U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5220

National Water-Quality Assessment Program

Use of Classes Based on Redox and Groundwater Age

to Characterize the Susceptibility of Principal Aquifers

to Changes in Nitrate Concentrations, 1991 to 2010
Use of Classes Based on Redox and 

Groundwater Age to Characterize the 

Susceptibility of Principal Aquifers 

to Changes in Nitrate Concentrations, 

1991 to 2010

By P.B. McMahon

National Water-Quality Assessment Program

Scientific Investigations Report 2012–5220

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

McMahon, P.B., 2012, Use of classes based on redox and groundwater age to characterize the susceptibility of 

principal aquifers to changes in nitrate concentrations, 1991 to 2010: U.S. Geological Survey Scientific Investigations 

Report 2012–5220, 41 p.
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable 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.gov/). Information 

on the Nation’s water resources is critical to ensuring long-term availability 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, 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 water-quality management 

and policy (http://water.usgs.gov/nawqa). The NAWQA Program is designed to answer: What is the quality 

of our Nation’s streams and groundwater? How are conditions changing over time? How do natural features 

and human activities affect the quality of streams and groundwater, 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/studies/study_units.html ).

In the second decade of the Program (2001–2012), a major focus is on regional assessments of waterquality 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 groundwater, and by determining water-quality 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, 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.

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 water-resource 

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. 

William H. Werkheiser

USGS Associate Director for Water

v

Contents

Foreword ........................................................................................................................................................iii

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

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

Methods...........................................................................................................................................................2

Well Selection .......................................................................................................................................2

Sources of Data.....................................................................................................................................3

Aquifer Groups and Geology...............................................................................................................3

Redox Classification.............................................................................................................................3

Groundwater-Age Classification........................................................................................................3

Evaluation of Redox-Age Classes ...............................................................................................................6

Relation Between Redox-Age Classes and Changes in Nitrate Concentrations 

in Trend-Well Networks ..................................................................................................................9

Redox-Age Classes in Principal Aquifers................................................................................................11

Susceptibility to Changes in Nitrate Concentrations Near the Water Table 

in Agricultural Areas.............................................................................................................11

Susceptibility to Changes in Nitrate Concentrations in Parts of Aquifers 

that Provide Domestic Water Supplies..............................................................................19

Susceptibility to Changes in Nitrate Concentrations in Parts of Aquifers 

that Provide Public Water Supplies ...................................................................................26

Summary and Conclusions.........................................................................................................................31

Acknowledgments.......................................................................................................................................33

References Cited..........................................................................................................................................33

Appendix 1.....................................................................................................................................................37

Figures

1. Map showing location of selected principal aquifers in the United States........................4

2. Change in nitrate concentration for pairs of samples collected from selected 

wells in the United States at near decadal time scales in relation to the 

redox-age class of the more recently collected sample .......................................................8

3. Change in nitrate concentration for pairs of samples collected from selected 

wells in the United States at near decadal time scales in relation to the change 

in redox condition .........................................................................................................................9

4. Map showing central locations of well networks sampled at near decadal 

time scales by the National Water-Quality Assessment Program.....................................10

5. Median change in nitrate concentration in relation to the percentage of 

samples that were classified as oxic-potentially young in well networks 

that were sampled at near decadal timescales and the susceptibility of 

the networks to changes in nitrate concentrations..............................................................14

6. Concentrations of nitrate in pairs of samples collected from selected networks 

of major-aquifer study wells in the United States at near decadal time scales ..............15

7. Map showing central locations of networks of shallow monitoring wells 

in agricultural areas and the susceptibility of the networks to changes in 

nitrate concentrations. ..............................................................................................................16

8. Median percentage of samples assigned to the four redox-age classes for 

principal aquifers that have at least two networks of shallow monitoring 

wells in agricultural areas, and the susceptibility of the aquifers to changes 

in nitrate concentrations ...........................................................................................................18
vi

9. Percentage of samples assigned to the four redox-age classes for networks 

of shallow monitoring wells in agricultural areas in the Central Valley aquifer 

system, High Plains aquifer, and the West-central glacial aquifers, and the 

susceptibility of the networks to changes in nitrate concentrations ................................19

10. Map showing central locations of networks of domestic wells and the 

susceptibility of the networks to changes in nitrate concentrations ................................20

11. Median percentage of samples assigned to the four redox-age classes 

for principal aquifers that have at least two networks of domestic wells, 

and the susceptibility of the aquifers to changes in nitrate concentrations....................24

12. Variability in the percentage of samples classified as oxic-potentially young 

for principal aquifers that have at least two networks of domestic wells, 

and the susceptibility of the networks to changes in nitrate concentrations..................25

13. Percentage of samples classified as oxic-potentially young in collocated 

networks of shallow monitoring wells in agricultural areas and domestic wells, 

and the susceptibility of the networks to changes in nitrate concentrations..................27

14. Map showing central locations of networks of public-supply wells and the 

susceptibility of the networks to changes in nitrate concentrations ................................28

15. Median percentage of samples assigned to the four redox-age classes for 

principal aquifers that have at least two networks of public-supply wells, 

and the susceptibility of the aquifers to changes in nitrate concentrations....................30

16. Percentage of samples classified as oxic-potentially young in collocated 

networks of public-supply and domestic wells, and the susceptibility of the 

networks to changes in nitrate concentrations ....................................................................31

Appendix Figure

1–1. Map showing the central locations of networks of shallow monitoring wells 

in urban areas and the susceptibility of the networks to changes in nitrate 

concentrations ............................................................................................................................39

Tables

1. Criteria used to assign redox-age classes to groundwater samples ..................................6

2. Redox-age classes for water samples collected from principal aquifers in 

the United States and median well depth, well type, and aquifer confinement ................7

3. Redox-age classes for water samples collected from selected well networks 

in the United States at near decadal time scales, median change in nitrate 

concentration for each network, and the statistical significance of the change 

in concentration..........................................................................................................................12

4. Redox-age classes for water samples collected from networks of shallow 

monitoring wells in agricultural areas in the United States and the susceptibility 

of the networks to changes in nitrate concentrations .........................................................17

5. Redox-age classes for water samples collected from networks of domestic 

wells in the United States and the susceptibility of the networks to changes 

in nitrate concentrations ...........................................................................................................21

6. Redox-age classes for water samples collected from networks of public-supply 

wells in the United States and the susceptibility of the networks to changes in 

nitrate concentrations ...............................................................................................................29
vii

Appendix Table

1–1. Redox-age classes for water samples collected from networks of shallow 

monitoring wells in urban areas in the United States and the susceptibility 

of the networks to changes in nitrate concentrations .........................................................40

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 0.2642 gallon (gal)

Mass Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L). Concentrations of tritium are given in tritium units (TU).

Use of Classes Based on Redox and Groundwater Age 

to Characterize the Susceptibility of Principal Aquifers 

to Changes in Nitrate Concentrations, 1991 to 2010

By P.B. McMahon

nitrate concentrations. For the parts of aquifers that provide 

domestic water supplies, the aquifers most susceptible to 

changes in nitrate concentrations were the Northern Atlantic 

Coastal Plain aquifer system and the Early Mesozoic Basin, 

Valley and Ridge carbonate-rock, and Piedmont and Blue 

Ridge crystalline-rock aquifers in the eastern United States; 

the Ozark Plateaus aquifer system in parts of Missouri and 

Arkansas; and the Central Valley, Columbia Plateau basalticrock, and Snake River Plain basaltic-rock aquifer systems 

in the West. The least susceptible aquifers were the Texas 

Coastal Uplands and Denver Basin aquifer systems. For 

the parts of aquifers that provide public water supplies, the 

aquifers most susceptible to changes in nitrate concentrations 

were the Eastern glacial aquifers and the California Coastal 

Basin, Basin and Range basin-fill, and High Plains aquifers 

in the West. The least susceptible aquifer was the CambrianOrdovician aquifer system in the upper Midwest.

Principal-aquifer lithology groups with the largest 

percentage of domestic-well networks considered to have 

a high susceptibility to changes in nitrate concentrations 

were the basaltic- and other volcanic-rock aquifer systems, 

carbonate-rock aquifers, and crystalline-rock aquifers. The 

lithology groups with the smallest percentage of networks 

considered to have a high susceptibility to changes in nitrate 

concentrations were the glacial aquifers and sandstone 

aquifers. There are important geologic differences between 

the aquifer lithology groups with high and low susceptibilities 

to changes in nitrate concentrations. The relatively large 

percentage of high-susceptibililty networks in the basaltic- and 

other volcanic-rock aquifer systems, carbonate-rock aquifers, 

and crystalline-rock aquifers may indicate the importance of 

fractures and karst features in promoting the rapid movement 

of oxic-potentially young groundwater in those aquifers. The 

relatively small percentage of high-susceptibility networks 

in the glacial and sandstone aquifers reflects geologic 

characteristics of those aquifers that support anoxic redox 

conditions (high electron-donor content) and inhibit water 

movement (fine-grained confining layers).

For networks of monitoring and domestic wells that were 

approximately collocated, the monitoring-well networks had 

the higher percentage of samples classified as oxic-potentially 

Abstract

The National Water-Quality Assessment (NAWQA) 

Program of the U.S. Geological Survey is using multiple 

approaches to measure and explain trends in concentrations of 

nitrate in principal aquifers of the United States. Near decadal 

sampling of selected well networks is providing information 

on where long-term changes in nitrate concentrations have 

occurred. Because those studies do not include all the 

NAWQA well networks, a determination has yet to be made 

as to what might be expected in networks from which timeseries data have not been collected. Characterizing aquifer 

susceptibility to changes in nitrate concentrations on the basis 

of data collected from all the NAWQA well networks would 

be a step toward extrapolating findings from those studies to 

broader regions.

In this study, water samples collected from 6,593 wells 

in 39 principal aquifers and 5 alluvial aquifers (collected from 

1991 to 2010) were assigned to four redox-age classes on 

the basis of concentrations of dissolved oxygen and various 

indicators of groundwater age. The redox-age assignments 

were then used to characterize the susceptibility of principal 

aquifers to changes in nitrate concentrations. Aquifer areas 

(as defined by well networks) in which at least 75 percent 

of the samples were classified as oxic-potentially young 

were considered to have a high susceptibility to changes 

in nitrate concentrations. Aquifer areas were considered 

to have a medium susceptibility if at least 25 percent and 

less than 75 percent of the samples were classified as oxicpotentially young. Aquifer areas were considered to have a 

low susceptibility if less than 25 percent of the samples were 

classified as oxic-potentially young.

The three primary well types sampled by NAWQA 

(shallow monitoring wells near the water table, domestic 

wells, and public-supply wells) generally represent different 

depth zones and (or) areas of the principal aquifers. For 

the parts of aquifers near the water table in agricultural 

areas, the aquifers most susceptible to changes in nitrate 

concentrations were the Columbia Plateau basin-fill aquifers, 

Eastern glacial aquifers, and the West-central glacial aquifers. 

None of the aquifers had a low susceptibility to changes in 
2 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

young, indicating that susceptibility tended to be higher at 

the shallower depths of the monitoring wells. For networks 

of domestic and public-supply wells that were approximately 

collocated, the public-supply wells had the higher percentage 

of samples classified as oxic-potentially young, indicating 

that susceptibility tended to be higher in the vicinity of publicsupply wells than in the vicinity of domestic wells even 

though the public-supply wells had larger median well depths. 

Previous studies found that high rates of pumping in publicsupply wells with long screens induced more rapid downward 

movement of young groundwater than did domestic wells, 

which had shorter screens and were less heavily pumped. 

The data from this study are generally consistent with 

those findings.

Introduction

Is groundwater quality getting better or worse, why, and 

what will happen in the future? These are some of the important questions being addressed by the National Water-Quality 

Assessment (NAWQA) Program of the U.S. Geological 

Survey (USGS). The NAWQA Program is using multiple 

approaches to measure and explain trends in concentrations of 

nitrate in principal aquifers of the United States. Near decadal 

sampling of selected NAWQA well networks has provided a 

preliminary determination of where changes in concentrations 

of chloride, dissolved solids, and nitrate in groundwater have 

occurred (Rupert, 2008; Lindsey and Rupert, 2012). Because 

those studies do not include all the NAWQA well networks, a 

determination has yet to be made as to what might be expected 

in networks from which time-series data have not been collected. Characterizing aquifer susceptibility to changes in 

nitrate concentrations on the basis of data collected from all 

the NAWQA well networks would be a step toward extrapolating findings from those studies to broader regions.

Long-term changes in concentrations of nitrate in 

groundwater are controlled by factors such as nitrogen input 

history at the land surface, denitrification in the aquifer, and 

location in the flow system (Clark and others, 2008; Burow 

and others, 2008a,b; Kauffman and others, 2001; McMahon 

and others, 2008a,b). Nitrate concentrations in groundwater 

increased over several decades in many agricultural areas of 

the United States following the dramatic increase in fertilizer 

usage that began in the late 1940s (see summary by Puckett 

and others, 2011). Denitrification is the microbial reduction of nitrate to nitrogen gas (N2

) and in some aquifers it is 

an important process for decreasing nitrate concentrations 

(Böhlke and others, 2002; Green and others, 2008; Tesoriero 

and Puckett, 2011). Aquifers containing oxic shallow groundwater are more susceptible to changes in nitrate concentrations 

than aquifers containing anoxic deep groundwater (Dubrovsky 

and others, 2010). This pattern occurs because nitrate persists in oxic groundwater and is removed by denitrification in 

anoxic groundwater, and deep groundwater generally is older, 

sometimes predating nitrogen inputs by humans at the land 

surface, and contains a broader mix of water of differing ages 

and sources than shallow groundwater. Thus, redox-age classifications could provide a framework for characterizing the 

susceptibility of aquifers to changes in nitrate concentrations 

and such a framework could be used for interpreting timeseries monitoring data.

To a certain extent, this type of redox-age assessment has 

already been done using statistically based models of groundwater vulnerability (Rupert, 1998; Nolan and Hitt, 2006; 

Gurdak and Qi, 2006; Rupert and Plummer, 2009). Vulnerability models sometimes incorporate redox and groundwaterage information, but use surrogate variables, such as soil type 

and well depth (Nolan and Hitt, 2006). More direct measures 

of redox conditions and groundwater age are available, such 

as concentrations of dissolved oxygen, chlorofluorocarbons, 

and sulfur-hexafluoride, and detections of tritium, pesticide 

compounds, or volatile organic compounds (VOCs).

The purpose of this report is to characterize the susceptibility of selected principal aquifers of the United States to changes 

in nitrate concentrations on a basis of the redox-age classification scheme developed in this report. Redox classes are defined 

by concentrations of dissolved oxygen. Groundwater-age 

classes are defined by concentrations of tritium, nitrate, pesticide compounds, VOCs, chlorofluorocarbons, sulfur hexafluoride, and (or) helium. The redox-age classification scheme uses 

NAWQA water-quality data collected from 39 principal aquifers 

and 5 alluvial aquifers from 1991 to 2010.

Methods

This section describes the well networks and waterquality data used in the study, and the principal aquifers in 

which the networks are located. In addition, the redox and 

groundwater-age classes used to characterize the aquifers 

are defined.

Well Selection

Wells included in this study were used in NAWQA studies designed to describe the quality of water withdrawn from 

principal aquifers and used for drinking (termed major-aquifer 

studies or MASs, and source-water studies or DWGSs), and 

studies of shallow groundwater within specific land-use settings (termed land-use studies or LUSs). MASs focused on 

the quality of groundwater resources without being linked to a 

specific land use and used data mostly from existing domestic 

wells. DWGSs focused on the quality of groundwater from 

public-supply wells. Water samples from MAS and DWGS 

wells were collected before any treatment or pressure tanks 

and therefore do not represent water consumed for drinking. 

LUSs targeted the uppermost recently recharged groundwater 
Methods 3

to identify the effects of the overlying land use and used data 

mostly from monitoring wells and some production wells. 

