science for a changing world

Prepared in cooperation with the 

CEDAR FALLS UTILITIES

Simulation of Ground-Water Flow 

in the Silurian-Devonian Aquifer, 

Cedar Falls, Iowa

Water-Resources Investigations Report 02-4081

U.S. Department of the Interior 

U.S. Geological Survey

Date: July 22,2002

Mail Stop 415

Memorandum 

To:

From: 

Subject:

U.S. Geological Survey, Information Services, Bldg. 810

Box 25286, MS 306, Denver Federal Center, Denver, Colorado

District Chief, Colorado District, Water Resources Division

New U.S. Geological Survey Water-Resources Investigations Report

The following report was authorized by _____ _____Tohn Helgesen________ for the

Director on _____February 27,2002____ for distribution. A reproducible copy

is enclosed.

TITLE: Simulated Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa

AUTHOR: Michael J. Turco

CONTENTS: : 40 over-size sheets (for examples, larger than 

8 1/2 x 11 inches)_______________

Depositories: See monthly list of "New Publications of the U.S. Geological Survey"

for addresses of libraries and Earth Science Information Centers (ESIC).

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William F. Horak, Jr.
Simulation of Ground-Water Flow 

in the Silurian-Devonian Aquifer, 

Cedar Falls, Iowa

By Michael J. Turco

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations Report 02-4081

Prepared in cooperation with the 

CEDAR FALLS UTILITIES

Iowa City, Iowa 

2002
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, 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. Geological Survey.

For additional information write to:

District Chief

U.S. Geological Survey

Water Resources Division

P.O. Box 1230

Iowa City, Iowa 52244

Copies of this report can be purchased 

from:

U.S. Geological Survey 

Information Services 

Box 25286

Denver Federal Center 

Denver, CO 80225
CONTENTS

Abstract..................................................................................................................^^ 1

Introduction.........................................................................................................................................................^ 1

Description of Study Area........................................................................................................................................... 2

Acknowledgments....................................................................................................................................................... 2

Methods of Investigation ....................................................................................................................................................... 2

Surface-Water Measurements...................................................................................................................................... 2

Ground-Water-Level Measurements............................................................................................................................ 4

Well Construction and Nomenclature.......................................................................................................................... 4

Aquifer Properties..............................................................................................................................................^ 5

Hydrogeology............................................................................................................................_^ 5

Geology and Water-Bearing Characteristics................................................................................................................ 8

Surface Water............................................................................................................................................................... 9

Ground Water............................................................................................................................................................... 9

Simulation of Ground-Water Flow ........................................................................................................................................ 10

Model Description and Boundary Conditions ............................................................................................................. 11

Model Parameters.........................................................................................~^ 11

Model Calibration........................................................................................................................................................ 15

Sensitivity Analysis..................................................................................................................................................... 19

Model Limitations ....................................................................................................................................................... 22

Simulation Results....................................................................................................................................................... 23

Summary...................................................................... 26

References................................................... 27

Appendix....................................................................................................................._^ 29

FIGURES

1. Map showing location of study area ......................................................................................................................... 3

2. Map showing extent of digital model, location of data-collection sites, and potentiometric surface constructed

from mean measured water levels for the Silurian-Devonian aquifer, April 1998 to February 1999....................... 4

3. Graph showing Cedar River stage at Waterloo, Iowa, and measured water levels in three wells near

Cedar Falls, Iowa....................................................................................................................................................... 5

4-9. Maps showing:

4. Orientation of model grid, grid discretization, and stress-related model parameters used in layer 1............... 12

5. Orientation of model grid, grid discretization, and stress-related model parameters used in layer 2............... 13

6. Orientation of model grid, grid discretization, and stress-related model parameters used in layer 3............... 14

7. Distribution of horizontal and vertical hydraulic conductivity, layer 1............................................................. 16

8. Distribution of horizontal and vertical hydraulic conductivity, layer 2............................................................. 17

9. Distribution of horizontal and vertical hydraulic conductivity, layer 3............................................................. 18

10. Graph showing proportion of simulated inflow and outflow from river leakage and general head-boundaries

as a result of varying riverbed conductance and corresponding root-mean-squared error........................................ 22

11. Map showing simulated potentiometric surface in the Devonian-age rock .............................................................. 24

12. Map showing simulated potentiometric surface in the Silurian-age rock................................................................. 25

TABLES

1. Well data and statistical summary of measured water levels, April 1998-February 1999...................................... 6

2. Hydrogeologic units in the study area and their water-bearing characteristics....................................................... 8

3. Estimated horizontal hydraulic conductivity in the study area ............................................................................... 10

4. Simulated ground-water level and deviation from the mean measured water level in wells located within the

modeled area............................................................................................................................................................ 20

CONTENTS iii
5. Results of the sensitivity analyses of the calibrated steady-state ground-water flow model for the

Silurian-Devonian aquifer near Cedar Falls, Iowa.................................................................................................. 21

6. Simulated water budget under steady-state conditions ........................................................................................... 21

7. Well characteristics, nitrate levels, and pumpage information for active Cedar Falls Utilities municipal-supply

wells, 1999 .....................................................................................................................................................^ 26

Al. Iowa Department of Natural Resources-Geological Survey Bureau (IDNR-GSB) well logs used to estimate the

elevation of the top of each rock-age interval and the thickness of water-bearing units in the study area............. 31

CONVERSION FACTORS AND VERTICAL DATUM

Multiply

inch

foot (ft) 

mile (mi) 

square mile (mi2 ) 

gallon (gal)

million gallons (Mgal) 

foot per day (ft/d) 

cubic foot per second (ft3/s) 

gallon per minute (gal/min) 

gallon per day (gal/d)

million gallons per day (Mgal/d) 

foot squared per day (ft2/d)

By

25.4

0.3048 

1.609 

2.590 

3.785

3,785 

0.3048 

0.02832 

0.06309 

0.003785

0.04381 

0.09290

To obtain

millimeter

meter 

kilometer 

square kilometer 

liter

cubic meter 

meter per day 

cubic meter per second 

liter per second 

cubic meter per day

cubic meter per second 

meter squared per day

Temperature in degrees Fahrenheit (°F) can be converted to degrees Celsius (°C) as follows:

op _ (°F-32) 

L ~ 1.8

Abbreviated water-quality units used in this report: Chemical concentrations are given in metric 

units. Chemical concentration is given in milligrams per liter (mg/L) or micrograms per liter 

(|ig/L). Milligrams and micrograms per liter are units expressing the concentration of chemical 

constituents in solution as weight (milligrams or micrograms) of solute per unit volume (liter) 

of water. For concentrations less than 7,000 mg/L, the numerical value of milligrams per liter is 

the same as for concentrations in parts per million. The numerical value of micrograms per liter 

is the same as for concentrations in parts per billion.

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

(NAVD 88).

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

83).

Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft /d)/ft ]ft. In this report, the mathematically reduced form, foot 

squared per day (ft/d), is used for convenience.

iv CONTENTS
Simulation of Ground-Water Flow in the 

Silurian-Devonian Aquifer, Cedar Falls, Iowa

By Michael J. Turco

Abstract

The Silurian-Devonian bedrock aquifer in 

the Cedar Falls, Iowa, area provides large quantities of good quality ground water for municipal 

water suppliers as well as private residential users. 

The highly transmissive nature of the SilurianDevonian aquifer material, due to fractures and 

karst features in the area and areas of thin, overlying Quaternary deposits, results in a groundwater supply vulnerable to contamination. To 

address these concerns, the U.S. Geological 

Survey, in cooperation with Cedar Falls Utilities, 

conducted a study from 1998 to 2001 to evaluate 

the hydrogeology and simulate the ground-water 

flow in the Silurian-Devonian bedrock aquifer in 

the Cedar Falls area.

A steady-state, ground-water flow model 

was constructed for a 200-square-mile area 

including Black Hawk County and small portions 

of Benton, Bremer, Buchanan, Butler, Grundy, 

and Tama Counties in northeast Iowa. The 

Silurian-Devonian aquifer was modeled using 

ground-water and surface-water data collected 

from April 1998 to February 1999 to help conceptualize the ground-water flow system. A potentiometric surface map constructed using the mean 

water-level from April 1998 to February 1999 

shows predominant ground-water flow is toward 

the Cedar River. The modeled area was 

discretized into a 220-row by 180-column grid 

with cells measuring 500 feet by 500 feet, 

including the cities of Cedar Falls, Waterloo, and 

Evansdale. Three model layers were used to simulate flow in the surficial Quaternary aquifers, the 

Devonian-age rock, and the Silurian-age rock.

The shape of the simulated potentiometric 

surfaces and the direction and magnitude of the 

simulated ground-water flow is similar to the 

potentiometric surface and flow directions interpreted from the mean measured water levels. The 

simulated ground-water flow is predominantly 

toward the Cedar River. The primary sources of 

inflow to the Quaternary and Silurian-Devonian 

aquifers are recharge from precipitation and 

leakage from the Cedar River. The primary 

sources of outflow from the flow system are 

municipal ground-water withdrawals (pumpage) 

and leakage to the Cedar River and its tributaries. 

With the exception of the main pumping centers, 

where increased withdrawals may cause localized 

mixing, there is little evidence of mixing between 

the Devonian-age and Silurian-age rock units.

INTRODUCTION

The Silurian-Devonian aquifer in the Cedar 

Falls, Iowa, area provides large quantities of good 

quality ground water for municipal water suppliers as 

well as private residential users. Ground-water yields 

at individual wells can be more than 2,000 gal/min. 

However, the highly transmissive nature of this 

bedrock aquifer material due to the high density of 

fractures and karst features in the area, and areas of 

thin overlying unconsolidated deposits (20 to 100 ft) 

result in a ground-water supply vulnerable to contamination. Both historical and current land uses result in 

the potential for contamination of the aquifer. A longterm trend toward larger nitrate concentrations 

(Schaap, 1999) and an increased number of detections 

of trace organic compounds have been reported for 

specific municipal water-supply wells (Paul Mallinger, 

Cedar Falls Utilities, written commun., 1997).

Abstract
To address these concerns, the U.S. Geological 

Survey (USGS), in cooperation with Cedar Falls 

Utilities, conducted a study from April 1998 to 

September 2001 to evaluate the hydrogeology of the 

Silurian-Devonian aquifer in the Cedar Falls area. The 

objectives of the study were to (1) evaluate the hydrogeology and (2) simulate the ground-water flow in the 

Silurian-Devonian aquifer under current (1998) 

pumping conditions. The purpose of this report is to 

describe the results of the study. Hydrogeologic and 

water-quality data used in this report were collected 

from April 1998 to February 1999.

The results of the study can be used by Cedar 

Falls Utilities (CPU) to establish a wellhead protection 

program and manage the development of the groundwater resource. The establishment of a wellhead 

protection program can educate people living and 

working in areas contributing recharge to wells about 

practices that prevent ground-water contamination and 

encourage the protection of the water supply for future 

generations. This study advances the understanding of 

ground-water flow to pumping wells in a fractured 

carbonate aquifer, and results of the study can be used 

by other water managers and planners using water 

supplies from similar hydrogeologic systems.

