U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007–5036

The Association of Arsenic With Redox Conditions, Depth, 

and Ground-Water Age in the Glacial Aquifer System of the 

Northern United States

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM
Front-cover image: Mixed land uses are spread across this glacial landscape in Dane County, south-central Wisconsin. 

Water samples from 56 wells throughout Wisconsin were analyzed for the study described in this report. (File photograph from the USGS Wisconsin Water Science Center, Middleton, Wis.)
The Association of Arsenic With Redox 

Conditions, Depth, and Ground-Water 

Age in the Glacial Aquifer System of the 

Northern United States

By Mary Ann Thomas

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

 Scientific Investigations Report 2007–5036

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Thomas, M.A., 2007, The association of arsenic with redox conditions, depth, and ground-water age in the glacial 

aquifer system of the Northern United States: U.S. Geological Survey Scientific Investigations Report 2007–5036, 

26 p. 
iii

FOREWORD

The U.S. Geological Survey (USGS) is committed to providing the Nation with credible scientific 

information that helps to enhance and protect the overall quality of life and that facilitates 

effective management of water, biological, energy, and mineral resources (http://www.usgs.

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of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish 

and wildlife. Population growth and increasing demands for water make the availability of that 

water, now measured in terms of quantity and quality, even more essential to the long-term 

sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 to 

support national, regional, State, and local information needs and decisions related to waterquality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program is 

designed to answer: What is the condition of our Nation’s streams and ground water? How are 

conditions changing over time? How do natural features and human activities affect the quality 

of streams and ground water, and where are those effects most pronounced? By combining 

information on water chemistry, physical characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based insights for current and emerging water issues 

and priorities. From 1991 to 2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation’s 

river basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studyu.

html). 

In the second decade of the Program (2001–2012), a major focus is on regional assessments 

of water-quality conditions and trends. These regional assessments are based on major river 

basins and principal aquifers, which encompass larger regions of the country than the Study 

Units. Regional assessments extend the findings in the Study Units by filling critical gaps in 

characterizing the quality of surface water and ground water, and by determining status and 

trends at sites that have been consistently monitored for more than a decade. In addition, the 

regional assessments continue to build an understanding of how natural features and human 

activities affect water quality. Many of the regional assessments employ modeling and other 

scientific tools, developed on the basis of data collected at individual sites, to help extend 

knowledge of water quality to unmonitored, yet comparable areas within the regions. The 

models thereby enhance the value of our existing data and our understanding of the hydrologic 

system. In addition, the models are useful in evaluating various resource-management scenarios 

and in predicting how our actions, such as reducing or managing nonpoint and point sources of 

contamination, land conversion, and altering flow and (or) pumping regimes, are likely to affect 

water conditions within a region.

Other activities planned during the second decade include continuing national syntheses of 

information on pesticides, volatile organic compounds (VOCs), nutrients, selected trace elements, and aquatic ecology; and continuing national topical studies on the fate of agricultural 

chemicals, effects of urbanization on stream ecosystems, bioaccumulation of mercury in stream 

ecosystems, effects of nutrient enrichment on stream ecosystems, and transport of contaminants to public-supply wells.
iv

The USGS aims to disseminate credible, timely, and relevant science information to address 

practical and effective water-resource management and strategies that protect and restore 

water quality. We hope this NAWQA publication will provide you with insights and information 

to meet your needs, and will foster increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all waterresource issues of interest. External coordination at all levels is critical for cost-effective management, regulation, and conservation of our Nation’s water resources. The NAWQA Program, 

therefore, depends on advice and information from other agencies—Federal, State, regional, 

interstate, Tribal, and local—as well as nongovernmental organizations, industry, academia, and 

other stakeholder groups. Your assistance and suggestions are greatly appreciated.

Robert M. Hirsch 

Associate Director for Water
Contents

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

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

 Background ......................................................................................................................................2

 Previous Studies ..............................................................................................................................2

 Methods.............................................................................................................................................4

 Description of the Study Area ......................................................................................................4

Arsenic Concentrations and Related Factors in the Glacial Aquifer System .....................................7

 Arsenic Concentrations ................................................................................................................7

 Redox Conditions .............................................................................................................................9

 Well Depth and Well Type .............................................................................................................9

 Ground-Water Age .......................................................................................................................12

 Other Water-Quality Properties ..................................................................................................16

 Relative Significance of Factors Related to Arsenic .............................................................17

Regional Variation of Arsenic and Redox Conditions Within the Glacial Aquifer System...............19 

 Regional Framework of the Glacial Aquifer System ..............................................................19

 Comparison of Arsenic and Related Factors Among Framework Areas..............................21

 Relation to Conceptual Model for High-Arsenic Environments.............................................21 

Summary and Conclusions.........................................................................................................................24

References Cited..........................................................................................................................................25

Figures

1-3. Maps showing—

1. Location of wells in the glacial aquifer system sampled for arsenic by the National 

Water-Quality Assessment Program, 1991–2003.....................................................................3

2. Bedrock beneath glacial deposits. .........................................................................................................5

3. Thickness and texture of glacial deposits near land surface . .........................................................6

4. Graph showing requency of arsenic concentrations in samples from the glacial 

aquifer system...............................................................................................................................7

5. Map showing arsenic concentrations in samples from the glacial aquifer system ......................8

6-11. Graphs showing—

6. Frequency of estimated redox conditions of samples from the glacial aquifer system, 

and arsenic concentrations for samples in each redox category . ...................................10

7. Relation of arsenic concentration and redox condition to well depth for the three types 

of wells sampled. ........................................................................................................................11

8. Relation of arsenic concentrations to estimated ground-water age determinations from 

chlorofluorocarbons, sulfur hexafluoride, or tritium/helium-3 methods ...........................13

9. Relation of arsenic concentrations to estimated ground-water age determinations from 

tritium concentrations................................................................................................................14

10. Relation of arsenic concentration and redox condition to well depth for young and 

old waters ....................................................................................................................................15

11. Percentage of samples with elevated arsenic concentrations for young and old waters 

in three redox categories ..........................................................................................................16
vi

12. Map showing framework areas of the glacial aquifer system ......................................................20

13-15. Graphs showing—

13. Arsenic in relation to redox and well-depth relations for framework areas of the 

glacial aquifer sysem.................................................................................................................22

14. Comparison of arsenic concentration and redox-related characteristics among 

framework areas of the glacial aquifer system: percentage of samples with 

elevated arsenic concentrations, percentage of samples with strongly 

reducing conditions, median concentrations of dissolved organic carbon 

and median concentrations of ammonia ................................................................................23

15. Comparison of oxic samples among framework areas of the glacial aquifer system: 

percentage of oxic samples, median arsenic concentrations in oxic samples, 

median orthophosphate concentrations in oxic samples, and median pH 

in oxic samples............................................................................................................................23

Tables

1. Concentrations of water-quality constituents used to classify redox conditions of 

samples from the glacial aquifer system..................................................................................7 

2. Results of Spearman’s rho tests for the 12 water-quality constituents that have the 

best correlations with arsenic..................................................................................................17

3. Relative significance of individual variables as indicators of elevated arsenic 

concentrations, based on results of univariate logistic regression .................................18

Conversion Factors and Abbreviations

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Volume

liter (L) 0.2642 gallon (gal)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

Concentration of chemical constituents in water are given in either 

milligrams per liter (mg/L) or micrograms per liter (μg/L).
Abstract

More than 800 wells in the glacial aquifer system of the 

Northern United States were sampled for arsenic as part of 

U.S. Geological Survey National Water-Quality Assessment 

(NAWQA) studies during 1991–2003. Elevated arsenic concentrations (greater than or equal to 10 micrograms per liter) 

were detected in 9 percent of samples. 

Elevated arsenic concentrations were associated with 

strongly reducing conditions. Of the samples classified as 

iron reducing or sulfate reducing, arsenic concentrations were 

elevated in 19 percent. Of the methanogenic samples, arsenic 

concentrations were elevated in 45 percent. In contrast, concentrations of arsenic were elevated in only 1 percent of oxic 

samples. 

Arsenic concentrations were also related to ground-water 

age. Elevated arsenic concentrations were detected in 34 

percent of old waters (recharged before 1953) as compared to 

4 percent of young waters (recharged since 1953). For samples 

classified as both old and methanogenic, elevated arsenic concentrations were detected in 62 percent of samples, as compared to 1 percent for samples classified as young and oxic. 

Arsenic concentrations were also correlated with well 

depth and concentrations of several chemical constituents, 

including (1) constituents linked to redox processes and (2) 

anions or oxyanions that sorb to iron oxides. 

Observations from the glacial aquifer system are consistent with the idea that the predominant source of arsenic 

is iron oxides and the predominant mechanism for releasing 

arsenic to the ground water is reductive desorption or reductive dissolution. Arsenic is also released from iron oxides 

under oxic conditions, but on a more limited basis and at lower 

concentrations. 

