U.S. Department of the Interior

U.S. Geological Survey

National Water-Quality Assessment Program

Trace Elements and Radon in Groundwater 

Across the United States, 1992–2003

Scientific Investigations Report 2011–5059
Cover. Left photograph: Outcrop of crystalline bedrock. (Photograph by James Degnan, U.S. Geological Survey). Middle photograph: 

Domestic well in crystalline bedrock aquifer. Right photograph: Drinking water from a domestic well.
Trace Elements and Radon in Groundwater 

Across the United States, 1992–2003

By Joseph D. Ayotte, Jo Ann M. Gronberg, and Lori E. Apodaca

National Water-Quality Assessment Program

Scientific Investigations Report 2011–5059

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

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. (Also available at 

http://pubs.usgs.gov/sir/2011/5059.)
iii

Foreword

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

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

water, biological, energy, and mineral resources (http://www.usgs.gov/). Information on the Nation’s water 

resources is critical to ensuring long-term availability of water that is safe for drinking and recreation and is 

suitable for industry, irrigation, and fish and wildlife. Population growth and increasing demands for water 

make the availability of that water, measured in terms of quantity and quality, even more essential to the 

long-term sustainability of our communities and ecosystems.

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

national, regional, State, and local information needs and decisions related to water-quality management 

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

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

and human activities affect the quality of streams and groundwater, and where are those effects most 

pronounced? By combining information on water chemistry, physical characteristics, stream habitat, and 

aquatic life, the NAWQA Program aims to provide science-based insights for current and emerging water 

issues and priorities. From 1991 to 2001, the NAWQA Program completed interdisciplinary assessments 

and established a baseline understanding of water-quality conditions in 51 of the Nation’s river basins and 

aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/studies/study_units.html).

National and regional assessments are ongoing in the second decade (2001–2012) of the NAWQA Program 

as 42 of the 51 Study Units are selectively reassessed. These assessments extend the findings in the Study 

Units by determining water-quality status and trends at sites that have been consistently monitored for 

more than a decade, and filling critical gaps in characterizing the quality of surface water and groundwater. 

For example, increased emphasis has been placed on assessing the quality of source water and finished 

water associated with many of the Nation’s largest community water systems. During the second decade, 

NAWQA is addressing five national priority topics that build an understanding of how natural features and 

human activities affect water quality, and establish links between sources of contaminants, the transport 

of those contaminants through the hydrologic system, and the potential effects of contaminants on humans 

and aquatic ecosystems. Included are studies on the fate of agricultural chemicals, effects of urbanization 

on stream ecosystems, bioaccumulation of mercury in stream ecosystems, effects of nutrient enrichment on 

aquatic ecosystems, and transport of contaminants to public-supply wells. In addition, national syntheses of 

information on pesticides, volatile organic compounds (VOCs), nutrients, trace elements, and aquatic ecology 

are continuing. 

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

effective water-resource management and strategies that protect and restore water quality. We hope this 

NAWQA publication will provide you with insights and information to meet your needs, and will foster 

increased citizen awareness and involvement in the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national assessment by a single program cannot address all water-resource 

issues of interest. External coordination at all levels is critical for cost-effective management, regulation, 

and conservation of our Nation’s water resources. The NAWQA Program, therefore, depends on advice 

and information from other agencies—Federal, State, regional, interstate, Tribal, and local—as well as 

nongovernmental organizations, industry, academia, and other stakeholder groups. Your assistance and 

suggestions are greatly appreciated.

 William H. Werkheiser

USGS Associate Director for Water
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v

Contents

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

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

The National Water-Quality Assessment Program.........................................................................2

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

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

Water-Quality Benchmarks for Human Health ................................................................................3

Methods...........................................................................................................................................................5

Well Selection .......................................................................................................................................5

Sample Collection and Analysis.........................................................................................................5

Study Design..........................................................................................................................................5

Climate...........................................................................................................................................6

Aquifer Groups and Geology......................................................................................................6

Chemical Evolution and Groundwater Age ...........................................................................11

Redox and pH..............................................................................................................................11

Data Handling and Statistical Methods ..........................................................................................14

Quality Control.....................................................................................................................................15

Occurrence of Trace Elements in Groundwater.....................................................................................17

Trace-Element Occurrence and Concentrations in Monitoring and Drinking-Water Wells..17

Trace-Element Occurrence and Concentrations in Wells in Agricultural and 

Urban Areas ...........................................................................................................................36

Comparisons of Trace-Element Concentrations in Groundwater to Human-Health 

Benchmarks ...........................................................................................................................36

Trace-Element Occurrence and Concentrations in Groundwater by Major Aquifer Group ..38

Unconsolidated Sand and Gravel Aquifers (USG)................................................................39

Glacial Unconsolidated Sand and Gravel Aquifers (GLA)...................................................43

Semiconsolidated Sand Aquifers (SCS).................................................................................44

Sandstone Aquifers (SAN) .......................................................................................................44

Sandstone and Carbonate-Rock Aquifers (SCR)..................................................................44

Carbonate-Rock Aquifers (CAR)..............................................................................................45

Basaltic- and other Volcanic-Rock Aquifers (BAV) .............................................................45

Crystalline-Rock Aquifers (CRL) ..............................................................................................45

Relation of Selected Trace Elements to pH and Redox State......................................................46

Iron and Manganese.................................................................................................................46

Aluminum, Copper, Lead, and Zinc..........................................................................................46

Arsenic, Chromium, Molybdenum, and Selenium ................................................................47

Uranium and Radon...................................................................................................................47

Relation to Groundwater Age ...........................................................................................................57

Analysis of Multiple Factors Affecting Trace-Element Occurrence ...................................................57

Cationic Elements ...............................................................................................................................65

Oxyanion- and Ion-Complex-Forming Elements ............................................................................65

Other Elements....................................................................................................................................68

Co-occurrence of Trace Elements at Concentrations Greater than Human-Health 

Benchmarks ....................................................................................................................................68
vi

Figures

1. Maps showing locations of (A) major aquifer study (MAS) wells, and (B) land-use 

study (LUS) wells sampled for trace elements by the National Water-Quality 

Assessment Program, 1992–2003...............................................................................................4

2. Maps showing (A) generalized categories of oxidation-reduction potential (redox) 

in samples from wells in humid and dry regions of the United States, and (B) pH of 

precipitation in 2001 .....................................................................................................................7

3. Maps showing (A) principal aquifers of the United States and major aquifer groups 

used in the current study, and (B) wells sampled by the National Water-Quality 

Assessment Program, 1992−2003, color coded by aquifer..................................................12

4. Maps showing geographic distribution of trace-element concentrations in 

groundwater collected from wells as part of the National Water-Quality Assessment 

Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, 

(E) beryllium, (F) boron, (G) cadmium, (H) chromium, (I) cobalt, (J) copper, (K) iron, 

(L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, 

and (X) zinc ..................................................................................................................................22

5. Boxplots showing trace-element concentrations in groundwater across the United 

States by climate region............................................................................................................34

6. Trilinear diagrams showing major-ion chemistry by redox class for (A) unconsolidated 

sand and gravel aquifers, (B) glacial unconsolidated sand and gravel aquifers, 

(C) semiconsolidated sand aquifers, (D) sandstone aquifers, (E) sandstone and 

carbonate-rock aquifers, (F) carbonate-rock aquifers, (G) basaltic- and other 

volcanic-rock aquifers, and (H) crystalline-rock aquifers...................................................40

7. Boxplots showing distributions of pH by major aquifer group............................................43

8. Boxplots showing the percentage of groundwater samples with trace-element 

concentrations greater than or equal to the reporting level, by redox and pH 

categories, for the major aquifer groups: (A) aluminum, (B) antimony, (C) arsenic, 

(D) barium, (E) beryllium, (F) boron, (G) cadmium, (H) chromium, (I) cobalt, (J) copper, 

(K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and 

(X) zinc ..........................................................................................................................................48

Summary........................................................................................................................................................71

Acknowledgments.......................................................................................................................................72

References Cited..........................................................................................................................................72

Appendix 1. Selected National-Scale Studies of Trace Elements in Groundwater......................80

Appendix 2. Wells Sampled by the NAWQA Program, 1992–2003, by Study Type, Land Use, 

Major Aquifer Group and Aquifer Name ....................................................................................81

Appendix 3. Summary Statistics and Percent Exceedences of Human-Health Benchmarks 

and Nonhealth Guidelines for Trace Elements in Groundwater Across the United 

States Sampled by the NAWQA Program, 1992–2003, by Major Aquifer Group..................87

Appendix 4. Summary of the Percent Exceedences of Human-Health Benchmarks and 

Nonhealth Guidelines for Trace Elements in Groundwater Across the United States 

Sampled by the NAWQA Program, 1992–2003, by pH, Redox State, and Major 

Aquifer Group..................................................................................................................................95

Appendix 5. Summaries of Logistic Regression Analyses of Selected Trace Elements by 

Major Aquifer Group....................................................................................................................103
vii

9. Boxplots showing the percentages of groundwater samples with trace-element 

concentrations greater than or equal to 1 microgram per liter for (A) aluminum, 

(B) lead, (C) copper, and (D) zinc, by pH and redox category .............................................54

10. Boxplots showing the percentages of groundwater samples with trace-element 

concentrations greater than or equal to 1 microgram per liter for (A) aluminum, 

(B) lead, (C) copper, and (D) zinc in dry- and humid-region unconsolidated sand and 

gravel aquifers ............................................................................................................................54

11. Boxplots showing the percentages of samples with trace-element concentrations 

greater than or equal to 1 microgram per liter for (A) arsenic, (B) chromium, 

(C) molybdenum, and (D) selenium, by pH and redox category..........................................55

12. Boxplots showing (A) concentrations of arsenic in glacial unconsolidated sand 

and gravel aquifers (GLA), (B) percentages of samples with arsenic concentrations 

greater than or equal to 1 microgram per liter in GLA aquifers, (C) concentrations of 

arsenic in unconsolidated sand and gravel aquifers (USG), and (D) percentages of 

samples with arsenic concentrations greater than or equal to 1 microgram per liter 

in USG aquifers ...........................................................................................................................56

13. Boxplots showing the distribution of (A) dissolved oxygen, (B) well depth, and 

(C) pH in groundwater samples by category of age of recharge .......................................58

14. Boxplots showing percentages of groundwater samples with trace-element 

concentrations greater than or equal to 1 microgram per liter (μg/L) (300 μg/L for iron 

and 50 μg/L for manganese) by category of age of recharge for samples from (A) dry 

and (B) humid regions................................................................................................................59

15. Boxplots showing the percentages of groundwater samples with concentrations 

greater than reporting level for (A) aluminum (Al), (B) manganese (Mn), (C) iron (Fe), 

and (D) nickel (Ni), by major covariates from logistic regression models ........................66

16. Boxplots showing the percentages of groundwater samples with concentrations 

greater than or equal to 1 microgram per liter for (A) arsenic (As), (B) molybdenum 

(Mo), (C) selenium (Se), and (D) uranium (U), by major covariates from logistic 

regression models ......................................................................................................................67
viii

Tables

1. Generalized description of the major aquifer groups sampled for the NAWQA 

Program across the United States.............................................................................................8

2. Threshold concentrations for classification of generalized groundwater redox 

conditions and number of samples collected for the NAWQA Program across the 

United States by pH category, 1992–2003...............................................................................14

3. Summary of blank data for trace elements in groundwater sampled for the NAWQA 

Program, 1992–2003, across the United States .....................................................................16

4. Summary statistics with human-health-benchmark and nonhealth-guideline 

exceedences for trace elements in groundwater sampled for the NAWQA Program, 

1992–2003, across the United States.......................................................................................18

5. Summary statistics with human-health-benchmark and nonhealth-guideline 

exceedences for trace elements in groundwater sampled for the NAWQA Program 

by climate region, 1992–2003, across the United States......................................................19

6. Frequency and distribution statistics for the occurrence of trace elements in 

groundwater sampled for the NAWQA Program by climate region, 1992–2003, 

across the United States ...........................................................................................................21

7. Group-comparison statistics for concentrations of trace elements and selected field 

parameters in groundwater samples collected for the NAWQA Program by climate 

region and well type, 1992–2003, across the United States ................................................35

8. Group-comparison statistics for concentrations of trace elements and selected field 

parameters in groundwater sampled for the NAWQA Program by climate region and 

land-use type, 1992–2003, across the United States ............................................................37

9. Percentage of samples collected across the United States for the NAWQA Program 

from which one or more trace elements exceeded human-health benchmarks by 

primary water use and study type, 1992–2003 .......................................................................38

10. Summary of the percentages of exceedences of human-health benchmarks or 

nonhealth guidelines in samples collected for the NAWQA Program across the 

United States, 1992–2003, by major aquifer group ................................................................42

11. Summary of logistic regression models for selected trace elements in groundwater 

sampled for the NAWQA Program across the United States by element and major 

aquifer group, 1992–2003...........................................................................................................60

12. Number of groundwater samples collected for the NAWQA Program across the 

United States and number of multiple human-health benchmark exceedences by 

aquifer group, 1992–2003...........................................................................................................69

13. Co-occurrence of trace elements at concentrations greater than human-health 

benchmarks in samples collected for the NAWQA Program across the United States 

from the unconsolidated sand and gravel aquifer group, 1992–2003 ................................70

14. Co-occurrence of trace elements with boron and strontium at concentrations 

greater than human-health benchmarks in groundwater samples collected for the 

NAWQA Program across the United States, 1992–2003 ......................................................70
ix

Conversion Factors, Datum, Acronyms, and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Radioactivity

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

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.

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 either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

Concentrations of radioisotopes in water are given in picocuries per liter (pCi/L).

Chemical symbols

Al Aluminum

Sb Antimony

As Arsenic

Ba Barium

Be Beryllium

B Boron

Cd Cadmium

Cr Chromium

Co Cobalt

Cu Copper

Fe Iron

Pb Lead

Li Lithium

Mn Manganese

Mo Molybdenum
x

Ni Nickel

Se Selenium

Ag Silver

Sr Strontium

Tl Thallium

U Uranium

V Vanadium

Zn Zinc

Rn Radon

Aquifer groups

USG Unconsolidated sand and gravel aquifers

GLA Glacial unconsolidated sand and gravel aquifers

SCS Semi-consolidated sand aquifers

SAN Sandstone aquifers

SCR Sandstone and carbonate-rock aquifers

CAR Carbonate-rock aquifers

BAV Basaltic- and other volcanic-rock aquifers

CRL Crystalline-rock aquifers

USGD Unconsolidated sand and gravel aquifers from dry climates

USGH Unconsolidated sand and gravel aquifers from humid climates

Other abbreviations

α	 		 Acceptable statistical significance level for rejecting null hypothesis

AL Action level

AMCL Alternative maximum contaminant level (applies only to proposed radon 

 standard)

CDC U.S. Centers for Disease Control and Prevention

CFC Chlorofluorocarbon

DO Dissolved oxygen

GCMS Gas chromatography/mass spectrometry

HA Health advisory

HBSL Health-based screening level

HHB Human-health benchmark
xi

HPLC High performance liquid chromatography

ICP Inductively-coupled plasma

ICP–AES Inductively-coupled plasma–atomic emission spectroscopy

ICP–MS Inductively-coupled plasma–mass spectrometry

LRL Laboratory reporting level

LT–MDL Long-term method-detection level

LUS Land-use study

MAS Major aquifer study

MCL Maximum contaminant level

N number of samples

NAWQA National Water-Quality Assessment

NWIS National Water Information System

p Attained statistical significance level

QC Quality control

R2 Coefficient of determination, or fraction of variance explained by regression

ROS Regression-on-order statistics

RPD Relative percent difference

RSD Relative standard deviation

TDS Total dissolved solids

TU Tritium units

SMCL Secondary maximum contaminant level

SSMDC Sample-specific minimum detectable concentration

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

VOC Volatile organic compound
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Abstract

Trace-element concentrations in groundwater were 

evaluated for samples collected between 1992 and 2003 from 

aquifers across the United States as part of the U.S. Geological 

Survey National Water-Quality Assessment Program. This 

study describes the first comprehensive analysis of those data 

by assessing occurrence (concentrations above analytical 

reporting levels) and by comparing concentrations to humanhealth benchmarks (HHBs). Data from 5,183 monitoring and 

drinking-water wells representing more than 40 principal 

and other aquifers in humid and dry regions and in various 

land-use settings were used in the analysis. Trace elements 

measured include aluminum (Al), antimony (Sb), arsenic 

(As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd), 

chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), 

lithium (Li), manganese (Mn), molybdenum (Mo), nickel 

(Ni), selenium (Se), silver (Ag), strontium (Sr), thallium (Tl), 

uranium (U), vanadium (V), and zinc (Zn). Radon (Rn) gas 

also was measured and is included in the data analysis.

Climate influenced the occurrence and distribution of 

trace elements in groundwater whereby more trace elements 

occurred and were found at greater concentrations in wells in 

drier regions of the United States than in humid regions. In 

particular, the concentrations of As, Ba, B, Cr, Cu, Mo, Ni, Se, 

Sr, U, V, and Zn were greater in the drier regions, where processes such as chemical evolution, ion complexation, evaporative concentration, and redox (oxidation-reduction) controls 

act to varying degrees to mobilize these elements. Al, Co, Fe, 

Pb, and Mn concentrations in groundwater were greater in 

humid regions of the United States than in dry regions, partly 

in response to lower groundwater pH and (or) more frequent 

anoxic conditions. In groundwater from humid regions, concentrations of Cu, Pb, Rn, and Zn were significantly greater in 

drinking-water wells than in monitoring wells. 

Samples from drinking-water wells in dry regions had 

greater concentrations of As, Ba, Pb, Li, Sr, V, and Zn, than 

samples from monitoring wells. In humid regions, however, 

concentrations of most trace elements were greater in monitoring wells than in drinking-water wells; the exceptions 

were Cu, Pb, Zn, and Rn. Cu, Pb, and Zn are common trace 

elements in pumps and pipes used in the construction of 

drinking-water wells, and contamination from these sources 

may have contributed to their concentrations. Al, Sb, Ba, B, 

Cr, Co, Fe, Mn, Mo, Ni, Se, Sr, and U concentrations were 

all greater in monitoring wells than in drinking-water wells in 

humid regions. 

Groundwater from wells in agricultural settings had 

greater concentrations of As, Mo, and U than groundwater 

from wells in urban settings, possibly owing to greater pH in 

the agricultural wells. Significantly greater concentrations of 

B, Cr, Se, Ag, Sr, and V also were found in agricultural wells 

in dry regions. Groundwater from dry-region urban wells had 

greater concentrations of Co, Fe, Pb, Li, Mn, and specific conductance than groundwater from agricultural wells. 

The geologic composition of aquifers and aquifer geochemistry are among the major factors affecting trace-element 

occurrence. Trace-element concentrations in groundwater 

were characterized in aquifers from eight major groups based 

on geologic material, including (1) unconsolidated sand and 

gravel; (2) glacial unconsolidated sand and gravel; (3) semiconsolidated sand; (4) sandstone; (5) sandstone and carbonate 

rock; (6) carbonate rock; (7) basaltic and other volcanic rock; 

and (8) crystalline rock. The majority of groundwater samples 

and the largest percentages of exceedences of HHBs were 

in the glacial and nonglacial unconsolidated sand and gravel 

aquifers; in these aquifers, As, Mn, and U are the most common trace elements exceeding HHBs. 

Overall, 19 percent of wells (962 of 5,097) exceeded 

an HHB for at least one trace element. The trace elements 

with HHBs included in this summary were Sb, As, Ba, Be, 

B, Cd, Cr, Cu, Pb, Mn, Mo, Ni, Se, Ag, Sr, Tl, U, and Zn. Mn 

occurred most often at concentrations greater than its humanhealth benchmark (12 percent), followed by As (7.0 percent), 

Sr (4.3 percent), U (4.0 percent), B (1.9 percent), and Mo 

(1.5 percent). Rn occurred at concentrations greater than the 

U.S. Environmental Protection Agency (USEPA) proposed 

maximum contaminant level of 300 pCi/L in more than 

65 percent of water samples, and concentrations of Rn in 

2.7 percent of samples were greater than the USEPA-proposed 

alternate maximum contaminant level of 4,000 pCi/L. There 

were more exceedences of HHBs in the dry-region groundwater than in the humid-region groundwater.

Trace Elements and Radon in Groundwater Across the 

United States, 1992–2003

By Joseph D. Ayotte, Jo Ann M. Gronberg, and Lori E. Apodaca
2 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Groundwater pH and redox conditions were significant 

factors for the occurrence of many trace elements. Low pH 

(less than 7) was a significant factor in the occurrence of many 

cationic metals, such as Al, Fe, Mn, and Ni; these metals, as 

well as Cu, Pb, and Zn, adsorb more strongly to aquifer materials as pH increases. Anoxic conditions often were related to 

the increased occurrence of many oxyanion-forming elements, 

such as As, Cr, and Mo, whereas oxic conditions often were 

related to higher Se occurrence. Groundwater redox and pH 

effects were evident for As, Cr, Mo, and Se. Based on all 

samples, As occurrence generally increased as geochemical 

conditions became increasingly anoxic for groundwater with 

pH less than 7, but was consistently high for samples with 

pH greater than 7. A similar pattern was evident for Mo and, 

to a lesser extent, Cr. For groundwater in aquifers in glacial 

unconsolidated sand and gravel, however, As occurrence 

increased as groundwater became increasingly anoxic and as 

pH increased, indicating that redox is an important process 

throughout the range of pH. Al, Cu, Pb, and Zn occurred 

more often in low-pH groundwater and, except for Al, in 

oxic conditions.

In general, older waters (mostly pre-1953) had more 

occurrences of trace elements, greater pHs, were from deeper 

wells, and had lower concentrations of dissolved oxygen 

than younger waters (defined as waters containing a fraction 

younger than 1953). Most oxyanion-forming trace elements 

occurred more frequently in old groundwater. However, 

although U occurrence was greater in older water collected 

from dry-region aquifers, U occurred more often in young 

water in humid-region aquifers. This difference may be related 

to old, humid-region groundwater having lower concentrations 

of dissolved oxygen, a condition which can inhibit U mobility.

Overall, As co-occurred primarily with silica (SiO2) and 

Mo in water with slightly high pH. About 12 percent of water 

samples without SiO2 or Mo had concentrations of As greater 

than or equal to 1 μg/L. This number increased to about 

45 percent if either Mo or silica were present and to about 

85 percent if both Mo and SiO2 were present. From a subset 

of water samples selected from 2,714 wells for co-occurrence 

analysis, samples from 572 wells (21.1 percent) had HHB 

exceedences of one or more trace elements. Of the 2,714 

samples, 62 (2.3 percent) had two or more trace elements 

that exceeded HHBs. Fifty of the 62 (80 percent) were from 

unconsolidated sand and gravel aquifers and involved at least 

As and Mn, Mn and U, or As and U. 

Introduction

In 2000, groundwater accounted for about 46 percent 

of water used in the United States, with about 31 percent of 

use for public supply and about 15 percent from domestic 

wells (also known as private wells). Total withdrawals 

from groundwater have generally increased over the past 

5 decades along with increases in population (Hutson and 

others, 2004). For example, groundwater withdrawals of 

freshwater increased by 9 percent from 1995 to 2000, and 

population increased by 7 percent over the same period 

(Hutson and others, 2004). Trace elements in groundwater 

used for drinking have long been a concern because of 

potential adverse effects on human health. Many trace 

elements in drinking water can have adverse health effects at 

high concentrations, whereas others may present aesthetic or 

nuisance problems. 

As used in this report, the term “trace elements” refers to 

metals and semimetallic elements that typically are found in 

concentrations less than 1 mg/L in natural waters (Hem, 1985). 

These elements are mobilized primarily from rock weathering, 

and their concentrations in groundwater reflect their presence 

and abundance in aquifer materials, geochemical conditions 

(such as pH and oxidation-reduction potential), the presence 

and abundance of complexing ions and organic matter, and 

attenuation processes such as adsorption (Hem, 1985; Salbu 

and Steinnes, 1994). Human activities, such as mining and 

waste disposal, also can affect concentrations of trace elements 

in groundwater.

Studies worldwide have shown that high concentrations 

of trace elements can have adverse effects on human health 

(Wu and others, 1989; Haglund and others, 1996; HopenhaynRich and others, 1996; Cantor, 1997; Karagas and others, 

1998; National Academy of Sciences, 1999; Chappell and 

others, 2001; Karagas and others, 2001; Smedley and others, 2001; Kendall and Smith, 2002; Smedley, 2003; Seiler, 

2004; Selinus and others, 2005; Wasserman and others, 2006; 

Meliker and others, 2007; Walker and others, 2008). Several 

trace elements, including arsenic, cadmium, chromium, lead, 

and radon gas, have known and established toxicity (Selinus 

and others, 2005). Other trace elements, such as copper and 

selenium, in too small an amount, can cause adverse health 

outcomes (Selinus and others, 2005; Chrosniak and others, 

2006). Because many trace elements are widely dispersed in 

rocks and unconsolidated materials, the individual occurrence 

rate of a specific trace element may be low, but the overall 

chance of having at least one trace element of concern in a 

water sample may be relatively high. Because of differences 

in well characteristics and regulations, concentrations of trace 

elements in groundwater from domestic wells could differ 

from those in groundwater withdrawn from public wells. Thus, 

the distinction between drinking waters supplied by public and 

domestic wells may be a significant factor for health scientists 

assessing exposure to trace elements through drinking water.

The National Water-Quality Assessment 

Program

Knowledge of the quality of the Nation’s streams and 

aquifers is important because of the effect on human and 

aquatic health and the substantial costs associated with decisions involving land and water management, conservation, and 

regulation. Understanding regional patterns and environmental 
Introduction 3

factors affecting trace-element concentrations in streams and 

groundwater is essential for effectively developing programs 

to manage and protect these water resources and human 

health. The U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program was implemented 

in 1991 to improve the scientific and public understanding of 

water quality in the Nation’s major river basins and groundwater systems. The goals of the NAWQA Program are to describe 

current water-quality conditions and trends in the Nation’s 

rivers and groundwater to determine the natural characteristics 

and human influences that affect water quality (Gilliom and 

others, 1995). 

Groundwater quality in the United States has been characterized by the NAWQA Program through studies that use 

nationally consistent methods and are distributed within 51 

major river basins and aquifers nationwide (figs. 1A and B). 

The analysis in this report is based on NAWQA Program data 

collected from 1992 to 2003. More information on the design, 

components, and current activities of the NAWQA Program 

is available (Gilliom and others, 1995; Lapham and others, 

1995; Rosen and Lapham, 2008). Prior to 1999, trace-element 

sampling in groundwater was mainly done for NAWQA major 

aquifer studies (MASs) and to a lesser extent for agricultural 

and urban land-use studies (LUSs). Trace-element sampling 

was fully implemented in the NAWQA Program in 1999.

Purpose and Scope

The primary purpose of this report is to describe the 

occurrence and distribution of 23 trace elements and radon 

(Rn) in water from wells sampled by the USGS NAWQA 

Program from 1992 to 2003. The term occurrence is used 

in this report to describe concentrations of trace elements 

above a specified analytical reporting level—in most cases 

1 μg/L, unless otherwise noted. The analyses presented are 

based on single samples from 5,183 wells. The trace elements 

evaluated include aluminum (Al), antimony (Sb), arsenic 

(As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd), 

chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), 

lithium (Li), manganese (Mn), molybdenum (Mo), nickel 

(Ni), selenium (Se), silver (Ag), strontium (Sr), thallium 

(Tl), uranium (U), vanadium (V), and zinc (Zn). Rn, a direct 

decay product of 226radium, was included because of its 

known adverse effects on human health (U.S. Environmental 

Protection Agency, 2010). Physical properties (pH, dissolved 

oxygen (DO) concentration, specific conductance, and 

water temperature) and concentrations of major inorganic 

constituents and nutrients also were measured. 

The occurrence and distribution of trace elements in 

groundwater is described at the national scale and by major 

aquifer group (groups of principal and other aquifers) with a 

focus on areas where concentrations of trace elements from 

wells exceed health-based standards. As part of this objective, 

measured trace-element concentrations in groundwater are 

compared to U.S. Environmental Protection Agency (USEPA) 

drinking-water standards and proposed standards (for Rn)—

including maximum contaminant levels (MCL), health 

advisories (HA), and action levels (AL)—and to HealthBased Screening Levels (HBSLs) published by the USGS 

(Toccalino and Norman, 2006). Collectively, these are referred 

to as human-health benchmarks (HHBs). Also compared are 

selected secondary maximum contaminant levels (SMCL), 

which are nonhealth guidelines.

Additional objectives include (1) the identification of 

factors that relate to the occurrence of selected trace elements, 

such as geochemistry, redox, and age of groundwater; and 

(2) the analysis of co-occurrence of selected trace elements 

in individual wells where more than one trace element 

exceeds HHBs. 

Previous Studies

It is beyond the scope of this report to provide a comprehensive bibliography of trace-element studies in the United 

States. The reader is referred to several resources that provide 

an excellent overview of previous studies of trace-element 

sources and mobility (Freeze and Cherry, 1979; Drever, 

1988; Hem, 1985). Few systematic evaluations of multiple 

trace elements in groundwater at the scale of this study have 

been undertaken in the United States. Selected national- and 

regional-scale studies that are particularly relevant to the current analysis are listed in appendix 1. 

Previous studies of trace elements in groundwater 

throughout the United States have identified important relations among controls on occurrence and mobility. Also, the 

literature on factors that govern trace-element sources and 

mobility in specific regions of the United States is extensive, 

and some studies have compared data across regions. In most 

studies, however, only one trace element was assessed, or 

a few trace elements from a particular group (such as As or 

selected radionuclides) were evaluated, leaving gaps in the 

available information to assess trace-element occurrence and 

no opportunity to assess the co-occurrence of trace elements. 

Water-Quality Benchmarks for Human Health

The human-health benchmarks used in this study include 

MCLs and ALs established by the USEPA for public-water 

supplies (U.S. Environmental Protection Agency, 2004) and 

HBSLs derived from the analysis of toxicology data for 

specific contaminants (Toccalino and Norman, 2006). In most 

cases, the HBSLs are equivalent to USEPA HAs, although 

HBSLs are updated more often as new toxicological data 

become available. Although not all of the wells analyzed for 

this report were used for drinking water, they were all selected 

with the aim of characterizing trace-element concentrations in 

aquifers across the United States. Thus, comparisons of traceelement concentrations to HHBs are not intended to represent 

potential exposure but rather to indicate where and under what 

conditions groundwater in important aquifers is affected. The 
4 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

0 200 400 KILOMETERS

0 200 400 MILES

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

EXPLANATION

NAWQA study areas

Major aquifer study wells

Land-use study wells

B

A

Figure 1. Locations of (A) major aquifer study (MAS) wells, and (B) land-use study (LUS) wells sampled for trace elements by the 

National Water-Quality Assessment Program, 1992–2003.
Methods 5

USEPA has established enforceable MCLs for Sb, As, Ba, Be, 

Cd, Cr, Se, Tl, and U. ALs have been established for Cu and 

Pb, and exceedence of these ALs triggers possible treatment or 

other requirements for a water system. Currently an MCL and 

alternative MCL (AMCL) have been proposed for Rn. HBSLs, 

similar to USEPA HAs, are acceptable levels based on health 

information (nonenforceable) and have been established for B, 

Mn, Mo, Ni, Ag, Sr, and Zn (Toccalino and Norman, 2006). 

Nonhealth guidelines, such as SMCLs, are nonenforceable and 

pertain to water aesthetics, such as for Al and Fe. Three of the 

trace elements discussed in this report (Co, Li, and V) do not 

have USEPA drinking-water standards (U.S. Environmental 

Protection Agency, 2004) or HBSLs. 

Methods

This study used data collected by the USGS NAWQA 

Program to characterize trace-element concentrations in major 

aquifer groups across the United States. The data were analyzed primarily by determining their distributions and comparing those distributions to factors such as climate, composition 

of geologic materials, well types, well depths, pH, and redox 

conditions. A variety of methods were used to develop the data 

used in the analyses and to identify factors influencing trace 

element concentrations in groundwater.

Well Selection

Wells included in this study are from NAWQA studies 

designed to describe the quality of water withdrawn from 

major aquifers and used for drinking (termed major aquifer 

studies or MASs) and studies of shallow groundwater within 

specific land-use settings (termed land-use studies or LUSs) 

(Gilliom and others, 1995). MASs focused on the quality 

of groundwater resources throughout an aquifer and used 

data mostly from existing supply wells irrespective of depth. 

LUSs targeted the uppermost recently recharged groundwater 

to identify the effects of the overlying land use and used 

data mostly from monitoring wells and some supply wells. 

Individual MASs and LUSs are described in reports for 

individual NAWQA study areas (see http://water.usgs.gov/

nawqa/). The sampling network for each study comprised 

about 30 wells that were randomly selected within the target 

area by a geographically distributed sampling method (Scott, 

1990). A total of 5,183 wells from which geologic and 

chemical data had been collected were from networks in 48 

NAWQA study areas across the United States (fig. 1). The 

actual numbers of wells used in the analyses were based on 

whether or not a particular trace element was analyzed in 

water from a well or on the availability of ancillary data for 

that analysis. Additional information on sampling networks 

and wells is shown in appendix 2. 

Specific well-selection protocols designed by the 

NAWQA Program were used in the design of the MASs and 

LUSs (Lapham and others, 1995). Ancillary data for each well 

were collected according to NAWQA protocols (Koterba and 

others, 1995) and included latitude and longitude, topographic 

setting, well depth, open interval, casing diameter and 

material, geologic materials and (or) aquifers contributing 

water to the well, static water level, potential point sources of 

contamination near the well, and land use within 50 and 500 m 

of the well. 

Sample Collection and Analysis

Groundwater was collected and analyzed for trace elements, major ions, nutrients, tritium, chlorofluorocarbons 

(CFCs), and other constituents in accordance with NAWQA 

protocols (Koterba and others, 1995; Lapham and others, 

1995). Prior to collection, wells were pumped until the water 

temperature, DO, specific conductance, and pH were stable 

(Koterba and others, 1995). Cleaned polytetrafluoroethylene 

(PTFE) tubing with stainless-steel fittings was connected to 

either a submersible pump or to a water line close to the 

wellhead. Groundwater was collected at a flow rate of 

0.5 L/min (0.0083 L/s) in a portable sampling chamber. 

Samples for trace elements were passed through a 0.45-μm 

disposable polypropylene capsule filter; the trace-element 

samples were then acidified with ultra pure nitric acid to a 

pH of less than 2. Trace elements were analyzed at the USGS 

National Water Quality Laboratory in Denver, Colo., by using 

inductively coupled plasma atomic-emission spectrometry 

(ICP–AES), inductively coupled plasma mass spectrometry 

(ICP–MS), graphite-furnace atomic-absorption spectrometry 

(AA), or hydride generation atomic-absorption spectrometry 

(Fishman and Friedman, 1989; Faires, 1993; McLain, 1993; 

Ivahnenko and others, 1996; Garbarino, 1999; Ivahnenko and 

others, 2001). Laboratory reporting levels (LRLs) generally 

ranged from 0.2 to 1 μg/L, except for the LRLs for Fe and B, 

which were higher. Concentrations of Rn were analyzed by 

liquid scintillation methods; LRLs for Rn ranged from 

52 to 80 pCi/L (Prichard and Gessell, 1977; Mullin and 

Wanty, 1991).

Study Design

Several factors affect trace-element solubility and mobility. In this study, data were analyzed in relation to climate, the 

geology of aquifer materials, aquifer geochemistry, and the 

redox potential and pH of the groundwater. The ancillary data 

used in the data analyses represent regional- to national-scale 

conditions. Depending on the scale of the analysis, variables 

such as climate, geology, pH, and redox can be used to explain 

differences in trace-element concentrations in groundwater. 

However, when concentrations of trace elements in groundwater are analyzed at the major aquifer scale, the effect of climate 

becomes diminished because most aquifer groups are limited 

in spatial extent and are in either predominantly dry or 

humid regions. 
6 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Climate

In general, concentrations of trace elements are greater in 

drier climates of the United States, owing to processes such as 

redox condition, ion competition, complexation, evaporative 

concentration, and to more chemically evolved groundwater 

conditions (Hem, 1985; Welch and others, 2000; McMahon 

and Chapelle, 2008). To characterize general climatic conditions in the United States, a climate variable indicating generally dry or generally humid conditions for North America was 

used (fig. 2A) (Commission for Environmental Cooperation 

Working Group, 1997). With a few exceptions, the western 

United States has a generally dry climate, whereas the east 

is generally humid (fig. 2A). Local studies have shown that 

the effects of drier climates can favor increased groundwater 

concentrations of trace elements, such as Se (Dubrovsky and 

others, 1993; Seiler and others, 2003), As (Seiler, 2004; Thiros 

and Manning, 2004), B, Mo, and U (Fujii and Swain, 1995; 

Welch and Lico, 1998). In humid regions, by contrast, larger 

groundwater fluxes and shorter groundwater residence times 

may result in lower pH, favoring mobility of trace elements 

such as Al, Fe, Mn, Pb, and Zn to a greater extent than in dry 

regions. In addition, precipitation in much of the humid east 

has been more acidic than in the west (National Atmospheric 

Deposition Program, 2001) (fig. 2B); greater acidity can lead 

to increased weathering of geologic materials containing trace 

elements and potentially greater mobility of those elements in 

groundwater young enough (post-1960) to be affected by that 

precipitation (Jacks, 1993). 

