U.S. Department of the Interior 

U.S. Geological Survey 

Water Quality and Environmental Isotopic Analyses of 

Ground-Water Samples Collected from the Wasatch and 

Fort Union Formations in Areas of Coalbed Methane 

Development—Implications to Recharge and GroundWater Flow, Eastern Powder River Basin, Wyoming 

By Timothy T. Bartos and Kathy Muller Ogle 

Water-Resources Investigations Report 02-4045 

Prepared in cooperation with the 

WYOMING STATE ENGINEER’S OFFICE and the 

BUREAU OF LAND MANAGEMENT 

Cheyenne, Wyoming 

2002 
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

Any use of trade, product, or firm names in this publication is for 

descriptive purposes only and does not imply endorsement by the 

U.S. Government 

For additional information write to: 

District Chief


U.S. Geological Survey, WRD


2617 E. Lincolnway, Suite B


Cheyenne, Wyoming 82001-5662


Copies of this report can be purchased from: 

U.S. Geological Survey


Branch of Information Services


Box 25286, Denver Federal Center


Denver, Colorado 80225

CONTENTS


Page 

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

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

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

Numbering system for wells and springs.................................................................................................................... 4 

Methods of investigation............................................................................................................................................. 6 

Description of study area ...................................................................................................................................................... 7 

Climate ........................................................................................................................................................................ 7 

Geographic and geologic setting................................................................................................................................. 7 

Hydrogeologic setting ................................................................................................................................................. 10 

Resource extraction..................................................................................................................................................... 19 

Acknowledgments....................................................................................................................................................... 20 

Water-level measurements .................................................................................................................................................... 20 

Ground-water quality ............................................................................................................................................................ 24


Quality assurance and quality control......................................................................................................................... 26


Major-ion chemistry.................................................................................................................................................... 27


Classification of ionic composition and comparisons between aquifers .......................................................... 30


Areal distribution .............................................................................................................................................. 35


Vertical distribution within aquifers.................................................................................................................. 38


Vertical distribution between aquifers............................................................................................................... 40


Geochemical processes ..................................................................................................................................... 41


Environmental isotopes............................................................................................................................................... 53


Tritium............................................................................................................................................................... 53


Stable isotopes of oxygen and hydrogen........................................................................................................... 56


Hydrogeologic implications of observed major-ion chemistry and environmental isotopes................................................ 63 

Comparison of water quality to regulatory standards ........................................................................................................... 69 

Public supply and domestic use .................................................................................................................................. 69 

Agricultural use........................................................................................................................................................... 70 

Summary ............................................................................................................................................................................... 74 

Selected references................................................................................................................................................................ 75 

Appendix—Water-quality analyses....................................................................................................................................... 85 

Figures 

1. Map showing location of the study area in the Powder River Basin, Wyoming ................................................... 3 

2. Diagram showing systems for numbering springs and wells ................................................................................ 5 

3.	 Generalized stratigraphic column of the Wasatch and Fort Union Formations in the central Powder River 

Basin, Wyoming .............................................................................................................................................. 8 

4.	 Map showing generalized geology, clinker in the study area, lines of geologic cross sections, and 

ground-water sampling locations in the eastern Powder River Basin, Wyoming, 1999 ................................. 11 

5.	 Diagram showing correlation of coal beds in the Powder River coal field, eastern Powder River 

Basin, Wyoming .............................................................................................................................................. 12 

6. Geologic cross sections, eastern Powder River Basin, Wyoming.......................................................................... 14 

7.	 Chart showing relation of Cenozoic stratigraphic units to hydrogeologic units, eastern Powder 

River Basin, Wyoming..................................................................................................................................... 16 

8.	 Map showing generalized geology, potentiometric contours for Wyodak-Anderson coalbed aquifer, 

and ground-water sampling locations in the study area, eastern Powder River Basin, Wyoming, 1999 ........ 17 

9.	 Map showing coal-lease areas, coalbed methane permitted or production wells, and ground-water 

sampling sites in the study area, eastern Powder River Basin, Wyoming, 1999............................................. 21 

CONTENTS iii 
Figures--Continued 

Page 

10-12. Graphs showing: 

10. Physical characteristics of ground water, eastern Powder River Basin, Wyoming, 1999 ............................ 30 

11. General mineral characteristics of ground water, eastern Powder River Basin, Wyoming, 1999................ 31 

12. Concentrations of major ions in ground water, eastern Powder River Basin, Wyoming, 1999 ................... 32 

13. Trilinear diagram showing water types for ground-water samples collected from springs, Wasatch aquifer, 

and coalbed aquifers, eastern Powder River Basin, 1999................................................................................. 34 

14. Stiff diagrams for ground-water samples collected from coalbed methane production wells, monitoring 

wells, and springs, and generalized geology in the study area, eastern Powder River Basin, 

Wyoming, 1999 ................................................................................................................................................ 37 

15-19. Graphs showing: 

15. Calcium and sulfate concentrations in relation to well depth, Wasatch aquifer, eastern Powder River 

Basin, Wyoming, 1999 ............................................................................................................................ 39 

16. Potassium concentrations in relation to well depth, coalbed aquifers, eastern Powder River Basin, 

Wyoming, 1999 ....................................................................................................................................... 40 

17. Calcium, magnesium, sodium, bicarbonate, and sulfate concentrations in relation to the clustered 

wells W5, W4, W3, and C12 in the eastern Powder River Basin, Wyoming, 1999................................ 42 

18. Calcium, magnesium, sodium, bicarbonate, and sulfate concentrations in relation to the clustered 

wells W8 and C17 in the eastern Powder River Basin, Wyoming, 1999 ................................................ 44 

19. Calcium, magnesium, sodium, bicarbonate, and sulfate concentrations in relation to the clustered 

wells W7 and C16 in the eastern Powder River Basin, Wyoming, 1999 ................................................ 46 

20. Diagram of conceptual model of major-ion composition in relation to hydrogeologic units, eastern 

Powder River Basin, Wyoming ........................................................................................................................ 50 

21-23. Graphs showing: 

21. Relation between δ18O (oxygen-18/oxygen-16 isotopic ratio) and δ2H (deuterium/hydrogen isotopic 

ratio) for ground-water samples collected from springs discharging from clinker, the Wasatch 

aquifer, and coalbed aquifers, eastern Powder River Basin, Wyoming, 1999......................................... 59 

22. δ2

H (deuterium/hydrogen isotopic ratio), δ18O (oxygen-18/oxygen-16 isotopic ratio), and major-ion 

chemistry in relation to the clustered wells W5, W4, W3, and C12 in the eastern Powder River 

Basin, Wyoming, 1999 ............................................................................................................................ 60 

23. δ2

H (deuterium/hydrogen isotopic ratio), δ18O (oxygen-18/oxygen-16 isotopic ratio), and major-ion 

chemistry in relation to the clustered wells W7 and C16 and clustered wells W8 and C17 in the 

eastern Powder River Basin, Wyoming, 1999 ......................................................................................... 64 

24-25. Maps showing: 

24. δ2

H (deuterium/hydrogen isotopic ratio) values for coalbed aquifers in the study area, eastern 

Powder River Basin, Wyoming, 1999 ..................................................................................................... 66 

25. δ2H (deuterium/hydrogen isotopic ratio) values for springs discharging from clinker and for wells 

completed in the Wasatch aquifer in the study area, eastern Powder River Basin, Wyoming 1999 ....... 67 

26. Diagram showing suitability of water for use in irrigation based on analyses of water from springs, 

Wasatch aquifer, and coalbed aquifers, eastern Powder River Basin, Wyoming, 1999 ................................... 73 

Tables 

1. Identification numbers, well-completion information, water levels, geologic units, and assigned 

hydrogeologic units for wells sampled in the study area ................................................................................. 22 

2. Differences in water-level elevations and hydraulic potential for vertical ground-water flow in monitoringwell clusters ...................................................................................................................................................... 24 

3. Source or cause, and significance of dissolved-mineral constituents and physical properties of water................ 25 

4. Calculated relative-percent differences between normal environmental and replicate ground-water samples ..... 27 

5. Summary of water-quality data for ground-water samples collected from wells completed in the 

Wasatch aquifer ................................................................................................................................................ 28 

iv WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Tables--Continued 

Page 

6. Summary of water-quality data for ground-water samples collected from wells completed in coalbed aquifers. 29


7. Results of rank-sum tests between Wasatch aquifer and coalbed aquifers............................................................ 35


8.	 Kendall’s tau correlation coefficients and resulting probabilities (p-values) between dissolved solids or 


major ions and well depth, Wasatch aquifer..................................................................................................... 38


9.	 Kendall’s tau correlation coefficients and resulting probabilities (p-values) between dissolved solids or 


major ions and well depth, coalbed aquifers .................................................................................................... 38


10. Tritium concentrations in ground-water samples collected from springs and wells ............................................. 55


11.	 δ2H (deuterium/hydrogen isotopic ratio) and δ18O (oxgen-18/oxygen-16 isotopic ratio) values for ground-


water samples collected from springs discharging from clinker and wells completed in the Wasatch 


aquifer and coalbed aquifers............................................................................................................................. 57


12.	 Comparison of ground-water-quality samples collected from wells with selected U.S. Environmental 


Protection Agency maximum and secondary maximum contaminant levels for public drinking-


water supplies ................................................................................................................................................... 69


13.	 Comparison of ground-water-quality samples collected from wells with selected State of Wyoming 


standards for domestic use ............................................................................................................................... 70


14. Hardness classification of ground-water-quality samples collected from wells.................................................... 70


15.	 Comparison of ground-water-quality samples collected from wells with selected State of Wyoming 


standards for agricultural (irrigation) and livestock use................................................................................... 71


CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS 

Multiply By To obtain 

Length 

inch (in) 25.40 millimeter (mm) 

inch (in) 72.540 centimeter (cm) 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

Area 

square mile (mi2) 2.590 square kilometer (km2

) 

Volume 

liter (L) 33.82 ounce (oz) 

milliliter (mL) 0.0338 ounce (oz) 

microliter (μL) 0.0000338 ounce (oz) 

Hydraulic Conductivity 

foot per day (ft/d) 0.3048 meter per day (m/d) 

Transmissivity 

foot squared per day (ft2/d) 0.0929 meter squared per day (m2

/d) 

Concentration 

milligram per liter (mg/L) 1.0 part per million (ppm) 

microgram per liter (μ g/L) 1.0 part per billion (ppb) 

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

°F = 9/5 (°C) + 32


°C = 5/9 (°F - 32) 

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929--a geodetic datum derived from a general 

adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929. 

CONTENTS v 
Abbreviated water-quality units used in this report: 

‰ 

δ2H 

δ18O 

meq/L 

mg/L 

μg/L 

μm 

μS/cm 

pCi/L 

TU 

Abbreviations used in this report: 

°C 

°F 

2H 

3

H 

16O 

18O 

ANC


BLM


Ca


CaCO3


CH4


Cl


cm


CO2


F


ft


GMWL


H


H2S


H2O


HCO3


K


m


MCL


Mg


Na


NWQL


RPD


SAR


Si


SMCL


SO4


TDS


USEPA


USGS


VSMOW


WSEO


per mil or parts per thousand


deuterium/hydrogen isotopic ratio


oxygen-18/oxygen-16 isotopic ratio


milliequivalents per liter


milligram per liter


microgram per liter


micrometer


microsiemens per centimeter at 25 degrees Celsius


picocuries per liter


tritium units


degrees Celsius


degrees Fahrenheit


deuterium


tritium


oxygen-16


oxygen-18


acid-neutralizing capacity


Bureau of Land Management


carbon


calcium


calcium carbonate


methane


chloride


centimeter


carbon dioxide


fluoride


foot


Global meteoric water line


hydrogen


hydrogen sulfide


water


bicarbonate


potassium


meter


maximum contaminant level


magnesium


sodium


National Water Quality Laboratory of U.S. Geological Survey


relative-percent difference


sodium-adsorption ratio


silica


secondary maximum contaminant level


sulfate


total dissolved solids


U.S. Environmental Protection Agency


U.S. Geological Survey


Vienna Standard Mean Ocean Water


Wyoming State Engineer’s Office


vi WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 

C 
Water Quality and Environmental Isotopic Analyses of 

Ground-Water Samples Collected from the Wasatch and 

Fort Union Formations in areas of Coalbed Methane 

Development—Implications to Recharge and GroundWater Flow, Eastern Powder River Basin, Wyoming 

By Timothy T. Bartos and Kathy Muller Ogle 

ABSTRACT 

Chemical analyses of ground-water samples 

were evaluated as part of an investigation of lower 

Tertiary aquifers in the eastern Powder River Basin 

where coalbed methane is being developed. 

Ground-water samples were collected from two 

springs discharging from clinker, eight monitoring 

wells completed in the Wasatch aquifer, and 

13 monitoring or coalbed methane production 

wells completed in coalbed aquifers. The groundwater samples were analyzed for major ions and 

environmental isotopes (tritium and stable isotopes 

of hydrogen and oxygen) to characterize the composition of waters in these aquifers, to relate these 

characteristics to geochemical processes, and to 

evaluate recharge and ground-water flow within 

and between these aquifers. This investigation was 

conducted in cooperation with the Wyoming State 

Engineer’s Office and the Bureau of Land Management. 

Water quality in the different aquifers was 

characterized by major-ion composition. Samples 

collected from the two springs were classified as 

calcium-sulfate-type and calcium-bicarbonatetype waters. All ground-water samples from the 

coalbed aquifers were sodium-bicarbonate-type 

waters as were five of eight samples collected from 

the overlying Wasatch aquifer. 

Potential areal patterns in ionic composition 

were examined. Ground-water samples collected 

during this and another investigation suggest that 

dissolved-solids concentrations in the coalbed 

aquifers may be lower south of the Belle Fourche 

River (generally less than 600 milligrams per liter). 

As ground water in coalbed aquifers flows to the 

north and northwest away from an inferred source 

of recharge (clinker in the study area), dissolvedsolids concentrations appear to increase. 

Variation in ionic composition in the vertical 

dimension was examined qualitatively and statistically within and between aquifers. A relationship 

between ionic composition and well depth was 

noted and corroborates similar observations by 

earlier investigators in the Powder River Basin in 

both Wyoming and Montana. This relationship 

results in two different water-quality zones with 

different characteristics – a shallow zone, comprising the upper part of the Wasatch aquifer, characterized by mixed cation composition and either 

sulfate or bicarbonate as the dominant anion; and a 

deeper zone, comprising the lower (deeper) part of 

the Wasatch aquifer and the underlying coalbed 

aquifers, characterized by sodium-bicarbonatetype waters. The zonation appears to be related to 

geochemical processes described by earlier investigators such as dissolution and precipitation of 

minerals, ion exchange, sulfate reduction, and 

mixing of waters. Qualitative and statistically significant differences were observed in sulfate concentrations between the coalbed aquifers and the 

overlying Wasatch aquifer. Ionic composition suggests that bacterially mediated redox processes 

such as sulfate reduction were probably the dominant geochemical processes in the anaerobic coalbed aquifers. 

ABSTRACT 1 
Tritium was used to qualitatively estimate 

the time of ground-water recharge. Tritium concentrations in both springs suggests that both were 

recharged after 1952 and contain modern water. 

Tritium was not detected at concentrations suggestive of modern water in any ground-water samples 

collected from the coalbed aquifers or in six of 

eight ground-water samples collected from the 

overlying Wasatch aquifer. Tritium concentrations 

in the remaining two wells from the Wasatch aquifer suggest a mixture between submodern 

(recharged before 1952) and modern water, 

although the low concentrations suggest that 

ground water in these two wells have very little 

modern water. The relative absence of modern 

water in all aquifers in the study area suggests that 

recharge processes to these aquifers are probably 

very slow. 

Paired δ2H (deuterium/hydrogen isotopic 

ratio) and δ18O (oxygen-18/oxygen-16 isotopic 

ratio) values for samples collected from the springs 

and all aquifers are close to the Global Meteoric 

Water Line, a meteoric water line for North American continental precipitation, and an estimated 

local meteoric water line, suggesting the water in 

the aquifers is of meteoric origin. The δ2H and 

δ18O values suggest that the waters were recharged 

in a colder climate or temperature, mid-latitudes, 

and mid-continent. In general, the samples do not 

form separate groups based on aquifer origin; this 

suggests either intermixing of the waters in the 

aquifers or that the different aquifers are subject to 

similar recharge and/or evolutional paths for the 

water. However, examination of the differences in 

the values of δ2

H and δ18O, in combination with 

major-ion chemistry at three monitoring-well 

clusters, suggest that changes in the values with 

depth may represent different timing or sources of 

recharge to the different aquifers. 

The areal distribution of δ2H was examined 

and an apparent break in the δ2

H along a northwest 

to southeast trend was observed. In the coalbed 

aquifers, all but one ground-water sample (collected from the Big George coal bed), show a pattern where the δ2H values become more negative 

towards the center of the Powder River Basin and 

values greater (less negative) than an arbitrary reference value of -140 ‰ (per mil or parts per thousand) were observed near the outcrop area of the 

Wyodak-Anderson coal zone. In the overlying 

Wasatch aquifer, the δ2

H values became less negative towards the center of the basin. The values 

more negative than -140 ‰ are near the outcrop 

area and the values that are less negative than 

-140 ‰ are closer to the basin center. It is unclear 

if this pattern is a result of sample size, different 

recharge mechanisms, geochemical processes, or 

if the processes producing these differences are 

independent or unrecognized. 

Results of water-quality sampling were 

compared with selected regulatory and nonregulatory standards as well as commonly-used 

guidelines for proposed water uses. Dissolved solids was the measure that most frequently exceeded 

U.S. Environmental Protection Agency public 

water-supply standards and State of Wyoming 

domestic-use standards in ground-water samples 

collected from the Wasatch aquifer and coalbed 

aquifers. The State of Wyoming agricultural standards (irrigation) for sulfate and dissolved solids 

were exceeded in some samples collected from the 

Wasatch aquifer and coalbed aquifers. The State of 

Wyoming livestock standard for pH was exceeded 

in some samples collected from the Wasatch aquifer. Water from the Wasatch aquifer ranged from 

soft to very hard, and water from the coalbed aquifers ranged from moderately hard to very hard. 

Samples collected from wells completed in both 

the Wasatch aquifer and coalbed aquifers plotted in 

a wide range of both sodium- and salinity-hazard 

classes, but most samples clustered in or near the 

combined medium-sodium-hazard—high-salinityhazard classes. 

INTRODUCTION 

The Powder River Basin (fig. 1) in northeastern 

Wyoming is an important source of energy resources 

for the entire United States. Oil, conventional natural 

gas, uranium, coal, and most recently, coalbed methane 

(i.e., natural gas), are all developed in the basin. Since 

1997, the development of coalbed methane has rapidly 

2 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
INTRODUCTION 3
increased as demand for natural gas has increased in 

the United States. Coalbed methane is produced by 

pumping water from wells installed in coal beds that 

contain methane. The removal and discharge of this 

water has renewed interest in hydrological and 

geochemical processes that control the water quality 

and volumes of produced water in the coalbed aquifers. Consequently, the U.S. Geological Survey 

(USGS), in cooperation with the Wyoming State Engineer’s Office (WSEO) and the Bureau of Land Management (BLM), conducted an investigation to 

enhance the understanding of the characteristics of the 

aquifers associated with the development of this natural resource in the Powder River Basin. This report 

documents the results of the investigation. 

Purpose and Scope 

The purpose of this investigation was to increase 

understanding of the aquifers in the Wasatch and Fort 

Union Formations of lower Tertiary age in the eastern 

Powder River Basin in northeastern Wyoming. The 

scope of the investigation included the collection of 

ground-water samples from springs and wells and subsequent analysis of those samples for major ions, tritium, and oxygen and hydrogen isotopes to meet the 

following objectives: 

•	 Determine possible hydrologic and geochemical 

relationships between different aquifers and/or 

aquifer systems in the lower Tertiary formations, 

using qualitative and quantitative (statistical) 

comparisons of major-ion chemistry and environmental isotopic composition between aquifers 

•	 Determine spatial (areal) and vertical variation of 

major-ion chemistry and environmental isotopic 

composition within each aquifer 

•	 Examine major-ion chemistry in the context of other 

studies of the same aquifers in the Powder River 

Basin 

•	 Qualitatively estimate the time of ground-water 

recharge 

•	 Determine what can be inferred about ground-water 

recharge by comparing concentrations of stable 

isotopes of oxygen and hydrogen with the global 

meteoric water line, the meteoric water line for 

North American continental precipitation, and an 

estimated local meteoric water line 

•	 Determine what can be inferred about potential 

ground-water flowpaths and recharge characteristics of the different aquifers using major-ion 

chemistry and environmental isotopic composition 

•	 Characterize water quality in relation to waterquality standards using major-ion chemistry 

In addition, a bibliography was compiled as part of 

this investigation. While all of these references may 

not be directly cited in this report, they are listed in the 

Selected References section of this report to assist readers searching for additional information about coalbed 

methane and the geology and hydrogeology of lower 

Tertiary formations in the Powder River Basin. 

Numbering System for Wells and Springs 

Wells and springs are identified by location using a 

15 digit USGS well-identification number consisting of 

latitude, longitude, and a sequence number (fig. 2). For 

example, site 433652105075501 is the first site inventoried at a location having a latitude of 43 degrees, 36 

minutes, and 52 seconds, and a longitude of 

105 degrees, 7 minutes, and 55 seconds. The last two 

digits in the well-identification number (01) are a 

sequence number indicating the order of inventory. 

A local well number also is assigned to wells and 

springs using the Federal township-range system of 

land subdivision. An example of a local number established for a well used in this report is 45-072-03dbc01 

(fig. 2). The first number (45) denotes the township 

(T), the second number (072) denotes the range (R), 

and the third number (03) denotes the section. The first 

letter following the section number denotes the quarter 

section (160-acre tract); the second letter, the quarterquarter section (40-acre tract); and the third letter, if 

shown, the quarter-quarter-quarter section (10-acre 

tract). These subsections are designated a, b, c, and d 

in a counter-clockwise direction, beginning in the 

northeast quadrant. The last two digits in the local 

number are a sequence number indicating the order of 

inventory. For example, well 45-072-03dbc01 is the 

first well inventoried in the southwest quarter of the 

northwest quarter of the southeast quarter of section 3, 

T. 45 N., R. 072 W. 

4 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
INTRODUCTION 5 
Abbreviated site numbers are often used to describe 

well locations in place of USGS well-identification 

numbers, local well numbers, or local well names for all 

wells discussed in this report. For example, W1 indicates well W1 is completed in the Wasatch aquifer, and 

C1 indicates well C1 is completed in a coalbed aquifer. 

Site numbers, USGS well-identification numbers, and 

local well numbers are listed in the tables. Only site 

numbers are used in illustrations. 

Methods of Investigation 

Ground-water samples for water-quality analyses 

were collected during June 1999 from 27 locations in or 

near areas of coalbed methane development. Samples 

were collected from two springs discharging from clinker, eight BLM or WSEO monitoring wells completed 

in the Wasatch aquifer, seven BLM or WSEO monitoring wells completed in the coalbed aquifers, and ten 

coalbed methane production wells completed in the 

coalbed aquifers. 

Different sampling devices and techniques were 

used to collect the various ground-water samples. Samples were collected from the springs by submerging 

sampling containers as close as possible to the point of 

discharge. In addition, samples were not collected from 

the springs if it had rained several days prior to arrival at 

the site to ensure that the samples represented discharge 

from the springs and were not influenced by 

surface-water runoff. Submersible pumps were used to 

collect samples from all but one monitoring well; the 

remaining well had sufficient gas and artesian pressure 

(hydraulic head) to sample without a pump. Coalbed 

methane production wells were sampled using submersible pumps already present in the wells. Attempts were 

made to remove (purge) a minimum of three casing volumes of water from each monitoring well and to allow 

onsite measurements (pH, specific conductance, and 

temperature) to stabilize prior to sample collection to 

ensure that the sample collected represented the ground 

water rather than water stored in the well casing. During purging at several wells, gas-lock of the pump or 

water-level drawdown prevented removal of three casing volumes; these wells were sampled after the pump 

could be restarted or after sufficient time was allowed 

for water-level recovery (because of drawdown). Purging, or removal of three casing volumes of water, was 

not conducted at coalbed methane production wells 

because the wells are actively pumping water to produce methane; however, the wells were allowed to discharge water for some period of time prior to sample 

collection and onsite measurements of field parameters 

were monitored for stability. 

