U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 02–4004

Prepared in cooperation with

MUSCATINE POWER AND WATER, 

MUSCATINE, IOWA

Simulation of Ground-Water Flow and 

Delineation of Areas Contributing 

Recharge to Municipal Water-Supply 

Wells, Muscatine, Iowa
U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 02–4004

Simulation of Ground-Water Flow 

and Delineation of Areas Contributing 

Recharge to Municipal Water-Supply 

Wells, Muscatine, Iowa

By Mark E. Savoca, Keith J. Lucey, and Brian D. Lanning

Iowa City, Iowa

2002

Prepared in cooperation with

MUSCATINE POWER AND WATER,

MUSCATINE, IOWA
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: Copies of this report can be purchased

U.S. Geological Survey

Information Services

Box 25286

Denver Federal Center

Denver, CO 80225

from:

District Chief

U.S. Geological Survey

Water Resources Division

P.O. Box 1230

Iowa City, Iowa 52244
CONTENTS III

CONTENTS

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

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

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

Description of Study Area ........................................................................................................................................... 4

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

Methods of Investigation ....................................................................................................................................................... 4

Simulation of Ground-Water Flow ........................................................................................................................................ 4

Model Description and Boundary Conditions ............................................................................................................. 4

Model Parameters ........................................................................................................................................................ 7

Model Calibration........................................................................................................................................................ 8

Sensitivity Analysis ..................................................................................................................................................... 9

Model Limitations ....................................................................................................................................................... 12

Results of Simulation................................................................................................................................................... 13

Delineation of Areas Contributing Recharge......................................................................................................................... 16

Zones of Transport and Potential Sources of Contamination ...................................................................................... 16

Effects of Changing Climatic Conditions on Areas Contributing Recharge ............................................................... 18

Summary................................................................................................................................................................................ 25

References.............................................................................................................................................................................. 26

FIGURES

1–4. Maps showing:

1. Location of study area .................................................................................................................................... 3

2. Model boundary conditions for layers 1 and 2 ............................................................................................... 6

3. Simulated water-table surface in upper alluvium, February 1993.................................................................. 10

4. Measured water-table surface in upper alluvium, February 26, 1993 ............................................................ 11

5. Graph showing root mean squared error between measured and simulated water levels 

as a result of varying model input parameters ......................................................................................................... 13

6. Graph showing proportion of simulated inflow obtained from river leakage as a result 

of varying model input parameters .......................................................................................................................... 14

7–14. Maps showing:

7. Simulated water-table surface in upper alluvium, February 1993.................................................................. 15

8. Estimated areas contributing recharge to municipal well fields under 1998 

pumping conditions. ....................................................................................................................................... 17

9. Estimated 2-, 5-, and 10-year zones of transport for municipal well fields.................................................... 19

10. Estimated 2-, 5-, and 10-year zones of transport for wells in the Grandview well 

field and potential sources of contamination .................................................................................................. 20

11. Estimated 2-, 5-, and 10-year zones of transport for wells in the Main well field 

and potential sources of contamination .......................................................................................................... 21

12. Estimated 2-, 5-, and 10-year zones of transport for wells in the Progress Park 

well field and potential sources of contamination .......................................................................................... 22

13. Drainage basins of creeks within areas contributing recharge to the Grandview 

well field and potential sources of contamination .......................................................................................... 23

14. Estimated areas contributing recharge to municipal well fields under less-thanand greater-than-normal recharge from precipitation..................................................................................... 24

TABLES

1. Simulated water budgets.......................................................................................................................................... 12

2. Comparison of simulated traveltime and ground-water recharge date determined from 

chlorofluorocarbon and tritium analyses for samples collected May 4–7, 1999..................................................... 18
IV CONTENTS

CONVERSION FACTORS AND VERTICAL DATUM

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic datum derived 

from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Multiply By To obtain

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

square mile (mi2

) 2.590 square kilometer 

gallon (gal) 3.785 liter

million gallons (Mgal) 3,785 cubic meter 

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

 foot squared per day (ft2/d) 0.0929 meter squared per day

million gallons per day (Mgal/d) 3,785 cubic meters per day 

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

cubic foot (ft3

) 0.02832 cubic meter 

cubic foot per second (ft3

/s) 0.02832 cubic meter per second 

cubic foot per day (ft3

/d) 0.02832 cubic meter per day
Abstract 1

Simulation of Ground-Water Flow and Delineation 

of Areas Contributing Recharge to Municipal 

Water-Supply Wells, Muscatine, Iowa

By Mark E. Savoca, Keith J. Lucey, and Brian D. Lanning

Abstract

Mississippi River alluvium in the 

Muscatine, Iowa, area provides large quantities 

of good quality ground water for municipal, 

industrial, and agricultural supplies. Three municipal well fields for the City of Muscatine produce 

a total of about 27 million gallons per day from 

the alluvium. A previously published steady-state 

ground-water flow model was modified, and 

results from the model were used with particletracking software to delineate approximate areas 

contributing recharge to Muscatine Power and 

Water municipal supply wells and to determine 

zones of transport within the areas contributing 

recharge. 

Under steady-state conditions and 1998 

pumpage, primary sources of inflow to the 

ground-water flow system are recharge through 

infiltration of precipitation and upland runoff 

(53 percent) and Mississippi River leakage 

(41 percent). The primary components of outflow 

from the ground-water flow system are pumpage 

(39.6 percent), flow to drainage ditches in Illinois 

(32.9 percent), and Muscatine Slough leakage 

(24.7 percent). 

Several sources of water are present 

within estimated areas contributing recharge to 

Muscatine Power and Water municipal well fields 

including ground water from the alluvial aquifer, 

Mississippi River water, and recharge originating 

as runoff from two unnamed creeks in the 

northern part of the study area. Recharge originating from the Mississippi River accounts for 

about 46 percent of the total water discharged 

from the municipal well fields. The average 

simulated traveltime of particles tracked from 

recharge to discharge at the municipal well fields 

was 13.6 years. Particle-tracking results illustrate 

the influence of nearby industrial supply wells 

on the shape and size of the area contributing 

recharge to Muscatine Power and Water wells. 

Two large embayments into the area contributing 

recharge to municipal wells are present along the 

Mississippi River. These areas represent ground 

water that is unavailable to municipal wells 

due to withdrawals by industrial supply wells. 

Recharge originating from the Mississippi River 

accounts for about 98 percent of the total water 

discharged from the Muscatine Power and Water 

Main well field. However, recharge originating 

from the Mississippi River accounts for less 

of the total discharge from the Progress Park 

and Grandview municipal well fields (12 and 

34 percent, respectively).

The effects of changing climatic conditions 

on the size and shape of the 10-year zone of transport to Muscatine Power and Water municipal 

well fields were simulated by decreasing and 

increasing recharge from precipitation to the 

ground-water model to demonstrate the variability 

inherent in delineating these areas. Locations of 

potential sources of contamination within the 

zones of transport also are identified.

INTRODUCTION

The sands and gravels of the Mississippi River 

alluvium in the Muscatine, Iowa, area provide large 

quantities of good quality ground water for municipal, 

industrial, and agricultural supplies. Three municipal 
2 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

well fields contain 26 water-supply wells that produce 

about 27 Mgal/d from the alluvial aquifer for the City 

of Muscatine. The general plan for development 

of the ground-water resource calls for doubling 

municipal withdrawals during the next 20 years 

(J. Doering, Muscatine Power and Water, oral 

commun., December 1996). The highly permeable 

nature of the alluvium and varied land use in the area 

result in a ground-water supply potentially vulnerable 

to contamination. 

A cooperative study of the Mississippi River 

alluvium near Muscatine, Iowa, was conducted 

by Muscatine Power and Water (MPW) and the 

U.S. Geological Survey (USGS) from 1992 to 1995. 

