Prepared in cooperation with the

Bureau of Land Management

Hydrogeologic Framework and Estimates of Ground-Water 

Volumes in Tertiary and Upper Cretaceous Hydrogeologic 

Units in the Powder River Basin, Wyoming

Scientific Investigations Report 2005-5008

U.S. Department of the Interior

U.S. Geological Survey
Hydrogeologic Framework and Estimates 

of Ground-Water Volumes in Tertiary and 

Upper Cretaceous Hydrogeologic Units in 

the Powder River Basin, Wyoming

By Kurt Hinaman

Prepared in cooperation with the

Bureau of Land Management

Scientific Investigations Report 2005-5008

U.S. Department of the Interior

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

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

reproduce any copyrighted materials contained within this report.
iii

Contents

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

Introduction ................................................................................................................................................... 1

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

Previous Investigations ...................................................................................................................... 2

Hydrogeologic Framework ......................................................................................................................... 2

Methods Used to Estimate Ground-Water Volumes ............................................................................... 6

Development of a Geographic Information System ....................................................................... 6

Computation of Ground-Water Volumes .......................................................................................... 8

Sensitivity Analysis Methods ........................................................................................................... 10

Estimates of Ground-Water Volumes ...................................................................................................... 11

Hydraulic Properties Used in Estimating Ground-Water Volumes ........................................... 11

Ground-Water Volumes Derived from Porosity ............................................................................ 12

Ground-Water Volumes Derived from Specific Yield and Specific Storage ............................ 12

Sensitivity Analysis ............................................................................................................................ 12

Comparison with a Previous Study ................................................................................................. 13

Discussion of Ground-Water Volumes ........................................................................................... 15

Approaches to Increase Accuracy of Ground-Water Volume Estimates ......................................... 16

Summary ...................................................................................................................................................... 16

References ................................................................................................................................................... 17

Figures

1. Map showing hydrogeologic units of the Powder River structural basin in Wyoming 

and Montana ............................................................................................................................... 3

2. Generalized stratigraphic column of Tertiary and upper Cretaceous formations in 

the southern Powder River structural basin, Wyoming (modified from Flores and 

Bader, 1999) .................................................................................................................................. 4

3. West to east hydrogeologic section of the Tertiary and upper Cretaceous hydrogeologic units showing the asymmetric nature of the Powder River structural 

basin, Wyoming ............................................................................................................................ 5

4. Sketch showing example contours, outcrop, and triangulated irregular networks 

(TINS). A, original contours from Lewis and Hotchkiss (1981), B, TINS based on 

original contours, C, original contours and contours extended and added by 

manual methods, and D, revised TINS based on extended and added contours ............. 7

5. Schematic showing relations among land surface, water table, hydrogeologic units, 

and saturated section of the hydrogeologic units in the study area, Powder River 

structural basin, Wyoming ......................................................................................................... 9

6. Sketch showing how the original values of contours (A) were increased by 10 percent to form another set of values (B) that were used to create TINS to study the 

effect of a systematic error in contour values ...................................................................... 10

7. Sketch showing how the values of inflection lines were changed in the center of 

the closed contour, with the value used in the base analysis (A), and showing the 

changed value of the inflection line (B), which was used in the sensitivity analysis .... 11
iv

Figures—Continued

8. Sensitivity of calculated ground-water volumes to selected factors used to 

calculate ground-water volumes ............................................................................................ 15

Tables

1. Hydraulic properties of hydrogeologic units in the Powder River Basin used to 

estimate ground-water volumes ............................................................................................. 12

2. Estimated ground-water volumes in Tertiary and upper Cretaceous hydrogeologic 

units in the Powder River Basin based on porosity ............................................................. 13

3. Estimated ground-water volumes in Tertiary and upper Cretaceous hydrogeologic 

units in the Powder River Basin based on specific yield and specific storage .............. 14

4. Sensitivity of calculated ground-water volumes to selected factors used to 

calculate the ground-water volumes ..................................................................................... 14

5. Comparison of estimates of available volumes of ground water from Applied 

Hydrology Associates, Inc. and Greystone Environmental Consultants, Inc. (2002) 

to the estimates of ground-water volumes determined in this study from storativity 

factors ......................................................................................................................................... 15
Hydrogeologic Framework and Estimates of Ground-Water 

Volumes in Tertiary and Upper Cretaceous Hydrogeologic 

Units in the Powder River Basin, Wyoming

By Kurt Hinaman

A total ground-water volume of 2.0x1014 ft3 (cubic feet) 

was calculated using porosity values, and a total ground-water 

volume of 3.6x1013 ft3 was calculated using specific yield 

and specific storage values. These results are consistent with 

retention properties, which would have some of the total water 

being retained in the sediments.

Sensitivity analysis shows that the estimates of groundwater volume are most sensitive to porosity. The estimates also 

are sensitive to confined thickness and saturated thickness. 

Better spatial information for hydrogeologic units could help 

refine the ground-water volume estimates.

Introduction

The Powder River structural basin in Wyoming and Montana (fig. 1) is an important source of energy resources for the 

United States. Coalbed natural gas (Johnson and Flores, 1998; 

Miller, 1999; Montgomery, 1999) is contained in Tertiary and 

upper Cretaceous hydrogeologic units (fig. 2) in the Powder 

River structural basin. The Powder River structural basin is 

asymmetric, with the deepest part of the basin west of the 

basin center (fig. 3).

Coalbed natural gas, referred to as coalbed methane (CBM), is released when water pressure in coals is 

lowered, usually by pumping ground water (Rice and Nuccio, 

2000). After this water is pumped out of the ground-water 

system it must be disposed. Many issues related to disposal 

and use of by-product water from CBM production have 

developed, in part due to the uncertainty of basic hydrologic 

and geochemical properties of Tertiary and upper Cretaceous 

hydrogeologic units, such as the volume of ground water in 

storage and the chemical composition of that water. Some 

studies have started to define these properties, such as groundwater chemistry (Rice and others, 2000, 2002; Bartos and 

Ogle, 2002), whereas other properties, such as the volume 

of ground water in the basin, remain poorly defined. One 

hydrologic property of primary interest is the amount of water 

contained in Tertiary and upper Cretaceous hydrogeologic 

units in the Powder River structural basin. 

Abstract

The Powder River Basin in Wyoming and Montana is an 

important source of energy resources for the United States. 

