U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3041

June 2009

National Water-Quality Assessment Program

Redox Conditions in Selected Principal Aquifers 

of the United States

Printed on recycled paper

Introduction

Reduction/oxidation (redox) processes affect the quality 

of groundwater in all aquifer systems. Redox processes can 

alternately mobilize or immobilize potentially toxic metals 

associated with naturally occurring aquifer materials, contribute 

to the degradation or preservation of anthropogenic contaminants, and generate undesirable byproducts such as dissolved 

manganese (Mn2+), ferrous iron (Fe2+), hydrogen sulfide (H2

S), 

and methane (CH4

). Determining the kinds of redox processes 

that occur in an aquifer system, documenting their spatial distribution, and understanding how they affect concentrations of 

natural or anthropogenic contaminants are central to assessing 

and predicting the chemical quality of groundwater.

Microorganisms that catalyze redox processes in natural systems compete for limited resources. Those microbial 

processes that couple the most efficient electron donors to the 

most efficient electron acceptors have a competitive advantage. 

Dissolved and particulate organic carbon are typically the most 

common electron donors available in groundwater systems, 

although reduced forms of nitrogen, iron, sulfur, and possibly 

other species could be important electron donors. There is also 

variability in the kinds and amounts of available electron acceptors. Dissolved oxygen (O2

) produces more energy per mole 

of organic carbon oxidized than any other commonly available 

electron acceptor, and thus it is used preferentially by subsurface microorganisms. Under anoxic conditions, the next most 

energetically favorable naturally available electron acceptor is 

nitrate (NO3

–

), followed by manganese (IV) [Mn(IV)], ferric 

iron [Fe(III)], sulfate (SO4

2–), and finally carbon dioxide (CO2

). 

This order of preferential electron acceptor utilization—O2

 > 

NO3

–

 > Mn(IV) > Fe(III) > SO4

2– > CO2

—is referred to as the 

ecological succession of terminal electron-accepting processes. 

One consequence of this ecological succession is that redox 

processes have a tendency to segregate into zones dominated 

by a single electron-accepting process.

Documenting the consumption of soluble electron acceptors and production of distinctive byproducts along defined 

flow path segments is the most reliable method for identifying 

the distribution of redox processes in a groundwater system 

(Chapelle and others, 1995). But diagnostic redox byproducts 

like excess nitrogen gas (excess N2

), H2S, CH4, and hydrogen 

(H2

), which are important components of hierarchical schemes 

for identifying redox processes (Chapelle and others, 1995; 

Christensen and others, 2000), are not routinely measured 

in regional water-quality assessments. Thus, McMahon and 

Chapelle (2008) developed a framework for assessing redox 

processes based on water-quality parameters that are commonly 

measured in regional water-quality assessments. The redox 

framework subsequently was used to assess redox conditions 

in 15 principal aquifer systems (PAs) of the United States 

using data from the U.S. Geological Survey (USGS) National 

Water-Quality Assessment (NAWQA) Program (McMahon and 

Chapelle, 2008). The USGS has identified 62 PAs in the United 

States (U.S. Geological Survey, 2003; Lapham and others, 

2005). This Fact Sheet extends the analysis of McMahon and 

Chapelle (2008) to additional PAs by applying the framework 

to a larger set of water-quality data from the USGS national 

water databases.

Framework for Assessing Redox Processes

The redox framework of McMahon and Chapelle (2008) 

is based on the dissolved concentrations of five water-quality 

parameters (O2

, NO3

–

, Mn2+, Fe2+, and SO4

2–) that are relatively 

inexpensive and easy to measure. The framework is a modification of those developed by Chapelle and others (1995) and 

Paschke and others (2007). Although the framework has the 

advantages of broad applicability and internal consistency, it 

also is subject to certain limitations that are discussed in detail 

by McMahon and Chapelle (2008). One limitation of the framework is its inability to distinguish between Fe(III) and SO4

2–

reduction. Subsequently, Chapelle and others (2009) proposed 

an addition to the framework in which Fe2+/H2

S ratios are used 

to distinguish between Fe(III) and SO4

2– reduction. The updated 

redox framework is presented in table 1. Jurgens and others 

(2009) created a Microsoft Excel workbook that automates 

application of the redox framework to large data sets.