Hydrogeology and water quality were investigated in 

an 80-mi2

 area in Muscatine and Louisa Counties in 

Iowa and Rock Island and Mercer Counties in Illinois 

(fig. 1), and the results were documented in a report by 

Lucey and others (1995). A ground-water flow model 

was constructed by Lucey and others (1995) using the 

USGS computer program MODFLOW (McDonald 

and Harbaugh, 1988) to simulate February 1993 

hydrologic conditions, which were considered to 

be an acceptable estimate of the ground-water system 

at equilibrium (steady-state condition). Steady-state 

conditions occur when inflow to the ground-water 

flow system equals outflow, resulting in a relatively 

stable water-table surface and a constant amount of 

water in storage in the aquifer. The ground-water flow 

model consists of three layers to represent the alluvium and part of the bedrock. A 30-row by 24-column 

grid was used to discretize each model layer into 

2,000-ft by 2,000-ft cells. Model results were used to 

develop an improved understanding of the complex 

ground-water flow system and to quantify sources 

of water to the alluvium. Lucey (1997) used this model 

to describe effects of hypothetical future pumping 

scenarios on water levels and sources of water in the 

Mississippi River alluvium.

There is a need to delineate areas contributing 

recharge to municipal water-supply wells in the study 

area described by Lucey and others (1995) to aid in the 

development of a wellhead protection plan (WHPP) 

for the Muscatine area. A WHPP helps protect a 

municipal ground-water supply from contamination 

and can provide many benefits that include: (1) protection of public health by preventing contamination of 

the water supply, (2) protection of the community’s 

investment in its water supply, (3) ensuring a clean 

water supply necessary for continued economic 

growth, (4) preserving the ground-water resource for 

future generations, and (5) the possible reduction in 

regulatory water-quality monitoring costs (State of 

Ohio Environmental Protection Agency, 1994). To 

help address this need, the USGS, in cooperation 

with MPW, initiated a study in 1999 to delineate areas 

contributing recharge to municipal water-supply wells 

and to determine the 2-, 5-, and 10-year zones of transport within the areas contributing recharge.

Results of the study can aid water managers 

in directing wellhead protection efforts within 

technically defensible areas contributing recharge 

to municipal water-supply wells. Knowledge will be 

gained about ground-water flow and areas contributing 

recharge in alluvium and about the application of 

simulation techniques used to delineate areas contributing recharge to wells constructed in alluvium.

Purpose and Scope

The purpose of this report is to describe the 

methods used to delineate areas contributing recharge 

to municipal water-supply wells in Muscatine, Iowa, 

and to present the results of those delineations for 

1998 pumping conditions. The steady-state groundwater flow model constructed by Lucey and others 

(1995) was modified to discretize the three model 

layers into 200-ft by 200-ft cells. Smaller cell size 

provides a more accurate representation of the groundwater flow system, which improves the accuracy of 

contributing areas determined by model simulation. 

Smaller cell size allows for a more detailed representation of model features such as drains, streams, and 

rivers and provides a more accurate representation of 

the spatial distribution of well withdrawals by eliminating the need to simulate multiple wells in a single 

larger cell. Results from the modified flow model were 

used with the particle-tracking software MODPATH 

(Pollock, 1994) to delineate approximate areas 

contributing recharge to wells and to determine 2-, 

5-, and 10-year zones of transport within the areas 

contributing recharge. The report also presents delineations based on less-than-average and greater-thanaverage recharge conditions to demonstrate the variability inherent in delineating areas contributing 

recharge to wells. Locations of potential sources 

of contamination within the zones of transport are 

identified.
INTRODUCTION 3

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

River, stream, drainage ditch, or gravel pit Upland area

EXPLANATION

0 1 2 3 MILES

0 1 2 3 KILOMETERS

Muscatine-Louisa

pump station

Transportation (road or railroad) Pump station

Rock Island County

Mercer County

drainage

ditches

Keating

 Creek

Muscatine County

Louisa County

IOWA

ILLINOIS

MISSISSIPPI

RIVER

Copperas Creek

drainage

ditches Muscatine Slough

Lock and Dam

Muscatine

41°20'

41°25'

91°10' 91°05' 91°

WISCONSIN

IOWA ILLINOIS

Study area 

MINNESOTA

MISSOURI

NEBRASKA

SOUTH DAKOTA Missouri River

Mississippi River

Figure 1. Location of study area.
4 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

Description of Study Area

The study area covers approximately 80 mi2

in southeast Iowa and northwest Illinois and includes 

parts of Muscatine and Louisa Counties in Iowa and 

Rock Island and Mercer Counties in Illinois (fig. 1). 

The relatively flat alluvial plain of the Mississippi 

River Valley in this area generally has an altitude of 

535 to 550 ft above sea level and gradually rises to 

590 ft at the base of bluffs on either side of the valley. 

These steep bluffs separate the valley from the upland 

areas that rise to 750 ft. Alluvium overlies bedrock 

consisting of limestone, dolomite, and shale in the 

Mississippi River Valley, and the thickness of the 

alluvium varies from about 40 ft in the northeast to 

more than 140 ft in the southern and western parts 

of the study area. Glacial till (predominantly clay) 

overlies bedrock units in the upland areas. Detailed 

descriptions of aquifer characteristics, geology, and 

hydrology of the study area are given by Hansen 

and Steinhilber (1977) and Lucey and others (1995). 

The U.S. Army Corps of Engineers controls 

the Mississippi River stage with a series of lock and 

dam structures, and channel depth is maintained by 

dredging operations to facilitate commercial navigation. The Muscatine Slough and its associated tributaries drain 52.3 mi2 in Iowa. Streamflow and stage in 

the slough are controlled by a pump station located at 

the south end of the slough (fig. 1). The lowlands in 

the river valley are protected from Mississippi River 

flooding by a levee complex extending along both 

sides of the Mississippi River.

Land use in the Mississippi River valley is 

primarily agricultural. Sandy soils on the Iowa side of 

the river often require supplemental irrigation, which 

uses ground-water withdrawals. Industrial activity 

includes manufacturing, electric power generation, 

and sand and gravel extraction.

Acknowledgments

Information and assistance were provided by 

MPW during this study. A field inventory of potential 

sources of contaminants was conducted by MPW 

personnel, and the cooperation and assistance of 

property owners during the field inventory is appreciated. Water-supply well withdrawals for 1998 were 

obtained from records provided by Grain Processing 

Corporation, Iowa-Illinois Gas and Electric Power 

Company, Monsanto Corporation, and MPW.

METHODS OF INVESTIGATION

The ground-water flow model constructed by 

Lucey and others (1995) to simulate February 1993 

steady-state conditions was modified for this study. In 

the modified model, the model cell size was decreased 

from 2,000 ft by 2,000 ft to 200 ft by 200 ft, hydraulic 

conductivity for selected cells was changed, a larger 

number of drainage ditches in Illinois and tributaries 

to Muscatine Slough in Iowa were simulated, and gravel 

pits were simulated. Results for the modified model 

were verified for similarity to results for the model 

constructed by Lucey and others (1995) after a model 

calibration process. The calibrated model was then 

further modified to simulate average 1998 municipal 

and industrial pumpage for delineation of areas contributing recharge to MPW municipal water-supply wells 

and to determine the 2-, 5-, and 10-year zones of transport within the areas contributing recharge.

Ground-water samples were collected from 

three MPW wells. Sample collection followed USGS 

protocols (Koterba and others, 1995). Prior to sample 

collection, wells were purged of at least three well 

casing volumes using a submersible pump. Pumping 

continued until field measurements of water temperature, pH, specific conductance, and dissolved oxygen 

stabilized, at which time water samples were collected 

for the analysis of tritium (Ostlund and Dorsey, 1977) 

and chlorofluorocarbons (Busenberg and Plummer, 

1992). Tritium- and CFC-based ground-water recharge 

dates were compared to model-simulated traveltimes 

to evaluate model performance.

