U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 01–4056

Uranium and Radon in Ground Water 

in the Lower Illinois River Basin

National Water-Quality Assessment Program
Uranium and Radon in Ground Water 

in the Lower Illinois River Basin

By William S. Morrow

Urbana, Illinois

2001

Water-Resources Investigations Report 01–4056

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY
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. Government.

For additional information write to:

District Chief

U.S. Geological Survey

221 North Broadway Avenue

Urbana, IL 61801

Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286
FOREWORD

The U.S. Geological Survey (USGS) is committed 

to serve the Nation with accurate and timely scientific 

information that helps enhance and protect the overall 

quality of life, and facilitates effective management of 

water, biological, energy, and mineral resources. Information on the quality of the Nation’s water resources 

is of critical interest to the USGS because it is so 

integrally linked to the long-term availability of water 

that is clean and safe for drinking and recreation and 

that is suitable for industry, irrigation, and habitat for 

fish and wildlife. Escalating population growth and 

increasing demands for the multiple water uses make 

water availability, now measured in terms of quantity 

and quality, even more critical to the long-term 

sustainability of our communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program to support 

national, regional, and local information needs and 

decisions related to water-quality management and 

policy. Shaped by and coordinated with ongoing 

efforts of other Federal, State, and local agencies, the 

NAWQA Program is designed to answer: What is the 

condition of our Nation’s streams and ground water? 

How are the conditions changing over time? How do 

natural features and human activities affect the quality 

of streams and ground water, and where are those 

effects most pronounced? By combining information 

on water chemistry, physical characteristics, stream 

habitat, and aquatic life, the NAWQA Program aims 

to provide science-based insights for current and 

emerging water issues. NAWQA results can contribute 

to informed decisions that result in practical and effective water-resource management and strategies that 

protect and restore water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. Collectively, these 

Study Units account for more than 60 percent of the 

overall water use and population served by public 

water supply, and are representative of the Nation’s 

major hydrologic landscapes, priority ecological 

resources, and agricultural, urban, and natural sources 

of contamination. 

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The assessments thereby build local knowledge 

about water-quality issues and trends in a particular 

stream or aquifer while providing an understanding 

of how and why water quality varies regionally and 

nationally. The consistent, multi-scale approach helps 

to determine if certain types of water-quality issues are 

isolated or pervasive, and allows direct comparisons 

of how human activities and natural processes affect 

water quality and ecological health in the Nation’s 

diverse geographic and environmental settings. 

Comprehensive assessments on pesticides, nutrients, 

volatile organic compounds, trace metals, and aquatic 

ecology are developed at the national scale through 

comparative analysis of the Study-Unit findings.

The USGS places high value on the communication and dissemination of credible, timely, and relevant 

science so that the most recent and available knowledge about water resources can be applied in management and policy decisions. We hope this NAWQA 

publication will provide you the needed insights and 

information to meet your needs, and thereby foster 

increased awareness and involvement in the protection 

and restoration of our Nation’s waters. 

The NAWQA Program recognizes that a national 

assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for a fully integrated understanding of watersheds and for cost-effective 

management, regulation, and conservation of our 

Nation’s water resources. The Program, therefore, 

depends extensively on the advice, cooperation, and 

information from other Federal, State, interstate, 

Tribal, and local agencies, non-government organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch

Associate Director for Water
Contents V

CONTENTS

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

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

Purpose and Scope....................................................................................................................................................... 3

Description of Study Area ........................................................................................................................................... 3

Acknowledgments ....................................................................................................................................................... 3

Study Design and Sampling, and Analytical Methods .......................................................................................................... 3

Well Selection and Installation .................................................................................................................................... 6

Uranium and Radon Sampling and Analysis Methods................................................................................................ 7

Hydraulic Conductivity Analysis................................................................................................................................. 8

Quality-Assurance/Quality-Control Plan .................................................................................................................... 8

Statistical Analysis....................................................................................................................................................... 9

Uranium ................................................................................................................................................................................. 9

Geochemistry............................................................................................................................................................... 9

Sources of Uranium ..................................................................................................................................................... 10

Radon..................................................................................................................................................................................... 10

Decay Series ................................................................................................................................................................ 10

Sources of Radon......................................................................................................................................................... 12

Uranium Detections and Correlations ................................................................................................................................... 12

Mahomet Buried Bedrock Valley (MBBV) ................................................................................................................. 12

Galesburg/Springfield Plain Not Overlying a Buried Bedrock Valley (GSP N/O BV) ............................................... 12

Bloomington Ridged Plain Not Overlying a Buried Bedrock Valley (BRP N/O BV) ................................................ 17

Bloomington Ridged Plain Overlying the Mahomet Buried Bedrock Valley (BRP O/L MBBV) .............................. 17

Radon Detections and Correlations ....................................................................................................................................... 20

Mahomet Buried Bedrock Valley (MBBV) ................................................................................................................. 20

Galesburg/Springfield Plain Not Overlying a Buried Bedrock Valley (GSP N/O BV) ............................................... 20

Bloomington Ridged Plain Not Overlying a Buried Bedrock Valley (BRP N/O BV) ................................................ 22

Bloomington Ridged Plain Overlying the Mahomet Buried Bedrock Valley (BRP O/L MBBV) .............................. 22

Factors Affecting Uranium and Radon Distribution and Concentrations.............................................................................. 22

Type of Glacial Deposit ............................................................................................................................................... 22

Well Locations............................................................................................................................................................. 23

Sampling Method ........................................................................................................................................................ 23

Significance of Results ................................................................................................................................................ 26

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

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

FIGURES

1–3. Maps showing:

1. Location of the lower Illinois River Basin ......................................................................................................... 2

2. Physiography in the lower Illinois River Basin ................................................................................................. 4

3. Locations of aquifers and wells sampled in the lower Illinois River Basin....................................................... 5

4. Diagram showing radioactive decay series of uranium-238 with half-lives............................................................... 11

5–6. Maps showing:

5. Generalized geologic radon potential for indoor air concentrations of Illinois, based on permeability 

and uranium content of geologic deposits.......................................................................................................... 13

6. Locations of wells and ranges of uranium concentrations in ground water in the lower Illinois 

River Basin......................................................................................................................................................... 14
VI Contents

7–9. Graphs showing:

7. Concentration of uranium as a function of the concentration of (A) nickel, (B) sulfate, and (C) nitrogen 

as ammonia, and (D) barium in water from the Galesburg/Springfield Plain not overlying a 

buried bedrock valley (GSP N/O BV), lower Illinois River Basin, with Spearman Rank Correlation ............. 16

8. Concentration of uranium as a function of the concentration of (A) nickel, (B) tritium, and (C) sulfate 

in water from the Bloomington Ridged Plain not overlying a buried bedrock valley (BRP N/O BV), 

lower Illinois River Basin, with Spearman Rank Correlation ........................................................................... 18

9. Concentration of uranium as a function of the concentration of (A) nickel, (B) sulfate, and 

(C) manganese in water from the Bloomington Ridged Plain overlying the Mahomet Buried 

Bedrock Valley (BRP O/L MBBV), lower Illinois River Basin, with Spearman Rank Correlation ................. 19

10–11. Maps showing:

10. Locations of wells and ranges of radon concentrations in ground water in the lower Illinois 

River Basin ........................................................................................................................................................ 21

11. Locations of wells in the lower Illinois River Basin and Wisconsinan moraine locations................................ 24

TABLES

1. Relations of aquifers with glacial stage, physiographic area, well depth, and well type 

in the lower Illinois River Basin ............................................................................................................................. 6

2. Well dimensions for sampled wells in the lower Illinois River Basin .................................................................... 7

3. Sampled inorganic constituents, field parameters, and well characteristics correlated with uranium 

and radon using the Spearman Rank Correlation test............................................................................................. 8

4. Relations of uranium concentrations with dissolved oxygen in ground water....................................................... 10

5. Uranium concentrations, range, mean, and median, by aquifer ............................................................................. 15

6. Radon concentrations, range, mean, and median, by aquifer ................................................................................. 22
VII Contents

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

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.

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

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or

micrograms per liter (μg/L).

Abbreviations

AMCL Alternate Maximum Contaminant Level

BRP Bloomington Ridged Plain

BRP N/O BV Bloomington Ridged Plain not overlying a buried bedrock valley

BRP O/L MBBV Bloomington Ridged Plain overlying the Mahomet Buried Bedrock Valley

GSP N/O BV Galesburg/Springfield Plain not overlying a buried bedrock valley

MBBV Mahomet Buried Bedrock Valley

MCL Maximum Contaminant Level

ml milliliter

MRL Maximum Reporting Limit

Rn radon

μm micrometer

U uranium

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square foot (ft2) 0.09290 square meter

square mile (mi2) 2.590 square kilometer 

Volume

gallon (gal) 3.785 liter

Flow rate

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

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

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter 

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day
Introduction 1

Uranium and Radon in Ground Water 

in the Lower Illinois River Basin

By William S. Morrow

Abstract

Uranium and radon are present in ground 

water throughout the United States, along with 

other naturally occurring radionuclides. The occurrence and distribution of uranium and radon are of 

concern because these radionuclides are carcinogens that can be ingested through drinking water. 

As part of the U.S. Geological Survey (USGS) 

National Water-Quality Assessment (NAWQA) 

program, water samples were collected and 

analyzed for uranium and radon from 117 wells 

in four aquifers in the lower Illinois River Basin 

(LIRB) from 1996 to 1997. The aquifers were the 

shallow glacial drift deposits of the Bloomington 

Ridged Plain (BRP) not overlying a buried bedrock 

valley (BRP N/O BV), shallow glacial drift deposits of the BRP overlying the Mahomet Buried 

Bedrock Valley (BRP O/L MBBV), shallow 

glacial drift deposits of the Galesburg/Springfield 

Plain not overlying a buried bedrock valley (GSP 

N/O BV), and the deep glacial drift deposits of the 

Mahomet Buried Bedrock Valley (MBBV).

