Water Quality in Basin-Fill Aquifers of the 

Southwestern United States: Arizona, California, 

Colorado, Nevada, New Mexico, and Utah, 1993–2009

The Quality of Our Nation’s Waters

U.S. Department of the Interior

U.S. Geological Survey

Circular 1358

National Water-Quality Assessment Program
Cover. View of basin area looking north near Layton, Utah, with Wasatch Mountains to the east.
Water Quality in Basin-Fill Aquifers of 

the Southwestern United States: Arizona, 

California, Colorado, Nevada, New Mexico, 

and Utah, 1993–2009

By Susan A. Thiros, Angela P. Paul, Laura M. Bexfield, and David W. Anning

National Water-Quality Assessment Program

Circular 1358

U.S. Department of the Interior

U.S. Geological Survey

The Quality of Our Nation’s Waters
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

visit http://www.usgs.gov/pubprod

To order this and other USGS information products, visit http://store.usgs.gov

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Thiros, S.A., Paul, A.P., Bexfield, L.M., and Anning, D.W., 2014, The quality of our Nation’s waters—Water quality in 

basin-fill aquifers of the southwestern United States: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 

1993–2009: U.S. Geological Survey Circular 1358, 113 p., http://dx.doi.org/10.3133/cir1358.

ISSN 1067-084X (print)

ISSN 2330-5703 (online) 

ISBN 978-1-4113-3756-5

Library of Congress Cataloging-in-Publication Data

Thiros, Susan A.

The quality of our nation’s waters---water quality in basin-fill aquifers of the southwestern United States : Arizona, 

California, Colorado, Nevada, New Mexico, and Utah, 1993–2009 / by Susan A. Thiros, Angela P. Paul, Laura M. 

Bexfield, and David W. Anning.

pages cm. -- (Circular / U.S. Geological Survey ; 1358)

Includes bibliographical references.

ISBN 978-1-4113-3756-5

1. Water quality--Southwestern States. 2. Groundwater -- Quality -- Southwestern States . I. Paul, Angela P. 

II. Bexfield, Laura M. III. Anning, David W. IV. Geological Survey (U.S.) V. Title. VI. Series: U.S. Geological Survey 

circular ; 1358.

 TD223.9.T45 2014

 628.1’140979--dc23

2014001604
National Water-Quality Assessment Program
iv

Foreword

The United States has made major investments in assessing, managing, regulating, and conserving natural resources, such as water and a variety of ecosystems. Sustaining the quality of the 

Nation’s water resources and the health of our diverse ecosystems depends on the availability 

of sound water-resources data and information to develop effective, science-based policies. 

Effective management of water resources also brings more certainty and efficiency to important 

economic sectors. Taken together, these actions lead to immediate and long-term economic, 

social, and environmental benefits that make a difference to the lives of millions of people

(http://water.usgs.gov/nawqa/applications/ ).

Two decades ago, Congress established the U.S. Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program to meet this need. Since then NAWQA has served as a primary 

source of nationally consistent information on the quality of the Nation’s streams and groundwater, on ways in which water quality changes over time, and on the natural features and 

human activities affecting the quality of streams and groundwater. Objective and reliable data, 

systematic scientific studies, and models are used to characterize where, when, and why the 

Nation’s water quality is degraded—and what can be done to improve and protect the water for 

human and ecosystem needs. This information is critical to our future because the Nation faces 

an increasingly complex and growing need for clean water to support people, economic growth, 

and healthy ecosystems. For example, NAWQA findings for public-supply wells, which provide 

water to about 105 million people, showed that 22 percent of source-water samples contained 

at least one contaminant at levels of potential health concern. Similarly, 23 percent of samples 

from domestic (or privately owned) wells, which supply untreated water to an additional 43 million people, also had contaminant levels of potential concern. 

This report is one of a collection of publications that describe water-quality conditions in 

selected Principal Aquifers of the United States (http://water.usgs.gov/nawqa/studies/praq/). 

The collection is part of the series “The Quality of Our Nation’s Waters,” which describes major 

findings of the NAWQA Program on water-quality issues of regional and national concern 

and which provides science-based information for assessing and managing the quality of our 

groundwater resources. Other reports in this series focus on occurrence and distribution of 

nutrients, pesticides, and volatile organic compounds in streams and groundwater, the effects 

of contaminants and streamflow alteration on the condition of aquatic communities in streams, 

and the quality of untreated water from private domestic and public-supply wells. Each report 

builds toward a more comprehensive understanding of the quality of regional and national water 

resources (http://water.usgs.gov/nawqa/nawqa_sumr.html ). All NAWQA reports are available 

online at http://water.usgs.gov/nawqa/bib/.

The information in this series primarily is intended for those interested or involved in resource 

management and protection, conservation, regulation, and policymaking at regional and national 

levels. In addition, the information should be of interest to those at a local level who wish to 

know more about the general quality of streams and groundwater in areas near where they live 

and how that quality compares with other areas across the Nation. We hope this 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.

Jerad Bales

Acting Associate Director for Water

U.S. Geological Survey
v

High Plains aquifer system—Circular 1337

Glacial aquifer system—Circular 1352

Northern Atlantic Coastal Plain 

surficial aquifer system—Circular 1353

Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge siliciclasticrock aquifers

Valley and Ridge carbonaterock aquifers

Early Mesozoic basin aquifers 

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Mississippi embayment–Texas coastal 

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Southwest Principal Aquifers—Circular 1358

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system 

Western Volcanics—Circular 1359

Hawaiian volcanic-rock aquifers

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and 

basaltic-rock aquifers

Principal Aquifers and Water-Quality Summary Reports

National summary of groundwater quality in Principal Aquifers—Circular 1360
vi

Contents

Chapter 1: Overview of Major Findings and Implications....................................................................1

Chapter 2: NAWQA Approach to Assessing Groundwater Quality....................................................7

Chapter 3: Background: Environmental Setting, Hydrogeology, Population, Land Use, and 

Water Use...............................................................................................................................15

Chapter 4: The Hydrology of Southwest Basin-Fill Aquifers and How Human Alterations

Affect Groundwater Quality.................................................................................................25

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation in Southwest 

Study Basins...........................................................................................................................39

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest 

Basin-Fill Aquifers.................................................................................................................49

References Cited .........................................................................................................................................95

Glossary.......................................................................................................................................................100

Appendixes.................................................................................................................................................105
Basin-fill deposits form Southwest basin-fill aquifers.

vii

Introduction to This Report

This report presents information from a regional assessment of the water 

quality in the Southwest basin-fill aquifers. It is one of a series of reports by the 

U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) 

Program. The assessment explains where and why specific groundwaterquality conditions occur in the Southwest. Understanding the quality of water 

in the basin-fill aquifers and the natural and human-related factors that affect 

it is important because of consequences for human health and for sustained 

availability of groundwater for desired uses in the Southwest. This information 

is provided to help water managers estimate groundwater-quality conditions in 

unmonitored areas, assess the susceptibility and vulnerability of groundwater 

under different basin-development scenarios, and develop cost-effective 

groundwater-monitoring programs.

Water-quality conditions in the Southwest basin-fill aquifers, referred to and 

summarized in this report as the Southwest Principal Aquifers, are discussed 

in greater detail in other reports that are listed in appendix 1. Detailed technical 

information, data and analyses, collection and analytical methodology, models, 

graphs, and maps that support the findings presented in this report, in addition to reports in this series from other Principal Aquifers, can be accessed at 

http://water.usgs.gov/nawqa/.
viii

Predicted nitrate and arsenic concentrations in basin-fill 

aquifers of the southwestern United States

By David W. Anning, Angela P. Paul, Tim S. McKinney, 

Jena M. Huntington, Laura M. Bexfield, and Susan A. Thiros

U.S. Geological Survey Scientific Investigations Report 2012–5065 

(available at http://pubs.usgs.gov/sir/2012/5065/)

Effects of natural and human factors on groundwater quality of 

basin-fill aquifers in the southwestern United States—

Conceptual models for selected contaminants

By Laura M. Bexfield, Susan A. Thiros, David W. Anning, 

Jena M. Huntington, and Tim S. McKinney

U.S. Geological Survey Scientific Investigations Report 2011–5020 

(available at http://pubs.usgs.gov/sir/2011/5020/)

Conceptual understanding and groundwater quality of selected 

basin-fill aquifers in the southwestern United States

Edited by Susan A. Thiros, Laura M. Bexfield, David W. Anning, 

and Jena M. Huntington

U.S. Geological Survey Professional Paper 1781

(available at http://pubs.usgs.gov/pp/1781/)

Companion studies of these aquifers are discussed in the 

following reports.
The Southwest Principal Aquifers are a vital groundwater resource that 

underlies a vast area in the southwestern United States. More than 

46 million people—15 percent of the Nation’s population—reside in the 

arid and semiarid area that encompasses the Southwest Principal Aquifers. The 

California Coastal Basin aquifers, Central Valley aquifer system, Basin and 

Range basin-fill aquifers, and Rio Grande aquifer system, all basin-fill aquifers, 

are grouped together to form the Southwest Principal Aquifers. Groundwater is 

an important source of drinking and irrigation water in many Southwest basins. 

Groundwater quality and the effects of contaminant movement are of critical 

importance to the 1.4 million people in mostly rural areas who obtain their 

water from domestic wells.

The San Bernardino Mountains 

tower above the urbanized 

Santa Ana Basin in southern 

California. Basin-fill aquifers 

are an important source of 

drinking water for residents 

of this and other Southwest 

basins.

Photograph by Don Graham

Southwest

Principal Aquifers

Chapter 1: Overview of Major Findings 

Chapter 1

and Implications
WY

CA

NV

AZ

UT CO

WA

OR ID

MT

NM

Many basins contain 

agricultural 

and urban areas

Basin-fill aquifers 

are an important 

source of water 

in many parts of 

the Southwest High rates of artificial 

recharge and well 

pumping enhance the 

movement of some 

contaminants to deeper 

parts of an aquifer

Public-supply well

NOT TO SCALE

Water Quality of the Southwest Basin-Fill Aquifers

Contaminants in water from one of every three drinking-water wells 

sampled are a potential human-health concern 

1

Water from about 42 percent of domestic wells and 26 percent of public-supply wells 

in the Southwest contained at least one contaminant at a concentration that was greater than 

its human-health benchmark. Arsenic, nitrate, and uranium exceeded their respective U.S. 

Environmental Protection Agency Maximum Contaminant Levels (MCLs) more than twice as 

frequently in Southwest drinking-water wells than in drinking-water wells nationwide. 

The widespread detection of contaminants in domestic wells and the lack of 

required monitoring or 

treatment underscores 

the need for public 

education on where 

contaminants are likely 

to occur and testing and 

treatment options.

Volcanic and granitic rocks are geologic sources 

of arsenic and uranium in groundwater. Water sampled 

from 14 percent of domestic wells and 19 percent of 

public-supply wells in the Southwest exceeded the 

MCL for arsenic. Exposure to arsenic and uranium 

contributes to an increased risk of cancer. Overall, 

contaminants in basin-fill aquifers from geologic 

sources are a greater concern for drinking water than 

are contaminants from human-related sources.

A statistical model estimates that arsenic concentrations in groundwater in almost half of the basin area 

in the Southwest are equal to or greater than the MCL. 

Understanding the factors that affect concentrations of 

arsenic and other contaminants with geologic sources 

in groundwater can help water suppliers prioritize 

areas for new groundwater development and reduce 

treatment costs.

2 Arsenic and uranium derived from geologic sources are potential drinking-water concerns

The Southwest Principal Aquifers consist of many basin-fill aquifers in

California, Nevada, Utah, Arizona, New Mexico, and Colorado. Demands 

for irrigation and drinking water have substantially increased groundwater 

withdrawals and irrigation return flow to some of these aquifers. These changes 

have increased the movement of contaminants from geologic and human 

sources to depths used to supply drinking water in several basin-fill aquifers in 

the Southwest.