Aquifer Groups and Geology

Most trace elements in groundwater are derived 

from minerals in the rocks and sediment through which 

groundwater flows; thus, regional and subregional differences 

in geology can have implications for trace-element occurrence 

in groundwater. For example, some Cretaceous-age deposits 

of the central Great Plains have been shown to contain labile 

concentrations of As, B, Cr, Cu, Ni, Se, V, and U (Tourtelot, 

1956). Some materials derived from these deposits have 

been shown to be related to high concentrations of Se in 

groundwater and surface water in parts of the western United 

States (Seiler and others, 2003). Arsenic occurrence in 

groundwater may be similarly controlled by availability in 

aquifer materials, as well as by groundwater geochemistry 

(Welch and others, 2000; Smedley and Kinniburgh, 2002; 

Welch and Stollenwerk, 2003; Thomas, 2007). Aquifers in 

some carbonate lithologies, such as the Floridan aquifer 

system and parts of the Edwards-Trinity aquifer system in 

Texas, have high concentrations of Sr that ultimately affect the 

concentrations in the associated groundwater (Hanshaw and 

Back, 1979; Back and others, 1988; Musgrove and Banner, 

2004). Although most trace elements in groundwater are from 

natural sources, it is important to note that the sources of some 

trace elements in groundwater may be anthropogenic, and 

that some naturally occurring trace elements can be mobilized 

by anthropogenic activities (Böhlke, 2002; Seiler and others, 

2003; Stollenwerk and Colman, 2004; Eberts and others, 2005; 

deLemos and others, 2006; Dubrovsky and others, 2010; 

Ayotte and others, 2011). 

For this study, more than 40 aquifers were divided into 

8 major aquifer groups based on the geologic materials from 

which they derive water (table 1; fig. 3). These groups are 

based on important aquifers in the United States (Miller, 2000) 

and the principal aquifers of the United States map (U.S. 

Geological Survey, 2003). Principal aquifers are defined as 

aquifers that are regionally extensive and can yield useable 

quantities of water (U.S. Geological Survey, 2003). The eight 

major aquifer groups used in this report are (1) unconsolidated 

sand and gravel aquifers; (2) glacial unconsolidated sand and 

gravel aquifers; (3) semiconsolidated sand aquifers; (4) sandstone aquifers; (5) sandstone and carbonate-rock aquifers; 

(6) carbonate-rock aquifers; (7) basaltic- and other volcanicrock aquifers; and (8) crystalline-rock aquifers.

In most cases, wells in the major aquifer groups used in 

this study are predominantly in either dry or humid regions. 

The primary exceptions are the wells in the unconsolidated 

sand and gravel aquifers and the sandstone and carbonate-rock 

aquifer groups, in which the number of wells is split more 

evenly between dry and humid regions of the United States 

(table 1). 

Some principal aquifers (and aquifer systems) contain 

deposits from multiple types of geologic materials. In this 

study, however, the composition of geologic materials was 

a primary factor in analyzing the water chemistry data so 

that individual wells from the same principal aquifer could 

be grouped into different categories based on rock type. 

An example of this is the Valley and Ridge aquifers, which 

are listed as sandstone and carbonate-rock aquifers on the 

principal aquifer map of the United States (U.S. Geological 

Survey, 2003) and described together in the Ground Water 

Atlas of the United States (Miller, 2000). For this analysis, the 

Valley and Ridge aquifers were separated into the sandstone 

aquifer group and the carbonate-rock aquifer group. This was 

done to account for differences in geochemical conditions, 

which can differ widely between these rock types. In addition, 

crystalline-rock aquifers in New York and New England 

(Miller, 2000) and in the Rocky Mountain Front Range, 

which are described as “other” aquifers on the principal 

aquifers map of the United States (U.S. Geological Survey, 

2003), were included in this report as unofficially named 

aquifers in the crystalline-rock aquifer group. Also in this 

report, unconsolidated deposits were separated into groups of 

unconsolidated aquifers of glacial origin and unconsolidated 

aquifers of nonglacial origin. The glacial sand and gravel 

aquifers include deposits of both Laurentide and Cordilleran 

continental glaciations (Warner and Arnold, 2006). Additional 

information about aquifer groups, principal aquifers, and other 

aquifers is available at http://water.usgs.gov/nawqa/studies/

praq/ or http://pubs.usgs.gov/ha/ha730/gwa.html.
Methods 7

Figure 2. (A) Generalized categories of oxidation-reduction potential (redox) in samples from wells in humid and dry regions of 

the United States (Commission for Environmental Cooperation Working Group, 1997), and (B) pH of precipitation in 2001 (National 

Atmospheric Deposition Program, 2001).

Redox Category

Climate Category

A

B

EXPLANATION

EXPLANATION

Humid

Dry

pH of precipitation, in 

 standard units

Greater than 5.0 to 5.3

Greater than 5.3 to 5.8 

4.2 to 4.7

Greater than 4.7 to 5.0

0 200 400 KILOMETERS

0 200 400 MILES

Mixed

Anoxic

Oxic
8 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 1. Generalized description of the major aquifer groups sampled for the NAWQA Program across the United States.—Continued[Groups were compiled and modified from the U.S. Geological Survey Ground Water Atlas of the United States (Miller, 2000) and Principal Aquifer Map of the United States (U.S. Geological Survey, 2003)]

Aquifer 

group 

code and 

identifier

Major aquifer group (bold) and principal 

and other aquifers Geologic framework Hydrologic system

Percentage of 

wells in dry or 

humid climate 

region

Percentage of 

wells inferred 

to be of oxic, 

mixed, or 

anoxic redox 

state

USG Unconsolidated sand and gravel aquifers

1 Basin and Range basin-fill aquifers The unconsolidated sand and gravel aquifers 

can be grouped into the following categories: basin-fill (valley-fill) aquifers and sand and gravel aquifers. Also, stream-valley aquifers are located beneath channels, flood plains, and terraces in the valleys of major streams. Unconsolidated sand and gravel aquifers are characterized by intergranular porosity, and all contain water primarily under unconfined or water-table conditions and can occupy different hydrogeologic settings.

These aquifers mostly contain water under 

unconfined or water-table conditions but confined conditions exist locally where the aquifers contain beds of low-permeability silt, clay, or marl. The High Plains aquifer is the most intensively pumped aquifer in the United States. During the 1990s, the pumping rate was about 15 billion gallons per day.

2 California Coastal Basin aquifers3 Central Valley aquifer system4 Columbia Plateau basin-fill aquifers5 High Plains aquifer6 Mississippi River Valley alluvial aquifer7 Northern Rocky Mountains Intermontane 

Basins aquifer system

8 Rio Grande aquifer system9 Snake River Plain basin-fill aquifers

10 Surficial aquifer system

11 Willamette Lowland basin-fill aquifers

A Stream valley aquifers (not a principal 

aquifer)

GLA Glacial unconsolidated sand and gravel aquifers

12 Glacial aquifer system Large areas of the central and northeastern 

United States as well as Alaska are covered with sediments that were deposited during several cycles of continental glaciation. The GLA aquifers are unconsolidated sand and gravel characterized by intergranular porosity, contain water primarily under unconfined, or water-table conditions, and can occupy different hydrogeologic settings. Includes some alluvial aquifers.

Local movement of water in the glacial 

aquifers generally is from the valley walls toward streams; regional movement of water is down the valley. Yields of wells completed in the glacial-deposit aquifers are as much as 3,000 gallons per minute where the aquifers consist of thick sand and gravel.

13 Unconsolidated-deposit aquifers (Alaska)

SCS Semiconsolidated sand aquifers

14 Coastal lowlands aquifer system Sediments that primarily consist of semiconsolidated sand, silt, and clay, interbedded 

with some carbonate rocks, underlie the Coastal Plains that border the Atlantic Ocean and the Gulf of Mexico. Depositional environments include fluvial, deltaic, and shallow marine. The rocks are Cretaceous and Tertiary in age. In general, older rocks crop out farthest inland.

Many local aquifers and confining units 

are in the area underlain by this aquifer system. Based on similarities in their hydraulic characteristics and water levels, sequences of local aquifers and confining units can be grouped into regional aquifers and confining units.

15 Mississippi embayment aquifer system16 Northern Atlantic Coastal Plain aquifer 

system

17 Southeastern Coastal Plain aquifer system18 Texas coastal uplands aquifer system

DRY HUMID 0

80604020

100

OxiMixed

c

Anoxic

DRY HUMID 0

80604020

100

OxiMixed

c

Anoxic

0

80604020

100

DRY HUMID

OxiMixed

c

Anoxic
Methods 

9

Table 1. Generalized description of the major aquifer groups sampled for the NAWQA Program across the United States.—Continued[Groups were compiled and modified from the U.S. Geological Survey Ground Water Atlas of the United States (Miller, 2000) and Principal Aquifer Map of the United States (U.S. Geological Survey, 2003)]

Aquifer 

group 

code and 

identifier

Major aquifer group (bold) and principal 

and other aquifers Geologic framework Hydrologic system

Percentage of 

wells in dry or 

humid climate 

region

Percentage of 

wells inferred 

to be of oxic, 

mixed, or 

anoxic redox 

state

SAN Sandstone aquifers

19 Cambrian-Ordovician aquifer system Aquifers in sandstone are more widespread 

than those in all other kinds of consolidated rocks. Sandstone aquifers commonly grade laterally into fine-grained, low-permeability rocks such as shale or siltstone. Many sandstone aquifers are parts of complex interbedded sequences of sedimentary rocks.

Sandstones retain some primary porosity 

unless cementation has filled the pores; most of the porosity consists of secondary openings such as joints, fractures, and bedding planes. Groundwater movement primarily is along bedding planes, but the joints and fractures cut across bedding and facilitate vertical flow.

20 Early Mesozoic basin aquifers21 Lower Tertiary aquifers22 Pennsylvanian aquifers23 Valley and Ridge aquifers (sandstone, 

unofficial name)

B Woodbine aquifer (not a principal aquifer)

SCR Sandstone and carbonate-rock aquifers

24 Edwards-Trinity aquifer system Aquifers in sandstone and carbonate rocks are 

most widespread in the eastern U.S., but also occur in Texas, Oklahoma, Arkansas, Montana, Wyoming, and South Dakota. These aquifers consist of interbedded sandstone and carbonate rocks.

The carbonate rocks are the most productive 

aquifers, whereas the interbedded sandstones yield less water. The presence or absence of solution openings affects aquifer recharge and discharge and is reflected by the water levels in wells completed in different rock types.

25 Mississippian aquifers

CAR Carbonate-rock aquifers

26 Biscayne aquifer Aquifers in carbonate rocks are prominent in 

the central and southeastern parts of the Nation, but are also in southeastern California and northeastern Maine and in Puerto Rico. The rocks that compose these aquifers range in age from Precambrian to Miocene. Most of the carbonate-rock aquifers consist of limestone, but dolomite and marble are local sources of water.

The water-yielding properties of carbonate rocks are highly variable; some yield 

almost no water and are considered to be confining units, whereas others are among the most productive aquifers. Vertical passages, usually developed at the intersections of joints, connect the horizontal bedding plane openings.

27 Floridan aquifer system28 Ordovician aquifers29 Ozark Plateaus aquifer system30 Piedmont and Blue Ridge carbonate-rock 

aquifers

31 Silurian-Devonian aquifers32 Valley and Ridge aquifers (carbonate) 

(unofficial name)

OxiMixed

c

Anoxic

DRY HUMID 0

80604020

100

OxiMixed

c

Anoxic

DRY HUMID

0

80604020

100

OxiMixed

c

Anoxic

DRY HUMID

0

80604020

100
10 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 1. Generalized description of the major aquifer groups sampled for the NAWQA Program across the United States.—Continued[Groups were compiled and modified from the U.S. Geological Survey Ground Water Atlas of the United States (Miller, 2000) and Principal Aquifer Map of the United States (U.S. Geological Survey, 2003)]

Aquifer 

group 

code and 

identifier

Major aquifer group (bold) and principal 

and other aquifers Geologic framework Hydrologic system

Percentage of 

wells in dry or 

humid climate 

region

Percentage of 

wells inferred 

to be of oxic, 

mixed, or 

anoxic redox 

state

BAV Basaltic- and volcanic-rock aquifers

33 Columbia Plateau basaltic-rock aquifers Volcanic rocks have a wide range of chemical, 

mineralogic, structural, and hydraulic properties. The variability of these properties is related largely to rock type and the way the rock was ejected and deposited.

The permeability of basaltic rocks depends 

on the cooling rate of the lava flow, the character of interflow zones, and the thickness of the flow. Fractures and joints develop in the upper and lower parts, but few open spaces develop in the center because it cools slowly.

34 Hawaiian volcanic-rock aquifers35 Snake River Plain basaltic-rock aquifers36 Early Mesozoic basin aquifers (volcanic)

CRL Crystalline-rock aquifers

37 New York and New England crystallinerock aquifers (unofficial name) The igneous and metamorphic rocks are permeable where fractured and generally 

yield only small amounts of water to wells. In many places they are the only reliable source of water supply. Crystalline rocks are in northern Minnesota and northeastern Wisconsin, northeastern New York, and the New England, Piedmont, and Blue Ridge Physiographic Provinces.

The permeability of igneous and metamorphic rocks depends on secondary porosity, where joints and fractures transmit 

and store water that can flow to a well. Hydraulic conductivity is low compared to other aquifers but can vary by orders or magnitude. In the fracture-flow system, storage is typically low and groundwater velocities high.

38 Piedmont and Blue Ridge crystalline-rock 

aquifers

C Rocky Mountain Front Range crystallinerock aquifers (unofficial name)

OxiMixed

c

Anoxic

DRY HUMID 0

80604020

100

OxiMixed

c

Anoxic

DRY HUMID 0

80604020

100
Methods 11

Chemical Evolution and Groundwater Age

The geochemistry of groundwater evolves over time in 

large part because of contact with aquifer materials. Groundwater is rarely in chemical equilibrium with the minerals in 

the aquifer through which it flows; it is constantly changing 

along its flow path through reactions with minerals in what 

has been described as chemical evolution (Plummer and Back, 

1980; Hem, 1985). In general, the longer the water has been in 

an aquifer, the greater the concentrations of dissolved solids, 

and the more chemically evolved the water is considered to 

be. Chemical evolution also may be affected by factors other 

than age (which is related to the rate of flow of groundwater 

through an aquifer). Factors that affect chemical evolution 

include the geologic and geochemical composition of the 

aquifer, the geochemical composition of the groundwater, and 

biogeochemical processes in the aquifer. 

Trilinear diagrams, a method for showing ratios of major 

cations and anions in water samples, are used to indicate water 

type information about samples from various aquifer groups 

in this report. Trilinear diagrams have been used successfully 

to assess water type and infer chemical evolution (Hanshaw 

and Back, 1979; Back and others, 1988; Drever, 1988; Rogers, 

1989; Hem, 1985; Welch and Lico, 1998). 

The occurrence of many trace elements in groundwater is 

related to the length of time that the water has been in contact 

with the soil and underlying aquifer materials—that is, the 

time elapsed since recharge. Measurement of the time water 

is in an aquifer (apparent age of the water) is complicated by 

the potential mixing of waters of different ages. For wells with 

short open intervals, like many monitoring wells, the effects 

of mixing may be minimized. Groundwater samples that were 

collected by the NAWQA Program for age dating were analyzed for several constituents, including , tritium-helium, and 

CFCs (Plummer and others, 1993). In this report, only samples 

that were analyzed for tritium (number of samples (N) equal to 

1,932) were used to estimate the age of groundwater. 

The most basic use of tritium analysis in groundwater is 

to indicate whether it is present at measurable concentrations 

in the water sample; if it is, at least some of the water in the 

sample postdates the advent of nuclear testing in about 1953 

(Plummer and others, 1993). Groundwater samples recharged 

after 1953 are defined as “young” waters, and those recharged 

before 1953 are defined as “old”. Concentrations of tritium 

in recharge water derived entirely from precipitation prior 

to nuclear testing are estimated to have been less than about 

0.2 to 0.8 tritium units (TU) (0.65 to 2.6 pCi/L) by the early 

1990s (Plummer and others, 1993) and about 0.12 to 0.5 TU 

(about 0.39 to 1.6 pCi/L) by the early 2000s (Rupert and 

Plummer, 2005). In this report, the presence of tritium above 

the common reporting limit of 2.5 pCi/L was considered to 

indicate that at least some young groundwater was present in 

the sample. Groundwater samples with tritium activities less 

than or equal to 2.5 pCi/L are interpreted as containing water 

predominantly recharged before 1953.

Redox and pH

Other factors, such as redox status or pH, may have a 

controlling effect on trace-element concentrations despite the 

degree of chemical evolution of the water sample. Methods 

for inferring redox state from commonly measured chemical 

constituents, including concentrations of DO, may provide 

a useful context for evaluating trace-element occurrence 

on a national and regional scale. Redox is a measure of the 

oxidation-reduction state of the groundwater, which in natural 

systems can be measured by the hierarchical progression of 

terminal electron acceptors for the reduction of compounds in 

groundwater (Froelich and others, 1979; Chapelle and others, 

1995; Chapelle and others, 2002; Paschke, 2007; McMahon 

and Chapelle, 2008; McMahon and others, 2009). Redox 

conditions for the water in each well in this analysis were 

determined from a scheme that evaluates concentrations of 

DO, nitrate, Mn, Fe, and sulfate, and methane (Paschke, 2007; 

McMahon and Chapelle, 2008). For this study, redox was 

generalized into three categories: (1) groundwater that was O2

or nitrate-reducing was considered generally oxic; (2) groundwater that was Mn(IV) reducing, Fe(III)/sulfate-reducing, 

or methanogenic was considered generally anoxic; and 

(3) groundwater without a distinct redox signal, or for which 

more than one redox process criterion was met, was considered mixed. Water samples used in this study from dry climates have redox conditions that are generally oxic, whereas 

humid-region samples are more often anoxic (fig. 2A). The 

generalized redox classification and results for wells in this 

study are presented in table 2. 

Groundwater pH is another important geochemical control that can affect the solubility and mobility of trace metals, 

often through sorption/desorption processes (Drever, 1988; 

Welch and Stollenwerk, 2003; Paschke, 2007). For many metals, such as Al, Cu, Fe, Mn, and Zn, solubility in water often 

decreases with increasing pH. For others, such as As and U, 

solubility can increase with increasing pH over the normal pH 

range of most natural waters. Thus, consideration of pH along 

with redox conditions is important to the overall distribution 

of trace elements in groundwater. For this analysis, samples 

are grouped into two categories: less than 7 (low pH) and 

greater than or equal to 7 (high pH), based on the midpoint 

of the pH scale and the control of reactions involving trace 

elements by processes that predominate at pH values either 

smaller or greater than 7. 
12 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 3. (A) Principal aquifers of the United States and major aquifer groups used in the current study, and (B) wells sampled by the 

National Water-Quality Assessment Program, 1992−2003, color coded by aquifer (see explanation). Areal extent of aquifers from U.S. 

Geological Survey (2003).

0 100 MILES

0 100 KILOMETERS

34

13

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 200 400 KILOMETERS

0 200 400 MILES

1

2

3

4

5

6

7

8

9

10

11

12

14

15

15

16

17

18

19

22

22

22

23

24

25

25

25

26

27

27

28

29

30 31

31

32

33

35

B

C

A

A

A

A

A

21

20/36

38

37

28
Methods 13

Figure 3. (A) Principal aquifers of the United States and major aquifer groups used in the current study, and (B) wells sampled 

by the National Water-Quality Assessment Program, 1992−2003, color coded by aquifer (see explanation). Areal extent of 

aquifers from U.S. Geological Survey (2003).—Continued

Columbia Plateau basaltic-rock aquifers

Hawaiian volcanic-rock aquifers

Snake River Plain basaltic-rock aquifers

Sandstone aquifers

Cambrian-Ordovician aquifer system

Early Mesozoic basin aquifers

Lower Tertiary aquifers

Pennsylvanian aquifers

Valley and Ridge aquifers (sandstone)

Woodbine aquifer (not a principal aquifer)

Carbonate-rock aquifers

Biscayne aquifer

Floridan aquifer system

Ordovician aquifers

Ozark Plateaus aquifer system

Piedmont and Blue Ridge carbonate-rock aquifers

Silurian-Devonian aquifers

Valley and Ridge aquifers (carbonate)

Edwards-Trinity aquifer system

Mississippian aquifers

Unconsolidated sand and gravel aquifers

Basin and Range basin-fill aquifers

California Coastal Basin aquifers

Central Valley aquifer system

Columbia Plateau basin-fill aquifers

High Plains aquifer

Mississippi River Valley alluvial aquifer

Northern Rocky Mountains Intermontane 

 Basins aquifer system

Rio Grande aquifer system

Snake River Plain basin-fill aquifers

Surficial aquifer system

Willamette Lowland basin-fill aquifers

stream valley aquifers (not a principal aquifer)

Crystalline-rock aquifers

New York and New England crystalline-rock aquifers 

 (unofficial name)

Piedmont and Blue Ridge crystalline-rock aquifers

Rocky Mountain Front Range crystalline-rock aquifers 

 (unofficial name)

Glacial unconsolidated sand and gravel aquifers 

Glacial aquifer system 

Unconsolidated-deposit aquifers (Alaska)

Semiconsolidated sand aquifers

Coastal lowlands aquifer system

Mississippi embayment aquifer system

Northern Atlantic Coastal Plain aquifer system

Southeastern Coastal Plain aquifer system

Texas coastal uplands aquifer system

Sandstone and carbonate-rock aquifers

Basaltic- and other volcanic-rock aquifers

EXPLANATION

Aquifers and well locations—Aquifers are shown by major aquifer group 

(colored areas) and are identified by number. Well location is shown by colored 

circles (colored triangles if the well is completed in the material beneath the 

uppermost aquifer).

12

13

1

2

3

4

5

6

7

8

9

10

11

A

12

13

14

15

16

17

18

19

20

21

22

23

B

25

26

27

28

29

30

31

32

33

34

35

37

38

C

24

36 Early Mesozoic basin aquifers (volcanic)
14 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Data Handling and Statistical Methods

Trace-element data used in this report were retrieved 

from the USGS NAWQA data warehouse in 2006 (Bell and 

Williamson, 2006). The NAWQA data warehouse contains 

water-quality data, field parameters, and ancillary information 

for all groundwater sites sampled as part of the NAWQA 

Program. The original data also are available at http://water.

usgs.gov/nawqa/data. The trace-element dataset includes 

5,183 sites; however, the number of trace elements analyzed 

differs from site to site.

A large proportion of the trace-element and other waterchemistry data were censored at various LRLs. The censoring level for each trace element and Rn was adjusted to the 

respective Long Term Method Detection Limit (LT–MDL; a 

value that is usually one-half the LRL), thus providing a lower 

reporting level for each element (Childress and others, 1999). 

For this report, data censored to the LT–MDL were used for 

distributional data summaries; the censored data are available 

with this report (at http://water.usgs.gov/nawqa/trace/pubs/

sir2011-5059). Because LRLs change over time, multiple 

reporting levels have been defined for many trace elements. 

For this report, the data were recensored to the most common 

reporting level for each trace element to allow comparisons 

among data from different groups. The common reporting 

level was 1 μg/L for most trace elements and 80 pCi/L for Rn. 

Reporting levels for B, Fe, and Sr were 12, 10, and 0.9 μg/L, 

respectively. Statistical analyses were based on all data values, 

including censored data. Maps were used to show the geographic distribution for all trace elements. 

Summary statistics, which include the minimum, 

maximum, and common percentiles, were computed for all 

trace elements by using regression-on-order statistics (ROS)—

statistics appropriate for data with a large proportion (up to 

80 percent) of censored values (Helsel and Hirsch, 1992; 

Helsel, 2005). Data summaries that refer to the proportion 

of samples with concentrations above specified common 

reporting levels are not affected by the recensoring because 

the reporting levels are equal to or greater than the LT–MDLs. 

Nonparametric statistics, which do not require distributional assumptions about the censored data, were used to 

describe distributions or compare data among groups. The 

Wilcoxon and Kruskal-Wallis tests were used to determine 

whether the distributions of the data from two or more groups 

were significantly different (Helsel and Hirsch, 1992; SAS 

Institute, 1999; Helsel, 2005). If multiple-comparison tests 

indicated significant differences, the Tukey test was used on 

the ranks of the data to identify which group means were 

significantly different (Helsel and Hirsch, 1992; SAS Institute, 

1999; Helsel, 2005). Although other methods are available 

for comparing groups, such as substitution and distributional 

methods (Helsel, 2005), nonparametric methods offer several 

benefits, including no assumption of normally distributed data; 

good statistical power; and the use of data below the censoring 

Table 2. Threshold concentrations for classification of generalized groundwater redox conditions and 

number of samples collected for the NAWQA Program across the United States by pH category, 1992–2003.

[<, less than; ≥, greater than or equal to; mg/L, milligrams per liter; O2, oxygen; NO3

-

, nitrate; Mn, manganese; Fe, iron; SO4

2-, 

sulfate; >, greater than; ≤, less than or equal to; --, not applicable]

Reduction process

Number of samples Criteria 

(mg/L)

pH < 7 pH ≥ 7 O2 NO3

- Mn, 

total

Fe, 

total SO4

2-

Oxic

O2 949 1,764 > 0.5 > 0.5 ≤ 0.05 ≤ 0.1 --

NO3

- 46 264 ≤ 0.5 > 0.5 ≤ 0.05 ≤ 0.1 --

Mixed

Mixed 495 420 Multiple criteria met

Anoxic

Mn 32 113 ≤ 0.5 ≤ 0.5 > 0.05 ≤ 0.1 --

Fe/high SO4

2- 250 359 ≤ 0.5 ≤ 0.5 > 0.05 > 0.1 > 4

Fe/low SO4

2- 63 116 ≤ 0.5 ≤ 0.5 > 0.05 > 0.1 ≤ 4

Total 1,835 3,036
Methods 15

level without having to create values for them, resulting in 

an accurate portrayal of the censored information (Helsel and 

Hirsch, 1992; Helsel, 2005). Spearman correlation coefficients 

(rho) were computed to measure the strength of the relation 

between environmental factors (ancillary data) and concentrations of selected trace elements (Helsel and Hirsch, 1992). The 

significance level (α) for all statistical tests was equal to 0.05. 

Thus, for p-values less than 0.05, the null hypothesis can be 

rejected in favor of the alternate hypothesis.

Logistic regression was used to help identify processes 

and factors affecting trace-element concentrations in 

groundwater for each aquifer group. For logistic regression 

methods, the dependent variable is a binary response variable, 

and the regression model predicts the probability of an event 

outcome, or the probability that a response is observed. In 

this way, logistic regression models can be used with data that 

are highly censored without violating assumptions about the 

distribution of the data (Helsel, 2005). Independent variables 

were either continuous or binary variables. For logistic 

regression models, common reporting levels and HHBs for 

selected trace elements were used as cut points for binary 

dependent and independent variable development, unless 

otherwise noted. Models were evaluated by consideration 

of the Hosmer-Lemeshow goodness of fit (Hosmer and 

Lemeshow, 2000). Higher p-values indicate better fit because 

the null hypothesis is that the model fits the data. Also model 

discrimination was evaluated with the c statistic, which is an 

estimate of the area under the receiver operating characteristic 

curve—values greater than 0.7 are considered acceptable, and 

values greater than 0.8 are considered excellent (Hosmer and 

Lemeshow, 2000). 

All statistical tests and models were computed by SAS 

System software (SAS Institute, 1999). Statistical summaries 

were computed by robust ROS methods (Helsel, 2005) by 

using a SAS macro for such a method (Practical Stats, 2007). 

Quality Control

As part of the quality control (QC) for groundwater samples collected by the NAWQA Program, additional samples 

were collected and analyzed to determine if the measured 

concentrations of trace elements were representative of environmental concentrations and were reproducible. Two primary 

types of QC samples for evaluation of the environmental data 

included blanks and replicates. Blanks are samples of water 

free of constituents of interest, are collected prior to collection 

of an environmental sample, and indicate if trace elements 

were introduced to the QC sample (and by association, the 

environmental sample) during collection, handling, or analysis. Replicates are also collected sequentially after the environmental sample and indicate variability in reported results 

for a given constituent. The analysis of blank samples was 

performed by using analytical methods with detection limits at 

or below those for the environmental samples (Apodaca and 

others, 2006). 

Potential sample contamination levels, at the 95-percent 

confidence level, were determined from the blank data for 

each trace element (Apodaca and others, 2006). For a given 

trace element, the percentages of blank concentrations greater 

than the common reporting levels used in this report were 

used to evaluate the possible effects of contamination on 

the dataset. The results of the trace-element-blank analysis 

indicate that Sb, Be, Mo, and Tl in groundwater were largely 

unaffected by contamination (fewer than 1 percent of samples 

had concentrations greater than 1 μg/L) (table 3). Limited 

numbers of blanks for Li and V in groundwater did not allow 

for a reliable assessment of the potential contamination 

associated with these trace elements. Blank samples for Al and 

Zn had the largest potential for contamination, with 50 percent 

or more of blanks having concentrations greater than 1 μg/L. 

In this study, a common reporting level of 12 μg/L was used 

for B, and 15 percent of the blank samples exceeded this 

value (table 3).

Potential contamination, at the 95-percent upper confidence level, was identified in blank samples for 17 trace 

elements (Al, As, Ba, B, Cd, Co, Cr, Cu, Fe, Pb, Mn, Ni, Se, 

Ag, Sr, U, and Zn) at the common reporting levels used in this 

report (Apodaca and others, 2006). About 3 percent or fewer 

of blank samples for As, Cd, Co, Pb, Ni, Se, Ag, and U were 

contaminated. Four to 10 percent of blanks for Ba, Cr, Fe, Mn, 

and Sr were possibly contaminated. Al, B, Cu, and Zn were 

the most affected, with possible contamination detected in 15 

to more than 50 percent of blank samples (table 3). 

Contamination of samples can also be viewed in terms 

of its effect on evaluation of samples exceeding HHBs or 

nonhealth guidelines. The contamination levels for most of 

the trace elements reported here are at concentrations that are 

less than 10 percent of HHBs or nonhealth-guideline concentrations; therefore, potential contamination of samples would 

not affect the comparison of concentrations of trace elements 

in groundwater with benchmarks or guidelines (Apodaca and 

others, 2006). The exceptions are possibly Al, Cd, and Pb. 

Potential contamination identified for these trace elements is 

greater than 10 percent of the HHBs or nonhealth guidelines, 

but affects only 5 percent or fewer of the Cd and Pb samples. 

For most trace elements, the level of potential contamination 

is not large enough to affect the measured concentration of the 

environmental sample significantly.

Replicate groundwater-sample concentration differences for all trace elements were less than 15 percent except 

for Tl (18.3 percent) and V (15.2 percent). This indicates that 

sampling, handling, and analysis of samples had a relatively 

small effect on the reported value and that reproducibility of 

sample concentrations was generally good (Apodaca and others, 2006). 

The analysis of the QC data indicated that the potential 

contamination from sample processing and analysis for most 

trace elements of interest was small and did not significantly 

affect data interpretation for most elements. For this reason, all 

concentration data were used unaltered.
16 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 3. Summary of blank data for trace elements in groundwater sampled for the NAWQA Program, 1992–2003, 

across the United States.

[μg/L, micrograms per liter; pCi/L, picocuries per liter for radon; UCL, upper confidence level; >, greater than; <, less than; --, no data]

Element Number of 

blank samples

 Common reporting 

level used 

in this report 

(μg/L or pCi/L)

Percentage of blank samples with 95-percent UCL 

contamination concentrations (modified from Apodaca 

and others, 2006, table 1) equal to or exceeding common 

reporting levels used in this report

Aluminum 330 1 > 50

Antimony 347 1 < 1

Arsenic 329 1 1

Barium 344 1 7

Beryllium 346 1 < 1

Boron 164 12 15

Cadmium 353 1 2

Chromium 343 1 6

Cobalt 343 1 2

Copper 345 1 15

Iron 423 10 10

Lead 343 1 1

Lithium 37 1 < 8

Manganese 546 1 10

Molybdenum 341 1 < 1

Nickel 339 1 3

Radon 0 80 --

Selenium 302 1 2

Silver 343 1 1

Strontium 186 0.9 8

Thallium 151 1 < 1

Uranium 368 1 1

Vanadium 37 1 < 8

Zinc 341 1 50
Occurrence of Trace Elements in Groundwater 17

Occurrence of Trace Elements in 

Groundwater

The occurrence of trace elements in groundwater from 

wells across the United States was highly variable. Concentrations of Ba, B, Li, Rn, and Sr were above their reporting levels 

in nearly all samples; however, as noted in the section on quality control, measured concentrations of some elements, such 

as Al, B, Cu, Fe, and Zn, may be excessive on the basis of data 

from blank samples, which suggest systematic low-level bias 

in the dataset. Concentrations of Al, Ba, B, Cr, Li, Mn, Ni, Rn, 

Sr, V, and Zn exceeded the common reporting level in more 

than 50 percent of samples (median) compared to Sb, As, Be, 

Cd, Co, Cu, Fe, Pb, Mo, Se, Ag, Tl, and U, which were equal 

to or above their common reporting level in fewer than 

50 percent of samples (table 4). As, B, Mn, Mo, Rn, Sr, and U 

occurred at concentrations greater than HHBs in more than 

1 percent of water samples, and Fe and Mn occurred at concentrations greater than nonhealth guidelines in 19 and 

31 percent of water samples, respectively (table 4).

Most trace elements (particularly common oxyanion or 

ion-complex forming elements) occurred in greater concentrations in dry regions than in humid regions (table 5). This was 

true for As, Ba, B, Cr, Cu, Li, Mo, Ni, Se, Sr, U, V, and Zn 

(tables 5 and 6; figs. 4 and 5). In contrast, concentrations of 

elements that are mobile in anoxic and (or) low-pH groundwater conditions (such as Al, Co, Fe, Mn, and Pb) are commonly 

greater in the humid regions than in the dry regions (tables 5 

and 6; figs. 4 and 5). These large-scale differences may reflect 

several processes, including differences in precipitation, the 

effects of irrigation, ion competition, complexation, redox, and 

evaporative concentration in the dry parts of the United States. 

Processes associated with dry climates, especially in conjunction with irrigation, can lead to characteristically high concentrations of total dissolved solids (TDS) in groundwater (Hem, 

1985; Seiler and others, 2003; Anning and others, 2007). 

Trace-Element Occurrence and Concentrations 

in Monitoring and Drinking-Water Wells

Well-specific factors, such as depth and type in addition 

to specific land uses, can influence concentrations of trace 

elements in groundwater. The concentrations of trace elements 

were compared among monitoring wells in areas representing 

primarily agricultural or urban land uses and drinking-water 

wells, by climate region (table 7).

In humid regions, 14 trace elements had greater concentrations in monitoring wells than in drinking-water wells; 

however, only Cu, Pb, Rn, and Zn concentrations were greater 

in drinking-water wells than in monitoring wells in humid 

regions (table 7). Concentrations of trace elements commonly 

associated with low-pH recharge and (or) oxic groundwater 

are expected to be greater in monitoring wells (shallow wells) 

in humid regions than in drinking-water wells (generally deep 

wells). Accordingly, concentrations of Al, Sb, Ba, B, Cr, Co, 

Fe, Li, Mn, Mo, Ni, Se, Sr, and U were all greater in monitoring wells than in drinking-water wells in humid regions of 

the United States (table 7). The greater concentrations of Cu, 

Pb, and Zn in drinking-water wells in humid regions (and Pb 

and Zn in dry regions) may be an indication that these water 

samples were commonly collected through existing pumps and 

plumbing, which can contain these metals in parts, fittings, 

and valves (Fahlquist and Ardis, 2004; Groschen and others, 

2008). Concentrations of Fe and Mn, which occur more often 

in humid regions than in dry, were greater in monitoring wells 

than in drinking-water wells in dry climate and humid regions 

(table 7). 