SIMULATION OF GROUND-WATER FLOW

Model Description and Boundary 

Conditions

The model consists of three layers to represent 

the alluvium and part of the bedrock. The relation 

between the geologic units and the equivalent layers 

in the ground-water flow model is described in Lucey 

and others (1995). The alluvium is divided into layers 1 

and 2, which represent the upper and lower alluvium, 

respectively. Layer 3 represents the bedrock. The alluvium is modeled in this manner because the groundwater withdrawals at the pumping centers are from the 

lower alluvium and because differences in hydraulic 

conductivities between the upper and lower alluvium 

are interpreted from geologic data (Lucey and others, 
SIMULATION OF GROUND-WATER FLOW 5

1995). Flow in layer 1 is modeled as unconfined (under 

water-table conditions), whereas flow in layers 2 and 3 

is modeled as confined. Shale in the northeast part of 

the study area (Lucey and others, 1995, fig. 4) forms 

a confining unit, where present in layer 3.

A 305-row by 250-column grid was used 

to divide the area of study into a grid of 200-ft by 

200-ft cells. The grid was used to discretize each of 

the model layers. The cell area was identical in each 

layer, but its vertical dimension varied with layer 

thickness. The active cells of the model coincide 

with the river valley (where the alluvium is present) 

and the inactive cells coincide with the upland areas. 

Previous modeling (Lucey and others, 1995) has 

shown that the upland areas contribute insignificant 

amounts of ground water to the alluvial ground-water 

flow system. Therefore, upland areas are modeled as 

inactive or no-flow cells. The model code calculates a 

hydraulic head (ground-water-level altitude) at the 

center, or node, of each active cell and a ground-water 

flux across each cell face based on water-level gradients between cells. Cells are identified by a row, 

column, layer designation.

The Mississippi River, Copperas Creek, and 

Keating Creek are simulated by river cells that allow 

leakage through the river bottom to or from layer 1 

based on the difference in hydraulic heads (between 

river and layer 1) and riverbed conductance (fig. 2). 

Conductance is the product of the vertical hydraulic 

conductivity of the bed material, and the length and 

width of the reach in the cell, divided by bed-material 

thickness (McDonald and Harbaugh, 1988). An estimated bed-material thickness of 1 ft was used for river, 

drain, and stream cells. The vertical hydraulic conductivity of bed materials was initially estimated based on 

expected lithologies and modified during model calibration. A river cell will provide or receive as much 

water as the model requires to reach a mathematical 

solution. These perennial streams are expected to 

continue to flow even if the water table in the alluvium 

is lower than the river bottom. A riverbed thickness 

of 1 ft was assumed. Riverbed vertical hydraulic 

conductivities of 1 ft/d and 0.5 ft/d were used for 

cells representing the Mississippi River and cells 

representing the creeks, respectively. A larger vertical 

hydraulic conductivity was used for the Mississippi 

River because of periodic dredging operations to 

maintain a navigation channel.

Drainage ditches in Illinois are simulated by 

drain cells that allow leakage only from layer 1 to the 

drain. It is assumed that the stage in the drain reflects 

the altitude of the water table and that flow in these 

drainage ditches is predominantly from ground water. 

A drainbed thickness of 1 ft and a vertical hydraulic 

conductivity of 5 ft/d were used for the drainbed 

conductance calculation.

The Muscatine Slough is represented by 

stream cells and is simulated using a computer code 

that calculates leakage between layer 1 and the slough 

while accounting for streamflow entering or leaving 

each defined stream cell (Prudic, 1989). Stream cells 

function like river cells by allowing leakage from 

or to layer 1 based on the difference in water levels. 

However, stream cells can cease to flow rather than 

provide an infinite supply of water like a river cell. 

Large ground-water withdrawals could cause parts of 

Muscatine Slough or its associated tributaries to cease 

flowing during periods of low recharge to the alluvium. Stream cells give the model the flexibility to 

simulate this situation. A streambed thickness of 1 ft 

and a streambed vertical hydraulic conductivity of 

5 ft/d were used in the streambed conductance calculation. Detailed descriptions of channel location, length, 

width, stage, and discharge estimates or measurements 

for the Mississippi River, Copperas Creek, Keating 

Creek, drainage ditches in Illinois, and Muscatine 

Slough are given in Lucey and others (1995). Stage 

was assigned to previously undefined drain and 

slough cells based on stage measurements obtained 

by Lucey and others (1995) and estimates from 

USGS 7.5-minute topographic maps.

The upper boundary of the model area is a free 

surface that represents the water table. A specifiedflux boundary is used to represent areal recharge to 

layer 1. No-flow boundaries are used to simulate the 

limits of the model area where ground-water flow is 

assumed to be insignificant or in areas where aquifer 

material is absent. The bottom of the modeled system 

is at the top of the relatively impermeable Maquoketa 

Formation and is represented by a no-flow boundary 

at the bottom of layer 3. The lateral hydrologic boundaries formed by the relatively impermeable glacial 

till adjacent to the alluvium (layers 1 and 2) establish 

logical hydrologic limits for modeling ground-water 

flow in the alluvium. These boundaries are modeled 

as no-flow boundaries. A no-flow boundary was 

used across the narrow part of the Mississippi River 

Valley in the northeast part of the model area because 

subsurface flow from the north is considered insignificant to the overall water budget (Lucey and others, 

1995).
6 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

41°15'

41°20'

41°25'

91°10' 91°05' 91°

River (layer 1) Stream (layer 1)

Pumping (layer 2) Drain (layer 1)

General-head (layers 1 and 2) Outer model boundary (no flow) (layers 1 and 2)

EXPLANATION

0 1 2 3 MILES

0 1 2 3 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

Note: Features on map are shown larger than individual model-cell size for visibility and clarity

MISSISSIPPI RIVER

Grain

Processing

Corporation

well field

MPW

Main well field

Monsanto

Corporation

well field

Iowa–Illinois

Gas and Electric

Company well field

MPW

Grandview well field

Muscatine Power and 

Water (MPW)

Progress Park well field

Figure 2. Model boundary conditions for layers 1 and 2.
SIMULATION OF GROUND-WATER FLOW 7

The model area is not a closed hydrologic 

system, and general-head boundaries are used to 

simulate lateral model boundaries where ground 

water enters or leaves the system. Flow across the 

boundary is proportional to the differences between 

hydraulic head in the cells at the model boundary 

and hydraulic head assigned at a distance outside 

the model. General-head boundaries are used at the 

southern limits of layers 1 and 2 to simulate subsurface flow out of the study area through the alluvium 

down the Mississippi River Valley. General-head 

boundaries also are used at the perimeter of layer 3 

to represent ground water entering or leaving the 

system through the bedrock in proportion to relative 

hydraulic head differences between the cells at the 

model boundary and the regional potentiometric 

surface outside the model. In both cases, a constanthead source is placed 5 miles from the closest active 

cell in the model, and the hydraulic conductivity of the 

laterally adjacent unit is used in the computation of 

ground-water flux across the boundary. The groundwater level at the boundary was derived by projecting 

regional ground-water gradients in the alluvium for 

layers 1 and 2 and in the bedrock for layer 3.

Model Parameters

Individual cells of the model were assigned 

values for each of the parameters used by the model to 

solve the ground-water flow equations. Parameters are 

specified at the node of each active cell and represent 

an average for the entire cell. Spatial variation in 

parameter value is represented by assigning appropriate values to individual cells in the model array. 

Spatially distributed parameters used in the model 

include the geometry of the model layers, horizontal 

and vertical hydraulic conductivity, recharge from 

infiltration of precipitation and of runoff from upland 

areas, and ground-water pumpage. The model code 

uses transmissivity to solve equations and simulate 

ground-water flow. Horizontal hydraulic conductivity 

is multiplied by the layer thickness to calculate transmissivity at each node throughout the model grid. 