0 5 10 15

Arsenic

Nitrate

Uranium

Percentage of samples from drinking-water wells with 

concentrations greater than human-health benchmarks

Aquifers nationwide

Southwest basin-fill aquifers

Los Angeles

Phoenix

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

NEVADA

OREGON IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Reno

San Francisco

Pacif ic Ocean 0 100 200 MILES

0 100 200 KILOMETERS

Estimated arsenic

concentration, in 

micrograms per liter

<1.0

1.0 to <5.0

5.0 to <10.0

≥10.0

Area not modeled
Excess irrigation water and groundwater withdrawals 

for irrigation and public supply can increase the movement 

of contaminants from human-related sources to deeper parts 

of basin-fill aquifers. Contaminants include nitrate from 

fertilizers and manure, and volatile organic compounds 

and pesticides used in urban and agricultural areas. Nitrate 

concentrations exceeded the MCL in 18 percent of domestic 

wells and almost 4 percent of public-supply wells in the 

Southwest. Surprisingly, artificial recharge and groundwater 

withdrawals also are contributing to the release of naturally 

occurring uranium from aquifer sediments—concentrations 

in one of four domestic wells in the eastern Central Valley, 

California, exceeded the MCL.

Artificial recharge and groundwater 

withdrawals are moving contaminants to 

deeper parts of basin-fill aquifers

4

Dissolved-solids concentrations in deep wells used for 

public supply typically are low, but substantial increases in 

concentrations over time in some areas of the Southwest raise 

questions about the long-term viability of the groundwater 

resource for drinking. Dissolved solids, which can impart an 

3 Dissolved-solids concentrations in groundwater are increasing in some areas

unpleasant taste to water, were measured at concentrations that 

exceeded the recommended maximum value in one of every 

four drinking-water wells (31 percent of domestic wells and 

19 percent of public-supply wells).

circ1358-blockdiagram_recharge

0 5 10 15 20 25

Dieldrin

Dibromochloro- propane (DBCP)

Nitrate

Uranium

Arsenic

Urban monitoring wells

Agricultural monitoring wells

Domestic wells

Public-supply wells

Direction of generally 

increasing well depth

Percentage of groundwater samples with concentrations

greater than human-health benchmarks

A typical sediment-filled basin 

is surrounded by mountains

Domestic well Monitoring 

well
1 Contaminants in water from one of every three drinking-water 

wells sampled are a potential human-health concern

Water from 35 percent of drinking-water wells in the Southwest (42 percent of 

domestic wells and 26 percent of public-supply wells) sampled as part of NAWQA 

studies contained at least one contaminant at a concentration that was greater than its 

human-health benchmark. Arsenic, uranium, and nitrate are the contaminants in Southwest basin-fill aquifers of the most concern for human health. The U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for these three 

contaminants were exceeded more than twice as frequently in Southwest drinking-water 

wells than in drinking-water wells nationwide. For example, nitrate exceeded its MCL in 

29 percent of domestic wells sampled in the Central Valley aquifer system in California 

compared to 4 percent nationwide.

Treatment or blending of water can reduce concentrations of some contaminants 

to below MCLs, but adds cost. Such treatment is required for public-supply wells, 

which are regulated under the Safe Drinking Water Act, but is not required for domestic 

wells—the source of water to about 1.4 million people in the Southwest. The widespread 

detection of contaminants in domestic wells and the lack of required monitoring or 

treatment underscores the need for public education about where contaminants are likely 

to occur and what testing and treatment options are available. See chapters 5 and 6.

2 Arsenic and uranium derived from geologic sources are potential 

drinking-water concerns

Volcanic and granitic rocks are geologic sources of arsenic and uranium in 

groundwater. Arsenic-containing rocks are widespread in the Southwest, and samples 

from 16 percent of drinking-water wells (14 percent of domestic wells and 19 percent 

of public-supply wells) contained arsenic at a concentration exceeding its MCL of 

10 micrograms per liter (μg/L). Uranium-containing rocks are less widespread, but 

uranium concentration exceeded its MCL of 30 μg/L in 7 percent of the drinking-water 

wells sampled (12 percent of domestic wells, but only 1 percent of public-supply 

wells). Exposure to arsenic and uranium contributes to an increased risk of cancer. 

Mineral dissolution, geochemical interactions, high rates of evapotranspiration, and 

other processes that typically occur in the arid Southwest can increase concentrations of 

arsenic and uranium in groundwater. Overall, contaminants in basin-fill aquifers from 

geologic sources are a greater concern for drinking water than are contaminants from 

human-related sources.

A statistical model estimates that arsenic concentrations in groundwater in almost 

half of the basin area in the Southwest are equal to or greater than the MCL. As population growth and scarce water resources in this part of the country stimulate the search 

for additional sources of groundwater, water treatment or blending may be necessary to 

reduce arsenic concentrations to below the MCL. Understanding the factors that affect 

concentrations of arsenic and other contaminants with geologic sources in groundwater 

can help water suppliers prioritize areas for new groundwater development and reduce 

treatment costs. See chapters 4 and 6.

4 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Overview of Major Findings and 

Water-Quality Issues for the Southwest Basin-Fill 
3 Dissolved-solids concentrations in groundwater are increasing 

in some areas

Shallow groundwater in agricultural and urban areas often has high dissolvedsolids concentrations as a result of evapotranspiration, mineral dissolution, and 

contributions from human-related sources, such as wastewater and urban runoff. 

Concentrations in deep wells used for public supply typically are low, but substantial 

increases in concentrations over time in some areas of the Southwest raise concerns 

about the long-term viability of the groundwater resource for drinking. Confining 

layers naturally protect deep groundwater, but wells can provide pathways that rapidly 

transport groundwater across confining layers and, thus, short circuit the natural 

groundwater flow system. Dissolved solids, which can impart an unpleasant taste to 

water, were measured at concentrations that exceeded the recommended maximum 

value of 500 milligrams per liter (mg/L) in one of every four drinking-water wells 

sampled (31 percent of domestic wells and 19 percent of public-supply wells). See 

chapters 4 and 6.

4 Artificial recharge and groundwater withdrawals are moving 

contaminants to deeper parts of basin-fill aquifers

Excess irrigation water and groundwater withdrawals for irrigation and public 

supply can increase the movement of contaminants from human-related sources to 

deeper parts of basin-fill aquifers that are pumped for drinking water. Contaminants 

include nitrate from fertilizers and manure, volatile organic compounds (VOCs) and 

pesticides used in urban and agricultural areas, and dissolved solids from irrigation 

water that has become more concentrated with salts by evapotranspiration. Nitrate 

concentrations exceeded the MCL of 10 mg/L in 18 percent of domestic wells and 

almost 4 percent of public-supply wells in the Southwest. At least one VOC, pesticide, 

or pesticide degradate was detected in 57 percent of drinking-water wells, mostly at a 

low but measurable concentration. Surprisingly, artificial recharge and groundwater 

withdrawals also are contributing to the release of naturally occurring uranium from 

aquifer sediments—uranium concentrations in one of four domestic wells in the 

eastern Central Valley, California, exceeded the MCL.

Increased development of water resources in the Southwest likely will increase 

the movement of contaminants to the parts of aquifers used for drinking-water supply. 

Groundwater can take years or decades to travel to these depths, and samples of deep 

groundwater collected now likely do not reflect the full effect of past and present 

human activities. Water-quality samples from shallow and deeper parts of an aquifer 

can provide an early warning of potential future water-quality concerns. Groundwater 

remediation across large areas is costly, slow, and generally impractical, so management practices that prevent the mobilization of contaminants are likely to be a more 

effective way to maintain the quality of groundwater in the Southwest. See chapters 

4 and 6.

Chapter 1

Chapter 1: Overview of Major Findings and Implications 5

Implications

Aquifers
Race Track Playa, Death Valley, California.

“Results from the NAWQA groundwater investigations 

provide an excellent overview of aquifer conditions in 

the Southwest that will prove invaluable in my ambient 

groundwater studies in Arizona.”

—Douglas Towne, Groundwater Hydrologist

Arizona Department of Environmental Quality
This chapter summarizes 

the study design used to 

investigate water quality 

in Principal Aquifers, 

including basin-fill 

aquifers in the Southwest.

Groundwater studies conducted during 1993–2009 as part of the 

NAWQA Program in the California Coastal Basin aquifers, the Central 

Valley aquifer system, the Basin and Range basin-fill aquifers, and the 

Rio Grande aquifer system were designed to answer broad questions about the 

occurrence, fate, and transport of contaminants in aquifers used as sources of 

drinking water. The Southwest Principal Aquifers study area contains basin-fill 

aquifers spread across more than 400 basins (fig. 2–1). Groundwater quality 

was studied in 15 basins that represent a wide range of agricultural and urban 

development; several basins have had some of the highest water-use rates in the 

Southwest for a relatively long time, while others have only recently become 

more developed. 

Understanding study results

Important aspects of the NAWQA Principal Aquifer assessments:

•	 Water samples were collected at the wellhead (see diagram in sidebar, Anatomy 

of a well, p. 20) prior to any treatment. They represent the quality of the groundwater resource but not necessarily the quality of tap water.

•	 The focus of the assessments is the condition of the total resource, including 

groundwater in a wide range of hydrologic and land-use settings across 

the Nation, rather than conditions at specific sites with known waterquality concerns.

•	 The assessments are guided by a nationally consistent study design, and all 

assessments use the same methods of sampling and analysis. Findings apply to 

water quality of a particular aquifer but also contribute to the larger picture of 

how and why water quality varies regionally and nationally. This consistent 

approach helps to determine if a water-quality issue is isolated or widespread. 

(See http://water.usgs.gov/nawqa/about.html for more information.)

•	 The assessments focus on aquifers used for water supply or on shallow 

groundwater that underlies an area with a particular type of land use. Because 

the NAWQA groundwater study areas do not cover the full spatial extent of the 

targeted Principal Aquifer, the findings might not represent the effects of the full 

range of geology, climate, and land use present.

•	 Analytical methods used by USGS chemists for assessments of water quality in 

Principal Aquifers are designed to measure constituents at as low a concentration as feasible. As a result, constituents frequently are detected at concentrations far below human-health benchmarks for drinking water (see sidebar, 

Human-health benchmarks and other guidelines used in this assessment, p. 43). 

Low-level detections allow scientists to identify and evaluate emerging issues 

and to track contaminant levels over time.

Chapter 2

Domestic wells (an example is shown), 

public-supply wells, and monitoring wells in 

urban and agricultural areas were sampled to 

characterize the quality of water in Southwest 

basin-fill aquifers.

Chapter 2: NAWQA Approach to Assessing 

Groundwater Quality
circ1358ch02 fig2-1

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal Basin

Middle Rio Grande Basin

Southwest Principal Aquifers

study area boundary

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pacif ic

Ocean

 EXPLANATION

Principal Aquifers

 Basin and Range basin-fill aquifers 

 California Coastal Basin aquifers

 Central Valley aquifer system

 Rio Grande aquifer system

Area without basin-fill aquifers

Study basin 

N

Principal Aquifers and study basins

Southwest

Principal Aquifers

Figure 2–1. As part of the NAWQA Program, water-quality conditions were studied in 15 basin-fill aquifers that are part of four 

Principal Aquifers in the southwestern United States. The basins containing these aquifers were selected for study because 

groundwater in these areas is an important source of water for drinking and irrigation.

8 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Assessing Water Quality in Southwest BasinFill Aquifers

How does one go about characterizing the groundwater 

over an area as large as that covered by basin-fill aquifers 

in the Southwest, let alone the whole United States? The 

approach taken by the USGS is to use different types of 

groundwater studies to gain a better understanding of how 

and why water quality varies. These groundwater studies are 

the building blocks of NAWQA’s water-quality assessments 

of Principal Aquifers. Groundwater studies were designed 

to answer the following questions: How does land use affect 

groundwater quality? How does water quality change as 

it moves through the aquifer? What is the quality of the 

drinking-water resource? Groundwater studies designed to 

broadly assess water-quality conditions in aquifers used for 

drinking-water supply were conducted to sample networks 

of existing pumping wells—in the NAWQA Program these 

studies are called major-aquifer studies (fig. 2–2). Agricultural 

and urban land-use studies designed to characterize and 

explain the quality of recently recharged groundwater (generally less than 10 years old(1)) in these land-use settings were 

conducted to sample mostly shallow monitoring wells that 

were installed as part of the NAWQA Program. Other studies 

focused on the groundwater quality of public water supplies 

and the transport of contaminants to public-supply wells.(2, 3)

Figure 2–2. The Southwest Principal Aquifers assessment characterized the quality of water from basin-fill aquifers used as a 

source for drinking—water pumped from domestic and public-supply wells—by grouping samples collected mostly for major-aquifer 

and contaminant-transport studies. Shallow groundwater quality in agricultural and urban settings was assessed using samples 

collected for land-use studies. Samples also were collected from Basin and Range carbonate-rock aquifers to assess the water quality 

(see sidebar, Water quality of Basin and Range carbonate-rock aquifers, Nevada and Utah, p. 14).

