SUMMARY OF GROUND-WATER HYDROLOGY OF THE

CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM IN THE

NORTHERN MIDWEST, UNITED STATES

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1405-A
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Summary of Ground-Water Hydrology 

of the Cambrian-Ordovician Aquifer 

System in the Northern Midwest, 

United States

By H.L. YOUNG

REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1405-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1992
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

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

descriptive purposes only and does not imply endorsement by the

U.S. Government

Library of Congress Cataloging in Publication Data

Young, H.L.

Summary of ground-water hydrology of the Cambrian-Ordovician aquifer system in the northern Midwest, United States / by 

H.L. Young.

p. cm. (Regional aquifer-system analysis) (U.S. Geological Survey professional paper ; 1405-A)

Includes bibliographical references.

Supt. of Docs, no.: I 19.16:1405-A

1. Water, Underground Middle West. 2. Aquifers Middle West. 3. Geography, Stratigraphic Cambrian. 4. Geography,

Stratigraphic Ordovician. I. Title. II. Series. III. Series: U.S. Geological Survey professional paper. 

GB1017.Y68 1991

551.49'0977-dc20 90-3099

CIP

For sale by Book and Open-File Report Sales, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started in 

1978 following a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States. The RASA Program 

represents a systematic effort to study a number of the Nation's most 

important aquifer systems, which in aggregate underlie much of the country 

and which represent an important component of the Nation's total water 

supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political 

subdivisions to which investigations have often arbitrarily been limited in the 

past. The broad objective for each study is to assemble geologic, hydrologic, 

and geochemical information, to analyze and develop an understanding of the 

system, and to develop predictive capabilities that will contribute to the 

effective management of the system. The use of computer simulation is an 

important element of the RASA studies, both to develop an understanding of 

the natural, undisturbed hydrologic system and the changes brought about in 

it by human activities, and to provide a means of predicting the regional 

effects of future pumping or other stresses.

The final interpretive results of the RASA Program are presented in a series 

of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number, 

and where the volume of interpretive material warrants, separate topical 

chapters that consider the principal elements of the investigation may be 

published. The series of RASA interpretive reports begins with Professional 

Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available.

Dallas L. Peck 

Director

CONTENTS

Foreword......................................................................... Ill

Abstract........................................................................... Al

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

Hydrogeology................................................................... 4

Geologic Setting.......................................................... 4

Hydrogeologic Units .................................................... 5

Recharge, Ground-Water Movement, and Discharge.......... 15

Geochemistry of Water....................................................... 20

General Chemical Characteristics Related to the

Hydrologic System........................................... 21

Hydrochemical Facies............................................. 21

Dissolved Solids .................................................... 21

Trace Elements and Minor Constituents.................... 25

Effects of Pleistocene Glaciation on Ground-Water

Quality................................................................. 25

Ground-Water Development and Its Effects........................... 30

Ground-Water Development.......................................... 30

Effects on the Aquifer System...................................... A33

Ground-Water Flow Simulation........................................... 36

Regional Model .......................................................... 36

Conceptual Model................................................. 36

Steady-State Simulation......................................... 38

Transient Simulation............................................. 38

Subregional Models..................................................... 40

Chicago-Milwaukee Area Model .............................. 43

Steady-State Simulation................................... 43

Transient Simulation....................................... 46

Topics for Further Investigation ......................................... 50

Hydraulic Characteristics of Confining Units................... 50

Relation of Confining Units to Water Quality.................. 50

Other Topics.............................................................. 51

Conclusions...................................................................... 51

References Cited.............................................................. 52

Project Publications.......................................................... 54

ILLUSTRATIONS

FIGURES 1-5. Maps showing:

1. Location and general features of the area of study of the Cambrian-Ordovician aquifer system........................ A3

2. Extent of major glaciations in the northern Midwest during the Pleistocene Epoch ....................................... 5

3. General bedrock geology of the northern Midwest.................................................................................... 6

4. Structural surface of the Precambrian basement in the northern Midwest.................................................... 7

5. Altitude of the top of the St. Peter Sandstone in the northern Midwest....................................................... 8

6-9. Maps of northern Midwest showing thickness of:

6. Maquoketa confining unit in the northern Midwest................................................................................... 12

7. St. Peter-Prairie du ChienJordan aquifer in the northern Midwest............................................................ 13

8. Ironton-Galesville aquifer in the northern Midwest.................................................................................. 14

9. Mount Simon aquifer in the northern Midwest......................................................................................... 16

10. Map showing generalized water table and major surface-water drainage network in the northern Midwest.................... 17

11. Generalized hydrogeologic section across southern Wisconsin illustrating typical predevelopment flow systems in the

northern Midwest .................................................................................................................................... 18

12-19. Maps of northern Midwest showing:

12. Approximate predevelopment potentiometric surface for the St. Peter-Prairie du ChienJordan aquifer in the

northern Midwest......................................................................................................................... 19

13. Sulfate distribution in the St. Peter-Prairie du ChienJordan aquifer in the northern Midwest........................ 22

14. Hydrochemical facies of ground water in the Cretaceous and drift aquifers in the northern Midwest................ 23

15. Hydrochemical facies of ground water in the Mount Simon and St. Peter-Prairie du ChienJordan aquifers

in the northern Midwest................................................................................................................ 24

16. Dissolved-solids distribution in the Mount Simon aquifer in the northern Midwest......................................... 26

17. Dissolved-solids distribution in the St. Peter-Prairie du ChienJordan aquifer in the northern Midwest............ 27

18. Distribution of boron in the Mount Simon and St. Peter-Prairie du ChienJordan aquifers in the northern

Midwest...................................................................................................................................... 28

19. Distribution of 8180 ratio in the Cambrian-Ordovician aquifer system in the northern Midwest........................ 29

20. Graphs showing ground-water withdrawals from the Cambrian-Ordovician aquifer system in the Chicago and Twin

Cities areas, 1864-1980.............................................................................................................................. 31
VI CONTENTS

Page

21-23. Maps showing:

21. Major areas of ground-water withdrawal from the Cambrian-Ordovician aquifer system in the northern

Midwest, 1979-1980..................................................................................................................... A32

22. Generalized potentiometric surface for the St. Peter-Prairie du Chien-Jordan aquifer in the northern

Midwest, 1980............................................................................................................................. 34

23. Decline in head in the composite Cambrian-Ordovician aquifer system in the Chicago-Milwaukee area,

1864-1980................................................................................................................................... 35

24-27. Maps of northern Midwest showing:

24. Location and areal extent of regional and subregional ground-water flow models of the Cambrian-Ordovician

aquifer system in the northern Midwest.......................................................................................... 37

25. Simulated predevelopment head in the St. Peter-Prairie du Chien-Jordan aquifer in the northern Midwest...... 39

26. Simulated 1980 head in the St. Peter-Prairie du Chien-Jordan aquifer in the northern Midwest...................... 41

27. Simulated decline in head in the St. Peter-Prairie du Chien-Jordan aquifer, 1861-1980.................................. 42

28. Schematic diagram of simulated water budget for the Cambrian-Ordovician aquifer system in the northern Midwest..... 43

29. Geohydrologic section along Chicago-Milwaukee area model column 21.................................................................... 44

30. Map showing simulated predevelopment head and vertical flow direction in the St. Peter-Prairie du Chien-Jordan

aquifer in the Chicago-Milwaukee area ........................................................................................................ 45

31. Schematic diagram of simulated predevelopment ground-water flow across the top of the St. Peter-Prairie du ChienJordan aquifer within the area of the Chicago-Milwaukee flow system .............................................................. 47

32. Map showing simulated 1985 head and vertical flow direction in the composite Cambrian-Ordovician aquifer system

in the Chicago-Milwaukee area................................................................................................................... 48

33. Hydrograph for well Wk-07/17E/05-0020 completed in the St. Peter-Prairie du Chien-Jordan aquifer in northwestern

Waukesha County, Wisconsin..................................................................................................................... 49

34. Schematic diagram of simulated 1985 ground-water flow across the top of the St. Peter-Prairie du Chien-Jordan

aquifer within the area of the Chicago-Milwaukee flow system ........................................................................ 49

TABLE

TABLE 1. Hydrogeologic-unit nomenclature used in the study of the Cambrian-Ordovician aquifer system ......................................... A10

METRIC CONVERSION FACTORS AND VERTICAL DATUM

For readers who wish to convert measurements from the inch-pound system of units to the metric 

system of units, the conversion factors are listed below:

Multiply inch-pound units By To obtain metric units

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

square mile (mi2 2.590 square kilometer (km2)

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

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06308 liter per second (L/s)

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

foot squared per day (ft2/d) 0.09290 meter squared per day (m,2/d)

Sea Level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD 

of 1929) a geodetic datum derived from a general adjustment of the first-order level nets of both the 

United States and Canada, formerly called Sea Level Datum of 1929.
REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

SUMMARY OF GROUND-WATER HYDROLOGY OF THE CAMBRIANORDOVICIAN AQUIFER SYSTEM IN THE NORTHERN MIDWEST,

UNITED STATES

By H.L. YOUNG

ABSTRACT

The Cambrian-Ordovician aquifer system contains very productive 

aquifers throughout an area of about 161,000 square miles in the 

northern Midwest. The aquifer system is used extensively for industrial and rural water supplies and is the primary source of water for 

many municipalities in most of its area of occurrence, except in Indiana, 

central and southern Illinois, and western Iowa, where the aquifer 

system contains saline water. About 680 million gallons per day was 

withdrawn from drilled wells in the aquifer system in 1980.

Rocks of Cambrian and Ordovician age, mainly marine sandstones 

and carbonate rocks, constitute most of the bedrock and subcrop 

beneath glacial drift in southeastern Minnesota, northeastern Iowa, 

Wisconsin, northern Illinois, and extreme northwestern Indiana. These 

strata dip generally to the south and east off the Transcontinental arch 

in Minnesota and the Wisconsin arch, which are structurally high areas 

of the Precambrian basement, into the structural lows of the Forest 

City basin of southwestern Iowa, the Illinois basin, and the Michigan 

basin.

The Cambrian and Ordovician rocks are buried by younger rocks in 

the remainder of Iowa, Illinois, and Indiana and in most of northern 

Missouri. Silurian and Devonian carbonate rocks immediately overlie 

the Cambrian and Ordovician rocks in those areas and are termed the 

"Silurian-Devonian aquifer" in this study. The balance of the Devonian 

rocks and the overlying Mississippian and Pennsylvanian rocks generally are fine-grained sediment or dense carbonate rocks and collectively 

are considered to be a regional confining unit. Most of the area is 

covered by a veneer of glacial drift, which, along with Cretaceous 

sandstone in northwestern Iowa, is treated as a regional water-table 

aquifer.

The Cambrian-Ordovician aquifer system is composed of six hydrogeologic units, which are, in descending order, the Maquoketa confining unit, St. Peter-Prairie du Chien-Jordan aquifer, St. LawrenceFranconia confining unit, Ironton-Galesville aquifer, Eau Claire 

confining unit, and Mount Simon aquifer. The uppermost confining unit 

is the least permeable; it consists primarily of the Maquoketa Shale but 

includes the dense carbonate rocks of the Galena Dolomite and the 

Decorah, Platteville, and Glenwood Formations where they are overlain by the Maquoketa Shale. The presence of the Maquoketa in Iowa, 

eastern Wisconsin, northeastern Illinois, and Indiana effectively confines the entire aquifer system below.

The aquifer system is a leaky-artesian system in which movement of 

ground water is controlled partly by the internal confining units. In the 

northern outcrop area, unconfined conditions prevail in shallow parts of 

the aquifer system and where the system is thin. Much of the recharge 

in upland areas discharges to streams through local flow systems, 

which are no more than a few miles in length. The remainder of the 

recharge moves slowly downward to deeper formations and downgradient to form or join the regional flow system.

Computer simulations of regional ground-water flow improve understanding of the regional character of the Cambrian-Ordovician aquifer 

system. Ground-water flow in the confined part of the aquifer system is 

mainly horizontal, away from the structural highs in the north, toward 

the structural basins in the south and east. The rate of ground-water 

movement is very slow, and the flux along flow paths into the basins 

decreases because of a progressive loss of head and small but widespread upward leakage. Saline water in the basins restricts movement 

of freshwater into the deeper parts of the basins, thereby forcing flow 

upward through confining units. Principal regional discharge areas are 

the Mississippi and Missouri Rivers, the Illinois and Michigan structural basins, and Lake Michigan. However, the lake is not in direct 

hydraulic connection with the Cambrian-Ordovician aquifer system and 

receives flow primarily from the Silurian-Devonian aquifer, which it 

directly overlies. The longest regional flow paths originate in recharge 

areas in northwestern Iowa and extend southeastward as much as 400 

miles toward the Illinois basin.

Simulated predevelopment recharge and discharge for the CambrianOrdovician aquifer system balance at 351 million gallons per day.

Development of the aquifer system began in various parts of the 

northern Midwest in the 1860's and 1870's with the drilling of deep, 

generally flowing artesian wells near Lake Michigan in eastern Wisconsin and northeastern Illinois and along the valleys of the Mississippi 

River and its tributaries. Initial heads of 186 and 130 feet above Lake 

Michigan at Milwaukee and Chicago, respectively, have been reported. 

Large-scale pumping has produced cones of depression in these two 

areas, with respective head declines of as much as 375 and 900 feet. 

Other major pumping centers generally have had much smaller 

declines. The largest withdrawals from the aquifer system were about 

180 million gallons per day in each of the major metropolitan areas of 

Chicago and Minneapolis-St. Paul (Twin Cities). However, the total 

decline in head in the St. Peter-Prairie du Chien-Jordan aquifer in the 

Twin Cities by 1980 was only 90 feet because the aquifer is unconfined.

Al
A2 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

Most of the eastern two-thirds of Iowa, where the aquifer system is 

tightly confined, is characterized by more than 50 feet of head decline, 

with 200 feet or more at Mason City and the Quad Cities.

Pumpage from the Cambrian-Ordovician aquifer system throughout 

the study area averaged 683 million gallons per day for the period 

1976-80. Results of a transient-model simulation show that recharge 

increased over predevelopment recharge by 447 million gallons per 

day. Natural discharge decreased by 99 million gallons per day, and 137 

million gallons per day was released from aquifer storage.

Mineralization of ground water in the aquifer system increases from 

slightly mineralized calcium magnesium bicarbonate water in the 

northern recharge areas, through more mineralized, mixed water types 

with increased sodium and sulfate, to highly mineralized sodium 

chloride brines in the deeper parts of the structural basins.

INTRODUCTION

Much of the sedimentary rock that overlies the Precambrian crystalline basement rock in the northern 

Midwest consists of the marine sandstone and carbonate 

rocks that make up the Cambrian and Ordovician Systems. The entire Cambrian and Ordovician sequence 

forms the major regional aquifer system of the area. The 

Cambrian-Ordovician aquifer system is essentially continuous and contains very productive aquifers throughout an area of about 161,000 mi2 (square miles) in 

southeastern Minnesota, Wisconsin, Iowa, northern Missouri, and northern Illinois (fig. 1). Many metropolitan 

areas depend on the aquifer system for municipal and 

industrial water supplies. It also is used extensively for 

self-supplied industrial, rural, and domestic water supplies.

Development of the Cambrian-Ordovician aquifer system in the northern Midwest began as early as 1864 when 

a flowing well 711 ft (feet) deep was drilled in Chicago. 

By the late 1800's many wells had been drilled, especially 

where flowing wells were obtainable. Withdrawal of 

ground water from the aquifer system has increased 

almost steadily in relation to the growth in population, 

attaining rates of about 180 Mgal/d (million gallons per 

day) in 1980 in the Chicago and Minneapolis-St. Paul 

areas. The hydraulic head in the aquifer system has 

declined hundreds of feet since the late 1800's in the 

Chicago and Milwaukee areas and to a somewhat lesser 

extent in other areas, such as Minneapolis-St. Paul, 

Green Bay, Wis., and Des Moines, Mason City, and 

Cedar Rapids, Iowa. Increased ground-water withdrawals in the future will cause continued or increased waterlevel declines.

The aquifer system contains highly mineralized water 

in several places, especially in its deepest parts, generally coinciding with regional discharge or structurally 

low areas. This mineralized water is unsuitable for most 

uses and limits ground-water development in some 

areas. It is a possible source of contamination to freshwater supplies in areas of large ground-water withdrawals, such as northeastern Illinois, southeastern Wisconsin, and central Iowa.

In October 1978, the U.S. Geological Survey (USGS) 

began a regional assessment of the Cambrian-Ordovician 

aquifer system in the northern Midwest (Steinhilber and 

Young, 1979,) as part of its national program of Regional 

Aquifer-System Analysis (RASA) (See Foreword of this 

report, and Bennett, 1979).

Although many parts of the aquifer system have been 

studied in each State within the project area as part of 

the USGS's Cooperative Program and by State and local 

agencies, the regional study has the obvious benefit of 

assessing the aquifer system in its entirety. The major 

goals of the RASA program are to (1) gain an understanding of each hydrogeologic system, including the 

nature of the hydrogeologic units and the ground-water 

flow system, and the chemical quality of the water, and 

(2) describe the regional interaction of components of the 

system, especially as affected by large-scale withdrawal 

of ground water.

The border of the area of study of the CambrianOrdovician aquifer system (fig. 1) delimits either the 

natural physical or hydrologic boundaries of the aquifer 

system or places where aquifers in the system are not 

used because of water containing extremely high dissolved solids (greater than 10,000 mg/L (milligrams per 

liter)). The northern boundary, from northwestern Iowa 

to northeastern Wisconsin, delineates the erosional edge 

of Cambrian-age rocks overlying crystalline basement 

rocks of Precambrian age. The Missouri River is a 

ground-water discharge line and forms the western and 

southwestern boundary. Beyond the eastern and southeastern boundary in Michigan, Illinois, and Indiana, 

water in the aquifer system is highly mineralized.

The rocks of the aquifer system subcrop beneath 

glacial drift or crop out in much of the northern part of 

the study area, but in most of the remainder of the area 

they are overlain by the Maquoketa Shale, the uppermost Ordovician rock unit, which has very low permeability. The aquifer system is bounded below by impermeable Precambrian basement rock. Other rocks in the 

study area are minor aquifers regionally, but can be 

important locally. The major local aquifers consist of 

Silurian and Devonian carbonate rocks, Cretaceous sandstone, and Quaternary sand and gravel. Some wells 

developed in the Cambrian-Ordovician aquifer system 

also are open to the Silurian and Devonian carbonate 

rocks; thus, the Silurian-Devonian aquifer was also 

included in the study.

The purpose of this report is to summarize the findings 

of the regional study of the Cambrian-Ordovician aquifer 

system. The report includes discussions of the hydrogeology, geochemistry, and development of ground water
INTRODUCTION A3

'Lake oSheboygan 

Winnebago/ .

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 U.S. GEOLOGICAL SURVEY TEST WELL DRILLED FOR THE 

NORTHERN MIDWEST REGIONAL AQUIFER-SYSTEM 

ANALYSIS

FIGURE 1. Location and general features of the area of study of the Cambrian-Ordovician aquifer system.

in the system and suggestions for future studies. 

Detailed discussions of the aquifer system are presented 

in a series of separate chapters of USGS Professional 

Paper 1405: chapter A (this report) is the summary, 

chapter B describes the regional hydrogeologic framework and general water quality (Young, in press), chapter C describes a three-dimensional ground-water flow 

model of the study area (Handle and Kontis, in press), 

chapter D describes the geochemistry (Siegel, 1989), and 

chapter E describes a detailed three-dimensional ground-
A4 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

water flow model of the Chicago-Milwaukee area (H.L. 

Young and A.J. MacKenzie, USGS, written commun., 

1988).

Other published reports and abstracts of papers presented at scientific meetings by the central staff and by 

personnel of the subprojects and contractors are listed in 

the section, "Project Publications."

Data needed for the study were generated from existing files and publications, as well as from newly gathered 

and interpreted data, from several State agencies, and 

USGS district offices. A computerized data base was 

developed (Kontis and Mandle, 1980) to store, edit, and 

access the data efficiently.

