Contribution from the National Water-Quality Assessment Program

U.S. Department of the Interior

U.S. Geological Survey

Analysis of Nutrients, Selected Inorganic 

Constituents, and Trace Elements in Water from 

Illinois Community-Supply Wells, 1984–91

Water-Resources Investigations Report 99–4152
Analysis of Nutrients, Selected Inorganic 

Constituents, and Trace Elements in Water from 

Illinois Community-Supply Wells, 1984–91

By Kelly L. Warner

Water-Resources Investigations Report 99–4152

Urbana, Illinois

2000
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to:

District Chief

U.S. Geological Survey

221 North Broadway Avenue

Urbana, Illinois 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 mission of the U.S. Geological Survey 

(USGS) is to assess the quantity and quality of the 

earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound 

decisions. Assessment of water-quality conditions and 

trends is an important part of this overall mission.

One of the greatest challenges faced by waterresources scientists is acquiring reliable information 

that will guide the use and protection of the Nation’s 

water resources. That challenge is being addressed by 

Federal, State, interstate, and local water-resource 

agencies and by many academic institutions. These 

organizations are collecting water-quality data for a 

host of purposes that include: compliance with permits 

and water-supply standards; development of remediation plans for specific contamination problems; operational decisions on industrial, wastewater, or watersupply facilities; and research on factors that affect 

water quality. An additional need for water-quality 

information is to provide a basis on which regionaland national-level policy decisions can be based. Wise 

decisions must be based on sound information. As a 

society we need to know whether certain types of 

water-quality problems are isolated or ubiquitous, 

whether there are significant differences in conditions 

among regions, whether the conditions are changing 

over time, and why these conditions change from 

place to place and over time. The information can be 

used to help determine the efficacy of existing waterquality policies and to help analysts determine the 

need for and likely consequences of new policies.

To address these needs, the U.S. Congress appropriated funds in 1986 for the USGS to begin a pilot program in seven project areas to develop and refine the 

National Water-Quality Assessment (NAWQA) Program. In 1991, the USGS began full implementation of 

the program. The NAWQA Program builds upon an 

existing base of water-quality studies of the USGS, as 

well as those of other Federal, State, and local agencies. 

The objectives of the NAWQA Program are to:

• Describe current water-quality conditions for a 

large part of the Nation’s freshwater streams, 

rivers, and aquifers.

• Describe how water quality is changing over 

time.

• Improve understanding of the primary natural 

and human factors that affect water-quality

conditions.

This information will help support the development 

and evaluation of management, regulatory, and monitoring decisions by other Federal, State, and local 

agencies to protect, use, and enhance water resources. 

The goals of the NAWQA Program are being 

achieved through ongoing and proposed investigations 

of 59 of the Nation’s most important river basins and 

aquifer systems, which are referred to as study units. 

These study units are distributed throughout the 

Nation and cover a diversity of hydrogeologic settings. 

More than two-thirds of the Nation’s freshwater use 

occurs within the 59 study units and more than twothirds of the people served by public water-supply 

systems live within their boundaries.

National synthesis of data analysis, based on 

aggregation of comparable information obtained from 

the study units, is a major component of the program. 

This effort focuses on selected water-quality topics 

using nationally consistent information. Comparative 

studies will explain differences and similarities in 

observed water-quality conditions among study areas 

and will identify changes and trends and their causes. 

The first topics addressed by the national synthesis are 

pesticides, nutrients, volatile organic compounds, and 

aquatic biology. Discussions on these and other waterquality topics will be published in periodic summaries 

of the quality of the Nation’s ground and surface water 

as the information becomes available.

This report is an element of the comprehensive 

body of information developed as part of the NAWQA 

Program. The program depends heavily on the advice, 

cooperation, and information from many Federal, 

State, interstate, Tribal, and local agencies and the 

public. The assistance and suggestions of all are 

greatly appreciated.

Robert M. Hirsch

Chief Hydrologist
Contents IV

CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . III

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

1. Introduction and Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Well Selection and Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Water Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4. Statistical Analyses of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

5. Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6. Nitrate and Nitrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

8. Sulfate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

9. Arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

10. Boron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

11. Chromium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

12. Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

13. Cyanide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

14. Iron. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

15. Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

16. Manganese . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

17. Mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

18. Selenium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

19. Silver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

20. Zinc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

21. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

22. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Contents V

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

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

°C = (°F - 32) / 1.8

Vertical datum: 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.

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

micrograms per liter (μg/L).

Abbreviations used in this report:

IEPA Illinois Environmental Protection Agency

LIRB lower Illinois River Basin

MCL Maximum Contaminant Level

MCLG Maximum Contaminant Level Goal

MRL method reporting limit

NAWQA National Water-Quality Assessment

SMCL Secondary Maximum Contaminant Level

USEPA United States Environmental Protection Agency

USGS United States Geological Survey

Multiply By To obtain

Length

foot (ft) 0.3048 meter

Area

square mile (mi2) 2.590 square kilometer 

Flow rate

million gallons per day (Mgal/d) 0.04381 cubic meter per second
Abstract 1

Analysis of Nutrients, Selected Inorganic Constituents, and 

Trace Elements in Water from Illinois Community-Supply 

Wells, 1984–91

By Kelly L. Warner

Abstract

The lower Illinois River Basin (LIRB) 

study unit is part of the National Water-Quality 

Assessment program that includes studies of most 

major aquifer systems in the United States. Retrospective water-quality data from communitysupply wells in the LIRB and in the rest of Illinois 

are grouped by aquifer and depth interval. Concentrations of selected chemical constituents in 

water samples from community-supply wells 

within the LIRB vary with aquifer and depth of 

well. Ranked data for 16 selected trace elements 

and nutrients are compared by aquifer, depth 

interval, and between the LIRB and the rest of 

Illinois using nonparametric statistical analyses. 

For all wells, median concentrations of nitrate and 

nitrite (as Nitrogen) are highest in water samples 

from the Quaternary aquifer at well depths less 

than 100 ft; ammonia concentrations (as Nitrogen), however, are highest in samples from well 

depths greater than 200 ft. Chloride and sulfate 

concentrations are higher in samples from the 

older bedrock aquifers. Arsenic, lead, sulfate, 

and zinc concentrations are appreciably different 

between samples from the LIRB and samples 

from the rest of Illinois for ground water from the 

Quaternary aquifer. Arsenic concentration is highest in the deep Quaternary aquifer. Chromium, 

cyanide, lead, and mercury are not frequently 

detected in water samples from communitysupply wells in Illinois.
2 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

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

full-scale National Water-Quality Assessment (NAWQA) program. 

The long-term goals of the NAWQA program are to describe the 

status of and trends in the quality of a large, representative part of 

the Nation’s surface- and ground-water resources and to identify 

the major natural and human factors that affect the quality of these 

resources. In addressing these goals, the program will produce a 

wealth of water-quality information that will be useful to policy 

makers and water managers at the National, State, and local levels.

The NAWQA program emphasis is on regional-scale 

water-quality issues. The program will not diminish the need 

for smaller-scale studies and monitoring currently designed 

and implemented by Federal, State, and local agencies to meet 

specific needs. The NAWQA program, however, will provide a 

large-scale framework for understanding the regional and national 

water-quality conditions that cannot be acquired from small-scale 

programs and studies.

Study-unit investigations of more than 50 hydrologic 

systems that include parts of most major river basins and aquifer 

systems throughout the Nation are the building blocks of the 

national assessment. The study units range in size from 1,000 

to more than 60,000 mi2

 and represent 60–70 percent of the 

Nation’s water use and population served by public water 

supplies. Twenty study-unit investigations were started in 1991, 

16 additional investigations were started in 1994, and 15 more 

began in 1997. The lower Illinois River Basin (LIRB) was selected 

by the USGS as 1 of 16 study units to begin investigation in 1994.

The LIRB study unit includes 18,000 mi2

 of central and 

western Illinois (fig. 1–1.). The LIRB extends from the downstream end of the 11,000 mi2 upper Illinois River Basin at Ottawa, 

Ill., to the confluence of the Illinois and Mississippi Rivers at 

Grafton, Ill. Major rivers in the LIRB include 240 river miles 

of the main stem of the Illinois River and the Vermilion (1,330 mi2 

drainage area), Mackinaw (1,140 mi2 drainage area), Spoon 

(1,860 mi2

 drainage area), Sangamon (5,420 mi2 drainage area), 

and La Moine (1,350 mi2 drainage area) Rivers.

The major aquifers in the LIRB are composed of rocks 

of Quaternary and Pennsylvanian-Mississippian age. Many of 

the major aquifers are sand and gravel, or till of Quaternary age 

(Quaternary aquifer) in buried bedrock valleys. Streams in central 

Illinois commonly are underlain by buried bedrock valleys, such as 

the lower Illinois buried bedrock valley (fig. 1–3.). The Quaternary 

aquifers supply about 90 percent of the ground water used for 

community supply.

The LIRB is mostly in the Till Plains Section of the 

Central Lowland physiographic province (Warner, 1998). The 

four subsections of the Till Plains are the Bloomington Ridged 

Plains, Galesburg, Springfield, and Kankakee (fig. 1–2.). The 

Galesburg and Springfield Plains are hydrogeologically similar. 

Both plains are covered by Illinoian glacial deposits. Buried 

bedrock valleys are confined to the Illinois River valley and 

some small tributaries. Most wells in these plains are less than 

150 ft deep, and the tills do not yield large amounts of water. The 

Bloomington Ridged Plain is covered by Wisconsinan glacial 

deposits. Glacial deposits in the Bloomington Ridged Plain are up 

to 500 ft thick in some areas. Three major buried bedrock valleys 

are filled with thick Wisconsinan and Illinoian deposits (fig. 1–3.). 

Wells in the areas of buried bedrock valleys average 200 to 300 ft 

in depth. The sequence of tills and sand lenses in the Bloomington 

Ridged Plain is thicker than in the other plains.

The author acknowledges the statistical support of David 

Frothingham and Cathy Nguyen and spatial data support of 

David Fazio, U. S. Geological Survey. The Illinois Environmental 

Protection Agency is acknowledged, specifically David McMillan, 

for help in collecting and reviewing the data.

1. INTRODUCTION AND ACKNOWLEDGMENT

THE LOWER ILLINOIS RIVER BASIN STUDY UNIT IS PART OF THE NATIONAL WATER-QUALITY ASSESSMENT 

PROGRAM

The National Water-Quality Assessment program includes studies of most major aquifer systems in the United States. 
39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

Figure 1–1. Location of the lower Illinois River Basin.

Figure 1–2. Physiography in the lower Illinois River Basin 

(modified from Willman and others, 1975).

Figure 1–3. Major bedrock valleys in the lower Illinois River 

Basin (modified from Herzog and others, 1994).

Introduction and Acknowledgment 3
4 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

From 1984 to 1991, the USGS, in cooperation with the 

Illinois Environmental Protection Agency (IEPA), collected 

untreated ground water from community-supply wells in Illinois. 

The IEPA laboratories analyzed all samples in accordance with 

U.S. Environmental Protection Agency (USEPA) standards for the 

Clean Water Act data collection (U.S. Environmental Protection 

Agency, 1986). The analysis of these data is the framework for the 

design of a basinwide water-quality monitoring network of the 

UIRB. The distribution of 16 inorganic constituents in ground 

water from community-supply wells were evaluated statistically. 

Results of the analysis of the first water sample collected at each 

well by the IEPA or USGS are stored in the USGS National Water 

Information System (NWIS) and used in this study. Inorganic constituents discussed in this report are ammonia, nitrate and nitrite, 

chloride, sulfate, arsenic, boron, chromium, copper, cyanide, iron, 

lead, manganese, mercury, selenium, silver, and zinc. All analyses 

were performed on whole water samples and analytical results are 

expressed as total concentrations. Some constituents have more 

than one method reporting limit (MRL). Wells are widely distributed in counties within the LIRB.

In 1984, the IEPA and USGS began sampling untreated 

and unfiltered water from community-supply wells. As part of a 

pilot network to assess ground-water quality in community-supply 

wells, the USGS sampled 100 wells quarterly from 1984–85. 

(Voelker, 1986). Approximately 2,000 additional wells were 

sampled by IEPA from 1985 to 1987 (Voelker, Oberg, and Grober, 

1988) in order of priority assigned by the Illinois State Water 

Survey (ISWS) (O’Hearn and Schock, 1984). The priority was 

based on potential yield of the aquifer and susceptibility of the 

aquifer to contamination. An additional 516 wells were sampled 

from 1988 to 1991.

Water-quality data from 2,616 community-supply wells 

sampled by IEPA and USGS prior to 1991 are stored in the USGS 

water-quality data base. Approximately 2,400 well logs had information on depth of well and aquifer. A subset of more than 600 of 

these wells are in the LIRB (fig. 2–1.) and 1,800 are outside the 

LIRB. The first sample from each well analyzed for at least 5 inorganic constituents was selected to represent the inorganic water 

quality of the aquifer at the well site. This representative sample 

was selected to minimize the bias associated with statistics for 

wells with multiple analyses. For the 16 constituents, the water 

quality within the LIRB is compared with the water quality of the 

rest of Illinois. Concentrations of inorganic constituents are summarized by aquifer and by seven arbitrarily defined intervals of 

depth.

Quality assurance included the IEPA Laboratory participation in the USGS’s analytical evaluation program. During the 

period 1984–91, there were changes in most of the laboratory 

methods that resulted in multiple MRL’s (table 2–1.). The IEPA 

Laboratory analyzed standard reference samples for trace constituents, major constituents, nutrients, and mercury. All analyses since 

1988 had standard deviations from 0 to 1.01 from a known concentration, which indicates satisfactory quality assurance for the laboratory. Quality assurance on the data-collection process changed 

during the period from 1984 to 1991. Prior to 1990, qualityassurance samples were collected at least once during a field trip 

but, after review of the analytical data, were not retained for future 

redundant analysis. The quality-assurance procedures used since 

1990 include the analyses of blind samples and trip blanks with no 

incidence of contamination. These quality-assurance data are 

being retained for future reference.

The wells are divided into six general aquifer groups on 

the basis of the age of the rocks in which wells are completed: 

undefined, Quaternary, Mississippian-Pennsylvanian, SilurianDevonian, Cambrian-Ordovician, and multiple aquifers 

(table 2–2.). Wells that are screened in more than one aquifer are 

included in the “multiple” group. Wells that have no geologic log 

are included in the “undefined” group. A similar age and material 

classification of aquifers was used by the ISWS for a ground-water 

quality summary of Illinois (Gibb and O’Hearn, 1980). The agerelated aquifer classification also has been used by the ISWS for 

water-use reports (Kirk, 1987) and the USGS (Avery, 1995).

In the tables of this report, the division of well samples 

into the Quaternary and “other” aquifers is included to indicate 

the differences between glacial drift (Quaternary) and bedrock 

aquifers. The Quaternary, in the tables, is identical to the Quaternary aquifer group previously defined. The previously defined 

Mississippian-Pennsylvanian, Silurian-Devonian, CambrianOrdovician, and multiple aquifer groups are combined and referred 

to as “other” aquifers—all wells that are known to tap bedrock 

aquifers.

Sample analyses are arbitrarily grouped into seven categories by well depth, representing 50-ft intervals to the depth of 

300 ft (table 2–3.) and a group of wells deeper than 300 ft. The 

total depth of wells was reported in driller’s logs and in ISGS 

reports. Although screened intervals of some wells differ from total 

depths, for statistical purposes, well depths are used to divide the 

samples into intervals. Approximately 62 percent of wells are less 

than 300 ft deep. The distribution and correlation of inorganic concentrations in ground water for depths less than 300 ft were given 

more emphasis in the analysis because of the vulnerability of 

shallow aquifers to contamination and a network design that 

emphasizes the water quality of shallow wells (O’Hearn and 

Schock, 1984). In tables, the depth-related groups are combined 

into only three groups—depths less than 50 ft, depths equal to or 

greater than 50 ft, and wells with no reported depth information.

2. WELL SELECTION AND CHARACTERISTICS

COMMUNITY-SUPPLY WELLS DEFINED BY SIX AQUIFER GROUPS AND SEVEN DEPTH INTERVALS

Water quality of community-supply wells within the lower Illinois River Basin is compared with water quality of community 

supplies for the rest of Illinois. 
2. Well Selection and Characteristics 5

Figure 2–1. Wells sampled from 1984 to 1991.