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley

Spanish

Springs

Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana

Coastal Basin

Middle Rio Grande Basin

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pacif ic

Ocean

Los Angeles

Phoenix

Sacramento

San Diego

Southwest Principal Aquifers 

study area boundary

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Fresno

Reno

San Francisco

Modesto

Carson Desert

Rincon-Hatch area

Northern Coast Ranges area

Major-aquifer study well

Agricultural land-use study well

Urban land-use study well

Contaminant-transport study well

Groundwater samping site

Approximate boundary of Basin and

Range carbonate-rock aquifers

EXPLANATION

Principal Aquifers

 Basin and Range basin-fill aquifers

 California Coastal Basin aquifers

 Central Valley aquifer system

 Rio Grande aquifer system

Area without basin-fill aquifers

Study basin

N

Sacramento 

Valley

San Joaquin 

Valley

Groundwater sampling sites in Southwest Principal Aquifers

Southwest

Principal Aquifers

Chapter 2

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 9
Table 2–1. Water samples and ancillary information were collected from 1,078 wells in the Southwest Principal Aquifers study 

area from 1993 to 2009. Major-aquifer and contaminant-transport studies were conducted to sample wells screened at depths that 

represent parts of the aquifers used for drinking water, whereas the intent of land-use studies was to intercept water that recharged 

through a particular land use, and the wells typically were screened just below the water table. Information for the NAWQA studies, 

such as study location, sampling frequency, and data collected, is listed in appendix 1.

Characteristic Major-aquifer study Agricultural 

land-use study

Urban 

land-use study

Contaminanttransport study

Study design objective

Assess quality of water 

in aquifers used for 

drinking

Assess quality of recently 

recharged groundwater 

associated with 

agricultural land use

Assess quality of recently 

recharged groundwater 

associated with urban 

land use

Assess quality of water in 

aquifers used for public supply

Land use in study area Mixed Agricultural Residential and 

commercial Mostly urban

Number of studies in 

Southwest Principal 

Aquifers study area

17 9 7 3

Number of wells per 

study (typical) 20–30 20–30 20–30 14–39

Type of wells sampled

Mostly domestic 

and public-supply 

wells, but also some 

irrigation, stock, and 

industrial wells

Monitoring wells, except 

for studies of the 

San Joaquin Valley, 

California, for which 

domestic wells also 

were sampled

Monitoring wells 

Public-supply wells in Modesto, 

California; Eagle and Spanish 

Springs Valleys, Nevada; and 

Albuquerque, New Mexico

Well depth range, 

in feet 15–1,805 11–390 11–200 195–2,070

Median well 

depth, in feet 300 38 34 620

Number of wells 

sampled 508 294 207 69

This Principal Aquifer assessment brings together and 

interprets results from all of the NAWQA groundwater studies 

in the Southwest Principal Aquifers study area. Throughout 

this assessment, the results are grouped as those that characterize the groundwater used as a drinking-water supply and 

those that characterize the quality of groundwater that has 

recently recharged in either an agricultural or an urban landuse setting. The groundwater generally used for drinking is 

from deeper wells and recharged the aquifers many years ago, 

in some cases long before any manmade chemicals were used 

on the land surface. The results of the land-use studies allow 

us to evaluate the effect of recent human activities on shallow 

groundwater quality and may provide an early warning for 

potential future contamination of drinking-water resources.

To assess water-quality conditions of the groundwater 

used as a drinking-water resource, one sample was analyzed 

from each of about 656 existing pumping wells for major 

and trace inorganic constituents, nutrients, VOCs, and 

pesticides (see appendix 2, table A2–1, and sidebar, NAWQA 

assessments use a wide range of geochemical data and site 

information, p. 13). Most of these wells were sampled as part 

of major-aquifer studies, with 508 wells distributed across 

16 basins (or groups of basins in the case of the northern 

Coast Ranges area in California) (fig. 2–2; table 2–1). The 

other samples were collected from 69 public-supply wells 

in Modesto, California; Eagle and Spanish Springs Valleys, 

Nevada; and Albuquerque, New Mexico, as part of contaminant-transport studies that focused on the quality of public 

drinking water in those areas and from 79 domestic wells 

in the San Joaquin Valley, California, as part of agricultural 

land-use studies. About 48 percent of the existing pumping 

wells sampled were domestic wells and 39 percent were 

public-supply wells. The others were mostly irrigation wells, 

but also included stock and industrial wells.

10 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
To assess the quality of recently recharged groundwater 

underlying urban areas, one groundwater sample was collected 

from each of 209 monitoring wells installed by the USGS in 

seven basins (fig. 2–2). The urban land-use studies focused 

on areas similar to those where many people work and live—

single- and multi-family residential and commercial development—in the Las Vegas (Las Vegas Valley), Carson City 

(Eagle Valley), Reno/Sparks (Truckee Meadows), Sacramento 

(Sacramento Valley in the Central Valley), Albuquerque 

(Middle Rio Grande Basin), Orange County (Santa Ana 

Coastal Basin), and Salt Lake City (Salt Lake Valley) metropolitan areas. The wells were relatively shallow—median 

depth of 34 feet (ft)—and tapped groundwater near the water 

table. The water quality of these samples is an indication of 

how the chemicals we use in day-to-day life—pesticides, 

solvents, fertilizers—might affect the quality of groundwater 

used for drinking in the future.

To assess the quality of recently recharged groundwater 

underlying agricultural areas, one groundwater sample was 

collected from each of 294 monitoring and domestic wells 

as part of nine land-use studies (fig. 2–2). Domestic wells, 

ranging from 55 to 390 ft deep (median depth of 150 ft), were 

sampled in addition to monitoring wells in the San Joaquin 

Valley agricultural land-use studies. The monitoring wells 

sampled in the agricultural areas were relatively shallow—

median depth of 33 ft—and tapped groundwater near the 

water table. Factors considered in determining what land uses 

to study include the importance of the land-use setting to 

groundwater use in the studied basin and the contamination 

potential of the targeted land use. Different crops grown 

in the Southwest receive different pesticides and amounts 

of irrigation. The agricultural land-use studies in Carson 

Valley, Carson Desert, and San Luis Valley targeted alfalfa 

and pasture areas, major crops in the Southwest that do not 

require heavy use of fertilizers or pesticides. Cotton, a crop 

that requires multiple applications of fertilizers and pesticides 

during the year, was the principal crop in the West Salt River 

Valley study area. Flood-irrigated alfalfa, peppers, onions, 

wheat, cotton, and pecans were studied in the Rincon-Hatch 

area in the Rio Grande floodplain. Rice fields that typically 

are flooded for 6 months at a time were the studied setting 

in Sacramento Valley. Almond orchards, vineyards, and a 

combination of corn, alfalfa, and vegetable/row crops were 

the settings targeted for study in the San Joaquin Valley.

Chapter 2

Urban land-use studies focused on the quality of shallow 

groundwater underlying areas of recently developed residential 

and commercial land (area around a monitoring well in Salt Lake 

Valley, Utah, is shown). Samples from monitoring wells in this 

setting provide information about the effects of human activities 

on shallow groundwater quality.

Monitoring well

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 11
Land-use-study monitoring wells were used to sample water that has recently infiltrated to the water table. In the Southwest, 

the median depth of these wells below the water table is 16 feet. Domestic wells, which are used to supply drinking water to 

individual households, mostly were deeper—median depth below the water table of 123 feet—than the monitoring wells. Publicsupply wells are used to supply drinking water to at least 25 people(48) and typically are deeper—median depth below the water 

table of 364 feet—than domestic or monitoring wells. Public-supply wells commonly are open to multiple water-producing parts 

of the aquifer. As a result, public-supply wells commonly have open intervals that are longer and deeper and yields that are larger 

than those of domestic wells.

circ1358 ch02fig3

Domestic well open to

intermediate depths

Public-supply 

well open

to intermediate 

and deeper 

parts of aquifer

Shallow monitoring well installed 

in agricultural or urban land-use 

settings to characterize the quality 

of recently recharged groundwater

Pumping water level

Public-supply and domestic 

well used to characterize 

the drinking-water resource

Public-supply well and domestic 

well used to characterize the 

drinking-water resource

NOT TO SCALE

Two types of wells that supply drinking water were 

sampled: domestic (private) wells and public-supply wells. 

Domestic wells typically are shallower than public-supply 

wells and, therefore, pump water that is nearer to sources 

of manmade contaminants, such as fertilizers and pesticides 

applied at the land surface. Domestic wells commonly are 

located in rural areas, so they are more likely than public-supply 

wells to be vulnerable to contamination from agricultural 

chemicals. Public-supply wells pump water from deeper in the 

aquifer and, therefore, are more likely to have higher concentrations of constituents from geologic sources, because deeper 

water generally has been in contact with the aquifer materials 

(sediments) for a longer period of time. Public-supply wells 

commonly are located in suburban and urban areas, so they are 

more likely than domestic wells to be vulnerable to contamination from chemicals associated with urban activities. Publicsupply wells have larger pumps and longer screened intervals 

than domestic wells and are pumped for longer periods of time. 

As a result, public-supply wells pump much larger volumes of 

water than domestic wells and capture groundwater from a 

What types of wells were sampled, and how might that affect water quality?

larger area; therefore, public-supply wells are more vulnerable 

than domestic wells to manmade contamination from distant 

sources. If the amount of water withdrawn is large enough, the 

flow direction and velocity of the groundwater can be changed, 

which can, in turn, affect the groundwater geochemistry and 

the constituents contained therein. Water from domestic wells 

is not regulated, and homeowners are responsible for testing, 

maintenance, or treatment of the water from their domestic 

well. Water from public-supply wells is required to be tested 

by the well operator on a routine basis to help assure that 

the water provided to consumers meets Federal and State 

water-quality standards.

Almost one in four of the groundwater samples collected 

from Southwest basin-fill aquifers were from monitoring wells. 

Monitoring wells are not pumped for water supply—they are 

used for measuring water levels or occasionally collecting 

water samples, but are not used for drinking water, irrigation, 

or to supply water for other uses. Monitoring wells sampled 

as part of the NAWQA groundwater studies were installed 

expressly for that purpose.

12 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Chemists at the USGS National Water Quality Laboratory analyze 

groundwater samples using an array of sophisticated techniques.

Additional information (often called “ancillary information”) 

about the well and the surrounding environment complements 

the chemical data measured. This additional information often 

is key to making sense of the chemical data. For example, 

the information might be used to determine that shallow 

groundwater is more (or less) vulnerable to contamination 

than deep groundwater, that domestic wells are more (or less) 

vulnerable to contamination than public-supply wells, or that 

urban land use is associated with different types of groundwater 

contamination than is agricultural land use. Chemical data 

without accompanying ancillary data are much less useful for 

understanding factors that affect groundwater quality. 

Constituents measured in samples from most wells

Constituent group Examples

Water-quality properties pH, specific conductance, dissolved 

oxygen, temperature

Major ions (filtered) Bromide, calcium, chloride, 

magnesium, sodium, sulfate

Trace elements (filtered) Arsenic, boron, iron, manganese, 

selenium, uranium

Nutrients (filtered) Ammonia, nitrate, phosphorus

Pesticides (filtered) Herbicides, insecticides, fungicides

Volatile organic compounds Solvents, gasoline hydrocarbons, 

refrigerants, trihalomethanes, 

fumigants

Organic carbon (filtered)

Additional constituents measured in samples from some wells

Constituent group Examples

Radionuclides Radon

Groundwater age tracers Tritium, chlorofluorocarbons

Stable isotopes Oxygen-18, hydrogen-2

Microorganisms Escherichia coli and total coliforms

Additional site information

Use of well

Well depth

Depth to water

Well-construction data

Principal Aquifer

Land-surface elevation at well

Land use within a 500-meter 

(1,640-foot) radius buffer

Estimates of nutrient inputs

Estimates of pesticide use

NAWQA assessments use a wide range of geochemical data and site information

Chapter 2

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 13
Most of the data on groundwater quality in the Southwest 

Principal Aquifers study area are from basin-fill aquifers. 