Coalbed methane gas is contained in Tertiary and upper Cretaceous hydrogeologic units in the Powder River Basin. This 

gas is released when water pressure in coalbeds is lowered, 

usually by pumping ground water. Issues related to disposal 

and uses of by-product water from coalbed methane production have developed, in part, due to uncertainties in hydrologic 

properties. One hydrologic property of primary interest is the 

amount of water contained in Tertiary and upper Cretaceous 

hydrogeologic units in the Powder River Basin. The U.S. 

Geological Survey, in cooperation with the Bureau of Land 

Management, conducted a study to describe the hydrogeologic 

framework and to estimate ground-water volumes in different 

facies of Tertiary and upper Cretaceous hydrogeologic units in 

the Powder River Basin in Wyoming.

A geographic information system was used to compile 

and utilize hydrogeologic maps, to describe the hydrogeologic 

framework, and to estimate the volume of ground water in 

Tertiary and upper Cretaceous hydrogeologic units in the Powder River structural basin in Wyoming. Maps of the altitudes 

of potentiometric surfaces, altitudes of the tops and bottoms 

of hydrogeologic units, thicknesses of hydrogeologic units, 

percent sand of hydrogeologic units, and outcrop boundaries for the following hydrogeologic units were used: Tongue 

River-Wasatch aquifer, Lebo confining unit, Tullock aquifer, 

Upper Hell Creek confining unit, and the Fox Hills-Lower 

Hell Creek aquifer. Literature porosity values of 30 percent for 

sand and 35 percent for non-sand facies were used to calculate 

the volume of total ground water in each hydrogeologic unit. 

Literature specific yield values of 26 percent for sand and 

10 percent for non-sand facies, and literature specific storage 

values of 0.0001 ft-1 (1/foot) for sand facies and 0.00001 ft-1

for non-sand facies, were used to calculate a second volume of 

ground water for each hydrogeologic unit. Significant figure 

considerations limited estimates of ground-water volumes to 

two significant digits.
The U.S. Geological Survey (USGS), in cooperation with 

the Bureau of Land Management (BLM), conducted a study 

to describe the hydrogeologic framework and to estimate 

ground-water volumes in different facies of Tertiary and upper 

Cretaceous hydrogeologic units in the Powder River structural 

basin in Wyoming. These estimates provide the BLM with 

information about the maximum possible amount of water that 

could be produced by CBM production in the Powder River 

structural basin over the next 10 to 15 years (U.S. Department 

of the Interior, 2002).

Purpose and Scope

The purpose of this report is to describe the hydrogeologic framework and to estimate ground-water volumes in 

Tertiary and upper Cretaceous hydrogeologic units in the 

Powder River structural basin in Wyoming. The sensitivity of 

ground-water volume estimates to various hydrologic properties also is described.

Previous Investigations

Many previous investigations have been done in the 

Powder River Basin and an exhaustive discussion of previous 

work is beyond the scope of this report. For a general overview of work, readers are referred to Bartos and Ogle (2002) 

which includes a review of previous geologic investigations. 

Another compilation of references is the 1999 assessment of 

coal in this area (Fort Union Coal Assessment Team, 1999). 

Additional information can be found in numerous guidebooks 

about the Powder River Basin or coalbed methane (Cooper 

and others, 1963; Diedrich and others, 1988; Miller, 1999; 

Schwochow and Nuccio, 2002).

Hydrogeologic Framework 

In previous investigations, 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, where another structural feature, the Cedar 

Ridge anticline, separates it from the Williston Basin (not 

shown in fig. 1). The entire basin covers an area of more than 

22,000 square miles (Flores and others, 1999).

Tertiary and upper Cretaceous hydrogeologic units 

within the Powder River Basin in Wyoming are the focus of 

this study. The Tertiary and upper Cretaceous formations that 

contain the hydrogeologic units of interest are described first 

in this section of the report.

The Lewis Shale and its equivalents (fig. 2), which 

are upper Cretaceous-age marine units (Steidtmann, 1993; 

Merewether, 1996), form the basal confining layer. Overlying 

this shale is the upper Cretaceous-age Fox Hills Sandstone, 

which represents a sandy shoreline as the seaway retreated 

to the northeast (Steidtmann, 1993; Mereweather, 1996), and 

the Lance Formation of which about one-third is composed 

of channel sandstones and the rest of the formation is finergrained interfluvial sedimentary rock (Conner, 1992). Crossbedding, channel orientation, and orientation of well-cemented 

sandstones having log-like forms show deposition from eastward-flowing streams on both sides of the Bighorn Mountains 

and on the east side of the Powder River Basin, indicating that 

those mountains had yet to emerge, and that the Powder River 

Basin had yet to form by the end of the Cretaceous period 

(Conner, 1992).

The lowest Tertiary unit is the Paleocene-age Fort Union 

Formation, which is subdivided into three members—the 

Tullock, Lebo Shale, and Tongue River Members (fig. 2). 

The Tullock Member is composed of about one-third channel 

sands and two-thirds finer grained overbank deposits (Brown, 

1993). In the northwest part of the Powder River Basin, the 

Tullock Member contains carbonate clasts in its lowermost 

part, which are interpreted to mean that the Bighorn Mountains were starting to emerge during the time of its deposition 

(Brown, 1993). At the same time, coal-forming environments 

were forming and being deposited from the southeast to the 

northwest (Brown, 1993). The Lebo Shale Member is mainly 

a mudstone with some minor channel sands (Whipkey and 

others, 1991). The depositional environment for the Lebo 

Member was described as lacustrine by Ayers (1986), whereas 

others (summarized in Flores and Bader, 1999) indicate that 

fine-grained sediments are from the shedding of fine-grained 

marine Cretaceous shales from the rising Bighorn Mountains. 

The Tongue River Member contains considerable channel 

sandstones and coals. The Tongue River Member transitions to 

the Eocene-age Wasatch Formation with no distinct marker or 

lithologic change at the contact between the two units (Bartos 

and Ogle, 2002).

Some of the hydrogeologic units described in this report 

correspond with geologic formations, whereas other hydrogeologic units combine geologic formations, and other hydrogeologic units have contacts within stratigraphic units (fig. 2). 

This study uses terms for hydrogeologic units defined by 

Lewis and Hotchkiss (1981) and further described by Hotchkiss and Levings (1986). The uppermost hydrogeologic unit is 

the Tongue River-Wasatch aquifer, which averages 55 percent 

sand (Lewis and Hotchkiss, 1981). Below the Tongue RiverWasatch aquifer is the Lebo confining unit, which averages 

2 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
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Figure 1. Hydrogeologic units of the Powder River structural basin in Wyoming and Montana. 