Table 2–1. The method reporting limits of 

selected water-quality constituents in the 

lower Illinois River Basin

[mg/L, milligrams per liter; μg/L, micrograms per 

liter; multiple reporting limits resulted from the use 

of different methods of analysis]

Water-quality

constituent

Method

reporting

limit

Units of

measure

Ammonia 0.10 mg/L

Arsenic 1.0 μg/L

Boron 50 μg/L

Chloride 1.0 mg/L

Chromium 5.0 μg/L

Copper 5.0 μg/L

Cyanide .005 mg/L

.010 mg/L

Iron 50 μg/L

Lead 5.0 μg/L

Manganese 5.0 μg/L

Mercury .01 μg/L

.005 μg/L

.10 μg/L

Nitrate and nitrite .10 mg/L

Selenium 1.0 μg/L

Silver 3.0 μg/L

Sulfate 10 mg/L

Zinc 50 μg/L

100 μg/L

Table 2–2. Aquifer groups discussed in this 

report

Aquifer

Undefined

Quaternary

Mississippian-Pennsylvanian

Silurian-Devonian

Cambrian-Ordovician

Multiple

Table 2–3. Depth intervals below land surface 

for ground-water data

[ft, foot]

Depth interval (ft)

Less than 50

50–99

100–149

150–199

200–249

250–300

Greater than 300
6 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

The largest freshwater withdrawals in Illinois, excluding 

the Chicago area, are in the LIRB. In 1988, the total freshwater 

use for the LIRB, including surface and ground water, was 

4,100 Mgal/d (Avery, 1995). Surface water is used mostly for 

thermoelectric power. Ground water is used mostly for community 

supply and irrigation (Avery, 1995). Other uses of ground water, 

in order of most to least use, include self-supplied industrial, 

self-supplied domestic, self-supplied livestock, and self-supplied 

commercial and mining. Avery (1995) defined these use categories 

and computed the water-use rates for each.

The highest ground-water use is in La Salle, Mason, 

Peoria, and Tazewell Counties (fig. 3–1.). Mason and Tazewell 

Counties have high ground-water use because of irrigation. 

The largest withdrawals in the State for irrigation are in Mason 

(100 Mgal/d) and Tazewell (29 Mgal/d) Counties. These counties 

irrigate crops of corn, soybean, and many specialty crops. Groundwater use in Peoria and La Salle Counties is mostly for community 

supply (60 percent) and commercial facilities (30 percent).

More than one-half of the drinking water in the LIRB is 

from ground water. In 1988, the total community-supply delivery 

was 190 Mgal/d, of which 108 Mgal/d or 57 percent was for 

domestic use. In addition to community supplies, 25 percent 

of the population in counties within the LIRB use self-supplied 

ground water. The Quaternary aquifer supplies 90 percent of the 

population served by community supply. In 1988, the communitysupply withdrawals, by county, in the LIRB were greatest for 

Peoria (19 Mgal/d), Tazewell (14 Mgal/d) and La Salle 

(11 Mgal/d) Counties (Avery, 1995). More than 5 Mgal/d of 

ground water is used for community supply in McLean and 

Scott Counties. A large amount of ground-water use for community supply in Champaign County is outside of the LIRB.

The 1986 water use for Standard Metropolitan Statistical 

Areas (SMSA), which include the largest cities in the LIRB, is 

summarized in Kirk (1987). Peoria and Normal use ground water 

for primary water supply. Decatur, Bloomington, and Springfield 

mostly use surface-water sources. Lake Decatur supplies the city 

of Decatur on the Sangamon River; Big Creek, Long Creek, 

Friends Creek, and Sand Creek are tributaries. Ground water from 

sand and gravel aquifers is pumped into Friends Creek to maintain 

the water supply in Lake Decatur.

3. WATER USE

GROUND WATER IS USED FOR DRINKING WATER AND IRRIGATION

More than one-half of the drinking water in the lower Illinois River Basin is from ground water with the largest amounts 

withdrawn in La Salle, Mason, Peoria, and Tazewell Counties.
3. Water Use 7

39 ̊

40 ̊

41 ̊

91 ̊ 90 ̊ 89 ̊ 88 ̊

Figure 3–1. Total ground-water use, by county, in the lower Illinois River Basin (weighted by percent area of each county within the 

lower Illinois River Basin).
8 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Censored data—those data below the MRL—comprise 

more than 50 percent of the observations for 10 of the16 constituents evaluated in this report. The lack of concentrations above the 

MRL makes it impossible to calculate a simple mean and standard 

deviation and determine the distribution of the data. Percentiles 

were used to characterize the distribution of the data. Parametric 

tests that use parameters such as the mean and standard deviation 

to summarize data and compute test statistics were avoided 

because a normal distribution of the data is assumed in these 

tests. Instead, values were ranked from lowest to highest for each 

constituent, and nonparametric tests were used on these rankings 

for the statistical analysis because nonparametric tests do not 

assume a normal distribution. When nearly 50 percent or more 

of the data are censored, even nonparametric tests cannot detect 

differences in central values (Helsel and Hirsch, 1992). If more 

than 80 percent of the values are below the detection limit, the 

statistics are not reliable, and method reporting limits are given.

The statistical analyses used in this study begin with the 

null hypothesis—that the data sets being compared are identical 

in the specified attribute, such as data distribution. A test statistic 

then is computed according to the equations of the particular test. 

Associated with each test statistic is a probability (p-value) of 

obtaining the computed test statistic or one even less likely by 

chance alone. If the p-value is less than the significance level 

chosen (p ≤ 0.05), then the null hypothesis is rejected, leading to 

the conclusion that there is at least one difference among two or 

more of the data sets; this difference is statistically significant 

because the probability of it occurring by chance alone is small. 

It is possible for a test to incorrectly indicate a difference when 

there is actually no difference among the data sets being compared. 

The significance level represents the risk level or the acceptable 

probability of making this mistake and is usually set at 0.05 (Helsel 

and Hirsch, 1992, p. 106). Thus, a significance level of 0.05 indicates a 5 percent chance that the conclusion, no difference between 

samples, will be erroneous.

The Wilcoxon rank-sum test, also known as the MannWhitney test, was used to compare concentrations between two 

independent data sets, such as samples from the Quaternary aquifer 

and those from other aquifers. The purpose of applying this test is 

usually to determine whether the two groups come from the same 

population, same median and other percentiles, or alternatively 

whether they differ only in location, central value, or median 

(Helsel and Hirsch, 1992). This test is the nonparametric analog 

to the 2-sample t-test and is 95 percent as powerful as the t-test; 

however, when the assumptions of the t-test are violated, as in the 

case of dissimilar variances, the Wilcoxon rank-sum test is much 

more powerful than the t-test (Zar, 1984).

Comparisons of concentrations among three or more 

independent data sets requires the Kruskal-Wallis test, which 

is in essence an analysis of variance (ANOVA) procedure applied 

on the ranking of the data. The Kruskal-Wallis test is 95 percent 

as effective as the parametric single-factor ANOVA test or 

more effective for data with dissimilar variances or non-normal 

distributions (Zar, 1984). The Kruskal-Wallis test assumes the null 

hypothesis—that all samples have identical distributions, for 

example, normal or lognormal. If the null hypothesis is rejected, 

then at least one sample differs from the rest. In practice, the 

Kruskal-Wallis test usually is performed for a more specific 

purpose—to determine whether all groups have the same median 

or whether at least one median is different. This form requires that 

all other characteristics of the data distributions, such as spread or 

skewness, be identical, although not necessarily in the original 

units (Helsel and Hirsch, 1992).

Spearman’s rank correlation procedure was used to identify 

monotonic (but not necessarily linear) correlations among constituents, providing a measure of the intensity of association between 

two variables. Spearman’s correlation coefficient (r) is the linear 

correlation coefficient computed on the ranks of data instead of 

actual values. This coefficient ranges from −1 to +1; a negative 

coefficient indicates that the higher ranks of one variable are 

related to the lower ranks of the other variable. The closer the absolute value of r is to 1, the greater is the correlation between the two 

variables. A small r value, however, can still be significant, 

depending on the associated p-value. If this p-value is less than the 

significance level (p=0.05), then the null hypothesis of no correlation (or r=0) is rejected, and the correlation coefficient computed is 

considered statistically significant. Spearman’s rank correlation is 

91 percent as effective as the parametric form (Zar, 1984).

In ranking the data, values below the same MRL were 

given averaged, equivalent ranks; for example, five observations 

below the MRL would each have a rank of three (the average of 

ranks one to five), whereas the next observation lying above the 

MRL would be ranked sixth. For constituents with multiple 

MRL’s, censored data with lower MRL’s had lower ranks than 

those with higher MRL’s. Other studies have ranked all censored 

data for a constituent equally, using the maximum MRL (Helsel 

and Hirsch, 1992).

In this study, percentiles (when falling between values) 

were interpolated and calculated by the following equation from 

Helsel and Hirsch (1992, p. 451):

,

where

n is the number of observations of Xi

, and

j is the fraction of data less than or equal 

to the percentile value Pj.

For example, the second observation in a data set of seven points 

would constitute the 25th percentile, the value not exceeded by 

25 percent of the data, because (7+1)∗0.25=2.

SAS, a commercial statistical software package, was used 

for statistical computations in this study. Because censored values 

Pj X( ) n + 1 = • j

4. STATISTICAL ANALYSES OF DATA

STATISTICAL APPROACH USED FOR DATA ANALYSIS

Statistical analysis of each constituent was done by nonparametric tests on ranked data and using exploratory analysis.
4. Statistical Analyses of Data 9

Figure 4–1. General boxplot and definitions of features.

Figure 4–2. General dot chart for ground-water-quality data.

Data value(s) exceeding upper quartile plus 1.5 times the 

interquartile range but less than the upper quartile plus 3 

times the interquartile range

Largest data values less than or equal to the upper quartile 

plus 1.5 times the interquartile range

Upper quartile (75th percentile)

Median (50th percentile)

Lower quartile (25th percentile)

Smallest data value greater than or equal to the lower 

quartile minus 1.5 times the interquartile range

Data value(s) exceeding upper quartile plus 3 times the 

interquartile range

EXPLANATION

GROUP OF VALUES WITH THE 

SAME CONCENTRATIONS FOR THE 

AREA OF ILLINOIS THAT IS NOT IN 

THE LOWER ILLINOIS RIVER BASIN

GROUP OF VALUES WITH THE 

SAME CONCENTRATIONS FOR THE 

LOWER ILLINOIS RIVER BASIN

CONCENTRATION

GROUP

123 0

5

10
10 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

make up at least 15 percent of the data for 15 of the 16 constituents 

examined, the program mdl3.exe (written by Dennis Helsel of the 

USGS) was used to estimate the mean and standard deviation from 

log-probability regression. The 25th, 50th, and 75th percentiles 

were estimated from the lognormal maximum likelihood estimate 

(MLE) technique (Helsel and Hirsch, 1992). In this report, the 

MLE generated percentiles are given in statistics tables for each 

constituent. Probability plot procedures are used in this technique 

to fit a lognormal distribution to the values above the MRL. Values 

below the MRL are then extrapolated to estimate the summary 

statistics. If extrapolated values were more than one order of 

magnitude below the MRL, then a value of less than the MRL 

was substituted in the percentiles. If more than 80 percent of the 

analyses were below the detection limit, then the MLE technique is 

not reliable and the MRL is given for the percentiles. For example, 

55 percent of the arsenic values for samples from the Quaternary 

aquifer were above the MRL, so the MLE technique was used to 

determine percentiles. Only 13 percent of the arsenic values for 

samples from other aquifers were above the MRL; therefore, the 

MRL was substituted for the percentiles.

Simple substitution methods, such as substituting the MRL 

or one-half its value for measurements below the MRL, were not 

used in estimating the mean because studies have shown that such 

methods performed poorly in comparison with other methods. The 

results of substitution methods also are biased—either low or high 

depending on the substitution values, which are arbitrary (Helsel 

and Hirsch, 1992).

Boxplots and dot charts are graphical presentations of the 

data. The boxplots represent the rank-sum test results and show 

the 25th, 50th, and 75th percentiles as calulated using robust 

log-probability regression (fig. 4–1.). Boxplot percentiles (logprobability regression) may be slightly different than percentiles 

(MLE) given in the statistics tables in this report because of differences in methods of interpolation. Side-by-side boxplots are 

convenient for determining differences in medians and similarity 

in spreads. Modified dot charts are high precision bar charts that 

show position along an aligned (common datum) scale (Helsel 

and Hirsch, 1992). Each dot symbol represents a group of data at 

that value. Therefore, one dot symbol may represent multiple 

analytical values. Because of the number of censored values, only 

noncensored data are displayed on the dot charts.The dot charts 

were paired to conveniently compare LIRB results with the rest 

of Illinois results (fig. 4–2.).

High concentrations of constituents were plotted on maps 

of the LIRB. High concentrations are defined as either exceeding 

the USEPA Maximum Contaminant Level (MCL), Maximum 

Contaminant Level Goal (MCLG), or Secondary Maximum 

Contaminant Level (SMCL); or IEPA Class I or Class II groundwater standards (table 4–1.).

4. STATISTICAL ANALYSES OF DATA—Continued

STATISTICAL APPROACH USED FOR DATA ANALYSIS

Statistical analysis of each constituent was done by nonparametric tests on ranked data and using exploratory analysis.
4. Statistical Analyses of Data 11

Table 4–1. Selected Federal and Illinois ground-water-quality standards and maximum contaminant levels

[USEPA, U.S. Environmental Protection Agency; IEPA, Illinois Environmental Protection Agency; mg/L, milligrams per liter; –, no standard; 

p, proposed standard; t, regulated at the tap]

Water-quality

constituent

USEPA maximum

contaminant level

(mg/L)

USEPA maximum

contaminant

level goal

(mg/L)

USEPA secondary

maximum

contaminant level

(mg/L)

IEPA class I

ground-water

standard

(mg/L)

IEPA class II

ground-water

standard

(mg/L)

Ammonia – – –– –

Nitrate and 

nitrite

10 10 –– –

Chloride – – 250 – –

Sulfate 500p 500p 250 400 400

Arsenic .05 – – .05 .2

Boron ––– 2 2

Chromium .1 .1 – .1 1

Copper – 1.3t 1 .65 .65

Cyanide .2 .2 – .2 .6

Iron – – .3 5 5

Lead .015t 1.3t – .0075 .1

Manganese – – .05 .15 10

Mercury .002 .002 – .002 .01

Selenium .05 .05 – .05 .05

Silver – – .1 – –

Zinc – – 5 5 10
12 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Ammonia is a nonpersistent, noncumulative toxic 

substance. Ammonia is toxic to aquatic organisms, however, 

because it reduces the oxygen-carrying capacity of the blood 

(McNeely, Neimanis, and Dwyer, 1979). Ammonia is not regulated 

by the Safe Drinking Water Act. In Illinois, the maximum total 

ammonia concentration allowable for general-use surface waters 

(those waters for which there is no specific designation) depends 

on water temperature and pH but at no time may exceed 15 mg/L 

(Illinois Environmental Protection Agency, 1990). Although there 

is no ground-water standard for ammonia, 15 mg/L is considered a 

high concentration for this report. Only 4 samples of 609 collected 

in the LIRB for this study had concentrations above 15 mg/L 

(fig. 5–1.). In the rest of Illinois, 9 samples of the 1,891 collected 

had ammonia concentrations over 15 mg/L.

In most natural waters, ammonia is present primarily in 

its ionized form (NH4+) with a very small percentage of neutral 

ammonia (NH3); the sum of both concentrations constitutes total 

ammonia. A high percentage of the nitrogen in fertilizers applied 

in Illinois occurs as (or converts to) NH4+, which sorbs strongly 

to mineral surfaces and organic matter rather than leaching 

(University of Illinois, 1994). Bacteria and fungi contribute 

ammonia to the soil by converting the nitrogen in organic matter 

to ammonia (ammonification). Other natural and anthropogenic 

sources of ammonia include the hydrolysis of urea, microbial 

reduction of nitrite or nitrate under anaerobic conditions and 

biological fixation of atmospheric nitrogen (N2), precipitation, 

sewage effluent, paper mills, metal refineries, and a variety of 

industrial and cleaning operations (Canadian Council of Resource 

and Environment Ministers, 1992). Ammonia may be lost from the 

soil by plant uptake, erosion, volatilization, or microbial conversion to nitrate (nitrification).

In the LIRB, the concentrations of ammonia in 73 percent 

of the analyses were above the MRL (0.10 mg/L), and the range 

of concentrations was from less than 0.10 to 20 mg/L (table 5–1.). 

There is a significant difference in ammonia concentration by 

aquifer and depth. Nonparametric statistical techniques indicated 

a significant difference among aquifer groups (fig. 5–2.). The 

other aquifers had higher median concentrations (1.1 mg/L) and 

concentrations at the 25th percentile (0.80 mg/L) than did the 

Quaternary aquifer (0.49 and 0.08 mg/L, respectively).

Ninety-seven percent of ammonia values for samples from 

the other aquifers were above the MRL (0.10 mg/L) compared 

with 69 percent of the ammonia values for the Quaternary aquifer. 

Maximum concentrations in ground water decrease with increasing aquifer age; the highest values and greatest variability are in 

the Quaternary aquifer. Other aquifers tend to lie deeper, where 

there are few or no plant roots to remove ammonia and where 

possibly low oxygen levels limit nitrification. There is a significant 

difference (p ≤ 0.05) in ammonia concentrations by depth interval. 