Quality-assurance procedures involved calibration 

of all field meters, collection of quality-control samples 

(replicates), and use of standardized USGS sampling, 

processing, and analytical techniques. Field meters 

were calibrated prior to sampling at each well. Qualitycontrol samples consisted of two replicate (duplicate) 

ground-water samples. Water-quality samples were 

collected, preserved, and shipped according to standardized USGS techniques described in Book 9 of the USGS 

National Field Manual for the Collection of WaterQuality Data (Wilde and others; 1998; Wilde and 

Radtke, 1998; Wilde and others, 1999). Water-quality 

samples were analyzed at USGS laboratories using 

standardized USGS laboratory analytical methods and 

laboratory quality-assurance procedures as documented 

in Fishman and Friedman (1989). Ground-water samples were analyzed by the inductively coupled plasma 

method with atomic emission spectrometry for major 

ions (Fishman and Friedman, 1989; Fishman, 1993), tritium by electrolytic enrichment with gas counting 

(Ostlund and Dorsey, 1977), δ2H (deuterium/hydrogen 

isotopic ratio) by hydrogen equilibrium (Coplen and 

others, 1991) and δ18O (oxygen-18/oxygen-16 isotopic 

ratio) by carbon dioxide equilibrium (Epstein and Mayeda, 1953). For this report, dissolved refers to constituents in water samples that passed through a 

0.45-micrometer disposable cartridge filter during sample collection. 

Each well was located using a military-grade Global Positioning System (GPS) receiver to obtain the latitude and longitude. The well was then plotted on the 

appropriate USGS Quadrangle maps (1:24,000 scale) to 

obtain land-surface elevations. 

Water levels in monitoring wells were measured 

prior to sampling. Each monitoring well was equipped 

with a dedicated water-level recorder that was removed 

prior to water-quality sampling. Water levels measured 

by the recorders were noted immediately prior to 

removal. Water levels could not be measured in coalbed 

methane production wells, so water-level measurements 

from these wells were obtained from WSEO well permits. Well-completion information (i.e., well depths, 

casing material, screened or opened intervals) also was 

obtained from WSEO well permits. 

6 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Water-quality data commonly are not normally distributed and commonly contain censored data values 

(unquantified data reported as being less than some 

value, such as a minimum reporting level (MRL)). In 

addition, nonnormality is difficult to detect with small 

sample sizes. Nonparametric statistics do not rely on an 

assumption that data are normally distributed, are resistant to the effects of outliers, and are more appropriate 

when sample sizes are small (Helsel and Hirsch, 1992). 

Therefore, nonparametric statistical methods are used 

exclusively to describe and analyze water-quality data in 

this report. Individual methods are described in the sections where used. 

DESCRIPTION OF STUDY AREA 

Climate 

The climate of the study area is classified as a 

steppe climate typical of semiarid grassland prairies 

(Martner, 1986). The average mean temperature in 

Gillette for 1961 to 1990 was 44.2 degrees Fahrenheit, 

but temperature and precipitation vary widely, depending upon the season (Western Regional Climate Center, 

2001). Mean annual precipitation for 1961 through 

1990 in Gillette was 16.61 inches (Western Regional 

Climate Center, 2001). Estimated mean annual evaporation greatly exceeds mean annual precipitation in the 

study area; mean annual evaporation estimated from 

Class-A pans ranges from about 65 to 70 inches per year, 

while mean annual evaporation estimated from lakes 

ranges from about 45 to 50 inches per year (Martner, 

1986, p. 177, and references therein). Estimated mean 

annual potential evapotranspiration also exceeds precipitation in most of the study area for 1951 through 1980 

and was about 22 to 23 inches, based on the Thornthwaite method (Martner, 1986, p. 182). Wind in the study 

area is primarily from the west, and the average hourly 

wind velocity in the area is about 13 miles per hour 

(Lowry, Wilson, and others, 1986). 

Geographic and Geologic Setting 

The name Powder River Basin has been used to 

refer to both a structural basin and a drainage basin. The 

structural basin and the drainage basin do not coincide 

and both are frequently used interchangeably to define 

the area. In this report, Powder River Basin refers to the 

structural basin. The Powder River Basin is an asymmetrical syncline formed during the Laramide orogeny 

(Late Cretaceous to early Tertiary age). The axis of the 

basin trends from southeast to northwest near the western margin of the basin (fig. 1), far from its geographic 

center. In Wyoming, the Powder River Basin is bounded 

by the Black Hills uplift in the northeast, the Hartville 

uplift in the southeast, the Laramie Mountains in the 

south, the Casper arch in the southwest, and the Bighorn 

Mountains in the west (fig. 1). The basin continues 

northward into Montana (not shown in fig. 1), where 

another structural feature, the Cedar Ridge anticline, 

separates it from the Williston Basin. The entire basin 

covers an area of more than 22,000 square miles (Flores 

and others, 1999). The Powder River drainage basin 

refers to the area in Wyoming and Montana drained by 

the perennial Powder River and its tributaries. The Powder River Basin includes not only the Powder River and 

associated tributaries, but also the upstream parts of the 

Belle Fourche River, Cheyenne River, and Tongue River 

drainage basins (fig. 1). The study area is in the eastern 

part of the Powder River Basin, within the unglaciated 

part of the Missouri Plateau of the Northern Great Plains 

and the Missouri River drainage basin. In the study area, 

the perennial Powder River and Little Powder River flow 

northward and the perennial Belle Fourche River flows 

northeastward (fig. 1). Tributaries to these rivers are primarily ephemeral, with streamflow resulting from precipitation (Lowry, Wilson, and others, 1986). Grasscovered plains, rolling hills, and tablelands dominate the 

landscape. Rivers and streams have eroded deeply in the 

area and have wide, flat streambeds and broad floodplains. Isolated flat-topped buttes, mesas, and ridges 

commonly are present several hundred feet above valley 

floors. 

Within the study area, the geologic units of interest 

are lower Tertiary in age. In ascending stratigraphic 

order, these geologic units are the Fort Union Formation 

of Paleocene age and the Wasatch Formation of Eocene 

age (fig. 3). Sediments comprising both formations have 

been interpreted to be continental in origin. The Fort 

Union Formation in the Powder River Basin was 

deposited by northeastward-flowing fluvial systems 

consisting of braided, meandering, and anastomosed 

streams in the basin center; these fluvial systems were 

fed by alluvial fans at the basin margin associated with 

surrounding ancestral uplifts (Flores and Ethridge, 

1985; Flores, 1986). The Wasatch Formation overlies 

the Fort Union Formation and is exposed at the surface 

DESCRIPTION OF STUDY AREA 7 
8 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS
throughout much of the Powder River Basin in Wyoming, including the study area (fig. 4). The Fort Union 

Formation is exposed primarily along the margins of 

the basin in Wyoming, including the eastern edge of the 

study area (fig. 4). The Wasatch Formation was deposited in conditions similar to those of the Fort Union 

Formation, primarily fluvial, lacustrine, and swampy 

environments (Seeland, 1992; Ellis and others, 1999a). 

The Fort Union Formation in the Powder River 

Basin ranges in thickness from 2,300 to 6,000 ft (feet) 

(Curry, 1971) and is subdivided into three members – 

the Tullock, Lebo, and Tongue River Members (fig. 3). 

Along the eastern margin of the Powder River Basin, 

including the study area, the Fort Union Formation is 

nearly flat and dips to the west about 2 to 3 degrees 

(Glass, 1997). Near the western margin, the Fort Union 

Formation dips to the east from 10 to 25 degrees (Glass, 

1997). The Tullock Member ranges in thickness from 

370 to about 1,440 ft (Brown, 1993) and consists primarily of lenticular, discontinuous, fine to medium-grained 

sandstone beds interbedded with fine-grained sediments 

such as siltstone, claystone, mudstone, carbonaceous 

shale, rare limestone, and thin coal beds (Curry, 1971). 

The Lebo Member ranges in thickness from about 499 

to about 1,700 ft (Law, 1975; Lewis and Hotchkiss, 

1981) and consists primarily of shale or mudstone interbedded with lesser amounts of sandstone, siltstone, and 

sparse, very thin coal beds. Like the Tullock Member, 

the Tongue River Member (as much as 1,860 ft thick 

(Curry, 1971)) also consists primarily of lenticular, discontinuous, fine to medium-grained sandstone beds 

interbedded with fine-grained sediments such as siltstone, claystone, mudstone, carbonaceous shale, and 

limestone; however, in contrast, coal beds are more 

common, thicker, and laterally continuous in the 

Tongue River Member. 

While there are many thick coal beds in the Tongue 

River Member, the thickest and most laterally continuous coal beds are associated with a coal zone present 

basinwide in Wyoming and Montana known as the 

Wyodak-Anderson coal zone (Averitt, 1975; Glass, 

1980). The Wyodak-Anderson coal zone is composed 

of different coal beds (fig. 3) that split, merge, re-split, 

or pinch out (fig. 5). Goolsby and Finley (2000) recently 

suggested that the coal beds of the Wyodak-Anderson 

coal zone and some coal beds below the zone (fig. 3) 

may represent a single lithologic unit that was deposited 

in a corkscrew-type structure as the result of a depositional center that migrated over time. As many as six of 

these coal beds can comprise the coal zone in any given 

location (Ellis, 1999). Currently, 11 individual coal beds 

have now been identified in the coal zone (Flores and 

others, 1999). The coal beds of the WyodakAnderson coal zone “were deposited in low-lying peat 

swamps and raised mires, and associated detrital rocks 

were deposited by trunk-tributary, meandering, anastomosed, and braided streams” (Flores, 1999, p. 6, and 

references therein). Historically, the naming of these 

many beds has been extremely complicated and the correlation of the beds controversial (Flores, 1999). 

Despite this complexity and as a result of much work by 

many investigators over many years, the WyodakAnderson coal zone has been correlated throughout the 

Powder River Basin in Wyoming. Many of the beds 

comprising the zone have been correlated throughout 

the basin and are laterally continuous over large distances (Glass, 1980, 1997; Ellis and others, 1999a; 

Flores, 1999; Flores and others, 1999) (fig. 5). 

The Wyodak-Anderson coal zone and the individual beds comprising the zone vary widely in thickness 

and depend on geographic location. Although estimates 

vary widely, the most recent estimate suggests that the 

Wyodak-Anderson coal zone in the Powder River Basin 

in Wyoming and Montana “has a maximum net coal 

thickness (total thickness of all coal beds greater than 

2.5 ft thick) of 284 ft” and that “the entire zone is more 

than 600 ft thick (measured from the top of the uppermost coal to the base of the lowermost coal) in the center part of the basin” (Ellis, 1999, p. 44). Throughout 

the basin, coal beds comprising the coal zone “average 

25 ft in thickness, and are separated by clastic sedimentary rocks ranging from a few feet to 150 ft in thickness” 

(Ellis, 1999, p. 44). The coal beds of the WyodakAnderson coal zone outcrop or subcrop along the eastern margin of the basin (fig. 4). Consequently, this area 

is where most of the coal mining in the basin occurs. In 

the vicinity of the city of Gillette, several of the coal 

beds of the Wyodak-Anderson coal zone merge to form 

a single thick coal bed known as the Wyodak coal bed 

(location C, fig. 5). The Wyodak coal bed splits into 

several seams south, east, and north of Gillette (fig. 5). 

The maximum thickness of a single merged coal bed 

within the Wyodak-Anderson coal zone in the Gillette 

coal field is 145 ft (Ellis and others, 1999a). The complexity of the Wyodak-Anderson coal zone and associated coal beds is illustrated in several cross sections 

within the study area (fig. 6). The reader is referred to 

Flores (1999) and Flores and Bader (1999) for a detailed 

DESCRIPTION OF STUDY AREA 9 
historical review, discussion, and interpretation of the 

Wyodak-Anderson coal zone in the Powder River Basin 

in Wyoming and Montana. 

Along the eastern part of the study area where the 

Wyodak-Anderson coal zone outcrops or subcrops 

(fig. 4), the coal beds have burned and baked the sediments around the beds to form rock known as clinker 

(also frequently referred to as scoria). Clinker is a distinct orange to red color throughout the basin and in the 

study area. The clinker covers about a 250-square mile 

area along the eastern boundary of the Gillette coal 

field east of Gillette and Wright (Heffern and Coates, 

1997). The clinker is resistant to erosion and caps 

many of the topographically high locations in the area 

(Heffern and Coates, 1997). 

The Wasatch Formation, like the Tongue River 

Member of the Fort Union Formation, consists primarily of fine- to coarse-grained lenticular, discontinuous 

sandstone beds interbedded with fine-grained sediments such as shale, siltstone, claystone, and mudstone. Consequently, the Wasatch Formation is 

difficult to distinguish from the underlying Fort Union 

Formation, especially in the subsurface, because of 

similarity both in lithology and in depositional history 

(Ellis and others, 1999a). The contact between these 

two formations and its relationship to the PaleoceneEocene boundary remains controversial (Flores, 1999), 

but the contact commonly is placed above a coal bed, 

the Roland coal (Olive, 1957) (figs. 3 and 5). Palynostratigraphic differences (Tschudy, 1976; Nichols, 1994, 

1998; Nichols and Brown, 1992) and mineralogical differences (Denson and others, 1989a, 1989b) typically 

are used to identify the two formations and locate the 

contact in the subsurface. The reader is referred to 

Flores (1999) and Flores and Bader (1999) for an overview and discussion of the contact between the 

Wasatch and Fort Union Formations. In the study area, 

many of the coal mines and coalbed methane developers consider the top of the Wyodak-Anderson coal zone 

as the contact, even though some of the Fort Union Formation may be present above the coal zone and below 

the overlying Wasatch Formation; this convention will 

be followed in this report. Dip of the Wasatch Formation is shallow, generally less than 4 degrees (Glass, 

1997). The Wasatch Formation contains many coal 

beds, with the thickest beds in the western and central 

parts of the Powder River Basin outside the study area, 

particularly near Lake De Smet (fig. 5) (Glass, 1980, 

1997). Clinker is present in the Wasatch Formation in 

many locations (fig. 4) and frequently forms caprocks 

because it is more resistant to weathering than surrounding rock (Heffern and others, 1996; Heffern and 

Coates, 1999). 

Methane (CH4) is found in the coal beds of the 

study area and sometimes in lithological units, such as 

sandstone lenses, located immediately above or below 

the coal beds (Jones and De Bruin, 1990; Law and others, 1991; Rice, 1993). Coalbed methane is created by 

a series of processes (discussed in greater detail later in 

this report) that convert organic matter present in coal 

into methane. Coalbed methane in the Powder River 

Basin is composed almost entirely of methane, with a 

small amount of carbon dioxide (1.5 to 2 percent) (Law 

and others; 1991; Rice, 1993; De Bruin and others, 

2000); composition is suspected to be very uniform 

throughout the basin (Rice, 1993). Based on the rank 

of the coal and the isotopic composition of the methane 

in the lower Tertiary coal beds of the Powder River 

Basin, it is believed that the coalbed methane was primarily generated from bacterial activity (biogenic) as 

opposed to heating (thermogenic) (Boreck and Weaver, 

1984; Law and others, 1991; Rice and Flores, 1990, 

1991; Rice, 1993; Gorody, 1999). Coalbed methane is 

believed to be stored in coal beds in four possible ways 

(Yee and others, 1993): (1) as free gas within small 

pores (micropores) and fractures (cleats); (2) as gas dissolved in water within the coal; (3) as gas adsorbed on 

surfaces of organic material, micropores, and cleats; 

and (4) as gas absorbed within the molecular structure 

of the coal molecules. De Bruin and others (2000) suggest the gas in the shallow coal beds of the study area is 

probably stored primarily as free gas and dissolved gas 

where good cleat development is present. The reader is 

referred to the recent publication by De Bruin and others (2000) for a discussion of coalbed methane in both 

the Powder River Basin and the State of Wyoming and 

recent publications by Rice (1997) and Nuccio (2000) 

for discussions of coalbed methane in the U.S. 

Hydrogeologic Setting 

The relations of Cenozoic stratigraphic units to 

assigned hydrogeologic units in this study are shown in 

figure 7. Only the hydrogeology of the geologic units 

overlying and associated with the Wyodak-Anderson 

10 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 

12 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
coal zone (Wasatch and Fort Union Formations) will 

be discussed. Convention in this report will be to use 

the geologic unit (i.e., member, formation, coal bed or 

coal zone) as the hydrogeologic unit or aquifer name, 

even though multiple aquifers may be present in any 

one geologic unit. 

The Wyodak-Anderson coal zone and its associated coal beds comprise the most continuous hydrogeologic unit or aquifer in lower Tertiary deposits in the 

study area (Bloyd and others, 1986; Lowry, Wilson, 

and others, 1986; Martin and others, 1988). Because of 

the complicated history of the naming and correlation 

of the various coal beds comprising the WyodakAnderson coal zone, the aquifer within the coal zone 

has been referred to by many names, most frequently as 

the Wyodak coal aquifer or the Wyodak-Anderson coal 

or coalbed aquifer. The convention used in this report 

will be to use Wyodak-Anderson coalbed aquifer 

(fig. 7). Martin and others (1988) suggested that the 

aquifer can consist of not only the coal beds within the 

zone, but also the interbedded sandstone beds and the 

clinker along the coal outcrop. Aquifers present in 

other coal beds below the Wyodak-Anderson coal zone 

in the Tongue River Member of the Fort Union Formation will be combined with the coal zone in this report 

and will be collectively referred to as coalbed aquifers. 

Water in the Wyodak-Anderson coalbed aquifer is 

typically unconfined near the outcrop along the eastern 

margin of the basin in the study area (Davis, 1976; Heffern and Coates, 1999) but becomes confined to the 

west, away from the outcrop and downdip (Davis, 

1976; Martin and others, 1988). The aquifer is confined above by overlying low permeability shales or 

other fine-grained units in the Tongue River Member of 

the Fort Union Formation or Wasatch Formation and 

below by low permeability shale or other fine-grained 

units in the Tongue River Member (Martin and others, 

1988). Artesian conditions can exist; Whitcomb and 

others (1966) also noted that the gas present within the 

coal beds or associated geologic materials (for example, sandstone lenses above or below the coal bed) can 

contribute significantly to hydraulic head in wells in the 

Powder River Basin and may cause water levels to rise 

higher than if only artesian pressure were present. 

The amount of hydraulic connection under natural 

conditions between the Wyodak-Anderson coalbed 

aquifer and sandstone lenses comprising aquifers in the 

overlying Wasatch or Fort Union Formations is unclear. 

Many investigators have suggested that downward vertical flow or leakage from overlying geologic units to 

the Wyodak-Anderson coalbed aquifer is small because 

of low vertical hydraulic conductivity of the overlying 

rocks (Davis and Rechard, 1977; Feathers and others, 

1981; Bloyd and others, 1986), even though a downward vertical gradient between the coalbed aquifer and 

the overlying Wasatch aquifer is commonly present 

(Ground-Water Subgroup, 1974; Davis, 1976; Bureau 

of Land Management, 1999). However, some leakage 

probably occurs downward where the hydraulic gradient allows for downward vertical ground-water flow 

and where sandstone lenses may directly overlie the 

coalbed aquifer (Bureau of Land Management, 1999). 

The Wyodak-Anderson coalbed aquifer is anisotropic; ground-water flow primarily occurs through 

fractures (cleats) in the coal bed (Ground-Water Subgroup, 1974; Brown, 1980; Rehm and others, 1980; 

Martin and others, 1988). Flow direction in this aquifer 

may change where the coal bed splits into other beds 

and is interbedded with other lithological units and 

where differences in the distribution and density of the 

fractures (cleats) in the coal occur (Martin and others, 

1988). Anisotropy in coal beds in the Wasatch and Fort 

Union Formations of the Powder River Basin in both 

Wyoming and Montana has been examined or discussed by Stone and Snoeberger (1977), Rehm and others (1980), Stoner (1981), Dobson (1996), and Peacock 

(1997). In general, these investigators suggest that 

anisotropy in coals from both the Wasatch and Fort 

Union Formations in the Powder River Basin appears 

to be related to cleat orientation; cleat orientation may 

be oriented to major structural features (e.g., basin axis, 

lineaments, and faults). 

Hydraulic conductivity values from aquifer tests 

conducted in the Wyodak-Anderson coalbed aquifer 

were summarized in Martin and others (1988) and Peacock (1997). Martin and others examined 357 aquifer 

tests conducted in the Wyodak-Anderson coalbed aquifer at coal mines in the Powder River Basin in Wyoming and noted hydraulic conductivity values approximated a lognormal distribution with a geometric mean 

of 0.8 ft per day. Peacock (1997) reported a geometric 

mean of about 0.5 ft per day for 166 aquifer tests conducted in the Wyodak-Anderson coalbed aquifer in the 

study area. Rehm and others (1980) summarized 

hydraulic conductivities from 193 aquifer tests conducted in Paleocene coal beds (Fort Union Formation 

or Group) in the Northern Great Plains (including the 

DESCRIPTION OF STUDY AREA 13 
14 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 

16 ER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 

Powder River Basin in Wyoming and Montana). The 

investigators reported the frequency distribution of 

coalbed conductivity values approximated a lognormal 

distribution with a geometric mean of about 0.9 ft per 

day.

Regional flow within the Wyodak-Anderson coalbed aquifer is suspected to be to the north and northwest, based on a potentiometric-surface map prepared 

by Daddow (1986) (fig. 8). gional ground-water 

flow in the area near Porcupine Creek (fig. 8) may be to 

the north (Martin and others, 1988). e relationship 

between regional ground-water flow in the coalbed 

aquifers and the Fort Union Formation in the Powder 

River Basin has not been examined.

Recharge to the aquifer is suspected to occur primarily along the outcrop areas of clinker associated 

with the Wyodak-Anderson coal zone (Lowry and 

Cummings, 1966; Whitcomb and others, 1966; Davis, 

1976; Martin and others, 1988; Heffern and Coates, 

1999). In the study area, the clinker associated with the 

Wyodak-Anderson coalbed aquifer is located along the 

eastern margin, near the outcrop area (fig. 4). Recharge

also may occur locally where the coal bed subcrops 

below the alluvium in the stream valleys (Davis, 1976; 

Davis and Rechard, 1977; Martin and others, 1988). 

Discharge is suspected to primarily occur in areas 

where the coal subcrops under the base of alluvium in

valleys (Brown, 1980; Martin and others, 1988) or as

downward leakage (Brown, 1980). 

WAT

Re

Th

Clinker associated with and considered part of the 

Wyodak-Anderson coalbed aquifer by some investigators is highly permeable, may store large quantities of 

water, and may function locally as an unconfined or 

confined aquifer (Davis, 1976; Heffern and others, 

1996; Heffern and Coates, 1999). Recharge of the clinker occurs vertically from precipitation, from streams 

that cross the clinker, and from lateral inflow from clinker or unburned coal and overburden updip (Heffern 

and Coates, 1999). Recharge rates to the clinker are 

suspected to be very high, especially compared to surrounding geologic units (Heffern and Coates, 1999). 

Transmissivities of the clinker are very high, and based 

on tests conducted at coal mines in the area, range from 

10,000 to more than 1,000,000 feet squared per day 

(Heffern and Coates, 1999). 

