Water Availability and Use Science Program

Generalized Hydrogeologic Framework and Groundwater 

Budget for a Groundwater Availability Study for the Glacial 

Aquifer System of the United States

Scientific Investigations Report 2017–5015

U.S. Department of the Interior

U.S. Geological Survey
Cover: Glacial outwash near Ely, Minnesota. Photograph by Christopher Hoard, 

U.S. Geological Survey.
Generalized Hydrogeologic Framework and 

Groundwater Budget for a Groundwater 

Availability Study for the Glacial Aquifer 

System of the United States 

By H.W. Reeves, E.R. Bayless, R.W. Dudley, D.T. Feinstein, M.N. Fienen, 

C.J. Hoard, G.A. Hodgkins, S.L. Qi, J.L. Roth, and J.J. Trost

Water Availability and Use Science Program

Scientific Investigations Report 2017–5015

U.S. Department of the Interior

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

RYAN K. ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Deputy Director exercising the authority 

of the Director

U.S. Geological Survey, Reston, Virginia: 2017

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Suggested citation:

Reeves, H.W., Bayless, E.R., Dudley, R.W., Feinstein, D.T., Fienen, M.N., Hoard, C.J., Hodgkins, G.A., Qi, S.L., Roth, 

J.L., and Trost, J.J., 2017, Generalized hydrogeologic framework and groundwater budget for a groundwater availability study for the glacial aquifer system of the United States: U.S. Geological Survey Scientific Investigations Report 

2017–5015, 49 p., https://doi.org/10.3133/sir20175015.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-4196-8
iii

Contents

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

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

Background............................................................................................................................................1

Study Area..............................................................................................................................................2

Hydrogeology Overview .............................................................................................................3

Climate...........................................................................................................................................4

Purpose and Scope ..............................................................................................................................4

Groundwater Availability Issues for Glacial Aquifer System .................................................................6

Heterogeneity of Glacial Deposits ............................................................................................6

Importance of Material with Low Hydraulic Conductivity ....................................................6

Groundwater Quality Limitations...............................................................................................7

Groundwater/Surface-Water Interaction................................................................................7

Importance of Land Drainage Systems....................................................................................7

Importance of Spatial Scale ......................................................................................................7

Approach.........................................................................................................................................................8

Hydrogeologic Frameworks................................................................................................................8

Background...................................................................................................................................9

Hydrogeologic Framework Based on Aquifer Types and Hydrophysiographic 

Regions ............................................................................................................................9

Water-Well Record Analysis....................................................................................................10

Generalized Groundwater Budget Components............................................................................10

Recharge and Discharge..........................................................................................................10

Water Withdrawals and Water Use .......................................................................................12

Storage ........................................................................................................................................13

Hydrogeologic Framework and Groundwater Budget ..........................................................................14

Generalized Hydrogeologic Frameworks .......................................................................................14

Hydrogeologic Framework Based on the Quaternary Geologic Atlas..............................14

Hydrogeologic Framework Based on National Water-Quality Assessment 

Regions ..........................................................................................................................21

Summary of Hydrogeologic Frameworks ..............................................................................21

Generalized Groundwater Budget ...................................................................................................21

Recharge and Discharge..........................................................................................................25

East Region ........................................................................................................................30

Central Region...................................................................................................................30

West-Central Region ........................................................................................................30

West Region.......................................................................................................................30

Water Withdrawals ...................................................................................................................30

Storage and Changes in Storage ............................................................................................30

Groundwater Budgets...............................................................................................................40

Summary........................................................................................................................................................42

Acknowledgments.......................................................................................................................................42

References....................................................................................................................................................42
iv

Figures

1. Map showing study area for glacial aquifer study includes areas glaciated by the

Laurentide and Cordilleran ice sheets ......................................................................................3

2. Map showing location of continental ice sheets relevant to the glacial aquifer

system regional groundwater availability study......................................................................4

3. Map showing the four regions defined for the glacial aquifer system for the

National Water-Quality Assessment (NAWQA) Project principal aquifer analysis ..........5

4. Map showing base-flow index grid clipped to glacial extent...............................................8

5. Boxplot showing difference between the estimated base-flow index (BFI) for

various base-flow separation techniques for streamgages in the Great Lakes Basin

and BFI in the Geospatial Attributes of Gages for Evaluating Streamflow dataset

determined using the BFI method............................................................................................11

6. Boxplot showing difference between the estimated base-flow index (BFI) for

various base-flow separation techniques and BFI determined using the BFI method 

for reference gages in the Geospatial Attributes of Gages for Evaluating Streamflow 

dataset..........................................................................................................................................11

7. Graph showing base-flow index (BFI) computed using the HYSEP3 method plotted

against the estimated BFI using the BFI method for streamgages in the Great Lakes

Basin from the Geospatial Attributes of Gages for Evaluating Streamflow

(GAGES II) dataset......................................................................................................................12

8. Graph showing base-flow index (BFI) computed using the HYSEP3 method plotted

against the estimated BFI using the BFI method for 391 reference gages in the

glacial aquifer study area from the Geospatial Attributes of Gages for Evaluating

Streamflow (GAGES II) dataset................................................................................................12

9. Graph showing base-flow index (BFI) computed using the HYSEP3 method plotted

against the estimated BFI using the BFI method for 391 reference gages in the

glacial aquifer system study area from the Geospatial Attributes of Gages for

Evaluating Streamflow (GAGES II) dataset ............................................................................13

10. Pie chart showing distribution of estimated groundwater withdrawals in million

gallons per day from source aquifers in the Lake Michigan Basin groundwater-flow

model, 2001–5 ..............................................................................................................................13

11. Map showing stratified sand and gravel aquifers in the Northeastern United States...15

12. Map showing potential aquifer material based on classified map units of

Quaternary geologic atlas.........................................................................................................16

13. Map showing Minnesota Quaternary hydrogeology indicating three major aquifer

types: alluvium, lake deposits, and outwash..........................................................................19

14. Map showing potential aquifer material based on classified map units for potential

aquifer from the Quaternary geologic atlas ...........................................................................20

15. Map showing North Dakota surficial aquifers map..............................................................22

16. Map showing potential aquifer material based on classified map units of

Quaternary geologic atlas.........................................................................................................23

17. Map showing estimated sand and gravel thickness for North Dakota from

interpolation of water-well records.........................................................................................24

18. Map showing hydrogeologic framework for the glacial aquifer system study based

on classification of mapped units derived from the Quaternary geologic atlas ..............25

19. Map showing hydrogeologic framework for the glacial aquifer system study based

on classification of mapped units derived from the Quaternary geologic atlas ..............26
v

20. Map showing sand and gravel thickness for the glacial aquifer system derived

from analysis of water-well records........................................................................................27

21. Diagram showing idealized groundwater-flow paths in a valley-fill aquifer system

with till and bedrock uplands showing various recharge mechanisms............................28

22. Map showing estimated mean annual recharge for the conterminous glacial

aquifer system study area from a grid of the conterminous United States ......................29

23. Map showing estimated mean annual precipitation for the period 1981–2010 and

estimated recharge for the East region of the study area...................................................31

24. Map showing estimated mean annual precipitation for 1981–2010 and estimated

recharge for the Central region of the study area ................................................................32

25. Map showing estimated mean annual precipitation for 1981–2010 and estimated

recharge for the West-Central region of the study area .....................................................33

26. Map showing estimated mean annual precipitation for 1981–2010 and estimated

recharge for the West region of the study area....................................................................34

27. Graph showing estimated fresh groundwater withdrawals from the glacial aquifer

system by glacial availability study region.............................................................................35

28. Graphs showing estimated fresh groundwater withdrawals from the glacial aquifer

system by type and glacial availability study region ............................................................36

29 Graphs showing changes in terrestrial water storage derived from GRACE satellite

data from 2002 to 2014................................................................................................................39

30. Block diagrams showing estimated storage, recharge, discharge, and groundwater

withdrawals for the four glacial aquifer system regions .....................................................41

Tables

1. Characterization of National Water-Quality Assessment Project regions used in

the principal aquifer study ..........................................................................................................6

2. Quantitative comparison of Quaternary geologic atlas classification to stratified

sand and gravel aquifers in the Northeastern United States..............................................16

3. Quantitative comparison of stratified sand and gravel aquifer types of the

Northeastern United States to classified mapped units in the Quaternary geologic

atlas...............................................................................................................................................17

4. Quantitative comparison of Quaternary geologic atlas classification to the

Minnesota Quaternary hydrogeology map.............................................................................21

5. Quantitative comparison of Minnesota Quaternary hydrogeology map to classified

mapped units in the Quaternary geologic atlas.....................................................................21

6. Mapped unit and representative thickness and specific yield and specific storage 

values assigned for estimation of groundwater in storage.................................................37

7. Estimated groundwater storage in the glacial aquifer system...........................................38

8. Summary of generalized annual water budgets for four regions of glacial aquifer

system study................................................................................................................................40
vi

Conversion Factors

[U.S. customary units to International System of Units]

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Volume

cubic foot (ft3) 28.32 cubic decimeter (dm3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic mile (mi3) 4.168 cubic kilometer (km3) 

Flow rate

acre-foot per year (acre-ft/yr) 0.01427 cubic meter per second (m3/s)

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

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3/yr)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

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

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

[International System of Units to U.S. customary units]

Multiply By To obtain

Length

kilometer (km) 0.6214 mile (mi)

Volume

cubic kilometer (km3) 0.2399 cubic mile (mi3) 

Flow rate

meter per year (m/yr) 3.281 foot per year ft/yr) 
vii

Datum

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

(NAVD 88).

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

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

Abbreviations

BFI Base-flow index

GAGES II Geospatial Attributes of Gages for Evaluating Streamflow, version II

GRACE Gravity Recovery and Climate Experiment

HYSEP Hydrograph separation program

NAWQA National Water-Quality Assessment

RASA Regional Aquifer-System Analysis

SMCL Secondary Maximum Contaminant Level

SWB Soil Water Balance

USGS U.S. Geological Survey

Generalized Hydrogeologic Framework and Groundwater 

Budget for a Groundwater Availability Study for the 

Glacial Aquifer System of the United States

By H.W. Reeves, E.R. Bayless, R.W. Dudley, D.T. Feinstein, M.N. Fienen, C.J. Hoard, G.A. Hodgkins, S.L. Qi, 

J.L. Roth, and J.J. Trost

Abstract

The glacial aquifer system groundwater availability study 

seeks to quantify (1) the status of groundwater resources in the 

glacial aquifer system, (2) how these resources have changed 

over time, and (3) likely system response to future changes 

in anthropogenic and environmental conditions. The glacial 

aquifer system extends from Maine to Alaska, although the 

focus of this report is the part of the system in the conterminous United States east of the Rocky Mountains. The glacial 

sand and gravel principal aquifer is the largest source of public 

and self-supplied industrial supply for any principal aquifer 

and also is an important source for irrigation supply. Despite 

its importance for water supply, water levels in the glacial 

aquifer system are generally stable varying with climate and 

only locally from pumping. The hydrogeologic framework 

developed for this study includes the information from waterwell records and classification of material types from surficial 

geologic maps into likely aquifers dominated by sand and 

gravel deposits. Generalized groundwater budgets across the 

study area highlight the variation in recharge and discharge 

primarily driven by climate. 

Introduction

The Glacial Aquifer System Groundwater Availability 

Study assesses groundwater availability for the expansive and 

diverse glacial aquifer system of the United States. The glacial 

aquifer system is present in parts of 26 States and is subject to 

a range of climatic conditions: humid to semiarid, maritime to 

continental to arctic. Groundwater availability in the system 

may be constrained by climatic conditions, poor water quality 

from natural or anthropogenic constituents, hydrogeology, 

competing uses, or the discharge needed to maintain or restore 

environmental streamflows. The glacial aquifer system is a 

major source of water for public, self-supplied domestic and 

industrial, and irrigation water use (Maupin and Barber, 2005). 

The study seeks to quantify (1) the status of groundwater 

resources in the glacial aquifer system, (2) how these resources 

have changed over time, and (3) likely system response to future 

changes in anthropogenic and environmental conditions.

Background

The U.S. Geological Survey (USGS) Groundwater 

Resources Program (now incorporated into the Water Availability and Use Science Program) began an assessment of 

groundwater availability in the principal aquifers of the United 

States in 2004. Several studies have been completed: Northern 

Atlantic Coastal Plain Aquifer System, Columbia Plateau 

Regional Aquifer System, High Plains Aquifers, Mississippi 

Embayment Regional Aquifer Study, Great Basin Carbonate 

and Alluvial Aquifer System, Central Valley Aquifer, North 

and South Carolina Atlantic Coastal Plain Aquifer System, 

Denver Basin Aquifer, and Middle Rio Grande Basin Study; 

several are underway in addition to the Glacial Aquifer 

System: Pacific Northwest Volcanic Aquifer System, Pennsylvanian and Mississippian Aquifer System of the Appalachian Plateaus, Ozark Plateaus Aquifer System, Hawaii 

Volcanic-Rock Aquifers, Williston and Powder River Basins, 

and Floridan Aquifer System (U.S. Geological Survey, 2017). 

These regional studies aim to provide the required information for a national assessment of groundwater resources. The 

glacial aquifer system often begins at land surface, which 

makes it vulnerable to contamination from surface activities 

and important in providing base flow to streams. The vast 

extent, heterogeneity of aquifer material, and range of climatic 

conditions for the glacial aquifer system impose challenges to 

this regional groundwater availability study.

The glacial geology of North America has been studied 

for more than 150 years: “The literature of American glacial 

geology is already very extensive, and every year is adding 

to its bulk * * *. It is no exaggeration to say, that, the whole 

surface of North America, from the shores of the Arctic 

Ocean to the latitude of New York, and from the Pacific to the 

Atlantic, has been scarped, scraped, furrowed, and scoured 

by the action of ice” (Giekie, 1874). Despite changes in the 
2 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

interpretation of the mechanisms of glaciation that led to 

modern conditions, the work of scientists interpreting the 

glaciated landscape, including Hitchcock (1841), Agassiz 

(1876), Chamberlain (1894, 1895), Leverett and Taylor 

(1915), and numerous others, is the foundation of scientific 

interpretation of glacial geology. Importantly, the practice 

of interpreting glacial geology based on the modern-day 

landforms resulting from glacial action emanates from 

these early studies and shapes modern approaches based on 

morphostratigraphy (Frye and Willman, 1962) or morphosequences (Koteff and Pessl, 1981; Stone and Stone, 2005). 

