.S. Department of the Interior 

.S. Geological Survey

Regional Ground-Water Flow and Geochemistry 

in the Midwestern Basins and Arches Aquifer System 

in Parts of Indiana, Ohio, Michigan, and Illinois

Regional Aquifer-System Analysis

'rofessional Paper 1423-C

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Regional Ground-Water Flow and 

Geochemistry in the Midwestern 

Basins and Arches Aquifer System 

in Parts of Indiana, Ohio, Michigan, 

and Illinois

By SANDRA M. EBERTS awdLORI L. GEORGE

REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1423-C
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, 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.

Reston, Virginia 2000

Library of Congress Cataloging-in-Publication Data

Eberts, Sandra M.

Regional ground-water flow and geochemistry in the Midwestern basins and arches 

aquifer system in parts of Indiana, Ohio, Michigan, and Illinois / Sandra M. Eberts and 

Lori L. George.

p. cm. (Regional aquifer-system analysis midwestern basins and arches) (U.S. 

Geological Survey professional paper; 1423-C) 

Includes bibliographical references. 

ISBN 0-607-92669-4

1. Groundwater flow Middle West. 2. Water chemistry Middle West. I. George, Lori 

L. II. Title. III. Series. IV. Series: U.S. Geological Survey professional paper ; 1423-C

GB1197.7.E23 2000 

551.49'0977 dc21

For sale by U.S. Geological Survey, Branch of Information Services, Box 25286,

Federal Center, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (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 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 

beginning with Professional Paper 1400.

Charles G. Groat 

Director

CONTENTS

	Page

Forward......................................................................................... Ill

Abstract......................................................................................... Cl

Introduction................................................................................... Cl

Purpose and scope................................................................... C2

Approach.................................................................................. C2

Acknowledgments.................................................................... C2

Geohydrology................................................................................. C3

Geologic setting....................................................................... C3

Hydrologic setting................................................................... CIO

Ground-water use ............................................................. CIO

Aquifers and confining units ............................................ C12

Ground water........................................................................... C13

Hydraulic characteristics ................................................. C13

Levels................................................................................. C15

Recharge............................................................................ C17

Discharge........................................................................... C17

Regional ground-water flow.......................................................... C26

Conceptual model.................................................................... C26

Numerical model..................................................................... C28

Model design...................................................................... C28

Discrete geohydrologic framework............................. C29

Boundaries, sources, and sinks.................................. C34

No-flow boundaries............................................... C34

Specified-head boundaries.................................... C35

Specified-flux boundary......................................... C35

Head-dependent flux boundaries......................... C35

Approach to mapping regional recharge and

discharge................................................................ C35

Parameterization......................................................... C36

	Page

Model calibration............................................................... C37

Procedure..................................................................... C37

Estimates of parameter values................................... C40

Simulated hydraulic heads......................................... C41

Simulated flows........................................................... C43

Model discrimination.................................................. C45

Reliability of parameter estimates............................. C46

Simulated regional ground-water flow................................... C47

Geochemistry................................................................................. C56

Spatial distribution of dissolved-solids concentrations and

hydrochemical fades......................................................... C58

Dissolved-solids concentrations........................................ C58

Hydrochemical facies........................................................ C66

Relation between dissolved-solids concentrations and

hydrochemical facies................................................... C73

Geochemical and hydrologic processes, by water type.......... C73

Calcium-magnesium-bicarbonate water.......................... C73

Calcium-magnesium-sulfate water.................................. C75

Sodium-enriched water..................................................... C80

Sodium-chloride and sodium-calcium-chloride water..... C81

Insights into regional flow provided by geochemical data.... C82

Major solutes..................................................................... C82

Isotopes.............................................................................. C82

Sulfur isotopes............................................................. C85

Oxygen and hydrogen isotopes................................... C88

Tritium and carbon isotopes....................................... C88

Variation in ground-water chemistry with depth............ C94

Summary and conclusions............................................................ C95

References cited............................................................................. C98

ILLUSTRATIONS

FIGURES 1,2. Maps showing:

1. Location of study and modeled areas...................................................................................................................... C4

2. Generalized bedrock geology and location of geohydrologic sections A-A' and B-B'............................................ C5

3. Generalized sections A-A' and B-B' showing geologic and hydrologic units that compose the Midwestern Basins and

Arches aquifer system......................................................................................................................................................... C7

4-7. Maps showing:

4. Thickness of carbonate rocks of Silurian and Devonian age in the Midwestern Basins and Arches Region...... C8

5. Generalized surficial geology and location of geologic section C-C'....................................................................... C9

6. Shaded relief of land-surface topography (geomorphology) in the Midwestern Basins and Arches Region,

from digital data................................................................................................................................................ CIO

7. Generalized thickness of glacial deposits in the Midwestern Basins and Arches Region.................................... Cll

8. Generalized hydrogeologic section C-C' showing typical relation between glacial aquifers and glacial confining units

in the Midwestern Basins and Arches aquifer system...................................................................................................... C12

9-14. Maps showing:

9. Transmissivity of glacial aquifers............................................................................................................................ C14

10. Transmissivity of the carbonate-rock aquifer......................................................................................................... C16

11. Composite regional potentiometric surface in glacial deposits constructed from water levels on drillers'

logs.......................................................................................................................................^ CIS

12. Regional potentiometric surface in the carbonate-rock aquifer, July 1990, and general directions of regional

ground-water flow.............................................................................................................................................. C19

13. Estimated ground-water discharge to selected stream reaches for long-term steady-state conditions in the

Midwestern Basins and Arches aquifer system............................................................................................... C20

14. Mean ground-water discharge as a percentage of mean streamflow, and mean sustained ground-water

discharge as a percentage of mean ground-water discharge for selected stream reaches............................ C21
VI CONTENTS

FIGURE 15. Diagrams showing the effect of recharge from precipitation on the configuration of a water table and associated

ground-water flow................................................................................................................................................................... C23

16. Graph showing base-flow duration curves for various ground-water recharge conditions for a streamflow-gaging

station on the Kankakee River.............................................................................................................................................. C24

17. Map showing relative amounts of mean ground-water discharge and mean sustained ground-water discharge to

streams in selected surface-water drainage basins ............................................................................................................. C25

18. Diagrammatic conceptual model of the Midwestern Basins and Arches aquifer system showing flow paths associated 

with local, intermediate, and regional flow systems and flow systems simulated by the regional ground-water 

flow model............................................................................................................................................................................... C27

19. Diagram showing effect of model-cell spacing (model scale) on the amount of flow in an aquifer system that can be

simulated with a numerical model........................................................................................................................................ C29

20, 21. Maps showing:

20. Density of perennial streams that drain the modeled area at the scale of 1:100,000 and streams that are

explicitly represented in the regional ground-water flow model by use of stream cells .................................. C30

21. Model layers used to simulate the Midwestern Basins and Arches aquifer system, and the location of model

sections D-D' andE-E': areal extent and boundaries....................................................................................... C31

22. Sectional diagrams showing the areal extent and boundaries of the model layers used to simulate the Midwestern

Basins and Arches aquifer system........................................................................................................................................ C33

23, 24. Maps showing:

23. Zones used for parameterization: model layer 1 and model layer 2......................................................................... C38

24. Simulated and measured (observed) hydraulic heads in the carbonate-rock aquifer and the upper

weathered zone water-bearing unit (model layer 2)........................................................................................... C42

25. Graph showing weighted residuals of hydraulic heads and flows plotted against weighted simulated values from the

regional ground-water flow model for the Midwestern Basins and Arches aquifer system.............................................. C43

26. Map showing simulated ground-water discharge to selected stream reaches from regional flow systems and estimated

mean sustained ground-water discharge (observed flow) to the reaches............................................................................ C44

27. Diagrams of two regional ground-water flow models used to test whether the carbonate rocks in the Midwestern

Basins and Arches Region are transmissive throughout their entire thickness................................................................ C46

28-36. Maps showing:

28. Simulated regional recharge and discharge areas.................................................................................................... C50

29. Percentages of the surface area of selected surface-water drainage basins simulated as regional recharge

and discharge areas ............................................................................................................................................. C52

30. Simulated patterns of advective regional flow in the Midwestern Basins and Arches aquifer system................. C53

31. Simulated relative magnitudes of horizontal regional flow in the Midwestern Basins and Arches aquifer

system: glacial deposits and bedrock.................................................................................................................. C54

32. Locations of wells sampled during this investigation, carbonate-rock and glacial cores sampled for isotopic

analysis, and geochemical sections A-A',B-B', C-C', andD-D'.... .................................................................... C57

33. Dissolved-solids distribution in ground-water from the glacial aquifers in the Midwestern Basins and

Arches aquifer system.......................................................................................................................................... C64

34. Dissolved-solids distribution in ground-water from the carbonate-rock aquifer in the Midwestern Basins

and Arches aquifer system................................................................................................................................... C65

35, 36. Trilinear diagrams showing:

35. Water chemistry of the Midwestern Basins and Arches aquifer system, categorized by dissolved-solids 

concentration: glacial aquifers and carbonate-rock aquifer where it does not underlie the upper 

confining unit....................................................................................................................................................... C67

36. Water chemistry in the carbonate-rock aquifer of the Midwestern Basins and Arches aquifer system where

it underlies the upper confining unit, categorized by structural basin............................................................. C69

37, 38: Maps showing:

37. Hydrochemical facies of ground water from the glacial aquifers in the Midwestern Basins and Arches

aquifer system...................................................................................................................................................... C71

38. Hydrochemical facies of ground water from the carbonate-rock aquifer in the Midwestern Basins and

Arches aquifer system.......................................................................................................................................... C72

39^6: Graphs showing:

39. Concentration of calcium plus magnesium as a function of bicarbonate concentration in ground water from

the glacial aquifers in the Midwestern Basins and Arches aquifer system...................................................... C74

40. Concentration of calcium plus magnesium as a function of bicarbonate concentration in ground water from

the carbonate-rock aquifer in the Midwestern Basins and Arches aquifer system.......................................... C75

41. Saturation indices of calcite and dolomite as a function of dissolved-solids concentration for water from

wells in the Midwestern Basins and Arches aquifer system............................................................................. C76

42. Concentration of calcium plus magnesium as a function of concentration of sulfate plus one-half bicarbonate

in ground water from the glacial aquifers in the Midwestern Basins and Arches aquifer system................. C77

43. Concentration of calcium plus magnesium as a function of concentration of sulfate plus one-half bicarbonate

in ground water from the carbonate-rock aquifer in the Midwestern Basins and Arches aquifer system..... C78
CONTENTS VII

Page

FIGURES 39-46. CONTINUED

44. Saturation indices of gypsum as a function of dissolved-sulfate concentration for water from wells in

the Midwestern Basins and Arches aquifer system.............................................................................. C79

45. Dissolved-solids concentration in water from wells in the Midwestern Basins and Arches aquifer

system along geochemical sections A-A', B-B', C-C', and D-D'................................................................ C83

46. Sulfate concentration in water from wells in the Midwestern Basins and Arches aquifer system along

geochemical sections A-A', B-B', C-C', and D-D'....................................................................................... C84

47, 48. Maps showing:

47. 834S and A34S values in water from wells in the Midwestern Basins and Arches aquifer system................. C86

48. 8 18O values in water from wells in the Midwestern Basins and Arches aquifer system............................... C89

49. 8 18O - 8D relation for ground water in the Midwestern Basins and Arches aquifer system............................................ C90

50. Tritium concentrations and carbon-14 activities in water from wells in the Midwestern Basins and Arches aquifer

system.............................................................................................................................................................. C91

TABLES

Page

TABLE 1. Relation between geologic and hydrologic units of the Midwestern Basins and Arches aquifer system and model layers

used to simulate regional flow within the aquifer system...................................................................................................... C6

2. Summary of hydraulic characteristics of aquifers and confining units in the Midwestern Basins and Arches aquifer

system.................................................................................................................................................................... CIS

3. Parameter estimates and reliability of the optimal parameter set from the calibrated final model of regional flow in the

Midwestern Basins and Arches aquifer system....................................................................................................................... C41

4. Simulated ground-water budget of regional flow systems in the Midwestern Basins and Arches aquifer system.................... C48

5. Simulated ground-water budget of regional flow systems in the carbonate-rock aquifer in the Midwestern Basins and

Arches aquifer system............................................................................................................................................................... C48

6. Well-construction information for wells sampled during investigation of the Midwestern Basins and Arches aquifer

system, 1991-92....................................................................................................................................................... C59

7. Chemical analyses of selected constituents in water from wells sampled during investigation of the Midwestern Basins

and Arches aquifer system, 1991-92............................................................................................................................. C60

8. Analyses of isotopes in water from wells sampled during investigation of the Midwestern Basins and Arches aquifer

system, 1991-92 ....................................................................................................................................................... C62

9. Analyses of sulfur and carbon isotopes in carbonate rocks and glacial deposits from the Midwestern Basins and Arches

Region ................................................................................................................................................................^ C63

10. Major water types, their specific cation and anion compositions, and their abundance in the glacial and carbonate-rock

aquifers in the Midwestern Basins and Arches aquifer system......................................................................................... C70

11. Summary of 14C age estimates for ground water sampled during investigation of the Midwestern Basins and Arches

aquifer system, 1991-92............................................................................................................................................................ C93

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATED WATER-QUALITY UNITS

Multiply inch-pound unit

inch (in)

foot (ft)

mile (mi)

square mile (mi 2 )

inch per year (in/yr)

foot per day (ft/d)

foot squared per day (ft2/d)

cubic feet per second (ft3/s)

gallon per minute (gal/min)

gallon per day (gal/d)

million gallons per day (Mgal/d)

By

25.4

0.3048

1.609

2.590

25.4

0.3048

0.09290

28.32

0.06309

4.381 x 1Q-8

0.04381

To obtain SI unit

millimeter

meter

kilometer

square kilometer

millimeter per year

meter per day

meter squared per day

liter per second

liter per second

cubic meter per second

cubic meter per second
VIII CONTENTS

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".

Water-quality units used in this report: Concentrations of dissolved consituents are given in milligrams per liter 

(mg/L) or micrograms per liter (|ig/L), units expressing the concentration of chemical constituents as mass of solute 

per unit volume of water.
REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

REGIONAL GROUND-WATER FLOW AND

GEOCHEMISTRY IN THE MIDWESTERN BASINS AND

ARCHES AQUIFER SYSTEM IN PARTS OF INDIANA,

OHIO, MICHIGAN, AND ILLINOIS

BY SANDRA M. EBERTS AND LORI L. GEORGE

ABSTRACT

This report synthesizes information on the regional ground-water flow 

and geochemistry in the Midwestern Basins and Arches aquifer system in 

parts of Indiana, Ohio, Michigan, and Illinois. Aquifers that compose this 

water-table aquifer system include glacial aquifers and an underlying, areally 

extensive carbonate-rock aquifer.

Water within the aquifers is most commonly a Ca-Mg-HCO3 type or a 

Ca-Mg-SO4 type. In general, the distribution of hydrochemical facies within 

the aquifer system is controlled by the mineralogy of the aquifer material, 

rather than by a chemical evolution of water along general directions of 

regional ground-water flow.

Some ground-water flow systems within the aquifer system provide base 

flow to streams in response to ground-water recharge events. Other (often 

deeper) ground-water flow systems respond minimally to variations in groundwater recharge from precipitation and provide a fairly constant supply of water 

to streams. Streamflow hydrographs and base-flow duration curves were used 

to estimate such components of base flow in selected streams for long-term 

steady-state conditions in the aquifer system. Mean sustained ground-water 

discharge (discharge from fairly stable ground-water flow systems) ranges 

from 3 to 50 percent of mean ground-water discharge (discharge from all 

ground-water flow systems) to the selected stream reaches. These percentages 

indicate that 50 to 97 percent of base flow in the streams within the study area 

can be attributed to transient ground-water flow systems, which typically have 

a major component of local-scale flow. Because ground-water flow across the 

external boundaries of the aquifer system is minimal, such percentages indicate that most ground-water flow in the aquifer system is associated with seasonally transient local flow systems.

Results of a ground-water flow model that was calibrated by use of 

regression methods and that simulates regional flow systems within the aquifer 

system (approximately 10 percent of total ground-water flow in the aquifer 

system) indicate that most water (99 percent) in simulated regional flow systems is from recharge at the water table. Most water (78 percent) discharges 

from simulated regional flow systems to the principal streams. Less than 3 percent of water in simulated regional flow systems discharges to the Ohio River, 

Lake Erie, or downdip areas in the Illinois (structural) Basin.

Simulations also indicate that most of the Midwestern Basins and Arches 

aquifer system is characterized by alternating regional recharge and discharge 

areas at intervals of less than every 10 miles along the dominant regional 

trends of the potentiometric surfaces in the aquifers. Such alternating regional 

recharge and discharge areas result in the absence of long flow paths from the 

very highest regional potentiometric levels to the very lowest regional potentiometric levels. The presence of tritiated ground water (less than 50 years old)

across most of the aquifer system also indicates that the aquifer system 

receives recharge across most of the study area.

The northeastern part of the aquifer system near Lake Erie differs from 

the rest of the system with respect to regional ground-water flow and chemistry. Specifically, part of the northeastern part of the aquifer system can be 

characterized as a broad area (tens of miles) of weak regional discharge (less 

than 0.5 inch per year). Results of the regional ground-water flow model indicate that regional flow systems have a limited ability to carry ground water 

away from this area; thus precipitation is prevented from recharging the 

regional flow systems in this part of the aquifer system. Some ground water 

recharged during Pleistocene glaciation was found in this area. Sulfide concentrations and sulfur isotope data, which indicate that extensive sulfate reduction has occurred in the aquifer system within this area, confirm that only 

minimal recharge of this part of the aquifer system has taken place over a long 

period of time.

The longest simulated ground-water flow paths within the aquifer system 

(nearly 50 miles) were also identified in the northeastern part of the aquifer 

system. These flow paths terminate at Lake Erie. On the basis of carbon-14 

data, some of the oldest waters (approximately 13,000 years) were found at 

the downgradient end of these particularly long flow paths. The area near these 

flow paths is the only area within the aquifer system in which a systematic 

increase in ground-water ages was observed in the general direction of 

regional ground-water flow. Not all of the regional flow paths in the northeastern part of the aquifer system are particularly long. The very oldest waters that 

were found in the aquifer system (approximately 38,000-45,000 years) are 

associated with comparably short ground-water flow paths (approximately 10 

miles). These waters are present beneath the Maumee River Basin and indicate 

that parts of the aquifer system beneath the Maumee River Basin are fairly 

stagnant.

INTRODUCTION

Quaternary glacial deposits and underlying Devonian and 

Silurian carbonate rock in parts of Indiana, Ohio, Michigan, 

and Illinois compose the Midwestern Basins and Arches aquifer system investigated as part of the Regional Aquifer-System Analysis (RASA) program of the U.S. Geological Survey 

(USGS) (Sun and Johnston, 1994). Objectives of the Midwestern Basins and Arches RASA project were to describe

Cl
C2 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

the geohydrology, ground-water flow, and geochemistry in 

this aquifer system (Bugliosi, 1990). Large subregions of the 

Midwestern Basins and Arches aquifer system were previously investigated (Ohio Department of Natural Resources, 

Division of Water, 1970; Norris and Fidler, 1973; Bloyd, 

1974; Weist, 1978; Indiana Department of Natural Resources, 

1988, 1990); these investigations, however, were limited in 

areal extent by political boundaries (state lines) or surfacewater drainage divides. The RASA program provided an 

opportunity to synthesize earlier work and to study the full 

areal extent of the aquifer system.

Principal results of the Midwestern Basins and Arches 

RASA project are presented in Professional Paper 1423. 

Chapter A is a summary of the aquifer system. Chapter B 

describes the geohydrologic framework of the aquifer system. 

Chapter C (this chapter) describes regional ground-water flow 

and geochemistry in the aquifer system.

PURPOSE AND SCOPE

This report presents information on the geohydrology of 

and regional flow and geochemistry in the Midwestern Basins 

and Arches aquifer system in parts of Indiana, Ohio, Michigan, and Illinois. Specific report objectives are to describe (1) 

physical and hydraulic boundaries of the aquifer system; (2) 

regional relations between surface water and ground water; 

(3) a numerical model used to compute a regional groundwater budget, regional recharge and discharge areas, and patterns of regional ground-water flow; and (4) ground-water 

chemistry in relation to these patterns of regional flow. In the 

"Summary and Conclusions" section of the report, the hydrological and geochemical information are integrated to present 

a comprehensive analysis of the occurrence and flow of water 

in the aquifer system.

Regional flow, as defined for this report, is ground-water 

flow associated with flow systems that are minimally affected 

by seasonal variations in ground-water recharge from precipitation. Although local-scale flow systems that readily respond 

to variations in ground-water recharge are numerous and 

important throughout the aquifer system, it was outside the 

scope of the RASA project to investigate such flow systems. 

Specifically, local flow systems are not explicitly simulated 

by the numerical ground-water flow model.

APPROACH

A steady-state quasi-three-dimensional finite-difference 

ground-water flow model (numerical model) was constructed, 

calibrated, and used to describe regional flow in the Midwestern Basins and Arches aquifer system. Before this regional 

ground-water flow model was constructed, calibration targets 

were established. These calibration targets are based on measured ground-water levels and estimated ground-water discharge from regional flow systems, as approximated from 

measured streamflows, and were used to judge how well the 

numerical model simulated field conditions in the aquifer system. The numerical model (a mathematical representation of 

regional flow in the aquifer system) was built from a conceptual model (a simplified description of the aquifer system). 

The conceptual model was developed by synthesizing information on the thickness and areal extent of the aquifers and 

confining units within the aquifer system, as well as information on hydraulic characteristics, boundary conditions, flow 

regimes, sources and sinks of water, and general directions of 

ground-water flow. A geographic information system (GIS) 

was used to prepare a geographically referenced data base of 

this information, which was then converted for input into the 

numerical model by use of an interface program (Van Metre, 

1990). Available computer codes MODFLOW (McDonald 

and Harbaugh, 1988) and MODFLOWP (Hill, 1992) were 

used to construct and calibrate the numerical model on the 

basis of a nonlinear-regression method developed by Cooley 

and Naff (1990). Nonlinear regression was used to automatically adjust parameter values that represent hydraulic characteristics of the aquifers and confining units, as well as annual 

recharge, so that the calibration targets were matched as 

closely as possible. Aspects of the model other than the 

parameter values were calibrated by trial and error adjustment. A postprocessing routine (Harbaugh, 1990) was used to 

help quantify ground-water budgets. Output from the calibrated numerical model was also used in conjunction with the 

GIS to map regional recharge and discharge areas. The particle-tracking program MODPATH (Pollock, 1989) was used to 

determine and illustrate simulated advective regional groundwater flow patterns. An additional postprocessing routine was 

used to map discharge vectors (Scott, 1990).

Geochemistry data were used to describe the relations 

among ground-water chemistry, aquifer mineralogy, and 

present and past patterns of regional flow in the Midwestern 

Basins and Arches aquifer system. The spatial variability in 

the concentrations of major solutes in the ground water was 

evaluated and related to the source aquifers and general patterns of regional ground-water flow. Chemical and isotopic 

analyses of ground water and aquifer material along general 

directions of regional ground-water flow were used to evaluate the important hydrologic and geochemical processes controlling ground-water chemistry and to qualitatively estimate 

ground-water ages.

ACKNOWLEDGMENTS

The authors gratefully acknowledge numerous landowners for their cooperation in allowing water samples and other 

hydrologic information to be collected on their properties. In 

addition, the authors thank Frank Voss, Robert Hanover, and
GEOHYDROLOGY C3

Anthony Robinson, who helped with many aspects of the 

work presented herein.

GEOHYDROLOGY

The area of principal hydrologic interest of the Midwestern Basins and Arches RASA project encompasses approximately 44,000 mi , most of which is in the Midwestern 

Basins and Arches Region as defined in Shaver (1985). 

Boundaries of this study area (fig. 1) are coincident with the 

contact between Devonian limestones and younger Devonian 

shales (fig. 2) or surface-water bodies.

GEOLOGIC SETTING

The Midwestern Basins and Arches aquifer system generally lies between the Appalachian, the Illinois, and the Michigan (structural) Basins and is located along the axes of the 

Cincinnati, the Findlay. and the Kankakee Arches in parts of 

Indiana, Ohio, Michigan, and Illinois (fig. 2). The sedimentary rocks within the area range in age from Precambrian 

through Mississippian; however, bedrock units of primary 

interest range in age from Ordovician (Cincinnatian) through 

Lower Mississippian (table 1). The oldest bedrock units 

exposed at the bedrock surface are generally found along the 

axis of the Cincinnati Arch in the south-central part of the 

study area, owing to several periods of erosion (figs. 2 and 3). 

In general, units exposed at the bedrock surface are progressively younger with distance from the axes of the arches. Four 

faults or fault zones partially dissect these sedimentary rocks 

within the region (fig. 2).

The bedrock units of Ordovician age (Cincinnatian) consist of interbedded shales and limestones. Shales predominate 

in these units; less than one-quarter of the sequence is made 

up of limestones (Gray, 1972). This sequence of interbedded 

shales and limestones thickens eastward from the western 

border of Indiana toward Ohio and is overlain by carbonate 

rocks (limestones and dolomites) of Silurian and Devonian 

age. These carbonate rocks locally contain some evaporite 

deposits in northwestern Ohio and northern Indiana (French 

and Rooney, 1969; Janssens, 1977); they contain sulfide minerals in an area associated with the Findlay Arch (Botoman 

and Stieglitz, 1978). The carbonate rocks of Silurian and 

Devonian age range in thickness from 0 ft at the contact with 

the rocks of Ordovician age to 2,500 ft in southeastern Michigan (Casey, 1994) (fig. 4). Erosion has resulted in the loss of 

hundreds of feet of carbonate rock from across the central 

part of the study area. The carbonate-rock sequence has been 

completely eroded in places by the ancient Teays-Mahomet 

River system, described in Melhorn and Kempton (1991). As 

a result of this erosion, the older shales and limestones of 

Ordovician age are present at bedrock surface in sinuous 

exposures north of their principal area of exposure (fig. 2).

The carbonate rocks are overlain by shales of Devonian and 

Mississippian age along the margins of the structural basins. 

Erosion has resulted in the loss of the shale sequence 

throughout the central part of the study area except for an 

area approximately 50 mi northwest of Columbus, Ohio (fig. 

2). This shale outlier is referred to herein as the "Bellefontaine Outlier."

The bedrock is overlain by Quaternary glacial deposits 

throughout most of the study area (fig. 5 and table 1). These 

deposits directly overlie the carbonate rocks in the central 

part of the area (subcrop area of the carbonate rocks) and 

overlie the younger shales along the margins of the structural 

basins. Glacial deposits mask the ancient bedrock topography 

and bury numerous valleys in the bedrock surface.

The Quaternary glacial deposits the result of multiple 

glacial advances range in age from Kansan (oldest) to Wisconsinan (youngest) (Bennison, 1978). The deposits of Kansan and Illinoian age are not widespread within the study area 

and typically are present beyond the limit of the Wisconsinan 

ice sheet (fig. 5). The Kansan and Illinoian deposits are also 

thinner than the more widespread deposits of Wisconsinan 

age (Goldthwait and others, 1965; Geosciences Research 

Associates, 1982; Soller, 1986). The Wisconsinan ice sheet 

eroded much of these earlier glacial deposits; this resulted in 

landforms that contain material from multiple glacial 

advances. The resultant geomorphology is illustrated in figure 

6; a photograph of a shaded relief generated from digital 

topographic data for every 30 seconds of latitude and longitude (U.S. Geological Survey, 1987).

The glacial deposits include ground- and end-moraine 

deposits, glaciolacustrine deposits, and outwash deposits (fig. 

5); ice-contact stratified drift is present within the moraine 

deposits. The glacial deposits range in thickness from 0 to 

approximately 400 ft (fig. 7) (Mozola, 1969, 1970; Fleck, 

1980; Gray, 1983; Soller, 1986). The areas dominated by 

ground- and end-moraine deposits are characterized by broad, 

low ridges with smooth, gentle slopes separated by flat, gently undulating plains (Mickelson and others, 1983-). End 

moraines are close together where they abut highlands, such 

as the Bellefontaine Outlier (Young and others, 1985). The 

mineral composition of the moraines reflects local bedrock; 

about 4 percent of the material in Ohio was transported from 

the Canadian Shield north of the study area (Strobel and 

Faure, 1987).

Surficial glaciolacustrine deposits are present in the lowlands adjacent to Lake Michigan and Lake Erie and are the 

result of glacial lakes that formed along the margins of the 

retreating Wisconsinan ice (Young and others, 1985) (figs. 5 

and 6). These glaciolacustrine deposits are dominated by lake 

bottom silts and clays. Minor sands and gravels mark the 

beaches of ancient shorelines (Goldthwait and others, 1965); 

some lakebed sands in Michigan just west of Lake Erie have 

been mapped (Western Michigan University, Department of 

Geology, 1981).
C4 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

(VIRGINIA

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40

I ' 

0 20 40 60 KILOMETERS

EXPLANATION

60 MILES Basin boundaries from The National Atlas otthe United States, 

 I Hydrologic Units map, 1:3,500,000.1998

Boundary of study area

 Boundary of modeled area

 Boundary of major surface-water drainage basin

FIGURE 1. Location of study and modeled areas.
GEOHYDROLOGY C5

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

60 MILES

Geologic time units

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

20 40 60 KILOMETERS

EXPLANATION

Geologic time units not shown

Geology generalized by G.D. Casey (U.S. Geological Survey, 

written commun., 1994) from Bownocker, 1920; Willman and 

others, 1967; Burger and others, 1971; Gray and others, 1972, 

Johnson and Keller, 1972; Gray and others, 1987

 ^ Fault or fault zone

A'

Contact between carbonate rocks and 

younger shales

Line of geohydrologic section 

Shown in figure 3

FIGURE 2. Generalized bedrock geology and location of geohydrologic sections A-A' and B-B'.
C6 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

T.VBLE 1. Relation between geologic and hydrologic units of the Midwestern Basins and Arches aquifer system and

model layers used to simulate regional flow within the aquifer system 

[Fm, Formation; Ls, Limestone; equiv, equivalent; modified from Casey, 1992, fig. 3]

Generalized Geologic Units

SYSTEM

QUATERNARY

<

Q_ 

Q_

00 

00 

00

^

-Z. 

<

O

LLJ 

Q

-Z. 

< 

CC

^

00

\ ORDOVICIAN

SERIES

PLEISTOCENE

LOWER

UPPER

MIDDLE

LOWER

CAYUGAN

NIAGARAM

ALEXANDRIAN

CINCINNATIAN

MIDDLE /^X.

Southeast 

Indiana

Northweste

jrn t° 

east-centr

Indiana

Borden 

Group

Mew Alba 

Shale

> 

Muscatat 

Group

Salina5 

Group

Salamoni 

Dolomite gi

Brassfield 

Limestone

3*f

Maquoketa 

Group

a

Trenton c 

Limestone

^

Border \ 

Group \

trrT r \

New \ , 

"V Albany s 

Shale ' 

Am 

Sh

i i i i | | | ]-["[ [ JjCsLick Muscatatu 

Group

Salina5

Group

e5 Salamonit 

_,-,- Dolomite

^spm <f CreeK 5 

\l Brassred 

\ Lmestone

PI

Maquoket

, .° Ml |

i. CD 1 1 1 1 M 1 1 1 1 1 

; E Trenton

" ,p Limestone

rn Northern 

Indiana 

al

Northwestern 

Ohio

Cenlra|- South 

western «

Ohio

GLACIAL DEPOSITS

iiJJi __

Coldwater 

Shale

\ 2

Ellsworth

hale I "

rim Antrim 

ale Shale

r-C

 Traverse E 

Formation i

ck Illllllllll t

Detroit ^ 

River | 

Formation =«-

Salina5 

Group

; 5 Salamonie 

Dolomite

E E

LJ_ U_

0 II 'o

ro en

 Brassfeld 2

CD CD

u L mestone o

|il

a Maquoketa 

Group

III Illllllllllll

Trenton 

Limestone \X^\

1

Bedford Shale

Antrim 

Shale

i" Traverse 

- Formation

2 Detroit 

I River 

I Formation

Salina

Uroup

Lockport 

Dolomite

JJJJJJJJ-T^l 

Brassfield | 1 

Limestone "3 £

^ Undifferentiated 

Cincinnatian 

rocks

[H^ 

Trenton 

Limestone

,^/~\

Bedfo

Ohio Q 

Shale §

Olentangy Shale oient.

Delaware 

Limestone

Columbus 

Limestone

Detroit River 

Group

Salina -1 LLLL

Group Salln

Guelph 6 

t g Dolomite LOC

_§- ^ 3oat Island 6 QQ 

" -5 Do omite

±j Q GasportG 

Do om te

r_r_-___ 7 j- Rocheste

£Qj]XmFrp Dayton

Brassfeld s 2 

Limestone 5 = Brassfie

Undifferentiated Undifff 

Cincinnatian Cine 

rocks r

n^ 

Trenton Tr 

Limestone Lim

Generalized 

Hydrologic 

Units

western 

hio

North South

GLACIAL 

AQUIFERS 

AMD 

CONFINING 

UNITS

UPPER 

CONFINING

rd Shale UNIT

hio 

nale

ngy Sha e

Sa idstone

CARBONATE- 

........ ROCK

a Group AQUIFER

kport 

omite

Shale equiv.

Limestone

d Limestone

9

rentiated BASAL 

nnatian CONFINING 

Dcks UN!T

enton 

estone -~\

Model 

Layers

LAYER 1

LAYER 2

^ockford Shale 

Sunbury Shale 

Delaware Limestone

4 Columbus Limestone

5 Follows usage of the Indiana Geological Survey (Gray and others, 1985)

II I I I I II Nondeposition or erosion

I I Intervals not included in investigation 6Follows usage of the Ohio Geological Survey (Hull, 1990; Larsen, 1991) 

'Rochester Shale equivalent 

Dayton Limestone 

9Upper weathered zone water-bearing unit (not an aquifer)
GEOHYDROLOGY C7

SEA LEVEL

VERTICAL SCALE GREATLY EXAGGERATED

B

SEA LEVEL

SEA LEVEL

Geology generalized from Bownocker (1920), Willman 

and others (1967), U.S. Geological Survey (1970,1971), 

Burger and others (1971), Gray and others (1972), 

Johnson and Keller (1972), Gray and others (1987), 

Bunner (1993), and G.D. Casey (U.S. Geological Survey, 

written commun., 1993)

-500

VERTICAL SCALE GREATLY EXAGGERATED

SEA LEVEL

100 MILES

100 KILOMETERS

EXPLANATION

Geologic time units

Pleistocene

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Hydrologic units 

G Glacial aquifers and confining units

U Upper confining unit

C Carbonate-rock aquifer

w Upper weathered zone water-bearing unit

B Basal confining unit

? :vv.::.:.: .> :::;' Water within the carbonate-rock aquifer with a dissolved-solids 

concentration greater than 10,000 milligrams per liter 

Queried where exact extent unknown

 Boundary of hydrologic unit

FIGURE 3. Generalized sections A-A and B-B' showing geologic and hydrologic units that compose the Midwestern Basins and Arches

aquifer system (faults not shown; lines of section shown in fig. 2).
C8 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES Contours from Casey, 1994

0 20 40 60 KILOMETERS

EXPLANATION

_ 200 _ Thickness of carbonate rocks Lines of

 400 equal thickness dashed where inferred.

 soo Interval 200 feet. Darker colors indicate 

\'°200 Skater thickness

1,400 

 1,600 

 1,800 

 2,000 

 2,200 

Rocks of Ordovician age Upper weathered zone waterbearing unit. Not an aquifer. Silurian and Devonian 

carbonate rocks absent

Fault zone

Fault Hachures on downthrown side

Boundary of study area

FIGURE 4. Thickness of carbonate rocks of Silurian and Devonian age in the Midwestern Basins and Arches Region.
GEOHYDROLOGY C9

38 -

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

Geology modified from Bennison, 1978

0 20 40 60 KILOMETERS

EXPLANATION

Glacial deposits

Ground-moraine deposits 

End-moraine deposits 

Glaciolacustrine deposits 

Outwash deposits 

Bedrock

 - - Limit of Wisconsinan ice sheet (Flint, 1959)

 Limit of all glacial advances (Flint, 1959)

 Boundary of study area

C Cp Line of geologic section Shown in figure 8

FIGURE 5. Generalized surficial geology and location of geologic section C C".
CIO REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

42' r.