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Figure 2. Generalized stratigraphic column of Tertiary and upper Cretaceous formations in the 

southern Powder River structural basin, Wyoming (modified from Flores and Bader, 1999).

4 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
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Figure 3. West to east hydrogeologic section of the Tertiary and upper Cretaceous hydrogeologic units 

showing the asymmetric nature of the Powder River structural basin, Wyoming.

31 percent sand (Lewis and Hotchkiss, 1981); however, 

predominant mudstones give it confining unit characteristics. 

Below the Lebo confining unit is the Tullock aquifer, which 

includes basal sands of the Lebo Shale Member. This aquifer 

averages 53 percent sand (Lewis and Hotchkiss, 1981) and is 

confined at its base by the upper Hell Creek confining unit. 

This unit typically acts as a confining unit, but with sand content ranging from 9 to 88 percent, and a mean of 35 percent, 

wells completed in it can flow (Hotchkiss and Levings, 1986). 

This confining unit is the upper part of the Lance Formation 

in Wyoming. Below this unit is the basal aquifer of interest in 

this report, the Fox Hills-Lower Hell Creek aquifer. Average 

sand content is 50 percent and it is a good source of water in 

the Powder River Basin (Hotchkiss and Levings, 1986). This 

aquifer includes the lower part of the Lance Formation and the 

Fox Hills Sandstone. At its base is the basal confining unit, 

the Lewis Shale, which is equivalent to the Bearpaw Shale in 

Hotchkiss and Levings (1986).

For all of the hydrogeologic units of interest, the general 

direction of ground-water flow is from south to north with the 

influence of the major rivers as discharge areas (Hotchkiss and 

Levings, 1986; Whitehead, 1996). In parts of the basin, this 

general direction of flow is influenced by a flow component 

from topographically high areas, such as the Bighorn Mountains to the west, Laramie Range to the south, and Black Hills 

to the east. An example is shown on the potentiometric map 

of the Wyodak-Anderson coal bed by Daddow (1986), which 

only shows the eastern limb of the flow system and has a more 

southeast to northwest flow pattern. From a more regional 

view, the general flow is from south to north.

Hydrogeologic Framework 5
Methods Used to Estimate GroundWater Volumes

 Following is a description of methods used to estimate 

the volume of water in Tertiary and upper Cretaceous hydrogeologic units in the Powder River Basin in Wyoming. The 

first task was to develop a geographic information system 

(GIS) to display and utilize data sets for use in calculating volume estimates. The second task was to determine the volume 

of water. Estimates of ground-water volumes were determined 

by two methods: (1) a total volume based on porosity and 

saturated thickness, and (2) a volume that could be removed 

by pumping based on specific yield, specific storage, and saturated thickness. The hydraulic properties used in the estimates 

are described in the Hydraulic Properties Used in Estimating 

Ground-Water Volume section of this report. The third task 

was to determine the sensitivity of the ground-water volume 

estimates to the factors used to calculate these estimates.

Development of a Geographic Information 

System

Data were prepared for input into a GIS. Folded paper 

maps of three themes: (1) the altitude of the base of the hydrogeologic units (shown in fig. 3); (2) percent sand of each unit; 

and (3) the total thickness of each unit (Lewis and Hotchkiss, 

1981) were carefully ironed to flatten creases caused by folds. 

Each map was then placed flat onto a large table and covered 

with transparent film. The theme of each map was traced onto 

a separate piece of transparent film along with at least nine 

registration points, which were either latitude and longitude 

tics, or locations of major cities or towns. Care was taken to 

ensure that each theme was bounded by tics. Separate themes, 

on transparent film, were taken to the University of Wyoming American Heritage Center in Laramie, Wyoming and 

scanned on a large format scanner. In GIS, a tic-point coverage 

was created and then the scanned images were imported and 

converted to grids. Additional themes of water levels for various hydrogeologic units were scanned and processed through 

the steps previously described. A map of the water table of 

the Tongue River-Wasatch aquifer (Hotchkiss and Levings, 

1986) was traced onto transparent film along with registration 

points. Potentiometric surface maps of other hydrogeologic 

units (Hotchkiss and Levings, 1986) were prepared in a similar 

manner.

Each theme was converted to a grid, formatted for use 

in GIS, and georeferenced using ARCMAP1. The tic and grid 

coverages were imported, and the grid was zoomed to overlay the tic coverage. The georeferencing tool was used to add 

control points to the grid by clicking on a control point on the 

grid, then selecting the corresponding control point in the tic 

coverage. Grids were first registered on the outside of the grid, 

and final control points were registered inside of the grid. A 

third order polynomial was used to transform the grid, and the 

grid was rectified. The projection file of the tic coverage was 

copied for use as the projection file of the rectified grid. A new 

grid was created by selecting pixels in the rectified grid that 

represented contour lines, then converting the grid to a line 

coverage. The line coverage was edited by deleting unneeded 

tic marks and adding values to the contour lines. In a search 

for errors during verification of the results, some themes were 

converted to the scale and projection of the original paper 

maps. Paper copies of these maps were plotted, laid over the 

original maps on a light table, and any errors were corrected.

Coverage projections were changed to Albers equal-area 

projection, with a North American Datum of 1927 (NAD 27). 

All maps used feet as the units. Surfaces were then converted 

to triangulated irregular networks (TINS2). Arcs were used as 

the input coverage, the contour value (altitude, thickness, or 

percent sand, depending upon the theme) was used as the z 

factor, and the contours were used as hard lines.

To create a point coverage to sample the TINS, a grid was 

made from the polygon coverage of the Fox Hills Sandstone 

outcrop which has the outermost extent of the hydrogeologic units of interest. This grid was made with a cell size of 

1,000 feet (ft) on a side, which was chosen as a compromise 

between detail and reasonable computation times (days). This 

grid was then converted to a point coverage, which was a 

series of 419,327 points with each point separated by 1,000 ft 

from another point. The TINS of the themes were sampled 

in GIS by use of the “TINSPOT” command, which sampled 

the TIN at each point with the linear option. At each point the 

altitude of the base of the unit, the unit’s thickness, and the 

percentage sand of the unit were sampled.

Considerable time was spent correcting negative values. 

