Water Quality in the

Lower Illinois River Basin

Illinois, 1995–98

U.S. Department of the Interior

U.S. Geological Survey

Circular 1209
POINTS OF CONTACT AND ADDITIONAL INFORMATION

Front cover: Summer wildflowers at Indian Creek near Wyoming, Ill. (photograph by David J. Fazio).

Back cover: Left, hydrologists collecting a water sample from Panther Creek near El Paso, Ill. (photograph by Paul 

J. Terrio); right, biologists collecting invertebrate samples from the Illinois River at Valley City, Ill. (photograph by 

David J. Fazio).

The companion Web site for NAWQA summary reports:

http://water.usgs.gov/nawqa/

Lower Illinois River Basin contact and Web site:

USGS State Representative

U.S. Geological Survey

Water Resources Division

221 N. Broadway Ave., Urbana, IL 61801

e-mail: dc_il@usgs.gov

http://il.water.usgs.gov/proj/lirb

National NAWQA Program:

Chief, NAWQA Program

U.S. Geological Survey

Water Resources Division

12201 Sunrise Valley Drive, M.S. 413

Reston, VA 20192

http://water.usgs.gov/nawqa/

River Basin Assessments

Albemarle-Pamlico Drainage Basin (Circular 1157)

Allegheny and Monongahela River Basins (Circular 1202)

Apalachicola-Chattahoochee-Flint River Basin (Circular 1164)

Central Arizona Basins (Circular 1213)

Central Columbia Plateau (Circular 1144)

Central Nebraska Basins (Circular 1163)

Connecticut, Housatonic and Thames River Basins (Circular 1155)

Eastern Iowa Basins (Circular 1210)

Georgia-Florida Coastal Plain (Circular 1151)

Hudson River Basin (Circular 1165)

Kanawha - New River Basins (Circular 1204)

Lake Erie - Lake Saint Clair Drainages (Circular 1203)

Las Vegas Valley Area and the Carson and Truckee River Basins

(Circular 1170)

Long Island-New Jersey Coastal Drainages (Circular 1201)

Lower Susquehanna River Basin (Circular 1168)

Mississippi Embayment (Circular 1208)

Ozark Plateaus (Circular 1158)

Potomac River Basin (Circular 1166)

Puget Sound Basin (Circular 1216)

Other NAWQA summary reports

Red River of the North Basin (Circular 1169)

Rio Grande Valley (Circular 1162)

Sacramento River Basin (Circular 1215)

San Joaquin-Tulare Basins (Circular 1159)

Santee River Basin and Coastal Drainages (Circular 1206)

South-Central Texas (Circular 1212) 

South Platte River Basin (Circular 1167)

Southern Florida (Circular 1207)

Trinity River Basin (Circular 1171)

Upper Colorado River Basin (Circular 1214) 

Upper Mississippi River Basin (Circular 1211)

Upper Snake River Basin (Circular 1160)

Upper Tennessee River Basin (Circular 1205)

Western Lake Michigan Drainages (Circular 1156)

White River Basin (Circular 1150)

Willamette Basin (Circular 1161)

National Assessments

The Quality of Our Nation’s Waters—Nutrients and Pesticides (Circular 1225)
U.S. GEOLOGICAL SURVEY CIRCULAR

Water Quality in the

Lower Illinois River Basin, Illinois, 1995–98

1209

By George E. Groschen, Mitchell A. Harris, Robin B. King, Paul J. Terrio, and 

Kelly L. Warner
Library of Congress Cataloging-in-Publications Data

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

does not constitute endorsement by the U.S. Government.

Free on application to the

U.S. Geological Survey

Information Services

Box 25286 Federal Center

Denver, CO 80225

Or call: 1-888-ASK-USGS

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

2000

Water quality in the Lower Illinois River Basin, Illinois, 1995–98 / by George E. Groschen...[et al.].

p. cm. -- (U.S. Geological Survey Circular ; 1209)

Includes bibliographical references.

ISBN 0-607-95423-X (alk. paper)

1. Water quality--Illinois--Illinois River Watershed. I. Groschen, G. E. II. Geological Survey (U.S.)

III. Series.

TD224.I3 W37 2000

363.739’42’097735--dc21

00-049464
CONTENTS

Contents III

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM ............................................................... IV

SUMMARY OF MAJOR FINDINGS.................................................................................................... 1

Stream and river highlights ........................................................................................................... 1

Ground-water highlights................................................................................................................ 2

INTRODUCTION TO THE LOWER ILLINOIS RIVER BASIN............................................................. 3

Land use ....................................................................................................................................... 3

Study design in relation to land use.............................................................................................. 4

Water use...................................................................................................................................... 4

Hydrologic conditions during the study period .............................................................................. 4

The lower Illinois River.................................................................................................................. 5

MAJOR FINDINGS ............................................................................................................................. 6

Nutrient concentrations in streams and rivers are a major concern ............................................. 6

Stream and river samples often exceeded the nitrate drinking-water standard and 

phosphorus guideline................................................................................................................. 7

Nitrate concentrations were lower than in other areas of the Corn Belt during late summer........ 7

Nitrate concentrations differed upstream and downstream from water-supply reservoirs ............ 8

NATIONAL PERSPECTIVE—Algal index indicates comparatively poor water quality in corn belt 9

Riparian buffer zones influence the quality of Midwestern streams and rivers ............................. 10

NATIONAL PERSPECTIVE—Nutrient concentrations were among the highest in the Nation..... 11

Nitrate concentrations in some shallow wells exceeded the drinking-water standard ................. 12

Some pesticides were always detected in streams and rivers...................................................... 12

Pesticide concentrations in streams periodically exceeded drinking-water standards ................. 13

Pesticide concentrations in streams have a seasonal pattern ...................................................... 14

Many factors affect the presence and concentration of pesticides in streams.............................. 14

Concentrations of agricultural chemicals generally reflect application rates................................. 15

Stream size affects pesticide concentrations................................................................................ 15

Concentrations of alachlor and cyanazine in the Illinois River decreased .................................... 16

Pesticide breakdown products were detected more frequently than the parent compound.......... 16

Pesticides were not frequently detected in ground water.............................................................. 17

Insecticides discontinued in the 1970s were still detected in fish and sediment........................... 17

NATIONAL PERSPECTIVE—Basin and national results were similar for pesticides ................... 19

Other organochlorine compounds are persistent in sediments and fish....................................... 20

Common stream animals indicate poor water-quality conditions.................................................. 20

Highest concentrations of polycyclic aromatic hydrocarbons were in bed sediment near 

urban areas................................................................................................................................ 21

Arsenic exceeded the drinking-water standard in the Mahomet aquifer ....................................... 22

NATIONAL PERSPECTIVE—Arsenic in ground water is a national concern............................... 22

Radon exceeded the proposed drinking-water standard in almost one-half of shallow wells....... 23

Volatile organic compounds were detected in ground water at low concentrations...................... 24

Fish and sediment in the Illinois River have high concentrations of trace elements ..................... 24

STUDY UNIT DESIGN........................................................................................................................ 25

GLOSSARY........................................................................................................................................ 27

REFERENCES ................................................................................................................................... 28

APPENDIX—WATER QUALITY FROM THE LOWER ILLINOIS RIVER BASIN IN A NATIONAL 

CONTEXT........................................................................................................................................... 30
NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

IV National Water-Quality Assessment Program 

1991–95

1994–98

1997–2001

Not yet scheduled

High Plains Regional

Ground-Water

Study,

1999–2004

NAWQA Study Units—

Assessment schedule

Lower Illinois River Basin

THIS REPORT summarizes major findings about water quality in the lower Illinois River Basin that emerged 

from an assessment conducted between 1995 and 1998 by the U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program. Water quality is discussed in terms of local and regional issues and 

compared to conditions found in all 36 NAWQA study areas, called Study Units, assessed to date. Findings are 

also explained in the context of selected national benchmarks, such as those for drinking-water quality and the 

protection of aquatic organisms. The NAWQA Program was not intended to assess the quality of the Nation’s 

drinking water, such as by monitoring water from household taps. Rather, the assessments focus on the quality of 

the resource itself, thereby complementing many ongoing Federal, State, and local drinking-water monitoring 

programs. The comparisons made in this report to drinking-water standards and guidelines are only in the context 

of the available untreated resource. Finally, this report includes information about the status of aquatic 

communities and the condition of in-stream habitats as elements of a complete water-quality assessment.

Many topics covered in this report reflect the concerns of officials of State and Federal agencies, water-resource 

managers, and members of stakeholder groups who provided advice and input during the lower Illinois River Basin 

assessment. Basin residents who wish to know more about water quality in the areas where they live will find this 

report informative as well. 

THE NAWQA PROGRAM seeks to improve scientific and public understanding of water quality in the Nation’s 

major river basins and ground-water systems. Better understanding facilitates effective resource management, 

accurate identification of water-quality priorities, and successful development of strategies that protect and restore 

water quality. Guided by a nationally consistent study design and shaped by ongoing communication with local, 

State, and Federal agencies, NAWQA assessments support the investigation of local issues and trends while 

providing a firm foundation for understanding water quality at regional and national scales. The ability to integrate 

local and national scales of data collection and analysis is a unique feature of the USGS NAWQA program.

The lower Illinois River Basin is one of 51 water-quality assessments initiated since 1991, when the U.S. 

Congress appropriated funds for the USGS to begin the NAWQA Program. As indicated on the map, 36 

assessments have been completed, and 15 more assessments will conclude in 2001. Collectively, these assessments 

cover about one-half of the land area of the United States and include water resources that are available to more 

than 60 percent of the U.S. population.
Summary of Major Findings 1 

SUMMARY OF MAJOR FINDINGS

Ottawa Marseilles Marseilles

Henry

Peoria

Kingston Kingston Mines

Havana

Valley City

Hardin

Grafton Grafton

Colmar

El Paso El Paso

Green

Valley

Monticello Monticello

Springfield Springfield

Bloomington Bloomington

Normal

Decatur Decatur

Oakford Oakford

Wyoming Wyoming

Pontiac Pontiac

Streator Streator

LA SALLE LA SALLE

BUREAU

PUTNAM

LIVINGSTON LIVINGSTON

MARSHALL MARSHALL

STARK

KNOX

PEORIA

FULTON MCDONOUGH MCDONOUGH

HANCOCK HANCOCK

SCHUYLER SCHUYLER

BROWN

ADAMS

PIKE

GREENE

JERSEY

MACOUPIN MACOUPIN

SCOTT MORGAN

CASS MENARD

SANGAMON SANGAMON

CHRISTIAN CHRISTIAN

MACON

DE WITT DE WITT

PIATT LOGAN

MASON

TAZEWELL TAZEWELL

MCLEAN

WOODFORD WOODFORD

FORD

Bloomington Ridged Plain Subsection

Galesburg Plain Subsection

Springfield Plain Subsection

CITY

PHYSIOGRAPHY

EXPLANATION

COUNTY BOUNDARY

STREAM

Illinois River

La Moine Riv er

Sangamo n River

Panther Cr

Ma ckinaw River Spo on River

Vermili o n River

0 10 20 30 40 50MILES

0 10 20 30 40 50 KILOMETERS

ILLINOIS

Lower

Illinois

River

Basin

Stream and river highlights

During the past century, agricultural runoff, channel and 

drainage modifications, urbanization, and other activities 

have altered water quality, aquatic biological communities, 

and aquatic habitat in the lower Illinois River Basin. The 

water quality of large rivers, such as the Illinois and 

Sangamon Rivers, was more likely to meet drinking-water 

standards than water quality of small streams during 1995–

98. In spring, concentrations of nitrate—especially in small 

streams—exceeded the Maximum Contaminant Level 

(MCL) of 10 mg/L (milligrams per liter) for drinking water. 

Some of these streams replenish drinking-water reservoirs. 

In samples collected during runoff from spring and early 

summer storms, concentrations of herbicides and a few 

insecticides exceeded drinking-water standards or guidelines, or guidelines to protect aquatic life. In a few samples 

from small streams, concentrations of commonly used 

agricultural pesticides were among the highest nationally. 

Although most concentrations were low with respect to 

existing drinking-water standards or guidelines, criteria 

for the protection of human health or wildlife have not 

been established for more than one-half of the chemicals 

detected.

• Nitrogen and phosphorus concentrations were among 

the highest in the Nation. The highest concentrations in 

the basin were found in small streams in agricultural 

areas. The MCL for nitrate was exceeded in 15 percent 

of samples from all streams and rivers. The concentration 

of total phosphorus in most samples (79 percent) from 

all streams and rivers exceeded the 0.1-mg/L guideline 

recommended by U.S. Environmental Protection Agency 

(USEPA) to prevent excess algal growth in streams. (See 

pages 6–8.)

• Nitrate concentrations in the Illinois River at the 

inflow to the basin (Ottawa) and outflow from the basin 

(Valley City) were similar; however, approximately twice 

the amount of nitrogen was transported out of the basin 

(124,000 tons per year) as was transported into the basin 

(66,000 tons per year). (See page 6.)

• During August 1997, concentrations of nitrate in 

streams were lower in the basin than in other NAWQA 

study-area streams of the upper Midwest. Algal communities may have incorporated much of the instream nitrogen, resulting in lower nitrate concentrations in the water 

during late summer. (See pages 7–10.)

• Three herbicides commonly used by farmers to 

protect corn and soybean crops—atrazine, metolachlor, 

and cyanazine—were detected in every sample collected 

during 1995–98. During periods of spring runoff, these 

herbicides exceeded drinking-water standards or guidelines or aquatic-life guidelines. Another herbicide, 

acetochlor, was detected in most samples (81 percent). 

(See pages 12–17.)

• Pesticide breakdown products were detected much 

more frequently than the parent compound, and generally 

at higher concentrations and for a longer period of time 

after application. (See pages 16–17.)

• Organochlorine compounds (including insecticides no 

longer used) were detected in fish tissue and sediment, 

and polycyclic aromatic hydrocarbons were detected in 

sediments at levels of concern. (See pages 17–22.)

• The biological communities of streams in the basin 

comprised many organisms that are tolerant of poor water 

quality; however, some high-velocity streams had fairly 

diverse invertebrate communities. (See pages 20–21.)

Trends in stream-water quality

For decades, fertilizers and many pesticides have been 

applied to crops and land. The persistence of pesticides and 

breakdown products in the soil, water, and sediment within 

a watershed is not well understood. Concentrations of the 

herbicides alachlor and cyanazine, however, decreased from 

the 1991–92 period to the 1996–98 period in the Illinois 

River because of reduced application rates since the early 

1990s. (See page 16.)

The lower Illinois River Basin occupies approximately 18,000

square miles in central and west-central Illinois. The basin lies

almost entirely within the Till Plains physiographic section. The

glacial materials account for the flat prairie landscape and the

thick, rich soils. The three physiographic subsections influence water quality indirectly by influencing the type and intensity of agricultural activities. [1]
2 Water Quality in the Lower Illinois River Basin

Small streams Major rivers

Pesticides1

Agricultural Agricultural Mixed

land use

—

Semivolatile

organics6

Phosphorus2

Arsenic4

a

a

—

Percentage of samples with concentrations equal to or

greater than a health-related national guideline for

drinking water, aquatic life, or water-contact recreation;

or above a national goal for preventing excess algal

growth.

Percentage of samples with concentrations less than a

health-related national guideline for drinking water,

aquatic life, or water-contact recreation; or below a

national goal for preventing excess algal growth.

Percentage of samples with no detection. (a Percentage is 1 or less and may not be clearly visible)

Not assessed

Selected Indicators of Stream Quality

Other

trace

elements4

Organochlorines5

Nitrate3

Shallow

ground water Water-supply wells

Western and

southern

shallow aquifers

Mahomet

(deep

aquifer) Agricultural

Pesticides1

—

Radon

Arsenic

Selected Indicators of Ground-Water Quality

Volatile

organics7

Nitrate3

Other

trace

elements

—

Percentage of samples with concentrations equal to or

greater than a health-related national guideline for

drinking water

Percentage of samples with concentrations less than a

health-related national guideline for drinking water

Percentage of samples with no detection

Not assessed

Ground-water highlights

In contrast to the water quality of streams and rivers in 

the basin and the quality of ground water in other areas 

across the Nation, agricultural chemicals in ground-water 

samples from shallow monitoring wells (generally less 

than 100 feet deep) and drinking-water wells only rarely 

exceeded the nitrate MCL. Except for radon and nitrate, 

shallow ground water in the lower Illinois River Basin 

generally met drinking-water standards or guidelines. 

Except for radon and arsenic, the water quality in the 

Mahomet aquifer (greater than 200 feet deep) meets all 

drinking-water standards or guidelines.

• Major corn and soybean herbicides were not as frequently detected in ground-water samples as they were in 

stream-water samples. No ground-water sample exceeded 

drinking-water standards or guidelines for pesticides. 

(See page 17.)

• Naturally occurring arsenic exceeded the current 

MCL of 50 μg/L (micrograms per liter) in 2 of 30 wells 

sampled in the Mahomet aquifer, a major drinking-water 

source. If the MCL is lowered to 5 μg/L, as proposed by 

the USEPA, samples from 60 percent (18 of 30) of the 

domestic (private household) and public-supply (publicly 

owned wells generally serving a community) wells sampled would probably exceed the lower standard. (See 

page 22.)

• Geologic materials underlying the basin indicate that 

it is an area of potentially high radon concentrations in 

ground water. In about one-half of the samples of shallow 

ground water, radon exceeded the proposed MCL of 

300 pCi/L (picocuries per liter). Only 2 of 30 samples 

from the Mahomet aquifer exceeded the proposed radon 

MCL. (See pages 23–24.)

• Volatile organic compounds (VOCs) were detected in 

samples from 83 percent of shallow domestic and publicsupply wells, but no samples exceeded drinking-water 

standards or guidelines. VOCs were detected in 80 percent of samples from the Mahomet aquifer but at concentrations near the method detection limit and well below 

drinking-water standards and guidelines. (See page 24.)