The samples from wells deeper than 300 ft had more detections of 

ammonia, but the wells from 200 to 300 ft had the highest median 

concentrations (0.79 to 0.95 mg/L). The highest ammonia concentrations in the LIRB samples (based on the mean, and the 75th and 

90th percentiles) were from the eastern part of the study area in 

proximity to the Mahomet buried bedrock valley (fig. 1–3.). Within 

the LIRB, ammonia is correlated with arsenic, boron, fluoride, 

iron, and nitrate and nitrite and has a correlation coefficient greater 

than 0.50 and a significance level of p ≤ 0.05.

Comparison of ammonia concentrations in ground water in 

the LIRB with ammonia concentrations in ground water outside 

the LIRB shows some differences. Ammonia concentrations are 

higher in the other aquifers of the LIRB than in corresponding 

aquifers outside the LIRB, as suggested by higher values for the 

mean; median; and 25th, 75th, and 90th percentiles in the LIRB. 

Nonparametric techniques indicated a significant difference 

between the LIRB and the rest of Illinois for aquifers older than the 

Quaternary. Ammonia concentrations in water from the Quaternary 

aquifers are not statistically different between the LIRB and the 

rest of Illinois. Median concentrations and percentage of samples 

above the MRL, including all samples, are similar for the LIRB 

and the rest of Illinois. In the area outside of the LIRB, ammonia 

and boron is the only correlation greater than 0.50 and has a significance level of p ≤ 0.05.

5. AMMONIA

AMMONIA CONCENTRATIONS INCREASE WITH DEPTH AND AQUIFER

Ammonia concentrations differ by aquifer and depth within the lower Illinois River Basin. There is significant difference in 

the concentrations of ammonia in the Quaternary aquifer between the lower Illinois River Basin and the rest of Illinois.
5. Ammonia 13

Figure 5–1. Ammonia concentrations in ground water above 

the reporting limit for Quaternary or other aquifers, and 

concentrations in undefined aquifers in the lower Illinois 

River Basin.

Figure 5–2. Distribution of ammonia concentrations in ground 

water by aquifer group and depth interval. 

Table 5–1. Summary statistics for ammonia within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above the 

method reporting limit; percentiles and maximums are in milligrams per 

liter; other aquifers include wells open to multiple aquifers and bedrock 

aquifers]

Pecentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 609 443 0.12 0.74 1.7 20

Quaternary aquifers 472 324 .08 .49 2.1 20

Other aquifers 62 60 .80 1.1 1.6 5.6

Undefined aquifers 75 59 .17 1.1 1.5 8.2

Depths less than 50 feet 73 32 .02 .08 .37 5.2

Depths 50 feet 

or greater 527 404 .12 .83 2.1 20

Unreported depths 9 7 .30 2.1 4.1 8.2

Rest of Illinois

Overall 1,885 1,406 0.12 0.41 0.85 60

Quaternary aquifers 543 360 .07 .44 1.6 60

Other aquifers 910 746 .18 .43 .66 11

Undefined aquifers 432 300 .09 .32 .70 23

Depths less than 50 feet 79 35 .02 .08 .48 9.1

Depths 50 feet 

or greater 1,772 1,350 .11 .42 .88 60

Unreported depths 34 21 .06 .26 .73 3.0

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

AQUIFER GROUP

0

70

10

30

50

AMMONIA CONCENTRATIONS,

IN MILLIGRAMS PER LITER AS NITROGEN

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

0

70

10

30

50

AMMONIA CONCENTRATIONS, 

IN MILLIGRAMS PER LITER AS NITROGEN

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN
14 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

High levels of nitrate (NO3

-

) or nitrite (NO2

-

) in drinking 

water can reduce the oxygen carrying capacity of blood and lead 

to anoxia or death, particularly in infants (Canadian Council of 

Resource and Environment Ministers, 1992). Most cases of methemoglobinemia, also known as blue baby syndrome, are associated 

with water containing more than 10 mg/L of nitrate as nitrogen 

(Canadian Council of Resource and Environment Ministers, 1992). 

The MCL’s and MCLG’s are both 10 mg/L for nitrate, 1 mg/L for 

nitrite, and 10 mg/L for nitrate and nitrite as nitrogen (U.S. Environmental Protection Agency, 1996). In this study, total nitrate and 

nitrite is referred to as nitrate because most of the total is nitrate. 

Although nitrite is considerably more toxic than nitrate, nitrite concentration in water usually is low because it is easily converted to 

nitrate in the presence of oxygen. The IEPA Class I standard for 

potable resource ground water is 10 mg/L, and the Class II standard for general resource ground water is 100 mg/L. Eleven of the 

610 samples had nitrate and nitrite above 10 mg/L (fig. 6–1). In 

the LIRB, all 11 measurements exceeding the MCL for nitrate 

were in the Quaternary aquifer at depth intervals less than 150 ft.

As part of the nitrogen cycle, microorganisms convert 

ammonium to nitrite and then to nitrate, which is the form of 

nitrogen most easily absorbed by plants. Nitrate may be lost 

from the soil by microbial conversion to nitrogen gas (N2) in the 

absence of oxygen, or by leaching into ground water. Thus, high 

nitrate concentrations are possible in deep ground water. Natural 

and anthropogenic sources of nitrate include igneous rocks, 

volcanic emissions, organic matter, animal excrement, atmospheric 

deposition, chemical fertilizers, leakage from septic tanks, sewage 

effluent, and industrial discharges (McNeely, Neimanis and 

Dwyer, 1979).

In the LIRB, the concentrations of nitrate in 36 percent of 

the analyses were above the MRL (0.10 mg/L), and the range of 

concentrations were from .1 to 29 mg/L (table 6–1.). There is a 

significant difference in nitrate concentration between the Quaternary aquifer group and other aquifers, among aquifer groups, and 

among various depth intervals (fig. 6–2.). In the LIRB, the range of 

nitrate concentrations is smaller in the other aquifers than in the 

Quaternary aquifer. Over 90 percent of the concentrations are at 

the MRL (0.10 mg/L) in the other aquifers. The median concentration of nitrate was at the MRL for all aquifers, but the median concentration was greater than 0.10 mg/L for depth intervals less than 

100 ft. The percentage of samples with concentrations above the 

MRL for depths less than 50 ft (61 percent) is almost twice that for 

depths greater than 50 ft (33 percent). The lack of nitrate in the 

depth interval greater than 300 ft indicates the presence of confining layers that impede nitrate from leaching deeper into the ground 

water or denitrification. There is no noticeable spatial trend of 

nitrate concentrations in ground water, but there are a number of 

nitrate detections near the Illinois River south of Peoria. Spearman’s correlation coefficient is highest between nitrate and ammonia (0.65), and nitrate and iron (0.53) with a high probability of 

significance. There also is a high probability of significance that 

nitrate is correlated with arsenic, boron, selenium, and sulfate, but 

the correlation is low (from 0.30 to 0.40).

Statistical tests show that nitrate concentrations are not 

significantly different between the LIRB and the rest of Illinois for 

the Quaternary or other aquifers. For the rest of Illinois, the eight 

observations exceeding 10 mg/L of nitrate occur in the Quaternary 

aquifer and the other aquifers, at various depths. In the rest of 

Illinois, there is a significant difference by aquifer group and depth 

interval. The Quaternary has a higher percentage of values above 

the MRL than the other aquifers, although the median is 0.1 mg/L 

for both. In the rest of Illinois, nitrate correlates highest with 

ammonia (0.46) or fluoride (0.39).

6. NITRATE AND NITRITE

NITRATE CONCENTRATIONS VARY BY AQUIFER AND DEPTH INTERVAL

There is a difference in nitrate concentration by aquifer and depth interval within the lower Illinois River Basin. There is 

no significant difference in nitrate concentrations in the Quaternary samples and other aquifer samples between the lower 

Illinois River Basin and the rest of Illinois.
6. Nitrate and Nitrite 15

Figure 6–1. Nitrate and nitrite concentrations above the 

method reporting limit for Quaternary, other aquifers, and 

undefined aquifers in the lower Illinois River Basin.

Figure 6–2. Distribution of nitrate and nitrite concentrations 

in ground water by aquifer group and depth interval.

(U.S. Environmental Protection Agency Maximum Contaminant Level is 

10 milligrams per liter.)

Table 6–1. Summary statistics for nitrates and nitrites within and 

outside the lower Illinois River Basin

[N, number of observations; narl, number of observations above the 

method reporting limit; percentiles and maximums are in milligrams per 

liter; <, less than; gray cells are interpolated values; other aquifers include 

wells open to multiple aquifers and bedrock aquifers]

Percentile

Aquifer or 

depth interval N narl 25 50(median) 75 Maximum

Lower Illinois River Basin

Overall 610 222 <0.01 0.03 0.39 29

Quaternary aquifers 473 191 .01 .05 1.3 29

Other aquifers 62 5 <.10 <.10 <.10 5.0

Undefined aquifers 75 26 <.01 .03 .84 8.0

Depths less than 50 feet 72 44 .04 .25 3.9 14

Depths 50 feet 

or greater 529 175 <.01 .02 .28 29

Unreported depths 9 3 <.01 .02 2.9 5.0

Rest of Illinois

Overall 1,889 496 <0.01 0.01 0.12 20

Quaternary aquifers 543 199 <.01 .04 .78 18

Other aquifers 912 167 .10 .10 .10 14

Undefined aquifers 434 130 <.01 .02 .17 20

Depths less than 50 feet 79 44 .03 .20 3.0 9.4

Depths 50 feet 

or greater 1,776 436 <.01 .01 .1 20

Unreported depths 34 16 .01 .08 1.1 9.4

0

30

5

10

15

20

25

CONCENTRATION OF NITRATES AND NITRITES, 

IN MILLIGRAMS PER LITER AS NITROGEN

0

30

5

10

15

20

25

CONCENTRATION OF NITRATES AND NITRITES,

IN MILLIGRAMS PER LITER AS NITROGEN

150–199200–249250–300

Greater than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

MCL

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

MCL
16 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

The primary concern associated with chloride is the 

undesirable taste it may leave in drinking water. Unlike chlorine 

(Cl2), chloride (Cl-

) is not toxic to humans and is important in the 

osmotic activity of extracellular fluid. In the environment, chloride 

ions are very mobile and relatively unreactive because they do not 

appreciably enter into oxidation or reduction reactions, do not form 

salts of low solubility, are not significantly adsorbed on mineral 

surfaces, and are not biochemically active (Hem, 1985). Because 

of their large size, however, chloride ions may be held back by 

the minute pore size of clay or shale as water seeps through these 

layers (Hem, 1985). The USEPA SMCL is 250 mg/L. The IEPA 

Class I standard for potable resource ground water and Class II 

standard for general resource ground water for chloride are both 

200 mg/L. There are 58 samples with chloride concentrations 

above 200 mg/L in the LIRB (fig. 7–1.). Concentrations are highest 

in the Cambrian-Ordovician aquifer where the median concentration (300 mg/L) exceeded IEPA Class I standard for potable 

resource ground water and Class II standard for general resource 

ground water and the USEPA SMCL.

Chloride is widely distributed in the environment and 

occurs most commonly as sodium chloride (NaCl), which is table 

salt; calcium chloride (CaCl2), which is used as road salt; and 

potassium chloride (KCl) (Canadian Council of Resource and 

Environment Minister, 1992). Although chloride levels in the 

atmosphere generally are low, except near coastal areas and salt 

mines, concentrations may reach 19,300 mg/L in seawater and 

200,000 mg/L in brines (Canadian Council of Resource and Environment Minister, 1992). Chlorides also may be leached from 

sedimentary rocks, particularly evaporites. The Wapsipinicon 

Limestone of Devonian age and the St. Louis Limestone of 

Mississippian age are present in the LIRB and contain anhydrite 

and gypsum beds (Willman and others, 1975). Anthropogenic 

sources of chloride include road salt, irrigation drainage, effluents 

from chemical industries, oil well operations, and sewage. It has 

been estimated that 25 to 50 percent of the applied road salt can 

enter ground water (Canadian Council of Resource and Environment Minister, 1992).

In the LIRB, the concentrations of chloride in 97 percent of 

the chloride analyses were above the MRL (1.0 mg/L), and the 

range of concentrations was from 1.0 to 1,700 mg/L (table 7–1.). 

There is a statistically significant difference in chloride concentration by aquifer and depth within the LIRB (fig. 7–2.). In the LIRB, 

median chloride concentrations are much higher in the other aquifers (164 mg/L) than in the Quaternary aquifer (16 mg/L). These 

concentrations may be related to the presence of brine deep in the 

bedrock of the LIRB. Statistical tests also show a significant difference in chloride concentrations among depth intervals. Median 

chloride concentrations are highest for samples from wells greater 

than 300 ft deep. No spatial pattern of chloride concentrations in 

ground water was discerned. There is a significantly high correlation (0.58) between chloride and sulfate. There are significant 

(p ≤ 0.05) correlations between chloride and iron, arsenic, and 

boron; however, the coefficients for these correlations are less than 

0.50.

A significant difference in chloride concentration for 

the other aquifers between the LIRB and the rest of Illinois was 

determined. The median chloride concentration in the other 

aquifers in the rest of Illinois is much lower. In contrast to the 

LIRB, chloride levels for the rest of Illinois were not significantly 

different between the other aquifers and the Quaternary aquifer; 

however, there are significant differences by aquifer group. The 

percentage of observations exceeding the 200 mg/L standard was 

much higher for the other aquifers in the LIRB (41 percent) than in 

the rest of Illinois (4.5 percent). The rest of Illinois had the highest 

median concentration in the less than 50-ft interval. There is a 

significant difference by depth interval for the rest of Illinois. In 

the rest of Illinois, chloride significantly correlates with sulfate 

(0.45). 

7. CHLORIDE

CHLORIDE CONCENTRATIONS ARE SIGNIFICANTLY HIGHER FOR AQUIFERS OLDER THAN THE QUATERNARY

There is a difference in chloride concentration by aquifer and depth within the lower Illinois River Basin. There is a 

significant difference in chloride concentrations in the older aquifers between the lower Illinois River Basin and the 

rest of Illinois. 
7. Chloride 17

Figure 7–1. Chloride concentrations above the method reporting limit for Quaternary, other aquifers, and undefined aquifers 

in the lower Illinois River Basin.

Figure 7–2. Distribution of chloride concentrations in ground 

water by aquifer group and depth interval.

(See figure 4–1. for explanation of boxplots. U.S. Environmental Protection 

Agency Secondary Maximum Contaminant Level is 250 milligrams per liter.)

Table 7–1. Summary statistics for chloride within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limit; percentiles and maximums are in milligrams per liter; other 

aquifers include wells open to multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 613 594 6.0 18 53 1,700

Quaternary aquifers 475 456 5.0 16 34 450

Other aquifers 63 63 14 110 330 890

Undefined aquifers 75 75 6.9 30 300 1,700

Depths less than 50 feet 74 74 16 22 46 410

Depths 50 feet

or greater 530 511 5.4 18 56 1,700

Unreported depths 9 9 4.6 18 42 290

Rest of Illinois

Overall 1,889 1,739 3.1 14 37 820

Quaternary aquifers 542 505 5.3 16 30 820

Other aquifers 911 841 3.3 15 47 540

Undefined aquifers 436 393 2.3 7.3 28 540

Depths less than 50 feet 79 79 14 22 39 420

Depths 50 feet

or greater 1,776 1,627 3 13 37 820

Unreported depths 34 33 3.1 15 34 110

 LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

 LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

1

1,800

2

5

10

20

50

100

200

500

1,000

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

AQUIFER GROUP

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

1

1,800

2

5

10

20

50

100

200

500

1,000

1

1,800

2

5

10

20

50

100

200

500

1,000

1

1,800

2

5

10

20

50

100

200

500

1,000

SMCL

SMCL

SMCL

SMCL
18 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Sulfate (SO4

-2) occurs in combination with various ions, 

forming compounds that can cause gastrointestinal irritation or 

produce an unpleasant taste in water. Among these compounds, 

sodium sulfate has one of the lower taste-thresholds at 

200–500 mg/L (American Waterworks Association, 1990). 

At concentrations from 150 to 500 mg/L, some sulfate compounds 

may cause diarrhea, which can lead to dehydration in infants and 

young children (McNeely, Neimanis and Dwyer, 1979). This effect 

is temporary; however, residents in areas with high sulfate levels 

adapt easily with no ill effects (American Waterworks Association, 

1990). The IEPA Class I standard for potable resource ground 

water and the Class II standard for general resource ground water 

for sulfate, except when due to natural causes, are 400 mg/L 

(Illinois Environmental Protection Agency, 1990). The USEPA 

proposed MCL is 500 mg/L and SMCL is 250 mg/L (U.S. Environmental Protection Agency, 1996). The MRL for sulfate is 

10 mg/L. Thirty of 613 samples in the LIRB had concentrations 

greater than 400 mg/L and most were from the older aquifers 

(fig. 8–1.).

