National Water Availability and Use Program

Estimate of Ground Water in Storage in the Great 

Lakes Basin, United States, 2006

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2006–5180

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Estimate of Ground Water in Storage in the 

Great Lakes Basin, United States, 2006

By William F. Coon and Rodney A. Sheets

National Water Availabilty and Use Program

Scientific Investigations Report 2006–5180

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources,:

natural hazards, and environment:

World Wide Web: http://www.usgs.gov/

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Coon, W.F., and Sheets, R.A., 2006, Estimate of ground water in storage in the Great Lakes Basin, United States, 

2006: U.S. Geological Survey Scientific Investigations Report 2006–5180, 19 p.
iii

Contents

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

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

Hydrogeology..................................................................................................................................................2

Major Aquifer Systems ........................................................................................................................6

Minor Aquifer Systems ........................................................................................................................8

Estimate of Ground-Water Storage ............................................................................................................8

Methods of Computation ...................................................................................................................11

Assumptions and Limitations Inherent in Storage Estimate........................................................12

Storage Volume...................................................................................................................................13

Uncertainty in Storage Estimate ......................................................................................................15

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

References Cited..........................................................................................................................................16

Additional References.................................................................................................................................18

Glossary.........................................................................................................................................................19

Figures

1–4. Maps Showing —

1. Drainage area of the Great Lakes Basin, United States ........................................................3

2. Regional bedrock aquifers in the Great Lakes Basin, United States ...................................4

3. Surficial aquifer system in the Great Lakes Basin, United States........................................5

4. Movement of simulated ground-water divide in the Cambrian-Ordovician

aquifer in response to intensive pumping in the Milwaukee, Wisconsin 

area........................................................................................................................................14

Tables

1. Hydrogeologic characteristics of the regional aquifer systems in the 

United States part of the Great Lakes Basin.....................................................................9

2. Estimation of ground water stored in the regional aquifer systems in the 

United States part of the Great Lakes Basin...................................................................10

3. Uncertainty in the estimate of ground-water storage in the United States 

part of the Great Lakes Basin............................................................................................15
iv

Conversion Factors, Datums, and Abbreviations

Inch-Pound to International System (SI) Units

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Area

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2)

Volume

gallon (gal) 3.785 liter (L)

cubic mile (mi3) 4.166 cubic kilometer (km3)

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Hydraulic conductivity

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

Transmissivity

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)

Dissolved concentrations are reported in milligrams per liter (mg/L).
Estimate of Ground Water in Storage in the Great Lakes 

Basin, United States, 2006

By William F. Coon and Rodney A. Sheets

Abstract

Hydrogeologic data from Regional Aquifer System 

Analyses (RASA) studies by the U.S. Geological Survey in 

the Great Lakes Basin, United States, during 1978–95, were 

compiled and used to estimate the total volume of water that is 

stored in the many aquifers of the basin. These studies focused 

on six regional aquifer systems: the Cambrian-Ordovician 

aquifer system in Wisconsin, Illinois, and Indiana; the Silurian-Devonian aquifers in Wisconsin, Michigan, Illinois, Indiana, and Ohio; the surficial aquifer system (aquifers of alluvial 

and glacial origin) found throughout the Great Lakes Basin; 

and the Pennsylvanian sandstone and carbonate-rock aquifers 

and the Mississippian sandstone aquifer in Michigan. Except 

for the surficial aquifers, all of these aquifer systems are 

capable of yielding substantial quantities of water and are not 

small aquifers with only local importance. Individual surficial 

aquifers, although small in comparison to the bedrock aquifers, collectively represent large potential sources of ground 

water and therefore have been treated as a regional system.

 Summation of ground-water volumes in the many 

regional aquifers of the basin indicates that about 1,340 cubic 

miles of water is in storage; of this, about 984 cubic miles is 

considered freshwater (that is, water with dissolved-solids 

concentration less than 1,000 mg/L). These volumes should 

not be interpreted as available in their entirety to meet watersupply needs; complete dewatering of any aquifer is environmentally undesirable. The amount of water that is considered 

available on the basis of water quality and environmental, 

economic, and legal constraints has not been determined. The 

effect of heavy pumping in the Chicago, Ill., and Milwaukee, 

Wis., areas, which has caused the regional ground-water divide 

in the Cambrian-Ordovician aquifer system to shift westward, 

has been included in the above estimates. This shift in the 

ground-water divide has increased the amount of water in 

storage in the deep-bedrock aquifers of the Great Lakes Basin 

by about 36 cubic miles; however, this water is removed by 

wells and, after use, is mostly discharged to the Mississippi 

River Basin rather than to the Great Lakes Basin. The corresponding decrease in ground-water storage that has resulted 

from lowering of the potentiometric surface due to this heavy 

pumping (0.059 cubic miles) is negligible compared to the 

total estimated storage.

Introduction

Freshwater is vital to the economic and environmental 

well-being of the United States. Besides the need to maintain 

streamflows and lake levels for ecological purposes, freshwater is required to meet the public, domestic, commercial, and 

industrial needs of the American people. As developed areas 

expand and the competition for limited water resources intensifies in heavily populated areas and where freshwater is in 

limited quantities, the need for conservation, protection from 

pollution, and sound development of water resources becomes 

more important. The Nation’s freshwater needs are met by 

withdrawals from streams, lakes, reservoirs, and groundwater systems. As water demands increase, the role of ground 

water to meet these demands becomes increasingly important. 

Ground water is the dominant source of drinking water for 

most rural areas, the largest source of water for irrigation and 

other uses in arid and most semiarid regions, and an important 

source of water for urban, industrial, and irrigation uses in 

humid areas (Heath, 1985).

Estimation of the volume of ground water that is stored 

in an aquifera water-bearing layer of rock or unconsolidated 

material that will yield a usable quantity of water to a well 

(Heath, 1983) is an essential first step for assessing groundwater resources in a given area. Storage calculations require 

hydrogeologic data on the water-storage capacity and watertransmission capability of the aquifer, which are controlled by 

the rock type or composition of the unconsolidated materials 

that form the aquifer, and the amount of voids (openings), the 

degree of fracturing, and the interconnectedness of voids and 

fractures in the aquifer. Storage properties of aquifers determine the amount of water that can be released from storage. 

An aquifer whose upper surface is free to fluctuate under 

atmospheric pressure (an unconfined aquifer) can yield a 

greater volume of water per unit change in hydraulic head than 

one that is similar in all respects but is completely filled with 

water that is under pressure because overlying material or a 

rock unit restricts the movement of water (a confined aquifer). Additionally, calculations of storage are complicated by 

water-supply wells that can increase the area that contributes 

ground water to the wells and that can lower the water level 

and remove water from storage, at least to the point that a new 

equilibrium condition is attained.
   Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

In 2005, the U.S. Geological Survey (USGS) began the 

National Water Availability and Use Program to describe 

the status and trends in the availability and use of freshwater resources, which would enable informed policy decisions regarding the economic and environmental uses of the 

Nation’s water resources (U.S. Geological Survey, 2002a). 

The Great Lakes Basin was selected as the pilot study area for 

that program (U.S. Geological Survey, 2005a). The American 

public and its decision makers are becoming increasingly 

concerned about water availability in the United States part 

of the Great Lakes Basin, but the ground-water resources of 

this basin have not been comprehensively assessed since the 

early 1970s (Great Lakes Basin Commission, 1975). Many 

conditions have changed since that assessmentcompetition 

for water has increased, new water sources and technologies 

have been developed, and water resources in some areas of 

the basin have been depleted (U.S. Geological Survey, 2005a). 

USGS studies of regional aquifer systems within and adjacent to the Great Lakes Basin during 1978–95 have greatly 

increased our knowledge of the ground-water resources of 

the area (Young, 1992; Westjohn and Weaver, 1998; Bugliosi, 

1999; Randall, 2001) and have provided hydrogeologic data 

that characterize the many aquifer systems of the region.

 To assess the present status of ground-water resources 

in the Great Lakes Basin and to provide a basis for assessing 

future trends in these resources, the volume of ground water 

that is stored in the basin needs to be estimated. The regional 

aquifer studies mentioned above include the hydrogeologic 

data on which such an estimate can be based. Therefore, as 

part of the Great Lakes Basin pilot study, this report (1) summarizes the hydrogeologic characteristics of the major and 

minor aquifers in the basin and (2) presents an estimate of the 

volume of ground water that is stored in these aquifers and, of 

this total volume and within the limitations set by water-quality criterion, the volume of freshwater. These estimates do 

not take into consideration any environmental, economic, or 

legal constraints that would limit the availability of this water 

for the many and varied uses in the basin. The only previous 

estimate of ground-water storage in the Great Lakes Basin 

(United States part) was “at least 1,000 mi3” by Grannemann 

and others (2000); no data supporting this value were given 

with the estimate, however.

