Groundwater Availability of the Williston Basin, 

United States and Canada

Water Availability and Use Science Program 

U.S. Department of the Interior

U.S. Geological Survey

Professional Paper 1841

Canada

Saskatchewan Manitoba

United States

Montana

North Dakota

South Dakota

Groundwater Availability of the Williston 

Basin, United States and Canada

By Andrew J. Long, Joanna N. Thamke, Kyle W. Davis, and Timothy T. Bartos 

Water Availability and Use Science Program

Professional Paper 1841

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

Long, A.J., Thamke, J.N., Davis, K.W., and Bartos, T.T., 2018, Groundwater availability of the Williston Basin, United 

States and Canada: U.S. Geological Survey Professional Paper 1841, 42 p., https://doi.org/10.3133/pp1841.

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Library of Congress Cataloging-in-Publication Data

Names: Long, Andrew J., author. | Geological Survey (U.S.), issuing body.

Title: Groundwater availability of the Williston Basin, United States and 

 Canada / Andrew J. Long [and three others].

Other titles: U.S. Geological Survey professional paper ; 1841. 1044-9612

Description: Reston, Virginia : U.S. Department of the Interior, U.S. 

 Geological Survey, 2018. | Series: Professional paper, ISSN 1044-9612 ; 

 1841 | Includes bibliographical references.

Identifiers: LCCN 2018040131 | ISBN 9781411342545 (paperback)

Subjects: LCSH: Groundwater--Williston Basin--Management. | 

 Aquifers--Williston Basin. | Water-supply--Williston Basin. | Basins 

 (Geology)--Great Plains. | Basins (Geology)--Manitoba. | Basins 

 (Geology)--Saskatchewan. | Williston Basin.

Classification: LCC TD403 .L66 2018 | DDC 628.1/140978--dc23 | SUDOC I 

 19.16:1841

LC record available at https://lccn.loc.gov/2018040131

ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

ISBN 978-1-4113-4254-5
iii

Foreword

Although often overlooked, groundwater is increasingly important to all our lives. Groundwater 

is the Nation’s primary reserve of freshwater. It provides half of the drinking water in the 

United States, is essential to food production, and facilitates business and industrial activities. 

Groundwater also is an important source of water for sustaining the ecosystem health of rivers, 

wetlands, and estuaries throughout the country.

Large-scale development of groundwater resources and their resulting declines in groundwater 

levels, as well as other effects of pumping, have led to concerns about the future availability 

of groundwater to meet our Nation’s needs. The compounding effects of recent droughts 

emphasize the need for an updated status of the Nation’s groundwater resources. Assessments 

of groundwater resources provide the science and information needed by the public and by 

policy makers to evaluate water availability and its effects on the water supply, and to manage 

and use the water resources responsibly. Adding to this already complex task of resource 

assessment is the analysis of potential future effects due to climate variability, which can 

exacerbate an already challenging situation.

The U.S. Geological Survey’s Water Availability and Use Science Program is doing large-scale 

multidisciplinary regional groundwater availability studies (including the study described herein) 

of the uppermost principal aquifer systems of the Williston Basin. The regional studies are 

intended to inform citizens, communities, and natural resource managers about the status of 

the Nation’s groundwater resources and about how changes in land use, water use, and climate 

have affected those resources. The studies also are intended to aid development of tools to 

forecast how these resources may change. The findings from these groundwater assessments 

of principal aquifer systems will be combined in an integrated national assessment. Results 

derived from these studies will help answer questions about the Nation’s ability to meet current 

and future demands for groundwater.

Donald Cline

Associate Director for Water
iv

Acknowledgments 

This project was funded and supported by the U.S. Geological Survey Water Availability and Use 

Science Program, which includes the former Groundwater Resources Program. The authors thank 

the following U.S. Geological Survey personnel for their assistance and guidance throughout 

the project: Brian Clark, Geoff Delin (retired), Kevin Dennehy (retired), Daniel Feinstein, Mike 

Fienen, Seth Haines, Arlen Harbaugh, Randy Hunt, Chris Langevin, Howard Reeves, and Tom 

Reilly (emeritus). The authors gratefully acknowledge Matthew Bachmann for his assistance 

with the model scenarios, Katherine Chase for her assistance with research and development 

for precipitation text and illustrations, and DeAnn Dutton for her assistance with illustration 

support. The authors also thank Sue Kahle and Joshua Valder for providing excellent technical 

reviews of the report.

Sincere appreciation goes to Yves Michaud and Alfonso Rivera (Geological Survey of Canada), 

Graham Phipps (Manitoba Sustainable Development), Jon Reiten and Kevin Chandler (Montana 

Bureau of Mines and Geology), Attila Folnagy (Montana Department of Natural Resources 

and Conservation), Kimberly Fischer (North Dakota State Water Commission), and Kei Lo 

(Saskatchewan Water Security Agency), for their participation in monthly technical conference 

calls as part of the Williston Area Aquifers Modeling Consortium to discuss the groundwater 

resources of the Williston Basin.
v

Contents

Foreword ........................................................................................................................................................iii

Acknowledgments ......................................................................................................................................iv

Executive Summary.......................................................................................................................................1

Introduction.....................................................................................................................................................3

Purpose and Scope ..............................................................................................................................3

Description of Study Area ...................................................................................................................3

Energy Resources and Water Use............................................................................................3

Physiography and Climate..........................................................................................................5

Hydrogeologic Framework.........................................................................................................5

Modeling.................................................................................................................................................9

Conceptual Model of Groundwater Flow.................................................................................9

Summary of Numerical Modeling ...........................................................................................11

Assessing Groundwater Availability.........................................................................................................11

Analysis of Precipitation and Recharge ..................................................................................................13

Comparison of Conceptual and Numerical Models...............................................................................15

Groundwater Flow ..............................................................................................................................15

Average Groundwater-Flow Budget................................................................................................16

Simulated Transient Groundwater-Flow Budget ....................................................................................18

Simulated Groundwater Response to Flowing Artesian Wells ............................................................20

Hydrology of Flowing Artesian Wells ..............................................................................................20

Scenario 1: Flowing Artesian Wells.................................................................................................20

Simulated Groundwater Responses during Drought.............................................................................25

Scenario 2: Drought with Steady Groundwater Withdrawals.....................................................25

Scenario 3: Drought with Increased Groundwater Withdrawals...............................................25

Consumption Estimate ..............................................................................................................25

Supply Estimate..........................................................................................................................28

Simulation of Scenario 3...........................................................................................................28

Improving Hydrologic Monitoring Networks...........................................................................................29

Numerical Groundwater-Flow Model Uses, Limitations, and Challenges .........................................38

References Cited..........................................................................................................................................38
vi

Figures

1. Map showing location of the Williston Basin and hydrogeologic unit layers, United

States and Canada ......................................................................................................................4

2. Diagram showing lithostratigraphic and corresponding hydrogeologic units of the

Williston Basin, United States and Canada..............................................................................6

3. Three-dimensional view of the top of the basal confining unit in the Williston and

Powder River Basins, underlying the Upper Cretaceous aquifer system, United

States and Canada .......................................................................................................................7

4. Hydrogeologic cross section A–A’ showing the glacial, lower Tertiary, and Upper

Cretaceous aquifer systems in the Williston Basin, United States and Canada ...............8

5. Conceptual model of groundwater flow in the Williston Basin, United States and

Canada .........................................................................................................................................10

6. Graphs showing annual cumulative departure from the mean for precipitation

and simulated precipitation recharge as a spatial average within the active model

area, and precipitation for selected weather stations ........................................................12

7. Map showing locations of weather stations and long-term precipitation

(1981–2011) data in the study area, Williston Basin, United States and Canada.............14

8. Diagram and pie charts showing simulated groundwater-flow budget, 

Williston Basin, United States and Canada. ..........................................................................17

9. Graphs showing simulated groundwater-flow budget by stress period

(predevelopment-2005) for the glacial aquifer system, lower Tertiary aquifer

system, and Upper Cretaceous aquifer system for the control volume, 

Williston Basin, United States and Canada............................................................................19

10. Graph showing groundwater withdrawals from wells, by aquifer system, 

Williston Basin, United States and Canada............................................................................20

11. Map showing scenario 1 simulated hydraulic-head change from 1961 to 2005 for

the Fox Hills aquifer, Williston Basin, United States and Canada .....................................22

12. Map showing scenario 1 simulated hydraulic-head change from 1961 to 2035 for

the Fox Hills aquifer, Williston Basin, United States and Canada .....................................23

13. Graphs showing scenario 1 simulated and measured hydrographs of hydraulic

head for four monitoring wells open to the Fox Hills aquifer and total flow from

flowing artesian wells, Williston Basin, United States and Canada..................................24

14. Map showing scenario 2 simulated hydraulic-head change from 2005 to 2015 

for the glacial aquifer system (model layer 2), Williston Basin, United States 

and Canada..................................................................................................................................26

15. Map showing scenario 2 simulated hydraulic-head change from 2005 to 2015 

for the upper Fort Union aquifer, Williston Basin, United States and Canada .................27

16. Map showing scenario 3 simulated drawdown of hydraulic head resulting from

groundwater withdrawals for energy-resource production for 2015 in the glacial

aquifer system (model layer 2), Williston Basin, United States and Canada....................30

17. Map showing scenario 3 simulated drawdown of hydraulic head resulting from

groundwater withdrawals for energy-resource production for 2015 in the upper

Fort Union aquifer (model layer 3), Williston Basin, United States and Canada..............31

18. Map showing scenario 3 simulated drawdown of hydraulic head resulting from 

groundwater withdrawals for energy-resource production for 2015 in the middle 

Fort Union hydrogeologic unit (model layer 4), Williston Basin, United States and 

Canada..........................................................................................................................................32
vii

Figures—Continued

19. Graph showing scenario 3 simulated change in the groundwater-flow budget 

from 2005 to 2015 for the active model area, Williston Basin, United States and 

Canada..........................................................................................................................................33

20. Map showing results of data-worth analysis for hydraulic-head forecasts at 

site H6, Williston Basin, United States and Canada .............................................................34

21. Map showing results of data-worth analysis for hydraulic-head forecasts at 

site H2, Williston Basin, United States and Canada .............................................................35

22. Map showing results of data-worth analysis for hydraulic-head forecasts at 

site H1, Williston Basin, United States and Canada .............................................................36

23. Map showing results of data-worth analysis for streamflow forecasts at 

site F3, Williston Basin, United States and Canada..............................................................37

Table

1. Estimated and simulated average groundwater-flow budget for 1981–2005 within a

control volume of the active model area. ..............................................................................15

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

cubic foot (ft3

) 28.32 cubic decimeter (dm3) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3

/s)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

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

foot per year (ft/yr) 0.3048 meter per year (m/yr)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)
viii

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is refereced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum. 
Groundwater Availability of the Williston Basin, 

United States and Canada

Executive Summary

The Williston Basin of the Northern Great Plains is a 

sedimentary basin—a geologic bowl-like structure filled 

with layered sedimentary rocks dating as far back as the 

Paleozoic age. The basin, which is nationally important for 

the production of energy resources, spans Montana, North 

Dakota, and South Dakota in the United States, and Manitoba 

and Saskatchewan in Canada. The three uppermost principal 

aquifer systems are the glacial, lower Tertiary, and Upper 

Cretaceous aquifer systems. As deep as 3,000 feet (ft) at 

the center of the basin, these are the most accessible aquifer 

systems in the basin and are the primary sources of potable 

groundwater in much of this area. The glacial aquifer system 

consists of Quaternary-age unconsolidated till, silt, clay, 

outwash sand and gravel, and occasional cobbles and boulders. 

The lower Tertiary and Upper Cretaceous aquifer systems 

consist primarily of sandstone, siltstone, mudstone, shale, and

coal. 

As energy demands have increased in the basin, 

horizontal drilling and hydraulic-fracturing have been used 

(especially since 2005) to develop previously inaccessible 

formations—namely, the Bakken and Three Forks Formations. 

The basin has yielded a large supply of domestic oil and 

natural gas since the 1950s, but the technologies required to 

extract those materials use large amounts of freshwater. The 

increasing freshwater demands of energy production in the 

Williston Basin, in addition to population growth, have led to 

a need for new tools to assess groundwater resources.

Average annual precipitation ranges from about 6 inches 

in the southwestern part of the Williston Basin to more than 30 

inches in the eastern part, with much less change from north 

to south. The spatial distribution of groundwater recharge 

is consistent with that of precipitation. Average long-term 

precipitation recharge to groundwater (1985–2011) is 10 

inches or less and results partly from snowmelt during spring 

(March–April). Groundwater recharge also results from 

streams losing flow as they infiltrate the ground and from 

reservoir water seeping into the ground. Groundwater loses 

flow to streams and reservoirs by exfiltration through the 

streambed material or reservoir-bottom sediment. Exfiltration 

to streams provides important base flow that sustains streams 

during dry seasons. Groundwater withdrawals are used for 

By Andrew J. Long, Joanna N. Thamke, Kyle W. Davis, and Timothy T. Bartos

agriculture, public supply, industry, and domestic needs. 

Declining groundwater levels resulting from continuously 

flowing artesian wells have occurred in the Upper Cretaceous 

aquifer system. 

The availability of groundwater for human and ecological 

benefit is dependent on accessibility, volume, and the rate of 

replenishment, which are related to other interconnected and 

uncertain factors. These factors include climate variability, 

human use of groundwater and surface water, aquifer 

characteristics, continual changes in groundwater storage, and 

changes in the ease of extracting groundwater. Analyses in 

this report made use of an existing numerical groundwaterflow model that recently was described and documented for 

the Williston Basin. This model was calibrated to temporally 

varying conditions for 1961‒2005 and was designed to 

forecast the outcomes of different hydrologic scenarios, 

examples of which are presented in this report. 

A groundwater-flow budget that quantifies all inflows 

and outflows for the overall groundwater system is a valuable 

starting point for assessment of groundwater availability. 

Comparison of estimated and simulated groundwater-flow 

budgets indicates noteworthy similarities and differences. 

Differences resulting primarily from limitations inherent to 

estimating and simulating regional-scale groundwater flow 

help provide ranges of uncertainty in each flow component. 

The long-term averages for estimated and simulated 

precipitation recharge for the Williston Basin were 26 and 

60 percent, respectively, of total groundwater recharge. 

Estimated and simulated stream infiltration to groundwater 

were 72 and 29 percent, respectively, of all groundwater 

recharge. Estimated and simulated stream exfiltration 

from groundwater into streams were 97 and 90 percent, 

respectively, of all groundwater discharge. 

The lower Tertiary aquifer system has the largest overall 

groundwater-flow budget and the greatest variability of the 

three aquifer systems, whereas the Upper Cretaceous aquifer 

system has the smallest groundwater-flow budget and least 

variability. The Upper Cretaceous aquifer system has, on 

average, a groundwater-flow budget that is less than one-third 

as large as each of the other two aquifer systems but accounts 

for about 70 percent of the total well withdrawals; this largely 

is because flowing artesian wells discharging from the Upper 

Cretaceous aquifer system account for about 50 percent of all 

well withdrawals.
2 Groundwater Availability of the Williston Basin, United States and Canada

The numerical groundwater-flow model of the Williston 

Basin was used to simulate three scenarios. Scenario 1 focused 

on flowing artesian wells, which were simulated as headdependent boundary cells; that is, the rate of well discharge is 

influenced by hydraulic head surrounding the well. The model 

was used to simulate 1960‒2035 hydraulic-head changes that 

would result if none of the flowing artesian wells were capped 

or plugged during this period and other conditions remained 

constant. A conservative steady groundwater pumping rate 

was assumed for 2006‒35, which was the period projected by 

the model because the model was calibrated for 1960‒2005. 

