U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4103

July 2003

1

In cooperation with the Illinois Environmental Protection Agency

Arsenic in Illinois Ground Water–Community and 

Private Supplies

By Kelly L. Warner, Angel Martin, Jr., and Terri L. Arnold

Introduction

Assessing the distribution 

of arsenic in ground water from 

community-water supplies, private supplies, or monitoring wells 

is part of the process of determining the risk of arsenic contamination of drinking water in Illinois. 

Lifestyle, genetic, and environmental factors make certain 

members of the population more 

susceptible to adverse health 

effects from repeated exposure to 

drinking water with high arsenic 

concentrations (Ryker, 2001). In 

addition, such factors may have 

geographic distribution patterns 

that complicate the analysis of 

the relation between arsenic in 

drinking water and health effects. 

For example, arsenic may not 

be the only constituent affecting 

the quality of drinking water in a 

region (Ryker, 2001); however, 

determining the extent and distribution of arsenic in ground water 

is a starting place to assess the 

potential risk for persons drinking 

from a community or private supply. Understanding the potential 

sources and pathways that mobilize arsenic in ground water is a 

necessary step in protecting the 

drinking-water supply in Illinois 

(fig. 1). Figure 1. Location of community water-supply wells and the Mahomet Buried 

Bedrock Valley in Illinois.
Arsenic in Illinois Ground Water–Community and Private Supplies 2 Water-Resources Investigations Report 03-4103 

Relation Between 

High Arsenic 

Concentrations and 

Health Effects 

The metallic element 

arsenic has a long history as 

a poison. Albertus Magnus 

(Albert the Great), in approximately 1250, is the first to have 

recorded producing pure arsenic 

(MacRae, 2002), which usually 

was found in a mixed mineral. 

It is believed that Napoleon was 

poisoned with arsenic (Weider 

and Forshufvud, 1995). In the 

early 1800s in Italy, there were 

over a thousand unexplained 

deaths of young children who 

died in their living rooms. The 

deaths were determined to be 

the result of the release of poisonous arsenic gas from Paris 

green wallpaper that lined the 

living rooms and this heavierthan-air gas accumulated at 

lower levels where young children were more likely to breathe 

(King, 2002). Up to the 1940s, 

arsenic successfully was used 

to treat syphilis and leprosy. 

Arsenic was popularized as the 

poison of choice in many murder-mystery novels and movies, 

such as Arsenic and Old Lace, 

released in 1944. Today, the 

threat of arsenic poisoning is 

real in many parts of the world, 

such as Bangladesh, India, and 

China, where in the late 1990s 

many people consumed or 

inhaled toxic amounts of arsenic 

(West Bengal and Bangladesh, 

2002). Arsenic in ground water 

also is a public-health issue in 

Illinois (fig. 1) and other parts 

of the Nation. Understanding 

the distribution, fate, and transport of arsenic in ground water 

and defining susceptible areas 

needing further investigation 

helps water-resource managers 

assess the risk of arsenic contamination of wells, perhaps, by 

geographic location and aquifer.

Dissolved arsenic is found 

in ground water across Illinois. The U.S. Environmental 

Protection Agency (USEPA) 

drinking-water standard for 

arsenic has been 50 micrograms 

per liter (μg/L) since 1942 

(U.S. Environmental Protection Agency, 2002). In January 

2001, the standard for arsenic 

in drinking water was lowered 

by USEPA to 10 μg/L and must 

be implemented at all community-supply facilities by 2006 

(U.S. Environmental Protection 

Agency, 2002; Dawn Shellenberger, U.S. Environmental 

Protection Agency, written 

commun., 2002). The cost is 

substantial for remediating 

high arsenic concentrations. 

The Illinois Environmental 

Protection Agency (IEPA) has 

estimated that the total cost to 

reduce arsenic concentrations to 

below 10 μg/L for 50 selected 

community-water supplies with 

elevated arsenic concentrations 

in Illinois (fig. 2) could reach 40 

million dollars, with the highest costs associated with small 

community supplies (Illinois State Water Survey, 

2002). Private wells are 

not regulated for arsenic 

concentration.

The purpose of this 

report, prepared by the 

U.S. Geological Survey 

(USGS), in cooperation with the IEPA, is to 

describe the known distribution of arsenic in ground 

water in Illinois and to 

Figure 2. Projected number and cost of remediating arsenic from community-water 

supplies in Illinois (<, less than) (Modified from Illinois State Water Survey, 2002; oral 

commun., Rick Cobb, Illinois Environmental Protection Agency, 2002).
Arsenic in Illinois Ground Water–Community and Private Supplies 3 Water-Resources Investigations Report 03-4103

describe use of a geostatistical 

technique to estimate arsenic 

concentrations in areas where 

little or no data are available. 