Uranium was detected in water samples 

from all aquifers except the MBBV and ranged 

in concentration from less than 1 microgram 

per liter (μg/L) to 17 μg/L. Uranium concentrations did not exceed 20 μg/L, the proposed 

U.S. Environmental Protection Agency (USEPA) 

Maximum Contaminant Level (MCL) at the time 

of sampling (1996–97). The current (2001) 

promulgated MCL is 30 μg/L (U.S. Environmental 

Protection Agency, 2000). The highest median 

uranium concentration (2.0 μg/L) among the four 

aquifers was in the BRP N/O BV. Uranium most 

often occurred in oxidizing and sulfate-rich water.

Radon was detected in water samples from 

all aquifers in the LIRB. Radon concentrations in 

all aquifers ranged from less than 80 picocuries 

per liter (pCi/L) to 1,300 pCi/L. Of 117 samples, 

radon concentrations exceeded 300 pCi/L (the 

proposed USEPA MCL) in 34 percent of the 

samples. Radon concentrations exceeded 

300 pCi/L in more than one-half of the samples 

from the GSP N/O BV and the BRP O/L MBBV. 

No sample exceeded the proposed Alternative 

Maximum Contaminant Level (AMCL) of 

4,000 pCi/L. Concentrations of uranium and 

radon were not correlated.

INTRODUCTION

In 1991, the U.S. Geological Survey (USGS) 

implemented the full-scale National Water-Quality 

Assessment (NAWQA) program to assess the quality 

of the ground- and surface-water resources of the 

United States, characterize the trends in the quality of 

these resources, and identify natural and anthropogenic 

factors that affect water quality. The lower Illinois 

River Basin (LIRB) (fig. 1) is one of more than 50 study 

units throughout the United States where water-quality 

data are being collected in the NAWQA program.

Uranium (U) and radon (Rn) are among 

naturally occurring radionuclides present in ground 

water throughout the United States (Szabo and 

Zapecza, 1987). The occurrence and distribution of 

U and Rn in ground water are of concern because these 

radionuclides are carcinogens that can be ingested 

through drinking water and because naturally 

occurring radionuclides, such as radium, have been 
2 Uranium and Radon in Ground Water in the Lower Illinois River Basin

ILLINOI

M

S

ISSOURI

I L L I

N

O I S

I O W

A

39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

Figure 1. Location of the lower Illinois River Basin.
Study Design and Sampling, and Analytical Methods 3

found at levels above drinking-water standards in 

Illinois (Kay, 1999). Uranium concentrations, however, 

are low in northern Illinois (Gilkeson and Cowart, 

1982). From 1996 to 1997, the USGS collected and 

analyzed samples from 117 wells in the LIRB to evaluate the distribution of U and Rn in ground water.

Purpose and Scope

This report presents the results of sampling and 

analysis of ground water for concentrations of U and 

Rn in the LIRB, 1996–97. Radon and U concentration 

data may be useful for designing (1) future monitoring 

in problem areas and (2) maps and programs for multimedia mitigation programs for Rn similar to the maps 

and programs suggested by dePaul and others (2000), 

as well as a U abatement program if necessary. Samples 

were collected from 117 randomly selected wells 

completed in four separate physiographic areas 

within the LIRB. For this study, radon-222 (222Rn) 

and uranium-238 (238U) were collected and analyzed. 

Relations among U and Rn with sampled inorganic 

chemical constituents and physical properties of the 

water were examined.

Description of Study Area

The LIRB covers an area of 18,000 mi2 in 

western and central Illinois and consists of the drainage 

area from Ottawa, Ill., to the confluence of the Illinois 

and Mississippi Rivers at Grafton, Ill. (fig. 1). The 

primary land use is agricultural—mostly corn and 

soybeans. The largest population centers in the basin 

are Peoria, Springfield, Bloomington-Normal, and 

Decatur. The major aquifers underlying the basin 

are composed of glacial deposits of Quaternary age 

and Pennsylvanian- to Mississippian-age bedrock. 

Some major Quaternary aquifers are present in buried 

bedrock valleys (Warner, 1998).

The LIRB is part of the Till Plains Section 

of the Central Lowland Province and the Ozark 

Plateaus Province. The main physiographic areas 

(about 98 percent) of the LIRB are the Bloomington 

Ridged Plain subsection, the Galesburg Plain subsection, and the Springfield Plain subsection; all are within 

the Till Plains Section. About 2 percent of the LIRB 

comprises the Kankakee Plain subsection (also within 

the Till Plains Section), the Lincoln Hills Section 

(Ozark Plateaus Province), and the Salem Plateau 

Section (Ozark Plateaus Province) (fig. 2) (Warner, 

1998).

The Bloomington Ridged Plain (BRP) is in the 

northeast region of the LIRB and is surficially covered 

by Wisconsinan glacial drift with prominent glacial 

moraines and topography. The Galesburg Plain and 

Springfield Plain are in the southwest region of the 

LIRB and are surficially covered with Illinoian glacial 

drift. Most Illinoian glacial features are subdued, 

although some localized glacial topography is evident 

(Willman and others, 1975). Buried bedrock valleys are 

underneath the overlying surficial glacial deposits in 

areas of these subsections. The four aquifers described 

in this report are within the Bloomington Ridged Plain 

and the Galesburg/Springfield Plain subsections in the 

LIRB (fig. 3, table 1).

Acknowledgments

Thanks are extended to the private homeowners 

and land managers in the LIRB who allowed the USGS 

to sample their supply wells or allowed access to install 

and sample monitoring wells. Thanks also are extended 

to USGS hydrologists Zoltan Szabo and Robert Kay for 

extensive colleague reviews and numerous constructive 

comments.

STUDY DESIGN AND SAMPLING, 

AND ANALYTICAL METHODS

Four aquifers were defined by depth and physiography. The well depth and physiographic relations 

are shown in table 1. The first defined aquifer is the 

shallow glacial drift in the Bloomington Ridged Plain 

not overlying a buried bedrock valley (BRP N/O BV), 

and is in the northeastern part of the basin. The 

BRP N/O BV consists of the shallowest waterproducing Wisconsinan-age deposits in the BRP that 

are not over buried bedrock valleys (fig. 3, table 1). 

This aquifer is of interest because many communities 

within the area have difficulty obtaining potable ground 

water and, commonly, are dependent on surface-water 

supply. These shallow deposits, used for primarily 

private domestic supply, may become more important 

as surface-water supplies approach maximum potential 

use. The BRP N/O BV underlies approximately 21 percent of the LIRB.

The second aquifer is the shallow glacial drift in 

the Bloomington Ridged Plain overlying the Mahomet 
4 Uranium and Radon in Ground Water in the Lower Illinois River Basin

39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

EXPLANATION

PHYSIOGRAPHY

TILL PLAINS SECTION

Bloomington Ridged Plain

 subsection

Galesburg Plain subsection

Springfield Plain subsection

Kankakee Plain subsection

LINCOLN HILLS SECTION

SALEM PLATEAU SECTION

BASIN BOUNDARY

Figure 2. Physiography in the lower Illinois River Basin.
Study Design and Sampling, and Analytical Methods 5

ILLINOI

M

S

ISSOURI

I L L I

N

O I S

I O W

A

39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

Figure 3. Locations of aquifers and wells sampled in the lower Illinois River Basin.
6 Uranium and Radon in Ground Water in the Lower Illinois River Basin

Buried Bedrock Valley (BRP O/L MBBV) (fig. 3, 

table 1). The BRP O/L MBBV is near the eastern edge 

of the basin. The aquifer consists of the shallowest 

water-producing Wisconsinan-age deposits in the BRP 

overlying the Mahomet Buried Bedrock Valley. This 

aquifer is of interest because shallow ground water may 

be recharging the deep glacial aquifer in the Mahomet 

Buried Bedrock Valley, a very productive and heavily 

used water supply. The BRP O/L MBBV underlies 

approximately 6 percent of the LIRB.

The third aquifer is the shallow glacial drift in 

the Galesburg/Springfield Plain not overlying a buried 

bedrock valley (GSP N/O BV). The GSP N/O BV is in 

the southwestern part of the basin. The GSP N/O BV 

consists of the shallowest water-producing Illinoianage deposits capable of domestic supply (fig. 3, 

table 1). This aquifer is of interest because many 

communities have difficulty obtaining potable ground 

water from the deeper bedrock; the water from the 

bedrock aquifers is commonly highly mineralized. 

The GSP N/O BV underlies approximately 42 percent 

of the LIRB.

The fourth aquifer is the deep glacial drift in 

the Mahomet Buried Bedrock Valley (MBBV) in 

the Bloomington Ridged Plain (fig. 3, table 1). The 

MBBV lies near the eastern edge of the LIRB and 

trends towards southwestern Tazewell County in the 

center of the LIRB, where the MBBV joins another 

buried bedrock valley beneath the Illinois River. 

The MBBV comprises primarily pre-Illinoian glacial 

deposits and is overlain by Wisconsinan glacial deposits at the surface. Confining clays and silts in the 

Wisconsinan, Illinoian, and pre-Illinoian drift overlying the deep glacial drift aquifer are commonly greater 

than 100 ft thick. The aquifer underlies approximately 

6 percent of the LIRB but is the largest source of ground 

water for irrigation, industrial, and municipal water 

supplies in east-central Illinois. At least 40 municipalities and water districts withdraw water from the MBBV 

(Kempton and others, 1991).

Well Selection and Installation

Samples were collected from 60 existing supply 

wells and 57 monitoring wells installed specifically 

for this study (fig. 3). In each aquifer, water from 28 to 

30 wells was sampled . All wells were in rural agricultural areas. Wells or well locations were selected for 

sampling on the basis of a computer-based random 

site-selection method developed by Scott (1990).

