Ground Water near Pella and in Selected Parts

of Jasper, Mahaska, and Marion Counties, Iowa

Water-Resources Investigations Report 02–4118

Prepared in cooperation with the

CITY OF PELLA, IOWA

U.S. Department of the Interior

U.S. Geological Survey
Front cover: USGS hydrologic technician Kenneth Hedmark (facing camera) and hydrologist Michael Turco (left) are usin

drill rig to install a test well in the South Skunk alluvial aquifer near Pella, Iowa (photograph by Jim Caldwell, USGS).
U.S. Department of the Interior

U.S. Geological Survey

Ground Water near Pella and in

Selected Parts of Jasper,

Mahaska, and Marion Counties,

Iowa

By JAMES P. CALDWELL and ERIC M. SADORF

Water-Resources Investigations Report 02–4118

Prepared in cooperation with the

CITY OF PELLA, IOWA

Iowa City, IA

2002
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

For additional information write to: Copies of this report can be purchased fro

District Chief U.S. Geological Survey

U.S. Geological Survey Information Services

P.O. Box 1230 Building 810

400 S. Clinton St., Rm. 269 Box 25286, Federal Center

Iowa City, IA 52244–1230 Denver, CO 80225–0286

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsem

by the U.S. Government.
Content

CONTENTS

Abstract.................................................................................................................................................................................. 

Introduction ........................................................................................................................................................................... 

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

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

Previous Investigations ................................................................................................................................................ 

Acknowledgments ....................................................................................................................................................... 

Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties.................................................. 

Surficial Aquifers......................................................................................................................................................... 

Alluvial Aquifers ............................................................................................................................................... 

Glacial-Drift Aquifers........................................................................................................................................ 

Buried-Channel Aquifers................................................................................................................................... 

Bedrock Aquifers......................................................................................................................................................... 

Pennsylvanian Aquifer....................................................................................................................................... 

Mississippian Aquifer........................................................................................................................................ 

Silurian-Devonian Aquifer ................................................................................................................................ 

Cambrian-Ordovician Aquifer System.............................................................................................................. 

Description of South Skunk River Alluvial Deposits near Pella........................................................................................... 

Collection of Test-Hole and Observation-Well Data................................................................................................... 

Thickness and Lithology ............................................................................................................................................. 

Aquifer Properties........................................................................................................................................................ 

Water Quality............................................................................................................................................................... 

Summary................................................................................................................................................................................ 

References ............................................................................................................................................................................. 

FIGURES

1–7. Maps showing:

1. Location of ground-water availability study and Pella North and Pella East investigation areas near

Pella, Iowa ......................................................................................................................................................... 

2. Location of data-collection sites at Pella North investigation area, South Skunk River alluvial valley near

Pella, Iowa ......................................................................................................................................................... 

3. Location of data-collection sites at Pella East investigation area, South Skunk River alluvial valley near

Pella, Iowa ......................................................................................................................................................... 

4. Interpreted thickness of alluvial deposits in Pella North investigation area near Pella, Iowa ........................... 

5. Interpreted thickness of alluvial deposits in Pella East investigation area near Pella, Iowa.............................. 

6. Altitude and configuration of glacial-drift surface in Pella North investigation area near Pella, Iowa ............. 

7. Altitude and configuration of glacial-drift surface in Pella East investigation area near Pella, Iowa................ 

8. Geologic sections A–A' through C–C ' based on test-hole data .............................................................................. 

TABLES

1. Description of hydrogeologic units in south-central Iowa ...................................................................................... 

2. Description of drilled test holes and geologic information used in study of ground water near Pella, Iowa.......... 

3. Horizontal hydraulic-conductivity values estimated using slug tests for South Skunk River alluvial aquifer

near Pella, Iowa, 2001 ............................................................................................................................................. 
IV Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

TABLES—Continued

4. Selected water-quality characteristics and constituent concentrations in samples collected from observat

wells in South Skunk River alluvial aquifer near Pella, Iowa, July 12, 2001..............................................

Temperature can be converted to degrees Celsius (oC) or degrees Fahrenheit (oF) by the equations:

o

C = 5/9 (oF - 32) o

F = 9/5 (oC)+32 .

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

milligrams per liter (mg/L) and micrograms per liter (μg/L). A milligram per liter expresses the

concentration of chemical constituents in solution as weight (milligrams) of solute per unit volume

(liter) of water. A microgram per liter expresses the concentration of chemical constituents in solution

as weight (micrograms) of solute per unit volume (liter) of water. Microsiemens per centimeter

(μS/cm) at 25 degrees Celsius (oC) expresses the capability of a unit volume of water to conduct an

applied electrical current.

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 the "Sea Level Datum of 1929."

Altitude: As used in this report, refers to distance above or below sea level.

CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Multiply By To obtain

Length

inch (in) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

millimeter (mm) 0.03937 inch

Area

square mile (mi2) 2.590 square kilometer

Volume

acre-foot (acre-ft) 1,233 cubic meter

gallon (gal) 3.785 liter

million gallons (Mgal) 3,785 cubic meter

Volume per unit time (includes flow)

cubic foot (ft3) 28.32 liter

cubic foot per second (ft3

/s) 0.02832 cubic meter per second

foot per second (ft/s) 0.3048 meter per second

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

gallon per minute per foot

[(gal/min)/ft]

4.831 liter per second per meter

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

Ground Water near Pella and in Selected Parts of Jasper,

Mahaska, and Marion Counties, Iowa

By James P. Caldwell and Eric M. Sadorf

Abstract

The U.S. Geological Survey, in cooperation

with the city of Pella, Iowa, conducted a study of

the ground-water resources in selected parts of

Jasper, Mahaska, and Marion Counties near Pella,

Iowa, during 2000–01. The purpose of the study

was to describe the ground-water resources in the

Pella, Iowa, area.

Following a review of available hydrologic

and geologic information, the study focused on

investigating the alluvial deposits along the South

Skunk River within a 10-mile radius of the city of

Pella. Thickness, lithologic, and water-quality

data for the alluvial deposits in the South Skunk

River Valley were collected at selected sites from

August 2000 through July 2001.

The South Skunk River alluvial aquifer near

Pella consists of stratified deposits of sand and

gravel of glacial and fluvial origin. The upper

15 to 20 feet of the alluvial deposits are interbedded with flood-plain clay and silt lenses and are

overlain by a developed soil horizon. The alluvial

aquifer is underlain by a thin (less than 50 feet

thick) layer of glacial drift. Glacial drift, as it is

defined in the study area, is a heterogeneous,

unsorted, unstratified, unconsolidated, relatively

impermeable deposit consisting of clay, sand, and

gravel. The bedrock surface beneath the glacial

drift consists primarily of shale and limestone of

Pennsylvanian or Mississippian age. The alluvial

sand and gravel deposits along the South Skunk

River near Pella range from less than 30 to more

than 80 feet thick. Saturated thickness of the alluvial aquifer ranges from 15 to about 70 feet.

Water-quality samples were collected from

four observation wells installed in the South

Skunk River alluvial aquifer in the study area

Results of the water-quality sampling show th

the chemical quality of the ground water is ge

ally similar at all of the sampling sites. All

ground-water samples were low in dissolved o

gen, which resulted in high concentrations of 

and manganese and reduced forms of nitrogen

INTRODUCTION

The city of Pella, Iowa, obtains its municipal

water supply from the alluvial aquifer adjacent to 

Des Moines River and a deep bedrock well compl

in the Cambrian-Ordovician aquifer (locally refer

to as the Jordan aquifer). A radial collector well a

four vertical wells completed in the alluvium are

located 3 mi south of Pella adjacent to the channe

the Des Moines River, approximately 1.5 mi south 

Red Rock Dam (fig. 1). Yields from the alluvial w

are affected by the river stage (Veenstra and Kimm

Inc., written commun., 2001). These wells are cla

fied as "ground water under the influence of surfa

water" by the Iowa Department of Natural Resour

(IDNR). This classification requires a greater leve

treatment than water from wells unaffected by stre

or lakes. Pella's Cambrian-Ordovician aquifer wel

adjacent to the water-treatment plant, yields water 

is relatively high in dissolved solids and ammonia

nitrogen.

Concerns over the quality of water from the C

brian-Ordovician aquifer and increasing demands 

water supply due to residential, commercial, and

industrial development created a need for an evalu

tion of additional sources of public water supply in 
2 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

Index map

Newton

JASPER COUNTY

MARION COUNTY

POWESHIEK COUNTY

MAHASKA COUNTY

Oskaloosa

Knoxville

Pella

Red

Rock

Dam

Pella

North

Pella

East

South

Skunk

River

Moines

Des

River

Existing alluvial

well field

Area of

figure 2

Area of

figure 3

Lake

Red

Rock

10-mile radius

from city of Pella

41°22'30"

Base from U.S. Geological Survey digital data 1:100,000, 1991

Universal Transverse Mercator projection

Zone 15

0

0

2

2

 4 MILES

4 KILOMETERS

41°37'30"

93°07'30" 92°37'30"

Minnesota

Wisconsin

Illinois

Missouri

Nebraska

South

Dakota

Missouri

Riv

er

Mississippi

River

Iowa

Des Moines

Burlington

Keokuk

Tri-county area

Figure 1. Location of ground-water availability study and Pella North and Pella East investigation areas near

Pella, Iowa. The investigation areas lie within parts of Jasper, Mahaska, and Marion Counties.
Introductio

Pella area. During 2000, municipals withdrawals from

all water sources averaged 1.6 Mgal/d (Charles Keuning, Water and Wastewater Director, city of Pella, oral

commun., 2000). Because the Pella metropolitan area

is experiencing significant growth in population and

industry, city officials estimate that water use may

increase to 3.4 Mgal/d by 2010 (Veenstra and Kimm,

Inc.,written commun., 2001).