Negative values were the result of contours from Lewis and 

Hotchkiss (1981) not extending to the outermost extent of the 

outcrop (fig. 4A). When TINS were made from these contours, 

the TINS did not extend past the outcrop areas (fig. 4B) and 

“no sample” values (-9999) were assigned to sample points. 

In these areas, contours were extended or added by manual 

methods past the outcrop extent (fig. 4C). Extended and added 

contours were used to make revised TINS, which extended 

past the outcrop extent (fig. 4D). When sampled, revised TINS 

then gave positive values.

Areas with structural arches, closed structural highs, and 

elongated or closed structural lows were a concern because 

the TIN process assigns a uniform value for all samples within 

these areas. Although having uniform values is a possible 

interpretation of a surface between contours with the same 

values, a more plausible interpretation is that there is an inflection point or line between two contours with the same values. 

Values would change between the last known contour and 

this inflection point or line, and then the rate of change would 

1

Copyrighted term of ESRI, Inc. 2TINS and TIN are used somewhat interchangeably, with TIN usually referring to one theme, such as “a TIN of the water table.”

6 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
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Figure 4. Example contours, outcrop, and triangulated irregular networks (TINS). A, original contours from Lewis and Hotchkiss 

(1981), B, TINS based on original contours, C, original contours and contours extended and added by manual methods, and 

D, revised TINS based on extended and added contours.

Methods Used to Estimate Ground-Water Volumes 7
change sign as one continued from the inflection point or line 

to the other known contour. In other words, the inflection point 

or line represents the crest of a hill or an arch, or the lowest 

point in a valley or a depression. In these areas, a line was 

added by manual methods and assigned a value (fig. 4C). The 

resulting TIN uses values from this line, and the area inside 

the closed contour (or between the contour ridge or trough) no 

longer has uniform values. Although values assigned to these 

lines were subjective, an analysis based on the resulting values 

is “less incorrect” than an analysis based on uniform values 

between contours. The result is “less incorrect” because the 

added line adds values that agree with an interpretation of a 

curved high (or low) instead of a flat plateau inside the closed 

contour (or between the contour ridge or trough). Values were 

selected to be consistent with contour intervals or the available data outside the high or low. An analysis of the possible 

error introduced by this method is described in the Sensitivity 

Analysis Methods section, and shows that any error due to this 

procedure is minor compared to other possible errors.

Computation of Ground-Water Volumes

The computation of volume of water in storage is 

illustrated with a hypothetical schematic of the basin (fig. 5). 

This schematic shows three of the six hydrogeologic units as 

nested bowls, with the bowl in the center of the basin, labeled 

as Tongue River-Wasatch aquifer, as the uppermost unit. The 

hydrogeologic units increase in age with depth and outward 

from the center of the basin. This schematic shows the water 

table not intersecting the land surface; however, areas may 

exist in the basin where this may happen. At any location, the 

saturated thickness for the Tongue River-Wasatch aquifer is 

given by a, which is the distance between the water table and 

the base of the Tongue River-Wasatch aquifer. The saturated 

thickness for the next lowest units, part of the Fort Union 

Formation, is more complicated. In areas where the potentiometric surface of Tullock aquifer is above the base of the 

Tongue River-Wasatch aquifer, the Tullock aquifer is confined 

and the saturated thickness of the Tullock aquifer, b, also is 

the apparent thickness of the Tullock aquifer. With increasing distance from the center of the basin, an area is reached 

where the water table is below the contact between the Tongue 

River-Wasatch aquifer and the Lebo Confining Unit (B). In 

this area (B), the Tongue River-Wasatch aquifer is completely 

unsaturated or absent. The amount of saturated thickness, c, 

is the difference between the water table and the base of the 

Tullock aquifer.

The transition of aquifers between confined and unconfined conditions is poorly understood. On figure 5, the Tullock 

aquifer is confined at b. Applied Hydrology and Greystone 

Environmental Consultants, Inc. (2002, Appendix B) report 

that aquifer test results in alluvial sediments, which are part 

of the Tongue River-Wasatch aquifer as described in this 

report, indicate that at some depth even these materials act in 

a confined manner. In this study, an assumption was made that 

when a hydrogeologic unit has more than 50 feet of saturated 

thickness, it is confined. Thus, specific yield was used as the 

storage factor for a saturated thickness of 50 feet or less, and 

specific storage was used as the storage factor for a saturated 

thickness greater than 50 feet.

Water-table data were from Hotchkiss and Levings 

(1986). The water table in the center of the basin (A in fig. 5) 

is from the approximate potentiometric surface in the Tongue 

River-Wasatch aquifer, whereas the water table (B in fig. 5) in 

stratigraphically lower units, represented by the Tullock aquifer in figure 5, is from potentiometric surface maps of other 

hydrogeologic units used in this report and given in Hotchkiss 

and Levings (1986).

In some areas of the study, potentiometric contours of 

Hotchkiss and Levings (1986) did not extend to the edge of 

the outcrop. In these areas, water levels in shallow wells were 

obtained from the USGS Ground-Water Site Inventory database and a point coverage was made of these water levels. A 

TIN was assembled from this coverage. This was done only in 

areas where contoured water levels of Hotchkiss and Levings 

(1986) were not available.

Any perched water-table zones were ignored in the calculation of ground-water volumes. The assumption was made 

that the extent of perched zones is small when compared to the 

entire area of the basin.

For all volume estimates of saturated rock using the first 

method to estimate ground-water volume, the total volume of 

water was calculated by summing a component of water in 

sands and a component of water in non-sands. For the unconfined part of the aquifers, equation 1 was used to calculate the 

total amount of water in the sands:

sandwater = 1,000 * 1,000 * (sattk * ( %sand/100) * sand-porosity)

(1)

where:

sandwater = the total amount of water in the sands, in cubic feet, = the total amount of water in the sands, in cubic feet,

1,000 * 1,000 = the area of the grid cell, in square feet,

sattk = the saturated thickness (a in figure 5), in feet,

%sand = the percentage of sand, and

sand-porosity = the porosity of the sand.

Equation 2 was used to calculate the total amount of 

water in the non-sand part of the cell:

nonsandwater = 1,000 * 1,000 * (sattk * ( ( 100 - %sand) / 100) * 

non-sand porosity)

(2)

where:

nonsandwater = the total amount of water in the non-sand component = the total amount of water in the non-sand component 

of the cell, in cubic feet,

1,000 * 1,000 = the area of the grid cell, in square feet,

sattk = the saturated thickness (a in figure 5), in feet,

%sand = the percentage of sand, and

non-sand porosity = the porosity of the non-sand component.