Alternatively, a transmissivity can be assigned to 

model cells.

A uniform transmissivity of 0.3 ft2/d was 

assigned to nodes in layer 3 (bedrock) and calculated 

by assuming a thickness of 300 ft and a hydraulic 

conductivity of 0.001 ft/d. A uniform transmissivity 

was selected on the basis of slug-test results (Lucey 

and others, 1995, table 4) and limited hydrogeologic 

information about the bedrock. Shale in the northeast 

part of the study area is modeled as a confining unit 

beneath the alluvium, so horizontal flow in that unit 

is not represented in the model.

The initial hydraulic conductivity distributions 

used in the model arrays for layers 1 and 2 were 

derived from the calibrated model of Lucey and others 

(1995) and were based on lithology and expected 

ranges of hydraulic conductivity values determined 

from the literature and aquifer-test analyses (Lucey 

and others, 1995, table 4, figs. 13 and 14). Initial 

values were modified during model calibration, and 

final hydraulic conductivities used in the model arrays 

for the upper and lower alluvium ranged from 150 to 

1,000 ft/d. The final hydraulic conductivity distributions closely resemble those of Lucey and others 

(1995), the major difference being several cells within 

the “gravel and coarse sand” area of the upper alluvium in which initial hydraulic conductivity values of 

600 ft/d were increased to 1,000 ft/d. Gravel pits were 

simulated in the modified model. The water table is 

exposed at several active and abandoned gravel pits 

in the northeast part of the study area (fig. 1). Large 

hydraulic conductivities (10,000 ft/d) were assigned 

to cells in layers 1 and 2 that represent gravel pits. 

The smaller cell size in the modified model facilitates 

simulation of gravel pits and provides a more realistic 

simulation of ground-water flow paths near these 

features.

Vertical leakance is required by the model to 

simulate ground-water flow between layers. Vertical 

leakance between two adjacent model layers is calculated from the thickness of each layer between its 

node and the common layer contact and the vertical 

hydraulic conductivity of each layer (McDonald and 

Harbaugh, 1988, eq. 51). Vertical leakance between 

two model layers with an intervening confining unit 

is calculated with the above properties and the vertical 

hydraulic conductivity and thickness of the confining 

unit (McDonald and Harbaugh, 1988, eq. 52). Vertical 

hydraulic conductivities used to calculate vertical 

leakance ranged from 1.5 to 10 ft/d in the alluvium, 

1 ft/d in the limestone and dolomite, and 0.005 ft/d 

in the shale. Calculation of vertical leakance in the 

alluvium assumed that the ratio of vertical hydraulic 

conductivity to horizontal hydraulic conductivity 

was 1 to 100. The small hydraulic conductivity for 

the alluvium in the vertical direction compared to the 

horizontal direction represents impediment to flow 

from intervening clay or silt layers and small-scale 
8 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

depositional features. The larger vertical than horizontal hydraulic conductivity in the limestone and 

dolomite is used to simulate the effective hydraulic 

connection with the overlying alluvium. Vertical flow 

through the shale is accounted for with a small vertical 

leakance between layers 2 and 3 where the confining 

unit is present in the northeast part of the study area.

Inflow to the ground-water system includes 

infiltration of precipitation, infiltration of runoff from 

upland areas to the river valley, leakage from rivers 

and streams, and flow across general-head boundaries. 

The model calculates leakage from rivers and streams 

and subsurface flow across outer boundaries. An array 

to represent inflow from infiltration of precipitation 

and infiltration of runoff from upland areas was 

prepared as model input. A daily net inflow rate from 

precipitation of 0.003 ft/d was used in the model to 

simulate the assumed steady-state February 1993 

conditions. Runoff from upland areas was accounted 

for by increasing the precipitation inflow rate at the 

model cells along the boundary between the river 

valley and the upland area. Methods used to compute 

precipitation and runoff inflow rates are described in 

Lucey and others (1995). Infiltration of precipitation 

and the infiltration of runoff from upland areas will 

be referred to as recharge from precipitation in this 

report.

Outflow from the ground-water system includes 

pumpage, leakage to rivers, streams, and drains, and 

subsurface flow across outer boundaries. The model 

calculates all of these fluxes except pumpage, for 

which an array was constructed for input to the model. 

Average daily withdrawals from the ground-water 

system during February 1993 (6,150,000 ft3/d, or 

about 46 Mgal/d) were assigned to cells containing 

pumping wells (fig. 2). Average daily pumpage during 

February 1993 was obtained from records provided by 

well-field operators.

Model Calibration

The smaller cell size in the modified model 

resulted in some changes in the representation of 

hydrologic features when compared to the model 

constructed by Lucey and others (1995). Gravel pits 

were simulated in the northeast part of the study area, 

a larger number of drainage ditches in Illinois and tributaries to Muscatine Slough in Iowa were simulated, 

and the extent of the Mississippi River and associated 

island areas were represented in greater detail. These 

changes required a calibration of the modified model 

to the assumed February 1993 steady-state conditions 

described in Lucey and others (1995).

The steady-state model was calibrated through 

an iterative process by varying hydraulic conductivity, 

vertical leakance, recharge from precipitation, and 

drainbed, streambed, and riverbed conductance 

during numerous simulations. As these parameters 

were adjusted within reasonable limits, model 

output was analyzed to ascertain whether the 

adjustment improved the match between measured 

(February 26, 1993) and simulated ground-water 

levels and measured (February 1993) and simulated 

discharge from Muscatine Slough. The goal of the 

model calibration process is to minimize differences 

between the measured and simulated values while 

maintaining realistic values of hydraulic properties. 

The calibration process continued until further incremental adjustments to model input parameters 

produced no perceivable improvement in model 

results.

During the calibration process, improvements 

in the model output were evaluated by calculating 

the average head difference (AVEH) and the root 

mean squared error (RMSE) between measured 

and simulated ground-water levels. The AVEH 

is a measure of systematic error; it approaches zero 

when the sum of the differences between measured 

and simulated ground-water levels that are greater 

than zero equals the sum of the differences that are 

less than zero. The RMSE is a measure of the magnitude of error between measured and simulated groundwater levels over the entire model area (Anderson and 

Woessner, 1992).

A total of 60 comparisons of measured and 

simulated water levels were made—39 in layer 1, 15 

in layer 2, and 6 in layer 3. The calibration values of 

AVEH and RMSE were, respectively, 0.1 ft and 0.9 ft 

in layers 1 and 2, 1.4 ft and 3.6 ft in layer 3, and 0.2 ft 

and 1.4 ft in all three model layers. The RMSE for all 

three model layers represents 8.6 percent of the range 

of measured water levels (16.2 ft).

The calibration process also attempted to minimize the percentage of error between estimated and 

simulated mean daily discharge for February 1993 

from the Muscatine Slough. The mean daily discharge 

from Muscatine Slough simulated by the calibrated 

model is 4,042,000 ft3/d, which is 20 percent less than 

the amount estimated from pump operation time and 
SIMULATION OF GROUND-WATER FLOW 9

rated pump capacity (5,080,000 ft3/d). Model input 

data that affect flow to the slough, such as streambed 

conductance, vertical leakance, horizontal hydraulic 

conductivity, and recharge from precipitation, have 

been adjusted through a reasonable range of hydrologic values during calibration to maximize simulated 

flow in the slough while minimizing AVEH and 

RMSE.

A comparison of measured and simulated 

ground-water-level altitudes shows how accurately the 

calibrated model simulates the ground-water system. 