Generally, MAS networks covered larger geographic areas and 

their wells were deeper than LUSs. Gilliom and others (1995) 

presented a general discussion of NAWQA well networks. 

Individual MASs, DWGSs, and LUSs are described in reports 

for individual NAWQA study areas (U.S. Geological Survey, 

2011). Data for a total of 6,593 wells from networks in 39 

principal aquifers and 5 alluvial aquifers were used in this 

study. Locations of the principal aquifers are shown in fig. 1.

Sources of Data

Many of the NAWQA wells have been sampled more 

than once, but the water-quality data used in this study primarily represent the most recently collected water sample from 

each well. Water samples included in this study were collected 

from 1991 to 2010. Methods for collecting and analyzing 

groundwater samples for the NAWQA program are well documented (U.S. Geological Survey, 2011) and are not repeated 

here. Water-quality and groundwater-age data of primary 

interest in this study are concentrations of dissolved oxygen, 

nitrate, pesticide compounds, volatile organic compounds 

(VOCs), chlorofluorocarbons, sulfur hexafluoride, tritium, 

and helium. Dissolved oxygen was measured in the field at 

the time of sample collection. Nitrate, pesticide compounds, 

and VOCs were analyzed at the USGS National Water Quality 

Laboratory in Denver, Colorado. Chlorofluorocarbons and 

sulfur hexafluoride were analyzed at the USGS Chlorofluorocarbon Laboratory in Reston, Virginia. Tritium was measured 

at the USGS Tritium Laboratory in Menlo Park, California, 

the USGS Noble Gas Laboratory in Denver, Colorado, or 

the Noble Gas Laboratory of the Lamont-Doherty Earth 

Observatory in Palisades, New York. Helium was measured 

at the USGS Noble Gas Laboratory in Denver, Colorado, 

or the Noble Gas Laboratory of the Lamont-Doherty Earth 

Observatory in Palisades, New York. The data can be found in 

the USGS National Water Information System (NWIS) or in 

Hinkle and others (2010).

Aquifer Groups and Geology

This study primarily examined 39 principal aquifers 

(fig. 1). Principal aquifers are defined as aquifers that are 

regionally extensive and can yield useable quantities of 

water (U.S. Geological Survey, 2003). The aquifers are 

broadly grouped into eight lithologic groups: basaltic and 

other volcanic rocks, carbonate rocks, crystalline rocks, 

glacial sand and gravel, sandstone and carbonate rocks, 

sandstone, semiconsolidated sand, and unconsolidated sand 

and gravel (U.S. Geological Survey, 2003). Five relatively 

small alluvial aquifers also were examined and they are 

considered to be part of the unconsolidated sand and gravel 

lithology group.

Redox Classification

Redox conditions in many of the principal aquifers 

were described on a regional basis (McMahon and others, 

2009) using the redox framework developed by McMahon 

and Chapelle (2008). That framework uses a dissolvedoxygen concentration of 0.5 milligram per liter (mg/L) as the 

threshold between oxic and anoxic conditions. The framework 

has additional redox subclasses for anoxic conditions, but for 

the purposes of this report only two redox classes are considered––oxic (dissolved oxygen concentration greater than 

or equal to 0.5 mg/L) and anoxic (dissolved oxygen concentration less than 0.5 mg/L). Consideration of just two redox 

classes is appropriate for this report because oxygen reduction 

typically is the first redox process to occur in groundwater 

and denitrification typically is the first anoxic redox process 

to follow oxygen reduction when nitrate is present in groundwater (Chapelle and others, 1995; McMahon and Chapelle, 

2008). The susceptibility of aquifers to changes in nitrate 

concentrations would be greater in oxic groundwater than in 

anoxic groundwater (Rupert, 2008), although the susceptibility also could be high in anoxic groundwater that is actively 

undergoing denitrification.

The use of a dissolved-oxygen concentration of 0.5 mg/L 

as the threshold for onset of denitrification probably is conservative. As discussed by Green and others (2010), mixing 

in heterogeneous aquifers and in well screens can result in the 

co-occurrence of geochemical indicators of denitrification with 

dissolved oxygen concentrations greater than 0.5 mg/L. Several field studies have reported apparent threshold concentrations for the onset of denitrification in the range of about 1 to 

2 mg/L (Böhlke and others, 2002; McMahon and others, 2004; 

Böhlke and others, 2007; Green and others, 2008; Tesoriero 

and Puckett, 2011).

Groundwater-Age Classification

The substantial increase in fertilizer usage in the United 

States beginning in about the late 1940s is an important event 

in the context of this study because fertilizer represents the 

largest single anthropogenic source of nitrogen in the country 

(Dubrovsky and others, 2010), and an increase in usage of 

fertilizer has been linked to increased concentrations of nitrate 

in groundwater (see review by Puckett and others, 2011). 

Given this history, a useful tracer of groundwater age for this 

study would differentiate between water recharged before and 

after the early 1950s. For the purposes of this report these 

waters are referred to as old and young groundwater, respectively. The susceptibility of aquifers to changes in nitrate 

concentrations would be greater in young groundwater than 

in old groundwater.

Tritium was used to differentiate between old and 

young groundwater in this study. Tritium is a radioactive 

isotope of hydrogen with a half-life of 12.32 years (Lucas 
4 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Figure 1. Location of selected principal aquifers in the United States (modified from U.S. Geological Survey, 2003).

EXPLANATION

Aquifers are shown by major lithology group (colored areas) and are identified by number 

Basin and Range basin-fill aquifers

California Coastal Basin aquifers

Central Valley aquifer system

Columbia Plateau basin-fill aquifers

High Plains aquifer

Mississippi River Valley alluvial aquifer

Northern Rocky Mountains Intermontane

 Basins aquifer system

Rio Grande aquifer system

Snake River Plain basin-fill aquifers

Surficial aquifer system

Willamette Lowland aquifer system

Unconsolidated sand and gravel aquifers

1

10

2

3

4

5

6

7

8

9

11

Glacial sand and gravel aquifers

12e

12c

12wc

12w

12e Eastern glacial aquifers

12c Central glacial aquifers

12wc West-central glacial aquifers

12w Western glacial aquifers

Cambrian-Ordovician aquifer system

Coastal Lowlands aquifer system

Denver Basin aquifer system

Early Mesozoic Basin aquifers

Lower Tertiary aquifers

Mississippi Embayment aquifer system

Northern Atlantic Coastal Plain aquifer system

Pennsylvanian aquifers

Southeastern Coastal Plain aquifer system

Texas Coastal Uplands aquifer system

Valley and Ridge clastic-rock aquifers

Semiconsolidated sand aquifers

13

14

15

16

17

Sandstone aquifers

18

19

20

21

22

23

Edwards-Trinity aquifer system

Mississippian aquifers

Sandstone and carbonate-rock aquifers

24

25

Biscayne aquifer

Castle Hayne aquifer

Columbia Plateau basaltic-rock aquifer system

Floridan aquifer system

Ordovician aquifers

Ozark Plateaus aquifer system

Piedmont and Blue Ridge carbonate-rock aquifers

Piedmont and Blue Ridge crystalline-rock aquifers

Silurian-Devonian aquifers

Snake River Plain basaltic-rock aquifer system

Valley and Ridge carbonate-rock aquifers

New York and New England crystalline-rock aquifers

 (unofficial name)

Basaltic- and other volcanic-rock aquifers

Carbonate-rock aquifers

27

28

29

30

31

32

33

34

Crystalline-rock aquifers

35

36

38

39

37 Hawaiian volcanic-rock aquifers

26 Basin and Range carbonate-rocks aquifers

1

2

3

6

8

10

13

14

14

16

17

19

24

25

26

27

28

29

30

31

39

4

5

7

9

11

12e

12c

12wc

12w

15

18

20

21

22

23

25

30

32 33

33

34

35

36

38

37

12w12w

HAWAII

ALASKA
Methods 5

and Unterweger, 2000). Small concentrations of tritium are 

produced naturally by interactions between the atmosphere 

and cosmic rays. It is an excellent tracer of water movement 

because it is part of the water molecule. In general, tritium in 

groundwater originates from precipitation. Because tritium 

is radioactive, its concentration in groundwater decreases 

over time as a result of radioactive decay. Before the onset 

of atmospheric testing of nuclear weapons in about 1953 

(prebomb), the tritium content of precipitation in the conterminous United States probably ranged from about 2 to 8 tritium 

units (TU) (Kaufman and Libby, 1954; Thatcher, 1962). As a 

result of radioactive decay, groundwater derived from precipitation that fell before 1953 would have contained less than 

0.5 TU tritium in 2010 (the most recent samples used in this 

study), but it could have contained upwards of about 1 TU in 

1991 (the earliest samples used in this study). The tritium content of precipitation increased substantially after the onset of 

atmospheric nuclear weapons testing but has slowly decreased 

from its peak in the early 1960s. Even with the variability in 

tritium content of precipitation over time, most groundwater 

in the United States exclusively derived from precipitation 

that fell since 1953 (postbomb) contained more than 0.5 TU 

in 2010. On the basis of this information, water samples with 

tritium concentrations less than 0.5 TU were considered to 

be potentially old groundwater (recharged before the early 

1950s), and water samples with tritium concentrations greater 

than or equal to 0.5 TU were considered to be potentially 

young groundwater (recharged after the early 1950s). For 

comparison, several other studies have used tritium concentrations ranging from about 0.2 to 1 TU as the cutoff between 

old and young groundwater (Michel and Schroeder, 1994; 

Plummer and others, 2004; Manning and others, 2005; Landon 

and others, 2010a).

Because only 39 percent of the samples that were 

assigned to a groundwater-age class had tritium data, other 

indicators of groundwater age were used for the samples that 

did not have tritium data. These indicators include detections 

of pesticide compounds and (or) VOCs; elevated concentrations of nitrate; and (or) dating with chloroflurocarbons, 

sulfur hexafluoride, or tritium/helium-3 (Plummer and others, 

1993; Kolpin and others, 1995; Busenberg and Plummer, 

2000; Shelton and others, 2001; Manning and others, 2005; 

Plummer and others, 2008). The water-quality data set used in 

this study contained information for as many as 155 pesticide 

compounds and 85 VOCs. Minimum detection levels for the 

pesticide compounds ranged from 0.000057 to 0.021 microgram per liter (μg/L). For the VOCs, minimum detection 

levels ranged from 0.001 to 0.3 μg/L. Samples that did not 

have tritium data but had a detection of a pesticide compound 

or a VOC were considered to be potentially young groundwater. Samples that did not have tritium data but had a nitrate 

concentration greater than 1.3 milligrams of nitrogen per 

liter (mg-N/L) also were considered to be potentially young 

groundwater. This concentration represents the 75th percentile 

concentration of nitrate in groundwater samples with tritium 

concentrations less than 0.5 TU. For comparison, Nolan and 

Hitt (2003) proposed a national background nitrate concentration of about 1 mg-N/L and Mueller and Helsel (1996) 

proposed a background concentration of 2 mg-N/L. Background nitrate concentrations in groundwater are likely to vary 

regionally and locally, but that variability was not taken into 

account in this study. Some samples were dated using chloroflurocarbons, sulfur hexafluoride, or tritium/helium-3 (Hinkle 

and others, 2010), and those data were used to determine the 

presence of young groundwater in samples for which tritium data were unavailable. For samples that had no tritium, 

chlorofluorocarbon, sulfur-hexafluoride, and tritium/helium-3 

data, or detections of a pesticide compound or VOC, and had 

a nitrate concentration of less than or equal to 1.3 mg-N/L, the 

age was considered to be potentially old. The criteria used to 

assign redox-age classes to groundwater samples are summarized in table 1.

The approach for classifying groundwater ages does 

not consider mixing, which is why the age determinations 

are qualified as being potentially old or young. It is likely 

that some groundwater classified as being potentially old 

contained a component of young groundwater, and vice 

versa (Weissmann and others, 2002; Manning and others, 

2005; Plummer and others, 2008). For example, 54 percent 

of the samples that were classified as potentially old on the 

basis of tritium concentrations less than 0.5 TU would have 

been classified as potentially young using just the pesticide 

compound, VOC, and nitrate data. Twenty-one percent of 

the samples that were classified as being potentially young 

on the basis of tritium concentrations greater than or equal to 

0.5 TU would have been classified as potentially old using just 

the pesticide compound, VOC, and nitrate data. Thus, some 

samples overlapped the age classes used in this report. Another 

potential limitation is that some VOCs could be present 

in groundwater recharged before 1950 either from natural 

sources or solvent and fuel use in the early 20th century. 

Chloroform, the most commonly detected VOC in the Nation’s 

groundwater (Zogorski and others, 2006), has both natural 

and man-made sources (McCulloch, 2003). Although the 

presence of tritium, pesticide compounds, VOCs, or elevated 

concentrations of nitrate in a sample generally indicates 

that the sample contained a fraction of young groundwater 

(Plummer and others, 2008), it does not indicate how much. 

Techniques are available for estimating the fractions of old 

and young groundwater in samples (Plummer and others, 

2003; Manning and others, 2005), but the data required for 

that analysis were not available for most of the samples used 

in this study. Ideally, one would analyze all the water samples 

for a comparable set of tracers that characterize groundwater 
6 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

age at multiple time scales to distinguish between water that 

is completely old or young, or is mixed (Landon and others, 

2010a). Despite the limitations of the approach used in this 

report to classify groundwater age, the approach still provides 

useful information at the regional scale examined in this 

report, as is described in the next section.

Evaluation of Redox-Age Classes

Two approaches were used to evaluate whether the redoxage classes described in table 1 could provide useful information on the susceptibility of principal aquifers to changes in 

nitrate concentrations. In the first approach, redox-age classes 

assigned to water samples from as many as 6,489 wells were 

evaluated in relation to well depth, well type, and aquifer 

confinement to see if the redox-age classes made sense hydrologically. In the second approach, redox-age classes assigned 

to water samples from 1,111 trend wells that were sampled 

at near decadal time scales (Lindsey and Rupert, 2012) were 

evaluated in relation to changes in nitrate concentrations in 

those samples to determine which redox-age classes had the 

largest and smallest changes in nitrate concentrations.

Oxic-potentially young water was mostly associated with 

relatively shallow monitoring and domestic wells completed 

in unconfined aquifers, whereas anoxic-potentially old water 

was mostly associated with deeper domestic wells completed 

in unconfined and confined aquifers (table 2). In general, the 

median well depths associated with each of the four primary 

redox-age classes increased in the order of anoxic-potentially 

young, oxic-potentially young, oxic-potentially old, and 

anoxic-potentially old (table 2). The fact that potentially 

young water came from shallower wells than potentially 

old water makes sense hydrologically and is consistent with 

what is known about groundwater-age stratigraphy in the 

principal aquifers (McMahon and others, 2011; Puckett and 

others, 2011).

Within a given redox class, the difference in median 

well depths between age criteria (a) and (b) was smaller than 

the difference between criteria (c) and (d) (tables 1 and 2). 

This could mean that age criteria (a) and (b), used to classify 

potentially young water, are more comparable than age 

criteria (c) and (d), used to classify potentially old water. For 

potentially old water, the median well depth was 1.6 to 2.0 

times greater for samples classified using criterion (c) than 

it was for samples classified using criterion (d). This could 

indicate that small nitrate concentrations and the absence 

of detections of pesticide compounds and VOCs (criterion 

(d)) is not always indicative of old water. In some instances, 

the smaller median well depth for samples classified using 

criterion (d) might indicate young groundwater that was 

not impacted by anthropogenic chemicals, in which case 

criterion (d) would overestimate the amount of potentially 

old water in an aquifer. For the purpose of characterizing 

the susceptibility of an aquifer to changes in nitrate 

concentrations, overestimation of the amount of potentially 

old water is probably less of an issue in anoxic water than 

oxic water because of the higher denitrification potential in 

Table 1. Criteria used to assign redox-age classes to groundwater samples. Young groundwater is defined as water 

recharged since the early 1950s, and old groundwater is defined as water recharged before the early 1950s. Age classes 

are labeled as potentially young or old because some samples probably represent a mixture of ages and the fractions of 

young and old water in them are unknown.