Major influences on streams and rivers

• Subsurface (tile) drainage from agricultural areas

• Agricultural and urban land-surface runoff

• Drainage modification to streams and channels

Major influences on ground water

• Agricultural and urban land uses

• Permeability of soil and aquifers

• Minerals in geological materials

1 Insecticides, herbicides, and pesticide breakdown products, sampled in water.

2 Total phosphorus, sampled in water.

3 Nitrate (as nitrogen), sampled in water.

4 Arsenic, mercury, and trace metals, sampled in sediment.

5 Organochlorine compounds including DDT and PCBs, sampled in sediment.

6 Miscellaneous industrial chemicals and combustion by-products, sampled in sediment.

7 Solvents, refrigerants, fumigants, and gasoline compounds, sampled in water.
INTRODUCTION TO THE LOWER ILLINOIS RIVER BASIN

Introduction to the Lower Illinois River Basin 3

0 10 20 30 40 50 60 70 80 90 100

PERCENTAGE OF TOTAL AREA

Lower Illinois River Basin EXPLANATION

La Moine River Basin

Sangamon River Basin

Indian Creek Basin

Panther Creek Basin

AGRICULTURAL LAND

FOREST LAND

URBAN OR BUILT-UP LAND

ALL OTHER

0 25

50

MILES

0 25 KILOMETERS

50

40o

41o

89o

90o

91o

39o

LAND USE

Urban

Agriculture

Grassland

Forest

Water

Wetland

EXPLANATION

Lower

Illinois

River

Basin

ILLINOIS

The lower Illinois River Basin 

(the basin) encompasses 

18,000 mi2 (square miles) of central and west-central Illinois 

between the upper end at Ottawa 

and the confluence of the Illinois 

River with the Mississippi River 

near Grafton. The basin includes 

all of 22 counties and parts of 19 

counties.

As of 1990, 1.3 million people 

lived in the basin. More than 50 

percent of the population lived in 

the counties of Macon, McLean, 

Peoria, Sangamon, and Tazewell. 

The four most populated cities are 

Peoria, Springfield, Decatur, and 

Bloomington. The basin population 

decreased by 7 percent from 1980 

to 1990 [2].

Land use

Agriculture is the predominant 

land use—typically corn and soybean row crops. Agriculture 

accounts for 88 percent of the 

overall land area, whereas forests 

account for 7 percent and urban 

areas account for about 2 percent 

(fig. 1). The remaining land use, 

about 3 percent, is mostly grassland, wetland, or water.

The small-stream basins that 

were sampled during 1995–98 have 

even higher percentages of land 

devoted to farming; for example, 

the Panther Creek Basin is 

99 percent agricultural land (fig. 2). 

The lower Illinois River Basin has 

some of the most highly productive 

and intensively cultivated farmland in the world. As a result, 

application rates of farm-related 

chemicals are among the highest in 

the country in support of corn and 

soybean production. Runoff from 

these vast, flat farm areas carries 

relatively high concentrations of 

nutrients and pesticides into the Figure 2. Proportion of land devoted to agriculture varies only slightly among 

basins—from 88 percent in the entire lower Illinois River Basin to 99 percent in 

the Panther Creek Basin.

Figure 1. Land use in the lower Illinois River Basin is predominantly 

agriculture. Farming is most intensive in the part east of the Illinois River. 

Streambanks generally are steeper west of the Illinois River and are more 

forested than streambanks east of the river.
4 Water Quality of the Lower Illinois River Basin

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

STREAMFLOW, IN CUBIC

FEET PER SECOND

WATER YEAR 1996 WATER YEAR 1998 WATER YEAR 1997

O NDJ F A M MJ J A S O NDJ F A M MJ J A S O NDJ F A M MJ J A S

25th percentile of flow

75th percentile

of flow

Streamflow

 MONTHLY PRECIPITATION,

 IN INCHES

PRECIPITATION AT PEORIA,

ILLINOIS

8

7

6

5

4

3

2

1

0

TOTAL

 PUBLICSUPPLY

DOMESTIC SUPPLY

COMMERCIAL

INDUSTRIALAND MINING

IRRIGATION AND LIVESTOCK

 THERMOELECTRIC

0

50

100

150

200

250

WATER WITHDRAWALS,

IN MILLIONS OF GALLONS PER DAY

PERCENTAGE OF TOTAL

WATER WITHDRAWALS

(1,017)

(325)

GROUND WATER

SURFACE WATER

Population

served, in

thousands

3,660

3,400

3,160

3,150

0

1

2

3

4

5

6

86%

93%

100%

ESTIMATED WATER

WITHDRAWALS IN 1995

small streams and major rivers of 

the basin.

Animal farming is also a significant industry in the basin [2], and 

runoff from manure spreading is a 

contributor to some of the highest 

levels of nitrate and total nitrogen 

found in the rivers and streams of 

the United States.

Study design in relation to 

land use

Chemical and biological samples 

were collected from a range of 

river and stream sizes in different 

landscape types and from shallow 

aquifers. A deep aquifer (Mahomet 

aquifer) was sampled to assess the 

overall water quality. A group of 

the shallow large-bore wells used 

for domestic (private household) 

supply was sampled, as well as 

other shallow domestic wells, to 

assess overall water quality and 

determine the occurrence of 

human-induced contaminants and 

the extent of land-use effects on 

recently recharged ground water.

Water use

In the lower Illinois River 

Basin, 48 percent of the publicor municipal-supply water is drawn 

from ground-water sources and 

52 percent from surface-water 

sources (fig. 3). All private-supply 

water, usually for domestic use, is 

from ground water. Rural residents 

(about 25 percent of the population) generally are self-supplied 

from ground water [2]. In most 

of the western and southern parts 

of the basin, homeowners rely on 

cisterns or large-diameter dug or 

bored wells [3]. These large-diameter wells are highly susceptible to 

contamination from surface-water 

runoff and shallow ground water.

Irrigation, which is confined to 

sandy soils near the center of the 

basin, is entirely supplied by 

ground water.

Hydrologic conditions during 

the study period

Rainfall generally was near 

normal during the data-collection 

period (fig. 4). Typically, the largest rainstorms are during April to 

July. The generally normal rainfall 

had exceptions; in particular, water 

year 1997 (October 1996–September 1997) was slightly drier than 

Figure 4. Hydrologic conditions during the data-collection period were near normal. 

Some relatively dry periods resulted in below-normal flow, and several wet periods 

resulted in above-normal flow on the Illinois River at Valley City.

Figure 3. Water 

withdrawals in the 

lower Illinois River 

Basin are dominated 

by thermoelectric 

withdrawals (cooling 

water for power 

generation). Drinking 

water is supplied 

roughly equally from 

surface-water 

reservoirs and 

ground-water 

supplies. 
Introduction to the Lower Illinois River Basin 5

4

6

8

10

12

14

16

18

20

2

0

DEPTH TO WATER, IN FEET

BELOW LAND SURFACE

0

10

20

30

40

50

60

ANNUAL PRECIPITATION, IN INCHES

Normal precipitation

PRECIPITATION AT PEORIA, ILLINOIS

GROUND-WATER LEVELS IN SOUTHERN PART OF BASIN

Period of data

collection

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

MEAN ANNUAL STREAMFLOW

(in cubic feet per second)

25,000

5,000

15,000

EXPLANATION

COUNTY

BOUNDARY

STREAM

Lower

Illinois

River

Basin

ILLINOIS

39o

40o

91o

41o

90o

89o

BUREAU

LA SALLE

PUTNAM

STARK

MARSHALL

KNOX

PEORIA WOODFORD

LIVINGSTON

MCLEAN

FORD

TAZEWELL FULTON

MCDONOUGH

HANCOCK

MASON

SCHUYLER

ADAMS

BROWN

CASS MENARD

LOGAN

DE WITT

PIATT

MACON

SANGAMON

SCOTT

MORGAN

CHRISTIAN

MACOUPIN GREENE

JERSEY

PIKE

OTTAWA

GRAFTON

PEORIA

VALLEY CITY

0 25 50 MILES

0 25 50 KILOMETERS

Fox River

normal, and spring 1996 had fewer 

storms than normal.

Ground-water levels reflected 

the near-normal rainfall conditions, 

particularly in the southern part of 

the basin (fig. 5). Shallow ground 

water typically is recharged in late 

winter and early spring, then the 

level falls during the summer growing season. Water level in a representative well is shown in figure 5, 

along with 10 years of annual rainfall data collected near Peoria, 

which lies near the center of the 

basin.

The lower Illinois River

The Illinois River is a navigable 

waterway that connects the Mississippi River to Chicago and the 

Great Lakes. Barges carrying grain 

and other commodities are a significant influence on suspended sediment and water quality. The mean 

annual flow of the Illinois River 

increases from about 12,600 ft3

/s 

(cubic feet per second) at Ottawa to 

22,600 ft3

/s at Valley City [2]. Five 

major tributaries—Vermilion 

River, Spoon River, Mackinaw 

River, La Moine River, and Sangamon River—and many smaller 

streams join the Illinois River 

(fig. 6). Flow gaging and sampling 

of the outflow from the basin is 

done at Valley City because the 

water level of the Mississippi 

River, at times, affects the flow of 

the Illinois River downstream from 

Valley City.

Figure 5. Ground-water conditions were 

near normal during 1989–98, including 

the data-collection period. In 1993, the 

observation-well water level shows the 

effect of the significant 1993 flood on the 

shallow ground water. (Data from K. 

Hlinka, Illinois State Water Survey, and 

National Weather Service.)

Figure 6. The lower Illinois River has five major tributaries. The inflow 

to the basin is at Ottawa, Ill., where the Fox River joins the Illinois 

River. Where the Illinois River leaves the basin and enters the 

Mississippi River near Grafton, Ill., the outflow is almost double that of 

the inflow. 
6 Water Quality in the Lower Illinois River Basin

MAJOR FINDINGS

Figure 8. Mean nitrate concentrations generally are lowest 

in the Illinois River. Nitrate concentrations in the three largest 

rivers show a downward trend with increasing streamflow 

from the Sangamon River near Oakford to the Illinois River 

at Ottawa and to the Illinois River at Valley City. The 

La Moine River, which is the only river or stream sampled 

that lies entirely within the Galesburg Plain physiographic 

subsection, does not fit the general trend. The nitrate 

concentrations are flow-weighted means.

1

10

100

1,000

10,000

100,000

0 2 4 6 8 10 12 14 16

STREAMFLOW, IN CUBIC FEET PER SECOND

MEAN NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

Illinois River at Valley City

Indian Creek

near Wyoming Panther Creek

near El Paso

Sangamon River

at Monticello

Illinois River at Ottawa Sangamon River

near Oakford

Mackinaw River

near Green Valley La Moine River at Colmar

INFLOW

(Illinois River at Ottawa)

OUTFLOW

(Illinois River at Valley City)

Nitrate

Total phosphorus

140

AVERAGE ANNUAL LOAD,

IN THOUSANDS OF TONS120

100

80

60

40

20

0

Figure 9. About twice as much nitrate and total phosphorus 

leaves the lower Illinois River as enters it. The loads for the 

upper Illinois River Basin—the basin upstream from 

Ottawa—are about one-half of the respective loads for the 

lower Illinois River Basin.

Nutrient concentrations in streams and rivers 

are a major concern

Agriculture is the most important activity in the 

lower Illinois River Basin from an economic and 

water-quality perspective (fig. 7). Application rates 

of nitrogen and phosphorus in synthetic fertilizer and 

manure are among the highest in the Nation. The high 

application rates of agrichemicals in the basin were 

reflected by the kinds and amounts of contaminants 

found in streams, rivers, and ground water. Nevertheless, contaminants associated with urban or industrial 

activities also were detected in some streams, rivers, 

and wells.

Flow-weighted concentrations of nitrate (nitrate as 

nitrogen) were generally highest in small streams in 

intensive agricultural areas and east of the Illinois 

River (fig. 8). The lowest flow-weighted mean concentration of nitrate was for the Illinois River at Valley 

City—the outflow from the basin. Concentrations 

of nutrients were similar at the basin inflow (Illinois 

River at Ottawa) and the basin outflow (Illinois River 

at Valley City). Urban sources, primarily wastewater 

effluent, however, were major contributors at the 

inflow [4], whereas agriculture provided most of the 

nutrient contribution to the water resources within the 

basin.

The nitrate load (amount of nitrate in water that 

moves past a location on the river) measured at the 

outflow from the basin, 124,000 ton/yr (tons per year), 

was nearly twice the load measured at the inflow, 

66,000 ton/yr (fig. 9). Flow-weighted mean concentrations of nitrate are similar between the two Illinois 

River locations; therefore, the difference in loads is 

directly related to the difference in streamflow (fig. 6). 

The lower Illinois River Basin contributes roughly the 

same amount of nutrients to the Mississippi River as 

does the upper Illinois River Basin, which includes the 

Figure 7. Farming and related businesses are 

major activities in the lower Illinois River Basin. 

Agriculture has a significant effect on water quality 

and wildlife habitat. (Photograph by David J. Fazio, USGS.)
Major Findings 7

Chicago urban area and rural areas 

in Illinois, Indiana, and Wisconsin.

The annual nitrogen yield 

(amount of nitrogen in water 

that flows out of the watershed 

divided by the area of the watershed) measured at the river and 

stream locations ranged from 4.6 to 

12 (ton/mi2

)/yr (tons per square 

mile per year), the smallest yields 

being measured in the largest river 

and the largest yields measured in 

the smallest streams. The overall 

yield for the basin (Illinois River at 

Valley City) was 4.6 (ton/mi2

)/yr 

during 1995–98.

Substantial differences among 

nutrient concentrations in small 

agricultural basins and the lower 

nutrient concentrations in large 

rivers indicate that hydrological 

and biochemical processes reduce 

the nitrate concentration as nitrate 

moves through the basin and into 

the Mississippi River. Instantaneous concentrations of nutrients

varied with season: nitrate concentrations were highest during the 

winter and spring and lowest in 

the late summer and fall.

Sites on the largest rivers—the

Illinois River at Ottawa and Valley 

City and the Sangamon River near 

Oakford—had the highest flowweighted mean concentrations of 

phorus, and samples from Panther 

Creek near El Paso had the smallest. Indian Creek near Wyoming 

is downstream from a small 

wastewater-treatment plant, which 

is likely the reason for the higher 

total phosphorus at this site.

Nitrate concentrations were 

lower than in other areas of 

the Corn Belt during late 

summer

During August 1997, concentrations of dissolved nitrate and total 

nitrogen were found to be lower 

in streams and rivers of the lower 

Illinois River Basin than in some 

other areas of the Midwest. A synoptic study was done at 70 sites 

with drainage areas between 100 

What Was Assessed?

The NAWQA Program was not 

intended to assess the quality of the 

Nation's drinking water, such as by 

monitoring water from household 

taps. Rather, NAWQA assessments 

focus on the quality of the resource 

itself, thereby complementing many 

ongoing Federal, State, and local 

drinking-water monitoring programs. 

Comparisons made in this report to 

drinking-water standards and guidelines are made only in the context of 

the available untreated resource.

Streamflow-Weighted 

Concentrations

Drinking-water standards are generally based on the yearly average of 

samples collected quarterly. This is a 

legal definition of yearly average concentrations. Truly representative 

“average” water-quality conditions of 

a stream would require daily samples 

for some streams because of the variability in the concentration and in 

streamflow from day to day—especially over a period of storm runoff. 

During a storm, the levels of contaminants in a stream can vary 10 times or 

more from beginning to end. A method 

is needed to address this variability 

and “weigh” the concentrations found 

in individual “point” samples from a 

stream. A process is used to estimate 

the daily load or amount of a chemical 

that flows past a point. The daily loads 

are summed and then divided by the 

amount of streamflow to get an average for the period of interest. This flow 

weighting produces a more accurate 

mean concentration of a specific 

chemical for a period such as a year or, 

as for this report, for the 2–3 year 

period when samples were collected.

total organic nitrogen, ammonia 

nitrogen, and phosphorus; the latter 

two are indicators of urban wastewater effluent [4]. The Sangamon 

River near Oakford had the highest 

flow-weighted mean concentration 

of total organic nitrogen, ammonia 

nitrogen, and phosphorus. Samples from middle-sized La Moine 

River at Colmar generally had 

higher flow-weighted mean concentrations of most nutrients compared with samples from the 

Sangamon River at Monticello, 

which also is middle sized but is at 

the eastern edge of the basin. Flowweighted mean concentrations of 

nitrate in samples from the Sangamon River at Monticello, however, 

were greater than those in samples 

from the La Moine River at Colmar. Panther Creek near El Paso, a 

small-basin site, generally had 

higher concentrations of most 

nutrients than did Indian Creek 

near Wyoming, another small-basin 

site.

Stream and river samples 

often exceeded the nitrate 

drinking-water standard and 

phosphorus guideline

Flow-weighted mean concentrations of nitrate for five of the eight 

sample locations in the basin, 

including two locations on the 

Sangamon River (used for public 

water supplies), were higher than 

the Maximum Contaminant Level 

(MCL) of 10 mg/L nitrate as nitrogen established for drinking water.

The concentration of total 

phosphorus in most samples 

(80 percent) from all streams 

and rivers exceeded the 0.1 mg/L 

guideline recommended by USEPA 

to prevent eutrophication in 

streams. Samples from Indian 

Creek near Wyoming had the highest concentrations of total phos-
8 Water Quality in the Lower Illinois River Basin

DATE SAMPLE COLLECTED

NITRATE CONCENTRATION,

IN MILLIGRAMS PER LITER

Drinking-water standard

Sangamon River at Monticello

Sangamon River near Oakford

20

18

16

14

12

10

8

6

4

2

0

9/1/96

12/1/96

3/1/976/1/97

9/1/97

12/1/97

3/1/986/1/989/1/98

and 1,000 mi2 in Illinois, Iowa, and Minnesota in 

August 1997, when streamflow had approximately 

reached the annual minimum [5]. Median concentrations of dissolved nitrate in streams generally 

increased northwestward, from Illinois (0.52 mg/L) to 

Iowa (1.5 mg/L) to Minnesota (3.5 mg/L). No significant relations between nitrate concentrations and streambank characteristics or soil permeability were 

identified. Streams that have formed on windblown 

glacial materials, however, generally had lower concentrations than streams formed on glacial till 

(unsorted material left behind by continental glaciers). 