In addition, potential sources of 

arsenic in Illinois are described. 

In Illinois, the most abundant 

data on arsenic in ground water 

are from community supplies 

(fig. 1). Assessment of the 

spatial distribution of arsenic 

in community supplies and 

comparison to other water-quality data available from private 

supplies can allow inferences 

to be made about the extent and 

distribution of arsenic in private 

supplies.

Because arsenic is common in ground water in Illinois, 

the patterns and correlations of 

arsenic with other constituents 

can be used to identify areas 

of concern, highlight potential indicators of high arsenic 

concentrations, and determine 

where additional research is 

needed to understand the fate 

and transport of arsenic in 

ground water. In addition, the 

study of community-water supplies will help the owners of 

private, unregulated wells make 

informed decisions on whether 

to have their well tested for 

arsenic.

Arsenic 

Concentrations 

on a National and 

Regional Scale

Elevated arsenic concentrations (above drinking-water 

standards) are a national and 

regional concern. An environmental research program by the 

USGS (Ryker, 2001) compiled 

arsenic analyses from 31,000 

wells and springs in 49 States 

for the national map (fig. 3). 

Scientists with the USGS and 

State agencies collected and 

analyzed these data mainly from 

Figure 3. Areas of equal arsenic concentrations in ground water, United States (from Ryker, 2001). In 

2001, the U.S. Geological Survey database included trace-element data from 31,000 wells and springs.
Arsenic in Illinois Ground Water–Community and Private Supplies 4 Water-Resources Investigations Report 03-4103 

private wells, monitoring wells, 

and community-supply wells. 

These samples were collected 

for studies on the quality of the 

Nation’s potable ground-water 

resources. The data set contains 

no explicit information on the 

rural population that does not 

use ground water from public supply. The national map 

includes arsenic concentrations in drinking-water data 

from monitoring and research 

programs in the United States. 

Monitoring of community-supply systems is required for compliance with State and Federal 

water-quality standards. The 

national map data set provides 

an important basis for estimating how many community-supply systems have arsenic concentrations above the present 

standard, or what proportion of 

the urban population obtains 

water from community-supply 

systems with arsenic concentrations above the drinking-water 

standard (Ryker, 2001).

More than 99 percent of the 

Nation’s rural population relies 

on ground water for drinking 

water. Approximately 50 percent of community supplies in 

Illinois are from ground water; 

approximately 90 percent of 

private supplies are from ground 

water. Because private wells are 

unregulated, no national regulatory database is available to fill 

this data gap on rural private 

wells.

Additional data often are 

available at statewide or local 

scales for more specific comparison. Although the national 

map shows that Illinois has 

three areas with arsenic concentrations at or above 5 μg/L 

in ground water (central, 

northeastern, and northwestern Illinois), larger areas with 

appreciably higher arsenic concentrations are present in other 

parts of the Nation. 

Arsenic 

Concentrations in 

Ground Water in

Illinois

Known Distribution

The most extensive data 

set of arsenic concentrations 

in ground water in Illinois has 

been collected by the IEPA as 

part of compliance monitoring 

programs. Approximately 8,200 

samples collected by IEPA 

from 2,771 community-water 

supplies in Illinois provide a 

detailed picture of the arsenic distribution in most of the 

aquifers underlying Illinois. The 

USGS National Water-Quality 

Assessment (NAWQA) program has collected data from 

225 monitoring and private 

wells screened in the glacial 

and alluvial aquifer system. 

The glacial and alluvial aquifer 

system, as defined in this report, 

includes all aquifers above 

bedrock (fig. 4). In the central Mahomet Buried Bedrock 

Valley (fig. 1), the aquifer of 

concern is the deep portions of 

the glacial and alluvial aquifer system (Mahomet aquifer) 

(fig. 4). NAWQA samples are 

collected for studies of the 

quality of the Nation’s surfaceand ground-water resources. 

This program uses a nationally 

consistent network design, sampling program, and methods of 

data analyses for low levels of 

over 300 chemical constituents. 

