampling sites and gas-production wells.

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WHITE

COUNTY

POPE

COUNTY

YELL

COUNTY

STONE

COUNTY

PERRY

COUNTY

NEWTON

COUNTY

SEARCY

COUNTY

CONWAY

COUNTY FAULKNER

COUNTY

VAN BUREN

COUNTY JACKSON

COUNTY

CLEBURNE

COUNTY

PRAIRIE

COUNTY

PULASKI COUNTY

INDEPENDENCE

COUNTY

WOODRUFF

COUNTY

LONOKE

COUNTY

JOHNSON

COUNTY

IZARD

COUNTY

MONROE

COUNTY

SHARP

COUNTY

LOGAN

COUNTY

SALINE COUNTY

LAWRENCE

COUNTY

BAXTER COUNTY

ST FRANCIS

COUNTY

MADISON COUNTY

Area

enlarged Gas-production

area

N

ARKANSAS

MISSOURI

OKLAHOMA

TEXAS

LOUISIANA

MISSISSIPPI

TENNESSEE

Area

enlarged Gas-production

area

Hot Springs

!

!

Urban area

County boundary

Sampled well

Historical sampled well

Gas well

EXPLANATION

!

151050 20 MILES

0 5 10 15 20 KILOMETERS

Base modified from Shuttle Radar Topography Mission shaded relief, 2006

Datum World Geodetic System 1984

Conway

Morrilton

Heber

Springs

Jacksonville

Cabot

9

10 60

9

95

40

40

65

64

67

65

92

16 16

9

27

25

25

5

16

92

27

16

YELL

COUNTY

WHITE COUNTY

VAN BUREN COUNTY

STONE COUNTY

SEARCY COUNTY

PULASKI COUNTY

POPE

COUNTY

PERRY COUNTY

NEWTON

COUNTY

LONOKE

COUNTY

FAULKNER COUNTY CONWAY COUNTY

CLEBURNE COUNTY

!

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93°00' 92°30' 92°00'

35°30'

35°00'

Greers Farry

 Lake

Lake

Conway

Arkansas RiverArkansas River
4 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

The study area is characterized by a rugged and 

mountainous landscape to the north and rugged and rolling 

landscape to the south. Van Buren County is mostly rural 

with a population density of 24 persons per square mile and 

has an economy dominantly based on small industry, cattle 

farming, and tourism (U.S. Census Bureau, 2012a). Although 

Faulkner County has a much larger population density of 175 

persons per square mile, three colleges, and comparatively 

larger towns than Van Buren County, Faulkner County remains 

50 percent rural (U.S. Census Bureau, 2012b). As such, both 

counties have outlying rural areas where water is supplied 

solely by domestic water wells.

Overview of Hydrogeologic Setting

Surface and subsurface geological formations of 

interest in the study area are composed of Pennsylvanian 

and Mississippian sandstone and shale formations with 

occasional thin limestone units. From oldest to youngest, these 

formations include the Mississippian Fayetteville Shale and 

the Bloyd, Hale, and Atoka Formations of Pennsylvanian age 

(fig. 2).

The exposed and shallow subsurface geologic formations 

serving as local aquifers for Van Buren and Faulkner Counties 

are a series of dominantly shale and sandstone strata of the 

Hale, Bloyd, and Atoka Formations (fig. 2). Subsurface 

geology, particularly with respect to lateral facies within the 

Fayetteville Shale, was poorly defined prior to natural-gas 

development. The Pitkin Limestone conformably overlies 

the Fayetteville Shale along the Ozark uplift, but is absent 

on well logs that were examined in the eastern Arkoma 

Basin (Ratchford and others, 2006). As such, shales of the 

Hale Formation rest on top of the Fayetteville Shale, which 

presented challenges to early mapping activities in selecting 

the top of the Fayetteville Shale; however, consistent 

characteristic response on gamma ray, resistivity, and 

conductivity logs eventually provided confident determination 

of the contact, allowing for a better definition of the 

subsurface geology in the gas-production area (Ratchford and 

others, 2006). 

The Fayetteville Shale is a black, fissile, concretionary, 

clay shale, which contains pyrite and silica replacement fossils 

in some intervals (McFarland, 2004). The Fayetteville Shale 

ranges in thickness from 50 to 550 feet in the gas-production 

area and dips from north to south in the subsurface at altitudes 

of 0 feet above the National Geodetic Vertical Datum of 1929 

(NGVD 29) to -6,500 feet NGVD 29 in the western part of the 

study area, and approximately -1,500 to -7,500 feet NGVD 29 

in the eastern part of the study area. The highly organic-rich 

facies within the Fayetteville Shale is present in the middle 

and lower part of the formation. Vitrinite reflectance falls 

within 1.93 to 5.09 percent, which corresponds to the dry-gas 

window (Ratchford and others, 2006). 

The Hale Formation is made up of two members: the 

lower is the Cane Hill Member, which is typically composed 

of silty shale interbedded with siltstone and thin-bedded, finegrained sandstone, and the upper is the Prairie Grove Member, 

which is composed of thin to massive limey sandstone. The 

Hale Formation is up to 300 feet in thickness (McFarland, 

2004). The Cane Hill Member of the Hale Formation is 

exposed in the extreme northern part of Van Buren County 

(fig. 2).

The Bloyd Formation in its type locality in northwestern 

Arkansas is formally divided into five members, two of 

which are limestone. The limestone members are absent in 

the study area, and the Bloyd Formation will be discussed as 

undifferentiated for purposes of this report. The lower twothirds of the Bloyd Formation consist dominantly of very 

thin- to thinly-bedded sandstone with shale interbeds. The 

upper Bloyd Formation is dominantly shale with interbedded 

sandstone that is commonly calcareous; the sandstone units 

can reach thicknesses of up to 80 feet (Rains and Hutto, 2011). 

Total thickness for the Bloyd Formation can exceed 400 feet in 

the study area. Exposures of the Bloyd Formation are found in 

northern Van Buren County (fig. 2).

The Atoka Formation in the study area consists of a 

sequence of thick shales that are interbedded with typically 

thin-bedded, very fine-grained sandstone. The Atoka 

Formation is unconformable with the underlying Bloyd 

Formation and can reach a thickness of up to 25,000 feet in the 

Ouachita Mountains (McFarland, 2004). The Atoka Formation 

is exposed throughout Faulkner County and the southern part 

of Van Buren County (fig. 2).

Unlike large regional aquifer systems in Arkansas that 

yield large quantities of water for multiple uses and span 

across several States, shallow groundwater in the study area 

is derived from formations that together comprise the Western 

Interior Plains Confining System for the underlying Ozarks 

Plateaus aquifer system. Regionally, geohydrologic properties 

for the collective formations within this confining system are 

represented dominantly by low-permeability shale with minor 

occurrences of relatively permeable sandstone, limestone, 

and coal (Imes and Emmett, 1994). No one formation within 

this confining system, even where representing a source of 

local water supply, forms a distinct aquifer regionally, and 

the regional designation as a confining unit indicates that on 

a regional scale these formations impede the vertical flow of 

water and confine the underlying aquifers.

Locally exposed rocks in the study area have an upper, 

relatively permeable surface formed by fractures providing 

secondary permeability, whereas approximately 300 feet 

below the land surface this situation is contrasted by the 

consolidated, compacted, and less permeable lower part of 

the confining system. Generally, groundwater is replenished 

by precipitation that infiltrates the ground in upland areas and 

percolates to the water table, where it then flows downgradient 

toward lowland areas and discharges into perennial streams 

(Imes and Emmett, 1994). As such, a conceptual groundwater 

flow model for the study area is one controlled by expansion 

fractures with limited groundwater storage and yields 

sufficient for use almost solely as domestic supply, and flow 
Introduction 5

Figure 2. Geology of the Fayetteville Shale study area with generalized geologic cross section.

-8,000

-6,000

-4,000

-2,000

NGVD 29

2,000

FEET

-8,000

-6,000

-4,000

-2,000

2,000

NGVD 29

FEET

SOUTH NORTH

IPa

IPbh Mpfb

Pre Mpfb

VERTICAL EXAGGERATION x5

A A' FAULKNER COUNTY VAN BUREN COUNTY VAN BUREN COUNTY STONE COUNTY

ARKANSAS

MISSOURI

OKLAHOMA

TEXAS

LOUISIANA

MISSISSIPPI

TENNESSEE

Area

enlarged

Mpfb Pitkin Limestone and Fayetteville

 Shale

IPjv Johns Valley Shale

IPhc Cane Hill Member of Hale

 Formation

IPbh Bloyd Formation, and Prairie Grove 

 Member of Hale Formation

IPa Atoka Formation

IPhs Hartshorne Sandstone

IPma McAlester Formation

Tm Midway Group

Tw Wilcox Group

Qt Terrace deposits—Alluvial deposits on one or 

 more terrace levels

Qso Alluvium—Alluvial deposits of local streams or 

 of overbank flow of major streams. Includes

 deposits in abandoned meanders of major 

 streams in some areas

Qal Alluvium—Alluvial deposits of present streams

!

!

Urban area

Line of section

County boundary

Sampled well

Historical sampled well

Gas well

EXPLANATION

A A'

151050 20 MILES

0 5 10 15 20 KILOMETERS

151050 20 MILES

0 5 10 15 20 KILOMETERS

Base modified from U.S. Geological Survey digital data Geologic data modified from Haley and others, 1976

Datum World Geodetic System 1984

Conway

Morrilton

Heber

Springs

Jacksonville

Cabot

9

10 60

9

95

40

40

65

64

67

65

92

16 16

9

27

25

25

5

16

92

27

16

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93°00’ 92°30’ 92°00’

35°30’

35°00’

YELL

COUNTY

WHITE COUNTY

VAN BUREN COUNTY

STONE COUNTY

SEARCY COUNTY

PULASKI COUNTY

POPE

COUNTY

PERRY COUNTY

NEWTON

COUNTY

LONOKE

COUNTY

FAULKNER COUNTY CONWAY COUNTY

CLEBURNE COUNTY

Tm Tw

Mpfb

Qso

Qso

Qt

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IPjv

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A'

A

Greers Farry

 Lake

Lake

Conway

Arkansas RiverArkansas River

NGVD 29, National Geodetic Vertical Datum of 1929
6 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

paths confined by small-scale topographic boundaries with 

short flow paths from elevated areas to valley floors in small 

stream systems. 