In the dry regions of the United States, concentrations 

of As, Ba, Pb, Li, Sr, V, and Zn were greater in drinkingwater wells than in monitoring wells. Some of these elements 

commonly form oxyanions (As, Cr, and V) under typical 

groundwater conditions and are soluble in oxic or anoxic 

alkaline environments (Smedley and Kinniburgh, 2002). For 

As, this may be related to the fact that pH values are greater 

in drinking-water wells in this region than in monitoring wells 

(table 7).
18 Trace Elements and Radon in Groundwater Across the United States, 1992–2003 Table 4. Summary statistics with human-health-benchmark and nonhealth-guideline exceedences for trace elements in groundwater sampled for the NAWQA Program, 1992– 2003, across the United States. [MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey from USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trig- ger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression-on-order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level. USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guide- line to address cosmetic or aesthetic considerations of drinking water] Element Number of samples Percentage of samples with censored values Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline 10th 25th Median 75th 90th Maximum Benchmark value Type Percentage exceeding benchmark Guideline value (SMCL) Percentage exceeding SMCL Aluminum 2,826 26.1 0.43 0.98 3.0 4.9 11 1,100 -- -- -- 50 or 200 3.2, 1.1 Antimony 3,009 89.2 < 1 < 1 < 1 < 1 < 1 6.3 6 MCL 0.030 -- -- Arsenic 3,396 49.4 0.079 0.23 0.79 3.0 7.4 550 10 MCL 7.0 -- -- Barium 3,122 1.3 9.0 24 54 120 220 5,100 2,000 MCL 0.10 -- -- Beryllium 3,025 97.0 < 1 < 1 < 1 < 1 < 1 18 4 MCL 0.13 -- -- Boron 1,174 10.1 8.5 17 35 82 220 3,400 1,000 HBSL 1.9 -- -- Cadmium 3,124 92.3 < 1 < 1 < 1 < 1 < 1 16 5 MCL 0.26 -- -- Chromium 3,123 37.9 0.41 0.68 1.2 3.0 5.2 150 100 MCL 0.030 -- -- Cobalt 3,026 72.3 0.026 0.063 0.17 0.48 1.1 680 -- -- -- -- -- Copper 3,144 36.7 0.20 0.47 1.0 3.0 8.5 2,000 1,300 AL 0.030 1,000 0.030 Iron 5,131 40.6 0.095 0.81 7.9 93 1,500 81,000 -- -- -- 300 19 Lead 3,120 78.9 0.005 0.018 0.070 0.27 1.0 480 15 AL 0.16 -- -- Lithium 936 6.0 0.60 2.0 6.0 18 54 1,200 -- -- -- -- -- Manganese 4,976 30.5 0.14 0.85 7.0 84 360 28,000 300 HBSL 12 50 31

Molybdenum 3,063 42.4 0.13 0.32 1.0 3.3 8.0 4,700 40 HBSL 1.5 -- --Nickel 3,076 36.7 0.15 0.35 1.1 2.5 4.9 670 100 HBSL 0.23 -- --Radon 3,877 1.4 140 240 430 790 1,500 220,000 300 or 4,000 MCL1/AMCL2 65, 2.7 -- --Selenium 3,166 72.3 0.041 0.12 0.34 1.0 3.0 94 50 MCL 0.19 -- --Silver 2,988 99.4 < 1 < 1 < 1 < 1 < 1 8.0 100 HBSL 0 100 0Strontium 1,309 0.2 46 100 270 680 1,700 44,000 4,000 HBSL 4.3 -- --Thallium 867 89.7 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --Uranium 3,541 48.8 0.018 0.087 0.52 3.6 12 550 30 MCL 4 -- --Vanadium 833 30.9 0.11 0.295 1.4 11 27 190 -- -- -- -- --Zinc 3,006 16.3 0.43 1.6 4.8 18 69 3,300 2,000 HBSL 0.03 5,000 0

1 USEPA proposed maximum contaminant level (MCL) for radon in water.2 USEPA proposed alternative maximum contaminant level (AMCL) for radon in water: a proposed higher allowable MCL if accompanied by a multimedia mitigation program to address radon risk in 

indoor air.
Occurrence of Trace Elements in Groundwater 19

Table 5. Summary statistics with human-health-benchmark and nonhealth-guideline exceedences for trace elements in groundwater sampled for the NAWQA Program by climate region, 1992–2003, across the United States.—Continued[Regions shown in figure 2A. MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics (ROS); percentiles not estimated if percentage of censored values is greater than 80; if percentiles are not estimated, actual values are shown for percentiles greater than or equal to the assessment level. USEPA Secondary Maximum Contaminant Level (SMCL) is a non-enforceable federal guideline to address cosmetic or aesthetic considerations of drinking water]

Element

Number 

of 

samples

Percentage 

of samples 

with 

censored 

values

Concentration percentile, 

in μg/L or pCi/L Human-health benchmark Nonhealth guideline

10th 25th Median 75th 90th Maximum Benchmark Type

Percentage 

exceeding 

benchmark

Guideline 

value 

(SMCL)

Percentage 

exceeding 

SMCL

Dry regions

Aluminum 998 34.0 0.50 0.91 2.0 3.9 5.0 270 -- -- -- 50 or 200 0.8, 0.2Antimony 1,115 83.8 < 1 < 1 < 1 < 1 < 1 6.3 6 MCL 0.1 -- --Arsenic 1,252 29.8 0.27 0.69 2.0 5.5 11 550 10 MCL 11 -- --Barium 1,178 0.1 21 36 66 120 200 990 2,000 MCL 0 -- --Beryllium 1,121 99.0 < 1 < 1 < 1 < 1 < 1 18 4 MCL 0.2 -- --Boron 363 2.2 22 37 65 150 300 2,600 1,000 HBSL 0.83 -- --Cadmium 1,188 89.2 < 1 < 1 < 1 < 1 < 1 16 5 MCL 0.3 -- --Chromium 1,198 33.1 0.42 0.70 1.3 3.1 5.4 150 100 MCL 0.080 -- --Cobalt 1,125 68.5 0.034 0.071 0.16 0.42 1.0 680 -- -- -- -- --Copper 1,204 27.3 0.30 0.59 1.3 3.0 6.0 480 1,300 AL 0 1,000 0Iron 1,706 52.4 0.034 0.28 4.0 17 360 38,000 -- -- -- 300 11Lead 1,193 79.0 0.003 0.010 0.043 0.15 0.61 12 15 AL 0 -- --Lithium 354 0.0 7.6 12 20 53 120 580 -- -- -- -- --Manganese 1,660 44.4 0.020 0.15 1.4 46 350 5,400 300 HBSL 11 50 24Molybdenum 1,159 20.9 0.41 0.95 2.5 5.9 11 4,700 40 HBSL 1.9 -- --Nickel 1,168 30.3 0.20 0.46 1.3 3.0 5.0 670 100 HBSL 0.3 -- --Radon 1,440 1.46 170 270 430 720 1,100 5,600 300 or 4,000 MCL1/AMCL2 69, 0.4 -- --Selenium 1,231 51.3 0.11 0.27 0.84 2.6 6.8 94 50 MCL 0.4 -- --Silver 1,109 98.7 < 1 < 1 < 1 < 1 < 1 8.0 100 HBSL 0 100 0Strontium 524 0.2 100 250 540 1,200 3,600 44,000 4,000 HBSL 9.2 -- --Thallium 346 83.9 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --Uranium 1,228 23.3 0.21 0.75 3.2 8.8 22 550 30 MCL 7.5 -- --Vanadium 348 9.5 0.47 1.8 9.6 21 39 190 -- -- -- -- --Zinc 1,154 12.5 0.58 2.0 5.4 26 87 1,400 2,000 HBSL 0 5,000 0

1 USEPA proposed maximum contaminant level (MCL) for radon in water.2 USEPA proposed alternative maximum contaminant level (AMCL) for radon in water: a proposed higher allowable MCL if accompanied by a multimedia mitigation program to address radon risk in 

indoor air.
20 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 5. Summary statistics with human-health-benchmark and nonhealth-guideline exceedences for trace elements in groundwater sampled for the NAWQA Program by climate region, 1992–2003, across the United States.—Continued[Regions shown in figure 2A. MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics (ROS); percentiles not estimated if percentage of censored values is greater than 80; if percentiles are not estimated, actual values are shown for percentiles greater than or equal to the assessment level. USEPA Secondary Maximum Contaminant Level (SMCL) is a non-enforceable federal guideline to address cosmetic or aesthetic considerations of drinking water]

Element

Number 

of 

samples

Percentage 

of samples 

with 

censored 

values

Concentration percentile, 

in μg/L or pCi/L Human-health benchmark Nonhealth guideline

10th 25th Median 75th 90th Maximum Benchmark Type

Percentage 

exceeding 

benchmark

Guideline 

value 

(SMCL)

Percentage 

exceeding 

SMCL

Humid regions

Aluminum 1,828 21.9 0.49 1.2 3.3 5.8 16 1,100 -- -- -- 50 or 200 4.4, 1.6Antimony 1,894 92.3 < 1 < 1 < 1 < 1 < 1 5.0 6 MCL 0 -- --Arsenic 2,144 60.8 0.037 0.12 0.41 1.6 5.0 340 10 MCL 4.5 -- --Barium 1,944 2.0 5.5 18 47 110 240 5,100 2,000 MCL 0.2 -- --Beryllium 1,904 95.8 < 1 < 1 < 1 < 1 < 1 16 4 MCL 0.1 -- --Boron 811 13.7 7.3 12 26 52 160 3,400 1,000 HBSL 2.8 -- --Cadmium 1,936 94.2 < 1 < 1 < 1 < 1 < 1 7 5 MCL 0.2 -- --Chromium 1,925 41.0 0.41 0.68 1.2 3.0 5.0 17 100 MCL 0 -- --Cobalt 1,901 74.5 0.021 0.055 0.16 0.50 1.3 210 -- -- -- -- --Copper 1,940 42.5 0.14 0.40 1.0 3.2 11 2,000 1,300 AL 0.05 1,000 0.05Iron 3,425 35.2 0.18 1.49 11 220 2,100 81,000 -- -- -- 300 23.5Lead 1,927 78.9 0.011 0.033 0.12 0.40 1.2 480 15 AL 0.3 -- --Lithium 582 9.6 0.50 1.1 2.8 5.7 11 1,200 -- -- -- -- --Manganese 3,316 23.5 0.37 1.4 13 98 370 28,000 300 HBSL 11.8 50 33.8Molybdenum 1,904 56.1 0.056 0.16 0.54 2.0 5.0 1,500 40 HBSL 1.3 -- --Nickel 1,908 40.6 0.12 0.30 1.0 2.2 4.5 170 100 HBSL 0.2 -- --Radon 2,437 1.4 130 220 430 860 2,100 220,000 300 or 4,000 MCL1/AMCL2 63.4, 4.1 -- --Selenium 1,935 85.6 < 1 < 1 < 1 < 1 1.1 56 50 MCL 0.05 -- --Silver 1,879 99.8 < 1 < 1 < 1 < 1 < 1 1.0 100 HBSL 0 100 0Strontium 785 0.1 36 74 160 370 890 16,000 4,000 HBSL 1 -- --Thallium 521 91.0 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --Uranium 2,313 62.3 0.006 0.025 0.17 1.2 4.5 440 30 MCL 2.1 -- --Vanadium 485 46.2 0.038 0.13 0.4 2.0 10 120 -- -- -- -- --Zinc 1,852 18.7 0.36 1.4 4.1 15 56 3,300 2,000 HBSL 0.05 5,000 0

1 USEPA proposed maximum contaminant level (MCL) for radon in water.2 USEPA proposed alternative maximum contaminant level (AMCL) for radon in water: a proposed higher allowable MCL if accompanied by a multimedia mitigation program to address radon risk in 

indoor air.
Occurrence of Trace Elements in Groundwater 21

Table 6. Frequency and distribution statistics for the occurrence of trace elements in groundwater sampled for the NAWQA 

Program by climate region, 1992–2003, across the United States.

[Climate regions shown in figure 2A. Yellow shading indicates higher dry-region concentrations; blue shading indicates higher humid-region concentrations; concentrations of elements in same Tukey group do not differ significantly at alpha = 0.05. No shading indicates concentrations that do 

not differ significantly between well types. Reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms 

per liter), radon (80 picocuries per liter), and strontium (0.9 micrograms per liter). ≥, greater than or equal to; <, less than; N is the number of wells 

sampled; number of trace element samples varies with availability of grouping variable data and with censoring level]

Element

Number of samples and percentage ≥ reporting level by climate region

Dry region Humid region

p-value

N Percentage Tukey group N Percentage Tukey group

Aluminum 998 65 B 1,828 78 A < 0.0001

Antimony 1,115 1.0 A 1,894 1.1 A 0.8571

Arsenic 1,252 67 A 2,144 34 B < 0.0001

Barium 1,178 100 A 1,944 98 B < 0.0001

Beryllium 1,121 0.4 A 1,904 0.7 A 0.1936

Boron 363 97 A 811 76 B < 0.0001

Cadmium 1,188 1.3 A 1,936 0.6 A 0.0600

Chromium 1,198 58 A 1,925 53 B 0.0148

Cobalt 1,125 8.4 B 1,901 13 A 0.0004

Copper 1,204 61 A 1,940 51 B 0.0272

Iron 1,706 32 B 3,425 50 A < 0.0001

Lead 1,193 9.4 B 1,927 13 A 0.0015

Lithium 354 97 A 539 78 B < 0.0001

Manganese 1,660 51 B 3,316 75 A < 0.0001

Molybdenum 1,159 74 A 1,904 39 B < 0.0001

Nickel 1,168 62 A 1,908 54 B < 0.0001

Radon 1,440 98 A 2,437 96 A 0.7276

Selenium 1,201 46 A 1,854 9.7 B < 0.0001

Silver 1,109 0.6 A 1,879 0.2 A 0.0674

Strontium 524 100 A 785 100 B < 0.0001

Thallium 346 0.0 A 521 0.0 A 1.0000

Uranium 1,228 71 A 2,313 28 B < 0.0001

Vanadium 348 81 A 485 35 B < 0.0001

Zinc 1,154 87 A 1,852 81 B < 0.0001
22 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc. 

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 200 400 KILOMETERS

0 200 400 MILES

< 1

≥ 1 and ≤ 6

> 6 

Antimony, in micrograms

 per liter

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

A

B

Aluminum, in micrograms 

 per liter

< 1

≥ 1 and ≤ 50

> 50 and ≤ 200

> 200

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
Occurrence of Trace Elements in Groundwater 23

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS 0 200 400 KILOMETERS

0 200 400 MILES

D

Barium, in micrograms 

 per liter

< 1

≥ 1 and ≤ 2,000

> 2,000 

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

C

Arsenic, in micrograms

 per liter

< 1

≥ 1 and ≤ 10

> 10

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
24 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 200 400 KILOMETERS

0 200 400 MILES

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

< 12

≥ 12 and ≤ 1,000

> 1,000 

Boron, in micrograms 

 per liter

F

Beryllium, in micrograms 

 per liter

< 1

≥ 1 and ≤ 4

> 4 

E

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
Occurrence of Trace Elements in Groundwater 25

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS 0 200 400 KILOMETERS

0 200 400 MILES

< 1

≥ 1 and ≤ 100

> 100

G

H

Chromium, in micrograms 

 per liter

Cadmium, in micrograms 

 per liter

< 1

≥ 1 and ≤ 5

> 5

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
26 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

J

I

0 200 400 KILOMETERS

0 200 400 MILES

< 1

≥ 1 and ≤ 1,300

> 1,300 

Copper, in micrograms 

 per liter

< 1

≥ 1 

Cobalt, in micrograms 

 per liter

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
Occurrence of Trace Elements in Groundwater 27

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 200 400 KILOMETERS

0 200 400 MILES

< 10

≥ 10 and ≤ 300

> 300

L

K

Iron, in micrograms 

 per liter

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

< 1

≥ 1 and ≤ 15

> 15 

Lead, in micrograms 

 per liter

!!!!

!!!!!!!!!!!

!

!
28 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 200 400 KILOMETERS

0 200 400 MILES

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

< 1

≥ 1 and ≤ 300

> 300 

Manganese, in micrograms 

 per liter

N

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

< 1

≥ 1 and ≤ 6

> 6 and ≤ 54

> 54 

Lithium, in micrograms 

 per liter

M
Occurrence of Trace Elements in Groundwater 29

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS 0 200 400 KILOMETERS

0 200 400 MILES

< 1

≥ 1 and ≤ 100

> 100

Nickel, in micrograms 

 per liter

P

< 1

≥ 1 and ≤ 40

> 40

Molybdenum, in micrograms 

 per liter

O

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
30 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS 0 200 400 KILOMETERS

0 200 400 MILES

< 1

≥ 1 and ≤ 50

> 50 

Selenium, in micrograms 

 per liter

R

< 80

≥ 80 and ≤ 300

> 300 and ≤ 4,000

> 4,000

Radon, in picocuries

 per liter

Q

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
Occurrence of Trace Elements in Groundwater 31

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 200 400 KILOMETERS

0 200 400 MILES

T

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

Strontium, in micrograms 

 per liter

< .9

≥ .9 and ≤ 4,000

> 4,000

< 1

≥ 1 and ≤ 100

Silver, in micrograms 

 per liter

S
32 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 200 400 KILOMETERS

0 200 400 MILES

Uranium, in micrograms

 per liter

< 1

≥ 1 and ≤ 30

> 30

V

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

Thallium, in micrograms 

 per liter

< 1

U
Occurrence of Trace Elements in Groundwater 33

Figure 4. Geographic distribution of trace-element concentrations in groundwater collected from wells as part of the National WaterQuality Assessment Program, 1992–2003: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, (G) cadmium, 

(H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, (R) selenium, 

(S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc.—Continued

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 100 MILES

0 100 KILOMETERS

0 500 MILES

0 500 KILOMETERS

0 200 400 KILOMETERS

0 200 400 MILES

X

< 1

≥ 1 and ≤ 2,000

> 2,000 

Zinc, in micrograms 

 per liter

W

Vanadium, in micrograms 

 per liter

< 1

≥ 1 and ≤ 1.4

> 1.4 and ≤ 27

> 27 

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]

EXPLANATION

[Triangle represents well

completed in the material

beneath the uppermost aquifer]
34 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 5. Trace-element concentrations in groundwater across the United States by climate region. [Concentrations below 

the common reporting level (1 μg/L, unless otherwise noted) were estimated by using regression-on-order statistics (see text 

for explanation). Elements with more than 80 percent of censored data (table 5) are not shown. μg/L, micrograms per liter; pCi/L, 

picocuries per liter (for radon)]

EXPLANATION

90th percentile

Number of samples

75th percentile

median

25th percentile

10th percentile

Humid climate

Dry climate

Values below common

reporting level

interquartile

range

As, humid

As, dry

Ba, humid

Ba, dry

B, humid

B, dry

Cr, humid

Cr, dry

Cu, humid

Cu, dry

Fe, humid

Fe, dry

Li, humid

Li, dry

Mn, humid

Mn, dry

Mo, humid

Mo, dry

Ni, humid

Ni, dry

Pb, humid

Pb, dry

Rn, humid

Rn, dry

Se, dry

Sr, humid

Sr, dry

U, humid

U, dry

V, humid

V, dry

Zn, humid

Zn, dry

Reporting level = 1 μg/L, except for B=12 μg/L, Fe=10 μg/L, and Sr=0.9 μg/L; Rn = 80 pCi/L

0.1

0.01

0.001

0.0001

0.00001

1

10

100

1,000

10,000

100,000

0.1

0.01

0.001

0.0001

0.00001

1

10

100

1,000

10,000

100,000

Conccentration, in micrograms per liter (picocuries per liter for radon)

Reporting level = 1 μg/L, except for B=12 μg/L, Fe=10 μg/L, and Sr=0.9 μg/L; Rn = 80 pCi/L

1,904

Trace element and climate

Al, humid

Al, dry

3,316 1,660 1,904 1,159 1,908 1,168

1,927 1,193

2,437 1,440 1,231 785 524 2,313 1,228 485 348 1,852 1,154

1,828 998 2,144 1,252 1,944 1,178 811 363 1,925 1,198

Co, humid

Co, dry

1,901 1,125 1,940 1,204 3,425 1,706

582 354
Occurrence of Trace Elements in Groundwater 35

Table 7. Group-comparison statistics for concentrations of trace elements and selected field parameters in groundwater samples collected for the NAWQA Program by climate region and well type, 1992–2003, across the United States.[Climate regions shown in figure 2A. Yellow shading indicates higher monitoring-well concentrations; blue shading indicates higher drinking-water well concentrations; concentrations of elements in the same Tukey group are not different significantly at alpha = 0.05. No shading indicates concentrations that do not differ significantly between well types. Reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon (80 picocuries per liter), and strontium (0.9 micrograms per liter); N is the number of wells sampled; number of trace element samples varies with availability of grouping variable data; ≥, greater than or equal to; --, not applicable]

Element

Dry region Humid region

Monitoring wells Drinking-water wells

p-value

Monitoring wells Drinking-water wells

p-value N Percentage ≥ reporting 

level

Tukey 

group

N

Percentage 

≥ reporting 

level

Tukey 

group

N

Percentage 

≥ reporting 

level

Tukey 

group

N

Percentage 

≥ reporting 

level

Tukey 

group

Aluminum 422 70 A 523 58 A 0.2049 579 90 A 1,078 71 B < 0.0001Antimony 438 1.4 A 611 0.7 A 0.2427 598 2.3 A 1,122 0.5 B 0.0004Arsenic 489 61 B 668 70 A 0.0028 616 37 A 1,344 33 A 0.0775Barium 484 100 B 620 100 A 0.0301 618 100 A 1,151 97 B 0.0004Beryllium 443 0.9 A 611 0.0 B 0.0189 602 1.0 A 1,128 0.6 A 0.3857Boron 136 94 A 220 99 A 0.2918 262 87 A 515 70 B < 0.0001Cadmium 445 2.5 A 565 0.6 B 0.0093 602 1.2 A 1,158 0.4 A 0.0770Chromium 464 49 A 645 62 A 0.1746 609 65 A 1,142 46 B < 0.0001Cobalt 447 19 A 612 1.3 B < 0.0001 603 25 A 1,124 4.6 B < 0.0001Copper 469 79 A 649 51 B < 0.0001 610 42 B 1,154 56 A < 0.0001Iron 677 39 A 914 28 B < 0.0001 1,325 55 A 1,834 45 B < 0.0001Lead 447 2.9 B 657 13 A < 0.0001 599 5.5 B 1,152 17 A < 0.0001Lithium 133 92 B 214 100 A 0.0039 166 85 A 348 75 B 0.0161Manganese 629 78 A 916 35 B < 0.0001 1,293 90 A 1,761 62 B < 0.0001Molybdenum 470 76 A 616 74 B < 0.0001 607 48 A 1,123 36 B < 0.0001Nickel 479 83 A 616 46 B < 0.0001 611 78 A 1,123 42 B < 0.0001Radon 491 100 A 847 97 B < 0.0001 617 97 B 1,638 96 A < 0.0001Selenium 451 44 A 661 48 A 0.5767 614 12 A 1,099 8.6 B 0.0067Silver 438 1.1 A 606 0.2 B 0.0402 584 0.3 A 1,125 0.2 A 0.5046Strontium 177 100 B 316 99 A 0.0013 275 100 A 476 100 B < 0.0001Thallium 128 0.0 A 212 0.0 A 1.0000 170 0.0 A 326 0.0 A 1.0000Uranium 524 72 A 631 72 B < 0.0001 682 34 A 1,425 27 B 0.0075Vanadium 129 71 B 212 88 A < 0.0001 145 33 A 315 35 A 0.4239Zinc 443 76 B 632 94 A < 0.0001 587 68 B 1,089 90 A < 0.0001pH 667 -- B 926 -- A < 0.0001 1,327 -- B 1,831 -- A < 0.0001Dissolved oxygen 661 -- B 858 -- A < 0.0001 1,794 -- A 1,286 -- A 0.5481Specific conductance 621 -- A 925 -- B < 0.0001 1,329 -- A 1,828 -- B < 0.0001
36 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Comparisons of Trace-Element Concentrations 

in Groundwater to Human-Health Benchmarks

HHBs were defined for many trace elements to ensure 

that high-quality safe drinking water is provided to people 

using publicly supplied water. The results in this section are 

not intended to be interpreted as estimates of potential exposure, but rather as a simple comparison to identify geographic 

areas where concentrations of trace elements in groundwater 

were found by the NAWQA Program to be greater than established or proposed HHBs. Not all water samples collected for 

this study were from drinking-water wells; about 40 percent 

were monitoring wells. The trace elements with HHBs are Sb, 

As, Ba, Be, B, Cd, Cr, Cu, Pb, Mn, Mo, Ni, Se, Ag, Sr, Tl, U, 

and Zn; however, it is important to note that, for this summary, 

not all trace elements were analyzed in samples from all of the 

selected wells. Comparison of these data to HHBs is intended, 

in part, to provide a context for analysis of specific trace elements in specific areas of the United States. 

For the analysis presented here, wells were selected 

irrespective of well type or land use (4,527 wells). In 

19 percent of samples collected from those wells, at least 

one trace element exceeded an HHB (table 9). Wells with 

HHB exceedences were widespread across the United States; 

they occurred in all aquifer groups and in both humid and 

dry regions. 

For trace elements, wells in urban settings had greater 

rates of HHB exceedences than agricultural wells or drinkingwater wells in varied land-use settings (table 9). In addition, 

monitoring wells tended to have greater HHB-exceedence 

rates than drinking-water wells. For some contaminants in 

groundwater, such as nitrate and volatile organic compounds 

(VOCs), differences in land use (as an indicator of contaminant source) can have a profound effect on rates of HHB 

exceedences. For trace elements, however, the differences 

may be more related to redox, pH, and other conditions than 

to source indicators, although anthropogenic activities can 

change redox conditions and thereby can change concentrations of naturally occurring trace elements in groundwater. 

In general, the wells in the urban and agricultural categories 

(dominantly monitoring wells) are shallow water-table wells, 

where the pH in groundwater is commonly low and DO 

high. Water in a greater percentage of wells in major aquifers (dominantly drinking-water wells) was found to have a 

higher pH and lower DO than water sampled from the shallow 

wells. These differences may influence trace-element HHB 

exceedences, depending on the element.

An important finding from this analysis was that traceelement contamination in groundwater far outpaces contamination in most other commonly assessed groups, such 

as VOCs, pesticides, and nutrients. In drinking-water wells, 

one or more trace elements exceeded HHBs in untreated 

water from 13 percent of public-supply wells and 13 percent 

of domestic-supply wells (table 9); for pesticides, these rates 

were 2.2 and 0.76 percent, respectively (Gilliom and others, 

2006). Similarly, about 2 percent of public-supply wells and 

Trace-Element Occurrence and Concentrations 

in Wells in Agricultural and Urban Areas

Contaminants that are introduced at the land surface 

can affect water quality in underlying aquifers. Some trace 

elements have anthropogenic sources that are a direct result 

of land use. An example of this is the use of B in fertilizer in 

agricultural settings. Land use may not have a direct effect 

but can result in changes in groundwater pH, TDS, or redox 

conditions, which in turn can affect the solubility of naturally 

occurring trace elements. These changes can either enhance 

or inhibit the solubility of trace elements in groundwater. For 

example, Se can be concentrated by evaporation or by irrigation return flow (Seiler and others, 2003). Moreover, studies 

in agricultural areas in the northeastern United States show 

that organic As added to poultry feed is converted to inorganic 

As and loaded to the soil zone, where it may be soluble under 

favorable redox conditions (Rutherford and others, 2003; Stolz 

and others, 2007). Arsenic also has historically been used as 

a pesticide for orchard and row crops (D’Angelo and others, 

1996; Hinkle and Polette, 1999; Loiselle and others, 2001). 

Many studies acknowledge that anthropogenic sources of As 

in groundwater cannot be ruled out, but most suggest that 

groundwater As sources are predominantly natural (Hinkle 

and Polette, 1999; Welch and others, 2000; Ayotte and others, 

2003; Peters and Blum, 2003; Welch and Stollenwerk, 2003; 

Robinson and Ayotte, 2006; Robinson and Ayotte, 2007). Geochemical conditions near waste sites, such as landfills, however, may be altered sufficiently to allow Fe and Mn hydroxide 

dissolution, resulting in the mobilization of otherwise sequestered soil As and other metals (Stollenwerk and Colman, 2004; 

deLemos and others, 2006).

The concentrations of trace elements were compared 

between wells representing primarily agricultural and urban 

land uses (table 8). In this comparison, water samples from 

wells in agricultural settings have greater concentrations 

of some trace elements, such as As, Mo, and U, than 

samples from wells in urban settings, regardless of climate. 

Concentrations of DO and pH values also were greater in 

water samples from agricultural wells than in samples from 

urban wells. Significantly greater concentrations of B, Cr, Se, 

Ag, Sr, and V also were found in samples from agricultural 

wells in dry regions. Concentrations of Pb were greater in 

water samples from wells in agricultural settings than in 

samples from urban wells in humid regions, but the converse 

was true for dry regions.

Water sampled from urban wells in both climate regions 

generally had greater concentrations of Fe, Li, and Mn than 

water from agricultural wells, possibly because urban-well 

water also had lower pH and DO (table 8). Collectively, the 

urban-well samples were more anoxic than agricultural-well 

samples, and this result may partly explain why the urban-well 

samples had fewer HHB or guideline exceedences of oxyanion-forming elements, and why trace elements that are more 

mobile under anoxic conditions, such as Fe and Mn, were 

generally prevalent. 
Occurrence of Trace Elements in Groundwater 37

Table 8. Group-comparison statistics for concentrations of trace elements and selected field parameters in groundwater sampled for the NAWQA Program by climate region and land-use type, 1992–2003, across the United States.[Yellow shading indicates higher agricultural land use well concentrations; blue shading indicates higher urban land use well concentrations; Concentrations of elements with same Tukey group do not differ significantly at alpha = 0.05. No shading indicates concentrations that do not differ significantly between well types. Reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon (80 picocuries per liter), and strontium (0.9 micrograms per liter). N is the number of wells sampled; number of trace element samples varies with availability of grouping variable data; ≥, greater than or equal to; <, less than; --, not applicable]

Element

Dry region Humid region

Agricultural land-use wells Urban land-use wells

p-value

Agricultural land-use wells Urban land-use wells

p-value N Percentage ≥ reporting 

level

Tukey 

group

N

Percentage 

≥ reporting 

level

Tukey 

group

N

Percentage 

≥ reporting 

level

Tukey 

group

N

Percentage 

≥ reporting 

level

Tukey 

group

Aluminum 171 74 A 189 68 A 0.1080 258 92 A 341 90 A 0.5417Antimony 161 1.9 A 211 1.4 A 0.7416 261 3.5 A 353 1.1 A 0.0513Arsenic 191 77 A 225 56 B < 0.0001 302 49 A 363 32 B < 0.0001Barium 190 100 A 225 100 A 0.0889 264 100 A 363 100 A 0.1237Beryllium 163 0.0 A 213 1.9 A 0.0803 261 1.5 A 353 0.3 A 0.0886Boron 38 100 A 58 100 B < 0.0001 106 88 A 146 87 A 0.7145Cadmium 164 2.4 A 212 3.3 A 0.6302 261 2.7 A 353 0.3 B 0.0099Chromium 181 56 A 213 45 B 0.0009 263 65 A 357 63 A 0.9566Cobalt 165 6.7 B 216 26 A < 0.0001 262 21 A 357 28 A 0.0534Copper 176 79 A 223 72 A 0.2735 263 48 A 362 48 A 0.6096Iron 480 27 B 268 41 A < 0.0001 922 42 B 540 56 A < 0.0001Lead 162 0.6 B 218 4.6 A 0.0235 261 14 A 353 4.0 B < 0.0001Lithium 40 98 B 58 98 A < 0.0001 61 77 B 86 98 A < 0.0001Manganese 462 57 B 234 75 A < 0.0001 895 75 B 541 90 A < 0.0001Molybdenum 185 90 A 217 67 B < 0.0001 263 60 A 360 45 B 0.0001Nickel 186 86 A 224 86 A 0.5004 263 72 B 362 78 A 0.0493Radon 367 98 A 183 99 A 0.1939 425 96 A 300 99 A 0.1750Selenium 176 52 A 219 38 B 0.0003 262 11 A 331 13 A 0.4383Silver 163 3.1 A 210 0.0 B 0.0112 259 0.0 A 338 0.3 A 0.3836Strontium 49 100 A 88 100 B < 0.0001 163 100 B 154 99 A < 0.0001Thallium 37 0.0 A 57 0.0 A 1.0000 56 0.0 A 97 0.0 A 1.0000Uranium 202 83 A 222 63 B < 0.0001 455 42 A 377 30 B 0.0002Vanadium 37 95 A 58 78 B < 0.0001 58 28 A 70 36 A 0.4626Zinc 177 84 A 195 69 A 0.0736 263 73 A 339 65 A 0.1179pH 456 -- A 277 -- B <0.0001 922 -- A 543 -- B < 0.0001Dissolved oxygen 475 -- A 253 -- B < 0.0001 893 -- A 528 -- B 0.0005Specific conductance 483 -- B 275 -- A 0.0029 925 -- A 542 -- A 0.4707
38 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

1 percent of domestic-supply wells had one or more VOCs 

greater than HBSLs (Zogorski and others, 2006). Nitrate 

exceeded its MCL in 3 percent of public-supply-well samples 

and 7 percent of domestic-supply-well samples (Dubrovsky 

and others, 2010), yet still did not approach the exceedence 

rate for trace elements.

HHB exceedences varied by element. Overall, 17 of 19 

trace elements with defined HHBs had some exceedences 

(table 4). Seven of the 19 exceeded benchmarks in at least 

1 percent of water samples (table 4). Mn occurred most often 

at concentrations greater than its HHB (12 percent), followed 

by As (7.0 percent), Sr (4.3 percent), U (4.0 percent), B 

(1.9 percent) and Mo (1.5 percent). Rn is a special case 

because it has a proposed two-tier benchmark that accommodates state-specific programs related to the mitigation 

of Rn in air. In the case of Rn, 65 percent of samples had 

concentrations greater than the USEPA-proposed MCL of 

300 pCi/L, and 2.7 percent had concentrations greater than 

the USEPA-proposed AMCL of 4,000 pCi/L (table 4). For all 

other elements with defined HHBs, concentrations greater than 

those benchmarks occurred in fewer than 1 percent of samples. 

Similar rates of exceedence of HHBs for trace elements were 

found in a study that used only domestic wells sampled by 

the NAWQA Program (DeSimone, 2009). Radionuclides, As, 

Mn, Sr, and B were the contaminants exceeding HHBs most 

frequently in a national study of source-water samples from 

public wells (Toccalino and others, 2010). 

Because trace elements tend to occur at different 

concentrations in dry and humid climates, the rates of HHB 

exceedence also vary by climate. For example, in dry regions, 

concentrations of As exceeded the MCL of 10 μg/L in 

11 percent of samples compared to fewer than 5 percent in 

humid regions (table 5). Sr concentrations were greater than 

the HBSL of 4,000 μg/L in about 9 percent of dry-region 

samples compared to 1 percent in humid regions. U exceeded 

the 30 μg/L MCL in 7.5 percent of dry-region samples and 

in only 2.1 percent of humid-region samples (table 5). Se 

concentrations in groundwater were not commonly greater 

than the USEPA MCL of 50 μg/L but were greater than the 

MCL about 8 times more frequently in the dry-region samples 

than in the humid-region samples (0.4 and 0.05 percent, 

respectively). As discussed previously, these differences may 

reflect the generally high mobility of some of these elements 

(As, Se, U) in high-pH, high-TDS groundwater. 

Trace-Element Occurrence and Concentrations 

in Groundwater by Major Aquifer Group

Groundwater trace-element occurrences in relation 

to aquifer geology are described for eight major groups of 

aquifers in the United States. Most wells in aquifers sampled 

Table 9. Percentage of samples collected across the United States for the NAWQA Program from which one or more trace elements 

exceeded human-health benchmarks by primary water use and study type, 1992–2003. 

[TE, trace elements; VOCs, volatile organic compounds; --, not applicable]

Study type

Public-supply wells Domestic-supply wells Monitoring wells All wells

Number 

sampled

Percentage 

of samples 

exceeding 

benchmark

Number 

sampled

Percentage 

of samples 

exceeding 

benchmark

Number 

sampled

Percentage 

of samples 

exceeding 

benchmark

Number 

sampled

Percentage 

of samples 

exceeding 

benchmark

Trace-element studies

Agricultural land use 0 -- 402 9.5 904 26 1,306 21

Urban land use 9 33 17 29 740 28 766 28

Major aquifer study 372 12 1,895 13 188 26 2,455 14

All TE studies 381 13 2,314 13 1,832 27 4,527 19

Other contaminant groups

All VOCs studies1 340 2.1 1,606 1.1 1,060 1.5 3,006 1.4

All pesticides studies2 364 2.2 2,356 0.76 2,293 2.5 5,013 1.7

All nitrate studies3 384 3.0 2,388 7.0 1,902 12 4,674 8.7

1

Zogorski and others, 2006.