To help address concerns about future sources of

water supply, the U.S. Geological Survey (USGS), in

cooperation with the city of Pella, conducted a study

to provide information about the sources of ground

water (unconsolidated and bedrock aquifers) in parts

of Jasper, Mahaska, and Marion Counties, particularly

in the Pella area. The objective of the study was to

describe the unconsolidated and bedrock aquifers and

their water quality in the Pella area using available

information and data collected during 2000-01 by

reconnaissance drilling, test-well installation, and

water-quality sampling. The extent of the study area

was limited to within a 10-mi radius of the city of

Pella (fig. 1).

Purpose and Scope

The purpose of this report is to describe the results

of the study of ground-water resources in parts of Jasper, Mahaska, and Marion Counties in the Pella, Iowa,

area. Available hydrologic and geologic data were

compiled from scientific literature and previous studies. Bedrock aquifer systems in the study area, including the Pennsylvanian, Mississippian, SilurianDevonian, and Cambrian-Ordovician aquifers, are

briefly described. The surficial (alluvial and buriedchannel) aquifer systems adjacent to the Des Moines

and South Skunk River alluvial valleys were the main

focus of this study. Thickness, lithologic, and waterquality data for the unconsolidated alluvial deposits

were collected as part of this study during 2000–01

from parts of the South Skunk River Valley north and

east of Pella and are presented in this report.

Information from this study will contribute to

understanding the flow systems of local and regional

aquifers. Collection of hydrogeologic data from alluvial aquifers will enhance data resources needed for

constructing flow models simulating interactions

between ground and surface water, provide critical

results needed by managers for planning and operation

of public-water supplies, and provide information that

is transferable to present and (or) future public-w

supplies utilizing alluvial aquifer sources.

Description of Study Area

The city of Pella is in south-central Iowa betw

the Des Moines and South Skunk River Valleys a

35 mi southeast of Des Moines. Pella is located in 

northeast corner of Marion County, about 2.5 mi

northeast of the Des Moines River Valley and app

mately 3 mi south of the South Skunk River Valle

Jasper County borders Marion County 8 mi north 

Pella; Mahaska County borders Marion County 2.

to the east of the city. The study area includes par

Jasper, Mahaska, and Marion Counties (hereafter

referred to as the "tri-county area") and is contain

within a 10-mile radius of the city of Pella (fig. 1)

The study area lies within the Southern Iowa 

Plain landform region, characterized by relatively

thick glacial drift with a thick loess cover (Prior,

1991). Post-Pleistocene (Holocene) erosional pro

cesses have reshaped the glacial drift to produce l

upland divides, steep hill slopes, and broad lowla

valley floors. The land-surface altitude of the Des

Moines and South Skunk River Valleys in this are

varies from about 715 to 725 ft above sea level; la

surface altitude in adjacent upland areas is about

900 ft.

The Des Moines and South Skunk Rivers, the 

major drainage systems that receive runoff from t

tri-county area, originate in north-central Iowa. T

Des Moines River discharges into the Mississippi

River at Keokuk. Lake Red Rock, created by the 

struction of Red Rock Dam (1969) on the Des Mo

River 2.5 mi southwest of Pella (fig. 1), has an

impoundment storage capacity of 1,700,000 acre-

Red Rock Dam, operated by the U.S. Army Corp

Engineers (USCOE), maintains a minimum down

stream flow of 300 ft3

/s (Lucey, 1991).

The confluence of the South Skunk and North

Skunk Rivers is near Oskaloosa, Iowa. The Skunk

River system drains to the southeast and discharg

into the Mississippi River south of Burlington.

Selected reaches of the South Skunk River near P

were channelized and straightened during 1900–2

(Buchmiller, 2001). Within the study area, the So

Skunk River Valley varies in width from approximately 0.5 to 3 mi.

Streamflow in the South Skunk River is meas

about 25 mi upstream of the study area at a USGS
4 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

streamflow-gaging station on the South Skunk River at

Colfax (05471050)1. Annual mean daily streamflow

(October 1985 through September 2000) at this site is

about 622 ft3/s. However, during October 1999

through September 2000, mean daily streamflow averaged 96.3 ft3

/s due to less-than-normal precipitation in

central Iowa (Nalley and others, 2001). A USGS

streamflow-gaging station, South Skunk River at Oskaloosa (05471500)1

, is located approximately 18 mi

south of the study area. Annual mean daily streamflow

(October 1946 through September 2000) at this site is

about 1,039 ft3/s. However, during October 1999

through September 2000, mean daily streamflow averaged 290 ft3/s due to less-than-normal precipitation

in central and south-central Iowa (Nalley and

others, 2001).

Unconsolidated material of alluvial or glacial origin covers most of the tri-county area. The thickness

of the unconsolidated materials in this area varies from

0 ft where the bedrock outcrops along the river valley

walls to about 220 ft in areas above a buried channel

that lies between the Des Moines and South Skunk

River Valleys (Cagle, 1973). The valleys of these two

rivers contain significant deposits of alluvial material.

The underlying bedrock in the tri-county area consists of a thick sequence of sedimentary rocks. The

major bedrock aquifers within this sedimentary

sequence include several water-bearing sandstone and

carbonate layers within Pennsylvanian strata, and the

Mississippian aquifer, the Silurian-Devonian aquifer,

and the Cambrian-Ordovician aquifer system. The

uppermost bedrock in the study area consists of Pennsylvanian and/or Mississippian sedimentary rocks.

The geologic framework that contains these hydrogeologic units is described in table 1.

Following an initial reconnaissance investigation,

this study focused on describing the water resources of

the South Skunk River alluvial aquifer system. Two

areas along the South Skunk River Valley, north and

east of Pella, were selected for additional data collection. These two areas, referred to as the Pella North

and Pella East investigation areas, were selected

because of the areal extent of the South Skunk River

alluvial valley at those locations (fig. 1).

Previous Investigations

Few comprehensive studies have been co

to characterize all surficial and bedrock water

resources in the Pella area; most have focused 

alluvial aquifer of the Des Moines River Vall

the deep Cambrian-Ordovician aquifer, know

as the Jordan aquifer. The geology, physiogra

drainage of the tri-county area are described b

(1894), Miller (1900), and Williams (1904). 

water resources of the tri-county area are desc

Simpson and Norton (1912), Norton and othe

(1912), and Simpson (1912a,b). Information 

occurrence, availability, quality, and utilizatio

water in central, south-central, and southeast 

presented in Twenter and Coble (1965), Cagl

Heinitz (1978), and Coble and Roberts (1971

tively. The bedrock topography of central, sou

central, and southeast Iowa was mapped by H

(1985), Cagle (1973), and Hansen (1973), res

tively. Bruner and Hallberg (1987) describe th

ground-water quality of the entire Skunk Rive

particularly with regard to the occurrence of n

Information about water quality and geology 

able for the Mississippian aquifer (Horick and 

hilber, 1973), the Silurian-Devonian aquifer (

1984), and the Jordan aquifer part of the Cam

Ordovician aquifer (Horick and Steinhilber, 1

The recharge to, and ground-water movemen

Jordan (Cambrian-Ordovician) aquifer was ev

by Burkart and Buchmiller (1990). A compre

summary of geology and hydrologic character

the major aquifers in Iowa was presented by O

(1992). The effects of the Red Rock Dam on 

Des Moines River alluvial water-supply wells 

supplied water to the city of Pella until 1973 w

lyzed by W.L. Steinhilber (USGS, written com

1971). Lucey (1991) evaluated the ground-wa

system, geochemistry, and underseepage near 

Rock Dam near Pella. The effects of remedial 

on the ground-water flow system at Red Rock 

were investigated by Linhart and Schaap (200

Areas of the Des Moines River alluvial aq

near the present municipal-supply wells were 

ously investigated by the city of Pella to deter

occurrence of sand and gravel deposits of suf

thickness to construct additional municipal w

(Bennett and Williams, Inc., written commun

1989). Sixteen borings (10 test borings and 6 

tion wells) were installed in the Des Moines R

alluvium in 1988 at selected locations within 

1Real-time stage and discharge data for these streamflowgaging stations can be accessed on the World Wide Web at URL:

http://wwwdiaiwc.cr.usgs.gov/data.html
Introductio

southeast of the present well field (Bennett and Williams, Inc., written commun., 1989). Results from this

drilling indicated a relatively thin layer of alluvial sand

and gravel overlying shallow bedrock. Lithologic logs

from selected borings reported encountering wood,

coal, and other organic debris within the aquifer (Bennett and Williams, Inc., written commun., 1989). Geologic logs from eight of the borings identified

limestone as the uppermost bedrock underlying the

alluvial aquifer; geologic logs from three of the borings identified shale as the uppermost bedrock; two

identified sandstone, and three did not specify bedrock

type. Depths to bedrock varied from 16.5 to 37.5 ft

below land surface. Thickness of the sand and gravel

deposits ranged from 15 to 28 ft. In 1988, the satur

thickness of the Des Moines River alluvial aquife

ranged from 11 to 22 ft, which is considered insu

cient to sustain water yield rates required by the ci

Pella (Veenstra and Kimm, Inc., written commun

2001).