Hydrogeology

The Great Lakes Basin, which covers nearly 

300,000 mi2, encompasses Lakes Superior, Michigan, Huron, 

Erie, and Ontario, and straddles the border between the United 

States and Canada. Eight states border the Great Lakes, parts 

of which represent 59 percent of the basin on the U.S. side 

(fig. 1); the province of Ontario represents the 41 percent 

of the basin that lies on the Canadian side (Neff and others, 

2005). The lakes themselves occupy nearly one-third 

(94,000 mi2) of the basin area (U.S. Environmental Protection 

Agency, 2005).

The Great Lakes Basin is underlain almost entirely by 

a thick sequence of sedimentary rocks. Sandstone or carbonate-rocks, where present (fig. 2), can be reliable sources of 

ground water. Glacial and alluvial deposits overlie the bedrock 

throughout much of the basin and form productive aquifers 

where the deposits are well sorted and composed primarily 

of sand and gravel (fig. 3), especially where precipitation is 

appreciable.

Ground water is the source of most of the water stored 

in the Great Lakes. Precipitation that infiltrates the soils and 

percolates to the water table is the main source of recharge 

to the aquifers. Ground water discharges directly to the Great 

Lakes as seepage or indirectly as base flow (dry-weather flow) 

in streams and rivers. Direct seepage is believed to take place 

near the shores (Grannemann and others, 2000) and is a relatively small component of the total water inflow to the lakes 

(Croley and Hunter, 1994). Base flow, after direct precipitation on the lakes, accounts for the second largest influx of 

water to the lakes. Using 5,735 years of daily streamflow data 

from 195 streams in the U.S. part of the Great Lakes Basin, 

Holtschlag and Nicholas (1998) estimated that the groundwater component of lake inflow from streams and rivers averages 67.3 percent of the total flow. Neff and others (2005) did 

a basinwide analysis of data from 959 streamflow-monitoring 

stations with a combined 28,784 years of daily streamflow 

record to estimate the ground-water component of streamflow 

for the United States and Canadian areas of the Great Lakes 

Basin. According to data reported in Neff and others (2005), 

the ground-water component of streamflow averages 66 percent of total streamflow for all parts of the basin.

Four studies of regional aquifer systems in the United 

States part of the Great Lakes Basin have been conducted by 

the USGS since 1975. The Great Lakes Basin contains three 

major aquifer systems: the Cambrian-Ordovician aquifer 

system (in Wisconsin, Illinois, and Indiana), the SilurianDevonian aquifers (in Wisconsin, Michigan, Illinois, Indiana, 

and Ohio), and the surficial aquifer system (aquifers of alluvial 

and glacial origin that are found throughout the Great Lakes 

Basin). The basin also contains three minor aquifer systems, 

all in Michigan: the Pennsylvanian sandstone aquifer, the 

Pennsylvanian sandstone and carbonate-rock aquifer, and 

the Mississippian sandstone aquifer (U.S. Geological Survey, 2003). Other small aquifer systems are present in many 

locations in the basin but are less important as ground-water 

sources on a regional basis and were not included in the 

ground-water-storage calculations.

The hydrogeologic data generated by the USGS regional 

aquifer-system analyses (RASA) have increased our knowledge of the bedrock and surficial aquifers in the Great Lakes 

Basin and provide a basis for estimating the amount of ground 

water in storage in the six aquifers named above. From the 

total estimated storage volume, the usable quantity of ground 

water—that is, the amount that contains freshwater with dissolved-solids concentrations less than 1,000 mg/L—also can 

be computed.
Hydrogeology 

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New York

Illinois

Indiana Ohio

Pennsylvania

Lake Superior

Lake Huron

Lake Michigan

Lake Erie

Lake Ontario

90° 85° 80° 75° 70°

40°

45°

50°

Base from U.S. Geological Survey digital data 

1:2,000,000; Geographic Coordinate System; decimal degrees

EXPLANATION

Wisconsin

Michigan

Duluth

Minnesota

Milwaukee

Chicago

Detroit

Cleveland

Syracuse

Grand R

Fox R

Maumee R

Ohio R

Mississippi

Great Lakes surface-water drainage basin

0 100 200 MILES

0 100 200 KILOMETERS

CANADA

Figure 1. Drainage area of the Great Lakes Basin, United States.
 Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

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90° 85° 80° 75°

40°

45°

EXPLANATION

Lake Superior

Lake Michigan

Lake Huron

Lake Erie

Lake Ontario

Principal bedrock aquifers

Great Lakes Basin surface-water divide 

New York and New England 

carbonate-rock aquifers

Pennsylvanian aquifers

Marshall aquifer

Principal aquifers from U.S. Geological Survey, 2003; 

scale 1:2,500,000. 

0 50 100 200 MILES

0 50 100 200 KILOMETERS

Base from U.S. Geological Survey digital data 

1:2,000,000; Geographic Coordinate System; decimal degrees

CANADA

Mississippian aquifers

Other rocks/Confining units

Silurian-Devonian aquifers

Cambrian-Ordovician aquifer system

New York sandstone aquifers

Figure 2. Regional bedrock aquifers in the Great Lakes Basin, United States.
Hydrogeology 

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90° 85° 80° 75°

40°

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EXPLANATION

Lake Superior

Lake Michigan

Lake Huron

Lake Erie

Lake Ontario

Great Lakes Basin surface-water divide 

Southern limit of Wisconsinan glaciation (U.S. Geological Survey, 2005c)

Base from U.S. Geological Survey digital data 

1:2,000,000; Geographic Coordinate System; decimal degrees

0 50 100 200 KILOMETERS

0 50 100 200 MILES

Surficial Aquifer System

Aquifers from U.S. Geological Survey, 2002b; 

scale 1:2,500,000. 

CANADA

Figure 3. Surficial aquifer system in the Great Lakes Basin, United States.
   Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

Major Aquifer Systems

The Cambrian-Ordovician aquifer system (fig. 2) 

underlies an area of about 161,000 mi2 in parts of Minnesota, 

Iowa, Missouri, Wisconsin, Illinois, and Indiana (Young, 

1992). The part of this system within the Great Lakes Basin is 

in eastern Wisconsin, northeastern Illinois, and northwestern 

Indiana. It primarily comprises three units: the St. Peter-Prairie 

du Chien-Jordan aquifer, the Ironton-Galesville aquifer, and 

the Mount Simon aquifer. These units are confined by several 

formations, the main component of which is the Maquoketa 

Shale (Young, 1992).

The St. Peter-Prairie du Chien-Jordan aquifer consists 

of sandstone and dolomite. The part of this aquifer within 

the Great Lakes Basin ranges from 0 to 500 ft in thickness 

(Young, 1992) and has storage coefficients of 7.5 ×10-5 to 

1.2 × 10-3 (Young and Siegel, 1992). Transmissivity varies 

greatly, depending mainly on the degree of fracturing and 

solution enlargement of the dolomite, and commonly ranges 

from 500 to 5,000 ft2/d (Young, 1992). Hydraulic conductivity 

values used in a RASA ground-water model of the ChicagoMilwaukee area ranged from 1.7 to 5.4 ft/d (Young and Siegel, 

1992).

The Ironton-Galesville Sandstone aquifer is the most productive unit of the Cambrian-Ordovician system of northeastern Illinois and southeastern Wisconsin (Young, 1992). This 

unit ranges from 0 to more than 200 ft in thickness but generally is 50 to 150 ft thick within the Great Lakes Basin (Young, 

1992). Storage coefficient ranges from 1.0 × 10-6 to 7.5 × 10-5, 

and hydraulic conductivity ranges from 1.0 to 31 ft/d (with 

a median 8.4 ft/d) (Young and Siegel, 1992). The RASA 

ground-water models of this aquifer used narrower ranges of 

these values5 × 10-5 to 9 × 10-4 for storage coefficient, and 

2.6 to 8.6 ft/d for hydraulic conductivity (Mandle and Kontis, 

1992). Reported transmissivity values range from 71 to 

11,000 ft2/d, but most range from the upper hundreds to about 

3,000 ft2/d (Young, 1992).

The Mount Simon Sandstone aquifer is the most extensive aquifer in the Cambrian-Ordovician aquifer system 

(Young, 1992). Its thickness exceeds 2,000 ft in northeastern 

Illinois but decreases to 100 ft near and north of Milwaukee, 

Wis. (Young, 1992). Ground-water yields are generally not 

commensurate with the aquifer’s great thickness because only 

the upper few hundred feet of the aquifer, if penetrated at all, 

can be tapped for water supply; water in the lower part of the 

aquifer is saline near Lake Michigan (Young, 1992). Storage 

coefficient for this unit ranges from 1 × 10-6 to 1 × 10-2 (Young 

and Siegel, 1992), but the RASA models used a smaller range 

of 5 × 10-5 to 9 × 10-4 (Mandle and Kontis, 1992). Hydraulic 

conductivity ranges from 0.027 to 23 ft/d (Young and Siegel, 

1992); the RASA models used a much narrower rangefrom 

0.5 to 8.6 ft/d (Mandle and Kontis, 1992). Reported transmissivity of the Mount Simon aquifer ranges from 270 to 

9,400 ft2/d (Young, 1992).

