GEOHYDROLOGY AND SIMULATION OF

STEADY-STATE FLOW CONDITIONS IN

REGIONAL AQUIFER SYSTEMS IN CRETACEOUS

AND OLDER ROCKS UNDERLYING KANSAS,

NEBRASKA, AND PARTS OF ARKANSAS,

COLORADO, MISSOURI, NEW MEXICO, OKLAHOMA,

SOUTH DAKOTA, TEXAS, AND WYOMING

U.S. GEOLOGICAL SURREY PROFESSIONAL PAPER 1414-C
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Geohydrology and Simulation of Steady-State Flow 

Conditions in Regional Aquifer Systems in Cretaceous 

and Older Rocks Underlying Kansas, Nebraska, and 

Parts of Arkansas, Colorado, Missouri, New Mexico, 

Oklahoma, South Dakota, Texas, and Wyoming

By DONALD C. SIGNOR, JOHN O. HELGESEN, DONALD G. JORGENSEN, and 

ROBERT B. LEONARD

REGIONAL AQUIFER-SYSTEM ANALYSISCENTRAL MIDWEST

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1414 C
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

The use of firm, trade, and brand names in this report is for 

identification purposes only and does not constitute 

endorsement by the U.S. Government.

Library of Congress Cataloging in Publications Data 

Signer, Donald C.

Geohydrology and simulation of steady-state flow conditions in regional aquifer systems in Cretaceous and older rocks underlying Kansas, Nebraska, and parts of Arkansas, Colorado, Missouri, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming/ 

by Donald C. Signer...[et al.].

105 p. cm.-(Regional aquifer-system analysis Central Midwest) (U.S. Geological Survey Professional Paper; 1414-C)

Includes bibliographical references (p. 103-105). 

Supt. of Docs. no. I 19.10:1414-C

1. Groundwater Middle West. 2. Groundwater flow Middle West. 

I. Signer, Donald C., 1933- . II. Series. III. Series: U.S. 

Geological Survey professional paper: 1414-C.

GB1017.G46 1996. 96-28445 

551.49'0978 dc20 CIP

ISBN 0-607-87033-8

For sale by U.S. Geological Survey, 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.

Gordon P. Eaton 

Director

CONTENTS

Page

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

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

Purpose and Scope ............................................................... 2

Physical Setting ................................................................... 3

Acknowledgments................................................................. 4

Geologic Setting ......................................................................... 6

Definition of Geohydrologic Characteristics Methodology ....... 10

Geophysical Logs ................................................................. 11

Dissolved Solids ................................................................... 11

Hydraulic Head.................................................................... 13

Porosity................................................................................ 15

Hydraulic Conductivity and Intrinsic Permeability ............. 16

Geohydrology............................................................................. 18

Plains Subregion................................................................... 19

Western Interior Plains Aquifer System ......................... 19

Western Interior Plains Confining System ..................... 26

Great Plains Aquifer System .......................................... 33

Overlying Units.............................................................. 35

Ozark Subregion................................................................... 35

Ozark Plateaus Aquifer System ...................................... 36

St. Francois Aquifer .............................................. 36

St. Francois Confining Unit .................................. 41

Ozark Aquifer ....................................................... 43

Ozark Confining Unit............................................ 45

Springfield Plateau Aquifer ................................... 47

Simulation of Ground-Water Flow ............................................. 47

Model Framework................................................................ 48

Relation of Model Layers and Geohydrologic Units ....... 48

Boundary Conditions of Model Layers .......................... 48

Layer 1 ................................................................. 48

Layer 2 ................................................................. 54

Layer 3 ................................................................. 54

Layer 4 ................................................................. 57

Layer 5 ................................................................. 59

Simulation of Variable-Density Flow ................................... 59

Model Calibration................................................................ 59

Calibration Criteria........................................................ 59

Hydraulic Head .................................................... 61

Effects of Variable Density ............................ 69

Recharge........................................................ 72

Sensitivity Analysis .............................................. 78

Error Analysis....................................................... 81

Results of Simulations ................................................................ 86

Flow..................................................................................... 86

Model Budget ...................................................................... 95

Water Supply and Water-Storage Potential ............................... 97

Great Plains Aquifer System ................................................ 97

Western Interior Plains Aquifer System ............................... 97

Ozark Plateaus Aquifer System ............................................ 99

Summary.................................................................................... 99

References Cited......................................................................... 103

ILLUSTRATIONS

[Plates are in pocket]

PLATE 1. Map showing predevelopment equivalent freshwater head in Cambrian- and Ordovician-age rocks (lower units of the Western

Interior Plains and Ozark Plateaus aquifer systems) in parts of Arkansas, Colorado, Kansas, Missouri, Nebraska,

New Mexico, Oklahoma, South Dakota, and Texas 

2. Map showing predevelopment equivalent freshwater head in Mississippian-age rocks (upper unit in the Western Interior Plains

aquifer system and Springfield Plateau aquifer in the Ozark Plateaus aquifer system) in parts of Arkansas, Colorado, Kansas,

Missouri, Nebraska, New Mexico, Oklahoma, Texas, and Wyoming 

FIGURE 1-8. Map showing: Page

1. Location and extent of study area ................................................................................................................................. 3

2. Regional aquifer-system studies contiguous to the Central Midwest regional aquifer-system analysis ........................... 4

3. Physiographic subdivisions within CMRASA study area ............................................................................................... 5

4. Generalized land-surf ace altitude ................................................................................................................................... 6

5. Climatic characteristics of central United States, 1931-60 ............................................................................................. 7

6. Estimated mean annual recharge to aquifer-outcrop areas in CMRASA geohydrologic units, 1951-80 .......................... 8

7. Major structural features of central United States ........................................................................................................ 9

8. Regional geology of central United States ..................................................................................................................... 12

9. Generalized geologic sections across study area .................................................................................................................... 13

10. Map showing major faults in study area ............................................................................................................................... 14

11-18. Graph showing:

11. Relation of regional to reported oil- and gas-reservoir porosity of Jurassic and Lower Cretaceous sandstone ................ 15

12. Relation of regional to reported oil- and gas-reservoir porosity of Cambrian and Devonian rocks ................................. 16
VI CONTENTS

Page

FIGURE 13. Relation of regional to reported oil- and gas-reservoir porosity of Mississippian limestone ........................................... 16

14. Relation of regional to reported oil- and gas-reservoir porosity of Pennsylvanian and Permian rocks ........................... 17

15. Relation of estimated regional to reported oil- and gas-reservoir intrinsic permeability of Cambrian and Ordovician

dolostone..................................................................................................................................................................... 18

16. Relation of estimated regional to reported oil- and gas-reservoir intrinsic permeability of Mississippian limestone ....... 18

17. Relation of estimated regional to reported oil-and gas-reservoir intrinsic permeability of Pennsylvanian and Permian

sandstone and limestone.............................................................................................................................................. 19

18. Relation of estimated regional to reported oil- and gas-reservoir intrinsic permeability of Jurassic and Lower Cretaceous

sandstone.................................................................................................................................................................... 19

19. Cross section showing geohydrologic units from central Colorado to eastern Missouri ......................................................... 20

20-31. Maps showing:

20. Extent of major aquifer systems in study area .............................................................................................................. 21

21. Estimated lateral intrinsic permeability of lower units in the Western Interior Plains aquifer system and the Ozark and

St. Francois aquifers in the Ozark Plateaus aquifer system ......................................................................................... 22

22. Estimated lateral intrinsic permeability of upper unit in the Western Interior Plains aquifer system and the Springfield

Plateau aquifer in the Ozark Plateaus aquifer system ................................................................................................. 23

23. Regional porosity of lower units in the Western Interior Plains aquifer system and the Ozark and St. Francois aquifers in

the Ozark Plateaus aquifer system .............................................................................................................................. 24

24. Dissolved-solids concentrations in water from lower units in the Western Interior Plains aquifer system and the Ozark

aquifer, the St. Francois confining unit, and the St. Francois aquifer in the Ozark Plateaus aquifer system .............. 25

25. Regional porosity of upper unit in the Western Interior Plains aquifer system and the Springfield Plateau aquifer in the

Ozark Plateaus aquifer system .................................................................................................................................... 26

26. Dissolved-solids concentrations in water from upper unit in the Western Interior Plains aquifer system and the Springfield

Plateau aquifer in the Ozark Plateaus aquifer system ................................................................................................. 27

27. Estimated porosity of sandstone in the Western Interior Plains confining system ......................................................... 28

28. Estimated porosity of carbonate rocks in the Western Interior Plains confining system ................................................ 29

29. Estimated lateral intrinsic permeability of the Western Interior Plains confining system ............................................. 30

30. Thickness of evaporite deposits in the Western Interior Plains confining system .......................................................... 31

31. Approximate equivalent freshwater head in buried parts and near-surface freshwater head in outcrop area of the Western

Interior Plains confining system .................................................................................................................................. 32

32. Geohydrologic section showing the Maha and Apishapa aquifers and intervening Apishapa confining unit in the Great

Plains aquifer system ............................................................................................................................................................ 33

33-44. Map showing:

33. Great Plains aquifer system in North America .............................................................................................................. 34

34. Predevelopment potentiometric surface of the Maha aquifer in the Great Plains aquifer system .................................. 36

35. Estimated porosity of the Apishapa and Maha aquifers ................................................................................................ 37

36. Estimated lateral intrinsic permeability of the Apishapa and Maha aquifers ................................................................ 38

37. Dissolved-solids concentrations in water from the Maha aquifer in the Great Plains aquifer system ............................ 39

38. Geohydrologic units overlying regional aquifer systems in Plains subregion .................................................................. 40

39. Thickness of the Great Plains confining system ............................................................................................................. 41

40. Differences in hydraulic head between the High Plains aquifer and the Great Plains aquifer system ............................ 42

41. Physiographic features of Ozark subregion .................................................................................................................... 43

42. Shale content of the St. Francois confining unit ............................................................................................................ 44

43. Predevelopment potentiometric surface and dissolved-solids concentrations in water from the Ozark aquifer ............... 46

44. Location of model grid and study area .......................................................................................................................... 49

45. Diagram showing generalized model layers and relation to principal geohydrologic units in Plains and Ozark subregions ... 50 

46-62. Map showing:

46. Model layer-1 grid and areal extent of alluvium and glacial drift, the High Plains aquifer, and the Great Plains confining

system......................................................................................................................................................................... 51

47. Potentiometric surface of model layer 1 ......................................................................................................................... 52

48. Model layer-2 grid representing areal extent and boundary conditions of the Great Plains aquifer system ................... 55

49. Model layer-3 grid representing areal extent, outcrop area, and boundary conditions of the Western

Interior Plains confining system .................................................................................................................................. 56

50. Model layer-4 grid representing areal extent and boundary conditions of the upper unit in the Western Interior Plains

aquifer system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system .......................................... 58

51. Model layer-5 grid representing areal extent and boundary conditions of the lower units in the Western Interior Plains

aquifer system and the Ozark and St. Francois aquifers in the Ozark Plateaus aquifer system ................................... 60

52. Model-computed and field hydraulic head for model layer 2 representing the Great Plains aquifer system ................... 62

53. Difference between model-computed and field hydraulic head for model layer 2 representing the Great Plains aquifer

system..................................... ........ .. .......... . 63

54. Model-computed and field hydraulic head for model layer 4 representing the upper unit in the Western Interior Plains

aquifer system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system .......................................... 64
CONTENTS VII

Page 

FIGURE 55. Difference between model-computed and field hydraulic head for model layer 4 representing the upper unit in the Western

Interior Plains aquifer system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system ................... 65

56. Model-computed and field hydraulic head for model layer 5 representing the lower units in the Western Interior Plains

aquifer system and the Ozark and St. Francois aquifers in the Ozark Plateaus aquifer system ................................... 66

57. Difference between model-computed and field hydraulic head for model layer 5 representing the lower units in the Western

Interior Plains aquifer system and the Ozark and St. Francois aquifers in the Ozark Plateaus aquifer system ........... 67

58. Difference between simulated steady-state hydraulic head with and without effects of variable density for model layer 2 70

59. Difference between simulated steady-state hydraulic head with and without effects of variable density for model layer 4 71

60. Difference between simulated steady-state hydraulic head with and without effects of variable density for model layer 5 73

61. Effects of variable density on simulated flow vectors for model layer 4 ......................................................................... 74

62. Effects of variable density on simulated flow vectors for model layer 5 ......................................................................... 75

63. Graph showing computed mean annual recharge versus mean annual precipitation, by model-grid element ......................... 76

64-71. Graphs showing sensitivity of difference between model-computed and field hydraulic head to:

64. Change in aquifer hydraulic conductivity in CMRASA geohydrologic units ................................................................. 79

65. Change in leakance coefficients between layers in CMRASA geohydrologic units ......................................................... 79

66. Change in recharge rate in CMRASA geohydrologic units ............................................................................................ 80

67. Change in recharge rate in CMRASA geohydrologic units, Ozark subregion ................................................................. 80

68. Simultaneous change in recharge rate and hydraulic conductivity in CMRASA model layer 4 ..................................... 81

69. Simultaneous change in recharge rate and hydraulic conductivity in CMRASA model layer 5 ..................................... 82

70. Simultaneous change in leakance and recharge rate in CMRASA model layer 4 ........................................................... 83

71. Simultaneous change in leakance and recharge rate in CMRASA model layer 5 ........................................................... 84

72-79. Map showing:

72. Subregional model grid and relation of the Great Plains aquifer system subregional model to regional model layer 2 ... 85

73. Lateral flow-velocity vectors for model layer 2 representing the Great Plains aquifer system ....................................... 87

74. Lateral flow-velocity vectors for model layer 4 representing the upper unit in the Western Interior Plains aquifer system

and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system .................................................................. 88

75. Lateral flow-velocity vectors for model layer 5 representing the lower units in the Western Interior Plains aquifer system 

and the combined Ozark aquifer, St. Francois confining unit, and St. Francois aquifer in the Ozark Plateaus aquifer 

system......................................................................................................................................................................... 89

76. Vertical flow directions for model layer 1 simulating water movement between surficial units and underlying units,

principally between the High Plains aquifer and the Great Plains aquifer system represented by model layer 2 ........ 90

77. Vertical flow directions between model layers 2 and 3 simulating water movement principally between the Great Plains

aquifer system and the Western Interior Plains confining system ............................................................................... 91

78. Vertical flow directions between model layers 3 and 4 simulating water movement between the Western Interior Plains 

confining system and the upper unit in the Western Interior Plains aquifer system and between the Western Interior 

Plains confining system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system ............................. 93

79. Vertical flow directions between model layers 4 and 5 simulating water movement between the upper and lower units in 

the Western Interior Plains aquifer system and between the Springfield Plateau aquifer and the combined Ozark and 

St. Francois aquifers in the Ozark Plateaus aquifer system ......................................................................................... 94

80. Diagram showing simulated regional hydrologic budget ....................................................................................................... 96

TABLES

Page

TABLE 1. Generalized correlation of geohydrologic units to stratigraphic units in most of the Plains subregion .................................. 10

2. Generalized correlation of geohydrologic units to stratigraphic units in most of the Ozark subregion ................................... 11

3. Commonly observed ranges of permeability in several rock types ......................................................................................... 53

4. Statistical comparison of model-computed hydraulic head to field hydraulic head ................................................................ 61

5. Simulated recharge to outcrops of the regional model area represented by model layers 2, 4, and 5 for

several interception factors ................................................................................................................................................. 77

6. Results of statistical analyses comparing hydraulic-head data from the regional and subregional models of

the Great Plains aquifer system ........................................................................................................................................ 86

7. Water in storage in geohydrologic units in the regional study area ....................................................................................... 98
VIII CONTENTS

CONVERSION FACTORS

Multiply By To obtain

inch 25.40

foot (ft) 0.3048

square foot (ft2 ) 0.09290

mile (mi) 1.609

square mile (mi2 ) 2.590

millidarcy (mD) 9.87 x 10 '16

foot per second (ft/s) 0.3048

foot per mile (ft/mi) 0.1894

cubic foot per second (ft3/s) 0.02832

cubic foot per day (ft3/d) 0.02832

cubic foot per year (ft3/yr) 0.02832

cubic foot per square mile per year 0.0109

[(ft3/mi2 )/yr]

gallon per day per square foot [(gal/d)/ft2] 40.7427

inch per year (in/yr) 25.40 

acre-foot (acre-ft) 1,233

millimeter

meter

meter squared

kilometer

square kilometer

square meter

meter per second

meter per kilometer

cubic meter per second

cubic meter per day

cubic meter per year

cubic meter per square kilometer per year

liter per day per square meter 

millimeter per year 

cubic meter

Chemical concentrations and water temperatures are given in metric units. Chemical concentration is given in milligrams per 

liter (mg/L). Water temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by the 

following equation:

°F = 1.8 ( °C) + 32.

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

GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW 

CONDITIONS IN REGIONAL AQUIFER SYSTEMS IN CRETACEOUS AND

OLDER ROCKS UNDERLYING KANSAS, NEBRASKA, AND PARTS OF 

ARKANSAS, COLORADO, MISSOURI, NEW MEXICO, OKLAHOMA, SOUTH

DAKOTA, TEXAS, AND WYOMING

Bi DONALD C. SIGNOR, JOHN O. HELGESEN, DONALD G. JORGENSEN, AND ROBERT B. LEONARD

ABSTRACT

Regional aquifer systems of Cambrian through Lower Cretaceous sedimentary rocks were the focus of an investigation termed 

the Central Midwest regional aquifer-system analysis. The study 

area consists of about 370,000 square miles and extends from the 

foothills of the Rocky Mountains in Colorado to the Mississippi 

River in eastern Missouri and from South Dakota to the Ouachita, 

Arbuckle, and Wichita Mountains of Arkansas and Oklahoma.

Geologically, the study area lies within the stable interior of the 

North American continent and is dominated structurally by broad 

basins and arches. Substantial crustal deformation along the Ouachita, Arbuckle, and Wichita Mountains in the south and the Rocky 

Mountains in the west was the major factor in defining the southern 

and western boundaries of the study area.

There are two subregions in the study area, the Plains subregion 

and the Ozark subregion, that have three regional flow systems. Two 

regional flow systems separated by a thick confining system are 

present in the Plains subregion; one regional flow system exists in the 

Ozark subregion.

Within the Plains subregion, one flow system is in Cambrian 

through Mississippian rocks; the second flow system is in Cretaceous 

sandstone. The regional geohydrologic units in the Plains subregion 

are the basement confining unit (Precambrian rocks), the Western 

Interior Plains aquifer system (Cambrian and Mississippian rocks), 

the Western Interior Plains confining system (Upper Mississippian 

and Jurassic rocks), the Great Plains aquifer system (Lower Cretaceous rocks), the Great Plains confining system (Upper Cretaceous 

and younger rocks), and the High Plains aquifer (Tertiary and Quaternary rocks). The Great Plains confining system and the High 

Plains aquifer were studied only as they relate hydrologically to the 

Great Plains aquifer system.

In the Ozark subregion, a freshwater flow system in Lower Paleozoic rocks is laterally adjacent to the saline-water flow system of the 

Western Interior Plains aquifer system. The regional geohydrologic 

units in the Ozark subregion are the basement confining unit, the 

Ozark Plateaus aquifer system, and the Western Interior Plains confining system. The Ozark Plateaus aquifer system consists of the

St. Francois aquifer, the St. Francois confining unit, the Ozark aquifer, the Ozark confining unit, and the Springfield Plateau aquifer.

The distribution of hydraulic head in the Western Interior Plains 

aquifer system indicates flow generally west to east and southeast. 

Hydraulic heads in the Ozark Plateaus aquifer system indicate 

nearly radial outflow toward the Plains subregion and subregion 

boundaries of the Missouri River, Mississippi River, and crustal 

deformation areas toward the south. Upward leakage occurs near 

the saltwater-freshwater transition zone where the aquifers in the 

Plains and Ozark subregions are laterally adjacent.

A numerical ground-water flow model was used to test the conceptualization of flow in the regional aquifers and aquifer systems in 

the study area. The numerical model used is a finite-difference 

model termed "The U.S. Geological Survey Modular Ground-Water 

Flow Model." Five principal geohydrologic units in the regional 

study area were represented in the model by five model layers. The 

Western Interior Plains aquifer system and the Ozark Plateaus aquifer system were represented by two model layers; the Western Interior Plains confining system and the Great Plains aquifer system by 

one model layer each; and a combined Great Plains confining system 

and High Plains aquifer by one model layer. Calibration of the 

model was based on closeness of fit of computed hydraulic head to 

field hydraulic head, analyses of regional outflow and inflow at the 

boundaries, and comparison to subregional-model hydraulic head and 

flow.

Comparisons of model-computed hydraulic head to field hydraulic head were made for model layers 2, 4, and 5. Model layer 2 simulated the Great Plains aquifer system. Model layer 4 simulated the 

upper unit of the Western Interior Plains aquifer system in the 

Plains subregion and the Springfield Plateau aquifer in the Ozark 

subregion. Model layer 5 simulated the lower units of the Western 

Interior Plains aquifer system in the Plains subregion and the St. 

Francois confining unit and St. Francois aquifer in the Ozark subregion. The average deviation of computed hydraulic head from field 

hydraulic head was less than 1 foot for the three layers. Effects of 

variable density due to brines increased average deviation of 

computed hydraulic head from field hydraulic head to 6 feet lower

Cl
C2 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

for model layer 2, 30 feet higher for layer 4, and 41 feet higher for 

layer 5.

A sensitivity analysis to evaluate the effects of changes of 

hydraulic conductivity, leakance between layers, and recharge on 

computed hydraulic head showed that computed hydraulic head was 

very sensitive to changes in hydraulic conductivity, slightly sensitive 

to leakance changes between model layers 4 and 5, but more sensitive 

to leakance changes between layers 1 and 2, and only moderately 

sensitive to recharge for the overall model. Changes in recharge data 

affected computed hydraulic-head values in the Ozark subregion to a 

large degree. Simultaneously varying recharge and permeability values in the model showed that more than one solution of the model 

could be computed that would meet the criterion of computed 

hydraulic head matching calibration hydraulic head. This result was 

also true of simultaneously varying recharge and leakance between 

layers. Thus, the model solution was not unique.

Because of the large model-node spacing of 28 miles for the 

regional-model cells, an error analysis comparing the differences 

between computed heads and field heads for two node spacings was 

made. The area used for comparing hydraulic-head data was the 

Great Plains aquifer system. The Great Plains aquifer system was 

studied using a subregional model with a uniform node spacing that 

was one-half of the node spacing and the same orientation as the 

regional model. The hydraulic characteristics of the regional and 

subregional models were discretized from the same maps and data 

bases. The same computer model was used in both regional and subregional applications. From a statistical analysis, the model-node 

spacing relationship between regional and subregional models did not 

appear to make a significant difference in computed hydraulic head. 

The analysis also showed that there was no significant difference 

between the field hydraulic-head values of the two models. In conclusion, the modeling analysis indicated that:

1. The steady-state flow simulation of the study area reasonably represented the regional flow system.

2. An analysis using only freshwater hydraulic head where variabledensity brines are present may lead to erroneous conclusions. 

The potential for ground-water development in the study area 

can be summarized as follows:

1. Water in the Great Plains aquifer system is of questionable or 

unknown quality, and very little ground-water development 

has occurred to date. However, the quantity of water in storage is very large, and the aquifer system may be a supplemental source of water to augment the declining supply in the 

overlying High Plains aquifer.

2. Very little of the Western Interior Plains aquifer system contains 

water with less than 5,000 milligrams per liter dissolved solids, 

and therefore, the potential for ground-water development is 

slight. However, very poor water quality and very sluggish 

flow indicate that the aquifer system contains potential sites 

for waste storage or disposal.

3. Large quantities of water can be obtained from the Ozark Plateaus aquifer system. It contains freshwater, and there is a 

very large amount of water in storage. Simulated recharge to 

the aquifer system is about 7,000 cubic feet per second, or 

about 13 percent of the total recharge reaching the water table 

(55,000 cubic feet per second). The rejected recharge is discharged locally to streams. The large quantity of rejected 

recharge represents a potential ground-water resource;

however, withdrawals of ground water will affect the base flow 

of streams draining the Ozark subregion.

INTRODUCTION

The study area of the Central Midwest regional 

aquifer-system analysis (CMRASA) includes about 

370,000 mi 2 (fig. 1). It extends from the foothills of 

the Rocky Mountains in Colorado and Wyoming to 

the Mississippi River in eastern Missouri, and from 

South Dakota to the Ouachita, Arbuckle, and 

Wichita Mountains of Arkansas and Oklahoma.

Three important regional aquifer systems containing both freshwater and saline water were studied. In 

much of the area, little was known previously about 

the regional flow and hydrochemistry within the aquifer systems or about hydrologic relations among them.

Within the central United States, four other 

regional aquifer-system analyses (RASA) overlie or 

share geographic or hydrologic boundaries with the 

CMRASA study. These RASA studies, conducted 

either prior to or concurrent with the CMRASA, are 

the High Plains (Weeks, 1978), the Northern Great 

Plains (Dinwiddie, 1979), the Northern Midwest 

(Steinhilber and Young, 1979), and the Gulf Coast 

(Grubb, 1984) (fig. 2).

The background, scope, objectives, and approach 

of the CMRASA are described in the project-planning 

report by Jorgensen and Signer (1981). The findings 

of the CMRASA are reported in five chapters: chapter 

A is the summary chapter, which will collate the 

important findings reported in other chapters; chapter 

B describes the geohydrologic framework; chapter C 

(this report) describes the geohydrology and modeling 

analysis of the regional aquifer systems; chapter D 

describes the geohydrologic and model analyses of the 

Ozark Plateaus aquifer system; and chapter E 

describes the geohydrologic and model analyses of the 

Great Plains aquifer system.

PURPOSE AND SCOPE

The purpose of the work discussed in this report is 

to describe the aquifers, aquifer systems, confining 

units, and confining systems, and to test a conceptualization of regional flow within the study area by a 

computer simulation. The scope of the study is limited to rocks ranging in age from Cambrian through 

Late Cretaceous. The study does not include the flow 

regime in the Tertiary and Quaternary rocks constituting the High Plains aquifer; however, the computer 

flow model simulates the effects of Late Cretaceous 

and younger rock as a boundary condition.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C3

105

100 200 MILES

_I

100 200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

FIGURE 1. Location and extent of study area.

PHYSICAL SETTING

The study area includes parts of two major physiographic divisions, the Interior Plains and the Interior 

Highlands (fig. 3). The Interior Plains includes the 

Great Plains and the Central Lowlands, and the Interior Highlands includes the Ozark Plateaus and the 

Ouachita Province. The Ouachita Province includes 

the Arkansas Valley, which is between the 

Boston Mountains of the Ozark Plateaus and the 

Ouachita Mountains south of the study area. The 

study area is bounded laterally by either major geologic features or by major rivers. Geologic features 

include the Rocky Mountain Uplifts on the west, the 

Siouxana Arch on the northeast, a series of uplifts on 

the south, and the Fall Line (the boundary with the 

Mississippi Alluvial Plain) on the southeast. Major 

rivers are the Mississippi and the Missouri Rivers.

Land-surface altitude ranges from less than 500 ft 

on the Mississippi Alluvial Plain and along the Arkansas Valley to a maximum of about 10,000 ft in the 

extreme western part of the area in the foothills of the 

Rocky Mountains (fig. 4). Most of the Interior Plains 

is relatively flat, whereas the topography of the 

Ozark Plateaus is hilly and rugged. The Interior 

Plains area is mainly a mixture of grassland and row 

crops; much of the Ozark Plateaus is covered by hardwood forest.

Climatic characteristics, including runoff, precipitation, and potential evapotranspiration, are shown in 

figure 5. Mean annual precipitation for 1931-60, 

which included two major droughts ^ 1933-37 and 

1952-57), ranged from 12 in. in eastern Colorado to 

more than 48 in. in the Ozark Plateaus (Eagleman, 

1976, fig. 3). Mean annual potential
C4 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

35

0 200 400 KILOMETERS

EXPLANATION

CENTRAL MIDWEST REGIONAL AQUIFER-SYSTEM ANALYSIS

HIGH PLAINS REGIONAL AQUIFER-SYSTEM ANALYSISPartly overlies Central Midwest RASA study area

NORTHERN GREAT PLAINS REGIONAL AQUIFER-SYSTEM ANALYSIS 

NORTHERN MIDWEST REGIONAL AQUIFER-SYSTEM ANALYSIS 

GULF COAST REGIONAL AQUIFER-SYSTEM ANALYSIS

FIGURE 2. Regional aquifer-system studies contiguous to the Central Midwest regional aquifer-system

analysis.

evapotranspiration generally exceeds precipitation 

(fig. 5). Another C MR ASA study (Dugan and Peckenpaugh, 1985) to determine the amount of water passing the root zone and reaching the water table 

indicates that less than 1 in. is available in Colorado, 

whereas more than 15 in. of water are available annually for recharge in the Ozark Plateaus (fig. 6). The 

eastern part of the study area has abundant water 

resources, especially surface water. Mean annual surface-water runoff during 1931-60 ranged from 1 in. or 

less in western Kansas and eastern Colorado to more 

than 15 in. in the Ozark Plateaus (fig. 5). Mean 

annual runoff exceeded 27 in. during 1951-80 in the 

Ozark Plateaus (Hedman and others, 1987). Most of 

the western part of the study area is deficient in surface water but has substantial ground-water resources,

mainly in the High Plains aquifer, which overlies some 

of the aquifers described in this report (fig. 2).

ACKNOWLEDGMENTS

The plan of this investigation was developed with 

the assistance of personnel from the Arkansas 

Geological Commission, the Colorado Geological Survey, the Kansas Geological Survey, the Missouri Division of Geology and Land Survey, the Nebraska 

Conservation and Survey Division, the Oklahoma 

Geological Survey, and the South Dakota Geological 

Survey.