In water, sulfur occurs primarily as sulfate in the presence 

of oxygen. Under anaerobic conditions, bacteria reduces sulfate 

to sulfide, most commonly hydrogen sulfide, which has a rotten 

egg odor. Sulfide concentrations in ground water usually are low 

because metal sulfides have low solubility (Canadian Council of 

Resource and Environment Minister, 1992). Sulfate is produced 

naturally by volcanic emissions; the weathering of sedimentary 

rocks; and bacterial oxidation of sulfur-containing compounds 

such as hydrogen sulfide, metal sulfides, and organo-sulphur compounds. Anthropogenic sources of sulfur include mine-drainage 

wastes; coal combustion; and industrial discharges from tanneries, 

paper mills, textile plants, and metal-working industries (McNeely, 

Neimanis and Dwyer, 1979). Rock containing pyrite (iron sulfide) 

can be oxidized to release sulfur, with microorganisms acting as 

a catalyst and mediating oxidation. Sulfur oxidation is the source 

of the acidic water that drains many mined areas (Fetter, 1993).

In the LIRB, the concentrations of sulfate in 69 percent of 

the sulfate analyses were above the MRL, and the range of concentrations were from 10 to 1,500 mg/L (table 8–1.). Median sulfate 

concentrations in the LIRB are higher in the older aquifers than in 

the Quaternary aquifer, with the exception of the MississippianPennsylvanian aquifers, which have lower values for the median 

and quartiles (fig. 8–2.). Within the LIRB, there is a significant 

difference between the Quaternary and other aquifer groups and 

by depth interval. The samples from wells in the Quaternary 

aquifer that are 50 ft or less in depth had a median concentration 

of 69 mg/L, whereas aquifers older than the Quaternary and 

greater than 300 ft deep had median concentrations of 150 mg/L. 

Median sulfate concentrations are highest in water from wells less 

than 50 ft below land surface, although samples from ground water 

deeper than 300 ft had the maximum concentration. Water from 

wells with elevated sulfate concentrations show the same spatial 

pattern as those with elevated chloride concentration. There is 

a significant correlation (p ≤ 0.05) between sulfate and chloride 

(0.58), arsenic (0.57), ammonia (0.47), and iron (0.46) concentrations in ground water in the LIRB.

There is a significant difference in sulfate concentration 

in the Quaternary aquifer group between the LIRB and the rest of 

Illinois. Concentrations of sulfate in the rest of Illinois were highest in the Silurian-Devonian aquifers. These high concentrations 

may be due to the distribution of the Silurian-Devonian aquifers, 

which are more widely used in the northern part of the State. 

Sulfate concentration is not significantly different by depth in 

the rest of Illinois. Correlations within the rest of Illinois between 

sulfate concentrations and other constituents are similar to those 

found in the LIRB.

8. SULFATE

SULFATE CONCENTRATIONS ARE HIGHEST IN THE OLDER THAN QUATERNARY AQUIFERS

There is a difference in sulfate concentration by aquifer and depth interval within the lower Illinois River Basin. There is a 

significant difference in sulfate concentration in the Quaternary aquifer between the lower Illinois River Basin and the rest 

of Illinois.
8. Sulfate 19

Figure 8–1. Sulfate concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in 

the lower Illinois River Basin.

Figure 8–2. Distribution of sulfate concentrations in groundwater samples by aquifer group and depth interval.

(See figure 4–1. for explanation of boxplots. The U.S. Environmental Protection 

Agency proposed Maximum Contaminant Level is 500 milligrams per liter (mg/L) 

and the Secondary Maximum Contaminant Level is 250 mg/L.)

Table 8–1. Summary statistics for sulfate within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limit; percentiles and maximums are in milligrams per liter; gray 

cells are interpolated values; other aquifers include wells open to multiple 

aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 612 422 9.4 46 95 1,500

Quaternary aquifers 474 308 8.2 39 79 1,300

Other aquifers 63 55 22 110 310 1,200

Undefined aquifers 75 59 14 54 230 1,500

Depths less than 50 feet 74 73 49 68 99 860

Depths 50 feet

or greater 529 344 7.3 38 94 1,500

Unreported depths 9 5 3.6 42 290 520

Rest of Illinois

Overall 1,888 1,535 16 53 140 1,400

Quaternary aquifers 542 395 12 43 91 1,400

Other aquifers 910 780 19 80 200 1,200

Undefined aquifers 436 360 16 39 110 860

Depths less than 50 feet 79 79 51 77 150 800

Depths 50 feet

or greater 1,775 1,427 15 51 140 1,400

Unreported depths 34 29 19 53 110 440

LOWER ILLINOIS RIVER BASIN

1

10,000

2

5

10

20

50

100

200

500

1,000

2,000

5,000

0.2

2,000

0.5

1

2

5

10

20

50

100

200

500

1,000

LOWER ILLINOIS RIVER BASIN

0.2

2,000

0.5

1

2

5

10

20

50

100

200

500

1,000

1

10,000

2

5

10

20

50

100

200

500

1,000

2,000

5,000 REST OF ILLINOIS

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

REST OF ILLINOIS

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

AQUIFER GROUP

Proposed MCL

Proposed MCL

Proposed MCL

Proposed MCL

SMCL

SMCL

SMCL

SMCL
20 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Arsenic has been classified as a human carcinogen (group 

A) by the USEPA. Although arsenic is found in many foods and 

may be a dietary requirement, excessive amounts of arsenic causes 

cardiac damage and acute gastrointestinal damage. Chronic doses 

may lead to vascular disorders, such as blackfoot disease (American Waterworks Association, 1990), and the accumulation of 

arsenic in body tissues can result in symptoms of severe poisoning. 

The trivalent form of arsenic, arsenite (AsO3

3-), is more toxic than 

the pentavalent form, arsenate (AsO4

3-), because arsenite is more 

effectively bioaccumulated (McNeely, Neimanis and Dwyer, 

1979). Organo-arsenic compounds generally are less toxic than 

inorganic arsenic (American Waterworks Association, 1990). 

Common analytical procedures, however, report only total arsenic 

concentrations. Long-term use of water with 210 μg/L total arsenic 

has been reported as poisonous (Hem, 1985). High concentrations 

of arsenic also may reduce crop yield by destroying chlorophyll. 

IEPA Class I standard for potable resource ground water and 

Class II standard for general resource ground water for arsenic are, 

respectively, 50 and 200 μg/L, and the USEPA MCL is 50 μg/L. In 

the LIRB, 16 of the 613 samples had arsenic concentrations above 

50 μg/L (fig. 9–1.).

Inorganic forms of arsenic prevail in most natural waters 

and may bind to colloidal humic matter where organic content is 

high. Total dissolved arsenic in ground water, as used in this report, 

is usually a combination of arsenite and arsenate. Arsenite is found 

mostly in reducing conditions, in absence of hydrogen sulfide. 

Arsenite is the most toxic and mobile form of arsenic and is found 

in association with iron in the soluble ferrous state (Fetter, 1993). 

Arsenic associated with sulfate indicates oxidized sulfide minerals 

and an oxidized form of arsenic, arsenate. Under strongly reducing 

conditions, if iron and hydrogen sulfide are present, then arsenic 

sulfide would coprecipitate with iron sulfide (Fetter, 1993). Arsenate constitutes the stable form of arsenic in aerobic water, whereas 

arsenite dominates under anaerobic conditions. Arsenate tends to 

adsorb on or coprecipitate with hydrous iron oxides, aluminum 

hydroxide, and clay. Arsenite has a strong affinity for sulfur, 

readily adsorbing on and coprecipitating with other metal sulfides 

(Canadian Council of Resource and Environment Minister, 1992). 

These affinities may partly explain the correlations found between 

arsenic and other constituents in the study. Arsenic enters the 

environment from pesticides, coal combustion, the smelting of 

nonferrous metal ores, embalming fluid, volcanic gasses, geothermal water, and the weathering of arsenic minerals, which are 

widely distributed, particularly those arsenic minerals formed by 

sulfide, iron, and nickel (Canadian Council of Ministers of the 

Environment, 1992; Fetter, 1993). In the 1930’s, poison pesticide 

baits utilizing arsenic were used in the Midwest to counter grasshopper infestation. Poison baits were buried and later showed up in 

a water supply in concentrations of 21,000 μg/L (21 mg/L) (Fetter, 

1993).

In the LIRB, the concentrations of arsenic in 48 percent 

of the analyses were above the MRL (1.0 μg/L), and the range of 

concentrations was from 1 to 110 μg/L (table 9–1.). The median 

concentration for all aquifers is at the MRL (1.0 μg/L). Application 

of the Kruskal-Wallis test indicates significant difference in arsenic 

concentrations between the Quaternary and older aquifers. The 

Quaternary aquifer has a median concentration of arsenic above 

the MRL at depths greater than 150 ft. The highest concentration 

of arsenic was from wells in the Quaternary aquifer (fig. 9–2.). The 

percent of values above the MRL decreased with increasing aquifer age. There are significant differences among depth intervals. 

Arsenic concentrations are higher for depths from 150 to 300 ft 

deep. Spatially, there are more detections of arsenic in ground 

water east of the Illinois River in the proximity of the buried 

Mahomet Valley. There are significant correlations (p ≤ 0.05) 

between arsenic and sulfate (−0.59), iron (0.56), fluoride (0.47), 

boron (0.40), and chloride (−0.26) for the Quaternary aquifer and 

the older aquifers of the LIRB. In the Quaternary aquifer, arsenic 

correlates with ammonia (0.68) and nitrate (0.51).

There is a significant difference in arsenic concentrations 

for the Quaternary and older aquifers between the LIRB and the 

rest of Illinois. For the rest of Illinois, the Quaternary aquifer that 

are deeper than 300 ft below land surface and the older aquifers 

shallower than 540 ft deep have median arsenic concentrations 

above the MRL. There is a significant difference in arsenic concentrations by aquifer group and depth intervals for the rest of Illinois. 

In the rest of Illinois, results from Spearman’s rank correlation 

procedure did not show a strong correlation between arsenic and 

sulfate, fluoride, boron, or chloride, but there is a significant correlation between arsenic and iron (0.49) or manganese (0.32).

9. ARSENIC

ARSENIC CONCENTRATIONS ARE HIGHEST IN THE DEEP QUATERNARY AQUIFER

There is a difference in arsenic concentration by aquifer and depth interval within the lower Illinois River Basin. There is a 

significant difference in arsenic concentration in the Quaternary and older aquifers between the lower Illinois River Basin 

and the rest of Illinois.
9. Arsenic 21

Figure 9–1. Arsenic concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in 

the lower Illinois River Basin.

Figure 9–2. Distribution of arsenic concentrations in ground water 

by aquifer group and depth interval.

(U.S. Environmental Protection Agency Maximum Contaminant Level is 50 micrograms 

per liter.

Table 9–1. Summary statistics for arsenic within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above the method 

reporting limit; percentiles and maximums are in micrograms per liter; 

<, less than; gray cells are interpolated values; other aquifers include wells 

open to multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 613 293 0.18 0.95 7.5 110

Quaternary aquifers 475 262 .32 1.5 10 110

Other aquifers 63 8 <1.0 <1.0 <1.0 66

Undefined aquifers 75 23 .06 .29 1.0 45

Depths less than 50 feet 74 29 .20 .63 2.0 28

Depths 50 feet

or greater 530 259 .19 1.0 8.3 110

Unreported depths 9 5 .29 1.0 29 51

Rest of Illinois

Overall 1,891 616 0.09 0.37 1.5 96

Quaternary aquifers 543 267 .22 .96 5.0 96

Other aquifers 912 221 .07 .26 .92 80

Undefined aquifers 436 128 .16 .44 1.2 60

Depths less than 50 feet 79 24 .06 .28 1.0 85

Depths 50 feet

or greater 1,778 589 .10 .39 1.6 96

Unreported depths 34 3 <1.0 <1.0 <1.0 2.0

0

120

20

40

60

80

100

ARSENIC CONCENTRATIONS,

IN MICROGRAMS PER LITER

120

20

40

60

80

100

ARSENIC CONCENTRATIONS,

IN MICROGRAMS PER LITER

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

MCL

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

MCL

0
22 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Boron does not appear to accumulate in human tissues, but 

large doses can cause digestive difficulties and affect the central 

nervous system (McNeely, Neimanis and Dwyer, 1979). Although 

boron is essential to plant growth in trace amounts, concentrations 

as high as 1 mg/L may be toxic to some plants, such as citrus crops 

(Hem, 1985). Boron concentrations range from generally less than 

a few hundred micrograms per liter in surface and ground water to 

48,000 μg/L in brines and hot springs (Hem, 1985). Both IEPA 

Class I and Class II standards for boron in ground water are 

2,000 μg/L, and there is no USEPA MCL. Four of the 613 samples 

had boron concentrations above 2,000 μg/L (fig. 10–1.).

Elemental boron is not found in nature and little is known 

about the environmental chemistry of boron compounds. The more 

important boron solutes are unlikely to adsorb extensively to mineral surfaces because the surfaces tend to be anionic or uncharged 

(Hem, 1985). Boric acid (H3BO3) is probably the predominant 

boron species in fresh water because it is moderately soluble and 

does not readily dissociate. Boron is derived naturally from volcanic activity; soil leaching; and the weathering of igneous and sedimentary rocks, particularly granites, pegmatite, gabbros, and 

serpentine rocks. Anthropogenic sources include domestic sewage, 

agricultural runoff, and industrial discharges because of the use of 

boron in water softeners, cleaning agents, pesticides, and a variety 

of industrial operations (Canadian Council of Resource and Environment Minister, 1992).

In the LIRB, the concentrations of boron in 71 percent 

of the analyses were above the MRL (50 μg/L), and the range of 

concentrations was from 50 to 2,100 μg/L (table 10–1.). In the 

LIRB, there is a significant difference between aquifer groups 

and depth intervals (fig. 10–2.). All samples from Silurian or older 

aquifers had boron concentrations above the MRL of 50 μg/L. The 

Kruskal-Wallis test showed a significant difference in boron concentrations between the Quaternary aquifer and the older aquifers. 

The median boron concentration was much higher for the older 

aquifers (802 μg/L) than the Quaternary aquifer (130 μg/L).

In the LIRB, the Kruskal-Wallis test indicated a significant 

difference in boron concentration by depth interval. Boron 

concentrations increased with increasing depth. Median boron 

concentrations were lowest in samples from wells within 50 ft 

of the land surface and highest in samples from wells with depths 

greater than 300 ft. Approximately 85 percent of samples from the 

older aquifers were from wells greater than 300 ft deep. These 

samples had the highest median concentration of 940 μg/L. In the 

LIRB, there is no spatial pattern of boron concentration in ground 

water. There is a significant correlation (p ≤ 0.05) of boron with 

fluoride (0.73), ammonia (0.56), manganese (−0.46), nitrite/nitrate 

(0.38), chloride (0.31), selenium (0.19), and arsenic (0.13) in the 

LIRB.

The boron concentration in water samples from the rest 

of Illinois are significantly different from the LIRB for the older 

aquifers but similar for the Quaternary aquifer. For the Quaternary 

aquifer, boron concentrations in ground water in the LIRB and the 

rest of Illinois had comparable values for the mean, percentiles, 

and the percentage of values above the MRL but had substantially 

lower values for the other aquifers in the rest of Illinois. There are 

significant differences by aquifer group and depth interval in the 

rest of Illinois. There are not as many samples from other aquifers 

within the LIRB (63) compared with the rest of Illinois (908). The 

relative scarcity of samples from other aquifers in the LIRB may 

account for some of the differences in statistical test results. There 

are significant correlations (p ≤ 0.05) of boron with fluoride (0.70) 

and ammonia (0.58) in the rest of Illinois. 

10. BORON

BORON CONCENTRATIONS ARE SIGNIFICANTLY DIFFERENT BETWEEN THE QUATERNARY AND OLDER 

AQUIFERS

There is a difference in boron concentration by aquifer group and depth interval within the lower Illinois River Basin. There 

is a significant difference in boron concentration in the other aquifers between the lower Illinois River Basin and the rest of 

Illinois.
10. Boron 23

Figure 10–1. Boron concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in 

the lower Illinois River Basin.

Figure 10–2. Distribution of boron concentrations in ground water 

by aquifer group and depth interval.

(See figure 4–1. for explanation of boxplots. Method reporting limit is 50 micrograms 

per liter.)