Shaded relief modified from Theliri and Pike (1991)

D

I 

) 50

50

I

100 MILES 

1

I 

100 KILOMETERS

FIOURK 6. Shaded relief of land-surface topography (geomorphology) in the Midwestern Basins and Arches Region, from digital data.

Outwash deposits commonly fill the ancient drainage systems, which served as channels for the deposition of such 

washed and sorted material. In many places, these out wash 

deposits underlie principal streams that currently drain the 

area.

HYDROLOGIC SETTING

The study area has a distinctly seasonal humid temperate 

climate. Precipitation in Indiana is greatest from March 

through July (Glatfelter and others, 1991). The wettest 

months in Ohio tend to be April through August, whereas 

February and October tend to be the driest (Sherwood and 

others, 1991). Mean annual precipitation computed from stations with at least 50 years of data ranges from 33 to 43 in.

across the study area (E.F. Bugliosi, U.S. Geological Survey, 

written commun., 1993). Approximately 26 in/yr are consumed by evapotranspiration in Indiana (Clark, 1980). Todd 

(1969) notes that potential evapotranspiration exceeded precipitation from mid-May through mid-September over a 30- 

year period in southwestern Ohio.

Parts of three major river systems the Ohio, the St. 

Lawrence, and the Upper Mississippi drain the study area 

(fig- 1).

GROUND-WATER USE

Ground water is plentiful throughout much of the study 

area and serves as an important resource. Approximately 433 

Mgal/d of ground water was reported to have been withdrawn 

from the Midwestern Basins and Arches aquifer system in
GEOHYDROLOGY Cll

Lake 

Michigan

- \Chii.-qo

>gton .

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES

0 20 40 60 KILOMETERS

EXPLANATION

Glacial thickness data from Mozda, 1969,1970; 

Fleck, 1980; Grey, 1983; Seller, 1986

Thickness of glacial deposits In feet

0-100

101-200 

| 201-400 

 Greater than 400

 Limit of Wisconsinan ice sheet (Flint, 1959)

 Limit of all glacial advances (Flint. 1959)

 Boundary of study area

FIGURE 7. Generalized thickness of glacial deposits in the Midwestern Basins and Arches Region.
C12 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Indiana and Ohio during the 1990 calendar year (Beary, 

1993). Only 15 percent (67 Mgal/d) of this water was withdrawn from the carbonate rocks. Of the remaining 85 percent, 

much of the water was withdrawn from outwash deposits thai 

underlie principal streams (E.A. Beary, U.S. Geological Survey, written commun., 1993). These pumpage figures reflect 

only ground-water withdrawals reported by users capable of 

pumping 100,000 gal/d or greater, and not all of this withdrawn water is consumed. Regardless of the actual amount of 

pumpage from the aquifer system, the system is not heavily 

stressed at the regional scale, as is apparent when the pumpage figures are compared to the amount of ground water that 

discharges to streams within the study area. Eberts (1999) 

estimates that, over a period of long-term steady-state conditions in the aquifer system, greater than 13,000 Mgal/d discharges from the aquifer system to streams within the study 

area.

No regional-scale cones of depression are present within 

the aquifer system. At the subregional scale, irrigation pumpage in northwestern Indiana results in seasonal water-level 

declines in the carbonate rocks. On an annual basis, however, 

the carbonate-rock aquifer appears to be able to support highcapacity irrigation pumpage without significant long-term 

depletion (Indiana Department of Natural Resources, 1990).

AQUIFERS AND CONFINING UNITS

The water table within the Midwestern Basins and Arches 

aquifer system generally is within Quaternary alluvium or 

glacial deposits. Glacial aquifers typically consist of sands 

and gravels that compose outwash deposits (fig. 5) or discontinuous lenses of ice-contact stratified drift within groundand end-moraine deposits (fig. 8). These aquifers are most 

commonly unconfined where the outwash deposits are 

present along principal streams and are locally semiconfined 

or confined by clayey till elsewhere in the region. Because the 

glacial aquifers are not normally extensive, individual aquifers can supply large yields of ground water only locally 

(Ohio Department of Natural Resources, Division of Water, 

1970).

The shale sequence of Mississippian and Devonian age 

functions as a confining unit. Specifically, the shale sequence 

restricts the flow of ground water between the glacial aquifers 

and the underlying carbonate-rock aquifer along the margins 

of the structural basins (fig. 3). In this report, these shales are 

referred to as the "upper confining unit."

The carbonate-rock aquifer directly underlies the upper 

confining unit along the margins of the structural basins and 

underlies the glacial deposits, which collectively function as a 

semiconfining unit, within the central part of the study area.

VERTICAL SCALE GREATLY EXAGGERATED Modified from Fenelon and others (1994, p. 149)

500

10 MILES

0510 KILOMETERS

EXPLANATION

Glacial aquifers (sands and gravels) ~~l

[- Ground- and end-moraine deposits 

Glacial confining units (clayey till) J

Carbonate-rock aquifer (carbonate rocks)

Basal confining unit (interbedded shales and limestones)

Unknown glacial deposits

FIGURE 8. Generalized hydrologic section C-C' showing typical relation between glacial aquifers and glacial confining units in the 

Midwestern Basins and Arches aquifer system (line of section shown in fig. 5).
GEOHYDROLOGY C13

The carbonate-rock aquifer is unconfined in areas where it is 

locally exposed at the land surface. Lateral boundaries of the 

carbonate-rock aquifer generally are coincident with the 

occurrence of waters that have a dissolved solids concentration of 10,000 mg/L or greater (see fig. 34, p. C65) or where 

the aquifer pinches out in the south-central part of the study 

area (fig. 4).

The carbonate-rock aquifer is confined below by a basal 

confining unit that is composed of interbedded shales and 

limestones of Ordovician age. Gupta (1993) demonstrates 

that these shales significantly limit the flow of ground water 

into or out of the bottom of the carbon ate-rock aquifer but 

that some water moves across the basal confining unit of the 

Midwestern Basins and Arches aquifer system to become part 

of an even larger aquifer system. There is some evidence that 

the bottom of the carbonate-rock aquifer may actually be 

within the Silurian and Devonian carbonate rocks in some 

areas. Arihood (1994) notes that fractures in the carbonate 

rocks in a few fully penetrating wells in northwest Indiana are 

not productive in the bottom 60 to 400 ft of the wells.

Although the Ordovician rocks (basal confining unit) 

underlie the carbonate-rock aquifer throughout most of the 

study area, they are laterally contiguous with the aquifer 

along the axis of the Cincinnati Arch in the south-central part 

of the area. The contact between Silurian and Ordovician 

rocks, where it is exposed at the bedrock surface, has been 

described throughout the literature as a spring horizon. The 

role of the shales along this contact was summarized succinctly by Norris and others (1950, p. 23): "The chief importance of the impervious Ordovician shale with respect to 

ground water is that it deflects the water to the surface as 

springs." These interbedded shales and limestones, however, 

are used locally as a source of water in the south-central part 

of the study area where they are exposed at the bedrock surface and other aquifers are absent. Weathering has increased 

secondary porosity and permeability within this area and has 

allowed water circulation to increase at shallow depths. 

Yields from wells completed in the interbedded shales and 

limestones within this area are typically less than 10 gal/min, 

drawdowns commonly are extreme, and dry holes are common (Indiana Department of Natural Resources, 1988). In 

this report, the interbedded shales and limestones of Ordovician age that are exposed at the bedrock surface are referred 

to as the "upper weathered zone water-bearing unit." The 

upper weathered zone water-bearing unit is not considered to 

be an aquifer but may be hydraulically connected to the carbonate-rock aquifer.

GROUND WATER

The occurrence and flow of ground water in the Midwestern Basins and Arches aquifer system are controlled by the 

geohydrologic framework of the aquifer system and by the

distribution and rate of recharge and discharge. Recharge and 

discharge also affect the long-term availability of ground 

water.

HYDRAULIC CHARACTERISITCS

The productivity of the glacial aquifers varies spatially 

within small distances because of variations in the composition, continuity, and structure of the deposits (Strobel, 1993). 

On the basis of data from 101 aquifer tests, transmissivities of 

the glacial aquifers within the study area range from 300 to 

69,700 ft2/d (fig. 9 and table 2). Storage coefficients for the 

same material range from 0.00002 to 0.38 (Joseph and Eberts, 

1994). Transmissivities at two wells within the study area that 

are completed in clayey till (not considered to be aquifer 

material) are 1.5 and 2.1 ft2/d (Strobel, 1993).

On the basis of available aquifer-test data, the vertical 

hydraulic conductivity at wells completed in glacial deposits 

within the study area ranges from 0.0001 to 0.77 ft/d (Norris, 

1959, 1979, 1986; Fleming, 1989; Strobel, 1993). Strobel 

(1993) notes that clayey till within the study area may be 

fractured at shallow depths as a result of desiccation, biological action, oxidation of minerals, or isostatic rebound after 

the retreat of the last ice sheet. He observed fractures in till 

within the study area to depths of 15 ft, and he suggests that 

the intersection of such fractures with one another and with 

sand and gravel lenses within the clayey till can result in vertical hydraulic conductivities greater than those commonly 

considered restrictive to ground-water flow.

Vertical and horizontal hydraulic conductivities of the 

shales of Mississippian and Devonian age range from 10"7 to 

10 ft/d, as determined from laboratory analysis of core samples (G.D. Casey, U.S. Geological Survey, written commun., 

1993). Because these values do not account for fractures in 

the shales, effective hydraulic conductivities that represent 

field conditions may be orders of magnitude larger (Freeze 

and Cherry, 1979, p. 158).

Water in the carbonate-rock aquifer is primarily present in 

fractures, joints, bedding planes, and solution channels within 

the rock. These openings are due, in part, to the effects of 

weathering during the period of geologic history when the 

carbonate-rock aquifer was exposed at the land surface before 

glaciation (Ohio Department of Natural Resources, Division 

of Water, 1970). The productivity of the aquifer varies with 

the concentration of openings within the rock, which seldom 

approach conditions associated with karst terranes. These 

openings are interconnected on an areal basis. Previous 

researchers (Ohio Department of Natural Resources, Division 

of Water, 1970) noted that the hydraulic characteristics of the 

carbonate-rock aquifer approach those of a regionally homogeneous medium as the study area increases. Arihood (1994) 

also notes that aquifer tests that create drawdown cones over 

several miles depict the carbonate-rock aquifer as an equivalent porous medium. On the basis of data from 171 aquifer
C14 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Lake

Michigan

, Lake

- {Chicago

40"

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

I , ' 

0 20 40 60 KILOMETERS

EXPLANATION

 Boundary of study area

Well location and transmissivity Transmissivity range in feet squared per day 

o 100 to 1,000 

O Greater than 1,000 to 10,000 

O Greater than 10,000 to 100,000

FIGURE 9. Transmissivity of glacial aquifers.
GEOHYDROLOGY

TYBLE 2. Summary of hydraulic characteristics of aquifers and confining units in the Midwestern Basins and Arches

aquifer system 

I , data not available; ft/d, feet per day; ft 2 /d, feet squared per day]

Aquifer or confining unit

Glacial aquifers

Glacial confining units

Upper confining unit

Carbonate-rock aquifer

Upper weathered zone 

water-bearing unit

Basal confining unit

Range of horizontal 

hydraulic conductivities 

(ft/d)

-

-

d irr7

-

f0.0016- 12

6 10 7

Range of vertical 

hydraulic conductivities 

(ft/d)

b n nnni n 77

- io-5

-

--

- 10 5

Range of transmissivities 

(fr/d)

a300 - 69,700

C 1.5,2.1

-

a'e70- 52,000

-

Range of storage 

coefficients

an nnnrn n ^8

-

aO.OOOO 1-0.05

_

--

a Joseph and Eberts (1994).

b Norris (1959, 1979, 1986), Fleming (1989), Strobel (1993).

c Strobel(1993).

G.D. Casey (U.S. Geological Survey, written commun., 1993). From laboratory analyses of core samples. 

e Geometric mean, 1,912 ft2 /d. 

f Dumouchelle(1992). 

B Lawrence Wickstrom (Ohio Geological Survey, written commun., 1991). From laboratory analyses of core samples.

tests, transmissivities of the carbonate-rock aquifer range 

from 70 to 52,000 ft2/d (fig. 10). These data were tested for 

normality by use of the Shapiro-Wilk test and were found to 

follow a lognormal distribution; they have a geometric mean 

of 1,912 ft2/d. Storage coefficients range from 0.00001 to 

0.05 (Joseph and Eberts, 1994). No pumped-well test data are 

available for estimation of vertical-hydraulic conductivities 

of the carbonate-rock aquifer.

Very little information is available to describe the hydraulic characteristics of the interbedded shales and limestones 

that function as a basal confining unit to the Midwestern 

Basins and Arches aquifer system. Analyses of core collected 

from the upper part of the interbedded shale and limestone 

sequence in southwestern Ohio provide estimates of vertical 

and horizontal hydraulic conductivities that range from IO"7 

to 10 ft/d (Lawrence Wickstrom, Ohio Geological Survey, 

written commun., 1991). These values do not account for secondary porosity within the rocks. Shales of Ordovician age, 

however, are considered favorable for underground storage of 

liquefied natural gas in southern Indiana, where they underlie 

the carbonate-rock aquifer, because of their low hydraulic 

conductivities (Droste and Vitaliano, 1976); thus, these shales 

can be assumed to be very restrictive to ground-water flow in 

this area. Hydraulic conductivities are likely to be higher in 

the upper weathered zone water-bearing unit where the shales 

have been exposed at the land surface. On the basis of slugtest data at four wells completed in the upper part of the interbedded shale and limestone sequence near the upper weathered zone water-bearing unit, horizontal hydraulic 

conductivities range from 0.0016 to 12 ft/d (Dumouchelle, 

1992).

LEVELS

Review of historical ground-water-level data indicates a 

long-term steady-state condition in the aquifer system. 

(Long-term steady-state conditions refer to a state of dynamic 

equilibrium in which no net change in storage in the aquifer 

system occurs over a long-term period. The long-term period 

referred to herein is a minimum of 10 years and includes wet 

and dry periods.) Extensive ditching to drain swampland in 

low-lying areas in northwestern Ohio (Kaatz, 1955) and 

northwestern Indiana in the late 1800's and early 1900's 

resulted in some dewatering of shallow glacial deposits (5 to 

7 ft) in Indiana (Rosenshein, 1963; Indiana Department of 

Natural Resources, 1990) and possibly similar dewatering in 

Ohio; however, a new equilibrium has been established in 

these areas (Eberts, 1999). Annual ground-water-level fluctuations related to ground-water recharge range from 3 to 7 ft in 

the aquifer system (Clark, 1980; Shindel and others, 199la, 

b). Annual high water levels are reached between March and 

June, and annual low water levels are reached near the end of 

the growing season.

The altitude of the water table, which typically is in glacial deposits, is a controlling factor for regional flow in the 

Midwestern Basins and Arches aquifer system. Most regional 

variation in water-table altitude is a consequence of the variation in land-surface altitude, and depth to the water table varies predictably at the regional scale. The specific relation 

between land-surface altitude and water levels in glacial 

deposits was determined by use of a least-squares method of 

linear regression (Eberts, 1999). Depth to water is greatest in 

topographically high areas and decreases in areas such as 

stream valleys. A composite regional potentiometric-surface 

map of the glacial deposits was constructed from water levels
C16 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

38«

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES

O

O

O

0 20 40 60 KILOMETERS

EXPLANATION 

Boundary of study area 

Well location and transmissivity Transmissivity range in feet squared per day

1 to 100

Greater than 100 to 1,000

Greater than 1,000 to 10,000

Greater than 10.000 to 100.000

FIGURE 10. Transmissivity of the carbonate-rock aquifer.
GEOHYDROLOGY C17

reported on drillers' logs and is shown in figure 11. Also 

shown in figure 11 is the lack of available drillers' logs for 

most areas south of the limit of the Wisconsinan ice sheet. 

Perhaps so few domestic wells have been completed in glacial deposits within these areas because the units are not productive.

The regional potentiometric surface in the carbonate-rock 

aquifer is a subdued reflection of the land surface and further 

illustrates the effect of variations in land-surface altitude on 

the aquifer system. A regional potentiometric-surface map of 

the carbonate-rock aquifer (fig. 12) was constructed from 

water levels synoptically measured during July 1990 (Eberts, 

1999). Potentiometric highs are in west-central Ohio near the 

Bellefontaine Outlier and near the southern limit of the carbonate-rock aquifer along the border between Indiana and 

Ohio. Potentiometric lows less than or equal to 600 ft are 

along the Wabash and the Ohio Rivers and Lake Erie.

RECHARGE

Ground-water recharge at the water table of the Midwestern Basins and Arches aquifer system is primarily from infiltration of precipitation and the associated flow away from the 

water table within the saturated zone. Recharge from precipitation at the water table varies seasonally because evapotranspiration, which can intercept infiltrating precipitation, varies 

seasonally. Recharge to the carbonate-rock aquifer is primarily from percolation of ground water through overlying 

units. The rate at which the carbonate-rock aquifer is 

recharged by percolation depends on the permeability and 

thickness of the overlying deposits and the difference 

between the water table in the overlying deposits and the 

potentiometric surface in the carbonate-rock aquifer.

Very few estimates of recharge to the Midwestern Basins 

and Arches aquifer system have been made. Daniels and others (1991) estimated recharge rates through unsaturated glacial till to be 1.4 and 1.8 in/yr from a tritium profile obtained 

from a core collected in Indiana, but they stated that such 

rates are more applicable to a local scale than a regional 

scale. Walton and Scudder (1960) report recharge rates of 12 

in/yr through outwash deposits and 8 in/yr through glacial till 

on uplands within parts of the Great Miami River Basin. All 

other available recharge estimates, excluding those determined by use of previously constructed numerical groundwater flow models, are for recharge to the carbonate-rock 

aquifer. These estimated recharge rates, based on analyses of 

flow nets and on cones of influence of pumped wells, range 

from 0.14 to 6.3 in/yr (Rosenshein, 1963; Watkins and Rosenshein, 1963; Rowland and Kunkle, 1970; Cravens and others, 

1990; Roadcap and others, 1993).

DISCHARGE

Ground-water discharge from the aquifer system includes 

discharge to streams, ditches, lakes, and wetlands and the

removal of water from the saturated zone by evapotranspiration and pumping. Ground water discharges to a stream if the 

water table or potentiometric surface is above the stage of the 

stream, whereas the stream loses water to the aquifers if the 

water table is below the stream stage. Ground-water discharge to streams (base flow) can be estimated from streamflow data by separating streamflow hydrographs into directrunoff and base-flow components.

Streamflow data were used to estimate ground-water discharge from the aquifer system to streams that drain the study 

area. Daily mean base flows for the period associated with 

unregulated or only minimally regulated low flow were computed for selected streamflow-gaging stations by means of the 

local-minimum method of hydrograph separation (Pettyjohn 

and Henning, 1979). A computer program (R.A. Sloto, U.S. 

Geological Survey, written commun., 1988) was used in this 

investigation to automate the local-minimum method of 

hydrograph separation; use of a computer program ensured 

that the separation technique was applied consistently.

Mean ground-water discharge to stream reaches above 

each selected streamflow-gaging station was estimated from 

the daily mean base flows. This was accomplished for each 

station by computing the average of all the daily mean base 

flows for the period selected for analysis (Eberts, 1999). 

Mean ground-water discharge to stream reaches between 

streamflow-gaging stations was estimated by computing the 

difference between mean ground-water discharge estimates 

for adjacent stations (fig. 13). These means describe the central tendency of ground-water discharge to the selected 

streams within the study area for long-term steady-state conditions in the aquifer system.

Mean ground-water discharge ranges from 17 to 80 percent of mean streamflow for the 43 selected stream reaches 

for which streamflow data were analyzed. (These values are 

the upper numbers in figure 14.) Mean ground-water discharge as a percentage of mean streamflow increases with 

distance downstream in about half of the principal surfacewater drainage basins. Stated another way, ground water generally makes up a greater proportion of streamflow at the bottom of these drainage basins than in areas higher up in the 

basins. Unusually large percentages of mean ground-water 

discharge occur along stream reaches that drain areas underlain by large amounts of outwash deposits (figs. 5 and 14). In 

contrast, a notable decrease in mean ground-water discharge 

as a percentage of mean streamflow with distance downstream occurs where the Maumee River drains an area underlain by glaciolacustrine deposits (figs. 5 and 14). A decrease 

in mean ground-water discharge as a percentage of mean 

streamflow also occurs in the south-central part of the study 

area where the streams flow over areas where the carbonaterock aquifer is absent (figs. 4 and 14). Cross (1949) also 

reported a relation between geology and base flow in Ohio.

Fluctuations of base flow in streams within the study area 

result from changes in hydraulic gradients in the Midwestern
CIS REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

(Chicago

Lake 

Michigan

40°

EvansvUie

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

 1,200 

0 20 40 60 MILES

I i i______I I I I i

0 20 40 60 KILOMETERS

EXPLANATION

Potentiometric contour Line of equal water-

 500 level altitude. Dashed where glacial deposits 

. 700 are absent. Contour interval 100 feet. Datum 

^H.^~ 's sea level. Darker blues indicate lower altitude

 Boundary of study area

 Limit of all glacial advances (Flint, 1959)

^^ Flowline Used to define boundaries of regional 

ground-water flow model

» Observation well

Limit of Wisconsinan ice sheet (Flint, 1959)

FIGURE 11. Composite regional potentiometric surface in glacial deposits constructed from water levels on drillers' logs.
GEOHYDROLOGY C19

38" V

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES Modified from Eberts, 1999

J_______L

1.200 

 {,150 

 1,100 

 1,050 

 1,000 

 950 

 900 

 -850 

 800 

 750 

 700 

__ ccn __

l/*/L/

0 20 40 60 KILOMETERS

EXPLANATION

Potentiometric contour Shows altitude at which 

water level would have stood in tightly cased wells, 

July 1990. Dashed where approximately located. 

Contour interval 50 feet. Datum is sea level. 

Darker blues indicate lower altitude

Upper weathered zone water-bearing unit 

Not an aquifer. Carbonate-rock aquifer absent

General direction of regional ground-water flow

Flowline Used to define boundaries of regional 

ground-water flow model

Boundary of study area

FIGURE 12. Regional potentiometric surface in the carbonate-rock aquifer, July 1990, and general directions of regional ground-water flow
C20 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

38C Evansvtlie

\Humingtoo

WEST 

<VlRGJNJA

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES

0 20 40 60 KILOMETERS

EXPLANATION

Boundary of study area

^ A Ground-water discharge to selected stream reach Upper number is mean ground-water discharge, in

]g cubic feet per second. Lower number is mean sustained ground-water discharge, in cubic feet per second bo

A Streamflow-gaging station

FIGURE 13. Estimated ground-water discharge to selected stream reaches for long-term steady-state conditions 

in the Midwestern Basins and Arches aquifer system.
GEOHYDROLOGY C21

38°

Lake 

Michigan

IChiragu

Ev

Base from U.S. Geological Survey digital data, 

1:2,000.000.1972

0 20 40 60 MILES

I I 

0 20 40 60 KILOMETERS

EXPLANATION

Boundary of study area

36

25

Discharge percentages for selected stream reach Upper number is mean ground-water discharge as a 

percentage of mean streamflow. Lower number is mean sustained ground-water discharge as a percentage 

of mean ground-water discharge. Percentages represent long-term steady-state conditions in the Midwestern 

Basins and Arches aquifer system

A Streamflow-gaging station

FIGURL 14. Mean ground-water discharge as a percentage of mean streamflow, and mean sustained ground-water discharge 

as a percentage of mean ground-water discharge for selected stream reaches.
C22 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Basins and Arches aquifer system. Winter (1983) illustrates 

the effects of ground-water recharge from precipitation on 

hydraulic gradients in a water-table aquifer. He shows that 

hydraulic gradients in the areas of a water-table aquifer nearest a surface-water body are the first to respond to a recharge 

event because the water table is closest to land surface in 

these areas. Eventually, hydraulic gradients in the upland 

areas are affected by the recharge event (fig. 15). Seasonal 

fluctuations of base flow in streams are related to seasonal 

variations in ground-water recharge and the corresponding 

changes in hydraulic gradients at the water table. Some 

ground-water flow systems within an aquifer system are minimally affected by recharge events; these more stable groundwater flow systems provide a relatively constant source of 

ground-water discharge to streams over the course of a year 

and throughout long periods (fig. 15). The term "sustained 

ground-water discharge" is used herein to refer to this relatively constant source of base flow.

Base-flow duration curves constructed from daily mean 

base flows can be used to identify the component of base flow 

that is sustained during long periods, which include the driest 

periods (Eberts, 1999). (Base-flow duration curves are cumulative frequency curves that show the percentage of time during which specified base flows were equaled or exceeded in a 

given period; they are constructed by use of the method 

described by Searcy (1959), except that daily mean base 

flows are used instead of daily mean streamflows.)

Base-flow duration curves for streamflow-gaging stations 

along the principal streams within the study area are generally made up of two limbs when plotted on log-probability 

paper (fig. 16). The upper limb of each curve is commonly 

concave, whereas the lower limb of each curve is commonly a 

straight line, representing a flattening of the overall curve. 

This two-limb shape indicates the presence of at least two 

sources of ground-water discharge to these streams. Superimposed base-flow duration curves, constructed from periods of 

record that represent different ground-water recharge conditions, provide insight into the sources of ground-water discharge that result in the upper and lower limbs of the curves 

(fig. 16). Specifically, superimposed curves constructed from 

(1) the entire period of record, (2) from only summer months, 

when potential evapotranspiration exceeds precipitation 

(Todd, 1969), and (3) from a period of drought (U.S. Geological Survey, 1991) for single streamflow-gaging stations 

within the study area show that daily mean base flows that 

make up the upper limbs of the curves are from a groundwater source that readily responds to variations in groundwater recharge from precipitation. Conversely, the daily mean 

base flows that make up the lower limbs are from a groundwater source not greatly affected by variations in groundwater recharge (sustained ground-water discharge), as evident 

from the minimal differences in the lower limbs of the same 

curves. The base flows that make up the upper limb of each 

curve are likely to include a major component of transient,

local-scale ground-water flow, because ground-water levels in 

local ground-water flow systems commonly decline in the 

summer and during droughts in response to a decrease in 

ground-water recharge from precipitation. The base flows that 

make up the lower limb of each curve are from a more stable 

ground-water flow, and likely represent a more dominant 

influence of intermediate- and regional-scale ground-water 

flow systems.

Lower limbs are absent or indistinct on base-flow duration 

curves constructed for streams within the study area that 

cease or nearly cease to flow during periods of dry weather. 

Such streams include small tributaries and streams that drain 

areas underlain by poorly permeable rocks. The absence of a 

lower limb on a base-flow duration curve cannot be used in 

itself to infer that stable flow systems are absent in the underlying aquifer system. Rather, relatively stable ground-water 

flow systems may be present in the underlying aquifer system 

but may simply discharge at some other point.

Mean sustained ground-water discharge to the stream 

reaches above selected streamflow-gaging stations was estimated by constructing base-flow duration curves and computing the average, for each curve, of all the daily mean base 

flows that make up the lower limb (Eberts, 1999). These baseflow duration curves were constructed, with the aid of a computer program developed by Lumb and others (1990), for the 

same period of record used to estimate mean ground-water 

discharge to the streams. Mean sustained ground-water discharge to stream reaches between streamflow-gaging stations 

was estimated by computing the difference between mean 

sustained ground-water discharge estimates for adjacent stations (fig. 13). These means describe the central tendency of 

sustained ground-water discharge to the streams for longterm steady-state conditions in the aquifer system.

Mean sustained ground-water discharge ranges from 3 to 

50 percent of mean ground-water discharge for the 43 

selected stream reaches. (These values are the lower numbers 

in figure 14.) Mean sustained ground-water discharge as a 

percentage of mean ground-water discharge increases with 

distance downstream along many stream reaches. Notable 

exceptions are stream reaches in the south-central part of the 

study area, where the carbonate-rock aquifer is absent, and 

stream reaches along the Maumee and Sandusky Rivers, 

which drain into Lake Erie.

Relative amounts of mean ground-water discharge and 

mean sustained ground-water discharge to streams are illustrated by principal drainage basin in figure 17. These values 

are for the entire area above the most downstream streamflow-gaging station in each selected drainage basin. Circles 

are used to illustrate the relative volumes of ground-water 

discharge within the basins. The sizes of the circles were 

determined by use of an exponential-curve-scaling method 

because the range of volumes was too large to be represented 

effectively by linear scaling. Estimated mean ground-water 

discharges are noted in the figure for reference. Also repre-
GEOHYDROLOGY C23

Land surface

Conditions before recharge event Some ground water 

discharges to surface water

A.

Initial effects of recharge event Water table mounds near 

surface-water bodies where infiltration distance is smallest; 

ground-water discharge to surface water increases

B.

Later effects of recharge event Elevated water table mimics 

land surface; ground-water discharge to surface water remains 

elevated as compared to pre-recharge conditions

C.

Conditions after effects of recharge event Ground-water 

discharge to surface water decreases

D.

EXPLANATION

Direction of ground-water flow Light arrows are associated with shallow, 

transient ground-water flow systems. Heavy arrows are associated with 

more stable ground-water flow systems

Water level

FIGURE 15. Diagrams showing the effect of recharge from precipitation on the configuration of a water table and associated groundwater flow (modified from Winter, 1983).
C24 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

10,000

£ 1,000

100

0.5 20 50 80 99.5 

PERCENTAGE OF TIME BASE FLOW WAS EQUALED OR EXCEEDED

EXPLANATION

Base-flow duration curves Dashed lines represent 

upper limb, solid lines represent lower limb

 Entire period, 1916-88

 - - Summer months, 1916-88

 Drought, 1933-36

FIGURE 16. Base-flow duration curves for various ground-water recharge conditions for a streamflow-gaging station

on the Kankakee River.

sented in figure 17 are pie slices in each of the circles that 

shows the percentage of mean ground-water discharge to 

streams attributed to mean sustained ground-water discharge. 

The greatest volume of ground-water discharge to streams 

is within the Wabash River Basin. This basin also has the 

highest mean sustained ground-water discharge as a percentage of mean ground-water discharge to the streams. Large 

amounts of outwash deposits are present within the basin. It 

is also the largest drainage basin within the study area. Bedrock crops out locally along the main stem of the Wabash 

River. In addition, the Wabash River is relatively deeply 

incised, and it has nearly the lowest base level within the 

study area. These factors likely contribute to the availability 

of ground-water recharge, resulting in a large volume of 

ground-water flow beneath the basin, and facilitate the interception of flow paths associated with the more stable groundwater flow systems.

The surface-water drainage basins with the smallest mean 

sustained ground-water discharge to streams as a percentage 

of mean ground-water discharge include the basins in the 

southeastern part of the study area. This may be due to the 

relatively small size of the basins, as well as their substantial 

local relief; transient, local ground-water flow systems generally dominate in such areas. These basins also differ from the 

Wabash River Basin in that glacial deposits are thin or absent 

within this area.

Flows of streams that drain into Lake Erie also consist of 

small percentages of mean sustained ground-water discharge. 

Hydraulic gradients within the aquifer system beneath the 

Maumee River Basin are fairly low. Because this basin is near 

Lake Erie and the base level of the aquifer system, the low 

hydraulic gradients may limit the capacity of the aquifer system to carry ground water away from recharge areas. As a 

result, much of the precipitation that potentially would
GEOHYDROLOGY C25

38°

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

0 20 40 60 KILOMETERS

EXPLANATION

37 Mean sustained ground-water discharge to streams within selected surface-water drainage basins Represented by 

pie slice. Number is discharge, in cubic feet per second, for long-term steady-state conditions in the aquifer system

291 --' Mean ground-water discharge to streams within selected surface-water drainage basins Represented by entire circle. 

(3.16) Numbers are discharge, in cubic feet per second and inches per year (in parentheses), for long-term steady-state conditions 

in the aquifer system

 Boundary of surface-water drainage basin

 Boundary of study area

FIGURE 17. Relative amounts of mean ground-water discharge and mean sustained ground-water discharge 

to streams in selected surface-water drainage basins.
C26 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

recharge stable ground-water flow systems in these areas may 

be forced to discharge locally by means of drainage tile or 

shallow, transient ground-water flow systems. The Maumee 

River is also incised only a few feet, which may prevent it 

from intercepting flow from some stable ground-water flow 

systems. Poorly permeable glaciolacustrine sediments may 

also impede discharge from the carbonate-rock aquifer to the 

Maumee River. In general, glacial deposits in the Maumee 

River Basin are thin, absent, or poorly permeable. Toth 

(1963) notes that low ground-water discharge to streams 

within a drainage basin can be due to other areas of groundwater discharge within the basin. Before ditching in the early 

1900's, much of the Maumee River Basin was swampland. 

Norris (1974) notes that the historic Black Swamp in this area 

resulted from a combination of poor drainage and groundwater discharge from regional ground-water flow into what 

was a relatively stagnant area of surface water and ground 

water.

The Sandusky River Basin is also associated with a fairly 

low percentage of sustained ground-water discharge to 

streams. Much of the ground water that flows through this 

drainage basin is likely to discharge to Lake Erie rather than 

to the streams within the basin.

REGIONAL GROUND-WATER FLOW

General concepts regarding flow within an aquifer system 

are reviewed herein to facilitate discussions of the conceptual 

and numerical models of the Midwestern Basins and Arches 

aquifer system. An aquifer system can comprise local, intermediate, and regional ground-water flow systems (fig. 18). In 

a local system of ground-water flow, recharge and discharge 

areas are adjacent to each other. In an intermediate groundwater flow system, recharge and discharge areas are separated 

by one or more topographic highs and lows. In a regional 

ground-water flow system, recharge areas are along groundwater divides, and discharge areas lie at the bottom of major 

drainage basins. Not all types of ground-water flow are 

present in every aquifer system (Toth, 1963).

The greatest amount of ground-water flow in an aquifer 

system is commonly in local flow systems. Ground-water levels and flow in local flow systems are the most affected by 

seasonal variations in recharge because recharge areas of 

these relatively shallow, transient ground-water flow systems 

make up the greatest part of the surface of a drainage basin 

(Toth, 1963). Regional flow systems are less transient than 

local and intermediate flow systems. For the remainder of this 

report, the term "regional flow systems" is used to describe 

flow systems that are minimally affected by seasonal variations in ground-water recharge and are capable of providing a 

fairly constant source of discharge to streams (sustained 

ground-water discharge). Although this use of the term 

"regional flow systems" refers, in large part, to intermediate

and regional flow systems as defined by Toth (1963), some 

local-scale flow also may be included.

CONCEPTUAL MODEL

A conceptual model of an aquifer system is a simplified, 

qualitative description of the physical system. A conceptual 

model may include a description of the aquifers and confining 

units that make up the aquifer system, boundary conditions, 

flow regimes, sources and sinks of water, and general directions of ground-water flow. The conceptual model of the Midwestern Basins and Arches aquifer system presented herein is 

based on information presented in the "Geohydrology" section of this report.