Because of the interbedded nature of the sandstone, 

siltstone, and shale rocks, the lack of detailed geologic 

maps, infrequent outcrops, and poor to nonexistent welllog records, difficulties arise when attempting to determine 

rock type penetrated by individual domestic water wells, in 

addition to identifying the rock type for the dominant waterproducing zone for each well. Water-quality data collected for 

this study, in addition to field observations, led to a cursory 

understanding of water type related to possible rock types, and 

this is discussed in greater detail in the section titled “RockWater Interaction.”

Where information was available for domestic wells 

sampled during this study, water wells range in depth from 

approximately 25 feet to 385 feet and average approximately 

85 feet in the study area. Because the shallow aquifer system 

is formed in dominantly sandstone and shale strata of the 

Hale, Bloyd, and Atoka Formations with limited primary 

porosity, fractures formed from expansion of these rocks 

with uplift and weathering provide secondary fracture 

porosity and yields that are sufficient only for domestic 

water supply. Several wells were described by owners as 

going dry during excessive pumping, particularly during 

drought periods, based on information obtained during 

reconnaissance and sampling activities for this study. 

Separation distance is often used in shale-gas production 

areas to describe the distance between the zone of gas 

production and the zone of freshwater production for shallow 

wells. Because the maximum known depth for any one 

well was 385 feet and assuming a conservative estimate of 

500 feet for possible deeper wells in the area, then based 

on the cited production depths for production from the 

Fayetteville Shale (1,500 – 6,500 feet; Southwestern Energy, 

2012), the separation distance between shallow groundwater 

and the gas producing zone varies from approximately 1,000 

to 6,000 feet.

Water Sample Collection and Analysis 

Methods

Well selection for this study was dictated largely by 

project cost considerations. To reduce costs in the form of 

reconnaissance activities, designated persons from each 

county collected pertinent information from well owners 

interested in having their well sampled and participating in 

the study. Because well owners concerned with perceived 

changes in the quality of water from their well water (taste, 

appearance, and other aesthetic effects) were the most likely 

residents to participate, as evidenced in conversations at the 

time of sampling, the study exhibited some degree of bias 

toward the sampling of potentially affected wells in the gasproduction area.

Water-quality samples were collected from a total of 127 

domestic wells; 71 wells in Van Buren County were sampled 

in July 2011 and 56 wells in Faulkner County were sampled 

in October and November 2011. All samples were collected 

by U.S. Geological Survey (USGS) personnel using standard 

methods described by Radtke and others (2005), Rounds 

(2006), and Wilde (2004, 2006a, 2006b). Field measurements 

made at the time of sampling included pH, temperature, 

and specific conductance. All samples were analyzed for 

major cations and anions, and trace elements by Duke 

University, Durham, N.C. Major anions were determined by 

ion chromatography, major cations by direct current plasma 

optical emission spectrometry (DCP-OES), and trace-elements 

by VG PlasmaQuad-3 inductively coupled plasma massspectrometer (ICP-MS). Dissolved solids concentrations 

were computed as the sum of the major constituents: calcium, 

sodium, magnesium, nitrate as nitrogen, sulfate, bicarbonate, 

silica, and iron. 

Additional samples for dissolved methane analysis were 

collected from 20 wells in Van Buren County and 31 wells 

in Faulkner County. All samples were collected following 

an Isotech Laboratories protocol (Isotech Laboratories, 

Inc., 2012a), and all bottles were supplied by Isotech. After 

purging the well until stabilization of pH, temperature, and 

specific conductance, a 5-gallon bucket was filled with water. 

Sample bottles were filled from the sample supply hose 

and then immersed in the bucket, where a minimum of two 

volumes of water was displaced from each bottle prior to 

capping underwater with a septum cap. This process ensured 

that no atmospheric methane contamination occurred in the 

sample bottle. After collection, samples were packed on 

ice and shipped overnight to the laboratory. Samples from 

Van Buren County were analyzed by Isotech Laboratories 

in Champaign, Ill., using chromatographic separation. For 

samples with sufficient methane, the carbon isotopic ratio 

(

13C/12C) was analyzed by combustion and dual-inlet isotope 

ratio mass spectrometry. This ratio is reported as δ13C, in 

parts per thousand (0/

00, or “per mil”). Detailed laboratory 

methodology and quality control information is available from 

Isotech Laboratory, Inc. (2012b). Samples from Faulkner 

County were analyzed at the Duke Environmental Isotope 

Laboratory. Methane and δ13C were determined by cavity 

ring-down spectroscopy (CRDS). Headspace equilibrations 

and extractions and concentration calculations were performed 

by a modification of the method of Kampbell and Vandegrift 

(1998). 

Data quality for inorganic chemical constituents was 

based on laboratory results for performance-evaluation 

samples provided by the USGS Standard Reference Sample 

(SRS) program. The USGS distributes SRS samples semiannually for laboratory performance comparison purposes. 

Approximately 100 laboratories participated in the SRS 

program during 2012 (U.S. Geological Survey, 2012a). 

Results from these laboratories are compiled and evaluated 

using non-parametric statistics. For each inorganic constituent, 

the median reported concentration is considered the most 
Shallow Groundwater Quality and Geochemistry 7

probable value (MPV), and the spread of data is estimated 

by F-pseudosigma (Fps), which is the interquartile range 

divided by 1.349. If the data distribution is normal, Fps 

equals standard deviation for a normal distribution, but is not 

as strongly affected by outliers. A Z-value is calculated for 

each result:

Z = (lab result – MPV) / Fps

If Z is greater than 3 or less than -3, the laboratory result 

is significantly different from the MPV (p<0.01), and the 

accuracy of analysis for this constituent at this laboratory is 

questionable. The Duke University laboratory participated 

in the spring and fall 2012 SRS events; results of which 

are shown in appendix 1. Z-values were less than -3 for 

molybdenum and antimony in the spring and for silver in the 

fall. Therefore, all results for these three constituents were 

deleted from the data set discussed in this report, and none 

of these results were entered into the USGS National Water 

Information System (NWIS) database (U.S. Geological 

Survey, 2012b).

Performance-evaluation results were not available 

for methane and δ13C, so data quality was evaluated based 

on reproducibility. For Isotech Laboratories, the standard 

deviation of internal standards for δ13C was 0.081‰ and for 

duplicates was 0.077‰ for 2011. Methane concentrations 

in water samples are calculated using Henry’s Law from 

the compositional analysis of methane in the headspace 

gas. As such, quality control data are available only for 

methane in the gas phase. The standard deviation of internal 

standards for methane gas for 2011 by Isotech Laboratory was 

0.281 mole percent. For the Duke Environmental Isotopes 

Laboratory, relative standard deviation of dissolved methane 

concentrations determined by CRDS on field duplicates was 

9.8%. Standard deviation of δ13C measurements determined 

by CRDS for eight field duplicate samples ranged from a 

minimum of 0.07‰ to a maximum of 1.0‰ with a mean 

0.55‰, resulting in a relative standard deviation of 1.7%. 

All these results were considered to be an indication of 

acceptable accuracy.

The non-parametric Wilcoxon rank-sum test analysis 

(Helsel and Hirsch, 1992) was used to identify significant 

differences between concentrations in two groups of samples. 

This analysis does not depend on the distributions of data 

within each group and is not affected by outliers. For 

computing the test statistic, censored values are assigned 

the lowest rank. A p-value of less than 0.05 for the test was 

considered sufficient evidence of a significant difference 

between the groups.

Linear correlation was used as a primary means to 

evaluate the relation between pairs of variables in the 

historical data set. The squared correlation coefficient (R2), 

computed as a value between 0 and 1, is a measure of the 

percent of variation in one variable that is accounted for by 

variation in the other variable. The closer R2 is to 1, where 

a linear relation is appropriate based upon inspection of the 

graph, the better the fit and accounting of the variation in 

the response variable (Helsel and Hirsch, 1992). None of 

the variables included in this correlation analysis had any 

censored values.

Shallow Groundwater Quality and 

Geochemistry

Numerous challenges exist in defining background 

groundwater quality and natural variation and range for 

shallow groundwater constituents in the study area. These 

challenges include differentiating natural from anthropogenic 

sources of contamination, identifying sources of potential 

effects on groundwater quality, identifying transport pathways 

for potential contaminants, understanding natural variation 

from chemical reactions and microbially mediated processes 

in the aquifer, and numerous other variables involved in 

groundwater contamination studies. Compounding these 

challenges in the Fayetteville Shale gas-production area is 

the paucity of existing groundwater-quality data by which 

to establish background groundwater-quality conditions. 

Additionally, many existing sources of contaminants 

are present throughout the area that could affect shallow 

groundwater quality, including domestic, agricultural, small 

industrial, and urban sources. 

Because little to no monitoring had been conducted prior 

to drilling and gas production in the study area, interpretation 

of water-quality data collected from individual wells after 

gas production relies heavily on comparative analysis. One 

approach is to compare contaminant source-water chemistry 

to that of shallow groundwater to establish indicator 

constituents unique to the source water. The difficulty with 

this type of comparison is that many of the shallow domestic 

wells are completed in shale or influenced by shale chemistry, 

as are waters from the gas-producing formation. Two other 

comparative analysis approaches include: (1) statistical 

comparison of historical, pregas development shallow 

groundwater-quality data collected in the gas-production 

area to water-quality data collected for this study, and 

(2) comparison of groundwater-quality data in similar 

geologic and topographic areas outside of the gas-production 

area to data collected from wells within the gas-production 

area. To achieve the most rigorous characterization of 

groundwater-quality conditions and identify potential changes 

to water quality, all three of these approaches were applied in 

this study.
8 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Historical Groundwater Quality and 

Geochemistry

Historical groundwater-quality data were retrieved 

from the USGS NWIS database (U.S. Geological Survey, 

2012b) for six counties that contain the bulk of permitted and 

active gas production wells: Cleburne, Conway, Faulkner, 

Independence, Van Buren, and White Counties. The NWIS 

database yielded 43 sites with samples collected from 1951 

through 1983 (table 1; appendix 2) prior to any shale-gas 

drilling activities. No data were noted in Independence 

County and one of the wells plotted in Johnson County. 