Table 10–1. Summary statistics for boron within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limit; percentiles and maximums are in micrograms per liter; 

gray cells are interpolated values; other aquifers include wells open to 

multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 613 438 50 170 390 2,100

Quaternary aquifer 475 324 44 130 290 2,100

Other aquifers 63 57 390 802 1,100 2,000

Undefined aquifers 75 57 61 400 790 2,100

Depths less than 50 feet 74 39 22 51 120 1,400

Depths 50 feet

or greater 530 391 58 200 400 2,100

Unreported depths 9 8 140 450 740 1,400

Rest of Illinois

Overall 1,879 1,354 53 180 440 2,300

Quaternary aquifer 540 346 33 89 330 2,300

Other aquifers 908 724 77 270 540 1,600

Undefined aquifers 431 284 39 110 350 1,500

Depths less than 50 feet 78 27 10 28 72 500

Depths 50 feet

or greater 1,767 1,306 58 200 460 2,300

Unreported depths 34 21 36 150 380 840

REST OF ILLINOIS

REST OF ILLINOIS

BORON CONCENTRATION, IN MICROGRAMS PER LITER

AQUIFER GROUP

BORON CONCENTRATION, IN MICROGRAMS PER LITER

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

5

2,400

10

20

50

100

200

500

1,000

 LOWER ILLINOIS RIVER BASIN

5

2,400

10

20

50

100

200

400

1,000

5

2,400

10

20

50

100

200

500

1,000

5

2,400

10

20

50

100

200

500

1,000

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

MRL

MRL

MRL

MRL
24 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Chromium is present primarily in its trivalent (Cr3+) and 

hexavalent (Cr6+) form in aqueous systems. Whereas, Cr3+ is nontoxic and nutritionally essential, Cr6+ is toxic, causing liver and 

kidney damage, internal hemorrhage, and respiratory disorder, as 

well as chronic and subchronic effects such as dermatitis and skin 

ulceration (American Waterworks Association, 1990). The USEPA 

has classified chromium as a human carcinogen (group A) because 

Cr6+ has been shown to cause cancer in humans and animals by 

inhalation exposure. Chromium also is toxic to aquatic organisms 

and will bioaccumulate. The 1992 Canadian Water Quality Guidelines (Canadian Council of Resource and Environment Minister, 

1992) suggest chromium limits of 20 μg/L to protect fish and 

2 μg/L to protect aquatic life, in general. The USEPA MCL and the 

IEPA Class I standard for potable resource ground water for chromium are 100 μg/L. In this study, chromium concentrations constitute total chromium. There are no detections above 100 μg/L and 

very few concentrations above the MRL of 5.0 μg/L (fig. 11–1.).

Hexavalent chromium is soluble and not significantly 

sorbed by soils or particulate matter (Canadian Council of 

Resource and Environment Minister, 1992). Under anaerobic conditions or in the presence of oxidizable substances, usually organic 

molecules, Cr6+ will be reduced to Cr3+, which tends to sorb on 

particulate matter or precipitate out of solution (Canadian Council 

of Resource and Environment Minister, 1992; McNeely, Neimanis 

and Dwyer, 1979). Dissolved chromium concentrations often are 

low in aqueous systems, particularly in well aerated waters with 

little organic matter. Federal surveys from 1969 to 1980 show 

mean chromium concentrations of 10 μg/L for surface water and 

16 μg/L for ground water (American Waterworks Association, 

1990).

Chromium enters the environment naturally through 

weathering, although chromite (the predominant chromiumbearing mineral) is highly resistant to weathering. Anthropogenic 

sources of chromium include metal-plating operations; combustion 

of fossil fuels; tanning; old mining operations; and the manufacturing of ceramics, paints, dyes, and paper (American Waterworks 

Association, 1990; Canadian Council of Resource and Environment Minister, 1992).

In the LIRB, the concentrations of chromium in 1 percent 

of the analyses were above the MRL, and the range of concentrations was from 5 to 40 μg/L (table 11–1.). There is a difference in 

the range of concentrations between the Quaternary and older 

aquifers, although the median for all aquifers is at the MRL (5.0 

μg/L). This difference also is reflected in the percentage of concentrations greater than the MRL, which is lowest for the Quaternary 

aquifer. The maximum concentration of 40 μg/L was from a sample in the older aquifer at a depth greater than 300 ft below land 

surface (fig. 11–2.). Although there are differences in ranges of 

concentrations, there is no significant difference among aquifer 

groups in the LIRB. There is a significant difference between the 

Quaternary aquifer and other aquifers. Within the LIRB, there is 

not a significant difference in chromium concentration by depth 

interval. Chromium detections in ground water do not appear to 

have a spatial pattern and, thus, may be a result of local land-use 

effects. The largest Spearman correlation coefficient with significance (p ≤ 0.05) is between chromium and silver (0.32).

Chromium concentrations generally were low in the LIRB 

and the rest of Illinois. Only 7 and 10 percent of the observations in 

the rest of Illinois and within the LIRB, respectively, have chromium detections. The mean and 25th and 75th percentiles are 

equivalent for the LIRB and the rest of Illinois. Unlike the LIRB, 

chromium concentrations within the rest of Illinois did not differ 

significantly between the Quaternary aquifer and the other aquifers. There is no significant difference between the LIRB and the 

rest of Illinois for the Quaternary aquifer, but there are differences 

between the two areas by depth interval of the sampled wells. The 

percentage of values above the MRL was higher for the older aquifers of the LIRB than in the rest of Illinois, 22 and 6 percent, 

respectively. The largest Spearman correlation coefficient with significance in the rest of Illinois is between chromium and silver 

(0.29). 

11. CHROMIUM

CHROMIUM CONCENTRATIONS ARE LOW

There are no significant differences in chromium concentration by aquifer group or depth interval within the lower Illinois 

River Basin. There is a significant difference in chromium concentration in the older than Quaternary aquifers between the 

lower Illinois River Basin and the rest of Illinois.
11. Chromium 25

Figure 11–1. Chromium concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in the 

lower Illinois River Basin.

Figure 11–2. Distribution of chromium concentrations in ground 

water by aquifer group and depth interval.

(Method reporting limit is 5.0 micrograms per liter.)

Table 11–1. Summary statistics for chromium within and outside 

the lower Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limit; percentiles and maximums are in micrograms per liter; 

<, less than; gray cells are interpolated values; other aquifers include wells 

open to multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 613 62 <5.0 <5.0 <5.0 40

Quaternary aquifers 475 39 <5.0 <5.0 <5.0 22

Other aquifers 63 14 1.4 2.5 4.5 40

Undefined aquifers 75 9 <5.0 <5.0 <5.0 9

Depths less than 50 feet 74 4 <5.0 <5.0 <5.0 22

Depths 50 feet

or greater 530 57 <5.0 <5.0 <5.0 40

Unreported depth 9 1 <5.0 <5.0 <5.0 7

Rest of Illinois

Overall 1,880 128 <5.0 <5.0 <5.0 36

Quaternary aquifers 541 45 <5.0 <5.0 <5.0 36

Other aquifers 908 63 <5.0 <5.0 <5.0 30

Undefined aquifers 431 20 <5.0 <5.0 <5.0 32

Depths less than 50 feet 78 5 <5.0 <5.0 <5.0 7

Depths 50 feet

or greater 1,768 120 <5.0 <5.0 <5.0 36

Unreported depths 34 3 <5.0 <5.0 <5.0 6

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

0

50

10

20

30

40

CHROMIUM CONCENTRATIONS,IN MICROGRAMS PER LITER

0

50

10

20

30

40

CHROMIUM CONCENTRATIONS, IN MICROGRAMS PER LITER

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

150–199200–249250–300

Greater

than 300

100–149

50–99

Less than50

AQUIFER GROUP

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

MRL

MRL
26 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Copper (Cu) is not known to be toxic to man at moderate 

levels, but water becomes distasteful to drink at concentrations 

of copper between 1,000 to 5,000 μg/L (Canadian Council of 

Resource and Environment Ministers, 1992). Lack of copper in the 

diet has been associated with iron deficiency and reproductive 

abnormalities. Very large doses of copper over time may result in 

liver and renal damage or anemia (American Waterworks Association, 1990). The IEPA Class I standard for potable resource ground 

water and the IEPA Class II standard for general resource ground 

water, except due to natural causes, are 650 μg/L. The recommended USEPA MCLG is 1,300 μg/L at the tap for copper and the 

SMCL is 1,000 μg/L. Water from five wells had concentrations of 

copper above 650 μg/L (fig. 12–1.).

Copper may occur in solution in either the Cu2+ or Cu 

oxidation stage. The redox conditions in oxygenated water and the 

tendency of Cu ions to disproportionate favor the more oxidized 

form (Hem, 1985). Water with lower concentrations of copper are 

readily explainable as results of coprecipitation by oxides or 

adsorption on mineral surfaces (Hem, 1985). Copper is a common 

heavy metal constituent of natural water, but little copper in water 

is of natural origin because most copper minerals are relatively 

insoluble (McNeely, Neimanis, and Dwyer, 1979). It is estimated 

that anthropogenic sources of copper to aquatic environments 

provide 33 to 60 percent of the total annual global input (Canadian 

Council of Resource and Environment Ministers, 1992). Copper is 

common in the earth’s crust as stable sulfide minerals.

In the LIRB, the concentrations of copper in 28 percent 

of the analyses were above the MRL (5.0 μg/L), and the range of 

concentrations was from 5 to 2,700 μg/L (table 12–1.). There is no 

significant difference, in median concentration of copper, between 

the Quaternary and older aquifers or among individual aquifer 

groups. The median concentration is below the MRL for all aquifers. The range of concentrations for the LIRB was from 5 to 

2,700 μg/L with the highest concentrations in the Quaternary 

aquifer (fig. 12–2.). The Kruskal-Wallis test showed no significant 

difference in concentration of copper between depth intervals. The 

median concentration for all depth intervals was less than 5 μg/L, 

which is the MRL. The spatial distribution of copper concentrations above the MRL does not show a pattern. Water samples from 

a cluster of wells near Peoria in the Quaternary aquifer have copper 

concentrations above the MRL, but this cluster probably represents 

the greater density of wells in the area. The highest Spearman correlation coefficient with significance is between copper and lead 

(0.40).

Copper concentrations in the rest of Illinois are similar 

to the LIRB. Further Kruskal-Wallis tests show no significant 

difference between the LIRB and the rest of Illinois in copper 

concentrations by depth interval or aquifer group. In the rest of 

Illinois, the highest Spearman correlation coefficient with significance is lead (0.32).

12. COPPER

COPPER CONCENTRATIONS DO NOT VARY SIGNIFICANTLY BY DEPTH INTERVAL OR AQUIFER

There is no difference in copper concentration by aquifer or depth interval within the lower Illinois River Basin. There is no 

significant difference in copper concentration in the Quaternary and older aquifers between the lower Illinois River Basin 

and the rest of Illinois.
12. Copper 27

Figure 12–1. Copper concentrations above the method reporting limit for Quaternary, other aquifers, and undefined aquifers 

in the lower Illinois River Basin.

Figure 12–2. Distribution of copper concentrations in ground 

water by aquifer group and depth interval.

(U.S. Environmental Protection Agency Maximum Contaminant Level Goal is 

1,300 micrograms per liter (μg/L) and Secondary Maximum Contaminant Level is 

1,000 μg/L. Illinois Environmental Protection Agency Class I and Class II standards 

are 650 μg/L.)

Table 12–1. Summary statistics for copper within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limit; percentiles and maximums are in micrograms per liter; 

gray cells are interpolated values; other aquifers include wells open to 

multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin:

Overall 613 174 0.25 1.2 6.0 2,700

Quaternary aquifers 475 129 .22 1.1 6.0 2,700

Other aquifers 63 19 .27 1.3 6.0 650

Undefined aquifers 75 26 .52 2.2 9.0 190

Depths less than 50 feet 74 26 .53 2.0 6.0 1,400

Depths 50 feet

or greater 530 145 .21 1.1 6.0 2,700

Unreported depths 9 3 .58 2.2 13 68

Rest of Illinois:

Overall 1,880 421 .31 1.1 4.2 1,300

Quaternary aquifers 541 124 .31 1.2 4.4 340

Other aquifers 908 206 .31 1.2 4.3 1,100

Undefined aquifers 431 91 .28 1.0 3.8 1,300

Depths less than 50 feet 78 26 .63 2.2 7.3 420

Depths 50 feet

or greater 1,768 384 .29 1.1 4.0 1,300

Unreported depths 34 11 .63 2.2 9.3 110

0

3,000

500

1,000

1,500

2,000

2,500

0

3,000

500

1,000

1,500

2,000

2,500

COPPER CONCENTRATIONS, IN MICROGRAMS PER LITER COPPER CONCENTRATIONS, IN MICROGRAMS PER LITER

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

MCLG

SMCL

CLASS I and II standards

MCLG

SMCL

CLASS I and II standards
28 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Cyanide in drinking water may cause damage to the 

thyroid and nervous system (U.S. Environmental Protection 

Agency, 1994). Cyanide combines with cell cytochrome; prevents 

oxygen transport; and is readily absorbed from the lungs, intestines, and skin (American Waterworks Association, 1990). The 

toxicity of cyanide varies, depending on its chemical form. 

Although high doses can be fatal, low, chronic amounts may be 

detoxified in the liver (American Waterworks Association, 1990). 

Cyanide concentrations generally are low in drinking water. A 

1970 USEPA survey of 969 community water-supply systems 

throughout the nation showed the maximum cyanide concentration 

of 0.008 mg/L (U.S. Department of Health, Education, and 

Welfare, 1970). Because cyanide is biodegradable and readily 

forms compounds with metal ions (such as copper and iron), largescale distribution is unlikely (McNeely, Neimanis and Dwyer, 

1979). The IEPA Class I and Class II standards for cyanide in 

ground-water resources are 0.2 and 0.6 mg/L, respectively. The 

USEPA MCL is 0.2 mg/L. Cyanide was detected in two samples 

at concentrations below the MCL (fig. 13–1.).

Cyanides, defined by the presence of a carbon-nitrogen 

group (CN), occur in a variety of chemical forms, from the simple 

ion (CN-) and undissociated form (HCN) to salts (for example, 

sodium cyanide and potassium cyanide) to organic and metal 

complexes. The cyanide ion has a strong affinity for many metal 

ions but is not strongly adsorbed or retained in soils because of the 

ion’s negative charge. Some metallocyanide complexes, such as 

iron-cyanide, may dissociate as a result of sunlight when pH is less 

than 8.0 (Canadian Council of Resource and Environment Ministers, 1992). In waters at pH below 8.0, HCN generally will be the 

predominate dissolved form. Cyanide is generated primarily from 

industrial operations, such as electroplating, metal cleaning, steel 

plants, and oil refineries. Concentrations in effluent can range from 

an average of 3 mg/L for electroplating to 30–60 mg/L for steel 

mills.

In the LIRB, the concentrations of cyanide in less than 

1 percent of the analyses were above the MRL (0.005 or 

0.01 mg/L) and, therefore, no statistical tests were performed 

on these data. The range of concentrations was from 0.005 to 

0.03 mg/L (table 13–1.). The only two samples above the MRL 

were taken from the Quaternary aquifer in 1986 (fig. 13–2.). The 

first sample was from ground water (0.01 mg/L) in Christian 

County at a depth of 42 ft, and the second sample (0.03 mg/L) 

was from ground water in Champaign County at a depth of 270 ft. 

Statistical tests for correlation are not useful with only two detections of cyanide.

In the rest of Illinois, 28 (1 percent) of the samples were 

above the MRL, whereas 78 percent were less than 0.01 mg/L and 

21 percent were less than 0.005 mg/L. Although concentrations 

ranged up to 0.10 mg/L, most were below the MRL.

13. CYANIDE

CYANIDE DETECTED IN ONLY TWO GROUND-WATER SAMPLES

There is not a statistical difference in cyanide concentration by aquifer or depth interval within the lower Illinois River Basin 

because less than 1 percent of analyses had a detection of cyanide. There is no significant difference in cyanide concentration in the Quaternary and other aquifers between the lower Illinois River Basin and the rest of Illinois because less than 

1 percent of analyses had a detection of cyanide. 
13. Cyanide 29

Figure 13–1. Cyanide concentrations above the method reporting limit for Quaternary, other aquifers, and undefined aquifers 

in the lower Illinois River Basin.

Figure 13–2. Distribution of cyanide concentrations in ground 

water by aquifer group and depth interval.

(Method reporting limit varies depending on analytical method, 0.005 or 

0.01 milligram per liter.)

Table 13–1. Summary statistics for cyanide within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limits; percentiles and maximums are in milligrams per liter; 

<, less than; other aquifers include wells open to multiple aquifers and 

bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 612 2 <0.01 <0.01 <0.01 0.03

Quaternary aquifers 475 2 <.01 <.01 <.01 .03

Other aquifers 63 0 <.01 <.01 <.01 <.01

Undefined aquifers 74 0 <.01 <.01 <.01 <.01

Depths less than 50 feet 74 1 <.01 <.01 <.01 <.01

Depths 50 feet

or greater 529 1 <.01 <.01 <.01 .03

Unreported depths 9 0 <.01 <.01 <.01 <.01

Rest of Illinois

Overall 1,890 28 <0.01 <0.01 <0.01 0.10

Quaternary aquifers 542 5 <.01 <.01 <.01 .02

Other aquifers 912 17 <.01 <.01 <.01 .10

Undefined aquifers 436 6 <.01 <.01 <.01 .09

Depths less than 50 feet 79 0 <.01 <.01 <.01 <.01

Depths 50 feet

or greater 1,777 28 <.01 <.01 <.01 .10

Unreported depths 34 0 <.01 <.01 <.01 <.01

MRL

0

0.12

0.02

0.04

0.06

0.08

0.10

CYANIDE CONCENTRATIONS, IN MILLIGRAMS PER LITER

0

0.12

0.02

0.04

0.06

0.08

0.10

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

150–199200–249

250–300

Greater than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

CYANIDE CONCENTRATIONS, IN MILLIGRAMS PER LITER

MRL
30 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Iron in drinking water does not pose a health threat and, 

therefore, is not regulated under the Safe Drinking Water Act. In 

high enough concentrations, however, iron forms reddish-brown 

precipitates that encrust pipes and stain laundry and plumbing 

fixtures. Iron also produces an unpleasant taste in drinking water 

and may promote bacterial growth in pipes and service mains. 