8 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
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Figure 5. Schematic showing relations among land surface, water table, hydrogeologic units, and saturated section 

of the hydrogeologic units in the study area, Powder River structural basin, Wyoming.

Significant figures limit the number of meaningful digits 

in calculations. Common practice is to limit the number of significant figures in a mathematical operation to the number of 

significant figures in the least precise number (Hansen, 1991). 

For these calculations, the least precise numbers are porosity 

and percent sand in each of the units, which are given to two 

digits.

For hydrogeologic units below the Tongue River-Wasatch 

aquifer, similar formulas were used to calculate the volume of 

water, with special attention paid to the saturated thickness at 

each cell point. Conceptually, in areas where the unit is confined, the saturated thickness is given as the apparent thickness 

of the hydrogeologic unit, such as b in figure 5. With proximity to the edge of the basin, the saturated thickness transitions 

from that shown in b in figure 5 to the saturated thickness 

shown as c in figure 5, which is where the water table in the 

hydrogeologic unit is the top boundary of the saturated thickness of that unit. Transitioning from the conceptual idealization to the practical implementation required careful consideration of selection criteria when designing the GIS procedure to 

carry out this task.

A second method to estimate ground-water volumes is 

to consider specific yield and specific storage in the calculations. For the unconfined part of the aquifers, the volume of 

water that could be recovered was calculated by multiplying 

the saturated thickness of the aquifer by the specific yield and 

the area of the grid cell. For confined parts of the aquifers, an 

assumption was made that only water in the saturated thickness of the Tongue River-Wasatch aquifer would be removed 

by pumping. For example, equation 3 was used to calculate 

the amount of water released from confined storage in the 

sand part of the Fox Hills-Lower Hell Creek aquifer if only the 

Tongue River-Wasatch aquifer is dewatered:

sfhvs = 1,000 * 1,000 * (fhtk * (fhsnd / 100) * Ssnd) * (trwsattk)

(3)

where:

sfhvs = the volume of water released from confined storage in the 

sand part of the Fox Hills-Lower Hell Creek aquifer, in cubic feet, 

1,000 * 1,000 = the area of the grid cell, in square feet,

fhtk = the confined thickness of the Fox Hills-Lower Hell Creek = the confined thickness of the Fox Hills-Lower Hell Creek 

aquifer, in feet,

fhsnd = the percentage of sand in the Fox Hills-Lower Hell Creek = the percentage of sand in the Fox Hills-Lower Hell Creek 

aquifer,

Ssnd = specific storage for sand, in feet = specific storage for sand, in feet-1, and

trwsattk trwsattk = the saturated thickness in the Tongue River-Wasatch aqui- = the saturated thickness in the Tongue River-Wasatch aquifer, in feet.

The procedure using equation 3 for the Fox Hills-Lower 

Hell Creek aquifer was repeated for each hydrogeologic unit. 

Estimated ground-water volumes were determined for the sand 

Methods Used to Estimate Ground-Water Volumes 9
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Figure 6. Sketch showing how the original values of contours (A) were increased by 10 percent 

to form another set of values (B) that were used 

to create TINS to study the effect of a systematic 

error in contour values.

and non-sand parts of each unit using appropriate values of 

confined thickness, percentage sand, and specific storage.

Toward the edge of the basin, the Tongue River-Wasatch 

aquifer is not present (fig. 3). In these areas, the saturated 

thickness of the outcropping unit was assumed to be pumped 

dry and the change in head relative to the base of the outcropping unit was used to calculate the amount of water released 

from storage from each hydrogeologic unit underlying the 

outcropping unit. Thus, if the Lebo confining unit was the 

outcropping unit, then the water released from the Tullock 

aquifer, the Upper Hell Creek confining unit, and the Fox 

Hills-Lower Hell Creek aquifer was calculated by using the 

saturated thickness of the Lebo confining unit as the change 

in hydraulic head in the lower units. Toward the outer edge of 

the basin, older units became the outcropping units and the 

procedure just described was repeated.

Sensitivity Analysis Methods

Estimates of error can be highly theoretical; for example, 

see articles in Mower and Congalton (2000) and Lowell and 

Jaton (1999). A sensitivity analysis was conducted on selected 

factors used to calculate the ground-water volume by use of 

porosity and by recording how sensitive the ground-water 

volume estimates were to these selected factors. Although 

some hydrogeologic parameters are only known to within an 

order of magnitude difference, for this analysis, an error of 

10 percent was used for most parameters. For example, if the 

thickness of a unit was 300 ft, then a value of 330 ft was used 

in the analysis, or if the porosity was 30 percent, then a value 

of 33 percent was used in the analysis. These values were then 

plotted in a graph to show how sensitive the volume of ground 

water was to these factors.

Two types of errors were studied for factors that were 

based on contour maps. The first type of possible error is a 

systematic error in all of the values. Figure 6 shows how this 

error was evaluated. Figure 6A shows a group of contours with 

original values. Figure 6B shows each of these contours with 

each value increased by 10 percent. These contours were then 

used to make a TIN that was sampled, and the sampled values 

were used to calculate a ground-water volume, which was then 

compared to a base value of ground-water volume. The difference between these two volumes was calculated and differences were compiled to determine the sensitivities of possible 

systematic errors.

The second type of possible error is from the values of 

the extended or added inflection lines. As discussed previously, inflection lines occur between contours that have the 

same values. The values selected for these inflection lines 

were subjective and were the focus of this analysis. Figure 7A shows the inflection line with a value of 62, which 

was assigned by the author. What if another value had been 

used? Figure 7B shows that the value of the inflection line 

was increased about 10 percent to 68. In this analysis, all of 

the inflection lines of a theme were varied in a similar man10 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
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Figure 7. Sketch showing how the values of 

inflection lines were changed in the center of the 

closed contour, with the value used in the base 

analysis (A), and showing the changed value 

of the inflection line (B), which was used in the 

sensitivity analysis.

ner, then the theme was used to make a TIN of that modified 

theme. This TIN was sampled and the sampled values were 

used to calculate a ground-water volume, which was then 

compared to the base ground-water volume. The difference 

between these two volumes was calculated, which was used 

as a measure of the sensitivity of the values of the inflection 

lines.