2

Gilliom and others, 2006.

3

Dubrovsky and others, 2010.
Occurrence of Trace Elements in Groundwater 39

in this study were in humid regions, although the basaltic- and 

other volcanic-rock (BAV) aquifers were in predominantly 

dry regions. Only aquifers in unconsolidated sand and gravel 

(USG) and in sandstone and carbonate rock (SCR) had fewer 

than 85 percent of wells in one climate region. For samples 

from the SCR group, 79 of the 222 (36 percent) were in humid 

regions. Groundwater samples in the USG aquifer group (the 

largest group of samples in this study) were more evenly split 

between dry and humid regions, with 723 of the 1,801 wells 

(40 percent) in humid regions. 

Dominant water types, based on major-ion chemistry, 

were determined to provide a description of each major 

aquifer group (fig. 6). Because some ions are geochemically 

active or redox sensitive, water type is useful in analyzing 

the occurrence of some trace elements. Trace-element and 

Rn concentrations in groundwater were compared to current 

and proposed HHBs. In many cases, median concentrations 

of a trace element in the aquifer groups were less than the 

corresponding HHBs, so higher percentiles, such as the 

75th or 95th percentiles, were compared. In selected cases, 

sources and processes controlling trace-element concentrations 

and, if applicable, unusually high or low concentrations 

were examined.

Summary statistics (percentiles) of the concentrations 

of trace elements and Rn and the percentages of samples in 

which concentrations exceeded current and proposed HHBs 

and nonhealth guidelines for each major aquifer group are 

shown in appendix 3. The median concentrations for all trace 

elements in all of the major aquifer groups were less than 

established HHBs and nonhealth guidelines (table 10). Radon 

concentrations, however, were greater than the USEPA-proposed MCL of 300 pCi/L in more than 50 percent of wells in 

five of the eight aquifer groups (table 10). 

Groundwater concentrations of 12 different elements 

were above current or proposed HHBs (or nonhealth guidelines) in at least 1 percent of samples in at least one aquifer 

group (table 10, blue shading). Fe, Mn, and Rn exceeded 

HHBs (or nonhealth guidelines) in some water samples in 

all aquifer groups. The USG, GLA, and SAN aquifer groups 

had at least eight elements that exceeded current or proposed 

HHBs or nonhealth guidelines in at least 1 percent of water 

samples, and Al, As, Fe, Mn, Rn, and Sr concentrations 

exceeded HHBs or nonhealth guidelines in all three aquifer 

groups. The SCS, SCR, CAR, and BAV groups had five or 

fewer elements that exceeded current or proposed HHBs or 

nonhealth guidelines in at least 1 percent of water samples 

(combinations of As, Fe, Pb, Mn, Rn, or Sr). The CRL aquifer 

group was similar but also included HHB or nonhealth-guideline exceedences of U and Al. Sr exceeded HHBs or nonhealth 

guidelines in at least 1 percent of the water samples from the 

USG, GLA, SAN, and SCR aquifer groups, but in the SCR, 

more than 26 percent exceeded the HHBs or nonhealth guidelines. Mo exceeded HHBs or nonhealth guidelines in more 

than 1 percent of the samples in the USG and GLA aquifer 

groups (table 10). 

Unconsolidated Sand and Gravel Aquifers (USG)

Groundwater samples from the USG aquifer group were 

predominantly of the calcium-magnesium-bicarbonate 

(Ca-Mg-bicarbonate or mixed bicarbonate) type (fig. 6A) 

with a median pH of 7.3 (fig. 7). These samples had the largest 

variability in water type, however, with many samples in the 

sodium-chloride (Na-Cl) and Ca-Mg-sulfate fields; this variability may be related to differences in the lithologic compositions of the original source materials from which the sand and 

gravel was derived, as well as climate differences between the 

sampling locations. 

Generally, trace-element concentrations in groundwater 

samples from USG wells are among the highest of any of 

the aquifer groups described in this report. Concentrations of 

As, B, Li, Mo, Se, U, and V (and HHB exceedences) were 

generally greater in the USG than in any other aquifer groups 

(table 10). 

About 9 percent of water samples in the USG have 

U concentrations above the USEPA drinking water MCL 

(table 10). This is the greatest rate among all of the aquifer 

groups for U and accounts for about 95 percent of all U 

HHB exceedences. Sr exceeded the HHB of 4,000 μg/L in 

5.2 percent of samples (mostly in western aquifers), which 

accounts for about 45 percent of Sr benchmark exceedences 

for all aquifer groups combined. 

Concentrations of As and U in all groundwater samples 

in the USG aquifer group were most often above the USEPA 

drinking water MCLs of 10 and 30 μg/L, respectively, in the 

western principal aquifers, such as the Basin and Range basinfill aquifers, California Coastal Basin aquifers, Central Valley 

aquifer system, Rio Grande aquifer system, and High Plains 

aquifer (figs. 3, 4C and V). In many cases, several oxyanionand ion-complex-forming trace elements are correlated 

(rho > 0.5) with each other and also with bicarbonate, 

sulfate, and TDS. Many of these samples had relatively high 

groundwater pH, which can enhance the solubility of trace 

elements such as As, B, Mo, Se, and U (Dubrovsky and others, 

1991; Fujii and Swain, 1995; Welch and others, 2000; Focazio 

and others, 2001; Smedley and Kinniburgh, 2002; Welch and 

Stollenwerk, 2003).

In the generally humid eastern regions, As concentrations 

were greatest in groundwater samples from the Mississippi 

River Valley alluvial aquifer and the surficial aquifer system 

(southern Florida). In the Mississippi River Valley alluvial 

aquifer, the occurrence of As appears to be associated with the 

reductive dissolution of Fe oxide coatings on aquifer materials 

possibly coupled with the oxidation of lignite and peat (Kresse 

and Fazio, 2003). Uranium occurs in groundwater in the surficial aquifer system (Florida) and may be related to phosphate 

deposits containing U in the limestone-derived sediments of 

the Tamiami Formation (Meyer, 1989). U in shallow oxic 

groundwaters may form soluble complexes with bicarbonate 

and phosphate in the aquifer (Meyer, 1989). 
40 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 6. Major-ion chemistry by redox class for (A) unconsolidated sand and gravel aquifers, (B) glacial unconsolidated sand and 

gravel aquifers, (C) semiconsolidated sand aquifers, (D) sandstone aquifers, (E) sandstone and carbonate-rock aquifers, (F) carbonaterock aquifers, (G) basaltic- and other volcanic-rock aquifers, and (H) crystalline-rock aquifers.

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Percentage of total milliequivalents per liter

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Percentage of total milliequivalents per liter

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Percentage of total milliequivalents per liter

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Percentage of total milliequivalents per liter

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Magnesium

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Sulfate

Percent

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Calcium plus magnesium

C. Semiconsolidated sand aquifers (SCS) D. Sandstone aquifers (SAN)

A. Unconsolidated sand and gravel aquifers (USG) B. Glacial unconsolidated sand and gravel aquifers (GLA)

EXPLANATION

Oxic Mixed Anoxic
Occurrence of Trace Elements in Groundwater 41

Figure 6. Major-ion chemistry by redox class for (A) unconsolidated sand and gravel aquifers, (B) glacial unconsolidated sand and 

gravel aquifers, (C) semiconsolidated sand aquifers, (D) sandstone aquifers, (E) sandstone and carbonate-rock aquifers, (F) carbonaterock aquifers, (G) basaltic- and other volcanic-rock aquifers, and (H) crystalline-rock aquifers.—Continued

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Carbonate plus bicarbonate

Sulfate plus chloride

Calcium

plus magnesium

Chloride plus fluoride plus nitrite plus nitrate

Percentage of total milliequivalents per liter

Calcium

Magnesium

Sodium

plus potassium

Sulfate

Percent

Percent

Carbonate plus bicarbonate

Sulfate plus chloride

Calcium

plus magnesium

Chloride plus fluoride plus nitrite plus nitrate

Percentage of total milliequivalents per liter

Calcium

Magnesium

Sodium

plus potassium

Sulfate

Percent

Percent

Carbonate plus bicarbonate

Sulfate plus chloride

Calcium

plus magnesium

EXPLANATION

Oxic Mixed Anoxic

E. Sandstone and carbonate-rock aquifers (SCR) F. Carbonate-rock aquifers (CAR)

G. Basaltic- and volcanic-rock aquifers (BAV) H. Crystalline-rock aquifers (CRL)
42 Trace Elements and Radon in Groundwater Across the United States, 1992–2003 Table 10. Summary of the percentages of exceedences of human-health benchmarks or nonhealth guidelines in samples collected for the NAWQA Program across the United States, 1992–2003, by major aquifer group. [Elements in which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom of the table; USG, unconsolidated sand and gravel; GLA, glacial unconsolidated sand and gravel; SCS, semiconsolidated sand; SAN, sandstone; SCR, sandstone and carbonate rock; CAR, carbonate rock; BAV, basaltic and other volcanic rock; CRL, crystalline rock; --, no data; N is the number of wells sampled; %, percentage; ≥, more than or equal to; benchmark units are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter]Element Bench- mark or guideline Major aquifer group USG GLA SCS SAN SCR CAR BAV CRL

N

%

N

%

N

%

N

%

N

%

N

%

N

%

N

%

Aluminum 200 1,226 1.0 485 1.0 301 3.0 229 1.7 166 0.60 195 0.00 31 0.00 193 1.0Antimony 6 1,318 0.00 498 0.00 338 0.00 256 0.39 186 0.00 189 0.00 30 0.00 194 0.00Arsenic 10 1,507 12 512 6.8 373 0.80 282 1.4 189 1.1 229 1.3 81 6.2 223 5.4Barium 2,000 1,376 0.07 513 0.00 343 0.29 283 0.35 189 0.00 189 0.00 35 0.00 194 0.00Beryllium 4 1,318 0.00 511 0.00 338 0.29 258 0.78 186 0.00 189 0.00 31 0.00 194 0.52Boron 1,000 552 3.4 274 1.1 181 0.00 86 0.00 -- -- -- -- 30 0.00 51 0.00Cadmium 5 1,351 0.37 511 0.00 338 0.00 274 1.1 186 0.00 189 0.00 81 0.00 194 0.00Chromium 100 1,367 0.07 491 0.00 338 0.00 275 0.00 188 0.00 190 0.00 80 0.00 194 0.00Copper 1,300 1,372 0.07 501 0.00 339 0.00 280 0.00 189 0.00 189 0.00 80 0.00 194 0.00Iron 300 1,769 12 1,190 29 570 29 389 32 222 7.7 480 19 232 3.4 279 9.3Lead 15 1,355 0.00 498 0.00 339 1.2 277 0.36 187 0.00 189 0.00 81 0.00 194 0.00Manganese 300 1,700 12 1,148 20 564 9.8 376 17 219 3.2 463 1.9 228 2.6 278 3.6Molybdenum 40 1,368 2.4 499 2.0 339 0.00 257 0.78 188 0.53 189 0.00 30 0.00 194 0.00Nickel 100 1,369 0.00 502 0.00 341 0.29 262 2.3 189 0.00 189 0.00 30 0.00 194 0.00Radon 300 1,481 77 763 54 243 39 358 65 136 44 431 61 192 43 273 92Selenium 50 1,395 0.29 511 0.20 342 0.00 282 0.35 189 0.00 183 0.00 74 0.00 190 0.00Silver 100 1,297 0.00 507 0.00 339 0.00 264 0.00 184 0.00 169 0.00 35 0.00 194 0.00Strontium 4,000 483 5.2 259 1.2 155 0.00 59 3.4 98 26 140 0.71 31 0.00 84 0.00Thallium 2 366 0.00 202 0.00 154 0.00 57 0.00 -- -- -- -- -- -- 58 0.00Uranium 30 1,409 8.7 640 0.47 339 0.00 336 0.30 187 0.53 318 0.31 33 0.00 279 3.9Zinc 2,000 1,335 0.07 494 0.00 308 0.00 238 0.00 178 0.00 190 0.00 70 0.00 193 0.00Number of elements with 

≥ 1% exceedence

9 8 5 8 5 4 4 6
Occurrence of Trace Elements in Groundwater 43

Glacial Unconsolidated Sand and Gravel 

Aquifers (GLA)

The water type of the GLA groundwater samples was 

predominantly Ca-Mg bicarbonate but with some samples 

having a Na-Cl influence (fig. 6B). The median pH of samples 

from this aquifer group was 7.1 (fig. 7). The GLA aquifer 

group had the greatest percentage of mixed and anoxic samples (and the lowest percent of oxic) of all of the categories in 

this report (fig. 6B; table 1). Some of the GLA groundwater 

samples had relatively high percentages of Na and Cl, and 

most of those samples were oxic, possibly because of shallow 

groundwater that had been affected to some extent by land use 

(deicing chemicals, septic use, urbanization, and agriculture). 

The sources of trace elements, aquifer geochemistry, and 

redox varied greatly within this aquifer group, in part because 

of the wide array of source rocks that affected the composition 

of the glacial sediments. 

Mn exceeded the HBSL (300 μg/L) in about 20 percent of 

all water samples in the GLA—more than in samples from any 

other major aquifer group (table 10). This rate of exceedence 

may be related to the relatively high percentage of mixed 

and anoxic samples in the GLA as well as commonly low pH 

conditions (compare figs. 2, 3A, 6B, and 4N). Arsenic also 

is a common trace element with concentrations above HHBs 

in water samples from the GLA aquifer group, as are Al, B, 

Fe, Mo, Rn, and Sr. Arsenic occurred at concentrations above 

10 μg/L in nearly 7 percent of 512 samples (table 10). U was 

greater than 30 μg/L in 0.5 percent of 640 samples, and most 

of these samples came from the west-central region, especially 

North Dakota (table 10; fig. 4V). U concentrations in 

groundwater in this region may be related to the incorporation 

of uraniferous Cretaceous-age shale in the glacial sediments 

(Tourtelot, 1956; Seiler and others, 2003; Ayotte and others, 

2007; Groschen and others, 2008). Groundwater samples 

from glacial sediments derived in part from crystalline 

rocks, primarily in Minnesota, had comparatively low U 

concentrations. Arsenic, by contrast, appears to be more 

strongly related to geochemical controls such as redox and 

pH (Warner, 2001; Thomas, 2003, 2007). 

Mo and Sr concentrations in water from wells in the GLA 

were above HBSLs in 2 and 1.2 percent of samples, respectively (table 10). These Mo samples were from groundwater 

primarily in the central portion of the GLA, including Illinois 

and Indiana (fig. 4O). Mo correlates most strongly with bicarbonate (rho equal to 0.58, p less than 0.001, N equal to 149), 

Sr (rho equal to 0.53, p less than 0.0001, N equal to 245), and 

As (rho equal to 0.47, p less than 0.0001, N equal to 470). 

Arsenic and Mo commonly occur as oxyanions in water and, 

as such, may be released by redox- or pH-driven desorption 

from mineral oxides. The correlation of As with Mo concentrations has been noted in studies of other areas in Illinois and 

Canada (Warner, 2001; Thomas, 2003). Sr concentrations were 

greatest in water samples in the east-central part of the GLA, 

primarily in Illinois, Indiana, and Ohio (fig. 4T). Sr is relatively insoluble in water except when it forms complexes with 

anions, such as sulfate or carbonate; in water samples from the 

GLA, Sr was correlated with bicarbonate (rho equal to 0.64, 

p less than 0.0001, N equal to 95) and sulfate (rho equal to 

0.44, p less than 0.0001, N equal to 256). 

1,798 1,190 482 399 222 482 230 279

B D F BC E C A E

Major aquifer group code

4

5

6

7

8

USG GLA SCS SAN SCR CAR BAV CRL

9

pH, in standard units

EXPLANATION

90th percentile

Number of samples

75th percentile

Median

25th percentile

10th percentile

230

Figure 7. Distributions of pH by major aquifer group. Letters representing nonparametric 

Tukey group are shown below boxes. Groups with different letters are significantly different 

(alpha = 0.05). USG, unconsolidated sand and gravel aquifers; GLA, glacial unconsolidated 

sand and gravel aquifers; SCS, semiconsolidated sand aquifers; SAN, sandstone aquifers; 

SCR, sandstone and carbonate-rock aquifers; CAR, carbonate-rock aquifers; BAV, basalticand other volcanic-rock aquifers; and CRL, crystalline-rock aquifers.
44 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Semiconsolidated Sand Aquifers (SCS)

Groundwater samples from the SCS aquifer group consisted of a wide mix of water types (fig. 6C). Most samples 

from the SCS had either bicarbonate or Cl as the predominant 

anion, but some waters had a strong sulfate component. The 

bicarbonate-chloride-sulfate signature of these waters may be 

a reflection of the marine depositional history of these eastern 

and southern coastal plain sediments. Groundwater samples 

from the Northern Atlantic Coastal Plain aquifer system were 

predominantly of the sulfate type, whereas samples from the 

Mississippi Embayment aquifer system, Coastal Lowlands 

aquifer system, and the Texas Coastal uplands aquifer system 

were predominantly of the bicarbonate type.

The median pH of water samples from this aquifer 

group (5.5) was the lowest among all the aquifer groups 

(fig. 7); however, there was variability within this aquifer 

group. For example, the water samples from the Northern 

Atlantic Coastal Plain aquifer system and the Southeastern 

Coastal Plain aquifer system collectively had a median pH of 

5.1, whereas the samples from the Coastal Lowlands aquifer 

system, Mississippi embayment aquifer system, and Texas 

coastal uplands aquifer system had median pH values of 6.6, 

6.3, and 7.6, respectively. Differences in pH may affect the 

solubility of certain trace elements.

In water samples from the SCS aquifer group, Al, Fe, Pb, 

Mn, and Rn concentrations were above current or proposed 

HHBs or nonhealth guidelines in 1 percent or more of samples 

(table 10). The greatest concentrations of Al, Fe, Pb, and Mn 

were measured in water samples from the Northern Atlantic 

Coastal Plain and Southeastern Coastal Plain aquifer systems, where low pH values were likely to be a factor in their 

mobility (figs. 4A, K, L, and N). Concentrations of Co also 

were greatest in the water samples from the Northern Atlantic 

Coastal Plain aquifer system. The same was true for Ni and 

Zn; these results were consistent with the low pH of the water 

in this aquifer (figs. 4I, P, and X). 

Concentrations of As were greatest in water samples from 

the Coastal Lowlands and Northern Atlantic Coastal Plain 

aquifer systems of the SCS (figs. 3, 4C). Concentrations of U 

were below HHBs in all water samples from the SCS aquifers, 

but concentrations above 5 μg/L were found in water samples 

from the Coastal Lowlands, Southeastern Coastal Plain, and 

the Texas Coastal Uplands aquifer systems (figs. 3, 4V). Concentrations of U were correlated with concentrations of many 

dissolved ions; this correlation is consistent with mobilization 

by ion complexation, which occurs in other aquifer groups. 

Sandstone Aquifers (SAN)

The sandstone aquifer group covers large areas of the 

east- and west-central parts of the United States (fig. 3A). The 

water type of samples from the SAN aquifers was variable 

but predominantly of the Ca-Mg-bicarbonate type. Some 

samples had a substantial percentage of Na and proportionally 

more sulfate (fig. 6D). Differences in water type may reflect 

differences in climate and (or) local differences in aquifer 

chemistry. The Na-dominated samples, for example, were 

mainly from the Lower Tertiary sandstone aquifers in 

Wyoming, where processes involving ion exchange of Ca 

for Na on aquifer materials and sulfate reduction may have 

been responsible for differences in water type (Bartos and 

others, 2005).

The pH of water samples from the SAN aquifers also varied locally with a median value of 7.3. Groundwater samples 

from Pennsylvanian aquifers and from the Woodbine aquifer 

(Dallas-Fort Worth metropolitan area) (fig. 3A) had substantially lower pH values (medians of 6.3 and 6.8, respectively) 

than other SAN water samples. Groundwater samples from 

the Lower Tertiary aquifers (primarily in the Bighorn Basin) 

had a median pH of 8.1 and tended to be of the Na-bicarbonate 

type, which transitioned to Na-sulfate type as TDS increased 

(Bartos and others, 2005). The median TDS value for all of the 

water samples from the Lower Tertiary aquifers and the 

Woodbine aquifer was more than 1,300 mg/L (compared to 

less than 300 mg/L for all other SAN water samples). 

Groundwater samples from the SAN aquifer group 

had eight elements (Al, As, Cd, Fe, Mn, Ni, Rn, and Sr ) 

with more than 1 percent of concentrations above current 

and proposed HHBs and nonhealth guidelines; however, 

percentages were less than 2 percent, except for Fe, Mn, Ni, 

Rn, and Sr (table 10). Concentrations of As were greatest in 

groundwater samples from the northeastern parts of the Early 

Mesozoic basin aquifers and the Valley and Ridge aquifers 

(75th-percentile value greater than 3 μg/L) and also in the 

Woodbine aquifer and Lower Tertiary aquifers (75th-percentile 

value greater than 1 μg/L). Concentrations of As from all 

samples were most strongly correlated with Mo (rho equal to 

0.41, p less than 0.001, N equal to 256). 

Concentrations of U were greatest in the Woodbine 

aquifer, Early Mesozoic basin aquifers, and Lower Tertiary 

aquifers (75th-percentile value greater than 3 μg/L). At the 

principal aquifer scale, sulfate, alkalinity, bicarbonate, specific 

conductance, and Mo are among the most significant correlates with U. 

Sandstone and Carbonate-Rock Aquifers (SCR)

Groundwater samples from the SCR aquifer group—the 

Edwards-Trinity aquifer system in south-central Texas and the 

Mississippian aquifers primarily in Tennessee and Alabama 

were predominantly of the Ca-Mg-carbonate type (figs. 3A, 

6E). A few water samples, mostly from the Mississippian aquifers, had a Na-Cl component. The pH of waters from the SCR 

aquifers is variable, with the median from the Edwards-Trinity 

aquifer system (dry climate) equal to 7.0 and that from the 

Mississippian aquifers (humid climate) equal to 6.6 (fig. 7). 

Sr exceeded the HBSL of 4,000 μg/L (table 10) in more 

than 25 percent of samples from the aquifers in SCR—

much greater than in other aquifer groups—however, these 

Sr concentrations were not evenly distributed across this 

aquifer group. For example, Sr concentrations exceeded the 
Occurrence of Trace Elements in Groundwater 45

HBSL in about 10 percent of water samples from wells in the 

Edwards aquifer and in about 50 percent of water samples 

from the Trinity aquifer in south-central Texas (figs. 3, 4T). 

The source of the Sr is likely the argillaceous carbonate rocks 

that underlie the aquifer system (Oetting and others, 1996; 

Musgrove and Banner, 2004). Sr in groundwater samples 

from the Trinity aquifer correlated most strongly with specific 

conductance, sulfate, TDS, and K (rho equal to 0.64 to 0.55, 

p less than 0.001, N equal to 29). Median concentrations of 

Rn were near the USEPA-proposed MCL of 300 pCi/L in 

groundwater samples from the Trinity aquifer and about 

150 pCi/L in groundwater samples from the Edwards aquifer. 

Median Rn concentrations groundwater samples from the 

Mississippian aquifers were about 600 pCi/L. 

Carbonate-Rock Aquifers (CAR)

Groundwater samples from wells in carbonate-rock aquifer group were predominantly of the Ca-Mg bicarbonate type 

(fig. 6F). The chemistry of most samples indicated recharging 

groundwater (Ca bicarbonate) to evolved groundwater (dissolution of dolomite and gypsum) (Hanshaw and Back, 1979; 

Back and others, 1988). 

Most of the groundwater samples were collected from 

these aquifers in humid parts of the United States, and very 

few elements (As, Fe, Mn, Rn) above current or proposed 

HHBs or nonhealth guidelines were detected (table 10). Al, 

As, Rn, Sr, and U concentrations were elevated locally in 

the carbonate-rock aquifers. Al, for example, was above 

50 μg/L in samples from only the Ozark Plateaus aquifer system, whereas As concentrations were greatest in the samples 

from the Biscayne aquifer and Floridan aquifer system. Rn 

was above the USEPA-proposed AMCL of 4,000 pCi/L in only 

one groundwater sample from the Floridan aquifer system, but 

median concentrations exceeded the USEPA-proposed MCL 

of 300 pCi/L in most of the samples from this aquifer group.

Sr, a common element in carbonate-aquifer waters, was 

analyzed in water samples from the carbonate rocks of only 

the Valley and Ridge aquifers and Biscayne aquifers. The 

95th-percentile value for Sr in these samples exceeded 

1,000 μg/L, and the median in water samples from the 

Biscayne aquifer was greater than 750 μg/L. U also 

occurred in some water samples in the Biscayne aquifer, 

with the 95th-percentile value exceeding 24 and 15 μg/L in 

samples collected from shallow and deep positions in the 

aquifer, respectively.

By contrast, As concentrations in 30 groundwater 

samples from a dry-region carbonate aquifer in the Great 

Basin (Nevada and Utah) ranged from 0.7 to 45.7 μg/L with 

a median value of 9.7 μg/L (Schaefer and others, 2006). 

Processes identified as leading to the mobilization of As in 

groundwater include geothermal heating and long time of 

travel along a flow path. For example, in the central part of 

this carbonate-rock aquifer, groundwater flow is generally 

from north to south, and As concentrations also increase in this 

direction (Schaefer and others, 2006).

Basaltic- and other Volcanic-Rock Aquifers 

(BAV)

The fewest groundwater samples were collected from 

the BAV aquifer group, partly because of the smaller areal 

distribution of these deposits. Fewer than 100 water samples 

were collected for each trace element except Fe (N equal to 

232) and Mn (N equal to 228) (table 10). The water type of the 

groundwater samples from BAV aquifer group was mixed but 

tended toward either Ca-Mg bicarbonate or Ca-Mg chloride 

(fig. 6G). The median pH of water samples from this aquifer group (7.7) was the highest among the aquifer groups in 

this report (fig. 7). The pH of water in these aquifers may be 

increased by processes such as silicate hydrolysis and carbonic 

acid weathering (Wood and Fernandez, 1988). The former process results in the addition of hydroxyl ions to the water, raising the pH, whereas in the latter, bicarbonate ions are added. 

Ion exchange of Na for Ca can further increase the dissolution 

of calcite, resulting in still higher pH (Wood and Fernandez, 

1988). The water samples from the BAV aquifer group had 

only four elements above HHBs or nonhealth guidelines 

(table 10), although As was found above 10 μg/L in 6.2 percent of water samples (all from the Upper Snake River basin), 

and Rn was above the USEPA-proposed MCL of 300 pCi/L in 

43 percent of water samples (table 10).

Crystalline-Rock Aquifers (CRL)

The water type of samples from the CRL aquifer group 

was predominantly Ca bicarbonate to mixed Ca-Mg-Na 

bicarbonate, but with some samples having a stronger Na-Cl 

influence (fig. 6H). The median pH of water samples from 

crystalline-rock aquifers was about 6.5 (fig. 7); however, pH 

was slightly alkaline (7.3) in New York and New England and 

the Rocky Mountain Front Range crystalline-rock aquifers, 

and somewhat acidic (5.9) for the Piedmont and Blue Ridge 

crystalline-rock aquifers. This variation was evident from the 

large interquartile range for pH (1.6 pH units) for the CRL 

water samples (fig. 7). Concentrations of six elements (Al, As, 

Fe, Mn, Rn, and U) were greater than HHBs and nonhealth 

guidelines in at least 1 percent of the groundwater samples 

from the CRL aquifer group (table 10). Arsenic exceedences of 

HHBs in the CRL were 5.4 percent but jumped to 17 percent 

in samples from the CRL aquifers in the eastern part of New 

England. U was greatest in groundwater from CRL aquifers 

in the Northeast (New York and New England) and the Rocky 

Mountain Front Range (95th-percentile values of 18 and 

220 μg/L, respectively), whereas 95th-percentile concentrations in samples from the Piedmont and Blue Ridge CRL aquifers ranged from 1.9 to 6.7 μg/L. Water from the CRL aquifers 

had the greatest Rn concentrations of any aquifer group. For 

example, 92 percent of Rn concentrations from this aquifer 

group were greater than the USEPA-proposed MCL of 

300 pCi/L, and 28 percent of concentrations from CRL 

samples were greater than the USEPA-proposed AMCL of 
46 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

4,000 pCi/L (table 10; appendix 3). Rn concentrations were, 

by far, greater in the water samples from CRL aquifers in New 

York and New England (fig. 4Q).

Concentrations of As in water samples from the CRL 

were correlated most strongly with pH (rho equal to 0.32, 

p less than 0.0001, N equal to 223) and redox as indicated by 

DO concentration (rho equal to -0.34, p less than 0.0001, 

N equal to 221). U in the samples from the CRL aquifers was 

correlated with many water-quality and well-construction 

parameters, including Mo, Ca, pH, specific conductance, TDS, 

Na, and well depth, collectively indicating possible complexation of U with bicarbonate. U concentrations were also correlated with Rn in the CRL aquifers, but Rn was correlated with 

no other elements at the aquifer-group level. In water from 

the CRL aquifers in New York and New England, however, 

Rn correlated weakly with Pb (rho equal to 0.38, p less than 

0.0005, N equal to 82).

Relation of Selected Trace Elements to pH and 

Redox State

The effects of redox and pH account for substantial 

additional variability in the concentrations of trace elements 

in groundwater among the major aquifer groups. Other factors 

such as water type, TDS, organic matter, and biological 

activity also may influence trace-element concentrations and 

may account for unexplained variability in the expected redoxand pH-related behavior of some trace elements. Most trace 

elements are sensitive to redox conditions in groundwater, 

either because that element can occupy multiple redox states 

in natural waters, or because the redox state controls the 

concentration of soluble complexes. Variation in pH has an 

effect on the adsorption of many trace elements, as well as 

on the solubility of some concentration-limited hydroxide 

solid phases (such as Fe and Mn). Some redox-sensitive trace 

elements are more mobile under low pH and oxic conditions. 

For example, Cd, Cu, Pb, and Zn can increase from 0 to 

100 percent adsorption on oxide-hydroxides over a pH 

range of 2 standard units (Salbu and Steinnes, 1994). Other 

elements, such as Fe and Mn, are mobile under low pH 

and (or) anoxic conditions. Some oxyanion-forming trace 

elements, by contrast, are generally more mobile under high 

pH conditions and include As, Cr, Mo, Se, and V. Arsenic 

sorption, for example, decreases as pH increases (Smedley and 

Kinniburgh, 2002; Stollenwerk, 2003).

In this report, groundwater samples are classified into 

three simplified redox states: oxic, mixed, and anoxic 

(table 2) based on a scheme that uses commonly measured 

ions and properties of the water sample to assess the redox 

state (Chapelle and others, 1995; Chapelle and others, 2002; 

Paschke, 2007; McMahon and Chapelle, 2008; McMahon 

and others, 2009). Most samples were oxic according to this 

scheme (fig. 2A). About 70 percent or more of the water samples in the USG, SCR, CAR, BAV, and CRL aquifer groups 

were classified as oxic. By contrast, more than 50 percent 

of groundwater samples in the GLA, SCS, and SAN aquifer 

groups were classified as mixed or anoxic (table 1). Generally, anoxic and mixed redox conditions were more common 

in well water in the humid parts of the United States, whereas 

oxic conditions were more common in well water in the dry 

regions (fig. 2A). Additional information on Fe and other 

trace-element guideline exceedences for categories of pH and 

redox are shown in appendix 4. 

The median pH varies by aquifer group and is above or 

equal to 7 for the samples from the USG, GLA, SAN, CAR, 

and BAV groups and below 7 for the samples from the SCS, 

SCR, and CRL groups (fig. 7). The pH of precipitation is 

lower in the eastern part of the United States than in the west 

(fig. 2B), and acidification of groundwater from external factors, such as acid deposition related to increased fossil-fuel 

burning, can be associated with the mobility of trace metals in 

recently recharged (post-1960) groundwater (Jacks, 1993). 

Iron and Manganese

Based on the redox classification scheme used in this 

report, samples with high Fe or Mn (concentrations greater 

than 0.1 or 0.05 mg/L, respectively) were considered to be 

anoxic (table 2). Therefore, Fe and Mn occurred most often 

in mixed and anoxic samples and least often in oxic samples 

(figs. 8K, N). For samples with data sufficient for the inference of redox status, 19 percent had DO less than or equal to 

0.5 mg/L and high Fe/Mn, 62 percent had low Fe/Mn and DO 

greater than 0.5 mg/L, and 19 percent had high Fe/Mn and DO 

greater than 0.5 mg/L. Thus, 81 percent of the time, Fe/Mn 

and DO were mutually exclusive under the above conditions. 

Aluminum, Copper, Lead, and Zinc

Al, Cu, Pb, and Zn are common elements in the environment and are found in materials and products used in drinkingwater-well construction and plumbing. It is therefore difficult 

to determine the extent to which the concentrations of these 

elements in water samples from drinking-water wells are 

related to these materials or to other sources. Additionally, bias 

caused by systematic contamination of samples was identified for Al and Zn through analysis of water initially free of 

these elements (Apodaca and others, 2006). Thus, results from 

analyses of these elements at concentrations near the reporting 

levels must be considered in light of the potential bias. 

Redox effects on trace-element occurrence and distribution are often, but not always, similar among the major aquifer 

groups. In groundwater samples from the USG, Al occurrence 

generally increased as samples became more anoxic in both 

pH categories (figs. 8A and 9A). Al exceeded the nonhealth 
Occurrence of Trace Elements in Groundwater 47

guideline in up to 3 percent of water samples from the major 

aquifer groups and most commonly exceeded guidelines under 

low-pH and mixed or anoxic conditions (table 10). Cu also 

was sensitive to redox and pH, occurring more frequently 

under low-pH and oxic conditions in about half of the aquifer 

groups (fig. 8J). Pb, based on solubility considerations, is more 

mobile under low-pH conditions (Hem, 1985), and this was 

true for most water samples (figs. 9B and 8L). 

Additional variation in redox and pH effects within the 

same aquifer group may be related to climate. For example, in 

the humid-region USG aquifers, Pb in groundwater occurred 

more frequently and was more strongly redox controlled in the 

high-pH samples than in the low-pH samples (fig. 10B). In the 

dry-region USG aquifers, redox and pH effects for groundwater Pb occurrence were less discernible. Further, Cu occurrence in groundwater was not sensitive to pH in either region 

but was redox sensitive in humid-region samples (fig. 10C). 

Zn occurrence in groundwater showed a clear sensitivity to 

redox in the humid-region USG aquifers but not in dry-region 

aquifers (fig. 10D). 

Arsenic, Chromium, Molybdenum, and Selenium

As, Cr, Mo, and Se commonly form oxyanions in groundwater that are affected by redox and pH, and thus the chemical 

behavior of these elements is somewhat predictable. The percentage of samples with As, for example, generally increased 

from oxic to anoxic redox conditions when pH was less than 

7, but was consistently greater than 50 percent when pH was 

greater than or equal to 7 (fig. 11A). A similar pattern was seen 

for Mo (fig. 11C) and, to a lesser extent, Cr (fig. 11B). The 

pattern was opposite and applied in both pH categories for Se, 

which is more soluble under oxic conditions (fig. 11D). For 

As, Cr, and Mo, the lack of redox sensitivity at high pHs may 

be related to pH-driven desorption or ion competition, which 

either obscures or reduces the redox effects that were seen in 

the low-pH samples. 

Some deviation from these general patterns occurred by 

aquifer group and by aquifer, but the general patterns may 

indicate the relative importance of different processes in specific aquifer settings. For example, in the GLA aquifer group, 

As concentrations and occurrence increased significantly as 

samples became increasingly anoxic and as pH increased 

(figs. 12A and B). These results are similar to findings from 

other regional studies (Thomas, 2007). For groundwater 

samples from the USG aquifer group, however, As occurrence increased significantly as pH increased, but only for 

oxic and mixed conditions (figs. 12C and D). In the high-pH 

USG samples, there were no differences in the groundwater 

As concentrations or occurrence rates among redox categories. 

This result suggests that As mobility in some settings may be 

less controlled by redox when pH is greater than 7.