The water-table surface and general flow directions 

(interpreted from water-level contours) in the upper 

alluvium are similar for maps prepared from simulated 

(fig. 3) and measured (February 26, 1993, fig. 4) water 

levels. Flow is approximately perpendicular to watertable contours in the direction of decreasing hydraulic 

head. The influent nature (losing reach) of Copperas 

Creek as it enters the river valley from the adjacent 

upland area is apparent in each case. A ground-water 

divide located between the southern part of Muscatine 

Slough and the cone of depression caused by the 

pumping centers also is apparent in each case. Flow 

is toward the pumping centers north of the groundwater divide in Iowa and toward the drainage ditches 

in Illinois.

Differences between the water-table surface 

in the upper alluvium constructed from measured 

and simulated data are most apparent east of the 

Mississippi River in Illinois. There are few measured 

water levels in Illinois, resulting in a more detailed 

depiction of the simulated water-table surface than is 

possible from the measured data. Differences between 

the simulated water-table surface in the upper alluvium from Lucey and others (1995, fig. 15) and the 

modified model (fig. 3) also are apparent east of the 

Mississippi River in Illinois. The smaller cell size used 

in the modified model allows for a more accurate 

depiction of changes in drain elevation and nearby 

water levels.

The calibrated model calculates a groundwater flux between cells from which a simulated water 

budget was computed (table 1). Differences between 

this budget and the budget from Lucey and others 

(1995) are attributed to the greater extent and detail to 

which the drain, slough, and river networks are represented in the modified model as a result of decreased 

cell size. Increased slough and drain outflows and 

increased river inflows in the calibrated model support 

this explanation.

The steady-state model was considered calibrated when the following criteria were met:

1. Incremental changes in model input parameters did 

not produce an AVEH closer to zero or a smaller 

RMSE for all layers in the model;

2. The RMSE represented a small percentage of the 

range in measured ground-water levels;

3. Incremental changes in model input parameters did 

not decrease the percentage difference between 

estimated and simulated discharge from the 

Muscatine Slough; and

4. Simulated lateral ground-water flow directions 

closely resembled flow directions interpreted 

from the water-table map in the upper alluvium 

constructed using water levels measured on 

February 26, 1993. 

Sensitivity Analysis

The calibrated model is influenced by 

uncertainty resulting from limited knowledge of 

the spatial variation of parameter values and uncertainty associated with the definition of boundary 

conditions. A sensitivity analysis establishes the effect 

of parameter uncertainty on the calibrated model by 

documenting the response of the model (simulated 

water levels and discharge) to incremental changes 

in parameter values. The model is sensitive to a 

parameter when changes in the parameter value 

produce substantial changes in model response. If 

improvement in the model is desired, additional data 

collection could be directed toward refining the most 

sensitive parameters.

Simulated water-level response to incremental 

changes in selected input parameters is shown in 

figure 5. The RMSE is plotted against the multiplication factor used to vary the parameter. The calibrated 

model parameters are used as comparison and are 

represented by a multiplication factor of 1. The multiplication factor was applied uniformly to the entire 

model for the indicated parameter and ranged from 

0.1 to 2.0. The parameter being tested was adjusted 

while the remaining model parameters were held at the 

calibrated values. Water levels were most sensitive to 

horizontal hydraulic conductivity in layers 1 and 2 and 

to recharge from precipitation. Water levels were 

insensitive to transmissivity in layer 3, drainbed and 

streambed conductance, riverbed conductance, and 

vertical leakance.
10 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

Upland area

River, stream, drainage ditch, or gravel pit

EXPLANATION

Simulated water-table contour—Contour interval 

2 feet. Datum is sea level

0 1 2 3 MILES

0 1 2 3 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

41°20'

41°25'

91°10' 91°05'

532

532

530

532

532

532

534

534

524

534

526

530

530

538

536

526

526

528

530

532

532

532

530

536

536

Muscatine County

Louisa County

Rock Island County

Mercer County

MISSISSIPPI

RIVER

Muscatine

91°

Muscatine Slough

Lock and Dam

Figure 3. Simulated water-table surface in upper alluvium, February 1993 (modified model with 1993 

package).
SIMULATION OF GROUND-WATER FLOW 11

Upland area

River, stream, drainage ditch, or gravel pit

Ground-water-level measuring site

EXPLANATION

Water-table contour–Dashed where approximately

located. Contour interval 2 feet. Datum is sea level

0 1 2 3 MILES

0 1 2 3 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

41°20'

41°25'

91°10' 91°05'

532

536

530

530

530

534

536

520 524

528

530

532 536 538

526

538

532

534

528

526

534

530

534

532

91°

Lock and Dam

Rock Island County

Mercer County

Muscatine County

Louisa County

Muscatine Slough

MISSISSIPPI

RIVER

IOWA

ILLINOIS

Figure 4. Measured water-table surface in upper alluvium, February 26, 1993 (from Lucey and others, 1995).
12 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

The sensitivity of simulated river leakage was 

evaluated by varying model input parameters and 

determining the proportion of simulated inflow to the 

ground-water system obtained from river leakage. The 

proportion of simulated inflow obtained from river 

leakage was most sensitive to recharge from precipitation and horizontal hydraulic conductivity in layers 1 

and 2, whereas transmissivity of layer 3, drainbed and 

streambed conductance, changing vertical leakance, 

and riverbed conductance had less of an effect (fig. 6).

An evaluation of the sensitivity of simulated 

discharge from Muscatine Slough was made by 

varying horizontal hydraulic conductivity in layers 1 

and 2, drainbed and streambed conductance, recharge 

from precipitation, and vertical leakance. Of these 

parameters, simulated discharge from Muscatine 

Slough was most sensitive to recharge from precipitation and horizontal hydraulic conductivity in layers 1 

and 2.

The sensitivity of the general-head boundaries 

to changes in hydraulic conductivity affecting flow 

entering or leaving the model through the bedrock was 

evaluated. The hydraulic conductivity was increased, 

and the flow contributed by the general-head boundary 

was compared to the total amount of ground-water 

flow entering the model. The calibrated model uses 

a hydraulic conductivity of 0.001 ft/d for the bedrock 

and general-head boundary, and flow entering the 

model from the general-head boundary is less than 

0.2 percent of the total. Even by increasing the 

hydraulic conductivity to 100 ft/d, which is unreasonably large for the limestone and dolomite bedrock in 

this area (Lucey and others, 1995), the ground-water 

contribution from the bedrock remains less than 

3 percent of the total. 

Model Limitations

The ground-water flow model constructed by 

Lucey and others (1995) and modified for use in this 

study estimates the effects of ground-water withdrawals 

from the alluvium. However, several model limitations 

should be considered. Model input parameters, such as 

Table 1. Simulated water budgets

[Inflow, water added to the ground-water system; ft3/d, cubic feet per day; outflow, water removed from the ground-water system; slough leakage, 

from or to Muscatine Slough and its associated drain network in Iowa; pumpage—municipal, ground-water withdrawals by Muscatine Power and Water; 

pumpage—industrial, ground-water withdrawals by Grain Processing Corporation, Iowa-Illinois Gas and Electric Company, and Monsanto Corporation; 

drain leakage, to the drainage ditch network in Illinois]

Budget component Lucey and others

(1995)

Modified model

(calibrated)

Modified model

(1998 pumpage)

Inflow (ft3/d)

Recharge from precipitation 9,025,000 9,022,000 9,022,000

River leakage—Mississippi River 5,411,000 6,938,000 7,066,000

River leakage—Copperas and Keating Creeks 236,000 360,000 360,000

Slough leakage 425,000 680,000 682,000

Subsurface flow across outer boundaries a5.0 22,000 22,000

Pumpage—Municipal 0 0 0

Pumpage—Industrial 0 0 0

Drain leakage 0 0 0

Total inflow a15,097,000 17,022,000 17,152,000

Outflow (ft3/d)