These data were limited with respect to trace metal chemistry 

and many of the major ions. The most frequently analyzed 

constituent was chloride, which can be important as an 

indicator of migration of production waters into shallow 

groundwater. 

Production water commonly is termed “flowback” or 

“produced” by the Fayetteville Shale gas industry, which 

basically are accounting terms used by the industry to denote 

early and later return of emplaced hydraulic fracturing fluids. 

Produced water normally is a term reserved for formation 

water associated with oil and gas reserves that are brought to 

the surface with oil or gas and must be separated and removed 

at the surface. Fluid recovery rates following hydraulic 

fracturing are highest during the first 2-3 weeks, and diminish 

greatly after that time (King, 2012). Because of the shorter 

residence time in the producing formation (less rock-water 

interaction), initial flowback is normally of lower salinity than 

the low-volume, later-return produced water. The Fayetteville 

Shale is a dry formation, and yet water brought to the surface 

with gas is probably a blend of injected hydraulic fracturing 

fluids, residual formation salts, and naturally occurring 

formation brine. The produced water salinity is a key factor to 

determine the relative proportion of low saline flowback water 

and typically highly saline formation water. A progressive 

increase in salinity of the produced water with pumping 

time of the shale-gas well reflects an increasing fraction of 

the producing formation water component. Over a longer 

time period, the saline formation water component consists 

of a larger fraction of the return flow. This report refers to 

‘flowback water” as an intermediate stage following hydraulic 

fracturing relative to “produced water” that represents longterm flow fluid production associated with gas production. 

Chloride can serve as an early indicator of infiltration 

of production water into the shallow aquifer system for 

several reasons: (1) chloride is a conservative constituent 

in groundwater; it does not react with other minerals or 

adsorb to clays or organics, and also is highly soluble; and 

(2) chloride is elevated in gas-production waters associated 

with the Fayetteville Shale—flowback water has chloride 

concentrations that vary from approximately 2,500 to 

5,000 mg/L and produced water can range upward to greater 

than 20,000 mg/L (Arkansas Oil and Gas Commission, 

unpublished data, April 4, 2012), similar to that of seawater. 

All but one historical sample included analysis of chloride.

The highest chloride concentration from the historical 

dataset was 378 mg/L; two samples had concentrations greater 

than 200 mg/L, and six samples had concentrations between 

100 and 200 mg/L. The median chloride concentration 

was 20 mg/L (table 1). Nineteen of the historical samples 

were from wells identified as completed in the Atoka 

Table 1. Descriptive statistics for selected water-quality constituents from historical data collected from 1951 through 1983 from 43 

wells in the Fayetteville Shale gas-production area, north-central Arkansas. 

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than]

Parameter Number 

of samples

Number of 

censored 

values1

Minimum 25th 

percentile Median 75th percentile Maximum

pH (standard units) 27 0 5.9 6.6 6.8 7.4 8.2

Conductance (μS/cm) 41 0 32 236 378 602 1,840

Calcium (mg/L) 23 0 7.9 19 23 43 107

Magnesium (mg/L) 23 0 0.30 5.8 9.1 20 211

Sodium (mg/L) 23 0 2.0 13 24 51 145

Bicarbonate (mg/L) 24 0 6.0 71 125 256 980

Sulfate (mg/L) 26 0 0.8 4.4 10 22 255

Chloride (mg/L) 42 0 1.6 5.6 20 64 378

Iron (μg/L) 20 10 <10 <10 < 10 85 6,300

Manganese (μg/L) 8 0 10 58 120 188 600

1

Data are reported as less than a censoring limit. 
Shallow Groundwater Quality and Geochemistry 9

Formation, whereas 18 samples were associated with the 

Bloyd Formation. The median chloride concentration for 

samples from the Atoka Formation was 22 mg/L, which was 

similar to the median chloride concentration of 19 mg/L for 

samples from the Bloyd Formation. Statistical analysis of the 

difference in chloride concentrations for samples from each 

formation using Wilcoxon rank-sum test analysis (Helsel and 

Hirsch, 1992) revealed no significant differences (p-value 

= 0.78) based on geology, suggesting that the groundwater 

geochemistry in each formation results from similarities in 

the rock type and mineralogy for these formations (that being 

interbedded sandstone and shale).

A full suite of major ions was available for only 16 

of the 43 samples from the historical data. A comparison 

of calculated dissolved solids concentration and specific 

conductance demonstrated a good linear relation with a 

R-squared (R2) value of 0.96 (fig. 3). More importantly, a 

cation/anion balance was calculated for the 16 samples with 

complete major ion chemistry with all but 2 samples having 

less than 3.5 percent charge-balance error. A linear relation 

was established between total cations and total anions in 

milliequivalents per liter with a R2 value of 1.00 and a slope 

of 0.99 (fig. 4). These quality-assurance checks warrant a 

high degree of confidence in the laboratory analyses. The 

groundwater generally is characterized by bicarbonate as the 

major anion, ranging from a calcium-bicarbonate to sodiumbicarbonate water type. Three of the samples had chloride as 

the major anion (greater than 50 percent of the total anions 

in milliequivalents per liter), and one sample was a calciumsulfate water type. 

Current Study Groundwater Quality and 

Geochemistry

Groundwater Quality and Spatial and Temporal 

Comparative Analysis

The inorganic water-quality analyses for 127 groundwater 

samples collected for this study were compared to national 

drinking water standards (U.S. Environmental Protection 

Agency, 2009). A statistical summary of selected constituents 

from the water-quality data collected for this study is 

reported in table 2, and a complete list of the water-quality 

analyses is available in appendix 3. The primary drinkingwater maximum contaminant level for nitrate (10 mg/L as 

nitrogen) was exceeded in 2 of the 127 samples. Sources of 

nitrate in the study area observed during reconnaissance and 

sampling activities included septic systems, animal waste, and 

fertilizers. Secondary drinking-water standards were exceeded 

0 200 400 600 800 1,000 1,200 1,400 1,600

Dissolved solids, in milligrams per liter

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

Specifc conductance, in microsiemens per centimeter at 25 degrees Celsius

R2 = 0.96

Figure 3. Relation between dissolved solids concentrations and specific conductance values for historical data (1951–83) in the 

Fayetteville Shale gas-production area, north-central Arkansas.
10 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

for iron in 55 samples and manganese in 74 samples, with 

standards of 0.3 mg/L and 0.05 mg/L, respectively. These 

secondary standards are non-enforceable limits based on 

aesthetic qualities of water including taste, odor, and staining 

properties of water. Iron and manganese are found in most 

rock types, have solubility characteristics that are controlled 

dominantly by changes in reduction-oxidation (redox) 

reactions, and are found in elevated concentrations in aquifers 

throughout Arkansas (Kresse and Fazio, 2002). Chloride 

has a secondary drinking-water standard of 250 mg/L; all 

concentrations were less than this concentration established 

for taste threshold. In general, shallow groundwater in Van 

Buren and Faulkner Counties is of good quality with respect 

to use as a drinking-water source, though taste and staining 

problems are unavoidable where untreated water contains 

elevated concentrations of iron, manganese, and other metals 

as a result of geochemical and microbiological processes in 

the aquifer.

Water-quality data collected for this study were compared 

to historical analyses, and comparisons using water-quality 

data for this study also were made between wells located 

less than 2 miles and wells located greater than 2 miles from 

active gas-production wells to assess potential effects from 

gas-production activities. Chloride, as stated in the previous 

section, can be an important indicator constituent and would 

be one of the first breakthrough constituents in a plume 

of contamination from gas-production waters. Chloride 

concentrations from data collected for this study ranged 

from approximately 1.0 mg/L to 70 mg/L, with a median 

concentration of 3.7 mg/L (table 2). Boxplots were constructed 

from data collected for this study and from historical analyses 

for an overall comparison of water quality between the two 

sets (fig. 5), and generally show higher concentrations for all 

major ions in the historical analyses compared to analyses 

determined from this study. Maximum and median chloride 

concentrations in groundwater samples collected for this 

study were substantially lower than historical groundwater 

maximum and median chloride concentrations (tables 1 

and 2). Application of the Wilcoxon rank-sum test showed 

significantly higher chloride concentrations (p-value < 0.001) 

in the historical data compared to chloride concentrations 

for this study. Other major ion and trace metal concentrations 

also were lower in samples collected for this study than in 

samples collected historically (tables 1 and 2; appendixes 2 

and 3). 

Much of the difference for the two data sets can be 

attributed to sampling a larger geographic area for the 

historical data (six counties compared to two counties for 

this study), although sampling sites for both data sets were 

collected from the same shallow aquifer system (dominantly 

the Atoka and Bloyd Formations). Only eight historical 

sites are located in the study area, which provide too small 

0 5 10 15 20 25 30

Total cations in milliequivalents per liter

0

5

10

15

20

25

30

Total anions, in milliequivalents per liter

R2 = 1.00

Figure 4. Relation between total cation and anion milliequivalent concentrations for historical data (1951–83) in the Fayetteville Shale 

gas-production area, north-central Arkansas.
Shallow Groundwater Quality and Geochemistry 11

a number of samples for a meaningful statistical analysis. 

The historical data for the larger spatial extent of the gasproduction area provide a snapshot of the range and variability 

of groundwater geochemistry, but should not be used for strict 

comparison to current data as the same wells were not sampled 

for both periods—thus no inference should be made that water 

quality has improved since the earlier period represented by 

the historical data. The historical data contain concentrations 

for chloride (and other constituent concentrations) that are 

larger than that of the new data and reveal the wider range 

of constituent concentrations that occur in groundwater from 

similar rock formations in the gas-production area of northcentral Arkansas. As such, the historical data can be used 

to establish a general benchmark with which to examine 

possible variations in the water chemistry induced from 

natural processes and compare them to potential effects from 

contamination by shale-gas production waters.