The Midwestern Basins and Arches aquifer system is in a 

state of dynamic equilibrium with respect to hydrologic variations over the long-term period. As a result, the aquifer system may be adequately described on the basis of long-term 

average water levels and ground-water discharges. In addition, annual ground-water-level fluctuations are quite small 

(less than 10 ft) compared to the thickness of the aquifer system (hundreds of feet).

The water table within the aquifer system generally is 

within alluvium or glacial deposits; glacial aquifers can supply large yields of ground water in only a limited number of 

places. The glacial deposits are underlain by an aieally extensive carbonate-rock aquifer, which is semiconfined or locally 

confined by the glacial deposits across most of the study area. 

The carbonate-rock aquifer is confined by shale along the 

margins of the aquifer system. Very little water is produced 

from the carbonate-rock aquifer under the shales because 

shallower freshwater sources are generally available.

Spatial patterns in hydraulic characteristics of the glacial 

aquifers or the carbonate-rock aquifer are not readily apparent from the available transmissivity data (figs. 9 and 10); 

however, some of the highest transmissivities in the glacial 

aquifers are associated with outwash deposits along the principal streams (figs. 5 and 9). Despite the spatial variability of 

hydraulic characteristics within the carbonate-rock aquifer, 

the aquifer functions as a single hydrologic unit at a regional 

scale (Arihood, 1994).

The upper boundary of the aquifer system coincides with 

the water table. The lower boundary generally coincides with 

the contact between the carbonate-rock aquifer and interbedded shales and limestones of Ordovician age where they 

underlie the aquifer. Where the carbonate-rock aquifer is hundreds of feet thick, the lower boundary of the aquifer system 

may be within the carbonate rocks. Lateral boundaries of the 

carbonate-rock aquifer include the limit of potable water 

(waters that contain dissolved-solids concentrations less than 

10,000 mg/L; U.S. Environmental Protection Agency, 1984)) 

to the north, east, and west (fig. 34), Lake Erie to the north-
REGIONAL GROUND-WATER FLOW C27

NOT TO SCALE

EXPLANATION 

Glacial deposits

Carbonate-rock aquifer

Basal confining unit

Local ground-water flow path

Intermediate ground-water flow path

Regional ground-water flow path

Indicates flow simulated by the regional ground-water 

flow model constructed for this investigation

FIGURE 18. Diagrammatic conceptual model of the Midwestern Basins and Arches aquifer system showing flow paths associated with 

local, intermediate, and regional flow systems (modified from Toth, 1963) and flow systems simulated by the regional

ground-water flow model.

east, and the Ohio River and the upper weathered zone waterbearing unit to the south.

Several types of ground-water flow systems are present 

within the Midwestern Basins and Arches aquifer system, as 

evidenced by base-flow duration curves constructed for 

selected streamflow-gaging stations within the study area 

(discussed previously in the "Discharge" section). The 

amount of ground-water discharge to streams from fairly stable ground-water flow systems (regional, as defined in this 

report) relative to the amount of discharge from all scales of 

ground-water flow systems (local, intermediate, and regional) 

within the aquifer system (fig. 17) indicate that local flow systems dominate ground-water discharge to streams within the 

Midwestern Basins and Arches Region. Unless a large 

amount of ground water flows across lateral boundaries of the 

aquifer system or large volumes of ground water discharges 

to places other than streams, figures 14 and 17 can be used to 

infer that local flow systems dominate ground-water flow in 

the aquifer system.

The amount of recharge to regional flow systems (as 

defined in this report) within the Midwestern Basins and 

Arches aquifer system is approximately equal to mean sustained ground-water discharge to streams, ditches, lakes, and

wetlands and losses from the relatively stable parts of the 

aquifer system by means of evapotranspiration and pumping. 

Recharge to the deepest parts of the aquifer system occurs 

predominantly in the upland areas.

The amount of ground water available to sustain streams 

during the driest periods (discharge from regional flow systems) within the study area is related to a number of factors. 

These factors include the availability of recharge, the geology 

and hydraulic gradients within the aquifer system, the position of the streams within the drainage basins and relative to 

the aquifer system in general, the relative incisement of the 

streams, and the presence of other places of discharge within 

the drainage basins.

Ground water flows from recharge areas, which are associated with high ground-water levels, to discharge areas, 

which are associated with low ground-water levels. General 

directions of regional flow in the aquifer system are away 

from several potentiometric highs toward the principal 

streams and Lake Erie. Most active flow of freshwater (less 

than 10,000 mg/L dissolved solids) in the carbonate-rock 

aquifer within the aquifer system is confined to the subcrop 

area of the aquifer.
C28 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

NUMERICAL MODEL

A numerical model was constructed to test and improve 

upon the conceptual model of regional ground-water flow 

within the Midwestern Basins and Arches aquifer system. 

Concepts that were tested include the assumption that the carbonate rocks are (are not) productive throughout their entire 

thickness and hypotheses about what contributes to the differences in the percentages of mean sustained ground-water discharge to streams across the study area. The numerical model 

was also used to investigate the absence of systematic 

increases in ground-water ages along general directions of 

regional flow throughout most of the study area and the presence of isotopically distinct ground water beneath the 

Maumee River Basin (see "Geochemistry" section). Various 

aspects of the qualitative conceptual model were also quantified by use of the numerical model. Specifically, a regional 

ground-water budget was computed; rates and patterns of 

recharge and discharge to and from regional flow systems 

were mapped; and natural regional ground-water flow patterns and relative magnitudes of regional ground-water flow 

were determined.

In a numerical model, aquifers and confining units within 

an aquifer system are represented by cells organized into layers. Hydraulic heads and flow in each layer and the exchange 

of water between adjacent layers and across boundaries are 

computed simultaneously. These calculations are most commonly accomplished by use of a computer code that solves 

finite-difference or finite-element approximations of the partial differential equations (three-dimensional ground-water 

flow equation, boundary conditions, and initial conditions) 

that form the numerical model (Anderson and Woessner, 

1992, p. 20).

The specific computer code used in this investigation is a 

three-dimensional modular model that solves a finite-difference approximation of the partial differential equations that 

describe ground-water flow (MODFLOW) (McDonald and 

Harbaugh, 1988). In the governing ground-water flow equation represented by this model, the density of water is 

assumed to be constant. Although the density of the water in 

the aquifer system may change within the carbonate-rock 

aquifer along the margins of the Michigan and Appalachian 

(structural) Basins, the effects of the density variations on 

ground-water flow within the modeled part of the aquifer system were assumed to be small enough that a variable-density 

flow model was considered unnecessary. This is a reasonable 

assumption because most of the aquifer system that was modeled is miles from these margins, stresses on the aquifer system do not affect the lateral limits of freshwater, and any 

affect density variations may have on model estimates of 

hydraulic conductivity or simulated hydraulic heads are likely 

to be within the confidence limits of the estimated hydraulic 

conductivity or the error associated with the hydraulic-head 

observations.

MODEL DESIGN

The numerical model built as part of this investigation 

was designed to simulate steady-state regional flow systems 

within the aquifer system. These are flow systems that are 

minimally affected by seasonal variations in ground-water 

recharge from precipitation and are capable of sustaining discharge to the streams during the driest periods. On the basis 

of stream base-flow estimates, much less than 50 percent of 

ground-water flow in the aquifer system is associated with 

these regional flow systems (figs. 14 and 17).

The model was not designed to simulate the preponderant 

local flow systems that are juxtaposed on the regional flow 

systems (figs. 15 and 18). Such local flow systems are too 

small and numerous to be adequately represented with a 

regional-scale model. Specifically, the model cell spacing 

chosen for this investigation (4 mi on a side) is not fine 

enough to capture the curvature of the water table associated 

with such local flow systems. This effect of scale results in 

the simulation of less cross-sectional area and hydraulic gradient than actually exists in the aquifer system; thus, the 

model cannot simulate the corresponding flow (fig. 19). 

Because local flow systems cannot be explicitly simulated in 

a model with a coarse regional-scale cell spacing, a boundary 

condition (discussed later in this section) was used to simulate the influence of the local flow systems on the deeper 

regional flow systems. The simulated regional ground-water 

budget, therefore, represents the budget of just the regional 

flow systems and not all flow systems within the aquifer system. Because water in local flow systems moves from 

recharge areas to the nearest stream valley, a map of the density of perennial streams that drain the modeled area is 

included to help illustrate the relative number of local flow 

systems that may be present within the aquifer system but are 

not included in the model (fig. 20).

Some flow systems may cross the basal confining unit of 

the Midwestern Basins and Arches aquifer system and 

become part of an even larger aquifer system. These flow systems also are not simulated by the regional ground-water flow 

model (fig. 18).

Unlike some areally extensive aquifer systems elsewhere 

in the Nation, the Midwestern Basins and Arches aquifer system is not subject to regional-scale pumping stresses. It was 

therefore unnecessary to construct a transient model that is 

capable of simulating changes in ground-water levels, discharge, and storage with time to represent regional groundwater flow (see "Conceptual Model" section). As a result, 

once a steady-state calibration was achieved, the numerical 

model was not calibrated to transient conditions, nor was it 

used to make predictions about the effects of future pumpage 

on regional ground-water flow. Known volumes of pumpage 

were also not included in the model because reported pumpage from the aquifer system is approximately 3 percent of 

total flow in the aquifer system. Although it is not known how
REGIONAL GROUND-WATER FLOW C29

Hydraulic head

Actual hydraulic gradient associated 

with local flow systems

Hydraulic head

Hydraulic gradient 

represented by a 

model with a coarse 

regional-scale cell 

spacing

DARCY'S LAW

Q,-KAf

Q, Flow

K, Hydraulic conductivity 

A, Area 

~ji~, Hydraulic gradient

A model with a coarse-regional-scale cell spacing often 

cannot capture the actual cross-sectional area and 

hydraulic gradients associated with local flow systems. 

On the basis of Darcy's Law, if K is constant, simulated 

flow (Q) in such a simulation will be less than actual flow 

in the aquifer system because simulated area (A) and 

hydraulic gradients (-=-) will be less than actual area 

and hydraulic gradients.

FIGURE 19. Effect of model-cell spacing (model scale) on the amount of flow in an aquifer system that can be simulated with a

numerical model.

much of this reported pumpage is associated with the predominant local flow systems as opposed to the simulated 

regional flow systems, most pumpage is assumed to be from 

the local flow systems. Even if all reported pumpage from the 

carbonate-rock aquifer is associated with regional flow systems, pumpage from the carbonate-rock aquifer (67 Mgal/d) 

would be less than 5 percent of flow in the regional flow systems based on estimates of mean sustained ground-water discharge to streams (fig. 17). Although some pumpage from 

glacial aquifers is also likely to be from regional flow systems, many of the largest water users produce water from outwash deposits along principal streams that are commonly 

associated with local flow systems, which are not simulated 

in the model.

DISCRETE GEOHYDROLOGK FRAMEWORK

The numerical regional ground-water flow model is a 

quasi-three-dimensional two-layer model (table 1) structured 

within a 65-row by 61-column finite-difference grid (fig. 21). 

The upper model layer (layer 1) is used to simulate hydraulic 

heads and flow through glacial and other surficial deposits 

(fig. 22). Effective hydraulic conductivities were used to

account for the heterogeneities with the glacial deposits. (An 

effective hydraulic conductivity is a hydraulic conductivity of 

an equivalent homogeneous formation for which the mean 

flux is equal to that prevailing in the heterogeneous formation 

(Indelman and Dagan, 1993).) The lower model layer (layer 

2) is used to simulate hydraulic heads and flow through the 

bedrock. A single layer was considered sufficient to simulate 

flow in the carbonate-rock aquifer because vertical hydraulichead gradients within the aquifer are small and the carbonaterock aquifer functions as a single hydrologic unit at a regional 

scale. Where the carbonate-rock aquifer is absent in the 

south-central part of the study area, parameter values in 

model layer 2 were chosen to simulate hydraulic heads and 

flow in the upper weathered zone water-bearing unit.

Because relatively little horizontal flow occurs within the 

shale that separates the glacial deposits and the carbonaterock aquifer along the margins of the modeled area, this 

upper confining unit is not represented as a separate layer in 

the model. Rather, a quasi-three-dimensional approach is 

used. With such an approach, only the resistance of the upper 

confining unit to vertical flow between the glacial deposits 

and the carbonate-rock aquifer is simulated (figs. 21A and 

22).
C30 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Lake

Michigan OlMrARIO/"

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES Stream data from U.S. Geological Survey, 1989

I I ' 

0 20 40 60 KILOMETERS

EXPLANATION 

Perennial stream

^ Boundary of modeled area

 Perennial stream simulated by use of stream cells Ground-water flow systems associated with 

smaller streams are not explicitly represented in the regional ground-water flow model

* Regional ground-water flow See inset 

- Local ground-water flow See inset

FIGURE 20. Density of perennial streams that drain the modeled area at the scale of 1:100,000 and streams that are 

explicity represented in the regional ground-water flow model by use of stream cells.
REGIONAL GROUND-WATER FLOW C31

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

D D

0 20 40 60 KILOMETERS

EXPLANATION 

Model cell

Area of upper confining unit Not a separate model layer 

Boundary of model layer 1 

Boundary of model layer 2 

Line of model section

FIUUIE 21 Model layers used to simulate the Midwestern Basins and Arches aquifer system, and the location of

model sections D-D' and E-E': (A) areal extent.
C32 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

B

Base from U.S. Geological Survey digital data, 0 20 40 60 MILES

1:2,000,000,1972 I . ' , .' '

I I I T

EXPLANATION ° 20 40 60 KILOMETERS 

] Model cell

 Specified-head boundary

 Head-dependent flux boundary (stream cell) Used to simulate principal streams

LJ General head-dependent flux boundary Used to simulate exchange of water at the 

regional water table. Recharge applied to model cell.

 Specified-flux boundary

No-flow boundary

 Line of model section

FIGURE 21. CONTINUED Model layers used to simulate the Midwestern Basins and Arches aquifer system, and the location of

model sections D-D' and E-E': (B) boundaries.
Surface grid spacing is 4 miles

MOD i i iM10 D

EL

j

p

*

LAi iLA

YY

E\Ri iER

1

2

1 11

NOT TO SCALE

Surface grid spacing is 4 miles

NOT TO SCALE

E'

EXPLANATION 

Model cellSpecified-head boundary

Head-dependent flux boundary (stream cell) Used to simulate principal streamsGeneral head-dependent flux boundary Used to simulate exchange of water at the regional water table. 

Recharge applied to model cell

| I! Moraine deposits 

Outwash deposits Carbonate-rock aquifer 

[ y Upper weathered zone water-bearing unit 

^^^ n No-flow boundary

| Limit of water in carbonate-rock aquifer with dissolved-solids concentration less than 10,000 milligrams per liter

Upper confining unit Shows where confining-unit resistance to vertical flow is simulated

Contact between hydrologic unit parameter zones

Trace of section

FIGURE 22. Sectional diagrams showing the areal extent and boundaries of the model layers used to simulate the Midwestern Basins and Arches aquifer system

(lines of section on figure 21). n
C34 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Each model cell is 4 mi on a side. This cell spacing was 

chosen to allow the curvature in the regional potentiometricsurface map of the carbonate-rock aquifer in figure 12 to be 

represented by the model. It was assumed that flow in the 

fractured carbonate-rock aquifer behaves as flow in a porous 

medium at this simulation scale. It is noted in the "Hydraulic 

Characteristics" section of this report that this is assumption 

is probably valid.

The model grid is oriented so that it parallels the edge of 

the carbonate-rock subcrop along the margin of the Illinois 

(structural) Basin. This is because it was outside the scope of 

the investigation to study flow in the carbonate-rock aquifer 

within the Illinois Basin; a hydraulic boundary which is 

easiest to simulate parallel to a finite-difference grid rather 

than a physical boundary is simulated along this edge of the 

model. Anisotropy was not a consideration for model-grid 

orientation; it is assumed that no principal direction of horizontal anisotropy dominates regional flow within the aquifer 

system.

BOUNDARIES, SOURCES, AND SINKS

The following types of boundaries are included in the 

numerical model and are represented on figures 21B and 22. 

Model boundaries were set for a given model construction 

and were not automatically adjusted by the nonlinear regression.

No-flow boundaries

Part of the eastern and all of the northern and northwestern boundaries of model layer 1, which represents glacial 

aquifers and confining units, coincide with principal surfacewater drainage divides that are assumed to coincide with 

ground-water divides in the glacial deposits. The northern 

part of the eastern boundary and most of the western boundary of model layer 1 coincide with regional flowlines in the 

glacial deposits (fig. 11); ground water flows parallel to and 

not across flowlines. The southern boundary of model layer 1 

coincides with the limit of the Wisconsinan ice sheet (fig. 5). 

Glacial deposits are thin or absent, and few glacial aquifers 

are present south of this limit. Horizontal flow in glacial 

deposits beyond the limit of the Wisconsinan ice sheet is 

assumed to be negligible at the regional scale. In addition, the 

limit of the Wisconsinan ice sheet coincides with regional 

flowlines in the glacial deposits throughout much of the modeled area.

The eastern, northern, and part of the western boundaries 

of model layer 2, which represents the carbonate-rock aquifer 

or the upper weathered zone water-bearing unit, coincide 

with flowlines or the position of water in the carbonate-rock 

aquifer with a dissolved-solids concentration of 10,000 mg/L 

or greater (fig. 12; see also fig. 34). Water in the carbonaterock aquifer is assumed to move slowly where it becomes 

saline (greater than 10,000 mg/L dissolved solids) and, for

the purposes of this investigation, no water is assumed to flow 

across these boundaries. In addition, the position of the saline 

waters is assumed to have remained constant over the short 

period of time (tens of years) represented by the model calibration targets. Part of the northwestern boundary of model 

layer 2 coincides with a ground-water divide in the carbonaterock aquifer.

Most of the water in the carbonate-rock aquifer along the 

northeastern model boundary is likely to flow upward into 

overlying glacial deposits and ultimately into Lake Erie. It is 

assumed that no water in the carbonate-rock aquifer flows laterally beyond the shore of the lake. Hanover (1994) notes that 

discharge from the carbonate-rock aquifer to Lake Erie is 

concentrated near the lakeshore. The assumption of no flow 

along this boundary was tested by constructing an alternative 

model with a specified-head boundary in model layer 2. The 

results of this alternative model are nearly identical to the 

results of the calibrated final model in which the no-flow 

boundary forces simulated ground water in the carbonaterock aquifer to discharge through the overlying glacial deposits. This finding indicates that the no-flow boundary does not 

limit the amount of simulated regional ground-water flow that 

can leave the aquifer system at this point.

Ground water flows toward part of the western boundary 

of model layer 2 (fig. 12); however, some of this western 

boundary is simulated as a no-flow boundary (fig. 21B). This 

decision was made because a specified-head boundary in 

model layer 1 (fig. 2\B) was considered sufficient to simulate 

the outward flux from the glacial deposits and the carbonaterock aquifer; at the regional scale, the hydraulic heads in 

these hydrologic units are very similar along this boundary, 

owing to the absence of the upper confining unit. In addition, 

available ground-water-chemistry data (see "Tritium and Carbon Isotopes" section) indicate a notable increase in the age 

of water in the carbonate-rock aquifer just west of the model 

boundary where the aquifer dips beneath the upper confining 

unit. This combination of factors may indicate that much of 

the water in the carbonate-rock aquifer discharges through 

the overlying glacial deposits rather than moving downdip 

into the Illinois (structural) Basin.

The carbonate-rock aquifer is directly underlain by poorly 

permeable interbedded shales and limestones. As a result, a 

no-flow boundary condition is assigned to the lower model 

boundary where it coincides with the bottom of the carbonate-rock aquifer. Simulation of an alternative conceptual 

model, however (see "Model Discrimination" section), indicates that the lower limit of active freshwater flow may actually be within the carbonate rocks of Silurian and Devonian 

age where these rocks are hundreds of feet thick. The lower 

model boundary coincides with the top of the upper confining 

unit where water in the carbonate rock beneath the upper confining unit is saline.

It is assumed that flow in the upper weathered zone waterbearing unit is restricted to shallow depths (tens of feet) and
REGIONAL GROUND-WATER FLOW C35

that any exchange of water between this zone and deeper bedrock units is negligible. Therefore, the lower model boundary 

beneath the upper weathered zone water-bearing unit is simulated as a no-flow boundary.

Specified-head boundaries

The northeastern model boundary in model layer 1 coincides with the shore of Lake Erie. It is assumed that groundwater levels in the glacial deposits along this boundary are 

near lake level and that they can be represented by the average lake level for the long-term period. A specified-head 

boundary condition is also imposed on the southern part of 

the western boundary of model layer 1 where the boundary is 

coincident with the 700-ft equipotential line on the groundwater-level map of the glacial deposits (fig. 11).

Water-level data reported by Eberts (1999) indicate that 

ground-water flow in the bedrock on both sides of the Ohio 

River is toward the river. As a result, the southern boundary 

of model layer 2 coincides with the position of the Ohio 

River. Specified hydraulic heads used to simulate this boundary were derived from 1:250,000 topographic maps. A specified-head boundary condition also is imposed along a small 

section of the western boundary of model layer 2 where notable glacial aquifers, and thus model layer 1, are absent. This 

boundary coincides with the 700-ft equipotential line on the 

potentiometric-surface map of the carbonate-rock aquifer in 

figure 12.

Specif ied-f lux boundary

A specified- flux boundary condition is imposed on the 

most northwestern boundary of model layer 2. Ground-water 

flow across this boundary is approximated by use of Darcy's 

Law,

where K is horizontal hydraulic conductivity,

A is the cross-sectional area through which flow 

 occurs, and 

 is the hydraulic gradient approximated from the

hydraulic-head contours in figure 12.

This boundary flow was manually recalculated and a new 

amount of flow was specified in the model after transmissivity estimates were updated by the model during calibration.

Head-dependent flux boundaries (sources and sinks)

Principal streams that drain the modeled area are explicitly simulated by use of head-dependent-flux boundary conditions (figs. 21B and 22). Although these streams only partially 

penetrate the glacial deposits or carbonate-rock aquifer, analysis of streamflow data indicates that they are discharge

points for regional flow within the aquifer system. Hydraulic 

heads along these stream cells are set equal to the stage of the 

streams.

The upper boundary of the aquifer system coincides 

with the water table, which is generally present in glacial 

deposits but is locally present in bedrock where glacial 

deposits are thin or absent. A discussion of the treatment of 

flow across the water-table boundary in the model can be 

found in the following section.

APPROACH TO MAPPING REGIONAL RECHARGE AND 

DISCHARGE

Anderson and Woessner (1992, p. 152) note that no universally applicable method has been developed for estimating 

ground-water recharge across a water table and that most proposed methods have been used with limited success. 

Although recent investigations have demonstrated that spatial 

variation in the rate of recharge across the water table of an 

aquifer system can be significant (Stoertz and Bradbury, 

1989), modelers have traditionally assumed a spatially uniform recharge rate to simulate the water-table flux across 

areas of similar surficial geology. Such an approach prohibits 

adequate representation of flow across the water table 

because ground-water basins often include areas where the 

net flux is upward (Anderson and Woessner, 1992, p. 152).

A few recently published concepts, which have been used 

by other researchers to simulate a water-table flux, are summarized below. These ideas were considered during construction of the numerical model of regional flow in the 

Midwestern Basins and Arches aquifer system.

Jorgensen and others (1989a, b) and Stoertz (1989) demonstrate that the water-table flux, which is appropriate for 

simulation of an aquifer system, is scale dependent. If the 

size of a model cell is larger than the length of some flow 

paths within the aquifer system, some ground water recharges 

and discharges within the area represented by a single model 

cell. The result is a need to reduce the amount of net recharge 

applied at the water-table boundary of the model to simulate 

the aquifer system correctly at the desired scale. Buxton and 

Modica (1992) show that despite uniformity of surficial geology (and thereby recharge rates) in the physical aquifer system across a modeled area, net recharge may vary across the 

modeled area because the water-table boundary combines the 

effects of recharge from precipitation and ground-water discharge to streams. Stoertz (1989) also notes that a model-cell 

spacing that captures the general water-table curvature is necessary in order to equate simulated recharge with basin yield. 

An additional observation by Stoertz (1989) is that simulated 

patterns of recharge and discharge are not affected if the permeability of the entire basin is changed; however, simulated 

recharge and discharge rates are affected. To map recharge 

and discharge areas and to simultaneously estimate appropriate rates, the modeler must constrain the model solution with 

some measurements of flow such as streamflow or pumpage.
C36 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

In the current investigation, the assumption was made that 

the amount of net recharge appropriate for simulation of 

regional ground-water flow equals the amount of water necessary to maintain the regional trend of the water table and to 

simultaneously supply the principal streams with a base flow 

equal to long-term average ground-water discharge from 

fairly stable flow systems within the aquifer system (mean 

sustained ground-water discharge). This net recharge 

excludes recharge across the water table that discharges near 

the point of recharge by means of evapotranspiration or by 

means of local-flow-system discharge to small tributary 

streams.

Because net regional recharge results from the combined 

effects of recharge from precipitation and local ground-water 

discharge, net regional recharge is simulated in the numerical 

model by applying a uniform rate of recharge to areas of similar surficial geology and allowing recharge in excess of the 

appropriate net regional recharge to discharge by application 

of a general head-dependent-flux boundary condition above 

the uppermost active model layer (figs. 21 and 22). This general head-dependent-flux boundary condition is not coincident with the head-dependent-flux boundary condition used 

to simulated the principal streams (stream cells); no recharge 

is applied to stream cells because the principal streams are 

areas of known regional discharge, and estimates of discharge 

from regional flow systems to these streams are used to constrain the model solution.

Hydraulic heads specified for the general head-dependent-flux boundary condition used to help simulate the 

exchange of water at the regional water table are equal to the 

altitude of the regional water table. These altitudes were estimated by a method in which digital topographic data and 

empirical equations relate water-table altitudes and land-surface topography (Williams and Williamson, 1989). The conductance term for the general head-dependent-flux boundary 

condition is defined to be proportional to the total length of 

small tributary streams in each model cell (fig. 20) because 

such streams are assumed to dominate the exchange of water 

at this boundary.

The inclusion of the general head-dependent-flux boundary condition in the numerical model allows for simulation of 

some discharge from regional flow systems to areas that are 

not coincident with the principal streams. Such upward net 

flux across the regional water table may include water that 

flows from the point of recharge by way of regional flow systems and subsequently leaves the aquifer system through 

evapotranspiration or discharge to springs, seeps, ditches, and 

streams smaller than those represented by the stream cells in 

the model. This approach to simulation of the water-table 

boundary also allows for simulation of horizontal flow in the 

water-table aquifer.

Regional recharge and discharge areas were mapped on a 

cell-by-cell basis by computing the difference between the 

amount of recharge applied to the uppermost active model

layer and the amount of water lost by means of the headdependent-flux boundary conditions. In localized areas near 

regional potentiometric highs, recharge to the deep regional 

flow systems may be higher than the amount of recharge 

applied to the model in areas with similar surficial geology. In 

these places, additional water may enter the simulated 

regional flow systems by means of the general-head-dependent-flux boundary condition. No net regional recharge or 

discharge was simulated or mapped where layer 2 is the 

uppermost active model layer and the carbonate-rock aquifer 

is isolated from the water table by the upper confining unit 

(fig. 21).

Head-dependent-flux boundary conditions have been used 

by other modelers to simulate the flux across a regional 

water-table boundary (Williamson and others, 1990; Leahy 

and Martin, 1993). Because the Midwestern Basins and 

Arches aquifer system is a relatively unstressed steady-state 

system at the regional scale, the application of a head-dependent-flux boundary condition in this investigation had to differ slightly from previous applications. Specifically, the 

approach taken in this investigation, as described above, 

allows net regional recharge to be computed by a steady-state 

model on a cell-by-cell basis while horizontal flow in the 

water-table aquifer is simulated. This is possible because, in 

addition to observations of hydraulic head, base-flow observations along the stream cells are included in the model. The 

combination of hydraulic-head and base-flow observations 

was necessary to prevent the general head-dependent-flux 

boundary condition from overly constraining the model solution.

PARAMETERIZATION

To simulate steady-state regional ground-water flow in the 

aquifer system, the modeler specified the following system 

characteristics: (1) horizontal hydraulic conductivity or transmissivity, (2) vertical hydraulic conductivity, (3) streambed 

hydraulic conductivity, (4) recharge, and (5) a conductance 

term for the general head-dependent-flux boundary condition 

used to help simulate flux at the regional water table. These 

quantities were calculated by means of 16 parameters (a 

quantity that is estimated by use of trial and error or nonlinear 

regression) because it was found that regional ground-water 

flow in the aquifer system could be reasonably simulated with 

this few parameters. In addition, for reliable estimation of 

parameter values, the number of parameters must be a fraction of the number of observations of ground-water levels and 

flows used to estimate them (Hill, 1992, p. 15).

Horizontal hydraulic conductivity in layer 1, used to simulate glacial deposits, is simulated with three parameters. The 

corresponding parameter zones (areas over which a parameter 

value is applied uniformly) are shown in figure 23A and represent areas of moraine deposits, outwash deposits, and glaciolacustrine deposits. The horizontal hydraulic conductivities
REGIONAL GROUND-WATER FLOW C37

are effective values that represent the combined effects of 

sands and gravels (glacial aquifers) and clayey till (glacial 

confining units) on regional ground-water flow. (These horizontal hydraulic conductivities are multiplied within the computer program by specified saturated thicknesses to compute 

transmissivity.) Transmissivity in layer 2 is simulated with 

two parameter zones representing the carbonate-rock aquifer 

and the upper weathered zone water-bearing unit (fig. 23B). 

(In the "Model Discrimination" section of this report, results 

are presented for an alternative model in which the parameter 

value for the carbonate-rock aquifer zone is horizontal 

hydraulic conductivity rather than transmissivity. In this alternative model, the carbonate-rock aquifer's transmissivity varies systematically with aquifer thickness.)

The vertical hydraulic conductivity between the glacial 

deposits (layer 1) and the bedrock (layer 2) (fig. 23) is simulated with four parameters. One parameter is used to represent the vertical hydraulic conductivity of the upper confining 

unit. The other three represent the effective vertical hydraulic 

conductivities of the glacial deposits and the underlying bedrock where the shale is absent. The associated parameter 

zones coincide with areas of moraine deposits underlain by 

bedrock, outwash deposits underlain by bedrock, and glaciolacustrine deposits underlain by bedrock.

Streambed hydraulic conductivity is simulated by use of 

two parameters. One parameter is used to simulate most 

streams within the modeled area, and the other is used to simulate the effect of the upper confining unit where it separates 

the streams and the carbonate-rock aquifer in the southeastern 

part of the modeled area (fig. 21). Streambed thickness and 

area for each stream cell are specified.

Recharge from precipitation is simulated with four 

parameters. The principal recharge zone represents recharge 

to moraine deposits (ground- and end-moraine deposits) or 

locally to the carbonate-rock aquifer. The other smaller zones 

represent recharge to outwash deposits, glaciolacustrine 

deposits, or the upper weathered zone water-bearing unit 

directly (fig. 23).

Finally, the conductance term of the general head-dependent-flux boundary condition used to help simulate the 

exchange of water at the regional water table (fig. 2\B) is 

simulated by use of one parameter. This conductance parameter is multiplied by the lengths of small streams present 

within each respective model cell to attain the conductance 

needed by the head-dependent boundary package of MODFLOWP.

MODEL CALIBRATION

Calibration of a numerical ground-water flow model is the 

process of finding a set of boundary conditions, parameter 

values, and stresses that produce simulated ground-water levels and flows that match field-based measurements or estimates within a preestablished range of error (Anderson and 

Woessner, 1992, p. 223). The difference between the 

observed and simulated ground-water levels and flows are 

hydraulic-head and flow residuals, respectively. The observed 

values used for the regional ground-water flow model include 

389 synoptic measurements of ground-water levels in the carbonate-rock aquifer and the upper weathered zone waterbearing unit, and 43 estimates of mean sustained groundwater discharge to principal streams that represent long-term 

steady-state conditions in the aquifer system. (These data are 

discussed in the "Levels and Discharge" sections of this 

report.) No observed ground-water levels in the glacial deposits were included in the model.

An estimate of the standard deviations for the errors in 

these observations was made in advance of model simulations 

to calculate weights for the regression, discussed below in the 

"Procedure" section. The estimated standard deviations for 

the errors in the ground-water-level data include the error 

associated with determination of the measuring-point elevations from topographic maps, deviation of measured values 

from long-term average ground-water levels (Eberts, 1999). 

and vertical hydraulic gradients in the aquifer system due to 

measurement of open-hole wells that may not represent water 

levels strictly associated with the regional flow systems. 

These sources of error were evaluated for each measurement; 

standard deviations of the errors ranged from 6 to 12 ft.

Estimates of mean sustained ground-water discharge to 

selected streams were assumed to be appropriate calibration 

values for simulation of steady-state regional flow in the aquifer system. These means range from 88 to 98 percent streamflow duration streamflow that is equaled or exceeded 88 to 

98 percent of the time and all but four of the means fall 

between 88 and 94 percent streamflow duration. (Previous 

researchers (Cross, 1949; Schneider, 1957) have used streamflow that is exceeded 90 percent of the time as an approximate index of dry-weather flow in Ohio.)

For calculating the weights in the regression (see below), 

it is assumed that the error associated with the estimates of 

mean sustained ground-water discharge has a 90 percent 

chance of being 20 percent of the estimated discharge. Estimation of standard deviations associated with these values 

followed procedures described in Hill (1992, p. 49).

PROCEDURE

An automated nonlinear-regression approach to calibration developed by Cooley and Naff (1990) and extended for 

complicated three-dimensional problems by Hill (1992) was 

used in this investigation. Specifically, parameter values were 

automatically adjusted to achieve the smallest possible value 

of the objective function. The objective function in this 

method is the weighted sum of squared differences between 

observed and simulated hydraulic heads and flows:
C38 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Lake 

Michnicn

Evsnsvi!

Base from U.S. Geological Survey digital data. 

1:2,000,000,1972

0 20 40 60 MILES

I 

0 20 40 60 KILOMETERS

EXPLANATION 

Glacial deposits

Moraine deposits Ground and end moraines 

Glaciolacustine deposits 

Outwash deposits

Upper confining unit Shows where confining-unit resistance to 

vertical flow is simulated between model layers

Boundary of model layer 1 

Boundary of model layer 2

FIGURE 23. Zones used for model parameterization: (A) model layer 1
REGIONAL GROUND-WATER FLOW C39

B

Lake 

Michigan

- »\Ch .nit

\Huntlngt o»

W6ST 

V If! U INI A

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES

0 20 40 60 KILOMETERS

EXPLANATION 

Carbonate-rock aquifer

Upper weathered zone water-bearing unit

Upper confining unit Shows where confining-unit resistance to 

vertical flow is simulated between model layers

Boundary of model layer 1 

Boundary of model layer 2

FIGIRK 23. Continued Zones used for model parameterization: (B) model Layer 2
C40 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

SSE = £[w/ /2^.] 2 i = (2)

where ef is the difference between the observed and calculated values of measurement /, 

Wf 2 is the square root of the weight assigned to the 

error in the observed value of measurement /;

I /9 Wj Cj is the weighted residual corresponding to measurement /; and

n is the number of observations.

The weights in this equation reflect the assumed reliability 

(standard deviations) of the hydraulic-head measurements or 

flow estimates (observations) and account for the different 

units of measure associated with hydraulic heads (L) and 

flows (L /T). The weights equal 1 divided by the variance of 

the observation error. The parameter values that correspond to 

the smallest SSE possible for the parameterization and 

boundary conditions imposed on the model are called the 

optimal parameter values.

Scaled sensitivities for each of the model parameters also 

can be computed by use of the nonlinear regression method. 