The windblown materials typically have slightly 

higher permeability than the glacial till and are more 

likely to allow rainwater to infiltrate rather than run off 

to streams.

In Illinois, rainfall and thus runoff to streams was 

low before and during synoptic sampling; therefore, 

most streamflow was derived from ground-water 

seepage. Thus the low nutrient concentrations in 

ground water in the basin—much lower than those 

in streams and rivers—are a major reason why low 

nitrate concentrations were detected in the streams 

during August 1997 in the basin.

Differences in moisture conditions and precipitation 

during the 3-month period before the August sampling 

(wettest in Minnesota and driest in Illinois) probably 

also contributed to the differences in concentrations 

across the three-State region. The percentages of total 

nitrogen present as organic nitrogen and ammonia 

were higher in Illinois than in other parts of the study 

area. Nitrogen incorporation in algae also was a factor 

in the distribution of dissolved nitrate. Algal communities, particularly phytoplankton, were more stable 

and productive in Illinois than in other parts of the 

three-State synoptic study, and much of the nitrogen in 

the aquatic system was incorporated into these plant 

communities in the basin.

Nitrate concentrations differed upstream and 

downstream from water-supply reservoirs

Dissolved nitrate concentrations in samples from 

the Sangamon River (at Monticello) were found to be 

more variable upstream from several water-supply 

reservoirs than downstream from the reservoirs 

(fig. 10). From April through June, nitrate concentrations were frequently above the MCL of 10 mg/L 

(highest concentration, 17 mg/L) in the Sangamon 

River at Monticello, 29 mi upstream from Lake Decatur, the drinking-water source for the city of Decatur. 

The city of Decatur often must treat water for nitrate 

above the MCL during spring and early summer or 

mix the high-nitrate surface water with ground water 

to lower the nitrate concentration. Nitrate concentrations also were highest in the spring in the Sangamon 

River near Oakford, which is downstream from Lake 

Decatur and other water-supply reservoirs used by the 

city of Springfield. Concentrations in samples from 

the Sangamon River near Oakford, however, seldom 

exceeded the nitrate MCL.

Conversely, concentrations of nitrate usually were 

less than 1 mg/L in the Sangamon River at Monticello 

from August through November, but typically 

remained higher than 1 mg/L at the Oakford site 

during the same period. Data from Scribner and others 

[6] indicate that nitrate concentrations of reservoirs in 

corn- and soybean-producing areas of the Midwest 

have nutrient concentrations and patterns similar to 

those in the Sangamon River near Oakford.

Figure 10. Nitrate concentration in the Sangamon River is 

affected by reservoirs and other factors. Monticello lies 

upstream from several water-supply reservoirs and the cities 

of Springfield and Decatur. Oakford lies downstream from 

the reservoirs. The reservoirs are for water supply, and 

during spring they are replenished by river water that 

exceeds the drinking-water standard for nitrate. Water 

leaving the reservoirs usually does not exceed the nitrate 

standard and has less variability in nitrate concentrations.
Major Findings 9

Corn Belt

Lower Illinois River Basin

Small streams in the Corn Belt have both poorer water quality, 

as indicated by the algal status index, and higher nitrate concentrations than small streams in other parts of the Nation. The algal 

status index reflects a greater relative abundance of benthic algal 

species that are considered 

tolerant of poor water-quality 

conditions, including high 

nutrient concentrations. 

Results for this study are from 

data collected in 16 NAWQA 

Study Units from 1996 to 

1998.

Algae grow on virtually all 

stream surfaces receiving 

light. Algal communities are 

the foundation of the food 

web of many streams, serving 

as a food source for invertebrates and fish. Nuisance algal growth can result from excess 

nutrients and can lead to large variations in dissolved oxygen concentrations.

In large streams, flow-weighted mean concentrations of nitrate 

were also higher in the Corn Belt than in the rest of the Nation, but 

the algal status index was not significantly higher. The dynamics 

of the algal community change with stream size, and the importance of phytoplankton (floating algae) increases as streams get 

larger. The algal status index does not reflect the composition of 

the phytoplankton community, and this may explain the differences observed between small and large streams.

Chemical, physical, and biological factors all influence the 

algal community and local stream conditions. The algal status 

index at individual lower Illinois River Basin streams was similar 

to that at other streams in the Corn Belt.

Algal index indicates comparatively 

Excess nutrients often cause algal blooms that can be seen in 

streams and rivers in the basin. Algae reduce the amount of 

nutrients that remain in the water by converting nutrients into 

algal biomass. (Photograph by Paul J. Terrio, USGS.)

The lower Illinois River Basin lies

within the eastern one-half of the

Corn Belt.

Algal status index values and nitrate concentrations for the lower Illinois River Basin, 

and for Study Units in the Corn Belt, are 

higher than mean values for Study Units in 

the Nation.

EXPLANATION

Algal index for samples from lower

Illinois River Basin streams or rivers

Nitrate mean for samples from lower

Illinois River Basin streams or rivers

Algal index mean

for Corn Belt or U.S.

Nitrate mean for

Corn Belt or U.S.

Two standard errors

of the mean

Two standard errors

 of the mean

ALGAL STATUS INDEX

DECREASING WATER QUALITY

70

60

50

40

30

20

10

0

GEOGRAPHIC REGION AND STREAM BASIN SIZE NITRATE CONCENTRATION, IN MILLIGRAMS PER LITER

16

14

12

10

8

6

4

2

0

Corn U.S. U.S.

Belt

Corn

Belt

SMALL (Less

than 1,000

square miles)

LARGE (1,000

square miles or

greater)

(12) (74) (12) (42)

Numbers in parentheses are samples in each group

poor water quality in Corn Belt 
10 Water Quality in the Lower Illinois River Basin

Riparian buffer zones influence the quality of midwestern streams and rivers

Despite similar land use throughout 

the Corn Belt region of the Midwest, 

streams flowing through cropland 

differ considerably in their ecological 

characteristics, in part because of 

differences in riparian buffer zones (see 

text boxes). This conclusion is based on 

an investigation of 70 streams and rivers within three NAWQA Study Units 

in the upper Midwest during August 

1997 (figure at right) [5; 7]. Specifically, increases in tree cover in buffer 

zones were associated with aquatic biological communities indicative of good 

stream quality, reduced nuisance algal 

growths, and maintenance of sufficient 

dissolved oxygen concentrations to 

support diverse communities of aquatic 

organisms. For example, the number of 

aquatic insects indicative of good 

stream quality tended to increase with 

increases in percentage of tree cover, 

especially in sites where streamflow 

and dissolved oxygen conditions were 

favorable. Fish communities, sampled 

at 24 sites in the UMIS Study Unit, also 

indicated better overall conditions in 

streams with wooded riparian zones 

than those with more open canopy [8]. 

Organic enrichment resulting from 

excessive algal production in some 

midwestern streams may reduce 

dissolved oxygen concentrations and 

be detrimental to other requirements of 

aquatic organisms.

Shading from tree cover in riparian 

buffer zones may influence nutrient 

concentrations indirectly by reducing 

the growth of phytoplankton. In 

streams where phytoplankton were 

abundant (often where buffer zones 

were thin or lacking), dissolved nitrate 

concentrations were significantly lower 

(figure below) [9]. The lower nutrient 

concentrations may result from uptake 

by the abundant phytoplankton. Thus, 

assessments of eutrophication would 

benefit from consideration of biological 

communities and the riparian zone, 

rather than being based solely on nutrient concentrations in the water.

Streams with less tree cover, and 

thus less shading, contained relatively 

large growths of phytoplankton (algae 

suspended in the water) at levels considered indicative of eutrophication [9]. 

The influence of riparian buffer zones on the 

quality of 70 midwestern streams and rivers was 

evaluated in the Upper Mississippi River (UMIS), 

Eastern Iowa (EIWA), and lower Illinois River 

Basins (LIRB).

Dissolved nutrient concentrations decreased in 

eutrophic streams with excessive algal productivity. Rates of nutrient uptake by the algae can 

exceed rates at which nutrients are transported 

by streams during low-flow conditions.

Digital images derived from USGS topographic maps were used to estimate the 

percentage of trees in a riparian buffer zone (a 100-meter width on each side of the 

stream) for 2- to 3-mile segments upstream from each sampling site, supplemented by 

vegetation surveys at the sampling site [5].

Resource agencies, including the U.S. 

Department of Agriculture, encourage 

maintenance of strips of trees or grass 

between cropland and streams as a best 

management practice. These “riparian 

buffer zones” are thought to intercept 

runoff of sediment and chemicals from 

fields, promote bank stability, and provide 

shading and habitat for aquatic life [10]. 

Riparian buffer zones should be considered 

along with other important factors that 

affect chemical and biological indicators 

of stream quality, such as soil drainage 

properties and stream hydrology [7].

UMIS

LIRB

EIWA

Wisconsin

Minnesota

Iowa

Illinois

Mississippi River

15

10

5

0

0 50 100 150 200

Dissolved nitrite + nitrate (mg/L)

Phytoplankton abundance

Chlorophyll-a ( g/L)
Major Findings 11

Nutrient concentrations were among the highest 

Concentrations, loads, and yields of nutrients in the lower 

Illinois River Basin were among the highest in the Nation. 

When compared with data from NAWQA studies nationwide, nitrate concentrations in the lower Illinois River Basin 

were in the highest 10 percent at every sampling location, 

and total phosphorus concentrations were in the highest 

25 percent at seven of the eight locations.

Of the 10 largest rivers monitored by NAWQA (greater 

than 20,000 ft3/s mean daily streamflow), the lower Illinois 

River Basin site at Valley City had the highest flow-weighted 

mean nitrate and total nitrogen concentrations (see “Streamflow-Weighted Concentrations,” p. 7). The concentrations 

were greater than twice the concentrations for both nitrate 

and total nitrogen of the next highest ranked location—

Mississippi River at Hastings, Minnesota—and of a 

comparable-size river location—Mississippi River at 

Red Wing, Minnesota

Goolsby, Battaglin, and others [11, 12] concluded that the 

basin is one of the areas of highest nitrogen yield (amount of 

nitrogen runoff into streams per unit area) contributing to the 

Mississippi River. Alexander and others [13] estimated that 

50 percent to greater than 90 percent of the nitrogen exported 

from much of the basin is transported to the Gulf of Mexico, 

where excess nutrients help create hypoxia—oxygen concentrations below the level required by most aquatic life in the 

Gulf. The Illinois River contributes from 15 to 20 percent of 

the total nitrogen that goes into the Gulf of Mexico from the 

Mississippi River [14, 12].

Roughly between 61,000 and 124,000 tons of nitrogen is 

transported annually from the lower Illinois River Basin to 

the Gulf of Mexico. More than one-half of the excess nitrogen entering the Gulf from the Mississippi River is thought to 

come from the upper Mississippi Basin—that portion 

upstream from St. Louis, Missouri. The Illinois River joins 

the Mississippi River just north of St. Louis.

TOTAL NITROGEN IN STREAMS

AGRICULTURAL AREAS

MIXED LAND USE AREAS

LOWER ILLINOIS RIVER BASIN

LOWER ILLINOIS RIVER BASIN

EXPLANATION

AVERAGE ANNUAL TOTAL NITROGEN INPUT, IN

POUNDS PER ACRE, BY COUNTY, FOR 1995–98

INPUTS ARE FROM FERTILIZER, MANURE, AND

THE ATMOSPHERE

GREATER THAN 25 POUNDS PER ACRE

6 TO 25 POUNDS PER ACRE

LESS THAN 6 POUNDS PER ACRE

AVERAGE ANNUAL CONCENTRATION OF TOTAL

NITROGEN, IN MILLIGRAMS PER LITER

HIGHEST (GREATER THAN 2.9)

LOWEST (LESS THAN 0.64)

MEDIUM (0.64 TO 2.9)

Soybeans, which are a major crop in the lower Illinois 

River Basin, are important for nutrient management and 

the nitrogen balance of agricultural watersheds. The roots 

of soybeans harbor bacteria that fix nitrogen from the 

atmosphere into the soil in a form that can be used by the 

plant, and by other crops the following year. (Photograph by 

Kelly L. Warner, USGS.)

in the Nation
12 Water Quality in the Lower Illinois River Basin

Nitrate concentrations in 

some shallow wells 

exceeded the drinking-water 

standard

The 117-well network sampled 

in the basin consisted of 57 shallow 

monitoring wells installed for the 

study (fig. 11) and 60 water-supply 

wells of various types. Although 

the concentrations of nitrate in 

none of the 30 samples from the 

deep Mahomet aquifer exceeded 

the MCL (all concentrations were 

below 0.10 mg/L), nitrate concentrations in the shallow wells 

exceeded the MCL in 7 percent (6 

of 87) of the shallow-well samples. 

Concentrations above the MCL 

ranged from 11 to 77 mg/L. All 

exceedances of the nitrate MCL 

were in samples from wells less 

than 50 feet deep.

An important reason for fewer 

detections and much lower concentrations of nitrate and other 

agrichemicals in ground water is 

artificial drainage (where present) 

and the geological materials from 

which the soils developed. Most 

farmland in Illinois is artificially 

drained by ditches and subsurface 

drains (tile drains) because of the 

poor natural drainage of the soils 

and their parent materials (fig. 12). 

Artificial drainage short circuits 

infiltrating water to streams 

through the subsurface drains 

rather than allowing the water to 

move downward to the water table 

or to deeper aquifers [15]. Subsurface drainage also decreases the 

amount of potential runoff across 

land surface to streams because the 

soil is less saturated and ponding is 

less likely [15]. Another factor that 

helps prevent agrichemicals from 

reaching wells is that the finegrained materials may be conducive to rapid breakdown of many of 

the chemicals.

One-half of the shallow wells 

installed for monitoring recently 

recharged ground water (28 of 57) 

were installed in the geological 

materials overlying the Mahomet 

aquifer. The shallow ground water 

in this area has a high potential for 

eventually recharging the underlying Mahomet aquifer [2]. Sampling 

results from these wells indicate 

the Mahomet aquifer is fairly well 

protected from contamination at 

land surface.

Some pesticides were always 

detected in streams and 

rivers

Herbicides are applied during 

spring planting to virtually all 

corn and soybean crops in the 

basin. Insecticides are applied 

during the summer to about 10 

to 30 percent of the corn crop, 

depending on weather conditions. 

Approximately 6,000 to 6,700 tons 

of agricultural pesticides were 

applied annually in the lower 

Illinois River Basin during 1996–

98 (data from [16–18]).

Atrazine, which is commonly 

applied to corn in the basin, and its 

breakdown products (hydroxyatrazine, deisopropylatrazine, and 

deethylatrazine) were the most 

frequently detected pesticide and 

related compounds in streams 

throughout the basin and shallow 

ground water beneath cropland. 

Atrazine was detected in every 

stream sample collected from the 

basin, even during winter. Prometon, which is not used in appreciable amounts on cropland but 

commonly is applied for weed 

control in rights-of-way in the 

basin, was detected frequently in 

streams (93 percent of samples) 

and infrequently in shallow ground 

water (7 percent of samples).

In samples collected at Illinois 

River at Valley City (basin outflow), 40 pesticides or pesticide 

breakdown products were detected. 

Atrazine and metolachlor were 

always present, and cyanazine 

was detected in 97 percent of the 

samples.

Because the method detection 

limit varies widely from one 

pesticide or related compound to 

another, a common reporting level 

is used to compare detections of 

the pesticides and compounds on 

an equal basis. In figure 13, the 

frequency of detection of pesticides 

in stream and river samples and 

ground-water samples is shown 

Figure 12. Subsurface drainage allows 

water with high concentrations of 

agrichemicals to be transported directly 

to ditches and streams. The outlet pipe 

from a subsurface drain at center of 

photograph is draining into the stream.

(Photograph by Kelly L. Warner, USGS.)

Figure 11. Study-Unit staff installed 57 

wells to sample recently recharged 

ground water. Assistance and 

geological expertise was provided by 

the Illinois State Geological Survey. 

(Photograph by William S. Morrow, Jr., USGS.)
Major Findings 13

10 8 6 4 2 0 0 10 20 30 40 50 60 70 80 90 100

DETECTION FREQUENCY, IN PERCENT EQUAL TO OR

GREATER THAN 0.05 MICROGRAM PER LITER

Wells Rivers and streams

Bromoxynil

Carbaryl

Dicamba

Diuron

Atrazine

Metolachlor

Cyanazine

Prometon

Acetochlor

Simazine

Alachlor

Bentazon

Diazinon

Metribuzin

Tebuthiuron

2,4-D

Pendimethalin

Triallate

Carbofuran

Butylate

Propachlor

Acifluorfen

Chlorpyrifos

Color indicates type of

 pesticide

Herbicide

Insecticide

Prometon primarily is used

 on nonagricultural land

Figure 13. Many commonly applied pesticides were frequently 

detected in streams and rivers in the lower Illinois River Basin. 

Detections in ground water were far fewer, and some of the pesticides frequently detected in surface water (such as cyanazine, acetochlor, and alachlor) were not detected in shallow ground water at 

or above the common reporting level of 0.05 micrograms per liter.

metolachlor aquatic-life guideline (7.8 μg/L) were concurrent with an 

exceedance of the atrazine MCL. In contrast, diazinon concentrations twice 

exceeded the Great Lakes water-quality objective and one chlorpyrifos 

concentration exceeded the aquatic-life guideline (0.041 μg/L) independent of all other pesticide exceedances.

In samples from the La Moine River at Colmar and the Sangamon River 

at Monticello, for every pesticide other than atrazine, concentrations 

exceeded a drinking-water standard or guideline concurrent with an atrazine MCL exceedance. The lone exception was cyanazine, which exceeded 

the HAL independent of all other standard or guideline exceedances.