[≥, greater than or equal to; >, greater than; ≤, less than or equal to; < less than; mg/L, milligrams per liter; mg-N/L, milligrams of nitrogen per 

liter; TU, tritium units]

Class Criteria

Redox class

Oxic Dissolved oxygen ≥0.5 mg/L

Anoxic Dissolved oxygen <0.5 mg/L

Age class

Potentially young (a) If tritium data are available

Tritium concentration ≥0.5 TU

(b) If no tritium data are available

Detection of at least one pesticide compound or

Detection of at least one volatile organic compound or

Nitrate concentration >1.3 mg-N/L or

Dated using chlorofluorocarbons, sulfur hexafluoride, or tritium/helium-3

Potentially old (c) If tritium data are available

Tritium concentration <0.5 TU

(d) If no tritium, chlorofluorocarbon, sulfur hexafluoride, or tritium/helium-3 data are available

No detection of pesticide compounds and

No detection of volatile organic compounds and

Nitrate concentration ≤1.3 mg-N/L
Evaluation of Redox-Age Classes 

7

Table 2. Redox-age classes for water samples collected from principal aquifers in the United States and median well depth, well type, and aquifer confinement.[Refer to table 1 for the definition of age criteria a through d; m, meters; %, percentage of samples]

Redox-age class

Well depth below

land surface Well type Aquifer confinement

Number of

samples

Median

(m)

Number of

samples

Shallow

monitoring

wells

(%)

Domestic

wells

(%)

Publicsupply wells

(%)

Number of

samples

Confined

(%)

Unconfined

(%)

Oxic-potentially young, criterion (a) 1,489 31 1,393 46 43 10 1,106 18.1 81.9

Oxic-potentially young, criterion (b) 2,540 22 2,380 46 41 13 1,720 12.5 87.5

Oxic-potentially old, criterion (c) 321 70 301 16 66 18 254 20.5 79.5Oxic-potentially old, criterion (d) 319 43 305 25 55 20 181 21.5 78.5

Anoxic-potentially young, criterion (a) 470 18 428 49 43 8 312 24.4 75.6

Anoxic-potentially young, criterion (b) 761 19 732 45 42 14 493 26.8 73.2

Anoxic-potentially old, criterion (c) 267 93 249 12 61 27 202 77.7 22.3

Anoxic-potentially old, criterion (d) 322 46 313 24 55 20 197 33.5 66.5
8 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

anoxic water. For each redox class, the median well depth 

for potentially old water classified using criterion (d) still 

was larger than the median well depth for potentially young 

water, regardless of whether age criterion (a) or (b) was used 

to classify that water. Overall, age criterion (d) probably is 

indicative of old water in some instances and young water 

in others; of the eight possible redox-age classes (tables 1 

and 2), the oxic-potentially old classification based on age 

criterion (d) probably has the greatest uncertainty with respect 

to characterizing aquifer susceptibility to changes in nitrate 

concentrations. The oxic-potentially old, criterion (d), redoxage class was assigned to 5 percent of the 6,593 samples used 

in this study.

Overall, samples classified as oxic-potentially young 

showed the largest changes in nitrate concentrations for pairs 

of samples collected at near decadal time scales, whereas 

samples classified as anoxic-potentially old showed the 

smallest changes (fig. 2). This result is consistent with what 

would be predicted on the basis of the discussions in the 

“Redox Classification” and “Groundwater-Age Classification” 

sections of this report. Changes in nitrate concentrations for 

the samples classified as oxic-potentially old and anoxicpotentially young were intermediate in scale. Despite the 

relatively large uncertainty in age that could be associated with 

the oxic-potentially old classification based on age criterion 

(d), it actually had a smaller interquartile range for changes in 

nitrate concentrations than the same redox-age class based on 

age criterion (c) (fig. 2).

Although nitrate concentration was one of the criteria 

used to assign groundwater-age classes (table 1), it is unlikely 

that using nitrate-concentration data in this manner biased 

the comparison between redox-age classes and changes in 

nitrate concentrations in paired samples (fig. 2). Age classes 

were assigned to just one of the paired samples (typically 

the more recently collected sample), and then compared to 

changes in nitrate concentrations. A high nitrate concentration 

in a single groundwater sample may be an indicator of 

recently impacted groundwater, and that is the point of using 

it as an age indicator, but it is not a guarantee that changes 

in nitrate concentrations occurred at the time scale of the 

trends sampling.

In general, only the more recently collected sample 

was assigned a redox-age class for pairs of samples collected 

at near decadal time scales. An attempt was made to assign 

redox-age classes to both pairs of samples, but it quickly 

became apparent that the age assignments would not be 

comparable in most cases. This is because the paired samples 

usually were not analyzed for the same suite of pesticide 

compounds and VOCs, and often only one of the samples 

was analyzed for tritium.

Because data for dissolved oxygen typically were 

available for both pairs of samples, changes in nitrate 

concentrations were compared to changes in redox 

classification. The largest changes in nitrate concentrations 

occurred in pairs of samples that were both classified as 

oxic (fig. 3). This indicates that most of the changes in 

Figure 2. Change in nitrate concentration for pairs of samples collected from selected wells in the United States at near 

decadal time scales in relation to the redox-age class of the more recently collected sample.

10th percentile

25th percentile

50th percentile

75th percentile

90th percentile

EXPLANATION

108 Number of samples

Change in nitrate concentration, 

in milligrams of nitrogen per liter 

6

4

2

0

–2

–4

481 276

69

25 108 73 40 39

Oxic Anoxic

Potentially young Potentially old

Age class

(a) (b) (c) (d)

Potentially young Potentially old

(a) (b) (c) (d)

(a) through (d) Age criteria defined in table 1
Relation Between Redox-Age Classes and Changes in Nitrate Concentrations in Trend-Well Networks 9

nitrate concentrations were not a result of changes in redox 

conditions in the aquifer but were more likely a result of 

changing nitrogen inputs at the land surface and (or) changing 

fractions of young and old water in the sample pairs.

On the basis of the evaluation presented above, the 

redox-age classes in table 1 can provide useful information 

on aquifer susceptibility to changes in nitrate concentrations. 

For the remainder of this report, results for age criteria (a) 

and (b) were combined by redox class and the same was done 

for age criteria (c) and (d). This results in four redox-age 

classes (oxic-potentially young, oxic-potentially old, anoxicpotentially young, and anoxic-potentially old) instead of eight.

Relation Between Redox-Age Classes 

and Changes in Nitrate Concentrations 

in Trend-Well Networks

The data in figure 2 indicate a strong relation between 

redox-age class and the change in nitrate concentration in 

pairs of samples collected from individual wells at near 

decadal time scales. The relation between redox-age classes 

and changes in nitrate concentrations also was examined at 

the well-network level because the well networks, unlike 

single wells, were designed to be statistically representative 

of large aquifer areas. Lindsey and Rupert (2012) analyzed 

nitrate concentrations in water samples collected at near 

decadal time scales from 56 NAWQA well networks (fig. 4) 

and found statistically significant changes in concentrations at 

greater than a 90-percent confidence level in 18 (32 percent) 

of them. Lindsey and Rupert (2012) did not analyze nitrogen 

input histories in those 18 networks, but presumably the 

significant changes in nitrate concentrations were related 

to changes in nitrogen inputs at the land surface in some 

of them. Other factors such as variations in recharge rates, 

depth to groundwater, or pumping also could have affected 

nitrate concentrations (Rosen and others, 2008). Redox-age 

classes assigned to water samples from the LUS and MAS 

networks analyzed by Lindsey and Rupert (2012) are shown 

in table 3.

Results from 6 of 25 agricultural LUS networks for which 

near decadal changes could be evaluated showed significant 

increases in nitrate concentrations and 2 networks showed 

significant decreases (Lindsey and Rupert, 2012) (fig. 5 and 

table 3). Agricultural networks that showed significant increases 

in concentrations were located in the Central Valley aquifer 

system, Central glacial aquifers, Floridan aquifer system, Snake 

River Plain basaltic-rock aquifer system, and the South Platte 

River alluvial aquifer (fig. 4 and table 3). At least 75 percent 

of the samples in 7 of the 8 networks that showed significant 

changes in nitrate concentrations were classified as oxicpotentially young (fig. 5), and no more than 4 percent of the 

samples were classified as anoxic-potentially old (table 3). 

Samples from urban LUSs generally showed similar results, 

with 4 of 13 networks showing significant increases in nitrate 

concentrations and 1 showing a significant decrease. At least 

Figure 3. Change in nitrate concentration for pairs of samples collected from selected wells in the United 

States at near decadal time scales in relation to the change in redox condition.

Oxic to oxic Oxic to anoxic Anoxic to oxic Anoxic to anoxic

6

4

2

0

–2

–4

Change in nitrate concentration, 

in milligrams of nitrogen per liter 

791

58 109 209

10th percentile

25th percentile

50th percentile

75th percentile

90th percentile

EXPLANATION

109 Number of samples

Change in redox condition
10 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Figure 4. Central locations of well networks sampled at near decadal time scales by the National Water-Quality Assessment Program.

tn17

tn50

tn43tn6

tn5

tn24 tn23

tn4

tn30

tn44

tn1

tn3

tn2

tn29

tn28

tn27

*tn25

*tn56

tn7

tn45

tn8

tn31

tn40

tn53 tn35

tn21

*tn55

tn33 tn14

tn39

tn49

tn15

tn16

tn34

tn33 tn14

tn11 tn54

tn41

tn20 tn10

tn52

tn9

tn42

tn19

tn32

tn48

tn22

tn46

tn51

tn36

tn18

tn37

tn47

tn39

tn13

tn38

tn49

tn15

tn16

tn34

tn12

tn26

EXPLANATION

Trend-well network 

Unconsolidated sand and gravel aquifers *

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone aquifers

Sandstone and carbonate-rock aquifers

Basaltic- and other volcanic-rock aquifers

Carbonate-rock aquifers

Aquifer lithology

Agricultural land-use network

Urban land-use network

Major aquifer network

Significance of change in nitrate 

 concentrations (Lindsey and 

 Rupert, 2012)

No change

Decrease

Increase

tn1

Network not in a principal aquifer

Network identifier (see table 3)

75 percent of the samples in 4 of the 5 networks that showed 

significant changes in nitrate concentrations were classified as 

oxic-potentially young (fig. 5), and no more than 5 percent of 

the samples were classified as anoxic-potentially old (table 3). 

For 11 of the 17 agricultural LUSs and 7 of the 8 urban LUSs 

that showed no significant changes in nitrate concentrations, at 

least 75 percent of their samples also were classified as oxicpotentially young (table 3). Only about 11 percent of the LUS 

networks studied by Lindsey and Rupert (2012) had more than 

10 percent of their samples classified as potentially old (table 3), 

which is not surprising given that NAWQA land-use studies 

typically targeted the most recently recharged groundwater.

Three of 18 MASs showed significant increases in nitrate 

concentrations, and two showed significant decreases (Lindsey 

and Rupert, 2012) (fig. 4 and table 3). Networks that showed 

significant increases in nitrate concentrations were located 

in the Central Valley, Northern Atlantic Coastal Plain, and 

Floridan aquifer systems. Only one of the MASs (acfbsus1) 

that showed a significant change in nitrate concentrations 

had more than 75 percent of the samples classified as oxicpotentially young. For the other four networks, 0 to 54 percent 

of the samples were classified as oxic-potentially young 

(fig. 5 and table 3). On closer inspection, the LUS and MAS 

results are not necessarily inconsistent because most of the 

wells with large changes in nitrate concentrations in these 

MAS networks with significant changes were oxic-potentially 

young. For the two MAS networks (lirbsus1 and santsus2) that 

showed significant decreases in nitrate concentrations, 62 to 

100 percent of the samples were classified as anoxic and 69 

to 79 percent of the samples were classified as potentially old 
Redox-Age Classes in Principal Aquifers 11

(table 3). The network-level changes in nitrate concentrations 

for lirbsus1 and santsus2 were –0.04 and –0.05 mg-N/L, 

respectively (fig. 5 and table 3), and the change in concentration 

for almost all the sample pairs was less than 0.1 mg-N/L 

(figs. 6A, B). More than 50 percent of the data were pairs of 

nondetects (Lindsey and Rupert, 2012). The one sample in 

those two networks than did show a relatively large change 

in concentration (greater than 1 mg-N/L) was classified as 

oxic-potentially young (fig. 6B). The predominance of small 

changes in nitrate concentrations in these two networks would 

be expected for aquifers that contained large percentages 

of anoxic and (or) potentially old water. The two networks 

(sacrsus1 and dlmvsus1) that showed significant increases in 

nitrate concentrations, but for which the percentage of samples 

classified as oxic-potentially young was less than 75 percent 

(48 to 54 percent), had larger absolute changes in nitrate 

concentrations (0.14 mg-N/L) than lirbsus1 and santsus2 

(fig. 5). The concentration changes for sacrsus1 and dlmvsus1, 

however, were smaller than the change for the MAS network 

acfbsus1 (0.32 mg-N/L) that had more than 75 percent of its 

samples classified as oxic-potentially young (fig. 5 and table 3). 

Closer inspection of the data from sacrsus1 and dlmvsus1 shows 

that most of the large changes in concentrations occurred in 

samples that were classified as oxic-potentially young (figs. 6C, 

D). Three samples from the dlmvsus1 study that were classified 

as anoxic-potentially young showed concentration changes 

of 4.9 to 7.5 mg-N/L (fig. 6D); redox conditions in these 

samples apparently were anoxic but did not result in complete 

denitrification.

Two of the 13 MASs that showed no significant 

changes in nitrate concentrations had at least 75 percent of 

their samples classified as oxic-potentially young (table 3). 

The remaining 11 networks had 32 to 71 percent of their 

samples classified as oxic-potentially young. About 83 

percent of the MAS networks studied by Lindsey and Rupert 

(2012) had more than 10 percent of their samples classified as 

potentially old, which is a considerably larger percentage than 

for the agricultural LUS networks. This finding was expected 

because wells used in the MAS networks typically were 

deeper than those used in the LUS networks.

The redox-age results for pairs of samples collected 

from individual wells show that the largest changes in nitrate 

concentrations primarily occurred in samples that were 

classified as oxic-potentially young (figs. 2 and 6). A generally 

similar pattern was observed when samples were aggregated 

to the level of well networks. For LUS and MAS networks 

that showed significant changes in nitrate concentrations, 

the median changes in concentrations were 0.28, 0.14, and 

–0.05 mg-N/L for networks that had at least 75 percent, 

at least 25 percent and less than 75 percent, and less than 

25 percent of the samples classified as oxic-potentially young, 

respectively. On the basis of the data shown in figures 2 and 5, 

aquifer areas (as defined by well networks) in which at least 

75 percent of the samples were classified as oxic-potentially 

young were considered to have a high susceptibility to 

changes in nitrate concentrations (fig. 5). Aquifer areas were 

considered to have a medium susceptibility to changes in 

nitrate concentrations if at least 25 percent and less than 

75 percent of the samples were classified as oxic-potentially 

young (fig. 5). Aquifer areas were considered to have a low 

susceptibility to changes in nitrate concentrations if less than 

25 percent of the samples were classified as oxic-potentially 

young (fig. 5). These definitions of high, medium, and low 

are used to characterize aquifer susceptibility to changes in 

nitrate concentrations throughout the remainder of the report. 

The degree of susceptibility is not intended to indicate that 

significant changes in concentrations of nitrate will or will 

not be detected in the future in those areas. Other factors 

such as nitrogen input history at the land surface, mixing, 

and lag times related to nitrate transport could cause nitrateconcentration trends to develop over longer time scales than 

the near decadal time scale examined by Lindsey and Rupert 

(2012), or not at all.