Scaled sensitivities equal

a*, - 1,11,7 = (3)

where b: is one of the model parameters and

y} is a calculated hydraulic head or flow. 

A comparison of the scaled sensitivities for various parameters (hydraulic conductivity, transmissivity, recharge, conductance terms) in a specific model provides information on the 

relative effect of each parameter in the regression. Relatively 

small scaled sensitivities are associated with parameters that 

have little effect on simulated results and cannot be estimated 

by nonlinear regression.

Application of the nonlinear-regression procedure of 

model calibration in this investigation ensures that model 

error is due to model design rather than to suboptimal parameter values. This enables comparison of various model 

designs so that various aspects of the numerical and conceptual models can be tested. In general, the best models have (1) 

the smallest parameter coefficients of variation, (2) parameter 

correlations of less than 0.95, (3) the smallest calculated error 

variance (SSE divided by the difference between the number 

of observations and the number of estimated parameters 

(Draper and Smith, 1981)), and (4) weighted residuals that 

are normal, independent, and of equal variance. On the basis 

of these standards, the calibrated final model presented herein 

is the best representation of regional ground-water flow in the 

Midwestern Basins and Arches aquifer system among the

alternatives tested. (A brief discussion of what was learned 

from two alternative models is found in the section "Model 

Discrimination.")

ESTIMATES OF PARAMETER VALUES

For 8 of the 16 model parameters described in the 

"Parameterization" section of this report, scaled sensitivities 

are large enough for the parameter values to be estimated by 

nonlinear regression. The number and location of observations used for the model calibration affect these scaled sensitivities and are directly responsible for which parameters can 

be estimated. Estimated parameters in the calibrated final 

model, ordered from highest to lowest in terms of sensitivity, 

include (L) transmissivity of the carbonate-rock aquifer, (2) 

horizontal hydraulic conductivity of the moraine deposits, (3) 

recharge applied to the moraine deposits, (4) effective vertical 

hydraulic conductivity of the combined moraine/bedrock 

areas, (5) the conductance term for the general head-dependent-flux boundary condition used to simulate the regional 

water table, (6) hydraulic conductivity of streambeds 

throughout most of the modeled area, (7) horizontal hydraulic 

conductivity of the outwash deposits, and (8) vertical hydraulic conductivity of the upper confining unit.

The other model parameters were assigned values from 

available data in the literature and were held constant during 

the regression. Any adjustments to these values were made by 

trial and error. These values include an effective vertical 

hydraulic conductivity of 0.1 x 10"2 ft/d for the combined glaciolacustrine/bedrock deposits and 0.1 ft/d for the combined 

outwash/bedrock deposits. The value for the hydraulic conductivity of the streambeds that are underlain by the upper 

confining unit is set at 0.1 x 10"2 ft/d. Horizontal hydraulic 

conductivities of the glaciolacustrine deposits and the upper 

weathered zone water-bearing unit are set at 0.05 and 0.06 

ft/d, respectively. Values for recharge applied to the glaciolacustrine deposits, the upper weathered zone water-bearing 

unit, and the outwash deposits range from 0.1 x 10"2 to 11.8 

in/yr.

Estimated values for the optimal parameter set from the 

calibrated final model are listed in table 3. Each of the parameter estimates falls within the range of published field values 

for the Midwestern Basins and Arches aquifer system where 

data are available (table 2). The estimated transmissivity for 

the carbonate-rock aquifer not only is within the range of 

field-determined estimates of transmissivity but also is within 

16 percent of the geometric mean of these values. The estimated effective horizontal hydraulic conductivity for the 

moraine deposits falls within the range of textbook values for 

these materials (Freeze and Cherry, 1979). The estimated 

recharge value in table 3 does not represent net regional 

recharge to the regional flow systems nor does it represent all 

recharge to the entire aquifer system, which would include 

recharge to local, intermediate, and regional flow systems.
REGIONAL GROUND-WATER FLOW C41

TABLE 3. Parameter estimates and reliability of the optimal parameter set from the calibrated final model of regional flow

in the Midwestern Basins and Arches aquifer system 

[ft 2 /d, feet squared per day; ft/d, feet per day; in/yr, inches per year]

Parameter

Transmissivity of the carbonate-rock aquifer

Horizontal hydraulic conductivity of the moraine deposits

Recharge applied to the moraine deposits b

Effective vertical hydraulic conductivity of the combined

Parameter 

estimate

I,610ft 2 /d

21.3 ft/d

2.15 in/yr

0.375 xl(T2 ft/d

Approximate 95 -percent 

linear confidence interval

1,030-

13.7-

1.41-

0.139xlO-2-

2,500 ft2 /d

-33.1 ft/d

2.88 in/yr

0.101 x IQ-'ft/d

Relative 

parameter 

reliability8

0.23

.23

.18

.59

moraine/bedrock areas

Conductance term for the general head-dependent flux 0.259 ft 2 /d 

boundary condition used to simulate the regional water table b

Hydraulic conductivity of streambeds throughout most of 0.0149 ft/d 

the modeled areac

Horizontal hydraulic conductivity of the outwash deposits 168 ft/d 

Vertical hydraulic conductivity of the upper confining unit 0.466 x 10"3 ft/d

0.161-0.418 ft 2 /d .25

0.0045 - 0.05 ft/d .77

46.1-620 ft/d .87

0.645 x 10^- 0.338 x 10" ft/d 1.80

aCoefficient of variation for the recharge parameter; comparable measure of reliability for the other parameters, which were log-transformed for the

regression. Smallest values indicate greatest parameter reliability. 

The estimated recharge value is not net recharge to regional flow systems; net recharge to regional flow systems is computed by subtracting the flux

associated with the head-dependent flux boundary conditions from this estimated value of the recharge parameter on a cell-by-cell basis (fig. 28). 

c Reported values range from 0.0007 - 18.7 ft/d (Meyer, 1978; Smith and others, 1985; Cunningham, 1992, Dumouchelle and others, 1993).

Rather, it represents the recharge rate applied to the moraine 

deposits that may be used in conjunction with the effects of 

the general head-dependent-flux boundary condition to estimate regional recharge rates. In a few areas near the regional 

potentiometric highs, the estimated recharge value was not 

great enough to balance observations of hydraulic heads and 

flow used to constrain the model solution, so water entered 

the model by means of the general head-dependent-flux 

boundary condition. This result was expected because the 

amount of recharge that reaches the deepest parts of an aquifer system is typically greatest near regional potentiometric 

highs. Net values of regional recharge or discharge are not 

apparent from table 3 but are presented in map form later in 

this report.

Model output indicates that no parameter correlations 

exceed 0.90. The greatest correlation (0.87) is between the 

recharge parameter and the conductance parameter associated 

with the general head-dependent-flux boundary condition.

SIMULATED HYDRAULIC HEADS

A total of 389 measured ground-water levels in the carbonate-rock aquifer and the upper weathered zone waterbearing unit (model layer 2) were used as observations in the 

regression. No water levels in the glacial deposits (model 

layer 1) were used because the available data, which are from

drillers' logs, are likely to reflect a local water table associated with local flow systems not explicitly simulated in the 

model. In addition, estimates of the regional water table, 

which is typically present in glacial deposits, were included 

as part of the general head-dependent-flux boundary condition.

A comparison of simulated and measured potentiometric 

surfaces in the carbonate-rock aquifer is illustrated in figure 

24. Simulated equipotential lines closely follow equipotential 

lines contoured from measured ground-water-level data. 

Observation locations are coded on the map to indicate locations where the simulated and measured (observed) hydraulic 

heads differ by less than three times the standard deviation of 

the errors associated with the observation. Locations where 

simulated hydraulic heads are above or below this range also 

are noted. Figure 24 indicates that the simulated hydraulic 

head most commonly differs from the observed hydraulic 

head by more than three times the standard deviation of the 

errors associated with the observation in the areas along the 

Great Miami River and along the northeastern and southeastern edges of the model. Although these patterns indicate 

some lack of model fit in these areas, the overall model fit is 

good.

A graph of weighted residuals plotted against weighted 

simulated values (Draper and Smith, 1981; Hill, 1994) (fig. 

25) shows that the hydraulic-head residuals are indeed ran-
C42 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

38° -

Base from U.S. Geological Survey digital data. 

1:2,000,000,1972

EXPLANATION 20 40 60 KILOMETERS

Upper weathered zone water-bearing unit 

Not an aquifer. Carbonate-rock aquifer absent

Potentiometric contour Carbonate-rock aquifer. 

Contour interval 100 feet, datum is sea level

- 700- - - Simulated

 ?w- Measured

 Boundary of model layer 2

Observation location

o Reasonably predicted. Simulated and observed hydraulic

heads differ by less than three times the standard deviation 

of the errors associated with the observation

 Underpredicted. Simulated hydraulic head less than observed 

hydraulic head by more than three times the standard deviation of the errors associated with the observation

© Overpredicted. Simulated hydraulic head greater than observed 

hydraulic head by more than three times the standard deviation of the errors associated with the observation

FIGURE 24. Simulated and measured (observed) hydraulic heads in the carbonate-rock aquifer and the upper weathered zone

water-bearing unit (model layer 2).
REGIONAL GROUND-WATER FLOW C43

20 30 40 50 60 70 80 90 100

WEIGHTED SIMULATED VALUES, DIMENSIONLESS

EXPLANATION 

+ Weighted hydraulic-head residuals

O Weighted flow residuals

FIGURE 25. Weighted residuals of hydraulic heads and flows plotted against weighted simulated values from the regional ground-water

flow model for the Midwestern Basins and Arches aquifer system.

dom and have equal variance. Results of a runs test printed by 

MODFLOWP show that the hydraulic-head residuals are also 

independent. [The runs test takes into account the order of the 

residuals; too few runs commonly indicates positive serial 

correlation between residuals at individual locations (Hill, 

1992).]

The root mean squared (RMS) error associated with 

hydraulic heads, which is the average of the squared differences in measured and simulated hydraulic heads, is another 

measure of model fit. Anderson and Woessner (1992, p. 241) 

note that if the ratio of the RMS error to the total head loss in 

the system is small, then the errors are only a small part of the 

overall model response. The RMS error computed from measured and simulated hydraulic heads in the regional groundwater flow model is 40 ft. The total head loss from the highest 

recharge area to the lowest discharge area in the model is 710 

ft. The ratio of the RMS error to the total head loss in the system is 0.06. In summary, the model errors are only a small 

part of the overall model response; thus the model satisfactorily approximates ground-water-level observations.

SIMULATED FLOWS

Simulated and observed flows (estimates of mean sustained ground-water discharge) along 43 stream reaches were 

compared to help evaluate overall model response. Estimates 

of mean sustained ground-water discharges and simulated 

flows are listed by stream reach in figure 26. These values are 

difficult to compare without knowledge of the error associated with the observation for each stream reach. This is 

because each streamflow-gaging station that bounds a 

selected stream reach is assumed to contribute the same 

amount of error to the observation; some stream reaches are 

bounded by one streamflow-gaging station, whereas others 

are bounded by as many as five. Mean sustained groundwater discharges to stream reaches bounded by five streamflow-gaging stations are less well known than observations 

for other reaches bounded by fewer gaging stations. Stream 

reaches in figure 26 are coded to indicate locations where the 

simulated and observed flows differ by less than three times 

the standard deviation of the errors associated with the observation. Most simulated flows fall within this range. Reaches
C44 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

WEST

VIRGINIA

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES

0 20 40 60 KILOMETERS

EXPLANATION 

Boundary of modeled area

Ground-water discharge to selected stream reach Color corresponds to deviation of simulated from observed flow. Upper number is estimated mean 

sustained ground-water discharge (observed flow), in cubic feet per second. Lower number is simulated ground-water discharge, in cubic feet per second

~ Reasonably predicted Simulated and observed flows differ by less than three times the standard deviation of the errors associated with the observation 

 Underpredicted Simulated flow less than observed flow by more than three times the standard deviation of the errors associated with the observation

Overpredicted Simulated flow greater than observed flow by more than three times the standard deviation of the errors associated with the observation 

Streamflow-gaging station

FIGURE 26. Simulated ground-water discharge to selected stream reaches from regional flow systems and 

estimated mean sustained ground-water discharge (observed flow) to the reaches.
REGIONAL GROUND-WATER FLOW

where simulated flows are above or below this range are also 

noted.

The graph of weighted residuals plotted against weighted 

simulated values in figure 25 indicates that the flow residuals 

are approximately random and have nearly equal variance 

except for weighted simulated values greater than about 40. 

These values are consistently less than observed values. 

(Negative weighted flow residuals calculated by MODFLOWP indicate underprediction of flow because the convention within the model is to represent ground-water losses to 

streams as negative values.) Simulated flows underpredict the 

flow observations slightly more often than they predict and 

overpredict them. All of the underpredicted stream reaches 

are in the upbasin areas; all of the streams at the bottom of the 

basins are well predicted considering the error on the observations. An alternative model that was constructed to test a 

hypothesis about why some upstream reaches are underpredicted in the regional ground-water flow model is discussed 

below. Results of a runs test for combined hydraulic-head and 

flow residuals, however, indicate randomness among 

weighted residuals.

MODEL DISCRIMINATION

Model discrimination is the process of comparing different hypotheses about an aquifer system by comparing results 

of models constructed using the different hypotheses (Hill, 

1992). Two alternative models were developed to test two 

hypotheses used in the construction of the regional groundwater flow model. For the first alternative model, it was 

hypothesized that flow to streams in the upbasin areas may be 

underpredicted because a notable amount of ground water 

that sustains flow in the principal streams during the driest 

periods may be recharged at the water table within the area of 

the stream cells. No recharge is applied to the stream cells in 

the calibrated final model, therefore, this intracell flow is not 

represented in the model. Such a model design would have a 

greater affect on model calibration along upstream reaches as 

compared to downstream reaches because the area represented by the stream cells makes up a greater proportion of 

the drainage basins associated with upstream reaches.

To test whether some upstream reaches were underpredicted simply because no recharge is applied to the stream 

cells, an additional recharge parameter that represents 

recharge to stream cells was added to the model. The optimal 

recharge rate for this new recharge parameter is virtually 

zero, and the same stream reaches are underpredicted by this 

new model. In other words, the results of the new model are 

similar to the results of the model without the additional 

recharge parameter. It was concluded that lack of recharge to 

stream cells in the calibrated final model is not a factor that 

affects the overall model response. Locally, however, the 

underprediction of upstream reaches that flow along highly 

permeable outwash valleys, such as the Mad River (a tributary to the Great Miami River), may be related to this lack of 

simulated recharge.

Because simulation of an increased amount of curvature 

at the water table equates with simulation of an increased 

amount of flow within an aquifer system (fig. 19), the underprediction of some upstream reaches in the calibrated final 

model is possibly related to the cell spacing and the inability 

of the selected spacing to capture the curvature of the water 

table necessary to balance mean sustained ground-water discharge to the underpredicted stream reaches. Such a scale 

effect would be smallest in relation to the most downstream 

reaches because a greater proportion of sustained groundwater discharge to these streams is associated with the most 

regional trends of the water table, which are well represented 

by the coarse cell spacing of the calibrated final model. In 

other words, the mean sustained ground-water discharges 

used as observations in the regional ground-water flow model 

may be slightly high for some of the upstream reaches 

because of the cell spacing chosen for this investigation.

The calibrated final model presented in this report, however, is a reasonable representation of regional ground-water 

flow in the Midwestern Basins and Arches aquifer system. 

This is demonstrated, in part, by the estimated transmissivity 

for the areally extensive carbonate-rock aquifer, which is 

within 16 percent of the geometric mean of reported transmissivities. On the basis of Darcy's Law, calculated flows 

vary in direct proportion to aquifer transmissivity and hydraulic gradient. If the mean sustained ground-water discharges to 

streams used to help calibrate the numerical model were not 

generally appropriate as calibration targets, transmissivities 

and hydraulic conductivities could not have been so reasonably estimated while hydraulic gradients were so well predicted. Stated another way, if flow observations and thereby 

simulated flows were not generally appropriate for the scale 

of the model, transmissivities or hydraulic gradients would 

have to have been inappropriately adjusted to accommodate 

the associated excess or missing flow.

A second alternative model was used to test whether the 

transmissivity of the carbonate-rock aquifer varies systematically with the thickness of the carbonate rocks. In this model, 

horizontal hydraulic conductivity rather than transmissivity 

of the carbonate-rock aquifer is estimated and multiplied by 

the thickness of the carbonate rocks to determine optimal 

transmissivities. This alternative model construction is graphically depicted in figure 27. It differs from the calibrated final 

model in that the lower part of the carbonate rocks is represented in this second alternative model, whereas the use of a 

single transmissivity in the calibrated final model would be 

similar to a model with a third layer of near-zero transmissivity used to represent the deepest part of the carbonate rocks.

The optimal parameter set for this second alternative 

model includes an estimated vertical hydraulic conductivity 

for the upper confining unit that is orders of magnitude higher 

than is considered reasonable. In addition, vertical hydraulic
C46 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Upper

confining

unit

CarbonatQz

Basal confining unit

Section taken from along A-A'\n figures 2 and 3

Upper confining 

unit simulation

Model layer 1

Model layer 2

Flow not simulated'

Upper confining 

unit simulation

Model layer 1

Model layer 2

PHYSICAL SYSTEM

CALIBRATED FINAL MODEL

Transmissivity of the carbonate-rock aquifer 

is constant. The deepest parts of the carbonate 

rock are not simulated because the horizontal 

hydraulic conductivity is assumed to be near 

zero at depth, owing to lack of productive 

fractures.

SECOND ALTERNATIVE MODEL

Transmissivity of the carbonate-rock aquifer 

increases systematically with increased 

thickness of the carbonate rocks. Horizontal 

hydraulic conductivity is assumed to be 

constant throughout the entire thickness of 

carbonate rock.

EXPLANATION 

Glacial deposits (model layer 1)

Upper confining unit

Carbonate-rock aquifer (model layer 2)

Basal confining unit

FIGURE 27. Diagrams of two regional ground-water flow models used to test whether the carbonate rocks in the Midwestern Basins and

Arches Region are transmissive throughout their entire thickness.

gradients between the carbonate-rock aquifer and the glacial 

deposits are tens of feet greater than gradients considered reasonable. Results of the runs test indicate that the residuals for 

this alternative model are not independent. In general, the 

largest head residuals are associated with the areas where the 

carbonate rock is very thick and computed transmissivities 

are thereby quite large.

To summarize, the second alternative model cannot match 

field conditions and simultaneously accommodate the 

ground-water flows associated with the large simulated transmissivities in areas where the carbonate rocks are very thick. 

It is concluded that the entire thickness of the carbonate rocks 

may not contribute substantially to the transmissivity of the 

carbonate-rock aquifer. This is consistent with findings that 

fractures at depth in the carbonate rocks may not be transmissive (Arihood, 1994). In addition, local anhydrite deposits are 

present at depths greater than 150 ft in the Sandusky Bay 

area, an area where the carbonate rock is very thick (Carlson, 

1991). The presence of anhydrite is indicative of little active 

freshwater flow in the carbonate rock at these depths in at 

least one part of the aquifer system.

RELIABILITY OF PARAMETER ESTIMATES

Confidence intervals on estimated parameter values can 

help indicate the reliability of the estimates. Linear confidence intervals on the parameters can be computed if the 

model is correct and linear in the vicinity of the optimal set of 

values and if the parameters are normally distributed (Hill, 

1994). Beale's measure and its critical values can be used to 

test model linearity (Cooley and Naff, 1990). The Beale's
REGIONAL GROUND-WATER FLOW C47

measure for the calibrated final model is 0.38. This value falls 

between the critical value of 0.046 (a value below which 

Beale's measure would indicate model linearity) and 0.52 

(the value above which would indicate model nonlinearity).

If weighted residuals are independently distributed, the 

model is likely to be correct; if weighted residuals are normally distributed and the model is linear, the estimated 

parameter values are generally normally distributed (Hill, 

1994; Seber and Wild, 1989). To test whether the weighted 

residuals are independent and normally distributed, they were 

compared with expected independent values from a standard 

normal distribution. The RN2 statistic (Hill, 1992) for the calibrated final model is 0.948, which is less than the critical 

value of 0.987 at the 95-percent confidence level. This statistic indicates that the weighted residuals may not be independent and normally distributed. However, this test is more 

restrictive than the less powerful Kolmogorov test (Hill, 

1992, p 63). On the basis of the Kolmogorov test, Yager 

(1993) demonstrated that a RN2 statistic of 0.946 indicates 

that weighted residuals are independent and normally distributed at the 99-percent confidence interval for a model with a 

similar number of parameters and observations as for the calibrated final model. These results indicate that the weighted 

residuals from the calibrated final model are at least nearly 

independent and normally distributed. The distribution of the 

residuals was not investigated further.

Beale's measure indicates that the calibrated final model 

is at least slightly nonlinear; moreover, figure 24 indicates 

some spatial patterns in the weighted residuals for hydraulic 

head, and figure 25 indicates some nonrandomness of the 

weighted residuals for flow. As a result, the confidence intervals given in table 3 should be considered approximate.

The relative reliability of estimated parameter values were 

compared by use of coefficients of variation. Smaller coefficients of variation indicate greater reliability than do larger 

coefficients of variation. For parameters that are log-transformed during the regression, a substitute for the coefficient 

of variation (pseudo coefficient of variation) can be calculated by determining the difference between the upper and 

lower confidence limits divided by the exponential of the estimated parameter value and dividing this result by two times 

the critical value from the Students-t probability distribution 

used to compute the confidence limits. This measure is 

exactly the coefficient of variation for the parameters that 

were not log-transformed and is a comparable measure of 

reliability for those parameters that were log-transformed. All 

of the parameter values except the recharge parameter in the 

calibrated final model of this investigation were log-transformed during calibration. (Hydraulic-conductivity measurements in various geohydrologic situations are commonly 

lognormally distributed (Hill, 1992, p. 18)). Coefficients of 

variation and pseudo coefficients of variation for the optimal 

parameter set of the calibrated final model are listed in table 

3. The parameter values for recharge applied to the moraine

deposits, transmissivity of the carbonate-rock aquifer, and 

horizontal hydraulic conductivity of the moraine deposits are 

the most precisely known parameter values, whereas the vertical hydraulic conductivity of the upper confining unit is the 

least precisely known parameter value.

It should be noted once more that the recharge parameter 

estimate listed in table 3 does not represent net recharge to 

regional flow systems. Because net regional recharge (or net 

regional discharge) was computed by subtracting the simulated flux associated with the head-dependent-flux boundary 

conditions from the flux associated with this estimated value 

of recharge applied to the moraine deposits (see fig. 28), the 

reliability of net recharges or net discharges is not known.

SIMULATED REGIONAL GROUND-WATER FLOW

The calibrated final model can be used to quantify various 

aspects of the conceptual model and to draw conclusions 

about regional ground-water flow in the Midwestern Basins 

and Arches aquifer system. A ground-water budget that quantifies flow associated with the regional flow systems represented by the model is given in table 4; simulated net 

recharges or net discharges across the regional water table 

were used in the computations.

The calibrated final model represents the movement of 

1,292 Mgal/d of water through the parts of the aquifer system 

not greatly affected by seasonal variations in ground-water 

recharge from precipitation; this is approximately 10 percent 

of the total flow in the aquifer system. Ninety-nine percent of 

this water is from recharge at the water table. Seventy-eight 

percent of the water (1,006 Mgal/d) leaves the system by 

means of the principal streams that were explicitly represented in the model by use of the stream cells. Nineteen percent of simulated regional ground-water flow discharges by 

means of seeps, springs, ditches, small streams, or evapotranspiration discharge that was simulated by use of the 

general head-dependent-flux boundary condition. Two percent of the water (24 Mgal/d) leaves the system along the 

margin of the Illinois (structural) Basin. Much of this water 

probably discharges to streams just beyond the model boundary, but some of it may move downdip into the Illinois Basin. 

(Previous work (Cartwright, 1970) demonstrates, by means of 

temperature data, that some water enters the Illinois Basin 

and that water in the deep parts of the Illinois Basin ultimately discharges near the center of the basin through fracture zones associated with faults and anticlines in the basin.) 

Approximately 1 percent (18 Mgal/d) of the water associated 

with regional ground-water flow represented in the model discharges to the Ohio Valley aquifer that follows the Ohio River 

(local-scale alluvial aquifer not represented in the calibrated 

final model) and ultimately discharges to the Ohio River. 

Most of this discharge is from the carbonate-rock aquifer; 

less than 1 Mgal/d is from the upper weathered zone water-
C48 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

bearing unit (not an aquifer). Discharge attributed to regional 

ground-water flow in these bedrock units is a very small percentage of total ground-water discharge to the Ohio River; 

discharge directly to the Ohio River from the Ohio Valley 

aquifer along the boundary of the modeled area has been 

computed at approximately 1,400 Mgal/d (C.G. Norman, 

Ohio River Valley Water Sanitation Commission, written 

commun., 1989). These relative amounts of discharge to the 

Ohio River are consistent with a conceptual model in which 

local flow systems dominate flow in the Midwestern Basins 

and Arches aquifer system. Less than 1 percent of the 

regional ground-water flow represented in the model discharges to Lake Erie.

A ground-water budget that quantifies regional flow 

within the carbonate-rock aquifer, as represented by the

TABLE 4. Simulated ground-water budget of regional flow

systems in the Midwestern Basins and Arches aquifer system

[Mgal/d, million gallons per day]

 .Flow

(Mgal/d)

Percent

«*h"6e ° 

discharge

Recharge

Across the regional trend of the 1,277 99 

water table

From losing stream reaches 15 1 

Total 1,292 100

Discharge

To principal streams 1,006 78

Across the regional trend of the

water table to seeps, springs, 242 19

ditches, small streams, or by

means of evapotranspiration

Along the margin of the 24 2 

Illinois (structural) Basin

From the carbonate-rock 18 1 

aquifer to the Ohio River

To Lake Erie 1 <1

Across the northwestern 1 <1 

boundary of the modeled area

From the upper weathered

zone water-bearing unit to <1 <1

the Ohio River

Total 1,292 100

model, was also computed (table 5). Of the 386 Mgal/d of 

water that moves along simulated regional flow paths in the 

carbonate-rock aquifer, 85 percent enters the aquifer by 

means of percolation through the overlying glacial deposits. 

Fifteen percent is associated with recharge by precipitation 

directly onto the carbonate-rock aquifer. Most of the water 

that enters the carbonate-rock aquifer flows back into the 

overlying glacial deposits. Eight percent of simulated 

regional flow in the carbonate-rock aquifer discharges to 

seeps, springs, ditches, and streams smaller than those represented in the model by use of the stream cells. Less than 1

TABLE 5. Simulated ground-water budget of regional flow

systems in the carbonate-rock aquifer in the Midwestern

Basins and Arches aquifer system

[Mgal/d, million gallons per day]

Flow 

(Mgal/d)

Percent

recharge or

discharge

15

<1

100

84

Recharge

From percolation of water 327 85 

through glacial deposits

Across the regional trend of the

water table from precipitation 59

directly onto the aquifer

From the upper weathered <1 

zone water-bearing unit

Total 386

Discharge

By means of flow into 322 

overlying glacial deposits

Across the regional trend of the

water table to seeps, springs, 32

and small streams

To the Ohio River 18 

To principal streams 9

Along the margin of the 5 

Illinois (structural) Basin

Into the upper weathered zone <1 

water-bearing unit

Across the northwestern <1 

boundary of the modeled area

Total 386 100
REGIONAL GROUND-WATER FLOW C49

percent of the simulated ground-water flow in the carbonaterock aquifer is into the upper weathered zone water-bearing 

unit.

The simulated amount of water that reaches the carbonate-rock aquifer by means of regional flow paths is only 30 

percent of simulated regional ground-water flow in the aquifer system. The rest of the water remains within the glacial 

deposits. Because glacial deposits contribute so much to 

regional ground-water flow as defined in this report, it is not 

surprising that drainage basins where glacial deposits are 

thin, absent, or poorly permeable are associated with small 

amounts of sustained ground-water discharge to streams (see 

"Discharge" section). Such basins include those that drain to 

Lake Erie and those in the southeastern part of the study area 

(fig. 17).

One of the principal objectives of this investigation, 

which was met by use of the calibrated final model, was to 

map regional recharge and discharge areas. This was an 

important objective because water and contaminants that 

enter the aquifer system in regional recharge areas are likely 

to traverse a greater length of aquifer than water that enters 

the system at local recharge areas (Stoertz, 1989). Computed 

net amounts of recharge to and discharge from regional flow 

systems, as represented by the model, were used to construct 

the regional recharge and discharge map shown in figure 28. 

This map is regional in scale; therefore, the map is not meant 

to imply that recharge to flow systems too small to be represented in the model is not possible in areas designated as 

regional discharge areas and that discharge from local flow 

systems is not possible in areas designated as regional 

recharge areas. The map simply implies that more water 

recharges the fairly stable flow systems than discharges from 

such systems within areas mapped as regional recharge areas. 

The opposite holds true for regional discharge areas.

The Midwestern Basins and Arches aquifer system is 

characterized by alternating regional recharge and discharge 

areas, typically on a scale of less than 10 mi, except in the 

northeastern part of the modeled area (fig. 28). Ground water 

generally does not move from recharge areas associated with 

the very highest potentiometric levels (figs. 11 and 12) along 

long, continuous flow paths to areas associated with the very 

lowest potentiometric levels, such as the Wabash and Ohio 

Rivers and Lake Erie, while remaining isolated from additions of recharge. Rather, regional recharge and discharge 

areas are present all along the regional potentiometric gradient depicted by the potentiometric-surface maps in figures 11 

and 12, except in the northeastern part of the study area. 

These patterns of regional recharge and discharge may 

explain the differences in ground-water ages between the 

northeastern part of the study area and the rest of the aquifer 

system (see "Geochemistry" section).

The regional potentiometric high near the Bellefontaine 

Outlier (fig. 6) is associated with some of the highest 

recharge rates. Another area associated with high recharge

rates is the area of extensive outwash deposits north of the 

Wabash River in Indiana (fig. 5). The lowest regional 

recharge rates are associated with the area where the upper 

weathered zone water-bearing unit is exposed at the land surface or is overlain by thin glacial deposits.

High discharge rates are commonly associated with the 

principal streams within the modeled area. Mapped discharge 

areas, however, are not limited to the width of the stream 

cells. Specifically, a broad area (tens of miles) of regional discharge was simulated in the northeastern part of the modeled 

area. This area likely represents an area in which water that 

follows regional ground-water flow paths leaves the system 

by means of ditches, small streams, or evapotranspiration 

because ground-water levels are near land surface. The finegrained glaciolacustrine deposits in the area may be associated with a thick capillary fringe that could help facilitate 

evapotranspiration. In addition, the hydraulic gradient 

towards the Maumee River is minimal. Toth (1963) notes that 

ground-water discharge in basins characterized by low relief, 

such as the Maumee River Basin in the lowlands near Lake 

Erie (fig. 6), takes place between the midline and the bottom 

of the drainage basin. In addition, he notes that only a small 

proportion of the ground water discharges as base flow in the 

principal streams in such basins. This pattern is similar to 

what is observed and simulated for the drainage basins in the 

northeastern part of the modeled area near Lake Erie. 

Because some regional ground-water flow discharges before 

it reaches the streams, estimates of base flow in the streams 

cannot be equated with recharge to the aquifer system in this 

area.

Rates of simulated discharge associated with this broad 

regional discharge area are extremely low. Typically, discharge rates are less than 0.5 in/yr in these areas. It is noteworthy that this broad area of weak discharge is largely 

coincident with an area characterized by the highest concentrations of dissolved sulfide within the modeled area, which 

also indicates that oxygenated recharge is not readily available to this part of the aquifer system (Ohio Department of 

Natural Resources, 1970). In addition, low recharge rates 

may also explain the presence of isotopically distinct ground 

water at depth beneath the Maumee River Basin (see 

"Geochemistry" section).

Areas that are not designated as regional recharge or discharge areas are evident on the regional recharge and discharge map (fig. 28). These are areas in which model layer 2 

was the uppermost active layer of the model and the carbonate-rock aquifer is isolated from the water table by means of 

the upper confining unit. Neither recharge nor the general 

head-dependent-flux boundary condition was applied to these 

model cells.

On an areal map, discharge areas commonly constitute a 

smaller part of the surface area of a watershed than recharge 

areas do (Freeze and Cherry, 1979, p. 197). Percentages of the 

surface area simulated as regional recharge and discharge
C50 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Lake 

Michigan

) Lake { 

Detroit '/St. Clair)

 ^(Chicago

40°

Evansvilie

 »

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

20 40 60 MILES

0 20 40 60 KILOMETERS

EXPLANATION 

Simulated regional recharge, in inches per year

D 0 to 0.5

D Greater than 0.5 to 4.0

 Greater than 4

CH No simulated regional recharge or discharge

Simulated regional discharge, in inches per year

n o to 0.5

D Greater than 0.5

^ Boundary of model layer 1

^^ Boundary of model layer 2

FIGURE 28. Simulated regional recharge and discharge areas. (These areas do not necessarily coincide with recharge and discharge

areas of local flow systems.)
REGIONAL GROUND-WATER FLOW C51

areas were computed for eight selected surface-water drainage basins that drain the modeled area (fig. 29). Regional 

recharge areas are larger than regional discharge areas across 

most of the modeled area; however, regional discharge areas 

predominate in the northeastern part of the study area.

Regional discharge areas that are associated with the largest simulated upward hydraulic gradients are near the downstream end of the Wabash and Scioto Rivers, where model 

layers 1 and 2 are present, and just east of the mouth of the 

Sandusky River. Simulated hydraulic heads in the carbonaterock aquifer in a few of these model cells were above land 

surface. The area east of the mouth of the Sandusky River 

was previously mapped as an area of flowing wells and large 

springs (Breen, 1989).

Patterns of advective regional ground-water flow that 

were computed from the model output are shown in figure 30. 

The flow paths represent flow in glacial deposits and bedrock 

units rather than flow associated with individual aquifers, 

although some individual flow paths may represent flow 

solely in either the glacial deposits or the bedrock units. 

Regional ground-water flow is generally from regional 

recharge areas to adjacent regional discharge areas. Some 

simulated regional flow paths on this figure bypass an adjacent regional discharge area and indicate discharge in an area 

further down the potentiometric gradient. Such areas worth 

noting include the downstream end of the Wabash River (A), 

the areas west of the Scioto River (B) and near Lake Erie (C), 

the area north of the Maumee River (D). and the areas near 

the highest regional potentiometric levels (E) (fig. 30). These 

regional flow paths that bypass adjacent regional discharge 

areas cannot be determined from two-dimensional potentiometric-surf ace maps of the aquifer system. Such flow paths 

develop because regional recharge is available across most of 

the aquifer system; the result is a three-dimensional flow 

field.

The flow paths can also be used to identify areas where 

regional ground-water flow does not discharge to principal 

streams. One example is in the northeastern part of the modeled area (F, fig. 30). Many of the flow paths that discharge in 

this area are relatively short and may be associated with discharge to ditches and streams that are too small to be represented in the model by use of the stream cells or by means of 

evapotranspiration. A second example is the south-central 

part of the modeled area (G, fig. 30). These simulated flow 

paths likely indicate discharge to springs and seeps, which are 

common in the interbedded shales and limestones of the 

upper weathered zone water-bearing unit. They could also 

indicate discharge to small streams.

A ground-water divide is noted in the southeastern part of 

the modeled area (H, fig. 30). Water south of a certain point is 

diverted away from the Scioto River and discharges to the 

Ohio River.

Simulated flow paths near Lake Erie indicate that the 

Lake diverts water away from some of the principal streams

within the area (I, fig. 30). Flow paths near Lake Erie also 

indicate that recharge at the potentiometric high along the 

eastern boundary of the modeled area is likely to be the 

source of water that discharges in the area characterized by 

flowing wells and large springs east of the mouth of the 

Sandusky River (J, fig. 30; see also figs. 12 and 28).