In a large part of the study area, the Wasatch Formation is exposed at land surface and overlies the Wyodak-Anderson coal zone. As stated previously and 

illustrated in figure 7, convention in this report will be 

to define the top of the Wyodak-Anderson coal zone 

and the aquifer contained within as the contact between 

the Fort Union Formation and the overlying Wasatch 

Formation and aquifer. The Wasatch aquifer consists 

primarily of discontinuous lenticular sandstone beds 

surrounded by fine-grained sedimentary rocks with low 

permeability such as siltstone, claystone, mudstone, 

carbonaceous shale, and limestone, although coal beds 

may be locally important if present. The discontinuous, lenticular sandstone beds are the actual geologic 

materials that primarily yield water to wells and comprise the aquifer (Littleton, 1950; Hagmaier, 1971; 

Hodson and others, 1973; Ground-Water Subgroup, 

1974; Bloyd and others, 1986; Martin and others, 1988; 

Lowry and others, 1993). 

Fine-grained sedimentary rocks in the Wasatch 

Formation do not yield sufficient quantities of water to 

be considered as an aquifer, even though they may be 

saturated and static water levels in wells completed in 

the fine-grained rocks may be the same as in adjacent 

beds of sandstone (Brown, 1980; Martin and others, 

1988). The sandstone lenses have been considered as 

individual aquifers because they are surrounded by low 

permeability sedimentary rocks and they may be relatively hydraulically isolated from each other (Lowry 

and others, 1993). The yield of wells completed in the 

Wasatch aquifer is directly related to the number and 

thickness of sandstone lenses or beds penetrated by the 

well. Unconfined and confined conditions can exist in 

the Wasatch aquifer (Littleton, 1950; Whitcomb and 

others, 1966; Davis, 1976), although the sandstone 

beds comprising the aquifer may be primarily confined 

(Lowry and others, 1993). 

Hydraulic conductivity values for aquifer tests conducted in the sandstones comprising the Wasatch aquifer were summarized in Martin and others (1988) and 

Peacock (1997). Martin and others (1988, p. 19) noted 

that the frequency distribution “of the hydraulic conductivity of the Wasatch aquifer has a lognormal distribution with a geometric mean of 0.2 ft per day.” 

Similarly, Peacock also noted a lognormal distribution 

and reported a geometric mean of about 0.3 ft per day 

for the Wasatch aquifer. Rehm and others (1980) 

reported a very similar geometric mean (0.35 ft per 

day) for sandstones of the Fort Union Group in the 

Northern Great Plains. The investigators also noted 

that fine-grained sedimentary rocks (e.g., siltstones, 

claystones, and mudstones) associated with the Fort 

Union Group were much less permeable and reported a 

geometric mean of 0.007 ft per day (Rehm and others, 

1980). 

Recharge to the Wasatch aquifer is suspected to be 

primarily from areal infiltration and percolation of precipitation (King, 1974; Ground-Water Subgroup, 1974; 

Brown, 1980). Infiltration to the Wasatch aquifer may 

be enhanced by clinker throughout the area (Davis, 

1976; Bloyd and others, 1986; Martin and others, 

1988). 

Ground-water flow in the Wasatch aquifer is suspected to be primarily local and related to topography, 

with discharge occurring primarily along streams adjacent to local recharge areas (Hagmaier, 1971; King, 

1974; Bloyd and others, 1986; Martin and others, 1988) 

and possibly underflow out of the study area (Bloyd 

and others, 1986). Brown (1980) also noted that discharge to clinker may occur in some areas. Investigators differ on whether regional ground-water flow in 

the Wasatch aquifer occurs and the relationship 

between local ground-water flow and possible intermediate and regional ground-water flow is unclear (see 

discussions in Fogg and others (1991) and LindnerLunsford and Wilson (1992)). However, Martin and 

others (1988) suggest regional ground-water flow in 

the Wasatch aquifer occurs and flow is to the north, 

although the volume of flow is suspected to be small 

and the flow rate probably slow because of the low permeability of many of the rocks in the Wasatch Formation. 

18 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
In the northern part of the study area, lithologic 

units comprising the Tongue River Member of the Fort 

Union Formation are exposed at land surface and 

directly overlie the Wyodak-Anderson coalbed aquifer 

(fig. 8). As discussed earlier, the Wasatch Formation 

and the Tongue River Member of the Fort Union Formations are very similar, both lithologically and hydrogeologically, and properties are expected to be similar. 

Consequently, some investigators combine the Wasatch 

Formation and the upper part of the Fort Union Formation (i.e., Tongue River Member, excluding the 

Wyodak-Anderson and other coalbed aquifers) into a 

single aquifer or hydrogeologic unit (Feathers and others, 1981; Lewis and Hotchkiss, 1981; Koch and others, 

1982; Slagle and others, 1985; Hotchkiss and Levings, 

1986; Bloyd and others, 1986). Convention in this 

report will be to assume that properties of the upper 

lithologic units of the Tongue River Member of the Fort 

Union Formation (i.e., above the Wyodak-Anderson 

coalbed aquifer) are similar to the Wasatch aquifer and 

the Tongue River Member above the WyodakAnderson coalbed aquifer will be considered as part of 

the Wasatch aquifer for all comparisons with the underlying coalbed aquifers (fig. 7). 

Resource Extraction 

The study area is located within the Powder River 

coal field. The Powder River coal field coincides with 

the boundary of the Powder River Basin (fig. 1) and has 

the largest coal resources of any coal field in the contiguous United States (Glass, 1997). Coals are present in 

both Cretaceous and lower Tertiary geologic formations 

in the basin, but the thickest and most frequently mined 

are the two formations of lower Tertiary age discussed 

previously—the Wasatch Formation of Eocene age and 

the Fort Union Formation of Paleocene age. Most of the 

coal mining occurs in the Wyoming part of the Powder 

River Basin; much of the mining in Wyoming occurs 

along the eastern margin and includes the study area, as 

indicated by the large number of coal-lease areas 

(fig. 9). Most of the coal in the basin and in the study 

area is mined from the Wyodak-Anderson coal zone in 

the Tongue River Member of the Fort Union Formation, 

the most important and well known coal bed or coal 

zone within the Powder River coal field. 

The coal is primarily subbituminous in rank and 

low in trace elements (Stricker and others, 1998); the 

coal also is low in sulfur and ash content (Ellis and others, 1998). Consequently, coal from the WyodakAnderson coal zone is very suitable for coal-fed power 

plants concerned with meeting increasingly stringent 

air-quality standards and is shipped to many states in the 

U.S. The coal is removed using strip-mining methods 

because the Wyodak-Anderson coal zone is present at or 

near the land surface in the eastern Powder River Basin. 

Recently, there has been increased interest in the 

potential for coalbed methane production in the Powder 

River Basin. Methane suspected to be related to coal 

beds in the Powder River Basin in Wyoming was first 

reported by Olive (1957). Subsequently, Whitcomb and 

others (1966) and Lowry and Cummings (1966) published gas analyses from several water wells in the Powder River Basin in Wyoming indicating high concentrations of methane. Commercial extraction of coalbed 

methane began in the mid- to late-1980’s in the Powder 

River coal field (Jones and De Bruin, 1990). Extraction 

greatly increased around 1997, when over 200 coalbed 

methane wells were in production and monthly methane 

production reached one billion cubic feet (De Bruin and 

Lyman, 1999). Since then, development continues rapidly as “the coalbed methane play in the Powder River 

Coal Field is currently the most active gas play in the 

United States” (De Bruin and Lyman, 1999, p. 67). 

More than 3,000 wells have been drilled as of July 2000, 

many more have been permitted (fig. 9), and thousands 

more are expected to be installed in the future. 

Methane is removed from the coal by pumping 

water (dewatering) from coalbed methane production 

wells installed in coal beds. The wells are completed by 

installing well casing to the top of the coal bed, reaming 

the coal bed, and then leaving the coal bed open to the 

hole. Water is then removed from the well by installing 

a submersible pump in the open coal bed; pumping 

reduces hydrostatic pressure in the coal that allows the 

methane to be released (desorption). The gas is transported to the surface in the space between the tubing 

attached to the submersible pump and the well casing 

and the produced water is discharged to the surface via 

the tubing. 

Because of the importance of the coal and gas 

resources in the area, many studies of the geology and 

hydrogeology of the area have been, and continue to be 

conducted. The rapid development of the WyodakAnderson coal zone as a source of coalbed methane has 

renewed interest in hydrologic processes throughout the 

Powder River Basin. 

DESCRIPTION OF STUDY AREA 19 
Acknowledgments 

The authors would like to thank all the people who 

supported this study, assisted with collection of groundwater-quality samples, and assisted with preparation of 

the report. Richard G. Stockdale of the WSEO and 

Mike Brogan and Joe Meyer of the BLM are gratefully 

acknowledged for their support of this study. Special 

thanks are given to Barrett Resources (now Williams 

Energy Services) who kindly gave permission to sample 

their coalbed methane production wells and provided 

assistance with the sampling of the wells. Ray Murphy 

and Larry Porter of the WSEO, and Joe Meyer and Mike 

Brogan of the BLM, kindly provided assistance with the 

sampling of the monitoring wells in the study. Ed Heffern (BLM) provided data and invaluable assistance for 

preparation of the map showing the distribution of clinker in the study area. Finally, the authors would like to 

thank U.S. Geological Survey colleagues Greg Boughton and Laura Hallberg for assisting in the collection 

and compilation of data used in the report, Cynthia A. 

Rice and Michael Cannon for providing colleague 

reviews, and Suzanne C. Roberts and Emily A. Sabado 

for preparation of illustrations and editorial assistance, 

respectively. 

WATER-LEVEL MEASUREMENTS 

Well-completion information, water levels, and the 

identified aquifer for all wells sampled as part of this 

study are summarized in table 1. If a well was completed in the Wyodak-Anderson coal zone, individual 

coal beds within the zone were identified from well permits if possible. All but one of seventeen wells completed in a coalbed aquifer were in the WyodakAnderson coal zone or equivalent zone or bed (i.e., one 

well was completed in the Big George coal bed); well 

C15 was suspected to be completed in a coal bed located 

below the zone. Well locations are shown on figure 8. 

Water levels were measured in all monitoring wells 

prior to sampling, including one monitoring-well cluster 

comprising three wells completed successively deeper 

in the Wasatch aquifer and five monitoring-well clusters 

comprising two wells each (one well completed in the 

Wasatch aquifer and one well completed successively 

deeper in the Wyodak-Anderson coalbed aquifer) 

(table 1). A monitoring-well cluster is a series of wells 

located close to each other where each well is drilled 

and completed at a different depth. Water levels measured in successively deeper wells completed as part of 

a well cluster can be used to evaluate the hydraulic 

potential (differences in hydraulic head) for vertical 

ground-water flow at the location. There was a relatively strong vertical gradient within the Wasatch aquifer at the site of the well cluster (table 2) containing 

wells W3, W4, and W5. The decreasing hydraulic head 

with depth indicates that there was a hydraulic potential 

for downward ground-water flow in the Wasatch aquifer 

in the vicinity of this location if geologic conditions 

allow. There was a downward vertical gradient from the 

Wasatch aquifer to the Wyodak-Anderson coalbed aquifer as measured in three of the four well clusters (table 

2) that were completed in both zones. This decreasing 

hydraulic head between aquifers indicated the potential 

for downward vertical flow from the Wasatch aquifer to 

the underlying Wyodak-Anderson coalbed aquifer. The 

fourth well cluster indicated hydraulic head increases 

with depth at one well cluster (table 2, wells W8 and 

C17), indicating an apparent hydraulic potential for 

upward ground-water flow, based on a water level measured below land surface in a well completed in the 

Wasatch aquifer (W8) and a flowing well discharging a 

mixture of gas and water completed deeper in the underlying Canyon coal bed of the Wyodak-Anderson coalbed aquifer (C17). The hydraulic potential for vertical 

ground-water flow could not be evaluated in one well 

cluster (wells W6 and C14) because the well completed 

in the Wasatch aquifer (W6) had just been installed and 

well development recently completed; therefore, the 

water level was not used because it did not represent the 

static water level in the aquifer. 

It should be noted that all of these monitoring-well 

clusters are located within active coalbed methane production fields (fig. 9) and areas of coal mining. Significant drawdown has been observed in the WyodakAnderson coalbed aquifer in some of these areas as a 

result of both of these activities (Meyer, 1999); therefore, water levels measured in these monitoring wells 

may have been affected depending on hydraulic connection between the individual wells in the clusters. However, a downward vertical gradient in the Wasatch and 

Fort Union Formations in the study area prior to much 

development has been noted previously (Ground-Water 

Subgroup, 1974; Davis, 1976). In addition, the waterlevel measurements represent a discrete measurement at 

one point in time and water levels may vary with time. 

20 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 

--

Table 1. Identification numbers, well-completion information, water levels, geologic 

[P, perforated well casing; S, commercial well screen; O, open hole; L, well liner; --, water 

Land-surface Elevation of 

Site U.S. Geological Survey elevation Depth of well bottom of well Date of waternumber site-identification (feet above sea (feet below (feet above sea level measure­

(fig. 8) number Local well number level) land surface) level) ment 

W1 440253105385702 47-073-16cbc02 4,850 910 3,940 6-16-99 

W2 440542105351802 48-073-36bcc02 4,740 700 4,040 6-17-99 

W3 440724105291301 48-072-22adc01 4,574 410 4,164 6-24-99 

W4 440724105291302 48-072-22adc02 4,574 185 4,385 6-25-99 

W5 440724105291303 48-072-22adc03 4,574 80 4,490 6-25-99 

W6 441019105414502 49-074-36ddc02 4,915 1,100 3,815 

W7 441451105375502 1,249-073-03cc 02 4,775 750 4,025 6-17-99 

W8 443241105360801 53-073-26dba01 4,045 3200 3,844 6-29-99 

C1 433408105270101 242-072-36db 01 4,998 765 4,233 45-17-98 

C2 434252105264301 243-072-12da 01 4,861 5650 4,211 45-08-99 

C3 434941105412001 45-074-36dad01 4,883 61,220 3,663 6-22-99 

C4 435411105294001 45-072-03dbd01 4,876 7703 4,173 412-20-96 

C5 435915105242201 46-071-05dda01 4,625 340 4,285 42-18-97 

C6 440139105253301 47-071-30aaa01 4,709 399 4,310 46-18-96 

C7 440253105385701 47-073-16cbc01 4,850 1,219 3,631 6-15-99 

C8 440358105331401 247-072-07ac 01 4,776 853 3,603 44-22-98 

C9 440542105351801 48-073-36bcc01 4,740 905 3,835 6-16-99 

C10 440623105364101 48-073-27dac01 4,836 81,057 3,779 45-21-99 

C11 440808106070601 248-077-16ac 01 4,093 91,224 2,869 46-18-99 

C12 440756105293301 248-072-15dc 01 4,613 565 4,048.4 45-24-93 

C13 441017105352201 248-073-02aa 01 4,718 842 3,876.3 42-04-98 

C14 441019105414501 49-074-36ddc01 4,915 101,234 3,681 6-23-99 

C15 441047105535401 49-075-32adb01 4,439 111,795 2,598 6-24-99 

C16 441451105375501 249-073-03cc 01 4,775 905 3,870 6-17-99 

C17 443241105360802 53-073-26dba02 4,044 297 3,747 410-9-98 

1

Plots in section 4 when using Global Positioning System (GPS) location; well is assigned to section 3 on well permit, so this location was 

used. 2

Well located in center of quarter-quarter section. 3

Well was drilled to a depth of 201 feet below land surface but was completed to a depth of 200 feet below land surface. 4

Water level from Wyoming State Engineer’s Office well permit. 5

Well was drilled to a depth of 675 feet below land surface but was completed to a depth of 650 feet below land surface. 6

Well was cased to a depth of 1,220 feet below land surface but was perforated 1,050 to 1,070 feet below land surface. 7Well was drilled to a depth of 722 feet below land surface but was completed to a depth of 703 feet below land surface. 8

Well was drilled to a total depth of 1,066 feet below land surface, and well casing was installed to a depth of 980 feet below land surface; 

well is open from 980 to 1,057 feet below land surface. 9Well is open to sandstone and shale from 1,003 to about 1,070 feet below land surface and coal from 1,070 to 1,224 feet below land surface. 10Well was drilled to a total depth of 1,263 feet below land surface but was completed to a depth of 1,234 feet below land surface. 11Well was drilled to a total depth of 1,868 feet below land surface, plugged back to 1,841 feet below land surface, cased to 1,841 feet below 

land surface, and perforated from 1,728 to 1,795 feet below land surface. 

22 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
units, and assigned hydrogeologic units for wells sampled in the study area 

level not recovered from well installation; LS, water level at land surface] 

Water level Water-level 

(feet below elevation Open interval Type of Date conland (feet above (feet below open struction Hydrogeologic 

surface) sea level) land surface) interval completed Geologic unit unit 

365.9 4,484.1 850-910 P 10-06-98 

260.2 4,479.8 670-700 P 10-24-98 

94.6 4,479.4 340-410 P 02-07-93 

36.0 4,534.0 140-180 S 04-22-98 

17.5 4,552.5 15-80 S 04-22-98 

-- -- 1,050-1,100 P 06-18-99 

291.3 4,483.7 690-750 P 10-14-98 

21 4,024.0 3140-200 S 05-21-98 

4

406 44,592.0 667-765 O 05-17-98 

4

272 44,589.0 5531-650 O 05-08-99 

241.7 4,641.3 61,050-1,070 P 12-11-98 

4

319.5 44,556.5 7661-703 O 12-20-96 

4155 44,470.0 275-340 O 02-18-97 

4

338 44,371.0 340-399 O 06-18-96 

410.9 4,439.1 1,187-1,219 P 10-26-98 

4

278 44,498.0 760-853 O 05-28-99 

307.6 4,432.4 860-905 P 10-22-98 

4

360 44,476.0 8980-1,057 O 05-21-99 

4LS 44,093 9

1,003-1,224 O 06-11-99 

4

203 44,410.4 482-565 O 05-24-93 

4

703 44,015.3 768.5-842.0 O 02-04-98 

481.4 4,433.6 101,156-1,234 L 11-17-98 

133.16 4,305.8 111,728-1,795 P 02-10-90 

328.9 4,446.1 845-905 L 10-11-98 

4

32.8 44,011.2 221-297 O 07-06-96 

Wasatch Formation Wasatch aquifer 

Wasatch Formation Wasatch aquifer 

Wasatch Formation Wasatch aquifer 

Wasatch Formation Wasatch aquifer 

Wasatch Formation Wasatch aquifer 

Wasatch Formation Wasatch aquifer 

Wasatch Formation Wasatch aquifer 

Tongue River Member of the Fort Union Wasatch aquifer 

Formation above Wyodak-Anderson coal zone 

Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak coal bed of the Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak-Anderson coal zone Coalbed aquifers 

Upper Wyodak coal bed of the Wyodak-Anderson coal Coalbed aquifers 

zone 

Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak coal bed of the Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak coal bed of the Wyodak-Anderson coal zone Coalbed aquifers 

Big George coal bed Coalbed aquifers 

Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak coal bed of the Wyodak-Anderson coal zone Coalbed aquifers 

Wyodak-Anderson coal zone Coalbed aquifers 

Coal bed or zone below the Wyodak-Anderson coal Coalbed aquifers 

zone 

Upper Wyodak coal bed of the Wyodak-Anderson coal Coalbed aquifers 

zone 

Canyon coal bed of the Wyodak-Anderson coal zone Coalbed aquifers 

WATER-LEVEL MEASUREMENTS 23 
--

Table 2. Differences in water-level elevations and hydraulic potential for vertical ground-water flow in monitoring-well 

clusters 

[--, water level not recovered from well installation; NC, not calculated; NA, not applicable because difference was not calculated because one well is 

discharging a mixture of gas and water] 

Site 

number 

(fig. 8) Geologic unit 

Hydrogeologic 

unit 

Water-level 

elevation 

Difference in 

water-level 

elevations 

Hydraulic potential for 

vertical ground-water flow 

W1 Wasatch Formation Wasatch aquifer 4,484.1 45 

Downward 

C7 Wyodak-Anderson coal bed of Coalbed aquifer 4,439.1 

the Fort Union Formation 

W2 Wasatch Formation Wasatch aquifer 4,479.8 47.4 

Downward 

C9 Wyodak-Anderson coal bed of Coalbed aquifer 4,432.4 

the Fort Union Formation 

W6 Wasatch Formation Wasatch aquifer Cannot evaluate because 

C14	 Wyodak-Anderson coal bed of Coalbed aquifer 4,433.6 NC water level not recovered 

the Fort Union Formation from installation of well 

W6 prior to measurement 

W7 Wasatch Formation Wasatch aquifer 4,483.7 37.6 

Downward C16 Wyodak-Anderson coal bed of Coalbed aquifer 4,446.1 

the Fort Union Formation 

W8 Tongue River Member of the Wasatch aquifer 4,024.0 

Fort Union Formation NA Upward 

C17 Wyodak-Anderson coal bed of Coalbed aquifer Flowing 

the Fort Union Formation 

W5 Wasatch Formation Wasatch aquifer 4,552.5 18.5 

Downward 

W4 Wasatch Formation Wasatch aquifer 4,534.0


W3 Wasatch Formation Wasatch aquifer 4,479.4 54.6
 Downward 

GROUND-WATER QUALITY 

Many natural factors can affect ground-water quality; however, the primary factors include the source and 

chemical composition of recharge water, the lithological and hydrological properties of the geologic unit, the 

various chemical processes occurring within the geologic unit, and the amount of time the water has 

remained in contact with the geologic unit (residence 

time). All of these factors can affect the type and quantities of dissolved constituents in ground water. The 

most abundant dissolved constituents measured are the 

major ions, which can be both positively charged (cations) and negatively charged (anions). Because of the 

requirements of electroneutrality, cations and anions are 

present at equal concentrations in water and comprise 

most of the dissolved solids in ground water. The most 

abundant cations present in water are calcium (Ca), 

magnesium (Mg), sodium (Na), and potassium (K); the 

most abundant anions are bicarbonate (HCO3), chloride 

(Cl), and sulfate (SO4). By measuring the concentrations of these ions in ground-water samples, the ionic 

composition of the water is determined and the chemical quality of the water can be characterized and 

described. A brief summary of the source or cause of 

these and other dissolved constituents and physical and 

general mineral characteristics commonly present in 

ground water is presented in table 3. 

24 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Table 3. Source or cause, and significance of dissolved-mineral constituents and physical properties of water 

(modified from Popkin, 1973, p. 85) [mS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter] 

Constituent or 

property Source or cause 

Specific Mineral content of the water. 

conductance (μS/cm) 

GROUND-WATER QUALITY 

pH Hardness as calcium 

carbonate (CaCO3) 

Calcium (Ca) and 

magnesium (Mg) 

Sodium (Na) and potassium (K) Bicarbonate (HCO3) and carbonate (CO3) Sulfate (SO4) Chloride (Cl) Fluoride (F) Silica (SiO2) 

Acids, acid-generating salts, and free carbon dioxide lower the pH. Carbonates, bicarbonates, hydroxides, phosphates, silicates, and borates raise the pH. In most water, nearly all the hardness is due to calcium and magnesium. All metallic cations other than the alkali metals also 

cause hardness. 

Dissolved from many rocks and soil, but especially from limestone, dolomite, and gypsum. Calcium and magnesium are 

detected in large quantities in some brines. Magnesium is present 

in large quantities in seawater. 

Dissolved from many rocks and soil; also in ancient brines, seawater, industrial brines, and sewage. Action of carbon dioxide in water on carbonate rocks such as 

limestone and dolomite. 

Dissolved from rocks and soil containing gypsum, iron sulfides, 

and other sulfur compounds. Commonly present in mine water 

and in some industrial wastes. 

Dissolved from rocks and soil. Present in sewage and found in 

large concentrations in ancient brines, seawater, and industrial brines. 