Description of Study Area

f\

The study area covers approximately 200 mi in 

northeast Iowa and includes Black Hawk County and 

small portions of Benton, Bremer, Buchanan, Butler, 

Grundy, and Tama Counties (fig. 1). The topography 

of the alluvial valley along the Cedar River in the 

study area is relatively flat. Uplands consisting of preIllinoian glacial deposits extend to the northeast and 

southwest. Land use in the study area is predominantly 

agricultural, including large areas of cropland and 

small livestock or poultry farms. Total average annual 

precipitation at Waterloo, 1961-90, just southeast of 

Cedar Falls, was 33.70 inches. During the same time 

interval, average monthly precipitation ranged from 

0.80 inch during January to 4.88 inches during July, 

and average monthly temperature ranged from 14.6°F 

in January to 73.9°F in July (National Oceanic and 

Atmospheric Administration, 1998).

Acknowledgments

The author thanks the many well owners and 

operators that allowed USGS personnel to measure 

their wells during this study, specifically Cedar Falls 

Utilities, the University of Northern Iowa, and 

Waterloo Water Works, without which this study 

would not have been possible.

METHODS OF INVESTIGATION

Ground-water and surface-water data were 

collected during the study to help define the hydrogeology of the Silurian-Devonian aquifer and to assist 

in constructing a ground-water flow model. Data were 

collected at 63 ground-water sites (observation, 

municipal-supply, industrial-supply, and residentialsupply wells) and one surface-water site (fig. 2). Previously conducted aquifer tests, completed at the time of 

well construction by the well driller or owner, were 

evaluated to determine aquifer hydraulic properties. 

Wells used for this study were selected on the basis of 

their location and primary aquifer, with an emphasis 

on spatial distribution throughout the study area of 

wells open to the Silurian-Devonian aquifer.

Ground-water flow in the study area was simulated using the USGS-developed MODFLOW model 

(McDonald and Harbaugh, 1988). MODFLOW model 

parameters were input using the Department of 

Defense Groundwater Modeling System (GMS) 

(Brigham Young University, 1998) preprocessor. The 

model was used to obtain a better understanding of the 

ground-water flow patterns of the Silurian-Devonian 

aquifer in the study area and provide a quantitative 

estimate of the water budget in the study area.

Surface-Water Measurements

Streamflow and stage data were collected periodically from 1998 to 1999 as part of the USGS 

Streamflow network in Iowa (May and others, 1998; 

Nalley and others, 1999). Discharge and stage data 

were used to calibrate the ground-water flow model 

and estimate the stage of the rivers used in the model. 

Discharge and stage measurements on the Cedar River 

were made at Waterloo, Iowa (fig. 3), southeast of 

Cedar Falls, and on Beaver Creek were made at New 

Hartford, Iowa, west of the study area.

2 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
92°30' 92 20' 92°10'

42°40'

42°30' -

42 =: 20' -

Base from U.S. Geological Survey digital 

data, 1:100,000, Universal Transverse 

Mercator projection, Zone 15

12 MILES

\ I I I

6 12 KILOMETERS

EXPLANATION

Areas consisting of primarily 

alluvial deposits

 Major rivers and streams

 Extent of digital model

Figure 1. Location of study area.

METHODS OF INVESTIGATION
92°40' 92°30' 92°20' 92°10'

42 40' -

42°30'

42°20' -

/ Beaver Creek at 

' / New Hartford, IA 

I

Base from U.S. Geological Survey digita 

data, 1:100,000, Universal Transverse 

Mercator projection. Zone 15

12 MILES

12 KILOMETERS

EXPLANATION

 Major rivers and streams ^

 -- Extent of digital model _ 29

 850--- Potentiometric contour Shows altitude 

at which water level would have stood 

in tightly cased wells. Dashed were estimated. 

Contour interval, in feet, is variable. 

Datum is sea level

Figure 2. Extent of digital model, location of data-collection sites, and potentiometric surface constructed 

from mean measured water levels for the Silurian-Devonian aquifer, April 1998 to February 1999.

USGS gaging station and name

Well used to eonstruct potentiometric 

surface of Silurian-Devonian aquifer 

and map number

Ground-Water-Level Measurements

Ground-water levels (table 1) were measured 

bimonthly from April 1998 to February 1999 with a 

calibrated steel tape or an airline. Ground-water levels 

were recorded to the nearest 0.01 ft when using a steel 

tape and to the nearest 1 ft when using an airline. 

Ground-water levels were used to evaluate seasonal 

variations in horizontal and vertical components of

flow directions, to help conceptualize the groundwater flow system, and to aid in the calibration of the 

ground-water flow model.

Well Construction and Nomenclature

Ground-water-level data were collected at wells 

constructed by private drillers prior to this study. Wells

4 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
HI 

LLJ

UJ

DC DQ 

0< 

LU 

O

870

865

860

855

850

DC LU 

SW 845

840

835

830 -

825

EXPLANATION

 Cedar River stage

- - 423042092265801 (map number 7)

 - 423341092273001 (map number 11)

-m- 423411092260401 (map number 37)

Figure 3. Cedar River stage at Waterloo, Iowa, and measured water levels in three wells near Cedar Falls, 

Iowa (see table 1 and figure 2 for location of wells and river gages).

completed in the Silurian-Devonian aquifer were 

constructed with open holes in the bedrock. All wells 

used in this study had casing material consisting of 

polyvinylchloride (PVC) or steel.

All wells were surveyed with a global positioning system (GPS) to determine their latitude and 

longitude. Differentially corrected GPS was used to 

establish an altitude for each well's measuring point so 

that all water levels could be converted to a common 

datum (sea level). Wells used in this study are designated by a unique 15-digit station-identification 

number that was assigned in the USGS Ground-Water 

Site Information database.

Aquifer Properties

Specific-capacity data for wells with previous 

aquifer-test data were evaluated to estimate hydraulic 

conductivity of the geologic material adjacent to the 

open interval of the well. In this report, hydraulic

conductivity refers to horizontal hydraulic conductivity unless specifically referred to as vertical 

hydraulic conductivity. The hydraulic conductivity 

was calculated using a modified Theis equation for 

estimating transmissivity from specific-capacity data, 

and the available thickness of the aquifer adjacent to 

the well (Theis and others, 1963).

HYDROGEOLOGY

Hydrogeologic information relevant to the study 

of the ground-water flow system and description of the 

conceptual ground-water flow model is presented in 

the following section. The geology of the bedrock 

units in the study area is discussed in more detail in 

reports by Anderson (1983), Horick (1984), Hansen 

(1975), Olcott (1992), and Witzke (1988). Geologic 

units within the study area and their water-bearing 

characteristics are summarized in table 2.

HYDROGEOLOGY
Table 1. Well data and statistical summary of measured water levels, April 1998-February 1999

Simulation ofGround-Wal

<D

 n 

o

5'

(D 

COE"

55'

6

(D 

0

53'

3

.Q

<0

O<0

a. 

o>

 n

2L

6"

I

[USGS, U.S. Geological Survey; ddmmss, degrees, minutes, seconds]

Map 

number 

(fig- 2)

1

2

3 

4

5 

6

7

8

9 

10

11

12

13

14 

15

16

17

18 

19

20

21

22

23

24

25

26

27

28

29

30

31

32

USGS site 

identification

423401092373601

422516092193501

422452092190601 

422451092201401

423139092261401 

423053092244701

423042092265801

422948092281201

423143092275801 

423045092283401

423341092273001

423341092273301

422956092245101

422838092034201 

421857092115601

423131092224201 

422818092212801

423200092224001 

422819092212701

423002092231101

422801092191401

422638092164001

422810092250401

423131092195201

422439092270101

422009092285201

422217092203601

421939092175201

421608092183401

421917092085501

421859092061201

422300092134801

Latitude 

(ddmmss)

423400

422516

422452 

422451

423140 

423055

423045

422950

423140 

423045

423340

423340

422956

422838 

421855

423130 

422820

423135 

422820

423000

422800

422640

422810

423131

422439

422009

422217

421939

421608

421917

421859

422300

Longitude 

(ddmmss)

923750

921935

921906 

922014

922620 

922445

922710

922750

922800 

922835

922730

922735

922451

920342 

921155

922210 

922130

922215 

922125

922315

921915

921640

922505

921952

922701

922852

922036

921752

921834

920855

920612

921348

Altitude of land 

surface 

(feet above sea 

level)

897.86

940.96

909.24 

954.70

858.80 

934.54

881.28

959.93

941.90 

941.59

876.12

876.26

901.76

988.75 

836.11

859.04 

951.43

857.45 

951.83

925.51

850.89

834.90

879.00

871.74

881.89

999.51

953.90

913.84

975.80

811.65

823.47

822.93

Maximum 

altitude of 

measured 

water level 

(feet above sea 

level)

882.86

843.12

839.59 

844.53

848.80 

846.54

853.78

844.93

845.90 

841.59

854.12

854.26

849.76

927.05 

793.11

843.04 

833.43

852.45 

835.83

842.51

844.89

828.90

857.00

848.74

875.90

876.54

861.86

873.53

857.99

802.40

796.79

814.15

Mean altitude of 

measured 

water level 

(feet above sea 

level)

881.29

840.70

837.27 

842.39

846.80 

843.54

851.38

843.93

843.90 

840.59

851.12

850.26

846.76

856.65 

748.91

840.04 

830.43

843.45 

828.83

839.51

834.89

824.90

853.00

843.74

872.37

873.13

858.99

859.27

854.32

796.42

791.41

810.5

Minimum altitude of 

measured 

water level 

(feet above sea 

level)

879.86

839.65

836.12 

841.34

845.80 

840.54

847.28

839.93

841.90 

838.59

846.12

839.26

844.76

812.75 

736.96

832.04 

827.43

842.45 

823.83

837.51

824.89

820.90

850.00

839.74

868.89

869.51

856.63

850.29

851.77

794.37

788.43

808.96

Dep

w

(fe

bel

la

surf

16

1

8

14

15

18

14

22

17

21

27

27

20

36

25

19

20

20

21

20

13

14

22

15

23

24

23

24

36

12

1

8
Table 1. Well data and statistical summary of measured water levels, April 1998-February 1999 Continued

[USGS, U.S. Geological Survey; ddmmss, degrees, minutes, seconds]

HYDROGEOLOGY

 o

Map 

number

(fig. 2)

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56 

57 

58 

59 

60 

61 

62

63

USGS site 

identification

422542092075801

423047092085701

423702092104601

423343092234701

423411092260401

423431092272301

423556092283601

423346092282901

423423092241301

423303092271201

423225092270401

423210092265201

423601092300601

423216092312801

422658092320201

423254092274801

422904092292601

422959092201701

423416092095701

422804092165301

422737092153501

423902092272502

423039092272401

423043092274701 

423059092273101 

423051092274001 

423055092273001 

423046092273001 

423156092285701 

423027092220601

423030092215701

Latitude 

(ddmmss)