Source of Redox Data

Data for this analysis were retrieved from the USGS Data 

Warehouse (DW) and State databases of the USGS National Water 

Information System (NWIS) between January and December 2008. 

Duplicate samples were removed in instances when data for a 

well were present in both databases. Only the most recent sample 

with complete data for all five redox parameters was retained if a 

particular well was sampled on multiple dates or times.

The analysis in this report includes the 15 PAs and 

1,692 wells studied by McMahon and Chapelle (2008), plus 

an additional 13 PAs and 3,443 wells (fig. 1 and table 2). 

Only data from domestic wells were considered in this analysis, which is consistent with the approach of McMahon and 
Chapelle (2008). Domestic wells were used for this purpose 

because they had better spatial distributions in the PAs than the 

other well types, although it is recognized that none of the well 

networks is completely representative of entire PAs (fig. 1). 

Furthermore, only PAs with at least 30 samples were included 

in the analysis, resulting in a sample size of 5,135 wells. This 

analysis separates the carbonate-rock and crystalline-rock 

aquifers of the Piedmont and Blue Ridge and separates the 

Columbia Plateau and Snake River Plain basaltic-rock aquifers 

of the western volcanic-rock PAs, whereas those pairs of aquifers were combined by McMahon and Chapelle (2008). Redox 

assignments in the dataset were based on the framework in 

table 1, except that Fe2+/H2

S ratios were not used to distinguish 

between Fe(III) and SO4

2– reduction because only a relatively 

small number of samples has H2

S data and because the exception maintained consistency with the previous analysis of redox 

processes in PAs (McMahon and Chapelle, 2008).

Redox Processes in Selected 

Principal Aquifers

The framework presented in table 1 allows for a generalized 

but internally consistent comparison of redox processes between 

PAs of the United States. Percentages of water samples from 

domestic wells that were characterized as being oxic, suboxic, 

anoxic, or as having mixed redox processes are illustrated spatially in figure 2. The anoxic classification is subdivided further 

in table 2. Some broad spatial patterns in the distribution of redox 

processes are apparent in figure 2 that are consistent with the patterns described by McMahon and Chapelle (2008). Oxygen reduction was the predominant redox process in 87 and 84 percent of the 

samples from nonglacial sand and gravel PAs (excluding alluvial 

aquifers) and volcanic-rock PAs in the Western United States, 

respectively (fig. 2 and table 2). Whereas the number of samples 

in which O2

 reduction was the predominant redox process ranged 

from 43 (glacial sand and gravel PAs) to 63 percent (carbonate/

sandstone and crystalline-rock PAs) in the remaining PA lithologic 

groups, many of which were located in the central and eastern 

parts of the country. Several of those PAs contained relatively large 

percentages of anoxic samples, with the Silurian-Devonian, central 

United States glacial sand and gravel, and Cambrian-Ordovician 

PAs in the upper Midwest containing the largest percentages 

(fig. 2). Ferric iron/SO4

2– reduction was the most common anoxic 

redox process in those three PAs (table 2). Suboxic conditions were 

common in some aquifers containing karst features (Floridan PA) 

or fractured rocks (New England crystalline-rock PA), as well as in 

some sandstone aquifers (Denver Basin and Lower Tertiary PAs) 

(fig. 2 and table 2). Redox conditions were highly variable within 

some PA lithologic groups. Among the carbonate aquifers, the 

number of samples that were O2

 reducing ranged from 13 to about 

90 percent and the number that were Fe(III)/SO4

2– reducing ranged 

from 0 to 60 percent.

The spatial variability of redox processes shown in figure 2 

likely reflects differences in geology, climate, and hydrology, 

among other factors, across the United States (McMahon and 

Chapelle, 2008). Geology, climate, and hydrology influence 

redox processes primarily through their controls on groundwater residence time and electron acceptor/donor availability. 

Superimposed on those natural factors are anthropogenic factors 

like land-use change and chemical applications/releases at the 

land surface which also can affect residence time (by altering 

recharge) and electron acceptor/donor availability (for example, 

agricultural nitrogen applications or gasoline spills).

