U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2020–5078

National Water Quality Program

Water Quality of Groundwater Used for Public Supply in 

Principal Aquifers of the Western United States
Cover: High-density housing and Oquirrh Mountains, Riverton, Utah, March 16, 2014. Photograph 

taken by Alan Cressler, U.S. Geological Survey.
Water Quality of Groundwater Used for 

Public Supply in Principal Aquifers of the 

Western United States

By Celia Z. Rosecrans and MaryLynn Musgrove

National Water Quality Program

Scientific Investigations Report 2020–5078

U.S. Department of the Interior

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

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Rosecrans, C.Z., and Musgrove, M., 2020, Water Quality of groundwater used for public supply in principal 

aquifers of the western United States: U.S. Geological Survey Scientific Investigations Report 2020–5078, 142 p., 

https://doi.org/10.3133/sir20205078.

Associated data for this publication:

Arnold, T.L., DeSimone, L.A., Bexfield, L.M., Lindsey, B.D., Barlow, J.R., Kulongoski, J.T., Musgrove, M., 

Kingsbury, J.A., and Belitz, K., 2016, Groundwater quality data from the National Water Quality Assessment Project, 

May 2012 through December 2013: U.S. Geological Survey data release, https://doi.org/10.5066/F7HQ3X18.

Arnold, T.L., Bexfield, L.M., Musgrove, MaryLynn, Lindsey, B.D., Stackelberg, P.E., Barlow, J.R., DeSimone, 

L.A., Kulongoski, J.T., Kingsbury, J.A., Ayotte, J.D., Fleming, B.J., and Belitz, Kenneth, 2017, Datasets from 

groundwater-quality data from the National Water-Quality Assessment Project, January through December 2014 

and select quality-control data from May 2012 through December 2014: U.S. Geological Survey data release, 

https://doi.org/10.5066/F7W0942N.

Arnold, T.L., Bexfield, L.M., Musgrove, M., Lindsey, B.D., Stackelberg, P.E., Lindsey, B.D., Barlow, J.R., Kulongoski, J.T., 

and Belitz, K., 2018, Datasets from groundwater-quality and select quality-control data from the National WaterQuality Assessment Project, January through December 2015 and previously unpublished data from 2013 to 2014: 

U.S. Geological Survey data release, https://doi.org/10.5066/F7XK8DHK.

Arnold, T.L., Sharpe, J.B., Bexfield, L.M., Musgrove, M., Erickson, M.L., Kingsbury, J.A., Degnan, J.R., Tesoriero, A.J., 

Kulongoski, J.T., and Belitz, K., 2020, Datasets from groundwater-quality and select quality-control data from the 

National Water-Quality Assessment Project, January through December 2016, and previously unpublished data from 

2013 to 2015: U.S. Geological Survey data release, https://doi.org/10.5066/P9W4RR74.

Rosecrans, C.Z., 2020, Data for groundwater-quality and select quality-control data for the Colorado Plateaus Principal 

Aquifer: U.S. Geological Survey data release, https://doi.org/10.5066/P916H748.

ISSN 2328-0328 (online)
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................4

Principal Aquifer Descriptions and Settings .............................................................................................6

Basin and Range Carbonate-Rock Aquifers ....................................................................................6

Basin and Range Basin-Fill Aquifers .................................................................................................8

Rio Grande Aquifer System ...............................................................................................................10

High Plains Aquifer .............................................................................................................................11

Colorado Plateaus Aquifers ..............................................................................................................13

Columbia Plateau Basaltic-Rock Aquifers .....................................................................................14

Methods.........................................................................................................................................................14

Sample Collection and Analysis .......................................................................................................16

Data Reporting and Quality-Assurance and Quality-Control Methods .....................................16

Status Assessment .............................................................................................................................18

Benchmarks for Evaluating Groundwater Quality and Relative Concentrations ............18

Selection of Constituents for the Understanding Assessment ..........................................19

Aquifer-Scale Proportions ........................................................................................................19

Understanding Assessment ..............................................................................................................19

Assessment Levels ....................................................................................................................19

Statistical Tests of Correlation .................................................................................................30

Potential Explanatory Factors ....................................................................................................................30

Depth to Bottom of Well Perforation ...............................................................................................31

Groundwater Age Category ..............................................................................................................31

Land Use...............................................................................................................................................32

Aquifer Lithology .................................................................................................................................34

Hydrologic Conditions ........................................................................................................................34

Geochemical Conditions ....................................................................................................................37

Correlation Between Potential Explanatory Factors ....................................................................40

Status and Understanding Assessments of Water Quality ...................................................................46

Inorganic Constituents .......................................................................................................................46

Trace Elements ...........................................................................................................................48

Factors Affecting Antimony ............................................................................................51

Factors Affecting Arsenic ...............................................................................................51

Factors Affecting Cobalt ..................................................................................................77

Factors Affecting Fluoride ...............................................................................................77

Factors Affecting Manganese and Iron ........................................................................79

Factors Affecting Molybdenum ......................................................................................79

Factors Affecting Selenium .............................................................................................81

Factors Affecting Strontium ............................................................................................81

Factors Affecting Thallium ..............................................................................................83

Factors Affecting Uranium ..............................................................................................83

Radionuclides .............................................................................................................................83

Gross Alpha Activity .........................................................................................................85
iv

Radium Activity .................................................................................................................85

Radon Activity ...................................................................................................................85

Salinity Indicators ......................................................................................................................90

Nutrients ......................................................................................................................................95

Organics and Special Interest Constituents ..................................................................................98

Pesticides ....................................................................................................................................98

Volatile Organic Compounds (VOCs) ....................................................................................109

Special Interest Constituent: Perchlorate ...........................................................................113

Summary......................................................................................................................................................115

Acknowledgments.....................................................................................................................................117

References Cited........................................................................................................................................118

Appendix 1 Quality-Control Data and Analysis .................................................................................129

Figures

1. Map showing six western U.S. principal aquifers selected for assessment 

of the quality of groundwater used for public supply, sampled by the U.S. 

Geological Survey National Water Quality Assessment Project, 2013–2017 .....................3

2. Maps showing the western U.S. physiographic provinces coincident with 

selected western principal aquifer boundaries by lithology .................................................7

3. Map showing the Basin and Range carbonate-rock aquifers, land use, and 

springs sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 .................................................................................................8

4. Map showing the Basin and Range basin-fill aquifers, land use, and 

groundwater wells sampled by the U.S. Geological National Water-Quality 

Assessment Project, 2013–2017 .................................................................................................9

5. Map showing the Rio Grande aquifer system, land use, and groundwater wells 

sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017 ......................................................................................................................10

6. Map showing the High Plains aquifer, land use, and groundwater wells 

sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017 ......................................................................................................................12

7. Map showing the Colorado Plateaus aquifers, land use, and groundwater wells 

sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017 ......................................................................................................................13

8. Map showing the Columbia Plateau basaltic-rock aquifers, land use, 

and groundwater wells sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017, and sub-provinces of the 

Columbia Plateaus regional aquifer system ...........................................................................15

9. Map showing equal-area grid and sampled groundwater wells and springs 

for six western U.S. principal aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017 .....................................................17

10. Boxplots showing depth to bottom of well perforation by principal aquifer 

in wells sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................31

11. Boxplots showing depth to bottom of well perforation by available groundwater 

age category for six western U.S. principal aquifers sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017 ...................32
v

12. Pie charts showing comparison of the 1974 percentage of land use to the 2012 

percentage of land use calculated within the 500-m buffer zone and the 2012 

percentage of land use calculated within the principal aquifer boundary of six 

western U.S. principal aquifers sampled by the USGS National Water-Quality 

Assessment Project, 2013–2017. Note that the boundary for the Columbia 

Plateau aquifer system includes the area of the Columbia Plateau basaltic-rock 

aquifers and the Columbia Plateau basin-fill aquifers .........................................................33

13. Map showing irrigated land-use for six western U.S. principal aquifers, and 

pie charts showing comparison of the 2012 percentage of irrigated land-use 

calculated within the 500-m buffer zone and within the principal aquifer boundary ......35

14. Graphs showing relations between elevation and aridity index at well and 

spring sites and map showing regional climate class values mapped for six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 ................................................................... 36

15. Graphs showing relations for six western U.S. principal aquifers of dissolved 

oxygen concentration with depth to bottom of well perforation and aquifer 

lithology and pH values with depth to bottom of well perforation and aquifer 

lithology sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................38

16. Graphs showing relations for six western U.S. principal aquifers of bicarbonate 

concentration by calcium concentration, aquifer lithology and groundwater 

age category, and pH, aquifer lithology, and groundwater age category 

category in springs and wells sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................39

17. Graphs showing relations for six western U.S. principal aquifers of total 

dissolved solids with depth to bottom of well perforation, aquifer lithology, 

and groundwater age category in springs and wells sampled with the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017 ...................41

18. Graphs showing maximum relative concentrations of constituents with 

health-based benchmarks and considered in the understanding assessment

for trace elements, radionuclides, salinity indicators, and nutrients in six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................47

19. Pie charts of aquifer scale proportions for inorganic constituents with 

health-based and non-health-based benchmarks in six western U.S. principal 

aquifers sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................49

20. Graphs showing concentrations of trace elements relative to health-based 

and non-health-based benchmarks measured in six principal aquifers of the 

western U.S. sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................50

21. Maps showing concentrations of arsenic, fluoride health-based (HB) 

benchmark, fluoride non-health-based (SMCL) benchmark, cobalt, manganese 

HB benchmark, manganese SMCL benchmark, iron HB benchmark, iron SMCL 

benchmark, molybdenum, selenium, strontium, and uranium in six western 

U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................58

22. Graphs showing arsenic concentrations by pH, redox and aquifer lithology 

and TDS, groundwater age, and aquifer lithology in six western U.S. principal 

aquifers sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................71
vi

23. Graphs showing rank correlations for trace elements arsenic, cobalt, fluoride, 

manganese, iron, molybdenum, selenium, strontium, and uranium in six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................72

24. Graphs showing fluoride concentrations by pH, aquifer lithology, and 

groundwater age and calcium concentrations, aquifer lithology, and 

groundwater age in six western U.S. principal aquifers sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017 ...................78

25. Graphs showing molybdenum concentrations by pH, aquifer lithology, and 

redox and total dissolved solids concentrations, aquifer lithology, and climate 

class in six western U.S. principal aquifers sampled by the U.S. Geological 

Survey National Water-Quality Assessment Project, 2013–2017 .......................................80

26. Graphs showing strontium concentrations by total dissolved solids, aquifer 

lithology, and groundwater age and graphs showing pH values, aquifer 

lithology, and groundwater age in six western U.S. principal aquifers sampled 

by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017 .....................................................................................................................................82

27. Graphs showing uranium concentrations by total dissolved solids 

concentrations, aquifer lithology, and redox condition and graphs showing 

bicarbonate concentration, aquifer lithology, and redox conditions in six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................84

28. Maps showing concentrations of 30-day gross alpha activity, radium-226 and 

radium-228 (combined), and radon-222 in six western U.S. principal aquifers 

sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017 ......................................................................................................................86

29. Graphs showing rank correlations for radionuclides 30-day gross alpha 

activity, radium-226 and radium-228 (combined), and radon-222 in six western 

U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................89

30. Graphs showing relations of radium-226 and radium-228 (combined) with 

measured pH values by aquifer lithology and redox condition in six western 

U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................90

31. Graphs showing concentrations of salinity indicators relative to benchmarks 

and rank correlations for total dissolved solids in six western U.S. principal 

aquifers sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ..............................................................................................91

32. Maps showing concentrations of TDS, chloride, and sulfate in six western 

U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................92

33. Map showing concentrations of nitrate in six western U.S. principal aquifers 

sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017 ......................................................................................................................96

34. Graphs showing rank correlations for nitrate in six western U.S. principal 

aquifers sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................97

35. Graphs showing measured nitrate concentrations with dissolved oxygen 

concentration, by groundwater age and redox class, for six western 

U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................98
vii

36. Graphs showing detection frequency and maximum relative concentration of 

selected organic and special interest constituents detected in groundwater 

samples collected in six western U.S. principal aquifers sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017 .................100

37. Maps showing concentrations of selected pesticides atrazine, 

didealkylatrazine, bromacil, propazine, and propoxur in six western 

U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 ...................................................................103

38. Graphs showing rank correlations for pesticides in two western U.S. principal 

aquifers sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 .............................................................................................108

39. Maps showing concentrations of selected volatile organic compounds 

chloroform and bromodichloromethane in six western U.S. principal aquifers 

sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017 ....................................................................................................................110

40. Graphs summarizing rank correlations for chloroform and 

bromodichloromethane in six western U.S. principal aquifers sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017 .........112

41. Map showing concentrations of special interest constituent perchlorate in 

the Rio Grande aquifer system and High Plains aquifer sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017 .................114

42. Graphs showing rank correlations for special interest constituent perchlorate 

in the Rio Grande aquifer system and High Plains aquifer sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017 .................115

Tables

1. Summary of aquifer characteristics, descriptive information, and groundwater 

use for six western U.S. principal aquifers sampled by the USGS National 

Water-Quality Assessment Project, sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project 2013–2017 ........................................................5

2. Benchmark type and value for constituents selected for additional evaluation 

in six western U.S. principal aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017 .....................................................20

3. Constituents detected in samples but not selected for the status assessment

in six western U.S. principal aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017 .....................................................22

4. Groundwater age category by principal aquifer in wells and springs sampled 

by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017 .....................................................................................................................................32

5. Results of Kruskal-Wallis and Wilcoxon rank-sum test by principal aquifer 

for differences in values of selected potential explanatory factors among 

groundwater age categories in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017 ....42

6A. Spearman's rho (ρ) correlation results between potential explanatory 

factors in the Basin and Range carbonate-rock aquifers sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017 ...................43
viii

6B. Spearman’s rho (ρ) correlation results between potential explanatory factors in 

the Basin and Range basin-fill aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017 .....................................................43

6C. Spearman's rho (ρ) correlation results between potential explanatory factors in 

the Rio Grande aquifer system sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................44

6D. Spearman's rho (ρ) correlation results between potential explanatory factors 

in the High Plains aquifer sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................44

6E. Spearman's rho (ρ) correlation results between potential explanatory factors in 

the Colorado Plateaus aquifers ................................................................................................45

6F. Spearman's rho (ρ) correlation results between potential explanatory factors 

in the Columbia Plateau basaltic-rock aquifers sampled by the U.S. Geological 

Survey National Water-Quality Assessment Project, 2013–2017 .......................................45

7. Summary of aquifer-scale proportions for inorganic constituent groups with 

health-based benchmarks in six western U.S. principal aquifers sampled 

in springs and wells by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................49

8. Summary of aquifer-scale proportions for inorganic constituent groups with 

non-health-based benchmarks in six western U.S. principal aquifers sampled 

in springs and wells by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................51

9A. Detection frequency and aquifer-scale proportions for inorganic constituents 

that met criteria for additional evaluation by the status assessment, for Basin 

and Range carbonate-rock aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017 .....................................................52

9B. Detection frequency and aquifer-scale proportions for inorganic constituents 

that met criteria for additional evaluation by the status assessment, for Basin 

and Range basin-fill aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................53

9C. Detection frequency and aquifer-scale proportions for inorganic constituents 

that met criteria for additional evaluation by the status assessment, for Rio 

Grande aquifer system sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................54

9D. Detection frequency and aquifer-scale proportions for inorganic constituents 

that met criteria for additional evaluation by the status assessment, for High 

Plains aquifer sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017 ...............................................................................................55

9E. Detection frequency and aquifer-scale proportions for inorganic constituents 

that met criteria for additional evaluation by the status assessment, for 

Colorado Plateaus aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017 .....................................................................56

9F. Detection frequency and aquifer-scale proportions for inorganic constituents 

that met criteria for additional evaluation by the status assessment, for 

Columbia Plateau basaltic-rock aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017 .....................................................57

10. Summary of aquifer-scale proportions for organic constituents with 

health-based benchmarks and for the special interest constituent, perchlorate, 

in six western U.S. principal aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017 .....................................................99
ix

11. Summary of aquifer-scale proportions for pesticides selected for further 

evaluation in six western U.S. principal aquifers sampled by the U.S. Geological 

Survey National Water-Quality Assessment Project, 2013–2017 .....................................101

12. Summary of aquifer-scale proportions for volatile organic compounds (VOCs) 

selected for further evaluation in six western U.S. principal aquifers sampled 

by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017 ...................................................................................................................................102

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain

 Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

 Area

acre 4,047 square meter (m2)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2)

acre 0.004047 square kilometer (km2)

section (640 acres or 1 square mile) 259.0 square hectometer (hm2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

 Volume

acre-foot (acre-ft) 1,233 cubic meter (m3)

 Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

 Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L)

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

°C = (°F – 32) / 1.8.

Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.
x

Supplemental Information

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm 

at 25 °C).

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

micrograms per liter (μg/L), or nanograms per liter (ng/L).

Activities for radioactive constituents in water are given in picocuries per liter (pCi/L).

Results for measurements of stable isotopes of an element (with symbol E) in water, solids, 

and dissolved constituents commonly are expressed as the relative difference in the ratio of 

the number of the less abundant isotope (iE) to the number of the more abundant isotope of a 

sample with respect to a measurement standard.

Abbreviations

°F degrees Fahrenheit

AMCL alternative maximum contaminant level

EPA U.S. Environmental Protection Agency

GAMA Groundwater Ambient Monitoring and Assessment

HB health-based

HBSL health-based screening level

HCO3 bicarbonate

MCL maximum contaminant level

MDL method detection limit

NAWQA National Water Quality Assessment

PA principal aquifer

PSWs public supply wells

QC quality control

RL reporting limit

RSD relative standard deviation

SD standard deviation

SMCL secondary-maximum contaminant level

TDS total dissolved solids

THMs trihalomethanes

USGS U.S. Geological Survey

VOCs volatile organic compounds
Water Quality of Groundwater Used for Public Supply in 

Principal Aquifers of the Western United States

By Celia Z. Rosecrans and MaryLynn Musgrove

Abstract

Groundwater provides nearly half of the Nation’s 

drinking water. As the Nation’s population grows, the 

importance of (and need for) high-quality drinking-water 

supplies increases. As part of a national-scale effort to 

assess groundwater quality in principal aquifers (PAs) that 

supply most of the groundwater used for public supply, the 

U.S. Geological Survey National Water-Quality Assessment 

(NAWQA) Project staff sampled six principal aquifers in 

the western United States between 2013 and 2017: (1) the 

Basin and Range carbonate-rock aquifers, (2) Basin and 

Range basin-fill aquifers, (3) Rio Grande aquifer system, 

(4) High Plains aquifer, (5) Colorado Plateaus aquifers, and 

(6) Columbia Plateau basaltic-rock aquifers. These six PAs 

supply a large part of the Nation’s drinking water and cover a 

large geographic extent of the western conterminous United 

States. Groundwater samples were analyzed for a large suite 

of water-quality constituents including major ions, nutrients, 

trace elements, volatile organic compounds (VOCs), pesticide 

compounds, radioactive constituents, age tracers, and, in 

selected PAs, perchlorate. Two types of assessments were 

made: (1) a status assessment that describes the quality of 

the groundwater resource at time of collection and (2) an 

understanding assessment that evaluates relations between 

groundwater quality and potential explanatory factors 

that represent characteristics of the aquifer system. The 

assessments characterize untreated groundwater quality, which 

might be different than the quality of drinking water delivered 

to consumers. The assessments are based on water-quality data 

collected from 352 wells and 6 springs using an equal-area 

grid sampling design. This sampling approach allows for 

the estimation of the proportion of high, moderate, or low 

concentrations relative to federal water-quality benchmarks 

of selected constituents in the area of each PA. Results were 

compared to established benchmarks for drinking-water 

quality to provide context for evaluating the quality of 

untreated groundwater: Federal regulatory benchmarks 

for protecting human health, non-regulatory human-health 

benchmarks, and non-regulatory benchmarks for nuisance 

chemicals. Not all constituents that were analyzed have 

benchmarks and thus were not considered for assessments. 

Concentrations are characterized as high if they are greater 

than their benchmark. Concentrations are considered moderate 

if they are greater than one-half their benchmark (for inorganic 

constituents), or greater than one-tenth their benchmark (for 

organic constituents). Concentrations are considered low if 

they are less than moderate or the constituent was not detected.

Status assessment results indicated that inorganic 

constituents more commonly occurred at high and moderate 

concentrations in the six PAs than organic constituents, 

and organic constituents predominately occurred at low 

concentrations. Inorganic constituents that exceeded 

health-based benchmarks (high concentrations) were present 

in all six PAs; aquifer-scale proportion were 30 percent in 

the Rio Grande aquifer system, 22 percent in the Basin and 

Range basin-fill aquifers, 20 percent in the Basin and Range 

carbonate-rock aquifers, 19 percent in the High Plains aquifer, 

16 percent in the Colorado Plateaus aquifers, and 8 percent in 

the Columbia Plateau basaltic-rock aquifers. Arsenic, fluoride, 

manganese, and total dissolved solids were the constituents 

most commonly present at high concentrations. Organic 

constituents with human-health benchmarks (pesticide 

compounds and VOCs) did not occur at high concentrations 

and moderate concentrations were infrequent; aquifer-scale 

proportions ranged from 0 to 5 percent. Detections of organic 

compounds at low concentrations, however, occurred in all six 

PAs, with detection frequencies ranging from 10 to 26 percent 

for pesticide compounds and from 10 to 46 percent for VOCs. 

Specific organic constituents with detection frequencies 

greater than 10 percent were four herbicides (atrazine, 

didealkylatrazine, bromoform, and propazine), one insecticide 

(propoxur), and two VOCs (the trihalomethanes chloroform 

and bromodichloromethane). Where collected—in the Rio 

Grande aquifer system and High Plains aquifer—perchlorate 

did not occur at high concentrations; moderate aquifer-scale 

proportions were 3 and 11 percent, respectively.
2 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

The understanding assessment included statistical tests 

to evaluate relations between constituent concentrations and 

potential explanatory factors to identify natural and human 

factors that affect groundwater quality. Potential explanatory 

factors included depth to bottom of well perforation, 

groundwater age category, land use, aquifer lithology, 

hydrologic conditions, and geochemical conditions. Higher 

concentrations of trace elements, radioactive constituents, 

and constituents with non-health-based benchmarks generally 

were associated with unconsolidated sand and gravel aquifer 

lithologies, premodern groundwater age, greater aridity, 

and more alkaline pH. Organic constituents with detection 

frequencies greater than 10 percent generally were associated 

with urban land use, shallower well depths, and higher total 

dissolved solids concentrations. The results for the six western 

PAs provide important insights into the quality of groundwater 

that is used for drinking water in the western United States, 

as well as natural and human factors that affect groundwater 

quality in this region.

Introduction

Groundwater provides nearly half of the Nation’s 

drinking water. The increasing population places a demand on 

the Nation’s water resources, and specifically on groundwater 

as a source for high-quality public water supplies. An 

estimated 76,500 million gallons per day (Mgal/day) of fresh 

groundwater was withdrawn from 66 principal aquifers (PAs) 

in the United States (U.S.) in 2000 (Maupin and Barber, 

2005). This total includes groundwater withdrawals used 

for irrigation, public supply, and self-supplied industry. 

Groundwater withdrawals used for irrigation account for the 

largest proportion (74 percent), whereas withdrawals used for 

public and self-supplied industry account for the remaining 

proportion (26 percent). The U.S. Geological Survey (USGS) 

has mapped 62 of the uppermost PAs of the United States 

(U.S. Geological Survey, 2003) and identified 20 PAs that 

supply approximately 75 percent of the groundwater used 

for public supply and 85 percent of the groundwater used for 

domestic supply (Arnold and others, 2016). Public-supply 

wells provide water to public water systems serving at least 

15 service connections (or serving an average of at least 

25 people for at least 60 days per year) and are subject 

to federal regulatory and non-regulatory water-quality 

benchmarks (after treatment and before distribution), 

whereas private wells (considered domestic in this work) 

are not regulated by federal regulatory and non-regulatory 

water-quality benchmarks; private well owners are responsible 

for the quality of their water (U.S. Environmental Protection 

Agency, 2017, 2019a). From 2012 to 2021, the USGS National 

Water-Quality Assessment (NAWQA) Project is evaluating 

the water quality of source water (that is, untreated water) 

from public-supply wells (PSWs) in 20 PAs that are important 

sources of drinking water. The NAWQA Project objectives 

are to describe water-quality conditions of the Nation’s water 

resources, assess how water quality is changing with time, 

and improve understanding of the natural and human factors 

that affect water quality. The water-quality evaluations of 

PAs are referred to as principal aquifer studies and focus on 

public-supply wells, which often access deeper groundwater 

resources than shallower domestic-supply wells. Each 

principal aquifer study evaluates water quality by sampling 

spatially distributed PSWs or springs using an equal-area grid 

sampling approach (Belitz and others, 2010).

This report assesses the quality of groundwater used for 

public supply in six western U.S. PAs (fig. 1):

• the Basin and Range carbonate-rock aquifers

• the Basin and Range basin-fill aquifers

• the Rio Grande aquifer system

• the High Plains aquifer

• the Colorado Plateaus aquifers

• the Columbia Plateau basaltic-rock aquifers

The assessments consider the physical setting and the 

susceptibility and vulnerability to contamination for each PA. 

Two types of assessments were made for each PA: (1) status:

an assessment that describes the quality of the groundwater 

resource at time of collection and (2) understanding:

an evaluation of the natural and human factors affecting 

the quality of groundwater, including an explanation of 

statistically significant associations between water quality and 

selected explanatory factors. These assessments characterize 

untreated groundwater quality, which might be different than 

the quality of drinking water delivered to consumers. The 

purpose of the USGS -principal aquifer studies is to evaluate 

the quality and availability of drinking water in the Nation’s 

aquifers, and to improve our understanding of where and 

why water quality is degraded and how it might respond to 

changes in climate and land use (Burow and Belitz, 2014). 

The understanding assessment objective is to improve 

understanding about why specific constituents are present in 

the groundwater at moderate to high concentrations or are 

frequently detected. The understanding assessment does not, 

however, identify sources of constituents to individual wells 

or springs.
Introduction 3

Seattle

Spokane

Yakima

Kennewick Portland

Salt Lake

City

San

Francisco Denver

Grand

Junction

Las Vegas

Flagstaff Los

Angeles Oklahoma

City

Albuquerque

Amarillo

Phoenix

Lubbock

Dallas

Tucson

Midland El Paso

San Antonio

0 150 300 MILES

0 150 300 KILOMETERS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH DAKOTA

SOUTH DAKOTA

NEBRASKA

KANSAS

IOWA MINNESOTA

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock aquifer

Basin and Range basin-fill aquifer

Rio Grande aquifer sytem

High Plains aquifer (McMahon

 and others, 2007)

Colorado Plateaus aquifers

Colombia Plateau aquifers

Map area

Figure 1. Six western U.S. principal aquifers selected for assessment of the quality of groundwater used for public 

supply, sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
4 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Western U.S. aquifers were selected for sampling and 

assessment based on their national ranking as a source of 

groundwater used for public supply and other characteristics 

(table 1). Four of the six western PAs rank in the top 

20 aquifer systems providing public supply. Additionally, 

the Colorado Plateaus aquifers are included because of 

their large geographic extent in the conterminous western 

United States, and the Basin and Range carbonate-rock 

aquifers are included because of their hydraulic connection 

to portions of the Basin and Range basin-fill aquifers, which 

collectively constitute a significant regional groundwater 

resource (Harrill and Prudic, 1998). The California Coastal 

Basin aquifers and Central Valley aquifer system, which 

rank in the top 20 of aquifer systems providing public 

supply, have been extensively studied by the California 

Groundwater Ambient Monitoring and Assessment (GAMA) 

Program (https://www.waterboards.ca.gov/gama) and are 

not considered herein. The California GAMA Priority Basin 

Project design provides statistically robust assessments of 

untreated groundwater quality in the primary aquifer systems 

used for drinking water in California (Fram and Belitz, 2012). 

The assessment methods and results presented herein are 

comparable to those of the GAMA Priority Basin Project.

Purpose and Scope

The purpose of this report is to provide:

• A description of the hydrogeologic setting of the six 

PAs in the western United States recently sampled by 

the NAWQA Project

• A status assessment summarizing the quality 

of groundwater used for drinking water at 

public-supply depths

• An understanding assessment identifying natural 

and human factors affecting groundwater quality and 

relations between water quality and selected potential 

explanatory factors

Untreated groundwater samples in the six PAs were 

collected from a total of 352 PSWs and 6 springs used for 

public supply between March 2013 and December 2017. 

Samples were analyzed for numerous water-quality 

constituents including major ions, trace elements, nutrients, 

radioactive constituents, volatile organic compounds (VOCs), 

pesticide compounds (which include pesticide degradants), 

selected isotopes and age tracers, and perchlorate (in selected 

PAs). To provide context for water-quality data, constituent 

concentrations of untreated groundwater were compared 

to available water-quality benchmarks. The quality of 

water received by consumers can be different because after 

withdrawal, groundwater might be treated before delivery. 

Federal regulatory benchmarks for protecting human health 

(maximum contaminant levels [MCLs]; U.S. Environmental 

Protection Agency [EPA] primary drinking water regulations; 

U.S. Environmental Protection Agency, 2018a) were used for 

this evaluation. Additionally, non-regulatory human-health 

benchmarks (health-based screening levels [HBSLs]; Norman 

and others, 2018; U.S. Geological Survey, 2018a); and 

federal non-regulatory benchmarks for nuisance chemicals 

(EPA secondary maximum contaminant levels [SMCLs]; 

U.S. Environmental Protection Agency, 2018b) were used. 

This report was written to consider benchmarks in the context 

of health-based (MCLs and HBSLs) and non-health-based 

(SMCLs) benchmarks.
Introduction 5

Table 1. Summary of aquifer characteristics, descriptive information, and groundwater use for six western U.S. principal aquifers 

sampled by the U.S. Geological Survey National Water-Quality Assessment Project 2013–2017.

[mi2, square miles; <, less than; —, not applicable; Mgal/day, million gallons per day]

Principal aquifer 

name

Aquifer characteristics and descriptive information

Lithology1

Aquifer sampling Land use2

Number 

of cells in 

network

Sample 

collection 

(fiscal year)

Buffered 

cell size3

(mi2)

Total aquifer 

area4

(mi2)

Natural

(percent)

Urban

(percent)

Agriculture

(percent)

Irrigated 

agriculture

(percent)

Basin and range 

carbonate-rock 

aquifers

Carbonate rock 520 2013 and 

2015

3.6 16,000 99 <1 <1 <1

Basin and range 

basin-fill 

aquifers

Unconsolidated to 

semiconsolidated 

sand and gravel

78 2013 7.2 148,000 92 4 4 3

Rio Grande 

aquifer system

Unconsolidated to 

semiconsolidated 

sand and gravel

60 2014 1.8 29,000 91 4 5 8

High Plains 

aquifer

Unconsolidated to 

semiconsolidated 

sand and gravel

80 2015 and 

2016

— 170,000 58 1 41 12

Colorado Plateaus 

aquifers

Sandstone 60 2017 7.2 140,000 97 1.0 2 1

Columbia Plateau 

basaltic-rock 

aquifers

Igneous and 

metamorphic rock

60 2016 — 42,000 76 1 23 7

Principal aquifer name

Groundwater use6

Public supply Irrigation

Total of public supply, 

irrigation, and selfsupplied industrial

Withdrawal

(Mgal/day) Rank Withdrawal(Mgal/day) Rank Withdrawal(Mgal/day) Rank

Basin and range carbonate-rock aquifers 71.2 35 0.5 62 73.6 47

Basin and range basin-fill aquifers 1,010 4 4,550 4 5,620 4

Rio Grande aquifer system 240 18 867 11 1,120 11

High Plains aquifer 389 13 17,000 1 17,500 1

Colorado Plateaus aquifers 102 28 81.9 34 198 36

Columbia Plateau basaltic-rock aquifers 223 19 674 13 933 15

1As defined by the U.S. Geological Survey Principal Aquifers Map (U.S. Geological Survey, 2003).

2Natural, urban, agricultural, and mixed land use totals are for 2012 from Falcone (2015); 2012 Irrigated land use from U.S. Geological Survey (2014). 

Differences in agricultural land use and irrigated land use might result from different sources used to quantify land-use types.

3For sparsely populated regions, where wells are not evenly distributed the study area was delineated by placing a buffer around existing wells; an equal-area 

grid was then defined within the buffered area.

4Source of aquifer areas: Basin and Range carbonate-rock aquifers and Rio Grande aquifer system (Arnold and others, 2018); Basin and Range basin-fill 

aquifers (Arnold and others, 2016); High Plains aquifer and Colorado Platueau and Columbia Plateau (Arnold and others, 2020).

5Includes both public-supply wells and springs used for public supply.

6Groundwater use withdrawals for public supply, irrigation, and self-supplied industrial for year 2000 are from Maupin and Barber (2005), adjusted for 

stream-valley from Sargent and others (2008). National rankings for public supply and total public supply, irrigation, and self-supplied industrial are from 

Arnold and others (2016).
6 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Principal Aquifer Descriptions 

and Settings

The geographic setting, aquifer characteristics, and 

hydrogeology for each of the six PAs included in this study 

are presented later in the report, and background information 

that can be useful for understanding water-quality results is 

provided. Aquifer characteristics, descriptive information, 

and groundwater use are summarized in table 1. The PAs lie 

within (or nearly coincident with) physiographic provinces 

(fig. 2A) and have lithology classes defined in the USGS 

PA map (U.S. Geological Survey, 2003) as either carbonate 

rock; unconsolidated to semiconsolidated sand and gravel 

(hereinafter referred to as unconsolidated sand and gravel); 

sandstone; or igneous and metamorphic rock (fig. 2B). 

Except for the High Plains PA, aquifer extents are defined 

by the USGS PA map (U.S. Geological Survey, 2003). In 

this work, the High Plains PA extent used is that defined by 

McMahon and others (2007), which does not include glacial 

unconsolidated sediments—an area of overlap with the Glacial 

principal aquifer study in northeastern Nebraska (fig. 2B). 

The water quality of the Glacial principal aquifer study, which 

includes this northeastern part of the High Plains, is reported 

in Stackelberg (2017).

Basin and Range Carbonate-Rock Aquifers

The Basin and Range carbonate-rock aquifers underlie 

an area of about 16,000 square miles (mi2; Arnold and 

others, 2018) in parts of Utah, Nevada, Idaho, California, 

and Arizona (fig. 3). The aquifers rank 35th in the Nation 

as a source of groundwater for public supply (Arnold and 

others, 2016; table 1). The approximate land use overlying 

the Basin and Range carbonate-rock aquifers in 2012 was 

nearly 99-percent natural with less than 1 percent categorized 

as urban and agricultural land use, respectively (Falcone, 

2015). Although the dominant land use overlying the aquifer is 

largely undeveloped, the cities of Las Vegas, Nevada, and Salt 

Lake City, Utah, are located in the area of the carbonate-rock 

aquifers and have estimated populations of 2.1 million and 

1.2 million, respectively (U.S. Census Bureau, 2018). The 

carbonate-rock aquifer system is a potential source of water 

for these cities and anticipated population growth will likely 

increase demand on groundwater resources.

The carbonate-rock aquifers are within the Basin and 

Range Province (fig. 2A; Dettinger and others, 1995; Harrill 

and Prudic, 1998; Anning and Konieczki, 2005). The Basin 

and Range Province has a characteristic topographic pattern 

of elongated mountain ranges alternating with long expanses 

of flat, dry basins that resulted from a dynamic fault system. 

The Province includes the Basin and Range carbonate-rock 

aquifers and the Basin and Range basin-fill aquifers (described 

later). Only small parts of the Basin and Range carbonate-rock 

aquifers are mapped at the surface because they are commonly 

overlain by basin-fill sediments (fig. 3). The lateral extent of 

the carbonate-rock aquifers is approximated in some areas, 

and the stratigraphic thickness varies owing to thrusting and 

structural compression and extension (Harrill and Prudic, 

1998; Schaefer and others, 2005). The carbonate-rock aquifers 

generally are considered to be stratigraphically below the 

basin-fill aquifers; however, during parts of the Paleozoic 

and Mesozoic Eras, tectonic compression of the Great Basin 

resulted in thrusted sequences of carbonate and clastic 

rocks (Planert and Williams, 1995). The thickness of these 

sequences ranges from about 0 to 30,000 feet (ft) in eastern 

Nevada and western Utah (Schaefer and others, 2005).

The carbonate-rock aquifers are composed of thick 

sequences of Paleozoic and Mesozoic limestone and dolomite 

with lesser amounts of shale, sandstone, and quartzite that 

underlie several topographic basins (Planert and Williams, 

1995). In general, the overall thickness of the carbonate rocks 

is greater than the clastic rocks (Harrill and Prudic, 1998), 

with estimates of carbonate rock thickness up to 25,000 ft in 

parts of southern Nevada (Dettinger and others, 1995). The 

carbonate-rock units can extend across topographic basins 

and mountain blocks, forming multi-basin groundwater 

flow systems (Eakin, 1966) that result in extensive aquifers 

that can store and transmit large quantities of water along 

faults and fractures. Dettinger and others (1995) estimated 

the volume of water stored in the carbonate-rock aquifer of 

southern Nevada to be approximately 800 million acre-feet 

(acre-ft). Discharge from these aquifers can be in the form 

of large springs, extensive wetlands, and evapotranspiration 

(Schaefer and others, 2005). Evapotranspiration is the largest 

source of groundwater discharge in the Great Basin (Harrill 

and Prudic, 1998). Prudic and others (1995) simulated 

discharge by evapotranspiration in the carbonate-rock 

aquifers of the Great Basin as part of the USGS Great Basin 

Regional Aquifer-System Analysis (RASA) study. Simulated 

discharge from this work estimated evapotranspiration at 

1,213,000 acre-feet per year (acre-ft/yr) with an additional 

211,000 acre-ft/yr estimated to discharge from regional 

springs, much of which was lost to evapotranspiration.

The climate generally is arid to semiarid; however, 

the differences of altitude from the many mountain ranges 

cause climatic variations. Mean annual precipitation ranges 

from less than 5 inches (in.) in the valleys to more than 

60 in. in mountain regions (Prudic and others, 1995). Most 

of the precipitation falls during the winter months as snow 

in the mountains. The snowpack melts during the spring and 

summer, which provides essential groundwater recharge to the 

carbonate-rock aquifers (Schaefer and others, 2005). Water 

that recharges the carbonate-rock aquifers in the mountains 

can travel through or beneath several basins and ranges before 

reaching a discharge area (Schaefer and others, 2005). Mean 

annual temperatures range from 30 degrees Fahrenheit (°F) in 

some northern valleys to about 77 °F in the extreme southern 

valleys (Prudic and others, 1995; U.S. Climate Data, 2020). 

The low humidity and fairly constant light to moderate winds 

cause high rates of evapotranspiration (Robson and Banta, 

1995; Schaefer and others, 2005).
Principal Aquifer Descriptions and Settings 7

48°

46°

44°

42°

40°

38°

36°

34°

32°

30°

28°

26°

24°

128° 124° 120° 116° 112° 108° 104° 100°

Base modified from U.S. Geological Survey and other Federal and State 

digital data, various scales; Albers Equal-Area Conic projection, standard 

parallels are 29°30' N. and 45°30' N.; North American Datum of 1983

0 200 400 600 800 MILES

0 200 400 600 800 KILOMETERS

B

Principal aquifer lithology 

 (U.S. Geological Survey, 2003)

Unconsolidated sand and gravel

Glacial unconsolidated sediments

Carbonate rock

Sandstone

Igneous and metamorphic rock

Province boundary

EXPLANATION

CANADA

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

48°

46°

44°

42°

40°

38°

36°

34°

32°

30°

28°

26°

24°

128° 124° 120° 116° 112° 108° 104° 100°

EXPLANATION

Province (Robson and Banta,

 1995, 1996)

Basin and Range

Southern Rocky Mountains

Great Plains

Colorado Plateaus

Wyoming Basin

Columbia Plateau

A

Base modified from U.S. Geological Survey and other Federal and State 

digital data, various scales; Albers Equal-Area Conic projection, standard 

parallels are 29°30' N. and 45°30' N.; North American Datum of 1983

0 200 400 600 800 MILES

0 200 400 600 800 KILOMETERS

CANADA

MEXICO MEXICO

Map area

Figure 2. A, Western U.S. physiographic provinces coincident with B, selected western principal aquifer boundaries by lithology. 
8 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

IDAHO

NEVADA

UTAH

ARIZONA

COLORADO

NEW MEXICO

TEXAS

WYOMING

Rio Grande

Colorado

River

Salt Lake

City

Las Vegas

Phoenix

122° 120° 118° 116° 114° 112° 110° 108° 106° 104°

42°

40°

38°

36°

34°

32°

30°

28°

Base modified from U.S. Geological Survey and other Federal and State digital data, various scales; Albers EqualArea Conic projection, standard parallels are 29°30' N. and 45°30' N.; North American Datum of 1983

Land use, 2012 (Falcone, 2015)

Natural

Urban

Agricultural

Principal aquifer boundary (U.S.

 Geological Survey, 2003)

Basin and Range basin-fill aquifer 

Basin and Range carbonate-rock aquifer

Great Basin section

Approximated carbonate rock extent

(Schaefer and others, 2005)

Basin and Range Province

Carbonate-rock spring site

Carbonate-rock well site

EXPLANATION

0 125 250 375 500 MILES

0 125 250 375 500 KILOMETERS

Map area

Figure 3. Basin and Range carbonate-rock aquifers, land use, and groundwater wells and springs sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

Basin and Range Basin-Fill Aquifers

The Basin and Range basin-fill aquifers underlie an 

area of about 148,000 mi2 (Arnold and others, 2016) and 

consist of approximately 330 sediment-filled basins separated 

by mountain ranges in Nevada, California, Arizona, Utah, 

and adjacent states (fig. 4; Anning and Konieczki, 2005; 

Thiros and others, 2014). Land use overlying the Basin 

and Range basin-fill aquifers in 2012 was approximately 

92-percent natural, 4-percent agricultural, and 4-percent 

urban (Falcone, 2015). The basin-fill aquifers rank fourth in 

the Nation as a source of groundwater for public supply and 

rank fourth in the Nation as a source for irrigation (Arnold 

and others, 2016; table 1). Groundwater development for 

public supply is expected to increase for some of the most 

populous cities in the Basin and Range Province—Phoenix, 

Arizona (4.1 million); Las Vegas, Nevada (2.2 million); 

and Salt Lake City, Utah (1.2 million; U.S. Census Bureau, 

2018). In some basins, groundwater withdrawals for public 

supply and irrigation is the primary component of discharge 

and have altered directions of groundwater flow (Thiros 

and others, 2014). In 2012, approximately 3 percent of the 

land cover overlying the Basin and Range basin-fill aquifers 

was mapped as irrigated lands (U.S. Geological Survey, 

2014). Additionally, groundwater withdrawals from deep 

wells typically used for public supply have enhanced the 

movement of groundwater from shallower to deeper parts 

of the basin-fill aquifers. The development of water from 

the deeper parts of the aquifer might result in susceptibility 

of water-quality degradation by human activities at the land 

surface and increased vulnerability to contaminants where 

sources are present (Bexfield and others, 2011). Further, 

increased groundwater withdrawals have led to overdraft of 

aquifers resulting in land subsidence in many areas of the arid 

southwestern United States (Arizona Department of Water 

Resources, 2013, 2014, 2017, 2019; Leake, 2016).
Principal Aquifer Descriptions and Settings 9

122° 120° 118° 116° 114° 112°

42°

40°

38°

36°

34°

32°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital data, various scales; 

Albers Equal-Area Conic projection, standard parallels are 29°30' N. and 45°30' N.; North American 

Datum of 1983

Land use, 2012 (Falcone, 2015)

Natural

Urban

Agricultural

Principal aquifer boundary—

 Basin and Range basin-fill aquifer

 (U.S. Geological Survey, 2003)

Sampled well

0 100 200 MILES

Map area 0 100 200 KILOMETERS

Colorado

River

Salt Lake

City

Las Vegas

Phoenix

ARIZONA

UTAH

IDAHO

NEVADA

OREGON

CALIFORNIA

Figure 4. Basin and Range basin-fill aquifers, land use, and 

groundwater wells sampled by the U.S. Geological National 

Water-Quality Assessment Project, 2013–2017.

The basin-fill aquifers are within the Basin and 

Range Province–a structural province characterized by 

numerous north-south trending mountain ranges separated 

by intervening basins (Dettinger and others, 1995). The 

topographic basins generally overlie the Basin and Range 

carbonate-rock aquifers in the Great Basin area (fig. 3) 

of the Basin and Range Province and, in some places, 

could be hydraulically connected to the adjacent and 

underlying carbonates (Schaefer and others, 2005). The 

basins were created in response to an east-northeast to 

west-southwest regional extension field that resulted in a 

network of north to northwest-trending horst and grabens 

bound by high-angle normal faults (Harrill and Prudic, 

1998). The subsidence of these grabens and subsequent 

erosion resulted in late Cenozoic basin-fill deposits 

(Harrill and Prudic, 1998). Basin-fill materials vary from 

unconsolidated to well-consolidated deposits of cobble, 

gravel, silt, and clay and can contain evaporite deposits or 

Quaternary basalt flows (Anning and Konieczki, 2005). The 

thickness of many of the alluvial basins is unknown, but 

many basin fills range in thickness from 1,000 to 5,000 feet 

and can be more than 10,000 feet in the middle of some 

basins in Utah and Arizona (Robson and Banta, 1995). The 

basin-fill aquifers are unconfined in most areas, but can be 

confined locally (Anning and Konieczki, 2005).

The climate of the Basin and Range Province 

generally is arid to semiarid. Average annual precipitation 

ranges from less than 4 inches in southwestern Arizona 

to about 16 inches in Utah (Robson and Banta, 1995). 