The Illinois State Water Survey 

(ISWS) also collects arsenic 

data for studies in the State. In 

a study by the ISWS, in cooperation with the Illinois Waste 

Management and Research 

Center, the variability of arsenic concentrations with depth 

is being studied in the glacial 

and alluvial aquifer system 

in central Illinois. In another 

study, approximately 30 small 

community-water supplies are 

being sampled for arsenic and 

arsenic species concentrations 

by the ISWS. In addition, 150 

non-community wells are being 

sampled for arsenic and other 

chemical constituents. All these 

programs and studies provide 

useful information, but it also is 

important to combine the available data sets. IEPA and USGS 

provide data on arsenic concentrations in community and 

private wells.
Arsenic in Illinois Ground Water–Community and Private Supplies 5 Water-Resources Investigations Report 03-4103

Concentrations of total (dissolved and suspended) arsenic 

samples collected from community supplies and concentrations 

of dissolved arsenic in samples 

collected from monitoring 

and private wells may not be 

comparable. Two major issues 

concerning these data sets are: 

(1) the difference in pumping 

volumes; (2) the difference in 

filtering procedures; samples 

from community-supply wells 

are unfiltered, whereas samples 

from monitoring and private 

wells are filtered. Because 

community-supply wells generally pump larger amounts than 

monitoring or private wells, 

water flows from a larger area 

in an aquifer (capture zone) 

to community-supply wells; 

therefore, resulting arsenic 

concentrations may differ even 

when community-supply, and 

monitoring or private wells are 

close to each other (Ohio Environmental Protection Agency, 

2002). The filtering issue is well 

documented (Horowitz and others, 1996). Comparing arsenic 

concentrations from filtered 

and unfiltered samples is complicated by the adsorption of 

arsenic to clay particles and colloids within the water sample. 

Filtration will remove these particles and the adsorbed arsenic, 

decreasing the apparent concentration of arsenic in the sample. 

Turbidity is a rough estimate 

of the amount of particles in a 

water sample. When it is low, 

few particulates are present in 

samples, and dissolved and total 

arsenic concentrations should 

be similar. Turbidity in both the 

community- and private-supply 

samples was low, so comparing 

arsenic concentrations in ground 

water from these two well types 

should not be biased because of 

turbidity.

Arsenic concentrations in 

ground water usually are the 

result of the interaction between 

water and geologic materials. 

The geochemical environment in the aquifer is one of 

the biggest factors affecting 

arsenic concentrations. Factors such as ground-water age, 

depth to water, and land use 

affect the fate and transport 

of constituents introduced by 

humans, but have limited effects 

on the concentration of arsenic 

in ground water. Arsenic data 

from community and private 

supplies were compared in 

Illinois and other parts of the 

country (Warner, 2001; Bruce 

and Oelsner, 2001). Comparison of arsenic concentrations 

in private supplies (filtered) 

and arsenic concentrations in 

community-water supplies 

(unfiltered) in the deep porFigure 4. Relation of time stratigraphy, general rock stratigraphy, 

and hydrostratigraphy of glacial deposits in central Illinois (modified from Willman and Frye, 1970; Hansel and Johnson, 1996; and 

Warner, 2001).
Arsenic in Illinois Ground Water–Community and Private Supplies 6 Water-Resources Investigations Report 03-4103 

tions of the glacial and alluvial 

aquifer system in Illinois were 

similar for a data set containing 

less than 100 samples (Warner, 

2001). The deep portions of 

the glacial and alluvial aquifer 

system are defined as the deep 

glacial drift aquifer (fig. 4) by 

Warner (2001). In the deep portions of the glacial and alluvial 

aquifer system, filtered arsenic 

concentrations for ground-water 

samples from private wells 

ranged from less than 1.0 to 

84 μg/L, with a median concentration of 1.5 μg/L; whereas 

the range of unfiltered arsenic 

concentrations for ground water 

from community-water supplies ranged from less than 1 

to 110 μg/L, with a median of 

1.0 μg/L (Warner, 2001). A 

study comparing filtered private and unfiltered community 

supplies in the High Plains 

aquifer in central Colorado did 

not show a statistical difference in arsenic concentration 

(Bruce and Oelsner, 2001). 

The median concentration in 

the High Plains aquifer from 

private and community supplies was 2.04 and 1.55 μg/L, 

respectively. On a national 

basis, arsenic concentrations 

in samples not from community-supply wells tended to 

be higher than concentrations 

in samples from communitysupply samples (Welch and 

others, 1999).

The IEPA collected arsenic 

data at 2,771 community-water 

supply wells (prior to treatment) 

in various aquifers in Illinois 

from 1978 through 2001. Some 

wells were sampled multiple 

times. Arsenic was not detected 

in 60 percent of samples (5031 

of 8180 samples; fig. 5).