The supply wells selected for sampling were 

completed in glacial drift and constructed after 1980 to 

ensure that modern well-construction techniques were 

used. Of the 60 wells sampled, 59 were private-supply 

wells and 1 was a public-supply well. Water samples 

were obtained prior to passing through residential or 

public water-treatment systems to avoid any chemical 

modifications introduced by these systems.

Of the 60 existing supply wells, 30 were 200 ft or 

greater in depth and screened in the deep glacial drift 

aquifer. Wells selected were in the Mahomet Buried 

Bedrock Valley area where the top of the bedrock was 

below an altitude of 500 ft. Wells were completed in the 

sands and gravels of the glacial drift. Well depths 

ranged from 200 to 365 ft, with a median depth of 

256 ft. The well diameters ranged from 4 to 5 inches; 

80 percent were 5-inch-diameter wells. Well-screen 

lengths ranged from 4 to 12 ft, with a median screen 

length of 5.5 ft (table 2).

The remaining 30 supply wells were less than 

110 ft deep and screened in the shallow glacial drift 

Table 1. Relations of aquifers with glacial stage, physiographic area, well depth, and well type in the lower Illinois River Basin

Aquifer Aquifer

abbreviation Glacial stage Physiographic area

Range of

well depth,

in feet below

land surface

Well type

Bloomington Ridged Plain 

not overlying a buried 

bedrock valley

BRP N/O BV Wisconsinan Bloomington Ridged Plain 12–59 Monitoring

Bloomington Ridged Plain 

overlying the Mahomet 

Buried Bedrock Valley

BRP O/L MBBV Wisconsinan Bloomington Ridged Plain 13–59 Monitoring

Galesburg/Springfield Plain 

not overlying a buried 

bedrock valley

GSP N/O BV Illinoian Galesburg and Springfield Plains 30–108 Supply

Mahomet Buried Bedrock Valley MBBV Pre-Illinoian Bloomington Ridged Plain 200–365 Supply
Study Design and Sampling, and Analytical Methods 7

of the Galesburg/Springfield Plain, where no buried 

bedrock valleys were located. Wells were completed 

primarily in the sands and gravels, but also in siltysands of the glacial drift. Well depths ranged from 30 to 

108 ft, with a median value of 50 ft. Well diameters 

ranged from 4 to 36 inches. Seventeen wells were 36 

inches in diameter, and one well was 24 inches in diameter. The large-diameter (24 and 36 inches) wells were 

constructed because the water-producing units in this 

area are thin and poorly permeable. Well-screen lengths 

ranged from open ended casing to 78 ft, with a median 

screen length of 26 ft (table 2).

During the study, 57 monitoring wells were 

installed. Well locations were selected by use of 

the random site-selection program and the well sited 

within 0.5 mi of the selected point. Occasionally, going 

slightly beyond 0.5 mi was necessary if the local hydrogeologic conditions were consistent. All monitoring 

wells were constructed of 2-inch-diameter polyvinyl 

chloride (PVC) with 5-ft-long screens. Wells were 

constructed according to NAWQA protocols (Lapham 

and others, 1995) using the hollow-stem augering 

drilling method at the first water-producing unit and 

installed on private land and rural agricultural roadsides. Wells were completed in primarily sands and 

gravels but also were completed in less productive silts 

and clayey-silts of the glacial drift.

In the shallow glacial drift of the BRP N/O BV, 

29 monitoring wells were installed, with well depths 

ranging from 12 to 59 ft and a median well depth of 

26.5 ft, whereas 28 wells were installed in the shallow 

glacial drift of the BRP O/L MBBV, with well depths 

ranging from 13 to 59 ft and a median well depth of 

19.0 ft (table 2).

Uranium and Radon Sampling and Analysis 

Methods

Water samples for U and Rn analysis were 

collected after the field-measured parameters (pH, 

dissolved oxygen, water temperature, and turbidity) 

had stabilized to ensure that a representative sample 

from the aquifer was obtained. The readings usually 

stabilized after three well volumes were purged or, in 

the case of some large-diameter wells, approximately 

2 hours of purge time had elapsed. Where sampling 

wells by bailing was necessary, the wells also were 

purged by bailing. After sufficient water had recharged 

the well, samples were collected using primarily Teflon 

and stainless-steel sampling equipment. Samples also 

were analyzed for a comprehensive set of nutrient, trace 

element, and other inorganic constituents (table 3).

Samples for analysis of U were obtained by 

collecting 250 ml of water through a 0.45-μm highcapacity tortuous flow polysulfone capsule filter and 

preserved with 1 ml of nitric acid (HNO3). Samples 

were shipped to the USGS National Water-Quality 

Laboratory (NWQL) in Denver, Colo., and analyzed for 

238U (Natural U consists almost entirely of the 

238U isotope (Berlin and Rudell, 1986).) by inductively 

coupled plasma-mass spectrometry (ICPMS), with a 

minimum reporting level (MRL) of 1 μg/L (Faires, 

1993, Ivahnenko and others, 1996).

Water samples for Rn analysis consisted of 10 ml 

of water injected under a mineral-oil cocktail to eliminate contact with the atmosphere (Mullin and Wanty, 

1991). The 10-ml samples were obtained by collecting 

15 ml of water with a hypodermic syringe. To prevent 

degassing of Rn by aeration of the sample, the needle 

was inserted through a Teflon septum on the sampling 

line. After filling and purging the syringe twice, 15 ml 

of water was collected again and 5 ml was eliminated 

from the syringe. If the sampled well was not capable 

Table 2. Well dimensions for sampled wells in the lower Illinois River Basin

Aquifer Aquifer

abbreviation

Well depth,

in feet below

land surface

Median

well depth,

in feet

Well

diameter,

in inches

Well

screen

length,

in feet

Well

screen

median,

in feet

Bloomington Ridged Plain 

not overlying a buried bedrock valley

BRP N/O BV 12–59 26.5 2 5 5

Bloomington Ridged Plain 

overlying the Mahomet Buried Bedrock Valley

BRP O/L MBBV 13–59 19.0 2 5 5

Galesburg/Springfield Plain 

not overlying a buried bedrock valley

GSP N/O BV 30–108 50 4–36 0–78 26

Mahomet Buried Bedrock Valley MBBV 200–365 256 4–5 4–12 5.5
8 Uranium and Radon in Ground Water in the Lower Illinois River Basin

of being pumped at a rate less than or equal to recharge, 

the sample was collected using a Teflon bailer from the 

well by filling the bailer with water and inserting a 

hypodermic syringe into the lower quarter of the bailer 

to minimize aeration and atmospheric contact. The 

sample was then injected under a mineral-oil cocktail 

and shipped overnight to the NWQL. All Rn samples 

were analyzed for 222Rn using liquid scintillation 

methodology, with an MRL of 80 pCi/L (Prichard 

and Gessell, 1977, Mullin and Wanty, 1991). 

Hydraulic Conductivity Analysis

Hydraulic conductivity results were obtained 

by conducting slug tests on 42 of the 57 (74 percent) 

monitoring wells. A 3-ft long, 1-inch diameter PVC rod 

was inserted and removed from the water in the well 

casing to effect a water-level change; the change in 

water level was measured by use of a pressure transducer. Hydraulic conductivity values were calculated 

from water-level data change over time using the 

Bouwer and Rice (1976) method of analysis for 

unconfined aquifer conditions, and Cooper and others 

(1967) method of analysis for confined aquifers.

Quality-Assurance/Quality-Control Plan

A quality-assurance plan for samples analyzed 

for U was developed and consisted of 32 qualityassurance samples collected in the field, including 

17 field blanks, 3 equipment blanks, and 12 replicates. 

The quality-assurance plan for samples analyzed for 

Rn consisted entirely of 13 quality-assurance replicate 

samples collected in the field. Blanks were not collected because of the short half-life of Rn and its degassing properties, which leads to low, not high, bias.

No U was detected at an MRL of 0.2 μg/L in 

any of the 17 field blanks. No U was detected in the 

two bailer blank samples. No U concentration bias 

due to random sampling contamination is expected.

The range of the percent difference for the 

12 replicate sample pairs for U concentrations was 

from 0 to 20 percent. Median and mean percent 

differences were 0 and 2 percent, respectively. All 

but one pair had 0 percent differences between the 

pairs, with identical replicate sample concentrations 

ranging from less than 1 μg/L to 7 μg/L. The only 

pair that differed in U concentration had an environmental sample concentration of 2 μg/L, with a replicate 

concentration of 1.6 μg/L (a difference of 20 percent). 

The one replicate concentration pair collected by 

bailing had 0 percent difference. Precision of U 

concentrations, therefore, should not differ substantially between pumped or bailed samples.

The range of the percent difference for the 

13 replicate samples for Rn concentrations was 0 to 

17 percent. Median and mean percent differences 

were 4 and 5 percent, respectively. The one replicate 

concentration pair collected by bailing had a 4 percent 

difference. Precision of Rn concentrations did not differ 

substantially between pumped or bailed samples.

Table 3. Sampled inorganic constituents, field parameters, and 

well characteristics correlated with uranium and radon using the 

Spearman Rank Correlation test

Inorganic constituents Field parameters and 

well characteristics

Aluminum Alkalinity

Antimony Dissolved oxygen

Arsenic pH

Barium Specific conductivity

Beryllium Turbidity

Bromide Water temperature

Cadmium Well depth

Calcium Well diameter

Chloride

Chromium

Cobalt

Copper

Fluoride

Iron

Lead

Magnesium

Manganese

Molybdenum

Nickel

Nitrogen, ammonia

Nitrogen, ammonia + organic nitrogen

Nitrogen, nitrite

Nitrogen, nitrite + nitrate

Phosphate, ortho

Phosphorus

Potassium

Selenium

Silica

Silver

Sodium

Sulfate

Zinc
Uranium 9

Statistical Analysis

Although sample sizes ranged from 28 to 

30 samples for each aquifer, the Rn and U data were 

not normally distributed; therefore, nonparametric 

methods were used in the statistical analyses.