Most recharge to the basin-fill aquifers typically occurs 

along mountain fronts through infiltration of mountain 

stream runoff and inflow from fractured rock (Robson 

and Banta, 1995). In some basins, irrigation is a major 

source of recharge and also some rivers regionally, 

such as the Colorado River in California (Planert and 

Williams, 1995; Thiros and others, 2014). Although natural 

discharge of some basin-fill aquifers occurs through 

streams and springs, the largest component of discharge is 

evapotranspiration, and most spring discharge is consumed 

by evapotranspiration (Robson and Banta, 1995; Harrill and 

Prudic, 1998).
10 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Rio Grande Aquifer System

The Rio Grande aquifer system underlies an area of 

about 29,000 mi2 in parts of Colorado, New Mexico, and 

Texas (fig. 5; Arnold and others, 2017). Approximate land 

use overlying the Rio Grande aquifer system primarily is 

natural land cover (91 percent) with relatively small areas 

of agriculture (5 percent) and urban (4 percent) land use 

(Falcone, 2015). The two largest cities are Albuquerque, 

New Mexico (population of about 890,000), and El Paso, 

Texas (population of about 800,000; U.S. Census Bureau, 

2018). The Rio Grande aquifer system ranks 18th in 

the Nation for groundwater withdrawals used for public 

supply and ranks 11th for withdrawals used for irrigation 

in 2000 (Arnold and others, 2016; table 1). In some basins, 

groundwater withdrawals for irrigation and public supply 

have become a substantial discharge component altering 

groundwater flow directions (Robson and Banta, 1995; 

Bexfield and others, 2011; Thiros and others, 2014). 

In 2012, mapped irrigated land cover accounted for 

approximately 5,320 mi2 (8 percent) of the area overlying 

the Rio Grande aquifer system (U.S. Geological Survey, 

2014), whereas agricultural land accounted for 5 percent 

(Falcone, 2015); differences likely resulted from different 

sources and methods used to quantify land-use types.

The Rio Grande aquifer system is within the Basin 

and Range Physiographic Province and the southern part 

of the Southern Rocky Mountains Province (fig. 2A). 

The aquifer system is characterized by alluvial fans near 

the base of mountains that slope toward the relatively 

flat basin floor (Robson and Banta, 1995) and consists 

of a network of hydraulically interconnected basin-fill 

aquifers in about 20 alluvial basins extending along the 

Rio Grande Valley and nearby valleys (Robson and Banta, 

1995; Wilkins, 1998). The basin-fill alluvial aquifers of 

the Rio Grande generally are unconfined and water loss 

to evapotranspiration is an important factor affecting 

groundwater quality in areas of irrigated agriculture or 

areas of shallow depth to water (Bexfield and others, 

2011; DeSimone and others, 2015). Evapotranspiration 

(evaporation of water from soil and transpiration of water 

from plants) leaves solutes behind, resulting in an increase 

to the dissolved-solids concentration in the remaining 

groundwater through time.

ARIZONA

COLORADO

UTAH

TEXAS

Albuquerque

El Paso

110° 108° 106°

38°

36°

34°

32°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital data, various scales; 

Albers Equal-Area Conic projection, standard parallels are 29°30' N. and 45°30' N.; North American 

Datum of 1983

0 50 100 MILES

0 50 100 KILOMETERS

Land use, 2012 (Falcone, 2015)

Natural

Urban

Agricultural

Principal aquifer boundary—

 Rio Grande aquifer system (U.S.

 Geological Survey, 2003)

Sampled well

Map area

NEW

MEXICO

Rio Grande

San Luis

Valley

Figure 5. Rio Grande aquifer system, land use, and groundwater 

wells sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017.
Principal Aquifer Descriptions and Settings 11

The principal water-yielding units are the Tertiary and 

Quaternary Santa Fe Group and younger unconsolidated 

deposits of sand, silt, and clay interbedded with Quaternary 

gravel (Robson and Banta, 1995; Wilkins, 1998). The 

Santa Fe Group primarily is unconsolidated to moderately 

consolidated deposits of gravel, sand, and clay interbedded 

with volcanic-flows and tuff in some areas (Wilkins, 1998). 

The thickness of the basin fill varies greatly, with estimates 

as much as 30,000 ft in the San Luis Valley, 20,000 ft near 

Albuquerque, New Mexico, and approximately 2,000 ft 

near El Paso, Texas (Robson and Banta, 1995). The basins 

are bounded by Precambrian-, Mesozoic- and Cenozoic-age 

sedimentary, igneous, and metamorphic bedrock formations 

(Wilkins, 1998). The Rio Grande Rift, a northward-trending 

fault-bounded structural rift zone, is the principal geologic 

feature in the area (Robson and Banta, 1995). The rift consists 

of uplifted blocks on the east and west and down-dropped, 

alluvial-filled grabens in the middle (Wilkins, 1998). The 

configuration of the rift in the bounding highlands affects 

precipitation, runoff, groundwater recharge, source material 

of the basin fill, aquifer characteristics, and water quality 

(Robson and Banta, 1995).

Recharge to the Rio Grande aquifer system primarily 

originates as precipitation in the mountainous areas that 

surround the basins (Wilkins, 1998). Runoff from snowmelt 

or rainfall enters the basins by two pathways: (1) flow across 

alluvial fans and subsequent percolation downward through 

streambeds or (2) mountain-front recharge in which fractures 

or permeable layers in bedrock aquifers discharge directly to 

basin-fill aquifers in the subsurface at the mountain front or 

discharge to mountain streams as baseflow near the mountain 

front (Robson and Banta, 1995). The movement of water 

from recharge to discharge areas can take thousands of years 

because of the distance traveled and the aquifer characteristics 

(Anderholm and others, 1995). The flood plain of the Rio 

Grande River is a major point of discharge for deeper regional 

and shallower local flow systems. Regional groundwater 

flow is from the basin margins toward the Rio Grande 

River and southward from basin to basin. Groundwater 

discharge mostly occurs near the center of the basins or in 

the subsurface to the adjacent downstream alluvial basin and 

primarily occurs through evapotranspiration and groundwater 

pumping (Bexfield and Anderholm, 1997). Discharge of 

groundwater through evapotranspiration in the flood plain of 

the Rio Grande is substantial because of shallow depths to 

groundwater, which results in salts concentrated at and near 

the land surface and in shallow groundwater (Bexfield and 

Anderholm, 1997; Thiros and others, 2014). Local recharge 

and discharge flow systems have developed in the flood plain 

due to irrigation practices in which surface water is diverted 

from the Rio Grande River and delivered to fields through 

unlined canals and laterals (Bexfield and Anderholm, 1997). 

Irrigated fields, canals, and laterals provide local recharge 

of surface water to the aquifer, whereas localized discharge 

occurs through irrigation wells, evapotranspiration from fields, 

and constructed groundwater-tiles that are intended to keep 

the water table below a certain level (Bexfield and Anderholm, 

1997). The climate of the region (south-central Colorado, 

central New Mexico, and west Texas) is arid to semiarid 

and is characterized by warm average daily temperatures, 

low relative humidity, and a high rate of evaporation that 

substantially exceeds mean annual precipitation (Thiros and 

others, 2014). Pan evaporation rates in New Mexico range 

from near 56 inches in the north-central mountains to more 

than 110 inches in southeastern valleys (Western Regional 

Climate Center, 2018). Most precipitation in the alluvial basins 

falls between July and October. Mean annual precipitation 

ranges from less than 10 inches over much of the Rio Grande 

Valley to more than 20 inches at higher elevations in New 

Mexico (Western Regional Climate Center, 2018).

High Plains Aquifer

The High Plains aquifer underlies an area of about 

170,000 mi2 in parts of eight western States—Colorado, 

Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, 

Texas, and Wyoming (fig. 6; Arnold and others, 2018). The 

High Plains aquifer is the principal source of water in one of 

the major agricultural areas of the United States and ranks first 

in the Nation for groundwater withdrawals used for irrigation 

and total withdrawals (Arnold and others, 2016; table 1). In 

2000, withdrawals from the High Plains aquifer accounted for 

21.2 percent of groundwater withdrawn in the United States 

(Hutson and others, 2004), making it the most intensively used 

PA in the Nation. Population estimates for the three largest 

cities in the High Plains aquifer boundary are for Lubbock, 

Texas (about 317,000), Amarillo, Texas (about 265,000), and 

Midland, Texas (about 171,000; U.S. Census Bureau, 2018). 

In 2012, land use overlying the High Plains aquifer was 

approximately 58-percent natural, 41-percent agricultural, 

and 1-percent urban (Falcone, 2015). The agricultural land 

use was dominated by crop production (39 percent), followed 

by pasture/hay (1 percent), and grazing potential (1 percent). 

Crops grown in the High Plains are a substantial percentage of 

total crop production for the United States (U.S. Department 

of Agriculture, 2014). Historically, the primary crop was 

wheat, followed by corn, sorghum, hay/alfalfa, cotton, 

soybeans, and peanuts (McMahon and others, 2007). In 2012, 

approximately 12 percent of the land cover overlying the High 

Plains aquifer was mapped as irrigated lands (U.S. Geological 

Survey, 2014). The differences in the percentage of 

agricultural land use and the percentage of irrigated land use 

reflected dryland farming (table 1). Dryland farming is a type 

of farming practiced in arid and semiarid regions in which 

drought resistant crops are grown without the use of irrigation 

(Peterson, 2005; Stewart, 2016).
12 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Pecos River

Mississippi River

Denver

Oklahoma

Amarillo City

Lubbock

Midland

Wichita

TEXAS

COLORADO

NEW OKLAHOMA

MEXICO

NEBRASKA

KANSAS

WYOMING SOUTH DAKOTA

0 50 100 150 MILES

0 50 100 150 KILOMETERS

106° 104° 102° 100° 98°

42°

40°

38°

36°

34°

32°

EXPLANATION

Base modified from U.S. Geological 

Survey and other Federal and State digital 

data, various scales; Albers Equal-Area Conic

projection, standard parallels are 29°30' N. and 

45°30' N.; North American Datum of 1983

Land use, 2012 (Falcone, 2015)

Natural

Urban

Agricultural

Principal aquifer boundary—

 High Plains aquifer (McMahon 

 and others, 2007)

Sampled well

Map

area

Figure 6. High Plains aquifer, land use, and groundwater wells 

sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017.

The aquifer occupies the higher elevations of the 

Great Plains Province (fig. 2A), an area known as the 

High Plains. The Great Plains Province, which extends 

from the Canadian border south into Texas, contains most 

of the High Plains aquifer (fig. 2B). The topography of 

the High Plains aquifer is characterized by flat to gently 

rolling terrain, a remnant of the vast plain formed by 

sediments that were deposited by ancestral streams flowing 

eastward from the Rocky Mountains (Weeks and others, 

1988). The High Plains aquifer consists of near-surface 

sedimentary deposits of Tertiary and Quaternary age that 

form six hydraulically connected hydrogeologic units of the 

regionally unconfined aquifer (Gutentag and others, 1984; 

Dennehy and others, 2002; McMahon and others, 2007). Of 

these six hydrogeologic units, the Ogallala Formation has 

the largest areal extent of the geologic units (134,000 mi2; 

Luckey and others, 1986) and composes a large part of the 

High Plains aquifer where saturated (McMahon and others, 

2007). Water in the underlying bedrock can contain large 

concentrations of dissolved solids that could affect the 

quality of water in the High Plains aquifer (McMahon and 

others, 2007). The average saturated thickness across the 

aquifer is about 200 ft and is greatest in central Nebraska 

where it ranges between 600 and 1,000 ft (McMahon and 

others, 2007). Regional groundwater flow generally is 

west to east; however, variability exists near high-capacity 

pumping wells and major rivers that locally affect hydraulic 

gradients and regional groundwater flow (McMahon and 

others, 2007). Recharge to the High Plains aquifer occurs 

by infiltration of irrigation water, areally diffuse infiltration 

from precipitation, focused infiltration of storm- and 

irrigation-runoff, and upward movement of water from 

underlying aquifers (McMahon, 2001). Discharge occurs 

primarily by irrigation wells, discharge to streams and 

underlying aquifers, groundwater flow across the eastern 

boundary of the aquifer, and evapotranspiration (Luckey and 

others, 1986).

Because of the large areal extent of the High Plains 

aquifer, the climate is diverse; mean annual temperature 

increases from north to south, ranging from 39 °F in the 

north to 64 °F in the south, and mean annual precipitation 

increases from west to east, from about 12 to 33 inches 

(McMahon and others, 2007). Most of the High Plains has 

a middle-latitude, dry continental climate with abundant 

sunshine, moderate precipitation, frequent winds, low 

humidity and high rates of evaporation (Weeks and others, 

1988). Evaporation rates are among the highest in the 

United States because of high summer air temperatures and 

persistent winds, which leaves little available precipitation 

to recharge the aquifer (Gutentag and others, 1984; Weeks 

and others, 1988). Evaporation rates range from 60 inches 

in the north to 109 inches in the south (Gutentag and others, 

1984). Elevations range from about 1,000 ft along the 

eastern boundary to about 8,000 ft along the northwestern 

boundary (McMahon and others, 2007).
Principal Aquifer Descriptions and Settings 13

Colorado Plateaus Aquifers

The Colorado Plateaus aquifers cover an area of about 

140,0000 mi2 in southwestern Wyoming, southeastern 

Utah, western Colorado, northwestern New Mexico, and 

northeastern Arizona (fig. 7). Land use overlying the 

Colorado Plateaus aquifers is approximately 97-percent 

natural, 2-percent agricultural, and 1-percent urban 

(Falcone, 2015). The Colorado Plateaus aquifers rank 

28th in the Nation for groundwater withdrawals used 

for public supply (Arnold and others, 2016; table 1). In 

2012, 1 percent of the land cover overlying the Colorado 

Plateaus aquifers was mapped as irrigated agriculture (U.S. 

Geological Survey, 2014). The two largest cities in the PA 

boundary are Grand Junction, Colorado (population of 

about 152,000), and Flagstaff, Arizona (population of about 

141,000; U.S. Census Bureau, 2018).

The Colorado Plateaus aquifers are nearly coincident 

with the extent of the Colorado Plateau Province, except 

that they include the Wyoming Basin Province and the 

western margins of the Southern Rocky Mountains Province 

(fig. 2A). The Colorado Plateaus Province is characterized 

by vast regions of plateaus, mesas, and deep canyons. 

The Colorado Plateaus aquifers generally are composed 

of permeable, moderate to well-consolidated sedimentary 

rocks that vary greatly in thickness, lithology, and hydraulic 

characteristics (Robson and Banta, 1995, 1996). The 

Colorado Plateaus aquifers are a complex sequence of 

water-yielding layers resulting from structural deformation 

and erosion since deposition of the aquifer sediments 

(Robson and Banta, 1995). For example, in uplifted areas, 

younger rocks have eroded away, resulting in aquifer units 

present only in older rocks that underlie more extensive 

units in the Colorado Plateaus Province, whereas younger 

aquifer units are present only in the Wyoming Basin 

Province (fig. 2A).

Groundwater recharge to the Colorado Plateaus 

aquifers generally occurs in areas of higher altitude that 

receive more precipitation than the lower elevation basins. 

Recharge also occurs along basin margins. Groundwater 

flow generally is toward the center of structural basins that 

consist of broad basins and uplifts but is complex where 

aquifers have been intensely folded and faulted (Robson 

and Banta, 1995, 1996). Groundwater discharges from the 

aquifers directly to streams, springs, and seeps, or indirectly 

by transpiration from vegetation growing along stream 

valleys and evaporation from shallow groundwater by upward 

movement through confining layers into overlying aquifers, 

or by withdrawal from wells (Robson and Banta, 1995). The 

climate of the Colorado Plateaus Province, the southwest 

part of the Wyoming Province and the western margins of 

the Southern Rocky Mountains Province, is arid to semiarid; 

Colorado

River

Green

River

Flagstaff

Salt Lake

City

Grand

Junction

Albuquerque

110° 108°

42°

40°

38°

36°

34°

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital data, various 

scales; Albers Equal-Area Conic projection, standard parallels are 29°30' N. and 45°30' N.; 

North American Datum of 1983

Land use, 2012 (Falcone, 2015)

Natural

Urban

Agricultural

Principal aquifer boundary—

 Colorado Plateaus aquifers

 (U.S. Geological Survey, 2003)

Sampled well

Map

area

UTAH

ARIZONA

COLORADO

NEW

MEXICO

WYOMING

Figure 7. Colorado Plateaus aquifers, land use, and groundwater 

wells sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017.

annual precipitation (1900–2000) in the Colorado Plateaus 

Province (fig. 2A) has ranged from 5.4 to 26.3 inches per 

year (in/yr), with a median of 11.8 in/yr (Hereford and others, 

2002). Most precipitation falls in winter and spring with brief 

and intense summer thunderstorms that produce 20–40 percent 

of annual precipitation (Robson and Banta, 1995).
14 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Columbia Plateau Basaltic-Rock Aquifers

The Columbia Plateau Province, in the northwest corner 

of the western United States (fig. 2A) is characterized by 

extensive basalt formations and is overlain by unconsolidated 

to semiconsolidated sedimentary deposits. The Province 

contains the Columbia Plateau regional aquifer system 

(fig. 8A), which is composed of the Columbia Plateau 

basaltic-rock aquifers overlain by the Columbia Plateau 

basin-fill aquifers (Kahle and others, 2009; fig. 2B). The 

Columbia Plateau basaltic-rock aquifers rank 19th in the 

Nation as a source of groundwater used for public supply 

(Arnold and others, 2020; table 1). The basin-fill aquifers rank 

67th for public supply and were not sampled for this study. 

The Columbia Plateau basaltic-rock aquifers cover an area of 

approximately 42,000 mi2 and extend across small portions of 

northwestern Idaho, northeastern Oregon, and a large part of 

southeastern Washington (Arnold and others, 2020).

The natural land cover of the Columbia Plateau regional 

aquifer system largely determines corresponding land use. 

Except for the forested mountains in the highlands and 

barren rock at mid-elevation, the regional aquifer system 

is predominately shrub and grasslands in which dryland 

agricultural includes winter and spring lentils and wheat 

(Kahle and others, 2011). In 2012, land use overlying the 

regional aquifer system was approximately 76-percent 

natural, 23-percent agricultural, and 1-percent urban 

(Falcone, 2015). In 2012, about 7 percent of the land use 

was irrigated agriculture (U.S. Geological Survey, 2014) 

which predominantly occurred overlying the basin-fill 

aquifers (fig. 8B). Irrigated crops include the Nation’s largest 

production of apples and hops, as well as potatoes, onions, 

mint, and increasingly, wine grapes (Kahle and others, 

2011). Population estimates for the three largest cities in the 

regional aquifer system boundary are Spokane, Washington 

(about 564,000), tri-cities area, Washington (about 290,000), 

and Yakima, Washington (about 250,000; U.S. Census 

Bureau, 2018).

The Columbia Plateau basaltic-rock aquifers lie in a 

structural and topographic basin, the Columbia Plateau, 

which is a wide basalt plateau bounded on the west by the 

Cascade Range, on the east by the Rocky Mountains, and 

on the north by the Okanogan Highlands (Kahle and others, 

2011). The Plateau consists of a series of basalt flows, the 

Columbia River Basalt Group (CRBG), which were extruded 

during the Miocene era between 17 million and 6 million 

years ago (Whiteman and others, 1994). More than 300 flow 

events have been identified, with thicknesses from 10 ft to 

more than 300 ft (Tolan and others, 1989; Drost and others, 

1990). Reidel and others (2002) reported total thickness of the 

CRBG to be greater than 14,000 ft near Pasco, Washington. 

The Columbia Plateau regional aquifer system is divided into 

four informal physiographic sub-provinces that represent 

structural regions—the Yakima Fold Belt, Blue Mountains, 

Palouse Slope, and the Clearwater Embayment (Reidel and 

others, 2002; fig. 8B). Variations in stratigraphy, depositional 

environment, and post-depositional folding and faulting 

result in unique groundwater characteristics and important 

flow barriers associated with each of these structural regions. 

Miocene- to Holocene-aged sedimentary deposits overlie 

large areas of the Columbia Plateau and range in thickness 

from 0 to 1,300 ft, with a median thickness of 47 ft (Kahle 

and others, 2009). These deposits comprise the Columbia 

Plateau basin-fill aquifers, which are unconsolidated to 

semiconsolidated sediments and minor basalt and andesite 

(Drost and others, 1990; Vaccaro, 1999; Kahle and others, 

2009; fig. 8).

Regional groundwater flow is from areas of high altitude 

to surface drainage features in the lowlands—principally the 

Columbia River and its tributaries. Groundwater discharge is 

to surficial features such as rivers, lakes, waterways, and to 

subsurface features such as drains and wells. Groundwater 

flows laterally and vertically in the basaltic-rock aquifers. 

Lateral movement generally is confined to basalt-interflow 

areas, defined as the brecciated areas of basalt flow tops and 

bottoms that commonly have open and highly connected pore 

structures (Whiteman and others, 1994). Basalt-interflow 

zones are separated by low hydraulic conductivity flow 

interiors in which most of the fractures are cooling joints that 

are mostly orientated vertically (Burns and others, 2011), 

thereby enabling a mostly vertical movement of groundwater.

Much of the Columbia Plateau region is semiarid. 

Elevation in the central part of the Plateau ranges from 350 

to 2,000 ft with precipitation ranging from 7 to 15 in/year 

(Kahle and others, 2011). The surrounding forested mountains 

have elevations ranging from 2,000 to 3,500 ft, where annual 

precipitation can exceed 45 inches (Kahle and others, 2011). 

Mean monthly temperatures in the low-lying areas range from 

37 to 86 °F, and from 27 to 54 °F in the higher mountainous 

areas (Vaccaro, 1999).

Methods

This section describes sample collection and 

quality-assurance methods. Quality-assurance results are 

detailed in appendix 1. This section also describes methods 

used in the status and understanding assessments. The status 

assessment methods include descriptions of water-quality 

benchmarks used for evaluating groundwater quality, the 

criteria for selecting constituents for further evaluation, 

and methods used to calculate aquifer-scale proportions for 

selected constituents. The understanding assessment methods 

include a description of the treatment of concentrations below 

method detection limits (MDL) and the statistical methods 

used to identify significant correlations between potential 

explanatory factors and selected water-quality constituents.
Methods 15

Snake River

Columbia River

Yakima

Fold

Belt

Blue

Mountains

Palouse

Slope

Clearwater

Embayment

0 25 50 75 100 MILES

0 25 50 75 100 KILOMETERS

Spokane

Yakima

WASHINGTON

OREGON

IDAHO

Pasco

Kennewick

122° 120° 118°

46°

44°

B

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

Map

area

EXPLANATION

Principal aquifer (U.S. Geological Survey, 2003)

Columbia Plateau basaltic-rock aquifers

Columbia Plateau basin-fill aquifers

Irrigated agriculture (U.S. Geological Survey, 2014)

Columbia Plateau sub-provinces (Kahle and others, 

 2009

Approximated Quincy-Pasco subunit (Quincy-Pasco 

 subunit delineation, Williamson and others, 1998)

Sampled well

Snake River

Columbia River

0 25 50 75 100 MILES

0 25 50 75 100 KILOMETERS

Spokane

Yakima

WASHINGTON

OREGON

IDAHO

122° 120° 118°

46°

44°

A

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

EXPLANATION

Land use, 2012 (Falcone, 2015)

Natural

Urban

Agricultural

Principal aquifer (U.S. Geological Survey, 2003)

Columbia Plateau basaltic-rock aquifers

Columbia Plateau basin-fill aquifers

Sampled well

Map

area

Pasco

Kennewick

Figure 8. A, Columbia Plateau basaltic-rock aquifers, land use, and groundwater wells sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017; and B, sub-provinces of the Columbia Plateaus regional aquifer system.
16 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Sample Collection and Analysis

The NAWQA Project has developed rigorous protocols 

to ensure that the data collected are of known and high quality. 

Water-quality samples are collected by using established 

USGS protocols and procedures to obtain a representative 

sample and to avoid contamination (Koterba and others, 1995). 

The network of principal aquifer studies consists of PSWs 

with some springs in the Basin and Range carbonate-rock 

aquifers that yield water from the part of the aquifers used 

for public drinking-water supply (Burow and Belitz, 2014). 

Hereafter, the discussion of principal aquifer study networks 

described as PSWs also includes these public-supply spring 

samples. Each PA was divided into equal-area grid cells 

(fig. 9); a method that allows for evaluation of a constituent 

concentration at a regional scale (Belitz and others, 2010) and 

is described in more detail in the “Status Assessment” section. 

One well per grid cell was randomly selected for sampling 

from a population of existing PSWs (Burow and Belitz, 2014). 

For some PAs, parts of the region are sparsely populated, and 

PSWs are not equally distributed (specifically the Basin and 

Range basin-fill aquifers, Basin and Range carbonate-rock 

aquifers, Rio Grande aquifer system, and Colorado Plateaus 

aquifers). In these PAs, the study area was delineated by 

placing buffers around the existing PSWs and an equal area 

grid was then defined within the buffered area. Buffer sizes 

varied and ranged from 3.1 miles (mi; 5 kilometers[km]) for 

the Rio Grande aquifer system, to 12.4 mi (20 km; fig. 9). 

Sample sites in the Basin and Range carbonate-rock aquifers 

included a mix of PSWs (total number, n; n=14) and springs 

(n=6) because springs are commonly used for public supply 

in this PA (table 1). Sample sites for the Colorado Plateaus PA 

included two PSWs located outside the extent of the buffered 

area, but within the PA boundary, based on availability of the 

PSWs for sampling (fig. 7).

Water-quality samples were analyzed for most 

constituents at the USGS National Water Quality Laboratory 

(NWQL) in Denver, Colorado. Analytes included major and 

minor ions, nutrients and dissolved organic carbon, pesticide 

compounds, VOCs, radionuclides, selected isotopes and age 

tracers, and perchlorate (in selected PAs). Not all measured 

constituents are described in this report. Constituents 

discussed in this report that were measured at other 

laboratories are radionuclides, tritium (3H), and perchlorate. 

Radionuclides were analyzed by Test America in Richland, 

Washington, for samples collected from the Basin and Range 

basin-fill aquifers, Basin and Range carbonate-rock aquifers, 

and Rio Grande aquifer system. Radionuclide samples from 

the High Plains aquifer, the Colorado Plateaus aquifers, and 

the Columbia Plateau basaltic-rock aquifers were analyzed 

by ALS Environmental in Fort Collins, Colorado. Perchlorate 

samples were analyzed by Weck Laboratories, Inc., in 

Industry, California. Tritium samples were analyzed at the 

USGS Tritium Laboratory in Menlo Park, California, or at the 

University of Miami Tritium Laboratory in Miami, Florida. 

The detection level was 0.1 tritium units (TU) or better at both 

labs (Thatcher and others, 1977).

Data Reporting and Quality-Assurance and 

Quality-Control Methods

Groundwater-quality data and quality-assurance and 

quality-control (QA-QC) results for data collected by the 

NAWQA Project during each year are published in USGS 

Data Series reports. Analytical results for the western PAs are 

publicly available as follows:

• Basin and Range basin-fill aquifers, Arnold and 

others (2016)

• Rio Grande aquifer system, Arnold and others (2017)

• Basin and Range carbonate-rock aquifers and the High 

Plains aquifer (excluding samples collected in Texas), 

Arnold and others (2018)

• High Plains aquifer (samples collected in Texas) and 

the Columbia Plateau basaltic rock aquifers, Arnold 

and others (2020)

• Colorado Plateaus aquifers, as a USGS data release 

(Rosecrans, 2020).

The quality of environmental samples was determined by 

the analysis of blank, replicate, and spike samples collected 

in each PA. The quality-assurance plan for the NAWQA 

Project groundwater samples was derived from previous 

NAWQA studies (Koterba and others, 1995) and the USGS 

National Field Manual (U.S. Geological Survey, variously 

dated). Blanks are collected to determine if water samples 

might become contaminated during sample collection, field 

processing transport, or laboratory analysis (Mueller and 

others, 2015). Sequential replicate samples were collected 

to assess the precision of the water-quality data and to 

evaluate whether the differences between concentrations in 

samples were from differences in groundwater quality or 

from variability resulting from the collection, processing, and 

laboratory analysis of the samples. Spike samples are used to 

indicate the accuracy of reported results. Spike samples are 

prepared by adding a known concentration of target analyte 

to a known volume of water collected from groundwater and 

indicate effects of matrix interference and sample handling on 

recovery of the analyte. Quality-assurance and quality-control 

results for all western principal aquifer study samples except 

for the Colorado Plateaus aquifers, are detailed in Arnold and 

others (2016, 2017, 2018, and 2020). Quality-assurance and 

quality-control results for the Colorado Plateaus aquifers are 

detailed in appendix 1.
Methods 17

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

0 150 300 MILES

0 150 300 KILOMETERS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

Grid cell

Buffered

 cell area

Basin and Range carbonate-

 rock aquifers

Basin and Range basin-fill

 aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon

 and others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-

 rock aquifers

Principal aquifer (U.S. Geological Survey, 2003) Well

Spring

Map area

Figure 9. Equal-area grid and sampled groundwater wells and springs for six western U.S. principal aquifers sampled 

by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
18 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Status Assessment

The status assessment quantifies groundwater quality 

at the aquifer scale relative to drinking water benchmarks. 

This section describes the methods used to (1) describe 

groundwater quality using established drinking-water 

standards, (2) select constituents for further evaluation 

by the understanding assessment, and (3) calculate 

aquifer-scale proportions.

Benchmarks for Evaluating Groundwater Quality 

and Relative Concentrations

To provide context for evaluating the quality of 

untreated groundwater at the depth zone used for public 

supply, groundwater samples are compared to established 

drinking-water standards (health-based and non-health-based). 

The quality of water received by the consumer can 

be different than untreated groundwater because after 

withdrawal, groundwater might be treated before delivery. 

The concentrations of constituents in groundwater samples 

collected in the NAWQA principal aquifer studies are 

compared to three water-quality benchmarks: (1) regulatory 

EPA MCLs (U.S. Environmental Protection Agency, 2018a); 

(2) non-regulatory EPA and non-regulatory USGS HBSLs 

(Norman and others, 2018; U.S. Geological Survey, 2018a); 

and (3) non-regulatory EPA SMCLs (U.S. Environmental 

Protection Agency, 2018b). Maximum contaminent levels 

are enforceable standards that represent the maximum level 

of a contaminant allowed in drinking water used for public 

supply and are established to protect the consumer against 

drinking-water contaminants that have potential human-health 

effects. Health-based screening levels are non-enforceable 

water-quality benchmarks used to evaluate water-quality 

data in a human-health context (Norman and others, 2018). 

Health-based screening levels were developed by the USGS 

and EPA using EPA methodologies for establishing drinking 

water standards and the most recent EPA peer-reviewed, 

publicly available human-health toxicology information 

(Norman and others, 2018). Health-based screening levels 

can be used to prioritize contaminants of concern that could 

warrant further monitoring and provide an early indication 

of contaminant concentrations of potential human-health 

concern in water resources (Toccalino, 2007). Of the more 

than 50 inorganic constituents evaluated for water quality, 

28 had human-health benchmarks, and out of the more than 

300 organic constituents evaluated for water quality, 170 had 

human-health benchmarks. Secondary maximum contaminant 

levels are non-enforceable guidelines that assist public water 

systems in managing their drinking water for aesthetic effects 

(such as taste, odor, or color), technical effects (such as 

corrosivity, scaling, and sedimentation), and cosmetic effects 

(such as tooth or skin discoloration).

Groundwater quality was defined in terms of relative 

concentration (RC), which relates measured concentrations in 

groundwater samples to water-quality benchmarks:

 RelativeConcentration=

sampleconcentraion  _________

benchmarkconcentration (1)

Relative concentrations were classified as high, moderate, 

or low based on the following criteria:

Category

Relative concentration 

for 

inorganic constituents

Relative concentration 

for organic constituents 

and perchlorate

High >1 >1

Moderate >0.5 and ≤1 >0.1 and ≤1

Low ≤0.5 ≤0.1

Organic constituents and perchlorate have a lower 

threshold because they generally are less prevalent and have 

smaller concentrations relative to benchmarks than inorganic 

constituents (Toccalino and others, 2004). Concentrations 

are considered low if they are less than moderate or the 

constituent was not detected. Several inorganic constituents 

have MCL and SMCL benchmarks. For example, fluoride 

has a health-based MCL benchmark of 4 micrograms per liter 

(μg/L) and a SMCL of 2 μg/L; therefore, where applicable, 

inorganic results can be discussed for both health-based 

and non-health-based benchmarks. Relative concentrations 

are discussed throughout this report as high, moderate, or 

low concentrations.
Methods 19

Selection of Constituents for the 

Understanding Assessment

Almost 400 inorganic and organic constituents were 

analyzed for samples from the six western PAs assessed in 

this report. Only a subset, however, are considered herein 

for additional evaluation by application of an understanding 

assessment; the selection criteria were different for inorganic 

and organic constituents. For inorganic constituents, only 

those detected at high or moderate concentrations were 

selected. For organic constituents, the selection criteria 

included constituents with detection frequencies greater 

than or equal to 10 percent. Samples for the special interest 

constituent, perchlorate, were collected in only two PAs―the 

High Plains aquifer and the Rio Grande aquifer system―for 

which the same selection criteria were applied as for organic 

constituents. The selection criteria for an application of an 

understanding assessment identified 21 inorganic constituents, 

7 organic constituents, and perchlorate (table 2). An additional 

28 inorganic and 118 organic constituents were measured at 

detectable concentrations but were not selected for further 

evaluation. Inorganic constituents that were not selected 

include those that do not have benchmarks or that were only 

detected at low concentrations, whereas organic constituents 

not selected include those with detection frequencies less than 

10 percent or those that do not have benchmarks (table 3). 

Most of the organic constituents detected and listed in table 3

were single occurrence detections at low concentrations.

Aquifer-Scale Proportions

The principal aquifer studies included the full extent 

of the PAs described in the “Principal Aquifer Description 

and Settings” section and used an equal-area grid sampling 

approach (Belitz and others, 2010). The one-well-per-cell 

sampling design provides a spatially unbiased estimate of 

the aquifer-scale proportion that is equal to the observed 

detection frequency of a concentration above a given threshold 

or benchmark (Belitz and others, 2010). Thus, aquifer-scale 

proportions allow for estimating the percentage of a PA with 

low, moderate, or high concentrations for selected constituents 

or constituent groups. A constituent could be considered 

more important or notable not because it has a higher 

median concentration or higher concentration but because its 

concentration is high in a larger part of the aquifer (Belitz and 

others, 2010).

Understanding Assessment

The understanding assessment identifies natural and 

human factors affecting groundwater quality and evaluates 

statistically significant correlations—among potential 

explanatory factors and between potential explanatory 

factors and water quality—to provide physical and chemical 

context for groundwater quality. Potential explanatory factors 

considered in this report include depth to bottom of well 

perforation, groundwater age category, land use, aquifer 

lithology, hydrologic conditions, and geochemical conditions. 

Potential explanatory factors are described in more detail in 

subsequent sections.

Assessment Levels

A common data “assessment level” was assigned for all 

data for statistical analysis. Water-quality results from the PAs 

had numerous concentrations reported as below the MDL with 

a ‘<’ remark code (that is, a nondetection). Additionally, some 

constituents are reported as estimated and assigned a remark 

code of ‘E’. A value coded with ‘E’ typically is less than the 

MDL and refers to values that were quantified detections, but 

the concentration has more uncertainty because it approaches 

the lower analytical capability of the instrumentation. 

The ‘E’ coded values reported above the MDL for a given 

constituent were considered at their estimated value and 

ranked for nonparametric statistical tests accordingly; ‘E’ 

coded values reported at less than the MDL were considered 

as nondetections. If multiple MDLs were reported for 

individual constituents with ‘E’ coded values, only ‘E’ coded 

values above the highest reported MDL were considered at 

their estimated value and were ranked accordingly. Except 

for pesticide compounds, the assessment level applied to 

statistical analysis using ranks (nonparametric statistics) 

accounted for nondetections by setting the concentration 

below the lowest MDL before ranking. The assessment level 

applied for pesticide compounds was slightly different and 

based on a method used in other national assessments of 

pesticides (Gilliom and others, 2006). When ranking pesticides 

for a statistical test, all pesticide nondetections were assigned a 

rank below the lowest reported ‘E’ value, and results with ‘E’ 

values and quantified detections were assigned ranks on the 

basis of their reported value.
20 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 2. Benchmark type and value for constituents selected for additional evaluation in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[Inorganic constituents selected for understanding assessment included those with maximum concentrations greater than 0.5 times a benchmark concentration. 

Organic constituents selected if frequency of detection, at any concentration, was greater than or equal to 10 percent. Benchmark type: Regulatory, health-based 

benchmark: MCL, EPA maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening 

level; HHBP-NC, EPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, EPA secondary 

maximum contaminant level. Benchmark units: μg/L, microgram per liter; mg/L, milligram per liter; pCi/L picocurie per liter; ng/L, nanograms per liter. 

Abbreviations: D, detected but not selected for additional evaluation in understanding assessment; D-eval, detected and selected for additional evaluation in 

understanding assessment ―, not detected; na, not analyzed for in respective principal aquifer]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basin-fill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Inorganic constituents with health-based benchmarks

Trace elements

Antimony Naturally 

occuring

MCL 6 μg/L D-eval D D D D D

Arsenic Naturally 

occuring

MCL 10 μg/L D-eval D-eval D-eval D-eval D-eval D-eval

Cobalt Naturally 

occuring

HBSL-NC 2 μg/L D D-eval D D-eval D-eval D-eval

Fluoride Naturally 

occuring

MCL 4 mg/L D-eval D-eval D-eval D-eval D-eval D-eval

Iron Naturally 

occuring

HBSL-NC 4,000 μg/L D D D D-eval D-eval D

Manganese Naturally 

occuring

HBSL-NC 300 μg/L D D-eval D-eval D-eval D-eval D-eval

Molybdenum Naturally 

occuring

HBSL-NC 30 μg/L D D-eval D-eval D-eval D D-eval

Selenium Naturally 

occuring

MCL 50 μg/L D D-eval D D-eval D D

Strontium Naturally 

occuring

HBSL-NC 4,000 μg/L D-eval D-eval D-eval D-eval D-eval D

Thallium Naturally 

occuring

MCL 2 μg/L D-eval D D D D D

Uranium Naturally 

occuring

MCL 30 μg/L D D-eval D-eval D-eval D-eval D-eval

Nutrients

Nitrate, as nitrogen Natural, 

fertilizer, 

sewage

MCL 10 mg/L D D-eval D D-eval D D-eval

Radionuclides

Gross alpha-particle 

activity

Naturally 

occuring

MCL 15 pCi/L D D-eval D-eval D-eval D-eval D-eval

Radium-226 and 

radium-228 

(combined)

Naturally 

occuring

MCL 5 pCi/L D-eval D D-eval D D-eval D

Radon-222 Naturally 

occuring

MCL-proposed 4,000 pCi/L D-eval D-eval D-eval D-eval D-eval D-eval
Methods 21

Table 2. Benchmark type and value for constituents selected for additional evaluation in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Inorganic constituents selected for understanding assessment included those with maximum concentrations greater than 0.5 times a benchmark concentration. 

Organic constituents selected if frequency of detection, at any concentration, was greater than or equal to 10 percent. Benchmark type: Regulatory, health-based 

benchmark: MCL, EPA maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening 

level; HHBP-NC, EPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, EPA secondary 

maximum contaminant level. Benchmark units: μg/L, microgram per liter; mg/L, milligram per liter; pCi/L picocurie per liter; ng/L, nanograms per liter. 

Abbreviations: D, detected but not selected for additional evaluation in understanding assessment; D-eval, detected and selected for additional evaluation in 

understanding assessment ―, not detected; na, not analyzed for in respective principal aquifer]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basin-fill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Inorganic constituents with secondary maximum constaminant level benchmarks

Chloride Naturally 

occuring and 

anthropogenic

SMCL 250 μg/L D-eval D-eval D-eval D-eval D-eval D

Fluoride Naturally 

occuring and 

anthropogenic

SMCL 2 mg/L D D-eval D-eval D-eval D-eval D-eval

Iron Naturally 

occuring and 

anthropogenic

SMCL 300 μg/L D D-eval D-eval D-eval D-eval D-eval

Manganese Naturally 

occuring and 

anthropogenic

SMCL 50 μg/L D-eval D-eval D-eval D-eval D-eval D-eval

Sulfate Naturally 

occuring and 

anthropogenic

SMCL 250 mg/L D-eval D-eval D-eval D-eval D-eval D-eval

Total dissolved solids 

(TDS)

Naturally 

occuring and 

anthropogenic

SMCL 500 mg/L D-eval D-eval D-eval D-eval D-eval D-eval

Organic and special-interest constituents with health-based benchmarks

Pesticides

Atrazine Herbicide MCL 3,000 ng/L ― D ― D-eval ― D-eval

Bromacil Herbicide HBSL-NC 100,000 ng/L ― D D D-eval ― D

Didealkylatrazine Herbicide 

degradate

HHBP-NC 12,000 ng/L D D ― D-eval D ―

Propazine Herbicide HBSL-NC 40,000 ng/L ― D ― D-eval ― D

Propoxur Insecticide HBSL-NC 2,000 ng/L ― D ― D D D-eval

Volatile organic compounds

Chloroform Disinfection 

by-product

MCL1 80 μg/L D-eval D-eval D-eval D-eval D-eval D-eval

Bromodichloromethane Disinfection 

by-product

MCL1 80 μg/L D-eval D D D-eval D D

Constituents of special interest

Perchlorate Natural, 

fertilizer, 

rocket fuel

HBSL-NC 15 μg/L na na D-eval D-eval na na

1EPA MCL for trihalomethanes is the sum of chloroform, bromodichloromethane, dibromochloromethane, and bromoform.
22 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Inorganic constituents with benchmarks

Aluminum Naturally 

occurring

SMCL 200 μg/L D D D D D D

Aluminum Naturally 

occurring

HBSLNC6,000 μg/L D D D D D D

Barium Naturally 

occurring

MCL 2,000 μg/L D D D D D D

Beryllium Naturally 

occurring

MCL 4 μg/L D D D D D D

Boron Naturally 

occurring

HBSLNC6,000 μg/L D D D D D D

Cadmium Naturally 

occurring

MCL 5 μg/L D D D D D D

Chromium Naturally 

occurring

MCL 100 μg/L D D D D D D

Copper Naturally 

occurring

MCL 1,300 μg/L D D D D D D

Gross beta-particle activity Naturally 

occurring

MCL 50 pCi/L D D D D D D

Lead Naturally 

occurring

MCL 15 μg/L D D D D D D

Nickel Naturally 

occurring

HBSLNC100 μg/L D D D D D D

Nitrite, as nitrogen Natural, 

fertilizer, 

sewage

MCL 1 mg/L D D D D D D

Silver Naturally 

occurring

HBSLNC100 μg/L D D D ― D ―

Zinc Naturally 

occurring

HBSLNC2,000 μg/L D D D D D D

Inorganic constituents with no benchmarks

Ammonia, as nitrogen Natural, 

fertilizer, 

sewage

None None mg/L D D D D D D

Bicarbonate Naturally 

occurring

None None mg/L D D D D D D
Methods 23

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Bromide Naturally 

occurring

None None mg/L D D D D D D

Calcium Naturally 

occurring

None None mg/L D D D D D D

Carbonate Naturally 

occurring

None None mg/L D D D D D D

Cobalt Naturally 

occurring

None None μg/L D D D D D D

Dissolved organic carbon Naturally 

occurring

None None mg/L D D D D D D

Lithium Naturally 

occurring

None None μg/L D D D D D D

Magnesium Naturally 

occurring

None None mg/L D D D D D D

Nitrogen, total Natural, 

fertilizer, 

sewage

None None mg/L D D D D D D

Orthophosphate Naturally 

occurring

None None mg/L D D D D D D

Potassium Naturally 

occurring

None None mg/L D D D D D D

Silica Naturally 

occurring

None None mg/L D D D D D D

Sodium Naturally 

occurring

None None mg/L D D D D D D

Vanadium Naturally 

occurring

None None μg/L D D D D D D

Organic constituents with benchmarks

Bromodichloromethane 

(THM)

Disinfection 

by-product

MCL 80 μg/L ― D D ― D D

Bromoform (THM) Disinfection 

by-product

MCL 80 μg/L ― D D D D D

Dibromochloromethane 

(THM)

Disinfection 

by-product

MCL 80 μg/L ― D D D D D

1,2-Dichloropropane 

(1,2-DCP)

Fumigant MCL 5 μg/L ― D ― D D D

1,4-Dichlorobenzene Fumigant MCL 75 μg/L ― D ― ― D D
24 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Bromomethane Fumigant HHBPNC140 μg/L ― ― ― ― D ―

Metalaxyl Fungicide HHBPNC474,000 ng/L ― D ― ― ― ―

Propiconazole Fungicide HHBPNC600,000 ng/L ― D ― ― ― ―

1H-1,2,4-Triazole Fungicide 

degradate

HHBPNC30,000 ng/L ― ― ― D ― ―

Carbendazim Fungicide 

degradate

HHBPC160,000 ng/L ― ― ― ― D ―

2,4-Dichlorophenoxyacetic 

acid (2,4-D)

Herbicide MCL 70,000 ng/L ― D ― ― D ―

Acetochlor Herbicide HHBPNC100,000 ng/L ― ― D ― ― ―

Asulam Herbicide HHBPNC230,000 ng/L ― ― ― D ― ―

Atrazine Herbicide MCL 3,000 ng/L ― D ― ― D ―

Bentazon Herbicide HBSLNC1,000,000 ng/L ― ― ― ― ― D

Bromacil Herbicide HBSLNC100,000 ng/L ― D D ― ― D

Chlorsulfuron Herbicide HHBPNC300,000 ng/L ― ― ― ― ― D

Dimethenamid Herbicide HHBPNC300,000 ng/L ― ― ― D ― ―

Diuron Herbicide HBSL-C 20,000 ng/L ― D ― D D D

Fluometuron Herbicide HBSL-C 40,000 ng/L ― D ― ― D ―

Hexazinone Herbicide HBSLNC300,000 ng/L ― D ― D ― D

Imazaquin Herbicide HHBPNC1,600,000 ng/L ― D ― ― ― ―

Imazethapyr Herbicide HHBPNC16,000,000 ng/L ― D ― ― ― ―

Linuron Herbicide HHBPNC49,000 ng/L ― D ― ― ― ―

Metolachlor Herbicide HBSLNC600,000 ng/L ― D ― D ― ―
Methods 25

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Nicosulfuron Herbicide HHBPNC8,000,000 ng/L ― D ― ― ― ―