Redox-Age Classes 

in Principal Aquifers

The three primary well types sampled by NAWQA 

generally represent different depth zones and (or) areas of the 

principal aquifers. Shallow monitoring wells are completed 

near the water table, whereas domestic and public-supply 

wells are mostly completed in deeper zones in the aquifers. 

In this section of the report, redox-age classes assigned 

to networks of shallow monitoring wells in agricultural 

areas, domestic wells, and public-supply wells were used 

to characterize the susceptibility to changes in nitrate 

concentrations of these different depth zones and (or) areas 

of the principal aquifers. The susceptibility to changes in 

nitrate concentrations near the water table in urban areas 

is not considered here because of the generally small area 

represented by those networks of shallow monitoring wells, 

however, redox-age classes and susceptibility rankings for 

those networks are shown in the Appendix.

Susceptibility to Changes in Nitrate 

Concentrations Near the Water Table 

in Agricultural Areas

Redox-age classes were assigned to samples collected 

from 40 networks of shallow monitoring wells in agricultural 

areas (fig. 7 and table 4). Most networks (58 percent) had a 

high susceptibility to changes in nitrate concentrations because 

at least 75 percent of their samples were classified as oxicpotentially young. Only 10 percent of the networks had a low 

susceptibility to changes in nitrate concentrations because less 

than 25 percent of their samples classified as oxic-potentially 

young (table 4).
12 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Table 3. Redox-age classes for water samples collected from selected well networks in the United States at near decadal time scales, 

median change in nitrate concentration for each network, and the statistical significance of the change in concentration. 

[usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; car, carbonate rock; bav, basaltic and other volcanic rock; 

san, sandstone; scr, sandstone and carbonate rock; alus, agricultural land-use study; mas, major-aquifer study; ulus, urban land-use study; mg-N/L, milligrams 

of nitrogen per liter; shading is used to differentiate between alus, mas, and ulus studies; bold indicates a statistically significant change in nitrate concentrations 

at greater than a 90-percent confidence level]

Aquifer

number

Aquifer

lithology Aquifer name Studytype

Network name

(number of wells)

Network identifier

(see figure 4)

3 usg Central Valley aquifer system alus sanjlusor2a (19) tn1

3 usg Central Valley aquifer system alus sanjlusor1a (17) tn2

3 usg Central Valley aquifer system alus sanjluscr1a (18) tn3

3 usg Central Valley aquifer system alus sacrluscr1 (21) tn4

4 usg Columbia Plateau basin-fill aquifers alus ccptlusag2b (16) tn5

4 usg Columbia Plateau basin-fill aquifers alus ccptlusor1b (19) tn6

8 usg Rio Grande aquifer system alus riogluscr1 (12) tn7

8 usg Rio Grande aquifer system alus rioglusag1 (25) tn8

10 usg Surficial aquifer system alus santluscr1 (19) tn9

10 usg Surficial aquifer system alus gaflluscr1 (20) tn10

10 usg Surficial aquifer system alus sofllusor1 (17) tn11

11 usg Willamette Lowland aquifer system alus willlusag3 (24) tn12

12c gla Central glacial aquifers alus wmiclusag2 (26) tn13

12c gla Central glacial aquifers alus whitluscr1 (20) tn14

12wc gla West-central glacial aquifers alus umisluscr1 (22) tn15

12wc gla West-central glacial aquifers alus eiwaluscr1 (30) tn16

12w gla Western glacial aquifers alus pugtluscr1 (19) tn17

15 scs Northern Atlantic Coastal Plain aquifer system alus dlmvluscr1 (16) tn18

15 scs Northern Atlantic Coastal Plain aquifer system alus albelusag1 (12) tn19

29 car Floridan aquifer system alus acfbluscr3 (19)1 tn20

31 car Ozark Plateaus aquifer system alus ozrklusag2a (20) tn21

34 car Valley and Ridge carbonate-rock aquifers alus potolusag1 (24) tn22

36 bav Snake River Plain basaltic-rock aquifer system alus usnkluscr2 (26) tn23

36 bav Snake River Plain basaltic-rock aquifer system alus usnkluscr3 (28) tn24

--3 usg South Platte River alluvial aquifer alus spltluscr1 (29) tn25

1 usg Basin and Range basin-fill aquifers mas nvbrsus2 (16) tn26

1 usg Basin and Range basin-fill aquifers mas cazbsus1a (24) tn27

2 usg California Coastal Basin aquifers mas sanasus2 (14) tn28

3 usg Central Valley aquifer system mas sanjsus1 (26) tn29

3 usg Central Valley aquifer system mas sacrsus1 (28) tn30

5 usg High Plains aquifer mas hpgwsus1a (30) tn31

12c gla Central glacial aquifers mas lerisus1 (27) tn32

12c gla Central glacial aquifers mas lirbsus1 (29) tn33

12wc gla West-central glacial aquifers mas eiwasus2 (30) tn34

13 scs Coastal Lowlands aquifer system mas trinsus3 (17) tn35

15 scs Northern Atlantic Coastal Plain aquifer system mas linjsus2 (24) tn36

15 scs Northern Atlantic Coastal Plain aquifer system mas dlmvsus1 (23) tn37

18 san Cambrian-Ordovician aquifer system mas umissus3 (22) tn38

18 san Cambrian-Ordovician aquifer system mas wmicsus1 (25) tn39

24 scr Edwards-Trinity aquifer system mas sctxsus1 (23) tn40

29 car Floridan aquifer system mas acfbsus1 (20) tn41

29 car Floridan aquifer system mas santsus2 (29) tn42

35 bav Columbia Plateau basaltic-rock aquifer system mas ccptsus1b (30)2 tn43

3 usg Central Valley aquifer system ulus sacrlusrc1 (18) tn44

8 usg Rio Grande aquifer system ulus rioglusrc1 (10) tn45

12e gla Eastern glacial aquifers ulus necblusrc1 (21) tn46

12c gla Central glacial aquifers ulus uirblusrc1 (18) tn47

12c gla Central glacial aquifers ulus lerilusrc1 (29) tn48

12wc gla West-central glacial aquifers ulus umislusrc1 (26) tn49

12w gla Western glacial aquifers ulus pugtlusrs1 (24) tn50

15 scs Northern Atlantic Coastal Plain aquifer system ulus linjlusrc1 (27) tn51

16 scs Southeastern Coastal Plain aquifer system ulus santlusrc1 (17) tn52

24 scr Edwards-Trinity aquifer system ulus sctxlusrc1 (30) tn53

27 car Biscayne aquifer ulus sofllusrc1a (17) tn54

 --3 usg Alluvial aquifer in Memphis, Tennessee ulus miselusrc1 (20) tn55

 --3 usg Alluvial aquifers in the Colorado Rocky Mountains ulus ucollusrc1 (16) tn56
Relation Between Redox-Age Classes and Changes in Nitrate Concentrations in Trend-Well Networks 13

Table 3. Redox-age classes for water samples collected from selected well networks in the United States at near decadal time scales,

median change in nitrate concentration for each network, and the statistical significance of the change in concentration.—Continued

[usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; car, carbonate rock; bav, basaltic and other volcanic rock; 

san, sandstone; scr, sandstone and carbonate rock; alus, agricultural land-use study; mas, major-aquifer study; ulus, urban land-use study; mg-N/L, milligrams 

of nitrogen per liter; shading is used to differentiate between alus, mas, and ulus studies; bold indicates a statistically significant change in nitrate concentrations 

at greater than a 90-percent confidence level]

Aquifer

number

Redox-age class (percentage of samples)4 Data from Lindsey and Rupert (2012)

Oxicpotentially youngOxicpotentially oldAnoxicpotentially youngAnoxicpotentially oldMedian change in nitrate 

concentration (mg-N/L)

Statistical significance of

change in nitrate concentration

3 95 0 5 0 0.62 No change

3 88 6 6 0 0.07 No change

3 82 0 18 0 1.0 Increase

3 10 0 86 5 –0.04 No change

4 94 6 0 0 –0.77 No change

4 89 0 11 0 0.05 No change

8 75 8 17 0 –0.11 No change

8 40 8 48 4 0.13 No change

10 95 0 5 0 –0.63 Decrease

10 90 10 0 0 0.84 No change

10 41 0 53 6 –0.02 No change

11 46 0 54 0 –0.04 Decrease

12c 85 0 12 4 0.84 Increase

12c 45 0 50 5 0.00 No change

12wc 100 0 0 0 0.88 No change

12wc 97 3 0 0 –0.02 No change

12w 79 0 21 0 1.1 No change

15 56 0 44 0 –0.10 No change

15 33 0 58 8 0.03 No change

29 89 11 0 0 0.29 Increase

31 81 0 19 0 0.02 No change

34 96 0 4 0 –0.09 No change

36 100 0 0 0 0.26 Increase

36 96 4 0 0 0.06 Increase

--3 83 0 17 0 2.0 Increase

1 69 19 0 13 0.05 No change

1 38 63 0 0 0.14 No change

2 50 7 21 21 –0.05 No change

3 81 8 4 8 0.45 No change

3 54 18 25 4 0.14 Increase

5 43 57 0 0 0.01 No change

12c 33 22 19 26 0.01 No change

12c 0 0 21 79 –0.04 Decrease

12wc 67 27 3 3 –0.04 No change

13 35 59 6 0 0.00 No change

15 71 0 13 17 –0.01 No change

15 48 0 52 0 0.14 Increase

18 55 9 32 5 –0.04 No change

18 32 4 36 28 0.01 No change

24 91 0 9 0 –0.04 No change

29 100 0 0 0 0.32 Increase

29 10 28 21 41 –0.05 Decrease

35 69 7 3 21 0.02 No change

3 83 0 17 0 –0.03 No change

8 30 0 30 40 0.05 No change

12e 90 0 10 0 –0.09 Decrease

12c 93 0 7 0 –0.01 No change

12c 72 0 28 0 0.28 Increase

12wc 88 0 12 0 0.04 Increase

12w 92 0 4 4 0.18 No change

15 93 0 7 0 –0.09 No change

16 82 12 6 0 0.04 No change

24 100 0 0 0 0.36 Increase

27 82 6 12 0 0.02 No change

--3 75 0 20 5 0.21 Increase

--3 75 6 19 0 –0.03 No change

1

Network has wells in the Floridan and Southeastern Coastal Plain aquifer systems.

2

Network has wells in the Columbia Plateau basaltic-rock and basin-fill aquifer systems.

3

Network not in a principal aquifer.

4

Redox-age classes where determined for the more recently collected samples in each network, and redox-age percentages may not sum to 100 percent 

because of rounding.
14 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

For principal aquifers that had at least 2 networks of 

wells, median percentages of samples classified as oxicpotentially young ranged from about 57 to 96 percent (fig. 8). 

On the basis of these data, for the parts of aquifers near the 

water table in agricultural areas, the aquifers most susceptible 

to changes in nitrate concentrations were the Columbia 

Plateau basin-fill aquifers, Eastern glacial aquifers, and the 

West-central glacial aquifers (fig. 8). None of the aquifers 

had a low susceptibility to changes in nitrate concentrations, 

which would be indicated by a median percentage of samples 

classified as oxic-potentially young that was less than 

25 percent. The High Plains aquifer had the highest median 

percentage of samples classified as oxic-potentially old, which 

generally reflects the low organic-carbon content of sediment 

and relatively low recharge rates in the aquifer (McMahon and 

others, 2007). The Central Valley and Surficial aquifer systems 

had the highest median percentages of samples classified 

as anoxic-potentially young (fig. 8). Only the Rio Grande 

aquifer system and the Central glacial aquifers had median 

percentages of samples classified as anoxic-potentially old that 

were greater than zero.

Although the median percentages in figure 8 provide a 

general comparison of redox-age classes between principal 

aquifers, they do not indicate the substantial redox-age 

variability that can occur within an aquifer. For the three well 

networks in the Central Valley aquifer system, the percentage 

of samples classified as oxic-potentially young ranged from 

13 to 90 percent (table 4 and fig. 9), and the percentage of 

samples classified as anoxic-potentially young ranged from 10 

to 83 percent. In the High Plains aquifer, median percentages 

of samples classified as oxic-potentially old ranged from 10 to 

41 percent (fig. 9). In the West-central glacial aquifer, median 

percentages of samples classified as anoxic-potentially old 

ranged from 0 to 35 percent (fig. 9). Large intraaquifer redoxage variability was observed in most of the aquifers that had 

multiple networks of shallow monitoring wells in agricultural 

areas (table 4).

Distinct patterns in the spatial distribution of networklevel susceptibilities are apparent in some of the aquifers. 

Networks in the Central glacial aquifers of Indiana and 

parts of southern Michigan and Wisconsin had medium 

susceptibilities, whereas networks in glacial aquifers to the 

Figure 5. Median change in nitrate concentration in relation to the percentage of 

samples that were classified as oxic-potentially young in well networks that were 

sampled at near decadal timescales (concentration data from Lindsey and Rupert, 

2012), and the susceptibility of the networks to changes in nitrate concentrations.

0 25 50 75 100

–0.5

0.0

0.5

1.0

1.5

2.0

2.5

acfbsus1

dlmvsus1 sacrsus1

santsus2

Percentage of samples classified as oxic-potentially young

lirbsus1

Low

susceptibility

Medium

susceptibility

High

susceptibility

–1.0

Median change in nitrate concentration,

in milligrams of nitrogen per liter

Agricultural land-use network

Urban land-use network

Major aquifer network

EXPLANATION

Significance of change in nitrate 

 concentrations (Lindsey and 

 Rupert, 2012)

No change

Decrease

Increase

Trend-well network
Redox-Age Classes in Principal Aquifers 15

Figure 6. Concentrations of nitrate in pairs of samples collected from selected networks of major-aquifer study wells in the 

United States at near decadal time scales; (A) lirbsus1, (B) santsus2, (C) sacrsus1, and (D) dlmvsus1 networks.

0.0 0.1 0.2 0.3 0.4 0.5

0.0

0.1

0.2

0.3

0.4

0.5

0 2 4 6

0

2

4

6

Nitrate concentration in 2007, 

in mg-N/L

Nitrate concentration in 1996, 

in mg-N/L

Nitrate concentration in 1998, 

in mg-N/L

Nitrate concentration in 2006, 

in mg-N/L

A B

C

0 5 10 15 20 0 5 10 15 20

Nitrate concentration in 1996,

in mg-N/L

Nitrate concentration in 1988, 

in mg-N/L

lirbsus1

n = 29

∆NO3

 = –0.04 mg-N/L

santsus2

n = 29

∆NO3

 = –0.05 mg-N/L

sacrsus1

n = 28

∆NO3 = 0.14 mg-N/L

dlmvsus1

n = 23

∆NO3 = 0.14 mg-N/L

0

5

10

15

20

Nitrate concentration in 2008, 

in mg-N/L

0

5

10

15

20

Nitrate concentration in 2001, 

in mg-N/L

D

EXPLANATION

Redox-age class

Oxic-potentially young

Oxic-potentially old

Anoxic-potentially young

Anoxic-potentially old

Change in nitrate concentration,

 in milligrams of nitrogen per 

 liter (mg-N/L)

0 ±1

±5

sacrsus1

±0.1

Network name

n Number of samples

∆NO3 Median change in nitrate 

 concentration for the network 

 (Lindsey and Rupert, 2012)
16 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Figure 7. Central locations of networks of shallow monitoring wells in agricultural areas and the susceptibility of the networks to 

changes in nitrate concentrations.

