Prepared in cooperation with the Illinois Environmental Protection Agency

Hexavalent and Total Chromium at Low Reporting Concentrations 

in Source-Water Aquifers and Surface Waters Used for 

Public Supply in Illinois, 2013

Scientific Investigations Report 2015–5020

U.S. Department of the Interior

U.S. Geological Survey
Front cover and other images: Front cover shows collection of groundwater samples, Union-York Water District (intake 00251). This and 

other photography by Patrick Mills, U.S. Geological Survey.
Hexavalent and Total Chromium at Low 

Reporting Concentrations in Source-Water 

Aquifers and Surface Waters Used for 

Public Supply in Illinois, 2013

By Patrick C. Mills and Richard P. Cobb

Prepared in cooperation with the Illinois Environmental Protection Agency

Scientific Investigations Report 2015–5020

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Mills, P.C., and Cobb, R.P, 2015, Hexavalent and total chromium at low reporting concentrations in source-water aquifers and surface waters used for public supply in Illinois, 2013: U.S. Geological Survey Scientific Investigations Report 

2015–5020, 58 p., http://dx.doi.org/10.3133/sir20155020.

ISSN 2328-0328 (online)
iii

Contents

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

Introduction.....................................................................................................................................................2

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

Previous Studies ...................................................................................................................................3

Chemistry of Hexavalent Chromium ...........................................................................................................4

Study Methods ...............................................................................................................................................7

Well, Intake, and Treated-Sample Site Selection............................................................................7

Sample Collection and Laboratory Analyses .................................................................................12

Quality Assurance...............................................................................................................................14

Data Analysis.......................................................................................................................................16

Accuracy of Chromium Data......................................................................................................................17

Occurrence and Distribution of Chromium..............................................................................................19

Frequency of Detection and Concentration ...................................................................................19

Hexavalent Chromium...............................................................................................................19

Total Chromium and Relation to Hexavalent Chromium......................................................22

Comparison to Regulatory Standards and Results of Other Studies .........................................22

Geochemical and Physical Factors Associated with Occurrence of Chromium..............................24

pH Conditions.......................................................................................................................................24

Land Use...............................................................................................................................................25

Geohydrology and Well Construction..............................................................................................28

Groundwater-Related Factors .................................................................................................28

Surface-Water-Related Factors..............................................................................................35

Other Factors.......................................................................................................................................36

Effects of Treatment and Distribution on Sample Concentrations ......................................................37

Sampling Factors Associated with Analytical Results of Chromium ..................................................39

Sample Temperature and Holding Time..........................................................................................39

Sample Tap Composition ...................................................................................................................40

Summary and Conclusions.........................................................................................................................41

References Cited..........................................................................................................................................42

Appendix 1. Table of well and surface-water intake information, field characteristics of 

water quality, and concentrations of hexavalent chromium and total chromium in 

samples from selected source-water aquifers and surface waters used for public 

supply in Illinois, 2013 ....................................................................................................................48

Appendix 2. Protocols for collecting representative samples of untreated source water 

and treated water. A, Hexavalent chromium. B, Total chromium...........................................58

Appendix 3. Example Illinois Environmental Protection Agency field sampling record ..................60

Appendix 4. Example analytical result report and quality-assurance documentation 

provided by UL laboratory.............................................................................................................65
iv

Figures

1. Map showing principal source-water aquifers and surface waters used for public 

supply, selected wells and intakes, and extent of episodic glaciation in Illinois ..............5

2. Diagram showing relation of chromium speciation in water to oxidation-reduction 

(redox) conditions and pH...........................................................................................................6

3. Diagram showing general classification of lithostratigraphic and hydrostratigraphic units that compose source-water aquifers in Illinois.....................................9

4. Map showing locations and designations of selected public-supply wells and 

intakes that tap source-water aquifers and surface waters in Illinois .............................10

5. Photos showing typical quality-assurance measures and collection of samples 

for chromium at selected public-water-supply facilities in Illinois, 2013......................... 13

6. Graph showing frequency of detection and maximum concentrations of 

hexavalent chromium and total chromium in public-supply source waters in 

Illinois, 2013..................................................................................................................................19

7. Map showing locations of selected wells and intakes that tap public-supply 

source-water aquifers and surface waters in Illinois, and the occurrence and 

relative concentrations of hexavalent chromium [Cr(VI)] in untreated water 

samples, 2013 ..............................................................................................................................20

8. Graphs showing frequency of detection and maximum concentrations of 

hexavalent chromium and total chromium in public-supply source-water 

aquifers and surface waters in Illinois, by source type, 2013 .............................................21

9. Graphs showing hexavalent chromium and total chromium in samples from 

public-supply source-water aquifers and surface waters in Illinois, by source 

type, 2013......................................................................................................................................23

10. Graphs showing relation of concentrations of hexavalent chromium to pH in 

source waters used for public supply in Illinois, 2013..........................................................26

11. Box-and-whisker plots of concentrations of hexavalent chromium in publicsupply source waters in Illinois, by urban and rural land use, 2013 ..................................28

12. Box-and-whisker plots of concentrations of hexavalent chromium in publicsupply source-water aquifers in Illinois, by glacial drift aquifer and alluvial 

aquifer, 2013.................................................................................................................................29

13. Graph showing frequency of detection of hexavalent chromium in public-supply 

source-water aquifers in Illinois, by designation of aquifer confinement, 2013 ..............30

14. Graph showing frequency of detection of hexavalent chromium in public-supply 

source-water aquifers in Illinois, by aquifer depth, 2013.....................................................31

15. Graphs showing hexavalent chromium in public-supply source-water aquifers in Illinois, by age and lithology of aquifer, 2013..............................................................................34

16. Graph showing frequency of detection of hexavalent chromium in public-supply 

source-water aquifers in Illinois, by well-casing depth, 2013.............................................35

17. Graph showing relation of concentrations of hexavalent chromium to total 

chromium in intake, tap , and distribution samples from public-supply surfacewater sources in Illinois, 2013 ..................................................................................................37

18. Graph showing concentrations of hexavalent chromium in intake and tap samples 

from public-supply surface-water sources in Illinois, 2013.................................................38

19. Graph showing relation of concentration of hexavalent chromium in tap samples 

from public-supply surface-water sources in Illinois, soon after treatment and 

near the endpoint of distribution, 2013....................................................................................39
v

20. Box-and-whisker plots of concentrations of hexavalent chromium 

in public-supply source waters in Illinois, by sample-tap material, 2013..........................40

21. Graph showing concentrations of hexavalent chromium in public-supply source waters 

in Illinois, by chrome-plated and brass sample-tap material, 2013....................................41

Tables

1. Potential reactions resulting in transformations of trivalent and hexavalent 

chromium .......................................................................................................................................7

2. Criteria and related data used for the stratified–random selection of public-supply 

intakes for sampling and analysis of chromium in Illinois surface waters, 2013.............11

3. Summary statistics for detections of hexavalent chromium and total chromium 

in samples from source-water aquifers and surface waters used for public 

supply in Illinois, 2013.................................................................................................................17

4. Summary statistics for detections of hexavalent chromium in samples from 

various unconsolidated and bedrock aquifers used for public water supply in 

Illinois, 2013..................................................................................................................................32

Acknowledgments

Various persons and agencies are acknowledged for their contributions to the study. Anthony 

Dulka, Alan Fuhrmann, Edward Wagner, Ryan Bennett, Gregory White, and Joseph Konczyk of 

the Illinois Environmental Protection Agency, Division of Public Water Supplies, participated in 

sample-site selection, quality-assurance training, sample collection, and (or) data management 

and analysis. Well operators, owners, and public and private facilities freely provided access to 

the wells, surface-water intakes, and (or) distribution taps for sampling. The laboratory staff of 

the UL company (formerly known as Underwriters Laboratories), including Jessie Varab, helped 

to develop sampling protocols and provided analytical and quality-assurance support services. 

Without their efforts, this project could not have been completed.
vi

Conversion Factors and Abbreviations

Inch-Pound to International System Units

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

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

Yield

gallon per day per 

square mile (gal/d/mi2

) 

0.00144 cubic meter per day per 

square kilometer

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

°F = (1.8 × °C) + 32

Abbreviated water-quality units used in this report: Concentrations of chemical constituents in 

water are given in micrograms per liter (μg/L) and milligrams per liter (mg/L). Micrograms per 

liter are considered equivalent to parts per billion at the reported concentrations. Milligrams per 

liter are considered equivalent to parts per million at the reported concentrations.

Concentrations of chemical buffers are given in millimole (mM), a unit of metric measurement 

that is equal to one thousandth (10-3) of a mole (the amount of a substance that corresponds to 

its formula mass in milligrams or one thousandth of a gram-molecule).

Specific conductance (SC) of water is given in microsiemens per centimeter at 25 degrees 

Celsius (μS/cm at 25 °C).

pH of water is given in standard units.

Abbreviated concentrations of solids used in this report: Concentrations of chemical 

constituents in solids are given in milligrams per kilogram (mg/kg).

Additional abbreviations: micrometer (μm), millivolt (mv), milliliter (mL), millimeter (mm).
Hexavalent and Total Chromium at Low Reporting 

Concentrations in Source-Water Aquifers and Surface 

Waters Used for Public Supply in Illinois, 2013

By Patrick C. Mills1 and Richard P. Cobb2

Abstract

On the basis of their recent review of the human health 

effects of hexavalent chromium [Cr(VI)] in public drinking 

water, the U.S. Environmental Protection Agency is considering the need for Federal regulation of Cr(VI). Presently, only 

total chromium is regulated, at a Maximum Contaminant 

Level (MCL) of 100 micrograms per liter (μg/L). The occurrence of Cr(VI) in groundwater and surface waters generally 

is attributed to industrial sources, but can be of natural origin. 

California’s recently established MCL for Cr(VI) of 10 μg/L 

illustrates the drinking-water concerns associated with Cr(VI). 

To improve understanding of the possible impact of a Cr(VI)-

specific standard that approximates the California level on the 

management of Illinois’ public drinking water, the U.S. Geological Survey, in cooperation with the Illinois Environmental 

Protection Agency, assessed the occurrence and distribution of 

Cr(VI) in the State’s public-water supplies.

During 2013, untreated water samples were collected 

to be analyzed for Cr(VI) and total chromium [Cr(T)] at 119 

water-supply wells and 32 surface-water intakes; also, 32 

treated surface-water samples were collected near the point 

of treatment and 32 near the furthest point of distribution. 

Public-supply sample sites were selected by a stratified random method. Samples typically were analyzed within 24 hours 

of collection at reporting limits of 0.02 μg/L for Cr(VI) and 

0.1 μg/L for Cr(T). The occurrence of Cr(VI) was compared 

with selected geophysical, physical, and sampling factors that 

might more fully explain its distribution and magnitude of 

concentrations. 

The maximum concentration of Cr(VI) in groundwater 

was 2.1 μg/L. Maximum concentrations in untreated and 

treated surface water were 0.29 μg/L and 2.4 μg/L, respectively. All sample concentrations were below the California 

MCL; only 35 percent were below that State’s non-enforceable 

1

 U.S. Geological Survey

2

 Illinois Environmental Protection Agency

public health goal of 0.02 of μg/L. Cr(VI) was undetected in 

43 percent of untreated groundwater samples, with a median 

of 0.06 μg/L when detected. All but two (94 percent) of 

untreated surface-water samples had detections. In untreated 

surface water, the median concentration was 0.09 μg/L, 

whereas in treated (tap and distributed) water the median was 

0.20 μg/L. Surface waters treated with lime for softening 

typically had the greatest Cr(VI) concentrations (maximum, 

2.4 μg/L; median, 1.2 μg/L). 

The maximum concentration of Cr(T) in groundwater 

was 1.8 μg/L. Maximum concentrations in untreated and 

treated surface water were 1.8 μg/L and 2.5 μg/L, respectively. 

All sample concentrations were below the Federal MCL. Total 

chromium was detected in 65 percent of untreated groundwater samples, with a median of 0.40 μg/L, when detected. All 

but one (97 percent) of untreated surface-water samples had 

detections. In untreated surface water, the median concentration was 0.40 μg/L, whereas in treated (tap and distributed) 

water the median was 0.30 μg/L. As with Cr(VI), surface 

waters treated with lime typically had the greatest Cr(T) 

concentrations.

Examination of factors that might account for or be associated with the occurrence of Cr(VI) in public-supply source 

waters found few clearly evident factors. Associations in 

frequencies of occurrence and range of concentrations indicate 

that surface waters and groundwaters of shallow, unconsolidated, unconfined aquifers, particularly alluvial aquifers, are 

possibly most commonly affected by anthropogenic sources of 

Cr(VI). Groundwaters of deep (greater than 500 feet) bedrock 

aquifers, particularly the Cambrian-Ordovician aquifer system, 

are possibly most commonly affected by geologic sources of 

Cr(VI). Additional study, with supporting geologic and geochemical data that were not collected in this study, would be 

necessary to verify these associations. 

There was a weak positive relation (ρ = 0.23) between 

concentrations of Cr(VI) and Cr(T) in untreated water samples, with a much stronger positive relation (ρ = 0.86 and ρ = 

0.90, respectively) in samples collected soon after treatment 

and near the endpoint of distribution. The stronger relation 

and greater similarity between Cr(VI) and Cr(T) concentrations in treated water samples indicate that Cr(VI) represents a 

greater proportion of the measured concentrations of Cr(T) in 
2

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

treated waters than in untreated waters. The analysis of spikes 

and other quality-assurance samples indicate uncertainties 

associated with obtaining or confirming consistently accurate 

analytical results for Cr(VI) at near the applied reporting limit 

of 0.02 μg/L. 

Introduction

The U.S. Environmental Protection Agency (USEPA) 

regularly reevaluates public drinking-water standards. Following the release of toxicity studies by the U.S. Department 

of Health and Human Services’ National Toxicology Program 

in 2008 (U.S. Environmental Protection Agency, 2013a), 

the USEPA launched a comprehensive review of the health 

effects of hexavalent chromium [Cr(VI)] in public drinking waters. On the basis of this review and consideration of 

recently available toxicity studies pertaining to ingestion of 

Cr(VI), the USEPA has proposed to classify Cr(VI) as likely 

to cause cancer in humans when ingested over a lifetime (U.S. 

Environmental Protection Agency, 2010a). In 2010, a draft of 

this human-health assessment (U.S. Environmental Protection Agency, 2012a) was released for public comment and 

external peer review. Upon finalization of the assessment, its 

conclusions and other relevant information will be considered 

to determine the necessity of establishing a specific regulatory 

standard for Cr(VI) in public drinking waters. 

The USEPA has regulated Cr(VI) as part of the total chromium [Cr(T)] standard under the Federal Safe Drinking Water 

Act (SDWA) (U.S. Environmental Protection Agency, 2012b) 

since 1992. This standard addresses all forms of chromium, 

including Cr(VI) and trivalent chromium [Cr(III)]. The trivalent form occurs most commonly in waters used for public 

supply (Frey and others, 2004) and is generally nontoxic at 

levels typically found in public drinking water. The present 

(2014) Maximum Contaminant Level (MCL) for Cr(T) is 100 

micrograms per liter (μg/L) (U.S. Environmental Protection 

Agency, 2013b). In 2014, California established an MCL of 10 

μg/L for Cr(VI) (National Ground Water Association, 2014) 

and, in 2011, established a nonenforceable public health goal 

of 0.02 μg/L (California Department of Public Health, 2014). 

New Jersey reportedly is considering a regulatory standard of 

about 0.07 μg/L (New Jersey Drinking Water Quality Institute, 

2010). Additionally, the Agency for Toxic Substances and 

Disease Registry of the U.S. Department of Health and Human 

Services has a nonenforceable children’s drinking water health 

guideline of 10 μg/L and an adult water health guideline of 

40 μg/L (Illinois Environmental Protection Agency, 2011a). 

These state and Federal guidelines illustrate the drinking-water 

concerns associated with Cr(VI) and suggest that, if established, the Federal drinking water standard for Cr(VI) might 

fall within the range of 0.02–40 μg/L and possibly close to 

10 μg/L.

The occurrence of Cr(VI) in groundwater and surface 

waters generally is attributed to industrial sources, but it 

can be of natural origin (McNeill and others, 2011). Water 

geochemistry, principally oxic and alkaline pH conditions, 

enhances the transformation of chromium from the generally 

nontoxic form Cr(III) to more soluble and toxic Cr(VI). This 

transformation process has important implications regarding 

changes in the occurrence and concentration of Cr(VI) during 

water treatment and distribution. Additionally, there are indications that drinking water might be inadvertently affected by 

trace amounts of Cr(VI) included in water-treatment additives 

(Anthony Dulka, Illinois Environmental Protection Agency, 

written commun., 2011). Accurate representation of in situ 

concentrations can be difficult because of the time-related 

instability of the oxidation-reduction (redox) species of chromium (Ball, 2003) and the potential for cross contamination 

when analyzing at the extremely low concentrations required 

for present and possible future water-quality regulation. Many 

of the sampling and processing methods can be quite intricate, 

requiring strict attention to procedural guidelines. Under the 

influence of these factors, there may be bias for underestimation or overestimation of dissolved concentrations.

 Although the reasonable likelihood of Cr(VI) occurrence 

in many of the Nation’s public-supply waters is well documented (as will be subsequently discussed), none of the available studies specifically address the potential for statewide 

occurrence of Cr(VI) in the source and drinking waters for 

public supply in Illinois. Additionally, there remains an incomplete understanding of the factors that affect Cr(VI) distribution in source waters and detection in drinking-water samples, 

including the potential contributions to Cr(VI) detection of 

sampling methodology and chromium-species transformation 

during water treatment and distribution.

The U.S. Geological Survey (USGS), in cooperation 

with the Illinois Environmental Protection Agency (IEPA), 

conducted a comprehensive assessment of the occurrence of 

Cr(VI) in the source and treated public-supply waters of Illinois in an attempt to improve understanding of 

1. the occurrence of Cr(VI) in Illinois’ waters used for 

public supply, 

2. selected factors that affect its occurrence, and 

3. the impact of a specific regulatory standard for Cr(VI) 

at a level of 10 μg/L or less on management of Illinois’ 

public drinking waters. 

During 2013, untreated water samples were collected to 

be analyzed for Cr(VI) and Cr(T) at 119 water-supply wells 

and 32 surface-water intakes distributed throughout the State; 

also, 32 treated surface-water samples were collected near 

the point of treatment and 32 near the endpoint of distribution (the furthest point, approximating the longest insystem 

residence time). A reporting limit at the lower end (0.02 μg/L) 

of those that might be expected for Federal regulation of 

Cr(VI) was applied in the assessment; a similarly low reporting limit (0.1 μg/L) was applied for assessment of Cr(T). 

Quality-assurance samples were evaluated along with laboratory analytical results to consider the accuracy of the results 
Introduction

and related uncertainty associated with sample collection and 

analysis at the relatively low reporting limits of this study. 

The occurrence of Cr(VI) was examined along with selected 

geochemical, physical, and sampling factors that might more 

fully explain the distribution and magnitude of concentrations. 

Data particularly relevant to improved understanding of the 

occurrence of Cr(VI) with respect to aqueous geochemistry , 

including dissolved-oxygen (DO) concentrations and redox 

potentials, were either not available or of insufficient accuracy 

in this study. Also, other redox-sensitive chemical constituents were not routinely analyzed in all samples; assessment 

of redox conditions on the basis of those constituent data that 

were sufficiently available was beyond the scope of the study.

Purpose and Scope

This report presents the analytical results for Cr(VI) and 

Cr(T) from 215 samples collected by the IEPA at public water 

facilities in Illinois during January–December 2013. The 

samples represent untreated source water from water-supply 

wells and surface-water intakes, as well as treated water at the 

location of treatment and near the endpoint of distribution. 

Limited analysis is provided of the (1) geochemical factor 

pH and selected physical factors that may account for or be 

associated with the occurrence and distribution of Cr(VI) in 

the State’s source and treated waters used for public supply, 

(2) selected sampling factors that may be associated with the 

analytical results of Cr(VI), and (3) the effect of lime as a 

treatment additive on Cr(VI) concentrations in drinking water. 

Physical factors that were evaluated included aspects of land 

use, geohydrology, well construction, and water treatment. 

Sampling factors included aspects of the sample processing 

prior to analysis and of the material composition of the watersupply delivery system. Additionally, quality-assurance factors 

were examined that address the accuracy of the analytical 

results and the uncertainty associated with sample collection 

and analysis at the reporting limits of 0.02 μg/L for Cr(VI) and 

0.1 μg/L for Cr(T). Results are summarized statistically and 

presented in figures and tables.

Previous Studies

Hexavalent chromium has been detected in drinking 

water or groundwater at concentrations above the 100 μg/L 

MCL for Cr(T) at a number of geographically restricted locations within the United States, including Hinkley, California 

(Zimmerman, 2014) and Midland, Texas (Texas Commission 

on Environmental Quality (2013); in large part those elevated 

concentrations are attributed to industrial contamination. 

Several reconnaissance studies on a larger scale have documented the occurrence of Cr(VI) in groundwaters and surface 

waters used as sources for public supply. In samples from 

Lake Michigan, Cr(VI) concentrations consistently have been 

detected in the range of 0.2–0.3 μg/L (Central Lake County 

Joint Action Water Agency, 2011); the lake is a public-water 

source for over 7,000,000 people in and near Chicago, Illinois 

and Milwaukee, Wisconsin (Mills and Sharpe, 2010). In a 

study of chromium occurrence in the western Mojave Desert, 

Calif., Cr(VI) was detected in about 94 percent of the samples 

from 133 public-supply, domestic, and observation wells, 

with maximum and median concentrations of 60 μg/L and 5.5 

μg/L, respectively (Ball and Izbicki, 2004). The samples were 

analyzed by a field-speciation method with a reporting limit 

of 0.1 μg/L. The concentration of 25 percent of the samples 

was greater than 10 μg/L. A statewide assessment of Cr(VI) in 

drinking water obtained from about 6,500 public-supply wells 

in California found detectable concentrations above 1 μg/L 

in about 55 percent of well-water samples and above 10 μg/L 

in 4.5 percent of samples (California State Water Resources 

Control Board, 2011). In a study of the drinking-water supply 

facilities of 35 U.S. cities, the Environmental Working Group 

(Sutton, 2010) found Cr(VI) at concentrations above 0.06 μg/L 

in the tapwater samples from 25 cities; the greatest concentration was 12.9 μg/L. More than 26 million people are served 

by the 31 facilities with detected concentrations of Cr(VI). 

The largest areal reconnaissance study for Cr(VI) in drinking 

water of the United States, the National Chromium and Boron 

Occurrence Survey (NCBOS), consisted of the collection of 

almost 350 public-supply tap samples from groundwater and 

surface-water sources (Frey and others, 2004). The samples, 

most of which were chlorinated or otherwise treated, were 

collected from about 40 states. Cr(VI) was detected above the 

method detection3

 or reporting limit of 0.2 μg/L in 42 percent 

of these samples, with a maximum concentration of about 53 

μg/L. The study included almost 20 supply facilities and about 

30 source water bodies in Illinois. The mean of the sample 

population was 1.1 μg/L, and the median was less than the 

reporting limit. Concentrations in groundwater samples, representing 67 percent of the source-water samples, typically were 

greater than those in surface-water samples; respective 95thand 75th-percentile concentrations were about 6 μg/L and 1 

μg/L in groundwater and 0.5 μg/L and 0.25 μg/L in surface 

water. Untreated tapwaters (typically from groundwater) had 

notably greater concentrations than treated waters.

The IEPA indirectly has considered the statewide potential for elevated concentrations of Cr(VI) in Illinois’ public 

drinking waters. Although IEPA has not previously sampled 

and analyzed for Cr(VI), the agency has reviewed untreated 

source water (ambient) and treated (finished) water sample 

results for Cr(T) from the State’s 1,750 or so public-supply 

facilities (Illinois Environmental Protection Agency, 2011a). 

Cr(T) was detected in only 4 percent of treated water samples 

collected during the 1993–2010 review period, with no concentrations exceeding the MCL of 100 μg/L for Cr(T). The 

3

 Hereafter the reference to level of detection will be uniformly described 

as “above the reporting level.” The method detection limit (MDL), as defined 

by USEPA (1986), is the lowest concentration of an analyte reportable with 

99 percent confidence that the value is greater than zero. The reporting level, 

or minimum reporting level (MRL), is the minimum concentration that can be 

reported by a laboratory as a quantified value (U.S. Environmental Protection 

Agency, 2010b).
4

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

maximum reporting limit for Cr(T) applied in the analysis of 

these treated drinking water samples from groundwater and 

surface-water sources was 7 μg/L. The median detectable 

concentration in treated drinking water was 11 μg/L, with a 

75th-percentile concentration of 22 μg/L. The median detectable concentration in treated water from groundwater sources 

was about 10 μg/L and from surface-water sources was about 

14 μg/L. Median detectable concentrations in untreated 

groundwaters and surface waters were less than those in 

treated waters (about 8 μg/L and 1 μg/L, respectively). 

Chemistry of Hexavalent Chromium

Chromium is a metallic element which is odorless 

and tasteless and can be present in various oxidation states. 

The most common forms of chromium in the environment 

are Cr(III), Cr(VI), and the metal form Cr(0). Chromium is 

found naturally in rocks, plants, soil and volcanic dust, and 

humans and other animals. Naturally occurring Cr(III) that 

can generate Cr(VI) is prevalent in ultramafic igneous rocks 

near convergent plate boundaries (Stowe, 2006). Reportedly, 

Cr(VI) also may be found in certain limestone deposits (Frias 

and others, 1994; Scientific Analytical Institute, 2014). Manganese minerals, including manganese oxide (MnO2

), that are 

commonly found in association with chromium-bearing rocks 

have been indicated as a potential oxidant for the transformation of geochemically inert and immobile Cr(III) to Cr(VI), 

the more soluble and mobile form in aqueous settings (Chung 

and others, 2001; Oze and others, 2006). Phosphate additions 

to soils for nutrient amendment are identified as a geochemical mechanism for release of Cr(VI) from enriched soils 

(Becquer and others, 2003). Ultramafic, chromium-rich rock 

units or soils generally are not found in Illinois, except to a 

limited extent in the unglaciated southeastern part as peridotite 

associated with dikes and sills near structural features (Nelson, 

1995). It is possible that eroded chromium-bearing materials 

from the Canadian shield region could be distributed among 

the Illinois and Wisconsin Episode glacial deposits that blanket most of the State (fig. 1). However, brief review of readily 

available scientific literature found no indication of elevated 

or widespread concentrations of chromium-bearing minerals 

in these glacial deposits. A survey of near-surface soils (depth 

less than about 3 feet [ft]) found chromium concentrations in 

Illinois typically in the range of about 35 to 55 milligrams per 

kilogram (mg/kg), with the national median about 30 mg/kg; 

concentrations were as great as about 3,060 mg/kg in the eastern United States (Maryland) and 4,620 mg/kg in the western 

United States (California) (Smith and others, 2014). To what 

extent these near-surface samples represent concentrations at 

depth in aquifers used for supply of drinking water is uncertain. Tourmaline, which in some forms can include chromium 

as a trace constituent, is a minor component (1–5 percent) of 

till deposits that are older than the Illinois Episode and either 

exposed in a small area of western Illinois or buried in parts 

of western and southern Illinois (Willmann and others, 1963; 

Kolata and Nimz, 2010). These older till deposits do not serve 

as groundwater or surface-water source deposits for the State’s 

public water supply. Limestones are prevalent throughout the 

State, but brief review of readily available scientific literature 

found no discussion or data regarding Cr(VI) or other forms 

of chromium associated with Illinois limestones. Manganese 

is commonly found in Illinois groundwaters, particularly 

where concentrations are elevated in the glacial drift aquifers4

(Voelker and Clarke, 1987).

Trivalent chromium occurs naturally in many vegetables, 

fruits, meats, grains, and yeast. Cr(VI) and Cr(0) generally are 

associated with industrial processes. Major industrial uses of 

Cr(VI) are associated with steel and pulp mills, metal plating, 

leather tanning, wood preservation, and production of dyes 

and pigments. Occurrences of Cr(VI) in drinking water from 

these uses are attributed to leakage, poor storage, and (or) 

improper discharge or disposal (U.S. Environmental Protection Agency, 2010a). Erosion of natural deposits of Cr(III) 

also can contribute to Cr(VI) in drinking water. At least 60 

industrial and landfill sites in Illinois have been identified as 

locations of operational use or disposal of chromium (Agency 

for Toxic Substances and Disease Registry, 2011), although to 

what extent this represents the use or disposal of the Cr(VI) 

form is uncertain.

Chromium is complex in its behavior in drinking-water 

systems because of its possible occurrence in various oxidation states. The most common states are the cationic Cr(III) 

form and the anionic Cr(VI) form. The anionic Cr(VI) species 

are chromate (HCrO4

-

 or CrO4

-2) and dichromate (Cr2

O7

-2). At 

the low chromium concentrations typical of drinking waters 

(substantially below 1,000 milligrams per liter [mg/L]), the 

Cr(VI) chromate species occurs as monovalent HCrO4

- below 

pH 6.5 and divalent CrO -2 

4 between pH 6.5 and pH 10 (Tong 

and King, 1953; Rai and others, 1987) (fig. 2). The dichromate 

species would only be present in drinking water under the 

atypical condition of Cr(VI) concentrations greater than 1,000 

mg/L. Such conditions might possibly result from an unexpected industrial discharge. Cr(III) forms a series of hydroxide 

complexes as pH increases above 4.0 (that is, becomes less 

acidic and then increasingly alkaline above pH 7.0). Its most 

predominant anionic species in waters within the pH range 

common to drinking water (pH 6.5–8.5; U.S. Environmental Protection Agency, 2013c) is Cr(OH) 0 

3 (Frey and others, 

2004).

Cr(VI) becomes increasingly prevalent in waters with 

DO concentrations greater than about 1–2 mg/L (Ball and 

Izbicki, 2004; Hawley and others, 2004). In highly oxygenated 

waters, or in the presence of oxidant additives such as chlorine 

or chloramine, Cr(VI) is expected to predominate. Cr(III) is 

4

 Hydrostratigraphic classifications in this report follow the use of the U.S. 