In addition to comparison of historical water-quality 

data to data collected for this study, comparisons of chloride 

concentrations collected for this study were made between 

94 domestic-well sample sites less than 2 miles from active 

gas-production wells (63 wells less than 0.5 mile from 

a gas-production well; 29 of which were less than 0.25 

mile from a gas-production well) and 33 domestic-well 

sample sites greater than 2 miles (maximum distance of 

16 miles) from gas-production wells. The 2-mile threshold 

was chosen as a conservative estimate for the length of 

possible plume migration based on average and maximum 

gasoline-contaminantion plume lengths of 0.06 and 1.5 miles, 

respectively (Ruiz-Aguilar and others, 2003; Falta, 2004), 

in addition to the fact that average and maximum length of 

horizontally-drilled and fractured lateral wells are 0.8 and 

1.5 miles, respectively (Arkansas Oil and Gas Commission, 

2012). A Wilcoxon rank-sum test showed no significant 

difference between the two groups (p-value = 0.71). The 

median chloride concentration for wells less than 2 miles 

from the nearest production wells was 3.7 mg/L; whereas, the 

median chloride concentration for wells greater than 2 miles 

from the nearest production well was 3.7 mg/L. As such, using 

two comparative analysis methods (historical and distance) 

as initial screening tools for potential effects to water quality 

from gas-production activities within the western third of the 

shale-gas production area showed no evidence of migration of 

gas-production water into shallow groundwater based solely 

on the data gathered from the wells sampled in this study.

Groundwater Geochemistry and Geochemical 

Evolution

In addition to the comparative analysis discussed in 

the above section, geochemical analyses can help identify 

potential outliers in the data and potential effects from gasproduction activities as compared to natural geochemical 

processes. Data from individual well sites may be inferred to 

represent the chemistry of groundwater at a specific location 

along a groundwater flow path, and together with data from 

numerous other wells, provide information on geochemical 

evolution in the shallow groundwater system in the study area.

Table 2. Descriptive statistics for selected water-quality constituents from data collected in 2011 from 127 wells in Van Buren and 

Faulkner Counties, Arkansas. 

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than]

Parameter Number 

of samples

Number of 

censored 

values1

Minimum 25th 

percentile Median 75th percentile Maximum

pH (standard units) 127 0 4.2 5.9 6.5 7.1 8.6

Conductance (μS/cm) 127 0 13 101 250 339 909

Calcium (mg/L) 127 16 <2.1 5.6 17 35 88

Magnesium (mg/L) 127 4 <0.6 2.1 4.2 8.5 46

Sodium (mg/L) 127 20 <2.1 3.4 9.1 24 159

Bicarbonate (mg/L) 127 4 <2.0 47 130 196 527

Sulfate (mg/L) 127 10 <0.5 1.6 4.3 9.7 151

Chloride (mg/L) 127 0 0.94 2.3 3.7 7.6 70

Iron (mg/L) 127 38 <0.05 <0.05 0.19 0.51 5.7

Manganese (μg/L) 127 29 <15 19 96 286 4,370

1

Data are reported as less than a censoring limit.
12 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Figure 5. Selected constituents for A, historical data and B, data from 127 wells in the Fayetteville Shale gas-production area, northcentral Arkansas.

Sodium Calcium Magnesium

Constituent

Bicarbonate Chloride Sulfate

0

100

Concentration, in milligrams per liter

200

300

400

0

100

200

300

400

24 23 23 24 42 26

127 127 127 127 127 127

A

B

50th Percentile

75th Percentile

25th Percentile

10th Percentile

90th Percentile

Number of values

EXPLANATION

24
Shallow Groundwater Quality and Geochemistry 13

Describing the geochemical evolution of groundwater 

resulting from physical, chemical, and microbial processes 

in shallow aquifer systems with respect to increasing rockwater interaction provides a framework to understand 

geochemical and hydrological processes. A valuable proxy 

for increased rock-water interaction is increasing dissolved 

solids concentrations, particularly constituents that are not 

derived from only evapotranspiration of recharged water. 

Thus, how individual chemical constituents or relations 

between constituents change with increasing dissolved solids 

concentrations can reveal an overall pattern of geochemical 

conditions, water-chemistry controls, and specific chemical 

and biochemical reactions along a continuum of increased 

rock-water interaction. Chemical reactions are predicated on 

traveltime (slower traveltime leads to increased opportunity 

for rock-water processes to occur), availability of reactive 

minerals, electron donors and acceptors, charged substrates, 

and other variables (Appelo and Postma, 1999). The following 

section investigates relations between various chemical 

constituents to establish one possible model for the evolution 

of geochemistry in the shallow groundwater system in the 

study area. 

Rainwater in Arkansas has a mean pH value of 4.7 and 

average dissolved solids concentrations below 3 mg/L (Kresse 

and Fazio, 2002). Rainwater is highly undersaturated with 

respect to most mineral phases and as such dissolves minerals 

as it percolates through soil and moves through aquifer host 

rock, thus increasing the dissolved solids concentration of soil 

pore water and groundwater. In aquifers with sufficient labile 

organic matter and reduced mineral phases such as pyrite, 

as is typical of surficial shale formations in the study area, 

oxygen carried to the subsurface with the percolating rain 

water is consumed as carbon and mineral phases are oxidized, 

and groundwater will tend to become more reduced along the 

flow path (Chapelle, 2001). Geochemical evolution of shallow 

groundwater in the study area can be visualized to represent a 

transition with increased rock-water interaction (represented 

by increasing dissolved solids concentration) of less 

geochemically evolved groundwater to more geochemically 

evolved groundwater and from a more oxidized water to a 

more reduced water (fig. 6).

Rock-Water Interaction

Most aquifer systems in Arkansas exhibit a strongly 

calcium- or sodium-bicarbonate groundwater type (Arkansas 

Department of Environmental Quality, 1996). As limestone 

and other carbonate rocks are dissolved along the flow 

path, acids are buffered and pH increases concurrently with 

dissolved load. This process may be illustrated graphically by 

the relation between increasing pH values and dissolved solids 

concentrations (fig. 7). The interpretation that this process 

is controlled by dissolution of carbonate rock is supported 

by the relation between bicarbonate and dissolved solids 

concentration (fig. 8a), which shows a positive linear relation 

of increasing bicarbonate concentrations with increasing 

dissolved solids concentrations. Generally increasing, but 

weakly linear, trends were noted for sulfate and chloride 

compared to increasing dissolved solids concentrations, as 

well. Sulfate concentrations generally are less than 20 mg/L 

for dissolved solids concentrations less than 300 mg/L, with 

elevated concentrations up to 151 mg/L occurring at dissolved 

solids concentrations greater than 300 mg/L (fig. 8b); chloride 

concentrations greater than 20 mg/L occur only where 

dissolved solids concentrations exceed 200 mg/L (fig. 8c) with 

concentrations upward to 70 mg/L.

The relation of major cations (calcium, magnesium, and 

sodium) with dissolved solids concentrations shows that, 

similar to bicarbonate, calcium (fig. 8d) and magnesium 

(fig. 8e) correlate more tightly with increasing dissolved 

solids concentrations than sodium (fig. 8f); carbonate rocks 

dominantly have calcium and magnesium as the major 

cations constituting rock chemistry. Summing calcium 

with magnesium yielded only a slight improvement for the 

relation and linearity of fit with increasing dissolved solids 

concentrations. A better relation should be expected after 

combining the constituents based on the strong linearity 

of the bicarbonate-dissolved solids relation, if dissolution 

of a calcium plus magnesium carbonate is proposed as the 

dominant source for these constituents. Cation exchange—

specifically the exchange of calcium for sodium on the 

clay mineral exchange sites—can be shown to account for 

the variability of calcium and sodium in solution and the 

poor relation between calcium and magnesium with 

dissolved solids.

In addition to dissolution and precipitation of minerals 

in the aquifer source rock, cation exchange is one of the 

more important rock-water interactions affecting chemical 

constituents in solution. This process occurs where 

groundwater is in contact with clays and weathered shale 

surfaces (Langmuir, 1997). Clays carry a negative electrostatic 

charge, which gives these materials a measurable capacity 

to hold cations. Cation exchange capacity (CEC) is highly 

dependent upon pH—higher pH gives a higher CEC; thus 

dissolution of carbonate minerals increases CEC by buffering 

pH and drives exchange of calcium for sodium as calcium and 

magnesium are released during carbonate mineral dissolution. 

Generally, cations with higher valence will preferentially 

Increasing dissolved solids concentration

Less geochemically evolved More geochemically evolved

More oxidized More reduced

Figure 6. Conceptual model of geochemical evolution of 

groundwater with increased rock-water interaction represented 

by increasing dissolved solids concentrations.
14 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

occupy exchange sites over those of a lower valence. As such, 

calcium ions in solution (2+ charge) will replace sodium ions 

(1+ charge) at the exchange site, releasing the sodium ion into 

solution. As this process continues along a flow path, more 

and more calcium ions exchange for sodium at the exchange 

site, resulting in an increase of sodium ions in solution at the 

expense of calcium.

Concentrations reported in milliequivalents per liter 

take both the charge and weight of an individual ion into 

consideration, allowing a comparison of the number of ions in 

solution. Dissolution of a sodium-chloride salt in water will 

yield an equal number of sodium and chloride ions in solution, 

thus having an equivalent ratio of one; similar stoichiometric 

relations also hold for dissolution of carbonate rock and other 

minerals. Assuming most sodium and chloride is derived 

from dissolution of the parent salt and calcium, magnesium, 

and bicarbonate are derived from dissolution of carbonate 

minerals, a comparison of ratios for the common cations and 

anions theoretically would plot at 1.0 on both axes, given that 

no reactions take place, adding or removing ions to or from 

solution. In aquifer systems, however, rock-water interaction 

can appreciably alter the chemical composition of the 

groundwater.

A review of the relation between sodium/chloride 

and calcium plus magnesium/bicarbonate equivalent ratios 

suggests that cation exchange is actively occurring in the 

shallow aquifer (fig. 9a), with increasing sodium/chloride 

ratios resulting in decreasing calcium plus magnesium/

bicarbonate ratios. Sodium concentration in solution will 

continue to increase as cation exchange progresses; chloride 

is a conservative ion that does not participate in reactions 

in the system and remains invariant in concentration unless 

further dissolution occurs. As such, increasing cation 

exchange results in sodium/chloride ratios greater than 

60 times what is expected from simple dissolution of sodiumchloride salts (fig. 9a). The increase in sodium from cation 

exchange observed in samples for this study was large 

enough to result in 28 samples being categorized as sodiumbicarbonate water type. With respect to chemical evolution 

along the continuum of increasing rock-water interaction 

in the aquifer, sodium/chloride ratios are found to increase 

with increasing dissolved solids concentrations (fig. 9b). 