Some of the simulated ground-water flow paths that terminate at Lake Erie are the longest within the modeled area 

(at nearly 50 mi). Simulated flow paths that begin just west of 

the Sandusky River and continue to Lake Erie are associated 

with a recharge area characterized by thin or absent glacial 

deposits. This recharge area has been previously referred to 

as the "limestone ridge area" and is the site of an exposed fossil coral reef (K, fig. 30). Other researchers have noted that 

this area of the carbonate-rock aquifer is vulnerable to contamination; nitrate contamination of ground water in the 

vicinity of this regional recharge area has been recognized 

since 1965 (Richards, 1990).

A comparison of regional ground-water flow paths and 

the position of the continental divide within the modeled area 

(the surface-water drainage divide that separates streams that 

flow toward the Atlantic Ocean from those that flow toward 

the Gulf of Mexico) illustrates the relation between surfacewater and ground-water drainage basins (fig. 30). Groundwater divides associated with regional flow in the aquifer system are generally coincident with surface-water divides. 

Locally, however, deep regional ground-water flow paths can 

cross even major surface-water drainage divides (L, fig. 30). 

The amount of ground water that flows across the major surface-water drainage divides is likely to be a very small percentage of water that moves through the aquifer system 

because very few flow paths cross these divides.

Relative magnitudes (or volumes) of regional groundwater flow are not apparent from the map of regional groundwater flow patterns. Discharge vectors that illustrate the relative magnitude and resultant direction of horizontal regional 

flow within each cell of each model layer are shown in figure 

31. The lengths of these discharge vectors are scaled linearly; 

units are feet cubed per day.

The greatest magnitudes of horizontal regional groundwater flow in the glacial deposits are associated with the most 

extensive outwash deposits. These outwash deposits tend to 

be concentrated along the principal streams within the modeled area. Notably large magnitudes of horizontal regional 

flow can be found in areas of outwash deposits within the 

Wabash River Basin. This drainage basin has the highest 

mean sustained ground-water discharge as percentage of 

mean ground-water discharge to the streams within the modeled area. In the northeastern part of the study area, magnitudes of horizontal regional flow in the glacial deposits are so 

small the vectors do not show up in figure 3L4. Glacial 

deposits in this region are thin, locally absent, or poorly permeable. In addition, horizontal hydraulic gradients are fairly 

low. Such low magnitudes of horizontal regional flow are
C52 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

86°

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

 '

0 20 40 60 KILOMETERS

EXPLANATION 

 ^ Boundary of surface-water drainage basin and basin number

Boundary of modeled area

8

Basin 

Number Basin Name

Percent Regional 

Recharge Area

Percent Regional 

Discharge Area

1 Wabash River Basin 58

2 Maumee River Basin 44

3 Sandusky River Basin 48

4 White River Basin 59

5 Whitewater River Basin 63

6 Great Miami River Basin 62

7 Little Miami River Basin 67

8 Scioto River Basin 52

42

56

52

41

37

38

33

48

FIGURE 29. Percentages of the surface area of selected surface-water drainage basins simulated as regional recharge and discharge areas.
REGIONAL GROUND-WATER FLOW

Lake 

Michigan

38°

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

I ,i i______l I I I i

0 20 40 60 KILOMETERS

EXPLANATION

~~ Boundary of model layer 1 

^^^^" Boundary of model layer 2

 ;> Simulated regional ground-water flow path Shows path from point of regional recharge 

to point of regional discharge

 - Continental divide Shows line of separation of streams that flow toward the Atlantic and 

streams that flow toward the Gulf of Mexico

D Label used to facilitate discussion of regional flow paths

FIGURE 30. Simulated patterns of advective regional flow in the Midwestern Basins and Arches aquifer system.
C54 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

r

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

I ' 

0 20 40 60 KILOMETERS

EXPLANATION 

Boundary of model layer 1

Discharge vector Length of arrow shows relative magnitude and arrow points 

to resultant direction of horizontal flow in each model cell in layer 1

FIGURE 31. Simulated relative magnitudes of horizontal regional flow in the Midwestern Basins and Arches aquifer system:

(A) glacial deposits.
REGIONAL GROUND-WATER FLOW C55

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 KILOMETERS

EXPLANATION

 Boundary of model layer 2

 - Discharge vector Length of arrow shows relative magnitude and arrow points 

to resultant direction of horizontal flow in each model cell in layer 2

FIGURE 31. Continued Simulated relative magnitudes of horizontal regional flow in the Midwestern Basins and Arches aquifer system:

(B) bedrock.
C56 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

consistent with the hypothesis that regional recharge to this 

part of the aquifer system could be limited by the inability of 

the aquifer system to carry ground water away from the area. 

This area is largely coincident with the area of weak regional 

discharge (fig. 28).

Simulated discharge vectors indicate high magnitudes of 

horizontal regional flow in the carbonate-rock aquifer in the 

areas around the regional potentiometric highs (fig. 3 IB). 

High magnitudes of horizontal regional flow are also associated with the downstream end of the Wabash and White Rivers, the margin of the Illinois (structural) Basin, the Ohio 

River, an area west of the Scioto River, and the area east of 

the Sandusky River. Discharge vectors along part of the Lake 

Erie shore indicate that the magnitude of horizontal regional 

flow in the carbonate-rock aquifer in this area is fairly small. 

Ground-water flow may be predominantly vertical in this area 

because it is an area of regional ground-water discharge. Simulated discharge vectors were computed for the upper weathered zone water-bearing unit, but the relative magnitudes of 

flow in this poorly permeable unit are so small that the vectors do not show up at the scale of figure 31B.

It should be noted that the discharge vectors show only 

relative magnitudes of horizontal regional ground-water flow 

and do not indicate flow velocities. Additional information on 

the effective porosity of the aquifers would be necessary to 

compute flow velocities. Appropriate effective-porosity data 

for fractured carbonate rock are difficult to obtain and were 

not available for this investigation. Ground-water ages presented in the following section, however, provide insight into 

ground-water residence times.

The calibrated final model was not used to simulate 

potential effects of future pumpage on regional ground-water 

flow in the aquifer system. Data on future pumpage needs at 

the regional scale are not available, and any simulations of 

future pumpage at this time would be contrived. It is noteworthy, however, that only a small percentage of current pumpage is associated with the regional flow systems explicitly 

simulated with this model. Therefore, more water associated 

with such regional flow systems almost certainly could be 

used. The quality of the ground water associated with some 

parts of the aquifer system, however, may limit its use.

GEOCHEMISTRY

Geochemical data were collected from the Midwestern 

Basins and Arches aquifer system to investigate the relations 

among ground-water chemistry, aquifer mineralogy, and 

present and past patterns of regional flow. The data include a 

synthesis of basic data from more than 1,300 ground-water 

analyses of water samples from the aquifer system, as well as 

detailed chemical and isotopic analyses of ground water and 

aquifer material along general directions of regional flow. The 

analyses represent two hydrologic units (table 1) within the

Midwestern Basins and Arches aquifer system aquifers 

within glacial deposits and the carbonate-rock aquifer and 

were obtained from records in the U.S. Geological Survey's 

National Water Information System (NWIS) data base; files 

of the Indiana Department of Natural Resources, the Ohio 

Department of Natural Resources, and the Ohio Environmental Protection Agency; various published reports; and samples 

collected as part of this investigation. The data were compiled 

and analyzed to investigate the ground-water chemistry of the 

aquifer system on a regional scale. Ground-water chemistry 

of subregional areas of the Midwestern Basins and Arches 

aquifer system is described in the following reports: in Ohio, 

by Ohio Department of Natural Resources, Division of Water 

(1970), Norris and Fidler (1973), Norris (1974), Deering and 

others (1983), Breen and Dumouchelle (1991); and in Indiana, by Geosciences Research Associates, Inc. and Purdue 

University, Water Resources Research Center (1980) and 

Indiana Department of Natural Resources (1988, 1990). 

Analyses of brines from rocks of Silurian and Devonian age 

are found in Stout and others (1932), Lamborn (1952), 

Walker (1959), Stith (1979), Keller (1983) and Wilson and 

Long(1993a, b).

Data compiled from the literature and the available data 

bases were selected on the basis of the following criteria: (1) 

major-ion concentrations (Ca, Mg, Na, Cl, SO4, and HCO3 ) 

were determined, (2) the analyses balanced electrochemically 

within 10 percent and, (3) lithologies of the water-producing 

units were determined. In cases where multiple analyses were 

available for a well, the most recent analysis that met the 

above criteria was selected. The dissolved-solids data for 

most of the analyses that were used in this report were calculated by summing the concentrations of all major constituents 

according to the method described in Fishman and Friedman 

(1989). Dissolved-solids concentrations for waters in the Illinois and Michigan Basins were estimated from borehole geophysical data where available laboratory determinations were 

sparse (DJ. Schnoebelen, U.S. Geological Survey, written 

commun., 1993).

New data that were collected during this investigation 

include detailed chemical and isotopic analyses of ground 

water from the aquifer system along general directions of 

regional ground-water flow, as determined from the map of 

the potentiometric surface of the carbonate-rock aquifer (fig. 

12), and isotopic analyses of aquifer material collected from 

cores of glacial deposits and carbonate rock. The locations of 

the ground-water and aquifer-material samples are shown in 

figure 32. At each sampling location along four transects 

across the aquifer system, ground-water samples were collected from the carbonate-rock aquifer, and, where possible, 

from a glacial aquifer. Sampling was restricted to existing 

domestic wells or test wells; wells with short open intervals 

in the deep parts of the aquifer were generally not available. 

At each sampling location, an attempt was made to sample 

the deepest available well in the carbonate-rock aquifer in
GEOCHEMISTRY C57

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972 I I 

0 20 40 60 KILOMETERS

EXPLANATION

D'

Boundary of study area 

Location of geochemical section

Well sampled for chemical analysis during this investigation 

Number refers to tables 6,1. and 8 ©

Glacial aquifers 557« 

Carbonate-rock aquifer, shallow

Carbonate-rock aquifer, deep

Core sampled for isotopic analysis Number refers 

to table 9

Core samples collected during this investigation 

Mineral samples collected by Botomon and Fauer (1976)

FIGLRE 32. Locations of wells sampled during this investigation, carbonate-rock and glacial cores sampled for isotopic analysis, and

geochemical sections A-A', B-B', C-C', and D-D'.
REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

order to intersect the dominant regional ground-water flow 

paths in the aquifer system. The deep bedrock wells are completed from 100 to nearly 450 ft into the carbonate-rock aquifer. To evaluate the effects of sampling the deep wells with 

long open intervals, RASA investigators also sampled a 

nearby well with a short open interval. These wells were shallow and completed in the top 5 to 50 ft of the carbonate-rock 

aquifer. Well depth and length of open interval for each well 

sampled are listed in table 6. Deep bedrock wells that are 

cased through the top part of the carbonate-rock aquifer are 

also noted in table 6. Where possible, a glacial well, a shallow 

bedrock well, and a deep bedrock well within several miles of 

each other were identified and grouped as a sample site.

Ground-water samples were collected during 1991-92 and 

analyzed for major and minor constituents according to the 

methods described in Fishman and Friedman (1989). Field 

determinations were made for pH, temperature, and alkalinity 

according to methods described in Wood (1976). Dissolved 

sulfide was determined in the field with a Hach DR-2000 

spectrophotometer according to the procedure described in 

the instrument manual (Hach Chemical Company, 1989).

Samples were collected and analyzed for dissolved gas 

concentrations; stable isotope ratios of oxygen, hydrogen, 

carbon and sulfur; and the radioactive isotopes tritium (~H) 

and carbon-14 ( 14C) by use of standard methods. Samples of 

aquifer material were collected from cores in the core repositories of the Indiana Geological Survey and the Ohio Geological Survey. The samples were analyzed for carbon and sulfur 

isotopes by use of standard methods. By convention, stable 

isotope ratios are expressed in delta units (8) as per mil (parts 

per thousand, or per mil) differences relative to the appropriate standard: Standard Mean Ocean Water (SMOW) for oxygen (8 O) and hydrogen (8D), Pee Dee Formation belemnite 

for carbon (8 13C), and Canyon Diablo troilite for sulfur 

(8' S). Tritium concentrations are expressed in tritium units 

(TU), and C values are presented in percent modern carbon 

(pmc). The chemical and isotopic data are listed in tables 7-9.

SPATIAL DISTRIBUTION OF DISSOLVED-SOLIDS

CONCENTRATIONS 

AND HYDROCHEMICAL FACIES

Knowledge of the spatial distribution of dissolved-solids 

concentrations and hydrochemical facies can help in distinguishing recharge and discharge areas, identifying areas of 

ground-water mixing, and estimating relative rates of groundwater flow. Water chemistry in the glacial and carbonate-rock 

aquifers was classified by calculating the dissolved-solids 

concentrations and the percentages of major cations and 

anions composing the total dissolved cations and anions, 

respectively. Maps were prepared showing the areal distribution of dissolved solids and hydrochemical facies indicated 

by the predominant cations and anions. These maps were

used to delimit zones of potable water in the carbonate-rock 

aquifer, to examine the relation between the glacial aquifers 

and the carbonate-rock aquifer, and to evaluate possible 

geochemical and hydrological processes that control the distribution of major dissolved solutes.

Generally, dissolved-solids concentrations in ground 

water increase along flow paths from the surface to the saturated zone and through the aquifer because of dissolution 

of minerals (Freeze and Cherry, 1979). Additional processes 

such as evaporation or evapotranspiration can increase concentrations of dissolved solutes. Some ground waters, notably 

brines, contain extremely high concentrations of dissolved 

solids because of combined effects of mineral dissolution and 

evaporation. Thus, dissolved-solids concentrations generally 

can be used as an indicator of the degree of chemical evolution of ground water in an aquifer system.

The predominant major ions in ground water can also be 

used as indicators of the important chemical and hydrologic 

processes in an aquifer system. Chebotarev (1955) characterized regional changes in the dominant anion species in 

ground water. The general evolutionary sequence that he 

observed for the dominant anions was HCO3 to SO4 to Cl, 

which correlated with increasing distance along the flow path 

and an increasing age of water. This anion sequence also corresponded to an increase in dissolved-solids concentration. 

Water containing predominantly bicarbonate is generally 

present in areas of active ground-water flushing, water containing predominantly sulfate is present where ground-water 

circulation is less active, and water containing predominantly 

chloride is generally present where ground-water flow is very 

sluggish and flushing of the aquifer is minimal. Back (1960) 

introduced the concept of hydrochemical facies based on the 

dominance of cations and anions. Using hydrochemical facies 

and dissolved-solids concentrations, he inferred the general 

sequence of reactions and hence the general directions of 

regional ground-water flow.

DISSOLVED-SOLIDS CONCENTRATIONS

Dissolved-solids concentrations of ground water from the 

Midwestern Basins and Arches aquifer system are shown in 

figures 33 and 34. Dissolved-solids concentrations in water 

from the glacial aquifers (fig. 33) range from less than 100 to 

greater than 2,600 mg/L; the median is 378 mg/L. In more 

than 80 percent of available water analyses for the glacial 

aquifers, dissolved-solids concentrations are less than 500 

mg/L. Ground water with dissolved-solids concentrations less 

than 500 mg/L is found in the glacial aquifers across most of 

the study area, as is ground water with concentrations ranging 

from 500 to 1,000 mg/L. Concentrations greater than 1,000 

mg/L, however, are restricted primarily to the northeastern 

part of the study area. Few analyses of water from the glacial 

aquifers are available for this area because many of the wells 

are completed in the underlying carbonate-rock aquifer rather
GEOCHEMISTRY C59

TABLE 6. Well construction information for wells sampled during investigation of the Midwestern Basins and Arches

aquifer system, 1991-92

[Map number is location in figure 32; ft, feet; , not applicable; N.A., data not available; G, glacial aquifer; C-R, carbonate-rock aquifer;

all depth data are in feet below land surface]

Well name

Ross

Bin

Ben Logan H.S.

Searfoss

Rife

Schoenberger

Beasley

Hill

Gillig

Lamalie

S-18

Finnegan

Wilson

Spencer

Stair

Francis

Augustine

Richard

Kern

Auckerman

Rice

City ofWeston

Hartman

Lacy

Hummel

Jones

Harrison

Martin

Lockhart

Mattingly

Merchant

Gates

Bickford

Bartlett

Desoto Substation

Staggs

Ice

Underwood

Cohee

Ellis

Harmon

Fox

Skiles

Justice

Lee

Starbuck

Geradot

Stenzel

Rees

Map 

number

10 G

10 S

10D

15 G

15 S

15 D

16 G

16 S

16 D

17 G

17 D

18 D

11G

us

11D

12 S

12 D

13 G

13S

13 D

14 S

14 D

19

4G

4S

4D

3S

3D

2G

2S

2D

ID

5G

5S

5D

6G

6S

6D

7G

7S

7D

8G

8D

9G

9S

9D

22

21

20

Well 

depth (ft)

102

86

340

60

39

240

81

80

310

75

340

185

50

43

250

62

320

81

102

400

35

500

144

67

121

201

117

200

140

150

310

320

45

106

265

116

110

302

99

102

293

89

202

62

113

182

260

260

230

Depth to 

bedrock (ft)

76

97

-

34

33

--

52

45

70

2

-

16

18

40

6

-

73

78

32

65

114

--

88

119

108

135

140

170

88

74

88

-

92

161

49

51

63

65

76

72

170

160

Depth to top of 

open interval (ft)

N.A.

80

160

N.A.

34

36

61

63

55

71

180

26

N.A.

22

21

43

9

N.A.

75

109

32

410

114

64

90

122

108

138

136

140

170

88

41

74

91

113

92

162

84

55

53

84

64

59

67

79

70

170

160

Depth to bottom of 

open interval (ft)

N.A.

86

340

N.A.

39

240

81

80

310

75

340

185

N.A.

43

250

62

320

N.A.

102

400

35

500

144

67

121

201

117

200

140

150

310

320

45

106

265

116

110

302

89

102

293

89

202

62

113

182

260

260

230

Length of open 

interval (ft)

N.A.

6

a !80

N.A.

5

205

20

17

255

4

b !60

159

N.A.

21

229

20

311

N.A.

27

C 291

3

d 90

30

3

31

79

9

62

4

10

140

e 232

4

32

174

3

18

140

5

47

240

5

138

3

46

103

190

90

70

Aquifer

G

C-R

C-R

G

C-R

C-R

G

C-R

C-R

G

C-R

C-R

G

C-R

C-R

C-R

C-R

G

C-R

C-R

C-R

C-R

C-R

G

C-R

C-R

C-R

C-R

G

C-R

C-R

C-R

G

C-R

C-R

G

C-R

C-R

G

C-R

C-R

G

C-R

G

C-R

C-R

G,C-R

C-R

C-R

a Well is cased through the top 83 ft of bedrock. 

D Well is cased through the top 110 ft of bedrock. 

: Well is cased through the top 31 ft of bedrock. 

Well is cased through the top 345 ft of bedrock. 

5 Well is open to the New Albany Shale (upper confining unit) from 88 to 204 ft below land surface.
C60 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

TABLE 7. Chemical analyses of selected constituents in water from wells sampled during investigation of the Midwestern

Basins and Arches aquifer system, 1991-92

[Map number is location in figure 32; uS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; ug/L,

micrograms per liter; , data not collected]

Well name

Ross

Birt

BenLoganH.S.

Searfoss

Rife

Schoenberger

Beasley

Hill

Gillig

Lamalie

S-18

Finnegan

Wilson

Spencer

Stair

Francis

Augustine

Richard

Kern

Auckerman

Rice

City of Weston

Hartman

Lacy

Hummel

Jones

Harrison

Martin

Lockhart

Mattingly

Merchant

Gates

Bickford

Bartlett

Desoto Substation

Staggs

Ice

Underwood

Cohee

Ellis

Harmon

Fox

Skiles

Justice

Lee

Starbuck

Geradot

Stenzel

Rees

Map 

number

10 G

10 S

10 D

15 G

15 S

15 D

16G

16 S

16D

17 G

17 D

18 D

11G

US

11D

12 S

12 D

13 G

13 S

13 D

14 S

14D

19

4G

4S

4D

3S

3D

2G

2S

2D

ID

5G

5S

5D

6G

6S

6D

7G

7S

7D

8G

8D

9G

9S

9D

22

21

20

Date of 

collection

08-06-91

08-07-91

07-28-92

08-15-91

08-12-91

08-14-91

08-19-91

08-20-91

08-21-91

08-22-91

08-27-91

06-13-92

08-03-92

07-29-92

08-04-92

08-05-92

08-06-92

07-30-92

08-07-92

08-13-92

08-12-92

08-11-92

09-10-92

09-06-91

09-05-91

09-03-91

10-07-91

09-04-91

09-09-91

09-11-91

09-10-91

10-08-91

09-03-92

09-01-92

08-19-92

08-20-92

09-02-92

08-21-92

09-04-92

08-24-92

08-25-92

08-29-92

08-26-92

08-27-92

08-28-92

05-29-92

08-28-92

08-30-91

08-29-91

Specific 

onductance 

((iS/cm)

2,360

1,010

707

2,400

2,400

2,720

772

1,760

1,660

2,450

2,690

765

1,470

1,010

1,070

991

1,260

2,080

1,610

1,690

1,180

3,250

1,330

670

953

695

595

570

1,140

1,060

908

1,210

958

765

738

723

713

653

673

746

711

683

782

736

834

655

942

588

603

pH

(standard 

units)

6.83

7.10

7.35

7.08

6.93

6.95

7.32

7.37

7.40

7.18

6.99

7.15

7.38

7.02

7.21

7.30

7.34

7.85

7.75

7.20

7.41

7.25

7.00

7.42

7.22

7.19

7.49

7.40

7.23

7.32

7.30

7.41

7.17

7.26

7.26

7.37

7.31

7.44

7.35

7.34

7.31

7.15

7.44

7.20

7.31

7.22

7.24

7.40

7.48

Eh 

(millivolts)

121

512

397

145

58

52

70

126

102

72

3

82

35

176

153

-36

-79

67

127

-132

-88

-96

-

72

95

126

77

56

104

70

-84

97

143

149

128

132

124

76

123

87

124

-1

113

125

-28

76

30

-68

Temperature Dissolved 

(degrees oxygen (mg/L) 

Celsius) (mg/L) °

12.1

11.5

13.8

11.5

11.1

11.2

10.5

11.5

11.6

12.1

12.6

11.8

11.3

12.8

11.4

12.1

11.3

11.5

12.3

12.4

12.8

12.9

13.5

7.4

11.3

11.3

12.2

13.5

12.4

12.1

13.2

13.7

13.0

12.0

13.0

11.4

12.6

11.7

12.5

12.0

12.3

11.9

13.3

12.4

11.6

11.3

13.3

11.8

12.1

0.1

4.5

.9

<.l

.1

<.l

.1

.1

<.l

<.l

<.l

-

<.l

.1

.1

-

<.l

.1

.1

<.l

.1

<.l

<.l

.1

<.l

.2

<.l

<.l

.1

<.l

<.l

.1

.2

.1

<.l

.1

<.l

<.l

<.l

.1

<.l

.1

.1

.1

<.l

<.l

<.l

290

120

87

490

490

540

88

200

220

400

490

100

190

140

140

120

140

250

170

190

130

540

100

86

110

98

76

77

110

110

100

44

110

100

96

100

88

79

95

96

98

75

92

100

100

75

110

62

59

Magnesium Sodium Potassium 

(mg/L) (mg/L) (mg/L)

200

55

34

75

69

79

34

75

66

140

160

47

72

50

56

48

62

88

57

73

62

180

67

36

58

35

26

26

35

46

43

27

44

42

37

32

35

32

32

34

31

29

34

32

39

35

47

29

22

38

16

13

62

46

53

15

71

65

40

27

1.4

47

16

21

19

41

120

120

69

45

74

53

11

22

8.8

14

16

87

43

31

170

32

8.5

10

10

12

19

10

13

7.1

18

17

6.2

13

11

26

22

45

4.5

2.4

1.6

3.9

4.6

5.0

1.4

4.0

4.0

4.2

4.6

1.4

3.5

2.4

3.0

3.6

3.8

4.1

3.2

7.0

2.8

3.7

9.1

1.2

2.0

1.8

1.3

1.0

3.4

2.5

2.3

8.2

1.9

1.5

1.5

1.3

3.2

2.1

2.1

2.2

1.5

1.4

0.90

1.0

2.9

3.4

3.0

2.0

5.1
GEOCHEMISTRY C61

TABLE 7. Chemical analyses of selected constituents in water from wells sampled during investigation of the Midwestern

Basins and Arches aquifer system, 1991 92 Continued

[Map number is location in figure 32; jiS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; ng/L,

micrograms per liter; , data not collected]

Well name

Ross

Bin

Ben Logan H.S.

Searfoss

Rife

Schoenberger

Beasley

Hill

Gillig

Lamalie

S-18

Finnegan

Wilson

Spencer

Stair

Francis

Augustine

Richard

Kern

Auckerman

Rice

City of Weston

Hartman

Lacy

Hummel

Jones

Harrison

Martin

Lockhart

Mattingly

Merchant

Gates

Bickford

Bartlett

Desoto Substation

Staggs

Ice

Underwood

Cohee

Ellis

Harmon

Fox

Skiles

Justice

Lee

Starbuck

Geradot

Stenzel

Rees

Map 

number

10 G

10 S

10 D

15 G

15 S

15 D

16 G

16 S

16 D

17 G

17 D

18 D

11G

US

11D

12 S

12 D

13 G

13 S

13 D

14 S

14 D

19

4G

4S

4D

3S

3D

2G

2S

2D

ID

5G

5S

5D

6G

6S

6D

7G

7S

7D

8G

8D

9G

9S

9D

22

21

20

Bicarbonate, 

as HCO3

(mg/L)

533

397

383

224

266

229

346

168

179

222

280

528

285

463

490

454

478

89

144

283

227

196

472

432

536

466

390

397

427

410

395

407

439

476

420

412

439

441

401

421

376

472

371

377

388

440

395

371

384

Sulfate, 

asSC-4 

(mg/L)

1,200

240

60

1,600

1,600

1,500

120

850

860

1,500

1,600

19

610

180

190

170

180

1,100

750

530

440

1,900

65

33

130

6.5

0.10

<.10

49

40

28

19

83

43

74

48

42

5.0

40

65

65

.40

43

71

85

6.9

190

30

16

Sulfide 

(mg/L)

<0.01

<.01

<.01

.02

<.01

.01

.01

<.01

<.01

.02

0.35

1.1

.04

.02

<.01

.28

3.3

.02

<.01

23.3

2.3

7.4

.03

<.01

<.01

.01

.01

<.01

.02

<.01

4.1

.02

.02

<.01

<.01

.03

.03

.02

.01

<.01

<.01

.11

<.01

<.01

1.8

.01

.03

.13

Chloride 

(mg/L)

7.0

7.2

20

23

6.3

17

6.4

16

15

37

44

2.2

10

28

13

27

100

34

35

110

12

45

100

9.5

9.2

4.4

5.5

5.6

150

140

85

210

61

6.0

4.9

21

5.4

8.6

5.1

12

14

5.3

54

26

39

7.6

35

1.9

2.9

Bromide

(mg/L)

0.12

.04

.04

.08

.08

.15

.05

.19

.25

.45

.36

.02

.09

.08

.07

.12

.16

.32

.31

.51

.09

.38

.06

.09

.04

.10

.04

.09

.20

.06

1.20

.04

.02

.02

.03

.03

.04

.02

.01

.03

.03

.06

.04

.07

.05

.05

.02

.04

Silica 

(mg/L)

22

11

10

17

17

16

13

12

12

17

12

7.2

15

15

20

16

16

11

8.4

6.5

13

9.0

9.5

19

20

15

16

20

12

14

14

7.0

15

18

15

20

17

16

17

16

16

19

17

18

15

10

15

16

11

Dissolved 

solids, 

sum of 

constituents 

(mg/L)

1,930

665

427

2,400

2,430

2,340

454

1,330

1,350

2,270

2,490

439

1,110

668

702

656

784

1,670

1,240

1,150

836

2,860

826

416

630

402

334

344

658

603

501

694

566

459

453

439

426

383

403

452

421

393

445

442

489

368

628

357

355

Iron 

(Hg/U

9,000

4

3

5,200

3,100

2,100

520

1,500

2,200

2,600

780

12

2,700

720

3,100

770

14

1,700

990

13

52

350

180

2,800

2,200

1,200

1,400

1,000

1,600

2,700

64

7

2,200

2,300

1,600

1,900

1,100

890

2,000

2,000

1,800

4,000

2,100

2,300

1,700

8

1,500

370

250

Strontium 

(Jig/U

16,000

13,000

8,900

12,000

12,000

11,000

4,400

14,000

12,000

13,000

12,000

200

17,000

5,000

13,000

27,000

4,700

14,000

22,000

18,000

17,000

10,000

7,100

3,200

11,000

1,300

620

530

220

1,100

1,100

1,800

430

3,000

5,200

1,500

4,800

1,500

1,600

3,000

860

930

510

190

1,500

1,100

5,300

9,700

2,400

Dissolved 

organic 

carbon 

(mg/L)

2.0

1.0

.60

1.3

1.2

1.3

1.1

1.6

1.6

1.3

1.6

2.4

1.0

1.5

1.1

1.3

1.3

1.1

.80

.90

1.5

1.0

3.8

1.1

1.7

1.3

1.7

1.6

1.0

1.0

1.1

1.4

.90

1.4

1.2

1.2

1.2

1.5

1.1

1.2

1.2

3.6

1.7

1.2

.90

1.0

1.8

1.3

1.6
C62 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

TABLE 8. Analyses of isotopes in water from wells sampled during investigation of the Midwestern Basins and Arches

aquifer system, 1991 92 

[Map number is location in figure 32; delta notation (5) explained on p. C92; TU, tritium units; , data not available]

Well name

Ross

Birt

Ben Logan H.S.

Searfoss

Rife

Schoenberger

Beasley

Hill

Gillig

Lamalie

S-18

Finnegan

Wilson

Spencer

Stair

Francis

Augustine

Richard

Kern

Auckerman

Rice

CityofWeston

Hartman

Lacy

Hummel

Jones

Harrison

Martin

Lockhart

Mattingly

Merchant

Gates

Bickford

Bartlett

Desoto Substation

Staggs

Ice

Underwood

Cohee

Ellis

Harmon

Fox

Skiles

Justice

Lee

Starbuck

Geradot

Stenzel

Rees

Map 

number

10 G

10 S

10 D

15 G

15 S

15 D

16 G

16 S

16 D

17 G

17 D

18 D

11G

us

11 D

12 S

12 D

13 G

13 S

13 D

14 S

14 D

19

4G

4S

4D

3S

3D

2G

2S

2D

ID

5G

5S

5D

6G

6S

6D

7G

7S

7D

8G

8D

9G

9S

9D

22

21

20

5 18 0

(per mil)

-8.35

-8.60

-8.55

-8.10

-8.00

-8.10

-8.55

-7.90

-7.85

-8.40

-8.45

-9.05

-7.95

-8.15

-8.15

-8.20

-8.05

-9.65

-9.90

-14.35

-8.40

-10.05

-8.45

-7.85

-7.70

-7.90

-7.15

-7.40

-7.50

-7.40

-7.40

-6.95

-7.70

-7.60

-7.75

-7.75

-7.45

-7.40

-7.65

-7.65

-7.65

-7.25

-7.75

-7.65

-7.75

-7.40

-7.75

-7.60

-7.50

5D

(per mil)

-51.5

-54.0

-54.5

-51.0

-50.0

-50.0

-56.5

^8.5

^9.0

-54.5

-54.5

-59.0

-50.0

-51.0

-50.5

-51.5

-51.0

-65.5

-67.5

-99.5

-55.0

-67.5

-55.5

^7.5

^6.5

^9.0

^4.5

^6.0

^5.5

^6.0

^5.0

^3.5

^7.0

^7.5

^7.5

^6.5

^6.5

^4.5

^7.5

^6.5

^6.5

^5.0

^7.0

-48.5

^7.5

^5.5

^9.0

^7.5

^6.5

8 BC

(per mil)

-14.9

-14.2

-14.1

-16.6

-15.4

-15.7

-13.2

-14.7

-13.7

-10.8

-8.5

-10.0

-15.4

-13.8

-15.0

-14.1

-15.3

-17.2

-13.3

-10.7

-14.1

-7.1

-1.2

-14.6

-14.9

-14.3

-12.7

-14.4

-14.1

-13.8

-13.8

-11.0

-14.9

-14.4

-13.8

-14.5

-15.0

-15.6

-14.7

-15.1

-14.2

-8.8

-13.7

-13.8

-14.2

-3.1

-16.2

-14.9

-17.9

5 34S, 

sulfate 

(per mil)

-10.40

-5.80

4.80

4.00

3.20

3.20

-9.60

-7.60

-9.60

23.30

26.70

-3.20

-9.20

-9.80

-7.70

1.50

-1.80

.30

6.60

29.00

4.40

25.90

20.50

3.10

-7.90

.40

2.40

2.50

2.00

-.80

-7.90

-10.00

-2.10

-2.20

-5.90

-5.50

-3.90

1.90

-.20

3.00

-8.10

9.40

5.50

5 34S, 

sulfide 

(per mil)

-

-

-

^2.40

-22.90

-

--

-53.20

-51.50

-27.80

-33.00

^2.50

--

--

2.2

-

^3.40

-37.80

-12.20

-27.60

Tritium 

(TU)

1.7

9.1

11.2

3.4

<.l

<.l

<1

<.l

<1

<.l

<.l

.5

<.l

10.9

6.2

11.6

10.6

.5

<.l

<1

<.l

<.l

.6

3.0

<.l

<.l

<.l

<.l

16.9

19.1

14.1

<.l

20.6

.3

.2

7.8

.7

<.l

.4

1.6

10.9

<.l

20.0

18.1

15.0

<.l

12.2

<.l

<.l

Carbon- 14 

(percent 

modern)

49.1

58.9

69.5

37.9

30.3

24.5

54.6

6.2

8.6

3.9

4.7

47.7

18.5

58.1

55.7

45.0

62.0

1.3

0.2

8.8

1.4

5.5

60.6

56.1

56.9

43.4

45.6

100.0

78.1

67.5

1.2

78.3

61.8

42.3

66.8

61.8

35.7

58.0

56.9

68.0

44.2

51.3

72.3

65.3

28.1

74.3

35.1

20.8
GEOCHEMISTRY C62

TABLE 9. Analyses of sulfur and carbon isotopes in carbonate rocks and glacial deposits from the Midwestern Basins and

Arches Region 

[Core locations shown in figure 32; delta notation (5) explained on p. C92; , data not available]

Core 

number

a546

a550

a557

Gl

G2

G3

G4

G5

Cl

C2

Geologic unit

Detroit River Group-pyrite

Salina Group-anhydrite

Lockport dolomite-pyrite

Glacial deposits

Glacial deposits

Glacial deposits

Glacial deposits

Glacial deposits

Columbus Limestone

Salina Group

Salina Group

Detroit River Group

Salina Group

Salina Group

Salina Group

Salina Group

Salina Group-bedded gypsum

Lockport Dolomite

Interval sampled 

(feet below land 

surface)

-

-

3-25

5-60

18-70

10-180

12-130

235

273 - 375

425 - 575

35

100

150

301

350

320 - 465

650

8*S, 

sulfate 

(per mil)

24.9

-14.8

-9.8

-9.8

-9.9

-8.2

9.6

-1.0

8.2

26.4

6.5

-.7

28.3

9.2

28.7

14.6

SJ34 C

o S, 

sulfide 

(per mil)

-21.9

-

7.0

-13.1

-14.6

-12.8

-8.0

-14.0

-24.9

-7.8

-1.9

-14.0

-8.8

-7.1

-9.9

-10.0

--

2.4

6 nC

(per mil)

--

-.45

-.05

-.55

.00

1.45

--

.10

-3.20

--

-.25

--

1.30

C3 Salamonie Dolomite

C4 Salina Group

Salina Group-pyrite

105

20-173 

173

18.3

-2.3

3.6 1.35

aData from Botoman and Faure (1976).

than the less productive glacial deposits, which were derived 

from clay-rich lacustrine sediments.