Because the Cambrian-Ordovician aquifer system is 

entirely confined, except for a small area in southeastern 

Wisconsin (J.T. Krohelski, U.S. Geological Survey, written 

commun., 2006) and in relatively small areas where the aquifer has been dewatered, specific yields have not been calculated for individual aquifers nor for the system as a whole. 

Porosity values of the various stratigraphic units in eastern 

Wisconsin, northeastern Illinois, and western Michigan have 

been measured in 36 boreholes; the average porosity of the 

sandstone units is 15.2 percent (Carlson, 2000). Specific yields 

are typically somewhat lower than porosities, and an estimated 

specific yield of 5 percent has been applied to these units in 

ground-water-flow models of southeastern Wisconsin (Feinstein and others, 2005a); this value was not based on measured 

data, however (D.T. Feinstein, U.S. Geological Survey, written 

commun., 2006).

The Silurian-Devonian aquifers (fig. 2) underlie parts 

of Wisconsin, Michigan, Illinois, Indiana, and Ohio, although 

the part that lies in Indiana is outside the Great Lakes drainage 

area. Water occurs primarily in fractures, bedding planes, and 

other openings, and most of the active flow within fractures is 

within the upper 100 ft of the aquifers (Bugliosi, 1999). The 

RASA study boundaries of these aquifers in the Indiana-OhioMichigan area were partly defined on the basis of dissolvedsolids concentration, which is representative of the degree of 

salinity and varies greatly in this areafrom less than 500 to 

greater than 1,000 mg/L (Bugliosi, 1999). Areas where dissolved-solids concentrations in the Silurian-Devonian aquifers 

in southern Michigan and northeastern Ohio exceed 

10,000 mg/L were excluded from the RASA study of this area 

and from the ground-water-storage estimates presented in this 

report.

The Silurian-Devonian aquifers in eastern Wisconsin 

and northeastern Illinois range from 0 to more than 1,000 ft in 

thickness but generally are 200 to 500 ft thick in their subcrop 

areas (Young, 1992). In this area, the aquifers are confined 

by Pennsylvanian, Mississippian, and Devonian rock units 

(Young, 1992). Aquifer thickness ranges from about 200 to 

1,400 ft across northern Michigan (Mandle and Kontis, 1992) 

and from 200 to more than 2,500 ft elsewhere in the Great 

Lakes Basin, primarily northwestern Ohio and southeastern 

Michigan (Casey, 1996), although the average thickness in 

this area is about 900 ft. In northern Indiana and Ohio, these 

aquifers are semiconfined by surficial deposits—mostly 

till—or confined by overlying shales (Bugliosi, 1999). Storage 

coefficient ranges from 9 × 10-5 to 4.8 × 10-3 in the Wisconsin-Illinois area (Young and Siegel, 1992) and from 1 × 10-5

to 5 × 10-2 in the Indiana-Ohio-Michigan area (Joseph and 

Eberts, 1994). The median storage coefficient for these aquifers in Ohio and Indiana is 1 × 10-4 (Joseph and Eberts, 1994, 

table 7). Specific yield ranges from 0.017 to 0.03 in northeastern Illinois (Prickett and others, 1964; Sasman and others, 

1981) and from 0.01 to 0.05 in Ohio (Bloyd, 1974).

Hydraulic conductivity of the Silurian-Devonian aquifers 

in Wisconsin and Illinois ranges from 0.068 to 40 ft/d; the 

RASA models used values of 0.17 to 7.9 ft/d (Young and Siegel, 1992). Hydraulic conductivity in Indiana and Ohio ranges 

from 0.0016 to 12 ft/d (Joseph and Eberts, 1994). Reported 
Hydrogeology 

transmissivities in Wisconsin and Illinois range from 67 to 

360,000 ft2/d (Young, 1992), but most values range from about 

670 to 2,000 ft2/d (Young and Siegel, 1992). Transmissivity 

values obtained from aquifer tests in Indiana and Ohio ranged 

from 70 to 52,000 ft2/d (Joseph and Eberts, 1994). Transmissivity is low where the aquifers are confined or have few fractures or solution openings and high in areas with large solution 

openings or extensive fractures (Young and Siegel, 1992).

The surficial aquifer system (fig. 3) consists of discontinuous sand-and-gravel valley-fill aquifers of glacial and 

alluvial origin that are found throughout the Great Lakes 

Basin. Although individual surficial aquifers do not cover 

large areas similar to those covered by the Cambrian-Ordovician and Silurian-Devonian bedrock aquifers, their ubiquity 

has a regional effect on ground-water resources and, therefore, 

allows them to collectively be treated as a regional system. 

To be considered a surficial aquifer, the glacial deposits must 

have adequate thickness and capacity to transmit water to meet 

local water demands at a sustained level. The presence, areal 

extent, and thickness of these deposits vary widely across the 

basin.

A RASA study of glacial (stratified-drift) deposits 

throughout the northeastern United States (Kontis and others, 2004) provided detailed information on surficial aquifers 

in the RASA study area, as well as a basis for transferring 

glacial-deposits’ characteristics from the northeastern United 

States to other areas of the Great Lakes Basin where similar 

data were missing. Kontis and others (2004) identified more 

than 5,000 sand-and-gravel aquifers that were created by or in 

glacial meltwater. These surficial aquifers, which were defined 

as those with saturated thicknesses of at least 10 ft and(or) the 

potential for sustained yields of at least 10 gal/min to wells, 

underlie 12.6 percent (15,400 mi2) of the glaciated Northeast 

(Kontis and others, 2004). More than 80 percent of these 

aquifers are valley-fill aquifers with a saturated thickness of 10 

to 150 ft in valleys from about 1,000 to 10,000 ft wide (Kontis 

and others, 2004). The porosity of these deposits ranges from 

27 to 45 percent, with a median value of about 38 percent 

(Kontis and others, 2004). Laboratory measurements of 

specific yield (or the long-term unconfined storage coefficient) 

range from 16 to 47 percent, with a median value of 33 percent; values obtained from pumping tests range from 3 to 13 

percentmuch lower than those obtained by laboratory methods (Kontis and others, 2004). Storage coefficients reported 

for confined surficial aquifers in the glaciated Northeast range 

from 10-4 to 10-2 (Kontis and others, 2004). Horizontal hydraulic conductivity typically ranges from about 50 to 500 ft/d but 

can be much greater, whereas vertical hydraulic conductivity 

is far lower than horizontal conductivitycommonly only a 

tenth as large (Kontis and others, 2004). Transmissivities for 

these surficial aquifers typically range from 1,000 to 

50,000 ft2/d (Kontis and others, 2004).

Surficial aquifers are generally absent where Precambrian 

crystalline rocks are at or near the land surface (Young and 

Siegel, 1992), as in the parts of the basin that lie in Minnesota, 

northern Wisconsin, and the western part of the Upper Peninsula of Michigan, but are estimated to cover about 40 percent 

of eastern Wisconsin and northeastern Illinois (on the basis 

of a surficial-aquifer map presented in Olcott (1992)). Where 

surficial aquifers exist in Wisconsin and adjacent states, 

aquifer thickness generally ranges from 50 to 100 ft, but in 

buried bedrock valleys the thickness commonly ranges from 

200 to 400 ft (Young and Siegel, 1992). These aquifers, which 

are potential sources of water in Wisconsin and Illinois, typically were not developed in the past because deep wells that 

mostly tap the Cambrian-Ordovician aquifer system described 

previously have yielded a reliable and abundant water supply 

(Young, 1992). Degradation of the quality of water extracted 

from the Cambrian-Ordovician system in recent years, however, has resulted in increased withdrawals from the surficial 

aquifers, at least in southeastern Wisconsin (J.T. Krohelski, 

U.S. Geological Survey, written commun., 2006). The specific 

yield for these aquifers has been estimated at 0.15 (Feinstein 

and others, 2005a).

Surficial deposits are absent in some parts of the Lower 

Peninsula of Michigan but form the largest reservoir of fresh 

ground water in other parts of the State (Westjohn and Weaver, 

1998). These deposits range in thickness from 0 to 400 ft in 

the eastern and southern parts of the Lower Peninsula where 

sand and gravel make up from 25 to 75 percent of the deposits (Westjohn and others, 1994). Aquifer tests indicate that 

these “thin” deposits are typically partly or fully confined; 

the median storage coefficient is 8 × 10-4 (C.J. Hoard, U.S. 