Norflurazon Herbicide HHBPNC96,000 ng/L ― D ― D ― D

Pendimethalin Herbicide HHBPNC2,000,000 ng/L ― ― ― D ― ―

Prometon Herbicide HBSLNC300,000 ng/L ― D D D ― ―

Propazine Herbicide HBSLNC40,000 ng/L ― D ― ― ― D

Simazine Herbicide MCL 4,000 ng/L ― D D D ― D

Sulfentrazone Herbicide HHBPNC900,000 ng/L ― ― ― ― ― D

Sulfometuron-methyl Herbicide HHBPNC1,760,000 ng/L ― ― ― ― ― D

Sulfosulfuron Herbicide HHBPNC1,500,000 ng/L ― D ― ― ― ―

Tebuthiuron Herbicide HBSLNC900,000 ng/L ― ― ― D D D

Terbuthylazine Herbicide HBSLNC2,000 ng/L ― D ― ― ― ―

Triclopyr Herbicide HHBPNC300,000 ng/L ― ― ― ― D ―

2-Hydroxyatrazine (OIET) Herbicide 

degradate

HHBPNC60,000 ng/L ― ― ― D ― D

Didealkylatrazine (CAAT) Herbicide 

degradate

HHBPNC12,000 ng/L D D ― ― D ―

Bifenthrin Insecticide HHBPNC70,000 ng/L ― D ― ― ― ―

Carbaryl Insecticide HBSL-C 4,000,000 ng/L D ― ― ― ― ―

Diflubenzuron Insecticide HHBPNC100,000 ng/L ― D ― ― ― ―

Methoxyfenozide Insecticide HHBPNC600,000 ng/L ― ― ― ― ― D

Pyridaben Insecticide HHBPNC30,000 ng/L ― ― D ― ― ―
26 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Tebufenozide Insecticide HHBPNC120,000 ng/L ― ― ― D ― ―

Desulfinylfipronil Insecticide 

degradate

HBSLNC1,000 ng/L ― ― D ― ― ―

Piperonyl butoxide Pesticide 

synergist

HHBPNC992,000 ng/L ― ― ― ― ― ―

1,1,1-Trichloroethane 

(TCA)

Solvent MCL 200 μg/L ― ― D ― D ―

1,1,2-Trichloroethane Solvent MCL 5 μg/L ― ― ― ― D ―

1,2,4-Trichlorobenzene Solvent MCL 70 μg/L ― ― ― ― D ―

1,2-Dichlorobenzene Solvent MCL 600 μg/L ― D ― D D ―

1,2-Dichloroethane Solvent MCL 5 μg/L ― ― ― ― D ―

1,1-Dichloroethane Solvent HBSLNC1,000 μg/L ― D ― ― D ―

1,4-Dioxane Solvent HBSL-C 30 μg/L ― D ― ― ― ―

Carbon tetrachloride 

(tetrachloromethane)

Solvent MCL 5 μg/L ― ― ― D D D

Chlorobenzene Solvent MCL 100 μg/L ― D ― ― D ―

cis-1,2-Dichloroethylene Solvent MCL 70 μg/L ― D ― D ― ―

Dichloromethane 

(methylene chloride)

Solvent MCL 5 μg/L ― D ― D D ―

Tetrachloroethylene (PCE) Solvent MCL 5 μg/L ― D D D D D

trans-1,2-Dichloroethylene Solvent MCL 100 μg/L ― ― ― ― D ―

Trichloroethylene (TCE) Solvent MCL 5 μg/L ― D D D D D

Isopropyl alcohol Solvent HBSLNC10,000 μg/L ― D D D D D

1,1-Dichloroethylene Solvent, 

organic 

synthesis

MCL 7 μg/L ― ― ― D D ―

Cyclohexanone Solvent, 

organic 

synthesis

HBSLNC30,000 μg/L ― ― ― D D ―

Carbon disulfide Natural, 

organic 

synthesis

HBSLNC600 μg/L ― D D D D D
Methods 27

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Bromochloromethane Organic 

synthesis, 

fire retardant

HBSLNC60 μg/L ― ― ― ― D ―

1,2,3-Trichloropropane 

(1,2,3-TCP)

Organic 

synthesis, 

former 

fumigant

HBSLNC30 μg/L ― D ― ― ― D

1,2,4-Trimethylbenzene Gasoline 

hydrocarbon

HBSLNC60 μg/L ― D D D ― D

Benzene Gasoline 

hydrocarbon

MCL 5 μg/L ― D ― ― D ―

Ethylbenzene Gasoline 

hydrocarbon

MCL 700 μg/L ― ― ― D ― ―

m- and p-Xylene Gasoline 

hydrocarbon

MCL 10,000 μg/L ― ― ― D ― ―

o-Xylene Gasoline 

hydrocarbon

MCL 10,000 μg/L ― ― ― ― D ―

Toluene Gasoline 

hydrocarbon

MCL 1,000 μg/L ― D ― D D D

Organic constituents with no benchmarks

N-(3,4-Dichlorophenyl)-N

’-methylurea (DCPMU)

Herbicide 

degradate

None None ng/L ― D ― D D ―

4-Hydroxychlorothalonil Fungicide 

degradate

None None ng/L ― ― ― D ― D

2-Isopropyl-6-methyl4-pyrimidinolInsecticide 

degradate

None None ng/L ― D ― D D ―

Acetochlor oxanilic acid Herbicide 

degradate

None None ng/L ― ― ― D ― ―

Alachlor oxanilic acid Herbicide 

degradate

None None ng/L ― ― ― ― ― D

Deisopropylatrazine 

(CEAT)

Herbicide 

degradate

None None ng/L ― D D D ― ―

Deethylatrazine (CIAT) Herbicide 

degradate

None None ng/L ― D D D ― D

Dechlorometolachlor Herbicide 

degradate

None None ng/L ― ― ― D ― ―

Demethyl hexazinone B Herbicide 

degradate

None None ng/L ― D ― D ― ―
28 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Demethyl norflurazon Herbicide 

degradate

None None ng/L ― ― ― ― ― D

Desamino metribuzin Herbicide 

degradate

None None ng/L ― ― ― ― ― D

Desamino-diketo 

metribuzin

Herbicide 

degradate

None None ng/L ― ― ― ― ― D

Desulfinylfipronil amide Insecticide 

degradate

None None ng/L ― ― D ― ― ―

Didemethyl hexazinone F Herbicide 

degradate

None None ng/L ― ― D ― ― ―

Diketonitrile-isoxaflutole Herbicide 

degradate

None None ng/L ― ― D ― ― ―

Dimethenamid sulfonic 

acid

Herbicide 

degradate

None None ng/L ― ― ― D ― ―

Dimethenamid 

sulfinylacetic acid

Herbicide 

degradate

None None ng/L ― D D ― ― ―

Disulfoton sulfone Insecticide 

degradate

None None ng/L ― D ― ― ― ―

Demethyl fluometuron 

(DMFM)

Herbicide 

degradate

None None ng/L ― D ― D ― ―

Fenamiphos sulfone Insecticide 

degradate

None None ng/L ― ― ― ― D ―

Fipronil amide Insecticide 

degradate

None None ng/L ― ― ― D ― ―

2-(1-Hydroxyethyl)-6-met

hylaniline

Herbicide 

degradate

None None ng/L ― D ― ― ― ―

Hydroxyacetochlor Herbicide 

degradate

None None ng/L ― ― ― D ― ―

Hydroxyalachlor Herbicide 

degradate

None None ng/L ― ― ― D ― ―

Hydroxy monodemethyl 

fluometuron

Herbicide 

degradate

None None ng/L ― ― D ― ― ―

Hydroxymetolachlor Herbicide 

degradate

None None ng/L ― D ― D ― ―

Metolachlor oxanilic acid Herbicide 

degradate

None None ng/L ― ― ― D ― ―
Methods 29

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Metolachlor sulfonic acid Herbicide 

degradate

None None ng/L ― ― D D D D

Deethylhydroxyatrazine 

(OIAT)

Herbicide 

degradate

None None ng/L ― D ― D ― D

Tebuthiuron 

Transformation Product 

109

Herbicide 

degradate

None None ng/L ― ― D ― ― ―

Alachlor sulfonic acid Herbicide 

degradate

None None ng/L ― ― ― D ― D

Chloromethane 

(Methylchloride)

Solvent, 

organic 

synthesis

None None μg/L ― ― ― ― D ―

Chlorodifluoromethane 

(HCFC-22)

Refrigerant 

and 

propellant

None None μg/L ― D ― D ― D

Acetonitrile Solvent, 

organic 

synthesis

None None μg/L ― D ― ― D ―

tert-Butyl alcohol (TBA) Gasoline 

oxygenate 

and 

degradate

None None μg/L ― D D D D ―

n-Pentanal Flavoring 

agent

None None μg/L ― ― ― ― D ―

Chloropicrin Fumigant None None μg/L ― ― ― D ― ―

Methyl tert-butyl ether 

(MTBE)

Gasoline 

oxygenate 

and 

degradate

None None μg/L ― D ― D D D

1,1-Dichloro-2-propanone Disinfection 

by-product

None None μg/L ― ― ― ― D ―

Butane Fuel, 

refrigerant 

and 

propellant

None None μg/L ― D ― ― D ―

Dimethoxymethane Solvent, 

organic 

synthesis

None None μg/L ― D ― D ― ―
30 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 3. Constituents detected in samples but not selected for the status assessment in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.—Continued

[Detected constituents were not selected for understanding assessment if they had no benchmarks, or for inorganic constituents if the maximum concentration 

measured was less than 0.5 times benchmark concentration, or for organic constituent if the detection frequecy at any concentration was less than 10 percent and 

(or) maximum concentration measured was less than 0.1 times benchmark concentration. Benchmark type: Regulatory, health-based benchmark: MCL, USEPA 

maximum contaminant level; Non-regulatory health-based benchmarks: HBSL-NC, USGS noncancer health-based screening level; HBSL-C, USGS cancer 

health-based screeing level, HHBP-NC, USEPA Chronic noncancer human health benchmarks for pesticides; Non-regulatory, aesthetic-based benchmarks: SMCL, 

USEPA secondary maximum contaminant level. Benchmark units: μg/L, microgram per liter; pCi/L picocurie per liter; mg/L, milligram per liter; ng/L, nanograms 

per liter. Abbreviations: D, detected but not selected for understanding assessment; ―, not detected]

Constituent Typical use or 

source

Benchmarks Principal aquifer

Type Value Units

Basin and 

Range 

carbonaterock 

aquifers

Basin 

and 

Range 

basinfill 

aquifers

Rio 

Grande 

aquifer 

system

High 

Plains 

aquifer

Colorado 

Plateaus 

aquifers

Columbia 

Plateau 

basalticrock 

aquifers

Hexane Solvent None None μg/L ― ― ― ― D ―

n-Pentane Blowing agent, 

solvent

None None μg/L ― D ― D D D

1-Chloro-1,1-

difluoroethane

Refrigerant 

and 

propellant

None None μg/L ― ― ― ― D ―

Statistical Tests of Correlation

Because water-quality data generally are not normally 

distributed (Helsel and Hirsch, 2002), nonparametric statistical 

methods were used to identify significant correlations. For 

all results discussed in this report, the significance level (p) 

was compared to an alpha (α) value of 0.05 to determine if 

a relation was statistically significant (p<α); p values below 

0.05 are considered significant and only those correlations 

are reported herein. Explanatory factors considered included 

categorical (for example, groundwater age category) and 

discrete variables (for example, depth to bottom of well 

perforation, pH, aridity index). In this report, we define 

discrete variables from the perspective of sample collection, 

that is, the value used for the statistical test sample was 

collected at a discrete time (or depth) rather than the statistical 

perspective that defines these types of measurements as 

continuous for the respective statistical testing methods 

applied. The two nonparametric statistical tests used for the 

categorical and discrete samples were:

• Spearman’s rho, which was used to evaluate 

correlations between discrete variables. The rank-order 

coefficient, ρ (rho), was reported if correlations were 

statistically significant.

• The Kruskal-Wallace test and the Wilcoxon rank-sum 

test were used to detect differences among groups 

of data. The Wilcoxon-rank sum is equivalent to the 

Kruskal-Wallace when there are only two groups of 

data. The null hypothesis for both tests is that the 

median values of the continuous variables grouped 

by the categorical variables are not different from one 

another. If a Kruskal-Wallace test indicated statistical 

significance among groups, the nonparametric Tukey’s 

test was used to determine the ordering of groups. 

Consistent with the nonparametric approach, the 

Tukey’s test was performed based on ranks; results are 

based on the group means of ranks.

Potential Explanatory Factors

Potential explanatory factors (depth to bottom of well 

perforation, groundwater age category, land use, aquifer 

lithology, hydrologic conditions, and geochemical conditions) 

assigned to wells or springs in each of the six PAs are 

described in this section. Significant correlations among 

these explanatory factors that might affect relations with 

water-quality constituents also are described. As noted in 

the previous section, potential explanatory factors are either 

categorical (groundwater age category) or discrete, where a 

value is assigned for each groundwater sample or sample site 

(depth to bottom of well perforation, land use, hydrologic 

conditions, and geochemical conditions). Aquifer lithology, 

although a categorical variable, is assumed to be consistent for 

all sites in a PA.
Potential Explanatory Factors 31

Depth to Bottom of Well Perforation

Available construction information for sampled wells 

generally included depth to bottom of well perforation (that 

is, well screen opening). For instances where the depth to 

bottom of well perforation was not available, well depth was 

substituted. Median well-bottom perforation depths for the six 

PAs ranged from 248 feet below land surface (ft bls) for the 

High Plains aquifer to 595 ft bls for the Rio Grande aquifer 

system (fig. 10). Well-bottom perforation depths for the High 

Plains aquifer were notably shallower than the other PAs and 

primarily reflected the depth of the Ogallala Formation, a 

near-surface geologic unit that composes a large part of the 

High Plains aquifer. Median well-bottom perforation depths 

also are relatively deep (more than 500 ft bls in the Basin and 

Range carbonate-rock aquifers, Basin and Range basin-fill 

aquifers, and Colorado Plateaus aquifers).

Groundwater Age Category

Groundwater age refers to the residence time of 

groundwater in an aquifer, measured as the time elapsed since 

the water was in contact with the atmosphere and recharged 

the aquifer. Groundwater samples were classified as modern, 

premodern, or mixed based on measured 3H concentrations 

(Lyndsey and others, 2019). Atmospheric concentrations of 

3H increased sharply above natural background levels from 

above-ground nuclear weapon testing in the middle part of 

the 20th century. Groundwater age categories were based on 

curves of 3H in local precipitation (Michel and others, 2018) 

for samples predominantly recharged after 1953 (modern), 

prior to 1953 (premodern), or a mixture of modern and 

premodern groundwater (mixed). Available groundwater 

age categories for the six western PAs are detailed in table 4

and figure 11 (relative to available depth to bottom of 

well perforation).

Deeper wells in the western PAs generally are associated 

with older, premodern groundwater (fig. 11). The Basin 

and Range basin-fill aquifers, Rio Grande aquifer system, 

and Colorado Plateaus aquifers had the largest proportions 

(greater than 50 percent) of wells classified as premodern 

(table 4). Groundwater wells classified as mixed were highest 

in the Columbia Plateau basaltic-rock aquifers (50 percent) 

and the High Plains aquifer (39 percent). Groundwater wells 

classified as modern were associated with the shallowest 

depths to bottom of well perforation for all PAs (and included 

all spring samples from the Basin and Range carbonate-rock 

aquifers). Although springs sampled in the Basin and Range 

carbonate-rock aquifers have an assigned depth of zero for the 

bottom of well perforation (fig. 3), this might not adequately 

reflect complexities of the carbonate rocks where water is 

transmitted along faults and fractures.

14

78 60 80 51 56

Basin and

Range

carbonate-rock

aquifers

Basin and

Range

basin-fill

aquifers

Rio Grande

aquifer

system

High

Plains

aquifer

Colorado

Plateaus

aquifers

Columbia Plateau

basaltic-rock

aquifers

0

1,000

2,000

3,000

4,000

Depth to bottom of well perforation, in feet below land

surface (North American Vertical Datum of 1988)

45 Number of values

75th percentile

50th percentile (median)

25th percentile

Interquartile

range

Largest value within 1.5 times interquartile 

range above 75th percentile

Smallest value within 1.5 times interquartile 

range below 25th percentile

Outside value—Value is greater than 1.5 and

 less than 3 times the interquartile range

 beyond either end of the box. No values 

 equal to or greater than 3 times the

 interquartile range were measured.

EXPLANATION

Spring site and number of values 6

6

Figure 10. Depth to bottom of well perforation by principal aquifer in wells sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017.
32 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 4. Groundwater age category by principal aquifer in wells and springs sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017.

Principal aquifer

Number of 

sample sites 

in network1

Number of 

samples 

with age 

category1

Groundwater age category

Number Percent

Modern Mixed Premodern Modern Mixed Premodern

Basin and Range carbonate-rock aquifers 20 19 9 3 7 47 16 37

Basin and Range basin-fill aquifers 78 77 16 21 40 21 27 52

Rio Grande aquifer system 60 60 13 8 39 22 13 65

High Plains aquifer 80 80 21 31 28 26 39 35

Colorado Plateaus aquifers 60 58 16 11 31 28 19 53

Columbia Plateau basaltic-rock aquifers 60 60 8 30 22 13 50 37

1Differences between number of samples in network and those with an age category assigned reflect samples for which tritium was not sampled for or analyzed.

4 3 7

16 21 40

13

8 39

21 31 28 11 9 29 7 28 21

Basin and Range

carbonate-rock

aquifers

Basin and Range

basin-fill

aquifers

Rio Grande

aquifer

system

High Plains

aquifer

Colorado

Plateaus

aquifers

Columbia Plateau

basaltic-rock

aquifers

0

1,000

2,000

3,000

4,000

Depth to bottom of well perforation, in feet below land

surface (North American Vertical Datum of 1988)

5 Modern Mixed Premodern

16 Number of values

75th percentile

50th percentile

(median)

25th percentile

Interquartile

range

Largest value within 1.5 times 

 interquartile range above 

 75th percentile

Smallest value within 1.5 times 

 interquartile range below 

 25th percentile

Outside value—Value is greater than 

 than 1.5 and less than 3 times the 

 interquartile range beyond either 

 end of the box. No values equal to 

 or greater than 3 times the 

 interquartile range were measured. 

Spring site and number of

 values

5

Groundwater age

EXPLANATION

Figure 11. Depth to bottom of well perforation by available groundwater age category for six western U.S. principal aquifers sampled 

by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

Land Use

Land use was quantified as the percentage of three land 

covers—natural, urban, and agricultural—calculated for areas 

(1) within a radius of 500 m (500-m buffer) around each 

sample site for each PA (fig. 12A) and (2) within the extent of 

each PA boundary (fig. 12B). Land-use values are based on 

the conterminous U.S. wall-to-wall anthropogenic land-use 

trends (NWALT), 1974–2012 (Falcone, 2015) and might 

differ slightly from other PA summaries (for example, Arnold 

and others, 2016, 2017, 2018, 2020). NWALT classifications 

were grouped into natural (water, low use, very low use, and 

natural resource-production classes that included mining/

extraction and timber and forest activities), urban (developed 

and semi-developed), and agricultural (production of crops, 

pasture/hay, and grazing potential) categories. The fourth 

land-use category, mixed, was assigned to sites where there 

was not a dominant category (that is, land-use percentages 

within the 500-m buffer were less than 50 percent for each of 

the natural, urban, or agricultural categories).
Potential Explanatory Factors 33

98

1

92

4 4

91

4 5

58

1

41

76

1

23

97

1

2

Basin and Range

carbonate-rock

aquifers

Colorado Plateaus

aquifers

Basin and Range

basin-fill

aquifers

Columbia Plateau

aquifers

High Plains

aquifer

RioGrande

aquifer

system

B Percent land use (2012) within

principal aquifer boundary

1

80

5

15

65

20

15

92

3 5

85

10

5

58

28

13

1

42

46

9 3

22

38

32

8

20

40

32

8

43

29

23

5

40

35

20

5

62

30

7

1

48

42

7 3

Percent land use (1974)

within 500-meter buffer

Basin and Range

carbonate-rock

aquifers

Colorado Plateaus

aquifers

Basin and Range

basin-fill

aquifers

Columbia Plateau

aquifers

High Plains

aquifer

RioGrande

aquifer

system

Percent land use (2012)

within 500-meter buffer

A

EXPLANATION

Natural

Agricultural

Urban

Mixed

EXPLANATION

Natural

Agricultural

Urban

Figure 12. A, Comparison of the 1974 percentage of land use to the 2012 percentage of land use calculated within the 500-m buffer 

around sampled wells and springs; and B, the 2012 percentage of land use calculated within the principal aquifer boundary of six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017. Note 

that the boundary for the Columbia Plateau aquifer system includes the area of the Columbia Plateau basaltic-rock aquifers and the 

Columbia Plateau basin-fill aquifers.
34 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Available land-use data from 1974 and 2012 indicated 

that some land-use changes occurred during this period 

(fig. 12), with an overall decrease in natural land use 

accompanied by an increase in urban land use. Decreases 

in natural land use within the 500-m buffer ranged from 

16 percent in the Basin and Range basin-fill aquifers to 

2 percent in the Columbia Plateau basaltic-rock aquifers. 

Corresponding increases in urban land use ranged from 

18 percent in the Basin and Range basin-fill aquifer to 

2 percent in the Columbia Plateau basaltic-rock aquifers. 

Agricultural land use within the 500-m buffer during the about 

40-year period remained unchanged in most PAs, with less 

than a 5 percent decrease in two PAs, the Basin and Range 

basin-fill aquifers and the High Plains aquifer.

There are notable differences between the 2012 land use 

within the 500-m buffer zone (fig. 12A) and the overall land 

use for each PA (fig. 12B). Urban land use was consistently 

higher for the 500-m buffer zone estimation, which likely 

reflects the spatial distribution of PSWs in populated areas, 

resulting in a bias toward urban land use. Urban land use 

for each PA overall generally was low, with the highest 

value (4 percent) occurring in the Basin and Range basin-fill 

aquifers and the Rio Grande aquifer system. Agricultural land 

use was highest for the High Plains aquifer (41 percent) and 

Columbia Plateau basaltic-rock aquifers (23 percent) and 

was 4 percent or less in the other PAs. For the understanding 

assessment, only land use within the 500-m buffer zone was 

considered as a potential explanatory factor.

Irrigated land use, associated with irrigated agriculture, 

also was considered as a potential explanatory factor. The 

Moderate Resolution Imaging Spectrodiometer (MODIS) 

Irrigated Agriculture Dataset for the United States 

(MIrAD-US; U.S. Geological Survey, 2014) was used to 

quantify the percentage of irrigated land for (1) individual 

500-m well buffer zones and (2) the extent of each PA 

(fig. 13A). Differences in the proportion of irrigated land 

use between the 500-m buffer zone and the overall land use 

for each PA likely reflect the bias of PSWs generally being 

in populated (that is, urban land-use) areas (fig. 13B). For 

example, the High Plains aquifer had the largest percentage 

of irrigated land use by PA (12 percent), whereas the 

estimate for the 500-m buffer zone was only 7 percent. For 

the understanding assessment, only irrigated land use for 

the 500-m well buffer zone was considered as a potential 

explanatory factor.

Aquifer Lithology

The six western PAs are composed of four defined 

lithology classes (U.S. Geological Survey, 2003): carbonate 

rock, unconsolidated to semiconsolidated sand and gravel, 

sandstone, and igneous and metamorphic rock (table 1; 

fig. 2B). Groundwater samples were dominantly collected 

from unconsolidated sand and gravel (61 percent), with less 

from sandstone (17 percent), igneous and metamorphic rocks 

(16 percent), and carbonate rocks (6 percent, including the six 

spring samples).

Hydrologic Conditions

Hydrologic conditions were represented by two 

potential explanatory factors: the land-surface elevation of 

each sample site and a calculated aridity index (fig. 14A). 

Land-surface elevation (elevation) is considered a proxy for 

the relative hydrologic position within a PA flow system, 

with higher elevation generally associated with distal 

boundaries of a flow system. The aridity index is calculated 

by dividing average annual evapotranspiration by average 

annual precipitation; values were extracted for each sample 

site from the Consortium for Spatial Information Global 

Aridity Index dataset (Trabucco and Zomer, 2019). The 

aridity index is considered an indicator of climate and was 

grouped into climate classes (arid, semiarid, dry subhumid, 

and humid, fig. 14B) using the generalized classification 

scheme for global-aridity values (United Nations Environment 

Programme, 1997). Differences in the climate classifications 

can be attributed to well elevation, variations in north-to-south 

trending regional evapotranspiration rates, and west-to-east 

trending precipitation rates. For example, samples from the 

Columbia Plateau basaltic-rock aquifers followed a general 

trend of increasing aridity index (from arid to humid) with 

increasing elevation, except for two wells. These two wells 

are along the westernmost edge of the PA and coincide 

with the western slopes of the Pacific Northwest Cascade 

Range; this area likely receives relatively high precipitation 

compared to the central and eastern parts of the PA. Sample 

sites for the three PAs in the Basin and Range Province 

(fig. 2A)—the Basin and Range carbonate-rock and basin-fill 

aquifers and the Rio Grande aquifer system—are semiarid 

to arid with aridity generally increasing from the northeast 

to southwest (fig. 14B). In the High Plains aquifer, aridity 

generally decreases from west to east, from semiarid to dry 

subhumid, consistent with a west to east increase in mean 

annual precipitation from about 12 to 33 inches (McMahon 

and others, 2007). The climate of the High Plains aquifer is 

unique among the studied PAs in that it does not follow the 

generalized association of higher precipitation occurring at 

higher land-surface elevation or latitude. Elevation across 

the High Plains aquifer generally decreases from west to 

east from about 8,000 ft at the northwest boundary to about 

1,000 ft at the eastern boundary (McMahon and others, 

2007); this elevation gradient results in the highest amounts 

of precipitation falling on the lower elevations of the PA. The 

climate of the Colorado Plateaus aquifers generally is arid to 

semiarid, with dry subhumid conditions on the eastern and 

western boundaries (fig. 14B).
Potential Explanatory Factors 35

EXPLANATION

Irrigated agriculture Not irrigated

B

0.1

99.9

Basin and Range

carbonate-rock

aquifers

7

93

5

95

5

95

7

93

3

97

7

93

3

97

Basin and Range

basin-fill aquifers

8

92

RioGrande

aquifer system

12

88

High Plains

aquifer

1

99

Colorado Plateaus

aquifers

7

93

Columbia Plateau

aquifers

Percent irrigated land

use within a principal

aquifer boundary

Percent irrigated land

use within 500-meter

buffer

Seattle

Spokane Yakima

Kennewick

Portland

Salt Lake

City

Denver

Grand

Junction

Las Vegas

Flagstaff

Los

Angeles Oklahoma City Albuquerque

Amarillo Phoenix

Lubbock

Dallas Tucson

Midland El Paso

San

Antonio

120° 115° 110° 105° 100°

45°

40°

35°

30°

A

0 150 300 450 600 MILES

0 150 300 450 600 KILOMETERS

EXPLANATION

Irrigated land

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and others, 2007)

Colorado Plateaus aquifers

Colombia Plateau aquifers

Groundwater wells

Groundwater springs

Base modified from U.S. Geological Survey and other Federal and State digital data, various scales; Albers 

Equal-Area Conic projection, standard parallels are 29°30' N. and 45°30' N.; North American Datum of 1983

Map area

Figure 13. A, Irrigated land use for six western U.S. principal aquifers; and B, comparison of the 2012 percentage of irrigated land use 

calculated within the 500-m buffer around sampled wells and springs and within the principal aquifer boundary.
36 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Seattle

Spokane

Yakima

Kennewick

Portland

Salt Lake

City

Denver Grand

Junction

Las Vegas

Flagstaff

Los

Angeles

Oklahoma Albuquerque City

Phoenix Amarillo

Lubbock

Dallas

Tucson

Midland El Paso

San

Antonio

120° 115° 110° 105°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital data, 

various scales; Albers Equal-Area Conic projection, standard parallels are 29°30' N. 

and 45°30' N.; North American Datum of 1983

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and others, 2007)

Colorado Plateaus aquifers

Colombia Plateau aquifers

Well climate class

Arid

Semiarid

Dry subhumid

Humid

B

0.0

0.3

0.6

0.9

0.0

0.3

0.6

0.9

Aridity index Aridity index

0 2,500 5,000 7,500 0 2,500 5,000 7,500 0 2,500 5,000 7,500

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Elevation of sample site, in feet above North American Vertical Datum of 1988

Climate class

Site type

Well Spring

Arid

Semiarid

Dry subhumid

Humid

EXPLANATION

A

0 150 300 450 600 MILES

0 150 300 450 600 KILOMETERS

Map area

Figure 14. A, Relations between elevation and aridity index at well and spring sites; and B, regional climate class values mapped for 

six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Potential Explanatory Factors 37

Geochemical Conditions

Geochemical conditions in groundwater result from a 

broad combination of influences: flow through an aquifer 

and reaction with surrounding minerals, soil, and bedrock; 

groundwater age, land use, aquifer lithology, and hydrologic 

conditions. Geochemical conditions can strongly affect the 

mobility and occurrence of many constituents, including 

those with human-health benchmarks. Knowledge of 

geochemical conditions in an aquifer can provide insight 

into concentrations of constituents. Several expressions 

of geochemical conditions were considered as potential 

explanatory factors: oxidation-reduction (redox), pH, 

bicarbonate (HCO3) concentration, and total dissolved solids 

(TDS) concentration.

Groundwater redox and pH are often primary controls 

on contaminant fate in an aquifer, affecting the degree of 

natural attenuation through chemical transformation or 

sorption (Appelo and Postma, 1999; McMahon and Chapelle, 

2008; McMahon and others, 2009). Redox conditions were 

assessed based on the concentration of dissolved oxygen: 

A concentration greater than or equal to 0.5 milligrams per 

liter (mg/L) was considered oxic and a concentration less 

than 0.5 mg/L was considered anoxic. Oxic groundwater 

conditions were prevalent across all PAs (81 percent of 

samples), regardless of well depth (fig. 15A). Groundwater pH 

is affected by the rates and types of rock weathering processes, 

which can vary across aquifer lithologies (DeSimone and 

others, 2015). Values of pH ranged from 6.2 to 9.8 (fig. 15B). 

The Colorado Plateaus aquifers had the largest range of pH 

values (6.2–9.8), whereas the Basin and Range carbonate-rock 

aquifers had the smallest range (6.7–7.6). Some samples in 

most of the PAs had relatively high pH values (greater than 

8.5), excluding the Basin and Range carbonate-rock aquifers. 

The narrow range in the Basin and Range carbonate-rock 

aquifers likely reflects the carbonate-rock lithology, which 

buffers pH, resulting in near-neutral conditions. Relatively low 

pH values (less than 6.5) were measured in only two PAs, the 

Basin and Range basin-fill aquifers and the Colorado Plateaus 

aquifers. Values of pH have a recommended (SMCL) range of 

6.5 to 8.5 (U.S Environmental Protection Agency, 2018b).

Alkalinity is a measure of the capacity of water to 

neutralize acid. In most groundwater, alkalinity is dominantly 

produced by concentrations of carbon dioxide species, 

carbonate (CO3) and HCO3 and is specifically dominated 

by HCO3 at the measured pH range in western PAs (Hem, 

1985). Bicarbonate concentration, as a measure of alkalinity, 

is considered herein as a potential explanatory factor. There is 

no benchmark for alkalinity or HCO3 (table 3). Bicarbonate 

concentrations for the six western PAs are considered 

relative to calcium concentrations and pH and categorized 

by groundwater age (fig. 16). Variations in groundwater 

alkalinity and HCO3 are often related to weathering reactions 

and changes in pH. Groundwater in carbonate-rock lithologies 

and unconsolidated aquifers with carbonate sediments 

typically have near-neutral pH or alkaline conditions (pH 

greater than 7) because of the buffering capacity of carbonate 

minerals (DeSimone and others, 2015). The weathering of 

silicate minerals, a major component in source rock of the 

unconsolidated sand and gravel aquifers in western U.S., also 

consumes acidity, but silicate weathering occurs much slower 

than carbonate dissolution (DeSimone and others, 2015); as 

a result, groundwater in unconsolidated sand and gravel or 

volcanic-rock lithologies in the western United States with 

long residence times (that is, a premodern age category) tends 

to be alkaline.

Total dissolved solids, which is a measure of groundwater 

salinity, also was considered as a potential explanatory factor 

because of (1) its prevalence at elevated concentrations in 

the PAs of the arid west (Dennehy and others, 2007; Thiros 

and others, 2010; Bexfield and others, 2011; DeSimone 

and others, 2015) and (2) numerous mechanisms that can 

result in elevated TDS concentrations in groundwater. 

All groundwater naturally contains dissolved constituents 

resulting from the weathering and dissolution of minerals 

in rocks and sediments. Concentrations of TDS can be high 

because of natural factors or because of human activities. 

Natural sources of elevated TDS include mineral dissolution 

and evaporative concentration in shallow groundwater, or 

alternatively can result from prolonged water-rock interaction 

in old groundwater. Human activities, such as applications 

to the land surface of road salt, fertilizers, or other chemicals 

in urban and agricultural areas can result in elevated TDS 

concentrations (Hem, 1985). Human alteration of groundwater 

flow systems and irrigation water are two factors that 

contribute to elevated TDS concentrations (Anning and others, 

2007; Bexfield and others, 2011; DeSimone and others, 2015). 

Irrigation water in arid climates has the potential to evaporate, 

thereby increasing salinity; dissolved salts in irrigation 

water are left behind when water evaporates or is taken up 

by plants, and percolation of excess irrigation water to the 

water tables carries these salts to groundwater (DeSimone 

and others, 2015). Correlation between TDS and other 

potential explanatory factors can provide insight into factors 

contributing to relative high TDS at the PA scale.
38 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

7 8 9 10 6 7 8 9 10 6 7 8 9

pH, in standard units

Depth to bottom of well perforation, in feet below land

surface (North American Vertical Datum of 1988)

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

6 7 8 9 10 7 8 9 10 6 7 8 9

6 10

6 10

Basin and Range

carbonate-rock aquifer

Basin and Range

basin-fill aquifer

Rio Grande

aquifer system

B

0

1,000

2,000

3,000

4,000

0

1,000

2,000

3,000

4,000

EXPLANATION

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

pH concentration, in standard units

<6.8 (Low)

6.8 to 8.5 (Moderate)

>8.5 (High)

0

1,000

2,000

3,000

4,000

Depth to bottom of well perforation, in feet below land

surface (North American Vertical Datum of 1988)

0 5 10

0 5 10 0 5 10 0 5 10

0 5 10 0 5 10

Dissolved oxygen, in milligrams per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifer

Basin and Range

basin-fill aquifer

Rio Grande

aquifer system

A

0

1,000

2,000

3,000

4,000

EXPLANATION

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

Redox category

Anoxic

Oxic

Figure 15. Relations for six western U.S. principal aquifers of A, dissolved oxygen concentration with depth to bottom of well 

perforation and aquifer lithology; and B, pH values with depth to bottom of well perforation and aquifer lithology sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Potential Explanatory Factors 39

10

9

8

7

6

Bicarbonate, in milligrams per liter

pH, standard units

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0 200 400 600 0 200 400 600 0 200 400 600

10

9

8

7

6

0 200 400 600 0 200 400 600 0 200 400 600

B

Bicarbonate, in milligrams per liter

Calcium, in milligrams per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

200

100

0

300

0 200 400 600 0 200 400 600 0 200 400 600

0 200 400 600 0 200 400 600 0 200 400 600

0

100

200

300

A

EXPLANATION

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

EXPLANATION

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

Figure 16. Relations for six western U.S. principal aquifers of bicarbonate concentration with A, calcium concentration, aquifer 

lithology and groundwater age category; and B, pH, aquifer lithology, and groundwater age category in springs and wells sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
40 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

The SMCL for TDS is 500 mg/L (U.S Environmental 

Protection Agency, 2018b). Concentrations above 1,500 to 

2,600 mg/L can be problematic for irrigation of crops with low 

median salt tolerance (Ayers and Westcott, 1994; Maas and 

Grattan, 1999; Grieve and others, 2012). High TDS (exceeding 

the SMCL) results were relatively common in western PAs, 

with most exceedances being in PAs with unconsolidated 

sand and gravel or sandstone lithologies (figs. 2B, 17). High 

aquifer-scale proportions occurred in about a third of samples 

from the Basin and Range basin-fill aquifers (32 percent), the 

Rio Grande aquifer system (35 percent), and the Colorado 

Plateaus aquifers (37 percent); about a quarter of samples from 

the High Plains aquifer (26 percent) and the Basin and Range 

carbonate-rock quarter (25 percent) also had high aquifer-scale 

proporitons (tables 9A–E). Concentrations of TDS above 

the 500 mg/L SMCL benchmark were generally observed at 

shallower depths (that is, depth to bottom of well perforation) 

in the Basin and Range basin-fill, Rio Grande, and High Plains 

aquifers (fig. 17).

Correlation Between Potential 

Explanatory Factors

The interpretation of explanatory factors and their 

correlation with water-quality constituents might be affected 

by correlation between potential explanatory factors. 

Correlations between potential explanatory factors and 

groundwater age category are presented for each PA in 

table 5. Correlations between other potential explanatory 

factors for each of the six PAs are presented in tables 6A–F, 

respectively. Aquifer lithology, which is defined for each PA 

rather than each sample (table 1; fig. 2B), is not included in 

this comparison of potential explanatory factors. Relations 

between aquifer lithology and other potential explanatory 

factors are briefly discussed here where applicable and 

detailed subsequently when evaluating individual constituents 

or constituent groups.

There were numerous differences among groundwater 

age categories with potential explanatory factors (table 5). In 

some PAs, these differences were systematic with significant 

differences among the three groundwater age categories. For 

example, in the High Plains aquifer, all three age-category 

groups were significantly different with respect to depth to 

the bottom of well perforation and the median values for 

depth to bottom of well perforation increased from modern, 

to mixed, to premodern. This relation is consistent with the 

general observation that groundwater age (as measured by the 

groundwater age categories modern, mixed and premodern) 

tended to increase with increasing depth to bottom of well 

perforation (fig. 11). In other PAs, differences between 

groundwater age and explanatory factors were causative. 

Premodern groundwater age generally was associated with 

higher pH values (table 5) and might reflect the weathering 

of silicate minerals over long groundwater residence times 

in several PAs (the Basin and Range basin-fill aquifers, 

Rio Grande aquifer system, High Plains aquifer, and the 

Colorado Plateaus aquifers), for example. Differences of TDS 

with groundwater age also can be explained by causative 

associations. Higher TDS concentrations were associated 

with modern groundwater in the High Plains aquifer and the 

Columbia Plateau basaltic-rock aquifers system (table 5); 

this relation might reflect irrigated land use and the large 

amounts of agriculture associated with these PAs, where 

agricultural land use and irrigated land use were both 

relatively high (table 1). Relatively high TDS values were 

associated with premodern groundwater in the Basin and 

Range carbonate-rock aquifers (table 5; fig. 17), perhaps 

reflecting longer residence time allowing for dissolution of 

carbonate minerals.

Non-causative differences between groundwater age and 

percent land use occurred in the Basin and Range basin-fill 

aquifers and the Rio Grande aquifer system (table 5). 

For example, in the Basin and Range basin-fill aquifers, 

correlations of percent agricultural land use with modern 

groundwater might result from modern recharge occurring 

at shallower depths and subsequent movement into deeper 

parts of the aquifer used for water supply. Recharge in the 

basin-fill aquifers generally occurs along mountain fronts 

through infiltration of mountain stream runoff and inflow from 

fractured rock (Robson and Banta, 1995), and groundwater 

withdrawals from deep wells in the basin-fill aquifers have 

enhanced the movement of groundwater from shallower 

to deeper parts of the aquifers (Bexfield and others, 2011). 

Additionally, in the Rio Grande aquifer system, urban 

land use is correlated with premodern groundwater age 

(table 5) and depth to bottom of well perforation (table 6C); 

PSWs commonly located in urban areas access deeper and 

older groundwater.

Significant correlations between hydrologic 

conditions and geochemical conditions varied across the 

PAs and generally are attributed to causative associations 

(tables 6A–F). For example, relations between agricultural 

land use and irrigated land use in the High Plains aquifer are 

positive; the corresponding relations between natural land use 

and agricultural and irrigated land are negative (table 6D).

The aridity index and sample site elevation were positively 

correlated in all the PAs except for the High Plains aquifer 

(tables 6A–F). The aridity index typically increases with 

elevation and latitude because of increasing precipitation; 

however, in the High Plains aquifer (table 6D), a negative 

correlation reflects the west to east decrease in elevation 

and increase in precipitation for the High Plains Province 

(McMahon and others, 2007). Positive correlations between 

dissolved oxygen, well elevation, and the aridity index in 

the Basin and Range basin-fill aquifers (table 6B) likely 

reflect recent recharge near the proximal boundary of basins 

where redox reactions have not consumed dissolved oxygen. 
Potential Explanatory Factors 41

Total dissolved solids, in milligrams per liter

Depth to bottom of well perforation, in feet below land surface

(North American Vertical Datum of 1988)

High Plains

aquifer

Colorado

Plateaus aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0

1,000

3,000

2,000

4,000

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

0

1,000

3,000

2,000

4,000

EXPLANATION

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

Figure 17. Relations for six western U.S. principal aquifers of total dissolved solids with depth to bottom of well perforation, aquifer 

lithology, and groundwater age category in springs and wells sampled with the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017.

Causative relations also likely account for inverse correlations 

between pH and the aridity index in PAs with unconsolidated 

sand and gravel aquifers (tables 6B–D) or sandstone 

lithologies (table 6E). Recent recharge to groundwater in 

more arid areas (with a higher aridity index) generally has 

lower pH values resulting from low pH of rainfall controlled 

by equilibrium between atmospheric carbon dioxide and 

carbonic acid, as well as limited time for hydrolysis reactions 

to occur with silicate minerals in the aquifer lithology. 

A similar explanation might account for inverse correlations 

between TDS and the aridity index and well elevation 

observed in PAs with unconsolidated sand and gravel 

(tables 6B–D) or igneous and metamorphic-rock lithologies 

(table 6F). Groundwater recharge from rainfall at relatively 

high elevations and high aridity index values likely is dilute 

with limited weathering and dissolution reactions having 

occurred, resulting in lower TDS concentrations. Many other 

complex patterns of correlation between aquifer lithology, 

land use, hydrologic conditions, or geochemical conditions 

are evident in correlations between potential explanatory 

factors (tables 6A–F). Selected examples are discussed in 

subsequent sections that address individual constituents or 

constituent groups.

Relations between potential explanatory factors also 

could be related to specific groundwater processes. Oxic 

groundwater conditions were prevalent in all PAs (fig. 15A). 

Dissolved oxygen concentrations generally decreased with 

well depth, except for the Basin and Range carbonate-rock 

aquifers, Rio Grande aquifer system, and Columbia Plateau 

basaltic-rock aquifers as demonstrated by inverse correlation 

with depth to bottom of well perforation (tables 6A, C, F). 

Redox reactions likely consume dissolved oxygen along 

flow paths associated with deeper and older groundwater. 

Differences among groundwater age categories generally were 

not significant for dissolved oxygen, except in the Basin and 

Range carbonate-rock aquifers (table 5; fig. 15A).
Table 5. Results of Kruskal-Wallis and Wilcoxon rank-sum test by principal aquifer for differences in values of selected potential explanatory factors groundwater age categories in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project[Only significant differences between groundwater age categories and the discrete potential explanatory factors are reported (p<0.05). Median values for potential explanatory fain parenthesis for a given groundwater age category. Statistical tests of significance for given land use type were done on percent of a given land use type within the 500-meter bsampled well. Abbreviations: ns, no significance; >, greater than; ft, feet; mg/L, milligrams per liter] Potential explanatory factor Principal aquifer Basin and Range carbonate-rock aquifers Basin and Range basin-fill aquifers Rio Grande aquifer system High Plains aquifer Colorado Plateaus aquifers Columb

basaltic-

Depth to bottom of well perforation ns Premodern (540 ft) > Modern (285 ft); Mixed (510 ft) > Modern (285 ft) Premodern (640 ft) > Modern (255 ft) Premodern (340 ft) > Modern (188 ft); Premodern (340 ft) > Mixed (220 ft) Premodern (805 ft) > Modern (327 ft); Premodern (805 ft) > Mixed (460 ft) Percent natural land use Modern (100 percent) > Mixed (2 percent); Premodern (57 percent) > Mixed (2 percent) ns ns ns Premodern (99 percent) > Mixed (80 percent) Percent urban land use ns ns Premodern (48 percent) > Modern (9 percent) Modern (62 percent) > Mixed (3 percent) ns Percent agricultural land use ns Modern (8.5 percent) > Premodern (0 percent) ns ns ns Percent irrigated land use ns ns ns ns ns Aridity index Modern (0.34) > Premodern (0.30) ns Modern (0.22) > Premodern (0.16) ns Modern (0.34) > Premodern (0.22) Premo

> Mod

Land surface elevation of well Modern (5,760 ft) > Premodern (4,493 ft); Modern (5,760 ft) > Mixed (4,545 ft) ns Modern (6,525 ft) > Premodern (4,826 ft) ns Modern (6,860 ft) > Mixed (5,881 ft) Mixed 

> Mode

Premod

> Mod

Dissolved oxygen Modern (7.6 mg/L) 

> Premodern (3.8 mg/L)

ns ns ns ns 

pH ns Premodern (7.6) 

> Modern (7.3); 

Premodern (7.6) 

> Mixed (7.3)

Premodern (8.0) 

> Modern (7.6)

Premodern (7.6) 

> Modern (7.3); 

Mixed (7.4) 

> Modern (7.3)

Premodern (7.8) 

> Modern (7.4)

Bicarbonate (HCO3) ns Modern (263 mg/L) 

> Premodern (154 mg/L); 

Modern (263 mg/L) 

> Mixed (176 mg/L)

ns Modern (266 mg/L) 

> Premodern (182 mg/L); 

Modern (266 mg/L) 

> Mixed (201 mg/L)

ns 

Total dissolved solids (TDS) Premodern (573 mg/L) 

> Modern (219 mg/L); 

Premodern (573 mg/L) 

> Mixed (293 mg/L)

ns ns Modern (533 mg/L) 

> Premodern (270 mg/L); 

Modern (533 mg/L) 

> Mixed (341 mg/L)

ns Modern 

> Premode
Potential Explanatory Factors 43

Table 6A. Spearman's rho (ρ) correlation results between potential explanatory factors in the Basin and Range carbonate-rock 

aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[rho (ρ), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated (p-values less than 

0.05). Abbreviation: ns, no significance between factors]

rho (ρ)

Depth to 

bottom 

of well 

perforation

Percent 

natural 

land 

use

Percent 

urban 

land 

use

Percent 

agricultural 

land use

Percent 

irrigated 

land use

Aridity 

index

Elevation 

of well

Dissolved 

oxygen pH Bicarbonate (HCO3)

Total 

dissolved 

solids 

(TDS)

Depth to 

bottom 

of well 

perforation

ns ns ns ns ns ns –0.71 ns ns 0.50

Percent natural land use –0.77 –0.58 ns ns ns ns ns ns ns

Percent urban land use ns ns ns ns ns ns ns ns

Percent agricultural land use ns ns ns ns ns ns ns

Percent irrigated land use1 ns ns ns ns ns ns

Aridity index 0.61 ns ns ns ns

Elevation of well ns ns ns –0.57

Dissolved oxygen ns ns –0.66

pH ns ns

Bicarbonate (HCO3) ns

Total dissolved solids (TDS)

1Based on values for irrigated land use within the 500-meter well buffer zone of each sampled well.