However, the carbonate-rock aquifers, which store a large 

amount of water and lie below and (or) adjacent to several Basin 

and Range basin-fill aquifers, are viewed as a potential future 

source of water for expanding cities like Las Vegas.(49) The Basin 

and Range carbonate-rock aquifers cover an area of about 

92,000 square miles (mi2

), mostly in Nevada and Utah (see map at 

right). The carbonate-rock aquifers consist of thick sequences 

of Paleozoic limestone and dolomite interbedded with shale, 

sandstone, and quartzite. Depth to groundwater can be several 

hundreds of feet and, at present, relatively few wells tap these 

aquifers. A regional assessment of the availability of groundwater 

in both the basin-fill deposits and carbonate rocks is presented by 

Heilweil and Brooks.(50)

Data were collected to describe baseline water quality in 

the carbonate-rock aquifers and can be used to assess potential 

future changes in water quality. Water was sampled from 30 wells 

and springs tapping carbonate-rock aquifers (20 in Nevada and 

10 in Utah) in 2003.(51) Major findings from the study include

•	 Groundwater near outcrops of a carbonate-rock aquifer 

can have a component of recently recharged water. As a 

result, human- or land-surface-related contaminants, such 

as pesticides, nitrate, and fecal indicator bacteria, were 

detected in some samples.

•	 The quality of water from sites located in discharge areas 

or wells with screens at a great depth below the land 

surface generally shows no effect of land-use activities. 

This groundwater was recharged thousands of years ago.

•	 Arsenic concentrations in sampled groundwater increased 

along regional flow paths. Long residence times and 

geochemical processes within the aquifer likely contribute 

to the elevated arsenic concentrations, although 

geothermal resources in the region could also be sources 

of arsenic.

Carbonate rocks are highly fractured in some outcrops, 

which allows groundwater to rapidly recharge the aquifer.

Photograph by Michael Rosen, USGS

Water quality of Basin and Range carbonate-rock aquifers, Nevada and Utah

Las Vegas

Salt Lake City

Great Salt Lake

Southwest Principal Aquifers

study area boundary

Map area

Approximate boundary of Basin 

and Range carbonate-rock aquifers

NEVADA

OREGON IDAHO

CALIFORNIA

ARIZONA

UTAH

 EXPLANATION

Carbonate-rock outcrop

Groundwater sample site for 

 Basin and Range carbonate-rock 

 aquifer study

N

Basin and Range carbonate-rock aquifers underlie Basin 

and Range basin-fill aquifers in a large part of Nevada and 

Utah. Population growth and demand for water could lead to 

development of the carbonate-rock aquifers as a source of 

water supply.

14 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
The Southwest is a land of contrasts, with its arid deserts and rocky 

canyons, emerald-green expanses of irrigated agriculture, and fastgrowing cities and suburbs. All these features—climate, geology, 

agriculture, and population growth—affect the groundwater flow system and 

groundwater quality. Understanding these key background features is essential 

for assessing the vulnerability of Southwest basin-fill aquifers to contamination. 

Chapter 3

Chapter 3: Background: Environmental Setting, 

Hydrogeology, Population, Land Use, and 

Water Use 

Basin-fill aquifers in the Southwest mostly are overlain by dry undeveloped land, irrigated 

agricultural fields, cities and towns, or a combination of these. Race Track Playa, left; 

Salinas Valley, center; San Fernando Valley, right; all in California.

This chapter summarizes 

background information 

for basin-fill aquifers 

in the Southwest and 

provides the context for 

understanding findings 

about water quality in 

this group of Principal 

Aquifers. The chapter 

covers the environmental 

and hydrogeologic setting 

and information about 

population, land use, and 

water use.

Much of the information presented in this chapter is from the report

“Conceptual understanding and groundwater quality of selected

basin-fill aquifers in the southwestern United States” (available at 

http://pubs.usgs.gov/pp/1781/).

Photographs: Left, David Anning, USGS; center, Edwin Remsberg, U.S. Department of Agriculture; right, Chris Austin, www.aquafornia.com
Environmental Setting

The Southwest Principal Aquifers study area covers 

about 409,000 mi2 and encompasses much of California and 

Nevada and parts of Utah, Arizona, New Mexico, and Colorado 

(fig. 2–1). The study area is characterized by sediment-filled 

basins or valleys that are separated by mountains composed of 

consolidated rock. Basin-fill aquifers, which contain permeable 

unconsolidated deposits, are the primary source of water for 

many cities and agricultural communities in this arid to semiarid 

area. In addition, the aquifers provide base flow to streams that 

support important aquatic and riparian communities.

Large variations in size, altitude, and climate in the 

Southwest contribute to differences in groundwater quality 

among basins. The generally north-south trending study 

basins range in size from about 23 mi2

 for Eagle Valley, 

Nevada, to about 20,000 mi2 for the Central Valley, California. 

Study-basin-floor altitudes range from near sea level in the 

California coastal basins and Central Valley to more than 

8,000 ft in the San Luis Valley, Colorado. Mountain altitudes 

surrounding the basins often exceed 10,000 ft and can reach 

more than 14,000 ft in the Sierra Nevada, California, and in 

the San Juan Mountains, Colorado, resulting in considerable 

topographic relief within a drainage basin. The wide range in 

Southwest Principal Aquifers at a glance

People living in the Southwest Principal Aquifers study area

Maximum average annual precipitation, in inches, falling on 

most Southwest basins

Number of unconsolidated sediment-filled basins in the Southwest

Extent of the study area in square miles

People in the study area who depend on domestic wells for drinking water

46,600,000

1,400,000

409,000

425

12

1⁄4 Proportion of groundwater withdrawals for irrigation and public supply

in the United States that is from Southwest basin-fill aquifers

16 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 3–1. Precipitation in the Southwest is sparse—less than 12 inches per year over much of the region. Mountains 

receive more precipitation than basins, and snowmelt and runoff from mountainous areas contribute much of the recharge to 

the basin-fill aquifers. In many areas, streams flow only intermittently, and basin-fill aquifers are the major source of water for 

drinking and irrigation.

altitude causes large variations in precipitation, evaporation, 

temperature, and vegetation within the study area, all of which 

can affect water quality.

The climate of the sediment-filled basins is arid to 

semiarid—average precipitation in the study basins ranges from 

about 6 inches per year (in/yr) in Las Vegas Valley, Nevada, 

to almost 22 in/yr in the northern Central Valley, California.(4)

The surrounding mountains can receive much more precipitation than the basins, much of it as snow (fig. 3–1). Warm 

average daily temperatures in the basins combined with low 

humidity result in potential evapotranspiration that greatly 

exceeds average annual precipitation. As a consequence of 

evapotranspiration, salts are concentrated at and near land 

surface and in shallow groundwater.

Large-scale patterns of climate variability strongly, but 

irregularly, control weather in the Southwest, causing large 

year-to-year variations in precipitation and groundwater recharge. 

The latest drought affecting the region began in about 1999, but 

much longer droughts, on the scale of decades, are indicated by 

paleoclimate record.(5) On a longer time scale, conditions in much 

of the region were cooler and wetter prior to about 10,000 years 

ago. Paleorecharge, that is, groundwater recharge that occurred 

during the last glacial period, is a major source of present-day 

groundwater in several of the more arid basins.(6)

NEVADA

WYOMING 

OREGON

IDAHO

CALIFORNIA ARIZONA NEW MEXICO

UTAH

COLORADO

Pacif ic

Oc ean

Los Angeles

Phoenix

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

San Francisco

Southwest Principal Aquifers

study area boundary

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal Basin

Middle Rio Grande Basin

From McKinney and Anning(4)

Average annual precipitation

N

Southwest

Principal Aquifers

0 to < 12

12 to < 24

24 to < 36

36 to < 48

48 to < 60

60 to < 72

72 to 171

Study basin

Average annual 

precipitation, 

in inches

EXPLANATION

Chapter 3

Chapter 3: Background: Environmental Setting, Hydrogeology, Population, Land Use, and Water Use 17
Figure 3–2. The Southwest Principal Aquifers study area contains more than 400 basins that are filled with unconsolidated deposits 

derived from the adjacent mountains. Differences among the rock types that are the source of the deposits contribute to differences in 

groundwater quality in the basins.

Hydrogeologic Setting

Southwest basin-fill aquifers are similar hydrogeologically because they all are composed of unconsolidated to 

semiconsolidated gravel, sand, silt, and clay that are saturated 

with water. An important factor that affects the quality of 

groundwater in Southwest basin-fill aquifers is the type of 

rock that the water flows over or through (fig. 3–2). The 

consolidated rocks in mountainous areas are the primary 

source of the sediment deposited in the basins. Some rocks 

weather (erode) more easily than others and, therefore, 

contribute more dissolved constituents to groundwater. Intrusive igneous rocks and metamorphic rocks generally are less 

easily eroded and less soluble in groundwater than volcanic 

and marine-deposited sedimentary rocks. The concentration 

of constituents with geologic sources, such as arsenic and 

Unconsolidated basin-fill deposits exposed in a gravel pit range 

from fine sand and silt to gravel and cobbles.

circ1358 ch03fig2

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pacif ic

Ocean

Southwest Principal Aquifers

study area boundary

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal Basin

Middle Rio Grande Basin

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal Basin

Middle Rio Grande Basin

Modified from McKinney and Anning(4)

Consolidated rock types and unconsolidated deposits 

N

Southwest

Principal Aquifers

Consolidated rock types

Metamorphic or intrusive 

 Volcanic

 Carbonate

Sedimentary formations

Unconsolidated deposits

 Alluvium and surficial deposits

Study basin

EXPLANATION

18 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
uranium, in groundwater depends on the type of rock through 

which the groundwater flows.

Other geologic features also can affect the quality of 

groundwater in the basin fill. For example, faults facilitate 

localized upward movement of geothermal and (or) highly 

mineralized groundwater from greater depths to shallower 

parts of aquifers in some basins. This mineralized groundwater 

frequently has elevated concentrations of contaminants with 

geologic sources, such as dissolved solids and arsenic, that 

impair water quality.

Geologic characteristics of a basin control the recharge 

and movement of groundwater (fig. 3–3). The basins are 

bounded by faults and mountains, and most contain thick 

(thousands of feet) sequences of sediment. The sediments are 

more compact with depth, resulting in fewer connected pore 

spaces for groundwater to flow through and lower permeability in deeper parts of the aquifer. As a result, water flows 

more slowly through the aquifer at depth and, therefore, has 

more time to interact with the aquifer sediments. Coarsegrained sediment has been deposited by high-energy streams 

in alluvial fans and stream channels along the mountain 

fronts, where the water table (top of unconfined aquifer) can 

be several hundred feet below land surface. The alluvial fans 

transition to a flat valley floor where deposition in ancient 

lakes and floodplains created layers of fine-grained, less 

permeable sediment interbedded with more permeable layers 

of sand and gravel. These confining layers of silt and clay 

result in confined conditions in the aquifer or a confined 

aquifer (see sidebar, Aquifer, p. 20).

Confining layers in many basins typically occur within 

about 100 ft of the land surface, although they commonly are 

thin and not widespread. A shallow aquifer can form above the 

uppermost confining layer. The quality of shallow groundwater 

commonly is affected by human activities at the land surface 

and by natural processes, such as evaporative concentration. 

Confining layers and upward movement of groundwater prevent 

the infiltration of water that could contain contaminants from 

the land surface, thus partially protecting the water quality of 

deeper parts of basin-fill aquifers. This deeper groundwater is 

the groundwater resource most used—water-supply wells in 

basin-fill aquifers of the Southwest typically pump water from 

between about 200 to 1,000 ft below land surface. Publicsupply wells and irrigation wells typically are open to a greater 

thickness of aquifer than domestic wells because of the need for 

larger yields (see sidebar, What types of wells were sampled, 

and how might that affect water quality?, p. 12).