Within the past 10 years, the city of Pella has

observed a decline in the capacity of the radial co

tor well and the four shallow alluvial wells that cu

rently serve as the primary source of supply for th

city. This decline in capacity is most apparent dur

low-flow conditions in the Des Moines River asso

ated with dry weather or reduction of flow from th

conservation pool at Red Rock Dam.

Table 1. Description of hydrogeologic units in south-central Iowa

Hydrogeologic unit1

1Modified from Twenter and Coble (1965) and Cagle and Heinitz (1978).

Approximate thickness in vicinity of

Pella, Iowa (feet)1 Age of rock unit2

2Age classification of rock units are those of the Iowa Department of Natural Resources, Geological Survey Bureau.

Potential well yield

(gallons per minute) Lithology1

Alluvial, glacial-drift, and

buried-channel aquifers

0–100 Quaternary Less than 20 to more

 than 500

Sand, gravel, silt, clay,

and boulders.

Pennsylvanian aquifer3

3

Pennsylvanian aquifer and confining units are thin or absent in some locations within the tri-county area.

0–50 Pennsylvanian Less than 10 to 100 Sandstone and limeston

Confining unit 0–100 Pennsylvanian Very small Shale, sandstone, thin

limestone, and coal.

Mississippian aquifer4

4

Mississippian aquifer includes both upper and lower aquifer units.

250–400 Mississippian Less than 20 to 100 Limestone, dolomite, an

shale (gypsum and

anhydrite occur

locally).

Confining unit 200–250 Devonian

and

Mississippian

Very small Shale, siltstone,

limestone, and

dolomite.

Silurian-Devonian aquifer5

5

Silurian rock units are thin or absent in south-central Iowa. However, aquifer is still referred to as the “Siliurian-Devonian aquifer.”

450–500 Silurian

and

Devonian

Less than 20 to 100 Limestone, dolomite, an

shale.

Confining unit 500–550 Ordovician

and

Silurian

Very small Dolomite, shale, chert,

limestone, and

sandstone.

Cambrian-Ordovician System

Cambrian-Ordovician

aquifer6

Dresbach aquifer7

6

Jordan aquifer is principal water-bearing strata in Cambrian-Ordovician aquifer; the St. Peter aquifer is usually cased off in water-supply

wells. Thickness given is for the Jordan aquifer. 7Dresbach aquifer is not utilized in south-central Iowa. Hydrologic and water-quality properties are not known (NA).

500–550

20–50

NA

Cambrian

and

Ordovician

More than 1,000

NA

Sandstone and dolomite

Confining unit 350–550 Cambrian Very small Sandstone, shale,

siltstone, and dolomit

Confining unit8

8Precambrian confining unit hydrogeologic and water-quality properties are undetermined for tri-county area.

Not determined Precambrian Not determined Crystalline igneous and

metamorphic rocks.
6 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

Prior to this study, few wells or test borings had

been completed in the South Skunk River alluvial

aquifer near Pella. Although several household and

farmstead wells have been completed in the South

Skunk River alluvial aquifer in this area, little geologic

or hydrologic information has been reported.

Acknowledgments

The authors thank the city of Pella Water Department staff for their assistance in gathering technical

information and providing field assistance. The

authors also thank Ken Hedmark, Mike Turco, and

Ron Kuzniar, USGS, for assistance during the installation of test borings and monitoring wells, and during

data collection and compilation. Dan Christiansen,

USGS, provided valuable assistance with data collection, compilation, and preparation of figures. Brian

Lanning, Von Miller, Doug Schnoebelen, and Aimee

Donnelly, USGS, aided in data compilation and interpretation. Mark Savoca, USGS, provided guidance in

the collection and interpretation of aquifer test data.

Vernon Greenwood, USCOE, provided information

about the construction and maintenance of Red Rock

Dam, and information about the geology and hydrology near Lake Red Rock. The authors are grateful to

landowners in the Pella area who generously granted

access to their property for collection of data.

GROUND WATER NEAR PELLA AND IN SELECTED

PARTS OF JASPER, MAHASKA, AND MARION

COUNTIES

The tri-county area is underlain by aquifers that

can be divided into two main types—surficial and bedrock. Surficial aquifers in the study area consist of

unconsolidated materials and can be divided into three

types—alluvial, glacial drift, and buried channel. The

major bedrock aquifers in the study area, which are the

Pennsylvanian, Mississippian, Silurian-Devonian, and

Cambrian-Ordovician aquifers, consist of sedimentary

rock. Beneath the combined surficial and sedimentary

rock sequences lie Precambrian crystalline igneous

and metamorphic rocks, referred to as the basement

complex. The basement complex is not believed to

contain water in south-central Iowa and, therefore, is

considered to be the base of the ground-water

reservoir (Cagle and Heinitz, 1978).

Ground-water-yield characteristics and w

quality varies both between the aquifers and w

each aquifer. Generally, well yield is directly 

tional to the thickness of the aquifer. Alluvial 

will yield more water to wells on a sustained 

where the deposits are exposed or near the sur

can be recharged by infiltration of precipitatio

surface water. Glacial-drift and buried-channe

fers can have moderately high yields but usua

cannot sustain high pumping rates (greater th

300 gal/min) because recharge to these deeper 

is limited by the water-transmitting character

the overlying materials (Buchmiller, 2001).

In areas where carbonate (limestone and d

bedrock is near the land surface and is not co

confining units, the rock may be highly fractu

to weathering, erosion, and glacial loading an

unloading processes prior to burial by younge

logic materials. Fractures in bedrock aquifers 

permeability and improve water-transmitting

characteristics to wells.

 Water quality also affects whether groun

can be used for human consumption. Dissolv

and naturally occurring minerals, such as sulf

the use of ground water in some areas becaus

drinking-water regulations, aesthetic reasons, 

treatment costs.

The aquifers that can consistently yield su

quantities and that may have acceptable wate

for water-supply development in the Pella are

alluvial aquifers and the Cambrian-Ordovicia

system. The Mississippian aquifer and the Si

Devonian aquifer may be capable of yielding

sufficient quantities for development as a grou

water resource, but the variable chemical qual

water from these aquifers generally precludes 

water supply. In south-central Iowa, water fro

cial-drift and buried-channel aquifers is used 

by private, rural wells.

Surficial Aquifers

The surficial aquifers in the tri-county are

Quaternary water-bearing sand and gravel dep

that lie between the land surface and the bedr

face, except for a few areas of bedrock outcrop

widespread, but irregular, deposits are divided 

three classifications (alluvial, glacial drift, an

channel) on the basis of areal and vertical dis

and water-bearing characteristics.
Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Countie

Alluvial Aquifers

Alluvial aquifers are the shallowest and therefore

most readily available sources of potential groundwater supply. Alluvial aquifers are contained in unconsolidated materials deposited by fluvial processes

within the stream valleys. In the tri-county area, alluvial aquifers consist mostly of sand and gravel.

Recharge to the alluvial aquifers is by infiltration

of precipitation, stream seepage, and seepage from

underlying glacial-drift or bedrock aquifers

(W.L. Steinhilber, USGS written commun., 1971).

Available information on the alluvial deposits in

the Des Moines River Valley near Pella is limited to

the areas near Red Rock Dam and the current (2002)

and abandoned (1973) city of Pella well fields. In the

vicinity of the Red Rock Dam on the Des Moines

River (fig. 1), many test holes and observation wells

were completed in and through the alluvial deposits by

the USCOE as part of an exploratory and continual

monitoring program to assess the geologic, hydrologic, and geochemical conditions at the dam and reservoir site (W.L. Steinhilber, USGS, written commun.,

1971; Lucey, 1991; Linhart and Schaap, 1999). Information also is available for the current and abandoned

Pella municipal well fields on the east bank of the Des

Moines River channel, within 0.25 to 1.5 mi downstream of Red Rock Dam (Ranney Water Systems,

written commun., 1967; W.L. Steinhilber, USGS, written commun., 1971; Bennett and Williams, Inc., written commun., 1989). The thickness and lithology of

the Des Moines River alluvial deposits near Pella are

described in the "Previous Investigations" section of

this report.

Prior to this study, very little data had been collected pertaining to the South Skunk River alluvial

aquifer near Pella. Data collected during this study

show that the South Skunk alluvial aquifer near Pella

consists of stratified deposits of sand and gravel of glacial and fluvial origin. The thickness and lithology of

the South Skunk River alluvial deposits in the Pella

North and Pella East investigation areas are described

in a following section of this report.