The nonparametric Spearman Rank method (rs) 

was used to calculate the coefficient of correlation 

between U and Rn with inorganic constituents, field 

parameters, and well characteristics (table 3). For the 

purposes of this report, the terms used to describe 

statistically significant correlations are strong (±0.50 to 

±1.00), moderate (±0.35 to ±0.49), and weak, or not 

correlating (less than ±0.35). Correlations are evaluated 

as to the degree of significance (p) and reported with the 

correlation value.

Relations between U and Rn with type of 

deposit, well location, and sampling method were 

evaluated using the nonparametric Mann-Whitney 

test with a null hypothesis of equal medians (Helsel 

and Hirsch, 1995).

URANIUM

Geochemistry

Uranium is present in ground water in three 

naturally occurring isotopes: 238U, uranium-234 

(

234U), and uranium-235 (235U). The predominant 

forms, in terms of activity, found in natural waters 

are 238U and 234U. Uranium-238 has a half-life of 

4.5 × 109

 years; U-234 has a half-life of 247,000 years. 

The 238U isotope constitutes the majority of the mass 

(99 percent) of natural U (Aieta and others, 1987), but, 

in general, a higher 234U to 238U activity ratio is 

found in ground water relative to the source material 

(Osmond and Cowant, 1976). The higher 234U to 238U 

activity ratio in ground water than in adjoining rock is 

hypothesized to be a result of 234U being ejected into 

solution from alpha (α) recoil from decay of 238U 

and thorium-234 (234Th) parents in the source rock 

(Osmond and Cowant, 1976; Aieta and others, 1987). 

Natural U also includes 235U, but it constitutes only 

0.7 percent by weight of natural U. Uranium-235 is 

important for use in nuclear reactors and nuclear 

weapons (Wrenn and others, 1987).

The solubility of U in ground water is affected 

by local oxidizing-reducing conditions. Uranium is 

most soluble in oxidizing conditions. Uranium in the 

+6 oxidation state becomes mobile and occurs in the 

form of uranyl (UO2

2+) complexes, or where bicarbonate is present (most ground water), as UO2(CO3)3

–4

complexes (Hsi and Langmuir, 1985). In reducing 

conditions, U in the +4 or +5 oxidation state is immobile. Urananite (UO2) is usually the end product of U 

precipitation. The precipitation of U where oxidationreduction potential greatly decreases over a short 

distance has resulted in the formation of economically 

important U-rich deposits (Hostetler and Garrels, 

1962).

Concentrations of dissolved oxygen were used 

in this study as an indication of the oxidizing-reducing 

conditions of the aquifers and as an indicator of the 

potential solubility of U. In northern New Jersey, where 

U concentrations greater than 20 μg/L in ground water 

were present, dissolved-oxygen concentrations were 

greater than 1 mg/L in all but one case. Uranium 

concentrations less than 1 μg/L occurred more 

frequently in ground water containing less than 1 mg/L 

of dissolved oxygen than in ground water containing 

greater than 1 mg/L of dissolved oxygen (Szabo and 

Zapecza, 1991). In the LIRB, U concentrations did 

not exceed 20 μg/L in any of the 117 samples; however, 

U concentrations equaling or exceeding 1 μg/L were 

more frequent (64 percent) where dissolved-oxygen 

concentrations were greater than 1 mg/L. Uranium 

concentrations less than 1 μg/L were more frequent 

(86 percent) where dissolved-oxygen concentrations 

were less than 1 mg/L. The highest U concentration 

detected in the LIRB, however, was in a sample with a 

dissolved-oxygen concentration less than 1 mg/L. The 

distribution of U relative to oxygen-rich and oxygenpoor waters is similar for the LIRB and other areas, 

such as northern New Jersey (Szabo and Zapecza, 

1991). The concentration range, median, and percentage of concentrations greater than 1 μg/L for oxygenrich and oxygen-poor water is shown in table 4.

The solubility of U in ground water is also 

affected by pH. The pH of all samples was between 

6.3 and 8.0, with 85 percent between 6.7 and 7.3. At 

these pH levels, uranyl ions form dissolved complexes, 

predominantly with carbonates and biophosphates. 

Adsorption of uranyl ions may occur on some aquifer 

materials in oxidizing conditions, especially iron 

hydroxides (Hsi and Langmuir, 1985), with adsorption 

strongest at near-neutral pH (7.0 pH) (Wanty and 

Schoen, 1991). Complexation can mobilize U even in 

the presence of adsorptive iron hydroxides (Hsi and 

Langmuir, 1985).
10 Uranium and Radon in Ground Water in the Lower Illinois River Basin

Sources of Uranium

Geologic sources for U in water for the four 

aquifers are the glacial drift deposits. For the wells 

completed in the deep aquifer, the bedrock surrounding 

the glacial deposits also may be a source. Gilkeson and 

Cowart (1982), in their assessment of U in water in 

northern Illinois, suggest that glacial sediments may 

be a significant source of U. The glacial deposits in 

the shallow glacial drift aquifers are Wisconsinan and 

Illinoian in age; source rocks may have originated 

locally or from the north/northeast. Origin of glacial 

deposits may be as far north as areas north of Lake 

Michigan and Lake Superior in Canada. Wisconsinan 

glacial deposits in the LIRB are from mainly the Lake 

Michigan lobe, which advanced from the Hudson 

Bay region. The tills are from primarily shales, sandstones, and carbonate rocks of southern Wisconsin, 

the western Michigan Basin, and the northern Illinois 

Basin (Schumann, 1993). Illinoian glacial deposits in 

central Illinois advanced mainly south-southwesterly 

from the Lake Michigan Basin. These Illinoian tills 

generally contain greater amounts of illite and dolomite 

(Killey, 1998; Matsch, 1976). Based on the occurrence 

of U in the tills, some glacial material containing U of 

Canadian origin (Lang and others, 1970) may be a 

partial source for the Wisconsinan and Illinoian glacial 

deposits. Gilkeson and Cowart (1982) also suggest that 

local black shale deposits ground up and incorporated 

into glacial debris could be a significant source of soluble U.

The age of glacial deposits in the MBBV is 

primarily pre-Illinoian. The source direction for these 

pre-Illinoian glacial deposits probably is from the 

northeast (Killey, 1998). Supply wells in the MBBV 

usually are screened to glacial drift deposits composed 

of sand from glaciofluvial sources. Sands contain abundant hornblende, garnet, and epidote composition 

similar to that of overlying tills (Kempton and others, 

1991).

The bedrock surrounding the glacial deposits 

may affect water quality of the MBBV. In the MBBV, 

the bedrock is primarily Pennsylvanian shale with some 

sandstone and limestone, mainly from the Mattoon, 

Bond, and Modesto Formations (Kempton and others, 

1991). The average U content for Pennsylvanian shales 

throughout Illinois is slightly more than 0.004 percent 

equivalent U. The Pennsylvanian shale in the basin has 

been found to contain U in the range from 0.000 to 

0.014 percent equivalent U (Ostrom and others, 1955). 

Gilkeson and Cowart (1982) suggest that the grinding 

of black shale to fine debris may liberate some U.

In the LIRB, U may be derived from phosphatebearing fertilizers used for agricultural purposes. 

Uranium can be present in many phosphate-bearing 

fertilizers and can contribute dissolved U through 

leaching (Zielinski and others, 1997). Uranium 

concentrations in fertilizers have been found to 

correlate positively with phosphorus compounds 

in the Mississippi River drainage system (Spalding 

and Sackett, 1972).

RADON

Decay Series

Radon is a radioactive gas that occurs naturally 

in ground water as an intermediate step in the decay of 

238U to lead-206 (206Pb). Uranium-238 decays through 

a number of steps to radium-226 (226Ra), which decays 

to 222Rn by releasing an alpha particle from its nucleus. 

Radon-222 decays through a number of radionuclides 

with short half-lives until stable 206Pb is formed (fig. 4). 

Radon is soluble in water, odorless, and tasteless. The 

only Rn isotope analyzed in this study was 222Rn. 

The other two isotopes of Rn, 220Rn and 219Rn, were 

not analyzed. Their occurrence in ground water is 

difficult to determine because their half-lives are short 

Table 4. Relations of uranium concentrations with dissolved oxygen in ground water

[μg/L, micrograms per liter]

Uranium concentration variables

Water with dissolved

 oxygen less than or equal to 

1 milligram per liter

Water with

dissolved oxygen

greater than

1 milligram per liter

Uranium concentration range, in μg/L 0–17 1–15

Uranium concentration median, in μg/L 0 1

Percentage of uranium concentrations equal or greater than 1 μg/L 14 64
Radon 11

Figure 4. Radioactive decay series of uranium-238 with half-lives.
12 Uranium and Radon in Ground Water in the Lower Illinois River Basin

(seconds), whereas 222Rn has a comparatively long 

half-life at 3.82 days.

Sources of Radon

The glacial drift immediately surrounding 

the well borehole is the primary source for Rn 

in ground-water samples. The Rn source is in the zone 

immediately surrounding the borehole because the 

slow traveltime of typical ground water, coupled with 

the relatively short half-life (3.82 days) of Rn, limits 

the distance that Rn can travel in ground water before 

decay (Hoehn and others, 1992). The glacial drift 

deposits in the LIRB may have originated from local 

sources or as distant as the original area of glacier 

formation, such as Canada. The origin of the deposits 

is partly dependent on the type of glacial deposits 

encountered: subglacial, end moraine, or other types 

of glacier deposition. Typical glacial sediments are 

rarely homogeneous; this heterogeneous composition, 

coupled with variable U concentrations, results in 

varying Rn concentrations.