Upper limits of 50 μg/L and 300 μg/L have been recommended 

for iron, the first for more palatable water (McNeely, Neimanis 

and Dwyer, 1979) and the latter to reduce staining (Hem, 1985). 

Although iron serves as a minor plant nutrient, concentrations 

exceeding 20,000 μg/L have been shown to be toxic to some plants 

(McNeely, Neimanis, and Dwyer, 1979.). The IEPA Class I standard for potable resource ground water and Class II standard 

for general resource ground water for iron are 5,000 μg/L. The 

USEPA SMCL level is 300 μg/L. There are 37 samples with 

concentrations of iron above 5,000 μg/L (fig. 14–1.).

Iron generally occurs in either its ferric (Fe3+) or ferrous 

(Fe2+) form, with each having different chemical properties. When 

exposed to oxygen, ferrous iron (which is present under reducing 

conditions and is more mobile than ferric iron) is oxidized to the 

ferric form, which tends to precipitate. Thus, ground water that is 

clear when first drawn from a well may soon turn brown from precipitating ferric hydroxide (Hem, 1985). Various microorganisms 

also affect the form in which iron occurs. Iron enters the environment naturally through the weathering of sulfide minerals and 

igneous, sedimentary, and metamorphic rocks. Anthropogenic 

sources of iron include coal burning, acid mine drainage, mineral 

processing, sewage, landfill leachates, and iron-related industries 

(Canadian Council of Resource and Environment Ministers, 1992). 

Whereas, iron concentrations usually are less than 500 μg/L in 

well aerated surface water, concentrations in ground water and 

thermal hot springs may range from 10,000 to 100,000 μg/L 

(Canadian Council of Ministers of the Environment, 1992). For 

this study, samples were analyzed for total iron concentration.

In the LIRB, the concentrations of iron in 85 percent of the 

analyses were above the MRL (50 μg/L), and the range of concentrations was from 50 to 21,000 μg/L (table 14–1.). The KruskalWallis test indicated a significant difference in iron concentrations 

between the Quaternary aquifer and the other aquifers, as indicated 

by the substantially higher median and 25th and 75th percentiles in 

the Quaternary aquifer. The highest values occurred in the Quaternary and Mississippian-Pennsylvanian aquifers, and within 150 ft 

of the land surface (fig. 14–2.). The lowest medians were determined for aquifers other than the Quaternary and depths greater 

than 50 ft. The highest medians and 25th percentiles were for 

depths of 151–300 ft within the LIRB. Iron concentrations are 

significantly different among aquifer groups and among depth 

intervals. Ground water with iron concentrations above 5,000 μg/L 

are more often detected east of the Illinois River. Arsenic, ammonia, nitrate/nitrite, manganese, and sulfate each had significant 

(p ≤ 0.05) correlation coefficients, exceeding 0.45, with iron.

Compared with the LIRB, iron concentrations for the rest 

of Illinois spanned an even greater range of values from less than 

50 to greater than 45,000 μg/L. There are significant differences by 

aquifer group and depth interval for the rest of Illinois. Although 

the highest values overall were in the rest of Illinois, the median 

and percentiles were lower in the rest of Illinois than in the LIRB. 

Between the LIRB and the rest of Illinois, the Kruskal-Wallis test 

indicated no significant difference in iron concentrations for either 

the Quaternary aquifer or the other aquifers, but unlike the LIRB, 

the median and 75th percentile generally decreased with depth 

interval in the rest of Illinois. In the rest of Illinois, the highest 

significant (p ≤ 0.05) correlation is between iron and arsenic (0.49) 

and manganese (0.69).

14. IRON

IRON CONCENTRATIONS ARE SIGNIFICANTLY DIFFERENT BETWEEN THE QUATERNARY AND OTHER AQUIFERS

There is a difference in iron concentration by aquifer and depth interval within the lower Illinois River Basin. There is no 

significant difference in iron concentration in the Quaternary and other aquifers between the lower Illinois River Basin and 

the rest of Illinois.
14. Iron 31

Figure 14–1. Iron concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in 

the lower Illinois River Basin.

Figure 14–2. Distribution of iron concentrations in ground water 

by aquifer group and depth interval.

(See figure 4–1. for explanation of boxplots. Method reporting limit is 50 micrograms per 

liter (μg/L). U.S. Environmental Protection Agency Secondary Maximum Contaminant 

Level is 300 μg/L.)

Table 4–1. Summary statistics for iron within and outside the lower 

Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limit; percentiles and maximums are in micrograms per liter; 

gray cells are interpolated values; other aquifers include wells open to 

multiple aquifers and bedrock aquifers]

Percentiles

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 612 521 250 1,400 2,600 21,000

Quaternary aquifer 474 400 410 1,600 2,800 13,000

Other aquifers 63 54 120 250 1,100 21,000

Undefined aquifers 75 67 190 500 1,800 6,000

Depths less than 50 feet 74 55 75 700 2,100 21,000

Depths 50 feet

or greater 529 457 290 1,500 2,600 13,000

Unreported depths 9 9 750 2,000 2,800 5,100

Rest of Illinois

Overall 1,880 1,588 120 450 1,500 45,000

Quaternary aquifer 541 476 400 1,500 2,900 40,000

Other aquifers 907 762 96 290 820 45,000

Undefined aquifers 432 350 86 340 1,100 17,000

Depths less than 50 feet 78 59 92 1,100 3,300 40,000

Depths 50 feet

or greater 1,768 1,506 120 450 1,400 45,000

Unreported depths 34 23 35 130 860 11,000

 LOWER ILLINOIS RIVER BASIN

MRL

SMCL

SMCL

REST OF ILLINOIS

IRON CONCENTRATION, IN MICROGRAMS PER LITER

0.1

45,100

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

10,000

20,000

0.1

45,100

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

10,000

20,000

0.1

45,100

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

10,000

20,000

AQUIFER GROUP

 LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

0.1

45,100

0.2

0.5

1

2

5

10

20

50

100

200

500

1,000

2,000

5,000

10,000

20,000

IRON CONCENTRATION, IN MICROGRAMS PER LITER

MRL

MRL

MRL

SMCL

SMCL
32 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Lead exposure, as measured by blood test levels, is associated with a range of adverse health effects, including interference 

with heme synthesis necessary for formation of red blood cells, 

anemia, kidney damage, impaired reproductive function, interference with vitamin D metabolism, impaired cognitive performance, 

delayed neurological and physical development, and elevations 

in blood pressure (American Waterworks Association, 1990). 

Lead can have a deleterious effect on the cognitive development 

of infants and young children (Shannon and Graef, 1987), who 

absorb ingested lead more readily than do older children and 

young adults. Listed as a probable human carcinogen by the 

USEPA, lead also accumulates in aquatic organisms, including 

benthic bacteria, fresh water plants, invertebrates, and fish, but 

does not become amplified at successive levels of the food chain 

(Canadian Council of Resource and Environment Ministers, 1992). 

The IEPA Class I standard for potable ground-water resources and 

Class II standard for general ground-water resources are 7.5 μg/L 

and 100 μg/L, respectively. The USEPA MCL is 15 μg/L and the 

MCLG is 1,300 μg/L for lead concentrations in drinking water. 

There are 37 samples with concentrations above 7.5 μg/L and 7 of 

these samples are above 100 μg/L (fig. 15–1.).

Concentrations of dissolved lead generally are low in 

surface and ground water because of the tendency of lead to sorb 

onto sediment surfaces and form insoluble compounds with 

carbonates, hydroxides, sulfides, sulfates, and various oxides 

(Canadian Council of Resource and Environment Ministers, 1992). 

Lead in drinking water primarily comes from the corrosion of lead 

pipes and solder (American Waterworks Association, 1990), especially if water is poorly buffered or has a pH below neutrality. 

Although the concentrations of lead in drinking water commonly 

are less than 50 μg/L, concentrations can be as high as 2,600 μg/L 

in tap water left unused overnight in lead plumbing (Canadian 

Council of Resource and Environment Ministers, 1992).

Lead enters the environment naturally through the weathering of rock, particularly sulfide minerals. Anthropogenic sources, 

which contribute more lead to the hydrosphere than weathering, 

include mining, milling, smelting, and refining operations; atmospheric deposition; street runoff; and industrial and municipal 

wastewater discharges (Canadian Council of Resource and Environment Ministers, 1992).

In the LIRB, the concentrations of lead in 8 percent of the 

analyses were above the MRL (5.0 μg/L) and, therefore, statistical 

tests for differences among data sets are not valid. The range of 

concentrations was from 5 to 220 μg/L (table 15–1. and fig. 15–2.). 

The 25th, 50th, and 75th percentile concentrations of lead for all 

aquifers and specified depth intervals are at the MRL of 5 μg/L. 

The density of ground-water samples with detections of lead are in 

Peoria and Tazewell Counties near the Illinois River. In the LIRB, 

lead is significantly correlated with zinc (0.42) and copper (0.46), 

which are often found together in sulfide minerals in the carbonates of Illinois and are both constituents that may be dissolved 

from metal pipes and solder.

The rest of Illinois had 5 percent of observations above the 

MRL. The range of concentrations in the rest of Illinois was from 5 

to 200 μg/L. The 25th, 50th, and 75th percentile concentrations of 

lead for all aquifers and depth intervals are at the MRL of 5 μg/L. 

Lead is not significantly correlated to other trace elements for the 

rest of Illinois, except for zinc (0.22) and copper (0.32), which 

have low correlation coefficients but high significance (p ≤ 0.05).

15. LEAD

LEAD CONCENTRATIONS ARE LOW

There is no difference in lead concentration by aquifer or depth interval within the lower Illinois River Basin. There is a 

significant difference in lead concentration in the Quaternary aquifers between the lower Illinois River Basin and the rest 

of Illinois.
15. Lead 33

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

CLASS II standard

MCL

MRL

Figure 15–1. Lead concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in 

the lower Illinois River Basin.

Figure 15–2. Distribution of lead concentrations in ground water 

by aquifer group and depth interval.

(U.S. Environmental Protection Agency Maximum Contaminant Level is 

15 micrograms per liter (μg/L). Illinois Environmental Protection Agency Class II 

standard is 100 μg/L. Method reporting limit is 5.0 μg/L.)

Table 15–1. Summary statistics for lead within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above the 

method reporting limit; percentiles and maximums are in micrograms per 

liter; <, less than; gray cells are interpolated values; other aquifers include 

wells open to multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 613 46 <5.0 <5.0 <5.0 220

Quaternary aquifers 475 33 <5.0 <5.0 <5.0 220

Other aquifers 63 5 <5.0 <5.0 <5.0 190

Undefined aquifers 75 8 <5.0 <5.0 <5.0 190

Depths less than 50 feet 74 6 <5.0 <5.0 <5.0 200

Depths 50 feet

or greater 530 37 <5.0 <5.0 <5.0 220

Unreported depths 9 3 .42 2.1 22 80

Rest of Illinois

Overall 1,891 94 <5.0 <5.0 <5.0 200

Quaternary aquifers 543 24 <5.0 <5.0 <5.0 48

Other aquifers 912 41 <5.0 <5.0 <5.0 50

Undefined aquifers 436 29 <5.0 <5.0 <5.0 200

Depths less than 50 feet 79 3 <5.0 <5.0 <5.0 33

Depths 50 feet

or greater 1,778 88 <5.0 <5.0 <5.0 200

Unreported depths 34 3 <5.0 <5.0 <5.0 62

0

250

50

100

150

200

0

250

50

100

150

200

LEAD CONCENTRATION, IN MICROGRAMS PER LITER LEAD CONCENTRATION, IN MICROGRAMS PER LITER

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

CLASS II standard

MCL

MRL
34 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Manganese is an essential trace element for microorganisms, plants, and animals, and is unlikely to be toxic at concentrations found in natural waters. Manganese is regulated primarily 

for its tendency to deposit black oxide stains on laundry, cooking 

utensils, plumbing fixtures, and pipes. An upper limit of 10 μg/L 

has been recommended to minimize staining, and concentrations 

exceeding 200 μg/L may cause an unpleasant taste in drinking 

water (McNeely, Neimanis and Dwyer, 1979). The IEPA Class I 

standard for potable resource ground water and Class II standard 

for general resource ground water for manganese (Mn) are 150 and 

10,000 μg/L, respectively. The USEPA SMCL is 50 μg/L. Within 

the LIRB, 26 percent of the 613 sample concentrations exceeded 

the Class I standard for potable resource ground water, whereas 

none exceeded the Class II standard for general resource ground 

water. In the LIRB, 161 analyses are above the IEPA Class I standard (150 μg/L). Ten percent of the 1,881 observations in the rest 

of Illinois were greater than 150 μg/L (fig. 16–1.).

In the absence of oxygen, under reducing conditions, 

manganese is present primarily as dissolved Mn+2 ions. Exposure 

to air will oxidize Mn+2 to its tetravalent form (Mn+4), which tends 

to precipitate as manganese oxides. Manganese commonly occurs 

in association with iron (Canadian Council of Resource and Environment Ministers, 1992). Manganese is widely distributed in 

soils, sediments, and metamorphic and sedimentary rocks, which 

provide an appreciable natural source of Mn through weathering. 

The iron and steel industry and acid mine drainage, in particular, 

release a large portion of the manganese found in the environment 

(Canadian Council of Resource and Environment Ministers, 1992).

In the LIRB, the concentrations of manganese in 84 percent of the analyses were above the MRL (5.0 μg/L), and the range 

of concentrations was from 5 to 1,500 μg/L (table 16–1.). Application of the Kruskal-Wallis test showed significant differences 

between aquifers and by depth interval in the LIRB (fig. 16–2.). 

Compared with the other aquifers within the LIRB, the highest 

quartiles (25th, 50th, and 75th) and percentage of concentrations 

above the MRL were in the Quaternary aquifer. Only the Mississippian-Pennsylvanian aquifer had comparable values for the 

mean, median, and maximum. Manganese concentrations are highest within 100 ft of the land surface and lowest for depths greater 

than 300 ft. The mean and 75th and 90th percentiles for manganese 

generally decreased with increasing depth interval. Ground water 

with manganese concentrations above 150 μg/L are most common 

in the southern part of the LIRB in the shallow Quaternary aquifer. 

Boron and iron had the highest significance values (p ≤ 0. 05) 

for Spearman’s rank-order correlation coefficients for manganese 

(−0.46 and 0.47, respectively).

Comparison between the rest of Illinois and the LIRB 

shows that there is not a significant difference in the Quaternary 

or older aquifers. Among the aquifer groups in the rest of Illinois, 

manganese concentrations were highest in the Quaternary aquifer. 

Overall, the median concentration of manganese in ground water 

in the rest of Illinois is lower than in the LIRB. Manganese concentrations in the rest of Illinois appeared to decrease with increasing 

depth interval, with the highest concentrations occurring within 

100 ft of the land surface. In the rest of Illinois, only iron is significantly correlated with manganese (0.69).

16. MANGANESE

MANGANESE CONCENTRATIONS ARE HIGHEST IN THE QUATERNARY AQUIFER

There is a difference in manganese concentration by aquifer and depth interval within the lower Illinois River Basin. There 

is no significant difference in manganese concentration in the Quaternary and older aquifers between the lower Illinois River 

Basin and the rest of Illinois.
16. Manganese 35

Figure 16–1. Manganese concentrations above the method 

reporting limit for Quaternary, other aquifers, and undefined 

aquifers in the lower Illinois River Basin.

Figure 16–2. Distribution of manganese concentrations in ground 

water by aquifer group and depth interval. 

(See figure 4–1. for explanation of boxplots. Illinois Environmental Protection Agency 

Class I standard is 150 micrograms per liter (μg/L). U.S. Environmental Protection Agency 

Secondary Maximum Contaminant Level is 50 μg/L. Method reporting limit is 5.0 μg/L.)