The study was aided greatly by a Liaison Committee made up of representatives from State agencies 

and the U.S. Geological Survey. The Liaison Committee not only disseminated information concerning
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C5

105

Ouachita 

Mountains

OUACHITA i

PROVINCE

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

BOUNDARY OF PHYSIOGRAPHIC DIVISON-Dashed where approximate 

BOUNDARY OF PHYSIOGRAPHIC PROVINCE-Dashed where approximate

 BOUNDARY OF PHYSIOGRAPHIC SECTION

FIGURE 3. Physiographic subdivisions within CMRASA study area (modified from Fenneman, 1946).

the study to State agencies but also helped identify 

sources of data, denned areas of concern, and made 

valuable suggestions that aided the study. Committee 

members included: Orville Wise, Arkansas Geological 

Commission; Robert Longenbaugh, Office of Colorado 

State Engineer; William Hambleton, Kansas Geological Survey; Donald Miller, Missouri Division of Geology and Land Survey; Verlon Vrana, Nebraska 

Natural Resources Commission; and Charles Mankin 

represented by Robert Arndt, Oklahoma Geological 

Survey.

The investigation also was served by a stratigraphy advisory group that included: Orville Wise, 

Arkansas Geological Commission; Richard Pearl, Colorado Geological Survey; P. Alien Macfarlane, Kansas 

Geological Survey; Thomas Thompson, Missouri Division of Geology and Land Survey; Marvin Carlson, 

Nebraska Conservation and Survey Division; and 

Charles Mankin, Oklahoma Geological Survey. This 

group, along with Claire Davidson of the Geologic 

Names Committee of the U.S. Geological Survey, contributed significantly to the resolution of problems 

related to stratigraphic correlation and nomenclature.
C6 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105'

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 4000 TOPOGRAPHIC CONTOUR-Shows altitude of land surface. Contour 

interval, in feet, is variable. Datum is sea level

FIGURE 4. Generalized land-surface altitude.

GEOLOGIC SETTING

Gently dipping sedimentary rocks that form broad 

uplifts and basins occur over most of the study area. 

However, stratigraphic nomenclature differs markedly 

among States and structural areas (fig. 7). As noted 

by Jorgensen and others (1993) hundreds of names 

have been assigned to rock units within the thick 

stratigraphic interval. The principal stratigraphic 

units composing the geohydrologic units discussed in 

this report are listed in tables 1 and 2.

The study area lies within the stable interior of the 

North American continent. From Late Proterozoic to 

present, or since Cambrian time to present, most of 

the study area has undergone relatively gentle

deformation, which involves upwarp and downwarp of 

the Earth's crust over large areas. Structurally, the 

study area has been dominated by broad basins and 

uplifts. Accordingly, most folding of sedimentary 

rocks has been gentle, and few major fault zones of 

regional importance occur. However, along the southern and western margins of the study area, substantial 

crustal deformation of mountain-building force 

resulted in intense folding and faulting. The lateral 

change from simple to complex geologic structure was 

the major factor in defining the southern and western 

boundaries of the study area. At the boundaries, the 

change in structure is relatively abrupt in most locations but transitional in others.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C7

JFMAMJJASOND 

MONTH 200 MILES

200 KILOMETERS

EXPLANATION

CLIMATIC CHARACTERISTICS GRAPHS 

Moisture deficit

Moisture surplus

Potential evapotranspiration

Precipitation

Actual evapotranspiration

LINE OF EQUAL ANNUAL RUNOFF-Interval, in inches, varies 

LINE OF EQUAL ANNUAL PRECIPITATION-Interval 12 inches 

LINE OF EQUAL ANNUAL POTENTIAL EVAPOTRANSPIRATION-Interval 10 inches

FIGURE 5. Climatic characteristics of central United States, 1931-60 (modified from Eagleman, 1976; 

Farnsworth and Peck, 1982, map 3; Moody and others, 1986; Gerbert and others, 1987).

 to 

 24 

 40-
C8 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Base modified from U.S. Geological Survey 

1:7,500.000, National Atlas, 1970

EXPLANATION

RANGE OF MEAN ANNUAL RECHARGE Values in inches 

| | Less than 0.1 [ | 0.5-1.0 ^H 5 - 10

RECHARGE AREA WHERE CENTRAL MIDWEST REGIONAL 

____ AQUIFER-SYSTEM ANALYSIS GEOHYDROLOGIC UNITS 

ARE NOT EXPOSED

FIGURE 6. Estimated mean annual recharge to aquifer-outcrop areas in CMRASA geohydrologic units, 1951-80 (from Dugan

and Peckenpaugh, 1985).

The present areal distribution of major 

time-stratigraphic units that crop out in the Central 

Midwest study area is shown in figure 8. Cross sections (fig. 9) illustrate the general regional continuity 

of the units; the greatest structural deformation is 

along the southern and western margins of the study 

area. Faulting (fig. 10) was substantial in these areas

and caused significant offset of rock units. Most 

faults in the interior of the study area have much less 

offset than the faults along the southern and western 

margins.

Precambrian rocks, consisting mainly of igneous 

and metamorphic rocks, form a basement complex 

that underlies the Paleozoic and Mesozoic
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C9

105" 100° 95° 90°

' JBIack , - , 

 ' jtiiR?''

>llft /Boundary of study ar» a \B

"S, /-> L - . , x . ^nf

V\ \ ^/ *,'

__J

40'

35e

^ «/ ^^~>» ' Sedgwick , ,S / Cherokee \ 1

/ ^ I - /

Basin /2 \ Basin ./

»^i _ \^~- --Y -V & e* 

U iArkg^Basin //^ |

X>--^^/T^

I AmarmoUptrft jfe~~^\ s\-*^ 

J____TCXAS k>W- U

100 200 MILES

100 200 KILOMETERS

"'* ^rbC^

L s Uplift L^H--J M

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

EXTENT OF STRUCTURAL FEATURE

FIGURE 7. Major structural features of central United States (modified from Condra and Reed, 1959; Carlson, 1963; Merriam,

1963; Oetkingand others, 1966,1967; Renfro, 1973).

sedimentary rocks of interest in this study (tables 1, 

2). Deep burial of these rocks at most locations has 

precluded detailed knowledge of their nature. Merriam (1963) describes Precambrian sedimentary rocks 

that are only slightly metamorphosed in northeastern 

Kansas. Seismic reflection studies (Brown and others, 

1983) suggest layering and complex structures in the 

same area. Along the southern boundary in Oklahoma, the basement includes extrusive rhyolite rocks 

or intrusive granite of Early to Middle Cambrian age 

(Ham and others, 1964).

Major Precambrian faults in the study area are 

oriented to the present northeast (fig. 10). The

alignment nearly parallels a fault belt called the Colorado lineament (not shown in figs. 7 or 10) described 

by Warner (1980, fig. 3) and the Sierra Grande, Las 

Animas, and Siouxana Arches (fig. 7). Other major 

faults in the area trend northwest in the same general 

area as the Central Kansas Uplift and the Cambridge 

and Chadron Arches (figs. 7, 10). Information concerning faults and fractures in Precambrian rocks is of 

importance because the faults mark weak zones that 

were sometimes reactivated during later geologic time 

and the fractures are major paths of ground-water 

flow in well-indurated rocks.
CIO REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

TABLE 1. Generalized correlation of geohydrologic units to stratigraphic units in most of the Plains subregion

[From Jorgensen and others, 1993]

Geohydrologic unit

High Plains aquifer

Great Plains confining system

Great 

Plains 

aquifer 

system

Maha aquifer

Apishapa 

confining unit

Apishapa 

aquifer

Western 

Interior 

Plains 

confining 

system

Western 

Interior 

Plains 

aquifer 

system

Upper aquifer 

unit

Confining unit

Lower aquifer 

units

Basement confining unit

Principal rock-stratigraphic unil(s)

Ogallala Formation and unconsolidated 

deposits

Pierre Shale, Niobrara Formation, 

Carlile Shale, Greenhorn Limestone, 

Graneros Shale (includes Lower 

Cretaceous)

Dakota Sandstone, "D" sandstone, 

"J" sandstone, and equivalent of 

Newcastle Sandstone

Kiowa Shale and equivalent of 

Skull Creek Shale

Cheyenne Sandstone and equivalent of Fall 

River Sandstone and 

Lakota Sandstone

Morrison Formation, Sundance Formation, 

Entrada Sandstone, Dockum Formation, 

Elk City Sandstone, Doxey Shale, Big 

Basin Sandstone, Cloud Chief Formation, 

Day Creek Dolomite, Whitehorse 

Sandstone, NippewalJa Group, Sumner 

Group, Chase Group, Council Grove 

Group, Admire Group, Wabaunsee Group, 

Shawnee Group, Douglas Group, Lansing 

Group, Kansas City Group, Pleasanton 

Group, Marmaton Group, Cherokee Group, 

Atokan rocks, Morrowan rocks, and 

Springer Group

Meramecian, Osagean, and 

Kinderhookian rocks

Chattanooga Shale and Woodford Shale

Hunton Group, Sylvan Shale, equivalent of 

Galena Dolomite, Viola Limestone, 

Simpson Group, Arbuckle Group, and 

Reagan Group

Mostly igneous and metamorphic rocks

Time-stratigraphic unit

Quaternary and 

Tertiary

Upper Cretaceous

Lower 

Cretaceous

Jurassic 

through 

Upper 

Mississippian 

(Chesterian)

Upper 

Mississippian 

through 

Upper 

Cambrian

Cambrian and 

Precambrian

Dake (1930, p. 194) stated that in the Ozark subregion, rhyolite lava and ash, along with granite, granite 

porphyry, and basic dikes, formed a large land mass 

during the Precambrian. He further stated that the 

mass, of nearly 2,000-ft elevation, was deeply eroded 

and faulted by Cambrian time. Precambrian faulting 

in the Ozark subregion is reported also by Bridge 

(1930, p. 136). The core of the St. Francois Mountains is an epizonal granite batholith that was 

emplaced 1.5 billion years ago and was concomitant 

with the lowering and raising of sea level.

DEFINITION OF GEOHYDROLOGIC 

CHARACTERISTICS METHODOLOGY

A description of the geohydrologic framework 

includes the extent, thickness, and altitude of the geohydrologic units. It is useful to define the

geohydrologic units within the framework of lithostratigraphic units because the rock units are, in general, well established and well known, especially as 

compared to geohydrologic units. Definition of geohydrologic units (aquifers, aquifer systems, confining 

units, and confining systems) within the study area is 

based largely on hydrologic relations and hydraulic 

properties of the rocks. Aquifers and aquifer systems 

that contain relatively freshwater have a characteristic potentiometric surface that is continuous and relatively smooth. Therefore, the potentiometric surface 

of water-bearing units is useful in defining a geohydrologic unit. In addition, maps showing hydrochemical 

characteristics are useful in delineating a geohydrologic unit because many aquifers have a characteristic 

water chemistry.

Interpretive maps prepared for this study are 

based on many sources, and density of data varies 

considerably depending on the geographic area and
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS Cll

TABLE 2. Generalized correlation of geohydrologic units to stratigraphic units in most of the Ozark subregion

[From Jorgensen and others, 1993]

Geohydrologic unit

Western Interior Plains 

confining system

Plateaus 

aquifer

system

Springfield 

Plateau 

aquifer

Ozark 

confining unit

Ozark aquifer

St. Francois 

confining unit

St. Francois 

aquifer

Basement confining unit

Principal rock-stratigraphic unit(s)

Marmaton Group, Cherokee Group, 

Atokan rocks, Bloyd Shale, Hale 

Formation, Morrowan rocks, Pitkin 

Limestone, Fayetteville Shale, and 

Batesville Sandstone

Moorefield Formation, St. Louis 

Limestone, Salem Limestone, Warsaw 

Limestone, Boone Formation, St. Joe 

Limestone Member of Boone Formation, 

Keokuk Limestone, Burlington Limestone, 

and Fern Glen Limestone

Chouteau Group 1 and 

Chattanooga Shale, Northview Shale, 

and Hannibal Shale

Clif ty Limestone, Penters Chert, Lafferty 

Limestone, St. Clair Limestone, Brassfield 

Limestone, Cason Shale, Fernvale 

Limestone, Kimmswick Limestone, 

Plattin Limestone, Joachim Dolomite, 

St. Peter Sandstone, Everton Formation, 

Smithville Formation, Powell Dolomite, 

Cotter Dolomite, Jefferson City Dolomite, 

Roubidoux Formation, Gasconade 

Dolomite, Gunter Sandstone Member of 

Gasconade Dolomite, Eminence Dolomite, 

and Potosi Formation

Elvins Group, Derby and Doe Run 

Dolomites, Davis Formation

Bonneterre Dolomite, 

Lamotte Sandstone, 

and Reagan Sandstone

Mostly igneous and metamorphic rocks

Time-stratigraphic unit

Middle 

Pennsylvanian 

through Upper 

Mississippian 

(Chesterian)

Mississippian

Lower Mississippian 

and Upper Devonian

Middle 

Devonian 

through 

uppermost 

Cambrian

Upper Cambrian

Precambrian

Designated Chouteau Limestone by U.S. Geological Survey.

the type of information being mapped. Many maps in 

this report show few or no data points in some areas 

where previously prepared interpretations were relied 

upon.

GEOPHYSICAL LOGS

Compilation of maps defining the framework of 

geohydrologic units was based largely on electric and 

lithologic logs from test holes for petroleum. However, in areas where the rocks contain freshwater, 

lithologic and geophysical logs from wells or test holes 

drilled for water were used. Logs that were 

representative of an area were selected to be included 

in the CMRASA data base (Helgesen arid Hansen,

1989). Ideally, the log record included rocks of Cretaceous through Precambrian age. That is, logs were 

selected to be representative of both the areal and vertical scope of the study. In areas for which numerous 

logs are available, such as near oil and gas reservoirs, 

only a few of the numerous logs available were 

selected. In other areas, where data are scarce, most 

available logs were selected.

DISSOLVED SOLIDS

Dissolved-solids concentration is a measure of the 

water chemistry of a geohydrologic unit. Many 

aquifers have a characteristic hydrochemistry that is 

the result of many relations, including the chemistry
C12 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Mz

UPz

MPz

LPz

200 KILOMETERS

EXPLANATION

QUATERNARY DEPOSITS

TERTIARY DEPOSITS

MESOZOIC ROCKS Cretaceous, Jurassic, and Triassic age

UPPER PALEOZOIC ROCKS Permian, Pennsylvanian, and Mississippian age

MIDDLE PALEOZOIC ROCKS Devonian and Silurian age

LOWER PALEOZOIC ROCKS Ordovician and Cambrian age

PRECAMBRIAN ROCKS

A'

TRACE OF SECTION Shown in figure 9 

CONTACT

FIGURE 8. Regional geology of central United States (modified from Kinney, 1966).
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C13

FEET A Zone of Q

10.000^

5.000-

SEA 

LEVEL

5,000 

10,000-

complex o 

geology U

Quaternary and Tertiary deposits

\ ,- ,. i Precambrian rocks ,"- . v i j ,> i ^, .

s

FEET

5.000 

SEA 

LEVEL

5,000 _

10,000 

15,000 

20,000 

25,000 -

30,000 

II

OS«§

35,000

complex Quaternary and Tertiary deposits 

geology / Mesozoic rocks

,". ' " ' ^Precambrian rocks ,'"*' 

- ^v^'--. - x '.T'-^ I^.x"

^\/;<. / ,-\ f-^M ;c / ;-.>. -M/x /

' -. C " %/ , *- ^ *N v' -',' _' ^s v v

VERTICAL SCALE GREATLY EXAGGERATED

0 100 200 300 MILES 

| i i____l

0 100 200 300 KILOMETERS

EXPLANATION 

FAULT Arrows show direction of movement

FIGURE 9. Generalized geologic sections across study area. 

Traces of sections shown in figure 8.

of the water recharging the aquifers. Maps showing 

preliminary interpretations of the dissolved-solids concentrations were prepared from data collated into files 

for the CMRASA (Helgesen and Hansen, 1989).

Most of the chemical analyses of saline water were 

obtained from oil- and gas-well tests. Large differences between the reported concentrations of dissolved 

solids typically characterized samples from adjacent 

sites. The cause of these differences is difficult to 

determine. It is not known whether the differences 

are related to areal, vertical, and temporal variability, 

or to errors resulting from the sampling procedure or 

methods of chemical analysis.

The exact source of many of the water samples is 

unknown. Some samples were obtained from 

drill-stem tests; others are "production" water. Some 

may be mixtures of formation water with drilling fluids, or with water from zones above or below the zone 

of interest. Chemical analyses of freshwater from 

adjacent wells usually did not show large variability.

Hydrochemical data from many sources were 

entered in the CMRASA "Hydrochemical File." 

Sources of data for the computer file included the 

U.S. Geological Survey's National Water Data Storage and Retrieval System (WATSTORE) (Baker and 

Foulk, 1975), National Uranium Resource Evaluation 

(NURE) project files (Arendt and others, 1979, p. 11), 

Petroleum Data System (PDS) brine file maintained 

at the University of Oklahoma, as well as information 

from existing publications and other sources. Selected 

data from this large file were placed in a "Hydrochemical Data Base."

Selection of analyses for inclusion in the data base 

generally was governed by ionic balance, supporting 

data, and implied continuity with adjacent analyses. 

In the absence of analytical data throughout an extensive area, the concentration of dissolved solids was 

estimated from wireline-geophysical logs by measurement of spontaneous potential or by cross plotting 

resistivity and porosity data. The procedure for estimating water resistivity and dissolved-solids concentration is described by Jorgensen (1988).

HYDRAULIC HEAD

The configuration of the potentiometric surface is 

useful in evaluating the continuity of an aquifer and 

to show areas of recharge and discharge. The potentiometric surface is defined by the altitude of the water 

level in tightly cased wells. In rocks containing variable-density fluids in which density is not a function 

of pressure, no potential field can be defined (Jorgensen and others, 1982). However, a surface defined 

by equivalent freshwater head is useful in evaluating 

the lateral hydraulic continuity of the water-yielding 

rocks.
C14 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105'

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

THRUST FAULT Sawteeth on overthrust side 

NORMAL FAULT Hachures on downthrown side

 CONCEALED FAULT

FIGURE 10. Major faults in study area (modified from King, 1968; Sims, 1985; Krueger and Keller, 1986).

Altitudes of equivalent freshwater head were determined from shut-in pressures of drill-stem tests or by 

measurements of the altitude of the water level in 

wells containing freshwater. The following equation 

was used to calculate equivalent freshwater head (he ):

(1)

where

z is the altitude at the centerline of the rock 

interval tested (L);

p is the pressure at the centerline of the rock

section (FL~2);

d is density of freshwater (ML ~3 ); and 

g is the acceleration due to gravity (LT ~2 ).

Pressure values from the drill-stem tests were adjusted 

to the centerline of the rock section. The adjustment 

to pressure ( pa ) was calculated by

(2)

where

is the density of the formation water; and
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C15

2g is the altitude of the pressure gage used for 

the drill-stem test.

Density of water is a function of the concentration of 

dissolved solids and temperature. In shallow wells 

containing freshwater, the correction for density is 

trivial, and the equivalent freshwater head is considered equal to the altitude of the water level within the 

well.

Accuracy of the calculated freshwater-head values 

using results from drill-stem tests is difficult to evaluate because it is dependent on the accuracy of the 

recording gage and the conditions of the test, including the hydraulic character of the rock. Comparison 

of data from adjoining drill-stem tests indicates 

considerable differences. The differences may be due 

either to unknown inaccuracies as reported in the 

drill-stem test results, uncertainties in fluid density 

and temperature, or to areal, vertical, or temporal 

variations, such as the effects of petroleum production 

in nearby wells.

Potentiometric surfaces mapped for this study are 

estimated predevelopment hydraulic-head distributions. In relatively shallow, freshwater parts of the 

aquifers, considerable water-level data were available, 

and development generally has not caused large 

regional water-level declines. In deeper, saline parts 

of the aquifers, data are scarce in some areas and 

reflect the complexities of petroleum development, as 

discussed above, in other areas. Thus, varying degrees 

of interpretation were required to estimate the predevelopment potentiometric surfaces.

Data of selected water-level measurements in wells 

were stored in the Ground-Water Site Inventory 

(GWSI) data base of the U.S. Geological Survey. 

Results of drill-stem tests were stored in a CMRASA 

file (Helgesen and Hansen, 1989), "Reservoir 

Parameter File."

POROSITY

Porosity of saturated rock is the ratio of the volume of interstices in the rock to its total volume 

(Lohman, 1972, p. 3). Porosity is an important element in both storage and transmissive properties of a 

geohydrologic unit.

Porosity values were available throughout most of 

the study area from analyses of the results of 

drill-stem tests, data from laboratory analyses of 

test-hole cores, and analyses of production data from 

oil and gas reservoirs. However, these data were not 

considered to be definitive of regional values. Cores 

are very small samples, and for fractured rock, they 

may not be in their original configuration and they

rarely if ever indicate the size of major openings along 

fractures. Erroneous values may be obtained, especially in regard to repacked samples of unconsolidated 

material. Reported porosity values for many oil and 

gas reservoirs are of unknown accuracy and determined by unknown methods. Formation porosity can 

be determined from borehole wireline-geophysical 

measurements, such as density (gamma-gamma), neutron, and sonic logs (MacCary, 1978, p. 9).

For the CMRASA, porosity values were determined from borehole-compensated dual-porosity logs 

(density and neutron) where available. Those data 

were used to prepare preliminary maps of regional 

porosity. Porosity values determined from logs were 

considered to better represent geohydrologic units in 

which they were made because, generally, the values 

include the effect of the thickness of the entire unit. 

The porosity values determined from geophysical logs 

were for entire rock sections with similar lithologic 

and hydraulic characteristics and usually with several 

hundred feet or more of thickness.

A comparison between reported porosities for oil 

and gas reservoirs and log-derived values on the preliminary regional porosity maps was made for different lithologies and different stratigraphic units. 

Mapped values correlated well with porosity values of 

reservoir-rock sections that consisted of Jurassic and 

Lower Cretaceous sandstone (fig. 11). The good correlation likely results from the dominance of primary

30

20

10

"0 10 20 30 

LOG-DERIVED POROSITY FROM REGIONAL MAPS, IN PERCENT

FIGURE 11. Relation of regional (log-derived) to reported oiland gas-reservoir porosity of Jurassic and Lower Cretaceous 

sandstone.
C16 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

porosity that is more homogeneous in nature than 

fracture or secondary porosity.

Reservoir porosity values reported in the literature 

for Cambrian and Devonian rocks, Mississippian limestone, and Pennsylvanian and Permian rocks did not 

correlate well with regionalized (log-derived) porosity 

values (figs. 12-14). The poor correlations are attributed partly to the effects of heterogeneous secondary 

porosity that resulted principally from fracturing and 

dissolution of rock material. Oil and gas reservoirs in 

a heterogeneous rock section typically have larger 

porosity values than average values for the rock section (Ray and others, 1985). In general, oil and gas 

reservoirs have larger porosity than the regional rock 

unit within which they occur. This is especially true 

in fractured rocks. The larger porosity in oil and gas 

reservoirs generally occurs in thin vertical sections 

that are usually of limited lateral extent. Therefore, 

reported porosity values for oil and gas reservoirs are 

generally site specific and may not be representative 

of the entire section of the rock unit, either vertically 

or areally.

Porosity data indicate a general trend of decreasing porosity with depth. This is consistent with 

Davis' (1969, p. 59) statement and supporting data 

that permeability and porosity of dense rocks decrease 

with depth. The relation is well defined by sandstone 

in the study area, which generally has dominant primary porosity; however, the trend is less defined in

 Carbonate rock 

o Sandstone .

o o

«D

0 10 20 3 

LOG-DERIVED POROSITY FROM REGIONAL MAPS, IN PERCENT

FIGURE 12. Relation of regional (log-derived) to reported oiland gas-reservoir porosity of Cambrian and Devonian rocks.

carbonate rocks, which are dominated by secondary 

porosity.

For the deepest rocks in the study area, porosity 

data of any type or source were not available. Estimates of porosity in these rocks were made based on 

porosity data of similar rocks at lesser depths and the 

relation of decreasing porosity with depth. These 

porosity estimates were used to supplement the geophysical-log porosity values on the preliminary porosity maps of the geohydrologic units. From the 

combined data, final porosity maps were prepared.

HYDRAULIC CONDUCTIVITY AND 

INTRINSIC PERMEABILITY

Hydraulic conductivity is the primary criterion for 

distinguishing between aquifers, which convey water, 

and confining units, which restrict or confine water 

movement. Hydraulic-conductivity data for aquifers 

containing freshwater are largely from aquifer tests 

using one or more wells that completely penetrate a 

single aquifer and from estimates based on the specific 

capacity of the pumping well. Specific capacity is the 

rate of well discharge per foot of drawdown and is a 

function of transmissivity and well efficiency. Available specific-capacity data were from wells that were 

usually open to one or more aquifers but that generally did not penetrate the complete thickness of the

35

£ 30

20

10 . /.

.' / 

* / 

/

10 20 

LOG-DERIVED POROSITY FROM REGIONAL MAPS, IN PERCENT

30

FIGURE 13. Relation of regional (log-derived) to reported oiland gas-reservoir porosity of Mississippian limestone.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C17

30

20

10

 Carbonate rock / 

oSandstone .

0 ! ° o ° o o //

 o /

V °

0 10 20 30 

LOG-DERIVED POROSITY FROM REGIONAL MAPS, IN PERCENT

FIGURE 14. Relation of regional (log-derived) to reported oiland gas-reservoir porosity of Pennsylvanian and Permian 

rocks.

lowermost aquifer. Thus, not all specific-capacity 

data provided accurate estimates for an individual 

aquifer. Hydraulic-conductivity values determined 

from aquifer tests or specific-capacity tests are stored 

in the U.S. Geological Survey's Ground-Water Site 

Inventory (GWSI) data base.

In rocks containing saline water, virtually 

no aquifer-test or specific-capacity data were available. In these areas, intrinsic permeability data were 

obtained from analyses of results from drill-stem tests, 

from laboratory analyses of rock cores, or from 

oil-production tests. Commercial quantities of oil are 

found in reservoirs or traps of relatively permeable 

rock material. Because nearly all drill-stem tests are 

conducted in these traps or reservoirs of permeable 

material, permeability values determined from 

drill-stem tests may not be representative of the effective regional permeability that is needed for evaluating regional aquifer systems.

Permeability values from cores of fractured rock 

are difficult to evaluate as to their representativeness 

of effective regional permeability. For example, permeability values from a 20-ft core of fractured and 

vuggy dolostone within the study area ranged from 

less than l.lxlO'15 ft 2 to more than 3.2x10 -11 ft 2 (Jorgensen and others, 1993). The permeability of the 

fractures in the rock core was not determined because 

it was not possible to arrange the fractured pieces in 

the laboratory within the same spacing or in-situ orientation. Both laboratory tests and drill-stem tests 

generally are conducted on relatively thin rock

sections of 20 ft or less. In general, laboratory tests of 

the thin intervals tested are not representative of the 

thick regional geohydrologic units. Therefore, a 

method was developed to estimate the permeability of 

geohydrologic units using data from boreholes with a 

specific suite of wireline geophysical logs. 

The permeability relation used was:

n

m + 2

(3)

where e is a rock constant;

n is porosity;

m is the cementation factor; and 

Ss is the specific surface area (Jorgensen, 1988). 

The second set of terms to the right of the equal sign 

of equation 3 is termed the porosity factor (p) from 

which an empirical equation for intrinsic permeability 

was developed. The equation is:

k = (1.828 x!05 )/> 1.10 (4)

Intrinsic-permeability values from aquifer 

and specific-capacity tests and calculated estimates of 

intrinsic permeability using geophysical-log data and 

equation 4 were plotted on maps, and lines of equal 

value were drawn to show regionalized intrinsic permeability. These values were the initial estimates of 

intrinsic permeability for a digital finite-difference 

flow model of the geohydrologic units. The modeling 

procedure itself can be used to refine estimates of rock 

characteristics, such as permeability, if adequate data 

are available. The modeling procedure, which will be 

discussed in detail later in this report, therefore was 

used to further refine initial estimates.

Regionalized permeability values were compared to 

site-specific permeability values reported for oil and 

gas reservoirs, as shown in figures 15-18. The figures 

show the poor correlation between reported reservoir 

permeability and regionalized permeability and that 

differences of two orders of magnitude are common. 

Permeability values for reservoirs are site specific and 

were measured in relatively thin sections of rock, such 

as 20 ft or less, whereas the regionalized permeability 

values calculated from geophysical-log data were for 

thick sections, such as 100 to l,000,ft. Similar to the 

comparison of regional porosity to reservoir porosity, 

it is concluded that permeability values determined 

from maps of estimated permeability based on geophysical-log data more accurately represent regional 

permeability than reported values of oil- and 

gas-reservoir permeability.
CIS REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

>-d

r~ CO

,

10' 10 ' 10" 10' 1 10"

ESTIMATED REGIONAL PERMEABILITY, IN SQUARE FEET

FIGURE 15. Relation of estimated regional (log-derived) to 

reported oil- and gas-reservoir intrinsic permeability of 

Cambrian and Ordovician dolostone.

GEOHYDROLOGY

Geohydrology of the study area is described for 

two subdivisions, which for clarity and convenience 

are referred to as the Plains subregion and the 

Ozark subregion. Jorgensen and others (1993) present 

a geologic description of the major geohydrologic units 

in the two subregions and discuss the geologic and 

hydrologic history of the regions. Jorgensen and others (1993) define six regional geohydrologic units in 

the Plains subregion and three regional geohydrologic 

units in the Ozark subregion. The correlation of these 

regional geohydrologic units to rock- and 

time-stratigraphic units is summarized in tables 1 and 

2; subsurface relations and areal extent of the geohydrologic units are shown in figures 19 and 20. Geohydrologic units that were studied in detail are the Great 

Plains aquifer system, the Western Interior Plains 

confining system, which is common to both the Plains 

and Ozark subregions, the Western Interior Plains 

aquifer system, and the Ozark Plateaus aquifer system. Maps showing the thickness of and altitude of 

the tops of the principal aquifers and confining units 

composing these four regional systems are presented 

by Jorgensen and others (1993). The remaining geohydrologic units were not studied in detail for the 

CMRASA; however, the High Plains aquifer was studied as the subject of a separate regional aquifer-system 

analysis (fig. 2) (Weeks, 1978).