In general, the geologic Rn potential for indoor 

air concentrations, based on the permeability and U 

content of geologic deposits for the LIRB, is “high”

(fig. 5) (Schumann, 1993). In areas of glacial deposits, 

soil permeability (and presumably porosity) typically 

is high, allowing the Rn to be mobile (Gundersen and 

others, 1992; Gundersen and Schumann, 1996). For Rn 

in water in porous media, however, high porosity tends 

to correspond to low Rn content (Wanty and others, 

1992). In the LIRB, the factors most likely to result 

in high Rn content in ground water are high U content 

of the sediment, or high Rn emanation coefficient, or 

both. Neither factor is well defined for these shallow 

sediments. Despite the heterogeneous nature of glacial 

deposits, the critical variables that affect Rn concentration in ground water, such as U content, porosity, and 

emanation coefficient (Wanty and others, 1992), are 

distributed fairly uniformly resulting in little variation 

in Rn content.

URANIUM DETECTIONS AND CORRELATIONS

Uranium was analyzed in water samples 

collected from 117 wells completed in four separate 

aquifers. Of the 117 wells, 22 were sampled by bailing 

and 95 were sampled by pumping. Of the 22 wells 

bailed, all were completed in the BRP N/O BV and 

the BRP O/L MBBV. Uranium concentrations ranged 

from less than 1 μg/L to 17 μg/L. The median and mean 

values were 0 and 1.4 μg/L, respectively. The maximum measured concentration of 17 μg/L did not 

exceed the USEPA proposed Maximum Contaminant 

Level (MCL) drinking-water standard at the time of 

sampling (1996–97) of 20 μg/L (U.S. Environmental 

Protection Agency, 1996). The current (as of December 

2000) promulgated MCL is 30 μg/L (U.S. Environmental Protection Agency, 2000). Locations of wells that 

were sampled for analysis of U and ranges of U concentrations are shown in fig. 6. Uranium distribution by 

aquifer is given in table 5.

Mahomet Buried Bedrock Valley (MBBV)

Uranium concentrations were less than 1 μg/L 

in the 30 samples from the MBBV. This lack of 

detection at a concentration of less than 1.0 μg/L is 

interpreted to be the result of reducing conditions in 

the aquifer. Under reducing conditions, U is precipitated in the immobile +4 valence state. The presence 

of reducing conditions in all sampled wells of the 

aquifer was inferred on the basis of dissolved-oxygen 

concentrations at less than 0.1 mg/L throughout the 

aquifer.

Uranium is likely present, but precipitated or 

sorbed onto the aquifer matrix, as indicated by the 

presence of Rn in all water samples from the MBBV. 

This presence implies that there are radionuclide 

parents (U and Rn) in the aquifer material in concentrations great enough to produce the measured concentrations of Rn.

Galesburg/Springfield Plain Not Overlying a 

Buried Bedrock Valley (GSP N/O BV)

Uranium was detected at concentrations greater 

than 1.0 μg/L in 10 of 30 (33 percent) samples collected 

from the GSP N/O BV. Concentrations ranged from 

less than 1.0 μg/L to 17 μg/L. Oxidizing conditions, 

which enable U to become mobile in the +6 valence 

state, generally were present in at least some parts of 

the GSP N/O BV as indicated by dissolved-oxygen 

concentrations. Dissolved-oxygen concentrations 

ranged from less than 0.1 to 6.8 mg/L. Dissolvedoxygen concentrations were greater than 0.1 mg/L in 

18 of 30 samples (60 percent) and were greater than 

1.0 mg/L in 15 of 30 samples (50 percent). The median 
Uranium Detections and Correlations 13

EXPLANATION

GEOLOGIC RADON POTENTIAL

LOW = Less than 2 picocuries per liter

MEDIUM = 2 to 4 picocuries per liter

HIGH = Greater than 4 picocuries per liter

Lower Illinois

River Basin

ILLINOIS ILLINOIS

N

0 40 80 120 160

0 25 50 75 100 Miles

Kilometers

Figure 5. Generalized geologic radon potential for indoor air concentrations of Illinois, 

based on permeability and uranium content of geologic deposits (modified from 

Schumann, 1993).
14 Uranium and Radon in Ground Water in the Lower Illinois River Basin

ILLINOI

M

S

ISSOURI

I L L I

N

O I S

I O W

A

39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

Figure 6. Locations of wells and ranges of uranium concentrations in ground water in the lower Illinois River Basin.
Uranium Detections and Correlations 15

dissolved-oxygen concentration was 1.4 mg/L and the 

mean was 2.0 mg/L.

Concentrations of U did not correlate significantly with those of Rn (rs = 0.31, p = 0.11). 

Uranium and Rn concentrations did not correlate 

above an rs of an absolute value of 0.28 (p = 0.16) in 

any of the other aquifers. This result is likely because 

of the different chemical behavior of these elements. 

The concentration and mobility of Rn is related to the 

concentration of U in the sediment, specifically the 

sources of U near the well screen. The solubility of Rn 

is not dependent upon the oxidation/reduction potential 

of the water, whereas the mobility of U is dependent on 

the oxidation/reduction potential and pH. Under similar 

hydrologic flow conditions and favorable oxidizing 

alkaline conditions, U can travel substantial distances 

relative to Rn because the half-life of U is so much 

longer than that of Rn. Thus, under oxidizing conditions where U is mobile, it is possible that U can be 

found at higher concentrations in ground water where 

the aquifer matrix has little U in the aquifer matrix than 

in areas where the water is reducing, even if plenty of U 

is in the aquifer matrix. The Rn concentrations, which 

are primarily proportional to the U content of the aquifer matrix, may be low in the areas of rapidly flowing 

oxidizing water because of the potential long-term 

leaching of U from the aquifer matrix, especially that 

in soluble grain coatings. Conversely, under reducing 

conditions, Rn may be present in high concentrations 

in an area of uraniferous deposits, whereas U would 

not be present in the water because U is immobilized, 

possibly as a precipitate in the aquifer matrix. Szabo 

and Zapecza (1991) noticed a lack of correlation 

between U and Rn concentrations in water from 

fractured rock aquifers in northern New Jersey. They 

further indicated that long distance transport of U 

occurs in oxidizing waters, resulting, on occasion, in 

elevated concentrations of U in areas with moderate or 

low U content in the aquifer matrix and relatively low 

Rn concentration in the water.

Uranium correlated strongly with sulfate and 

nickel (rs = 0.63 (p = 0.0002) and rs = 0.52 (p = 0.0036), 

respectively; see fig. 7). The correlation of sulfate 

with U is an indicator of oxidizing conditions where U 

is soluble. Sulfate is the stable form of dissolved sulfur 

in oxidizing water. In northern New Jersey, a similar 

strong correlation is noted between the concentrations 

of sulfate and U; Szabo and Zapecza (1991) concluded 

that both accumulated (increasing in concentration 

along the flow path) in oxidizing water.

The correlation of U and nickel concentrations 

could be an indication that the source rock is the same 

for both and potentially contains elevated concentrations of both. The southern Precambrian Canadian 

Shield area, the Superior and Grenville Provinces, 

are the probable origin of glacial debris transported 

from the north. Rocks in these areas contain U and 

nickel deposits (Lang and others, 1970). Glacial sediments derived from this source area, thus, may contain 

abundant U and nickel. Extremely high and variable 

concentrations of U in rock and concentrations of Rn 

in water have been documented in the southwestern 

portion of the Canadian Shield (Steck, 1987), the likely 

source of some glacial debris in the LIRB. Another 

possibility is that the oxidizing conditions are favorable 

for U and nickel to be in solution as these trace 

elements are among the most mobile of trace elements. 

The weak correlations of dissolved-oxygen concentrations with U (rs = 0.35, p = 0.059) and of nickel 

(rs = −0.22, p = 0.24) indicate the oxidizing geochemical conditions may not be the primary reason for the 

correlation between U and nickel concentrations. 

Also, possibly, the correlation between the concentrations of U and nickel is simply fortuitous with no 

Table 5. Uranium concentrations, range, mean, and median, by aquifer

[μg/L, micrograms per liter]

Aquifer Aquifer

abbreviation

Uranium

concentration

range

(μg/L)

Uranium

concentration,

median value

(μg/L)

Uranium

concentration,

mean value

(μg/L)

Bloomington Ridged Plain 

not overlying a buried bedrock valley

BRP N/O BV Less than 1.0–15 2.0 2.8

Bloomington Ridged Plain 

overlying the Mahomet Buried Bedrock Valley

BRP O/L MBBV Less than 1.0–12 1.0 1.6

Galesburg/Springfield Plain 

not overlying a buried bedrock valley

GSP N/O BV Less than 1.0–17 1.0 1.4

Mahomet Buried Bedrock Valley MBBV Less than 1.0 Less than 1.0 Less than 1.0
16 Uranium and Radon in Ground Water in the Lower Illinois River Basin

Figure 7. Concentration of uranium as a function of the concentration of (A) nickel, (B) sulfate, (C) nitrogen as ammonia, and (D) barium in water from the 

Galesburg/Springfield Plain not overlying a buried bedrock valley (GSP N/O BV), lower Illinois River Basin, with Spearman Rank Correlation.
Uranium Detections and Correlations 17

hydrologic or geochemical significance. Correlation 

between these two elements in ground water is not 

widely documented in the scientific literature.

Strong inverse correlations were noted among 

the concentrations of U with those of barium 

(rs = −0.50, p = 0.0054) and ammonia (rs = −0.50, 

p = 0.032). In general, in water when one of these 

constituents is present, U is absent (fig. 7). Ammonia 

is a reduced species of nitrogen that is most stable 

under reducing conditions, conditions that favor U 

precipitation. The divalent cations, barium and radium, 

are often released into solution under reducing conditions when the iron hydroxides, to which these cations 

are sorbed, dissolve (Szabo and Zapecza, 1991); again, 

these conditions are likely to immobilize U.