Dissolved in minute to small concentrations from most rocks and 

soil. Added to most water by fluoridation of municipal supplies. 

Dissolved from many rocks and soil, commonly less than 30 mg/L. Large concentrations, as much as 250 mg/L, generally occur in alkaline water. 

Significance 

Indicates degree of mineralization. Specific conductance is 

water to conduct an electric current. Varies with temperatu

ization of the constituents. 

pH is a measure of the activity of hydrogen ions. A pH of 7 

Values higher than 7 denote increasing alkalinity; values low

ity. Corrosiveness of water generally increases with decreas

line water also may be corrosive. 

Consumes soap before a lather will form and deposits soap 

scale in boilers, water heaters, and pipes. Hardness equival

and carbonate concentration is called carbonate hardness. A

called noncarbonate hardness. Water with hardness of 60 m120 mg/L, moderately hard; 121 to 180 mg/L, hard; more th

Causes most of the hardness and scale-forming properties o

ness). 

Large concentrations, in combination with chloride, give a s

have little effect on the usefulness of water for most purposein steam boilers. A large sodium concentration may limit th

Bicarbonate and carbonate produce alkalinity. Bicarbonate

pose in steam boilers and hot-water facilities to form scale a

gas. In combination with calcium and magnesium, causes c

Sulfate in water containing calcium forms hard scale in stea

sulfate in combination with other ions gives bitter taste to w

on some people. Some calcium sulfate is considered benefi

In large concentrations in combination with sodium, gives s

concentrations increases the corrosiveness of water towards 

Fluoride in drinking water reduces the incidence of tooth de

during the period of enamel calcification. However, it may 

dysfunction, depending on the concentration of fluoride, the 

ing water consumed, and susceptibility of the individual. 

Forms hard scale in pipes and boilers. Transported in steam 

deposits on blades of turbines. Inhibits deterioration of zeo

25 Dissolved solids Primarily mineral constituents dissolved from rocks and soil. Water containing more than 1,000 mg/L dissolved solids is 
--------------------------------------------------

--------------------------------------------------------

The ionic composition of water is used to classify 

it into ionic types based on the dominant dissolved cation and anion, expressed in milliequivalents per liter 

(meq/L). A milliequivalent (meq) is a measurement of 

the molar concentration of the ion, normalized by the 

ionic charge of the ion. The dominant dissolved ion 

must be greater than 50 percent of the total. For example, water classified as a sodium-bicarbonate-type 

water contains more than 50 percent of the total cation 

milliequivalents as sodium and more than 50 percent of 

the total anion milliequivalents as bicarbonate. If no 

cation or anion is dominant (greater than 50 percent), 

the water is classified as mixed and the two most common cations or anionsin decreasing order of abundance 

are used to describe the water type. For example, a 

water containing 45 percent sodium, 35 percent calcium, and 20 percent magnesium, and 55 percent bicarbonate, 30 percent sulfate, and 15 percent chloride 

would be classified as a sodium-calcium-bicarbonatetype water. 

Quality Assurance and Quality Control 

A review of water-quality analyses (appendix table 

1) in this study suggested that four sampled monitoring 

wells (C3, C7, C9, and C14, fig. 8) were contaminated 

by grout during well completion. The pH values for 

ground-water samples collected from these wells were 

very alkaline (pH = 9.6, 10.5, 11.3, and 11.7), were 

much higher than for similar wells completed in the 

coalbed aquifers in the study area (Rice and others, 

2000), and were uncommonly high for natural aquifers 

(Hem, 1985). High pH values in this range are typical 

for wells contaminated with alkaline grout (cement 

and/or bentonite) used to seal the annular space during 

well completion (Nielson, 1991). Nielson (1991, 

p. 585) described four ways that very alkaline water 

from annular seals can enter well screen areas and be 

sampled: “(1) Wells located in low permeability units 

with strong vertical gradients (i.e., grout bleeds directly 

downward); (2) Grout injected into the screened area of 

the well; (3) Bentonite seals too thin or ineffective; and 

(4) Fractured rock providing channels around bentonite 

seals.” It is unclear which of these may have affected 

the water-quality analyses, but the approach used in 

this investigation was to remove all water-quality analyses for samples from these four wells. 

Ion balances were calculated and examined for 

each ground-water sample as a quality-assurance check 

of the chemical analyses. The ion balance was calculated (in meq/L) as the total dissolved-cation concentration minus the total dissolved-anion concentration 

divided by the total concentration of ions dissolved in 

solution. The total cation concentration was calculated 

as the sum of calcium, magnesium, sodium, and potassium; the total anion concentration was calculated as 

the sum of acid-neutralizing capacity, chloride, fluoride, and sulfate. Inorganic nitrogen species (nitrite 

plus nitrate and ammonium) were not included in the 

calculations because neither was measured in the 

ground-water samples. Nineteen of twenty-one 

ground-water samples (about 90 percent) had ion balances within the +/- 6 percent range, indicating that the 

major-ion analyses were of good quality. The remaining two samples had ion balances of about 9 and 12 percent; these samples are still included and considered 

acceptable in this study because the ion balances are 

still relatively low, and unmeasured constituents such 

as organic anions, nutrients, and trace metals may contribute to higher ion balances (Hem, 1985). 

A replicate sample (duplicate) is a ground-water 

quality-control sample collected sequentially after a 

regular sample (normal environmental sample); both 

are analyzed for the same constituents to assess the 

combined effects of field and laboratory procedures on 

measurement variability. Constituents measured in 

both samples are compared by calculating the relativepercent difference (RPD) using the equation given 

below. The RPD cannot be calculated for compounds 

that are below the minimum reporting level (MRL). 

 

RPD = absolute value 

 sample1-sample2 -

 × 100 (1) sample1 + sample2  2 

Two replicate samples were collected during this 

study. Results of the RPD calculations are shown in 

table 4. RPD values greater than zero but less than 

1 percent are reported as less than 1 percent (< 1 percent) in the table. 

For the first replicate sample, RPDs for all but two 

measured constituents were less than 1 percent in one 

normal environmental sample and associated replicate 

sample, indicating very good precision (reproducibility) (table 4). The RPD was 4 percent for calcium and 

26 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Table 4. Calculated relative-percent differences between normal environmental and replicate ground-water samples 

[mg/L, milligrams per liter; CaCO3, calcium carbonate; TU, tritium units; < MRL, less than minimum reporting level; NC, not calculated because both 

normal environmental sample and replicate sample were less than minimum reporting level; ‰, per mil or parts per thousand] 

Sample 1 Sample 2 

Normal Relative- Normal Relativeenvironmental Replicate percent environmental Replicate percent 

Characteristic or constituent sample sample difference1 sample sample difference1 

Acid-neutralizing capacity (ANC), 580 580 0 267 267 0 

laboratory (mg/L as CaCO3) 

Dissolved solids, calculated sum 624 622 < 1 4,020 4,020 0 

(mg/L) 

Hardness, total (mg/L as CaCO3) 120 120 0 2,500 2,500 0 

Sodium-adsorption ratio (SAR) 

Calcium, dissolved (mg/L) 

Magnesium, dissolved (mg/L) 

Sodium, dissolved (mg/L) 

Potassium, dissolved (mg/L) 

Bicarbonate, calculated from 

ANC2 (mg/L) 

Chloride, dissolved (mg/L) 

Sulfate, dissolved (mg/L) 

Fluoride, dissolved (mg/L) 

Silica, dissolved (mg/L) 

Tritium, TU 

δ2H (‰) 

δ18O (‰) 

8 8 0 2 2 0 

25 26 4 550 550 0 

14 14 0 280 270 14 

190 190 0 270 260 14 

13 12 8 14 14 0 

707 707 0 326 326 0 

7 7 0 18 17 3 

1.2 1.2 0 2,700 2,700 0 

1.1 1.1 0 0.3 0.3 0 

20 20 0 17 16 6 

< MRL < MRL NC 6 9 40 

-151.0 -150.3 < 1 -147.8 -147.2 < 1 

-19.99 -19.94 < 1 -19.03 -19.02 < 1 

1Value rounded to nearest percent. 2

Equivalent bicarbonate concentrations calculated from acid-neutralizing capacity (ANC) concentrations by dividing ANC concentrations (expressed 

as milligrams per liter of calcium carbonate) by 0.8202 (Hem, 1985, p. 57). 

8 percent for potassium. In the second sample, RPDs 

for all but five constituents were less than 1 percent. 

RPDs for magnesium, sodium, chloride, and silica were 

about 3 to 6 percent. The RPD for tritium is high 

(about 40 percent), indicating poor precision between 

the two sample types; the source of the poor precision is 

unclear. Therefore, precision between all constituents 

but tritium is very good in the second set of samples. 

Major-Ion Chemistry 

Ground-water-quality data for samples collected 

from springs and wells are presented in the Appendix, 

and summary statistics for samples collected from wells 

are presented in tables 5 and 6. Visual descriptions and 

comparisons of data collected from springs and wells in 

the Wasatch aquifer are provided by plotting individual 

data values because the sample sizes are less than 10 

(figs. 10, 11, and 12). Percentiles and boxplots were 

constructed for characteristics and constituents measured, analyzed, or calculated in samples collected from 

coalbed aquifers. 

Dissolved solids are reported as a concentration 

value in mg/L (milligrams per liter) calculated by summing all major ions; dissolved-solids concentrations 

often are used to compare water quality between different groups of water samples. Dissolved-solids concentrations were used to compare data collected as part of 

this study to data collected by several other investigations with larger sample sizes (n > 30). 

GROUND-WATER QUALITY 27 
Table 5. Summary of water-quality data for ground-water samples collected from wells completed in the Wasatch aquifer 

[MRL, minimum reporting level; oC, degrees Celsius; NA, not applicable; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; CaCO3, calcium 

carbonate] 

Number of 

detections/ 50th 

number of percentile 

Characteristic or constituent samples Highest MRL Minimum (Median) Maximum 

Physical characteristics 

Water temperature, field measurement (oC) 8/8 NA 14 18 20 

pH, field measurement (standard units) 8/8 0.1 7.2 7.8 8.9 

General mineral characteristics 

Acid-neutralizing capacity (ANC), laboratory 7/7 1.0 129 378 1,020 

(mg/L as CaCO3) 

Specific conductance, field measurement 8/8 1.0 438 1,382 4,330 

(μS/cm at 25oC) 

Dissolved solids, calculated sum (mg/L) 7/7 NA 263 1,010 4,020 

Hardness, total (mg/L as CaCO3) 8/8 NA 22 97.5 2,500 

Sodium-adsorption ratio (SAR) 8/8 NA 2 9 13 

Major ions, in milligrams per liter, dissolved (sample filtered through 0.45-micrometer disposable cartridge filter) 

Calcium 8/8 0.02 6.2 15.5 550 

Magnesium 8/8 0.01 1.5 15.4 280 

Sodium 8/8 0.09 97 225 480 

Potassium 8/8 0.24 2.2 9.4 18 

Bicarbonate, calculated from ANC1 7/7 NA 157 461 1,244 

Chloride 7/8 0.3 5 9.6 18 

Sulfate 8/8 0.3 10 130 2,700 

Fluoride 7/8 0.1 0.3 0.85 1.5 

Silica 8/8 0.1 8 10 17 

1

Equivalent bicarbonate concentrations calculated from acid-neutralizing capacity (ANC) concentrations by dividing ANC concentrations (expressed as 

milligrams per liter of calcium carbonate) by 0.8202 (Hem, 1985, p. 57). 

Despite small sample size (n = 7), the median 

dissolved-solids concentration for ground-water samples collected from wells completed in the Wasatch 

aquifer in the study area (1,010 mg/L) is the same 

(1,010 mg/L) as reported by Lowry, Wilson, and others 

(1986) for the Powder River drainage basin in Wyoming and Montana, and similar to the median 

dissolved-solids concentration (1,220 mg/L) reported 

by Larson (1984) for samples collected from the 

Wasatch aquifer in Campbell County (Wyoming), suggesting good comparability. In contrast, the median 

dissolved-solids concentration (1,010 mg/L) is about 

one-half the median dissolved-solids concentration 

(2,215 mg/L) reported by Martin and others (1988) for 

samples collected from wells completed in the Wasatch 

Formation in the Powder River Basin. The difference 

may be attributable to the location and depth of the 

wells examined by Martin and others (1988). Those 

wells were located at coal mines at relatively shallow 

depths along the eastern margin of the study area. As 

will be discussed later in this report, the ionic composition of the shallow part of the Wasatch Formation is 

believed to be different than deeper parts of the formation; this apparent relationship between depth and ionic 

composition in the Wasatch Formation may explain the 

observed differences in median dissolved-solids concentrations between this study and the study by Martin 

and others (1988). 

28 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Table 6. Summary of water-quality data for ground-water samples collected from wells completed in coalbed aquifers 

[MRL, minimum reporting level; oC, degrees Celsius; NA, not applicable; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; CaCO3, calcium carbonate; N

percentile is less than highest minimum reporting level] 

Characteristic or constituent 

Number of 

detections 

/number of 

samples 

Highest 

MRL Minimum 

10th 

percentile 

25th 

percentile 

50th 

percentile 

(Median) 

75th 

percentile 

Physical characteristics 

Water temperature, field measurement (oC) 13/13 NA 14 15.2 17 19 20 

pH, field measurement (standard units) 13/13 0.1 6.9 6.9 7.1 7.2 7.4 

General mineral characteristics 

Acid-neutralizing capacity (ANC), laboratory 13/13 1.0 346 349.6 432 584 788 (mg/L as CaCO3) Specific conductance, field measurement 

(μS/cm at 25oC) 

13/13 1.0 665 672.6 839 1,070 1,610 

Dissolved solids, calculated sum (mg/L) 13/13 NA 382 387.2 483 644 959 

13/13 NA 74 75.2 100 150 245 

Sodium-adsorption ratio (SAR) 13/13 NA 5 5.4 6 7 8 

Hardness, total (mg/L as CaCO3) 

Major ions, in milligrams per liter, dissolved (sample filtered through 0.45-micrometer disposable cartridge 

Calcium 13/13 0.02 15 15.8 20 36 50 Magnesium 13/13 0.01 8.4 8.5 11.4 15 28 Sodium 13/13 0.09 120 124 160 210 300 

Potassium 13/13 0.24 4.9 5.2 7.3 12 13 

Bicarbonate, calculated from ANC1 

GROUND-WATER QUALITY 

13/13 NA 422 426 527 712 1,103 

Chloride 13/13 0.29 6.9 6.9 8 9.1 14 Sulfate 5/13 0.3 < 0.3 NC NC NC 0.95 Fluoride 13/13 0.1 0.6 0.68 1.0 1.2 1.4 Silica 13/13 0.1 7.4 8.1 9.8 10 11 

1Equivalent bicarbonate concentrations calculated from acid-neutralizing capacity (ANC) concentrations by dividing ANC concentrations (expressed as milligrams 

0.8202 (Hem, 1985, p. 57). 

29 
The median dissolved-solids concentration for 

13 ground-water samples collected from wells completed in the coalbed aquifers (644 mg/L) is lower than 

reported by Martin and others (1988), 1,310 mg/L for 

379 samples collected from 88 wells at coal mines. The 

88 wells were located at the coal mines at relatively shallow depths along the eastern margin of the study area. 

Water quality of the coalbed aquifers in this area may be 

different because of the location of the wells in relation 

to clinker deposits and/or the burn line (the contact 

between the unburned coal or overburden) (Heffern and 

Coates, 1999); this will be discussed later in the report. 

In contrast, the median dissolved-solids concentration 

(644 mg/L) in this study is very similar to the median 

dissolved-solids concentration calculated for groundwater samples (680 mg/L) collected from 36 wells completed in coalbed aquifers in the study area as part of 

another investigation (Rice and others, 2000). That 

dataset represents a better comparison because groundwater samples were collected from coalbed aquifers 

throughout the study area. 

Classification of Ionic Composition and 

Comparisons Between Aquifers 

The relative ionic composition of ground-water 

samples collected from springs and wells in the study 

area are plotted on a trilinear diagram (fig. 13). A trilinear diagram, also frequently referred to as a Piper diagram (Piper, 1944), provides a convenient method to 

classify and compare water types based on the ionic composition of different water samples (Hem, 1985). Cation 

and anion concentrations for each ground-water sample 

are converted to total meq/L and plotted as percentages 

of their respective totals in two triangles (fig. 13). The 

cation and anion relative percentages in each triangle are 

then projected into a quadrilateral polygon that describes 

the water type or hydrochemical facies. 

Samples collected from two springs in the study area 

both had the same dominant cation, calcium, but different dominant anions, sulfate and bicarbonate (fig. 13); 

samples collected from these springs would be classified 

as calcium-sulfate-type and calcium-bicarbonate-type 

waters. 

30 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
GROUND-WATER QUALITY 31
32 ER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 

‘

WAT
GROUND-WATER QUALITY 33
34 ER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS WAT
Ground-water samples collected from wells had a 

different ionic composition than samples collected from 

the springs, but had very similar compositions with 

respect to each other, regardless of aquifer (fig. 13). All 

ground-water samples collected from the coalbed aquifers were sodium-bicarbonate-type waters and only 

three of eight samples collected from the Wasatch aquifer were not classified the same. Coalbed aquifers in the 

Powder River Basin in Wyoming and Montana are characterized by sodium-bicarbonate-type waters, as noted 

by other investigators (Law and others, 1991; VanVoast, 

1991; Rice and others, 2000; Rice, 2000). Two of the 

three samples collected from the overlying Wasatch 

aquifer that were not sodium-bicarbonate-type waters 

were similar in ionic composition, with mixed cations, 

and with the same dominant anion, sulfate. The other 

sample was a mixed type, and plots in the sodium-magnesium-sulfate-bicarbonate part of the trilinear diagram. 

The similarity in water type between most ground-water 

samples suggests that similar geochemical processes 

may be controlling major-ion chemistry in these aquifers and that the waters had the same or similar origins. 

The two-sided version of the rank-sum test (Helsel 

and Hirsch, 1992), also known as the Wilcoxon ranksum test (Wilcoxon, 1945) or Mann-Whitney test 

(Mann, 1945), is a nonparametric hypothesis test that 

was used to determine the probability that the median 

dissolved-solids and major-ion concentrations between 

aquifers were the same. The rank-sum test, like most 

nonparametric hypothesis tests, requires no assumptions about the population distribution, is resistant to 

outliers, and is more appropriate for small sample sizes 

where nonnormality is more difficult to detect. The null 

hypothesis of identical median concentrations between 

aquifers was rejected if the probability (p-value) of 

obtaining identical medians by chance was less than 

0.05. Results of the hypothesis testing are provided in 

table 7 and in figure 12. 

Median concentrations of all but two major ions 

were statistically the same between coalbed and overlying aquifers (p-values > 0.05). Fluoride concentrations 

were significantly different between aquifers 

(p-value = 0.0221). It is unclear why fluoride concentrations differ between aquifers, although it has been 

suggested that differences in fluoride concentrations 

within the Wasatch and Fort Union Formations may be 

due to local lithological variation (Wyoming Water 

Development Commission, 1985). While median sulfate concentrations are low in the overlying Wasatch 

aquifer (130 mg/L), sulfate concentrations are very low 

in samples collected from the coalbed aquifers, as indicated by greater than 50 percent of data values at concentrations less than detectable limits. This contrast is 

easily seen on the boxplot and plotting of individual 

data values presented earlier (fig. 12). Results of the 

rank-sum test indicate that these differences are statistically significant (p-value = 0.0001) between aquifers. It 

should be noted that the severe censoring of sulfate concentrations in samples collected from the coalbed aquifers may reduce the ability of the rank-sum test to reject 

the null hypothesis when actually false (Helsel and Hirsch, 1992); however, the very low p-value (0.0001) for 

sulfate would indicate that this is not the case. 

Table 7. Results of rank-sum tests between Wasatch 

aquifer and coalbed aquifers 

[Bold text indicates a statistically significant difference with a p-value 

less than 0.05] 

Rank-sum test probability 

Constituent or major ion (p-value) 

Dissolved solids 0.5261 

Calcium 0.4913 

Magnesium 0.7997 

Sodium 0.9710 

Potassium 0.4659 

Bicarbonate 0.1538 

Chloride 0.5622 

Sulfate 0.0001 

Fluoride 0.0221 

Silica 0.9706 

Areal Distribution 

The ionic composition of ground-water samples 

can be represented by another type of water-quality 

diagram – the stiff diagram (Stiff, 1951). Stiff diagrams 

are used to compare the ionic composition of water 

samples between different locations, depths, or aquifers. The stiff diagram is a polygon created from three 

horizontal axes extended on both sides of a vertical axis. 

Cations are plotted on the left side of the axis and anions 

are plotted on the right side, both in meq/L. A greater 

distance from the vertical axis represents a larger ionic 

concentration. The cation and anion concentrations are 

connected to form an asymmetric polygon known as a 

GROUND-WATER QUALITY 35 
stiff diagram, where the size is a relative indication of 

the dissolved-solids concentration. 

The areal distribution of stiff diagrams constructed 

for all water samples collected as part of this study are 

shown in figure 14. The horizontal axis units are the 

same on all stiff diagrams to aid comparisons between 

different sampling locations and aquifers. The color of 

the stiff diagrams indicates the aquifer and the number 

above each diagram is the calculated dissolved-solids 

concentration. 

Like the trilinear diagram presented and discussed 

earlier, the stiff diagrams show and reinforce the similarity in ionic composition (sodium-bicarbonate-type 

water) observed in most ground-water samples, regardless of aquifer. Stiff diagrams constructed for samples 

collected from wells completed in the Wasatch aquifer 

show no areal pattern in water type (all but three are 

sodium-bicarbonate-type waters) or obvious areal pattern in dissolved-solids concentrations; this suggests 

that well location, in addition to aquifer, may not be 

related to water type. However, there may be a relationship between well depth and ionic composition in the 

Wasatch aquifer. 

Stiff diagrams constructed for ground-water samples collected from wells completed in the coalbed aquifers also show no spatial pattern because the water type 

is the same (sodium-bicarbonate type) for all samples. 

The dissolved-solids concentration in the sample collected from well C11 (fig. 14), the most western site in 

the study area, is much higher than all other samples 

collected from wells completed in coalbed aquifers; this 

sample was collected from a well completed in the Big 

George coal bed, a coal bed considered equivalent to the 

Wyodak-Anderson coal zone (Flores and Bader, 1999). 

The Big George coal bed is both deeper and possibly 

older (Flores, 1999) than other coal beds in the 

Wyodak-Anderson coal zone (fig. 5). It should be noted 

that this well also is completed in sandstone and shale 

overlying the coal bed, and the effect on resulting water 

quality cannot be determined. However, another waterquality sample collected from this coal bed in the same 

area was similar in both water type (sodium-bicarbonate 

type) and high dissolved-solids concentration (Rice and 

others, 2000). 

On the basis of ground-water samples collected 

during this study and additional samples collected in the 

area as part of another study (Rice and others, 2000), 

dissolved-solids concentrations in the WyodakAnderson coalbed aquifer appear to be lower (generally 

less than 600 mg/L) south of the Belle Fourche River, 

near the southern Rochelle Hills (an area with clinker in 

the Lebo Member of the Fort Union Formation). Clinker in this area has been speculated to provide much 

recharge to coalbed aquifers downdip to the west in the 

study area (Davis, 1976; Daddow, 1986; Lowry, Wilson, 

and others, 1986; Martin and others, 1988; Heffern and 

others, 1996; Bureau of Land Management, 1999; Heffern and Coates, 1999). 