Although extensive geologic data on each rock unit are 

available, very little information exists on the vertical 

variation in hydraulic properties, head distribution, or 

water-quality characteristics within the aquifer system, 

except in areas where the units crop out. To obtain data 

on individual hydrogeologic units at various locations, 

deep test wells were drilled at seven sites (fig. 1). Data 

on hydraulic conductivity, hydraulic heads, and water 

quality were obtained from these test wells and from 

several existing wells by using inflatable hydraulic packers, pressure transducers, and submersible pumps. Two 

piezometers were installed near each of the two wells in 

Houston County, Minn. After completion of packer tests 

on the other five test wells, nests of three or four 

piezometers separated by grout plugs were installed at 

different depths in the wells. The piezometers are now 

part of the USGS's ground-water-level and water-quality 

monitoring programs. The deepest well, 3,475 ft deep 

near Zion, 111., has been described by Nicholas and 

others (1987).

Water levels were measured or obtained from existing 

records for the study in the fall of 1980 and spring of 1981 

for about 800 deep wells in northern Illinois by the 

Illinois State Water Survey (Visocky and others, 1985) 

and for almost 200 wells in southeastern Wisconsin by 

the USGS. These data were used to prepare potentiometric surface maps for the Cambrian-Ordovician aquifer system in the very important, heavily pumped 

Chicago-Milwaukee area. The measured heads are 

mostly from wells open to more than one of the aquifers 

in the aquifer system and thus generally are a composite 

of heads in the aquifer system; however, they probably 

are most representative of heads in the St. Peter-Prairie 

du ChiennJordan aquifer.

More than 3,000 chemical analyses of ground water 

were available from the USGS's WATSTORE (Water 

Data Storage and Retrieval System) data base, publications of State and Federal agencies, and other scientific 

reports. Almost 200 ground-water samples were collected during the study to obtain water-chemistry data 

representative of single aquifers and to obtain special

data in particular locations. The new samples were 

analyzed for the customary major constituents and several trace constituents. In addition, the following special 

determinations were made: about 120 ratios of the stable 

isotopes of oxygen, hydrogen, and carbon, 155 analyses 

of radium and uranium concentration, and 36 carbon-14 

ages.

HYDROGEOLOGY

GEOLOGIC SETTING

The present land surface of the northern Midwest is 

largely the result of Pleistocene continental glaciation 

that deposited a veneer of unconsolidated glacial drift on 

the eroded bedrock surface until as recently as 10,000 yr 

B.P. (years before present), during Wisconsin time. 

Glaciers advanced and retreated numerous times during 

the Pleistocene Epoch; the extent of the Wisconsin, 

Illinoian, and pre-Illinoian glaciations in the area is 

shown in figure 2. The topography is formed by a variety 

of depositional and erosional features in the drift, which 

generally is 50 to 100 ft thick but is as much as several 

hundred feet thick in end moraines and buried bedrock 

valleys. Drift commonly is less than 50 ft thick on some 

upland areas and is absent from the Driftless Area of 

southwestern Wisconsin and adjacent areas (fig. 2).

Consolidated sedimentary rocks beneath the drift 

range in age from Precambrian to Cretaceous (fig. 3). 

They crop out in generally concentric, arcuate patterns 

away from the Transcontinental and Wisconsin arches, 

which are topographically high areas of the Precambrian 

basement (fig. 4) in northern Minnesota and Wisconsin, 

respectively. Cambrian and Ordovician rocks also crop 

out near the Missouri and Mississippi Rivers in eastern 

and southern Missouri, where they dip downward to the 

north away from the Ozark uplift a high on the Precambrian surface in southeastern Missouri. The configuration of the basement surface shows the strongly downwarped character of the structural basins, which also is 

reflected in the surfaces of most of the overlying sedimentary formations.

The thickness of the sedimentary rocks increases away 

from the arches toward flanking structural basins: the 

Forest City basin in southwestern Iowa, the Illinois 

basin in southeastern Illinois, and the Michigan basin 

centered on the Lower Peninsula of Michigan. The 

maximum thickness is about 14,000 and more than 15,000 

ft in the Illinois and Michigan basins, respectively. 

Figure 5 depicts the top of the Middle Ordovician St. 

Peter Sandstone, which shows the direct influence of the 

structural features of the Precambrian basement and is 

representative of the general configuration of most of the 

Paleozoic sedimentary strata in the area.
HYDROGEOLOGY A5

95°

45e

MICHIGAN

Scale 1:6.000,000

50 100 MILES

_____I________I

50 100 KILOMETERS

Base enlarged from 

U.S.Geological Survey 

1:7,500,000. 1970

FIGURE 2. Extent of major glaciations in the northern Midwest during the Pleistocene Epoch. (From Young, in press.)

HYDROGEOLOGIC UNITS

Rocks in the northern Midwest are regionally divisible 

into five primary aquifers, each underlain by a less 

permeable confining unit (table 1). In most of the study 

area, the uppermost aquifer is the drift aquifer, which 

contains the water table and consists of unconsolidated 

glacial drift. The Cretaceous shale and sandstone in 

western Iowa are not differentiated for this study, but 

are broadly termed the "Cretaceous aquifer" and locally 

are included with the drift aquifer as an unconfined

aquifer layer in the regional flow model. These watertable aquifers were not included in the present study. 

However, the Cretaceous Dakota Formation becomes a 

major aquifer in the Central Midwest RASA study area, 

which adjoins the present study area on the west and 

south (Jorgensen and Signer, 1981).

The sequence of rocks ranging in age from Pennsylvanian through Late Devonian, primarily shales and carbonate rocks, is regarded as the uppermost regional 

confining unit (table 1). This unit, the PennsylvanianMississippian-Devonian confining unit, contains some
A6 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

Base enlarged from 

U.S. Geological Survey 

1:7,500,000. 1970

^^^Hl ^^ ^ Cretaceous rocks

_J_| Jurassic rocks

_P_) Permian rocks

p~| Pennsylvanian rocks

M

EXPLANATION

GEOLOGIC UNITS 

Mississippian rocks 

Devonian rocks 

Silurian rocks

Modified from Professional Paper 1405-B

~^ 1 Ordovician rocks older than 

_ J the Maquoketa Shale ^?

 6 , Cambrian rocks

3! S.r?

Precambrian sandstone

Om | Ordovician Maquoketa ' p x Precambrian crystalline 

Shale rocks

 CONTACT

A A' LINE OF HYDROGEOLOGIC SECTION Section shown in figure 11 

FIGURE 3. General bedrock geology of the northern Midwest. (Modified from Young, in press.)
HYDROGEOLOGY A7

95'

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 soo STRUCTURE CONTOUR Shows altitude of the top of

Precambrian rocks. Contour interval, in feet, is variable. 

Datum is sea level

M

AREA OF OUTCROP OR SUBCROP OF PRECAMBRIAN ROCKS 

BENEATH GLACIAL DRIFT

FIGURE 4. Structural surface of the Precambrian basement in the northern Midwest. (From Young, in press.)
A8 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 soo STRUCTURE CONTOUR Shows altitude of the top of the St. Peter 

Sandstone. Includes areas of eroded surface. Contour interval, 

in feet, is variable. Datum is sea level

EROSIONAL BOUNDARY OF ST. PETER SANDSTONE

FIGURE 5. Altitude of the top of the St. Peter Sandstone in the northern Midwest. (From Young, in press.)
HYDROGEOLOGY A9

local aquifers that supply small to moderate well yields, 

but generally it has low permeability compared with the 

Cambrian-Ordovician aquifer system and can be considered a regional confining unit. This unit forms the 

bedrock surface in most of the southern and western 

parts of the study area. It thickens from its erosional 

edge to more than 3,000 ft in the Illinois and Michigan 

basins, and is absent in Minnesota, Wisconsin, northeastern Iowa, northern Illinois, and northwestern Indiana.

Rocks of Middle Devonian through Silurian age, 

mainly dolomite and limestone, form the SilurianDevonian aquifer, which immediately overlies the 

Cambrian-Ordovician aquifer system except where Silurian and Devonian rocks are absent on the Wisconsin 

arch. The permeability of this aquifer is highly variable 

and depends primarily on the extent and degree of 

intersection of fractures and joints in the rock, and on the 

subsequent solutional enlargement of these openings by 

weathering and ground-water movement. This secondary permeability is high in the outcrop and subcrop areas 

of the aquifer. Reported transmissivities range from 67 

to 360,000 ft2/d (feet squared per day), but more commonly they are less than 500 ft2/d. Except for the outcrop 

area, the aquifer is overlain and confined by younger 

rocks of the Pennsylvanian-Mississippian-Devonian confining unit. The Silurian-Devonian aquifer thickens from 

its erosional edge to more than 1,000 ft in the Illinois and 

Michigan basins, but generally it is 200 to 500 ft thick in 

its outcrop areas.

The remaining sedimentary rocks older than Silurian 

(mostly Cambrian and Ordovician in age) are the main 

subject of this study. The Cambrian-Ordovician aquifer 

system consists of three successive pairs of alternating 

aquifers and overlying confining units (table 1). The 

hydrogeologic units are named for their dominant stratigraphic components in the northern part of the study 

area but consist of stratigraphic and hydrologic equivalents in the southern part. The components of the 

Cambrian-Ordovician aquifer system are defined for this 

study as follows:

Maquoketa confining unit. This unit provides the 

primary confinement to the Cambrian-Ordovician aquifer >>ystem. The Maquoketa Shale is the main component 

of the unit, but where the Maquoketa overlies dolomite 

and shale of the Galena Dolomite and Decorah, Platteville, and Glenwood Formations, these formations also 

are an important part of the confining unit. Where the 

Maquoketa is absent, these formations are more likely to 

have a higher permeability because of the effects of 

weathering, and locally they are moderately productive 

aquifers. In southeastern Minnesota, the Maquoketa 

Formation is mainly limestone and is included with 

limestone of the overlying Middle Devonian Cedar Valley 

Formation the only younger rock in the area and the

underlying dolomitic limestones of the Dubuque and 

Galena Formations to form the local upper carbonate 

aquifer. The Maquoketa confining unit thickens from the 

north to the south and east. It commonly is 400 to 600 ft 

thick in eastern Iowa, eastern Wisconsin, and northeastern Illinois (fig. 6).

St. Peter-Prairie du Chien-Jordan aquifer: This 

aquifer is an important source of ground water, especially in the western part of the study area, in Iowa and 

Minnesota. In those States, the aquifer consists of the 

generally uniform Jordan Sandstone and the hydraulically connected dolomite of the overlying Prairie du 

Chien Group. The transmissivity of the Jordan aquifer 

depends on the degree of development of secondary 

permeability in the dolomite due to fractures and solutional enlargement by weathering. A very large range in 

transmissivity, from 42 to 33,000 ft2/d, has been 

reported, but a more common range is about 500 to 5,000 

ft2/d. The aquifer is well confined in most of Iowa, but in 

much of southeastern Minnesota the rocks are near the 

surface, are dissected by valleys, and are only partly 

confined. In Wisconsin, the aquifer subcrops in uplands 

in the northwest and crops out in the uplands of the 

Driftless Area in the west and southwest. Because the 

water table is very deep beneath the ridges, the upper 

part of the aquifer is unsaturated beneath much of the 

uplands. In the eastern part of the study area, the 

aquifer is not as important a source of water because the 

Jordan Sandstone is not present and the Prairie du Chien 

is thin or absent owing to pre-St. Peter erosion in much 

of southern Wisconsin and northern Illinois. In northern 

Missouri, the aquifer consists of the St. Peter and 

underlying carbonate equivalents of the Prairie du Chien 

and Jordan the Roubidoux Formation and the Gasconade and Eminence Dolomites. The Roubidoux and Gasconade contain some sandstone that makes them productive aquifers.

The aquifer increases in thickness away from the 

northern eroded edge to more than 1,500 ft in the Illinois 

basin (fig. 7).

St. Lawrence-Franconia confining unit. The two 

formations that form this unit generally are very silty 

and shaly, fine-grained, poorly sorted, dolomitic sandstones. Although they contribute water locally to some 

multiaquifer wells, their low permeability and anisotropy 

make them more important a^ a regional confining unit. 

These formations grade laterally southward and eastward into the Potosi and Derby-Doerun Dolomites and 

the Davis Formation (a shaly, silty dolomite) in Missouri, 

Illinois, and Indiana. The unit thickens southward much 

as the Maquoketa confining unit does (fig. 6). It generally 

is about 100 to 200 ft thick in the northern, freshwater 

part of the aquifer system, but is less than 100 ft thick in 

most of Wisconsin.
A10 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

TABLE l. Hydrogeologic-unit nomenclature used in the study of the Cambrian-Ordovician aquifer system 

[Aquifer units are shaded. Gp, Group; Fm, Formation; Ss, Sandstone; Ls, Limestone; Dol, Dolomite. From Young, in press]

Time-stratigraphic unit

System

Quaternary

Cretaceous

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Precambrian

Series

Holocene

Pleistocene

Virgilian 

Missourian 

Desmoinesian 

Atokan 

Morrowan

Chesterian

Meramecian

Osagean

Kinderhookian

Upper

Middle

Lower

Cayugan

Niagaran

Alexandrian

Cincinnatian

Mohawkian

Chazyan

Canadian

St. Croixan

Rockstratigraphic 

unit *

Undifferentiated

Undifferentiated

Undifferentiated

Ste. Genevieve Ls. 

St. Louis Ls. 

Salem Ls. 

Warsaw Ls.

Keokuk Ls. 

Bur ngton Ls.

Und fferentiated

Undifferentiated

Und fferentiated

Und fferentiated

Und fferentiated

Maquoketa 

Shale

Galena Dol. 

Decorah Fm. 

Platteville Fm.

Glenwood Fm.

St. Peter Ss.

-g£ Shakopee 

<u^ Fm.

'(5'F Oneota 

olo Dol.

Jordan Ss.

St. Lawrence Fm.

Franconia Fm.

Ironton Ss.

Galesville Ss.

Eau Claire 

Formation

Mount Simon 

Sandstone

H ^ Hinckley Ss.

,g Fond du Lac Fm

o

c ci Solor Church 

ECS Fm.

Crystalline rocks

References

Southeastern 

Minnesota

Undifferentiated 

drift aquifer

Cretaceous aquifer (local)

Absent

Devonian ..... ..... .... ... ....... ... ^

c 

Absent ^a>

(D'5 

°CT

a5«

Q.

Maquoketa, Dubuque, "^ 

and Galena Fm.

Decorah-PlattevilleGlenwood confining bed

St. Peter aquifer

Basal St. Peter confining bed

Prairie du ChienJordan aquifer

St. LawrenceFranconia confining 

bed

Ironton-Galesville 

aquifer

Eau Claire 

confining bed

Mount SimonHinckley aquifer

? ?

Confining bed

Delin and Woodward (1984)

Wisconsin

Sand and gravel aquifer

Absent

0)

Absent §

o3 11

3 

V)

Maquoketa Shale

Galena-Platteville 

aquifer

Sandstone 

aquifer

Lake Superior 

sandstones and 

lava flows

Confining unit

Kammerer(1984)

Iowa

Surficial aquifer

Cretaceous aquifer

Confining beds

Absent ;

Mississippian 

aquifer

Confining bed

Absent Silurian-Devonian 

aquifer

Confining bed

Water bearing

Confining bed

Water bearing

Confining bed

Jordan 

aquifer

Cambrian-Ordovician 

aquifer system

Confining bed

Water bearing

Confining bed

Water bearing

aquifer Dresbach 

(little information)

Confining bed

Horick and Steinhilber 

(1973, 1978); 

Horick (1984)

General geologic nomenclature of the upper Mississippi Valley
HYDROGEOLOGY All

TABLE l. Hydrogeologic-unit nomenclature used in the study of the Cambrian-Ordovician aquifer system Continued 

[Aquifer units are shaded. Gp, Group; Fm, Formation; Ss, Sandstone; Ls, Limestone; Dol, Dolomite. From Young, in press]

Norther

Glacial drift 

aquifers

Absent

Shallow bedrock 

aquifers

Confining bed

Cambrian-Ordovician

aquifer

Confining bed

Elmhurst-Mount Simon 

aquifer

Absent

Confining bed

Schicht and others (1976); 

Sasman and others (1982)

n Illinois

Aquigroup

Prairie

Upper Bedrock

^

i

 i 

<

Q

f

Valley Bedrock

Mississippi

i j

5 >

D

Midwest

Bedrock Basal 

Aquifer or 

aquitard

Pleistocene

Absent

Pennsylvanian

Absent

St. Louis-Salem 

aquifer

Confining unit

Keokuk-Burlington aquifer

Confining unit

Devonian

Silurian Dolomite 

aquifer

Maquoketa 

confining unit

GalenaPlatteville unit

Ancell aquifer

~ Prairie du Chien 

3 Group

TO

C

"c 

g Eminence Fm. 

o Potosi Dol.

T3 

T3

Franconia Fm.

IrontonGalesville 

aquifer

Eau Claire 

confining unit

ElmhurstMount Simon 

aquifer

Absent

Confining unit

Visocky and others (1985)

Northern 

Missouri

Drift (minor aquifers 

in buried valleys)

Absent

(Poor aquifer)

Absent

(Poor aquifer)

Mississippian aquifer

(Confining bed)

(Poor aquifer)

(Confining bed)

Kimmswick Fm. (minor aquifer)

Decorah Fm. 

Plattin Fm. 

Joachim Dol. 

(confining bed)

St. Peter Ss. 

Everton Dol. 

(moderate aquifer)

Confining bed

Roubidoux Fm. 

Gasconade Dol.

Eminence Dol. 

Potosi Dol. 

(good aquifer)

Derby-Doerun Dol. (poor aquifer)

Davis Formation

T3 

CD 

.Q

TO

Upper confini

-Ordovician aquifer

Cambriar

LOWGT confining bed

Bonneterre Fm. 

(little information)

Lamotte Ss. 

(probable aquifer)

Absent (?)

Confining bed

lmes(1985)

Hydrogeologic unit *

Drift aquifer

Cretaceous aquifer

PennsylvanianMississippianDevonian confining unit

Silurian-Devonian 

aquifer

aquifer system

c

CD '(J

0 O

6

C 

CD

.Q

E

CD

O

Maquoketa 

confining unit

St. Peter-Prairie du 

Chien-Jordan aquifer

St. Lawrence-Franconia 

confining unit

Ironton-Galesville 

aquifer

Eau Claire 

confining unit

Mount Simon aquifer

Confining unit

Layer number 

in digital 

flow model *

Aquifer layer 

5

Confining layer 

4-5

Aquifer layer 

4

Confining layer 

3-4

Aquifer layer 

3

Confining layer 

2-3

Aquifer layer 

2

Confining layer 

1-2

Aquifer layer 

1

Impermeable 

boundary 

layer

Mandle and 

Kontis 

(in press)

Nomenclature of the Northern Midwest Regional Aquifer-System Analysis
A12 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

90°

40e

Scale 1:6,000,000

50 100 MILES

50 100 KILOMETERS

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 eoo LINE OF EQUAL THICKNESS OF THE MAQUOKETA CONFINING 

UNIT Dashed where inferred. Interval 200 feet

FIGURE 6. Thickness of the Maquoketa confining unit in the northern Midwest. (From Mandle and Kontis, in press.)

fronton -Galesville aquifer, This aquifer is the most 

productive unit of the Cambrian-Ordovician aquifer system in the east-central part of the study area, in northeastern Illinois and southeastern Wisconsin. It consists 

of moderately well sorted quartzose sandstones. 

Reported values of transmissivity for the IrontonGalesville aquifer range from 71 to 11,000 ft2/d, but most 

of the values range from the upper hundreds to about 

3,000 ft2/d an indication of the overall uniformity of the 

aquifer. The aquifer thickens southward from its eroded 

edge, reaching maximums of 150 to 200 ft in northwestern and east-central Illinois, but its thickness generally is
HYDROGEOLOGY A13

95'

40

Base enlarged from 

U.S.Geological Survey 

1:7,500.000, 1970

EXPLANATION

 500 LINE OF EQUAL THICKNESS OF THE ST. PETER-PRAIRIE DU CHIENJORDAN AQUIFER Dashed where inferred. Interval 250 feet

FIGURE 7. Thickness of the St. Peter-Prairie du ChiennJordan aquifer in the northern Midwest. (From Handle and

Kontis, in press.)