Concentrations of U do not correlate with 

those of dissolved phosphorus (rs = −0.08, p = 0.64). 

Phosphorus sorbs strongly to soil. Phosphorus was 

detected in 33 percent of the samples with a maximum 

concentration of 0.28 mg/L. The water-quality data do 

not indicate the source of dissolved U is leachate from 

anthropogenic phosphate fertilizer application.

Bloomington Ridged Plain Not Overlying a Buried 

Bedrock Valley (BRP N/O BV)

Uranium was present at concentrations greater 

than 1.0 μg/L in 18 of the 28 (64 percent) water samples 

collected from the BRP N/O BV. Concentrations 

ranged from less than 1.0 μg/L to 15 μg/L. The median 

value was 2 μg/L, and the mean was 2.8 μg/L.

Oxidizing conditions generally were present 

as indicated by dissolved-oxygen concentrations. 

Dissolved-oxygen concentrations ranged from 0.3 

to 5.9 mg/L. Dissolved-oxygen concentrations were 

greater than 0.1 mg/L in all samples (100 percent) 

and were greater than 1.0 mg/L in 19 of 27 samples 

(70 percent). The median and mean concentrations 

were 1.4 and 2.9 mg/L, respectively. The oxidizing 

conditions allow U to become mobile in the +6 valence 

state.

Uranium had a strong (rs = 0.73, p = < 0.0001) 

correlation with nickel (fig. 8). This correlation may be 

an indication of a similar source for the U and nickel as 

the GSP N/O BV source. The deposits containing U 

and nickel from the Superior and Grenville Provinces 

in the general pre-Cambrian Canadian Shield area 

(Lang and others, 1970) are considered a possible 

origin because the Wisconsinan glaciation came from 

a generally north/northeast direction. Local black 

shales ground up in the till may be another possible 

U-containing source. Often, black shales also contain 

high concentrations of trace elements (Leventhal and 

Hosterman, 1982), including U and nickel. Gilkeson 

and Cowart (1982) suggest the black shale component 

in glacial debris could be a source of dissolved U. 

Dissolved oxygen correlated weakly with U (rs = 0.26, 

p = 0.19), but dissolved oxygen did correlate strongly 

with nickel (rs = 0.63, p = 0.0004). This result indicates 

that oxidizing conditions may affect the occurrence and 

co-occurrence of nickel and U.

Although the correlation of U with sulfate was 

moderate (rs = 0.39, p = 0.038), a noticeable general 

trend of increasing concentrations of both is evident 

(fig. 8). Tritium had a strong (rs = 0.50, p = 0.0084) 

correlation with U (fig. 8). This correlation is assumed 

to indicate the proximity between oxidizing conditions 

near the land surface (favorable to dissolved U and 

sulfate) and young water with high tritium concentrations.

Concentrations of U do not correlate with those 

of dissolved phosphorus (rs = −0.20, p = 0.19). Phosphorus sorbs strongly to soil. Phosphorus was detected 

in 14 percent of the samples with a maximum concentration of 0.06 mg/L. The water-quality data do not 

indicate the source of dissolved U is leachate from 

anthropogenic phosphate fertilizer application.

Bloomington Ridged Plain Overlying the Mahomet 

Buried Bedrock Valley (BRP O/L MBBV)

Uranium was detected in the BRP O/L MBBV 

in 17 of the 29 (59 percent) samples collected. Concentrations ranged from less than 1.0 μg/L to 12 μg/L. 

The median and mean values were 1.0 and 1.6 μg/L, 

respectively.

Oxidizing conditions generally were present 

as indicated by dissolved-oxygen concentrations. 

Dissolved-oxygen concentrations ranged from 0.1 to 

7.4 mg/L. All but 1 of 29 samples analyzed (96 percent) 

had concentrations greater than 0.1 mg/L and greater 

than 1.0 mg/L in 23 of 29 (79 percent) samples, indicating oxidizing conditions that would allow U to be 

dissolved and mobile in the +6 valence state. The 

median and mean values were 3.1 and 3.2 mg/L, 

respectively.

Uranium shows no correlation with Rn 

(rs = 0.00, p = 0.99), nor with nickel (rs = 0.18, 

p = 0.36) (fig. 9). Neither concentrations of U nor 

nickel correlate with the concentrations of dissolved 
18 Uranium and Radon in Ground Water in the Lower Illinois River Basin

Figure 8. Concentration of uranium as a function of the concentration of (A) nickel, (B) tritium, and (C) sulfate in water from the Bloomington Ridged Plain not overlying a buried bedrock valley (BRP N/O BV), lower Illinois River Basin, with Spearman Rank Correlation.
Uranium Detections and Correlations 19 Figure 9. Concentration of uranium as a function of the concentration of (A) nickel, (B) sulfate, and (C) manganese in water from the Bloomington Ridged Plain overlying the Mahomet Buried Bedrock Valley (BRP O/L MBBV), lower Illinois River Basin, with Spearman Rank Correlation.
20 Uranium and Radon in Ground Water in the Lower Illinois River Basin

oxygen. Oxidizing conditions are prevalent throughout 

the BRP O/L MBBV, indicating no gradient from 

oxidizing to reducing conditions in this aquifer. The 

concentration of U does correlate strongly with that of 

sulfate (rs = 0.52, p = 0.0043) (fig. 9). This correlation 

is again indicative of the accumulation of U and sulfate 

with increasing mineralization of the water in oxidizing 

environments (see previous discussions and Szabo and 

Zapecza, 1991).

The absence of correlation of the concentrations 

of U and nickel may be the result of the different distribution of wells in the BRP O/L MBBV relative to the 

other glacial deposits of the LIRB. The majority of the 

sampled wells in the BRP O/L MBBV (24 of 29, or 

83 percent) were completed in off-moraine lobe 

deposits as opposed to the BRP N/O BV, where 13 of 

28 wells (46 percent) were completed in off-moraine 

lobe deposits. Glacial deposits from the moraine lobes 

may contain more sediment from northern (Grenville 

Province, Canadian Shield) areas that may contain U 

and nickel. The moraines contain supraglacial deposits 

with material from the accumulation zone that may 

have originated from northern areas. The glacial 

deposits off the moraine lobes may contain a greater 

proportion of local bedrock debris than on the moraine 

lobes. The absence of correlation between U and nickel 

concentrations are believed to reflect the absence of the 

potential source material for these trace elements, the 

U- and nickel-rich material from the Canadian Shield.

The concentrations of U also correlated strongly 

with that of manganese (rs =0.52, p = 0.0035) in the 

BRP O/L MBBV (fig. 9). Manganese is soluble in 

reducing environments (Hem, 1985) where U would 

tend to be insoluble. The aquifer, however, is oxidizing. 

Reduced forms of manganese may be stabilized in solution by organic carbon in oxidizing water (Hem, 1985). 

Manganese also can be solubilized in mildly acidic 

water. Manganese can be stable in solutions with less 

extreme geochemical conditions (in terms of reduction 

potential and acidity) than can iron. Thus, possibly, the 

correlation between U and manganese concentrations 

does not contradict known geochemical behavior of 

either U or manganese.

RADON DETECTIONS AND CORRELATIONS

Radon was analyzed in water samples collected 

from 117 wells completed in four separate aquifers. 

Of the 117 wells, 22 were sampled by bailing and 

95 were sampled by pumping. Of the 22 wells bailed, 

all were completed in the BRP N/O BV and the 

BRP O/L MBBV. Radon concentrations ranged from 

less than 80 pCi/L to 1,300 pCi/L. The median and 

mean values were 225 and 304 pCi/L, respectively. At 

the time of sampling (1996–97), an MCL of 300 pCi/L 

was proposed by the U.S. Environmental Protection 

Agency (USEPA) for the drinking-water standard. 

This proposal was rescinded by the USEPA on July 30, 

1997. The current proposed (as of October 1999) 

standard is 300 pCi/L for an MCL, with an additional 

Alternative Maximum Contaminant Level (AMCL) 

of 4,000 pCi/L if there is a developed multimedia 

mitigation (MMM) program. An MMM program 

also addresses the Rn risks in indoor air (U.S. Environmental Protection Agency, 1999). Concentrations of 

greater than 300 pCi/L were detected in 40 samples 

(34 percent); no samples had concentrations greater 

than 4,000 pCi/L. One of the most critical inputs to any 

MMM program is the development of regional maps 

delineating concentrations in ground water (dePaul and 

others, 2000). Locations of wells sampled for Rn and 

ranges of Rn concentrations are shown in figure 10. 

Radon distribution by aquifer is given in table 6.

Mahomet Buried Bedrock Valley (MBBV)

Radon was detected in all 30 samples from the 

MBBV. Radon concentrations ranged from 110 to 

730 pCi/L. Concentrations of greater than 300 pCi/L 

were detected in two samples (7 percent); no sample 

had concentrations greater than 1,000 or 4,000 pCi/L. 

The median and mean values were 160 and 192 pCi/L, 

respectively (table 6). Radon did not significantly 

correlate with any chemical constituent.

Galesburg/Springfield Plain Not Overlying a 

Buried Bedrock Valley (GSP N/O BV)

Radon was detected in 29 of 30 (97 percent) 

samples taken from wells in the GSP N/O BV. Radon 

concentrations ranged from less than 80 to 1,200 pCi/L. 