Dissolved-solids concentrations represent total 

ions dissolved in solution; concentrations typically are 

lower near recharge sources where ground-water residence times (time in contact with aquifer materials) are 

relatively short and increase as ground water flows 

away from the source of recharge and residence times 

increase. As stated earlier, potentiometric contours suggest regional ground-water flow in the WyodakAnderson coalbed aquifer may be towards the west and 

northwest, away from clinker in the area (Daddow, 

1986) (fig. 8). If the clinker in this area is the source of 

most of the recharge to the coalbed aquifers in the study 

area, dissolved-solids concentrations should increase as 

ground water flows away from the clinker deposits, a 

pattern that may be present and may explain the apparent increase in dissolved-solids concentrations as 

ground water flows north of the Belle Fourche River. 

Alternatively, the lower dissolved-solids concentrations 

in the area may not represent a true spatial pattern and 

may simply be the result of natural or random variation, 

different processes in the area, or a product of small 

sample size in the area. 

Dissolved-solids concentrations in the WyodakAnderson coalbed aquifer appears to increase northeast 

of the Belle Fourche River and outside of the study area. 

Rice and others (2000) noted that dissolved-solids concentrations in the Wyodak-Anderson coalbed aquifer 

appears to increase from south to north and from east to 

west in the Powder River Basin in Wyoming. The investigators suggest that “this trend may be a result of 

increased water-rock interaction along a flowpath, an 

increase or change in composition of the ash content of 

the coal, or other factors not yet recognized” and “the 

increase in TDS (total dissolved solids) is generally a 

result of an increase in sodium and bicarbonate content 

of the water” (Rice and others, 2000, p. 4). 

36 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 

Vertical Distribution Within Aquifers 

Dissolved-solids and major-ion concentrations in 

ground-water samples collected from wells were compared with well depth to determine if there was a statistically significant relationship. Kendall’s tau test 

statistic (Helsel and Hirsch, 1992), a nonparametric 

measure of the monotonic correlation between two continuous variables, was calculated to measure the 

strength of this relationship. Kendall’s tau is a correlation coefficient calculated using the ranks of the data 

rather than actual data values; therefore, the test statistic 

is resistant to outliers and can be used for datasets with 

moderate censoring. Tau is dimensionless and ranges 

between –1 and 1; when one variable increases as the 

second increases, tau is positive. If the variables vary in 

opposite directions, tau is negative. If a strong correlation is observed, tau, like any statistical correlation coefficient, does not explain the cause of the relationship. 

Kendall’s tau correlation coefficients were considered 

statistically significant when probabilities 

(p-values) were less than 0.05; the null hypothesis of no 

correlation between dissolved-solids or major-ion concentrations and well depth was rejected if the probability of obtaining this correlation by chance was less than 

this value. The exact form of the test was used for the 

samples collected from the Wasatch aquifer because the 

sample size was less than 10; the large sample approximation was used for samples collected from the coalbed 

aquifers because sample size was greater than 10. 

Kendall’s tau correlation coefficients and resulting 

probabilities (p-values or attained levels of significance) 

between dissolved solids or major ions and well depth 

for eight samples collected from the Wasatch aquifer are 

shown in table 8. Bicarbonate and fluoride were positively correlated with well depth; all other major ions 

were negatively correlated with well depth. Correlation 

between calcium and well depth is very strong (tau = -

0.93) and statistically significant (p-value = 0.0004); 

correlation between sulfate and well depth was strong 

(tau = -0.79) and also is statistically significant (p-value 

= 0.0056). These statistically significant correlations 

between calcium and sulfate and well depth are illustrated in scatterplots (figure 15). The correlation 

between dissolved solids and well depth is relatively 

strong (tau = -0.62) and close to statistical significance 

(p-value = 0.07). Similar nonsignificant correlations 

were observed between magnesium and well depth (tau 

= -0.57, p-value = 0.062) and fluoride and well depth 

(tau = 0.54, p-value = 0.084). 

Table 8. Kendall’s tau correlation coefficients and resulting 

probabilities (p-values) between dissolved solids or major 

ions and well depth, Wasatch aquifer 

[Bold text indicates a statistically significant correlation with a p-value less 

than 0.05] 

Kendall’s tau 

correlation Probability 

Constituent or major ion coefficient (p-value) 

Dissolved solids -0.62 0.07 

Calcium -0.93 0.0004 

Magnesium -0.57 0.062 

Sodium -0.46 0.144 

Potassium -0.43 0.178 

Bicarbonate 0.24 0.562 

Chloride -0.36 0.360 

Sulfate -0.79 0.0056 

Fluoride 0.54 0.084 

Silica -0.14 0.72 

Kendall’s tau correlation coefficients and resulting 

probabilities (p-values) between dissolved solids or 

major ions and well depth for 13 samples collected from 

coalbed aquifers are shown in table 9. Chloride and fluoride were negatively correlated with well depth; all 

other major ions were positively correlated with well 

. 

Table 9. Kendall’s tau correlation coefficients and resulting 

probabilities (p-values) between dissolved solids or major 

ions and well depth, coalbed aquifers 

[Bold text indicates a statistically significant correlation with a p-value less 

than 0.05; NC, not calculated because greater than 50 percent of data 

values are less than minimum reporting level] 

Kendall’s tau 

correlation Probability 

Constituent or major ion coefficient (p-value) 

Dissolved solids 0.179 0.393 

Calcium 0.115 0.5822 

Magnesium 0.217 0.297 

Sodium 0.167 0.4228 

Potassium 0.462 0.0256 

Bicarbonate 0.231 0.2721 

Chloride -0.256 0.2224 

Sulfate NC NC 

Fluoride -0.026 0.902 

Silica 0.308 0.1301 

38 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
GROUND-WATER QUALITY 39 
The strongest correlation was observed between potassium and well depth; this positive correlation (tau = 

0.462) is statistically significant (p-value = 0.0256), 

but not strong. The statistically significant correlation 

between potassium and well depth is illustrated in a 

scatterplot (fig. 16). Kendall’s tau was not calculated 

for sulfate because greater than 50 percent of the data 

values were censored (less than the MRL). 

Vertical Distribution Between Aquifers 

Major-ion chemistry in the vertical dimension was 

examined qualitatively in three areas with monitoringwell clusters, including one area with a monitoring-well 

cluster where three wells are completed in the Wasatch 

aquifer (W5, W4, W3) and overlie a nearby well (C12) 

completed in the Wyodak-Anderson coalbed aquifer 

(fig. 14); each well is completed progressively deeper 

(fig. 17). Water-level measurements indicated the 

hydraulic potential for downward vertical ground-water 

flow at this location. Major-ion composition was similar in ground-water samples collected from the two 

shallowest wells (W5 and W4) with both having mixed 

cations (calcium, magnesium, and sodium) and the 

same dominant anion, sulfate. The ionic composition 

was similar in samples collected from the two deepest 

wells (W3 and C12)—both were sodium-bicarbonatetype waters. As illustrated in figure 17, calcium, magnesium, and sulfate concentrations decrease with 

increasing well depth; these decreases correspond with 

a large decrease (from 4,020 mg/L to 382 mg/L) in 

dissolved-solids concentrations with increasing depth. 

Sodium concentrations increase with depth between 

the two shallowest wells (W5 and W4) but decrease 

with increasing well depth in all the other wells (W4, 

W3, and C12) completed successively deeper. Bicarbonate concentrations decrease with increasing well 

depth in the two shallowest wells (W5 and W4), 

increase with depth between the next two deepest wells 

(W4 and W3), and then decrease between the two deepest wells (W3 and C12). 

40 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Major-ion chemistry in the vertical dimension 

between the Wasatch aquifer and the underlying coalbed 

aquifers also was examined at two additional 

monitoring-well clusters composed of one well completed in the Wasatch aquifer and one well completed in 

the underlying coalbed aquifers (figs. 18 and 19). The 

major-ion chemistry in these two well clusters show a 

similar pattern to that of the well cluster previously discussed (Wells W5, W4, W3, and C12) where shallowest 

wells have a different ionic composition (i.e., water 

type) than the well completed deeper in the Wasatch 

aquifer or underlying coalbed aquifer. The groundwater samples collected from the monitoring-well cluster composed of wells W8 and C17 (fig. 18) indicate the 

water in the shallow well completed in the Wasatch 

aquifer is a mixed type and the water in the underlying 

coalbed aquifer is a sodium-bicarbonate type; calcium 

concentrations are about the same in the two wells, but 

magnesium, sulfate, and dissolved solids decrease with 

depth while sodium and bicarbonate concentrations 

increase. The ground-water samples collected from the 

well cluster composed of wells W7 and C16 (fig. 19) 

indicate the water in the well completed in the Wasatch 

aquifer is a sodium-bicarbonate-sulfate type and the 

water in the underlying coalbed aquifer is a sodiumbicarbonate type; calcium, magnesium, bicarbonate, and 

dissolved-solids concentrations increase with increasing 

depth, and sodium and sulfate concentrations decrease 

with depth (fig. 19). 

All ground-water samples collected from coalbed 

aquifers during this study were sodium-bicarbonatetype waters. Ground-water samples collected from the 

overlying Wasatch aquifer were either mixed-type 

waters or sodium-bicarbonate-type waters; the mixedtype waters in these aquifers were from relatively shallow wells, generally completed to depths less than about 

200 feet, and the sodium-bicarbonate waters were from 

deeper wells . Results from the rank-sum statistical test 

(table 7) indicated statistically significant differences in 

median concentrations of both fluoride and sulfate 

between ground-water samples collected from the 

Wasatch aquifer and ground-water samples collected 

from the underlying coalbed aquifers. Statistically significant correlations were observed between calcium 

and sulfate concentrations and well depth for groundwater samples collected from the Wasatch aquifer 

(table 8); nonsignificant correlations were observed 

between dissolved solids, magnesium, fluoride, and well 

depth. A statistically significant correlation was 

observed between potassium and well depth in samples 

collected from the coalbed aquifers (table 9). All of 

these comparisons, both qualitative and statistical, show 

the most striking differences in ionic composition 

between the Wasatch aquifer and the underlying coalbed 

aquifers are observed in sulfate concentrations; this difference appears to be related to depth. 

Geochemical Processes 

Qualitative and statistical examination of the majorion chemistry of ground-water samples collected during 

this study suggests there is a relationship between depth 

and ionic composition in the Wasatch aquifer overlying 

the coalbed aquifers. A relationship between well depth 

and major-ion composition of ground water from lower 

Tertiary aquifers in the Powder River Basin has been 

noted by many other investigators. The relationship is 

reflected by the presence of two major-ion compositional systems—generally, a shallow system with mixed 

cation composition (calcium, magnesium, and lesser 

amounts of sodium) and either bicarbonate or sulfate as 

the dominant anion, and a deeper, underlying system 

where sodium and bicarbonate are the dominant ions. 

This apparent “zonation” in major-ion composition was 

first noted by Whitcomb and others (1966) for groundwater samples collected from the Fort Union Formation 

in Johnson County, Wyoming; the investigators reported 

that “the quality of water in the Fort Union Formation 

seems to be related to the depth of the water-bearing 

zone” (Whitcomb and others, 1966, p. 69). They noted 

that water from wells less than 500 feet deep in Johnson 

County, Wyoming, is typically of a sodium-sulfate, calcium-sodium sulfate, or calcium-sulfate type with high 

dissolved-solids concentrations, and water from deeper 

wells is sodium-bicarbonate type with lower dissolved 

solids. 

In a study of the geochemistry of waters in the 

Wasatch and Fort Union Formations in the Powder River 

Basin within Wyoming, Hagmaier (1971) noted changes 

in major-ion chemistry with depth in lower Tertiary 

aquifers and suggested three “divisions” or “zones” with 

different water types—“shallow ground water, deep 

ground water from the Wasatch Formation, and deep 

ground water from the Fort Union Formation” (Hagmaier, 1971, p. 6). In Hagmaier’s classification, shallow 

ground water is typically represented by 

calcium-sodium-sulfate-bicarbonate-type water in 

recharge areas 

GROUND-WATER QUALITY 41 
42 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
GROUND-WATER QUALITY 43 
44 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
GROUND-WATER QUALITY 45 
46 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
GROUND-WATER QUALITY 47 
and sodium-calcium-bicarbonate-sulfate-type water in 

local discharge areas; deep ground water from the 

Wasatch Formation is typically represented by 

sodium-calcium-sulfate-bicarbonate-type water; and 

deep ground water from the Fort Union Formation is 

typically represented by a sodium-bicarbonate-type 

water. Similarly, a study conducted by the GroundWater Subgroup (1974) suggested that water from 

shallow aquifers in the Wasatch and Fort Union Formations in the Gillette area is generally a “calcium 

and/or magnesium-sulfate water” and “as depth to the 

aquifer increases, calcium and magnesium ions give 

way to sodium ions, and sulfate is at least partially 

replaced by bicarbonate ions,” resulting in a sodiumbicarbonate-type water (Ground-Water Subgroup, 

1974, p. 35). Lee (1981), in a comprehensive evaluation of the geochemistry of waters in the Fort Union 

Formation of the Powder River Basin in southern 

Montana, noted the presence of two distinct geochemical systems—a shallow, localized system generally 

less than about 200 ft deep that is chemically dynamic 

with highly variable water chemistry, and a deeper, 

regional system greater than about 200 ft deep that is 

chemically stagnant with water chemistry characterized by sodium and bicarbonate. The shallow 

geochemical system probably coincides with local 

ground-water flow, while the deeper geochemical system probably is dominated by regional ground-water 

flow (Lee, 1981; Slagle and others, 1985). Feathers 

and others (1981) also noted that ionic composition in 

waters from the Wasatch and Fort Union Formations in 

the Powder River Basin in Wyoming have a relationship to well depth. Lowry, Wilson, and others (1986, 

p. 99, fig. 8.4.3-2) noted the same compositional 

change for waters from lower Tertiary and Upper Cretaceous aquifers in the Powder River drainage basin in 

Wyoming and Montana occurs and at a depth similar 

to that noted by Lee (about 200 feet). Rankl and 

Lowry (1990) examined major-ion composition with 

depth in lower Tertiary and Upper Cretaceous aquifers 

in the Powder River Basin in Wyoming and Montana 

and noted the same changes in ionic composition, 

although at a depth up to 500 feet (same as Whitcomb 

and others (1966)). Conceptually, and using nomenclature similar to that presented by Lee (1981), the 

relation of water type to aquifers in the study area is 

shown in figure 20. 

Although some of these investigators were often 

discussing water-quality characteristics and geochemical processes in the Fort Union Formation, or combining the Wasatch and Fort Union Formations into a 

single hydrogeologic unit in the Powder River Basin, 

the Wasatch Formation, as discussed earlier, is very 

similar to and essentially indistinguishable from the 

Fort Union Formation in depositional and lithological 

composition (Lowry and Cummings, 1966; Davis, 

1976; Flores and others, 1999), although it has been 

suggested that coal beds and sandstone deposits may be 

less common in the Wasatch Formation (Lowry and 

Cummings, 1966, p. 37). Consequently, many of these 

investigators have noted that the same geochemical 

processes probably control water chemistry in both 

lower Tertiary formations (Lowry and Cummings, 

1966; Ground-Water Subgroup, 1974; Feathers and 

others, 1981; Lowry, Wilson, and others, 1986; Rankl 

and Lowry, 1990). It also should be noted that many of 

these discussions are referring to the entire Fort Union 

Formation or combined Fort Union and Wasatch Formations and that processes may differ or be somewhat 

altered within a single coal bed or zone. 

Cation exchange is the chemical reaction frequently cited to explain the high percentage of sodium 

compared to calcium and magnesium in water from the 

Fort Union Formation in the Powder River Basin 

(Renick, 1924; Riffenburg, 1925; Lowry and Cummings, 1966; Whitcomb and others, 1966; Hagmaier, 

1971; Lee, 1981; Woessner and others, 1981; Lowry, 

Wilson, and others, 1986; VanVoast and Reiten, 1988; 

VanVoast, 1991) in Wyoming and Montana, and correspondingly in the Wasatch Formation in Wyoming 

(Lowry and Cummings, 1966; Whitcomb and others, 

1966; Hagmaier, 1971). Cation exchange is a reaction 

in which the calcium and magnesium in the water are 

exchanged for sodium that was adsorbed to aquifer solids such as clay minerals, resulting in higher sodium 

concentrations and softer water (decreased calcium and 

magnesium concentrations). The generalized reactions 

(Hem, 1985, p. 13) are as follows: 

Na2X + Ca2+ →← CaX + 2Na+ (2) 

Na2X + Mg2+ →← MgX + 2Na+ (3) 

Where Na+ = sodium ion 

Ca2+ = calcium ion 

Mg2+ = magnesium ion 

X = aquifer solid 

48 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Hagmaier (1971) and Lee (1981) both noted that calcium and magnesium cations change from dominant to 

subordinate cations in relation to total cations with 

increasing depth and as ground water flows away from 

sources of recharge. Similar conclusions were reached 

earlier by Hamilton (1970) in western North Dakota 

and Riffenburg (1925) in the Northern Great Plains of 

the United States. 

Sulfate reduction in water in the Fort Union Formation in Wyoming and Montana was suggested by investigators based on the relatively high percentage of 

bicarbonate in comparison to sulfate and by the detection of hydrogen sulfide and hydrocarbon gases such as 

methane in the water at many locations (Riffenburg, 

1925; Lowry and Cummings, 1966; Whitcomb and others, 1966; Hagmaier, 1971; Dockins and others, 1980; 

Lee, 1981; Woessner and others, 1981; Lowry, Wilson, 

and others, 1986; VanVoast and Reiten, 1988; VanVoast, 

1991). These observations, in combination with 

geochemical modeling at two coal mines, led Martin 

and others (1988) to conclude that sulfate reduction is 

likely an ongoing process at some locations in the Powder River Basin. Sulfate reduction is a process where 

bacteria obtain energy for metabolism from oxidation of 

relatively simple organic compounds. That oxidation is 

coupled to sulfate reduction. Byproducts of this activity 

are bicarbonate and hydrogen sulfide. This process generally occurs when no free oxygen is available (anaerobic or reducing conditions). The generalized reaction 

(Drever, 1997, p. 161) for sulfate reduction is as follows: 

- SO 2-

4 + 2Corganic + 2H2O = H2S + 2HCO3 (4) 

The presence of hydrogen sulfide (H2S) is often 

indicated by the presence of a “rotten-egg” odor; this 

odor was frequently detected in ground-water samples 

collected from the Wasatch aquifer and coalbed aquifers 

during this study. Dockins and others (1980), using stable isotopes of sulfur, showed that bacteriallymediated sulfate reduction is probably responsible for 

low sulfate concentrations and most of the sulfide production in the Fort Union Formation in the Powder 

River Basin in southeastern Montana. The investigators 

also noted that sulfate-reducing bacteria were detected 

in 25 of 26 of their ground-water samples. 

Most of these investigators suggest both processes 

in combination may explain the observed ionic composition and zonation of water from the Wasatch and Fort 

Union aquifers in the Powder River Basin of both Wyoming and Montana. The general underlying assumption 

is that ground water associated with recharge (probably 

areas 1, 2, and 3 in the Wyodak-Anderson coal zone and 

area 4 in the shallow geochemical zone, fig. 20) is represented by water dominant in calcium, magnesium, 

and sulfate, with lesser amounts of sodium and bicarbonate (Hagmaier, 1971; Lee, 1981; Woessner and others, 1981). As ground water flows away from the source 

of recharge, the interaction between water and rock 

increases. Sodic lithologic units are encountered as the 

ground water moves along a flowpath and calcium and 

magnesium ions are exchanged for sodium ions 

attached to aquifer solids. Anaerobic sulfate-reducing 

bacteria, in the presence of organic material in coals and 

carbonaceous shales, also act on the ground water as it 

moves along a flowpath. Therefore, both reactions 

result in a decrease in calcium, magnesium, and sulfate, 

and a corresponding increase in sodium and bicarbonate 

as ground water flows away from the source of recharge 

and results in a water that evolves to a sodium-bicarbonate type (areas 5 and 7 in the deep geochemical zone and 

area 6 in the Wyodak-Anderson coal zone, fig. 20). 

With the possible exception of Hagmaier (1971), 

Lee’s evaluation of the water quality in the Fort Union 

Formation in the Powder River Basin in southern Montana is still the most comprehensive examination of the 

water chemistry in relation to hydrogeologic and 

geochemical conditions. However, it should be noted 

that many of the conclusions reached by Lee (1981) are 

very similar to conclusions reached by Hagmaier (1971) 

for the geochemistry of waters in the same aquifers in 

the Powder River Basin in Wyoming, even though both 

investigators appear to have reached their conclusions 

independently. As discussed previously, Lee suggests 

the presence of two distinct geochemical systems: a 

shallow, localized, chemically dynamic system generally less than 200 feet (area 4 in the shallow geochemical zone, fig. 20) and an underlying deeper, regional, 

chemically static system (areas 5 and 7 in the deep 

geochemical zone and area 6 in the Wyodak-Anderson 

coal zone, fig. 20). At topographically high locations 

and from springs, water from the shallow system probably represents recharge waters and the dominant ions 

are magnesium, calcium, sodium, and bicarbonate, with 

moderate amounts of sulfate and low concentrations of 

chloride. At topographically low locations, water in the 

shallow system is dominant either in sodium and sulfate 

GROUND-WATER QUALITY 49 
50 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
with lower concentrations of magnesium, calcium, and 

bicarbonate, or is dominant in sodium and bicarbonate 

with lower concentrations of magnesium, calcium, and 

sulfate. 

Lee proposed three general trends in the major-ion 

composition of ground water as it flows away from 

recharge areas as well as the geochemical processes that 

might produce the trends. The three trends are: 

(1) enrichment in sodium and sulfate, (2) enrichment in 

sodium and depletion of sulfate, and (3) depletion of 

sodium and sulfate. Lee found the first trend was the 

most common trend in the shallow system, although all 

three trends were observed. Lee concluded that the primary chemical reactions that contribute to sodium 

enrichment are dissolution of minerals such as sodium 

feldspars by infiltrating recharge waters in combination 

with cation-exchange on clay minerals, although other 

reactions of less significance also may occur. Direct sulfate enrichment may be attributed to the weathering of 

pyrite and dissolution of gypsum (also noted in Dockins 

and others (1980)), or indirectly by precipitation of calcium and magnesium carbonates that remove bicarbonate from solution (also suggested by Hagmaier (1971)). 

As ground water continues to flow away from the source 

of recharge, the trend towards sulfate enrichment is 

reversed and relative depletion of sulfate is commonly 

observed (second trend). 

Bacterially mediated sulfate reduction is the chemical reaction suspected to explain the decreases in sulfate 

in both the shallow and deeper systems. Lee proposed 

that the observed trend of sodium and sulfate depletion 

(third trend) may be explained by mixing of infiltrating 

ground water with older ground water that results in dilution. In addition, water in the deeper system, typically 

with sodium-bicarbonate-type waters, may locally leak 

upward into the shallow system and decrease the relative 

percentage of sulfate. In the chemically stagnant deeper 

system, Lee suggests bacterially mediated sulfate reduction was the dominant geochemical process and results 

in water that is primarily sodium-bicarbonate type. 

Carbonate equilibria also may be controlling the 

amount of dissolved calcium and magnesium in solution. The presence of dissolved constituents at relatively 

high concentrations (i.e., sodium and bicarbonate concentrations in most samples collected from all aquifers in 

the study area) can control the amount of calcium and 

magnesium in solution through the precipitation of carbonate minerals. The reader is referred to Hem (1985), 

Stumm and Morgan (1996), and Drever (1997) for more 

detailed discussions of carbonate equilibria and equilibrium water chemistry. 