This section briefly describes the geology of the 

principal aquifers and confining units making up the

10'

/

 .. x x-

 x ' 

/ x.

 - x " *

* 

1Q-' 6 10' 15 10 " 10'1J 10-" 10" 10" 

ESTIMATED REGIONAL PERMEABILITY, IN SQUARE FEET

FIGURE 16. Relation of estimated regional (log-derived) to 

reported oil- and gas-reservoir intrinsic permeability of 

Mississippian limestone.

four regional systems and presents information on 

their hydraulic properties, hydraulic heads, and 

dissolved-solids concentrations. The basement confining unit of Precambrian rocks is discussed first, and 

then the two subregions are discussed separately 

beginning with the geohydrologic units containing the 

oldest rocks.

The lowermost confining unit in both the Plains 

and Ozark subregions is the basement confining unit, 

composed mostly of crystalline rocks (tables 1, 2). 

The rocks are fractured and yield small quantities of 

water to wells at many locations, such as in the mountains west of Denver, Colorado, in southeastern South 

Dakota, and locally where the rocks crop out in the 

St. Francois Mountains. However, on a regional 

basis, they are assumed to form the base of 

ground-water flow in the Plains and Ozark subregions. 

The top of the basement confining unit ranges from 

less than 34,000 ft below sea level in southwestern 

Oklahoma to more than 1,000 ft above sea level in the 

St. Francois Mountains in eastern Missouri. It is 

about 500 ft above sea level in southeastern South 

Dakota and more than 7,000 ft below sea level along 

the Rocky Mountains in central Colorado 

(Jorgensen and others, 1993, pi. 3).
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C19

10 1

 j \U

< UI

£ E 10 ]

10

10- 10'= 10" 10-" 10" 10" 10' 

ESTIMATED REGIONAL PERMEABILITY, IN SQUARE FEET

FIGURE 17. Relation of estimated regional (log-derived) to 

reported oil- and gas-reservoir intrinsic permeability of 

Pennsylvanian and Permian sandstone and limestone.

PLAINS SUBREGION

The principal geohydrologic units (table 1) in the 

Plains subregion are the basement confining unit, the 

Western Interior Plains aquifer system, the Western 

Interior Plains confining system, the Great Plains 

aquifer system, the Great Plains confining system, and 

the High Plains aquifer. These geohydrologic units, 

with the exception of the previously described basement confining unit, are discussed in the following 

paragraphs.

WESTERN INTERIOR PLAINS AQUIFER SYSTEM

The Western Interior Plains aquifer system, at 

most locations, consists of aquifers of permeable limestone, dolostone, and sandstone of Late Cambrian 

through Late Mississippian age that are separated by 

slightly permeable shale or dolostone all of which 

overlay the basement confining unit. The stratigraphic units included in the aquifer system are listed 

in table 1. Collectively, the aquifer system contains 

permeable and slightly permeable beds that function 

regionally as a hydraulic unit.

The Western Interior Plains aquifer system 

extends from the Rocky Mountains eastward to a 

transitional boundary with the Ozark aquifer system 

(fig. 20). The aquifer system is not present over large 

areas of western Nebraska and northern Colorado. 

The southern boundary of the system along the 

faulted and folded area in southern Oklahoma is considered to be mostly a no-flow boundary. The regional

10"

10"

10 1

n r

'

«^_

Id 16 10 1S 10" 10'" 10" 10" 10' 

ESTIMATED REGIONAL PERMEABILITY, IN SQUARE FEET

FIGURE 18. Relation of estimated regional (log-derived) to 

reported oil- and gas-reservoir intrinsic permeability of 

Jurassic and Lower Cretaceous sandstone.

study boundary in southeast Oklahoma and northwest 

Arkansas also approximates a no-flow boundary.

The Western Interior Plains aquifer system consists of an upper unit, a confining unit, and lower 

units. The three units are reasonably distinct at most 

locations but are within the same hydrologic system. 

This conceptualization of upper and lower geohydrologic units is implied from equivalent freshwater heads 

and water chemistry.

Estimated lateral intrinsic permeability of rocks in 

the Western Interior Plains aquifer system is very 

small in the western part of the system (figs. 21 and 

22) but increases toward the east by about six to 

seven orders of magnitude.

The top of the lower units in the Western Interior 

Plains aquifer system generally dips away from the 

Ozark Uplift and Missouri River toward the Anadarko, Arkoma, and Denver Basins (fig. 7). The lowest altitude is in the Anadarko Basin at more than 

26,000 ft below sea level. Near the Arkoma Basin, the 

altitude of the top is more than 10,000 ft below sea 

level and near Denver, Colorado, more than 7,000 ft 

below sea level.

In northwestern Nebraska, northeastern Colorado, 

and scattered areas of central Kansas, the lower units 

are absent on the Cambridge Arch, the Chadron Arch, 

and the Central Kansas Uplift. The lower units are 

separated laterally from similar carbonate rocks in 

South Dakota and Wyoming by the Chadron Arch 

and the Siouxana Arch. The thickest section of the 

lower units exceeds 10,000 ft in southwestern
C20 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

WEST o;

o <e 

o £

-Zone of complex geology. Geohydrologic 

relations have not been defined with 

certainty and thus are not shown 

I

EAST

(0 (0

o.E

O CD

Great Plains confining system

Transition zone between 

flow systems

NOT TO SCALE

EXPLANATION

 > DIRECTION OF GROUND-WATER FLOW 

FIGURE 19. Cross section showing geohydrologic units from central Colorado to eastern Missouri.

Oklahoma and generally exceeds 1,000 ft in 

Oklahoma, southern Kansas, and eastern Nebraska.

Estimated lateral intrinsic permeability ranges 

from IxlO"11 ft2 in southeastern Kansas to less than 

1x10 ft2 in the western Anadarko Basin in Oklahoma and less than 1x10 ~ 18 ft2 in the central Denver 

Basin in Colorado (fig. 21). A lateral intrinsic permeability of 1x10 ~ 12 ft2 for rocks making up the aquifer 

on the Central Kansas Uplift was determined by a 

ground-water flow model analysis by Carr and others 

(1986). Figure 21 shows the estimated mean lateral 

intrinsic-permeability values for that area of the Central Kansas Uplift to be about IxlO'12 to 1x10 ~ 15 ft2, 

a difference of three orders of magnitude. The lateral 

intrinsic permeability values shown in figure 21 were 

used as initial estimates for model simulation of the 

lower units in the Western Interior Plains aquifer 

system, and the range of variability is discussed in a 

later section. The very small permeability values are 

due to the effect of depth on permeability. In general, 

values of lateral intrinsic permeability less than 

1x10"15 ft2 are considered typical of confining material. However, the lower units are considered to be 

regionally distinct as an aquifer in the Western Interior Plains aquifer system. Regionalized porosity generally ranges from 1 to 5 percent for the lower units 

(fig. 23).

Dissolved-solids concentrations in water from the 

lower units range from 280,000 mg/L in central Oklahoma to 10,000 mg/L at the western limit and less 

than 1,000 mg/L in the transition zone to the freshwater flow system of the Ozark sub region (fig. 24). The 

most distinctive characteristic of the water chemistry 

in the lower units in the Western Interior Plains aquifer system is the extensive area in which the water 

contains extremely large concentrations of dissolved 

solids.

Lines of equal predevelopment equivalent freshwater head for the lower units (plate 1) show a large 

hydraulic gradient east and northeastward from the 

Rocky Mountains toward the Central Kansas Uplift; 

a southeastward gradient between the Central Kansas 

Uplift and the Missouri River in eastern Nebraska; 

generally eastward gradients transverse to the 

Nemaha Uplift in eastern Kansas and east-central 

Oklahoma; and an area of relatively low equivalent 

freshwater head in southeastern Kansas and northeastern Oklahoma. Two additional distinct features 

are apparent from the lines of equivalent freshwater 

head (plate 1). The first is an area of high equivalent 

freshwater head in the vicinity of the Anadarko Basin 

in western Oklahoma and the Oklahoma and Texas 

Panhandles, which likely is related to an overpressure 

zone in the sandstone of the Western Interior Plains
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C21

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

APPROXIMATE EXTENT OF MAJOR AQUIFER SYSTEMS 

J//////J Great Plains aquifer system

|____ Western Interior Plains aquifer system

I ! ' I

Ozark Plateaus aquifer system

- APPROXIMATE BOUNDARY BETWEEN PLAINS AND OZARK 

SUBREGIONS

FIGURE 20. Extent of major aquifer systems in study area.

confining system. The second is the area of contorted 

contours of equivalent freshwater head in eastern Missouri and adjacent Arkansas, which results from natural recharge to the near-surface rocks and local 

discharge to streams in this area.

The intermediate confining unit that separates the 

lower units from the upper unit in the Western Interior Plains aquifer system limits regional flow between 

the two units. The confining unit is present over most

of the study area in Oklahoma, except in the panhandle; in the eastern one-half of Kansas, except over the 

Nemaha Uplift; and in the extreme southeastern corner of Nebraska. The confining unit is thin relative to 

other regional units in the study area. The thickness 

varies generally from about 300 ft thick in northeastern Kansas and southeastern Nebraska to zero where 

absent.
C22 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105 95°

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

LINE OF EQUAL ESTIMATED REGIONAL PERMEABILITY-Value is the 

thickness-weighted mean horizontal intrinsic permeability. Interval, in feet 

squared, is variable

LIMIT OF CAMBRIAN AND ORDOVICIAN ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 21. Estimated lateral intrinsic permeability of lower units in the Western Interior Plains aquifer system and the Ozark

and St. Francois aquifers in the Ozark Plateaus aquifer system.

The top of the upper unit in the Western Interior 

Plains aquifer system generally slopes away from the 

Missouri River and the Ozark subregion toward the 

Denver, Arkoma, and Anadarko Basins; the lowest 

altitude is less than -22,000 ft in the Anadarko Basin 

of southwestern Oklahoma. The upper unit in the 

study area is absent from areas in southeastern 

Nebraska and northeastern Kansas over the Nemaha 

Uplift, in the extreme southeastern corner of South

Dakota, and from about central Nebraska west almost 

to the Wyoming State line. It is also absent from 

northeastern Colorado to central Kansas where the 

upper unit is eroded from the Cambridge Arch and 

Central Kansas Uplift.

The thickest section of the upper unit exceeds 

2,500 ft in western Oklahoma where maximum deposition has occurred, but generally the unit has a thickness of several hundreds of feet elsewhere.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C23

SOUTH DAKOTA 

Boundar

PLAINS 

SUBREGION ; -, 

NEBRASKAX

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 10- LINE OF EQUAL ESTIMATED REGIONAL PERMEABILITY-Value is the

thickness-weighted mean horizontal intrinsic permeability. Interval, in feet 

squared, is variable

i i i i i i LIMIT OF MISSISSIPPIAN ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 22. Estimated lateral intrinsic permeability of upper unit in the Western Interior Plains aquifer system and the 

Springfield Plateau aquifer in the Ozark Plateaus aquifer system.

Additionally, the rocks in the deep basins have not 

been exposed to erosion and diagenesis associated with 

uplift. Accordingly, the rock sections in the Anadarko 

and Denver Basins are less permeable than rocks in 

the same stratigraphic units in other areas (fig. 22). 

In contrast, the dolostone and limestone in and adjacent to the Ozark Uplift, the Cambridge 

Arch-Central Kansas Uplift, and the Nemaha Uplift 

in southeastern Nebraska and northeastern Kansas 

are fractured, have solution openings, and are very

permeable because they have been exposed many 

times to weathering for extensive periods.

Estimated values of lateral intrinsic permeability 

for the upper unit range from 1x10 ~ 12 ft 2 in eastern 

Kansas to less than 1x10 ~ 16 ft 2 in the Anadarko Basin 

in Oklahoma and IxlO"18 ft 2 in the Denver Basin in 

Colorado (.fig. 22). These values were used to estimate 

hydraulic-conductivity values for model simulation of 

the upper unit, and their range of variability is discussed in later sections. As with the lower units in the
C24 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Base modified from U.S. Geological Survey -,r». . »T ATISVK.T 

1:7,500,000, National Atlas, 1970 EXPLA1NAI ION

 5 LINE OF EQUAL POROSITY-Interval, In percent, 

is variable

i i i i i i i i i LIMIT OF CAMBRIAN AND ORDOVICIAN 

ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 23. Regional porosity of lower units in the Western Interior Plains aquifer system and the Ozark and St. Francois

aquifers in the Ozark Plateaus aquifer system.

Western Interior Plains aquifer system, the very small 

mean lateral intrinsic-permeability values occur in the 

deeply buried parts of the unit. Comparison of the 

intrinsic permeability and porosity of the lower units 

(figs. 21, 23) to the intrinsic permeability and porosity 

of the upper unit (figs. 22, 25) indicates that the upper 

unit is much more permeable and porous than the 

lower units over much of the study area, although the 

values are nearly the same where both units are 

deeply buried.

Regional porosity of the upper unit (fig. 25) shows 

the same effect of depth of burial as described for permeability. The porosity is small (about 1 percent) in 

and near major basins not subjected to uplift and 

erosion and increases to 10-15 percent near the Central Kansas, Nemaha, and Ozark Uplifts.

Dissolved-solids concentrations of water in the 

upper unit range from more than 250,000 mg/L in 

small areas of north-central Oklahoma and 

south-central Kansas to less than 10,000 mg/L in the
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C25

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970 EXPLANATION

DISSOLVED-SOLIDS CONCENTRATION Values in milligrams per liter

100,000 - 200,000

More than 200,000

0-1,000 

1,000-20,000 

20,000- 100,000

50- LINE OF EQUAL DISSOLVED-SOLIDS CONCENTRATION Dashed where 

approximate. Interval, in thousands of milligrams per liter, is variable

| | | | | LIMIT OF CAMBRIAN AND ORDOV1CIAN ROCKS

FIGURE 24. Dissolved-solids concentrations in water from lower units in the Western Interior Plains aquifer system and the 

Ozark aquifer, the St. Francois confining unit, and the St. Francois aquifer in the Ozark Plateaus aquifer system.

transition zone adjacent to the freshwater flow system 

in the Ozark subregion and in east-central Nebraska 

(fig. 26). These large concentrations were expected 

because the water may represent a paleomarine 

evaporative brine introduced in the geologic past, a 

hypothesized, slow flow rate and long contact time, 

and minimal recharge through the overlying confining

system at most locations. The velocity of flow is very 

slow because in much of the area both intrinsic permeability and hydraulic gradient are small.

Gradients in the upper unit shown on the map of 

equivalent freshwater head (pi. 2) are similar to those 

described for the lower units. Gradients are generally 

east-northeast in the western part of the unit and
C26 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105° 100'

l "^

>j ; MINNESOTA

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 5 LINE OF EQUAL POROSITY-Inteival, in percent, 

is variable

i i i i i i i i i LIMIT OF MISSISSIPPIAN ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 25. Regional porosity of upper unit in the Western Interior Plains aquifer system and the Springfield Plateau aquifer in

the Ozark Plateaus aquifer system.

south-southeast in the northern and eastern parts. 

Moderately small equivalent freshwater-head values 

occur in southeastern Kansas and eastern Oklahoma, 

and areas of large hydraulic head associated with the 

Anadarko Basin in western Oklahoma and the eastern 

part of the Texas Panhandle are coincident with an 

overlying geopressure zone in Morrowan rocks 

(table 1).

WESTERN INTERIOR PLAINS CONFINING SYSTEM

The Western Interior Plains confining system separates the Great Plains aquifer system and the Western 

Interior Plains aquifer system. The system restricts 

vertical flow to and from the Western Interior Plains 

aquifer system. It is the most extensive unit in the 

CMRASA study area. The confining system is
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C27

105'

SOUTH DA%TA " ' "-""--V~%

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

5.000 LINE OF EQUAL DISSOLVED-SOLIDS CONCENTRATION IN WATER 

FROM UPPER UNIT IN THE WESTERN INTERIOR PLAINS AQUIFER 

SYSTEM OR SPRINGFIELD PLATEAU AQUIFER IN THE OZARK 

PLATEAUS AQUIFER SYSTEM-Dashed where approximate. Interval, 

in milligrams per liter, is variable

i i i i i i i i i LIMIT OF MISSISSIPPIAN ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 26. Dissolved-solids concentrations in water from upper unit in the Western Interior Plains aquifer system and the 

Springfield Plateau aquifer in the Ozark Plateaus aquifer system.

bounded on the west by the Rocky Mountains and 

extends eastward to the Ozark subregion. The confining system also extends north and northeast beyond 

the study area and south through the faulted and 

folded zone in southern Oklahoma. At the southeastern limit of the study area, the confining system dips 

steeply into the Arkoma Basin. The altitude of the

top ranges from more than 6,000 ft in northeastern 

New Mexico to less than -3,000 ft in the Denver 

Basin. In general, in western Missouri, eastern 

Kansas, and eastern Oklahoma, there is a west-to-east 

slope toward a broad physiographic low near the outcrop area adjacent to the western perimeter of the 

Ozark subregion. The thickness of the confining
C28 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

PLAINS 

SUBREGION

KANSAS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 15 LINE OF EQUAL POROSITY-Interval, in percent, 

is variable

i I i I i I i n LIMIT OF PENNSYLVAN1AN ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 27. Estimated porosity of sandstone in the Western Interior Plains confining system.

system ranges from zero in northeastern Nebraska and 

near the Ozark subregion to more than 20,000 ft in the 

Anadarko Basin of southwestern Oklahoma.

The rocks composing the confining system are of 

Late Mississippian through Jurassic age but are 

mostly of Pennsylvanian and Permian age. The 

Pennsylvanian rocks consist mostly of shale separated 

by limestone and sandstone layers. In the deeper part 

of the Anadarko and Arkoma Basins, the ratio of 

sandstone to limestone increases. In the shallower 

areas, the ratio of limestone to sandstone exceeds 1.

Adjacent to the Amarillo Uplift and Sierra Grande 

Arch, permeable arkosic gravel is present and is 

termed "granite wash" because it is believed to have 

been eroded from a granite uplift.

The Permian rocks of the confining system consist 

mostly of shale with evaporite deposits, sandstone, 

and limestone. Adjacent to uplifts, such as the Front 

Range of the Rocky Mountains, the Sierre Grande, 

and the Amarillo, permeable arkosic gravel is present. 

Triassic and Jurassic rocks are mostly shale, sandstone, and limestone.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C29

In general, except in the deeper part of the Arkoma 

and Anadarko Basins, the sandstone and limestone 

deposits are aquifer units. The estimated porosity of 

sandstone, porosity of carbonate rocks, and estimated 

thickness-weighted mean lateral intrinsic permeability 

of the confining system are shown in figures 27-29. 

The aquifer units in the Western Plains confining system are less deeply buried and more permeable than 

aquifer units in the underlying Western Interior 

Plains aquifer system; however, leakage through the 

confining system at most locations is very small

because the system includes thick layers of shale and 

extensive layers of nearly impermeable evaporite 

deposits (mainly halite and gypsum).

Halite and gypsum deposits are present in the 

western one-half of Kansas, parts of northwestern 

Oklahoma, the Oklahoma and Texas Panhandles, 

eastern Colorado, western Nebraska, and eastern 

Wyoming (fig. 30). Although not continuous, there 

appears to be sufficient thickness of overlapping units 

that the evaporite deposits severely restrict vertical 

leakage through the confining system; hence, leakage

'.MINNESOTA

SOIJTH DAKOTA ' 

Boundary of study area

PLAINS 

SUBREGION

OZARK 

SUBREGION

NEW MEXICO" "

0 100 200 KILOMETERS

35

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

-15-

EXPLANATION

LINE OF EQUAL POROSITY-Interval, in percent, 

is variable

LIMIT OF PENNSYLVANIAN ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 28. Estimated porosity of carbonate rocks in the Western Interior Plains confining system.
C30 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105 100 95° 90°

40'

35'

/

WYOMING

SOUTH DAKOTA ' -- " "*" 

\ MINNESOTA

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 70 LINE OF EQUAL ESTIMATED PERMEABILITY-Value is the thicknessweighted mean horizontal intrinsic permeability. Interval, in feet squared, 

is variable

i i i i i i EXTENT OF WESTERN INTERIOR PLAINS CONFINING SYSTEM

 APPROXIMATE BOUNDARY BETWEEN PLAINS AND OZARK 

SUBREGIONS

FIGURE 29. Estimated lateral intrinsic permeability of the Western Interior Plains confining system.

may approach zero. Because of the confining system's 

large thickness and extent, it not only functions as a 

confining system but also may transmit small quantities of lateral flow where there is slight intrinsic permeability (fig. 29). However, such lateral flow cannot 

be defined or quantified in the confining system.

The pressure of pore water in the permeable units 

of the confining system varies both vertically and horizontally because these units are not in a reasonably 

distinct hydraulic system (Jorgensen and others, 

1993). Very high pressures exist in a geopressure zone

near the base of the confining system in the Anadarko 

Basin in Oklahoma. At this location, equivalent 

freshwater head exceeds 6,000 ft in Morrowan sand 

and shale. The head in the underlying upper unit in 

the Western Interior Plains aquifer system near the 

Anadarko Basin also is about 6,000 ft because of the 

overpressure. Pressure also is transferred and attenuated into the underlying lower units in the Western 

Interior Plains aquifer system where equivalent freshwater head is more than 3,000 ft in the slightly permeable rocks.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C31

SOUTH DAKOTA 

-.- - .. /

Boundary of

Base modified from U.S. Geological Survey 

1:7.500,000, National Atlas, 1970

EXPLANATION

-700 LINE OF EQUAL THICKNESS OF EVAPORITE 

DEPOSITS IN THE WESTERN INTERIOR 

PLAINS CONFINING SYSTEM-Dashed where 

approximate. Interval, in feet, is variable

i i i i i i LIMIT OF PENNSYLVANIAN ROCKS

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 30. Thickness of evaporite deposits in the Western Interior Plains confining system.

The geohydrologic characteristics of a confining 

system are such that, where a vertical hydraulic gradient exists between overlying and underlying units, a 

single hydraulic-head value in the confining unit cannot represent the entire vertical section nor can a laterally continuous surface of equivalent freshwater 

head be representative of the confining system. 

Therefore, even though pressure data from drill-stem 

tests are available for computing an equivalent freshwater head in parts of the confining system, a valid

map of the regional equivalent freshwater head for the 

entire thickness of the confining system cannot be 

made.

Although the Western Interior Plains confining 

system is conceived as a confining system, very generalized equivalent-freshwater-head data referenced to 

the centerline of the confining system were used to 

map an approximation of freshwater head for the system (fig. 31). Lines of equal equivalent freshwater
C32 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 7000 LINE OF EQUAL APPROXIMATE EQUIVALENT FRESHWATER HEAD 

OR NEAR-SURFACE FRESHWATER HEAD-Shows altitude at which 

water would have stood in a tightly cased well penetrating rocks of the 

Western Interior Plains confining system. Dashed where approximately 

located. Interval, in feet, is variable. Datum is sea level

LIMIT OF WESTERN INTERIOR PLAINS CONFINING SYSTEM

 APPROXIMATE BOUNDARY BETWEEN PLAINS AND OZARK 

SUBREGIONS

FIGURE 31. Approximate equivalent freshwater head in buried parts and near-surface freshwater head in outcrop area of the

Western Interior Plains confining system.

head in figure 31 are interpretations and serve two 

purposes, which are to:

1. Provide the altitude of a constant-head boundary in 

the outcrop area for computer simulation; and

2. Indicate that freshwater-head values in vertically 

adjacent layers are consistent; that is, the hydraulic head in buried parts of the confining system

should be intermediate between hydraulic head in

the overlying and underlying units. 

The data in the buried part of the confining system 

indicate consistency. However, in the outcrop area 

overlying the Anadarko Basin in western Oklahoma, 

the overpressure in the lower part of the confining system (Morrowan sandstone of earliest Pennsylvanian
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C33

WEST EAST

Great Plains confining system 

(Upper Cretaceous rocks)

equivalents) *- Maha aquifer *" (Dakotabanosto

Apishapaconfining unit

uivalents) ^- Apishapa aquifer

Plains confining system 

through Jurassic rocks) (Cambrian through 

Mississippian rocks)

Basement confining unit

NOT TO SCALE

EXPLANATION

 >- DIRECTION OF GROUND-WATER FLOW

FIGURE 32. Geohydrologic section showing the Maha and Apishapa aquifers and intervening Apishapa 

confining unit in the Great Plains aquifer system.

age) cannot be related to hydraulic head in the outcrop or subjacent layers.

At locations where the confining system crops out, 

data are presented for the near-surface freshwater 

head (fig. 31). In general, these heads represent or are 

affected substantially by the near-surface water table. 

The altitude of the water table varies a few feet per 

year, but for the scale of this study can be assumed to 

be at a constant altitude (fig. 31). The outcrop area 

of the confining system is extensive and includes both 

recharge and discharge areas to the underlying aquifers, which depend primarily on land-surface topography and hydraulic-head relations.

GREAT PLAINS AQUIFER SYSTEM

The Great Plains aquifer system consists of two 

aquifers separated at most locations by a confining 

unit. These are the Maha and Apishapa aquifers and 

the intervening Apishapa confining unit (table 1, 

fig. 32). The aquifer system, which consists mostly of 

water-bearing Lower Cretaceous sandstone, is one of 

the most extensive in North America (Helgesen and 

others, 1982) and extends from near the Arctic Circle 

in Canada to New Mexico (fig. 33). In the United 

States, water in the Maha and Apishapa aquifers generally flows from west to east. This was recognized by 

Darton (1905); however, at that time available data 

were inadequate for clearly differentiating between 

the two aquifers at most locations. These two aquifers have been termed the "Dakota aquifer" or

"Dakota aquifer system" by various investigators. 

Others have restricted the term "Dakota aquifer" to 

either the upper (Maha; aquifer or the lower 

(Apishapa) aquifer. In this report, the aquifers that 

consist mostly of Lower Cretaceous sandstone and 

that are part of the regional flow system are termed 

the Great Plains aquifer system as defined by Helgesen and others (1993).

The altitude of the top of the aquifer system 

ranges from more than 6,000 ft in northeastern New 

Mexico, a recharge area, to 1,000 ft in southeastern 

South Dakota, a discharge area, to less than -4,000 ft 

in the Denver Basin.

The Great Plains aquifer system has some large 

outcrop areas in southeastern Colorado and 

east-central Kansas. Smaller outcrop areas occur 

along the Front Range in the Rocky Mountains in 

Colorado and near the Black Hills Uplift in southwestern South Dakota. In terms of regional flow, the 

aquifer system receives recharge through the western 

outcrops. There are indications that vertical leakage 

through the overlying Upper Cretaceous shale provides some recharge (Neuzil, 1980). Outflow occurs in 

the eastern outcrops and where the Missouri River 

incises the unit in southeastern South Dakota, eastern 

Nebraska, and western Iowa. Also, there is evidence 

of inflow to and outflow from the overlying material 

near peripheral areas of the eastern extent of the unit, 

such as in eastern Nebraska and central Kansas. At 

those locations, the unit pinches out and is in direct 

contact with permeable material of overlying units.
C34 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

140° 130° 120° 110° 100° 90° 80° 70°_____60°

40<

3o<

Base from Central Intelligence Agency, 

North America, 1:38,700,000.1992 

Lambert Conformal Conic projection 

Standard parallels 37° and 65°

250 500 750 MILES

0 250 500 750 KILOMETERS 

Contours modified from Hitchon (1969) 

Lobmeyer (1980). and others

EXPLANATION

 3000 POTENT1OMETRIC CONTOUR-Shows altitude of potentiometric surface of the 

Great Plains aquifer system. Hachures indicate depression. Contour interval is 

500, 1000, and 2000 feet. Datum is sea level

- LIMIT OF GREAT PLAINS AQUIFER SYSTEM

FIGURE 33. Great Plains aquifer system in North America (modified from Helgesen and others,

1982).

The Great Plains aquifer system occurs beyond the 

boundary of the CMRASA study area along the 

northern State line of Nebraska. The State line is a 

common border with the Northern Great Plains 

RASA. The potentiometric surface of the Maha aquifer in the Great Plains aquifer system suggests that 

flow lines are approximately parallel to the State line, 

indicating little flow across that boundary, a condition 

that is consistent with the results of the Northern 

Great Plains RASA study (Downey, 1986, figs. 40, 45, 

pi. 3).

The Apishapa aquifer is the lower of the two major 

aquifers in the Great Plains aquifer system and is

composed of permeable, loosely cemented, medium- to 

very fine-grained sandstone of the Cheyenne Sandstone and equivalents, such as the Fall River and 

Lakota Sandstone (fig. 32, table 1). The name 

Apishapa refers to the water-yielding sandstone layers 

that occur in the vicinity of the Apishapa River and 

other adjacent areas in southeastern Colorado, northeastern New Mexico, and southwestern Kansas. The 

maximum thickness of the Apishapa aquifer exceeds 

400 ft; however, the typical thickness is between 100 

and 200 ft.

The Apishapa confining unit is composed of very 

slightly permeable shale layers, which at most
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C35

locations consist of the Kiowa Shale and the equivalent Skull Creek Shale. The confining unit, which separates the Apishapa aquifer from the overlying Maha 

aquifer, is not as extensive as the Maha aquifer. The 

Apishapa confining unit is thin, with thicknesses ranging from 10 to 270 ft. The altitude of the top is, in 

general, about 200 ft less than the altitude of the top 

of the Great Plains aquifer system.

The Maha aquifer is the upper aquifer unit in the 

Great Plains aquifer system and is more extensive 

than the underlying Apishapa aquifer (fig. 32). The 

aquifer at most locations consists of permeable, 

loosely cemented, medium- to fine-grained sandstone 

of the Dakota Sandstone, "D" and "J" sandstones of 

informal usage, and equivalent Newcastle Sandstone. 