Currently (2000), drinking-water standards or guidelines and aquatic life 

guidelines have been established only for individual pesticides. Furthermore, USEPA drinking-water MCLs or HALs are established for only 25 

of the 40 pesticides detected in the lower Illinois River Basin. Pesticides, 

however, commonly are found in mixtures of as many as 21 compounds in 

surface water (fig. 15). Although most samples of shallow ground water 

contained relatively few detectable concentrations of pesticides compared 

to samples from streams and rivers, two or more pesticides were detected 

at a common reporting level of 

0.05 μg/L. Of the pesticides 

detected in 1 percent or more of 

samples, those shown in figure 13, 

only 2,4-D had a higher method 

detection limit than 0.05 μg/L; 

therefore the detection frequency 

of 2,4-D may be underestimated 

in figure 13. Even at the common 

reporting level, atrazine was 

detected in almost all stream and 

river samples.

Pesticide concentrations in 

streams periodically 

exceeded drinking-water 

standards

Concentrations of atrazine and 

cyanazine in some individual samples exceeded USEPA drinkingwater standards or guidelines. 

Annual average concentrations of 

each compound in streams and 

rivers, however, were below their 

respective MCL (atrazine: 3 μg/L) 

or lifetime health advisory level 

(HAL) (cyanazine: 1 μg/L). Carbofuran, diazinon, chlorpyrifos, and 

carbaryl were the only insecticides 

detected. Diazinon concentration 

exceeded the Great Lakes waterquality objective (0.08 μg/L) [19] 

in one sample at Illinois River at 

Valley City during August 1998.

Some pesticide concentrations 

that exceeded human health 

standards or guidelines also were 

potentially toxic to aquatic life. 

For example, an atrazine concentration of 110 μg/L in a single sample 

from the La Moine River at Colmar 

(fig. 14) exceeded not only the 

USEPA MCL (3 μg/L) but also the 

Canadian guideline for the protection of freshwater aquatic life 

(1.8 μg/L) [20]. In samples from 

the Illinois River at Ottawa and at 

Valley City and the Sangamon 

River at Oakford, all exceedances 

of the cyanazine HAL and the 
14 Water Quality in the Lower Illinois River Basin

in all samples where any of pesticides were detected at 

all. The health effects of such combinations of pesticides in drinking water, even in low concentrations, 

are not well understood.

Pesticide concentrations in streams have a 

seasonal pattern

Herbicide concentrations in stream and river 

samples are highest in late spring to early summer 

(fig. 16), and insecticide concentrations typically peak 

in midsummer. The highest pesticide concentrations 

typically occur during the first one or two periods 

of storm runoff after pesticide application. During 

seasonal peaks, concentrations of pesticides in 

samples of stream water exceeded drinking-water 

standards or guidelines or aquatic-life guidelines. For 

example, for about 6 weeks after herbicide application 

in the spring, concentrations of atrazine at Illinois 

River at Valley City were greater than the MCL of 

3 μg/L. The annual mean atrazine concentration in the 

Illinois River at Valley City for 1996 through 1998, 

however, never exceeded the MCL. Peak concentrations of the insecticides diazinon and chlorpyrifos 

were measured during July or August of 1997 or 1998. 

Insecticides are often applied later in the growing 

season than herbicides.

Many factors affect the presence and 

concentration of pesticides in streams

The presence and concentration of pesticides in 

streams in the lower Illinois River Basin are influenced 

Metolachlor

Cyanazine

Acetochlor

Atrazine

1

10

100

1,000

0.001

0.01

0.1

1,000

0.1

1

10

100

10,000

STREAMFLOW, IN CUBIC FEET PER SECOND

DATE SAMPLE COLLECTED

CONCENTRATION, IN MICROGRAMS

PER LITER

APRIL MAY JUNE JULY

1997

Streamflow

EXPLANATION

ATRAZINE MCL (3 MICROGRAMS PER LITER)

ACETOCHLOR REGISTRATION LIMIT (2 MICROGRAMS PER LITER)

CYANAZINE HAL (1 MICROGRAM PER LITER)

METOLACHLOR ALC (1.8 MICROGRAMS PER LITER)

Figure 14. During late spring and early summer, many 

pesticides exceeded drinking-water standards or guidelines 

or aquatic-life guidelines. The graph shows concentrations of 

selected pesticides in the La Moine River at Colmar during 

part of 1997. (Registration limit for acetochlor is from [21]; 

ALC, Aquatic-Life Criterion from [20]; HAL, Health Advisory 

Level.)

Less than 5 5 to 9 10 or more

NUMBER OF PESTICIDE COMPOUNDS

DETECTED IN ONE SAMPLE

PERCENTAGE OF SAMPLES

60

50

40

30

20

10

0

Figure 15. Most samples from streams 

and rivers contained multiple pesticides 

and related compounds. Five or more 

compounds were detected in more than 

90 percent of samples. The maximum 

number of compounds detected at or 

above the method detection limit in one 

sample was 21.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

JANUARY

FEBRUARY

MARCH

APRIL

MAY

JUNE

JULY

AUGUST

SEPTEMBER

OCTOBER

NOVEMBER

DECEMBER

MONTHLY MEAN TOTAL PESTICIDE

CONCENTRATION, IN MICROGRAMS

PER LITER

Figure 16. Pesticide concentrations vary seasonally. 

The sum of pesticide concentrations in samples 

peaks in late spring during runoff from rainstorms. 

Often, runoff from spring rains supplies reservoirs on 

some rivers and streams with a substantial amount of 

the water storage for the remainder of the year. 

Monthly mean totals are for all locations sampled for 

pesticides during October 1996–September 1998.
Major Findings 15

by land use and agricultural practices (fig. 17) such as 

pesticide application rate, drainage of crop land, and 

cropping methods. Natural factors, such as the size of 

the streams, type of terrain, and soil characteristics, 

also are important determinants. In addition, the distribution of individual pesticides in streams throughout a 

watershed depends on the physical and chemical properties of the pesticides that are in use—properties such 

as sorption (the ability of a pesticide to stick to soil 

particles), transformation (the ease with which a pesticide breaks down in the environment), and volatilization (the tendency of a pesticide to become gaseous 

and rise into the atmosphere).

Concentrations of agricultural chemicals 

generally reflect application rates

The most heavily applied pesticides (more than 

1 million pounds of active ingredient per year; 

acetochlor, atrazine, cyanazine, and metolachlor) 

were generally the most frequently detected in streams 

and shallow aquifers. Herbicides such as alachlor and 

simazine, which are no longer applied as heavily as in 

previous decades, still are detected at relatively high 

rates. Bentazon is not applied as heavily as other herbicides, yet it is frequently detected in stream and well 

samples (fig. 18).

Pesticide concentrations in water also were commonly related to drainage of water over and through 

agricultural soils, pesticide uptake by plants and 

microbes, and attachment to soil particles. Pesticide 

and nutrient persistence and regional differences in 

soils, geology, and climate govern the distribution of 

these water-quality indicators in the lower Illinois 

River Basin.

The relatively infrequent detection of metolachlor in 

well samples may be related to the following factors:

1. Metolachlor is applied after weeds sprout and emerge, 

and it is taken up by plants and is converted to other 

compounds.

2. Soil microbes effectively degrade metolachlor.

3. Soils retain metolachlor instead of allowing it to run off 

or seep downward to ground water.

Stream size affects pesticide concentrations

In the lower Illinois River Basin, pesticide concentrations in small streams typically range from the 

highest to the lowest observed concentrations, whereas 

concentrations in large rivers are moderate and less 

variable. For example, during the period when most 

pesticides are washed into streams (May through 

July), the maximum concentration of atrazine measured in the Sangamon River at Monticello was about 

twice as high as that measured in the Illinois River. 

The highest concentrations of several pesticides, 

including atrazine (110 μg/L; May 1997), acetochlor 

(12 μg/L; May 1998), cyanazine (16 μg/L; May 1997), 

and 2,4-D (7.3 μg/L; May 1998) were detected in samples from the La Moine River at Colmar. In contrast, 

0

200400600800

1,0001.2001,4001,6001,800

PERCENTAGE OF SAMPLES WITH

DETECTIONS

TONS OF PESTICIDE APPLIED PER YEAR

IN THE LOWER ILLINOIS RIVER BASIN

0

20

40

60

80

100

AlachlorSimazine

BentazonPendimethalinCyanazineAcetochlorMetolachlorAtrazine

DETECTIONS IN WATER SAMPLES AT OR ABOVE THE

METHOD DETECTION LEVEL FOR EACH COMPOUND

STREAM OR RIVER SAMPLES

GROUND-WATER SAMPLES

EXPLANATION

Figure 18. Detection rates in streams are 

mainly affected by the amount of pesticide 

applied in the basin. Detection rates in 

ground water, even for those pesticides that 

are heavily applied, are much lower.

Figure 17. Pesticides are used to protect crops 

and increase production. Pesticides are often 

sprayed directly on the plants in the field. Other 

pesticides are incorporated into the soil, partly to 

limit the amount of chemical that leaves the 

field. (Photograph by Kelly L. Warner, USGS.)
16 Water Quality in the Lower Illinois River Basin

in months other than May or June, 70 percent (35 of 

50) of samples from Sangamon River at Monticello 

had total pesticide concentrations that were less than 

1 μg/L, as compared with 79 percent (26 of 33) of the 

samples from Illinois River at Valley City. Samples for 

analysis of pesticides were not collected from the 

smallest streams monitored.

Concentrations of alachlor and cyanazine in 

the Illinois River decreased

Application rates of cyanazine decreased during the 

1996–98 sampling period in anticipation of its use 

being eliminated in 2003. Alachlor use has declined 

since the early 1990s—in fact, no use of alachlor was 

reported for 1998 in the basin. The use of alachlor, by 

voluntary agreement [21], is decreasing as acetochlor 

replaces it (fig. 19).

Cyanazine was detected in 97 percent of the river 

and stream samples and alachlor was detected in 

74 percent of the samples during 1996–98. A 

comparison of mean monthly concentrations of those 

detections, however, shows a different story. During 

1991–92, samples that were comparable to those collected as part of this NAWQA study were collected 

from the Illinois River at Valley City. The mean 

monthly concentrations for the two periods, separated 

by about 4–5 years or longer, indicate that concentrations of alachlor and cyanazine have substantially 

decreased (fig. 20). Pesticide breakdown products were detected 

more frequently than the parent compound

In stream and river samples (fig. 21) and groundwater samples (fig. 22), pesticide breakdown products, 

such as metolachlor ethanesulfonic acid (a breakdown 

product of metolachlor) and deethylatrazine (a breakdown product of atrazine) were detected more frequently at higher concentrations than their respective 

parent compounds. The toxicity of most of these 

breakdown products generally is unknown. Also 

unknown is the combined effect of parent compounds 

and the breakdown products. Drinking-water standards or guidelines or aquatic-life guidelines have 

been established for only a few pesticide breakdown 

products.

Most of the residue of metolachlor detected in 

ground water consisted of breakdown products 

(fig. 22). Metolachlor was frequently detected in 

streams and rivers. Because primarily breakdown 

products of metolachlor were found in ground water, 

metolachlor must break down before it can reach 

ground water.

Acetochlor

Alachlor

Cyanazine

2,4-D

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

PERCENTAGE OF ILLINOIS CORN

CROP TREATED

YEAR

0

5

10

15

20

25

30

Figure 19. Use of herbicides on corn and 

soybeans in Illinois changed during 1988–97. Use 

of three common herbicides has declined from their 

peak use, but the use of acetochlor has increased 

substantially. (Data from [22].)

Sampled from April 1991

through March 1992

Sampled from Oct. 1996

through Sept. 1998

MONTHLY MEAN CONCENTRATION,

IN MICROGRAMS PER LITER

Detection limit

for 1991–92

samples

MONTHLY MEAN CONCENTRATION,

IN MICROGRAMS PER LITER

Sampled from April 1991

through March 1992

Sampled from Oct. 1996

through Sept. 1998

Detection limit

for 1991–92

samples

MONTH SAMPLES COLLECTED

J FM AMJ JA S OND

0

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.0

0

0.5

1.5

2.5

3.5

3.0

2.0

1.0

Cyanzine

Alachlor

Figure 20. Monthly mean concentrations of cyanazine 

and alachlor declined substantially in water samples 

collected from the Illinois River at Valley City.
Major Findings 17

Pesticides were not frequently detected in 

ground water

Pesticides are much less common in ground 

water than in streams. Six compounds, all herbicides 

(atrazine, metolachlor, prometon, bentazon, cyanazine, and dicamba), were detected in monitoring and 

water-supply wells. No insecticides were detected in 

ground-water samples. Only four of these pesticides 

were detected more than once. The maximum number 

of pesticides detected in a single well sample was four. 

Concentrations of all pesticides detected were very 

low (less than 0.44 μg/L). In ground-water samples, 

no pesticide came close to exceeding an existing 

drinking-water standard or guideline.

Insecticides discontinued in the 1970s were 

still detected in fish and sediment

Several organochlorine compounds (dieldrin, heptachlor epoxide, chlordane, and DDT) were commonly 

detected in fish tissue. Fish samples usually comprised 

8 specimens of common carp, but at several locations 

Figure 21. Breakdown products of common herbicides 

account for most of the analyzed herbicide residue in river 

water. Long after the parent compound drops below 

detection, breakdown products can still be detected. Earlier in 

the spring, acetochlor concentrations peaked at 9 micrograms 

per liter at this location, Sangamon River at Monticello. (ESA, 

ethanesulfonic acid) CONCENTRATION, IN MICROGRAMS PER LITER

DATE SAMPLE COLLECTED

8/4/989/8/98

6/29/98

7/6/98

7/14/987/20/987/27/988/12/988/17/988/24/988/31/989/14/98

EXPLANATION

METOLACHLOR

METOLACHLOR ESA

ALACHLOR ESA

ALACHLOR

ACETOCHLOR ESA

ACETOCHLOR

0

1

2

3

4

5

6

7

8

9

10

CONCENTRATION, IN MICROGRAMS

PER LITER

WELL 1-05

WELL 2-03WELL 2-06WELL 2-09WELL 2-12WELL 2-14WELL 2-15WELL 2-16WELL 2-22WELL 2-29

0

5

10

15

20

25

30

35

40

45

50

METOLACHLOR

METOLACHLOR OXANILIC ACID

METOLACHLOR ETHANESULFONIC ACID

EXPLANATION

Figure 22. Breakdown products account for most of the 

residue of metolachlor in ground water. In these 10 shallow 

wells, metolachlor or its residue was detected. Metolachlor 

never exceeded the HAL (70 μg/L) in well samples; no 

drinking-water standards or guidelines have been 

established for the breakdown products.

Standards for water, biota, and sediment quality

The USEPA [23–25] is responsible for setting drinking-water 

standards and guidelines to protect human health. Maximum Contaminant Level (MCL) is the maximum allowable concentration of a 

contaminant in water delivered to any user of public water systems. 

MCLs are enforceable standards based on an average annual concentration taken from four quarterly samples of finished drinking water. 

Health Advisory Level (HAL) is a nonenforceable, risk-based guideline. HALs indicate contaminant exposures below which no short- or 

long-term human-health effects are expected, based on drinking a 

specific amount of water for a specific period of time. Risk of illness 

increases with exposure time and concentration. Of 88 pesticides 

analyzed for in water, 14 have MCLs and 38 have HALs.

Guidelines to protect aquatic life have been set by several agencies including the USEPA, Environment Canada [26, 20], and the 

International Joint Commission (IJC) [19]. The aquatic-life guidelines developed by USEPA are based on 4-day average concentrations, are intended to protect 95 percent of the aquatic species, and 

should not be exceeded more than once in 3 years. The Canadian and 

IJC aquatic-life guidelines, which are more stringent than those of 

the USEPA, indicate a single maximum concentration that should 

never be exceeded. Aquatic-life guidelines have been established for 

only 18 pesticides.

Probable Effect Levels (PELs), are used to indicate contaminant 

concentrations in freshwater sediment associated with adverse effects 

on aquatic life for the Great Lakes region. The PEL is an estimate of 

the concentration above which adverse biological effects frequently 

occur [19, 27]. PELs are established for 18 of the 43 organochlorine 

and semivolatile organic compounds and 8 of 29 trace elements analyzed for this study.
18 Water Quality in the Lower Illinois River Basin, 1995–98.

white suckers were collected. These 

compounds are insecticides or 

insecticide breakdown products. 

Most of the organochlorine insecticides are no longer used because 

their registration was canceled by 

USEPA in the 1970s. Dieldrin, 

which was applied to corn and 

around buildings against termites, 

is a transformation product of aldrin, another insecticide that was 

applied to corn before use was discontinued during the early 1970s 

[28].

Dieldrin was detected in every 

tissue sample and one-third of the 

bed-sediment samples collected 

Heptachlor epoxide was detected 

throughout the basin but at only 

one location in the Illinois River. 

The concentration of heptachlor 

epoxide in all fish tissue samples 

from the lower Illinois River Basin 

in which the compound was 

detected exceeded the concentration in more than 95 percent of all 

NAWQA samples. Heptachlor 

epoxide was not detected in any 

bed-sediment samples.

Chlordane was found in every 

tissue sample and in two bedsediment samples. No concentrations exceeded applicable guidelines. Total chlordane, as defined 

for this report, is the sum of five 

individual components of technical 

chlordane [28]. Among NAWQA 

studies nationwide, the highest 

concentration of oxychlordane (a 

breakdown product of chlordane) 

in fish tissue was found in fish 

from the Sangamon River at Monticello.

DDT was once a widely used 

insecticide in agricultural and 

urban areas of the United States. 

Legal use of DDT ended in 1972. 

Adverse effects of DDT on fish 

and wildlife include growth 

impairment, reproductive failure, 

and inhibition of thyroid activity, 

and recent studies have shown 

DDT to be disruptive of reproductive cycles in animals [28].