Figure 3–3. In an undeveloped Southwest basin, the principal source of recharge to the basin-fill aquifer is precipitation that 

falls on the surrounding mountains and drains into the basin. Groundwater moves from the basin margin to the basin lowland. 

Natural discharge typically occurs as evapotranspiration, flow to streams and springs, and subsurface outflow to an adjacent 

basin. In a developed basin, in contrast, irrigation and groundwater pumping are the primary sources of recharge and discharge 

(see chapter 4 for more discussion).

circ1358 ch03fig3

EXPLANATION

Precipitation Recharge

Discharge

Groundwater movement

Seepage from stream

Seepage to stream

Evapotranspiration

Subsurface

outflow to 

adjacent basin

Subsurface

inflow from

adjacent basin

Subsurface inflow from

consolidated rock

NOT TO SCALE

Saturated basin-fill

deposits

Consolidated 

rock

Unconfined part of aquifer

Unsaturated zone

Confined part of aquifer

Confining layer

(clay and silt)

Fault

Water-filled fractures

Coarse-grained

sediment (mostly

sand and gravel)

Fine-grained 

sediment

(mostly sand, 

silt, and clay)

Water table

Basin lowland

Basin margin

Alluvial fan

Chapter 3

Chapter 3: Background: Environmental Setting, Hydrogeology, Population, Land Use, and Water Use 19
An underground layer of saturated permeable materials (rock, gravel, sand, or silt) that will yield a 

useful quantity of water to a well.

Anatomy of a well

A well is simply a hole in the ground (well bore or borehole) 

from which water can be removed. The well bore is lined with 

a well casing, such as a pipe, to prevent the well bore from 

collapsing. The casing, along with a sealant (called grout), also 

prevents water from flowing into the well from the land surface 

or from parts of the aquifer where the water quality may be less 

desirable. The casing can be open at the bottom or perforated at 

a specific depth with a screen, to allow water to flow into the well 

where it can be pumped to the surface. Coarse sand or gravel 

(called sand pack or gravel pack) can be placed around the well 

screen to help improve the flow of water into the well. Some wells 

are cased only near the land surface, allowing water to flow into 

the well from nearly the entire length of the well bore.

The unsaturated zone is the area below the land 

surface and above an aquifer. In addition to soil, 

rocks, and air, the unsaturated zone contains water 

from the land surface (such as rain) that is slowly 

moving downward to the water table of the aquifer. 

An unconfined aquifer is bounded at its top by the 

water table, below which water fills all the pore 

spaces in the rock. Water from the land surface can 

move down into an unconfined aquifer.

A confining layer is a layer of material (often clay) 

through which water does not easily flow, creating a 

boundary between aquifers.

A confined aquifer is bounded at its top by a confining 

layer. Water enters or “recharges” confined aquifers 

where the confining layer is not present. Where the 

confining layer is not continuous or is breached (for 

example, by a well), flow between the unconfined and 

confined aquifer can occur.

The pressure within a confined aquifer can be 

greater than that in the overlying unconfined aquifer 

if the source of the water in the confined aquifer is 

at a higher elevation than the unconfined aquifer. 

In that case, water in a well in a confined aquifer 

will rise to a higher level than that in the overlying 

unconfined aquifer.

Sedimentary aquifer 

Groundwater storage and flow 

between grains of sediment

Bedrock aquifer

Groundwater storage

and flow in fractures

Carbonate aquifer

Groundwater storage and flow

in solution cavities or fractures

Aquifer (aq.ui.fer)—  ̆

When pumped, groundwater flows through the 

screened interval and 

up and out of the well

Unsaturated

Saturated

Water table

Screen

Well bore

Water level

in well

Wellhead

Well casing

Pump

Grout

Gravel pack

Confining layer

Confined aquifer

Unconfined aquifer

Unsaturated zone

Water table Wells

Surface water

Screen

20 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Population, Land Use, and Water Use

About 46.6 million people live in the Southwest Principal 

Aquifers study area,(7) mostly in urban metropolitan areas, but 

also in rural agricultural communities that are associated with 

about 22,500 mi2 (14.4 million acres) of cropland. California 

is the most populous state in the Nation, with more than 

37 million people (2010 census) and a population density of 

239 people per square mile (13th highest nationally). Population in the Southwest is growing rapidly, putting increased 

pressure on much of the area’s limited water resources. During 

1990–2010, the population growth rate in Nevada, Arizona, 

and Utah was the largest in the United States. By 2030, the 

population in the Southwest is projected to increase by about 

15 million from that of 2010.(8) An example of explosive 

Figure 3–4. Urban and irrigated agricultural lands are focused mostly in areas where water (surface water, groundwater, or both) 

is accessible. Sagebrush and other rangeland and desert vegetation cover much of the alluvial basins in the Southwest. Forests are 

restricted to mountainous areas.

Southwest Principal Aquifers

study area boundary

NEVADA

WYOMING 

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pacif ic

Ocean

Urban

 Land Use

Agriculture

Rangeland

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal Basin

Middle Rio Grande Basin

Forest or other

EXPLANATION

Study basin

From Homer and others (83)

Land use

Circ1358 ch03fig 4

Southwest

Principal Aquifers

N

Chapter 3

population growth is Spanish Springs Valley, Nevada, where 

the population increased from less than 800 people in 1979 

to about 45,000 in 2008.(9) Another example of increasing 

population is the Santa Ana Coastal Basin in southern 

California, where what was once agricultural land has been 

almost completely urbanized with a population density of 

about 7,000 people per square mile in 2005 and less than 

5 percent undeveloped land. Groundwater is used for drinking 

by almost 2.6 million people in the Santa Ana Coastal Basin, 

and water quality is a major management concern.

Prior to people moving to the Southwest, the land was 

primarily rangeland, and rangeland still covers most of the 

alluvial basins (fig. 3–4). People live and grow crops where 

water is available, either from streams or aquifers. Most 

crops are cultivated on the relatively flat basin floors and, in 

Chapter 3: Background: Environmental Setting, Hydrogeology, Population, Land Use, and Water Use 21
“By the use of all the perennial streams during the 

season of irrigation, by the storage of the surplus water 

that runs to waste in seasons when irrigation is not 

practiced, by the impounding of the storm-waters, by 

the recovery of the floods accumulated in valley sands, 

and by the utilization of the artesian fountains, a vast 

area of the arid lands will ultimately be reclaimed and 

millions of men, women and children will find happy, 

rural homes in the sunny lands.” 

—John Wesley Powell, The Irrigable Lands of the Arid 

Region published in 1890 in the Century Illustrated 

Monthly Magazine.

particular, along rivers and streams that are used to supply 

irrigation water to fields. Irrigation is required to grow crops 

in most areas of the Southwest and even prehistoric farmers, 

such as the Hohokam culture in central and southern Arizona, 

diverted water to their fields.(10) Among the study basins, agricultural land area ranges from less than 2 percent of the Las 

Vegas Valley to about 61 percent of the Central Valley. Agricultural production in the Central Valley in 2007 was valued 

at $28 billion, more than 75 percent of the total value of crop 

production for California and more than that for any other 

State in the Nation.(11) Irrigated agricultural land is rapidly 

being urbanized in much of the Southwest. For example, as 

more people have moved to the Central Valley—its population 

was more than 6 million in 2005—the towns and cities have 

encroached onto what previously was agricultural land. As 

another example, cropland in Maricopa County, Arizona, 

which is where the metropolitan Phoenix area is located, was 

reduced by 140 mi2 or 67 percent from 1997 to 2007.(12)

The Las Vegas Valley, Nevada, population has grown from 

273,000 people in 1972 to more than 2 million in 2010. Areas of 

irrigated vegetation are shown in red and impervious surfaces, 

such as roads, are shown in gray on these Landsat satellite 

images. The large irrigated areas mostly are golf courses and 

city parks.

USGS Earth Explorer by the NASA/Goddard Space Flight Center

22 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 3–5. About 98 percent of the water pumped from 

Southwest Principal Aquifers is used for irrigation and public 

supply.(13) In 2005, however, about 1.4 million people in mostly rural 

areas relied on domestic wells that tap basin-fill aquifers—these 

wells are not routinely monitored for water quality.(52)

Much of the population of the arid to semiarid 

Southwest relies on good-quality groundwater 

because of limited surface-water supplies.

0

2,000

4,000

6,000

8,000

Irrigation Public supply Self-supplied

industrial

Self-supplied

domestic

Basin and Range basin-fill aquifers

EXPLANATION

Groundwater withdrawals in Southwest Principal Aquifers

California Coastal Basin aquifers

Central Valley aquifer system

Rio Grande aquifer system

Groundwater withdrawals,* in millions of gallons per day 

*Self-supplied domestic withdrawal data 

as of 2005; all other withdrawal data as of 2000.

Chapter 3

Because the amount of surface water available in the arid 

to semiarid area is limited, cultural and economic activities 

in the region are particularly dependent on good-quality 

groundwater. In 2000, about 20,000 million gallons per 

day (23.0 million acre-feet per year) of groundwater was 

used in the Southwest Principal Aquifers study area. Water 

pumped from the basin-fill aquifers for irrigation and public 

supply (fig. 3–5) accounted for about one-quarter of the total 

withdrawals for these purposes from all aquifers in the United 

States.(13) Although the primary uses of groundwater in the 

Southwest are irrigation and public supply, water use varies 

locally by basin, and withdrawals for industrial uses, mining, 

and electric power generation are substantial in some areas.

Chapter 3: Background: Environmental Setting, Hydrogeology, Population, Land Use, and Water Use 23
Sprinkler irrigation is common in the San Luis Valley of south-central Colorado and in many other basins in the Southwest where 

agriculture is prevalent. Pumping for irrigation has caused groundwater-level declines in some basins, and recharge of excess irrigation 

water can transport agricultural chemicals that are applied on the cropland to the groundwater below.
Groundwater in Southwest basin-fill aquifers flows from recharge 

areas near the mountains to discharge areas in the valley lowlands 

in hundreds of individual basins. Each basin is unique, but natural 

hydrologic conditions and geochemical processes affect groundwater flow 

and quality in similar ways. The susceptibility of the aquifers to contaminants 

from geologic sources and from the land surface is controlled by these factors. 

Human activities in some basins—especially irrigation and pumping—have 

profoundly altered natural aquifer systems by increasing groundwater flow 

volumes and rates and introducing new contaminants.

Agricultural and urban development in many Southwest basins has changed the water quality of 

the basin-fill aquifers. Groundwater recharged to and pumped from the aquifers has enhanced the 

movement of water and chemicals from the land surface to parts of the aquifers used for water 

supply. The Arizona Canal and irrigated fields near Scottsdale, Arizona, are shown.

Photograph by Tim Roberts, www.shutterstock.com

Much of the information presented in this chapter is from the report “Effects of natural and human factors on 

groundwater quality of basin-fill aquifers in the southwestern United States—Conceptual models for selected 

contaminants” (available at http://pubs.usgs.gov/sir/2011/5020/ ).

Chapter 4

Chapter 4: The Hydrology of Southwest BasinFill Aquifers and How Human Alterations Affect 

Groundwater Quality 

 This chapter explains 

and discusses the hydrologic and geochemical 

processes and human 

activities that affect the 

movement and quality of 

groundwater in Southwest 

basin-fill aquifers.
Natural Hydrologic Settings and Geochemical 

Processes Are Key to Groundwater Flow 

and Quality 

Under natural conditions—that is, where human development has not altered the hydrology—recharge to basin-fill 

aquifers originates mostly as precipitation on the surrounding 

mountains that runs off and infiltrates into the coarse-grained 

stream-channel and alluvial-fan deposits near the basin 

margins (figs. 3–3 and 4–1A). Little of the precipitation that 

falls on the basin floor in drier areas becomes groundwater 

recharge because most of the water evaporates or is transpired 

by plants (evapotranspiration) before it infiltrates down to 

the water table. In wetter areas, such as parts of the Central 

Valley, there is more groundwater recharge from precipitation 

on the basin. Groundwater discharge under natural conditions 

typically is through evapotranspiration and seepage to streams 

and springs in basin lowlands. The presence of groundwaterfed streams or springs that flowed year round in the early days 

of the settlements are why the cities of Las Vegas, Nevada; 

Tucson, Arizona; and San Bernardino, California, are located 

where they are.