The altitude of the top of the Maha aquifer and the 

altitude of the top of the Great Plains aquifer system 

are the same except where the Maha is absent and the 

Apishapa aquifer is present in a few locations in 

southeastern Colorado and northeastern New Mexico. The aquifer thickens from less than 100 ft in 

eastern Colorado and along its eastern limit to about 

650 ft in northeastern Nebraska. Maximum thicknesses exceed 900 ft in small areas of northcentral and central Nebraska.

Nearly all of the available hydrologic information 

relating to permeability, water chemistry, and potentiometric head of the Great Plains aquifer system are 

for the Maha aquifer. The limited information available for the Apishapa aquifer indicates that potentiometric head does not differ greatly between the two 

aquifers. The regional predevelopment potentiometric 

surface for the Maha aquifer is shown in figure 34. 

The head in the Apishapa aquifer is believed to be 

similar and closely related. However, in some areas 

there may be a considerable difference; for example, in 

South Dakota just north of the study area, a substantial potentiometric-head difference occurs between the 

aquifers. Also in the recharge areas in southeastern 

Colorado, the Apishapa aquifer has a lower head than 

the Maha.

Estimated porosity and lateral intrinsic permeability for both aquifers (figs. 35, 36) are considered to be 

primary because the aquifer material is mostly a 

loosely cemented sandstone. Both porosity and permeability decrease with depth.

A generalized distribution of dissolved-solids concentrations in water from the Maha aquifer (Helgesen 

and Leonard, 1989) is shown in figure 37. Limited 

water-quality information within the study area 

indicates that water from the Apishapa in some locations may have a larger dissolved-solids concentrations than water from the Maha aquifer.

OVERLYING UNITS

The uppermost geohydrologic units for the Plains 

subregion of the CMRASA consist principally of the 

Great Plains confining system and the High Plains 

aquifer, with smaller areas of glacial drift and loess 

deposits (fig. 38). The geohydrology and digital simulation of the High Plains aquifer are treated in detail 

by Gutentag and others (1984) and Luckey and others 

(1984).

Below the High Plains aquifer, the Great Plains 

confining system, which has a combined thickness of 

as much as 8,000 ft (fig. 39), is composed of shale 

including the Upper Cretaceous Graneros, Pierre, and 

Carlile Shales, as well as very slightly to slightly permeable Tertiary clay and silt. The confining system 

also includes two extensive but minor aquifers in the 

Greenhorn Limestone and Niobrara Formation. The 

confining system at most locations effectively restricts 

flow between the Great Plains aquifer system and the 

unconfined High Plains aquifer or other unconfined 

units. Vertical movement of water from the High 

Plains aquifer through the Upper Cretaceous shale is 

dependent on the hydraulic head in the High Plains 

aquifer, the vertical permeability and thickness of the 

shale, and the hydraulic head in the Great Plains 

aquifer system. Leonard and others (1983, fig. 8) 

show that in the coincident areas of the High Plains 

aquifer and the Great Plains aquifer system, hydraulic 

head in the High Plains aquifer is generally higher 

than in the Great Plains aquifer system (fig. 40). 

Peripheral to the Upper Cretaceous shale in the Great 

Plains confining system are areas where permeable 

materials of the High Plains aquifer are in direct contact with permeable materials of the Great Plains 

aquifer system. Glacial drift and loess deposits in 

eastern Kansas and Nebraska in part lie directly on 

rocks older than Early Cretaceous. The glacial drift is 

largely heterogeneous material of variable hydraulic 

character. Loess deposits also are present as a surficial unit and consist of uniform silt-sized permeable 

material. The glacial drift and loess deposits probably 

act as leaky confining materials relative to the underlying units.

OZARK SUBREGION

The Ozark subregion generally corresponds to the 

Ozark Plateaus physiographic province (fig. 41). 

Ground-water flow, in general, is outward from a 

topographic high along an axis from the St. Francois 

Mountains in southeastern Missouri to northwestern 

Arkansas. The regional flow system is in 

Paleozoic sedimentary rocks, which are underlain by
C36 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

40'

35'

105° 100° 95° 90°

WYOMING SOyTH DA%TA \

""-- -~-Y

-r 

>

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

-J000 POTENTIOMETRIC CONTOUR-Shows altitude at which water would have 

stood in tightly cased wells. Contour interval, in feet, is variable. Datum is 

sea level

i i i i i i LIMIT OF GREAT PLAINS AQUIFER SYSTEM

FIGURE 34. Predevelopment potentiometric surface of the Maha aquifer in the Great Plains aquifer system.

the basement confining unit that is composed mostly 

of igneous and metamorphic rocks.

The geohydrologic units and model analysis of 

ground-water flow in the Ozark subregion are 

described by lines and Emmett (1994). The following 

description of the geohydrologic framework of the 

Ozark Plateaus aquifer system in the Ozark subregion 

is based on the aquifer and confining-unit designations 

of that report.

OZARK PLATEAUS AQUIFER SYSTEM

Most ground water in the Ozark subregion is in the 

Ozark Plateaus aquifer system. The aquifer system 

contains freshwater and is laterally adjacent to the 

Western Interior Plains aquifer system of the Plains 

subregion. The geologic units of the Ozark Plateaus 

aquifer system are approximately equivalent to those 

of the Western Interior Plains aquifer system
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C37

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 20 LINE OF EQUAL POROSITY-lnterval, in percent, 

is variable

III I I I I M LIMIT OF GREAT PLAINS AQUIFER SYSTEM

 APPROXIMATE BOUNDARY BETWEEN PLAINS 

AND OZARK SUBREGIONS

FIGURE 35. Estimated porosity of the Apishapa and Maha aquifers.

(tables 1, 2; fig. 19). The two aquifer systems are separated by a no-flow hydrologic boundary, not rock 

boundaries. In general, the rocks of the St. Francois 

aquifer, the St. Francois confining unit, and the Ozark 

aquifer are age equivalents to the Paleozoic rocks of 

the lower units in the Western Interior Plains aquifer 

system. The rocks of the Springfield Plateau aquifer 

are equivalent to the Mississippian rocks of the upper 

unit in the Western Interior Plains aquifer system. 

The intervening Ozark confining unit is principally the 

Chattanooga Shale.

The Ozark Plateaus aquifer system is overlain 

along the western and southern borders by the Western Interior Plains confining system. The Ozark Plateaus aquifer system consists of rocks that range in 

age from Late Cambrian through Late Mississippian 

(table 2). Tops and bottoms of the geohydrologic 

units do not always conform to geologic-time divisions 

or boundaries of rock-stratigraphic units but are chosen to delineate groups of rocks having similar 

hydrologic properties. The geohydrologic units that 

comprise the Ozark Plateaus aquifer system crop out
C38 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105

0 100 200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

-JO LINE OF EQUAL ESTIMATED PERMEABILITY-Value is the thicknessweighted mean horizontal intrinsic permeability. Interval, in feet squared, 

is variable

LIMIT OF GREAT PLAINS AQUIFER SYSTEM

FIGURE 36. Estimated lateral intrinsic permeability of the Apishapa and Maha aquifers.

in an approximately circular pattern around the 

St. Francois Mountains.

ST. FRANCOIS AQUIFER

The St. Francois aquifer, the lowermost aquifer in 

the Ozark Plateaus aquifer system, is composed of 

Upper Cambrian rocks, primarily sandstone in the 

western one-third of the Ozark subregion and sandstone overlain by coarse-grained dolostone in the eastern two-thirds of the subregion.

The St. Francois aquifer extends to the boundaries 

of the Ozark Plateaus aquifer system (fig. 20).

Beyond the relatively small outcrop area near the St. 

Francois Mountains (fig. 41), the aquifer dips into the 

subsurface and is buried beneath the other units of the 

Ozark Plateaus aquifer system. The aquifer dips most 

steeply to the east toward the Illinois Basin (fig. 7) 

and south toward the Mississippi Alluvial Plain 

(fig. 41) where it is buried to a depth of more than 

5,000 ft. The aquifer occurs in most of the Ozark subregion, although it rarely is penetrated by wells in the 

southeast part of the area (Arkansas). The St. Francois aquifer is thickest (greater than 1,000 ft) in eastern Missouri beneath the Mississippi Alluvial Plain
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C39

105° 100° 95° 90°

35C

WYOMING

N, ( v:

SOUTH i DAKOTA DAKOTA S : I

-- , i

MINNESOTA

W1SCUNS1N

i r

0 100 200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500.000. National Atlas, 1970

EXPLANATION

DISSOLVED-SOLIDS CONCENTRATION, 

IN MILLIGRAMS PER LITER

Less than 1,000 

1,000 to 10,000 

Greater than 10,000

LIMIT OF GREAT PLAINS 

AQUIFER SYSTEM

FIGURE 37. Dissolved-solids concentrations in water from the Maha aquifer in the Great Plains aquifer system (modified from

Helgesesn and Leonard, 1980).

but generally is between 100 and 300 ft thick in northern Arkansas.

Estimates of hydraulic conductivity based 

on specific-capacity data in and near the outcrop area 

near the St. Francois Mountains range from

IxlO"4 ft/s to less than 1x10 "6 ft/s and tend to decrease 

away from the outcrop.

Few data are available on water chemistry of the 

St. Francois aquifer except in the outcrop area. 

Chemical analyses of water samples collected from the
C40 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105'

! TEXAS \ . 

 v^-vj

0 100 200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

WESTERN INTERIOR PLAINS CONFINING SYSTEM 

GREAT PLAINS CONFINING SYSTEM 

HIGH PLAINS AQUIFER 

GLACIAL DRIFT AND LOESS

 APPROXIMATE BOUNDARY BETWEEN PLAINS AND OZARK 

SUBREGIONS

FIGURE 38. Geohydrologic units overlying regional aquifer systems in Plains subregion.

aquifer in the outcrop area indicate dissolved-solids 

concentrations from less than 100 to more than 400 

mg/L. Dissolved-solids concentrations in water samples from wells that penetrate the deeply buried St. 

Francois aquifer are not large, but actual 

concentrations in water from the St. Francois aquifer 

are not known because the wells are open to overlying 

geohydrologic units.

Where the St. Francois aquifer crops out, 

ground-water levels are affected primarily by topography. The aquifer is recharged from infiltration of precipitation through the soil zone. The aquifer is 

recharged also by flow from the fractured Precambrian rocks near the outcrop area. Hydraulic-head 

data in the immediate vicinity of the St. Francois 

Mountains indicate that water from the St. Francois
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C41

105'

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 2000 LINE OF EQUAL THICKNESS OF GREAT PLAINS CONFINING SYSTEMInterval, in feet, is variable

i i i i i i LIMIT OF GREAT PLAINS CONFINING SYSTEM

FIGURE 39. Thickness of the Great Plains confining system.

aquifer discharges upward to overlying aquifers 

through confining units downdip from the outcrop 

area and discharges to the St. Francis and Black Rivers to the south and the Big River to the north 

(fig. 1). In that area, leakance of the confining unit is 

not sufficiently small to hydrologically isolate the 

St. Francois aquifer from topographic effects.

ST. FRANCOIS CONFINING UNIT

The St. Francois confining unit restricts flow 

between the St. Francois aquifer and the overlying 

Ozark aquifer. The confining unit is composed of

shale, dolostone, and limestone. No quantitative measurements of the hydraulic conductivity of the St. 

Francois confining unit are known (Imes and Emmett, 

1994). Imes and Emmett (1994) cite evidence that 

the hydraulic conductivity near fracture zones in the 

unit is too large to hydrologically isolate the underlying St. Francois aquifer in those localities. However, 

in areas where the rocks are not fractured, the unit 

forms an effective confining unit.

The total outcrop area of the confining unit is 

about 400 mi 2. The confining unit dips steeply and 

radially from the St. Francois Mountains (as much as 

150 ft/mi), except to the west where the unit dips
C42 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

MINNESOTA 

!..____W-~- 

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

GREAT PLAINS AQUIFER SYSTEM

HIGH PLAINS AQUIFER

 UOO LINE OF EQUAL DIFFERENCE IN HYDRAULIC HEADShows alitiude of the potentiometric surface of High Plains 

aquifer minus (-) the altitude of the potentiometric surface 

of the Great Plains aquifer system. Interval 200 feet

FIGURE 40. Differences in hydraulic head between the High Plains aquifer and the Great Plains aquifer system (modified from

Leonard and others, 1983).

more gently into the subsurface. Smaller isolated 

areas within the Ozark subregion where the unit does 

not occur in the subsurface usually are at or near 

areas where the St. Francois aquifer is also absent.

The maximum thickness of the St. Francois confining unit, as determined from well logs, is about 900 ft 

in southeastern Missouri beneath the northwestern 

edge of the Mississippi Alluvial Plain. The confining 

unit generally is thicker in the eastern and northeastern parts of the Ozark subregion. The confining unit

is thinner in western Missouri, ranging from about 100 

to 300 ft, and pinches out a few miles west of the 

Ozark subregion in eastern Kansas and northeastern Oklahoma.

Shale in the St. Francois confining unit, which in 

general is nearly impermeable, may occur as distinct 

layers but usually is distributed throughout the limestone and dolostone beds. The percentage-of-shale 

content of the St. Francois confining unit ranges from 

0 to 60 percent and generally is less than 30 percent
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C43

EXPLANATION

 LIMIT OF OZARK SUBREGION

 BOUNDARY OF PHYSIOGRAPHIC PROVINCES

 BOUNDARY OF PHYSIOGRAPHIC SUBDIVISIONS 

FIGURE 41. Physiographic features of Ozark subregion (modified from Fenneman, 1946).

(fig. 42). The confining ability of the unit is dependent not only on the shale content but also on the 

degree of cementation of the carbonate rock and the 

abundance of fractures and solution openings.

The relatively fine-grained nature of the dolostone and the lack of significant secondary porosity 

are the probable reasons for the small permeability of 

the confining unit even in regions devoid of shale. 

However, the confining unit is more likely to be leaky 

in the dolomitic areas than in areas with a large shale 

content. There is evidence from well cores that the 

upper part of the confining unit is more permeable 

along a northwest-trending reef zone passing through 

Wright and Douglas Counties, Missouri. It is possible

that the St. Francois and Ozark aquifers are more 

hydraulically connected along the reef zone. Fracture 

zones are evident and likely result in increased leakage 

locally, especially in the vicinity of faults.

OZARK AQUIFER

The Ozark aquifer, the most areally extensive and 

most intensively pumped aquifer in the Ozark subregion, is composed of a sequence of formations ranging 

in age from Late Cambrian to Middle Devonian 

(table 2) that vary considerably in water-yielding 

capability but collectively act as a regional aquifer. 

Where buried west and south of the extensive outcrop 

area, the base of the Chattanooga Shale
C44 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

-20-

EXPLANATION

AREA WHERE PERCENTAGE OF SHALE IS ZERO

LINE OF EQUAL PERCENTAGE OF SHALE IN 

ST. FRANCOIS CONFINING UNIT-Interval 10 

and 20 percent

LIMIT OF ST. FRANCOIS CONFINING UNIT

FIGURE 42. Shale content of the St. Francois confining unit (modified from Jorgensen and others, 1993).

stratigraphically represents the upper boundary of the 

Ozark aquifer. The aquifer is exposed over a large 

part of the Ozark subregion and is dissected by 

streams where exposed. The rocks are highly fractured and moderately faulted.

Where the aquifer dips into the subsurface, it generally is overlain by Upper Devonian rocks; however, 

in the northern part of the Ozark subregion (Gasconade County, Missouri, and vicinity), the aquifer is in 

direct contact with as much as 200 ft of Pennsylvanian-age shale. To the southeast in the Mississippi 

Alluvial Plain, the aquifer is overlain by thick deposits of Cretaceous, Tertiary, and Quaternary sediment.

The Ozark aquifer is the thickest geohydrologic 

unit in the Ozark subregion. In southeastern Kansas, 

southwestern Missouri, and northeastern Oklahoma, 

aquifer thicknesses generally range from about 800 to 

1,500 ft. Thicknesses are less uniform over short distances in Oklahoma where Precambrian topographic 

relief affected the thickness of rocks that typically are 

part of the Ozark aquifer.

The hydraulic conductivity of the Ozark aquifer 

has been estimated from specific-capacity data and 

inferred from density of fractures and development of 

solution openings. In general, the more the rocks have 

been uplifted, the more they have been fractured, the
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C45

more dissolution has occurred along fractures, and correspondingly, the greater the hydraulic conductivity. 

Areas of greatest hydraulic conductivity are concentrated along an east-west line passing through the 

St. Francois Mountains where the hydraulic conductivity may be as much as IxlO"3 ft/s. In areas where 

dissolution has occurred and springs are numerous, the 

hydraulic conductivity is extremely large and locally 

exceeds IxlO"1 ft/s (Jorgensen and others, 1993). The 

hydraulic conductivity decreases to the south to as 

small as 1x10 "8 ft/s near the southern boundary of the 

study area.

The Ozark aquifer is a complex sequence of formations (table 2) of different lithologies and a wide range 

of porosity and permeability values. The lithologies 

include dolostone, limestone, sandstone, chert, and 

shale. Dolostone is the predominant rock type. Dissolution of carbonate rock along fractures and bedding 

planes, including karst development, has enhanced the 

permeability of the aquifer. Sandstone is present as 

massive bodies in some formations. The sandstone is 

usually clean, well sorted, and permeable; however in 

Arkansas, thick clean sandstone is well cemented and, 

where unfractured, is much less permeable.

The limestone and chert of Silurian and Devonian 

age are the uppermost rocks in the Ozark aquifer in 

the north-central and southern part of the Ozark subregion. The rocks are less permeable than the older 

rocks in the aquifer and generally are not a source of 

water supply. The smaller permeability reflects a lack 

of secondary porosity.

Water in most of the Ozark aquifer is unconfined, 

and its flow is controlled by the altitude of the water 

table, which generally is related closely to the 

land-surface altitude. Throughout the outcrop area, 

ground water moves toward the major rivers and their 

tributaries (fig. 43). Hundreds of springs have developed. Flow to most of the springs originates in the 

upland intervalley areas. Water moves through solution channels along fractures downward to the water 

table near the valleys and discharges along streams. 

The Mississippi River Valley is a major discharge area 

for the aquifer. The Missouri River Valley along the 

northern edge and east of Saline County, Missouri, is 

also a major discharge area. To the west of Saline 

County, the aquifer is confined, and the Missouri 

River at that location is not a major discharge area. 

In west-central Missouri, there is evidence that water 

from the Ozark aquifer discharges into the Marais des 

Cygnes and Osage Rivers (fig. 43) (Imes and Emmett, 

1994). In eastern Kansas and southward to northeastern Oklahoma, flow in the Ozark aquifer is outward to 

the boundary of the Ozark subregion. Westerly flowing ground water discharges through overlying

deposits into the Neosho River Valley (fig. 43). The 

western boundary of the aquifer is also approximately 

coincident with the eastward extent of Pennsylvanian 

shale; this shale affects, in part, the vertical leakage 

upward out of the Ozark Plateaus aquifer system. 

South of the Boston Mountains, there is limited flow 

southward toward the Arkansas River Valley (fig. 43). 

The quantity of flow southward is limited because of 

the slight permeability of the rocks in that area. The 

dissolved-solids concentrations (fig. 43) indicate the 

extent of freshwater around parts of the periphery of 

the Ozark aquifer. Concentrations increase toward 

the west and are approximately coincident with the 

eastern extent of the Pennsylvanian shale and the 

approximate boundary between the Plains and Ozark 

subregions (fig. 24).

OZARK CONFINING UNIT

The Ozark confining unit is the uppermost confining unit of the Ozark Plateaus aquifer system and consists mostly of Upper Devonian Chattanooga Shale 

and Lower Mississippian Chouteau Limestone. The 

semipermeable rocks of the Ozark confining unit 

impede the movement of water between the Ozark 

aquifer and the overlying Springfield Plateau aquifer. 

However, local lithologic and structural variations can 

increase the leakance to the degree that confinement is 

minimal and does not restrict flow between the Ozark 

and the Springfield Plateau aquifers to any significant 

degree. The leakance is the result of the large permeability of the rocks or thinning of the confining unit. 

The confining unit is not present everywhere in the 

Ozark subregion; it is missing from the Salem Plateau, 

an area that includes approximately the eastern 

two-thirds of the Ozark subregion (fig. 41). In 

extreme eastern Missouri, formations of Ordovician, 

Silurian, and Devonian age are included as part of the 

confining unit. It is probable that some of these formations in eastern Missouri, several of which are thick 

limestone formations, constitute the western edge of 

another geohydrologic unit in another flow system 

outside of the Ozark subregion.

The confining unit does not occur in the subsurface 

in small areas of southeastern Kansas, southwestern 

Missouri, northeastern Oklahoma, and northern 

Arkansas, probably due to nondeposition. The confining unit does not occur in the subsurface over a large 

area in the northern part of the area (Gasconade 

County, Missouri, and vicinity) and along the Mississippi Alluvial Plain where the rocks that typically 

form the unit have been removed by erosion.

Thickness of the Ozark confining unit ranges from 

0 to more than 1,600 ft in extreme eastern Missouri.
C46 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

35'

OKLAHOMA

30 60 KILOMETERS

EXPLANATION

AQUIFER ABSENT

POTENTIOMETRIC CONTOUR-Shows altitude at which water level would 

have stood in a tightly cased well. Dashed where approximately. Contour 

interval 200 feet. Datum is sea level

2000 LINE OF EQUAL DISSOLVED-SOILDS CONCENTRATION-Interval 1,000 

milligrams per liter

MAJOR DIRECTION OF REGIONAL FLOW IN OZARK AQUIFER

-'"' * - MAJOR DIRECTION OF FLOW IN WESTERN INTERIOR PLAINS AQUIFER SYSTEM 

FIGURE 43. Predevelopment potentiometric surface and dissolved-solids concentrations in water from the Ozark aquift

However, at most locations, the unit is less than 

100 ft thick and is relatively thin in comparison to 

other geohydrologic units in the Ozark subregion.

Along the northwestern boundary of the Ozark 

subregion, the confining unit is lithologically more 

complex than elsewhere in the subregion. Limestone 

predominates in this sequence, but shale also is 

present in large quantities. Sandstone is present in 

small quantities. The thickness of the major shale

formations, the Maquoketa Shale, Orchard Creek 

Shale, Chattanooga Shale, and the Hannibal Shale, 

may total as much as 100 ft; additionally, many of the 

limestone formations also contain shale. In the northern and northwestern parts of the Ozark subregion, 

the lithology varies from mostly shale to mostly limestone. Southward, the confining unit is represented 

solely by the Chattanooga Shale, which is predominantly shale with small amounts of sandstone.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C47

The shale content of the Ozark confining unit, 

which is an indicator of the confining unit's effectiveness, ranges from near 0 to 100 percent. In southwestern Missouri and southeastern Kansas, the percentage 

of shale in the confining unit changes abruptly within 

short distances and ranges from less than 20 to 

100 percent. In much of southwestern and southern 

parts of the Ozark subregion, shale constitutes 

100 percent of the confining unit.

SPRINGFIELD PLATEAU AQUIFER

The Springfield Plateau aquifer is composed of a 

sequence of permeable Mississippian limestone. The 

aquifer is the uppermost unit of the Ozark Plateau 

aquifer system and at most locations overlies the 

Ozark confining unit.

The outcrop of the aquifer corresponds approximately with the Springfield Plateau (fig. 41) from 

which it derives its name. The aquifer is used extensively as a source of stock and domestic water in the 

outcrop area, usually in combination with the underlying Ozark aquifer. The Springfield Plateau aquifer 

is present also in the subsurface south of the Boston 

Mountains. In that area, the aquifer contains water 

with more than 1,000 mg/L dissolved solids probably 

because near-surface permeability has not developed 

and because slow-moving water dissolves minerals as 

it passes through the slightly permeable rocks. The 

aquifer is unused in that area because of both the 

small yield to wells and large dissolved-solids 

concentrations.

The aquifer crops out around the western and 

southern perimeter of the Salem Plateau and along the 

southwestern extension of the Ozark Uplift axis. The 

aquifer is not present in the area adjacent to the Mississippi Alluvial Plain and in the subsurface in small 

areas near the western edge of the Springfield Plateau 

and the southern and southwestern edge of the Ozark 

subregion in Arkansas and Oklahoma. The Springfield 

Plateau aquifer has a relatively uniform thickness of 

100 to 400 ft throughout most of the subregion but is 

much thicker to the northeast in St. Louis County, 

Missouri, and in a narrow area along the Mississippi 

River in Perry County where a thickness of 1,500 ft is 

shown.

Hydraulic-conductivity values from aquifer tests 

or specific-capacity tests of the Springfield Plateau 

aquifer are not available (Jorgensen and others, 1993).

The Springfield Plateau aquifer is recharged in its 

outcrop areas. In general, water moves locally from 

high intervalley recharge areas to low discharge areas 

along the streams that dissect the area. Regionally, 

lateral flow in the Springfield Plateau aquifer is 

believed to be similar to that in the underlying Ozark

aquifer. In the western part of the Ozark subregion, 

the aquifer receives upward discharge from the Ozark 

aquifer. In the eastern part of the subregion, 

hydraulic gradients are reversed, and water is discharged downward to the underlying Ozark aquifer.

The Springfield Plateau aquifer is overlain along 

the western and southern boundaries by confining 

material, which is mostly Pennsylvanian shale. This 

overlying confining material is the eastern edge of the 

Western Interior Plains confining system. This confining system is composed of layers of very slightly permeable shale and layers of moderately permeable 

limestone, sandstone, and coal. The limestone layers 

have some fractures and slight permeability except 

near the land surface where dissolution has increased 

the permeability.

SIMULATION OF GROUND-WATER FLOW

A numerical ground-water flow model was used to 

test the conceptualization of flow in the regional aquifers and aquifer systems in the study area. Numerical 

flow models simulate ground-water flow and response 

of a ground-water system to stress by using information on aquifer properties, boundary conditions, and 

human-induced development to solve mathematical 

equations that quantify directions and rates of 

ground-water flow, water-level changes, streamaquifer interactions, and effects of wells (Bachmat 

and others, 1980, p. 19).

The numerical model selected for this study is a 

finite-difference model termed, "The U.S. Geological 

Survey Modular Ground-Water Flow Model" 

(McDonald and Harbaugh, 1988). It is designed to 

simulate three-dimensional movement of ground water 

of constant density through porous material. Hereafter in this report, the numerical model will be referred 

to as the model.

The aquifer or aquifer systems simulated may be 

composed of heterogeneous and anisotropic materials, 

may have steady-state or transient flow regimes, and 

may have irregular boundaries. The model has the 

capability of using either of two solution procedures 

[strongly implicit (SIP) or slice-successive overrelaxation (SSOR)] to approximate the solution of the following equation:

3~ 

6y

_ ~ dh

where

x, y, and z are Cartesian coordinates aligned 

along the major axes of hydraulic 

conductivity Kxx, Km , Kzz (LT"1 );
C48 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

h is the hydraulic head (L); 

W is volumetric flux per unit volume and 

represents sources or sinks of water

(T1 );

<SS is the specific storage of the porous

media (L"1 ); and 

t is time (T).

The term "modular" refers to the computerprogram structure of the model, which consists of a 

main program and a series of independent subroutines 

called modules. The modules are grouped into packages that deal with specific features of the hydrologic 

system to be simulated. Packages included in the 

model are those for simulating block-centered flow, 

rivers, natural recharge, wells, drains and springs, 

evapotranspiration, and head-dependent flux boundaries (termed general-head boundaries in the model 

documentation).

MODEL FRAMEWORK

The solution of equation 5 requires that the Cartesian coordinates of the model grid be aligned with the 

principal directions of the hydraulic-conductivity tensor. Ward (1968, p. 21) indicates a major structural 

lineament orientation in south-central Kansas that 

strikes N. 50°-70°E., with a mean strike of N. 60°E. 

A second set of lineaments oriented at an average 

value of 93° from the first set of lineaments strikes 

northwest between extremes of N.24°-56°W., with a 

mean and mode strike of N. 35°W. Ward (1968) also 

points out that the joint system studied appears to 

extend from southern Oklahoma to northeastern Kansas and western Missouri. Ward (1968) concluded 

that the joints may have formed as a result of northwest, horizontal, compressive forces. The lineaments 

are, in general, fractures. Because hydraulic conductivity tends to be greatest along the directions of 

greatest fracturing, the model grid was oriented to 

match the two principal directions of the fractures.

The model grid was oriented to coincide with the 

principal directions of lineaments, N. 35°W., from a 

point in east-central Kansas at the juncture of 

latitude 39°N. and longitude 96° W. As a consequence, 

the orthogonal column orientation from that juncture 

is N. 55°E. It is believed that the grid orientation for 

the model reasonably corresponds to the principal 

hydraulic-conductivity tensor of the aquifer system to 

be simulated. The model grid has 28 rows, 33 columns, and a uniform node spacing of 28 mi (fig. 44).

RELATION OF MODEL LAYERS AND 

GEOHYDROLOGIC UNITS

Five principal geohydrologic units in the regional 

study area are represented in the model as layers 

(fig. 45). From top to bottom:

Layer 1. Surficial units, which consist of the High 

Plains aquifer (Gutentag and others, 1984), glacial 

drift, and the Great Plains confining system. 

Layer 2. The Great Plains aquifer system. 

Layer 3. The Western Interior Plains confining

system.

Layer 4. The upper unit in the Western Interior 

Plains aquifer system in the Plains subregion and the 

Springfield Plateau aquifer in the Ozark subregion. 

Layer 5. The lower units in the Western Interior 

Plains aquifer system in the Plains subregion and a 

combined unit in the Ozark subregion consisting of 

the Ozark aquifer, the St. Francois confining unit, 

and the St. Francois aquifer.

The regional model layer 4, the upper unit in the 

Western Interior Plains aquifer system, continues into 

the Ozark subregion as an equivalent layer simulating 

the Springfield Plateau aquifer. The rocks that are 

equivalent to the lower units in the Western Interior 

Plains aquifer system are the Ozark aquifer, the St. 