422542

423047

423702

423343

423411

423431

423556

423346

423423

423303

423225

423210

423601

423216

422658

423254

422904

422959

423415

422805

422737

423900

423039

423043 

423059 

423051 

423055 

423046 

423156 

423030

423030

Longitude 

(ddmmss)

920758

920857

921046

922347

922604

922723

922836

922829

922413

922712

922704

922652

923006

923128

923202

922748

922926

922017

920955

921655

921535

922725

922724

922747 

922731 

922740 

922730 

922730 

922857 

922205

922155

Altitude of land 

surface 

(feet above sea 

level)

968.23

992.54

1011.08

864.05

872.54

877.30

926.48

867.68

873.90

862.70

857.95

873.99

888.41

970.82

965.22

862.06

942.50

845.00

949.78

837.82

836.79

887.47

887.52

914.49 

932.72 

922.87 

941.12 

913.63 

964.35 

853.13

858.91

Maximum 

altitude of 

measured 

water level 

(feet above sea 

level)

869.66

907.63

972.48

857.35

857.80

860.80

871.89

859.21

852.64

852.23

850.31

849.58

874.63

884.25

882.32

849.92

868.98

827.09

907.78

823.82

795.79

869.47

847.02

846.99

847.72 

847.62 

848.87 

846.88 

852.35 

828.13

843.16

Mean altitude of 

measured 

water level 

(feet above sea 

level)

867.04

907.07

971.70

855.10

856.45

859.22

869.15

856.94

851.12

849.89

847.62

847.11

871.23

878.72

878.00

849.75

868.12

826.53

893.78

782.82

787.79

866.27

843.45

839.69 

844.02 

843.67 

845.26 

843.63 

849.25 

822.13

840.74

Minimum altitude of 

measured 

water level 

(feet above sea 

level)

863.23

906.53

969.32

851.05

855.24

856.20

864.48

855.25

850.40

848.27

846.48

845.34

869.54

872.82

876.27

849.58

867.32

825.46

881.78

762.82

779.79

861.47

836.52

815.49 

837.47 

840.62 

839.87 

838.13 

841.19 

819.13

838.66

Dep

w

(f

be

l

sur

2

2

2

1

1

1

1

1

1

2

1

1

1

1

1

1

'1

3

3

3

1

1

1

1

2

1

1

2

2

'Depth unknown, but estimated on the basis of similar nearby wells.
Table 2. Hydrogeologic units in the study area and their water-bearing characteristics

[gpm, gallons per minute]

Hydrogeologic 

unit1

Alluvial, glacial-drift,

General 

thickness1 

(feet)

20-250

Age of rock 

unit2

Quaternary

Potential well yield

Well yields variable, 3-25 gpm,

Lithology1

Medium- to coarse-grained

Equivalent 

layer in the 

ground-water 

flow model

Layer 1

and buried-channel 

aquifers

Silurian-Devonian 

aquifer

Confining unit

10-200 Devonian

10-300 Silurian

100-300 Ordovician

not a widely used source of 

water.

Permeability is assumed to vary 

dependent upon proximity to 

the Cedar River. Well yields in 

excess of 2,500 gpm with 

minimal drawdown.

Permeability dependent upon the 

number and density of fractures and degree of dolomitization. Well yields typically 

300 gpm.

Well yields very small; regional 

confining unit.

sand; fine-grained sand and 

silt.

Highly fractured limestone, 

dolomite, and shale. May 

locally have a karst topography (Horick, 1984).

Dolomite with some limestone 

and chert.

Shale and dolomite.

Layer 2

Layer 3

Basal 

(no-flow) 

boundary

1 Modified from Horick (1984) and Olcott (1992).

2Age classifications of rocks are those of the Iowa Department of Natural Resources, Geological Survey Bureau.

Geology and Water-Bearing 

Characteristics

The Quaternary-age deposits comprise a surficial aquifer system that ranges in thickness from about 

20 ft to 250 ft in the study area (Iowa Geological 

Survey Bureau, 1999). Quaternary-age deposits 

include alluvial, glacial-drift, and buried-channel 

materials with variable permeabilities both vertically 

and horizontally. The glacial deposits in Iowa are 

rich in clay, which may indicate a lower permeability 

in some locations (Olcott, 1992). The "sand" and 

"coarse-sand" alluvial units described in the 

descriptive well logs for CPU water-supply wells 

(Paul Mallinger, Cedar Falls Utilities, written 

commun., 1997) may provide only limited protection 

against contaminants in surface water and shallow 

ground water from leaching to the underlying bedrock 

units (Hallberg and others, 1996).

The uppermost bedrock consists of rocks of 

Devonian age (table 2). The Devonian-age rocks 

unconformably overlie Silurian-age rocks and consist 

primarily of limestone, dolomite, and shale but locally 

include minor amounts of sandstone (Witzke and 

others, 1988). In the study area, the thickness of the 

Devonian-age rocks varies from about 10 ft in the

northeast to about 200 ft to the southwest. This rock 

unit has a significant occurrence of fractures. Fractures 

allow ground-water flow along discrete paths, 

resulting in increasing solution activity along the fractures (Knochenmus and Robinson, 1996). Fracturing 

and dissolution of aquifer material is known as 

secondary permeability. The relative amount of 

secondary permeability appears to be greatest nearest 

the Cedar River. The primary orientation of the fractures is assumed to coincide with the orientation of the 

Cedar River, which may flow along this regional fracture pattern. Video well logs (Paul Mallinger, Cedar 

Falls Utilities, written commun., 1998) show a high 

density and occurrence of fractures in the Devonianage rocks, some of them large, in CPU municipalsupply wells. The transmissivity in the Devonian-age 

rock, calculated using available specific capacity data, 

decreases as distance from the Cedar River increases. 

Most production wells in the study area are completed 

in the highly fractured rocks of Devonian age because 

large amounts of good quality water have been available historically.

The Silurian-age rocks consist primarily of 

dolomite, with some limestone and minor amounts of 

chert (table 2) (Iowa Geological Survey Bureau, 

1999). The elevation of the contact between the

8 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
Silurian-age rocks and Devonian-age rocks was estimated by using descriptive well logs from the Iowa 

Geological Survey Bureau (1999). Altitudes derived 

from these logs were used to estimate the uppermost 

occurrence of Silurian-age rock in the study area. The 

thickness of the Silurian-age rocks varies from about 

10 to 300 ft. Silurian-age rocks are assumed to be fractured to a lesser degree than Devonian-age rocks 

because they are not the uppermost bedrock unit. In 

instances where the Silurian-age portion of the 

Silurian-Devonian aquifer is the uppermost bedrock 

unit, such as in the Cedar River Valley south of the 

study area, the estimated amount of secondary permeability is comparable to the fractured Devonian rock in 

this study area.

The rocks of Devonian and Silurian age 

commonly are considered to be one hydrogeologic 

unit because they are often in hydraulic connection. 

Silurian-age rocks are usually the primary source of 

water to wells completed in this aquifer system 

throughout its main use area in Iowa, so the unit 

became locally known as the Silurian-Devonian 

aquifer (Horick, 1984). The Silurian-Devonian aquifer 

is absent in extreme northeastern Iowa and is more 

than 700 ft thick in southwestern Iowa (Horick, 1984). 

The thickness of the Silurian-Devonian aquifer, in the 

study area, is estimated to be about 20 to 500 ft.

Transmissivity of the Silurian-Devonian aquifer 

is highly variable, depending on the degree of interconnection between the fractures and bedding planes 

(Schaap, 1999). Transmissivity in the study area 

reaches a maximum of about 360,000 ft2/d in areas 

where the Silurian-Devonian aquifer underlies Quaternary deposits of the Cedar River alluvium. Transmisf\

sivity may be about 1,200 ft/d where the aquifer is 

confined (Olcott, 1992) by Mississippian-age rock 

units outside the study area.

Rocks of Ordovician age underlie the SilurianDevonian aquifer. The uppermost Ordovician-age 

rocks consist of shale, dolomite, and limestone 

(Olcott, 1992). The uppermost occurrence of Ordovician-age rocks was estimated using descriptive well 

logs from the Iowa Geological Survey Bureau (1999). 

Due to the lithology of this rock unit, it is considered 

to be a regional confining unit, restricting vertical 

ground-water flow into or from the Silurian-Devonian 

aquifer in the study area.

Surface Water

The Cedar River is the major surface-water 

feature in the study area. The river has its headwaters 

in southern Minnesota and flows southeasterly to its 

confluence with the Mississippi River in southeast 

Iowa. The fall, or change in stage, from Janesville, 

Iowa, to Waterloo, Iowa, is about 60 ft. Large variations in river stage are reflected in the water level of 

Wells near the river (fig. 3). A potentiometric surface 

map by Horick (1984) shows that the Cedar River, in 

general, receives ground-water discharge from the 

Silurian-Devonian aquifer throughout most of the 

study area. However, the Cedar River is a slightly 

losing reach north of Cedar Falls, from just south of 

Janesville, Iowa, to just north of Cedar Falls, Iowa 

(Mark Savoca, U.S. Geological Survey, written 

commun., 2001). Base flow at Waterloo, Iowa, from 

April 1998 to February 1999 was estimated to be 

between 600 ft3/s and 1,100 ft3/s by hydrograph separation.

Ground Water

Rocks of Devonian and, to a lesser extent, 

Silurian age contain numerous fractures which provide 

openings for ground-water movement and storage. 

Hydraulic conductivity describes the ability of 

geologic materials to transmit water and depends upon 

the size, orientation, density, and the degree of connection of fractures and pores. Driscoll (1986) and Freeze 

and Cherry (1979) provide a general range of 

hydraulic conductivities for various lithologies 

(table 3). Horick (1984) estimated the transmissivity 

for the Silurian and Devonian rocks to range from 

200 ft2/d to 361,000 ft2/d statewide. Assuming a 

uniform thickness of 300 ft, hydraulic conductivities 

in the Silurian- and Devonian-age rocks range from 

about 1 ft/d to 1,200 ft/d statewide. Transmissivity 

values for this study were estimated using specificcapacity data available for each of the network wells 

used in this study. Estimated transmissivities 

throughout the Silurian-Devonian aquifer in the study 

area vary over several orders of magnitude, with larger 

values near the Cedar River. The probable cause for 

this heterogeneity is due to the spatial variability of 

increased secondary permeability near the Cedar 

River.