Geological Survey (2014), which in most cases conform to the usage of the 

Illinois State Geological Survey (ISGS) (Visocky, 1997). Lithostratigraphic 

classifications and references to periods of glaciation follow the use of the 

ISGS (Visocky, 1997; Kolata and Nimz, 2010).
JO DAVIESS

STEPHENSON

WINNEBAGO

LAKE

MCHENRY

BOONE

OGLE CARROLL

COOK

KANE

DE KALB

DU PAGE

WHITESIDE

LEE

ROCK ISLAND WILL

KENDALL

LA SALLE

HENRY

BUREAU

GRUNDY MERCER PUTNAM

KANKAKEE

STARK

KNOX

MARSHALL LIVINGSTON HENDERSON

WARREN

IROQUOIS

FORD

PEORIA

WOODFORD

MCLEAN

TAZEWELL

FULTON

HANCOCK

MCDONOUGH

VERMILION

MASON CHAMPAIGN

LOGAN SCHUYLER DE WITT

PIATT

ADAMS

MENARD BROWN CASS MACON

SANGAMON EDGAR MORGAN DOUGLAS

PIKE CHRISTIAN SCOTT MOULTRIE

COLES SHELBY

GREENE MACOUPIN

MONTGOMERY

CLARK CALHOUN

CUMBERLAND

JERSEY

FAYETTE

EFFINGHAM CRAWFORD

JASPER BOND

MADISON

CLAY LAWRENCE RICHLAND MARION

CLINTON

ST CLAIR

WAYNE

WABASH

EDWARDS

MONROE

WASHINGTON

JEFFERSON

WHITE

HAMILTON

RANDOLPH

PERRY

FRANKLIN JACKSON

GALLATIN WILLIAMSON SALINE

HARDIN

POPE UNION

JOHNSON

MASSAC ALEXANDER

PULASKI

Chicago

Springfield

Ohio

River

River

Kaskaskia

Mississippi

River

River

River

Fox

Illinois

Wabash

River

River

Kankakee

IOWA

MISSOURI

KENTUCKY

INDIANA

Lake

Michigan

0 25 50 75 100 MILES

0 50 100 KILOMETERS

Base from U.S. Geological Survey 1:24,000 and 1:100,000 Digital Data,

Albers Equal-Area Conic projection,

Standard parallels 33° and 45°, central meridian -89°

92° 91° 90° 89° 88°

42°

41°

40°

39°

38°

37°

EXPLANATION

Unconsolidated (sand and gravel)

 

Shallow bedrock (depth to top of 

 aquifer less than or equal to 500 feet)

Deep bedrock (depth to top of aquifer 

 greater than 500 feet)

Unglaciated area in Illinois

Illinois Episode

Wisconsin Episode

Well

Surface-water intake

Public-water-supply sample location

Principal source-water aquifers

Extent of glaciation

Chemistry of Hexavalent Chromium

Figure 1. Principal source-water aquifers and surface waters used for public supply, selected wells and intakes, 

and extent of episodic glaciation in Illinois (modified from O’Hearn and Schock, 1984, fig. 1, and Joseph Konczyk, 

Illinois Environmental Protection Agency, written commun., 2003).
Cr+3

CrOH+2

Cr(OH)3

0

CrO4

-2

HCrO4

-

Cr(OH)4

-

Redox condition

Anoxic Oxic

pH, in units

0 2 4 6 8 10 12 14

Hexavalent chromium [Cr(VI)]

Trivalent chromium [Cr(III)]

EXPLANATION

6Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

Figure 2. Relation of chromium speciation in water to oxidationreduction (redox) conditions and pH (modified from McNeill and 

others, 2013, fig. 2.2).

expected to predominate in waters with very limited oxygen, 

as characterized by DO concentrations less than about 2 mg/L 

and (or) redox potential below 0 millivolt (mv) in the pH 

range common to drinking waters (McNeill and others, 2011). 

Findings of the NCBOS study (Frey and others, 2004) 

indicate that Cr(III) typically is the principal source of chromium concentrations in the Nation’s drinking waters. Cr(III) 

was found to be the predominant chromium form in samples 

from surface-water sources but less prevalent in samples from 

groundwater sources. Often, Cr(VI) was the principal source 

of chromium concentrations in groundwater samples. Alkaline 

pH, which can be common to carbonate aquifers (Lindsey and 

others, 2008), possibly contributes to the more frequent occurrence of Cr(VI) in drinking waters from groundwater sources 

than surface-water sources. In Illinois, carbonate aquifers are 

the source of about 10–20 percent of the public water supply 

from aquifers and for tens of thousands of private well owners 

(U.S. Geological Survey, 2014a). These aquifers are predominantly in the northern part of the State (fig. 1).

The following discussion summarizes the detailed 

description of the chemistry of chromium by McNeill and others (2011), particularly in regards to its transformable behavior 

in treated-water distribution systems. Under changing conditions of water chemistry, Cr(III) can transform to the more 

soluble and toxic Cr(VI) form, as well as back to the Cr(III) 

form. The water chemistry, in part, can be affected by treatment additives and the material composition of the distribution infrastructure. Conventional treatment processes, such as 

filtering and chlorination, may be effective in removing Cr(III) 

from drinking water, but not Cr(VI) (U.S. Environmental 

Protection Agency, 2011).

Trivalent chromium can be nearly insoluble between 

pH 6–10, with minimum solubility at about pH 8. This near 

insolubility of Cr(III) within the range of pH that encompasses 

that representative of drinking water largely accounts for the 

predominance of Cr(VI) in many untreated (raw5

) and treated 

(finished) drinking waters. Dissolved or soluble Cr(III) in 

water can sorb to oxide surfaces, complex with natural organic 

matter, form precipitate solids, and (or) be fixed in solids. 

Although with its great tendency toward insolubility, the soluble fraction of Cr(III) can range up to 100 percent, depending 

on various factors including levels of organic matter, oxides 

such as Fe(OH)3, and pH, among others. Although Cr(VI) can 

undergo reactions similar to those for Cr(III), such as sorption or precipitation, there is a much greater tendency for it to 

remain soluble. Cr(VI) typically does not form precipitates at 

levels found in drinking water and does not bond strongly to 

natural organic matter. Therefore, generally present in drinking water as a soluble anion, its potential as a human toxin is 

much greater than that of Cr(III).

Virtually all transformations between Cr(III) and Cr(VI) 

associated with the collection, treatment, and distribution of 

drinking water are influenced by constituents that are either 

naturally present or purposefully added to the water. The various reactions that might occur and the rates of those reactions 

are well described in table 1. Upon entry to water-treatment 

and distribution systems, naturally present concentrations of 

Cr(VI) can be boosted rapidly (within a few minutes or more) 

with the transformation of any soluble Cr(III) after treatment, 

primarily from the addition of oxidants for disinfection. This 

transformation is problematic to facility operators, because 

even if Cr(VI) is fully removed from the incoming raw source 

water, it might reoccur. After treatment, Cr(VI) concentrations 

could be further boosted during distribution upon oxidant 

contact with plumbing fixtures that contain chromium. Studies have documented increases in chromium concentrations, 

including Cr(VI), from incoming untreated water or point of 

treatment to the endpoint of distribution; a 17-percent increase 

in chromium concentrations was documented in one study 

(Craun and McCabe, 1975) and as much as 10 μg/L in at least 

one sample in another (Frey and others, 2004). A reduction 

in concentration of 20 μg/L also was documented in the latter 

study (Frey and others, 2004).

As indicated from the study by Frey and others (2004), 

reduction reactions that transform Cr(VI) to Cr(III) also can be 

of great importance to the treatment and distribution process, 

because they can effectively limit Cr(VI) in drinking water. 

Such reactions often are manipulated intentionally, as the initial component of the treatment process. By this reaction, dissolved Cr(VI) that might be present can be converted to particulate or sorbed solids that can be settled or filtered for removal 

5

 In this study, all untreated water is considered raw water from an aquifer 

or surface-water source. Few, if any, potable waters for public consumption 

from facilities in Illinois, and particularly from the facilities included in this 

study, distribute untreated water.
Study Methods 7

Table 1. Potential reactions resulting in transformations of trivalent and hexavalent chromium.

[μg/L, microgram per liter. Modified from McNeill and others, 2011, table 1; p. 18–19 provides references to the various reaction 

dynamics]

Reaction Occurs in the presence of— Typical location or condition

Trivalent chromium [Cr(III)] oxidation to hexavalent chromium [Cr(VI)]

Fast (minutes to hours)

Manganese dioxide (MnO solids) 2 Oxygenated high pH groundwater

Chlorine, hydrogen peroxide (H O ), 2 2

potassium permanganate (KMnO )

4

Water treatment, distribution system

Slower (days to years)

Oxygen Groundwater, distribution system

Chloramine Distribution system

Cr(VI) reduction to Cr(III)

Fast (minutes to hours)

Ferrous iron (Fe+2), stannous chloride 

(SnCl ), sulfites 2

Lower-dissolved-oxygen groundwater; 

water treatment, distribution system

Slower (days to years)

Absence of dissolved oxygen, sulfides, 

numerous bacteria

Groundwater; potentially in iron mains and 

dead ends

Conversion of soluble Cr to particulate Cr

Fast (seconds to hours)

Water pH greater than 5 Possible whenever soluble Cr(III) 

is above about 1 μg/L

Iron or aluminum oxides Addition of coagulant 

from the water. Ferrous iron is commonly used to induce the 

reduction. Beyond intentional treatment, iron solids that might 

be present in distribution lines also can induce Cr(VI) reduction; such sources of iron can include, among others, hematite, 

magnetite, ilmenite, and green rust. Other potential reductants 

that might be included naturally in the untreated source water 

or as additives are certain reduced sulfur compounds, organic 

compounds, microbes, and stannous chloride. 

Several other factors further contribute to the complexity associated with Cr(VI) occurrence and transformation 

in drinking waters and with meaningful characterization 

of Cr(VI) occurrence and transformation for scientific and 

compliance-monitoring purposes. Chromium may be a trace 

contaminant in treatment additives including alum coagulants 

(Eyring and others, 2002) and possibly lime used for water 

softening (Missouri Department of Natural Resources, 2010). 

Other potential sources of chromium include leaching from 

materials used in treatment-plant and distribution components, 

such as cast iron, cement, stainless steel, and chrome-plated 

pipes and sample taps. 

Finally, a number of studies have documented that the 

results of inductively coupled plasma–mass spectrometry 

(ICP–MS) analysis for Cr(T) may not always relate well to 

those of ion chromatographic (IC) analysis for Cr(VI), with 

reported Cr(VI) concentrations sometimes greater than those 

of Cr(T) in the same water samples. For example, a study 

of more than 1,500 drinking-water samples found that for samples with concentrations of Cr(T) greater than 1 μg/L, more 

than half of the samples had greater concentrations of Cr(VI) 

than Cr(T). From that study, Eaton and others (2001) surmised 

that there might be differences in the behavior of Cr(III) and 

Cr(VI) in ICP–MS analysis or possibly problems with sample 

preservation. Other studies indicate iron may play a role in 

Cr(T) analysis, limiting full accounting of dissolved chromium 

in drinking-water samples (Parks and others, 2004). For full 

review of the chemistry of chromium, including Cr(VI), and 

its possible geochemical response in natural and treated-water 

distribution systems under varying conditions of pH, oxygenation, and other factors, the reader is referred to McNeill and 

others (2011) and Frey and others (2004).

Study Methods

The methods used for well, surface-water intake, and 

sample-tap selection; land-use determination; sample collection; laboratory analysis; quality assurance; and data analysis 

are described in the following sections of the report. Qualityassurance methods include onsite (sampling) and laboratory 

methods. 

Well, Intake, and Treated-Sample Site Selection

Untreated (raw) water samples in this study were collected from water-supply wells and surface-water intakes 

selected by stratified random methods. Separate methods were 
8

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

used for selection of groundwater and surface-water sample 

sites.

The initial 119 wells selected for sampling in this study 

represented a collection of wells which previously composed 

the IEPA Radon Network (Dave McMillan, Illinois Environmental Protection Agency, oral commun., 2001) and was used 

similarly in an assessment of herbicides and their transformation products in source aquifers for public water supply in 

Illinois (Mills and McMillan, 2004). The wells in the Radon 

Network were randomly selected from the larger IEPA Ambient Network of 362 wells compiled by a stratified random 

method (Illinois Environmental Protection Agency, 2011b). 

Criteria used for the stratification included aquifer type, 

geologic-based vulnerability to contamination, and well depth. 

The selected wells represent principal aquifers, as classified 

by O’Hearn and Schock (1984) (fig. 1); that is, the aquifers 

have a potential yield of 100,000 gallons per day per square 

mile (gal/d/mi2) or more and an area of at least 50 square miles 

(mi2). The wells tap the major types of source-water aquifers 

in Illinois, including unconsolidated glacial drift and alluvial 

aquifers and Cambrian- to Pennsylvanian-age bedrock aquifers 

(generally classified as the Pennsylvanian, Mississippian, Silurian or Silurian-Devonian aquifers and Cambrian-Ordovician 

aquifer system6) (fig. 3).

Among the wells in the Radon and Herbicide Network 

initially selected prior to 2001, 53 (45 percent) were no longer 

available for use in the present Cr(VI) study (well abandoned; 

well for emergency use only; facility no longer a public supplier) or were inappropriate for purposes of this study (pump 

cycle time insufficiently short; sample tap difficult to access 

or exposed to possible atmospheric contamination). Typically, replacement wells were selected from the pool of IEPA 

Ambient Network wells and matched as closely as possible 

the stratified-selection characteristics and location of the 

well being replaced. All replacement wells were selected by 

IEPA personnel. Given the large number of wells requiring 

replacement and the periodic necessity to select a replacement 

within a short timeframe, the wells were considered to reasonably match those they replaced. No replacement wells were 

selected with consideration of expected potential for detection 

of Cr(VI).

Fifty-six of the selected wells tap unconsolidated aquifers 

(well depths 28–404 ft), of which 22 are considered alluvial 

aquifers (well depths 36–145 ft) and the remainder glacial drift 

aquifers. Sixty three wells tap bedrock aquifers (well depths 

45–2,665 ft), of which 3 are open to the Pennsylvanian aquifer 

(well depths 215–280 ft), 3 are open to the Mississippian 

aquifer (well depths 70–300 ft), 24 are open to the SilurianDevonian or Silurian aquifer (45–555 ft), and 33 are open 

to the Cambrian-Ordovician aquifer (well depths 150–2,665 

ft). Alluvial aquifers consist of sand and gravel deposits of 

streams and rivers; these deposits in glacial drift aquifers 

result from glacial meltwater. The bedrock aquifers principally consist of carbonates (dolomite and limestone) and (or) 

6

 Hereafter referred to as the Cambrian-Ordovician aquifer.

sandstone. The 119 wells selected for sampling are listed in 

appendix 1, and their locations shown in figures 1 and 4.

The network of 32 surface-water intakes selected for 

sampling was compiled by a stratified random method established for this study. At the time of compilation (around 2010), 

there were approximately 150 actively used intakes (nonemergency intakes) at about 80 public water-supply facilities in 

Illinois. A statistically representative network of 32 intakes 

was selected from a population of about 111 “unique” intakes 

considered to be routinely used. The following selection 

guidelines were applied:

1. Multiple intakes of the same supply facility open to the 

same water body were grouped in a subset population 

where separated by a distance of less than 4 mi.

2. Multiple intakes of separate supply facilities open to the 

same water body were grouped in a subset population 

where separated by a distance of less than 2 mi.

3. Multiple intakes of the same supply facility open to 

similar, but separate water bodies (a lake and reservoir, 

for example), were grouped in a subset population where 

separated by a distance of less than 2 mi.

4. Intakes from the same facility or from separate facilities, and individually open to different types of water 

bodies (a river and lake, for example), were individually 

included in the selection population regardless of the 

distance separating the intakes.

5. Comparatively small side-channel reservoirs adjacent to 

streams with comparatively small watersheds were considered a river-type water body for application of intakedistance criteria (stream water is presumed to represent 

the bulk of the water contained in the reservoir).

6. Comparatively large side-channel reservoirs adjacent to 

streams or rivers with comparatively large watersheds 

were considered a lake-type water body for application 

of intake-distance criteria (spring discharge, surface 

runoff, and direct precipitation are presumed to represent 

an appreciable percentage of the water contained in the 

reservoir, in addition to the stream water). 

Regardless of satisfying applicable intake-distance criteria, not all Lake Michigan supply facilities were included in 

the selection population. There are a disproportionately large 

number of facilities (16) that withdraw from the lake, many 

with multiple intakes as necessary to serve nearly 3.5 million 

area residents (Timothy Bryant, Illinois State Water Survey, 

Illinois Water Inventory Program, written commun., 2010). 

To provide reasonable representation of the Lake Michigan 

facilities and the large population they serve, 10 spatially distributed facilities with 11 intakes (2 from the City of Chicago 

facility) were included in the selection population.
Quaternary

Tertiary and

Cretaceous

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

Precambrian

Pleistocene

Undifferentiated

Undifferentiated

Undifferentiated

Undifferentiated

Undifferentiated

Undifferentiated

Undifferentiated Undifferentiated

Valmeyeran

Kinderhookian

Niagaran

Alexandrian

Cincinnatian

Mohawkian

Chazyan

Canadian

St. Croixian

St. Louis Ls.

Salem Ls.

Warsaw Ls.

Keokuk Ls.

Burlington Ls.

Port Byron Fm.

Racine Fm.

Waukesha Ls.

Joliet Ls.

Kankakee Ls.

Edgewood Ls.

Maquoketa

Shale Group

Galena Group

 Decorah Sub-

 group

Platteville Group Ancell Gr Glenwood Fm.

St. Peter

Sandstone

Prairie du 

Chien GroupShakopee Dol.

New 

 Richmond Ss.

Oneota Dol.

Gunter Ss.

Jordan Ss.

Eminence Fm.-

Potosi Dol.

Franconia Fm.

Ironton Ss.

Galesville Ss.

Eau Claire Fm.

Mt. Simon Fm.

Cambrian-Ordovician aquifer Silurian-Devonian aquifer Mississippian aquifer

 Sand and gravel

(glacial drift and alluvial)

Aquifer or 

confining unit

Aquifer or 

confining unit

Aquifer or confining unit

Pennsylvanian

aquifer or

confining unit

St. Louis-Salem

aquifer

KeokukBurlington aquifer

Devonian aquifer 

or confining unit

Silurian aquifer

Maquoketa 

confining unit

Galena-Platteville

aquifer or 

confining unit

Ancell aquifer

Prairie du

Chien

Middle confining unit

EminencePotosi

Franconia

Ironton-Galesville

aquifer

Eau Claire

aquifer or

confining unit

Elmhurst-Mt. Simon

aquifer

Deep bedrock Shallow bedrock Uncon- solidated

Unconsolidated glacial deposits; pebbly

clay (till) silt, and gravel. Loess 

(windblown silt), and alluvial silts, sands

and gravels.

Sand and silt.

Mainly shale and thin sandstone,

limestone, and coal beds.

Limestone; cherty limestone; green,

brown, and black shale; silty dolomite.

Shale, calcareous; limestone beds, thin.

Dolomite, silty at base, locally cherty.

Shale, gray or brown; locally dolomite

and (or) limestone, argillaceous.

Dolomite and (or) limestone, cherty.

Dolomite, shale partings, speckled.

Dolomite and (or) limestone, cherty, sandy 

at base.

Sandstone, fine- and coarse-grained;

little dolomite; shale at top.

Sandstone, fine- to medium-grained;

locally cherty red shale at base.

Dolomite, sandy, cherty (oolitic), 

sandstone. Sandstone, interbedded with 

dolomite.

Dolomite, white to pink, coarse-grained,

cherty (oolitic), sandy at base.

Dolomite, white, fine-grained, geodic

quartz, sandy at base.

Dolomite, sandstone, and shale,

glauconitic, green to red, micaceous.

Sandstone, fine- to medium-grained,

well sorted, upper part dolomitic.

Shale and siltstone, dolomite, glauconitic;

sandstone, dolomitic, glauconitic.

Sandstone, coarse-grained, white, red in 

lower half; lenses of shale and siltstone, 

red, micaceous.

Granite and other crystalline rock.

Dol. = Dolomite

Fm. = Formation

Gr. = Group

Ls. = Limestone

Ss. = Sandstone

Aquifer absent

SYSTEM SERIES

LITHOSTRATIGRAPHIC

UNIT

HYDROSTRATIGRAPHIC

UNIT LOG

PRINCIPAL

AQUIFER

TYPE

DESCRIPTION

Study Methods

Figure 3. General classification of lithostratigraphic and hydrostratigraphic units that compose source-water aquifers in Illinois.

(Lithostratigraphic classifications follow the usage of the Illinois State Geological Survey (ISGS) (Visocky, 1997); hydrostratigraphic 

classifications follow the usage of the U.S. Geological Survey (2014), which in most cases conform to the usage of the ISGS; figure 

modified from Visocky, 1997, fig. 2.) 
JO DAVIESS

STEPHENSON

WINNEBAGO

MCHENRY LAKE

BOONE

OGLE CARROLL

COOK

KANE

DE KALB

DU PAGE

WHITESIDE

LEE

ROCK ISLAND WILL

KENDALL

LA SALLE

HENRY

BUREAU

GRUNDY MERCER PUTNAM

KANKAKEE

STARK

KNOX

MARSHALL LIVINGSTON HENDERSON

WARREN

IROQUOIS

FORD

PEORIA WOODFORD

MCLEAN

TAZEWELL

FULTON

HANCOCK

MCDONOUGH

VERMILION

MASON

CHAMPAIGN

LOGAN SCHUYLER DE WITT

PIATT

ADAMS

MENARD BROWN CASS MACON

SANGAMON

EDGAR MORGAN DOUGLAS

PIKE CHRISTIAN SCOTT MOULTRIE COLES

SHELBY

GREENE MACOUPIN

MONTGOMERY

CLARK CALHOUN

CUMBERLAND

JERSEY

FAYETTE

EFFINGHAM CRAWFORD

JASPER BOND

MADISON

CLAY RICHLAND LAWRENCE

MARION CLINTON

ST CLAIR

WAYNE

WABASH

MONROE

EDWARDS

WASHINGTON

JEFFERSON

WHITE

HAMILTON

RANDOLPH PERRY

FRANKLIN JACKSON

GALLATIN WILLIAMSON SALINE

HARDIN

POPE UNION

JOHNSON

MASSAC ALEXANDER

PULASKI

Ohio

River

River

Kaskaskia

Mississippi

River

River

River

Fox

Illinois

Wabash

River

River

Kankakee

Chicago

Springfield

IOWA

MISSOURI

KENTUCKY

INDIANA

Lake

Michigan

00757

47532

50224

52122

11904

11347

45174

01416

47687

11363

31853

45081

01463

71531

11367

00706

11318

20447

00351

11560

60065

50203

22040

01187

11557

11629

20442

20029

60058

01029

20328

11514

00727

20389

00155

20609

47542

45263

00546

11456

20458

60053

11782

31409

11470

11914

47518

47776

50376

11438

11327

31873

11640

01536

20297

50383

00337

20412

20613

50351

00622

11707

22077

20181

50051

60089

31820 20436

01143

11419

47592

11678

01079

47561

11508

11706

50094

00937

20019

11562

31428

47692

45155

20180

01537

31713

47501

20136

50138

47583

00395

22148

00766

01118

20141

50251

11891

47774

11621

31307

47811

00251

60127

01113

11570

52095

50003 01443

11778

52056

50060

50057

50308

00366

47819

11894

31745

01670

01357

70550

52100

60181

60173

00107

70290

47614

70620

52126

70010

52079

45170

60068

00341

70811

52140

22155

60184

01299

01851

52099

52097

60043

60049

47698

58088 01952

01305

58059

22082

20207

00757

70010

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0 25 50 75 100 MILES

0 50 100 KILOMETERS

Base from U.S. Geological Survey 1:24,000 and 1:100,000 Digital Data,

Albers Equal-Area Conic projection,

standard parallels 33o and 45o

, central meridian -89o

EXPLANATION

Well with identifier

Surface-water intake with identifier

Public-water-supply sample location

92° 91° 90° 89° 88°

42°

41°

40°

39°

38°

37°

10Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

Figure 4. Locations and designations of selected public-supply wells and intakes that tap source-water aquifers and 

surface waters in Illinois.
Study Methods

A single representative intake was selected randomly 

from the grouped subset populations for inclusion in the 

larger selection population of unique intakes. For this subset 

selection, a random number generator was used (Random.

org, 2012a). The population of unique intakes was stratified 

on the primary basis of region and age of glaciation (glacial 

provenance) and type of water body in which the intake was 

located. Glacial provenances were regions of Illinois Episode 

glaciation, Wisconsin Episode glaciation, and unglaciated 

regions (fig. 1). Types of water bodies were lakes, rivers, 

Lake Michigan, and the Mississippi River. These criteria 

were selected on the basis of the possible influence of terrestrial geology, spatial scale of contributions to the water-body 

chemistry, and water-body residence time. Subsequently, 

eight categories were selected for stratified random selection. 

These categories and the number of included intakes in each 

category are given in table 2. The number of intakes selected 

for sampling from each category was based on the percentage 

of the total selection population of intakes represented by the 

category, with this percentage applied to a total of 32 intakes 

to be sampled. One adjustment was made to accommodate 

the single river facility operated in an unglaciated region of 

the State (Ohio River; intake 70010) (fig. 1). This facility 

or category represents less than 1 percent of all intakes and 

thus was not statistically represented in the proportionalized 

accounting of 32 intakes for sampling. To include this facility 

or category, the number of intakes to be selected for the largest 

category listing (14 intakes; Illinois Episode glaciation–lake) 

was reduced by one intake. A random sequence generator 

then was used to select the representative intakes for sampling 

from an ordered listing of intakes (Random.org, 2012b). Each 

intake in a category was assigned a rank number up to the 

total number of intakes in the category. The final selection of 

intakes for sampling was determined by the sequential order of 

the randomly shuffled rankings. 

Periodically a selected intake was determined to be 

unavailable or inappropriate for sampling. In such instances, 

the next available sequentially ordered intake in the same 

stratification category served as a replacement for sampling. 

The 32 intakes selected for sampling are listed in appendix 1, 

and their locations shown in figures 1 and 4.

Inspection of the spatial distribution of the randomly 

selected wells and surface-water intakes (fig. 1) indicates that 

some parts of the State are not well represented in this assessment of Cr(VI) in the source waters for public supply. Of particular note, there is a relative absence of wells in the southern 

half of the State and intakes in the northwestern part. In actuality, the random selection provided a reasonable distribution 

of wells and intakes in the parts of the State in which groundwater and surface-water supplies presently are developed. 

In the southern half of the State, relatively few bedrock and 

unconsolidated aquifers (particularly glacial drift aquifers) are 

present. In the northern half, both bedrock and unconsolidated 

aquifers are generally prevalent and serve as the predominant 

source for public supply. In future years, should increasing 

demand for public water lead to development of the available 

but underused, surface-water sources, additional assessment of 

Cr(VI) in these surface-water sources might be warranted.

Treated water samples were collected from (1) a tap close 

to the point of treatment and (2) a tap at or near the endpoint 

of distribution for the public-supply facility. The untreated 

water and near-treatment tap locations were dictated by facility design. The distribution tap location was one at which 

regulatory samples routinely are collected under the USEPA 

Disinfectants and Disinfection Byproducts Rule (Stage 1 and 

2) (U.S. Environmental Protection Agency, 2014). At a few 

locations, the sampling conditions were not conducive to the 

Table 2. Criteria and related data used for the stratified random selection of public-supply intakes for sampling and analysis of 

chromium in Illinois surface waters, 2013.

Stratification category Number of intakes 

in category

Percentage of 

total intakes

Proportional number of 

intakes per category 

for 32 total intakes

Final number of 

intakes per category 

for sampling

Illinois Episode glaciation–lake 47 42.3 14 13

Illinois Episode glaciation–river 18 16.2 5 5

Illinois Episode glaciation–

Mississippi River

10 9.0 3 3

Wisconsin Episode glaciation–lake 10 9.0 3 3

Wisconsin Episode glaciation–river 9 8.1 3 3

Wisconsin Episode glaciation–

Lake Michigan

11 9.9 3 3

Unglaciated–lake 5 4.5 1 1

Unglaciated–Ohio River 1 0.9 0 1
12

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illino

study’s sampling method. For these infrequent instances, a 

nearby alternative sample location was selected. Final selection of sample taps for treated water samples was made jointly 

by personnel of the IEPA and water-treatment facilities.

Sample Collection and Laboratory Analyses

Untreated (raw), treated tap, and distribution water 

samples were collected by IEPA personnel in accordance 

with standard presampling protocol of the Illinois IEPA and 

prescribed methods of analysis for Cr(VI) (USEPA Method 

218.6) and Cr(T) (USEPA Method 200.8) (appendix 2). The 

IEPA protocol, which generally conforms to that of the USGS 

(U.S. Geological Survey, variously dated), is detailed in the 

IEPA Quality Assurance Project Plan (QAPP). For inspection 

of the QAPP, contact the IEPA, Bureau of Water, Division of 

Public Water Supplies. 

IEPA personnel also collected quality-assurance duplicate 

samples and equipment blanks. USGS personnel prepared and 

collected quality-assurance spike samples. Details of collection protocols for quality-assurance samples are discussed in 

the following report section, “Quality Assurance.”

Prior to collection of each sample, water was purged 

from the wells and intakes in a manner to ensure that the 

samples of untreated source water (groundwater and surface 

water) would be representative of source-water quality. Wells 

and intakes typically were operated for 30 minutes or longer 

prior to site arrival by a sampler for collection of untreated 

water samples. Upon arrival, wells and intakes were purged 

through a raw-water tap (before chlorination and other treatment) for an additional 15 minutes or longer to further remove 

water stored in the well casing and (or) delivery piping and to 

condition (flush) the sample tap. By use of a calibrated waterquality meter with a flowthrough cell attached to the sample 

tap, the values of pH, water temperature, and specific conductance were monitored for stabilization of these field-based 

characteristics of water-quality (hereafter referred to as “field 

parameters”) (fig. 5A). The stabilized field-parameter values 

ensure the collection of samples representative of source-water 

quality. Concentrations of DO were not monitored because the 

water-quality meters used by the IEPA personnel responsible 

for public-water-supply data collection do not contain DO 

sensors. A sensor for measurement of redox potential (or Eh, 

in millivolts) is included in the water-quality meters used by 

IEPA to determine if conditions are stabilized prior to sampling; however, the sensor is not calibrated prior to use, as its 

restricted use limited to monitoring relative field-parameter 

values for stability satisfies the agency’s programmatic objectives. These redox potential data, which are not archived in the 

IEPA water-quality database, were of insufficient accuracy for 

the evaluation of Cr(VI) occurrence in this study. 

For collection of treated water samples, water was 

purged from the delivery piping for a minimum of 10 minutes, 

primarily to condition the sample tap. In many cases, delivery 

piping to the tap located soon after treatment was purged for 

a minimum of 15 minutes because these taps were generally close to the raw-water taps. Field parameters were not 

monitored for stabilization prior to collection of treated water 

samples; however, for about 60 percent of these samples a 

synoptic assessment of field values was made after a few minutes of purging to ensure instrument acclimation to the purgedwater stream. These field values along with those obtained 

during purging for collection of untreated water samples, 

allowed limited consideration of possible relations between 

water chemistry and Cr(VI) concentrations in untreated, 

treated, and distributed waters.

Observations regarding site conditions that might affect 

the sample results and data recorded prior to sampling to 

ensure the collection of representative samples are documented by sampling personnel in an IEPA field sheet and UL 

company (formerly known as Underwriters Laboratories) 

chain-of-custody form. Examples of an IEPA field sheet and 

UL form are presented in appendixes 3 and 4. Among other 

observations, recorded information and data include pump run 

time prior to sampling, location of raw-water tap relative to 

the well or intake, sample-tap material composition, and the 

sequential listing of field parameters during purging. 