Francois aquifer, and the intervening St. Francois 

confining unit. In some areas there is a predominantly 

shale confining unit that impedes water flow between 

layers 4 and 5. This unit includes the Chattanooga 

Shale that occurs between the upper and lower units 

of the Western Interior Plains aquifer system and the 

Ozark confining unit that occurs between the 

Springfield Plateau aquifer and the Ozark aquifer.

BOUNDARY CONDITIONS OF MODEL LAYERS

Four types of boundary conditions affecting flow 

were used in the regional model. They were: (1) no 

flow, (2) specified head, (3) general-head boundary 

(head-dependent flux), and (4) specified flow. A 

description of actual geohydrologic boundary conditions and the assignment of corresponding lateral and 

vertical boundary conditions for each layer in the 

model are described in the following sections.

LAYER 1

The top layer of the flow model (fig. 46) simulates 

a composite hydrologic unit consisting of small areas 

of alluvium and glacial drift, the High Plains aquifer, 

plus shale of the underlying Great Plains confining 

system. Layer 1 is treated as a water source or sink 

and a confining upper boundary to the Great Plains 

aquifer system. Therefore, a specified-head boundary 

condition was used for all cells in layer 1. The data
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C49

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 44. Location of model grid and study area.

for the specified hydraulic heads were obtained by discretizing a predevelopment potentiometric-surface 

map of the geohydrologic unit overlying the regional 

aquifer systems. These data are represented by a 

machine plot of the potentiometric surface of model 

layer 1 (fig. 47).

The leakance coefficient between layer 1 and layer 

2 was computed at all locations where the Great 

Plains confining system is present using confining-system thickness and the value for vertical hydraulic conductivity. The thickness of the confining system was 

computed as the difference between the altitude of the 

base of the High Plains aquifer (Gutentag and others,
C50 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

SURFICIAL 

UNITS

GREAT 

PLAINS 

AQUIFER 

SYSTEM

WESTERN

INTERIOR

PLAINS

CONFINING 

SYSTEM

WESTERN 

INTERIOR 

PLAINS 

AQUIFER 

SYSTEM

OZARK

PLATEAUS

AQUIFER

SYSTEM

BASEMENT

CONFINING

UNIT

PRINCIPAL GEOHYDROLOGIC UNITS 

PLAINS SUBREGION

HIGH PLAINS AQUIFER and 

OTHER SURFICIAL UNITS

GREAT PLAINS 

CONFINING SYSTEM

MAHA AQUIFER

APISHAPA CONFINING UNIT

APISHAPA AQUIFER

(No subunits delineated)

Upper unit

Confining unit

Lower unit

(No subunits)

OZARK SUBREGION I ___ PL

EQUIVALENT MODEL UNITS 

PLAINS SUBREGION OZARK SUBREGION |

(No subunits delineated)

SPRINGFIELD PLATEAU AQUIFER

Confining unit

OZARK AQUIFER

ST. FRANCOIS CONFINING UNIT

ST. FRANCOIS AQUIFER

(No subunits)

LAYER 1

(Specified

head)

LAYER 2 

(Active)

LAYERS

(Partly active, partly 

specified head)

LAYER 4

(Active) |_____ 

/////// / / / / /Y / / / / /

LAYER 5 

(Active)

NO-FLOW BOUNDARY

EXPLANATION

LEAKANCE ARRAY-Calculated as harmonic 

mean of vertical hydraulic conductivities 

of adjacent layers

LEAKANCE ARRAY-Calculated as ratio of 

estimated vertical hydraulic conductivity of 

confining unit to thickness of confining unit

OUTCROP AREA-Area of direct recharge

FIGURE 45. Generalized model layers and relation to principal geohydrologic units in Plains and Ozark subregions 

(boundary conditions of model layers shown in figures 46 and 48-51).

1984, fig. 6, p. 18) and the top of the Great Plains 

aquifer system (Helgesen and others, 1993, pi. 4). 

The glacial drift is predominantly slightly permeable 

clay; therefore, it also is considered a confining unit. 

Where glacial drift is present and where the Upper 

Cretaceous shale crops out, the confining-system 

thickness is calculated as the difference between the 

potentiometric head in layer 1 (fig. 47) and the altitude of the top of one of the three underlying geohydrologic units the Great Plains aquifer system, the 

Western Interior Plains confining system, or the 

Western Interior Plains aquifer system.

An estimate of the vertical hydraulic conductivity 

of the Great Plains confining system needed for computation of leakance factors was obtained by multiplying the lateral hydraulic conductivity of the Great 

Plains aquifer system (fig. 36) by IxlO'6 . This constant multiplier is suggested by consideration of the

4-to-6 order-of-magnitude difference between hydraulic conductivity of shale and cemented sandstone 

(table 3). Davis (1969, p. 70, table 4) gives a value of 

LlxlO"10 ft/s for the hydraulic conductivity of Cretaceous shale (lateral or vertical not specified). 

Bredehoeft and others (1983, p. 20, table 3) give a 

modeled best-fit value for vertical hydraulic conductivity of Cretaceous shale in South Dakota as 

5xlO"n ft/s. The hydraulic conductivity of the Great 

Plains aquifer system generally would require a multiplier of about 1x10 "6 to approximate the published 

estimates of hydraulic conductivity of shale, which is 

consistent with the information in table 3.

The intrinsic permeability of the Great Plains 

aquifer system tends to decrease with depth; that is, 

the thicker the overburden, the smaller the intrinsic 

permeability (figs. 36, 39). A similar permeabilitydepth relation also exists for Upper Cretaceous shale
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C51

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

Extent of High 

Plains aquifer

Extent of Great Plains 

confining system

ALLUVIUM AND GLACIAL DRIFTHigh Plains aquifer absent

HIGH PLAINS AQUIFER-Great 

Plains confining system absent

HIGH PLAINS AQUIFER AND GREAT 

PLAINS CONFINING SYSTEM

GREAT PLAINS CONFINING SYSTEMHigh Plains aquifer absent

SURFICIAL UNITS (Layer 1) ABSENT

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 46. Model layer-1 grid and areal extent of alluvium and glacial drift, the High Plains aquifer, and the Great Plains

confining system.
C52 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

40°

35

105°

A .» iv Vi

100°

I ..* . .J,

95° 90°

)\«-r

0 100 200 KILOMETERS

-s

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

MODEL LAYER BOUNDARY

INTERIOR MODEL CELL WHERE LAYER 1 IS 

ABSENT

 2000 PREDEVELOPMENT POTENTIOMETRIC CONTOURShows alitiude at which water level would have 

stood in tightly cased wells. Contour interval, in 

feet, is variable. Datum is sea level

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 47. Potentiometric surface of model layer 1 (machine plot of digitized data).
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C53

Table 3. Commonly observed ranges of permeability in several rock types (from Davis, 1969)

[Hydraulic conductivity (K) at 1.005 centipoise]

,-16 r!2 v9 1 fl- Square meter 10' 19 10' 10 10' 1Z 10

Square foot 10' 18 10'° 10' 11 10'8 10

Millidarcy 10'4 10'3 10'2 10' 1 1 101 102 103 104 105 106 107 10

,-6

Unconsolidated 

rocks

Consolidated 

rocks

Permeability 

characteristics

1

Unweathered clay 

and unweathered 

glacial drift.

Shale at depths 

greater than 250 

feet; evaporite 

deposits; limestone 

at depths greater 

than 15,000 feet.

Very slightly 

permeable.

1 1

Very fine sand; silt; 

mixtures of sand, silt, and 

clay; unstratified clay.

Sandstone; dolostone; 

limestone; fractured rocks; 

shale at depths less than 

250 feet.

Slightly permeable.

"Clean" sand; 

mixtures of "clean" 

sand and gravel.

Extensively 

fractured rock; 

weathered 

limestone or 

dolostone.

Permeable

1 "Clean" 

well-sorted 

gravel or 

cobble.

Rocks with 

large 

openings.

Very 

permeable.

Gallons per day 

per square foot

Foot per day 

Foot per second

lC-5 10'4 10'3 10'2 10' 1 1 101 102 103 104 105 106

10'6 10'5 10'4 10'3 10'2 10' 1 1 101 102 103 104 105

1Q-10 1Q-9 1Q-8 1Q-7 1Q-6 1Q-5 1Q-4 1Q-2

(Neuzil and others, 1984, p. 116, fig. 5). Assuming 

that the relation of permeability to depth for both 

shale and sandstone of the Great Plains aquifer system generally is valid, multiplying lateral hydraulic 

conductivity of the Great Plains aquifer system by a 

constant to determine vertical hydraulic conductivity 

of the Great Plains confining system also relates vertical hydraulic conductivity of the confining system to 

depth of burial.

Where the Great Plains confining system and glacial drift are not present, and permeable units such as 

alluvium and the High Plains aquifer are in direct 

contact with permeable materials of the Great Plains 

aquifer system and other sediments in layer 2, the 

computed leakance coefficient is the harmonic mean of 

vertically adjacent layers for the uniformly spaced 

model grid. In those locations, the thickness of layer 1 

is computed also as the difference between the potentiometric head in layer 1 and the altitude of the top of 

the immediately underlying unit.

The vertical hydraulic conductivity used in computing leakance coefficients where the Great Plains 

confining system (Upper Cretaceous shale) and glacial 

drift are not present (and, therefore, permeable materials of layers 1 and 2 are in direct contact) was based 

on assumed ratios of vertical-to-horizontal permeability. Generally, vertical permeability is less than horizontal permeability in unf ractured sedimentary rocks.

Gutentag and others (1984, p. 23) conclude that, 

on a regional scale, the sediments comprising the High 

Plains aquifer are distributed randomly in the vertical 

section. Cronin (1964, p. 33) presents observations 

that the individual beds or lenses of silt, sand, gravel, 

and clay of the Ogallala Formation (the major geologic unit of the High Plains aquifer) in the southern 

High Plains of Texas and New Mexico are not continuous over wide areas. This is generally true of the 

High Plains aquifer. Because there are clay lenses that 

apparently are not areally extensive, a vertical-to-lateral ratio of hydraulic conductivity in model layer 1 

was specified as 0.1. This hydraulic-conductivity ratio
C54 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

was applied to areas in layer 1 where permeable materials directly overlie the Great Plains aquifer system 

or other underlying aquifers.

LAYER 2

The second layer of the flow model (fig. 48) simulates flow in the Great Plains aquifer system, which 

includes the Maha and Apishapa aquifers and the 

Apishapa confining unit. General-head, constant-flow 

(recharge), and no-flow boundaries are used in layer 2 

for simulation of the Great Plains aquifer system.

Along the eastern extent of the Great Plains aquifer system in northeastern Nebraska, central and 

western Kansas, the Oklahoma Panhandle, and the 

Texas Panhandle, the aquifer system pinch-out is well 

defined. No-flow boundaries in model layer 2 simulate 

that condition. For each cell that simulates outcrop 

areas of the Great Plains aquifer system, general-head 

and constant-flow (recharge) boundaries are used to 

account for both stream controls and direct recharge 

from precipitation. The aquifer system crops out in a 

small area at its eastern extent and in a much larger 

area in southeastern Colorado and northeastern 

New Mexico (fig. 48).

Model cells representing areas along the Front 

Range of the Rocky Mountains in New Mexico, 

northern Colorado, and southeastern Wyoming are 

no-flow boundaries (fig. 48) because those cells represent the extent or lateral termination of the Great 

Plains aquifer system. Those cells representing outcrop locations in those areas are general-head and 

recharge boundaries. Outcrop locations in northern 

Colorado and southeastern Wyoming are in narrow 

bands where the rocks of the aquifer system were 

uplifted and exposed. There are no data available to 

indicate the amount of inflow that occurs, but because 

of the small amount of surface exposure, inflow or 

recharge is likely to be small.

At the northwest corner of Nebraska and southwest corner of South Dakota, and in the Black Hills 

Uplift just north of the study area where the Great 

Plains aquifer system crops out, the potentiometric-surface map (fig. 34) indicates the potential for 

recharge. Miller and Rahn (1974) estimate about 

5 ft3/s of recharge for the entire outcrop area of the 

Dakota Sandstone (the major component of the Maha 

aquifer) on the east side of the Black Hills; however, 

most of that recharge remains in South Dakota and 

does not enter the study area. Case (1984, p. 158) estimated ground-water movement from Wyoming into 

South Dakota in the aquifer south of the Black Hills 

as 0.7 ft 3/s. Cells representing that area are specified

as general-head boundaries; however, recharge is 

expected to be small.

The Great Plains aquifer system extends beyond 

the limit of the regional study area along the State 

line between Nebraska and South Dakota and at the 

northeast corner of Nebraska. In general, the potentiometric-surface map (fig. 34) implies flow parallel to 

or nearly parallel to the South Dakota-Nebraska State 

line, indicating little or no flow across the Nebraska 

State line. Therefore, a no-flow boundary condition is 

specified for cells in layer 2 along the South DakotaNebraska State line.

The ratio of vertical-to-lateral hydraulic conductivity for layer 2 was assumed to be 1:20. The basis 

for this assumption is the stratification of the Great 

Plains aquifer system. Helgesen and others (1982) 

indicate that the Great Plains aquifer system has substantial lithologic heterogeneity consisting of interbedded sandstone, siltstone, and mudstone, which 

commonly exhibit local facies changes associated with 

terrestrial and nearshore marine environments of deposition. Because of these conditions, vertical hydraulic 

conductivity would be relatively small as compared to 

lateral hydraulic conductivity. The ratio of vertical-to-lateral hydraulic conductivity is required to 

compute the vertical hydraulic conductivity and thus 

the leakance coefficient between layer 2 and overlying 

units where permeable materials are in direct hydraulic contact. However, for most of the area of the 

Great Plains aquifer system, vertical leakage is controlled by the Great Plains confining system (fig. 32) 

represented in model layer 1 and evaporite deposits 

included in the Western Interior Plains confining system represented in layer 3 (fig. 30).

LAYERS

The third layer of the numerical model represents 

the Western Interior Plains confining system, a thick 

section of mostly Pennsylvanian and Permian rocks 

(fig. 49). The rocks are interbedded shale, limestone, 

sand, and evaporite deposits. The predominant 

hydraulic characteristic of the confining system is very 

small vertical permeability owing to rocks such as 

extensive halite and gypsum deposits. Because of the 

generally very small vertical permeability, the Western Interior Plains confining system is simulated by 

model layer 3 as a vertical-flow restriction in model 

cells representing areas where the system is buried. 

For cells representing outcrop areas, the model layer is 

an upper confining boundary and specified head. Zero 

leakance between layer 2 and layer 3 is used in the 

model to simulate areas where evaporite deposits are 

present (fig. 30). A non-zero leakance coefficient
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C55

105° 100 95°

35

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

OUTCROP AREAS

-'- \

jjf General-head and recharge boundaries

ACTIVE CELL WITHOUT A BOUNDARY CONDITION 

INACTIVE CELL

BURIED AREAS-Any cell that has a common APPROXIMATE BOUNDARY BETWEEN THE 

face with an inactive cell and has no other PLAINS AND OZARK SUBREGIONS 

imposed boundary condition is termed a 

"no-flow" boundary

O General-head boundary 

\^ No-flow boundary

FIGURE 48. Model layer-2 grid representing areal extent and boundary conditions of the Great Plains aquifer system.
C56 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Base modified from U.S. Geological Survey 

1:7,500.000. National Atlas, 1970

EXPLANATION

O OUTCROP AREA-Specified-head boundary 

O GENERAL-HEAD BOUNDARY

^ NO-FLOW BOUNDARY-Any cell that has a common 

face with an inactive cell and has no other imposed 

boundary condition is termed a "no-flow" boundary

\ ACTIVE CELL WITHOUT A BOUNDARY 

CONDITION

INACTIVE CELL

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 49. Model layer-3 representing areal extent, outcrop area, and boundary conditions of the Western Interior

Plains confining system.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C57

between layers 2 and 3 is computed on the basis of 

mapped intrinsic permeability of the Western Interior 

Plains confining system (fig. 29) to represent an area 

where evaporite deposits are not present, assuming a 

vertical-to-lateral hydraulic-conductivity ratio of 

I:1.67xl06. However, leakance coefficients between 

layer 3 and underlying layer 4 were computed only on 

the basis of the Western Interior Plains confining system vertical-to-lateral hydraulic-conductivity ratio of 

I:4.17xl07 and the thickness of layer 3.

Layer 3 of the model simulates flow in an outcrop 

area second in size only to layer 1. The specified head 

in the cells that represent the outcrop area are the 

shallow ground-water levels (approximate water table) 

(fig. 31). An extremely small leakance coefficient is 

used between layers 3 and 4. Thus, the specified 

heads of layer 3 can simulate a source or sink of water 

to or from the near-surface water table or to or from 

underlying aquifers represented by layer 4 or 5. 

Because the initial hydraulic head for cells simulating 

the outcrop area is specified, that area of the model 

"intercepts" all other inputs or outputs; thus no flows 

related to springs, drains, streams, recharge, or withdrawals affect the simulation in these cells.

The computed hydraulic head represents the entire 

thickness of the confining system at its centerline. 

The hydraulic head in a confining unit actually varies 

in the vertical, and a single value cannot represent the 

hydraulic head throughout the entire thickness. 

Therefore, the model-computed hydraulic head cannot 

be correlated directly to measured water levels unless 

the measured data were acquired at the centerline of 

the confining unit. Simulation of steady-state conditions should result in computed hydraulic-head values 

that are midway between the values in the overlying 

and underlying layers where a vertical connection is 

allowed. However, where no vertical connection with 

layer 2 above the confining unit is allowed, computed 

hydraulic head is affected slightly by the calculated 

leakage to or from underlying layer 4.

General-head boundaries are specified at the lateral 

extent of layer 3 from the northwest corner of 

Nebraska southward to northern New Mexico. Also, 

cells representing the extent of the confining system 

across the Texas Panhandle and 30 mi into Oklahoma 

are general-head boundaries. In eastern Nebraska and 

northeastern Kansas, general-head boundaries were 

used for cells near the Missouri River where water levels are nearly constant and hydraulic continuity 

exists. Along the State line between Nebraska and 

South Dakota, the study boundary was simulated as 

no flow.

The rationale for these boundary conditions was 

based on preliminary data from drill-stem tests that

show hydraulic gradients from west to east generally 

paralleling the western extent of the confining system 

in the Panhandle of Texas. The hydraulic-head contours are normal to the Nebraska-South Dakota State 

line, indicating flow approximately parallel to the 

State line. There is no information on inflows or outflows at that location in the Western Interior Plains 

confining system, and specification of general-head 

boundaries allowed the model to compute flow rates 

that were consistent with the available hydraulic-head 

data.

Data collected for the Western Interior Plains confining system show a geopressure area in the Anadarko 

Basin of west-central Oklahoma. Specifically, the 

geopressure exists in the lowest rocks in a very thick 

section of the Western Interior Plains confining system. The source of the high pressure is not simulated 

in model layer 3, but in layer 4 as explained below.

LAYER 4

The upper unit in the Western Interior Plains 

aquifer system and the Springfield Plateau aquifer in 

Mississippian-age rocks are simulated in the model by 

layer 4 (fig. 50). The Mississippian-age rocks are predominantly limestone with some sandstone and shale 

layers. The rocks have slight permeability where 

deeply buried and are permeable in outcrop areas in 

the Ozark subregion.

Cells that simulate outcrop areas in layer 4 are 

subjected to a constant-flow boundary condition by 

the recharge option of the model. No-flow boundaries 

are specified for the cells at the limit of layer 4 

(fig. 50), principally where the rocks of the upper unit 

in the Western Interior Plains aquifer system pinch 

out. General-head boundaries (fig. 50) were used 

along the southern and western edges where the upper 

unit generally extends to the study-area limits.

In the Anadarko Basin of west-central Oklahoma, 

there is a geopressure zone (pi. 2) that results from 

overpressuring in the overlying layer 3. The overpressuring cannot be readily explained. Because the pressure exists immediately above the upper unit in the 

Western Interior Plains aquifer system and vertically 

much closer to the centerline of the rocks simulated in 

layer 4 than to the centerline of the rocks simulated 

by layer 3, the pressure source is more effectively simulated in layer 4. Simulation is by a general-head 

boundary using the hydraulic-head values digitized 

from map data (pi. 2) as the controlling hydraulic 

head and an arbitrary large head-dependent flux coefficient. Using the general-head option of the model 

allows head-dependent lateral flow in layer 4 and vertical flow to layers 3 and 5 due to large hydraulic-head
C58 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105'

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

x ACTIVE CELL WITHOUT A BOUNDARY CONDITION 

INACTIVE CELL

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

OUTCROP AREAS

(5!'' General-head and recharge boundaries 

<£ Recharge boundary

BURIED AREAS-Any cell that has a common 

face with an inactive cell and has no other 

imposed boundary condition is termed a 

"no-flow" boundary

0> General-head boundary

^ Overpressure, general-head boundary

V- Geohydrologic unit absent

^ No-flow boundary

FIGURE 50. Model layer-4 grid representing areal extent and boundary conditions of the upper unit in the Western 

Interior Plains aquifer system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C59

values from an overpressure. The imposed conditions 

produce the effect of overpressure on the simulated 

flow in layer 4.

LAYER 5

The lower units in the Western Interior Plains 

aquifer system are simulated by model layer 5 

(fig. 51). The rocks simulated are primarily thick 

sections of dolostone, limestone, and sandstone of 

Devonian, Silurian, Ordovician, and Cambrian age. 

Permeability is very small in deep sections in the 

western part of the study area and in the Anadarko 

Basin, increasing by about three orders of magnitude 

across Kansas and Nebraska from south to north to 

slightly permeable (fig. 20). The rocks are more permeable in the Ozark Uplift area, particularly where 

the Ozark and St. Francois aquifers crop out and have 

been subjected to weathering.

No-flow boundaries are specified at the northwestern extent of layer 5 in northeastern Colorado and 

central and northeastern Nebraska. Toward the 

northeastern part of the study area, the lower units in 

the Western Interior Plains aquifer system continue 

beyond the study area under the Missouri River 

trench into western Iowa and central Missouri. 

Where the Missouri River is incised directly into these 

lower units, hydraulic connection exists between the 

river and these units. The exchange of water between 

the river and the lower units is simulated using a general-head boundary. General-head boundaries also are 

used to simulate the continuation of the lower units 

generally to the southern and western extents of the 

study area. Recharge (constant-flow) cells, ranging 

from -393 to 544 ft3/s, simulate the outcrop area, the 

rates of which are based on an analysis presented by 

Jorgensen and others (1989a, b).

SIMULATION OF VARIABLE-DENSITY FLOW

The modeled region includes geohydrologic units 

that contain water with large concentrations of dissolved solids. Water at temperatures exceeding mean 

annual temperature also is present, especially at great 

depths. Large concentrations of dissolved solids in 

water cause it to have greater density than freshwater. 

Because temperatures at great depths generally exceed 

the mean annual temperature, water density is 

reduced. These two conditions may have offsetting 

effects on deeply buried aquifers containing water 

having large concentrations of dissolved solids; however, it is more likely that these conditions result in a 

variable fluid density that is different from uniform-temperature freshwater conditions. The flow

equation (eq. 5) is based on a mathematical development valid only for a constant, uniform-density fluid. 

Weiss (1982) developed a computer program that generates source and sink input to ground-water models 

to enable them to simulate the effect of variabledensity ground-water flow. For application to the 

CMRASA digital model, Weiss1 code was modified to 

handle a multilayer system; however, the computational technique was unchanged.

MODEL CALIBRATION

Calibration of the model herein refers to the process in which the model-input information needed for 

simulation is modified (or calibrated) such that the 

model is able to simulate measured or independently 

calculated data of the geohydrologic system being 

modeled. Neither is it meant that the model simulations are unique nor that the model will accurately 

simulate conditions other than those specified for the 

calibration. Calibration was accomplished by matching model-computed hydraulic-head values to either 

measured hydraulic head or, in lieu of measured head, 

independently estimated hydraulic head. Thus, the 

measured or estimated head, collectively referred to 

herein as "field hydraulic head," is that which is compared to the model-computed hydraulic head to evaluate a simulation. The field hydraulic-head 

distributions used for calibration are assumed to represent steady-state (predevelopment) conditions. As 

discussed in the section entitled "Hydraulic Head," 

considerable water-level data are available in the shallow parts of aquifers. However, head data are scarce 

in deeper, saline parts of the aquifers, and the predevelopment potentiometric surfaces are, in part, 

estimated.

CALIBRATION CRITERIA

Calibration criteria of the model were based on 

closeness of fit of model-computed hydraulic head to 

field hydraulic head. Regional outflow and inflow at 

the boundaries and subregional-model hydraulic-head 

values and flows also were considered. A criteria of 

average difference from the field hydraulic head 

of ±1 ft and a standard deviation of 300 ft was specified for calibrating hydraulic head in layers 2, 4, and 

5. Layer 1 is a specified-head source-sink layer 

(fig. 47); therefore, computed hydraulic-head values 

for that layer were the field hydraulic-head values. In 

layer 3, the outcrop area was a specified-head boundary (fig. 49); accordingly, the field hydraulic-head values were the computed hydraulic-head values for those 

particular nodes. Hydraulic-head values in layer 3
C60

N N V N * " XV__V-->-; 

N ^ \ N N cJ^

Vrv*^ x & ^ev^er7^

Base modified from U.S. Geological Survey 

1:7,500,000. National Atlas, 1970

EXPLANATION

OUTCROP AREAS

General-head and recharge boundaries 

Recharge boundary

BURIED AREAS-Any cell that has a common 

face with an inactive cell and has no other 

imposed boundary condition is termed a 

"no-flow" boundary

General-head boundary 

Geohydrologic unit absent 

No-flow boundary

x ACTIVE CELL WITHOUT A BOUNDARY CONDITION 

INACTIVE CELL

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 51. Model layer-5 grid representing areal extent and boundary conditions of the lower units in the Western 

Interior Plains aquifer system and the Ozark and St. Francois aquifers in the Ozark Plateaus aquifer system.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C61

where it is a subsurface unit were computed by the 

model. However, adjustments to the leakance arrays 

controlling leakage to and from layer 3 were made 

with the objective of obtaining computed hydraulichead values in layers 2, 4, and 5 within the calibration 

criteria, and consideration of the computed hydraulichead values in layer 3 was incidental to that objective.

The largest estimated regional water-budget components were for surface recharge. Estimated maximum values of recharge are indicative of a 

water-resource potential, but it is unlikely that 

amount of recharge would occur on a sustained basis. 

Lateral flow at aquifer boundaries simulated by general-head boundaries was fairly large at the boundary 

near the Mississippi Alluvial Plain and the Missouri 

and Mississippi Rivers. Vertical leakage, although 

generally small for individual cells, was significant for 

entire layers, particularly in aquifers in the Ozark subregion where a vigorous, freshwater flow system predominates.

Calibration criteria for water-budget components 

were not defined because of the extent of the regional 

study area and the lack of data. However, if the calibration criteria for matching simulated hydraulic 

heads to field hydraulic heads can be met using aquifer-characteristic values within the prototypeto-model range of variability, the model-computed 

components of the budget, such as outflow to rivers 

and extent boundaries, may be assumed to reasonably 

simulate the water budget. From the recharge analysis in the Ozark subregion, quantities of recharge and 

discharge were established on a cell-by-cell basis.

Geohydrologic units represented by layers 4 and 5 

crop out in the Ozark subregion, and the outcrop areas 

are recharged directly from precipitation. Most of the 

net flow to the aquifers in the Ozark subregion is subsequently discharged to major rivers, such as the 

Missouri, Mississippi, and Arkansas, and to vertical 

leakage in the western part of the Ozark subregion 

and across the boundary between the Ozark subregion 

and the Mississippi Alluvial Plain.

In recharge areas of the Great Plains aquifer system represented by the regional model layer 2, net 

recharge values were uniformly reduced by the same 

factor as for the Ozark subregion. The stream-aquifer 

interaction values, however, were unknown. Some 

outcrop-area recharge rates, such as those occurring in 

hogback areas along the Rocky Mountain Front 

Range, were analyzed by Helgesen and others (1993), 

and those values were used.

HYDRAULIC HEAD

Comparison of computed hydraulic heads from the 

model to the field hydraulic heads for layers 2, 4, and 

5, was made numerically (table 4) and graphically 

(figs. 52-57). The effects of variable density will be 

discussed separately.

For layer 2, the average deviation from field 

hydraulic-head values (table 4) is less than 0.2 ft. 

The standard deviation of about 160 ft indicates, considering the average deviation from field values 

(0.2 ft) is in essence zero, that 95 percent of all deviations from the field hydraulic head are less than 

320 ft. Another way to evaluate the variability is 

that 95 percent of all values of computed hydraulic 

head lie in the interval of the average plus or minus 

two standard deviations (Snedecor, 1957, p. 36).

There are four areas of major deviation from field 

hydraulic heads for layer 2, as shown in figures 52 and 

53. In two areas, model-computed head values are 

300 to 400 ft higher than the field values. These areas 

are located in east-central and northern Colorado 

where the Great Plains aquifer system is deeply buried. Two areas have computed head values significantly smaller than the field head values. One area is a 

strip in central Colorado along the Rocky Mountains 

where most of the lateral boundaries for layer 2 are 

no-flow boundaries. Along the mountains, the Great 

Plains aquifer system dips toward the east, and the 

field hydraulic-head values may not truly define the 

hydraulic-head distribution in the layer. Also, the 

McDonald and Harbaugh (1984) model assumes

TABLE 4. Statistical comparison of model-computed hydraulic head to field hydraulic head

Statistics for differences between computed and field hydraulic head

Average deviation *

(feet)

Model 

layer

2 

4 

5

Number of 

values

248 

301 

382

Constant 

density

0.17 

-.82 

-.13

Variable 

density 2

-5.87 

30.22 

40.82

Mean square deviation 

(square feet)

Constant 

density

24,956 

54,785 

90,561

Variable 

density

24,331 

48,720 

80,276

Standard deviation

(feet)

Constant 

density

157.97 

234.06 

300.93

Variable 

density

155.98 

220.73 

283.33

'Negative average indicates that the computed hydraulic-head average is less than the field hydraulic head. 