NAWQA program wells 

in the glacial and 

alluvial aquifer system

All IEPA routine samples 

from community 

supplies in all aquifers

All IEPA routine samples 

from community-supply 

wells in the glacial and 

alluvial aquifer system

Number of analyses 268 2036 886

Minimum concentration (μg/L) < .2 < 1 < 1

Maximum concentration (μg/L) 128 100 100

25 percent of analyses above this concentration (μg/L) < .2 < 1 < 1

75 percent of analyses above this concentration (μg/L) 3.9 2 6

Median concentration (μg/L) .5 < 1 < 1

Percent arsenic detections 58 43 47

Percent above 10 μg/L 14 11 19

Table 1. Comparison of arsenic concentrations in ground water from private and monitoring wells within the glacial and alluvial aquifer system, all routinely sampled community-supply wells, and community-supply wells within the glacial and alluvial aquifer system.

[NAWQA, National Water-Quality Assessment; IEPA, Illinois Environmental Protection Agency; μg/L, micrograms per liter; <, less than]

Figure 5. Arsenic concentrations in community-supply well samples in Illinois, 

1978-2001.
Arsenic in Illinois Ground Water–Community and Private Supplies 7 Water-Resources Investigations Report 03-4103

As previously discussed, community-supply 

well samples are collected 

routinely or as part of 

special studies within the 

IEPA (table 1). Routine 

samples (2,036 samples in 

the data set) are samples 

collected by the community-supply well operator before any treatment 

for compliance. Samples 

for arsenic generally are 

collected every 5 years 

unless there are compliance issues requiring more 

sampling. Special studies 

can include a wide variety 

of programs from ambient monitoring to compliance. 

Of the community- and private-supply samples, 11 and 14 

percent, respectively, exceeded 

the 10 μg/L arsenic standard. 

The highest arsenic concentrations and most frequent detections in samples from community-supply wells were from the 

deep portions of the glacial and 

alluvial aquifer system underlying central Illinois (Mahomet 

aquifer) (fig. 4).

With large water-quality 

data sets (for example, the community-supply data set from 

IEPA), there can be statistical 

bias because of the number of 

samples in any given year and 

the laboratory methods used for 

analysis. The largest number of 

samples from community-water 

supplies in Illinois was collected 

each year from 1982 to 1987 

(fig. 6) after the implementation 

of the Clean Water Act in 1980. 

The percentage of samples 

each year with concentrations 

greater than or equal to 10 μg/L 

and less than 50 μg/L remained 

relatively constant at 9 percent 

across these years, and samples 

greater than 50 μg/L ranged 

from 1 to 2 percent over this 

time period. Laboratory reporting levels (lrl) have ranged from 

0.001 to 5 μg/L. Generally, the 

lower reporting levels are for 

more recent samples because 

analytical machines and methods with higher precision were 

used in more recent years. If 

all the samples were screened 

to the highest reporting level 

of 5 μg/L, then only 15 percent 

of all wells would have arsenic 

detections.

Figure 6. Sampling program for community-supply wells in Illinois, 1978-2001

The glacial and alluvial 

aquifer system, which is the 

primary aquifer system used 

for community-water supplies 

in Illinois, had the greatest 

number of arsenic samples. 

Almost 50 percent of the 

community-supply wells in 

Illinois are open to this aquifer system (fig. 7). Similar 

ranges and arsenic detections 

are present for samples taken 

from either community-supply (less than 1.0-100 μg/L) or 

private/monitoring (less than 

0.2-128 μg/L) wells open to this 

aquifer system (table 1). 

Estimated 

Distribution

Possible arsenic sources 

may be determined by establishing a relation between arsenic 
Arsenic in Illinois Ground Water–Community and Private Supplies 8 Water-Resources Investigations Report 03-4103 

concentration and concentrations of other chemical constituents and/or physical properties 

(such as well depth). The relation of arsenic with depth is not 

straightforward. Arsenic sources 

may be at various depths and 

geochemical conditions affecting arsenic mobilization also 

vary with depth. A study by the 

USGS in the central Mahomet 

Buried Bedrock Valley in 

Illinois (fig. 1) found arsenic 

concentrations and frequency 

of detection to be highest in the 

deep glacial drift aquifer (fig. 4) 

(median depth of 256 feet) 

compared to the shallow drift 

aquifer (fig. 4) (median depth 

of 33 feet) (Warner, 2001). 