For low dissolved solids waters (less than approximately 

50 mg/L dissolved solids), the sodium/chloride ratio is 

approximately 1.0, suggesting dissolution of a sodiumchloride salt with no cation exchange occurring between 

calcium and sodium in the aquifer system. The ratios begin 

to increase markedly at dissolved solids concentrations 

greater than 50 mg/L, showing that cation exchange 

progresses with increases of sodium in solution with 

increasing rock-water interaction. Sodium/chloride ratios 

near 1.0 for the lower dissolved solids concentration waters 

are attributed to sandstone as the source rock, as explained in 

the following section.

0 100 200 300 400 500 600 700 800

Dissolved solids, in milligrams per liter

pH, in standard units

3

4

5

6

7

8

9

Figure 7. Relation between dissolved solids concentrations and pH values from data collected in the Fayetteville Shale gas-production 

area, north-central Arkansas.
Shallow Groundwater Quality and Geochemistry 15

0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700 800 0 100 200 300 400 500 600 700 800

Dissolved solids, in milligrams per liter Dissolved solids, in milligrams per liter

0

20

40

60

80

100

120

140

160

180

0

20

40

60

80

100

120

140

160

0

10

20

30

40

50

60

70

80

90

100

0

50

100

150

200

250

300

350

400

450

500

0

10

20

30

40

50

60

70

80

0

5

10

15

20

25

30

35

40

45

50

Sodium, in milligrams per liter Sulfate, in milligrams per liter Calcium, in milligrams per liter

Bicarbonate, in milligrams per liter Chloride, in milligrams per liter Magnesium, in milligrams per liter

A

F

C

B

E

D

Figure 8. Relation between dissolved solids concentrations and A, bicarbonate, B, sulfate, C, chloride, D, calcium, E, magnesium, and 

F, sodium concentrations from data collected in the Fayetteville Shale gas-production area, north-central Arkansas.
16 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Figure 9. Relation between sodium/chloride ratios and A, calcium plus magnesium/bicarbonate ratios constructed using 

milliequivalent concentrations and B, dissolved solids concentrations from data collected in the Fayetteville Shale gas-production area, 

north-central Arkansas. 

0.00 0.50 1.00 1.50 2.00 2.50 3.00

0 100 200 300 400 500 600 700 800

Calcium+magnesium/bicarbonate, equivalent ratio

Dissolved solids, in milligrams per liter

0

10

20

30

40

50

60

70

0

10

20

30

40

50

60

70

Sodium/chloride, equivalent ratio Sodium/chloride, equivalent ratio

A

B
Shallow Groundwater Quality and Geochemistry 17

Water type was determined for each of the samples using 

milliequivalent concentrations to calculate percentage of total 

cations for each of the major cations (calcium, magnesium, 

and sodium) and percentage of total anions for each of the 

major anions (bicarbonate, chloride, and sulfate). A trilinear 

diagram was constructed to show the general distribution 

and trend for the major cations and anions for each sample 

(fig. 10). The trilinear diagram, particularly the lower left 

triangle, shows the general distribution of ions and an overall 

trend from a calcium-bicarbonate type water to a sodiumbicarbonate water.

A general plotting of water types along the defined 

continuum of increasing dissolved solids concentrations shows 

that the major ion chemistry of the shallow groundwater is 

not random but follows a predictable regular pattern based 

on rock type and rock-water interaction. The pH values were 

very low, ranging from 4.2 to 5.8, in groundwater samples 

with dissolved solids concentrations less than approximately 

30 mg/L (fig. 11). In addition, these low dissolved solids 

samples are represented dominantly by water types with 

chloride as the major anion (all but two samples with chloride 

as the major anion occur where dissolved solids concentration 

was less than 30 mg/L) and, together with the low dissolved 

solids and low pH values, resemble an evaporated rainwater 

with addition of silica and minor trace metals (Kresse and 

Fazio, 2002). A comparison of percent chloride (of the total 

anions in milliequivalents per liter) and increasing dissolved 

solids concentrations shows the dominance of chloride in the 

lower dissolved solids groundwater, whereas percent chloride 

generally declines as carbonate dissolution, and resultant 

increasing bicarbonate concentrations, begins to dominate the 

chemistry of the groundwater with increasing dissolved solids 

concentrations (fig. 12).

Chloride

Calcium + magnesium

Sulfate + chloride

Sodium

Sulfate

Calcium

Magnesium

Bicarbonate

PERCENT

PERCENT

PERCENT

20

40

60

80

100

0

20

40

60

80

100

0

100

80

60

40

20

0

20

40

60

80

100

0

20

0

40

60

80

100

20

0

40

60

80

100

80

100

60

40

20

0

80

60

40

20

100

0

80

60

40

20

100

0

20

40

60

80

0

100

Figure 10. Percentage distribution of major ions from data collected in the Fayetteville Shale gas-production area, north-central 

Arkansas.
18 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

0 100 200 300 400 500 600 700 800

Dissolved solids, in milligrams per liter

3

4

5

6

7

8

9

pH, in standard units

Dissolved

solids Water type 

EXPLANATION

Less than 30

31 to 95

96 to 249

Greater than 250

Sodium-chloride plus mixed-chloride

Calcium-bicarbonate plus mixed-bicarbonate

Calcium-bicarbonate plus transitional

Sodium-bicarbonate plus calcium-bicarbonate

Figure 11. Relation between dissolved solids concentrations and pH values with a depiction of the evolution of chemical water types 

from data collected in the Fayetteville Shale gas-production area, north-central Arkansas.

0 100 200 300 400 500 600 700 800

Dissolved solids, in milligrams per liter

0

10

20

30

40

50

60

70

80

90

Chloride, percentage of equivalent anions

Figure 12. Relation between dissolved solids concentration and chloride as percent of total anions in milliequivalents per liter from 

data collected in the Fayetteville Shale gas-production area, north-central Arkansas. 
Shallow Groundwater Quality and Geochemistry 19

Many of the sites from which the lower dissolved 

solids samples were derived had outcropping sandstone 

formations nearby, and the wells appear to have been 

completed in sandstone rock with water chemistry that 

corroborates this observation. The sandstone is assumed to 

have little carbonate material in the host rock or weathered soil 

above the sandstone, thus having little to no buffering capacity 

and little opportunity for cation exchange as discussed in the 

preceding paragraph. Some samples were below the mean pH 

of 4.7 for rainwater in Arkansas (Kresse and Fazio, 2002), 

possibly as a result of entrainment of carbon dioxide in the soil 

zone leading to formation of carbonic acid. Similar findings 

were noted to the south of the study area near Hot Springs, 

Arkansas, where well-defined, thick sequences of quartz 

formations and shale formations allowed differentiation of 

groundwater chemistry from each rock type. Samples taken 

from wells completed in the quartz formations revealed a 

soft, acidic groundwater (7 of 11 samples with pH values less 

than 4.7; 3 samples less than 4.0), indicating a quartz source 

rock with little to no buffering capacity (Kresse and Hays, 

2009). The interpretation established for groundwater data 

gathered in Hot Springs is thought to explain the observed 

low dissolved solids groundwater chemistry in Van Buren and 

Faulkner Counties.

Shale formations in the study area have abundant 

carbonate material, the source sediments for these rocks being 

accumulated in marine environments (McFarland, 2004). 

Groundwater moving through shale rock is quickly buffered 

along a given flow path with increasing pH and dissolved 

solids concentrations with dissolution of carbonate minerals. 

The groundwater samples with dissolved solids concentrations 

greater than 30 mg/L had bicarbonate as the dominant anion 

with the exception of three samples, two of which were a 

sodium-chloride water type and one calcium-sulfate water 

type. Samples with dissolved solids concentrations between 

approximately 31 and 95 mg/L exhibited a mixture of 

calcium-bicarbonate (each ion greater than 50 percent of total 

cations and anions, respectively) and mixed-bicarbonate (no 

dominance of calcium, magnesium, or sodium) water types 

(fig. 11).

For samples with dissolved solids concentrations between 

approximately 96 and 249 mg/L, calcium-bicarbonate was 

the dominant water type (24 of 40 of the samples), whereas 

the majority of the remaining samples had increasing sodium 

percentages and sodium as one of two major cations, thus 

representing a transitional water toward sodium as the 

dominant cation. For all samples with dissolved solids 

concentrations greater than 250 mg/L, 20 of 51 samples were 

a strongly sodium-bicarbonate water type—each constituent 

greater than 50 percent of the total cations or anions, 

respectively—with sodium percentages ranging from 52 to 

99 percent of the total cations. The remaining samples were 

calcium-bicarbonate water types with a few transitional water 

types (fig. 11).

In summary, the model proposed for the predominant 

rock-water interactions controlling major-ion groundwater 

chemistry observed in the study area is primarily mineral 

dissolution and cation exchange. Rock-water interaction 

alters the chemical composition of shallow groundwater 

in the study area along a continuum represented by 

increasing dissolved solids concentrations. Groundwater 

samples from the relatively carbonate-free sandstone 

formations are represented by a low dissolved solids 

concentration (less than 30 mg/L), slightly acidic water 

type. Sandstones are predominated by low-solubility quartz 

with little or no carbonate minerals to buffer pH, resulting 

in the characteristic water chemistry. Groundwater from the 

shale formations in the study area contain abundant soluble 

carbonate minerals, which undergo dissolution to produce 

groundwater with a maximum dissolved solids concentrations 

of 686 mg/L and water type evolving from a dominantly 

mixed-bicarbonate and calcium-bicarbonate to a strongly 

sodium-bicarbonate water type as cation exchange reaches 

a maximum. This understanding of the chemical evolution 

of groundwater with increased opportunity for rock-water 

interaction has important implications to the occurrence of 

natural methane in study area groundwater, which is discussed 

in the following section.

Reduction-Oxidation Processes

The subsurface environment hosts thriving microbial 

ecosystems, and these microbes sustain their life functions 

by participating in electron transfer through redox reactions. 