Potential evapotranspiration—the amount of water 

that would evaporate and transpire if unlimited water were 

available—can greatly exceed the average annual precipitation 

in most Southwest basins. For example, potential evapotranspiration in Las Vegas Valley in southern Nevada is almost 

Large deposits of borax, a 

mineral that contains boron, 

have formed by repeated 

evaporation of saline lakes in 

closed basins in desert regions 

in California and Nevada. The 

use of borax in a wide variety 

of commercial products was 

popularized by the famous 20 

Mule Team Borax trademark, 

which was based on the early 

use of mules to haul borax from 

the mines.

Photograph from Rio Tinto Minerals

100 in/yr, but average annual precipitation is less than 12 in/yr 

(fig. 3–1). This means that all of the water from precipitation 

in this basin potentially can evaporate, leaving behind any 

dissolved minerals that the water contained. Over time, these 

minerals build up in the soil and unsaturated zone. Recharge 

from large rains or from irrigation moves the minerals down 

through the unsaturated zone, delivering pulses of highconcentration solutes to the water table.

There are few, if any, ways that surface water and 

groundwater can leave constricted or closed basins other than 

by evapotranspiration, and so constituents accumulate in 

these basins and aquifers. Less accumulation occurs in open 

basins, where dissolved constituents can be flushed out of the 

basin and aquifer by discharge to streams or by subsurface 

outflow. Of the 15 Southwest study basins, only parts of the 

Central Valley and San Jacinto Basin in California and the 

San Luis Valley in Colorado are closed under most conditions. 

However, more than 50 basins in the southwestern United 

States have no natural outflow other than by evapotranspiration(14) and, therefore, are especially vulnerable to the accumulation of dissolved constituents, some of which are potentially 

harmful at elevated concentrations.

26 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Evaporative concentration of dissolved 

constituents can contaminate groundwater.
Figure 4–1. Before extensive modification of some groundwater systems began in the 1800s and early 1900s, the quantity of 

water that came into Southwest basin-fill aquifers was about the same as the quantity that went out. An example is shown here of 

the Central Valley in California. A, Before development, natural processes in the basin and surrounding mountains controlled the 

hydrology. B, Agricultural and urban activities have greatly altered the modern hydrologic system in the basin, increasing downward 

groundwater flow.

Corcoran clay

(confining layer)

Fine-grained deposits

Water table

Sierra

Nevada

Groundwater

movement

Precipitation

75%

RECHARGE

Stream loss

25%

Coast Ranges

A. Predevelopment conditions in the Central Valley, California—Balanced groundwater budget

B. Modern conditions in the Central Valley, California—As a result of pumping and irrigation, about seven times 

 more groundwater now moves through the aquifer than before development of water resources

Consolidated rock Marine deposits

Evapotranspiration

85%

DISCHARGE

Discharge

to streams,

springs, and

seeps

15%

Precipitation

11%

RECHARGE

Stream loss

20%

Excess

irrigation

water and 

other sources

69%

Evapotranspiration

20%

DISCHARGE

Wells

64%

Discharge

to streams,

springs, and

seeps

16%

Estimated recharge and discharge 2,000,000 acre-feet per year

Estimated recharge 13,300,000 acre-feet per year

Estimated discharge 14,600,000 acre-feet per year

Modified from Faunt and others;(81) Thiros and others(9)

Stream

loss

Precipitation

Precipitation

RECHARGE DISCHARGE

Stream

gain

Evapotranspiration

Evapotranspiration

Stream

loss Irrigation

loss

RECHARGE DISCHARGE

Stream Pumping gain wells

NOT TO SCALE

NOT TO SCALE

Chapter 4

Chapter 4: The Hydrology of Southwest Basin-Fill Aquifers and How Human Alterations Affect Groundwater Quality 27
Hydrogeologic Factors Control the Susceptibility 

of Groundwater to Contamination

Groundwater and the dissolved constituents it contains 

flow relatively rapidly through coarse-grained sediment, 

such as sand and gravel. Basin-fill aquifers in the Southwest 

generally contain more sand and gravel near the basin margins 

and along stream channels and more fine-grained layers 

in and near the basin lowlands (fig. 3–3). Confining layers 

or lenses of silt and clay slow the downward movement of 

groundwater and contaminants. As a result, in parts of basins 

where confining layers are present, deep groundwater is 

less susceptible to contamination from the land surface than 

shallow groundwater.

Rates of recharge to and discharge from aquifers and the 

presence of confining layers are natural controls on the general 

direction and amount of groundwater flow. Groundwater 

moves downgradient from higher altitude recharge areas, 

which in Southwest basins occur along basin margins, to lower 

altitude discharge areas, which occur in the basin lowlands 

(fig. 3–3). Confining layers can create artesian conditions in 

underlying parts of an aquifer, resulting in upward hydraulic 

gradients (upward flow) in discharge areas (see sidebar, 

Aquifer, p. 20). Upward hydraulic gradients inhibit the 

downward movement of contaminants, whereas downward 

gradients enhance contaminant movement to deeper parts of 

an aquifer.

In the early 1900s, several uncapped artesian wells (wells that tap 

a confined part of the aquifer) near the center of Las Vegas Valley, 

Nevada, flowed freely. The Eglington well was flowing at about 

615 gallons per minute when this photograph was taken in 1912 

(from Carpenter(53)). Intensive groundwater withdrawals in the 

basin, first for irrigation and then for public supply, lowered the 

pressure in the aquifer and caused flow from the artesian wells 

and most springs to cease by the early 1970s.(54)

The thickness of the 

unsaturated zone, which 

corresponds to the depth to 

the water table (see sidebar, 

Aquifer, p. 20), influences 

subsurface water flow and 

contaminant movement in 

Southwest basins. Under 

natural conditions, groundwater in Southwest basins 

is up to several hundred 

feet below land surface in 

upland and mountain front areas near the basin margins and 

generally less than 50 ft beneath the valley floor in the basin 

lowlands. Because the unsaturated zone typically is thin in 

the basin lowlands, contaminants at the land surface in these 

areas can move readily to shallow parts of an aquifer if there 

is any recharge. However, an upward hydraulic gradient 

and confining layers in discharge areas often impede the 

downward movement of groundwater to depths used for 

water supply. Near basin margins, recharge water travels 

downward through the thick unsaturated zone to the water 

table. A potentially long travel time through the unsaturated 

zone increases the likelihood that some contaminants will 

degrade or be removed from the water by interaction with 

aquifer solids during infiltration. On the other hand, the lack 

of substantial fine-grained layers coupled with a downward 

hydraulic gradient in areas near the basin margin can make the 

water table and unconfined part of the aquifer more susceptible to contamination by chemicals that might be transported 

through the unsaturated zone.

A low natural groundwater recharge rate, typical of 

many basins in the Southwest, combined with a large aquifer 

storage capacity, results in a long groundwater residence 

time. Groundwater residence time refers to the length of 

time water remains in an aquifer from recharge to discharge. 

Long residence times allow more time for groundwater to 

interact with aquifer sediments, resulting in concentrations of 

dissolved constituents that increase along groundwater flow 

paths. Groundwater throughout large parts of Southwest basins 

can be quite old—on the order of tens of thousands of years 

(fig. 4–2)—implying that some of the water was recharged 

during the last ice age when conditions in the Southwest were 

considerably cooler and wetter. This old groundwater was 

recharged prior to human development in the basin and has 

been isolated from human-related chemicals used at the land 

surface. If contamination of this deep groundwater occurs, 

however, the effects on groundwater quality can continue for a 

long time.

28 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Groundwater with a long 

residence time in basin-fill 

aquifers is more likely to 

have acquired elevated 

concentrations of constituents, such as arsenic, from 

a geologic source (see 

discussion on arsenic in 

chapter 6).
Figure 4–2. Groundwater throughout much of the Middle Rio Grande Basin in New Mexico is at least 10,000 to 20,000 years 

old, as determined from carbon-14 data. Some parts of the basin contain young groundwater (recharged since about 1950), 

particularly along the mountain fronts, near arroyos (intermittent or ephemeral streams), and in the Rio Grande inner valley 

where recharge occurs and the water table is generally less than about 30 feet below land surface. This young groundwater 

is more vulnerable to the effects of human-related activities at the land surface than is the very old groundwater.

Arroyo Tonque

Arroyo Chico

Rio

Puerco

Rio

Salado

Jemez

River

Santa Fe River

Creek

Galisteo

Rio

Grande

Tijeras

Arroyo

Grande Rio

Rio

Puerco

Abo Arroyo

Rio Salado

Jemez

River

R io

San Jose

EXPLANATION

Radiocarbon age of 

groundwater,

in years

>20,000

10,000 to 20,000

2,000 to <10,000

<2,000

circ1358 ch04fig2

Jemez

Mountains Española Basin

Albuquerque

Sandia Mountains

Mountains

Manzanita

Sierra Lucero

Los Pinos

Mountains

Ladron

Peak

Manzano Mountains

San Juan Basin

Estancia Basin

Albuquerque

municipal boundary

Middle Rio Grande

Basin boundary

Modified from Plummer and others(78)

N

Middle 

Rio Grande

Basin

Chapter 4

Chapter 4: The Hydrology of Southwest Basin-Fill Aquifers and How Human Alterations Affect Groundwater Quality 29
Geochemical Conditions Are Important Controls 

on Contaminant Transport

Geochemical conditions, such as pH (a measure of 

acidity) and oxic or anoxic conditions, are important controls 

on contaminant transport and fate in groundwater. Under 

natural conditions, pH and reduction/oxidation (redox) 

processes (see sidebar, How do redox reactions work?, p. 31) 

in groundwater are determined largely by the chemical 

composition and organic-carbon content of surrounding 

rocks and sediments and by rates of recharge and discharge. 

Volcanic rocks typically cause groundwater to have a high pH 

(more basic). Low rates of natural recharge to many Southwest 

basin-fill aquifers result in long groundwater residence times 

in parts of the aquifer and ample 

time for water/rock reactions 

that tend to increase pH.

Oxic conditions predominate in Southwest basin-fill 

aquifers, whereas anoxic 

(reducing) conditions occur 

more commonly in glacial 

sand and gravel aquifers and 

consolidated rock aquifers in 

the central and eastern parts of 

the Nation.(15) Groundwater is 

considered oxic if the dissolvedoxygen concentration is equal to or greater than 0.5 mg/L and 

anoxic if the concentration is less than 0.5 mg/L. Groundwater 

was characterized as oxic in about 70 percent of wells sampled 

in the Southwest for which water-level and dissolved-oxygen 

data were collected (fig. 4–3). Oxic conditions are associated 

with high concentrations of some contaminants, such as nitrate 

and uranium, in groundwater, but with low concentrations 

Figure 4–3. Groundwater in Southwest basin-fill aquifers is more 

commonly oxic than anoxic, especially at shallow depths. Anoxic 

conditions are more common at depth because the older, deeper 

groundwater has had more time for subsurface microorganisms to 

consume the dissolved oxygen.

of others, such as petroleum hydrocarbons, by promoting 

their degradation. Under anoxic conditions, bacteria that 

thrive in groundwater with low concentrations of oxygen can 

degrade nitrate and some human-related compounds, such as 

solvents.(16) High pH or anoxic conditions promote desorption 

(detachment) of arsenic from aquifer sediments, elevating 

its concentration in groundwater (see Arsenic section in 

chapter 6).

Percentage of oxic and anoxic water samples

Depth below water table, in feet

Anoxic conditions—Dissolved-oxygen 

 concentration less than 0.5 milligram 

 per liter 

Oxic conditions—Dissolved-oxygen 

 concentration equal to or greater than 

 0.5 milligram per liter

0 20 40 60 80

Less than 25

(697 samples)

Data from the USGS 

National Water 

Information System

25–99.9

(519 samples)

100–200

(331 samples)

Greater than 200

(490 samples)

EXPLANATION

30 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Groundwater was 

characterized as oxic 

in about 70 percent of 

wells sampled in the 

Southwest. Oxic conditions are associated with 

high concentrations of 

nitrate and uranium in 

groundwater.
How do redox reactions work?