From the perspective of groundwater resources, depositional 

features have attendant aquifer properties and the hydrogeologic framework for the glacial aquifer system relies on 

interpretation of glacial geology informed by previous work in 

hydrogeology across parts of the glacial aquifer system (Eyles, 

1983; Eyles and others, 1985; Anderson, 1989). 

Climatic conditions, including average precipitation, 

temperature, and wind speed, directly affect groundwater 

availability. The recharge to groundwater in semiarid or arid 

regions may be so low that withdrawals from the system lead 

to groundwater mining and depletion of the resource; however, 

the effects of climatic conditions in parts of the glacial aquifer 

system may be more subtle. Seasonal, annual, or decadal 

variations in climate can lead to changes in the recharge to the 

groundwater system and the demands on the system for public, 

domestic, irrigation, and industrial supply. The combined 

effect of lower recharge and higher demand may be transient, 

but it can have undesired consequences such as land subsidence, temporary streamflow depletion, water levels dropping 

below the level of existing pumps or wells, and migration of 

poor-quality water into the aquifer. Understanding climatic 

variations and the effects on the groundwater system is important because these undesired conditions may occur in an area 

that appears to have sufficient surface-water and groundwater 

resources to meet demands if only average conditions are 

considered. 

Study Area

The glacial aquifer system in the United States extends 

from Maine to Alaska (fig. 1). The system is made of material deposited by Laurentide and Cordilleran ice sheets (fig. 2) 

that extended into North America in series of advances and 

retreats of continental glaciers between 2.5 million and 12,000 

years ago (Dorr and Eshman, 1970; Booth and others, 2003; 

Marshall and others, 2003; Mickelson and Colgan, 2003). In 

some areas, later glaciation removed old material and only 

material from the last advance is found; but, in other areas, 

glacial deposits from sequential advances and retreats are 

present. The dynamics of deglaciation and the resulting landforms and glacial geology are subjects of much research that 

can help provide a foundation for the glacial aquifer system 

study. The glacial landforms and land systems have characteristic material that affects groundwater yield, aquifer hydraulic 

properties, and, ultimately, groundwater availability at both 

local and regional scales (Eyles, 1983; Eyles and others, 1985; 

Anderson, 1989).

The deposits that form the glacial aquifer system are 

dominated by material deposited by continental glaciation; 

however, material of more recent deposition is present in the 

study area. Much of the alluvial deposits in the study area are 

comprised of material originally deposited by glaciers that has 

subsequently been transported and reworked. Classifying these 

alluvial deposits separately from glacial deposits at the scale 

of the study area would unnecessarily complicate the regional 

analysis. All unconsolidated material north of the extent of 

glaciation, including recent glacial deposits in areas such as 

Alaska, will be considered as part of the glacial aquifer system 

even if the material has been deposited more recently by other 

mechanisms. 

Large continental glaciers, including those that deposited 

the material in the glacial aquifer system study area, affect 

groundwater systems in ways other than deposition of material 

(Callegary and others, 2013). These continental glaciers can 

profoundly change hydraulic pressures and force freshwater 

into the underlying unconsolidated and consolidated materials, and this introduction of water can lead to geochemical 

alteration (Person and others, 2007). High hydraulic pressures 

and loading by glaciers can deform geologic units, fracture 

bedrock, and change porosity and permeability. These other 

features of glacial interaction with groundwater are not 

discussed as part of the framework.

The glacial sand and gravel principal aquifer as defined 

by Miller (1999) and studied as part of the northeast Regional 

Aquifer-System Analysis (RASA) (Haeni, 1995; Randall, 

2001; Kontis and others, 2004) was defined to include sand 

and gravel deposits from the Laurentide ice sheet and not the 

Cordilleran deposits in the western continental United States 

and Alaska; therefore, the glacial sand and gravel principal 

aquifer is contained within the glacial aquifer system. This 

groundwater availability study follows the study area established for the National Water-Quality Assessment (NAWQA) 

Project (Warner and Arnold, 2006) and includes the entire 

glacial aquifer system. This definition of the study area is 

broader than the glacial sand and gravel principal aquifer 

because (1) the regional contributions of the material from the 

Cordilleran ice sheet to estimates of groundwater availability 

are included, and (2) low conductivity material associated with 

glacial deposits may be important in contributing to groundwater availability. The study area is quite broad because, 

for areas glaciated by either the Laurentide or Cordilleran 

ice sheet, water from domestic wells for potable supply and 

local wells for irrigation or industrial use may be produced 

from glacial deposits with low hydraulic conductivity or local 

isolated sand or gravel units that are typically not mapped 

as sand and gravel aquifers. Deposits with lower hydraulic 

conductivity also contribute important storage to the system 

and can be important to water availability and water-quality 

characteristics anywhere in the system.
Introduction 3

 

 

 

 

0

0

250

250

50 MILES

500 KILOMETERS

Extent of glacial 

 deposits

EXPLANATION

D.R. Soller, P.H. Packard, and C.P. Garrity, 2012;

R.F. Flint, W.C. Alden, E.T. Apfel, H.S. Bostock, S.R. Capps, J.W. Goldthwait, L.M. Gould, 

G.F. Kay, M.M. Leighton, Frank Leverett, Paul MacClintock, D.A. Nichols, H. Norman, 

F.T. Thwaites, G.W. White, and G.A. Young, 1945

Figure 1. Study area for glacial aquifer study includes areas glaciated by the Laurentide and Cordilleran ice sheets.

The study area was divided into four regions for the 

NAWQA glacial principal aquifer study (fig. 3) (Warner 

and Ayotte, 2014). The NAWQA regions were developed 

to classify the glacial aquifer system based on two factors: 

(1) intrinsic susceptibility to contamination, and (2) vulnerability of the aquifer to contamination. Intrinsic susceptibility 

depends only on the physical characteristics of the aquifer and 

the ease at which contaminants may move through the aquifer 

system. Vulnerability, however, considers both intrinsic 

susceptibility and the potential for contamination based on 

sources, the characteristics of potential contaminants, and the 

geochemical conditions of the aquifer (Warner and Arnold, 

2006). The characteristics of these regions are summarized 

in table 1. These regions will be retained in this report to 

facilitate future incorporation of the water-quality characteristics of the glacial aquifer system noted by Warner and Ayotte 

(2014) into water budget analysis presented in this report. 

The groundwater availability analysis additionally will benefit 

from the previous NAWQA regional assessment (Warner and 

Ayotte, 2014), and results from the availability analysis may 

be used to support continued regional work (Burow and Belitz, 

2014).

Hydrogeology Overview

The hydrogeology of the glacial aquifer system varies 

as reflected by regional spatial landforms and has notable 

heterogeneity at local scales. The materials deposited by 

glaciers range in size from clay particles to boulders. Drift 

refers to all material transported and ultimately deposited by 

glaciers through various mechanisms (Foster, 1983). Depositional features fall broadly into two classes: (1) till features, 

which comprise material deposited by glacial ice as unsorted 

mixtures of sizes; and (2) outwash features, which are deposited by glacial meltwater, tend to be more sorted, and may be 

coarse grained (Dorr and Eshman, 1970). In this report, till 

will refer to all unsorted to poorly sorted deposits associated 

with glaciers from various mechanisms. Till features tend to 

have relatively low hydraulic conductivity and may locally 

serve as confining units, whereas outwash features often have 

higher hydraulic conductivity and may be local aquifers. Variations in local terminology, however, can lead to contradictions of these generalizations. In some parts of the study area, 

units described as coarse tills may be marginal aquifers. In all 

parts of the study area, deposits often show abrupt changes 

in lithology at local scales, and aquifers may be difficult to 

identify and correlate in space. 
4 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

150°

Cordilleran

 Ice Sheet

Laurentide Ice Sheet

140° 130° 120° 110° 100° 90° 80° 70° 60°

40°

50°

60° AK

WA NH

MA

RI

CN

NJ

DE

MD

ME

NY

CO

UT

NV

WY

ID

CA

OR

PA

OH

MI

KY

VA WV

IL

WI

IN

MO

MN

IA

SD

NE

KS

MT

AZ

ND

VT

NC

CANAD A

Base map of states from U.S. Geological Survey 1:2,000,000 Digital Data Albers Equal-Area Conic projection, 

Standard parallels 29° 30' and 45° 30', central meridian -96°, latitude of origin 23°

Mapped glacial regions are from U.S. Geological Survey DDS–38, Geological Society of America Glacial 

Map of North America (1945), and the Geological Survey of Canada's Late Wisconsinin and Holocene 

Retreat of the Laurentide Ice Sheet (1986)

2500 500 KILOMETERS

0 250 500 MILES

EXPLANATION

Glaciated areas

Unglaciated areas within 

the United States

Figure 2. Location of continental ice sheets relevant to the glacial aquifer system regional groundwater availability study (modified 

from Bayless and others, 2017).

A glaciated area in North Dakota provides one example 

of this complexity as exhibited in a prairie wetland complex. 

This wetland complex is located on the Coteau du Missouri, a 

glacial moraine formed by highly heterogeneous glacial tills 

that can contain individual fluvial deposits of sand and gravel 

within a clay matrix of low permeability. This setting helps 

control the location and behavior of the wetland complex 

that often has seasonal flow reversals in the groundwater 

system between aquifers and local wetlands (Winter, 2003). 

Other glacial settings across the glacial aquifer system exhibit 

different depositional features and different hydrogeologic 

controls on groundwater flow. Challenges to this study include 

synthesizing numerous studies of the geology and hydrogeology conducted at various spatial scales across the glacial 

aquifer system to assess regional groundwater availability. 

Climate

Climate varies substantially across the glacial aquifer 

system. For example, the average annual precipitation from 

1981 to 2010 was less than 10 inches (in.) in parts of Montana 

and North Dakota and over 70 in. in the Northwest Pacific and 

parts of Alaska (National Oceanic and Atmospheric Administration, 2017a, b). This variation is coupled with hydrogeologic conditions to provide a major control on the groundwater 

availability of the system.

Purpose and Scope

In this report, several hydrogeologic frameworks 

for the glacial aquifer system will be assessed for use in 

understanding groundwater availability of the aquifer. The 

framework is designed to focus the study on aspects of the 

system that control groundwater availability. The developed 

framework also will be used to relate groundwater availability analysis from a water budget and potential constraint 

framework to previous synthesis of groundwater quality for 

the glacial aquifer system. In addition to the hydrogeologic 

framework, estimated regional and subregional groundwater 

budgets are presented and discussed. The framework and 
Introduction 5

130° 120° 110° 100° 90° 80° 70°

40°

30°

20°

EXPLANATION

Glacial aquifer system regions

East

Central

West-Central

West

AK

160°W

160°W 150°W 140°W 130°W

170°E 180° 170°W

80°N

70°N

60°N

50°N

40°N

0 250 500 MILES

0 250 500 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

2500 500 KILOMETERS

0 250 500 MILES

WA

NH

MA

CN

NJ

RI

DE

MD

ME

NY

CO

UT

NV

WY

ID

CA

OR

PA

OH

MI

KY

WV VA

IL

TN

GA

FL

WI

IN

MO

AR

MS

LA

AL

MN

IA

SD

NE

KS

OK

MT

AZ NM

TX

ND

VT

SC

NC

Figure 3. The four regions defined for the glacial aquifer system for the National Water-Quality Assessment (NAWQA) Project principal 

aquifer analysis (modified from Warner and Ayotte, 2014).
6 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

Table 1. Characterization of National Water-Quality Assessment Project regions used in the principal aquifer study (modified from 

Warner and Ayotte, 2014).

Characterization of 

glacial regions East Central West-Central West

Topography Mountainous Flat Flat Mountainous

Climate Humid Humid Arid/humid Arid/humid

Thickness Moderately thin Thick Thick Moderately thick

Characterization of sediment Mixed—more coarse than 

fine

Mixed—more fine than 

coarse

Mixed – more fine than 

coarse

Mixed—more coarse 

than fine

Common bedrock Crystalline Carbonate Shale and Carbonate Crystalline

Major glacial aquifer use Drinking water Drinking water Drinking water and irrigationDrinking water and 

irrigation

Characteristic land use Urban and forested Agriculture Agriculture Forested and urban

Other principal aquifers 

underlying the glacial 

aquifer system

New York-New England 

carbonate-rock aquifers

New England crystalline 

rock aquifer

Early Mesozoic basin 

aquifer

Cambrian-Ordovician 

aquifer

Silurian-Devonian aquifer

Mississippian aquifer

Pennsylvanian aquifer

Cambrian-Ordovician 

aquifer

Silurian-Ordovician 

aquifer

Mississippian aquifer

High Plains aquifer

Lower Tertiary sandstone

Upper Cretaceous sandstone

none

groundwater budgets provide the foundation for numerical 

groundwater-flow models that can be developed as part of the 

study to quantify the response of parts of the glacial aquifer 

system to changes in groundwater withdrawals and climate. 

Groundwater-flow models are essential to groundwater availability studies because the effect of development on the system 

is estimated using these models, and groundwater availability 

may ultimately be determined through such an assessment 

of these effects. The scope of the report is the glacial aquifer 

system of the United States (fig. 1), although the major focus 

is on the part of the system in the conterminous United States 

east of the Rocky Mountains.

Groundwater Availability Issues for 

Glacial Aquifer System

The glacial aquifer system is unique among principal 

aquifers because of several factors including its large size, 

the range of climatic conditions across the system, and the 

diversity of hydrogeologic conditions within the system. The 

groundwater availability study must consider parts of the 

system that may be over 1,000 feet (ft) thick or less than a few 

feet thick, the hydraulic conductivity may range over several 

orders of magnitude, and the arrangement of the materials can 

vary from nearly uniform layers of gravel, sand, silt, or clay to 

poorly sorted mixtures of these materials. These factors lead to 

several issues confronting the groundwater availability study 

of the glacial aquifer system. 

Heterogeneity of Glacial Deposits

Glacial deposits are quintessentially heterogeneous, and 

site-specific layers or lenses ranging from less than a foot to 

several feet thick and extending from tens to hundreds of feet 

may have profound consequences on local groundwater availability, especially in determining water-quality conditions that 

may either limit or provide potable water (Brusseau, 1994). 

Groundwater availability on a regional scale is less influenced 

by local heterogeneity because the general spatial patterns and 

temporal trends can be identified at the regional scale. Heterogeneity is acknowledged as crucial in the local interpretation 

of these patterns and trends; notably, local conditions may 

vary from the regional setting because of local depositional 

processes and the interaction between hydrogeology, climate, 

and local stresses such as pumping wells or land-use change.