Table 16–1. Summary statistics for manganese within and outside 

the lower Illinois River Basin

[N, number of observations; narl, number of observations above the 

method reporting limit; percentiles and maximums are in micrograms per 

liter; gray cells are interpolated values; other aquifers include wells open to 

multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 613 512 12 46 160 1,500

Quaternary aquifers 475 417 23 64 190 1,500

Other aquifers 63 37 1.8 7.0 15 1,400

Undefined aquifers 75 58 6.0 15 62 370

Depths less than 50 feet 74 58 9.8 180 370 1,400

Depths 50 feet

or greater 530 447 12 42 140 1,500

Unreported depths 9 8 .19 31 88 140

Rest of Illinois

Overall 1,881 1,498 5.7 15 45 2,200

Quaternary aquifers 541 494 27 59 180 2,200

Other aquifers 908 675 4.6 10 21 1,200

Undefined aquifers 432 329 5.0 11 36 1,100

Depths less than 50 feet 78 71 47 170 410 1,500

Depths 50 feet

or greater 1,769 1,400 5.5 14 42 2,200

Unreported depths 34 27 5.0 10 .27 280

REST OF ILLINOIS

1

2,300

2

5

10

20

50

100

200

500

1,000

LOWER ILLINOIS RIVER BASIN

1

2,300

2

5

10

20

50

100

200

500

1,000

REST OF ILLINOIS

1

2,300

2

5

10

20

50

100

200

500

1,000

LOWER ILLINOIS RIVER BASIN

MANGANESE CONCENTRATION, IN MICROGRAMS PER LITER

1

2,300

2

5

10

20

50

100

200

500

1,000

MANGANESE CONCENTRATION, IN MICROGRAMS PER LITER

AQUIFER GROUP

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

150–199200–249250–300

Greater than 300

100–149

50–99

Less than 50

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

SMCL

MRL

CLASS I standard

MRL

MRL

MRL

SMCL

SMCL

SMCL

CLASS I standard

CLASS I standard

CLASS I 

standard
36 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

The methylmercury ion is about 50 times more toxic to 

mammals than inorganic mercury salts. Mercury compounds have 

a long retention time in the human body (McNeely, Neimanis and 

Dwyer, 1979). The ingestion of mercury, particularly as part of an 

organic compound such as methylmercury, can result in permanent 

brain damage (McNeely, Neimanis, and Dwyer, 1979). The IEPA 

Class I standard for potable resource ground water and the Class II 

standard for general resource ground water for mercury, except due 

to natural causes, are 2 and 10 μg/L, respectively. The MCL for 

mercury (inorganic) is 2 μg/L. There are no samples with mercury 

concentrations above the MCL, Class I, or Class II standards. 

Overall, there are few detections—only 298 detections out of 

2,485 samples.

The most stable form of mercury in most natural water 

systems is the free metal, Hg (aq). Concentrations of mercury 

in a ground-water system closed to the atmosphere are likely 

to be higher than concentrations in water open to the atmosphere 

because mercury tends to vaporize (Hem, 1985). Although mercury is rare, its natural tendency to volatilize widely disperses it. 

Organic complexes, such as methylmercury, (HgCH3) and other 

similar forms, can be produced by methane-generating bacteria in 

contact with metallic mercury in sediments (Hem, 1985). Higher 

mercury concentrations in unfiltered samples, such as those collected for this study, generally are associated with unidentified solids. Concentrations of mercury in filtered samples usually are very 

low (Hem, 1985). Low concentrations of inorganic mercury favor 

the formation of dimethylmercury, whereas high concentrations 

favor the formation of monomethylmercury (McNeely, Neimanis, 

and Dwyer, 1979).

Mercury in the earth’s crust occurs only in minor quantities 

as the mineral cinnabar. A number of sulfide minerals, such as 

sphalerite, chalcopyrite, and galena, may contain mercury. Various 

industries have used mercury in products such as paints, electrical 

equipment, agricultural applications, and dental amalgams. In 

1960, the use of mercury for treatment of seed grain and various 

other applications was banned in the United States. The burning of 

fossil fuels also can introduce mercury compounds to the environment (McNeely, Neimanis, and Dwyer, 1979).

In the LIRB, the concentrations of mercury in 12 percent 

of the analyses were above the MRL (0.005, 0.01, or 0.10 μg/L) 

(fig. 17–1.), and the range of concentrations was from 0.01 to 

1.10 μg /L (table 17–1. and fig.17–2.). The water from wells that 

were open to multiple or unidentified aquifers had 56 percent of 

the samples above the MRL. Most detections are in the east central 

part of the LIRB. A low but statistically significant (p ≤ 0.05) 

Spearman’s correlation coefficient between mercury and cyanide 

(0.31) or copper (0.26) indicates a slight correlation within the 

LIRB.

In the rest of Illinois, the concentrations of mercury in 

12 percent of the analyses were above the MRL (0.005, 0.01, 

or 0.10 μg/L), and the range of concentrations was from 0.01 to 

1.60 μg /L. The depth interval of less than 50 ft had a slightly 

higher percent (20 percent) of samples above the MRL compared 

with the other depth intervals, which ranged from 9 to 13 percent. 

The Spearman’s correlation between mercury and cyanide (0.27) 

or copper (0.11) is significant for the rest of Illinois. 

17. MERCURY

MERCURY CONCENTRATIONS DO NOT EXCEED 2 MICROGRAMS PER LITER

There is a difference in mercury concentration by aquifer and depth interval within the lower Illinois River Basin. There is 

no significant difference in mercury concentration in the Quaternary and other aquifers between the lower Illinois River 

Basin and the rest of Illinois.
17. Mercury 37

Figure 17–1. Mercury concentrations above the method 

reporting limits for Quaternary, other aquifers, and undefined 

aquifers in the lower Illinois River Basin.

Figure 17–2. Distribution of mercury concentrations in ground 

water by aquifer group and depth interval.

(U.S. Environmental Protection Agency Maximum Contaminant Level is 

2.0 micrograms per liter (μg/L); method reporting limits vary among 0.1, 0.01, 

and 0.005 μg/L depending on the analytical method.)

Table 17–1. Summary statistics for mercury within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above the 

method reporting limits; percentiles and maximums are in micrograms per 

liter ; other aquifers include wells open to multiple aquifers and bedrock 

aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 606 71 0.01 0.05 0.05 1.1

Quaternary aquifers 469 56 .01 .05 .05 1.1

Other aquifers 63 11 .01 .05 .05 .12

Undefined aquifers 74 14 .01 .05 .05 .18

Depths less than 50 feet 72 7 .01 .01 .05 .06

Depths 50 feet

or greater 526 63 .01 .05 .05 1.1

Unreported depths 8 1 .03 .05 .05 .05

Rest of Illinois

Overall 1,879 227 0.01 0.05 0.05 1.6

Quaternary aquifers 541 65 .01 .01 .05 .86

Other aquifers 903 90 .01 .05 .05 1.6

Undefined aquifers 435 72 .01 .05 .05 .21

Depths less than 50 feet 79 16 .01 .05 .05 .49

Depths 50 feet

or greater 1,766 208 .01 .05 .05 1.6

Unreported depths 34 3 .05 .05 .05 .15

0

2.0

0.5

1.0

1.5

0

2.0

0.5

1.0

1.5

MERCURY CONCENTRATION, IN MICROGRAMS PER LITER

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

MERCURY CONCENTRATION, IN MICROGRAMS PER LITER
38 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Low levels of selenium are essential to human health. 

There are some correlations between reduced levels of breast 

cancer and heart disease at less than acute levels (Illinois State 

Water Survey, 1977b). Most intake of selenium is from food, 

which reflects the local soil conditions (American Waterworks 

Association, 1990). The concentration of selenium in drinking 

water typically is low. In 1978, USEPA surveys of the water 

quality of rural and community ground water showed 43 of 329 

(13 percent) ground-water samples had selenium concentrations 

greater than 5 μg/L, and 10 of 329 samples (3 percent) had selenium concentrations greater than 10 μg/L (American Waterworks 

Association, 1990). The IEPA Class I and the Class II standards for 

selenium, except due to natural causes, are 50 μg/L. The USEPA 

drinking water standard MCL and MCLG also are 50 μg/L. There 

were many detections of selenium in the LIRB (fig. 18–1.). There 

were no concentrations above the standard for selenium for the 

LIRB or rest of Illinois in the samples collected for this study.

Dissolved selenium in the aquatic environment occurs in 

oxygenated water as selenite (SeO3

2-) or selenate (SeO42-) ionic 

species but is readily reduced to elemental and relatively insoluble 

Se0

 (Hem, 1985). Selenium is frequently found in large amounts in 

deposits of native sulfur and sulfide ores of heavy metals and in 

coal (Canadian Council of Resource and Environment Ministers, 

1992). Selenium occurs naturally in water in trace amounts as a 

result of geochemical processes such as weathering of rocks and 

erosion of soil (Canadian Council of Resource and Environment 

Ministers, 1992). Selenium is found at low concentrations in the 

soils of Illinois. Anthropogenic sources of selenium in water 

include effluents from copper and lead refineries and municipal 

sewage (Canadian Council of Resource and Environment Ministers, 1992).

In the LIRB, the concentrations of selenium in 21 percent 

of the analyses were above the MRL (1.0 μg/L), and the range of 

selenium concentrations was from 1 to 7 μg/L (table 18–1.). In the 

LIRB, there are differences among aquifers and by depth interval 

of selenium concentrations as determined using the Kruskal-Wallis 

test (fig. 18–2.). There is a significant difference in concentrations 

between the Quaternary aquifer and older aquifers. The median 

concentrations for the Quaternary and older aquifers are below 

the MRL of 1 μg/L, but there was a larger percent of samples 

(24 percent) with concentrations above the MRL for the Quaternary aquifer compared with the older aquifers (3 percent). The 

water from wells that are less than 50 ft deep had more samples 

with concentrations above the MRL (34 percent) than samples 

from wells greater than 50 ft deep (19 percent). The concentrations 

of selenium in ground water over 2 μg/L are concentrated along 

the Illinois River upstream from the city of Peoria. Selenium 

concentrations were compared with other trace elements, nutrients, 

and inorganic constituents using the Spearman’s correlation coefficient. The correlations between selenium and nitrate (0.40) and 

ammonia (−0.31) had the most significance. There are small but 

significant (p ≤ 0.05) correlations among selenium and iron, fluoride, arsenic, boron, and sulfate.

There is no significant difference in selenium concentrations between the LIRB and the rest of Illinois for the Quaternary 

and older aquifers or by depth interval. A higher percent of concentrations above the MRL were found in the samples from the LIRB 

(21 percent) compared with the rest of Illinois (10 percent). The 

median for the rest of Illinois and the LIRB is 1 μg/L, which is at 

the MRL.

18. SELENIUM

SELENIUM DETECTED IN SHALLOW QUATERNARY AQUIFER

There is a difference in selenium concentration by aquifer and depth interval within the lower Illinois River Basin. There is 

no significant difference in selenium concentration in the Quaternary and other aquifers between the lower Illinois River 

Basin and the rest of Illinois.
18. Selenium 39

Figure 18–1. Selenium concentrations above the method 

reporting limit for Quaternary, other aquifers, and undefined 

aquifers in the lower Illinois River Basin.

Figure 18–2. Distribution of selenium concentrations in 

ground water by aquifer group and depth interval. 

(Method reporting limit is 1.0 microgram per liter.)

Table 18–1. Summary statistics for selenium within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above the 

method reporting limit; percentiles and maximums are in micrograms per 

liter; <, less than; gray cells are interpolated values; other aquifers include 

wells open to multiple aquifers and bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median)

75 Maximum

Lower Illinois River Basin

Overall 613 128 <1.0 <1.0 <1.0 7.0

Quaternary aquifers 475 115 .39 .60 .94 7.0

Other aquifers 63 2 <1.0 <1.0 <1.0 1.0

Undefined aquifers 75 11 <1.0 <1.0 <1.0 4.0

Depths less than 50 feet 74 25 .49 .74 1.0 5.0

Depths 50 feet

or greater 530 101 <1.0 <1.0 <1.0 7.0

Unreported depths 9 2 <1.0 <1.0 <1.0 1.0

Rest of Illinois

Overall 1,885 195 <1.0 <1.0 <1.0 39

Quaternary aquifers 542 94 <1.0 <1.0 <1.0 25

Other aquifers 908 58 <1.0 <1.0 <1.0 5.0

Undefined aquifers 435 43 <1.0 <1.0 <1.0 39

Depths less than 50 feet 78 21 .15 .39 1.0 25

Depths 50 feet

or greater 1,773 172 <1.0 <1.0 <1.0 13

Unreported depths 34 2 <1.0 <1.0 <1.0 39

0

40

10

20

30

SELENIUM CONCENTRATION, IN MICROGRAMS PER LITER

0

40

10

20

30

SELENIUM CONCENTRATION, IN MICROGRAMS PER LITER

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

MRL

MRL
40 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Silver (Ag) is not considered a health threat by the USEPA 

because silver does not impair bodily functions or cause physiological problems, and there is no evidence of carcinogenicity or 

mutagenicity (American Waterworks Association, 1990). Accumulations of 1 to 5 grams of silver can result in argyria (a permanent, 

blue-gray discoloration of the skin and organs), but no known 

cases of argyria from drinking water have been reported (American 

Waterworks Association, 1990). The IEPA Class I standard for 

silver is 50 μg/L, but there is no IEPA Class II standard for silver. 

There is no USEPA MCL or MCLG for silver, but the SMCL is 

100 μg/L. There are low detections of silver (3–31 μg/L) in ground 

water in the LIRB (fig. 19–1.) and the rest of Illinois but no concentrations above the SMCL.

The solubility of silver is limited to less than 10 μg/L 

in most natural water due to pH-EH conditions (Hem, 1985). 

Silver occurs naturally in its elemental form and in ores, such as 

argentite (Ag2S) and cerargyrite (AgCl). Silver also may be found 

in association with sulfur, chlorides, and ammonia salts. Silver 

is released into the environment by weathering; volcanic activity; 

and discharges from photographic, electronic, and electroplating 

industries. Most of the world’s silver production comes as a byproduct of lead-zinc, copper, and gold mining (Canadian Council 

of Resource and Environmental Ministers, 1992). Concentrations 

of silver generally are low in the aquatic environment because of 

low crustal abundance, precipitation as solids, and sorption to solids, particularly by manganese dioxide. Silver iodide has been used 

in seeding clouds with condensation nuclei to induce rainfall, but 

resulting concentrations of silver in rainfall are too low to have 

substantial effects on ground water (Hem, 1985).

In the LIRB, the concentrations of silver in 12 percent of 

the analyses were above the MRL (3.0 μg/L), and the range of 

concentrations was from 3 to 13 μg/L (fig. 19–2. and table 19–1.). 

The ranges of concentrations for the Quaternary and older aquifers 

are similar. The Quaternary aquifer had the lowest percentage of 

concentrations above the MRL (10 percent), whereas 17 percent 

for the older aquifers were above the MRL. There does not appear 

to be a spatial pattern of silver detections in ground water. Chromium (0.32) and copper (0.25) are significantly correlated with 

silver.

There is a greater range of silver concentrations in the rest 

of Illinois, but the 75th percentile was still at the MRL. Samples 

from older aquifers in the rest of Illinois had less than 1 percent 

silver concentrations above the MRL compared with 17 percent of 

silver concentrations from older aquifers above the MRL for the 

LIRB. There is a higher percentage of detections of silver in the 

water from wells less than 50 ft deep. Similar to the LIRB, there is 

a significant correlation of silver with chromium (0.29) and copper 

(0.15).

19. SILVER

SILVER CONCENTRATIONS VARY BY AQUIFER

There is a difference in silver concentration by aquifer within the lower Illinois River Basin. There is a significant difference 

in silver concentration in the older aquifers between the lower Illinois River Basin and the rest of Illinois.
19. Silver 41

Figure 19–1. Silver concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in 

the lower Illinois River Basin.

Figure 19–2. Distribution of silver concentrations in ground 

water by aquifer group and depth interval. 

(Method reporting limit is 3.0 micrograms per liter.)

Table 19–1. Summary statistics for silver within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above the 

method reporting limit; percentiles and maximums are in micrograms per 

liter; <, less than; other aquifers include wells open to multiple aquifers and 

bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median) 75 Maximum

Lower Illinois River Basin

Overall 612 72 <3.0 <3.0 <3.0 13

Quaternary aquifers 474 48 <3.0 <3.0 <3.0 13

Other aquifers 63 11 <3.0 <3.0 <3.0 12

Undefined aquifers 75 13 <3.0 <3.0 <3.0 10

Depths less than 50 feet 74 7 <3.0 <3.0 <3.0 10

Depths 50 feet

or greater 529 64 <3.0 <3.0 <3.0 13

Unreported depths 9 1 <3.0 <3.0 <3.0 11

Rest of Illinois

Overall 1,880 132 <3.0 <3.0 <3.0 31

Quaternary aquifers 541 45 <3.0 <3.0 <3.0 31

Other aquifers 908 53 <3.0 <3.0 <3.0 27

Undefined aquifers 431 34 <3.0 <3.0 <3.0 19

Depths less than 50 feet 78 15 <3.0 <3.0 <3.0 13

Depths 50 feet

or greater 1,768 115 <3.0 <3.0 <3.0 31

Unreported depths 34 2 <3.0 <3.0 <3.0 19

0

40

10

20

30

SILVER CONCENTRATION, IN MICROGRAMS PER LITER

0

40

10

20

30

SILVER CONCENTRATION, IN MICROGRAMS PER LITER

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

AQUIFER GROUP

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

MRL

MRL
42 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

Zinc is relatively nontoxic to man (McNeely, Neimanis, 

and Dwyer, 1979). There are no known health effects from excess 

zinc intake, but there are some adverse effects of a too low intake 

of zinc (American Waterworks Association, 1990). Excessive 

amounts of zinc are regulated on the basis of taste and smell 

considerations. The IEPA Class I and the Class II standards for 

zinc, except due to natural causes, are 5,000 and 10,000 μg/L, 

respectively. The USEPA SMCL is 5,000 μg/L. Two samples had 

zinc concentrations above the SMCL (fig. 20–1.). There were no 

samples above the Class I standard for samples from wells in the 

rest of Illinois.