Recharge from precipitation 0 0 0

River leakage—Mississippi River 121,000 303,000 288,000

River leakage—Copperas and Keating Creeks 291,000 69,000 69,000

Slough leakage 3,360,000 4,722,000 4,252,000

Subsurface flow across outer boundaries 155,000 121,000 121,000

Pumpage—Municipal 3,136,000 3,195,000 3,602,000

Pumpage—Industrial 2,955,000 2,955,000 3,193,000

Drain leakage 5,095,000 5,651,000 5,651,000

Total outflow 15,113,000 17,016,000 17,176,000

a

Total not precise due to rounding.
SIMULATION OF GROUND-WATER FLOW 13

1.0

3.8

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

ROOT MEAN SQUARED ERROR, IN FEET

HORIZONTAL HYDRAULIC CONDUCTIVITY

(LAYERS 1 AND 2)

RECHARGE FROM PRECIPITATION

VERTICAL LEAKANCE

RIVERBED CONDUCTANCE

DRAINBED AND STREAMBED CONDUCTANCE

TRANSMISSIVITY (LAYER 3)

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

MULTIPLICATION FACTOR

horizontal and vertical hydraulic conductivity and 

recharge from precipitation, are specified at the node of 

each active cell and represent an average for the entire 

cell. The assumptions of uniformity for the entire cell 

introduce inaccuracies because of the heterogeneous 

nature of geologic materials and the variability of 

climatic conditions. The steady-state model assumes 

that inflows to the ground-water system equal outflows. 

If this was not the case in February 1993, the resultant 

change in ground-water storage would be a source of 

model error. For example, water levels could have been 

either rising or falling during the assumed equilibrium 

conditions. The steady-state flow model does not 

account for dynamic (transient) conditions (natural or 

anthropogenic). The steady-state model does not indicate time needed to reach equilibrium conditions. 

Attaining equilibrium might take a substantial period of 

time and is complicated by varying climatic and hydrologic conditions, noncontinuous pumping and pumping 

that is cycled among well fields, and changing and 

seasonally varying irrigation pumpage (not included 

in this model). 

Results of Simulation

The calibrated model was modified to simulate 

1998 municipal and industrial supply well withdrawals 

from the ground-water system. Average daily withdrawals during 1998 (6,795,000 ft3

/d, or about 

51 Mgal/d) represent an increase of 11 percent since 

1993 and reflect recent ground-water usage. It was 

assumed that steady-state conditions (February 1993) 

described in Lucey and others (1995) are still an 

appropriate representation of the system. Therefore, 

1998 ground-water levels, and river, slough, and drain 

stages were not collected.

The model calculates a ground-water-level 

altitude at the node of each 200-ft by 200-ft cell 

from which a simulated water-table surface in the 

upper alluvium (fig. 7) was constructed. The model 

also calculates a ground-water flux between cells 

from which a simulated water budget was computed 

(table 1, 1998 pumpage). Ground-water flow 

directions derived from analysis of the computed 

Figure 5. Root mean squared error between measured and simulated water levels as a result of varying 

model input parameters.
14 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

20

90

30

40

50

60

70

80

PROPORTION OF INFLOW OBTAINED FROM RIVER 

LEAKAGE, IN PERCENT

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

MULTIPLICATION FACTOR

HORIZONTAL HYDRAULIC CONDUCTIVITY

(LAYERS 1 AND 2)

RECHARGE FROM PRECIPITATION

VERTICAL LEAKANCE

RIVERBED CONDUCTANCE

DRAINBED AND STREAMBED CONDUCTANCE

TRANSMISSIVITY (LAYER 3)

ground-water-level altitudes, and inflows and outflows 

quantified in the water budget, assist in developing 

an improved understanding of the ground-water 

system. The sources of water recharging the 

alluvium can be identified from an analysis of 

the water budget (table 1). Under assumed steadystate conditions and 1998 pumpage, the model 

calculated 17,152,000 ft3

/d of inflow to the groundwater system and 17,176,000 ft3

/d of outflow from 

the system. There is a 0.14 percent discrepancy 

between the calculated inflow and outflow due 

to model error in approximating a solution to the 

mathematical equations.

Primary sources of inflow are infiltration of 

precipitation and upland runoff (53 percent) and 

Mississippi River leakage (41 percent). Minor 

amounts of inflow occur through leakage from 

Muscatine Slough and its associated drainage network 

(3.9 percent) and through leakage from Copperas and 

Keating Creeks (2.0 percent) in Illinois. All of these 

sources of inflow enter the system through the upper 

alluvium. 

The primary components of outflow from the 

ground-water system are pumpage (39.6 percent), 

flow to drainage ditches in Illinois (32.9 percent), 

and slough leakage (24.7 percent). Pumpage is from 

the lower alluvium, whereas drain and slough leakage 

leave the system from the upper alluvium. Flow across 

the general-head boundaries of the model, which 

primarily represents subsurface flow through the alluvium down the river valley, accounts for less than 

1.0 percent of the total.

Differences in simulated water budgets (table 1) 

for the calibrated model (1993 pumpage) and the 

model with 1998 pumpage illustrate model response to 

increased pumpage. Average daily supply-well withdrawals during 1998 (6,795,000 ft3

/d) represent an 

increase of 11 percent (645,000 ft3

/d) since 1993. 

Increased pumpage (outflow) from the groundwater system results in an increased inflow from 

the Mississippi River (128,000 ft3/d) and Muscatine 

Slough (2,000 ft3

/d) and a reduction in outflow from 

the Mississippi River (15,000 ft3

/d) and Muscatine 

Slough (470,000 ft3

/d).

Figure 6. Proportion of simulated inflow obtained from river leakage as a result of varying model input 

parameters.
SIMULATION OF GROUND-WATER FLOW 15

Upland area

River, stream, drainage ditch, or gravel pit

EXPLANATION

Water-table contour–Contour interval 2 feet. 

Datum is sea level

0 1 2 3 MILES

0 1 2 3 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

41°20'

41°25'

91°10' 91°05'

532

532

530

532

530

526

522

524

524

528

526

530

534

536

536

532

528

526

530

530

534

538

536

538

538

540

542

532

530

91°

Lock and Dam

Rock Island County

Mercer County

Muscatine County

Louisa County

Muscatine Slough

MISSISSIPPI

RIVER

IOWA

ILLINOIS

Figure 7. Simulated water-table surface in upper alluvium, February 1993 (modified model with 1998 

pumpage).
16 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

DELINEATION OF AREAS CONTRIBUTING 

RECHARGE

Results from the ground-water flow model 

(MODFLOW) were used with the particle-tracking 

software MODPATH (Pollock, 1994) to delineate 

approximate areas contributing recharge to MPW well 

fields and to determine the 2-, 5-, and 10-year zones 

of transport under steady-state conditions and 1998 

pumpage within contributing areas. MODPATH calculates flow paths and traveltimes for hypothetical water 

particles in the saturated zone using a velocity distribution derived from simulated ground-water-level 

altitudes and flow rates (MODFLOW results) and 

estimates of aquifer effective porosity. Limitations 

of particle-tracking analysis are discussed by Pollock 

(1994) and are largely dependent on the accuracy of 

the ground-water levels and flow rates computed by 

the ground-water flow model. Particle flow paths and 

traveltimes calculated by MODPATH are based on 

advective ground-water flow and do not reflect the 

effects of dispersion, diffusion, or chemical and microbial retardation, which would be required to fully 

describe contaminant transport.

Estimates of effective porosity were assigned to 

model cells based on lithology and published values of 

porosity (Freeze and Cherry, 1979; Driscoll, 1986). 

Estimates of effective porosity ranged from 0.15 to 

0.24 in layer 1 and 0.15 to 0.20 in layer 2. A constant 

effective porosity value of 0.03 was assigned to cells in 

layer 3. Hypothetical water particles were distributed 

on the faces of each cell containing an active MPW 

municipal water-supply well, and a backward-tracking 

analysis was conducted in which particles were 

tracked backward in time along flow paths to their 

points of recharge (for example, water table, river, or 

other model boundary). The final location of particles 

defines the approximate extent of the area contributing 

recharge to the cell containing the pumping well.