Importance of Material with Low Hydraulic 

Conductivity

Previous regional studies on groundwater availability for 

parts of the glacial aquifer system include Vaccaro and others 

(1998), Randall (2001), and Kontis and others (2004), and the 

regional study in the northeastern United States (Randall, 2001; 

Kontis and others, 2004) focused on stratified sand and gravel 

aquifers. The USGS NAWQA Project synthesis for the glacial 

principal aquifer considered all unconsolidated material north 

of the extent of glaciation including clays and silts in addition 

to sands and gravels (Warner and Arnold, 2006). This report 

adopts the latter approach in order to include both productive 

sand and gravel deposits and less productive glacial deposits, 
Groundwater Availability Issues for Glacial Aquifer System 7

all parts of the glacial aquifer system, to provide a context for 

groundwater availability analysis across the study area. 

Groundwater Quality Limitations

Water-quality characteristics identified by the USGS 

NAWQA Project for the glacial aquifer system include (1) 

contaminants from geologic sources, in particular arsenic and 

manganese, are a potential concern for human health; (2) in 

agricultural areas, concentrations of nitrate and pesticides 

were usually low in groundwater associated with fine-grained 

glacial deposits but could be high in groundwater associated with coarse-grained glacial deposits; (3) in urban areas, 

chloride concentrations in groundwater tend to be increasing; 

and (4) 75 percent of samples from drinking-water wells in 

the glacial aquifer system had concentrations exceeding a 

U.S. Environmental Protection Agency secondary maximum 

contaminant level (SMCL) (Warner and Ayotte, 2014). The 

SMCLs are nonregulatory guidelines developed to advise 

decision makers and the public about issues such as unpleasant 

taste or odor; staining of skin or teeth; and staining of laundry, 

dishes, or plumbing fixtures. Iron was the most common 

constituent that exceeded the SMCL in the samples. Other 

constituents associated with exceedances in SMCL include 

manganese, chloride, sodium, sulfate, and aluminum. Water 

with exceedances in SMCL often also has high dissolved 

solids and hardness (Warner and Ayotte, 2014). Current 

NAWQA studies in the glacial aquifer system are aimed at 

monitoring to analyze trends and patterns and the development 

of tools to inform decision makers about potential waterquality issues and effects of management actions, particularly 

for groundwater accessed by public-supply wells (Rowe and 

others, 2013).

Groundwater/Surface-Water Interaction

Aquifers in glacial deposits are expected to be the shallowest aquifers across the study area; therefore, the interaction 

between groundwater and surface water is an important aspect 

of the glacial aquifer system. Groundwater provides base flow 

to streams across the study area (Winter and others, 1998; 

Healy and others, 2007; Reilly and others, 2008), and the relative amount of base flow to total streamflow for streams in the 

study area varies according to the hydrogeology of the glacial 

deposits and climatic factors. The base-flow index (BFI), 

ratio of base flow to total streamflow, across the conterminous 

United States is estimated to range from almost 6 percent to 

over 90 percent (fig. 4; Wolock, 2003a). 

The importance of base flow in many parts of the study 

area has motivated a great deal of interest in potential streamflow depletion by pumping wells (Barlow and Leake, 2012), 

and several States in the study area, including Massachusetts, 

Michigan, Rhode Island, and Washington, have adopted regulations or management goals to maintain environmental flows 

in streams that consider the potential for streamflow depletion 

by wells (Kendy and others, 2012; State of Washington 

Department of Ecology, 2015).

Importance of Land Drainage Systems

Many parts of the study area, often where base flow index 

is shown to be low in figure 4, have been artificially drained 

by the installation of agricultural drainage tiles. This drainage 

lowers the average water table, and even though the lowering 

may be relatively small, on the order of 1–10 ft, the cumulative 

effect on the volume of groundwater stored in the glacial aquifer 

system can be quite large (Konikow, 2013). Drained glacial 

systems also have been recognized as important in the transport 

of nutrients and other constituents to streams (Dubrovsky and 

others, 2010; Minnesota Pollution Control Agency, 2013). As 

previously noted, in agricultural parts of the study area that are 

dominated by fine-grained material, groundwater may be low in 

nitrate and pesticides, but streams may have elevated concentrations because of transport of these constituents in tile systems. 

In parts of the system dominated by coarse-grained deposits, 

the aquifer system is more likely to have elevated nitrate or 

pesticide concentrations, and groundwater can transport these 

constituents to streams. Drainage tiles continue to be installed 

because of the agricultural benefits offered by controlling soil 

moisture, and the installation has been extended to areas where 

drains typically have not been used (Associated Press, 2013). 

Attention has been directed to the water-quality effects of 

agricultural land and the potential for streamflow depletion by 

pumping wells. To address concerns for both of these topics, 

control of runoff from tile drains coupled with appropriate irrigation has been proposed to increase food production, maintain 

environmental flows, and decrease runoff of nutrients linked 

to water-quality degradation in the Midwestern United States 

(Baker and others, 2012).

Importance of Spatial Scale

Understanding spatial scale and the relation between 

spatial scale, potential groundwater availability constraints, data 

requirements, and appropriate analytical approaches is very 

important for regional groundwater availability studies in order 

to match the scale of the analysis to the questions that can be 

addressed. Because of the large size of the study area, conditions and stresses in one part of the study area do not affect 

the entire study area. Questions related to the spatial scale of 

heterogeneities within glacial deposits or the effects of pumping 

from an aquifer on nearby surface-water features are appropriately addressed at the local scale. The study of site-specific 

groundwater availability conditions across the entire study area 

is infeasible; therefore, regional summaries will be sought in 

the analysis, and methods to estimate local responses within 

regional systems will be tested. The data and analysis used for 

the hydrogeologic frameworks and the groundwater availability 

questions addressed will be consistent with the scale of the 

analysis.
8 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

EXPLANATION

Base-flow index

High : 90

Low : 6

0 300 600 MILES

0 300 600 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

130° 120° 110° 100° 90° 80° 70°

40°

30°

WA

NH

MA

CN

NJ

RI

DE

MD

ME

NY

CO

UT

NV

WY

ID

CA

OR

PA

OH

MI

KY

WV VA

IL

TN

GA

FL

WI

IN

MO

AR

MS

LA

AL

MN

IA

SD

NE

KS

OK

MT

AZ NM

TX

ND

VT

SC

NC

Figure 4. Base-flow index grid clipped to glacial extent (from Wolock, 2003a).

Approach 

Groundwater availability depends on several factors: 

hydrogeology; climate; existing water use; and imposed social, 

economic, or legal constraints on groundwater extraction. The 

hydrogeology of a system determines how groundwater moves 

through the system, governs how quickly the system will 

respond to changes in external conditions such as changes in 

climate or pumping, and controls the volume of groundwater 

in storage. Climate determines the amount of water that might 

be available for recharge to the system. Climate also affects 

seasonal and long-term variations in groundwater levels. 

The response of the groundwater system to change, current 

water use, and constraints such as instream flow requirements 

determines the amount of groundwater that is economically 

available for use without an unacceptable effect on existing 

users or ecosystems. 

Groundwater availability studies typically propose a 

hydrogeologic framework of the study area to define the 

system and focus efforts on the parts of the system that control 

groundwater availability (Masterson and others, 2013). The 

range of climatic and geologic conditions across the glacial 

aquifer system motivates development of a broad hydrogeologic framework that captures important factors that may 

control groundwater availability. A framework includes hydrogeology and components of the groundwater budget related to 

climate and water use.

Hydrogeologic Frameworks

Hydrologic frameworks for the glacial aquifer system are 

developed to describe the important features of the system that 

provide or limit groundwater across the study area. The frameworks are used to identify the features of the system used to 

quantify groundwater availability. Two main approaches to the 

framework are considered. The first approach is to adopt the 

four regions used by the USGS NAWQA Project in the recent 

regional assessment of groundwater quality in the glacial 

aquifer system (fig. 3). The second approach is to develop 

hydrogeologic settings across the study area that build on the 

study of stratified sand and gravel aquifers in the northeastern 

part of the study area (Randall, 2001; Kontis and others, 

2004). In this approach, the glacial aquifer system will be 

subdivided into units with similar hydrogeology to allow for 

analysis of similar units across the system. This analysis will 

be used to contrast the effects of hydrogeology and climatic 

variation across the system on groundwater availability.
Approach 9

Background

Meinzer (1923) classified groundwater resources for the 

United States by physiographic regions and the age of the 

aquifer material. That work lays the groundwork for subsequent hydrogeologic frameworks used through the present 

day. Meinzer (1923) states the importance of Quaternary-age 

deposits and notes that glacial deposits are one of class of 

these deposits: 

The Quaternary is by far the important system in the 

United States with respect to water supply. Indeed, 

it would probably not be an exaggeration to say that 

it is as important as all other systems taken together. 

It lies at the surface throughout the largest area, 

supplies the most wells, and affords the greatest 

quantities of water.

The Quaternary deposits of the United States are for 

the most part included in three groups—glacial drift 

[glacial deposits], the valley fill of the West, and the 

deposits of the Atlantic Coastal Plain. Both glacial 

drift and valley fill are of especial importance as 

sources of water, the drift being the principal source 

of ground water in the northern part of the country 

and the fill being the principal source in the western 

part.

Meinzer (1923) continues the discussion of glacial 

drift deposits by citing 84 reports written at the State level 

describing water resources within the glacial aquifer system; 

the importance of the glacial aquifer system to water supply 

was clearly recognized and described starting in the late 19th 

century.

The groundwater regions based on physiography and age 

(Meinzer, 1923) were merged and simplified into 10 regions 

by Thomas (1951). These 10 regions were subsequently used 

by McGuinness (1951, 1963) in summaries to Congress on 

groundwater conditions in the United States. The current study 

area is covered by 3 of the 10 regions: Western Mountain 

Ranges, Glaciated Central region, and Glaciated Appalachian 

region. Alaska was included in the State summary section 

of the 1963 report, but it was not assigned to a groundwater 

region (McGuinness, 1963). 

Heath (1984) modified the work by Thomas (1951) 

and added Alluvial Valley, Southeast Coastal Plain, Hawaii, 

Alaska, and Puerto Rico and U.S. Virgin Islands to create 15 

groundwater regions. Note that the glaciated and unglaciated Appalachian regions were included by Heath (1984) in 

the glaciated and unglaciated Central regions. The relevant 

regions for this framework report are the Western Mountain 

Ranges, Alluvial Basins, Glaciated Central region, Northeast 

and Superior Uplands, and Alaska. The addition of an Alluvial Valley region that includes deposits in the glaciated and 

nonglaciated parts of the United States highlights the importance of valley-fill deposits to water supply. 

North America was later classified into 28 groundwater 

regions in part of a volume commemorating the “Decade of 

North American Geology” (Heath, 1988). Relevant summaries of glaciated areas in this volume include Farvolden and 

Cherry (1988), Krothe and Kempton (1988), Lennox and 

others (1988), Randall and others (1988), Rosenshein (1988), 

Sloan and van Everdingen (1988), and Stephenson and others 

(1988). Glacial deposits such as glacial alluvial and valley fill 

(for example, in buried bedrock valleys) were again noted as 

important features that could yield large amounts of groundwater to wells across the different regions. Other glacial 

features associated with aquifers that yield moderate to high 

volumes are ice-contact and outwash deposits (Stephenson and 

others, 1988). Tills and other poorly sorted deposits resulting 

from several depositional mechanisms are noted over glaciated parts of the continent. These deposits may be aquitards 

or locally important as capable of supplying low to moderate 

yields for domestic and, perhaps, agricultural supply, which 

depends on the presence of coarse material, poorly stratified 

lenses, weathering, or fractures (Meinzer, 1923; Heath, 1984; 

Stephenson and others, 1988). 

The USGS initiated the RASA Program in 1978 to quantify groundwater resources within major aquifer systems (Sun 

and Johnston, 1994). Rather than attempting to study groundwater resources by the previously described groundwater 

regions, each of which could have several aquifers present 

and used in different parts of the region, the RASA Program 

was organized upon the study of major aquifer systems. 

The aquifer systems were “from two general types: (1) an 

aquifer system comprised of an extensive set of aquifers and 

confining units that may be discontinuous locally, but which 

act hydrologically as a single system on a regional scale; and 

(2) a system consisting of a set of independent aquifers that

share many common characteristics hydrologically” (Sun

and Johnston, 1994). These major aquifer systems became

known as principal aquifers (Miller, 1999). Glacial-deposit

aquifers are from the second type of major aquifer systems.

The glacial aquifer system examined herein is formed from

independent aquifers and low-permeability units comprised

of unconsolidated deposits that share common origins across

North America despite differences in climate or details of the

specific local hydrogeologic conditions.

Hydrogeologic Framework Based on Aquifer 

Types and Hydrophysiographic Regions

One approach in developing a hydrologic framework for 

the study is to propose hydrophysiographic regions or settings 

built from idealized aquifer types or local hydrogeologic 

components for the study area. This approach is an extension of the previous work in the Northeastern United States 

(Kontis and others, 2004) and Southwestern United States 

(Anning and Konieczki, 2005). The approach is to group 

aquifer types defined at the local level into flow systems and 

ultimately define hydrophysiographic regions. The hydrophysiographic regions may be built from the local level or 

be the large regions from previous studies (for example, 
10 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

McGuinness, 1963; Heath, 1988). At the local and subregional 

scale, mapping and interpretation of the glacial geology would 

be necessary to develop base geology maps, identify aquifer 

types, and scale to hydrogeologic flow systems. Because of 

the size of the study area, new mapping will not be part of this 

study, and the geographic analysis will be used to develop the 

classification based on existing maps and reports. 

Water-Well Record Analysis

To complement the analysis using existing geologic 

maps and analysis of hydrogeology in parts of the study area, 

the study takes advantage of recent availability of electronic 

water-well records in many States across the glacial aquifer 

system (Bayless and others, 2017). A USGS internal standardized database of water-well records assembled from individual 

State databases and data from USGS will inform three-dimensional analysis of the glacial aquifer system by providing 

information on lithology with depth. For inclusion in the 

internal database, water-well records meet location and several 

other broad quality-control checks. Several interpolated maps 

based on the data have been produced, such as total thickness of glacial deposits, total sand and gravel thickness, and 

estimated effective horizontal and vertical hydraulic conductivities based on literature values assigned to lithologic terms 

(Bayless and others, 2017). The internal water-well record 

database is one of the few available datasets that provide 

information on glacial deposits with depth. Because of the 

heterogeneous nature of these deposits, these data may prove 

very powerful in the development of numerical and statistical 

models of groundwater availability or groundwater quality. 