A comparison of private supplies in this same area found 

concentrations to be similar for 

comparable depths (Warner, 

2001). In contrast, the ISWS 

found, in general, higher arsenic 

concentrations in samples from 

the shallow portions than in 

the deep portions of the glacial 

and alluvial aquifer system in 

Tazewell County (Illinois State 

Water Survey, 2002). This result 

is based on 590 samples collected by the Tazewell County 

Health Department and interpolated well-depth information. 

In Illinois, many groundwater supplies are hard (hardness as CaCO3

 greater than 

120 milligrams per liter (mg/L); 

Heath, 1984). Therefore, most 

community and some private 

supplies treat the drinking water 

to remove calcium and magnesium, as well as iron, manganese, and other constituents. 

The removal of these constituents from drinking water also 

may reduce arsenic concentrations because arsenic adsorbs to 

iron and manganese oxides (Illinois State Water Survey, 2002). 

Data from 1,449 communitywater supplies that utilize the 

glacial and alluvial aquifer system were analyzed for arsenic, 

iron, and manganese concentrations. Co-kriging, 

a geostatistical method, 

was used to estimate 

arsenic concentrations 

in ground water across 

the State by interpolating 

between data points with 

known arsenic, iron, and 

manganese concentrations 

(figs. 8a, b). More information on these methods 

can be found in Isaaks 

and Srivastava (1989) 

and Kitanidis (1997). 

Where the estimates are based 

on relatively few data points 

(compared to other parts of 

the State), there is more uncertainty in the estimated arsenic 

concentrations (fig. 8b). The 

geostatistical methods provide a 

good estimate when compared 

to known arsenic concentrations 

but tend to underestimate the 

highest concentrations and tend 

slightly to underestimate the 

variability of the concentrations 

(the multiple R2 coefficient of 

determination was 0.64 between 

measured and estimated values). The darker shaded areas 

on figure 8a indicate the highest 

estimated arsenic concentrations 

in association with iron and 

manganese. These areas also 

may indicate where community 

and some private supplies likely 

are treating for iron and manganese, which may remove some 

of the arsenic. The area of high 

arsenic, iron, and manganese 

Figure 7. Number of community-supply samples analyzed for arsenic by aquifer system in 

Illinois.
Arsenic in Illinois Ground Water–Community and Private Supplies 9 Water-Resources Investigations Report 03-4103

Figure 8. (A) Estimated arsenic concentrations in association with iron and manganese and (B) uncertainty of arsenic 

concentration estimates, Illinois.
Arsenic in Illinois Ground Water–Community and Private Supplies 10 Water-Resources Investigations Report 03-4103 

concentrations in central Illinois 

is associated with part of the 

Mahomet Buried Bedrock Valley (fig. 1). Water-quality data 

from private and monitoring 

wells also indicated high arsenic 

concentrations in this area.

Potential Sources of 

Arsenic in Illinois

In most of Illinois, arsenic sources are natural. Three 

potential sources considered 

here for high arsenic concentrations in Illinois are: 1) dissolution from geologic material; 

2) mobilization of adsorbed or 

coprecipitated arsenic; and/or 

3) bedrock recharge.

The dissolution and mobilization of arsenic from geologic 

materials will occur if there is a 

source of arsenic in the geologic 

materials and if the geochemical conditions are conducive to 

arsenic transport. There are geographically extensive geologic 

and geochemical conditions that 

produced widespread arsenic 

enrichments in the earth’s crust 

in the formation of arsenic-rich 

pyrite (Goldhaber and others, 

2002). These bedrock deposits usually are in contact with 

overlying glacial sediments 

and structural features in the 

bedrock may provide conduits 

for arsenic transport. Another 

source of arsenic is dark shale 

and coal containing pyrite. 

Some coal deposits in Illinois, 

Indiana, and Kentucky 

contain high arsenic concentration (fig. 9).

Geochemical rock 

analyses done on the 

glacial deposits and 

underlying bedrock in the 

Central Mahomet Buried 

Bedrock Valley indicate 

that the highest arsenic 

concentrations are in the 

organic-rich bedrock 

underlying the valley. 

This organic-rich bedrock 

usually is shale or dark 

limestone. The sand and 

gravel making up the deep 

portions of the glacial and 

alluvial aquifer system 

had the lowest arsenic 

concentration of the sediments above the bedrock. The 

till overlying the aquifer in this 

area had arsenic concentrations 

higher than the sand and gravel, 

but substantially less than the 

organic-rich bedrock (Jeff Catalano, U.S. Geological Survey, 

written commun., 1999). Thus, 

there is an arsenic source in the 

geologic materials of the sediments and bedrock composing 

the aquifers, but the organicrich bedrock has the highest 

concentration of arsenic.