Table 1. Threshold concentrations for identifying redox processes in groundwater (modified from McMahon and Chapelle, 2008; Chapelle 

and others, 2009).

[O2, dissolved oxygen; NO3

–

-N, dissolved nitrate as nitrogen; Mn2+, dissolved manganese; Fe2+, dissolved iron; SO4

2–, dissolved sulfate; H2

S, hydrogen sulfide; Mn(IV), 

oxidized manganese; Fe(III), ferric iron; mg/L, milligrams per liter; —, not applicable; ≥, greater than or equal to; <, less than]

General

redox

category

Predominant

redox process

Distinguishing Fe(III)–

from SO4

2–-reduction

Water-chemistry criteria

(mg/L) Fe2+/H2

S

mass ratio Comments

O2 NO3

–

-N Mn2+ Fe2+ SO4

2–

Oxic O2 reduction — ≥0.5 — <0.05 <0.1 — — —

Suboxic — — <0.5 <0.5 <0.05 <0.1 — — (1)

Anoxic NO3

–

 reduction — <0.5 ≥0.5 <0.05 <0.1 — — —

Mn(IV) reduction — <0.5 <0.5 ≥0.05 <0.1 — — —

Fe(III)/SO4

2– reduction — <0.5 <0.5 — ≥0.1 ≥0.5 — —

— Fe(III) reduction <0.5 <0.5 — ≥0.1 ≥0.5 >10 —

— Mix – Fe(III)/SO4

2– reduction <0.5 <0.5 — ≥0.1 ≥0.5 ≥0.3 and ≤10 —

— SO4

2– reduction <0.5 <0.5 — ≥0.1 ≥0.5 <0.3 —

Methanogenesis — <0.5 <0.5 — ≥0.1 <0.5 — —

Mixed — — — — — — — — (2)

1

Further definition of redox processes not feasible.

2

Criteria for more than one redox process are met.
EXPLANATION

Principal aquifers and reference number

Semiconsolidated sand aquifers

 Coastal Lowlands (18)

 Mississippi Embayment (19)

 Northern Atlantic Coastal Plain (20)

Unconsolidated sand and gravel aquifers, glacial

 Eastern glaciated region (27)

 Central glaciated region (28)

 Western glaciated region (29)

Carbonate/sandstone aquifers

 Cambrian-Ordovician (6)

 Edwards-Trinity (7)

 Mississippian (8)

 Valley and Ridge (9)

Sandstone aquifers

 Central Oklahoma (12)

 Colorado Plateau (13)

 Denver basin (14)

 Early Mezosoic basins (15)

 Lower Tertiary (16)

 Pennsylvanian (17)

Western volcanic-rock aquifers

 Columbia Plateau basaltic rock (30)

 Snake River Plain basaltic rock (31)

Crystalline-rock aquifers

 New England (10)

Carbonate aquifers Piedmont and Blue Ridge (11)

 Floridan (1)

 Ordovician (2)

 Ozark Plateau (3)

 Piedmont and Blue Ridge (4)

 Silurian-Devonian (5)

Unconsolidated sand and gravel aquifers, nonglacial

 Basin and Range basin fill (21)

 Central Valley (22)

 High Plains—northern (23)

 High Plains—central (24)

 High Plains—southern (25)

 Northern Rocky Mountains intermontain basins (26)

Domestic well

18

10

26 29

30

22

21

31

28

6

3

19

7

25

12

24

13

14

23

9

11

1

17

8

27

16

5

2

4

15

20

29

Figure 1. Location of domestic wells and principal aquifers included in the redox analysis. The glacial deposit aquifer 

was subdivided into eastern, central, and western deposits on the basis of geological source of the glacial material 

(Warner and Arnold, 2005). As defined here, the glacial deposit aquifer in the central United States extends from Ohio 

to central Montana.