2Model computation includes "pseudosources" (Weiss, 1982).
C62 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

MODEL-LAYER BOUNDARY

 2500 . MODEL-COMPUTED HYDRAULIC-HEAD 

CONTOUR-Interval, in feet, is variable. 

Datum is sea level

 2500 FIELD HYDRAULIC-HEAD CONTOURInterval, in feet, is variable. Datum is 

sea level

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 52. Model-computed and field hydraulic head for model layer 2 representing the Great Plains aquifer system

(predevelopment steady-state conditions).
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C63

105° 100° 951

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

MODEL-LAYER BOUNDARY

 '°° LINE OF EQUAL DIFFERENCE BETWEEN MODEL-COMPUTED 

AND FIELD HYDRAULIC HEAD (MODEL-COMPUTED HEAD IS 

BELOW FIELD HEAD)-Interval, infect, is variable

 ico LINE OF EQUAL DIFFERENCE BETWEEN MODEL-COMPUTED 

AND FIELD HYDRAULIC HEAD (MODEL-COMPUTED HEAD IS 

ABOVE HELD HEAD)-Interval 100 feet

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 53. Difference between model-computed and field hydraulic head for model layer 2 representing the Great Plains

aquifer system.
C64 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

V

EXPLANATION

MODEL-LAYER BOUNDARY

 ,o»~ MODEL-COMPUTED HYDRAULIC-HEAD 

CONTOUR-Interval, in feet, is variable. 

Datum is sea level

 1000 FIELD HYDRAULIC-HEAD CONTOURInterval, in feet, is variable. Datum is 

sea level

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 54. Model-computed and field hydraulic head for model layer 4 representing the upper unit in the Western Interior 

Plains aquifer system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system (predevelopment steady-state 

conditions).
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C65

105° 100° 95° _9fi!

40'

35'

V> >M ; 4 .i A»t| 'VMSi**

100 200 MILES

I I I

0 100 200 KILOMETERS

^v^^-^V\^^A^ I

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

MODEL-LAYER BOUNDARY

_,:0 UNE op EQUAL DIFFERENCE BETWEEN MODEL-COMPUTED 

AND FIELD HYDRAULIC HEAD (MODEL-COMPUTED HEAD IS 

BELOW FIELD HEAD)-Interval, in feet, is variable

ro LINE OF EQUAL DIFFERENCE BETWEEN MODEL-COMPUTED 

AND FIELD HYDRAULIC HEAD (MODEL-COMPUTED HEAD IS 

ABOVE FIELD HEAD)-Interval, in feet, is variable

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 55. Difference between model-computed and field hydraulic head for model layer 4 representing the upper unit in the 

Western Interior Plains aquifer system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system.
C66 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105° 100° 95°

35'

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

/\/\

EXPLANATION

MODEL-LAYER BOUNDARY

 2000 MODEL-COMPUTED HYDRAULIC-HEAD 

CONTOUR-Interval, in feet, is variable. 

Datum is sea level

 2000 FIELD HYDRAULIC-HEAD CONTOURInterval, in feet, is variable. Datum is 

sea level

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 56. Model computed and field hydraulic head for model layer 5 representing the lower units in the Western Interior 

Plains aquifer system and the Ozark and St. Francois in the Ozark Plateaus aquifer system (predevelopment steady-state 

conditions).
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C67

40°

35'

105° 100° 95° 90°

0 100 200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

\

EXPLANATION

MODEL-LAYER BOUNDARY

 100 LINE OF EQUAL DIFFERENCE BETWEEN MODEL-COMPUTED 

AND FIELD HYDRAULIC HEAD (MODEL-COMPUTED HEAD IS 

BELOW FIELD HEAD)- Interval, in feet, is variable

 ;oo LINE OF EQUAL DIFFERENCE BETWEEN MODEL-COMPUTED 

AND FIELD HYDRAULIC HEAD (MODEL-COMPUTED HEAD IS 

ABOVE FIELD HEAD)-Interval, in feet, is variable

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 57. Difference between model-computed and field hydraulic head for model layer 5 representing the lower units in the 

Western Interior Plains aquifer system and the Ozark and St. Francois aquifers in the Ozark Plateaus aquifer system.
C68 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

lateral flow, which is not true for steeply dipping 

strata. Therefore, the poor match of model-computed 

hydraulic head to field hydraulic head along the 

Rocky Mountain Front Range is expected. The second area of smaller computed heads is in north-central 

Nebraska along the Nebraska-South Dakota State 

line. The model cells along the State line are specified 

also as no-flow boundaries because it was assumed 

that flow in the aquifer system is parallel or nearly 

parallel to the State line. The flow direction or path 

was inferred from the predevelopment potentiometric 

surface (fig. 34). Field hydraulic-head values indicate 

inflow from the outcrop area near the Black Hills 

Uplift at the extreme northwest corner of Nebraska, 

and the boundary conditions of the model specified 

that condition. However, the model-computed 

hydraulic-head values did not match the field head 

values. It is concluded that, along the western 

one-half of the Nebraska-South Dakota State line, 

inflow across the line may be adequate to maintain 

higher heads in layer 2 than those calculated by the 

model. Over the remaining area of layer 2, differences 

between field and model-computed head values are 

less than 100 ft. In general, it is concluded that the 

flow system was simulated accurately in regard to 

matching field hydraulic heads.

The average deviation of model-computed head 

values from field heads for layer 4 is about -0.8 ft 

(table 4). The standard deviation is slightly more 

than 234 ft, indicating that 95 percent of all 

model-computed head values are within 468 ft of the 

field head values. Field head values are matched 

well over most of Missouri, Kansas, and parts of 

Oklahoma and Nebraska, but model-computed values 

in large areas in Oklahoma, Colorado, and Nebraska 

differ from field values by two standard deviations or 

more (figs. 54, 55). In north- central Oklahoma, western Oklahoma, the Texas Panhandle, Colorado, and 

southwestern Nebraska, the computed-head values 

poorly matched the field values. However, the configuration of the two head distributions are similar 

(fig. 54). In central Oklahoma, a major discrepancy 

between computed and field head values occurs on the 

eastern flank of the Anadarko Basin, extending northward from an area where the aquifer material represented by layer 4 is not present. Model-computed 

head is less than field head by at least 600 ft in this 

vicinity. North of the Anadarko Basin south into the 

Palo Duro Basin of the Texas Panhandle and in 

northeastern Colorado and southwestern Nebraska, 

there are areas where the model-computed head is 

400 ft less than the field head.

In the areas of west-central Oklahoma and the 

eastern Texas Panhandle peripheral to the Anadarko

Basin, the field hydraulic-head values are based on 

data for an area in which geopressure exists; thus, the 

computed hydraulic-head values do not match for that 

reason. In the Anadarko Basin itself (west-central 

Oklahoma and the eastern Texas Panhandle), the 

model-computed heads match the field heads almost 

exactly because of the general-head boundary condition imposed on the model nodes in that locality. A 

match of head values using the general-head boundary 

condition indicates that a very small inflow or outflow 

will maintain the model-computed head at the field 

head. The small permeability (figs. 21, 29) of the geohydrologic units making up layer 4 in the Anadarko 

Basin causes very small flows to have a large effect on 

the hydraulic head because the units have only a 

slight capacity for transmitting water. Of the 16 cells 

in the Anadarko Basin area with a general-head 

boundary condition, 4 cells have flows exceeding 

1x10"2 ft 3/s, with a maximum single node flow of 

2.6xlO-2 ft 3/s. The net flow of about 2xlO"2 ft 3/s into 

the 16-cell area (about 12,600 mi 2 ) of the Anadarko 

Basin is negligible for that large an area; that small a 

flow rate is beyond the limits of accuracy for the 

model input.

Where the upper unit in the Western Interior 

Plains aquifer system, represented by model layer 4, 

pinches out because of the Cambridge Arch, Central 

Kansas Uplift, and Nemaha Uplift (figs. 7, 50), 

no-flow boundaries are specified in the model. In 

northeast Nebraska north and west of the Nemaha 

Uplift, the computed head values are as much as 400 

ft less than the field head values (fig. 55). In the 

model area representing the Central Kansas Uplift, 

the model-computed head is as much as 300 ft higher 

than the field head. Field head values in northeastern 

Nebraska and north-central Kansas indicate fairly 

uniform hydraulic head with a head change from west 

to east of only about 100 ft for that area (fig. 54). 

Freeze and Witherspoon (1967, p. 634) conclude that 

buried stratigraphic pinch-outs can cause recharge or 

discharge areas where they would not be anticipated 

on the basis of water-table configuration. However, 

specifying general-head boundary conditions at the 

extent of layer 4 to represent the area surrounding the 

Nemaha Uplift would fix or control the head in almost 

70 percent (31 of 46) of the cells in that area. Computed heads at nodes could readily be forced to fit the 

field heads to provide model-computed inflow or outflow data for the pinch-out boundaries. However, 

there are no measured flow data for comparison with 

model-computed flow rates.

Computed heads for layer 5 have an average deviation of 0.13 ft less than the field heads (table 4). The 

standard deviation is about 301 ft, the largest
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C69

standard deviation of the three calibrated layers. The 

match of model-computed heads to field heads of the 

lower units in the Western Interior Plains aquifer system is fairly good over the regional study area, with 

the exception of the Anadarko Basin in west-central 

Oklahoma and the area in the eastern Panhandle of 

Texas and east-central Colorado. In those two locations, localized extremes of deviation occur (figs. 56, 

57). For example, in the Anadarko Basin, computed 

hydraulic heads in two cells are more than 1,600 ft less 

than the field heads. However, at that location in the 

Anadarko Basin, a significant thickness of Devonian 

or Mississippian shale (Chattanooga Shale) is present. 

Because the simulated vertical leakance of the shale 

unit is related by a constant to the hydraulic conductivity of the underlying layer, the possibility arises 

that the hydraulic-conductivity values used in layer 5 

for the cells representing the Anadarko Basin are too 

small. Similarly, the constant relating lateral hydraulic conductivity in layer 5 to the vertical hydraulic 

conductivity of the shale may be too small.

The effect of the Chattanooga Shale may be illustrated by another example. Where the Chattanooga 

Shale is present, the difference in computed hydraulic 

head between layers 4 and 5 in vertically adjacent 

model cells representing the aquifer in the Anadarko 

Basin area is almost 7,200 ft. The extreme field head 

difference at that location between layers 4 and 5 is 

6,200 ft. A model-computed head difference of 

7,200 ft is maintained primarily by the small vertical 

permeability of the intervening confining unit. However, in vertically adjacent cells of layers 4 and 5 in 

east- central Colorado, the computed heads are the 

same because of the absence of the Chattanooga 

Shale.

The model-computed head values (fig. 57) are 

smaller than the field head values in cells representing 

layer 5 in northwest Arkansas near the eastern State 

line of Oklahoma. This is an area where the Ozark 

confining unit is present between the Ozark and 

Springfield Plateau aquifers, and the Western Interior 

Plains confining system (layer 3) is also present. The 

area is a transition area where layer 4 crops out, and 

in laterally adjacent cells, layer 3 overlies layer 4. 

However, for one cell, layers 3, 4, and 5 are all 

present, but layer 5 crops out in a laterally adjacent 

cell. The model-computed and measured head values 

are well matched in layer 4 (fig. 54) but not matched 

for layer 5 (fig. 56). This mismatch may indicate that 

the leakance specified for the Ozark confining unit for 

that area is too small. That the leakance for the confining unit is too small is substantiated because the 

field head values in layer 4 are larger than those in 

layer 5 and more leakage between layers 4 and 5

would increase the hydraulic head in layer 5. Because 

the model- computed heads in outcrop areas of layer 5 

are fairly well matched to field heads, the transmissivity and recharge values are considered sufficiently 

accurate, which further supports a conclusion that the 

Ozark confining unit may be more leaky 

than simulated.

Effects of Variable Density

Model computations show little difference between 

hydraulic head computed with and without specification of variable density for layer 2. The average deviation of computed head values is almost -6 ft (table 

4). The standard deviation for the variable-density 

simulation is almost 2 ft less than the constantdensity computation. The differences for layer 2 in 

the two sets of calculated hydraulic-head values are 

only slightly discernible on a plot of hydraulic head at 

the scales shown in figure 52. The differences in the 

two sets of head values is illustrated in figure 58, 

which shows that the effects of variable density occur 

only in northeastern Colorado, extreme southwestern 

Nebraska, and southeastern Wyoming. The 

model-computed heads for that area are generally 

smaller than the field hydraulic head, with a maximum difference of about 130 ft at a cell representing 

an area on the Front Range of the Rocky Mountains 

in Colorado. The area of layer 2 affected by variable 

density represents a deeply buried, thick section of the 

Great Plains aquifer system containing water with 

large concentrations of dissolved solids. Also, the permeability of the aquifers represented is slight (fig. 36), 

which amplifies the effect of even small values of variable density. For the remainder of layer 2, there are 

several reasons that variable density has little effect 

on the computation of hydraulic head. These are: (1) 

the lateral density differences represented by the data 

are small (fig. 37); (2) the density differences between 

the freshwater of layer 1 and layer 2 are small; (3) the 

Great Plains confining system above and the Western 

Interior Plains confining system below isolate a major 

part of the aquifer material represented by model 

layer 2 from the effect of vertical density differences; 

and (4) the specified-head boundary assigned everywhere in model layer 1 produces a general effect.

Model computation of hydraulic head for layer 4 

that included the effects of variable density resulted in 

an average difference between computed hydraulic 

head and field hydraulic head of about 30 ft (table 4). 

Principal areas of hydraulic-head differences between 

values calculated with and without the effects of variable density are located in southeastern Kansas and 

eastern and central Oklahoma (fig. 59).
C70 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

>. 

xv\

EXPLANATION

MODEL-LAYER BOUNDARY

 so LINE OF EQUAL DIFFERENCE IN HYDRAULIC HEADDifference is simulated hydraulic head without variabledensity effects minus simulated hydraulic head with 

variable-density effects (pseudosources). Interval, in 

feet, is variable

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 58. Difference between simulated steady-state hydraulic head with and without effects of variable density

(pseudosources) for model layer 2.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C71

40°

35'

105° 100° 95" 90"

I 

» - -

WSOMIV

, * > ,

vi'»|HN>;\kr-i* V v .* > ' 

'"..Hi>« S; Ri"/'' 11 ; ^\

I ^1-,' (^IN^IN

100

I I \

0 100 200 KILOMETERS

:>. 

 ^

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

MODEL-LAYER BOUNDARY

 ,00 LINE OF EQUAL DIFFERENCE IN HYDRAULIC HEADDifference is simulated hydraulic head without variabledensity effects minus simulated hydraulic head with 

variable-density effects (pseudosources). Interval, in 

feet, is variable

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 59. Difference between simulated steady-state hydraulic head with and without effects of variable density

(pseudosources) for model layer 4.
C72 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

In eastern Oklahoma, the maximum head difference between computed and field hydraulic head 

exceeded 250 ft at a location where the flux due to 

variable density was not particularly large (1.67 ft 3/s). 

However, the flux value was derived principally from 

lateral density differences in the freshwater-saltwater 

transition zone between the Ozark subregion and the 

Plains subregion. An example of the effect of slight 

permeability is in north-central Oklahoma where 

model layer 4 represents an aquifer in the Anadarko 

Basin. A calculated hydraulic head for one cell of the 

model affected by variable density exceeded the 

hydraulic head calculated without this effect by 

slightly more than 300 ft. This hydraulic-head difference due to variable density is greater than that in 

eastern Oklahoma, but it is a hydraulic-head difference that resulted from a much smaller variabledensity effect represented by a pseudosource 

(2.1xlO~3 ft3/s). Although the variable-density effects 

in the Anadarko Basin area are represented by very 

small computed flux values, the resulting computed 

hydraulic-head values for the Anadarko Basin area 

were a major contribution to the greater-thanaverage difference between model-computed and field 

hydraulic head than was the case with computed 

hydraulic head that did not include the effects of variable density.

The average difference between the computed and 

field hydraulic-head values for layer 5 (when the 

model included the effects of variable density) is 

about 41 ft (table 4). Hydraulic-head differences 

between values calculated with and without the 

effects of variable density are shown in figure 60. 

Hydraulic-head differences for layer 5 result predominantly from lateral density differences in extreme 

southeastern Kansas, eastern Oklahoma, and almost 

all of the study area in northwestern Arkansas. 

Fluxes of less than 1.0 ft 3/s that result from vertical 

density differences, although small, cause fairly large 

differences in calculated hydraulic head. The large 

hydraulic-head differences between the two calculation techniques not only result from the effect of variable density but also from the effect of extremely 

small permeability. The calculated hydraulic head 

also can be smaller than that calculated without consideration of variable density, which was the result in 

model layer 5 representing an area of an aquifer in the 

Anadarko Basin in west-central Oklahoma.

The areal configuration of computed hydraulic 

head for the study with consideration of variable density differs little from the configuration of hydraulic 

head computed without considering variable density, 

except in areas of very small hydraulic conductivity. 

Computation of hydraulic head for the two conditions,

however, result in some "local" hydraulic- head differences that would cause different lateral flow directions 

for the two conditions. Flow vectors show no direction differences for model layer 2, but in layers 4 and 

5 there are changes illustrated by figures 61 and 62. 

For layer 4 (fig. 61), simulation of the area between 

the Nemaha Uplift and the Central Kansas Uplift 

(fig. 7) and the area southeast of the Central Kansas 

Uplift toward eastern Oklahoma exhibits flow-direction changes. For layer 5 (fig. 62), flow-direction 

changes are limited to cells representing the area in 

the Central Kansas Uplift and extreme eastern Oklahoma at the freshwater-saltwater transition zone. The 

only flow-direction changes that would indicate a significant change in the overall flow regime are those in 

layer 5 over the Central Kansas Uplift. Flow-direction reversals imply a compensating vertical-flow 

change. For the flow simulation without consideration 

of variable density, the majority of vertical-flow vectors in western Kansas and eastern Colorado west of 

the Central Kansas Uplift indicated flow from layer 4 

to layer 5. However, considering variable density by 

introducing pseudosources, the majority of verticalflow vectors indicated flow from model layer 5 to layer 

4 in that same area.

Evaluating the effect of variable density by the 

introduction of pseudosources (Weiss, 1982) in the 

regional model clearly indicated that not including 

variable density and only using freshwater head in 

computation may lead to conclusions different from 

those when variable density is considered regarding 

flow and possibly erroneous conclusions as postulated 

by Jorgensen and others (1982). Even with the large 

cell size of the regional model, both lateral and vertical flow-direction changes were indicated in some 

areas, which supports the necessity of considering 

variable density.

Recharge

A recharge model for the regional study area was 

developed by Dugan and Peckenpaugh (1985). Inputs 

to the recharge model were data on soil permeability, 

available water capacity of the soil, topography, consumptive water requirements of various vegetation 

types, monthly precipitation, and computed monthly 

potential evapotranspiration (PET). The PET values 

were obtained using the Jensen-Haise method (Jensen 

and Haise, 1963). Recharge was computed using type 

vegetation environments for the study area during a 

30-year period, 1951-80. The first environment was a 

representation of vegetation conditions during the 

period based on 1978 vegetation statistics. The second vegetation environment for the period was termed
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C73

,

r- X V

35'

105° 100° 95° 90°

N"FU

I I I

0 100 200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

/\/\ MODEL-LAYER BOUNDARY

 10 LINE OF EQUAL DIFFERENCE IN HYDRAULIC HEADDifference is simulated hydraulic head without variabledensity effects minus simulated hydraulic head with 

variable-density effects (pseudosources). Interval, in 

feet, is variable

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 60. Difference between simulated steady-state hydraulic head with and without effects of variable density

(pseudosources) for model layer 5.
C74 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