Most of the tri-county area is located in the

south-central part of the State. Possible yields for alluvial wells in south-central Iowa range from 5 to

500 gal/min (Cagle and Heinitz, 1978). Available well

yields for the Des Moines River alluvial aquifer in the

study area (five wells) range from 150 to 530 gal/min

(Iowa Department of Natural Resources, 2001).

Alluvial aquifers yield the least mineralized w

of all ground-water sources in south-central Iowa

(Cagle and Heinitz, 1978). The water quality of th

alluvial aquifers reflects the characteristics of wat

from wells that are completed in sand and gravel 

sisting mostly of siliceous materials of low solubi

Hardness (as CaCO3) averages about 400 mg/L, a

alkalinity (as CaCO3) averages about 250 mg/L

(Twenter and Coble, 1965; Cagle and Heinitz, 19

Dissolved solids concentration usually is less tha

500 mg/L. Majors ions are calcium and bicarbona

but sulfate and sodium also are important constitu

Glacial-Drift Aquifers

Glacial drift underlies the alluvial aquifers wi

the stream valleys and is exposed in the upland ar

throughout most of the tri-county area. The maxim

thickness of glacial drift in south-central Iowa is a

430 ft (Cagle and Heinitz, 1978), but in the tri-co

area, the maximum thickness is approximately 20

The glacial-drift aquifers occur in discontinuous s

and gravel lenses contained within the clay-rich m

of the drift deposits. These lenses of aquifer mate

seldom exceed 5 ft in thickness and are local in o

rence (Cagle and Heinitz, 1978). The specific cap

(yield per unit of drawdown) for glacial-drift well

the tri-county area ranges from 0.3 to 5 (gal/min)/

(Iowa Department of Natural Resources, 2001). P

ble yields for wells completed in the glacial-drift 

fer in south-central Iowa range from 1 to 5 gal/mi

(Cagle and Heinitz, 1978).

Water from glacial-drift aquifers in south-cen

Iowa generally is very hard and may contain unde

able concentrations of dissolved solids, sulfate, nit

and iron (Cagle and Heinitz, 1978). The water qu

in glacial-drift aquifers varies significantly with d

with larger concentrations of hardness (CaCO3), d

solved solids, sodium, sulfate, and iron present in 

deeper sand and gravel lenses. However, nitrate (N

concentrations vary inversely with depth due to th

infiltration of nitrate-bearing surface water into th

shallower aquifer units. Hardness concentrations 

age 539 mg/L (CaCO3), dissolved solids concentr

tions average about 970 mg/L, sulfate concentrati

average 336 mg/L, nitrate concentrations average

about 60 mg/L (NO3), and iron concentrations ave

2,000 μg/L (Cagle and Heinitz, 1978).
8 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

Buried-Channel Aquifers

The sand and gravel comprising buried-channel

aquifers were deposited in glacial drift or bedrock

valleys, which were covered by later glacial deposits.

Buried-channel aquifers can, but do not always, coincide with present stream valleys and are usually concealed by surficial deposits (Thompson and Kemmis,

1990; Olcott, 1992). Water-bearing deposits in the

buried-channel aquifers tend to be more widely distributed than the water-bearing lenses of glacial-drift

aquifers. A buried-channel aquifer was mapped in

the tri-county area between the South Skunk and

Des Moines River Valleys (Hansen, 1985). Wells completed in this aquifer range in depth from 200 to 215 ft.

The thickness of the water-bearing gravel deposits

ranges from 0 to 20 ft. Possible yields for wells completed in the buried-channel aquifers in south-central

Iowa range from 5 to 25 gal/min (Cagle and Heinitz,

1978); the specific capacity for these wells range from

0.09 to 7.14 (gal/min)/ft (Iowa Department of Natural

Resources, 2001).

Buried-channel aquifers in south-central Iowa

yield hard water that is highly mineralized and similar

to the chemical composition of the deep glacial-drift

aquifers (Cagle and Heinitz, 1978). Hardness concentrations average 868 mg/L (CaCO3), dissolved solids

concentrations average about 2,350 mg/L, sulfate concentrations average 1,254 mg/L, nitrate concentrations

average 6.7 mg/L as NO3, and iron concentrations

average 3,400 μg/L (Cagle and Heinitz, 1978).

Bedrock Aquifers

The water-bearing characteristics of the consolidated sedimentary rocks within the tri-county area are

highly variable. The bedrock units that yield water to

wells, in order of increasing depth below land surface,

are the Pennsylvanian aquifer, the Mississippian aquifer, the Silurian-Devonian aquifer, and the CambrianOrdovician aquifer.

High dissolved mineral concentrations limit the

use of water from bedrock aquifers in most parts of

south-central Iowa and are a deterrent to more extensive development of these resources (Cagle and Heinitz, 1978). The natural chemical quality of water from

the principal bedrock aquifers in south-central Iowa is

highly variable and is affected primarily by the mineralogy of aquifer materials and the length of time that

the water is in contact with these minerals (Olcott,

1992). Because chemical concentrations tend 

increase along ground-water flow paths, grou

in outcrop and recharge areas is the least min

Ground water in deep, confined aquifers whe

water movement is slow tends to be the most 

ized (Cagle and Heinitz, 1978; Buchmiller, 20

Of the bedrock resources described for th

only the Cambrian-Ordovician aquifer is con

capable of yielding large quantities of potable 

suitable for public water-supply wells (Veens

Kimm, Inc., written commun., 2001). Howev

expense of constructing a water-supply well i

deep aquifer (greater than 2,000 ft below surf

coupled with high maintenance, operation, an

treatment costs due to high mineralization of 

water, may limit development of this aquifer (

and Kimm, Inc., written commun., 2001).

Pennsylvanian Aquifer

Consolidated sedimentary rocks of Penns

age form the bedrock surface in much of the tr

area. These stratified deposits consist primari

shale and thin, discontinuous layers of sandst

limestone, siltstone, and coal. Several Pennsy

limestone and sandstone units contain aquifer

limited in areal extent and thickness. Althoug

information is available for the Pennsylvanian 

in the tri-county area, estimated yields from t

fer in most places in south-central Iowa are le

25 gal/min, typically 3 to 10 gal/min, with a f

yielding 100 gal/min (Cagle and Heinitz, 197

In most areas of south-central Iowa, the P

vanian aquifer yields water that contains unde

concentrations of minerals. Chemical analysis 

samples collected from this aquifer shows the 

ing average concentrations of constituents: ha

(869 mg/L as CaCO3), dissolved solids (4,53

sulfate (1,088 mg/L), nitrate (3.5 mg/L as NO

iron (3,200 μg/L) (Cagle and Heinitz, 1978).

Pennsylvanian strata form a regional confi

unit that, in some areas, separate overlying un

dated materials from the Mississippian aquife

some localized areas, a limited water supply c

developed in bedrock of Pennsylvanian age, b

mineralization and low yield preclude large-s

development as a water supply.
Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Countie

Mississippian Aquifer

Mississippian strata underlie the entire tri-county

area and in some places form the bedrock surface. The

Mississippian aquifer includes two main aquifers,

locally referred to as the upper and lower aquifers,

which occur within the limestone, dolomite, and localized interbeds of gypsum and anhydrite. The presence

of these evaporite minerals may increase dissolved

solids and sulfate concentrations in water from

this source.

Well yields are highest in areas where the Mississippian strata form the bedrock surface and aquifer

recharge is from overlying surficial aquifers (Cagle

and Heinitz, 1978). In the tri-county area, well yields

range from 5 to 20 gal/min where Mississippian strata

are overlain by Pennsylvanian strata and from 5 to

60 gal/min where the Mississippian strata form the

bedrock surface. Specific capacities calculated for

165 wells in south-central Iowa indicate a

median value of 0.15 (gal/min)/ft for the upper, and

0.10 (gal/min)/ft for the lower aquifer (Cagle and

Heinitz, 1978).

Within the tri-county area, the quality of water

from the Mississippian aquifer is highly variable and

quite mineralized, but water of acceptable quality may

be available from both the upper and lower aquifers in

limited areas of Marion County near Pella. In Marion

County, the best quality water occurs in isolated areas

where the Pennsylvanian bedrock has been eroded and

the Mississippian strata are the bedrock surface (Cagle

and Heinitz, 1978).

Within these isolated areas, the average concentration of hardness is 449 mg/L (CaCO3), dissolved

solids is 674 mg/L, sulfate is 217 mg/L, nitrate ranges

(three samples only) from 0.10 to 4.3 mg/L as NO3,

and iron is 2,200 μg/L. In the remainder of the tricounty area, water from the Mississippian aquifer

system is very highly mineralized. The average concentration of hardness is 923 mg/L (CaCO3),

dissolved solids is 4,274 mg/L, sulfate is 2,385 mg/L,

nitrate is 8.4 mg/L as NO3, and iron is 7,600 μg/L

(Cagle and Heinitz, 1978). In localized areas near

Pella, a limited water supply could be developed from

the Mississippian aquifer; however, the likelihood of

excess mineralization and low yield probably preclude

large-scale development of this aquifer.