Geological Survey, written commun., 2005). The thickness 

of the surficial deposits increases northwestward across the 

Lower Peninsula from 400 ft to more than 1,000 ft (Olcott, 

1992), and the percentage of sand and gravel increases to 

more than 75 percent in the northwestern quarter of the Lower 

Peninsula (Westjohn and others, 1994). These “thick” deposits 

are assumed to be mostly unconfined; sparse data in this area 

prevents verification of this assumption, however (C.J. Hoard, 

U.S. Geological Survey, written commun., 2005). Despite 

the ubiquitous presence of sand-and-gravel deposits, visual 

inspection of a surficial-aquifer map in Olcott (1992) indicates 

that only about 50 percent of the thin-deposits area and about 

60 percent of the thick-deposits area are covered by surficial 

aquifers. Freshwater is found in most of the surficial deposits 

of the Lower Peninsula, except within a 1,600-mi2 area of the 

Saginaw Lowlands, where deposits are thin (less than 200 ft 

thick) and have a low percentage of sand and gravel (less than 

25 percent) and where saline water is common (Westjohn and 

Weaver, 1996b).

Thickness of the surficial deposits in the Great Lakes 

Basin of Indiana and Ohio ranges from less than 100 ft to 

more than 400 ft (Casey, 1996). About 90 percent of northwestern Indiana was estimated to be covered by surficial aquifers (Olcott, 1995a). Elsewhere in northern Indiana and Ohio, 

about 13 percent of the area—similar to the Northeast United 

States RASA value (Kontis and others, 2004)—was estimated 

to be covered by surficial aquifers; this determination was 

based on comparison of surficial-aquifer maps of Indiana-Ohio 

(Olcott, 1995a) and the northeastern United States (Olcott, 
 Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

1995b). Storage coefficients for confined conditions in these 

states range from 2 × 10-5 to 0.3; the median value is 

6 × 10-4 (Joseph and Eberts, 1994, tables 1 and 2). Specific 

yields for unconfined conditions range from 1.6 × 10-3 to 0.38; 

the median value is 0.13 (Joseph and Eberts, 1994, table 1). 

Horizontal hydraulic conductivity in this area ranges from 0.33 

to 1,000 ft/d, and transmissivity ranges from 300 to 

69,700 ft2/d (Joseph and Eberts, 1994, ). Similar to the bedrock aquifer in this area, dissolved-solids concentrations in 

ground water vary widely—from less than 500 to greater than 

1,000 mg/L (Bugliosi, 1999).

Minor Aquifer Systems

The Pennsylvanian and Mississippian aquifers in 

the Lower Peninsula of Michigan (fig. 2) consist of three 

bedrock aquifersthe Saginaw aquifer (Pennsylvanian sandstone), the Parma-Bayport aquifer (Pennsylvanian sandstone 

and carbonate), and the Marshall aquifer (Mississippian sandstone). The total areal extent of these three aquifers is about 

22,000 mi2 (Westjohn and Weaver, 1996c). Saline ground 

water underlies freshwater-bearing aquifers everywhere in the 

Lower Peninsula of Michigan (Westjohn and Weaver, 1998). 

Therefore, the usable supply of freshwater (defined as water 

that has a maximum dissolved-solids concentration of 

1,000 mg/L) is limited to those parts of each aquifer that 

underlie, and are in direct hydraulic connection with, glacial 

deposits. Elsewhere, the remaining parts of each aquifer are 

overlain by confining rock layers, dissolved-solids concentrations exceed 1,000 mg/L, and saline water or brine is present. 

Most aquifer tests indicate that these aquifers are either partly 

or fully confined; the median storage coefficient is 3 × 10-4

(C.J. Hoard, U.S. Geological Survey, written commun., 2005).

The Saginaw aquifer encompasses about 10,400 mi2

(Westjohn and Weaver, 1998) and about 5,000 mi2 contains 

freshwater (Westjohn and Weaver, 1996b). Aquifer thickness 

ranges from 100 to 370 ft (Westjohn and Weaver, 1996a), and 

porosity ranges from 3 to 34 percent; the median porosity is 

20 percent (Westjohn and others, 1990). Transmissivity ranges 

from 130 to 2,700 ft2/d, and vertical hydraulic conductivity 

ranges from 0.0001 to 55 ft/d (Westjohn and Weaver, 1996a). 

Hydrogeologic characteristics of this sandstone unit are matrix 

controlled; that is, the hydraulic values are generally a function of type and degree of cementation (Westjohn and Weaver, 

1998).

The Parma-Bayport aquifer encompasses about 

11,000 mi2 (Westjohn and Weaver, 1998), of which 8,700 mi2

contains freshwater and 2,300 mi2 contains brine (Westjohn 

and Weaver, 1996b). Aquifer thickness generally ranges 

from 100 to 150 ft (Westjohn and Weaver, 1996a). Porosity of 

the upper part of the sandstone component of this sandstone 

and carbonate-rock aquifer ranges from 25 to 35 percent and 

decreases as the sandstone becomes increasingly consolidated 

with depth and as the carbonate-rock component increases; 

porosity of the lower part of the aquifer ranges from 2 to 25 

percent (Westjohn and Weaver, 1996a). In general, porosity 

and hydraulic conductivity values for this aquifer are similar to 

those reported for the Saginaw aquifer (Westjohn and Weaver, 

1996a).

The Marshall aquifer encompasses about 22,000 mi2, of 

which about 12,000 mi2 contains freshwater and about 

10,000 mi2 contains brine (Westjohn and Weaver, 1996c). 

Aquifer thickness typically ranges from 75 to 175 ft but 

exceeds 200 ft in the northwestern part of the aquifer (Westjohn and Weaver, 1996c). Porosity ranges from 16 to 25 percent; the median porosity is 21 percent (Westjohn and others, 

1990). Transmissivity of highly fractured sandstone can range 

from 3,000 to 29,000 ft2/d (Westjohn and Weaver, 1998) and 

hydraulic conductivity from 150 to 550 ft/d (Westjohn and 

Weaver, 1996c). In contrast, transmissivity of well-cemented, 

unfractured sandstone can range from 7 to 50 ft2/d and hydraulic conductivity from 0.2 to 1.8 ft/d (Westjohn and Weaver, 

1998).

The Mississippian and Upper Devonian aquifers in 

northeastern Ohio consist of rock units within the Cuyahoga 

Group (Lower Mississippian), Cussewago and Berea Sandstones (Upper Devonian; Pashin and others, 1995). Aquifers in 

these units are also present to a much lesser extent in northwestern Pennsylvania (fig. 2). Altogether, Mississippian and 

Upper Devonian units underlie about 7,790 mi2 of the Great 

Lakes Basin. The Cuyahoga Group has the largest areal extent 

(4,650 mi2) and is the thickest of these units (up to 

170 ft), but it is sometimes considered a leaky confining unit 

on a regional basis because few of the sandstone or shale units 

with the Cuyahoga Group are highly permeable (Eberts and 

others, 1990). Of the aquifers in the remaining units, as much 

as 25 percent produce water considered to be non-potable, 

with dissolved-solids concentrations greater than 1,000 mg/L 

(Rau, 1969; Eberts and others, 1990; Barton and Wright, 

1997). In fact, in parts of northeastern Ohio, the Cussewago 

Sandstone and Berea Sandstone are also tapped for oil and 

gas production (Barton and Wright, 1997). For this reason, 

the Mississippian and Upper Devonian aquifers in Ohio were 

excluded from any of the RASA studies (Bugliosi, 1999), and 

because the total storage in these aquifers is estimated to be 

small compared to the total storage in the Great Lakes Basin, 

these aquifers also were excluded from the ground-water-storage estimates presented in this report.

Estimate of Ground-Water Storage

The estimate of the volume of water stored in the aquifers 

of the Great Lakes Basin, described herein, was based mainly 

on data from RASA reports, which focused on regional aquifer 

systems that are capable of yielding substantial quantities of 

water. The many bedrock aquifers that are small or incapable 

of meeting water needs beyond those of domestic or smallcommunity uses were excluded from the storage computation. Among these small bedrock aquifers are the Jacobsville 
Estimate of Ground-Water Storage 

Table 1. Hydrogeologic characteristics of the regional aquifer systems in the United States part of the Great Lakes Basin.