35

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

LATERAL FLOW VECTORS-Length of shah 

indicates relative magnitude

- » Variable density 

~~~ Constant density

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 61. Effects of variable density on simulated flow vectors for model layer 4.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C75

105° 100° 95°

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

LATERAL FLOW VECTORS-Length of shaft 

indicates relative magnitude

" Variable density 

" Constant density

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 62. Effects of variable density on simulated flow vectors for model layer 5.
C76 REGIONAL AQUIFER-SYSTEM ANALYSIS -CENTRAL MIDWEST

"pre-agricultural" (Dugan and Peckenpaugh, 1985, 

p. 68), with woodland the same as in 1978 but with 

cultivated land or cropland treated as grassland. The 

results show that recharge differences between the two 

vegetation environments are not significant except in 

localized areas.

Recharge patterns within the region indicate that 

climate, particularly precipitation, is a controlling factor. Data show that the proportion of precipitation 

that potentially may be recharged decreases as precipitation decreases. A scatter diagram of computed 

mean annual recharge versus precipitation for 

356 regional model-grid cells shows a close relation 

between precipitation and recharge (fig. 63). The relation becomes approximately linear where mean annual 

precipitation exceeds 35 in. and recharge exceeds 

5 in/yr. The spatial relation of recharge to rainfall is 

illustrated by figures 5 and 6. These figures confirm 

the similar patterns for rainfall and recharge, and figure 6 shows a maximum of about 15 in. of annual 

recharge in central Arkansas where mean annual precipitation is about 48 in. (fig. 5). In Colorado and 

New Mexico, the recharge model predicted extremely 

small recharge. Dugan and Peckenpaugh (1985, p. 72) 

conclude that the small recharge is closely related to 

large PET, solar radiation, percentage of possible sunshine, and small relative humidity.

Outcrops in the Ozark subregion are the principal 

recharge areas simulated in the regional flow model. 

In modeling extensive aquifers, it has been observed 

that the effective amount of flow to the water table 

required for a successful simulation of observed water 

levels may be significantly less than that calculated 

from independent data sources. For example, Jorgensen and others (1989b, p. 683) use Dugan and 

Peckenpaugh's (1985) data to estimate that for the 

Ozark subregion, the annual precipitation is approximately 190,000 ft 3/s, and from that, potential 

recharge to the water table in the aquifer system of 

the Ozark subregion is about 45,000 ft3/s.

Ground-water flow discharging from aquifers to 

10 major streams in southern Missouri, which likely is 

only part of the discharge in the Ozark subregion, was 

reported as 4,169 ft 3/s (Harvey, 1980, p. 50, table 4) 

(that does not include discharge to the Missouri or 

Mississippi Rivers). Therefore, one can conclude that 

recharge probably exceeds 10 percent and may be as 

much as 20 percent of the potential recharge. Thus, 

recharge was expected to exceed 4,500 ft3/s and be 

9,000 ft 3/s or more for the Ozark subregion in this 

study.

Using the technique developed by Jorgensen and 

others (1989a) for determining the net amount of 

recharge within a model cell that leaves the cell as

13

12

CO 10

LLJ

I

(J

LLI

DC 7

10 20 25 30 35 40 45 

PRECIPITATION. IN INCHES

55

FIGURE 63. Computed mean annual recharge versus mean 

annual precipitation, by model-grid element (from Dugan and 

Peckenpaugh, 1985, fig. 35, p. 73).

regional flow, the minimum net recharge to regional 

aquifer units in the Ozark subregion was determined 

to be only about 2,100 ft3/s. Thus, the net recharge 

was computed to be about 5 percent of the potential 

recharge for a three-layer model of the Ozark Plateaus 

aquifer system. The model-grid spacing was 

14 by 14 mi. An advantage of the technique is that 

the recharge is distributed in the model cells on a 

rational basis of topographic effects on runoff and 

stream-aquifer interaction. The 2,100 ft3/s (the minimum) represents recharge that cannot (and thus, has 

no potential to) be intercepted by sinks within the 

model cells but would move to the aquifer as part of 

net recharge. The remaining recharge, in general, is
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C77

intercepted and carried away in streamflow. However, there is part of recharge that does have the 

potential to be intercepted, yet it still may become a 

part of the net recharge and enter the regional aquifer 

flow system. In the recharge analysis by 

Jorgensen and others (1989a, b), the part of actual 

recharge that becomes net recharge instead of being 

intercepted by streams was varied by applying what is 

termed herein an "interception factor." The interception factor defines that part of total water-table 

recharge within a model-cell area that exits the cell 

and thus is not a part of the simulated aquifer flow. 

The factor is unknown and, initially, can only be estimated. Its maximum value would be 1.

Values for recharge to the water table in outcrop 

areas modeled in the region are listed in table 5. The 

values were derived from recharge modeling by Dugan 

and Peckenpaugh (1985) for the whole region and the 

analyses of Jorgensen and others (1989a,b) applied to 

the Ozark subregion. The net flow shown for the outcrop areas modeled is water-table recharge minus local 

aquifer discharge to streams. The data imply that 

although the recharge through the outcrops of the 

Ozark and St. Francois aquifers (represented by model 

layer 5) are always more than twice the recharge for 

the Springfield Plateau aquifer (represented by model

layer 4), the net flow amounts are similar when the 

interception factor is less than 1.

The analysis gives a recharge budget (table 5) for 

the regional study outcrop areas for field purposes 

ranging between 7,399 and 10,135 ft3/s. For the Ozark 

subregion, independently determined aquifer-stream 

interaction outflow for the Springfield Plateau, Ozark, 

and St. Francois aquifers (sum of the outflows from 

layer 4 and 5 only) is 3,761 ft3/s, giving a net flow 

range for calibration purposes of 3,536 to 6,223 ft 3/s. 

Note that an interception factor of 1 giving a recharge 

rate of 1,836 ft 3/s was not believed to provide a field 

recharge value but does provide a minimum possible 

value. Net flow to the water table through outcrops 

for aquifers simulated by layer 5 is negative for an 

interception factor of 1 (table 5). For that case, local 

aquifer discharge to streams exceeds recharge to the 

water table. This condition could exist only if 

ground-water discharge would be supplied from a 

remote source, which is not the hydrologic case for the 

system.

The recharge budget that gave the best simulation 

based on matching computed hydraulic head to field 

hydraulic head is given also in table 5. The total 

recharge for the region as a model input is 6,947 ft 3/s. 

The upper bracket of the recharge calibration values,

TABLE 5. Simulated recharge to outcrops of the regional model area represented by model layers 2,

4, and 5 for several interception factors

Model layer

2

4

5 

Total

2

4

5 

Total

2

4

5 

Total

2

4

5 

Total

Interception Recharge (cubic 

factor1 feet per second)

1.00 0

499

1,337

1,836

0.85 102

2,227

5,070

7,399

0.78 151

3,077

6,907

10,135

Final Simulation Values

0.86 93

2,087

4,767

6,947

Aquifer-stream 

interaction 

outflow2 (cubic 

feet per second)

80

446

3,315

3,841

80

446

3,315

3,841

80

446

3,315

3,841

80

446

3,315

3,841

Net flow (cubic feet 

per second)

-80

43

-1,978

-2,005

22

1,781

1,755

3,558

71

2,631

3,592

6,294

13

1,641

1,452

3,106

lrThe interception factor defines that part of total recharge that exits a cell and does not become recharge that is simulated 

in the model.

2Aquifer-stream interaction data for layer 2 from Helgesen and others (1993). For layers 4 and 5, data are from Hedman 

and others (1987) and the analysis of Jorgensen and others (1989a, b).
C78 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

10,135 ft 3/s, obtained by using an interception factor 

of 0.78, exceeded the best model simulation value by 

46 percent. Use of the upper-bracket recharge value 

gave such extremely large model-computed hydraulic 

heads that unrealistic transmissivity adjustments 

would have been necessary to simulate field hydraulic 

heads in the Ozark subregion. The lower bracket of 

the calibration recharge also gave model-computed 

heads greater than the field hydraulic heads, but only 

minor adjustments of recharge values were necessary 

to obtain a reasonable match between computed and 

field head values in the Ozark subregion.

In the central and western part of the regional 

study, outcrop areas of the Great Plains aquifer system represented by layer 2 generally are more scattered. They were not subjected to the netflow-per-cell analysis, but modified recharge data of 

Dugan and Peckenpaugh (1985) were entered directly 

into the model. Total recharge to layer 2 of the model 

(table 5) is about 1 percent of the total for the region 

or 93 ft 3/s, of which 83 ft 3/s occurred in cells representing areas in north-central Kansas and 10 ft 3/s 

through the remaining areas in Oklahoma, Colorado, 

New Mexico, Wyoming, and Nebraska. In the western part of the study area, recharge rates were generally less than 1 ft 3/s per model cell, values consistent 

with the recharge-precipitation relation shown in 

figure 63.

SENSITIVITY ANALYSIS

The effect on model results of systematically varying data entered into the model illustrates the relative 

importance of the data to the calibration criteria of 

the model. The primary calibration criterion of the 

regional study was the closeness of fit of model-computed to field hydraulic head. A measure of the closeness of fit is the mean head difference between 

computed and field head values. Luckey and others 

(1984, p. 48) point out that the mean hydraulic-head 

difference (water-level residual) is only one of several 

possible field criteria. A small mean hydraulic-head 

difference could be obtained from a model simulation 

as a result of a balance of errors, but the possibility 

exists that compensating errors cannot be identified. 

The mean water-level residual would give no indication of the magnitude of those errors, and even though 

the field criterion was met, the model may not be calibrated properly. The standard deviation of the differences between the model-computed head and the field 

head is a measure of hydraulic-head-difference variability that is not affected by compensating errors. 

Also, the standard deviation of the differences

emphasizes the larger differences (Luckey and others, 

1984, p. 48).

A sensitivity analysis was performed on the 

CMRASA regional model to evaluate the changes in 

the mean and standard deviation of the hydraulic-head differences that would result from general 

changes in model inputs. Hydraulic conductivity, leakance between layers, and recharge were changed over 

a range of values after first determining a calibration 

in which the average difference of model-computed 

head from field head was less than 1 ft. The input 

data were changed by using a single-value multiplier; 

thus, areal distribution of the data was not affected. 

The sensitivity of model-computed hydraulic-head 

values to a general change of model inputs is shown by 

the changes in mean hydraulic-head differences and 

standard deviations, figures 64, 65, 66.

Model-computed head is quite sensitive to changes 

in aquifer hydraulic conductivity (fig. 64). Figure 64 

shows that computed hydraulic head is much more 

sensitive to decreases in hydraulic conductivity than 

to increases in hydraulic conductivity.

The mean difference (between simulated and field 

head) and standard deviation of the hydraulic-head 

difference are not minimal at the same hydraulic conductivity for model layer 5. The standard deviation, 

as a sensitivity indicator, shows that computed 

hydraulic head becomes very sensitive primarily at 

hydraulic-conductivity reductions from calibration 

hydraulic conductivity of 40 percent or more.

Standard deviations for model layers 2 and 4 are 

near minimum at the field hydraulic conductivity and 

are fairly insensitive to hydraulic-conductivity 

increases from that point. However, the standard 

deviations are sensitive to hydraulic-conductivity 

reductions of 40 percent or more as in the case of 

model layer 5.

Computed hydraulic-head values in model layers 4 

and 5 are not very sensitive to changes in leakance 

coefficients (fig. 65). The principal reasons that the 

variation of leakance coefficient does not greatly affect 

computed hydraulic head in model layers 4 and 5 are 

the simulations of evaporite deposits in model layer 3 

as a restriction to vertical flow, the large outcrop area 

simulated by model layer 5, and the relatively small 

differences in hydraulic head between layers 4 and 5.

Layer 2 is very sensitive to leakage from layer 1 

(fig. 65). Increasing the leakance coefficient causes 

more vertical inflow than can be dissipated laterally 

without an excessive increase of hydraulic head. This 

situation occurs partly because model layer 3 represents a zero-leakage layer where evaporites are present 

in an area coinciding with most of layer 2. Conversely, 

the computed hydraulic-head values are smaller when
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C79

co uLU Z

400

350

300

u.uJ 250

5<

§^ 

II

200

150

250

UJ It UJ

D LU 

Q. U.

150

Z <

100

50

-50

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

RATIO OF TESTED HYDRAULIC CONDUCTIVITY (K) TO CALIBRATION 

HYDRAULIC CONDUCTIVITY (Kc), K/K c

FIGURE 64. Sensitivity of difference between modelcomputed and field hydraulic head to change in aquifer 

hydraulic conductivity in CMRASA geohydrologic units.

the leakance coefficient, and thus inflow, are 

decreased. The sensitivity of computed hydraulic head 

to increased or decreased leakage may be termed typical for a confined aquifer.

Model-computed hydraulic-head values are only 

moderately sensitive to changes in recharge for the 

regional area (fig. 66). Because constant-flow and 

general-head boundaries were specified in layer 2 in all 

recharge areas (fig. 48), varying recharge had no effect 

on hydraulic-head computation for that layer. There 

were major changes in computed hydraulic head, however, in the Ozark subregion where recharge was 

applied to cells in layers 4 and 5 that represent outcrops of the Ozark Plateaus aquifer system. Among 

cells of layers 4 and 5 representing confined parts of

350

.

01 9 

O <

300

5< 25°

giti 200

P £<

Q Q

QE 150

. 

52O~ 

O Q

z<uiU!

UJ I

§ U

Q u.

2 i 

< Z

UJ <

100

100

50

I I I I

-50

-100

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

RATIO OF TESTED LEAKANCE (U TO 

CALIBRATION LEAKANCE (tfi), L/L r.

FIGURE 65. Sensitivity of difference between modelcomputed and field hydraulic head to change in leakance 

coefficients between layers in CMRASA geohydrologic 

units.

the aquifer system, only those laterally adjacent to 

cells representing outcrop areas were affected. Therefore, the mean differences between computed and field 

hydraulic head were small because differences over the 

whole area were included when computing the mean.

Mean differences in the Ozark subregion alone are 

much greater (fig. 67). In the Ozark subregion, for 

values of recharge used in the regional-model field simulation (where the ratio of tested recharge rate to field 

recharge rate is 1.0), the mean difference between 

computed and field hydraulic head is almost 23 ft 

greater than the value for layer 4, representing the 

whole regional area, and almost 10 ft greater than the 

value for layer 5 (fig. 67). To obtain the mean difference between computed and field hydraulic head of 

less than 1 ft in the regional model, the mean differences for the Ozark subregion are cancelled by a negative mean difference for the remainder of the regional 

area.
C80 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

uj Q 

O <

z x

£y t =

350

300

200

150

100

100

I T

Modellayer V 

number 5 

o -.

O Q

Z <

uluj 

ui X 

§ O

OQ< 

Ul DC 

O Q

50

-50

-100

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

RATIO OF TESTED RECHARGE RATE (ff) TO CALIBRATION 

RECHARGE RATE (ffc). Ft/ft c

FIGURE 66. Sensitivity of difference between modelcomputed and field hydraulic head to change in recharge rate 

in CMRASA geohydrologic units.

Varying two model inputs simultaneously and plotting the mean difference between computed and field 

hydraulic head illustrates the nonunique characteristic 

of a model calibration. Varying the hydraulic conductivity and recharge simultaneously shows an almost 

linear effect on the difference between model-computed and field hydraulic head for layers 4 and 5. 

Mean differences of less than 1 ft result for recharge 

and hydraulic-conductivity ratios ranging from about 

0.4 and 0.6, respectively, to about 1.4 and 1.6, respectively (figs. 68, 69).

Varying recharge and leakance simultaneously 

showed that two values of leakance would give zero 

mean differences between computed and field hydraulic head for different values of recharge (figs. 70, 71). 

For layer 4, the relation is almost symmetrical. A 

similar relation exists for the two model inputs in 

layer 5 (fig. 71), but the results are not as symmetrical 

as for layer 4.

gjffi 200

OQ ~ 

O <

150

Z Q

Six" 100

50

I 1 1 1

100

OQ<

Ul CC

£ui -50

Q "-

-100 I___I

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

RATIO OF TESTED RECHARGE RATE (ff) TO CALIBRATION 

RECHARGE RATE (ffc). ff/W c

FIGURE 67. Sensitivity of difference between modelcomputed and field hydraulic head to change in recharge rate 

in CMRASA hydrologic units, Ozark subregion.

The large outcrop area simulated by cells in layer 5 

affects the leakance-recharge relation because leakance 

would not occur in the cells representing the outcrop 

area of the Ozark and St. Francois aquifers in the 

Ozark subregion. Therefore, leakance would have less 

effect on mean differences between computed and field 

hydraulic head; however, recharge would have a 

greater effect.

Summarizing the relative importance of 

model-input data to field data, the sensitivity analysis 

curves (figs. 64-66) show that changes in hydraulicconductivity values from those used for the calibration resulted in the greatest change in model-computed values of hydraulic head. Changes of leakance 

coefficient resulted in significant changes in the mean 

difference between computed and field values of 

hydraulic head for model layer 2 and standard deviation from the mean for model layer 4. However, the 

leakance coefficient was generally less important than 

changes in hydraulic conductivity. Differences 

between computed and field hydraulic head were least
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C81

2.2 

O o

£^ 2.0

E ^ 1.8

O > 1.6

S D 1.4 8|

y o -i 2

_l CJ

2 ^ 1.0

> <

x Q 0.8

Q >-

"" 0.6 

0.4

c/3 uj

O £9

0.2

0

Mean difference between computed 

and field hydraulic head for 

an individual simulation, in feet

-15.7 -1.7

-21.2 -20

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 

RATIO OF TESTED RECHARGE RATE (R) TO CALIBRATION RECHARGE RATE (Rc), R/RC

FIGURE 68. Sensitivity of difference between model-computed and field hydraulic head to simultaneous change in recharge

rate and hydraulic conductivity in CMRASA model layer 4.

sensitive to change in recharge relative to calibration 

values.

ERROR ANALYSIS

McDonald and Harbaugh (1988) point out that 

any property, such as hydraulic conductivity, of a 

modeled aquifer associated with a node applies to or is 

distributed uniformly over the extent of a cell area. 

This is of concern for large nodes as it could introduce 

error. The node spacing for the regional study of 

28 mi is large because the study area is very large. 

Additionally, the data available for deep geohydrologic units are sparse. Also affecting selection of a 

large node spacing was the fact that two subregional 

studies, a model of the Great Plains aquifer system 

and a model of the Ozark Plateaus aquifer system, 

were made at a node spacing of one-half the regional 

node spacing, allowing study in greater detail. In general, more data are available for those aquifer systems 

in the subregional areas.

The question of discretization errors resulting from 

the large node spacing of this study was raised by the 

investigators during the course of the study. Karplus 

(1958, p. 104) points out that one would be tempted

to assume that the accuracy of any finite-difference 

solution to a flow equation would be increased by 

decreasing the node spacing. However, not only the 

grid spacing but also the nature of the function being 

analyzed affect the accuracy of the solution. An analysis shows that, if the derivatives of the fourth order 

or greater of a continuous function are zero (or very 

small), the second derivative of a function, such as the 

flow equation (eq. 5) represents an accurate expression 

regardless of the node spacing (Karplus, 1958, 

p. 104-105). Estimating the error inherent in a second-order finite-difference approximation of the solution of a differential equation can be accomplished by 

computing an estimate of the fourth-order derivative 

(Karplus, 1958, p. 106):

o) ' (6) )

where

d

X

is the differential operator; 

is the potential (L); 

is the dimensional direction (L); 

is the node spacing (L); and
C82 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

2.2

O o

I- ^

£^ 2.0 

E ^ 1.8 B ij n"

y o T 2

_l CJ

a: ^ 1.0 

x § 0.8 

0.6 

0.4

C/D 

LU

O E9

0.2

0

Mean difference between computed 

'and field hydraulic head for 

an individual simulation, in feet

-49.7

-45.2

-39.6

90.4

-.1

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 

RATIO OF TESTED RECHARGE RATE {/?) TO CALIBRATION RECHARGE RATE (Rc), R/RC

FIGURE 69. Sensitivity of difference between model-computed and field hydraulic head to simultaneous change in recharge

rate and hydraulic conductivity in CMRASA model layer 5.

subscripts "9, 10, 1, 2, and 0" designate five potential 

values along the x-dimension AJf distance apart, 

with O0 between O^ and <J>2 an^ $9 and <J>i 0 beyond 

0>! and 0>2 (Karplus, 1958, p. 100, fig. 4.20). In 

general, the hydraulic-head gradients encountered in 

the regional study area are sufficiently small so that 

the sum of the potentials in equation 6 are very small 

in relation to A^"4. Therefore, the value of equation 6 

is very small. From the preceding premise concerning 

the fourth-order derivative, the error in solving the 

flow equation of the model using a large node spacing 

is sufficiently small that it can be ignored.

However, Weiss (1986, p. 31-35, and p. 39) made 

an analysis of errors due to node spacing and concluded that the difference in simulated hydraulic head 

that could be attributed to different node size was surprisingly large considering the small cell size 

selected. The original model used by Weiss (1986) 

had a 3-mi cell size. Simulated hydraulic heads using 

a grid of 1- by 1-mi cells were as much as 90 ft less 

than simulated hydraulic heads using the larger node 

spacing in an outcrop area. Recharge 50 percent 

greater than that of the 3- by 3-mi grid model was 

required by the 1- by 1-mi grid model to simulate

approximately the same hydraulic head near 

the outcrop.

Ames (1977, p. 24) emphasizes in an error analysis 

that truncation error is of the finite-difference equation and not of the solution. This point is emphasized 

because the boundary conditions are essential to the 

correct solution just as they are when using analytical 

methods. It is further pointed out that replacement of 

a continuous function by discretely spaced variables 

leads to discretization error (Ames, 1977, p. 24). Also, 

one can readily observe that curved physical boundaries of a study area approximated by a series of 

square or rectangular cells can be better approximated 

by a series of small cells than by a few large cells. The 

errors evoked by large cells at a physical boundary 

compared to error in a simulation using smaller cells 

for the same study area are also discretization errors.

An analysis comparing model-computed and field 

hydraulic-head differences for different node spacings 

was made for this study. The hydraulic characteristics of the model cells in the regional and subregional 

numerical models were drawn and discretized from the 

same maps and data bases. The same computer code 

was used in all cases for computation.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C83

2.2 

2.0

t :£ 1.8 £t

9 > 1.6

o ^ 1-4 8i 

^8 «

3° 23 1.0

DC

0) 

UJ

rSO DC

O £9

0.8 

0.6

0

-9.5

i i I 

Mean difference between computed 

and field hydraulic head for 

an individual simulation, in feet

6.0 8.8

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 

RATIO OF TESTED RECHARGE RATE (R) TO CALIBRATION RECHARGE RATE (Ftc), R/RC

FIGURE 70. Sensitivity of difference between model-computed and field hydraulic head to simultaneous change in 

leakance and recharge rate in CMRASA model layer 4.

The area chosen for comparing simulation results 

that were computed with different node spacing is 

that encompassing the Great Plains aquifer system. 

The area in regional model layer 2 in relation to the 

subregional model grid is shown by figure 72. The 

Great Plains aquifer system model grid, 46 rows and 

35 columns, is included in the area. The data in the 

Great Plains aquifer system model were discretized to 

represent model cell areas 14 mi on a side located in 

quadrants of the regional-model cell areas of 28 mi on 

a side. Thus, there are no data points in the Great 

Plains aquifer system model that have exactly the 

same coordinates of data in the regional model. A 

data point "located" at a cell center or node in the 

regional model would be approximated by an average 

of the four subregional model-cell values within the 

regional model-cell area.

Comparison of simulated hydraulic-head values 

computed by the two models was made using a statistical analysis. Field hydraulic-head values for the two 

systems also were compared using the same analysis. 

The comparison method used was a test of differences 

between hydraulic-head values by pairing computed 

head values and pairing field head values representing

the same hydrologic-unit area. An assumption concerning the differences between paired values is that 

the differences represent a population with a normal 

distribution of values. The objectives of the analyses 

were to learn the size of the mean value of each of the 

two populations of hydraulic-head difference and, particularly, whether the means are different or not different from zero (Snedecor, 1957, p. 49).

The results of three analyses of each of two sets of 

data are shown in table 6. The largest number of 

hydraulic-head comparisons in the regional model was 

termed "all nodes." For every regional-model node 

where one to four subregional-node values were 

present, the hydraulic-head value in the regionalmodel node was compared to the average of the values 

in the subregional-model nodes. "Full-data nodes" are 

those where all subregional nodes within a regionalmodel node have data, and "interior nodes" are comparisons for regional-model nodes that are not located 

on the unit-extent or study boundaries. The vast 

majority of interior nodes are nodes that also have 

data in each of the subregional-model nodes.

The mean difference between field hydraulic-head 

values for both models on a cell-by-cell basis is small,
C84 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

2.2

oS 2-°

£ ^ 1.8

O > 1.6 li -

yg 1.2 

2 j 1.0

X £ 0.8 

0.6 

0.4

</> z uj o

O £9

0.2

0.2

I I i 

Mean difference between computed 

and field hydraulic head for 

an individual simulation, in feet

39.4

27.4

20.5

21.9

41.1

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

RATIO OF TESTED RECHARGE RATE (R) TO CALIBRATION RECHARGE RATE (Rc), R/RC

FIGURE 71. Sensitivity of difference between model-computed and field hydraulic head to simultaneous change in leakance

and recharge rate in CMRASA model layer 5.

ranging from 0.57 to 1.42 ft, with standard deviations 

ranging from 40.28 to 65.88 ft. The field hydraulichead values are from the same maps and data bases, 

so the mean differences should approach zero. The 

standard deviations of the differences represent variations due to judgment and technique of different individuals involved in digitizing and applying the data to 

represent model-cell areas. The small mean differences show that field values have approximately compensating positive and negative differences. The 

means of the differences for computed hydraulic head 

are more variable but range only from 2.91 ft for all 

nodes to 12.50 ft for full-data nodes. The standard 

deviations of the differences are similar, ranging from 

171.48 to 215.79 ft. The mean difference for "all 

nodes," which include boundary nodes, is the least 

because in the model, boundary nodes are constrained 

by imposed boundary conditions; therefore, they vary 

less from field values. Conversely, results for the 

"full-data" and "interior" nodes are more variable 

because of fewer constraints imposed on those cells in 

the simulation.

In these cases, the statistical inference from the 

analyses is that the hypothesis of zero value for the 

mean of the differences between hydraulic-head values 

in the models with different-size nodes may be 

accepted with a significant probability that the 

hypothesis is correct. The model node-spacing relation between the regional and subregional models does 

not appear to significantly affect the conclusions that 

can be made about the computed hydraulic-head values. Thus, the computed hydraulic-head values for 

the two models do not appear to be significantly different, and it is concluded that the use of a model 

simulation for the regional area with a smaller node 

spacing on the order of one-half the spacing used 

would not have significantly affected the accuracy of 

the results, a conclusion consistent with the analysis 

of Karplus (1958, p. 104-105). Luckey and Stephens 

(1987, p. 31), in a similar analysis comparing 10-mi 

node spacing (100-mi 2 grids) to 5-mi node spacing 

(25-mi2 grids) in a regional study of the High Plains 

aquifer, determined that "* * * the same general conclusions about the operation of the hydrologic system
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C85

VJ ,^^v7V^

200 MILES

100 200 KILOMETERS

EXPLANATION

SUBREGIONAL MODEL REGIONAL MODEL LAYER 2 

® RECHARGE CELL IN OUTCROP AREA . - RECHARGE CELL 

o STREAM CELL IN OUTCROP AREA /N/S MODEL EXTENT

x N CELL THAT CONNECTS VERTICALLY 

ADJACENT LAYERS

BOUNDARY OF STUDY AREA

FIGURE 72. Subregional model grid and relation of the Great Plains aquifer system subregional model to regional model layer 2.
C86 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

TABLE 6. Results of statistical analyses comparing hydraulic-head data from the regional and subregional models of the Great

Plains aquifer system

[The statistics are of hydraulic-head differences of paired data. >, greater than]

Hydraulic-head 

data

Field hydraulic 

heads

Model-computed 

hydraulic heads

Field hydraulic 

heads

Model-computed 

hydraulic heads

Field hydraulic 

heads

Model-computed 

hydraulic heads

Number of 

data pieces

254

250

216

216

217

214

Mean difference 

(X) , in feet

1.42

2.91

.57

12.50

1.33

11.61

Standard 

deviation (S), in 

feet

All nodes

40.28

171.48

Full-data nodes

65.88

193.22

Interior nodes

56.18

215.79

Sample of 

standard error

(Sx)

2.53

10.845

4.480

13.15

3.81

14.75

Probability of 

greater mean 

Computed I1 difference

0.562 >0.50

.268 >.50

.126 >.50

.951 .33

.346 >.50

.79 .43

1 r

t = (Snedecor, 1957, p. 50). 

ij^

would have been reached," if the smaller grid spacing 

would have been used instead of the larger spacing.

RESULTS OF SIMULATIONS

FLOW

Computed head values have the same general areal 

distribution as field hydraulic-head values developed 

from data (figs. 52, 54, 56). Therefore, the simulated 

regional flow system generally conforms to the initial 

conception of the flow patterns. A lateral flowvelocity vector and vertical flow-velocity vector were 

determined at each node for each model cell for all of 

the units that were simulated. All velocities referred 

to are Darcy velocities (specific discharges). In the 

case of lateral flow, flow velocities at model nodes in 

each direction were derived from simulated volumetric 

flow rates divided by the harmonic mean of crosssectional areas at individual vertical cell faces. Then, 

the two velocity values thus determined for opposing 

cell faces were averaged. Lateral velocities along the 

two "horizontal" grid directions then were used to 

compute a resultant velocity vector. The length of 

the vector arrows on the flow-direction plots (figs. 

73-75) are scaled logarithmically to indicate magnitude. This applies only to the "shaft" of the arrow 

(the arrowheads or points are all the same size). 

Therefore, a "shaft" of a flow vector that is twice the 

length of another vector indicates one order of 

magnitude (10 times) greater velocity. These data 

from the model simulation for layers 2, 4, and 5 are 

shown in figures 73-75. Vertical flow directions are 

shown in figures 76-79. The relative magnitude of 

flow also is indicated.

For model layer 2 (fig. 73) representing the Great 

Plains aquifer system, simulated flow is mostly 

eastward across Nebraska, with northeasterly components across eastern Colorado and in the western parts 

of Kansas. Recharge areas are indicated for 

southeastern Colorado, along the Front Range of the 

Rocky Mountains, and in the Black Hills Uplift area 

in southwestern South Dakota. In north-central Kansas, positive recharge was input to model cells representing the outcrop area. However, general-head 

boundary conditions imposed on the cells allowed flow 

towards the outcrop cells, and thus a net discharge is 

indicated by the flow vectors. This is consistent with 

the model flow budget that indicates an outflow for 

that area of about 53 ft3/s.

Vertical flows for layer 2 (figs. 76, 77) indicate 

upward flow to layer 1 coincident with most of the 

eastern extent of the Great Plains aquifer system and 

along the Missouri River in northeastern Nebraska.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C87

105° 90°

40

35

100

_L

200 MILES

I ' 

0 100 200 KILOMETERS

Base modified from U.S. Gaological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

 LATERAL FLOW VECTOR-Length of shaft 

indicates relative magnitude

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 73. Lateral flow-velocity vectors for model layer 2 representing the Great Plains aquifer system.

Vertical flows are indicated beyond the extent of the 

Great Plains aquifer system in northeastern Kansas, 

southeastern Nebraska, and a short distance in the 

alluvium along the Missouri River. This is an area of 

glacial drift and alluvium represented by layer 1, and

simulated vertical flows are to or from underlying layers that represent subcrops, such as model layer 3.

Downward flow to layer 2 is greatest near the layer 

boundaries. The greatest vertical flow is in those 

locations because the Great Plains confining system is
C88 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

.^'V

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970 ^ ,

EXPLANATION

 LATERAL FLOW VECTOR-Length of shaft 

indicates relative magnitude

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 74. Lateral flow-velocity vectors for model layer 4 representing the upper unit in the Western Interior Plains aquifer 

system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system.

thin or not present. Where the permeable materials of 

the High Plains aquifer and the Great Plains aquifer 

system are in direct contact, the simulation indicates 

that the interchange readily occurs.

Vertical leakage of the Great Plains aquifer system, between layer 2 and layer 3, is shown in 

figure 77. Most of the area has no flow indicated 

because zero vertical leakance is specified to simulate
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C89

105° 100° 95° 90°

40'

35'

\ivtiMiv. .

0 100 200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

LATERAL FLOW VECTOR-Length of shaft 

indicates relative magnitude

APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 75. Lateral flow-velocity vectors for model layer 5 representing the lower units in the Western Interior Plains aquifer 

system and the combined Ozark aquifer, St. Francois confining unit, and St. Francois aquifer in the Ozark Plateaus aquifer

system.
C90 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105° 100' 95°

35'

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

VERTICAL FLOW DIRECTIONS-Size of symbol 

indicates relative magnitude of flow

Q_> Upward 

K Downward

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 76. Vertical flow directions for model layer 1 simulating water movement between surficial units and underlying units, 

principally between the High Plains aquifer and the Great Plains aquifer system represented by model layer 2.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C91

105° 100

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

EXPLANATION

VERTICAL FLOW DIRECTIONS-Size of symbol 

indicates relative magnitude of flow

'v J Upward 

^>K Downward

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 77. Vertical flow directions between model layers 2 and 3 simulating water movement principally between Great Plains 

aquifer system and the Western Interior Plains confining system (large no-flow area in model layer 2 results from assigning zero 

vertical leakance for evaporite deposits in model layer 3).
C92 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

evaporite deposits in the Western Interior Plains confining system. Where evaporite deposits are not 

present, vertical flow is indicated, but the quantity is 

small. An area of significant vertical flow from layer 3 

to layer 2 is indicated in north-central Nebraska. As 

an example to illustrate the small vertical flow, the 

largest volume of vertical flow in a single model cell 

between layers 2 and 3 was computed to be about 

2x10"2 ft 3/s. On a daily basis, that amount is about 

1,700 ft 3/d or about 631,000 ft 3/yr. Cell size is 28 by 

28 mi or 784 mi 2. The leakage then would be about 

805 (ft 3/mi2 )/yr. A surface-penetration depth for 

805 (ft3/mi2 )/yr is about 3.5x10 "4 in/yr. For some 

considerations, such as geochemical reactions occurring over geologic time, this quantity could 

be significant.

Simulated lateral flow in the upper unit in the 

Western Interior Plains aquifer system (layer 4) shows 

the diversity of flow resulting from irregular 

layer-extent boundaries (fig. 74). Flow in eastern Colorado and western Kansas is principally toward the 

Cambridge Arch and the Central Kansas Uplift. In 

eastern Nebraska, flow is to the southeast, but flow 

"splits" around the Nemaha Uplift where the upper 

unit of the aquifer system does not occur. Along the 