Table 6B. Spearman's rho (ρ) correlation results between potential explanatory factors in the Basin and Range basin-fill aquifers 

sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[rho (ρ), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated (p-values less than 

0.05). Abbreviation: ns, no significance between factors]

rho (ρ)

Depth to 

bottom 

of well 

perforation

Percent 

natural 

land 

use

Percent 

urban 

land 

use

Percent 

agricultural 

land use

Percent 

irrigated 

land use

Aridity 

index

Elevation 

of well

Dissolved 

oxygen pH Bicarbonate (HCO3)

Total 

dissolved 

solids 

(TDS)

Depth to 

bottom 

of well 

perforation

ns 0.25 ns ns ns ns ns 0.26 ns ns

Percent natural land use 0.88 ns ns ns 0.36 ns 0.28 ns ns

Percent urban land use –0.31 –0.23 ns –0.36 ns ns ns ns

Percent agricultural land use 0.66 ns ns ns ns 0.35 0.27

Percent irrigated land use1 ns ns –0.30 ns 0.37 ns

Aridity index 0.74 0.55 –0.30 ns –0.44

Elevation of well 0.51 ns ns –0.46

Dissolved oxygen ns ns –0.31

pH –0.60 ns

Bicarbonate (HCO3) 0.33

Total dissolved solids (TDS)

1Based on values for irrigated land use within the 500-meter well buffer zone of each sampled well.
44 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 6C. Spearman's rho (ρ) correlation results between potential explanatory factors in the Rio Grande aquifer system sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[rho (ρ), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated (p-values less than 

0.05). Abbreviation: ns, no significance between factors]

rho (ρ)

Depth to 

bottom 

of well 

perforation

Percent 

natural 

land 

use

Percent 

urban 

land 

use

Percent 

agricultural 

land use

Percent 

irrigated 

land use

Aridity 

index

Elevation 

of well

Dissolved 

oxygen pH Bicarbonate (HCO3)

Total 

dissolved 

solids 

(TDS)

Depth to 

bottom 

of well 

perforation

–0.27 0.28 ns ns ns ns –0.27 ns –0.26 ns

Percent natural land use –0.89 ns 0.26 0.30 ns ns ns 0.26 ns

Percent urban land use ns ns ns ns ns ns ns ns

Percent agricultural land use 0.66 ns ns ns ns 0.25 ns

Percent irrigated land use1 ns 0.31 ns –0.44 ns –0.31

Aridity index 0.87 0.38 –0.26 ns –0.49

Elevation of well 0.26 ns ns –0.65

Dissolved oxygen –0.27 ns ns

pH –0.44 ns

Bicarbonate (HCO3) 0.40

Total dissolved solids (TDS)

1Based on values for irrigated land use within the 500-meter well buffer zone of each sampled well.

Table 6D. Spearman's rho (ρ) correlation results between potential explanatory factors in the High Plains aquifer sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[rho (ρ), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated (p-values less than 

0.05). Abbreviation: ns, no significance between factors]

rho (ρ)

Depth to 

bottom 

of well 

perforation

Percent 

natural 

land 

use

Percent 

urban 

land 

use

Percent 

agricultural 

land use

Percent 

irrigated 

land use

Aridity 

index

Elevation 

of well

Dissolved 

oxygen pH Bicarbonate (HCO3)

Total 

dissolved 

solids 

(TDS)

Depth to 

bottom 

of well 

perforation

ns ns ns ns ns ns ns 0.24 ns –0.36

Percent natural land use –0.61 –0.32 –0.28 ns ns ns ns ns –0.25

Percent urban land use –0.43 ns ns ns ns ns ns ns

Percent agricultural land use 0.57 ns ns ns ns ns ns

Percent irrigated land use1 0.22 ns ns ns ns ns

Aridity index –0.74 –0.31 –0.23 –0.27 –0.40

Elevation of well 0.32 0.40 ns ns

Dissolved oxygen 0.38 ns ns

pH –0.46 0.69

Bicarbonate (HCO3) ns

Total dissolved solids (TDS)

1Based on values for irrigated land use within the 500-meter well buffer zone of each sampled well.
Potential Explanatory Factors 45

Table 6E. Spearman's rho (ρ) correlation results between potential explanatory factors in the Colorado Plateaus aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[rho (ρ), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated (p-values less than 

0.05). Abbreviation: ns, no significance between factors]

rho (ρ)

Depth to 

bottom 

of well 

perforation

Percent 

natural 

land 

use

Percent 

urban 

land 

use

Percent 

agricultural 

land use

Percent 

irrigated 

land use

Aridity 

index

Elevation 

of well

Dissolved 

oxygen pH Bicarbonate (HCO3)

Total 

dissolved 

solids 

(TDS)

Depth to 

bottom 

of well 

perforation

0.31 –0.38 ns ns ns ns ns ns ns ns

Percent natural land use –0.76 –0.54 –0.35 ns ns ns ns ns ns

Percent urban land use ns ns ns ns ns ns ns ns

Percent agricultural land use 0.54 ns ns ns ns ns ns

Percent irrigated land use1 ns ns ns ns ns ns

Aridity index 0.71 ns –0.35 ns ns

Elevation of well ns ns ns ns

Dissolved oxygen ns –0.47 –0.47

pH ns ns

Bicarbonate (HCO3) 0.73

Total dissolved solids (TDS)

1Based on values for irrigated land use within the 500-meter well buffer zone of each sampled well.

Table 6F. Spearman's rho (ρ) correlation results between potential explanatory factors in the Columbia Plateau basaltic-rock aquifers 

sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[rho (ρ), Spearman’s correlation statistic; ρ values are shown for tests in which the variables were determined to be significantly correlated (p-values less than 

0.05). Abbreviation: ns, no significance between factors]

rho (ρ)

Depth to 

bottom 

of well 

perforation

Percent 

natural 

land 

use

Percent 

urban 

land 

use

Percent 

agricultural 

land use

Percent 

irrigated 

land use

Aridity 

index

Elevation 

of well

Dissolved 

oxygen pH Bicarbonate (HCO3)

Total 

dissolved 

solids 

(TDS)

Depth to 

bottom 

of well 

perforation

ns 0.32 ns ns ns ns –0.41 0.30 ns ns

Percent natural land use –0.57 ns ns ns ns ns ns ns ns

Percent urban land use –0.52 ns –0.27 –0.34 ns 0.30 ns ns

Percent agricultural land use 0.37 ns ns ns ns 0.37 ns

Percent irrigated land use1 ns ns 0.29 –0.27 ns ns

Aridity index 0.69 ns ns ns –0.37

Elevation of well ns ns ns –0.30

Dissolved oxygen –0.42 ns ns

pH ns ns

Bicarbonate (HCO3) 0.80

Total dissolved solids (TDS)

1Based on values for irrigated land use within the 500-meter well buffer zone of each sampled well.
46 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Status and Understanding 

Assessments of Water Quality

As described in the “Methods” section, the status 

assessment quantifies groundwater quality at the aquifer scale 

relative to drinking-water benchmarks (table 2) and it includes 

a summary of detection frequencies, aquifer-scale proportions, 

and graphical representations of the concentrations of selected 

constituents that met the criteria for further evaluation. The 

understanding assessment identifies natural and human factors 

affecting groundwater quality and evaluates statistically 

significant correlations among potential explanatory factors 

and between potential explanatory factors and water quality 

to provide physical and chemical context for groundwater 

quality. The understanding assessment objective is to improve 

understanding about why specific constituents are present in 

the groundwater at moderate to high concentrations or are 

frequently detected. The understanding assessment does not, 

however, identify sources of constituents to individual wells 

or springs. The understanding assessment includes discussion 

of the relations between potential explanatory factors and 

selected constituents for each principal aquifer study and 

is presented after the status assessment for individual 

constituents or groups of constituents. Relatively weak 

correlations (Spearman’s rho values less than 0.3) generally 

are not discussed because the goal is to focus the discussion 

on stronger correlations that are more likely to represent 

controlling factors for constituents detected at moderate 

and high concentrations. Relatively weak correlations also 

could represent an artifact of other potential factors that were 

not considered.

Inorganic Constituents

Most inorganic constituents in groundwater are 

present naturally because groundwater reacts with minerals 

in rocks, soils, and sediment. Inorganic constituents in 

groundwater also could be altered by human activities such 

as irrigation and groundwater pumping, or by natural factors 

such as geochemical conditions. Inorganic constituents are 

described later in the report by four groups―trace elements, 

radionuclides, salinity indicators, and nutrients. Of the 

almost 50 inorganic constituents analyzed, 28 had regulatory 

or non-regulatory health-based benchmarks and 7 had 

non-regulatory aesthetic benchmarks. Of the 35 inorganic 

constituents with benchmarks, 18 were selected for additional 

evaluation (table 2) because they were detected at moderate 

or high concentrations in at least one PA (fig. 18); remaining 

constituents either had no benchmarks, no detections, or were 

detected only at low concentrations (table 3).

Aquifer scale proportions of high, moderate, and low 

relative concentrations for inorganic benchmarks, such as 

health-based (HB) and non-health-based benchmarks (SMCL), 

are summarized in pie charts for each PA (fig. 19). The 

proportion of each aquifer’s area that had high concentrations 

for inorganic constituents with HB benchmarks ranged 

from 8 percent in the Columbia Plateau basaltic-rock 

aquifers to 30 percent in the Rio Grande aquifer system 

(fig. 19A). Trace elements with HB benchmarks accounted 

for the largest proportion of high concentrations across all 

PAs (table 7). The proportion of high concentrations for 

inorganic constituents with SMCL benchmarks ranged from 

20 percent in the Columbia Plateau basaltic-rock aquifers 

to 48 percent in the Colorado Plateaus aquifers (fig. 19B). 

Salinity indicators accounted for the largest proportion of 

high concentrations of constituents with SMCL benchmarks 

across all PAs except for the Columbia Plateau basaltic-rock 

aquifers (table 8). The constituent pH has an SMCL range 

defined by two benchmarks—less than 6.5 or greater than 8.5; 

aquifer-scale proportions were calculated as the percentage 

less than or greater than, respectively. Values of pH less than 

the lower SMCL benchmark occurred in 2 percent or less 

of the Basin and Range basin-fill the and Colorado Plateaus 

PAs. Exceedances of pH values above the upper pH SMCL 

benchmark occurred in all of the PAs except for the Basin 

and Range carbonate-rock aquifers and ranged from 1 to 

22 percent in the PAs (table 8). Aquifer-scale proportions 

of inorganic constituents with HB benchmarks (table 7) and 

SMCL benchmarks (table 8) selected for further evaluation are 

summarized for each PA and discussed for each constituent 

group later in the report.
Status and Understanding Assessments of Water Quality 47

0.01

0.1

1

10

100

0.01

0.1

1

10

100

Maximum relative concentration, dimensionless

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

Colorado Plateaus

aquifers

Columbia Plateau

aquifers

Maximum relative

concentration, dimensionless

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

Colorado Plateaus

aquifers

Columbia Plateau

aquifers

High Plains

aquifer

0.1

1

10

Trace elements

High Plains

aquifer

Antimony

Arsenic

Fluoride

Manganese

Molybdenum Selenium

Strontium

Thallium

Uranium

Antimony

Arsenic

Fluoride

Manganese

Molybdenum

Selenium

Strontium

Thallium

Uranium

Antimony

Arsenic

Fluoride

Manganese

Selenium

Thallium

Uranium

Antimony

Arsenic

Fluoride

Manganese

Selenium

Strontium

Thallium

Uranium

Antimony

Arsenic

Fluoride

Manganese

Molybdenum

Selenium

Strontium

Thallium

Uranium

Fluoride

Iron

Manganese

Fluoride

Iron

Manganese

Fluoride

Iron

Manganese

Fluoride

Iron Manganese

Fluoride

Iron

Manganese

Gross Alpha

Radium*

Radon−222

Gross Alpha

Radium*

Radon−222

Gross Alpha

Radium*

Radon−222

Gross Alpha

Radium* Gross Alpha Radon−222

Radium*

Radon−222

Gross Alpha

Radium*

Radon−222

Iron

Cobalt

Cobalt

Iron

Cobalt

Molybdenum

Cobalt

Iron

Molybdenum

Strontium

Iron

Cobalt

Iron

Fluoride

Iron Manganese

Antimony

Thallium

Arsenic

Fluoride

Manganese

Strontium

Uranium

Cobalt

Iron

Molybdenum

Selenium

EXPLANATION

EXPLANATION

Arsenic Iron Constituent with health-based

 benchmarks—center of 

 symbol is location of data

Constituents with non-health-based

 benchmarks—center of symbol is 

 location of data

Relative concentration

High

Moderate

Low

Radium* Constituent with health-based benchmarks—

 center of symbol is location of data

Relative concentration

High Moderate Low *Radium 226 and Radium 228

A

B Radionuclides

Figure 18. Maximum relative concentrations of constituents with health-based benchmarks and considered in the Understanding 

assessment for A, trace elements; B, radionuclides; C, salinity indicators; and D, nutrients in six western U.S. principal aquifers 

sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
48 Water Quality of Groundwater Used for Public Supply in Principal Aquifers Maximum relative concentration, dimensionless

Basin and Range

carbonate-rock

aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

Colorado Plateaus

aquifers

Columbia Plateau

aquifers

High Plains

aquifer

Maximum relativeconcentration, 

dimensionless

Basin and Range

carbonate-rock

aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

Colorado Plateaus

aquifers

Columbia Plateau

aquifers

High Plains

aquifer

0.1

1

10

0.1

1

10

C Salinity indicators

D

Nitrate

Nitrate

Nitrate

Nitrate

Nitrate Nitrate

Chloride

Sulfate

TDS Chloride

Sulfate

TDS

Chloride

Sulfate

TDS

Chloride

Sulfate

TDS

Chloride

Sulfate

TDS Chloride

Sulfate

TDS

Nutrients

EXPLANATION

Radium Constituent with health-based benchmarks—

 center of symbol is location of data

Relative concentration

High Moderate Low

EXPLANATION

Nitrate Constituent with health-based benchmarks—

 center of symbol is location of data

Relative concentration

High Moderate Low

Figure 18. —Continued

Trace Elements

The trace element group includes metallic and 

non-metallic constituents that are typically present in 

groundwater at concentrations less than 1 mg/L (Hem, 1985). 

Concentrations of trace elements in groundwater often 

reflect their presence and abundance in aquifer materials and 

are mobilized primarily by rock weathering, geochemical 

conditions, the presence and abundance of complexing 

ions and organic matter, or attenuated by processes such as 

adsorption (Hem, 1985; Salbu and Steinnes, 1994). Human 

activities, such as mining, agricultural land use, and waste 

disposal, also can affect concentrations of trace elements in 

groundwater (Ayotte and others, 2011). Eleven trace elements 

were selected for evaluation by the status assessment―

antimony, arsenic, cobalt, fluoride, manganese, iron, 

molybdenum, strontium, selenium, thallium, and uranium―

because they were detected at moderate or high concentrations 

in at least one PA (fig. 20). Manganese, iron, and fluoride 

were evaluated for HB and SMCL benchmarks. The trace 

element constituent group had the highest proportions of 

moderate and high concentrations among inorganic groups 

(table 7). Concentrations of trace elements tended to be higher 

in unconsolidated sand and gravel aquifers (tables 1, 7). The 

proportion of each aquifer having at least one trace element 

with a high concentration ranged from 5 percent (Columbia 

Plateau basaltic-rock aquifers) to 27 percent (Rio Grande 

aquifer system), and the proportion of each aquifer having 

at least one trace element with a moderate concentration 

ranged from 7 percent (Columbia Plateau basaltic-rock 

aquifers) to 50 percent (High Plains aquifer, table 7). High 

concentrations of trace elements relative to SMCL benchmarks 

ranged from 10 percent (Basin and Range carbonate-rock 

aquifers) to 26 percent (High Plains aquifers), and moderate 

concentrations ranged from 5 percent (Basin and Range 

carbonate-rock aquifers) to 16 percent (High Plains aquifer 

and Columbia Plateau basaltic-rock aquifers, table 8).
Status and Understanding Assessments of Water Quality 49

Inorganic health-based

benchmark

Basin and Range

carbonate-rock aquifers

Colorado Plateaus

aquifers

Basin and Range

basin-fill aquifers

Columbia Plateau

basaltic-rock aquifers

High Plains

aquifer

RioGrande

aquifer system

Inorganic non-health-based

benchmark

A B

Basin and Range

carbonate-rock aquifers

Colorado Plateaus

aquifers

Basin and Range

basin-fill aquifers

Columbia Plateau

basaltic-rock aquifers

High Plains

aquifer

RioGrande

aquifer system

EXPLANATION

Low Moderate High

Proportion of principal aquifer at public-supply depths 

with relative concentrations in each category

60 20

20

31

47

22

42

28

30

25

56

19

57 27

16

82

10

8

30

45

25

31

33

36

30

28

42

16

49

35

22

30

48

42

38

20

Figure 19. Aquifer scale proportions for inorganic constituents 

with A, health-based (HB); and B, non-health-based (SMCL) 

benchmarks in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017.

Table 7. Summary of aquifer scale proportions (low, moderate, 

or high concentrations) for inorganic constituent groups with 

health-based benchmarks in six western U.S. principal aquifers 

sampled in springs and wells by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017.

[Concentration categories: high, concentration of at least one constituent 

greater than water-quality benchmark; moderate, concentration of at least 

one constituent greater than 0.5 of benchmark but less than benchmark; low, 

concentration of all constituents less than or equal to 0.5 of benchmark]

Principal aquifer

Number

of 

samples

Aquifer scale proportion

(percent)

Low Moderate High

Nutrients with health-based benchmarks

Basin and Range carbonaterock aquifers20 100 0 0

Basin and Range basin-fill 

aquifers

78 91 9 0

Rio Grande aquifer system 60 98 0 2

High Plains aquifer 80 86 11 3

Colorado Plateaus aquifers 60 98 2 0

Columbia Plateau basalticrock aquifers60 91 7 2

Trace elements with health-based benchmarks

Basin and Range carbonaterock aquifers20 65 20 15

Basin and Range basin-fill 

aquifers

78 40 38 22

Rio Grande aquifer system 60 48 25 27

High Plains aquifer 80 34 50 16

Colorado Plateaus aquifers 60 65 22 13

Columbia Plateau basalticrock aquifers60 88 7 5

Radioactive constituents with health-based benchmarks

Basin and Range carbonaterock aquifers20 90 5 5

Basin and Range basin-fill 

aquifers

78 85 15 0

Rio Grande aquifer system 60 85 10 5

High Plains aquifer 80 84 15 11

Colorado Plateaus aquifers 60 82 13 5

Columbia Plateau basalticrock aquifers60 95 2 3

All inorganic constituents with health-based benchmarks

Basin and Range carbonaterock aquifers20 60 20 20

Basin and Range basin-fill 

aquifers

78 47 31 22

Rio Grande aquifer system 60 42 28 30

High Plains aquifer 80 25 56 19

Colorado Plateaus aquifers 60 57 27 16

Columbia Plateau basalticrock aquifers60 82 10 8

1Represents high concentration of 72-hour gross alpha activity detected 

in one well; however only 30-day gross alpha activities were evaluated and 

discussed in the report.
50 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

100

10

1

0.1

0.01

100

10

1

0.1

0.01

100

10

1

0.1

0.01

Columbia Plateau

aquifers

Colorado Plateaus

aquifers

High Plains aquifer

Rio Grande

aquifer system

Basin and Range

basin- fill aquifers

Basin and Range

carbonate-rock aquifers

Columbia Plateau

aquifers

Colorado Plateaus

aquifers

High Plains aquifer

Rio Grande

aquifer system

Basin and Range

basin- fill aquifers

Basin and Range

carbonate-rock aquifers

Columbia Plateau

Colorado Plateaus

aquifers

High Plains aquifer

Rio Grande

aquifer system

Basin and Range

basin- fill aquifers

Basin and Range

carbonate-rock aquifers

Columbia Plateau

Colorado Plateaus

aquifers

High Plains aquifer

Rio Grande

aquifer system

Basin and Range

basin- fill aquifers

Basin and Range

carbonate-rock aquifers

Iron SMCL

Selenium

Arsenic

Iron HBSL

Molybdenum

Thallium

Antimony

Fluoride SMCL

Manganese SMCL Strontium

Fluoride HB

Manganese HBSL

Cobalt

Uranium

100

10

1

0.1

0.01

EXPLANATION

Relative concentration

Low

Moderate

High

Well

Spring

Figure 20. Concentrations of trace elements relative to health-based (HB) and non-health-based benchmarks (SMCL) measured 

in six principal aquifers of the western U.S. sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017.
Status and Understanding Assessments of Water Quality 51

Table 8. Summary of aquifer-scale proportions (low, moderate, 

or high concentrations) for inorganic constituent groups with 

non-health-based benchmarks in six western U.S. principal 

aquifers sampled in springs and wells by the U.S. Geological 

Survey National Water-Quality Assessment Project, 2013–2017.

[Concentration categories: high, concentration of at least one constituent 

greater than water-quality benchmark; moderate, concentration of at least 

one constituent greater than 0.5 of benchmark but less than benchmark; low, 

concentration of all constituents less than or equal to 0.5 of benchmark. 

Abbreviation: SMCL, secondary maximum contaminant level]

Principal aquifer

Number

of 

samples

Aquifer scale proportion 

(percent)

Low Moderate High

Trace elements with SMCL benchmarks

Basin and Range carbonaterock aquifers20 85 5 10

Basin and Range basin-fill 

aquifers

78 72 9 19

Rio Grande aquifer system 60 75 10 15

High Plains aquifer 80 58 16 26

Colorado Plateaus aquifers 60 67 8 25

Columbia Plateau basalticrock aquifers60 67 16 17

Salinity indicators with SMCL

Basin and Range carbonaterock aquifers20 30 45 25

Basin and Range basin-fill 

aquifers

78 32 36 32

Rio Grande aquifer system 60 33 32 35

High Plains aquifer 80 20 54 26

Colorado Plateaus aquifers 60 28 35 37

Columbia Plateau basalticrock aquifers60 56 37 7

All constituents with SMCL benchmarks

Basin and Range carbonaterock aquifers20 30 45 25

Basin and Range basin-fill 

aquifers

78 31 33 36

Rio Grande aquifer system 60 30 28 42

High Plains aquifer 80 16 49 35

Colorado Plateaus aquifers 59 22 30 48

Columbia Plateau basalticrock aquifers60 42 38 20

<6.5 >8.5 sum1

pH

Basin and Range carbonaterock aquifers20 0 0 0

Basin and Range basin-fill 

aquifers

78 1 5 5

Rio Grande aquifer system 60 0 7 4

High Plains aquifer 80 0 1 1

Colorado Plateaus aquifers 60 2 22 14

Columbia Plateau basalticrock aquifers

60 0 7 4

1Sum of pH exceedances less than 6.5 and (or) greater than 8.5 standard pH 

units.

Factors Affecting Antimony

Antimony (Sb) is a rare heavy metal that occurs at about 

0.2 part per million (ppm) in the Earth’s crust (Butterman 

and Carlin, 2004; Seal and others, 2017). It is an important 

mineral commodity that is used worldwide to produce alloys 

in industrialized societies (Miller, 1973; Butterman and Carlin, 

2004; U.S. Geological Survey, 2016). Stibnite (Sb2S3) is the 

most common antimony ore mineral (and over 100 minerals 

also contain antimony; Miller, 1973; Butterman and Carlin, 

2004; Seal and others, 2017). In the U.S., the leading use 

for antimony (as antimony trioxide) is as a fire retardant in 

adhesives, paints, papers, plastics, and sealants; secondary 

uses are in alloys for batteries and solders (Seal and others, 

2017). Natural sources of antimony in soil and water originate 

from mineral weathering; anthropogenic sources include 

mining, industrial processes, and pesticide application of some 

metal-containing pesticides (Arai, 2010).

The EPA MCL benchmark for antimony in drinking water 

is 6 μg/L (U.S. Environmental Protection Agency, 2018a). 

Antimony was detected at moderate concentrations in only 

one well (5 percent) of the Basin and Range carbonate-rock 

PA (table 9A). Antimony had significant negative correlations 

with dissolved oxygen (Spearman’s rho= –0.66) and no 

other explanatory factors. The geochemistry of antimony 

has similar traits to arsenic and often coexists with arsenic 

in the natural environment (Arai, 2010; Wilson and others, 

2010). Both trace elements most commonly occur in relativity 

oxic environments as antimonates and arsenates or in anoxic 

environments as antimonites and arsenites (Wilson and 

others, 2010). Of the 14 wells and 6 springs sampled in 

the Basin and Range carbonate-rock aquifers, only 2 wells 

had anoxic groundwater conditions, one of which had a 

moderate antimony concentration. A low concentration of 

arsenic co-occurred in this same well indicating the anoxic 

geochemical conditions favored the release of antimony and 

arsenic from the surrounding aquifer material.

Factors Affecting Arsenic

Arsenic (As), which is toxic to humans in small 

amounts, occurs in many minerals, such as metal arsenides 

and sulfides (Hem, 1985). Moderate and high concentrations 

of arsenic occurred in all six PAs and accounted for the 

highest trace-element group detection frequencies in most of 

the PAs (fig. 21A; tables 9A–F). Natural sources of arsenic 

in groundwater include the dissolution of arsenic-bearing 

minerals, desorption of arsenic from mineral surfaces, and 

mixing with hydrothermal fluids (Welch and others, 2000; 

Smedley and Kinniburgh, 2002; Barringer and Reilly, 2013). 

Potential anthropogenic sources include copper ore smelting, 

coal combustion, arsenical pesticides, arsenical veterinary 

pharmaceuticals, and wood preservatives (Fram and Belitz, 

2012). Natural sources of arsenic in drinking water adversely 

affect large human populations across broad regional areas 

(Welch and others, 2000; Smedley and Kinniburgh, 2002).
52 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 9A. Detection frequency and aquifer-scale proportions (low, moderate, or high concentrations) for inorganic constituents 

that met criteria for additional evaluation by the status assessment for the Basin and Range carbonate-rock aquifers sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[Concentration categories: high, concentrations greater than water quality benchmark; moderate, concentrations greater than or equal to 0.5 of water quality 

benchmark but less than benchmark; low, concentrations less than 0.5 of bechmark. Abbreviation: SMCL, secondary maximum contaminant level]

Constituent

Raw detection frequency Raw detection frequency

(percent)

Aquifer-scale proportion

(percent)

Number of 

wells with 

analysis

Detection 

frequency

(percent)

Number of 

springs with 

analysis

Detection 

frequency

(percent)

Number

of 

samples

Low Moderate High

Nutrients

Nitrate, as nitrogen 14 100 6 100 20 100 0 0

Trace elements

Antimony 14 79 6 50 20 95 5 0

Arsenic 14 100 6 100 20 85 5 10

Cobalt 14 86 6 50 20 100 0 0

Fluoride 14 100 6 100 20 90 10 0

Iron 14 57 6 0 20 100 0 0

Manganese 14 36 6 0 20 100 0 0

Molybdenum 14 100 6 100 20 100 0 0

Selenium 14 100 6 100 20 100 0 0

Strontium 14 100 6 100 20 90 10 0

Thallium 14 36 6 17 20 95 0 5

Uranium 14 100 6 100 20 100 0 0

Inorganic constituents with SMCL

Chloride 14 100 6 100 20 85 10 5

Fluoride 14 100 6 100 20 90 0 10

Iron 14 57 6 0 20 100 0 0

Manganese 14 36 6 0 20 95 5 0

Sulfate 14 100 6 100 20 80 15 5

Total dissolved solids (TDS) 14 100 6 100 20 30 45 25

Radioactive constituents

Gross alpha activity (30-day) 14 100 6 100 20 100 0 0

Radium-226 and radium-228 (combined) 14 100 6 100 20 94 6 0

Radon activity1 14 100 5 100 19 95 0 5

1Radon-222 was evaluated at the proposed maximum contaminant level of 4,000 picocuries per liter.
Status and Understanding Assessments of Water Quality 53

Table 9B. Detection frequency and aquifer-scale proportions (low, moderate, or high 

concentrations) for inorganic constituents that met criteria for additional evaluation 

by the status assessment for the Basin and Range basin-fill aquifers sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.5 of water quality benchmark but less than 

benchmark; low, concentrations less than 0.5 of bechmark. Abbreviation: SMCL, secondary maximum 

contaminant level]

Constituent

Raw detection 

frequency

Aquifer-scale proportion

(percent)

Number

of

samples

Detection 

frequency

(percent)

Low Moderate High

Nutrients

Nitrate, as nitrogen 78 91 82 9 0

Trace elements

Antimony 78 77 77 0 0

Arsenic 78 100 52 35 13

Cobalt 78 80 99 1 0

Fluoride 78 100 89 5 6

Iron 78 51 100 0 0

Manganese 78 72 92 5 3

Molybdenum 78 100 87 9 4

Selenium 78 99 100 0 0

Strontium 78 100 95 5 0

Thallium 78 9 100 0 0

Uranium 78 100 96 1 3

Inorganic constituents with SMCL

Chloride 78 100 78 10 12

Fluoride 78 100 81 8 11

Iron 78 51 96 3 1

Manganese 78 72 91 1 8

Sulfate 78 100 73 14 13

Total dissolved solids (TDS) 78 100 32 36 32

Radioactive constituents

Gross alpha activity (30-day) 78 100 99 1 0

Radium-226 and radium-228 (combined) 67 100 100 0 0

Radon activity1 76 100 95 5 0

1Radon-222 was evaluated at the proposed maximum contaminant level of 4,000 picocuries per liter.
54 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 9C. Detection frequency and aquifer-scale proportions (low, moderate, or high 

concentrations) for inorganic constituents that met criteria for additional evaluation by the 

status assessment for the Rio Grande aquifer system sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.5 of water quality benchmark but less than 

benchmark; low, concentrations less than 0.5 of bechmark. Abbreviation: SMCL, secondary maximum 

contaminant level]

Constituent

Raw detection 

frequency

Aquifer-scale proportion

(percent)

Number

of 

samples

Detection 

frequency

(percent)

Low Moderate High

Nutrients

Nitrate, as nitrogen 60 85 98 0 2

Trace elements

Antimony 60 72 100 0 0

Arsenic 60 93 60 22 18

Cobalt 60 63 100 0 0

Fluoride 60 100 88 8 4

Iron 60 48 100 0 0

Manganese 60 28 98 2 0

Molybdenum 60 100 95 3 2

Selenium 60 88 100 0 0

Strontium 60 100 95 2 3

Thallium 60 3 100 0 0

Uranium 60 100 90 7 3

Inorganic constituents with SMCL

Chloride 60 100 82 8 10

Fluoride 60 100 78 10 12

Iron 60 48 47 2 0

Manganese 60 28 23 2 3

Sulfate 60 100 70 23 7

Total dissolved solids (TDS) 60 100 33 32 35

Radioactive constituents

Gross alpha activity (30-day) 60 100 98 2 0

Radium-226 and radium-228 (combined) 60 100 98 2 0

Radon activity1 60 100 92 5 3

1Radon-222 was evaluated at the proposed maximum contaminant level of 4,000 picocuries per liter.
Status and Understanding Assessments of Water Quality 55

Table 9D. Detection frequency and aquifer-scale proportions (low, moderate, or high 

concentrations) for inorganic constituents that met criteria for additional evaluation by 

the status assessment for the High Plains aquifer sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.5 of water quality benchmark but less than 

benchmark; low, concentrations less than 0.5 of bechmark. Abbreviation: SMCL, secondary maximum 

contaminant level]

Constituent

Raw detection 

frequency

Aquifer-scale proportion

(percent)

Number

of 

samples

Detection 

frequency

(percent)

Low Moderate High

Nutrients

Nitrate, as nitrogen 80 95 86 11 3

Trace elements

Antimony 80 92 100 0 0

Arsenic 80 99 49 44 6

Cobalt 80 49 99 1 0

Fluoride 80 100 85 12 3

Iron 80 38 99 1 0

Manganese 80 32 98 1 1

Molybdenum 80 99 99 1 0

Selenium 80 96 97 3 0

Strontium 80 99 89 7 4

Thallium 80 0 100 0 0

Uranium 80 98 77 15 8

Inorganic constituents with SMCL

Chloride 80 100 94 2 4

Fluoride 80 100 67 18 15

Iron 80 38 99 0 1

Manganese 80 33 89 0 11

Sulfate 80 100 79 11 10

Total dissolved solids (TDS) 80 100 20 54 26

Radioactive constituents

Gross alpha activity (30-day) 80 100 96 4 0

Radium-226 and radium-228 (combined) 80 100 100 0 0

Radon activity1 79 100 99 1 0

1Radon-222 was evaluated at the proposed maximum contaminant level of 4,000 picocuries per liter.
56 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 9E. Detection frequency and aquifer-scale proportions (low, moderate, or high 

concentrations) for inorganic constituents that met criteria for additional evaluation by 

the status assessment for the Colorado Plateaus aquifers sampled by the U.S. Geological 

Survey National Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.5 of water quality benchmark but less than 

benchmark; low, concentrations less than 0.5 of bechmark. Abbreviation: SMCL, secondary maximum 

contaminant level]

Constituent

Raw detection 

frequency

Aquifer-scale proportion

(percent)

Number

of 

samples

Detection 

frequency 

(percent)

Low Moderate High

Nutrients

Nitrate, as nitrogen 60 62 98 2 0

Trace elements

Antimony 59 29 100 0 0

Arsenic 59 83 78 14 8

Cobalt 59 25 100 0 2

Fluoride 60 100 95 5 0

Manganese 60 58 93 4 3

Iron 60 43 93 7 0

Molybdenum 60 97 98 2 0

Selenium 59 59 100 0 0

Strontium 59 100 93 7 0

Thallium 59 5 100 0 0

Uranium 59 93 98 2 0

Inorganic constituents with SMCL

Chloride 60 100 91 7 2

Fluoride 60 100 88 7 5

Iron 60 43 77 7 16

Manganese 60 58 80 7 13

Sulfate 60 100 72 12 16

Total dissolved solids (TDS) 60 100 28 35 37

Radioactive constituents

Gross alpha activity (30-day) 60 17 98 2 0

Radium-226 and radium-228 (combined) 45 44 94 2 4

Radon activity1 60 100 95 5 0

1Radon-222 was evaluated at the proposed maximum contaminant level of 4,000 picocuries per liter.
Status and Understanding Assessments of Water Quality 57

Table 9F. Detection frequency and aquifer-scale proportions (low, moderate, or high 

concentrations) for inorganic constituents that met criteria for additional evaluation by the 

status assessment for the Columbia Plateau basaltic-rock aquifers sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.5 of water quality benchmark but less than 

benchmark; low, concentrations less than 0.5 of bechmark. Abbreviation: SMCL, secondary maximum 

contaminant level]

Constituent

Raw detection 

frequency

Aquifer-scale proportion

(percent)

Number

of 

samples

Detection 

Frequency 

(percent)

Low Moderate High

Nutrients

Nitrate, as nitrogen 60 67 91 7 2

Trace elements

Antimony 60 37 100 0 0

Arsenic 60 77 93 2 5

Cobalt 60 18 98 2 0

Fluoride 60 100 97 3 0

Iron 60 60 100 0 0

Manganese 60 43 98 2 0

Molybdenum 60 100 98 2 0

Selenium 60 63 100 0 0

Strontium 60 100 100 0 0

Thallium 60 2 100 0 0

Uranium 60 77 98 2 0

Inorganic constituents with SMCL

Chloride 59 100 100 0 0

Fluoride 59 100 88 9 3

Iron 59 42 90 3 7

Manganese 59 61 76 14 10

Sulfate 59 100 95 3 2

Total dissolved solids (TDS) 59 100 56 37 7

Radioactive constituents

Gross alpha activity (30-day) 60 100 97 0 2

Radium-226 and radium-228 (combined) 60 100 100 0 0

Radon activity1 60 100 97 0 3

1Radon-222 was evaluated at the proposed maximum contaminant level of 4,000 picocuries per liter.
58 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

A

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

0 150 300 MILES

0 150 300 KILOMETERS

Arsenic, in micrograms per liter

High (>10)

Moderate (5 to 10)

Low (<5)

Not detected

Spring Groundwater

well

—

—

Map area

Figure 21. Concentrations of A, arsenic; B, fluoride health-based (HB) benchmark; C, fluoride non-health-based (SMCL) 

benchmark; D, cobalt; E, manganese HB benchmark; F, manganese SMCL benchmark; G, iron HB benchmark; H, iron SMCL 

benchmark; I, molybdenum; J, selenium; K, strontium and L, uranium in six western U.S. principal aquifers sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 59

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Fluoride (HB), in milligrams per liter

High (>4)

Moderate (2 to 4)

Low (<2)

Not detected

Spring Groundwater

well

—

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

B

Figure 21. —Continued
60 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

C

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Fluoride (SMCL), in milligrams per liter

High (>2)

Moderate (1 to 2)

Low (<1)

Not detected

Spring Groundwater

well

—

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 21. —Continued
Status and Understanding Assessments of Water Quality 61

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Cobalt, in micrograms per liter

High (>2)

Moderate (1 to 2)

Low (<1)

Not detected

Spring Groundwater

well

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

D

Figure 21. —Continued
62 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Manganese (HB), in micrograms per liter

High (>300)

Moderate (150 to 300)

Low (<150)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

E

Figure 21. —Continued
Status and Understanding Assessments of Water Quality 63

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Manganese (SMCL), in micrograms per liter

High (>50)

Moderate (25 to 50)

Low (<25)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

F

Map area

Figure 21. —Continued
64 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Iron (HB), in micrograms per liter

G

High (>4,000)

Moderate

(2,000 to 4,000)

Low (<2,000)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

—

Map area

Figure 21. —Continued
Status and Understanding Assessments of Water Quality 65

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Iron (SMCL), in micrograms per liter

H

High (>300)

Moderate (150 to 300)

Low (<150)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 21. —Continued
66 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Molybdenum, in micrograms per liter

I

High (>30)

Moderate (15 to 30)

Low (<15)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 21. —Continued
Status and Understanding Assessments of Water Quality 67

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Selenium, in micrograms per liter

High (>50)

Moderate (25 to 50)

Low (<25)

Not detected

Spring Groundwater

well

—

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

J

Figure 21. —Continued
68 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Strontium, in micrograms per liter

High (>4,000)

Moderate

(2,000 to 4,000)

Low (<2,000)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

K

Figure 21. —Continued
Status and Understanding Assessments of Water Quality 69

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

L

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Uranium, in micrograms per liter

High (>30)

Moderate (15 to 30)

Low (<15)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 21. —Continued
70 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

The EPA MCL for arsenic in drinking water is 10 μg/L 

(U.S. Environmental Protection Agency, 2018a). Large 

regions of the continental United States have groundwater 

concentrations that exceed the MCL, especially in California 

and the southwestern states (Welch and others, 2000). Elevated 

concentrations of arsenic in groundwater generally are not 

related to high arsenic concentrations in rocks or aquifer 

sediments, but they are attributed to geochemical conditions 

that promote arsenic mobility and hydrologic conditions 

that favor accumulation of arsenic in groundwater (Smedley 

and Kinniburgh, 2002). Previous studies have identified two 

geochemical mechanisms associated with the occurrence 

of elevated arsenic in groundwater: (1) desorption from, or 

inhibition of sorption to, aquifer materials with increasing 

pH and (2) release of arsenic from dissolution of iron or 

manganese oxyhydroxides under iron- or manganese-reducing 

conditions (Welch and others, 2000; Smedley and 

Kinniburgh, 2002; McMahon and Chapelle, 2008, and many 

references therein).

High and moderate arsenic concentrations occurred more 

often under oxic groundwater conditions at pH values greater 

than 7.0 relative to anoxic groundwater conditions (fig. 22A). 

Of the 36 samples with high concentrations of arsenic, 31 

were oxic. For the oxic samples, pH ranged from 7.0 to 9.3; 

conversely, in four of the five anoxic samples pH ranged 

from 8.0 to 8.9—indicating desorption from, or inhibition 

of sorption to, aquifer materials with increasing pH as an 

important mechanism for elevated arsenic concentrations. 

The remaining anoxic, low pH sample also had moderate 

and high concentrations of manganese and iron, respectively, 

indicating reductive dissolution of iron or manganese oxides 

as the potential source of elevated arsenic concentration. 

The primary mechanism for increasing pH of groundwater 

under oxic conditions are hydrolysis of silicate minerals or 

dissolution of calcite, which also increases TDS (Fram and 

Belitz, 2012). In the high pH samples, there was no difference 

in arsenic concentration among redox categories, suggesting 

that at pH values greater than 7.0, arsenic mobility could be 

less controlled by redox (fig. 22A). A similar finding was 

reported by Ayotte and others (2011) in a national-scale 

evaluation of trace elements grouped by geologic material that 

included unconsolidated sand and gravel, sandstone, carbonate 

rock, and basaltic and other volcanic-rock lithologies.

The accumulation of arsenic in groundwater generally 

requires favorable hydrologic conditions. Groundwater with 

long residence time allows for reactions to occur with aquifer 

materials, minimizing the amount of flushing and maximizing 

reaction times (Smedley and Kinniburgh, 2002). High arsenic 

concentrations were mostly associated with premodern 

or mixed groundwater age categories and elevated TDS 

concentrations (greater than 500 mg/L). Groundwater flow is 

often from the basin margins toward low-lying valley floors 

in long flow paths with slow flow rates, resulting in sufficient 

time for chemical reactions to release arsenic from the 

sediments (Anning and others, 2012). Arsenic concentrations 

often increase as groundwater moves from mountain fronts 

to lowlands in the center of the basins (Anning and others, 

2012). Basins that have no natural surface outflow, that is, 

water only leaves the basins through evapotranspiration or 

deep groundwater underflow, are particularly vulnerable to 

accumulation of arsenic and other constituents from geologic 

sources (Welch and others, 2000; McMahon and Chapelle, 

2008; Bexfield and others, 2011). Significant correlations 

of arsenic with explanatory factors in the Basin and Range 

basin-fill aquifers and Rio Grande aquifer system reflected 

geochemical and hydrologic conditions that favor arsenic 

mobility and accumulation of concentrations in groundwater 

(fig. 23A). For example, in these two PAs, arsenic was 

positively correlated with TDS and (or) pH and negatively 

correlated with aridity index and well elevation. Additionally, 

the positive correlation between arsenic and natural land 

use is consistent with the hydrogeologic nature of natural 

land use; natural land use in the southwestern United States 

tends to have relatively low rates of groundwater recharge 

rate and relatively high rates of evapotranspiration that likely 

correspond to relatively low flushing of solutes out of a basin 

(as compared to urban and agricultural land use with higher 

rates of recharge; Anning and others, 2012).

In the Basin and Range carbonate-rock aquifers, 

arsenic was positively correlated with TDS, and negatively 

correlated with aridity index and well elevation. These 

relations could, in part, reflect the inverse relation between 

TDS and well elevation in this PA (table 6A). Groundwater 

recently recharged from precipitation occurring at higher 

well elevations generally has higher aridity index values 

and relatively low TDS. Recharge occurring at these higher 

elevations has not had sufficient residence time for dissolution 

of carbonate minerals to produce the increased pH values (and 

TDS) that favor arsenic mobility. Premodern groundwater 

age also correlated with TDS in the Basin and Range 

carbonate-rock aquifers (table 5).