Most of the simulated decline occurred during 1960‒2005 for 

the area surrounding the flowing artesian wells; this drawdown 

area continued to expand through 2035, with simulated 

hydraulic-head declines of as much as 170 ft. 

The total rate of outflow for flowing artesian wells 

increased from 1960 to 1980 to a high of about 61 cubic 

feet per second (ft3/s), as the number of wells increased, and 

then varied between about 57 and 61 ft3/s until 2003. After 

2003, when few additional wells were installed, the simulated 

outflow decreased to about 54 ft3/s in 2035; this decrease in 

outflow was steepest for the 2006 stress period and gradually 

flattened through the remainder of the simulation, which is 

consistent with a flattening of the simulated hydraulic-head 

declines, particularly after 2010. This simulation indicates 

that the installation of flowing artesian wells initiated the 

hydraulic-head declines, which then resulted in decreases 

in outflow rates because of decreasing hydraulic pressure 

within the wells, and that outflows were moving toward an 

equilibrium state with hydraulic head. A second reason for 

the flattening of the simulated hydraulic-head decline was 

that many of the wells ceased to flow. Of the 300 flowing 

artesian wells simulated, 75 wells ceased to flow by the end of 

2035 because hydraulic head had declined below the landsurface. Although scenario 1 indicates decreasing severity 

of hydraulic-head decline for the future, different scenarios 

also should be considered, such as continued increase in 

groundwater pumping rates and drier climatic conditions, 

including long-term drought.

Scenarios 2 and 3 offer two additional examples, 

but many other possibilities could be simulated with the 

groundwater-flow model. Scenario 2 simulated 10-year 

drought for 2006‒15, with no increases in groundwater 

pumping after 2005. For most of the western part of the active 

model area, simulated groundwater declines were small, in 

the range of 0‒1 ft, for the glacial and lower Tertiary aquifer 

systems. Much of the eastern areas show declines in the range 

of 1‒5 ft, with more than 5 ft of decline in many areas and 

more than 200 ft in some parts of the glacial aquifer system. 

These results show the change that followed a period of low 

to moderate precipitation-recharge rates; a drought simulation 

following higher recharge rates would result in much larger 

groundwater declines than those resulting from scenario 

2. Some of the central and western areas show simulated 

groundwater increases, primarily in the range of 0‒1 ft, which 

is a result of spatio-temporal variability of precipitation. 

Climatically wet eastern areas are more susceptible to 

groundwater declines during drought than are the dry western 

areas, and groundwater increases might occur in some western 

areas locally during drought because of spatio-temporal 

variability of precipitation. 

Scenario 3 was identical to scenario 2, except that it also 

applied the increased groundwater withdrawals necessary 

to fill the needs of energy-resource production (ERP) for 

2006‒15. The freshwater used for ERP was estimated by 

two separate methods. The first method focused strictly on 

the estimated total freshwater needs for ERP in the Williston 

Basin within the United States. The second method focused 

on groundwater-withdrawal data obtained from water-use 

permits. Results of the two methods were consistent, which 

reduced uncertainty of these estimates. A combination of 

groundwater and surface water was used to supply the needs 

of ERP; however, if an extended drought occurred during this 

period, low streamflows and emptying of ephemeral lakes 

and ponds might have caused less surface-water availability. 

Therefore, the purpose of this scenario was to estimate the 

effects on the groundwater supply if ERP water demands were 

totally supplied by groundwater during a drought. 

For scenario 3, the simulated drawdown was less than 

25 ft. Most of the pumping wells added were in the glacial and 

Fort Union aquifer systems, and areas of drawdown coincide 

with these well locations. Much of the simulated drawdown 

occurred within areas of the Little Muddy aquifer, which is 

a sand and gravel aquifer within the glacial aquifer system; 

numerous permitted wells added in scenario 3 withdraw water 

from this aquifer. The total area affected by pumping wells 

added in scenario 3 is small but affects large parts of local 

areas. The simulated change from 2005 to 2015 included a 

decrease in precipitation recharge of 64 percent, which was 

the primary forcing that caused a 28 percent decrease in the 

net outflow to surface water. Simulated pumping-well outflow 

increased by 34 percent, and flowing-well outflow decreased 

by 4 percent. 

Numerical groundwater-flow models are used to inform 

water-management decisions, and a useful model application 

is to design a monitoring network that would provide data 

that best improves the predictive ability of a model through 

reduction of uncertainty. Uncertainty of simulated hydrologic 

forecasts, or “predictive uncertainty,” can be reduced by 

providing additional data for model calibration. If the new 

data were to come from a new observation well, for example, 

the degree of reduced uncertainty would depend on the 

specific location of the new well. One way to determine the 

best locations for new observation wells is to apply a method 

referred to as a “data-worth analysis.” Examples of how the 

numerical model can be used to assist in selecting locations for 

new observation wells were applied and described.
Introduction 3

Introduction

Purpose and Scope

The increasing freshwater demands of energy 

production in the Williston Basin necessitate new tools to 

assess groundwater resources. Because of the importance of 

groundwater in this energy-rich area, the U.S. Geological 

Survey (USGS) began a study in 2011 to assess and provide 

better tools that can be used to manage groundwater 

availability in the Williston Basin. The purpose of this report 

is to describe the results of that study. Specifically, this 

report describes (1) the current groundwater availability of 

the three uppermost principal aquifer systems—the glacial, 

lower Tertiary, and the Upper Cretaceous—in the Williston 

Basin, (2) how these resources have changed over time, and 

(3) tools to assess system response to anthropogenic stresses 

and climate variability. The scope of this report is focused on 

the Williston Basin and summarizes previous publications 

that resulted from this study describing a hydrogeologic 

framework, a conceptual model and groundwater-flow budget, 

and a numerical model of groundwater flow (Long and others, 

2014; Thamke and others, 2014; Davis and Long, 2018a). 

This report presents hydrologic forecasts simulated with this 

numerical model to assess the potential for drought, pumped 

groundwater, and free-flowing artesian wells to affect future 

groundwater availability. An example of the use of this model 

to help determine high-value locations to establish new 

hydrologic-monitoring stations also is presented. The study 

described in this report has benefitted from numerous studies 

and publications dating from the 1960s until present, most of 

which were summarized by Long and others (2014), Thamke 

and others (2014), and Davis and Long (2018a). A brief 

summary of the key findings from this report is in Thamke and 

others (2018).

This report is one in a series of reports summarizing the 

findings of USGS assessments of groundwater availability 

of principal aquifers throughout the United States. These 

include assessments of the Middle Rio Grande Basin, New 

Mexico (Bartolino and Cole, 2002); the Southwest Alluvial 

Basins (Tillman and others, 2008); the Central Valley Aquifer, 

California (Faunt, 2009); the Atlantic Coastal Plain of North 

and South Carolina (Campbell and Coes, 2010); the Great 

Basin Carbonate and Alluvial Aquifer System (Heilweil and 

Brooks, 2010); the U.S. Great Lakes Basin (Reeves, 2010); 

the Denver Basin Aquifer System, Colorado (Paschke, 2011); 

the Mississippi Embayment (Clark and others, 2011); the 

Columbia Plateau Regional Aquifer System (Vaccaro and 

others, 2015); the North Atlantic Coastal Plain Aquifer System

(Masterson and others, 2016); and the Glacial Aquifer System 

of the United States (Reeves and others, 2017). A map of 

principal aquifers of the United States is accessible from U.S. 

Geological Survey (2018).

Description of Study Area

The Williston Basin is a sedimentary basin—a geologic 

bowl-like structure filled with layered sedimentary rocks. 

The basin is as deep as 10,000 feet (ft) and consists of 

Paleozoic, Mesozoic, and Cenozoic-age formations that 

overlie Precambrian-age rocks (Sandberg, 1962; Downey 

and Dinwiddie, 1988; Dolton and others, 1990). The basin is 

nationally important for the production of energy resources, 

spanning Montana, North Dakota, and South Dakota in the 

United States, and Manitoba and Saskatchewan in Canada 

(Thamke and others, 2014). The three uppermost principal 

aquifer systems in the basin are the glacial, lower Tertiary, 

and Upper Cretaceous. They are the most accessible aquifer 

systems in the basin and are the primary sources of potable 

groundwater in much of the area (fig. 1). The glacial aquifer 

system, present in the northern and eastern parts of the 

Williston Basin, is composed primarily of glacial drift. 

The lower Tertiary and Upper Cretaceous aquifer systems, 

throughout most of the basin, are composed of sedimentary 

rocks including shale, silt, sandstone, and coal. In the 

Williston Basin, these three aquifer systems are as deep 

as 2,850 ft below land surface and overlie 800–2,000 ft of 

relatively impermeable Upper Cretaceous marine shale that 

serves as a basal confining unit that impedes groundwater flow 

(Anna, 1986; Downey, 1986; Downey and Dinwiddie, 1988; 

Thamke and others, 2014). This report provides a broad-scale 

assessment of the complex topic of groundwater availability in 

the Williston Basin and describes tools that can be applied to 

help manage this resource.

Energy Resources and Water Use

The lower Tertiary and Upper Cretaceous aquifer systems 

extend into the Powder River Basin of Montana and Wyoming 

(fig. 1) and are part of the Northern Great Plains aquifer 

system (Whitehead, 1996). This larger aquifer system consists 

of all bedrock aquifers in North and South Dakota, eastern 

Montana, and northeastern Wyoming. Whitehead (1996) 

did not describe a Canadian extent of this aquifer system. In 

the United States, the Northern Great Plains aquifer system 

provides 101 million gallons per day (Mgal/d) of water to 

irrigation (66.6 Mgal/d), public supply (33 Mgal/d), and selfsupplied industrial use (1.62 Mgal/d) (Maupin and Barber, 

2005). Most of this water is obtained from the lower Tertiary 

and Upper Cretaceous aquifer systems (Wesolowski, 1991). 
4 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig01

F

F

F

A

A′

Nesson anticline

POWDER

RIVER

BASIN

WILLISTON

BASIN

LFU

UFU

UHC

MFU

UHC

LHC

LFU

LFU

LHC

MFU

UFU

FH

UFU

FH

FH

MFU

LFU

UFU

Miles City arch

Cedar Creek anticline

CANADA

UNITED STATES

MANIT

O

B

A

SASKAT

C

H

E

WAN

NORTH DAKOTA

SOUTH DAKOTA

MONTANA

WYOMING

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

Bismarck

Williston

Regina

Miles City

Jame sRiver

Little Missouri River

Missouri River

Big Muddy Creek

Cheyenne River

O'Fallon

Creek

Boxelder Creek

Rosebud Creek

Grand River

Cedar Creek

Knife River

Tong ue River

Souris River

Heart River

Moreau River

Milk

River

Cannonball River

PowderRiver

Yellowstone River

Littl e Missouri River Redwater Rive r

Yellowstone River

South S askatchewan River

Big MuddyC reek

Bighorn

River

Big Coulee

Little Muddy River

Moose Mountain Creek

Moose Jaw River

Souris River

SourisRiver

As si niboineRiver

Qu'Appelle River

Jame sRiver

Little Missouri River

Missouri River

Cheyenne River

O'Fallon

Creek

Boxelder Creek

Rosebud Creek

Grand River

Cedar Creek

Knife River

Tong ue River

Souris River

Heart River

Moreau River

River

PowderRiver

Yellowstone River

Big Coulee

Missouri River

South S askatchewan River

Frenchma n River

Bighorn

River

Moose Mountain Creek

Moose Jaw River

Souris River

SourisRiver

As si niboineRiver

Qu'Appelle River

Lake Oahe

Lake Fort

Peck

Lake

Sakakawea

Old

Wives

Lake

0 30 60 90 120 KILOMETERS

0 30 60 90 120 MILES

108° 106° 104° 102° 100°

50°

48°

46°

F

Upper Cretaceous

aquifer system

Lower Tertiary

aquifer system

Fox Hills aquifer

Lower Hell Creek aquifer

Upper Hell Creek hydrogeologic unit

Lower Fort Union aquifer

Middle Fort Union hydrogeologic unit

Upper Fort Union aquifer

Hydrogeologic units 

modified from Thamke and others (2014)

Glacial aquifer system

Anticline or arch

Active model area

EXPLANATION

Geologic structure

FH

LHC

UHC

LFU

MFU

UFU

Williston Basin

(Hamke and 

others, 1966)

Map

area

Lower Tertiary and

Upper Cretaceous 

aquifer systems

Figure 1. Location of the Williston Basin and hydrogeologic unit layers, United States and Canada. Cross section A–A' is 

shown in figure 4. Hydrogeologic units and geologic structures modified from Thamke and others (2014).
Introduction 5

Since 2005, as energy demands have increased, 

horizontal drilling and hydraulic-fracturing methods have 

been used to develop previously inaccessible formations 

in the Williston Basin—namely, the Bakken and Three 

Forks Formations (Gaswirth and others, 2013; U.S. Energy 

Information Administration, 2013). The Williston Basin has 

yielded a large supply of domestic oil and natural gas since the 

1950s, but the technologies required to extract those materials 

use large amounts of freshwater, with the largest increases 

occurring since 2005 (Schuh, 2010; Anna and others, 2011; 

Haines and others, 2017; Barnhart and others, 2018). Water 

use for hydraulic-fracturing per oil well increased by about 

six times from 2005 to 2014, totaling 24.5 × 109 gallons for 

about 10,140 wells (Scanlon and others, 2016). The increased 

demand for freshwater resulted in an increase of water 

pipelines by hundreds of miles and water-permit transfers 

from irrigation to oil production (Scanlon and others, 2016). 

Geologic units of early Tertiary and Late Cretaceous age 

contain most of the nation’s reserves of coal, in the form of 

lignite and coalbed natural gas (Bluemle, 1998). Strip mining 

of coal consists of the removal of large volumes of host rock, 

thereby affecting aquifers, and coal bed natural gas extraction 

requires the removal of large volumes of groundwater from 

coal beds to release stored gases (Thamke and others, 2014). 

Continued development in the region, including alternative 

energy, industry, irrigation, and growing demands for domestic 

and municipal water in the Northern Great Plains, depends on 

the quantity and quality of groundwater available from these 

shallow and accessible aquifers. 

Physiography and Climate

The study area includes the Williston Basin (fig. 1), the 

extent of which is not precisely defined but has been estimated 

by Hamke and others (1966), Pitman and others (2001), and 

Pollastro and others (2013). Topography in the study area 

is characterized by relatively low relief, except near large 

river channels, with a gently rolling land surface underlain 

by glacial drift and sedimentary rocks composed primarily 

of sandstone, coal, and shale. Large river systems such as the 

Missouri and Yellowstone Rivers erode the relatively soft 

sedimentary rocks and create several hundred feet of local 

topographic relief. 

Surface-water resources in the Williston Basin include 

rivers, streams, lakes, and wetland ponds. In this area, the 

Missouri River flows toward the east and southeast, with the 

Yellowstone and Little Missouri Rivers entering from the 

south (fig. 1). The study area includes three large surfacewater reservoirs: Fort Peck Lake, Lake Sakakawea, and Lake 

Oahe, all located along the Missouri River (fig. 1). Several 

other tributaries in the southeastern part of the Williston 

Basin flow easterly and enter the Missouri River from the 

west. Numerous natural lakes and wetland ponds are scattered 

across the landscape in glacial deposits north of the Missouri 

River. Much of this area is characterized by a non-integrated 

drainage pattern, where few streams cross. Surface water is 

heavily appropriated in most of the study area (Schuh, 2010), 

and this water supply is not always dependable in upper 

ephemeral stream reaches.