Logistic regression was used to investigate the relative 

significance of redox, ground-water age, depth, and other 

water-quality constituents as indicators of elevated arsenic 

concentrations in the glacial aquifer system. The single variable that explained the greatest amount of variation in the data 

was redox. Multivariate models that included a redox variable 

overestimated the percentage of samples with elevated arsenic 

concentrations because, even though elevated arsenic concentrations were associated with strongly reducing samples, not 

all strongly reducing samples had elevated arsenic concentrations. 

Arsenic concentrations and redox conditions differed 

among four broad areas of the glacial aquifer system. For the 

East, Central, and West-Central north areas, there was a trend 

of increasing arsenic concentrations that corresponded to an 

increase in reducing conditions. For the West-Central south 

area, arsenic concentrations in oxic samples were higher than 

for the other areas, possibly because of high concentrations of 

orthophosphate, which is linked to desorption of arsenic from 

iron oxides under oxic conditions. 

The observed differences in arsenic concentrations among 

broad areas of the glacial aquifer system were generally 

consistent with a conceptual model developed by Smedley and 

Kinniburg1

, who studied or reviewed studies of widespread 

arsenic contamination in Bangladesh, India, China, Vietnam, 

Hungary, Argentina, northern Chile and the Southwestern 

United States. 

Introduction 

The U.S. Environmental Protection Agency (USEPA) 

recently lowered the Maximum Contaminant Level (MCL) 

for arsenic from 50 μg/L to 10 μg/L in recognition of the 

detrimental health effects associated with arsenic in drinking 

water, including bladder, skin, and lung cancers; diabetes; and 

neurological dysfunction (National Research Council, 1999). 

The new MCL went into effect January 2006 for public watersupply systems but not for domestic wells (U.S. Environmental Protection Agency, 2006). Domestic wells are not routinely 

tested for arsenic in most parts of the country, so homeowners 

may not know whether their well water has arsenic concentrations greater than the MCL. 

1

Smedley, P.L., and Kinniburg, D.G., 2002, A review of the source, behaviour, and distribution of arsenic in natural waters: Applied Geochemistry, 

v. 17, p. 517–568.

The Association of Arsenic With Redox Conditions, Depth, 

and Ground-Water Age in the Glacial Aquifer System of 

the Northern United States

By Mary Ann Thomas
Arsenic concentrations in ground water have considerable 

spatial variability and are difficult to predict on a well-by-well 

basis; however, the risk of high concentrations is greater in 

some areas than in others (Smedley and Kinniburg, 2002). If 

the most susceptible aquifers (or parts of aquifers) could be 

identified, efforts related to monitoring or education could be 

targeted to areas of greatest need. 

The U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program has been systematically sampling the Nation’s ground water since 1991. Between 

1991 and 2003, NAWQA analyzed for arsenic and other 

water-quality properties in 813 samples from the glacial aquifer system in the Northern United States (fig. 1). The samples 

were selected on the basis of a stratified random design, to be 

representative of important hydrogeologic settings within the 

glacial aquifer system. A synthesis of these data provides a 

large-scale survey of arsenic in ground water, reflecting comparable methods of collection, analysis, and quality control. 

The goals of this report are to (1) document arsenic 

concentrations in samples from the glacial aquifer system; (2) 

describe relations between arsenic concentrations and redox 

conditions, well depth, ground-water age, and other waterquality variables; and (3) document variation in arsenic and 

related factors within the glacial aquifer system. 

Background

Arsenic is relatively abundant in nature. Although arsenic 

in water can come from anthropogenic sources, most instances 

of widespread arsenic contamination in ground water are 

from naturally occurring minerals (Welch, and others, 2000; 

Smedley and Kinniburg, 2002). Arsenic can occur in multiple 

minerals, including sulfides (especially pyrite), hydrous metal 

oxides (especially iron oxides), coal, ironstones, clays, phosphates, silicates, and carbonates. The most important sources 

of elevated arsenic in ground water are pyrite and iron oxides, 

partly because of their abundance in aquifers (Smedley and 

Kinniburg, 2002). Pyrite is an iron sulfide mineral, and arsenic 

can be present as part of the mineral structure. Iron oxides 

occur as coatings on mineral grains; arsenic can coprecipitate 

with iron oxides or sorb to their surface. Pyrite and iron oxides 

can be enriched in distinct horizons or disseminated throughout the aquifer matrix. 

Arsenic concentrations in the aquifer solid phase do not 

always correspond to arsenic concentrations in ground water; 

multiple studies have found little or no correlation between 

the two (Warner, 2001; Welch and others, 2000; Kolker and 

others, 2003; Smedley and Kinniburg, 2002; Erickson and 

Barnes, 2005). Most instances of widespread arsenic enrichment in the ground water are in areas where arsenic concentrations in the solid phase are within the range of normal concentrations (Smedley and Kinniburg, 2002). 

In ground water, arsenic has two common oxidation 

states. The predominant form in oxic waters is arsenate, an 

oxyanion with the +5 oxidation state. In reduced waters, the 

predominant form is arsenite, an uncharged species with a -3 

oxidation state. Because arsenite is uncharged, it is the more 

mobile in solution. Arsenite is also considered to be the more 

toxic form. 

Because of its two common oxidation states, arsenic can 

be released from minerals to ground water (mobilized) under 

either oxic or reducing conditions. Under oxic conditions, 

arsenic can be mobilized by dissolution of pyrite or by desorption from iron oxides due to an increase in pH or competition 

with other anions (Welch and others, 2000; Smedley and Kinniburg, 2002). Under reducing conditions, arsenic associated 

with iron oxides can be mobilized by reductive desorption or 

reductive dissolution. Reductive desorption occurs when (1) 

arsenate is reduced to arsenite, which is less strongly sorbed to 

iron oxides, or (2) the charge on the mineral surface changes 

and allows arsenate to be desorbed. Reductive dissolution of 

iron oxides releases arsenic that is part of the mineral structure 

or sorbed to the surface. 

Pyrite and iron oxides are also important sinks for arsenic, so the formation of these minerals can decrease arsenic 

concentrations in ground water. 

Previous studies

Arsenic concentrations in ground water of the United 

States were documented by Welch and others (2000) and 

Ryker (2003). Their dataset included about 30,000 samples 

from varied aquifers and springs in 48 states; of those, about 

11 percent had arsenic concentrations greater than the MCL 

of 10 μg/L. Highest arsenic concentrations were detected in 

the West, but concentrations in the Midwest were higher than 

previously recognized. 

In New England, elevated arsenic concentrations in 

glacial valley-fill deposits and the underlying crystalline 

bedrock have been linked to desorption related to an increase 

in pH (Ayotte and others, 2003). Widespread contamination of 

ground water by arsenic from oxidation of sulfides has been 

documented in parts of eastern Wisconsin, an area where glacial deposits overlie a Paleozoic sandstone aquifer (Schreiber 

and others, 2003). Reductive desorption or dissolution of iron 

oxides was linked to elevated arsenic concentrations in glacial 

deposits and (or) the underlying bedrock in Ohio (Matisoff 

and others, 1982), the Midwest (Korte, 1991; Welch and others, 2000; Thomas, 2003), southeastern Michigan (Kolker and 

others, 2003), central Illinois (Kirk and others, 2004; Kelly 

and others, 2005), and North Dakota and Minnesota Erikson 

and Barnes, 2005). 

Smedley and Kinniburg (2002) synthesized common 

features of widespread arsenic contamination in ground 

water from around the world, including Bangladesh, India, 

Vietnam, China, Hungary, Argentina, northern Chile, and the 

Southwestern United States. They developed a conceptual 

model for widespread arsenic contamination of ground water 

in low-temperature, nonmineralized areas. In most areas of 

widespread arsenic contamination, the predominant source of 

 Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
arsenic is iron oxides. For elevated arsenic concentrations in 

ground water to develop, a geochemical trigger is needed to 

release arsenic from minerals. One possible trigger is the onset 

of strongly reducing conditions, which result from degradation of organic carbon. A confining layer near land surface can 

help maintain reducing conditions by retarding the transport 

of oxygen (or other electron acceptor) from the land surface. 

Arsenic can also be released from iron oxides under oxic 

conditions due to an increase in pH and (or) competitive sorption by phosphate or other anions. After arsenic is mobilized, 

it can accumulate in ground water if the rate of flushing is low 

relative to the rate of mobilization. Therefore, arsenic risk is 

high in areas of sluggish ground-water flow (in either oxic or 

reducing conditions). 