In the Columbia Plateau basaltic-rock aquifers, three of 

the four samples with moderate or high arsenic concentrations 

occurred with oxic groundwater conditions and elevated TDS 

values (fig. 22A, B). In this PA, arsenic also was positively 

correlated with dissolved oxygen and negatively correlated 

with the aridity index (fig. 23A). Arsenic concentrations 

were not correlated with potential explanatory factors in 

the High Plains aquifer and had slight negative correlation 

with TDS in the Colorado Plateaus aquifers. Elevated 

arsenic concentrations in the Colorado Plateaus aquifers 

alternatively could be associated with coal deposits; the 

Colorado Plateaus Province hosts large coal-bearing strata 

(Kirschbaum and Biewick, 2016). In 2016, the Colorado 

Division of Reclamation, Mining and Safety reported nearly 

13 million short tons of coal production that provided more 

than 60 percent of Colorado’s electricity generation (Colorado 

Division of Reclamation, 2016). Arsenic in bituminous coal 

occurs primarily in pyrite, a common mineral composed of 

iron and sulfur, which can contain a small amount of arsenic 

(Kolker and others, 2006). Pyrite in coal rapidly oxidizes 

to arsenate, which is readily leached by precipitation and 

groundwater (Huggins and others, 2002).
Status and Understanding Assessments of Water Quality 71

Total dissolved solids, in milligrams per liter

Arsenic concentration, in micrograms per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0.01

0 500 1,500 2,500

0.01

0.1

1

10

100

0.1

1

10

100

1,000 2,000 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

B

pH, in standard units

Arsenic concentration, in micrograms per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

7 8 9 7 8 9 7 8 9

0.01

0.1

1

10

100

0.01

0.1

1

10

100

7 8 9 7 8 9 7 8 9

A

Aquifer lithology Redox category

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Anoxic Oxic

EXPLANATION

Low

Moderate

High

Relative concentration

Low

Moderate

High

Relative concentration

EXPLANATION

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

Figure 22. Arsenic concentrations by A, pH, redox, and aquifer lithology; and B, total dissolved solids, groundwater age, and aquifer 

lithology in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017.
72 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

pH

Total

dissolved

solids

−0.31

−0.55

0.36 0.37 0.25

0.55

−0.34

−0.40

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

High Plains aquifer

Colorado Plateaus aquifers

Depth to 

bottom of well

perforation

pH

Basin and Range basin-fill aquifers

HCO pH 3 Percent

irrigated

land use

rho

rho

rho

B

−0.55

Percent

agricultural

land use

Percent

natural

land use

0.29

0.53

Explanatory factor

Cobalt

Total

dissolved

solids

Total

dissolved

solids

HCO3

Colorado Plateaus aquifers

A

−0.69

0.46

0.23 0.37

−0.27 −0.36

0.51

Explanatory factor

rho

Arsenic

−0.56 −0.36

0.41

−0.53

0.37

−0.53

–1.0

–0.5

0.0

0.5

1.0

Aridity pH

index

Well

elevation

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Aridity

index

Well

elevation

HCO3 pH Percent

natural

land use

Aridity

index

Columbia Plateau aquifers

Basin and Range carbonate-rock aquifers

Basin and Range basin-fill aquifers

Rio Grande

rho

rho rho

Total

dissolved

solids

Total

dissolved

solids

Dissolved

oxygen

Dissolved

oxygen

Explanatory factor

Explanatory factor

−0.27

–1.0

–0.5

0.0

0.5

1.0

Figure 23. Spearman’s rho results for trace elements A, arsenic; B, cobalt; C, fluoride; D, manganese; E, iron; F, molybdenum; 

G, selenium; H, strontium; and I, uranium in six western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 73

D Manganese

−0.28

0.60

−0.53 −0.37

0.25 0.45

−0.37 −0.35 −0.33 −0.26

–1.0

–0.5

0.0

0.5

1.0

Aridity

index

Well

elevation

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Dissolved

oxygen

High Plains aquifer

Columbia Plateau aquifers

Rio Grande aquifer system

rhorho rho

−1.0

−0.5

0.0

1.0

0.5

Colorado Plateaus aquifers

Total

dissolved

solids

HCO3

rho

Total

dissolved

solids

Dissolved

oxygen

Explanatory factor Explanatory factor

Aridity

index

Well

elevation

Total

dissolved

solids

HCO3

Dissolved

oxygen

Percent

irrigated

land use

pH Total

dissolved

solids

HCO3

−0.36

0.33 0.43 0.49

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Aridity

index

Well

elevation

Dissolved

oxygen

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Dissolved

oxygen

Dissolved

oxygen

Percent

agricultural

land use

Percent

irrigated

land use

High Plains aquifer

Basin and Range carbonate-rock aquifers Basin and Range basin-fill aquifers

Rio Grande aquifer system

rho

rhorho

rho

Dissolved

oxygen

Aridity

index

Well

elevation

Percent

agricultural

land use

Basin and Range carbonate-rock aquifers

–1.0

–0.5

0.0

0.5

1.0

rho

Dissolved

oxygen

Total

dissolved

solids

Well

elevation

Basin and Range basin-fill aquifers

−1.0

−0.5

0.0

1.0

0.5

HCO pH 3 Percent

irrigated

land use

Dissolved

oxygen

Aridity

index

Total

dissolved

solids

Explanatory factor

Total

dissolved

solids

Total

dissolved

solids

Total

dissolved

solids

Depth to

bottom of 

well perforation

Explanatory factor

C Fluoride

−0.71

−0.36

0.62 0.65

−0.30 −0.37

0.31 0.37 0.28 0.40 0.50

0.53

−0.68

−0.53

0.33 0.31

−0.31 −0.32 −0.31

0.51

−0.39 −0.40 −0.37

0.34

0.54 0.52

Figure 23. —Continued
74 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

−0.53 −0.35

0.26

−0.66

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

High Plains aquifer Colorado Plateaus aquifers

Columbia Plateau basaltic-rock aquifers

Aridity

index

Well

elevation

Dissolved

oxygen

Depth to 

bottom of

well perforation

pH

Basin and Range basin-fill aquifers Rio Grande aquifer system

Dissolved

oxygen

Percent

irrigated

land use

Percent

urban

land use

rho

rho

rho rho

rho

D—Continued Manganese

Percent

agricultural

land use

Percent

agricultural

land use

Dissolved

oxygen

Dissolved

oxygen

Well

elevation

0.27

0.41

−0.59 −0.49 −0.48

0.29

−0.30 −0.64 −0.68 −0.33

0.56

−0.27

Aridity

index

Well

elevation

Dissolved

oxygen

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

HCO3

Basin and Range basin-fill aquifers Rio Grande aquifer system

rho

rho

Dissolved

oxygen

Aridity

index

Well

elevation

pH Total

dissolved

solids

Total

dissolved

solids

Percent

agricultural

land use

Iron

Explanatory factor

E

Explanatory factor

Explanatory factor

Total

dissolved

solids

Total

dissolved

solids

Total

dissolved

solids

F Molybdenum

Explanatory factor

−0.72

–1.0

–0.5

0.0

0.5

1.0

Columbia Plateau aquifers

rho

−1.0

−0.5

0.0

1.0

0.5

Dissolved

oxygen

Colorado Plateaus aquifers

Total

dissolved

solids

HCO3

rho

Explanatory factor

Dissolved

oxygen

Depth to

bottom of 

well perforation

0.29

Explanatory factor

Total

dissolved

solids

−0.24 −0.25

0.24

0.33

−0.27

0.37

−0.28

0.41

−0.26

−0.47 −0.53

0.50

−0.71

0.35 0.35

Figure 23. —Continued
Status and Understanding Assessments of Water Quality 75

−0.46 −0.34

0.42 0.53

0.22

−0.27 −0.39

0.52 0.37

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

High Plains aquifer

Columbia Plateau aquifers

rho rho

Well

elevation

Aridity

index

HCO3 Percent

agricultural

land use

Percent

natural

land use

Total

dissolved

solids

Aridity

index

pH Total

dissolved

solids

H Strontium

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Aridity

index

Well

elevation

Percent

irrigated

land use

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Dissolved

oxygen

Dissolved

oxygen

pH

High Plains aquifer

Basin and Range carbonate-rock aquifers Basin and Range basin-fill aquifers

Rio Grande aquifer system

rho

rhorho

rho

HCO3 Aridity

index

Well

elevation

Total

dissolved

solids

Total

dissolved

solids

Total

dissolved

solids

Well

elevation

HCO pH 3 Aridity

index

HCO3

Explanatory factor

Explanatory factor

F—Continued Molybdenum

–1.0

–0.5

0.0

0.5

1.0

Total

dissolved

solids

Colorado Plateaus aquifers

rho

Depth to bottom of

well perforation

HCO3

Explanatory factor

–1.0

–0.5

0.0

0.5

1.0

High Plains aquifer

rho

Aridity

index

HCO3 Total

dissolved

solids

Dissolved

oxygen

Depth to

bottom of

well perforation

G Selenium

Explanatory factor

Explanatory factor

0.83

−0.46

0.63

0.42

−0.38

0.75

−0.39 −0.27

−0.47

0.73

−0.50 −0.51

−0.25

−0.31

0.27

−0.59

0.83

0.32

−0.30

0.39

−0.52

−0.29

0.28

0.55

0.28

Figure 23. —Continued
76 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

−0.30 −0.25

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

High Plains aquifer Colorado Plateaus aquifers

Columbia Plateau aquifers

Aridity

index

Total

dissolved

solids

rho rho

rho

HCO3

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

Percent

agricultural

land use

Basin and Range basin-fill aquifers Rio Grande aquifer system

rho

rho

Well

elevation

Well

elevation

Percent

agricultural

land use

Percent

natural

land use

Aridity

index

HCO pH 3 Total

dissolved

solids

Percent

agricultural

land use

Dissolved

oxygen

Dissolved

oxygen

Depth to 

bottom of

well

perforation

Percent

irrigated

land use

Percent

agricultural

land use

HCO3

Explanatory factor

Explanatory factor

I Uranium

H—Continued Strontium

−1.0

−0.5

0.0

1.0

0.5

Colorado Plateaus aquifers

Total

dissolved

solids

HCO3

rho

pH

Explanatory factor

−0.29

0.30

0.31

−0.37 −0.38 −0.33

0.59 0.53 0.38

0.24

−0.29

−0.48

0.39 0.61 0.46

−0.32

0.52

−0.51

0.36 0.27

Figure 23. —Continued
Status and Understanding Assessments of Water Quality 77

Factors Affecting Cobalt

Cobalt (Co) is a naturally occurring trace element 

found in relatively low abundance in the Earth’s crust and in 

natural waters. It can substitute for iron in ferromagnesian 

rock minerals (Hem, 1985). Cobalt was historically 

produced as a by-product of nickel and copper mining, but 

more recently, worldwide production of mined cobalt has 

increased because of the growth of rechargeable batteries and 

aerospace industries (Ma and Hooda, 2010; U.S. Geological 

Survey 2018b). In 2017, worldwide production of cobalt was 

approximately 110,000 million tons (U.S. Geological Survey, 

2018b). Apart from mining and smelting activities, cobalt can 

enter soils from phosphorus-containing fertilizers, sewage 

sludges, and atmospheric deposition (Ma and Hooda, 2010). 

The behavior of cobalt in soils is largely influenced by the 

presence of manganese and iron oxides. Coprecipitation or 

adsorption by these oxides is an important factor in controlling 

cobalt in natural water (Hem, 1978, 1980). The solubility 

of cobalt, like other cation trace elements, decreases with 

increasing pH (Ma and Hooda, 2010).

Cobalt is an essential element for humans as a 

constituent of vitamin B12 (Agency for Toxic Substances 

and Disease Registry, 2004). Cobalt has a HBSL of 2 μg/L 

(U.S. Geological Survey, 2018a). The concentration of cobalt 

in surface and groundwater in the United States generally is 

low—between 1 and 10 parts per billion (ppb) in populated 

areas; in most drinking water, cobalt concentrations are less 

than 1–2 ppb (Agency for Toxic Substances and Disease 

Registry, 2004). Elevated cobalt concentrations were 

infrequent: moderate concentrations occurred in 2 percent 

or less of the Basin and Range basin-fill aquifers, High 

Plains aquifer, and Columbia Plateau basaltic-rock aquifers 

(tables 9B, D, F, respectively); high concentrations occurred in 

2 percent of the Colorado Plateaus aquifers (fig. 20; table 9E). 

Cobalt was inversely correlated with pH and positively 

correlated with TDS and HCO3 (fig. 23B).

Factors Affecting Fluoride

Fluoride (the anion of the element fluorine; F) in 

groundwater has natural and anthropogenic sources. Natural 

sources include the dissolution of fluoride-bearing minerals 

such as fluorite and fluorapatite (Fram and Belitz, 2012). 

The main anthropogenic source of fluoride in water is the 

fluoridation of public water supplies through the addition of 

sodium fluoride or hexafluorosilic acid during drinking-water 

treatment. The addition of fluoride to drinking water is a 

public health measure to reduce tooth decay. Fluorine is an 

important nutrient for the structure of teeth and bones in 

higher life forms (Hem, 1985). There are two water-quality 

benchmarks for fluoride, a regulatory HB benchmark of 

4 mg/L and a non-regulatory SMCL benchmark of 2 mg/L 

(U.S. Environmental Protection Agency, 2018a, 2018b). 

Fluoride was evaluated herein for both benchmarks (fig. 20). 

Chronic exposure to fluoride in drinking water above the 

HB benchmark can result in skeletal bone disorders or 

tooth discoloration (U.S. Environmental Protection Agency, 

2018a). The SMCL benchmark is a guideline for an upper 

boundary where high fluoride levels occur naturally and is 

based on balancing the beneficial effects of protection against 

tooth decay and undesirable effects of tooth discoloration 

(U.S. Environmental Protection Agency, 2018b).

High concentrations of fluoride relative to the HB 

benchmark (4 mg/L) were measured in less than 7 percent of 

samples and occurred only in the unconsolidated sand and 

gravel PAs―the Basin and Range basin-fill aquifers, the Rio 

Grande aquifer system, and the High Plains aquifer (fig. 21B; 

tables 9B–D). High fluoride concentrations relative to the 

SMCL benchmark (2 mg/L) occurred in all PAs (fig. 21C) with 

the greatest aquifer proportions (10–15 percent) occurring in 

unconsolidated sand and gravel aquifers (tables 9B–D) and the 

carbonate-rock aquifers (table 9A); high concentrations were 

less frequent (5 percent or less) in sandstone and basaltic-rock 

lithologies (Colorado Plateaus aquifers and Columbia Plateau 

basaltic-rock aquifers; tables 9E, F).

Fluoride had mostly similar significant correlations 

among selected potential explanatory factors across all PAs 

(fig. 23C), with the most common being concentrations of 

TDS, dissolved oxygen, and HCO3. Differences in positive 

or negative correlations among these explanatory factors 

reflect relations of groundwater age, pH, and calcium and 

fluoride concentrations among the different aquifer lithologies 

(figs. 24A, B). Fluoride concentrations in groundwater 

typically are limited by the low solubility of fluorite 

(CaF2) in waters containing calcium; that is, lower calcium 

concentrations result in higher fluoride concentrations because 

of the dissolution of fluorite (Nordstrom and others, 1989; 

Kim and Jeong, 2005). Elevated fluoride concentrations are 

associated with groundwater with long residence times as a 

result of increased interaction with aquifer materials, higher 

TDS concentrations, and increases in pH that result from 

mineral reactions contributing to the precipitation of calcite. 

The unconsolidated sand and gravel aquifers (Basin and Range 

basin-fill aquifers, Rio Grande aquifer system, and High Plains 

aquifer) and sandstone aquifers (Colorado Plateaus aquifers) 

had the highest fluoride concentrations at pH values greater 

than 7.0 (fig. 24A) and low calcium concentrations (fig. 24B). 

Groundwater compositions that primarily are controlled 

by the weathering of silicate minerals tend to be alkaline 

(as those reactions consume acidity), which is consistent 

with the occurrence of alkaline conditions associated with 

unconsolidated sand and gravel and volcanic rock lithologies 

of the western U.S. (DeSimone and others, 2015). In contrast, 

groundwater compositions that primarily are controlled by 

carbonate reactions, which also consume acidity, tend to 

have higher concentrations of HCO3 and calcite (DeSimone 

and others, 2015). In the Basin and Range carbonate-rock 

aquifers, two samples with high concentrations relative to 

the SMCL benchmark were alkaline with relatively high 

calcium concentrations.
78 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Calcium concentration, in milligrams per liter

Fluoride concentration, in milligrams per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0 300

0

2

4

6

8

0

2

4

6

8

0 100 200 300 0 100 200 300 100 200

0 100 200 300 0 100 200 300 0 100 200 300

B

pH, in standard units

Fluoride concentration, in milligrams per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

6 7 8 9 10 6 7 8 9 10 6 7 8 9 10

0

2

4

6

8

0

2

4

6

8

6 7 8 9 10 6 7 8 9 10 6 7 8 9 10

A

EXPLANATION

Low

Moderate

High

Relative concentration

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

EXPLANATION

Low

Moderate

High

Relative concentration

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

Figure 24. Fluoride concentrations by A, pH, aquifer lithology, and groundwater age; and B, calcium concentrations, aquifer lithology, 

and groundwater age in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017.
Status and Understanding Assessments of Water Quality 79

Fluoride concentration was negatively correlated with 

well elevation and aridity index in the Basin and Range 

basin-fill aquifers and Rio Grande aquifer system (fig. 23C); 

these PAs had a combination of relatively high aridity and 

lower elevations. In the High Plains aquifer, fluoride also was 

negatively correlated with aridity index, but, conversely, was 

positively correlated with well elevation; as noted earlier, 

these correlationslikely reflects the climate pattern overlying 

the High Plains aquifer, where precipitation and elevation 

gradients are directionally opposite. Negative correlations 

with percentage of irrigated land use in the Basin and Range 

basin-fill aquifers and the Columbia Plateau basaltic-rock 

aquifers indicated that fluoride concentrations were lowest in 

areas of irrigated land use, where flushing of solutes associated 

with higher groundwater recharge likely occurs relative to 

natural land use.

Factors Affecting Manganese and Iron

Sources of manganese and iron in groundwater include 

dissolution and weathering of minerals in soils, sediments, 

and rocks. Manganese and iron are discussed together because 

manganese and iron-oxide minerals both commonly coat 

mineral and sediment grains, and manganese commonly 

substitutes for iron in silicate and oxide minerals (Fram, 

2017). Both metals participate in redox processes and their 

solubilities are strongly dependent on groundwater redox 

conditions (Hem, 1985). Manganese (VI) and iron (III) 

occur in oxidizing conditions, whereas manganese (II) and 

iron (II) occur in reducing conditions. The reduced species 

are significantly more soluble than the oxidized species 

(Hem, 1978). Anthropogenic sources of manganese include 

municipal wastewater discharges, sewage sludge, mining 

and mineral processing, emissions from alloy, steel, and iron 

production, and combustion of fossil fuels (Howe and others, 

2004). Anthropogenic sources of iron include the iron and 

steel industry, sewage, and dust from mining (Reimann and de 

Caritat, 1998).

Manganese and iron are considered essential nutrients 

for the human body at low levels, but exposures at high 

levels are toxic (U.S. Department of Human Health Services, 

2012). Manganese has two benchmarks: a HBSL of 300 μg/L 

(Norman and others, 2018; U.S. Geological Survey, 2018a) 

and a SMCL of 50 μg/L (U.S. Environmental Protection 

Agency, 2018b). Iron also has two benchmarks: a HBSL 

of 4,000 μg/L (Norman and others, 2018; U.S. Geological 

Survey, 2018a) and a SMCL of 300 μg/L (table 2). Manganese 

and iron, when at concentrations above their SMCL 

benchmarks, are considered nuisance constituents in drinking 

water, causing unpleasant taste, odor, and appearance. 

High and moderate concentrations of manganese and iron 

relative to their HBSLs were both infrequent and occurred 

in 7 percent or less of samples from each PA (figs. 21E, G; 

tables 9A–F). Concentrations exceeding manganese and iron 

SMCL benchmarks were more common but mostly occurred 

in 15 percent or less of samples from each PA (figs. 21F, H,

tables 9A–F).

Manganese and iron concentrations were similarly 

correlated with the potential explanatory factors of dissolved 

oxygen concentrations (negative), TDS (positive), well 

elevation (negative), and aridity index (negative; figs. 23D, E). 

Correlations with land use in the Basin and Range basin-fill 

aquifers might reflect higher concentrations of manganese 

occurring in agricultural and irrigated land use in this PA. 

Manganese is considered to be an essential micronutrient for 

crops, and manganese fertilizer is commonly applied to boost 

crop production (Mousavi and others, 2011).

Factors Affecting Molybdenum

Molybdenum (Mo) is an uncommon element in the 

Earth’s crust but occurs frequently as an accessory mineral in 

many metal ores (Hem, 1985). Most molybdenum ore deposits 

are in the form of molybdenite (MoS2) and are associated with 

porphyry granite or quartz monzonite plutons (Misra, 2000). 

Shale, clay schists, and granite are the primary geogenic 

sources in soils (Evans and Barabash, 2010). Organic-rich 

sediments and sedimentary deposits in sulfidic environments 

could have high concentrations of molybdenum (Crusius 

and others, 1996), however. Anthropogenic sources of 

molybdenum to groundwater include industrial sources and 

surface application as biosolids (Evans and Barabash, 2010). 

Molybdenum has a HBSL of 30 μg/L (Norman and others, 

2018; U.S. Geological Survey, 2018a). Molybdenum is an 

essential trace nutrient for animals and plants (particularly 

legumes for nitrogen fixation); however, high levels in humans 

and animals can interfere with copper uptake (Vyskočil and 

Viau, 1999; Evans and Barabash, 2010).

High or moderate concentrations of molybdenum 

occurred in less than 10 percent of the Basin and Range 

basin-fill aquifers (table 9B; fig. 21I), whereas moderate 

concentrations occurred in 3 percent or less of the Rio 

Grande aquifer system, High Plains aquifer, Colorado 

Plateaus aquifers, and the Columbia Plateau basaltic-rock 

aquifer (tables 9C–F). The primary controls on molybdenum 

concentrations in groundwater are redox conditions, pH, 

and availability of molybdenum from aquifer materials 

(Smedley and others, 2014). Molybdenum differs from most 

other trace metals in that it occurs as a negatively charged 

species (as a molybdenate oxyanion or hydroxyanion) in soil 

solution at pH values greater than about 5.0, resulting in a 

decrease in molybdenum sorption with increasing pH (Evans 

and Barabash, 2010). Moderate and high concentrations of 

molybdenum most often occurred in the Basin and Range 

basin-fill and Rio Grande aquifer system, under alkaline, 

oxic groundwater conditions (fig. 25A), whereas moderate 

concentrations occurred under anoxic conditions in the High 

Plains aquifer and Columbia Plateau basaltic-rock aquifers. 
80 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

Total dissolved solids concentration, in milligrams per liter

Molybdenum concentration, in micrograms per liter

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

0

10

20

30

40

50

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

0

10

20

30

40

50

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

B

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Molybdenum concentration, in micrograms per liter

0

10

20

30

40

50

0

10

20

30

40

50

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

pH, in standard units

7 8 9 10 6 7 8 9 10 6 7 8 9 10

7 8 9 10 6 7 8 9 10 6 7 8 9 10

A

Aquifer lithology Redox category

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Anoxic Oxic

EXPLANATION

Low

Moderate

High

Relative concentration

Aquifer lithology Climate class

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Arid Semiarid

Dry

subhumid Humid

—

EXPLANATION

Low

Moderate

High

Relative concentration

— —

—

Figure 25. Molybdenum concentrations by A, pH, aquifer lithology, and redox; and B, total dissolved solids concentrations, aquifer 

lithology, and climate class in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 81

Under reducing conditions, molybdenum can accumulate in 

groundwater through its release from aquifer materials through 

reductive dissolution of iron and manganese oxides (Schlieker 

and others, 2001; Bennett and Dudas, 2003). For the anoxic 

samples with moderate or near moderate molybdenum 

concentrations, elevated concentrations of manganese and 

iron co-occurred, indicating molybdenum release from aquifer 

materials under iron- or manganese-reducing conditions.

Molybdenum was correlated with aridity index (negative) 

and TDS (positive) in the Basin and Range basin-fill 

aquifers, Rio Grande aquifer system, High Plains aquifer, and 

Columbia Plateau basaltic-rock aquifers (fig. 23F), which 

likely reflects the generally elevated TDS concentrations 

(greater than 500 mg/L) in groundwater and aridity common 

to these areas of the western United States (fig. 25B). The 

positive correlation with HCO3 in the High Plains aquifer and 

Colorado Plateaus aquifers likely results from the positive 

correlation with TDS because HCO3 was the dominant anion 

in these aquifers (fig. 23F).

Factors Affecting Selenium

Selenium (Se) is found as selenide minerals, selenate 

and selenite salts, and as substitution for sulfur in sulfide 

minerals. It is naturally concentrated in soils that overlie 

bedrock with high selenium concentrations (Stillings, 2017). 

Selenium in groundwater results from weathering and leaching 

of rocks and the dissolution or oxidation of soluble salts in 

soils (Kunmar and Riyazuddin, 2011). Anthropogenic sources 

of selenium include mining, processing, use in industrial 

and agricultural applications, and waste (Stillings, 2017). 

The EPA MCL for selenium in drinking water is 50 μg/L 

(U.S. Environmental Protection Agency, 2018a). Selenium 

is an essential micronutrient in animal and human diets 

but is toxic at elevated levels or from long-term exposure 

in areas with high levels of soil selenium (Saha, 2017; 

U.S. Environmental Protection Agency, 2018a). Selenium 

occurred at moderate concentrations in 3 percent of the High 

Plains aquifer (table 9D); all other concentrations were low 

(fig. 21J). Mobilization of selenium from bedrock or soils 

to groundwater occurs under oxidizing, slightly alkaline 

conditions (McNeal and Balistrieri, 1989; Presser and Swain, 

1990; Piper and others, 2000). Leaching of soil selenium 

is enhanced by anionic constituents such as nitrate, sulfate, 

and phosphate from anthropogenic sources such as fertilizer 

in agricultural land-use areas (Cary and Gissell-Nielsen, 

1973). Selenium concentrations in the High Plains aquifer 

were inversely correlated with hydrologic conditions (aridity 

index and depth to bottom of well perforation) and positively 

correlated with geochemical conditions (TDS, dissolved 

oxygen, and HCO3; fig. 23G); these relations are consistent 

with leaching of selenium from soils within the predominately 

alkaline, oxic (figs. 15A, B), and moderate- to elevated-TDS 

conditions that occur in the High Plains aquifer.

Factors Affecting Strontium

Strontium (Sr) is a fairly common element in the 

Earth’s crust, with chemistry similar to that of calcium 

(Hem, 1985). Natural sources of strontium in groundwater 

are the weathering of silicate minerals—particularly 

feldspars, dissolution of carbonate and sulfate minerals, 

and mixing with seawater and brines (McNutt, 2000; 

Faure and Mensing, 2005). Strontium has a HBSL of 

4,000 μg/L (Norman and others, 2018; U.S. Geological 

Survey, 2018a). This report evaluates concentrations of 

natural strontium (which are isotopically stable) and does 

not consider radioactive strontium-90 (90Sr). Radioactive 

strontium-90 is widely dispersed in the environment and 

the food chain from atmospheric testing of nuclear weapons 

in the 1950s and 1960s; it occurs in radioactive waste 

and nuclear power production waste and is regulated in 

drinking water by the EPA (U.S. Environmental Protection 

Agency, https://ofmpub.epa.gov/tdb/pages/contaminant/

contaminantOverview.do?contaminantId=10600, accessed 

May 22, 2020).

Strontium was detected at moderate and high 

concentrations in all PAs except for the Columbia Plateau 

basaltic-rock aquifers (fig. 21J). Moderate concentrations 

occurred in about 2–10 percent of the Basin and Range 

carbonate-rock aquifers, Basin and Range basin-fill aquifers, 

Rio Grande aquifer system, High Plains aquifer, and Colorado 

Plateaus aquifers (tables 9A–E). High concentrations occurred 

in 4 percent or less of the Rio Grande aquifer system and 

High Plains aquifer (tables 9C, D). Strontium was positively 

correlated with TDS in all PAs where moderate or high 

concentrations occurred, and negatively correlated with pH in 

the Basin and Range basin-fill aquifers, Rio Grande aquifer 

system, and Colorado Plateaus aquifers (fig. 23H). Strontium 

becomes increasingly soluble with increasing TDS, and 

relatively higher concentrations might result from the effects 

of competitive ions on dissolved constituents as they interact 

with mineral surfaces (Wright and others, 2015). Moderate 

and high concentrations of strontium primarily occurred at 

elevated TDS values (fig. 26A) and at pH values between 7.0 

and 7.9 (fig. 26B).
82 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

pH, in standard units

Strontium concentration, in micrograms per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0

2,000

4,000

6,000

8,000

0

2,000

4,000

6,000

8,000

6 7 8 9 10 6 7 8 9 10 6 7 8 9 10

6 7 8 9 10 6 7 8 9 10 6 7 8 9 10

B

Total dissolved solids concentration, in milligrams per liter

Strontium concentration, in micrograms per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0

2,000

4,000

6,000

8,000

0

2,000

4,000

6,000

8,000

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

A

EXPLANATION

Low

Moderate

High

Relative concentration

EXPLANATION

Low

Moderate

High

Relative concentration

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

Aquifer

lithology

Groundwater age

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Modern Mixed

Premodern

Not

measured

Figure 26. Strontium concentrations by A, total dissolved solids, aquifer lithology, and groundwater age; and B, pH values, aquifer 

lithology, and groundwater age in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality 

Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 83

Factors Affecting Thallium

Thallium (Tl) is a heavy metal and is mostly associated 

with potassium minerals in clays, soils, and granite (Kaplan 

and Mattigod, 1998; U.S. Geological Survey, 2018c). Several 

thallium minerals occur as sulfide or selenide complexes with 

antimony, arsenic, copper, lead, and silver (U.S. Geological 

Survey, 2018c). Thallium generally is present at low 

levels in the environment, but anthropogenic sources have 

resulted in increased concentrations near mineral smelters 

and coal-burning facilities (Karbowska, 2016). The MCL 

for thallium in groundwater is 2 μg/L (U.S. Environmental 

Protection Agency, 2018a). Thallium is toxic to humans, 

plants, animals, and microorganisms (Kazantzis, 2000; 

Peter and Viraraghavan, 2005), and tends to bioaccumulate 

(Karbowska, 2016). Thallium mobility in groundwater is 

largely controlled by redox conditions; it can mobilize under 

anoxic (reducing) and oxic (oxidizing) conditions. Thallium 

in soils has an affinity for iron and manganese oxyhydroxides 

and is mobilized by reductive dissolution of iron-manganese 

oxides. Conversely, in oxidizing conditions, thallium might be 

mobilized by gradual oxidation of thallium-bearing sulfides 

(Belzile and Chen, 2017). High concentration of thallium 

occurred in one well in the Basin and Range carbonate-rock 

PA (5 percent).

Factors Affecting Uranium

Uranium (U) is a radioactive trace element enriched 

in crystalline rocks such as granite and sediments derived 

from them. Sources of uranium to groundwater include 

dissolution of uranium-bearing minerals such as uraninite, 

zircon, and titanite and desorption of uranium from mineral 

surfaces in the presence of bicarbonate (Hem, 1985; Jurgens 

and others, 2010). In areas of agricultural land use, uranium 

contamination of soils has resulted from high or prolonged 

application of phosphate fertilizers, which can have high 

uranium concentrations (Rothbaum and others, 1979; 

Romero Guzmán and others, 2006). Uranium mobility in 

groundwater is largely controlled by redox conditions as 

well as pH. Uranium is highly soluble in its oxidized forms 

and only slightly soluble in its reduced forms; it can form 

bonds with other ions and remain in solution over a wider 

range of conditions than otherwise possible (DeSimone and 

others, 2015). Uranium is sometimes found in anoxic, low 

pH groundwater, possibly because of ion complexation or 

competition (Ayotte and others, 2011). The MCL for uranium 

is 30 μg/L (U.S. Environmental Protection Agency, 2018a). 

Uranium is weakly carcinogenic and chemically toxic; 

elevated concentrations in drinking water can cause kidney 

damage (Kurttio and others, 2002; U.S. Environmental 

Protection Agency, 2018a). The radioactive decay of uranium 

produces daughter products which also are of concern for 

human health with drinking-water benchmarks (for example, 

radium; U.S. Environmental Protection Agency, 2018a).

Moderate concentrations of uranium occurred in five 

PAs, all except for the Basin and Range carbonate-rock PA, 

with aquifer-scale proportions ranging from 1 to 15 percent; 

the highest proportion occurred in the High Plains aquifer 

(tables 9B–F). High concentrations of uranium, with 

aquifer-scale proportions ranging from 3 to 8 percent, occurred 

in the Basin and Range basin-fill aquifers, Rio Grande aquifer 

system, and High Plains aquifer; the highest proportion 

occurred in the High Plains aquifer (tables 9B–D). Moderate 

and high concentrations of uranium occurred most frequently 

in the unconsolidated sand and gravel aquifers, usually in 

association with oxic conditions, elevated TDS, and HCO3

concentrations ranging from 155 to 484 mg/L (figs. 27A, B). 

High concentrations of uranium previously have been noted 

throughout the High Plains aquifer, particularly in shallow 

groundwater (Nolan and Weber, 2015). Low recharge rates, 

oxic conditions, long flow paths, and high concentrations of 

naturally occurring uranium in rocks and soils contribute to 

the accumulation of uranium in the unconsolidated sand and 

gravel aquifers of the western United States (DeSimone and 

others, 2015).

Uranium concentrations were correlated with land use, 

hydrologic conditions (well elevation, depth to bottom of well 

perforation, and aridity index), and geochemical conditions 

(HCO3, pH, dissolved oxygen, and TDS; fig. 23I). Differences 

in positive or negative correlations in the PAs reflect 

groundwater conditions that favor the release of uranium from 

aquifer materials or reflect correlations among the explanatory 

factors (tables 6A–F).

Radionuclides

Radionuclides in groundwater primarily are from 

geologic sources and include uranium (note that uranium is 

considered with trace elements and discussed in the previous 

section, “Trace Elements”), radon (Rn), radium (Ra), 

polonium (Po), and lead (Pb). Radionuclides are produced 

naturally by the decay of radioactive parent materials, 

predominantly uranium and thorium (DeSimone and others, 

2015). Radionuclides in water (with the exception of uranium) 

typically are considered relative to their decay energy or 

radioactive activity, which is measured in picocuries per liter 

(piC/L). Radioactive activities for three radionuclides with 

health-based benchmarks (gross alpha, radium, and radon-222 

[radon]) were selected for evaluation by the understanding 

assessment in this report (table 2). Gross beta activity 

also has a health-based benchmark (4 millirems per year; 

U.S. Environmental Protection Agency, 2018a); gross beta 

activity occurred at low concentrations in all PAs and thus, 

was not further evaluated (table 3). Moderate concentrations 

of radionuclides occurred in 15 percent of the Basin and 

Range basin-fill aquifers and High Plains aquifer; moderate 

concentrations occurred in 2–13 percent of the remaining 

PAs (table 7). High-concentrations of radionuclides occurred 

in 1–5 percent of the PAs, excluding the Basin and Range 

basin-fill aquifers where no high concentrations occurred.
84 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Bicarbonate concentration, in milligrams per liter

Uranium concentration, in micrograms per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0

25

50

75

100

0

25

50

75

100

0 200 400 600 0 200 400 600 0 200 400 600

0 200 400 600 0 200 400 600 0 200 400 600

B

Total dissolved solids concentration, in milligrams per liter

Uranium concentration, in micrograms per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0

25

50

75

100

0

25

50

75

100

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500 0 500 1,000 1,500 2,000 2,500

Columbia Plateau

basaltic-rock aquifers

A

Aquifer lithology Redox category

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Anoxic Oxic

EXPLANATION

Low

Moderate

High

Relative concentration

Aquifer lithology Redox category

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Anoxic Oxic

EXPLANATION

Low

Moderate

High

Relative concentration

Figure 27. Uranium concentrations by A, total dissolved solids concentrations, aquifer lithology, and redox condition; and B,

bicarbonate concentration, aquifer lithology, and redox conditions in six western U.S. principal aquifers sampled by the U.S. 

Geological Survey National Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 85

Gross Alpha Activity

Gross alpha activity is a measure of radioactive 

decay from uranium-238. Activity is measured as 

72-hour and 30-day activities, with an MCL of 15 pCi/L 

(U.S. Environmental Protection Agency, 2018a). Only 

30-day gross alpha activities are evaluated in this report. 

Moderate and high concentrations of gross alpha activity 

were relatively uncommon and occurred in 4 percent or less 

of samples from each of the PAs; there were no moderate or 

high concentrations in the Basin and Range carbonate-rock 

aquifers (fig. 28A; tables 9B–F). Gross alpha activity had 

positive correlations with TDS in the Columbia Plateau 

basaltic-rock aquifers and HCO3 in the Rio Grande aquifer 

system (fig. 29A), which likely reflects geochemical 

conditions that favor release of the parent material uranium to 

groundwater (that is, oxic groundwater with elevated TDS and 

HCO3 concentrations; figs. 27A, B). Gross alpha activity was 

positively correlated with depth to bottom of well perforation 

in the High Plains aquifer and could reflect naturally occurring 

uranium within the aquifer lithology (Hall and others, 2019) 

or uranium-enriched surface-water irrigation that enters deeper 

regional groundwater (Böhlke and others, 2007; McMahon 

and Böhlke, 2007).

Radium Activity

Radium (Ra) can enter groundwater by dissolution 

of aquifer materials, by desorption from rock or sediment 

surfaces, and by release from minerals during radioactive 

decay (DeSimone and others, 2015). Radium-226, derived 

from radioactive decay of uranium-238, is the most common 

isotope of radium. Radium-228, derived from the radioactive 

decay of thorium, is less common. Both isotopes are 

carcinogenic. The MCL for radium is 5 pCi/L and is based 

on their combined activity (radium-226 plus radium-228; 

U.S. Environmental Protection Agency, 2018a). Radium 

evaluation in this report is based on their combined activity. 

Radium in drinking water is a human-health concern because 

it accumulates in bone and other tissues, thereby increasing 

lifetime cancer risks (DeSimone and others, 2015).

Moderate and high concentrations of radium were 

relatively uncommon (fig. 28B). High concentrations only 

occurred in the Colorado Plateaus aquifers (4 percent of 

PA); moderate concentrations prop occurred in the Basin 

and Range carbonate-rock aquifers (6 percent), Rio Grande 

aquifer system (2 percent), and the Colorado Plateaus 

aquifers (2 percent; tables 9A, C, E). Geochemical conditions 

that favor the release of radium into groundwater include 

anoxic conditions, low pH, and high concentrations of TDS 

(DeSimone and others, 2015). Although radium-226 decays 

from uranium, the mechanisms that favor the mobility of 

uranium are quite different than those that favor radium 

mobility. Under oxic groundwater conditions, uranium 

is highly soluble in its oxidized forms and only slightly 

soluble when reduced. In contrast, radium in oxic, low TDS 

water strongly sorbs to iron and manganese oxyhydroxides 

and to clays formed from feldspar weathering (Szabo and 

Zapecza, 1991; Thomas and others, 1993). High radium 

concentrations occurred in two samples in the Colorado 

Plateuas aquifers under anoxic groundwater conditions at pH 

values ranging from 6.9 to 7.2 (fig. 30). Radium correlated 

with dissolved oxygen (negative) and with depth to bottom 

of well perforation (positive) in the Basin and Range 

carbonate-rock aquifers, indicating that lower dissolved 

oxygen concentrations associated with deeper wells favored 

the release of radium to groundwater (fig. 29B). The positive 

correlation with TDS in the Basin and Range basin-fill 

aquifers and negative correlation with pH in the Colorado 

Plateaus aquifers reflect favorable geochemical conditions for 

radium release.

Radon Activity

Radium-226 decays into radon-222 (radon) gas. Radon 

in groundwater occurs as a dissolved gas of which most 

is released into the air. Inhalation of radon poses a risk 

of lung cancer (U.S. Environmental Protection Agency, 

2019b). Radon has two proposed regulatory water-quality 

benchmarks: (1) an MCL of 300 pCi/L and (2) an alternative 

maximum contaminant level (AMCL) of 4,000 pCi/L 

(U.S. Environmental Protection Agency, 2018c). The lower 

MCL was proposed to apply in states and to public water 

systems that do not have programs to address health risks from 

radon in indoor air, whereas the AMCL would apply to states 

with established indoor air programs (U.S. Environmental 

Protection Agency, 2018c). The AMCL (4,000 pCi/L) is 

considered in this report.

Moderate and high concentrations of radon were 

relatively uncommon across the PAs, occurring at aquifer-scale 

proportions up to 5 percent (fig. 28C; tables 9A–F). Significant 

correlations with potential explanatory factors occurred in 

four PAs―the Basin and Range carbonate-rock aquifers, 

Rio Grande aquifer system, High Plains aquifer, and 

Colorado Plateaus aquifers (fig. 29C). Correlations of radon 

with aridity index were common to the four PAs; positive 

correlations occurred among the the PAs with arid climates 

(the Basin and Range carbonate-rock aquifers, Rio Grande 

aquifer system, and Colorado Platueaus aquifers, fig. 14A), 

and negative correlation with aridity index occurred in the 

High Plains aquifer. In the Rio Grande aquifer system and 

Colorado Plateaus aquifers, elevated concentrations of radon 

were associated with geochemical conditions of low TDS 

and elevated HCO3 concentrations, which occurred at higher 

elevations in the northern part of the Rio Grande Valley 

and the eastern boundary of the Colorado Plateaus aquifers 

associated with recharge fronts (fig. 28C). These correlations 

could reflect radon associations with source rock or recharge 

fronts that are represented by the selected explanatory factors.
86 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

A

30-day gross alpha activity, in picocuries per liter

High (>15.0)

Moderate (7.5 to 15.0)

Low (<7.5)

Not detected

Spring Groundwater

well

—

—

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

0 150 300 MILES

0 150 300 KILOMETERS

Map area

Figure 28. Concentrations of A, 30-day gross alpha activity; B, radium-226 and radium-228 (combined); and C, radon-222 in six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 87

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

B

Radium 226 and Radium 228 (combined),

in picocuries per liter

High (>5.0)

Moderate (2.5 to 5.0)

Low (<2.5)

Not detected

Spring Groundwater

well

—

—

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 28. —Continued
88 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Radon-222, in picocuries per liter

High (>4,000)

Moderate

(2,000 to 4,000)

Low (<2,000)

Not detected

Spring Groundwater

well

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

C

Figure 28. —Continued
Status and Understanding Assessments of Water Quality 89

−0.28

0.33

−0.26

0.28

−0.40 −0.34

0.45

0.58

−0.74

0.31

0.47 0.32

−0.32

−1.0

−0.5

0.0

1.0

0.5

rho

Dissolved

oxygen

Depth to

bottom of

well perforation

Basin and Range carbonate-rock aquifers

Explanatory factor

−1.0

−0.5

0.0

1.0

0.5

High Plains aquifer

Depth to

bottom of

well perforation

rho

pH

−1.0

−0.5

0.0

1.0

0.5

Rio Grande aquifer system

rho

−1.0

−0.5

0.0

1.0

0.5

Columbia Plateau aquifers

Basin and Range basin-fill aquifers

rho

Aridity

index

−1.0

−0.5

0.0

1.0

0.5

rho

Explanatory factor

Well

elevation

Percent

irrigated

land use

Percent

natural

land use

Total

dissolved

solids

Explanatory factor

−1.0

−0.5

0.0

1.0

0.5

rho

−1.0

−0.5

0.0

1.0

0.5

rho

Basin and Range carbonate-rock aquifers Rio Grande aquifer system

Aridity

index

Aridity

index

B

A

C

Radium 226 and Radium 228 (combined)

30-day gross alpha activity

Radon-222

HCO3

Total

dissolved

solids

HCO3

−0.34

−1.0

−0.5

0.0

1.0

0.5

Colorado Plateaus aquifers

pH

rho

−0.34

−1.0

−0.5

0.0

1.0

0.5

Colorado Plateaus aquifers

pH

rho

Explanatory factor

Total

dissolved

solids

Explanatory factor Explanatory factor

0.25 0.29

Figure 29. Spearman’s rho results for radionucludes A, 30-day gross alpha activity; B, radium-226 and radium-228 (combined); and 

C, radon-222 in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017.
90 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

pH, in standard units

Radium 226 and Radium 228 (combined), in picocuries per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0

2

4

6

0

2

4

6

6 7 8 9 10 6 7 8 9 10 6 7 8 9 10

6 7 8 9 10 6 7 8 9 10 6 7 8 9 10

Aquifer lithology Redox category

Carbonate rock

Unconsolidated

sands and

gravels

Sandstone

Basaltic rock

Anoxic Oxic

EXPLANATION

Low

Moderate

High

Relative concentration

Figure 30. Relations of radium-226 and radium-228 (combined) with measured pH values by aquifer lithology and redox 

condition in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment 

Project, 2013–2017.

Salinity Indicators

Total dissolved solids, sulfate, and chloride 

concentrations are considered salinity indicators in this 

report. These constituents have SMCL benchmarks (table 2) 

and can affect the aesthetic properties of water, such as 

taste, odor, or color (U.S. Environmental Protection Agency, 

2018b). The SMCL benchmark for TDS is 500 mg/L. The 

SMCL benchmark for sulfate and chloride is 250 mg/L. 

Natural sources and processes that increase TDS, chloride, 

and sulfate in groundwater include dissolution of minerals 

in soils, sediments and rock; concentration of solutes in 

shallow groundwater by evapotranspiration; mixing with 

hydrothermal fluids or saline or brackish groundwater; and 

mixing with saline or brackish water from the ocean, estuaries 

or saline lakes (Hem, 1985). Anthropogenic sources of these 

constituents include water used for irrigation, wastewater 

discharge, brines from petroleum extraction activity, or direct 

application (road salts, fertilizers, urban runoff) to the land 

surface. Sulfate and chloride are major ions that typically are 

important contributors to TDS.