Reduction/oxidation (redox) processes require one chemical species that donates electrons and another chemical species that 

accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts electrons it is 

“reduced.” Redox processes typically are facilitated by microbes (bacteria), which use the energy produced by the processes. In 

groundwater, organic carbon is the most common electron donor. If dissolved oxygen is present, it is the preferred electron acceptor, 

because reduction of dissolved oxygen produces more energy than reduction of other chemical species that commonly occur in 

groundwater. The atmosphere is the source of the dissolved oxygen, so the redox conditions in an aquifer near where recharge occurs 

usually are oxic (defined here as having a concentration of dissolved oxygen of at least 0.5 mg/L).

As groundwater moves through the aquifer along a flow path, the dissolved oxygen in the groundwater gradually is consumed by 

redox processes. Once all of the dissolved oxygen is consumed, other chemical species can accept electrons and become reduced. 

If nitrate is present, it will become the preferred electron acceptor until it in turn is completely consumed. This pattern continues, with 

manganese, iron, sulfate, and finally carbon dioxide acting as electron acceptors until they are consumed, in that order. This order 

of use of electron acceptors has important implications for the preservation, degradation, and even production of contaminants in 

groundwater. Because redox reactions occur in a sequence, it can take a long time for strongly reducing conditions to develop. For this 

reason, anoxic groundwater commonly is older than oxic groundwater, and, within the anoxic category, strongly reducing groundwater 

commonly is older than mildly reducing groundwater. 

From a water-quality perspective, denitrification—the reduction of nitrate to nitrogen gas—is one of the most important redox 

processes that occurs in groundwater. Nitrate is a concern for human health and, where it discharges to surface water, can impair 

aquatic communities. Conversion of nitrate by denitrification to harmless nitrogen gas, the same gas we breathe in the atmosphere, is 

the primary way that nitrate is removed from water.

Oxygen Nitrate Manganese Iron Sulfate Carbon dioxide

Oxic Anoxic

Oxic Mildly

reducing

Strongly

reducing

Very strongly

reducing

Order of consumption of electron acceptors

Chapter 4

Chapter 4: The Hydrology of Southwest Basin-Fill Aquifers and How Human Alterations Affect Groundwater Quality 31
Human Activities Have Substantially Altered 

Groundwater Movement and Geochemistry 

The balance between recharge and discharge has changed 

in many basins because of the addition of artificial recharge 

water, increased groundwater withdrawals from pumping, or a 

combination of both (fig. 4–1B). Human activities associated 

with agricultural and urban uses have increased recharge 

across broad areas, caused waters from different sources and 

depths to mix, and enhanced water movement from the land 

surface down to aquifers used for water supply. Many human 

activities have introduced new sources of contaminants that, 

in combination with changes to the groundwater system, have 

made the groundwater more vulnerable to contamination.

Artificial Recharge is a Major Source of 

Groundwater Recharge and Contaminants in 

Developed Basins 

There are many sources of artificial recharge to basin-fill 

aquifers in developed Southwest basins. These include the 

infiltration of excess irrigation water applied to crops and 

lawns; seepage and leakage from canals, water distribution 

pipes, sewer pipes, and septic-tank systems; infiltration of 

stormwater runoff from retention basins and recharge basins; 

seepage of treated wastewater through streambeds and 

irrigated fields; and infiltration of surface water and imported 

water from recharge ponds and injection wells. Artificial 

recharge has caused groundwater flow rates to increase, flow 

directions to change, and saturated thickness to increase or 

form “groundwater mounds” (and therefore thinner unsaturated zones) in some basin-fill aquifers. In some areas, this 

additional recharge results in more groundwater discharge 

to streams (see sidebar, Groundwater discharge can deliver 

contaminants to surface water, p. 36). 

Excess irrigation water from agricultural and urban areas is a 

source of recharge to basin-fill aquifers. Top, flood irrigation on a 

wheat field in Yuma, Arizona; bottom, sprinkler irrigation on a lawn 

in Las Vegas Valley, Nevada.

Photographs: Top, Jeff 

Vanuga; bottom, Lynn 

Betts; both with the Natural 

Resources Conservation 

Service, U.S. Department of 

Agriculture 

An inflatable rubber dam along the Santa Ana River in 

California impounds water to be diverted into recharge 

ponds. Artificially recharged water, such as this, can move 

contaminants from treated wastewater, stormwater, and 

agricultural runoff into the aquifer.

Photograph by Carmen Burton, USGS

32 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

In some basins, the amount of groundwater recharge from 

natural and artificial sources is about seven times that of 

natural recharge prior to water development.
Figure 4–4. Water development in the Southwest study basins has caused increases in groundwater recharge, 

mostly from excess irrigation water, and discharge, by well pumping. As a result, downward movement of water has 

increased, increasing the vulnerability of the aquifers to contamination from chemicals used at the land surface. The 

distance along the diagonal from lower left to upper right indicates the amount of change in recharge and discharge 

since development of water resources in the basin. In most cases, increases in discharge exceed increases in 

recharge (yellow area of graph), which causes groundwater-level declines.

Irrigation, which is used on essentially all cropland in 

the arid to semiarid Southwest, enhances the movement of 

fertilizers, manure, and pesticides applied to crops through 

the soil zone and down into groundwater. Evaporative 

concentration of irrigation water increases concentrations 

of dissolved solids and, in some areas, other constituents in 

artificial recharge. For example, if crops are irrigated in areas 

where soils contain arsenic, the arsenic can be released from 

the soils into the shallow groundwater. In the southern Carson 

Desert, Nevada, groundwater levels have risen by about 15 ft 

as a result of recharge from widespread irrigation in the area 

since the early 1900s. Evaporative concentration of irrigation 

water and release of arsenic from aquifer sediments have 

resulted in high concentrations of arsenic in the area’s shallow 

groundwater.(17, 18)

Human activities associated with urban and suburban 

land use can contribute contaminants from the land surface 

to groundwater. Stormwater runoff is a potential source of 

a variety of contaminants to groundwater recharging across 

broad areas or through streambeds and flood retention basins. 

Contamination also can occur from point sources, such as 

leaking storage or septic tanks, landfills, or accidental spills.

Artificial recharge has become a major source—and, in 

some cases, the primary source—of groundwater recharge 

in most of the 15 Southwest study basins.(19) Recharge from 

irrigation canals, irrigated agricultural fields, and urban turf 

areas currently provides at least half the recharge to basin-fill 

aquifers in the Central Valley, California; San Luis Valley, 

Colorado; and West Salt River Valley, Arizona. Nearly half 

of the groundwater recharge to the Santa Ana Coastal Basin 

in California (about 165,000 acre-feet per year) comes from 

engineered recharge facilities along and near the Santa Ana 

River. In the West Salt River Valley and Central Valley, the 

amount of groundwater recharge from natural and artificial 

sources is about seven times that of natural recharge prior to 

water development (fig. 4–4).

circ1358 ch04fig4

1 2 3 4 5 6 7

7

6

5

4

3

2

Factor by which groundwater recharge has increased

1

Factor by which groundwater discharge has increased

1

3

5

2

Increase in recharge

greater than

increase in discharge Increase in discharge

greater than

increase in recharge

Increase in recharge 

and discharge 

4

6

7

8

11 12

9

10

13

14

16

15

 1 West Salt River Valley

 2 Central Valley

 3 Middle Rio Grande Basin

 4 San Luis Valley

 5 Spanish Springs Valley

 6 Las Vegas Valley

 7 Santa Ana Inland Basin–Chino subbasin

 8 Santa Ana Coastal Basin

 9 Santa Ana San Jacinto Basin

10 Carson Valley

11 Upper Santa Cruz Basin

12 Sierra Vista subbasin

13 Salt Lake Valley

14 Truckee Meadows

15 Santa Ana Inland Basin–Bunker Hill subbasin

16 Eagle Valley

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system

Study basin

Principal Aquifer

Even though recharge in the West Salt River Valley and 

Central Valley increased almost sevenfold from 

predevelopment conditions, discharge increased even 

more, resulting in large water-level declines in parts of 

these aquifers 

Modified from Bexfield and others(19)

Increases in groundwater recharge and discharge 

in study basins between predevelopment 

and modern conditions EXPLANATION

Chapter 4

Chapter 4: The Hydrology of Southwest Basin-Fill Aquifers and How Human Alterations Affect Groundwater Quality 33
Pumping Changes Natural Groundwater 

Flow Systems

Pumping wells have become the primary way that 

groundwater discharges from many Southwest basins, 

resulting in a decrease in natural discharge to streams and 

evapotranspiration. In most of the Southwest study basins, 

water pumped from wells accounted for more than half of the 

discharge from the basin-fill aquifers in 2000 (fig. 4–5). In 

many basins with pumping wells, the amount of groundwater 

stored in the aquifers has decreased because high pumping 

rates exceed any increases in recharge (fig. 4–4)—the groundwater discharge is from storage, and pumping is not sustainable. The largest estimated decrease in aquifer storage is in the 

Central Valley, where in some places groundwater levels have 

dropped more than 400 ft. Groundwater levels have dropped 

between 300 and 400 ft in part of the West Salt River Valley, 

and declines of at least 100 ft have been documented in parts 

of basins in Nevada (Las Vegas Valley), New Mexico (Middle 

Rio Grande), Arizona (Upper Santa Cruz), and elsewhere in 

California (Santa Ana Inland). Estimated groundwater residence times in the study basins under developed conditions—

affected by artificial recharge and groundwater pumping—are 

shorter than under predevelopment conditions.(19) As a result, 

groundwater moves through the aquifer more quickly, potentially transporting contaminants down to the groundwater 

supply. Geochemical conditions, such as redox and pH, can 

be altered by oxygen-rich artificial recharge and water-level 

declines caused by pumping wells. Wells also can facilitate 

the movement of groundwater within an aquifer and allow the 

transport of oxygenated water to confined parts of an aquifer 

that previously was anoxic (see sidebar, Wells can “short 

circuit” groundwater flow paths and increase contaminant 

movement, p. 37).

Turbine pumps capable of lifting groundwater from great depths became available in about 1930. By pumping the vast reserves 

of groundwater stored in the thick basin-fill aquifer, farmers have developed the San Joaquin Valley, California, into a major 

agricultural region.

Photograph by Richard Thornton, www.shutterstock.com

34 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 4–5. Pumping from wells is the primary way that groundwater discharges in 13 of 16 basins or subbasins with 

basin-fill aquifers studied in the Southwest. The largest withdrawals by pumping in 2000 occurred in the Central Valley 

of California.

Central

Valley West Salt River

 Valley

Middle Rio

Grande Basin

San Luis

Valley

Salt Lake Valley

Santa Ana Coastal Basin

Bunker Hill

subbasin

Chino

subbasin

Santa Ana

San Jacinto

Basin

Las Vegas

Valley

Eagle

Valley

Carson

Valley

Sierra Vista

subbasin

Truckee Meadows Spanish Springs Valley

Upper Santa

Cruz Basin

Santa Ana

Inland Basin

NEVADA

CALIFORNIA

ARIZONA

NEW MEXICO

UTAH COLORADO

N

Estimated groundwater pumping from basin-fill aquifers (2000) and type of groundwater discharge in study basins

Estimated groundwater pumping (2000),

 in acre-feet per year

 10,000

 10,000 to <50,000

 50,000 to <100,000

 100,000 to <500,000

 500,000 to <1,000,000

 ≥1,000,000

Boundary of hydrogeologic area

Study basin

Evapotranspiration

Stream gain Subsurface

outflow

Springs and drains

Pumping wells

Groundwater discharge type in study basins—

 Size of pies is relative to discharge in the 

 Central Valley (14,600,000 acre-feet per year)

EXPLANATION

Estimated groundwater pumping in 2000 (U.S. Geological Survey;(79) McKinney and Anning(4))

Chapter 4

Chapter 4: The Hydrology of Southwest Basin-Fill Aquifers and How Human Alterations Affect Groundwater Quality 35
Ditches in the irrigated part of New Mexico’s Rio Grande 

floodplain return shallow groundwater to the river, which 

is an important part of the public water supply in the 

Albuquerque area. The groundwater is recharged by excess 

irrigation water, which can contain elevated concentrations 

of dissolved solids.