High-arsenic environments include deltas or alluvial 

plains, where large volumes of young sediment have undergone rapid burial and strongly reducing conditions can 

develop. These environments also tend to have associated 

fine-grained sediments that can form a confining or semiconfining layer close to land surface. In addition, the land surface 

is typically flat, low lying and poorly drained, and hydraulic 

gradients are low. As a result, arsenic is not flushed away 

faster than it can be mobilized. Inland or closed basins in arid 

or semiarid areas are also at risk because high pH can develop 

Figure 1. Location of wells in the glacial aquifer system sampled for arsenic by the National Water-Quality Assessment Program, 

1991–2003.

Introduction 
and lead to arsenic desorption. In addition, the low flushing 

rates associated with arid or semi-arid environments allow 

arsenic to accumulate in the ground water.

Data from the glacial aquifer system were not explicitly 

included in the development of the conceptual model. Therefore, it might be useful to examine the model in light of data 

from the current study. 

Methods

Data were collected in accordance with NAWQA protocols for study design and site selection (Gilliom and others, 

1995; Scott, 1990; Squillace and Price, 1996), well selection 

or installation (Lapham and others, 1995), sample collection, 

sample preservation, and quality-assurance/quality-control 

methods (Koterba and others, 1995; U.S. Geological Survey, 

variously dated). Ground-water samples were analyzed at the 

USGS National Water Quality Laboratory for major ions and 

trace elements (Fishman, 1993; Garbarino, 1999), dissolved 

organic carbon (Brenton and Arnett, 1993), tritium (Ostlund 

and Dorsey, 1977), CFCs (Plummer and others, 1993), and 

SF6 (Busenberg and Plummer, 2000). Tritium-helium-3 was 

analyzed for at Lamont-Doherty Earth Observation Noble 

Gas Facility (Ekwurzel and others, 1994). Multiple organic 

constituents, including volatile organic compounds and 

pesticides, also were analyzed for as part of NAWQA studies 

(http://water.usgs.gov/nawqa), but results of these analyses are 

not included in this report.

The laboratory reporting limit for arsenic and several 

other constituents changed between 1991 and 2003. To 

account for this, concentrations were censored to the highest common reporting level, which was 1 μg/L for arsenic. 

Because concentrations of arsenic and several other constituents did not follow a normal statistical distribution, statistical 

analysis was done by means of nonparametric methods (Helsel 

and Hirsch, 2002). Median and 75th- percentile values were 

used to summarize concentrations of water-quality constituents in groups of data. Kruskal-Wallis and Tukey tests were 

used to determine whether concentrations of constituents were 

significantly different among groups of samples. Spearman’s 

rho was used to identify significant monotonic correlations 

between arsenic and other variables. Logistic regression was 

used to evaluate the relative significance of variables as indicators of elevated arsenic concentrations (Hosmer and Lemeshow, 1989). 

Description of the Study Area

The glacial aquifer system is one of the most important 

sources of ground water in the Northern United States. Glacial 

deposits extend across parts of 26 states and were the source 

of drinking water for 41 million people during 2000 (Warner 

and Arnold, 2005). For this report, the glacial aquifer system 

is considered to include unconsolidated deposits above bedrock north of the line of continental glaciation (fig. 1; Warner 

and Arnold, 2005). Areas where glacial deposits are part of 

alluvium or flood deposits that extend south of the boundary 

of continental glaciation are not considered in this report. 

A large proportion of glacial deposits is derived from 

local bedrock or bedrock that is upgradient relative to the 

direction of movement of the glacial lobes. Bedrock beneath 

glacial deposits includes sedimentary and crystalline rocks of 

Precambrian through Tertiary age (fig. 2). The largest proportion of the study area is underlain by Paleozoic and Cretaceous 

sedimentary bedrock (predominantly carbonates, shale, and 

sandstone). In the northeastern part of the study area, bedrock 

is predominantly crystalline (igneous) and metamorphosed 

sedimentary rock. Crystalline rock also subcrops in part of 

Wisconsin, Michigan, Minnesota, and Alaska. 

Glacial deposits in the study area range in thickness from 

less than 1 to more than 600 ft (fig. 3). In general, glacial 

deposits are thinner and less extensive in the eastern part of the 

study area. Glacial deposits also vary in terms of texture, from 

clay to boulders. For this report, glacial deposits are referred 

to as “coarse grained” (predominantly sand or gravel) or “fine 

grained” (predominantly till or clay). The distribution of 

coarse- and fine-grained deposits near land surface is shown in 

figure 3. Sediment texture is not typically uniform with depth, 

so coarse-grained deposits can underlie fine-grained deposits 

and vice versa. 

Ground water is typically produced from coarse-grained 

deposits, which can be at land surface or beneath fine-grained 

deposits. The coarse-grained deposits typically occur as layers, lenses, or linear valley-fill deposits. Sediment texture 

near land surface and aquifer geometry are related to intrinsic 

susceptibility, or the ease with which water enters and moves 

through the glacial aquifer system (Warner and Arnold, 2005). 

Additional details of the glacial aquifer system are discussed 

in a later section of this report (“Regional Variation of Arsenic 

and Redox Conditions Within the Glacial Aquifer System”). 

Water-quality data collected as part of NAWQA studies 

are not uniformly distributed across the glacial aquifer system 

(figs. 1–5). Instead, data are from 26 well networks, each of 

which includes 20–30 wells. Well locations were determined 

by a stratified random design (Gilliom and others, 1995). As a 

result, well networks are representative of hydrogeologic and 

(or) land-use settings in the glacial aquifer system. Networks 

of domestic or public-supply wells are representative of 

hydrogeologic settings that are important in terms of local or 

regional water supply. Monitor-well networks are installed just 

below the water table, and the stratification includes land use 

and hydrogeologic setting. 

 Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
Figure 2. Bedrock beneath glacial deposits. (Data not shown for Alaska.)

Introduction 
Figure 3. Thickness and texture of glacial deposits near land surface. (Data not shown for Alaska.)

 Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
Arsenic Concentrations and Related 

Factors in the Glacial Aquifer System 

This section of the report documents arsenic concentrations and related factors in the glacial aquifer system as 

a whole, and the analyses include all samples with arsenic 

determinations. Later sections of the report consider subsets of 

the data. 

Arsenic Concentrations

The frequency distribution of arsenic concentrations is 

shown in figure 4. The median concentration was less than 

1 μg/L. About 9 percent of samples had arsenic concentrations 

greater than or equal to the USEPA MCL of 10 μg/L. (In this 

report, arsenic concentrations greater than or equal to 10 μg/L 

are referred to as “elevated.”) About 1 percent of samples had 

concentrations greater than or equal to 50 μg/L, which was the 

USEPA MCL prior to January 2006. The maximum arsenic 

concentration was 340 μg/L. This might be considered an 

outlier because it was almost 4 times higher than the secondhighest concentration of 84 μg/L; however, the maximum 

concentration was confirmed by analysis of a duplicate sample 

(Andrews and others, 1998).

Arsenic concentrations were measured in water samples 

from 813 wells in 18 states. The spatial distribution of arsenic 

concentrations is shown in figure 5. In most areas, the samples 

had a wide range of concentrations. There is no evident spatial 

pattern that links elevated arsenic concentrations to a particular type of bedrock geology (fig. 2) or glacial deposit (fig. 3). 

500

250

300

0

400

550

30%

61%

1%

NUMBER OF SAMPLES

<1 1 to <10 10 to <50 50 to 340

 Maximum 

8%

 Level = 10 Pg/L 

ARSENIC, IN MICROGRAMS PER LITER 

100

450

200

150

350

50

 Common reporting

 limit = 1 Pg/L Contaminant

Figure 4. Frequency of arsenic concentrations in 

samples from the glacial aquifer system.

Table 1. Concentrations of water-quality constituents used to classify redox conditions of samples from the glacial aquifer system of 

the Northern United States. 

[Modified from Paschke and others, in press; mg/L, milligrams per liter; μg/L, micrograms per liter; “-”, constituent concentration not specified]

Redox classification1

Abbreviation 

used in this 

report

Oxygen 

(mg/L)

Nitrate

(mg/L as N)

Manganese

(μg/L)

Iron 

(μg/L)

Sulfate 

(mg/L)

Oxygen reducing Oxic ≥0.5 - <50 <100 -

Nitrate reducing NO3 <.5 ≥0.5 <50 <100 -

Manganese reducing Mn <.5 <.5 ≥50 <100 -

Iron or sulfate reducing2 Fe/SO4 <.5 < .5 - ≥100 ≥4

Methanogenic2 Meth <.5 <.5 - ≥100 <4

1

Samples that do not fall into one of these categories are referred to as “mixed or unknown.” 

2

These classifications are collectively referred to as “strongly reducing.” 

Arsenic Concentrations and Related Factors in the Glacial Aquifer System 
Figure 5. Arsenic concentrations in samples from the glacial aquifer system.

 Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
Well Depth and Well Type

The water samples were collected from 411 monitor 

wells, 277 domestic wells, 104 public-supply wells, and 21 

other well types. (Hereafter, the “other” wells are included 

in the “public-supply” category.) The wells were from 8 to 

425 ft deep, and the median depth was 44 ft. For the dataset 

as whole, the relation between arsenic concentrations and 

well depth was significant (p<0.0001) but weak (Spearman’s 

rho=0.24). When the data were subdivided by well type, 

arsenic concentrations were correlated with well depth for 

domestic wells (p<0.0001; rho=0.37) and public-supply wells 

(p<0.0001; rho=0.51) but not for monitor wells (p=0.32). 

Relations among arsenic concentration, redox condition, 

and well depth for each well type are shown in figure 7. Of the 

three well types, monitor wells were shallowest (median depth 

of 26 ft) and had the lowest percentage of samples with elevated arsenic concentrations (4 percent) and strongly reducing 

conditions (18 percent). Compared to monitor wells, domestic 

wells were deeper (median depth of 82 ft) and had a higher 

percentage of samples with elevated arsenic concentrations (14 

percent) and strongly reducing conditions (47 percent). 

Public-supply wells were similar to domestic wells in 

terms of median depth (80 ft) but differed in terms of redox 

conditions; samples from public-supply wells had a lower 

percentage of strongly reducing conditions and higher percentages of oxic and mixed redox conditions (fig. 7). In general, 

samples from public-supply wells had redox conditions that 

were intermediate between those of monitor and domestic 

wells. Public-supply wells also had longer open intervals and 

higher discharge rates than domestic (or monitor) wells (fig. 

7). The long open intervals and higher discharge rates of public-supply wells can cause mixing of ground water from multiple depths, with differing redox conditions and (or) groundwater ages (Landon and others, 2006). For this reason, further 

discussion of relations among depth, redox, and ground-water 

age are limited to the subset of data from monitor and 

domestic wells. 

Redox Conditions

Arsenic can be released to (or removed from) the ground 

water as a result of redox reactions, a term used as a simplification for terminal electron accepting processes (TEAPS). 

Redox reactions involve the transfer of electrons from an electron donor to an electron acceptor. In ground water, the electron donor is typically organic carbon; however, sulfide minerals and some manufactured organic compounds also can act 

as electron donors. Electron acceptors include oxygen, nitrate, 

manganese, ferric iron, sulfate, and carbon dioxide. During 

the transfer of electrons, organic carbon is oxidized, and the 

electron acceptors are reduced in sequential reactions that are 

mediated by microbes. In general, ground-water systems with 

very low concentrations of organic carbon (or other electron 

donor) are oxic, whereas systems with high concentrations of 

organic carbon (or other electron donor) are reducing.

For the current study, redox conditions of ground-water 

samples were classified on the basis of concentrations of 

five chemical constituents—dissolved oxygen (DO), nitrate, 

manganese, ferrous iron, and sulfate (table 1; modified from 

Paschke and others, in press). This system, adopted by the 

NAWQA Program, uses concentrations thresholds for five 

constituents to define five redox categories. Samples that did 

not correspond to any of the categories were classified as 

“mixed or unknown.” The term “strongly reducing” is used 

hereafter as a substitute for “iron reducing or sulfate reducing 

or methanogenic.” 

The fact that the redox classification system does not distinguish between iron-reducing and sulfate-reducing categories 

is limiting because arsenic concentrations can vary between 

the two redox conditions. In iron-reducing conditions, arsenic can go into solution as a result of reductive dissolution or 

desorption from metal oxides. By contrast, in sulfate-reducing 

conditions, arsenic can be removed from solution as a result of 

coprecipitation with metal sulfide minerals. 

The frequency distribution of redox conditions of samples 

from the glacial aquifer system is shown in figure 6A; 38 

percent of samples were oxic, 4 percent were nitrate reducing, 

9 percent were manganese reducing, 23 percent were iron or 

sulfate reducing, 6 percent were methanogenic, and 20 percent 

were mixed or unknown. 

In general, as waters become more reducing, arsenic 

concentrations were detected more frequently (fig. 6B). For 

example, 1 percent of oxic waters had elevated arsenic concentrations, as compared to 19 percent of iron or sulfate reducing 

waters and 45 percent of methanogenic waters. In addition, 

arsenic concentrations were higher in more reducing waters, 

based on the 75th-percentile values (table in fig. 6).

Relations shown in fig. 6 are consistent with the hypothesis that the predominant mechanism for arsenic release is 

reductive desorption or dissolution. However, a small percentage of the elevated arsenic concentrations were in oxic 

samples, which suggests that another mechanism releases 

arsenic to ground water in oxic conditions. 

Arsenic Concentrations and Related Factors in the Glacial Aquifer System 
Oxic Fe/SO Meth 4

90

80

70

60

10

20

30

40

50

ARSENIC, IN MICROGRAMS PER LITER

 0

NUMBER OF SAMPLES

38%

Oxic NO3 Fe/SO4 Meth Mix

4%

20%

23%

6%

N=813

0

50

100

150

200

250

300

350

NO3

REDOX

Mn

9%

Mn

A

B

Mix*

 Maximum

Contaminant Level

 

1 Pg/L, micrograms per liter. 

Summary statistics Oxic NO3 Mn Fe/SO4 Meth Mix 

Number of samples 311 30 73 188 51 160 

Percentage of samples 

with arsenic>10 Pg/L1 1 0 4 19 45 5 

Arsenic (Pg/L)1 

 Median <1 <1 <1 2 7 <1 

 75th percentile <1 <1 2.0 6.5 29 2.1 

 Maximum 15 8 22 53 84 340

=10 ug/L

Figure 6. A, Frequency of estimated redox conditions of samples from the glacial aquifer system. B, Arsenic 

concentrations for samples in each redox category.

10 Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
 Other

WELL DEPTH, IN FT

ARSENIC, IN MICROGRAMS PER LITER

450

1 10 100

0

200

100

300

400

Monitor Domestic *

Maximum

Contaminant

Level

1 100

EXPLANATION

Redox conditions:

x

Oxic

NO3

Mn

Mix Methanic

Fe/SO4

10

Well type Well and sample characteristics Monitor Domestic Public supply 

Number of samples 411 277 125 

Well depth (median, in feet) 26 82 80 

Open interval (median, in feet) 5 4 12 

Well discharge (median, in gallons per minute) 1 18 376 

Percentage of samples with arsenic >10 Pg/L1 4 14 12 

Percentage of samples with redox classification: 

 Oxic 43 32 37 

 Mixed or unknown 24 13 22 

 Strongly reducing 18 47 28 

1 Pg/L, micrograms per liter. 

1 10 100

Figure 7. Relation of arsenic concentration and redox condition to well depth for the three types of wells sampled. (See table 1 

for explanation of redox abbreviations. *Not shown is sample with arsenic concentration of 340 μg/L and mixed redox conditions 

at 18 feet.)

Arsenic Concentrations and Related Factors in the Glacial Aquifer System 11
Ground-Water Age 

Of the 688 samples from monitor or domestic wells, 

ground-water age was determined for 483. The term “groundwater age” refers to the time since water recharged the groundwater system (or was isolated from the atmosphere). Ages 

were determined by analysis for one or more of the following 

environmental tracers: chlorofluorocarbons (CFCs), sulfur 

hexafluoride (SF6), tritium/helium-3 (3H/3He), or tritium. CFCs 

and SF6 are anthropogenic compounds with known inputs, 

whereas 3H/3He and tritium methods are based on radioactive 

decay since the time of a known event (Plummer and others, 

1993). 

For 219 samples from monitor and domestic wells, age 

dates were based on CFCs, SF6, or 3H/3He tracers. The samples were collected over a range of years, so the recharge year 

was subtracted from the collection year to estimate the age of 

the sample. Elevated arsenic concentrations (≥10 μg/L) were 

detected only in samples older than 15 years (fig. 8). Samples 

less than 5 years old were predominantly oxic (81 percent), 

and most of the young, oxic waters had arsenic concentrations less than 1 μg/L. As the age of the water increases, redox 

conditions become more reducing, and arsenic concentrations 

increase. Samples older than 15 years had elevated arsenic 

concentrations in 7 to 8 percent of samples. 

The tritium method was used to determine ages of 264 

samples from monitor and domestic wells. The tritium agedating method differs from the other three methods in that it 

provides a binary indication of ground-water age—whether 

a sample was recharged before or after 1953. If measurable tritium (>1 tritium unit) is detected in a sample, it is 

assumed that at least some fraction of the water was recharged 

since 1953. For this report, waters recharged before 1953 

are referred to as “old” and those recharged since 1953 are 

referred to as “young.” 