[mi2, square miles; ft, feet; S, southern; E, eastern; N, northern; NE, northeastern; NW, northwestern; <, less than; >, greater than]

Aquifer Area 

(mi2)

Area of saline water

(mi2)

Aquifer thickness (ft)Porosity 

(percent)

Specific 

yield

Storage 

coefficient References

Major aquifers

Cambrian-Ordovician aquifer system

St. Peter-Prairie du Chien- Jordan aquifer 24,951 0–500 0.05 7.5 × 10-5 –

1.2 × 10-3 1, 2, 3

Ironton-Galesville aquifer 33,116 50–150 0.05 5.0 × 10-5 –

9.0 × 10-4 1, 2, 3, 4

Mount Simon aquifer 33,116 a100–2,000 0.05 5.0 × 10-5 – 9.0 × 10-4 1, 2, 3, 4

Silurian-Devonian aquifer system

Parts of Ohio and S. Michigan only 8,616 (

b

) 200–1,600 0.01–0.05 1.0 × 10-5 –

5.0 × 10-2 5, 6

E. Wisconsin and N.E. Illinois 9,695 200–500 0.017–0.03 9.0 × 10-5 –

4.8 × 10-3 1, 2

N. Michigan 2,913 200–1,400 c

 9.0 ×10-5 – 4.8 × 10-3 4

Surficial aquifer system

New York and Pennsylvania d2,730 10–150 27 –45 0.16–0.47 1.0 × 10-4 –

1.0 × 10-2 7

Minnesota and N. Wisconsin (

e

) 2

E. Wisconsin and N.E. Illinois f12,680 50–400 0.15 (

g

) 2, 3, 8, 9

N.E. Indiana d395 200–400 0.0016–0.38 2 × 10-5 –

3 × 10-1 6, 9

N. Ohio d1,500 100–400 0.0016–0.38 2 × 10-5 – 3 × 10-1 5, 6

N.W. Indiana h514 100–200 0.0016–0.38 2 × 10-5 –

3 × 10-1 6, 9

Michigan, Lower Peninsula:

Deposits generally < 400 feet thick i13,950 j800 50–400 (

g

) 8.0 × 10-4 10, 12, 14

Deposits generally > 400 feet thick i7,500 400–1,000 (

g

) 8.0 × 10-4 8, 12

Minor aquifers (in the central Lower Peninsula of Michigan)

Pennsylvanian sandstone

Saginaw aquifer 10,400 5,400 100–370 3–34 3.0 × 10-4 10, 11, 12, 14

Pennsylvanian sandstone and carbonate rock

Parma-Bayport aquifer 11,000 2,300 100–150 2–35 3.0 × 10-4 10, 11, 12, 14

Mississippian sandstone

Marshall aquifer 22,000 10,000 75–175 16–25 3.0 × 10-4 10, 11, 13, 14

a

 Water in the Mount Simon sandstone is generally saline below the upper few hundred feet in eastern Wisconsin and northeastern Illinois (Young, 1992).

b

 Not included in storage estimate.

c

 Same values as for eastern Wisconsin.

d

 Thirteen percent of total area is considered surfical aquifer system; that is, sand-and-gravel glacial deposits with a saturated thickness of at least 10 ft 

 and(or) the potential for sustained yields of at least 10 gallons per minute to wells (Kontis and others, 2004).

e Surfical aquifer system is generally absent where Precambrian crystalline rocks are at or near the land surface (Young and Siegel, 1992).

f

 Forty percent of total area is assumed to be covered by surfical aquifer system, as defined in footnote d.

g

 Values for the stratified-drift aquifers in New York and Pennsylvania (from Kontis and others, 2004).

h

 Ninety percent of total area is assumed to be covered by surfical aquifer system, as defined in footnote d.

i

 Data presented by Westjohn and Weaver (1996b) indicate that the area of Michigan in which “thin” glacial deposits (deposits generally less than 400 ft thick) 

 occur represents about 69 percent (about 27,900 mi2) of the Lower Peninsula; the remaining 31 percent (about 12,500 mi2) is assumed to be covered 

 by “thick” glacial deposits (generally greater than 400 ft thick). Of these total areas, 50 percent of the thin-deposits area and 60 percent of the thick-

 deposits area are assumed to be covered by surfical aquifer system as defined in footnote d.

j

 Ground water in 1,600 mi2 of the area covered by thin glacial deposits (the Saginaw Lowlands) is saline. Fifty percent of this area is assumed to consist of 

 surfical aquifer system, as explained in footnote i.

REFERENCES: (1) Young, 1992. (2) Young and Siegel, 1992. (3) Feinstein and others, 2005a. (4) Mandle and Kontis, 1992. (5) Casey, 1996. (6) Joseph and 

Eberts, 1994. (7) Kontis and others, 2004. (8) Olcott, 1992. (9) Lloyd and Lyke, 1995. (10) Westjohn and Weaver, 1998. (11) Westjohn and Weaver, 1996a. 

(12) C.J. Hoard, U.S. Geological Survey, written commun., 2005. (13) Westjohn and Weaver, 1996. (14) Westjohn and Weaver, 1996c.
10 Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

Table 2. Estimation of ground water stored in the regional aquifer systems in the United States part of the Great Lakes Basin.

[ft, feet; mi2, square miles; mi3, cubic miles; S, southern; E, eastern; N, northern; NE, northeastern; NW, northwestern; <, less than; >, greater than]

Aquifer

Area of aquifer (mi2) Average

thickness 

of aquifer

(ft)

Hydraulic

head

(ft)

Specific

yield

Storage

coefficient

Storage (mi3)

Total With freshwater Total Freshwater

Major aquifers

Cambrian-Ordovician aquifer system

St. Peter-Prairie du Chien- Jordan aquifer 24,951 250 750 0.05 a3.0 × 10-4 60.13 60.13

Ironton-Galesville aquifer 33,116 100 600 .05 a2.12 × 10-4 32.16 32.16

Mount Simon aquifer:

Entire aquifer 33,116 1,050 1550 .05 a2.12 × 10-4 331.34

Freshwater part 33,116 b

300 800 .05 a2.12 × 10-4 95.14

Silurian-Devonian aquifer system

Parts of Ohio and S. Michigan only 8,616 900 1000 .03 c

1.0 × 10-4 44.22 44.22

E. Wisconsin and N.E. Illinois 9,695 350 450 .02 a6.6 × 10-4 11.47 11.47

N. Michigan 2,913 800 900 .03 a6.6 × 10-4 13.57 13.57

Surficial aquifer system

New York and Pennsylvania: 

Unconfined d1,365 80 80 .33 a1.0 × 10-3 6.84 6.84

 Confined d1,365 80 180 .33 a1.0 × 10-3 6.87 6.87

E. Wisconsin and N.E. Illinois: 

Unconfined e6,340 225 225 .15 f1.0 × 10-3 20.09 20.09

Confined e6,340 225 325 .15 f1.0 × 10-3 20.14 20.14

N.E. Indiana and N. Ohio 

Unconfined d947 250 250 .13 c6.0 × 10-4 5.86 5.86

 Confined d947 250 350 .13 c6.0 × 10-4 5.87 5.87

N.W. Indiana g514 150 150 .13 c6.0 × 10-4 1.91 1.91

Michigan, Lower Peninsula

 Deposits generally > 400 ft thick and 

 assumed to be mostly unconfined

h

7,500 700 700 f.33 c8.0 × 10-4 328.92 328.92

 Deposits generally < 400 ft thick and 

 assumed to be mostly confined

i

13,950 i13,150 225 325 f.33 c8.0 × 10-4 196.86 185.57

Minor aquifers (in the central Lower Peninsula of Michigan)

Pennsylvanian sandstone

Saginaw aquifer:

Confined 5,400 235 435 j.2 c3.0 × 10-4 48.20

Partly confined 5,000 5,000 235 235 j.2 c3.0 × 10-4 44.57 44.57

Pennsylvanian sandstone and carbonate rock

Parma-Bayport aquifer:

 Confined 2,300 125 325 j.2 c3.0 × 10-4 10.93

 Partly confined 8,700 8,700 125 125 j.2 c3.0 × 10-4 41.26 41.26

Mississippian sandstone

Marshall aquifer:

Confined 10,000 125 325 j.21 c3.0 × 10-4 49.90

Partly confined 12,000 12,000 125 125 j.21 c3.0 × 10-4 59.74 59.74

TOTAL 1,340.86 984.34

a Average of log-transformed range of values.

b Water in the Mount Simon sandstone is generally saline below the upper few hundred feet in eastern Wisconsin and northeastern Illinois (Young, 1992). 