The climate is semiarid, with monthly precipitation 

exceeding monthly potential evapotranspiration by less than 

5 inches per year (in/yr) (Reilly and others, 2008). Within 

the study area, precipitation ranges from 6 in/yr in the 

southwestern part to more than 30 in/yr in the eastern part 

(Thornton and others, 1997, 2012; Long and others, 2014). 

Pasture and hayland covers 70 percent of the study area 

(Multi-Resolution Land Characteristics Consortium, 2011). 

Except for a few towns, population density generally is less 

than 10 people per square mile (Statistics Canada, 2001; 

U.S. Census Bureau, 2001).

Hydrogeologic Framework

Available groundwater resources used in the Williston 

Basin in the United States and Canada primarily are in three 

uppermost principal aquifer systems composed of Late 

Cretaceous, Tertiary, and Quaternary-age lithostratigraphic 

units (fig. 2). Thamke and others (2014) constructed a 

three-dimensional hydrogeologic framework that defined 

and described these three aquifer systems in the Williston 

Basin for the United States and Canada, with emphasis on 

the lower Tertiary and Upper Cretaceous bedrock aquifer 

systems. Davis and Long (2018a) further developed the 

hydrogeologic framework for the glacial aquifer system by 

spatially defining five glacial-material zones and extending 

this analysis to the edges of the active model area (fig. 1). This 

section summarizes the hydrogeologic framework described 

by Thamke and others (2014) and Davis and Long (2018a), 

which include greater detail of the lithostratigraphic and 

hydrogeologic units. 

From shallowest (youngest) to deepest (oldest), the 

three principal aquifer systems are the glacial, lower Tertiary, 

and Upper Cretaceous aquifer systems, with the latter two 

contained within bedrock (fig. 2). These three principal aquifer 

systems are hydraulically separated from deeper aquifers by 

a basal confining unit composed primarily of 800–3,000 ft 

of low-permeability marine shale of Upper Cretaceous age; 

the altitude of the top is shown in figure 3. The bowl-shaped 

structure and layered geology of the Williston Basin results 

in bedrock hydrogeologic units that are exposed to the land 

surface or are in contact with the overlying glacial aquifer 

system near the outer parts of the basin (figs. 1, 4). The 

glacial aquifer system overlies parts of the lower Tertiary and 

Upper Cretaceous aquifer systems in the northern part of the 

Williston Basin (fig. 1).
6 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig02

Diagram is not to scale relative to geologic time and stratigraphic thickness.

FmMbr

Boissevain FormationjBoissevain Formationj

Canada

Ravenscrag Formationj

Ravenscrag Formationj,m

Ravenscrag Formationj

Ravenscrag Formationj

aAge estimates of boundaries in mega-annum (Ma, million years) before present. Age estimates are boundary 

estimates from the International Commission on Stratigraphy (Cohen and others, 2017).

bSpatial distribution of lithostratigraphic units from Bluemle (1983); Love and Christiansen (1985); Macdonald 

and Slimmon (1999); Martin and others (2004); Vuke and others (2007); Murphy and others (2009); and Nicolas and others (2010).

c Modified from Whitehead (1996). dGenerally located north of the Missouri River.e Includes locally occuring alluvium where both types of deposits are present.f Includes upper Tertiary units (Pliocene, Miocene, and Oligocene) that exist only in a small part of study area.g International stratigraphic community has replaced the term “Tertiary” as a system/period name with, in ascending 

order, the Paleogene Period (Paleocene-Oligocene Series/Epoch) and the Neogene Period (Miocene and Pliocene Series/Epochs) (Cohen and others, 2017). The lithostratigraphic units of interest for this study are Paleogene in age, but the use of “Tertiary” is retained herein because of widespread use in key hydrogeologic literature for the study area.

h This name applies only in the United States.

i Present in central part of Williston Basin and a small contiguous area in Montana. Eocene and Paleocene Golden 

Valley Formation also is present in local areas.

j Divided formation to correspond to hydrogeologic units in this regional study.k Pinches out north of the Missouri River. Mapped as Tongue River Formation in South Dakota by Martin and others 

(2004).

l Cannonball Member present in the southern and eastern parts of the Williston Basin. Slope Member is above the 

Cannonball Member in southern North Dakota. Ludlow Member is present in the southern part of North Dakota.

mLower Ravenscrag Formation is part of the Upper Hell Creek hydrogeologic unit but is part of the Tertiary System.nAlluvium added to shallowest aquifer/aquifer system where glacial deposits are absent.oAlso known as Fox Hills Formation.

Till Glacial aquifer systeme

r eppU suoecat er C met sys r efi uqa n

Golden Valley Fm

Basal confining

unit Basal confining unit Pierre Formation Pierre Formation Bearpaw Formation Bearpaw Formation Pierre Shale Pierre Shale Bearpaw Shale Bearpaw Shale

Pierre Formation

Coulter Mbr Fox Hills aquifer Eastend Formation Fox Hills Sandstoneo

Montana Grouph

Lower Hell Creek

aquifer Frenchman Formationj Hell Creek Formation (lower part)

Upper Hell Creek

hydrogeologic unit Boissevain Formationj Frenchman Formationj Hell Creek Formation (upper part)

s (part) uoecat er Cr eppU

(66–100.5 Ma)

Cenozoic Mesozoic (part)

Erathem/

era

s (part) uoecat er C

Lower Fort Union aquifer Turtle Mountain Formation Cannonball and Ludlow

Members

l Ludlow and Tullock Members

Middle Fort Union

hydrogeologic unit Lebo Shale Member (Slope Fmk

) Lebo Shale Member

Tongue River Member (Bullion Creek Formation) Tongue River Member e necoel aP

(56-66 Ma)

Sentinel Butte Memberi

ormation F noi nUtr oF h

yr aitr eTr e woL

aquifer systemn

Upper Fort Union aquifer 

y (Paleogene) r aitr eT f,g

Glacial

aquifers

Glacial depositse Glacial depositsd, e

Manitoba Saskatchewan South Dakota North Dakota Montana

United States

Principal aquifer systemc Hydrogeologic unit 

Lithostratigraphic units 

Generalized spatial distribution of lithostratigraphic unit within Williston Basinb

Group/ formation

Series/

Epoch (age estimate, in Ma)a System/ period

Holocene 

and Pleistocene 

(0–2.6 Ma)

Quaternary

Eocenef

(33.9–56 Ma)

Bold lines separate aquifer systems, and colors separate hydrogeologic units. Some lithostratigraphic units are split between two or more hydrogeologic units, as indicated by the colors.

EXPLANATION

Gray shading indicates unit is absent 

due to erosion or nondeposition.

FormationMember

Figure 2. Lithostratigraphic and corresponding hydrogeologic units of the Williston Basin, United States and Canada (modified from Thamke and others, 2014).
Introduction 7

tac18-1237_fig03

500 750

1,000

1,250

1,500

1,750

2,250 2,000

2,500

2,750

1,500

2,750

Upper Cretaceous

aquifer system

Lower Tertiary

aquifer system

0

Altitude, in feet above 

North American Vertical Datum of 1988

-6,000

-3,000

3,000

6,000

9,000

Glacial aquifer system

Upper Fort Union aquifer

Middle Fort Union hydrogeologic unit

Lower Fort Union aquifer

Upper Hell Creek hydrogeologic unit

Lower Hell Creek aquifer (combined with Fox Hills

in the Powder River basin) 

Fox Hills aquifer (combined with the lower Hell

Creek in the Powder River basin) 

POWDER RIVER

BASIN

WILLISTON

BASIN

100° 102°30’ 105° 107°30’

47°30’

45°

VERTICAL EXAGGERATION 20

EXPLANATION

Hydrogeologic units Structure contour—Shows altitude of top of basal 

confining unit. Contour interval 250 feet. Datum 

is North American Vertical Datum of 1988 

(NAVD 88)

×

Figure 3. Three-dimensional view of the top of the basal confining unit in the Williston and Powder River Basins, underlying the Upper 

Cretaceous aquifer system, United States and Canada. Selected hydrogeologic units are shown, with selected hydrogeologic cross 

sections (from Thamke and others, 2014).
8 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig04

Upper Cretaceous

aquifer system

Lower Tertiary

aquifer system

FH

LHC

UHC

LFU

MFU

UFU

Fox Hills aquifer

Lower Hell Creek aquifer

Upper Hell Creek hydrogeologic unit

Lower Fort Union aquifer

Middle Fort Union hydrogeologic unit

Upper Fort Union aquifer

Glacial aquifer system

5,000

4,500

4,000

3,500

3,000

2,500

2,000

1,500

1,000

500

0

Altitude, in feet above

North American Vertical Datum of 1988

Cedar Creek anticline

Nesson anticline

Miles City arch

FH

LHC

UHC

LFU

MFU

UFU

WILLISTON BASIN POWDER RIVER BASIN

A A′

0 50 7525 100 KILOMETERS

0 25 50 75 100 MILES

NORTHEAST

VERTICAL SCALE IS 145X THE HORIZONTAL SCALE

SOUTHWEST

Lake Sakakawea

Hydrogeologic units

EXPLANATION

Figure 4. Hydrogeologic cross section A–A' showing the glacial, lower Tertiary, and Upper Cretaceous aquifer systems in the 

Williston Basin (from Long and others, 2014), United States and Canada. Location of cross section A–A' is shown in figure 1. 

The glacial aquifer system consists of Quaternaryage unconsolidated till, silt, clay, outwash sand and gravel, 

and occasional cobbles and boulders. Reeves and others 

(2017) provided a generalized hydrogeologic framework 

and groundwater budget for the glacial aquifer system of 

the United States. Estimated volume of the glacial aquifer 

system in the study area is 150 trillion cubic feet (ft3; Thamke 

and others, 2014). Widely varying lithologic characteristics 

result in an aquifer system that is characterized by multiple 

disconnected, locally productive sand and gravel aquifers 

buried beneath till and other glacial or surficial deposits and 

typically located in pre-glacial valleys that exist on the buried 

bedrock surface (Kehew and Boettger, 1986; Cummings and 

others, 2012; Pugin and others, 2014; Thamke and others, 

2014). Thickness of the glacial aquifer system in the Williston 

Basin varies locally; maximum thickness is estimated to 

be about 750 ft and thickest in northern North Dakota and 

Canada (Thamke and others, 2014). Hydraulic characteristics 

of the glacial aquifer system vary widely because of highly 

variable lithology. Hydraulic conductivity values estimated by 

Thamke and others (2014) for the glacial aquifer system range 

from 0.01 to 25 feet per day, varying more than for the lower 

Tertiary and Upper Cretaceous aquifer systems. 

Davis and Long (2018a) separated the glacial aquifer 

system into two layers and subdivided these layers 

horizontally to represent different materials present in the 

glacial drift on the basis of geologic maps, aquifer maps, 

borehole logs, and locations of buried pre-glacial valleys. 

The upper layer was subdivided into three material zones: 

(1) low-permeability material consisting of till; (2) mediumpermeability material consisting of glaciolacustrine and 

glaciotectonic deposits; and (3) high-permeability material 

consisting of glaciofluvial, loess, and eolian deposits. 

The lower layer was subdivided into two material zones: 

(1) low-permeability material consisting of till and (2) highpermeability material consisting of sand and gravel that filled 

pre-glacial valleys. These valley-fill aquifers may be covered 

by different materials of low, medium, or high permeability. 

The spatial distribution of these five glacial-material zones is 

shown in Davis and Long (2018a). Additional background on 

glacial geomorphology is described in Aber and others (1989), 

Benn and Evans (1998), and Hart and Boulton (1991). Other 

geologic and hydrologic terms are described in Bates and 

Jackson (1984) and Wilson and Moore (1989). 
Introduction 9

The remainder of this section is summarized from 

Thamke and others (2014). The lower Tertiary aquifer 

system in the Williston Basin consists of the upper and 

lower Fort Union aquifers separated by the middle Fort 

Union hydrogeologic unit (fig. 2). The lower Tertiary aquifer 

system is as thick as 2,250 ft, and estimated volume is 

1,000 trillion ft3 in the Williston Basin. Rocks composing the 

lower Tertiary aquifer system represent many depositional 

environments, most commonly continental depositional 

environments (fluvial, deltaic, tidal, and barrier-shoreface) and 

less commonly marine depositional environments (Flores and 

Bader, 1999).

The upper Fort Union aquifer is as thick as 1,920 ft and 

composed of crossbedded light-yellow to light-yellow-gray 

sandstone, sandy mudstone, gray shale, carbonaceous shale, 

and thick coal beds and associated clinker deposits (permeable 

rocks created by the natural burning of coal beds). Thickness 

of the upper Fort Union aquifer is greatest in the central part 

of the Williston Basin and thins at the edges where erosion has 

removed most of the unit (fig. 4).

Present throughout the central part of the Williston Basin, 

the middle Fort Union hydrogeologic unit thins toward the 

northeast and is not present in the northeastern one-third of the 

basin (fig. 4). Composed of as much as 520 ft of thickness of 

alternating beds of sandstone, siltstone, mudstone, claystone, 

and lignite, rocks in the middle Fort Union hydrogeologic 

unit generally are finer-grained and darker-colored than the 

overlying upper Fort Union aquifer and underlying lower Fort 

Union aquifer. Because of spatially variable lithology, the 

middle Fort Union hydrogeologic unit may act as a confining 

unit in some areas and as an aquifer in other areas. The lower 

Fort Union aquifer is composed of as much as 670 ft of 

thickness of yellow-weathering sandstones and light-grayweathering sandy mudstones interfingering with alternating 

brown and gray beds of sandstone, siltstone, claystone, 

mudstone, and lignite deposited in continental and marine 

environments.

The Upper Cretaceous aquifer system is the deepest 

and most areally extensive of the three aquifer systems 

and consists of, from top to bottom, the upper Hell Creek 

hydrogeologic unit, lower Hell Creek aquifer, and the Fox 

Hills aquifer (fig. 2). The volume of this aquifer system is 

about 1,000 trillion ft3 in the Williston Basin. The upper 

Hell Creek hydrogeologic unit is composed of as much as 

740 ft of thickness of alternating layers of gray and brown 

mudstone, siltstone, sandstone, and sparse lignite beds 

deposited by meandering streams with point bars and channel 

plugs. Because of spatial variability, this lithology may act 

as a confining unit in some areas and as an aquifer in other 

areas. The general lithology of the lower Hell Creek aquifer 

is similar to the upper Hell Creek hydrogeologic unit, except 

that the latter has a smaller percentage of sandstone. Streamchannel deposits and erosional surfaces are common in the 

lower Hell Creek aquifer (Flores, 1992), with a maximum 

thickness of as much as 550 ft in the Williston Basin. The 

Fox Hills aquifer is the most areally extensive of the units, 

with as much as 420 ft of thickness of interbedded sandstone, 

siltstone, and mudstone.

Modeling

Conceptual Model of Groundwater Flow

Average long-term precipitation recharge (1985–2011) 

for the study area ranges from 0 to about 10 in/yr (Long and 

others, 2014), generally with low values in the west and 

high values in the east. Precipitation in the form of snow 

primarily is stored during winter and infiltrates during spring 

melting periods. Groundwater recharge in the study area also 

results from stream reaches that lose flow as they infiltrate 

the ground and reservoir water that potentially seeps into 

the ground (Whitehead, 1996; Aurand, 2013; Bednar, 2013; 

Long and others, 2014). Groundwater loses flow to streams 

and reservoirs by exfiltration through the streambed material 

or reservoir sediments (Long and others, 2014). Exfiltration 

to streams provides important base flow that sustains many 

streams during dry seasons (fig. 5). 