Arsenic concentrations were significantly higher in oldwater samples than in young-water samples (fig. 9). Of the 

203 young-water samples, 5 percent had elevated arsenic concentrations. Of the 61 old samples, 43 percent of had elevated 

arsenic concentrations. Young and old waters also differed 

in terms of redox conditions (fig. 9). For young waters, the 

highest proportion of samples (33 percent) were oxic and had 

arsenic concentrations less than 1 μg/L. For old waters, less 

than 10 percent were oxic. Most old waters (66 percent) were 

strongly reducing. Of the old and strongly reducing samples, 

60 percent had elevated arsenic concentrations.

 The two types of age data were combined by categorizing age dates from CFCs, SF6

, and 3H/3He methods according 

to whether the recharge year was before or after 1953. For 

this combined dataset, elevated arsenic concentrations were 

detected in 4 percent of young-water samples and 34 percent 

of old-water samples (fig. 10). 

Relations between depth, redox and age are not simple 

(fig. 10). As a generalization, old and reducing water is more 

prevalent at greater depths. However, elevated arsenic concentrations can be detected in samples from a range of depths 

because (1) old waters can occur at shallow depths, (2) old, 

deep waters can be oxic, and (3) young, shallow waters can be 

reducing. For example, the maximum arsenic concentration of 

340 μg/L was detected in a sample that was young water from 

a shallow well (18 ft), and the redox condition was categorized 

as mixed. 

The percentages of samples with elevated arsenic, by 

redox and age categories are shown in figure 11. For young 

and old waters alike, the percentage of samples with elevated 

arsenic concentrations was greater in more reducing conditions, but the trend is more pronounced for old waters. For 

example, 17 percent of young and methanogenic samples had 

elevated arsenic concentrations, as compared to 62 percent of 

old and methanogenic samples. 

12  Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
Less than 5 years 5 to <15 years 

15 to <30 years 

PERCENTAGE OF SAMPLES

0

20

30

40

50

80

70

60

10

90

 Less than 5 5 to <15 15 to <30 30-58 

ARSENIC, IN MICROGRAMS PER LITER

0

20

30

40

50

80

70

60

10

90

 

N=219

Oxic NO3 Mn Fe/SO4 Meth Mix

0

20

30

40

50

80

70

60

10

90

0

20

30

40

50

80

70

60

10

90

0

20

30

40

50

80

70

60

10

90

PERCENTAGE OF SAMPLES

 EXPLANATION

Arsenic, in micrograms per liter

<1 1 to <10 >10

 based on chloroflurocarbon, sulfur hexafluoride, 

Oxic NO3 Mn Fe/SO4 Meth Mix

Oxic NO3 Mn Fe/SO4 Oxic NO Meth Mix 3 Mn Fe/SO4 Meth Mix

Age, in years, 

 MCL

30 to 58 years 

REDOX REDOX

REDOX REDOX

Less 

than 5 

5 to

<15 

15 to 

<30 

30 to

58

Number of 

samples 48 63 59 49 

Percentage of 

samples with 

arsenic >10 Pg/L1

0 0 7 8 

1

μg/L, micrograms per liter 

 or tritium/helium-3 data 

Figure 8. Relation of arsenic concentrations to estimated ground-water age determinations from chlorofluorocarbons, 

sulfur hexafluoride, or tritium/helium-3 methods. (See table 1 for explanation of redox abbreviations. MCL is Maximum 

Contaminant Level.)

Arsenic Concentrations and Related Factors in the Glacial Aquifer System 13
1 μg/L, micrograms per liter 

Young 

(recharged 

since 1953) 

Old 

(recharged 

before 1953) 

Number of samples 200 64

Percentage of samples 

with arsenic >10 Pg/L1 5 43 

YOUNG* OLD

 

 

 Young

PERCENTAGE OF SAMPLES

Less than 1

1 to <10

Greater than or equal to 10

Arsenic, in micrograms per liter

EXPLANATION

ARSENIC, IN MICROGRAMS PER LITER

Age based on tritium concentrations

 

REDOX REDOX

N=264

0

20

30

50

80

70

60

10

90

40

10

20

40

30

0

 Figure 9. Relation of arsenic concentrations to estimated ground-water age determinations from 

10

20

40

30

0

tritium concentrations. (See table 1 for explanation of redox abbreviations. MCL is Maximum 

 Oxic NO3 Mn Fe/SO4 Oxic NO Meth MIx 3 Mn Fe/SO4 Meth MIx

MCL

Contaminant Level. *Not shown is sample with arsenic concentration of 340 Pg/L). 

 Old

Figure 9. Relation of arsenic concentrations to estimated ground-water age determinations from tritium concentrations. 

(See table 1 for explanation of redox abbreviations. MCL is Maximum Contaminant Level. *Not shown is sample with arsenic 

concentration of 340 μg/L.)

14  Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
Young 

(recharged since 1953) 

Old

(recharged before 1953) 

Number of samples 403 80

Percentage of samples with 

arsenic >10 Pg/L1 4 34 

1

 μg/L, micrograms per liter 

Figure 10. Relation of arsenic concentration and redox condition to well depth for young and old

Young Old *

ARSENIC, IN MICROGRAMS PER LITER

WELL DEPTH, IN FT

0

50

400

350

300

250

200

150

100

EXPLANATION

 Oxic NO3 Mn Mix Fe/SO4 x Meth

Redox

 waters. (See table 1 for explanation of redox abbreviations. *Not shown is sample with arsenic 

 Maximum

Contaminant

 Level = 10 Pg/L

0

50

400

350

300

250

200

150

100

concentration of 340 Pg/L and mixed redox conditions at 18 ft). 

1 10 100 1 10 100

Figure 10. Relation of arsenic concentration and redox condition to well depth for young and old waters. (See 

table 1 for explanation of redox abbreviations. *Not shown is sample with arsenic concentration of 340 μg/L 

and mixed redox conditions at 18 feet.)

Arsenic Concentrations and Related Factors in the Glacial Aquifer System 15
Figure 11. Percentage of samples with elevated arsenic concentrations for young and old 

waters in three redox categories. (Elevated arsenic concentration, greater than or equal to 

10 micrograms per liter. See table 1 for explanation of redox abbreviations.) PERCENTAGE OF SAMPLES WITH

REDOX CONDITIONS

Ox, NO3, Mn, or Mix Fe/SO4 Meth

Old, recharged before 1953

Young, recharged since 1953

Ox, NO3, Mn,

or Mix Fe/SO4 Meth

Young (N=403)

Number of samples 306 85 12

Percentage of samples

with arsenic >10 μg/L 1 13 17

Old (N=80)

Number of samples 28 31 21

Percentage of samples

with arsenic >10 μg/L 11 35 62

Figure 11. Percentage of samples with elevated arsenic concentrations for young and old

waters in three redox categories. (Elevated arsenic concentration, greater than or

 equal to 10 micrograms per liter. See table 1 for explanation of redox abbreviations.)

0

20

40

60

80

ELEVATED ARSENIC CONCENTRATIONS

Strongly reducing conditions

Other Water-Quality Properties

Spearman’s rho, a nonparametric test for monotonic correlation, was used to investigate possible correlations between 

arsenic and 30 other water-quality properties and constituents: 

pH, dissolved oxygen, alkalinity, major ions (calcium, magnesium, sodium, potassium, iron, manganese, chloride, bromide, 

sulfate, silica, fluoride, and dissolved solids), nutrients (nitrate, 

ammonia, and orthophosphate), trace elements (aluminum, 

barium, chromium, cobalt, copper, molybdenum, nickel, selenium, uranium, and zinc), dissolved organic carbon (DOC), 

and radon. 

A total of 12 constituents had correlations with arsenic 

that were statistically significant (p<0.0001), with Spearman’s 

rho values of at least 0.33 (table 2). Ammonia, molybdenum, 

iron, fluoride, strontium, lithium, silica, orthophosphate, and 

DOC were directly correlated with arsenic, whereas dissolved 

oxygen, nitrate, and radon were inversely correlated with 

arsenic. 

Correlations of arsenic with iron, ammonia, DOC, nitrate, 

and dissolved oxygen are consistent with the observation that 

arsenic is associated with reducing conditions. Orthophosphate, silica, molybdenum, and fluoride are anions or oxyanions that sorb to iron oxides, as does arsenic (Smedley and Kinniburg, 2002). Radon is a radioactive gas with a short half-life 

that is associated with young waters, so the inverse correlation 

with arsenic is consistent with the observation that arsenic is 

generally higher in older waters. It is unclear why strontium 

and lithium were correlated with arsenic. 

16  Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
Table 2. Results of Spearman’s rho tests for the 12 water-quality constituents that have the 

best correlations with arsenic in the glacial aquifer system of the Northern United States.