 Used 300 ft for thickness of freshwater part of aquifer. 

c Median value.

d

 Total aquifer area is 13 percent of the surface-water drainage area (from Kontis and others, 2004). This area was divided equally between unconfined and 

 confined aquifers. 

e

 Forty percent of total area is considered surfical aquifer system. This area was divided equally between unconfined and confined aquifers. Of the area 

 affected by the shift in the ground-water divide, 13 percent of the estimated ground water in the surficial aquifers is assumed to recharge the Great Lakes 

 Basin bedrock aquifers and to flow toward the pumping centers in the Chicago-Milwaukee area (U.S. Geological Survey, 2004). The remainder of the water

 discharges to local streams that drain to basins outside of the Great Lakes Basin. 

f

 Used values for the surfical aquifer system in the glaciated northeastern United States (Kontis and others, 2004).

g

 Ninety percent of total area is considered surfical aquifer system; all of which was assumed to be unconfined.

h

 Sixty percent of total area is considered surfical aquifer system. 

i

 Fifty percent of total area is considered surfical aquifer system. 

j

 Porosity value.
Estimate of Ground-Water Storage 11

sandstone aquifer in northern Michigan (Olcott, 1992) and 

the carbonate-rock and sandstone aquifers in western and 

central New York (Olcott, 1995b). Many individual surficial 

aquifers fit this exclusionary definition but were collectively 

treated as components of a regional surficial aquifer system, so 

their contributions were included in the ground-water-storage 

estimates. The parts of the Silurian-Devonian carbonate-rock 

aquifer in northern Indiana, northwestern and eastern Ohio, 

and southern Michigan that were excluded from the Midwestern Basins and Arches RASA study on the basis of dissolvedsolids concentrations greater than 10,000 mg/L (Bugliosi, 

1999), also were excluded from the storage calculations.

Methods of Computation

Ground-water storage volume is a function of storativity, the volume of water released from (or taken into) storage per unit surface area of aquifer per unit decline (or rise) 

in hydraulic head (Freeze and Cherry, 1979). In a confined 

aquifer, storativity, also referred to as the storage coefficient, is 

the product of specific storage and aquifer thickness. Storage 

volume in a confined aquifer is solely a function of the compressibility of pore water and the mineral matrix of the aquifer. 

In an unconfined aquifer, where storativity can be several 

orders of magnitude larger than if the aquifer is confined, the 

amount of water stored owing to the compressibility of water 

and the aquifer is negligible. Therefore, in an unconfined 

aquifer, storativity essentially equals specific yield and reflects 

the volume of water that will drain from an aquifer under the 

influence of gravity.

The volume of ground water (V) stored in each of the 

Great Lakes aquifer systems was computed as the product of 

storativity, aquifer area, and the change in hydraulic head 

(Fetter, 1980):

V = S A ∆h      (1)

where

S is storativity (or storage coefficient), dimensionless,

A is area underlain by an aquifer, in square feet, and

∆h is change in hydraulic head, in feet.

As mentioned above, storativity is defined differently on 

the basis of whether the aquifer is confined or not. For a confined aquifer, and until a confined aquifer becomes unconfined 

as a result of drawdown of the potentiometric surface below 

the top of the aquifer, storativity is defined as follows (Fetter, 

1980):

S = b Ss(2)

where

b is aquifer thickness, in feet, and

Ss is specific storage, per foot.

Combining equations 1 and 2, the volume of water stored in a 

confined aquifer becomes

    V = (b Ss) A ∆h (3)

For an unconfined aquifer, storativity is defined as follows (Fetter, 1980):

    S = Sy + h Ss(4)

where

Sy is specific yield, dimensionless,

h is hydraulic head, which in the case of an unconfined 

aquifer, equals the saturated thickness of the aquifer, in feet, 

and

Ss is as previously defined.

Combining equations 1 and 4, the volume of water stored in an 

unconfined aquifer becomes

    V = (Sy + h Ss) A ∆h (5)

To compute the total storage available in an aquifer, ∆h 

must be replaced with different terms depending on whether 

confined or unconfined conditions prevail. In a confined aquifer, ∆h would equal the height of the potentiometric surface 

above the bottom of the aquifer (H). In an unconfined aquifer, 

∆h would equal the saturated thickness (h), and the maximum 

h could be approximated by the aquifer thickness (b), as would 

be the case when an aquifer changes from a confined to an 

unconfined condition. Computation of total storage is the sum 

of “expansion water”water released due to compression 

of the aquifer solids and expansion of water that accompanies a decline in head under both confined and unconfined 

conditionsand (2) “gravitational water”water released 

due to gravitational drainage under unconfined conditions. In 

a confined aquifer, gravitational drainage will not occur until 

the potentiometric surface drops below the top of the aquifer and the aquifer becomes unconfined; that is, an aquifer 

whose upper surface is a water table, free to fluctuate under 

atmospheric pressure. Therefore, the total storage volume 

(Vt) of a confined aquifer is the sum of expansion water when 

the potentiometric surface is above the top of the aquifer and 

“gravitational plus expansion” water when the potentiometric 

surface is below the top of the aquifer:

Vt = [(b Ss) A (H – b)] + [(Sy + b Ss) A b]

 = [(b Ss) A (H – b)] + [Sy A b] + [b Ss A b]

 = [(b Ss) A (H – b + b)] + [Sy A b]

 = [b Ss A H] + [Sy A b]. (6)

The total storage volume (Vt) of an unconfined aquifer is the 

sum of gravitational and expansion water in the saturated 

thickness (h) of the aquifer. Although gravitational water 

dominates the water released from storage in an unconfined 

aquifer, the expansion-water component is included in the following equation so as to be consistent with equation 6:

Vt = (Sy + h Ss) A h = [Sy A h] + [Ss A h2]. (7)
12 Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

Assumptions and Limitations Inherent in 

Storage Estimate

The hydrogeologic characteristics of the many aquifers 

included in estimation of the volume of water stored in the 

Great Lakes Basin vary widely between aquifers and within 

a given aquifer (table 1). Therefore the total ground-water 

storage for each aquifer was calculated from average, median, 

or other representative values of these characteristicsthickness, porosity, specific yield, and storage coefficientfor the 

parts of each aquifer that lie within the Great Lakes Basin 

(table 2). Assumptions and transference of data from one area 

to another were necessary to provide reasonable parameter 

values for a given aquifer in an area that was not studied and 

to allow inclusion of all the principal aquifers in the storage 

estimate. Many of the RASA reports discuss the development 

of ground-water models and present the parameter values used 

therein; these model values generally constrained the range of 

parameter values that were deemed most reasonable for the 

specific region and, where given, were preferentially used in 

the calculations of storage.

Hydraulic head can vary greatly across an aquifer; therefore, representative values of the height of the potentiometric 

surface above the top of the confined aquifers in the study 

area were estimated from generalized hydrogeologic sections 

presented in the RASA reports. These values were estimated 

as the average distance from the top of the aquifer to the land 

surface, and are considered gross approximations in that large 

discrepancies can be noted at any specific point in a particular 

aquifer; nonetheless, they provide a basis for simplifying a calculation that otherwise would be unmanageable. The following representative values were added to the average thickness 

of their respective aquifers and were used in the calculation 

of ground-water storage in the confined aquifers of the Great 

Lakes Basin: 500 ft for the Cambrian-Ordovician aquifer 

system (Young, 1992), 100 ft for the Silurian-Devonian 

(Bugliosi, 1999) and the surficial aquifer systems (T.S. Miller, 

U.S. Geological Survey, oral commun., 2005), and 200 ft for 

the minor aquifer systems of Michigan (Westjohn and Weaver, 

1998). To compute the maximum volume of water stored in 

an unconfined aquifer, the aquifer thickness (b) was used as 

an approximation of the saturated thickness (h). In reality, the 

saturated thickness of an unconfined aquifer would be some 

value less than the total thickness of the aquifer.

Specific-yield values are applicable to unconfined conditions; however, these values were required to compute the 

total storage volume for confined aquifers in the case where 

pumping might lower the potentiometric surface below the 

top of an aquifer, thus creating an unconfined condition. The 

possibility of this actually occurring on a widespread scale in 

any of the bedrock aquifers included in this study is unanticipated. Median values of specific yield and storage coefficients 

for a given aquifer were used when available. Alternatively, 

averages of the maximum and minimum values in a range 

of published values were used. Because storage-coefficient 

values can span several orders of magnitude, averaging was 

performed on log-transformed values. For the minor bedrock 

aquifers in the Lower Peninsula of Michigan, where specific yields were unavailable, porosities (which are typically 

slightly larger than specific yields) were substituted in the storage calculations.

Calculation of ground-water storage in surficial aquifers was complicated by the wide range in aquifer thickness, 

extent, and composition and by lack of data in many parts 

of the Great Lakes Basin. The problem of missing data was 

resolved by assuming that the surficial-aquifer characteristics 

documented by the RASA study of the northeastern United 

States (Randall, 2001; Kontis and others, 2004) would be 

comparable, and therefore transferable, to surficial aquifers 

elsewhere in the Great Lakes Basin. Only about 13 percent of 

the glaciated northeastern United States is covered by deposits 

that are, or could be considered, potential aquifers, and more 

than 80 percent of these are valley-fill aquifers (Randall, 2001; 

Kontis and others, 2004). Therefore, the values found for 

valley-fill stratified-drift aquifers in the glaciated Northeast 

were applied to the sand-and-gravel surficial aquifers found 

elsewhere in the Great Lakes Basin unless otherwise stipulated 

in a published report. The areal extent of surficial aquifers was 

estimated from surficial-aquifer maps in Olcott (1992; 1995a) 

in comparison with a surficial-aquifer map of northeastern 

United States (Olcott, 1995b), in which surficial aquifers had 

been estimated to cover about 13 percent of the land area. 