Long and others (2014) estimated the groundwater-flow 

budget, or balance of total groundwater inflows and outflows, 

for a control volume within the study area; this control volume 

was defined as a volume of the earth consisting of the lower 

Tertiary and Upper Cretaceous aquifer systems and the glacial 

aquifer system directly overlying these bedrock aquifer 

systems in the Williston Basin. Davis and Long (2018a) 

modified this control volume slightly near the Miles City arch 

(fig. 1). The base of the Upper Cretaceous aquifer system 

within the active model area (fig. 1) defines the horizontal 

extent and bottom of the control volume, and the land surface 

defines the top. 

The glacial aquifer system contains productive buried 

sand and gravel aquifers that are the source of water for 

thousands of shallow wells (Whitehead, 1996; Long and 

others, 2014). The glacial aquifer system has a wide range of 

hydraulic conductivities and is characterized by disconnected 

local flow systems. The underlying bedrock aquifers 

commonly are under confined or partially confined conditions, 

except along the basin margins or in the shallowest aquifers 

(Long and others, 2014). Groundwater flow is exchanged, 

in both directions, between the glacial aquifers system and 

the underlying bedrock aquifers (Davis and Long, 2018a). 

Groundwater flow in the study area, particularly in the deeper 

aquifers, generally is from west and southwest to east, where 

discharge to streams and springs occurs; in the shallower 

aquifers, however, groundwater flow largely is controlled by 

land-surface topography, with steeper potentiometric surfaces 

than in the underlying aquifers (Long and others, 2014). 

Further descriptions of groundwater flow characteristics are in 

section, “Comparison of Conceptual and Numerical Models.”
10 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig05

Basal confining unit

(thickness not to scale)

 Nesson 

anticline

Upper Cretaceous

aquifer system

Lower Tertiary

aquifer system

Fox Hills aquifer

Lower Hell Creek aquifer

Upper Hell Creek hydrogeologic unit

Lower Fort Union aquifer

Middle Fort Union hydrogeologic unit

Upper Fort Union aquifer

FH

LHC

UHC

LFU

MFU

UFU

Glacial aquifer system, showing sand and 

gravel aquifers within large areas of till

Cedar Creek anticline

FH

LHC

UHC

LFU

MFU

UFU

NORTHEAST

SOUTHWEST

Souris River

Lake Sakakawea

Missouri River

Yellowstone River

Hydrogeologic units

EXPLANATION

Groundwater-flow direction

Figure 5. Groundwater flow in the Williston Basin, United States and Canada (modified from Long and others, 2014).

Declining groundwater levels resulting from continuously 

flowing artesian wells have occurred in the Fox Hills and 

lower Hell Creek aquifers; these wells, installed for domestic 

and agricultural use, flow continuously because of hydrostatic 

aquifer pressure (Fischer, 2013; Montana Department of 

Natural Resources and Conservation, 2014; North Dakota 

State Water Commission, 2015). Fischer (2013) described 

521 flowing artesian wells in North Dakota that are open to 

the Fox Hills and lower Hell Creek aquifers, primarily near 

the Yellowstone, Little Missouri, and Knife Rivers (fig. 1). 

Flowing artesian wells may have, in part, caused groundwaterlevel declines of about 1 foot per year (ft/yr) since the 1970s 

in the Fox Hills and lower Hell Creek aquifers near the 

Yellowstone River (Smith and others, 2000; Fischer, 2013). 

Collectively, these wells were estimated to have a total flow 

rate of about 1.6 ft3/s in 2008 on the basis of direct flow 

measurements from a subset of the wells (Fischer, 2013). 

However, because of uncertainty of this estimate, Fischer 

(2013) applied numerical modeling to improve the estimate 

and to better estimate the discharge from individual wells, 

including unmeasured wells. The total simulated withdrawal 

for all pumped and flowing artesian wells combined was in 

the range of 3–6 times higher than that originally estimated 

by Fischer (2013) for 1980‒2008. Because most of the 

uncertainty in groundwater withdrawal is associated with the 

flowing artesian wells, numerical modeling indicates that the 

original estimate of 1.6 ft3/s probably was low. 
Assessing Groundwater Availability 11

Summary of Numerical Modeling

Two calibrated numerical groundwater-flow models 

were described by Davis and Long (2018a), and the digital 

files associated with the models are available in Davis and 

Long (2018b). The first model represents average hydrologic 

conditions for 1981‒2005 executed in steady-state mode 

(Harbaugh, 2005) and is referred to as the “steady-state 

model” in this report. The second model was calibrated to 

temporally varying conditions for 1961‒2005 and is referred 

to as the “transient model” in this report. The main purpose 

of the steady-state model was to provide a structure and 

starting point for the transient model. The transient model was 

designed to be a tool for forecasting the outcomes of different 

hydrologic scenarios. Thamke and others (2014) provided the 

bedrock hydrogeologic framework and initial (uncalibrated) 

aquifer-property values that were used in numerical modeling. 

Long and others (2014) described a conceptual model of 

groundwater flow, including a groundwater-flow budget, that 

provided a basis for numerical modeling. Another purpose 

of the steady-state model was to test this conceptual model 

and to calibrate many of the adjustable parameters, such as 

hydraulic conductivity, the values of which were then used in 

the transient model (Davis and Long, 2018a).

The transient model is not fully continuous with respect 

to time but rather represents distinct blocks of time called 

“stress periods,” within which all specified model inflows 

are constant. All model output in this report represents the 

end of each stress period. The transient model consists of 

an initial steady-state stress period to simulate a long-term 

period prior to 1961 when groundwater withdrawals were 

small; this is referred to as the “predevelopment period” in this 

report. Following this predevelopment stress period are four 

5-year stress periods representing calendar years 1961–65, 

1966–70, 1971–75, and 1976–80, which are followed by 25 

annual stress periods representing each of the calendar years, 

1981– 2005. This transient model was used in this study to 

simulate groundwater responses to hydrologic scenarios 

involving flowing artesian wells, drought, and increased 

groundwater withdrawal, which are described in sections that 

follow. These scenarios were simulated by adding additional 

stress periods to the transient model and thereby extending the 

simulation period through 2035.

The steady-state and transient models had identical 

spatial dimensions and grid layout (Davis and Long, 2018a). 

The model grid in the horizontal dimension consisted of 

square cells, about 1 mile (mi) (1,600 meters) on each side, 

with a total of about 657,000 cells in the active model area. 

The models consisted of eight layers: two layers representing 

the glacial aquifer system (layers 1 and 2) and six layers 

representing the six underlying bedrock hydrogeologic units 

(layers 3–8) (figs. 4, 5).

Assessing Groundwater Availability

Groundwater availability of a regional groundwater 

system is complex, dependent of many interconnected and 

uncertain factors. These factors include climate variability, 

human use of groundwater, complex aquifer characteristics, 

continual changes in groundwater storage, and changes in the 

ease of extracting groundwater. For example, aquifers that are 

highly productive but small in volume are most vulnerable 

to severe dewatering if local water demands are high. The 

groundwater stored in a regional aquifer overall might be 

minimally affected by local groundwater withdrawal but 

heavily affected locally in the withdrawal area. If the saturated 

zone of an unconfined aquifer is several hundred feet thick, a 

lowered water table might cause minimal change as a fraction 

of total groundwater storage but leads to other problems, 

such as (1) reduced streamflow and lake levels because of 

reduced groundwater input; (2) increased difficulty extracting 

groundwater from the deepening source; (3) possible 

decreased water quality at deeper levels; and (4) possible 

aquifer compaction resulting from reduced hydrostatic 

pressure in the aquifer’s structural matrix (Anderson and 

Woosley, 2005; Barlow and Leake, 2012). Confined aquifers 

are affected in similar ways. The total volume of groundwater 

stored in a confined or unconfined aquifer may be of interest 

to some water managers but provides little information 

that is useful for managing groundwater because of the 

aforementioned problems.

A groundwater-flow budget that quantifies all inflows 

and outflows for the overall groundwater system is a valuable 

starting point to assess groundwater availability for human 

and ecological needs. The change in groundwater storage that 

results from changing inflows and outflow is a useful metric to 

assess the long-term sustainability of an aquifer, and a monthly 

or annual groundwater-flow budget provides an estimate of 

this metric. A groundwater-flow budget also is useful for 

comparing the relative flow magnitudes of different inflows 

and outflows. Because natural inflows to an aquifer generally 

are balanced by natural outflows, consumptive groundwater 

use ideally should be small in comparison to total groundwater 

recharge for long-term sustainability.

Quantification of the hydrogeologic framework defines 

the properties and characteristics of an aquifer system and can 

be used, along with a groundwater-flow budget, to develop 

a conceptual understanding, or model, of groundwater flow. 

The hydrogeologic framework, groundwater-flow budget, and 

conceptual model described by Long and others (2014) and 

Thamke and others (2014) were used to construct a numerical 

model of groundwater flow (Davis and Long, 2018a). To 

assess groundwater availability in the Williston Basin, this 

model was used to assess changes in groundwater storage and 

groundwater levels that potentially would result from human 

activity and climate variability.
12 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig06

Richardton Abbey

Willow City

Dupree

Glendive

Crosby

Regina

Glasgow

Bismarck

Simulated precipitation recharge

Precipitation

A

B

Precipitation, in inches

1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015

50

25

0

−25

−50

−75

5.0

2.5

0.0

−2.5

−5.0

−7.5

Recharge, in inches

50

25

0

−25

−50

−75

Climatically

dry period

Figure 6. Annual cumulative departure from the mean for (A) precipitation and simulated 

precipitation recharge as a spatial average within the active model area (Davis and Long, 

2018b), and (B) precipitation for selected weather stations. Climatically dry periods are 

shown in gray. Weather station locations are shown in figure 7. 
Analysis of Precipitation and Recharge 13

Analysis of Precipitation and Recharge

An analysis of precipitation patterns indicates that 

climate within the study area is characterized by large spatial 

and temporal variation. An analysis of the annual cumulative 

departure from the mean (ACDM) for precipitation is one 

way to visualize multi-year or decadal pattern in precipitation 

rates across the study area. The ACDM, representing the 

spatial average of spatially-distributed (or gridded) annual 

precipitation estimates for the study area, was calculated for 

1930–2014 (fig. 6A). The ACDM is the cumulative summation 

through time of the positive or negative difference between 

each annual precipitation value and the long-term mean, or 

average, value. Daily precipitation values from the PRISM 

Climate Group (Northwest Alliance for Computational 

Science and Engineering, 2018) averaged over the study 

area were accessed through the USGS GeoData Portal (U.S. 

Geological Survey, 2018). However, the PRISM data were 

not available for the Canadian part of the study area. Maurer 

and others (2002) provided a similar dataset with spatially 

distributed precipitation estimates for the entire study area, 

including Canada, but for a shorter period (1950–99). Because 

comparison of total annual precipitation for these two datasets 

indicates similarity, PRISM data were used to assess long-term 

changes by analysis of the ACDM (fig. 6A), which represents 

the United States only. 

Downward slopes in the ACDM (fig. 6A) result from 

multiple years of below-average precipitation, whereas 

upward slopes indicate wet periods. Three distinct dry periods 

that lasted at least 4 years since 1930 are evident for periods 

where the slope of the ACDM for precipitation is relatively 

steep and downward overall (fig. 6A). These periods consist 

of most of the 1930s, the late 1950s, and the late 1980s. The 

wettest periods include the early 1940s, the early 1960s, and 

2007‒14. 

The ACDM for simulated groundwater recharge has 

less interannual variability than the ACDM for precipitation 

(fig. 6A), which indicates that the soils store infiltrating 

precipitation for periods long enough to damp the variability 

of precipitation that becomes recharge. The volume of 

recharge is much less than the volume of precipitation as a 

result of evapotranspiration and direct runoff, which is evident 

in figure 6A, where the ACDM for recharge is exaggerated by 

a factor of 10 in relation to precipitation. 

The ACDM was calculated individually for eight weather 

stations in the study area (figs. 6B, 7). Daily precipitation 

values for climate stations in the study area for 1930–2006 

were available from the Global Historical Climatology 

Network (National Oceanic and Atmospheric Administration, 

2018b). These data were supplemented with precipitation data 

for the same stations from Climate Data Online (National 

Oceanic and Atmospheric Administration, 2018a) to extend 

the period of record to 2015. The daily values were used to 

calculate ACDM for each station for 1930–2014 (fig. 6B). 

There are spatial differences in the occurrence of wet and dry 

periods between the northwest and southeast parts of the study 

area. However, all the stations experienced mostly dry years 

during the three dry periods that were indicated for the spatial 

average (fig. 6A). The Glasgow and Regina stations in the 

northwest experienced wet conditions during the 1940s and 

early 1950s, compared to the other stations. The Richardton 

Abbey station overall was most different from the other 

stations, particularly after 1975, where the ACDM generally 

increased until 1987 and decreased until 2015. The Bismarck 

station was almost opposite to the Richardton Abbey station 

after 1961. 

A map of the long-term precipitation rate indicates a large 

change in total precipitation from west to east but less change 

from north to south (fig. 7), which is similar to the spatial 

distribution of groundwater recharge (Long and others, 2014). 

Differences in the ACDM for precipitation between stations 

indicate that temporal patterns of precipitation are related to 

the spatial distribution of precipitation. A spatio-temporal 

analysis of precipitation, therefore, provides insight by proxy 

into the spatio-temporal characteristics of groundwater 

recharge. 

Consistent with the spatial distribution of precipitation 

are the ACDM curves for the weather stations, which also 

have more variability from west to east than from north to 

south (figs. 6B, 7). Furthermore, the timing of dry and wet 

periods is not consistent everywhere in the study area. For 

example, the Glasgow and Regina stations, located farthest 

to the northwest, overall have similar ACDM curves; 

however, the Glendive and Crosby stations have similar 

curve characteristics that differ from Glasgow and Regina 

(fig. 6B). The next station to the east, Richardton Abbey, has 

a unique ACDM and might represent a climatic transition 

from west to east. Separating these weather-station groupings 

into categories of climatically “driest,” “moderately dry,” 

“moderate,” “moderately wet,” and “wettest” helps to 

visualize this pattern in relation to the long-term precipitation 

map (fig. 7). Although the Willow City and Dupree stations are 

spaced far apart, they have similar ACDM characteristics and 

are in the “moderately wet” zone. The Bismarck station, in the 

“wettest” zone, also differs from the other stations. Therefore, 

the ACDM curves for the weather stations indicate climatic 

differences from west to east but similarities from north to 

south; this is consistent with long-term average precipitation 

that has a gradient from west to east. 
14 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig07

Jame sRiver

Little Missouri River

Missouri River

BigMuddy Creek

Cheyenne River

O'Fallon

Creek

Boxelder Creek

Rosebud Creek

Grand River

Cedar Creek

Knife River

Tong ue River

Souris River

Heart River

Moreau River

Milk

River

Cannonball River

PowderRiver

Yellowstone River

Littl e Missouri River Redwater Rive r

Yellowstone Ri ver

South S askatchewan River

Frenchma n River

Big MuddyC reek

Bighorn

River

Big Coulee

Little Muddy River

Moose Mountain Creek

Moose Jaw River

Souris River

SourisRiver

As si niboineRiver

Qu'Appelle River

Cheyenne River

' Moreau River

Missouri River

o

River

Lake Oahe

Lake Fort

Peck Lake

Sakakawea

Old

Wives

Lake

CANADA

UNITED STATES

MANIT

O

B

A

SASKAT

C

H

E

WAN

NORTH DAKOTA

SOUTH DAKOTA

MONTANA

WYOMING

Bismarck

Williston

Regina

Miles City

0 30 60 90 120 KILOMETERS

0 30 60 90 120 MILES

108° 106° 104° 102° 100°

50°

48°

46°

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

GLASGOW

CROSBY

RICHARDTON ABBEY

BISMARCK

WILLOW

CITY

DUPREE

GLENDIVE

REGINA

Wettest

Moderate

Moderately wet

Moderately dry

Driest

DUPREE

EXPLANATION

Weather station

Lines separating climatically 

wet and dry areas, based on 

annual cumulative departure

from the mean precipitation

> 24 to 35

> 21 to 24

> 18 to 21

> 15 to 18

> 12 to 15

> 9.0 to 12

6.3 to 9.0

Precipitation, in

inches per year

Figure 7. Locations of weather stations and long-term precipitation (1981–2011) data in the study area, Williston Basin, 

United States and Canada. Precipitation data from Long and others (2014).
Comparison of Conceptual and Numerical Models 15

Comparison of Conceptual and 

Numerical Models

A comparison of the groundwater-flow budget from the 

conceptual model (Long and others, 2014) and the steadystate numerical model (Davis and Long, 2018a) serves as a 

qualitative evaluation of both models because these models 

should be reasonably consistent. Differences between these 

two groundwater-flow budgets help to quantify the range of 

uncertainty of both budgets (table 1).