west and south sides of the Nemaha Uplift, the simulated flow moves southeast and along the southern 

boundary of the uplift and the model-layer boundary 

northeastward toward the Missouri River in northeastern Kansas. However, flow also is indicated 

toward the southeast along the east side of the Central Kansas Uplift into east-central Oklahoma. Flow 

toward the northeast in western Oklahoma is shown 

into Kansas and then south back toward eastern Oklahoma. The radial flow of the Ozark Plateaus aquifer 

system from the Ozark Uplift toward the west and 

south is illustrated by the vector plot and shows the 

area in southeastern Kansas where flow directions converge. This results in the freshwater-saltwater transition zone that occurs at the confluence of the Ozark 

Plateaus aquifer system and the Western Interior 

Plains aquifer system.

Vertical flows simulated for layer 4 are shown in 

figures 78 and 79. Because layer 3 is treated as a confining system, flow volumes between layer 3 and 4 

(fig. 78) generally are small. The simulation shows 

upward flow from layer 4 to layer 3 in an extensive 

area of the western Ozark subregion, throughout most 

of Oklahoma, all of the Texas Panhandle, and southeastern Colorado. Simulated upward flow over a 

significant part of that area (western Oklahoma and 

eastern Texas Panhandle) is due to modeling the overpressure zone in the Anadarko Basin, a zone which 

also is overlain by evaporite deposits. Upward flow to

layer 3 from layer 4 is, however, particularly large at 

the periphery of the Ozark subregion. This is the area 

where the Springfield Plateau aquifer, which is represented by model layer 4, discharges water upward. In 

east-central Colorado, most of Kansas, and the eastern one-half of Nebraska, vertical flow is downward. 

However, there is a significant area in the central part 

of Nebraska and the Central Kansas Uplift where the 

upper unit in the Western Interior Plains aquifer system is not present, and simulated vertical flows shown 

are between layer 3 and layer 5.

Vertical flow directions simulated between aquifers 

represented by model layers 4 and 5 are shown in figure 79. There is a clear pattern of large upward flows 

to layer 4 at the boundary of the Ozark subregion and 

large downward flows to layer 5 from layer 4 in the 

interior of the Ozark subregion. Along the Missouri 

River in eastern Nebraska, a majority of nodes show 

upward flow to layer 4 indicating discharge to the 

river. (Recall that flow from layer 3 down to layer 4 

predominated along the Missouri River in that locality.) From the extreme southeastern corner of 

Nebraska along the Missouri River to about 60 mi 

east of Kansas City (fig. 1), the simulation shows 

downward flow from layer 4 to layer 5, but thereafter, 

with only one exception, fairly large upward flows 

occur from layer 5 to layer 4 along the Missouri and 

Mississippi Rivers, which simulates discharge to those 

rivers.

Flow from layer 5 to layer 4 (fig. 79) is indicated in 

central Oklahoma extending in a narrow band across 

central Kansas into Nebraska. Location of the band 

is approximately bracketed between the Central Kansas Uplift and the Nemaha Uplift. Along that band 

from south-central Nebraska through north-central 

Kansas, the upper unit in the Western Interior Plains 

aquifer system is present. Just west of the band of 

upward flow cells is the Central Kansas Uplift where 

the rocks represented by layer 4 are not present. 

Thus, the simulated downward flow is from the Western Interior Plains confining system represented by 

layer 3 to layer 5 representing the lower units in the 

Western Interior Plains aquifer system.

Because layer 4 is modeled as overpressured in the 

Anadarko Basin of west-central Oklahoma, simulated 

flow is into layer 5. In the Texas Panhandle and a 

large area in southwestern Kansas, some upward flow 

is indicated from layer 5. However, the simulated 

hydraulic-head values for those areas of layer 4 and 

layer 5 are nearly the same (figs. 54, 56;, implying 

that vertical flow is small. The reason these simulated 

hydraulic-head relations exist in the panhandle area is 

that the areal extents of layers 4 and 5 across the 

Texas Panhandle are coincident and are modeled using
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C93

100 200 MILES

, ' 

200 KILOMETERS

Base modified from U.S. Geological Survey 

1:7,500,000, National Atlas, 1970

V

EXPLANATION

VERTICAL FLOW DIRECTIONS-Size of symbol 

indicates relative magnitude of flow

\ J Upward 

)K Downward

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 78. Vertical flow directions between model layers 3 and 4 simulating water movement between Western Interior Plains 

confining system and the upper unit in the Western Interior Plains aquifer system and between the Western Interior Plains 

confining system and the Springfield Plateau aquifer in the Ozark Plateaus aquifer system.
C94 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

105°

100 200 MILES 

200 KILOMETERS

Base modified from U.S. Geological Survey 

'

1:7,500,000, National Atlas, 1970

EXPLANATION

VERTICAL FLOW DIRECTIONS-Size of symbol 

indicates relative magnitude of flow

(~Jl Upward 

/K Downward

 APPROXIMATE BOUNDARY BETWEEN THE 

PLAINS AND OZARK SUBREGIONS

FIGURE 79. Vertical flow directions between model layers 4 and 5 simulating water movement between the upper and lower 

units in the Western Interior Plains aquifer system and between the Springfield Plateau aquifer and the combined Ozark and 

St. Francois aquifers in the Ozark Plateaus aquifer system (no-flow area in the Ozark subregion is where model layer 5 

simulates an outcrop area).
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C95

general-head boundaries to simulate continuation of 

the units beyond the extent of the study area (figs. 50, 

51). Therefore, the initial condition in which field 

hydraulic heads in layer 5 slightly exceeded heads in 

layer 4 persists when the hydraulic heads are computed in the simulation. The extent of model layers 

and boundary conditions imposed in eastern Colorado 

and northwestern Kansas results in both upward and 

downward flow in adjacent cells in that area. Variations of aquifer characteristics, steep hydraulic-head 

gradients, and the occurrence of mountain and basin 

conditions affect the simulation accuracy and consistency because of the large cell areas.

Lateral flow-velocity vectors for layer 5 shown in 

figure 75 have a pattern very similar to those for layer 

4 (fig. 74) in the western part of the regional area. 

This is expected because the computed hydraulic 

heads for the two layers are similar and because, in 

locations of the model where the confining unit is 

absent between the upper and lower units in the Western Interior Plains aquifer system, the layers are modeled with a large leakance coefficient between them. 

The lateral flow vectors in layer 5 are affected by 

large permeability and recharge in the outcrop area 

causing the outflow pattern from the Ozark subregion. 

The same flow pattern near the Ozark subregion 

boundary in southeastern Kansas shown for layer 4 

also is shown by the flow pattern for layer 5 and 

results from the simulation of that area as a boundary 

between separate freshwater and saltwater 

flow systems.

A comparison can be made between the magnitude 

of lateral flow vectors in layers 4 and 5 in southwestern Kansas where the units represented by these layers are deeply buried and the magnitude of lateral 

flow-velocity vectors in the Ozark subregion where 

these units are closer to the surface. A representation 

of flow in the deep sections by model layers 4 and 5 

indicates nearly stagnant or extremely sluggish flow, 

whereas flow velocities in the Ozark subregion are 

many orders of magnitude greater (figs. 74, 75). For 

example, an average interstitial velocity across a 

model-cell face for layer 5 located in western Kansas 

was computed to be 3.2xlO~10 ft/s compared to an 

average interstitial velocity of 1.4xlO~5 ft/s in the 

Ozark subregion, a difference of about five orders of 

magnitude.

Lateral flow for layer 5 in Kansas and Nebraska is 

affected by the Cambridge Arch and Central Kansas 

Uplift. From southwestern Kansas, flow appears 

almost uniform toward the northeast, then at the Central Kansas Uplift, flow turns toward the southeast 

paralleling the uplift (fig. 75). In northeastern Kansas, the flow moves to the northeast toward the Missouri River. In the eastern one-half of Nebraska, flow 

is indicated to the south just east of the Cambridge 

Arch moving east and southeast toward the Missouri 

River, and also is affected by the Nemaha Uplift. 

Flow in Oklahoma is generally toward the northeast, 

meeting with the outflow from the Ozark subregion. 

Upward flow from layer 5 to layer 4 is shown in eastern Oklahoma (fig. 79), primarily as a result of the 

model simulation of the- boundary conditions of the 

two flow systems.

MODEL BUDGET

The model-computed budget results from the 

imposed aquifer properties, geometry, and boundary 

conditions, coupled with the computed hydraulic 

head. The budget for all layers is shown in figure 80.

The specified-head condition for the surficial units 

(layer 1) resulted in a simulated downward flow of 

224 ft3/s from layer 1 to layer 2 and simulated upward 

flow from layer 2 to layer 1 of 219 ft 3/s.

The overall hydrologic budget for layer 2 (the 

Great Plains aquifer system) is about 361 ft3/s in and 

about the same amount out (fig. 80). The representation of the Great Plains aquifer system (layer 2) has 

about 93 ft3/s recharge imposed (table 5), most of 

which (83 ft3/s) is in north-central Kansas with general-head boundaries specified to allow the simulation 

to compute recharge values necessary to balance the 

flow. The model simulation, from specification of general-head boundaries, computed an additional 

recharge of 42 ft3/s, a major part of which (27 ft3/s) 

was for the southeastern Colorado, northeastern New 

Mexico area. In north-central Kansas, discharge at 

outcrops was computed to be 136 ft3/s for a net outflow at those model cells of 53 ft3/s.

The Western Interior Plains confining system 

(layer 3) has only about 1 ft3/s interchange with the 

Great Plains aquifer system represented by layer 2, 

and there are very small lateral flows. The major 

interchanges from layer 3 occur in cells that simulate 

the outcrop area of constant-head boundary conditions near the Ozark subregion. There, vertical leakance allows upward movement of water from the 

Ozark Plateaus aquifer system and some from the 

Western Interior Plains aquifer system (fig. 78).

In regard to the constant-head cells (fig. 80), inflow 

to the system (to layer 4) is 363 ft 3/s, and outflow 

from the system (to layer 3) is 854 ft 3/s for a net leakage from layer 4 to layer 3 of about 491 ft 3/s. All but 

about 1 ft3/s of the flow from layer 4 to layer 3 is 

within and in proximity to the Ozark subregion.
C96 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Layer 

number Recharge

/ 

/ 

/

1,

fer 

F1*67

F^

1*31

y 154"

1| Water table / General 

(Constant head) , head 

1224 '/ I

U 

|219 1 142 Net recharge

Net re

3 1 f t Wa*er table < |l 4^ 492 (Consent / 1M3

I 363 /

U t Rivers \FallLine 

1^30" ^37\

Layer 5 outcrops

charge 

_ . Net flows

3,069 - inflow 

1,452 

1,617 - outflow

5 I t Rivers \fallline

I 169 97T / y\ 

*225 ^\

EXPLANATION

1f1^" DIRECTION OF FLOW-Number indicates 

rate of flow, in cubic feet per second

Note: Fall Line refers to the boundary with 

the Mississippi Alluvial Plain

FIGURE 80. Simulated regional hydrologic budget.

Recharge to that part of layer 4 that represents the 

Springfield Plateau aquifer in the Ozark subregion was 

input as 1,643 ft 3/s (fig. 80). A major outflow determined from the simulation for the Springfield Plateau 

aquifer is 1,130 ft 3/s to rivers, 772 ft 3/s to the Missouri 

and Mississippi Rivers, and about 355 ft 3/s from 

model cells in the vicinity of the Canadian and 

Arkansas Rivers in eastern Oklahoma and northwestern Arkansas (fig. 1). An outflow across the Fall Line 

(toward the Mississippi Alluvial Plain) of 137 ft 3/s is 

indicated. Vertical-flow vectors (fig. 79) between 

layer 4 and layer 5 representing flow between the 

Springfield Plateau aquifer and the Ozark and 

St. Francois aquifers in the Ozark subregion and 

within about 40 mi beyond the western Ozark subregion boundary indicate a large vertical exchange. In 

this area, the model-computed flow between layers 4 

and 5 is 1,366 ft3/s from layer 4 to 5 and 1,527 ft3/s 

from layer 5 to layer 4, resulting in net upward flow of 

161 ft3/s. The model budget for the region (fig. 80) 

indicates a net vertical flow from layer 5 to layer 4 of 

169 ft3/s; therefore, most of that vertical flow for the 

regional model occurs in the cells representing the 

Ozark subregion and its vicinity.

Model budget results for layer 5 (fig. 80), the combined Ozark aquifer, St. Francois confining unit, and 

St. Francois aquifer in the Ozark subregion, indicates

a combined aquifer discharge to the Missouri and Mississippi Rivers of 941 ft3/s and combined inflow from 

rivers of 225 ft3/s. Simulated inflow and outflow at 

the study boundary for layer 5 (the lower units in the 

Western Interior Plains aquifer system) are 154 and 

31 ft3/s, respectively. These flows are principally 

along the study-area boundaries in Oklahoma, the 

Texas Panhandle, and southeastern Colorado (fig. 51). 

Flow was generally out of the Ozark subregion; the 

net simulated flow across the Fall Line (fig. 3) to the 

Mississippi Alluvial Plain for layers 4 and 5 is 

827 ft3/s. lines and Emmett (1994) present evidence 

that most ground water entering the Mississippi Alluvial Plain across the Fall Line discharges upward into 

alluvial material within a short distance. Permeable 

rocks of the Ozark aquifer (represented by layer 5 in 

the Ozark subregion) are in direct contact with permeable alluvium and Cretaceous-age sand in a narrow 

band approximately 10-mi wide along the Fall Line. 

Southeast of the band, a nearly impermeable Cretaceous clay separates the permeable Cretaceous sand 

and underlying Paleozoic rocks from the alluvial 

material. Some freshwater may flow beneath the clay 

and eventually leak upward into the alluvium, but it 

is likely that most ground water moves into the alluvium. Also, the few dissolved-solids measurements 

indicate freshwater does not travel far beneath the
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C97

alluvial plain (Imes and Emmett, 1994). Water that 

crosses the Fall Line from the Ozark Plateaus aquifer 

system probably discharges into marshes, shallow 

water tables, drains, and the Black and White Rivers 

(fig. 1) that are adjacent and practically parallel to 

the Fall Line for a major part of its length.

WATER SUPPLY AND WATER-STORAGE 

POTENTIAL

GREAT PLAINS AQUIFER SYSTEM

The Great Plains aquifer system in the regional 

study area is generally overlain by a thick, slightly 

permeable confining material and by the very productive High Plains aquifer. Because large quantities of 

water of usable quality are available from the High 

Plains aquifer and water in the Great Plains aquifer 

system is of questionable or unknown quality, very little development for water supply has occurred in relation to the amount of water stored. Also, because the 

Great Plains aquifer system is deeply buried in northeastern Colorado, northwestern Kansas, and throughout most of Nebraska, it has not been considered prior 

to this study to have significant potential for development (Helgesen and others, 1982, p. 410). However, a 

unit saturated thickness exceeding 700 ft in Nebraska 

indicates a very large quantity of water in storage. 

Water stored in the Great Plains aquifer system based 

on thickness and porosity (figs. 34, 35) is estimated to 

be almost 12 billion acre-ft (table 7) in the regional 

study area. The amount of water that the aquifer system might yield is estimated to be one-fourth that 

amount or about 3 billion acre-ft if nonyielding clay 

layers and specific yield are considered still a very 

large resource. (Note: In table 7 the amount listed for 

South Dakota, 190 million acre-ft, is not representative of the storage in the Great Plains aquifer system 

for the whole State but represents only the small part 

of the State included in the CMRASA.) Helgesen and 

others (1982, p. 410) point out that in the regional 

study area, the Great Plains aquifer system (historically referred to in the literature as the "Dakota") has 

been an important source of water for irrigation, 

domestic, or stock use for many years in parts of Kansas and southeastern Colorado. The droughts of the 

1970's increased attention to the Great Plains aquifer 

system as a source for irrigation water in Kansas, 

eastern Nebraska, and northwestern Iowa.

Water from the Great Plains aquifer system may 

be a source to supplement water withdrawn for irrigation from the High Plains aquifer. This conclusion is

based on small dissolved-solids concentrations (fig. 37) 

and simulated vertical flow between the surficial units 

(layer 1) and the Great Plains aquifer system (layer 2) 

(fig. 76). Potable water occurs along the eastern and 

southeastern extent of the Great Plains aquifer system, a condition consistent with the vertical exchange 

(both upward flow and downward flow) of water 

between the two layers as indicated by the 

steady-state model simulation. Along the eastern 

extent of the Great Plains aquifer system, the natural 

flow system may be effective in mixing natural 

recharge, water in the Great Plains aquifer system, 

and water in the High Plains aquifer. This aspect is 

particularly important to a location in Kansas where 

the largest area of interchange is indicated by the 

model simulation in an area of large irrigation demand 

and concern for dwindling supplies. Additional study 

of the effect on the quantity and quality of water 

flowing into and out of the High Plains aquifer would 

be of value at locations where it is presently occurring 

and at locations where interchange might be induced 

further. Previous studies have dealt with parts of the 

issue to a large degree (U.S. Geological Survey, 1966; 

Lobmeyer and Weakly, 1979; Kume and Spinazola, 

1982; Spinazola and Dealy, 1983; Kume, 1984; Watts, 

1989), but in light of information developed in the 

present study, further investigations would be of 

value.

WESTERN INTERIOR PLAINS AQUIFER SYSTEM

Ground water stored in the Western Interior Plains 

aquifer system, based on saturated thickness and 

regionalized porosity (figs. 23, 25), amounts to more 

than 7 billion acre-ft of water (table 7). Because very 

little of the aquifer system contains water with less 

than 5,000 mg/L dissolved solids, the water has very 

limited use, and its potential for development as a 

water supply is slight.

Results of flow simulation in model layers 4 and 5 

representing units in the Western Interior Plains aquifer system, however, indicate extremely slow fluid 

velocities in the deeply buried units and that the system is nearly isolated. Because of the degree of isolation, very slow-moving water, and poor water quality, 

the aquifer system at least in some areas has characteristics that indicate potential for waste storage or 

disposal. Although this study did not consider waste 

disposal, consideration of the depth, slow groundwater movement, and extremely large dissolved-solids 

concentrations in the ground water (brines) suggests 

that waste storage or disposal is feasible. As an example, lateral flow velocities simulated in layer-5 model
C98 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

TABLE 7. Water in storage in geohydrologic units in the regional study area

[Data in millions of acre-feet; , no data. Total ground water in storage. Drainage water would be less if 

nonyielding clay beds and specific yield of permeable units are taken into consideration]

Geohydrologic unit

Western Interior Plains aquifer system

f^~nr.t D1«;«c,

State

Arkansas

Colorado

Kansas

Missouri

Nebraska

New Mexico

South Dakota

Oklahoma

Texas

Wyoming 

Total

aquifer system Upper unit

Plains subregion

...

1,553 188

2,717 818

29

7,035 174

54 8

190

9 625

137

244 51

11,802 2,030

Lower units

71

223

1,631

121

766

9

8

2,099

215

...

5,143

Geohydrologic unit

State

Arkansas

Arkansas5

Kansas

Missouri

Oklahoma

Total

Springfield 

Plateau St. Francois 

aquifer1 Ozark aquifer aquifer

Ozark subregion'5

30 1,775 51

50 1,113 39

25 127 7

118 3,329 544

45 445 8

268 6,789 649

Combined 

St. FrancoisOzark aquifer2

1,825

1,152

134

3,873

454

7,438

1Regional model layer 4. 2Regional model layer 5.

3 Data from Leo F. Emmett, U.S. Geological Survey, written commun., 1987.

4Freshwater. 5Saline water.

cells in southwestern Kansas are extremely slow. The 

model computed a quantity of flow across a model-cell 

face in the northeasterly direction to be DxlO"6 ft3/s 

(layer 5, row 21, column 16; flow from row 21 to 20 

along column 16). The simulated face area of that 

model cell is 1.53x10 8 ft2, and porosity is about 5 percent (fig. 23). Therefore, a pore velocity for modeled 

flow would be about 1.2xlO"12 ft/s, which is 4xlO'5 ft/yr 

or about 40 ft in a million years. Even for three 

orders of magnitude greater velocity, the travel would 

be only about 7.6 mi in a million years.

In considering long-term waste storage, other 

important factors besides velocity of travel should be 

evaluated. Plans for storage of wastes near areas of 

prior tectonic activity would need to include the 

potential for future tectonic activity. There is general 

tectonic stability in the Western Interior Plains; however, tectonic activity has occurred in the Rocky

Mountains and across the southern boundary of the 

regional study area, such as the Cimarron Uplift, the 

Amarillo-Wichita-Criner Uplifts, and the Arbuckle 

Mountains. Also, the Central Kansas Uplift and 

similar features indicate areas of potential tectonic 

activity as a result of prior tectonic events. Although 

the overpressure zone in the Anadarko Basin is in an 

area of very slight permeability and water movement 

is extremely slow, the potential for upward movement 

exists because of overpressure. Waste storage in an 

overpressured zone probably is not desirable. However, the integrity of the overlying confining material, 

the very slow water movement, and the thickness of 

the confining system overlying the Western Interior 

Plains aquifer system indicate that the aquifer system 

has potential for long-term waste storage or disposal 

in some areas.
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C99

OZARK PLATEAUS AQUIFER SYSTEM

The results of this study quantify the large potential of the water resources of the Ozark subregion. 

Freshwater in storage in the Ozark subregion of the 

CMRASA is estimated to exceed 7.5 billion acre-ft 

(table 7) (L.F. Emmett, U.S. Geological Survey, written commun., 1987). Also, a total model-recharge 

input (table 5) of 6,947 ft3/s is but a small part of the 

approximately 55,000 ft3/s of potential recharge from 

precipitation through the soil zone to the water table 

for the entire Ozark subregion. Thus, more than 87 

percent of the potential recharge is "rejected," that is, 

intercepted locally by streams as indicated by the 

regional model analysis. The rejected water is a 

ground-water resource that could be utilized. 

Increased pumping of ground water would result in 

increased recharge to the regional aquifers and 

decreased discharge to streams in their outcrop areas.

A major point to be considered is that, of the water 

resources actually available for planning and development in the Ozark subregion, the model evaluates only 

a relatively small component of the water that reaches 

the water table and is not discharged locally (not 

intercepted within model cells). Water budgets from 

ground-water models have been used as a basis for 

planning and development; however, serious errors can 

result if available resources not accounted for in the 

model are ignored. The Ozark subregion is not unique 

in that some tens of thousands of cubic feet per second 

of rainfall and rejected or intercepted recharge could 

be withdrawn and removed from the ground-water 

flow system without materially affecting the 

ground-water flow system. However, such withdrawals would affect the surface-water resource. For example, if the 6,947 ft3/s of recharge that was simulated 

by the model was used in the future, this would not 

mean that all the aquifer contribution to the base flow 

of stream would be eliminated. Instead, less recharge 

from infiltration to the water table would be intercepted by streams. Ground-water contribution to 

streams would be reduced, and the overland part of 

the streamflow would be affected only slightly. The 

ground-water resources of the area generally are little 

used. Harvey (1980, p. 1) concluded that because 

only a small part of available ground water in the 

Springfield-Salem Plateaus of southern Missouri and 

northern Arkansas is used (and large increases in use 

are not predicted), it is unlikely that declines in 

ground-water levels of extended duration will occur in 

the near future.

The regional model computed flow to the Missouri 

and Mississippi Rivers of 1,488 ft3/s in the Ozark subregion. Flows were 1,088 ft3/s to the Missouri River

and 400 ft3/s to the Mississippi River. These computed flows are minor in relation to actual mean 

annual streamflows, such as 177,800 ft3/s, the mean 

annual discharge of the Missouri River at St. Louis 

for 1951-80 (Hedman and Jorgensen, 1990, table 2). 

However, results of an analysis of the gain in mean 

annual flow to the Missouri River from the Missouri 

River Valley aquifer between Waverly and St. Louis, 

Missouri, for that 30-year period averaged 1,110 ft 3/s 

(Hedman and Jorgensen, 1990, table 2), a value comparable to the 1,088 ft 3/s computed by the model for 

subjacent regional geohydrologic units. Analysis by 

Hedman and Jorgensen (1990) includes gaged inflow, 

ungaged inflow, valley recharge, city discharge, city 

intake, water-surface evaporation, consumptive use, 

and Missouri River Valley aquifer underflow. Large 

boundary sources and sinks (such as the Missouri and 

Mississippi Rivers) often are simulated as unknowns 

so that the resulting computed flows compensate for 

other imposed conditions, such as specified-head and 

head-dependent flux. The implications are that analyses of gains and losses of major streams should be 

carefully analyzed particularly if the sources or sinks 

are boundaries between studies. The comparability of 

model-computed river gain to an independent analysis 

of river gain (gain of the Missouri River between 

Waverly and St. Louis, Missouri) is an indication of 

good correspondence between the model and the independent streamflow study.

SUMMARY

The study area of the Central Midwest regional 

aquifer-system analysis (CMRASA) extends from the 

foothills of the Rocky Mountains in Colorado to the 

Mississippi River in eastern Missouri, and from South 

Dakota to the Ouachita, Arbuckle, and Wichita 

Mountains of Arkansas and Oklahoma. The purpose 

of this report is to describe the aquifers, aquifer systems, confining units, confining systems, and the 

results of computer simulation of regional flow. The 

study is limited to rocks ranging from Cambrian 

through Lower Cretaceous age and, thus, does not 

include the flow regime in younger rocks constituting 

the High Plains aquifer and other surficial units. 

However, effects of Upper Cretaceous and younger 

units are included in the simulation as an upper 

boundary.

Structural deformation is not severe over most of 

the study area, which lies within the stable interior of 

the North American continent. Geologic structure is 

dominated by broad basins and arches; however, along 

the southern and western boundaries, substantial
C100 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

crustal deformation resulted in intense folding and 

faulting. These complex structures define the southern and western boundaries of the study area. Generally, time-stratigraphic units are continuous where 

present without severe structural deformation and offset of rock units except at those boundaries.

Regional estimates of porosity were determined 

from geophysical logs. A comparison between 

reported porosity (from analyses of drill-stem tests, 

data from laboratory analyses of rock cores, and analyses of production data from oil and gas reservoirs) 

and the regionalized porosity (from geophysical logs) 

indicated good correlation for loosely cemented fineto medium-grained sandstone with dominantly primary porosity but poor correlation for rocks such as 

limestone in which secondary porosity dominates. 

Reported porosity values generally are not representative of entire stratigraphic sections because testing 

was done in specific oil and gas reservoirs, which typically have greater than average porosity. Preliminary 

maps of porosity were made and modified to provide 

an estimate of porosity for basins based on decreases 

in porosity with depth.

Few permeability data were available for most of 

the regional study area. In rocks containing saline 

water, permeability data were obtained from analyses 

of drill-stern tests, laboratory analysis of rock cores, 

and oil-production tests. A method was developed to 

estimate the permeability of geohydrologic units from 

geophysical logs that provided estimated values of 

porosity. Permeability values from aquifer tests and 

calculated permeability values using data from 

geophysical logs were plotted and contoured on maps 

to show regionalized permeability. Correlation 

between site-specific permeability values from oil-reservoir tests and calculated permeability values using 

geophysical logs was poor. Differences of two orders of 

magnitude were not unusual because site-specific data 

represent small sections of rock, whereas geophysical 

logs may represent the entire section of the geohydrologic unit.

Geohydrology of the study area is described on the 

basis of two subdivisions referred to as the Plains subregion and the Ozark subregion. In the Plains subregion, six regional geohydrologic units have been 

defined, and in the Ozark subregion, three regional 

geohydrologic units have been defined. Geohydrologic 

units that were studied in detail were the Great Plains 

aquifer system, the Western Interior Plains confining 

system, which is common to both the Plains and 

Ozark subregions, the Western Interior Plains aquifer 

system, and the Ozark Plateaus aquifer system.

Hydraulic-head distribution indicates that flow in 

the Western Interior Plains and Great Plains aquifer

systems is generally west to east and southeast in the 

regional saline-water system. Hydraulic head in the 

Ozark Plateaus aquifer system indicate nearly radial 

outflow from the Ozark Uplift toward the Plains 

subregion, the Missouri River, the Mississippi River, 

and the southern boundary of the study area. 

Recharge in the Plains subregion occurs at the western 

boundary near the Rocky Mountains and Black Hills 

Uplift and through some outcrop areas of the Great 

Plains aquifer system in southeastern Colorado and 

north-central Kansas. Comparatively large quantities 

of water are recharged in the Ozark subregion through 

the outcrop areas. Upward movement from both 

freshwater and saline-water flow systems probably 

occurs near the saltwater-freshwater transition zone in 

western Missouri, southeastern Kansas, northeastern 

Oklahoma, and northwestern Arkansas.

A numerical ground-water flow model was used to 

test the conceptualization of flow in the regional aquifers and aquifer systems in the study area. The model 

used was the U.S. Geological Survey Modular 

Ground-Water Flow Model, which was designed to 

simulate three-dimensional movement of ground water 

of constant density through porous media. The model 

grid for the study area utilized 28 rows and 33 columns uniformly spaced at 28 mi. Rows are oriented 

coincident with the principal direction of structural 

lineaments, N. 35°W. from a point in east-central 

Kansas. The lineaments are, in general, fractures, and 

it is believed that the grid orientation reasonably corresponds to the principal hydraulic-conductivity tensor of aquifers in the flow system to be simulated.

Five geohydrologic units in the regional study area 

are represented in the model as layers. They are: 

Layer 1. High Plains aquifer, glacial drift, and the

Great Plains confining system; 

Layer 2. Great Plains aquifer system. 

Layer 3. Western Interior Plains confining system. 

Layer 4. The upper unit in the Western Interior 

Plains aquifer system in the Plains subregion and the 

Springfield Plateau aquifer in the Ozark Plateaus 

aquifer system in the Ozark subregion. 

Layer 5. The lower units in the Western Interior 

Plains aquifer system in the Plains subregion and the 

combined Ozark aquifer, the St. Francois confining 

unit, and the St. Francois aquifer in the Ozark Plateaus aquifer system in the Ozark subregion.

The model was based on a mathematical development for which a constant, uniform density was 

assumed. However, some deep geohydrologic units 

contain water with large concentrations of dissolved 

solids. Because this saline water has greater density 

than freshwater, consideration was given to the effects 

of variable density on flow. The effect of
REGIONAL AQUIFERS GEOHYDROLOGY AND SIMULATION OF STEADY-STATE FLOW CONDITIONS C101

variable-density water on flow was simulated using a 

technique that compensates for variable density in the 

constant-density flow computation.

Calibration of the model was based on comparison 

of model-computed hydraulic head to measured or 

estimated hydraulic head (collectively referred to in 

the report as "field hydraulic head"). The general 

areal configuration of computed hydraulic-head values 

showed little difference between simulations that 

included the effect of variable density and simulations 

that assumed constant density. However, some 

"local" hydraulic-head differences caused computed 

flow-velocity vectors to indicate significant differences 

in direction. No direction differences were indicated 

for model layer 2, but in layers 4 and 5 there were 

changes primarily in areas of large density differences 

and very small permeability. Indication of flowdirection changes supports the necessity of considering 

the effects of variable density.

A major component of the regional steady-state 

hydrologic budget is potential recharge in the Ozark 

subregion. A recharge model for the regional study 

area developed areal recharge potential based on 

hydrologic properties of the soil, vegetation type, 

monthly precipitation, and computed monthly potential evapotranspiration for a 30-year period (1951-80). 

Using a technique of distributing recharge potential 

within a model cell, the minimum recharge in the 

Ozark subregion was determined to be about 5 percent 

of the potential recharge; however, stream-discharge 

data attributed to ground-water flow for only part of 

the Ozark subregion indicated that recharge probably 

exceeds 10 percent of potential recharge and may be 

as much as 20 percent. Recharge potential distributed 

within a model cell coupled with cell-by-cell estimates 

of outflow or inflow because of stream-aquifer interactions gave an estimate of net flow per cell that was 

used as input to the model. The recharge budget that 

gave the best-fit model simulation for the whole region 

was a recharge of about 7,000 ft3/s, of which 93 ft3/s 

were into layer 2, about 2,100 ft3/s were into layer 4,

o

and about 4,800 ft /s were into layer 5. Outflow to 

rivers and streams for layers 4 and 5 totaled about 

3,800 ft3/s; thus, the net recharge to the region was 

about 3,200 ft3/s. Only about 1 percent of the net 

recharge modeled for the region goes into layer 2.

A sensitivity analyses of the model showed that 

average differences between model-computed and field 

hydraulic-head values were most sensitive to changes 

in hydraulic conductivity. Differences were not very 

sensitive to changes in leakance between layers 

because computed hydraulic-head differences between 

layers 4 and 5 were small. Also, for much of the 

regional study area, evaporite deposits in the Western

Interior Plains confining system were simulated as 

impermeable in layer 3; thus, layers 4 and 5 were isolated from layer 2. The changes in leakance between 

layers in and near the Ozark subregion resulted in a 

small effect because the large outcrop area of the 

Ozark aquifer (layer 5) offered little area for vertical 

leakage.

Computed hydraulic heads were moderately sensitive to recharge for the overall regional model. 

Because of the modeling technique for layer 2, 

hydraulic-head values were insensitive to changes in 

recharge; any change was compensated for by the general-head boundary condition. The effects of recharge 

were major for changes in computed hydraulic head in 

the Ozark subregion where recharge was applied to 

cells in layers 4 and 5 that represented outcrop areas 

of the Springfield Plateau aquifer and the Ozark aquifer. The buried parts of those layers were affected 

only near the outcrop areas. Therefore, the effects of 

recharge on the average difference between computed 

and field hydraulic head were small because differences for the whole region were included when computing the average.

Varying two model characteristics simultaneously, 

such as hydraulic conductivity and recharge, illustrated a nonunique characteristic of the model calibration. For model layers 4 and 5, average differences 

between computed and field hydraulic head of less 

than 1 ft were obtained with recharge rates from 

about 40 percent of the calibration values and with 

hydraulic conductivity from about 60 percent of the 

calibration values to about 160 percent for both values. Varying recharge and leakance values simultaneously produced a double-value relation of 

differences between computed and field hydraulic 

head. The relation was symmetrical for layer 4 in 

that for a recharge of 80 percent of the calibration 

value, a leakance factor of about 40 percent or 160 

percent gave an average difference between computed 

and field hydraulic head of about 1 ft. A similar relation occurred for layer 5; however, it was somewhat 

less symmetrical.

The question of discretization errors resulting from 

the large (28 mi) node spacing of the regional model 

was investigated. An error analysis compared computed and field hydraulic heads of the regional model 

with 28-mi node spacing to computed and field 

hydraulic heads of the Great Plains aquifer system 

subregional model with 14-mi node spacing. There 

were no significant differences between model results 

using the two different node spacings.

Model-computed hydraulic heads indicated 

regional flow systems that generally conformed to 

conceptual flow patterns. Variations of flow directions
C104 REGIONAL AQUIFER-SYSTEM ANALYSIS CENTRAL MIDWEST

Harvey, E.J., 1980, Ground water in the Springfield-Salem Plateaus 

of southern Missouri and northern Arkansas: U.S. Geological 

Survey Water-Resources Investigations Report 80-101, 66 p.

Hedman, E.R., and Jorgensen, D.G., 1990, Surface- and groundwater interaction and hydrologic budget of the Missouri River 

Valley aquifer between Yankton, South Dakota, and St. 

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