Generalized Groundwater Budget Components

Generalized groundwater budgets will be estimated for 

the glacial aquifer system of the continental United States 

and the four NAWQA regions for the glacial aquifer system 

(excluding Alaska). The groundwater budgets alone do 

not provide enough information to determine groundwater 

availability because availability also depends on constraints; 

however, the groundwater budgets are necessary to build the 

analysis (Bredehoeft and others, 1982; Bredehoeft, 1997; 

Bredehoeft, 2002; Healy and others, 2007). These budgets can 

aid in future development of groundwater-flow models used to 

quantify the response of the groundwater budget in response 

to development or climate change. The system and regional 

budgets also set the context for discussing groundwater availability, and spatial variation in groundwater-flow budgets 

across the study area reveals the importance of climate differences across the system in determining regional and local 

groundwater availability.

Recharge and Discharge

An initial estimate of recharge and discharge for the 

glacial aquifer study area in the conterminous United States is 

made using the BFI and recharge grids developed by Wolock 

(2003a, b). These estimates are based on analysis at reference 

streamgages in the Geospatial Attributes of Gages for Evaluating Streamflow, version II (GAGES II) dataset (Falcone 

and others, 2010; Falcone, 2011) interpolated using inversedistance weighting interpolation to a 1 kilometer (km)×1 km 

grid across the conterminous United States. Three issues arise 

with this estimate of recharge for the glacial aquifer system 

study: (1) whether the method used to perform base-flow separation at gages is consistent with other estimates, (2) whether 

the inverse-distance weighting interpolation of the discharge 

values at selected streamgages across the region used to indicate recharge satisfactorily reflects the distribution of recharge, 

and (3) whether long-term average recharge estimates can be 

improved by also considering the annual variation in recharge 

and providing this variability to users of the information. 

The first issue is addressed in this report, the other two issues 

require additional study.

The BFI method (Wahl and Wahl, 1995) was used to 

estimate base-flow index for the GAGES II dataset. Subsequent analysis from the literature with a synthetic watershed model indicated that this method may underestimate 

recharge (Partington and others, 2012). To assess BFI results 

in the study area, regional estimates of recharge computed 

for the Great Lakes Basin (Neff and others, 2005; Neff and 

others, 2006) using several methods were examined. The 

methods used were BFLOW (Arnold and Allen, 1999), PART 

(Rutledge, 1998), UKIH (Piggott and others, 2005), and 

HYSEP fixed-interval (HYSEP1), HYSEP sliding-interval 

(HYSEP2), and HYSEP local-minimum (HYSEP3) (Sloto 

and Crouse, 1996). For the Great Lakes estimates, the BFI, 

UKIH, and BFLOW methods produce similar results; the 

HYSEP3 estimates are intermediate between the BFI method 

and the estimates using the HYSEP1, HYSEP2, and PART 

methods with BFI estimating the lowest value of the five techniques (fig. 5); similar results are observed for the reference 

streamgages in the GAGES II dataset across the glacial aquifer 

system study area (fig. 6).

Given these results for the Great Lakes part of the study 

area, and despite inherent uncertainties associated with baseflow separation techniques (Dingman, 2002; Partington and 

others, 2012), the BFI-based estimate (Wolock 2003a, b) 

will be increased to produce estimates closer to the HYSEP 

local-minimum method (HYSEP3) (Sloto and Crouse, 1996). 

Graphing was used to compare BFI values using HYSEP3 

method against BFI-method-derived values for streamgages 

in the Great Lakes Basin. Determining the best-fit line forced 

through zero yields a slope of 1.21 (fig. 7). The same approach 

using all annual BFI values at reference streamgages gives 

a slope of 1.19 (fig. 8), and using the period of analysis 

(1980–2013) for the 391 reference streamgages gives a slope 

of 1.21 (fig. 9). Based on these results, the BFI grid and 
Approach 11

0.8

0.6

0.4

0.2

0

0.2

-0.4

-0.6

-0.8

Great Lakes base-flow index minus GAGES II base-flow index

Base-flow separation method

BFLOW HYSEP1 HYSEP2 HYSEP3 PART UKIH

Outlier

Outlier

Maximum value or upper quartile plus 

1.5 times (upper quartile minus lower quartile)

Minimum value or lower quartile minus

1.5 times (upper quartile minus lower quartile)

Upper quartile

Lower quartile

95 percent confidence interval for median 

EXPLANATION

Median

Figure 5. Difference between the estimated base-flow index (BFI) for various base-flow separation techniques for streamgages 

in the Great Lakes Basin (Neff and others, 2005) and BFI in the Geospatial Attributes of Gages for Evaluating Streamflow (GAGES II) 

dataset (Falcone and others, 2010) determined using the BFI method. Negative values indicate that the other method produces a higher 

estimate of base flow than the BFI method used for GAGES II.

Outlier

Outlier

Maximum value or upper quartile plus 

1.5 times (upper quartile minus lower quartile)

Minimum value or lower quartile minus

1.5 times (upper quartile-lower quartile)

Upper quartile

Lower quartile

95 percent confidence interval for median 

EXPLANATION

BFI method minus other method

0.8

0.6

0.4

0.2

0

0.2

-0.4

-0.6

-0.8

HYSEP1 HYSEP2 HYSEP3 PART

Base-flow separation method

Median

Figure 6. Difference between the estimated base-flow index (BFI) for various base-flow separation techniques and BFI 

determined using the BFI method for reference gages in the Geospatial Attributes of Gages for Evaluating Streamflow (GAGES II)

dataset (Falcone and others, 2010). Negative values indicate that the other method produces a higher estimate of base flow than 

the BFI method used for GAGES II. Each complete year of data at each reference gage is considered a separate observation in this 

boxplot.
12 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

Base-flow index from GAGES II, BFI method

Base-flow index from HYSEP3 method

0

0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

0.8

1.0

1.0

EXPLANATION

Great Lakes estimates (Neff and others, 2005)

1:1 line

Fit through zero, slope equals 1.21

Figure 7. Base-flow index (BFI) computed using the HYSEP3 

(local minimum) method plotted against the estimated BFI using 

the BFI method for streamgages in the Great Lakes Basin from 

the Geospatial Attributes of Gages for Evaluating Streamflow 

(GAGES II) dataset.

Base-flow index from GAGES II, BFI method

Base-flow index from HYSEP3 method

0

0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

0.8

1.0

1.0

EXPLANATION

Annual estimate at reference streamgage

1:1 line

Fit through zero, slope equals 1.19

Figure 8. Base-flow index (BFI) computed using the HYSEP3 

(local minimum) method plotted against the estimated BFI using 

the BFI method for 391 reference gages in the glacial aquifer 

study area from the Geospatial Attributes of Gages for Evaluating 

Streamflow (GAGES II) dataset. Each point on the graph 

represents the BFI computed using a calendar year of data at a 

reference gage. associated recharge estimate grid from Wolock (2003a, b) will 

be increased by a factor of 1.2 to estimate the recharge to the 

glacial aquifer system study area.

Water Withdrawals and Water Use

Water withdrawals from an aquifer system alter the 

groundwater budget. New withdrawals first remove water 

from storage as reflected in lowering of water levels in wells. 

After an initial period when all of the water produced by a 

well is from storage, the system transitions so that changes 

in recharge and discharge eventually balance the new withdrawal (Bredehoeft and others, 1982; Bredehoeft, 2002; 

Barlow and Leake, 2012). If the total change in recharge and 

discharge cannot balance the withdrawal, then groundwater 

in the system is being mined and long-term production from 

the well is not sustainable. Groundwater-flow modeling is 

required to quantify the rate that the system responds to new 

pumping and the magnitudes of changes in storage, discharge, 

and recharge (Bredehoeft, 2002); understanding withdrawals 

imposed on the aquifer system and the observed response 

of the system to these withdrawals are crucial in developing 

a groundwater-flow model. In addition, estimates of water 

withdrawals are needed to complete the groundwater budget 

for a region with production wells, and, in some areas, water 

withdrawals may be a significant part of the groundwater 

budget (Healy and others, 2007; Faunt, 2009).

Estimation of groundwater withdrawals from the glacial 

aquifer system is confounded by lack of information on the 

source aquifer of reported groundwater use. In parts of the 

system, the bedrock aquifers underlying the glacial aquifer 

system are important for water supply, and the distribution of 

withdrawals between different aquifers may not be known. 

In some areas, however, the distribution among aquifers can 

be quantified. For example, water use by source aquifer was 

estimated for a groundwater flow model of the Lake Michigan 

Basin (Buchwald and others, 2010); in that study, approximately 55 percent of the groundwater withdrawals for the 

2001–5 period were attributed to the glacial aquifer system 

(fig. 10). To estimate water withdrawals from the glacial 

aquifer system, a nationwide study disaggregating groundwater withdrawals by principal aquifer for calendar year 2000 

(Maupin and Barber, 2005) may be used to estimate water 
Approach 13

Base-flow index from GAGES II, BFI method

Base-flow index from HYSEP3 method

0

0

0.2

0.2

0.4

0.4

0.6

0.6

0.8

0.8

1.0

1.0

EXPLANATION

Period of analysis estimate at reference streamgage

1:1 line

Fit through zero, slope equals 1.21

Figure 9. Base-flow index (BFI) computed using the HYSEP3 

(local minimum) method plotted against the estimated BFI using 

the BFI method for 391 reference gages in the glacial aquifer 

system study area from the Geospatial Attributes of Gages for 

Evaluating Streamflow (GAGES II) dataset. Each point on the 

graph represents the period of analysis estimate of BFI at a 

reference gage using records with at least 1 calendar year of data 

in the period from 1980–2013.

Quaternary

700.88

54.67 percent

Cambrian-Ordovician

404.31

31.53 percent

Silurian-Devonian

91.51

7.14 percent

Jurassic-Mississippian

83.69

6.53 percent

Mixed

1.73

0.13 percent

withdrawals from the glacial aquifer system for 2005 and 

2010. For each State in the glacial aquifer system, the ratio of 

estimated withdrawals from the glacial aquifer system to total 

groundwater withdrawals in the part of the State in the glacial 

aquifer system was estimated using information from Hutson 

and others (2004) and Maupin and Barber (2005). These State 

ratios were then applied to estimated withdrawals by county, 

or the part of the county, in the glacial aquifer system for 

2005 (Kenny and others, 2009) and 2010 (Maupin and others, 

2014) to produce consistent estimates of withdrawals from the 

glacial aquifer system across the study area.

Storage

The final component of the groundwater budget to be 

estimated is the groundwater in storage in the system. Groundwater is a major freshwater reservoir; in fact, groundwater 

may be the only reasonable reservoir in many places. Groundwater storage can mitigate the effects of seasonal changes 

Figure 10. Distribution of estimated groundwater withdrawals in 

million gallons per day from source aquifers in the Lake Michigan 

Basin groundwater-flow model, 2001–5, data from appendix 2 

of Buchwald and others (2010). Quaternary indicates the glacial 

aquifer system.

in rainfall or moderate droughts that would affect riverine 

or surface-water reservoirs by providing a stable source of 

water for various uses. Total storage estimates, however, do 

not reflect the available groundwater in the system (Alley, 

2007): (1) not all the water in storage can be feasibly accessed, 

(2) changes in storage can affect other users, (3) changes in

storage can induce flow from adjacent aquifers and lead to

deleterious water-quality changes, and (4) even relatively

small changes in storage can affect surface-water flows by

decreasing discharge from the aquifer or inducing recharge

from surface water to the aquifer (Kraft and others, 2012).

The stable and accessible supply offered by groundwater has

led to overexploitation in some areas around the world where

groundwater levels have been reduced beyond the ability of

the system to meet demands (Schwartz and Ibaraki, 2011);

therefore, change in storage with time is an important feature

of the flow system to consider. Literature values for storage

coefficients will be used with mapped glacial thicknesses to

estimate patterns of groundwater in storage across the study

area. Reported water-level information from monitoring wells

(Bartolino and Cunningham, 2003; Coon and Sheets, 2006),

areas of agricultural drainage (Konikow, 2013), and Gravity

Recovery And Climate Experiment (GRACE) satellite data

(Huang and others, 2012) will be explored to estimate the

change in storage in the system.

For the United States part of the Great Lakes Basin, 

storage in the glacial aquifer system was estimated to be on 
14 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

the order of 580 cubic miles (mi3) (8.6 × 1013 cubic feet), 

approximately the volume of a sixth Great Lake of storage 

(Coon and Sheets, 2006). This estimate only considered 

aquifer units and did not account for storage in tills or other 

low-permeability deposits. A similar approach was adopted 

in this study using thickness and distribution of surficial units 

from Soller and others (2011) for the part of the system east 

of the Rocky Mountains. This base map (Soller and others, 

2011) was used rather than the “Quaternary Geologic Atlas of 

the United States” (U.S. Geological Survey and others, 2013), 

hereafter referred to as the Quaternary geologic atlas, because 

it has estimated thicknesses; although, note that use of this 

map limits the estimate of storage to the part of the study area 

east of the Rocky Mountains. Use of this base map was tested 

by repeating the estimate of Coon and Sheets (2006) for the 

United States part of the Great Lakes Basin.

Aquifer storage for unconfined aquifers including specific 

yield, storage release by drainage of the pore space in the 

aquifer material, and specific storage, storage release by the 

expansion of water and compression of the aquifer matrix, was 

estimated by Coon and Sheets (2006) using:

Vt=(Sy+hSs )Ah=[Sy

 Ah]+[Ss Ah2] (1)

where,

Vt is total volume of groundwater in storage, in 

cubic feet,

Sy is specific yield of aquifer, dimensionless,

h is saturated thickness of aquifer, in feet,

Ss is specific storage of aquifer, per foot, and

A is area of aquifer, in square feet.

Hydrogeologic Framework and 

Groundwater Budget

Despite a wealth of local studies and information (Wiltshire and others, 1986; Kahle and Futornick, 2012), integration of information into a comprehensive hydrogeologic 

framework for the study area proved to be very difficult; the 

foundational information is not available uniformly across 

the study area and analysis of existing information required 

a simpler approach. In particular, regional studies and maps 

tend to be available for the part of the study area east of the 

Rocky Mountains (Kontis and others, 2004; Soller and others, 

2011; U.S. Geological Survey and others, 2013), whereas 

glacial aquifer studies west of the Rocky Mountains, including 

Alaska, tend to be local in scope (Kahle and others, 2011; 

Callegary and others, 2013). Because of data limitations, the 

hydrologic framework discussion focuses on the study area 

east of the Rocky Mountains. The two approaches outlined for 

hydrogeologic frameworks are developed, and groundwater 

budgets for each framework are discussed.