50 to 150 ft (fig. 8). The aquifer terminates farther 

southward as the sandstones grade into carbonate rocks 

of the Davis Formation in central Iowa, Missouri, southern Illinois, and north-central Indiana.

In eastern Wisconsin north of Milwaukee, the rock 

units that make up the Elk Mound Group of Ostrom 

(1967), the Ironton and Galesville Sandstone Members in 

Ostrom's Wonewoc Sandstone and the Eau Claire and
A14 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

95'

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 100 LINE OF EQUAL THICKNESS OF THE IRONTON-GALESVILLE 

AQUIFER Dashed where inferred. Interval 50 feet

FIGURE 8. Thickness of the Ironton-Galesville aquifer in the northern Midwest. (From Mandle and Kontis, in press.)

Mount Simon Sandstones generally cannot be differentiated in well cuttings and geophysical logs. Thus, the 

Ironton-Galesville is not identified there as a separate 

aquifer.

Eau Claire confining unit. This is an important 

confining unit throughout much of the study area. Its

effectiveness as a confining unit, which is directly 

dependent on the proportion of relatively impermeable 

shale, siltstone, and dolomite to sandstone, generally 

increases toward the south. The confining unit is not 

identifiable north of Milwaukee, where the members of 

the Elk Mound Group generally cannot be differentiated.
HYDROGEOLOGY A15

Dolomite content increases southward and westward, 

where the Eau Claire Formation is replaced laterally by 

the Bonneterre Formation in northwestern Missouri and 

western Iowa. In northern Illinois, the confining unit 

consists of the two uppermost members of the Eau Claire 

Formation, as the latter is defined by the Illinois State 

Geological Survey (Willman and others, 1975). From 50 

to 125 ft of fine-grained sediment usually is evident on 

borehole gamma logs and geologic logs of well cuttings 

from extreme northern Illinois, southern Wisconsin, and 

southeastern Minnesota. The overall thickness of the 

unit increases steadily from its northern outcrop area to 

about 200 to 300 ft in central Iowa and north-central 

Illinois and to more than 750 ft in the Illinois basin.

Mount Simon aquifer. This is the most extensive 

aquifer in the northern Midwest because of its basal 

position on the Precambrian surface. It consists mainly of 

the Mount Simon Sandstone in the north and its equivalent in northern Missouri, the Lamotte Sandstone. However, the underlying Precambrian Hinckley Sandstone is 

included in Minnesota, and the overlying Elmhurst Sandstone Member of the Eau Claire Formation as used in 

northern Illinois is also included (table 1). The aquifer is 

dominantly sandstone that is poorly to moderately well 

sorted. It contains variable amounts of fine-grained 

sediment, including zones of lenticular shale beds, which 

are prominent a few hundred feet below the top of the 

Mount Simon in southeastern Wisconsin, northeastern 

Illinois, and northwestern Indiana. A zone as much as 

300 to 500 ft thick contains abundant interbedded red 

shale and forms a local confining unit within the Mount 

Simon. The hydraulic conductivity of the aquifer is 

somewhat low, especially where deeply buried and confined; however, reported transmissivity values range 

from 270 to 9,400 ft2/d because of the great thickness of 

the aquifer.

The Mount Simon aquifer crops out or subcrops 

beneath glacial drift in large areas of northern Wisconsin 

and eastern Minnesota and generally dips smoothly to 

the south and east toward the structural basins. Its top 

is about 2,500 ft below sea level in the Forest City basin 

and along the northern perimeter of the Illinois basin in 

north-central Illinois, but its base in these areas is, 

respectively, about 2,700 and 4,500 ft below sea level. 

This difference in the depth to the base is due to the 

much greater depth of the Precambrian surface in the 

Illinois basin than in other areas. The top of the aquifer 

is more than 12,000 ft below sea level in the center of the 

Illinois and Michigan basins.

The great thickness of the aquifer (fig. 9) in Illinois 

(more than 2,500 ft just south of Chicago), western 

Indiana, and extreme southeastern Wisconsin is not, and 

generally cannot be, used for water supply for several 

reasons. First, adequate yields generally are obtained

from multiaquifer wells that penetrate only the overlying 

Cambrian and Ordovician rocks, or that penetrate the 

Mount Simon aquifer only a few hundred feet. Second, 

water in the lower part of the aquifer is saline near Lake 

Michigan, and water in the entire aquifer is progressively more saline toward the Illinois and Michigan 

basins, where brines are present. The aquifer also is not 

used extensively in Iowa for essentially the same reasons: In the western and southern parts of the State, it is 

deeply buried and contains saline water.

The nearly impermeable crystalline rocks of the Precambrian basement beneath the Cambrian-Ordovician 

aquifer system are a very effective confining unit, and 

their upper surface marks the practical lower limit of 

ground-water availability.

RECHARGE, GROUND-WATER MOVEMENT, AND 

DISCHARGE

The concept of multiple flow systems of different areal 

and subsurface extent (Toth, 1962; Freeze and Witherspoon, 1967) applies to ground-water flow in the northern 

Midwest. The flow systems, which are driven by the 

hydraulic head at the water table (fig. 10), range from 

local to intermediate to regional in scope. These conceptual flow systems are illustrated in figure 11 as generalized predevelopment flow paths along an east-west 

hydrogeologic section across southern Wisconsin. The 

water table is used for the top of the section.

The flow paths in figure 11 are highly diagrammatic 

and should not be construed as representative of exact 

conditions. They are based on the premise that the 

section is parallel to the direction of most ground-water 

flow. This generally is true in the eastern half of the 

section, but is not true in the western half. Thus, the flow 

paths shown in the west are idealized, especially in the 

deep intermediate and regional flow systems.

Development of the ground water in the northern 

Midwest generally has caused little change in the altitude 

of the water table and in flow directions in local systems. 

Thus, the average present-day water-table conditions 

are similar to predevelopment conditions. However, 

large withdrawals from confined zones of intermediate 

and regional flow systems have caused major head 

declines and subsequent changes in direction of flow. 

These changes are discussed later in the report.

Most local flow systems are less than a few miles in 

length from areas of recharge to areas of discharge (see 

fig. 11). Local systems represent most of the flow within 

the water-table aquifers, which in turn account for a 

large part of the overall volume of ground-water 

recharge and discharge in the northern Midwest. Most of 

this flow does not enter the bedrock aquifers, or penetrates only to shallow depths, and discharges from glacial
A16 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

95

40'

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

soo LINE OF EQUAL THICKNESS OF THE MOUNT SIMON AQUIFER 

Dashed where inferred. Interval, in feet, is variable

FIGURE 9. Thickness of the Mount Simon aquifer in the northern Midwest. (From Mandle and Kontis, in press.)

drift into surface-water bodies or is lost by evapotranspiration. Where the glacial drift is thin or absent, 

notably in the Driftless Area (fig. 2) of southwestern 

Wisconsin and adjacent uplands along the Mississippi 

River, local flow systems also are present in the uppermost bedrock. The depth to the water table commonly is

less than 50 ft in most of the study area, but the water 

table is 200 to 300 ft deep beneath the highest ridges in 

the Driftless Area. Shallower, perched ground water is 

present beneath the ridges where low-permeability 

rocks restrict downward percolation of water through 

the thick unsaturated zone.
HYDROGEOLOGY A17

90°

45e

Scale 1:6,000,000 

0 50 100 MILES

0 50 100 KILOMETERS

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 soo WATER-TABLE CONTOUR Shows altitude of the water table, 1980. 

Contour interval 200 feet. Datum is sea level

FIGURE 10. Generalized water table and major surface-water drainage network in the northern Midwest. (From

Young, in press.)

Ground-water movement in most of the rest of the 

bedrock aquifers occurs in intermediate or regional flow 

systems, where flow paths are deeper and traverse much 

longer distances from recharge areas to discharge areas 

and movement is slow. The general patterns of predevelopment regional ground-water flow within the 

Cambrian-Ordovician aquifer system are illustrated by 

the potentiometric surface map for the St. Peter-Prairie 

du Chien-Jordan aquifer (fig. 12). The map is based on 

water levels in the earliest wells, generally constructed
00

VERTICAL SCALE GREATLY EXAGGERATED

HYDROGEOLOGIC UNITS 

Drift aquifer

I I

25

I I I

50 MILES

15 50 KILOMETERS

EXPLANATION

Geological section modified from 

L.C. Trotta (U.S. Geological Survey, 

written commun., 1983)

Silurian-Devonian aquifer

Maquoketa confining unit Consists of Maquoketa 

Shale, Galena Dolomite, and the Decorah, 

Platteville, and Glenwood Formations where overlain by the Maquoketa Shale

Galena Dolomite and Decorah, Platteville, 

and Glenwood Formations Included 

in model water-table layer

St. Peter-Prairie du Chien-Jordan aquifer

St. Lawrence-Franconia confining unit 

Ironton-Galesville aquifer

Eau Claire confining unit

Mount Simon aquiferPrecambrian crystalline rocks (confining unit)

CONTACT Dashed where approximate

GENERALIZED GROUND-WATER FLOW PATH

+- Local flow system

 *- Intermediate flow system

 *- Regional flow system 

A-A' LINE OF SECTION SHOWN IN FIGURES

2It-1 I

do

Hi

1

COI

ts1w 

23

FIGURE 11. Generalized hydrogeologic section across southern Wisconsin illustrating typical predevelopment flow systems in the northern Midwest.
HYDROGEOLOGY A19

95

40'

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

 500 

EXPLANATION

POTENTIOMETRIC CONTOUR Shows approximate altitude of the 

predevolpment potentiometric surface in the late 1800's for the 

St. Peter-Prairie du Chien-Jordan aquifer. Hachures indicate area 

of lower head. Dashed where inferred. Contour interval, in feet, is 

variable. Datum is sea level

GENERALIZED DIRECTION OF GROUND-WATER FLOW

AQUIFER BOUNDARY

BOUNDARY OF MAQUOKETA SHALE

FIGURE 12. Approximate predevelopment potentiometric surface for the St. Peter-Prairie du Chien-Jordan aquifer in

the northern Midwest. (Modified from Young, in press.)
A20 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

in the late 1800's. The major areas of recharge to regional 

flow in the aquifer system were in the unconfined, 

interbasin highlands in northwestern Iowa; southeastern 

Minnesota; western, southern, and eastern Wisconsin; 

and northern Illinois. Although the flow rates were 

small, a significant volume of recharge also leaked downward from the water table through the Maquoketa 

confining unit where the water table was higher than the 

head in the Cambrian-Ordovician aquifer system.

Downward flow of ground water in recharge areas is 

impeded, in general, by zones of lower vertical hydraulic 

conductivity and, in particular, by confining units. 

Hydraulic head is lost with increasing depth along downward flow paths in recharge areas. This decline in head 

was observed in several packer tests during the study in 

areas of direct recharge from glacial drift to the 

Cambrian-Ordovician aquifer system. In upland areas of 

western Wisconsin and near the Mississippi River in 

southeastern Minnesota, the heads measured in the St. 

Peter-Prairie du Chien-Jordan aquifer were 200 to 245 ft 

higher than the heads in the deeper Ironton-Galesville 

aquifer. The head in the Mount Simon aquifer was 95 to 

135 ft lower than the head in the Galena-Platteville unit 

in wells tested in the recharge area west of Milwaukee. 

Head losses with depth decreased along the flow paths 

away from the recharge areas as the water-table altitude 

decreased. Ultimately, the head gradient reversed at 

some distance from the recharge area where the water 

table was lower than the head beneath the Maquoketa 

confining unit and flow was upward from the CambrianOrdovician aquifer system.

Ground water in most of the Cambrian-Ordovician 

aquifer system is confined and, prior to development, 

moved laterally away from the recharge areas, either 

toward regional discharge areas, the major river valleys 

and Lake Michigan, or downdip toward the structural 

basins. Lake Michigan, however, is completely underlain 

by Silurian and younger rocks; thus, upward flow from 

the aquifer system was controlled by the Maquoketa 

confining unit. The lake did not receive direct discharge 

from the Cambrian-Ordovician aquifer system but, 

rather, from the Silurian-Devonian aquifer, only a small 

part of which was derived from upward leakage across 

the Maquoketa confining unit. The longest flow paths in 

the Cambrian-Ordovician aquifer system originated in 

the recharge areas of northwestern Iowa and extended 

southeastward as much as 400 mi (miles) toward the 

Illinois basin or to the Mississippi and Missouri River 

valleys near their confluence (fig. 12).

Other major patterns of confined flow in the CambrianOrdovician aquifer system consisted of the dominantly 

horizontal flow from eastern Wisconsin eastward to the 

Michigan basin (see fig. 3) and from northeastern Illinois 

southward to the Illinois basin. The composite predevelopment head in the aquifer system at Chicago and 

Milwaukee was estimated to be 130 and 186 ft above 

Lake Michigan (580 ft above sea level) by Suter and 

others (1959) and Weidman and Schultz (1915), respectively. Packer tests during this study showed only slight 

head differences between aquifers in the confined parts 

of the aquifer system in eastern Wisconsin, northeastern 

Illinois, and northwestern Indiana. Although these 

heads are probably equalized in part because of large 

regional drawdown from heavy pumping of wells in the 

Cambrian-Ordovician aquifers, simulation with the 

regional flow model also indicates little difference in 

predevelopment heads. Water-level measurements from 

packer tests and deep piezometers installed in the 3,475- 

ft-deep test well at Zion, 111., show that the present head 

is about 50 ft higher beneath a 500-ft-thick zone of 

interbedded shales that begins about 100 ft below the top 

of the Mount Simon (Nicholas and others, 1987). This 

higher head within the Mount Simon probably is a 

remnant of the predevelopment artesian conditions that 

prevailed within the entire Cambrian-Ordovician aquifer 

system near the western shore of Lake Michigan.

Regional predevelopment ground-water discharge 

from the Cambrian-Ordovician aquifer system occurred 

mainly as slow, upward leakage from the confined aquifers along flow paths toward the structural basins. Very 

saline water around the basins and brines within the 

basins restricted regional flow of ground water into the 

basins, thereby forcing the confined water upward 

through overlying rocks. Head in the confined aquifers 

gradually dissipates because of this upward leakage and 

because of frictional losses along the flow paths into the 

basins and upward through confining units. Frictional 

losses eventually may decrease the head until no appreciable upward flow occurs from deep within the basins 

except through fractures (Bond, 1972). Ground water in 

intermediate flow systems discharged upward from the 

Cambrian-Ordovician aquifers to the major river valleys, 

such as the Mississippi River valley in the Twin Cities 

artesian basin.

GEOCHEMISTRY OF WATER

Ground water in the Cambrian-Ordovician aquifer 

system in the northern Midwest is characterized by a 

large range of mineralization and chemical types, but its 

quality is good and suitable for most uses in most of the 

study area. Dissolved-solids concentrations generally are 

lowest in the recharge areas in the northern and central 

parts of the area and are highest in the structural basins 

to the south and east. The water generally is hard.

Investigation of the evolution of the chemical character 

of ground water in the Cambrian-Ordovician aquifer
GEOCHEMISTRY OF WATER A21

system is complicated because of numerous geochemical 

processes, which, through geologic time, have produced 

the present water quality. Ground water in the deepest 

parts of the aquifer system has moved long distances 

from the recharge areas, and its quality is a product of its 

very long time of contact with the rocks, and of the 

character of the sediment and minerals that form the 

rocks, in addition to other processes that are described 

later. Interpretation of the present chemical character of 

the ground water in terms of hydrochemical facies and 

other water-quality characteristics indicates some residual effects of the large-scale continental glaciations during the Pleistocene on ground-water quality in the northern Midwest. For example, there is evidence from stable 

isotope ratios that ground water in much of the deeper 

part of the Cambrian-Ordovician aquifer system represents recharge of meltwater from Pleistocene glaciers.

Detailed evaluation of water quality in the CambrianOrdovician aquifer system is limited by the availability of 

analyses that represent specific aquifers, because many 

wells are finished in more than one aquifer. Water 

quality generally is similar in all units of the aquifer 

system in the large areas of mainly unconfined conditions 

in Minnesota, Wisconsin, northeastern Iowa, and northcentral Illinois; therefore, this study also has included 

analyses of ground water from multiaquifer wells in 

those areas. In the confined areas in Iowa, Missouri, 

Illinois, and eastern Wisconsin, water-quality data for a 

single aquifer are available mainly for the St. PeterPrairie du ChienJordan aquifer; thus, most of the following discussion and presentation of area: distribution 

of water-quality properties in those areas pertains to this 

aquifer.

GENERAL CHEMICAL CHARACTERISTICS RELATED TO 

THE HYDROLOGIC SYSTEM

Ground water in the Cambrian-Ordovician aquifer 

system contains all the major ions that commonly dominate the water quality of natural waters: calcium, magnesium, and sodium cations and bicarbonate, sulfate, and 

chloride anions. The distribution of sulfate (fig. 13) is 

representative of the general pattern of distribution of 

the other major ions and dissolved solids in the aquifer 

system. The areal distribution and predominance of 

these ions were used (Siegel, 1989) to classify the water 

in the aquifer system into hydrochemical facies according 

to the method of Back (1966).

HYDROCHEMICAL FACIES

Most ground water, especially in the unconfined areas, 

is a calcium magnesium bicarbonate type (fig. 14), 

derived mainly from dissolution of carbonate minerals in

calcareous glacial drift by carbon dioxide-enriched 

recharge water. As previously stated, water quality 

tends to be similar in individual units of the aquifer 

system in the unconfined areas and, logically, is very 

similar to that in overlying water-table aquifers. Where 

the Cretaceous aquifer in northwestern Iowa and southwestern Minnesota directly overlies and recharges the 

Cambrian-Ordovician aquifer system, the water in both 

is a calcium sodium sulfate bicarbonate type (fig. 14); the 

sulfate is derived from oxidation of pyrite in shales of the 

Cretaceous Dakota Formation.

The chemical character of ground water in the confined 

parts of the aquifer system is much more variable. In 

general, the distribution of sulfate, chloride, and sodium 

concentrations closely resembles the distribution of dissolved solids. The areal distribution of water types in the 

aquifer system where it is overlain by the Maquoketa 

confining unit is related mainly to distance from recharge 

areas and to depth within the regional flow system. The 

most pronounced differences are along confined flow 

paths to the south and southeast from the recharge area 

of northwestern Iowa. As shown by the map of composite 

hydrochemical facies for the Mount Simon and St. PeterPrairie du ChienJordan aquifers (fig. 15), water type 

changes from a calcium sodium sulfate bicarbonate type 

in the recharge area of northwestern Iowa to sodium 

calcium sulfate bicarbonate and sodium-mixed anion (no 

dominant anion) types to a sodium chloride brine in the 

Illinois and Forest City basins (fig. 3).

Similarly, the calcium magnesium bicarbonate-type 

water from the recharge area of eastern Wisconsin 

changes to the east toward the Michigan basin. It 

changes first to a calcium sodium sulfate chloride type 

and then to a sodium sulfate chloride type, in transition 

to calcium sulfate and calcium chloride brines in the 

Michigan basin. From northern Illinois, the calcium 

magnesium bicarbonate-type water grades into a mixed 

cation (no dominant cation) bicarbonate chloride type 

before becoming a sodium chloride brine in the Illinois 

basin. These changes may result from membrane filtration, which would cause retention of calcium and magnesium ions as sodium and chloride ions are forced upward 

across shale layers (Graf, 1982).

DISSOLVED SOLIDS

The dissolved-solids content of ground water in the 

aquifer system generally increases in the direction of 

ground-water movement generally in the direction of 

dip of the rocks and with depth and increased thickness 

of the aquifers, especially where they are confined by the 

Maquoketa confining unit. The increase in dissolved 

solids is due mainly to the residence time in a flow path. 