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone and carbonate-rock aquifers

Carbonate-rock aquifers

Aquifer lithology Susceptibility to changes in nitrate 

 concentrations, networks of shallow 

 monitoring wells in agricultural areas

High

Medium

Low

am1 Network identifier (see table 4)

* Network not in a principal aquifer

EXPLANATION

ALASKA

Basaltic- and other volcanic-rock aquifers

am31

am5

am6

am4

am3

am2

am1

am10

am9

am8

*am38

*am39 am7

am29

am30 am28

am26

am27

am18

am16

am17

am21

am23

am22

am25 am19

am14 am26

am27

am18

am11

am32

*am40

am36

am12

am37

am13

am35

am34

am33 am16

am17

am21

am24

am20 am20 am23

am22

am25 am19

am14

am15

HAWAII
Redox-Age Classes in Principal Aquifers 17

Table 4. Redox-age classes for water samples collected from networks of shallow monitoring wells in agricultural areas in the United States and the susceptibility of the networks to changes in nitrate concentrations (only networks with at least 10 wells are listed).[usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; scr, sandstone and carbonate rock; car, carbonate rock; shading is used to differentiate between aquifer lithologies]

Aquifer

number

Aquifer

lithology Aquifer name Network name (number of wells) Network identifier (see figure 7) Redox-age class (percentage of samples)3 Susceptibility to changes in nitrate concentrations Oxic- potentially young Oxic- potentially old Anoxic- potentially young Anoxic- potentially old

1 usg Basin and Range basin-fill aquifers nvbrlusag1 (13) am1 15 15 31 38 Low3 usg Central Valley aquifer system sanjlusor1b (10) am2 90 0 10 0 High3 usg Central Valley aquifer system sanjluscr1b (10) am3 60 0 40 0 Medium3 usg Central Valley aquifer system sacrluscr1 (30) am4 13 0 83 3 Low4 usg Columbia Plateau basin-fill aquifers ccptlusag2b (27) am5 93 4 4 0 High4 usg Columbia Plateau basin-fill aquifers ccptlusor1b (25) am6 88 0 12 0 High5 usg High Plains aquifer hpgwlusag3 (30) am7 90 10 0 0 High5 usg High Plains aquifer hpgwlusag1 (27) am8 74 26 0 0 Medium5 usg High Plains aquifer hpgwlusag2 (27) am9 59 41 0 0 Medium

8 usg Rio Grande aquifer system riogluscr1 (76) am10 87 1 12 0 High

8 usg Rio Grande aquifer system rioglusag1 (34) am11 32 9 50 9 Medium10 usg Surficial aquifer system santluscr1 (30) am12 93 0 7 0 High10 usg Surficial aquifer system sofllusor1 (38) am13 21 0 76 3 Low12e gla Eastern glacial aquifers hdsnlusag1 (12) am14 100 0 0 0 High12e gla Eastern glacial aquifers connlusag1 (32) am15 91 3 6 0 High12c gla Central glacial aquifers lirbluscr1 (22) am16 95 0 5 0 High12c gla Central glacial aquifers lirbluscr2 (25) am17 88 0 12 0 High12c gla Central glacial aquifers wmiclusag2 (29) am18 86 0 10 3 High12c gla Central glacial aquifers miamluscr1 (21) am19 86 0 14 0 High12c gla Central glacial aquifers wmiclusag1a (23) am20 74 4 13 9 Medium12c gla Central glacial aquifers whitluscr2 (20) am21 65 10 20 5 Medium12c gla Central glacial aquifers uirbluscr1 (29) am22 59 0 41 0 Medium12c gla Central glacial aquifers leriluscr1 (30) am23 57 13 30 0 Medium

12c gla Central glacial aquifers whitluscr3a (24) am24 50 0 46 4 Medium

12c gla Central glacial aquifers whitluscr1 (26) am25 46 0 50 4 Medium

12wc gla West-central glacial aquifers umisluscr1 (27) am26 100 0 0 0 High

12wc gla West-central glacial aquifers eiwaluscr1 (31) am27 97 3 0 0 High

12wc gla West-central glacial aquifers rednlusag1 (21) am28 90 0 10 0 High

12wc gla West-central glacial aquifers cnbrluscr1 (26) am29 88 0 12 0 High

12wc gla West-central glacial aquifers rednlusag2 (20) am30 15 5 45 35 Low

12w gla Western glacial aquifers pugtluscr1 (20) am31 75 0 25 0 High

13 scs Coastal Lowlands aquifer system acadluscr1 (21) am32 100 0 0 0 High

15 scs Northern Atlantic Coastal Plain aquifer system linjluscr1 (15) am33 100 0 0 0 High

15 scs Northern Atlantic Coastal Plain aquifer system dlmvluscr1 (27) am34 67 0 33 0 Medium

15 scs Northern Atlantic Coastal Plain aquifer system albelusag1 (30) am35 50 0 43 7 Medium

25 scr Mississippian aquifers ltenlusag1 (29) am36 97 0 3 0 High

29 car Floridan aquifer system acfbluscr3 (24)1 am37 92 8 0 0 High

--2 usg Denver Basin alluvial aquifers spltluscr2 (21) am38 100 0 0 0 High

--2 usg South Platte River alluvial aquifer spltluscr1 (30) am39 83 0 17 0 High--2 usg Valley and Ridge alluvial aquifers utenluscr1 (30) am40 63 0 33 3 Medium 1Network has wells in the Floridan and Southeastern Coastal Plain aquifer systems.

2Network not in a principal aquifer.3Redox-age percentages may not sum to 100 percent because of rounding.
18 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

east and west had high susceptibilities. The areas of medium 

susceptibility in Indiana had relatively high percentages of 

samples classified as anoxic-potentially young compared to 

the surrounding networks with high susceptibilities (fig. 7 and 

table 4). This difference in redox-age classes between the two 

areas could indicate shallower depths to water and (or) finer 

grained sediment in the medium-susceptibility area relative 

to the high-susceptibility areas, both of which could result in 

anoxic groundwater. Other studies have reported an increase 

in concentrations of dissolved organic carbon, probably from 

the soil zone, and a decrease in concentrations of dissolved 

oxygen in groundwater with decreasing depths to the water 

table (Pabich and others, 2001; McMahon and Chapelle, 

2008). In the High Plains aquifer, network susceptibility 

decreased from high in the north to medium in the central 

and southern parts of the aquifer (fig. 7). This change in 

susceptibility corresponds to a north-to-south increase in 

the percentage of samples classified as oxic-potentially old 

(fig. 9 and table 4), an increase that is probably related to the 

north-to-south decrease in recharge and increase in depth to 

the water table (McMahon and others, 2007). In the Central 

Valley aquifer system, network susceptibility increased from 

low in the north to medium and high in the south (fig. 7). The 

percentage of samples classified as anoxic-potentially young 

in the northern network was about 2 to 8 times greater than the 

percentages in the southern networks where oxic-potentially 

young groundwater predominated (fig. 9 and table 4). The 

common occurrence of anoxic groundwater in the north may 

be related to the much shallower depths to the water table in 

the northern network of wells (median depth 1.1 m) than in the 

southern networks (median depths 14 to 20 m).

Figure 8. Median percentage of samples assigned to the four redox-age classes for principal aquifers that have at 

least two networks of shallow monitoring wells in agricultural areas, and the susceptibility of the aquifers to changes in 

nitrate concentrations.

0 25 50 75 100 0 25 50 75 100

Oxic-potentially young Oxic-potentially old

Susceptibility to changes 

in nitrate concentrations

Low Medium High

0 25 50 75 100

Median percentage of samples

0 25 50 75 100

Median percentage of samples

Anoxic-potentially young Anoxic-potentially old

Central Valley aquifer system (3)

Columbia Plateau basin-fill aquifers (2)

High Plains aquifer (3)

Rio Grande aquifer system (2)

Surficial aquifer system (2)

Eastern glacial aquifers (2)

Central glacial aquifers (10)

West-central glacial aquifers (5)

Northern Atlantic Coastal Plain aquifer system (3)

Central Valley aquifer system (3)

Columbia Plateau basin-fill aquifers (2)

High Plains aquifer (3)

Rio Grande aquifer system (2)

Surficial aquifer system (2)

Eastern glacial aquifers (2)

Central glacial aquifers (10)

West-central glacial aquifers (5)

Northern Atlantic Coastal Plain aquifer system (3)

EXPLANATION

(3) Number of well networks

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Aquifer lithology
Redox-Age Classes in Principal Aquifers 19

Figure 9. Percentage of samples assigned to the four redox-age classes for networks of shallow monitoring wells in agricultural areas 

in the Central Valley aquifer system, High Plains aquifer, and the West-central glacial aquifers, and the susceptibility of the networks to 

changes in nitrate concentrations.

Susceptibility to Changes in Nitrate 

Concentrations in Parts of Aquifers 

that Provide Domestic Water Supplies

Redox-age classes were assigned to samples collected 

from 105 networks of domestic wells (fig. 10 and table 5). 

Thirty-one percent of the networks were considered to have 

a high susceptibility to changes in nitrate concentrations 

and 17 percent of the networks were considered to have low 

susceptibilities (table 5). In comparison, 58 percent of the 

networks of shallow monitoring wells in agricultural areas 

were considered to have a high susceptibility to changes in 

nitrate concentrations and 10 percent were considered to have 

a low susceptibility.

For principal aquifers that had at least 2 networks of 

domestic wells, the median percentage of samples classified 

as oxic-potentially young ranged from about 6 to 100 percent 

(fig. 11), compared to about 57 to 96 percent for the shallow 

monitoring wells (fig. 8). For the parts of aquifers that provide 

domestic water supplies, the aquifers most susceptible to 

changes in nitrate concentrations were the Northern Atlantic 

Coastal Plain aquifer system and the Early Mesozoic Basin, 

Valley and Ridge carbonate-rock, and Piedmont and Blue 

Ridge crystalline-rock aquifers in the eastern United States; 

the Ozark Plateaus aquifer system in parts of Missouri and 

Arkansas; and the Central Valley, Columbia Plateau basalticrock, and Snake River Plain basaltic-rock aquifer systems in 

the West (figs. 10 and 11). For this analysis, western states 

are considered to be those located west of Minnesota, Iowa, 

Missouri, Arkansas, and Louisiana. The least susceptible 

aquifers were the Texas Coastal Uplands and Denver Basin 

aquifer systems (figs. 10 and 11).

Relatively large intraaquifer variability in redox-age 

classes was observed in some of the principal aquifers. For 

the five well networks sampled in the Floridan aquifer system, 

the percentage of samples classified as oxic-potentially young 

ranged from 10 to 100 percent (fig. 12). Aquifer confinement 

probably is an important control on redox-age variability in the 

Floridan aquifer system. More than 90 percent of the wells in 

network santsus2 were completed in the confined part of the 

aquifer and only 10 percent of its samples were classified as 

oxic-potentially young (table 5). Only 20 percent of the wells 

in network acfbsus1 were completed in the confined part of the 

aquifer and 100 percent of its samples were classified as oxicpotentially young. The Central glacial aquifers also showed 

large redox-age variability (fig. 12), which could be attributed 

to the diversity of depositional environments represented by 

well networks in those aquifers. Wells in network uirbsus1 were 

completed in glacial-moraine sands and gravels and 67 percent 

of their samples were classified as oxic-potentially young. Wells 

in network uirbsus2 were completed in glacial-till deposits and 

48 percent of their samples were classified as oxic-potentially 

young. Wells in network lirbsus1 were completed in confined 

buried-bedrock-valley deposits and 0 percent of their samples 

were classified as oxic-potentially young. Not all of the aquifers 

exhibited large variability in redox-age classes. Networks in 

the Columbia Plateau and Snake River Plain basaltic-rock 

aquifers had consistently high percentages of samples classified 

as oxic-potentially young (fig. 12). In contrast, networks in the 

Texas Coastal Uplands aquifer system had consistently low 

percentages of samples classified as oxic-potentially young. The 

number of well networks in each of those aquifers, however, 

was relatively small compared to the Floridan aquifer system 

and Central glacial aquifers (fig. 12).

0

25

50

75

100

Percentage of samples

Central Valley aquifer system

sanjlusor1b

sanjluscr1b

sacrluscr1

Redox-age class

Oxicpotentially

young

Oxicpotentially

old

Anoxicpotentially

young

Anoxicpotentially

old

0

25

50

75

100

Redox-age class

Oxicpotentially

young

Oxicpotentially

old

Anoxicpotentially

young

Anoxicpotentially

old

hpgwlusag3

hpgwlusag1

hpgwlusag2

High Plains aquifer

umisluscr1

0

25

50

75

100

West-central glacial aquifers

Redox-age class

Oxicpotentially

young

Oxicpotentially

old

Anoxicpotentially

young

Anoxicpotentially

old

cnbrluscr1

rednlusag2

EXPLANATION

Susceptibility to changes in nitrate 

 concentrations

High Medium Low
20 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Figure 10. Central locations of networks of domestic wells and the susceptibility of the networks to changes in nitrate concentrations.

d42

EXPLANATION

Susceptibility to changes 

 in nitrate concentrations, 

 networks of domestic wells

High

Medium

Low

d1 Network identifier (see table 5) 

* Network not in a principal aquifer

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone aquifers

Sandstone and carbonate-rock aquifers

Basaltic- and other volcanic-rock aquifers

Carbonate-rock aquifers

Crystalline-rock aquifers

Aquifer lithology

HAWAII

ALASKA

d41

d95 d93d94

d23

d4

d5

d16

d14

*d105

d59, d60,

d61, d62

d17

d19

d18, d39

d66

d25

d20

d21

d91

d13

d92

d12 d6

d9

d11

d10 d8

d2

d7

d3

d1

d22 d15

d74

d72

d73

d55

d40

d57

d38

d86

d31

d37

d82

d83

d84

d81

d47

d48

d54

d80

d53

d52

d44 d43

d45

d46

d90

d56

d33

d30

d32

d58

d36

d35

d34

d75

d41

d24

d77

d76

d78

d79

d101

d103 d104

d68

d69 d89 d64

d102 d51

d96

d65

d63

d85

d100 d49 d50

d98

d97 d29

d28 d27

d26

d67 d88 d71

d87

d70

d99 d95 d93d94

d23

d4

d5

d16

d14

*d105

d59, d60,

d61, d62

d17

d19

d18, d39

d66

d25

d20

d21

d91

d13

d92

d12 d6

d9

d11

d10 d8

d2

d7

d3

d1

d22 d15

d74

d72

d73

d55

d40

d57

d38

d86

d31

d37

d82

d83

d84

d81

d47

d48

d54

d80

d53

d52

d44 d43

d45

d46

d90

d56

d33

d30

d32

d58

d36

d35

d34

d75

d41

d24

d77

d76

d78

d79

d101

d103 d104

d68

d69 d89 d64

d102 d51

d96

d65

d63

d85

d100 d49 d50

d98

d97

d29

d28 d27

d26

d67 d88 d71

d87

d70

d99
Redox-Age Classes in Principal Aquifers 21

Table 5. Redox-age classes for water samples collected from networks of domestic wells in the United States and the susceptibility of the networks to changes in nitrate concentrations (only networks with at least 10 wells are listed).—Continued[usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and other volcanic rock; cry, crystalline rock; shading is used to differentiate between aquifer lithologies]

Aquifer

number

Aquifer

lithology Aquifer name Network name (number of wells) Network identifier (see figure 10) Redox-age class (percentage of samples)5 Susceptibility to changes in nitrate concentrations Oxic- potentially young Oxic- potentially old Anoxic- potentially young Anoxic- potentially old