Silurian-Devonian Aquifer

The Silurian-Devonian aquifer of Iowa is contained in a thick succession of carbonate (limestone

and dolomite) rocks of Silurian and Devonian age

Although the carbonate units that compose the Si

urian-Devonian aquifer in Iowa are distinct geolo

entities, it is common to treat them as one hydrolg

logic unit because of their hydraulic connection

(Horick, 1984). Within the tri-county area, howev

Silurian rocks are thin or absent. In the study area

Silurian-Devonian aquifer mainly occurs within D

nian-age carbonate rocks. To remain consistent w

established geologic nomenclature, the term "Silu

Devonian aquifer" is used in this respect despite t

localized absence of Silurian-age rocks.

The Silurian-Devonian aquifer is separated fr

the overlying Mississippian aquifer by a thick con

ing unit, which usually is shale. Although compo

primarily of limestone and dolomite, the SilurianDevonian aquifer also may contain evaporite min

such as gypsum and anhydrite. The typical thickn

of the Silurian-Devonian aquifer in the tri-county 

is about 500 ft (Twenter and Coble, 1965; Coble a

Roberts, 1971; Cagle and Heinitz, 1978).

Few wells have been completed in the Siluria

Devonian aquifer in south-central Iowa. Projected 

yields range from 20 to 100 gal/min in this area (C

and Heinitz, 1978). Evaporite minerals that occur

locally in the Silurian-Devonian strata have a maj

effect on water quality. Where these minerals occ

degradation of water quality can be caused by hig

concentrations of sulfate, sodium, potassium, and

chloride. Dissolved solids concentrations in this a

fer are in excess of 5,000 mg/L throughout most o

tri-county area (Twenter and Coble, 1965; Coble 

Roberts, 1971; Cagle and Heinitz, 1978; Olcott, 19

The naturally occurring poor water quality and lo

yield limit the use of water from the Silurian-Devo

aquifer within the tri-county area.

Cambrian-Ordovician Aquifer System

The Cambrian-Ordovician aquifer system is a

complex multi-aquifer system with individual aqu

separated by leaky confining units. In south-centr

Iowa, the aquifer system consists of an upper unit

Cambrian-Ordovician aquifer (as differentiated fr

the Cambrian-Ordovician aquifer system), and a l

part, the Dresbach aquifer. The two aquifer units 

separated by a thick confining unit of Cambrian-a

strata (Olcott, 1992; Turco, 1999). The Cambrian

Ordovician aquifer is separated from the overlyin

urian-Devonian aquifer by a thick Ordovician sha

and dolomite sequence.
10 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

The principal source of ground water to wells

within the Cambrian-Ordovician aquifer system is the

Cambrian-Ordovician aquifer (Turco, 1999). The

Cambrian-Ordovician aquifer occurs within a dolomite and sandstone sequence; two sandstone units that

function as aquifers are part of this hydrogeologic

unit. The upper sandstone unit, which composes the

St. Peter aquifer, is very friable and is usually cased

off in water-supply wells in south-central Iowa (Cagle

and Heinitz, 1978; Turco, 1999; Bill Bunker, Iowa

Geological Survey Bureau, oral commun., 2002). The

lower sandstone unit, which composes the Jordan

aquifer, is the principal water-bearing unit and

accounts for the high yields attributed to the

Cambrian-Ordovician aquifer. In the study area, the

depth to the top of the Jordan aquifer ranges from

2,045 to 2,220 ft below the land surface.

In south-central Iowa, hydrologic data are

lacking for the Dresbach aquifer, the lower part of

the Cambrian-Ordovician aquifer system (Cagle and

Heinitz, 1978). Because the depth to the top of the

Dresbach aquifer is greater than 2,200 ft below land

surface in the Pella area and the hydrologic and waterquality properties are unknown, the Dresbach aquifer

was not considered in this study.

The Cambrian-Ordovician aquifer, with potential

well yields in excess of 1,000 gal/min, has the highest

yield of any of the aquifers in the study area (Cagle

and Heinitz, 1978). In the eastern one-third of southcentral Iowa, which contains most of the tri-county

area, the Cambrian-Ordovician aquifer yields fair

quality water. Dissolved solids concentrations average

1,000 mg/L, sulfate concentrations average 285 mg/L,

and chloride concentrations average 88 mg/L (Cagle

and Heinitz, 1978).

DESCRIPTION OF SOUTH SKUNK RIVER

ALLUVIAL DEPOSITS NEAR PELLA

The alluvial deposits associated with the South

Skunk River Valley near Pella consist of stratified

sand and gravel deposits that are the result of postPleistocene (Holocene) valley fill and modern fluvial

processes. The upper 12 to 20 ft of the alluvial deposits are interbedded with silt, very fine sand, and clay

lenses and are overlain by a developed, organic-rich

soil horizon. The alluvial deposits beneath this thin

veneer of fine-grained materials generally become

coarser with depth.

Although all the unconsolidated material

described in available geologic logs could be 

to yield water, only those materials that are d

as having a particle size equal to or larger tha

medium-grained sand (approximately 0.25 to 

in diameter) are considered to be the alluvial 

for the purposes of this report. Therefore, the t

of the alluvial aquifer, with respect to its wate

resource development potential, is defined as 

val thickness of sand and gravel below the ba

surficial flood-plain silt, clay, and soil layer an

the top of the underlying glacial drift.

Collection of Test-Hole and Observation-W

Data

Test holes and observation wells were ins

the USGS to collect new geologic and hydrol

information for the unconsolidated deposits in 

Pella North and Pella East investigation areas

ing completion of the reconnaissance test-bor

activities, observation wells for measuring wa

els, determining hydraulic characteristics, and 

ing water-quality samples were constructed ad

selected test holes. The latitude and longitude 

observation wells and test holes were determi

global positioning system (GPS). Horizontal 

nate information is referenced to the North A

Datum of 1983 (NAD83).

A total of 13 test holes, with an average d

64.5 ft, were drilled by USGS in 2000 in the 

North and Pella East areas (figs. 2 and 3). All 

holes were drilled with 4-in. outside-diameter 

solid-stem augers. Borings were advanced to 

refusal, defined by slow penetration rate and h

bit pressure, or to the depth limits (approxima

85 ft) of the drill rig being used. Samples of a

tings were collected at major lithologic chang

the auger flights as they were withdrawn from 

borehole after reaching total depth, a techniqu

as "profile augering."

The test holes indicate that the thickness 

alluvial deposits ranges from about 55 to mor

80 ft. The upper 15 to 20 ft of the alluvial dep

interbedded with flood-plain clay and silt lens

are overlain by a developed organic-rich soil 

The deposits composing the alluvial aquifer a

ing-upward sequence of sand and gravel that 

appear to have significant interbedding of clay 

lenses beneath a depth of about 20 ft. Althoug
Description of South Skunk River Alluvial Deposits near Pella 

41°29'

Base from U.S. Geological Survey digital data 1:100,000, 1991

Universal Transverse Mercator projection

Zone 15

41°31'

93° 92°57'30"

JASPER COUNTY

MARION COUNTY

0

0

0.5

0.5

1.0 MILE

1.0 KILOMETER

South

Skunk

River

Van Zante Creek

A A'

A A'

B

B'

PN–09

PN–08

PN–07

PN–01

PN–01

PN–02

PN–02

PN–04

PN–03 PN–05

PN–06

Trace of geologic section shown in figure 8

USGS test hole and identifier

USGS observation well and identifier—

 Installed adjacent to test hole

EXPLANATION

mineralogy of the alluvial aquifer was not examined in

detail, the principal mineral constituents appear to be

mostly quartz, with minor limestone and lithic fragments. Wood and organic debris frequently were

observed in well cuttings. USGS boring logs (table 2)

indicate the alluvium is underlain by clay-rich

glacial drift.

After reviewing the stratigraphic information from

the test holes, observation wells were constructed at

three locations within the Pella North area (fig. 2) 

at one location in the Pella East area (fig. 3). The 

observation wells were constructed adjacent to th

original test holes to maintain subsurface control. 

ings for the observation wells were advanced usin

3.25-in. inside-diameter (ID) continuous-flight

hollow-stem augers. Samples of drill cuttings retu

to land surface during drilling were collected at m

lithologic changes, as were samples collected from 

Figure 2. Location of data-collection sites at Pella North investigation area, South Skunk River alluvial valley near

Pella, Iowa.
12 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

41°23'

41°24'

Base from U.S. Geological Survey digital data 1:100,000, 1991

Universal Transverse Mercator projection

Zone 15

92°46'30" 92°43'30"

0

0

0.5

0.5

1.0 M

1.0 KILOMETER

C C'

PE–01

PE–01

PE–02

PE–02

PE–03

PE–04

Trace of geologic section shown in figure 8

USGS test hole and identifier

USGS observation well and identifier— Installed adjacent to test hole

EXPLANATION

C

C'

South

Skunk

River

auger flights as previously described for test-hole drilling. Observation wells were constructed by lowering

2-in. inside-diameter (ID) schedule-40, flush-threaded

polyvinyl chloride (PVC) well casing, equipped with

5-ft, 0.010-in. slot-size well screens, into the annulus

of the hollow-stem auger assembly. Well screens were

placed within an interval of the aquifer that consisted

of medium or larger grained-sized materials. The

annular space (7.25-in. nominal borehole size) around

the screen and riser was allowed to fill with native

materials during withdrawal of the hollow-stem auger

assembly, providing a "natural" filter pack. The upper

4 ft of the annular space was filled with bentonite clay

to prevent surface runoff from entering the borehole.