The number of samples with mixed redox processes 

ranged from about 5 to 20 percent among the eight PA lithologic groups in table 2. In this study and that of McMahon and 

Chapelle (2008) the semiconsolidated sand PAs and glacial sand 

and gravel PAs had some of the largest percentages of mixedredox samples, whereas the nonglacial sand and gravel PAs 

and the western volcanic-rock PAs had some of the smallest 

percentages of mixed-redox samples. Both studies also found 

that, on average, wells in the semiconsolidated sand PAs and 

glacial sand and gravel PAs had shorter screens than wells in the 

other PAs. Thus, samples with mixed redox processes were not 

necessarily a result of mixing waters in long screen intervals but 

could reflect the inherent spatial heterogeneity of redox processes 

in a given hydrologic system.
Table 2. Redox classification of water samples from domestic wells in selected principal aquifers. Only aquifers with at least 30 samples are 

shown. The High Plains and glacial aquifers, each subdivided in this table, are considered single principal aquifers. Thus, 28 principal aquifers 

are listed in the table.

[O2, dissolved oxygen; NO3

–

, dissolved nitrate; Mn(IV), oxidized manganese; Fe(III), ferric iron; SO4

2–, dissolved sulfate]

Principal aquifer (map reference number)

Percentage of samples

Number

of samples

Oxic

conditions Suboxic

conditions

Anoxic conditions Mixedredox

conditions O2

reduction

NO3

–

reduction

Mn(IV)

reduction

Fe(III)/SO4

2–

reduction

Methanogenesis

Carbonate aquifers

Floridan1

 (1) 203 49.75 23.15 2.46 2.46 10.35 3.94 7.89

Ordovician (2) 36 50.00 19.44 2.78 0.00 16.67 0.00 11.11

Ozark Plateau1

 (3) 169 84.02 5.92 2.37 0.00 3.55 0.00 4.14

Piedmont and Blue Ridge2

 (4) 106 89.62 4.72 0.94 0.00 0.00 0.00 4.72

Silurian-Devonian (5) 199 13.07 11.06 1.51 2.01 60.29 3.52 8.54

Carbonate/sandstone aquifers

Cambrian-Ordovician1

 (6) 179 40.78 3.91 1.12 1.12 37.98 0.56 14.53

Edwards-Trinity1

 (7) 69 73.91 11.59 1.45 0.00 11.60 0.00 1.45

Mississippian (8) 57 71.93 8.77 1.75 0.00 3.51 0.00 14.04

Valley and Ridge (9) 463 66.09 2.38 0.86 4.75 9.29 0.00 16.63

Crystalline-rock aquifers

New England1

 (10) 95 50.53 23.16 1.05 3.16 12.63 0.00 9.47

Piedmont and Blue Ridge2

 (11) 743 74.97 1.88 0.67 2.02 5.25 0.00 15.21

Sandstone aquifers

Central Oklahoma (12) 82 82.93 1.22 2.44 0.00 0.00 0.00 13.41

Colorado Plateau (13) 37 75.68 0.00 2.70 0.00 5.41 0.00 16.21

Denver Basin (14) 74 33.78 39.19 0.00 5.41 13.51 1.35 6.76

Early Mesozoic basins (15) 314 76.11 5.10 2.23 3.50 6.05 0.00 7.01

Lower Tertiary (16) 30 43.33 26.66 6.67 6.67 10.00 0.00 6.67

Pennsylvanian (17) 197 24.37 4.06 1.52 3.05 26.90 1.52 38.58

Semiconsolidated sand aquifers

Coastal lowlands1

 (18) 81 54.32 4.94 0.00 2.47 8.64 2.47 27.16

Mississippi embayment (19) 85 34.12 36.47 0.00 0.00 15.29 4.71 9.41

Northern Atlantic Coastal Plain1

 (20) 392 46.17 4.08 1.02 0.26 23.22 1.02 24.23

Unconsolidated sand and gravel aquifers, nonglacial

Alluvial3 82 69.51 0.00 1.22 0.00 6.10 0.00 23.17

Basin and Range basin fill1

 (21) 121 88.43 2.48 2.48 0.00 2.48 0.83 3.30

Central Valley1

 (22) 136 80.88 3.68 4.41 2.21 2.21 0.74 5.87

High Plains aquifer–northern1

 (23) 190 77.37 3.16 2.11 3.16 5.26 0.00 8.94

High Plains aquifer–central1

 (24) 87 95.40 0.00 1.15 0.00 2.30 0.00 1.15

High Plains aquifer–southern1

 (25) 46 93.48 0.00 0.00 2.17 0.00 0.00 4.35

Northern Rocky Mountains Intermontane basins (26) 57 87.72 1.75 3.51 1.75 5.27 0.00 0.00