[p<0.0001 and rho>0.33] 

Type of 

correlation

Water-quality 

constituent

Number of 

samples

 Spearman’s 

rho p-value

Direct Ammonia 677 0.46 <0.0001

Molybdenum 646 0.45 < .0001

Iron 685 0.43 < .0001

Fluoride 675 0.41 < .0001

Strontium 406 0.40 < .0001

Lithium 365 0.40 < .0001

Silica 687 0.36 < .0001

Orthophosphate 666 0.34 < .0001

Dissolved organic

 carbon 650 0.33 < .0001

Inverse Dissolved oxygen 676 -0.40 < .0001

Nitrate 677 -0.39 < .0001

Radon 497 -0.35 < .0001

monitor and domestic wells with ground-water age determinations (N=483). 

The redox variable was a better indicator of elevated arsenic than the age or depth variables (table 3). Of the variables 

related to concentrations of other water-quality constituents, 

the ones with the highest Likelihood Ratios were ammonia, 

molybdenum, fluoride, and orthophosphate. 

As a second step, multivariate logistic regression was 

used to investigate the simultaneous influence of variables 

that are individually significant. Variables from the best-fitting 

univariate models were used to build a multivariate model by 

means of combined forward and backward selection methods 

(Hosmer and Lemeshow, 1989). Whether an added variable 

improved a model was determined by the whether the Likelihood Ratio and Wald’s statistic had a p-value <0.05 (Hosmer 

and Lemeshow, 1989). 

The predictive ability of the univariate redox model was 

improved by adding either age or depth variables, but not both. 

Age improved the predictive ability of the redox model more 

than depth did, but less than variables based on concentrations 

of molybdenum, ammonia, or fluoride did. Several combinations of the variables from table 3 created multivariate models 

that were relatively similar in terms of Likelihood Ratios and 

Wald’s statistics. 

Another way to evaluate multivariate logistic regression models is to compare predicted conditions to observed 

conditions. Often, a logistic regression model is created using 

one dataset and tested using a second dataset. An alternative 

is jackknifing, a method whereby each observation is omitted, 

one at a time; each observation is then classified as an “event” 

or “nonevent” using the model that omits that particular 

Relative Significance of Factors Related to 

Arsenic 

The relative significance of redox, ground-water age, well 

depth, and concentrations of the above mentioned 12 constituents as indicators of elevated arsenic concentrations was investigated by use of logistic regression. Logistic regression is a 

statistical method suitable for analysis of censored data (data 

that include values marked as less than the reporting limit), 

and it has been successfully used to investigate nitrate in 

ground water (Tesoriero and Voss, 1997; Nolan, 2001; Rupert, 

2003). The method differs from linear regression in that linear 

regression predicts a response, whereas logistic regression 

predicts the probability of a response. For this study, logistic 

regression was used to model the probability of detecting 

arsenic at a concentration greater than or equal to 10 μg/L. The 

goal was not to create a predictive equation for its own sake, 

but to use the predictive capability of logistic regression to 

investigate the relative significance of factors related to arsenic 

in the glacial aquifer system.

As a first step, univariate logistic regression was used 

to test the significance of individual variables as indicators 

of elevated arsenic concentrations. Univariate models were 

judged on the basis of the Likelihood Ratio and its associated 

probability (p-value) (Hosmer and Lemeshow, 1989). The 

Likelihood Ratio is also a measure of the goodness-of-fit of a 

model, with a higher value indicating a better model (Tesoriero and Voss, 1997). A positive slope coefficient indicates a 

direct relation between the explanatory variable and the probability of detecting elevated arsenic concentrations, whereas a 

negative slope indicates an inverse relation between the two. 

The univariate analysis was done for the subset of data from 

Arsenic Concentrations and Related Factors in the Glacial Aquifer System 17
Table 3. Variables that are the best indicators of elevated arsenic concentrations in the glacial aquifer system of the 

Northern United States, based on results of univariate logistic regression.

[N, number of samples. See text for description of Likelihood Ratio, p-value, and sign of regression coefficient.]

Name Description Type N Likelihood

ratio p-value

Sign of 

regression 

coefficient

Redox_var3 Simplified redox classification: 

1=iron or sulfate reducing; 

2=methanogenic; 

3=other redox conditions 

Nominal 483 68.6 <0.0001 Positive

Fe_var Iron >100 ug/L Binary 483 65.5 <.0001 Positive

NH4

_var Ammonia >0.15 mg/L Binary 479 64.4 <.0001 Positive

Redox_var2 Strongly reducing 

(iron or sulfate reducing or 

methanogenic) 0=no 1=yes

Binary 483 59.7 <.0001 Positive

DO Dissolved oxygen concentration, 

in mg/L Continuous 477 53.2 <.0001 Negative

Age_var Recharged before 1953 

0=no 1=yes Binary 483 51.5 <.0001 Positive

Depth Well depth, in ft Continuous 483 49.1 <.0001 Positive

Mo_var Molybdenum > 5 μg/L 

0=no 1=yes Binary 449 46.9 <.0001 Positive

F_var Fluoride > 0.03 mg/L

0=no 1=yes Binary 481 47.8 <.0001 Positive

Ba Barium concentration, in μg/L Continuous 452 42.0 <.0001 Positive

PO4

_var Orthophosphate > 0.05 mg/L

0=no 1=yes Binary 478 21.9 <.0001 Positive

Sr Strontium concentration, in μg/L Continuous 254 18.4 <.0001 Positive

18  Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
Appalachian Highlands physiographic division, and bedrock 

is predominantly crystalline and metamorphosed sedimentary 

rock. The glacial sediment was deposited by the latest advance 

(Wisconsin) of the Laurentide ice sheet. Much of the water 

used for public supply comes from glacial valley-fill deposits 

(Ayotte and others, 2003). The valley fill typically has three 

facies: two coarse-grained facies plus an intervening layer of 

finer-grained deposits (Randall, 2001). Uplands are covered by 

a layer of till with an average depth of 25 ft (Randall, 2001). In 

most areas, coarse-grained outwash is limited to valleys and is 

less than 200 ft thick (fig. 3). 

The Central area includes data from 443 monitor and 

domestic wells in Ohio, Indiana, Illinois, Michigan, and 

Wisconsin. This is part of the Interior Plains and Laurentian 

Upland physiographic divisions. Most of the area is underlain 

by Paleozoic shales, carbonates, and sandstone, but crystalline 

rocks underlie the northwestern part of the Central area (fig. 

2). Glacial sediments were deposited by the Great Lakes lobes 

of the Laurentide ice sheets. With the exception of Western 

Illinois, glacial deposits at land surface are of Wisconsinan 

age. Older glacial deposits underlie Wisconsinan deposits in 

many areas, especially in deep buried valleys. Glacial deposits 

in upland areas are about 50 to 150 ft thick, and coarse-grained 

outwash deposits are more than 400 ft thick in some areas 

(fig. 3). 

The West-Central area includes data are from 105 monitor and domestic wells in Minnesota, North Dakota, and 

Nebraska. The area is part of the Interior Plains physiographic 

division. Cretaceous sedimentary bedrock underlies most 

the area, except for the northwestern part, where crystalline 

bedrock subcrops (fig. 2). The glacial deposits were from 

Northwest Provenance lobes of the Laurentide ice sheet. The 

layer of till in upland areas is about 100 to 200 ft thick, and 

coarse-grained outwash deposits are more than 400 ft thick in 

some areas (fig. 3).

The West area includes data from 22 domestic wells in 

Alaska. The area is part of the Pacific Mountain System, and 

bedrock is predominantly volcanic and sedimentary. Glacial 

sediment is of Wisconsinan age and was deposited by the 

Corderillan ice sheet. 

For this report, a modified framework was used (fig. 12). 

Data from the West area were not considered because of the 

small number of samples. In addition, the West-Central area 

was subdivided by age of the glacial deposits. In the northern 

part of the West Central (WC-north) area, glacial deposits at 

land surface are Wisconsinan. Data are from 59 monitor and 

domestic wells in the North Dakota and Minnesota. In the 

southern part (WC-south), glacial deposits are older (preIllinoian). Data are from 46 monitor and domestic wells in 

Nebraska. 

observation (SAS Institute, 1995). A SAS procedure that 

approximates jackknifing was used to evaluate the predictive 

accuracy of multivariate models (SAS Institute, 1995). 

The model with the best predictive accuracy included 

three variables: redox, depth, and molybdenum concentrations. 

Using a probability cutpoint of 0.50, this three-variable model 

had an overall correct prediction rate of 99 percent. However, 

this rate may be misleading. All multivariate models that 

included the redox variable had a low rate of false-negative 

predictions and a high rate of false-positive predictions. For 

the three-variable model based on redox, depth, and molybdenum, the false negative rate was 4 percent, indicating that 

the model accurately predicted where elevated arsenic did not 

occur. This is consistent with the observation that elevated 

arsenic concentrations were uncommon in samples that were 

not strongly reducing. Most samples (about 90 percent) did 

not have elevated arsenic concentrations, so an accurate rate 

of predicting nonevents led to a high rate of overall correct 

predictions. On the other hand, the three-variable model had 

a much higher rate of false-positive predictions (26 percent), 

indicating that it overestimated the occurrence of elevated 

arsenic concentrations. This is consistent with the observation 

that, although arsenic was associated with strongly reducing 

conditions, not all strongly reducing samples had elevated 

arsenic concentrations. 