Concentrations greater than 300 pCi/L were detected 

in 16 of 30 (53 percent) samples, one sample had a 

concentration greater than 1,000 pCi/L, and no sample 

was greater than 4,000 pCi/L. The median and mean 

were 340 and 387 pCi/L, respectively (table 6). Again, 

concentrations of Rn did not significantly correlate 

with any chemical constituent.
Radon Detections and Correlations 21

ILLINOI

M

S

ISSOURI

I L L I

N

O I S

I O W

A

39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

Figure 10. Locations of wells and ranges of radon concentrations in ground water in the lower Illinois River Basin.
22 Uranium and Radon in Ground Water in the Lower Illinois River Basin

Bloomington Ridged Plain Not Overlying a Buried 

Bedrock Valley (BRP N/O BV)

Radon was detected in 25 of 28 (89 percent) 

samples taken from wells in the BRP N/O BV. Radon 

concentrations ranged from less than 80 to 680 pCi/L. 

Concentrations greater than 300 pCi/L were detected 

in 5 of 28 (18 percent) samples, and no sample was 

greater than 1,000 or 4,000 pCi/L. The median and 

mean were 215 and 251 pCi/L, respectively (table 6). 

Radon concentrations did not correlate significantly 

with any chemical constituent.

Bloomington Ridged Plain Overlying the Mahomet 

Buried Bedrock Valley (BRP O/L MBBV)

Radon was detected in 28 of 29 (97 percent) 

samples taken from wells in the BRP O/L MBBV. 

Radon concentrations ranged from less than 80 to 

1,300 pCi/L. Concentrations greater than 300 pCi/L 

were detected in 17 of 29 (59 percent) samples, one 

sample was greater than 1,000 pCi/L, and no sample 

was greater than 4,000 pCi/L. The median and mean 

were 350 and 387 pCi/L, respectively (table 6). 

Radon concentrations did not correlate significantly 

with any chemical constituent.

It is not surprising that most investigations 

find no relation between the concentrations of Rn 

and those of other constituents in water (Szabo and 

Zapecza, 1991; Brutsaert and others, 1981). Radon 

is chemically inert. Radon concentration in water is 

controlled by the solubility of Rn, the amount of 

U present in the aquifer, and an emanation or transfer 

factor from sediment to water (Wanty and others, 

1992). The geochemical environment does not affect 

Rn concentrations unless it affects the distribution 

of the U and radium precursor of the Rn. Possibly, 

the Ra-226 precursor of the Rn is mobilized (desorbed) 

in anoxic (reducing) environments, as documented 

in northern New Jersey (Szabo and Zapecza, 1991). 

Sorbed radium at mineral-grain surfaces generally 

contribute the most Rn to water (Tanner, 1964).

FACTORS AFFECTING URANIUM AND RADON 

DISTRIBUTION AND CONCENTRATIONS

Type of Glacial Deposit

The data sets of U concentrations from the 

Wisconsinan age BRP N/O BV and BRP O/L MBBV 

were combined (57 wells) and compared with U 

concentrations from the 30 wells sampled from the 

Illinoian age GSP N/O BV. The Mann-Whitney test 

indicated a statistically significant difference between 

the two groups (p = 0.013). This difference is an indication of different geochemistry of the deposits. The 

median and mean of the Wisconsinan (BRP N/O BV 

and BRP O/L MBBV) U concentration data sets 

(1 and 2.2 μg/L, respectively) are higher than that of 

the Illinoian (GSP N/O BV) U concentration data sets 

(less than 1.0 and 1.4 μg/L, respectively). The result 

is consistent with oxidizing geochemical conditions in 

the Wisconsinan glacial deposits and mixed oxidizing/

reducing conditions (40 percent of samples have less 

than or equal to 0.1 mg/L dissolved oxygen in the 

GSP N/O BV) in water from the Illinoian glacial 

deposits.

The combined Wisconsinan age BRP Rn 

concentration data set (median = 240 pCi/L) was 

compared with the Illinoian age GSP N/O BV Rn 

concentration data set (median = 340 pCi/L). The 

Mann-Whitney test (p = 0.15) indicated no significant 

difference at the 95th percent confidence level. If only 

Table 6. Radon concentrations, range, mean, and median, by aquifer

[pCi/L, picocuries per liter]

Aquifer Aquifer

abbreviation

Radon

concentration

range

(pCi/L)

Radon

concentration

median value

(pCi/L)

Radon

concentration

mean value

(pCi/L)

Bloomington Ridged Plain 

not overlying a buried bedrock valley

BRP N/O BV Less than 80–680 215 251

Bloomington Ridged Plain 

overlying the Mahomet Buried Bedrock Valley

BRP O/L MBBV Less than 80–1,300 350 387

Galesburg/Springfield Plain 

not overlying a buried bedrock valley

GSP N/O BV Less than 80–1,200 340 387

Mahomet Buried Bedrock Valley MBBV 110–730 160 192
Factors Affecting Uranium and Radon Distribution and Concentrations 23

the Rn concentration data set from the BRP N/O BV 

(median = 215 pCi/L) was used in the comparison, the 

Mann-Whitney test did indicate a statistically significant difference (p = 0.01) with the GSP N/O BV 

(median = 340 pCi/L).

The combined Wisconsinan age BRP Rn 

concentrations (median = 240 pCi/L) compared with 

the MBBV Rn concentrations (median = 160 pCi/L) 

were significantly different (p = 0.0007). The 

BRP O/L MBBV, which directly overlies the MBBV, 

has a median Rn concentration of 350 pCi/L, which is 

greater than the median Rn concentration of 160 pCi/L 

in the MBBV (p = <0.0001). The Illinoian age 

GSP N/O BV deposits also contained Rn concentrations (median = 340 pCi/L) that were significantly 

greater (p = < 0.0001) than those from the MBBV 

deposits (median = 160 pCi/L). These differences 

likely indicate the highest concentration of U-series 

radionuclides are in the shallow Wisconsinan and 

Illinoian sediments. The degree that the highest Rn 

in the shallowest youngest glacial deposits also is a 

function of the permeability and porosity of these 

deposits and high Rn emanation capacity is not clear.

The concentration of U in the water is a poor 

indicator of the U concentration of the glacial deposits 

surrounding the well screen because of the great mobility of U in oxidizing environments but immobility in 

reducing environments. The presence of high concentrations of U in the ground water may indicate the 

presence of abundant U concentration in the sediment 

but only under oxidizing conditions. Furthermore, U 

concentrations may be elevated in mineralized sulfaterich oxidizing water even if the sediment is not strongly 

enriched in U. Nevertheless, the presence of high U 

concentrations in shallow oxidizing water in Wisconsinan deposits indicates the geochemical solubility of 

U in this environment is likely a major factor in U 

distribution in the LIRB.

Well Locations

Examination of well locations in relation to 

glacial moraines indicated that 54 percent of the 

BRP N/O BV deposits were on a moraine, whereas 

only 17 percent of the BRP O/L MBBV deposits were 

on a moraine (fig. 11). Wells were grouped by their 

location relative to being on a moraine (ON) or off a 

moraine (OFF), and their relations were evaluated. Well 

depth means (ON = 30.1 ft, OFF = 22.5 ft) and medians 

(ON = 26.5 ft, OFF = 19.0 ft) differed significantly 

(p = 0.0066).

Uranium concentrations from the BRP N/O BV 

and the BRP O/L MBBV were not significantly different (p = 0.17) among water from wells, whether the 

wells were on or off moraines. Uranium concentrations 

show a strong correlation with those of nickel 

(rs = 0.72, p = 0.0003) for water from wells on a 

moraine, whereas for water from wells off a moraine, 

the correlation (rs = 0.36, p = 0.0312) is only moderate. 

This result indicates that the on-moraine materials 

deposited as an end moraine may contain U and nickel 

in more easily soluble forms from a different source 

than the off-moraine materials.

Radon concentrations in the BRP N/O BV and 

BRP O/L MBBV were significantly different between 

the ON/OFF data sets (p = 0.002). The median concentrations are ON = 160 pCi/L and OFF = 330 pCi/L. 

Either the U concentrations, physical properties associated with porosity, Rn emanation, or a combination of 

these factors are significantly different between the two 

deposit types.

Slug tests were conducted on 42 of the 57 

(74 percent) monitoring wells on and off moraines to 

estimate the hydraulic conductivity. Wells screened 

in locations off a moraine (31 wells) had an average 

hydraulic conductivity of 31 ft/d. Wells screened 

in locations on a moraine (11 wells) had an average 

hydraulic conductivity of 3.3 ft/d, approximately an 

order of magnitude lower than wells screened in 

locations off a moraine. This result indicates that the 

physical properties of the deposits are different. For 

example, poorly sorted but porous deposits could 

result in physical conditions that may result in lower 

Rn concentrations than other sediments with similar 

U concentrations.

Sampling Method

Differences in U and Rn concentrations between 

pumped and bailed samples were significant. Uranium 

samples collected by pumping had median and mean 

values of less than 1 μg/L and 1.6 μg/L, respectively, 

whereas U samples collected by bailing had median 

and mean values of 1.5 and 3.2 μg/L, respectively. The 

population concentrations differed significantly for U 

(p = 0.025). The wells with lower producing capacity 

may be completed in clays and silts that have high U 

content. As long as the water is oxidizing, the U can 

solubilize.
24 Uranium and Radon in Ground Water in the Lower Illinois River Basin

ILLINOI

M

S

ISSOURI

I L L I

N

O I S

I O W

A

39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

Figure 11. Locations of wells in the lower Illinois River Basin and Wisconsinan moraine locations.
Factors Affecting Uranium and Radon Distribution and Concentrations 25

Bailed samples can deviate from pumped 

samples in turbidity but also by altering the sampling 

zone through introduction of freshly exposed reactive 

surfaces by precipitation, particle detachment, or 

disaggregation. Turbid samples generally contain 

higher concentrations of trace elements than nonturbid 

samples (Gibs and others, 2000; Puls and Powell, 1992; 

Puls and others, 1992). The higher U concentrations in 

bailed as opposed to pumped samples may be an artifact of the bailing process.