Breakdown products of DDT 

were detected in every fish-tissue 

sample and in almost one-half the 

bed-sediment samples. The highest 

total DDT concentrations in the 

basin in fish tissue and sediment 

were in samples from Illinois River 

locations. Concentrations in fish 

tissue also were high at two Sangamon River locations (in Monticello 

and downstream from Springfield) 

compared to national NAWQA 

data. In bed sediment, excluding 

Table 1. Summary of total chlordane, total DDT, dieldrin, heptachlor epoxide, 

DDD, DDE, and total PCB concentrations in fish tissue (carp or white suckers) and bed 

sediment. The detections are compared to guidelines for the protection of fish-eating 

wildlife. DDD and DDE are breakdown products of DDT

[μg/kg, micrograms per kilogram (equivalent to parts per billion); NYSDEC, New York State Department 

of Environmental Conservation [27]; PEL, Probable Effect Level [26]; –, not applicable]

Fish Tissue

Compound

detected in fish 

tissue

Number of sites

with detections

(20 sites 

sampled)

Minimum

detected 

concentration

(μg/kg)

Maximum

detected 

concentration

(μg/kg)

NYSDEC 

whole-fish noncarcinogenic criteria 

recommended 

for the 

protection of

fish-eating 

wildlife 

(μg/kg)

Number of 

sites with 

concentrations

in excess of

NYSDEC 

criteria

Dieldrin 20 24 300 120 6

Heptachlor

epoxide

13 13 44 200 0

Chlordane, total 20 20 310 500 0

DDT, total 20 11 550 200 1

PCBs, total 15 130 4,400 110 15

Bed Sediment

Compound

detected in 

stream

sediments

Number of sites

with detections

(21 sites 

sampled)

Minimum

detected 

concentration

(μg/kg)

Maximum

detected 

concentration

(μg/kg)

Canadian

guidelines PEL

(μg/kg)

Number of sites

exceeding PEL

Dieldrin 7 1.4 5.5 6.7 0

Heptachlor

epoxide

0 – – 2.7 0

Chlordane,

total

2 2.6 3.4 8.9 0

DDD 5 1.1 4.1 8.5 0

DDE 5 1.0 2.6 6.8 0

DDT 0 – – 4.8 0

PCBs, total 1 57 57 277 0

in the lower Illinois River Basin 

(table 1). The highest dieldrin 

concentration in fish tissue in the 

NAWQA Program across the 

Nation was detected in fish from the 

Sangamon River. Every concentration detected in the basin was in the 

highest 10 percent of the national 

NAWQA results. Aquatic-life 

guidelines were exceeded in 

30 percent of the samples.

Heptachlor epoxide is a breakdown product of the insecticide 

heptachlor. The highest heptachlor 

epoxide concentration in fish tissue 

in the NAWQA Program nationally 

was detected in the La Moine River. 
Major Findings 19

National results from river and stream sampling reflect 

results found for pesticides in the lower Illinois River Basin 

[29]. Atrazine and metolachlor were the most frequently 

detected herbicides nationally and in the basin. Prometon 

and simazine were the next most frequently detected in the 

Nation, whereas cyanazine, prometon, and acetochlor were 

the next most frequently detected in the basin. Prometon has 

no listed agricultural use and is usually applied to roadways 

and rights-of-way.

The frequent detections of these herbicides in streams 

and rivers is due, at least in part, to their intensive application to crops and land areas. However, the major differences 

between the national results and the basin results were in 

that the frequencies of detections and levels of those detections were generally much higher in the basin than nationally (see Appendix). The Corn Belt, of which the basin is a 

part, has the highest rates of certain herbicide applications 

in the United States. Comparing only watersheds that are 

primarily agricultural, the results of the basin are similar to 

those from the rest of the Nation in frequency of detection.

For ground water, national and lower Illinois River Basin 

pesticide-sampling results also are similar. Nationally, 

detections of pesticides are less frequent and the concentrations are lower in wells than in streams. However, the detections of pesticides in ground water in the basin are even less 

frequent than in well samples for other agricultural areas in 

the United States. Most of the lower Illinois River Basin is 

underlain by glacially derived material that is poorly 

drained. Subsurface drainage diverts water to streams rather 

than allowing it to move downward (p. 12). The finegrained, glacially derived material is conducive to rapid 

breakdown of many of the pesticides. These factors help 

prevent pesticides from reaching wells, thereby keeping the 

concentrations lower than those found in ground water in 

other parts of the Nation.

HERBICIDES

EXPLANATION

HERBICIDE USE, IN POUNDS PER

ACRE OF AGRICULTURAL LAND

Highest (greater than 0.46)

Medium (0.16–0.46)

Lowest (less than 0.16)

No reported use STREAMS WITH MIXED LAND USE AREAS

13

15

14

6

22 10

41

5

13

4 46

26

5

29

36

35

20

38

7 7

24

21

27

2123

LOWER ILLINOIS RIVER BASIN

EXPLANATION

SUM OF HERBICIDE CONCENTRATIONS

(75TH PERCENTILE, IN MICROGRAMS PER LITER)

AQUATIC-LIFE GUIDELINES

Bold outline indicates exceedance by one or more herbicides. Number is

percentage of samples that exceeded a guideline within a one-year period.

7

AGRICULTURAL STREAMS

LOWER ILLINOIS RIVER BASIN

URBAN AREA

15

21

3

28

31

10 33

3

7 16

16

Middle 50 percent (0.11–1.3)

Highest 25 percent (greater than 1.3)

Lowest 25 percent (less than 0.11)

HERBICIDES

SHALLOW GROUND WATER IN AGRICULTURAL AREAS

LOWER ILLINOIS RIVER BASIN

MAJOR AQUIFERS

LOWER ILLINOIS RIVER BASIN

EXPLANATION

HERBICIDE USE, IN POUNDS PER

ACRE OF AGRICULTURAL LAND

Highest (greater than 0.46)

Medium (0.16–0.46)

Lowest (less than 0.16)

No reported use

4

13

7 3

7

1

EXPLANATION

HIGHEST DETECTION FREQUENCY

DRINKING-WATER STANDARDS OR GUIDELINES

Highest 25 percent (greater than 62)

Middle 50 percent (19–62)

Lowest 25 percent (less than 19)

Bold outline indicates exceedance by one or more herbicides. Number is

percentage of wells that exceeded a standard or guideline

7

Basin and national results were similar for pesticides
20 Water Quality in the Lower Illinois River Basin

Illinois River locations, DDT 

breakdown products were detected 

only in a small stream in Decatur.

Other organochlorine 

compounds are persistent 

in sediments and fish

Polychlorinated biphenyls

(PCBs) are a mixture of organochlorine compounds used in 

industrial processes. Even though 

production and use of PCBs was 

banned in the United States in 

1979, PCBs commonly are found 

in fish and sediments because large 

amounts were produced, they 

degrade slowly, and they are transported in the atmosphere [28]. 

PCBs were detected in 75 percent 

of the fish-tissue samples in the 

basin, and these detections were 

geographically widespread. All 

detections in the basin exceeded 

the water-quality guidelines for the 

protection of fish-eating wildlife 

developed by the New York State 

Department of Environmental Conservation [27]. The highest PCB 

concentrations in fish tissue were 

in samples from Illinois River locations, with concentrations decreasing with increasing downstream 

distance from Chicago. PCBs were 

detected at only one location in the 

bed sediment.

In sufficiently high concentrations, most organochlorine compounds can be toxic to fish and 

other animals. These compounds 

bioaccumulate in animal tissue and 

may result in tumors, hormonal and 

behavioral problems, immune- and 

respiratory-system suppression, 

and abnormal development. Available evidence indicates that concentrations of these compounds 

have decreased over time and may 

be leveling off nationally [30, 28].

Common 

stream 

animals 

indicate poor 

water-quality 

conditions

Degradation 

of streams and 

habitat is caused 

by more than 

just chemical 

contamination. 

Biological 

community 

measures are 

used to assess 

water quality 

because 

the living components of stream ecosystems 

reflect the physical, chemical, and 

biological attributes of a stream. 

Measures of the algal, benthicmacroinvertebrate (fig. 23), and 

fish communities in streams each 

respond to different aspects of 

water quality over differing time 

periods and geographic areas.

Algal, benthic macroinvertebrate, and fish indices (table 2) 

were used to assess water quality 

as part of the NAWQA Program 

[32]. The higher the index, the 

more degraded the stream conditions. (See Appendix.) The algal 

status index reflects the relative 

abundance of benthic algal 

species that are considered tolerant 

of poor water-quality conditions, 

including high nutrient concentrations. In some regions, this index 

correlates with transport and deposition of silt.

Eleven community characteristics were used in the invertebrate 

community status index to assess 

water quality. The fish status indicator is the sum of scores of four 

fish-population characteristics 

(percent tolerant species, percent 

omnivorous fish, percent nonnative fish, and individuals with 

external anomalies) that increase in 

concert with water-quality degradation (fig. 24).

In relation to the rest of the 

Nation, the invertebrate status 

index in the lower Illinois River 

Basin was intermediate at all sites 

except one (table 2). Macroinvertebrate communities of streams in 

Figure 23. Freshwater mussels are 

relatively long-lived animals that are 

indicators of long-term water quality. 

These mussels were from the North 

Fork Vermilion River in central Illinois. 

Mussels bury themselves in the bottom 

where they filter organic matter from the 

water. During the twentieth century, 

freshwater mussel populations declined 

drastically in the lower Illinois River 

Basin. (Photograph by David J. Fazio, USGS.)

Illinois River at Valley City

Sangamon River near Oakford

Mackinaw River near Green Valley

La Moine River at Colmar

Sangamon River at Monticello

Indian Creek near Wyoming

Panther Creek near El Paso

Fish

status

Invertebrate

status

Algal

status Location sampled

Highest 25 percent nationally (most degraded locations)

Middle 50 percent nationally

Lowest 25 percent nationally (least degraded locations)

Table 2. Selected biological indicators of water 

quality—comparison of lower Illinois River Basin sites to 

national network sites1

1 Represents 140 sites in the NAWQA national network that have algal, 

invertebrate, and fish data.
Major Findings 21

PERCENTAGE OF SAMPLES WITH DETECTIONS AT OR ABOVE 50

 MICROGRAMS PER KILOGRAM

Samples above PEL

0 10 20 30 40 50

1-Methylpyrene*

Benz[a]anthracene

Acenaphthylene

Benzo[a]pyrene

Benzo[g,h,i]perylene*

Benzo[k]fluoranthene*

Benzo[b]fluoranthene*

Chrysene

Dibenz[a,h]anthracene

Fluoranthene

Indeno[1,2,3-c,d]pyrene*

Phenanthrene

Pyrene

the basin are dominated by many species that are 

widely distributed, most of which are tolerant of the 

effects of excess nutrients. Excessive quantities of 

nutrients in runoff to streams can be harmful to intolerant species by stimulating algal growth that may 

decrease dissolved-oxygen concentrations at night. 

The highest proportions of sensitive macroinvertebrate 

species were in streams with comparatively swift currents because increased water velocity can increase 

rates of stream aeration and thus maintain higher dissolved-oxygen concentrations (fig. 25).

Algae index values were either high or intermediate 

in relation to the entire Nation. The algal status index 

indicates that general algal conditions (as an indicator 

of water quality) in the Corn Belt are poor relative to 

the rest of the country. (See pages 9–10.)

One of the lowest fish-index scores in the country 

(indicating a healthy fish community) was at Panther 

Creek near El Paso. This small, shaded stream held a 

variety of habitats and a diverse fish population. All 

the biological index scores were high (degraded community) for the Sangamon River near Oakford. Little 

habitat diversity was found at this site, and concentrations of nutrients were high.

Highest concentrations of polycyclic aromatic 

hydrocarbons were in bed sediment near 

urban areas

Polycyclic aromatic hydrocarbons (PAHs) were 

detected in concentrations at or above a common 

reporting level of 50 μg/kg in as much as 50 percent 

of the bed-sediment samples (fig. 26). PAHs are byproducts of combustion; sources include fires, manufacturing, power generation, and vehicle emissions. 

With sufficient exposure, many PAHs are carcinogenic, causing tumors in fish and other animals, and 

many PAHs are toxic to some organisms [33]. In the 

basin, the highest PAH concentrations were in the bed 

sediment of the Illinois River at locations upstream 

from Peoria. The highest PAH concentrations at other 

sites in the basin were near urban areas: specifically, in 

Figure 26. Four polycyclic aromatic 

hydrocarbons (PAHs) were detected at 

concentrations that may be detrimental 

to wildlife that live in bed sediments. 

Many PAHs are prevalent in sediments 

of the basin. Shown are percent 

detections at or above 50 micrograms 

per kilogram in bed-sediment samples 

and percent detections that exceed 

Canadian guidelines for protection of 

aquatic life [26]. [*, no guideline 

available; PEL, Probable Effect Level]

Figure 24. Longnose gar from the Sangamon 

River in central Illinois. In the basin, the 

Longnose gar is the most common gar found in 

small rivers. Many fish were collected, 

measured, and released to determine the health 

of the fish communities in the streams and rivers 

sampled. (Photograph by David J. Fazio, USGS.)

Figure 25. Basin streams with fast current had 

the most diverse macroinvertebrate communities. 

(Photograph by David J. Fazio, USGS.)
22 Water Quality in the Lower Illinois River Basin

Alaska EXPLANATION

Counties with arsenic concentrations exceeding 10

micrograms per liter in 10 percent or more of samples

Counties with arsenic concentrations exceeding 5

micrograms per liter in 10 percent or more of samples

Counties with arsenic concentrations exceeding 3

micrograms per liter in 10 percent or more of samples

Counties with fewer than 10 percent of samples exceeding

3 micrograms per liter, or areas of lowest concentration

Counties with insufficient data to make estimates

Lower

Illinois

River

Basin

0 5 10 15 20

0 5 10 15 20MILES

KILOMETERS

EXPLANATION

WELLS

MAHOMET

AQUIFER

Arsenic concentration greater than or

equal to 5 micrograms per liter and

less than 50 micrograms per liter

in samples

Arsenic concentration greater

than or equal to 50 micrograms

per liter in samples

a small stream in the city of Decatur, in the Vermilion 

River near its confluence with the Illinois River 

(downstream from Streator and Pontiac), and in the 

Sangamon River downstream from Springfield. Four 

detections of PAH compounds exceeded the Canadian 

interim freshwater sediment-quality guidelines for the 

protection of aquatic life [26].

Arsenic exceeded the drinking-water 

standard in the Mahomet aquifer

The Mahomet aquifer is overlain by more than 

100 feet of clay-rich glacial material. Contamination 

of this aquifer by human activities is minimal [31]. 

Although arsenic resulting from human activities can 

contaminate water, high concentrations of arsenic in 

the Mahomet aquifer are from natural sources. Arsenic 

concentrations in parts of the Mahomet aquifer 

exceeded the USEPA current MCL of 50 μg/L. In 

1999, the National Academy of Science [34] concluded that this current standard does not sufficiently 

protect public health. The USEPA has proposed lowering the MCL to 5 μg/L. If the arsenic MCL were lowered to 5 μg/L, then water from more than 60 percent 

of the wells sampled (18 of 30) would exceed the new 

level.

Within the lower Illinois River Basin, the percentage of private-well samples with concentrations of 

arsenic above 50 μg/L was approximately the same as 

that of samples from public-supply wells. Statewide 

data from the Illinois Environmental Protection 

Agency [31], however, indicate the percentage of concentrations above 5 μg/L was different between the 

public wells and private wells statewide.

overlying the Mahomet aquifer, where water from about 

7 percent of wells sampled (2 of 30) exceeded the current 

MCL (50 μg/L) and 60 percent would exceed the lower 

MCL. Only few a concentrations of arsenic above 5 μg/L 

and none above 50 μg/L in shallow wells (less than 100 feet 

deep) were found at wells in the western and southern shallow aquifers and other areas in the lower Illinois River 

Basin.

Nationally, water from 1 percent of wells sampled 

exceeded the current MCL [35], and only 13 percent would 

exceed the proposed new standard. In Illinois, if they do not 

treat the water for arsenic, people who draw water from the 

Mahomet aquifer have greater risk of drinking arsenic in 

water than ground-water users in the rest of the United 

States.

Arsenic is a naturally occurring trace element in natural 

waters. With sufficient exposure, arsenic can cause skin, lung, 

bladder, liver, and kidney cancer in humans [34, 2]. It also has 

been reported to impair development and hearing; to adversely 

affect the vascular, gastrointestinal, and nervous systems; and 

to be associated with diabetes. The current (2000) arsenic MCL 

applies to the sum of all arsenic forms in finished drinking 

water. The USEPA has proposed a new MCL of 5 μg/L.

Areas in the Western United States and upper Midwest have 

relatively high arsenic concentrations in ground water. In Illinois, the dark areas on the map generally indicate the counties 

Arsenic in ground water is a national concern
Major Findings 23

39o

40o

91o

41o

90o

89o

EXPLANATION

Water-supply well

Shallow monitor

 well

WELLS

AQUIFERS

Area of second ground-

 water study

Western and southern

 shallow aquifers

Mahomet aquifer (deep sand and gravel)

0 25 50 MILES

0 25 50 KILOMETERS

ILLINOIS RIVER: A history of degradation and recovery

Radon exceeded the proposed drinkingwater standard in almost one-half of shallow 

wells

The bedrock underlying the glacial materials in the 

lower Illinois River Basin and especially the glacial 

material itself contain a high concentration of the 

source elements that decay into radon. As a result, the 

potential for high concentrations of radon in ground 

water and soil gas in the basin is greater than the 

national average (W.S. Morrow, Jr., U.S. Geological 

Survey, written commun., 2000).

Radon was detected in all 30 water samples from 

the Mahomet aquifer (fig. 27). Concentrations of 

radon ranged from 110 to 730 picocuries per liter 

(pCi/L), and the mean concentration was 190 pCi/L. 

In Illinois, 7 percent of the fish species have disappeared since about 1900, and many more species are 

in decline [36]. Smith [37] listed eight conditions 

responsible for the loss of many fish populations in 

Illinois streams. Some of these conditions can be 

found in the basin: siltation; drainage of natural lakes, 

swamps, and prairie marshes; desiccation during 

drought; species interactions, including competition 

from non-native fish; industrial, domestic, and agricultural pollution other than siltation; dams and reservoirs; and increased water temperature.