Groundwater Flow

Long and others (2014) described general groundwaterflow directions and other characteristics within the context 

of an overall conceptual model of the study area. In this 

description, the estimated potentiometric surfaces for aquifers 

near the land surface closely resemble the undulating land 

topography, and flow directions are from upland areas toward 

streams. In the deepest aquifers, groundwater flows from 

southwest to northeast, with minimal influence from the land 

surface and has a smoother potentiometric surface. The lower 

Fort Union aquifer, with a middle range of depth among the 

three bedrock aquifers, has a potentiometric surface with 

characteristics that are a mixture of the deeper and shallower 

bedrock aquifers. Davis and Long (2018a) showed that the 

steady-state model simulated characteristics similar to those 

described by the conceptual model and further quantified 

similarities and differences of simulated and measured 

hydraulic-head values, spatially and temporally. Estimated 

groundwater-flow directions for the upper Fort Union aquifer, 

lower Fort Union aquifer, and Upper Cretaceous aquifer 

system were visually similar to simulated flow directions 

(Long and others; 2014; Davis and Long, 2018a).

Long and others (2014) did not provide a potentiometric 

surface for the glacial aquifer system, which therefore 

cannot be compared with the numerical model. However, the 

numerical-model simulation results are consistent with the 

locations of permitted groundwater wells in North Dakota, 

available from the North Dakota State Water Commission 

(2017). The numerical model generally simulated upland 

areas of the glacial aquifer system as unsaturated, where the 

simulated hydraulic head is below the bottoms of the cells 

(Davis and Long, 2018a), and the permitted wells generally 

are clustered in areas simulated as saturated. This indicates 

that modeling was consistent with the permit locations because 

water production wells would be in saturated areas where 

groundwater is available. 

Table 1. Estimated and simulated average groundwater-flow budget for 1981–2005 within a control volume of the active model area. 

[The last two columns show an alternate budget in which reservoir exfiltration is quantified as a net gain from groundwater, and reservoir infiltration is, 

therefore, zero (modified from Davis and Long, 2018a). Percent: The percentage of total recharge or total discharge. The totals may not add to 100 due to 

rounding. Abbreviations: ft3/s, cubic foot per second; –, not estimated; <, less than]

Recharge or discharge component and variable

A B C D E F

Estimated Simulated (quantified 

by MODFLOW)

Simulated (net flow 

for reservoir)

(ft3/s) (percent) (ft3/s) (percent) (ft3/s) (percent)

Groundwater recharge (inflow)

Precipitation recharge RP 1,205 26 1,723 35 1,723 60

Irrigation recharge RI 98 2 98 2 98 3

Stream infiltration SFin 3,302 72 815 16 815 29

Reservoir infiltration SFin

10 0 2,094 42 10 0

Groundwater inflow from the Powder River structural 

basin

GWin 8 <1 84 2 84 3

Groundwater inflow from the glacial aquifer system GWin – – 134 3 134 5

Total recharge 4,613 100 4,948 100 2,854 100

Groundwater discharge (outflow)

Stream exfiltration SFout 4,477 97 2,633 52 2,633 90

Reservoir exfiltration SFout 110 <1 2,128 42 134 1

Well withdrawal Wout 126 3 86 2 86 3

Groundwater outflow to the glacial aquifer system GWout – – 185 4 185 6

Total discharge 4,613 100 5,032 100 2,938 100

1Assumes a net value for groundwater infiltration and exfiltration.
16 Groundwater Availability of the Williston Basin, United States and Canada

Average Groundwater-Flow Budget

The groundwater-flow budget is described by equation 

1, in which inflows and outflows are assigned positive and 

negative values, respectively. 

(1)

where

RP is precipitation recharge;

RI is irrigation recharge (excess irrigation that 

infiltrates the aquifer);

SFin is stream and reservoir infiltration to 

groundwater;

GWin is groundwater inflow to the control volume;

SFout is stream and reservoir exfiltration from 

groundwater;

GWout is groundwater outflow from the control 

volume;

Wout is well withdrawal and flowing artesian wells; 

and

ΔS is storage change (positive or negative).

Long and others (2014) estimated the groundwaterflow budget for average conditions for a 25-year period 

(1981‒2005), as illustrated conceptually in figure 8A. Because 

this control volume was slightly modified by Davis and 

Long (2018a), as previously described, the estimated budget 

also was adjusted for consistency with the modified control 

volume (column A, table 1). Long and others (2014) did not 

consider possible storage change (ΔS) over the 25-year period. 

Simulated groundwater-flow budgets presented herein for 

the predevelopment period through 2005 consist of values 

from Davis and Long (2018a). Similarly to the estimated 

groundwater-flow budget, the simulated groundwater-flow 

budget for average conditions is dominated by SFin and SFout

and secondarily by RP; minor components consist of Wout, 

GWin, and GWout (fig. 8B; table 1). RP is a major forcing 

component that affects streamflow, which likewise affects 

SFin; the combination of RP and SFin are the major forcings 

that affect SFout. Consequently, RP, SFin, and SFout account 

for 94 percent of the pie-chart area for average conditions 

(fig. 8B). Of the three reservoirs in the study area, Lake 

Sakakawea is the only reservoir within the active model area 

(fig. 1).

Comparison of the estimated and simulated groundwaterflow budgets indicates noteworthy similarities and differences 

(table 1). Differences result primarily from limitations inherent 

in estimating and simulating regional-scale groundwater 

flow and help provide ranges of uncertainty in each flow 

component. Long and others (2014) estimated precipitation 

recharge for the entire study area by using the Soil-Water 

R R SF GW SF GW W S P I + + in in + + out out out + + + ∆ = 0

Balance (SWB) model (Westenbroek and others, 2010). Davis 

and Long (2018a) initially applied these SWB estimates to 

the numerical model and then adjusted the original estimates 

upward during model calibration; this increase was supported 

by comparison to two other recharge estimation methods for 

selected parts of the study area, in which the SWB estimates 

were at the lower range of estimates from the other methods 

(Aurand, 2013; Long and others, 2014). These methods were 

the chloride mass-balance and water-table-fluctuation methods 

(Healy and Cook, 2002; Healy, 2010). For the steady-state 

model (1981‒2005), precipitation recharge was 1.4 times 

higher than the estimated value (columns A and C, table 1). 

Column C in table 1 consists of the simulated budget 

as quantified by MODFLOW and is the sum of all flows for 

each model cell for each flow category. For the reservoir 

component, column C indicates large values for reservoir 

infiltration and exfiltration, which are nearly balanced 

(column C, table 1), because infiltration occurred for some 

reservoir cells and exfiltration occurred for others. Column E 

in table 1 shows results of an alternate way to quantify 

the simulated budget, one that is more consistent with the 

estimated budget. The estimated budget simplified the 

reservoir component by assuming a net reservoir exfiltration 

of 10 ft3/s (column A, table 1), with no attempt to estimate 

infiltration and exfiltration for different areas of the reservoir. 

This results in a net value of zero for reservoir infiltration. 

If a net groundwater flow for the reservoir is applied to the 

simulated budget, the net reservoir infiltration and exfiltration 

is 0 and 34 ft3/s, respectively (column E, table 1), and the 

estimated and simulated values are similar—1 percent or less 

of total groundwater discharge (columns B and F, table 1). 

An assessment of the recharge and discharge components 

as percentages of total recharge and discharge is a convenient 

way to compare the estimated and simulated groundwater-flow 

budgets (columns B and F, table 1). The long-term averages 

for estimated and simulated precipitation recharge were 26 

and 60 percent, respectively, of total groundwater recharge. 

Estimated and simulated stream infiltration to groundwater 

were 72 and 29 percent, respectively, of all groundwater 

recharge. Estimated and simulated stream exfiltration 

from groundwater into streams were 97 and 90 percent, 

respectively, of all groundwater discharge. Although simulated 

stream infiltration was much lower than the estimated value 

(table 1), uncertainty of the estimate was large, partly resulting 

from uncertainty in consumptive use of surface water, which 

primarily occurs during the irrigation season (Long and 

others, 2014). Estimated stream exfiltration relied heavily on 

measured streamflow and base-flow estimates during the postirrigation season (Long and others, 2014) and, therefore, was 

minimally affected by consumptive use. 
Comparison of Conceptual and Numerical Models 17

tac18-1237_fig08

 Control

volume inflow

and outflow 

Stream and 

reservoir exfiltration

Stream and 

reservoir infiltration

Low

permeability

marine shale

Well

 withdrawal

Precipitation

recharge

Irrigation 

recharge

8

6 7 5 4

3

2

1

7

7 6 54

3

2

1

6 4

3

2

1

B

A

Footnotes:

a. Predevelopment well withdrawal was not considered and is not shown on 

pie chart.

b. Storage change is zero for steady state.

EXPLANATION

Water budget pie charts

Glacial aquifer system

Lower Tertiary and Upper Cretaceous aquifer systems

Basal confining unit

Hydrogeologic units

7. Groundwater outflow from the control volume (GWout)

6. Groundwater inflow to the control volume (GWin)

5. Well withdrawal (Wout)

4. Irrigation recharge (RI

)

3. Precipitation recharge (RP)

2. Stream and reservoir exfiltration (SFout)

1. Stream and reservoir infiltration (SFin)

Average conditions 2005 Predevelopmenta b

(Pre-1961) (Steady state,1981–2005)

Groundwater table

8. Storage change (∆S)

Figure 8. Simulated groundwater-flow budget, Williston Basin, United States and Canada. (A) Conceptual profile of the groundwater 

system; (B) relative magnitudes of water-budget components for predevelopment (pre-1961), average conditions (steady state, 1981–

2005), and 2005. This groundwater-flow budget applies to the control volume. Values are from Davis and Long (2018b). 
18 Groundwater Availability of the Williston Basin, United States and Canada

Irrigation recharge is the excess irrigation that infiltrates 

the aquifer, for which the simulated and estimated values 

were identical, because this rate was specified in the model 

rather than calibrated. A relatively small amount (8 ft3/s) 

was estimated and simulated for groundwater flow from 

the Powder River Basin into the Williston Basin through 

connected hydrogeologic units (table 1). The glacial aquifer 

system extends beyond the limits of the control volume 

and, therefore, allows horizontal groundwater flow into 

and out of the control volume. The simulated groundwater 

inflow and outflow for the glacial aquifer system is 5 and 

6 percent, respectively, of the simulated groundwater-flow 

budget (column F, table 1); the estimated counterparts of this 

inflow and outflow were assumed to be balanced (Long and 

others, 2014) and, therefore, not considered in the estimated 

groundwater-flow budget. Well withdrawal was a small part 

of the estimated and simulated groundwater-flow budget 

(table 1); the estimated value was reduced by 32 percent in 

the steady-state model (1981‒2005) because of numerical 

instability during model calibration, as described by Davis 

and Long (2018a). However, this reduction of estimated 

well withdrawal was not applied to the transient model and, 

therefore, had no effect on simulating hydrologic scenarios. 

Infiltration to groundwater from domestic on-site sewage 

treatment, or septic, systems also was considered. About 

9 percent of all well withdrawals in the control volume are 

from domestic wells (Long and others, 2014). If as much as 

90 percent of the domestic groundwater use is presumed to be 

diverted to septic systems, the infiltration from domestic septic 

systems would be less than 0.3 percent of total groundwater 

recharge. Even if the error in this estimate is large, the septicsystem infiltration rate would be small enough to neglect. 

Simulated Transient Groundwater-Flow 

Budget

For transient simulations, storage change (ΔS) balances 

equation 1 so that the sum of all components equates to zero, 

and the sign of ΔS is determined on this basis. For example, 

if inflows exceed outflows, ΔS is negative and represents 

an increase in groundwater storage. The groundwater-flow 

budget for the predevelopment period differs little from that of 

average conditions (steady state, 1981‒2005) because average 

well withdrawals for 1981‒2005 were a small part of the total 

groundwater-flow budget (fig. 8B). 

Simulated precipitation recharge for 2005 was small 

in comparison to that of average conditions (fig. 8B), 

which is consistent with below-average precipitation 

recharge for 2000‒2005, as shown by a negative slope in 

the cumulative-departure curve (fig. 6A). Low precipitation 

recharge for 2005 resulted in total inflow that was only 

69 percent as large as total outflow and, consequently, large 

storage-change, ΔS (fig. 8B). 

A groundwater-flow budget for the transient model 

(predevelopment‒2005) provides additional insight into 

hydrologic change, as well as differences in the three aquifer 

systems (fig. 9). The time increments in figure 9 are variable, 

with 5-year increments for 1961‒80 and 1-year increments for 

1981‒2005. The total length of each bar represents the total 

magnitude of the groundwater-flow budget by stress period, 

which visually characterizes wet and dry periods; for example, 

1995‒99 was climatically wet and has the largest flows for 

all three aquifer systems. Generally, the wet and dry periods 

are visible and consistent for all three aquifer systems. These 

temporal changes are consistent with the cumulative departure 

from average precipitation recharge (RP) (fig. 6A): 1961‒75 

had moderately above-average RP; 1971‒86 had variable RP

that was above and below average; 1987‒94 represented low 

RP; 1995‒99 represented high RP; and 2000‒2005 represented 

low RP (fig. 9). In each stress period, ΔS is either positive 

or negative, and if RP is high, total inflows generally are 

greater than total outflows, resulting in negative ΔS (increased 

storage); whereas, low RP corresponds to positive ΔS

(decreased storage) (fig. 9).

Surface-water infiltration (SFin, positive) and exfiltration

(SFout, negative), shown in figure 9, together are the largest 

component of the groundwater-flow budget. SFout is larger 

than SFin for all three aquifer systems, particularly for the 

Upper Cretaceous aquifer system where SFin is negligible. 