The coverage of surficial aquifers in northeastern Indiana and 

northern Ohio appeared to be similar to that in the northeastern United States; therefore the percentage of coverage was 

estimated as 13 percent in these areas. Elsewhere in the Great 

Lakes Basin, surficial aquifers were assumed to cover 40 percent of eastern Wisconsin and northeastern Illinois; 90 percent 

of northwestern Indiana; and 60 and 50 percent of the thickdeposits and thin-deposits areas, respectively, of the Lower 

Peninsula of Michigan.

No estimate of the percentage of surficial aquifers that 

are confined (as opposed to unconfined) is available. This 

distinction strongly affects the calculation of ground-water 

storage because the amount of water that can be released from 

an unconfined aquifer (with a median specific yield of 0.33 

in the glaciated Northeast) is at least two orders of magnitude 

greater than that which can be released from a confined aquifer (with an average storage coefficient of 0.001 in the glaciated Northeast). Therefore, the estimated aquifer area in parts 

of the basin that were judged to contain a mix of unconfined 

and confined surficial aquifers was divided equally (50 percent 

each) between unconfined and confined conditions.

The confining units—that is, those rock layers (typically shales) or glacial deposits (typically till or lacustrine silt 

and clay) that impede the movement of water and overlie an 

aquifer—can also be potential sources of water. The hydrogeologic characteristics of confining units are much different from 

aquifers in that their hydraulic conductivity or rate of water 

transmission is very low. Although present, the water in the 

confining units is difficult to remove and is therefore consid-
Estimate of Ground-Water Storage 13

ered unavailable; these water volumes were not included in the 

storage calculations.

The aquifer areas were either obtained directly from a 

RASA report or calculated from the summation of subbasin 

drainage areas within a GIS (Geographic Information System) 

database of the Great Lakes Basin (U.S. Geological Survey, 

2005b). The boundaries of the aquifers were defined by the 

current known or assumed boundary of the Great Lakes Basin 

ground-water divide (Sheets and Simonson, 2006). In the 

absence of detailed information, the ground-water divide was 

assumed to approximate the surface-water divide.

In southeastern Wisconsin and northeastern Illinois, the 

ground-water divide of the deep Cambrian-Ordovician sandstone aquifer system is many miles west of the surface-water 

divide. In addition, recent studies have documented that heavy 

pumping has caused the cones of depression that surround the 

pumping centers in Chicago and Milwaukee to coalesce and 

to shift the divide westward 10 to 20 mi (Young, 1992; U.S. 

Geological Survey, 2004; Feinstein and others, 2005b; Sheets 

and others, 2005; fig. 4). This relocation of the ground-water 

divide has increased the area of the Cambrian-Ordovician 

aquifer system that currently contributes water to the pumping 

centers by about 3,220 mi2 (Sheets and Simonson, 2006). This 

water is captured within the area defined by the Great Lakes 

deep ground-water divide and therefore is included in the storage estimate. Based on a regional ground-water-flow model 

of southeastern Wisconsin (Feinstein and others, 2005b), 

however, this water actually does not contribute to the Great 

Lakes Basin because it is removed from the deep-bedrock 

aquifers by wells, most of which are located in the Mississippi 

River Basin. Assuming that well water, after use, is returned to 

surface-water bodies in the same basin in which a given well is 

located, about 5 percent of the pumped water is discharged to 

the Great Lakes Basin, whereas about 95 percent is discharged 

to the Mississippi River Basin (D.T. Feinstein, U.S. Geological 

Survey, written commun., 2006).

The shift in the deep-aquifer ground-water divide in 

southeastern Wisconsin and northeastern Illinois has little 

effect on the shallow aquifers, except where drawdown of 

the potentiometric surface of the deep aquifer induces downward movement of water from the shallow aquifers. The 

surficial glacial deposits that might be affected by this shift 

were estimated to cover about 40 percent of a 2,630-mi2 area 

(Young and others, 1989) or about 1,052 mi2. The estimate of 

the ground water stored in the surficial aquifers of this area 

was further complicated, not just by movement of the deep 

ground-water divide due to heavy pumping, but by the presence or absence of the confining rock layer, the Maquoketa 

Shale. One-hundred percent of the ground water stored in the 

surficial aquifers that lie within the area defined by the Great 

Lakes Basin surface-water divide was included in the storage estimate. Where surficial aquifers overlie the Maquoketa 

Shale between the surface-water divide and the pre-development deep-ground-water divide (fig. 4)–which approximates 

the location of the western edge of the Maquoketa Shale 

(D.T. Feinstein, U.S. Geological Survey, written commun., 

2006)–about 1.9 percent of the recharge to the surficial aquifers leaks through the Maquoketa Shale to the deep CambrianOrdovician aquifer system (U.S. Geological Survey, 2004). 

West of the Maquoketa Shale, between the pre-development 

and current deep-ground-water divides, about 12.8 percent 

of the recharge to the surficial aquifers leaks through to the 

deep aquifer system (U.S. Geological Survey, 2004). These 

percentages, rounded to 2 and 13 percent, respectively, were 

assumed to be reasonable approximations of the volume of 

ground water stored in the surficial aquifers that would supply 

the deep aquifers and were used for this purpose to calculate 

the contributions of these surficial aquifers to the Great Lakes 

Basin storage volume. The remaining water stored in these 

surficial aquifers discharges to local streams that drain to adjacent basins and was not included in the storage estimate.

Storage Volume

The total volume of ground water stored in the many 

aquifer systems in the Great Lakes Basin was estimated to be 

about 1,340 mi3 (table 2), which is greater than the earlier estimate of 1,000 mi3

 by Grannemann and others (2000) and more 

than the volume of Lake Michigan (1,180 mi3 at low-water 

datum; U.S. Environmental Protection Agency, 2005). Of this 

total, about 984 mi3 is considered freshwater; that is, water 

with dissolved-solids concentrations less than 

1,000 mg/L. Several parts of certain aquifers contain water 

that is saline (dissolved-solids concentrations exceeding 

1,000 mg/L); these include the surficial deposits in a 1,600-mi2

area of the Saginaw Lowlands in Michigan, below the upper 

few hundred feet of the Mount Simon Sandstone aquifer in 

Wisconsin and northeastern Illinois, and the confined parts of 

the sandstone aquifers in the Lower Peninsula of Michigan. 

The storage volumes of each of these aquifer parts, which 

were included in the total storage estimate, were not included 

in the freshwater estimate.

The movement of the ground-water divide along the 

western side of Lake Michigan has increased the volume of 

ground water stored within the Great Lakes Basin by about 

36 mi3. The major component of this increase—almost 90 percent—is the estimated ground water stored in the Mount Simon Sandstone aquifer; water below the upper few hundred 

feet is considered impotable, however. Therefore, of the total 

increase in storage, only about 13 mi3 —including 9.2 mi3 contained in the upper part of the Mount Simon aquifer—is considered freshwater. Heavy pumping in the Chicago-Milwaukee 

area has lowered the potentiometric surface and decreased the 

quantity of ground water stored in the confined CambrianOrdovician aquifer system. In southeastern Wisconsin this 

decrease in storage is reported to be about 65.4 billion gallons 

or 0.059 mi3 (U.S. Geological Survey, 2004) and represents 

less than 0.005 percent of the total estimated storage. Drawdown in the deep aquifers can cause an increase in downward 

leakage from shallow bedrock and surficial aquifers, however this downward leakage in Wisconsin is inhibited by the 
14    Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

!H Milwaukee

SHEBOYGAN

OZAUKEE

Illinois

Wisconsin

JEFFERSON CO

DODGE CO

WALWORTH CO

WASHINGTON CO

WAUKESHA CO

OZAUKEE

SHEBOYGAN

89° 88°30' 88° 87°30'

42°30'

43°

43°30'

Lake Michigan

EXPLANATION

Cambrian-Ordovician aquifer 

ground-water divide

Center of pumping, 1950 (16.9 million gallons per day)

Center of pumping, 2000 (33.5 million gallons per day)

Albany Pipes

Map Location

1860 (Predevelopment)

2002

Base from U.S. Geological Survey; scale 1:2,000,000.