One of the most notable effects of human activity on 

stream life occurred between 1900 and 1920 in the 

Illinois River downstream from Chicago, after the 

Chicago Sanitary and Shipping Canal opened. The 

biological repercussions of industrial and domestic 

wastes being diverted to the Illinois River peaked in 

about 1920, when plants, benthic organisms, and fish 

practically were eliminated from the Illinois River 

downstream from Chicago to a point downstream 

from Peoria [38]. Since the 1920s, water quality in 

this reach of the Illinois River has gradually improved 

because of the improved wastewater-treatment practices upstream. Monitoring by the Illinois Natural 

History Survey (INHS) has documented partial recovery of fish communities. Between 1957 and 1992, 

common carp and goldfish dominated fish communities in the main stem of the upper Illinois River. By 

1993, however, native species had returned to the 

main stem, and the relative abundance of common 

carp and goldfish had declined significantly [39]. In 

contrast, fish life in the bottomland lakes along the 

lower Illinois River has not increased to the degree 

anticipated from improvement in dissolved-oxygen 

conditions [40, 41].

Compared to urban areas upstream from the basin, 

streams in predominantly agricultural areas generally 

have higher biotic integrity and more diverse fish communities that commonly include various intolerant fish 

species [42, 43, and 37].

The Illinois 

River 

historically 

was an 

important 

wildlife 

resource. It 

is gradually 

recovering 

that status.

Figure 27. Ground-water studies encompassed most of the area of the 

lower Illinois River Basin. Two studies of water-supply wells and two 

studies of shallow, recently recharged ground water were completed. 

The first set of shallow monitor wells was drilled in the material overlying 

the Mahomet aquifer.
24 Water Quality in the Lower Illinois River Basin

The samples from the Mahomet aquifer, however, had 

the lowest average radon concentrations of the four 

groups of study wells. (See pages 25–26.) Concentrations in 7 percent of Mahomet aquifer samples (2 of 

30) exceeded the proposed USEPA radon MCL of 

300 pCi/L. No sample concentrations exceeded the 

USEPA-proposed Alternative Maximum Contaminant 

Level (AMCL) for radon of 4,000 pCi/L. The MCL is 

proposed for drinking water where there is no other 

indoor-air radon-mitigation strategy. The AMCL is 

proposed for drinking water where a multimedia 

indoor-air mitigation strategy is used to reduce radon 

exposure.

Radon concentration was higher in samples from 

the 57 shallow monitoring wells and the 30 watersupply wells in the western and southern shallow aquifers than in the Mahomet aquifer. In the samples from 

the western and southern shallow aquifers, 55 percent 

(16 of 29) exceeded the proposed MCL, but none 

exceeded the proposed higher AMCL. Radon was 

highest in the samples from shallow monitoring wells 

in the glacial materials overlying the Mahomet aquifer 

(fig. 27). In the shallow monitoring wells, 38 percent 

(22 of 57) of the samples exceeded the radon MCL, 

although no sample exceeded the AMCL.

Volatile organic compounds were detected in 

ground water at low concentrations

Samples were collected from 60 private and public 

water-supply wells in the basin to determine the extent 

of contamination by volatile organic compounds 

(VOCs); 30 of the wells were in the Mahomet aquifer 

and the other 30 were in the western and southern 

shallow aquifers. Detections were similar in samples 

from wells in the western and southern shallow aquifers (83 percent) and in the Mahomet aquifer (80 percent). One sample from a well in the area of western 

and southern shallow aquifers contained 0.57 μg/L of 

methyl tert-butyl ether (MTBE)—well below the HAL 

for MTBE of 20 μg/L. The concentration of VOCs in 

almost all of the samples was at or near the method 

detection limit and was usually 10 to 100 times lower 

than applicable standards or guidelines for drinking 

water. The source of these VOCs—especially those in 

the Mahomet aquifer, which is largely protected from 

atmospheric or land-use contamination—is uncertain.

Fish and sediment in the Illinois River have 

high concentrations of trace elements

The Canadian interim freshwater sediment-quality 

guidelines for the protection of aquatic life [26] were 

exceeded at four locations for various trace elements. 

Three of these locations were on the Illinois River 

close to Chicago. In the Illinois River, bed-sediment 

concentrations of seven metals (chromium, copper, 

lead, mercury, nickel, selenium, and zinc) were highest near Ottawa and decreased downstream [44] 

(fig. 28).

In many samples of fish tissue collected in the lower 

Illinois River Basin, cadmium concentrations were 

high compared to those in all samples analyzed in the 

rest of the Nation. (See Appendix.) Concentrations of 

trace elements were examined in carp liver collected at 

17 locations in the basin. The highest cadmium concentration in carp liver from across the Nation (52 milligrams per kilogram dry weight) was detected in the 

Vermilion River near its confluence with the Illinois 

River. Concentrations of cadmium in 10 carp-liver 

samples from the basin were in the upper one-third of 

the concentrations in all carp-liver samples collected 

in the Nation. Of those 10 samples, 6 were collected 

from the Illinois River main stem. Fitzpatrick and others [45] found cadmium at higher than background 

concentrations in sediment or biota at urban and agricultural locations in the upper Illinois River Basin. 

Long-term chronic exposure to cadmium can cause 

kidney and bone damage in humans. Mining and activities related to mining are the main sources of cadmium in the environment from human activities, but 

other sources include waste incineration, sewage 

sludge, fertilizers, batteries, tires, and many industrial 

processes [23].

Marseilles

Ottawa

Henry

Kingston Mines

Havana

Valley City

Hardin

Chromium

Lead Copper

0

40

80

120

160

250 200 150 100 50 0

THE FOX RIVER ENTERS THE ILLINOIS

RIVER AT OTTAWA

CONCENTRATION OF METAL IN

SEDIMENT, IN MILLIGRAMS PER

KILOGRAM

RIVER MILES UPSTREAM FROM

GRAFTON, ILLINOIS

Figure 28. Concentrations of three metals in 

bed sediments decreased in the Illinois River 

from Ottawa, Ill., to Kingston Mines, Ill. The 

concentrations of metals were highest near the 

confluence of the Fox River and the Illinois 

River at Ottawa.
Study Unit Design 25 

STUDY UNIT DESIGN

0 25 50 MILES

39 0 25 KILOMETERS o

40o

91o

41o

90o

89o

EXPLANATION

Basic

Intensive (2 years pesticide

data)

Agriculture land use

Mixed land use

SITES

PHYSIOGRAPHY

Bloomington Ridged Plain

Galesburg/Springfield Plain

La Moine

River at

Colmar

Illinois River

at Valley City

Sangamon

River at

Monticello

Illinois River

at Ottawa

Panther

Creek near

El Paso

Indian

Creek near

Wyoming

Sangamon

River near

Oakford

Mackinaw River

near Green Valley

50

39o

40o

91o

41o

90o

89o

EXPLANATION

Intensive assessment

Synoptic study

Bed sediment

Fish tissue

SITES

PHYSIOGRAPHY

Bloomington Ridged Plain

Galesburg/Springfield Plain

La Moine

River at Colmar

Illinois River

at Valley City

Sangamon River

at Monticello

Illinois River

at Ottawa

Panther Creek

near El Paso

Indian Creek

near Wyoming

Sangamon River

near Oakford

Mackinaw River

near Green Valley

0 25 50 MILES

0 25 50 KILOMETERS

Stream chemistry

The main objective of the stream-chemistry component of the study was to assess the relation between 

agricultural land use and basin size and chemical constituents of surface water. Stream and river sampling 

locations were distributed among subbasins and on the 

Illinois River. Sampling locations included Basic and 

Intensive Fixed Sites—sampled at regular intervals for 

a period of 2 to 3 years—and Synoptic Sites that were 

sampled during a 2-week period in August 1997.

Ground-water chemistry

One objective of the ground-water chemistry component of the study was to determine whether chemical constituents of ground water were related to 

agricultural land uses (land-use surveys). Another 

objective in two surveys of domestic (household) and 

public-supply (municipal) wells (Study-Unit surveys) 

was to assess the overall water quality in selected 

drinking-water-source aquifers (Mahomet aquifer and 

the western and southern shallow aquifers) and to 

determine whether they are affected by land-use practices. The full extent of the Mahomet aquifer (in the 

Mahomet bedrock valley) is not shown. Results for the 

Mahomet aquifer are representative only for that part 

shown.

Stream ecology

The main objective of the ecology component of the 

study was to assess surface-water quality by integrating the physical, chemical, and biological factors. 

Ecology Sites were distributed at the basic fixed sites 

and at smaller subbasins that are all predominantly 

agricultural land. Sites were classified as either intensive or synoptic on the basis of the level and schedule 

of sampling effort.

0 25 50 MILES

39 0 25 50 KILOMETERS o

40o

91o

41o

90o

89o

EXPLANATION

Study-unit survey

Land-use survey

WELLS

PHYSIOGRAPHY

Bloomington Ridged Plain - no bedrock valley

Bloomington Ridged Plain - other bedrock valley

Galesburg Plain -

no bedrock valley

/Springfield

Galesburg/Springfield Plain -

bedrock valley

Bloomington Ridged Plain - Mahomet bedrock valley
26 Water Quality in the Lower Illinois River Basin

Summary of data collection in the lower Illinois River Basin, 1995–98

Study

component What data were collected and why Types of sites sampled Number of sites

Sampling frequency 

and period

Stream Chemistry

Basic 

Sites—general water 

quality

Concentrations, seasonal variation, and annual loads. 

Data included streamflow, nutrients, major ions, 

organic carbon, suspended sediment, water temperature, specific conductance, alkalinity, pH, and dissolved oxygen.

Basic Fixed Sites: representative of 

common land uses, as well as 

major tributary and basin outflow 

sites.

8 Monthly plus storms; 

2 years

Intensive 

Sites—pesticides

Concentrations and seasonal variations in pesticides 

and breakdown products. Data included 75 pesticides and 13 breakdown products of the most 

heavily applied herbicides.

Subset of Basic Sites: Two watersheds 

with greater than 90 percent rowcrop agriculture and the basin 

inflow and outflow sites.

4 Weekly to monthly Jan. 

1996–June 1998

Synoptic 

Sites—water 

chemistry and 

pesticides

Spatial distribution of nutrients, pesticides, and pesticide breakdown products. Data included nutrients 

and 24 pesticides and pesticide breakdown products.

Basic Sites

Additional sites

4

17

Once in August 1997 during minimum flow

Contaminants in 

bed sediments

Occurrence and distribution of contaminants in bed 

sediment. Data included total PCBs, 32 organochlorine pesticides, 63 semivolatile organic compounds, 

and 44 trace elements.

Depositional zones of Basic Fixed 

Sites and some additional sites.

21 Once; June–October in 

1996–97

Contaminants in 

fish

Occurrence and distribution of contaminants in biota. 

Data included total PCBs, 30 organochlorine pesticides in whole fish, and trace elements in fish livers.

Most stream sites also sampled for 

bed sediments.

20 Once; June–October in 

1996–97

Stream Ecology

Intensive assessments

Relations among biological communities and water 

chemistry, physical habitat, and land use. Includes 

spatial relations, along with spatial variation. Data 

included algae, invertebrates, fish communities, and 

physical habitat.

Basic Fixed Sites 8

1 reach, 1 year (1996)

1 reach, 3 years (1996–98)

3 reaches, 1 year (1996 or 

1997)

Synoptic studies Better spatial assessment. Data collected were same as 

above.

Synoptic Sites 21 Once in August 1997

Ground-Water Chemistry

Aquifer survey—domestic and 

public-supply wells

Occurrence and distribution of chemicals in domestic 

and public-supply wells. Data included major ions, 

nutrients, 75 pesticides,13 pesticide breakdown 

products, 73 volatile organic compounds, dissolved 

organic carbon, trace elements, and radon. 

Domestic and public supply wells 

across the subunit aquifers

2 publicsupply wells 

and 58 domestic or private 

wells

Once; May–November in 

1996–97

Resample 

selected aquifer survey 

wells

Confirmation of low-level or questionable detections 

of pesticides, breakdown products, and age-dating 

chemicals.

A subset of the wells above 19 Once in 1998

Resample 

selected aquifer survey 

wells

Confirmation of high-level detections of arsenic and to 

determine the chemical nature of the dissolved 

arsenic. Data included major ions and selected dissolved metals in addition to two chemical forms of 

arsenic.

A subset of wells from the deep aquifer survey with arsenic that 

exceeded the drinking-water 

standard.

10 Once in 1997

Land-use 

effects—corn 

and soybean 

row crops

Describe the effects of agricultural land use on shallow 

ground water in the area underlain by most recent 

glacial geologic material. Data included major ions, 

nutrients, 75 pesticides, 13 pesticide breakdown 

products, dissolved organic carbon, trace elements, 

and radon.

Very shallow monitoring wells 

installed for this study.

57 Once in 1997
Glossary 27

GLOSSARY 

Algae—Chlorophyll-bearing nonvascular, primarily aquatic species 

that have no true roots, stems, or leaves; most algae are microscopic, but some species can be as large as vascular plants.

Aquifer—A water-bearing layer of soil, sand, gravel, or rock that 

will yield usable quantities of water to a well.

Bed sediment—The material that temporarily is stationary in the 

bottom of a stream or other watercourse.

Benthic—Refers to plants or animals that live on the bottom of 

lakes, streams, or oceans.

Breakdown product—A compound derived by chemical, biological, or physical action upon a pesticide. The breakdown is a 

natural process that may result in a more toxic or a less toxic 

compound and a more persistent or less persistent compound.

Concentration—The amount or mass of a substance present in a 

given volume or mass of sample. Usually expressed as microgram per liter (water sample) or micrograms per kilogram 

(sediment or tissue sample).

Cubic foot per second (ft3

/s, or cfs)—Rate of water discharge representing a volume of 1 cubic foot passing a given point during 1 second, equivalent to approximately 7.48 gallons per 

second or 448.8 gallons per minute or 0.02832 cubic meter 

per second.

Ground water—In general, any water that exists beneath the land 

surface, but more commonly applied to water in fully saturated soils and geologic formations.

Habitat—The part of the physical environment where plants and 

animals live.

Herbicide—A chemical or other agent applied for the purpose of 

killing undesirable plants. See also Pesticide.

Insecticide—A substance or mixture of substances intended to 

destroy or repel insects. See also Pesticide.

Intolerant species—Those species that are not adaptable to human 

alterations to the environment and thus decline in numbers 

where human alterations occur. See also Tolerant species.

Invertebrate—An animal having no backbone or spinal column.

Load—General term that refers to a material or constituent in solution, in suspension, or in transport; usually expressed in terms 

of mass or volume.

Mean—The average of a set of observations, unless otherwise 

specified.

Median—The middle or central value in a distribution of data 

ranked in order of magnitude. The median is also known as 

the 50th percentile.

Method detection limit—The minimum concentration of a substance that can be accurately identified and measured with 

present laboratory technologies.

Micrograms per liter (μg/L)—A unit expressing the concentration 

of constituents in solution as weight (micrograms) of solute 

per unit volume (liter) of water; equivalent to one part per billion in most stream water and ground water. One thousand 

micrograms per liter equals 1 milligram per liter.

Milligrams per liter (mg/L)—A unit expressing the concentration 

of chemical constituents in solution as weight (milligrams) of 

solute per unit volume (liter) of water; equivalent to one part 

per million in most stream water and ground water. One thousand micrograms per liter equals 1 milligram per liter.

Nitrate—An ion consisting of nitrogen and oxygen (NO3

–

). Nitrate 

is a plant nutrient and is very mobile in soils.

Organochlorine compound—Synthetic organic compounds containing chlorine. As generally used, this term refers to compounds containing mostly or exclusively carbon, hydrogen, 

and chlorine. Examples include organochlorine insecticides, 

polychlorinated biphenyls, and some solvents containing 

chlorine.

Periphyton—Organisms that grow on underwater surfaces; periphyton include algae, bacteria, fungi, protozoa, and other 

organisms.

Pesticide—A chemical applied to crops, rights-of-way, lawns, or 

residences to control weeds, insects, fungi, nematodes, 

rodents or other “pests.”

Phosphorus—A nutrient essential for growth that can play a key 

role in stimulating aquatic growth in lakes and streams.

Phytoplankton—See Plankton.

Picocurie (pCi)—One trillionth (10–12) of the amount of radioactivity represented by a curie (Ci). A curie is the amount of 

radioactivity that yields 3.7 x 1010 radioactive disintegrations 

per second (dps). A picocurie yields 2.22 disintegrations per 

minute (dpm) or 0.037 dps.

Plankton—Floating or weakly swimming organisms at the mercy 

of the waves and currents. Animals of the group are called 

zooplankton, and the plants are called phytoplankton.

Point source—A source at a discrete location such as a discharge 

pipe, drainage ditch, tunnel, well, concentrated livestock operation, or floating craft.

Relative abundance—The number of organisms of a particular 

kind present in a sample relative to the total number of organisms in the sample.

Riparian—Areas adjacent to rivers and streams often with a high 

density, diversity, and productivity of plant and animal species 

relative to nearby uplands.

Semivolatile organic compound (SVOC)—Operationally defined 

as a group of synthetic organic compounds that are solventextractable and can be determined by gas chromatography/mass spectrometry. SVOCs include phenols, phthalates, 

and polycyclic aromatic hydrocarbons (PAHs).

Sorption—General term for the interaction (binding or association) 

of a solute ion or molecule with a solid.

Subsurface drain—A shallow drain installed in an irrigated field to 

intercept the rising ground-water level and maintain the water 

table at an acceptable depth below the land surface.

Tolerant species—Those species that are adaptable to (tolerant of) 

human alterations to the environment and often increase in 

number when human alterations occur.

Trace element—An element found in only minor amounts (concentrations less than 1.0 milligram per liter) in water or sediment; includes arsenic, cadmium, chromium, copper, lead, 

mercury, nickel, and zinc.

Volatile organic compounds (VOCs)—Organic chemicals that 

have a high vapor pressure relative to their water solubility. 

VOCs include components of gasoline, fuel oils, and lubricants, as well as organic solvents, fumigants, some inert ingredients in pesticides, and some by-products of chlorine 

disinfection.

Yield—The mass of material or constituent transported by a river in 

a specified period of time divided by the drainage area of the 

river basin.
28 Water Quality in the Lower Illinois River Basin

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Niagara River biota contamination project—Fish flesh 

criteria for piscivorous wildlife: New York State 

Department of Environmental Conservation, Division 

of Fish and Wildlife, Bureau of Environmental Protection Technical Report 87–3, 182 p.