Samples for Cr(VI) and Cr(T) analysis were collected 

by using equipment included in sampling kits provided by 

UL. The kits consisted of a collection beaker, sample bottles, 

a syringe and 45-μm syringe filter, buffer solution, and blue 

refrigerant (ice) blocks. The kits were contained in a foaminsulated shipping box. Sampling protocols were developed 

and provided by UL, but slightly modified by the USGS. The 

UL protocols were developed to conform to the sampling and 

analysis requirements of USEPA Method 218.6 for Cr(VI) and 

USEPA Method 208.6 for Cr(T). These USEPA requirements 

are described in some detail at the conclusion of this section 

of the report. Specific details regarding sample collection are 

given in appendixes 2A [Cr(VI)] and 2B [Cr(T)]. The modified 

protocols consisted of adding additional progressive prerinsing 

of each sampling-equipment component with native sample 

water (untreated or treated, depending on the sample type) 

(fig. 5B). The repeated prerinsing used only water in contact 

with a previously prerinsed component in the sequence of its 

use, with the starting and endpoints a prerinsed sample-collection beaker and sample bottle, respectively. 

At all sample sites, efforts were made to limit the 

potential of sample contamination, including use of protective gloves and placement of all sample-collection equipment on an unused sheet of aluminum foil during sampling. 

Where possible, and necessary, an effort was made to prevent 

atmospheric contamination during sampling. Efforts included 

sampling indoors and closing outside doors if they connected 

to the sampling space. 

Sampling equipment was disposable (single use) and 

constructed of polypropylene. Samples were collected from 

sample taps at a flow rate of about 0.25–0.5 gallon per minute 

(gal/min). Untreated source waters for Cr(VI) analysis were 

collected in a nominal 100 milliliter (mL) beaker, passed 

through a 0.45-micrometer (μm) glass-microfiber syringe 
filter, and discharged into a prerinsed 125-mL sample container (fig. 5C). Samples of untreated water to be analyzed for 

Cr(T) and treated water to be analyzed for Cr(VI) and Cr(T) 

were collected directly from the sample tap into a 125-mL 

sample container. An ammonium sulfate and ammonium 

hydroxide buffering solution (1 mL) was then added to the 

untreated and treated water samples for Cr(VI) analysis. The 

sample volume for Cr(VI) analysis was 100 mL, with the 

sample volume for Cr(T) 100–125 mL.

About 95 percent of the wells and sample taps were 

enclosed in a well house, thus reducing the possibility of 

atmospheric contamination of samples and technical issues 

with sampling. Those samples collected outside were collected 

when weather conditions should not have affected collection 

of representative samples. The samples were shipped chilled 

(4 degrees Celsius [oC] or less) to UL the evening of the collection. Sample analysis was targeted for completion within 24 

hours of collection.

Laboratory-grade 

inorganic blank water

Injection of Cr(VI) spike 

into inorganic blank water

A

B D

C

Study Methods 13

Figure 5. Photos showing typical quality-assurance measures and collection of samples for chromium at selected public-water-supply 

facilities in Illinois, 2013: A, Monitoring of field water-quality characteristics (pH, temperature, specific conductance) for stabilization of 

values prior to collection of surface-water samples, City of Springfield (intake 52140), water-treatment laboratory. 

B, Rinsing sample equipment prior to collection of surface-water samples, City of Springfield, water-treatment laboratory. 

C, Collection of groundwater samples, Union-York Water District (intake 00251), water-treatment facility (garage). 

D, Preparation of a spike sample at a known concentration of hexavalent chromium [Cr(VI)], U.S. Geological Survey laboratory 

(See fig. 1 for location of facilities).
14

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illino

The following descriptions of USEPA Method 218.6 for 

Cr(VI) analysis and USEPA Method 200.8 for Cr(T) analysis 

are summarized from more detailed descriptions by McNeill 

and others (2011). The specific collection and analysis methods for Cr(VI) and Cr(T) used in this study are then discussed 

within the context of the evolving requirements, recommendations, and analytical capabilities of the described USEPA 

methods.

Method 218.6 combines a colorimetric (C) method 

used for decades with an IC method. By this enhanced C–IC 

method, Cr(VI) is separated from the sample matrix to minimize analytic interference and is concentrated to reduce the 

detection level. In its original development, filtering of water 

samples (at 0.45 μm) was required, as was the dropwise addition of a buffer solution of ammonium sulfate (250 millimoles 

[mM]) and ammonium hydroxide (100 mM) to bring the sampled to pH of 9–9.5. Within this pH range, Cr(VI) occurs as 

chromate (CrO4

-2), which is separated on an analytical column 

from other anions. The separation minimizes interferences 

with other anions that might affect the detection and assessed 

concentration of Cr(VI). With the addition of diphenylcarbazide (eluent) by use of a postcolumn mixing coil, a complex is 

formed which is measured with a flowthrough spectrometer. 

As listed, the reporting level of this method was 0.4 μg/L in 

drinking water and groundwater. This method is described in 

detail in U.S. Environmental Protection Agency (1994) and 

Dionex (1998).

The above-described method subsequently was modified to further lower the detection level. Method improvements included lowering the flow rates for the eluent and the 

postcolumn reagent, increasing the volume of sample injection 

and the size of the reaction coil, and lowering the concentration of the ammonium sulfate buffer solution by a factor of 10. 

The resulting reporting limit of this modified method was 0.06 

μg/L. The modifications are described in full detail in Dionex 

(2003). The method was once again modified, with adjustments in flow rates and column configuration. As a result, the 

reporting limit was further reduced to 0.019 μg/L (Dionex, 

2011). It is this modification and reporting limit that is the 

basis for the analytical method and reporting (0.02 μg/L) used 

by UL for this study. This and former versions of the method 

are not presently approved for compliance with the SDWA, 

because Cr(VI) is unregulated as a specific chemical constituent of drinking water under the guidelines of the SWDA 

(however, it is regulated as a component of Cr(T), at an MCL 

of 100 μg/L).

In Method 218.6, contamination, matrix interferences, 

and redox reactions of chromium are included as potential 

interferences to analysis. Contamination may be associated with reagents and glassware. Specific requirements are 

included for the cleaning of glassware and monitoring for 

reagent contamination. Because of the redox sensitivity of 

Cr(VI), it is essential to preserve the oxidation state of the 

chromium in a water sample at the time of its collection. It is 

also critical to properly buffer the samples to isolate Cr(VI) as 

chromate, thus limiting the potential of chemical interference 

during analysis from other anionic species of chromium. At 

pH 9, the oxidizing potential of Cr(VI) is too low to react with 

any reductants in the sample, and the ammonium in the buffer 

inhibits the oxidizing potential of chlorine. Although all versions of Method 218.6 require dropwise addition of buffer to 

obtain a pH of 9–9.5, a recent drinking-water specific update 

of the method—USEPA Method 218.7 (U.S. Environmental 

Protection Agency, 2011)—recommends adjustment to greater 

than pH 8 by using 1 mL of the buffer added to 100 mL of 

sample. The UL sampling protocol used in this study incorporates this approach to buffering.

In Method 218.6, 0.45-μm filtration of samples is 

required prior to preservation; however, current manufacturer 

protocol (Dionex, 2011) and guidelines of Method 218.7 

do not require field or preinjection filtering of samples. The 

UL sampling protocol used in this study incorporates the 

Method 218.6 approach to filtering. There are various conflicting requirements for sample holding time prior to analysis. 

Method 218.6 limits holding time to 24 hours; Federal guidelines (40 CFR 136) allow samples to be held up to 28 days, 

if preserved; near the start of planning for this study, USEPA 

allowed a holding time of 5 days (U.S. Environmental Protection Agency, 2011) for voluntary Cr(VI) monitoring; and 

Method 218.7, whereas recommending samples be analyzed as 

soon as possible, cites a holding time of 14 days for properly 

collected, preserved, shipped, and stored samples (U.S. Environmental Protection Agency, 2011). For this study, the most 

restrictive holding time of 24 hours was applied.

Method 200.8 for Cr(T) analysis using ICP–MS is 

approved for monitoring drinking-water quality under the 

SDWA, with an MDL of 0.08 μg/L (McNeill and others, 

2011). The MDLs of other approved methods for SDWA monitoring range up to 4 μg/L. For this study, a reporting level of 

0.1 μg/L was applied. Although the USEPA method designates 

a holding time of 6 months with preservation to below a pH 

2 with HNO3 acidification (Federal guidelines 40 CFR 136), 

the UL sampling protocol followed in this study differed, and 

samples were preserved only by chilling and typically were 

analyzed within 24 hours of collection.

Quality Assurance

The quality-assurance program for the data collected during this study focused on three components: training, sample 

collection and processing procedures, and laboratory analytical methods. The requirements of each of these components 

are described below. The results of the quality-assurance 

testing are presented in the following section “Accuracy of 

Chromium Data.” 

Two measures were taken to ensure samplers were 

fully aware of the sample-collection and handling and shipping requirements of the study: training held prior to the 

sampling effort and observation of the in-field performance of 

samplers. For the presampling training, the sample-collection 

methods detailed in appendix 2 were discussed as were the 
Study Methods

methods for collecting quality-assurance samples and packaging the chilled samples for overnight shipment and followingday analysis. Additionally, the IEPA QAPP, which summarizes 

all sample-collection and analysis aspects of the effort, was 

discussed. Each participating IEPA sampler then demonstrated 

the collection procedures for Cr(VI) and Cr(T) samples. After 

the sampling effort began, the USGS project chief observed 

each sampler in collection of samples at one or more sampling 

sites, offering or accepting and incorporating any necessary 

suggestions for representative data collection and handling.

Quality-assurance methods used during sample collection are described in the previous section “Sample Collection 

and Laboratory Analysis” and in the “National Field Manual 

for the Collection of Water-Quality Data” (U.S. Geological 

Survey, variously dated). Other onsite methods consisted of 

collecting equipment (or field) blanks and sample duplicates 

for analysis. A constraining factor in the selection of qualityassurance sample number and specific sites was the laboratory’s limit on accepting no more than 10 samples per day 

for analysis in order to satisfy the 24-hour criterion for their 

completed analysis. 

Five equipment blanks were collected (2 percent of field 

samples). A deliberative process was used to select blanksample sites to ensure the following considerations:

1. a blank sample was collected by each of the five samplers participating in the study, 

2. sample sites generally were distributed evenly throughout the 1-year time frame of the study,

3. sample sites did not overlap those with other qualityassurance samples collected, and 

4. sample sites represented the proportionally greater number of groundwater sites than surface-water sites. 

Additionally, the temporal distribution of sites allowed 

for at least two samples to be collected near the beginning of 

the study to provide early documentation of effective sampling 

methods (or recognition of any problems).

Each equipment blank was collected after prerinsing the 

disposable sampling and filtering equipment with laboratorycertified inorganic blank water. The blank water was not 

certified as free of Cr(VI); heavy metals (unspecified) were 

certified as below a concentration of 10 μg/L. The blanks were 

collected as if routine field samples with the inorganic blank 

water used for the samples. Analytical results of equipmentblank samples are used to evaluate the chromium-free nature 

of the disposable sampling equipment, effectiveness of routine 

equipment prerinse methods, and possibility of atmospheric 

contamination of the sampling equipment.

Ten duplicates were collected (5 percent of field samples). Similar considerations as those for equipment-blank 

sample sites were applied for selection of the duplicate 

sample sites. Additionally, each of the sample types (untreated 

groundwater, untreated surface water, treated surface water, 

distributed surface water) was represented in approximate 

proportion to its number of sample sites. Each duplicate 

sample was collected after its associated routinely scheduled field sample was collected, and a new set of disposable 

sampling equipment was used for the duplicate. Thus, the 

duplicate-sample site was treated as if it had not been visited 

previously. Analytical results of duplicates are used to evaluate the random variation of sampling. Differences in paired 

quality-assurance samples (duplicates) were evaluated by the 

relative percent difference (RPD) method (U.S. Environmental 

Protection Agency, 1989). For this method,

 (1)

where

C1 is the concentration of the quality-assurance 

sample, and 

C2 is the concentration of the field sample. 

Elevated RPDs may be indicative of inconsistent precision in analytical analysis of concentrations (Duwelius and 

others, 1996); however, laboratory spike samples provide a 

more definitive indicator of issues of laboratory precision. The 

RPD method also was used to quantify differences between 

Cr(VI) and Cr(T) results in a sample for those samples in 

which the reported concentration of Cr(T) was less than that of 

Cr(VI). For this application, C1 represented the Cr(T) concentration and C2 represented the Cr(VI) concentration.

Laboratory quality-assurance samples are used for evaluation of laboratory analytical methods, which include, among 

others, guidelines for sample holding times, quantitation 

reporting limits, instrument calibration, sample preparation, 

and sample analysis. Laboratory quality-assurance methods 

consisted of spikes prepared in the field and the laboratory and 

duplicates. Spikes are samples prepared to a known concentration and with minimal change to the matrix of the original 

sample. Specific methods for selecting and preparing laboratory quality-assurance samples can be obtained from the UL 

laboratory, on request. 

Field spikes were prepared by using a commercially 

provided Cr(VI) reagent with a concentration of 1,000 μg/L. 

A reagent at a lower, more appropriate concentration for 

expected sample concentrations of about 0.02–10 μg/L was 

not readily available. The USGS National Water Quality Laboratory, which generally prepares spikes of various analytes 

for USGS projects, could not do so for this project because 

of concerns about laboratory contamination by the elevated 

reagent concentration that was available. Spikes were prepared 

in the USGS Illinois Water Science Center laboratory in various ranges representing the expected sample concentrations. 

Accurate reagent dilution of 3–5 orders of magnitude to obtain 

representative sample concentrations was difficult with the 

available laboratory equipment. As such, the spikes prepared 

for the study are considered more as qualitative than quantitative indicators of the accuracy of sample analysis by the 

laboratory. The laboratory quality-assurance checks, including 

RPD

C C

C C = ( ) −

( ) +

1 2 100

1 2 2

x percent

/
16

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

their internally used spikes, are considered the primary indicator of the accuracy of their analytical results. 

Spike samples were prepared by USGS staff, with five 

samples (2 percent of field samples) analyzed for Cr(VI) and 

four samples (2 percent of field samples) analyzed for Cr(T). 

The preparations were made within 24 hours of their analysis 

on the following day. The samples included two at a presumed 

concentration of 0.0 μg/L and one each at 0.03, 0.98, and 3.90 

μg/L (with no 0.03-μg/L spike for Cr(T) analysis). Laboratorycertified inorganic blank water was used for the 0.0-μg/L 

spikes and in the preparation of spikes of greater concentration. The certified commercially provided reagent was used 

for Cr(VI) spikes with concentrations greater than 0.0 μg/L. 

As previously noted in the discussion of equipment blanks, the 

inorganic blank water was not specifically certified to be free 

of Cr(VI) below the concentration certification limit. Therefore, the remote possibility remains that Cr(VI) was present in 

the blank water at concentrations in the range of 0.02–10 μg/L. 

Data Analysis

The relational geochemical, physical, and sampling factors were analyzed by inspection of the analytical results for 

Cr(VI) and Cr(T) and associated data. Inspection included 

graphical and statistical measures. Analysis of geochemical factors was limited to consideration of pH. As previously 

noted, other geochemical data either were not available or 

appropriate for analysis or their analysis was beyond the scope 

of this study. Physical factors that were evaluated include 

aspects of land use, geohydrology, well construction, and 

water treatment. Sampling factors include aspects of sample 

shipment and processing and the material composition of 

water-distribution components. 

The nonparametic Spearman’s rank correlation test (Helsel and Hirsch, 1992) was used to determine the strength of 

association or covariance between Cr(VI) and Cr(T) concentrations; that is, for example, do Cr(T) concentrations tend to 

increase or decrease as Cr(VI) concentrations increase, or are 

their patterns of variation unrelated? This relational test also 

was used to determine, among others, the correlation between 

(1) Cr(VI) concentrations and pH, and (2) frequency of detection (representing occurrence) of Cr(VI) and depth-related 

factors, including depths to the top of aquifer and to the base 

of the well casing.

The Spearman’s rank test method uses ranks of data to 

determine a monotonic relation (a relation in one direction, 

but not necessarily linear nor causal) between two continuous 

variables. The statistics produced are the Spearman’s rho (ρ) 

and the p-value. The values of Spearman’s rho range from –1 

to +1, where larger absolute values indicate a stronger relation 

between the variables and ρ = 0 is indicative of no association 

(correlation) between the variables. A positive value of Spearman’s rho indicates a direct relation, where the value of one 

variable tends to increase as the value of the other increases. 

A negative value indicates an inverse relation, where the value 

of one variable tends to decrease as the value of the other 

increases. For statistical testing of the depth-related factors 

associated with frequency of detection, rank values were 

assigned to represent selected depth ranges. Ranks of 1 to 6 

were used to represent the following intervals of increasing 

depth: 0–50, 51–100, 101–200, 201–300, 301–500, and greater 

than 500 ft. For those correlations determined to be statistically significant, the corresponding p-values are included with 

the results.

The difference in concentration of Cr(VI) between 

selected sample populations were evaluated by using the 

Kolmogorov-Smirnov test (Chakravarti and others, 1967; 

College of St. Benedict and St. John’s University, 2014). 

This is a nonparametric test for the equality of continuous, 

one-dimensional probability distributions that can be used 

to determine the degree of similarity or difference between 

two variables (datasets). Cumulative distribution functions 

showing the distribution of a variable as a function of the rank 

order or exceedance probability are developed. The statistics 

produced are the maximum vertical difference between the 

cumulative distributions (plotted curves), reported as “D,” and 

the p-value. For those compared populations determined to be 

significantly different, the corresponding p-values are included 

with the results.

A confidence criterion (or significance level) of α = 

0.05 was used to determine the significance of statistical tests 

reported in this study. The significance level is the probability 

(p-value) of incorrectly rejecting the null hypothesis. The null 

hypothesis of a statistical test (for example, there is no difference between two groups of data) is rejected if the p-value is 

less than or equal to the α value of 0.05. A p-value less than or 

equal to 0.05 means there is a 1 in 20 or less chance of getting 

the observed results (that the two groups of data are different) 

if the null hypothesis is true. The smaller the p-value, the less 

likely the observed test statistic may have occurred by random 

chance when the null hypothesis is true, and the stronger 

the evidence for rejection of the null hypothesis (Helsel and 

Hirsch, 1992). Unless where indicated otherwise, p-values 

were greater than 0.05 for applied tests.

In this report, concentrations of Cr(VI) and Cr(T) 

determined to be less than their reporting limit are censored 

to (assigned) 0.0 μg/L. A null concentration was assigned to 

nondetections in this report to readily indicate the general proportion of nondetections in sample populations, particularly 

where summarized results that represent all concentrations and 

detected concentrations are compared (table 3) or are presented in figures such as box-and-whisker plots. All concentrations represent those which are equal to or greater than the 

analytical reporting limit, as well as those which are less than 

the reporting limit. Detected concentrations represent those 

which are equal to or greater than the reporting limit. Data 

summaries in this report that represent detected concentrations 

are specifically identified. 

Presenting statistically summarized results on the basis 

of detected concentrations is considered useful because the 

study was conducted to addresses concerns about the possible 
Accuracy of Chromium Data

Table 3. Summary statistics for detections of hexavalent chromium and total chromium in samples from source-water aquifers and 

surface waters used for public supply in Illinois, 2013.

[μg/L, microgram per liter; detected (all), samples with detected concentrations (all samples in category, concentrations designated as below the reporting limit 

censored to 0.0 μg/L); do., ditto. Reporting limit for hexavalent chromium, 0.02 μg/L; for total chromium, 0.1 μg/L. Maximum Contaminant Level for total 

chromium, 100 μg/L (U.S. Environmental Protection Agency, 2013b)]

Chromium analyte,

untreated (raw) 

water samples

Total number

of samples

in category

Detection

frequency,

in percent

Median

concentration,

in μg/L

[detected (all)]

Mean

concentration,

in μg/L

[detected (all)]

Maxiumum

detected

concentration,

in μg/L

Hexavalent chromium: all 151 64.9 0.07 (0.04) 0.14 (0.09) 2.1

Total chromium: all do. 71.5 0.40 (0.30) 0.51 (0.36) 1.8

Hexavalent chromium: 

groundwater

119 57.1 0.06 (0.02) 0.16 (0.09) 2.1

Total chromium: 

groundwater

do. 64.7 0.40 (0.20) 0.51 (0.33) 1.8

Hexavalent chromium: 

surface water

32 93.8 0.09 (0.09) 0.11 (0.10) 0.29

Total chromium: 

surface water

do. 96.9 0.40 (0.40) 0.49 (0.48) 1.8

Hexavalent chromium:

unconsolidated aquifer

56 42.9 0.14 (0.00) 0.27 (0.11) 2.1

Hexavalent chromium: 

bedrock aquifer

63 69.8 0.04 (0.03) 0.10 (0.07) 1.1

Hexavalent chromium: 

surface water (tap1

); 

no lime treatment

17 94.1 0.06 (0.06) 0.09 (0.08) 0.25

Hexavalent chromium: 

surface water (tap1

); 

lime treatment

15 100 1.2 (1.2) 1.2 (1.2) 2.4

1

 Treated water samples.

impact of elevated concentrations of Cr(VI) on public health 

and management of public drinking-water operations in Illinois. Use of the reporting limit of 0.02 μg/L for detected concentrations as a reference level represents the level presumed 

to be the lowest possible regulatory level (MCL) that might be 

established for Cr(VI). Estimation of the concentrations that 

might be less than the reporting limit and their distribution 

did not address the objectives of this study and was beyond 

its scope. For estimation of these left-censored data, statistical 

methods are considered most appropriate (Helsel, 2005) and 

could be applied by others.

Accuracy of Chromium Data

The quality-assurance tests of this study reflect the accuracy and representativeness of the obtained chromium data. 

These data could potentially have been adversely affected during sample collection and processing and laboratory analysis; 

these tests address the uncertainties associated with collection 

of samples and their analysis at the comparatively low detection and reporting levels of this study.

Cr(T) was not detected in any of the equipment blanks, 

a result that might be partly attributable to its comparatively 

greater analytical reporting limit of 0.1 μg/L. Cr(VI) was 

detected at concentrations of 0.02, 0.03, 0.05, and 0.21 μg/L 

in four of the blank samples and was undetected in the fifth 

sample. Water from the same preparation lot was used for 

the samples with concentrations of 0.02 μg/L and 0.05 μg/L; 

water from a separate lot was used for the remaining blank 

samples. It is uncertain whether the source of the Cr(VI) 

was the disposable sampling equipment, the blank water, 

or atmospheric contamination or whether the detections are 

an indication of the accuracy or difficulty of analyzing for 

this constituent at the low reporting level of 0.02 μg/L. The 

elevated concentration of 0.21 μg/L in one sample appears 

not to be related to the Cr contamination of the blank source 

water, because the concentrations of other blank samples 

using the same source lot were substantially lower (less than 
18

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illino

0.02–0.03 μg/L). Laboratory analytical issues likely account 

for the elevated concentration, given that the lab-reported 

Cr(VI) recoveries in the matrix spike (113 percent) and matrix 

duplicate spike (112 percent) samples were outside the acceptable limits of 90–110 percent. 

Blank-water samples that were collected as 0.0-μg/L 

Cr(VI) spike samples, in which there was no water contact 

with the sampling equipment other than the sample bottle, 

indicate possible difficulty in analyzing for this constituent at 

the low reporting level might account for the low-concentration detections in the various blank-water samples. The two 

0.0-μg/L spike samples, prepared from the same source lot, 

had reported concentrations of less than 0.02 μg/L and 0.04 

μg/L, respectively. In general, the method used to prerinse 

the disposable sampling equipment and use of the equipment 

itself were determined to contribute only minimally, at most, 

to the analytical results of the study. Most likely, prerinsing of 

sample equipment with the untreated sample water effectively 

conditioned the equipment for collection of representative 

samples.

Of 10 paired field and duplicate samples, 6 had detectable 

concentrations of Cr(VI); all 10 paired samples had detectable 

concentrations of Cr(T) in one (2 pairs) or both (8 pairs) of 

the paired samples. RPDs for the detectable concentrations of 

Cr(VI) in the paired samples ranged from –22 to +22; RPDs 

for the detectable concentrations of Cr(T) ranged from –155 

to +857. The RPDs are exaggerated by the low detectable 

concentrations of Cr(VI) (0.04–0.41 μg/L) and Cr(T) (0.1–1.0 

μg/L). When detected, the difference in Cr(VI) concentrations 

between paired samples was 0.01 μg/L or less for all pairs but 

one (+0.03 μg/L, duplicate); the difference in Cr(T) concentrations between paired samples was 0.1 μg/L or less for all 

pairs but two (–0.03, +0.04 μg/L, duplicates). The concentration discrepancy in these two samples is unexplained. For the 

most part, the measured differences in concentrations of paired 

samples were at or near the apparent variance of the analytical methods, as indicated by review of the laboratory quality 

assurance test reports (appendix 4). With minimal random 

variation associated with sample collection, the analytical 

results were determined to accurately represent the groundwater quality at the sample sites.

RPDs for the detectable concentrations of Cr(VI) in 

spikes ranged from –55 to +200; RPDs for the detectable 

concentrations of Cr(T) in spikes ranged from –12 to +5. The 

RPDs are exaggerated by the low Cr(VI) and Cr(T) concentrations [0.03–3.90 μg/L, Cr(VI); 0.98–3.90 μg/L, Cr(T)]. The 

analytical results of the two 0.0-μg/L Cr(VI) spikes indicated 

one at a concentration less than 0.02 μg/L and one at 0.04 

μg/L; respective results of the spikes at higher Cr(VI) concentration were less than 0.02, 1.0, and 4.0 μg/L. The analytical 

results of the two 0.0-μg/L Cr(T) spikes indicated both at a 

concentration less than 0.1 μg/L; respective results of the 

spikes at higher Cr(T) concentration were 1.1 μg/L and 4.1 

μg/L. The minor difference between the intended concentration of the 0.03-μg/L Cr(VI) spike and its analytical result of 

less than 0.02 μg/L is best explained by inaccuracies associated with spike preparation. The resulting Cr(VI) concentration of 0.04 μg/L is unexplained, but it does not seem to be 

associated with low concentrations of Cr(VI) in the inorganic 

blank water. Two other spikes and one equipment blank prepared from the same source lot of blank water at intended concentrations of 0.0–0.03 μg/L tested with a concentration less 

than 0.02 μg/L. The elevated concentration of the spike could 

be related to atmospheric contamination or analytical inaccuracy. Sample contamination seems unlikely, given the clean 

condition of the well-facility laboratory where the sample 

was collected. For the most part, the measured differences in 

concentrations of paired samples were at or near the apparent 

variance of the analytical methods, as indicated by review of 

the laboratory quality-assurance test reports (appendix 4). 

The results of the spike analyses, along with those of the 

equipment blanks and duplicates, indicate possible difficulties in obtaining or confirming consistently accurate analytical 

results for Cr(VI) at near the applied reporting limit of 0.02 

μg/L. Interferences might be associated with contamination 

introduced atmospherically or during sample collection and 

processing, but they might also perhaps be associated with 

laboratory analytical capability.

Comparison of the analytical results for Cr(VI) and Cr(T) 

found that for 5 percent of the untreated and treated samples, 

the concentration of Cr(VI) was greater than the concentration of Cr(T). Cr(VI) concentrations greater than those of 

Cr(T) should not be expected; however, this kind of result is 

not uncommon to such chromium studies, as was previously 

discussed in the section “Sample Collection and Laboratory 

Analysis.” Comparisons were made between results falling 

beyond the rounding limits of the separate and dissimilar 

reporting for Cr(VI) results (two decimal places) and Cr(T) 

results (one decimal place). RPDs for these proportionally 

unexpected Cr(VI)-Cr(T) pairs ranged from +11 to +135. The 

RPDs are exaggerated by the low concentrations [0.15–2.1 

μg/L, Cr(VI); less than 0.1–1.8 μg/L, Cr(T)]. The difference in 

concentrations between paired samples was less than was 0.1 

μg/L in three pairs, 0.1–0.3 μg/L in eight pairs. 

The discrepancy likely results because the sample concentrations approached the reporting limits of the separate 

analyses [0.02 μg/L, Cr(VI); 0.1 μg/L, Cr(T)] and at or near 

the expected variance of the analytical methods. Additionally, 

the laboratory analytical methods differ for the two analytes, 

and interferences may affect the analyses, as previously 

discussed.
2.1

1.8

0

20

40

60

80

100

Samples with chromium detected, in percent

Chromium analyte

EXPLANATION

Hexavalent chromium–Cr(VI) 

Total chromium–Cr(T)

Maximum concentration 

given at top of column.

Cr(VI): Reporting limit 

0.02 microgram per 

liter

Cr(T): Reporting limit 

0.1 microgram per liter 

Number of samples: 151

Occurrence and Distribution of Chromium

Occurrence and Distribution of 

Chromium

The analytical results of Cr(VI) and Cr(T) in untreated 

source-water aquifers and surface waters tapped for public 

supply in Illinois are herein presented. Included are findings 

regarding their frequencies of detection and spatial distribution, along with a statistical summary of the concentration 

data. Findings from this study are considered with respect to 

current and proposed regulatory standards and the results of 

other large-scale studies of Cr(VI) in public-supply waters. 

Frequency of Detection and Concentration

The occurrence of Cr(VI) and Cr(T) in the untreated 

source-water aquifers and surface waters of Illinois, as well as 

in treated waters, is represented by their frequency of detection and distribution of concentrations. Findings regarding the 

occurrence of Cr(VI) and Cr(T) are presented in the following 

discussions. The relation of between concentrations of Cr(VI) 

and Cr(T) is briefly explored. The summarized analytical 

results for the samples collected during 2013 are given in 

table 3. Additionally, these data are given in full in appendix 

1, along with their associated field parameters, location, and 

selected well-construction information. 

Hexavalent Chromium

Hexavalent chromium was prevalent in the State’s 

untreated source-water aquifers and surface waters, as indicated by the frequency (table 3; fig. 6) and spatial distribution 

of detections (fig. 7). Cr(VI) was detected in 65 percent of 

untreated water samples, with detection frequencies of 57 percent in 119 groundwater samples and 94 percent in 32 surfacewater samples (fig. 8A). Maximum and median detected 

concentrations for all untreated samples were 2.1 μg/L 

and 0.07 μg/L, respectively. In groundwater samples these 

respective concentrations were 2.1 μg/L and 0.06 μg/L, and 

in surface-water samples, 0.29 μg/L and 0.09 μg/L. Median 

concentrations represented by all groundwater and surfacewater samples were 0.02 μg/L and 0.09 μg/L, respectively. 