Waters in the coalbed aquifers in some areas may 

evolve to a sodium-bicarbonate-type water through a 

different combination of hydrological and geochemical 

processes. Heffern and Coates (1999) examined majorion composition in clinker associated with the WyodakAnderson coalbed aquifer along the eastern margin of 

the basin and study area (Rochelle Hills area) and speculated on the processes responsible for the composition 

of the waters in the deposits. As described earlier, the 

clinker in this area is often directly updip and in 

hydraulic connection with the WyodakAnderson coalbed aquifer (fig. 20) and the deposits in 

this area are suspected to be the source of much of the 

recharge to the aquifer in the study area. The investigators noted several trends in major-ion chemistry in the 

clinker: (1) Waters upgradient of the burn line (the contact between the clinker and unburned coal or overburden) are highly oxygenated with low dissolvedsolids concentrations and are primarily calciumbicarbonate-type waters (area 1 in the WyodakAnderson coal zone, fig. 20), and (2) waters “ponded” 

adjacent to or near the burn line are frequently characterized by high sulfate concentrations and high 

dissolved-solids concentrations (area 2 in the WyodakAnderson coal zone, fig. 20). Waters in the WyodakAnderson coalbed aquifer immediately downgradient 

of the burn line (and in hydraulic connection with the 

clinker) are frequently characterized by moderate 

dissolved-solids concentrations and sodiumbicarbonate-type waters typically seen in the coalbed 

aquifers in the study area (areas 3 and 6 in the WyodakAnderson coal zone, fig. 20). The investigators speculate on the processes that result in the observed chemistry. The following quotation is from Heffern and 

Coates (1999, p. 249) and explains their hypothesis: 

Mine permits show that wells drilled into ponded 

clinker water near this contact commonly have high levels of sulfate and TDS. We postulate that these high 

concentrations are due to dissolution of: 1) sulfates and 

pyrite in the coal near the burn line, and 2) minerals in 

the ash layer at the base of the clinker, as highly oxygenated ground water flows downdip through the clinker towards the coal. As this water moves downdip into 

and through the coal, calcium and sulfate precipitate 

out in fractures as gypsum, and magnesium and sulfate 

as epsomite (Davis, 1976). Water in the coal, and in 

clinker downdip from the coal, is typically of the 

sodium-bicarbonate type, with moderate TDS levels. 

GROUND-WATER QUALITY 51 
Coalbed methane in the Powder River Basin is 

believed to be biological in origin (biogenic) (Law and 

others, 1991; Rice, 1993; Gorody, 1999), a product of 

anaerobic bacterial decomposition of organic matter 

through a series of redox reactions. In samples collected from the coalbed aquifers during this study, the 

high percentage of sulfate nondetections (greater than 

50 percent) and low sulfate concentrations when 

detected is suggestive of waters that have undergone or 

are undergoing sulfate reduction. VanVoast (1991, p. 

1,143) noted that methane encountered in the Powder 

River Basin “invariably coincides with conditions of 

chemically reduced groundwater containing Na and 

HCO3 as the principal ions.” In addition, the author 

noted that “a chemical model accounting for methane 

and the associated water-quality conditions requires the 

exchange of cations Ca and Mg (to clays) for Na (to 

water) followed by: 4CH2O (coal) + SO4 + H2O →

H2S +3HCO3 + CH4" (VanVoast, 1991, p. 1,143). Rice 

and others (2000) also observed similar low concentrations and nondetections of sulfate in ground-water samples collected from 47 wells completed in the coalbed 

aquifers; similarly, the investigators noted that the low 

sulfate concentrations “are consistent with water in 

contact with a coal reservoir that has undergone or is 

undergoing methanogenesis” (Rice and others, 2000, 

p. 5). 

Methanogenesis is another type of redox chemical 

reaction. The anaerobic bacterial decomposition of 

organic matter occurs as a series of redox chemical 

reactions – each series or stage of redox reactions occur 

in a specific order (succession) and are mediated by different types of bacteria (Chapelle and others, 1993; 

Chapelle, 2001); this series of reactions essentially represents a microbial “food chain” where different types 

of bacteria utilize relatively simple organic matter or 

compounds as a source of energy (Chapelle and others, 

1993; Chapelle, 2001). Sulfate reduction (described 

earlier) is one of these reactions. After all or most sulfate has been consumed through the process of sulfate 

reduction, the anaerobic bacterial decomposition of 

organic matter occurs in three stages and each stage or 

reaction is mediated by a different type of bacteria – the 

final stage or process is known as methanogenesis 

(Rice, 1993) and the bacteria that mediate the process 

are known as “methanogens.” Although these redox 

chemical reactions occur in successive stages, both sulfate reduction and methanogenesis can exist within 

separate zones of the same aquifer (Chapelle and others, 1993; Chapelle, 2001). The methanogens utilize 

relatively simple organic compounds (produced in two 

prior stages of anaerobic decomposition by the breakdown of complex organic matter by acetogenic and fermentative bacteria such as hydrogen (H2), carbon 

dioxide (CO2), acetate, formate, and simple alcohols, 

to produce methane). There are two metabolic pathways used by bacteria to produce methane after sulfate 

reduction has ended—methyl-group fermentation and 

reduction of carbon dioxide (Schoell, 1980; Wolternate 

and others, 1984; Jenden and Kaplan, 1986; Whiticar 

and others, 1986). The two generalized reactions 

(Chapelle, 2001) showing the metabolic pathways for 

bacterial conversion of simple organic matter into 

methane are as follows: 

Methyl-group fermentation CH3COOH → CH4 + CO2 (5) 

acetic acid 

Carbon dioxide reduction CO2 + 4H2 → CH4 + 2H2O (6) 

Rice (1993) examined stable isotopes of gas and 

water and suggested that the second reaction, carbon 

dioxide reduction, was the metabolic pathway responsible for most (about 80 percent) of the biogenic generation of methane in coal beds of the Powder River 

Basin and methyl-group fermentation was responsible 

for the rest. Similarly, Gorody (1999) also examined 

the isotopic composition of gas and water from the coal 

beds and also concluded that reduction of carbon dioxide by methanogenic bacteria rather than methyl-group 

fermentation is probably responsible for most of the 

large quantities of methane present in the WyodakAnderson coal zone in the Powder River Basin. Rice 

(1993) suggests that biogenic methane generation in 

the Powder River Basin occurred between 10 and 

35 million years ago, the time of estimated maximum 

burial of the coal beds. In contrast, Gorody (1999) suggests that methane generation may have continued to at 

least the Pleistocene age. 

In the past, both sulfate reduction and methanogenesis have affected the chemistry of waters in the 

Wasatch aquifer and coalbed aquifers. On the basis of 

work in this and other studies described herein, sulfate 

reduction was likely the dominant geochemical process 

responsible for the sodium-bicarbonate-type waters in 

the deep aquifers in the study area. The low to nondetectable concentrations of sulfate in coalbed aquifers in 

the study area documented in this study and in Rice and 

others (2000), in combination with earlier work by 

52 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Dockins and others (1980) and other studies described 

earlier, suggests that sulfate reduction may be an ongoing process in some locations in aquifers in the study 

area. Presumably, the sulfate reduction is occurring 

primarily in the Wasatch aquifer overlying the coalbed 

aquifers (except possibly along eastern basin margin – 

see discussion in Heffern and Coates (1999) and discussion herein) because the near absence of sulfate in 

the coalbed aquifers indicates the waters in the coalbed 

aquifers are past sulfate reduction in terms of terminal 

electron-accepting processes (see discussion in 

Chapelle (2001)). Because of the absence of sulfate, 

waters in the coalbed aquifers can undergo methanogenesis if the organic precursors necessary for activity 

by methanogenic bacteria are present in the aquifers. 

Work to answer these questions is well outside the 

scope of this study. Future work will hopefully determine the location and extent of active sulfate reduction 

and determine if methanogenesis in the aquifers is 

occuring in any of the study area and throughout the 

Powder River Basin. 

Results from this and previous studies suggest that 

reducing conditions exist in significant portions of the 

Wasatch and coalbed aquifers. Very reducing conditions can affect the chemistry (i.e., mobility) of many 

other constituents, particularly redox-sensitive species 

(Hem, 1985; Stumm and Morgan, 1996; Drever, 1997). 

Iron and manganese are detected at relatively high concentrations in the Wasatch aquifer and coalbed aquifers, often at concentrations greater than regulatory 

standards (Martin and others, 1988; Rice and others, 

2000). Both iron and manganese are highly sensitive to 

redox conditions and both are more soluble in reducing 

conditions (Hem, 1985). 

Low to nondetectable concentrations of many 

trace elements in waters from the coalbed aquifers in 

the Powder River Basin in Wyoming have been 

reported by other investigators (Drever and others, 

1977; Martin and others, 1988; Rice and others, 2000); 

when detected, concentrations of many trace elements 

are well below any applicable USEPA regulatory standards. Hydrogen sulfide (H2S) produced by sulfate 

reduction can strongly affect (decrease) the solubility 

of many metals (Drever, 1997). In addition, the low to 

nondetectable sulfate concentrations in the coalbed 

aquifers can influence the solubility of trace elements 

such as barium. Rice and others (2000) noted that barium is frequently detected in the coalbed aquifers at 

concentrations higher than typically seen for most 

ground waters. The solubility of barium is primarily 

controlled by barite (barium sulfate) (Hem, 1985), so 

the absence of sulfate may be responsible for the relatively high concentrations observed in the waters. 

Environmental Isotopes 

Isotopes are atoms of the same element that have 

the same numbers of protons and electrons but different 

numbers of neutrons. The difference in the number of 

neutrons between different isotopes of the same element means they have the same atomic number but different atomic masses. Environmental isotopes are 

naturally occurring isotopes of elements commonly 

found in the natural environment (typically hydrogen, 

carbon, nitrogen, oxygen, and sulfur). Isotopes are 

called stable if not involved in any natural radioactive 

decay or called radioactive if they undergo radioactive 

decay. Stable isotopes are typically used in hydrogeology as “tracers of water, carbon, nutrient, and solute 

cycling” (Clark and Fritz, 1997, p. 5) whereas radioactive isotopes are typically used “to estimate the age or 

circulation of ground-water” (Clark and Fritz, 1997, 

p. 5). Both radioactive and stable isotopes were examined during this study. 

Tritium 

Tritium (3H) is a radioactive isotope of hydrogen 

(H) commonly used as a tool to provide an approximate 

age for ground water. Tritium atoms are unstable and 

undergo radioactive decay with a half-life of 

12.43 years (Plummer and others, 1993). Unlike other 

radioisotopes, tritium atoms can substitute for other 

hydrogen atoms in the water molecule. The incorporation of tritium directly into the water molecule makes 

tritium an excellent tool for qualitatively age-dating 

ground water. Tritium is produced by cosmic radiation 

naturally in the upper atmosphere at very low concentrations and is incorporated into precipitation. However, atmospheric testing of thermonuclear devices 

beginning in the early 1950’s and continuing through 

the early 1960’s increased tritium concentrations by 

several orders of magnitude above natural levels, 

resulting in a global atmospheric spike in tritium concentrations measured in precipitation. The atmospheric spike occurred during and after thermonuclear 

device testing and “created a tritium reservoir in the 

stratosphere which contaminated global precipitation 

GROUND-WATER QUALITY 53 
systems for over four decades” (Clark and Fritz, 1997, 

p. 177). A test-ban treaty was signed in 1963, effectively ending large-scale atmospheric testing of thermonuclear devices and the resulting large increases in 

atmospheric tritium. Non-signatory nations (France 

and China) conducted small atmospheric tests of thermonuclear devices until 1980 (Clark and Fritz, 1997) 

but these tests probably contributed little to tritium concentrations in precipitation (Michel, 1989). Atmospheric concentrations decreased after the test-ban 

treaty by decay in the atmosphere and loss to the hydrologic cycle (primarily the oceans). Tritium concentrations in precipitation have only recently (about 1990) 

approached natural levels. Concentrations probably 

will never completely return to natural levels because 

low concentrations of tritium continue to be released by 

other anthropogenic sources such as nuclear power 

plants and weapons plants (Clark and Fritz, 1997). 

Tritium in ground water is primarily from precipitation that infiltrated downward to the aquifer (groundwater recharge). Recharge to ground water originates 

as precipitation, therefore, increased levels of tritium 

present in precipitation during and after nuclear device 

testing can be measured in ground-water samples to 

qualitatively estimate the time that the water entered 

the aquifer and became isolated from the atmospheric 

source of tritium. Tritium is generally used as a qualitative dating tool because tritium concentrations have 

varied considerably, both spatially and temporally 

(Michel, 1989). The presence of elevated levels of tritium in ground water is considered an indicator of 

recent or active ground-water recharge that has 

occurred since the early 1950’s and has become the 

standard for defining “modern” ground water (Plummer and others, 1993; Clark and Fritz, 1997). Because 

modern ground water is defined by the very presence of 

tritium, water with detectable concentrations also is 

frequently referred to as post-bomb water or ground 

water with some component of recharge after 1952. 

Ground water with little or no detectable tritium is 

defined as “submodern” or older and often is referred 

to as pre-bomb water or ground water recharged prior 

to 1952. 

Tritium concentrations are expressed in picocuries 

per liter (pCi/L) or tritium units (TU) where 1 TU is 

equal to 1 3

H atom in 1018 atoms of H, or 1 TU is equal 

to about 3.24 pCi/L (Plummer and others, 1993). From 

a practical standpoint, tritium concentrations in ground 

water recharged prior to nuclear device testing (submodern or pre-bomb water) would be less than about 

0.8 TU or 2.6 pCi/L; greater concentrations would indicate modern water that has been recharged after atmospheric thermonuclear device testing began (modern or 

post-bomb water) or ground water that is a mixture of 

submodern and modern water (Plummer and others, 

1993; Clark and Fritz, 1997). 

Concentrations detected in the water samples discussed in this report are compared to qualitative interpretations of tritium concentrations in ground water for 

continental regions as presented in Clark and Fritz 

(1997, p. 185), and reproduced here in its entirety for 

convenience: 

< 0.8 TU Submodern – recharged prior to 1952 

0.8 to ~ 4 TU Mixture between submodern and 

recent recharge 

5 to 15 TU Modern (< 5 to 10 years) 

15 to 30 TU Some “bomb” tritium present 

> 30 TU Considerable component of recharge from 

the 1960’s or 1970’s 

> 50 TU Dominantly the 1960’s recharge 

Tritium concentrations in ground-water samples 

collected from springs, monitoring wells, and coalbed 

methane production wells as part of this study are presented in table 10; for convenience, concentrations are 

presented in both pCi/L and TU. 

Tritium concentrations in water samples collected 

from spring S1 (about 24 TU) and spring S2 (about 

16 TU) suggest that both were recharged after 1952 and 

contain modern or post-bomb water (table 10). The tritium concentrations are both within the “some bomb 

tritium present” category presented by Clark and Fritz 

(1997), indicating that both springs were recharged 

after 1952 and are likely discharging modern or postbomb water. The presence of modern water in both 

springs is indicative of recent recharge; both springs 

had calcium and magnesium as dominant cations and 

the presence of modern water confirmed suggestions 

by other investigators (discussed earlier) that these two 

cations would be dominant in recharge waters in the 

study area. 

Ground-water samples were collected from eight 

monitoring wells completed in the Wasatch aquifer to 

examine tritium concentrations in aquifers overlying the 

coalbed aquifers (table 10). Tritium was not detected in 

two of eight monitoring wells completed in the Wasatch 

aquifer. Detectable tritium concentrations in the 

54 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Table 10. Tritium concentrations in ground-water samples collected from springs and wells 

[pCi/L, picocuries per liter; TU, tritium units; < MRL, less than minimum reporting level; NA, not applicable because concentration is less than 

minimum reporting level] 

Site 

number Hydrogeologic unit 

S1	 Springs discharging 

from clinker 

S2	 Springs discharging 

from clinker 

W1 Wasatch aquifer 

W2 Wasatch aquifer 

W3 Wasatch aquifer 

W4 Wasatch aquifer 

W5 Wasatch aquifer 

W6 Wasatch aquifer 

W7 Wasatch aquifer 

W8 Wasatch aquifer 

C1 Coalbed aquifers 

C2 Coalbed aquifers 

C4 Coalbed aquifers 

C5 Coalbed aquifers 

C6 Coalbed aquifers 

C8 Coalbed aquifers 

C10 Coalbed aquifers 

C11 Coalbed aquifers 

C12 Coalbed aquifers 

C13 Coalbed aquifers 

C15 Coalbed aquifers 

C16 Coalbed aquifers 

C17 Coalbed aquifers 

Minimum Analytical Tritium Tritium 

reporting level uncertainty1 concentration concentration 

(pCi/L) (+/- pCi/L) (pCi/L) (TU) 

1 4 76 24 

1 3 52 16 

1 1 < MRL NA 

1 1 9 2.8 

1 1 2 0.6 

1 1 < MRL NA 

1 1 6 1.8 

1 1 2 0.6 

1 1 1 0.3 

1 1 < MRL NA 

1 1 < MRL NA 

1 1 < MRL NA 

1 1 1 0.3 

1 1 < MRL NA 

1 1 < MRL NA 

1 1 1 0.3 

1 1 2 0.6 

1 1 1 0.3 

1 1 < MRL NA 

1 1 < MRL NA 

1 1 < MRL NA 

1 1 < MRL NA 

1 1 1 0.3 

1

Also known as two-sigma precision estimate. 

GROUND-WATER QUALITY 55 
---------------------------------------------------------------------------------------------------

remaining six wells ranged from the MRL (about 

0.3 TU or 1 pCi/L) to about 2.8 TU. Therefore, tritium 

concentrations in six of eight water samples collected 

from wells completed in the Wasatch aquifer overlying 

the coalbed aquifers suggest the water is submodern or 

pre-bomb (tritium concentration less than 0.8 TU). Tritium concentrations in the remaining two samples are 

in the category (0.8 to 4 TU) that suggests a mixture 

between submodern (pre-bomb) and modern (postbomb) water, although the low concentrations suggest 

that ground water in these wells have very little modern 

water. 

Tritium was not detected in eight of thirteen 

ground-water samples collected from monitoring wells 

and coalbed methane production wells completed in 

coalbed aquifers (table 10). Of the five wells with 

detectable concentrations, tritium was detected at the 

MRL (about 0.3 TU) in four wells and at a concentration of 0.6 TU in one well. The absence of any tritium 

at concentrations above 0.8 TU suggests that ground 

water in the coalbed aquifers probably is submodern or 

pre-bomb, indicating that no recharge water is likely to 

have entered the aquifers and traveled to the vicinity of 

the sampled wells since at least the early 1950’s. 

Stable Isotopes of Oxygen and Hydrogen 

Ratios of the stable isotopes of oxygen (oxygen-18/ 

oxygen-16, or 18O /16O) and hydrogen (deuterium/ 

hydrogen, or 2

H/H) in the water samples were examined 

in relation to meteoric water lines, to vertical distribution in monitoring-well clusters, and to spatial distribution. Hydrogen-2 (deuterium or 2

H) is a stable heavy 

isotope of hydrogen containing an extra neutron, and 

oxygen-18 (18O) is a stable heavy isotope of oxygen 

containing two extra neutrons. The variation in the 

number of neutrons in an element results in a different 

mass. This difference in mass causes fractionation to 

occur by a process known as Rayleigh fractionation 

(Clark and Fritz, 1997). Isotopic data are reported as 

ratios because isotopic ratios are determined more precisely than actual abundances. The isotopic concentration or ratio is expressed as the difference between the 

measured ratios of the sample and the reference water 

divided by the ratio of the measured reference water. 

Vienna Standard Mean Ocean Water (VSMOW) is generally used as the standard reference water for δ18O and 

δ2

H. The ratio, commonly known as delta (δ), is 

expressed in units of parts per thousand or per mil (‰) 

because the differences are very small. For example, a 

sample with a δ18O of -20 ‰ is depleted in δ18O by 

2 percent, or 20 ‰ relative to the standard. This ratio is 

calculated as follows (example for δ18O): 

δ

18O sample (in ‰) = (7) 

 18 16 18 16  ( O/ O)

sample – ( O/ O)

VSMOW

  × 1000  18 16   ( O/ O)

VSMOW 

An equation of the same form is used to calculate 

δ2

H. The analytical precision for δ18O values in a given 

laboratory is usually about +/- 0.2 ‰ and for δ2H, the 

analytical precision is usually +/- 1.0 ‰ (Clark and 

Fritz, 1997). In this study, a field replicate collected at 

well C15 had a difference of 0.7 ‰ for δ2H and 0.05 ‰ 

for δ18O between the normal environmental sample and 

the field replicate (table 11). 

The values of δ18O and δ2H in precipitation 

become progressively lighter from the equator towards 

the poles, from lower to higher elevations, and with the 

distance inland from the ocean. When paired δ18O and 

δ2

H values of precipitation throughout the world are 

plotted, they fall closely to a straight line known as the 

global meteoric water line (GMWL) (Craig, 1961) 

which is defined as follows: 

δ2H = 8 δ18O + 10 (8) 

Similarly, paired δ18O and δ2H values for North American continental precipitation have been plotted and 

they also fall closely to a straight line that has the same 

slope but is shifted downward from GMWL (Gat, 

1980). 

δ2H and δ18O values in ground water are representative of values in precipitation that recharge the ground 

water, unless some process after the water reaches the 

earth’s surface as precipitation causes isotopic fractionation, and consequently, deviation from meteoric water 

lines. Some processes that cause fractionation are evaporation, exchange with the aquifer matrix, or recharge 

that occurred at a different temperature or under a different climate. Local meteoric water lines also may 

deviate from the GMWL. Shifts in δ18O values may be 

affected by interaction with the aquifer matrix. Calcite 

interactions in carbonate aquifers or more complex 

56 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
Table 11. δ2H (deuterium/hydrogen isotopic ratio) and δ18O (oxygen-18/oxygen-16 isotopic ratio) values for groundwater samples collected from springs discharging from clinker and wells completed in the Wasatch aquifer and coalbed 

aquifers 

[‰, parts per thousand or per mil] 

δ2H δ18O 

Site number Hydrogeologic unit (‰) (‰) 

S1 Springs discharging from clinker -138.5 -18.04 

S2 Springs discharging from clinker -142.6 -18.41 

W1 Wasatch aquifer 

W2 Wasatch aquifer 

W3 Wasatch aquifer 

W4 Wasatch aquifer 

W5 Wasatch aquifer 

W6 Wasatch aquifer 

W7 Wasatch aquifer 

W8 Wasatch aquifer 

C1 Coalbed aquifers 

C2 Coalbed aquifers 

C4 Coalbed aquifers 

C5 Coalbed aquifers 

C6 Coalbed aquifers 

C8 Coalbed aquifers 

C10 Coalbed aquifers 

C11 Coalbed aquifers 

C12 Coalbed aquifers 

C13 Coalbed aquifers 

C151 Coalbed aquifers 

C16 Coalbed aquifers 

C17 Coalbed aquifers 

-139.0 -18.03 

-124.0 -15.57 

-140.7 -18.40 

-148.6 -19.02 

-147.8 -19.03 

-131.1 -18.14 

-146.5 -18.95 

-138.9 -18.46 

-137.9 -17.84 

-141.9 -18.31 

-144.2 -18.54 

-138.1 -17.92 

-140.9 -18.25 

-143.5 -18.79 

-143.4 -18.51 

-132.9 -17.80 

-136.6 -18.56 

-141.3 -18.40 

-151.0 -19.99 

2-150.3 2

-19.94 

-134.9 -18.66 

-143.7 -18.78 

1A petroleum product is present in this well but it is unlikely to affect δ2H and δ18O values (Carol Kendall, oral comm., 2000). 2

Replicate sample. 

GROUND-WATER QUALITY 57 
exchanges with the rock matrix in geothermal systems 

are commonly observed causes of shifts in the δ18O 

values. In general, there are fewer shifts in the δ2

H in 

aquifer systems because fifty percent or more of the 

total oxygen in the system is usually resident in the 

rocks, but almost all of the hydrogen in the system is 

in the water (Drever, 1997). Coalbed aquifers are 

exceptions to that general rule, because significant 

amounts of hydrogen are tied up in the coal. In addition, interaction between gases and water may introduce a more complex model, particularly when 

methanogenesis is present. As described earlier, the 

biogenic production of methane is a complex process 

of a mixed bacterial population and results in the production of carbon dioxide and hydrogen gases as well 

as methane. δ2H values have been observed to shift in 

water samples collected from landfills because of 

active methane production (Siegel and others, 1990; 

Hackley and others, 1996; Clark and Fritz, 1997). 