Estimates of Ground-Water Volumes

Two different calculations were used to estimate groundwater volumes in Tertiary and upper Cretaceous hydrogeologic 

units in the Powder River Basin in Wyoming. The first method 

used the porosity of the units and the second method used specific yield and specific storage. Both calculations gave results 

for unconfined and confined parts of each hydrogeologic unit.

Hydraulic Properties Used in Estimating 

Ground-Water Volumes

Regional compilations of porosity are not available for 

the hydrogeologic units of interest in this study. Literature values were used to obtain approximate porosity values for both 

sand and non-sand parts of the hydrogeologic units. Textbooks 

give varying ranges of possible porosities. Freeze and Cherry 

(1979) give ranges for sand and non-sand porosities of 25 

to 50 percent and 25 to 70 percent, respectively, with higher 

values for clays and lower values for gravels. Fetter (1988) 

reports that porosities in clastic rocks can range from 3 to 

30 percent. Domenico and Schwartz (1990) report sand porosities that range from 26 to 53 percent and non-sand porosities 

that range from 24 to 60 percent, with higher porosities found 

in silts and clays. In this study, the following porosities were 

assumed: sand porosity of 30 percent and non-sand porosity of 

35 percent (table 1).

Storage parameters from a compilation of aquifer tests 

for the upper hydrogeologic section of the Powder River Basin 

(see Appendix B of Applied Hydrology Associates, Inc. and 

Greystone Environmental Consultants, Inc., 2002) are consistent with the hydrogeologic setting of the sediments. Specific 

yield values from textbooks (Fetter, 1988; Domenico and 

Schwartz, 1990) refer to work by Johnson (1967) and range 

from 3 percent for clay to 27 percent for sandstones, with 

higher values for lithologies that are not reported to be in the 

Powder River Basin. In this study, specific yields of 26 percent 

for sand and 10 percent for non-sand were used, and specific 

storages of 1x10-4/ft for sand and 1x10-5/ft for non-sand were 

used (table 1).

Estimates of Ground-Water Volumes 11
Table 1. Hydraulic properties of hydrogeologic units in the Powder River Basin used to estimate groundwater volumes.

Hydrogeologic unit Lithology

Porosity, 

in percent

Specific yield, 

in percent

Specific storage 

(1/feet)

Tongue River-Wasatch aquifer Sand 30 26 1x10-4

Non-sand 35 10 1x10-5

Lebo confining unit Sand 30 26 1x10-4

Non-sand 35 10 1x10-5

Tullock aquifer Sand 30 26 1x10-4

Non-sand 35 10 1x10-5

Upper Hell Creek confining layer Sand 30 26 1x10-4

Non-Sand 35 10 1x10-5

Fox Hills-Lower Hell Creek aquifer Sand 30 26 1x10-4

Non-sand 35 10 1x10-5

Ground-Water Volumes Derived from Porosity

For total ground-water volumes calculated using porosity 

values (table 2), the confined part of the aquifers contain much 

more water than the unconfined part of the aquifers. This is 

due to the difference in volume of rock between the confined 

and unconfined parts. The unconfined part of the aquifers was 

assumed to be only the upper 50 feet of the stratigraphic section. Under this thin veneer of unconfined conditions may be 

thousands of feet of rock containing water under confined conditions (fig. 3). However, much of this water is not available 

for two reasons. First, for fine-grained sediments, it is difficult 

to pump this water out of these sediments because of their low 

permeability. Second, some of the water is interstitial wetting 

fluid, and thus unrecoverable.

Ground-Water Volumes Derived from Specific 

Yield and Specific Storage

For the recoverable ground-water volumes derived from 

specific yield and specific storage calculations (table 3), the 

non-sand part of hydrogeologic units contains less water than 

the sand part of hydrogeologic units. For the unconfined part 

of the aquifers, the non-sand facies contain about 40–65 percent less water than the sand facies, and for the confined part 

of the aquifers, the non-sand facies contain 80–90 percent less 

water than the sand facies. The unconfined part of the Tongue 

River-Wasatch aquifer has more water than the confined part 

of the same aquifer. For all of the other hydrogeologic units, 

the confined part of the aquifers contains more water than the 

unconfined part.

Water volumes calculated using specific yield and 

specific storage are less than water volumes calculated using 

porosity. This relation follows from the fact that specific yield 

is equal to porosity minus specific retention (Lohman and others, 1972). Specific retention is moisture bound to the surfaces 

of particles and this moisture does not drain by gravity. For 

confined aquifers, the storage coefficient is the volume of 

water an aquifer releases from storage per unit surface area 

of the aquifer per unit change in head (Lohman and others, 

1972).

The following comparisons help place ground-water 

volume into a “real-world” context. The total ground-water 

volume estimated from storativity (3.6 x 1013 ft3) is less water 

than is held in Lake Ontario but more than twice the water 

held in Lake Erie (lake water volumes from U.S. Environmental Protection Agency, 2004).

Sensitivity Analysis

A sensitivity analysis of selected factors shows that 

ground-water volume is most sensitive to changes in porosity 

of sand and non-sand facies (table 4, figure 8). The next two 

most sensitive factors are confined thickness and saturated 

thickness. Ground-water volume is not very sensitive to the 

sand percentage, and ground-water volume has very low sensitivity to values applied to inflection lines used in making the 

TIN of the sand percentage.

The sensitivity analysis also provides an indication of 

uncertainty for the calculated ground-water volumes. This 

analysis varied porosity by 10 percent, so that 30 percent 

porosity became 33 percent and 35 percent porosity became 

38.5 percent. Because of the communicative nature of porosity in the calculation, a 10 percent change in porosity resulted 

12 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
Table 2. Estimated ground-water volumes in Tertiary and upper Cretaceous hydrogeologic units in the 

Powder River Basin based on porosity.