As evident, detections were substantially more prevalent in 

surface waters than groundwater and, as a population, concentrations in surface water were greater (p < 0.001). The greatest 

concentrations were found in groundwater, with six detections 

exceeding the maximum concentration recorded in surface 

water (fig. 9A). It might be speculated that surface-water bodies are more vulnerable to direct natural and anthropogenic 

contributions of Cr(VI) than groundwater, and because of their 

typically greater oxygenation, they provide more favorable 

geochemical conditions for supporting its transformation from 

Cr(III) and its retention in the hexavalent form. Natural contributions can be from erosion and deposition from adjacent 

land deposits. Anthropogenic contributions of Cr(VI) can be 

from direct discharge, overland flow, shallow groundwater 

discharge, and airborne deposition. The comparatively lower 

maximum concentrations detected in some surface-water 

samples might be attributed to the greater capacity for advective dispersal and dilution in surface waters because of their 

greater flow rates and meteoric interaction than groundwater. 

Occurrence of Cr(VI) in groundwater presumably more greatly 

represents contributions from natural sources than anthropogenic sources, particularly as depth to groundwater increases. 

It is speculated that, in many cases, elevated concentrations 

in groundwater are associated with and in close proximity to 

spatially isolated source deposits. As previously noted, the 

geologic setting of Illinois is not one readily predisposed to 

widely distributed and (or) elevated occurrences of Cr(VI).

Hexavalent chromium was also prevalent in the State’s 

treated drinking waters, as indicated by their frequency of 

detection (table 3). Cr(VI) was detected in 97 percent of tap 

samples collected soon after treatment and in 100 percent 

of samples collected near the endpoint of their distribution 

(appendix 1). Maximum and median concentrations for all tap 

samples were 2.4 μg/L and 0.21 μg/L, respectively. Similarly, 

maximum and median concentrations for all distribution 

samples were 2.4 μg/L and 0.19 μg/L. There was a strong 

positive relation (ρ = 0.99) between Cr(VI) concentrations in 

tap samples and distribution samples. As is evident, Cr(VI) 

concentrations were significantly greater in treated water 

samples than untreated water samples (p = 0.007).

Figure 6. Frequency of detection and maximum concentrations 

of hexavalent chromium and total chromium in public-supply 

source waters in Illinois, 2013.
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0 25 50 75 100 MILES

0 50 100 KILOMETERS

Base from U.S. Geological Survey 1:24,000 and 1:100,000 Digital Data,

Albers Equal-Area Conic projection,

standard parallels 33° and 45°, central meridian -89°

92° 91° 90° 89° 88°

42°

41°

40°

39°

38°

37°

EXPLANATION

Concentration of Cr(VI), in micrograms per liter

Less than 0.02

0.02 to 0.04

0.05 to 0.09

0.10 to 1.0

1.1 to 2.5

Well Surface-water 

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20Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illino

Figure 7. Locations of selected wells and intakes that tap public-supply source-water aquifers and surface waters in Illinois, 

and the occurrence and relative concentrations of hexavalent chromium [Cr(VI)] in untreated water samples, 2013.
Groundwater Groundwater

Unconsolidated UnconsolidatedWisconsin Episode Wisconsin EpisodeWisconsin Episode Wisconsin EpisodeIllinois Episode Illinois EpisodeIllinois Episode Illinois EpisodeBedrock BedrockSurface water Surface water

 2.1 

119 2.1

56

0.28

23

2.1

33

1.1

63

0.29

32

0.28

9

0.29

21

 1.8 

119

1.8

56

1.0

23

1.8

33

1.6

63

1.8

32

0.7

9

1.8

21

EXPLANATION

EXPLANATION

Aquifer samples

Surface-water samples

Aquifer samples

Surface-water samples

0

20

40

60

80

100

A

Samples with hexavalent chromium detected, in percent Samples with total chromium detected, in percent

Source-water type

0

20

40

60

80

100

B

Numbers at top of column are maximum concentration/number of samples

Cr(VI): Reporting limit 0.02 microgram per liter

Cr(T): Reporting limit 0.1 microgram per liter

Occurrence and Distribution of Chromium

Figure 8. Frequency of detection and maximum concentrations of A, hexavalent chromium, and B, total chromium, 

in public-supply source-water aquifers and surface waters in Illinois, by source type, 2013.
22

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illino

Total Chromium and Relation to Hexavalent 

Chromium

As indicated by the Cr(VI) findings, Cr(T) also was 

frequently detected in the source-water aquifers and surface 

waters of Illinois (table 3; figs. 5 and 8). Cr(T) was detected in 

72 percent of untreated water samples, with detection frequencies of 65 percent in groundwater samples and 97 percent in 

surface-water samples (fig. 8B). Where detected, respective 

maximum and median concentrations were 1.8 μg/L and 0.40 

μg/L in all samples, groundwater samples, and surface-water 

samples.

Cr(T) also was frequently detected in the State’s treated 

drinking waters (table 3). Cr(T) was detected in 81 percent of tap samples and 91 percent of distribution samples 

(appendix 1). Maximum and median concentrations for both 

tap and distribution samples were 2.5 μg/L and 0.30 μg/L. 

There was a strong positive relation (ρ = 0.93) between Cr(T) 

concentrations in tap samples and distribution samples. Cr(T) 

concentrations were significantly greater in treated (tap) 

samples than untreated samples (p = 0.003).

There was a weak positive relation (ρ = 0.23) between 

Cr(VI) and Cr(T) concentrations in untreated waters. This 

weak relation might be explained by differences in the proportion of Cr(III) in samples from groundwater and surface water. 

In 52 percent of groundwater samples, concentrations of Cr(T) 

were two times or greater than those of Cr(VI), whereas in 

81 percent of surface-water samples, concentrations of Cr(T) 

were two times or greater. The relation between concentrations 

of Cr(VI) and Cr(T) in groundwater and surface-water samples 

was determined not to be statistically significant, as was the 

difference between these concentrations within each sourcewater type. Yet, concentrations of Cr(T) were determined to 

be significantly higher in surface waters than groundwaters 

(p = 0.002). The observed relations between concentrations of 

Cr(T) and Cr(VI) in groundwater and surface-water samples 

are exemplified in figure 9B. The typically greater concentrations of Cr(T) in these samples suggest that Cr(III) is the predominant chromium form in most source surface waters and 

that Cr(VI) and Cr(III) are more equally distributed in source 

groundwaters. One might expect Cr(VI) to be more prevalent 

than Cr(III) in surface waters, because these waters typically 

are more oxygenated than groundwater.

In an attempt to improve understanding of the relation 

between water oxygenation and the occurrence of chromium 

forms in untreated source waters, the concentration ratios 

of Cr(VI) to Cr(T) (hereafter referred to as “VITR”) were 

considered. As has been illustrated in studies of herbicides and 

their transformation products (Adams and Thurman, 1991), 

concentration ratios of related constituents that are differently affected by oxygenation can be useful in discerning the 

conditions or environments that might explain their occurrence 

and magnitude. In the consideration of chromium forms and 

their contribution to Cr(T), one might expect a greater VITR 

for samples from typically more greatly oxygenated surface 

waters than from typically more poorly oxygenated groundwaters. For this study, VITRs were determined from the 70 

sample pairs in which the concentration of each pair member 

was above its respective reporting limit and the Cr(VI) concentration did not exceed that of Cr(T). The calculated VITRs 

for untreated groundwater samples (minimum, 0.02; maximum, 0.87) did not differ significantly from those of surfacewater samples (minimum, 0.06; maximum, 0.90); thus, this 

evaluation approach provided no additional understanding of 

the relation between oxygenation and chromium forms in various source waters. 

Comparison to Regulatory Standards and 

Results of Other Studies

No concentrations of Cr(VI) or Cr(T) in untreated or 

treated waters exceeded the Federal MCL of 100 μg/L for 

Cr(T), and no concentrations of Cr(VI) exceeded California’s 

drinking water MCL of 10 μg/L for Cr(VI). Concentrations of 

Cr(VI) were equal to or greater than California’s nonenforceable public health goal of 0.02 μg/L in 65 percent of untreated 

water samples and 97 percent of drinking-water (tap and 

distribution) samples from surface-water sources. Cr(VI) concentrations of 23 percent of groundwater samples exceeded the 

standard of 0.07 μg/L reportedly under consideration in New 

Jersey; 62 percent of surface-water samples exceeded this 

standard. Concentrations of 19 percent of untreated surfacewater samples that were less than the potential New Jersey 

standard were greater than that standard after treatment for 

distribution. The relation between chromium concentrations 

in untreated and treated water samples will be discussed in 

further detail in the section “Effects of Treatment and Distribution on Sample Concentrations.”

Where studies can be reasonably compared on the basis 

of somewhat similar spatial scales and reporting limits, the 

findings of the present study indicate that Cr(VI) in untreated 

source waters and treated drinking waters is present at lower 

concentrations in Illinois than in many other parts of the 

Nation. In nationwide studies (Frey and others, 2004; Sutton, 

2010), maximum concentrations were found in the range of 

13–53 μg/L, as compared to 2.1 μg/L in the present Illinois 

study. Concentrations exceeded 0.06 μg/L in 71 percent of 

samples in one of the studies (Sutton, 2010), and the mean 

concentration was 1.1 μg/L in the other (Frey and others, 

2004). In the Illinois study, the mean concentration for samples with Cr(VI) detected was 0.14 μg/L and for all samples 

was as low as 0.09 μg/L. Cr(VI) concentrations exceeded 0.06 

μg/L in 63 percent of the Illinois samples. Frequency of detection cannot easily be compared between the studies, given the 

data available from the nationwide studies; however, it appears 

that the detection frequency in the Illinois study (65 percent) 

might be less that in the other studies. In one of those studies 

(Sutton, 2010), the frequency seemingly was 89 percent; in the 

other (Frey and others, 2004), the reported frequency was 42 

percent, but the concentration reporting limit was an order of 
0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Cr(VI)_GW Cr(VI)_SW Cr(T)_GW Cr(T)_SW

A

Concentration of chromium, in micrograms per liter Concentration of total chromium, in micrograms per liter

Hexavalent chromium [Cr(VI)] and total chromium [Cr(T)]

in groundwater (GW) and surface water (SW)

0.0 0.5 1.0 1.5 2.0 2.5

0.0

0.5

1.0

1.5

2.0

2.5

B

Concentration of hexavalent chromium, in micrograms per liter

57

94

65 97

1:1 LINE

68

EXPLANATION

EXPLANATION

Source-water type

Groundwater

Surface water

Values below the reporting limit of 

0.02 microgram per liter (mg/L) hexavalent 

chromium and 0.1 mg/L total chromium are 

censored to 0.0 mg/L.

U.S. Environmental Protection Agency 

(2013) Maximum Contaminant Level: total 

chromium, 100 mg/L.

Values below the reporting limit of 

0.02 microgram per liter (mg/L) hexavalent 

chromium and 0.1 mg/L total chromium are 

censored to 0.0 mg/L.

Percentage of detections: 119 wells and 

32 intakes sampled

Outlier: values beyond upper inner fence

Upper inner fence: 75th percentile 

+ (1.5 * interquartile range)

75th percentile

Median

25th percentile

Lower inner fence: 25th percentile 

- (1.5 * interquartile range)

Occurrence and Distribution of Chromium

Figure 9. Hexavalent chromium and total chromium in samples from public-supply source-water aquifers and surface waters in 

Illinois, by source type, 2013. A, Box-and-whisker plots of concentrations. B, Relation between concentrations.
24

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illino

magnitude greater than that used in the Illinois study (0.2 μg/L 

compared to 0.02 μg/L).

Detected concentrations in the more regional studies 

conducted in California were substantially greater than those 

in Illinois. In one study (Ball and Izbicki, 2004), maximum 

and median concentrations were 60 μg/L and 5.5 μg/L, 

respectively; in the other (California State Water Resources 

Control Board, 2011), detectable concentrations above 1 μg/L 

were found in 55 percent of the samples and above 10 μg/L 

in 4.5 percent of the samples. The detection frequency of 94 

percent in the study by Ball and Izbicki (2004) was substantially greater than the frequency of 65 percent in the Illinois 

study. Although Cr(VI) was detected in only 25 percent of the 

samples in the study by the California State Water Resources 

Control Board (2011), the reporting limit in that study was 

greater (0.2 μg/L). The greater Cr(VI) concentrations in 

nationwide locations outside of Illinois, in part, can be attributed to geologic settings that include natural source deposits 

of chromium, such as the mafic deposits prevalent in parts 

of California (Ball and Izbicki, 2004). The geochemistry and 

hydrology associated with the samples sites, including favorable conditions of pH, redox, and the contact time of water 

with aquifer materials, also might contribute to the differences 

in the concentrations between studies. Additionally, sample 

sites in the other studies may have been closer to industrial 

source areas for chromium.

At least one nationwide study with substantial data from 

both groundwater and surface-water sources (Frey and others, 

2004) confirmed the findings of the Illinois study, indicating that the greatest Cr(VI) concentrations in untreated water 

samples were from groundwater sources. Seemingly, unlike 

the Illinois study, untreated waters (typically groundwater) 

were found to have notably greater concentrations than treated 

waters. Although the Illinois study was limited to analysis 

of surface-water samples, treated waters were found to have 

notably greater concentrations than untreated waters. This 

finding is discussed in detail in the section “Effects of Treatment and Distribution on Sample Concentrations.” 

From the perspective of an Illinois-based assessment, 

the findings of this study for Lake Michigan samples matched 

those of other assessments of Lake Michigan water used 

for public supply (Central Lake County Joint Action Water 

Agency, 2011). Untreated, treated, and finished-water samples 

collected from the three facilities in the present study indicated 

concentrations close to (0.19 μg/L) or within the consistently 

determined range of 0.2–0.3 μg/L indicated by others. Furthermore, both assessments used principally the same sampling 

method, analytical lab, and reporting limits. 

Geochemical and Physical Factors 

Associated With Occurrence of 

Chromium

Awareness of geochemical and physical factors that 

could be associated directly or indirectly with the occurrence 

of Cr(VI) might be useful in identifying aquifers and surfacewater bodies that are vulnerable to elevated concentrations 

of Cr(VI) and require its reduction for public distribution of 

drinking water. Additionally, this information could allow subsequent monitoring efforts to be focused on locations where 

the potential for elevated Cr(VI) concentrations appears to be 

greatest. Focused monitoring should reduce the time and costs 

associated with the regulatory management of this constituent 

of concern. Factors examined in this study that may relate to 

the occurrence and detection of Cr(VI) in Illinois’ public-supply source waters include (1) the pH of untreated and treated 

water; (2) land use, with a focus on urbanization; and (3) 

several aspects of geohydrology and well construction. 

pH Conditions

The occurrence of dissolved Cr(III) and Cr(VI) in 

untreated source water or drinking water is, in part, affected by 

aqueous pH. As previously discussed in the section “Chemistry of Hexavalent Chromium,” pH greatly influences the solubility of these chromium forms. Cr(III) generally is insoluble 

between pH 6–10. Cr(VI) is increasingly soluble above pH 6 

to virtually 100 percent soluble above pH 8 (Eaton and others, 

2011). In consideration of the role of pH in chromium speciation, its relation to the occurrence of Cr(VI) in the untreated 

and treated drinking-water samples of this study was explored.

In untreated water samples, values of pH ranged from 6.3 

to 8.6, with a median of 7.0. Fourteen percent of the values 

were less than pH 6.5–the typical lower value for public drinking water–and less than 1 percent were greater than pH 8.5–

the typical upper value for public drinking water. This range of 

2 pH represents the guidance for protection of public drinking 

water against nuisance chemicals under the Secondary Drinking Water Regulations of the U.S. Environmental Protection 

Agency (2013c). The values for treated water samples available from 19 supply facilities ranged from pH 6.7 to pH 9.6, 

with a median of pH 8.0. Twenty-one percent of the values 

were greater than pH 8.5 in both the tap and distribution 

samples of treated water; 37 percent were greater than pH 8.5 

in at least one of the paired samples. 
Geochemical and Physical Factors Associated With Occurrence of Chromium

With a range of pH 6.3 to pH 9.6 for the untreated and 

treated water samples of the study, conditions appear favorable 

for the occurrence of dissolved Cr(VI) in the drinking waters 

of Illinois. Yet, on closer examination, there was only a weak 

positive relation (ρ = 0.32) between pH and Cr(VI) concentrations in untreated waters (groundwater and surface water 

considered collectively) (fig. 10A). The concentration of these 

samples ranged from less than 0.02 μg/L to 2.1 μg/L, respectively. When considered separately, there was no statistically 

significant relation between pH and Cr(VI) concentrations in 

either untreated groundwater or surface waters. 

The pH values of the nine surface-water samples that 

were subsequently treated with lime were compared to those 

values after treatment to determine whether the pH of the 

water was notably affected as the result of the lime treatment. The small sample size for the pretreatment source-water 

samples and postreatment tap samples precluded statistical 

examination of the differences in these sample populations. 

The median value of pH increased from pH 7.8 in untreated 

samples to pH 8.3 in lime-treated samples. Also, the pH values 

of seven of nine samples increased and the maximum value 

increased from pH 8.6 to pH 9.6 after treatment. This general 

increase in pH as the possible direct result of lime treatment 

might contribute to the increase in Cr(VI) noted in treated 

water samples. Again, the small number of samples available 

for this evaluation precludes drawing firm conclusions about 

its results.

The relation of pH to Cr(VI) concentrations, as associated 

with differing water treatments, also was examined. For the 

surface-water samples with available data, trends in the relation between pH and Cr(VI) concentrations were compared 

for samples before treatment, at the tap soon after treatment, 

and near the endpoint of distribution. Additionally, treated 

samples that excluded the addition of lime (for example, 

chlorinated only) were compared to those that included lime. 

As previously noted, samples collected near the endpoint of 

distribution might not uniquely represent an initially treated 

water sample. The distributed waters might include water or 

waters from multiple sources and (or) released from storage 

after treatment but before the date of sample collection of this 

study. 

For the raw, untreated surface waters without the subsequent addition of lime, there was a strong positive relation (ρ 

= 0.86) determined between pH and Cr(VI) concentrations. 

The pH and concentration ranges of these samples were pH 

7.3–8.2 and 0.04–0.24 μg/L, respectively. However, for the 

raw, untreated waters that were subsequently treated with 

lime, there was no significant relation between pH and Cr(VI) 

concentrations. The pH and concentration ranges of these 

samples were pH 7.2–8.6 and 0.08–0.24 μg/L, respectively. 

Their median values of pH and concentration were virtually 

the same (pH 7.7–7.8; 0.10–0.09 μg/L). It is uncertain why the 

relation between pH and Cr(VI) concentrations differed so for 

the two sample sets. Both sets represent raw surface waters 

prior to undergoing any treatment.

For tapwaters, there was no significant relation between 

pH and Cr(VI) concentrations, regardless of the type of treatment. The pH and concentration ranges of waters not treated 

with lime were pH 7.2–9.0 and 0.04 –0.23 μg/L, respectively. 

For waters treated with lime, the pH and concentration ranges 

were pH 7.7–9.6 and 0.09–2.2 μg/L, respectively. There was 

an increase in the median concentration of Cr(VI) from 0.09 

μg/L to 1.2 μg/L and maximum concentration from 0.24 μg/L 

to 2.2 μg/L after treatment.

For waters near the endpoint of distribution, there also 

was no significant relation between pH and Cr(VI) concentrations, regardless of the type of treatment. The pH and concentration ranges of waters that were not treated with lime were 

pH 6.7–9.0 and 0.03–0.22 μg/L, respectively. For waters with 

the addition of lime, the pH and concentration ranges were pH 

7.5–9.1 and 0.14–2.2 μg/L, respectively. 

A moderate positive relation (ρ = 0.53) between pH and 

Cr(VI) concentrations was indicated when samples representing water prior to treatment (raw) and soon after treatment 

that included lime were examined as a collective dataset. 

This relation is evident when viewed graphically (fig. 10B). 

As indicated by the 14 available samples, concentrations of 

Cr(VI) were significantly higher (p < 0.001) in tapwaters soon 

after treatment with lime than in untreated source waters. It is 

uncertain whether added lime contains Cr(VI) as a trace constituent and accounts for the apparent increase in concentrations of Cr(VI) in the treated waters or whether the increased 

pH seemingly associated with the addition of lime establishes 

geochemical conditions favorable to the conversion of Cr(III) 

to Cr(VI). Additional consideration of the relation between 

lime additions in treated water to Cr(VI) concentrations is 

included in section “Effect of Treatment and Distribution on 

Sample Concentrations.”

Land Use

Land use was evaluated to determine a possible relation 

to the frequency of detection and concentration of Cr(VI) 

in public-supply source waters of Illinois. Frequencies and 

concentrations were considered with respect to extent of 

urbanization; that is, source waters were classified as either 

principally urban or principally rural. Industrial sources are 

often cited as a principal basis for Cr(VI) contributions to the 

environment. For the present evaluation, the assumption was 

made that industrial sources of chromium are more likely to 

be associated with urban settings than rural settings. Historically, industrial production and disposal facilities have tended 

to be most densely located in or near urban settings. Regardless of setting, contamination of natural waters from industrial 

use of chromium can, in part, result from direct discharge 

of wastes to surface and groundwaters, improper disposal in 

landfills with discharge to groundwater, indirect discharge 

to surface waters from shallow groundwater discharge, and 

direct deposition in surface waters as airborne discharge from 
EXPLANATION

EXPLANATION

Groundwater or surface water

Values of concentration below the reporting 

limit of 0.02 microgram per liter (μg/L) are 

censored to 0.0 μg/L.

EXPLANATION

No lime treatment-raw intake

No lime treatment-tap

Lime treatment-raw intake

Lime treatment-tap

Values of concentration below the reporting 

limit of 0.02 microgram per liter (μg/L) are 

censored to 0.0 μg/L.

Approximate pH at which Cr(VI) is virtually 100 percent soluble

Approximate pH at which Cr(VI) is

virtually 100 percent soluble

6.0 6.5 7.0 7.5 8.0 8.5 9.0

0.0

0.5

1.0

1.5

2.0

2.5

A

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

0.0

0.5

1.0

1.5

2.0

2.5

, B

Concentration of hexavalent chromium, in micrograms per liter

pH, in standard units

26Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illino

Figure 10. Relation of concentrations of hexavalent chromium to pH in source waters used for public supply in Illinois, 2013. 

A, Samples from groundwater and surface-water sources of untreated (raw) water. 

B, Samples from surface-water sources of untreated water and tapwater treated with lime. 
Geochemical and Physical Factors Associated With Occurrence of Chromium

exhaust stacks. Urbanization status was determined for the 119 

groundwater-sample sites and 32 surface-water sites. By their 

closer positioning to industrial sources and potential for multiple routes of chromium exposure, it was expected that surface 

waters in industrial urban settings would be more vulnerable 

to Cr(VI) contributions than groundwaters in these settings. 

For the urban-rural designations, a state-scale land-use 

map was prepared from the National Land Cover Database at 

a 30-meter resolution (U.S. Geological Survey, 2014b) and the 

location of each groundwater and surface-water site plotted 

on the map. Plotted land cover was limited to areas classified 

as developed, low intensity; developed, medium intensity, and 

developed, high intensity. Those sample-site areas satisfying 

all of the following criteria were designated as “urban” for the 

examination of land use as relational factor for Cr(VI) occurrence. The criteria included the following:

1. Land cover is mapped principally as “developed, 

medium intensity” or “developed, high Intensity,”

2. metropolitan areas and communities have a minimum 

nominal population of 10,000, and 

3. there is evidence of present or past industrial activity in 

the area. 

Sites falling in smaller-sized urban areas were inspected 

for evidence of industrial activity by using satellite imagery 

(Google Earth , 2014). All imagery that was used was from 

after 2011. Exceptions to this approach for land-use designations included three Lake Michigan sites in the Chicago region 

(intakes 01299; 00107; 01305) and a Will County site (intake 

00341) (fig. 4). The Lake Michigan sites were considered as 

within the land-based area of Chicago mapped as “developed, 

medium intensity” or “developed, high intensity.” Although 

the community population of the somewhat rural Will County 

site is only about 5,0007, there was evidence of past industrial 

activity, and the riverside community is about 18 miles (mi) 

downstream of the more populous (about 27,000) and industrialized city of Kankakee.

On the basis of the designation criteria for land use, 8 

surface-water sample sites were designated as urban and 24 as 

rural. Urban sites included the three Lake Michigan sites, the 

Kankakee River sites (intakes 22082; 00341, respectively) in 

Kankakee and Will Counties, a Fox River site (intake 22155) 

in Kane County, a Mississippi River site (intake 52099) in 

Adams County, and a lake site (intake 52140) in Sangamon 

County (fig. 4). Designation of urban surface-water sites was 

somewhat problematic because most of the sites included rural 

areas within their watersheds. In this regard, two of the sites 

designated as rural also were evaluated separately as urban 

sites. These sites are situated at or near the downstream-most 

location of the State’s two largest rivers and might represent 

the collective water chemistry associated with industrial activity dispersed throughout the upstream reaches and watersheds 

7

All community population data included this report are from the 2010 

Census (U.S. Census Bureau, 2014).

of these rivers. The Alexander County site (intake 70010) 

withdraws water from the Ohio River and the Randolph 

County site (intake 60049) withdraws from the Mississippi 

River (figs. 3 and 7). Of the groundwater sample sites, 32 were 

designated as urban and 87 as rural.

As previously noted, Cr(VI) was detected in all but 2 

of the 32 untreated water samples from surface-water sites, 

which includes 100 percent of the urban sites. Maximum 

concentrations did not distinctly differ between land-use types, 

with 0.29 μg/L recorded at a rural Clinton County site (intake 

01851) and 0.28 μg/L recorded at an urban Lake Michigan site 

(intake 01299) (figs. 4, 7, and 11). Median concentrations were 

nominally greater at the urban sites (0.12 μg/L) than rural sites 

(0.08 μg/L). A larger sample population for urban surfacewater sites would be necessary to more identify any statistically significant relation between Cr(VI) concentrations and 

land use. Concentrations of Cr(VI) in samples from the two 

sites that also were separately considered as urban sites were 

0.08 μg/L (intake 60049, Ohio River) and 0.11 μg/L (intake 

70010, Mississippi River). Thus, concentrations detected at 

these sites did not notably differ from those considered fully 

representative of urban sites or rural sites.

For untreated water samples from groundwater sites, 

detection frequencies for Cr(VI) were similar between urban 

and rural sites (53 and 59 percent, respectively). Median 

detected Cr(VI) concentrations in samples from urban and 

rural sites also were similar (0.04 μg/L and 0.07 μg/L, respectively), as were the median concentrations for all samples 

(0.02 μg/L and 0.03 μg/L, respectively). The maximum concentration detected at urban sites (0.87 μg/L; Tazewell County, 

well 50060) was less than that at rural sites (2.1 μg/L; Whiteside County, well 11894) (figs. 4, 7, and 11). Statistically, 

there was no significant difference (p > 0.05) in concentrations 

between untreated groundwater samples from urban and rural 

sites.

The Whiteside County site is within a small rural community (population about 900) with no ready evidence of 

present or past industrial land use or upgradient industrial land 

use. The site well, about 0.25 mi from the Mississippi River, 

withdraws water from the unconfined Mississippi River alluvial aquifer and also possibly from the river. It may be that the 

site water chemistry represents, in part, urban and (or) industrial contributions of Cr(VI) from sources distributed upstream 

along the river. About 1 mi west, across the river and upstream 

in the closely located towns of Camanche and Clinton, Iowa, 

is a riverside industrial complex. Inspection of satellite 

imagery (Google Earth, 2014) of the various facilities in the 

complex indicates operations that include surface storage and 

shipment of coal and scrap metal recycling. Both operations 

are possible sources of chromium (West Virginia Geological 

and Economic Survey, 2002; Trustworthy Metal, 2014).
28

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

100

8

EXPLANATION

Percentage of detections 

Number of samples

Outlier: values beyond upper inner fence

Upper inner fence: 75th percentile + (1.5 * interquartile range)

75th percentile

Median

25th percentile

Lower inner fence: 25th percentile - (1.5 * interquartile range)

100

8

92

24

53

32

59

87

1.5

2.0

2.5

1.0

Source-water type and land use

Concentration of hexavalent chromium, in micrograms per liter

Surface waterurbanSurface waterruralGroundwaterurbanGroundwaterrural

0.0

0.5

Values below the reporting limit of 0.02 microgram per liter 

(μg/L) hexavalent chromium are censored to 0.0 μg/L.

U.S. Environmental Protection Agency (2013b) Maximum

Contaminant Level: total chromium, 100 μg/L.

Figure 11. Plots of concentrations of hexavalent chromium 

in public-supply source waters in Illinois, by urban and rural 

land use, 2013.

Collectively, urban surface waters may be more vulnerable to anthropogenic sources of chromium than urban 

groundwater, as suggested by their greater concentrations 

and frequency of detection of Cr(VI). However, factors other 

than urbanization seem to account for this apparent difference 

in vulnerability. Separately, in both urban and rural landuse settings for surface waters and groundwaters, there was 

minimal evidence of differences in vulnerability attributable 

to the settings. Evaluation of the relation between land use and 

Cr(VI) occurrence might have provided more insightful results 

if additional information was examined, including (1) known 

or potential industrial sources in proximity to the sourcewater sample locations and (2) areas contributing recharge or 

inflow to the supply wells and intakes. Added detail, however, 

is not always beneficial to such examinations. For example, 

in considering the relation between land use and occurrence 

of herbicide compounds in groundwater, Kolpin and others 

(1997) report that even when land-use patterns are considered 

in great detail, the relation between land use and occurrence 

of herbicide compounds in groundwater seems to be strengthened only marginally. Many factors can contribute collectively 

to the occurrence of chromium in groundwater and surface 

waters, including historical trends in land use and industrial 

activity, groundwater age, local flow patterns, geology, and 

ambient water and chromium chemistry. 

Geohydrology and Well Construction

Geohydrologic factors that were examined for association with the occurrence of Cr(VI) included factors related 

to groundwater systems and to surface-water systems. The 

groundwater factors were (1) aquifer type (unconsolidated or 

bedrock), (2) aquifer confinement (unconfined or confined), 

(3) aquifer depth, and (4) aquifer age and lithology (considered collectively). Well-casing depth also was examined as a 

groundwater-related factor. The surface water-related factors 

that were examined for association with the occurrence of 

Cr(VI) were (1) type of surface-water body (lake or river) and 

(2) glacial provenance. 