HYDROGEOLOGY
Table 3. Estimated horizontal hydraulic conductivity in the study area

[--, data not available]

Horizontal hydraulic conductivity, in feet per day

Lithology

Glacial till 

Clay 

Silty sand 

Fine to coarse sand

Modified from 

Driscoll(1986)

3.2 x 10-7-30 

3.2 X 10~7-3.2 X 1Q-4

3.2 X 10-2-3.2xl03

Modified from 

Freeze and 

Cherry (1979)

2.8 X 10-6-2.8 

2.8 X lQ-3 -28 

2.8-280 

2.8-10,000

From aquifertest analysis 

in this study1

--

Parameter 

value in 

groundwater flow 

model

4 

90 

360^50

Sand and gravel

Limestone and dolomite (unjointed crystalline) 

Limestone and dolomite (highly fractured) 

Limestone and dolomite (moderately fractured) 

Limestone and dolomite (slightly fractured)

3.2XKT'-1,000 10-100,000 600-300,000 1,000

300-600 100-240 

0.1-300 10-40

Horizontal hydraulic conductivity estimated using specific capacity data (Theis and others, 1963) and uniform thickness

of the aquifer material.

There is an appreciable difference in the wateryielding characteristics between the Quaternary-age 

materials, Devonian-age rocks, and Silurian-age rocks. 

Most ground-water movement occurs in the Devonianage rocks (Horick, 1984), and although the lithology 

of the Silurian- and Devonian-age rocks are similar, 

the hydraulic properties, due to increased occurrences 

of secondary permeability in the Devonian-age rocks, 

are significantly different. For example, one municipal 

supply well, open to both Devonian and Silurian-age 

rocks, had about an 80-percent reduction in production 

when the section of the well open to the Devonian-age 

rocks was cased, negating their contribution (Paul 

Mallinger, oral commun., 1999). Due to this hydraulic 

difference, the Silurian-Devonian aquifer was evaluated as two separate hydrogeologic units in direct 

connection consisting of the Devonian-age and 

Silurian-age rocks.

The potentiometric surface, based on mean 

water-level altitudes from April 1998 to February 

1999, indicates that the regional ground-water flow in 

the Silurian-Devonian aquifer in the study area is 

toward the Cedar River from northeast to southwest 

and then toward the southeast down the Cedar River 

valley (fig. 2). There is some local flow to pumping 

centers in the Cedar Falls and Waterloo area and 

farther south near Evansdale. The potentiometric 

surface created for this report compares favorably in 

shape and gradient in the Cedar Falls area to a previously mapped potentiometric surface created by 

Horick (1984) in 1980 for the Silurian-Devonian 

aquifer.

Recharge to the ground-water flow system is 

predominantly from infiltration of precipitation. 

Discharge from the ground-water flow system 

includes, but is not limited to, flow to the Cedar River 

and pumping wells.

SIMULATION OF GROUND-WATER FLOW

The ground-water flow model described in this 

report is a simplified mathematical approximation of 

the complex physical system. Limited onsite observations and hydrogeologic data were used to conceptualize the ground-water flow system. However, a 

calibrated model can aid in understanding and quantifying the ground-water flow system. The model also 

can be used to estimate effects of varying stresses on 

ground-water levels and the recharge and discharge of 

ground water.

The flow model was constructed to simulate 

steady-state conditions. Steady-state conditions occur 

when the volumetric rate of water entering a system 

equals the volumetric rate of water flowing out of the 

system. Mean water levels within the study area from 

April 1998 to February 1999 were considered to be an 

acceptable estimate of steady-state conditions. 

Ground-water levels measured in most wells during 

this period had little variation. The stage of the Cedar 

River and Beaver Creek was simulated assuming baseflow conditions. Recharge, used to account for precipitation and evapotranspiration, was assumed to be the 

average daily recharge to the system.

10 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
Model Description and Boundary 

Conditions

The modeled area was discretized into a 

220-row by 180-column grid with cells measuring 

500 ft by 500 ft. The model grid covers an area of

0 _» 20.8 mi by 17.0 mi, or about 355 mi . The grid was 

aligned so that the Cedar River coincided with the 

y-axes. Three layers were used to simulate flow: 

layer 1 represents the Quaternary alluvial and glacial 

deposits, layer 2 represents the Devonian-age portion 

of the Silurian-Devonian aquifer, and layer 3 represents the Silurian-age portion of the Silurian-Devonian 

aquifer. The Silurian-Devonian aquifer was simulated 

using two layers because differences were observed in 

the well yields of the Silurian- and Devonian-age 

rocks, and because secondary permeability is assumed 

to be less of a factor in the deeper Silurian-age rocks. 

Cells within the model grid are identified by row, 

column, and layer. Ground-water flow in layer 1 was 

simulated as unconfined because of water-table conditions present in the surficial deposits, and groundwater flow in layer 2 and 3 was simulated as confined. 

A small number of cells in layer 1, associated with a 

thin amount of Quaternary material or a boundary, 

were simulated as dry,

Boundary conditions were specified for the 

model (figs. 4, 5, and 6) to simulate flow entering or 

leaving along the edge of the modeled area in relation 

to the features within the modeled area. Recharge from 

precipitation to the upper surface of the flow model 

was represented as a nonuniform specified-flux 

boundary. Recharge is discussed in more detail in the 

following section on "Model Parameters."

No-flow boundaries were used to simulate areas 

where lateral ground-water flow is interpreted to be 

parallel with the boundary, such as along ground-water 

flow lines, or is considered to be insignificant, such as 

a contact with a nonaquifer material. The bottom of 

the model is the top of the Ordovician-age rocks, 

which form a relatively impermeable regional 

confining unit. Flow lines constructed from the mean 

water levels during the study period define the southeastern no-flow boundary for all model layers.

Lateral ground-water flow from geologic materials laterally adjacent to the modeled aquifers was 

simulated with general-head boundaries. General-head 

boundaries were used along the northeastern, northwestern, and southwestern edges of layer 1 to simulate 

subsurface flow from the glacial deposits near the

model boundaries to the alluvium in the Cedar River 

Valley, and along northeastern, northwestern, and 

southwestern limit of layer 2 and layer 3 to simulate 

regional ground-water flow in the Devonian and 

Silurian aquifers, respectively. For all general-head 

boundaries, a constant-head source, estimated to be 

similar in magnitude to the measured potentiometric 

surface shown in figure 3, was specified 1 mi from the 

closest active cell in the model. The hydraulic conductivity of the area between the constant-head source and 

the active cell was assumed to be equal to the nearest 

active cell.

The Cedar River and Beaver Creek were simulated using river cells that allow leakage to and from 

layer 1. River stage is specified for each river cell. 

River stage was interpreted along the river reach from 

the USGS gaging data at Waterloo, Iowa, and New 

Hartford, Iowa, for the Cedar River and Beaver Creek, 

respectively, and the USGS Cedar Falls 7.5-minute 

topographic map. Riverbed elevation was assumed to 

be 5 ft below stage and the thickness of the riverbed 

material was assumed to be 1 ft throughout the model 

area. The riverbed conductance term is a function of 

the riverbed thickness, the length of the river reach, 

and the width of the river channel. The amount of 

leakage between the river cells and layer 1 is calculated using the head difference between the river cells 

and layer 1 and the riverbed conductance term. A 

vertical hydraulic conductivity of 3 ft/d was assumed 

for river-bed material for the Cedar River and Beaver 

Creek.

Model Parameters

Model parameters are numerical values assigned 

to individual cells in the model array and are used in 

the flow equations that simulate ground-water flow in 

the modeled area. Parameters are assigned to the 

center of each model cell and represent an average 

value for the entire cell. Uniform values of parameters 

can be assigned to groups of model cells to represent 

the spatial distribution of altitudes of the model layers, 

hydraulic conductivity, recharge, and ground-water 

pumpage. Model grid orientation and pumpage locations are shown in figures 4, 5, and 6.

Model layers were constructed using altitudes of 

geologic contacts interpolated from descriptive well 

logs recorded during well construction. Geologic 

contacts used in the model were the Quaternary and 

Devonian, the Devonian and Silurian, and the Silurian

SIMULATION OF GROUND-WATER FLOW 11
92'30' 92'20' 92M01

42'40' -

42'30' -L

42'20' -

Base from U.S. Geological Survey digital 

data, 1:100,000, Universal Transverse 

Mercator projection, Zone 15

10 MILES

0 10 KILOMETERS

EXPLANATION

 Extent of digital model 

D MODFLOW river cell 

D MODFLOW general-head boundary

Active model cells 

D Dry cells

Figure 4. Orientation of model grid, grid discretization, and stress-related model parameters used in layer 1.

12 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
92°20' 92 10'

42 40' -

Base from U.S. Geological Survey digital 

data, 1:100,000, Universal Transverse 

Mercator projection, Zone 15

10 KILOMETERS

EXPLANATION

 - Extent of digital model 

* MODFLOW well 

o MODFLOW general-head boundary 

Active model cells

Figure 5. Orientation of model grid, grid discretization, and stress-related model parameters used in layer 2.

SIMULATION OF GROUND-WATER FLOW 13
92°30' 92°20' 92°10'

42°40' -

42°30'

42°20' -

Base from U.S. Geological Survey digital 0 5

data, 1:100,000, Universal Transverse l___________________|_______________

Mercator projection, Zone 15 I I I

0 5 10 KILOMETERS

EXPLANATION

 - Extent of digital model 

« MODFLOW well 

o MODFLOW general-head boundary 

Active model cells

Figure 6. Orientation of model grid, grid discretization, and stress-related model parameters used in layer 3.

10 MILES

14 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
and Ordovician contacts. Altitudes of each contact 

were interpolated by the preprocessor using the 

inverse distance-weighted method (IDW) to the model 

grid.

Hydraulic conductivity in layer 1 was estimated 

using available aquifer-test data from private drillers or 

contractors or published representative hydraulic 

conductivities for similar geologic materials. 

Hydraulic conductivities used for model layers 2 and 3 

account for the estimated degree of secondary permeability present. Hydraulic conductivities in layer 1, 2, 

and 3 range from 4 to 450 ft/d, 10 to 1,000 ft/d, and 40 

to 100 ft/d, respectively (figs. 7, 8, and 9).

Vertical leakance is required by the model to 

control the rate of ground-water flow between layers. 

Vertical leakance between model layers in hydraulic 

contact, such as in this model, is calculated from the 

distance of each layer between its node and the 

common layer contact and the vertical hydraulic 

conductivity of each layer (McDonald and Harbaugh, 

1988, equation 51). Vertical hydraulic conductivities 

in the model were estimated to be about 10 percent of 

the horizontal hydraulic conductivity for layers 1 and 

3. Vertical hydraulic conductivities in all layers were 

adjusted during the calibration process, and some cells 

in layer 2 have a vertical hydraulic conductivity value 

less than or greater than 10 percent of the horizontal 

hydraulic conductivity. A no-flow boundary was simulated at the bottom of layer 3; therefore, no vertical 

leakance was simulated.