Zinc is a common metal in the environment. Zinc is found 

in rocks as a sulfide or carbonate associated with copper-zinc and 

copper-lead-zinc ores (McNeely, Neimanis, and Dwyer, 1979). 

These ores are present in Illinois. Zinc is used in coatings to 

protect iron and steel; in alloys for die casting; and in brass rolled 

sheets and strips for dry batteries, roofing and exterior fittings on 

buildings, and some printing processes (Canadian Council of 

Resource and Environment Ministers, 1992). Zinc is used in brass 

and other alloys used in plumbing systems (Illinois State Water 

Survey, 1977a).

Zinc (Zn) has only one important oxidation state in the 

aqueous system (Zn2+) and tends to be relatively soluble in water 

(Hem, 1985). The highest dissolved zinc concentration in water is 

possible at low pH, low alkalinity, and high ionic strength (Hem, 

1985). In natural waters, zinc can be found in many forms, such 

as simple hydrated ions, and inorganic and organic complexes 

(Canadian Council of Resource and Environment Ministers, 1992). 

In the presence of other dissolved solids, much of the zinc will be 

transported in solution as hydrated cations or complex species. 

In the presence of suspended solids, much of the zinc will be 

sorbed to suspended solids and colloidal particles (Canadian 

Council of Resource and Environment Ministers, 1992).

In the LIRB, the concentrations of zinc in 7 percent of the 

analyses were above the MRL (50 or 100 μg/L), and the range of 

concentrations was from 50 to 16,000 μg/L (fig. 20–2. and table 

20–1.). In the LIRB, 78 percent of the samples that were analyzed 

for zinc are from the Quaternary aquifer. The Quaternary aquifer 

also had the highest zinc concentration—16,000 μg/L. All aquifers 

have a median concentration of 50 μg/L, which is at the lowest 

MRL. There is no significant difference in zinc concentrations by 

aquifer group within the LIRB. The median concentration for zinc 

is at the MRL for all depth intervals. The samples from wells less 

than 50 ft deep in the LIRB had 16 percent detections, and wells 

greater than 50 ft deep had detections of zinc in only 5 percent of 

samples. Although the median and quartiles are the same between 

samples for less than 50 ft and deeper, the range of concentrations 

increases with increasing depth interval. The range of concentrations for the LIRB is from 50 to 16,000 μg/L, with the maximum 

concentrations in the Quaternary aquifer at a depth interval greater 

than 300 ft. There is a significant difference in zinc concentrations 

by depth interval for the LIRB. There is no spatial pattern of zinc 

concentration in ground water. Zinc is significantly correlated with 

lead (0.42) and copper (0.33). The correlation of zinc with cyanide 

and mercury is small, but there is a high probability that there is 

some correlation.

Zinc concentrations were significantly different in the rest 

of Illinois by aquifer and by depth interval. There is a significant 

difference between zinc concentrations in the Quaternary aquifer 

in the LIRB and the rest of Illinois but no significant difference for 

older aquifers. The correlations of zinc and other constituents in 

samples from the rest of Illinois show the highest correlations with 

copper (0.25) and lead (0.22).

20. ZINC

ZINC CONCENTRATIONS VARY SIGNIFICANTLY BY DEPTH

There is a difference in zinc concentration by depth interval but not by aquifer group within the lower Illinois River Basin. 

There is a significant difference in zinc concentration in the Quaternary aquifer between the lower Illinois River Basin and 

the rest of Illinois.
20. Zinc 43

Figure 20–1. Zinc concentrations above the method reporting 

limit for Quaternary, other aquifers, and undefined aquifers in 

the lower Illinois River Basin.

Figure 20–2. Distribution of zinc concentrations in ground water 

by aquifer group and depth interval.

(U.S. Environmental Protection Agency Secondary Maximum Contaminant Level is 

5,000 micrograms per liter.)

Table 20–1. Summary statistics for zinc within and outside the 

lower Illinois River Basin

[N, number of observations; narl, number of observations above method 

reporting limits; percentiles and maximums are in micrograms per liter; 

<, less than; other aquifers include wells open to multiple aquifers and 

bedrock aquifers]

Percentile

Aquifer or

depth interval N narl 25 50(median) 75 Maximum

Lower Illinois River Basin

Overall 612 43 <50 <50 <50 16,000

Quaternary aquifers 475 32 <50 <50 <50 16,000

Other aquifers 63 6 <50 <50 <50 12,000

Undefined aquifers 74 5 <50 <50 <50 4,600

Depths less than 50 feet 74 12 <50 <50 <50 1,700

Depths 50 feet

or greater 529 29 <50 <50 <50 16,000

Unreported depths 9 2 <50 <50 120 350

Rest of Illinois

Overall 1,879 148 <50 <50 <50 4,000

Quaternary aquifers 541 25 <50 <50 <50 1,600

Other aquifers 906 79 <50 <50 <50 2,400

Undefined aquifers 432 44 <50 <50 <50 3,900

Depths less than 50 feet 78 5 <50 <50 <50 770

Depths 50 feet

or greater 1,767 139 <50 <50 <50 3,600

Unreported depths 34 4 <50 <50 <50 3,900

0

20,000

5,000

10,000

15,000

0

20,000

5,000

10,000

15,000

ZINC CONCENTRATION, IN MICROGRAMS PER LITER ZINC CONCENTRATION, IN MICROGRAMS PER LITER

150–199

200–249

250–300

Greater

than 300

100–149

50–99

Less than50

AQUIFER GROUP

DEPTH INTERVAL, IN FEET BELOW LAND SURFACE

Undefined

Quaternary

MississippianPennsylvanian

SilurianDevonian

CambrianOrdovician

Multiple

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

REST OF ILLINOIS

LOWER ILLINOIS RIVER BASIN

SMCL

SMCL
44 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

The lower Illinois River Basin study unit is part of the 

National Water-Quality Assessment program, which includes 

most major aquifers in the United States. More than one-half of 

the drinking water in the lower Illinois River Basin is supplied 

from ground water. The study characterized and compared the 

source water quality from wells in four major aquifer groups—

Quaternary, Mississippian-Pennsylvanian, Silurian-Devonian, 

and Cambrian-Ordovician. Water-quality data from wells that 

tapped undefined or multiple aquifers also were examined. The 

water-quality data were further characterized and compared with 

respect to the depth interval of the sampled wells. Similar waterquality data from wells in the area of the State outside of the lower 

Illinois River Basin were characterized and compared with the data 

from the lower Illinois River Basin.

Constituent concentrations vary in ground water in similar 

aquifers between the lower Illinois River Basin and the rest of 

Illinois (table 21–1.). There is a significant difference in lead 

concentrations in water from wells in the Quaternary aquifer 

between the lower Illinois River Basin and the rest of Illinois. 

Three other constituents (arsenic, sulfate, and zinc) also had 

significant differences in concentrations from wells in the Quaternary aquifer between the lower Illinois River Basin and the rest of 

Illinois. One common source for arsenic, lead, sulfate, and zinc is 

from mining and coal combustion. The older rocks in the lower 

Illinois River Basin are mined for coal. The percentage of land area 

within the lower Illinois River Basin with coal-bearing bedrock is 

greater than the area outside the lower Illinois River Basin containing coal-bearing bedrock, especially for areas where ground water 

is the main drinking-water source. The bedrock materials that were 

eroded and redeposited by Quaternary continental glaciers are a 

source for minerals that contain arsenic, lead, sulfate, and zinc. 

Leaching of these minerals combined with coal processing byproducts make the Quaternary aquifer susceptible to high concentrations of these minerals. Therefore, the lower Illinois River Basin 

contains a source in the immediate area for arsenic, lead, sulfate, 

and zinc that could easily leach to the ground-water system.

The concentrations of ammonia, arsenic, boron, chloride, 

chromium, and silver in the aquifers other than Quaternary are 

significantly different between the lower Illinois River Basin and 

the rest of Illinois. The median concentrations for ammonia and 

chloride are 2 to 10 times higher for the lower Illinois River Basin. 

The median concentrations for chromium and silver are the same 

for the lower Illinois River Basin and the rest of Illinois. Approximately 10 percent of all samples analyzed are above the MRL for 

chromium and silver, but the percent of detections above the MRL 

for the lower Illinois River Basin older aquifers is higher than the 

rest of Illinois. Within the lower Illinois River Basin, most constituents had differences in concentrations by aquifer or depth interval 

(table 21–2.). Chromium, copper, and lead did not show a significant difference by aquifers or depth interval.

The differences in concentrations of three constituents—

silver, sulfate, and zinc—in the lower Illinois River Basin and the 

rest of Illinois were significant by either depth interval or aquifer 

but not both. A comparison of concentration by depth interval 

within an aquifer indicated that the concentration of manganese, 

nitrate, and selenium are highest in the shallow Quaternary aquifer, 

whereas concentrations of arsenic are highest in the deeper 

Quaternary aquifer. The older aquifers did not show higher concentrations of any constituent at shallow depths. The concentrations of boron, chloride, chromium, and sulfate did increase within 

the older aquifers as depth increased.

21. SUMMARY AND CONCLUSIONS
21. Summary and Conclusions 45

Table 21–1. Significant difference in concentrations of 

constituents between the Quaternary and other aquifers 

of the lower Illinois River Basin and the rest of Illinois by 

means of the Wilcoxon Rank-Sum statistical test

Differences between the lower Illinois River Basin and the

rest of Illinois

Water-quality

constituent

Quaternary

aquifer

Other

aquifers

Ammonia No Yes

Nitrates and nitrites No No

Chloride No Yes

Sulfate Yes No

Arsenic Yes Yes

Boron No Yes

Chromium No Yes

Copper No No

Cyanide Not

applicable

Not

applicable

Iron No No

Lead Not

applicable

Not

applicable

Manganese No No

Mercury No No

Selenium No No

Silver No Yes

Zinc Yes No

Table 21–2. Significant difference in concentrations of constituents by 

aquifer group and depth interval by means of the Wilcoxon Rank-Sum 

statistical test for the lower Illinois River Basin and the rest of Illinois

Water-quality

constituent

Lower Illinois River Basin Rest of Illinois

Aquifer

group

Depth

interval

Aquifer

group

Depth

interval

Ammonia Yes Yes Yes Yes

Nitrates and

nitrites

Yes Yes Yes Yes

Chloride Yes Yes Yes Yes

Sulfate Yes Yes Yes No

Arsenic Yes Yes Yes Yes

Boron Yes Yes Yes Yes

Chromium No No No No

Copper No No No No

Cyanide Not

applicable

Not

applicable

Not

applicable

Not

applicable

Iron Yes Yes Yes Yes

Lead Not

applicable

Not

applicable

Not

applicable

Not

applicable

Manganese Yes Yes Yes Yes

Mercury Yes Yes Yes Yes

Selenium Yes Yes Yes Yes

Silver Yes No Yes Yes

Zinc No Yes Yes Yes
46 Analysis of Nutrients, Selected Inorganic Constituents, and Trace Elements in Water from Illinois Community-Supply Wells, 1984–91

American Waterworks Association, 1990, Water quality 

treatment in Pontius, Fredrick W., ed., (4th ed.): 

New York, McGraw-Hill, 1194 p.

Avery, Charles, 1995, Estimated water withdrawals and use 

in Illinois, 1988: U.S. Geological Survey Open-File 

Report 95–309, 52 p.

Canadian Council of Resource and Environment Ministers, 

1992, Canadian water quality guidelines: Ottawa, 

Ontario, Task Force on Water Quality Guidelines, 

Water Quality Branch, Inland Waters Directorate, 

88 p.

Fetter, C.W., 1993, Contaminant Hydrogeology: New 

York, Macmillan Publishing Company, 458 p.

Gibb, J.P., and O’Hearn, Michael, 1980, Illinois 

Groundwater Quality Data Summary: Urbana, 

Illinois, Illinois State Water Survey Division, 

Summary for contact number 1-47-26-84-353-00, 

60 p.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods 

in water resources: New York, Studies in 

Environmental Science 49, Elsevier Science 

Publishers, 522 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water: U.S. Geological 

Survey Water-Supply Paper 2254, 263 p.

Herzog, B.L., Warner, K.L., Stiff, B.J., Sieverling, J.B., 

Chenoweth, C.A., and Avery, C.F., 1994, Buried 

bedrock surface of Illinois: Illinois State Geological 

Survey Illinois Map 5, 1:500,000 scale, 1 sheet.

Illinois Environmental Protection Agency, 1990, Title—35 

Environmental protection, subtitle C—Water 

pollution, chapter I—Pollution control board: 

Springfield, Ill., Illinois Environmental Protection 

Agency, 195 p.

Illinois State Water Survey, 1977a, Technical letter 22, 

Removal of water supply contaminants—Copper 

and zinc: Urbana, Illinois; 2 p.

———1977b, Technical letter 24, Removal of water 

supply contaminants selenium: Urbana, Illinois, 2 p.

Kirk, J.R., 1987, Water withdrawals in Illinois, 1986: 

Illinois State Water Survey Circular 167, 43 p.

McNeely, R.N., Neimanis, V.P., and Dwyer, L., 1979, 

Water quality source-book-A guide to water quality 

parameters: Ottawa, Canada, Environment Canada, 

Inland Waters Directorate, Water Quality Branch, 

88 p.

O’Hearn, Michael, and Schock, S.C., 1984, Design of a 

statewide groundwater monitoring network for 

Illinois: Urbana, Illinois, Illinois State Water Survey 

Contract Report 354, 76 p.

Shannon, M., and Graef, J.W., 1987, Lead intoxication 

from lead-contaminated water used to reconstitute 

infant formula: Clinical Pediatrics, v. 28, no. 8, 

p. 380–382.

U.S. Department of Health, Education, and Welfare, 1970, 

Community water supply study—Analysis of 

national survey findings: Public Health Service, 

Environmental Health Service, Bureau of Water 

Hygiene, 111 p.

U.S. Environmental Protection Agency, 1986, RCRA 

ground-water monitoring technical enforcement 

guidance document: Office of Waste Programs 

Enforcement and Office of Solid Waste and 

Emergency Response (OSWER-9950.1), variously 

paginated.

U.S. Environmental Protection Agency, 1994, National 

primary drinking water standards: EPA Document 

810-F-94-001A, February, 1994, variously 

paginated.

U.S. Environmental Protection Agency, 1996, Drinking 

water regulations and health advisories: EPA 

Document 822-B-96-002, October, 1996.

University of Illinois, 1994, Illinois agronomy handbook, 

1995–96: College of Agriculture, Cooperative 

Extension Service Circular 1333, 205 p.

Voelker, D.C., 1986, Observation-well network in 

Illinois, 1984: U.S. Geological Survey Open-File 

Report 86–416 (W), 108 p.

Voelker, D.C., Oberg, D.J., and Grober, M.J., 1988, Waterquality data from the observation-well network in 

Illinois, 1985–87: U.S. Geological Survey OpenFile Report 87–538, 725 p.

Warner, K.L., 1998, Water-quality assessment of the 

lower Illinois River Basin: Environmental setting: 

U.S. Geological Survey Water-Resources 

Investigation Report 97–4165, 50 p.

Willman, H.B., Atherton, Elwood, Buschbach, T.C., 

Collinson, Charles, Frye, J.C., Hopkins, M.E., 

Lineback, J.A., and Simon, J.A., 1975, Handbook of 

Illinois stratigraphy: Illinois State Geological 

Survey Bulletin 95, 261 p.

Zar, J.H., 1984, Biostatistical analysis: Englewood Cliffs, 

N.J., Prentice-Hall, 718 p.

22. REFERENCES
Glossary 47

GLOSSARY

Class I Standard for Potable Resource Ground Water. Maximum permissible level 

determined by the Illinois Environmental Protection Agency for all ground water 

that is 10 feet or more below the land surface and meets defined geologic criteria or 

is within 200 or 400 feet of a potable water supply.

Class II Standard for General Resource Ground Water. Maximum permissible level 

determined by the Illinois Environmental Protection Agency for all ground water 

that is not regulated by Class I standards.

Maximum Contaminant Level (MCL). Maximum permissible level of a contaminant in 

water that is delivered to any user of a public water system.

Maximum Contaminant Level Goal (MCLG). A nonenforceable concentration of a 

drinking water contaminant that is protective of adverse human health effects and 

allows an adequate margin of safety.

Secondary Maximum Contaminant Levels (SMCL). A nonenforceable Federal 

guideline for taste, odor, color, and certain other nonaesthetic aspects of drinking 

water.
District Chief

U.S. Geological Survey

221 North Broadway Avenue

Urbana, Illinois 61801

(217)344-0037

COMMUNITY-SUPPLY WELLS, 1984–91—U.S. Geological Survey Water-Resources Investigations Report 99–4152Warner—ANALYSIS OF NUTRIENTS, SELECTED INORGANIC CONSTITUENTS, AND TRACE ELEMENTS IN WATER FROM ILLINOIS