The microbes orchestrate electron transfer using organic 

carbon as an electron donor substrate or energy source 

and a sequence of decreasingly energy-yielding terminal 

electron acceptors, including in order of reaction energy: 

oxygen, nitrate, manganese oxides, iron oxyhydroxides, 

sulfate, and carbon dioxide (Appelo and Postma, 1999). 

These microbially mediated redox reactions affect many 

geochemical changes on solid-phase material and dissolved 

constituents in groundwater. Shale formations are rich in labile 

organic material, which drives the reduction of oxygenated 

water by aerobic and fermentation bacteria. Hydrogen is a 

waste product of the fermentation bacteria, which is then 

used by respiring microorganisms including nitrate reducers, 

manganese reducers, iron reducers, sulfate reducers, and 

methanogens (Chapelle, 2001). It is generally observed in both 

stream sediments and aquifers that redox processes proceed 

sequentially from the highest energy yield downward (Appelo 

and Postma, 1999). Therefore, as oxygenated water enters an 

aquifer system with abundant organic matter, oxygen will first 

be used as a terminal electron acceptor, followed by nitrate, 

manganese oxide, iron oxyhydroxide, sulfate, and finally 

carbon dioxide with the generation of methane. Depending on 

the available supply of electron donors and electron acceptors, 

redox zonation can stop at any point along this continuum or 

proceed through each sequence to methane production. 
20 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Evidence for various steps in the redox zonation 

described above is found in several relations observed 

for various dissolved constituents. Electron potential and 

hydrogen ion concentrations are commonly used surrogates 

to establish the redox zonation for groundwater within an 

aquifer system (Chapelle, 2001). Such data were not available 

for this study; however, general water-quality data often 

can provide strong corollary evidence for redox conditions. 

Evidence for reduction of iron and manganese oxides (and 

oxyhydroxides) often is inferred where each dissolved 

species has a concentration greater than 0.5 mg/L, as oxidized 

forms are extremely insoluble. Iron concentrations range 

upward to 5.7 mg/L with a median concentration of 0.19 

mg/L, and manganese concentrations range upward to 4.4 

mg/L with a median concentration of 0.12 mg/L. As such, 

elevated concentrations of these metals in solution indicate 

the presence of iron and manganese reduction throughout 

the shallow aquifer system. Elevated iron (fig. 13a) and 

manganese (fig. 13b) concentrations were mutually exclusive 

with elevated nitrate concentrations, confirming predicted 

redox zonation in the aquifer—nitrate-reducing bacteria take 

advantage of the more energetically favorable nitrate reduction 

reaction, thereby out-competing the manganese- and ironreducing bacteria until dissolved nitrate is effectively depleted. 

In cases where both manganese and iron exceed 0.5 mg/L, 

nitrate is absent; whereas nitrate concentrations greater than 

0.5 mg/L occur only where manganese and iron concentrations 

are less than 0.5 mg/L, indicating that onset of manganese 

and iron reduction has not occurred. Nitrate concentrations 

demonstrated an inverse relation with pH (fig. 14), suggesting 

that nitrate reduction is dominant only in the low dissolved 

solids concentration, low pH waters.

Evidence for sulfate reduction, while not directly 

measured by presence of sulfide gas species or hydrogenion concentration for this study, is found anecdotally in 

reports of sulfide odors from domestic well owners during 

reconnaissance and sampling activities. An interesting 

relation between iron and dissolved solids concentrations 

does provide some corollary geochemical evidence of 

sulfate reduction in the shallow aquifer system. In general, 

dissolved iron is absent (less than 0.5 mg/L) for dissolved 

solids concentrations less than 60 mg/L; it was shown that 

reduction of nitrate is dominant for these lower dissolved 

solids waters. Iron concentrations generally increase with 

increases in dissolved solids concentrations from 60 mg/L up 

to approximately 290 mg/L, at which point iron concentrations 

steadily begin to decline to low and nondetectable 

concentrations for dissolved solids concentrations exceeding 

500 mg/L (fig. 15a); arsenic concentrations (fig. 15b) 

behave similarly. Sulfate reduction in the study area is 

theorized to dominate over iron reduction for dissolved solids 

concentrations greater than approximately 290 mg/L, and 

free sulfide in solution combines with both iron and arsenic 

in solution to precipitate as iron-sulfide minerals (with 

varying amounts of arsenic and other trace metals), which are 

extremely stable in reducing conditions.

Methane and Carbon Isotopes

Methane concentration and carbon isotopic composition 

were analyzed in 51 of the 127 samples collected for this study 

to investigate the potential for contamination of the shallow 

aquifer system by upward migration of methane along natural 

pathways (faults, fractures and other permeable avenues) and 

artificially created pathways associated with gas production 

(for example, a poorly cemented annulus section of a gasproduction well). Osborn and others (2011) showed average 

and maximum methane concentrations in drinking wells in 

Pennsylvania increased to 19.2 and 64 mg/L, respectively, 

with proximity to gas-production wells, whereas dissolved 

methane in wells at neighboring nongas-production sites 

averaged 1.1 mg/L. Average carbon stable isotope, 13C/12C 

(δ13C), values of dissolved methane in shallow groundwater 

for the Osborn and others (2011) study were significantly less 

negative for active than for nonactive gas-production sites (-37 

± 7 per mil (‰) and -54 ± 11 ‰, respectively; p-value less 

than 0.0001). 

Methane is encountered in gas basins and shallow 

aquifers and can be derived from thermally or microbially 

driven reactions, generating thermogenic or biogenic gas, 

respectively (Bernard and others, 1977; Fuex, 1977; Schoell, 

1980; Rice and Claypool, 1981; Grossman and others, 1989). 

Mixing of the forms can occur as highly mobile gas migrates 

upward along primary or secondary permeable pathways. 

Thermogenic gas is produced by the thermal decomposition 

(cracking) of longer chain hydrocarbon kerogen or oil at 

high burial temperatures. Biogenic gas is generated in the 

subsurface under anaerobic conditions by fermentative 

bacteria, acetogenic bacteria, and the Archaea methanogens. 

Depending upon the particular biogenic reaction pathway, 

the carbon isotopic composition of biogenic methane can be 

very light—enriched in 12C and depleted in 13C—yielding very 

low δ13C values. Typical biogenic methane δ13C values are 

less than -55‰; thermogenic methane δ13C values typically 

are higher than -55‰ (Schoell, 1980; Rice and Claypool, 

1981; Grossman and others, 1989; Aravena and Wassenaar, 

1993; Whiticar, 1999), providing a very useful indicator of 

gas source or origin. Zumberge and others (2012) analyzed 

100 gas samples from gas-production wells in the Fayetteville 

Shale and reported δ13C values ranging from -41.9 to -35.0‰. 

Methane was detected above the detection limit of 

0.0002 mg/L in 32 of the 51 samples for this study, with 

concentrations ranging upward to 28.5 mg/L. Seven 

samples had methane concentrations greater than or equal to 

0.5 mg/L. The carbon isotopic composition for all methane 

concentrations greater than 0.5 mg/L for this study, including 

the highest concentration of 28.5 mg/L, showed the methane 

was likely biogenic in origin with δ13C values ranging from 

-57.6 to -74.7‰. 
Shallow Groundwater Quality and Geochemistry 21

0 2 4 6 8 10 12 14 16 18 20

0 2 4 6 8 10 12 14 16 18 20

Nitrate, in milligrams per liter as nitrogen

0

1

2

3

4

5

0

1

2

3

4

5

6

Manganese, in milligrams per liter Iron, in milligrams per liter

A

B

Figure 13. Relation between nitrate and A, iron and B, manganese concentrations from data collected in the Fayetteville Shale gasproduction area, north-central Arkansas. 
22 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Of note to the discussion of the evolution of groundwater 

along a continuum of increased rock-water interaction in the 

aquifer, increases in methane concentrations were related to 

increases in dissolved solids concentrations (fig. 16), with 

the highest methane concentration of 28.5 mg/L occurring in 

conjunction with the highest dissolved solids concentration 

of 686 mg/L. As such, methane production follows the 

continuum established earlier and illustrated in figure 6—

increased rock-water interaction in the shallow aquifer allows 

more time for geochemical processes, including cation 

exchange and a transition from a calcium- to a sodiumbicarbonate water type, and redox zonation transitioning 

through oxygen consumption, reduction of nitrate, iron and 

manganese oxides, sulfate, and eventually to the production of 

biogenic methane.

Although carbon isotope analyses revealed that methane 

in seven samples exhibiting concentrations greater than 0.5 

mg/L was biogenic in origin, thermogenic methane signatures 

were noted in five samples that exhibited extremely low 

methane concentrations ranging from 0.012 to 0.324 mg/L 

with δ13C values ranging from -31.96 to -49.87. Two of these 

five wells were at distances of 5.2 and 7.3 miles from the 

nearest active production well.

The most likely transport pathway for anthropogenically 

induced release of thermogenic methane into shallow 

groundwater occurs where methane gas travels up the well 

bore and out into shallow formations along poorly cemented 

annulus sections of the well bore near the surface. This 

situation has been described and confirmed by the gas 

industry (Atlantic Council, 2011; King, 2012) and in soil-gas 

studies, where methane concentrations in soils were noted 

to occur near production pads and diminish to nondetectable 

concentrations at distances of greater than 30 feet from the 

pad (Naftz and others, 1998), indicating that the most likely 

anthropogenic release of methane is near gas-well bores.

Thermogenic methane can migrate to shallow aquifers 

and to the surface naturally, following pathways offered 

by faults, fractures, and other permeable zones (Warner 

and others, 2012; King, 2012). In conventional natural gas 

reservoirs, thermogenic methane migrates upward and is 

trapped by a confining low-permeability zone. If a confining 

layer is not 100 percent effective, methane can seep to the 

land surface through fractures and other permeable pathways. 

Documented cases of thermogenic natural gas in aquifers 

abound in areas overlying gasfields with reports of methane 

in groundwater prior to any drilling activities (Lohman, 1937) 

and anecdotes of flaring of tapwater being common prior to 

any gas development throughout the country (King, 2012). 

In addition, one of the primary exploration techniques for 

new, unexplored basins is the use of gas “sniffers” that detect 

methane leaking into the atmosphere in aerial overflights 

(Jones and others, 1999; Plummer, 1993). The fact that low 

levels of thermogenic methane were found in groundwater 

from domestic wells sampled for this study at great distances 

from production wells suggests that upward migration of 

thermogenic gas occurs naturally in the study area and can 

produce low thermogenic methane concentrations in the 

shallow aquifer.