Water from the carbonate-rock aquifer (fig. 34) ranges in 

dissolved-solids concentration from 170 to more than 

350,000 mg/L. Where the carbonate-rock aquifer does not 

underlie the upper confining unit, dissolved-solids concentrations range from 170 to 3,830 mg/L; the median is 560 mg/L. 

Ground water with dissolved-solids concentrations less than 

500 mg/L is present in many areas of the carbonate-rock 

aquifer where it is does not underlie the upper confining unit; 

however, it is found only locally in the northeastern part of 

the study area. Ground water with dissolved-solids concentrations between 500 and 1,000 mg/L is present primarily in the 

northeastern part of the area. Water with dissolved-solids 

concentrations greater than 1,000 mg/L in the carbonate-rock 

aquifer where the upper confining unit is not present is 

restricted to the northeastern part of the study area.

Dissolved-solids concentrations for the carbonate-rock 

aquifer where it underlies the upper confining unit also are 

shown in figure 34. Lines of equal dissolved-solids concentration are drawn for those areas in the basins where dissolved-solids concentrations are greater than 10,000 mg/L. 

The dissolved-solids concentration data for water in the structural basins are from wells completed in oil- and gas-producing zones within the carbonate rocks of Silurian and 

Devonian age. Dissolved-solids concentrations were computed either from chemical data of brines that were reported 

in the literature or calculated from borehole geophysical data 

(D.J. Schnoebelen, U.S. Geological Survey, written commun., 1993).

The distribution of dissolved-solids concentrations in the 

carbonate rocks differs greatly among the three structural 

basins (Appalachian, Michigan, and Illinois) in the Midwestern Basins and Arches Region:

Appalachian Basin Dissolved-solids concentrations 

within the Appalachian Basin range from less than 10,000 to 

greater than 350,000 mg/L; all but two reported brine analyses list dissolved-solids concentrations greater than 200,000 

mg/L. The few analyses that are available from the Appalachian Basin where the dissolved-solids concentrations are 

less than 100,000 mg/L indicate that the zone of transition 

from freshwater to saltwater is narrow. Chloride concentra-
C64 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Lake 

Michigan

J

* Lake \ 

' St. Clair)

IChii aac

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

40 60 MILES

9

0 20 40 60 KILOMETERS

EXPLANATION 

Dissolved solids concentration, in milligrams per liter

Less than or equal to 500

Greater than 500 and less than or equal to 1.000

Greater than 1,000

FIGURE 33. Dissolved-solids distribution in ground water from the glacial aquifers in the Midwestern Basins and Arches aquifer system.
GEOCHEMISTRY C65

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 KILOMETERS

EXPLANATION

Subcrop area of the carbonate- rock aquifer

 10.000 Line of equal dissolved-solids concentration 

Dashed where approximately located. Interval, 

in milligrams per liter, is variable

Dissolved solids concentration, in milligrams per liter

o Less than or equal to 500

o Greater than 500 and less than or equal to 1,000 

O Greater than 1,000 and less than or equal to 3.000 

O Greater than 3,000 and less than or equal to 10,000 

A Greater than 10.000

FIGURE 34. Dissolved-solids distribution in ground water from the carbonate-rock aquifer 

in the Midwestern Basins and Arches aquifer system.
C66 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

tions increase from less than 50 to greater than 9,300 mg/L 

within approximately 7 mi downbasin from where the carbonate-rock aquifer becomes confined above by the upper 

confining unit (Schmidt, 1979).

Michigan Basin In the Michigan Basin, dissolved-solids 

concentrations of water from carbonate rocks of Devonian 

and Silurian age range from less than 10,000 to greater than 

350,000 mg/L. The transition zone from freshwater to highly 

concentrated brine is comparatively wider than in the Appalachian Basin. Brines with dissolved-solids concentrations 

greater than 350,000 mg/L in the Michigan Basin are first 

noted more than 75 mi downbasin from the contact between 

the carbonate-rock aquifer and the upper confining unit, 

where it is exposed at the bedrock surface, whereas similarly 

concentrated brines in the Appalachian Basin are found less 

than 50 mi downbasin from where the carbonate rocks are 

confined.

Illinois Basin Dissolved-solids concentrations of water 

from carbonate rocks of Devonian and Silurian age in the Illinois Basin range from less than 3,000 to greater than 100,000 

mg/L. The contrast in the distribution of dissolved-solids concentrations between the Illinois Basin and the Appalachian 

and Michigan Basins is striking. The largest dissolved-solids 

concentrations in the Illinois Basin are approximately 

150,000 mg/L, much lower than in the other two basins. 

Water with dissolved-solids concentrations less than 10,000 

mg/L is found more than 70 mi downbasin from where the 

carbonate-rock aquifer dips beneath the upper confining unit.

Generally, the dissolved-solids concentration of water in 

structural basins increases with depth. In the Midwestern 

Basins and Arches aquifer system, dissolved-solids concentrations increase as the carbonate rocks dip toward the center 

of each structural basin (fig. 34). However, the relation 

between dissolved-solids concentration and depth is not consistent among the three basins. In the Appalachian and Michigan Basins, water with dissolved-solids concentrations of 

approximately 10,000 mg/L is first noted where the carbonate 

rocks are about 300 to 800 ft below land surface; in the Illinois Basin, water with dissolved-solids concentrations of 

approximately 10,000 mg/L is first noted where the carbonate 

rocks are more than 2,000 ft below land surface. The presence 

of dilute water at such depths in the Illinois Basin is unusual 

and has been investigated by various researchers (Clayton and 

others, 1966; Rupp and Pennington, 1987; Stueber and others, 1987; and Stueber and Walter, 1991). Several hypotheses 

have been advanced to explain the presence of the dilute formation water at the margins of the Illinois Basin and include 

(1) dilution of formation water by meteoric water of Pleistocene age, (2) precipitation of solid phases during mixing of 

formation water with meteoric water, and (3) increased infiltration through fracture systems within the carbonate rocks. 

The origin of dilute formation water in the carbonate rocks of 

Devonian and Silurian age is unclear, and further research is 

necessary to explain more fully the chemical and hydrological processes that have occurred in the Illinois Basin. 

Although water with dissolved-solids concentrations less than 

10,000 mg/L is potentially potable (U.S. Environmental Protection Agency. 1984), the dilute formation water is produced 

as a by-product of oil and gas production and is not used as a 

source of drinking water.

The only regionally extensive areas of water with dissolved-solids concentrations greater than 10,000 mg/L in the 

Midwestern Basins and Arches aquifer system are beneath 

the upper confining unit in the structural basins. However, in 

a small area along Lake Erie near the mouth of the Sandusky 

River, ground water from the carbonate-rock aquifer contains 

dissolved-solids concentrations ranging from 10,000 to 

greater than 70,000 mg/L (Breen and Dumouchelle, 1991). 

Breen and Dumouchelle (1991) evaluated bromide and chloride data and concluded that the source of salinity in this area 

is halite dissolution. Although Janssens (1977) states that the 

extensive halite beds found in the Salina Group in eastern 

Ohio disappear more than 50 mi east of where the saline 

water is present, the data indicate that perhaps an isolated 

remnant of bedded halite may be present in the carbonate 

rocks in this area. All other available data indicate that water 

with dissolved-solids concentrations less than 10,000 mg/L is 

found throughout the total thickness of the carbonate-rock 

aquifer where it does not underlie the upper confining unit. 

Data are not available, however, in the northern part of the 

study area where the thickness of the carbonate-rock aquifer 

increases to more than 2,500 ft. Dilute ground water may not 

be present at depth in this area.

HYDROCHEMICAL FACIES

Identification of hydrochemical facies in the Midwestern 

Basins and Arches aquifer system was based on the distribution of water types in the glacial and carbonate-rock aquifers. 

Water types were defined by use of the trilinear plotting technique (Piper, 1944); the trilinear diagrams are shown in figures 35 and 36.

The seven water types that were identified in the Midwestern Basins and Arches aquifer system and their abundance are listed in table 10. The two most abundant water 

types in the glacial aquifers and in the carbonate-rock aquifer 

where it does not underlie the upper confining unit are 

Ca-Mg-HCO3 and Ca-Mg-SO4 types. Nearly 80 percent of all 

available ground-water analyses from the glacial and carbonate-rock aquifers would be classified as either of these two 

water types. The next most common ground-water types are 

Ca-Mg-Na-HCO3 and Ca-Mg-Na-SO4 and are distinguished 

from the more common Ca-Mg-HCO3 and Ca-Mg-SO4 

waters by an increased percentage of dissolved sodium. The 

Cl-influenced ground-water type, present in small but notable 

quantities, is distinguished by chloride percentages that range 

from at least 20 percent to no more than 60 percent of the 

total anion milliequivalents. Na-Cl or Ca-Na-Cl type water
GEOCHEMISTRY C67

c& <£> % r& O <Z> '"' ^ §) r§> ,S>

CALCIUM CHLORIDE 

PERCENT OF TOTAL MILLIEQUIVALENTS PER LITER

EXPLANATION 

Dissolved-solids concentration, in milligrams per liter

+ Less than or equal to 500

X Greater than 500 and less than or equal to 1,000

0 Greater than 1.000 *

FIGURE 35. Trilinear diagram of water chemistry of the Midwestern Basins and Arches aquifer system, categorized by dissolved-solids

concentration: (A) glacial aquifers.
C68 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

B

X

VVV \X V V ixf V V

CALCIUM CHLORIDE 

PERCENT OF TOTAL MILLIEQUIVALENTS PER LITER

EXPLANATION 

Dissolved-solids concentration, in milligrams per liter

+ Less than or equal to 500

X Greater than 500 and less than or equal to 1,000

 O Greater than 1,000 and less than or equal to 3,000

V Greater than 3,000 and less than or equal to 10.000

m Greater than 10,000

FIGURE 35. CONTINUED. Trilinear diagram of water chemistry of the Midwestern Basins and Arches aquifer system, categorized by 

dissolved-solids concentration: (B) carbonate-rock aquifer where it does not underlie the upper confining unit.
GEOCHEMISTRY C69

<§> <§>

CALCIUM CHLORIDE 

PERCENT OF TOTAL MILLIEQUIVALENTS PER LITER

EXPLANATION 

+ Appalachian basin

X Michigan basin 

Illinois basin

FIGURE 36. Trilinear diagram of water chemistry in the carbonate-rock aquifer of the Midwestern Basins and Arches aquifer system 

where it underlies the upper confining unit, categorized by structural basin.
C70 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

TABLE 10. Major water types, their specific cation and anion compositions, and their abundance in the glacial and

carbonate-rock aquifers in the Midwestern Basins and Arches aquifer system

[%, percent; , data not available]

Number (percentage) of samples, by aquifer

Water type

Ca-Mg-HCO3

Ca-Mg-SO4

Ca-Mg-Na-HCO3

Ca-Mg-Na-SO4

Na-HCO3

Na-Cl 

or Ca-Na-CI

Cl- influenced

Cation types

Ca,Ca-Mg

Ca. Ca-Mg

Ca-Mg-Na, 

Ca-Na

Ca-Mg-Na. 

Ca-Na

Na

Na, Ca-Na, 

Ca-Mg-Na

All cation types

Anion types

HCO3

HCO3 -SO4 

SO4

HCO3

HCO3 -SO4 , SO4

HCO3

Cl

HCO,-SO4 -C1, 

HCO, -Cl, 

SO4 -Cl

Glacial 

aquifers1

479 

(75%)

69 

(11%)

55 

(9%)

11

(2%)

3 

«1%)

2 

(<1%)

18

(3%)

Carbonate-rock aquifer

Does not underlie the 

upper confining unit 2

215 

(40%)

207 

(38%)

44 

(8%)

41 

(8%)

9

(2%)

3 

(<1%)

20 

(4%)

Underlies the upper 

confining unit d

1

(1%)

2 

(2%)

2 

(2%)

-

-

79 

(93%)

1 

(1%)

Total number of samples from the glacial aquifers is 637.

Total number of samples from the carbonate-rock aquifer where it does not underlie the upper confining unit is 539.

Total number of samples from the carbonate-rock aquifer where it underlies the upper confining unit is 85.

predominates in the carbonate-rock aquifer where it underlies 

the upper confining unit; however, this water type is rare in 

other parts of the carbonate-rock aquifer and in the glacial 

aquifers. Na-HCO3 water is uncommon in the aquifer system.

Hydrochemical facies (figs. 37 and 38) were identified for 

the glacial and carbonate-rock aquifers on the basis of the 

spatial distribution of water types in table 10. Large areas of 

the Midwestern Basins and Arches aquifer system are characterized by the presence of either the Ca-Mg-HCO3 or the CaMg-SO4 type; these areas are called Ca-Mg-HCO3 facies and 

Ca-Mg-SO4 facies. respectively. The remaining areas of the 

aquifer system are not characterized by a single water type; 

rather, they are distinguished by the presence of either CaMg-HCO3 water or Ca-Mg-SO4 water or both types, along 

with water types with increased percentages of sodium (CaMg-Na-HCO3 , Ca-Mg-Na-SO4, and Na-HCO3 waters) and, 

in some places, sodium and chloride (Na-Cl and Cl-influenced waters). These areas where several different water 

types are present together are called multiple-water-type 

facies.

Hydrochemical facies of water from glacial aquifers are 

shown in figure 37. A Ca-Mg-HCO3 facies in the glacial aquifers extends throughout much of the Midwestern Basins and 

Arches Region. A facies of Ca-Mg-SO4 water is present in 

the northeastern part of the aquifer system. Multiple-watertype facies are present in the glacial aquifers along the north 

and northwest boundaries between the underlying carbonaterock aquifer and the upper confining unit. Another multiplewater-type facies is present in the south-central part of the 

study area where the glacial deposits are directly underlain by 

Ordovician-age shales and limestones (the upper weathered 

zone water-bearing unit).

Hydrochemical facies of water from the carbonate-rock 

aquifer are shown on figure 38. Ca-Mg-HCO3 water is 

present in the carbonate-rock aquifer throughout much of the 

Midwestern Basins and Arches aquifer system except in the 

northeastern part, where it is highly localized. The predominant water type in the northeastern part of the study area is 

Ca-Mg-SO4. Multiple-water-type facies are present in the 

carbonate-rock aquifer along the north boundary between the 

carbonate-rock aquifer and the upper confining unit. Another 

multiple water-type facies is present along a part of the west 

contact between the carbonate-rock aquifer and the upper 

confining unit.
GEOCHEMISTRY C71

Lake \ 

St Ctair I

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

 

0 20 40 60 KILOMETERS

EXPLANATION 

Subcrop area of the carbonate-rock aquifer

 - - Hydrochemical-facies boundary Dashed where approximately located 

Well for which chemical analysis is available

FIGURE 37. Hydrochernical facies of ground water from the glacial aquifers in the Midwestern Basins and Arches aquifer system.
C72 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

42-

Lake 

Michigan

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 MILES

I , 

0 20 40 60 KILOMETERS

EXPLANATION

Subcrop area of the carbonate-rock aquifer

 - - Hydrochemical-facies boundary Dashed where approximately located 

Well for which chemical analysis is available

FIGURE 38. Hydrochemical facies of ground water from the carbonate-rock aquifer in the Midwestern Basins and Arches aquifer system.
GEOCHEMISTRY C73

Water in the carbonate-rock aquifer in the Appalachian 

Basin is predominantly the Na-Ca-Cl type with nearly equal 

milliequivalent percentages of Na and Ca. In the Michigan 

Basin, water is either a Na-Cl or Ca-Na-Cl type, depending 

on the specific bedrock unit within the carbonate rocks from 

which the water is produced. The predominant water type in 

the Illinois Basin is Na-Cl; however, an area of Ca-Mg-NaHCO3 water is present in the basin as far as 15 miles downdip 

from where the carbonate-rock aquifer becomes confined.

RELATION BETWEEN DISSOLVED-SOLIDS CONCENTRATIONS 

AND HYDROCHEMICAL FACIES

Examination of figures 33, 34, 37, and 38 indicates a 

strong relation between the distribution of dissolved-solids 

concentrations and the distribution of hydrochemical facies in 

the glacial and carbonate-rock aquifers. The trilinear diagrams (figs. 35 and 36) of water from the glacial aquifers and 

the carbonate-rock aquifer, where it is does not underlie the 

upper confining unit, also show that water type changes as 

dissolved-solids concentrations change. The Ca-Mg-HCO3 

water found throughout the aquifer system is characterized by 

dissolved-solids concentrations that are generally less than 

500 mg/L. Ca-Mg-SO4 water has dissolved-solids concentrations that range from about 500 to more than 3,000 mg/L. As 

figure 35 shows, dissolved-solids concentrations increase as 

the percentage of sulfate in water increases. The dissolvedsolids concentrations of ground water in the multiple-watertype facies are varied, as one would expect given the variability in water types in such facies.

Water type and dissolved-solids concentrations are also 

related in the carbonate-rock aquifer where it underlies the 

upper confining unit. Ground water from the carbonate-rock 

aquifer in each of the surrounding structural basins is categorized by basin on the trilinear diagram in figure 36. Based on 

the available analyses, waters from the Appalachian Basin 

have dissolved-solids concentrations that are generally 

greater than 200,000 mg/L and are Na-Ca-Cl type waters. 

Waters from the Michigan Basin range in dissolved-solids 

concentration from around 10,000 mg/L to more than 

300,000 mg/L; water type changes from a Na-Cl to a Ca-NaCl as dissolved-solids concentrations increase. The dissolvedsolids concentration in waters from the Illinois Basin is generally less than 100,000 mg/L, and the waters are primarily of 

the Na-Cl type.

Examination of figures 33, 34, 37, and 38 indicates that 

the spatial distribution of dissolved-solids concentrations and 

hydrochemical facies in the glacial and carbonate-rock aquifers is similar. The similarity likely reflects the hydraulic connection between the units. Ground water can flow through the 

glacial deposits and into the bedrock, then can discharge to 

streams and lakes back through the glacial deposits. Moreover, the source rock of the glacial deposits in the region is 

primarily the underlying bedrock (Strobel and Faure, 1987). 

Thus, the similarity in chemistry is also likely due to the presence of the same reacting mineral phases in the glacial and 

carbonate-rock aquifers.

GEOCHEMICAL AND HYDROLOGICAL PROCESSES, 

BY WATER TYPE

The trends in major-ion chemistry in the Midwestern 

Basins and Arches aquifer system are controlled, in a large 

part, by interactions between ground water and minerals that 

compose the aquifers. Important geochemical reactions that 

can affect solute concentrations in ground-water systems 

include mineral dissolution and precipitation, oxidation and 

reduction, and ion exchange. Additional processes such as 

evaporation and mixing of water from various sources can 

also be important controls of solute concentrations. The 

extent to which these processes control ground-water chemistry is affected by the paths and rates of ground-water flow and 

the sequence of reactions in an aquifer system. The geochemical and hydrologic processes responsible for the various 

water types in the Midwestern Basins and Arches aquifer system are discussed in the following sections.

CALCIUM-MAGNESIUM-BICARBONATE WATER

Ca-Mg-HCO3 water, the predominant type in the glacial 

and carbonate-rock aquifers, is generally produced by dissolution of the carbonate minerals calcite and dolomite. The 

reaction of calcite and dolomite with water and carbon dioxide can be written as follows:

CaCC-3 + H2O + CO2 <--> Ca2+ + 2HCCV (4) 

(calcite)

CaMg(CO3 )2 + 2H2O + 2CO2 <--> Ca2+ + Mg2+ + 4HCCV (5) 

(dolomite)

Typically, CO2 is derived from the oxidation of organic matter and root respiration in the soil zone and is dissolved by 

recharge water. Dolomite is more abundant than calcite in the 

aquifer system (Harrison, 1960; Steiger and Holowaychuk, 

1971; Wilding and others, 1971; Casey, 1994); if dolomite 

dissolves according to equation 5, the molar proportions of 

[Ca+Mg] to [HCO3] should be linear, with a slope of 0.5. A 

fairly strong relation exists between [Ca+Mg] and [HCO3 ] 

for Ca-Mg-HCO3 type water from the glacial and carbonaterock aquifers (figs. 39 and 40); however, the Ca-Mg-SO4 and 

Ca-Mg-Na-SO4 water types do not follow the trend predicted 

by the dolomite dissolution model and are highly enriched in 

calcium and magnesium relative to bicarbonate ([Ca+Mg]/ 

[HCO3 ] > 0.5). The Ca-Mg-Na-HCO3 waters plot slightly 

below the line and are depleted in calcium and magnesium 

relative to bicarbonate ([Ca+Mg]/[HCO3 ] < 0.5). (The mechanism by which calcium and magnesium are depleted is ion
C74 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

20

S 18 

^ 

cc 

£ 16

CO

LU

o 14

Hi

1 12 -^

^ 10

2

CO

LU

2 8

CD

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co 6

ZD 

D_

^ 4

5

o

2 2

0

D

D

B D

n n

O Theoretical dolomite 

dissolution line

0 2 4 6 8 10 12 14 

BICARBONATE, IN Ml 111 MOLES PER LITER

EXPLANATION 

Major water type

A Ca-Mg-HCO3 water

D Ca-Mg-SO4 water

O Ca-Mg-Na-HCO3 water

0 Ca-Mg-Na-SO4 water

FIGURE 39. Concentration of calcium plus magnesium as a function of bicarbonate concentration in ground water from the glacial

aquifers in the Midwestern Basins and Arches aquifer system.

exchange of calcium and magnesium for sodium and is discussed in the section on "Sodium-Enriched Water.")

The state of equilibrium between ground water and calcite, dolomite, and other minerals in the aquifer system can 

be evaluated by the computation of saturation indices (SI), 

expressed as

SI mineral log (IAP/KT), (6)

where IAP is the ion-activity product of the mineral calculated from analytical data, and KT is the thermodynamic equilibrium constant at the measured temperature of the water 

sample (Plummer and others, 1976). If the saturation index is 

negative, the water is undersaturated with respect to the particular mineral, and dissolution of the mineral is possible. If 

the saturation index is positive, the water is oversaturated 

with respect to the mineral, and precipitation of the mineral in 

the aquifer is possible. A saturation index of zero indicates 

that the mineral is in equilibrium with the water and that the 

tendency or rates of dissolution and precipitation should be 

equal. Thermodynamics can only indicate the potential for

such phase transfers. In reality, various kinetic factors may 

cause systems to behave differently than predicted (Nordstrom and Ball, 1989; Busby and others, 1991).

Saturation indices were calculated for calcite and dolomite (and other mineral phases discussed later in this report) 

by use of the computer model WATEQF (Plummer and others, 1976) and chemical data for the water collected as part of 

this investigation. The accuracy of the saturation index for 

predicting equilibrium of carbonate minerals depends largely 

on the accuracy of the field measurements of pH and alkalinity. Uncertainty in these and other analytical data produces an 

uncertainty estimate in the saturation index of ± 0.1 for calcite and ± 0.2 for dolomite (Nordstrom and Ball, 1989; Busby 

and others, 1991). Because the accuracy of the pH measurements could not be verified for much of the historical groundwater-chemistry data, saturation indices were not calculated 

for historical samples.

The saturation indices for calcite and dolomite generally 

do not correlate with dissolved-solids concentrations (fig. 

41). Nearly all the Ca-Mg-HCO3 water is in equilibrium or 

oversaturated with respect to calcite and dolomite, an indica-
GEOCHEMISTRY C75

25

20

15

10

D

QTheoretical dolomite 

dissolution line .

0 2 4 6 8 10 

BICARBONATE, IN MILLIMOLES PER LITER

EXPLANATION 

Major water type

A Ca-Mg-HCO3 water

D Ca-Mg-SO4 water

O Ca-Mg-Na-HCO3 water

 Ca-Mg-Na-SO4 water

FIGURE 40. Concentration of calcium plus magnesium as a function of bicarbonate concentration in ground water from the carbonaterock aquifer in the Midwestern Basins and Arches aquifer system.

tion that carbonate mineral equilibria could be controlling 

concentrations of calcium, magnesium, and bicarbonate in 

Ca-Mg-HCO3 water in the aquifer system. The tendency 

toward oversaturation with respect to calcite has been noted 

in other aquifers by Nordstrom and Ball (1989), who conclude that kinetic effects may inhibit calcite precipitation. 

Reactions that add calcium, magnesium, and bicarbonate or 

that consume carbon dioxide can promote oversaturation with 

calcite and dolomite. For example, dissolution of gypsum 

may add calcium at a rate that exceeds calcium removal by 

calcite precipitation, causing oversaturation with calcite. Several Ca-Mg-SO/ptype waters are saturated with calcite but are 

significantly undersaturated with respect to dolomite. Undersaturation with respect to dolomite is an important factor in 

dedolomitization, a process that can promote the formation of 

Ca-Mg-SO4-type water. (Dedolomitization is discussed further in the next section.)

Ca-Mg-HCO3 water is the predominant water type in the 

glacial and carbonate-rock aquifers because calcite and dolomite are abundant in these aquifers (Harrison, 1960; Steiger

and Holowaychuk, 1971; Wilding and others, 1971; Casey, 

1994) and recharge takes place across most of the study area. 

A steady supply of CO2 is available for carbonate dissolution 

in the glacial aquifers because ground-water recharge generally takes place through the organic-rich soils overlying these 

deposits. Dissolved CO2 and other solutes are also recharged 

to the underlying bedrock because the glacial aquifers provide most of the recharge to the carbonate-rock aquifer at the 

regional scale.

CALCIUM-MAGNESIUM-SULFATE WATER

The predominant water type in the northeastern part of the 

Midwestern Basins and Arches aquifer system is Ca-Mg-SO4. 

This water is characterized by high concentrations of dissolved solids, calcium, magnesium, and sulfate. The predominance of sulfate over bicarbonate and the lack of agreement 

of the Ca-Mg-SO4 water with the simple dolomite dissolution 

model (figs. 39 and 40) indicates that other processes are controlling the chemistry of this water type. Two reactions can 

produce this type of water: (1) dedolomitization, which
C76 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

0.4

0.3

x 11102« *-

Q

S 0.1 

<;

-0.1

-0.2

0.4

0.2

£ -0.2

-0.4

-0.6

-0.8

CALCITE

n -

Upper error limit rn rn

a

A Equilibrium

D

Lower error limit

1,000 2,000 

DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

3,000

DOLOMITE

Upper error limit

-LIB- Equilibrium

n n

A

Lower error limit

n

n

n

n

1,ooo 2,000 3,000 

DISSOLVED-SOLIDS CONCENTRATION, IN MILLIGRAMS PER LITER

EXPLANATION 

Major water type

A Ca-Mg-HCO3 water

Q] Ca-Mg-SO4 water

O Ca-Mg-Na-HCO3 water

 Ca-Mg-Na-SO4 water

FIGURE 41. Saturation indices of calcite and dolomite as a function of dissolved-solids concentration for water from wells in the

Midwestern Basins and Arches aquifer system. (Error limits are due to analytical and thermodynamic data uncertainties.

Points plotting within these lines are considered to be in equilibrium with the given mineral.)
GEOCHEMISTRY C77

involves dissolution reactions with carbonate minerals and 

gypsum, and (2) sulfuric acid neutralization, which involves 

dissolution of carbonate minerals with sulfuric acid generated 

by the oxidation of pyrite.

The results of investigations of other regional carbonaterock aquifers show that dedolomitization may be a controlling process (Pearson and Rightmire, 1980; Back and others, 

1983; Busby and others, 1991). Ground water that moves 

through the aquifer initially dissolves calcite, dolomite and 

gypsum (or anhydrite). Once calcite and dolomite reach saturation, gypsum is still undersaturated and continues to dissolve, adding calcium and sulfate to the water. Consequently, 

calcite becomes oversaturated and, as it precipitates, the 

bicarbonate concentration decreases. The decrease in bicarbonate concentration causes the water to be undersaturated 

with respect to dolomite and promotes dissolution of dolomite. Dissolution of dolomite causes increases in the concentration of magnesium in the water. The overall reaction can be 

written as:

CaMg(CO3 )2(s) + CaSO4 2H2O(s) + H+ <-> 

(dolomite) (gypsum)

CaC03 (s)+Ca2+ +Mg2+ +S042-+HC03 - (7)

(calcite)

As the reaction proceeds, Ca:Mg ratios in the water decrease 

to 1.0, and sulfate concentrations increase. The ratio of 

[Ca+Mg]/[HCO3 ] is 1:1, in contrast to reaction 5, in which 

the ratio of [Ca+Mg]/[HCO3 ] is 1:2 or 0.5. On a plot of 

[Ca+Mg] as a function of [SO4 + 0.5HCO3 ], the dedolomitization reaction yields a straight line with a slope of 1. Plots of 

the water-chemistry data for the glacial aquifers (fig. 42) and 

the carbonate-rock aquifer (fig. 43) show very close agreeTheoretical dedolomitization 

trend line

4 6 8 10 12 14 16 18 

SULFATE PLUS ONE-HALF BICARBONATE, IN MILLIGRAMS PER LITER

EXPLANATION 

Major water type

A Ca-Mg-HCO3 water

D Ca-Mg-SO4 water

O Ca-Mg-Na-HCO3 water

 Ca-Mg-Na-SO4 water

FIGURE 42. Concentration of calcium plus magnesium as a function of concentration of sulfate plus one-half bicarbonate in ground water 

from the glacial aquifers in the Midwestern Basins and Arches aquifer system.
C78 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

ment between the theoretical trend for the dedolomitization 

model and the trends for the Ca-Mg-HCO3 - and the Ca-MgSO4-type waters. The Ca-Mg-Na-HCO3 - and Ca-Mg-NaSO4-type waters tend to plot below the 1:1 line.

Gypsum saturation indices for ground water collected as 

part of this investigation (fig. 44) increase with increasing 

sulfate concentrations. A value of + 0.2 is used as the estimate 

of uncertainty in the saturation index of gypsum. Saturation 

with respect to gypsum generally is indicated for a few samples in which sulfate concentrations are greater than about 

1,500 mg/L. Gypsum would tend to dissolve in water that is 

undersaturated with respect to gypsum.

Dedolomitization is not the only process, however, that 

yields the relation between Ca, Mg, SO4, and HCO3 shown in 

figures 42 and 43. An alternative model involves dissolution

of dolomite by sulfuric acid (H2SO4) produced by the oxidation of pyrite (FeS2):

FeS2 +15/4 O2 + 7/2 H2O <-» Fe(OH)3(s) 2H2SO4 (8)

(Nordstrom and others, 1979). The sulfuric acid reacts with 

dolomite:

,2+ 2+ CaMg(C03)2(s)+H2S04^Ca/++Mgz++S04-+2HCO3-. (9)

The overall stoichiometry of the dissolved products in reaction 9 is identical to that of the dedolomitization reaction (7). 

Thus, either model could produce the trends shown in figures 

42 and 43, and gypsum and (or) pyrite could be the sources of 

sulfate in the Ca-Mg-SO4 water in the northeastern part of the 

aquifer system.

25

20

15

10

o 5

D

Theoretical dedolomitization 

trend line

0 5 10 15 20 25 

SULFATE PLUS ONE-HALF BICARBONATE, IN MILLIGRAMS PER LITER

EXPLANATION 

Major water type

A Ca-Mg-HCOs water

D Ca-Mg-SO4 water

O Ca-Mg-Na-HCC-3 water

 Ca-Mg-Na-SO4 water

FIGURE 43. Concentration of calcium plus magnesium as a function of concentration of sulfate plus one-half bicarbonate in ground water 

from the carbonate-rock aquifer in the Midwestern Basins and Arches aquifer system.
GEOCHEMISTRY

The mineralogy of the carbonate rocks indicates that gypsum and pyrite both are present in the northeastern part of the 

aquifer system. Gypsum beds as thick as 5.5 ft are present at 

or just below land surface in the rocks of the Salina Group in 

the Sandusky Bay area of northwestern Ohio (Carlson, 1991). 

Lesser quantities of gypsum are also found in carbonate rocks 

of Silurian and Devonian age throughout the northeastern part 

of the aquifer system. Pyrite is present in the Findlay Arch 

mineral district as defined by Carlson (1991) and is associated with a suite of ore minerals that includes celestite, fluorite, sphalerite, and galena. The Findlay Arch mineral district 

forms a northeasterly trending belt that extends from the Ohio 

and Indiana State line south of the Maumee River to the 

islands in Lake Erie. Structural controls on the mineralization 

in the Findlay Arch mineral district appear to be the Findlay 

Arch and the Bowling Green Fault zone (Carlson, 1991). Sulfide minerals are present throughout the carbonate rocks in 

the Findlay Arch mineral district, but they are generally concentrated at or just below surfaces of past erosion and in collapsed solution cavities (Botoman and Stieglitz, 1978). 

Abundant pyrite and marcasite are found in the Columbus 

Limestone in south-central Ohio and in the Devonian shales 

along the entire east boundary between the carbonate rocks 

and the younger shales (Carlson, 1991). Gypsum and sulfide 

minerals also may be present in the glacial deposits because 

the mineral composition of tills in the study area consists of 

fragments of the local bedrock (Strobel and Faure, 1987).

The isotopic composition of sulfur in mineral and dissolved phases can be useful in distinguishing sources of sulfate to ground water and determining whether dedolomitization, driven by gypsum dissolution, or sulfide-mineral oxidation is the predominant geochemical reaction in the

Upper error limit 

Equilibrium

Lower error limit

-BD

D

D

500 1,000 1,500 2,000 

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

EXPLANATION 

Major water type

A Ca-Mg-HCO3 water

D Ca-Mg-SO4 water

O Ca-Mg-Na-HCO3 water

 Ca-Mg-Na-SO4 water

FIGURE 44. Saturation indices of gypsum as a function of dissolved-sulfate concentration for water from wells in the

Midwestern Basins and Arches aquifer system. (Error limits are due to analytical and thermodynamic data uncertainties.

Points plotting within these lines are considered to be in equilibrium with gypsum.)
C80 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

aquifer system (Pearson and Rightmire, 1980). The sulfur 

isotope data presented in tables 8 and 9 and discussed more 

fully in the section "Sulfur Isotopes" indicate that pyrite is 

the primary sulfate source for the glacial and carbonate-rock 

aquifers in most of the aquifer system; however, both pyrite 

and gypsum contribute sulfate to the Ca-Mg-SO4-type ground 

water in the northeastern part of the aquifer system.

Deering and others (1988) noted that the chemistry of 

ground water in the carbonate-rock aquifer in northwestern 

Ohio is established chiefly by the mineralogy of the carbonate 

rocks. Other researchers (Norris and Fidler, 1973; Norris, 

1974) have discussed the distribution of water types in western Ohio in the context of chemical evolution of bicarbonatedominated water to sulfate-dominated water as ground water 

flows down the potentiometric gradient from the potentiometric high near the Bellefontaine Outlier to Lake Erie and to the 

Ohio River. The results of the ground-water flow model presented in the "Numerical Model" section, however, indicate 

that water in the aquifer system does not follow long, continuous paths from the highest potentiometric levels to Lake 

Erie or the Ohio River. Rather, ground-water recharge and 

discharge areas alternate across western Ohio. Thus, it is 

likely that the distribution of Ca-Mg-SO4 water in the northeastern part of the aquifer system is primarily controlled by 

the mineralogy of the aquifer material.