The lower Tertiary aquifer system has the largest overall 

groundwater-flow budget and the greatest variability of the 

three aquifer systems. The Upper Cretaceous aquifer system 

has the smallest groundwater-flow budget and least variability 

(fig. 9); the small groundwater-flow budget partly results 

from a land-surface exposure that is areally small relative to 

the other two aquifer systems (fig. 1). The Upper Cretaceous 

aquifer system has, on average, a groundwater-flow budget 

that is less than one-third as large as the other two aquifer 

systems but accounts for about 70 percent of the total well 

withdrawals for the control volume (fig. 10); this largely is 

because flowing artesian wells discharging from the Upper 

Cretaceous aquifer system account for about 50 percent of 

all well withdrawals for the control volume (Davis and Long, 

2018a). Groundwater pumped from the lower Tertiary and 

Upper Cretaceous aquifer systems was below average during 

1996‒2000 (fig. 10) because of high precipitation recharge 

(figs. 6, 9), which reduced demand. This high-recharge period 

also corresponds to a decrease in total well withdrawals as 

a percentage of groundwater inflow for the control volume, 

although this percentage has increased overall from the 

predevelopment period to 2005 (fig. 10). 
Simulated Transient Groundwater-Flow Budget 19

tac18-1237_fig09

A. Glacial aquifer system

B. Lower Tertiary aquifer system

C. Upper Cretaceous aquifer system

-4

-3

-2

-1

0

1

2

3

4

-5

-4

-3

-2

-1

0

1

2

3

4

5

Magnitude of groundwater-flow budget, in thousands of cubic feet per second -2

-1

0

1

2

Predevelopment period

1961–651966–701971–751976–80

1981

1982

19831984

19851986

19871988

198919901991

199219931994

1995

1996

1997

1998

1999

20002001

2002

2003

20042005

Stress period, in calendar years

Storage change

Well withdrawal

Inflow from (positive) and outflow to 

(negative) bedrock aquifer systems

Inflow from (positive) and outflow to 

(negative) the glacial aquifer system

Precipitation and irrigation recharge

EXPLANATION

Surface-water infiltration (positive) 

and exfiltration (negative)

Figure 9. Simulated groundwater-flow budget by stress period (predevelopment-2005) for the (A) glacial aquifer system, 

(B) lower Tertiary aquifer system, and (C) Upper Cretaceous aquifer system for the control volume, Williston Basin, United 

States and Canada. Values are from Davis and Long (2018b).
20 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig10

Year

Discharge, in cubic feet per second

Total inflow, in percent

20102005200019951990198519801975197019651960

70

60

50

40

30

20

10

0

5

4

3

2

1

0

Withdrawal as a percentage of total inflow 

to control volume

Upper Cretaceous aquifer system, flowing

Upper Cretaceous aquifer system, pumped

Lower Tertiary aquifer system

Glacial aquifer system

EXPLANATION

Figure 10. Groundwater withdrawals from wells, by aquifer system, 

Williston Basin, United States and Canada. Values from Davis and Long 

(2018b).

Simulated Groundwater Response to 

Flowing Artesian Wells

Hydrostatic pressure in the aquifer causes flowing 

artesian wells to flow without a pump. Fischer (2013) 

described flowing artesian wells in North Dakota, installed for 

domestic and agricultural use, that are open to the Fox Hills 

and lower Hell Creek aquifers of the Upper Cretaceous aquifer 

system. The wells primarily are near the Yellowstone, Little 

Missouri, and Knife Rivers, with additional flowing artesian 

wells near Lake Sakakawea (fig. 1). These wells are of concern 

because they generally are not capped or valved, resulting in 

continuous discharge of groundwater (Smith and others, 2000; 

Honeyman 2007a, 2007b, 2007c; Fischer, 2013). Flowing 

artesian wells also are located in parts of Montana (Wanek, 

2009). To better understand the potential effects of flowing 

artesian wells into the future, a set of assumptions was selected 

and applied to a model scenario; however, many different 

assumptions could be made about the future, all of which 

would affect flowing artesian wells. This scenario is only one 

example of the many sets of assumptions that could be applied 

to test the effects of flowing artesian wells. 

Hydrology of Flowing Artesian Wells

About 70 percent of the flowing artesian wells 

that were completed in the Fox Hills and lower Hell 

Creek aquifers were installed during 1960‒90 (Fischer, 

2013). Within most of the study area, groundwater 

levels generally were steady prior to 2000 (Thamke 

and others, 2014), but declining water levels have 

occurred locally in areas where flowing artesian wells 

are located because of their cumulative groundwater 

discharge. Near the Yellowstone River, flowing 

artesian wells contributed to groundwater-level 

declines of about 1 ft/yr since the 1970s in the Fox 

Hills and lower Hell Creek aquifers (Smith and others, 

2000; Fischer, 2013). Hydraulic-head changes, mostly 

declines, were in these two aquifers near the Little 

Missouri River for 1994–2006 (Honeyman, 2007a, 

2007b, 2007c). These rates of change ranged from 

–4.1 to 1.4 ft/yr, with a mean rate of about –1.28 ft/yr, 

which was projected to result in wells ceasing to flow 

during 2007–93, depending on the well (Honeyman, 

2007a, 2007b, 2007c). In addition to discharging 

to the land surface, flowing artesian wells near the 

Yellowstone and Little Missouri Rivers also might 

allow leakage into the overlying lower Tertiary aquifer 

system because of inadequate sealing or corrosion of 

well materials (Fischer, 2013) in combination with the 

upward hydraulic gradient that occurs in these areas 

(Long and others, 2014). 

Scenario 1: Flowing Artesian Wells

Flowing artesian wells were simulated as head-dependent 

boundary cells (Davis and Long, 2018a); that is, the rate of 

well discharge is influenced by hydraulic head surrounding 

the well and, therefore, is similar to spring discharge. These 

wells were simulated as being open to the Fox Hills and 

lower Hell Creek aquifers. The transient model was used to 

assess potential outcomes if none of the flowing artesian wells 

were capped or plugged through 2035, with other conditions 

remaining constant. The simulation results provide estimates 

of (1) when flowing artesian wells might cease to flow because 

of hydraulic-head declines, and (2) the spatial distribution 

of hydraulic-head declines. Model input and output for 

this scenario are available from Davis and Long (2018c). 

Scenario 1 was executed for 1961‒2035 and was identical to 

the original transient model for its simulation through 2005. 

Period 2006‒35 was simulated with 1-year stress periods. To 

extend this simulation through 2035, precipitation-recharge 

rates and conditions for the MODFLOW Streamflow-Routing 

Package that were applied to the steady-state model were 

applied in the flowing-wells scenario at steady rates for 

2006‒35. The groundwater pumping rates that originally were 
Simulated Groundwater Response to Flowing Artesian Wells 21

applied to the 2005 stress period were applied in the flowingwells scenario at steady rates for 2006‒35. This assumes 

no increased groundwater pumping for energy-resource 

production, which is highly uncertain into the future and, 

therefore, is a conservative approach. Applying these steady 

rates was an attempt to assess the effects of flowing artesian 

wells without interference from other changing hydrologic 

conditions; however, the assumptions made for the rates of 

pumping and other hydrologic conditions play a large role 

in the outcome. Because groundwater pumping is expected 

to continue long into the future to some degree, this scenario 

cannot isolate the effects of flowing artesian wells but is useful 

as a long-term assessment of flowing artesian wells, if it is 

assumed that groundwater pumping will continue. 

All simulated flowing artesian wells were applied equally 

to the Fox Hills and lower Hell Creek aquifers, and results are 

summarized for the Fox Hills aquifer. Most of the simulated 

hydraulic-head decline for the Fox Hills aquifer occurred for 

the 1961‒2005 period (fig. 11), but the simulation through 

2035 showed that this drawdown area continued to enlarge 

after 2005 (fig. 12). Simulated declines were largest in areas 

near the Yellowstone, Missouri, and Little Missouri Rivers, 

where flowing artesian wells are located (fig. 12). Simulated 

hydraulic-head decline was as much as 170 ft in some areas. 

Simulated and measured hydraulic head generally was inverse 

to simulated outflow from flowing artesian wells in areas 

where drawdowns occurred (fig. 13). The total rate of outflow 

for flowing artesian wells increased from 1965 to 1980 to a 

high of about 61 ft3/s, as the number of wells increased, and 

then varied between about 61 and 57 ft3/s until 2003 (fig. 13). 

After 2003, when few additional wells were installed, the 

simulated outflow decreased to about 54 ft3/s in 2035; this 

decrease in outflow was steepest for the 2006 stress period and 

gradually flattened through the remainder of the simulation, 

which is consistent with a flattening of the simulated 

hydraulic-head declines, particularly after 2010 (fig. 13). This 

simulation indicates that the installation of flowing artesian 

wells initiated the hydraulic-head declines, which then resulted 

in decreases in outflow rates because of decreasing hydraulic 

pressure within the wells, and that outflows were moving 

toward an equilibrium state with hydraulic head. 

A second reason for the flattening of the simulated 

hydraulic-head decline was that many of the wells ceased to 

flow. The total number of flowing artesian wells established in 

the model was 571 (Davis and Long, 2018a), and of these, 271 

wells did not flow at any time during the simulation because 

hydraulic head was always below the land-surface (fig. 12). 

As hydraulic head declined throughout the simulation, 68 of 

these wells responded by ceasing to flow by the end of 2005, 

another 7 wells ceased flowing by the end of 2035, and the 

remaining 225 simulated wells continued to flow at the end of 

the simulation (fig. 12). 

The simulated precipitation recharge for the 2006‒35 

period was high in relation to the earlier simulated period, 

which is another factor that partly resulted in a flattening of the 

hydraulic-head decline for that period (fig. 13). The average 

simulated recharge for 1981‒2005 was 1,723 ft3/s, which is 

low in comparison to the value of 3,174 ft3/s that was used for 

the steady-state model, as well as for the 2006‒35 period in 

scenario 1. The value of 3,174 ft3/s for steady state was based 

on the average estimated recharge for the 1981‒2011 period 

because this was considered to best represent the long-term 

average (Davis and Long, 2018a), and the last 6 years of this 

period (2006‒11) were unusually wet years (fig. 6A). Another 

factor influencing the flattening of the hydraulic-head decline 

(fig. 13) is the simulated overlying aquifers that helped to 

buffer the decline by supplying groundwater inflow to the 

Fox Hills and lower Hell Creek aquifers as hydraulic pressure 

was reduced in those aquifers. Although scenario 1 indicates 

decreasing severity of hydraulic-head decline for the future, 

different scenarios also should be considered. For example, 

if other conditions prevail in the future—such as continued 

increase in groundwater pumping rates, drier climatic 

conditions, or long-term drought—then hydraulic-head 

declines would be larger than those indicated by scenario 1.

Fischer (2013) took a somewhat different approach 

to simulate the Fox Hills and lower Hell Creek aquifers’ 

hydraulic-head declines resulting from flowing artesian wells; 

this model consisted of one layer and flowing artesian wells 

that were simulated with specified flow rates, independent of 

hydraulic head. The result of this simulation was a decline of 

hydraulic-head values of as much as 200 ft for 1942‒2009 and 

as much as 55 ft for a projected future scenario representing 

2009‒39. 

As described in section, “Conceptual Model of 

Groundwater Flow,” the total withdrawal for all pumped and 

flowing artesian wells combined was simulated by Fischer 

(2013) to be in the range of 3–6 times higher than what was 

originally estimated for 1980‒2008; this increase for the 

numerical model was necessary to simulate the measured 

hydraulic-head declines near flowing artesian wells. One 

reason for numerical modeling was to improve the uncertain 

estimate of the total flow rate from flowing artesian wells 

of 1.6 ft3/s (Fischer, 2013). Inadequate sealing or corrosion 

of these wells, which may result in leakage into the lower 

Tertiary aquifer system, contributes to this uncertainty 

(Fischer, 2013). For scenario 1 herein, the rate of flowing 

artesian wells was as high as 61 ft3/s, which also was much 

higher than the 1.6 ft3/s originally estimated. Therefore, two 

different models indicate that the total flow rate from flowing 

artesian wells is much higher than 1.6 ft3/s. 
22 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig11

Jame sRiver

Little Missouri River

Missouri River

Big Muddy Creek

Cheyenne River

O'Fallon

Creek

Boxelder Creek

Rosebud Creek

Grand River

Cedar Creek

Knife River

Tong ue River

Souris River

Heart River

Moreau River

Milk

River

Cannonball River

PowderRiver

Yellowstone River

Littl e Missouri River Redwater Rive r

Yellowstone River

South S askatchewan River

Frenchma n River

Big MuddyC reek

Bighorn

River

Big Coulee

Little Muddy River

Moose Mountain Creek

Moose Jaw River

Souris River

SourisRiver

As si niboineRiver

Qu'Appelle River

Cheyenne River

' Moreau River

Missouri River

o

River

Lake Oahe

Lake Fort

Peck Lake

Sakakawea

Old

Wives

Lake

CANADA

UNITED STATES

MANIT

O

B

A

SASKAT

C

H

E

WAN

NORTH DAKOTA

SOUTH DAKOTA

MONTANA

WYOMING

Bismarck

Williston

Regina

Miles City

0 30 60 90 120 KILOMETERS

0 30 60 90 120 MILES

108° 106° 104° 102° 100°

50°

48°

46°

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

7107

34455

29350

21400

Hydraulic-head monitoring well

Modeled extent of the Fox Hills aquifer

> 30 to 50

> 5 to 30

> 0 to 5

> -5 to 0

> -30 to -5

> -100 to -30

> -170 to -100

EXPLANATION

Change in simulated hydraulic head, 

Fox Hills aquifer (1961–2005), in feet. 

Negative values indicate head decline

Figure 11. Scenario 1 simulated hydraulic-head change from 1961 to 2005 for the Fox Hills aquifer, Williston Basin, United 

States and Canada.
Simulated Groundwater Response to Flowing Artesian Wells 23

tac18-1237_fig12

Jame sRiver

Little Missouri River

Missouri River

Big Muddy Creek

Cheyenne River

O'Fallon

Creek

Boxelder Creek

Rosebud Creek

Grand River

Cedar Creek

Knife River

Tong ue River

Souris River

Heart River

Moreau River

Milk

River

Cannonball River

PowderRiver

Yellowstone River

Littl e Missouri River Redwater Rive r

Yellowstone River

South S askatchewan River

Frenchma n River

Big MuddyC reek

Bighorn

River

Big Coulee

Little Muddy River

Moose Mountain Creek

Moose Jaw River

Souris River

SourisRiver

As si niboineRiver

Qu'Appelle River

Cheyenne River

' Moreau River

Missouri River

o

River

Lake Oahe

Lake Fort

Peck Lake

Sakakawea

Old

Wives

Lake

CANADA

UNITED STATES

MANIT

O

B

A

SASKAT

C

H

E

WAN

NORTH DAKOTA

SOUTH DAKOTA

MONTANA

WYOMING

Bismarck

Williston

Regina

Miles City

0 30 60 90 120 KILOMETERS

0 30 60 90 120 MILES

108° 106° 104° 102° 100°

50°

48°

46°

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

EXPLANATION

Simulated flowing artesian well

Change in simulated hydraulic head, 

Fox Hills aquifer (1961–2035), in feet. 

Negative values indicate head decline

Modeled extent of the Fox Hills aquifer

Well did not flow during the simulation period (271 wells)

Well that was flowing in 2035 (225 wells)

Well stopped flowing between 2005 and 2035 (7 wells)

Well stopped flowing prior to 2005 (68 wells)

> 30 to 50

> 5 to 30

> 0 to 5

> -5 to 0

> -30 to -5

> -100 to -30

> -170 to -100

Figure 12. Scenario 1 simulated hydraulic-head change from 1961 to 2035 for the Fox Hills aquifer, Williston Basin, United 

States and Canada.
24 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig13

Well 34455

Simulated well flow, in cubic feet per second

Hydraulic head, in feet above the North American Vertical Datum of 1988 

Well 21400

Well 7107

Year

Well 29350

0

10

20

30

40

50

60

70

1,940

1,960

1,980

2,000

2,020

2,040

2,060

2,080

0

10

20

30

40

50

60

70

2,040

2,060

2,080

2,100

2,120

2,140

2,160

2,180

0

10

20

30

40

50

60

70

2,040

2,060

2,080

2,100

2,120

2,140

2,160

2,180

1960 1970 1980 1990 2000 2010 2020 2030 2040

0

10

20

30

40

50

60

70

1,780

1,800

1,820

1,840

1,860

1,880

1,900

1,920

Simulated hydraulic

head

Measured hydraulic 

head

Simulated flow from

flowing artesian

wells

EXPLANATION

Figure 13. Scenario 1 simulated and measured hydrographs of hydraulic head for four monitoring 

wells open to the Fox Hills aquifer and total flow from flowing artesian wells, Williston Basin, United 

States and Canada. Locations of wells are shown in figure 11.
Simulated Groundwater Responses during Drought 25

Simulated Groundwater Responses 

during Drought

The increased freshwater demands for energy-resource 

production (ERP) from the Williston Basin were offset by 

precipitation rates that were higher than average during the 

2006‒15 period, as indicated by a steep increase in the annual 

cumulative departure from mean precipitation (fig. 6A). The 

resulting increases in streamflow and groundwater recharge 

allowed for the availability of temporary surface-water 

and groundwater permits in North Dakota (North Dakota 

State Water Commission, 2017). If, however, a drought 

had occurred during this period of increased groundwater 

withdrawal, how would this affect groundwater availability? 