Volume of ground water in facies (cubic feet)

Hydrogeologic Unit Sand Non-sand Total

Unconfined (for saturated thickness less than or equal to 50 feet)

Tongue River-Wasatch aquifer 1.7x1012 1.8x1012 3.5x1012

Lebo confining unit 3.1x1011 4.7x1011 7.8x1011

Tullock aquifer 1.7x1012 1.8x1012 3.5x1012

Upper Hell Creek confining layer 1.5x1011 2.9x1011 4.4x1011

Fox Hills–Lower Hell Creek aquifer 2.7x1011 3.7x1011 6.4x1011

Total for unconfined units 4.1x1012 4.8x1012 8.9x1012

Confined (for saturated thickness greater than 50 feet)

Tongue River-Wasatch aquifer 5.6x1013 6.7x1013 1.2x1014

Lebo confining unit 1.6x1013 9.1x1012 2.4x1013

Tullock aquifer 1.6x1013 6.5x1012 2.2x1013

Upper Hell Creek confining layer 4.3x1012 4.5x1012 8.9x1012

Fox Hills–Lower Hell Creek aquifer 7.8x1012 6.6x1012 1.5x1013

Total for confined units 9.9x1013 9.4x1013 1.9x1014

Total for confined and unconfined units 1.0x1014 9.9x1013 2.0x1014

in about a 10 percent change in ground-water volume. The 

30 percent and 35 percent porosity values used in the calculations could have easily been justified as 25 percent and 

40 percent or some other values with the corresponding 

change in calculated volume. Without having detailed maps of 

field-derived porosity values for the hydrogeologic units, it is 

difficult to quantify the uncertainty of the porosity values used. 

Although representative values were used in the calculations 

given in this report, the values used are certainly not the only 

values that could have been used. If one assumed an uncertainty in porosity of 33 percent, then the uncertainty of the calculated ground-water volumes also is about 33 percent. Given 

this possible uncertainty, readers should consider the groundwater volumes given in this report as order of magnitude 

ground-water volumes and not as exact ground-water volumes. 

Thus, in this report, the results are referred to as estimates.

Estimates of Ground-Water Volumes 13

Comparison with a Previous Study

The purpose of this section is to compare the results of 

this study with the results from a previous study. Applied 

Hydrology Associates, Inc. and Greystone Environmental 

Consultants, Inc. (2002) made an estimate of available water 

from units associated with CBM development. The estimates 

of ground water for the sandstones can be compared to estimates given in this report of ground-water volumes for sand 

units that are based on storativity factors.

For the sandstones of the Tongue River-Wasatch aquifer, there is about an order of magnitude difference in water 

volumes calculated by the two studies (table 5). For the 

sandstones in the Lebo confining unit and the Tullock aquifer, 

the agreement between the two studies is much better. These 

variations in results from the two studies demonstrate the 

uncertainty in estimates calculated by using available data.
Table 3. Estimated ground-water volumes in Tertiary and upper Cretaceous hydrogeologic units in the 

Powder River Basin based on specific yield and specific storage.

Volume of ground water in facies (cubic feet)

Hydrogeologic unit Sand Non-sand Total

Unconfined (for saturated thickness less than or equal to 50 feet)

Tongue River-Wasatch aquifer 1.4x1012 5.3x1011 2.0x1012

Lebo confining unit 2.7x1011 1.3x1011 4.0x1011

Tullock aquifer 2.5x1011 1.2x1011 3.7x1011

Upper Hell Creek confining layer 1.3x1011 8.2x1010 2.1x1011

Fox Hills–Lower Hell Creek aquifer 2.4x1011 1.0x1011 3.4x1011

Total for unconfined units 2.3x1012 9.7x1011 3.3x1012

Confined (for saturated thickness greater than 50 feet)

Tongue River-Wasatch aquifer 1.9x1010 1.9x109 2.0x1010

Lebo confining unit 1.1x1013 2.2x1012 1.3x1013

Tullock aquifer 1.2x1013 1.2x1012 1.3x1013

Upper Hell Creek confining layer 2.1x1012 3.9x1011 2.5x1012

Fox Hills–Lower Hell Creek aquifer 3.4x1012 4.1x1011 3.9x1012

Total for confined units 2.8x1013 4.2x1012 3.2x1013

Total for unconfined and confined units 3.0x1013 5.2x1012 3.6x1013

Table 4. Sensitivity of calculated ground-water volumes to selected factors used to calculate the ground-water volumes.

Volume factor

Total ground-water volume with changed 

parameter minus base total ground-water 

volumea, in cubic feet Percent errorb

Sensitivityc

(unitless)

Sand and non-sand porosity -4.5x1013 -22.5 2.25

Confined thickness -2.5x1013 -12.5 1.25

Saturated thickness -1.7x1013 -8.5 0.85

Percentage sand 2.9x1012 1.45 0.15

Values used for inflection lines in the 

sand porosity coveraged

1.84x1010 0.01b 0.001

a

Base total ground-water volume from table 2 is 2x1014 cubic feet.

b

Calculated as 100 x (total ground-water volume with changed parameter – base total ground-water volume) / base total ground-water volume.

c

Calculated as absolute value of percent error / 10.

d

See the discussion associated with figure 7 for a more complete discussion of this factor.

14 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
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Table 5. Comparison of estimates of available volumes of ground water from Applied Hydrology Associates, Inc. and Greystone 

Environmental Consultants, Inc. (2002) to the estimates of ground-water volumes determined in this study from storativity factors.

[AHA, Applied Hydrology Associates, Inc. and Greystone Environmental Consultants, Inc., estimates of water volume; ft3, cubic feet; 

1 acre-foot = 43,560 ft3]

Volume from AHA study Volume from this study

Acre-feet Cubic ft3

Unconfined 

(ft3)

Confined

(ft3) Total (ft3)

Difference 

between 

studies (ft3)

Percent 

difference1

Tongue River-Wasatch 

sandstones

743,121,790 3.2x1013 1.4x1012 1.9x1010 1.4x1012 3.1x1013 96

Lebo sandstones 227,137,336 9.9x1012 2.7x1011 1.1x1013 1.1x1013 -1.4x1012 -14

Tullock sandstones 447,246,784 2.0x1013 2.5x1011 1.2x1013 1.2x1013 7.8x1012 39

1

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, and b is “volume from this study” in ft3.

Discussion of Ground-Water Volumes

When considering ground-water volumes estimated in 

this report, the distinction between total water and recoverable 

water must be understood. Volume estimates from porosity 

are the total volume of water that these facies may hold and 

not the actual amount of water that is available if an attempt 

was made to remove all water from these hydrogeologic units. 

Volume estimates from the storativity factors are the amount 

of water that could be pumped given considerable dewatering 

of the basin. Thus, both estimates given in this report represent 

the extreme upper bounds, with the former a bound on the 

maximum amount of water in the basin and the latter a bound 

on the maximum amount of water that could be produced.

Several factors were not included in this analysis, mainly 

due to time constraints and the scope of this study. Two of 

these factors are the thickness of coals and clinker zones. 