Differences for Rn concentrations between 

pumped and bailed samples (p = 0.019) also were 

significant. Radon samples collected by pumping 

had median and mean values of 260 and 373 pCi/L, 

respectively; bailed Rn samples had median and 

mean values of 195 and 236 pCi/L, respectively. 

This result may be attributed to multiple causes. 

Most likely, differences caused by physical Rn 

loss is from aeration alteration. The low hydraulic 

conductivity that makes bailing necessary could 

generally result in increased Rn concentration, all 

other factors being equal, based on analogy with 

fractured rock (Rumbaugh, 1983; Folger and others, 

1996, Szabo and others, 1997). In porous media, the 

effect is not so clear cut. Impermeable sediments may 

be porous as well, a fact that may minimize, not maximize, Rn concentration in the water (Wanty and others, 

1992). Because the bailed samples contained the lowest 

Rn concentrations, Rn concentrations appear to be most 

affected by the physical processes involved with bailing. Agitation caused by collecting the Rn sample 

with a bailer increases the potential for Rn degassing. 

Furthermore, when samples were collected by bailing, 

recharge rates were insufficient to maintain the water 

level in the well. After the initial purge, water refilled 

the borehole during a lengthy interval. Possible cascading of recharging water with the well screen may 

have allowed degassing of Rn prior to sampling. The 

wells that can be pumped likely are screened in coarser 

material than that present in the wells that need to 

be bailed; differences in physical properties of the 

sediments may affect Rn emanation and porosity, 

both of which affect Rn concentration in the water.

Concentrations of U in samples from pumped 

wells indicated no statistical difference (p = 0.53) 

between the BRP N/O BV and the BRP O/L MBBV. 

Concentrations of U in samples from bailed wells, 

however, indicated a significant difference (p= 0.025) 

between the two aquifers. Uranium concentrations 

from the bailed wells of the BRP N/O BV had a higher 

median (4 μg/L) and mean (5.3 μg/L) U concentration 

than the U concentrations from the bailed wells of 

the BRP O/L MBBV median (1 μg/L) and mean 

(1.8 μg/L). This result may be an indication of differences in geologic material between the primarily 

“on-moraine” well locations of the BRP N/O BV 

and the primarily “off-moraine” well locations of the 

BRP O/L MBBV. The high U content of water from 

the lowest producing wells of the BRP N/O BV may 

indicate that the aquifer material surrounding the boreholes contain more uraniferous clay and silt than the 

possibly more porous and permeable aquifer material 

surrounding the boreholes of the more productive 

wells. Turbid samples contain more trace elements than 

nonturbid samples from the same well. Finer-grained 

deposits in the BRP N/O BV may be agitated into 

colloidal suspension by bailing that can pass through 

a 0.45-μm filter by bailing the wells, but pumping the 

wells with the low (approximately 0.1 gal/min) flow 

rate used may not agitate the samples enough to bring 

these colloids into suspension (Puls and other, 1992). 

The pumped wells also may be screened in a coarser 

aquifer material than in the wells that needed to be 

bailed. The differences in the physical properties of 

the sediments may affect Rn emanation and porosity 

(Wanty and others, 1992), and, thus, the concentration 

of Rn in these deposits.

The sample population Rn concentration 

medians for the BRP O/L MBBV collected by pumping 

(415 pCi/L) and bailing (250 pCi/L) were significantly 

greater than the BRP N/O BV pumped (230 pCi/L) and 

bailed (160 pCi/L) Rn concentration population 

medians. Bailed well water samples did not indicate 

significant differences in Rn concentration at the 

95th percent confidence level (p = 0.06) for samples 

from the BRP N/O BV and BRP O/L MBBV. Radon 

concentrations obtained by pumping did indicate a 

significant difference between the two aquifers 

(p = 0.02). If all Rn concentration data, regardless of 

sampling technique, are combined, a significant difference (p= 0.01) is exhibited between the concentrations 

from the BRP N/O BV and BRP O/L MBBV aquifers. 

Combining the Rn sample results obtained by pumping 

and bailing for comparison is based on the assumption 

that Rn is not affected by colloidal transport, which 

is true for Rn gas. Combining the Rn sample results 

also requires the assumption that any resulting changes 

from degassing that occur are not significantly different 

among the aquifers. If significant degassing has 

occurred, bailing has resulted in considerable low bias.
26 Uranium and Radon in Ground Water in the Lower Illinois River Basin

To study the differences in the factors that 

may affect the Rn concentrations is one way to 

assess the differences in Rn concentrations between 

the BRP O/L MBBV and the BRP N/O BV aquifers. 

Such data could include the U content of the aquifer 

materials (Sloto and Senior, 1998), areal gamma 

surveys (Gundersen and others, 1992; Gundersen 

and Schumann, 1996), gamma ray geophysical 

logging (Szabo and dePaul, 2000), determining 

indoor air concentrations (Steck, 1987), sediment 

mineralogy, and aquifer porosity. Some data may be 

available already and can be assembled onto regional 

maps for further evaluation.

Significance of Results

Radon exceeds the MCL of 300 pCi/L in more 

than one-half of the wells in the Illinoian deposits of 

the GSP N/O BV and the Wisconsinan deposits of the 

BRP O/L MBBV. This data indicates that public-supply 

wells in these deposits will likely be involved in an 

MMM program to achieve Rn risk reduction. Further 

investigation and monitoring is needed to better characterize the deposits.

Uranium does not exceed the current MCL 

of 30 μg/L in any well sample. Some oxidizing 

sulfate-rich water samples, however, have U 

concentrations of up to 17 μg/L, which may be of 

concern for some areas and may warrant further 

study for future water-supply development.

SUMMARY

The lower Illinois River Basin (LIRB) is one of 

more than 50 study units throughout the United States 

where water-quality data are being collected by the 

U.S. Geological Survey, National Water-Quality 

Assessment (NAWQA) program to assess water quality 

at a regional and national level. Uranium (U) and radon 

(Rn) are found in ground water throughout the United 

States, and the occurrence and distribution of these 

radionuclides are of concern because they are carcinogens that can be ingested through drinking water. Radionuclides are of special concern in Illinois because 

some, such as radium, have been detected at levels 

above drinking-water standards in Illinois.

Uranium and Rn were sampled in four aquifers 

defined by physiography and depth in the LIRB in 1996 

and 1997. The aquifers were the shallow glacial drift 

deposits of the Bloomington Ridged Plain (BRP) not 

overlying a buried bedrock valley (BRP N/O BV), the 

shallow glacial drift deposits of the BRP overlying the 

Mahomet Buried Bedrock Valley (BRP O/L MBBV), 

the shallow glacial drift deposits of the Galesburg/

Springfield Plain not overlying a buried bedrock valley 

(GSP N/O BV), and the deep glacial drift deposits of 

the Mahomet Buried Bedrock Valley (MBBV). A total 

of 117 wells were sampled: 57 monitoring wells 

installed for the study and 60 previously installed 

supply wells.

Uranium was detected in all aquifers except 

the deep glacial drift aquifer in the MBBV. Uranium 

concentrations for all aquifers ranged from less than 

1.0 μg/L to 17 μg/L. Maximum U concentrations did 

not exceed 30 μg/L, the proposed U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant 

Level (MCL). The absence of detectable U in water 

from the deep glacial drift is probably because reducing 

conditions were present as indicated by dissolvedoxygen concentrations. The presence of Rn indicates 

that radionuclides are present in the MBBV. Precipitated U may be present in the aquifer material.

Concentrations of U correlated strongly with 

those of sulfate in oxic environments. Generally, 

concentrations of U were highest in water where 

the water is oxidizing and the concentrations of U 

correlated with those of nickel. The source of these 

glacial deposits may have been U- and nickel-rich 

material to the north, such as the Grenville 

Province of the Canadian Shield.

Bailed well water samples have higher U concentrations than those samples collected by pumping from 

the same glacial deposit. Concentrations of U may be 

positively biased for samples collected by bailing from 

the lowest producing glacial deposits. Bailing tends to 

result in turbid samples; turbid water has been shown 

to have higher concentrations of trace elements than 

nonturbid samples.

Concentrations of Rn ranged from less than 80 

to 1,200 pCi/L. Of the 117 samples, 40 (34 percent) 

exceeded 300 pCi/L, the proposed USEPA MCL, 

but no sample exceeded the proposed Alternative 

Maximum Contaminant Level of 4,000 pCi/L. 

Concentrations of Rn were highest in the water from 

the Wisconsinan BRP O/L MBBV and Illinoian 

GSP N/O BV glacial deposits and were lowest in 

the water from the deposits in the MBBV. This 

distribution indicates that additional data needs 

necessary to develop a multimedia mitigation (MMM) 
References 27

program for Rn likely will center on the Wisconsinan 

and Illinoian glacial deposits.

Concentrations of U and Rn did not significantly 

correlate. This result is expected because of the different chemical properties and half-lives whereby, U, but 

not Rn, is available for long-distance transport.

Concentrations of U in water from the BRP N/O 

BV are highest because of the oxidizing geochemical 

environment. Concentrations of U in water from the 

MBBV are low because of the anoxic geochemical 

environment, but the corresponding low concentrations 

of Rn in these deposits indicate that these deposits may 

actually contain low concentrations of U relative to the 

younger glacial deposits.

The lowest individual Rn concentrations from 

any sample were in water from wells finished in 

low-permeability deposits in the BRP N/O BV and 

the BRP O/L MBBV where samples were bailed. 

The low concentrations probably were due to physical 

loss of Rn as a result of degassing because of bailing.

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Water-Resources Investigations Report 01–4056

Morrow—URANIUM AND RADON IN GROUND WATER IN THE LOWER ILLINOIS RIVER BASIN—U.S. Geological Survey 

U.S. GEOLOGICAL SURVEY

221 NORTH BROADWAY AVE.

URBANA, ILLINOIS 61801