Generalized Hydrogeologic Frameworks 

The development of a framework based on the type of 

aquifer and the geographic area relies on the aggregation of 

local-scale mapping. For the glacial aquifer system, State 

aquifer maps serve as an intermediate scale, and differences 

in aquifer mapping between States prevented development 

of a full aquifer-type hierarchy for the study area. In order 

to assemble a generalized hydrogeologic framework, maps 

available from the Quaternary geologic atlas (U.S. Geological 

Survey and others, 2013) were evaluated against previously 

published State and regional aquifer maps. The Quaternary 

geologic atlas coverage at the time of the study was only for 

the part of the study area east of the Rocky Mountains (U.S. 

Geological Survey and others, 2013). The resulting framework 

focuses attention on deposits that are likely to be dominated 

by sand and gravel and serve as aquifers. The second approach 

is based on NAWQA regions, which produces a more regional 

focus on the entire aquifer system and accounts for local aquifers. Important differences between State aquifer maps and the 

generalized frameworks are identified. 

Hydrogeologic Framework Based on the 

Quaternary Geologic Atlas

To test the use of the Quaternary geologic atlas as a base 

map for the hydrogeologic framework, the map units found on 

the quadrangles that intersected the glacial aquifer system study 

area were classified into broad categories and converted from 

polygons to grids. The categories are generalizations of those 

used by Fullerton and others (2004) and are valley fill; outwash; 

ice contact; till; lacustrine coarse; lacustrine fine; loess or other 

eolian; peat, muck, and mud; other sand; other nonaquifer; 

alluvial fines; colluvium coarse; colluvium fine; bedrock; and 

water. These broad categories were further assigned as potential 

aquifer (valley fill, outwash, ice contact, lacustrine coarse, other 

sand), nonaquifer (till; lacustrine fine; peat, muck, and mud; 

loess or other eolian), or other nonaquifer (bedrock, alluvial 

fines, colluvium fine, colluvium coarse, water). Aquifer maps 

from the Northeastern United States, Minnesota, and North 

Dakota were then converted to grids. The degree to which the 

Quaternary geologic atlas captured the aquifer distribution was 

determined by the overlap between the gridded atlas categories 

and the gridded aquifer maps.

The regional stratified-drift aquifer map (Kontis and 

others, 2004) has six categories (fig. 11), and the map of potential aquifer material from the Quaternary geologic atlas appears 

to visually match the distribution of these units (fig. 12). Qualitatively, the potential aquifer material map lacks some of the 

smaller features mapped by the 1:500,000-scale map by Kontis 

and others (2004). Examination of these features shows that the 

geologic deposits related to these units are not delineated on the 

1:1,000,000-scale Quaternary atlas. Quantitative comparison of 

the two maps is summarized in tables 2 and 3.
Hydrogeologic Framework and Groundwater Budget 15

85° 80° 75° 70° 65°

45°

40°

0 150 300 MILES

0 150 300 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. EXPLANATION

Aquifers with potential for sustained or seasonal withdrawals either smaller than that of the aquifers in the other categories or could 

not be determined from the information available

Headwater aquifers: surficial aquifers at or near watershed divides, drained only by small streams and composed 

of sand and gravel that probably exceeds 40 feet in saturated thickness in all or part of each aquifer under nonpumping conditions

Outwash plain or sand-plain aquifers: surficial sand and gravel, greater than 1 square mile in aquifer area

Surficial aquifers that are estimated to be capable of yielding at least 5 million gallons per day, sustained by induced infiltration

Surficial or buried aquifers capable of a sustained yield of at least 5 million gallons per day, according to a geohydrologic evaluation 

or model simulation

Well field, or multiple well fields within an area of about 1 square mile, where pumpage has at some time averaged 5 million gallons 

per day or more for at least a year

[Blank areas on map classified as 'not mapped as aquifer' in summary tables]

° VT

MA

NJ

CN

RI

NH

NY

PA

OH

ME

Figure 11. Stratified sand and gravel aquifers in the Northeastern United States (from Kontis and others, 2004).
16 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

85° 80° 75° 70° 65°

45°

40°

0 150 300 MILES

0 150 300 KILOMETERS

Potential aquifer based on classification of the map 

units from the Quaternary atlas of the United States 

(U.S. Geological Survey and others, 2013)

EXPLANATION

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

VT

MA

NJ

CN

RI

NH

NY

PA

OH

ME

Figure 12. Potential aquifer material based on classified map units of Quaternary geologic atlas (U.S. Geological Survey and 

others, 2013).

Table 2. Quantitative comparison of Quaternary geologic atlas (U.S. Geological Survey and others, 2013) classification to stratified 

sand and gravel aquifers in the Northeastern United States (Kontis and others, 2004).

[Mgal/d, million gallons per day]

Quaternary geologic 

atlas classification

Well field yielding 

at least 5 Mgal/d 

at some time

Percent of mapped grid cells in each aquifer category

Surficial 

aquifer

Modeled 

aquifer Unknown yield Headwater Outwash Nonaquifer

Not a potential aquifer 0.005 0.07 0.17 5.4 0.2 0.04 94.1

Potential aquifer 0.14 1.2 1.8 37.9 1.6 1.3 56
Hydrogeologic Framework and Groundwater Budget 17

Table 3. Quantitative comparison of stratified sand and gravel aquifer types of the Northeastern United States (Kontis and others, 

2004) to classified mapped units in the Quaternary geologic atlas (U.S. Geological Survey and others, 2013).

Stratified sand and gravel aquifers in 

the Northeastern United States 

Percent of mapped grid cells in each map unit category of the Quaternary geologic atlas

Valley fillOutwashIce contactTillLacustrine coarseLacustrine finePeat, muck, mudOther sandOther nonaquiferAlluvial fineColluvium coarseColluvium fineWater

Well field, or multiple well fields 

within an area of about 1 square 

mile, where pumpage has at some 

time averaged 5 million gallons per 

day or more for at least a year

30.6 28.2 17.8 7.4 3.7 4.5 1.7 0 2.3 0 0 0 3.8

Surficial aquifers that are estimated to 

be capable of yielding at least 5 million gallons per day, sustained by 

induced infiltration

28 22.2 18 16 4.8 6.3 0 0 1.8 0 0 0 3.6

Surficial or buried aquifers capable of 

a sustained yield of at least 5 million gallons per day, according to a 

geohydrologic evaluation or model 

simulation

4.3 9.1 43.5 23.2 5.7 7.6 1.4 0.4 1.5 0 0 0 3.1

Headwater aquifers: surficial aquifers 

at or near watershed divides, drained 

only by small streams and composed 

of sand and gravel that probably exceeds 40 feet in saturated thickness 

in all or part of each aquifer under 

nonpumping conditions

2.9 24.7 27.1 32.9 4.9 3.7 0.9 0 1.6 0 0 0.2 0.8

Outwash plain or large sand-plain 

aquifers: surficial sand and gravel, 

greater than 1 square mile in aquifer 

area

1 27 50 13.3 3.4 2.3 0 0.1 0.1 0.1 0 0 2.6

Aquifers whose potential for sustained 

or seasonal withdrawals is smaller 

than that of aquifers in the categories 

above or could not be determined 

from the available information

9 9 24 33 11 7.5 0.9 0.9 1.1 0 0 0.3 1.8

Examination of the tables indicates that the Quaternary 

geologic atlas classification captures the nonaquifer part of the 

map. The surficial geology in the region is dominated by sandy 

till, which is classified as a “nonaquifer,” and 94 percent of the 

cells classified as “nonaquifer” in the Quaternary atlas are not 

mapped as aquifer material in the stratified sand and gravel 

aquifer map. Only about 5 percent of the material classified as 

“nonaquifer” are mapped as a potential aquifer in the stratified sand and gravel map, and the mismatch is almost all for 

areas mapped with unknown yield. Focusing on the material 

classified as “potential aquifer” indicates more mismatch, just 

over half of the cells classified as “potential aquifer” material 

are not mapped in the stratified sand and gravel map. Some of 

the mismatch may be attributed to differences in map scales 

and details in shape of mapped units. Overall, the Quaternary geologic atlas seems to capture the essential features 

of the stratified sand and gravel aquifer map, although on a 

more regional scale. The advantage of using the Quaternary 

geologic atlas as a base map for the glacial aquifer system 

study as opposed to the stratified sand and gravel aquifer map 

is that it extends across more of the study area.

A similar comparison may be made to the State Quaternary hydrogeology map for Minnesota (Land Management 

Information Center and others, 2000). The Minnesota map was 

selected as an example because the classification of aquifers 

is fairly simple and has four classes: outwash, alluvium, lake 
18 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

deposits, and nonaquifer. Each of these classes is further 

subdivided by estimated yield, but these subdivisions are 

not considered in the comparison. Examination of the State 

Quaternary hydrogeology map (fig. 13) and map units classified as “potential aquifers” from the Quaternary geologic 

atlas (fig. 14) indicates correspondence similar to the stratified 

sand and gravel aquifers map. In this case, the units on the 

State map (fig. 13) tend to be a bit larger and more uniform 

compared to the corresponding units from the Quaternary 

geologic atlas (fig. 14). The State map also has a denser 

network of alluvial aquifers, especially in southern Minnesota. 

The overlap between gridded maps of Minnesota is 

similar to the results from the northeastern stratified-drift 

aquifer. Mapped units on the Quaternary geologic atlas that 

are classified predominately as “not a potential aquifer,” 91.4 

percent, overlap cells classified on the Minnesota Quaternary 

hydrology geology map as “nonaquifer” (table 4). Some cells 

in the “not a potential aquifer” class overlap with alluvium, 

and this mismatch is related to the denser network of alluvium 

cells in southern Minnesota (fig. 14.) Examination of the 

comparison from the distribution within categories from the 

Minnesota Quaternary hydrogeology map (table 5) indicates 

that mapped units from the Quaternary geologic atlas classified as “valley fill” correspond quite well to the alluvium 

category on the Minnesota map, valley-fill cells are less than 

1 percent of the other hydrogeologic categories. Till is the 

dominant classification for mapped units from the Quaternary 

geologic atlas, and till cells appear as a substantial percentage 

of cells in all aquifer categories. This mismatch is a reflection 

of the more irregular potential aquifer shapes on the Quaternary geologic atlas compared to the more regular shapes on 

the hydrogeology map. The mismatch also is caused by some 

areas where the hydrogeology map indicates an outwash 

aquifer but the Quaternary geologic atlas indicates till. One 

explanation for this discrepancy is that the surficial materials 

may be till but may overlie an outwash aquifer. In this case, 

the use of the Quaternary geologic atlas as a base map will 

miss some aquifers. The importance of potentially buried aquifers is explored in more detail in the last comparison between 

a State aquifer map and the Quaternary geologic atlas.

In contrast to the four-category Minnesota Quaternary 

hydrogeology map, the North Dakota surficial aquifer map 

(North Dakota State Water Commission, 2010) has 278 

mapped aquifers at 1:100,000-scale (fig. 15). North Dakota 

was selected as an example because of the Spiritwood aquifer 

complex, a large buried-valley complex (Winter and others, 

1984; Kehew and Boettger, 1986), represented on the State 

map that is not well identified through classification of the 

map units on the Quaternary geologic atlas (U.S. Geological 

Survey and others, 2013). The Spiritwood aquifer complex 

(figs. 15–17) includes the following aquifers from the North 

Dakota surficial aquifers map: Spiritwood, Spiritwood-Barnes, 

Spiritwood-Berlin, Spiritwood-Devils Lake, SpiritwoodGrand Rapids, Spiritwood-Griggs, Spiritwood-LaMoure SE, 

Spiritwood-Oakes, Spiritwood-Sheyenne River, SpiritwoodStutsman, and Spiritwood-Warwick. The mapped units of the 

Quaternary geologic atlas classified as potential aquifers show 

good correspondence to the North Dakota surficial aquifer 

map in several places (fig. 16), but the Spiritwood aquifer 

complex is notably missing in the potential aquifer map 

derived from the Quaternary geologic atlas. This aquifer is 

described as being the result of catastrophic drainage of glacial 

lakes eroding sediments and bedrock and subsequent filling 

by permeable sediments that can be overlain in areas by lake 

deposits and glacial till (Kehew and Boettger, 1986; Hobbs 

and Bluemle, 1987).

The inability of the classified map units from the Quaternary geologic atlas to identify the Spiritwood aquifer complex 

motivates application of methods to use digital water-well 

records to provide information with depth across the study 

area that can help identify these buried features on a regional 

scale. By analyzing water-well driller records in electronic 

databases compiled by States (see “Water-Well Record 

Analysis” section), insight into the distribution of glacial 

materials with depth can be obtained. For the glacial aquifer 

system study, the key steps in the analysis are (1) broad 

quality-control tests to eliminate records that are obviously 

mislocated or have errors in reported altitude, well depth, or 

lithology; (2) translation of lithologic terms reported on the 

various water-well records to a consistent set of terms; and 

(3) assignment of representative values for coarse materials

or textbook hydraulic conductivity values used to estimate the

percentage of coarse material or estimated effective hydraulic

conductivity for the layered system (Arihood, 2009; Bayless

and others, 2017). For the North Dakota example, interpretation of reported lithologies from water-well records into sand

and gravel thickness identifies much of the mapped Spiritwood

aquifer complex (fig. 17). Patterns of thicker sand and gravel

deposits, in a general way, match other mapped aquifers, but

some mapped units are not identified, and in some areas, the

distribution of sand and gravel appears to be larger than the

mapped units.

Despite complications from buried systems, the classified 

map derived from the Quaternary geologic atlas will be used 

as one of the hydrogeologic frameworks for part of this study 

area (figs. 18 and 19). For analysis within the system, sand and 

gravel thickness or other interpreted maps based on water-well 

records will augment the Quaternary geologic atlas framework by providing information on the distribution of aquifer 

material with depth (fig. 20). The water-well records analysis 

is not used as the primary data source for the hydrogeologic 

framework for several reasons: (1) in areas with sparse data, 

the resulting framework is not helpful, (2) some features are 

missing in the water-well records because only recent information is included in some State databases and older water-well 

records that could indicate the presence of aquifers are not 

included in the analysis, and (3) water-well records were not 

available in electronic format suitable for this analysis for 

several of the States in the study area (Bayless and others, 

2017). This framework focuses attention on the glacial aquifer 

system in a way similar to that used by Kontis and others 

(2004) and Miller (1999).
Hydrogeologic Framework and Groundwater Budget 19

95° 90°

49°

48°

47°

46°

45°

44°

43°

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

0 100

0 100 200 KILOMETERS

200 MILES

Aquifer type

Alluvium

Lake deposits

Outwash

EXPLANATION

Minnesota

Figure 13. Minnesota Quaternary hydrogeology indicating three major aquifer types: alluvium, lake deposits, and outwash (Land 

Management Information Center and others, 2000).
20 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

95° 90°

49°

48°

47°

46°

45°

44°

43°

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

0 100

0 100 200 KILOMETERS

200 MILES

Potential aquifer

Alluvium

Lake deposits

Outwash

EXPLANATION

Minnesota

Figure 14. Potential aquifer material based on classified map units for potential aquifer from the Quaternary geologic atlas (U.S. 