To minimize the turbidity of the water, wells 

developed by pumping until the discharge wa

clear to remove fine-grained sand and silt from 

natural filter pack. Well depths ranged from 4

57.5 ft below land surface. Test-hole location

associated geologic information are listed in t

Thickness and Lithology

Descriptive logs of the 13 USGS test hole

in the two investigation areas (table 2) indicat

rich glacial drift underlies the sand and grave

alluvial aquifer at these sites. The thickness o

cial drift in the investigation areas is unknown 

Figure 3. Location of data-collection sites at Pella East investigation area, South Skunk River alluvial valley near Pel

Iowa.
Description of South Skunk River Alluvial Deposits near Pella 

Table 2. Description of drilled test holes and geologic information used in study of ground water near Pella, Iowa

Test-hole

identifier1

(figs. 2 and 3)

Location land

net2

(latitude,

longitude)

Geologic

unit3

Depth below land surface4

(feet)

Drillers log/cuttings description

Pella North investigation area

PN–01 41o

29'39.8"

93o00'17.2"

Qa

Qag

0–5

5–20

20–35

35–82

82 (TD)

Dark brown silt, clayey.

Light brown-gray silt.

Light brown to gray sand, silty.

Clay, glacial drift.

PN–02 41o

29'39.7"

92o59'51.6"

Qa

Qag

0–5

5–20

20–60

60–62

62 (TD)

Dark brown silt, clayey.

Light brown to gray silt.

Clay, glacial drift.

Clay, glacial drift.

PN–03 41o

29'33.6"

92o59'10.8"

Qa

Qag

0–5

5–25

25–60

60–62

62 (TD)

Dark brown silt, clayey.

Light-medium brown to gray silt.

Clay, glacial drift.

Clay, glacial drift.

PN–04 41o29'39.9"

92o58'37.5"

Qa

Qag

0–5

5–25

25–57

57 (TD)

Dark brown silt.

Medium brown to gray silt, clayey.

Clay, glacial drift.

PN–05 41o29'39.6"

92o

58'02.9"

Qa

Qag

0–8

8–27

27–56

56 (TD)

Dark brown silt.

Medium brown to gray silt, sandy.

Clay, glacial drift.

PN–06 41o29'06.9"

92o59'10.5"

Qa 0–12

12–52

52 (TD)

Dark brown silt.

Medium brown to gray silt with occasional lenses of fine

sand.

Gray silt.

(blue background indicates

alluvial aquifer interval)

Light brown-gray sand, coarsening downward. Lenses of

small to coarse gravel appear at 35 feet and are presen

to total depth. Lenses of fine sand and silt are

interspersed in sandy alluvium.

Tan-brown-gray sand, coarsening downward. Dark

organic particulate and thin lenses of clay and silt are

present throughout sandy alluvium. Lenses of small

to coarse gravel appear at 35 feet and are present to

total depth. Lenses of fine sand and silt are

interspersed in sandy alluvium.

Tan-brown-gray sand, coarsening downward. Lenses of

small to coarse gravel appear at 35 feet and are presen

to total depth. Lenses of fine sand and silt are

interspersed in sandy alluvium.

Tan-brown-gray sand, coarsening downward. Lenses of

small to coarse gravel appear at 35 feet and are present

to total depth. Lenses of fine sand and silt are

interspersed in sandy alluvium.

Medium brown to gray sand, coarsening downward.

Lenses of small to coarse gravel appear at 35 feet and

are present to total depth. Lenses of fine sand and silt

are interspersed in sandy alluvium.
14 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

Pella North investigation area—Continued

PN–07 41o

29'56.6"

92o

59'10.9"

Qa

Qag

0–5

5–15

15–71

71 (TD)

Dark brown silt.

Medium brown to gray silt, sandy.

Clay, glacial drift.

PN–08 41o

30'26.3"

92o

59'11.1"

Qa

Qag

0–5

5–52

52–62 (TD)

Dark brown silt.

Clay, glacial drift.

PN–09 41o

30'55.3"

92o

59'10.7"

Qa

Qag

0–5

5–25

25–52

52 (TD)

Dark brown silt.

Medium brown to gray silt, sandy.

Clay, glacial drift.

Pella East investigation area

PE–01 41o

23'58.7"

92o

45'40.5"

Qa

ND5

0–5

5–12

12–68

 68–70

70 (TD)

Dark brown-black sandy silt, clayey.

Medium brown sandy silt.

Very coarse gravel or weathered bedrock.

PE–02 41o

23'36.5"

92o

45'39.8"

Qa

Qag

0–5

5–12

12–57

57–72

72 (TD)

Dark brown-black sandy silt, clayey.

Medium brown sandy silt.

Clay, glacial drift.

Clay, glacial drift.

PE–03 41o

23'22.8"

92o

45'39.8"

Qa

Qag

0–5

5–15

15–45

45–85

85 (TD)

Dark brown-black silt.

Medium-rusty brown sandy silt.

Medium-dark gray clay, glacial drift. Very dense, dr

Clay, glacial drift.

PE–04 41o

23'00.6"

92o

45'39.5"

Qa 0–5

5–52.5 (TD)

Dark brown silt, clayey.

1Test holes PN–01 to PN–09, and PE–01 to PE–04 were drilled by U.S. Geological Survey during October 30–November 8, 2000.

2Location is given by latitude (top sequence) and longitude (bottom sequence).

3Qa = Quaternary-age alluvium; Qag=Quaternary-age alluvium and glacial drift.

4TD = total depth.

5ND = not determined.

Table 2. Description of drilled test holes and geologic information used in study of ground water near Pella, Iowa

Test-hole

identifier1

(figs. 2 and 3)

Location land

net2

(latitude,

longitude)

Geologic

unit3

Depth below land surface4

(feet)

Drillers log/cuttings description

Medium brown to gray sand, coarsening downward.

Lenses of small to coarse gravel appear at 20 feet 

are present to total depth. Lenses of fine sand and 

are interspersed in sandy alluvium.

Medium brown to gray sand, coarsening downward.

Lenses of small to coarse gravel appear at 40 feet 

are present to total depth. Lenses of fine sand and 

are interspersed in sandy alluvium.

Medium brown to gray sand, coarsening downward.

Lenses of small to coarse gravel appear at 30 feet 

are present to total depth. Lenses of fine sand and 

are interspersed in sandy alluvium.

Light brown-gray sand, coarsening downward. Lense

small to coarse gravel appear at 56 feet and are pre

to total depth.

Very coarse gravel or weathered bedrock, slow

penetration rate.

Tan-brown-gray sand, coarsening downward. Dark

organic particulate and thin lenses of clay and silt 

present throughout sandy alluvium. Lenses of coar

to very coarse sand present from 42–57 feet.

Rusty brown very fine sand, becoming gradationally

coarser to very coarse at 40 feet.

(blue background indicates

alluvial aquifer interval)

Light-medium brown to gray fine sand becoming

gradationally coarser with depth. Light gray, medi

sand.
Description of South Skunk River Alluvial Deposits near Pella 

it was fully penetrated in only one of the borings.

However, published bedrock-altitude maps of central

and south-central Iowa indicate that the glacial-drift

thickness is less than 50 ft in the South Skunk River

alluvial valley (Hansen, 1972, 1973, 1985; Cagle,

1973). Depth to the top of the glacial drift in boreholes

and wells ranged from 45 to more than 80 ft. Within

the two investigation areas selected for subsurface

evaluation, the thickest alluvial deposits encountered

while drilling were in the west-central part of the Pella

North investigation area and in the central part of the

Pella East investigation area (figs. 4 and 5). As previously described, the total thickness of aquifer material

in the Pella North and Pella East investigation areas

was estimated as the thickness of geologic materials

between the base of the overlying flood-plain silt, clay,

and soil and the top of the underlying glacial drift.

Therefore, the thickness of the aquifer material in the

Pella North and Pella East areas is estimated to range

from approximately 25 to more than 60 ft. Additional

test drilling, using mud rotary or similar methods,

would be needed to confirm the interpreted depth to

and thickness of glacial drift, the altitude and nature of

the underlying bedrock, and for more accurate determination of the particle size and mineral composition

of the geologic materials composing the alluvial

deposits in areas where borehole data are not available.

Figures 6 and 7 show the altitude of the glacialdrift surface in the Pella North and Pella East

study areas. These maps were prepared from depth-toglacial-drift data collected from USGS wells and

test holes, and information extrapolated from

1:125,000-scale bedrock topographic maps by Hansen

(1972, 1973, and 1985) and Cagle (1973). The greatest

thicknesses of alluvial deposits, in excess of 60 ft,

coincide with the areas of lowest altitude of the

glacial-drift surface. The land-surface altitude of the

South Skunk River flood plain is relatively uniform.

Therefore, the deeper glacial-drift surface areas appear

to be erosional channels in the glacial drift that have

been subsequently filled by unconsolidated deposits.