Occurrences of Mo and to some extent Cr in groundwater from the GLA aquifer group also increased as the water 

became more anoxic for both low- and high-pH samples 

(figs. 8H, O, 11B and C). Se occurrence, by contrast, dropped 

off sharply from oxic and mixed to anoxic water samples 

(figs. 8R, 11D). Se occurred at concentrations greater than 

1 μg/L in fewer than 30 percent of water samples from most 

aquifer groups except the USG group, from which concentrations in as many as half of the samples in the oxic or mixed 

categories were greater than 1 μg/L (fig. 8R). This result is 

consistent with results from studies in the San Joaquin Valley 

in California, where Se mobility was greater under oxic conditions (Dubrovsky and others, 1993; Fujii and Swain, 1995). 

These studies showed that Se was controlled by redox conditions, with only selenate (SeO4

2-) being mobile under oxic 

conditions. B, and to some extent V, appeared redox sensitive 

(figs. 8F, W) and generally followed the behavior of As and 

Mo (figs. 8C, O).

Uranium and Radon

U occurrence in groundwater was redox and pH 

dependent, occurring most frequently in samples with oxic 

or mixed redox conditions and generally under high-pH 

conditions (fig. 8V). However, because U can form soluble 

complexes under a wide range of redox and pH conditions that 

are common in groundwater across the United States, U also 

is sometimes found in water samples that are characterized as 

anoxic and low pH, possibly because of ion complexation 

or competition. 

Concentrations of Rn, a nonreactive gas, showed relations to redox and (or) pH conditions in the water samples 

from several aquifer groups. In general, Rn occurred most 

often in low-pH, oxic water samples. In groundwater from 

the CRL aquifer group, Rn occurrence was nearly ubiquitous, 

owing to the abundant U-bearing minerals in the rocks of the 

region and to the associated presence of radium (226Ra, which 

decays directly to Rn) on or near fracture surfaces. In groundwater samples from the GLA aquifer group, however, both 

redox and pH were related to occurrence (fig. 8Q). The apparent redox relation may be due in part to the decreased solubility of 226Ra in oxic groundwater conditions. This suggests that 

the source strength and (or) emanation efficiency for radon is 

possibly enhanced in the shallow part of the aquifer, perhaps 

due to greater weathering of radionuclides from aquifer materials or the flushing or depletion of sources of radon (226Ra) 

in the deeper parts of the aquifer. Oxic, low-pH groundwater 

conditions are consistent with the relative immobility of 226Ra 

in the shallow aquifer and the possible depletion of 226Ra in the 

deeper, anoxic parts of the aquifer. Decreased solubility would 

favor adsorption of 226Ra and provide a source for the generation of Rn (Focazio and others, 2001). 
48 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 8. The percentage of groundwater samples with trace-element concentrations greater than or equal to the reporting level, by 

redox and pH categories, for the major aquifer groups: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, 

(G) cadmium, (H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc. Reporting level is 1 microgram per liter 

unless otherwise noted. USG, unconsolidated sand and gravel aquifers; GLA, glacial unconsolidated sand and gravel aquifers; SCS, 

semiconsolidated sand aquifers; SAN, sandstone aquifers; SCR, sandstone and carbonate-rock aquifers; CAR, carbonate-rock aquifers; 

BAV, basaltic- and other volcanic-rock aquifers; and CRL, crystalline-rock aquifers. Error bars represent the 95-percent confidence 

interval of the mean (binomial distribution). See appendix 4 for the number of samples in each category.

Oxic Mixed Anoxic

A Aluminum

0

20

40

60

80

100

Percentage of Al samples

greater than or equal to

1 microgram per liter

0

20

40

60

80

100

B Antimony

EXPLANATION

Percentage of Sb samples

greater than or equal to

1 microgram per liter

C Arsenic

0

20

40

60

80

100

0

20

40

60

80

100

D Barium

BAV, pH < 7.0

BAV, pH ≥ 7.0

CAR, pH < 7.0

CAR, pH ≥ 7.0

GLA, pH < 7.0

GLA, pH ≥ 7.0

CRL, pH < 7.0

CRL, pH ≥ 7.0

SAN, pH < 7.0

SAN, pH ≥ 7.0

SCR, pH < 7.0

SCR, pH ≥ 7.0

SCS, pH < 7.0

SCS, pH ≥ 7.0

USG, pH < 7.0

USG, pH ≥ 7.0

Percentage of As samples

greater than or equal to

1 microgram per liter

Percentage of Ba samples

greater than or equal to

1 microgram per liter
Occurrence of Trace Elements in Groundwater 49

Figure 8. The percentage of groundwater samples with trace-element concentrations greater than or equal to the reporting level, by 

redox and pH categories, for the major aquifer groups: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, 

(G) cadmium, (H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc. Reporting level is 1 microgram per liter 

unless otherwise noted. USG, unconsolidated sand and gravel aquifers; GLA, glacial unconsolidated sand and gravel aquifers; SCS, 

semiconsolidated sand aquifers; SAN, sandstone aquifers; SCR, sandstone and carbonate-rock aquifers; CAR, carbonate-rock aquifers; 

BAV, basaltic- and other volcanic-rock aquifers; and CRL, crystalline-rock aquifers. Error bars represent the 95-percent confidence 

interval of the mean (binomial distribution). See appendix 4 for the number of samples in each category.—Continued

Oxic Mixed Anoxic

E Beryllium

0

20

40

60

80

100

Percentage of Be samples

greater than or equal to

1 microgram per liter

0

20

40

60

80

100

F Boron

EXPLANATION

Percentage of B samples

greater than or equal to

12 micrograms per liter

G Cadmium

0

20

40

60

80

100

0

20

40

60

80

100

H Chromium

BAV, pH < 7.0

BAV, pH ≥ 7.0

CAR, pH < 7.0

CAR, pH ≥ 7.0

GLA, pH < 7.0

GLA, pH ≥ 7.0

CRL, pH < 7.0

CRL, pH ≥ 7.0

SAN, pH < 7.0

SAN, pH ≥ 7.0

SCR, pH < 7.0

SCR, pH ≥ 7.0

SCS, pH < 7.0

SCS, pH ≥ 7.0

USG, pH < 7.0

USG, pH ≥ 7.0

Percentage of Cd samples

greater than or equal to

1 microgram per liter

Percentage of Cr samples

greater than or equal to

1 microgram per liter
50 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Oxic Mixed Anoxic

I Cobalt

0

20

40

60

80

100

Percentage of Co samples

greater than or equal to

1 microgram per liter

0

20

40

60

80

100

J Copper

EXPLANATION

Percentage of Cu samples

greater than or equal to

1 microgram per liter

K Iron

0

20

40

60

80

100

0

20

40

60

80

100

L Lead

BAV, pH < 7.0

BAV, pH ≥ 7.0

CAR, pH < 7.0

CAR, pH ≥ 7.0

GLA, pH < 7.0

GLA, pH ≥ 7.0

CRL, pH < 7.0

CRL, pH ≥ 7.0

SAN, pH < 7.0

SAN, pH ≥ 7.0

SCR, pH < 7.0

SCR, pH ≥ 7.0

SCS, pH < 7.0

SCS, pH ≥ 7.0

USG, pH < 7.0

USG, pH ≥ 7.0

Percentage of Fe samples

greater than or equal to

10 micrograms per liter

Percentage of Pb samples

greater than or equal to

1 microgram per liter

Figure 8. The percentage of groundwater samples with trace-element concentrations greater than or equal to the reporting level, by 

redox and pH categories, for the major aquifer groups: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, 

(G) cadmium, (H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc. Reporting level is 1 microgram per liter 

unless otherwise noted. USG, unconsolidated sand and gravel aquifers; GLA, glacial unconsolidated sand and gravel aquifers; SCS, 

semiconsolidated sand aquifers; SAN, sandstone aquifers; SCR, sandstone and carbonate-rock aquifers; CAR, carbonate-rock aquifers; 

BAV, basaltic- and other volcanic-rock aquifers; and CRL, crystalline-rock aquifers. Error bars represent the 95-percent confidence 

interval of the mean (binomial distribution). See appendix 4 for the number of samples in each category.—Continued
Occurrence of Trace Elements in Groundwater 51

Figure 8. The percentage of groundwater samples with trace-element concentrations greater than or equal to the reporting level, by 

redox and pH categories, for the major aquifer groups: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, 

(G) cadmium, (H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc. Reporting level is 1 microgram per liter 

unless otherwise noted. USG, unconsolidated sand and gravel aquifers; GLA, glacial unconsolidated sand and gravel aquifers; SCS, 

semiconsolidated sand aquifers; SAN, sandstone aquifers; SCR, sandstone and carbonate-rock aquifers; CAR, carbonate-rock aquifers; 

BAV, basaltic- and other volcanic-rock aquifers; and CRL, crystalline-rock aquifers. Error bars represent the 95-percent confidence 

interval of the mean (binomial distribution). See appendix 4 for the number of samples in each category.—Continued

Oxic Mixed Anoxic

M Lithium

0

20

40

60

80

100

Percentage of Li samples

greater than or equal to

1 microgram per liter

0

20

40

60

80

100

N Manganese

EXPLANATION

Percentage of Mn samples

greater than or equal to

1 microgram per liter

O Molybdenum

0

20

40

60

80

100

0

20

40

60

80

100

P Nickel

BAV, pH < 7.0

BAV, pH ≥ 7.0

CAR, pH < 7.0

CAR, pH ≥ 7.0

GLA, pH < 7.0

GLA, pH ≥ 7.0

CRL, pH < 7.0

CRL, pH ≥ 7.0

SAN, pH < 7.0

SAN, pH ≥ 7.0

SCR, pH < 7.0

SCR, pH ≥ 7.0

SCS, pH < 7.0

SCS, pH ≥ 7.0

USG, pH < 7.0

USG, pH ≥ 7.0

Percentage of Mo samples

greater than or equal to

1 microgram per liter

Percentage of Ni samples

greater than or equal to

1 microgram per liter
52 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 8. The percentage of groundwater samples with trace-element concentrations greater than or equal to the reporting level, by 

redox and pH categories, for the major aquifer groups: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, 

(G) cadmium, (H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc. Reporting level is 1 microgram per liter 

unless otherwise noted. USG, unconsolidated sand and gravel aquifers; GLA, glacial unconsolidated sand and gravel aquifers; SCS, 

semiconsolidated sand aquifers; SAN, sandstone aquifers; SCR, sandstone and carbonate-rock aquifers; CAR, carbonate-rock aquifers; 

BAV, basaltic- and other volcanic-rock aquifers; and CRL, crystalline-rock aquifers. Error bars represent the 95-percent confidence 

interval of the mean (binomial distribution). See appendix 4 for the number of samples in each category.—Continued

Oxic Mixed Anoxic

Q Radon

0

20

40

60

80

100

Percentage of Rn samples

greater than or equal to

300 picocuries per liter

0

20

40

60

80

100

R Selenium

EXPLANATION

Percentage of Se samples

greater than or equal to

1 microgram per liter

S Silver

0

20

40

60

80

100

0

20

40

60

80

100

T Strontium

BAV, pH < 7.0

BAV, pH ≥ 7.0

CAR, pH < 7.0

CAR, pH ≥ 7.0

GLA, pH < 7.0

GLA, pH ≥ 7.0

CRL, pH < 7.0

CRL, pH ≥ 7.0

SAN, pH < 7.0

SAN, pH ≥ 7.0

SCR, pH < 7.0

SCR, pH ≥ 7.0

SCS, pH < 7.0

SCS, pH ≥ 7.0

USG, pH < 7.0

USG, pH ≥ 7.0

Percentage of Ag samples

greater than or equal to

1 microgram per liter

Percentage of Sr samples

greater than or equal to

0.9 micrograms per liter
Occurrence of Trace Elements in Groundwater 53

Figure 8. The percentage of groundwater samples with trace-element concentrations greater than or equal to the reporting level, by 

redox and pH categories, for the major aquifer groups: (A) aluminum, (B) antimony, (C) arsenic, (D) barium, (E) beryllium, (F) boron, 

(G) cadmium, (H) chromium, (I) cobalt, (J) copper, (K) iron, (L) lead, (M) lithium, (N) manganese, (O) molybdenum, (P) nickel, (Q) radon, 

(R) selenium, (S) silver, (T) strontium, (U) thallium, (V) uranium, (W) vanadium, and (X) zinc. Reporting level is 1 microgram per liter 

unless otherwise noted. USG, unconsolidated sand and gravel aquifers; GLA, glacial unconsolidated sand and gravel aquifers; SCS, 

semiconsolidated sand aquifers; SAN, sandstone aquifers; SCR, sandstone and carbonate-rock aquifers; CAR, carbonate-rock aquifers; 

BAV, basaltic- and other volcanic-rock aquifers; and CRL, crystalline-rock aquifers. Error bars represent the 95-percent confidence 

interval of the mean (binomial distribution). See appendix 4 for the number of samples in each category.—Continued

Oxic Mixed Anoxic

U Thallium

0

20

40

60

80

100

Percentage of Tl samples

greater than or equal to

1 microgram per liter

0

20

40

60

80

100

V Uranium

EXPLANATION

Percentage of U samples

greater than or equal to

1 microgram per liter

W Vanadium

0

20

40

60

80

100

0

20

40

60

80

100

X Zinc

BAV, pH < 7.0

BAV, pH ≥ 7.0

CAR, pH < 7.0

CAR, pH ≥ 7.0

GLA, pH < 7.0

GLA, pH ≥ 7.0

CRL, pH < 7.0

CRL, pH ≥ 7.0

SAN, pH < 7.0

SAN, pH ≥ 7.0

SCR, pH < 7.0

SCR, pH ≥ 7.0

SCS, pH < 7.0

SCS, pH ≥ 7.0

USG, pH < 7.0

USG, pH ≥ 7.0

Percentage of V samples

greater than or equal to

1 microgram per liter

Percentage of Zn samples

greater than or equal to

1 microgram per liter
54 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Oxic Mixed Anoxic

pH < 7 pH < 7

pH < 7 pH < 7

pH ≥ 7 pH ≥ 7

pH ≥ 7 pH ≥ 7

A Aluminum

C Copper D Zinc

B Lead

0

20

40

60

80

100

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Percentage of samples greater than 

or equal to 1 microgram per liter

0

EXPLANATION

Oxic Mixed Anoxic

A Aluminum

C Copper D Zinc

B Lead

0

20

40

60

80

100

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Percentage of samples greater than 

or equal to 1 microgram per liter

0

EXPLANATION

Humid region

pH < 7 pH ≥ 7 pH < 7 pH ≥ 7

Humid region Dry region

pH < 7 pH ≥ 7 pH < 7 pH ≥ 7

Dry region

Humid region

pH < 7 pH ≥ 7 pH < 7 pH ≥ 7

Humid region Dry region

pH < 7 pH ≥ 7 pH < 7 pH ≥ 7

Dry region

Figure 9. The percentages of groundwater samples with trace-element concentrations greater than or equal to 

1 microgram per liter for (A) aluminum, (B) lead, (C) copper, and (D) zinc, by pH and redox category. Error bars represent 

the 95-percent confidence interval of the mean (binomial distribution). <, less than; ≥, greater than or equal to.

Figure 10. The percentages of groundwater samples with trace-element concentrations greater than or equal to 

1 microgram per liter for (A) aluminum, (B) lead, (C) copper, and (D) zinc in dry- and humid-region unconsolidated sand 

and gravel aquifers. Error bars represent the 95-percent confidence interval of the mean (binomial distribution). <, less 

than; ≥, greater than or equal to.
Occurrence of Trace Elements in Groundwater 55

Oxic Mixed Anoxic

pH < 7 pH < 7

pH < 7 pH < 7

pH ≥ 7 pH ≥ 7

pH ≥ 7 pH ≥ 7

A Arsenic

C Molybdenum D Selenium

B Chromium

0

20

40

60

80

100

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

Percentage of samples greater than 

or equal to 1 microgram per liter

0

EXPLANATION

Figure 11. The percentages of samples with trace-element concentrations greater than or equal to 1 microgram per 

liter for (A) arsenic, (B) chromium, (C) molybdenum, and (D) selenium, by pH and redox category. Error bars represent the 

95-percent confidence interval of the mean (binomial distribution). <, less than; ≥, greater than or equal to.
56 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Unconsolidated sand and gravel aquifers (USG)

Glacial unconsolidated sand and gravel aquifers (GLA)

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

pH < 7.0

pH ≥ 7.0

A

C D

B

Percentage of samples with arsenic

 concentrations greater than or 

equal to 1 microgram per liter

Percentage of samples with arsenic

 concentrations greater than or 

equal to 1 microgram per liter

0

20

40

60

80

100

1

10

100

Arsenic concentration, 

in micrograms per liter

1

10

100

0

20

40

60

80

100

Arsenic concentration,

 in micrograms per liter

(n = 497) (n = 497)

(n = 1,439) (n = 1,439)

Oxic

Mixed

Anoxic 

EXPLANATION

90th percentile

Number of samples

75th percentile

Median

25th percentile

10th percentile

n

Figure 12. (A) Concentrations of arsenic in glacial unconsolidated sand and gravel aquifers (GLA), (B) percentages of samples 

with arsenic concentrations greater than or equal to 1 microgram per liter in GLA aquifers, (C) concentrations of arsenic in 

unconsolidated sand and gravel aquifers (USG), and (D) percentages of samples with arsenic concentrations greater than or equal 

to 1 microgram per liter in USG aquifers. Error bars represent the 95-percent confidence intervals of the mean (binomial distribution). 
Analysis of Multiple Factors Affecting Trace-Element Occurrence 57

Relation to Groundwater Age

The occurrence of trace elements in groundwater is 

related to the length of time that the water has been in contact 

with the soil and underlying aquifer materials—that is, the 

time elapsed since recharge. In this analysis, the presence of 

tritium above the common reporting level of 2.5 pCi/L was 

considered to indicate that at least some young groundwater 

was present in the sample. Groundwater samples with tritium 

activities less than or equal to 2.5 pCi/L are interpreted as 

containing water predominantly recharged before 1953. In 

general, old groundwater tends to come from deeper wells 

with higher pH and lower DO (figs. 13A–C). These factors 

are generally associated with deep, long flow paths and long 

contact time between groundwater and aquifer materials. 

Most oxyanion-forming trace elements have greater rates 

of occurrence in old waters (fig. 14) and can serve as markers of chemical evolution. Fe and Mn also occurred more 

frequently in older waters, although their occurrence was 

greater in samples collected from humid-region aquifers. U, 

which commonly forms soluble complexes under oxic and 

variable-pH conditions, occurred more frequently in samples 

collected from older water in the dry-region aquifers, but in 

humid-region aquifers, U occurred more often in young water. 

This may be indicative of processes that favor U solubility and 

mobility, such as oxic conditions. In dry regions, ion competition, evaporative concentration, and complexation with bicarbonate and sulfate may be controlling factors (Seiler and others, 2003), whereas in humid regions, deeper, older waters are 

more often anoxic and may inhibit U mobility. Alternatively, 

in humid regions, the generally low-pH recharge may more 

readily dissolve U-bearing minerals in the aquifer matrix. 

Al and Ni behave as expected for metals soluble at low 

pH and under oxic conditions and occurred more frequently 

in samples of young groundwater, especially in the dry-region 

aquifers. Based on a small number of samples from the USG 

aquifer group, B occurrence appeared independent of groundwater age. 

Analysis of Multiple Factors Affecting 

Trace-Element Occurrence

Potential factors affecting the distribution of selected 

trace elements, most of which are known to have humanhealth implications, were investigated by using logistic 

regression. Elements were selected for logistic regression 

analyses on the basis of the rates of exceedences of HHBs and 

nonhealth guidelines; in most cases, models were developed 

for elements that occurred at concentrations greater than 

benchmarks or guidelines in at least 1 percent of the samples 

in any given aquifer group (table 10). The elements modeled 

(as a dependent variable) included Al, As, B, Fe, Mn, Mo, Ni, 

Rn, Se, Sr, and U (table 11). The number of modeled elements 

differed among aquifer groups because the occurrence of some 

elements may have been too low to model, as was the case for 

B in the SCR, CAR, and CRL aquifer groups. Independent 

variables used in this analysis included other trace elements, 

DO, nitrate, ammonia, Mn, Fe, sulfate, pH, phosphate, majorion chemistry, and well depth.

The analysis was designed to identify factors related to 

trace-element occurrence and mobility and to identify cooccurrence and nonoccurrence among selected trace elements. 

Trace-element concentrations were modeled as binary variables because a large proportion of samples for most trace 

elements had concentrations reported as less than the LRL (or 

multiple LRLs). This approach allowed the data to be used 

directly without modification. 

In general, the models corroborate the findings comparing trace-element occurrence with redox and pH. For some 

elements, only the pH or the redox variable was significant, 

possibly indicating the dominance of one control over another. 

Some variation in the number of variables also is likely among 

aquifer groups because of the number of samples collected 

from any given aquifer group. Variation in the identified 

explanatory variables also may be influenced to some degree 

by which trace elements were measured for any given sample. 

Several explanatory variables were either positive or 

negative covariates with the modeled dependent variables, 

indicating that these covariates tended to co-occur or not 

co-occur with the modeled trace element. In some cases, 

different variables representing similar processes (for 

example, low DO compared to the presence of Fe) were 

selected for a given model on the basis of model-performance 

statistics. Because of the effect of climate, separate models 

were generated for humid-region unconsolidated sand and 

gravel samples (USGH) and dry-region unconsolidated sand 

and gravel samples (USGD). The variables associated with 

occurrence for the selected trace elements are shown by 

aquifer group in table 11. The models are summarized by the 

type of independent variable, and independent variables are 

arranged by groups (anions and ion complexes, cations, and 

other); correlations are indicated as positive or negative. More 

quantitative results for these models by aquifer group are 

shown in appendix 5. 
58 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 13. The distribution of (A) dissolved oxygen, (B) well depth, and (C) pH in groundwater samples by category of age of recharge. 

“Old” refers to water samples recharged prior to 1953 (tritium activity less than or equal to 2.5 picocuries per liter), and “Young” refers 

to samples with at least some fraction of water recharged after 1953 (tritium activity greater than 2.5 picocuries per liter).

EXPLANATION

90th percentile

75th percentile

Median

25th percentile

10th percentile

Old

Young

Dissolved oxygen, in milligrams per liter

A Dissolved oxygen

0

2

4

6

8

10

Well depth, in feet

B Well depth

10

100

1,000

C pH

Old Young

Old Young

Dry Humid

pH, in standard units

6

7

8

9
Analysis of Multiple Factors Affecting Trace-Element Occurrence 59

221

334

213

316 221

319

225

340

268

485

265 469 173

282

218

331

106

610

70

479

82

492

103

700

173

979

169

929

38 161 80 484

80

505

96 49

Arsenic 

Molybdenum

Selenium

Uranium 

Iron

Manganese

Boron

Aluminum

Nickel

0

B Humid region

0

20

40

60

80

100

Percentage of samples with trace element present

A Dry region

20

40

60

80

100

EXPLANATION

Old Young

Trace element

Figure 14. Percentages of groundwater samples with trace-element concentrations greater than or equal to 1 microgram per liter (μg/L) 

(300 μg/L for iron and 50 μg/L for manganese) by category of age of recharge for samples from (A) dry and (B) humid regions. Number 

above bar is number of samples. “Old” refers to water recharged prior to 1953 (tritium activity less than or equal to 2.5 picocuries per 

liter), and “Young” refers to water with at least some fraction of water recharged after 1953 (tritium activity greater than 2.5 picocuries 

per liter). Error bars represent the 95-percent confidence interval of the mean (binomial distribution).
60 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 11. Summary of logistic regression models for selected trace elements in groundwater sampled for the NAWQA Program across 

the United States by element and major aquifer group, 1992–2003.—Continued

[USG, unconsolidated sand and gravel; GLA, glacial unconsolidated sand and gravel; SCS, semiconsolidated sand; SAN, sandstone; SCR, sandstone and carbonate rock; CAR, carbonate rock; BAV, basaltic and volcanic rock; CRL, crystalline rock; B, binary; N, continuous; +, positive covariate; -, negative covariate; 

yellow shading indicates anions and ion complexes; blue shading indicates cations; green shading indicates other correlates]

Dependent 

variable Explanatory variable Type of variable

Major aquifer group

USG 

dry

USG 

humid GLA SCS SAN SCR CAR BAV CRL

Aluminum Chromium B + + + + + +

Phosphate B + -

Arsenic B +

Bromide B -

Fluoride B -

Selenium B -

Sulfate N -

Nickel B + + + + +

Calcium N - - +

Nitrogen as ammonia B + + +

Silica N - - -

Copper B + +

Iron B - +

Manganese B + +

Lead B +

Magnesium N -

Sodium N +

Zinc B +

pH N - - - -

Dissolved oxygen N - - +

Well depth N -

Radon B +

Temperature N +

Arsenic Molybdenum B + + + + +

Uranium B + + -

Phosphate B +

Sulfate N +

Silica N + + + + +

Aluminum B +

Barium B -

Boron B +

Cobalt B +

Copper B -

Nickel B +

Potassium N +

Zinc B -

pH N + + + + +

Dissolved oxygen N - - -

Temperature N - +

Radon B +

Well depth N +
Analysis of Multiple Factors Affecting Trace-Element Occurrence 61

Table 11. Summary of logistic regression models for selected trace elements in groundwater sampled for the NAWQA Program across 

the United States by element and major aquifer group, 1992–2003.—Continued

[USG, unconsolidated sand and gravel; GLA, glacial unconsolidated sand and gravel; SCS, semiconsolidated sand; SAN, sandstone; SCR, sandstone and carbonate rock; CAR, carbonate rock; BAV, basaltic and volcanic rock; CRL, crystalline rock; B, binary; N, continuous; +, positive covariate; -, negative covariate; 

yellow shading indicates anions and ion complexes; blue shading indicates cations; green shading indicates other correlates]

Dependent 

variable Explanatory variable Type of variable

Major aquifer group

USG 

dry

USG 

humid GLA SCS SAN SCR CAR BAV CRL

Boron Molybdenum B + +

Arsenic B +

Bromide B +

Phosphate B

Sulfate N +

Uranium B

Aluminum B +

Magnesium N +

Potassium N +

Silica N

Sodium N +

Total dissolved solids N + + -

pH N + +

Dissolved oxygen N -

Temperature N +

Well depth N

Iron Nitrate B - - - -

Uranium B - - - -

Arsenic B + +

Molybdenum B - +

Chromium B -

Sulfate N +

Copper B - - -

Manganese B + + +

Nitrogen as ammonia B +

Sodium N -

Dissolved oxygen N - - - - - - -

pH N - - - - -

Well depth N + + - -

Radon B -

Total dissolved solids N +

Manganese Chromium B - -

Nitrate B - -

Arsenic B +

Sulfate N +

Uranium B +

Phosphate B +

Iron B + + + + +

Copper B - -

Zinc B + -

Aluminum B -

Barium B +
62 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 11. Summary of logistic regression models for selected trace elements in groundwater sampled for the NAWQA Program across 

the United States by element and major aquifer group, 1992–2003.—Continued

[USG, unconsolidated sand and gravel; GLA, glacial unconsolidated sand and gravel; SCS, semiconsolidated sand; SAN, sandstone; SCR, sandstone and carbonate rock; CAR, carbonate rock; BAV, basaltic and volcanic rock; CRL, crystalline rock; B, binary; N, continuous; +, positive covariate; -, negative covariate; 

yellow shading indicates anions and ion complexes; blue shading indicates cations; green shading indicates other correlates]

Dependent 

variable Explanatory variable Type of variable

Major aquifer group

USG 

dry

USG 

humid GLA SCS SAN SCR CAR BAV CRL

Manganese-Cont. Cobalt B +

Lead B -

Nickel B +

Dissolved oxygen N - - - - - - -

pH N + - - -

Well depth N - - - -

Radon B -

Total dissolved solids N +

Temperature N -

Molybdenum Arsenic B + + + + +

Fluoride B + + + +

Uranium B + + + +

Bromide B - +

Chromium B -

Nitrate B -

Selenium B +

Sulfate N +

Manganese B + + +

Silica N - -

Aluminum B +

Iron B -

Nickel B +

Zinc B +

pH N + + + + +

Dissolved oxygen N - -

Total dissolved solids N + +

Well depth N + +

Specific conductance N +

Radon B + +

Nickel Bromide B - +

Chromium B + +

Molybdenum B +

Nitrate B +

Selenium B -

Uranium B +

Aluminum B + + + +

Cobalt B + + +

Copper B + + +

Silica N + - -

Zinc B + + +

Manganese B + +
Analysis of Multiple Factors Affecting Trace-Element Occurrence 63

Table 11. Summary of logistic regression models for selected trace elements in groundwater sampled for the NAWQA Program across 

the United States by element and major aquifer group, 1992–2003.—Continued

[USG, unconsolidated sand and gravel; GLA, glacial unconsolidated sand and gravel; SCS, semiconsolidated sand; SAN, sandstone; SCR, sandstone and carbonate rock; CAR, carbonate rock; BAV, basaltic and volcanic rock; CRL, crystalline rock; B, binary; N, continuous; +, positive covariate; -, negative covariate; 

yellow shading indicates anions and ion complexes; blue shading indicates cations; green shading indicates other correlates]

Dependent 

variable Explanatory variable Type of variable

Major aquifer group

USG 

dry

USG 

humid GLA SCS SAN SCR CAR BAV CRL

Nickel-Cont. Nitrogen as ammonia B - -

Sodium N + -

Calcium N +

Iron B -

Magnesium N -

Potassium N +

pH N - - - -

Specific conductance N - + +

Total dissolved solids N + +

Temperature N +

Well depth N +

Radon Molybdenum B - + + +

Nitrate B - + + +

Sulfate N - - -

Arsenic B - -

Chromium B - -

Uranium B + +

Bromide B +

Fluoride B +

Phosphate B -

Selenium B +

Iron B - +

Cobalt B +

Lead B +

Silica N -

Manganese B -

pH N + - -

Temperature N - - -

Dissolved oxygen N + -

Specific conductance N -

Tritium B +

Well depth N -

Selenium Uranium B + + + + +

Nitrate B + + +

Arsenic B + +

Sulfate N +

Bromide B +

Fluoride B +

Molybdenum B +

Iron B - - -

Manganese B - -
64 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 11. Summary of logistic regression models for selected trace elements in groundwater sampled for the NAWQA Program across 

the United States by element and major aquifer group, 1992–2003.—Continued

[USG, unconsolidated sand and gravel; GLA, glacial unconsolidated sand and gravel; SCS, semiconsolidated sand; SAN, sandstone; SCR, sandstone and carbonate rock; CAR, carbonate rock; BAV, basaltic and volcanic rock; CRL, crystalline rock; B, binary; N, continuous; +, positive covariate; -, negative covariate; 

yellow shading indicates anions and ion complexes; blue shading indicates cations; green shading indicates other correlates]

Dependent 

variable Explanatory variable Type of variable

Major aquifer group

USG 

dry

USG 

humid GLA SCS SAN SCR CAR BAV CRL

Selenium-Cont. Aluminum B -

Sodium N +

pH N -

Total dissolved solids N + +

Well depth N + +

Specific conductance N +

Dissolved oxygen N +

Strontium Molybdenum B +

Sulfate N +

Uranium B +

Calcium N + +

Manganese B -

Sodium N +

Specific conductance N + +

Dissolved oxygen N - -

Total dissolved solids N + +

pH N -

Uranium Molybdenum B + + + + + +

Selenium B + + +

Arsenic B + +

Bicarbonate N + +

Fluoride B + +

Nitrate B + -

Phosphate B - -

Chromium B +

Sulfate N +

Nitrogen as ammonia B - - - -

Iron B - - -

Calcium N + +

Copper B + +

Nickel B + +

Silica N - -

Lead B +

Magnesium N + +

Manganese B +

Potassium N +

Total dissolved solids N + +

Dissolved oxygen N -

pH N +

Well depth N -
Analysis of Multiple Factors Affecting Trace-Element Occurrence 65

Cationic Elements

Low pH was a significant factor in the occurrences of 

many cationic metals such as Al, Fe, Mn, and Ni. In general, 

these metals, as well as Cu, Pb, and Zn, adsorb more strongly 

to aquifer materials as pH increases (McLean and Bledsoe, 

1992; Salbu and Steinnes, 1994). Because of the similarity 

in behavior, many of these metals co-occurred in the samples 

used in this study. For example, Al covaried with Ni in 

groundwater from five of nine aquifer groups. Al covariation 

with Ni was generally found in aquifer groups with greater 

proportions of samples having anoxic conditions. Nitrogen 

as ammonia (an indicator of anoxic conditions) co-occurred 

with Al in the USGH, SCS, and SAN aquifer groups, whereas 

silica was inversely related to Al in the USGD, SCS, and CRL 

aquifer groups. In general, low pH was a strong geochemical 

predictor of groundwater Al occurrence, affecting the GLA, 

SCS, BAV, and CRL aquifer-group samples.

These findings are partly corroborated by analysis of the 

most significant covariates for Al across all aquifer groups. 

The percentage of samples with Al concentrations greater 

than or equal to 1 μg/L is less than 40 from samples collected 

from old water and without concentrations of Ni or Cr greater 

than or equal to 20 and 1 μg/L, respectively (fig. 15A). This 

percentage increases with the co-occurrence of Cr and Ni, 

but relations also are strengthened by the presence of young 

water (recharged after 1953) as compared to water that was 

recharged before 1953 (fig. 15A). Al concentrations in samples 

from wells in the SCR aquifer group varied with cations such 

as Cu, Mn, Na, and Zn (table 11). Al in groundwater samples 

from the GLA, SCS, and SAN aquifer groups covaried with 

elements or compounds common in anoxic conditions, such 

as Ni, nitrogen as ammonia, Fe, and Mn. Concentrations of Al 

in groundwater samples from the USGD and USGH aquifer 

groups, however, correlated directly with low-DO conditions. 

Models for Fe occurrence from eight of the nine aquifer 

groups indicate that it was controlled primarily by low DO in 

the water samples (table 11) and by low pH (five of the nine 

groups). Only samples from the CAR aquifer group did not 

have DO, pH, or both as variables correlated with Fe. Fe and 

Mn, although often thought of as co-occurring, exhibit complex behavior that does not necessarily lead to co-occurrence. 

Models for Fe (at concentrations greater than or equal to 

300 μg/L) showed co-occurrence with Mn (at concentrations 

greater than or equal to 50 μg/L) only in samples collected 

from the USGD and SCR aquifer groups, and Fe was inversely 

related to Mn in the CRL group. By contrast, Mn varied with 

Fe in models for five of the nine aquifer groups (table 11). 

Overall, Fe occurrence was controlled by both redox (DO) 

and pH, but redox appeared to control Mn occurrence more 

than pH. The models for Fe and Mn indicate that high Fe 

predicted the presence of high Mn, but the converse was not 

as often true. Thus, Mn occurred with Fe, but Fe generally 

occurred without reduced Mn. This result is consistent with 

the thermodynamic sequence of redox progression, whereby 

Mn is reduced first, followed by Fe (Froelich and others, 1979; 

McMahon and Chapelle, 2008), and also can be seen in the 

graph of Mn occurrence (greater than or equal to 50 μg/L) by 

categories of DO, pH, and Fe (fig. 15B). 

The percentage of groundwater samples with Fe greater 

than or equal to 300 μg/L decreased if DO was greater than 

or equal to 1 mg/L, and pH was greater than or equal to 7, 

conditions under which Fe is relatively insoluble (fig. 15C). 

In addition, samples characterized as young water had less Fe 

than samples characterized as old water. Similarly, the percentage of samples with Mn decreased from about 90 to less than 

30 as pH and DO increased (fig. 15B); however, Mn occurrence at concentrations of greater than or equal to 50 μg/L 

was consistently high (more than 90 percent of samples) and 

appeared independent of pH and DO in samples with 300 μg/L 

or more of Fe (fig. 15B). Low pH and low DO, as with Fe, 

are strong indicators of high Mn. Fe also inversely covaried 

with nitrate, U, or both in the USGD, USGH, GLA, SCS, and 

CRL aquifer groups. For water samples in the SAN and CAR 

aquifer groups, Fe covaried with As. Mn covaried with As and 

U (GLA) and with Ba (CAR), Co (GLA), Ni (USGD), and Zn 

(SCS) (table 11). 

Ni occurrence covaried with several trace elements, and 

most often was related to low pH (USGD, USGH, and SCR, 

and CRL aquifer groups) or with factors associated with low 

pH, such as the presence of Al, Co, Cu, and Zn (table 11). The 

percentage of samples with Ni was consistently high (greater 

than 90 percent) when Co was present (fig. 15D). 

Oxyanion- and Ion-Complex-Forming Elements

Models for oxyanion- and ion-complex-forming elements 

also were influenced by pH and redox variables. Unlike the 

cationic metals, however, increases in pH generally result in 

increased desorption of oxyanion-forming elements from aquifer materials, thus increasing the solubility of those metals. In 

particular, As and Mo occurred most often in high-pH conditions (five of nine aquifer groups in table 11). 