NAD83 Geographic Coordinate System; decimal degrees

0 4 8 MILES

0 4 8 KILOMETERS

Great Lakes surface-water drainage basin

Figure 4.  Movement of simulated ground-water divide in the Cambrian-Ordovician aquifer in response

to intensive pumping in the Milwaukee, Wisconsin area (modified from Feinstein and others, 2005b).
Estimate of Ground-Water Storage 15

Maquoketa Shale confining unit and most of the leakage is 

derived not from storage (drainage of pores in the overlying 

unconfined aquifers) but through capture of water that would 

otherwise flow toward and into streams and lakes. Therefore, 

the loss of storage in the shallow aquifers through the pumping of the deep aquifers is considered to be negligible and is 

not represented in the 0.059-mi3 loss from Cambrian-Ordovician aquifer system (D.T. Feinstein, U.S. Geological Survey, 

Milwaukee, Wis., written commun., 2005).

Uncertainty in Storage Estimate

The computed storages are estimated values on a regional 

basis and should not be interpreted to mean that this quantity 

of water is available in its entirety to meet water-supply needs. 

To do so would require the complete dewatering of an aquifer, 

an action that is environmentally undesirable. The amount of 

water that is considered available on the basis of water quantity and quality and on the basis of environmental, economic, 

and legal constraints has not been determined.

The original estimate of total ground-water storage—

1,340 mi3—was based on average, median, or other representative values of the hydrogeologic parameters that were 

included in the ground-water storage formulas 

(eqs. 6 and 7). As a measure of the uncertainty in the total 

storage that might have arisen from selection of these values, 

the storage was recalculated with low and high values that 

bracketed the respective ranges of aquifer thickness, hydraulic head, specific yield, and storage coefficient, as listed 

in table 1. For each recalculation, only one parameter was 

adjusted and in only one direction—either lower or higher; 

however, this adjustment was done for all aquifers in the basin. 

The only exception to this rule concerned aquifer thickness; 

for this parameter, a decrease or increase required a comparable adjustment to the hydraulic-head value as well. Because 

hydraulic heads were estimated to begin with, the assessment 

of uncertainty associated with this parameter was performed 

on hydraulic-head values that were 50 percent lower and 

50 percent higher than their original values. For parameters 

for which a range of values was not given in the reference 

documents, the original parameter value was retained. The 

eight recalculated storage values (table 3) ranged from 464 

to 2,217 mi3, both values of which pertained to changes in 

aquifer thicknesses and differed from the original estimate by 

65 percent in both directions. Changes in specific yields produced the next largest deviation from the original estimate—

about 38 percent. The remaining estimates departed from the 

original estimate by less than 14 percent.

Table 3. Uncertainty in the estimate of ground-water storage in the United States 

part of the Great Lakes Basin.

[Values are in cubic miles. The original estimate of storage—1,340 cubic miles—was based on average, median, or other representative values of the hydrogeologic parameters that are included in the 

ground-water storage formulas. (See equations 6 and 7.) The following estimates are based on low and 

high values of the indicated hydrogeologic parameter (table 1).]

Adjusted hydrogeologic parameter Storage estimate

Low value High value

Aquifer thickness (including required 

adjustment of hydraulic-head values) 464 2,217

Hydraulic head 1,156 1,345

Specific yield 830 1,819

Storage coefficient 1,337 1,480
16 Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006

Summary

Hydrogeologic data from Regional Aquifer System 

Analyses (RASA) studies that were conducted by the U.S. 

Geological Survey in the Great Lakes Basin, United States, 

during 1978–95 were compiled and used to estimate the 

volume of water stored in the many aquifers of the basin. Storage estimates focused on six regional aquifer systems. Three 

are major aquifer systems: the Cambrian-Ordovician aquifer 

system (in Wisconsin, Illinois, and Indiana), the SilurianDevonian aquifers (in Wisconsin, Michigan, Illinois, Indiana, 

and Ohio), and the surficial aquifer system (aquifers of alluvial 

and glacial origin that are found throughout the Great Lakes 

Basin). Three are minor aquifer systems: the Pennsylvanian 

sandstone aquifer, the Pennsylvanian sandstone and carbonate rock aquifer, and the Mississippian sandstone aquifer (all 

in Michigan) (U.S. Geological Survey, 2003). Parts of these 

aquifers that were excluded from the RASA studies due to 

poor water quality and other small aquifer systems throughout 

the basin that are less important as ground-water sources on a 

regional basis were not included in the storage estimates.

 Summation of ground-water volumes in the many 

regional aquifers of the basin indicates that about 1,340 mi3 of 

water is in storage; of this, about 984 mi3 is considered freshwater (that is, water with dissolved-solids concentration less 

than 1,000 mg/L). These volumes should not be interpreted as 

available in their entirety to meet water-supply needs; complete dewatering of any aquifer is environmentally undesirable. The amount of water that is considered available on the 

basis of water quality and environmental, economic, and legal 

constraints has not been determined. Heavy pumping in the 

Chicago and Milwaukee areas has caused the ground-water 

divide in the Cambrian-Ordovician aquifer system to shift 

westward and thereby has increased the area of the deep-bedrock aquifer that can potentially contribute water to the Great 

Lakes Basin. This increased contribution—about 36 mi3, of 

which about 13 mi3 is considered freshwater—was included in 

the ground-water-storage estimates above. The corresponding 

decrease in ground-water storage that has resulted from lowering of the potentiometric surface due to this heavy pumping 

(0.059 mi3) is less than 0.005 percent of the total estimated 

storage.

Uncertainty in the storage estimates was assessed by 

recalculating the storage with low and high values of selected 

parameters—aquifer thickness, hydraulic head, specific yield, 

and storage coefficient. The recalculated storages differed 

from the original value by 65 percent when aquifer thicknesses 

were varied by each aquifer’s respective range in thickness. 

The differences between the original and recalculated storages 

were less than 40 percent when values for the other parameters 

were varied.

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Illinois: U.S. Geological Survey Professional Paper 1423–C, 

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Basin water availability and use: U.S. Geological Survey 

Fact Sheet 2005–3113, 4 p.

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to investigations of sand and gravel aquifers in the glaciated 

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aquifer system in the northern Midwest, United States: U.S. 

Geological Survey Professional Paper 1405–D, 76 p.

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aquifer-system analysis program of the U.S. Geological 

Survey, 1978–91: U.S. Geological Survey Water-Resources 

Investigations Report 91–4122, 92 p.

Sun, R.J., Weeks, J.B., and Grubb, H.F., 1997, Bibliography of 

regional aquifer-system analysis program of the U.S. Geological Survey, 1978–96: U.S. Geological Survey WaterResources Investigations Report 97–4074, 63 p., accessed 

May 2005 at http://pubs.er.usgs.gov/pubs/wri/wri974074
Glossary 19

Glossary

Definitions adapted from Freeze and Cherry, 1979; Heath, 1983; Miller, 2004; or Kontis 

and others, 2004.

Confined aquifer (artesian aquifer) – an aquifer that is completely filled with water 

under pressure and that is overlain by material that restricts the movement of water.

Hydraulic gradient – change in hydraulic head per unit distance.

Hydraulic head – the height of the free surface of a body of water above a given point 

beneath the surface; composed of elevation head and pressure head. 

Hydraulic conductivity – the capacity of a geologic material or deposit to transmit water. 

It is expressed as the volume of water that will move in a unit time under a unit hydraulic gradient through a unit area.

Potentiometric surface – a surface that represents the total head in an aquifer; that is, 

it represents the height above a datum plane at which the water level stands in tightly 

cased wells that penetrate an aquifer.

Specific storage – the volume of water released from or taken into storage in a unit volume of aquifer per unit change in hydraulic head owing to the compressibility of pore 

water and the mineral matrix of the aquifer. 

Specific yield – the volume of water that will drain from a unit volume of an unconfined 

aquifer under the influence of gravity. It is the long-term unconfined storage coefficient.

Storativity or storage coefficient – the volume of water released from or taken into storage per unit surface area of aquifer per unit change in hydraulic head. The storage coefficient is the product of specific storage and aquifer thickness for a confined aquifer, 

and it essentially equals specific yield for an unconfined aquifer. 

Transmissivity – the rate at which a volume of water is transmitted through a unit width 

of an aquifer under a unit hydraulic gradient. It equals the hydraulic conductivity multiplied by the aquifer thickness.

Unconfined aquifer – an aquifer whose upper surface is a water table free to fluctuate 

under atmospheric pressure.

Valley-fill aquifer – proglacial valley that has been partly filled with glacial deposits that 

typically consist of 10 to 150 feet of saturated sand and gravel and may be underlain, 

bordered, or in part overlain by fine-grained deposits and is always bordered on two 

sides and underlain by relatively impermeable bedrock.

Water table – the water level in the saturated zone at which the pressure head is equal to 

the atmospheric pressure.
William F. Coon and Rodney A. Sheets—Estimate of Ground Water in Storage in the Great Lakes Basin—Scientific Investigations Report 2006–5180

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