1 usg Basin and Range basin-fill aquifers cazbsus3 (15) d1 93 7 0 0 High

1 usg Basin and Range basin-fill aquifers cazbsus1b (17) d2 82 0 18 0 High

1 usg Basin and Range basin-fill aquifers cazbsus2 (22) d3 77 18 5 0 High

1 usg Basin and Range basin-fill aquifers grslsus1a (17) d4 71 29 0 0 Medium

1 usg Basin and Range basin-fill aquifers grslsus1b (16) d5 63 13 19 6 Medium

1 usg Basin and Range basin-fill aquifers nvbrsus3 (10) d6 50 50 0 0 Medium

1 usg Basin and Range basin-fill aquifers cazbsus1a (21) d7 38 57 0 5 Medium

3 usg Central Valley aquifer system sanjlusor1a (28) d8 93 4 4 0 High

3 usg Central Valley aquifer system sanjlusor2a (26) d9 92 0 8 0 High

3 usg Central Valley aquifer system sanjluscr1a (24) d10 83 0 13 4 High

3 usg Central Valley aquifer system sanjsus1 (32) d11 81 6 6 6 High

3 usg Central Valley aquifer system sacrsus1 (26) d12 58 15 23 4 Medium

4 usg Columbia Plateau basin-fill aquifers ccptlusag2a (12)1 d13 100 0 0 0 High

5 usg High Plains aquifer hpgwsus2 (20) d14 90 0 10 0 High

5 usg High Plains aquifer hpgwsus1b (46) d15 70 26 4 0 Medium

5 usg High Plains aquifer hpgwsus1a (74) d16 46 53 1 0 Medium

5 usg High Plains aquifer hpgwsus1c (108) d17 45 45 4 6 Medium

5 usg High Plains aquifer hpgwsus4 (30)2 d18 43 23 23 10 Medium

5 usg High Plains aquifer hpgwsus5 (27) d19 41 37 11 11 Medium

7 usg Northern Rocky Mountains Intermontaine Basins aquifer system nroksus1 (29) d20 55 28 14 3 Medium

7 usg Northern Rocky Mountains Intermontaine Basins aquifer system nroksus2 (28) d21 54 39 7 0 Medium

8 usg Rio Grande aquifer system riogsus1 (24) d22 4 21 4 71 Low

9 usg Snake River Plain basin-fill aquifers usnkluscr1 (25) d23 64 0 32 4 Medium

10 usg Surficial aquifer system soflsus2 (30) d24 13 10 50 27 Low

11 usg Willamette Lowland aquifer system willsus1 (66) d25 52 5 32 12 Medium

12e gla Eastern glacial aquifers connsus2 (28) d26 57 4 32 7 Medium

12e gla Eastern glacial aquifers delrsus3 (12) d27 50 33 8 8 Medium

12e gla Eastern glacial aquifers almnsus2 (30) d28 50 0 43 7 Medium

12e gla Eastern glacial aquifers hdsnsus3 (23) d29 13 4 17 65 Low

12c gla Central glacial aquifers uirbsus1 (27) d30 67 0 33 0 Medium

12c gla Central glacial aquifers lirbsus2 (28) d31 57 0 29 14 Medium

12c gla Central glacial aquifers wmicsus2 (25) d32 48 12 20 20 Medium

12c gla Central glacial aquifers uirbsus2 (23) d33 48 0 52 0 Medium

12c gla Central glacial aquifers miamsus1 (30) d34 43 0 50 7 Medium

12c gla Central glacial aquifers lerisus1 (30) d35 30 20 23 27 Medium

12c gla Central glacial aquifers lerispcg1 (21) d36 19 10 57 14 Low

12c gla Central glacial aquifers lirbsus1 (26) d37 0 0 23 77 Low

12wc gla West-central glacial aquifers eiwasus2 (32) d38 69 25 3 3 Medium

12wc gla West-central glacial aquifers hpgwsus4 (30)2 d39 43 23 23 10 Medium

12wc gla West-central glacial aquifers rednsus2 (10) d40 10 0 50 40 Low

12w gla Western glacial aquifers pugtsus1 (29) d41 72 10 10 7 Medium

12w gla Western glacial aquifers cooksus1a (21) d42 52 0 38 10 Medium
22 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010 Table 5. Redox-age classes for water samples collected from networks of domestic wells in the United States and the susceptibility of the networks to changes in nitrate concentrations (only networks with at least 10 wells are listed).—Continued [usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and other volcanic rock; cry, crystalline rock; shading is used to differentiate between aquifer lithologies] Aquifer number Aquifer lithology Aquifer name Network name (number of wells) Network identifier (see figure 10) Redox-age class (percentage of samples)5 Susceptibility to changes in nitrate concentrations Oxic- potentially young Oxic- potentially old Anoxic- potentially young Anoxic- potentially old 13 scs Coastal Lowlands aquifer system acadsus2 (19) d43 74 11 16 0 Medium 13 scs Coastal Lowlands aquifer system acadsus1 (20) d44 65 15 10 10 Medium 13 scs Coastal Lowlands aquifer system trinsus4 (11) d45 27 64 0 9 Medium 13 scs Coastal Lowlands aquifer system trinsus3 (29) d46 24 59 7 10 Low 14 scs Mississippi Embayment aquifer system misesus4 (10) d47 40 40 0 20 Medium 14 scs Mississippi Embayment aquifer system acadsus3 (17)3 d48 18 6 24 53 Low 15 scs Northern Atlantic Coastal Plain aquifer system podlsus2 (16) d49 81 0 6 13 High 15 scs Northern Atlantic Coastal Plain aquifer system linjsus2 (26) d50 77 0 8 15 High 15 scs Northern Atlantic Coastal Plain aquifer system dlmvsus1 (13) d51 77 0 23 0 High 16 scs Southeastern Coastal Plain aquifer system moblsus3 (18) d52 72 0 22 6 Medium 16 scs Southeastern Coastal Plain aquifer system moblsus1 (22) d53 41 23 18 18 Medium 17 scs Texas Coastal Uplands aquifer system acadsus3 (13)3 d54 23 8 38 31 Low 17 scs Texas Coastal Uplands aquifer system sctxsus4 (26) d55 12 12 4 73 Low 18 san Cambrian-Ordovician aquifer system umissus3 (24) d56 54 8 33 4 Medium 18 san Cambrian-Ordovician aquifer system umissus4 (25) d57 44 8 24 24 Medium 18 san Cambrian-Ordovician aquifer system wmicsus1 (22) d58 41 0 27 32 Medium 19 san Denver Basin aquifer system spltsus2 (29) d59 34 34 3 28 Medium 19 san Denver Basin aquifer system spltsus3 (26) d60 12 15 19 54 Low 19 san Denver Basin aquifer system spltsus4 (10) d61 0 10 0 90 Low 19 san Denver Basin aquifer system spltsus5 (10) d62 0 20 20 60 Low 20 san Early Mesozoic Basin aquifers delrsus1 (25) d63 88 0 8 4 High 20 san Early Mesozoic Basin aquifers potosus2 (22) d64 77 0 14 9 High 20 san Early Mesozoic Basin aquifers linjsus3 (20) d65 70 10 5 15 Medium 21 san Lower Tertiary aquifers yellsus2 (21) d66 33 5 29 33 Medium 22 san Pennsylvanian aquifers almnsus1 (29) d67 48 3 45 3 Medium 22 san Pennsylvanian aquifers kanasus1 (16) d68 13 0 81 6 Low 23 san Valley and Ridge clastic-rock aquifers potolusag2 (22) d69 59 5 36 0 Medium 23 san Valley and Ridge clastic-rock aquifers delrsus2 (24) d70 58 21 17 4 Medium 23 san Valley and Ridge clastic-rock aquifers lsussus1 (29) d71 41 10 45 3 Medium 24 scr Edwards-Trinity aquifer system sctxsus1 (27) d72 85 7 7 0 High 24 scr Edwards-Trinity aquifer system sctxsus2 (24) d73 50 13 8 29 Medium 24 scr Edwards-Trinity aquifer system trinsus1 (14) d74 43 21 14 21 Medium 29 car Floridan aquifer system acfbsus1 (10) d75 100 0 0 0 High 29 car Floridan aquifer system gaflsus2 (30) d76 67 20 13 0 Medium 29 car Floridan aquifer system gaflsus3 (30) d77 60 10 17 13 Medium 29 car Floridan aquifer system gaflsus4 (26) d78 12 8 42 38 Low 29 car Floridan aquifer system santsus2 (29) d79 10 28 24 38 Low 30 car Ordovician aquifers ltensus2 (21) d80 19 19 52 10 Low 31 car Ozark Plateaus aquifer system ozrklusag1a (17) d81 94 0 6 0 High

31 car Ozark Plateaus aquifer system ozrksus2a (33) d82 85 12 3 0 High
Redox-Age Classes in Principal Aquifers 23

Table 5. Redox-age classes for water samples collected from networks of domestic wells in the United States and the susceptibility of the networks to changes in nitrate concentrations (only networks with at least 10 wells are listed).—Continued[usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and other volcanic rock; cry, crystalline rock; shading is used to differentiate between aquifer lithologies]

Aquifer

number

Aquifer

lithology Aquifer name Network name (number of wells) Network identifier (see figure 10) Redox-age class (percentage of samples)5 Susceptibility to changes in nitrate concentrations Oxic- potentially young Oxic- potentially old Anoxic- potentially young Anoxic- potentially old

31 car Ozark Plateaus aquifer system ozrklusag2a (16) d83 81 0 19 0 High

31 car Ozark Plateaus aquifer system ozrksus3a (16) d84 56 19 13 13 Medium32 car Piedmont and Blue Ridge carbonate-rock aquifers lsuslusag1 (29) d85 83 0 17 0 High33 car Silurian-Devonian aquifers eiwasus1 (25) d86 16 4 20 60 Low34 car Valley and Ridge carbonate-rock aquifers lsuslusag3 (29) d87 100 0 0 0 High34 car Valley and Ridge carbonate-rock aquifers lsuslusag2 (29) d88 97 0 3 0 High34 car Valley and Ridge carbonate-rock aquifers potolusag1 (32) d89 94 0 6 0 High34 car Valley and Ridge carbonate-rock aquifers utensus1 (18) d90 67 11 22 0 Medium35 bav Columbia Plateau basaltic-rock aquifer system ccptlusag1a (17) d91 82 6 0 12 High35 bav Columbia Plateau basaltic-rock aquifer system ccptlusag2a (16)1 d92 81 13 6 0 High36 bav Snake River Plain basaltic-rock aquifer system usnkluscr4 (15) d93 100 0 0 0 High36 bav Snake River Plain basaltic-rock aquifer system usnkluscr2 (28) d94 100 0 0 0 High36 bav Snake River Plain basaltic-rock aquifer system usnkluscr3 (28) d95 96 4 0 0 High38 cry New York and New England crystalline-rock aquifers linjsus1 (25) d96 88 8 4 0 High38 cry New York and New England crystalline-rock aquifers necbsus2 (30) d97 53 0 47 0 Medium38 cry New York and New England crystalline-rock aquifers necbsus1 (28) d98 46 0 54 0 Medium38 cry New York and New England crystalline-rock aquifers connsus1 (27) d99 44 33 4 19 Medium39 cry Piedmont and Blue Ridge crystalline-rock aquifers lsussus2 (29) d100 100 0 0 0 High39 cry Piedmont and Blue Ridge crystalline-rock aquifers santsus3 (29) d101 86 0 14 0 High39 cry Piedmont and Blue Ridge crystalline-rock aquifers potosus1 (21) d102 81 0 19 0 High39 cry Piedmont and Blue Ridge crystalline-rock aquifers kanasus2 (19) d103 74 5 16 5 Medium39 cry Piedmont and Blue Ridge crystalline-rock aquifers albesus8 (48) d104 67 10 21 2 Medium

--4 usg Alluvial aquifers in the Colorado Rocky Mountains ucolsus1 (23) d105 78 13 9 0 High

1Network has wells in the Columbia Plateau basaltic-rock aquifer system and Columbia Plateau basin-fill aquifers.2Network is part of the High Plains aquifer and West-central glacial aquifers.3Network has wells in the Mississippi embayment and Texas coastal uplands aquifer systems.4Network not in a principal aquifer.5Redox-age percentages may not sum to 100 percent because of rounding.
24 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Figure 11. Median percentage of samples assigned to the four redox-age classes for principal aquifers that have at least two 

networks of domestic wells, and the susceptibility of the aquifers to changes in nitrate concentrations.

0 25 50 75 100 0 25 50 75 100

0 25 50 75 100 0 25 50 75 100

Percentage of samples Percentage of samples

Basin and Range basin-fill aquifers (7)

Central Valley aquifer system (5)

High Plains aquifer (6)

Northern Rocky Mountains Intermontaine Basins aquifer system (2)

Eastern glacial aquifers (4)

Central glacial aquifers (8)

West-central glacial aquifers (3)

Western glacial aquifers (2)

Coastal Lowlands aquifer system (4)

Mississippi Embayment aquifer system (2)

Northern Atlantic Coastal Plain aquifer system (3)

Southeastern Coastal Plain aquifer system (2)

Texas Coastal Uplands aquifer system (2)

Cambrian-Ordovician aquifer system (3)

Denver Basin aquifer system (4)

Early Mesozoic Basin aquifers (3)

Pennsylvanian aquifers (2)

Valley and Ridge clastic-rock aquifers (3)

Edwards-Trinity aquifer system (3)

Floridan aquifer system (5)

Ozark Plateaus aquifer system (4)

Valley and Ridge carbonate-rock aquifers (4)

Columbia Plateau basaltic-rock aquifer system (2)

Snake River Plain basaltic-rock aquifer system (3)

New York and New England crystalline-rock aquifers (4)

Piedmont and Blue Ridge crystalline-rock aquifers (5)

Basin and Range basin-fill aquifers (7) Central Valley aquifer system (5) High Plains aquifer (6) Northern Rocky Mountains Intermontaine Basins aquifer system (2) Eastern glacial aquifers (4)

Central glacial aquifers (8) West-central glacial aquifers (3) Western glacial aquifers (2) Coastal Lowlands aquifer system (4) Mississippi Embayment aquifer system (2) Northern Atlantic Coastal Plain aquifer system (3) Southeastern Coastal Plain aquifer system (2) Texas Coastal Uplands aquifer system (2)

Cambrian-Ordovician aquifer system (3) Denver Basin aquifer system (4)

Early Mesozoic Basin aquifers (3) Pennsylvanian aquifers (2) Valley and Ridge clastic-rock aquifers (3) Edwards-Trinity aquifer system (3) Floridan aquifer system (5) Ozark Plateaus aquifer system (4) Valley and Ridge carbonate-rock aquifers (4) Columbia Plateau basaltic-rock aquifer system (2)

Snake River Plain basaltic-rock aquifer system (3) New York and New England crystalline-rock aquifers (4) Piedmont and Blue Ridge crystalline-rock aquifers (5)

Oxic-potentially young Oxic-potentially old

Anoxic-potentially young Anoxic-potentially old

EXPLANATION

(3) Number of well networks

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone aquifers

Sandstone and carbonate-rock aquifers

Basaltic- and other volcanic-rock aquifers

Carbonate-rock aquifers

Crystalline-rock aquifers

Aquifer lithology

Susceptibility to changes

in nitrate concentrations

Low Medium High
Redox-Age Classes in Principal Aquifers 25

Figure 12. Variability in the percentage of samples classified as oxic-potentially young for principal aquifers that have at least two 

networks of domestic wells, and the susceptibility of the networks to changes in nitrate concentrations.

Principal-aquifer lithology groups with the largest 

percentage of networks considered to have a high susceptibility 

to changes in nitrate concentrations were the basaltic- and 

other volcanic-rock aquifer systems (100 percent of networks), 

carbonate-rock aquifers (50 percent), and crystalline-rock 

aquifers (44 percent) (table 5 and figs. 10 and 12). These three 

lithology groups include five of the six domestic-well networks 

with 100 percent of their samples classified as oxic-potentially 

young. The lithology groups with the smallest percentage of 

networks considered to have a high susceptibility to changes 

in nitrate concentrations were the glacial aquifers (0 percent of 

networks) and sandstone aquifers (about 13 percent) (table 5 

and fig. 10). These two lithology groups include the three well 

networks with 0 percent of their samples classified as oxicpotentially young (table 5).