Thus, a long flow path provides more time for the water
A22 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

ndisbH \T\ /Milwaukee

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 soo LINE OF EQUAL SULFATE CONCENTRATION Dashed where 

approximate or inferred. Interval, in milligrams per liter, is 

variable

AQUIFER BOUNDARY

.1850 WELL YIELDING WATER WITH SULFATE CONCENTRATION THAT 

EXCEEDS THE CONCENTRATIONS ON WHICH THE LINES ARE 

BASED Number is sulfate concentration, in milligrams per liter

FIGURE 13. Sulfate distribution in the St. Peter-Prairie du Chien-Jordan aquifer in the northern Midwest. (Modified

from Young, in press.)
GEOCHEMISTRY OF WATER A23

95

Minneapolis - ( 

"St. Paul* '

0 Madison , Milwaukee

\

40

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

- FACIES-ZONE BOUNDARY Dashed where approximate

FIGURE 14. Hydrochemical facies of ground water in the Cretaceous and drift aquifers in the northern Midwest. (From

Siegel, 1989.)

to interact with rock minerals. However, some very high 

concentrations of dissolved solids may be a result of 

selective ion concentration by shale-membrane filtration 

or ion exchange. Transition to higher dissolved solids in 

the confined areas commonly is accompanied by substantial increases in sulfate concentration and the presence of 

calcium sodium sulfate-type water.

The range in concentration of dissolved solids in the 

Cambrian-Ordovician aquifer system is as extreme as 

that found in any regional aquifer system. Dissolved
A24 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

95

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

- ? FACIES-ZONE BOUNDARY Dashed where approximate. Queried 

where doubtful

GENERALIZED DIRECTION OF PREDEVELOPMENT GROUND-WATER 

FLOW

BOUNDARY OF MOUNT SIMON AQUIFER

FIGURE 15. Hydrochemical facies of ground water in the Mount Simon and St. Peter-Prairie du ChiennJordan aquifers

in the northern Midwest. (Modified from Siegel, 1989.)
GEOCHEMISTRY OF WATER A25

solids range from less than 400 mg/L in recharge areas in 

Wisconsin, southeastern Minnesota, northeastern Iowa, 

and northern Illinois, where the aquifer system crops out 

or subcrops beneath glacial drift, to more than 200,000 

mg/L in the Mount Simon aquifer in the Illinois basin 

(figs. 16, 17), where the aquifer system is covered with 

several thousand feet of younger rocks. The 500-mg/L 

line of equal dissolved-solids concentration roughly outlines the areas of local, unconfined flow systems in the 

northern half of the study area. In Illinois and Wisconsin, 

the line also is approximately parallel to the outcrop 

pattern of the Maquoketa Shale the major confining 

unit between the Cambrian-Ordovician aquifer system 

and the overlying units. The water in the unconfined 

aquifer system in those areas is derived from and is 

identical in type (calcium magnesium bicarbonate) to that 

in the overlying glacial drift.

The dissolved-solids concentration in the confined 

Cambrian-Ordovician aquifer system in Iowa generally 

increases in the direction of the dip of the rocks (perpendicular to the direction of ground-water flow (fig. 12)), 

toward the Forest City basin and northern Missouri 

(figs. 16, 17). In addition, in central Iowa, the 500- and 

1,000-mg/L lines define a "plume" of water, fresher than 

that in the surrounding area, oriented north-south across 

the Maquoketa Shale outcrop area.

Figures 16 and 17 show several occurrences of 

dissolved-solids concentration higher than the concentrations generally observed in the vicinity. The high concentrations in the Mount Simon aquifer along the Mississippi River (fig. 16) are in water from deep wells and are 

indicative of the very slow rate of ground-water movement beneath this regional discharge area. In northeastern Illinois, the very high values are from deep parts of 

the Mount Simon that are not used for water supply. The 

reason for the presence of anomalously high dissolvedsolids concentrations in the St. Peter-Prairie du ChienJordan aquifer in eastern Wisconsin (fig. 17) is not clear. 

One possible explanation is that the rate of upward 

leakage from the Mount Simon is higher at those locations because the Eau Claire confining unit is absent.

TRACE ELEMENTS AND MINOR CONSTITUENTS

A reconnaissance survey of the distribution of trace 

elements in ground water shows that concentrations tend 

to be higher in confined parts of the aquifer system a 

trend generally similar to that for dissolved solids. 

Concentrations of cadmium, arsenic, and selenium seem 

to be directly related to the proximity of the Dakota 

Formation, which overlaps the western edge of the 

aquifer system in northwestern Iowa and southwestern 

Minnesota. Iron, manganese, cobalt, cadmium, copper, 

and lead concentrations are higher near the Dakota

Formation in northwestern Iowa and on the edge of the 

Forest City basin in southwestern Iowa. Concentrations 

of beryllium and molybdenum generally are higher near 

the Forest City basin than elsewhere in the aquifer 

system. Silica concentrations seem to be directly related 

to ground-water temperature in south-central Iowa and 

northern Missouri; however, the solid phases that control 

the solubility of silica are unknown.

Concentrations of boron (fig. 18), fluoride, lithium, and 

strontium increase from northeast to southwest in Iowa 

into northern Missouri, perpendicular to the direction of 

ground-water flow (see fig. 12). The distribution patterns 

of lithium, radium-226, and bromide show areas of lower 

concentrations similar to those of dissolved solids and the 

stable isotopes of water, which are discussed in the next 

section. Uranium isotopes in the ground water are at 

disequilibrium and show unusually high ratios of the 

activity of uranium-234 to that of uranium-238, generally 

ranging from about 3 to 30 in Iowa and northern Illinois 

(Gilkeson and others, 1984; Siegel, 1989). The two largest 

ratios known are 40.7 in water from the St. Peter 

Sandstone in southwestern La Salle County, 111., and 100 

in water from a deep packer test in the Mount Simon 

Sandstone in eastern Jackson County, Iowa.

High barium concentrations in northeastern Illinois 

and eastern Wisconsin are present in a zone in the 

confined aquifer system where reducing conditions prevail and markedly lower the sulfate concentration. Barium concentrations increase until the solubility limit of 

barite (BaS04), a relatively insoluble mineral, is reached. 

Sulfate concentrations increase progressively eastward 

from this zone (fig. 13) (discussed in the next section).

High levels of dissolved radium are a special groundwater-quality problem in the Cambrian-Ordovician aquifer system, primarily in the confined parts of the system 

(Scott and Barker, 1961; Gilkeson and others, 1983; 

Hahn, 1984). These levels may be related to the unusually high disequilibrium between uranium-234 and 

uranium 238 in water in sandstones in the aquifer system, possibly because of enrichment of uranium-234 

carried in Pleistocene meltwater (discussed in the next 

section).

EFFECTS OF PLEISTOCENE GLACIATION ON GROUNDWATER QUALITY

The distribution patterns of dissolved solids and other 

chemical constituents in the Cambrian-Ordovician aquifer system can be interpreted as residual effects of 

large-scale continental glaciations during the Pleistocene. Analysis of the isotopic composition of the ground 

water strongly indicates that the source of much of the 

ground water in the confined part of the aquifer system 

was recharge from glacial meltwater. That water is
A26 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

95'

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 soo LINE OF EQUAL DISSOLVED-SOLIDS CONCENTRATION Dashed 

where approximate or inferred. Interval, in milligrams per liter, 

is variable

 AQUIFER BOUNDARY

,3040 WELL YIELDING WATER WITH DISSOLVED-SOLIDS

CONCENTRATION THAT EXCEEDS THE CONCENTRATIONS ON 

WHICH THE LINES ARE BASED Number is dissolved-solids 

concentration, in milligrams per liter. (L) indicates water sample 

from lower part of the aquifer

FIGURE 16. Dissolved-solids distribution in the Mount Simon aquifer in the northern Midwest. (From Young, in press.)
GEOCHEMISTRY OF WATER A27

95

^ 5000 {M21ANA

40

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 500 LINE OF EQUAL DISSOLVED-SOLIDS CONCENTRATION Dashed 

where approximate or inferred. Interval, in milligrams per liter, 

is variable

AQUIFER BOUNDARY

^2520 WELL YIELDING WATER WITH DISSOLVED-SOLIDS

CONCENTRATION THAT EXCEEDS THE CONCENTRATIONS ON 

WHICH THE LINES ARE BASED Number is dissolved-solids 

concentration, in milligrams per liter

FIGURE 17. Dissolved-solids distribution in the St. Peter-Prairie du Chien-Jordan aquifer in the northern Midwest. 

(Distribution in northeastern Missouri and southern Illinois adapted from Imes (1985) and Meents and others (1952), 

respectively.) (From Young, in press.)
A28 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

40

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

 200 LINE OF EQUAL CONCENTRATION OF BORON Interval, in 

micrograms per liter, is variable

BOUNDARY OF MOUNT SIMON AQUIFER

FIGURE 18. Distribution of boron in the Mount Simon and St. Peter-Prairie du ChienJordan aquifers in the northern

Midwest. (From Siegel, 1989.)

isotopically depleted in 8180 (fig. 19) and 8D with respect 

to modern precipitation an indication that the source of 

the water was precipitation in a climate much colder than 

the present. This is illustrated in the previously mentioned area where ground water in the St. Peter-Prairie

du Chien^Jordan aquifer in central Iowa contains low 

concentrations of dissolved solids. The dilute water contains dissolved-solids concentrations of 500 mg/L or less, 

but the adjacent water contains more than 1,000 mg/L 

(fig. 17). The depletion of 8180 and 8D and concentrations
GEOCHEMISTRY OF WATER A29

40

\ T 1 y \ . Madison

KANSAS

Scale 1:6,000,000 

0 50 100 MILES

50 100 KILOMETERS

Base enlarged from 

U.S.Geological Survey 

1:7,500.000. 1970

EXPLANATION

 -9 LINE OF EQUAL 818O RATIO Dashed where approximate. Interval, 

in parts per mil, is variable

BOUNDARY OF CAMBRIAN-ORDOVICIAN AQUIFER SYSTEM

FIGURE 19. Distribution of 8180 ratio in the Cambrian-Ordovician aquifer system in the northern Midwest. (From

Siegel, 1989.)

of lithium, radium-226, and bromide also correlate with 

the pattern of dissolved solids (Siegel, 1989). The northsouth orientation of the area of dilute water suggests a

flow system with recharge areas in south-central Minnesota and discharge areas in Missouri, probably because 

Pleistocene subglacial meltwater directly entered the
A30 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

aquifer in its outcrop area in southern Minnesota under 

very high hydrostatic head from the overlying load of 

glacial ice.

Gilkeson and others (1983) have postulated that subglacial recharge also accounts for the high disequilibrium 

between uranium-234 and uranium-238 in the confined 

aquifer system in northern Illinois. A relatively recent 

(in geologic terms) source of uranium-234 would be 

necessary to account for its lack of equilibration with 

uranium-238. Fine-grained sediment derived from abrasion and crushing of igneous and metamorphic rocks by 

the glaciers could have been a regional source of 

uranium-234 influx into the Cambrian-Ordovician aquifer 

system.

Evidence of isostatic loading by Pleistocene ice sheets 

may be the isotopic composition of sulfate in saline water 

in the aquifer system near Lake Michigan in eastern 

Wisconsin and northeastern Illinois. The first eastward 

increase in dissolved solids is caused primarily by 

increases in the concentration of calcium and sulfate. The 

834S values for sulfur in the sulfate are positive (Gilkeson 

and others, 1984; Siegel and Begor, 1989), indicating that 

the sulfate is evaporitic in origin. Evaporite minerals are 

abundant in various younger formations in the Michigan 

basin but are rare in the Cambrian and Ordovician 

formations west of Lake Michigan. It is believed that 

isostatic loading from glacial ice over the Michigan basin 

created a westward hydraulic gradient, opposite from 

the eastward gradient that prevailed prior to development and that would have prevailed prior to glaciation. 

This flow reversal caused saline water in the Michigan 

basin to discharge westward through the present 

recharge areas.

Apparent ages of water in the Cambrian-Ordovician 

aquifer system in southeastern Wisconsin, derived from 

carbon-14 dating, gradually increase along flow paths 

eastward from the recharge area, indicating the effects 

of postglacial recharge and ground-water movement. 

Dates in and near the present recharge area are about 

6,000 yr B.P. and near Lake Michigan are at or near the 

maximum limit of the method, about 40,000 yr B.P. (D.I. 

Siegel, Syracuse University, written commun., 1988). A 

similar increase in age along a flow path southeastward 

from the recharge area northwest of the Twin Cities 

artesian basin was reported by Alexander and others 

(1983).

GROUND-WATER DEVELOPMENT AND ITS 

EFFECTS

GROUND-WATER DEVELOPMENT

An abundant ground-water resource has been an 

important part of the economic development of the

northern Midwest for more than a century. The need for 

a reliable and abundant water supply was met by drilling 

deep wells, mainly in the Cambrian-Ordovician aquifer 

system. Deep wells may have been drilled prior to 1860, 

but the earliest documented deep well was drilled in 

1864, to a depth of 711 ft in the Galena-Platteville unit at 

Chicago (Shufeldt, 1865). The head in this well was 80 ft 

above land surface, or about 111 ft above Lake Michigan, 

and the well flowed at a rate of about 400 gal/min (gallons 

per minute).

Drilling of deep wells had proliferated throughout the 

northern Midwest by the 1880's and 1890's, especially 

where artesian flow was obtainable. Flowing wells were 

common in several topographically low areas in the 

Mississippi, Missouri, and Illinois River valleys, near 

Lake Michigan, and around Lake Winnebago in northeastern Wisconsin. Original heads of more than 100 ft 

above land surface were recorded at Sheboygan, Milwaukee, and Kenosha, Wis., and at Dubuque, Iowa. The 

supply from these wells initially was considered inexhaustible, and many wells were allowed to flow unregulated as "fountains" in gardens and parks. Overdraft and 

interference between closely spaced wells caused a 

decline in heads in deep wells, and many no longer flowed 

by the early 1900's. Inadequate depths of casing and 

corrosion of iron casing in wells commonly allowed discharge into upper aquifers where the head was lower, 

adding to the head decline.

Ground-water withdrawals from the CambrianOrdovician aquifer system have increased more or less 

steadily during the past century of development in direct 

relation to population growth, especially in population 

centers, and to economic and industrial development. 

Total withdrawal from the aquifer system in 1980 was 

about 680 Mgal/d. The largest concentrations of pumpage 

in 1979-80 were in the Chicago and Twin Cities metropolitan areas about 180 Mgal/d in each area (fig. 20). 

Elsewhere, a comparison of pumpage by county (fig. 21) 

shows that withdrawals of 10 Mgal/d were equaled or 

exceeded in only a few counties: Dane (43), Waukesha 

(18), Jefferson (12), and Fond du Lac (11) Counties, Wis., 

and Winnebago (22) and La Salle (14) Counties, 111. 

Withdrawals from individual aquifers in some areas are 

difficult to estimate because many wells are open to and 

yield water from more than one aquifer.

In most of the northern part of the study area, the 

Cambrian-Ordovician aquifer system generally is unconfined and consists of fairly permeable sandstones that are 

not deeply buried. In this area, moderate to large water 

supplies are obtained from the lower parts of the aquifer 

system, except where it thins along its northern eroded 

edge. The St. Peter-Prairie du Chien^Jordan aquifer is 

not a primary source of ground water in most of this 

area, because it is absent owing to erosion or is unsatur-
GROUND-WATER DEVELOPMENT AND ITS EFFECTS A31

£\>\J

Q

DC

LU 

Q_

0150

_i

(D

Z

0

_l _l

WITHDRAWAL, AVERAGE INMl

otn 

3 O

i i

8-COUNTY 

CHICAGO AREA 

(see fig. 21) 

Data from files of

I I I

_ Illinois State Water 

Survey, compiled for

Northern Midwest 

Regional AquiferSystem Analysis

-

I 

-

-

zuu

>

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D_

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WITHDRAWAL, . 

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(see fig. 21)

Data from Horn (1983)

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-

3OOOOOOOOOC ooior-cNro^Lnusi^o;

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REGIONAL MODEL STRESS PERIOD

FIGURE 20. Ground-water withdrawals from the Cambrian-Ordovician aquifer system in the Chicago and Twin Cities areas, 1864-1980. (From

Young, in press.)

ated owing to its high topographic position. A major 

exception is the Twin Cities basin in southeastern Minnesota, where the aquifer is highly productive and provides almost 80 percent of the ground water pumped in 

the metropolitan area. The local Mount Simon-Hinckley 

aquifer (table 1) contributes about 11 percent of the 

pumpage from the Cambrian-Ordovician aquifer system 

in the Twin Cities area.

The confined St. Peter-Prairie du Chien-Jordan aquifer also is a major and very dependable ground-water 

source in Iowa, where it is termed the "Jordan aquifer" 

(table 1), and is the only part of the aquifer system 

commonly used there for large supplies. The aquifer is 

very productive, and wells generally need not be drilled 

into underlying aquifers. In addition, the lower aquifers 

contain increasingly saline water downdip to the south 

and southwest that is not suitable for most uses.

All the aquifers in the system are productive in most of 

the confined area of eastern Wisconsin and northeastern 

Illinois, and most deep wells in these areas are uncased in 

all rocks that are penetrated below the Maquoketa Shale. 

The Ironton-Galesville aquifer is the most productive, 

especially in the Chicago area, where, estimates by 

Walton and Csallany (1962) show, the aquifer may contribute more than 30 percent of the water to a pumped 

well open to all the Cambrian and Ordovician rocks. In

general, the basal Mount Simon aquifer is not penetrated 

deeply, for the reasons given earlier. The maximum 

thickness of Mount Simon penetrated by water wells 

probably is at Waukesha, Wis. (just west of Milwaukee), 

where the aquifer's full thickness of about 1,000 to 1,300 

ft is open to several wells. As previously discussed, the 

Eau Claire Formation north of Milwaukee is not a 

confining unit, as it is to the south; thus, the entire Elk 

Mound Group as used in Wisconsin (Ostrom, 1967) produces water to wells.

In the remainder of northern Illinois, especially in the 

west, local equivalents of the St. Peter-Prairie du ChienJordan aquifer are the main sources of ground water. 

The Ancell aquifer and the Galena-Platteville unit (table 

1) are at or near the surface in those areas and are 

confined by the Maquoketa Shale in only a few places. 

Because very large well yields usually are not needed, 

these shallow bedrock aquifers generally provide an 

adequate supply of ground water.

Few wells in the Chicago area are open to the Mount 

Simon aquifer because it contains mostly saline water. 

Salinity increases with depth in the aquifer and toward 

the Illinois and Michigan basins, where brines are 

present. The depth to the Mount Simon and the salinity 

of its water have made the aquifer more suitable for uses 

other than water supply. The Mount Simon is used for
A32 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

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| I ^HCAHOE

JERSEY 5 I ERJLj FAY- I EfFING-l. «.__ ic RAW\ 

, _i_ L, ETTE I H.M JASPER |FOR» l

Base modified from U.S. Geological Survey 

National Atlas 1:7,500,000

Sources of data: Illinois, Kirk and others (1982); 

lowa, R. C. Buchmiller (U.S. Geological Survey, 

written commun., 1986); Minnesota, Horn (1983); 

Missouri, Imes (1985); Wisconsin, U.S. Geological 

Survey, Wisconsin State Water-Use Data System

EXPLANATION

2 8 AVERAGE PUMPAGE, IN MILLION GALLONS PER DAY Number is 

average total pumpage from the Cambrian-Ordovician aquifer 

system in those counties with 1 million gallons per day, or more

METROPOLITAN AREAS IN FIGURE 20

FIGURE 21. Major areas of ground-water withdrawal from the Cambrian-Ordovician aquifer system in the northern Midwest, 1979-1980.

(From Young, in press.)
GROUND-WATER DEVELOPMENT AND ITS EFFECTS A33

underground storage of natural gas within and on the 

perimeter of the Illinois basin, and for injection of 

industrial liquid wastes in a few wells, primarily in 

northwestern Indiana (Ackermann and others, 1974; 

Ohio River Valley Water Sanitation Commission, 1976). 