Moderate and high concentrations of salinity indicators 

occurred in all six western PAs except for chloride in the 

Columbia Plateau basaltic-rock aquifers (fig. 31A). High 

concentrations of TDS ranged from 25 to 36 percent and 

generally were in the arid Southwest and in the southern part 

of the High Plains aquifer (fig. 32A; tables 9A–F). Where 

precipitation is relatively low and evaporation relatively 

high, less groundwater recharge occurs and products of 

rock weathering can concentrate in soils, resulting in 

high groundwater TDS when precipitation is sufficient to 

cause recharge. Parts of the arid Southwest have shallow 

groundwater with high evaporation rates that also contribute 

to elevated salinity indicators. The unconsolidated sand 

and gravel aquifers of the Southwest include sedimentary 

rocks, carbonate rocks, and evaporites that are more soluble 

and easily weathered than the quartz-rich sediments. Total 

dissolved solids were negatively correlated with several 

explanatory factors representing hydrologic conditions—

aridity index, depth to bottom of well perforation, and well 

elevation—which likely are proxies for conditions associated 

with climate, shallow groundwater, and hydrologic position 

(fig. 31B). Natural upwelling of saline groundwater, pumping 

of deep groundwater, and irrigated agriculture are all potential 

sources of TDS in the High Plains aquifer (DeSimone and 

others, 2015). High TDS concentrations in the southern part 

of the High Plains aquifer (fig. 32A) likely result from mixing 

with saline groundwater from underlying geologic units (Nativ 

and Smith, 1987; Scanlon and others, 2009). Total dissolved 

solids were positively correlated with HCO3 in all of the PAs 

except for the Basin and Range carbonate-rock aquifers; this 

is likely a causative correlation because HCO3 is a primary 

component of TDS.
Status and Understanding Assessments of Water Quality 91

0.27

−0.44 −0.46 −0.31

0.33

−0.31

−0.49 −0.65

0.40

−0.25 −0.40 −0.41 −0.36

0.69

−0.37 −0.30

0.80

0.52

−0.57

−0.47

0.73

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

High Plains aquifer

Colorado Plateaus aquifers Columbia Plateau aquifers

Depth to

bottom of

well perforation

HCO3

rho

rho

rho

–1.0

–0.5

0.0

0.5

1.0

Aridity

index

Well

elevation

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Dissolved

oxygen

Percent

irrigated

land use

Basin and Range carbonate-rock aquifers Basin and Range basin-fill aquifers

Rio Grande aquifer system

rho

rho rho

Dissolved

oxygen

Aridity

index

Well

elevation

Percent

agricultural

land use

Explanatory factor Explanatory factor

Percent irrigated

land use

Aridity

index

Percent

irrigated

land use

Dissolved

oxygen

HCO3

HCO3 Aridity

index

Well

elevation

HCO3

HCO3 pH

Total dissolved solids

Salinity indicators

Depth to

bottom of

well perforation

Well

elevation

B

−0.66

0.47

Columbia Plateau aquifers

Colorado Plateaus aquifers

High Plains aquifer

Rio Grande aquifer system

Basin and Range basin-fill aquifers

Basin and Range carbonate-rock aquifers

Columbia Plateau aquifers

Colorado Plateaus aquifers

High Plains aquifer

Rio Grande aquifer system

Basin and Range basin-fill aquifers

Basin and Range carbonate-rock aquifers

Columbia Plateau aquifers

Colorado Plateaus aquifers

High Plains aquifer

Rio Grande aquifer system

Basin and Range basin-fill aquifers

Basin and Range carbonate-rock aquifers

0.01 0.1 1 10

Concentration, dimensionless

Total dissolved

solids Chloride Sulfate

A

EXPLANATION

Groundwater well Spring

Low Moderate High

Relative concentration

Figure 31. A, Concentrations of salinity indicators relative to benchmarks; and B, Spearman’s rho results for total dissolved solids in six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
92 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

A

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Total dissolved solids, in milligrams per liter

High (>500)

Moderate (250 to 500)

Low (<250)

Not detected

Spring Groundwater

well

—

— —

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 32. Concentrations of A, TDS; B, chloride; and C, sulfate in six western U.S. principal aquifers sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 93

Salt Lake

City

Las Vegas

Phoenix

Albuquerque

El Paso

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

Rio Grande

Colorado

River

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

B

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Chloride, in milligrams per liter

High (>250)

Moderate (125 to 250)

Low (<125)

Not detected

Spring Groundwater

well

—

— —

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 32. —Continued
94 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Sulfate, in milligrams per liter

High (>250)

Moderate (125 to 250)

Low (<125)

Not detected

Spring Groundwater

well

—

— —

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

C

Figure 32. —Continued
Status and Understanding Assessments of Water Quality 95

As noted earlier, sulfate and chloride are major ions that 

typically are important contributors to TDS. In addition to the 

natural sources described earlier in this section, anthropogenic 

sources of chloride include septic, sewage, and wastewater; 

chloride is relatively inert, that is, it doesn’t react with aquifer 

materials or other dissolved constituents in dilute water and is 

a useful tracer of human influence on water quality (DeSimone 

and others, 2015). High and moderate concentrations of 

chloride mostly coincide with urban areas in each of the 

PAs (for example, Salt Lake City, UT; urban areas along 

the Colorado River southwest of Phoenix, AZ; and urban 

areas along the Rio Grande River in New Mexico and Texas; 

fig. 32B). High and moderate concentrations also occurred in 

the southern part of the High Plains aquifer, presumably from 

mixing with underlying saline groundwater. Aquifer-scale 

proportions with high chloride ranged from about 2 percent 

in the Colorado Plateaus aquifers to 12 percent in the Basin 

and Range basin-fill aquifers (tables 9A–E). Sulfate and 

chloride concentrations were well correlated in all PAs 

(Spearman’s rho values greater than or equal to 0.75). Sulfate 

concentrations can be affected by redox, pH, and weathering 

of aquifer materials. Anthropogenic sources of sulfate include 

dry and wet atmospheric deposition from industrial and urban 

emissions and leaching of accumulated solutes in soils by 

irrigation (Hem, 1985). Aquifer-scale proportions with high 

sulfate ranged from about 2 percent in the Columbia Plateau 

basaltic-rock aquifers to 15 percent in the Colorado Plateaus 

aquifers (fig. 32C; tables 9A–F).

Nutrients

Nutrients are elements that are important to plant growth 

and survival. Nutrients in water—specifically nitrogen and 

phosphorus compounds—have both natural and anthropogenic 

sources. Natural sources include dissolution of organic 

material in soils, animal waste, and atmospheric deposition. 

Anthropogenic sources can be nonpoint or point: point sources 

include municipal and industrial discharge and concentrated 

animal feeding operations, whereas non-point sources include 

applications of commercial fertilizers on agricultural and 

residential lands, cultivation of nitrogen-fixing crops, septic 

systems, and combustion of fossil fuels (Dubrovsky and 

others, 2010). Excess nutrients in waterways can promote 

algal blooms (eutrophication). Both oxidized and reduced 

forms of nitrogen, nitrate (NO3) or nitrite (NO2), respectively, 

have drinking-water benchmarks (MCLs of 10 and 1 mg/L, 

respectively; U.S. Environmental Protection Agency, 

2018a). Nitrite only occurred at low concentrations and was 

not selected for further evaluation by the understanding 

assessment. Excess nitrate concentrations in drinking water 

can cause methemoglobinemia, or “blue baby” syndrome, 

in infants (U.S. Environmental Protection Agency, 2018a). 

Nitrate in groundwater most commonly occurs in oxic 

groundwater systems, and concentrations above the national 

background concentration in groundwater (1 mg/L) might 

result from anthropogenic sources (Nolan and Hitt, 2006; 

Dubrovsky and others, 2010). Background concentrations can 

vary locally; however (Dubrovsky and others, 2010), and are 

likely higher for the High Plains aquifer (for example, 4 mg/L 

in shallow groundwater; Gurdak and others, 2009).

Moderate and high concentrations of nutrients were 

relatively uncommon (table 7). High aquifer-scale proportions 

of nitrate occurred in 3 percent or less of the Rio Grande 

aquifer system, the High Plains aquifer, and the Columbia 

Plateau basaltic-rock aquifers (fig. 33; tables 9C, D, F). 

Moderate aquifer-scale proportions of nitrate ranged from 

0 to 11 percent of PAs, with the highest aquifer proportions 

occurring in the Basin and Range basin-fill aquifers 

(9 percent), the High Plains aquifer (11 percent), and the 

Columbia Plateau basaltic-rock aquifers (7 percent; tables 9B, 

D, F). Nitrate was most strongly correlated (positive) with 

dissolved oxygen in the five PAs with moderate or high 

concentrations (fig. 34). Moderate or high concentrations 

occurred in oxic groundwater that was predominately of 

modern or mixed groundwater age (fig. 35). The positive 

correlation between dissolved oxygen and irrigated land use in 

the Columbia Plateau basaltic-rock aquifers (table 6F) could 

reflect the influence of irrigated land use within Quincy-Pasco 

subunit (fig. 8B), an area of intensive irrigated agriculture 

that receives more than 2.5 million acre-feet of surface 

water per year from the Columbia Basin Irrigation Project 

(Williamson and others, 1998), resulting in oxygen-rich 

surface-water recharge to groundwater. Land use surrounding 

the Quincy-Pasco subunit is predominately non-irrigated 

dryland agriculture or rangeland grazing (Williamson and 

others, 1998). In the Basin and Range basin-fill aquifers, 

negative correlations for nitrate with irrigated and agricultural 

land indicated that agriculture is likely not a source of nitrate 

in this PA; other nitrate sources or long lag times between 

irrigation and GW recharge (delayed arrival of nitrate beneath 

agricultural lands) not represented by the potential explanatory 

factors considered in this report might contribute to the 

moderate nitrate concentrations that occur in this PA.
96 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Nitrate, in milligrams per liter

High (>10)

Moderate (5 to 10)

Low (<5)

Not detected

Spring Groundwater

well

—

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

Map area

Figure 33. Concentrations of nitrate in six western U.S. principal aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 97

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

−1.0

−0.5

0.0

1.0

0.5

High Plains aquifer Colorado Plateaus aquifers

Columbia Plateau aquifers

Depth to

bottom of 

well perforation

HCO3

rho rho

rho

Dissolved

oxygen

Percent

agricultural

land use

–1.0

–0.5

0.0

0.5

1.0

–1.0

–0.5

0.0

0.5

1.0

Rio Grande aquifer system

rho

rho

Dissolved

oxygen

Percent

agricultural

land use

Explanatory factor

Explanatory factor

Aridity

index

Total

dissolved

solids

Dissolved

oxygen

Percent

natural

land use

pH

Dissolved

oxygen

HCO3

Nitrate

Percent

irrigated

land use

Dissolved

oxygen

Percent

irrigated

land use

Percent

agricultural

land use

Well

elevation

Total

dissolved

solids

Basin and Range basin-fill aquifers

−0.25 −0.40

0.33

−0.28

0.76

−0.39 −0.37

−0.33

0.29

0.49

0.29 0.27

−0.43

0.82

−0.34 −0.38

0.55 0.35 0.23

Figure 34. Spearman’s rho results for nitrate in six western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017.
98 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Dissolved oxygen, in milligrams per liter

Nitrate as nitrogen, in milligrams per liter

High Plains

aquifer

Colorado Plateaus

aquifers

Columbia Plateau

basaltic-rock aquifers

Basin and Range

carbonate-rock aquifers

Basin and Range

basin-fill aquifers

Rio Grande

aquifer system

0 5 10 0 5 10 0 5 10

0

5

10

15

0

5

10

15

0 5 10 0 5 10 0 5 10

Groundwater

age

Redox category

Modern

Mixed

Premodern

Not

measured

Anoxic Oxic

EXPLANATION

Low

Moderate

High

Relative concentration

Figure 35. Relations of measured nitrate concentration with dissolved oxygen concentration, by groundwater age and redox 

class, for six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017.

Organics and Special Interest Constituents

Organic constituents did not occur at high concentrations 

in any of the six PAs (table 10). Moderate concentrations 

occurred in three PAs—the Basin and Range basin-fill aquifers 

(VOCs), High Plains aquifer (pesticides and VOCs) and 

Colorado Plateaus aquifers (VOCs). Five pesticides (atrazine, 

bromacil, didealkylatrazine, propazine, and propoxur) and 

two VOCs (chloroform and bromodichloromethane) were 

selected for further analysis by the understanding assessment

because they were detected relatively frequently (between 

10 and 43 percent) at low concentrations in at least one 

principal aquifer study (fig. 36; tables 11, 12). Additionally, 

perchlorate—a special interest constituent—is included in this 

discussion. Perchlorate occurred at moderate concentrations 

in the two PAs in which it was collected: the Rio Grande 

aquifer system and the High Plains aquifer; there were no 

high concentrations.

Pesticides

Pesticides and pesticide compounds (hereafter, pesticides) 

include herbicides, insecticides, and fungicides—and their 

degradates—that are used to control unwanted weeds, insects, 

fungi, and other pests in agricultural and urban settings. 

The five pesticides selected for further evaluation included 

four herbicides (atrazine, didealkylatrazine, bromacil, and 

propazine) and one insecticide (propoxur). These pesticides 

had detection frequencies greater than 10 percent in one or 

more of the PAs (at low concentrations relative to health-based 

benchmarks; table 11; figs. 37A–E, respectively). Only one 

moderate concentration of a pesticide occurred in any PA—the 

herbicide atrazine in the High Plains aquifer (1 percent of PA 

area). The High Plains aquifer also had the highest frequency 

of herbicide detections of all the PAs, and four herbicides 

were detected at low concentrations―43 percent for atrazine, 

28 percent for didealkylatrazine, 18 percent for propazine, 

and 10 percent for bromacil (table 11). Of these herbicides, 

didealkylatrazine, a degradate of atrazine, generally was 

detected at frequencies less than or equal to atrazine except 

in the Basin and Range carbonate-rock aquifers (table 11). 
Status and Understanding Assessments of Water Quality 99

Table 10. Summary of aquifer-scale proportions for organic constituents with 

health-based benchmarks and for the special interest constituent, perchlorate, in 

six western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.1 of water quality benchmark but less than benchmark; 

low, concentrations less than 0.1 of benchmark and includes non-detections]

Principal aquifer

Number

of

samples

Percent of 

samples with 

detections

Aquifer scale proportion

(percent)

Low Moderate High

Pesticides

Basin and Range carbonate-rock aquifers 20 10 100 0 0

Basin and Range basin-fill aquifers 78 26 100 0 0

Rio Grande aquifer system 60 10 100 0 0

High Plains aquifer 79 19 99 1 0

Colorado Plateaus aquifers 59 12 100 0 0

Columbia Plateau basaltic-rock aquifers 60 25 100 0 0

Volatile organic carbons

Basin and Range carbonate-rock aquifers 20 10 100 0 0

Basin and Range basin-fill aquifers 78 22 97 13 0

Rio Grande aquifer system 60 13 100 0 0

High Plains aquifer 80 20 99 21 0

Colorado Plateaus aquifers 59 46 95 35 0

Columbia Plateau basaltic-rock aquifers 60 23 100 0 0

Perchlorate

Rio Grande aquifer system 60 58 97 3 0

High Plains aquifer 79 82 89 11 0

1Includes single moderate concentration detections of 1,2-dichloropropane and tetrachloroethylene; 

respective detection frequencies were less than 10 percent.

2Includes a single moderate concentration detection of trichloroethylene; respective detection frequency 

was less than 10 percent.

3Includes single moderate concentration detections of dichloromethane, carbon tetrachloride, and chloroform; except for chloroform respective detection frequencies were less than 10 percent.
100 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

High Plains aquifer

Columbia Plateau aquifers

Colorado Plateaus aquifers

Basin and Range carbonate-rock aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

Organics and special interest

Detection frequency

Maximum relative concentration, dimensionless

Detection frequency

0.0001

0.01

1

0.001

Maximum relative concentration, dimensionless Maximum relative concentration, dimensionless

0.0001

0.01

1

0.001

0.0001

0.01

1

0.001

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100

Chloroform

Chloroform

Chloroform

Perchlorate

Chloroform

Atrazine

Chloroform

Propoxur

Atrazine

Bromodichloromethane

Chloroform

Didealkylatrazine

Perchlorate

Bromodichloromethane Bromacil

Propazine

Propoxur

Propoxur

Propoxur

EXPLANATION

Low Moderate High

Relative concentration Pesticide Special interest Volatile organic compound

Figure 36. Detection frequency and maximum relative concentration of selected organic and special interest constituents 

detected in groundwater samples collected in six western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 101

Table 11. Summary of aquifer-scale proportions for pesticides selected for further 

evaluation in six western U.S. principal aquifers sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.1 of water quality benchmark but less than benchmark; 

low, concentrations less than 0.1 of bechmark and includes non-detections]

Constituent and principal aquifer

Number

of

samples

Percent of 

samples with 

detections

Aquifer scale proportion

(percent)

Low Moderate High

Pesticides

Atrazine (herbacide)

Basin and Range carbonate-rock aquifers 20 0 100 0 0

Basin and Range basin-fill aquifers 78 6 100 0 0

Rio Grande aquifer system 60 0 100 0 0

High Plains aquifer 80 43 99 1 0

Colorado Plateaus aquifers 60 0 100 0 0

Columbia Plateau basaltic-rock aquifers 60 13 100 0 0

Didealkylatrazine (herbicide degradate)

Basin and Range carbonate-rock aquifers 20 5 100 0 0

Basin and Range basin-fill aquifers 78 6 100 0 0

Rio Grande aquifer system 60 0 100 0 0

High Plains aquifer 79 28 100 0 0

Colorado Plateaus aquifers 58 2 100 0 0

Columbia Plateau basaltic-rock aquifers 60 0 100 0 0

Bromacil (herbicide)

Basin and Range carbonate-rock aquifers 20 0 100 0 0

Basin and Range basin-fill aquifers 78 4 100 0 0

Rio Grande aquifer system 60 2 100 0 0

High Plains aquifer 79 10 100 0 0

Colorado Plateaus aquifers 58 0 100 0 0

Columbia Plateau basaltic-rock aquifers 60 8 100 0 0

Propazine (herbicide)

Basin and Range carbonate-rock aquifers 20 0 100 0 0

Basin and Range basin-fill aquifers 78 1 100 0 0

Rio Grande aquifer system 60 0 100 0 0

High Plains aquifer 79 18 100 0 0

Colorado Plateaus aquifers 58 0 100 0 0

Columbia Plateau basaltic-rock aquifers 60 2 100 0 0

Propoxur (insecticide)

Basin and Range carbonate-rock aquifers 20 0 100 0 0

Basin and Range basin-fill aquifers 78 1 100 0 0

Rio Grande aquifer system 60 0 100 0 0

High Plains aquifer 79 1 100 0 0

Colorado Plateaus aquifers 58 5 100 0 0

Columbia Plateau basaltic-rock aquifers 60 10 100 0 0
102 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Table 12. Summary of aquifer-scale proportions for volatile organic compounds (VOCs) 

selected for further evaluation in six western U.S. principal aquifers sampled by the 

U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

[Relative concentration categories: high, concentrations greater than water quality benchmark; 

moderate, concentrations greater than or equal to 0.1 of water quality benchmark but less than benchmark; 

low, concentrations less than 0.1 of bechmark and includes non-detections]

Constituent and principal aquifer

Number

of

samples

Percent of 

samples with 

detections

Aquifer scale proportion

(percent)

Low Moderate High

VOCs

Chloroform

Basin and Range carbonate-rock aquifers 20 20 100 0 0

Basin and Range basin-fill aquifers 78 26 100 0 0

Rio Grande aquifer system 60 15 100 0 0

High Plains aquifer 80 28 100 0 0

Colorado Plateaus aquifers 59 15 98 2 0

Columbia Plateau basaltic-rock aquifers 60 17 100 0 0

Bromodichloromethane

Basin and Range carbonate-rock aquifers 20 10 100 0 0

Basin and Range basin-fill aquifers 78 5 100 0 0

Rio Grande aquifer system 60 3 100 0 0

High Plains aquifer 80 10 100 0 0

Colorado Plateaus aquifers 59 8 100 0 0

Columbia Plateau basaltic-rock aquifers 60 2 100 0 0

Dibromochloromethane

Basin and Range carbonate-rock aquifers 20 0 100 0 0

Basin and Range basin-fill aquifers 78 1 100 0 0

Rio Grande aquifer system 60 2 100 0 0

High Plains aquifer 80 4 100 0 0

Colorado Plateaus aquifers 59 5 100 0 0

Columbia Plateau basaltic-rock aquifers 60 3 100 0 0

Bromoform

Basin and Range carbonate-rock aquifers 20 0 100 0 0

Basin and Range basin-fill aquifers 78 1 100 0 0

Rio Grande aquifer system 60 2 100 0 0

High Plains aquifer 80 6 100 0 0

Colorado Plateaus aquifers 59 2 100 0 0

Columbia Plateau basaltic-rock aquifers 60 5 100 0 0
Status and Understanding Assessments of Water Quality 103

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

A

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Atrazine, in nanograms per liter

High (>3,000)

Moderate

(300 to 3,000)

Low (<300)

Not detected

Spring Groundwater

well

—

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

— —

Map area

Figure 37. Concentrations of selected pesticides A, atrazine; B, didealkylatrazine; C, bromacil; D, propazine and E, propoxur 

in six western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 

2013–2017.
104 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

B 125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Didealkylatrazine, in nanograms per liter

High (>12,000)

Moderate

(1,200 to 12,000)

Low (<1,200)

Not detected

Spring Groundwater

well

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

— —

— —

Map area

Figure 37. —Continued
Status and Understanding Assessments of Water Quality 105

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

C 125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Bromacil, in nanograms per liter

High (>100,000)

Moderate

(10,000 to 100,000)

Low (<10,000)

Not detected

Spring Groundwater

well

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

— —

— —

Map area

Figure 37. —Continued
106 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

D 125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Propazine, in nanograms per liter

High (>40,000)

Moderate

(4,000 to 40,000)

Low (<4,000)

Not detected

Spring Groundwater

well

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

— —

— —

Map area

Figure 37. —Continued
Status and Understanding Assessments of Water Quality 107

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

E 125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Propoxur, in nanograms per liter

High (>2,000)

Moderate

(200 to 2,000)

Low (<200)

Not detected

Spring Groundwater

well

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

— —

— —

Map area

Figure 37. —Continued
108 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Degradate concentrations in groundwater might exceed parent 

compound concentrations for the triazine (which include 

atrazine) and chloroacetanilide (which include metolachlor) 

herbicides (Kolpin and others, 1998, 2000; Scribner and 

others, 2000). Herbicide degradates are often more water 

soluble than their parent compounds; therefore, herbicide 

degradates are leached more rapidly through the soil into 

the groundwater, causing the degradate concentrations to be 

higher (Scribner and others, 2000). In this report, the herbicide 

degradate didealkylatrazine was the only degradate included 

for the understanding assessment (because it occurred at 

detection frequencies greater than 10 percent); however, other 

atrazine degradate compounds were detected (table 3). For 

example, in the Columbia Plateau basaltic-rock aquifers, low 

concentrations of atrazine (13-percent detections) and one of 

its degradate, 2-hydroxyatrazine (2 percent) were detected.

Correlations with pesticide compounds and potential 

explanatory factors occurred only in the High Plains aquifer 

and the Columbia Plateau basaltic-rock aquifers (fig. 38). 

Herbicides in the High Plains aquifer were positively 

correlated with geochemical factors (TDS and HCO3

concentrations) as well as percentage of urban land use, 

and inversely correlated with natural land use. The inverse 

correlation with natural land use likely reflects that herbicides 

generally are not applied or used in natural land use areas. It 

is likely that surface application of herbicides, particularly in 

agricultural areas, results in their transport and leaching to the 

shallow groundwater. Atrazine and didealkylatrazine were also 

negatively correlated with depth to bottom of well perforation, 

indicating they occur more commonly in shallow groundwater. 

The insecticide propoxur did not have significant correlations 

with any of the potential explanatory factors considered in 

this report.

Total

dissolved

solids

pH Depth to

bottom of

well perforation

HCO3

−0.6

−0.3

0.0

0.3

0.6

rho

Percent

natural

land use

Percent

urban

land use

HCO3 Percent

natural

land use

Total

dissolved

solids

Percent

urban

land use

HCO3 Percent

natural

land use

Percent

urban

land use

Total

dissolved

solids

Depth to

bottom of

well perforation

HCO3 Percent

natural

land use

Percent

urban

land use

Total

dissolved

solids

Atrazine Bromacil

Didealkylatrazine Propazine

Explanatory factor

Pesticides

−0.6

−0.3

0.0

0.3

0.6

−0.6

−0.3

0.0

0.3

0.6

rho

Explanatory factor

−0.6

−0.3

0.0

0.3

0.6

−0.51

0.30

−0.30 −0.22

0.44 0.40

0.30

−0.30

0.33 0.26 0.31

−0.31

0.25

−0.40

0.28 0.45

−0.41

0.39 0.29 0.40

Principal aquifer

EXPLANATION

High Plains aquifer Columbia Plateau aquifers

rhorho

Figure 38. Spearman’s rho results for pesticides in two western U.S. principal aquifers sampled by the U.S. Geological Survey National 

Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 109

Volatile Organic Compounds (VOCs)

Volatile organic compounds are characterized by their 

tendency to volatilize or evaporate and are present in many 

household, commercial, industrial, and agricultural products; 

common VOCs include solvents, gasoline hydrocarbons, 

and trihalomethanes (THMs; which may originate as water 

chlorination by-products). Although moderate concentrations 

of VOCs occurred in several of the PAs (table 10), these 

were often single occurrences of one or more compounds and 

were not included in the understanding assessment. Only two 

VOCs―chloroform and bromodichloromethane―which are 

both THMs, were included in the understanding assessment

because they were detected at frequencies greater than or 

equal to 10 percent (fig. 36). Trihalomethanes, also known 

as disinfection by-products, are produced from the use of 

chlorine to disinfect drinking water; they include chlorinated 

and brominated compounds, most commonly chloroform, 

bromodichloromethane, dibromochloromethane, and 

bromoform (Ivahnenko and Barbash, 2004). Chloroform is the 

most commonly detected VOC in groundwater and drinking 

water in the United States; detection frequencies typically are 

higher in PSWs than in domestic wells (11 percent compared 

to 5 percent) and might be attributed to higher pumping rates 

for PSWs and their proximity to developed areas (Zogorski 

and others, 2006). Trihalomethanes in drinking water are 

associated with acute and chronic human-health problems 

including nausea, headaches, and damage to liver and kidneys 

from prolonged exposure (Agency for Toxic Substances and 

Disease Registry, 1997, 2018). The MCL for combined THMs 

is 80 μg/L (U.S. Environmental Protection Agency, 2018a).

Chlorinated drinking water and wastewater is an 

important source of chloroform and other THMs to 

groundwater (Ivahnenko and Barbash, 2004). Major sources of 

chlorinated water to groundwater include irrigation of lawns, 

golf courses and parks; leaking drinking-water distribution 

and sewer pipes; artificial recharge of wastewater; regulated 

discharges of cooling process blowdown water from electric 

power-generating plants; combined sewer overflows; 

unintended backflow of chlorinated water to supply wells; and 

domestic well disinfection with bleach (California Department 

of Health, 1990; Agency for Toxic Substances and Disease 

Registry, 1997; Zogorski and others, 2006). Several natural 

sources of chloroform include volcanic gases (Isidorov and 

others, 1990), soil fungi (McCulloch, 2003), and marine algae 

(Gribble, 1994; Laturnus and others, 2002). McCulloch (2003) 

estimated that natural sources of chloroform contribute about 

90 percent of the total global chloroform flux (McCulloch, 

2003). A national-scale study of VOCs in groundwater and 

drinking-water supply wells was inconclusive regarding the 

contribution of natural sources of chloroform to groundwater 

(Zogorski and others, 2006).

Chloroform was detected at low concentrations in all PAs 

(fig. 39A) with detection frequencies ranging from 15 percent 

in the Colorado Plateaus aquifers and Rio Grande aquifer 

system to 28 percent in the High Plains aquifer (table 12). 

Only one moderate concentration of chloroform occurred, 

which was in the Colorado Plateaus aquifers. The high 

detection frequencies at low concentrations were consistent 

with the national-scale findings of Zogorski and others (2006) 

in which chloroform concentrations generally occurred at less 

than 1 μg/L.

Bromodichloromethane also was detected at low 

concentrations in all of the PAs (fig. 39B), with the highest 

detection frequency (10 percent) occurring in the High 

Plains and the Basin and Range carbonate-rock PAs 

(table 12). Low concentration of the remaining two THMs 

considered―dibromochloromethane and bromoform―

had detection frequencies ranging from 1 percent to about 

6 percent in the PAs except for the Basin and Range 

carbonate-rock aquifers where they were not detected 

(table 12). Although dibromochloromethane and bromoform 

were not selected for the understanding assessment

evaluation, detection frequencies are included in table 12

because of their generation from chlorination. The trend 

in detection frequencies and concentrations of THMs 

(decreasing from chloroform to bromodichloromethane 

to dibromochloromethane to bromoform) is consistent 

with studies of the formation of multiple THMs during 

water chlorination that have shown that concentrations and 

detection frequencies decrease with increasing bromide 

concentration (Minear and Bird, 1980; Clark and others, 1996; 

Chellam, 2000).

Chloroform and bromodichloromethane had negative 

correlation with aridity index in the High Plains, whereas 

in the Columbia Plateau aquifer only chloroform was 

negatively correlated with aridity index (figs. 40A, B). The 

most significant correlations for chloroform in the remaining 

PAs were negative correlations with pH in the Basin and 

Range carbonate-rock aquifers, Rio Grande aquifer system, 

and Colorado Plateaus aquifers. These relations indicate 

that causative and non-causative correlations might not 

directly relate to THM production in groundwater and could 

be associated with the recharge of chlorinated water to 

groundwater in urban land use areas or shallow groundwater 

depths within the PAs instead.
110 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

A 125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Chloroform, in micrograms per liter

High (>80)

Moderate (8 to 80)

Low (<8)

Not detected

Spring Groundwater

well

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

— —

—

Map area

Figure 39. Concentrations of selected volatile organic compounds A, chloroform; and B, bromodichloromethane in six 

western U.S. principal aquifers sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 111

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

B 125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

Bromodichloromethane, in micrograms per liter

High (>80)

Moderate (8 to 80)

Low (<8)

Not detected

Spring Groundwater

well

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

—

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

— —

— —

Map area

Figure 39. —Continued
112 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

High Plains aquifer

Total

dissolved

solids

rho

Aridity

index

Percent

urban

land use

HCO3

−0.8

−0.4

0.0

0.4

B

−0.8

−0.4

0.0

0.4

−0.8

−0.4

0.0

0.4

High Plains aquifer

Colorado Plateaus aquifers

Columbia Plateau aquifers

Total

dissolved

solids

rho rho rho

pH

pH

Basin and Range carbonate-rock aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

rho rho rho

Percent urban

land use

Explanatory factor Explanatory factor

Aridity

index

Percent

urban

land use

HCO3

Chloroform

−0.8

−0.4

0.0

0.4

0.22

−0.8

−0.4

0.0

0.4

Well

elevation

Percent

natural

land use

pH

Aridity

index

Bromodichloromethane

Colorado Plateaus aquifers

rho

Explanatory factor

−0.8

−0.4

0.0

0.4

Depth to

bottom of

well perforation

−0.8

−0.4

0.0

0.4

−0.8

−0.4

0.0

0.4

A

−0.49

−0.24

0.28

−0.38

0.25 0.27 0.35

−0.30

−0.34 −0.28

0.26

−0.28

0.25 0.27

Explanatory factor

−0.36

Figure 40. Spearman’s rho results for A, chloroform; and B, bromodichloromethane in six western U.S. principal aquifers sampled by 

the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Status and Understanding Assessments of Water Quality 113

Special Interest Constituent: Perchlorate

Perchlorate in drinking water is a human-health 

concern because it can disrupt the normal function of the 

thyroid gland; as of 2018, there is no MCL for perchlorate 

(U.S. Environmental Protection Agency, 2018d), but it has a 

HBSL of 15 μg/L (Norman and others, 2018; U.S. Geological 

Survey, 2018a). Perchlorate in groundwater has synthetic 

and natural sources (Hatzinger and others, 2018). Synthetic 

sources include rocket propellants, road flares, fireworks, 

blasting agents, sodium chlorate, and perchloric acid (Aziz 

and Hatzinger, 2008). Natural sources include the apparent 

atmospheric formation and accumulation in vadose-zone 

soils in arid environments (Dasgupta and others, 2005). 

Anthropogenic sources include the historical application of 

the naturally occurring imported Chilean nitrate fertilizer. 

In the surficial deposits of the Atacama Desert of Chile, 

perchlorate co-occurs with sodium nitrate (NaNO3) at an 

average concentration of around 0.1 percent (by mass) 

of total soluble salt (Ericksen, 1981, 1983; Dasgupta and 

others, 2006). During the first half of the 20th century, 

processed nitrate deposits from the Atacama Desert were 

widely used in the United States as fertilizer. Post 2002, 

changes in manufacturing processes reduced the perchlorate 

concentrations to less than (or about) 0.01 percent (Dasgupta 

and others, 2006); however, previous applications are a 

potential source of perchlorate in groundwater and drinking 

water in the United States (Hatzinger and others, 2018). 

Perchlorate not associated with Chilean fertilizer has been 

detected in soil, groundwater, and mineral deposits from arid 

regions of the southern High Plains in Texas and New Mexico 

(Jackson and others, 2004, 2006; Dasgupta and others, 2005; 

Rajagopalan and others, 2006; Scanlon and others, 2008) 

and the middle Rio Grande Basin of New Mexico (Plummer 

and others, 2006). The postulated source of this perchlorate 

is long-term accumulation of atmospheric dry deposition 

and precipitation (Rajagopalan and others, 2006, 2009; 

Rao and others, 2007) forming in the atmosphere through 

photochemical reactions (Bao and Gu, 2004; Dasgupta and 

others, 2005; Kang and others, 2008; Sturchio and others, 

2009). The perchlorate accumulations can be mobilized and 

transported from the vadose zone to the water table. Irrigation 

also could be associated with transport of perchlorate 

to groundwater from crops treated with Atacama nitrate 

fertilizer (Böhlke and others, 2009; Sturchio and others, 

2014; Hatzinger and others, 2018). Perchlorate also can form 

in chlorine solutions used for drinking-water disinfection 

(Greiner and others, 2008).

Perchlorate samples only were collected in the High 

Plains aquifer and the Rio Grande aquifer system (fig. 41) and 

no high concentrations were observed. Moderate aquifer-scale 

proportions of perchlorate occurred in 11 percent of the 

High Plains aquifer and 3 percent of the Rio Grande aquifer 

system (table 10). Detected concentrations of perchlorate 

in collected samples were common, occurring in 82 percent 

of the High Plains aquifer and 58 percent of the Rio Grande 

aquifer system. In the High Plains aquifer, the most significant 

perchlorate correlations were with dissolved oxygen, TDS, 

and aridity index (fig. 42). Moderate concentrations of 

perchlorate predominately occurred in wells in the southern 

part of the High Plains aquifer, associated with drier climate 

and elevated TDS concentrations in shallower wells. Although 

dissolved oxygen was positively correlated with perchlorate, 

the correlation is likely non-causative because the High 

Plains aquifer was predominately oxic (fig. 15A). In the Rio 

Grande aquifer system, concentrations of perchlorate were 

negatively correlated with percentage of agricultural and 

irrigated land use and positively correlated with TDS and 

HCO3. Determining the source of perchlorate (natural or 

synthetic) was not possible using the potential explanatory 

factors applied in this study; however, considering the 

location of moderate concentrations occurring in the most 

arid regions of these two PAs and significant correlations with 

hydrologic conditions and land use, the sources of perchlorate 

are more likely attributed to natural atmospheric formation 

and accumulation in the vadose zone and Atacama nitrate 

fertilizer rather than synthetic sources or those associated 

with drinking-water disinfection. In the Rio Grande aquifer 

system, Plummer and others (2006) proposed that low 

concentrations of perchlorate were likely associated with 

natural accumulation in the vadose zone and subsequent 

transport to the water table. In the High Plains aquifer, 

accumulation in the vadose zone and subsequent transport to 

groundwater from crops treated with Atacama nitrate fertilizer 

could account for the occurrence of low and moderate 

perchlorate concentrations.
114 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

CALIFORNIA

OREGON

WASHINGTON

IDAHO

MONTANA

NEVADA

UTAH

ARIZONA

COLORADO

NEW

MEXICO

TEXAS

OKLAHOMA

WYOMING

NORTH

DAKOTA

SOUTH

DAKOTA

NEBRASKA

KANSAS

125° 120° 115° 110° 105° 100° 95°

45°

40°

35°

30°

EXPLANATION

Base modified from U.S. Geological Survey and other Federal and State digital

data, various scales; Albers Equal-Area Conic projection, standard parallels are 

29°30' N. and 45°30' N.; North American Datum of 1983

0 150 300 MILES

0 150 300 KILOMETERS

Perchlorate, in micrograms per liter

High (>15.0)

Moderate (1.5 to 15.0)

Low (<1.5)

Not detected

Groundwater

well

Principal aquifer (U.S. Geological Survey, 2003)

Basin and Range carbonate-rock

 aquifers

Basin and Range basin-fill aquifers

Rio Grande aquifer system

High Plains aquifer (McMahon and

 others, 2007)

Colorado Plateaus aquifers

Colombia Plateau basaltic-rock

 aquifers

Colombia Plateau basin-fill

 aquifers

—

Map area

Figure 41. Concentrations of special interest constituent perchlorate in the Rio Grande aquifer system and High Plains 

aquifer sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.
Summary 115

High Plains aquifer

Total

dissolved

solids

rho

Rio Grande aquifer system

rho

Aridity

index

HCO3

Perchlorate

Dissolved

oxygen

Depth to

bottom of

well perforation

−0.6

−0.3

0.0

0.3

0.6

−0.6

−0.3

0.0

0.3

0.6

Total

dissolved

solids

Percent

natural

land use

Well HCO3

elevation

Percent

irrigated

land use

Percent

agricultural

land use

−0.53

0.27

−0.28 −0.31

0.27 0.28

Explanatory factor

−0.52 −0.29

0.56 0.48 0.29

Explanatory factor

Figure 42. Spearman’s rho results for special interest constituent perchlorate in the Rio Grande aquifer system and High Plains aquifer 

sampled by the U.S. Geological Survey National Water-Quality Assessment Project, 2013–2017.

Summary

The U.S. Geological Survey (USGS) National 

Water-Quality Assessment (NAWQA) Project has been 

evaluating the water quality of source water (that is, untreated 

water) from principal aquifers that are important sources of 

drinking water. Groundwater quality at public-supply well 

depths was evaluated for inorganic and organic constituents in 

six principal aquifers (PAs) of the western United States that 

are important sources of public supply:

• the Basin and Range carbonate-rock aquifers

• the Basin and Range basin-fill aquifers

• the Rio Grande aquifer system

• the High Plains aquifer

• the Colorado Plateaus aquifers

• the Columbia Plateau basaltic-rock aquifers.

To provide context for water-quality data, constituent 

concentrations of untreated groundwater were compared 

with available water-quality benchmarks. The quality of 

water received by consumers can be different because 

after withdrawal, groundwater might be treated prior to 

delivery. The benchmarks that were considered were federal 

regulatory health-based benchmarks (U.S. Environmental 

Protection Agency Maximum Contaminant Levels [MCLs]), 

non-regulatory health-based benchmarks (health-based 

screening levels [HBSLs] developed by the U.S. Geological 

Survey), and non-regulatory secondary benchmarks 

(U.S. Environmental Protection Agency secondary maximum 

contaminant levels [SMCLs]). The PA studies evaluated 

water quality by sampling spatially distributed public-supply 

wells (PSWs) or springs using an equal-area grid sampling 

approach. This approach provides a spatially unbiased 

estimate of the aquifer-scale proportion that is equal to the 

observed detection frequency of a constituent concentration 

relative to a given threshold or benchmark (Belitz and 

others, 2010). This approach was used to estimate the 

percentage of a principal aquifer that has low, moderate, 

or high concentrations of constituents relative to their 

benchmarks. Two types of assessments were made: (1) status:

an assessment that describes the quality of the groundwater 

resource; and (2) understanding: an evaluation of the natural 

and human factors affecting the quality of groundwater, 

including an explanation of statistically significant 

associations between water quality and selected explanatory 

factors. The assessments were based on water-quality data 

collected from 352 PSWs and 6 springs. Potential explanatory 

factors considered in the understanding assessment were depth 

to bottom of well perforation, groundwater age category, land 

use, aquifer lithology, hydrologic conditions, and geochemical 

conditions. Constituents selected for assessment included 

inorganic constituents with moderate or high concentrations 

and organic constituents with detection frequencies greater 

than 10 percent. Inorganic constituents were grouped and 

described by four classes―trace elements, radionuclides, 

nutrients, and salinity indicators. Organic constituents were 

grouped and described by two classes, pesticides and volatile 

organic compounds (VOCs), and additionally included 

perchlorate, a special interest constituent.

Trace elements with health-based benchmarks were 

the largest group with moderate and high concentrations in 

all of the PAs―aquifer-scale proportions were Rio Grande 

aquifer system (27 percent), Basin and Range basin-fill 

aquifers (22 percent), High Plains aquifer (16 percent), Basin 

and Range carbonate-rock aquifers (15 percent), Colorado 

Plateaus aquifers (13 percent), and Columbia Plateau 

basaltic-rock aquifers (5 percent). The trace elements arsenic, 

fluoride, and manganese were those that most frequently 

occurred at moderate and high concentrations. Of these trace 

elements, arsenic was most frequently high, particularly 

in the unconsolidated sand and gravel aquifers of the arid 

Southwest. Elevated concentrations of arsenic were attributed 

to geochemical conditions that promote arsenic mobility and 

hydrologic conditions that favor accumulation of arsenic in 

groundwater. High and moderate arsenic concentrations were 

associated with oxic conditions, elevated pH values (greater 

than 7), and premodern groundwater age.
116 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Three radionuclides with health-based 

benchmarks―30-day gross alpha activity, radium (radium-226 

plus radium-228), and radon-222 (radon)―were selected for 

evaluation by the understanding assessment in this report. 

High radionuclide concentrations occurred in four PAs―the 

Basin and Range carbonate-rock aquifers (5 percent of PA), 

Rio Grande aquifer system (5 percent), Colorado Plateaus 

aquifers (5 percent), and Columbia Plateau basaltic-rock 

aquifers (3 percent). High concentrations of 30-day gross 

alpha were associated with geochemical factors favoring 

the release of the parent material uranium in groundwater 

(oxic groundwater with elevated total dissolved solids [TDS] 

and bicarbonate [HCO3] concentrations), whereas high 

concentrations of radium-226 and radium-228 generally 

occurred with anoxic or mixed groundwater conditions at 

pH values ranging from 7.4 to 7.7. High concentration of 

radon above the 4,000 pCi/L proposed benchmark occurred 

in 5 percent or less of the Basin and Range carbonate-rock 

aquifers, Rio Grande aquifer system, Colorado Plateaus 

aquifers, and Columbia Plateau basaltic-rock aquifers.

Moderate and high aquifer-scale proportions of salinity 

indicators (TDS, sulfate, and chloride) occurred in all six 

western PAs (excluding chloride in the Columbia Plateau 

basaltic-rock aquifers). The proportion of each PA having 

high concentrations of TDS ranged from 25 to 37 percent 

and generally was highest in the most arid regions of the 

unconsolidated sand and gravel PAs (the Rio Grande aquifer 

system and the Basin and Range basin-fill aquifers) and in 

the southern part of the High Plains aquifer. These arid to 

semi-arid regions are associated with low precipitation and 

high evaporation rates, which result in less groundwater 

recharge to dilute the products of rock weathering. Portions 

of the Southwest also have shallow groundwater with high 

evaporation rates that further contribute to elevated TDS 

concentrations. High concentrations of TDS in the southern 

part of the High Plains aquifer might result from mixing with 

saline groundwater from underlying geologic units. Irrigated 

agriculture also is a potential source of TDS in the High 

Plains aquifer.

Moderate and high concentrations of the nutrient nitrate 

(as nitrogen) were relatively uncommon in the western PAs; 

aquifer-scale proportions with moderate concentrations ranged 

from 0 to 11 percent, with the highest aquifer proportions 

occurring in the High Plains aquifer (11 percent), Basin and 

Range basin-fill aquifers (9 percent), and Columbia Plateau 

basaltic-rock aquifers (7 percent). High concentrations were 

present in 3 percent or less of the Rio Grande aquifer system, 

High Plains aquifer, and Columbia Plateau basaltic-rock 

aquifers. Nitrate was most strongly correlated (positive) 

with dissolved oxygen in the five PAs with moderate or high 

concentrations, which also tended to occur in modern or mixed 

age groundwater.

Organic constituents did not occur at high concentrations 

in any of the six PAs. Moderate concentrations occurred in 

three PAs—the Basin and Range basin-fill aquifers, High 

Plains aquifer, and Colorado Plateaus aquifers. Five pesticides 

(the herbicides atrazine, bromacil, didealkylatrazine, and 

propazine, and the insecticide propoxur) and two VOCs 

(chloroform and bromodichloromethane) were selected for 

further analysis by the understanding assessment because 

they were detected relatively frequently (between 10 and 

43 percent) in at least one PA. The High Plains aquifer had the 

highest detection rates, at low concentrations, of the pesticide 

constituent group―atrazine (43 percent), didealkylatrazine 

(28 percent), propazine (18 percent) and bromacil 

(10 percent). Atrazine also was detected in 1 percent of the 

High Plains aquifer at moderate concentrations. Atrazine 

and its degradate didealkylatrazine had significant negative 

correlations with well depth; presumably, surface application 

of the parent compound is followed by degradation and both 

compounds are transported or leached to shallow groundwater 

in the High Plains aquifer. The insecticide propoxur 

was detected at low concentrations in Columbia Plateau 

basaltic-rock aquifers (10 percent of PA), Colorado Plateaus 

aquifers (5 percent), High Plains aquifer (1 percent), and the 

Basin and Range basin-fill aquifer (1 percent); concentrations 

were not correlated with any of the potential explanatory 

factors considered in this report.

Only two VOC constituents―chloroform and 

bromodichloromethane―which are trihalomethanes, were 

selected for the understanding assessment. Chloroform was 

detected at low concentrations in all PAs at frequencies that 

ranged from 15 percent of samples in the Colorado Plateaus 

aquifers and Rio Grande aquifer system to 28 percent in 

the High Plains aquifer. The high detection frequencies at 

low concentrations were consistent with the findings of 

previous national-scale studies. Bromodichloromethane also 

was detected at low concentrations in all PAs, occurring 

at frequencies up to 10 percent in the High Plains aquifer 

and the Basin and Range carbonate-rock aquifers. Several 

potential explanatory factors were correlated with chloroform 

and bromodichloromethane (TDS, urban land use, and well 

depth), which likely reflect associations with the recharge of 

chlorinated water to groundwater in urban land use areas and 

shallow groundwater depths.

The special interest constituent perchlorate was only 

sampled in the High Plains aquifer and Rio Grande aquifer 

system. No high concentrations were observed. Moderate 

concentrations were observed in 11 percent of the High Plains 

aquifer and 3 percent of the Rio Grande aquifer system. 
Acknowledgments 117

Detections at low concentrations were common, occurring 

at frequencies of 82 percent in the High Plains aquifer and 

58 percent in the Rio Grande aquifer system. Moderate 

concentrations of perchlorate predominately occurred in 

the southern part of the High Plains aquifer, associated with 

shallow well depths (depth to bottom of well perforation), 

elevated TDS concentrations, and arid climates. In the Rio 

Grande aquifer system, perchlorate was negatively correlated 

with agricultural and irrigated land use and was positively 

correlated with TDS and HCO3. The distribution of detections 

at low concentration in the Rio Grande aquifer system likely 

reflects natural perchlorate accumulation (atmospheric 

deposition and accumulation in the vadose zone with 

subsequent leaching into shallow groundwater). Perchlorate 

detections in the High Plains aquifer also might reflect natural 

accumulation or could potentially be associated with transport 

of perchlorate to groundwater from crops treated with 

Atacama nitrate fertilizer.

Acknowledgments

The authors would like to thank the well and property 

owners that provided access to sampled wells and springs 

and the many personnel in the U.S. Geological Survey Water 

Science Centers that collected water samples and reviewed 

results. We also thank Laura Bexfield for guidance with the 

quality control analysis included in this report. This work was 

funded by the U.S. Geological Survey National Water-Quality 

Assessment Project.
118 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

References Cited

Agency for Toxic Substances and Disease Registry 

(ATSDR), 1997, Toxicological profile for chloroform: 

U.S. Department of Health and Human Services, Public 

Health Service, 343 p., accessed October 18, 2018, at 

https://www.atsdr.cdc.gov/toxprofiles/tp6.pdf.