Recharge in the Cedar Falls area was assumed to 

be about 10 percent less than the previously published 

recharge rate calculated by Schulmeyer and Schnoebelen (1998) in Cedar Rapids, Iowa, because the 

annual average precipitation in Waterloo, Iowa, is 

33.70 inches whereas the annual average precipitation 

in Cedar Rapids, Iowa, is 36.39 inches (National 

Oceanic and Atmospheric Administration, 1998). The

o

recharge rate of 1.6x10 ft/d (7.1 in/yr) was assigned 

cells in layer 1 that represent alluvial material in the 

Cedar River Valley and to account for infiltration of 

runoff from upland areas and larger vertical hydraulic 

conductivity of sandy soils (Schulmeyer and Schnoebelen, 1998). A recharge rate of 1.4xlO~3 ft/d 

(6.3 in/yr) was assigned to all remaining cells in layer 

1. The rates of recharge specified for this model have 

accounted for the effects of evapotranspiration. 

Types of discharge from the flow system 

included in the model were ground-water pumpage, 

river leakage, and flow across general-head boundaries. Pumpage during the study period varied 

depending on the season, with larger withdrawals 

during the spring and summer. Ground-water withdrawals were simulated using a specified negative flux 

located at a cell node associated with a producing well. 

Water users in the study area had pumpage that ranged 

from about 8,000 gal/d to 2.5 Mgal/d.

Model Calibration

The model calibration process used for this 

model minimized the differences between modelcalculated ground-water levels and measured groundwater levels by adjusting model parameters. The mean 

ground-water levels calculated for each network well 

from measured water levels during the study period 

were used as a basis for calibration. The mean water 

levels for the study period were used as the calibration 

target because most of the major pumpage data used in 

the model was averaged from the annual pumpage for 

1998 calendar year, from the total monthly pumpage 

during the 1998 calendar year, or from the latest available pumpage data; and although there is seasonal 

variation of water level in the Silurian-Devonian 

aquifer, historical trends show little variation during 

the study period. Hydraulic conductivity, vertical 

leakance, riverbed conductance, and general-head 

boundary conductance were parameters that were 

varied, within reasonable limits, during numerous 

simulations until the root-mean-squared error (RMSE) 

(Anderson and Woessner, 1992) between the mean 

measured water levels for all wells inside the model 

boundary and simulated water levels in respective 

corresponding model cells was less than 5 ft. Model 

calibration was further refined by continuing to vary 

model parameters until the average head difference 

(AVEH) and RMSE were minimized. The AVEH is an 

indicator of model bias and is the sum of the differences between simulated and measured water levels 

divided by the total number of measurements. The 

RMSE (eq. 1) indicates the magnitude of error 

between simulated and measured values.

RMSE= (1)

where

M is the measured water level,

SIMULATION OF GROUND-WATER FLOW 15
92 30' 9220' 92 10'

4240'

42 30' -i

42 20' -

Base from U.S. Geological Survey digita 

data, 1:100,000, UniversalTransverse 

Mercator projection. Zone 15

10 MILES

10 KILOMETERS

EXPLANATION

Zone of equal horizontal 

hydraulic conductivity (feet per day)

Zone of equal vertical 

hydraulic conductivity (feet per day)

4

90 - 360

450

2

18-40

15

Figure 7. Distribution of horizontal and vertical hydraulic conductivity, layer 1.

16 Simulation of Ground-Water Flow In the Silurian-Devonian Aquifer, Cedar Falls, Iowa
92°30' 92°20' 92°10'

42°40' -

42°30' -__._

42°20' -

Base from U.S. Geological Survey digita 

data, 1:100,000, Universal Transverse 

Mercator projection, Zone 15

0 5

EXPLANATION

Zone of equal horizontal Zone of equal vertical

hydraulic conductivity hydraulic conductivity

(feet per day) (feet per day)

10 KILOMETERS

10

40

240

1.000

'/////

^\^\:

1.0

10

100

Figure 8. Distribution of horizontal and vertical hydraulic conductivity, layer 2.

SIMULATION OF GROUND-WATER FLOW 17
92'30' 9220' 92"10'

42'40' -

42'30' -

42'20' -

Base from U.S. Geological Survey digital 

data, 1:100,000, Universal Transverse 

Mercator projection, Zone 15 r

0 5 10 KILOMETERS

EXPLANATION

Zone of equal horizontal Zone of equal vertical 

hydraulic conductivity hydraulic conductivity 

(feet per day) (feet per day)

0.07

40

 Extent of digital model

Figure 9. Distribution of horizontal and vertical hydraulic conductivity, layer 3.

18 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
S is the water level simulated by the model, and 

TV is the number of observations.

The model was considered calibrated when the 

following criteria were achieved:

1. Parameter value changes did not result in an AVEH 

closer to 0 and a smaller RMSE for model layers 

2 and 3.

2. The RMSE was less than 5 ft for layers 2 and 3.

3. The simulated ground-water flow directions in 

layers 2 and 3 compared favorably with those 

determined using the measured ground-water 

levels during the study period.

4. The simulated discharge to the Cedar River from 

the aquifer compared favorably with the estimated base flow.

The RMSE for the calibrated model was calculated using water-level data from most wells within the 

model boundary. Four wells located inside the model 

area were not included in the calibration process 

because they were located extremely close to a model 

boundary or were affected by pumpage not included in 

the simulation. The RMSE of the calibrated model is 

4.14 ft; the AVEH is 1.29 ft. The RMSE for the Devonian-age and Silurian-age rock is 4.11 ft and 

0.51 ft, respectively. The difference between the simulated and measured water levels likely is because the 

model is a simplified representation of a complex 

ground-water flow system (table 4).

The simulated potentiometric surface of the 

Devonian and the Silurian units is similar to the mean 

measured potentiometric map shown in figure 2. The 

direction and gradient of ground-water flow is similar 

throughout most of the modeled area with the exception of areas near the model boundaries, in which 

simulated gradients are less than the measured gradients.

Simulated discharge to the Cedar River is

Q

73 ft /s, which is about 10 percent of the estimated 

base flow. This is an adequate simulation of the 

connection between the Cedar River and the adjacent 

Quaternary aquifer because this model simulates only 

a small portion of the Cedar River Basin and does not 

simulate smaller tributary streams that contribute 

water to the Cedar River.

Sensitivity Analysis

The model was constructed using parameters to 

solve mathematical equations that simulate the

ground-water flow system in the Quaternary alluvial 

and glacial material and the Devonian- and Silurianage bedrock in the study area. A sensitivity analysis 

evaluates the response of the calibrated model to variations in parameter values and determines which 

parameters have the greatest effect on the results. All 

model parameters, with the exception of the generalhead boundary and riverbed conductance, were varied 

using a multiplication factor of 10 to 0.1. The generalhead boundary conductance and the riverbed conductance were varied using a multiplication factor of 

1,000 to 0.001, due to the relatively small effect of 

these parameters on the model at smaller factors (table 

6). In some instances, the application of a multiplication factor caused the model to fail to converge to a 

solution. Model sensitivity was measured with the 

RMSE (eq. 1) using the difference between simulated 

and measured ground-water levels in layers 2 and 3. In 

general, improvement in parameter measurements or 

data sets that have the most effect on the model will 

result in improvements in the model simulation.

Water levels were most sensitive to changes in 

horizontal hydraulic conductivity in all layers and 

recharge in layer 1. Moderate sensitivity was associated with river conductance and general-headboundary conductance decreases. Water levels were 

insensitive to vertical hydraulic conductivity in layers 

1 and 2. Varying the horizontal hydraulic conductivity 

in layer 1 and layer 2 had the greatest effect on the 

proportion of total inflow and outflow from the rivers 

(table 5). Increases in riverbed conductance and 

general-head-boundary conductance showed no significant change in the RMSE; however, decreases in the 

general-head-boundary conductance by 2 or more 

orders of magnitude resulted in a RMSE above 100 or 

nonconvergence of the solution.

The sensitivity of the model to the Cedar River 

riverbed conductance was evaluated. The conductance 

terms were increased and decreased by 3 orders of 

magnitude, and the change in flow in and out of the 

river and across general-head boundaries was 

compared to the total volumetric rate of water entering 

and leaving the model. In the calibrated model, Cedar 

River cells account for 15.70 percent of the total 

inflow and 41.90 percent of the total outflow. When 

conductance values were decreased by 3 orders of 

magnitude, percentage of total inflow decreased to 

near zero. When conductance values were increased 

by 3 orders of magnitude, total inflow increased to 

41.72 percent (fig. 10).

SIMULATION OF GROUND-WATER FLOW 19
Table 4. Simulated ground-water level and deviation from the mean measured water level in wells 

located within the modeled area

[USGS, U.S. Geological Survey]

Map number

(fig. 2)

2

3

4

5

6

7

8

9

10

11

12

13

16

.17

19

20

21

22

23

24

36

37

38

39

40

41

42

43

44

45

46

48

49

53

55

56

57

58

59

60

61

63

18

52

USGS site 

identification

422516092193501

422452092190601

422451092201401

423139092261401

423053092244701

423042092265801

422948092281201

423143092275801

423045092283401

423341092273001

423341092273301

422956092245101

423131092224201

422818092212801

422819092212701

423002092231101

422801092191401

422638092164001

422810092250401

423131092195201

423343092234701

423411092260401

423431092272301

423556092283601

423346092282901

423423092241301

423303092271201

423225092270401

423210092265201

423601092300601

423216092312801

423254092274801

422904092292601

422737092153501

423039092272401

423043092274701

423059092273101

423051092274001

423055092273001

423046092273001

423156092285701

423030092215701

423200092224001

422804092165301

Primary 

aquifer

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Devonian

Silurian

Silurian

Simulated 

altitude of 

water level 

(feet)

843.85

844.15

847.76

847.87

845.43

844.20

844.96

846.93

849.43

854.00

854.08

845.38

843.06

832.71

832.61

841.25

834.46

829.36

849.30

843.59

858.03

859.16

859.64

870.25

855.27

862.80

851.78

849.89

849.22

872.70

866.91

851.92

861.64

792.01

842.86

842.26

843.01

842.42

842.24

842.17

853.54

841.52

844.13

782.58

Deviation from 

mean measured 

water level 

(feet)

3.149

6.881

5.365

1.070

1.894

-7.179

1.027

3.028

8.841

2.879

3.823

-1.382

3.022

2.281

3.779

1.735

-0.434

4.458

-3.700

-0.150

2.925

2.710

0.417 '

1.099

-1.671

11.678

1.887

2.27

2.108

1.473

-11.815

2.171

-6.475

4.223

-0.589

2.569

-1.011

-1.246

-3.024

-1.464

4.288

0.779

0.677

-0.238

20 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
Table 5. Results of the sensitivity analyses of the calibrated steady-state ground-water flow model for the Silurian-Devonian 

aquifer near Cedar Falls, Iowa

Parameter

Calibrated flow model

Areal recharge rate

Layer 1 

Horizontal hydraulic 

conductivity

Vertical hydraulic conductivity

Layer 2 

Horizontal hydraulic 

conductivity

Vertical hydraulic conductivity

Layer 3 

Horizontal hydraulic 

conductivity

Vertical hydraulic conductivity

General-head boundary conductance 

Layer 1

Layer 2

Layer 3

Riverbed conductance (Cedar River 

and Beaver Creek)