Withdrawal from the aquifer system in northern Missouri is limited to a small area in the southeast near the 

Missouri River (fig. 21), where the water is not highly 

mineralized (fig. 17). The most productive part of the 

Cambrian-Ordovician aquifer system in this area is the 

sequence consisting of the Roubidoux Formation and the 

Gasconade, Eminence, and Potosi Dolomites (table 1), 

which are hydraulically equivalent to the regional St. 

Peter-Prairie du Chien Jordan aquifer.

EFFECTS ON THE AQUIFER SYSTEM

Large-scale pumping of ground water from the 

Cambrian-Ordovician aquifer system has produced large 

head declines and has altered the direction of groundwater flow in parts of the study area. Changes in water 

quality also may have occurred.

The 1980 potentiometric surfaces for aquifers of the 

Cambrian-Ordovician aquifer system (illustrated here as 

the potentiometric surface for the St. Peter-Prairie du 

ChienJordan aquifer, fig. 22), compared with predevelopment conditions (fig. 12), show little effect from 

ground-water withdrawal in most of the recharge or 

unconfined areas; however, major areas of decline are 

evident in the confined areas. The largest total declines 

in head and the largest areal extent of the declines are at 

Chicago, 111., Milwaukee and Green Bay, Wis., the Quad 

Cities of Iowa and Illinois, and Mason City, Iowa, where 

the aquifer system is confined by the Maquoketa confining unit. The head in the composite aquifer system 

declined more than 900 ft locally (not depicted in fig. 23) 

in the deepest cones of depression in the Chicago area 

from 1864 to 1980 and more than 375 ft in the cone of 

depression at Milwaukee from about 1880 to 1980 (fig. 

23). The decline has been 200 ft or more at Mason City 

and the Quad Cities.

More than 440 ft of head decline occurred at Green Bay 

between 1886 and 1957, when the city discontinued use of 

its deep wells and began using water from Lake Michigan. Withdrawal from the Cambrian-Ordovician aquifer 

system in the Green Bay area decreased from 13.1 to 5.3 

Mgal/d; as a result, the head recovered almost 300 ft by 

1960. Continued pumping nearby has decreased head in 

the area and has shifted the cone of depression a few 

miles to the south. The bottom of the cone of depression 

was only 100 ft higher in 1980 than in 1957.

The head decline caused by withdrawals from wellconfined aquifers is larger than that from unconfined or 

partly confined aquifers. This is illustrated by comparing

conditions in the Chicago area with those in the Twin 

Cities area, and conditions in the Mason City, Iowa, area 

with those in the Madison, Wis., area. The historical 

rates and trends of pumping from the CambrianOrdovician aquifers in the Chicago and Twin Cities areas 

are similar (fig. 20). However, the decline of the composite head below the Maquoketa Shale at Chicago was as 

much as 900 ft from 1864 to 1980 (Visocky, 1982), but in 

the Twin Cities area the decline in the Prairie du 

ChienJordan aquifer, which is incised by deep bedrock 

valleys beneath the Mississippi and Minnesota Rivers, 

was only about 90 ft from 1890 to 1980 (Schoenberg, 

1984). About 200 ft of decline occurred in the underlying 

Mount Simon-Hinckley aquifer in the Twin Cities area, 

where the aquifer is confined by the Eau Claire Formation. Similarly, pumpage from the confined Jordan aquifer at Mason City was about 6 Mgal/d in 1980, and total 

drawdown was 200 ft (M.R. Burkart and R.C. Buchmiller, USGS, written commun., 1987). However, pumpage from unconfined Cambrian sandstones at Madison 

was about 40 Mgal/d, and total drawdown was only about 

60 ft. The cone of depression at Madison induces 

recharge through deeply incised bedrock valleys and 

affects only a local area around the city (McLeod, 1975a), 

whereas the deeper cone at Mason City is part of a large 

area, which covers two-thirds of Iowa, where head has 

declined more than 50 ft (Horick and Steinhilber, 1978).

The large head declines in northeastern Illinois, eastern Wisconsin, and eastern Iowa are very apparent 

because of the increased cost to drill deeper wells and to 

install and operate pumps at deeper levels. The accompanying change in direction of ground-water flow in and 

near the pumping centers, however, is less apparent. 

Where the head in confined parts of the CambrianOrdovician aquifer system has declined to a point below 

the water table, the original upward flow across the 

Maquoketa confining unit is now reversed. This has 

induced downward flow to the aquifer system. This 

transition occurred roughly in the 1920's in the Chicago 

and Milwaukee areas. These changes in flow direction 

may have affected water quality by inducing water 

having a different composition into the aquifer system 

within the cones of depression.

The very large head declines in the CambrianOrdovician aquifer system in the Chicago-Milwaukee 

area (fig. 23) are the most impressive effects of groundwater withdrawal in the northern Midwest. In addition 

to the 180 Mgal/d pumped from the aquifer system in an 

eight-county Chicago area in 1980, about 33 Mgal/d was 

pumped in southeastern Wisconsin, mainly in Waukesha 

and bordering municipalities west of Milwaukee. The 

development and resulting head decline in the ChicagoMilwaukee area have been described by several investi-
A34 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

Chicago 7

s£~-

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

-500 

EXPLANATION

POTENTIOMETRIC CONTOUR Shows approximate altitude of the 

1980 potentiometric surface for the St. Peter-Prairie du ChienJordan aquifer. Dashed where inferred. Hachures indicate area of 

lower head. Contour interval, in feet, is variable. Datum is sea 

level

AQUIFER BOUNDARY

FIGURE 22. Generalized potentiometric surface for the St. Peter-Prairie du Chien-Jordan aquifer in the northern

Midwest, 1980. (From Young, in press.)
GROUND-WATER DEVELOPMENT AND ITS EFFECTS A35

41° i _________i 

50 KILOMETERS

Base modified from U.S. Geological Survey 

1:2,000,000, 1970

EXPLANATION

.200 LINE OF EQUAL HEAD DECLINE, 1864-1980 Dashed where 

approximate. Interval, in feet, is variable

MAJOR POTENTIOMETRIC DIVIDE

FIGURE 23. Decline in head in the composite Cambrian-Ordovician aquifer system in the 

Chicago-Milwaukee area, 1864-1980. (Modified from H.L. Young and A.J. MacKenzie, 

U.S. Geological Survey, written commun., 1988.)
A36 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

gators, but, in general, each study focused on only the 

part of the aquifer system within each State.

The deepest parts of the regional cone of depression in 

the Chicago area are in northeastern Du Page County 

and the adjacent parts of western and northern Cook 

County, and at Joliet in northwestern Will County. 

Nonpumping water levels in 1980 in some wells at Joliet 

and Elmhurst were, respectively, more than 800 and 900 

ft below land surface (more than 200 ft below sea level) 

(Sasman and others, 1982). The general pattern of the 

location of these local cones of depression in 1958 (Suter 

and others, 1959) and in 1980 (Sasman and others, 1982) 

are quite similar, but the decline in head during this 

period was greater in the westernmost cones.

The maximum head decline in the Milwaukee area was 

more than 375 ft by 1980, and was centered in eastcentral Waukesha County and western Milwaukee. In 

1950, a separate cone of depression was centered on the 

city of Waukesha, 13 mi west of the center of the main 

cone in central Milwaukee County (Foley and others, 

1953). By 1980, however, the Milwaukee cone had 

expanded westward to include eastern Waukesha 

County and the Waukesha cone (fig. 23), as pumping 

centers shifted westward. The total rate of withdrawal in 

southeastern Wisconsin has remained almost constant 

since about 1971, even though the population increased, 

because of water conservation and recycling.

The mutual interference of the pumping centers at 

Chicago and Milwaukee is shown by the coalescence of 

the cones of depression, which forms an east-west potentiometric divide in southeastern Wisconsin (fig. 23). This 

divide moved northward about 6 mi from 1961 to 1980, 

from the Racine-Kenosha County line (H.L. Young and 

A.J. MacKenzie, USGS, written commun., 1988) to 

central Racine County.

The city of Chicago and several nearby communities 

obtain their water supplies by diverting water from Lake 

Michigan at a rate that was fixed by a decree of the U.S. 

Supreme Court (1967). Illinois is allowed a maximum of 

1,700 ft3/s (cubic feet per second) (1,100 Mgal/d) of direct 

pumpage from the lake for domestic purposes; another 

1,500 ft3/s (970 Mgal/d) is accounted for by direct diversions to enhance navigation on the Chicago Sanitary and 

Ship Canal, and by the interception of surface runoff in 

the Lake Michigan drainage basin to the Chicago and 

Calumet Rivers, for a total of 3,200 ft3/s (2,070 Mgal/d).

Lake Michigan supplies most of the water needs of 

northeastern Illinois. However, the ground-water 

resource is an important supplemental supply, and in 

some areas is the only source of large supply. The 

previously mentioned 180 Mgal/d pumped from the 

Cambrian-Ordovician aquifer system in 1980 was 59 

percent of all ground water pumped in the eight-county 

area. As part of its management of the Lake Michigan

water diversion, the Illinois Division of Waters has a 

policy whereby the use of all ground-water resources 

must be optimized before an allotment from the diversion 

is granted; this optimization includes abandonment of 

deep wells to reduce the rate of head decline in the 

aquifer system. Completion of major parts of the Tunnel 

and Reservoir Plan of the Metropolitan Sanitary District 

of Greater Chicago will decrease the amount of water 

now diverted from the Lake Michigan basin used for 

stream dilution to meet water-quality standards. This 

will allow more diverted water to be allocated to publicsupply and industrial uses (Macaitis and others, 1977).

GROUND-WATER FLOW SIMULATION

Several computer models have been developed to 

simulate ground-water flow in the Cambrian-Ordovician 

aquifer system in all or parts of the northern Midwest 

(fig. 24). Many of these models have been used to predict 

head resulting from estimates of future rates of withdrawal of ground water.

REGIONAL MODEL

A major part of this study was development of a 

three-dimensional model to simulate regional groundwater flow in the Cambrian-Ordovician aquifer system in 

the northern Midwest (Mandle and Kontis, in press).

The finite-difference model developed by Trescott 

(1975) was used, but two major modifications were made 

to the computer code (Kontis and Mandle, 1988). First, 

the code was modified to simulate wells that are open to 

several different aquifers. Without this multiaquifer-well 

modification, estimates of the contribution of individual 

aquifer layers to total well discharge must be made prior 

to simulation. With this modification, only total well 

discharge is required. In addition, comparison of simulated composite heads with water-level measurements in 

multiaquifer wells is possible.

The other major modification to the computer code 

allows simulation of ground-water flow in areas where 

ground-water density is variable in space but not in time. 

This modification was made primarily to simulate the 

effects of the brines on the regional flow system. The 

model results indicate that ground water of high density 

is a partial barrier to fresh-ground-water flow, although 

ground water moves at extremely low flow rates within 

the areas of high-density ground water.

CONCEPTUAL MODEL

The regional ground-water flow system was idealized 

for simulation by combining adjacent geologic units having similar hydraulic properties into aquifers and confin-
GROUND-WATER FLOW SIMULATION A37

95

40

Base from

U.S. Geological Survey

1:7,500,000, 1970 EXPLANATION

|~ ~j REGIONAL GROUND-WATER FLOW MODEL

* -' (Mandle and Kontis, in press)

\ SUBREGIONAL GROUND-WATER FLOW MODEL

 1, NE Missouri (Imes, 1985)

2. NE Wisconsin (Emmons, 1987)

3. Iowa (Burkart and Buchmiller, in press)

4. Chicago-Milwaukee area (H.L. Young and A.J. MacKenzie, U.S. 

Geological Survey, written commun., 1988, and Young and 

others, 1989)

:5 SEPARATE STUDY AREAS

5. Minneapolis-St. Paul (Guswa and others, 1982)

6. Brown County, Wisconsin (Krohelski, 1986)

STUDY ARE A

FIGURE 24. Location and areal extent of regional and subregional ground-water flow models of the CambrianOrdovician aquifer system in the northern Midwest.
A38 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

ing units, as shown in table 1. The Cambrian and 

Ordovician rocks that were the focus of this study were 

subdivided into three pairs of aquifers and overlying 

confining units. The layering scheme was derived from 

the northern part of the study area, where the aquifers in 

the Cambrian-Ordovician aquifer system are primarily 

sandstones. The equivalent rocks contain more carbonate to the south and generally are less important as 

aquifers in most of Missouri, central and southern 

Illinois, and Indiana. To depict the regional characteristics of the flow system, aquifer and confining layers 

were discretized areally into 16- x 16-mi finite-difference 

blocks. Aquifer properties assigned were either averages of field data or estimates from rock properties and 

were assumed to be constant within each block. However, the property values can vary from block to block. 

Consequently, any variability in hydraulic properties or 

in the characteristics of the flow system on a scale 

smaller than the 16-mi spacing was not simulated.

Values of hydraulic properties were varied during 

calibration of the simulations until model results closely 

correlated with available field data for prepumping conditions prior to 1861 and for pumping conditions from 

1861 through 1980. The final set of hydraulic property 

values used in the simulation is not unique; other combinations of hydraulic property values could have been 

used during the calibration process, and would have 

given generally similar results. However, the hydraulic 

property values obtained during the calibration process 

in this study are consistent with the available field data, 

and generally depict the main features of the regional 

flow system and trends in water-level decline.

The flow equations used in the simulation describe 

horizontal flow in aquifers in which ground-water density 

is spatially variable but does not vary with time, and 

vertical flow between aquifers that occurs as flow 

through confining units and through multiaquifer wells. 

These flow equations do not accurately portray real flow 

conditions where (1) flow in aquifers is not strictly 

horizontal, such as near partially penetrating wells or 

streams, (2) flow in confining units is not strictly vertical, 

such as where confining units contain minor aquifers or 

locally their horizontal hydraulic conductivity exceeds 

the vertical hydraulic conductivity, or (3) movement of 

saline water has occurred and ground-water density 

varies with time. In addition, the simulation does not 

account for storage in confining units. This may be 

significant, especially if the confining units are very 

thick. Therefore, some degree of error in the simulation 

is to be expected.

STEADY-STATE SIMULATION

The model was calibrated for predevelopment steadystate conditions by comparing the simulated heads with 

the interpreted potentiometric surfaces derived from 

records of initial heads in the first wells drilled. Hydraulic conductivities of the aquifers and confining units were 

adjusted to produce a reasonable match between the 

simulated and derived initial heads.

Simulated steady-state conditions from the regional 

model show that regional predevelopment ground-water 

flow in the Cambrian-Ordovician aquifer system was 

generally to the southeast or east (fig. 25). The aquifer 

system was recharged in central Wisconsin, southcentral Minnesota, northwestern Iowa, northern Illinois, 

and central Missouri. Much of the water in the aquifer 

system discharged locally to rivers and streams where 

the aquifer system is fully or partly unconfined. Regionally, ground water flowed toward the Michigan basin 

from eastern Wisconsin and Illinois; toward the Mississippi River from western Wisconsin and Illinois and from 

eastern Minnesota and Iowa; toward the Missouri River 

from northern and south-central Missouri; and toward 

the Illinois basin from northwestern Iowa and northcentral Illinois. The principal regional discharge areas 

were the Mississippi and Missouri Rivers, the Illinois and 

Michigan basins, and Lake Michigan.

The simulation indicates that predevelopment 

recharge from the drift or Cretaceous aquifers into the 

Silurian-Devonian, St. Peter-Prairie du ChienJordan, 

Ironton-Galesville, and Mount Simon aquifers was about 

42, 450, 47, and 8 ft3/s, respectively. An additional 24 

ft3/s entered the Silurian-Devonian aquifer from the 

overlying Pennsylvanian-Mississippian-Devonian confining unit. Much of this water discharged from local flow 

systems to streams or to the glacial drift and Cretaceous 

rocks; discharge totaled 30, 370, 89, and 45 ft3/s from the 

Silurian-Devonian, St. Peter-Prairie du Chien-Jordan, 

Ironton-Galesville, and Mount Simon aquifers, respectively. An additional 37 fts/s discharged from the SilurianDevonian aquifer to the overlying PennsylvanianMississippian-Devonian confining unit. Thus, simulated 

total recharge was 571 ft3/s and was balanced by an 

equivalent rate of simulated total discharge.

TRANSIENT SIMULATION

Final calibration of the model was based on a transient 

simulation of the period 1861 to 1980, covering the period 

of development of the aquifer system, which was divided 

into 12 pumping periods. In addition to hydraulic conductivity, the storage coefficient of the aquifers was
GRO UND-WATER FLOW SIMULATION A39

40

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

-700 POTENTIOMETRIC CONTOUR Shows simulated predevelopment 

freshwater head in the St. Peter-Prairie du Chien-Jordan aquifer. 

Hachures indicate area of lower head. Contour interval 100 feet. 

Datum is sea level

AQUIFER BOUNDARY

FIGURE 25. Simulated predevelopment head in the St. Peter-Prairie du Chien-Jordan aquifer in the northern Midwest.

(From Mandle and Kontis. in press.)
A40 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

adjusted for transient calibration. The simulated 1980 

potentiometric surface for the St. Peter-Prairie du 

ChiennJordan aquifer (fig. 26) was compared with the 

potentiometric surface constructed from measured heads 

(fig. 22). Simulated cumulative head declines from 1861 

to 1980 in the St. Peter-Prairie du ChiennJordan aquifer 

(fig. 27) compare favorably with measured composite 

head declines in the Cambrian-Ordovician aquifer system. The largest differences between simulated and 

measured head declines occurred along the Mississippi 

River. A possible explanation for these differences is the 

discharge from a large number of flowing wells from the 

late 1800's and early 1900's, which was not accounted for 

in the water-use inventory or in the simulation.

Calibration also included comparison of the simulated 

heads at selected model nodes with the measured heads 

within the nodes at various times during the simulation 

period. The range in measured heads within a given node 

is large, and the simulated head generally was higher 

than most measured heads within the node. However, 

the simulated head declines follow the general trend of 

head declines shown by the measured heads.

Simulated transient conditions to 1980 show the large 

areas where water levels in the regional flow system 

declined (fig. 27) as a result of extensive development of 

ground water from the Cambrian-Ordovician aquifer 

system. Flow that would normally discharge to rivers or 

by leakage to overlying aquifers through confining units 

is captured by pumped wells within these large cones of 

depression. In addition to recharge areas identified in the 

steady-state simulations, induced transient leakage 

through the confining units due to reversals in direction 

of vertical gradients caused by pumping also is shown in 

much of Iowa, eastern Wisconsin, northeastern Illinois, 

and north-central Missouri.

For the final simulated pumping period, 1976-80, 

recharge from the glacial drift or Cretaceous rocks was 

217, 1,069, 65, and 9 ft3/s to the Silurian-Devonian, St. 

Peter-Prairie du ChiennJordan, Ironton-Galesville, and 

Mount Simon aquifers, respectively. In addition, 38 ft3/s 

entered the Silurian-Devonian aquifer from the overlying Pennsylvanian-Mississippian-Devonian confining 

unit. Thus, total recharge averaged 1,398 ft3/s for the 

period 1976-80. The increase in recharge from the 

steady-state simulation can be attributed to steeper 

hydraulic gradients near recharge areas or to a reduction 

in discharge from the different aquifer layers. Simulated 

discharge to the glacial drift or Cretaceous rocks 

decreased to 15, 266, 72, and 34 ft3/s from the SilurianDevonian, St. Peter-Prairie du ChiennJordan, IrontonGalesville, and Mount Simon aquifers, respectively. Discharge from the Silurian-Devonian aquifer to the 

overlying Pennsylvanian-Mississippian-Devonian confining unit decreased to 24 ft3/s. Pumpage was 1,208 ft3/s;

therefore, the total rate of discharge, 1,619 ft3/s, 

exceeded recharge by 221 ft3/s (released from aquifer 

storage), and water levels declined.