The selected factors could be considered, in part, to 

reflect the interrelation of water age, water source, aquifer 

type, aquifer depth, and oxygenation with the occurrence of 

Cr(VI). Surface water is younger than groundwater. Groundwater typically is younger in unconsolidated aquifers than in 

bedrock aquifers. This is particularly true for most unconsolidated aquifers used for public supply in Illinois (Robert Kay, 

U.S. Geological Survey, oral commun., 2104). The younger 

age of groundwater in unconsolidated aquifers can be attributed, in part, to the comparatively shallower depths and more 

rapid recharge rates of most unconsolidated aquifers (Kolpin 

and others, 2000). Associated with the comparatively younger 

age, shallower depths, and more rapid recharge rates is generally greater oxygenation (Holm and others, 1986). Where 

chromium is present, oxygenated waters are more likely to be 

enriched in Cr(VI) than Cr(III) (McNeill and others, 2011). 
Geochemical and Physical Factors Associated With Occurrence of Chromium

Thus, surface waters might be expected to be more vulnerable to occurrence of Cr(VI) than unconsolidated aquifers 

and unconsolidated aquifers more vulnerable than bedrock 

aquifers. These physical factors may be indirectly associated with the occurrence of Cr(VI) in natural water systems; 

however, with the exception of oxygenation, they should not 

be considered causative. The geochemistry of water systems 

including pH and oxygenation, in association with available 

geologic source materials of chromium, more directly account 

for the occurrence of dissolved Cr(VI) in water (John Izbicki, 

U.S. Geological Survey, written commun., 2014).

Groundwater-Related Factors

As previously noted, redox data, including DO, were not 

available for examination of their role in the occurrence of 

Cr(VI) in the source waters used for public supply in Illinois. 

Also, the limited scope of the study precluded analysis of possible alternative indicators of redox conditions (discussed in 

the following section “Other Factors”). In the absence of these 

DO and redox data and related analyses, the possible association of various aspects of geohydrology and well construction 

with Cr(VI) occurrence in the State’s source-water aquifers 

was briefly examined. These factors might relate indirectly to 

the geochemical conditions in these aquifers and (or) in other 

ways be associated with the occurrence of Cr(VI) in these 

aquifers.

Cr(VI) was detected more frequently in samples from 

bedrock aquifers (70 percent, representing 53 percent of 

sample locations) than from unconsolidated aquifers (43 

percent, representing 47 percent of sample locations) (fig. 9). 

Although the concentrations were not greatly different, statistical analysis of the distributions of Cr(VI) concentrations in 

samples from bedrock and unconsolidated aquifers indicated 

the concentrations in unconsolidated aquifers are significantly 

greater than those in bedrock aquifers (p = 0.02). Maximum 

concentrations were 2.1 μg/L in unconsolidated aquifers and 

1.1 μg/L in bedrock aquifers; mean concentrations were 0.11 

μg/L and 0.07 μg/L, respectively. Median concentrations were 

marginally greater in bedrock aquifers than unconsolidated 

aquifers (0.03μg/L and 0.0 μg/L, respectively). Additionally, 

only 6 percent of concentrations were greater than 0.20 μg/L 

in bedrock aquifers, whereas 14 percent were greater in unconsolidated aquifers. These findings might, in part, be explained 

by consideration of the depth of the aquifers. All but 12 of the 

63 bedrock aquifers in the sampling are designated as shallow bedrock aquifers (depth to the top of the aquifer is less 

than or equal to 500 ft below land surface). Depth to the top 

of shallow bedrock aquifers with detections of Cr(VI) ranged 

from land surface to 467 ft, with 45 percent of the detections 

from aquifers with depths of 200 ft or less. These depths did 

not notably differ from those associated with Cr(VI) detection in unconsolidated aquifers (ranging from land surface to 

200 ft). Detection frequency of Cr(VI) in samples from deep 

bedrock aquifers (depth greater than 500 ft) was 83 percent. 

This elevated frequency of detection suggests the geologic 

units that compose these deep aquifers might contain natural 

chromium source materials for Cr(VI).

The occurrence of Cr(VI) in unconsolidated aquifers 

appears to be associated with the type of aquifer. Glacial drift 

aquifers represent 59 percent of the unconsolidated aquifers 

in the study, and alluvial aquifers represent 41 percent. Cr(VI) 

was detected in 57 percent of samples from alluvial aquifers 

and in 33 percent from glacial drift aquifers. Concentrations of 

Cr(VI) were significantly greater (p = 0.02) in samples from 

alluvial aquifers (fig. 12). Maximum and median concentrations in alluvial aquifer samples were 2.1 μg/L and 0.04 μg/L, 

respectively; the median for detected concentrations was 0.19 

μg/L. In samples from glacial drift aquifers, the maximum and 

median concentrations were 0.31 μg/L and 0.0 μg/L, respectively; the median concentration for detected concentrations 

was 0.08 μg/L.

Values below the reporting limit of 0.02 microgram per liter (μg/L)

hexavalent chromium are censored to 0.0 μg/L.

U.S. Environmental Protection Agency (2013b) Maximum 

Contaminant Level: total chromium, 100 μg/L.

Outlier: values beyond upper inner fence

 Upper inner fence: 75th percentile + (1.5 * interquartile range)

 75th percentile

 Median

 25th percentile

Lower inner fence: 25th percentile - (1.5 * interquartile range)

Percentage of detections

Number of samples

33

33

EXPLANATION

33

33

57

23

0.0

0.5

1.0

1.5

2.0

2.5

Concentration of chromium, in micrograms per liter

Glacial drift

(sand and gravel)

Alluvial

(sand and gravel)

Type of unconsolidated aquifer

Figure 12. Plots of concentrations of hexavalent chromium in 

public-supply source-water aquifers in Illinois, by glacial drift 

aquifer and alluvial aquifer, 2013.
30

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

Figure 13. Graph showing frequency of detection of hexavalent 

chromium in public-supply source-water aquifers in Illinois, by 

designation of aquifer confinement, 2013.

2.1

40

0.38

79

EXPLANATION

Unconfined aquifer

Confined aquifer

Numbers at top of column are maximum

concentration/number of samples

Reporting limit 0.02 microgram per liter

0

40

60

80

100

20

Percentage of samples with hexavalent chromium detected 

Confinement designation

Generally, wells open to alluvial aquifers were shallower 

than those open to glacial drift aquifers. The maximum and 

median depths for wells that tapped alluvial aquifers were 

145 ft and 78 ft, respectively. For wells that tapped glacial 

drift aquifers, these respective depths were 118 ft and 404 

ft. The elevated concentrations of Cr(VI) in alluvial aquifer 

samples might reflect anthropogenic sources of chromium, as 

suggested by their relative proximity to land surface. Other 

possible contributions of chromium in these aquifers might 

include mineralogic differences in their composition and (or), 

in some cases, the contribution of surface waters entrained 

with the groundwater withdrawals. Most of the wells open 

to an alluvial aquifer are about a mile or less from a river or 

stream.

As with unconsolidated aquifers, a greater occurrence of 

Cr(VI) in unconfined aquifers than confined aquifers might 

be expected. Unconfined aquifers generally are shallower, 

the groundwater is of younger age, and the recharge rates 

are greater than those of confined aquifers; thus, unconfined 

aquifers more likely to be oxic. Confined aquifers typically are 

more likely to be anoxic (Yamanaka and others, 2006). Both 

unconsolidated and bedrock deposits can be unconfined, as are 

the shallow bedrock aquifers in northern Illinois in particular 

(fig. 1). 

Aquifers are considered confined when the potentiometric surface (water level) in the aquifer is above the top of the 

aquifer; confined aquifers are bounded above and below by 

lithologic units of distinctly lower permeability than that of the 

aquifer (U.S. Geological Survey, 1989). For this study, designation of aquifers as confined or unconfined relied on those of 

the IEPA. The status of aquifer confinement of wells included 

in the IEPA Ambient Network have been designated by policy 

in order to evaluate the vulnerability of Illinois’ source-water 

aquifers to contamination (Illinois Environmental Protection 

Agency, 1995). The primary approach used for these designations was a weighted-ranking method that includes five 

hydrogeologic criteria determined from well-construction logs 

and other data sources. For example, one criterion is, “Does at 

least one contiguous unit of impermeable geologic materials 

greater than 10 ft thick overlie the aquifer (excluding the top 

10 ft of soil materials)?” If insufficient data are available for 

the weighted-ranking method, the designations are based on 

alternative hydrogeologic-related vulnerability criteria (Berg 

and others, 1984), tritium data (Illinois Environmental Protection Agency, 2003; Mills and others, 2002, p. 9), evidence of 

human sources of contaminants (such as synthetic organic 

compounds), and (or) information from detailed hydrogeologic investigations of the aquifer (Wade Boring, Illinois 

Environmental Protection Agency, oral commun., 2003). Each 

of these approaches for designating aquifers as confined or 

unconfined is based on the assumption that groundwater from 

confined aquifers generally represents old recharge (about 50 

years or older) and groundwater in unconfined aquifers generally represents comparatively young recharge (younger than 

about 50 years).

On initial consideration, aquifer confinement does not 

appear to be readily associated with the occurrence of Cr(VI) 

in groundwater (fig. 13). Cr(VI) was detected in samples from 

58 percent of wells that tap aquifers designated as confined 

(representing 66 percent of wells or aquifers) and 55 percent 

of wells that tap aquifers designated as unconfined (representing 34 percent of wells or aquifers). However, statistical analysis of the distribution of Cr(VI) concentrations in 

samples from confined and unconfined aquifers found concentrations to be significantly greater in unconfined aquifers 

(p = 0.003). Maximum concentrations of Cr(VI) were 2.1 μg/L 

in unconfined aquifers and 0.38 μg/L in confined aquifers. 

Mean concentrations were 0.25 μg/L and 0.07 μg/L, respectively; median concentrations were 0.09 μg/L and 0.04μg/L. 

Furthermore, the five highest concentrations of Cr(VI) were 

found in samples from unconfined aquifers, with 35 percent 

of the detections greater than 0.10 μg/L. The unconfined 

aquifers included in the study population were predominantly 

unconsolidated (78 percent). Only 8 percent of detections 

were above 0.10 μg/L in samples from confined aquifers. The 

distribution of concentrations suggest that unconfined aquifers, 

which generally are shallower than confined aquifers, might 
Geochemical and Physical Factors Associated With Occurrence of Chromium

Figure 14. Frequency of detection of hexavalent chromium in 

public-supply source-water aquifers in Illinois, by aquifer depth, 

2013.

EXPLANATION

Maximum concentration

Number of samples

2.1

21

Reporting limit 0.02 microgram per liter

2.1

21 0.13

9

0.38

16

0.15

9

0.26

3

0.09

10

0-50 51-100 101-200 201-300 301-500 Greater

than 500

0

20

40

60

80

100

Samples with hexavalent chromium detected, in percent

Depth to top of aquifer below land surface, in feet

be more directly affected by anthropogenic sources of chromium than confined aquifers. Anthropogenic contaminants are 

typically the result of discharges at or near land surface. Other 

sources of the chromium detected in unconfined aquifers, 

including geologic, also might be possible.

Of the detections from aquifers designated as confined, 

83 percent were from bedrock aquifers; only 17 percent were 

from unconsolidated aquifers. Analysis of the distribution of 

Cr(VI) concentrations in samples from confined bedrock and 

confined unconsolidated aquifers indicated their concentrations were not significantly different. The small number of 

samples from unconfined bedrock deposits precluded similar 

statistical analysis for Cr(VI) concentrations in unconfined 

bedrock and unconsolidated aquifers. Although the extent of 

aquifer confinement does not appear to be solely associated 

with occurrence of Cr(VI), the elevated frequency of Cr(VI) 

detection in confined bedrock aquifers further suggests that 

source geology might partly contribute to the occurrence of 

Cr(VI) in certain source-water aquifers of the State. 

It is possible that extent of aquifer confinement is more 

directly associated with the occurrence of Cr(VI) than indicated, because designations of confinement used in this 

examination might not always be accurate. It is presumed 

that the weighted designation method (Illinois Environmental 

Protection Agency, 1995) relied greatly on lithologic information from the construction log of sampled wells. Designations 

based on such an approach might not always take into account 

the lateral extent and continuity of the confining units and the 

possible contribution of fractures in either bedrock or clay-rich 

deposits. The presence of fractures could allow preferential 

movement of water and Cr(VI) into a seemingly confined 

aquifer.

Depth to top of the aquifer was examined for a possible 

association with the occurrence of Cr(VI) in the source-water 

aquifers of the State. Mills and McMillan (2004) conducted 

a similar examination in their study of the occurrence of herbicides and their transformation products in Illinois aquifers. 

For the present examination, depth to the top of an aquifer 

(or aquifer depth) is defined as depth to the top of the aquifer 

material, regardless of whether the aquifer material is fully saturated. This depth factor might relate to the extent of oxygenation of the aquifer, physical and (or) hydraulic isolation from 

anthropogenic sources of chromium, as well as other possible 

factors that could be associated with the occurrence of Cr(VI).

There was no statistically significant relation between frequency of Cr(VI) detection and aquifer depth (from land surface) (fig. 14) or concentrations of Cr(VI) and aquifer depth. 

Regardless of aquifer depth, as classified in six ranges from 

0–50 ft to greater than 500 ft, detection frequencies fell within 

the relatively narrow range of 50–83 percent. The comparatively elevated frequency of detection (83 percent) of Cr(VI) 

in samples from the greatest depth interval further suggests 

geology (aquifer mineralogy) might substantially contribute 

to the occurrence of Cr(VI) in Illinois’ deepest source waters. 

These deep aquifers generally are composed of sandstone and 

(or) carbonate strata, with the sandstone units the principal 

source of water supply (Wilson, 2011).

Examination of concentrations above 0.1 μg/L also found 

that of the 20 detections of Cr(VI) above this level, 65 percent 

were of samples from the shallowest aquifers (depth 50 ft or 

less); 85 percent were of samples from aquifers with depths 

of 200 ft or less. Additionally, the four greatest concentrations 

(0.46–2.1 μg/L) were in samples from the shallowest aquifers. 

From this consideration, anthropogenic sources might plausibly account for much of the Cr(VI) occurrence in Illinois’ 

shallow source waters.

With the geology of the unconsolidated and bedrock 

aquifers possibly contributing to Cr(VI) occurrence, the 

related aspects of aquifer age and lithology were examined 

collectively for association with the frequency of detection 

and magnitude of Cr(VI) concentrations. For unconsolidated 

aquifers, the examination considered glacial provenance 

(Illinois Episode glaciation or Wisconsin Episode glaciation) 

(fig. 1). As a consequence of the somewhat different source 

areas within the Canadian Shield for the Illinois and Wisconsin Episode glacial advances, their glacial mineralogy differs 

(Willmann and others, 1963). For bedrock aquifers, the examination considered the age of an aquifer in conjunction with its 

principal lithology. 
32

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

The glacial provenances associated with the individual 

wells in this study may not always accurately represent the 

age of the glacial deposits that compose the aquifers tapped 

by the wells. Determining the age of deposits at the depth at 

which wells tap glacial aquifers was beyond the study scope. 

In most cases, the provenance designations are expected to 

be accurate. This is certainly true in areas of the State beyond 

the southward extent of Wisconsin Episode glaciation (fig. 1). 

Throughout the east-central to northeastern parts of the State, 

the Wisconsin Episode deposits overlie older Illinois Episode deposits. Of the 23 wells in this area of multiple glacial 

advances, the comparatively shallower wells are considered 

likely to tap only Wisconsin Episode or younger aquifers. 

The deepest wells (up to 404 ft), might be open in part or full 

to Illinois Episode aquifers. Also, for this examination, wells 

open to postglacial alluvial deposits were assigned to the glacial provenance of their mapped location; two alluvial wells 

in the far southwestern and southern unglaciated areas of the 

State (wells 61027; 00757, respectively) (figs. 1 and 4) were 

designated as within the region of Illinois Episode glaciation.

Hexavalent chromium was detected in samples from 

48 percent of wells in the area of Illinois Episode glaciation 

(representing 55 percent of unconsolidated aquifers) and 35 

percent of wells in the area of Wisconsin Episode glaciation 

(representing 41 percent of unconsolidated aquifers) (fig. 

8A). Included in the calculation of frequency of detection for 

Illinois Episode glaciation was the detection (0.67 μg/L) from 

a site (well 00757) in the unglaciated southern part of the State 

(figs. 1 and 4). 

Maximum concentration in the area of Illinois Episode 

glaciation was 2.1 μg/L (well 11894); this well taps younger 

Mississippi River alluvial deposits (fig. 1). Excluding the 

results of this site and those of another well that taps Illinois 

River alluvial deposits (0.87 μg/L; well 50060), the greatest 

detected concentration in glacial drift aquifers within the area 

of Illinois Episode glaciation was 0.31 μg/L (well 52095). 

Maximum concentration in the area of Wisconsin Episode 

glaciation was 0.28 mg/L (well 11894). Analysis of the distribution of Cr(VI) concentrations in samples from wells open 

to unconsolidated aquifers in the areas of Illinois and Wisconsin Episode glaciation indicates their concentrations do not 

significantly differ.

The relation of geology to occurrence of Cr(VI) also was 

examined in the context of the age and lithology of the unconsolidated and bedrock aquifers. These aquifers include (1) 

Quaternary sand and gravel aquifers, which can be classified 

as typically older glacial drift aquifers and typically younger 

alluvial aquifers, and (2) Paleozoic bedrock aquifers (fig. 3). 

The principal bedrock aquifers (or aquifer systems) in increasing age are the following:

1. Pennsylvanian aquifer, with sandstones the principal 

source of water, 

2. Mississippian aquifer, with limestones the principal 

source of water, 

3. Silurian and Silurian-Devonian aquifers, with dolomites 

and limestones the principal sources of water, and 

4. Cambrian-Ordovician aquifer, with sandstones and dolomites the principal sources of water.

A number of wells included in the study are open to specific subunits of the Cambrian-Ordovician aquifer, such as the 

Ironton-Galesville or Galena-Platteville aquifers. Generally, 

these subunits are composed principally of either sandstones 

or dolomites. For this examination of geology, wells open to 

Silurian and Silurian-Devonian aquifers were considered collectively as open to the Silurian-Devonian aquifer, and wells 

open to any of the subunits of the Cambrian-Ordovician aquifer were considered collectively as open to this aquifer system.

Hexavalent chromium detections in samples from various unconsolidated and bedrock source-water aquifers ranged 

from 33 percent for the glacial drift aquifers to 76 percent for 

the Cambrian-Ordovician aquifer (table 4; fig. 15). Disregarding the limited representation by Mississippian aquifers (about 

2 percent of aquifers), the maximum concentration of 2.1 μg/L 

and the greatest median concentration for detections of 0.18 

μg/L (and 0.07 μg/L for all concentrations) were recorded in 

samples from alluvial aquifers. As previously indicated from 

the examination of glacial provenance, elevated concentrations 

of Cr(VI) in alluvial aquifers might, in part, be associated with 

anthropogenic contributions. Generally, Cr(VI) concentrations 

were greatest in these aquifers and the detection frequency 

(59 percent) generally similar to that of the bedrock aquifers. 

Glacial drift aquifers possibly are somewhat less vulnerable to 

anthropogenic contributions of Cr(VI) than alluvial aquifers. 

Glacial drift aquifers generally are more deeply buried than 

alluvial aquifers and often beneath a sequence or sequences of 

finer-grained deposits.

The findings regarding geology as a relational factor 

associated with the occurrence of Cr(VI) also seem to support 

those of the previously discussed examinations of other possible factors; that is, geology (aquifer mineralogy) might contribute largely to the occurrence of Cr(VI) in Illinois’ bedrock 

aquifers. However, other unstudied factors, such as geochemical conditions, also may contribute in an important way to its 

occurrence. Geology as a possible factor in the occurrence 

of Cr(VI) in bedrock aquifers seems to be supported by the 

comparatively low frequency of detection of Cr(VI) associated 

with the glacial drift aquifers. Most bedrock aquifers, which 

have greater frequencies of detection and approximately similar concentrations of Cr(VI) relative to glacial drift aquifers, 

typically are overlain by glacial drift aquifers or low-permeability unconsolidated deposits. Although Cr(VI) concentrations often seem to be greater in alluvial aquifers than bedrock 

aquifers, these typically shallower aquifers are considered 

an unlikely primary source for the comparatively frequent 

occurrence and elevated concentration of Cr(VI) evidenced in 

the bedrock aquifers. Alluvial aquifers have a comparatively 

small spatial footprint, and most groundwater flow associated 

with them is likely to be as discharge to surficial water bodies 

rather than as recharge to the underlying bedrock systems. 
Geochemical and Physical Factors Associated With Occurrence of Chromium

Table 4. Summary statistics for detections of hexavalent chromium in samples from various unconsolidated and bedrock aquifers used 

for public water supply in Illinois, 2013.

[μg/L, microgram per liter; detected (all), samples with detected concentrations (all samples in category, concentrations designated as below the reporting limit 

censored to 0.0 μg/L); do., ditto. Reporting limit for hexavalent chromium, 0.02 μg/L. Maximum Contaminant Level for total chromium, 100 μg/L

(U.S. Environmental Protection Agency, 2013b)]

Aquifer

designation

Aquifer

type

Percentage of

aquifers

Detection

frequency,

in percent

Median

detected

concentration,

in μg/L

[detected (all)]

Mean

detected

concentration,

in μg/L

[detected (all)]

Maximum

detected

concentration,

in μg/L

Alluvial Unconsolidated 18.5 59.1 0.18 (0.07) 0.39 (0.23) 2.1

Glacial drift do. 28.6 32.4 0.07 (0.00) 0.12 (0.04) 0.31

Pennsylvanian Bedrock 2.5 66.7 0.09 (0.04) 0.09 (0.06) 0.14

Mississippian do. 2.5 66.7 0.21 (0.03) 0.21 (0.14) 0.38

Silurian-Devonian do. 20.2 62.5 0.03 (0.02) 0.11 (0.07) 1.1

Cambrian-Ordovician 

system

do. 27.7 75.8 0.05 (0.03) 0.08 (0.06) 0.46

Additionally, alluvial aquifers in the State are commonly 

underlain by low-permeability glacial till deposits that blanket 

most of their distributed area (Illinois State Geological Survey, 

2014). It is recognized that at some locations within the State 

withdrawals from the bedrock aquifer are suspected of inducing flow from streams and alluvial aquifers into the bedrock 

aquifers (O’Hearn and Gibb, 1980).

Throughout this report, including the following section 

“Effects of Treatment and Distribution on Sample Concentrations,” an apparent association between elevated Cr(VI) 

concentrations and water-treatment using processed limestone 

is discussed. The lime treatment, typically referred to as water 

softening, is used to reduce the natural hardness of the water 

associated with elevated concentrations of calcium and magnesium. In Illinois, Mississippian-age limestone reportedly 

is the source for the processed lime (Anthony Dulka, Illinois 

Environmental Protection Agency, written commun., 2012). 

However, neither an elevated frequency of Cr(VI) detection nor elevated concentrations were evidenced in untreated 

water samples collected from the Mississippian aquifer. These 

findings might easily be explained by the limitation of having 

collected only three samples collected from this aquifer. Also, 

it is possible that the limestone deposits near the few sample 

locations are mineralogically dissimilar to the source limestones used for the water-treatment additive.

Although well construction is not considered a factor that 

directly contributes to the occurrence of Cr(VI) in groundwater, certain aspects of construction might be indirectly associated with its occurrence. As groundwater systems deepen, the 

waters are increasingly likely to be anoxic and DO limited, 

particularly in Illinois groundwaters (Holm and others, 1986; 

Wilson, 2011). As is documented, geochemical conditions in 

groundwater systems are a controlling factor in chromium 

speciation. In consideration of a possible association between 

depth in groundwater systems and occurrence of Cr(VI), a 

particular aspect of well construction was examined: the depth 

below land surface to the base of the well casing. This is the 

depth to the top of the open or screened interval of a well, and 

thus, the shallowest depth at which groundwater may enter a 

well. Total depth of a well is often evaluated as a factor for its 

possible association with various water chemistries (Mills and 

McMillan, 2005; Wilson, 2011). For this study, well-casing 

depth was presumed to provide a more realistic indicator 

of distance from recharge areas (and thus, of younger, and 

possibly more oxic waters) than does total well depth. Thus, 

well-casing depth was examined in relation to the frequency of 

Cr(VI) detection and concentrations of Cr(VI).

There was a weak positive relation between wellcasing depth and frequency of Cr(VI) detection (ρ = 0.25) 

(fig. 16); although, this relation was not statistically significant (p = 0.09) given the marginally lower alpha criterion (α 

= 0.05) applied in this study. Also, there was no significant 

relation between well-casing depth and concentrations of 

Cr(VI). Regardless of casing depth, as classified in six ranges 

from 0–50 ft to greater than 500 ft, detection frequencies 

fell within the relatively narrow range of 47–68 percent. The 

absence of an apparent association between well-casing depth 

and occurrence of Cr(VI) possibly can be attributed to unique 

aspects of well construction. Many of the wells included in the 

study (53 percent) tap bedrock aquifers and thus are open to 

the aquifer from the base of the well casing to the base of the 

well (lengths of open intervals were up to 1,500 ft). Water may 
34

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

33

33

76

33

57

23

62

24

67

3

67

3

Values below the reporting limit of 0.02 microgram per 

liter (μg/L) hexavalent chromium are censored to 0.0 μg/L.

U.S. Environmental Protection Agency (2013b) Maximum 

Contaminant Level: total chromium, 100 μg/L.

Reporting limit of 0.02 microgram per liter

Percentage of detections

Number of samples

Outlier: values beyond upper inner fence

Upper inner fence: 75th percentile +

 (1.5 * interquartile range)

 75th percentile

 Median

 25th percentile

Lower inner fence: 25th percentile -

 (1.5 * interquartile range)

33

33

EXPLANATION

0.31

34

Maximum concentration

Number of samples

EXPLANATION

0.31

34

2.1

22

0.46

33

1.1

24

0.38

3

0.14

3

0.0

0.5

1.0

1.5

2.0

2.5

B

Concentration of hexavalent chromium,

 in micrograms per liter

 Samples with hexavalent chromium detected,

 in percent

A

Glacial drift

(sand and gravel)

Alluvial (sand

and gravel)

Source-water aquifer and principal lithology

CambrianOrdovician

System

(sandstone and

dolomite)

SilurianDevonian

(dolomite and

limestone)

Mississippian

(limestone)

Pennsylvanian

(sandstone)

0

20

40

60

80

100

Figure 15. Hexavalent chromium in public-supply source-water aquifers in Illinois, by age and lithology of aquifer, 2013. A, Frequency 

of detection. B, Box-and-whisker plots of concentration

enter these wells from various depths in the aquifer. Generally, open (screened) intervals of wells that tap unconsolidated 

aquifers more closely approximate total well depths than do 

open intervals of wells that tap bedrock aquifers. On statistical 

examination, there was no significant relation between well 

depth and frequency of Cr(VI) detection in unconsolidated 

aquifers. There was a weak inverse relation between well-casing depth and concentrations of Cr(VI) (ρ = –0.24) for wells 

tapping unconsolidated aquifers; although, this relation was 

not significant (p = 0.08). Neither well-casing depth nor total 

well depth appear to be associated with or useful indicators of 

the possible occurrence of Cr(VI) in aquifers.

There may be other possible explanations for the apparent 

absence of relations between the selected geohydrologic and 

well-construction factors and the occurrence of Cr(VI). Several explanations, among others, are that (1) any given sample 

water mass is a composite of waters of multiple ages, so mixing of only a small quantity of recently oxygenated recharge 
Geochemical and Physical Factors Associated With Occurrence of Chromium

1.1

19

2.1

36

0.38

24

0.46

18 0.26

10

0.09

12

EXPLANATION

1.1 Maximum concentration

19 Number of samples

Reporting limit 0.02 microgram per liter

0-50 51-100 101-200 201-300 301-500 Greater

than 500

0

20

40

60

80

100

Percentage of samples with hexavalent chromium detected

Depth of well casing below land surface, in feet

Figure 16. Frequency of detection of hexavalent chromium in 

public-supply source-water aquifers in Illinois, by well-casing 

depth, 2013.

in which Cr(VI) might be present with older recharge could 

result in detection of trace concentrations; (2) recent recharge 

may circumvent confining units and (or) move rapidly to depth 

in an aquifer by preferential flow through fractures or improperly constructed (grouted), damaged, or unused wells; and (3) 

the water chemistry that is intended to be representative of 

the aquifer might change upon withdrawal and collection as a 

sample.

Surface-Water-Related Factors

Glacial provenance also was examined as a factor that 

might be associated with the frequency of detection and 

magnitude of concentrations of Cr(VI) in surface waters used 

for public supply. The locations of surface-water intakes were 

designated as within the region of Illinois Episode glaciation, 

Wisconsin Episode glaciation, or an unglaciated region (only 

two intakes in this latter region) (fig. 1). Cr(VI) was detected 

in all samples from surface-water intakes located in the area of 

Illinois Episode glaciation (representing 66 percent of surfacewater sources); 88 percent of intakes in the area of Wisconsin 

Episode glaciation (representing 28 percent of surface-water 

sources) (fig. 8A); and in both samples from the unglaciated 

region in the southernmost part of the State (fig. 1). Maximum 

concentration in the area of Illinois Episode glaciation was 

0.29 μg/L (intake 01851). This intake taps a south-central Illinois lake in Clinton County (fig. 7). The maximum concentration in the area of Wisconsin Episode glaciation was 0.28 μg/L 

(intake 01299); this intake taps Lake Michigan. Maximum 

concentration in the unglaciated region was 0.08 μg/L. As 

with the frequency of detection and maximum concentrations, 

median detected concentrations (0.09 μg/L and 0.10 μg/L, 

respectively) were virtually indistinguishable between the 

regions of episodic glaciation; thus, glacial provenance seems 

to have limited bearing on the occurrence of Cr(VI) in surfacewater sources for public supply. The small number of samples 

from the region of Wisconsin Episode glaciation precluded 

statistical comparison of Cr(VI) concentrations in surface 

waters in regions of Wisconsin and Illinois Episode glaciation.

The similarity in Cr(VI) occurrence in regards to glacial 

provenance of surface-water sources may, in part, be related 

to minimal differences in the mineralogic composition of the 

glacial materials, particularly in what seems to be a paucity 

of chromium minerals. Additionally, about half of the intake 

locations are rivers or reservoirs fed by rivers, and several of 

these rivers—including the Mississippi, Kaskaskia, and Ohio 

Rivers—traverse both glacial regions; thus, there is notable 

potential for the mixing and homogenation of the water chemistry in many of these surface-water bodies.

To improve understanding of the relation between 

the type of source-water body and occurrence of Cr(VI) in 

surface-water sources used for public supply, sample results 

were examined with respect to whether the source-water 

sources were lakes or rivers (fig. 4). Sources designated as 

lakes varied widely in area and storage capacity and included 

spring-fed impoundments, riverside reservoirs for water withdrawn from a river, instream reservoirs, and Lake Michigan. 

Sources designated as rivers varied widely in length and flow 

capacity and included creeks, rivers that traversed distances 

representing only a few counties up to a large part of the State, 

and multistate rivers, such as the Mississippi and Ohio Rivers. 