To help answer this question, a hypothetical 10-year drought 

was simulated for 2006‒15, and two groundwater-withdrawal 

scenarios were applied as a comparison (scenarios 2 and 3). 

Scenarios 2 and 3 offer additional example scenarios, but 

many other possibilities could be simulated with this model. 

Model input and output for these scenarios are available from 

Davis and Long (2018c).

Scenario 2: Drought with Steady Groundwater 

Withdrawals

Estimated annual groundwater recharge for 1987‒91 

was the lowest 5 years of the estimated record for 1981‒2011 

(Long and others, 2014). To simulate a drought, precipitation 

recharge and flow conditions for the Streamflow-Routing 

Package that were applied to the transient model (Davis and 

Long, 2018a) for 1987‒91 were applied twice consecutively 

to simulate a 10-year drought for the 2006‒15 period. To 

simulate steady groundwater withdrawals, the groundwater 

pumping rates that originally were applied to the 2005 

stress period were applied in this scenario at steady rates 

for 2006‒15, which represents pumping rates prior to large 

increases in water use for ERP. 

For most of the western part of the active model area, 

simulated groundwater declines resulting from the 10-year 

drought were small, in the range of 0‒1 ft, for the glacial 

aquifer system and the upper Fort Union aquifer (figs. 14, 15). 

In the eastern areas, simulated declines in the range of 1‒5 ft 

were common, with more than 5 ft of decline in many areas 

(figs. 14, 15). More than 200 ft of simulated decline occurred 

for eastern parts of the glacial aquifer system (fig. 14). These 

results show the change from 2005 (the year prior to the 

simulated drought) to 2015, when simulated precipitation 

recharge was 1,437 and 516 ft3/s, respectively, for the active 

model area. This is only a moderate decrease in recharge 

compared to the long-term average rate of 3,174 ft3/s that was 

simulated in the steady-state model (Davis and Long, 2018a). 

The highest simulated precipitation-recharge rates were for 

1995 (5,846 ft3/s), 1997 (5,202 ft3/s), and 1999 (5,946 ft3/s). 

A drought simulation following any of these higher recharge 

rates would result in much larger groundwater declines. 

Some of the central and western areas show groundwater 

increases, primarily in the range of 0‒1 ft (figs. 14, 15). This 

results from the spatio-temporal variability of precipitation, 

as described in section, “Analysis of Precipitation and 

Recharge.” A noteworthy insight from this simulation is that 

the climatically wet eastern areas are more susceptible to 

groundwater declines during drought than are the dry western 

areas, and groundwater increases might occur in some western 

areas locally because of spatio-temporal variability. 

Scenario 3: Drought with Increased 

Groundwater Withdrawals

This scenario was identical to scenario 2, except that 

increased groundwater withdrawals to supply the needs of 

ERP for 2006‒15 also were applied. The freshwater used 

for ERP was estimated by two separate methods. The first 

method, the consumption estimate, focused strictly on the 

estimated freshwater needs for ERP in the Williston Basin 

within the United States and did not differentiate groundwater 

from surface water. The second method, the supply estimate, 

focused on groundwater-withdrawal data for North Dakota 

obtained from water-use permits (North Dakota State Water 

Commission, 2017). The supply estimate provided pumping 

rates, locations, and specific aquifers for individual wells that 

were used in the model simulation. Results of the two methods 

were consistent, which reduced uncertainty of these estimates. 

A combination of groundwater and surface water was 

used to supply the needs of ERP during 2006‒15 (North 

Dakota State Water Commission, 2017). However, if an 

extended drought occurred during this period, low streamflows 

might have resulted in less surface-water availability. The 

purpose of this scenario was to estimate the effects on the 

groundwater supply if ERP water demands were totally 

supplied by groundwater during a drought.

Consumption Estimate

Total freshwater use for ERP in the Williston Basin in 

the United States was obtained from Barnhart and others 

(2018), who tabulated the annual reported freshwater use for 

hydraulic fracturing (referred to as “treatments”) for the study 

area. Barnhart and others (2018) provided spatially distributed 

treatment data for a grid spacing of 1 mi but did not determine 

specific sources of water or differentiate groundwater sources 

from surface-water sources. Herein, these treatment volumes 

per grid cell per year were used to determine the consumption 

estimates. For 2006‒15 period, the total consumption estimate 

was 94,000 acre-feet (acre-ft) for the Williston Basin in the 

United States, with about 90,000 acre-ft of this in North 

Dakota and the remainder in Montana.
26 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig14

Jame sRiver

Little Missouri River

Missouri River

Big Muddy Creek

Cheyenne River

O'Fallon

Creek

Boxelder Creek

Rosebud Creek

Grand River

Cedar Creek

Knife River

Tong ue River

Souris River

Heart River

Moreau River

Milk

River

Cannonball River

PowderRiver

Yellowstone River

Littl e Missouri River Redwater Rive r

Yellowstone River

South S askatchewan River

Frenchma n River

Big MuddyC reek

Bighorn

River

Big Coulee

Little Muddy River

Moose Mountain Creek

Moose Jaw River

Souris River

SourisRiver

As si niboineRiver

Qu'Appelle River

Cheyenne River

' Moreau River

Missouri River

o

River

Lake Oahe

Lake Fort

Peck Lake

Sakakawea

Old

Wives

Lake

CANADA

UNITED STATES

MANIT

O

B

A

SASKAT

C

H

E

WAN

NORTH DAKOTA

SOUTH DAKOTA

MONTANA

WYOMING

Bismarck

Williston

Regina

Miles City

0 30 60 90 120 KILOMETERS

0 30 60 90 120 MILES

108° 106° 104° 102° 100°

50°

48°

46°

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

> 5 to 82

> 1 to 5

> 0 to 1

> -1 to 0

> -5 to -1

> -231 to -5

Change in simulated hydraulic head, 

glacial aquifer system (2005–2015), 

in feet

Extent of the glacial 

aquifer system. White 

indicates unsaturated 

areas.

EXPLANATION

Figure 14. Scenario 2 simulated hydraulic-head change from 2005 to 2015 for the glacial aquifer system (model layer 2), 

Williston Basin, United States and Canada. Negative values indicate decline.
Simulated Groundwater Responses during Drought 27

tac18-1237_fig15

Jame sRiver

Little Missouri River

Missouri River

Big Muddy Creek

Cheyenne River

O'Fallon

Creek

Boxelder Creek

Rosebud Creek

Grand River

Cedar Creek

Knife River

Tong ue River

Souris River

Heart River

Moreau River

Milk

River

Cannonball River

PowderRiver

Yellowstone River

Littl e Missouri River Redwater Rive r

Yellowstone River

South S askatchewan River

Frenchma n River

Big MuddyC reek

Bighorn

River

Big Coulee

Little Muddy River

Moose Mountain Creek

Moose Jaw River

Souris River

SourisRiver

As si niboineRiver

Qu'Appelle River

Cheyenne River

' Moreau River

Missouri River

o

River

Lake Oahe

Lake Fort

Peck Lake

Sakakawea

Old

Wives

Lake

CANADA

UNITED STATES

MANIT

O

B

A

SASKAT

C

H

E

WAN

NORTH DAKOTA

SOUTH DAKOTA

MONTANA

WYOMING

Bismarck

Williston

Regina

Miles City

0 30 60 90 120 KILOMETERS

0 30 60 90 120 MILES

108° 106° 104° 102° 100°

50°

48°

46°

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

> 5 to 23

> 1 to 5

> 0 to 1

> -1 to 0

> -5 to -1

> -41 to -5

Change in simulated hydraulic head, 

upper Fort Union aquifer

(2005–2015), in feet

Extent of the upper 

Fort Union aquifer. 

White indicates 

unsaturated areas.

EXPLANATION

Figure 15. Scenario 2 simulated hydraulic-head change from 2005 to 2015 for the upper Fort Union aquifer, Williston Basin, 

United States and Canada. Negative values indicate decline.
28 Groundwater Availability of the Williston Basin, United States and Canada

Supply Estimate

The supply estimate was used to assign well locations and 

pumping rates for ERP applications. This scenario was applied 

only within the area of North Dakota because the consumption 

estimate for North Dakota accounted for 96 percent of the total 

consumption estimate for the Williston Basin in the United 

States. Additionally, the State of North Dakota provided to 

the public detailed water-permitting data that could be applied 

directly to the model. Data for permits to use groundwater and 

surface water in North Dakota for ERP were obtained from the 

North Dakota State Water Commission (2017). In addition to 

the steady groundwater pumping rates applied in scenario 2, 

data from these groundwater permits were applied in scenario 

3 to account for the additional needs of ERP. Standard permits 

and temporary permits are two classes of permits that were 

obtained from the North Dakota State Water Commission. 

In North Dakota, standard permits generally apply for multiyear periods, whereas temporary permits are issued for 1 year 

or less. For standard permits, only those issued for use in 

ERP were used in this scenario. All temporary permits for 

2006‒2015 granted for industrial purposes were assumed to 

be used for ERP. Water-use rates reported by each permittee 

were available for the standard permits and were used in this 

scenario; for temporary permits, the permitted rates were used 

because the reported rates were not provided. A small number 

of permits were either outside of the active model area or open 

to formations below the Fox Hills aquifer and, therefore, were 

not included in this scenario. 

Although the consumption estimate did not separate 

groundwater use from surface-water use, a water-budget 

analysis of the two estimation methods for North Dakota 

indicated that the supply estimate was consistent with the 

consumption estimate. The total supply estimate for North 

Dakota groundwater was 51,000 acre-ft for 2006‒15, 

and the total consumption estimate for North Dakota was 

90,000 acre-ft for surface water and groundwater combined, 

which equates to 39,000 acre-ft of surface-water use. Using 

these estimates, the ratio of groundwater to surface water 

is about 1.3. As a comparison, the ratio of groundwater to 

surface-water usage for the North Dakota standard-permit data 

for ERP was about 1.4 for the same period. A limitation of 

this analysis is that groundwater might have been transported 

across the border of Montana and North Dakota; however, the 

transport volume was assumed to be cancelled by transport 

in both directions because of treatments that were needed in 

Montana. 

In many cases, the specific source aquifer was not 

specified in the groundwater-permit data. For example, in 

some cases the Fort Union Formation was listed as the source, 

and this formation was represented by model layers 3, 4, and 

5 (Davis and Long, 2018a). In these cases, the simulated well 

was assigned to model layer 3, 4, or 5, depending on which 

had the highest hydraulic conductivity to best accommodate 

the withdrawal rate. In some cases, the source was simply 

listed as groundwater, in which case the simulated well 

was assigned to the model layer with the highest hydraulic 

conductivity for standard permits; for temporary permits, 

however, the model layer was selected from layers 1–5 

because permits for 2006‒15 generally were applied to 

aquifers represented by these model layers. 

Although permitted wells with a glacial-aquifer source 

generally were clustered in areas simulated as saturated, a 

regional model cannot include a high level of spatial detail and 

accuracy and, therefore, adjustments to some well locations 

were made. If a well was at a location that had unsaturated 

model cells, the location was moved as much as 7 mi to a new 

location that could better accommodate simulated pumping. 

About 19 percent of the simulated pumping wells were moved 

in this way.

Simulation of Scenario 3

As described in the previous section, the ratio of 

groundwater to surface-water use for ERP in North Dakota 

was about 1.4. If, however, surface water was not available, 

and groundwater was to supply all ERP needs, the total 

groundwater-use rate of 51,000 acre-ft for 2006‒15 would 

have increased to about 87,400 acre-ft, or 1.71 times higher. 

Therefore, all ERP water-use data from the North Dakota 

permits were multiplied by 1.71 for this scenario. Each 

individual ERP permit site was added as a new pumping well 

in the simulation. All pumping wells from scenario 2 were 

assumed to supply non-ERP usage and remained unchanged 

for scenario 3. 

The difference in simulated hydraulic-head values 

between scenarios 2 and 3 represents the drawdown that would 

result from the new wells added to simulate groundwater 

pumping for ERP needs. The simulated drawdown that 

resulted from the new wells was less than 25 ft for 2015 

(figs. 16, 17, 18). Most of the pumping wells added in scenario 

3 were in the glacial aquifer system, upper Fort Union aquifer, 

and middle Fort Union hydrogeologic unit, and areas of 

drawdown coincide with these well locations (figs. 16, 17, 18). 

For many of the added pumping wells, drawdown in the 

pumped aquifer also resulted in drawdown in the underlying 

or overlying aquifer. For example, much of the simulated 

drawdown occurred within areas of the Little Muddy aquifer, 

which is a sand and gravel aquifer within the glacial aquifer 

system (fig. 16), and numerous permitted wells added in 
Improving Hydrologic Monitoring Networks 29

scenario 3 withdraw water from this aquifer. These simulated 

pumping wells also resulted in drawdown in the upper Fort 

Union aquifer and to lesser extent in the middle Fort Union 

hydrogeologic unit (figs. 17, 18). The total area affected by 

the added pumping wells for scenario 3 is small overall but 

represents a large part of the Little Muddy aquifer, in addition 

to other local areas (figs. 16, 17, 18). 

A comparison of the scenario 3 groundwater-flow 

budgets for 2005 and 2015 indicates noteworthy changes. 

Precipitation recharge decreased by 69 percent, which was the 

primary forcing that resulted in a 28 percent decrease in the 

net outflow to surface water. Simulated pumping-well outflow 

changed from 73.4 in 2005 to 98.3 ft3/s in 2015, an increase 

of 24.9 ft3/s (18,039 acre-ft/yr), or 34 percent (fig. 19). 

Although pumping wells do not account for a large part of the 

budget overall, local effects of these wells are an important 

consideration for groundwater management. Lowered 

hydraulic-head values caused flowing-well outflow to decrease 

by 4 percent. Constant-head inflow and outflow represent 

groundwater flow across the model-domain boundaries, with 

greater inflows than outflows. 

The stress periods for 2005 and 2015 were similar in 

having greater total outflows than inflows, the difference of 

which is shown by the storage change (fig. 19). This indicates 

that both these years were drier than the previous year (2004 

and 2014, respectively). Had the drought simulation followed 

a wet period, such as the late 1990s, changes would be much 

larger than shown in figure 19.

Improving Hydrologic Monitoring 

Networks

Because of the costs and resources needed to install 

hydrologic monitoring networks, their design should be well 

planned and have specific objectives. Numerical groundwaterflow models are used to inform water-management decisions, 

and a useful application is to design a monitoring network that 

would provide data that best improves the predictive ability 

of a model. Because any prediction is inherently uncertain, 

reducing this uncertainty is critical to improving this ability. 