Geologic sections showing the general stratigraphy of

the unconsolidated deposits in the Pella North and

Pella East investigation areas are shown in figure 8.

Aquifer Properties

In-situ hydraulic-conductivity (slug) tests were

performed in May 2001 in each observation well

within the Pella North and Pella East investigatio

areas. Slug tests, used to estimate aquifer hydraul

characteristics, were performed using the techniq

described by Lohman (1972). After measuring th

water level, each observation well was re-develop

by pumping for approximately 30 minutes, at a di

charge of 7.5 to 11 gal/min, to ensure that the wat

was relatively free of fine sediment. After pumpin

the water level was allowed to return to static con

tions before the slug test was performed.

During a slug test, water-level changes are m

sured in a well after displacing the initial water le

by introducing or removing a cylinder (slug) of kn

displacement. Water-level changes following intro

tion or removal of the slug were measured with a 

sure transducer and data recorder. Water levels we

initially recorded at intervals of 0.03 seconds, but 

recording interval was gradually increased as the 

test duration increased. The hydraulic conductivit

was calculated according to the Bouwer and Rice

method for partially penetrating wells (Bouwer, 19

Horizontal hydraulic-conductivity values ranged f

about 24 ft/d in observation well PN–07 to 54 ft/d 

observation well PN–04 (table 3).

Water Quality

Water samples were collected from the observ

tion wells on July 12, 2001. Results were used to

assess areal variations of constituent concentratio

and to identify areas where selected constituents o

at concentrations that may be undesirable for drin

water purposes. Specific conductance, pH, tempe

ture, dissolved oxygen, bicarbonate, carbonate, an

alkalinity were measured onsite at the time of sam

collection. Laboratory analyses, performed at the

USGS National Water-Quality Laboratory (NWQ

in Lakewood, Colorado, consisted of major ions

(calcium, magnesium, sodium, potassium, sulfate

chloride, fluoride, bromide, and silica), trace meta

(arsenic, iron, and manganese), and nutrients (nitr

nitrogen, nitrite plus nitrate nitrogen, ammonia ni

gen, and orthophosphorus). Concentrations of sele

triazine pesticides and pesticide-degradation prod

(acetochlor, alachlor, ametryn, atrazine, bromacil

butachlor, butylate, carboxin, cyanazine, cycloate

deethylatrazine, deisopropylatrazine, diphenamid

hexazinone, metolachlor, metribuzin, prometon, p

etryn, propachlor, propazine, simazine, simetryn, 

bacil, terbuthylazine, trifluralin, and vernolate) as 
16 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

Base from U.S. Geological Survey digital data 1:100,000, 1991

Universal Transverse Mercator projection

Zone 15

41°31'

92°57'30"

JASPER COUNTY

MARION COUNTY

0

0

0.5

0.5

1.0 MI

1.0 KILOMETER

South

Skunk

River

PN–09

PN–08

PN–07

PN–01 PN–02 PN–04

PN–03 PN–05

PN–06

41°29'

93°

50

60

70

80

60

50

PN–01

PN–02

Upland area

South Skunk River Valley

Line of equal thickness of alluvial 

 deposits. Interval is 10 feet

USGS test hole and identifier

USGS observation well and identifier—

 Installed adjacent to test hole

EXPLANATION

50

Van Zante Creek

Figure 4. Interpreted thickness of alluvial deposits in Pella North investigation area near Pella, Iowa (based on testhole and observation-well data).
Description of South Skunk River Alluvial Deposits near Pella 

50

50

40

40

40

70

60

60

41°23'

41°24'

Base from U.S. Geological Survey digital data 1:100,000, 1991

Universal Transverse Mercator projection

Zone 15

92°46'30" 92°43'30"

0

0

0.5

0.5

1.0 MILE

1.0 KILOMETER

PE–01

PE–01

PE–02

PE–02

PE–03

PE–04

W–9749

W–9749

South

Skunk

River

Upland area

South Skunk River Valley

Line of equal thickness of alluvial 

 deposits. Interval is 10 feet

USGS test hole and identifier

USGS observation well and identifier—

 Installed adjacent to test hole

Domestic well and Iowa Geological 

 Survey Bureau well code

EXPLANATION

50

Figure 5. Interpreted thickness of alluvial deposits in Pella East investigation area near Pella, Iowa (based on test-hole

and observation-well data).
18 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa

Base from U.S. Geological Survey digital data 1:100,000, 1991

Universal Transverse Mercator projection

Zone 15

41°31'

92°57'30"

0

0

0.5

0.5

1.0 MI

1.0 KILOMETER

South

Skunk

River

PN–09

PN–08

PN–07

PN–01

PN–02

PN–04

PN–03 PN–05

PN–06

41°29'

93°

PN–01

PN–02

Upland area

South Skunk River Valley

Subsurface contour—Shows altitude of 

 glacial-drift surface. Contour interval 

 10 feet. Datum is sea level

USGS test hole and identifier

USGS observation well and identifier—

 Installed adjacent to test hole

EXPLANATION

680

690

680

690

680

Van Zante Creek

JASPER COUNTY

MARION COUNTY

Figure 6. Altitude and configuration of glacial-drift surface in Pella North investigation area near Pella, Iowa (based

on test-hole and observation-well data, and information extrapolated from Hansen, 1972, 1973, 1985; Cagle, 1973).
Description of South Skunk River Alluvial Deposits near Pella 

41°23'

41°24'

Base from U.S. Geological Survey digital data 1:100,000, 1991

Universal Transverse Mercator projection

Zone 15

92°46'30" 92°43'30"

0

0

0.5

0.5

1.0 MILE

1.0 KILOMETER

PE–01

PE–02

PE–03

PE–04

W–9749

South

Skunk

River

PN–01

PN–02

Upland area

South Skunk River Valley

Subsurface contour—Shows altitude of 

 glacial-drift surface. Contour interval 

 10 feet. Datum is sea level

USGS test hole and identifier

USGS observation well and identifier—

 Installed adjacent to test hole

Domestic well and Iowa Geological 

 Survey Bureau well code

EXPLANATION

650

650

650

660

680

670

670

670

660

W–9749

Figure 7. Altitude and configuration of glacial-drift surface in Pella East investigation area near Pella, Iowa (based on

test-hole and observation-well data, and information extrapolated from Hansen, 1972, 1973, 1985; Cagle, 1973).
20 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa 650 640 Trace of sections shown in figures 2 and 3 Vertical scale exaggeration x 10 660 670 680 690 700 710 720 730 740 750 760 770 650 640 660 670 680 690 700 710 720 730 740 750 760 650 640 630 620 660 670 680 690 700 710 720 730

Altitude, in feet above sea level

A A'

B B'

PN–01

PN–02

PN–05

PN–05

PN–03

PN–04

Van Zante Creek

Van Zante Creek

PN–07

PN–08

PN–09

PN–03

PN–06

C 

Flood-plain silt and clay with  occasional thin beds of fine  sandFining-upward alluvial deposit sequencesGlacial driftUndifferentiated bedrock

Geologic contactEstimated top of bedrockTest hole or observation wel and identifier

EXPLANATION

Figure 8. Geologic sections A–A' through C–C ' based on test-hole data.
Description of South Skunk River Alluvial Deposits near Pella 

as dissolved organic carbon (DOC), and dissolved

solids were measured by NWQL.

Physical properties and constituent concentrations

for major ions, trace metals, nutrients, DOC, and dissolved solids detected in water samples collected from

the investigation area test wells are summarized in

table 4. Pesticides and pesticide-degradation products

were not detected in any samples above the NWQL

method detection limits and are not listed in table 4.

Samples from all test wells had relatively low

concentrations of dissolved oxygen. Measured values

of specific conductance, pH, temperature, alkalinity,

bicarbonate, dissolved solids, and DOC were similar

for all samples as were concentrations of calcium,

magnesium, sodium, potassium, fluoride, bromide,

silica, and manganese. Sulfate, chloride, orthophosphorus, arsenic, manganese, and iron concentrations

varied. Sulfate was present in samples from all

observation wells, ranging from 13 (observation

well PN–04) to 80 mg/L (observation well PN–01).

Chloride concentrations ranged from 4.0 (observation

well PE–01) to 21 mg/L (observation well PN-01).

Concentrations of nutrients, except orthophosphorus, were similar in all ground-water samples.

Ammonia nitrogen concentrations ranged from

0.47 (observation well PN–01) to 1.0 mg/L (observation well PN–07). Concentrations of nitrite nitrogen

and nitrite plus nitrate were near or less than laboratory method detection limits (0.05 mg/L). Orthophosphorus concentrations ranged from less than 0.02

(observation wells PN–07 and PE–01) to 0.16 mg/L

(observation well PN–04).

Arsenic concentrations ranged from less than

0.2 (observation well PN–04) to 8 μg/L (observation

well PE–01). Iron concentrations ranged from

1,800 (observation well PN–01) to 16,000 μg/L

(observation well PE–01). Manganese concentrat

ranged from 220 (observation well PN–07) to

940 μg/L (observation well PE–01).