Agency for Toxic Substances and Disease Registry 

(ATSDR), 2004, Toxicological profile for Cobalt: 

U.S. Department of Health and Human Services, Public 

Health Service, 486 p., accessed March 1, 2019, at 

https://www.atsdr.cdc.gov/toxprofiles/tp33.pdf.

Agency for Toxic Substances and Disease Registry 

(ATSDR), 2018, Toxicological profile for 

Bromodibromochloromethane: U.S. Department 

of Health and Human Services, Public Health 

Service, 237 p., accessed October 18, 2018, at 

https://www.atsdr.cdc.gov/toxprofiles/tp129.pdf.

Anderholm, S.K., Radell, M.J., and Richey, S.F., 1995, 

Water-quality assessment of the Rio Grande Valley study 

unit, Colorado, New Mexico, and Texas—Analysis of 

selected nutrient, suspended-sediment, and pesticide data: 

U.S. Geological Survey Water-Resources Investigations 

Report 94–4061, 203 p., https://doi.org/10.3133/wri944061.

Anning, D.W., and Konieczki, A.D., 2005, Classification of 

hydrogeologic areas and hydrogeologic flow systems in the 

Basin and Range Physiographic Province, southwestern 

United States: U.S. Geological Survey Professional 

Paper 1702, 37 p., https://doi.org/10.3133/pp1702.

Anning, D.W., Bauch, N.J., Gerner, S.J., Flynn, M.E., 

Hamlin, S.N., Moore, S.J., Schaefer, D.H., 

Anderholm, S.K., and Spangler, L.E., 2007, Dissolved 

solids in basin-fill aquifers and streams in the southwestern 

United States (ver. 1.1, March 22, 2010): U.S. Geological 

Survey Scientific Investigations Report 2006–5315, 168 p., 

https://doi.org/10.3133/sir20065315.

Anning, D.W., Paul, A.P., McKinney, T.S., Huntington, J.M., 

Bexfield, L.M., and Thiros, S.A., 2012, Predicted nitrate 

and arsenic concentrations in basin-fill aquifers of the 

southwestern United States: U.S. Geological Survey 

Scientific Investigations Report 2012–5065, 78 p., 

https://doi.org/10.3133/sir20125065.

Appelo, C.A.J., and Postma, D., 1999, Geochemistry, 

groundwater, and pollution: Rotterdam, the 

Netherlands, Balkema.

Arai, Y., 2010, Arsenic and antimony, chap. 16 of Hooda, 

P.S., ed., Trace elements in soils: John Wiley & Sons, 

p. 381–407, https://doi.org/10.1002/9781444319477.ch16.

Arizona Department of Water Resources, 2013, Land 

subsidence monitoring report no. 1: Arizona Department 

of Water Resources, 29 p., accessed September 19, 

2019, at https://new.azwater.gov/sites/default/files/

ADWRLandSubsidenceMonitoringReport_Number1_

Final.pdf.

Arizona Department of Water Resources, 2014, Land 

subsidence monitoring report no. 2: Arizona Department 

of Water Resources, 35 p., accessed September 19, 

2019, at https://new.azwater.gov/sites/default/files/

ADWRLandSubsidenceMonitoringReport_Number2_

Final.pdf.

Arizona Department of Water Resources, 2017, Land 

subsidence monitoring report no. 3: Arizona Department 

of Water Resources, 39 p., accessed September 19, 

2019, at https://new.azwater.gov/sites/default/files/

ADWRLandSubsidenceMonitoringReport_Number3_

Final.pdf.

Arizona Department of Water Resources, 2019, Land 

subsidence monitoring report no. 4: Arizona Department 

of Water Resources, 45 p., accessed September 19, 2019, 

at https://new.azwater.gov/sites/default/files/ADWR%20

Land%20Subsidence%20Monitoring%20Report_Number4_

Final.pdf.

Arnold, T.L., DeSimone, L.A., Bexfield, L.M., Lindsey, B.D., 

Barlow, J.R., Kulongoski, J.T., Musgrove, M., 

Kingsbury, J.A., and Belitz, K., 2016, Groundwater 

quality data from the National Water-Quality Assessment 

Project, May 2012 through December 2013 (ver. 1.1, 

November 2016): U.S. Geological Survey Data Series 997, 

56 p., https://doi.org/10.3133/ds997.

Arnold, T.L., Bexfield, L.M., Musgrove, M., Lindsey, B.D., 

Stackelberg, P.E., Barlow, J.R., DeSimone, L.A., 

Kulongoski, J.T., Kingsbury, J.A., Ayotte, J.D., 

Fleming, B.J., and Belitz, K., 2017, Groundwater-quality 

data from the National Water-Quality Assessment Project, 

January through December 2014 and select quality-control 

data from May 2012 through December 2014: 

U.S. Geological Survey Data Series 1063, 83 p., 

https://doi.org/10.3133/ds1063.
References Cited 119

Arnold, T.L., Bexfield, L.M., Musgrove, M., Stackelberg, P.E., 

Lindsey, B.D., Kingsbury, J.A., Kulongoski, J.T., 

and Belitz, K., 2018, Groundwater-quality and select 

quality-control data from the National Water-Quality 

Assessment Project, January through December 2015, 

and previously unpublished data from 2013 to 2014: 

U.S. Geological Survey Data Series 1087, 68 p., 

https://doi.org/10.3133/ds1087.

Arnold, T.L., Bexfield, L.M., Musgrove, M., Erickson, M.L., 

Kingsbury, J.A., Degnan, J.R., Tesoriero, A.J., 

Kulongoski, J.T., and Belitz, K., 2020, Groundwater-quality 

and select quality-control data from the National 

Water-Quality Assessment Project, January through 

December 2016, and previously unpublished data from 

2013 to 2015: U.S. Geological Survey Data Series 

Report 1124, 135 p., https://doi.org/10.3133/ds1124.

Ayers, R.S., and Wescot, D.W., 1994, Water quality for 

agriculture: Food and Agriculture Organization of the 

United Nations, FAO Irrigation and Drainage Paper 29, 

rev. 1, 174 p., accessed May 2, 2018, at http://www.fao.org/

DOCREP/003/T0234E/T0234E00.HTM.

Ayotte, J.D., Gronberg, J.M., and Apodaca, L.E., 2011, 

Trace elements and radon in groundwater across the 

United States, 1992–2003: U.S. Geological Survey 

Scientific Investigations Report 2011–5059, 115 p., 

https://doi.org/10.3133/sir20115059.

Aziz, C.E., and Hatzinger, P.B., 2008, Perchlorate sources, 

source identification, and analytical methods, in

Stroo, H.F., and Ward, C.H., eds., In Situ bioremediation 

of perchlorate: New York, Springer, p. 55–78, 

https://doi.org/10.1007/978-0-387-84921-8_4.

Bao, H., and Gu, B., 2004, Natural perchlorate has a unique 

isotopic signature: Environmental Science & Technology, 

v. 38, no. 19, p. 5073–5077, accessed October 2018 at 

https://doi.org/10.1021/es049516z.

Barringer, J.L., and Reilly, P.A., 2013, Arsenic in 

groundwater—A summary of sources and the 

biogeochemical and hydrogeologic factors affecting 

arsenic occurrence and mobility, in Bradley, P., ed., Current 

perspectives in contaminant hydrology and water resources 

sustainability: IntechOpen, https://doi.org/10.5772/55354.

Belitz, K., Jurgens, B., Landon, M.K., Fram, M.S., and 

Johnson, T., 2010, Estimation of aquifer scale proportion 

using equal area grids—Assessment of regional scale 

groundwater quality: Water Resources Research, v. 46, 

no. 11, 14 p., https://doi.org/10.1029/2010WR009321.

Belzile, N., and Chen, Y.-W., 2017, Thallium in 

the environment—A critical review focused on 

natural waters, soils, sediments and airborne 

particles: Applied GeoChemisty, v. 84, p. 218–243, 

https://doi.org/10.1016/j.apgeochem.2017.06.013.

Bennett, B., and Dudas, M.J., 2003, Release of arsenic and 

molybdenum by reductive dissolution of iron oxides 

in a soil with enriched levels of native arsenic: Journal 

of Environmental Engineering and Science, v. 2, no. 4, 

p. 265–272, https://doi.org/10.1139/s03-028.

Bexfield, L.M., and Anderholm, S.K., 1997, Water-quality 

assessment of the Rio Grande Valley, Colorado, New 

Mexico, and Texas—Ground-water quality in the 

Rio Grande flood plain, Cochiti Lake, New Mexico, 

to El Paso, Texas, 1995: U.S. Geological Survey 

Water-Resources Investigations Report 96–4249, 93 p., 

https://doi.org/10.3133/wri964249.

Bexfield, L.M., Thiros, S.A., Anning, D.W., Huntington, J.M., 

and McKinney, T.S., 2011, Effects of natural and human 

factors on groundwater quality of basin-fill aquifers in 

the southwestern United States—Conceptual models 

for selected contaminants: U.S. Geological Survey 

Scientific Investigations Report 2011–5020, 90 p., 

https://doi.org/10.3133/sir20115020.

Böhlke, J.K., Verstraeten, I.M., and Kraemer, T.F., 2007, 

Effects of surface-water irrigation on sources, fluxes, and 

residence times of water, nitrate, and uranium in an alluvial 

aquifer: Applied Geochemistry, v. 22, no. 1, p. 152–174, 

https://doi.org/10.1016/j.apgeochem.2006.08.019.

Böhlke, J.K., Hatzinger, P.B., Sturchio, N.C., Gu, B., 

Abbene, I., and Mroczkowski, S.J., 2009, Atacama 

perchlorate as an agricultural contaminant in groundwater—

Isotopic and chronologic evidence from Long Island, 

New York: Environmental Science and Technololgy, 

v. 43, no. 15, p. 5619–5625, accessed October 2018 at 

https://doi.org/10.1021/es9006433.

Burns, E.R., Morgan, D.S., Peavler, R.S., and Kahle, S.C., 

2011, Three-dimensional model of the geologic framework 

for the Columbia Plateau Regional Aquifer System, 

Idaho, Oregon, and Washington: U.S. Geological Survey 

Scientific Investigations Report 2010–5246, 44 p., 

https://doi.org/10.3133/sir20105246.

Burow, K.R., and Belitz, K., 2014, Groundwater studies—

Principal aquifer surveys: U.S. Geological Survey Fact 

Sheet 2014–3024, 2 p., https://doi.org/10.3133/fs20143024.
120 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Butterman, W.C., and Carlin, J.F., Jr., 2004, Mineral 

commodity profiles—Antimony: U.S. Geological 

Survey Open-File Report 2003–19, 35 p., 

https://doi.org/10.3133/ofr0319.

California Department of Health, 1990, Proposed identification 

of chloroform as a toxic air contaminant—Part A, Exposure 

assessment: California Air Resources Board, 70 p., accessed 

October 19, 2019, at https://www.arb.ca.gov/toxics/id/

summary/chloroform_A.pdf.

Cary, E.E., and Gissell-Nielsen, G., 1973, Effect of fertilizer 

anions on the solubility of native and applied selenium in 

soil: Soil Science Society of America Journal Abstract, 

v. 37, no. 4, p. 590–593, accessed January 2019 at 

https://doi.org/10.2136/sssaj1973.036159950037000400

34x.

Chellam, S., 2000, Effects of nanofiltration on trihalomethane 

and haloacetic acid precursor removal and speciation 

in waters containing low concentrations of bromide 

ion: Environmental Science & Technology, v. 34, no. 9, 

p. 1813–1820, https://doi.org/10.1021/es991153t.

Clark, R.M., Pourmoghaddas, H., Wymer, L.J., and 

Dressman, R.C., 1996, Modelling the kinetics of 

chlorination by-product formation—The effects of bromide: 

Journal of Water Supply: Research & Technology - Aqua, 

v. 45, no. 3, p. 112–119.

Colorado Division of Reclamation, Mining and Safety, 2016, 

2016 Yearly coal production summary: Colorado Division 

of Reclamation, Mining and Safety, accessed January 2019, 

at https://www.colorado.gov/drms.

Crusius, J., Calvert, S., Pedersen, T., and Sage, D., 

1996, Rhenium and molybdenum enrichments 

in sediments as indicators of oxic, suboxic and 

sulfidic conditions of deposition: Earth and 

Planetary Science Letters, v. 145, no. 1–4, p. 65–78, 

https://doi.org/10.1016/S0012-821X(96)00204-X.

Dasgupta, P.K., Martinelango, P.K., Jackson, W.A., 

Anderson, T.A., Tian, K., Tock, R.W., and Rajagopalan, S., 

2005, The origin of naturally occurring perchlorate—

The role of atmospheric processes: Environmental 

Science & Technology, v. 39, no. 6, p. 1569–1575, 

https://doi.org/10.1021/es048612x.

Dasgupta, P.K., Dyke, J.V., Kirk, A.B., and Jackson, W.A., 

2006, Perchlorate in the United States—Analysis of relative 

source contributions to the food chain: Environmental 

Science & Technology, v. 40, no. 21, p. 6608–6614, 

https://doi.org/10.1021/es061321z.

Dennehy, K.F., Litke, D.W., and McMahon, P.B., 2002, The 

High Plains aquifer, USA—Groundwater development 

and sustainability, in Hiscock, K.M., Rivett, M.O., and 

Davison, R.M., eds., Sustainable groundwater development: 

Geological Society, London, Special Publication 193, 

p. 99–119, https://doi.org/10.1144/GSL.SP.2002.193.01.09.

Dennehy, K.F., McMahon, P.B., Gurdak, J.J., and 

Bruce, B.W., 2007, Water quality and the availability and 

sustainability of water supplies in the High Plains aquifer, 

chap. 4 of McMahon, P.B., Dennehy, K.F., Bruce, B.W., 

Gurdak, J.J., and Qi, S.L., 2007, Water-quality assessment 

of the High Plains aquifer, 1999–2004: U.S. Geological 

Survey Professional Paper 1749, p. 105–136, 

https://doi.org/10.3133/pp1749.

DeSimone, L.A., McMahon, P.B., and Rosen, M.R., 2015, 

The quality of our Nation’s waters—Water quality in 

principal aquifers of the United States, 1991–2010: 

U.S. Geological Survey Circular 1360, 151 p., 

https://doi.org/10.3133/cir1360.

Dettinger, M.D., Harrill, J.R., Schmidt, D.L., and Hess, J.W., 

1995, Distribution of carbonate-rock aquifers and the 

potential for their development, southern Nevada and parts 

of California, Arizona, and Utah: U.S. Geological Survey 

Water-Resources Investigations Report 91–4146, 100 p., 

https://doi.org/10.3133/wri914146.

Drost, B.W., Whiteman, K.J., and Gonthier, J.B., 1990, 

Geologic framework of the Columbia Plateau Aquifer 

System, Washington, Oregon, and Idaho: U.S. Geological 

Survey Water-Resources Investigations Report 87–4238, 

10 p., https://doi.org/10.3133/wri874238.

Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., 

Hamilton, P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., 

Nolan, B.T., Puckett, L.J., Rupert, M.G., Short, T.M., 

Spahr, N.E., Sprague, L.A., and Wilber, W.G., 2010, 

The quality of our Nation’s waters—Nutrients in 

the Nation’s streams and groundwater, 1992–2004: 

U.S. Geological Survey Circular 1350, 174 p., 

https://doi.org/10.3133/cir1350.

Eakin, T.E., 1966, A regional interbasin groundwater system in 

the White River area, southeastern Nevada: Water Resources 

Research, v. 2, no. 2, p. 251–271, accessed January 3, 2018, 

at https://doi.org/10.1029/WR002i002p00251.

Ericksen, G.E., 1981, Geology and origin of the Chilean 

nitrate deposits: U.S. Geological Survey Professional 

Paper 1188, 37 p., accessed October 2018 at 

https://doi.org/10.3133/pp1188.
References Cited 121

Ericksen, G.E., 1983, The Chilean nitrate deposits: 

American Scientist, v. 71, no. 4, p. 366–374, 

https://www.jstor.org/stable/27852136.

Evans, L.J., and Barabash, S.J., 2010, Molybdenum, silver, 

thallium, and vanadium, chap. 22 in Hooda, P.S., ed., 

Trace elements in soils: Chichester, United Kingdom, 

John Wiley, p. 515–550, accessed August 7, 2018, 

at https://onlinelibrary.wiley.com/doi/pdf/10.1002/

9781444319477.

Falcone, J.A., 2015, U.S. conterminous wall-to-wall 

anthropogenic land use trends (NWALT), 1974–2012: 

U.S. Geological Survey Data Series 948, 33 p., 

https://doi.org/10.3133/ds948.

Faure, G., and Mensing, T.M., 2005, Isotopes—Principles and 

applications (3d ed.): New York, John Wiley, 928 p.

Fram, M.S., 2017, Groundwater quality in the western 

San Joaquin Valley study unit, 2010—California 

GAMA Priority Basin Project: U.S. Geological Survey 

Scientific Investigations Report 2017–5032, 130 p., 

https://doi.org/10.3133/sir20175032.

Fram, M.S., and Belitz, K., 2012, Status and understanding 

of groundwater quality in the Tahoe-Martis, Central Sierra, 

and Southern Sierra study units, 2006–2007—California 

GAMA Priority Basin Project: U.S. Geological Survey 

Scientific Investigations Report 2011–5216, 222 p., 

https://doi.org/10.3133/sir20115216.

Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, P.A., 

Martin, J.D., Nakagaki, N., Nowell, L.H., Scott, J.C., 

Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 

2006, The quality of our Nation’s waters—Pesticides 

in the Nation’s streams and ground water, 1992–2001: 

U.S. Geological Survey Circular 1291, 172 p., 

https://doi.org/10.3133/cir1291.

Greiner, P., McLellan, C., Bennett, D., and Ewing, A., 2008, 

Occurrence of perchlorate in sodium hypochlorite: Journal 

- American Water Works Association, v. 100, no. 11, p. 

68–74, https://doi.org/10.1002/j.1551-8833.2008.tb09773.x.

Gribble, G.W., 1994, The natural production of chlorinated 

compounds: Environmental Science & Technology, v. 28, 

no. 7, p. 310A–319A, https://doi.org/10.1021/es00056a712.

Grieve, C.M., Grattan, S.R., and Maas, E.V., 2012, Plant salt 

tolerance, chap. 13 of Wallender, W.W., and Tanji, K.K., 

eds., ASCE manual and reports on engineering practice 

no. 71: Reston, Virginia, Agricultural Salinity Assessment 

and Management, p. 405–459, https://www.ars.usda.gov/

ARSUserFiles/20360500/pdf_pubs/P2246.pdf.

Gurdak, J.J., McMahon, P.B., Dennehy, K.F., and Qi, S.L., 

2009, Water quality in the High Plains aquifer, Colorado, 

Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, 

Texas, and Wyoming, 1999–2004: U.S. Geological Survey 

Circular 1337, 64 p., https://doi.org/10.3133/cir1337.

Gutentag, E.D., Heimes, F.J., Krothe, N.C., Luckey, R.L., 

and Weeks, J.B., 1984, Geohydrology of the High Plains 

aquifer in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 1400–B, 63 p., 

https://doi.org/10.3133/pp1400B.

Hall, S.M., Van Gosen, B.S., Paces, J.B., Zielinski, R.A., 

and Breit, G.N., 2019, Calcrete uranium deposits 

in the southern High Plains, USA: Ore Geology 

Reviews, v. 109, p. 50–78, accessed August 2019 at 

https://doi.org/10.1016/j.oregeorev.2019.03.036.

Harrill, J.R., and Prudic, D.E., 1998, Aquifer systems in the 

Great Basin region of Nevada, Utah, and adjacent states—

Summary report: U.S. Geological Survey Professional 

Paper 1409–A, 66 p., https://doi.org/10.3133/pp1409A.

Hatzinger, P.B., Böhlke, J.K., Sturchio, N.C., Izbicki, J., and 

Teague, N., 2018, Four-dimensional isotopic approach 

to identify perchlorate sources in groundwater—

Application to the Rialto-Colton and Chino subbasins, 

southern California (USA): Applied Geochemistry, 

v. 97, p. 213–225, accessed October 2018 at 

https://doi.org/10.1016/j.apgeochem.2018.08.020.

Helsel, D.R., and Hirsch, R.M., 2002, Statistical methods in 

water resources: U.S. Geological Survey Techniques of 

Water-Resources Investigations, book 4, chap. A3, 510 p., 

https://doi.org/10.3133/tm4A3. 

Hem, J.D., 1978, Redox processes at surfaces of 

manganese oxide and their effects on aqueous metal 

ions: Chemical Geology, v. 21, no. 3–4, p. 199–218, 

https://doi.org/10.1016/0009-2541(78)90045-1.

Hem, J.D., 1980, Redox coprecipitation mechanisms 

of manganese oxides, in Kavanaugh, M.C., and 

Leckie, J.O., eds., Particulates in water—Advances 

in Chemistry Series, no. 189: Washington, D.C., 

American Chemical Society, v. 189, p. 45–72, 

https://doi.org/10.1021/ba-1980-0189.ch003.

Hem, J.D., 1985, Study and interpretation of chemical 

characteristics of natural water: U.S. Geological 

Survey Water Supply Paper 2254, 263 p., 

https://doi.org/10.3133/wsp2254.
122 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Hereford, R., Webb, R.H., and Graham, S., 2002, Precipitation 

history of the Colorado Plateau region, 1900–2000: 

U.S. Geological Survey Fact Sheet 119–02, 4 p., 

https://doi.org/10.3133/fs11902.

Howe, P.D., Malcolm, H.M., and Dobson, S., 2004, 

Manganese and its compounds—Environmental aspects: 

World Health Organization and International Programme 

on Chemical Safety, accessed June 20, 2018, at 

https://apps.who.int/iris/handle/10665/42992.

Huggins, F.E., Huffman, G.P., Kolker, A., Mroczkowski, S.J., 

Palmer, C.A., and Finkelman, R.B., 2002, Combined 

application of XAFS spectroscopy and sequential 

leaching for determination of arsenic speciation in 

coal: Energy & Fuels, v. 16, no. 5, p. 1167–1172, 

https://doi.org/10.1021/ef0200166.

Hutson, S.S., Barber, N.L., Kenny, J.F., Linsey, K.S., 

Lumia, D.S., and Maupin, M.A., 2004, Estimated use of 

water in the United States in 2000: U.S. Geological Survey 

Circular 1268, 46 p., https://doi.org/10.3133/cir1268.

Isidorov, V.A., Zenkevich, I.G., and Ioffe, B.V., 1990, 

Volatile organic compounds in solfataric gases: 

Journal of Atmospheric Chemistry, v. 10, p. 329–340, 

https://doi.org/10.1007/BF00053867.

Ivahnenko, T., and Barbash, J.E., 2004, Chloroform 

in the hydrologic system—Sources, transport, 

fate, occurrence, and effects on human health 

and aquatic organisms: U.S. Geological Survey 

Scientific Investigations Report 2004–5137, 34 p., 

https://doi.org/10.3133/sir20045137.

Jackson, W.A., Rainwater, K., Anderson, T., Lehman, T., 

Tock, R., Rajagopalan, S., and Ridley, M., 2004, 

Distribution and potential sources of perchlorate in the High 

Plains region of Texas—Final Report: Texas Commission 

on Environmental Quality, Austin, Texas, 170 p., 

https://clu-in.org/download/contaminantfocus/perchlorate/

final%20report.pdf.

Jackson, W.A., Anderson, T., Harvey, G., Orris, G., 

Rajagopalan, S., and Namgoo, K., 2006, Occurrence and 

formation of non-anthropogenic perchlorate in Gu, B., 

and Coates, J.D., eds., Perchlorate—Environmental 

occurrences, interactions, and treatment: Boston, 

Mass., Springer, p. 49–66, accessed October 2018 at 

https://doi.org/10.1007/0-387-31113-0_3.

Jurgens, B.C., Fram, M.S., Belitz, K., Burow, K.R., 

and Landon, M.K., 2010, Effects of groundwater 

development on uranium—Central Valley, California, 

USA: Groundwater, v. 48, no. 6, p. 913–928, accessed 

November 2012 at https://doi.org/10.1111/j.1745-

6584.2009.00635.x.

Kahle, S.C., Olsen, T.D., and Morgan, D.S., 2009, Geologic 

setting and hydrogeologic units of the Columbia Plateau 

Regional Aquifer System, Washington, Oregon, and Idaho: 

U.S. Geological Survey Scientific Investigations Map 3088, 

1 sheet, https://doi.org/10.3133/sim3088.

Kahle, S.C., Morgan, D.S., Welch, W.B., Ely, D.M., 

Hinkle, S.R., Vaccaro, J.J., and Orzol, L.L., 2011, 

Hydrogeologic framework and hydrologic budget 

components of the Columbia Plateau Regional Aquifer 

System, Washington, Oregon, and Idaho: U.S. Geological 

Survey Scientific Investigations Report 2011–5124, 66 p., 

https://doi.org/10.3133/sir20115124.

Kang, N., Jackson, W.A., Dasgupta, P.K., and 

Anderson, T.A., 2008, Perchlorate production by 

ozone oxidation of chloride in aqueous and dry 

systems: The Science of the Total Environment, 

v. 405, no. 1–3, p. 301–309, accessed October 2018 at 

https://doi.org/10.1016/j.scitotenv.2008.07.010.

Kaplan, D.I., and Mattigod, S.V., 1998, Aqueous geochemistry 

of thallium, in Nriagu, J.O., ed., Thallium in the 

environment: John Wiley & Sons, Inc., v. 2, p. 15–29.

Karbowska, B., 2016, Presence of thallium in the 

environment—Sources of contaminations, distribution 

and monitoring methods: Environmental Monitoring and 

Assessment, v. 188, no. 640, 19 p., accessed August 2018 at 

https://doi.org/10.1007%2Fs10661-016-5647-y.

Kazantzis, G., 2000, Thallium in the environment and 

health effects: Environmental Geochemistry and 

Health, v. 22, no. 4, p. 275–280, accessed August 2018, 

https://doi.org/10.1023/A:1006791514080.

Kim, K., and Jeong, G.Y., 2005, Factors influencing 

natural occurrence of fluoride-rich groundwaters—A 

case study in the southeastern part of the Korean 

Peninsula: Chemosphere, v. 58, no. 10, p. 1399–1408, 

https://doi.org/10.1016/j.chemosphere.2004.10.002.

Kirschbaum, M.A., and Biewick, L.R.H., 2016, A summary of 

coal deposits in the Colorado Plateau—Arizona, Colorado, 

New Mexico, and Utah, chap. B of Kirschbaum, M.A, 

Roberts, L.N.R, and Biewick, L.R.H, eds., Geologic 

assessment of coal in the Colorado Plateau—Arizona, 

New Mexico, and Utah: U.S. Geological Professional 

Paper 1625B, p. B1–B99, https://doi.org/10.3133/pp1625B.

Kolker, A., Palmer, C.A., Bragg, L.J., and Bunnell, J.E., 

2006, Arsenic in coal: U.S. Geological Survey Fact 

Sheet 2005–3152, 4 p., https://doi.org/10.3133/fs20053152.
References Cited 123

Kolpin, D.W., Thurman, E.M., and Linhart, S.M., 1998, The 

environmental occurrence of herbicides—The importance 

of degradates in ground water: Archives of Environmental 

Contamination and Toxicology, v. 35, p. 385–390, 

https://doi.org/10.1007/s002449900392.

Kolpin, D.W., Thurman, E.M., and Linhart, S.M., 2000, 

Finding minimal herbicide concentrations in ground 

water? Try looking for their degradates: The Science 

of the Total Environment, v. 248, no. 2–3, p. 115–122, 

https://doi.org/10.1016/S0048-9697(99)00535-5.

Koterba, M.T., Wilde, F.D., and Lapham, W.W., 1995, 

Ground-water data-collection protocols and procedures 

for the National Water-Quality Assessment Program—

Collection and documentation of water-quality samples 

and related data: U.S. Geological Survey Open-File 

Report 95–399, 114 p., https://doi.org/10.3133/ofr95399.

Kunmar, R.A., and Riyazuddin, P., 2011, Speciation 

of selenium in groundwater—Seasonal 

variation and redox transformations: Journal of 

Hazardous Materials, v. 192, no. 1, p. 263–269, 

https://doi.org/10.1016/j.jhazmat.2011.05.013.

Kurttio, P., Auvinen, A., Salonen, L., Saha, H., Pekkanen, J., 

Mäkeläinen, I., Väisänen, S.B., Penttilä, I.M., and 

Komulainen, H., 2002, Renal effects of uranium in drinking 

water: Environmental Health Perspectives, v. 110, no. 4, 

p. 337–342, https://doi.org/10.1289%2Fehp.02110337.

Laturnus, F., Haselmann, K.F., Borch, T., and Grøn, C., 2002, 

Terrestrial natural sources of trichloromethane (chloroform, 

CHCl3)—An overview: Biogeochemistry, v. 60, no. 2, 

p. 121–139, https://doi.org/10.1023/A:1019887505651.

Leake, S.A., 2016, Land subsidence from ground-water 

pumping: web page, accessed September 19, 2019, 

at https://geochange.er.usgs.gov/sw/changes/

anthropogenic/subside/.

Luckey, R.R., Gutentag, E.D., Heimes, F.J., and Weeks, J.B., 

1986, Digital simulation of ground-water flow in the High 

Plains aquifer in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S. Geological Survey Professional Paper 1400–D, 57 p., 

https://doi.org/10.3133/pp1400D.

Lyndsey, B.D., Jurgens, B.C., and Belitz, K., 2019, 

Tritium as an indicator of modern, mixed, and 

premodern groundwater age: U.S Geological Survey 

Scientific Investigations Report 2019–5090, 18 p., 

https://doi.org/10.3133/sir20195090.

Ma, Y., and Hooda, P.S., 2010, Chromium, nickel and cobalt, 

chap. 19 in Hooda, P.S., ed., Trace elements in soils: 

Chichester, United Kingdom, John Wiley, p. 461–480, 

accessed March 1, 2019, at https://onlinelibrary.wiley.com/

doi/pdf/10.1002/9781444319477.

Maas, E.V., and Grattan, S.R., 1999, Crop yields as affected 

by salinity, in Skaggs, R.W. and van Schilfgaarde, J., eds., 

Agricultural drainage: American Society of Agronomy, 

Monograph no. 38, p. 55–108, accessed May 2, 2018, at 

https://www.researchgate.net/publication/269112293_Crop_

yields_as_affected_by_salinity.

Maupin, M.A., and Barber, N.L., 2005, Estimated 

withdrawals from principal aquifers in the United States, 

2000: U.S. Geological Survey Circular 1279, 47 p., 

https://doi.org/10.3133/cir1279.

McCulloch, A., 2003, Chloroform in the 

environment—Occurrence, sources, sinks and 

effects: Chemosphere, v. 50, no. 10, p. 1291–1308, 

https://doi.org/10.1016/S0045-6535(02)00697-5.

McMahon, P.B., 2001, Vertical gradients in water chemistry 

in the central High Plains aquifer, southwestern Kansas 

and Oklahoma panhandle, 1999: U.S. Geological Survey 

Water-Resources Investigations Report 2001–4028, 47 p., 

https://doi.org/10.3133/wri014028.

McMahon, P.B., and Böhlke, J.K., 2007, Transport—Linking 

the chemistry of recharge and the used resource in the High 

Plains aquifer, chap. 2 of McMahon, P.B., Dennehy, K.F., 

Bruce, B.W., Gurdak, J.J., and Qi, S.L., 2007, Water-quality 

assessment of the High Plains aquifer, 1999–2004: 

U.S. Geological Survey Professional Paper 1749, 

p. 49–72, https://pubs.usgs.gov/pp/1749/downloads/pdf/

P1749ch2.pdf.

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

processes and water quality of selected principal aquifer 

systems: Ground Water, v. 46, no. 2, p. 259–271, 

https://doi.org/10.1111/j.1745-6584.2007.00385.x.

McMahon, P.B., Dennehy, K.F., Bruce, B.W., Gurdak, J.J., and 

Qi, S.L., 2007, Water-quality assessment of the High Plains 

aquifer, 1999–2004: U.S. Geological Survey Professional 

Paper 1749, 136 p., https://doi.org/10.3133/pp1749.

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

Jurgens, B.C., 2009, Redox conditions in selected principal 

aquifers of the United States: U.S. Geological Survey Fact 

Sheet 2009–3041, 6 p., https://pubs.usgs.gov/fs/2009/3041/.

McNeal, J.M., and Balistrieri, L.S., 1989, Geochemistry 

and occurrence of selenium—An overview, in

Jacobs, L.W., ed., Selenium in agriculture and the 

environment: SSSA Special Publication 23, p. 1–13, 

https://doi.org/10.2136/sssaspecpub23.c1.

McNutt, R.H., 2000, Strontium isotopes, chap. 8 of

Cook, P.G., and Herczeg, A.L., eds., Environmental tracers 

in subsurface hydrology: Boston, Kluwer Academic 

Publishers, p. 233–260.
124 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Michel, R.L., Jurgens, B.C., and Young, M.B., 2018, 

Tritium deposition in precipitation in the United 

States, 1953–2012: U.S. Geological Survey 

Scientific Investigations Report 2018–5086, 11 p., 

https://doi.org/10.3133/sir20185086.

Miller, M.H., 1973, Antimony, in Brobst, D.A., and 

Pratt, W.P., eds., United States mineral resources: 

U.S. Geological Survey Professional Paper 820, p. 45–50, 

https://doi.org/10.3133/pp820.

Minear, R.A., and Bird, J.C., 1980, Trihalomethanes—Impact 

of bromide ion concentration on yield, species distribution, 

rate of formation and influence of other variables, in

Jolley, R.L., ed., Water chlorination—Environmental 

impact and health effects v. 3: Ann Arbor, Mich., Ann Arbor 

Science Publishers Inc., p. 151–160.

Misra, K.C., 2000, Porphyry deposits, in Misra, K.C., 

Understanding mineral deposits: Dordrecht, Springer, 

p. 353–413, https://doi.org/10.1007/978-94-011-3925-0.

Mousavi, S.R., Shahsavari, M., and Rezaei, M., 2011, A 

general overview on Manganese (Mn) Importance for 

crops production: Australian Journal of Basic and Applied 

Sciences, v. 5, no. 9, p. 1799–1803.

Mueller, D.K., Schertz, T.L., Martin, J.D., and 

Sandstrom, M.W., 2015, Design, analysis, and interpretation 

of field quality-control data for water-sampling projects: 

U.S. Geological Survey Techniques and Methods, book 4, 

chap. C4, 54 p., https://doi.org/10.3133/tm4C4.

Nativ, R., and Smith, D.A., 1987, Hydrogeology and 

geochemistry of the Ogallala aquifer, Southern High 

Plains: Journal of Hydrology (Amsterdam), v. 91, no. 3–4, 

p. 217–253, https://doi.org/10.1016/0022-1694(87)90206-X.

Nolan, B.T., and Hitt, K.J., 2006, Vulnerability of shallow 

groundwater and drinking-water wells to nitrate in the 

United States: Environmental Science & Technology, v. 40, 

no. 24, p. 7834–7840, https://doi.org/10.1021/es060911u.

Nolan, J., and Weber, K.A., 2015, Natural uranium 

contamination in major U.S. aquifers linked to nitrate: 

Environmental Science & Technology Letters, v. 2, no. 8, 

p. 215–220. https://doi.org/10.1021/acs.estlett.5b00174.

Nordstrom, D.K., Ball, J.W., Donahoe, R.J., and 

Whittemore, D., 1989, Groundwater chemistry 

and water-rock interaction at Stripa: Geochimica 

et Cosmochimica Acta, v. 53, no. 8, p. 1727–1740, 

https://doi.org/10.1016/0016-7037(89)90294-9.

Norman, J.E., Toccalino, P.L., and Morman, S.A., 2018, 

Health-Based Screening Levels for evaluating water-quality 

data (2d ed.). U.S. Geological Survey web page accessed 

May 3, 2018, at https://doi.org/10.5066/F71C1TWP.

Peter, A.L.J., Viraraghavan, T., 2005, Thallium—A 

review of public health and environmental concerns: 

Environmental International, v. 31, no. 4, p. 493–501, 

https://doi.org/10.1016/j.envint.2004.09.003.

Peterson, G.A., 2005, Dryland Farming, in

Hillel, D., ed., Encyclopedia of soils in the 

environment: Academic Press, p. 414–417, 

https://doi.org/10.1016/B0-12-348530-4/00260-5.

Piper, D.Z., Skorupa, J.P., Presser, T.S., Hardy, M.A., 

Hamilton, S.J., Huebner, M., and Gulbrandsen, R.A., 

2000, The Phosphoria Formation at the Hot Springs Mine 

in southeast Idaho—A source of selenium and other trace 

elements to surface water, ground water, vegetation, and 

biota: U.S. Geological Survey Open-File Report 00–50, 

73 p., https://doi.org/10.3133/ofr0050.

Planert, M., and Williams, J.S., 1995, Ground water atlas 

of the United States—Segment 1, California, Nevada: 

U.S. Geological Survey Hydrologic Investigations Atlas 

730–B, 28 p., https://doi.org/10.3133/ha730B.

Plummer, L.N., Böhlke, J.K., and Doughten, M.W., 2006, 

Perchlorate in Pleistocene and Holocene groundwater 

in north-central New Mexico: Environmental Science 

& Technology, v. 40, no. 6, p. 1757–1763, accessed 

October 2018 at https://doi.org/10.1021/es051739h.

Presser, T.S., and Swain, W.C., 1990, Geochemical 

evidence for Se mobilization by the weathering of 

pyritic shale, San Joaquin Valley, California, U.S.A: 

Applied Geochemistry, v. 5, no. 5–6, p. 703–717, 

https://doi.org/10.1016/0883-2927(90)90066-E.

Prudic, D.E., Harrill, J.R., and Burbey, T.J., 1995, Conceptual 

evaluation of regional ground-water flow in the 

carbonate-rock province of the Great Basin, Nevada, Utah, 

and adjacent states: U.S. Geological Survey Professional 

Paper 1409–D, 102 p., https://doi.org/10.3133/pp1409D.

Rajagopalan, S., Anderson, T.A., Fahlquist, L., 

Rainwater, K.A., Ridley, M., and Jackson, W.A., 2006, 

Widespread presence of naturally occurring perchlorate 

in high plains of Texas and New Mexico: Environmental 

Science & Technology, v. 40, no. 10, p. 3156–3162, 

accessed October 2012 at https://doi.org/10.1021/es052155i.
References Cited 125

Rajagopalan, S., Anderson, T.A., Cox, S., Harvey, G., 

Cheng, Q., and Jackson, W.A., 2009, Perchlorate in 

wet deposition across North America: Environmental 

Science & Technology, v. 43, no. 3, p. 616–622, accessed 

October 2018 at https://doi.org//10.1021/es801737u.

Rao, B., Anderson, T.A., Orris, G.J., Rainwater, K.A., 

Rajagopalan, S., Sandvig, R.M., Scanlon, B.R., 

Stonestrom, D.A., Walvoord, M.A., and Jackson, W.A., 

2007, Widespread natural perchlorate in unsaturated 

zones of the southwest United States: Environmental 

Science & Technology, v. 41, no. 13, p. 4522–4528, 

https://doi.org/10.1021/es062853i.

Reidel, S.P., Johnson, V.G., and Spane, F.A., 2002, Natural 

gas storage in basalt aquifers of the Columbia Basin, 

Pacific Northwest USA—A guide to site characterization: 

Richland, Wash., Pacific Northwest National Laboratory, 

277 p., accessed January 3, 2018, at https://www.pnnl.gov/

main/publications/external/technical_reports/PNNL13962.pdf.

Reimann, C., and de Caritat, P., 1998, Chemical 

elements in the environment– Factsheets for 

the geochemist and environmental scientist: 

Springer-Verlag Bernlin Heidelberg, 398 p., 

https://link.springer.com/book/10.1007/978-3-642-72016-1.

Robson, S.G., and Banta, E.R., 1995, Ground water atlas of 

the United States—Arizona, Colorado, New Mexico, Utah: 

U.S. Geological Survey Hydrologic Atlas 730–C, accessed 

January 3, 2018, at https://pubs.usgs.gov/ha/ha730/ch_c/.

Robson, S.G., and Banta, E.R., 1996, Ground water 

atlas of the United States—Montana, North Dakota, 

South Dakota, Wyoming: U.S. Geological Survey 

Hydrologic Atlas 730–I, accessed January 3, 2018, at 

https://pubs.usgs.gov/ha/ha730/ch_i/.

Romero Guzmán, E.T., Ordoñez Regil, En., 

Reyes gutiérrez, L.R., Esteller Alberich, M.V., 

Rojas Hernández, A., and Ordoñez Regil, Ed., 

2006, Contamination of corn growing areas due to 

intensive fertilization in the high plane of Mexico: 

Water, Air, and Soil Pollution, v. 175, p. 77–98, 

https://doi.org/10.1007/s11270-006-9114-1.

Rosecrans, C.Z., 2020, Data for groundwater-quality and 

select quality-control data for the Colorado Plateaus 

principal aquifer: U.S. Geological Survey data release, 

https://doi.org/10.5066/P916H748.

Rothbaum, H.P., McGaveston, D.A., Wall, T., Johnston, A.E., 

and Mattingly, G.E.G., 1979, Uranium accumulation in 

soils from long-continued applications of superphosphate: 

Journal of Soil Science, v. 30, no. 1, p. 147–153. 

https://doi.org/10.1111/j.1365-2389.1979.tb00972.x.

Saha, U.K., 2017, Selenium—A vital element in 

soil-pant-animal/human continuum: Journal of 

Agricultural Science and Botany, v. 1, no. 1, p. 1–3, 

https://doi.org/10.35841/2591-7897.1.1.1-3.

Salbu, B., and Steinnes, E., 1994, Trace elements in natural 

waters: Boca Raton, Fla., CRC Press, 302 p.

Sargent, B.P., Maupin, M.A., and Hinkle, S.R., 2008, 

Estimated withdrawals from stream-valley aquifers and 

refined estimated withdrawals from selected aquifers 

in the United States, 2000: U.S. Geological Survey 

Scientific Investigations Report 2008–5003, 71 p., 

https://doi.org/10.3133/sir20085003.

Scanlon, B.R., Reedy, R.C., Jackson, W.A., and Rao, B., 

2008, Mobilization of naturally occurring perchlorate 

related to land‐use change in the southern High Plains, 

Texas: Environmental Science & Technology, v. 42, no. 23, 

p. 8648–8653. https://doi.org/10.1021/es800361h.

Scanlon, B.R., Nicot, J.P., Reedy, R.C., Kurtzman, D., 

Mukherjee, A., and Nordstrom, D.K., 2009, Elevated 

naturally occurring arsenic in a semiarid oxidizing 

system, Southern High Plains aquifer, Texas, USA: 

Applied Geochemistry, v. 24, no. 11, p. 2061–2071, 

https://doi.org/10.1016/j.apgeochem.2009.08.004.

Schaefer, D.H., Thiros, S.A., and Rosen, M.R., 2005, 

Groundwater quality in the carbonate-rock aquifer of the 

Great Basin, Nevada and Utah, 2003: U.S. Geological 

Survey Scientific Investigations Report 2005–5232, 

https://pubs.usgs.gov/sir/2005/5232/.

Schlieker, M., Schuring, J., Hencke, J., and Schulz, H.D., 

2001, The influence of redox processes on trace element 

mobility in a sandy aquifer—An experimental approach: 

Journal of Geochemical Exploration, v. 73, no. 3, p. 

167–179, https://doi.org/10.1016/S0375-6742(01)00195-9.

Scribner, E.A., Thurman, E.M., and Zimmerman, L.R., 2000, 

Analysis of selected herbicide metabolites in surface 

and ground water of the United States: The Science of 

the Total Environment, v. 248, no. 2–3, p. 157–167, 

https://doi.org/10.1016/S0048-9697(99)00539-2.
126 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Seal, R.R., II, Schulz, K.J., and DeYoung, J.H., Jr., 2017, 

with contributions from Sutphin, D.M., Drew, L.J., 

Carlin, Jr., J.F., and Berger, B.R., Antimony, chap. C 

of Schulz, K.J., DeYoung, J.H., Jr., Seal, R.R., II, and 

Bradley, D.C., eds., Critical mineral resources of the 

United States—Economic and environmental geology 

and prospects for future supply: U.S. Geological 

Survey Professional Paper 1802, p. C1–C17, 

https://doi.org/10.3133/pp1802C.

Smedley, P.L., and Kinniburgh, D.G., 2002, A review of the 

source, behavior, and distribution of arsenic in natural 

waters: Applied Geochemistry, v. 17, no. 5, p. 517–568, 

https://doi.org/10.1016/S0883-2927(02)00018-5.

Smedley, P.L., Cooper, D.M., Ander, E.L., Milne, C.J., 

and Lapworth, D.J., 2014, Occurrence of molybdenum 

in British surface water and groundwater—

Distribution, controls and implications for water 

supply: Applied Geochemistry, v. 40, p. 144–154, 

https://doi.org/10.1016/j.apgeochem.2013.03.014.

Stackelberg, P., 2017, Groundwater quality in the glacial 

aquifer system, United States: U.S. Geological Survey Fact 

Sheet 2017–3055, 4 p., https://doi.org/10.3133/fs20173055.

Stewart, B.A., 2016, Dryland farming, in

Smithers, G.W., ed., Reference module in food 

science: Elsevier, accessed September 2018 at 

https://doi.org/10.1016/B978-0-08-100596-5.02937-1.

Stillings, L.L., 2017, Selenium, chap. Q of Schulz, K.J., 

DeYoung, J.H., Jr., Seal, R.R., II, and Bradley, D.C., eds., 

Critical mineral resources of the United States—Economic 

and environmental geology and prospects for future supply: 

U.S. Geological Survey Professional Paper 1802, p. Q1–

Q55, https://doi.org/10.3133/pp1802Q.

Sturchio, N.C., Caffee, M., Beloso, A.D., Jr., Heraty, L.J., 

Böhlke, J.K., Hatzinger, P.B., Jackson, W.A., Gu, B., 

Heikoop, J.M., and Dale, M., 2009, Chlorine-36 as a tracer 

of perchlorate origin: Environmental Science & Technology, 

v. 43, no. 18, p. 6934–6938, accessed October 2018 at 

https://doi.org/10.1021/es9012195.