Cr(VI) was detected in samples from 91 percent of surfacewater intakes withdrawing water from lakes and 92 percent 

from rivers (representing 63 and 38 percent of surface-water 

sources, respectively). The maximum concentration in samples 

from lakes was 0.29 μg/L (intake 01851); this intake taps an 

instream reservoir on the Kaskaskia River (figs. 3 and 6). The 

maximum concentration in samples from rivers was 0.15 μg/L 

(intake 60043). Interestingly, this intake taps the Kaskaskia 

River just downstream of the aforementioned reservoir. With 

the frequency of detection, maximum concentrations, and 

median detected concentrations (0.10 μg/L and 0.09 μg/L, 

respectively) generally indistinguishable between lakes and 

rivers, the type of water body appears to have limited bearing on the occurrence of Cr(VI) in surface-water sources for 

public supply. 

On closer examination, there might be some difference 

in Cr(VI) occurrence associated with these water bodies. 
36

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

Concentrations vary more widely in the lake samples (range, 

less than 0.02 μg/L to 0.29 μg/L; standard deviation, 0.09 

μg/L) than the river samples (range, 0.05 μg/L to 0.15 μg/L; 

standard deviation, 0.03 μg/L). Yet, analysis of the distribution of Cr(VI) concentrations in samples from lakes and rivers 

indicates their apparent difference in concentrations is not 

significant. Possibly the small number of samples (lake, 22; 

river, 10) affected the statistical findings. The availability of 

an additional number of data for analysis could possibly reveal 

a stronger association between type of water body and Cr(VI) 

occurrence.

Included in the wider range of CR(VI) concentrations 

in lake samples were five concentrations (0.20–0.29 μg/L) 

that exceeded the maximum concentration in river samples. 

In addition to the sample collected from the reservoir on the 

Kaskaskia River (intake 01851), elevated concentrations 

were detected in three samples from Lake Michigan (intakes 

00107; 01305; 10299) and one from a small downstate 

reservoir (intake 01952) (figs. 3 and 6). It can be speculated 

that this subtle difference in Cr(VI) occurrence might result, 

in part, from the greater potential for atmospheric contribution of Cr(VI) to lakes and the more stable hydrologic and 

geochemical environments of lakes for maintaining relatively 

steady concentrations from whatever the contributing source. 

Subject to periodic flushing and dilution in response to storm 

events and climatic trends, rivers might tend toward consistently lower and narrower ranges of Cr(VI) concentrations 

than lakes, although there might also be a greater potential 

for short-lived spikes in concentrations at a given location in 

rivers.

Other Factors

Several other factors are recognized as affecting the 

occurrence of Cr(VI) and Cr(T) in natural waters and in the 

laboratory analysis for chromium in water samples. Although 

redox indicator measurements such as DO were unavailable 

for this study, redox conditions (including concentrations of 

DO) have been documented as substantial contributing factors 

to the occurrence of Cr(VI). In addition to the assessment of 

these characteristics of water chemistry, future studies could 

benefit from the assessment of various other redox-sensitive 

constituents. Redox reactions are generally driven by microbial activity (Wilson, 2011). The microorganisms that drive 

these reactions obtain energy by transferring electrons from an 

electron donor to an electron acceptor. The electron acceptor 

is reduced and the donor is oxidized. Dissolved oxygen (O2

)

is the preferred electron acceptor. Under anoxic conditions, 

naturally available electron acceptors can include, among others, nitrate (NO3-), manganese (Mn4+), and ferric iron (Fe3+). 

Dissolved and particulate organic carbon generally are the 

principal electron donors (Thurman, 1985). Reduced forms of 

nitrogen, iron, and manganese also can be important donors. 

Future studies could include analyses for these electron acceptors and donors.

Various anions also have been identified that can interfere 

in C–IC analysis for Cr(VI) as the chromate anion CrO4

2- and 

carbon and alkalinity can interfere in ICP–MS analysis for 

Cr(T) (McNeill and others, 2011). Dionex (2003) conducted 

bench tests with chloride and sulfate to evaluate their potential 

for anionic interference in C–IC analysis for Cr(VI). Both of 

these anions are commonly present in natural waters and are 

periodically found in elevated concentrations in these source 

waters and in drinking water (Iowa Department of Natural 

Resources, 2009; Kelly and others, 2012; U.S. Environmental 

Protection Agency, 2012c). On the basis of spike recoveries, 

the Dionex tests found an increasing bias toward underestimation of Cr(VI) concentrations if chloride and sulfate concentrations were greater than about 500 μg/L. However, it was also 

determined that spike recoveries were greater than 80 percent 

at anionic concentrations as high as 1,000 mg/L for chloride 

and 2,000 mg/L for sulfate. This recovery rate for the noted 

range of anionic concentrations indicates that C–IC analysis 

for Cr(VI) in the present study probably was not affected in 

any meaningful way by the possible presence of competing 

anions in the water samples. The typical chloride and sulfate 

concentrations in drinking water are less than about 250 μg/L 

(U.S. Environmental Protection Agency, 2013c; 2012c).

Specific conductance (SC) correlates with the sum of 

dissolved major-ion concentrations in water and often with 

a single dissolved-ion concentration (Hem, 1985). SC was 

examined as a possible indicator of the magnitude of concentrations of dissolved ions, such as chloride and sulfate, that 

might affect laboratory analyses of chromium. The examination also considered its possible relation with concentrations 

of Cr(VI). Values of SC in untreated groundwater and surface 

water ranged from 106 to 4,000 microsiemens per centimeter 

at 25 °C (μS/cm), with a median value of 647 μS/cm. Of the 

147 available values, 16 percent were greater than 1,000 μS/

cm. There was a weak inverse relation (ρ = –0.26) between 

SC and Cr(VI) concentrations. Concentrations in waters with 

a SC greater than 1,000 μS/cm did not differ significantly (p 

= 0.07) from those in waters with lower values of SC. On the 

basis of this examination, the laboratory results of this study 

for chromium do not appear significantly affected by ionic 

interferences during analysis. 

The Metropolitan Water District of Southern California 

(2000) identified the potential for false-positive determinations 

of Cr(VI) as the result of carbon mass interference in ICP–MS 

analysis. Further study by Eaton and others (2001) confirmed 

this potential bias, with the resulting apparent concentration of 

Cr(T) in at least one sample near the MCL of 100 μg/L. The 

study identified a strong correlation between alkalinity and 

false positives for Cr(T) analysis. As reported, the magnitude 

of error in Cr(T) analysis depends on factors additional to 

alkalinity; however, in general, waters with elevated alkalinities will be expected to result in a greater occurrence of false 

positives by ICP–MS if carbon is not removed before analysis 

or a carbon correction factor is not applied. At the time of its 

reporting (2001), the authors of that study indicated that this 
Effects of Treatment and Distribution on Sample Concentrations

potential for carbon interference was not identified as such in 

published ICP–MS analytical methods for Cr(T), including 

USEPA Method 200.8. It is uncertain whether this issue has 

now been addressed in the USEPA method used for the present 

study. A brief literature review suggests that current analytical 

protocols for chromium analysis recognize the potential for 

carbon interference and apply various procedures to inhibit or 

account for the interference in analytical results. Carbon and 

alkalinity data were not collected or analyzed for in the present 

study. Future studies of Cr(VI) occurrence might benefit from 

determination of the variously noted constituents of water 

samples that can interfere with the laboratory analyses for 

chromium.

Effects of Treatment and Distribution 

on Sample Concentrations

After withdrawal of raw waters from source aquifers and 

surface waters, the waters are typically treated before public 

distribution as potable water for drinking and other purposes. 

Treatment is required for various purposes including removal 

or reduction of undesirable dissolved or suspended constituents, including clay particles, bacteria, iron, manganese, 

organic compounds, and radionuclides, among others; disinfection; and softening (reducing hardness or the concentration 

of dissolved calcium and magnesium in water). Treatment 

procedures can, in part, include the following:

1. coagulation–flocculation and sedimentation by addition 

of agents such as alum (aluminum sulfate) or iron salts 

(such as iron(III) chloride); 

2. filtration by use of sand and membrane filters, as wells 

as activated carbon “filters”; 

3. processing with ultrafiltration membranes (less than 

0.2-millimeter pores), ion-exchange systems, and reverse 

osmosis using semipermeable membranes; 

4. addition of chlorine (as sodium hypochlorite or chloramine, for example), potassium permanganate, or ozone 

as oxidizing agents for disinfection; and 

5. fluoridation (addition of hexafluorosilicic acid) and conditioning with lime and soda ash as precipitating agents 

for reduction of hardness.

As a result of the various possible treatment processes to 

which the raw water may be subjected, as well as its subsequent storage and distribution, there is potential for changes 

in its chromium content (concentration and (or) valence). To 

evaluate these possible changes, three treated water relations 

were examined. The examinations considered (1) the relation 

of concentrations of Cr(VI) to Cr(T) in samples collected upon 

withdrawal (untreated raw water), soon after treatment, and 

near the endpoint of distribution; (2) the relation of Cr(VI) in 

untreated water to Cr(VI) in treated water; and (3) the relation 

of Cr(VI) in treated water to Cr(VI) in distributed water. Each 

of these examinations was completed by using the 32 sample 

sets collected from the surface-water sources selected for this 

study (figs. 1 and 3). The scope of the study did not allow 

for examination of the possible effects of the wide variety of 

likely treatment processes used in Illinois on Cr(VI) occurrence in treated water for public distribution. The examination 

was restricted to lime treatment for water softening on the 

basis of reports of lime additions enhancing Cr(VI) concentrations in treated public supply waters of Illinois and Missouri 

(Missouri Department of Natural Resources, 2010; Anthony 

Dulka, Illinois Environmental Protection Agency, written commun., 2012; Song and others, 2012).

Comparison of the concentrations of Cr(VI) to Cr(T) in 

samples collected upon withdrawal (untreated raw water), 

soon after treatment, and near the endpoint of distribution 

found notably different trends for each sample type (fig. 17). 

As previously discussed in the section “Total Chromium and 

Relation to Hexavalent Chromium,” concentrations of Cr(T) in 

the untreated water samples typically were greater than those 

of Cr(VI), indicating that both Cr(III) and Cr(VI) were present 

in most of the samples. There was a weak positive relation 

(ρ = 0.23) between Cr(VI) and Cr(T) concentrations, suggesting that this relation might reflect differences in the proportion 

1:1 LINE

EXPLANATION

Sample source

Intake (untreated)

Tap (treated)

Distribution (treated)

Concentration of total chromium, in micrograms per liter

Values below the reporting limit of 0.02 microgram per liter (μg/L) 

hexavalent chromium and 0.1 μg/L total chromium are censored 

to 0.0 μg/L.

U.S. Environmental Protection Agency (2013b) Maxiumum

Contaminant Level: total chromium, 100 μg/L.

Concentration of hexavalent chromium,

in micrograms per liter

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0 2.5

Figure 17. Relation of concentrations of hexavalent chromium 

to total chromium in intake (untreated), tap (soon after treatment), 

and distribution (near endpoint) samples from public-supply 

surface-water sources in Illinois, 2013.
38

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

of Cr(III) in samples from groundwater and surface-water 

sources. The calculated difference between concentrations of 

Cr(VI) and Cr(T) in these separate source-water types were 

determined to not be statistically significant. 

In contrast to the untreated water samples, there was a 

strong positive relation between Cr(VI) and Cr(T) concentrations in both the posttreatment (tap) (ρ = 0.86) and distribution (ρ = 0.90) samples from surface-water sources. Similar 

treatment and distribution samples were not collected from 

groundwater sources for comparison. The notable change in 

the correlation between Cr(VI) and Cr(T) following treatment indicates that some aspect or aspects of the various 

treatment processes that are deployed affect chromium forms 

and their concentrations in the samples. Other contributing 

factors should not be discounted, including source histories 

(routing and storage conditions and sampling times for the 

untreated and treated waters). Yet, the relatively short distance 

required for water to be routed from the raw-water tap to the 

treated-water sample tap at most sites and the small difference in the times samples were collected do not suggest that 

the samples represent uniquely different water volumes that 

might contribute substantially to the noted differences in 

chromium concentrations. This conclusion is supported by the 

strong similarity of Cr(VI) and Cr(T) concentrations in treated 

and distribution samples, as will be discussed subsequently 

in greater detail. Several conflicting trends were noted in the 

differing Cr(VI) and Cr(T) concentrations after treatment of 

the raw intake water. Concentrations of Cr(VI) and (or) Cr(T) 

either increased, decreased, or did not appreciably change after 

treatment. The most frequent and pronounced change was a 

substantial increase in both the Cr(VI) and Cr(T) concentrations, as observed in almost one half of the samples.

Examination of the relation of Cr(VI) concentrations in 

untreated water to those in treated water (fig. 18), along with 

the strong relation noted between Cr(VI) and Cr(T) in the 

treated water samples, further indicate that some aspect or 

aspects of treatment results in a significant increase in Cr(VI) 

concentrations. In all but two of the treated water samples, the 

concentrations of Cr(VI) exceeded the maximum concentration (0.25 μg/L) recorded in the samples that were not treated 

with the lime additive. Further investigation determined that 

15 public-supply treatment facilities included in the study 

soften their water by use of a lime-based additive. These facilities are among the approximate 20 facilities in Illinois that use 

a lime-based additive to soften surface waters and 25 facilities 

that use it to soften groundwater. The source location or locations of the limestone were not determined for this study, but 

limestone deposits of this age are readily present and quarried 

in southern, southwestern, and western Illinois and adjoining 

States.

Examination of the relation of Cr(VI) concentrations 

in treated tapwater to Cr(VI) concentrations in distribution 

water (fig. 19) indicates little difference in these concentrations between the point of treatment and near the endpoint of 

distribution. It is uncertain why the relative concentrations of 

these two chromium forms remain stable after treatment. This 

0.00 0.05 0.10 0.15 0.20 0.25 0.30

0.0

0.5

1.0

1.5

2.0

2.5

Concentration of hexavalent chromium in treated tapwater,

in micrograms per liter

Concentration of hexavalent chromium in untreated water,

in micrograms per liter

Values below the reporting limit of 0.02 microgram per liter (μg/L)

hexavalent chromium are censored to 0.0 μg/L.

U.S. Environmental Protection Agency (2013b) Maximum

Contaminant Level: total chromium, 100 μg/L.

EXPLANATION

No lime treatment

Lime treatment

Figure 18. Concentrations of hexavalent chromium in intake 

(untreated) and tap (soon after treatment) samples from publicsupply surface-water sources in Illinois, 2013.

stability might relate, in part, to use of the oxidant chloramine, 

which provides relatively persistent disinfection of treated 

water. More stable oxygenation of the water may contribute to 

more stable chromium concentrations and forms. Concentrations of Cr(VI) between treated- and distributed-water samples 

tended to vary more in samples in the upper range of concentrations (above about 1 μg/L) (fig. 19). There was no consistent trend in the variation however, because concentrations in 

some samples increased and others decreased after treatment. 

Factors that might account for the variation are unknown, but 

possibly include differences in the source histories for the 

“paired” treated- and distributed-water samples and (or) the 

precision of the laboratory analytical method. 

These results were in sharp contrast to those of the 

NCBOS study (Frey and others, 2004, p. 50–51). In that 

study, Cr(VI) concentrations were determined for 20 untreated 

samples and 321 samples individually treated by one of 10 

processes. The maximum concentration detected in treated 

samples conditioned by water softening (addition of lime and 

(or) soda ash; representing 6 percent of treated samples) was 

less than about 0.3 μg/L. These results were in very close 

agreement with the results representing all but one of the 

various other treatment processes that were evaluated and the 

results representing no predistribution treatment. Median and 
 

1:1 LINE

EXPLANATION

No lime treatment

Lime treatment

0.0

0.5

1.0

1.5

2.0

2.5

Concentration of hexavalent chromium in treated tapwater,

after distribution, in micrograms per liter

Concentration of hexavalent chromium in treated tapwater,

before distribution, in micrograms per liter

Values below the reporting limit of 0.02 microgram per liter (μg/L)

hexavalent chromium are censored to 0.0 μg/L.

U.S. Environmental Protection Agency (2013b) Maximum

Contaminant Level: total chromium, 100 μg/L.

0.0 0.5 1.0 1.5 2.0 2.5

Sampling Factors Associated with Analytical Results of Chromium 39

Figure 19. Relation of concentration of hexavalent chromium in 

tap samples from public-supply surface-water sources in Illinois, 

soon after treatment and near the endpoint of distribution, 2013.

95th-percentile concentrations in the more affected “disinfected only” samples (representing 44 percent of treated samples) were about 0.5 μg/L and 1 μg/L, respectively, and in the 

more affected untreated samples (representing about 6 percent 

of all samples) were about 7.5 μg/L and 17.5 μg/L, respectively. Thus, the water-softening process, with the assumed 

addition of lime, did not appear to notably affect Cr(VI) 

concentrations in the treated samples. In that study, the comparatively greater concentrations of Cr(VI) in the “affected” 

samples generally were associated with samples from groundwater sources. Potable waters from groundwater sources were 

much more likely to be untreated or only undergo disinfection 

treatment than potable waters from surface-water sources. As 

with the treated water samples included in the present Illinois 

study, those samples in the NCBOS study conditioned by 

water softening generally were associated with surface-water 

sources.

The role lime addition plays in the increase in Cr(VI) 

concentrations in the treated public-supply waters of Illinois 

is uncertain. The contrasting results regarding lime treatment 

found in this study and the NCBOS study (Frey and others, 

2004) suggest that Cr(VI) may be present as a trace constituent in only certain kinds of lime or other additives used 

throughout the United States by public-water suppliers. The 

presence of Cr(VI) seemingly would depend on the source 

deposits for the additives. Eyring and others (2002) found 

measureable levels of chromium in alum additives. Possibly, 

the addition of lime alters the geochemical conditions of the 

water, particularly pH, thus enhancing conditions conducive to 

transformation of Cr(III) to Cr(VI) (as previously discussed in 

the section “pH Conditions”). Questions remain regarding the 

manner in which lime additions affect Cr(VI) concentrations 

and why this addition seems to result in an increase in Cr(VI) 

concentrations in the treated water of only some drinkingwater systems. 

Sampling Factors Associated with 

Analytical Results of Chromium

Sampling factors associated with the analytical results of 

Cr(VI) were evaluated with respect to selected aspects of (1) 

sample shipment and processing, and (2) material composition 

of the infrastructure components of water distribution. Factors 

associated with shipment and processing included sample 

temperature at the time of arrival at the laboratory for analysis 

and the holding time between the time of sample collection 

and sample analysis. Evaluation of the material composition 

factor was limited to consideration of the composition of the 

sample tap at the point of sample collection. It is important to 

better understand the extent to which these sampling factors 

might affect sample integrity, and thus, influence understanding of laboratory analytics that are intended to solely represent 

the chemistries of waters intended for public distribution.

Sample Temperature and Holding Time

Sample temperature and holding time were considered as 

possible factors that might affect analytical results. Only six 

samples (about 3 percent of untreated, treated, and distributed 

samples) exceeded the target sample-preservation temperature 

of 4 °C. Of these, all but two were less than 2 °C above the 

target temperature. The untreated and treated samples from 

one facility (intake 70010) were at a temperature of about 

12 °C upon arrival at the laboratory. Consideration of the 

Cr(VI) and Cr(T) analytical results of the properly chilled 

distribution sample (0.09 μg/L and 0.1 μg/L, respectively) 

indicated the elevated temperatures had no evident effect on 

the analytical results for Cr(VI) and C(T) of the untreated 

(0.08 μg/L and 0.5 μg/L, respectively) and treated (0.06 μg/L 

and less than 0.1 μg/L, respectively) samples. Thus, in this 

study there was no indication that sample temperature at time 

of analysis affected Cr(VI) or Cr(T) sample results. A more 

controlled study, with a range of known concentrations and 

preservation temperatures would be necessary to improve the 

evaluation of the relation between sample temperature and 

analytical results.
40

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

Holding times in this study were somewhat more divergent from the target holding time of 24 hours than were temperatures from that target. About 35 percent of samples were 

analyzed more than 24 hours after their collection. However, 

of these, only five samples were analyzed beyond 30 hours; 

four of these samples (intakes 00395, 1 sample; 58059, 3 

samples) were analyzed about 31 hours after collection and 

one (well 20019) about 46 hours after collection. Inspection of the holding-time data, particularly those sample data 

with holding times of 30 hours or more provided no apparent 

evidence of a relation between the holding times of this study 

and analytical results. For those holding times greater than 30 

hours, the Cr(VI) concentrations were close to or below 75thpercentile concentrations. Of the four samples with concentrations greater than 0.5 μg/L, the greatest two were analyzed 

within 24 hours of sample collection and the remaining two 

within 25 hours. Presently, USEPA Method 218.7, the updated 

version of Method 218.6, cites a holding time of 14 days (U.S. 

Environmental Protection Agency, 2011), with no USEPA 

cited holding time for Method 218.6 lower than 5 days. As 

with the sample temperature assessment, a more controlled 

study with a range of known concentrations and holding times 

would be necessary to improve the evaluation of the relation 

between holding time and analytical results.

Sample Tap Composition

The potential effect of the composition of sample taps 

on the analytical results for Cr(VI) was considered to ensure 

the results were not noticeably biased by a tap-related Cr(VI) 

contribution or other alteration of the samples. The construction materials of 96 percent of the raw-water sample taps for 

groundwater and surface-water sources were inspected and 

recorded on the IEPA field sheet and UL chain-of-custody 

form (appendixes 3 and 4). In most cases, the inspections 

were brief and the determined materials not verified with 

input from the facility operator. Therefore, all identifications 

of materials are considered presumptive and may differ from 

the actual composition of the taps. In some cases, sample taps 

consisted of multiple parts, such as a brass valve with a rubber 

hose or copper tubing with a chrome-plated valve. It should 

be recognized that the taps are minor endpoint components of 

a substantially larger network of distribution lines, which can 

be composed of a variety of materials that are likely to differ 

from those of the taps. For these reasons, this assessment of 

sample-tap materials only is considered exploratory.

Sample-tap materials were found to fall into six general 

categories: synthetic, copper, brass, steel, iron, and chromeplated (or similarly appearing metallic plating). The materials 

are listed here in increasing order of their presumed potential to contribute to or affect the chromium composition of 

the water samples. Synthetic materials, representing about 6 

percent of raw-water taps, included polyvinyl chloride (PVC), 

polyethylene, rubber, and similar such materials. Although 

certain plastics can contain chromium in colorants (Garments 

Tech, 2013), it is unlikely that chromium is used in the whiteor beige-colored tap fixtures prevalent in potable-water-treatment systems and is likely to be fixed in the plastic, if present. 

Copper represented about 8 percent of taps and brass about 

39 percent. Brass may contain trace amounts of chromium. 

Iron material represented about 1 percent of taps, and was 

principally cast iron; chromium can be present in cast iron in 

amounts ranging from a trace up to about 28 percent (Acme 

Alloy, 2001). Steel materials represented about 3 percent 

of taps, and included stainless, galvanized, and black steel; 

about 10–20 percent of the metallic composition of stainless 

steel is chromium (Engineers Edge, 2014). Chrome-plated 

material represented about 43 percent of taps, and included 

60

57

88

8

73

11

100

4

100

2

62

63

Values below the reporting limit of 0.02 microgram per liter (μg/L)

hexavalent chromium are censored to 0.0 μg/L.

Synthetic* and Iron*: all samples are from surface-water sources.

Source-water samples from 115 wells and 30 intakes.

U.S. Environmental Protection Agency (2013b) Maximum

Contaminant Level: total chromium, 100 μg/L.

Percentage of detections

Number of samples

Outlier: values beyond upper inner fence

 Upper inner fence: 75th percentile + (1.5 * interquartile range)

 75th percentile

 Median

 25th percentile

Lower inner fence: 25th percentile - (1.5 * interquartile range)

88

8

EXPLANATION

1.0

1.5

2.0

2.5

Sampling tap material

0.0

0.5

Concentration of hexavalent chromium, in micrograms per liter

Synthetic*

(polyvinylchloride,

polyethylene,

or similar)

Copper Brass Iron*

(cast)

Steel

(stainless,

galvanized,

black)

Chrome

plated

Figure 20. Box-and-whisker plots of concentrations of 

hexavalent chromium in public-supply source waters in Illinois, by 

sample-tap material, 2013.
Summary and Conclusions

Values below the reporting limit of 0.02 

microgram per liter (μg/L) are censored 

to 0.0 μg/L.

EXPLANATION

Chrome-plated tap

Brass tap

38 percent of chrome-plated-tap samples

with undetected concentration below

the reporting limit.

40 percent of brass-tap samples with

undetected concentration below the

report limit.

0 10 20 30 40 50 60 70

0.0

0.5

1.0

1.5

2.0

2.5

Concentration of hexavalent chromium, in micrograms per liter

Sample count

Figure 21. Concentrations of hexavalent chromium in publicsupply source waters in Illinois, by chrome-plated and brass 

sample-tap material, 2013.

chrome-plated material of any base-material composition. 

For this proportional listing of tap materials and the following described findings, multicomponent taps of more than 

one composition were represented by the composition of the 

predominant part of the tap.

Graphical results of the sample-tap assessment are presented in figures 20 and 21. These results provide no apparent 

evidence of sample-tap composition affecting Cr(VI) occurrence or concentration in water samples. The frequency of 

greater concentrations of Cr(VI) associated with brass and 

chrome-plated taps suggests that these materials might more 

readily affect sample concentrations than the other tap materials. Yet, the median concentrations and frequencies of detection associated with the brass and chrome-plated materials are 

no more evident of a material effect on Cr(VI) concentrations 

than those statistics associated with the other materials. The 

median concentration of samples from potentially leachable 

chrome-plated taps (0.06 μg/L) was less than that of samples 

from all tap-material types (0.08–0.18 μg/L) other than copper 

(0.04 μg/L). Also, the small sample population of taps materials other than brass and chrome-plated limit the significance 

of this comparative analysis. Brass and chrome-plated sample 

taps represented about 83 percent of the study population.

Statistical examination of the concentrations of Cr(VI) 

associated with sample taps constructed primarily of chromebearing materials (stainless steel, chrome) and those of taps 

presumably constructed of non-chrome-bearing materials 

(synthetics, copper, brass, iron) indicated no significant difference between these concentrations. Although the second 

greatest maximum concentration of Cr(VI) (0.87 μg/L) was 

detected in a sample of a potentially leachable chrome tap, the 

greatest concentration (1.1 μg/L) was detected in a brass tap 

with presumably little to no leachable source of chromium. 

Condition (age) of the tap, the natural variability in Cr(VI) 

and pH of the source waters, or other factors appear to more 

strongly account for the variability of the sample results than 

the material composition of the taps. 

Some of the variability in sample results might be attributed to the common occurrence of somewhat elevated concentrations of Cr(VI) in surface waters. This is indicated by the 

comparatively elevated median concentrations and detection 

frequencies in samples from taps composed of synthetic and 

iron materials. Although small in number (10), all samples 

from these taps were from surface-water sources. Although 

the source history of the water samples was not expected 

to affect the tap-sample analysis to any notable extent, this 

history might have been a contributing factor to the variability of results. Source history includes the timespan between 

withdrawal and collection of the samples and the material 

composition of the pumps and transmission lines with which 

the water samples were in contact. Although not documented 

in this study, it is presumed that pump and transmission lines 

were not made of chrome-bearing materials and that transmission distances were relatively short (in the range of tens to 

hundreds of feet). With most well and intake pumps withdrawing water in the range of hundreds to thousands of gallons per 

minute, it is similarly presumed that the typical transmission 

times and contact times for the sampled waters were short.

Summary and Conclusions

On the basis of its recent review of the human health 

effects of Cr(VI) in public drinking water, the USEPA is considering the need for its specific Federal regulation. Presently, 

only Cr(T) is regulated, at an Maximum Contaminant Level 

(MCL) of 100 μg/L. The occurrence of Cr(VI) in groundwater 

and surface waters generally is attributed to industrial sources, 

but it can be of natural origin. Water geochemistry—principally oxic and elevated (alkaline) pH conditions—enhances 

the transformation of chromium from the generally nontoxic 

form Cr(III) to more soluble and toxic Cr(VI). This transformation process has important implications regarding changes 

in concentration during water treatment and distribution of 

the water to the public. California’s MCL of 10 μg/L Cr(VI) 

for drinking water established in 2014, and a lower limit of 

about 0.07 μg/L possibly under consideration in New Jersey, 

illustrate the drinking-water concerns associated with Cr(VI). 

To improve understanding of the possible impact of a Cr(VI)-

specific standard within the approximate range of the State 

limits on management of Illinois’ public drinking water, the 

USGS, in cooperation with the IEPA, assessed the occurrence 

of Cr(VI) in the State’s public water supplies.
42

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

During 2013, untreated water samples were collected at 

public-supply facilities from 119 water-supply wells and 32 

surface-water intakes; also, 32 treated surface-water samples 

were collected near the point of treatment and 32 near the 

endpoint of distribution. Sample sites were selected by a 

stratified random method. Samples typically were analyzed 

within 24 hours of collection at a reporting limit of 0.02 μg/L. 

The occurrence of Cr(VI) was compared with selected physical, geochemical, and sampling factors that might more fully 

explain its distribution and magnitude of concentrations. Certain geologic and geochemical data (redox potential and DO) 

of sufficient accuracy were not collected as part of this study; 

their availability would have contributed to a more complete 

understanding of the occurrence of Cr(VI) in the groundwater 

and surface waters of Illinois.

The maximum concentration of Cr(VI) in groundwater 

was 2.1 μg/L. Maximum concentrations in untreated and 

treated surface water were 0.29 μg/L and 2.4 μg/L, respectively. All sample concentrations were below the California 

MCL; only 35 percent were below that State’s non-enforceable 

public health goal of 0.02 of μg/L. Cr(VI) was undetected in 

43 percent of untreated groundwater samples, with a median 

of 0.06 μg/L, when detected. All but two (94 percent) of 

untreated surface-water samples had detections. In untreated 

surface water, the median concentration was 0.09 μg/L, 

whereas in treated water (tap and distributed) the median was 

0.20 μg/L. Surface waters treated with lime for softening 

typically had the highest Cr(VI) concentrations (maximum, 

2.4 μg/L; median, 1.2 μg/L). Concentrations of 19 percent of 

untreated surface-water samples that were less than the potential New Jersey standard were greater than that benchmark 

after treatment for distribution.

The maximum concentration of Cr(T) in groundwater 

was 1.8 μg/L. Maximum concentrations in untreated and 

treated surface water were 1.8 μg/L and 2.5 μg/L, respectively. 

All sample concentrations were below the Federal MCL. Total 

chromium was detected in 65 percent of untreated groundwater samples, with a median of 0.40 μg/L when detected. All but 

one (97 percent) of untreated surface-water samples had detections. In untreated surface water, the median concentration 

was 0.40 μg/L, whereas in treated water (tap and distributed) 

the median was 0.30 μg/L. As with Cr(VI), surface waters 

treated with lime typically had the greatest Cr(T) concentrations (maximum, 2.4 μg/L; median, 1.2 μg/L).