The relatively large area of Ca-Mg-HCO3 water within 

the Ca-Mg-SO4 facies in the carbonate-rock aquifer in northwestern Ohio corresponds to an area of thin or absent glacial 

deposits where the Lockport dolomite, a relatively pure dolomite (Janssens, 1977), crops out at the surface (fig. 5). The 

Ca-Mg-HCO3 water also generally corresponds to areas of 

active ground-water recharge (fig. 28). Breen and 

Dumouchelle (1991) showed that, in northwestern Ohio, 

water in the carbonate-rock aquifer where the glacial deposits 

are thin or absent has tritium concentrations indicative of 

recharge since the 1950's. Thus, active ground-water flushing 

favors the formation of Ca-Mg-HCO3 water in the carbonaterock aquifer in the northeastern part of the study area where it 

is at or near the surface and recharge to the aquifer system 

occurs.

SODIUM-ENRICHED WATER

Multiple-water-type facies described in the preceding section (figs. 37 and 38), are characterized by the presence of 

one or more of the following ground-water types: Ca-Mg-NaHCO3 , Ca-Mg-Na-SO4 , and Na-HCO3 . Many researchers 

have shown that an increase in sodium concentration in 

ground water without an associated increase in chloride can 

be caused by cation exchange of Ca~+ and Mg2+ ions for Na+ 

ions on clay minerals (Foster, 1950; Back, 1966; Freeze and 

Cherry, 1979; Thorstenson and others, 1979; Chapelle and 

Knobel, 1983). In an aquifer system with abundant carbonate

minerals, cation exchange may be accompanied by the dissolution of calcite and dolomite.

Na-HCO3 water can be produced from Ca-Mg-HCO3 

waters by cation exchange on clay minerals according to the 

reaction:

 + NavEx >2Na+ Ex,(10)

where 0 < x < 1. By combining reactions 4, 5, and 10, the 

overall stoichiometry of dissolution of calcite and dolomite 

and cation exchange may be represented as:

(Ca 1 .xMgx )CO3 + H2O + CO2 + Na2 Ex > 2Na+ +2HCO3 ~

+ (Cai.xMgx) Ex. (11)

As Ca2+ and Mg2+ are removed from solution and are 

replaced with Na+ by cation exchange, the solution becomes 

undersaturated with respect to calcite and dolomite. The carbonate minerals dissolve, providing more Ca2+ and Mg in 

solution to exchange with Na+ and causing HCO3 " concentrations to increase. The Ca-Mg-Na-HCO3 water in figures 39 

and 40 plots below the line predicted by the dolomite-dissolution model; this pattern, which indicates that the water is 

enriched in bicarbonate and depleted in calcium and magnesium relative to the dolomite dissolution model, is consistent 

with cation-exchange reactions. Ca-Mg-Na-SO4 water is 

associated with Ca-Mg-SO4 water in the glacial and carbonate-rock aquifers and is likely formed by the mixing of NaHCO3 and Ca-Mg-SO4 waters or incomplete cation exchange 

of the Ca-Mg-SO4 waters. The Ca-Mg-Na-SO4 water does 

not plot with the Ca-Mg-Na-HCO3 water in figures 39 and 40 

because of the additional calcium and magnesium from dedolomitization (7) or sulfuric acid neutralization (9).

Eberts and others (1990) and Lesney (1992) describe the 

presence of water ranging in composition from Ca-Mg-NaHCO3 to Na-HCO3 in bedrock units underlying shales and 

siltstones of Mississippian age in northeastern Ohio. Their 

research attributes this chemistry to mixing of Na-HCO3 

water, produced by cation exchange in the shales and siltstones, with water in the underlying aquifers. A similar water 

composition of Ca-Mg-Na-HCO3 to Na-HCO3 is present in 

the carbonate-rock aquifer in the Midwestern Basins and 

Arches aquifer system where the carbonate-rock aquifer is 

overlain by Devonian shales (upper confining unit) along the 

margins of the Michigan Basin. This similarity indicates that 

Na- and HCO3 -enriched water produced by cation exchange 

in the shales is mixing with water in the underlying carbonate-rock aquifer. However, significant areas of Ca-Mg-NaHCO3 water, associated with some Na-HCO3 water, are 

present in the carbonate-rock aquifer and the glacial aquifers 

where shales do not overlie the carbonate-rock aquifer south 

of the boundary between the aquifer and the upper confining 

unit. If Na-HCO3 water from the upper confining unit is the 

source of Na-enriched water south of this boundary, then it
GEOCHEMISTRY C81

must have flowed in an updip direction out of the shales and 

into the adjacent carbonate-rock aquifer and associated glacial deposits.

Research on regional aquifers in the northern Midwest 

(Siegel and Mandle, 1984; Siegel, 1989, 1991) suggests a 

mechanism by which Na-HCO3 water produced by cation 

exchange in the shales at the margin of the Michigan Basin 

may have moved into the carbonate-rock aquifer. On the basis 

of distributions of solutes and isotopes in water from the 

Cambrian-Ordovician aquifer in eastern Wisconsin, Siegel 

(1989, 1991) proposed that loading of glacial ice during 

Pleistocene glaciation may have reversed ground-water flow 

gradients and caused ground water from within the Michigan 

Basin to discharge at the west margin of the Michigan Basin 

in eastern Wisconsin. A numerical model that simulated 

ground-water flow during Late Wisconsinan glaciation in 

northern Illinois (Filley, 1985) showed that increased pore 

pressures may have caused water from within the Michigan 

Basin to discharge at the basin margins. Panno and Bourcier 

(1990) proposed that the formation of karst features in carbonate rocks adjacent to the Appalachian, Michigan, and Illinois Basins was caused by mixing of ground water and deepbasin saline water that was flushed from the basins during 

Pleistocene glaciation. A similar flow of ground water out of 

the Michigan Basin during the Pleistocene may have occurred 

at its southern margin, pushing through the Devonian shales 

and emplacing Na-HCO3 water in the carbonate-rock aquifer 

and overlying glacial aquifers.

An alternative hypothesis for the presence of multiplewater-type facies along the northern boundary of the carbonate-rock aquifer and upper confining unit is cation exchange 

within the glacial and carbonate-rock aquifers. Some bedrock 

units within the Salina Group and carbonate rocks of Devonian age in northern Indiana and northwestern Ohio contain 

numerous thin shales (Janssens, 1977; Turner, 1977; Doheny 

and others, 1975). Cation exchange within these shale-rich 

carbonate rocks may result in the formation of Na-enriched 

water. Mazor and others (1993) suggest that significant differences in water composition in an area indicate that water may 

be stored in distinct pockets within an aquifer and that such 

pockets are caused by changes in lithology. Thus, the presence of multiple-water-type facies may be the result of a transitional bedrock lithology in the aquifer along the subcrop 

boundaries.

The Ca-Mg-Na-HCO3 water found along the west subcrop boundary of the carbonate-rock aquifer is not associated 

with Na-HCO3 water, nor is it as enriched in sodium as the 

Ca-Mg-Na-HCO3 water along the north subcrop boundary of 

the carbonate-rock aquifer. The Ca-Mg-Na-HCO3 water 

found in the carbonate-rock aquifer beyond the west boundary of the carbonate-rock aquifer and upper confining unit 

increases in chloride concentration in a downdip direction 

and most likely results from mixing of Ca-Mg-HCO3 water 

with Na-Cl water from the Illinois Basin.

The multiple-water-type facies in the glacial aquifers in 

the south-central part of the study area is associated with the 

area where Ordovician shales and limestones (the upper 

weathered zone water-bearing unit) directly underlie the glacial deposits. Most of the water in this facies is the Ca-MgHCO3 type; however, some ground water that is enriched in 

sodium or chloride or both is present in this area. Research on 

glacial buried-valley aquifers in the Great Miami River Valley 

(Dumouchelle and others, 1993) indicates two potential 

sources of sodium and chloride to the glacial aquifers. In the 

Great Miami River Valley, shallow ground water in highly 

permeable outwash valleys contains elevated concentrations 

of sodium and chloride that are related to human activities. 

Moreover, small amounts of moderately saline Na-Cl water 

discharges from the shale bedrock to the glacial aquifers in 

the Great Miami River Valley. Thus, sodium and chloride 

from both sources may be affecting the chemistry of water in 

the glacial aquifers in the south-central part of the aquifer 

system.

Water with elevated percentages of sodium and chloride is 

also present in the carbonate-rock aquifer where it is not 

overlain by the upper confining unit. Na-Cl or Cl-influenced 

ground water is uncommon in the carbonate-rock aquifer; 

most of the few areas of such water are within the multiplewater-type facies that borders the Michigan Basin. Possible 

sources of sodium and chloride to these ground waters 

include the dissolution of halite from the carbonate rocks, 

mixing with small amounts of formation brine from deeper 

bedrock units, or surface contamination by water affected by 

road-deicing salts or formation brine associated with petroleum exploration and production activities (Breen and 

Dumouchelle, 1991). The presence of Na-Cl water in the carbonate-rock aquifer along the margin of the Michigan Basin 

may also be related to the hydrodynamic displacement of 

brine by Pleistocene glaciation.

In summary, the exact geochemical and hydrologic processes responsible for the multiple-water-type facies are 

unknown, especially along the north boundary of the Midwestern Basins and Arches aquifer system. The distribution 

of water types in the multiple-water-type facies however, is 

clearly related to the change in lithology, from carbonate 

rocks to shales, at the boundaries of the carbonate-rock aquifer. The multiple-water-type facies along the margin of the 

Michigan Basin may also be related to changes in the groundwater flow regime during Pleistocene glaciation.

SODIUM-CHLORIDE AND CALCIUM-SODIUM-CHLORIDE WATER

Na-Cl type water with dissolved-solids concentrations 

less than 10,000 mg/L is present in the carbonate-rock aquifer 

beneath the upper confining unit at the margins of the structural basins. Its presence likely results from mixing of freshwater with brines from within the basins. Processes 

responsible for the chemistry of brines in the Illinois, Michi-
C82 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

gan, and Appalachian Basins are complex (Stueber and others, 1987; Lowry and others, 1988; Walter and others, 1990; 

Stueber and Walter, 1991; Stueber and others, 1993; Wilson 

and Long, 1993a, b) and are beyond the scope of this report. 

Na-Cl, Na-Ca-Cl and Ca-Na-Cl type waters with dissolvedsolids concentrations greater than 10,000 mg/L are present 

just beyond the boundary between the carbonate-rock aquifer 

and the upper confining unit in the Michigan and Appalachian 

Basins, an indication that the upper confining unit prevents 

water from recharging the carbonate-rock aquifer in these 

areas. The presence of water with dissolved-solids concentrations less than 10,000 mg/L in the Illinois Basin more than 70 

mi downdip from the boundary of the carbonate-rock aquifer 

and the upper confining unit may be related to present or past 

patterns of flow in the carbonate-rock aquifer. Results of the 

regional ground-water flow model show that a small percentage of water may flow from the subcrop area down into the 

Illinois Basin. Such water could contribute to the current distribution of solutes. Other research on Illinois Basin brines 

(Clayton and others, 1966; Rupp and Pennington, 1987; Stueber and Walter, 1991) suggests that dilution of the brines may 

have occurred as a result of increased infiltration through 

fracture systems in the carbonate rocks or from meltwater 

that recharged the carbonate-rock aquifer during the Pleistocene. The origin of low dissolved-solids water deep in the 

Illinois Basin is uncertain, and more research would be 

needed for a full understanding of this part of the aquifer system.

INSIGHTS INTO REGIONAL FLOW PROVIDED BY 

GEOCHEMICAL DATA

Ground-water samples were collected from the glacial 

and carbonate-rock aquifers along four general directions of 

regional ground-water flow, as determined from the potentiometric surface map of the carbonate-rock aquifer (fig. 12). 

The locations of the sampling points are shown in figure 32. 

The data are listed in tables 7 and 8. Although the regional 

ground-water flow model indicates that ground water in the 

Midwestern Basins and Arches aquifer system does not follow long paths from the highest regional potentiometric levels to the lowest regional potentiometric levels, it is useful to 

evaluate the differences in ground-water chemistry along the 

general directions of regional ground-water flow.

MAJOR SOLUTES

Dissolved-solids and sulfate concentrations have been 

used as indicators of chemical evolution of ground water in 

various regional-scale carbonate-rock aquifers, such as the 

Floridan aquifer in Florida, the Edwards aquifer in Texas, and 

the Madison aquifer in parts of Montana, South Dakota and 

Wyoming (Plummer, 1977; Rye and others, 1981; and Busby 

and others, 1991). In these aquifers, concentrations of dissolved solids and sulfate generally increase along the principal directions of regional ground-water flow. Figures 45 and 

46 show dissolved-solids and sulfate concentrations along 

geochemical sections A-A', B-B', C-C' and D-D' in the Midwestern Basins and Arches aquifer system. In general, no 

consistent systematic change in dissolved-solids and sulfate 

concentrations is evident along the geochemical sections. The 

deep bedrock wells along section C-C' are the only subset of 

the data indicating consistent increases in dissolved-solids 

and sulfate concentrations in the direction of regional flow. 

This general absence of systematic trends in chemistry along 

the general directions of regional ground-water flow indicates 

that the aquifer system is not characterized by long flow paths 

that are isolated from additions of recharge, as have been postulated for regional carbonate-rock aquifers in other parts of 

the United States.

Figures 45 and 46 also can be used to compare the chemistry of ground water from the glacial aquifers and the carbonate-rock aquifer. In general, dissolved-solids and sulfate 

concentrations in the glacial and carbonate-rock aquifers are 

similar. The only notable exception to this is sample 10G (fig. 

32) from a glacial aquifer, which has considerably higher dissolved-solids and sulfate concentrations than samples 10S 

and 10D from the carbonate-rock aquifer.

In addition, figures 45 and 46 show the relation of chemistry between ground water from the shallow bedrock wells 

and ground water from the deep bedrock wells. In general, 

the dissolved-solids and sulfate concentrations in ground 

water from shallow bedrock wells were similar to those in 

ground water from deep bedrock wells. The exceptions are 

wells 4S and 4D and wells 14S and 14D. Of all the wells 

shown in figure 32, the wells that show the largest variations 

in ground-water chemistry with aquifer type or depth at a single sampling location (well groups 4, 10, and 14) are in the 

groups that have the greatest distance between the wells. 

Apparent variations with depth may be more related to spatial 

variability than to specific differences with depth.

ISOTOPES

Interpretation of isotopic data can often provide more 

insight into chemical and hydrologic processes than can be 

provided by interpretation of chemical data alone. The isotopic composition of ground water can be used to distinguish 

between competing reactions that contribute dissolved sulfur 

and carbon to water, to examine the possibility of different 

water masses mixing in an aquifer, and to estimate the relative or absolute ages of ground water in different parts of an 

aquifer system. Consequently, isotopic data can provide an 

independent confirmation of the interpretations made from 

the hydrologic and chemical data.

In this investigation, the stable isotopes 534S (for the sulfate and sulfide phases), 518O, 5D, 513C and the radioactive 

isotopes 14C and tritium were measured in ground water
GEOCHEMISTRY C83

800

600

400

200

700

cc

LLJ

=i 600 

cc.

LLJ 

Q_

^ 500

cc

j§ 400

i

- 300 

o

| 3,000

LJJ 

O

o

w 2,000

0... A

,-' ..B'

10 20 30 40 50 60 70 80

B 1 B

0.

-O

10 20 30 40 50 60 70 80 90

c

1,000

/

o---

Q10 20 30 40 50 60

D

3,000 l-i r-r2,000

70 80 

D'

1,000 O'X

0 10 20 30 40 50 60 70 80 90 100 

APPROXIMATE DISTANCE FROM REGIONAL POTENTIOMETRIC HIGH, IN MILES

EXPLANATION

-A-- Glacial aquifers Points not connected if data are missing

Carbonate-rock aquifer

-~Q Shallow

-Eh- Deep

FIGURE 45. Dissolved-solids concentration in water from wells in the Midwestern Basins and Arches aquifer system along geochemical

sections A-A', B-B', C-C', and D-D' (locations of wells shown in fig. 32).
C84

150

100

50

REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES 

A' A

A

fc

A 

.0

-Q 

i

o 10 20 30 40 50 60 70 80

150

100

en o

B1 B

<

or CD

Zf , , ,

10 20 30 40 50 60 70 80 90

1,500

1,000

500

A

A '""O

10 20 30 40 50 60 70 80

2,000

1,500

1,000

500

D D'

\

X..

0 10 20 30 40 50 60 70 80 90 100 

APPROXIMATE DISTANCE FROM REGIONAL POTENTIOMETRIC HIGH, IN MILES

EXPLANATION

 A-- Glacial aquifers Points not connected if data are missing

Carbonate-rock aquifer

 © " Shallow

-Eh Deep

FIGURE 46. Sulfate concentration in water from wells in the Midwestern Basins and Arches aquifer system along geochemical 

sections A-A', B-B', C-C", and D-D' (locations of wells shown in fig. 32).
GEOCHEMISTRY C85

along the four selected directions of regional ground-water 

flow. In addition. 8" S (for the sulfate and sulfide phases) and 

8 C values for aquifer material, including glacial deposits 

and carbonate rocks, were determined at several locations 

within the study area. The isotopic data are summarized in 

tables 8 and 9 and are discussed in the sections that follow.

SULFUR ISOTOPES

Aquifer-material and ground-water samples were analyzed for sulfur isotopes to determine the origin of dissolved 

sulfate and sulfide and to assist in the identification of the 

geochemical processes that affect ground-water chemistry in 

the Midwestern Basins and Arches aquifer system. Because 

various geochemical processes such as sulfate reduction or 

dedolomitization can affect the ground-water ages calculated 

from the carbon isotope data, as well as the associated interpretations made about regional ground-water flow, it is 

important to be able to distinguish the sources of sulfur in the 

ground water.

The most abundant sulfur-containing minerals in the 

study area include pyrite and, in the northeastern part of the 

area, gypsum. Sedimentary sulfide minerals such as pyrite 

can exhibit a wide range in 8 S values, from -70 to +70 per 

mil; however, 834S normally ranges from -30 to 10 per mil 

(Ohmoto and Rye, 1979). In gypsum derived from marine 

evaporites, S S values reflect the isotopic composition of 

dissolved sulfate in the ocean at the time that the gypsum was 

deposited. Gypsum of Silurian and Devonian age has 8 S 

values that range from about +16 to +30 per mil (Holser and 

Kaplan, 1966; Krouse, 1980).

Because such a wide range of 8 S values is possible for 

sources of sulfur, samples of aquifer material, including glacial deposits and carbonate rocks, and samples of pyrite and 

gypsum from the carbonate rocks were analyzed for 834S of 

the sulfate and sulfide fractions to constrain the variability in 

sulfur isotope values of the aquifer material. The data are 

shown in table 9, along with sulfur isotope data on pyrite and 

anhydrite samples from Ohio first given in Botoman and 

Faure(1976).

The 8" S^ su]fate ) values for gypsum and anhydrite samples, 28.7 per mil and 24.9 per mil, are in close agreement 

with those expected for marine sulfates of Silurian and Devonian age. The 834S(sulfide) values of the pyrite samples, -21.9 

per mil, -19.4 per mil, and 7.0 per mil, fall within the 

expected range for sedimentary sulfides. Organic sulfur, 

which has 8 S^if^ values similar to those of pyrite, was 

not specifically analyzed for, but it may be an additional 

source of reduced sulfur in the aquifer system.

The 8" S(sujfate ) values for the glacial deposits fall within 

a fairly narrow range, from -14.8 to -8.2 per mil, which is 

nearly identical to the range in 834S(suifidej values for the 

same material, -14.0 to -8.0 per mil. These 8~ 4S(sulfide) values 

are within the same range as the 8 S(sulfjde) values determined for the pyrite samples collected in the area and indicate 

that pyrite is a likely source of solid-phase sulfide in the glacial deposits. A likely explanation for the similarity in the

S(sulfate) and S S(SU]fjde) values for the glacial deposits is 

that sulfide (pyrite) in the glacial deposits oxidized to sulfate 

and, in the presence of dissolved calcium from the dissolution 

of carbonate minerals, precipitated as gypsum. This process 

has been shown to occur in the tills of the Interior Plains 

Region in Canada (Hendry and others, 1986; Keller and Van 

Der Kamp, 1988). These researchers based their conclusions 

on the similarity in sulfur isotope values between reduced 

organic sulfur in unoxidized tills and the sulfate from gypsum 

crystals found in the oxidized tills. They suggest that a largescale lowering of the water table during an extended dry climatic episode in western North America from 11,000 to 

3,000 years before present produced geochemical changes in 

the till that resulted in the formation of the gypsum and the 

subsequent development of fractures in the oxidized till. Similar secondary gypsum has also been found in glacial deposits 

near the Great Lakes. Desaulniers and others (1981) note that 

secondary gypsum usually is found in fractures of clayey tills 

and glaciolacustrine clays in southwestern Ontario near Lake 

Erie. Authigenic gypsum crystals have been found in lacustrine clays within Wisconsinan glacial deposits in northeastern Ohio (Carlson, 1991). Possible gypsum crystallization 

has been noted by Strobel (1990) along fractures in clayey till 

in northwestern Ohio. Therefore, given the similarity in 

834S(sulfate ) and 834S(suifide) values in the glacial deposits in 

the Midwestern Basins and Arches aquifer system, processes 

similar to those in the Interior Plains Region of Canada may 

have resulted in the formation of gypsum in the glacial deposits with an isotopically light sulfur signature.

The 834S(SMjfate) values of the carbonate rocks are highly 

variable. The 8" 4S(sulfate) values of core C3 and various intervals of core C2 range from 18.3 to 28.7 per mil and clearly 

reflect sulfate from marine evaporates of Silurian and Devonian age. However, 8 S(suifate ) values of all other samples of 

the carbonate rocks range from -2.3 to 14.6 per mil and do not 

indicate a clearly defined sulfur source. The wide range in 

§34S(sulfate) values of the carbonate rocks may reflect postdepositional processes, such as cementation, recrystallization, dolomitization. and weathering, that affected the 

carbonate rocks in the study area (Casey, 1994). The range in 

S34S(SUifide) values of the carbonate rocks, from -24.9 to 3.6 

per mil, is consistent with the 8" S(suifide ) values of the pyrite 

samples from the study area.

Sulfur isotope ratios in ground water in the Midwestern 

Basins and Arches aquifer system (fig. 47) are highly varied; 

S34S(sulfate) values range from -10.4 to +44.1 per mil, and 

834S(su|fide ) values range from -53.2 to -12.2 per mil. Variability in sulfur isotope values of the aquifer material is a major 

control on the distribution of sulfur isotope values in ground 

water from the aquifer system. For example, in the western 

part of the study area where little or no gypsum is contained
C86 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

 ^iChicuno

38°

Lake 

Michigan

tv

v> -

f Louiovmc 

^b ,1

'Hurrtifinto

i

\Y-*«oitft»/*

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

Boundary of study area

60 MILES

0 20 40 60 KILOMETERS

EXPLANATION

Sample locations from other studies

Well sampled for chemical analysis during this investigation 44J« Carbonate-rock aquifer. Data from Nichols and others 

Numbers represent 8 34Ssu|fate , 8 34Ssulfide , and A 34S values (1996). Number is 8 34Ssu|fate value 

(A34 S = 8 34Ssuifate - 8 34Ssu]f,de)- ~ indicates data not available 

or not applicable

Glacial aquifers __ 0 3/__/__ ^^ Group of wells 

Carbonate-rock aquifer, shallow B'B/-/-- (**} in close Proximity

C^£irt)Ori3t6~rOCK 3QUlI6r, Q66p*^'xx'" j ""x.

S Ssulfate 8 Ssu[fide A S

FIGURE 47. 834S and A34 S values in water from wells in the Midwestern Basins and Arches aquifer system.
GEOCHEMISTRY CSV

in the carbonate-rock aquifer, the highest §34S(suifate ) value 

in ground water is 9.4 per mil, an indication that the primary 

source of sulfate to ground water is from the oxidation of sulfide minerals or from the dissolution of gypsum formed by 

the oxidation of sulfide minerals. Ground water with S34S^sul_ 

fate) values consistent with the dissolution of marine gypsum 

or anhydrite of Silurian and Devonian age are found only in 

the northeastern part of the study area, coincident with the 

location of the only significant deposits of gypsum and anhydrite.

Sulfur isotope values in ground water reflect the 8 S values of the sulfur source but are also dependent on chemical 

reactions that can cause isotopic fractionation. For example, 

sulfate reduction can affect the isotopic composition of dissolved sulfate in ground water. Sulfate reduction is mediated 

by bacteria, which reduce sulfate to sulfide and oxidize 

organic carbon to CO2 under anaerobic conditions. The sulfate-reduction reaction can be written as

2-

+ Corganic + 2H2° <"> H2S + 2HCO3-. (12)

If the pH is greater than 7, HS" will predominate over H2 S as 

the sulfide product (Garrels and Christ, 1990). Bacterial sulfate reduction preferentially incorporates 32S into the sulfide 

product, causing the residual sulfate to be enriched in the 

heavier isotope, 34S. Thus, as sulfate reduction progresses, 

834S of sulfide will be less than that of sulfate, and §34S^sul_ 

fate) WU<1 increase relative to the original composition.

Bacterial sulfate reduction is the likely source of the dissolved sulfide in the Midwestern Basins and Arches aquifer 

system as indicated by the values of §34S<-suifide\ in ground 

water from the aquifers, which range from -53.2 to -12.2 per 

mil (fig. 47). Sulfate reduction has probably produced the 

extensive areas of the carbonate-rock aquifer in northwestern 

Ohio that are characterized by dissolved sulfide concentrations exceeding 3.0 mg/L (Ohio Department of Natural 

Resources, 1970); sulfide concentrations as high as 70 mg/L 

were noted in this area. Dissolved sulfide is less abundant in 

other parts of the aquifer system.

The differences between the S34S<-sulfate) values and the 

8 S(sulfide ) values, A^ 4 , are also shown in figure 47. The A34 

values range from 19.7 to 69.1 per mil. The largest A34 values 

in the aquifer system, 69.1 and 68.4 per mil, were found for 

water from the deep bedrock wells near the Maumee River 

and Sandusky Bay. Equilibrium fractionation between sulfate 

and sulfide is estimated to be between 65 and 74 per mil 

(Pearson and Rightmire, 1980). Thus, the large A34 values in 

the northeastern part of the study area may indicate isotopic 

equilibrium between dissolved sulfate and sulfide. In addition, the large A34 values in ground water near the Maumee 

River and Sandusky Bay may indicate a long residence time 

in the aquifer; in the Floridan aquifer of central and southern 

Florida, for example, A values were reported to be directly 

related to ground-water residence time in the aquifer. In addition, the largest A values were found in the areas of the 

Floridan aquifer farthest from the recharge area (Rye and others, 1981).

The sulfur isotope data were also used to evaluate the processes responsible for the elevated concentrations of dissolved sulfate in much of the northeastern part of the study 

area. Ground waters with sulfate concentrations greater than 

1,000 mg/L were found along geochemical sections C-C' and 

D-D\ These ground waters are saturated or nearly saturated 

with respect to gypsum, but their S34S(sulfate) values indicate 

several different sources of sulfate. The 8 S^sujfate ) values of 

25.9, 26.7, and 28.3 per mil in ground water from wells 14D, 

17D, and 17G (sulfate concentrations of 1,900, 1,600, and 

1,500 mg/L, respectively) are consistent with the dissolution 

of Silurian-Devonian-age gypsum (or anhydrite). However, 

ground water with similar sulfate concentrations but 534S<-sul_ 

fate) values that range from -10.4 to 4.0 per mil (wells 10G, 

13G, 15G, 15S, and 15D) cannot be explained by the dissolution of Silurian-Devonian age gypsum (or anhydrite) but are 

consistent with pyrite as the sulfur source. The limit on the 

amount of sulfate that can be produced in ground water by 

pyrite oxidation (reaction 8) in a closed system is less than 25 

mg/L owing to solubility constraints on dissolved-oxygen 

concentrations. Thus, closed-system pyrite oxidation could 

not generate the high sulfate concentrations that are found in 

the northeastern part of the aquifer system. Closed-system 

conditions are unlikely in much of the aquifer system, however, because the glacial aquifers are generally water-table 

aquifers, and the carbonate-rock aquifer is generally only 

semiconfined by the overlying glacial deposits. Thus, where 

pyrite is abundant in the glacial deposits, sulfate produced by 

the oxidation of pyrite (or other sources of reduced sulfur) in 

the glacial deposits may be continually added to the ground 

water as the water table rises and falls. Alternatively, high sulfate concentrations and isotopically light S~ S( SUifate^ values 

in ground water in the northeastern part of the aquifer system 

can be explained by the dissolution of gypsum composed of 

isotopically light sulfate, such has been noted in tills in Canada (Hendry and others, 1986; Keller and Van Der Kamp, 

1988).

The sulfur isotope data include a wide range in sulfur isotope signatures of sulfur sources in the Midwestern Basins 

and Arches aquifer system. In addition, sulfur isotope values 

for ground water in the aquifer system are highly varied, and 

the geochemical and hydrological processes affecting sulfur 

isotope values are not completely understood. As a consequence, use of the sulfur isotope data to help constrain the 

possible mass balance reactions may not be particularly 

effective, and large uncertainties in the 14C ages estimated for 

ground water in the aquifer system may result.
C88 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

OXYGEN AND HYDROGEN ISOTOPES

Variation in the isotope ratios of 18O/ 16O and 2H/ 1R 

(deuterium/hydrogen) in meteoric water (water that originates 

in the Earth's atmosphere) results from a combination of 

physical and chemical processes that change the isotopic 

composition from that of seawater. By definition, the isotopic 

composition of seawater, Standard Mean Ocean Water 

(SMOW), is 0 per mil for both 5 18O and 6D (Fritz and Fontes, 1980). Meteoric water is depleted in 8 18O and 8D relative 

to seawater because of isotopic fractionation; the lighter isotopes, O and H, evaporate preferentially, whereas the 

heavier isotopes, 18O and 2H, are concentrated in seawater. 

The distribution of 8 18O and 8D in modern precipitation is 

related to mean annual temperature and latitude (Dansgaard, 

1964). Generally, a greater amount of fractionation takes 

place at colder temperatures than at warmer temperatures; 

hence, 8 18O and 8D values decrease with a decrease in temperature, which corresponds to an increase in latitude or altitude. This covariance of 8 18O and 8D is reflected by the 

composition of meteoric water throughout the world and 

defines an average meteoric water line (Craig, 1961) defined 

as

= 8818O+10. (13)

Ground water recharged under present-day climatic conditions should have an isotopic composition similar to that of 

present-day meteoric water. However, if ground water was 

recharged during a colder climate, its isotopic composition 

should be lighter than that of present-day precipitation but 

should still plot on the meteoric water line.

Values for the stable isotopes of oxygen and hydrogen in 

ground water samples that were collected as part of this 

investigation are listed in table 8; 8 18O values are shown spatially in figure 48. All the data plot on or slightly above the 

meteoric water line (fig. 49), an indication that the ground 

water in the Midwestern Basins and Arches aquifer system 

originated as atmospheric precipitation. Surface-water samples collected at U.S. Geological Survey Benchmark Stations 

in Indiana, Michigan and Ohio (Tyler Coplen, U.S. Geological Survey, written commun., 1992) were used to determine 

the isotopic composition of present-day recharge in the area. 

Most of the ground water in the aquifer system has an isotopic composition similar to surface water in the study area and 

therefore was most likely recharged under similar climatic 

conditions as the present. A few ground-water samples collected from the northeastern part of the study area are isotopically light relative to the rest of the ground-water samples and 

present-day recharge. These relatively light waters probably 

recharged the aquifer system during a climate colder than the 

present.

Ground water with isotopically light SO values may 

represent either water that recharged aquifers beneath Wisconsinan-age ice sheets or meltwater from those ice sheets.

Estimates of the S18O value of Wisconsinan-age ice sheets 

range from -20 to -12 per mil (Yapp and Epstein, 1977; 

Desaulniers and others, 1981). Isotopically light ground 

water in areas around the Great Lakes has been previously 

found in east-central Michigan near Lake Michigan (Long 

and others, 1988) and southwestern Ontario near Lake Erie 

(Desaulniers and others, 1981; Clayton and others, 1966). 

The S18O values from these studies range from -18.5 to -7 per 

mil in Michigan and -17 to -9 per mil in southwestern 

Ontario.

The areas of isotopically light ground water in the Midwestern Basins and Arches aquifer system and in the areas 

discussed above are all associated with lowlands that were 

once occupied by late Wisconsinan glacial lakes (Wayne and 

Zumberge, 1965). The glacial deposits in these areas are typically lacustrine clays or clay-rich tills derived from the lacustrine sediments. The presence of clay-rich deposits in the 

areas once occupied by glacial lakes may inhibit groundwater recharge. In addition, glaciolacustrine clays in areas 

close to the Great Lakes are typically found near the base 

level of the various aquifer systems and are generally coincident with regional ground-water discharge areas. Vertical 

ground-water flow velocities through glaciolacustrine clays 

and clayey tills in southwestern Ontario (Desaulniers and others, 1981) were determined to be on the order of 40 to 80 ft in 

10,000 years. Pore waters in these deposits exhibit a distinct 

regular shift in S18O values with depth from SO values 

characteristic of present-day recharge at the water table to 

values between -14 to -17 per mil at depths of 60 to 100 ft. A 

similar decrease in 5 O values with depth is seen in the Midwestern Basins and Arches aquifer system in well groups 13 

and 14 and in the data presented by de Roche and Breen 

(1989). This decrease indicates that a lack of substantial 

recharge has likely preserved Pleistocene-age water in deeper 

parts of the aquifer system beneath the Maumee River Basin.

TRITIUM AND CARBON ISOTOPES

Tritium ( 3H) and carbon-14 ( 14C) are radioactive isotopes 

whose concentrations in ground water can be used as a guide 

to ground-water age. Estimated ground-water ages can help 

validate or refute interpretations made about ground-water 

flow based on other chemical and isotopic data or results of 

ground-water-flow modeling.

Because large quantities of tritium were introduced into 

the atmosphere during atmospheric testing of nuclear bombs 

from 1953 to the mid-1960's, tritium is a useful indicator of 

post-1953 recharge to ground water. Tritium concentrations 

in precipitation peaked at several thousand tritium units (TU; 

1 TU is equal to 1 tritium atom in 1018 hydrogen atoms or 

3.24 picocuries per liter, pCi/L) during 1963-64 (Michel, 

1989). Since the signing of the nuclear test ban treaty in the 

late 1960's, tritium concentrations in present-day precipitation have been decreasing to the pre-bomb levels of 2 to 8 tri-
GEOCHEMISTRY C89

38°

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 KILOMETERS

EXPLANATION

Boundary of study area Sample locations from other studies

Well sampled for chemical analysis during this investigation ~8-10. Nicholas and others (1996). Number is 5 18O value. 

Numbers is 8 18 O value, in per mil in per mil

_.__ .Group of wells ® de Roche and Breen (1989). Numbers represent the 

Carbonate-rock aquifer, shallow -9.90 (*%; in close proximity

Glacial aquifers _ _g 65 

aquifer, shallow -9.90 ( 

Carbonate-rock aquifer, deep ~1fl-35

through range in 5 18O values from 14 wells

-7.8

FIGURE 48. 818O values in water from wells in the Midwestern Basins and Arches aquifer system.
C90 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

-30 i 

-40

-50

-60

-70

-80

-90

-100

-100

-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5

8 "0, IN PER MILL

EXPLANATION 

O de Roche and Breen (1989)

D Nicholas and others (1996)

A Wells sampled during this investigation

FIGURE 49. 8 18 O - 8 D relation for ground water in the Midwestern Basins and Arches aquifer system. 