28. Wong, C.S., Capel, P.D., and Nowell, L.H., 2000, 

Organochlorine pesticides and PCBs in stream 

sediment and aquatic biota—Initial results from 

the National Water-Quality Assessment Program, 

1992–1995: U.S. Geological Survey Water-Resources 

Investigations Report 00–4053, 88 p.

29. Fuhrer, G.J., Gilliom, R.J., Hamilton, P.A., Morace, 

J.L., Nowell, L.H., Rinella, J.F., Stoner, J.D., and 

Wentz, D.A., 1999, The quality of our Nation’s 

waters—Nutrients and pesticides: U.S. Geological 

Survey Circular 1225, 82 p.

30. Nowell, L.H., Capel, P.D., and Dileanis, P.D., 1999, 

Pesticides in stream sediment and aquatic biota—

Distribution, trends, and governing factors: Boca 

Raton, Fla., CRC Press, Pesticides in the Hydrologic 

Systems series, v. 4, 1040 p.

31. Warner, K.L., 2000, Analysis of nutrients, selected inorganic constituents, and trace elements in water from 

Illinois community-supply wells, 1984–91: U.S. Geological Survey Water-Resources Investigations Report 

99–4152, 47 p.

32. Gilliom, J.G., Alley, W.M., and Gurtz, M.E., 1995, 

Design of the National Water-Quality Assessment 

Program—Occurrence and distribution of water-quality conditions: U.S. Geological Survey Circular 1112, 

33 p.

33. Eisler, Ronald, 1987, Polycyclic aromatic hydrocarbon 

hazards to fish, wildlife, and invertebrates—A synoptic review: Laurel, Md., Patuxent Wildlife Research 

Center, U.S. Fish and Wildlife Service, Biological 

Report 85 (1.11), 81 p.

34. National Research Council, 1999, Arsenic in drinking 

water: Washington D.C., National Academy Press, 

310 p.

35. Focazio, M.J., Welch, A.H., Watkins, S.A., Helsel, 

D.R., and Horn, M.A., 1999, A retrospective analysis 

on the occurrence of arsenic in ground-water 

resources of the United States and limitations in 

drinking-water-supply characterizations: U.S. Geological Survey Water-Resources Investigations Report 

99–4279, 21 p.

36. Illinois Department of Energy and Natural Resources 

and The Nature of Illinois Foundation, 1994, The 

changing Illinois environment: Critical trends; 

summary of the critical trends assessment project: 

Springfield, Ill., Illinois Department of Energy and 

Natural Resources, 89 p.

37. Smith, P.W., 1971, Illinois streams—A classification 

based on their fishes and an analysis of factors responsible for the disappearance of native species: Illinois 

Natural History Survey Biological Note, v. 76, p. 1–

14.

38. Richardson, R.E., 1928, The bottom fauna of the 

middle Illinois River, 1913–1925—Its distribution, 

abundance, valuation, and index value in the study of 

stream pollution: Illinois State Natural History Survey, v. 7, no. 2, p.189–192.

39. Sparks, R.E., and Lerczak, T.V., 1993, Recent trends in 

the Illinois River indicated by fish populations: 

Havana, Ill., Illinois Natural History Survey River 

Research Laboratory of the Forbes Biological Station, 

34 p.

40. Mills, H.B., Starrett, W.C., and Bellrose, F.C., 1966, 

Man’s effect on the fish and wildlife of the Illinois 

River: Illinois Natural History Survey Biological 

Notes no. 57, 24 p.

41. Sparks, R.E., and Starrett, W.C., 1975, An electrofishing survey of the Illinois River, 1959–1974: Illinois 

Natural History Survey Bulletin no. 31, p. 317–380.

42. Bertrand, W.A., 1984, Des Plaines River Basin fisheries 

assessment: Illinois Department of Conservation, 

Division of Fish and Wildlife Resources, 43 p.

43. Ruhl, P.M., 1995, Surface-water-quality assessment 

of the upper Illinois River Basin in Illinois, Indiana, 

and Wisconsin—Analysis of relations between fishcommunity structure and environmental conditions in 

the Fox, Des Plaines, and Du Page River Basins in 

Illinois: U.S. Geological Survey Water-Resources 

Investigations Report 94–4094, 50 p.

44. Colman, J.A., and Sanzalone, R.F., 1992, Geochemical 

characterization of streambed sediment in the Upper 

Illinois River Basin: Water Resources Bulletin, v. 28, 

no. 5, p. 933–950.

45. Fitzpatrick, F.A., Scudder, B.C., Crawford, J.K., 

Schmidt, A.R., Sieverling, J.B., and others, 1995, 

Surface water-quality assessment of the upper Illinois 

River Basin in Illinois, Indiana, and Wisconsin—

Major and trace elements in water, sediment, and 

biota, 1978–90: U.S. Geological Survey WaterResources Investigations Report 95–4045, 254 p.
30 Water Quality in the Lower Illinois River Basin

Streams in agricultural areas 

Streams in urban areas

Streams and rivers draining mixed land uses 

Shallow ground water in agricultural areas

Shallow ground water in urban areas 

Major aquifers 

Detected concentration in Study Unit

Frequencies of detection, in percent. Detection frequencies 

were not censored at any common reporting limit. The lefthand column is the study-unit frequency and the right-hand 

column is the national frequency 

Not measured or sample size less than two

Study-unit sample size. For ground water, the number of 

samples is equal to the number of wells sampled

National ranges of detected concentrations, by land use, in 36 

NAWQA Study Units, 1991–98—Ranges include only samples

in which a chemical was detected

Drinking-water quality (applies to ground water and surface water)

Protection of aquatic life (applies to surface water only)

Prevention of eutrophication in streams not flowing directly into 

lakes or impoundments

No benchmark for drinking-water quality

No benchmark for protection of aquatic life

*

**

66 38

CHEMICALS IN WATER

Concentrations and detection frequencies, lower Illinois River 

Basin, 1995–98—Detection sensitivity varies among chemicals and, 

thus, frequencies are not directly comparable among chemicals

Lowest

25 percent

Middle

50 percent

Highest

25 percent

National water-quality benchmarks

National benchmarks include standards and guidelines related to 

drinking-water quality, criteria for protecting the health of aquatic life, and 

a goal for preventing stream eutrophication due to phosphorus. Sources 

include the U.S. Environmental Protection Agency and the Canadian 

Council of Ministers of the Environment

|

|

|

--

This appendix is a summary of chemical concentrations 

and biological indicators assessed in the lower Illinois 

River Basin. Selected results for this basin are graphically 

compared to results from as many as 36 NAWQA Study 

Units investigated from 1991 to 1998 and to national 

water-quality benchmarks for human health, aquatic life, or 

fish-eating wildlife. The chemical and biological indicators 

shown were selected on the basis of frequent detection, 

detection at concentrations above a national benchmark, 

or regulatory or scientific importance. The graphs illustrate 

how conditions associated with each land use sampled in 

the lower Illinois River Basin compare to results from 

across the Nation, and how conditions compare among 

the several land uses. Graphs for chemicals show only 

detected concentrations and, thus, care must be taken to 

evaluate detection frequencies in addition to concentrations when comparing study-unit and national results. For 

example, acetochlor concentrations in lower Illinois River 

Basin agricultural streams were similar to the national 

distribution, but the detection frequency was much higher 

(83 percent compared to 33 percent).

CONCENTRATION, IN MICROGRAMS PER LITER

 0.0001 0.001 0.01 0.1 1 10 100 1,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Pesticides in water—Herbicides

Study-unit sample size

12

Acetochlor (Harness Plus, Surpass) * ** 83 33 130 -- 9 0 80 38 45

0 <1 56 -- <1 0 0 <1 60

Alachlor (Lasso, Bronco, Lariat, Bullet) **

64 44 | 130 -- 20 | 0 82 45 | 45

0 3 | 56 -- <1 | 0 0 1 | 60

Atrazine (AAtrex, Atrex, Atred, Gesaprim) 

100 88 | | 130 -- 86 | | 0 100 87 | | 45

27 40 | 56 -- 30 | 0 32 18 | 60

Bentazon (Basagran, Bentazone) ** 80 17 | 125 -- 1 | 0 46 12 | 35

14 4 | 56 -- 3 | 0 -- 2 | 0

Cyanazine (Bladex, Fortrol) 

98 44 | | 130 -- 14 | | 0 98 54 | | 45

4 1 | 56 -- 1 | 0 0 <1 | 60

2,4-D (Aqua-Kleen, Lawn-Keep, Weed-B-Gone) 

31 15 | | 125 -- 18 | | 0 23 11 | | 35

4 <1 | 55 -- 1 | 0 -- <1 | 0

Deethylatrazine (Atrazine breakdown product) * **

100 75 130 -- 62 0 100 75 45

30 39 56 -- 28 0 25 19 60

Metolachlor (Dual, Pennant) 

99 81 | | 130 -- 64 | | 0 100 83 | | 45

20 18 | 56 -- 9 | 0 7 5 | 60

Prometon (Pramitol, Princep) ** 92 44 | 130 -- 86 | 0 93 60 | 45

5 12 | 56 -- 21 | 0 5 5 | 60

Simazine (Princep, Caliber 90) 68 61 | | 130 -- 77 | | 0 87 74 | | 45

0 21 | 56 -- 18 | 0 2 5 | 60

APPENDIX—WATER-QUALITY DATA FROM THE LOWER ILLINOIS 

RIVER BASIN IN A NATIONAL CONTEXT

For a complete view of lower Illinois River Basin data and for additional information about specific benchmarks used, visit our Web site at 

http://water.usgs.gov/nawqa/. Also visit the NAWQA Data Warehouse for access to NAWQA data sets at http://water.usgs.gov/nawqa/data. 
Water-Quality Data in a National Context 31 

Tebuthiuron (Spike, Tebusan) 

19 22 | | 130 -- 39 | | 0 58 32 | | 45

0 3 | 56 -- 7 | 0 0 3 | 60

CONCENTRATION, IN MICROGRAMS PER LITER

 0.0001 0.001 0.01 0.1 1 10 100 1,000 

Other herbicides detected

Acifluorfen (Blazer, Tackle 2S) **

Benfluralin (Balan, Benefin, Bonalan) * **

Bromoxynil (Buctril, Brominal) * 

Butylate (Sutan +, Genate Plus, Butilate) **

DCPA (Dacthal, chlorthal-dimethyl) * **

Dacthal mono-acid (Dacthal breakdown product) * **

Dicamba (Banvel, Dianat, Scotts Proturf) 

2,6-Diethylaniline (Alachlor breakdown product) * **

Diuron (Crisuron, Karmex, Diurex) **

EPTC (Eptam, Farmarox, Alirox) * **

Linuron (Lorox, Linex, Sarclex, Linurex, Afalon) * 

MCPA (Rhomene, Rhonox, Chiptox) 

Metribuzin (Lexone, Sencor) 

Molinate (Ordram) * **

Pendimethalin (Pre-M, Prowl, Stomp) * **

Propachlor (Ramrod, Satecid) **

Terbacil (Sinbar) **

Triallate (Far-Go, Avadex BW, Tri-allate) * 

Triclopyr (Garlon, Grandstand, Redeem, Remedy) * **

Trifluralin (Treflan, Gowan, Tri-4, Trific) 

Herbicides not detected

Bromacil (Hyvar X, Urox B, Bromax) 

Chloramben (Amiben, Amilon-WP, Vegiben) **

Clopyralid (Stinger, Lontrel, Transline) * **

2,4-DB (Butyrac, Butoxone, Embutox Plus, Embutone) * **

Dichlorprop (2,4-DP, Seritox 50, Lentemul) * **

Dinoseb (Dinosebe) 

Ethalfluralin (Sonalan, Curbit) * **

Fenuron (Fenulon, Fenidim) * **

Fluometuron (Flo-Met, Cotoran) **

MCPB (Thistrol) * **

Napropamide (Devrinol) * **

Neburon (Neburea, Neburyl, Noruben) * **

Norflurazon (Evital, Predict, Solicam, Zorial) * **

Oryzalin (Surflan, Dirimal) * **

Pebulate (Tillam, PEBC) * **

Picloram (Grazon, Tordon) 

Pronamide (Kerb, Propyzamid) **

Propanil (Stam, Stampede, Wham) * **

Propham (Tuberite) **

2,4,5-T **

2,4,5-TP (Silvex, Fenoprop) **

Thiobencarb (Bolero, Saturn, Benthiocarb) * **

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Pesticides in water—Insecticides

Study-unit sample size

CONCENTRATION, IN MICROGRAMS PER LITER

 0.0001 0.001 0.01 0.1 1 10 100 1,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Volatile organic compounds (VOCs) in ground water

These graphs represent data from 16 Study Units, sampled from 1996 to 1998

Study-unit frequency of detection, in percent

National frequency of detection in percent Study-unit sample size

CONCENTRATION, IN MICROGRAMS PER LITER

 0.001 0.01 0.1 1 10 100 1,000 10,000 

Other insecticides detected 

Carbaryl (Carbamine, Denapon, Sevin) 

Carbofuran (Furadan, Curaterr, Yaltox) 

Dieldrin (Panoram D-31, Octalox, Compound 497) 

Fonofos (Dyfonate, Capfos, Cudgel, Tycap) **

Malathion (Malathion) 

Methyl parathion (Penncap-M, Folidol-M) **

Oxamyl (Vydate L, Pratt) **

Propargite (Comite, Omite, Ornamite) * **

Terbufos (Contraven, Counter, Pilarfox) **

Insecticides not detected

Aldicarb (Temik, Ambush, Pounce) 

Aldicarb sulfone (Standak, aldoxycarb) 

Aldicarb sulfoxide (Aldicarb breakdown product) 

Azinphos-methyl (Guthion, Gusathion M) * 

Disulfoton (Disyston, Di-Syston) **

Ethoprop (Mocap, Ethoprophos) * **

alpha-HCH (alpha-BHC, alpha-lindane) **

gamma-HCH (Lindane, gamma-BHC) 

3-Hydroxycarbofuran (Carbofuran breakdown product) * **

Methiocarb (Slug-Geta, Grandslam, Mesurol) * **

Methomyl (Lanox, Lannate, Acinate) **

Parathion (Roethyl-P, Alkron, Panthion, Phoskil) * 

cis-Permethrin (Ambush, Astro, Pounce) * **

Phorate (Thimet, Granutox, Geomet, Rampart) * **

Propoxur (Baygon, Blattanex, Unden, Proprotox) * **

Other VOCs detected

Bromodichloromethane (Dichlorobromomethane) 

2-Butanone (Methyl ethyl ketone (MEK)) * 

Carbon disulfide * 

Chlorobenzene (Monochlorobenzene) 

Chlorodibromomethane (Dibromochloromethane) 

Chloroethane (Ethyl chloride) * 

Chloromethane (Methyl chloride) 

Dichlorodifluoromethane (CFC 12, Freon 12) 

Dichloromethane (Methylene chloride) 

CONCENTRATION, IN MICROGRAMS PER LITER

 0.0001 0.001 0.01 0.1 1 10 100 1,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Chlorpyrifos (Brodan, Dursban, Lorsban) 

11 18 | | 126 -- 37 | | 0 14 20 | | 43

0 1 | 56 -- 1 | 0 0 <1 | 60

p,p'-DDE 1 8 | | 130 -- 2 | | 0 0 4 | | 45

0 4 | 56 -- 2 | 0 0 2 | 60

Diazinon (Basudin, Diazatol, Neocidol, Knox Out) 5 16 | | 130 -- 70 | | 0 76 39 | | 45

0 <1 | 56 -- 2 | 0 2 2 | 60

Methyl tert-butyl ether (MTBE) 

-- 4 | 0 -- 16 | 0 2 6 | 60
32 Water Quality in the Lower Illinois River Basin

CONCENTRATION, IN MILLIGRAMS PER LITER

 0.001 0.01 0.1 1 10 100 1,000 10,000 100,000 

CONCENTRATION, IN MILLIGRAMS PER LITER

 0.001 0.01 0.1 1 10 100 1,000 10,000 100,000 

Nutrients in water

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Dissolved solids in water

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

1,3 & 1,4-Dimethylbenzene (m-&p-Xylene) 

1-4-Epoxy butane (Tetrahydrofuran, Diethylene oxide) * 

1-Ethyl-2-methylbenzene (2-Ethyltoluene) * 

Hexachlorobutadiene 

Iodomethane (Methyl iodide) * 

4-Methyl-2-pentanone (Methyl isobutyl ketone (MIBK)) * 

Methylbenzene (Toluene) 

2-Propanone (Acetone) * 

Tetrachloroethene (Perchloroethene) 

Tetrachloromethane (Carbon tetrachloride) 

1,2,3,4-Tetramethylbenzene (Prehnitene) * 

1,2,3,5-Tetramethylbenzene (Isodurene) * 

Tribromomethane (Bromoform) 

1,2,3-Trichlorobenzene * 

1,1,1-Trichloroethane (Methylchloroform) 

Trichloromethane (Chloroform) 

1,2,3-Trimethylbenzene (Hemimellitene) * 

1,2,4-Trimethylbenzene (Pseudocumene) * 

VOCs not detected

tert-Amylmethylether (tert-amyl methyl ether (TAME)) * 

Benzene 

Bromobenzene (Phenyl bromide) * 

Bromochloromethane (Methylene chlorobromide) 

Bromoethene (Vinyl bromide) * 

Bromomethane (Methyl bromide) 

n-Butylbenzene (1-Phenylbutane) * 

sec-Butylbenzene * 

tert-Butylbenzene * 

3-Chloro-1-propene (3-Chloropropene) * 

1-Chloro-2-methylbenzene (o-Chlorotoluene) 

1-Chloro-4-methylbenzene (p-Chlorotoluene) 

Chloroethene (Vinyl chloride) 

1,2-Dibromo-3-chloropropane (DBCP, Nemagon) 

1,2-Dibromoethane (Ethylene dibromide, EDB) 