For reference, the quality of water in the Sout

Skunk alluvial aquifer was compared to the regul

tions established by the U.S. Environmental Prote

Agency (USEPA) Safe Drinking Water Act of 19

The USEPA regulations established maximum lev

for certain physical properties of, and chemical co

stituents in, finished (treated) drinking water. The

regulations are Maximum Contaminant Levels

(MCLs), Secondary Maximum Contaminant Leve

(SMCLs), and Health Advisory Levels (HALs). T

regulations apply to properties and constituents th

may cause adverse human health effects if presen

treated drinking water. MCLs are enforceable, he

based standards. SMCLs are established for prope

or constituents that can affect the aesthetic quality 

water (odor, appearance, taste) and may result in 

continued use of the water. HALs are nonregulato

levels that establish acceptable constituent concen

tions for a specified period of exposure (1-day, 10

long term, and lifetime). Lifetime HALs are estim

of concentrations that would result in no known o

anticipated adverse health effects (U.S. Environm

Protection Agency, 2002).

Raw water samples collected from all wells

exceeded USEPA SMCL regulations for iron

(SMCL = 300 μg/L) and manganese (SMCL =

50 μg/L). Arsenic was detected in a sample from

observation well PE–01 at a concentration of 8 μg

which is slightly less than the arsenic MCL of

10 μg/L. No other physical properties or chemica

constituents in samples from any of the wells

exceeded USEPA regulations. Note that the refere

USEPA regulations are for treated, not raw, water 

ples (U.S. Environmental Protection Agency, 199

Municipal well fields for the cities of Newton 

Oskaloosa (fig. 1) yield water from the South Sku

River alluvial aquifer north and south of Pella. A

review of historical analytical results of ground-w

samples from these well fields show that measure

values of specific conductance, pH, temperature, 

solved oxygen, alkalinity, and dissolved solids we

similar to results from observation wells in the Pe

area. Concentrations of major ions, trace metals, 

nutrients were very similar to the samples collecte

this study.

Table 3. Horizontal hydraulic-conductivity values estimated using

slug tests for South Skunk River alluvial aquifer near Pella, Iowa,

2001

Observation wells

(figs. 2 or 3)

Lithology near screened

interval of well

Estimated horizontal

hydraulic conductivity

(feet per day)

PN–01 Medium to coarse sand

with gravel.

Slug in: 46

Slug out: 53

PN–04 Medium to coarse sand

with gravel.

Slug in: 54

Slug in 49

PN–07 Medium to coarse sand

with gravel; thin silt

and clay lenses

within aquifer interval.

Slug in 33

Slug out: 24

PE–01 Medium to coarse sand

with gravel.

Slug in: 44

Slug out: 48
22 Ground Water near Pella and in Selected Parts of Jasper, Mahaska, and Marion Counties, Iowa Table 4. Selected water-quality characteristics and constituent concentrations in samples collected from observation wells in South Skunk River 

July12, 2001[μS/cm, microsiemens per centimeter at 25 degrees Celsius; oC, degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; <, less than; E, estimated v

Observation

wells

(figs. 2 or 3) Site identification

Time

(24-hour)

Specific

conductance

(μS/cm)

pH, water, whole,

onsite,

(standard units)

Temperature,

water

(

oC)

Oxygen,

dissolved

(mg/L)

Bicarbonate,

onsite

(mg/L as HCO3)

Carbonate, onsit

(mg/L as CO3)

PN–01 412940093001701 1300 576 7.4 14.3 1.0 220 0 PN–04 412941092583801 1605 622 7.3 13.4 .6 380 0 PN–07 413006092591001 1435 672 7.2 13.2 .8 380 0 

PE–01 412359092454101 1030 603 6.9 14.5 1.0 360 0 

Observation

wells

(figs. 2 or 3)

Magnesium,

dissolved

(mg/L)

Sodium,

dissolved

(mg/L

as Na)

Potassium,

dissolved

(mg/L)

Sulfate, dissolved

(mg/L as SO4)

Chloride,

dissolved

(mg/L as Cl)

Fluoride,

dissolved

(mg/L as F)

Bromide,

dissolved

(mg/L as Br)

Silica, dissolv

(mg/L as SiO

PN–01 26 11 0.9 80 21 0.3 0.12 30 PN–04 25 15 1.4 13 4.8 .3 .16 30 PN–07 30 13 1.6 42 5.9 .4 .11 30 

PE–01 24 5.6 1.8 35 4.0 E.1 .09 34 

 Observation wells

(figs. 2 or 3)

Manganese,

dissolved

(μg/L as Mn)

Nitrogen,

nitrite

(mg/L as N)

Nitrogen, nitrite

plus nitrate,

dissolved

(mg/L as N)

Nitrogen,

ammonia,

dissolved

(mg/L as N)

Orthophosphorus,

dissolved

(mg/L as P)

Carbon, organic,

dissolved

(mg/L as C)

Solids, re

180oC, d

(mg

PN–01 420 E0.003 E0.04 0.47 0.05 2.8 38PN–04 240 E.004 <.05 .97 .16 2.5 36PN–07 220 <.006 <.05 1.0 <.02 8.2 41

PE–01 940 .006 <.05 .65 <.02 3.0 40
References 

SUMMARY

The city of Pella currently obtains its municipal

water supply from the alluvial aquifer adjacent to the

Des Moines River and a Cambrian-Ordovician

(Jordan) aquifer well. Yields from the alluvial wells

are affected by the river stage, which is controlled by

discharge from Lake Red Rock. Additional sources of

water in the vicinity of Pella are needed to meet projected increased water needs.

To help address concerns about future sources of

water supply, the U.S. Geological Survey, in cooperation with city of Pella, conducted a study from

2000–01 to provide information about the sources of

ground water (unconsolidated and bedrock aquifers) in

selected parts of Jasper, Mahaska, and Marion Counties, within a 10-mi radius of the city of Pella. The

study focused on supplementing available information

for the alluvial deposits along the South Skunk River

Valley north and east of Pella.

Available hydrologic and geologic information

was compiled from the scientific literature and previous studies in the Pella area. Additional thickness and

lithology data for the alluvial deposits in the South

Skunk River Valley were collected for this study at

selected sites from October 2000 through March 2001.

Water-quality samples were collected in July 2001

from four observation wells installed in the South

Skunk River alluvial aquifer near Pella.

Test-hole and observation-well drilling were used

to collect geologic information for the unconsolidated

material in the South Skunk River Valley. Test holes

were drilled to determine the thickness and lithology

of the alluvial deposits and to construct observation

wells for measuring water levels, determining

hydraulic characteristics, and collecting waterquality samples.

The alluvial deposits along the South Skunk River

Valley near Pella consist of sand, gravel, silt, and clay

deposits of glacial and fluvial origin. The sand and

gravel are commonly interbedded with silt and clay

lenses in the upper 20 ft of the deposits. Deeper materials appear to be primarily sand and gravel. Within the

Pella North and Pella East investigation areas, the alluvial aquifer is underlain by clay-rich glacial drift.

Thicknesses of the alluvial deposits (flood-plain clay

and silt plus alluvial aquifer) in the South Skunk River

alluvial valley in the study area range from about 60 to

more than 80 ft. Thickness of the alluvial aquifer

materials in the study area range from about 25 to

60 ft.

Water quality in the South Skunk River alluvi

aquifer was similar among the samples collected 

four observation wells in the investigation areas. S

ples from all observation wells had relatively low

concentrations of dissolved oxygen. Measured va

of specific conductance, pH, temperature, alkalini

bicarbonate, dissolved solids, and dissolved organ

carbon were similar for all samples as were conce

tions of calcium, magnesium, sodium, potassium, 

ride, bromide, and silica. Sulfate was present in w

from all sampled wells, ranging from 13 to 80 mg

Chloride concentrations ranged from 4.0 to 21 mg

Concentrations of nutrients, except orthophospho

were similar in all ground-water samples. Ammo

nitrogen concentrations ranged from 0.47 mg/L to

1.0 mg/L. Concentrations of nitrite nitrogen and n

plus nitrate were near or less than laboratory meth

detection limits (0.05 mg/L). Orthophosphorus co

centrations ranged from less than 0.02 to 0.16 mg

Iron concentrations ranged from 1,800 to 16,000 μ

the U.S. Environmental Protection Agency (USEP

has established a Secondary Maximum Contamin

Level (SMCL) of 300 μg/L for iron. Manganese c

centrations ranged from 220 to 940 μg/L (SMCL 

50 μg/L). Arsenic concentrations ranged from les

than 0.2 to 8 μg/L which is slightly less than USE

Maximum Contaminant Level (MCL) of 10 μg/L

The chemical quality of water from observati

wells installed in the South Skunk River alluvial a

fer near Pella is similar to water from municipal w

installed in the South Skunk River alluvium at New

and Oskaloosa, located north and south of Pella,

respectively. Analyses of ground-water samples f

these well fields show that concentrations of majo

ions, nutrients, and trace metals were very similar 

the water samples collected for this study. Measu

values of specific conductance, pH, temperature, 

solved oxygen, alkalinity, and dissolved solids als

were similar.

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Department of Natural Resources—Geological Survey

Bureau Open-File Report 87–3, 36 p.

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