The analysis of spikes and other quality-assurance 

samples indicate possible difficulties in obtaining or confirming consistently accurate analytical results for Cr(VI) at near 

the applied reporting limit of 0.02 μg/L. Interferences might 

be associated with the collection and processing of samples 

or with laboratory analytical capability. The lack of readily 

available standards and inorganic blank water of sufficiently 

low concentrations of Cr(VI) hamper the preparation of field 

spikes and equipment blanks for quality assurance of environmental water samples. Cr(VI) concentrations of the quality 

assurance and environmental samples can often approach 

the relatively low reporting limit applied in this and similar 

studies. 

Examination of factors that might account for or be associated with the occurrence of Cr(VI) in public-supply source 

waters found few clearly evident factors. Associations in 

frequencies of occurrence and range of concentrations indicate 

that surface waters and groundwaters of shallow, unconsolidated, unconfined aquifers, particularly alluvial aquifers, are 

possibly most commonly affected by anthropogenic sources of 

Cr(VI). Groundwaters of deep (greater than 500 feet) bedrock aquifers, particularly the Cambrian-Ordovician aquifer, 

are possibly most commonly affected by geologic sources of 

Cr(VI). Additional study would be necessary to verify these 

associations.

There was a weak positive relation (ρ = 0.23) between 

concentrations of Cr(VI) and Cr(T) in untreated water samples, with a much stronger positive relation (ρ = 0.86 and ρ = 

0.90, respectively) in samples collected soon after treatment 

and near the endpoint of distribution. The stronger relation and 

greater similarity between Cr(VI) and Cr(T) concentrations in 

treated water samples indicate that Cr(VI) represents a greater 

proportion of the measured concentrations of Cr(T) in treated 

waters than in untreated waters. Examination of such factors 

as material composition of sample taps and sample temperature and holding time found no evidence that these sampling 

factors contributed to the occurrence or magnitude of Cr(VI) 

detections in the collected samples. 

Future related studies might consider examining the role 

of lime and related water softeners on occurrence of Cr(VI) in 

treated public waters. This examination could include determination of the source and mineralogic content of the carbonates 

from which the lime additives used in Illinois are produced. 

Studies that might further explore the factors related to occurrence of Cr(VI) in Illinois groundwater and surface waters 

could benefit from collection of site-specific data on Eh (redox 

potential) and DO in conjunction with the collection of waterquality samples. These data, along with an understanding of 

the mineralogic content of aquifer materials, also could allow 

the relative vulnerability of various aquifers to Cr(VI) occurrence and the determination of their principal source of Cr(VI) 

(anthropogenic or geologic) to be more fully addressed.

References Cited

Acme Alloys, 2001, Products, alloys, high alloy cast irons: 

4 p., accessed August 17, 2014, at http://www.acmealloys.

com/High%20Alloy%20Cast%20Irons.PDF.

Adams, C.D., and Thurman, E.M., 1991, Formation and transport of deethylatrazine in the soil and vadose zone: Journal 

of Environmental Quality, v. 20, p. 540–547.
References Cited

Agency for Toxic Substances and Disease Registry, 2011, 

Toxic substances portal—Chromium: Agency for Toxic 

Substances and Disease Registry Web page, accessed 

February 5, 2014, at http://www.atsdr.cdc.gov/substances/

toxsubstance.asp?toxid=17.

Ball, J.W., 2003, New cation-exchange method for field speciation of hexavalent chromium: U.S. Geological Survey 

Fact Sheet FS–006–03, 4 p.

Ball, J.W., and Izbicki, J.A., 2004, Occurrence of hexavalent 

chromium in ground water in the western Mojave Desert, 

California: Applied Geochemistry, v. 19, p. 1123–1135.

Becquer, T., Quantin, C., Sicot, M., and Boudot, J.P., 2003, 

Chromium availability in ultramafic soils from New 

Caledonia: Science of the Total Environment, v. 301, 

p. 251–261.

Berg, R.C., Kempton, J.P., and Cartwright, Keros, 1984, 

Potential for contamination of shallow aquifers in Illinois: 

Illinois State Geological Survey Circular 532, 30 p.

California State Water Resources Control Board, 2011, 

Hexavalent chromium: Division of Water Quality Groundwater Information Sheet, 8 p.

California Department of Public Health, 2014, Chromium-6 

fact sheet: 4 p., accessed February 5, 2014, at http://www.

cdph.ca.gov/certlic/drinkingwater/Documents/Chromium6/

Chromium-6FactSheet-Update-08-21-2013.pdf.

Central Lake County Joint Action Water Agency, 2011, Chromium in water: 3 p., accessed February 5, 2014, at http://

www.lakecountyil.gov/PublicWorks/PublicInformation/

Documents/Chromiumin%20in%20Drinking%20Water.pdf.

Chakravarti, I.M,, Laha, R.G., and Roy, J., 1967, Handbook 

of applied statistics, volume I: New York, John Wiley and 

Sons, p. 392–394.

Chung, J.B., Burau, R.G., and Zasoski, R.J., 2001, Chromate 

generation by chromate depleted subsurface materials: 

Water, Air, & Soil Pollution, v. 128, p. 407–417.

College of St. Benedict and St. John’s University, 2014, 

Kolmogorov-Smirnov test: College of St. Benedict and St. 

John’s University Web page, accessed February 28, 2014, at 

http://www.physics.csbsju.edu/stats/KS-test.html.

Craun, G.F., and McCabe, L.J., 1975, Problems associated 

with metals in drinking water: Journal of the American 

Water Works Association, v. 67, p. 593–599.

Dionex, 1998, Technical Note 26—Determination of Cr(VI) in 

water, waste water, and solid waste extracts: 6 p., accessed 

February 10, 2014, at http://www.dionex.com/en-us/

webdocs/4428-TN26_LPN034398-02.pdf.

Dionex, 2003, Application Update 144—Determination of 

hexavalent chromium in drinking water using ion chromatography: 7 p., accessed February 10, 2014, at http://www.

dionex.com/en-us/webdocs/4242-AU144_LPN1495.pdf.

Dionex, 2011, Application Update 179—Sensitive determination of hexavalent chromium in drinking water: 6 p., 

accessed February 10, 2014, at http://www.dionex.com/

en-us/webdocs/110230-AU179-IC-Hexavalent-ChromiumWater-AU70415_E.pdf.

Duwelius, R.F., Kay, R.T., and Prinos, S.T., 1996, Groundwater quality in the Calumet region of northwestern Indiana 

and northeastern Illinois, June 1993: U.S. Geological Survey Water-Resources Investigations Report 95–4244, 179 p.

Eaton, A.D., Ramirez, L.M., and Haghani, A., 2001, The Erin 

Brockovich factor—Analysis of total and hexavalent chromium in drinking waters, in Proceedings of the American 

Water Works Association Water Quality Technology Conference, Nashville, Tenn., November 11–15, 2001, p. 1–13.

Engineers Edge, 2014, Stainless steel specification and 

composition chart: Engineers Edge Web page, accessed 

February 13, 2014, at http://www.engineersedge.com/stainless_steel.htm.

Eyring, A.M., Weinberg, H.S., and Singer, P.C., 2002, Measurement and effect of trace element contaminants in alum: 

Journal of the American Water Works Association, v. 94, no. 

5, p. 135–146.

Frey, M.M.; Seidel, Chad; Edwards, M.A.; Parks, J.L.; and 

McNeill, L.S., 2004, Occurrence survey of boron and 

hexavalent chromium: AWWA [American Water Works 

Association] Research Foundation, 98 p.

Frías, M., Sánchez de Rojas, M.I., García, N., and Luxán, 

M.P., 1994, Contribution of toxic elements—Hexavalent 

chromium in materials used in the manufacture of cement: 

Cement and Concrete Research, v. 24, p. 533–541.

Garments Tech, 2013, Concern for chromium (Cr) and chrome 

(VI) use in leather, textiles (wool, cashmere, and angora), 

plastics item, metals item: Garments Tech Web page, 

accessed February 13, 2104, at http://garmentstech.com/tag/

effec-of-chromium-cr-in-plastics-product/.

Google Earth, 2014, Hello, Earth: Google Earth Web page, 

accessed March 15, 2014, at http://www.google.com/earth/.

Hawley, E.L., Deeb, R.A., Kavanaugh, M.C., and Jacobs, J.J., 

2004, Treatment technologies for chromium (VI), chap. 8 of 

Guertin, J., Jacobs, J.A., and Avakian, C.P., eds., Chromium 

(VI) handbook: Boca Raton, Fla., CRC Press, accessed 

February 11, 2014, at http://engr.uconn.edu/~baholmen/

docs/ENVE290W/National%20Chromium%20Files%20

From%20Luke/Cr(VI)%20Handbook/L1608_C08.pdf.
44

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

Helsel, D.R., 2005, Non-detects and data analysis: Hoboken, 

N.J., John Wiley & Sons, 250 p.

Helsel, D.R. and Hirsch, R.M., 1992, Statistical methods in 

water resources: New York, Elsevier, 529 p.

Hem, J.D., 1985, Study and interpretation of the chemical 

characteristics of natural water (3d ed.): U.S. Geological 

Survey Water-Supply Paper 2254, 263 p. 

Holm, T.R., George, G.K., and Barcelona, M.J., 1986, Dissolved oxygen and oxidation-reduction potentials in ground 

water: U.S. Environmental Protection Agency, EPA/600/2–

86/042, 56 p.

Illinois Environmental Protection Agency, 1995, Methodology 

for evaluating the vulnerability of community water supply 

wells: Unpublished document on file in Springfield office of 

the Illinois Environmental Protection Agency, 5 p.

Illinois Environmental Protection Agency, 2003, Use of tritium in assessing aquifer vulnerability: Illinois Environmental Protection Agency Web page, accessed February 7, 2003, 

at http://www.epa.state.il.us/water/tritium.html.

Illinois Environmental Protection Agency, 2011a, Assessment 

of total chromium in Illinois community water supplies: 8 

p., accessed February 6, 2014, at http://www.epa.state.il.us/

water/compliance/drinking-water/publications/chromiumfactsheet.pdf.

Illinois Environmental Protection Agency, 2011b: Ambient 

groundwater monitoring: Illinois Environmental Protection 

Agency Web page, accessed February 1, 2014, at http://

www.epa.state.il.us/water/groundwater/ambient-monitoring.

html.

Illinois State Geological Survey, 2014, Quaternary (ice 

age) deposits: Illinois State Geological Survey Web 

page, accessed March 15, 2014, at http://www.isgs.uiuc.

edu/?q=content/quaternary-ice-age-deposits.

Iowa Department of Natural Resources, 2009, Water quality standards review—Chloride, sulfate, and total dissolved solids: 79 p., accessed February 13, 2014, at http://

www.iowadnr.gov/portals/idnr/uploads/water/standards/

ws_review.pdf.

Kelly, W.R., Panno, S.V., and Hackley, Keith, 2012, The 

sources, distribution, and trends of chloride in the waters of 

Illinois: Illinois State Water Survey Bulletin B–74, 59 p.

Kolata, D.R., and Nimz, C.K., eds., 2010, Geology of Illinois: 

University of Illinois at Urbana-Champaign, Institute of 

Natural Resources Sustainability, and Illinois State Geological Survey, 530 p.

Kolpin, D.W., Kalkoff, S.J., Goolsby, D.A., Sneck-Fahrer, 

D.A., and Thurman, E.M., 1997, Occurrence of selected 

herbicides and herbicide degradation products in Iowa’s 

ground water, 1995: Ground Water, v. 35, p. 679–688.

Kolpin, D.W., Thurman, E.M., and Linhart, S.M., 2000, Finding minimal herbicide concentrations in ground water? Try 

looking for their degradates: Science of the Total Environment, v. 248, p. 115–122.

Lindsey, B.D., Berndt, M.P., Katz, B.G., Ardis, A.F., and 

Skach, K.A., 2008, Factors affecting water quality in 

selected carbonate aquifers in the United States, 1993–2005: 

U.S. Geological Survey Scientific Investigations Report 

2008–5240, 117 p.

McNeill, L.S., McLean, J.E., Edwards, M.A., and Parks, J.L., 

2011, State of the science of hexavalent chromium in drinking water: Water Research Foundation, 35 p.

McNeill, L.S., McLean, J.E., Edwards, M.A., and Parks, J.L., 

2013, Trace level hexavalent chromium occurrence and 

analysis: Water Research Foundation, 118 p.

Metropolitan Water District of Southern California, 2000, 

Potential interference in measuring total chromium by 

USEPA Method 200.8: Metropolitan Water District of 

Southern California Web page, accessed March 10 13, 2014, 

at http://www.safedrinkingwater.com/community/metchrom6memo.htm.

Mills, P.C., Halford, K.J., and Cobb, R.P., 2002, Delineation 

of the Troy Bedrock Valley and particle tracking analysis 

of ground-water flow underlying Belvidere, Illinois: U.S. 

Geological Survey Water-Resources Investigations Report 

02–4062, 46 p.

Mills, P.C., and McMillan, W.D., 2004, Herbicides and their 

transformation products in source-water aquifers tapped by 

public-supply wells in Illinois, 2001–02: U.S. Geological 

Survey Water-Resources Investigations Report 03–4226, 

57 p.

Mills, P.C., and Sharpe, J.B., 2010, Estimated withdrawals and 

other elements of water use in the Great Lakes Basin of the 

United States in 2005: U.S. Geological Survey Scientific 

Investigations Report 2010–5031, 95 p.

Missouri Department of Natural Resources, 2010, News 

Release 006—Trace levels of hexavalent chromium found 

in two cities’ drinking water: Missouri Department of Natural Resources Web page, accessed February 11, 2014, at 

http://dnr.mo.gov/newsrel/2010/nr10-006.htm.

National Ground Water Association, 2014, First drinking water 

standard for hexavalent chromium is now final: National 

Ground Water Association Web page, accessed July 17, 

2014, at URL http://newsmanager.commpartners.com/

ngwanz/issues/2014-07-03/index.html#1.
References Cited

Nelson, W.J., 1995, Structural features in Illinois: Illinois State 

Geological Survey Bulletin 100, 144 p. 

New Jersey Drinking Water Quality Institute, 2010, Testing 

subcommittee meeting minutes: 3 p., accessed February 5, 

2014, at http://www.state.nj.us/dep/watersupply/pdf/minutes100224.pdf.

O’Hearn, Michael, and Gibb, J.P., 1980, Groundwater discharge to Illinois streams: Illinois State Water Survey 

Contract Report 246, 31 p.

O’Hearn, Michael, and Schock, S.C., 1984, Design of a statewide ground-water monitoring network for Illinois: Illinois 

State Water Survey Contract Report CR–354, 76 p.

Oze, Christopher; Bird, D.K.; and Fendorf, Scott, 2006, Genesis of hexavalent chromium from natural sources in soil 

and groundwater: Proceedings of the National Academy of 

Sciences of the United States of America, v. 104, no. 6, p. 

6544–6549, accessed on November 23, 2011, at http://www.

pnas.org/content/104/16/6544.full.

Parks, J.L., McNeill, L.S., Frey, M.M., Eaton, A.D., Haghani, 

A., Rameriz, L., and Edwards, M.A., 2004, Determination 

of total chromium in environmental water samples: Water 

Research, v. 38, p. 2827–2838.

Rai, Dhanpat; Saas, B.M.; and Moore, D.A., 1987, Chromium 

(III) hydrolysis constants and solubility of chromium (III) 

hydroxide: Inorganic Chemistry, v. 26, p. 345–349.

Random.org, 2012a, What’s this fuss about true randomness?: 

Random.org Web page, accessed March 6, 2012, at http://

random.org/.

Random.org, 2012b, Random sequence generator: Random.

org Web page, accessed March 6, 2012, at http://random.

org/sequences/.

Scientific Analytical Institute, 2014, Hexavalent chromium 

analysis: Scientific Analytical Institute Web page, accessed 

February 26, 2014, at http://www.sailab.com/services/

hexavalent-chromium-analysis/.

Smith, D.B.; Cannon, W.F.; Woodruff, L.G.; Solano, Federico; 

and Ellefsen, K.J., 2014, Geochemical and mineralogical 

maps of the conterminous United States: U.S. Geological 

Survey Open-File Report 2014–1082, 386 p.

Song, R., Zhu, E., Fritz, E., Bobay, C., Bruce, S., Brammell, 

J., and Heitzman, G., 2012, Cr-6 minimization at Louisville 

Water Company–A successful story, in Proceedings of the 

American Water Works Association Annual Conference and 

Exposition, Dallas, Tex., June 10–14, 2012, p. 1247–1261.

Stowe, C.W., 2006, Compositions and tectonic settings of 

chromite deposits through time [abs.]: Economic Geology, v. 89, no. 3, p. 528–546, accessed February 5, 2014, at 

http://econgeol.geoscienceworld.org/content/89/3/528.short.

Sutton, Rebecca, 2010, Chromium-6 in U.S. tap water: Environmental Working Group, 23 p.

Thurman, E.M., 1985, Organic geochemistry of natural 

waters: Boston, Mass., Martinus Nijhoff/Dr. W. Junk Publishers, 497 p.

Texas Commission on Environmental Quality, 2013, West 

County Road 112, Midland, TX: Texas Commission on 

Environmental Quality Web page, accessed February 5, 

2014, at http://www.tceq.state.tx.us/remediation/sites/cr112.

html.

Tong, J.Y. –P., and King, E.L., 1953, A spectrophotometric 

investigation of the equilibria existing in acidic solutions of 

chromium(VI): Journal of the American Chemical Society, 

v. 75, p. 6180–6186.

Trustway Metal, 2014, What is chromium steel used for—

Metal items: Trustway Metal Web page, accessed August 7, 

2014, at http://trustwaymetal.com/what-is-chromium-steelused-for-metal-items/.

U.S. Census Bureau, 2014, American fact finder: U.S. Census 

Bureau Web page, accessed March 15, 2014, at http://factfinder2.census.gov/faces/nav/jsf/pages/community_facts.

xhtml.

U.S. Environmental Protection Agency, 1986, Appendix 

B—Definition and procedure for the determination of the 

method detection limit: 40 CFR Part 136, p. 317–320, 

accessed February 10, 2014, at http://www.epa.gov/region9/

qa/pdfs/40cfr136_03.pdf.

U.S. Environmental Protection Agency, 1989, Preparing perfect project plans—Pocket guide for preparation of quality 

assurance project plans: U.S. Environmental Protection 

Agency, EPA/600/0–89/087, 62 p.

U.S. Environmental Protection Agency, 1994, Method 218.6, 

Determination of dissolved hexavalent chromium in drinking water, groundwater, and industrial wastewater effluents 

by ion chromatography (revision 3.3): U.S. Environmental 

Protection Agency Web page, accessed February 10, 2014, 

at http://www.wcaslab.com/tech/218_6.pdf.

U.S. Environmental Protection Agency, 2010a, Chromium-6 in 

drinking water: U.S. Environmental Protection Agency Fact 

Sheet EPA 815–F–10–005: 3 p., accessed on February 13, 

2014, at http://water.epa.gov/drink/contaminants/basicinformation/upload/Chromium6inDrinkingWater.pdf.

U.S. Environmental Protection Agency, 2010b, The analysis 

of regulated contaminant occurrence data from public water 

systems in support of the second Six-Year Review of the 

National Primary Drinking Water Regulations: U.S. Environmental Protection Agency, EPA 815–B–09–006, 519 p.
46

Hexavalent and Total Chromium in in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinoi

U.S. Environmental Protection Agency, 2011, EPA’s recommendations for enhanced monitoring for hexavalent chromium (chromium-6) in drinking water: U.S. Environmental 

Protection Agency Web page, accessed on November 5, 

2011, at http://water.epa.gov/drink/info/chromium/guidance.

cfm. 

U.S. Environmental Protection Agency, 2012a, IRIS toxicological review of hexavalent chromium (2010 external 

review draft): U.S. Environmental Protection Agency Web 

page, accessed February 5, 2014, at http://cfpub.epa.gov/

ncea/iris_drafts/recordisplay.cfm?deid=221433.

U.S. Environmental Protection Agency, 2012b, Safe Drinking 

Water Act (SDWA): U.S. Environmental Protection Agency 

Web page, accessed July 17, 2014, at http://water.epa.gov/

lawsregs/rulesregs/sdwa/index.cfm.

U.S. Environmental Protection Agency, 2012c, Sulfate in 

drinking water: U.S. Environmental Protection Agency Web 

page, accessed February 13, 2014, at http://water.epa.gov/

drink/contaminants/unregulated/sulfate.cfm.

U.S. Environmental Protection Agency, 2013a, Water—

Chromium in drinking water: accessed February 5, 2014, 

at http://water.epa.gov/drink/info/chromium/guidance.

cfm#five.

U.S. Environmental Protection Agency, 2013b, Drinking water 

contaminants—National Primary Drinking Water Regulations: U.S. Environmental Protection Agency Web page, 

accessed February 3, 2014, at http://www.epa.gov/safewater/mcl.html.

U.S. Environmental Protection Agency, 2013c, Secondary Drinking Water Regulations—Guidance for nuisance 

chemicals: U.S. Environmental Protection Agency Web 

page, accessed February 3, 2014, at http://water.epa.gov/

drink/contaminants/secondarystandards.cfm.

U.S. Environmental Protection Agency, 2014, Disinfectants 

and disinfection byproducts rule (stage 1 & 2 DBPRs): 31 

p., accessed February 24, 2014, at http://www2.epa.gov/

sites/production/files/documents/DBPR_Presentation.pdf.

U.S. Geological Survey, 1989, Federal glossary of selected 

terms—Subsurface-water flow and solute transport: U.S. 

Geological Survey Office of Water Data Coordination, 38 p.

U.S. Geological Survey, 2014a, Water use in the United States: 

U.S. Geological Survey Web page, accessed August 14, 

2014, at http://water.usgs.gov/watuse/.

U.S. Geological Survey, 2014b, Multi-resolution land characteristics consortium (MRLC): U.S. Geological Survey Web 

page, accessed March 19, 2014, at http://www.mrlc.gov/.

U.S. Geological Survey, variously dated, National field manual for the collection of water-quality data: U.S. Geological 

Survey Techniques of Water-Resources Investigations, book 

9, chap. A1–A6, available online at http://pubs.water.usgs.

gov/twri9A.

Visocky, A.P., 1997, Water-level trends and pumpage in the 

deep bedrock aquifers in the Chicago region, 1991–1995: 

Illinois State Water Survey Circular 182, 45 p.

Voelker, D.C., and Clarke, R.P., 1987, Illinois ground-water 

quality: U.S. Geological Survey Open-File Report 87–723, 

9 p.

West Virginia Geological and Economic Survey, 2002, Trace 

elements in West Virginal coals—chromium (Cr): West Virginia Geological and Economic Survey Web page, accessed 

August 7, 2014, at http://www.wvgs.wvnet.edu/www/datastat/te/CrHome.htm.

Willmann, H.B., Glass, H.D., and Frye, J.C., 1963, Mineralogy of glacial tills and their weathering profiles in Illinois, 

part 1. Glacial tills: Illinois State Geological Survey Circular 347, 55 p.

Wilson, J.T., 2011, Water-quality assessment of the CambrianOrdovician aquifer system in the northern Midwest, United 

States: U.S. Geological Survey Scientific Investigations 

Report 2011–5229, 154 p.

Yamanaka, M., Nakano, R., and Tase, N., 2006, Sulfate reduction and sulfide oxidation in anoxic aquifers in northeastern 

Osaka Basin, Japan: Journal of Hydrology, v. 335, p. 55–67. 

Zimmerman, Janet, 2014, Hinkley—Environmental report 

issued on groundwater cleanup plan: Press-Enterprise 

[Riverside, Calif.], accessed February 5, 2014, at http://

www.pe.com/local-news/topics/topics-environmentheadlines/20120822-hinkley-environmental-report-issuedon-groundwater-cleanup-plan.ece.
Appendixes

Appendix 1. Table of well and surface-water intake information, field characteristics of water 

quality, and concentrations of hexavalent chromium and total chromium in samples from 

selected source-water aquifers and surface waters used for public supply in Illinois, 2013.

Appendix 2. Protocols for collecting representative samples of untreated source water and 

treated water. A, Hexavalent chromium. B, Total chromium. 

Appendix 3. Example Illinois Environmental Protection Agency field sampling record.

Appendix 4. Example analytical result report and quality-assurance documentation provided by 

UL laboratory.
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013. [IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable  N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

AQUIFER AND PRINCIPAL LITHOLOGY: AL (SG), Alluvial (sand and gravel); GD (SG), Glacial drift (sand and gravel); SD (DL), Si(dolomite and limestone); SI (DL), Silurian (dolomite and limestone); PV (S), Pennsylvanian (sandstone); MS (L), Mississippian (limestoneOrdovician (sandstone and dolomite) GLACIAL PROVENANCE AND SURFACE-WATER BODY: IE-L, Illinois Episode glaciation–lake; IE-R, Illinois Episode glaciation–rEpisode glaciation–Mississippi River WE-L, Wisconsin Episode glaciation–lake; WE-LM, Wisconsin Episode glaciation–Lake Michigan; W

glaciation–river; U-L, Unglaciated–lake; U-OR, Unglaciated–Ohio River Reporting limit for hexavalent chromium, 0.02 μg/L; for total chromium, 0.1 μg/L. Maximum Contaminant Level for total chromium: 100 μg/L (U.S. Environmental Protection Agency (2013b). IEP  well or intak  number USG  well or intake name Aquifer an  principal lithology or glacial provenance and surface- water body Sample date Depth of well, tota  (ft) Sample-tap material Lime1 treatment pH, water, whole, field (standard units) Temperature, water (°C) Specific conductance (μs/cm) 

00107 42N13E-22.4c IE-L 09/05/13 na Brass/cast iron na 8.2 21.9 303 

do. do. do. do. do. Chrome N 7.3 22.0 311 

do. do. do. do. do. Chrome N 7.3 22.6 311 

00155 11N 4W-34.2e1 GD (SG) 09/04/13 35 Brass na 6.8 13.4 811 

00251 9N11W-20.6h2 GD (SG) 02/27/13 115 Galv steel na 7.0 14.3 734 

00337 39N 9E-8.3d1 SI (DL) 05/13/13 350 Chrome na 6.6 11.8 1,100 

00341 33N 9E-26.1b WE-R 10/24/13 na ?? na 8.2 9.4 680 

do. do. do. do. do. Copper/brass Y 8.3 10.0 434 

do. do. do. do. do. Copper/brass Y 8.7 16.6 435 

00351 2N 1E-26.8g3 MS (L) 05/08/13 92 Brass na 6.9 13.9 934 

00366 44N10E-10.6g SI (DL) 11/20/13 250 Chrome na 7.1 11.1 781 

00395 20N 2E-35.8f1 GD (SG) 11/12/13 360 Brass na 7.2 13.3 900 

00546 45N10E-4.4g2 GD (SG) 08/28/13 150 Chrome na 6.9 11.7 685 

00622 39N 9E-28.2e1 SI (DL) 06/25/13 368 Copper/brass na 6.4 11.6 983 
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued[IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable; N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEPA 

well or 

intake 

number

USGS 

well or 

intake name

Aquifer and 

principal 

lithology 

or glacial 

provenance 

and surfacewater body

Sample 

date

Depth of 

well, total 

(ft)

Sample-tap 

material

Lime1

treatment

pH, water, 

whole, field 

(standard 

units)

Temperature, 

water (°C)

Specific 

conductance 

(μs/cm)

00706 12N13W-29.2h1 GD (SG) 04/25/13 90 Brass na 7.18 10.2 465 

00727 17N5E-24.2h1 GD (SG) 08/01/13 143 ?? na 7.2 15.6 792 

00757 14S 4E-1.5f1 AL (SG) 01/15/13 102 Brass na 7.4 14.1 435 

00766 32N 9E-5.6d CO (SD) 12/05/13 795 Brass na 7.0 14.7 1,880 

00937 44N10E-35.4b SI (DL) 01/22/13 262 Chrome? na 7.1 12.0 796 

01029 40N 9E-19.3a CO (SD) 07/15/03 1,425 Chrome na 6.5 14.6 537 

01079 20N10E-15.8d CO (SD) 10/30/13 1,115 Chrome na 6.4 12.2 584 

01113 3S 7W-8.8b3 MS (L) 04/04/13 300 Chrome na 7.0 13.6 572 

01118 1N 1W-33.6e AL (SG) 11/20/13 60 Brass na 6.8 13.0 857 

01143 30N 9E-27.3b SI (DL) 10/03/13 260 Brass na 7.4 12.3 726 

01187 4S 7W-20.1g1 AL (SG) 06/12/13 60 Brass na 6.7 14.3 623 

01299 42N13E-16.4g WE-LM 01/30/13 na Chrome na 8.0 2.9 305 

do. do. do. do. do. Chrome N ?? ?? ?? 

do. do. do. do. do. Chrome N do. do. do. 

01305 38N15E-15 WE-LM 10/23/13 na Chrome na 8.4 2.7 352 

do. do. do. do. do. Chrome N 7.3 14.6 296 

do. do. do. do. do. Chrome N 7.5 15.3 303 

01357 1N 3W-33.7g IE-L 09/03/13 na Brass na ?? ?? ?? 

do. do. do. do. do. Chrome Y? 8.4 28.2 324 

do. do. do. do. do. Brass Y? 8.0 25.4 321 

01416 16N13W-22.5g2 AL (SG) 03/14/13 90 Chrome na 7.1 14.7 771 
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued [IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable  N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEP  well or intak  number USG  well or intake name Aquifer an  principal lithology or glacial provenance and surface- water body Sampl  date Depth of well, tota  (ft) Sample-ta  material Lime1 treatment pH, water, whole, field (standard units) Temperature, water (°C) Specifi  conductance (μs/cm) 

01443 01463 01536 01537 01670 do. do. 01851 do. do. 01952 do. do. 11318 11327 11347 11363 11367 114

11438

11456

15N 7E-6.4h3

14N13W-6.6f

42N 6E-28.3c

40N 1E-14.3h

12N 9E-24.4f

do.

do.

 3N 1W-19.6f

do.

do.

 9N 4E-16.5a

do.

do.

45N 3E-27.6b1

15N10E-17.7g1

17N10E-29.5c1

18N 9E-9.5c2

16N 9E-16.6g3

38N 5E-14.4d1

37N 5E-32.1c2

36N 3E-18.4d1

AL (SG)

AL (SG)

CO (SD)

CO (SD)

WE-L

do.

do.

IE-L

do.

do.

IE-L

do.

do.

GD (SG)

SI (DL)

GD (SG)

GD (SG)

GD (SG)

CO (SD)

CO (SD)

CO (SD)

08/27/13

04/11/13

03/12/13

06/26/13

03/27/13

do.

do.

02/06/13

do.

do.

09/26/13

do.

do.

04/29/13

02/19/13

02/27/13

04/01/13

04/22/13

09/16/13

01/31/13

09/17/13

75

65

1,250

893

na

do.

do.

na

do.

do.

na

do.

do.

100

334

270

404

270

573

502

150

Copper

Brass/nylon

Chrome

Chrome

PVC

Chrome

Chrome

Cast iron

Chrome

Copper

PVC

Chrome

Brass

Chrome

Chrome

Chrome

Brass

Chrome

Brass

Chrome

Brass

na

na

na

na

na

Y?