There are important geologic differences between the 

aquifer lithology groups with high and low susceptibilities 

to changes in nitrate concentrations. The relatively large 

percentage of high-susceptibililty networks in the basalticand other volcanic-rock aquifer systems, carbonate-rock 

aquifers, and crystalline-rock aquifers may indicate the 

importance of fractures and karst features in promoting the 

0 25 50 75 100

Basin and Range basin-fill aquifers (7)

Central Valley aquifer system (5)

High Plains aquifer (6)

Northern Rocky Mountains Intermontaine Basins aquifer system (2)

Eastern glacial aquifers (4)

Central glacial aquifers (8)

West-central glacial aquifers (3)

Western glacial aquifers (2)

Coastal Lowlands aquifer system (4)

Mississippi Embayment aquifer system(2)

Northern Atlantic Coastal Plain aquifer system (3)

Southeastern Coastal Plain aquifer system (2)

Texas Coastal Uplands aquifer system (2)

Cambrian-Ordovician aquifer system (3)

Denver Basin aquifer system (4)

Early Mesozoic Basin aquifers (3)

Pennsylvanian aquifers (2)

Valley and Ridge clastic-rock aquifers (3)

Edwards-Trinity aquifer system (3)

Floridan aquifer system (5)

Ozark Plateaus aquifer system (4)

Valley and Ridge carbonate-rock aquifers (4)

Columbia Plateau basaltic-rock aquifer system (2)

Snake River Plain basaltic-rock aquifer system (3)

New York and New England crystalline-rock aquifers (4)

Piedmont and Blue Ridge crystallilne-rock aquifers (5)

Percentage of samples classified as oxic-potentially young

Median

Minimum Maximum

EXPLANATION

(3) Number of well networks

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone aquifers

Sandstone and carbonate-rock aquifers

Basaltic- and other volcanic-rock aquifers

Carbonate-rock aquifers

Crystalline-rock aquifers

Aquifer lithology

Susceptibility to changes in nitrate concentrations

Low Medium High
26 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

rapid movement of oxic-potentially young groundwater 

in those aquifers (Dubrovsky and others, 2010; McMahon 

and others, 2011). The relatively small percentage of highsusceptibility networks in the glacial and sandstone aquifers 

reflects geologic characteristics of those aquifers that support 

anoxic redox conditions (high electron donor content) and 

inhibit water movement (fine-grained confining layers).

Domestic-well networks in the eastern and western 

United States differed with respect to the percentage of 

samples assigned to certain redox-age classes. The 45 

networks located in the western United States had a larger 

median percentage (13 percent) of samples classified as 

oxic-potentially old than the 60 networks located in the East 

(4 percent). Previous studies already noted the presence of 

oxic groundwater that was sometimes thousands of years 

old in organic carbon-poor unconsolidated sand and gravel 

aquifers in the western United States, particularly in the 

Central Valley and Rio Grande aquifer systems (Plummer 

and others, 2004; Jurgens and others, 2008), and the Basin 

and Range basin-fill and High Plains aquifers (Winograd and 

Robertson, 1982; McMahon and others, 2004). Those aquifers 

typically have low natural recharge rates and large, thick flow 

systems. In contrast, networks located in the eastern United 

States had a larger median percentage (20 percent) of samples 

classified as anoxic-potentially young than networks located 

in the West (7 percent). This is not surprising considering the 

generally shallower depths to water, higher natural recharge 

rates, and smaller, shallower flow systems in the eastern 

United States than in the West (Wolock, 2003; Reilly and 

others, 2008; McMahon and others, 2011). Oxic-potentially 

old and anoxic-potentially young conditions both reduce 

aquifer susceptibility to changes in nitrate concentrations, 

but for different climatic, geologic, and hydrologic reasons.

Fifteen of the domestic-well networks were approximately collocated with networks of shallow monitoring 

wells in agricultural areas, which provides the opportunity to 

compare the susceptibility to changes in nitrate concentrations 

at different depths in the same aquifer area. The median 

depth of the domestic wells was greater than the median 

depth of the monitoring wells for each pair of well networks. 

Overall, the median difference in depth between domestic 

and monitoring wells was 13 m. For 10 of the 15 pairs of 

networks, the monitoring-well networks had the higher 

percentage of samples classified as oxic-potentially young 

(fig. 13), indicating that susceptibility tended to be higher at 

the shallower depths of the monitoring wells. For 7 of the 15 

pairs of nested networks, susceptibility was in fact higher in the 

monitoring wells than the domestic wells. Six of those seven 

pairs are in glacial aquifers (fig. 13). Only 3 of the 15 pairs of 

nested networks showed higher susceptibilities in the domestic 

wells than in the monitoring wells. For 5 of the 15 pairs of 

networks, susceptibilities were generally the same in both 

well types.

Susceptibility to Changes in Nitrate 

Concentrations in Parts of Aquifers that 

Provide Public Water Supplies

Redox-age classes were assigned to samples collected 

from 39 networks of public-supply wells (fig. 14 and table 6). 

Thirty-one percent of the networks were considered to have 

a high susceptibility to changes in nitrate concentrations 

and 26 percent of the networks were considered to have low 

susceptibilities (table 6). The percentage of high-susceptibility 

networks for public-supply wells was the same as for domestic 

wells, but the public-supply wells had a larger percentage of 

low-susceptibility networks than the domestic wells.

For principal aquifers that had at least 2 networks of 

public-supply wells, the median percentage of samples 

classified as oxic-potentially young ranged from about 7 to 

87 percent (fig. 15), compared to about 57 to 96 percent for the 

shallow monitoring wells (fig. 8) and about 6 to 100 percent for 

domestic wells (fig. 11). For the parts of aquifers that provide 

public water supplies, the aquifers most susceptible to changes 

in nitrate concentrations were the Eastern glacial aquifers and 

the California Coastal Basin, Basin and Range basin-fill, and 

High Plains aquifers in the West (figs. 14 and 15). The least 

susceptible aquifer was the Cambrian-Ordovician aquifer 

system in the upper Midwest (figs. 14 and 15).

Susceptibility to changes in nitrate concentrations in the 

networks of public-supply wells appeared to be controlled in 

part by aquifer confinement and well depth, as was the case 

for several of the networks of monitoring and domestic wells. 

Low-susceptibility networks in the Cambrian-Ordovician 

aquifer system had relatively large well depths (median 

depths of 124 to 558 m) and large percentages of wells 

completed in confined parts of the aquifer (median values of 

0 to 100 percent) compared to networks in the four aquifers 

with the highest susceptibilities. Well networks in the Eastern 

glacial aquifers, for example, had median well depths of 17 to 

118 m and percentages of wells completed in confined parts of 

the aquifer that ranged from 0 to 24 percent.

Well networks sanasus2 and sanasus3, in the California 

Coastal Basin aquifers (table 6 and fig. 14), had medium 

susceptibilities even though the median well depths (232 to 

294 m) were relatively large and 56 percent of the sanasus2 

wells were completed in confined parts of the aquifer. Several 

factors probably contributed to the susceptibility of those two 

networks. One factor is long well screens. Median well-screen 

lengths in the networks accounted for 60 to 68 percent of the 

well depths. Long well screens could increase the chances of 

mixing shallow, young water and deep, old water (Landon and 

others, 2010b). Another possible factor is artificial recharge 

that occurs in parts of the California Coastal Basin aquifers in 

southern California that could increase aquifer susceptibility 

to changes in nitrate concentrations by increasing recharge 

rates (Hamlin and others, 2002; McMahon and others, 2011). 

Pumping rate also may affect susceptibility but data were not 

available to evaluate this factor.
Redox-Age Classes in Principal Aquifers 27

Only four of the networks of public-supply wells were 

approximately collocated with networks of domestic wells. 

The median depth of the public-supply wells was greater than 

the median depth of the domestic wells for each pair of well 

networks. Overall, the median difference in depth between 

public-supply and domestic wells was 39 m, which is three 

times larger than the median difference in depth between the 

pairs of domestic- and monitoring-well networks. For three 

of the four pairs of networks, the public-supply wells had the 

higher percentage of samples classified as oxic-potentially 

young (fig. 16), indicating that susceptibility tended to be higher 

in the vicinity of public-supply wells than in the vicinity of 

domestic wells even though the public-supply wells had larger 

median well depths. Although the number of paired networks 

of public-supply and domestic wells was small, this finding is 

the opposite of what was observed for shallow monitoring wells 

and domestic wells (fig. 13). For one pair of public-supply (p24) 

and domestic-well networks (d45) (fig. 16), the percentage of 

samples classified as oxic-potentially young was higher for the 

domestic wells. This may be due to the fact that only 50 percent 

of the domestic wells were completed in confined parts of the 

aquifer (Coastal Lowlands aquifer system) whereas 80 percent 

of the public supply wells were completed in confined parts of 

the aquifer. Bruce and Oelsner (2001) studied closely located 

pairs of domestic and public-supply wells in the High Plains 

aquifer and found a more frequent occurrence of pesticide 

compounds and tritium in water from the public-supply wells 

than in water from the domestic wells. They concluded that 

high rates of pumping in public-supply wells with long screens 

induced more rapid downward movement of young groundwater 

than did domestic wells, which had shorter screens and were 

less heavily pumped. Jurgens and others (2008) studied a longscreened public-supply well in the Central Valley aquifer system 

and also found that well construction and operation induced 

downward movement of young groundwater. The data in figure 

16 are consistent with the idea that construction and operation 

characteristics of public-supply wells can enhance the downward 

movement of young groundwater (Landon and others, 2010b).

Figure 13. Percentage of samples classified as oxic-potentially young in collocated networks 

of shallow monitoring wells in agricultural areas and domestic wells, and the susceptibility of 

the networks to changes in nitrate concentrations.

0 25 50 75 100

Percentage of samples classified as oxic-potentially young,

networks of domestic wells

0

25

50

75

100

Percentage of samples classified as oxic-potentially young,

networks of shallow monitoring wells in agricultural areas

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Aquifer lithology

EXPLANATION

(d6, am1)

(d10, am3)

(d13, am5)

(d15, am9)

(d16, am8)

(d26, am15)

(d37, am17) (d32, am18)

(d34, am19)

(d33, am22)

(d35, am23)

(d38, am27)

(d40, am28)

(d44, am32)

(d51, am34)

1:1

Low susceptibility 

in both networks

Medium susceptibility

in both networks

High susceptibility 

in both networks

Higher susceptibility 

in monitoring-well

networks

Higher susceptibility 

in domestic-well

networks

d35

am23

Domestic-well network (see table 5)

Monitoring-well network (see table 4)
28 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Figure 14. Central locations of networks of public-supply wells and the susceptibility of the networks to changes in nitrate 

concentrations.

EXPLANATION

Susceptibility to changes in 

 nitrate concentrations, networks 

 of public-supply wells

High

Medium

Low

p1 Network identifier (see table 6)

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone aquifers

Sandstone and carbonate-rock aquifers

Basaltic- and other volcanic-rock aquifers

Carbonate-rock aquifers

Crystalline-rock aquifers

Aquifer lithology

p37

p6

p4

p9

p2

p3

p7 p5

p8

p1

p15

p14

p13

p10

p33

p24

p12

p26

p25

p22

p11

p31

p23

p29

p30

p21

p20

p19

p18

p17

p27 p39

p28

p35

p36

p16

p34

p32

p38

HAWAII

ALASKA
Redox-Age Classes in Principal Aquifers 29

Table 6. Redox-age classes for water samples collected from networks of public-supply wells in the United States and the susceptibility of the networks to changes in nitrate concentrations (only networks with at least 10 wells are listed). [usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and other volcanic rock; cry, crystalline rock; shading is used to differentiate between aquifer lithologies]

Aquifer

number

Aquifer

lithology Aquifer name Network name (number of wells) Network identifier (see figure 14) Redox-age class (percentage of samples)3 Susceptibility to changes in nitrate concentrations Oxic- potentially young Oxic- potentially old Anoxic- potentially young Anoxic- potentially old

1 usg Basin and Range basin-fill aquifers grslsus3 (30) p1 83 10 0 7 High

1 usg Basin and Range basin-fill aquifers nvbrsus2 (25) p2 60 32 0 8 Medium

1 usg Basin and Range basin-fill aquifers nvbrdwgs1 (14) p3 57 36 0 7 Medium

2 usg California Coastal Basin aquifers sacrsus3 (11) p4 82 0 9 9 High

2 usg California Coastal Basin aquifers sanasus1 (29) p5 79 21 0 0 High

2 usg California Coastal Basin aquifers sacrsus4 (17) p6 71 6 18 6 Medium

2 usg California Coastal Basin aquifers sanasus2 (18) p7 67 6 11 17 Medium

2 usg California Coastal Basin aquifers sanasus3 (17) p8 47 24 12 18 Medium

3 usg Central Valley aquifer system sanjdwgs1 (15) p9 100 0 0 0 High

5 usg High Plains aquifer hpgwspcg7 (15) p10 73 27 0 0 Medium

5 usg High Plains aquifer hpgwdwgs1 (15)1 p11 47 7 33 13 Medium

6 usg Mississippi River Valley alluvial aquifer misesus1 (10) p12 10 0 80 10 Low

8 usg Rio Grande aquifer system riogdwgs1 (16) p13 56 31 6 6 Medium

8 usg Rio Grande aquifer system riogtanc (23) p14 39 39 0 22 Medium

9 usg Snake River Plain basin-fill aquifers usnksus3 (12) p15 8 92 0 0 Low

10 usg Surficial aquifer system sofldwgs1 (15) p16 40 0 60 0 Medium

12e gla Eastern glacial aquifers linjdwgs1 (12) p17 100 0 0 0 High

12e gla Eastern glacial aquifers conndwgs1 (15) p18 87 0 13 0 High

12e gla Eastern glacial aquifers necbsus3 (29) p19 79 3 17 0 High

12c gla Central glacial aquifers miamspcb1 (15) p20 33 0 60 7 Medium

12c gla Central glacial aquifers whmidwgs1 (13) p21 23 0 54 23 Low

12wc gla West-central glacial aquifers hpgwdwgs1 (15)1 p22 47 7 33 13 Medium

12wc gla West-central glacial aquifers umisdwgs1 (15)2 p23 13 20 13 53 Low

13 scs Coastal Lowlands aquifer system trinsus4 (11) p24 18 36 18 27 Low

14 scs Mississippi Embayment aquifer system misesus4 (10) p25 40 50 10 0 Medium

14 scs Mississippi Embayment aquifer system misesus2 (30) p26 20 0 40 40 Low

15 scs Northern Atlantic Coastal Plain aquifer system dlmvspcg10 (30) p27 83 0 17 0 High

16 scs Southeastern Coastal Plain aquifer system santsus1 (20) p28 95 0 0 5 High

18 san Cambrian-Ordovician aquifer system umisdwgs1 (15)2 p29 20 20 40 20 Low

18 san Cambrian-Ordovician aquifer system uirbsus3 (29) p30 7 7 0 86 Low

18 san Cambrian-Ordovician aquifer system eiwasus3 (28) p31 0 41 3 55 Low

19 san Denver Basin aquifer system spltdwgs1 (12) p32 25 33 17 25 Medium

24 scr Edwards-Trinity aquifer system sctxsus3 (21) p33 95 0 5 0 High

27 car Biscayne aquifer soflsus1 (23) p34 13 0 87 0 Low

29 car Floridan aquifer system gafldwgs1 (14) p35 64 7 14 14 Medium

29 car Floridan aquifer system gafldwgs2 (14) p36 29 7 14 50 Medium

35 bav Columbia Plateau basaltic-rock aquifer system ccptsus1b (22) p37 55 18 0 27 Medium

37 bav Hawaiian volcanic-rock aquifers oahusus1 (23) p38 100 0 0 0 High

39 cry Piedmont and Blue Ridge crystalline-rock aquifers podldwgs1 (15) p39 100 0 0 0 High 1Network is part of the High Plains aquifer and West-central glacial aquifers.