Simulated changes in the components of the groundwater budget for the Cambrian-Ordovician aquifer system are summarized in figure 28. Natural predevelopment discharge of 544 ft3/s (351 Mgal/d) was balanced by 

the same amount of recharge. At the end of the simulation period (1980), pumping from the aquifer system 

totaled 1,058 ft3/s (683 Mgal/d), which was obtained from 

an increase in recharge of 692 ft3/s (447 Mgal/d), a 

decrease in natural discharge of 153 ft3/s (99 Mgal/d), and 

release from aquifer storage of 212 ft3/s (137 Mgal/d).

Simulated drawdown caused by large-scale regional 

pumping creates a series of coalescing cones of depression in the confined aquifer system. This is especially 

evident in the simulated drawdown in the St. PeterPrairie du ChiennJordan aquifer (fig. 27). Pumping centers in Iowa interfere with one another, as do pumping 

centers in eastern Wisconsin and northeastern Illinois. 

Simulations show that continued pumping at current or 

increased rates would increase the broadening and deepening of the existing regional cones of depression.

SUBREGIONAL MODELS

Two-dimensional flow models of the Chicago area 

(Prickett and Lonnquist, 1971), southeastern Wisconsin 

and the Chicago area (Young, 1976), and the Madison, 

Wis., area (McLeod, 1975a, 1975b) were made prior to 

this regional study. Two USGS cooperative program 

studies benefited from joint data-collection efforts with 

this study: they were three-dimensional flow model 

studies of the Minneapolis-St. Paul area (Guswa and 

others, 1982) and the Brown County, Wis., area (Krohelski, 1986). Four subregional areas were modeled as 

part of this regional study. Two-dimensional flow models 

were prepared of the Cambrian-Ordovician aquifer system in northern Missouri (Imes, 1985) and of the Jordan 

aquifer in Iowa (Burkart and Buchmiller, in press). 

Three-dimensional flow models of the aquifer system in 

northeastern Wisconsin (Emmons, 1987) and in the 

Chicago-Milwaukee area (discussed in the following section) also were prepared.

The two-dimensional model of the Chicago area by 

Prickett and Lonnquist (1971) was later used by Schicht 

and others (1976) and Visocky (1982). Mark Collins and 

Steven Burch (Illinois State Water Survey, written 

commun., 1990) have recently (1989) revised that model.

The three-dimensional model of the ChicagoMilwaukee area made during this regional study is an 

important subregional study and, therefore, is briefly 

discussed here.
GROUND-WATER FLOW SIMULATION A41

95

40° 

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

POTENTIOMETRIC CONTOUR Shows simulated 1980 freshwater 

head in the St. Peter-Prairie du Chien-Jordan aquifer. Hachures 

indicate area of lower head. Contour interval, in feet, is 

variable. Datum is sea level

AQUIFER BOUNDARY

FIGURE 26. Simulated 1980 head in the St. Peter-Prairie du Chien-Jordan aquifer in the northern Midwest. (From

Mandle and Kontis, in press.)
A42 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

90°

40C

Scale 1:6,000,000 

0 50 100 MILES

50 100 KILOMETERS

Base enlarged from 

U.S.Geological Survey 

1:7,500,000, 1970

EXPLANATION

LINE OF EQUAL SIMULATED FRESHWATER HEAD DECLINE IN THE 

ST. PETER-PRAIRIE DU CHIEN-JORDAN AQUIFER, 1861-1980 

Interval, in feet, is variable

AQUIFER BOUNDARY

FIGURE 27. Simulated decline in head in the St. Peter-Prairie du Chien-Jordan aquifer in the northern Midwest,

1861-1980. (From Mandle and Kontis, in press.)
GROUND-WATER FLOW SIMULATION A43

Natural

Recharge ° discharge

Pumpage-1058 i

5'' 14 Release >V

1,236 from storage^ *~

0

212

Cambrian-Ordovician

r

-*-

-5 44

-391

aquifer system

EXPLANATION

0 SIMULATED COMPONENTS OF THE GROUND- 

-1058 WATER BUDGET FOR THE CAMBRIANORDOVICIAN AQUIFER SYSTEM, DERIVED 

FROM THE REGIONAL FLOW MODEL 

Positive number indicates a source of water 

to the aquifer system, a negative number 

indicates a discharge from the aquifer system. 

Upper number is predevelopment, steady-rate 

and lower number is 1980 transient rate, in 

cubic feet per second

FIGURE 28. Simulated water budget for the Cambrian-Ordovician 

aquifer system in the northern Midwest.

CHICAGO-MILWAUKEE AREA MODEL

The last phase of the Northern Midwest RASA study 

consisted primarily of the preparation of a threedimensional model of the Chicago-Milwaukee area (H.L. 

Young and A.J. MacKenzie, USGS, written commun., 

1988; Young and others, 1989). This is the first simulation of three-dimensional flow in the aquifer system for 

this area. The previously mentioned models of the area 

simulate only two-dimensional flow and treat the 

Cambrian-Ordovician aquifer system as one uniform 

aquifer layer. The three-dimensional model uses a similar approach to model layers (fig. 29), multiaquifer wells, 

transient stress periods, and constant-head water table 

as used in the regional model (Mandle and Kontis, in 

press), but it does not account for variable water density. 

The eastern boundaries of both the regional and this 

subregional model extend to the center of the Michigan 

basin (fig. 24).

A major difference between the regional and ChicagoMilwaukee area models is the treatment of the

Maquoketa confining unit and the Galena-Platteville 

unit. In both models, the dolomitic Galena-Platteville 

unit is included as part of the Maquoketa confining unit 

where it is overlain by the Maquoketa Shale (fig. 11). In 

the regional model, the Galena-Platteville unit is considered to be an aquifer where it is not overlain by the 

Maquoketa. In the Chicago-Milwaukee area model, the 

Galena-Platteville unit is considered to be an aquifer 

where it is not overlain by the Maquoketa and also is less 

than 250 ft thick, in which case it is added to the St. 

Peter-Prairie du Chien^Jordan aquifer. This thickness 

was chosen arbitrarily to define areas where the confining characteristics of the Galena-Platteville unit probably are not diminished by secondary permeability as a 

result of surficial weathering and fracturing. The resulting western boundary of the modeled Maquoketa confining unit of the Chicago-Milwaukee area model is shown in 

figure 23.

The model was developed in two stages: a steady-state 

simulation to determine predevelopment conditions, and 

a transient simulation of the full period of development, 

1864-1985. The transient period was divided into 13 

stress periods of various lengths to best approximate the 

changes in rates of withdrawal and to accommodate key 

calibration dates. These calibration dates were 1961, 

1980, and 1985, when the most complete measured head 

data were available for both the Illinois and Wisconsin 

parts of the study area. As described for the regional 

model, most of the head data available are for multiaquifer wells; therefore, calibration was based on the composite heads in the aquifer system.

STEADY-STATE SIMULATION

Results of the steady-state simulation provide an 

understanding of the major components of the natural 

ground-flow system, especially in relation to flow patterns and interaquifer head differences. The general 

pattern of predevelopment flow in the CambrianOrdovician aquifer system can be interpreted from the 

simulated potentiometric surface and from areal distribution of vertical flow direction through the top of the St. 

Peter-Prairie du Chien^Jordan aquifer (fig. 30). Flow in 

the northern two-thirds of the area generally was to the 

east or west from the potentiometric divide that extends 

from eastern Dodge County, Wis., to western De Kalb 

County, 111. The bedrock valleys beneath the Illinois and 

Fox Rivers in central La Salle County, 111., are cut 

deeply into the St. Peter-Prairie du Chien^Jordan aquifer. Lack of confinement by the Maquoketa confining unit 

and the topographic low of the river valleys create a 

regional discharge area. The heads in the discharge areas 

are much lower in the St. Peter-Prairie du ChienJordan 

aquifer than in the aquifers below; thus, this discharge
A44 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

cc

I

CO

Drift aquifer

NORTH

SEA LEVEL

1000-

2000-

3000

4000 -

5000

^ ^fs-r^-"'-" -' -*" '-^^gsgs: *^*^-- ^tsxh?^ SHuria^r»-6^o"h'iari'aquifer^ 

Maquoketa confining unit ,,..:,.,-,. w-.^,.

St. Lawrence-Franconia confining unit

\

££%£;^^0£?%l£^ Mount Simon aquifer '*$l$l£&$ ^^^;;:;^^g|^@^ ';:!!'':'';^

£-&-^ffi$(£$&^t$£- ^.Zttytt^ '

Precambrian confining unit

VERTICAL SCALE GREATLY EXAGGERATED 25 50 MILES

T^ 

25 50 KILOMETERS

FIGURE 29. Geohydrologic section along Chicago-Milwaukee area model column 21. (From H.L. Young and A.J. MacKenzie, U.S.

Geological Survey, written commun., 1988.)

area controlled the flow pattern m the southern part of 

the area. These simulated patterns compare favorably 

with the estimated predevelopment conditions for this 

area shown in figure 12.

The patterns of vertical flow direction shown in figure 

30 are similar to the patterns for the aquifers below, with 

two main differences. The area of downward flow east of 

the potentiometric divide extended somewhat farther 

eastward for the Mount Simon aquifer, closer to Lake 

Michigan, because of slightly higher head in the aquifers 

above. Vertical flow through the top of the Mount Simon 

aquifer was upward in the entire eastern half and southern third of the study area, reflecting the good confinement of the Mount Simon by the Eau Claire.

Simulated vertical head differences were relatively 

small between each of the Cambrian-Ordovician aquifers

below the Maquoketa confining unit, but were much 

larger between the water table and these aquifers. 

Beneath the Maquoketa, head differences within the 

aquifer system generally were less than 5 ft, but to the 

west, where the confining unit is partly or completely 

absent, head differences were larger, generally ranging 

from 10 to 30 ft. The largest differences were in La Salle 

and Grundy Counties, 111., where heads in the St. 

Peter-Prairie du Chien-Jordan aquifer are depressed 

around a natural discharge area along the Illinois River 

valley.

The most striking feature was the high head in the 

Cambrian-Ordovician aquifers as much as 100 to 150 ft 

higher than the water table along the western shore of 

Lake Michigan. Heads of 750 to 800 ft above sea level in 

this area (fig. 30) were derived from the high water table
GROUND-WATER FLOW SIMULATION A45

Base modified from U.S. Geological Survey 

1:2,000,000, 1970

EXPLANATION

 soo POTENTIOMETRIC CONTOUR Shows simulated predevelopment 

head in the St. Peter-Prairie du Chien-Jordan aquifer. Hachures 

indicate area of lower head. Contour interval 50 feet. Datum is 

sea level

 - POTENTIOMETRIC DIVIDE 

AQUIFER ABSENT

DIRECTION OF SIMULATED VERTICAL FLOW THROUGH THE TOP OF 

THE AQUIFER

Upward 

Downward

FIGURE 30. Simulated predevelopment head and vertical flow direction in the St. 

Peter-Prairie du ChiennJordan aquifer in the Chicago-Milwaukee area. (From H.L. 

Young and A.J. MacKenzie, U.S. Geological Survey, written commun., 1988.)
A46 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

along the major potentiometric divide to the west. These 

conditions of high head were shown by the many flowing 

wells drilled during the late 1800's.

Rates of simulated steady-state horizontal flow within 

the Cambrian- Ordovician aquifers, represented as specific discharge or Darcian flow, ranged from less than 1 to 

more than 50 in/yr (inches per year). The rates generally 

were less than 10 in/yr in the confined aquifer system and 

decreased to the east the direction of ground-water 

movement and structural dip of the rocks (fig. 11). Rates 

were lowest in the Mount Simon aquifer, and were 

highest in the unconfined areas where hydraulic gradients steepen and the aquifers are relatively thin.

The simulated steady-state potentiometric surface of 

the St. Peter-Prairie du Chien-Jordan aquifer was used 

to subdivide the almost-71,000-mi2 modeled area into 

three zones, representing areas of major flow systems, to 

provide a better framework for analysis of modeled 

hydrologic budget components. One zone is an area of 

about 17,100 mi2 that encompasses deeply confined flow 

in the Michigan basin to the east. Its hydrologic budget 

is not important to the study, but, for modeling purposes, the area is a necessary extension of the confined 

aquifer system beyond the Chicago-Milwaukee area. 

Two flow systems account for the balance of the modeled 

area. One consists of about 6,800 mi2 of the Rock River 

basin west of the potentiometric divide shown in figure 

30. The model study focused on the Cambrian-Ordovician 

aquifer system in the remaining 47,100 mi2 to the east of 

the divide, which includes all the area affected by 

ground-water withdrawal in the Chicago-Milwaukee 

area. This area is referred to subsequently in this report 

as the Chicago-Milwaukee area flow system. The 

Cambrian-Ordovician aquifer system generally is overlain by the Maquoketa confining unit in that area, but is 

mainly unconfined or semiconfined in the Rock River 

basin to the west.

The steady-state simulation indicates that most predevelopment recharge and discharge of the CambrianOrdovician aquifer system in the Chicago-Milwaukee 

area flow system was directly between the drift aquifer 

and the St. Peter-Prairie du Chien-Jordan aquifer where 

the Maquoketa confining unit is absent (on the western 

side of the area, fig. 30). The boundary of the ChicagoMilwaukee area flow system, as delineated in the model, 

is not a pure zero-flux boundary, and a small amount of 

horizontal flow across the boundary is not accounted for. 

Recharge there averaged about 31.4 ft3/s (0.47 in/yr) 

through an area of 899 mi2 (fig. 31), and discharge 

averaged 32.0 fts/s (0.86 in/yr) through 506 mi2 . An 

additional 16.4 ft3/s (0.18 in/yr) entered the aquifer 

through the Maquoketa confining unit, where it is partly 

eroded and thin, over an area of 1,261 mi2 . Discharge 

under these conditions over an area of 1,269 mi2 was 9.81

ft3/s (0.10 in/yr). The simulation shows that 4.93 fts/s 

(0.003 in/yr) flowed downward through the confining unit 

where it is overlain by the Silurian-Devonian aquifer, an 

area of 23,975 mi2 . The size of the downward flow area is 

deceptive compared to the area in figure 30, because 

11,770 mi2 is in areas of very low rates of downward flow 

in the extreme southwestern and southeastern parts of 

the model grid, outside the area of figure 30. Upward 

discharge through the Maquoketa confining unit over an 

area of 19,175 mi2 averaged 11.3 fts/s (0.008 in/yr).

TRANSIENT SIMULATION

The calibrated steady-state model was the basis for a 

transient model, which simulates the entire period of 

development of the Cambrian-Ordovician aquifer system, from 1864 to 1985, in 13 stress periods. Calibration 

was achieved by adjusting hydraulic characteristics to 

obtain a reasonable match of simulated and measured 

potentiometric heads at the respective dates 1961, 1980, 

and 1985. In addition, comparison of simulated heads 

with long-term well hydrographs at several locations 

gave an indication of the cumulative accuracy of the 

simulation over time. The potentiometric surface derived 

from simulated 1985 composite heads in the aquifer 

system (fig. 32) closely matches the potentiometric surface derived from measured water levels, most of which 

are for multiaquifer wells.

Development of ground water from the aquifer system 

where it is confined has produced very large head 

declines, as previously described and shown in figure 23, 

that are centered on the major areas of withdrawal at 

Chicago and Milwaukee. The shape of the resultant cones 

of depression in the composite potentiometric surface 

(fig. 32) is a good example of boundary effects and the 

spread of drawdown in a confined aquifer. The steepening of the western side of the cone near the recharge 

areas on the west results from relatively high rates of 

vertical leakage in the recharge areas. Conversely, the 

eastern side of the cone slopes gently to the east away 

from the recharge areas, where vertical leakage is limited by the thick Maquoketa confining unit and by the 

very small vertical head gradients. Drawdown there is 

smaller, but more widespread, to satisfy the withdrawal 

rates in the cones.

Development of the large cones of depression has 

changed the directions of ground-water movement 

greatly. The major changes are the reversal of the 

original horizontal flow to the east and southeast toward 

the Michigan and Illinois basins and the reversal of the 

upward flow out of the Cambrian-Ordovician aquifer 

system through the Maquoketa confining unit near Lake 

Michigan. Within the Chicago-Milwaukee area flow sys-
GROUND-WATER FLOW SIMULATION A47

Model 

layer

Drift aquifer

32.0

0.86

i,

31.4

0.47

Silurian-Devonian 

aquifer

Maquoketa confining unit

9.81 11.3 

0.10 0.008

16.4

0.18

4.93

0.003

St. Peter-Prairie du Chien-Jordan aquifer

Modeled area, in square miles 899 506 1,261 1,269 23,975 19,175

31.4

0.47

EXPLANATION

SIMULATED PREDEVELOPMENT FLUX ACROSS TOP OF 

ST. PETER-PRAIRIE DU CHIEN-JORDAN AQUIFER 

Arrow shows direction of flow. Numbers are sums 

of fluxes. Upper number is cubic feet per second, 

lower number is inches per year

FIGURE 31. Simulated predevelopment ground-water flow across the top of the St. Peter-Prairie du Chien-Jordan 

aquifer within the area of the Chicago-Milwaukee flow system. (From H.L. Young and A.J. MacKenzie, U.S. 

Geological Survey, written commun., 1988.)

tern, upward flow now occurs only on the extreme 

perimeter of the area to the north and southwest (fig. 

32).

Less obvious are the changes caused by the shifting of 

potentiometric divides. The simulation shows that the 

location of the major north-south divide (fig. 30), which 

defines the western limit of the Chicago-Milwaukee flow 

system, probably has moved westward about 10 mi in 

northwestern Waukesha and southeastern Dodge Counties, Wis., and as much as 20 mi in southern Boone and 

northern De Kalb Counties, 111. (fig. 32). The lack of 

historical water-level data precludes accurate determination of the position of the predevelopment divide. The 

only long-term data relating to the position of the divide 

are the water-level records for an observation well in the 

St. Peter-Prairie du Chien-Jordan aquifer near the western line of Waukesha County (fig. 33). Water-level 

measurements from 1946 to 1987 seem to show a slight 

downward trend perhaps 5 ft of decline over the 41 

years. Simulated total head decline in that aquifer as of 

1985 also was about 5 ft, but in the underlying IrontonGalesville and Mount Simon aquifers the declines were 

about 30 ft. The larger declines in the underlying aquifers can be explained by the fact that the lower aquifers 

are well confined throughout the study area and in all of 

southern Wisconsin and northern Illinois, whereas the 

St. Peter-Prairie du Chien-Jordan aquifer is unconfined 

in the general area of the divide and farther to the west. 

Comparison of simulated 1985 flow into and out of the 

aquifer system in the main Chicago-Milwaukee area flow 

system (fig. 34) with those for predevelopment conditions (fig. 31) shows very large changes caused by the 

head decline. Downward flow across the Maquoketa 

confining unit increased by about 82 ft8/s, and upward 

flow decreased by about 14 ft8/s; downward flow from the 

drift aquifer increased by about 42 ft3/s, and discharge to 

the drift decreased by about 23 ft8/s. This is a net 

increase of about 161 ft8/s. The rates of upward discharge 

per unit area (shown in fig. 31 as inches per year) were 

about the same, but the areas decreased greatly because 

of flow reversal. Both the rates per unit area and the 

areas of downward flow are much larger for 1985 than for
A48 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

89° 88°

FIGURE 32. Simulated 1985 head and vertical 

flow direction in the composite Cambrian-Ordovician aquifer system in the Chicago-Milwaukee 

area. (From H.L. Young and A.J. MacKenzie, 

U.S. Geological Survey, written commun., 

1988.)

 800-

Base modified from U.S. Geological Survey 

1:2,000,000, 1970

EXPLANATION

POTENTIOMETRIC CONTOUR Shows simulated 1985 head in the 

composite Cambrian-Ordovician aquifer system. Hachures 

indicate area of lower head. Contour interval, in feet, is variable. 