Sturchio, N.C., Beloso, A.D., Jr., Heraty, L.J., Wheatcraft, 

S., and Schumer, R., 2014, Isotopic tracing of perchlorate 

sources in groundwater from Pomona, California: Applied 

Geochemistry, v. 43, p. 80–87, accessed October 2018 at 

https://doi.org/10.1016/j.apgeochem.2014.01.012.

Szabo, Z., and Zapecza, O.S., 1991, Geologic and 

geochemical factors controlling uranium, radium-226, and 

radon-222 in ground water, Newark Basin, New Jersey, in

Gunderson, L.C.S., and Wanty, R.B., eds., Field studies 

of radon rocks, soils, and water: U.S. Geological Survey 

Bulletin 1971, p. 243–265.

Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, 

Methods for determination of radioactive substances 

in water and fluvial sediments: U.S. Geological Survey 

Techniques of Water-Resources Investigations 05–A5, 95 p., 

https://doi.org/10.3133/twri05A5.

Thiros, S.A., Bexfield, L.M., Anning, D.W., and 

Huntington, J.M., eds., 2010, Conceptual understanding and 

groundwater quality of selected basin-fill aquifers in the 

Southwestern United States: U.S. Geological Professional 

Paper 1781, 288 p., https://doi.org/10.3133/pp1781.

Thiros, S.A., Paul, A.P., Bexfield, L.M., and Anning, D.W., 

2014, The quality of our Nation’s waters—Water quality in 

the basin-fill aquifers of the southwestern United States—

Arizona, California, Colorado, Nevada, New Mexico, and 

Utah, 1993–2009: U.S. Geological Survey Circular 1358, 

113 p., https://doi.org/10.3133/cir1358.

Thomas, J.M., Welch, A.H., Lico, M.S., Hughes, J.L., and 

Whitney, R., 1993, Radionuclides in ground water of the 

Carson River Basin, western Nevada and eastern California, 

U.S.A.: Applied Geochemistry, v. 8, no. 5, p. 447–471, 

https://doi.org/10.1016/0883-2927(93)90075-R.

Toccalino, P.L., 2007, Development and application 

of health-based screening levels for use in 

water-quality assessments: U.S. Geological Survey 

Scientific Investigations Report 2007–5106, 12 p., 

https://doi.org/10.3133/sir20075106.

Toccalino, P.L., Norman, J.E., Phillips, R.H., Kauffman, L.J., 

Stackelberg, P.E., Nowell, L.H., Krietzman, S.J., 

and Post, G.B., 2004, Application of health-based 

screening levels to ground-water quality data in 

a state-scale pilot effort: U.S. Geological Survey 

Scientific Investigations Report 2004–5174, 64 p., 

https://doi.org/10.3133/sir20045174.

Tolan, T.L., Reidel, S.P., Beeson, M.H., Anderson, J.L., 

Fecht, K.R., and Swanson, D.A., 1989, Revisions to the 

estimates of the areal extent and volume of the Columbia 

River Basalt Group, in Reidel, S.P., and Hooper, P.R., 

eds., Volcanism and tectonism in the Columbia River 

flood-basalt province: Geological Society of America 

Special Paper 239, p. 1–20, accessed January 3, 2018, at 

https://doi.org/10.1130/SPE239-p1.

Trabucco, A., and Zomer, R., 2019, Global Aridity Index and 

Potential Evapotranspiration (ET0) Climate Database v2: 

Figshare, https://doi.org/10.6084/m9.figshare.7504448.v3. 

United Nations Environment Programme, 1997, World 

atlas of desertification (2d ed.), Middleton, N.J., and 

Thomas, D.S.G., eds.: London, Arnold, 182 p.
References Cited 127

U.S. Census Bureau, 2018, Population Division, 

Annual Estimates of the Resident Population: April 1, 

2010, to July 1, 2017: accessed May 1, 2018, at 

https://data.census.gov/cedsci/.

U.S. Climate Data, 2020, accessed July 13, 2020 at 

https://www.usclimatedata.com/climate/death-valley/

california/united-states/usca0286

U.S. Department of Agriculture, 2014, 2012 Census of 

Agriculture United States Summary and State Data, v.1: 

U.S. Department of Agriculture, accessed January 2019 at 

https://www.nass.usda.gov/Publications/AgCensus/2012/

Full_Report/Volume_1,_Chapter_1_US/usv1.pdf.

U.S. Environmental Protection Agency, 2017, Drinking 

Water Requirements for States and Public Water Systems: 

Information about Public Water Systems, accessed 

September 16, 2019, at https://www.epa.gov/dwreginfo/

information-about-public-water-systems.

U.S. Environmental Protection Agency, 2018a, National 

primary drinking water regulations: accessed, May 3, 2018, 

at https://www.epa.gov/ground-water-and-drinking-water/

national-primary-drinking-water-regulations.

U.S. Environmental Protection Agency, 2018b, Secondary 

drinking water standards—Guidance for nuisance 

chemicals: accessed May 3, 2018, at https://www.epa.gov/

dwstandardsregulations/secondary-drinking-waterstandards-guidance-nuisance-chemicals.

U.S. Environmental Protection Agency, 2018c, Proposed 

radon in drinking water regulation: accessed May 2018 

at https://archive.epa.gov/water/archive/web/html/

regulations.html.

U.S. Environmental Protection Agency, 2018d, Perchlorate 

in Drinking water: accessed October 2018 at 

https://www.epa.gov/dwstandardsregulations/perchloratedrinking-water-frequent-questions.

U.S. Environmental Protection Agency, 2019a, Private 

drinking water well, accessed September 16, 2019, at 

https://www.epa.gov/privatewells.

U.S. Environmental Protection Agency, 2019b, 

Health risk of radon, accessed March 31, 2020, at 

https://www.epa.gov/radon/health-risk-radon.

U.S. Geological Survey, variously dated, The national 

field manual for the collection of water-quality data 

(version 7): U.S. Geological Survey Techniques and 

Method, book 9, chaps. A1–A9, accessed April 5, 2019, at 

https://water.usgs.gov/owq/FieldManual/.

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

the United States: Prepared by the U.S. Geological 

Survey for The National Atlas, scale 1:5,000,000, 

https://water.usgs.gov/ogw/aquifer/map.html.

U.S. Geological Survey, 2014, The 2012 Moderate 

Resolution Imaging Spectroradiometer (MODIS) irrigated 

agriculture 1-kilometer dataset for the conterminous 

United States (MIrAD-US)–Spatial distribution of 

irrigated lands: version 1, accessed January 24, 2018, at 

https://earlywarning.usgs.gov/USirrigation.

U.S. Geological Survey, 2016, Antimony statistics, in

Kelly, T.D., and Matos, G.R., comps., Historical statistics 

for mineral and material commodities in the United States 

(2016 version): U.S. Geological Survey Data Series 140, 

4 p., accessed June 14, 2016, at https://minerals.usgs.gov/

minerals/pubs/historical-statistics/.

U.S. Geological Survey, 2018a, Health-based screening levels 

for evaluating water-quality data: accessed December 2018 

at https://www2.usgs.gov/envirohealth/geohealth/articles/

2014-09-26-hsbl.html.

U.S. Geological Survey, 2018b, Cobalt mineral commodity 

summaries, January 2018: accessed Febraury 2019 at 

https://minerals.usgs.gov/minerals/pubs/commodity/cobalt/

mcs-2018-cobal.pdf.

U.S. Geological Survey, 2018c, Thallium mineral commodity 

summaries, January 2018: accessed July 2018 at 

https://minerals.usgs.gov/minerals/pubs/commodity/

thallium/mcs-2018-thall.pdf.

Vaccaro, J.J., 1999, Summary of the Columbia Plateau 

regional aquifer-system analysis, Washington, Oregon, and 

Idaho: U.S. Geological Survey Professional Paper 1413–A, 

51 p., https://doi.org/10.3133/pp1413A.

Vyskočil, A., and Viau, C., 1999, Assessment of molybdenum 

toxicity in humans: Journal of Applied Toxicology, v. 19, 

no. 3, p. 185–192, https://doi.org/10.1002/(SICI)1099-

1263(199905/06)19:3<185::AID-JAT555>3.0.CO;2-Z.

Weeks, J.B., Gutentag, E.D., Heimes, F.J., and Luckey, R.R., 

1988, Summary of the High Plains regional aquifer-system 

analysis in parts of Colorado, Kansas, Nebraska, New 

Mexico, Oklahoma, South Dakota, Texas, and Wyoming: 

U.S Geological Survey Professional Paper 1400–A, 30 p., 

https://doi.org/10.3133/pp1400A.

Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty, R.B., 

2000, Arsenic in groundwater of the United States—

Occurrence and geochemistry: Groundwater, v. 38, 

no. 4, p. 589–604, https://doi.org/10.1111/j.1745-

6584.2000.tb00251.x.
128 Water Quality of Groundwater Used for Public Supply in Principal Aquifers

Western Regional Climate Center, 2018, 

Climate of New Mexico: Western Regional 

Climate Center, accessed January 2018 at 

https://wrcc.dri.edu/Climate/narrative_nm.php.

Whiteman, K.J., Vaccaro, J.J., Gonthier, J.B., and Bauer, H.H., 

1994, The hydrogeologic framework and geochemistry of 

the Columbia Plateau aquifer system, Washington, Oregon, 

and Idaho: U.S. Geological Survey Professional Paper 

1413–B, 73 p., https://doi.org/10.3133/pp1413B.

Wilkins, D.W., 1998, Summary of the southwest alluvial 

basins regional aquifer system in parts of Colorado, New 

Mexico, and Texas: U.S. Geological Survey Professional 

Paper 1407–A, 49 p., https://doi.org/10.3133/pp1407A.

Williamson, A.K., Munn, M.D., Ryker, S.J., Wagner, 

R.J., Ebbert, J.C., and Vanderpool, A.M., 1998, Water 

quality in the Central Columbia Plateau: Washington and 

Idaho, 1992–95: U.S. Geological Survey Circular 1144, 

https://pubs.usgs.gov/circ/circ1144/ccpt.book.pdf.

Wilson, S.C., Lockwood, P.V., Ashley, P.M., and Tighe, M., 

2010, The chemistry and behavior of antimony in the soil 

environment with comparison to arsenic—A critical review: 

Environmental Pollution, v. 158, no. 5, p. 1169–1181, 

https://doi.org/10.1016/j.envpol.2009.10.045.

Wright, M.T., Fram, M.S., and Belitz, K., 2015, 

Geochemical conditions and the occurrence of selected 

trace elements in groundwater basins used for public 

drinking-water supply, Desert and Basin and Range 

hydrogeologic provinces, 2006–11—California 

GAMA Priority Basin Project: U.S. Geological Survey 

Scientific Investigations Report 2014–5173, 48 p., 

https://doi.org/10.3133/sir20145173.

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

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

Toccalino, P.L., 2006, The quality of our Nation’s waters—

Volatile organic compounds in the Nation’s ground water 

and drinking-water supply wells: U.S. Geological Survey 

Circular 1292, 101 p., https://pubs.usgs.gov/circ/circ1292/.
Appendix 1. Quality-Control Data and Analysis 129

Appendix 1. Quality-Control Data and Analysis

Samples

Quality-control (QC) samples are routinely collected with environmental groundwater samples. Quality-control samples 

help to identify data that best represent environmental conditions and data that might have been affected by contamination 

or bias during sample collection, processing, storage, transportation, or laboratory analysis (Bennett and Fram, 2014). The 

third cycle of the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project began in 2013, and 

during this cycle, groundwater samples were collected from four types of groundwater networks―principal aquifer study 

networks (six of which are the focus of this report), land-use study networks, major aquifer study networks, and enhanced trends 

networks. The NAWQA groundwater sampling has collected and evaluated three types of QC samples: (1) blank samples to 

assess positive bias from contamination during sample collection or sample analysis, (2) replicate samples to assess variability, 

and (3) matrix-spike tests to assess positive or negative bias. Quality-control results for five of the six principal aquifer study 

networks described in this report have been previously published in USGS data-series reports corresponding to collection 

periods of groundwater samples for the respective PAs (table 1.1); QC results for these five principal aquifer studies, therefore, 

are not discussed herein. Quality-control results for the Colorado Plateaus aquifers principal aquifer study will be included in a 

subsequent data-series report and also are described in this appendix.

Table 1.1. Principal aquifer study unit and respective U.S. Geological Survey (USGS) data-series reports that summarize quality-control 

results for groundwater samples collected by the National Water-Quality Assessment Project sampled by the U.S. Geological Survey 

National Water-Quality Assessment Project, 2013–2017.

Principal aquifer 

study unit

Sample 

collection 

year

USGS Data series report

Basin and Range 

basin-fill 

aquifers

2013 Arnold, T.L., DeSimone, L.A., Bexfield, L.M., Lindsey, B.D., Barlow, J.R., Kulongoski, J.T., Musgrove, M., 

Kingsbury, J.A., and Belitz, K., 2016, Groundwater quality data from the National Water-Quality 

Assessment Project, May 2012 through December 2013 (ver. 1.1, November 2016): U.S. Geological 

Survey Data Series 997, 56 p., https://doi.org/10.3133/ds997.

Basin and Range 

carbonate-rock 

aquifers

2013 and 

2015

Arnold, T.L., Bexfield, L.M., Musgrove, M., Stackelberg, P.E., Lindsey, B.D., Kingsbury, J.A., 

Kulongoski, J.T., and Belitz, K., 2018, Groundwater-quality and select quality-control data from the 

National Water-Quality Assessment Project, January through December 2015, and previously unpublished 

data from 2013 to 2014: U.S. Geological Survey Data Series 1087, 68 p., https://doi.org/10.3133/ds1087.

Rio Grande 

aquifers 

system

2014 Arnold, T.L., Bexfield, L.M., Musgrove, M., Lindsey, B.D., Stackelberg, P.E., Barlow, J.R., 

DeSimone, L.A., Kulongoski, J.T., Kingsbury, J.A., Ayotte, J.D., Fleming, B.J., and Belitz, K., 2017, 

Groundwater-quality data from the National Water-Quality Assessment Project, January through 

December 2014 and select quality-control data from May 2012 through December 2014: U.S. Geological 

Survey Data Series 1063, 83 p., https://doi.org/10.3133/ds1063.

High Plains 

aquifer

2015 and 

2016 

(sites in 

Texas)

Arnold, T.L., Bexfield, L.M., Musgrove, M., Stackelberg, P.E., Lindsey, B.D., Kingsbury, J.A., 

Kulongoski, J.T., and Belitz, K., 2018, Groundwater-quality and select quality-control data from the 

National Water-Quality Assessment Project, January through December 2015, and previously unpublished 

data from 2013 to 2014: U.S. Geological Survey Data Series 1087, 68 p., https://doi.org/10.3133/ds1087.

Columbia 

Plateau 

basaltic-rock 

aquifers

2016 Arnold, T.L., Bexfield, L.M., Musgrove, M., Erickson, M.L., Kingsbury, J.A., Degnan, J.R., Tesoriero, A.J., 

Kulongoski, J.T., and Belitz, K., 2020, Groundwater-quality and select quality-control data from the 

National Water-Quality Assessment Project, January through December 2016, and previously unpublished 

data from 2013 to 2015: U.S. Geological Survey Data Series 1124, 135 p., https://doi.org/10.3133/ds1124.

Colorado 

Plateaus 

aquifers

2017 Included in this report.
130 Water Quality of Groundwater used for Public Supply in Principal Aquifers

Blanks

Blank samples are used to determine if water samples could have been contaminated during sample collection, field 

processing transport, or laboratory analysis (Mueller and others, 2015). Blank samples are collected using blank water that 

has been certified by the USGS National Water Quality Laboratory (NWQL) to be free of detectable concentrations of the 

constituents of interest. Three types of blank samples were collected: equipment, field, and source-solution blanks. Equipment 

blanks generally are collected in a controlled environment (such as a laboratory) before field sampling begins to evaluate 

equipment and cleaning protocols for established data-quality requirements. A field blank is treated with the same sample 

collection, field processing, preservation, transportation, and laboratory handling procedures as an environmental sample to 

evaluate the potential of these procedures to be a source of contamination. A source-solution blank is a sample of the water 

used to collect the equipment and field blanks and is intended to verify that the blank water does not contain the constituents 

of interest. Five trace elements—barium, cobalt, copper, strontium, and zinc were detected at low concentrations in equipment 

blanks and field blanks, whereas only copper was detected in a source-solution blank (table 1.2). Copper and zinc are known 

to sometimes occur in field and equipment blanks because of their presence in the fittings and equipment used to collect 

groundwater samples (Olsen and others, 2010). Copper, zinc, and barium were not selected for analysis by the understanding 

assessment. Detections of trace elements in blank samples were less than 5 percent of their health-based benchmarks and were 

not considered to have affected the interpretation of the environmental samples collected in Colorado Plateaus principal aquifer 

study. Two major ions—calcium and chloride—were detected at low concentrations in field blanks; blank concentrations were 

two to three orders of magnitude below those of most environmental samples and thus were unlikely to significantly affect 

groundwater results. The volatile organic compounds (VOCs) isopropyl alcohol and carbon disulfide were detected in one field 

blank and one equipment blank, respectively (table 1.2), with no detections of these constituents in environmental samples. No 

pesticide compounds or nitrate were detected in any of the field blanks or equipment blanks (table 1.2).

Table 1.2. Constituents detected in groundwater blank samples from the Colorado Plateaus aquifers, U.S. Geological Survey National 

Water-Quality Assessment Principal Aquifer Studies, June 2017 to December 2017.

[μg/L, micrograms per liter; <, less than; nv, no value in category; mg/L, milligrams per liter; na, not applicable; E, estimated; ng/L, nanograms per liter]

Constituent Reporting 

level

Number of 

field blank 

detection/

total number 

of field blank 

sample

Concentration(s) 

detected in field 

blank sample(s)

Number of 

equipment 

blank detection/

total number 

of equipment 

blank sample

Concentration(s) 

detected in 

equipment blank 

sample(s)

Number of 

source solution 

blank detection/

total number of 

source solution 

blank sample

Concentration(s) 

detected in 

source solution 

blank sample(s)

Trace elements (μg/L)

Barium <0.1 1/5 0.21 2/5 0.13; 0.14; 0/4 nv

Cobalt <0.03 2/5 0.052; 0.081 1/5 0.081 0/4 nv

Copper <0.2 4/5 0.27; 0.27; 0.7; 0.84 3/5 0.29; 0.97; 1.5 1/4 0.28

Stontium <0.5 2/5 0.57; 1.76 1/5 0.57 0/4 nv

Zinc <2 1/5 8.1 1/5 5.2 0/4 nv

Nutrients (mg/L)

Nitrate as N <0.04 0/5 nv na na na na

Major and minor ions (mg/L)

Calcium <0.022 2/5 0.097; 0.097 na na na na

Chloride <0.02 1/5 0.08 na na na na

Volatile organic compounds (VOCs; μg/L)

Isopropyl 

alcohol

<0.6 1/5 1.84 0/6 nv 0/10 nv

Carbon 

disulfide

<0.1 0/5 nv 1/6 E 0.02 0/10 nv

Pesticides (ng/L)

na na 0/4 nv 0/1 nv 0/1 nv
Appendix 1. Quality-Control Data and Analysis 131

Replicates

Replicates are samples that are collected at the same time (sequential) as environmental samples and then processed, 

preserved, transported, and analyzed identically. Replicate samples assess water-quality data variability. Two methods were used 

to assess variability over the broad range of measured constituents―standard deviation (SD) for low concentrations and relative 

standard deviation (RSD) for high concentrations (Anderson, 1987; Mueller and Titus, 2005). The RSD is the SD divided by the 

mean concentration for each replicate pair, expressed as a percentage. A concentration of 5 times the constituent’s reporting limit 

(RL) defined the boundary between assessing the replicate concentration by SD or RSD. Replicate samples for all constituents 

were evaluated as follows:

• If concentrations were detected (that is, above the RL) in both the environmental and replicate sample pair, the SD or 

RSD was calculated. The SD was calculated for a constituent if the mean concentration was less than 5 times the RL 

(low concentration). The RSD was calculated for a constituent if the mean concentrations was greater than or equal to 

5 times the RL (high concentration). For low concentrations, the SD variability was considered acceptable if the SD 

was less than half of the RL. For high concentrations, the RSD variability was considered acceptable for RSDs less than 

10 percent:

• If both values were non-detections (that is, less than the RL), the variability was considered zero and acceptable.

• If one value in the sample pair was a non-detection and the other was a reported concentration less than the RL, a value 

of zero was substituted for the non-detection, resulting in the maximum estimate of variability for the sample pair.

• If one value in the sample pair was reported as a non-detection and the other was a reported concentration greater than 

the RL, the variability for the sample pair was considered unacceptable.

A small number of replicate sample pairs were collected in the Colorado Plateaus aquifers. Table 1.3 summarizes results 

for the single replicate analysis for inorganic and radioactive constituents for the Colorado Plateaus aquifers. The SD and RSD 

values were acceptable for inorganic and radioactive constituents. Table 1.4 summarizes the replicate pair analysis for organic 

constituents in groundwater samples—there were two pesticide and one VOC replicate sample pairs. No pesticide or VOC 

constituents were detected in the replicate sample pairs and results were considered acceptable.

Table 1.3. Summary of replicate samples for inorganic and radioactive constituents in groundwater samples collected from the 

Colorado Plateaus aquifers, U.S. Geological Survey National Water-Quality Assessment Principal Aquifer Studies, June 2017 to 

December 2017.

[Constituents for which all replicate samples were non-detection are not listed. Abbreviations: SD; percent standard deviation; RSD, percent relative standard 

deviation; RL, reporting limit; >, greater than; <, less than; ≤, less than or equal to; nv, no value in category]

Constituent

Number of non-detection or 

≤ coded replicate samples/

number of replicate samples

Number of SDs >1/2 RL/number 

of replicate samples with 

concentrations <5 times the RL

Number of RSDs >10 percent/

number of replicate samples 

with concentrations >5 times 

the RL

Trace elements

Aluminum 1/1 nv 0/1

Antimony 1/1 nv 0/1

Arsenic 0/1 nv 0/1

Barium 0/1 nv 0/1

Beryllium 1/1 nv 0/1

Boron 0/1 0/1 nv

Cadmium 1/1 nv 0/1

Cobalt 1/1 nv 0/1

Copper 0/1 nv 0/1

Iron 0/1 nv 0/1

Lead 1/1 nv 0/1

Lithium 0/1 nv 0/1
132 Water Quality of Groundwater used for Public Supply in Principal Aquifers

Table 1.3. Summary of replicate samples for inorganic and radioactive constituents in groundwater samples collected from the 

Colorado Plateaus aquifers, U.S. Geological Survey National Water-Quality Assessment Principal Aquifer Studies, June 2017 to 

December 2017.—Continued

[Constituents for which all replicate samples were non-detection are not listed. Abbreviations: SD; percent standard deviation; RSD, percent relative standard 

deviation; RL, reporting limit; >, greater than; <, less than; ≤, less than or equal to; nv, no value in category]

Constituent

Number of non-detection or ≤

coded replicate samples/number of replicate samples

Number of SDs >1/2 RL/number of 

replicate samples with concentrations <5 times the RL

Number of RDDs >10 percent/

number of replicate samples 

with concentrations >5 times 

the RL

Manganese 1/1 nv 0/1

Molybdenum 0/1 nv 0/1

Nickel 0/1 0/1 nv

Selenium 0/1 nv 0/1

Silver 1/1 nv 0/1

Strontium 0/1 nv 0/1

Thallium 1/1 nv 0/1

Uranium (natural) 0/1 nv 0/1

Vanadium 0/1 nv 0/1

Zinc 1/1 nv 0/1

Nutrients

Nitrate plus nitrate, as nitrogen1 0/1 nv 0/1

Major and minor ions and total dissolved solids (TDS)

Calcium 0/1 nv 0/1

Chloride 0/1 nv 0/1

Flouride 0/1 nv 0/1

Magnesium 0/1 nv 0/1

Potassium 0/1 nv 0/1

Sodium 0/1 nv 0/1

Sulfate 0/1 nv 0/1

Silica (as SiO2) 0/1 nv 0/1

TDS 0/1 nv 0/1

Radioactive constituents

Gross alpha radioactivity, 30-day 

count

0/1 0/1 nv

Radium-2262 0/1 0/1 nv

Radium-2282 0/1 0/1 nv

Radon-222 0/1 nv 0/1

1Nitrite plus nitrate (as nitrogen) is referred to as nitrate in the text for clarity.

2In this work, radium-226 and radium-228 activities are combined and reported as radium activity, but quality control is presented for individual constituents.
Appendix 1. Quality-Control Data and Analysis 133

Matrix Spikes

A matrix spike is the addition of a known concentration of constituent(s) (spike) to a replicate environmental sample that 

enables the laboratory to determine the effect of the groundwater matrix on the sample analysis. Known constituents added 

in matrix spikes are the same as those analyzed in the environmental sample and allow for an analysis of bias from matrix 

interference on a compound-by-compound basis. Low matrix-spike recovery can indicate that the compound might not be 

detected in some samples if it were present at very low concentrations. Low and high matrix-spike recoveries are of potential 

concern if a concentration in groundwater is close to the health-based benchmark. A low recovery could result in a falsely 

measured concentration less than the health-based benchmark, whereas a high recovery could result in a falsely measured 

concentration greater than the health-based benchmark (Bennett and Fram, 2014). The NAWQA Project considers median 

matrix-spike recoveries of 70–130 percent as acceptable, and results for constituents outside of this range were flagged as having 

unacceptable recoveries. Matrix spikes were performed for VOCs and pesticide compounds because the analytical methods for 

these constituents can be susceptible to matrix interference.

A single spike sample for pesticide compounds was collected in the Colorado Plateaus aquifers. Groundwater samples were 

spiked with 214 pesticides to calculate matrix-spike recoveries. Except for 38 pesticide compounds, matrix-spike recoveries 

were between 70 and 130 and considered acceptable (table 1.5). Of the 38 pesticides that had unacceptable matrix-spike 

recoveries, only one compound (didealkylatrazine) was detected at low concentrations in less than 2 percent of the Colorado 

Plateaus principal aquifer study (table 3). The matrix-spike recovery of didealkylatrazine was 149 percent, indicating that 

measured concentrations of this pesticide compound in the environmental sample could have been lower than the reported 

concentration. Didealkylatrazine was not selected for the status and understanding assessment; detections frequencies of less 

than 10 percent or maximum measured concentrations less than 0.1 of a given regulatory or non-regulatory health-based 

benchmark were not included in the assessments for organic constituents.

A single spike sample for VOCs was collected in the Colorado Plateaus aquifers, and the resulting matrix-spike recoveries 

are summarized in table 1.6. Groundwater samples were spiked with 86 VOCs to calculate matrix-spike recoveries. Except 

for 8 VOC constituents, matrix-spike recoveries were between 70 and 130 percent. The seven VOC constituents that had 

unacceptable matrix-spike recoveries—isophorone, n-pentane, tert-butyl alcohol, methyl tert-butyl ether, 2-propen-1-ol, 

alpha-Terpineol, chloropicrin, and butane—were not detected in any samples collected from the Colorado Plateaus aquifers.

Table 1.4. Summary of replicate samples for organic constituents in groundwater samples collected from the Colorado Plateaus 

aquifers, U.S. Geological Survey National Water-Quality Assessment Principal Aquifer Studies, June 2017 to December 2017.

[Constituents for which all replicate pairs were non-detects are not listed. Abbreviations: SD, percent standard deviation; >, greater than; RL, reporting limit; <, 

less than; —, no detections]

Constituent Number of non-detections/number of 

replicate pairs

Number of SDs >1/2 RL/ number of replicate pairs with 

concentrations <5 times the RL

Pesticides

—1 2/2 0/2

Volatile organic compounds (VOCs)

—1 1/1 0/1

1No detections of pesticide or volatile organic compound constituents in the replicate pair analysis; all replicate pairs consisidered acceptable for pesticides 

and volatile organic compounds.
134 Water Quality of Groundwater used for Public Supply in Principal Aquifers

Table 1.5. Quality-control summary for matrix-spike recoveries of pesticides 

in samples collected from groundwater at public supply depths in the Colorado 

Plateaus, U.S. Geological Survey National Water-Quality Assessment Principal 

Aquifer Studies, June 2017 to December 2017.

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

2-(1-Hydroxyethyl)-6-methylaniline 1 25

2,3,3-Trichloro-2-propene-1-sulfonic acid (TCPSA) 1 80

2,4-Dichlorophenoxyacetic acid (2,4-D) 1 83

2-[(2-Ethyl-6-methylphenyl)amino]-1-propanol 1 90

2-Aminobenzimidazole 1 75

2-Amino-N-isopropylbenzamide 1 101

2-Chloro-2,6-diethylacetanilide 1 94

2-Chloro-N-(2-ethyl-6-methylphenyl)acetamide 1 99

2-Hydroxyatrazine (OIET) 1 66

2-Isopropyl-6-methyl-4-pyrimidinol 1 113

3,4-Dichlorophenylurea (DCPU) 1 85

3-Hydroxycarbofuran 1 92

3-Phenoxybenzoic acid 1 79

4-Chlorobenzylmethyl sulfoxide 1 106

4-Hydroxy molinate 1 86

4-Hydroxychlorothalonil 1 78

4-Hydroxyhexazinone A 1 86

Acephate 1 69

Acetochlor 1 102

Acetochlor oxanilic acid 1 93

Acetochlor sulfonic acid 1 87

Acetochlor sulfynilacetic acid 1 88

Alachlor oxanilic acid 1 103

Alachlor sulfonic acid 1 132

Alachlor sulfynilacetic acid 1 106

Aldicarb 1 80

Aldicarb sulfone 1 89

Aldicarb sulfoxide 1 98

Ametryn 1 105

Asulam 1 43

Atrazine 1 88

Azinphos-methyl 1 95

Azinphos-methyl oxon 1 84

Azoxystrobin 1 92

Bentazon 1 105

Bifenthrin 1 26

Bromacil 1 77

Bromoxynil 1 96
Appendix 1. Quality-Control Data and Analysis 135

Table 1.5. Quality-control summary for matrix-spike recoveries of pesticides 

in samples collected from groundwater at public supply depths in the Colorado 

Plateaus, U.S. Geological Survey National Water-Quality Assessment Principal 

Aquifer Studies, June 2017 to December 2017.—Continued

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

Butralin 1 97

Butylate 1 93

Carbaryl 1 115

Carbendazim 1 100

Carbofuran 1 91

Carboxy molinate 1 78

Chlorimuron-ethyl 1 34

Chlorosulfonamide acid 1 10

Chlorpyrifos 1 86

Chlorpyrifos oxon 1 52

Chlorsulfuron 1 53

cis-Permethrin 1 66

Cyanazine 1 71

Dechlorofipronil 1 95

Dechlorometolachlor 1 92

Deethylatrazine (CIAT) 1 89

Deethylhydroxyatrazine (OIAT) 1 89

Deiodo flubendiamide 1 87

Deisopropyl prometryn 1 75

Deisopropylatrazine (CEAT) 1 69

Deisopropylhydroxyatrazine (OEAT) 1 84

Demethyl fluometuron (DMFM) 1 99

Demethyl hexazinone B 1 94

Desamino metribuzin 1 85

Desamino-diketo metribuzin 1 93

Desulfinylfipronil 1 92

Desulfinylfipronil amide 1 109

Diazinon 1 94

Diazoxon 1 75

Dichlorvos 1 57

Dicrotophos 1 97

Didealkylatrazine (CAAT) 1 149

Didemethyl hexazinone F 1 70

Diflubenzuron 1 91

Diflufenzopyr 1 36

Diketonitrile-isoxaflutole 1 112

Dimethenamid 1 98

Dimethenamid oxanilic acid 1 76
136 Water Quality of Groundwater used for Public Supply in Principal Aquifers

Table 1.5. Quality-control summary for matrix-spike recoveries of pesticides 

in samples collected from groundwater at public supply depths in the Colorado 

Plateaus, U.S. Geological Survey National Water-Quality Assessment Principal 

Aquifer Studies, June 2017 to December 2017.—Continued

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

Dimethenamid sulfinylacetic acid 1 72

Dimethenamid sulfonic acid 1 94

Dimethoate 1 98

Dimethoate oxon (Omethoate) 1 74

Disulfoton 1 72

Disulfoton oxon 1 87

Disulfoton oxon sulfone 1 83

Disulfoton oxon sulfoxide 1 100

Disulfoton sulfone 1 70

Disulfoton sulfoxide 1 115

Diuron 1 97

EPTC 1 86

EPTC degradate R248722 1 102

Ethoprophos 1 100

Etoxazole 1 81

Fenamiphos 1 97

Fenamiphos sulfone 1 79

Fenamiphos sulfoxide 1 99

Fentin 1 72

Fipronil 1 89

Fipronil amide 1 97

Fipronil sulfide 1 90

Fipronil sulfonate 1 74

Fipronil sulfone 1 92

Flubendiamide 1 94

Flumetsulam 1 105

Fonofos 1 88

Halosulfuron methyl 1 58

Hexazinone 1 96

Hexazinone Transformation Product C 1 90

Hexazinone Transformation Product D 1 87

Hexazinone Transformation Product E 1 70

Hexazinone Transformation Product G 1 75

Hydroxy didemethyl fluometuron 1 74

Hydroxy monodemethyl fluometuron 1 50

Hydroxyacetochlor 1 88

Hydroxyalachlor 1 95

Hydroxydiazinon 1 96
Appendix 1. Quality-Control Data and Analysis 137

Table 1.5. Quality-control summary for matrix-spike recoveries of pesticides 

in samples collected from groundwater at public supply depths in the Colorado 

Plateaus, U.S. Geological Survey National Water-Quality Assessment Principal 

Aquifer Studies, June 2017 to December 2017.—Continued

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

Hydroxymetolachlor 1 100

Hydroxyphthalazinone 1 88

Hydroxysimazine 1 97

Hydroxytebuthiuron 1 69

Imazamox 1 90

Imazaquin 1 87

Imazethapyr 1 85

Imidacloprid 1 81

Indoxacarb 1 69

Isoxaflutole 1 75

Isoxaflutole acid metabolite RPA 203328 1 70

Kresoxim-methyl 1 77

Lactofen 1 52

Linuron 1 92

Malaoxon 1 50

Malathion 1 76

Metalaxyl 1 96

Metconazole 1 88

Methamidophos 1 115

Methidathion 1 90

Methomyl 1 82

Methomyl oxime 1 54

Methoxyfenozide 1 91

Metolachlor 1 92

Metolachlor hydroxy morpholinone 1 107

Metolachlor oxanilic acid 1 80

Metolachlor sulfonic acid 1 108

Metribuzin 1 86

Metribuzin DK 1 105

Molinate 1 86

Myclobutanil 1 85

N-(3,4-Dichlorophenyl)-N’-methylurea (DCPMU) 1 100

Naled 1 0

Nicosulfuron 1 43

Norflurazon 1 77

Novaluron 1 96

O-Ethyl-O-methyl-S-propylphosphorothioate 1 97

O-Ethyl-S-methyl-S-propyl phosphorodithioate 1 94
138 Water Quality of Groundwater used for Public Supply in Principal Aquifers

Table 1.5. Quality-control summary for matrix-spike recoveries of pesticides 

in samples collected from groundwater at public supply depths in the Colorado 

Plateaus, U.S. Geological Survey National Water-Quality Assessment Principal 

Aquifer Studies, June 2017 to December 2017.—Continued

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

O-Ethyl-S-propylphosphorothioate 1 85

Orthosulfamuron 1 21

Oryzalin 1 88

Oxamyl 1 60

Oxamyl oxime 1 106

Oxyfluorfen 1 86

Paraoxon 1 95

Paraoxon-methyl 1 91

Pendimethalin 1 83

Phorate 1 77

Phorate oxon 1 67

Phorate oxon sulfone 1 67

Phorate oxon sulfoxide 1 74

Phorate sulfone 1 96

Phorate sulfoxide 1 99

Phthalazinone 1 58

Piperonyl butoxide 1 89

Profenofos 1 66

Prometon 1 84

Prometryn 1 82

Propanil 1 99

Propargite 1 70

Propazine 1 95

Propiconazole 1 83

Propoxur 1 112

Propyzamide 1 102

Prosulfuron 1 34

Pyraclostrobin 1 80

Pyridaben 1 82

Pyriproxyfen 1 80

sec-Acetochlor oxanilic acid 1 66

sec-Alachlor oxanilic acid 1 76

Siduron 1 95

Simazine 1 86

Sulfentrazone 1 71

Sulfometuron-methyl 1 55

Sulfosulfuron 1 62

Sulfosulfuron ethyl sulfone 1 72
Appendix 1. Quality-Control Data and Analysis 139

Table 1.5. Quality-control summary for matrix-spike recoveries of pesticides 

in samples collected from groundwater at public supply depths in the Colorado 

Plateaus, U.S. Geological Survey National Water-Quality Assessment Principal 

Aquifer Studies, June 2017 to December 2017.—Continued

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

Tebuconazole 1 95

Tebufenozide 1 91

Tebupirimfos 1 91

Tebupirimfos oxon 1 95

Tebuthiuron 1 94

Tebuthiuron Transformation Product 104 1 103

Tebuthiuron Transformation Product 106 1 85

Tebuthiuron Transformation Product 108 1 82

Tebuthiuron Transformation Product 109 1 82

Terbacil 1 87

Terbufos 1 69

Terbufos oxon 1 39

Terbufos oxon sulfone 1 76

Terbufos oxon sulfoxide 1 87

Terbufos sulfone 1 99

Terbufos sulfoxide 1 104

Terbuthylazine 1 97

Tetraconazole 1 106

Thiobencarb 1 95

trans-Permethrin 1 70

Triallate 1 74

Tribuphos 1 84

Triclopyr 1 79

Trifloxystrobin 1 78
140 Water Quality of Groundwater used for Public Supply in Principal Aquifers

Table 1.6. Quality-control summary for matrix-spike recoveries of volatile organic 

compounds (VOCs) in samples collected from groundwater at public supply 

depths in the Colorado Plateaus, U.S. Geological Survey National Water-Quality 

Assessment Principal Aquifer Studies, June 2017 to December 2017.

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

1,1-Dichloro-2-propanone 1 76

1,4-Dioxane 1 95

2,2-Dichloro-1,1,1-trifluoroethane (HCFC-123) 1 94

2-Nitropropane 1 86

Acetonitrile 1 75

Chlorodifluoromethane (HCFC-22) 1 91

Dichlorofluoromethane 1 96

Dimethoxymethane 1 91

Ethyl acetate 1 87

Hexane 1 96

Isophorone 1 65

Methyl acetate 1 84

Nitrobenzene 1 72

n-Nitroso-diethylamine (NDEA) 1 92

n-Pentane 1 175

tert-Butyl alcohol (TBA) 1 151

1,1,1,2-Tetrachloroethane 1 93

1,1,1-Trichloroethane (TCA) 1 95

1,1,2-Trichloroethane 1 107

1,1-Dichloroethane 1 98

1,1-Dichloroethylene 1 101

1,2,3-Trichloropropane (1,2,3-TCP) 1 93

1,2,4-Trichlorobenzene 1 93

1,2,4-Trimethylbenzene 1 101

1,2-Dibromo-3-chloropropane (DBCP) 1 90

1,2-Dibromoethane (EDB) 1 93

1,2-Dichlorobenzene 1 103

1,2-Dichloroethane 1 100

1,2-Dichloropropane (1,2-DCP) 1 93

1,4-Dichlorobenzene 1 105

Benzene 1 101

Bromochloromethane 1 99

Bromodichloromethane (THM) 1 99

Bromomethane 1 91

Carbon disulfide 1 97

Chlorobenzene 1 104

Chloromethane (Methylchloride) 1 92

cis-1,2-Dichloroethylene 1 102
Appendix 1. Quality-Control Data and Analysis 141

Table 1.6. Quality-control summary for matrix-spike recoveries of volatile organic 

compounds (VOCs) in samples collected from groundwater at public supply 

depths in the Colorado Plateaus, U.S. Geological Survey National Water-Quality 

Assessment Principal Aquifer Studies, June 2017 to December 2017.—Continued

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

cis-1,3-Dichloropropene 1 97

Dibromochloromethane (THM) 1 95

Dichloromethane (methylene chloride) 1 97

Ethylbenzene 1 101

m- and p-Xylene 1 106

Naphthalene 1 95

n-Propylbenzene 1 98

o-Xylene 1 101

sec-Butylbenzene 1 97

Styrene 1 94

Methyl tert-butyl ether (MTBE) 1 402

Tetrachloroethylene (PCE) 1 104

Carbon tetrachloride (tetrachloromethane) 1 94

Toluene 1 99

trans-1,2-Dichloroethylene 1 102

trans-1,3-Dichloropropene 1 96

Bromoform (tribromomethane) (THM) 1 92

Trichloroethylene (TCE) 1 100

Chloroform (trichloromethane) (THM) 1 100

Vinyl chloride (Chloroethylene) 1 103

1,3-Butadiene 1 110

2-Ethoxyethyl acetate 1 74

2-Propen-1-ol 1 67

alpha-Terpineol 1 69

Butanal 1 123

Chloropicrin 1 15

trans-Crotonaldehyde 1 119

1,1-Difluoroethane 1 78

1,2,3,4-Tetrahydronapthalene (Tetralin) 1 94

1,2-Dichloro-1,1,2,2-tetrafluoroethane (CFC-114) 1 74

1,3-Dioxolane 1 78

Butanol 1 79

1-Chloro-1,1-difluoroethane 1 73

1-Methoxy-4(2-propenyl)benzene 1 90

1-Octanol 1 107

2,6-Dimethyl-4-heptanone 1 116

2-Ethyl-1-hexanol 1 99

Isobutyl acetate 1 88
142 Water Quality of Groundwater used for Public Supply in Principal Aquifers

Table 1.6. Quality-control summary for matrix-spike recoveries of volatile organic 

compounds (VOCs) in samples collected from groundwater at public supply 

depths in the Colorado Plateaus, U.S. Geological Survey National Water-Quality 

Assessment Principal Aquifer Studies, June 2017 to December 2017.—Continued

[Acceptable recovery range is between 70 and 130 percent; bolded values have unacceptable 

recoveries]

Constituent (synonym or abbreviation)

Number of matrix 

spike samples 

collected

Recovery

(percent)

4-Methyl-2-pentanol 1 99

5-Methyl-2-hexanone 1 115

Butane 1 315

Cyclohexanone 1 99

Isopropyl acetate 1 87

Isopropyl alcohol 1 88

n-Pentanal 1 114

Propyl acetate 1 86

References Cited

Anderson, R.L., 1987, Practical statistics for analytical chemists: New York, Van Nostrand Reinhold Company, Inc., 315 p.

Arnold, T.L., DeSimone, L.A., Bexfield, L.M., Lindsey, B.D., Barlow, J.R., Kulongoski, J.T., Musgrove, M., Kingsbury, J.A., 

and Belitz, K., 2016, Groundwater quality data from the National Water-Quality Assessment Project, May 2012 through 

December 2013 (ver. 1.1, November 2016): U.S. Geological Survey Data Series 997, 56 p., https://doi.org/10.3133/ds997.

Arnold, T.L., Bexfield, L.M., Musgrove, M., Lindsey, B.D., Stackelberg, P.E., Barlow, J.R., DeSimone, L.A., Kulongoski, J.T., 

Kingsbury, J.A., Ayotte, J.D., Fleming, B.J., and Belitz, K., 2017, Groundwater-quality data from the Nationa Water-Quality 

Assessment Project, January through December 2014 and select quality-control data from May 2012 through December 2014: 

U.S. Geological Survey Data Series 1063, 83 p., https://doi.org/10.3133/ds1063.

Arnold, T.L., Bexfield, L.M., Musgrove, M., Stackelberg, P.E., Lindsey, B.D., Kingsbury, J.A., Kulongoski, J.T., and Belitz, K., 

2018, Groundwater-quality and select quality-control data from the National Water-Quality Assessment Project, January 

throught December 2015, and previously unpublished data from 2013 to 2014: U.S. Geological Survey Data Series 1087, 

68 p., https://doi.org/10.3133/ds1087.

Arnold, T.L., Bexfield, L.M., Musgrove, M., Erickson, M.L., Kingsbury, J.A., Degnan, J.R., Tesoriero, A.J., Kulongoski, J.T., 

and Belitz, K., 2020, Groundwater-quality and select quality-control data from the National Water-Quality Assessment 

Project, January through December 2016, and previously unpublished data from 2013 to 2015: U.S. Geological Survey Data 

Series Report 1124, 135 p., https://doi.org/10.3133/ds1124.

Bennett, G.L., V, and Fram, M.S., 2014, Groundwater-quality data in the North San Francisco Bay shallow aquifer 

study unit, 2012—Results from the California GAMA Program: U.S. Geological Sutvey Data Series 865, 94 p., 

https://doi.org/10.3133/ds865.

Mueller, D.K., and Titus, C.J., 2005, Quality of nutrient data from streams and ground water sampled during water years 

1992–2001: U.S. Geological Survey Scientific Investigations Report 2005–5106, 27 p., https://doi.org/10.3133/sir20055106.

Mueller, D.K., Schertz, T.L., Martin, J.D., and Sandstrom, M.W., 2015, Design, analysis, and interpretation of field 

quality-control data for water-sampling projects: U.S. Geological Survey Techniques and Methods, book 4, chap. C4, 54 p., 

https://doi.org/10.3133/tm4C4.

Olsen, L.D., Fram, M.S., and Belitz, K., 2010, Review of trace-element field-blank data collected for the California 

Groundwater Ambient Monitoring and Assessment (GAMA) Program, May 2004–January 2008: U.S. Geological Survey 

Scientific Investigations Report 2009–5220, 47 p., https://doi.org/10.3133/sir20095220.
For more information concerning the research in this report, contact the

Director, California Water Science Center

U.S. Geological Survey

6000 J Street, Placer Hall

Sacramento, California 95819

https://ca.water.usgs.gov

Publishing support provided by the 

U.S. Geological Survey Science Publishing Network, 

Sacramento Publishing Service Center
Rosecrans and Musgrove—Water Quality of Groundwater used for Public Supply in Principal Aquifers—SIR 2020–5078

ISSN 2328-0328 (online)

https://doi.org/10.3133/sir20205078