(Other sources of data include de Roche and Breen [19891, and Nicholas and others [1996]).

tium units estimated by Thatcher (1962). Ground water 

derived exclusively from pre-bomb precipitation would have 

a maximum tritium concentration of 0.8 TU by the early 

1990's if the maximum pre-bomb estimate of 8 TU in atmospheric precipitation is used (Plummer and others, 1993). For 

ground water with tritium concentrations greater than 0.8, 

some fraction of that water must have recharged the aquifer 

system since 1953.

Tritium concentrations for waters sampled as part of this 

investigation are listed in table 8 and shown spatially in figure 

50. Ground water with tritium concentrations greater than 0.8 

TU are found across most of the study area and indicate that 

post-1953 recharge has entered the aquifers in many parts of 

the Midwestern Basins and Arches aquifer system. Tritium 

concentrations vary somewhat predictably with depth; water 

from about half of the glacial and shallow bedrock wells has 

tritium concentrations greater than 0.8 TU, whereas only onethird of the deep bedrock wells produce tritiated water. Tritium concentrations are also spatially varied in the aquifer 

system. Ground water with tritium concentrations greater

than 0.8 TU is found in at least one well at nearly all sample 

locations; however, no ground-water samples from well 

groups 13, 14, 16, and 17 in the Maumee and Sandusky River 

Basins, including the wells completed in glacial aquifers, 

contain tritium. Breen and Dumouchelle (1991) determined 

that areas of the carbonate-rock aquifer in northwestern Ohio 

with tritium concentrations indicative of post-1953 recharge 

(fig. 50) are more likely to be found where the glacial deposits are absent or are less than 20 ft thick. A similar relation 

between tritium concentrations in ground water from the carbonate-rock aquifer and the thickness of the overlying glacial 

deposits is apparent in southeastern Michigan (Nicholas and 

others, 1996). This relation between tritium concentrations 

and thickness of glacial deposits is not apparent over the 

entire study area; the thickness of the overlying glacial deposits for wells producing water with tritium concentrations 

greater than 0.8 TU ranges from 6 to 160 ft. The thickness of 

the overlying glacial deposits for wells producing water with 

the highest tritium concentrations (greater than 15 TU) ranges 

from about 40 to 160 ft.
GEOCHEMISTRY

Base from U.S. Geological Survey digital data, 

1:2,000,000,1972

0 20 40 60 KILOMETERS

EXPLANATION

Areas of the carbonate-rock aquifer where tritium 

concentrations in ground water (Breen and 

Dumouchelle, 1991) indicate post-1953 recharge

Boundary of study area

Well sampled for chemical analysis during this investigation 

Numbers represent tritium concentrations, in tritium units, and 

carbon-14 activities, in percent modern carbon. indicates data 

not available

Glacial aquifers _ 05/__ ^ ^ Group of wells 

Carbonate-rock aquifer, shallow <o!i/i.s («%J in close proximity 

Carbonate-rock aquifer, deep- --^O-VO.2

Tritium concentration Carbon-14 activity

FIGURE 50. Tritium concentrations and carbon-14 activities in water from wells in the Midwestern Basins and Arches aquifer system.
C92 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

Tritium data can help in making qualitative estimates of 

ground-water ages on the order of tens of years. Alternatively, 

14C data can help date ground water that is thousands of years 

old. The maximum age that can be dated using 14C is about 

50,000 years given the isotope's half-life of 5,730 years 

(Freeze and Cherry, 1979). Ground-water samples were collected in the study area for C analysis to help constrain 

ground-water ages and to help describe regional groundwater flow in the Midwestern Basins and Arches aquifer system. Values for 6 C were also determined to provide information regarding the sources of dissolved inorganic carbon in 

the ground water and to help correct the measured 14C values 

for the dilutional effects caused by various geochemical processes. The data are listed in table 8, plotted in figure 50 and 

discussed below.

The radioactive decay expression for 14C can be 

expressed as

t = -8270 In (R), (14)

where t equals the decay age of the carbon, in years, and R 

equals the measured C activity of the sample expressed as 

a percentage of the 14C activity of modern carbon (Mook, 

1980). This equation indicates the time, in years, since the 

water was in contact with CO2 in the soil zone; use of the 

equation requires the assumption that there have been no 

other inputs of dissolved inorganic carbon. However, dissolution of carbonate minerals or oxidation of organic matter can 

contribute nonradioactive carbon to ground water, diluting 

the amount of C originally in the water and yielding an 

unadjusted age that is too old. To correct for nonradioactive 

carbon, an adjustment factor is added to the decay equation:

t = -8,270 In (R) + 8,270 In (Q). (15)

For waters in which all the inorganic carbon is produced by 

the dissolution of carbonate minerals in the soil zone, the Q 

value is 1.0. The Q value will become some fraction of 1.0 if 

dissolution continues below the water table where exchange 

with soil-gas CO2 cannot occur. Wigley (1975) has shown 

that in aquifers where CO2 is not produced below the water 

table, the minimum Q value will not be less than 0.5. Where 

gypsum dissolution or the exchange of Ca2+ and Mg2+ for 

Na+ drives the incongruent dissolution of dolomite under 

closed-system conditions (dedolomitization), or where CO2 is 

produced by the oxidation of old organic matter during bacterially mediated sulfate reduction, Q values may be quite small 

because of the introduction of significant amounts of C with 

zero activity. The Q values for ground water in the Madison 

Limestone in parts of Montana, South Dakota, and Wyoming, 

range from about 0.5, where minor dedolomitization has 

occurred, to less than 0.1, where ground waters have been 

affected by dedolomitization, cation exchange, and substantial sulfate reduction (Busby and others, 1991).

To produce accurate estimates of Q, one must use 

geochemical models that incorporate the effects of dedolomitization, sulfate-reduction, cation exchange, and isotopic 

exchange. This type of analysis, however, was beyond the 

scope of this investigation. As a first approximation, 14C ages 

were estimated from Q values that were selected somewhat 

arbitrarily. The raw-age estimates (Q=1.0) were corrected by 

use of Q values of 0.5, 0.3, and in some cases. 0.2 (table 11). 

A Q value of 0.5 is expected for closed-system dissolution of 

carbonate minerals where no additional source of CO2 is 

added and where no secondary mineral precipitates (Wigley, 

1975). Thus, a Q value of 0.5 can overestimate the age of 

ground water that has undergone dedolomitization or sulfate 

reduction. A Q value of 0.3 was used in this investigation to 

account for geochemical reactions that produce additional 

sources of nonradioactive carbon to the ground water system. 

A Q value of 0.2 was used for ground waters with small 14C 

activities and S13C values that are isotopically heavy. Much 

of the ground water in the aquifer system has 8 C values of 

around -14 or -15 per mil; however, ground water from wells 

ID, 13D, 14D, 17G, and 17D has 813C values that range from

-11.0 to -7.1 per mil, an indication that a greater proportion of 

the dissolved inorganic carbon in these waters was derived 

from the dissolution of carbonate rocks with 813C values of 

about 0 per mil than in the waters with 813C values of around

-14 or -15 per mil. Samples 8G, 9D, and 19 also have heavier 

813C values (-8.8, -3.1, and -1.3 per mil) than do most other 

samples. These waters, however, have likely been affected by 

methanogenesis, a reaction in which organic matter is metabolized by methanogenic bacteria to produce CO2 , methane 

(CH4), and dissolved organic compounds. The concentration 

of dissolved methane in well 8G was 18.65 mg/L (Eurybiades 

Busenberg, U.S. Geological Survey, written commun., 1992). 

Ground water from wells 9D and 19 were not analyzed for 

dissolved methane, but these samples had elevated concentrations of dissolved organic carbon and (or) little or no dissolved sulfate, a composition consistent with methanogenesis. The age of these waters cannot be accurately determined without measuring the 813C value of the dissolved 

methane.

Also listed in table 11 are ages calculated by use of the 

method proposed by Pearson and White (1967), in which 

813C values are used to correct for isotopic dilution due to 

congruent carbonate dissolution according to the equation

14C(adjusted) = (16)

14C(measured) x

carbonate

carbonate-
GEOCHEMISTRY C93

TABLE 11. Summary of 14C age estimates for ground water sampled during investigation of the Midwestern Basins and

Arches aquifer system, 1991-92

[Map number is location in figure 32. Q values are adjustment factors used to account for addition of nonradioactive carbon to the

ground-water system. Negative ages indicate that local geochemistry does not reflect the assumed conditions used in assigning the

Q value. Question mark indicates that age cannot be determined from available data. , value not determined]

Well name

Ross

Birt

Ben Logan H.S.

Searfoss

Rife

Schoenberger

Beasley

Hill

Gillig

Lamalie

S-18

Finnegan

Wilson

Spencer

Stair

Francis

Augustine

Richard

Kern

Auckerman

Rice

CityofWeston

Hartman

Lacy

Hummel

Jones

Harrison

Martin

Lockhart

Mattingly

Merchant

Gates

Bickford

Bartlett

Desoto Substation

Staggs

Ice

Underwood

Cohee

Ellis

Harmon

Fox

Skiles

Justice

Lee

Starbuck

Geradot

Stenzel

Rees

Map 

number

10 G

10 S

10 D

15 G

15 S

15 D

16 G

16 S

16 D

17 G

17 D

18 D

11G

US

11D

12 S

12 D

13 G

13 S

13 D

14S

14 D

19

4G

4S

4D

3S

3D

2G

2S

2D

1 D

5G

5S

5D

6G

6S

6D

7G

7S

7D

8G

8D

9G

9S

9D

22

21

20

Age, 

Q = 1.0

(years)

5,900

<50

<50

8,000

9,900

11,600

5,000

23,000

20,300

26,800

25,300

6,100

14,000

<50

<50

<50

<50

35,900

51,400

20,100

35,300

24,000

4,100

4,800

4,700

6,900

6,500

<50

<50

<50

36,600

<50

4,000

7,100

<50

4,000

8,500

4,500

4,700

<50

6,800

<50

<50

<50

10,500

<50

8,700

13,000

Age, 

Q = 0.5

(years)

200

<50

<50

2,300

4,100

5,900

-700

17,300

14,600

21,100

19,600

400

8,300

<50

<50

<50

<50

30,200

45,700

14,400

29,600

18,300

-1,600

-1,000

-1,100

1,200

800

<50

<50 ,

<50

30,800

<50

-1,800

1,400

<50

-1,800

2,800

-1,200

-1,100

<50

1,000

<50

<50

<50

4,800

<50

2,900

7,200

Age, 

Q = 0.3 

(years)

^1,100

<50

<50

-1,900

-100

1,700

-5,000

13,000

10,300

16,900

15,300

-3,900

4,000

<50

<50

<50

<50

26,000

41,400

10,100

25,300

14,000

-5,800

-5,200

-5,300

-3,100

-3,500

<50

<50

<50

26,600

<50

-6,000

-2,800

<50

-6,000

-1,400

-5,500

-5,300

<50

-3,200

<50

<50

<50

500

<50

-1,300

3,000

Age, 

Pearson- White

(years)

2,500

<50

<50

5,500

6,700

8,700

600

19,500

16,200

20,800

17,200

-600

10,800

<50

<50

<50

<50

31,600

45,200

16,200

25,800

-300

600

1,400

900

2,200

2,800

<50

<50

<50

30,700

<50

300

3,100

<50

600

5,500

1,000

1,400

<50

-1,000

<50

<50

<50

-5,900

<50

5,300

11,100

PearsonWhite 

Q value

0.66

-

.74

.68

.70

.59

.65

.61

.48

.38

.44

.68

.00

.00

.59

.48

.63

.32

.05

.65

.66

.64

.56

.64

-

.49

.64

.61

-

.67

.69

.65

.67

.39

.14

.66

.80

Q value(s) 

used for age 

estimate

0.66

-

.74

.68

.70

.59

.3 - .65

.3 - .61

.2 -.48

.2 -.38

.44

.3 - .68

-

.3 - .59

.2 - .48

.3 - .63

.2 - .32

-

.65

.66

.64

.56

.64

-

.2 - .49

.64

.61

-

.67

.3 - .69

.65

.67

.66

.3 - .80

Groundwater age 

(years)

<50- 2,500

<50

<50

<50- 5,500

6,700

8,700

600

13,000-19,500

10,300-16,200

13,500-20,800

12,000-17,200

<50

4,000 - 10,800

<50

<50

<50

<50

26,000-31,600

38,100-45,200

10,100-16,200

22,000 - 25,800

9

<50 - 600

1,400

900

2,200

2,800

<50

<50

<50

23,300-30,700

<50

300

3,100

<50

600

<50 - 5,500

1,000

<50- 1,400

<50

9

<50

<50

<50

9

<50

5,300

3,000-11,100
C94 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

where 14C (adjusted) is the 14C activity adjusted for dissolution of carbonate rocks with known 8 C values, 14C (measured) is the measured activity in the water sample, 8 C soji 

is the isotopic composition of soil-gas CO2, 813Ccarbonate is 

the isotopic composition of the carbonate minerals in the 

study area, and 813Cwater is the isotopic composition of the 

dissolved inorganic carbon in the water sample. Values of 

813C soi] in temperate climates vary from -25 to -18 per mil 

(Mook, 1980; Back and others, 1983); a value of -22.5 per 

mil was used in equation 16 for this investigation. Values of 

^ ^carbonate from samples of the carbonate-rock aquifer and 

the glacial aquifers are listed in table 9. The average 

S ^carbonate value of the 10 samples, 0.0 per mil, was used in 

equation 16.

A summary of ages calculated from equations 15 and 16 

is given in table 11. Ages were not calculated for the samples 

with tritium concentrations greater than 6 TU because such 

waters may contain a significant component of recent 

recharge with unknown amounts of bomb-derived C. On 

the basis of the range of calculated ages and the chemical and 

isotopic data, each ground water sample was assigned an 

approximate age. In some ground water samples, tritium concentrations are between 0.8 and 3 TU and may represent a 

mixture of old water and recent recharge. Such waters were 

assigned an age ranging from less than 50 years to the age 

calculated by use of equation 16. Waters that are saturated 

with respect to calcite and dolomite and apparently unaffected by dedolomitization or sulfate reduction were assigned 

the age estimated from equation 16. The range of adjustment 

factors (Pearson-White Q value) for these waters is 0.56 to 

0.67, which is in close agreement with the Q values calculated by Wigley (1975) for ground water that is undergoing 

congruent dissolution of calcite and dolomite in the soil zone 

and below the water table. Waters that have undergone dedolomitization or sulfate reduction were assigned a range of 

ages that bracket those calculated with a Q value of 0.3, or for 

waters with heavy 813C values, 0.2, and ages that were calculated by use of equation 16 (Pearson-White method).

The ground-water ages in the aquifer system are consistent with a conceptual model of regional ground-water flow in 

which the aquifer system alternately receives recharge and 

discharges across most of the study area except in the northeastern part, where the aquifer system does not receive a 

notable amount of recharge. Ground-water ages across most 

of the study area range from less than 50 years to several 

thousand years and generally do not follow any spatial trends. 

This pattern is consistent with a ground-water flow regime 

that is largely dominated by local flow systems that have 

short flow paths relative to the areal extent of the aquifer system. The oldest ground waters in the aquifer system range in 

age from at least 3,000 to possibly more than 40,000 years 

and are in the discharge areas in the northeastern part of the 

study area near the Maumee and Sandusky Rivers and Lake 

Erie. Relatively old waters were also found near the Wabash

River in northern Indiana and at the margin of the Illinois 

Basin where the upper confining unit overlies the carbonaterock aquifer.

Ground water from wells 13S, 1 3D, and 14D ranges in 

age from about 22,000 to more than 45,000 years and has 

818O and 8D values expected for water recharged during the 

Pleistocene. The age ranges of ground water from wells 14S, 

16S, 16D, 17G, and 17D are also Pleistocene; however, the 

8 O values are similar to those for present-day recharge. The 

retreat of Late-Wisconsinan glaciers in the Great Lakes area 

began about 13,300 years ago and was nearly complete by 

10,000 years ago (Barry, 1983). Isotopic data presented by 

Yapp and Epstein (1977) indicate that the 818O and SD values 

of precipitation during the interval between 10,000 and 9,500 

years before present were similar to those of present-day precipitation. Data on 818O values in mollusk shells from sediment cores in Lake Erie (Fritz and others, 1975) indicate a 

rapid increase in 818O values in water in Lake Erie after 

13,000 years before present because of a rapid rise in average 

land temperatures. The lower ages presented in table 11 for 

ground water from wells 14S, 16S, 16D, 17G, and 17D range 

from 10,300 to 13,500 years. Because the isotopic composition of precipitation during this time period may have been 

similar to that of today, the 818O and 8D data for these 

ground waters indicates that the lower age limits listed in 

table 11 for these waters may be the more reasonable. Thus, 

ground water from wells 13S, 13D, and 14D may represent 

water that recharged the aquifer system from beneath the glacial ice, whereas ground water from wells 14S, 16S, 16D, 

17G, and 17D may represent precipitation that entered the 

aquifer system at the end of glaciation. Alternatively, Siegel 

(1991) observed a similar disparity between 818O values and 

estimated 14C ages for Pleistocene-age ground water in the 

northern Midwest. He suggested that the calculated groundwater ages may be too great if 813C values of soil gas 

increased because of a reduction of the amount of organic 

matter that was present in the soil during the colder climate. 

As a result, the actual ages of ground water recharged during 

the Pleistocene may be less than those listed in table 11. 

Although considerable uncertainty is inherent in the absolute 

ages of ground water because of assumptions used in the calculations, the ages given in table 11 are helpful for evaluating 

the relative age of ground waters in the Midwestern Basins 

and Arches aquifer system.

VARIATION IN GROUND-WATER CHEMISTRY WITH DEPTH

The sampling methodology for this investigation, discussed earlier in this report, was designed to use existing 

wells to obtain as much information as possible on the variation in ground-water chemistry with depth in the Midwestern 

Basins and Arches aquifer system. Two types of deep bedrock 

wells were sampled: those that were open hole through the 

entire length of the well and those that were cased through
SUMMARY AND CONCLUSIONS C95

various lengths of the top section of the carbonate-rock aquifer (table 6). Data from these deep bedrock wells were compared to data from nearby shallow bedrock wells to assist in 

understanding the ground-water chemistry at depths as great 

as 450 ft below the top of the carbonate-rock aquifer.

A comparison of the chemical and isotopic data and the 

estimated ground water ages (tables 7, 8, and 11) between the 

shallow bedrock wells and the deep bedrock wells that are 

open hole through the entire length of the well shows that the 

chemistry and the ground-water ages between these wells are 

generally similar. The differences in ground-water ages in all 

these wells ranges from several hundred to less than 3,000 

years. Of the pairs of shallow and deep bedrock wells that 

have ages greater than 50 years, ground water in the deep 

bedrock well is older than in the shallow bedrock well in well 

groups 3, 5, and 15; whereas ground water in the shallow bedrock well is older than in the deep bedrock well in well 

groups 4 and 16. The similarity between ground water from 

wells that are open only to the shallow part of the carbonaterock aquifer and ground water from wells that are open to a 

much greater part of the carbonate-rock aquifer indicates that 

either (1) the chemistry and age of the ground water is similar 

throughout the sampled thickness of the carbonate-rock aquifer or (2) if ground water at depth has a distinctly different 

chemistry or older age, then the carbonate rocks are not transmissive enough at the greater depths to contribute significantly to the overall chemistry of the sampled water, and most 

ground water is produced from the upper part of the carbonate-rock aquifer.

Data from a shallow interval (less than 50 ft below the top 

of the aquifer) and an isolated deeper interval (as great as 450 

ft below the top of the aquifer) within the carbonate-rock 

aquifer are available for well groups 10, 13, 14, and 17. A 

comparison of ground-water chemistry and ages between the 

shallow and deep bedrock wells at well groups 13 and 14 

shows a systematic change in chemistry with depth and a distinctly different ground water in the deeper part of the carbonate-rock aquifer than in the shallow part. The chemical 

and isotopic data from these two well groups indicate that, in 

this area of the aquifer system, some recharge water enters 

the glacial aquifers and the shallow zones of the carbonaterock aquifer but does not penetrate deeply into the aquifer 

system. In contrast, a comparison of ground-water chemistry 

and ages at well groups 10 and 17 shows little difference 

between the shallow and deeper parts of the aquifer system. 

At well group 10, tritium concentrations indicative of post1953 recharge are found throughout the entire sampled thickness of the carbonate-rock aquifer. These wells are located at 

the highest potentiometric level in the aquifer system, where 

recharge rates to the deepest parts of the aquifer system 

should be relatively high. The chemical and isotopic data for 

ground water from wells 17G and 17D, located near the 

Sandusky River and Lake Erie, is quite similar. The age of the 

water in the overlying glacial aquifer is slightly older than

water from deeper within the carbonate-rock aquifer (13,500 

years compared to 12,000 years, using the lower age limit as 

discussed in the previous section). This slight decrease in age 

with depth may reflect an upward flow of ground water as it 

discharges the aquifer system.

Chemical and isotopic data are also available for ground 

water that was sampled from specific intervals in three deep 

wells that penetrate the entire thickness of the carbonate-rock 

aquifer in northwestern Indiana (DJ. Schnoebelen, U.S. Geological Survey, written commun., 1993). Ground water was 

sampled from various 10-ft intervals of the carbonate-rock 

aquifer by use of an inflatable packer system. Hydrogeologic 

data for these three wells and a description of the packer technology are given in Arihood (1994). The chemical and isotopic data from all the sampled intervals in each well are very 

similar, an indication that there is no stratification of groundwater chemistry with depth in this part of the aquifer system; 

however, the samples do not represent water from the deepest 

part of the carbonate-rock aquifer because the rocks in the 

bottom 60 to 400 ft of the wells are not necessarily transmissive (Arihood, 1994).

SUMMARY AND CONCLUSIONS

Aquifers in Quaternary glacial deposits and underlying 

Silurian and Devonian carbonate rock in parts of Indiana, 

Ohio, Michigan, and Illinois compose an integrated regionalscale water-table aquifer system. This aquifer system generally lies between the Appalachian, the Illinois, and the Michigan (structural) Basins and is located along the axes of the 

Cincinnati, the Findlay, and the Kankakee Arches.

Glacial deposits within the study area are the result of 

multiple glacial advances and directly overlie the carbonate 

rocks along the axes of the structural arches in the central part 

of the study area. These glacial deposits mask the ancient 

bedrock topography and bury numerous valleys in the bedrock surface. The glacial deposits and the carbonate rocks are 

separated by a shale sequence along the margins of and 

within the surrounding structural basins.

The water table within the Midwestern Basins and Arches 

aquifer system generally is within the glacial deposits. Glacial aquifers typically consist of sands and gravels in outwash 

deposits or as discontinuous lenses within ground or end 

moraines. Because such lenses are not necessarily areally 

extensive, the glacial aquifers can supply large yields of 

ground water only locally.

An areally extensive aquifer is present in the carbonate 

rocks that underlie the glacial deposits. At the regional scale, 

the carbonate-rock aquifer is semiconfined by the finer 

grained glacial deposits where the aquifer directly underlies 

these deposits (subcrop area of the carbonate-rock aquifer) 

but is confined where the aquifer underlies the intervening 

shale sequence, called the upper confining unit, along the
C96 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

margins of the aquifer system. Most active freshwater flow in 

the carbonate-rock aquifer is limited to the subcrop area of 

the aquifer because the upper confining unit inhibits recharge 

to the carbonate-rock aquifer. This lack of freshwater flow 

beneath the upper confining unit results in a transition from 

freshwater to saltwater in the carbonate-rock aquifer within 

the surrounding structural basins. Saline water is present 

within 5 mi of the subcrop boundary along the Michigan and 

Appalachian Basins. Freshwater extends as far as 70 mi 

downdip from the western subcrop boundary along the Illinois Basin; however, 14C data for one freshwater sample (ID) 

(fig. 32) collected from the aquifer just west of the subcrop 

boundary indicates that this water may be 23,000 to 31,000 

years old. The fact that this water is older than most waters 

throughout the subcrop area may indicate that freshwater in 

the carbonate-rock aquifer in the Illinois Basin is not associated with active freshwater flow.

The hydraulic characteristics of the aquifers within the 

Midwestern Basins and Arches aquifer system are varied. On 

the basis of available aquifer-test data, transmissivities of the 

glacial deposits span four orders of magnitude, ranging from 

1.5 to 69,700 ft /d, whereas transmissivities in the carbonaterock aquifer span three orders of magnitude, ranging from 70 

to 52,000 ft /d. Such variability in the carbonate-rock aquifer 

is attributed to the nonuniform distribution of fractures, 

joints, bedding planes, and solution channels in the rocks. 

The results of a numerical ground-water flow model, which 

simulates regional flow systems within the aquifer system, 

indicate that where the carbonate rocks are hundreds of feet 

thick, the entire thickness of the rocks may not contribute 

substantially to the transmissivity of the aquifer. Some of the 

available aquifer-test data support these model results. In 

addition, the local presence of anhydrite within the carbonate 

rocks at depths of only a few hundred feet near Sandusky Bay 

in the northeastern part of the aquifer system indicates that 

active freshwater flow may be restricted to the upper few hundred feet of the carbonate rocks in this area.

The types and distribution of hydrochemical facies of 

water within the glacial aquifers and the subcrop area of the 

carbonate-rock aquifer are similar. Water of a Ca-Mg-HCO3 

type is present throughout much of the area but is found only 

locally in the northeastern part of the study area, where CaMg-SO4 type water predominates. The dissolution of calcite 

and dolomite is responsible for the presence of the Ca-MgHCO3 water, whereas dedolomitization driven by gypsum 

dissolution and the oxidation of pyrite are responsible for the 

Ca-Mg-SO4 water. Multiple-water-type facies in the aquifer 

system, which are found near the margins of the Michigan 

and Illinois Basins, are characterized by the presence of CaMg-Na-HCO3 water or Ca-Mg-Na-SO4 water, or both; cation 

exchange in shales within the aquifer system is responsible 

for the presence of these Na-enriched waters. The distribution 

of these multiple-water-type facies may be related to past patterns of ground-water flow associated with Pleistocene glaciation. Water in the carbonate-rock aquifer under the upper 

confining unit in the Appalachian and Michigan Basins is a 

Na-Ca-Cl type, whereas the water under the upper confining 

unit in the Illinois Basin is a Na-Cl type. In general, the distribution of hydrochemical facies observed within the aquifer 

system is controlled by the mineralogy of the aquifer material 

rather than the evolution of water chemistry along paths of 

regional ground-water flow, as is commonly observed in 

closed aquifer systems.

Some ground-water flow systems within the Midwestern 

Basins and Arches aquifer system provide base flow to 

streams in response to ground-water recharge events, whereas 

other (often deeper) ground-water flow systems respond minimally to variations in ground-water recharge from precipitation. These latter, more stable flow systems have a more 

dominant influence of intermediate- and regional-scale flow 

than do the flow systems that readily respond to ground-water 

recharge, and they provide a fairly constant supply of water to 

streams over the course of a year and throughout long periods. Discharge to streams from such stable flow systems 

within the aquifer system is termed "sustained ground-water 

discharge" in this report.

Mean sustained ground-water discharge (discharge from 

fairly stable ground-water flow systems) to selected stream 

reaches within the study area ranges from 3 to 50 percent of 

mean ground-water discharge (discharge from all groundwater flow systems) to the stream reaches. These percentages 

indicate that the greatest amount of ground-water discharge 

to the streams (50 to 97 percent) is generally associated with 

transient flow systems, which typically have a major component of local-scale flow. Because results of the regional 

ground-water flow model indicate that most water in regional 

flow systems within the aquifer system discharges to streams 

within the study area, these percentages imply that seasonally 

transient local flow systems dominate flow in the Midwestern 

Basins and Arches aquifer system.

Values of mean sustained ground-water discharge as a 

percentage of mean ground-water discharge increase with 

distance downstream in nearly half of the principal surfacewater drainage basins; this pattern indicates that streams at 

the bottoms of these drainage basins receive more base flow 

from stable flow systems within the aquifer system than 

streams in the upbasin areas. Percentages decrease with distance downstream along stream reaches in the south-central 

part of the study area, where the carbonate-rock aquifer is 

absent and the underlying bedrock is not an aquifer, and 

along stream reaches in the Maumee and Sandusky River 

Basins.

Differences in the relative amounts of base flow in the 

principal surface-water drainage basins that can be attributed 

to discharge from fairly stable ground-water flow systems are 

notable for long-term steady-state conditions in the aquifer 

system. On the basis of historic streamflow data, the Wabash 

River Basin has the greatest percentage (33 percent) of base
SUMMARY AND CONCLUSIONS C97

flow that can be attributed to discharge from fairly stable flow 

systems. In contrast, less than 15 percent of the base flow in 

the principal surface-water drainage basins that drain into 

Lake Erie can be attributed to such stable flow systems.

The numerical model of regional flow systems within the 

aquifer system, which simulates approximately 10 percent of 

total ground-water flow in the system, indicates that most 

water (99 percent) in simulated regional flow systems is from 

recharge at the water table. Most water (78 percent) discharges from simulated regional flow systems to the principal 

streams within the area. Some water (19 percent) leaves simulated regional flow systems by means of evapotranspiration 

or discharge to seeps, springs, ditches, or small streams. Less 

than 3 percent of the water in simulated regional flow systems 

discharges to the Ohio River or Lake Erie or flows downdip 

into the Illinois Basin.

The Midwestern Basins and Arches aquifer system is generally characterized by alternating regional recharge and discharge areas, typically on a scale of less than 10 mi. Ground 

water does not follow long, continuous flow paths from 

recharge areas associated with the very highest potentiometric levels to discharge areas associated with the very lowest 

potentiometric levels while remaining isolated from additions 

of recharge. Rather, regional recharge areas exist all along the 

regional potentiometric gradient, except in the northeastern 

part of the aquifer system. Some of the highest regional 

recharge rates are associated with the Bellefontaine Outlier in 

Ohio.

The presence of tritiated water across most of the aquifer 

system can also be used to conclude that the aquifer system 

receives recharge across most of the study area. On the basis 

of tritium concentrations and modeled C activities, groundwater ages throughout most of the aquifer system range from 

less than 50 years to several thousand years. Significant 

amounts of post-1953 water is found more than 100 ft 

beneath the surface of the bedrock in the area of the Bellefontaine Outlier.

High rates of simulated discharge from regional flow systems are typically associated with the principal streams that 

drain the area underlain by the aquifer system. Simulated 

regional discharge areas, however, are not limited to the 

vicinity of these streams. Specifically, a broad area (tens of 

miles) of weak regional discharge (less than 0.5 in/yr) exists 

within the northeastern part of the study area. This broad 

regional discharge area likely represents an area in which 

water that moves through regional flow systems leaves the 

aquifer system by means of ditches, small streams, or evapotranspiration. The Maumee and Sandusky River Basins in the 

northeastern part of the study area are the only principal river 

basins where simulated regional discharge areas are larger 

than simulated regional recharge areas.

A number of factors help explain the existence of the 

broad, weak regional discharge area. The streams in this area 

are incised only a few feet, which limits their ability to intercept regional ground-water flow. In addition, the area has a 

very low hydraulic gradient, which results in the presence of 

regional discharge areas that are broader than the streams 

themselves. Results of the regional ground-water flow model 

indicate that the magnitude of regional-scale horizontal 

ground-water flow in the poorly permeable glaciolacustrine 

deposits within the area is lower than elsewhere in the aquifer 

system. In general, model results indicate that the regional 

flow systems have a limited ability to carry ground water 

away from the area, which prevents precipitation from 

recharging the regional flow systems. Previous researchers 

have noted that much of this area was swampland before 

being ditched in the early 1900's and that the historic Black 

Swamp resulted from poor surface drainage in addition to 

ground-water discharge from regional flow into what was a 

relatively stagnant area. Conclusions based on results of simulations are consistent with the low percentages of mean sustained ground-water discharge to streams relative to mean 

ground-water discharge to the streams in the area; much of 

the precipitation that is prevented from recharging the 

regional flow systems in the area is likely to be forced to discharge locally by means of drainage tile or shallow, transient 

local flow systems.

Chemical data also indicate that the aquifer system 

receives limited recharge in the northeastern part of the study 

area. Water with isotopically light 8 18O values (-10.05, - 

14.35) is found in the aquifer system near the Maumee River; 

these values indicate that the water may have entered the 

aquifer system beneath Wisconsinan ice sheets. On the basis 

of 14C ages, these are the oldest waters in the aquifer system. 

These data indicate minimal flushing of ground water since 

the last glaciation at depths greater than 100 ft below land 

surface in this part of the aquifer system. The occurrence of 

extensive sulfate reduction in this area, as indicated by sulfide 

concentrations and sulfur isotope data, confirms that only 

minimal recharge has taken place in this area over a long 

period of time.

A three-dimensional flow field exists within the aquifer 

system at the regional scale because regional recharge is 

available to the regional flow systems across most of the 

study area, as previously discussed. As a result, individual 

regional ground-water flow paths cannot be determined from 

two-dimensional potentiometric-surface maps of the aquifer 

system. They can be determined, however, by use of the 

three-dimensional regional ground-water flow model. Results 

of the model indicate that the longest regional ground-water 

flow paths within the aquifer system (at nearly 50 mi) terminate at Lake Erie.

Chemical data also indicate that water in the regional flow 

systems generally does not follow long flow paths (tens of 

miles). On the basis of dissolved-solids and sulfate concentrations in the ground water, there is a general absence of chemical evolution of ground water from the highest regional 

potentiometric levels to the lowest regional potentiometric
C98 REGIONAL AQUIFER-SYSTEM ANALYSIS MIDWESTERN BASINS AND ARCHES

levels. There also is no systematic increase in ground-water 

ages along the dominant regional trends of the potentiometric 

surfaces. An exception to this apparent absence of systematic 

increase in ground-water ages was identified along a 

geochemical section that terminates near Lake Erie.

Ground water at the downgradient end of the flow paths 

that terminate at Lake Erie is some of the oldest in the aquifer 

system (approximately 13,000 years). The very oldest ground 

water (approximately 38,000-45,000 years) was identified 

further west, beneath the Maumee River Basin. The A34S values (69.1 per mil, 68.4 per mil) for ground water sampled 

near Lake Erie and the Maumee River also indicate relatively 

long residence times in these parts of the aquifer system. 

Because the very oldest ground water, which is present 

beneath the Maumee River Basin, is associated with short 

(typically less than 10 mi) regional ground-water flow paths 

as compared to the flow paths that terminate at Lake Erie, it is 

likely that this part of the aquifer system is fairly stagnant. 

The old ground water near Lake Erie, however, is more likely 

to be associated with position along regional flow paths rather 

than with a stagnation point in the aquifer system. Results of 

the regional ground-water flow model indicate that flow in the 

aquifer system near Lake Erie at the end of relatively long 

flow paths may be predominantly vertical and may be associated with discharge from the deep parts of the aquifer system. 

Ground-water ages from 14C data for selected wells near 

Lake Erie increase slightly with decreasing depth, a pattern 

that is consistent with the modeled upward flow of ground 

water in this area.

No effects of future pumping on the aquifer system were 

investigated as part of the Midwestern Basins and Arches 

Regional Aquifer-System Analysis project. Nearly 433 

Mgal/d of ground water was reported to have been withdrawn 

from the aquifer system by users capable of pumping 100,000 

gal/d or greater during the 1990 calendar year. Only 15 percent of this water was reported to have been withdrawn from 

the carbonate-rock aquifer. Of the remaining 85 percent, most 

of the water was reported to have been withdrawn from outwash deposits that underlie principal streams. Estimates of 

mean ground-water discharge to streams for long-term 

steady-state conditions in the aquifer system indicate that 

more than 13,000 Mgal/d of water discharges from the aquifer system to streams within the study area. Current pumpage, 

therefore, is only a small percentage of the total amount of 

water that moves through the aquifer system. No regionalscale cones of depression are present within the aquifer system; the aquifer system is not heavily stressed at the regional 

scale, and much more of the water in the Midwestern Basins 

and Arches aquifer system could be used.

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