Several sources of water are present within estimated areas contributing recharge to MPW municipal 

well fields including ground water from the alluvial 

aquifer, Mississippi River and Muscatine Slough 

water, and recharge originating as runoff from two 

unnamed creeks in the northern part of the study area 

(fig. 8). Recharge originating from the Mississippi 

River accounts for about 46 percent of the total 

water discharged from the municipal well fields. 

The average simulated traveltime of particles tracked 

from recharge to discharge at the MPW municipal well 

fields was 13.6 years. Average simulated traveltimes 

of particles were compared to ground-water recharge 

dates determined from chlorofluorocarbon (CFC) 

and tritium analyses of water samples from three 

MPW wells (table 2). The presence of modern water 

(recharged since the 1950’s) as indicated by CFC and 

tritium analyses supports the simulated traveltimes.

Particle-tracking results illustrate the influence 

of nearby industrial supply wells on the shape and size 

of the area contributing recharge to MPW wells. Two 

large embayments into the area contributing recharge 

to MPW wells are present along the Mississippi River 

(fig. 8). These areas represent ground water that is 

unavailable to MPW wells due to withdrawals by 

Grain Processing Corporation (in the north) and 

Monsanto Corporation (in the south) supply wells. 

Differences in the source of recharge water to MPW 

wells due to the influence of industrial supply wells 

on the shape and size of the area contributing recharge 

may affect water quality at individual MPW well 

fields. Recharge originating from the Mississippi 

River accounts for about 98 percent of the total water 

discharged from the MPW Main well field. However, 

recharge originating from the Mississippi River 

accounts for less of the total discharge from the 

Progress Park and Grandview municipal well fields 

(12 and 34 percent, respectively).

Zones of Transport and Potential Sources 

of Contamination

The simulated 2-, 5-, and 10-year zones of transport for particles discharging at MPW municipal well 

fields (fig. 9) were computed by MODPATH using 

ground-water velocities and particle path lengths. The 

locations of potential sources of contamination were 

determined within the 2-, 5-, and 10-year zones of 

transport for the Grandview, Main, and Progress Park 

well fields (figs. 10, 11, and 12, respectively) by MPW 

personnel. Potential sources of contamination also 

were located within the drainage basins of two creeks 

(fig. 13) that discharge water to the alluvial aquifer 

within the 5- and 10-year zones of transport for the 

Grandview well field. Information about National 

Pollution Discharge Elimination System (NPDES) 

permitted sites within the zones of transport and 

drainage basins was obtained from the Iowa 

Department of Natural Resources. USGS personnel 

determined the horizontal location (latitude and longitude) of potential sources of contamination by using 

a hand-held global positioning system (GPS) unit.
DELINEATION OF AREAS CONTRIBUTING RECHARGE 17

41°20'

41°25'

91°05'

91°10'

Rock Island County

Mercer County

Muscatine County

Louisa County

Area of figure 1

Enlarged area

Estimated area contributing recharge

Industrial supply well

MPW municipal supply well

Outer model boundary

EXPLANATION

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

IOWA ILLINOIS

Muscatine

MISSISSIPPI

RIVER

Figure 8. Estimated areas contributing recharge to municipal well fields under 1998 pumping conditions.
18 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

Potential point sources of contamination were grouped by type into residential and 

industrial/commercial sites. The contamination potential associated with residential sites can originate from 

improperly functioning septic tanks and the rapid 

downward movement of surface water (and possible 

contaminants) into the underlying aquifer at improperly constructed domestic wells. Activities associated 

with industrial/commercial sites often include the 

handling and storage of a variety of chemicals and 

fuels. Waste streams from industrial/commercial 

processes (such as manufacturing and dry cleaning) 

also may create a potential for contamination if 

improperly handled. 

The potential for contamination of the alluvial 

aquifer within the 2-, 5-, and 10-year zones of transport also occurs along roads and railroads and from the 

Mississippi River and mining operations (gravel pits). 

Truck, train, and barge traffic often include the transport of chemicals and hazardous waste, and the unintended release of these materials could affect water 

quality. The presence of agricultural chemicals (pesticides and nitrogen fertilizers), derived from upland 

runoff into the Mississippi River, also could affect 

water quality in areas where pumpage from MPW 

well fields originates as recharge induced from the 

river. Open gravel pits may facilitate rapid infiltration 

of contaminants.

The delineation of zones of transport for MPW 

wells and the identification of potential contaminant 

sources within those zones provide important information that can be used in the development of a WHPP 

for MPW municipal wells. Additional efforts on 

the part of MPW to protect municipal water quality 

include the development and implementation of a 

ground-water and surface-water monitoring program 

(Jerry Doering, Muscatine Power and Water, written 

commun., 1998). Ground water from a network of 

observation wells located within areas contributing 

recharge to MPW wells and surface water from nearby 

streams will be periodically sampled and analyzed for 

a variety of anthropogenic constituents.

Effects of Changing Climatic Conditions 

on Areas Contributing Recharge

The delineation of areas contributing recharge 

to MPW municipal well fields is influenced by uncertainty resulting from limited knowledge of the 

parameter values, boundary conditions, and hydrologic stresses used in MODFLOW. The effects of lessthan- and greater-than-normal recharge from precipitation on the size and shape of the 10-year zone of transport to MPW municipal well fields were simulated by 

decreasing and increasing precipitation to the groundwater model by 20 percent. The resultant 10-year 

zones of transport (less-than- and greater-than-normal 

recharge from precipitation) were compared to the 

10-year zone of transport for normal recharge from 

precipitation used in the calibrated model (fig. 14). 

Areas where changes in the location of the 10-year 

zone of transport were determined are denoted in blue 

in figure 14. Decreasing the amount of recharge from 

precipitation reduces the amount of water available 

(per unit area) for municipal withdrawal and requires 

a 2-percent increase in the size of the 10-year zone 

of transport to maintain the same rate of withdrawal. 

Increasing the amount of recharge from precipitation 

has the opposite effect, resulting in a 3-percent reduction in the size of the zone of transport. This relation 

between recharge from precipitation and the size of the 

zone of transport is illustrated in areas A, B, and C of 

figure 14. 

Table 2. Comparison of simulated traveltime and ground-water recharge date determined from chlorofluorocarbon and tritium 

analyses for samples collected May 4–7, 1999

[ft, feet below land surface; simulated traveltime, average traveltime for all particles from well; CFC, chlorofluorocarbons; M, Main; PP, Progress Park; 

GV, Grandview]

Site ID / site name / well field Well depth

(ft)

Geologic

unit

Simulated

traveltime1

(years)

Ground-water

recharge date

CFC2 / tritium3

412329091041301 / MPW19 / M 83 Alluvium 9.6 Mid-1980’s / Post-1950’s

412215091065601 / MPW23 / PP 125 Alluvium 20.9 About 1990 / Post-1950’s

412331091051201 / MPW29 / GV 130 Alluvium 17.4 Early 1980’s / Post-1950’s

1

Based on results of MODPATH simulation.