Datum is sea level

POTENTIOMETRIC DIVIDE 

AQUIFER ABSENT

DIRECTION OF SIMULATED VERTICAL FLOW THROUGH THE TOP OF 

THE AQUIFER SYSTEM

Upward 

Downward

LOCATION OF WELL WK-07/17E/05-0020 Hydrograph in figure 33
GROUND-WATER FLOW SIMULATION A49

1945

FIGURE 33. Hydrograph for well Wk-07/17E/0&-0020 completed in the St. Peter-Prairie du Chien-Jordan aquifer in northwestern 

Waukesha County, Wisconsin. (Location shown in fig. 32. From H.L. Young and A.J. MacKenzie, U.S. Geological Survey, written 

commun., 1988.)

Model 

layer

Drift aquifer

8.51

0.78

73.3

0.79

Silurian-Devonian 

aquifer

Maquoketa confining unit

4.92 1.84 

0.11 0.00043

80.7

0.57

v

22.4 

0.0078

St. Peter-Prairie du Chien-Jordan aquifer

Modeled area, in square miles 1,257 148 1,923 607 38,853 4,297

73.3

0.79

EXPLANATION

SIMULATED 1985 FLUX ACROSS TOP OF ST.

PETER-PRAIRIE DU CHIEN-JORDAN AQUIFER 

Arrow shows direction of flow. Numbers are sums 

of fluxes. Upper number is cubic feet per second, 

lower number is inches per year

FIGURE 34. Simulated 1985 ground-water flow across the top of the St. Peter-Prairie du Chien-Jordan aquifer within the 

area of the Chicago-Milwaukee flow system. (From H.L. Young and A.J. MacKenzie, U.S. Geological Survey, written 

commun., 1988.)

the predevelopment simulation. Average pumpage from 

the Cambrian-Ordovician aquifers in the ChicagoMilwaukee area flow system was about 322 ft3/s (208 

Mgal/d) for the period 1981-85. Contribution from storage during this period accounted for about 116 ft3/s, and 

net increase from vertical flow was 161 ft3/s, leaving a 

difference of about 45 ft3/s, which is induced flow within

the aquifer system from the extended cone of depression 

west of the original potentiometric divide.

The indication that the major cones of depression have 

extended westward beyond the original potentiometric 

divide, especially within the fully confined IrontonGalesville and Mount Simon aquifers, is a significant 

finding of this simulation of the Chicago-Milwaukee area.
A50 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

Fortunately, little extension of the cones of depression is 

apparent in the St. Peter-Prairie du Chien-Jordan and 

drift aquifers, which are shallower and are under watertable conditions.

TOPICS FOR FURTHER INVESTIGATION

A number of areas of investigation warrant consideration in planning further studies of the CambrianOrdovician aquifer system in the northern Midwest. 

Those discussed here are related primarily to the occurrence and quality of ground water in heavily pumped 

areas in the confined parts of the aquifer system.

Confining units are important controlling factors in the 

regional flow system; thus, knowledge of their hydraulic 

properties and leakage capability, which affect water 

quality and quantity in adjacent aquifers, needs to be 

improved. Although the rate per unit area is small, 

induced transient leakage from a confining unit can be a 

major part of the volume of water released from storage 

during pumping of a confined aquifer because of the size 

of the confined area and the thickness of the confining 

unit. Ground water in confining units generally is of a 

different quality, commonly containing more dissolved 

solids than that in nearby aquifers, and its movement 

into these aquifers can alter water quality in the aquifers.

HYDRAULIC CHARACTERISTICS OF CONFINING UNITS

Vertical hydraulic conductivity and specific storage of 

confining units are the properties that are the most 

difficult to determine and generally are estimated. It is 

likely that vertical openings, such as joints and fractures, 

rather than the permeability of the solid, unfractured 

rock, account for much of the regional leakage through 

shale confining units. Detailed research on the confining 

units of the aquifer system is needed, similar in scope to 

the field and simulation studies by Bredehoeft and others 

(1983) of the effects of confining units on the Dakota 

aquifer in South Dakota. Data on the Maquoketa Shale 

and the underlying Galena-Platteville unit are especially 

important. The Galena-Platteville unit is known to be 

more of a confining unit than an aquifer where overlain 

by the Maquoketa, but its transition into a moderately 

productive, unconfined aquifer where the Maquoketa is 

absent has not been quantified. The lithostratigraphy of 

the Maquoketa in Iowa, northern Illinois, and Indiana 

has been studied in some detail (Parker, 1971; Gray, 

1972; Kolata and Graese, 1983), as has the GalenaPlatteville unit in northern Illinois (Willman and Kolata, 

1978). However, facies maps of the Maquoketa in eastern 

Wisconsin are not available.

The Mount Simon Sandstone in southeastern Wisconsin, northeastern Illinois, and northwestern Indiana contains much interbedded siltstone and shale, especially in 

the upper part of the formation. The extent and confining 

properties of these units have not been studied. A 

500-ft-thick zone containing interbedded shales begins 

about 100 ft below the top of the Mount Simon in the 

3,475-ft-deep test well at Zion, 111. (Nicholas and others, 

1987). Water-level measurements from packer tests and 

deep piezometers installed in the well show that the 

present hydraulic head is about 50 ft higher beneath the 

zone than above. The concentration of chloride in the 

ground water yielded during the air-rotary drilling process increased sharply as this zone was penetrated. The 

confinement by this zone undoubtedly controls the vertical distribution of saline water in the Mount Simon 

aquifer in the Chicago area. Historical records indicate 

that many wells originally drilled into the Mount Simon 

there have been backfilled to eliminate flow of saline 

water from that aquifer. Also, several deep wells in the 

Mount Simon in north-central Illinois and northwestern 

Indiana are used for disposal of liquid wastes. Therefore, 

knowledge of the extent and characteristics of this 

interbedded shale zone is important for future groundwater development and disposal of liquid wastes in these 

areas.

RELATION OF CONFINING UNITS TO WATER QUALITY

Back (1985) described the influence of confining units 

on aquifer geochemistry and pointed to the need for 

development of special techniques for collecting and 

interpreting data from confining units. Shales and siltstones are particularly important vehicles for waterquality change because of the susceptibility to solution 

and chemical reaction of their very small sediment particles and component mineral species. The particles in 

rocks of these types are dominantly clay minerals and 

quartz, but they contain variable amounts of other 

minerals, such as feldspar, mica, chlorite, pyrite, 

hydrated oxides of iron or aluminum, glauconite, or other 

minor minerals. Ground water readily dissolves or reacts 

with these minerals, thereby affecting the chemistry of 

water in confining units.

Ground-water quality also is believed to be altered by 

the selective ion filtration processes of chemical osmosis 

and reverse chemical osmosis by shale or clay layers that 

function as a semipermeable membrane, and by ion 

exchange with the clay minerals in shales. As summarized by Hanor (1983), these processes and others have 

been invoked by various investigators to explain the 

origin of brines in sedimentary basins.

The major concerns about water quality in the 

Cambrian-Ordovician aquifer system are the occurrences
CONCLUSIONS A51

of excessive radium concentrations and saline water. 

These are characteristics of the aquifer system primarily 

where it is confined by the Maquoketa, but high 

dissolved-solids concentrations also are present at depth 

along the Mississippi River valley and in areas of eastern 

Wisconsin just west of the western edge of the 

Maquoketa. Levels of radium that exceed the current 

primary drinking-water standard of 5 pCi/L (picocuries 

per liter) (U.S. Environmental Protection Agency, 1986) 

have been reported for water from many wells in the 

confined aquifer system in Iowa, northern Illinois, and 

eastern Wisconsin. The widespread occurrence of radium 

is not everywhere directly correlated with salinity; however, water salinity usually is directly related to radium 

concentrations. Field studies are needed to determine 

the exact source(s) of the radium within the aquifer 

system and the relation to the Maquoketa confining unit.

OTHER TOPICS

Other subjects related to the Cambrian-Ordovician 

aquifer system that need to be studied include the 

following:

 Effects on, and contribution to, discharge from multiaquifer wells and interformational flow by the SilurianDevonian aquifer in eastern Wisconsin and northeastern Illinois.

 Changes in water quality in the Cambrian-Ordovician 

aquifer system within the heavily pumped areas in 

eastern Wisconsin and northeastern Illinois during the 

past 60 years or so since reversal of the upward head 

gradient across the Maquoketa.

 Field delineation of potentiometric divides in individual aquifers west of the cones of depression in eastern 

Wisconsin and northeastern Illinois.

 The possibility of artificial recharge of the aquifer 

system in the Chicago area, suggested or proposed by 

Foley and Smith (1954), Suter and others (1959), 

Schicht and others (1976), and Bennett and others 

(1982).

 Effects on the regional flow system of major structural 

features, such as the Sandwich fault zone and La Salle 

anticline in northern Illinois and the Plum River fault 

zone of eastern Iowa.

 The role of ground water in the deposition of the 

Mississippi Valley-type ore deposits in southwestern 

Wisconsin and northwestern Illinois.

CONCLUSIONS

The Cambrian-Ordovician aquifer system is a very 

important regional resource in the northern Midwest. It 

provided about 680 Mgal/d of the ground-water needs to

parts of Illinois, Iowa, Minnesota, Missouri, and Wisconsin in 1980. About 180 Mgal/d was pumped in each of the 

metropolitan areas of Chicago and Minneapolis-St. Paul.

The aquifer system is composed of the entire thick and 

areally extensive sequence of Cambrian and Ordovician 

sandstone, siltstone, dolomite, and shale in the northern 

Midwest. Three aquifers, composed mainly of sandstone, 

are recognized in the aquifer system; the aquifers are 

separated by confining units. The major hydrogeologic 

units are the Maquoketa confining unit (the uppermost 

part of the aquifer system) and, in descending order, the 

St. Peter-Prairie du Chien-Jordan, Ironton-Galesville, 

and Mount Simon aquifers. The Cambrian and Ordovician strata dip gently off the elevated Precambrian 

basement of northern Minnesota and Wisconsin toward 

structural basins to the south and east.

Unconfined conditions prevail in the aquifer system in 

its northern outcrop area in the shallow parts of the 

aquifer system and where it is thin. Leaky-artesian 

conditions generally prevail elsewhere in the aquifer 

system.

Ground water moves in the aquifer system within flow 

systems of different areal and subsurface extent. Much of 

the ground-water recharge in upland areas discharges to 

streams through local, unconfined flow systems that are 

no more than a few miles in length. This flow is a large 

part of the overall volume of recharge and discharge in 

the study area. The remainder of the recharge moves 

slowly downward to deeper formations and downgradient to form or join the regional flow system. This deeper 

flow occurs within intermediate- or regional-scale flow 

systems toward the structural basins in the south and 

east. The rate of flow is very small and decreases 

progressively downgradient because of head loss from 

upward leakage through confining units. Saline water 

within the structural basins restricts movement of freshwater into the deeper parts of the basins; thus, the water 

is forced upward.

Principal regional discharge areas are the Mississippi 

and Missouri Rivers, the Illinois and Michigan basins, 

and Lake Michigan. However, the lake is not in direct 

hydraulic connection with the Cambrian-Ordovician 

aquifer system; rather, it receives flow from the SilurianDevonian aquifer, which directly underlies the lake and 

receives upward leakage from the Cambrian-Ordovician 

aquifer system.

The first deep wells were drilled into the aquifer 

system in the 1860's. Many were flowing artesian wells 

located near Lake Michigan in eastern Wisconsin and 

northeastern Illinois, and along the Mississippi River 

valley and its tributaries. Initial heads of 186 and 130 ft 

above Lake Michigan were reported at Milwaukee and 

Chicago, respectively.
A52 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

High rates of pumping from the aquifer system have 

caused large head declines and extensive cones of depression. The largest head declines are in areas where the 

aquifer system is well confined as much as 900 ft at 

Chicago, 375 ft at Milwaukee, and more than 200 ft at 

Mason City, Iowa, and the Quad Cities of Iowa and 

Illinois by 1980, and 440 ft at Green Bay, Wis., by 1957. 

However, the head decline in the upper, unconfined part 

of the aquifer system in Minneapolis-St. Paul was only 90 

ft by 1980.

Computer simulations of the regional ground-water 

flow system were made to approximate the predevelopment head in 1864 and to evaluate the flow system under 

both steady-state and transient conditions. The transient 

simulation through 1980 depicts the large water-level 

declines in the confined parts of the CambrianOrdovician aquifer system in Iowa, eastern Wisconsin, 

northeastern Illinois, and north-central Missouri and the 

associated changes in direction of ground-water movement. Ground-water withdrawals caused little change in 

the unconfined parts of the flow system. The simulation 

shows that total recharge to the aquifer system increased 

by 692 ft3/s from predevelopment conditions to a total of 

1,236 ft3/s in 1980. Natural discharge decreased by 153 

ft3/s, to a total of 391 ft3/s, and 212 ft3/s was released 

from aquifer storage.

A subregional simulation of the Cambrian-Ordovician 

aquifer system in the Chicago-Milwaukee area shows the 

effects of development there in more detail. The original 

artesian conditions caused upward flow from the aquifer 

system through the Maquoketa confining unit into the 

overlying Silurian-Devonian aquifer near Lake Michigan. However, by the 1920's in the areas of greatest head 

decline, the head in the aquifer system had begun to 

decline below that in the Silurian-Devonian aquifer. This 

head reversal caused the vertical flow to reverse, being 

induced downward across the Maquoketa. By 1985, the 

area of downward flow across the Maquoketa had 

increased to encompass most of the Chicago-Milwaukee 

area affected by pumping. A potentiometric divide that 

bounds the Chicago-Milwaukee area flow system on the 

west apparently has shifted westward as much as 20 mi 

in response to the expanding cones of depression. Downward flow across the Maquoketa in the ChicagoMilwaukee area flow system increased by 64 ft3/s from 

predevelopment conditions to a total of 81 ft3/s in 1985, 

and recharge to the aquifer system from the overlying 

drift aquifer increased by 42 ft3/s to a total of 73 ft3/s. 

Natural discharge decreased by 38 ft3/s to a total of 15 

ft3/s in 1985.

Ground water in the Cambrian-Ordovicican aquifer 

system is characterized by an extreme range of mineralization and chemical types, but its quality is good and 

suitable for most uses in most of the study area.

Dissolved-solids concentrations generally are low (less 

than 500 mg/L) in the recharge areas in the northern and 

central parts of the area and are high (brines containing 

as much as 200,000 mg/L) in the structural basins to the 

south and east. The water in the Cambrian-Ordovician 

aquifer system in most of the unconfined areas is a 

calcium magnesium bicarbonate type and is similar in 

quality in each of the aquifers.

Water quality in the confined parts of the aquifer 

system is much more variable. Concentrations of dissolved solids and the major ions in most places increase in 

direct proportion to the distance the water has traveled 

from areas of recharge. Transition to higher dissolved 

solids in the confined areas commonly is accompanied by 

substantial increases in sulfate concentration and the 

presence of calcium sodium sulfate-type water.

The residual effects on ground-water quality from 

continental glaciations during the Pleistocene are indicated by isotopic analysis of the ground water. Depletion 

of the oxygen-18 isotope in water indicates that the 

source of much of the ground water in the confined areas 

was subglacial recharge of meltwater during the Pleistocene. Effects from loading by Pleistocene ice sheets may 

be shown by the positive S^S values for sulfur in sulfate 

in saline water near Lake Michigan in eastern Wisconsin 

and northeastern Illinois. It is believed that isostatic 

loading from glacial ice over the Michigan basin created 

a westward hydraulic gradient, opposite from the natural 

eastward gradient that prevailed prior to development in 

the Chicago-Milwaukee area. This flow reversal by ice 

loading caused saline water from the Michigan basin, 

which contained sulfate with sulfur of an evaporitic 

origin, to flow westward and discharge through the 

present recharge areas.

Important research topics for future studies include 

the hydraulic characteristics and geochemical control of 

confining units in the Cambrian-Ordovician aquifer system, especially for the Maquoketa confining unit and a 

local confining unit in the upper part of the Mount Simon 

aquifer in the Chicago-Milwaukee area.

REFERENCES CITED

Ackermann, W.C., and others, 1974, Feasibility study on desalting 

brackish water from the Mt. Simon aquifer in northeastern Illinois: 

U.S. Department of the Interior, Office of Saline Water, Research 

and Development Progress Report 938 and Illinois State Water 

Survey Contract Report 153, 120 p.

Alexander, E.G., Jr., Siegel, D.I., and Milske, J.A., 1983, Isotopic 

studies of the Mt. Simon aquifer, southern Minnesota [abs.]: 

American Geophysical Union Transactions, EOS, v. 64, no. 18, 

p. 225.

Back, William, 1966, Hydrochemical facies and ground-water flow 

patterns in northern part of Atlantic Coastal Plain: U.S. Geological 

Survey Professional Paper 498-A, p. 1-42.
REFERENCES CITED A53

 1985, Geochemical significance of aquitards, in Hydrogeology of 

rocks of low permeability: International Association of Hydrogeologists Congress, 17th, Tucson, Ariz., 1985, Memoirs, v. 17, pt. 1, 

p. 358-369.

Bennett, G.D., 1979, Regional ground-water systems analyses: Water 

Spectrum, v. 11, no. 4.

Bennett, Truman, Gass, T.E., Lehr, J.H., Aller, Linda, and Nielsen, 

D.M., 1982, A cost effective water supply alternative for the 

Chicago suburban area: Artificial ground-water recharge: National 

Water Well Association research paper, 70 p.

Bond, B.C., 1972, Hydrodynamics in deep aquifers of the Illinois basin: 

Illinois State Geological Survey Circular 470, 72 p.

Bredehoeft, J.D., Neuzil, C.E., andMilly, P.C.D., 1983, Regional flow 

in the Dakota aquifer: A study of the role of confining layers: U.S. 

Geological Survey Water-Supply Paper 2237, 45 p.

Burkart, M.R., and Buchmiller, R.C., in press, Regional aquifer 

evaluation of hydrologic factors and effects of pumping, St. PeterJordan aquifer, Iowa: U.S. Geological Survey Water-Resources 

Investigations Report 90-4009.

Delin, G.N., and Woodward, D.G., 1984, Hydrogeologic setting and 

the potentiometric surfaces of regional aquifers in the Hollandale 

embayment, southeastern Minnesota, 1970-80: U.S. Geological 

Survey Water-Supply Paper 2219, 56 p.

Emmons, P.J., 1987, An evaluation of the bedrock aquifer system in 

northeastern Wisconsin: U.S. Geological Survey Water-Resources 

Investigations Report 85-4199, 48 p.

Fetter, C.W., Jr., 1981, Interstate conflict over ground water: 

Wisconsin-Illinois: Ground Water, v. 19, no. 2, p. 201-213.

Foley, F.C., and Smith, H.F., 1954, Ground-water recharge of a 

deeply buried artesian aquifer in Illinois and Wisconsin, U.S.A.: 

International Association of Scientific Hydrology Publication 37, 

pt. 2, p. 225-231.

Foley, F.C., Walton, W.C., and Drescher, W.J., 1953, Ground-water 

conditions in the Milwaukee-Waukesha area, Wisconsin: U.S. 

Geological Survey Water-Supply Paper 1229, 96 p.

Freeze, R.A., and Witherspoon, P.A., 1967, Theoretical analysis of 

regional groundwater flow: 2. Effect of water table configuration 

and subsurface permeability variation: Water Resources 

Research, v. 3, no. 2, p. 623-634.

Gilkeson, R.H., Cartwright, Keros, Cowart, J.B., and Holtzman, 

R.B., 1983, Hydrogeologic and geochemical studies of selected 

natural radioisotopes and barium in groundwater in Illinois: University of Illinois Water Resources Center Report 83-0180, Illinois 

State Geological Survey Contract/Grant Report 1983-6, 93 p.

Gilkeson, R.H., Perry, E.G., Jr., Cowart, J.B., and, Holtzman, R.B., 

1984, Isotopic studies of the natural sources of radium in groundwater in Illinois: University of Illinois Water Resources Center 

Research Report 84-187, 50 p.

Graf, D.L., 1982, Chemical osmosis, reverse chemical osmosis, and the 

origin of subsurface brines: Geochimica et Cosmochimica Acta, 

v. 46, p. 1431-1448.

Gray, H.H., 1972, Lithostratigraphy of the Maquoketa Group (Ordovician) in Indiana: Indiana Geological Survey Special Report 7, 

31 p.