Regional Variation of Arsenic and 

Redox Conditions Within the Glacial 

Aquifer System

The previous section of the report was an analysis of 

arsenic and related factors in the glacial aquifer system as a 

whole. The goal of this section is to document and compare 

arsenic concentrations and redox conditions among different 

parts of the glacial aquifer system. The analysis is limited to 

data from monitor and domestic wells. 

Regional Framework of the Glacial Aquifer 

System 

Warner and Arnold (2005) developed a regional framework to facilitate comparison of water-quality data among 

parts of the glacial aquifer system. Four framework areas were 

defined on the basis of physiography and origin of glacial 

lobes (fig. 12). The goal was to identify areas with similar 

geologic source material. Mineralogical or geochemical differences among the areas have not been documented, so these 

areas were presented as preliminary divisions that are subject 

to revision (Warner and Arnold, 2005). 

For the East area, data are from 118 monitor and domestic wells in Connecticut, Massachusetts, New Hampshire, Vermont, New York, and eastern Pennsylvania. This is part of the 

Regional Variation of Arsenic and Redox Conditions Within the Glacial Aquifer System 19
Framework 

areas1

Physiographic 

division2 Bedrock Ice sheet Lobes

Framework 

areas used 

in this report

Age of glacial 

deposits near 

land surface

East

Appalachian 

Highlands Crystalline; 

sedimentary Laurentide East Wisconsinan

Central

Interior Plains; 

Laurentian 

upland 

Sedimentary; 

crystalline Laurentide Great Lakes Central Wisconsinan

West-Central Interior Plains Sedimentary; 

cyrstalline Laurentide Northwest Province

WC-north Wisconsinan

WC-south Pre-Illinoian

West Pacific Mountain System

Crystalline; 

sedimentary Cordilleran - -

1

 Warner and Arnold (2005).

2

 Fenneman (1946.)

Figure 12. Framework areas of the glacial aquifer system. (Framework areas from Warner and Arnold, 2005.) 

20 Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
cially phosphate (Smedley and Kinniburg, 2002). For the WCsouth area, concentrations of orthophosphate are significantly 

higher than in the other areas, and these high concentrations 

might be linked to the higher arsenic concentrations in oxic 

samples. The link between arsenic and pH in oxic samples 

is not as clear; however, there is some evidence that pH and 

phosphate interact to affect sorption of arsenic to iron oxides 

(Smedley and Kinniburg, 2002). 

Relation to Conceptual Model for High-Arsenic 

Environments

Smedley and Kinniburg (2002) observe that, in most 

areas with widespread elevated arsenic concentrations, the 

arsenic content of the solid phase is within a normal range (1 

to 20 mg/kg). For the current study, there are not sufficient 

mineralogical data to determine whether regional differences 

in arsenic concentrations of the ground water are related to the 

arsenic content of the aquifer matrix. 

Smedley and Kinniburg (2002) propose two general 

requirements for the widespread development of elevated arsenic concentrations in ground water. One requirement is for a 

geochemical trigger to release arsenic from the aquifer matrix 

to the ground water. The two most important geochemical 

triggers are (1) development of strongly reducing conditions 

at near-neutral pH, which leads to release of arsenic from iron 

oxides by reductive desorption or dissolution, and (2) development of high pH (or competition with phosphate), which leads 

to desorption of arsenic from iron oxides. The second requirement is for arsenic to accumulate in the ground water faster 

than it is flushed away. Sluggish ground-water flow is often 

associated with flat, low-lying topography and thick accumulations of young, fine-grained sediment.

For the current study, the observed differences among the 

framework areas appear to be generally consistent with Smedley and Kinniburg’s conceptual model (2002). For example, 

arsenic concentrations were lowest in the East framework area. 

In the East, DOC concentrations in ground water are low, and 

strongly reducing conditions are not widespread; therefore, 

conditions are not conducive to arsenic release by reductive 

desorption or dissolution (fig. 14). Neither are geochemical conditions conducive to arsenic mobilization under oxic 

conditions because pH and concentrations of orthophosphate 

are low (fig. 15). Hydrogeology of the East area is also less 

conducive to arsenic accumulation in the ground water: the 

topographic relief is greater, and glacial deposits are generally 

thinner and more coarse-grained than in the Central and WestCentral region (fig. 3; Randall, 2001). The observation that 

arsenic concentrations were relatively low in glacial deposits 

of the East framework area is consistent with findings of Ayotte and others (2003). In New England, elevated arsenic concentrations were associated with bedrock aquifers, especially 

calcareous bedrock, but were much lower in glacial deposits.

Comparison of Arsenic and Related Factors 

Among Framework Areas

Results of Kruskal-Wallis and Tukey tests indicate that 

arsenic concentrations were significantly different (p<0.0001) 

among samples from the four framework areas considered in 

this report: East, Central, WC-north, and WC-south (fig. 13). 

Arsenic concentrations were lowest in the East area. None 

of the samples had elevated arsenic concentrations, and the 

median was less than 1 μg/L. Arsenic concentrations in the 

Central area were significantly higher than those in the East; 

9 percent of samples had elevated arsenic concentrations, 

although the median was still less than 1 μg/L. Arsenic concentrations were significantly higher in the two WestCentral areas than in the Central area. For the WC-north area, 

14 percent of samples had elevated arsenic concentrations, and 

the median was 2 μg/L. For the WC-south area, 11 percent of 

samples had elevated arsenic concentrations, and the median 

was 3.2 μg/L. Arsenic concentrations were not statistically 

different among the two WC areas; however, the percentage 

of elevated concentrations was higher in the WC-north area, 

whereas the median concentration was higher in the WC-south 

area (fig. 13). 

The framework areas also differed in terms of redox 

conditions (fig. 13). The percentage of strongly reducing 

samples increased from the East (15 percent) to the Central 

(33 percent) to the WC-north areas (47 percent). For these 

three areas, the increase in elevated arsenic concentrations 

corresponds to an increase in strongly reducing conditions 

(fig. 14). Plots of median DOC and ammonia concentrations 

show a similar trend (fig. 14). Organic carbon acts as an electron donor in redox reactions, and higher amounts of organic 

carbon are typically associated with more reducing conditions. Ammonia can also be derived from reduction of organic 

matter. The trends illustrated in fig. 14 are consistent with the 

idea that reductive desorption or dissolution from iron oxides 

is one of the most common mechanisms of arsenic mobilization in the glacial aquifer system. In the WC-south area, three 

of the four samples with elevated arsenic concentrations were 

strongly reducing, so reductive desorption or dissolution of 

arsenic from iron oxides is also probably an active mechanism 

for mobilizing arsenic in all four of the framework areas of the 

glacial aquifer system. 

A second mechanism for mobilizing arsenic may be 

active in the WC-south area. For the WC-south area, the percentage of oxic samples was similar to that for the East area, 

but arsenic concentrations in oxic samples were much higher 

in the WC-south than the East (or Central or WC-north) (fig. 

15). However, the majority of oxic samples in the WC-south 

area had midrange arsenic concentrations (1–9.9 μg/L) rather 

than elevated concentrations (equal to or greater than 10 μg/L) 

(fig. 15). 

Under oxic conditions, arsenic can be released to ground 

water by desorption from iron oxides as a result of increased 

pH or by competition for sorption sites by other anions, espeRegional Variation of Arsenic and Redox Conditions Within the Glacial Aquifer System  21
EXPLANATION

Redox conditions: Oxic NO x 3 Mn Mix Fe/SO4 Meth

East

WELL DEPTH, IN FT

ARSENIC, IN MICROGRAMS PER LITER

400

1 01 001 1 01 001 1 01 100

0

200

100

300

WC-north Central * WC-south

1 10 100

Maximum

Contaminant

Level

Well or sample characteristics East Central WC-north WC-south

selpmasforebmuN 811 344 95 64

Well depth (median, in feet) 30 39 29 45

Arsenic concentration (median, in μg/L1

) <1 <1 2.0 3.2

Percentage of samples with:

Arsenic >10 μg/L1 0 9 14 11

snoitidnoccixO 05 63 41 36

Strongly reducing conditions2

 15 33 47 11

1 μg/L, micrograms per liter

2 Strongly reducing conditions are iron or sulfate reducing or methanogenic.

PERCENTAGE OF SAMPLES

10

 0 

70

60

20

30

40

50

EXPLANATION

Arsenic, in micrograms per liter: <1 1 to <10 >10

East Central WC-north WC-south 

 Oxic NO3 Mn Fe/S Meth Mix

REDOX

 Oxic NO3 Mn Fe/S Meth Mix Oxic NO3 Mn Fe/S Meth Mix Oxic NO3 Mn Fe/S Meth Mix

Figure 13. Arsenic in relation to redox and well-depth relations for framework areas of the glacial aquifer system. (See table 1 

for explanation of redox abbreviations.) 