0 2 4 6 8 10 12 14 16 18 20

Nitrate, in milligrams per liter as nitrogen

pH, in standard units

4

5

6

7

8

9

Figure 14. Relation between nitrate concentrations and pH values from data collected in the Fayetteville Shale gas-production area, 

north-central Arkansas. 
Shallow Groundwater Quality and Geochemistry 23

0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700 800

Dissolved solids, in milligrams per liter

0

1

2

3

Arsenic, in micrograms per liter

0

1

2

3

4

5

6

Iron, in milligrams per liter

Reduction of iron3+ to iron2+ Reduction of sulfate

Formation of iron-arsenic sulfide

Reduction of iron3+ to iron2+ Reduction of sulfate

Formation of iron-arsenic sulfide

A

B

Figure 15. Relation between dissolved solids concentrations and A, iron and B, arsenic concentrations from data collected in the 

Fayetteville Shale gas-production area, north-central Arkansas. 
24 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Thermogenic gas also occurs in the Atoka Formation, 

which overlies the Fayetteville Shale, in isolated areas within 

the Fayetteville Shale gas-production area, although it has 

been actively produced and occurs in abundance in the Atoka 

Formation in the western part of the State (Arkansas Oil and 

Gas Commission, 2012). The first gas well producing from 

the lower Pennsylvanian Morrow Group rocks in western 

Arkansas was drilled in 1949 to a total depth of 5,000 feet 

and penetrated rocks of Pennsylvanian, Mississippian, 

Devonian, Silurian, and Ordovician age (Lantz, 1950)—the 

gas producing zone being the Atoka Formation within the 

Morrow Group. Although the production of methane gas 

from the Atoka Formation in the Fayetteville Shale gasproduction zone is limited (possibly loss of the methane over 

time through increased fracture pathways in the Fayetteville 

Shale production area), pockets of methane gas do occur in 

the Atoka Formation throughout the study area (Arkansas Oil 

and Gas Commission, 2012). As such, upward seepage of 

gas from the Atoka Formation is one possible source of the 

extremely low thermogenic methane concentrations observed 

in the shallow aquifer in the study area, as determined from 

the relatively heavy δ13C signatures.

Trace Metals

Trace metal concentrations appear to result from 

either rock-water interactions or the occurrence of low 

pH water—higher metal concentrations occurring with 

enhanced metal solubilities at low pH (Hem, 1989). Boron 

increased with increasing dissolved solids with the highest 

concentration observed being 420 micrograms per liter (μg/L). 

Boron was virtually absent (less than 5 μg/L) in samples 

with dissolved solids concentrations less than approximately 

50 mg/L, indicating that shale formations, rather than 

sandstone formations, are the source rock for boron. All boron 

concentrations greater than 100 μg/L occurred at dissolved 

solids concentrations exceeding 200 mg/L (fig. 17). Lithium 

and barium similarly showed positive, albeit weakly linear, 

trends with increasing dissolved solids concentration, and, 

similar to boron, tend to be related to groundwater in wells 

completed in shale formations.

Strontium geochemical behavior is very similar to 

calcium, and small amounts of strontium can be incorporated 

in carbonate rocks (Appelo and Postma, 1999). Groundwater 

analyses indicated that sandstone rocks in the study area were 

effectively free of carbonates; as such, strontium is largely 

absent (less than 20 μg/L) for samples with dissolved solids 

concentrations less than 75 mg/L. Strontium concentration 

increased with increasing dissolved solids concentrations; 

strontium concentrations generally were less than 50 μg/L 

for dissolved solids concentration less than 100 mg/L, and 

showed an upward trend thereafter with the greatest strontium 

concentration of 953 μg/L occurring at a dissolved solids 

concentration of 562 mg/L (fig. 18). 

0 100 200 300 400 500 600 700 800

Dissolved solids, in milligrams per liter

0

5

10

15

20

25

30

Methane, in milligrams per liter

Figure 16. Relation between dissolved solids and methane concentrations from data collected in the Fayetteville Shale gas-production 

area, north-central Arkansas. 
Shallow Groundwater Quality and Geochemistry 25

Dissolved solids, in milligrams per liter

0 100 200 300 400 500 600 700 800

0

50

100

150

200

250

300

350

400

450

Boron, in micrograms per liter

Figure 17. Relation between dissolved solids and boron concentrations from data collected in the Fayetteville Shale gas-production 

area, north-central Arkansas. 

Dissolved solids, in milligrams per liter

0 100 200 300 400 500 600 700 800

0

200

400

600

800

1,000

1,200

Strontium, in micrograms per liter

Figure 18. Relation between dissolved solids and strontium concentrations from data collected in the Fayetteville Shale production 

area, north-central Arkansas. 
26 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Appreciable concentrations of aluminum, cobalt, 

copper, lead, nickel, and zinc concentrations were observed 

only in low pH waters. Because low pH waters contained 

low dissolved solids concentrations, these metals also 

were observed in groundwater with low dissolved solids 

concentrations. Because these metals often are associated with 

metal household plumbing (Hem, 1989), a potential source 

for these metals in domestic well-water systems is leaching 

of metals from well casing, plumbing, and associated metalbased sealants. Most elevated concentrations for these trace 

metals occurred at pH values below approximately 5.5 to 6.0, 

and concentrations dropped near to or below detection limits 

for pH values greater than 6.0.

Limitations Associated with Data 

Interpretation

Although groundwater water-quality data from 

127 domestic well-water samples indicate that resulting 

geochemistry appear to result from natural processes 

within the shallow aquifer, there are certain limitations to 

interpretation of the data, in addition to the implication that 

there are no apparent effects from gas-production activities. 

Because every domestic well within the Fayetteville Shale 

gas-production area was not sampled in studies to date, these 

findings do not preclude the possibility that one or more 

domestic wells may have some mixing of water associated 

with gas production from a spill or other fluid release 

into shallow groundwater; however, no evidence of such 

contamination was found for the spatial extent and timeframe 

covered by this study. Further, although the hydrogeologic 

model is one of relatively short groundwater flow paths 

confined by small-scale topographic boundaries with 

groundwater flowing from higher altitudes to valley floors 

(Imes and Emmett, 1994), the possibility of slow groundwater 

traveltimes could result in contaminants that had not reached 

sampled wells during the course of this study.

The most likely sources and transport pathways for 

contamination from gas-production activities are related to 

water-handling processes at the land surface, including leakage 

from temporary holding lagoons, pipe breaks and leaks, spills, 

and other water-handling activities related to drilling, storage, 

and transport of gas-production water. Difficulties in detecting 

minor contamination events with low mixing proportions arise 

because of the lack of a clear and distinct fingerprint for the 

source water and the fact that local groundwater chemistry 

is highly variable—making determination of mixing endmember compositions difficult—and that many shallow 

domestic wells are completed in shale formations, thus having 

source rock and rock-water interaction processes similar to 

the Fayetteville Shale. As such, care must be taken in simply 

comparing chloride concentrations in samples from domestic 

wells for current conditions (postgas production) to historical 

(pregas production) concentrations. 

If a pregas-production chloride concentration was 

not known for a domestic well prior to postgas-production 

sampling, it is possible that minor mixing may have occurred 

with a release of production waters, but the final mix 

composition resulted in chloride concentrations within the 

range of historical analyses. For example, if groundwater 

extracted from a domestic well had a pregas-production 

average chloride concentration of 10 mg/L, simple mixing 

curves reveal that mixing of 10 percent gas-production water 

with a chloride concentration of 2,500 mg/L with 90 percent 

shallow aquifer water from the domestic well, would result in 

a postgas-production chloride concentration of approximately 

250 mg/L in the domestic water supply, which is within the 

range of historical concentrations. Although pregas-production 

water-quality data were lacking for the wells sampled for this 

study, geochemical data presented a well-defined pattern of 

geochemical evolution based on natural rock-water interaction 

and microbially mediated processes, suggesting that the 

current water quality is derived from these natural processes 

within the shallow aquifer with no apparent effects from gasproduction activities. 

It should also be noted that additives in water used for 

hydrofracturing in the form of various organic compounds 

were not analyzed on samples collected for this study. Various 

organic compounds are added to hydrofracturing water for 

use as friction reducers, disinfectants (biocide), surfactants, 

scale inhibitors, and corrosion inhibitors (U.S. Environmental 

Protection Agency, 2004; King, 2012). These chemicals are 

a small part of the water formulation used for fracturing 

purposes and comprise upward to about 0.5 percent of the 

water. Much of the volume of chemicals injected into the 

well is absorbed on pipe and in the production formation, and 

chemicals returning from a well after a fracturing treatment 

are at a fraction (upward to about 40 percent for polymers) 

of the total volume pumped down the well (Friedmann, 

1986). In transport with a contaminant plume, various 

retardation and destruction processes would further lessen the 

concentrations including biodegradation, dispersion, dilution, 

sorption, volatilization, chemical and biological stabilization, 

transformation, and destruction of contaminants (U.S. 

Environmental Protection Agency, 1999). As such, the chance 

of finding these compounds in a domestic well downgradient 

from a gas-production area is greatly reduced unless a 

domestic well is in close proximity to a gas-production area 

and the contaminant plume comprises a large part of the 

volume of the water in an affected well.

The Arkansas Department of Environmental Quality 

(ADEQ) operates a water-quality laboratory in North 

Little Rock, Arkansas, and analyzes samples using U.S. 

Environmental Protection Agency approved methods, in 

addition to participating in the biannual USGS SRS program. 

The ADEQ sampled 51 domestic wells (unpublished data) 

distributed throughout the Fayetteville Shale production area 

in 2011 for a suite of volatile organic compounds, including 

benzene, toluene, ethylbenzene, and xylene, using U.S. 

Environmental Protection Agency method 8260C, Volatile 
Summary 27

Organics by Gas Chromatography/Mass Spectrometry (U.S. 