Geological Survey and others, 2013). The map units are shaded by the underlying aquifer types from the Minnesota Quaternary 

hydrogeology map (Land Management Information Center and others, 2000).
Hydrogeologic Framework and Groundwater Budget 21

Table 4. Quantitative comparison of Quaternary geologic atlas 

(U.S. Geological Survey and others, 2013) classification to the 

Minnesota Quaternary hydrogeology map (Land Management 

Information Center and others, 2000).

Quaternary 

geologic atlas 

classification

Percent of mapped grid cells in each category 

from the Minnesota Quaternary hydrogeology map

Alluvium Lake 

deposits Outwash Nonaquifer

Not a potential 

aquifer

2.9 1.0 4.7 91.4

Potential 

aquifer

13.1 7.1 47.1 32.3

Hydrogeologic Framework Based on National 

Water-Quality Assessment Regions

The four regions used by the NAWQA glacial principal aquifer study (fig. 3) were used by Warner and Ayotte 

(2014) to discuss observed differences in water quality. The 

characteristics of these regions are summarized in table 1. 

These regions focus on the entire aquifer system and serve 

as a contrasting framework to that based on the Quaternary 

geologic atlas, which describes the mapped units with highest 

potential to serve as productive aquifers.

Summary of Hydrogeologic Frameworks 

The two hydrogeologic frameworks (1) based on classified map units from the Quaternary geologic atlas (U.S. 

Geological Survey and others, 2013) and (2) use of four 

NAWQA regions (Warner and Ayotte, 2014) represent 

contrasting views of the study area. The classified map units 

focus attention on large surficial deposits that are more likely 

to act as aquifers and support larger capacity wells; however, 

this approach has been shown to miss important buried 

aquifers formed by glacial processes and likely misses local 

aquifers that can support municipal, irrigation, or industrial 

wells that are not mapped at the 1:1,000,000-scale or result 

from local-scale heterogeneities within a regional depositional environment that is not expected to support wells with 

moderate to large yields. The focused approach also does not 

recognize that many low-yield domestic wells are located in 

areas designated as nonaquifer, and that the nonaquifer material may be important for groundwater storage or may provide 

base flow to streams. Conversely, the regional (NAWQA) 

approach recognizes that wells are placed in many areas of the 

glaciated part of North America that would not be considered 

aquifers from a public water supply perspective. This leads to 

inconsistencies in terminology because till or other low yield 

units are considered to be part of the aquifer system. Groundwater/surface-water interaction also may be highest in areas 

mapped as potential aquifers in the classification approach and 

not as important in nonaquifer parts of the regional approach. 

These two approaches will be retained in the study as each is 

appropriate for different groundwater availability questions. 

For example, base-flow and recharge estimates typically are 

made by looking at watershed characteristics and not restricted 

to classified map units. In developing groundwater-flow 

models to study groundwater availability, either framework 

will have to be augmented with depth-dependent information, 

from water-well records for example, and with local to statewide studies and maps.

Generalized Groundwater Budget

Generalized groundwater-budget components are estimated using existing data for the glacial aquifer system study 

area in the conterminous United States. The components 

are further subdivided into the four NAWQA regions (East, 

Central, West-Central, and West) to highlight variation in 

climate across the study area. 

Table 5. Quantitative comparison of Minnesota Quaternary hydrogeology map (Land Management Information Center and others, 

2000) to classified mapped units in the Quaternary geologic atlas (U.S. Geological Survey and others, 2013).

Minnesota 

Quaternary

hydrogeology map

Percent of mapped grid cells in each category in the Quaternary geologic atlas

Valley fillOutwashIce-contactTillLacustrine coarseLacustrine finePeat, muck, mudOther sandOther nonaquiferAlluvial finesColluvium coarseColluvium fineWater

Alluvium 20.2 31.9 0.1 26.3 4.2 7.8 0.9 0 0 0 4.1 0 4.6

Lake deposits 0.5 0.2 0 6.5 68.1 4.6 18.8 0 0 0 0 0 1.2

Outwash 0.8 65.3 0 19.8 4.7 1.1 1.3 0 0.1 0 0.2 0 6.7

Nonaquifer 0.5 5.4 0 66.7 3.1 8.7 7.5 0 0.2 0 3 0 4.9
22 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

104° 102° 100° 98° 96°

48°

46°

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

Surficial aquifer

Spiritwood aquifer complex

EXPLANATION

North

Dakota

0 75 150 KILOMETERS

0 75 150 MILES

Figure 15. North Dakota surficial aquifers map (North Dakota State Water Commission, 2010).
Hydrogeologic Framework and Groundwater Budget 23

EXPLANATION

Potential aquifer from classification of the

Quaternary geologic atlas

Spiritwood aquifer complex

104° 102° 100° 98° 96°

48°

46°

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

North

Dakota

0 75 150 KILOMETERS

0 75 150 MILES

Figure 16. Potential aquifer material based on classified map units of Quaternary geologic atlas (U.S. Geological Survey and others, 

2013).
24 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

104° 102° 100° 98° 96°

48°

46°

358

300

200

100

0

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

0 75 150 KILOMETERS

0 75 150 MILES

Spiritwood aquifer complex

EXPLANATION

Thickness of sand and gravel, 

in feet

North

Dakota

Figure 17. Estimated sand and gravel thickness for North Dakota from interpolation of water-well records using methods described 

by Arihood (2009) and Bayless and others (2017).
Hydrogeologic Framework and Groundwater Budget 25

110° 100° 90° 80° 70°

50°

40°

0 300 600 MILES

0 300 600 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. EXPLANATION

Potential aquifer

NH

MA

CN

NJ

RI

NY

MI

KY

PA

IL

WI

MI

IN

OH

MO

MN

IA

SD

NE

KS

MT ND

VT

ME

Figure 18. Hydrogeologic framework for the glacial aquifer system study based on classification of mapped units derived from the 

Quaternary geologic atlas (U.S. Geological Survey and others, 2013) grouped to show potential aquifer material.

Recharge and Discharge

The large size of the study area leads to a groundwater 

budget that is dominated by natural recharge and discharge. 

Traditionally, these two budget components are estimated by 

considering the long-term change in storage in the system 

to be negligible thereby assuming that long-term average 

recharge is equal to long-term average discharge. For the 

glacial aquifer system, the assumption that long-term change 

in storage is negligible is reasonable because groundwaterlevel data across the study area do not indicate long-term 

declines in water levels (Bartolino and Cunningham, 2003; 

Konikow, 2013, 2015). Long-term discharge is typically 

estimated using streamgage records. Records are analyzed 

to separate the base-flow component of streamflow from the 

runoff component. By analyzing several years of records 

with multiple storm events, the average long-term base flow 

from aquifer systems to streams may be derived (Gebert and 

others, 2007). This base-flow estimate may be directly used to 

represent long-term discharge, and thus recharge, or it may be 

augmented by modeling (Arnold and others, 2000) and waterquality information (Nolan and others, 2007; Gates and others, 

2014) to make an estimate of recharge. 

Recharge estimates for regional studies are challenging 

because recharge depends on local processes that differ across 

the region and because of the variety of methods that may 

be used to estimate recharge (Delin and others, 2007; Gebert 

and others, 2007). Regionalized methods produce spatially 

smoothed estimates, and local methods produce estimates with 

much more spatial variability. The processes controlling local 

recharge are expected to vary across the study area depending 

on the hydrogeologic setting. Consider the stratified sand and 

gravel aquifers in the Northeastern United States that typically are in valley-fill settings (fig. 21). Net recharge to these 
26 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

110° 100° 90° 80° 70°

50°

40°

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

0 300 600 MILES

0 300 600 KILOMETERS

Ice contact

Lacustrine coarse

Other sand

Outwash

Valley fill

EXPLANATION

NH

MA

CN

NJ

RI

NY

MI

KY

PA

IL

WI

MI

IN

OH

MO

MN

IA

SD

NE

KS

MT ND

VT

ME

Figure 19. Hydrogeologic framework for the glacial aquifer system study based on classification of mapped units derived from 

the Quaternary geologic atlas (U.S. Geological Survey and others, 2013) showing classes of aquifer material.

aquifers could include water from surface runoff and exchange 

with deeper regional systems. These other water sources and 

sinks are often mediated by fractures in upland tills, bedrock 

outcrops, and underlying rock units (Kontis and others, 

2004). In the Central region, estimates of recharge from Ohio 

(Dumouchelle and Schiefer, 2002) and Minnesota (Delin and 

others, 2007), for example, show recharge on areas mapped 

as till, emphasizing that on a regional scale, these areas may 

contribute to base flow and could be responsible for part of 

the recharge to the system. Recharge in the areas mapped 

as till, however, is less than recharge in the areas mapped as 

aquifer material consistent with the expected behavior of these 

settings. This lithologic control on recharge was explored for 

Nebraska by Gates and others (2014) who found that diffuse 

recharge in river valleys where till has been removed by 

erosion was significantly greater than diffuse recharge on areas 

of upland till and dominated overall recharge to the system. 

Recharge processes in the coteau du Missouri and similar 

parts of the West-Central region also occur in till areas and 

have been attributed to focused recharge in depressions in 

the till related to wetlands and ponds that can form a regional 

flow system (van der Kamp and Hayashi, 2009) and potentially deliver water to underlying bedrock aquifers (Long and 

others, 2014). The appropriate hydrologic framework from 
Hydrogeologic Framework and Groundwater Budget 27

 0 466

120° 110° 100° 90° 80° 70°

40°

30°

Base from U.S. Geological Survey digital data, 1:2,000,000, 1998

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96° W. 

180° 170° 150° 160°

60°

50°

0 250 500 MILES

0 250 500 KILOMETERS

EXPLANATION

Areas of insufficient data

Areas of incomplete analysis

Glacial extent boundary

0 250 500 MILES

0 250 500 KILOMETERS

Total thickness of coarse-grained 

sediment within the glacial 

deposits, in feet

WA

NH

MA

CN

NJ

RI

DE

MD

ME

NY

CO

UT

NV

WY

ID

CA

OR

PA OH

MI

KY

WV

VA

IL

TN

GA

FL

WI

IN

MO

AR

MS

LA

AL

MN

IA

SD

NE

KS

OK

MT

AZ NM

TX

ND VT

SC

NC

AK

Figure 20. Sand and gravel thickness for the glacial aquifer system derived from analysis of water-well records (from Bayless and 

others, 2017).

a recharge perspective appears to be a hybrid regional view 

with regional estimates made by considering net recharge, 

as indicated by base flow from catchments, combined with 

classified map units that indicate surficial aquifer material that 

could have higher recharge potential. If low permeability areas 

are neglected completely, the resulting recharge estimate for 

areas mapped as aquifer would be too high when compared to 

total precipitation or local observations. Note, however, that a 

mapped unit framework that neglects low-permeability areas 

can be adopted in the development of a numerical model as 

long as recharge attributed to bordering nonaquifer units is 

delivered to the boundary of the numerical model (DeSimone 

and others, 2002).

Figure 22 provides an estimate of mean annual recharge 

in inch per year on a 1-km grid for the glacial aquifer system 

study area except Alaska. Total recharge to the conterminous 

glacial aquifer system is estimated as 1×1013 cubic feet per 

year or a mean recharge of 6.3 inches per year [in/yr] over 

the study area. This serves as one estimate for recharge to the 

glacial aquifer system. The major assumption is that the base 
28 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

A

B

C

C

D

E

E

E

F 1

2

4

5

A

Drumlin Upland

valley

Upland kame terrace, largely unsaturated

Till-mantled bedrock hill

Ice-contact

deposit

Flood plain

Outwash

Major valley

EXPLANATION

Precipitation on valley floor, which infiltrates to water

table unless diverted as evapotranspiration or as storm

runoff from pavement or saturated soil

Runoff from adjacent till-covered hillsides at shallow

depth through sandy till, through soil horizons, and (or)

as surface rivulets

Continuous natural seepage losses from small tributaries

not incised to the water table

Lateral and upward flow from deep circulation systems

through bedrock

Induced infiltration from rivers near large-capacity wells

where the water table is lowered by pumping

D

C

E

A

B

F

1

2

3

4

5

Geologic material in vertical section

Till

Sand, gravel

Fine sand, silt, clay

Porous bedrock

Nonporous bedrock

Water table

Direction of groundwater flow

Well

Recharge to bedrock

Infiltration of precipitation through till in uplands

Seepage to river

Ground-water evapotranspiration where the water table is shallow

Underflow downvalley through stratified drift (not shown)

Pumpage from well screened in stratified drift

Discharge from stratified drift

Recharge to stratified drift

Discharge from bedrock

Pumpage from well that intersects fractures

Figure 21. Idealized groundwater-flow paths in a valley-fill aquifer system with till and bedrock uplands showing various recharge 

mechanisms (from Kontis and others, 2004).
Hydrogeologic Framework and Groundwater Budget 29

120° 110° 100° 90° 80° 70°

40°

30°

0 450 900 MILES

0 450 900 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

0.047 to 0.18

0.19 to 0.27

0.28 to 0.35

0.36 to 0.44

0.45 to 0.57

0.58 to 0.74

0.75 to 0.96

0.97 to 1.3

1.4 to 1.6

1.7 to 2.1

2.2 to 2.8

2.9 to 5.0

5.1 to 6.0

6.1 to 8.0

8.1 to 10

11 to 20

21 to 30

31 to 50

51 to 70

71 to 96

Estimated recharge, in inch per year

EXPLANATION

WA

NH

MA

CN

NJ

RI

DE

MD

ME

NY

CO

UT

NV

WY

ID

CA

OR

PA

OH

KY

VA

WV

IL

TN

GA

FL

WI

MI

IN

MO

AR

MS

LA

AL

MN

IA

SD

NE

KS

OK

MT

AZ NM

TX

ND

VT

SC

NC

Figure 22. Estimated mean annual recharge for the conterminous glacial aquifer system study area from a grid of the 

conterminous United States by Wolock (2003b) converted from millimeters per year to inches per year and increased by a 

factor of 1.2 to align the estimated base-flow index with other base-flow separation methods.
30 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

flow at streamgages in the glacial aquifer system study area 

is dominated by discharge from aquifers, and the amount of 

recharge to the glacial aquifer system that is exchanged with 

deeper aquifers either eventually appears as base flow or is 

small compared to the total recharge. An estimate by Wolock 

(2003b) based on the BFI method and adjusted by a factor of 

1.2 gives a representative value for various base-flow separation methods (figs.7–9).