22  Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
MEDIAN AMMONIA, PERCENTAGE OF SAMPLES

East

WC-south

WC-north

Central

WC-south

WC-north

East

East

15

9

3

0

Central

PERCENTAGE OF SAMPLES

WC-south

0

60

0

Elevated arsenic concentrations

Strongly reducing conditions

40

20

Dissolved organic carbon

3

WC-north

Central

FRAMEWORK AREAS

IN MIILLIGRAMS PER LITER REDUCING CONDITIONS ARSENIC CONCENTRATIONS

6

12

2

WITH ELEVATED WITH STRONGLY

 1

Ammonia

IN MIILLIGRAMS PER LITER

MEDIAN DOC,

WC-south

WC-north

East Central

Common reporting

level = 0.04 mg/L

0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

A

B

C

D

MEDIAN ORTHOPHOSPHATE,

East

WC-south

WC-south

East WC-north

4

1

Central

0

Orthophosphate in oxic samples

 

0.10

WC-north

Central

FRAMEWORK AREAS

Common reporting

IN MIILLIGRAMS PER LITER

2

3

 level = 0.02 mg/L

0.04

0.08

IN MICROGRAMS PER LITER

0

 

MEDIAN ARSENIC,

pH in oxic samples

 MEDIAN pH,

IN STANDARD UNITS

Common reporting

 level = 1 μg/L

WC-south WC-north

Central

East

0.02

6.0

7.5

6.5

7.0

Arsenic in oxic samples

0.06

WC-south

WC-north

Central

East

 Oxic conditions

PERCENTAGE OF

80

20

60

40

0

A

B

C

D

OXIC SAMPLES

Figure 14. Comparison of arsenic concentration and redoxrelated characteristics among framework areas of the glacial 

aquifer system: A, Percentage of samples with elevated 

arsenic concentrations (≥ 10 μg/L). B, Percentage of samples 

with strongly reducing conditions. C, Median concentrations 

of dissolved organic carbon. D, Median concentrations of 

ammonia.

Figure 15. Comparison of oxic samples among framework 

areas of the glacial aquifer system: A, Percentage of oxic 

samples. B, Median arsenic concentrations in oxic samples. 

C, Median orthophosphate concentrations in oxic samples. 

D, Median pH in oxic samples.

Regional Variation of Arsenic and Redox Conditions Within the Glacial Aquifer System  23
In the Central and WC-north areas, DOC concentrations 

were higher and reducing conditions were more prevalent than 

in the East area (fig. 14). In these area, glacial deposits are 

thicker, and fine-grained sediment is more common (not only 

near land surface but also as confining or semiconfining layers 

at depth). Erikson and Barnes (2005) investigated arsenic in 

glacial aquifers of North Dakota, Minnesota, Iowa, and South 

Dakota, and they linked elevated arsenic concentrations to 

the relatively high content of organic carbon and fine-grained 

sediment in Late Wisconsinan till from the Northwest Provenance glacial lobes. 

In the WC-south area, glacial deposits are thick and 

fine-grained sediment is prevalent, but DOC concentrations 

were lower and reducing conditions were less prevalent (fig. 

14). These observations could be related to the fact that glacial 

deposits are older (pre-Illinoian) and more heavily weathered. 

In the WC-south area, there is evidence that an additional 

mechanism mobilizes arsenic: desorption from iron oxides 

under oxic conditions due to competition with phosphate, possibly in combination with moderately high pH (fig. 15).

Summary and Conclusions

Arsenic concentrations were measured in 813 water 

samples from the glacial aquifer system of the Northern 

United States as part of USGS National Water-Quality Assessment studies during 1991–2003. Samples were collected from 

monitor, domestic, and public-supply wells that were 8 to 425 

ft deep. Arsenic was detected (at a concentration greater than 

or equal to 1 μg/L) in 39 percent of samples. Arsenic concentrations were elevated (greater than or equal to 10 μg/L) in 9 

percent of samples. 

Elevated arsenic concentrations were associated with 

strongly reducing conditions. Of the samples classified as 

iron reducing or sulfate reducing, arsenic concentrations were 

elevated in 19 percent. Of the methanogenic samples, arsenic 

concentrations were elevated in 45 percent. In contrast, concentrations of arsenic were elevated in only 1 percent of oxic 

samples. 

Elevated arsenic concentrations also were associated with 

older ground water. For the subset of data with ground-waterage determinations, arsenic concentrations were elevated in 34 

percent of old samples (recharged before 1953) as compared 

to 4 percent of young samples (recharged since 1953). For 

samples classified as both old and methanogenic, arsenic concentrations were elevated in 62 percent of samples, as compared to 1 percent for samples classified as young and oxic. 

Arsenic concentrations were correlated with concentrations of several other chemical constituents, including (1) constituents linked to redox processes (dissolved oxygen, nitrate, 

ammonia, iron, and dissolved organic carbon) and (2) anions 

or oxyanions that sorb to iron oxides (orthophosphate, silica, 

fluoride, and molybdenum.

Observations from the glacial aquifer system are consistent with the idea that the predominant source of arsenic 

is iron oxides and the predominant mechanism for releasing 

arsenic to the ground water is reductive desorption or reductive dissolution. Arsenic is also released from iron oxides 

under oxic conditions, but on a more limited basis and at lower 

concentrations. 

Logistic regression was used to investigate the relative 

significance of redox, ground-water age, depth, and other 

water-quality constituents as indicators of elevated arsenic 

concentrations in the glacial aquifer system. The single binary 

variable that explained the greatest amount of variation in the 

data was related to redox conditions. The best multivariate 

logistic regression models included a redox variable, but in 

general, these models overestimated the occurrence of elevated 

arsenic. This is consistent with the observation that, although 

arsenic was associated with strongly reducing conditions, not 

all samples with strongly reducing conditions had elevated 

arsenic concentrations. On the other hand, the redox-based 

multivariate models reliably predicted where elevated arsenic did not occur. This is consistent with the observation that 

elevated arsenic concentrations were uncommon in samples 

that were not strongly reducing. 

Comparisons of arsenic concentrations and redox conditions were made among four broad areas of the glacial aquifer 

system—East (Connecticut, Massachusetts, New Hampshire, 

Vermont, New York, and Pennsylvania), Central (Ohio, 

Indiana, Illinois, Michigan, and Wisconsin), West-Central 

north (North Dakota and Minnesota) and West-Central south 

(Nebraska). Arsenic concentrations were lowest in the East 

area, higher in the Central area, and highest in the West-Central north (WC-north) and West-Central south (WC-south) 

areas. 

For the East, Central, and WC-north areas, the increase in 

arsenic concentrations corresponds to an increase in strongly 

reducing conditions. In the East area, 15 percent of samples 

were strongly reducing, and none had elevated arsenic concentrations. For the Central area, 33 percent of samples were 

strongly reducing and 9 percent had elevated arsenic concentrations. For the WC-north area, 47 percent of samples were 

strongly reducing and 14 percent had elevated arsenic concentrations. There is a similar increase from East to Central 

to WC-north areas for median concentrations of dissolved 

organic carbon (DOC), the predominant electron donor in 

redox reactions. 

The WC-south area differed from the other areas; the 

median DOC concentration and the percentage of strongly 

reducing samples was relatively low, but the percentage of 

samples with elevated arsenic concentrations was relatively 

high. In addition, arsenic concentrations in oxic samples were 

higher than in the other areas. The higher arsenic concentrations in oxic samples in WC-south compared to the East could 

be the result of higher concentrations of orthophosphate and 

higher pH, both of which are linked to desorption of arsenic 

from iron oxides under oxic conditions. 

24  Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System
The observed differences in arsenic concentrations 

among broad areas of the glacial aquifer system were generally consistent with a conceptual model developed by Smedley 

and Kinniburg, who studied or reviewed studies of widespread 

arsenic contamination in Bangladesh, India, China, Vietnam, 

Hungary, Argentina, northern Chile and the Southwestern 

United States. 

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26  Association of Arsenic With Redox, Depth, and Ground-Water Age, Glacial Aquifer System

Thomas, M.A.—The Association of Arsenic With Redox Conditions, Depth, and Ground-Water Age in the Glacial Aquifer System of the 

Northern United States—Scientific Investigations Report 2007–5036

Science Center Director

USGS Ohio Water Science Center

6480 Doubletree Avenue

Columbus, OH 43229-1111

http://oh.water.usgs.gov