Guswa, J.H., Siegel, D.I., and Gillies, B.C., 1982, Preliminary evaluation of the ground-water-flow system in the Twin Cities metropolitan area, Minnesota: U.S. Geological Survey Water-Resources 

Investigations Report 82-44, 65 p.

Hahn, N.A., Jr., 1984, Radium in Wisconsin groundwater and removal 

methods for community water systems: Madison, Wisconsin 

Department of Natural Resources, 125 p.

Hanor, J.S., 1983, Fifty years of development of thought on the origin 

and evolution of subsurface sedimentary brines, in Boardman, 

S.J., ed., Revolution in the earth sciences: Dubuque, Iowa, 

Kendall/Hunt, 385 p.

Horick, P.J., 1984, Silurian-Devonian aquifer of Iowa: Iowa Geological

Survey Miscellaneous Map Series 10, 4 sheets. 

Horick, P.J., and Steinhilber, W.L., 1973, Mississippian aquifer of

Iowa: Iowa Geological Survey Miscellaneous Map Series 3,

3 sheets.

 1978, Jordan aquifer of Iowa: Iowa Geological Survey Miscellaneous Map Series 6, 3 sheets.

Horn, M.A., 1983, Ground-water-use trends in the Twin Cities metropolitan area, 1880-1980: U.S. Geological Survey Water-Resources 

Investigations Report 83-4033, 37 p.

Imes, J.L., 1985, The ground-water flow system in northern Missouri, 

with emphasis on the Cambrian-Ordovician aquifer: U.S. Geological Survey Professional Paper 1305, 61 p.

Jorgensen, D.G., and Signer, D.C., 1981, Plan of study for the Central 

Midwest Regional Aquifer-System Analysis in parts of Arkansas, 

Colorado, Kansas, Missouri, Nebraska, New Mexico, South 

Dakota, and Texas: U.S. Geological Survey Water-Resources 

Investigations Report 81-206, 28 p.

Kammerer, P.A., Jr., 1984, An overview of ground-water quality in 

Wisconsin: U.S. Geological Survey Water-Resources Investigations Report 83-4239, 58 p.

Kirk, J.R., Jarboe, Jacquelyn, Sanderson, E.W., Sasman, R.T., and 

Lonnquist, C.G., 1982, Water withdrawals in Illinois, 1980: Illinois 

State Water Survey Circular 152, 47 p.

Kolata, D.R., and Graese, A.M., 1983, Lithostratigraphy and depositional environments of the Maquoketa Group (Ordovician) in 

northern Illinois: Illinois State Geological Survey Circular 528, 

49 p.

Kontis, A.L., and Mandle, R.J., 1980, Data-base system for Northern 

Midwest Regional Aquifer-System Analysis: U.S. Geological Survey Water-Resources Investigations Report 80-104, 23 p.

 1988, Modifications of a three-dimensional ground-water flow 

model to account for variable water density and effects of multiaquifer wells: U.S. Geological Survey Water-Resources Investigations Report 87-4265, 78 p.

Krohelski, J.T., 1986, Hydrogeology and ground-water use and quality, Brown County, Wisconsin, with a section on bedrock geology 

by Bruce A. Brown: Wisconsin Geological and Natural History 

Survey Information Circular 57, 42 p.

Macaitis, Bill, Povilaitis, S.J., and Cameron, E.B., 1977, Lake Michigan diversion-stream quality planning: Water Resources Bulletin, 

v. 13, no. 4, p. 795-805.

Mandle, R.J., and Kontis, A.L., in press, Simulation of regional 

ground-water flow in the Cambrian-Ordovician aquifer system in 

the northern Midwest, United States: U.S. Geological Survey 

Professional Paper 1405-C.

McLeod, R.S., 1975a, A digital-computer model for estimating drawdowns in the sandstone aquifer in Dane County, Wisconsin: 

Wisconsin Geological and Natural History Survey Information 

Circular 28, 91 p.

 1975b, A digital-computer model for estimating hydrologic 

changes in the aquifer system in Dane County, Wisconsin: Wisconsin Geological and Natural History Survey Information Circular 30, 40 p.

Meents, W.F., Bell, A.H., Rees, O.W., and Tilbury, W.G., 1952, 

Illinois oil-field brines, their geologic occurrence and chemical 

composition: Illinois State Geological Survey, Illinois Petroleum 

No. 66, 38 p.

Nicholas, J.R., Sherrill, M.G., and Young, H.L., 1987, Hydrogeology 

of the Cambrian-Ordovician aquifer system at a test well in 

northeastern Illinois: U.S. Geological Survey Water-Resources 

Investigations Report 84-4165, 30 p.
A54 REGIONAL AQUIFER-SYSTEM ANALYSIS-NORTHERN MIDWEST

Ohio River Valley Water Sanitation Commission, 1976, Evaluation of 

the Ohio Valley region basal sandstone as a waste water injection 

interval: Cincinnati, Ohio River Valley Water Sanitation Commission, 30 p.

Ostrom, M.E., 1967, Paleozoic stratigraphic nomenclature for Wisconsin: Wisconsin Geological and Natural History Survey Information 

Circular 8, 1 sheet.

Parker, M.C., 1971, The Maquoketa Formation (Upper Ordovician) in 

Iowa: Iowa Geological Survey Miscellaneous Map Series 1, scale 

1:1,000,000, 6 sheets.

Prickett, T.A., and Lonnquist, C.G., 1971, Selected digital computer 

techniques for groundwater resource evaluation: Illinois State 

Water Survey Bulletin 55, 62 p.

Sasman, R.T., Benson, C.R., Ludwigs, R.S., and Williams, T.L., 1982, 

Water-level trends, pumpage, and chemical quality in the 

Cambrian-Ordovician aquifer in Illinois, 1971-1980: Illinois State 

Water Survey Circular 154, 64 p.

Schicht, R.J., Adams, R.J., and Stall, J.B., 1976, Water resources 

availability, quality, and cost in northeastern Illinois: Illinois State 

Water Survey Report of Investigations 83, 90 p.

Schoenberg, M.E., 1984, Water levels and water-level changes in the 

Prairie du Chien-Jordan and Mount Simon-Hinckley aquifers, 

Twin Cities metropolitan area, Minnesota, 1971-80: U.S. Geological Survey Water-Resources Investigations Report 83-4237, 23 p.

Scott, R.C., and Barker, F.B., 1961, Ground-water sources containing 

high concentrations of radium, in Geological Survey research 1961: 

U.S. Geological Survey Professional Paper 424-D, p. D357-D359.

Shufeldt, G.A., Jr., 1865, History of the Chicago artesian well: 

Chicago, Religio-Philosophical Publishing Assn., p. 1-20 (publ. in 

1897).

Siegel, D.I., 1989, Geochemistry of the Cambrian-Ordovician aquifer 

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

Survey Professional Paper 1405-D, 76 p.

Siegel, D.I., and Begor, K.F., 1989, The geochemistry of the sandstone 

aquifer, southern Wisconsin, in Swain, L.A., and Johnson, A.I., 

eds., Regional aquifer systems of the United States Aquifers of 

the midwestern area: American Water Resources Association 

Monograph 13, p. 73-82.

Steinhilber, W.L., and Young, H.L., 1979, Plan of study for the 

Northern Midwest Regional Aquifer-System Analysis: U.S. Geological Survey Water-Resources Investigations Report 79-44, 

20 p.

Suter, Max, Bergstrom, R.E., Smith, H.F., Emrich, G.H., Walton, 

W.C., and Larson, T.E., 1959, Preliminary report on groundwater 

resources of the Chicago region, Illinois: Illinois State Water 

Survey and State Geological Survey Cooperative Groundwater 

Report 1, 89 p.

Taylor, R.W., Butler, D.R., and Jansen, John, 1987, Geophysical 

logging of sandstone aquifers in Brookfield, Wisconsin [abs.]: 

Midwest Ground Water Conference annual meeting, 32d, Madison, 

Wis., October 28-30, 1987, Program with Abstracts, unpaginated.

Toth, J., 1962, A theory of ground water motion in small drainage 

basins in central Alberta, Canada: Journal of Geophysical 

Research, v. 67, no. 11, p. 4375-4387.

Trescott, P.C., 1975, Documentation of finite-difference model for 

simulation of three-dimensional ground-water flow: U.S. Geological Survey Open-File Report 75-438, 32 p.

U.S. Environmental Protection Agency, 1986, Maximum contaminant 

levels (subpart B of part 141, National interim primary drinkingwater regulations): U.S. Code of Federal Regulations, Title 40, 

Parts 100 to 149, revised as of July 1, 1986, p. 524-528.

U.S. Supreme Court, 1967, Wisconsin et al. v. Illinois et al.: U.S. 

Reports, v. 388, p. 426, June 12, 1967.

Visocky, A.P., 1982, Impact of Lake Michigan allocations on the 

Cambrian-Ordovician aquifer system: Illinois State Water Survey 

Contract Report 292, 36 p.

Visocky, A.P., Sherrill, M.G., and Cartwright, Keros, 1985, Geology, 

hydrology, and water quality of the Cambrian and Ordovician 

systems in northern Illinois: Illinois State Geological Survey, 

Illinois State Water Survey, and U.S. Geological Survey Cooperative Groundwater Report 10, 136 p.

Walton, W.C., and Csallany, Sandor, 1962, Yields of deep sandstone 

wells in northern Illinois: Illinois State Water Survey Report of 

Investigations 43, 47 p.

Weidman, Samuel, and Schultz, A.R., 1915, The underground and 

surface water supplies of Wisconsin: Wisconsin Geological and 

Natural History Survey Bulletin 35, 664 p.

Willman, H.B., Atherton, Elwood, Buschbach, T.C., Collinson, 

Charles, Frye, J.C., Hopkins, M.E., Lineback, J.A., and Simon, 

J.A., 1975, Handbook of Illinois stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.

Willman, H.B., and Kolata, D.R., 1978, The Platteville and Galena 

Groups in northern Illinois: Illinois State Geological Survey Circular 502, 75 p.

Young, H.L., 1976, Digital-computer model of the sandstone aquifer in 

southeastern Wisconsin: Southeastern Wisconsin Regional Planning Commission (Waukesha) Technical Report 16, 42 p.

 in press, Hydrogeology of the Cambrian-Ordovician aquifer 

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

Survey Professional Paper 1405-B.

Young, H.L., MacKenzie, A.J., and Mandle, R.J., 1989, Simulation of 

ground-water flow in the Cambrian-Ordovician aquifer system in 

the Chicago-Milwaukee area of the northern Midwest, in Swain, 

L.A., and Johnson, A.I., eds., Regional aquifer systems of the 

United States Aquifers of the midwestern area: American Water 

Resources Association Monograph 13, p. 39-72.

PROJECT PUBLICATIONS

The following publications are a result of the Northern 

Midwest Regional Aquifer-System Analysis.

ABSTRACTS

Alexander, E.G., Jr., Siegel, D.I., and Milske, J.A., 1983, Isotopic 

studies of the Mt. Simon aquifer, southern Minnesota: American 

Geophysical Union Transactions, EOS, v. 64, no. 18, p. 225.

Franz, K.E., and Siegel, D.I., 1984, Geochemical trends in the sandstone aquifer, eastern Wisconsin: American Geophysical Union 

Transactions, EOS, v. 65, no. 16, p. 212.

Shedlock, R.J., and Eikenberry, S.E., 1982, Saline formation waters in 

the Cambrian-Ordovician aquifer in northwest Indiana: Geological 

Society of America, North-Central Section, annual meeting, 16th, 

West Lafayette, Ind., 1982, Abstracts with Programs, v. 15, no. 5, 

p. 287.

Siegel, D.I., 1983, Sulfur isotope chemistry, Cambrian-Ordovician 

aquifer, North-Central United States: American Geophysical 

Union Transactions, EOS, v. 64, no. 45, p. 704.

 1987, Geochemical evidence for possible dedolomitization in the 

sandstone aquifer, eastern Wisconsin: Midwest Ground Water 

Conference annual meeting, 32d, Madison, Wis., October 28-30, 

1987, Program with Abstracts, unpaginated.

Siegel, D.I., and Franz, K.E., 1984, Isotopic studies of the sandstone 

aquifer, eastern Wisconsin: Geological Society of America annual 

meeting, 97th, Reno, Nev., 1984, Abstracts with Programs, v. 16, 

no. 6, p. 655.
PROJECT PUBLICATIONS A55

Siegel, D.I., Kontis, A.L., and Handle, R.J., 1982, The chemical 

evolution of water quality in the Cambrian-Ordovician aquifer, 

North-Central United States: American Geophysical Union Transactions, EOS, v. 63, no. 33, p. 612.

Siegel, D.I., and Handle, R.J., 1982, Geochemical evidence of Pleistocene glacial meltwater recharge to the Cambrian-Ordovician aquifer in the North-Central United States: American Geophysical 

Union Transactions, EOS, v. 63, no. 18, p. 325-326.

 1983, Geochemical evidence for glaciation of the driftless area, 

southwestern Wisconsin: Geological Society of America annual 

meeting, 96th, Indianapolis, Ind., 1983, Abstracts with Programs, 

v. 15, no. 6, p. 687.

Signer, D.C., Kontis, A.L., Jorgensen, D.G., and Handle, R.J., 1980, 

Ground-water flow regime based on preliminary data for regional 

aquifer systems: American Geophysical Union Transactions, EOS, 

v. 61, no. 46, p. 952.

Woodward, D.G., and Delin, G.N., 1982, Potentiometric surface of 

regional aquifers in the Hollandale embayment, southeastern 

Hinnesota: American Geophysical Union Transactions, EOS, v. 

63, no. 33, p. 612.

Young, H.L., and HacKenzie, A.J., 1987, The Cambrian-Ordovician 

aquifer system in the northern Hidwest A regional study: Hidwest Ground Water Conference annual meeting, 32d, Hadison, 

Wis., October 28-30, 1987, Program with Abstracts, unpaginated.

REPORTS

Barnes, H.J., 1985, The extent and behavior of the mineralized water 

in the Ht. Simon Formation, northeastern Illinois: H.S. thesis, 

Northern Illinois University (De Kalb), 127 p.

Bennett, G.D., Kontis, A.L., and Larson, S.P., 1982, Representation 

of multiaquifer well effects in three-dimensional ground-water flow 

simulation: Ground Water, v. 20, no. 3, p. 334-341.

Burkart, H.R., and Buchmiller, R.C., in press, Regional aquifer 

evaluation of hydrologic factors and effects of pumping, St. PeterJordan aquifer, Iowa: U.S. Geological Survey Water-Resources 

Investigations Report 90-4009.

Delin, G.N., and Woodward, D.G., 1984, Hydrogeologic setting and 

the potentiometric surfaces of regional aquifers in the Hollandale 

embayment, southeastern Hinnesota, 1970-80: U.S. Geological 

Survey Water-Supply Paper 2219, 56 p.

Emmons, P.J., 1987, An evaluation of the bedrock aquifer system in 

northeastern Wisconsin: U.S. Geological Survey Water-Resources 

Investigations Report 85-4199, 48 p.

Fassnacht, T.L., 1982, A seismic reflection study of the Precambrian 

basement along the Illinois-Wisconsin State line: H.S. thesis, 

Northern Illinois University (De Kalb), 103 p.

Franz, K.E., 1985, Geochemistry of the sandstone and Silurian aquifers 

in eastern Wisconsin: H.S. thesis, Syracuse University (Syracuse, 

N.Y.), 103 p.

Horn, H.A., 1983, Ground-water-use trends in the Twin Cities metropolitan area, Hinnesota, 1880-1980: U.S. Geological Survey 

Water-Resources Investigations Report 83-4033, 37 p.

Imes, J.L., 1985, The ground-water flow system in northern Hissouri, 

with emphasis on the Cambrian-Ordovician aquifer: U.S. Geological Survey Professional Paper 1305, 61 p.

Kontis, A.L., and Handle, R.J., 1980, Data-base system for Northern 

Hidwest Regional Aquifer-System Analysis: U.S. Geological Survey Water-Resources Investigations Report 80-104, 23 p.

 1988, Hodifications of a three-dimensional ground-water flow 

model to account for variable water density and effects of multiaquifer wells: U.S. Geological Survey Water-Resources Investigations Report 87^265, 78 p.

Handle, R.J., and Kontis, A.L., in press, Simulation of regional 

ground-water flow in the Cambrian-Ordovician aquifer system in 

the northern Hidwest, United States: U.S. Geological Survey 

Professional Paper 1405-C.

Hossler, J.H., 1983a, Bedrock topography and isopachs of Cretaceous 

and Quaternary strata, east-central and southeastern Hinnesota: 

Hinnesota Geological Survey Hiscellaneous Hap Series H-52, 

scale 1:500,000, 2 pis.

 1983b, Paleozoic lithostratigraphy of southeastern Hinnesota: 

Hinnesota Geological Survey Hiscellaneous Hap Series H-51, 

scale 1:500,000, 8 pis.

Nicholas, J.R., Sherrill, H.G., and Young, H.L., 1987, Hydrogeology 

of the Cambrian-Ordovician aquifer system at a test well in 

northeastern Illinois: U.S. Geological Survey Water-Resources 

Investigations Report 84-4165, 30 p.

Siegel, D.I., 1989, Geochemistry of the Cambrian-Ordovician aquifer 

system in the northern Hidwest, United States: U.S. Geological 

Survey Professional Paper 1405-D, 76 p.

Siegel, D.I., and Begor, K.F., 1989, The geochemistry of the sandstone 

aquifer, southern Wisconsin, in Swain, L.A., and Johnson, A.I., 

eds., Regional aquifer systems of the United States Aquifers of 

the midwestern area: American Water Resources Association 

Monograph 13, p. 73-82.

Siegel, D.I., and Handle R.J., 1984, Isotopic evidence for glacial 

meltwater recharge to the Cambrian-Ordovician aquifer, NorthCentral United States: Journal of Quaternary Research, v. 22, 

p. 328-335.

Steinhilber, W.L., and Young, H.L., 1979, Plan of study for the 

Northern Hidwest Regional Aquifer-System Analysis: U.S. Geological Survey Water-Resources Investigations Report 79-44, 

20 p.

Visocky, A.P., Sherrill, H.G., and Cartwright, Keros, 1985, Geology, 

hydrology, and water quality of the Cambrian and Ordovician 

systems in northern Illinois: Illinois State Geological Survey, 

Illinois State Water Survey, and U.S. Geological Survey Cooperative Groundwater Report 10, 136 p.

Woodward, D.G., 1984, Areal lithologic changes in aquifers in southeastern Hinnesota as determined from natural-gamma borehole 

logs: National Water Well Association Conference on Surface and 

Borehole Geophysical Hethods in Ground Water Investigations, 

San Antonio, Tex., February 7-9, 1984, Proceedings, p. 788-800.

 1985, Trends in municipal-well installations and aquifer utilization in southeastern Hinnesota, 1880-1980: U.S. Geological Survey 

Water-Resources Investigations Report 83-4222, 99 p.

-1986, Hydrogeologic framework and properties of regional

aquifers in the Hollandale embayment, southeastern Hinnesota: 

U.S. Geological Survey Hydrologic Investigations Atlas 677, 

2 sheets.

Young, H.L., in press, Hydrogeology of the Cambrian-Ordovician 

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

Geological Survey Professional Paper 1405-B.

Young, H.L., HacKenzie, A.J., and Handle, R.J., 1989, Simulation of 

ground-water flow in the Cambrian-Ordovician aquifer system in 

the Chicago-Hilwaukee area of the northern Hidwest, in Swain, 

L.A., and Johnson, A.I., eds., Regional aquifer systems of the 

United States Aquifers of the midwestern area: American Water 

Resources Association Honograph 13, p. 39-72.

Young, H.L., Handle, R.J., Kontis, A.L., and Siegel, D.I., 1989, The 

Cambrian-Ordovician regional aquifer system in the northern 

Hidwest A summary, in Swain, L.A., and Johnson, A.I., eds., 

Regional aquifer systems of the United States Aquifers of the 

midwestern area: American Water Resources Association Honograph 13, p. 5-37.



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