Environmental Protection Agency, 2012a). No organic 

compounds used as additives by the gas industry were 

detected in any of the samples. Chloroform, a common 

laboratory contaminant (Agency for Toxic Substances and 

Disease Registry, 1997), was detected in two wells, and 

three disinfection products (U.S. Environmental Protection 

Agency, 2012a), chloroform, dibromochloromethane, and 

bromodichloromethane, were found in one domestic well that 

used chlorine to disinfect the well (Roger Miller, Arkansas 

Department of Environmental Quality, written commun., 

September 18, 2012). These results provide supporting 

evidence that lack of organic chemistry data does not 

undermine investigative techniques using inorganic chemical 

constituents as a primary indication of contamination by 

waters used for hydrofracturing and other processes. 

If any cases of groundwater contamination currently exist 

from the excursion of fluids used and produced by the shalegas industry within the gas-production area, they are likely 

isolated and of limited extent. This supposition is supported by 

summary accounts of lengths of gasoline-contaminant plumes, 

which average less than approximately 300 feet (Ruiz-Aguilar 

and others, 2003; Falta, 2004) with maximum plume lengths 

of approximately 8,000 feet (Falta, 2004). Contamination 

of shallow groundwater by release of gas-production water 

of high salinity could result in greater plume lengths of the 

conservative chloride ion, which would depend on the sourcewater concentration, groundwater velocity, and dispersion 

values.

Implications of Study Results

Although there are data limitations in all environmental 

studies, certain implications can be made from the data and 

interpretations associated with this study. Methods used to 

evaluate the data (comparison of historical with current data, 

comparison of data at different distances from gas-production 

wells, evaluation of geochemical trends using inorganic 

constituents and methane) revealed no indication of systemic, 

regional effects on shallow groundwater quality from shalegas production. Comparative analyses demonstrated that 

maximum and median chloride concentrations for data 

from this study were below that of historical (prior to gas 

production) chloride concentrations, and, more importantly, 

that chloride concentrations for wells less than 2 miles 

from gas-production wells were not significantly different 

from chloride concentrations more than 2 miles from gasproduction wells. Additionally, groundwater-quality data 

collected for this study indicated that groundwater chemistry 

in the shallow aquifer system in the study area is a result 

of natural processes, controlled by geochemical rock-water 

interaction and microbially mediated redox reactions.

Relations between various inorganic chemical 

constituents suggest a natural evolution of chemistry from a 

low pH, low dissolved solids water in contact with sandstone 

formations that are poorly buffered with respect to carbonates 

to strongly calcium-bicarbonate and sodium-bicarbonate 

water types as a result of carbonate dissolution and cation 

exchange within shale formations. Redox reactions also 

proceed along this same evolutionary trend from more 

oxidized to more reducing conditions, resulting in the most 

strongly reduced condition in groundwater—production 

of methane—associated with the highest dissolved solids 

concentrations. This information indicated that natural 

processes can account for the geochemistry of the shallow 

groundwater in the study area.

The findings from this study were based on the sampling 

of 127 domestic wells in the western part of the shale-gas 

production area, representing approximately one-third of 

the entire gas-production area. These findings are similar 

to those of Nottmeier (2012), who sampled 100 wells 

distributed across the entire shale-gas production area in 

north-central Arkansas. Nottmeier (2012) also made 

comparisons to historical analyses and similarly attributed 

primary control of groundwater geochemistry to natural 

rock-water interaction. A review of groundwater inorganic 

chemistry with particular emphasis on chloride concentration 

was used for this study and that of Nottmeier (2012) to 

indicate that no regional effects on groundwater are apparent 

from activities related to gas production in the Fayetteville 

Shale in north-central Arkansas. 

Results from this study represent a timeframe 

relatively early in the gas-production life cycle, and any 

contaminants released during production activities may 

not have had sufficient time to reach the sampled wells. 

As such, groundwater-quality data from this study describe 

current conditions at the date of sampling and do not address 

potential legacy problems, if any should occur in the future; 

however, these data provide a baseline range and variation of 

geochemistry for groundwater in the study area, which can be 

used to assess future potential changes to groundwater quality 

in the area of gas production from the Fayetteville Shale.

Summary

The Mississippian Fayetteville Shale serves as an 

unconventional gas reservoir across north-central Arkansas, 

ranges in thickness from approximately 50 to 550 feet, and 

varies in depth from approximately 1,500 to 6,500 feet below 

the ground surface. Primary permeability in the Fayetteville 

Shale is severely limited, and successful extraction of the 

gas reservoir is the result of advances in horizontal drilling 

techniques and hydraulic fracturing that enhance and develop 

secondary fracture porosity and permeability. Drilling and 

production of gas wells began in 2004, with a steady increase 

in production thereafter. As of April 2012, approximately 

4,000 producing gas wells had been completed in the 

Fayetteville Shale in Arkansas.
28 Shallow Groundwater Quality and Geochemistry in the Fayetteville Shale Gas-Production Area, North-Central Arkansas

Gas-well drilling and completion activities in the 

Fayetteville Shale have the potential to affect water quality in 

shallow aquifers. Potential sources of contamination include 

fluids associated with the drilling operation and spent water 

from the fracturing process. Contaminant transport pathways 

include potential leakage from earthen pits used to store 

drilling mud and other process waters including hydraulic 

fracturing and flowback fluids, leakage from pipes, and losses 

from overflows, spills, and other unexpected releases. The 

process of fracture propagation during hydraulic fracturing 

adds a lesser but additional threat to shallow aquifer systems 

by creation of new fracture sets, enlargement of existing 

vertical fractures, and upward migration of pressurized fluids 

through poorly cemented annulus sections of the gas well. 

The potential for migration of gas-production fluids is greater 

where the gas-production zone is shallow or where deep 

faulting intersects the vertical or horizontal well bore. 

Surface and subsurface geological formations of 

interest in the study area are comprised of Pennsylvanian 

and Mississippian sandstones and shales with occasional thin 

limestone strata. From oldest to youngest, these formations 

include the Mississippian Fayetteville Shale, and the Hale, 

Bloyd, and Atoka Formations of Pennsylvanian age. Domestic 

wells are completed in these Pennsylvanian formations with 

limited yields from secondary fracture permeability. In Van 

Buren and Faulkner Counties, 127 wells were sampled and 

analyzed for major ions and trace metals with a subset of the 

samples analyzed for methane and carbon isotopes.

Because little to no monitoring had been conducted 

prior to drilling and gas production, interpretation of waterquality data from domestic well-water samples following 

gas production relies heavily on comparative analysis. One 

type of comparative analysis is to compare source-water 

chemistry to that of shallow groundwater in the search of 

indicator constituents unique to the source (gas production) 

water. Difficulties with this type of comparison are that many 

of the shallow domestic wells are completed in or influenced 

by shale chemistry, similar to the source rock for production 

of gas. Two types of additional comparative analyses 

include the statistical comparison of historical shallow 

groundwater quality collected within the gas-production area 

to water quality collected for this study, and comparison of 

groundwater quality in similar geologic and topographic areas 

within and outside of the gas-production area.

Historical groundwater quality data from 43 wells 

were extracted from the U.S. Geological Survey National 

Water Information System data base for the six counties that 

comprise the bulk of permitted and active gas-production 

wells: Cleburne, Conway, Faulkner, Independence, Van Buren, 

and White Counties. Chloride serves as an early indicator 

of potential effects from gas-production water for several 

reasons: (1) chloride is very conservative in its transport, it 

does not react with other minerals, adsorb to clays or organics, 

and chloride salts are soluble at extremely high concentrations; 

and (2) chloride is elevated in the gas-production waters—

initial flowback fluids vary from approximately 2,500 to 5,000 

milligrams per liter (mg/L) and later produced waters range 

upward to greater than 20,000 mg/L. The highest chloride 

concentration from the historical dataset was 378 mg/L; two 

samples had concentrations greater than 200 mg/L, and six 

samples had concentrations between 100 to 200 mg/L. The 

median chloride concentration was 20 mg/L. 

Chloride concentrations for the 127 groundwater samples 

collected for this study ranged from approximately 1.0 mg/L 

to 70 mg/L, with a median concentration of 3.7 mg/L and 

statistically were lower than chloride concentrations from 

the historical water-quality data. Major ions and trace metals 

additionally were lower in the data gathered for this study than 

in the historical analyses. Additionally, there was no statistical 

difference (Wilcoxon rank-sum p-value of 0.71) between 

groundwater quality for samples collected from 94 wells 

located less than 2 miles from gas-production wells and 33 

wells located more than 2 miles from gas-production wells.

Major ion chemistry was investigated to understand 

the effects of geochemical and reduction-oxidation (redox) 

processes on the shallow groundwater in the study area along 

a continuum of increased rock-water interaction represented 

by increases in dissolved solids concentration. Groundwater 

in sandstone formations that are relatively carbonate free is 

represented by a low dissolved solids (less than 30 mg/L), 

slightly acidic water type. With the dissolution of carbonates 

in shale formations in the study area, dissolved solids 

concentrations increase upward to 686 mg/L, and water type 

evolves from a dominantly mixed-bicarbonate and calciumbicarbonate to a strongly sodium-bicarbonate water type as 

cation exchange reaches a maximum. 

Methane concentration and carbon isotopic composition 

were analyzed in 51 of the 127 samples collected for this 

study. Methane was detected above a detection limit of 

0.0002 mg/L in 32 of the 51 samples, with concentrations 

ranging upward to 28.5 mg/L. Seven samples had methane 

concentrations greater than or equal to 0.5 mg/L. The carbon 

isotopic composition of these higher concentration samples, 

including the highest concentration of 28.5 mg/L, shows 

that the methane was likely biogenic in origin with stable 

carbon isotope ratio (δ13C) values ranging from -57.6 to 

-74.7‰. Methane concentrations correlated to dissolved solids 

concentrations, indicating that increasing reducing conditions 

correspond to more geochemically evolved groundwater with 

increasing rock-water interaction.

Although preproduction water-quality data were lacking 

for the wells sampled for this study, geochemical data 

presented a well-defined pattern of geochemical evolution 

based on natural rock-water and microbially mediated 

processes, strongly suggesting that the resulting water quality 

is derived from these natural processes with no effects from 

gas-production activities. Results from the groundwaterquality monitoring activities for this study provide a 

baseline range and variation of geochemistry for the shallow 

groundwater in the study area, which can be used to assess 

future potential changes to groundwater quality in the area of 

gas production from the Fayetteville Shale. 
References Cited 29

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