Collection of δ2H and δ18O in precipitation was 

not possible as part of this study and no local values 

were available; therefore, the δ2H and δ18O values in 

ground-water samples (table 11) were plotted and compared to published values for precipitation (fig. 21). 

The values plotted close to the GMWL, the meteoric 

water line for North American continental precipitation, and a local meteoric water line estimated by Gorody (1999) for the Powder River Basin. The close 

proximity to the meteoric water lines, including samples from two springs with modern water, suggests that 

the water was of meteoric origin, in contrast to water 

present in the aquifer materials during deposition (connate). The samples also plotted in the more negative 

area (i.e., left side) of the graph, which is appropriate 

for waters recharged at colder temperatures or in a 

colder climate, mid-latitudes, and mid-continent. 

Many of the values plotted slightly below all three 

meteoric water lines, despite different aquifers of origin, suggesting some isotopic fractionation, possibly 

by evaporation. The water samples were intermixed on 

the plot and did not group into separate areas of the 

graph based on the aquifer of origin. This could indicate that there was intermixing of the waters in the 

aquifers or it could indicate that the different aquifers 

were subject to similar recharge and or evolutional 

paths for the water, so that the net difference in the δ2H 

and δ18O values was minimal. Similar δ2

H and δ18O 

values were observed in four ground-water samples 

collected from the Wyodak-Anderson coalbed aquifer 

at two coal mines in the southern part of the Powder 

River Basin (Martin and others, 1988). 

δ2H and δ18O values were examined in the vertical 

dimension at the three monitoring-well clusters where 

major-ion chemistry was previously discussed. In the 

monitoring-well cluster composed of wells W5, W4, 

and W3, and nearby coalbed methane production well 

C12, depth to water increased with the depth of the well 

(fig. 22). The water level in the coalbed well was measured before the well was pumped to remove water for 

gas production, while the water levels in the wells completed in the Wasatch aquifer were collected after 

pumping of the coalbed methane production field 

began. The water level in the coalbed well (C12) probably had drawn down farther by 1999 than is presented 

in figure 22 because this well was a production well 

that produced water and gas in 1999. Hydraulic head 

(fig. 22) decreased with depth and indicated a hydraulic 

potential for downward ground-water flow if geologic 

conditions allow. 

δ2Η values generally increased (became less negative) with depth (fig. 22). δ2Η values in the two shallow 

Wasatch wells (W5and W4) are about the same, 

although the middle Wasatch well (W4) was slightly 

more negative than the uppermost well (W5); the difference between the two δ2Η values is probably within 

the precision of the δ2

Η analysis. δ2Η increased dramatically between the two shallow Wasatch wells (W5 

and W4) and the deepest Wasatch well (W3). Similarly, δ18Ο values were about the same between the two 

shallowest Wasatch wells but increased with depth 

between the middle Wasatch well (W4) and the deepest 

Wasatch well (W3). Physical and chemical processes 

such as methanogenesis and hydrogen-sulfide exchange 

reactions can shift δ2

Η values towards less negative values (deuterium enrichment) without corresponding 

shifts in δ18Ο values (Hackley and others, 1996, and references therein). If the sandstone lenses screened by the 

wells were assumed to be hydraulically connected, one 

of these processes could increase δ2

Η values as ground 

water moves downward. However, a shift in δ18Ο is 

observed between the two shallow Wasatch wells (W5 

and W4) and the deep Wasatch well (W3) and suggests 

that these processes have probably not affected the δ2Η

values between these wells. The shifts observed in δ2

Η

and δ18Ο between the two shallow Wasatch wells and 

the deepest Wasatch well almost parallels the meteoric 

water lines (fig. 21) and this type of deviation from the 

meteoric water lines may be more 

58 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
GROUND-WATER QUALITY 59 
60 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
GROUND-WATER QUALITY 61 
indicative of differences in recharge temperatures 

rather than processes such as methanogenesis or 

hydrogen-sulfide exchange reactions. In addition, it is 

interesting to note that the shift in δ2Η between the two 

shallowest Wasatch wells and the deepest Wasatch well 

corresponds to a change in water type (fig. 22) and a 

decrease in sulfate concentrations (figs. 17 and 22). 

δ2Η continued to increase with depth between the 

deepest Wasatch well (W3) and the underlying 

Wyodak-Anderson coalbed well (C12) (figs. 17 and 

22). Waters in both wells were sodium-bicarbonate 

type. Sulfate concentrations continued to decrease with 

depth, with a concentration of about 2,400 mg/L in the 

deep Wasatch well (W3) and less than the MRL of 

0.1 mg/L in the coalbed well (C12). In this case, δ2

Η

shifted to a less negative value without a corresponding 

shift in the δ18Ο values (fig. 22). As mentioned previously, this pattern is sometimes observed where waters 

have undergone or were undergoing methanogenesis or 

hydrogen-sulfide exchange reactions and might be 

observed if the waters in both wells were assumed to be 

originally isotopically the same, in hydraulic connection, and moving downward. Future work will hopefully determine if either geochemical process affected 

or currently affects the water chemistry in the deep 

wells. 

In the monitoring-well cluster composed of 

wells W7 (completed in Wasatch aquifer) and C16 

(completed in Wyodak-Anderson coalbed aquifer) 

(fig. 23), δ2Η increased (became less negative) with 

depth; δ18Ο also slightly increased (became less negative) with depth, but the difference in the δ18Ο values is 

probably within the precision of the analysis. Hydraulic 

head decreased with depth, indicating the potential for 

downward ground-water flow. Waters in both wells 

were sodium-bicarbonate type. Sulfate concentrations 

decreased with depth (fig. 19), with a concentration of 

about 160 mg/L in the Wasatch well (W7) and about 

1 mg/L in the coalbed well (C16). This pattern in stable 

isotope values, hydraulic gradient, and major-ion chemistry is very similar to the pattern just described for 

wells W3 and C12 in the monitoring-well cluster. 

In contrast, a very different pattern was observed in 

the monitoring-well cluster composed of wells W8 and 

C17 (fig. 23). Both δ2

Η and δ18Ο decreased (became 

more negative) with depth. Unlike the other two 

monitoring-well clusters, hydraulic head increased with 

depth, indicating the potential for upward ground-water 

flow. Major-ion chemistry also was different, as the 

water from the Wasatch well (W8) immediately overlying the coalbed aquifer was a mixed type, and the water 

in the coalbed well (C17) was a sodium-bicarbonate 

type. Sulfate concentrations decreased with depth 

(fig. 18), with a concentration of about 740 mg/L in the 

Wasatch well (W8) and less than 1 mg/L in the coalbed 

well (C17). In the other two monitoring-well clusters, 

the waters in samples collected from the Wasatch wells 

immediately overlying the coalbed aquifer were 

sodium-bicarbonate-type waters, the same as the waters 

in the underlying coalbed aquifers. 

Two explanations or hypotheses are proposed to 

explain the observed changes in stable isotope values 

and major-ion chemistry with increasing well depth at 

the three monitoring-well cluster locations. The first 

hypothesis requires an assumption of hydraulic connection between the intervals screened by successively 

deeper wells. In this case, the observed changes with 

depth are the result of geochemical processes (described 

earlier for the Powder River Basin) as ground water 

moves downward through the Wasatch Formation into 

the underlying coalbed aquifer. The second explanation 

is that there are two different aquifers or aquifer systems 

with minimal hydraulic connection – (1) the upper 

Wasatch Formation, represented by the two sandstone 

lenses screened by the two shallow Wasatch wells 

(wells W5 and W4) in the first monitoring-well cluster 

discussed, and (2) the coalbed aquifers and overlying 

deep Wasatch sandstone lenses, represented by the deep 

Wasatch well (W3) and coalbed well (C12) in the first 

monitoring-well cluster discussed, and Wasatch well 

W7 and coalbed well C16 in the second monitoringwell cluster discussed. At the third monitoring-well 

cluster examined, the shallow Wasatch well (W8) and 

underlying coalbed well (C17) would represent the 

same two different aquifers or aquifer systems. In this 

second hypothesis, each of the two aquifers or aquifer 

systems have different major-ion chemistry and stable 

isotope values because the waters in the different aquifers or aquifer systems are subject to different recharge 

and/or evolutional paths. 

The shallow aquifer or aquifer system would be 

part of the “shallow geochemical zone” described earlier for the Powder River Basin (and associated 

geochemical processes, primarily in the context provided by Lee (1981)) while the deeper aquifer or aquifer 

system would be part of the “deep geochemical zone” 

described earlier (see fig. 20 and related discussion in 

earlier section, Major-Ion Chemistry). 

62 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
δ2H values were examined spatially (figs. 24 and 

25) for both wells completed in the Wasatch aquifer 

and the coalbed aquifers. Springs discharging from 

clinker were included with the wells completed in the 

Wasatch aquifer. A break in δ2

H values can be 

observed along a northwest to southeast trend with both 

groups. An inferred -140 ‰ δ2

H value line is shown in 

both figures as an arbitrary reference value to examine 

relative increases or decreases in δ2

H values. Samples 

collected from wells completed in the coalbed aquifers, 

except for the sample from the Big George coal bed, 

followed a pattern where the more negative δ2H values 

plotted towards the center of the basin, whereas the less 

negative values plotted near the outcrop area. The pattern for the samples collected from the Wasatch aquifer 

and the springs was opposite. The values more negative than -140 ‰ plotted near the outcrop area, while 

the values less negative than -140 ‰ plotted towards 

the basin center. The δ2H -140 ‰ line in the coalbed 

aquifer and the δ2

H -140 ‰ line in the overlying 

Wasatch aquifer spatially were relatively close to each 

other given the size of the dataset. Further investigation 

of this possible spatial relation is required. The pattern 

may be the result of sample size, different recharge 

mechanisms, or geochemical processes, or the processes producing these differences may be independent. 

HYDROGEOLOGIC IMPLICATIONS OF 

OBSERVED MAJOR-ION CHEMISTRY 

AND ENVIRONMENTAL ISOTOPES 

Examination of data from this study, in combination with work by previous investigators, suggests several possibilities to help explain the composition of 

samples collected from the Wasatch aquifer and the 

underlying coalbed aquifers in this study. All interpretations of data were provided in previous sections, 

along with references to previous conclusions reached 

by other investigators. It should be noted that interpretations presented herein are a combination of past 

investigations and data collected as part of this investigation. Sample size for this study was limited, and as 

more data are collected in other areas of the basin, these 

interpretations may be modified or refined. 

In the previous section, two possible concepts 

were hypothesized to explain the observed major-ion 

chemistry and stable isotope values at three locations 

with monitoring-well clusters in the study area. The 

first concept proposes that the changes observed with 

depth at the three monitoring-well cluster locations are 

the result of geochemical processes that occur as 

ground water moves vertically through successively 

deeper, hydraulically connected sandstone lenses in the 

Wasatch Formation and finally, into the underlying 

coalbed aquifer. This investigation, along with earlier 

investigations described herein, has noted a hydraulic 

potential for downward ground-water flow within the 

Wasatch Formation. If geologic conditions are favorable to vertical ground-water flow, geochemical processes such as dissolution, precipitation, ion exchange, 

sulfate reduction, and mixing of waters are the processes that may occur as ground water moves downward through the Wasatch Formation and evolves the 

water to the sodium-bicarbonate type observed in the 

deeper part of the Wasatch Formation and the coalbed 

aquifers. 

The second concept assumes the presence of two 

different aquifers or aquifer systems to explain the differences in major-ion chemistry and stable isotope values observed at the three monitoring-well clusters. 

Three ground-water samples were collected from shallow wells in this study and all three were collected from 

the shallow part of the Wasatch Formation (less than 

about 200 ft below land surface). The wells had mixed 

cation composition (but generally dominant in calcium 

and magnesium) with either sulfate or bicarbonate as 

the dominant anion; all three wells were located at the 

monitoring-well clusters previously discussed. These 

wells could be part of a shallow aquifer or aquifer system represented by the “shallow geochemical zone” 

discussed previously. All ground-water samples collected from wells completed deeper in the Wasatch Formation and the underlying coal beds were sodiumbicarbonate-type waters; these wells could be representative of the underlying, deeper, chemically stagnant 

geochemical system described by Lee (1981) (described 

herein as the “deep geochemical zone” composed of the 

deep sandstone lenses present in the Wasatch Formation 

and underlying coal beds). In this explanation, little vertical hydraulic connection is present between successively deeper sandstone lenses in the Wasatch Formation 

and between the shallow sandstone lenses and the 

underlying coalbed aquifers; very little vertical flow, 

and therefore, intermixing of waters between the shallow and deep geochemical zones would occur. Heterogeneity and anisotropy, related to discontinuous 

HYDROGEOLOGIC IMPLICATIONS OF OBSERVED MAJOR-ION CHEMISTRY AND ENVIRONMENTAL ISOTOPES 63 
64 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
HYDROGEOLOGIC IMPLICATIONS OF OBSERVED MAJOR-ION CHEMISTRY AND ENVIRONMENTAL ISOTOPES 65 
66 ER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS WAT
HYDROGEOLOGIC IMPLICATIONS OF OBSERVED MAJOR-ION CHEMISTRY AND ENVIRONMENTAL ISOTOPES 67
sandstone lenses surrounded by a predominantly finegrained lithology present in the Wasatch Formation, 

could have a large effect relative to the actual groundwater flow direction and result in ground-water flow 

that is primarily horizontal. Other investigators such 

as Feathers and others (1981) and Lowry and others 

(1993) have suggested that there is very little vertical 

ground-water flow in the Wasatch and Fort Union Formations because of the predominantly fine-grained 

lithology, and that ground-water flow in these formations is primarily horizontal through the discontinuous 

sandstone lenses present. Therefore, well depth (and 

consequently, differences in ionic composition and stable isotope values) may simply reflect the relative distance water has flowed through the aquifer and 

different hydrological and geochemical origins and 

evolutional paths. Waters from the “shallow geochemical zone” may represent waters in local ground-water 

flow systems with relatively short flowpaths, whereas 

waters from the “deep geochemical zone” may be representative of a deep, regional ground-water flow system. This explanation also is consistent with 

differences in water chemistry noted in this and earlier 

studies, assuming very little vertical ground-water 

flow through the Wasatch Formation and into underlying hydrogeologic units such as the coalbed aquifers. 

Both of the proposed concepts can explain the 

observed composition of waters in the Wasatch Formation and sodium-bicarbonate composition of waters in 

the coalbed aquifers at the locations examined. In addition, the concept proposed by Heffern and Coates 

(1999) discussed earlier also can evolve the water in the 

coalbed aquifers to a sodium-bicarbonate type. At the 

basin scale, it is possible, and perhaps most likely, that 

all three concepts of the ground-water system are correct – the predominant hydrogeologic and geochemical 

processes at any given location are probably dependant 

on site-specific geologic and hydrogeologic conditions. 

In areas where many sandstone lenses are “vertically 

stacked” above coal beds and the hydraulic gradient 

allows for downward vertical flow, ground water may 

move downward through the Wasatch Formation and 

into the underlying coal beds. In other areas where the 

sandstones are relatively isolated with limited hydraulic connection, vertical ground-water flow is restricted 

and flow is primarily horizontal. Despite the localized 

differences in processes, the overall net effect at the 

basin scale is the system currently observed. The number of locations where vertical changes in major-ion 

chemistry and stable isotope values were examined 

during this study was limited to three locations. Examination of both major-ion chemistry and stable isotope 

values at additional locations throughout the Powder 

River Basin may help to refine or alter these proposed 

concepts of the ground-water system. 

Six of eight wells completed in the Wasatch aquifer had no post-bomb water, and two of the eight wells 

(W2 and W5) had concentrations suggesting a mixture 

of pre- and post-bomb water, although the low concentrations are suggestive of very little modern water 

(table 10). One of these two wells (W5) is the shallowest well completed in the Wasatch aquifer (probably the 

only water-table well). This well had very low concentrations of tritium, indicating that some post-bomb 

water may be present near the water table. Additional 

samples at the water table in the Wasatch aquifer 

should be collected to determine if some modern water 

is present at or near the water table at more locations in 

the basin. However, if there was a significant amount 

of areal recharge, it would be expected that post-bomb 

water would be distributed throughout the shallow zone 

of the Wasatch aquifer. The absence of post-bomb 

water in the shallow zone would suggest that processes 

responsible for recharge to the Wasatch aquifer in the 

Powder River Basin are probably very slow. As discussed earlier, most recharge to the coalbed aquifers is 

suspected to occur in or near clinker. It is possible that 

the majority of recharge from precipitation to the 

Wasatch aquifer may occur in the highly permeable 

clinker scattered throughout the basin; additional 

recharge also probably occurs from surface-water 

drainages in the study area. More accurate age-dating 

techniques and measurement of recharge rates would 

be required to understand recharge processes to the 

Wasatch aquifer. 

Based on the absence of any post-bomb water in 

samples collected from the coalbed aquifers, it appears 

that ground water may be flowing very slowly away 

from the suspected source of recharge, the clinker 

(which has modern or post-bomb water). Since no tritium data has been collected from wells completed in 

the coalbed aquifer adjacent to and immediately downgradient of suspected recharge areas (i.e., clinker), the 

rate at which water enters the coalbed aquifers from its 

recharge areas is not known. 

68 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
COMPARISON OF WATER QUALITY TO 

REGULATORY STANDARDS 

An important purpose in studying and describing 

the general and physical characteristics and ionic composition of water is to determine its suitability for proposed uses. In the study area, water from wells 

completed in the lower Tertiary aquifers (Wasatch aquifer and coalbed aquifers) and not used in relation to 

resource extraction (petroleum production, coal mining, 

or coalbed methane production) is used primarily for 

public supply, domestic, or agricultural purposes 

(Feathers and others, 1981; Wyoming Water Development Commission, 1985; Martin and others, 1988). To 

aid the determination of suitability for the intended 

uses, results of water-quality sampling are compared to 

U.S. Environmental Protection Agency (USEPA) and 

State of Wyoming water-quality levels and standards 

and several additional commonly used guidelines for 

the proposed uses. 

Public Supply and Domestic Use 

The U.S. Environmental Protection Agency (1991, 

1996) has established drinking-water regulations for 

public supplies of drinking water. The regulations specify maximum contaminant levels (MCLs) and secondary maximum contaminant levels (SMCLs). The 

MCLS are health-based and are legally enforceable 

standards while the SMCLs are nonenforceable, recommended standards. Although MCLs and SMCLs apply 

. 

only to public supplies of drinking water, not individual 

well owners, the levels are useful to determine the suitability of water for drinking. Standards also have been 

developed by the State of Wyoming to evaluate groundwater quality for domestic use (Wyoming Department 

of Environmental Quality, 1993). 

Results of ground-water-quality sampling of wells 

conducted as part of this study are compared to selected 

applicable MCLs and SMCLs in table 12 and State of 

Wyoming domestic standards in table 13. Dissolved 

solids was the constituent or characteristic with the 

highest percentage of exceedances. More than 50 percent of concentrations from both the Wasatch aquifer 

and coalbed aquifers exceeded USEPA levels and State 

of Wyoming standards. The ground-water sample collected from spring S1 (appendix table 1) does not 

exceed any levels or standards. The sample collected 

from spring S2 (appendix table 1) exceeds the MCL, 

SMCL, and Wyoming standard for both sulfate and dissolved solids. 

Hardness is another water-quality characteristic 

commonly used to characterize the suitability of water 

for public-supply and domestic use. Hardness usually is 

characterized on the basis of four classes (Hem, 1985, 

p. 158-159). Hardness values calculated for groundwater samples collected from wells as part of this study 

are summarized by aquifer in relation to the four classes 

in table 14. Both ground-water samples collected from 

springs (appendix table 1) were very hard (130 mg/L for 

sample collected from spring S1 and 790 mg/L for sample collected from spring S2). 

Table 12. Comparison of ground-water-quality samples collected from wells with selected U.S. Environmental Protection 

Agency maximum and secondary maximum contaminant levels for public drinking-water supplies 

(Contaminant levels from U.S. Environmental Protection Agency, 1991, 1996. Results of sampling individual wells are listed in appendix table 1.) 

[All constituent or characteristic concentrations are in milligrams per liter except as indicated; --, no established level; N/A, not applicable] 

Percentage and Percentage and Percentage and Percentage and 

number of samples number of samples Secondary number of samples number of samples 

Maximum collected from collected from maximum collected from collected from 

Constituent or contaminant Wasatch aquifer coalbed aquifers contaminant Wasatch aquifer coalbed aquifers 

characteristic level (MCL) that exceed MCL that exceed MCL level (SMCL) that exceed SMCL that exceed SMCL 

Chloride -- N/A N/A 250 0 0 

Fluoride 4 0 0 2 0 0 

Sulfate 500 38 (3/8) 0 250 38 (3/8) 0 

Dissolved solids -- N/A N/A 500 86 (6/7) 77 (10/13) 

pH, standard units -- N/A N/A 6.5-8.5 25 (2/8) 0 

HYDROGEOLOGIC IMPLICATIONS OF OBSERVED MAJOR-ION CHEMISTRY AND ENVIRONMENTAL ISOTOPES 69 
Table 13. Comparison of ground-water-quality samples collected from wells with selected State of Wyoming standards for 

domestic use 

(Standards from Wyoming Department of Environmental Quality, 1993, p. 9. Results of sampling individual wells are listed in appendix table 1.) 

[All constituent or characteristic concentrations are in milligrams per liter except as indicated] 

Percentage and number of Percentage and number of 

samples collected from samples collected from 

Constituent or Wasatch aquifer that exceed coalbed aquifers that exceed 

characteristic Domestic-use standard standard standard 

Chloride 250 0 0 

Fluoride 11.4-2.4 212 (1/8) 2 31 (4/13) 

Sulfate 250 38 (3/8) 0 

Dissolved solids 500 86 (6/7) 77 (10/13) 

pH, standard units 6.5-9.0 0 0 

1

Dependent on the annual average of the maximum daily air temperature: 1.4 milligrams per liter corresponds with a temperature range of 26.3 to 

32.5 degrees Celsius, and 2.4 milligrams per liter corresponds with a temperature of 12 degrees Celsius and below. 2

Percentage and number of samples presented for concentrations equal to or exceeding lower standard (1.4 milligrams per liter). 

Table 14. Hardness classification of ground-water-quality samples collected from wells 

(Hardness classification from Hem, 1985. Results of sampling individual wells are listed in appendix table 1.) 

[CaCO3, calcium carbonate; mg/L, milligrams per liter] 

Percentage1 and number of 

Hardness as CaCO3 samples collected from 

(mg/L) Hardness classes Wasatch aquifer within class 

0-60 Soft 38 (3/8) 

61-120 Moderately hard 25 (2/8) 

121-180 Hard 0 

more than 180 Very hard 38 (3/8) 

Percentage1 and number of 

samples collected from coalbed 

aquifers within class 

0 

38 (5/13) 

23 (3/13) 

38 (5/13) 

1

Individual percentages for each hardness class, when summed, do not equal 100 percent due to rounding. 

Agricultural Use 

Agricultural use includes water used to irrigate 

crops and water given to livestock. The State of Wyoming has established standards for both uses (Wyoming Department of Environmental Quality, 1993). 

Ground-water samples collected from wells are compared to selected applicable State of Wyoming agricultural standards in table 15. 

The ground-water sample collected from spring S1 

(appendix table 1) does not exceed any Wyoming agricultural (irrigation) or livestock standards. In contrast, 

the ground-water sample collected from spring S2 

(appendix table 1) exceeds the agricultural-use 

standard (irrigation) for sulfate and dissolved solids. 