2

E. Busenberg, U.S. Geological Survey, written commun., 2000.

3Plummer and others (1993).
DELINEATION OF AREAS CONTRIBUTING RECHARGE 19

Area of figure 1

Enlarged area

41°22'30"

91°05'

41°25'

91°07'30"

2-year zone of transport

5-year zone of transport

10-year zone of transport

Industrial supply well

MPW municipal supply well

EXPLANATION

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

Muscatine

MISSISSIPPI

RIVER

Outer model boundary

Figure 9. Estimated 2-, 5-, and 10-year zones of transport for municipal well fields.
20 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

41°22'30"

91°05'

41°25'

91°07'30"

2-year zone of transport

Potential sources of contamination

Mine (gravel pits)

Industrial, commercial

Residential (well, septic tank)

5-year zone of transport

10-year zone of transport

Industrial supply well

MPW municipal supply well

EXPLANATION

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

Area of figure 1

Enlarged area

MISSISSIPPI

RIVER

Outer model boundary

Figure 10. Estimated 2-, 5-, and 10-year zones of transport for wells in the Grandview well field and potential 

sources of contamination.
DELINEATION OF AREAS CONTRIBUTING RECHARGE 21

41°22'30"

91°05'

2-year zone of transport Potential sources of contamination

Mine (gravel pits)

Industrial, commercial

Residential (well, septic tank)

5-year zone of transport

10-year zone of transport

Industrial supply well

MPW municipal supply well

EXPLANATION

0 1/2 1 MILE

0 0.5 1 KILOMETER

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

Area of figure 1

Enlarged area

MISSISSIPPI

RIVER

Figure 11. Estimated 2-, 5-, and 10-year zones of transport for wells in the Main well field and potential 

sources of contamination.
22 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

41°22'30"

91°07'30" 91°05'

41°20'

2-year zone of transport

Potential sources of contamination

Mine (gravel pits)

Industrial, commercial

Residential (well, septic tank)

5-year zone of transport

10-year zone of transport

Industrial supply well

MPW municipal supply well

EXPLANATION

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

MISSISSIPPI

RIVER

Area of figure 1

Enlarged area

Muscatine County

Louisa County

Outer model boundary

Figure 12. Estimated 2-, 5-, and 10-year zones of transport for wells in the Progress Park well field and potential sources of 

contamination.
DELINEATION OF AREAS CONTRIBUTING RECHARGE 23

91°07'30" 91°05'

41°27'

41°25'

Basin boundary

Potential sources of contamination

Industrial, commercial

Residential (well, septic tank)

EXPLANATION

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

Area of figure 1

Enlarged area

Outer model boundary

Figure 13. Drainage basins of creeks within areas contributing recharge to the Grandview well field and potential 

sources of contamination.
24 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

41°22'30"

91°07'30" 91°05' 91°02'30"

Less-than-normal precipitation

Greater-than-normal precipitation

Different response

Industrial supply well

MPW municipal supply well

Area of change in areas contributing recharge

EXPLANATION

0 1 2 MILES

0 1 2 KILOMETERS

Base from U.S. Geological Survey, Blanchard Island 1:24,000, 1991;

Letts 1:24,000, 1965; Muscatine 1:24,000, 1991; and

Muscatine NW 1:24,000, 1965

MISSISSIPPI

RIVER

Area of figure 1

Enlarged area

Muscatine

B

C D

E F

A

Outer model boundary A

Extent of 10-year zone of transport

Figure 14. Estimated areas contributing recharge to municipal well fields under less-than- and greater-than-normal 

recharge from precipitation.
SUMMARY 25

A different response to changing climatic conditions was indicated in areas D, E, and F of figure 14. In 

these areas, the 10-year zone of transport for normal 

recharge from precipitation either extends beyond the 

zone of transport for the less-than-normal condition or 

is exceeded by the zone of transport for the greaterthan-normal condition. Areas of such response most 

likely result from model uncertainty and the complex 

nature of model response to changing parameter 

values or proximity to model boundaries. Changes in 

the location of the 10-year zone of transport were not 

apparent to the east (fig. 14) in areas where ground 

water is either unavailable to MPW wells due to 

nearby industrial withdrawals or the Mississippi River 

provides abundant recharge.

The Mississippi River is a major surface-water 

feature in the study area, and its stage has an important 

effect on ground-water levels in the Mississippi River 

alluvium (Lucey and others, 1995, fig. 10). An investigation of the potential effects of changing Mississippi 

River stage on areas contributing recharge to MPW 

well fields is beyond the scope of this study. However, 

it should be noted that river-induced changes in 

ground-water levels may alter model-calculated 

particle flow paths and traveltimes and affect the 

delineation of areas contributing recharge to MPW 

well fields.

SUMMARY

Mississippi River alluvium in the Muscatine, 

Iowa, area provides large quantities of good quality 

ground water for municipal, industrial, and agricultural supplies. Three municipal well fields for the 

City of Muscatine produce about 27 Mgal/d from the 

alluvium. The general plan for development of the 

ground-water resource calls for doubling municipal 

withdrawals during the next 20 years (J. Doering, 

Muscatine Power and Water, oral commun., 

December 1996). The highly permeable nature of 

the alluvium and varied land use in the area result in a 

ground-water supply potentially vulnerable to contamination. There is a need to delineate areas contributing 

recharge to municipal water-supply wells in the study 

area, described by Lucey and others (1995), to aid in 

the development of a wellhead protection plan 

(WHPP) for the Muscatine area.

A steady-state ground-water flow model 

constructed by Lucey and others (1995) was modified, 

and results from the model were used with the 

particle-tracking software MODPATH (Pollock, 1994) 

to delineate approximate areas contributing recharge 

to Muscatine Power and Water (MPW) municipal 

supply wells and to determine zones of transport 

within the areas contributing recharge. The model 

consists of three layers to represent the upper and 

lower alluvium and the bedrock. A 305-row by 

250-column grid was used to divide the area of study 

into an array of 200-ft by 200-ft cells. A uniform horizontal hydraulic conductivity of 0.001 ft/d was used in 

the model layer representing the bedrock. Horizontal 

hydraulic conductivities for the upper and lower alluvium ranged from 150 to 1,000 ft/d.

Under steady-state conditions and 1998 

pumpage, primary sources of model inflow are 

recharge through infiltration of precipitation and 

upland runoff (53 percent) and Mississippi River 

leakage (41 percent). The primary components of 

outflow from the ground-water flow system are 

pumpage (39.6 percent), flow to drainage ditches 

in Illinois (32.9 percent), and slough leakage 

(24.7 percent). 

Several sources of water are present within estimated areas contributing recharge to MPW municipal 

well fields including ground water from the alluvial 

aquifer, Mississippi River water, and recharge originating as runoff from two unnamed creeks in the 

northern part of the study area. Recharge originating 

from the Mississippi River accounts for about 

46 percent of the total water discharged from the 

municipal well fields. The average simulated traveltime of particles from recharge to discharge at the 

MPW municipal well fields is 13.6 years. Particletracking results illustrate the influence of nearby 

industrial supply wells on the shape and size of the 

area contributing recharge to MPW wells. Two large 

embayments into the area contributing recharge to 

MPW wells are present along the Mississippi River. 

These areas represent ground water that is unavailable 

to MPW wells due to withdrawals by industrial supply 

wells. Recharge originating from the Mississippi River 

accounts for about 98 percent of the total water 

discharged from the Muscatine Power and Water Main 

well field. However, recharge originating from the 

Mississippi River accounts for less of the total 

discharge from the Progress Park and Grandview 

municipal well fields (12 and 34 percent, respectively).

The effects of changing climatic conditions 

on the size and shape of the 10-year zone of transport 

to MPW municipal well fields were simulated by 

decreasing and increasing recharge from precipitation 
26 Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge to Municipal Water-Supply Wells, 

Muscatine, Iowa

to the ground-water model by 20 percent to demonstrate the uncertainty inherent in delineating these 

areas. Potential sources of contamination within the 

2-, 5-, and 10-year zones of transport to MPW municipal well fields were located and characterized. The 

delineation of zones of transport for MPW wells and 

the location of potential contaminant sources within 

those zones provide information used in the development of a WHPP for MPW municipal wells. Additional efforts on the part of MPW to protect municipal 

water quality include the development and implementaion of a ground-water and surface-water monitoring 

program.

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Savoca and others—Simulation of Ground-Water Flow and Delineation of Areas Contributing Recharge To Municipal USGS/WRIR 02–4004 

 Water-Supply Wells, Muscatine, Iowa 

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