Arsenic occurrence covaried with silica (USGD, USGH, 

GLA, SAN, and SCS aquifer groups), Mo (GLA, SCS, SAN, 

SCR, and CAR aquifer groups), and high pH (USGD, USGH, 

GLA, BAV, and CRL aquifer groups) (table 11). The percentage of samples with As concentrations greater than or equal to 

1 μg/L and with low Mo and silica concentrations was about 

12. This percentage increased to 40 to 45 percent if either 

Mo or silica were present and increased to about 85 percent 

if both Mo and silica were present (fig. 16A). The effect of 

pH on these relations is likely related to increased solubility 

through desorption, whereby the occurrence of As, associated 

with Mo or silica, was greater for high-pH samples than for 

low-pH samples. Arsenic co-occurred with U and phosphate 

in the USGH and GLA aquifer groups. Thus, competitive 

sorption also may play a role in the occurrence of As. Low 

DO correlated with As in the GLA, SCS, and CRL aquifer 

groups, indicating that in addition to the process of pH-driven 

desorption, redox may be an important control in some aquifer 

groups and (or) climates. 
66 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Figure 15. The percentages of groundwater samples with concentrations greater than reporting level for (A) aluminum (Al), (B) manganese 

(Mn), (C) iron (Fe), and (D) nickel (Ni), by major covariates from logistic regression models. Zero (0) indicates nonoccurrence of a covariate, 

and one (1) indicates occurrence except for age, for which zero is old (pre-1953), and one is young (post-1953) water. Number above bar 

is number of samples. Reporting level is 1 microgram per liter (μg/L) for aluminum and nickel; 300 μg/L for iron; and 50 μg/L for manganese. 

Error bars represent 95-percent confidence interval of the mean (binomial distribution). ≥, greater than or equal to; Cr, chromium; Co, cobalt.

37

19

173

161

276

336

212

640

47 259

707

1,762

103

526 767 483

291

770

1,430

361

56

91 39

107

76

142

411 73

A Aluminum

C Iron D Nickel

B Manganese

Covariates:

 Age (old or young)

 Cr ≥ 1 microgram per liter

 Ni ≥ 20 micrograms per liter

Age

Ni

Cr

Covariates:

 Fe ≥ 300 milligrams per liter

 pH ≥ 7, standard units

 Dissolved oxygen (DO) ≥ 1 milligrams per liter

Covariates:

 Age (old or young)

 pH ≥ 7, standard units

 Dissolved oxygen (DO) ≥ 1 milligram per liter

Covariates:

 Co ≥ 1 microgram per liter

 Al ≥ 1 microgram per liter

0, 0, 00, 0, 10, 1, 00, 1, 11, 0, 01, 0, 11, 1, 01, 1, 10, 0, 00, 0, 10, 1, 00, 1, 11, 0, 01, 0, 11, 1, 01, 1, 1

0, 0

0, 11, 0

1, 1

0 0

0, 0, 00, 0, 10, 1, 00, 1, 11, 0, 01, 0, 11, 1, 01, 1,

Age 1

pH

DO

Fe

pH

DO

Co

Al

Percentage with Al greater than

or equal to 1 microgram per liter

0

20

40

60

80

100

20

40

60

80

100

20

40

60

80

100

0

20

40

60

80

100

Percentage with Fe greater than 

or equal to 300 micrograms per liter

Percentage with Mn greater than 

or equal to 50 micrograms per liter

Percentage with Ni greater than 

or equal to 1 microgram per liter

EXPLANATION

EXPLANATION

EXPLANATION

EXPLANATION
Analysis of Multiple Factors Affecting Trace-Element Occurrence 67

Figure 16. The percentages of groundwater samples with concentrations greater than or equal to 1 microgram per liter for 

(A) arsenic (As), (B) molybdenum (Mo), (C) selenium (Se), and (D) uranium (U), by major covariates from logistic regression models. 

Zero (0) indicates nonoccurrence of a covariate, and one (1) indicates occurrence. Number above bar is number of samples. Error bars 

represent the 95-percent confidence interval of the mean (binomial distribution); ≥, greater than or equal to; Si, silicon; F, flouride.

911

545 636

873

1,073

514

525

825

553

158 972

1,166

1,243

143

911

543

0

20

40

60

80

100

0

20

40

60

80

100

Covariates:

 U ≥ 1 microgram per liter

 Nitrate ≥ 1 milligram per liter

Covariates:

 Mo ≥ 1 microgram per liter

 Se ≥ 1 microgram per liter

0, 00, 11, 01,

U 1

Nitrate 0, 0

0, 11, 01,

Mo 1

Se

Percentage with As greater than

or equal to 1 microgram per liter

Percentage with Se greater than

or equal to 1 microgram per liter

C Selenium D Uranium

Covariates:

 Mo ≥ 1 microgram per liter

 Si ≥ 20 milligrams per liter

Covariates:

 F ≥ 0.1 milligram per liter

 As ≥ 1 microgram per liter

0, 00, 11, 01,

Mo 1

Si 0, 0

0, 11, 01,

F 1

As

A Arsenic

0

20

40

60

80

100

B Molybdenum

Percentage with Mo greater than

or equal to 1 microgram per liter

0

20

40

60

80

100

Percentage with U greater than

or equal to 1 microgram per liter

EXPLANATION

EXPLANATION

EXPLANATION

EXPLANATION
68 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Mo in groundwater co-occurred with several trace elements, such as As, fluoride, Mn, and U. In groundwater from 

the CRL aquifer group, Mo co-occurred with fluoride, U, 

and high pH (table 11). These relations may be indicative of 

granitic lithologies, which are often enriched in these elements and can produce high pH in groundwater (Edmunds 

and Smedley, 2005). Mo was inversely related to Fe in the 

CRL aquifer group (table 11). In general, high pH and in some 

cases low DO or increasing well depth, appeared to provide a 

strong control for Mo. In samples with less than 0.1 mg/L of 

fluoride and less than 1 μg/L of As, the percentage with Mo 

greater than or equal to 1 μg/L was less than 10 (fig. 16B). 

This percentage increased to 40 to 50 percent if either fluoride 

or As was present and increased to more than 80 percent if 

both were present (fig. 16B). The effect of pH on these relations was similar to the effect on As in that the percentage of 

samples associated with As or silica was greater for high-pH 

samples than for low pH samples.

Se covaried with combinations of U, As, and nitrate (in 

the GLA, SAN, SCR, SCS, USGH, and USGD aquifer groups) 

(table 11). The percentages of samples with concentrations 

of Se greater than or equal to 1 μg/L increased with the 

occurrence of either nitrate or U (to about 13 and 20 percent, 

respectively) and to nearly 60 percent when both were present 

(fig. 16C). In the USGD aquifer group, Se covaried with 

bromide, fluoride, and sulfate in addition to U and nitrate. In 

four of nine aquifer groups, Se covaried inversely with Fe and 

(or) Mn. Se in groundwater from some aquifer groups also 

appears to be related to TDS and conductance (table 11). 

U was present in about half of all groundwater samples 

in which it was measured and covaried with more than one 

ion in most aquifer groups. U covaried with Mo in the USGD, 

USGH, SCS, SAN, SCR, and CRL aquifer groups (table 11). 

Se (in the USGD, GLA, SCR, aquifer groups) and As (in 

the USGH and CAR aquifer groups) also covaried with U, 

as did complexing ions such as bicarbonate (in the SCS and 

SCR aquifer groups) and sulfate (in the SCS aquifer group). 

Fluoride in groundwater samples from the GLA and CRL 

aquifer groups covaried with U, possibly indicating sources 

in granitic rocks. The presence of nitrogen as ammonia 

and Fe in samples, however, was always inversely related 

to U, possibly indicating sufficiently anoxic conditions to 

immobilize U. Mo and Se were the most common covarying 

trace elements in samples from this study and were strongly 

related to the overall presence of U in the samples. The 

percentages of samples with U at concentrations greater than 

or equal to 1 μg/L increased with the co-occurrence of Se (to 

nearly 60 percent) or Mo (to nearly 50 percent); however, the 

percentages of samples with U increased to about 95 if both Se 

and Mo were present (fig. 16D). 

The occurrence of trace elements such as As, Mo, Se, and 

U have been examined in detail in studies of unconsolidated 

aquifers (Deverel and Millard, 1988; Dubrovsky and others, 

1991; Fujii and Swain, 1995; Welch and Lico, 1998; Seiler 

and others, 2003). In general, these studies found that lithologic sources, redox chemistry, and evaporative concentration 

were significant processes affecting occurrence. In particular, 

some Upper Cretaceous marine sedimentary rocks in the western United States appeared to be enriched in Se, U, and other 

elements (Seiler and others, 2003). 

Other Elements

Rn covaried with some elements that are generally 

mobile under oxic conditions, including U (in the USGH and 

CRL aquifer groups), bromide (in the USGD aquifer group), 

fluoride (in the GLA aquifer group), and Se (in the SCR 

aquifer group). It also co-occurred with Mo in three aquifer 

groups. However, in the GLA group, Rn was inversely related 

to most covariates. Rn was generally related to factors indicative of recently recharged groundwater or groundwater from 

highly transmissive aquifers. These factors include low pH (as 

in the GLA and CAR aquifer groups), low water temperature 

(shallow water table), low specific conductance (short residence time), and shallow well depths. 

Boron exists predominantly as an uncharged ion in 

groundwater (Hem, 1985). In this study, B frequently occurred 

in samples from the major aquifer groups, but exceedences of 

HHBs were less common (table 11). B covaried with relatively 

few other elements, and the covariates differed according to 

aquifer group.

Co-occurrence of Trace Elements at 

Concentrations Greater than HumanHealth Benchmarks

Logistic regression analysis showed that several trace elements commonly occur together, and that some combinations 

of some trace elements are good predictors of the occurrence 

of other trace elements exceeding HHBs. Further analysis of 

the co-occurrence of trace elements that exceeded HHBs was 

done to identify samples with more than one trace-element 

exceedence of HHBs. In this way, it was possible to relate 

high concentrations of trace elements in water samples to 

potentially controlling factors and to identify which elements 

commonly co-occur. For example, in a study of domestic wells 

and multiple HHB exceedences, high TDS was shown to be 

related to the number of contaminants present in any given 

sample (DeSimone, 2009).

Recently, interest has increased in assessing contaminant 

mixtures as they relate to human health, partly to infer mechanisms of contamination and processes that control mobility. 

Characterizing all possible mixtures in a water sample is a 

difficult task; assessing the toxicity of any of these possible 

mixtures is even more challenging (Ryker and Small, 2008). 

Thus, for this analysis, which focused on trace elements, data 

on only selected trace elements were used in characterizing 

sample mixtures.
Co-occurrence of Trace Elements at Concentrations Greater than Human-Health Benchmarks 69

For this analysis, 2,714 groundwater samples with data 

on a specific set of trace elements—Sb, As, Ba, Be, Cd, Cr, 

Cu, Pb, Mn, Mo, Ni, Se, Ag, U, and Zn—were used. Radon 

was not included in the analysis because it occurs at concentrations exceeding the USEPA-proposed MCL of 300 pCi/L in 

65 percent of samples (table 4) and would therefore co-occur 

with other contaminants in the majority of samples. B, Co, 

Sr, and Tl were not included because concentrations were 

measurable in too few samples or because of the absence of 

regulation by the USEPA. In this subset of data, the rate of 

HHB exceedence by at least one or more trace elements was 

21.1 percent (572 of the 2,714 samples with at least one HHB 

exceedence). Of those, 62 samples (2.3 percent overall) had 

two or more trace elements that exceeded HHBs (table 12). 

Most of the multiple exceedences of HHBs occurred in 

the USG aquifer group. Of the 62 samples with multiple HHB 

exceedences, 50 were from the USG. The overall percentage 

of multiple HHB exceedences in the USG-aquifer samples was 

4.2 percent (50 of 1,187). Of the remaining 12 samples with 

multiple HHB exceedences, 6 were from the GLA, and 3 were 

from the CRL (table 12). 

The multiple HHB exceedences in samples from the 

USG aquifer group were dominated by co-occurrences of two 

of the following three elements: As, Mn, and U (table 13). 

Forty-two percent of the HHB exceedences consisted of the 

co-occurrence of As and Mn, 18 percent consisted of U and 

Mn, and 8 percent consisted of As and U. Mo was included in 

22 percent of the mixtures overall.

Multiple HHB exceedences in groundwater from the 

USG (42 of the 50) occurred in four areas: the northern High 

Plains, the Nevada Basin and Range, the California Central 

Valley, and the Santa Ana Basin. The sampled wells in these 

areas were predominantly in urban (17), agricultural (14), and 

mixed (11) land-use areas. Water samples from 21 of the same 

wells were classified as being mixed with respect to redox 

conditions: 13 were anoxic, and 6 were oxic, and 2 were not 

classified. Most of the elements that co-occur at concentrations 

greater than HHBs are redox sensitive and are more mobile 

under anoxic or mixed conditions; these conditions were more 

common in drinking-water wells and in shallow monitoring 

wells in urban areas than in wells in agricultural settings. 

Sr and B were measured along with the broader suite 

described above in a subset of 819 samples, primarily in water 

samples from the USG aquifer group. Concentrations of B 

and Sr at concentrations greater than HHBs in the High Plains 

aquifer commonly occur in various combinations with As, Mn, 

Mo, and U (table 14). In the USG aquifer-group samples from 

the Santa Ana Basin and the Coastal Range Province of 

southern California, As, B, Mn, Mo, and U frequently cooccur. Sr concentrations that exceed HHBs also are found with 

high concentrations of As and Mo in the GLA aquifer group 

(table 14), primarily in western Ohio and eastern Indiana.

Table 12. Number of groundwater samples collected for the NAWQA Program across the United 

States and number of multiple human-health benchmark exceedences by aquifer group, 1992–2003.

[HHB, human-health benchmark; BAV, basaltic and volcanic rock; CAR, carbonate rock; GLA, glacial unconsolidated 

sand and gravel; CRL, crystalline rock; SAN, sandstone; SCR, sandstone and carbonate rock; SCS, semiconsolidated 

sand; USG, unconsolidated sand and gravel]

Aquifer group Total number 

of samples

Samples with one or more 

HHB exceedences

Samples with two or more 

HHB exceedences

Number Percentage Number Percentage

BAV 30 0 0.0 0 0.0

CAR 161 4 2.5 0 0.0

GLA 464 101 21.8 6 1.3

CRL 188 26 13.8 3 1.6

SAN 214 49 22.9 2 0.9

SCR 173 3 1.7 0 0.0

SCS 297 45 15.2 1 0.3

USG 1,187 344 29.0 50 4.2

Total 2,714 572 21.1 62 2.3
70 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Table 13. Co-occurrence of trace elements at concentrations greater than human-health benchmarks in samples collected 

for the NAWQA Program across the United States from the unconsolidated sand and gravel aquifer group, 1992–2003.

[Total number of samples was 1,187; USG, unconsolidated sand and gravel; Mn, manganese; As, arsenic; U, uranium; Mo, molybdenum; Cd, cadmium; Se, selenium]

Number of trace 

elements exceeding 

human-health 

benchmarks

Number of samples 

with multiple 

exceedences 

(N = 50)

Percentage 

of multiple 

exceedences

Co-occurrence of trace elements in groundwater at concentrations 

above a human-health benchmark in the USG aquifer group

4 2 4 Mn As U Mo

3 2 4 Mn As U

1 2 As U Mo

1 2 Mn U Mo

2 21 42 Mn As

9 18 Mn U

4 8 As U

4 8 As Mo

3 6 U Mo

1 2 Mn Cd

1 2 As Se

1 2 U Se

Total exceedences by element 36 35 23 11 2 1

Table 14. Co-occurrence of trace elements with boron and strontium at concentrations greater than human-health 

benchmarks in groundwater samples collected for the NAWQA Program across the United States, 1992–2003.

[Total number of samples = 819; USG, unconsolidated sand and gravel; GLA, glacial unconsolidated sand and gravel; As, arsenic; B, boron; 

Mo, molybdenum; U, uranium; Mn, manganese; Sr, strontium]

Number of trace 

elements exceeding 

human-health 

benchmarks

Aquifer 

group

Number of samples 

with multiple 

exceedences 

(N = 14)

Co-occurrence of trace elements at concentrations above a humanhealth benchmarks in the USG and GLA aquifer groups

4 USG 1 As B Mo U

3 USG 1 As Mn Sr

USG 1 B Mo U

USG 1 Mo Sr U

2 USG 1 Mn Sr

USG 1 As B

GLA/USG 5 As Sr

GLA 1 Mo Sr

USG 1 B Mn

USG 1 Sr U

Total exceedences by element 8 4 3 4 10 4
Summary 71

Summary

Trace-element and Rn concentrations in groundwater 

were assessed for samples collected between 1992 and 2003 

in principal and other aquifers across the United States as 

part of the U.S. Geological Survey National Water-Quality 

Assessment Program (NAWQA). Water samples used for this 

study were collected primarily from shallow wells installed 

near the water table in particular land-use settings and from 

deeper public and domestic drinking-water wells. This study, 

the first comprehensive national assessment of NAWQA trace 

elements in groundwater, compares concentrations of trace 

elements at analytical reporting levels to assess occurrence 

and to identify geographic areas where trace elements exceed 

human-health benchmarks (HHBs) and may pose a humanhealth concern. The dataset contains trace element data from 

5,183 wells and represents more than 40 principal and other 

aquifers. Trace elements measured include aluminum (Al), 

antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), 

boron (B), cadmium (Cd), chromium (Cr), cobalt (Co), copper 

(Cu), iron (Fe), lead (Pb), lithium (Li), manganese (Mn), 

molybdenum (Mo), nickel (Ni), selenium (Se), silver (Ag), 

strontium (Sr), thallium (Tl), uranium (U), vanadium (V), and 

zinc (Zn). Radon (Rn) gas also was measured and is included 

in the data analysis.

Climate had an important influence on the occurrence and 

distribution of trace elements in groundwater. Concentrations 

of trace elements were commonly greater in the dry regions 

of the United States than in humid regions. In particular, trace 

elements that form oxyanions or ion complexes, such as As, 

Ba, B, Cr, Mo, Se, Sr, U, and V, were greater in groundwater 

in dry regions than in more humid regions. Cu, Ni, and Zn also 

were greater in dry regions. By contrast, concentrations of elements that are mobile in anoxic and (or) low-pH groundwater, 

such as Al, Co, Fe, Pb, and Mn, were greater in humid regions. 

Samples from drinking-water wells in dry regions had 

greater concentrations of As, Ba, Pb, Li, Sr, V, and Zn, than 

samples from monitoring wells. In humid regions, however, 

concentrations of most trace elements were greater in monitoring wells than in drinking-water wells; the exceptions 

were Cu, Pb, Zn, and Rn. Cu, Pb, and Zn are common trace 

elements in pumps and pipes used in the construction of 

drinking-water wells, and contamination from these sources 

may have contributed to their concentrations. Al, Sb, Ba, B, 

Cr, Co, Fe, Mn, Mo, Ni, Se, Sr, and U concentrations were all 

greater in monitoring wells than in drinking-water wells in the 

humid regions. 

Groundwater from wells in agricultural settings had 

greater concentrations of some trace elements, such as As, Mo, 

and U, than from wells in urban settings. Concentrations of 

DO and pH also were greater in groundwater from agricultural 

wells than from urban wells. Significantly greater concentrations of B, Cr, Se, Ag, Sr, and V were found in agricultural 

wells in the dry regions. Groundwater from urban wells had 

greater concentrations of Co, Fe, Mn, and specific conductance than groundwater from agricultural wells. 

Overall, groundwater in 19 percent of all wells (962 of 

5,097) exceeded an HHB for at least one trace element, including Sb, As, Ba, Be, B, Cd, Cr, Cu, Pb, Mn, Mo, Ni, Se, Ag, 

Sr, Tl, U, and Zn. HHB or nonhealth-guideline exceedences 

were common in samples collected from wells in all aquifer 

groups across the United States. Water from wells in urban 

settings had greater rates of HHB exceedence than from wells 

in agricultural settings or wells in varied land-use areas (major 

aquifer studies). As a group, monitoring wells generally had 

greater benchmark or guideline-exceedence rates than domestic or public wells. 

Mn occurred most often at concentrations greater 

than its HHB (12 percent), followed by As (7.0 percent), 

Sr (4.3 percent), U (4.0 percent), B (1.9 percent) and Mo 

(1.5 percent). Rn occurred at concentrations greater than 

the USEPA-proposed MCL of 300 pCi/L in 65 percent of 

samples, and 2.7 percent of Rn concentrations were greater 

than the USEPA-proposed AMCL of 4,000 pCi/L. In general, 

there were more exceedences of HHBs in groundwater in dry 

climates than in humid climates. 

Trace-element contamination in groundwater far outpaces 

the concentrations of most other commonly assessed contaminants, such as VOCs, pesticides, and nitrate. One or more 

trace elements exceeded HHBs in samples from 13 percent of 

public-supply wells and 13 percent of domestic-supply wells; 

for pesticides and VOCs, for example, these rates were 

2.2 percent or less. Nitrate exceeded its MCL in samples from 

3 percent of public-supply wells and 7 percent of domesticsupply wells.

The geology of aquifers also is a major factor affecting trace-element occurrence in the United States. Eight 

major aquifer groups were characterized based on the type of 

geologic material and included (1) unconsolidated sand and 

gravel; (2) glacial unconsolidated sand and gravel; (3) sandstone; (4) crystalline rock; (5) sandstone and carbonate rock; 

(6) carbonate rock; (7) semiconsolidated sand; and (8) basaltic 

and other volcanic rock. The majority of the water samples 

and the largest percentages of HHB exceedences occurred in 

groundwater from unconsolidated sand and gravel (USG) and 

the glacial unconsolidated sand and gravel (GLA) aquifers. 

As, Fe, Mn, and U were the most commonly occurring trace 

elements exceeding HHBs or nonhealth guidelines in both of 

these aquifer groups. 

Field measurements of pH and redox were related to 

trace-element occurrence and concentrations. Generally, 

anoxic and mixed redox conditions with low pH were more 

common in wells in the humid parts of the United States, 

whereas oxic, higher pH conditions prevailed in wells in the 

dry regions. Redox and pH effects were evident for As, Cr, 

Mo, and Se in all samples combined and from individual 

aquifer groups. Arsenic occurrence, for example, generally 

increased from oxic to anoxic groundwater when pH was less 

than 7; however, As occurrence was consistently high when 

pH was greater than 7, which may be related to pH-driven 

desorption. A similar pattern was seen for Mo and, to a lesser 

extent, Cr. In groundwater samples from the GLA aquifers, 
72 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

As concentrations increased as samples became increasingly 

anoxic and as pH increased. Also, the GLA aquifer group had 

the largest percentage of samples in the anoxic and mixed 

redox categories; thus redox-sensitive trace elements that are 

more mobile in anoxic conditions may occur more frequently. 

Arsenic, for example, exceeded 10 μg/L in 23 percent of 

samples of anoxic, high-pH groundwater and in 17 percent of 

samples of low-pH groundwater.

In general, older waters (mostly pre-1953) had more 

occurrences of trace elements, greater pHs, were from deeper 

wells, and had lower concentrations of dissolved oxygen 

than younger waters (defined as waters containing a fraction 

younger than 1953). Most oxyanion-forming trace elements 

occurred more frequently in old groundwater. However, 

although U occurrence was greater in older water collected from dry-region aquifers, in humid-region aquifers, U 

occurred more often in young water. This difference may be 

related to old, humid-region groundwater having lower concentrations of dissolved oxygen, a condition which can inhibit 

U mobility.

Logistic regression models were used to examine the 

relations of several trace elements to other chemical and ancillary data within selected major aquifer groups. The models 

showed that the pH and redox of groundwater were major 

controlling factors for many trace elements. In particular, low 

pH was found to be a significant factor in the occurrences of 

many cationic metals, such as Al, Fe, Mn, and Ni. In general, 

these metals, as well as Cu, Pb, and Zn, adsorb more strongly 

to aquifer materials as pH increases. The models for As occurrence were dominated by co-occurrence with silica, Mo, and 

high pH. About 12 percent of the samples with low silica or 

Mo had As concentrations greater than or equal to 1 μg/L. This 

proportion increased to about 45 percent if either Mo or silica 

was present and further to about 85 percent if both Mo and 

silica were present. The effect of increased pH on As concentrations is likely to increase solubility through desorption.

Co-occurrence of trace elements also was addressed by 

focusing on those that exceeded HHBs in any given sample. 

For this analysis, 2,714 samples that had data for Sb, As, Ba, 

Be, Cd, Cr, Cu, Pb, Mn, Mo, Ni, Se, Ag, U, and Zn were used. 

Because concentrations of Rn exceeded the USEPA-proposed 

MCL of 300 pCi/L in nearly 70 percent of samples, it would 

co-occur with most trace elements. For all selected samples 

from all aquifer groups, 21 percent (572 of 2,714 wells) had 

HHB exceedences of one or more trace elements. This was 

similar to the 19-percent (962 of 5,097 wells) rate of HHB 

exceedence for the complete dataset. Of the 2,714 wells, 

62 (2.3 percent) had two or more trace elements that 

exceeded HHBs. 

Multiple exceedences of HHBs occurred most often 

in the USG aquifer group. Of all of the samples with multiple exceedences, 50 of the 62 were from samples from 

the USG. The overall rate of multiple HHB exceedences in 

samples from the USG aquifer group was 4.2 percent and 

was dominated by combinations of As, Mn, and U. Eighty 

percent involved at least As and Mn, Mn and U, or As and U. 

Forty-two percent were only As and Mn, 18 percent were U 

and Mn, and 8 percent were As and U. Mo was involved in 

22 percent of the mixtures overall, and mostly in conjunction 

with As and U. 

B and Sr were measured in a subset of 819 samples 

(fewer samples than most trace elements) and sometimes cooccurred with other trace elements at concentrations greater 

than HHBs. These mixtures were primarily in samples from 

the USG aquifer group. 

Acknowledgments

The authors wish to thank several USGS employees who 

provided guidance to this study: Neil Dubrovsky, Dennis 

Wentz, B. Thomas Nolan, Patricia Toccalino, William Wilber, 

Michael Yurewicz, and Jeffrey Deacon. The authors also 

thank Michael Rosen, Stephen Hinkle, and Stewart Clark for 

their technical reviews; and the Pembroke Publishing Service 

Center for the following support: Christine Mendelsohn 

for illustrations, Mary Ashman for editing, and Ann Marie 

Squillacci for layout and design.

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Appendixes 1–5
80 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 1. Selected national-scale studies of trace elements in groundwater.

[As, arsenic; Cd, cadmium; F, fluoride; Hg, mercury; Pb, lead; Po, polonium; Ra, radium; U, uranium]

Trace element Geographic extent and 

design of study Hydrogeologic setting Number of wells Reference

As United States, various Interior plains, Appalachian 

highlands, Intermontane 

plateaus, Rocky Mountains, Pacific Mountains

20,000 (Welch and others, 2000)

As United States, various Various Many thousands (Focazio and others, 1999)

Various United States and Europe Various Various (Alley, 1993)

As United States Various Summary (Ryker, 2003)

Various United States, domestic 

wells

Various 2,171 (DeSimone, 2009)

226Ra, 228Ra, 210Po, 210Pb United States, various Various 99 (Focazio and others, 2001)

As, Cd, F, Pb, Hg, U, Rn United States, random and 

nonrandom

Various 3,397–15,495 (Focazio and others, 2006)

Ra, Rn, U, gross alpha United States, nonrandom Public wells Variable (Hess and others, 1985)

Ra, Rn United States Public wells Summary (Michel, 1990)

Rn United States Indoor air radon data Variable (Gundersen and others, 1992)

U United States, nonrandom Public wells 59,812 (Cothern and Lappenbusch, 1982)

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and geochemistry: Ground Water, v. 38, no. 4, p. 589-604.
Appendix 2 81

Appendix 2. Wells sampled by the NAWQA Program, 1992–2003, by study type, land use, major aquifer group, and aquifer name.

—Continued

[Aquifer number shown on figure 3. Number of wells = 5,147 of 5,183 (36 are missing well-depth data); usg, unconsolidated sand and gravel; gla, glacial 

unconsolidated sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and volcanic rock; igm, igneous and metamorphic rock]

Aquifer 

number Lithology Aquifer name Study network code

Number of 

wells

Median well 

depth

1 usg Basin and Range basin-fill aquifers cazbluscr1 9 43

1 usg Basin and Range basin-fill aquifers cazbsus1a 34 465

1 usg Basin and Range basin-fill aquifers cazbsus2 29 330

1 usg Basin and Range basin-fill aquifers cazbsus3 19 264

1 usg Basin and Range basin-fill aquifers grsllusrc1 30 69

1 usg Basin and Range basin-fill aquifers grslsus1a 26 239

1 usg Basin and Range basin-fill aquifers grslsus1b 29 240

1 usg Basin and Range basin-fill aquifers nvbrlusag1 20 19.5

1 usg Basin and Range basin-fill aquifers nvbrlusag2 10 18

1 usg Basin and Range basin-fill aquifers nvbrlusur1 28 30

1 usg Basin and Range basin-fill aquifers nvbrlusur2 27 29

1 usg Basin and Range basin-fill aquifers nvbrsus1 22 775

1 usg Basin and Range basin-fill aquifers nvbrsus2 18 394.5

1 usg Basin and Range basin-fill aquifers nvbrsus3 17 155

2 usg California Coastal Basin aquifers sanalusrc1 25 24

2 usg California Coastal Basin aquifers sanasus1 29 585

2 usg California Coastal Basin aquifers sanasus2 20 966

2 usg California Coastal Basin aquifers sanasus3 22 699.5

3 usg Central Valley aquifer system sacrluscr1 28 35

3 usg Central Valley aquifer system sacrlusrc1 19 63

3 usg Central Valley aquifer system sacrsus1 31 150

3 usg Central Valley aquifer system sanjluscr1 18 145

3 usg Central Valley aquifer system sanjlusor1 20 170

3 usg Central Valley aquifer system sanjlusor2 20 147.5

3 usg Central Valley aquifer system sanjsus1 30 182

4 usg Columbia Plateau basin-fill aquifers ccptlusag1 8 31

4 usg Columbia Plateau basin-fill aquifers ccptlusag2 32 69.5

4 usg Columbia Plateau basin-fill aquifers ccptlusor1 31 48

4 usg Columbia Plateau basin-fill aquifers ccptsus1b 13 90

5 usg High Plains aquifer cnbrsus2 27 47

5 usg High Plains aquifer hpgwlusag1 30 160

5 usg High Plains aquifer hpgwlusag2 29 170

5 usg High Plains aquifer hpgwlusur1 30 27.5

5 usg High Plains aquifer hpgwsus1a 74 262.5

5 usg High Plains aquifer hpgwsus1b 48 201

5 usg High Plains aquifer hpgwsus1c 104 207.5

5 usg High Plains aquifer hpgwsus2 20 73.5

5 usg High Plains aquifer hpgwsus4 27 190

6 usg Mississippi River Valley alluvial aquifer misesus1 29 110

6 usg Mississippi River Valley alluvial aquifer misesus3 24 102.5
82 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 2. Wells sampled by the NAWQA Program, 1992–2003, by study type, land use, major aquifer group, and aquifer name.

—Continued

[Aquifer number shown on figure 3. Number of wells = 5,147 of 5,183 (36 are missing well-depth data); usg, unconsolidated sand and gravel; gla, glacial 

unconsolidated sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and volcanic rock; igm, igneous and metamorphic rock]

Aquifer 

number Lithology Aquifer name Study network code

Number of 

wells

Median well 

depth

7 usg Northern Rocky Mountains Intermontane Basins aquifer nroksus1 31 137

7 usg Northern Rocky Mountains Intermontane Basins aquifer nroksus2 30 60

8 usg Rio Grande aquifer system rioglusag1 30 20

8 usg Rio Grande aquifer system riogluscr1 35 20

8 usg Rio Grande aquifer system rioglusur1 24 27

8 usg Rio Grande aquifer system riogsus1 30 169

9 usg Snake River Plain basin-fill aquifers usnkluscr1 27 34

9 usg Snake River Plain basin-fill aquifers usnkluscr3 1 180

9 usg Snake River Plain basin-fill aquifers usnksus1 9 67

9 usg Snake River Plain basin-fill aquifers usnksus2 23 150

9 usg Snake River Plain basin-fill aquifers usnksus3 10 119

10 usg Surficial aquifer system acfblusag1 16 41.5

10 usg Surficial aquifer system acfblusfo1 4 57.5

10 usg Surficial aquifer system gaflluscr1 23 27

10 usg Surficial aquifer system gafllusur3 16 23.5

10 usg Surficial aquifer system gaflsus1 32 27.5

10 usg Surficial aquifer system miselusrc1 24 48.5

10 usg Surficial aquifer system santluscr1 30 18

10 usg Surficial aquifer system sofllusfo1 5 10

10 usg Surficial aquifer system sofllusor1 31 13

11 usg Willamette Lowland basin-fill aquifers willsus1 70 60

12 gla Glacial aquifer system almnsus2 30 73

12 gla Glacial aquifer system connlusag1 40 21

12 gla Glacial aquifer system connlusfo1 40 17

12 gla Glacial aquifer system connlusur1 40 22

12 gla Glacial aquifer system delrsus3 16 114

12 gla Glacial aquifer system eiwaluscr1 31 18

12 gla Glacial aquifer system eiwalusrc1 30 23

12 gla Glacial aquifer system eiwasus2 32 52

12 gla Glacial aquifer system hdsnlusag1 14 19.5

12 gla Glacial aquifer system hdsnlusur1 27 29

12 gla Glacial aquifer system hdsnsus1 2 108

12 gla Glacial aquifer system leriluscr1 30 18

12 gla Glacial aquifer system lerilusrc1 30 25.5

12 gla Glacial aquifer system lerisus1 27 90

12 gla Glacial aquifer system linjsus1 1 152

12 gla Glacial aquifer system lirbluscr1 28 27.5

12 gla Glacial aquifer system lirbluscr2 29 20

12 gla Glacial aquifer system lirbsus1 30 245

12 gla Glacial aquifer system lirbsus2 30 50

12 gla Glacial aquifer system miamluscr1 26 25.5
Appendix 2 83

Appendix 2. Wells sampled by the NAWQA Program, 1992–2003, by study type, land use, major aquifer group, and aquifer name.

—Continued

[Aquifer number shown on figure 3. Number of wells = 5,147 of 5,183 (36 are missing well-depth data); usg, unconsolidated sand and gravel; gla, glacial 

unconsolidated sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and volcanic rock; igm, igneous and metamorphic rock]

Aquifer 

number Lithology Aquifer name Study network code

Number of 

wells

Median well 

depth

12 gla Glacial aquifer system miamlusrc1 25 39

12 gla Glacial aquifer system miamsus1 30 60

12 gla Glacial aquifer system necblusrc1 29 23

12 gla Glacial aquifer system necbsus3 30 56

12 gla Glacial aquifer system pugtluscr1 22 28.5

12 gla Glacial aquifer system pugtlusrs1 27 47

12 gla Glacial aquifer system pugtsus1 30 66.5

12 gla Glacial aquifer system rednlusag1 26 25.5

12 gla Glacial aquifer system rednlusag2 20 15

12 gla Glacial aquifer system rednsus1 27 40

12 gla Glacial aquifer system rednsus2 25 58

12 gla Glacial aquifer system rednsus3 15 37

12 gla Glacial aquifer system uirbluscr1 29 14

12 gla Glacial aquifer system uirblusrc1 26 29

12 gla Glacial aquifer system uirbsus1 27 72

12 gla Glacial aquifer system uirbsus2 23 45

12 gla Glacial aquifer system umisluscr1 29 25

12 gla Glacial aquifer system umislusfo1 15 19

12 gla Glacial aquifer system umislusrc1 30 19

12 gla Glacial aquifer system whitluscr1 23 28

12 gla Glacial aquifer system whitluscr2 20 22.5

12 gla Glacial aquifer system whitluscr3 24 20.5

12 gla Glacial aquifer system whitlusur1 25 29

12 gla Glacial aquifer system wmiclusag1 26 30

12 gla Glacial aquifer system wmiclusag2 30 42

13 gla Unconsolidated-deposit aquifers (Alaska) cooksus1a 29 90

14 scs Coastal Lowlands aquifer system acadluscr1 27 26

14 scs Coastal Lowlands aquifer system acadlusrc1 28 58.5

14 scs Coastal Lowlands aquifer system acadsus1 30 140

14 scs Coastal Lowlands aquifer system acadsus2 30 115

14 scs Coastal Lowlands aquifer system trinsus3 23 138

15 scs Mississippi Embayment aquifer system miselusrc2 8 98

15 scs Mississippi Embayment aquifer system misesus2 30 675

16 scs North Atlantic Coastal Plain aquifer system albelusag1 17 12

16 scs North Atlantic Coastal Plain aquifer system albelusur1 14 62.5

16 scs North Atlantic Coastal Plain aquifer system albesus1 11 20

16 scs North Atlantic Coastal Plain aquifer system albesus2 16 21.5

16 scs North Atlantic Coastal Plain aquifer system dlmvluscr1 28 21.5

16 scs North Atlantic Coastal Plain aquifer system dlmvsus1 27 45

16 scs North Atlantic Coastal Plain aquifer system linjluscr1 15 31

16 scs North Atlantic Coastal Plain aquifer system linjlusfo1 13 19

16 scs North Atlantic Coastal Plain aquifer system linjlusrc1 30 24

16 scs North Atlantic Coastal Plain aquifer system linjlusur1 20 24

16 scs North Atlantic Coastal Plain aquifer system linjsus2 30 100
84 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 2. Wells sampled by the NAWQA Program, 1992–2003, by study type, land use, major aquifer group, and aquifer name.