Uncertainty of simulated hydrologic forecasts, or 

predictive uncertainty, can be reduced by providing additional 

data to better inform model calibration. If the new data 

were to come from a new observation well, for example, 

the degree of uncertainty reduction for a specific forecast 

of interest depends on the location of this well within the 

model area. One way to determine the best locations for new 

observation wells is to apply a method described by Fienen 

and others (2010) and White and others (2016), referred to in 

this report as a “data-worth analysis.” For example, the model 

might be used to forecast the effects of a new pumping well 

on streamflow. If water-level data from a new observation well 

were used to update the model calibration, this could reduce 

model uncertainty associated with this forecast of interest, 

depending on the location of that well. The data-worth value 

is equal to the forecast uncertainty when a new observation 

well is included in model calibration divided by the forecast 

uncertainty without the new well. A data-worth map would 

show the relative value of additional groundwater-level data at 

different locations. 

As an example, the data-worth analysis was applied 

to hydrologic forecasts of interest for the Williston Basin 

simulated by the steady-state model (1981‒2005) described 

by Davis and Long (2018a). Therefore, the analysis applies to 

equilibrium conditions, but a similar analysis could be applied 

to scenarios simulating a finite time period. The analysis area 

includes the Clear Lake and Little Muddy aquifers, which 

are sand and gravel aquifers within the glacial aquifer system 

(fig. 20). A simulated pumping well was added to the area 

representing the Clear Lake aquifer in model layer 2 at a rate 

of 2.8 ft3/s, and six locations within or near this aquifer were 

selected as example sites to forecast the effects of the pumping 

well on hydraulic head (sites H1–H6, fig. 20). All input, 

output, and executable files for this analysis are in Davis and 

Long (2018c).

Figure 20 shows the results of the data-worth analysis 

applied to site H6. The simulated glacial aquifer system, 

represented by model layers 1 and 2, was unsaturated in 

upland areas, and therefore, results of the data-worth analysis 

primarily are restricted to lowland areas, where observation 

wells could be installed (fig. 20). Simulated groundwater flow 

is toward the southeast and south in the Clear Lake and Little 

Muddy aquifers. Northeast of a groundwater divide (fig. 20) 

that corresponds to a surface-water divide, groundwater flow 

is toward the north and northeast. The largest area showing 

high data worth is north of the northern tip of the Clear Lake 

aquifer, in the northwestern corner of North Dakota (fig. 20), 

with data-worth values as high as 30 percent. This indicates 

that it would be a good location for a new observation well 

and that observations near the upgradient divide, where no 

observation wells currently are located, would help inform 

the simulated predictions of hydraulic head at site H6. The 

area near a groundwater divide might be one of the best 

locations because this has the highest local hydraulic head that 

influences groundwater flow downgradient at site H6. 
30 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig16

Little

Missouri River

Souris

River

Little

Muddy River

Yellowston e

R

Knife

River

Missouri River

Lake 

Sakakawea

Little Muddy

aquifer

M

O

N

TA

N

A

N

O

RT

H

D

A

K

O

TA

UNITED STATES

SASKATCHEWAN CANADA

0 10 20 MILES

0 10 20 KILOMETERS

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

48°

49°

104° 103° 102°

EXPLANATION

> -24 to -15

> -15 to -0.5

Well added in the lower part

of the glacial aquifer system

Extent of glacial aquifer system.

White indicates unsaturated 

areas

Extent of Little Muddy aquifer

Change in simulated hydraulic head, 

in feet

WYOMING

MONTANA

SOUTH DAKOTA

NORTH DAKOTA

SASKATCHEWAN

MANITOBA

UNITED STATES

CANADA

Active Model area

Map 

area

Figure 16. Scenario 3 simulated drawdown of hydraulic head resulting from groundwater withdrawals for energyresource production for 2015 in the glacial aquifer system (model layer 2), Williston Basin, United States and Canada. 

Negative values indicate decline.
Improving Hydrologic Monitoring Networks 31

tac18-1237_fig17

48°

49°

104° 103° 102°

Little

Missouri River

Souris

River

Little

Muddy River

Yellowston e

R

Knife

River

Missouri River

Lake 

Sakakawea

M

O

N

TA

N

A

N

O

RT

H

D

A

K

O

TA

UNITED STATES

SASKATCHEWAN CANADA

0 10 20 MILES

0 10 20 KILOMETERS

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

EXPLANATION

> -24 to -15

> -15 to -0.5

Well added in the lower part

of the glacial aquifer system

Extent of upper Fort Union 

aquifer

Change in simulated hydraulic head, 

in feet

Figure 17. Scenario 3 simulated drawdown of hydraulic head resulting from groundwater withdrawals for energyresource production for 2015 in the upper Fort Union aquifer (model layer 3), Williston Basin, United States and 

Canada. Negative values indicate decline.
32 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig18

Little

Missouri River

Souris

River

Little

Muddy River

Yellowston e

R

Knife

River

Missouri River

Lake 

Sakakawea

M

O

N

TA

N

A

N

O

RT

H

D

A

K

O

TA

UNITED STATES

SASKATCHEWAN CANADA

0 10 20 MILES

0 10 20 KILOMETERS

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

48°

49°

104° 103° 102°

> -24 to -15

> -15 to -0.5

Well added in the lower part

of the glacial aquifer system

Extent of middle Fort Union

hydrogeologic unit

Change in simulated hydraulic head, 

in feet

EXPLANATION

Figure 18. Scenario 3 simulated drawdown of hydraulic head resulting from groundwater withdrawals for energyresource production for 2015 in the middle Fort Union hydrogeologic unit (model layer 4), Williston Basin, United 

States and Canada. Negative values indicate decline.
Improving Hydrologic Monitoring Networks 33

tac18-1237_fig19

Flow rate, in cubic feet per second

Storage change (outflows minus inflows)

Constant-head outflow

Flowing-well outflow

Pumping-well outflow

Net outflow to surface water

Constant-head inflow

Precipitation recharge

0 500 1,000 1,500 2,000 2,500

2015

2005

EXPLANATION

Figure 19. Scenario 3 simulated change in the groundwater-flow budget from 2005 to 2015 for the active 

model area, Williston Basin, United States and Canada.

The analysis was less clear for sites farther to the 

southwest of site H6. For site H2, the analysis indicated 

a slight preference for a new observation well in the far 

northeastern corner of figure 21, in the areas where data-worth 

values are in the range of 1–5 percent. Similar to site H6, 

this area is near the groundwater divide and has no current 

observation wells. For site H1, the analysis showed no clear 

area where an observation well would help inform predictions 

(fig. 22). Results for sites H3, H4, and H5 were similar to 

those of site H1. One factor influencing the low effectiveness 

of this analysis for sites H1–H5 might be the large areas 

simulated as unsaturated; this restricts groundwater flow to a 

narrow area that includes the Clear Lake aquifer. Furthermore, 

this area already contains observation wells (fig. 22). Site 

H6, however, is closer to the groundwater divide than the 

other sites in the Clear Lake aquifer area and is in an area less 

restricted to the narrow saturated area. 

A similar analysis was applied to streamflow predictions 

of interest. Seven locations for streamflow predictions within 

and near the Clear Lake aquifer area were selected (sites 

F1–F7, fig. 23), and the same pumping scenario was applied. 

Similar to the analysis for sites H1–H5, the analysis for site 

F3 showed no clear area where an observation well would 

help inform predictions (fig. 23), and the analyses for the other 

streamflow-prediction sites all were similar to the analysis for 

site F3. This probably was a result of the same factors as those 

related to the hydraulic-head predictions, because sites F1–F7 

all are farther southwest of site H6 (figs. 22 and 23). 
34 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig20

Sakakawea

Lake

MONTANA NORTH DAKOTA

UNITED STATES

CANADA

Little Muddy

aquifer

Clear Lake

aquifer

Groundwater divide

H1

H2

H3

H4

H5 H6

0 10 20 MILES

0 10 20 KILOMETERS

49°

48°

105° 104°

SASKATCHEWAN

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

EXPLANATION

Reduction in uncertainty for 

hydraulic-head forecast at 

site H6, in percent

H1

> 20 to 30

> 10 to 20

> 5 to 10

> 1 to 5

> 0 to 1

Modeled dry cells

Extent of Clear Lake and Little Muddy 

aquifers within the glacial aquifer system

Existing hydraulic-head observation in the 

lower part of the glacial aquifer system

Simulated pumping well in the lower part 

of the glacial aquifer system

Hydraulic-head forecast in the lower part 

of the glacial aquifer system

WYOMING

MONTANA

SOUTH DAKOTA

NORTH DAKOTA

SASKATCHEWAN

MANITOBA

UNITED

STATES

CANADA

Active Model area

Map

area

Figure 20. Results of data-worth analysis for hydraulic-head forecasts at site H6, Williston Basin, United States and 

Canada. The figure shows the relative data-worth values associated with a potential new observation well that, if installed, 

could reduce simulated uncertainty of a hydraulic-head forecast at site H6. 
Improving Hydrologic Monitoring Networks 35

EXPLANATION

Reduction in uncertainty for 

hydraulic-head forecast at 

site H2, in percent

H1

> 20 to 25

> 10 to 20

> 5 to 10

> 1 to 5

> 0 to 1

Modeled dry cells

Extent of Clear Lake and Little Muddy 

aquifers within the glacial aquifer system

Existing hydraulic-head observation in the 

lower part of the glacial aquifer system

Simulated pumping well in the lower part 

of the glacial aquifer system

Hydraulic-head forecast in the lower part 

of the glacial aquifer system

Sakakawea

Lake

MONTANA NORTH DAKOTA

UNITED STATES

CANADA

Little Muddy

aquifer

Clear Lake

aquifer

Groundwater divide

H1

H2

H3

H4

H5 H6

0 10 20 MILES

0 10 20 KILOMETERS

49°

48°

105° 104°

SASKATCHEWAN

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

Figure 21. Results of data-worth analysis for hydraulic-head forecasts at site H2, Williston Basin, United States and Canada. 

The map shows the relative data-worth values associated with a potential new observation well that, if installed, could reduce 

simulated uncertainty of a hydraulic-head forecast at site H2.
36 Groundwater Availability of the Williston Basin, United States and Canada

tac18-1237_fig22

Sakakawea

Lake

MONTANA NORTH DAKOTA

UNITED STATES

CANADA

Little Muddy

aquifer

Clear Lake

aquifer

Groundwater divide

H1

H2

H3

H4

H5 H6

0 10 20 MILES

0 10 20 KILOMETERS

49°

48°

105° 104°

SASKATCHEWAN

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

EXPLANATION

Reduction in uncertainty for 

hydraulic-head forecast at 

site H1, in percent

H1

> 20 to 53

> 10 to 20

> 5 to 10

> 1 to 5

> 0 to 1

Modeled dry cells

Extent of Clear Lake and Little Muddy 

aquifers within the glacial aquifer system

Existing hydraulic-head observation in the 

lower part of the glacial aquifer system

Simulated pumping well in the lower part 

of the glacial aquifer system

Hydraulic-head forecast in the lower part 

of the glacial aquifer system

Figure 22. Results of data-worth analysis for hydraulic-head forecasts at site H1, Williston Basin, United States and 

Canada. The map shows the relative data-worth values associated with a potential new observation well that, if installed, 

could reduce simulated uncertainty of a hydraulic-head forecast at site H1.
Improving Hydrologic Monitoring Networks 37

tac18-1237_fig23

Sakakawea

Lake

MONTANA NORTH DAKOTA

UNITED STATES

CANADA

Little Muddy

aquifer

Clear Lake

aquifer

Groundwater divide

0 10 20 MILES

0 10 20 KILOMETERS

49°

48°

105° 104°

SASKATCHEWAN

Base modified from U.S. Geological Survey and other Federal digital data, various scales.

EXPLANATION

Reduction in uncertainty for 

streamflow forecast at 

site F3, in percent

> 20 to 63

> 10 to 20

> 5 to 10

> 1 to 5

> 0 to 1

Modeled dry cells

Extent of Clear Lake and Little Muddy 

aquifers within the glacial aquifer system

Existing hydraulic-head observation in the 

lower part of the glacial aquifer system

Simulated pumping well in the lower part 

of the glacial aquifer system

F1 Streamflow forecast

F7

F6

F5

F4

F3

F1

F2

Figure 23. Results of data-worth analysis for streamflow forecasts at site F3, Williston Basin, United States and Canada. 

The map shows the relative data-worth values associated with a potential new observation well that, if installed, could 

reduce simulated uncertainty of a streamflow forecast at site F3.
38 Groundwater Availability of the Williston Basin, United States and Canada

Numerical Groundwater-Flow Model 

Uses, Limitations, and Challenges

The numerical groundwater-flow model described 

herein is useful for groundwater resource management and 

planning (Davis and Long, 2018a). Groundwater resources 

are affected by climate and the use of groundwater for human 

activities, for which numerous scenarios are possible during 

the coming years and decades. Scenario examples show how 

this model can be used to assess and plan for hydrologic 

stressors; however, numerous additional scenarios could be 

simulated. The digital files associated with this model and 

scenario simulations are available to government entities and 

the public for simulating other scenarios of interest (Davis and 

Long, 2018b, 2018c). The model also may be useful for the 

planning of data-collection networks, as described in section, 

“Improving Hydrologic Monitoring Networks.” 

All models are simplified approximations of the real 

world, but different models are designed and applied to 

different scales of space and time. Because the model 

described herein is regional, its strength is in regional 

applications and assessments. The horizontal dimension of 

each model cell is 1 mi on each side and, therefore, the model 

cannot simulate groundwater gradients and flows at less than 

this scale. However, the effects of a single pumping well can 

be simulated in a meaningful way, especially if the well’s 

zone of influence extends into multiple model cells. A similar 

scale of effective simulation applies to the interaction of 

groundwater and surface water. The time scale of this model 

is separated into annual and 5-year periods within which 

hydrologic stresses are constant, meaning seasonal, monthly, 

or daily changes cannot be simulated. The strength of this 

model is in the assessment of interannual or interdecadal 

changes, such as a 5-year drought or long-term groundwater 

decline resulting from groundwater extraction. Additional 

details on model limitations were described in Davis and Long 

(2018a), such as limited data availability related to flowing 

artesian wells. 

A considerable challenge in assessing groundwater 

availability for the Nation is that this resource primarily 

is of a regional nature. A map of the principal aquifers of 

the United States illustrates the regional extent of these 

aquifers and aquifer systems (U.S. Geological Survey, 2018). 

Numerical groundwater models provide the best available 

scientific tool for assessing groundwater availability under 

potential future scenarios and hydrologic stressors. These 

models represent regional flow systems that are stressed by 

extensive human activity and climatic fluctuations. Although 

processes occurring at fine scales of time and space contribute 

to the processes occurring at long-term and regional scales, 

the models must be simplified and discretized into coarse 

scales because of limitations of computational power, as 

well as limitations imposed on the scope and time frame of 

the overall investigation. Therefore, regional groundwater 

availability assessments favor the big picture over fine-scale 

processes, important as they may be. Nonetheless, these bigpicture assessments are critical to understanding and assessing 

groundwater availability for the Nation, now and in the future. 

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Publishing support provided by the U.S. Geological Survey

Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Wyoming-Montana Water Science Center

U.S. Geological Survey

3162 Bozeman Avenue

Helena, Montana, 59601

https://www.usgs.gov/centers/wy-mt-water
ISSN 1044-9612 (print)

ISSN 2330-7102 (online)

https://doi.org/10.3133/pp1841

Long and others—Groundwater Availability of the Williston Basin, United States and Canada

—Professional Paper 1841

Groundwater Availability of the 

Williston Basin, United States and Canada