The second possible source 

for the high arsenic concentrations considered here is the 

mobilization of coprecipitated 

arsenic. Consideration of this 

possible source is based on the 

premise that during glacial periFigure 9. Arsenic concentrations in coal from Illinois, Indiana, and Kentucky (from 

Goldhaber and others, 2002).
Arsenic in Illinois Ground Water–Community and Private Supplies 11 Water-Resources Investigations Report 03-4103

ods, waters with high arsenic 

concentration were in contact 

with clay within the till and the 

arsenic adsorbed onto these clay 

particles. As the glacial materials were buried, the geochemical environment became more 

reducing and the arsenic associated with the iron oxides and 

hydroxides was mobilized.

The third possible source 

for high arsenic concentrations 

considered here is bedrock 

recharge. This possible source 

was proposed by Panno and 

others (1994). Most of the work 

on arsenic in ground water in 

Illinois primarily has been in 

the deep portions of the glacial 

and alluvial aquifer system, 

where high arsenic concentrations are present in some areas 

and depths. In Piatt County in 

central Illinois (fig. 1), there are 

many bedrock structural features, such as faults and folds, 

which could provide direct pathways for ground water to flow 

from the deep bedrock to the 

deep portions of the glacial and 

alluvial aquifer system. Arsenic 

concentrations in the deep portions of the glacial and alluvial 

aquifer system increase along 

the ground-water-flow path just 

west of Piatt County (Warner, 

2001). Ground water from the 

bedrock may be recharging the 

aquifer with arsenic or may be 

altering geochemical conditions 

so that arsenic is mobilized in 

this area.

Summary and 

Conclusions

Dissolved arsenic is found 

in ground water across Illinois. 

Determining the extent and 

distribution of arsenic in ground 

water is a starting place to 

assess the potential risk for persons drinking from a community or private supply. Understanding the potential sources 

and pathways that mobilize 

arsenic in ground water is necessary in protecting the drinking-water supply in Illinois.

Arsenic was not detected 

in 60 percent of samples from 

community supplies considered in this study. The highest 

arsenic concentrations and most 

frequent detections in samples 

from community-supply wells 

were from the deep portions of 

the glacial and alluvial aquifer system underlying central 

Illinois (Mahomet aquifer). Of 

the community- and privatesupply samples, 11 and 14 

percent, respectively, exceeded 

the 10 μg/L arsenic standard; 

but, private-supply wells are not 

regulated for arsenic concentrations in ground water.

Arsenic sources may be at 

various depths and geochemical conditions affecting arsenic 

mobilization also vary with 

depth. Estimates of arsenic, 

iron, and manganese have been 

exptrapolated from measured 

concentrations. An area of high 

estimated arsenic concentration in ground water in central 

Illinois appears to be associated with the Mahomet Buried 

Bedrock Valley. Where the 

estimates are based on relatively 

few data points (compared to 

other parts of the State), there is 

more uncertainty in the estimated arsenic concentrations. 

The geostatistical methods 

provide a good estimate when 

compared to known arsenic 

concentrations but tend to 

underestimate the highest concentrations and tend slightly to 

underestimate the variability of 

the concentrations.

In most of Illinois, arsenic 

sources are natural. Limited 

geochemical rock analyses 

done on the glacial deposits and 

underlying bedrock in the Central Mahomet Buried Bedrock 

Valley indicate that the highest 

arsenic concentrations are in the 

organic-rich bedrock underlying the valley. Understanding 

the fate and transport of arsenic from the source to a community-supply well will need 

to involve simulation of the 

ground-water flow and chemical conditions of the glacial and 

alluvial aquifer system.
U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4103

July 2003

12

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S.A., 1999, Arsenic in ground 

water supplies of the United 

States, in W.R. Chappell, C.O. 

Abernathy and R.L. Calderon, 

eds., Arsenic exposure and 

health effects: Elsevier Sci- ence, New York, p. 9-17.

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For further information, 

please contact:

District Chief

U.S. Geological Survey

221 N. Broadway Avenue

Urbana, Illinois 61801

Telephone: 217-344-0037

Email: dc_il@usgs.gov

or visit the Illinois District Web site: 

il.water.usgs.gov