Factor used to 

change parameter

. --

x 0.10 

X 10.0

x 0.10 

x 10.0

x 0.10 

x 10.0

x 0.20 

X 5.0

X 0.10 

x 10.0

x 0.1 

x 10.0

x 0.1 

X 10.0

x 0.0001

x 1,000.0 

x 0.0001

x 1,000.0 

x 0.0001

x 1,000.0

x 0.0001 

XI, 000.0

Root mean 

square error 

(feet)

4.14

4.85 

32.25

6.87 

5.38

4.14 

4.14

36.25 

7.21

4.61 

4.67

29.48 

8.56

5.99

5.25

4.14

4.79 

4.19

4.08 

4.10

4.18

31.01 

4.26

Percentage 

of total flow 

IN from river 

leakage

15.70

40.59 

0.00

18.06 

11.99

17.21 

15.80

15.60 

10.04

15.66 

15.67

18.78 

7.99

14.70 

16.12

17.57

2.53 

15.89

15.75 

16.15

15.32

0.00

41.72

Percentage 

of total flow 

OUT to river 

leakage

41.90

23.32 

67.25

34.70 

59.61

41.62 

42.02

27.84 

64.62

41.45 

42.10

37.96 

57.87

40.64 

42.64

43.09

34.10 

43.20

41.04 

41.63

41.96

2.29 

58.55

Percentage 

of total IN 

from general 

head 

boundary

23.00

50.60 

0.06

17.45 

41.03

22.61 

23.07

17.05 

48.03

21.26 

23.51

17.92 

43.62

23.78 

22.99

16.69

71.46 

20.57

25.03 

21.34

24.88

17.79 

17.57

Percentage 

of total OUT 

to general 

head 

boundary

5.89

1.93

24.32

10.40 

0.17

5.54 

6.00

14.89 

0.09

4.91 

6.18

8.27 

1.01

7.05 

5.57

1.01

43.78 

2.76

8.58 

5.21

7.18

27.78 

6.78

Table 6. Simulated water budget under steady-state conditions

[Inflow, water added to the ground-water system; ft /d, cubic feet per day; outflow, water being removed from the ground-water system; 

pumpage, ground-water withdrawals by Cedar Falls Utilities, University of Northern Iowa, Waterloo Water Works, and City of Evansdale]

Budget component

Recharge from precipitation

River leakage Cedar River

River leakage Beaver Creek

Pumpage

General-head boundary (layer 1)

General-head boundary (layer 2)

General-head boundary (layer 3)

Total

Inflow 

(ft3/d)

9,561,000

2,453,000

0

0

2,132,000

690,800

771,800

15,608,600

Percentage 

of total inflow

61.3

15.7

0

0

13.7

4.4

4.9

100.0

Outflow 

(ft3/d)

0

6,330,000

221,000

8,130,000

43,000

724,000

161,000

15,609,000

Percentage 

of total 

outflow

0

40.6

1.4

52.1

0.3

4.6

1.0

100.0

SIMULATION OF GROUND-WATER FLOW 21
Ground-water flow IN from river leakage 

Ground-water flow OUT to river leakage

 * "Ground-water flow IN from general-head boundaries

 "Ground-water flow OUT to general-head boundaries 

RMSE

0.01 0.1 1 10

MULTIPLICATION FACTOR

100

o

1,000

Figure 10. Proportion of simulated inflow and outflow from river leakage and general-head boundaries as a result of varying 

riverbed conductance and corresponding root-mean-squared error.

Model Limitations

The ground-water flow model described in this 

report is a simulation of the physical ground-water 

system and is useful as an aid in evaluating the 

ground-water flow system in the study area. However, 

the following limitations to this model should be 

considered:

I. The presence of joints and fractures in the SilurianDevonian aquifer may introduce significant 

uncertainty into the model. The density and 

distribution of these features was not determined 

during this study. Orientations of the fractures 

can be inferred from the channel of the Cedar 

River, which is assumed to flow along these 

orientations. Fracture orientation was not 

included in the model parameters. The effect of 

fractures on hydraulic conductivity was assumed

to decrease as distance from the Cedar River 

increased and is included in the model.

2. Model input parameters are assigned to the node of 

each model cell. The nodal value is then used by 

the model as representative of conditions 

throughout the cell. This representation of uniformity throughout the cell is a potential source 

of model uncertainty in that model parameters 

probably are not uniform throughout a cell or 

group of cells. While the lithology across the 

modeled area is basically homogeneous, the 

distribution of hydraulic conductivity is heterogeneous and may change significantly across a 

modeled cell.

3. Small well withdrawals were not included in the 

model. The major pumpage from municipalities 

in the study area account for the main stress on 

the system, and minor pumpage from residences

22 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
and small industries is assumed to produce no 

significant effect on the modeled area.

4. The ground-water flow model was constructed to 

simulate steady-state conditions (the volumetric 

rate of water flowing into the system is the same 

as the volumetric rate of water flowing out of the 

system). Results of the ground-water flow model 

may not be valid when these conditions are not 

prevalent, as may be the case in the spring and 

early summer when Cedar River levels are the 

greatest and thunderstorms frequently produce 

large amounts of rainfall.

5. The steady-state flow model does not account for 

transient conditions. This steady-state model 

does not take into account the amount of time it 

may take to reach equilibrium. Varying climatic 

and hydrologic conditions and noncontinuous 

pumpage may complicate the amount of time it 

takes for this model to reach equilibrium.

6. Riverbed conductance values used in the model are 

derived from the iterative model calibration 

process rather than from field measurements. 

Measurements of riverbed conductance, if available in the future and used in this model, may be 

different than the estimated values used in this 

model and may produce different results.

Simulation Results

The model calculates a water level at each of the 

cell nodes and a ground-water flux across each cell 

face. Figure 11 shows the simulated potentiometric 

surface for layer 2 of the model. Model results for 

layer 2 indicate that ground water generally flows 

toward the Cedar River and slightly down the Cedar 

River Valley. The difference between the maximum 

and the minimum simulated water level in layer 2 is 

about 70 ft. Bends in the potentiometric surface 

contours coincide with the channel in the Cedar River 

and Beaver Creek, which indicates the effects of these 

streams on the ground-water flow patterns in this area. 

Water in the northern part of the simulated modeled 

area tends to flow away from the river near the model 

boundary, which may be caused by unaccounted for 

variations along the model boundary. Model results for 

layer 3, shown in figure 12, indicate the effects of 

pumpage on the potentiometric surface of the Silurian 

aquifer. Municipal pumping likely has caused some 

decline in ground-water levels in layer 3, as evidenced

by the closed 790-ft contour in the area near Evansdale, Iowa. Simulation results indicate that groundwater flow in layer 3 is toward the Cedar River and 

down the Cedar River Valley, much like in layer 2. The 

difference between the maximum and minimum simulated water levels in layer 3 is about 90 ft. The 

northern area of layers 2 and 3 exhibits a much shallower gradient than the southern area, which could be 

due to the lack of significant pumpage in those areas.

There are 43,200 data points used by the model 

to simulate potentiometric surfaces of the Devonian 

and Silurian portions of the Silurian-Devonian aquifer. 

The shape of the simulated potentiometric surfaces 

and the direction and magnitude of the simulated 

ground-water flow is similar to the potentiometric 

surface for the Silurian-Devonian aquifer and flow 

directions interpreted from the mean measured water 

levels. Simulated flow is predominantly horizontal in 

the Devonian- and Silurian-age rocks, so that little 

mixing may occur between the two units. Vertical 

ground-water flow is predominantly downward from 

the alluvium to the Devonian-age rock, probably due 

mainly to pumpage (table 7). Areas with significant 

pumpage, such as Cedar Falls and Waterloo, may 

cause localized mixing of water in the Devonian- and 

Silurian-age rocks.

The water budget for the calibrated flow model 

was used to evaluate the sources of inflow and outflow 

for the model and to determine if model results were 

consistent with the simplified conceptualization of the 

flow system used to construct the flow model. Total 

inflow to the modeled area was calculated to be 

15,609,000 ft3/d; total outflow from the modeled area 

was calculated to be 15,608,000 ft3/d, yielding a 

percent discrepancy of less than 0.01.

Primary sources of inflow to the model are 

precipitation (61.3 percent) and Cedar River leakage 

(15.7 percent). Infiltration of precipitation is predominantly through the overlying alluvial material.

Primary sources of outflow are pumpage 

(52.1 percent) and leakage to the Cedar River 

(40.6 percent) (table 6). River leakage is discharged 

from the Quaternary alluvial deposits; and municipal 

pumpage withdraws water from the Devonian- and 

Silurian-age rock units. The large volume of river 

leakage is consistent with the conceptualization of 

subsurface regional flow into the study area and infiltration of precipitation from the overlying Quaternary 

deposits, which then is discharged to the Cedar River 

and tributary streams.

SIMULATION OF GROUND-WATER FLOW 23
92°30' 92°20' 92 10'

42°40' -

42°30'

42 20' -

Base from U.S. Geological Survey digital 

data, 1:100,000, Universal Transverse 

Mercator projection, Zone 15 \ 

10 KILOMETERS

EXPLANATION

~850 Simulated potentiometric contour Shows 

altitude at which water would have stood 

in lightly cased well. Contour interval 10 

feet. Datum is sea level

Extent of digital model 

Active model cells 

Observation well

Figure 11. Simulated potentiometric surface in the Devonian-age rock.

24 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
92°30' 92 20' 92"10'

42 40' -

42°30' - _

42 C 20' -

Base from U.S. Geological Survey digital 

data, 1:100,000, Universal Transverse 

Mercator projection, Zone 15

10 MILES

_I

10 KILOMETERS

EXPLANATION

~ 850 Simulated potentiomelric contour Shows 

altitude at which water would have stood 

in tightly cased well. Contour interval 10 

feet. Datum is sea level

Extent of digital model 

Active model cells 

Observation well

Figure 12. Simulated potentiometric surface in the Silurian-age rock.

SIMULATION OF GROUND-WATER FLOW 25
Table 7. Well characteristics, nitrate levels, and pumpage information for active Cedar Falls Utilities municipal-supply wells, 

1999

[Data from Cedar Falls Utilities annual water report, 1999, and well logs; mg/L, milligrams per liter; N/A, data not available; <, less than; gpm, gallons per 

minute]

Cedar Falls 

Utilities 

municipal 

well number 

(fig. 14)

2

3

5

6

7

8

9

10

11

Year 

constructed

1956

1957

1961

1967

1967

1971

1971

1971

1993

Altitude of 

land surface 

(feet above 

sea level)

858

935

876

959

941

941

875

875

895

Total depth 

(in feet 

below land 

surface)

150

183

145

227

170

216

275

275

200

Thickness of 

alluvial 

material 

(feet)

10

94

31

115

100

105

30

30

60

Altitude of 

bottom of 

casing 

(feet above 

sea level)

818

769

786

825

870

800

707

707

815

Length of 

open hole 

(feet)

110

20

55

93

99

75

107

107

75

Average 

nitrite plus 

nitrate 

nitrogen, 

dissolved1 

(mg/L as N)

N/A

7.98

3.73

<1.00

<1.00 .