The estimated recharge grid based on inverse-distance 

weighting interpolation of estimates of BFI from the 

GAGES II (Wolock, 2003a) for the East region dataset is 

smooth and does not recognize patterns in soils or underlying 

geology in the estimation of recharge (fig. 22). The spatial 

pattern of recharge, however, has been shown to depend on 

landscape characteristics such as soils, geology, and topography (Arnold and others, 2000; Delin and others, 2007; Nolan 

and others, 2007; Santi and others, 2008; Gates and others, 

2014), even to the site scale (Fragalà and Parkin, 2010). For 

the glacial aquifer system, resolution of recharge patterns to 

the site scale is not feasible; however, application of a soil 

water balance (SWB) approach (Westenbroek and others, 

2009) appears to capture spatial patterns and allows for estimation across most of the study area. Recent work estimating 

recharge for Minnesota illustrates the performance of SWB 

(Smith and Westenbroek, 2015), and this type of application 

across the glacial aquifer system study area could be an important next step. Examining the recharge and discharge estimates 

by region helps emphasize differences between regions.

East Region

The spatial pattern in the estimated recharge mirrors the 

spatial distribution of precipitation for the East region (fig. 23). 

The estimated recharge for this region is 40 percent of the total 

estimated recharge for the entire glacial aquifer system study 

area (excluding Alaska), and the average recharge rate is 13 in/

yr (total recharge/total area). The estimated range is from 2.8 

to 26 in/yr.

Central Region

The spatial pattern in the estimated recharge mirrors 

the spatial distribution of precipitation for the Central region 

(fig. 24). The estimated recharge for this region is 32 percent 

of the total estimated recharge for the entire glacial aquifer 

system study area (excluding Alaska), and the average 

recharge rate is 6.3 in/yr. The estimated range is from less than 

1 to 17 in/yr.

West-Central Region

The spatial pattern in the estimated recharge mirrors the 

spatial distribution of precipitation for the West-Central region 

(fig. 25). Despite its large area, the estimated recharge for this 

region is only 11 percent of the total estimated recharge for the 

entire glacial aquifer system study area (excluding Alaska), 

and the average recharge rate is 1.6 in/yr. The estimated range 

is from less than 1 to 33 in/yr.

West Region

The spatial pattern in the estimated recharge mirrors 

the spatial distribution of precipitation for the West region 

excluding Alaska (fig. 26). Alaska is not included in the 

estimates because the data sources do not include Alaska. 

The estimated recharge for this region is the remaining 17 

percent of the total estimated recharge for the entire glacial 

aquifer system study area (excluding Alaska), and the average 

recharge rate is 20 in/yr. The estimated range is from less than 

1 to 96 in/yr. Note that the bibliography report published by 

Kahle and Futornick (2012) denotes reports with recharge 

estimates in the West region with a ”B” in the “Information 

Code” for the report.

Water Withdrawals

Groundwater withdrawals from the glacial aquifer 

system in 2000 were 5 percent of the total reported groundwater withdrawals (Maupin and Barber, 2005), and the study 

disaggregating groundwater withdrawals for the calendar year 

2000 national compilation by principal aquifer (Maupin and 

Barber, 2005) may be used to estimate water withdrawals 

from the glacial aquifer system for 2005 and 2010. Estimated 

withdrawals from the glacial aquifer system across the study 

area are greatest in the Central region. The withdrawals from 

the Central region are estimated to be 2–3 times larger than 

either the West-Central or East regions, and the West region 

has the lowest estimated withdrawal (fig. 27). Public supply 

withdrawals are the largest withdrawals in the Central and 

East. The largest withdrawals in the West are for public supply 

or aquaculture depending on the year. Irrigation withdrawals 

are the largest in the West-Central region and important in the 

Central region (fig. 28). Public supply withdrawals declined 

slightly from 2000 to 2010, which is consistent with the 

national trends (Maupin and others, 2014). Compared to other 

principal aquifers, the glacial aquifer system provides the 

most public supply and self-supplied industrial withdrawals 

(Maupin and Barber, 2005).

Storage and Changes in Storage

Groundwater storage can mitigate the effects of seasonal 

changes in rainfall or droughts that would affect riverine or 

surface-water water-supply reservoirs by providing a stable 

source of water for various uses. The glacial aquifer system 

study area is very large, and the resulting total storage, even 

for each of the four regions, is so large that total storage in the 

area does not inform groundwater availability. Local storage 

properties and the hydraulics associated with accessing this 

storage influence groundwater availability (Alley, 2007). 

Storage is also estimated in other regional studies, and, for 
Hydrogeologic Framework and Groundwater Budget 31

80° 75° 70° 65°

50°

45°

40°

80° 75° 70° 65°

EXPLANATION

Precipitation, in inch per year

1.8 to 10

11 to 14

15 to 18

19 to 24

25 to 31

32 to 37

38 to 44

45 to 48

49 to 54

55 to 260

Estimated recharge, in inch per year

0.047 to 0.18

0.19 to 0.27

0.28 to 0.35

0.36 to 0.44

0.45 to 0.57

0.58 to 0.74

0.75 to 0.96

0.97 to 1.3

1.4 to 1.6

1.7 to 2.1

2.2 to 2.8

2.9 to 5.0

5.1 to 6.0

6.1 to 8.0

8.1 to 10

11 to 20

21 to 30

31 to 50

51 to 70

71 to 96

0 150 300 MILES

0 150 300 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

NH

MA

CN

RI

NJ

ME

NY

PA

OH

VT

NH

MA

CN

RI

NJ

ME

NY

PA

OH

VT

Figure 23. Estimated mean annual precipitation for the period 1981–2010 (PRISM Climate Group; Oregon State University, 2004; 

Daly and others, 2008) and estimated recharge (from Wolock, 2003a) for the East region of the study area.
32 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

95° 90° 85° 80° 95° 90° 85° 80°

45°

50°

40°

0 125 250 MILES

0 125 250 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. EXPLANATION

Precipitation, in inch per year

1.8 to 10

11 to 14

15 to 18

19 to 24

25 to 31

32 to 37

38 to 44

45 to 48

49 to 54

55 to 260

Estimated recharge, in inch per year

0.047 to 0.18

0.19 to 0.27

0.28 to 0.35

0.36 to 0.44

0.45 to 0.57

0.58 to 0.74

0.75 to 0.96

0.97 to 1.3

1.4 to 1.6

1.7 to 2.1

2.2 to 2.8

2.9 to 5.0

5.1 to 6.0

6.1 to 8.0

8.1 to 10

11 to 20

21 to 30

31 to 50

51 to 70

71 to 96

MI

IL

OH

WI

IN

MN

IA

MI

IL

OH

WI

IN

MN

IA

Figure 24. Estimated mean annual precipitation for 1981–2010 (PRISM Climate Group; Oregon State University, 2004; Daly and 

others, 2008) and estimated recharge (from Wolock, 2003a) for the Central region of the study area.
Hydrogeologic Framework and Groundwater Budget 33

110° 100° 90°

50°

40°

30°

110° 100° 90°

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. 

0 250 500 MILES

0 250 500 KILOMETERS

EXPLANATION

Precipitation, in inch per year

1.8 to 10

11 to 14

15 to 18

19 to 24

25 to 31

32 to 37

38 to 44

45 to 48

49 to 54

55 to 260

Estimated recharge, in inch per year

0.047 to 0.18

0.19 to 0.27

0.28 to 0.35

0.36 to 0.44

0.45 to 0.57

0.58 to 0.74

0.75 to 0.96

0.97 to 1.3

1.4 to 1.6

1.7 to 2.1

2.2 to 2.8

2.9 to 5.0

5.1 to 6.0

6.1 to 8.0

8.1 to 10

11 to 20

21 to 30

31 to 50

51 to 70

71 to 96

MO

MN

IA

SD

NE

KS

MT ND

MO

MN

IA

SD

NE

KS

MT ND

Figure 25. Estimated mean annual precipitation for 1981–2010 (PRISM Climate Group; Oregon State University, 2004; Daly and others, 

2008) and estimated recharge (from Wolock, 2003a) for the West-Central region of the study area.
34 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

125° 120° 115°

50°

45°

125° 120° 115°

0 100 200 MILES

0 100 200 KILOMETERS

Base from U.S. Geological Survey digital data, 1:1,000,000, 2012

National Atlas of the United States of America

Albers Equal-Area Conic projection

Standard parallels 29°30’ N. and 45°30’ N.

Central meridian 96°00’ W. EXPLANATION

Precipitation, in inch per year

1.8 to 10

11 to 14

15 to 18

19 to 24

25 to 31

32 to 37

38 to 44

45 to 48

49 to 54

55 to 260

Estimated recharge, in inch per year

0.047 to 0.18

0.19 to 0.27

0.28 to 0.35

0.36 to 0.44

0.45 to 0.57

0.58 to 0.74

0.75 to 0.96

0.97 to 1.3

1.4 to 1.6

1.7 to 2.1

2.2 to 2.8

2.9 to 5.0

5.1 to 6.0

6.1 to 8.0

8.1 to 10

11 to 20

21 to 30

31 to 50

51 to 70

71 to 96

WA

ID

OR

MT

WA

ID

OR

MT

Figure 26. Estimated mean annual precipitation for 1981–2010 (PRISM Climate Group; Oregon State University, 2004; Daly and others, 

2008) and estimated recharge (from Wolock, 2003a), for the West region of the study area although neither data source includes 

estimates for Alaska.
Hydrogeologic Framework and Groundwater Budget 35

[2005 and 2010 withdrawals estimated from 

county-level withdrawals from groundwater 

prorated using withdrawals from glacial aquifer 

system in 2000 by State, Maupin and Barber (2005)]

2000 2005 2010

Year

3,500

3,000

2,500

2,000

1,500

1,000

500

0

Groundwater withdrawals from glacial aquifer system,

in million gallons per day

East

Central

West Central

West

EXPLANATION

Figure 27. Estimated fresh 

groundwater withdrawals from the 

glacial aquifer system by glacial 

availability study region (Hutson and 

others, 2004; Kenny and others, 2009; 

Maupin and others, 2014). 

purposes of comparison, simple estimates of total storage are 

presented.

Following the procedure of Coon and Sheets (2006), 

accounting for aquifer storage including both specific yield 

(storage release by drainage of the pore space in the aquifer 

material) and specific storage (storage release by the expansion of water and compression of the aquifer matrix) (eq. 1), 

requires estimates of aquifer material storage characteristics. 

As discussed, the base map by Soller and others (2011) was 

used in this estimate. Representative values from Coon and 

Sheets (2006) were used for Sy

 and Ss in equation 1, and the 

fraction of the area covered by aquifer material was estimated 

from attributes of the mapped units. For the current estimate, 

similar values of Sy

 and Ss were assigned to the mapped surficial units designated as coarse stratified (Soller and others, 

2011). The specific yield for the thickest units was set to zero 

following the assumption used by Coon and Sheets (2006) 

that storage for units with a thickness greater than 400 ft was 

primarily from confined materials (table 6).

For the initial estimation of storage, selection of only 

areas of coarse-stratified sediments of the glacial aquifer 

system was done to be consistent with the approach used 

by Coon and Sheets (2006). The groundwater storage was 

estimated using the values in table 6 and equation 1, and the 

total storage is 630 mi3

. This value is approximately 8 percent 

greater than the estimate from Coon and Sheets (2006), which 

was considered satisfactory for this analysis. 

Application of this method to regions of the glacial 

aquifer system yields estimates of storage for the East, 

Central, and West-Central glacial aquifer study regions 

(table 7). Storage was not estimated for the West region 

because few groundwater-level data were available and the 

glacial deposits in this region are often discontinuous. Because 

some water use, particularly self-supplied domestic, may 

access parts of the aquifer mapped as fine-grained stratified or 

even till, a second estimate was made including these materials. For this estimate, the specific yield of the units was set 

quite low, 0.01, assuming that water does not drain from these 

materials very well (table 6). This estimate is the difference 

between the aquifer material storage value and the total glacial 

aquifer system storage value in table 7.

Comparison of the aquifer material storage to annual 

water use indicates that total groundwater withdrawals are 

less than 1 percent and as low as 0.2 percent of the volume 

of groundwater in storage. As discussed by Coon and Sheets 

(2006), these estimates are general, and they serve primarily 

as a comparison to other systems. Importantly, groundwater mining, or continued loss in storage in response to 

pumping, is not observed across the study area (Bartolino and 

Cunningham, 2003; Konikow, 2013, 2015). Local storage loss 

and its effect on well owners or base flow can be important in 

parts of the study area (Alley, 2007; Kraft and others, 2012), 

but widespread loss is not documented.

Some principal aquifer systems are experiencing groundwater mining, or substantial declines in groundwater levels 

defined as losses in tens to hundreds of feet (Bartolino and 

Cunningham, 2003); these declines generally are not observed 

for the glacial aquifer system despite locally large withdrawals 

for various uses. There are three reasons why the glacial 

aquifer system has not experienced such dramatic changes in 

water levels:
36 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

0

200

400

600

800

1,000

1,200

0

200

400

600

800

1,000

1,200

2000 2005 2010 2000 2005 2010

Year Year

Public supply

Irrigation

Self-supplied industrial

Domestic

EXPLANATION

[2005 and 2010 withdrawals estimated from county-level withdrawals from groundwater prorated 

using withdrawals from glacial aquifer system in 2000 by State, Maupin and Barber (2005)]

East Central

West Central West

Groundwater withdrawals from glacial aquifer system,

in million gallons per day

Livestock

Aquaculture

Mining

Thermoelectric

Figure 28. Estimated fresh groundwater withdrawals from the glacial aquifer system by type and glacial availability study region

(Hutson and others, 2004; Kenny and others, 2009; Maupin and others, 2014).
Hydrogeologic Framework and Groundwater Budget 37

Table 6. Mapped unit (Soller and other, 2011) and representative thickness and specific yield and specific storage values assigned for 

estimation of groundwater in storage.