Several compilations of coal thickness are available, but 

those of the Wyoming State Geological Survey (James Case, 

oral commun., 2003) were not completed when work for this 

project was completed, and they do not cover the whole of the 

Wyoming part of the Powder River Basin. A map by Ellis and 

others (1999) provides the thickness for one of the coal zones 

in the Tongue River Member of the Fort Union Formation but 

not the total thickness of all coal zones. Clinker zones, which 

are areas of coals that have burned, have a smaller extent than 

coal zones and have few zones of fairly high porosity and 

specific yield (Coates and Heffern, 1999; Heffern and Coates, 

1999).

Significant figures limit the number of digits reported for 

estimates of ground-water volumes. Porosity and the thickness 

of sands are known only to two significant figures. Providing 

numbers with more than two significant digits only gives the 

reader an unfounded sense of the precision of the results.

Estimates of Ground-Water Volumes 15

Figure 8. Sensitivity of calculated ground-water volumes to 

selected factors used to calculate ground-water volumes.
Approaches to Increase Accuracy of 

Ground-Water Volume Estimates

The estimates of ground-water volumes in Tertiary and 

upper Cretaceous hydrogeologic units presented in this report 

could be refined if additional information was available. 

Although the collection and compilation of this information 

was beyond the scope of this study at least five approaches are 

available to obtain information that might increase the accuracy of ground-water volume estimates made in this report. 

They include (1) compilation of basin-wide maps of the tops 

and bottoms of coals, or maps of coal thickness, (2) compilation of basin-wide maps of storage factors for the hydrogeologic units, (3) compilation of basin-wide maps of porosity for 

the hydrogeologic units, (4) compilation of water-table maps 

either synoptically or temporally, and (5) inclusion of clinker 

zones in the analysis.

Compilation of basin-wide coal thickness maps is important; however, the extent of any given coal in the basin is often 

contentious, and gaining consensus among the many coal 

geologists practicing in the basin is needed. The advantage to 

this approach is that coal properties, including low storativity, fracturing, high hydraulic conductivities, and their lateral 

extent, would add greatly to future hydrogeologic and geologic 

studies, including revised estimates of ground-water volumes.

After coal thickness, knowing the distribution of porosity in the hydrogeologic units of interest would likely have 

the most effect on improving ground-water volume estimates. 

Porosity has been shown to have the most effect on these 

estimates, and detailed information about porosity distribution 

in the Powder River Basin is available in the public literature. 

Porosity is important not only in estimating ground-water 

volume, but also for simulation of ground-water flow, solute 

transport, and ground-water chemistry.

Basin-wide maps of storage factors would provide insight 

about where aquifers transit from unconfined to confined conditions. Storage factors could be correlated with local lithology to extend knowledge of relations in this area.

Water-table maps, either synoptic or temporal, would 

improve knowledge about the saturated thickness of the 

hydrogeologic units of interest, which directly relates to 

ground-water volume. These maps would help determine how 

ground-water volume changes, both seasonally and over long 

periods of time.

Finally, the addition of clinker zones to the analysis 

would add limited areas of high porosity and high specific 

yield. However, due to the limited extent of clinker zones, 

their contribution to estimates of ground-water volume might 

be minimal. Until such an analysis is undertaken, this assumption cannot be validated.

Summary

The Powder River Basin in Wyoming and Montana is an 

important source of energy resources for the United States. 

Coalbed methane gas is contained in Tertiary and upper Cretaceous hydrogeologic units in the Powder River Basin. This gas 

is released when water pressure in coalbeds is lowered, usually 

by pumping ground water. Issues related to disposal and uses 

of by-product water from coalbed methane production have 

developed, in part, due to uncertainties in hydrologic properties. One hydrologic property of primary interest is the amount 

of water contained in Tertiary and upper Cretaceous hydrogeologic units in the Powder River Basin. The U.S. Geological 

Survey, in cooperation with the Bureau of Land Management, 

conducted a study to describe the hydrogeologic framework 

and to estimate ground-water volumes in Tertiary and upper 

Cretaceous hydrogeologic units in the Powder River Basin in 

Wyoming.

The Tertiary and upper Cretaceous hydrogeologic units 

of interest for this study are (in descending order): Tongue 

River-Wasatch aquifer, which is contained in the Eocene-age 

Wasatch Formation and the Paleocene-age Fort Union Formation; the Lebo confining unit, which is contained in the upper 

part of the Lebo Shale Member of the Fort Union Formation; 

the Tullock aquifer, which is contained in the lower part of 

the Lebo Shale Member and in the Tullock Member of the 

Fort Union Formation; the Upper Hell Creek confining layer, 

which is contained in the upper part of the upper Cretaceousage Lance Formation; and the Fox Hills-Lower Hell Creek 

aquifer, which is contained in the lower part of the Lance 

Formation and the upper Cretaceous-age Fox Hills Sandstone. 

All of these hydrogeologic units contain varying amounts of 

sand and non-sand facies.

Maps from previous investigations, including altitudes 

of potentiometric surfaces, altitudes of formation tops and 

bottoms, thicknesses, and percentages of sand, were used in 

developing a geographic information system to estimate the 

volume of ground water in the hydrogeologic units of interest. Literature porosity values of 30 percent for sand and 35 

percent for non-sand facies were used to calculate the volume 

of total ground water in each hydrogeologic unit. Literature 

specific yield values of 26 percent for sand and 10 percent for 

non-sand facies, and literature specific storage values of 

1x10-4/ft for sand facies and 1x10-5/ft for non-sand facies were 

used to calculate a second volume of total ground water for 

each hydrogeologic unit.

A total ground-water volume of 2.0x1014 cubic feet was 

calculated using porosity values, and a total ground-water 

volume of 3.6x1013 cubic feet was calculated using specific 

yield and specific storage values. These results are consistent 

with retention properties, which would have some of the total 

water being retained.

Sensitivity analysis shows that estimates of ground-water 

volume are most sensitive to porosity. Estimates also are 

sensitive to confined thickness and saturated thickness. Given 

16 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
the uncertainty associated with the estimates presented in this 

report, the ground-water volumes should be considered as 

order of magnitude estimates.

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18 Hydrogeologic Framework and Estimates of Ground-Water Volume, Powder River Basin, Wyoming
Hinaman—Hydrogeologic Framework and Estimates of Ground-Water Volumes in the Powder River Basin, Wyoming—Scientifi c Investigations Report 2005-5008

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