<1.00

7.88

6.76

2.98

Average 

1999 

pumpage

(gpm)

0

2,614

2,407

2,125

1,673

1,849

769

2,228

1,516

'Nitrite plus nitrate concentrations provided by Keystone Lab, Cedar Rapids, Iowa, for Cedar Falls Utilities.

SUMMARY

The Silurian-Devonian bedrock aquifer in the 

Cedar Falls, Iowa, area provides large quantities of 

good quality ground water for municipal water 

suppliers as well as private residential users. The 

highly transmissive nature of the bedrock aquifer 

material, due to fractures and karst features in the area 

and areas of thin, overlying unconsolidated deposits, 

results in a ground-water supply vulnerable to contamination. To address these concerns, the U.S. Geological Survey (USGS), in cooperation with Cedar Falls 

Utilities, conducted a study from April 1998 to 

September 2001 to evaluate the hydrogeology of the 

Silurian-Devonian bedrock aquifer in the Cedar Falls 

area and simulate the ground-water flow in the 

Silurian-Devonian aquifer under current (1998) 

pumping conditions. The results of the study can be 

used by Cedar Falls Utilities (CPU) to establish a wellhead protection program and manage the development 

of the ground-water resource. The study area covers 

approximately 200 mi2 in northeast Iowa and includes 

Black Hawk County and small portions of Benton, 

Bremer, Buchanan, Butler, Grundy, and Tama 

Counties.

Ground-water and surface-water data were 

collected during the study to help define the hydrogeology of the Silurian-Devonian aquifer and to assist

in constructing a ground-water flow model. Groundwater levels were measured bimonthly from April 

1998 to February 1999 with a calibrated steel tape or 

an airline. Available well tests were used to estimate 

the hydraulic conductivity of the geologic material 

adjacent to the open interval of the well. Streamflow 

and stage data were collected periodically from 1998 

to 1999 as part of the USGS streamflow network in 

Iowa.

The Quaternary-age deposits comprise a surficial aquifer system that ranges in thickness from about 

20 ft to 250 ft and include alluvial, glacial-drift, and 

buried-channel materials with variable permeabilities 

both vertically and horizontally. The uppermost 

bedrock unit consists of Devonian-age rocks 

consisting primarily of limestone, dolomite, and shale 

but locally includes minor amounts of sandstone. This 

rock unit has a significant occurrence of fractures or 

secondary permeability. The relative amount of 

secondary permeability appears to be greatest nearest 

the Cedar River. The Silurian-age rocks consist primarily of dolomite with minor amounts of chert and 

are assumed to have less secondary permeability with 

respect to the Devonian-age rocks. Transmissivity of 

the Silurian-Devonian aquifer is highly variable, 

depending on the degree of interconnection between 

the fractures and bedding plane and the rock-age unit.

26 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
Ground-water flow in the study area was simulated using the USGS-developed MODFLOW model 

(McDonald and Harbaugh, 1988) to obtain a better 

understanding of the ground-water flow patterns of the 

Silurian-Devonian aquifer in the study area, to provide 

a quantitative estimate of the water budget in the study 

area.

The flow model was constructed to simulate 

steady-state conditions. Steady-state conditions occur 

when the volume of water entering a system equals the 

volume of water flowing out of the system. Mean 

water levels within the study area from April 1998 to 

February 1999 were considered to be an acceptable 

estimate of steady-state conditions.

The modeled area was discretized into a 

220-row by 180-column grid with cells measuring 

500 ft by 500 ft. The grid was aligned so that the Cedar 

River coincided with the y-axis. Three layers were 

used to simulate flow: layer 1 represents the Quaternary alluvial and glacial deposits, layer 2 represents 

the Devonian-age portion of the Silurian-Devonian 

aquifer, and layer 3 represents the Silurian-age portion 

of the Silurian-Devonian aquifer.

Natural boundaries to the model were not in 

close proximity to the study area to use as model 

boundaries. Boundary conditions were specified for 

the model to simulate flow along the edge of the 

modeled area in relation to the features within the 

modeled area. The upper surface of layer 1 represents 

unconfmed water-table conditions. Recharge from 

precipitation to the upper surface of the flow model 

was represented as a nonuniform specified-flux 

boundary. No-flow boundaries were used to simulate 

areas where lateral ground-water flow is interpreted to 

be parallel with the boundary, such as along groundwater flow lines, or is considered to be insignificant, 

such as a contact with a nonaquifer material. The 

bottom of the model is the top of the Ordovician-age 

rock, a relatively impermeable regional confining unit. 

Lateral ground-water flow from geologic materials 

adjacent to the model layers was simulated with a 

general-head boundary. The Cedar River and Beaver 

Creek were simulated using river cells that allow 

leakage to and from layer 1.

Model architecture and hydraulic properties 

were estimated using well log data from previously 

constructed private or municipal wells. Hydraulic 

conductivity in layer 1 was estimated using aquifer test 

data completed by private drillers or contractors 

during well construction or published average

hydraulic conductivities for similar geologic materials. 

Hydraulic conductivities for all model layers are a 

function of the estimated degree of secondary permeability present. Vertical leakance is required by the 

model to control the rate of ground-water flow 

between layers. Recharge in the Cedar Falls area was 

assumed to be about 10 percent less than the recharge 

in Cedar Rapids, Iowa.

The calibration process used in this model minimized the differences between simulated ground-water 

levels and measured ground-water levels by adjusting 

model parameters. The mean ground-water levels 

calculated for each network well from measured water 

levels during the study interval were used as a basis for 

calibration. The RMSE for the calibrated model, 

calculated from most network wells within the model 

boundary, is 4.14 ft; the AVEH is 1.29 ft. A difference 

between the simulated and measured water levels 

likely is because the model is a simplified representation of a complex ground-water flow system.

Water levels were most sensitive to changes in 

horizontal hydraulic conductivity in all layers and 

recharge in layer 1. Increases in riverbed conductance 

and general-head-boundary conductance showed no 

significant change in the RMSE; however, decreases in 

the general-head-boundary conductance by 2 or more 

orders of magnitude resulted in a RMSE above 100 or 

inability of the model to reach a solution.

The model calculates a water level at each of 

the cell nodes and a ground-water flux across each 

cell face. Primary sources of inflow to the model 

are precipitation (61.3 percent) and Cedar River 

leakage (15.7 percent). Primary sources of outflow are 

pumpage (52.1 percent) and river discharge 

(40.6 percent). There is little evidence of mixing 

between the Devonian- and Silurian-age rock units 

with the exception of the main pumping centers, where 

increased withdrawals may cause localized mixing 

between them.

REFERENCES

Anderson, M.P., and Woessner, W.W., 1992, Applied 

groundwater modeling Simulation of flow and 

advective transport: San Diego, Academic Press, Inc., 

381 p.

Anderson, W.I., 1983, Geology of Iowa Over two billion 

years of change: Ames, Iowa State University Press, 

268 p.

REFERENCES 27
Brigham Young University, 1998, The Department of

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Brigham Young University Engineering Computer 

Graphics Laboratory.

Driscoll, EG., 1986, Groundwater and wells (second

edition): St. Paul, Minn., U.S. Filter/Johnson Screens, 

1089 p.

Freeze, A.R., and Cherry, J.A. 1979, Groundwater: Englewood Cliffs, N.J., Prentice-Hall, 604 p.

Hallberg, G.R., Riley, D.G., Kantamneni, J.R., Weyer, P.J., 

and Kelley, R.D., 1996, Assessment of Iowa Safe 

Drinking Water Act monitoring data 1987 1995: 

University of Iowa Hygienic Laboratory Research 

Report 97-1, 132 p.

Hansen, R.E., 1975, Bedrock topography of northeast Iowa: 

U.S. Geological Survey Miscellaneous Investigations 

Map 1-717, 1 sheet, scale 1:125,000.

Horick, P.J., 1984, Silurian-Devonian aquifer in Iowa: State 

of Iowa, Iowa Geological Survey Miscellaneous Map 

Series 10.

Iowa Geological Survey Bureau, 1999, GEOSAM data 

retrieval system for various wells: accessed January 29, 

1999, at URL http://www.igsb.uiowa.edu/geosam/.

Knochenmus, L.A., and Robinson, J.L., 1996, Descriptions 

of anisotropy and heterogeneity and their effect on 

ground-water flow and areas of contribution to public 

supply wells in a karst carbonate aquifer system: 

U.S. Geological Survey Water-Supply Paper 2475, 

47 p.

Lucey, K.J., Kuzinar, R.L., and Caldwell, J.P., 1995, Hydrogeology and water quality of the Mississippi River 

Alluvium near Muscatine, Iowa, June 1992 through 

June 1994: U.S. Geological Survey Water-Resources 

Investigations Report 95-4047, 74 p.

May, J.E., Gorman, J.G., Goodrich, R.D., Miller, V.E.,

Turco, M.J., and Linhart, S.M., 1998, Water resources 

data, Iowa, water year 1998 v. 1. Surface water 

Mississippi River Basin: U.S. Geological Survey Water 

Data Report IA-98-1, 374 p.

McDonald, M.G. and Harbaugh, A.W., 1988, A modular 

three-dimensional finite difference ground-water flow 

model: U.S. Geological Survey Techniques of WaterResources Investigations, book 6, chap. Al, 586 p.

Nalley, G.M., Gorman, J.G., Goodrich, R.D., Miller, V.E., 

Turco, M.J., and Linhart, S.M., 1999, Water resources 

data, Iowa, water year 1999 v. 1. Surface water 

Mississippi River Basin: U.S. Geological Survey Water 

Data Report IA-99-1, 363 p.

National Oceanic and Atmospheric Administration, 1998, 

Climatological data, Iowa, annual summary: Asheville, 

N.C., National Climatic Data Center, v. 109, no. 13,

23 p.

Clcott, P.G., 1992, Ground water atlas of the United 

States Segment 9 (Iowa, Michigan, Minnesota, 

Wisconsin): U.S. Geological Survey Hydrologic Investigations Atlas 730-J, 31 p.

Parker, M.C., 1971, The Maquoketa Formation (Upper 

Ordovician) in Iowa: Iowa City, Iowa Geological 

Survey Miscellaneous Map Series!, 5 sheets, scale 

1:1,267,000.

Pollock, D.W., 1994, User's guide for

MODPATH/MODPATH-PLOT, version 3: A particle 

tracking post-processing package for MODFLOW, the 

U.S. Geological Survey finite-difference ground-water 

flow model: U.S. Geological Survey Open-File Report 

94-464, 6 chaps.