—Continued

[Aquifer number shown on figure 3. Number of wells = 5,147 of 5,183 (36 are missing well-depth data); usg, unconsolidated sand and gravel; gla, glacial 

unconsolidated sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and volcanic rock; igm, igneous and metamorphic rock]

Aquifer 

number Lithology Aquifer name Study network code

Number of 

wells

Median well 

depth

17 scs Southeastern Coastal Plain aquifer system moblluscr1 29 35

17 scs Southeastern Coastal Plain aquifer system mobllusrc1 30 41.5

17 scs Southeastern Coastal Plain aquifer system moblsus1 31 165

17 scs Southeastern Coastal Plain aquifer system santlusrc1 30 19.5

17 scs Southeastern Coastal Plain aquifer system santsus1 30 245

18 scs Texas coastal uplands aquifer system trinsus2 23 160

19 san Cambrian-Ordovician aquifer system umissus3 25 180

19 san Cambrian-Ordovician aquifer system umissus4 25 200

19 san Cambrian-Ordovician aquifer system wmicsus1 29 170

20 san Early Mesozoic basin aquifers delrsus1 30 160

20 san Early Mesozoic basin aquifers linjsus3 17 200

20 san Early Mesozoic basin aquifers potosus2 20 148.5

21 san Lower Tertiary aquifers yellsus2 29 160

22 san Pennsylvanian aquifers almnlusmi1 30 83.5

22 san Pennsylvanian aquifers almnsus1 30 105

22 san Pennsylvanian aquifers kanalusmi1 28 76.5

22 san Pennsylvanian aquifers kanasus1 20 77.5

22 san Pennsylvanian aquifers whitluscr2 2 23

23 san Valley and Ridge aquifers (sandstone) delrsus2 30 192.5

23 san Valley and Ridge aquifers (sandstone) lsussus1 29 155

23 san Valley and Ridge aquifers (sandstone) potolusag2 23 122

23 san Valley and Ridge aquifers (sandstone) potolusfo1 3 160

23 san Valley and Ridge aquifers (sandstone) utenluscr1 11 18

24 scr Edwards-Trinity aquifer system sctxlusrc1 30 261

24 scr Edwards-Trinity aquifer system sctxsus1 19 335

24 scr Edwards-Trinity aquifer system sctxsus2 25 420

24 scr Edwards-Trinity aquifer system sctxsus3 30 1,268

24 scr Edwards-Trinity aquifer system trinsus1 24 155

25 scr Mississippian aquifers kanasus1 10 191

25 scr Mississippian aquifers lerisus1 1 120

25 scr Mississippian aquifers ltenlusag1 32 37.5

25 scr Mississippian aquifers ltensus1 32 87

25 scr Mississippian aquifers ltensus2 4 99

26 car Biscayne aquifer sofllusrc1 30 15

26 car Biscayne aquifer soflsus1 30 99
Appendix 2 85

Appendix 2. Wells sampled by the NAWQA Program, 1992–2003, by study type, land use, major aquifer group, and aquifer name.

—Continued

[Aquifer number shown on figure 3. Number of wells = 5,147 of 5,183 (36 are missing well-depth data); usg, unconsolidated sand and gravel; gla, glacial 

unconsolidated sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and volcanic rock; igm, igneous and metamorphic rock]

Aquifer 

number Lithology Aquifer name Study network code

Number of 

wells

Median well 

depth

27 car Floridan aquifer system acfblusag1 3 44

27 car Floridan aquifer system acfbsus1 26 150

27 car Floridan aquifer system gaflsus1 4 69

27 car Floridan aquifer system santsus2 30 175

28 car Ordovician aquifers ltensus2 27 140

29 car Ozark Plateaus aquifer system ozrklusag1 20 167.5

29 car Ozark Plateaus aquifer system ozrklusag2 20 180

29 car Ozark Plateaus aquifer system ozrksus1 20 1,308.5

29 car Ozark Plateaus aquifer system ozrksus2a 30 196

29 car Ozark Plateaus aquifer system ozrksus3a 13 140

30 car Piedmont and Blue Ridge carbonate-rock aquifers lsuslusag1 30 160

31 car Silurian-Devonian aquifers eiwasus1 33 225

32 car Valley and Ridge aquifers (carbonate) kanasus2 1 325

32 car Valley and Ridge aquifers (carbonate) lsuslusag2 30 172.5

32 car Valley and Ridge aquifers (carbonate) lsuslusag3 30 159

32 car Valley and Ridge aquifers (carbonate) lsuslusur1 20 108

32 car Valley and Ridge aquifers (carbonate) potolusag1 29 144

32 car Valley and Ridge aquifers (carbonate) potolusag2 1 280

32 car Valley and Ridge aquifers (carbonate) utenluscr1 19 16

32 car Valley and Ridge aquifers (carbonate) utensus1 30 227.5

33 bav Columbia Plateau basaltic-rock aquifers ccptlusag1 18 101.5

33 bav Columbia Plateau basaltic-rock aquifers ccptlusag2 17 157

33 bav Columbia Plateau basaltic-rock aquifers ccptlusor1 9 170

33 bav Columbia Plateau basaltic-rock aquifers ccptsus1b 30 339

34 bav Hawaiian volcanic-rock aquifers oahusus1 30 512

35 bav Snake River Plain basaltic-rock aquifers usnkluscr2 29 225

35 bav Snake River Plain basaltic-rock aquifers usnkluscr3 28 202.5

35 bav Snake River Plain basaltic-rock aquifers usnkluscr4 15 360

35 bav Snake River Plain basaltic-rock aquifers usnksus1 34 315

35 bav Snake River Plain basaltic-rock aquifers usnksus2 13 232

36 bav Early Mesozoic basin aquifers (basalt and volcanic) linjsus3 4 167.5

36 bav Early Mesozoic basin aquifers (basalt and volcanic) potosus2 3 160

37 igm New England crystalline-rock aquifers (unofficial name) connsus1 30 235

37 igm New England crystalline-rock aquifers (unofficial name) linjsus1 29 202

37 igm New England crystalline-rock aquifers (unofficial name) necbsus1 28 187.5

37 igm New England crystalline-rock aquifers (unofficial name) necbsus2 30 303
86 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 2. Wells sampled by the NAWQA Program, 1992–2003, by study type, land use, major aquifer group, and aquifer name.

—Continued

[Aquifer number shown on figure 3. Number of wells = 5,147 of 5,183 (36 are missing well-depth data); usg, unconsolidated sand and gravel; gla, glacial 

unconsolidated sand and gravel; scs, semiconsolidated sand; san, sandstone; scr, sandstone and carbonate rock; car, carbonate rock; bav, basaltic and volcanic rock; igm, igneous and metamorphic rock]

Aquifer 

number Lithology Aquifer name Study network code

Number of 

wells

Median well 

depth

38 igm Piedmont and Blue Ridge crystalline-rock aquifers acfblusur1 21 31

38 igm Piedmont and Blue Ridge crystalline-rock aquifers kanasus2 29 205

38 igm Piedmont and Blue Ridge crystalline-rock aquifers lsussus2 30 146.5

38 igm Piedmont and Blue Ridge crystalline-rock aquifers potosus1 25 142

38 igm Piedmont and Blue Ridge crystalline-rock aquifers santsus3 30 200

99 igm Rocky Mountain Front Range (unofficial name) spltsus1 27 225

99 san Woodbine aquifer (not a principal aquifer) trinlusur1 20 25.5

99 usg Stream-valley aquifers–Bighorn, Wind, Powder River basins 

(unofficial name)

yelllusot1 29 15

99 usg Stream-valley aquifers–Bighorn, Wind, Powder River basins 

(unofficial name)

yellsus1 25 19

99 usg Stream-valley aquifers–South Platte River (unofficial name) spltluscr1 30 23

99 usg Stream-valley aquifers–South Platte River (unofficial name) spltlusrc1 30 27.5

99 usg Stream-valley aquifers–Valley and Ridge (unofficial name) potolusag2 1 71

99 usg Stream-valley aquifers–Upper Colorado River (unofficial name) ucollusrc1 25 24

99 usg Stream-valley aquifers–Upper Colorado River (unofficial name) ucolsus1 29 60

99 usg Stream-valley aquifers–Upper Colorado River (unofficial name) ucolsus2 13 49
Appendix 3 87

Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA Program, 1992–2003, by major aquifer group.[MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water]

Element

Number of 

samples

Percentage 

of samples 

with censored values

Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline

Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type

Percentage 

exceeding 

benchmark

Guideline 

value 

(SMCL)

Percentage 

exceeding 

SMCL

Unconsolidated sand and gravel aquifers

Aluminum 1,226 28.4 1,100 180 13 7.4 4.2 3.0 0.90 0.43 -- -- -- 50 or 200 2.2, 1.0Antimony 1,318 85.5 5.0 1.0 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL 0 -- --Arsenic 1,507 25.2 550 64 19 11 5.5 2.0 0.80 0.31 10 MCL 12 -- --Barium 1,376 0.20 5,100 620 350 220 130 71 36 20 2,000 MCL 0.1 -- --Beryllium 1,318 99.4 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL 0 -- --Boron 552 5.3 3,400 2,000 750 400 150 63 29 14 1,000 HBSL 3.4 -- --Cadmium 1,351 90.9 16 1.0 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 0.4 -- --Chromium 1,367 32.1 150 18 8.0 6.0 3.4 1.5 0.74 0.44 100 MCL 0.1 -- --Cobalt 1,326 71.1 18 4.4 1.5 0.77 0.33 0.14 0.065 0.033 -- -- -- -- --Copper 1,372 34.6 2,000 33 8.0 5.0 2.3 1.0 0.51 0.27 1,300 AL 0.1 1,000 0.1Iron 1,769 50.4 50,000 11,000 2,500 490 22 5.0 0.31 0.036 -- -- -- 300 12Lead 1,355 79.1 9.0 3.0 2.0 < 1 < 1 < 1 < 1 < 1 15 AL 0 -- --Lithium 375 0.05 580 280 150 110 42 17 7.1 1.7 -- -- -- -- --Manganese 1,700 39.6 9,800 1,900 740 380 43 1.9 0.17 0.025 300 HBSL 12 50 24Molybdenum 1,368 22.0 1,500 59 20 12 5.8 2.4 0.92 0.38 40 HBSL 2.4 -- --Nickel 1,369 37.9 58 18 7.0 4.9 2.7 1.0 0.34 0.16 100 HBSL 0 -- --Radon 1,481 0.07 40,000 4,000 1,800 1,300 830 510 310 230 300 or 4,000 MCL1/AMCL2 77, 1.1 -- --Selenium 1,395 56.3 94 35 11 6.2 2.3 0.72 0.23 0.086 50 MCL 0.30 -- --Silver 1,297 99.0 8.0 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL 0.00 100 0Strontium 483 0 26,000 9,200 4,100 2,400 1,000 530 290 130 4,000 HBSL 5.2 -- --Thallium 366 86.9 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --Uranium 1,409 22.0 550 140 47 26 9.4 3.4 0.78 0.21 30 MCL 8.7 -- --Vanadium 370 4.3 190 120 50 39 22 10 2.1 0.55 -- -- -- -- --Zinc 1,335 16.3 3,300 290 100 60 17 4.4 1.7 0.49 2,000 HBSL 0.1 5,000 0
88 Trace Elements and Radon in Groundwater Across the United States, 1992–2003 Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA program, 1992–2003, by major aquifer group.—Continued [MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water] Element Num- ber of samples Percentage of samples with cen- sored values Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type Percent- age exceeding benchmark Guideline value (SMCL) Percent- age exceeding SMCL Glacial unconsolidated sand and gravel aquifers Aluminum 485 27.8 820 200 31 14 5.0 3.0 0.99 0.41 -- -- -- 50 or 200 3.1, 1.0 Antimony 498 89.2 3.30 1.36 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL 0 -- -- Arsenic 512 55.3 340 42 17 6.52 1.47 0.41 0.12 0.030 10 MCL 6.8 -- -- Barium 513 0 1,100 660 290 210 110 65 29 11 2,000 MCL 0 -- -- Beryllium 511 97.7 1.24 < 1 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL 0 -- -- Boron 274 5.47 2,600 1,100 160 78 42 27 16 9.5 1,000 HBSL 1.1 -- -- Cadmium 511 93.9 1.00 < 1 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 0 -- -- Chromium 491 48.9 17 11 7.0 5.5 3.3 1.1 0.59 0.37 100 MCL 0 -- -- Cobalt 501 64.7 95 7.2 3.0 1.8 0.58 0.20 0.076 0.031 -- -- -- -- -- Copper 501 46.3 130 53 14 8.7 2.4 0.94 0.38 0.15 1,300 AL 0 1,000 0 Iron 1,190 28.1 38,000 12,000 3,800 2,100 620 17 4.0 0.47 -- -- -- 300 29.3 Lead 498 84.5 9.0 4.0 < 1 < 1 < 1 < 1 < 1 < 1 15 AL 0 -- --

Lithium 258 17.4 37 25 14 8.2 4.4 2.9 1.4 0.57 -- -- -- -- --

Manganese 1,148 11.8 28,000 2,200 1,000 550 230 49 5.2 1.2 300 HBSL 19.8 50 49.7Molybdenum 499 34.1 110 60 15 7.7 3.0 1.4 0.38 0.15 40 HBSL 2 -- --Nickel 502 24.9 52 17 9.0 5.5 3.0 1.6 0.59 0.26 100 HBSL 0 -- --Radon 763 0.79 3,700 2,200 1,200 840 520 320 200 150 300 or 4,000 MCL1/AMCL2 53.7, 0 -- --

Selenium 511 83.5 56 4.0 1.5 < 1 < 1 < 1 < 1 < 1 50 MCL 0.2 -- --

Silver 507 99.8 1.0 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL 0 100 0Strontium 259 0.39 9,100 6,700 1,200 580 270 130 84 43 4,000 HBSL 1.2 -- --Thallium 202 98.1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --

Uranium 640 53.8 140 16 5.8 3.3 1.2 0.36 0.088 0.025 30 MCL 0.5 -- --Vanadium 174 53.5 16 13 3.2 2.1 0.94 0.29 0.14 0.069 -- -- -- -- --Zinc 494 20.0 380 270 62 30 9.0 3.0 1.2 0.35 2,000 HBSL 0 5,000 0
Appendix 3 89

Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA program, 1992–2003, by major aquifer group.—Continued[MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water]

Element

Number of 

samples

Percentage 

of samples 

with censored values

Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline

Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type

Percentage 

exceeding 

benchmark

Guideline 

value 

(SMCL)

Percentage 

exceeding 

SMCL

Semiconsolidated sand aquifers

Aluminum 301 29.9 710 440 103 49 12 3.0 0.64 0.17 -- -- -- 50 or 200 10.2, 3.0Antimony 338 83.1 2.0 < 1 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL 0 -- --Arsenic 373 69.4 55 8.0 3.0 1.9 0.68 0.20 0.069 0.026 10 MCL 0.8 -- --Barium 343 0 2,000 820 390 280 140 68 25 9.7 2,000 MCL 0.3 -- --Beryllium 338 82.8 16 1.8 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL 0.3 -- --Boron 181 13.3 250 230 67 54 33 18 11 7.6 1,000 HBSL 0 -- --Cadmium 338 78.4 1.3 < 1 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 0 -- --Chromium 338 50 17 13 8.0 6.0 2.0 0.94 0.51 0.30 100 MCL 0 -- --Cobalt 339 48.1 210 31 6.9 4.1 1.1 0.28 0.079 0.028 -- -- -- --Copper 339 33.3 280 170 34 19 6.8 1.2 0.38 0.15 1,300 AL 0 1,000 0Iron 570 23.5 81,000 23,000 12,000 4,900 570 22 7.0 1.7 -- -- -- 300 29.2Lead 339 62.2 480 28 5.1 3.0 < 1 < 1 < 1 < 1 15 AL 1.2 -- --Lithium 155 2.6 62 56 29 20 13 4.0 1.0 0.35 -- -- -- -- --Manganese 564 5.3 7,200 1,100 450 300 120 37 8.6 1.6 300 HBSL 9.8 50 44Molybdenum 338 73 8.9 7.3 3.4 2.0 < 1 < 1 < 1 < 1 40 HBSL 0 -- --Nickel 341 36.7 160 39 11 5.3 2.6 1.0 0.27 0.091 100 HBSL 0.3 -- --Radon 243 0.8 2,200 1,500 1,000 780 430 260 150 87 300 or 4,000 MCL1/AMCL2 39.1, 0 -- --Selenium 342 75.4 35 9.7 2.1 1.3 < 1 < 1 < 1 < 1 50 MCL -- -- --Silver 338 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL 0 100 0Strontium 155 0 890 610 400 290 190 120 44 13 4,000 HBSL 0 -- --Thallium 154 87.7 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --Uranium 339 69.3 16 9.6 3.2 1.0 < 1 < 1 < 1 < 1 30 MCL 0 -- --Vanadium 154 41.6 8.1 7.3 4.8 3.9 2.0 0.33 0.15 0.062 -- -- -- -- --Zinc 308 15.6 1,200 750 270 110 23 5.1 1.5 0.37 2,000 HBSL 0 5,000 0
90 Trace Elements and Radon in Groundwater Across the United States, 1992–2003 Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA program, 1992–2003, by major aquifer group.—Continued [MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water] Element Num- ber of samples Percentage of samples with cen- sored values Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type Percent- age exceeding benchmark Guideline value (SMCL) Percent- age exceeding SMCL Sandstone aquifers Aluminum 229 22.7 970 630 25 7.0 4.0 3.0 1.3 0.59 -- -- -- 50 or 200 3.5, 1.7 Antimony 256 98.1 6.3 2.0 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL 0.4 -- -- Arsenic 282 67.7 57 17 5.4 3.9 1.1 0.58 0.23 0.10 10 MCL 1.4 -- -- Barium 283 1.4 2,700 1,800 530 350 190 63 21 6.8 2,000 MCL 0.4 -- -- Beryllium 258 96.9 18 4.0 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL 0.8 -- -- Boron 86 25.6 630 630 300 270 140 30 12 5.1 1,000 HBSL 0 -- -- Cadmium 274 97.1 9.4 6.0 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 1.1 -- -- Chromium 275 43.6 13 7.0 6.0 5.0 3.0 1.1 0.66 0.41 100 MCL 0 -- -- Cobalt 261 78.5 680 270 14 4.0 < 1 < 1 < 1 < 1 -- -- -- -- -- Copper 280 42.5 140 85 26 18 5.7 1.4 0.39 0.13 1,300 AL 0 1,000 0 Iron 389 30.6 38,000 24,000 7,700 3,700 740 30 4.0 0.53 -- -- -- 300 31.9 Lead 277 85.6 19 5.6 2.0 1.2 < 1 < 1 < 1 < 1 15 AL 0.4 -- --

Lithium 59 1.7 260 260 110 66 32 11 3.7 2.0 -- -- -- -- --Manganese 376 23.4 5,400 2,400 990 620 190 33 2.0 0.73 300 HBSL 17 50 44.7Molybdenum 257 67.7 4,700 40 7.7 4.0 1.0 0.42 0.12 0.039 40 HBSL 0.8 -- --Nickel 262 37 670 340 33 8.0 3.0 1.3 0.25 0.072 100 HBSL 2.3 -- --Radon 358 3.6 6,000 5,000 3,300 2,400 1,300 530 200 82 300 or 4,000 MCL1/AMCL2 64.5, 2.8 -- --Selenium 282 88.3 92 47 2.4 < 1 < 1 < 1 < 1 < 1 50 MCL 0.4 -- --Silver 264 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL 0 100 0Strontium 59 0 6,400 6,400 3,800 1,900 690 190 66 27 4,000 HBSL 3.4 -- --Thallium 57 94.7 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --Uranium 336 70.5 35 17 5.3 3.9 1.2 0.51 0.19 0.074 30 MCL 0.3 -- --Vanadium 57 75.4 12 12 7.4 3.4 < 1 < 1 < 1 < 1 -- -- -- -- --Zinc 238 9.2 870 580 260 140 42 8.1 2.0 1.0 2,000 HBSL 0 5,000 0
Appendix 3 91

Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA program, 1992–2003, by major aquifer group.—Continued[MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water]

Element

Number of 

samples

Percentage 

of samples 

with censored values

Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline

Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type

Percentage 

exceeding 

benchmark

Guideline 

value 

(SMCL)

Percentage 

exceeding 

SMCL

Sandstone and carbonate-rock aquifers

Aluminum 166 24.7 240 46 5.0 4.6 3.8 3.0 1.6 1.1 -- -- -- 50 or 200 0.6, 0.6Antimony 186 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL 0 -- --Arsenic 189 88.9 16 13 1.3 1.0 < 1 < 1 < 1 < 1 10 MCL 1.1 -- --Barium 189 0 990 600 200 120 56 37 29 14 2,000 MCL 0 -- --Beryllium 186 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL 0 -- --Boron 0 -- -- -- -- -- -- -- -- -- 1,000 HBSL -- -- --Cadmium 186 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 0 -- --Chromium 188 23.9 8.0 7.0 5.3 4.8 3.6 2.2 1.2 0.86 100 MCL 0 -- --Cobalt 186 96.8 11 2.6 < 1 < 1 < 1 < 1 < 1 < 1 -- -- 0 -- --Copper 189 26.5 57 41 11 8.0 4.9 2.0 0.98 0.49 1,300 AL 0 1,000 0Iron 222 59 5,800 2,300 410 120 11 2.4 0.25 0.038 -- -- -- 300 7.7Lead 187 72.7 9.0 6.8 4.0 2.7 1.0 0.44 0.18 0.082 15 AL 0 -- --Lithium 0 -- -- -- -- -- -- -- -- -- -- -- -- -- --Manganese 219 54.8 5,100 1,400 180 83 6.6 0.73 0.071 0.009 300 HBSL 3.2 50 13.2Molybdenum 188 72.3 55 31 6.0 3.1 1.0 0.26 0.070 0.021 40 HBSL 0.5 -- --Nickel 189 29.1 42 20 7.4 4.0 2.5 1.4 0.69 0.37 100 HBSL 0 -- --Radon 136 13.8 2,600 2,000 1,300 870 510 260 120 63 300 or 4,000 MCL1/AMCL2 44.1, 0.0 -- --Selenium 189 84.7 14 11 2.0 1.2 < 1 < 1 < 1 < 1 50 MCL 0 -- --Silver 184 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL -- 100 0Strontium 98 1 44,000 44,000 15,000 12,000 4,500 450 170 94 4,000 HBSL 25.5 -- --Thallium 0 -- -- -- -- -- -- -- -- -- 2 MCL -- -- --Uranium 187 84 93 13 3.2 1.4 < 1 < 1 < 1 < 1 30 MCL 0.5 -- --Vanadium 0 -- -- -- -- -- -- -- -- -- -- -- -- -- --Zinc 178 12.4 1,400 1,000 380 270 48 13 2.0 0.65 2,000 HBSL 0 5,000 0
92 Trace Elements and Radon in Groundwater Across the United States, 1992–2003 Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA program, 1992–2003, by major aquifer group.—Continued [MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water] Element Num- ber of samples Percentage of samples with cen- sored values Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type Percent- age exceeding benchmark Guideline value (SMCL) Percent- age exceeding SMCL Carbonate-rock aquifers Aluminum 195 9.2 130 54 15 12 6.0 4.2 2.6 1.0 -- -- -- 50 or 200 1, 0 Antimony 189 0.5 5.0 < 1 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL 0 -- -- Arsenic 229 81.7 19 12 3.0 2.0 < 1 < 1 < 1 < 1 10 MCL 1.3 -- -- Barium 189 7.9 540 260 120 69 33 19 10 2.9 2,000 MCL 0 -- -- Beryllium 189 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL -- -- -- Boron 0 -- -- -- -- -- -- -- -- -- 1,000 HBSL -- -- -- Cadmium 189 98.9 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 0 -- -- Chromium 190 31.6 9.0 7.7 6.0 5.0 4.0 2.0 0.84 0.54 100 MCL 0 -- -- Cobalt 189 88.4 5.0 1.6 < 1 < 1 < 1 < 1 < 1 < 1 -- -- 0 -- -- Copper 189 45.0 26 23 7.0 5.2 3.0 1.1 0.59 0.32 1,300 AL 0 1,000 0 Iron 480 44.2 13,000 4,900 1,800 1,000 63 5.0 0.48 0.055 -- -- -- 300 19 Lead 189 90.0 11 5.0 2.0 < 1 < 1 < 1 < 1 < 1 15 AL 0 -- --

Lithium 0 -- -- -- -- -- -- -- -- -- -- -- -- -- --Manganese 463 45.1 3600 810 130 52.00 11 1.1 0.25 0.048 300 HBSL 1.9 50 10Molybdenum 189 68.8 8.0 5.5 3.0 2.0 1.0 0.32 0.14 0.064 40 HBSL 0 -- --Nickel 189 38.1 9.7 8.0 4.0 3.0 2.0 1.0 0.65 0.37 100 HBSL 0 -- --Radon 431 1.6 6,000 2,400 1,300 900 650 410 180 110 300 or 4,000 MCL1/AMCL2 61, 0.2 -- --Selenium 183 98.9 2.8 1.3 < 1 < 1 < 1 < 1 < 1 < 1 50 MCL 0 -- --Silver 169 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL -- 100 0Strontium 140 0 15,000 2,700 1,500 1,200 900 540 200 65 4,000 HBSL 0.7 -- --Thallium 0 -- -- -- -- -- -- -- -- -- 2 MCL -- -- --Uranium 318 70.4 39 13 5.8 2.0 < 1 < 1 < 1 < 1 30 MCL 0.3 -- --Vanadium 0 -- -- -- -- -- -- -- -- -- -- -- -- -- --Zinc 190 16.3 830 780 220 65 15 4.2 1.6 0.36 2,000 HBSL 0 5,000 0
Appendix 3 93

Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA program, 1992–2003, by major aquifer group.—Continued[MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water]

Element

Number of 

samples

Percentage 

of samples 

with censored values

Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline

Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type

Percentage 

exceeding 

benchmark

Guideline 

value 

(SMCL)

Percentage 

exceeding 

SMCL

Basaltic- and volcanic-rock aquifers

Aluminum 31 58.1 20 20 8.7 2.7 1.4 0.48 0.16 0.074 -- -- -- 50 or 200 0, 0

Antimony 30 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL -- -- --Arsenic 81 42.0 38 38 11 8.0 3.0 1.0 0.40 0.17 10 MCL 6.2 -- --Barium 35 11.4 18 18 15 9.3 4.5 2.2 1.6 0.78 2,000 MCL 0 -- --Beryllium 31 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL 0 -- --Boron 30 0 190 190 180 120 59 42 30 23 1,000 HBSL 0 -- --Cadmium 81 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 0 -- --Chromium 80 35 13 13 3.0 2.5 1.9 1.3 0.78 0.53 100 MCL 0 -- --Cobalt 81 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 -- -- -- -- --Copper 80 45.5 10 10 7.4 6.6 2.9 1.3 0.64 0.34 1,300 AL 0 1,000 0Iron 232 65.5 4,100 970 59 12 5.0 0.66 0.10 0.019 -- -- -- 300 3.4Lead 81 76.5 4.0 4.0 1.9 1.5 0.92 0.53 0.31 0.19 15 AL 0 -- --Lithium 31 9.7 7.0 7.0 2.1 1.3 0.92 0.61 0.46 0.32 -- -- -- -- --Manganese 228 83.8 1,700 1,100 91 5.0 < 1 < 1 < 1 < 1 300 HBSL 2.6 50 6.1Molybdenum 30 100 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 40 HBSL 0 -- --Nickel 30 83.3 1.6 1.6 1.5 1.2 < 1 < 1 < 1 < 1 100 HBSL 0 -- --Radon 192 1.6 2,700 2,300 1,300 940 520 220 110 83 300 or 4,000 MCL1/AMCL2 42.7, 0 -- --Selenium 74 70.3 5.0 5.0 2.4 1.7 0.87 0.44 0.24 0.13 50 MCL 0 -- --Silver 35 97.1 1.0 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL -- 100 0Strontium 31 0 350 350 290 200 110 68 48 43 4,000 HBSL 0 -- --Thallium 0 -- -- -- -- -- -- -- -- -- 2 MCL 0 -- --Uranium 33 97.0 1.5 1.5 < 1 < 1 < 1 < 1 < 1 < 1 30 MCL 0 -- --Vanadium 31 0 58 58 56 48 41 25 15 12 -- -- -- -- --Zinc 70 4.3 700 700 210 110 47 15 6.0 3.5 2,000 HBSL 0 5,000 0
94 Trace Elements and Radon in Groundwater Across the United States, 1992–2003 Appendix 3. Summary statistics and percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in groundwater across the United States sampled by the NAWQA program, 1992–2003, by major aquifer group.—Continued [MCLs and SMCLs are U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) and Secondary Maximum Contaminant Levels (SMCLs) for public water supplies; HBSLs are Health-Based Screening Levels developed by the U.S. Geological Survey by using USEPA toxicity data and methods (Toccalino and Norman, 2006); Action levels (ALs) are concentrations which, if exceeded, trigger treatment or other requirements in USEPA regulations for public water supplies; --, not applicable; <, value is less than value shown. All benchmarks and guidelines are in micrograms per liter except radon, for which the proposed benchmark is in picocuries per liter; reporting level is 1 microgram per liter except for boron (12 micrograms per liter), iron (10 micrograms per liter), radon, (80 picocuries per liter), and strontium (0.9 micrograms per liter). Percentiles below multiple censoring levels were estimated by regression on order statistics; percentiles not estimated if percentage of censored values was greater than 80; if percentiles were not estimated, actual values are shown for percentiles greater than or equal to the assessment level; USEPA Secondary Maximum Contaminant Level (SMCL) is a nonenforceable federal guideline to address the cosmetic or aesthetic considerations of drinking water] Element Num- ber of samples Percentage of samples with cen- sored values Concentration percentile, in μg/L (pCi/L for radon) Human-health benchmark Nonhealth guideline Max 99th 95th 90th 75th 50th 25th 10th Benchmark Type Percent- age exceeding benchmark Guideline value (SMCL) Percent- age exceeding SMCL Crystalline-rock aquifers Aluminum 193 19.17 360 260 28 11 5.1 3.7 2.0 0.77 -- -- -- 50 or 200 3.6, 1.0 Antimony 194 94.85 4.8 3.2 < 1 < 1 < 1 < 1 < 1 < 1 6 MCL 0 -- -- Arsenic 223 78.48 51 29 11 4.0 1.00 0.26 0.064 0.018 10 MCL 5.4 -- -- Barium 194 6.7 320 200 99 82 43 12 3.8 1.3 2,000 MCL 0 -- -- Beryllium 194 96.91 7.5 1.7 < 1 < 1 < 1 < 1 < 1 < 1 4 MCL 0.5 -- -- Boron 51 56.86 130 130 52 27 16 12 7.5 5.2 1,000 HBSL 0 -- -- Cadmium 194 97.94 1.5 < 1 < 1 < 1 < 1 < 1 < 1 < 1 5 MCL 0 -- -- Chromium 194 43.3 8.0 6.9 4.0 3.0 1.8 1.0 0.62 0.395 100 MCL 0 -- -- Cobalt 194 90.21 12 3.0 < 1 < 1 < 1 < 1 < 1 < 1 -- -- -- -- -- Copper 194 23.71 420 330 51 22 10 3.1 0.92 0.31 1,300 AL 0 1,000 0 Iron 279 39.07 22,000 10,000 640 260 26 6.2 0.94 0.18 -- -- -- 300 9.3 Lead 194 79.38 13 8.0 3.2 2.1 0.55 0.15 0.040 0.014 15 AL 0 -- -- Lithium 58 1.72 1,200 1,200 35 20 8.5 5.0 2.6 0.98 -- -- -- -- --

Manganese 278 24.82 720 620 190 100 22 4.6 1.0 0.25 300 HBSL 3.6 50 16.2Molybdenum 194 62.89 34 19 6.0 4.0 1.8 0.57 0.21 0.081 40 HBSL 0 -- --Nickel 194 56.7 12 7.0 5.0 3.0 1.7 0.52 0.202 0.086 100 HBSL 0 -- --Radon 273 0.73 220,000 45,000 14,000 8,600 4,300 2,200 1,100 460 300 or 4,000 MCL1/AMCL2 91.9, 28.2 -- --Selenium 190 92.63 3.9 3.0 1.0 < 1 < 1 < 1 < 1 < 1 50 MCL 0 -- --Silver 194 98.45 1.0 < 1 < 1 < 1 < 1 < 1 < 1 < 1 100 HBSL 0 100 0Strontium 84 0 1,700 1,700 500 410 210 110 56 30 4,000 HBSL 0 -- --Thallium 58 94.8 < 1 < 1 < 1 < 1 < 1 < 1 < 1 < 1 2 MCL 0 -- --Uranium 279 67.38 430 150 26 12 1.7 0.27 0.045 0.009 30 MCL 3.9 -- --Vanadium 47 87.23 2.1 2.1 1.7 < 1 < 1 < 1 < 1 < 1 -- -- -- -- --Zinc 193 24.87 1,000 870 200 48 11 3.0 1.0 0.21 2,000 HBSL 0 5,000 0

1 USEPA proposed maximum contaminant level (MCL) for radon in water.2 USEPA proposed alternative maximum contaminant level (AMCL) for radon in water: a proposed higher allowable MCL if accompanied by a multimedia mitigation program to address radon risk in 

indoor air.
Appendix 4 95

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Unconsolidated sand and gravel aquifers (USG)

Aluminum 200 116 0.86 45 6.7 57 11 710 0.28 125 0.0 143 0.0

Antimony 6 133 0.0 43 0.0 55 0.0 787 0.0 123 0.0 145 0.0

Arsenic 10 168 3.6 54 7.4 63 9.5 837 11 147 17 170 19

Barium 2,000 141 0.0 51 0.0 57 0.0 802 0.0 138 0.0 152 0.66

Beryllium 4 133 0.0 43 0.0 55 0.0 787 0.0 123 0.0 145 0.0

Boron 1,000 59 3.4 17 18 12 8.3 352 1.1 63 7.9 42 7.1

Cadmium 5 133 0.75 43 0.0 55 0.0 787 0.25 123 0.81 146 0.68

Chromium 100 137 0.0 49 2.0 55 0.0 777 0.0 133 0.0 150 0.0

Copper 1,300 134 0.0 51 0.0 57 0.0 777 0.0 137 0.73 149 0.0

Iron 300 225 0.0 77 39 70 86 988 0.0 155 14 172 53

Lead 15 133 0.0 44 0.0 56 0.0 789 0.0 124 0.0 145 0.0

Manganese 300 199 0.0 73 32 67 42 968 0.0 149 36 162 54

Molybdenum 40 140 1.4 50 4.0 56 0.0 799 1.3 138 8.0 151 2.0

Nickel 100 141 0.0 50 0.0 56 0.0 798 0.0 138 0.0 151 0.0

Radon 300 186 82 49 84 46 57 883 81 112 73 141 54

Selenium 50 140 0.0 49 0.0 57 0.0 800 0.38 137 0.73 152 0.0

Silver 100 129 0.0 39 0.0 53 0.0 780 0.0 122 0.0 142 0.0

Strontium 4,000 54 5.6 19 16 25 4.0 309 3.9 40 13 28 0.0

Thallium 2 41 0.0 7 0.0 5 0.0 252 0.0 34 0.0 25 0.0

Uranium 30 160 6.9 53 15 58 0.0 806 7.1 138 23 155 7.1

Zinc 2,000 124 0.81 47 0.0 57 0.0 762 0.0 133 0.0 148 0.0

Number of elements with 

≥ 1% exceedence

6 10 7 6 8 7
96 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

—Continued

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Glacial unconsolidated sand and gravel aquifers (GLA)