Unconsolidated sand and gravel aquifers, glacial

Eastern glaciated region1

 (27) 180 48.33 5.56 2.22 5.00 20.56 2.22 16.11

Central glaciated region1

 (28) 321 20.25 2.18 0.31 0.00 42.68 9.97 24.61

Western glaciated region1

 (29) 100 60.00 4.00 1.00 8.00 9.00 4.00 14.00

Western volcanic-rock aquifers

Columbia Plateau basaltic rock4

 (30) 137 67.88 8.76 2.92 1.46 7.30 0.73 10.95

Snake River Plain basaltic rock4

 (31) 67 100.00 0.00 0.00 0.00 0.00 0.00 0.00

1

Included in analysis by McMahon and Chapelle (2008).

2

Included in analysis by McMahon and Chapelle (2008), but Piedmont and Blue Ridge carbonate-rock and crystalline-rock aquifers were previously combined.

3

Results not shown on figure 2.

4

Included in analysis by McMahon and Chapelle (2008), but Columbia Plateau and Snake River Plain basaltic-rock aquifers were previously combined.
Effect of Redox on Water Quality

A principal reason for assessing redox conditions in groundwater is to address practical water-quality issues. Two such issues 

are aquifer vulnerability to contamination and contamination 

of water-supply wells. McMahon and Chapelle (2008) used the 

redox framework to evaluate the occurrence of arsenic (As), NO3

–

, 

and volatile organic compounds (VOCs) in selected PAs. Within 

a given PA, they generally found As concentrations to be larger 

in samples that were Mn(IV) or Fe(III)/SO4

2– reducing than in 

samples that were O2

 reducing. Arsenic tends to adsorb on the 

surfaces of Mn(IV) and Fe(III) oxyhydroxides commonly present 

in aquifer materials under oxic conditions. Under Mn(IV)- and 

Fe(III)-reducing conditions, adsorbed As can be mobilized as metal 

oxyhydroxides are dissolved. In contrast to As, NO3

–

 concentrations were smaller in anoxic samples than in oxic samples, which 

is consistent with NO3

–

 degradation by denitrifying bacteria under 

anoxic conditions. Detection frequencies for chloroform and 

tetrachloroethene, two of the most commonly detected VOCs in 

groundwater (Zogorski and others, 2006), were higher under oxic 

conditions than under anoxic conditions. This finding is consistent 

with the fact that reductive dechlorination of these compounds 

occurs more readily under more strongly reducing conditions. 

These examples indicate that aquifer vulnerability to contamination 

from various natural and anthropogenic chemicals is dependent on 

redox conditions in groundwater.

Substantial redox heterogeneity in aquifers can have 

important effects on the distribution of natural and anthropogenic 

contaminants in groundwater and the placement of water-supply 

wells. For example, a transition from oxic to anoxic conditions 

over relatively short distances would not be beneficial to a well 

owner concerned about As contamination in Fe(III)-reducing 

sediments because it would be difficult to selectively screen the 

oxic zones. Conversely, a transition from oxic to anoxic conditions over relatively short distances would be beneficial to a well 

owner concerned about NO3

–

 contamination because it implies that 

NO3

–

 would not persist for long distances along flow paths in the 

aquifer. Thus, information on the spatial heterogeneity of redox 

processes in a given hydrologic system could be helpful when 

designing and operating water-supply wells.

Figure 2. Pie charts indicating the percentages of domestic well samples that were oxic, suboxic, anoxic, or diagnostic 

of mixed redox processes in selected principal aquifers. See figure 1 for explanation of map reference numbers. 