Schaap, B.D., 1999, Concentrations and possible sources of 

, nitrate in water from the Silurian-Devonian aquifer, 

Cedar Falls, Iowa: U.S. Geological Survey WaterResources Investigations Report 99 4106, 19 p.

Schulmeyer, P.M., and Schnoebelen, D.J., 1998, Hydrogeology and water quality in the Cedar Rapids area, 

Iowa, 1992-96: U.S. Geological Survey WaterResources Investigations Report 97 4261, 77 p.

Theis, C.V., Brown, R.H., and Meyer, R.R., 1963,

Estimating the transmissivity of aquifers from the 

specific capacity of wells, in Bentall, Ray, compiler, 

Methods of determining permeability, transmissibility, 

and drawdown: U.S. Geological Survey Water-Supply 

Paper 1536-1, p. 331-341.

Witzke, B.J., Bunker, B.J., and Rogers, F.S., 1988, Eifelian 

through Lower Frasnian stratigraphy and deposition in 

the Iowa area, central Midcontinent, U.S.A., in 

McMillan, N.J., Embry, A.F., and Glass, D.J., eds., 

Devonian of the world: Canadian Society of Petroleum 

Geologists, Memoir 14, v. I Regional syntheses, 

p. 221-250.

28 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
APPENDIX

Table A1. Iowa Department of Natural Resources-Geological Survey Bureau (IDNR-GSB) well logs used to estimate the 

elevation of the top of each rock-age interval and the thickness of water bearing units in the study area

[ddmmss, degrees, minutes, seconds; Qtr, Quaternary; Miss, Mississippian; Dev, Devonian; Sil, Silurian; Ord, Ordovician]

IDNRGSB well 

number1

26975

1

4879

20343

21112

22289

219

276

8959

3239

1689

9067

19439

11586

431

1932

15308

16010

23018

8515

2778

1735

2148

10616

5006

9599

4602

1938

30059

10419

14445

5370

2750

13835

30060

15788

7447

18612

9362

8576

1287

8519

9275

16109

9257

County1

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Benton

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Black Hawk

Latitude2 

(ddmmss)

420502

420518

420221

421213

421005

420745

421022

420950

420812

420838

421029

420727

421355

421329

421309

421431

422153

421930

421904

421842

421936

422028

421856

422451

422317

422540

422346

422517

422540

422421

423227

422958

422820

423002

423019

422955

422831

423058

423515

422927

423356

423504

423411

423649

423713

Longitude2 

(ddmmss)

921213

915252

921052

915123

915034

915502

920134

920043

920320

920849

921752

921119

915107

915246

915924

920802

920941

920459

921125

921531

922349

922550

923158

921623

921752

922122

922238

923054

923050

922736

921332

921555

921811

922313

922048

921959

922507

922735

921104

923053

920936

921421

921324

922104

922708

Altitude of 

land 

surface1 

(feet above 

sea level)

900

788

944

885,

865

905

780

813

925

868

980

978

905

908

890

924

893

875

825

949

991

988

1,007

835

928

965

960

986

1,010

890

931

925

839

926

851

846

930

922

997

997

981

996

988

981

876

Depth of 

well1 ( feet 

below land 

surface)

1,880

1,519

330

210

245

175

1,508

320

403

1,622

636

205

245

560

635

251

270

128

1,400

315

296

347

335

222

202

226

222

220

271

165

225

960

200

208

1,378

156

278

205

230

235

272

680

300

150

120

Distance to top of rock-age interval 1 (feet 

below land surface)

Qtr1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Miss1 Dev1

285

17

242

60

118

142

115

70

185

25

178

160

140

150

90

81

165

40

--

112

194

205

194

160

215

170

150

215

70

..

115

65

113

20

22

145

82

160

149

..

90

Sil 1

515

150

--

--

230

--

150

200

335

305

475

-

234

275

280

232

250

--

--

--

--

--

--

145

--

-

--

--

--

-

165

165

130

--

200

140

--

--

160

--

228

--

272

100

Ord1

697

435

--

--

'--

--

350

--

--

521

607

--

--

--

490

--

--

--

250

--

--

--

-

--

--

--

--

--

312

--

-

300

--

~

265

-

APPENDIX 31
Table A1. Iowa Department of Natural Resources-Geological Survey Bureau (IDNR-GSB) well logs used to estimate the 

elevation of the top of each rock-age interval and the thickness of water bearing units in the study area Continued

[ddmmss, degrees, minutes, seconds; Qtr, Quaternary; Miss, Mississippian; Dev, Devonian; Sil, Silurian; Ord, Ordovician]

IDNRGSB well 

number1

17624

10605

4591

13058

1544

5754

21048

2036

30066

20380

9780

70

4613

11754

15543

14461

14004

9037

6004

8773

3894

3714

1856

9382

27529

27535

14135

1578

16147

9469

5783

14428

7854

2024

8598

12971

824

14016

1246

4631

26318

22974

7201

1277

County1

Black Hawk

Black Hawk

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Bremer

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Buchanan

Butler

Butler

Butler

Butler

Butler

Butler

Butler

Fayette

Fayette

Fayette

Fayette

Fayette

Fayette

Latitude2 

(ddmmss)

42341 1

423353

424012

424250

424217

423914

424236

424119

424031

424008

424007

424336

424112

423859

424722

424632

424737

424458

422226

422025

421903

422836

422810

422854

423557

423707

423813

422718

422748

422540

422631

423640 .

423357

423753

423551

423852

424703

424440

423847

424051

424014

423940

424447

424629

Longitude2 

(ddmmss)

323108

922654

920754

921747

921330

921746

922423

922510

922011

921945

922138

922811

922902

922722

922444

922559

922822

923054

914030

914631

920015

915317

915316

920353

913828

915416

920243

915531

915401

915557

920229

923549

923744

924840

930026

924641

924006

914948

914856

915519

915459

920150

914025

914115

Altitude of 

land 

surface1 

(feet above 

sea level)

913

869

1,010

1,022

1,039

1,038

1,005

1,017

947

1,003

1,054

918

1,008

935

1,040

1,026

1,004

995

1,021

905

838

933

935

992

1,042

995

994

981

907

959

996

896

891

1,033

1,039

972

942

1,047

1,119

1,045

1,032

1,015

1,147

1,094

Depth of 

well1 ( feet 

below land 

surface)

158

150

180

375

952

880

150

804

410

191

220

1,720

220

150

372

370

129

145

200

107

405

293

' 307

380

1,195

1,201

1,292

352

265

180

235

3,595

165

332

440

186

230

205

205

1,315

1,200

665

1,312

155

Distance to top of rock-age interval 1 (feet 

below land surface)

Qtr1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

4)

0

0

0

0

0

Miss1 Dev1

1 13

115

--

._

80

..

108

50

..

115

135

10

72

35

--

--

--

70

181

80

15

11

25

40

..

..

28

..

15

160

78

122

65

110

45

160

35

145

--

--

--

5

--

--

Sil 1

--

--

161

-

120

257

135

128

--

135

168

90

165

95

--

183

94

115

-

--

181

120

100

185

75

47

114

155

65

162

230

316

--

--

--

--

--

--

175

50

25

15

89

15

Ord1

 --

--

--

235

150

285

--

146

162

185

--

110

198

148

270

185

--

--

--

--

385

--

295

375 

150

67

193

341

260

-

353

--

--

--

--

--

--

--

160

160

70

200

125

32 Simulation of Ground-Water Flow in the Silurian-Devonian Aquifer, Cedar Falls, Iowa
Table A1. Iowa Department of Natural Resources-Geological Survey Bureau (IDNR-GSB) well logs used to estimate the 

elevation of the top of each rock-age interval and the thickness of water bearing units in the study area Continued

[ddmmss, degrees, minutes, seconds; Qtr, Quaternary; Miss, Mississippian; Dev, Devonian; Sil, Silurian; Ord, Ordovician]

IDNRGSB well 

number1

1 3037

2407

8873

1 6863

12148

1 0025

2847

17048

20131

18145

17237

5639

25187

13810

8832

2109

9262

210

12199

12059

17954

13188

30488

30489

1825

10984

2141

2719

1611

4025

3626

1861

30967

30969

12665

14136

1707

2061

1424

1674

18143

4064

3626

County1

Fayette

Fayette

Fayette

Fayette

Fayette

Fayette

Fayette

Fayette

Fayette

Fayette

Fayette

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Grundy

Tama

Tama

Tama

Tama

Tama

Tama

Tama

Tama

Tama

Tama

Tama

Tama

Tama

Latitude2 

(ddmmss)

424412

424635

424619

424902

425348

425312

425327

425026

425001

425052

425519

421739

421326

421320

421721

421509

422221

421929

422156

422107

421755

421816

422236

422806

422636

422610

422857

423049

423019

422900

420540

42035 1

420626

420706

420955

421132

421056

420930

420747

420753

421417

421548

420540

Longitude2 

(ddmmss)

914155

915300

915247

920322

913802

914016

914703

914849

915726

920446

913759

924935

924913

924652

925859

925611

923212

923552

924558

924527

924004

925039

925718

923747

924337

925527

923534

924534

924721

924616

912037

921800

922857

923456

921821

922748

922555

922706

923743

924409

923616

924126

912037

Altitude of 

land 

surface1 

(feet above 

sea level)

,163

,093

,117

,111

,184

1,130

1,169

1,068

1,159

1,072

1,032

1,081

1,038

1,048

1,041

1,053

912

945

984

1,011

970

1,098

1,105

953

1,019

1,089

1,022

1,058

1,063

1,055

878

892

933

1,078

978

924

970

986

1,032

982

1,019

1,069

878

Depth of 

well1 ( feet 

below land 

surface)

150

126

850

175

216

770

294

1,238

152

360

60

500

118

160

 125

133

235

350

551

100

549

728

875

292

376

2,050

317

415

339

420

392

620

435

499

1,880

1,813

435

765

445

885

415

528

392

Distance to top of rock-age interval 1 (feet 

below land surface)

Qtr1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Miss1 Dev1

--

5

39

--

--

--

--

20

1 18

--

--

415

45 115

132 159

75

90

140

160

115

--

85

142

180

--

170

150

170

219

202

260

197

215

360

332

160

231

232

310

235

11 57

305

181

197

Sil 1 Ord1

135

80

110 200

155

60

400

60 136

65 150

125

220

5

-

--

--

-

-

-

--

--

--

--

--

-

-

--

775 845

--

--

-

--

-

460

-

-

464 625

505 673

-

-

-

775 880

-

--

--

'From Iowa Department of Natural Resources-Geological Survey Bureau virtual GEOSAM database; http://www.igsb.uiowa.edu/geosam_map/ last 

accessed 01/29/1999.

Calculated from public land survey description using IDNR-GSB conversion calculator; http://www.igsb.uiowa.edu/getutm/ last accessed 01/29/1999.

APPENDIX 33

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