[ft–1, 1/foot; ft, foot]

Unit

Representative 

saturated thickness, 

in ft

Specific yield, 

dimensionless

Specific storage, 

ft–1

Coarse-grained stratified sediment, Quaternary sediment 0–50 ft thick 30 0.13 0.00080

Coarse-grained stratified sediment, Quaternary sediment 100–200 ft thick 160 0.13 0.00080

Coarse-grained stratified sediment, Quaternary sediment 1,000–1,200 ft thick 1,150 0.13 0.00080

Coarse-grained stratified sediment, Quaternary sediment 200–400 ft thick 330 0.13 0.00080

Coarse-grained stratified sediment, Quaternary sediment 400–600 ft thick 525 0.00 0.00080

Coarse-grained stratified sediment, Quaternary sediment 50–100 ft thick 80 0.13 0.00080

Coarse-grained stratified sediment, Quaternary sediment 600–800 ft thick 720 0.00 0.00080

Coarse-grained stratified sediment, Quaternary sediment 800–1,000 ft thick 950 0.00 0.00080

Exposed bedrock, or sediment not of glacial origin 0–50 ft thick 0 0.00 0.00000

Values assigned to fine-grained material for second estimate of total storage, specific yield and specific storage set to zero in initial estimate

Fine-grained stratified sediment, Quaternary sediment 0–50 ft thick 30 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 100–200 ft thick 160 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 1,000–1,200 ft thick 1,150 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 1,200–1,400 ft thick 1,350 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 1,400–1,600 ft thick 1,550 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 200–400 ft thick 330 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 400–600 ft thick 525 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 50–100 ft thick 80 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 600–800 ft thick 720 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment 800–1,000 ft thick 950 0.01 0.00060

Fine-grained stratified sediment, Quaternary sediment more than 1,600 ft thick 1,600 0.01 0.00060

Organic-rich sediment, 0–50 ft thick 30 0.01 0.00010

Patchy Quaternary sediment, 0–50 ft thick 25 0.10 0.00010

Till, Quaternary sediment 0–50 ft thick 30 0.01 0.00010

Till, Quaternary sediment 100–200 ft thick 160 0.01 0.00010

Till, Quaternary sediment 1,000–1,200 ft thick 1,150 0.01 0.00010

Till, Quaternary sediment 1,200–1,400 ft thick 1,350 0.01 0.00010

Till, Quaternary sediment 200–400 ft thick 330 0.01 0.00010

Till, Quaternary sediment 400–600 ft thick 525 0.01 0.00010

Till, Quaternary sediment 50–100 ft thick 80 0.01 0.00010

Till, Quaternary sediment 600–800 ft thick 720 0.01 0.00010

Till, Quaternary sediment 800–1,000 ft thick 950 0.01 0.00010
38 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

Table 7. Estimated groundwater storage in the glacial aquifer 

system.

[--, storage for West region was not estimated]

Region

Aquifer material 

storage,

 in cubic miles

Fine-grained 

material storage, 

in cubic miles

Total glacial 

aquifer system 

storage, 

in cubic miles

East 67 30 97

Central 800 160 960

West-Central 610 370 980

West -- -- --

1. As the shallowest available aquifer generally composed

of unconsolidated sediment, the glacial aquifer system

acts as an unconfined aquifer in most parts of the study

area. Under unconfined conditions, storage release from

the aquifer material is a result of the drainage of the

pore space that is characterized by the aquifer specific

yield (Freeze and Cherry, 1979). Specific yield may be

on the order of 0.1 in contrast to the confined storativity

(specific storage × thickness) of an aquifer that may be

on the order of 0.001–0.0001; therefore, a water-level

change of 1 ft in an unconfined aquifer is equivalent to a

change of 100 ft in a confined aquifer.

2. To observe very large drawdowns in water levels, an

aquifer must be subjected to pumping and have hydraulic characteristics that lead to storage loss rather than

capture of water from discharge or other boundaries

(Bredehoeft, 2002). For example, observed changes in

the Cambrian-Ordovician system in northwestern Illinois

and southeastern Wisconsin require a fairly unique set

of circumstances: deep aquifers of sufficient quality for

desired use, large initial head, low specific storage, and

moderate transmissivity. For the bedrock aquifers below

Chicago, the initial static head in a bedrock aquifer at a

depth of 710 ft was as much as 80 ft above land surface

in the 1860s (Schufeldt, 1866) and had experienced

nearly 230 ft of drawdown from development by 1910

(Anderson, 1919). Deeper wells and continued use led

to drawdowns on the order of 900 ft by the mid-1960s

(Mandle and Kontis, 1992). There are no examples of

confined parts of the glacial aquifer system with geometry and hydraulic characteristics that could give this

type of behavior.

3. Finally, because the glacial aquifer system can be shallow and hydraulically well connected to surface water,

pumping in some parts of the glacial aquifer system

quickly is balanced by either capture of water that would

have discharged to surface water or capture of induced

recharge from surface water (Barlow and Leake, 2012).

Streamflow capture also is noted for confined aquifer

systems; therefore, confinement of glacial sediments 

does not preclude streamflow capture limiting waterlevel declines, rather confinement of the system may 

slow the time required to reach a steady state with no 

further declines (Leake, 2011; Barlow and Leake, 2012).

Despite not having dramatic changes in water levels in 

the aquifer system, significant loss of water from storage in the 

glacial aquifer system has been noted (Konikow, 2013). Some 

of the distributed storage loss in the glacial aquifer system 

is associated with drainage tiles that were installed in the 

mid-1800s to allow for transportation and agriculture in many 

parts of the study area (Kaatz, 1955; Dahl and Allord, 1999). 

Konikow (2013) estimates that land drainage in the conterminous United States has led to a storage loss of 13 mi3

. This 

drainage was critical for successful agriculture and westward 

expansion, and much of the drained land is within the study 

area; however, the effect on groundwater availability is likely 

minimal. Soils requiring tile drainage naturally dominate in 

areas that do not serve as productive surficial aquifers, and 

the local decline in groundwater level in these areas is on the 

order of 3–5 ft.

Storage change is observed in groundwater levels in 

monitoring wells. These observations may be augmented by 

the GRACE satellite system that provides remote-sensed data 

that may be useful in quantifying storage change (Chen and 

others, 2005). The GRACE data have been used to quantify 

water loss in the Central Valley of California (Famiglietti and 

others, 2011), the Amazon River (Chen and others, 2009), and 

other large systems, including the Great Lakes (Huang and 

others, 2012). The Great Lakes study in particular discusses 

the difficulty of detecting changes in groundwater storage 

using GRACE in a large humid system. The GRACE data 

indicate changes in the gravity field of the earth from several 

mechanisms, including changes in total water storage: surface 

water, soil moisture, snow, ice, and groundwater (Huang and 

others, 2012). The use of GRACE data to quantify changes 

in groundwater storage requires that the change in total water 

storage data be analyzed and distributed between these potential water sources. 

Data from the GRACE satellite system were analyzed 

to estimate changes in total water storage using published 

methods (Swenson and Wahr, 2006; Landerer and Swenson, 

2012), and the results for the four glacial aquifer system 

regions are shown in figure 29. The analysis confirmed the 

earlier discussion by Huang and others (2012) that the change 

in total terrestrial water storage is difficult to relate to changes 

in groundwater in storage. The GRACE satellite data are 

a significant independent data source that may be used to 

constrain or interpret regional (Zaitchik and others, 2008) or 

global (Döll and others, 2014) water-resources models, but 

these data require significant direct observations of groundwater level, soil moisture, surface-water storage, snowpack, 

and likely hydrologic models to resolve the various components of water storage. 
Hydrogeologic Framework and Groundwater Budget 39

−1,500

−1,000

−500

0

500

1,000

1,500

2,000

East

EXPLANATION

Loess fit

Monthly difference from average mass 

from January 2004 to December 2009 

Central

−1,500

−1,000

−500

0

500

1,000

1,500

2,000

Water equivalent, in centimeters

2004 2006 2008 2010 2012 2014 2016

Year

West Central

2004 2006 2008 2010 2012 2014 2016

Year

West

Figure 29 Changes in terrestrial water storage derived from GRACE satellite data from 2002 to 2014. GRACE land data available at 

https://grace.jpl.nasa.gov, supported by NASA MEaSUREs Program (Landerer and Swenson, 2012; Swenson, 2012; Swenson and 

Wahr, 2006).
40 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

Groundwater Budgets

Generalized groundwater budgets for each of the four 

regions in the glacial aquifer system were developed from 

(1) the long-term annual recharge and discharge based on

Wolock (2003b) as increased to be representative of various

base-flow separation estimates; (2) averaged withdrawals from

the glacial aquifer system for 2000, 2005, and 2010 estimated

by applying the ratio of groundwater withdrawals from the

glacial principal aquifer for 2000 to total groundwater withdrawals reported for 2000, 2005, and 2010; and (3) storage

estimated using representative thickness, specific yield, and

specific storativity values. Recharge estimates vary for the

four regions because of differences in climate and the area of

each region. The low recharge rate in the West-Central region

is offset by the large area of this region in the estimate of the

annual volume of recharge; conversely, high recharge rates in

the East region do not produce as large of an annual volume of

recharge because this region has the smallest area.

Water withdrawals in most of the four regions are 

orders of magnitude smaller than other discharges; therefore, 

estimated discharges and recharge are set equal (fig. 30, 

table 8). Pumping from the glacial aquifer system and underlying bedrock aquifers will change the local groundwater flow 

dynamics. Pumping can capture groundwater that would have 

been discharged to surface water or directly lost to evapotranspiration, changes in groundwater levels in the glacial 

aquifer system, or adjacent aquifers will change the rate of 

exchange of water between the systems. Groundwater-flow 

models are required to estimate the response of the system to 

pumping. For example, in the Central region, pumping in the 

Lake Michigan Basin changes the exchange of water between 

shallow and deep systems and decreases the discharge of 

groundwater to surface water (Feinstein and others, 2010). In 

the East region, changes in withdrawals also are anticipated 

to be balanced by capturing water that would have discharged 

to streams or inducing leakage from streams to the aquifer 

system. DeSimone and others (2002) presented detailed 

analysis for potential changes in base flow to stream reaches in 

response to changes in pumping or recharge. 

Table 8. Summary of generalized annual water budgets for four regions of glacial aquifer system study.

[--, storage for West region was not estimated]

East Central West Central West

Storage, in cubic miles 97 960 980 --

Recharge rate, in inches per year 13 6.3 1.6 20 

Annual recharge, in cubic miles 28.3 22.2 7.5 12.2

Annual discharge, in cubic miles 28.3 22.2 7.5 12.2

Annual withdrawals, in million gallons per day 

(average of 2000, 2005, and 2010 estimates)

4.6 12 1,010 562

Annual withdrawals, in cubic miles 

(average of 2000, 2005, and 2010 estimates)

0.32 0.85 0.33 0.19
Hydrogeologic Framework and Groundwater Budget 41

East Central

West Central West

Recharge 

420 x 1010 ft 3

/yr

Discharge

420 x 1010 ft 3

/yr

Storage

1,400 x 1010 ft 3

/yr

Storage

14,000 x 1010 ft 3/yr

Water withdrawals

4.6 x 1010 ft 3

/yr

Recharge 

330x 1010 ft 3

/yr

Discharge

330 x 1010 ft 3

/yr Water withdrawals

12 x 1010 ft 3

/yr

Exchange with underlying aquifers and lateral 

boundaries unknown except for local parts of 

the region where groundwater-flow models 

have been developed

Exchange with underlying aquifers and lateral 

boundaries unknown except for local parts of 

the region where groundwater-flow models 

have been developed

Trends in storage are 

being estimated in 

this project. 

Interannual and 

seasonal changes in 

storage are observed 

in long-term 

groundwater-level 

records

Trends in storage are 

being estimated in 

this project. 

Interannual and 

seasonal changes in 

storage are observed 

in long-term 

groundwater-level 

records

Trends in storage are 

being estimated in 

this project. 

Interannual and 

seasonal changes in 

storage are observed 

in long-term 

groundwater-level 

records

Trends in storage are 

being estimated in 

this project. 

Interannual and 

seasonal changes in 

storage are observed 

in long-term 

groundwater-level 

records

Recharge 

110 x 1010 ft 3

/yr

Discharge

~110 x 1010 ft 3

/yr

Storage

14,000 x 1010 ft 3

/yr

Storage

Unknown

Water withdrawals

4.6 x 1010 ft 3

/yr

Recharge 

180x 1010 ft 3

/yr

Discharge

180 x 1010 ft 3

/yr Water withdrawals

2.7 x 1010 ft 3

/yr

Exchange with underlying aquifers and lateral 

boundaries unknown except for local parts of 

the region where groundwater-flow models 

have been developed

Exchange with underlying aquifers and lateral 

boundaries unknown except for local parts of 

the region where groundwater-flow models 

have been developed

Figure 30. Estimated storage, recharge, discharge, and groundwater withdrawals (in cubic foot per year [ft3/yr]) for the four glacial 

aquifer system regions. Note that total withdrawals are small compared to recharge and discharge and were not considered to change 

the significant figures of the estimated values.
42 Hydrogeologic Framework and Groundwater Budget for a Groundwater Availability Study for the Glacial Aquifer System 

Summary

Water availability in the glacial aquifer system in the 

United States is quantified by (1) understanding the status of 

groundwater resources in the glacial aquifer system, (2) determining how these resources have changed over time, and (3) 

assessing likely system response to future changes in anthropogenic and environmental conditions. 

These three goals are common to related regional groundwater availability studies and serve as a basis of a national 

assessment of groundwater resources. The glacial aquifer 

system extends from Maine to Alaska, although the focus of 

this report is the part of the system in the conterminous United 

States east of the Rocky Mountains. The glacial aquifer system 

is the largest source for public and self-supplied industrial 

supply for any principal aquifer, and the system is also an 

important source for irrigation supply. Despite its importance 

for water supply, water levels in the glacial aquifer system are 

generally stable varying with climate and only locally from 

pumping. 

The need for information regarding the distribution of 

glacial deposits with depth was identified in the development 

of the hydrogeologic framework for the study. Many of the 

States in the study area have water-well records in digital 

databases, and the effort to assemble and interpret these data 

will be most useful for regional analyses. The hydrogeologic 

framework for this project includes the information from 

water-well records and classification of material types from 

the U.S. Geological Survey Quaternary geologic atlas into 

likely aquifers dominated by sand and gravel deposits.

Generalized groundwater budgets across the study area 

highlight the variation in recharge and discharge primarily 

driven by climate. Future efforts could focus on quantifying 

spatial and temporal patterns of recharge to provide water 

managers with more information on observed changes in the 

system.

Acknowledgments

This project required efforts of many USGS scientists, 

support staff, and administrative staff. The USGS Science 

Publishing Network staff provided expert assistance with 

editing, formatting, posting of online publications, and 

publishing reports. Kevin Dennehy, Groundwater Resources 

Program, and Sonya Jones, Water Availability and Use Science 

Program, provided management oversight and vision for 

this and related regional groundwater availability projects. 

The USGS Office of Groundwater and Water Science Field 

Team helped frame the initial project and provided reviews of 

progress and technical assistance throughout the duration of 

the project.

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