2Network has wells in the west-central glacial aquifers and Cambrian-Ordovician aquifer system.3Redox-age percentages may not sum to 100 percent because of rounding.
30 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

Figure 15. Median percentage of samples assigned to the four redox-age classes for principal aquifers that have at least two 

networks of public-supply wells, and the susceptibility of the aquifers to changes in nitrate concentrations.

Oxic-potentially young Oxic-potentially old

Anoxic-potentially young Anoxic-potentially old

0 25 50 75 100

Median percentage of samples

0 25 50 75 100

Median percentage of samples

EXPLANATION

(3) Number of well networks

Basin and Range basin-fill aquifers (3)

California Coastal Basin aquifers (5)

High Plains aquifer (2)

Rio Grande aquifer systems (2)

Eastern glacial aquifers (3)

Central glacial aquifers (2)

West-central glacial aquifers (2)

MIssissippi Embayment aquifer system (2)

Cambrian-Ordovician aquifer system (3)

Floridan aquifer system (2)

Basin and Range basin-fill aquifers (3)

California Coastal Basin aquifers (5)

High Plains aquifer (2)

Rio Grande aquifer systems (2)

Eastern glacial aquifers (3)

Central glacial aquifers (2)

West-central glacial aquifers (2)

MIssissippi Embayment aquifer system (2)

Cambrian-Ordovician aquifer system (3)

Floridan aquifer system (2)

0 25 50 75 100 0 25 50 75 100

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone aquifers

Carbonate-rock aquifers

Aquifer lithology

Susceptibility to changes 

in nitrate concentrations

Low Medium High
Summary and Conclusions 31

Summary and Conclusions

The National Water-Quality Assessment (NAWQA) 

Program of the U.S. Geological Survey is using multiple 

approaches to measure and explain trends in concentrations of 

nitrate in principal aquifers of the United States. Near decadal 

sampling of selected well networks is providing information 

on where long-term changes in nitrate concentrations have 

occurred. Because those studies do not include all the NAWQA 

well networks, a determination has yet to be made as to what 

might be expected in networks from which time-series data 

have not been collected. Characterizing aquifer susceptibility to 

changes in nitrate concentrations on the basis of data collected 

from all the NAWQA well networks would be a step toward 

extrapolating findings from those studies to broader regions.

The purpose of this report is to characterize the susceptibility of selected principal aquifers of the United States to 

changes in nitrate concentrations on a basis of the redox-age 

classification scheme developed in this report. In this study, 

water samples collected from 6,593 wells in 39 principal 

aquifers and 5 alluvial aquifers (collected from 1991 to 

2010) were assigned to four redox-age classes on the basis 

of concentrations of dissolved oxygen and various indicators of groundwater age. The redox-age classes are oxicpotentially young, oxic-potentially old, anoxic-potentially 

young, and anoxic-potentially old. The redox-age assignments 

were then used to characterize the susceptibility of principal 

aquifers to changes in nitrate concentrations. Aquifer areas 

(as defined by well networks) in which at least 75 percent 

of the samples were classified as oxic-potentially young 

were considered to have a high susceptibility to changes in 

nitrate concentrations. Aquifer areas were considered to have 

a medium susceptibility if at least 25 percent and less than 

75 percent of the samples were classified as oxic-potentially 

young. Aquifer areas were considered to have a low susceptibility if less than 25 percent of the samples were classified as 

oxic-potentially young.

For the parts of aquifers near the water table in agricultural 

areas, the aquifers most susceptible to changes in nitrate concentrations were the Columbia Plateau basin-fill aquifers, Eastern 

Figure 16. Percentage of samples classified as oxic-potentially young in collocated networks 

of public-supply and domestic wells, and the susceptibility of the networks to changes in 

nitrate concentrations.

0

25

50

75

100

Percentage of samples classified as oxic-potentially young,

networks of domestic wells

0 25 50 75 100

Percentage of samples classified as oxic-potentially young,

networks of public-supply wells

Unconsolidated sand and gravel aquifers

Semiconsolidated sand aquifers

Aquifer lithology

EXPLANATION

p24

d45

Public-supply well network (see table 6)

Domestic-well network (see table 5)

Crystalline-rock aquifers

(p10, d16)

(p24, d45)

(p27, d51)

(p39, d102)

Higher susceptibility 

in domestic-well

networks

1:1

Low susceptibility 

in both networks

Medium susceptibility 

in both networks

High susceptibility 

in both networks

Higher susceptibility 

in public-supply well

networks
32 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010

glacial aquifers, and the West-central glacial aquifers. None of 

the aquifers had a low susceptibility to changes in nitrate concentrations. Large intraaquifer redox-age variability was observed in 

most of the aquifers that had multiple networks of shallow monitoring wells in agricultural areas. For the three well networks in 

the Central Valley aquifer system, for example, the percentage of 

samples classified as oxic-potentially young ranged from 13 to 

90 percent, and the percentage of samples classified as anoxicpotentially young ranged from 10 to 83 percent.

For the parts of aquifers that provide domestic water supplies, the aquifers most susceptible to changes in nitrate concentrations were the Northern Atlantic Coastal Plain aquifer system 

and the Early Mesozoic Basin, Valley and Ridge carbonaterock, and Piedmont and Blue Ridge crystalline-rock aquifers in 

the eastern United States; the Ozark Plateaus aquifer system in 

parts of Missouri and Arkansas; and the Central Valley, Columbia Plateau basaltic-rock, and Snake River Plain basaltic-rock 

aquifer systems in the West. The least susceptible aquifers were 

the Texas Coastal Uplands and Denver Basin aquifer systems.

Relatively large intraaquifer variability in redox-age 

classes was observed in some of the principal aquifers. For the 

five networks of domestic wells sampled in the Floridan aquifer 

system, for example, the percentage of samples classified as 

oxic-potentially young ranged from 10 to 100 percent. Aquifer 

confinement probably is an important control on redox-age 

variability in the Floridan aquifer system. The Central glacial 

aquifers also showed large redox-age variability, which could be 

attributed to the diversity of depositional environments represented by well networks in those aquifers.

Principal-aquifer lithology groups with the largest percentage of domestic-well networks considered to have a high susceptibility to changes in nitrate concentrations were the basaltic- and other volcanic-rock aquifer systems, carbonate-rock 

aquifers, and crystalline-rock aquifers. These three lithology 

groups include five of the six domestic-well networks with 100 

percent of their samples classified as oxic-potentially young. 

The lithology groups with the smallest percentage of networks 

considered to have a high susceptibility to changes in nitrate 

concentrations were the glacial aquifers and sandstone aquifers. 

These two lithology groups include the three well networks with 

0 percent of their samples classified as oxic-potentially young. 

There are important geologic differences between the aquifer 

lithology groups with high and low susceptibilities to changes in 

nitrate concentrations. The relatively large percentage of highsusceptibililty networks in the basaltic- and other volcanic-rock 

aquifer systems, carbonate-rock aquifers, and crystalline-rock 

aquifers may indicate the importance of fractures and karst 

features in promoting the rapid movement of oxic-potentially 

young groundwater in those aquifers. The relatively small 

percentage of high-susceptibility networks in the glacial and 

sandstone aquifers reflects geologic characteristics of those 

aquifers that support anoxic redox conditions (high electron 

donor content) and inhibit water movement (fine-grained confining layers).

Domestic-well networks in the eastern and western 

United States differed with respect to the percentage of samples assigned to certain redox-age classes. The 45 networks 

located in the western United States had a larger median 

percentage (13 percent) of samples classified as oxic-potentially old than the 60 networks located in the East (4 percent). 

Previous studies already noted the presence of oxic groundwater that was sometimes thousands of years old in organiccarbon-poor unconsolidated sand and gravel aquifers in the 

western United States, particularly in the Central Valley and 

Rio Grande aquifer systems, and the Basin and Range basinfill and High Plains aquifers. Those aquifers typically have 

low natural recharge rates and large, thick flow systems. In 

contrast, networks located in the eastern United States had a 

larger median percentage (20 percent) of samples classified as 

anoxic-potentially young than networks located in the West 

(7 percent). This is not surprising considering the generally 

shallower depths to water, higher natural recharge rates, and 

smaller, shallower flow systems in the eastern United States 

than in the West. Oxic-potentially old and anoxic-potentially 

young conditions both reduce aquifer susceptibility to changes 

in nitrate concentrations, but for different climatic, geologic, 

and hydrologic reasons.

Fifteen of the domestic-well networks were approximately collocated with networks of shallow monitoring 

wells in agricultural areas, which provided the opportunity to 

compare the susceptibility to changes in nitrate concentrations 

at different depths in the same aquifer area. The median depth 

of the domestic wells was greater than the median depth of 

the monitoring wells for each pair of well networks. For 10 of 

the 15 pairs of networks, the monitoring-well networks had 

the higher percentage of samples classified as oxic-potentially 

young, indicating that susceptibility tended to be higher at the 

shallower depths of the monitoring wells.

For the parts of aquifers that provide public water supplies, the aquifers most susceptible to changes in nitrate concentrations were the Eastern glacial aquifers and the California 

Coastal Basin, Basin and Range basin-fill, and High Plains 

aquifers in the West. The least susceptible aquifer was the 

Cambrian-Ordovician aquifer system in the upper Midwest.

Only four of the networks of public-supply wells were 

approximately collocated with networks of domestic wells. 

The median depth of the public-supply wells was greater than 

the median depth of the domestic wells for each pair of well 

networks. For three of the four pairs of networks, the publicsupply wells had the higher percentage of samples classified 

as oxic-potentially young , indicating that susceptibility tended 

to be higher in the vicinity of public-supply wells than in the 

vicinity of domestic wells even though the public-supply wells 

had larger median well depths. Although the number of paired 

networks of public-supply and domestic wells was small, this 

finding is the opposite of what was observed for shallow monitoring wells and domestic wells. Previous studies found that 

high rates of pumping in public-supply wells with long screens 

induced more rapid downward movement of young groundwater than did domestic wells, which had shorter screens and were 

less heavily pumped. The data from this study are generally 

consistent with those findings.
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Acknowledgments

Karen Burow and Matthew Landon provided helpful 

comments on earlier versions of this report.

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Appendix 1

Appendix 1 39

um2 um1

um3

um4

um5

um6

um8

um9

*um37 um29

*um38

um24

um7

um10

um33

um11

um12

um14

um13

um15

um16

um17

um18

um20 um19

um21

um22

um23

um25

*um36

um26

um27

um28

um30

um31

um32

um34

um35

EXPLANATION

Susceptibility to changes in nitrate 

 concentrations, networks of shallow 

 monitoring wells in urban areas

High

Medium

Low

um1 Network identifier (see Appendix table 1)

* Network not in a principal aquifer

Unconsolidated sand and gravel aquifers

Glacial sand and gravel aquifers

Semiconsolidated sand aquifers

Sandstone aquifers

Sandstone and carbonate-rock aquifers

Carbonate-rock aquifers

Crystalline-rock aquifers

Aquifer lithology

ALASKA

Basaltic- and other volcanic-rock aquifers

HAWAII

Figure 1–1. Central locations of networks of shallow monitoring wells in urban areas and the susceptibility of the networks to changes 

in nitrate concentrations.
40 Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010 Table 1–1. Redox-age classes for water samples collected from networks of shallow monitoring wells in urban areas in the United States and the susceptibility of the networks to changes in nitrate concentrations (only networks with at least 10 wells are listed). [usg, unconsolidated sand and gravel; gla, glacial sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; cry, crystalline rock; shading is used to dif- ferentiate between aquifer lithologies] Aquifer number Aquifer lithology Aquifer name Network name (number of wells) Network identifier (see Appendix figure 1) Redox-age class (percentage of samples)2 Susceptibility to changes in nitrate concentrations Oxic- potentially young Oxic- potentially old Anoxic- potentially young Anoxic- potentially old

1 usg Basin and Range basin-fill aquifers grsllusrc1 (29) um1 100 0 0 0 High

1 usg Basin and Range basin-fill aquifers nvbrlusrc1 (16) um2 81 13 6 0 High1 usg Basin and Range basin-fill aquifers nvbrlusur1 (27) um3 70 11 19 0 Medium1 usg Basin and Range basin-fill aquifers nvbrlusur2 (20) um4 35 10 50 5 Medium2 usg California Coastal Basin aquifers sanalusrc1 (24) um5 67 0 25 8 Medium3 usg Central Valley aquifer system sacrlusrc1 (26) um6 77 12 12 0 High5 usg High Plains aquifer hpgwlusur1 (30) um7 60 0 40 0 Medium8 usg Rio Grande aquifer system rioglusur1 (24) um8 17 4 42 38 Low8 usg Rio Grande aquifer system rioglusrc1 (20) um9 15 0 45 40 Low

10 usg Surficial aquifer system gafllusrc1a (12) um10 58 0 42 0 Medium

12e gla Eastern glacial aquifers linjlusrc2 (26) um11 100 0 0 0 High12e gla Eastern glacial aquifers connlusur1 (39) um12 90 8 0 3 High12e gla Eastern glacial aquifers connlusrc1 (27) um13 81 0 19 0 High12e gla Eastern glacial aquifers necblusrc1 (29) um14 76 0 24 0 High12e gla Eastern glacial aquifers hdsnlusur1 (16) um15 50 19 19 13 Medium12c gla Central glacial aquifers uirblusrc1 (21) um16 95 0 5 0 High12c gla Central glacial aquifers lirblusrc1 (25) um17 80 16 4 0 High12c gla Central glacial aquifers lerilusrc1 (35) um18 77 0 23 0 High12c gla Central glacial aquifers miamlusrc1 (24) um19 75 0 25 0 High12c gla Central glacial aquifers whitlusur1a (25) um20 56 0 40 4 Medium12wc gla West-central glacial aquifers umislusrc1 (32) um21 88 0 13 0 High12wc gla West-central glacial aquifers eiwalusrc1 (29) um22 55 0 45 0 Medium13 scs Coastal Lowlands aquifer system trinlusrc1 (26) um23 65 0 31 4 Medium13 scs Coastal Lowlands aquifer system acadlusrc1 (25) um24 48 16 28 8 Medium14 scs Mississippi Embayment aquifer system miselusrc2 (10) um25 50 30 20 0 Medium15 scs Northern Atlantic Coastal Plain aquifer system linjlusur1 (20) um26 100 0 0 0 High15 scs Northern Atlantic Coastal Plain aquifer system linjlusrc1 (30) um27 90 0 7 3 High16 scs Southeastern Coastal Plain aquifer system santlusrc1 (30) um28 90 7 3 0 High19 san Denver Basin aquifer system spltlusrc2 (20) um29 75 5 15 5 High24 scr Edwards-Trinity aquifer system sctxlusrc1 (30) um30 100 0 0 0 High24 scr Edwards-Trinity aquifer system sctxlusrc2 (23) um31 78 9 4 9 High27 car Biscayne aquifer sofllusrc1a (30) um32 60 7 30 3 Medium29 car Floridan aquifer system gafllusrc1b (17) um33 6 12 65 18 Low39 cry Piedmont and Blue Ridge crystalline-rock aquifers podllusrc1 (30) um34 90 0 10 0 High39 cry Piedmont and Blue Ridge crystalline-rock aquifers acfblusur1 (15) um35 80 13 7 0 High--1 usg Pleistocene Terrace deposits miselusrc1 (26) um36 77 4 15 4 High

--1 usg Alluvial aquifers in the Colorado Rocky Mountains ucollusrc1 (25) um37 76 8 16 0 High

--1 usg Denver Basin alluvial aquifers spltlusrc1 (23) um38 52 4 43 0 Medium 1Network not in a principal aquifer.

2Redox-age percentages may not sum to 100 percent because of rounding.
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This report is available at: http://pubs.usgs.gov/sir/2012/5220
McMahon, P.B.—Use of Classes to Characterize Susceptibility of Principal Aquifers to Changes in Nitrate Concentrations, 1991 to 2010—SIR 2012–5220