Dibromomethane (Methylene dibromide) * 

trans-1,4-Dichloro-2-butene ((Z)-1,4-Dichloro-2-butene) * 

1,2-Dichlorobenzene (o-Dichlorobenzene) 

1,3-Dichlorobenzene (m-Dichlorobenzene) 

1,4-Dichlorobenzene (p-Dichlorobenzene) 

1,2-Dichloroethane (Ethylene dichloride) 

1,1-Dichloroethane (Ethylidene dichloride) * 

1,1-Dichloroethene (Vinylidene chloride) 

trans-1,2-Dichloroethene ((E)-1,2-Dichlorothene) 

cis-1,2-Dichloroethene ((Z)-1,2-Dichloroethene) 

1,2-Dichloropropane (Propylene dichloride) 

2,2-Dichloropropane * 

1,3-Dichloropropane (Trimethylene dichloride) * 

trans-1,3-Dichloropropene ((E)-1,3-Dichloropropene) 

cis-1,3-Dichloropropene ((Z)-1,3-Dichloropropene) 

1,1-Dichloropropene * 

Diethyl ether (Ethyl ether) * 

Diisopropyl ether (Diisopropylether (DIPE)) * 

1,2-Dimethylbenzene (o-Xylene) 

Ethenylbenzene (Styrene) 

Ethyl methacrylate * 

Ethyl tert-butyl ether (Ethyl-t-butyl ether (ETBE)) * 

Ethylbenzene (Phenylethane) 

1,1,1,2,2,2-Hexachloroethane (Hexachloroethane) 

2-Hexanone (Methyl butyl ketone (MBK)) * 

Isopropylbenzene (Cumene) * 

p-Isopropyltoluene (p-Cymene) * 

Methyl acrylonitrile * 

Methyl-2-methacrylate (Methyl methacrylate) * 

Methyl-2-propenoate (Methyl acrylate) * 

Naphthalene 

2-Propenenitrile (Acrylonitrile) 

n-Propylbenzene (Isocumene) * 

1,1,2,2-Tetrachloroethane * 

1,1,1,2-Tetrachloroethane 

1,1,2-Trichloro-1,2,2-trifluoroethane (Freon 113) * 

1,2,4-Trichlorobenzene 

1,1,2-Trichloroethane (Vinyl trichloride) 

Trichloroethene (TCE) 

Trichlorofluoromethane (CFC 11, Freon 11) 

1,2,3-Trichloropropane (Allyl trichloride) 

1,3,5-Trimethylbenzene (Mesitylene) * 

Trace elements in ground water

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Arsenic 

35 58 | 57 -- 36 | 0 58 37 | 60

CONCENTRATION, IN MICROGRAMS PER LITER

 0.01 0.1 1 10 100 1,000 10,000 100,000 

Ammonia, as N * **

63 84 195 -- 86 0 71 75 100

80 78 56 -- 71 0 83 70 60

Dissolved ammonia plus organic nitrogen, as N * ** 96 78 195 -- 74 0 97 62 100

50 28 56 -- 30 0 72 24 60

Dissolved nitrite plus nitrate, as N ** 95 95 | 195 -- 97 | 0 100 91 | 100

62 81 | 56 -- 74 | 0 60 71 | 60

Orthophosphate, as P * **

96 79 195 -- 72 0 93 74 100

25 59 56 -- 52 0 83 61 60

Total phosphorus, as P * ** 100 92 | 195 -- 90 | 0 100 88 | 100

Dissolved solids * ** 100 100 193 -- 100 0 100 100 101

100 100 57 -- 100 0 100 100 59
Water-Quality Data in a National Context 33 

Fish tissue from streams in agricultural areas

Fish tissue from streams in urban areas

Fish tissue from streams draining mixed land uses

Sediment from streams in agricultural areas 

Sediment from streams in urban areas 

Sediment from streams draining mixed land uses

Protection of fish-eating wildlife (applies to fish tissue)

Protection of aquatic life (applies to bed sediment)

No benchmark for protection of fish-eating wildlife

No benchmark for protection of aquatic life

|

|

**

CHEMICALS IN FISH TISSUE

AND BED SEDIMENT

Concentrations and detection frequencies, lower Illinois River 

Basin, 1995–98—Detection sensitivity varies among chemicals and, 

thus, frequencies are not directly comparable among chemicals. 

Study-unit frequencies of detection are based on small sample sizes; 

the applicable sample size is specified in each graph

Lowest

25 percent

Middle

50 percent

Highest

25 percent

National benchmarks for fish tissue and bed sediment

National benchmarks include standards and guidelines related to 

criteria for protection of the health of fish-eating wildlife and aquatic 

organisms. Sources include the U.S. Environmental Protection Agency, 

other Federal and State agencies, and the Canadian Council of 

Ministers of the Environment

*

CONCENTRATION, IN MICROGRAMS PER KILOGRAM

(Fish tissue is wet weight; bed sediment is dry weight)

 0.1 1 10 100 1,000 10,000 100,000 

CONCENTRATION, IN MICROGRAMS PER KILOGRAM

(Fish tissue is wet weight; bed sediment is dry weight)

 0.1 1 10 100 1,000 10,000 100,000 

National range of concentrations detected, by land use, in 36 

NAWQA Study Units, 1991–98—Ranges include only samples

in which a chemical was detected

Detected concentration in Study Unit

Frequencies of detection, in percent. Detection frequencies 

were not censored at any common reporting limit. The lefthand column is the study-unit frequency and the right-hand 

column is the national frequency

Not measured or sample size less than two

Study-unit sample size

66 38

--

12

Organochlorines in fish tissue (whole body)

and bed sediment

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Other trace elements detected

Lead 

Selenium 

Trace elements not detected

Cadmium 

CONCENTRATION, IN PICOCURIES PER LITER

Chromium 

98 85 | 57 -- 79 | 0 90 73 | 60

Radon-222 

93 99 | 57 -- 100 | 0 98 97 | 59

Uranium 

61 64 | 57 -- 35 | 0 17 33 | 60

Zinc 

19 28 | 57 -- 29 | 0 72 66 | 60

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

CONCENTRATION, IN MICROGRAMS PER LITER

 0.01 0.1 1 10 100 1,000 10,000 100,000 

 0.01 0.1 1 10 100 1,000 10,000 100,000 

Total Chlordane (sum of 5 chlordanes) 

100 38 | 7 -- 75 | 0 100 56 | 12

25 9 | 8 -- 57 | 0 8 11 | 12

o,p'+p,p'-DDD (sum of o,p'-DDD and p,p'-DDD) * 

43 49 7 -- 69 0 92 50 12

12 27 | 8 -- 50 | 0 25 20 | 12

p,p'-DDE * ** 100 90 7 -- 94 0 100 92 12

12 48 8 -- 62 0 25 39 12

o,p'+p,p'-DDE (sum of o,p'-DDE and p,p'-DDE) * 100 90 7 -- 94 0 100 92 12

12 48 | 8 -- 62 | 0 25 39 | 12

Total DDT (sum of 6 DDTs) **

100 90 | 7 -- 94 | 0 100 93 | 12

12 49 8 -- 66 0 25 41 12

Dieldrin (Panoram D-31, Octalox) * 

100 53 7 -- 42 0 100 38 12

38 13 | 8 -- 30 | 0 33 9 | 12
34 Water Quality in the Lower Illinois River Basin

CONCENTRATION, IN MICROGRAMS PER KILOGRAM, DRY WEIGHT

 0.1 1 10 100 1,000 10,000 100,000 CONCENTRATION, IN MICROGRAMS PER KILOGRAM, DRY WEIGHT

 0.1 1 10 100 1,000 10,000 100,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Semivolatile organic compounds (SVOCs)

in bed sediment

Study-unit sample size

Study-unit sample size

Other organochlorines detected

DCPA (Dacthal, chlorthal-dimethyl) * **

Hexachlorobenzene (HCB) **

Pentachloroanisole (PCA) * **

Organochlorines not detected

Chloroneb (Chloronebe, Demosan) * **

o,p'+p,p'-DDT (sum of o,p'-DDT and p,p'-DDT) * 

Endosulfan I (alpha-Endosulfan, Thiodan) * **

Endrin (Endrine) 

gamma-HCH (Lindane, gamma-BHC, Gammexane) * 

Total-HCH (sum of alpha-HCH, beta-HCH, gamma-HCH, and delta-HCH) **

Isodrin (Isodrine, Compound 711) * **

p,p'-Methoxychlor (Marlate, methoxychlore) * **

o,p'-Methoxychlor * **

Mirex (Dechlorane) **

cis-Permethrin (Ambush, Astro, Pounce) * **

trans-Permethrin (Ambush, Astro, Pounce) * **

Toxaphene (Camphechlor, Hercules 3956) * **

1 The national detection frequencies for total PCB in sediment are biased low because about 

30 percent of samples nationally had elevated detection levels compared to this Study Unit. 

See http://water.usgs.gov/nawqa/ for additional information.

CONCENTRATION, IN MICROGRAMS PER KILOGRAM

(Fish tissue is wet weight; bed sediment is dry weight)

 0.1 1 10 100 1,000 10,000 100,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Dieldrin+aldrin (sum of dieldrin and aldrin) **

100 52 | 7 -- 42 | 0 100 38 | 12

38 13 8 -- 29 0 33 9 12

Heptachlor epoxide (Heptachlor breakdown product) * 86 8 7 -- 9 0 58 14 12

0 1 | 8 -- 3 | 0 0 <1 | 12

Heptachlor+heptachlor epoxide ** 86 9 | 7 -- 9 | 0 58 14 | 12

0 1 8 -- 2 0 0 <1 12

Total PCB 1

29 38 | 7 -- 81 | 0 100 66 | 12

0 2 | 8 -- 21 | 0 8 9 | 12

Acenaphthylene 

12 23 | 8 -- 65 | 0 58 46 | 12

Anthraquinone **

12 21 8 -- 83 0 58 39 12

Benz[a]anthracene 

62 44 | 8 -- 94 | 0 83 62 | 12

9H-Carbazole **

25 19 8 -- 76 0 33 33 12

bis(2-Chloroethoxy)methane **

12 <1 8 -- 1 0 0 <1 12

Dibenz[a,h]anthracene 

25 8 | 8 -- 68 | 0 33 23 | 12

Dibenzothiophene **

25 12 8 -- 64 0 33 30 12

1,4-Dichlorobenzene (p-Dichlorobenzene) **

 -- 6 0 17 7 12

2,6-Dimethylnaphthalene **

38 65 8 -- 74 0 75 77 12

bis(2-Ethylhexyl)phthalate **

100 91 8 -- 99 0 100 95 12

Fluoranthene 

50 66 | 8 -- 97 | 0 92 78 | 12

N-Nitrosodiphenylamine **

0 2 8 -- 10 0 8 4 12

Phenol **

50 81 8 -- 82 0 50 80 12
Water-Quality Data in a National Context 35 

Frequency of detection, any concentration 2

Frequency of detection ≥ 1.0 μg/g (sediment) ≥ 0.1 μg/g (tissue) 3 SVOCs not detected 2

CONCENTRATION, IN MICROGRAMS PER GRAM

(Fish tissue is wet weight, bed sediment is dry weight)

 0.01 0.1 1 10 100 10,000 1,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent

Trace elements in fish tissue (livers) and 

bed sediment

Study-unit sample size

Other SVOCs detected

Acenaphthene 

Acridine **

Anthracene 

Benzo[a]pyrene 

Benzo[b]fluoranthene **

Benzo[ghi]perylene **

Benzo[k]fluoranthene **

Butylbenzylphthalate **

4-Chloro-3-methylphenol **

4-Chlorophenyl-phenylether **

Chrysene 

p-Cresol **

Di-n-butylphthalate **

Di-n-octylphthalate **

1,3-Dichlorobenzene (m-Dichlorobenzene) **

Diethylphthalate **

1,2-Dimethylnaphthalene **

1,6-Dimethylnaphthalene **

3,5-Dimethylphenol **

Dimethylphthalate **

2-Ethylnaphthalene **

9H-Fluorene (Fluorene) 

Indeno[1,2,3-cd]pyrene **

Isoquinoline **

1-Methyl-9H-fluorene **

2-Methylanthracene **

4,5-Methylenephenanthrene **

1-Methylphenanthrene **

1-Methylpyrene **

Naphthalene 

Nitrobenzene **

Phenanthrene 

Phenanthridine **

1,2,4-Trichlorobenzene **

2,3,6-Trimethylnaphthalene **

SVOCs not detected

C8-Alkylphenol **

Azobenzene **

Benzo[c]cinnoline **

2,2-Biquinoline **

4-Bromophenyl-phenylether **

2-Chloronaphthalene **

2-Chlorophenol **

1,2-Dichlorobenzene (o-Dichlorobenzene) **

2,4-Dinitrotoluene **

Isophorone **

N-Nitrosodi-n-propylamine **

Pentachloronitrobenzene **

Quinoline **

CONCENTRATION, IN MICROGRAMS PER KILOGRAM, DRY WEIGHT

 0.1 1 10 100 1,000 10,000 100,000 

Study-unit frequency of detection, in percent

National frequency of detection, in percent Study-unit sample size

Pyrene 

50 64 | 8 -- 95 | 0 92 76 | 12 Arsenic * 

50 56 6 -- 38 0 75 76 12

100 99 | 8 -- 98 | 0 100 97 | 12

Cadmium * 

67 77 6 -- 72 0 100 95 12

100 98 | 8 -- 100 | 0 100 98 | 12

Chromium * 33 62 6 -- 72 0 50 54 12

100 100 | 8 -- 99 | 0 100 100 | 12

Copper * 100 100 6 -- 100 0 100 100 12

100 100 | 8 -- 99 | 0 100 100 | 12

Lead * 

0 11 6 -- 41 0 67 41 12

100 100 | 8 -- 100 | 0 100 99 | 12

Mercury * 

50 71 6 -- 59 0 92 80 12

100 82 | 8 -- 97 | 0 100 93 | 12

Nickel * **

0 42 6 -- 44 0 33 50 12

100 100 8 -- 100 0 100 100 12

Selenium * 100 99 6 -- 100 0 100 99 12

100 100 | 8 -- 100 | 0 100 100 | 12

Zinc * 

100 100 6 -- 100 0 100 100 12

100 100 | 8 -- 99 | 0 100 100 | 12
36 Water Quality in the Lower Illinois River Basin

Biological indicator value, lower Illinois River Basin, by land 

use, 1995–98

Biological status assessed at a site

National ranges of biological indicators, 16 NAWQA Study 

Units, 1994–98

Streams in undeveloped areas

Streams in agricultural areas

Streams in urban areas

Streams in mixed-land-use areas

75th percentile

25th percentile

BIOLOGICAL INDICATORS

Higher national scores suggest habitat disturbance, water-quality 

degradation, or naturally harsh conditions. The status of algae, 

invertebrates (insects, worms, and clams), and fish provide a 

record of water-quality and stream conditions that waterchemistry indicators may not reveal. Algal status focuses on the 

changes in the percentage of certain algae in response to 

increasing siltation, and it often correlates with higher nutrient 

concentrations in some regions. Invertebrate status averages 11 

metrics that summarize changes in richness, tolerance, trophic 

conditions, and dominance associated with water-quality 

degradation. Fish status sums the scores of four fish metrics 

(percent tolerant, omnivorous, non-native individuals, and percent 

individuals with external anomalies) that increase in association 

with water-quality degradation

 0 10 20 30 40 50 60 70 80 90 100

 0 5 10 15 20

 Algal status indicator

Undeveloped

Agricultural

Urban

Mixed

 Invertebrate status indicator

Undeveloped

Agricultural

Urban

Mixed

 Fish status indicator

Undeveloped

Agricultural

Urban

Mixed
A COORDINATED EFFORT

Coordination with agencies and organizations in the lower Illinois River Basin was integral to the success of this 

water-quality assessment. We thank those who served as members of our liaison committee. 

Federal Agencies

U.S. Environmental Protection Agency

Natural Resources Conservation Service

U.S. Fish and Wildlife Service

State Agencies

Illinois Environmental Protection Agency

Illinois Natural History Survey

Illinois Department of Natural Resources

Illinois Sate Geological Survey

Illinois State Water Survey

Illinois Department of Agriculture

Illinois Hazardous Waste Research Center

Illinois Abandoned Lands Council

Illinois Department of Public Health

Local Agencies

Bloomington and Normal Water Reclamation District

Metropolitan Water Reclamation District of Greater 

Chicago

Bloomington Water Department

Greater Peoria Sanitary District

City of Bloomington

Springfield City Water, Light, and Power

Mahomet Valley Water Authority

Peoria Health Department

Sanitary District of Decatur

Universities

University of Illinois

Water Resources Center, University of Illinois

Natural Resources and Environmental Sciences, 

University of Illinois

College of Agriculture, Consumer and Environmental 

Sciences, University of Illinois

College of Engineering, University of Illinois

Other public and private organizations

Tri-County River Front Action Forum

The Nature Conservancy, Illinois Chapter

Heartland Water Resources

We thank the following individuals for contributing to this effort. 

Steve Gough, Gough and Associates, for assisting the biology team: field-data collection

David J. Fazio, U.S. Geological Survey, for most maps in this report

Kymm Akers, U.S. Geological Survey, for drafting two maps used in this report

William S. Morrow, Jr., David H. Dupré, Debbie L. Adolphson, Tim A. Brown and David G. Frothingham, 

U.S. Geological Survey, for data collection

Ken Hlinka, Illinois State Water Survey, for providing ground-water-level data

Sherry Reeves, U.S. Geological Survey, for editorial review and report layout

Jim Stark and Stephen Kalkhoff, U.S. Geological Survey, for review of manuscript

Matthew Short, Illinois Environmental Protection Agency, for review of manuscript

Gretchen Bonfert, McKnight Foundation, for review of manuscript

Michael Eberle, Leah Hout, and Joyce McClure, U.S. Geological Survey, for editorial review
Groschen and others— Water Quality in the Lower Illinois River Basin

U.S. Geological Survey Circular 1209

National Water-Quality Assessment (NAWQA) Program

Lower Illinois River Basin

NAWQA

Peoria

BloomingtonNormal

Decatur

Springfield

Illinois

River