Aluminum 200 99 0.0 78 5.1 42 2.4 85 0.0 73 0.0 94 0.0

Antimony 6 104 0.0 80 0.0 41 0.0 89 0.0 75 0.0 94 0.0

Arsenic 10 104 0.0 82 3.7 42 17 92 1.1 76 1.3 101 23

Barium 2,000 104 0.0 82 0.0 41 0.0 94 0.0 77 0.0 99 0.0

Beryllium 4 104 0.0 81 0.0 41 0.0 93 0.0 77 0.0 99 0.0

Boron 1,000 75 0.0 46 0.0 25 0.0 45 0.0 37 2.7 35 2.9

Cadmium 5 104 0.0 81 0.0 41 0.0 93 0.0 77 0.0 99 0.0

Chromium 100 100 0.0 81 0.0 40 0.0 88 0.0 75 0.0 92 0.0

Copper 1,300 104 0.0 80 0.0 42 0.0 91 0.0 75 0.0 94 0.0

Iron 300 220 0.0 167 29 90 77 264 0.0 188 29 220 75

Lead 15 104 0.0 80 0.0 41 0.0 89 0.0 75 0.0 94 0.0

Manganese 300 206 0.0 168 37 88 44 238 0.0 189 32 220 26

Molybdenum 40 104 0.0 80 0.0 41 0.0 90 3.3 75 5.3 94 1.1

Nickel 100 104 0.0 81 0.0 42 0.0 90 0.0 75 0.0 94 0.0

Radon 300 159 81 102 62 65 40 154 68 84 36 173 24

Selenium 50 103 0.0 81 0.0 42 0.0 95 1.1 76 0.0 99 0.0

Silver 100 103 0.0 81 0.0 39 0.0 92 0.0 77 0.0 99 0.0

Strontium 4,000 73 0.0 46 2.2 22 0.0 46 2.2 33 0.0 30 3.3

Thallium 2 58 0.0 42 0.0 10 0.0 36 0.0 26 0.0 22 0.0

Uranium 30 108 0.0 85 0.0 55 0.0 129 1.6 100 1.0 141 0.0

Zinc 2,000 98 0.0 78 0.0 42 0.0 89 0.0 74 0.0 99 0.0

Number of elements with 

≥ 1% exceedence

1 6 5 6 7 7
Appendix 4 97

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

—Continued

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Semiconsolidated sand aquifers (SCS)

Aluminum 200 100 1.0 68 10 44 2.3 7 0.0 14 0.0 18 0.0

Antimony 6 104 0.0 70 0.0 46 0.0 18 0.0 14 0.0 25 0.0

Arsenic 10 125 0.0 74 1.4 50 4.0 18 0.0 16 0.0 27 0.0

Barium 2,000 104 1.0 70 0.0 46 0.0 18 0.0 16 0.0 26 0.0

Beryllium 4 104 0.0 70 1.4 46 0.0 18 0.0 14 0.0 25 0.0

Boron 1,000 67 0.0 41 0.0 25 0.0 2 0.0 9 0.0 7 0.0

Cadmium 5 104 0.0 70 0.0 46 0.0 18 0.0 14 0.0 25 0.0

Chromium 100 103 0.0 69 0.0 46 0.0 18 0.0 16 0.0 26 0.0

Copper 1,300 104 0.0 70 0.0 46 0.0 18 0.0 15 0.0 25 0.0

Iron 300 195 0.0 142 37 73 79 28 0.0 19 63 37 62

Lead 15 104 2.9 70 1.4 46 0.0 18 0.0 15 0.0 25 0.0

Manganese 300 189 0.0 142 13 73 25 28 0.0 19 26 37 11

Molybdenum 40 104 0.0 70 0.0 46 0.0 18 0.0 14 0.0 25 0.0

Nickel 100 104 0.0 70 1.4 46 0.0 18 0.0 15 0.0 26 0.0

Radon 300 80 36 38 47 37 32 15 6.7 10 40 15 27

Selenium 50 104 0.0 70 0.0 46 0.0 18 0.0 16 0.0 26 0.0

Silver 100 104 0.0 70 0.0 46 0.0 18 0.0 14 0.0 25 0.0

Strontium 4,000 53 0.0 33 0.0 22 0.0 2 0.0 9 0.0 6 0.0

Thallium 2 53 0.0 33 0.0 22 0.0 2 0.0 9 0.0 6 0.0

Uranium 30 104 0.0 70 0.0 46 0.0 18 0.0 15 0.0 25 0.0

Zinc 2,000 98 0.0 67 0.0 45 0.0 9 0.0 16 0.0 20 0.0

Number of elements with 

≥ 1% exceedence

4 8 5 1 3 3
98 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

—Continued

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Sandstone aquifers (SAN)

Aluminum 200 20 0.0 27 11 31 0.0 78 0.0 14 0.0 50 0.0

Antimony 6 33 0.0 30 0.0 33 0.0 84 1.2 16 0.0 49 0.0

Arsenic 10 35 0.0 31 0.0 34 0.0 99 4.0 16 0.0 51 0.0

Barium 2,000 35 0.0 31 0.0 34 0.0 100 0.0 16 0.0 51 2.0

Beryllium 4 33 0.0 30 3.3 33 0.0 84 0.0 16 0.0 49 0.0

Boron 1,000 19 0.0 -- -- -- -- 46 0.0 4 0.0 13 0.0

Cadmium 5 35 0.0 30 3.3 33 0.0 99 1.0 16 0.0 49 0.0

Chromium 100 33 0.0 30 0.0 33 0.0 99 0.0 16 0.0 51 0.0

Copper 1,300 35 0.0 31 0.0 34 0.0 100 0.0 16 0.0 49 0.0

Iron 300 40 0.0 40 60 50 90 143 0.0 26 42 72 50

Lead 15 35 0.0 31 0.0 34 2.9 99 0.0 16 0.0 49 0.0

Manganese 300 39 0.0 37 54 49 53 135 0.0 27 15 74 11

Molybdenum 40 33 0.0 30 0.0 33 0.0 85 1.2 16 0.0 49 2.0

Nickel 100 33 0.0 31 3.2 34 2.9 84 0.0 16 0.0 49 0.0

Radon 300 35 86 34 44 46 24 131 86 26 62 71 55

Selenium 50 35 0.0 31 0.0 34 0.0 100 1.0 16 0.0 51 0.0

Silver 100 35 0.0 29 0.0 32 0.0 97 0.0 15 0.0 45 0.0

Strontium 4,000 14 0.0 -- -- -- -- 28 0.0 4 25 10 0.0

Thallium 2 14 0.0 -- -- -- -- 27 0.0 4 0.0 10 0.0

Uranium 30 36 0.0 34 2.9 43 0.0 117 0.0 24 0.0 62 0.0

Zinc 2,000 22 0.0 27 0.0 32 0.0 79 0.0 15 0.0 50 0.0

Number of elements with 

≥ 1% exceedence

1 8 5 6 4 5
Appendix 4 99

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

—Continued

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Sandstone and carbonate-rock aquifers (SCR)

Aluminum 200 80 1.3 8 0.0 5 0.0 64 0.0 2 0.0 6 0.0

Antimony 6 80 0.0 8 0.0 4 0.0 73 0.0 3 0.0 7 0.0

Arsenic 10 81 0.0 8 0.0 6 0.0 73 2.7 3 0.0 7 0.0

Barium 2,000 81 0.0 8 0.0 6 0.0 73 0.0 3 0.0 7 0.0

Beryllium 4 80 0.0 8 0.0 4 0.0 73 0.0 3 0.0 7 0.0

Boron 1,000 -- -- -- -- -- -- -- -- -- -- -- --

Cadmium 5 80 0.0 8 0.0 4 0.0 73 0.0 3 0.0 7 0.0

Chromium 100 80 0.0 8 0.0 6 0.0 73 0.0 3 0.0 7 0.0

Copper 1,300 81 0.0 8 0.0 6 0.0 73 0.0 3 0.0 7 0.0

Iron 300 91 0.0 20 30 7 57 76 0.0 7 0.0 8 75

Lead 15 80 0.0 8 0.0 5 0.0 73 0.0 3 0.0 7 0.0

Manganese 300 89 0.0 20 25 7 0.0 75 0.0 7 29 8 0.0

Molybdenum 40 81 0.0 8 0.0 5 0.0 73 1.4 3 0.0 7 0.0

Nickel 100 81 0.0 8 0.0 6 0.0 73 0.0 3 0.0 7 0.0

Radon 300 53 57 7 57 6 33 62 34 2 0.0 5 60

Selenium 50 81 0.0 8 0.0 6 0.0 73 0.0 3 0.0 7 0.0

Silver 100 80 0.0 8 0.0 4 0.0 72 0.0 2 0.0 7 0.0

Strontium 4,000 46 8.7 1 0.0 4 100 40 30 1 0.0 5 80

Thallium 2 -- -- -- -- -- -- -- -- -- -- -- --

Uranium 30 80 0.0 8 0.0 5 0.0 73 0.0 3 0.0 7 0.0

Zinc 2,000 80 0.0 8 0.0 6 0.0 69 0.0 3 0.0 7 0.0

Number of elements with 

≥ 1% exceedence

3 3 3 4 1 3
100 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

—Continued

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Carbonate-rock aquifers (CAR)

Aluminum 200 21 0.0 12 0.0 37 0.0 103 0.0 1 0.0 18 0.0

Antimony 6 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Arsenic 10 28 0.0 14 7.1 37 5.4 127 0.0 1 0.0 19 0.0

Barium 2,000 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Beryllium 4 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Boron 1,000 -- -- -- -- -- -- -- -- -- -- -- --

Cadmium 5 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Chromium 100 21 0.0 11 0.0 37 0.0 99 0.0 1 0.0 18 0.0

Copper 1,300 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Iron 300 58 0.0 22 41 49 90 286 0.0 9 22 49 73

Lead 15 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Manganese 300 52 0.0 22 14 49 6.1 276 0.0 9 0.0 48 6.3

Molybdenum 40 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Nickel 100 21 0.0 11 0.0 37 0.0 98 0.0 1 0.0 18 0.0

Radon 300 49 69 19 79 49 73 254 58 8 88 46 43

Selenium 50 20 0.0 11 0.0 37 0.0 93 0.0 1 0.0 18 0.0

Silver 100 20 0.0 11 0.0 30 0.0 90 0.0 1 0.0 14 0.0

Strontium 4,000 8 0.0 7 0.0 33 0.0 77 1.3 5 0.0 9 0.0

Thallium 2 -- -- -- -- -- -- -- -- -- -- -- --

Uranium 30 23 0.0 11 0.0 38 0.0 213 0.47 6 0.0 23 0.0

Zinc 2,000 21 0.0 11 0.0 37 0.0 99 0.0 1 0.0 18 0.0

Number of elements with 

≥ 1% exceedence

1 4 4 2 2 3
Appendix 4 101

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

—Continued

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Basaltic- and other volcanic-rock aquifers (BAV)

Aluminum 200 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Antimony 6 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Arsenic 10 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Barium 2,000 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Beryllium 4 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Boron 1,000 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Cadmium 5 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Chromium 100 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Copper 1,300 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Iron 300 20 0.0 1 100 1 100 148 0.0 9 33 4 50

Lead 15 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Manganese 300 19 0.0 1 0.0 1 100 146 0.0 9 33 4 0.0

Molybdenum 40 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Nickel 100 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Radon 300 19 11 1 0.0 -- -- 127 43 9 89 3 33

Selenium 50 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Silver 100 17 0.0 -- -- 1 0.0 16 0.0 -- -- -- --

Strontium 4,000 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Thallium 2 16 0.0 -- -- -- -- 14 0.0 -- -- -- --

Uranium 30 17 0.0 1 0.0 -- -- 15 0.0 -- -- -- --

Zinc 2,000 16 0.0 -- -- -- -- 15 0.0 -- -- -- --

Number of elements with 

≥ 1% exceedence

1 1 2 1 3 2
102 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 4. Summary of the percent exceedences of human-health benchmarks and nonhealth guidelines for trace elements in 

groundwater across the United States sampled by the NAWQA Program, 1992–2003, by pH, redox state, and major aquifer group.

—Continued

[Elements for which at least 1 percent of samples exceeded guidelines are shown with blue shading; the total number of these elements is shown at the bottom 

of the table; low pH is less than 7, whereas high pH is greater than or equal to 7; N, number of samples; %, percentage; --, no data; ≥, greater than or equal to;

benchmark or guidelines are in micrograms per liter for all elements except radon, for which the proposed benchmark is in picocuries per liter]

Element

Benchmark or 

guideline

Low pH High pH

Oxic Mixed Anoxic Oxic Mixed Anoxic

N % N % N % N % N % N %

Crystalline-rock aquifers (CRL)

Aluminum 200 91 0.0 15 13 4 0.0 62 0.0 2 0.0 16 0.0

Antimony 6 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Arsenic 10 102 0.0 16 0.0 4 0.0 76 11 3 0.0 19 16

Barium 2,000 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Beryllium 4 91 0.0 15 6.7 4 0.0 63 0.0 2 0.0 16 0.0

Boron 1,000 12 0.0 2 0.0 -- -- 26 0.0 -- -- 11 0.0

Cadmium 5 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Chromium 100 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Copper 1,300 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Iron 300 141 0.0 25 28 5 100 76 0.0 3 67 23 52

Lead 15 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Manganese 300 141 0.0 24 17 5 40 76 0.0 3 0.0 23 17

Molybdenum 40 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Nickel 100 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Radon 300 138 90 25 92 5 100 73 99 3 100 23 83

Selenium 50 90 0.0 15 0.0 4 0.0 60 0.0 2 0.0 16 0.0

Silver 100 91 0.0 15 0.0 4 0.0 63 0.0 2 0.0 16 0.0

Strontium 4,000 30 0.0 9 0.0 -- -- 31 0.0 1 0.0 12 0.0

Thallium 2 12 0.0 2 0.0 -- -- 31 0.0 1 0.0 12 0.0

Uranium 30 141 2.8 25 0.0 5 0.0 76 7.9 3 0.0 23 0.0

Zinc 2,000 91 0.0 15 0.0 4 0.0 62 0.0 2 0.0 16 0.0

Number of elements with 

≥ 1% exceedence

2 5 3 3 2 4
Appendix 5 103

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Unconsolidated sand and gravel aquifers–Dry climate

Aluminum Silica N 0.957 < 0.0001 0.811 0.0709

Dissolved oxygen N 0.872 0.0002

Phosphate B 1.830 0.0031

Nickel B 2.340 <0.0001

Lead B 6.280 0.0050

Selenium B 0.420 <0.0001

Chromium B 2.014 0.0007

Radon B 2.092 0.0008

Arsenic Boron B 2.765 0.0460 0.952 0.001

pH N 8.874 0.0073

Silica N 1.170 < 0.0001

Boron Arsenic B 5.112 0.0008 0.876 0.1068

Total dissolved solids N 1.010 < 0.0001

Iron pH N 0.100 0.0002 0.959 0.8788

Dissolved oxygen N 0.170 0.0007

Nitrate B 0.199 0.0081

Uranium B 0.195 0.0006

Manganese B 1.001 0.0016

Well depth N 1.003 0.0392

Manganese Nickel B 1.227 0.0002 0.915 0.0540

Dissolved oxygen N 0.507 < 0.0001

Well depth N 0.996 < 0.0001

Molybdenum Fluoride B 5.277 0.0017 0.825 0.3994

Uranium B 9.254 < 0.0001

Selenium B 2.174 0.0038

Dissolved oxygen N 0.782 < 0.0001

pH N 4.918 < 0.0001

Arsenic B 2.442 0.0014

Manganese B 2.289 0.0043

Nickel Total dissolved solids N 1.002 < 0.0001 0.807 0.1808

Aluminum N 2.749 < 0.0001

Copper B 1.686 0.0074

Uranium B 2.341 0.0007

pH N 0.456 0.0025

Radon Tritium B 3.147 0.0002 0.703 0.9576

Bromide B 3.839 < 0.0001

pH N 3.691 0.0123

Nitrate B 0.445 0.0147
104 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Unconsolidated sand and gravel aquifers–Dry climate—Continued

Selenium Specific conductance N 1.001 0.0005 0.884 0.1986

Bromide B 2.316 0.0018

Well depth N 1.003 < 0.0001

Nitrate B 4.376 < 0.0001

Aluminum B 0.377 < 0.0001

Fluoride B 13.452 0.0003

Manganese B 0.331 < 0.0001

Molybdenum B 2.188 0.0204

Uranium B 3.690 0.0010

Iron B 0.417 0.0018

Strontium Specific conductance N 1.005 < 0.0001 0.942 0.0828

Uranium Total dissolved solids N 1.002 0.0005 0.908 0.7753

Nitrogen as ammonia B 0.228 0.0001

Phosphate B 0.329 <0.0001

Copper B 2.441 0.0022

Iron B 0.434 0.0087

Molybdenum B 9.775 < 0.0001

Selenium B 5.292 < 0.0001

Chromium B 2.144 0.0046

Unconsolidated sand and gravel aquifers–Humid climate

Aluminum Calcium N 0.994 0.0079 0.877 0.9472

Nitrogen as ammonia B 5.796 0.0095

Dissolved oxygen N 0.834 0.0005

Arsenic B 2.166 0.0359

Fluoride B 0.251 0.0007

Nickel B 2.650 0.0085

Chromium B 6.924 < 0.0001

Arsenic pH N 1.695 0.0077 0.807 0.0118

Potassium N 1.229 0.0002

Silica N 1.042 < 0.0001

Phosphate B 2.602 < 0.0001

Aluminum B 2.826 0.0003

Copper B 0.405 0.0005

Uranium B 2.974 0.0001

Boron Total dissolved solids N 1.006 < 0.0001 0.901 0.0445

Aluminum B 2.751 0.0197

Molybdenum B 3.595 0.0024
Appendix 5 105

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Unconsolidated sand and gravel aquifers–Humid climate—Continued

Copper pH N 0.449 0.0003 0.832 0.4555

Bromide B 0.362 0.0002

Sulfate N 1.015 < 0.0001

Lead B 4.137 0.0004

Uranium B 3.194 0.0002

Zinc B 5.423 < 0.0001

Selenium B 2.273 0.0537

Iron Dissolved oxygen N 0.350 < 0.0001 0.939 < 0.0001

pH N 0.308 < 0.0001

Nitrate B 0.096 < 0.0001

Manganese Dissolved oxygen N 0.436 < 0.0001 0.884 0.8578

pH N 1.961 0.0014

Copper B 0.465 0.0056

Well depth N 0.998 0.00009

Molybdenum Total dissolved solids N 1.003 < 0.0001 0.903 0.1047

Silica N 0.981 0.0039

Well depth N 1.006 < 0.0001

Fluoride B 2.264 0.0002

Aluminum B 2.366 0.0305

Manganese B 2.559 0.0010

Uranium B 2.869 < 0.0001

Chromium B 0.489 0.0222

Nickel Temperature N 1.125 < 0.0001 0.842 0.0687

pH N 0.419 0.0001

Sodium N 1.011 0.0009

Nitrogen as ammonia B 0.420 0.0031

Aluminum B 3.589 0.0012

Selenium B 0.340 0.0074

Cobalt B 17.385 0.0003

Chromium B 3.465 < 0.0001

Radon Chromium B 0.168 < 0.0001 0.867 < 0.0001

Well depth N 0.998 0.0004

Uranium B 3.718 0.0002

Dissolved oxygen N 1.299 0.0006

Manganese B 0.284 0.0048

Selenium Total dissolved solids N 1.003 < 0.0001 0.866 0.0187

Uranium B 2.655 0.0119

Well depth N 1.002 < 0.0001
106 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Unconsolidated sand and gravel aquifers–Humid climate—Continued

Strontium Specific conductance N 1.006 < 0.0001 0.972 0.6053

Uranium pH N 2.577 0.0008 0.912 0.5779

Silica N 0.964 0.0002

Total dissolved solids N 1.004 < 0.0001

Nitrate B 3.367 0.0002

Nitrogen as ammonia B 0.102 < 0.0001

Copper B 1.865 0.0386

Arsenic B 3.571 0.0002

Iron B 0.456 0.0233

Molybdenum B 1.921 0.0336

Zinc Silica N 1.023 0.0068 0.852 0.1797

Phosphate B 0.526 0.0283

Copper B 9.011 < 0.0001

Chromium B 0.115 < 0.0001

Dissolved oxygen N 1.237 0.0002

Glacial unconsolidated sand and gravel aquifers

Aluminum pH N 0.530 0.0006 0.851 0.0904

Iron B 0.433 0.0087

Chromium B 16.366 < 0.0001

Manganese B 5.327 < 0.0001

Nickel B 3.096 < 0.0001

Arsenic pH N 2.312 0.0001 0.801 0.7528

Silica N 1.066 0.0007

Dissolved oxygen N 0.819 < 0.0001

Molybdenum B 2.518 0.0005

Uranium B 1.555 0.0966

Well depth N 1.008 0.0002

Boron pH N 2.695 0.0105 0.821 0.2896

Bromide B 4.036 0.0007

Molybdenum B 2.396 0.0337

Temperature N 1.144 0.0055

Potassium N 1.088 0.0251

Copper Potassium N 1.094 0.0337 0.799 0.6210

Nitrate B 3.068 < 0.0001

Sulfate N 1.011 < 0.0001

Aluminum B 0.380 < 0.0001

Iron B 0.478 0.0026

Molybdenum B 0.363 < 0.0001

Uranium B 2.160 0.0078

Zinc B 4.842 < 0.0001
Appendix 5 107

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Glacial unconsolidated sand and gravel aquifers—Continued

Iron pH N 0.326 < 0.0001 0.928 0.9070

Total dissolved solids N 1.002 0.0316

Well depth N 1.011 0.0018

Dissolved oxygen N 0.517 < 0.0001

Copper B 0.326 0.0017

Uranium B 0.311 0.0034

Chromium B 0.420 0.0161

Radon B 0.110 < 0.0001

Manganese pH N 0.607 0.0095 0.857 0.8400

Well depth N 0.986 < 0.0001

Dissolved oxygen N 0.812 < 0.0001

Arsenic B 2.385 0.0033

Iron B 2.252 0.0034

Uranium B 3.112 0.0002

Cobalt B 4.702 0.0001

Nitrate B 0.426 0.0043

Molybdenum Specific conductance N 1.002 < 0.0001 0.893 0.2575

pH N 3.651 < 0.0001

Silica N 0.911 0.0002

Bromide B 0.383 0.0040

Dissolved oxygen N 0.899 0.0415

Arsenic B 2.914 0.0016

Fluoride B 10 < 0.0001

Manganese B 4.157 0.0002

Nickel B 2.957 0.0004

Nickel Aluminum B 3.410 < 0.0001 0.768 0.0984

Molybdenum B 2.342 < 0.0001

Zinc B 2.862 0.0002

Cobalt B 6.181 < 0.0001

Lead Magnesium N 0.957 0.0108 0.825 0.5371

Chloride N 1.012 0.0003

Potassium N 0.582 0.0254

Copper B 1.063 < 0.0001

Radon pH N 0.416 0.0006 0.933 0.3919

Nitrate B 3.015 0.0004

Sulfate N 0.989 0.0025

Arsenic B 0.349 0.0012
108 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Glacial unconsolidated sand and gravel aquifers—Continued

Radon—Continued Fluoride B 2.356 0.0107

Iron B 0.325 0.0003

Molybdenum B 0.466 0.0211

Cobalt B 2.427 0.0213

Chromium B 0.542 0.0271

Selenium Nitrate B 4.384 0.0004 0.797 0.9472

Uranium B 5.161 < 0.0001

Manganese B 0.250 0.0009

Strontium Calcium N 1.020 < 0.0001 0.815 0.2493

Sodium N 1.010 0.0300

Dissolved oxygen N .0865 0.0105

Manganese B 0.280 0.0060

Uranium B 5.062 0.0001

Uranium Magnesium N 1.057 < 0.0001 0.876 0.8847

Silica N 0.918 0.0015

Well depth N 0.971 < 0.0001

Fluoride B 3.321 0.0012

Nickel B 3.173 0.0011

Manganese B 5.226 0.0005

Selenium B 4.092 0.0015

Zinc Temperature N 0.920 0.0028 0.753 0.8044

Specific conductance N 0.999 0.0117

Silica N 1.090 0.0063

Copper B 4.161 < 0.0001

Nickel B 2.247 0.0034

Radon B 0.404 0.0016

Cobalt B 3.764 0.0108

Semiconsolidated sand aquifers

Aluminum pH N 0.363 < 0.0001 0.856 0.7376

Silica N 0.938 < 0.0001

Nitrogen as ammonia B 2.974 0.0250

Nickel B 3.211 0.0031

Iron B 3.062 0.0063

Well depth N 0.997 0.0340

Arsenic Dissolved oxygen N 0.688 0.0003 0.824 0.2713

Molybdenum B 3.198 0.0013

Cobalt B 1.908 0.0653

Silica B 1.039 0.0019
Appendix 5 109

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Semiconsolidated sand aquifers—Continued

Boron Magnesium N 1.142 0.0006 0.784 0.2073

Dissolved oxygen N 0.888 0.0577

pH N 1.646 0.0192

Iron Dissolved oxygen N 0.692 < 0.0001 0.812 0.4138

Nitrate B 0.192 0.0011

Uranium B 0.248 0.0077

Manganese Well depth N 0.995 0.0010 0.817 0.4914

Dissolved oxygen N 0.702 < 0.0001

Phosphate B 3.446 0.0005

Zinc B 4.638 0.0006

Chromium B 0.519 0.0472

Molybdenum pH N 23.641 0.0007 0.971 0.9381

Total dissolved solids N 1.005 0.0728

Arsenic B 24.806 0.0022

Radon B 9.477 0.0079

Nickel Total dissolved solids N 1.001 0.0510 0.818 0.1158

Iron B 0.482 0.0120

Manganese B 4.4523 0.0039

Cobalt B 15.759 < 0.0001

Chromium B 2.803 0.0002

Radon Nitrate B 2.839 0.00018 0.692 0.9451

Molybdenum B 10.720 < 0.0001

Selenium Nitrate B 3.720 0.0016 0.785 0.0469

Total dissolved solids N 1.001 0.0096

Iron B 0.296 0.0020

Arsenic B 3.016 0.0133

Strontium Total dissolved solids N 1.011 < 0.0001 0.849 < 0.0001

Uranium Molybdenum B 17.527 < 0.0001 0.926 0.0022

Nitrogen as ammonia B 0.074 0.0009

Sulfate N 1.018 0.0109

Bicarbonate N 1.005 0.0056
110 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Sandstone aquifers

Aluminum Bromide B 0.142 0.0030 0.954 0.9446

Nitrogen as ammonia B 37.779 < 0.0001

Phosphate B 0.194 0.0049

Nickel B 8.611 0.0005

Chromium B 54.073 < 0.0001

Arsenic Silica N 1.040 0.0498 0.765 0.4879

Molybdenum B 5.719 < 0.0001

Uranium B 0.0278 0.0278

Boron Sodium N 2.0005 < 0.0001 0.972 0.7816

Iron pH N 0.231 0.0002 0.953 0.1861

Sodium N 0.991 0.0205

Nitrogen as ammonia B 28.063 < 0.0001

Dissolved oxygen N 0.525 < 0.0001

Arsenic B 3.895 0.0416

Molybdenum B 0.206 0.0398

Manganese pH N 0.189 < 0.0001 0.922 0.9936

Total dissolved solids N 1.001 0.0095

Nitrate B 0.129 0.0064

Iron B 17.920 < 0.0001

Copper B 0.128 < 0.0001

Molybdenum pH N 6.523 < 0.0001 0.929 0.0003

Bromide B 6.187 < 0.0001

Well depth N 1.007 0.0010

Arsenic B 6.655 0.0004

Uranium B 6.948 < 0.0001

Nickel Specific conductance N 0.999 0.0051 0.880 0.532

Calcium N 1.025 0.0003

Silica N 1.142 0.0013

Well depth N 1.006 0.0133

Aluminum B 10.755 < 0.0001

Copper B 6.989 < 0.0001

Manganese B 6.749 0.0015

Zinc B 8.674 0.0119

Radon Temperature N 0.798 0.0237 0.835 0.4521

Nitrate B 28.709 < 0.0001

Phosphate B 0.482 0.0551

Molybdenum B 9.670 < 0.0001
Appendix 5 111

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Sandstone aquifers—Continued

Selenium Sodium N 1.005 <0.0001 0.864 0.5721

Uranium B 6.970 < 0.0001

Iron B 0.084 0.0009

Strontium Total dissolved solids N 1.002 0.0118 0.814 0.0241

Uranium Magnesium N 1.087 < 0.0001 0.923 0.5177

Nitrogen as ammonia B 0.055 < 0.0001

Molybdenum B 24.836 < 0.0001

Sandstone and carbonate-rock aquifers

Aluminum Calcium N 0.986 0.0007 0.951 0.5763

Sodium N 1.094 0.0161

Copper B 8.843 0.0011

Manganese B 19.088 0.0019

Zinc B 21.719 < 0.0001

Chromium B 24.352 0.0003

Arsenic Temperature N 0.803 0.0010 0.827 0.8154

Molybdenum B 11.318 0.0007

Zinc B 0.103 0.0081

Boron --

Iron Copper B 0.215 0.0066 0.907 0.1299

Sulfate N 1.001 0.0375

Manganese B 46.877 0.0003

Manganese Temperature N 0.696 0.0001 0.906 0.0947

Iron B 23.787 < 0.0001

Chromium B 0.253 0.0472

Radon B 0.143 0.0112

Molybdenum --

Nickel Specific conductance N 1.006 < 0.0001 0.807 0.4762

pH N 0.219 0.0025

Sodium N 0.938 0.0020

Zinc B 19.108 0.0056

Radon Temperature N 0.823 0.0015 0.813 0.4555

Specific conductance N 0.999 0.0789

Silica N 0.825 0.0419

Iron B 5.048 0.0116

Selenium B 9.544 0.0051
112 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Sandstone and carbonate-rock aquifers—Continued

Selenium Dissolved oxygen N 1.525 0.0012 0.837 0.3103

Arsenic B 4.285 0.0519

Uranium B 65.706 < 0.0001

Strontium Sulfate N 1.072 0.0008 0.982 0.9075

Uranium Molybdenum B 6.734 0.0002 0.862 0.5618

Selenium B 1.932 0.0007

Bicarbonate N 1.007 0.0248

Calcium N 1.007 0.0159

Carbonate-rock aquifers

Aluminum Calcium N 1.024 < 0.0001 0.801 0.6415

Magnesium N 0.914 < 0.0001

Sulfate N 0.982 0.0693

Copper B 1.873 0.0818

Arsenic Temperature N 1.425 < 0.0001 0.906 0.0615

Molybdenum B 10.625 < 0.0001

Iron Well depth N 0.981 0.0003 0.941 0.4243

Arsenic B 5.437 0.0329

Copper B 0.027 < 0.0001

Molybdenum B 4.295 0.0371

Manganese Sulfate N 1.039 0.0210 0.922 0.2292

Dissolved oxygen N 0.728 0.0004

Barium B 12.365 0.0018

Iron B 8.533 < 0.0001

Lead B 0.080 0.0033

Molybdenum Nitrate B 0.363 0.0720 0.877 0.0025

Sulfate N 1.008 0.0124

Arsenic B 13.173 < 0.0001

Zinc B 4.497 0.0209

Radon B 8.615 0.0001

Nickel Specific conductance N 1.009 < 0.0001 0.909 0.0012

Silica N 0.762 < 0.0001

Bromide B 0.280 0.0141

Nitrate B 5.878 0.0032

Nitrogen as ammonia B 0.052 < 0.0001
Appendix 5 113

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Carbonate-rock aquifers—Continued

Radon pH N 0.168 0.0002 0.802 0.1311

Sulfate N 0.979 0.0445

Molybdenum B 9.741 < 0.0001

Strontium pH N 0.110 0.0145 0.884 0.0549

Dissolved oxygen N 0.718 < 0.0001

Selenium --

Uranium Potassium N 1.208 0.0041 0.854 0.2260

Dissolved oxygen N 0.814 0.0530

Phosphate B 0.141 0.0018

Arsenic B 7.540 0.0002

Nickel B 6.623 0.0007

Basaltic- and other volcanic-rock aquifers

Aluminum pH N 0.015 0.0557 0.863 0.3956

Dissolved oxygen N 58.869 0.0072

Arsenic Radon B 67.655 0.0112 0.986 0.6350

Sulfate N 1.218 0.0313

pH N 168.835 0.0123

Boron Total dissolved solids N 0.895 0.0288 0.957 0.9358

Sulfate N 6.917 0.0266

Iron pH N 0.074 0.0116 0.930 0.9393

Dissolved oxygen N 0.479 0.0003

Manganese Dissolved oxygen N 0.513 < 0.0001 0.906 0.4485

Molybdenum --

Nickel --

Radon Dissolved oxygen N 0.615 < 0.0001 0.823 0.0109

Selenium Sulfate N 1.076 0.0006 0.901 0.0198

pH N 0.044 0.0042

Strontium --

Uranium --
114 Trace Elements and Radon in Groundwater Across the United States, 1992–2003

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Crystalline-rock aquifers

Aluminum pH N 0.467 0.0009 0.792 0.6110

Silica N 0.888 < 0.0001

Chromium B 3.495 0.0022

Temperature N 1.161 0.0185

Arsenic Barium N 0.117 0.0082 0.821 0.3963

Nickel B 2.961 0.0244

pH N 1.962 0.0055

Dissolved oxygen N 0.720 0.0031

Copper pH N 0.342 0.0004 0.910 0.4821

Dissolved oxygen N 1.302 0.0035

Iron B 0.210 0.0027

Nickel B 6.361 0.0006

Zinc B 3.188 0.0342

Iron Dissolved oxygen N 0.650 0.0015 0.914 0.8912

Nitrate B 0.061 0.0210

Uranium B 0.199 0.0178

Manganese B 1.005 0.0035

Well depth N 0.995 0.0121

Manganese Dissolved oxygen N 0.608 0.0010 0.916 0.6589

pH N 0.209 0.0001

Iron B 11.674 < 0.0001

Zinc B 0.240 0.0414

Aluminum B 0.181 0.0121

Molybdenum pH N 4.058 < 0.0001 0.928 0.4879

Fluoride B 10.007 < 0.0001

Iron B 0.354 0.0573

Uranium B 3.447 0.0094

Nickel pH N 0.901 0.0691 0.878 0.1475

Magnesium N 0.636 < 0.0001

Potassium N 1.189 0.0039

Silica N 0.911 0.0004

Bromide B 5.575 0.0014

Copper B 10.189 0.0005

Lead Temperature N 1.163 0.0086 0.794 0.0345

Molybdenum B 0.142 0.0030

Uranium B 6.350 0.0050

Radon B 3.802 0.0077

Dissolved oxygen N 1.230 0.0140
Appendix 5 115

Appendix 5. Summaries of logistic regression analyses of selected trace elements by major aquifer group.—Continued

[Dependent variable, binary (0 for non-occurrence, 1 for occurrence, above reporting level); EXP(B), exponentiated model coefficient (equivalent to odds ratio, 

> 1 means direct relation and < 1 means inverse relation); c statistic is a measure of model discrimination (area under the receiver operating characteristics 

curve); Hosmer-Lemeshow1 statistic is a measure of model fit; N, continuous; B, binary; <, less than; --, not analyzed]

Dependent 

variable Explanatory variable Type of variable EXP(B) p-value c statistic (model discrimination)

Hosmer–

Lemeshow statistic

Crystalline-rock aquifers—Continued

Radon Temperature N 0.867 0.0292 0.832 0.6834

Sulfate N 0.917 0.0018

Arsenic B 0.162 0.0045

Uranium B 14.724 < 0.0001

Lead B 2.724 0.0438

Strontium Calcium N 1.099 0.0011 0.826 0.6674

Molybdenum B 8.883 0.0180

Selenium --

Uranium Calcium N 1.080 < 0.0001 0.923 0.3881

Nitrate B 0.266 0.313

Fluoride B 4.863 0.0073

Iron B 0.153 0.0011

Molybdenum B 14.865 < 0.0001

Lead B 11.696 0.0010

Zinc Temperature N 1.252 0.0251 0.863 0.0209

pH N 0.279 0.0001

Silica B 0.921 0.0129

Copper B 3.245 0.0126

Cobalt B 0.047 0.0326

1

 Hosmer, D.W., and Lemeshow, S., 2000, Applied logistic regression (2nd ed.): New York, John Wiley and Sons, 375 p.
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dc_nh@usgs.gov

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Ayotte and others—Trace Elements and Radon in Groundwater Across the United States, 1992–2003—Scientific Investigations Report 2011–5059