A classification system to evaluate the suitability 

of water for irrigation use was developed by the U.S. 

Salinity Laboratory Staff (1954). The classification 

system is based on two characteristics, the salinity hazard and sodium (alkali) hazard of the water. Salinity 

hazard is divided into four classes using the specific 

70 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
---------------------

------------------------

Table 15. Comparison of ground-water-quality samples collected from wells with selected State of Wyoming standards for 

agricultural (irrigation) and livestock use 

(Standards from Wyoming Department of Environmental Quality, 1993. Results of sampling individual wells are listed in appendix table 1.) 

[All constituent or characteristic concentrations are in milligrams per liter except as indicated] 

Agricultural-use 

Constituent or standard 

characteristic (irrigation) 

Chloride 100 

Sulfate 200 

Dissolved solids 2.000 

pH, standard units 4.5-9.0 

Percentage and Percentage and Percentage and Percentage and 

number of number of number of number of 

samples collected samples collected samples collected samples collected 

from Wasatch from coalbed from Wasatch from coalbed 

aquifer that aquifers that aquifer that aquifers that 

exceed standard exceed standard exceed standard exceed standard 

0 0 0 0 

75 (6/8) 8 (1/13) 0 0 

25 (2/8) 8 (1/13) 0 0 

0 0 25 (2/8) 0 

Livestock-use 

standard 

2,000 

3,000 

5,000 

6.5-8.5 

conductance of the water. The characteristics of the 

salinity-hazard classes and specific-conductance 

ranges are as follows: 

Specific 

Salinity- conductance 

hazard class (μS/cm)1 Characteristics 

Low 0-250	 Low-salinity water can be used 

for irrigation on most soil with 

minimal likelihood that soil salinity will develop. 

Medium 251-750	 Medium-salinity water can be 

used for irrigation if a moderate 

amount of drainage occurs. 

High 751-2,250 High-salinity water is not suitable 

for use on soil with restricted 

drainage. Even with adequate 

drainage, special management for 

salinity control may be required. 

Very high More than Very high-salinity water is not 

2,250	 suitable for irrigation under normal conditions. 

1μS/cm, microsiemens per centimeter at 25 degrees Celsius. 

tion with information about the soil characteristics and 

irrigation practices in the area being examined. The 

SAR is calculated by converting ion concentrations to 

meq/L and substituting into the SAR equation as follows: 

SAR = -

Ca + Mg 

2 

Na - (9) 

SAR values can then be compared to characteristics of 

the four sodium-hazard classes as follows: 

Sodium- Sodiumhazard adsorption 

class ratio (SAR) Characteristics 

Low 0-10	 Low-sodium water can be used for irrigation on most soil with minimal danger of 

harmful levels of exchangeable sodium. 

Medium >10-18	 Medium-sodium water will present an 

appreciable sodium hazard in finetextured soil having high cationexchange capacity. 

High >18-26	 High-sodium water may produce harmful 

levels of exchangeable sodium in most 

The sodium hazard also is divided into four classes soil. 

using the sodium-adsorption ratio (SAR). The SAR is Very high >26 Very high-sodium water is generally 

a dimensionless ratio that is calculated to indicate the unsatisfactory for irrigation purposes. 

tendency of sodium to replace calcium and magnesium 

in soils. The replacement of calcium and magnesium 

with sodium can damage the soil structure and reduce Typically, both salinity hazard and sodium hazard 

the permeability of the soil to water infiltration (Hem, are combined into a single plot to evaluate the suitabil1985). However, the SAR should be used in conjunc- ity of water for irrigation (U.S. Salinity Laboratory 

HYDROGEOLOGIC IMPLICATIONS OF OBSERVED MAJOR-ION CHEMISTRY AND ENVIRONMENTAL ISOTOPES 71 
Staff, 1954). Results from analyses of ground-water 

samples collected from springs and wells (appendix 

table 1) are shown in figure 26. Samples collected from 

both springs plot in the low sodium-hazard (S1) class 

but plot in the medium (C2) and high (C3) classes for 

salinity hazard. Ground-water samples collected from 

the Wasatch aquifer plot in the range from low (S1) to 

high (S3) sodium-hazard classes and medium (C2) to 

very high (C4) salinity-hazard classes. Ground-water 

samples collected from the coalbed aquifers also plot in 

the range from medium (C2) to very high (C3) salinityhazard classes, but plot in range from low (S1) to very 

high (S4) sodium-hazard classes. The sample that plots 

the highest in both the salinity- and sodium-hazard 

classes from all aquifers was collected from the Big 

George coal bed. Although this suggests that samples 

collected from wells completed in all aquifers plot in a 

wide range of both sodium and salinity-hazard classes, 

most samples cluster in or near the combined mediumsodium-hazard—high-salinity-hazard (S2-C3) class. 

72 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
HYDROGEOLOGIC IMPLICATIONS OF OBSERVED MAJOR-ION CHEMISTRY AND ENVIRONMENTAL ISOTOPES 73 
SUMMARY 

Ground-water samples collected from wells in 

coalbed aquifers of the Fort Union Formation and from 

springs and wells in overlying aquifers displayed distinct differences and trends between and within the 

aquifers and areally within the Powder River Basin. 

Data on water levels, major-ion composition, and isotopic composition of the water support the conclusion 

that the changes in water composition are likely the 

result of geochemical processes that occur as groundwater moves vertically and horizontally, as well as possible hydraulic connection between the aquifers. 

Major-ion composition of samples in this study 

varied, primarily in relation to depth or proximity to 

suspected recharge areas. Ground-water samples collected from springs discharging from clinker were classified as calcium-sulfate-type and calciumbicarbonate-type waters. Two ground-water samples 

collected from wells completed in the Wasatch aquifer 

had mixed cation composition with sulfate as the dominant anion, and one sample was a sodium-magnesiumsulfate-bicarbonate-type water; all three samples were 

collected from wells completed to depths less than 

200 feet. Ground-water samples collected from wells 

completed in the coalbed aquifers and 5 of 8 wells completed in the Wasatch aquifer were all sodiumbicarbonate-type waters. All five Wasatch wells with 

sodium-bicarbonate-type waters were collected from 

wells completed at depths greater than 200 feet. Statistically and qualitatively, major-ion composition of 

samples from the coalbed aquifers and the overlying 

Wasatch aquifer was not significantly different except 

for two constituents, sulfate and fluoride, with sulfate 

showing the most striking difference. The median sulfate concentration in the Wasatch aquifer was low 

(130 mg/L), but concentrations were much lower in the 

coalbed aquifers, with more than 50 percent of the values below detection limits. 

Major-ion data and water levels in monitoringwell clusters composed of wells completed in coalbed 

aquifers and the overlying Wasatch aquifer indicated 

changes in water composition that may be related to 

depth and possible hydraulic connection between aquifers. Water-level measurements at most of the 

monitoring-well clusters (4 out of 5) indicated a potential for downward ground-water flow from the Wasatch 

aquifer to the coalbed aquifers, with the one remaining 

well cluster indicating a potential for upward movement. Both increases and decreases in major ions (Ca, 

Mg, SO4, Na, HCO3) with depth and dissolved-solids 

concentrations were noted in the well clusters, with no 

consistent trend for any of the constituents except sulfate, which dramatically decreased with depth. In the 

Wasatch aquifer, statistically significant correlations 

were noted between calcium and sulfate concentrations 

and well depth, and correlations close to statistical significance were noted for dissolved solids, magnesium, 

and fluoride with increasing well depth. In the coalbed 

aquifers, a statistically significant correlation was 

found between potassium concentrations and well 

depth, but qualitatively, again the most striking difference was the decrease in sulfate with depth. 

No clear areal pattern in water type was noted 

because samples collected from most wells, regardless 

of aquifer type, were sodium-bicarbonate-type waters. 

However, a pattern in dissolved-solids concentrations 

in waters from the coalbed aquifers was noted. Samples from this study and another recent investigation 

(Rice and others, 2000) suggests that dissolved-solids 

concentrations may be lower (less than 600 mg/L) 

south of the Belle Fourche River and that concentrations appear to increase northward in the Powder River 

Basin in Wyoming. 

Tritium was used to qualitatively estimate the time 

of ground-water recharge. Tritium concentrations in 

water samples collected from two springs suggest that 

both were recharged after 1952 and contain modern (or 

post-bomb) water. Tritium concentrations in six of 

eight ground-water samples collected from wells completed in Wasatch aquifer overlying the coalbed aquifers suggest the water is submodern (or pre-bomb). 

Tritium concentrations in the remaining two wells suggest a mixture between submodern and modern water, 

although the low concentrations suggest that ground 

water in these wells has very little modern water. Tritium concentrations above pre-bomb concentrations 

were not detected in any wells completed in the coalbed 

aquifers, suggesting that ground water in the coalbed 

aquifers in the shallow coal beds of the Tongue River 

Member of the Fort Union Formation probably is submodern, and that no recharge water is likely to have 

reached the portions of the aquifers sampled in this 

study since at least the early 1950’s. This suggests 

recharge to the Wasatch aquifer and the coalbed aquifers is probably very slow. 

74 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
The δ2H and δ18O values from waters in this study 

suggest that the waters are of meteoric origin. Paired 

δ2H and δ18O values were plotted in relation to the 

Global Meteoric Water Line, a meteoric water line for 

North American continental precipitation, and a local 

meteoric water line constructed for the Powder River 

Basin (Gorody, 1999). The values plot close to all 

meteoric water lines, indicating that the water in 

ground-water samples collected during this investigation is of meteoric origin. The isotopic values suggest 

that the waters were recharged in a colder climate or at 

a cold temperature, mid-latitudes, and mid-continent. 

Samples do not group together based on aquifer origin; 

this suggests either intermixing of the waters in the 

aquifers or it suggests that the different aquifers are 

subject to similar recharge and/or evolutional paths for 

the water so that the difference in the δ2

H and δ18O values is minimal. 

The areal distribution of δ2H was examined and an 

apparent break in δ2

H values along a northwest to 

southeast trend was observed. In the coalbed aquifers, 

all but one ground-water sample (collected from the 

Big George coal bed), show a pattern where the δ2

H 

values become more negative towards the center of the 

Powder River Basin, and values greater than an arbitrary reference value of –140 ‰ (parts per thousand or 

per mil) were observed near the outcrop area of the 

Wyodak-Anderson coal zone. The pattern in samples 

collected from the overlying Wasatch aquifer was 

reversed. The values more negative than –140 ‰ are 

near the outcrop area, and the values that are less negative than –140 ‰ are closer to the basin center. It is 

unclear if this pattern is a result of sample size, different recharge mechanisms, methanogenesis, or if the 

processes producing these differences are independent. 

Results of ground-water-quality sampling were 

compared to selected USEPA and State of Wyoming 

regulatory and nonregulatory standards and several 

additional commonly used guidelines to determine the 

suitability of water for possible uses. The public 

drinking-water supply and domestic-use standards for 

dissolved solids were the standards most frequently 

exceeded in samples collected from both the Wasatch 

aquifer and the coalbed aquifers. The State of Wyoming agricultural-use (irrigation) standard for sulfate 

was exceeded in 75 percent of samples collected from 

the Wasatch aquifer and 8 percent of samples collected 

from the coalbed aquifers. The State of Wyoming 

agricultural-use (irrigation) standard for dissolved solids was exceeded in 25 percent of samples collected 

from the Wasatch aquifer and 8 percent of samples collected from the coalbed aquifers. The only State of 

Wyoming standard for livestock use exceeded was pH; 

25 percent of samples collected from the Wasatch aquifer exceeded the standard. Water from the Wasatch 

aquifer ranged from soft to very hard and water from 

the coalbed aquifers ranged from moderately hard to 

very hard. Samples collected from wells completed in 

both the Wasatch aquifer and coalbed aquifers plotted 

in a wide range of both sodium- and salinity-hazard 

classes, but most samples clustered in or near the combined medium-sodium-hazard—high-salinity-hazard 

classes. 

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84 WATER QUALITY AND ISOTOPIC CHARACTERISTICS OF WASATCH AND FORT UNION FORMATIONS 
APPENDIX


WATER-QUALITY ANALYSES


APPENDIX 85

Table 1. Physical properties, general mineral characteristics, and major ions in ground-water samples collected 

[μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; ANC, acid-neutralizing 

Specific 

U.S. Geological conducSurvey tance, pH, 

site- field mea- field 

Site identification Local well Hydrogeologic Date of Sample surement measurenumber number number Geologic unit unit collection time (μS/cm) ment 

S1 433652105075501 42-069-15dbb01 Clinker in Tongue River Member of the Fort Union Formation 

Springs discharging 

from clinker 

06-14-99 1545 303 7.8 

S2 442232105264101 51-071-30bac01	 Clinker in Tongue River Member of the Fort Union Formation 

Springs discharging 

from clinker 

06-21-99 1530 1,610 7.5 

W1 440253105385702 47-073-16cbc02 Wasatch Formation Wasatch aquifer 06-16-99 1000 438 8.6 

W2 440542105351802 48-073-36bcc02 Wasatch Formation Wasatch aquifer 06-17-99 1121 914 8.2 

W3 440724105291301 48-072-22adc01 Wasatch Formation Wasatch aquifer 06-24-99 1720 870 8.1 

W4 440724105291302 48-072-22adc02 Wasatch Formation Wasatch aquifer 06-25-99 0935 4,110 7.6 

W5 440724105291303 48-072-22adc03 Wasatch Formation Wasatch aquifer 06-25-99 1035 4,330 7.2 

W6 441019105414502 49-074-36ddc02 Wasatch Formation Wasatch aquifer 06-23-99 1024 1,850 7.6 

W7 441451105375502 2,349-073-03cc 02 Wasatch Formation Wasatch aquifer 06-17-99 1640 770 8.9 

W8 443241105360801 53-073-26dba01 Tongue River Member of the Wasatch aquifer 06-29-99 1535 2,490 7.3 

Fort Union Formation above 

Wyodak-Anderson coal zone 

C1 433408105270101 342-072-36dbc01 Wyodak-Anderson coal zone Coalbed aquifers 06-29-99 1150 688 7.2 

C2 434252105264301 343-072-12dac01 Wyodak coal bed of the Wyo- Coalbed aquifers 06-30-99 1420 684 7.1 

dak-Anderson coal zone 

C4 435411105294001 45-072-03dbd01	 Upper Wyodak coal bed of the 

Wyodak-Anderson coal zone 

Coalbed aquifers 06-30-99 1320 990 7.2 

C5 435915105242201 46-071-05dda01 Wyodak-Anderson coal zone Coalbed aquifers 06-30-99 1230 1,070 7.2 

C6 440139105253301 47-071-30aaa01 Wyodak-Anderson coal zone Coalbed aquifers 06-30-99 1135 1,060 7.1 

C8 440358105331401 347-072-07ac 01 Wyodak coal bed of the Wyo- Coalbed aquifers 06-30-99 1040 1,150 7.2 

dak-Anderson coal zone 

C10 440623105364101 48-073-27dac01 Wyodak-Anderson coal zone Coalbed aquifers 06-30-99 0955 1,210 7.4 

C11 440808106070601 3

48-077-16ac 01 Big George coal bed3 Coalbed aquifers 06-24-99 1330 4,180 6.9 

C12 440756105293301 348-072-15dcc01 Wyodak-Anderson coal zone Coalbed aquifers 06-25-99 0905 665 7.4 

C13 441017105352201 348-073-02aa 01 Wyodak coal bed of the Wyo- Coalbed aquifers 06-30-99 0915 1,520 7.0 

dak-Anderson coal zone 

C15 441047105535401 49-075-32adb01	 Coal bed or zone below the 

Wyodak-Anderson coal zone 

Coalbed aquifers 06-24-99 1110 1,060 7.3 

C16 441451105375501 349-073-03cc 01	 Upper Wyodak coal bed of the 

Wyodak-Anderson coal zone 

Coalbed aquifers 06-18-99 1005 1,700 7.3 

C17 443241105360802 53-073-26dba02	 Canyon coal bed of the Wyodak-Anderson coal zone Coalbed aquifers 06-29-99 1600 1,790 7.8 

1

Replicate sample.


2

Plots in section 4 when using Global Positioning System (GPS) location; well is assigned to section 3 on well permit, so this location was used.


3Well located in center of quarter-quarter section.


4

Big George coal bed considered equivalent to the Wyodak-Anderson coal zone (Flores and Bader, 1999)


86 WATER QUALITY AND ENVIRONMENTAL ISOTOPIC ANALYSES OF GROUND-WATER SAMPLES 
from springs discharging from clinker and wells completed in the Wasatch aquifer and coalbed aquifers 

capacity; CaCO3, calcium carbonate; NR, not reported because of problem with ANC analysis] 

Cal- Magne- Potas- Chlo- Bicar- Fluo- Dis- HardTemper- cium, sium, Sodium, Sodium- sium, ride, bonate, Sulfate, ride, Silica, solved ness, 

ature dis- dis- dis- adsorp- dis- dis- ANC, dis- dis- dis- dis- solids, (mg/L 

(degrees solved solved solved tion solved solved (mg/L as solved solved solved solved calculated as 

Celsius) (mg/L) (mg/L) (mg/L) ratio (mg/L) (mg/L) CaCO3) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) CaCO3) 

12 35 9.9 8.1 0.3 6.9 2.5 116 141 28 0.5 26 187 130 

10.5 210 65 46 0.7 25 12 160 195 720 0.8 30 1,200 790 

20 6.2 1.5 97 9 2.2 8.1 206 251 13 0.8 11 263 22 

20.5 12 6.7 180 10 6.9 17 378 461 100 0.9 10 562 57 

17.5 19 6.4 180 9 4.6 6.4 455 555 23 1 9.9 527 75 

14.5 450 93 480 5 12 5 129 157 2,400 < 0.1 9.2 3,490 1,500 

14.5 550 280 270 2 14 18 267 326 2,700 0.3 17 4,020 2,500 1550 1270 1260 12 114 117 1

267 1326 12,700 10.3 116 14,020 12,500 

18.5 7.7 24 330 13 13 0.3 1,020 1,244 10 1.5 10 1,010 120 

19 11 2.7 160 11 3.7 11 NR NR 160 0.9 8 NR 38 

16 6.5 94 280 5 18 14 723 882 740 0.5 15 1,660 550 

19.5 17 8.7 130 6 5.7 6.9 360 439 < 0.1 1.9 11 395 80 

19 17 8.4 130 6 4.9 9.3 355 433 1.8 1.2 10 396 77 

17 23 15 190 7 8.7 24 504 615 < 0.1 1.5 10 571 120 

17 32 14 210 8 8.3 15 584 712 < 0.1 1.4 9.1 644 140 

17 37 15 200 7 8.9 12 573 699 < 0.1 1.1 9.7 625 150 

20 40 22 210 6 13 8.5 632 771 < 0.1 1.2 11 680 190 

20 36 23 210 7 15 9.1 647 789 0.9 1.3 11 697 180 

26 51 39 1,000 26 48 21 2,570 3,134 < 0.3 1.2 7.4 2,720 290 

20 15 8.7 120 6 6.3 8.9 346 422 < 0.1 1.6 9.9 382 74 

18.5 49 27 270 8 13 7.7 853 1,040 < 0.1 0.8 10 888 230 

27 25 14 190 8 13 7 580 707 1.2 1.1 20 624 120 126 114 1

190 18 112 17 1580 1707 11.2 11.1 120 1622 1120 

17 54 29 340 9 13 8.3 956 1,166 1 0.9 10 1,030 260 

14 62 33 330 8 12 11 962 1,173 0.7 0.6 11 1,040 290 

APPENDIX 87 
88 

Table 2. Radioactive and stable-isotope values for ground-water samples collected from sprin

gs discharging fro

m clin

ker and wells co

and coalbed aquifers 

[pCi/L, picocuries per liter; δ2H, deuterium/hydrogen isotopic ratio; δ18O, oxygen-18/oxygen-16 isotopic ratio; ‰, parts per thousand or per mil; <MRL, value less tha

per liter] 

WATER QUALITY AND ENVIRONMENTAL ISOTOPIC ANALYSES OF GROUND-WATER SAMPLES 

U.S. Geological 

Survey 

site- 

Site identification Local well Hydrogeologic Date of Sample 

Tr

nu

mber nu

mber number Geologic unit unit collection time (p

S1 433652105075501 42-069-15dbb01 S2 442232105264101 51-071-30bac01 W1 440253105385702 47-073-16cbc02 W2 440542105351802 48-073-36bcc02 W3 440724105291301 48-072-22adc01 W4 440724105291302 48-072-22adc02 W5 440724105291303 48-072-22adc03 

W6 441019105414502 49-074-36ddc02 W7 441451105375502 3,449-073-03cc 02 

W8 443241105360801 C1 433408105270101 

C2 434252105264301 

C4 435411105294001 C5 435915105242201 C6 440139105253301 C8 440358105331401 

C10 440623105364101 C11 440808106070601 

C12 440756105293301 

C13 441017105352201 

C15 441047105535401 

C16 441451105375501 

C17 443241105360802 

53-073-26dba01 

442-072-36db 01 443-072-12da 01 

45-072-03dbd01 46-071-05dda01 

47-071-30aaa01 

447-072-07ac 01 

48-073-27dac01 

448-077-16ac 01 448-072-15dc 01 448-073-02aa 01 

49-075-32adb01 

449-073-03cc 01 

53-073-26dba02 

Clinker in Tongue River Member of the Fort Union Formation 

Clinker in Tongue River Member of the Fort Union Formation 

Wasatch Formation


Wasatch Formation


Wasatch Formation


Wasatch Formation


Wasatch Formation


Wasatch Formation


Wasatch Formation


Tongue River Member of the Fort Union Formation above 


Wyodak-Anderson coal zone


Wyodak-Anderson coal zone


Wyodak coal bed of the Wyodak-Anderson coal zone


Upper Wyodak coal bed of the 

Wyodak-Anderson coal zone


Wyodak-Anderson coal zone


Wyodak-Anderson coal zone


Wyodak coal bed of the WyodakAnderson coal zone


Wyodak-Anderson coal zone


Big George coal bed5


Wyodak-Anderson coal zone


Wyodak coal bed of the Wyodak-Anderson coal zone


Coal bed or zone below the 

Wyodak-Anderson coal zone


Upper Wyodak coal bed of the 

Wyodak-Anderson coal zone


Canyon coal bed of the WyodakAnderson coal zone


Springs discharging 06-14-99 1545 

fro

m clinker 

Springs discharging 06-21-99 1530 

fro

m clinker 

Wasatch aquifer 06-16-99 1000 <

Wasatch aquifer 06-17-99 1121 

Wasatch aquifer 06-24-99 1720 

Wasatch aquifer 06-25-99 0935 <

Wasatch aquifer 06-25-99 1035 

Wasatch aquifer 06-23-99 1024 

Wasatch aquifer 06-17-99 1640 

Wasatch aquifer 06-29-99 1535 <

Coalbed aquifers 06-29-99 1150 <

Coalbed aquifers 06-30-99 1420 <

Coalbed aquifers 06-30-99 1320 

Coalbed aquifers 06-30-99 1230 <

Coalbed aquifers 06-30-99 1135 <

Coalbed aquifers 06-30-99 1040 

Coalbed aquifers 06-30-99 0955 

Coalbed aquifers 06-24-99 1330 

Coalbed aquifers 06-25-99 0905 <

Coalbed aquifers 06-30-99 0915 <

Coalbed aquifers 06-24-99 1110 <

2<

Coalbed aquifers 06-18-99 1005 <

Coalbed aquifers 06-29-99 1600 

1Also known as two-sigma precision estimate.
 2Replicate sample.
 3Plots in section 4 when using Global Positioning System (GPS) location; well is assigned to section 3 on well per

mit, so this location was used.
 4Well located in center of quarter-quarter section.
 5Big George coal bed considered equivalent to the WyodakAnderson coal zone (Flores and Bader, 1999).