12

25

19

7 18

2 1

11

9

20

15

4

27

10

17

8

23

22 21

29

31

30

29

24

3

6 5

13 14

28

16

26

Redox state of aquifer,

as percentage of samples

Anoxic

Mixed

Suboxic

Oxic
Conclusion

The framework presented in table 1 allows for an 

analysis of redox processes in aquifer systems using five 

chemical parameters that are relatively inexpensive and easy to 

measure. A Microsoft Excel workbook was created to automate 

implementation of the framework to large data sets. The redox 

framework can be used to address practical water-quality issues 

such as aquifer vulnerability to contamination and contamination of water-supply wells. Given the interpretive power of 

the redox framework and relative ease of application, the five 

chemical parameters (O2

, NO3

–

, Mn2+, Fe2+, and SO4

2–) should 

be included in routine water-quality monitoring programs 

whenever possible.

References Cited

Chapelle, F.H., Bradley, P.M., Thomas, M.A., and McMahon, 

P.B., 2009, Distinguishing iron-reducing from sulfate-reducing 

conditions: Ground Water, v. 47, p. 300–305.

Chapelle, F.H., McMahon, P.B., Dubrovsky, N.M., Fujii, R.F., 

Oaksford, E.T., and Vroblesky, D.A., 1995, Deducing the distribution of terminal electron-accepting processes in hydrologically diverse groundwater systems: Water Resources 

Research, v. 31, p. 359–371.

Christensen, T.H., Bjerg, P.L., Banwart, S.A., Jakobsen, R., 

Heron, G., and Albrechtsen, H.-H., 2000, Characterization 

of redox conditions in groundwater contaminant plumes: 

Journal of Contaminant Hydrology, v. 45, p. 165–241.

Jurgens, B.C., McMahon, P.B., Chapelle, F.H., and Eberts, 

S.M., 2009, An Excel workbook for identifying redox processes in groundwater: U.S. Geological Survey Open-File 

Report 2009–1004, 8 p.

Lapham, W.W., Hamilton, P.A., and Myers, D.N., 2005, 

National water-quality assessment program—Cycle II 

regional assessments of aquifers: U.S. Geological Survey 

Fact Sheet 2005–3013, 4 p.

McMahon, P.B., and Chapelle, F.H., 2008, Redox processes and 

water quality of selected principal aquifer systems: Ground 

Water, v. 46, p. 259–271.

Paschke, S.S., Kauffman, L.J., Eberts, S.M., and Hinkle, 

S.R., 2007, Overview of regional studies of the transport of 

anthropogenic and natural contaminants to public-supply 

wells, section 1 of Paschke, S.S., ed., Hydrogeologic settings 

and groundwater flow simulations for regional studies of 

the transport of anthropogenic and natural contaminants to 

public-supply wells—studies begun in 2001: U.S. Geological 

Survey Professional Paper 1737–A, p. 1–1—1–18.

U.S. Geological Survey, 2003, Principal aquifers of the 48 

conterminous United States, Hawaii, Puerto Rico, and 

the U.S. Virgin Islands: accessed January 20, 2009 at 

http://www.nationalatlas.gov/mld/aquifrp.html.

Warner, K.L., and Arnold, T.L., 2005, Framework for regional 

synthesis of water-quality data for the glacial aquifer system 

in the United States. U.S. Geological Survey Scientific Investigations Report 2005–5223, 6 p.

Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara, Lapham, W.W., 

Moran, M.J., Rowe, B.L., Squillace, P.J., and Toccalino, P.L., 

2006, The quality of our Nation’s waters—Volatile organic 

compounds in the Nation’s ground water and drinking-water 

supply wells: U.S. Geological Survey Circular 1292, 101 p.

—P.B. McMahon, T.K. Cowdery, F.H. Chapelle, and B.C. Jurgens

Publishing support provided by:

Denver Publishing Service Center, Denver, Colorado

Manuscript approved for publication, May 18, 2009

Edited by Chelsea Carr

Graphics and layout by Joy Monson

For more information concerning this publication, contact:

Director, USGS Colorado Water Science Center

Box 25046, Mail Stop 415

Denver, CO 80225

(303) 236-4882

Or visit the Colorado Water Science Center Web site at:

http://co.water.usgs.gov/

This publication is available online at:

http://pubs.usgs.gov/fs/2009/3041/