Y?

na

N

N

na

Y

Y

na

na

na

na

na

na

na

na

6.6

7.0

6.6

6.5

8.3

??

do.

7.5

??

do.

8.6

8.0

8.7

6.4

7.5

6.5

7.0

7.0

6.6

6.6

6.8

12.0

13.1

14.3

12.0

5.4

??

do.

2.7

??

do.

21.9

21.8

21.0

10.7

13.3

11.8

12.0

11.9

11.2

12.2

12.5

817

735

523

509

371

??

do.

249

??

do.

361

256

258

740

3,330

660

485

537

550

597

563
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued[IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable; N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEPA 

well or 

intake 

number

USGS 

well or 

intake name

Aquifer and 

principal 

lithology 

or glacial 

provenance 

and surfacewater body

Sample 

date

Depth of 

well, total 

(ft)

Sample-tap 

material

Lime1

treatment

pH, water, 

whole, field 

(standard 

units)

Temperature, 

water (°C)

Specific 

conductance 

(μs/cm)

11470

11508

11514

11557

11560

11562

11570

11621

11629

11640

11678

11706

11707

11778

11782

11846

11891

11894

11904

11914

36N 4E-8.4h1

33N 1E-16.8a2

31N 5E-16.1d1

21N 9E-5.5a1

21N10E-1.6f1

21N10E-1.7g1

37N 2E-10.1c1

44N 1E-22.6c1

44N 2E-18.7a2

44N 2E-20.4h1

43N 2E-10.7d2

24N 6E-5.5e1

24N 6E-5.6d1

25N11E-32.6g1

40N 2E-23.2f2

40N 1E-20.1a1

29N 6E-22.2b1

21N2E-35.5h2

20N 5E-15.8b1

19N 5E-5.2a1

SI (DL)

CO (SD)

PV (S)

CO (SD)

SD (DL)

CO (SD)

CO (SD)

CO (SD)

GD (SG)

CO (SD)

CO (SD)

CO (SD)

CO (SD)

CO (SD)

CO (SD)

CO (SD)

CO (SD)

AL (SG)

CO (SD)

GD (SG)

10/29/13

12/03/13

07/22/13

06/17/13

05/14/13

01/30/13

07/01/13

02/04/13

06/18/13

02/04/13

11/06/13

08/06/13

07/16/13

08/12/13

10/08/13

03/26/13

12/09/13

01/28/13

02/20/13

10/28/13

230

2,665

280

1,870

298

769

1,053

1,600

295

1,457

530

1,100

1,082

740

723

345

355

88

820

176

Brass

Chrome

Chrome

Chrome

Chrome

Brass

Brass

Chrome

Chrome

Chrome

Chrome

Chrome

Brass

Chrome

Chrome

Chrome

Stainless 

steel

Brass

Chrome

Chrome

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

6.6

6.6

6.9

6.5

6.3

6.4

6.8

7.0

6.4

6.6

6.6

6.4

6.3

6.4

6.9

6.4

6.5

6.5

6.6

6.8

11.7

23.7

12.9

14.5

11.9

11.1

12.4

11.9

11.3

12.5

11.0

12.2

12.6

11.6

11.5

10.8

11.1

11.6

13.8

11.3

554

1,550

813

538

831

558

424

573

767

489

547

523

529

683

489

481

546

591

578

572
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued [IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable  N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEP  well or intak  number USG  well or intake name Aquifer an  principal lithology or glacial provenance and surface- water body Sampl  date Depth of well, tota  (ft) Sample-ta  material Lime1 treatment pH, water, whole, field (standard units) Temperature, water (°C) Specifi  conductance (μs/cm) 

20019 20029 20136 20141 20180 20181 20207 20297 20328 20389 20412 20436 20442 20447 20458 20609 20613 22040 220

22082

39N 8E-23.8f1

42N 8E-14.2h1

35N 6E-6.2f1

43N 8E-14.1e1

45N 9E-7.8d1

45N 9E-7.3e1

45N 8E-35.5a1

43N11E-34.5f1

35N12E-13.6e1

35N12E-9.4h1

35N 10E-13.8e2

33N12E-24.2g1

35N12E-34.6g1

36N10E-33.7e1

34N13E-17.5e1

41N 9E-34.1b1

40N 9E-3.5b1

33N 7E-4.4c1

30N10E-26.1h1

30N13W-8.3g

CO (SD)

GD (SG)

CO (SD)

CO (SD)

SI (DL)

SI (DL)

GD (SG)

SD (DL)

SI (DL)

SI (DL)

SI (DL)

SI (DL)

SI (DL)

SD (DL)

SI (DL)

SI (DL)

SI (DL)

CO (SD)

SI (DL)

WE-R

01/29/13

06/24/13

10/01/13

12/10/13

05/21/13

09/18/13

04/02/13

04/23/13

07/17/13

08/22/13

05/23/13

08/15/13

06/20/13

05/02/13

10/09/13

08/27/13

06/03/13

05/30/13

07/25/13

11/26/13

1,297

183

287

1,300

294

255

60

280

500

420

305

300

430

303

460

200

392

1,462

163

na

Brass

Chrome

Chrome

Chrome

Copper

Brass

Chrome

Chrome

Brass

Chrome

Brass

??

Brass

??

Brass

Chrome

Chrome

Chrome

Brass

Copper/

chrome

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

na

6.6

6.4

6.6

6.8

6.6

6.9

6.3

7.2

6.6

7.0

6.8

6.9

6.8

6.8

6.8

6.4

??

7.1

6.8

7.8

11.9

11.4

11.3

15.3

11.7

12.9

12.5

11.3

12.2

13.0

12.8

12.2

12.4

12.4

11.9

11.3

??

16.4

14.7

3.5

620

937

720

450

647

813

1,290

1,110

1,120

1,610

1,590

1,160

1,240

1,700

1,340

986?

??

769

990

677
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued[IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable; N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEPA 

well or 

intake 

number

USGS 

well or 

intake name

Aquifer and 

principal 

lithology 

or glacial 

provenance 

and surfacewater body

Sample 

date

Depth of 

well, total 

(ft)

Sample-tap 

material

Lime1

treatment

pH, water, 

whole, field 

(standard 

units)

Temperature, 

water (°C)

Specific 

conductance 

(μs/cm)

do.

do.

22148

22155

do.

do.

31307

31409

31428

31713

31745

31820

31853

31873

45081

45155

45170

do.

do.

45174

do.

do.

43N 8E-8.2c1

41N 8E-11.2g

do.

do.

30N 3W-26.1b3

26N 2E-5.7a1

28N 2E-7.5c2

15N 3E-7.3e1

18N 3E-33.7a2

15N 2W-27.8c1

17N 1W-26.4c1

18N 1E-32.7G2

20N 8E-33.8a1

11N11W-8.4a1

 8N 5E-25.4c

do.

do.

 8N 7E-11.8e1

do.

do.

CO (SD)

WE-R

do.

do.

AL (SG)

GD (SG)

CO (SD)

CO (SD)

SI (DL)

SD (DL)

SI (DL)

SI (DL)

GD (SG)

GD (SG)

IE-L

do.

do.

AL (SG)

do.

do.

11/19/13

12/10/13

do.

do.

02/07/13

10/16/13

02/07/13

08/13/13

06/04/13

08/05/13

04/09/13

03/11/13

04/11/13

03/14/13

07/16/13

do.

do.

03/13/13

do.

do.

1,300

na

do.

do.

50

120

2,005

1,377

45

604

555

445

338

68

na

do.

do.

36

Chrome

Copper/

chrome

Brass

Chrome

??

Chrome

Brass

Brass

Chrome

Brass

Brass

Brass

Brass

Chrome

Brass

Copper

PVC

Steel/nylon

Chrome

Brass

Y

Y

na

na

Y

Y

na

na

na

na

na

na

na

na

na

na

na

Y

Y

na

7.8

7.9

6.8

??

8.9

8.4

7.2

6.4

7.5

6.7

6.4

6.8

6.5

6.7

7.4

7.0

7.9

9.6

9.09

7.0

3.5

9.3

14.5

??

6.3

11.7

13.5

12.6

21.3

15.1

12.6

14.3

12.8

13.1

12.8

12.5

27.4

27.1

23.0

11.8

479

460

488

??

730

748

688

718

3,130

845

1,260

1,890

716

462

646

627

290

234

265

769
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued [IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable  N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEP  well or intak  number USG  well or intake name Aquifer an  principal lithology or glacial provenance and surface- water body Sampl  date Depth of well, tota  (ft) Sample-ta  material Lime1 treatment pH, water, whole, field (standard units) Temperature, water (°C) Specifi  conductance (μs/cm) 

45263 47501 47518 47532 47542 47561 47583 47592 47614 do. do. 47687 47692 47698 do. do. 47774 47776 47811 478

50003

23N12W-11.3e3

26N 8E-3.8g1

29N 6E-10.8e1

26N 9E-4.2f1

23N 9E-14.2g1

26N11W-2.4g1

24N10E-20.8g1

24N14W-1.6d2

25N 2E-1.4f

do.

do.

20N 7E-2.8c1

17N13W-27.6e1

19N11W-6.5e

do.

do.

11N 3E-35.6a2

 9N 6E-14.2f1

 8N11W-29.4h2

 7N11E-31.8a2

16N 6W-12.5g

GD (SG)

GD (SG)

CO (SD)

GD (SG)

GD (SG)

GD (SG)

GD (SG)

GD (SG)

WE-L

do.

do.

GD (SG)

GD (SG)

WE-L

do.

do.

AL (SG)

GD (SG)

AL (SG)

PV (S)

AL (SG)

08/29/13

09/18/13

11/25/13

01/16/13

09/05/13

12/04/13

10/17/13

10/24/13

02/20/13

do.

do.

03/14/13

03/12/13

07/11/13

do.

do.

01/22/13

12/17/13

02/27/13

12/19/13

07/11/13

104

100

1,940

79

340

116

226

176

na

do.

do.

283

28

na

do.

do.

63

65

32

269

55

Chrome

Steel

Brass

Copper

Brass

Brass

Brass

Copper

Plastic

Copper

Chrome

Chrome

Chrome

Chrome

Chrome

Chrome

Brass

Chrome

Copper

Chrome

Brass

na

na

na

na

na

na

na

na

na

Y

Y

na

na

na

Y

Y

na

na

na

na

na

7.3

7.4

6.8

7.0

7.4

7.3

7.3

7.4

8.2

??

do.

7.4

7.1

7.6

8.7

8.6

7.4

7.5

7.0

8.3

6.8

13.8

18.5

21.3

12.6

13.2

13.0

12.8

13.2

1.9

??

do.

12.4

13.4

23.8

25.0

22.7

12.9

14.8

13.2

14.3

12.8

692

1,040

2,300

760

672

542

801

746

610

??

do.

726

577

513

279

282

618

697

631

915

657
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued[IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable; N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEPA 

well or 

intake 

number

USGS 

well or 

intake name

Aquifer and 

principal 

lithology 

or glacial 

provenance 

and surfacewater body

Sample 

date

Depth of 

well, total 

(ft)

Sample-tap 

material

Lime1

treatment

pH, water, 

whole, field 

(standard 

units)

Temperature, 

water (°C)

Specific 

conductance 

(μs/cm)

50051

50057

50060

50094

50138

50203

50224

50251

50308

50351

50376

50383

52056

52079

do.

do.

52095

52097

do.

do.

52099

26N 4W-24.5a

24N 5W-3.3h1

25N 5W-35.4d1

 9N 5W-25.1c1

11N 6E-24.1e1

12N 3W-35.5d2

15N2W-12.6g1

19N 5W-13.2h2

21N 8W-6.8e

 9N 5E-7.6d1

25N 2W-16.2g1

25N 5W-12.5e

 8N 1E-15.a1

 4N 9W-4.5a

do.

do.

11N 5W-18.4h1

 7N 6W-20.3h

do.

do.

2S 9W-3.2f

GD (SG)

AL (SG)

AL (SG)

MS (L)

CO (SD)

GD (SG)

GD (SG)

GD (SG)

AL (SG)

CO (SD)

GD (SG)

AL (SG)

AL (SG)

IE-MR

do.

do.

GD (SG)

IE-L

do.

do.

IE-MR

07/25/13

11/14/13

10/10/13

12/10/13

09/25/13

05/21/13

01/29/13

12/17/13

11/05/13

06/11/13

12/09/13

05/07/13

09/24/13

06/25/13

do.

do.

07/02/13

04/30/13

do.

do.

04/04/13

260

91

145

70

1,680

85

70

145

96

1,572

335

78

66

na

do.

do.

44

na

do.

do.

na

Brass

Brass

Chrome

Chrome

Brass

Chrome

Brass?

Chrome

Chrome

Copper/

brass?

Chrome

??

Brass

PVC

Chrome

Chrome

Brass

Plastic

Plastic

Plastic

Chrome

na

na

na

na

na

na

na

na

na

na

na

na

na

na

N

N

na

na

N

N

na

7.4

7.0

6.8

6.6

7.4

6.8

7.3

7.1

7.2

7.8

7.2

6.9

6.9

7.8

7.4

7.3

6.8

7.35

??

do.

7.5

12.4

14.0

15.5

12.3

22.3

13.2

15.0

13.1

14.2

22.0

12.6

13.7

13.0

25.3

24.9

22.4

15.0

14.65

??

do.

9.9

770

947

1,320

768

2,710

639

806

508

519

4,000

583

1,270

651

417

556

428

636

3325

??

do.

469
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued [IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable  N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEP  well or intak  number USG  well or intake name Aquifer an  principal lithology or glacial provenance and surface- water body Sampl  date Depth of well, tota  (ft) Sample-ta  material Lime1 treatment pH, water, whole, field (standard units) Temperature, water (°C) Specifi  conductance (μs/cm) 

do. do. 52100 do. do. 52122 52126 do. do. 52140 do. do. 58059 do. do. 58088 do. do. 600

do.

do.

do.

do.

11N 2E30.6c

do.

do.

15N14W-12.3g2

 9N 4W-36.3h

do.

do.

15N 5W-13.3h

do.

do.

12N 8W-35.8d1

do.

do.

 9N 7W-10.7g

do.

do.

 2N 2W-19.5h

do.

do.

do.

do.

IE-L

do.

do.

AL (SG)

IE-L

do.

do.

IE-L

do.

do.

IE-L

do.

do.

IE-L

do.

do.

IE-R

do.

do.

do.

do.

03/21/13

do.

do.

01/30/13

04/03/13

do.

do.

11/26/13

do.

do.

11/05/13

do.

do.

09/24/13

do.

do.

06/04/13

do.

do.

do.

do.

na

do.

do.

85

na

do.

do.

do.

do.

na

do.

do.

na

do.

do.

na

do.

do.

Chrome

Brass

Chrome

Chrome

Chrome

Brass

Plastic

Chrome

Chrome

Chrome

Chrome

Brass

Brass/galv 

steel

Chrome

Chrome

Brass

Chrome

Chrome

Steel

Chrome

Chrome

Y

Y

na

N

N

na

na

N

N

na

Y

Y

na

N?

N?

na

Y

Y

na

N

N

??

do.

7.8

??

do.

7.1

8.2

??

do.

7.8

8.8

8.9

7.6

9.0

9.0

7.9

8.1

7.8

8.0

7.2

6.67

??

do.

4.5

??

do.

13.8

7.9

??

do.

8.3

9.2

16.8

12.3

15.0

17.0

22.8

23.5

25.0

22.2

22.5

20.9

??

do.

294

??

do.

635

310

??

do.

373

248

245

265

321

326

315

366

388

329

334

340
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued[IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable; N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEPA 

well or 

intake 

number

USGS 

well or 

intake name

Aquifer and 

principal 

lithology 

or glacial 

provenance 

and surfacewater body

Sample 

date

Depth of 

well, total 

(ft)

Sample-tap 

material

Lime1

treatment

pH, water, 

whole, field 

(standard 

units)

Temperature, 

water (°C)

Specific 

conductance 

(μs/cm)

60049

do.

do.

60053

60058

60065

60068

do.

do.

60089

60127

60173

do.

do.

60181

do.

do.

60184

do.

do.

70010

 7S 7W-25.4h

do.

do.

 3N 8W-31.2a1

 5N 9W-20.4h2

 4N 9W13.1c1

 5S 8W-13.7a

do.

do.

 3N 8W-5.6d1

 4S11W-1.5a1

 2N 2E-2.3d

do.

do.

 5S 5W-7.5b

do.

do.

 2S 7W-28.3e

do.

do.

17S 1W-23.1e

IE-MR

do.

do.

AL (SG)

AL (SG)

GD (SG)

IE-R

do.

do.

AL (SG)

AL (SG)

IE-L

do.

do.

IE-L

do.

do.

IE-R

do.

do.

U-OR

06/05/13

do.

do.

10/08/13

06/27/13

05/16/13

08/15/13

do.

do.

08/13/13

03/06/13

07/17/13

do.

do.

05/21/13

do.

do.

12/10/13

do.

do.

05/29/13

na

do.

do.

102

92

112

na

do.

do.

106

59

na

do.

do.

na

do.

do.

na

do.

do.

na

??

Copper

Chrome

Brass

Copper

Chrome

Chrome

Brass

Chrome

Copper/brass

Chrome

Chrome

PVC

Chrome

PVC

Chrome

Chrome

Cast iron

Brass

Chrome

Brass

na

Y

Y

na

na

na

na

N

N

na

na

na

N

N

na

N

N

na

Y

Y

na

7.2

8.4

7.9

6.7

6.8

7.0

7.5

7.3

6.9

6.8

6.5

7.4

7.3

7.3

7.7

??

do.

7.4

7.7

7.5

7.8

20.6

20.9

18.8

15.1

14.8

14.4

25.1

25.0

27.3

14.1

15.1

28.0

28.2

24.1

23.4

??

do.

2.8

3.6

7.7

23.4

304

263

285

947

819

631

357

358

391

919

872

290

234

265

348

??

do.

478

541

596

307
Appendix 1. Well and surface-water intake information, field characteristics of water quality, and concentrations of hexavalent chromium and total selected source-water aquifers and surface waters used for public supply in Illinois, 2013.—Continued [IEPA, Illinois Environmental Protection Agency; USGS, U.S. Geological Survey; ft, feet below land surface; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °Cditto; na, not applicable  N, no; Y, yes; ?, uncertain; ??, unrecorded; <, less than; chrome, chrome-plating or similarly appearing metallic plating; galv, galvanized; PVC, polyvin

IEP  well or intak  number USG  well or intake name Aquifer an  principal lithology or glacial provenance and surface- water body Sampl  date Depth of well, tota  (ft) Sample-ta  material Lime1 treatment pH, water, whole, field (standard units) Temperature, water (°C) Specifi  conductance (μs/cm) 

do. do. do. do. do. Chrome N ?? ?? ?? 

do. do. do. do. do. Brass N do. do. do. 

70290 5S 2E-35.3g IE-L 01/17/13 na Brass na 7.8 4.8 300 

do. do. do. do. do. Chrome Y? ?? ?? ?? 

do. do. do. do. do. Brass Y? do. do. do. 

70550 10S 2W-12.2a U-L 02/07/13 na Chrome na 7.9 5.6 106 

do. do. do. do. do. Chrome N ?? ?? ?? 

do. do. do. do. do. Chrome N do. do. do. 

70620 8S 3W-26.5h IE-L 02/28/13 na Brass na 7.6 5.5 162 

do. do. do. do. do. Chrome N ?? ?? ?? 

do. do. do. do. do. Copper N do. do. do. 

70811 13S 3E-4.8e IE-L 10/31/13 na Brass/rubber na 7.3 15.9 174 

do. do. do. do. do. Chrome N 8.1 18.0 209 

do. do. do. do. do. Chrome N 8.1 18.6 204 

71531 1N 9E-17.5g1 PV (S) 04/17/13 215 Brass na 7.7 14.5 1,780 

1Listing of facilities using lime differed slightly between the two references; for the questionable facilities, treatment was determined by change in water chemistry. 2For surface-water facilities, the first sample listing represents raw source water, the second listing represents treated water near the point of treatment, and the third listing re

end point of distribution. 3It is suspected that this recorded measurement is in error. Efforts to confirm were unsuccessful. 4Insufficient volume of buffer solution was added to sample upon collection; laboratory reports pH of sample was within acceptable range upon receipt. 5Measurements made without use of flow-through cell attached to the water-quality mete

6Sample temperature was 12.4 °C upon receipt at laboratory.
Appendix 2. Protocols for collecting representative samples of untreated 

source water and treated water 

[Developed by the UL1 wet chemistry laboratory and modified by the U.S. Geological 

Survey] 

A. Hexavelent Chromium (CrVI) (Method 218.6) 

Please read all the instructions before 

sampling to ensure accurate test results.

Before Sampling

• Check UL-supplied sampling kit for sample 

bottle, beaker, syringe, filter, and pH buffer 

solution. 

• Remove blue refrigerant (ice) packs from the 

UL-supplied foam-insulated shipping container

and place packs in freezer. They must be 

completely frozen before returning samples to 

the laboratory.

• Plan to have samples collected after 11 a.m. 

and prior to the normal pick-up time of your 

overnight carrier.

• Don protective gloves (nitrile or latex), 

replacing ones used for attaching/removing 

water-quality meter or handling spigot. 

• Use previously unused gloves ONLY for 

handling sampling equipment (after shipping 

box opened) and collecting sample. 

Sampling Steps for Finished Drinking 

Water

1. Flush the cold water line for 10-15 minutes 

prior to sample collection, while collecting 

stabilized field water-quality values. 

2. Remove the snap-cap security sample 

bottle from the bubble-wrap sleeve and open 

the sample bottle. Do not touch around the 

edge and interior of the sample bottle and 

bottle cap.

3a. Sample bottle rinse: fill 1/4 full, close cap, 

shake, and pour out rinse water; Repeat.

3b. Collect 100 mL sample (i.e. to the 100 mL 

mark). 

4. Pour the entire 1.0 mL pH buffer solution 

into the 100 mL sample.

5. Push the snap cap back to close the 

sample bottle and to put the security tab to the 

secure position. Secure the tab in place with 

the plastic locking strip.

1. The UL Company was formerly known as Underwriters Laboratory.

59

6. Indicate sampling date, time and name of 

sampler on both the bottle labels and the 

enclosed Chain of Custody form. Information 

on the Chain of Custody and labels must

match the sample locations and be complete.

7. Place sample bottle in sealed bubble-wrap 

sleeve and then into the shipping container. 

Sampling Steps for Raw Water

1. Flush the cold water line for 10-15 minutes 

prior to sample collection, while collecting 

stabilized field values. 

2. Do not touch around the edge and interior 

of the sample bottle, bottle cap, beaker, and 

syringe tip, and filter connections. 

3. Use the beaker to collect at least 20 mL

(1/4 full) sample and swirl the beaker to rinse 

all inner surfaces of the beaker. Repeat. 

4a. Fill beaker to about 100 ml. Pre-rinse 

filterless syringe by filling 1⁄2 full with water 

from beaker, shake, push water through 

syringe, Repeat.

4b. Draw up 10 mL of the sample from the 

beaker into a syringe by placing the tip of the 

syringe into the sample and pulling up the 

plunger slowly to the 10 mL mark.

5. Connect the 45-μ syringe filter to the 

syringe tip.

6. Push the plunger using gentle pressure to 

filter 10 mL of the sample through the filter as 

waste; place syringe and filter on clean piece 

of aluminum foil. 

7a. Remove the snap-cap security sample 

bottle from the bubble-wrap shipping sleeve

and open the sample bottle. Do not touch 

around the edge and interior of the sample 

bottle and bottle cap.
7b. Remove filter, placing on clean aluminum foil; 

Draw up 20 ml of the sample from the beaker into 

the syringe; replace the filter and discharge 10 ml 

(1/2 syringe volume) of water into the sample 

bottle; close cap, shake, discharge water; Repeat 

with remaining 10 ml of water in syringe. Make sure 

that no water from the syringe drops on the exterior 

of the filter and into the sample bottle.

7c. Dump any remaining sample from the beaker

as waste.

8. Use the rinsed beaker to recollect at least 100 

mL sample.

9. Disconnect the filter from the syringe tip and put 

the filter on a clean surface (e.g. a clean paper 

towel).

10. Draw up 20 mL of the sample from the beaker 

into the syringe by placing the tip of the syringe into 

the sample and pulling up the plunger slowly to the 

20 mL mark.

11. Connect the rinsed filter back to the syringe tip. 

Make sure that no water from the syringe drops on 

the exterior of the filter and into the sample bottle. 

12. Push the plunger using gentle pressure to filter 

the sample through the filter into the 120 mL 

sample bottle.

13. Repeat Steps 9-12 for 4 more times for a total 

volume of 100 mL. Make sure that the sample 

upper level is on the 100 mL mark.

14. Pour the entire 1.0 mL pH buffer solution into 

the 100 mL sample.

15. Push the snap cap back to close the sample 

bottle and to put the security tab to the secure 

position. Secure the tab in place with the plastic 

locking strip.

16. Indicate sampling date, time and name of 

sampler on both the bottle labels and the enclosed 

Chain of Custody form. Information on the Chain of 

Custody and labels must match the sample 

locations and be complete.

17. Place sample bottle in sealed bubble-wrap 

sleeve and then into refrigerated foam shipping 

container.

18. If filter clogs with sediment during steps 12-13, 

repeat steps 4b, 5, and 6 with extra filter(s) 

available from drinking-water sampling kits.

Shipping Instructions

• Place 2-3 frozen blue refrigerant packs and the 

Chain of Custody form into the shipping container 

along with the chromium (VI) and total chromium 

samples, and return the container and contents to 

the laboratory immediately after sample collection.

• Samples must be shipped to the laboratory by an 

overnight carrier or hand delivered.. 

• The laboratory must be notified on the day of 

sample collection of their scheduled arrival at the 

laboratory on the following day.

THERE WILL BE NO SATURDAY DELIVERIES

Additional Supplies: 

• Aluminum foil for placement of sampling 

equipment during sample collection (store 

aluminum foil in clean, sealed plastic bag). 

2-inch packing tape for sealing tops/bottoms of UL 

shipping boxes or other larger shipping boxes 

containing multiple samples.

Refrigerant Packs

• Freeze refrigerant packs prior to use in the foaminsulated shipping containers. Store with other

packs in larger cooler when transporting to the field 

for final use and shipment to help retain their frozen 

state.

Field Notes/Chain-of-Custody

• Chain of Custody form also will be used to list 

final stabilized field water-quality values and for 

field notes of conditions that might affect sample 

results, including air temperature, weather, 

composition and cleanliness of sample tap, 

conditions of sample location and facility, lot

number and expiration date of inorganic 

blank water. 

60
Appendix 2. Protocols for collecting representative samples of untreated 

source water and treated water—Continued 

[Developed by the UL1 wet chemistry laboratory and modified by the U.S. Geological 

Survey]

B. Total Chromium (Method 200.8) 

Please read all the instructions before sampling 

to ensure accurate test results.

Before Sampling

• Check UL-supplied sampling kit for sample bottle.

• Remove blue refrigerant (ice) packs from the ULsupplied foam-insulated shipping container and 

place packs in freezer. They must be completely 

frozen before returning samples to the laboratory.

• Plan to have samples collected after 11 a.m. and

prior to the normal pick-up time of your overnight

carrier.

• Don protective gloves (nitrile or latex), replacing 

ones used for attaching/removing water-quality 

meter or handling spigot. 

• Use previously unused gloves ONLY for handling 

sampling equipment (after shipping box opened) 

and collecting sample. 

Sampling Steps for Raw and Finished Drinking 

Water

1. Flush the cold water line for 10-15 minutes prior 

to sample collection, while collecting stabilized field 

water-quality values. 

2. Unscrew the cap from the sample bottle. Do not 

touch around the edge and interior of the sample 

bottle and bottle cap.

3a. Sample bottle rinse: fill 1/4 full, replace cap, 

shake, and pour out rinse water; Repeat.

3b. Collect 100 mL sample (to the neck of the 

bottle). 

4. Screw the cap back to close the sample bottle. 

5. Indicate sampling date, time and name of 

sampler on both the bottle label and the enclosed 

Chain of Custody form. Information on the Chain of 

Custody and labels must match the sample 

locations and be complete.

Shipping Instructions

• Place 2-3 frozen blue refrigerant packs and the 

Chain of Custody form into the shipping container 

along with the total chromium and chromium (VI) 

samples, and return the container and contents to 

the laboratory immediately after sample collection.

1. The UL Company was formerly known as Underwriters Laboratory.

• Sample bottles must be shipped to the laboratory

by an overnight carrier or hand delivered. 

• The laboratory must be notified on the day of 

sample collection of their scheduled arrival at the 

laboratory on the following day.

THERE WILL BE NO SATURDAY DELIVERIES

Additional Supplies: 

• Aluminum foil for placement of sampling 

equipment during sample collection (store foil in 

clean, sealed plastic bag). 

2-inch packing tape for sealing tops/bottoms of UL 

shipping boxes or other larger shipping boxes 

containing multiple samples.

Refrigerant Packs

• Freeze refrigerant packs prior to use in the foaminsulated shipping containers. Store with other

packs in larger cooler when transporting to the field 

for final use and shipment to help retain their frozen 

state.

Field Notes/Chain-of-Custody

• Chain of Custody form also will be used to list 

final stabilized field water-quality values and for 

field notes of conditions that might affect sample 

results, including air temperature, weather, 

composition and cleanliness of sample tap, 

conditions of sample location and facility, lot

number and expiration date of inorganic 

blank water. 

61
Appendix 3. Example Illinois Environmental Protection Agency field 

sampling record

USGS note: The recorded EH (redox) values represent readings from a water-quality meter sensor

that was not calibrated prior to use for measurement of field water-quality characteristics. Readings 

from this sensor only were used (along with the calibrated-sensor readings of other characteristics) to

determine stabilized field conditions necessary to ensure collection of water samples representative of

aquifer water quality. EH (redox) values are not archived in the IEPA water-quality database.

62 Hexavalent and Total Chromium at Low Reporting Concentrations in Source-Water Aquifers and Surface Waters 
Appendix 4. Example analytical result report and quality-assurance 

documentation provided by UL laboratory

	


	
	

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Page 6 of 10

USGS note: The recorded EH (redox) values represent readings from a water-quality meter sensor that was not calibrated prior to use for measurement of field water-quality characteristics. Readings from this sensor only were used (along with the calibrated-sensor readings of other characteristics) to determine stabilized field conditions necessary to ensure collection of water samples representative of aquifer water quality. EH (redox) values are not archived in the IEPA water-quality database. 
Page 7 of 10

69
Page 8 of 10

70
Page 9 of 10

71
Page 10 of 10

72

Mills and Cobb—Hexavalent and Total Chromium in Source-Water Aquifers and Surface Waters Used for Public Supply, Illinois—Scientific Investigations Report 2015–5020

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155020