Groundwater that discharges to streams can contain 

elevated concentrations of contaminants, such as dissolved 

solids and nitrate. Groundwater discharges naturally to 

streams in basin lowlands where the water table intersects the 

streambed. The amount of groundwater that discharges can be 

enhanced by irrigation in agricultural and urban areas, where 

large amounts of artificial recharge have caused groundwater 

levels to rise. The connection between groundwater and 

surface water is an important factor that can affect the quality 

of water in streams in Southwest basins.

In some irrigated agricultural areas, like the inner valley of 

the Rio Grande in New Mexico, ditches are used to intercept 

shallow groundwater, which contains high concentrations of 

dissolved solids. This water is diverted to the Rio Grande, thus 

increasing concentrations of dissolved solids in the river. In the 

Chino area of the Santa Ana Inland Basin, California, groundwater is a source of elevated concentrations of dissolved solids 

and nitrate in the Santa Ana River. Wells are used to decrease 

the volume of groundwater that discharges to the river, and 

desalination plants (desalters) are used to decrease concentrations of these constituents in the pumped groundwater.(55) (See 

Dissolved Solids section in chapter 6 for more information.)

Photograph by Laura Bexfield, USGS

Human-induced changes to the Central 

Valley Aquifer System, California

The intensely irrigated 

Central Valley, which includes 

the San Joaquin and Sacramento Valleys, is one of the 

most productive agricultural 

areas in the world—it 

produces 25 percent of the 

Nation’s table food on only 

1 percent of the country’s farmland.(56) Before large-scale 

irrigation began in the area, the 

Central Valley aquifer system 

received most of its recharge 

from precipitation and streams (fig. 4–1A). With development 

of water resources for irrigation and public supply, the 

infiltration of excess irrigation water has become the primary 

source of recharge to the aquifer system, and pumping from 

wells is now the primary discharge mechanism (fig. 4–1B). 

These human-induced changes—irrigation and groundwater 

pumping—have affected the geochemical conditions in many 

areas of the valley and increased the downward movement of 

shallow groundwater to deeper parts of the aquifer system.

High rates of groundwater 

pumping are causing the 

Central Valley to sink. By 

1970, half of the entire 

valley had subsided a foot 

or more. Subsidence of 

more than 28 feet in the San 

Joaquin Valley southwest of 

Mendota, California, is the 

most subsidence measured 

anywhere in the continental 

United States (signs on the 

pole show the approximate 

altitude of the land surface 

in 1925, 1955, and 1977). The 

location was identified by the 

research efforts of Joseph 

Poland, U.S. Geological Survey 

(pictured).

Central

Valley

Groundwater discharge can deliver 

contaminants to surface water

36 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Pumping can alter the direction of groundwater flow within an aquifer. More unexpectedly, however, wells—

particularly those used for public supply or irrigation—can also affect groundwater movement when the wells are 

not being pumped, by providing pathways for movement of water and contaminants from one part of the aquifer to 

another.(57) The long open intervals of supply wells commonly span depths of the aquifer that differ in groundwater 

age, geochemical conditions, and concentrations of dissolved constituents. Where hydraulic gradients are downward, recharge water and young groundwater can move down in and along wells into deeper parts of aquifers, 

carrying contaminants to depths used for public supply. Where hydraulic gradients are upward, nonpumping wells 

can provide pathways for deeper, older water to move upward. The deep water could have elevated concentrations 

of constituents from geologic sources.

Downward migration of contaminants was found to occur at a public-supply well in the eastern San Joaquin 

Valley, California.(58) The VOC and pesticide compounds chloroform, perchloroethene, and deethylatrazine were 

consistently detected in shallow 

parts of the aquifer. Only when 

the public-supply well was idle 

long enough to allow a substantial 

volume of water to migrate 

downward were these compounds 

also detected in water from a 

nearby deep monitoring well.

Groundwater flow can be “short circuited” through a well that is open to 

more than one depth in an aquifer. When the well is not being pumped, 

contaminants can move upward or downward, depending on the vertical 

hydraulic gradient, in and along the well, to parts of aquifer systems 

used for public supply. Downward flow in a well can occur even if there 

is a confining layer of clay within the aquifer if the well penetrates the 

confining layer.

Wells can “short circuit” groundwater flow paths and increase 

contaminant movement

Confining layer

Open interval

Water table

Higher

hydraulic

head

Lower

hydraulic

head

Well

Flow in well under

pumping conditions

Flow in well under 

nonpumping conditions 

Flow in 

aquifer

Modified from Bexfield and others(19)

circ1358 ch04custom sidebar

Chapter 4

Chapter 4: The Hydrology of Southwest Basin-Fill Aquifers and How Human Alterations Affect Groundwater Quality 37
Rio Grande near Albuquerque, New Mexico.

"NAWQA publications do a great job of presenting the 

overall groundwater quality issues with explanations for the 

Southwest. USGS NAWQA groundwater research addresses 

both the problems and reason for the problems, providing a 

generalized view of the basin-fill aquifers in the Southwest." 

—Bart Faris, Environmental Scientist

New Mexico Environmental Department, 

Ground Water Quality Bureau
Chapter 5: Quality of Groundwater Used for 

Drinking and Irrigation in Southwest Study Basins

Drinking water and irrigation are important uses of groundwater from 

the Southwest basin-fill aquifers. Does the quality of the groundwater 

support these uses? What are the constituents that might present 

human-health concerns when the water is used for drinking? What other 

constituents might adversely affect use of the water? In Southwest basin-fill 

aquifers, chemical constituents of concern for groundwater use originate from 

geologic sources (arsenic, uranium), human activities (pesticides, VOCs), or a 

combination of both natural and human sources (nitrate, dissolved solids).

Chapter 5

This chapter identifies 

and discusses constituents that were detected 

at a concentration 

greater than or near a 

human-health benchmark 

in water from drinkingwater supply wells 

tapping several Southwest basin-fill aquifers.

Public-supply wells (an 

example is shown) typically 

are deeper, pump larger 

volumes of groundwater, 

and draw from larger 

recharge (contributing) 

areas than domestic wells. 

Water pumped from both 

types of drinking-water 

wells can be susceptible 

to contamination associated 

with activities at the 

land surface.
About one in three (35 percent) of the drinking-water wells sampled in Southwest basinfill aquifers contained at least one constituent at a concentration greater than its human-health 

benchmark (table 5–1). Constituent concentrations exceeded benchmarks more frequently in 

domestic wells (42 percent) than in public-supply wells (26 percent). There were 257 instances 

in which a concentration exceeded a human-health benchmark in water sampled from more 

than 550 drinking-water wells (fig. 5–1; appendix 2, tables A2–2 and A2–3). Sixty-five percent 

of human-health benchmark exceedances were for a constituent from geologic sources—mostly 

arsenic and uranium, but also antimony, fluoride, manganese, molybdenum, radon using the 

proposed alternative MCL, and strontium. Twenty-four percent of human-health benchmark 

exceedances were for nitrate (natural and human-related sources), and 11 percent were for a 

manmade organic compound. Samples were collected prior to any water treatment or blending 

with other water sources to best represent water quality in the aquifer and do not necessarily 

represent the quality of water delivered to consumers.

Mixtures of contaminants from geologic sources, nitrate, VOCs, and pesticides at 

concentrations that exceeded benchmarks were present in many samples; 46 samples contained 

two contaminants and 11 samples contained three or four contaminants at a concentration that 

exceeded a benchmark (Appendix 2, fig. A2–1). Although human-health benchmarks have 

been developed for several individual contaminants, the health effects from drinking water 

with mixtures of contaminants are typically not known. Additional information for the primary 

contaminants—arsenic, uranium, nitrate, VOCs, and pesticides—measured at concentrations 

that exceeded human-health benchmarks and for dissolved solids is presented in chapter 6.

Water from about one in 

three drinking-water wells 

sampled in the Southwest 

Principal Aquifers study 

area contained at least one 

constituent at a concentration that exceeded its 

human-health benchmark.

Table 5–1. Twelve constituents were measured at concentrations greater than their human-health benchmarks in more than one drinking-water well 

quality in the aquifer and do not necessarily represent the quality of water delivered to consumers.

[MCL, Maximum Contaminant Level; μg/L, micrograms per liter; HBSL, Health-Based Screening Level; mg/L, milligrams per liter; pCi/L, picocuries per liter; AMCL, 

Human-health benchmark Number of 

drinkingwater wells 

sampled

Benchmark 1/10th of 

benchmark 

Type Value All aquifers in Southwest Principal 

Aquifers group

 Contaminant from 

Arsenic MCL 10 μg/L 559 16 73 

Uranium MCL 30 μg/L 559 7.3 46 

Manganese HBSL 300 μg/L 564 3.2 11 

Fluoride MCL 4 mg/L 564 1.2 30 

Molybdenum HBSL 40 μg/L 559 1.1 27 

Radon Proposed MCL 300 pCi/L 430 86 100 

Radon Proposed AMCL 4,000 pCi/L 430 0.5 71 

 Contaminant from 

*Nitrate MCL 10 mg/L as N 554 11 63 

Dibromochloropropane (DBCP) MCL 0.2 μg/L 555 3.2 4.0 

Dieldrin HBSL †0.002 μg/L 559 0.9 1.1 

Perchloroethene (PCE) MCL 5 μg/L 555 0.4 1.6 

Trichloroethene (TCE) MCL 5 μg/L 555 0.4 1.3 

* Nitrate also can occur naturally in groundwater from parts of Southwest basin-fill aquifers.

† Value is low end of range, associated with 10 –6 cancer risk(61).

40 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
sampled in the Southwest. Samples were collected prior to any water treatment or blending with other water sources to best represent water 

Alternative Maximum Contaminant Level; N, nitrogen; see figure 5–1 for the locations of the wells sampled]

Percentage of samples with concentrations that exceeded

Benchmark 1/10th of 

benchmark Benchmark 1/10th of benchmark Benchmark 1/10th of benchmark Benchmark 1/10th of benchmark

Basin and Range basin-fill 

aquifers Central Valley aquifer system California Coastal Basin aquifers Rio Grande aquifer system

geologic sources

22 74 7.3 82 2.7 46 39 94

2.6 42 22 61 0.9 32 1.5 51

0.4 3.9 2.6 10 7.1 21 7.5 19

2.6 39 0 2.0 0 27 1.5 73

1.3 27 0 10 1.8 37 1.5 46

94 100 96 100 64 100 72 100

0.5 82 0 83 1.1 45 0 41

human-related sources

7.1 66 26 80 6.3 62 0 21

0 0 10 13 1.8 1.8 0 0

0 0 2.6 3.3 0 0 1.6 1.6

0 0.9 0 0.7 1.8 5.4 0 0

0 0.4 0 0 1.8 5.4 0 0

What is a contaminant?

Contaminants have a wide range of sources, both manmade and geologic. Most organic chemicals in groundwater that are of 

concern for human health are manmade. In contrast, most inorganic constituents in groundwater have geologic or other natural sources, 

although their concentrations in groundwater may be altered by human activities, such as irrigation and groundwater pumping. Some 

contaminants have both manmade and natural sources. For example, nitrate in groundwater has many natural sources, but nitrate 

concentrations in groundwater underlying agricultural and urban areas commonly are higher than in other areas because of contributions from sources associated with human activities. 

But what exactly is a contaminant? The word means different things to different people. For example, a contaminant is defined 

by the Safe Drinking Water Act (SDWA) as “any physical, chemical, biological, or radiological substance or matter in water” 

(see http://www.epw.senate.gov/sdwa.pdf). This broad definition of contaminant includes every substance that may be found dissolved 

or suspended in water—everything but the water molecule itself. This is not a very practical definition because this would imply that all 

water is “contaminated.” Pure water that has nothing dissolved in it does not occur naturally—not even rainfall is pure water, because it 

contains, at a minimum, some dissolved gases. 

The U.S. Environmental Protection Agency defines a contaminant as “Any physical, chemical, biological, or radiological substance 

or matter that has an adverse effect on air, water, or soil” (see http://epa.gov/region04/superfund/qfinder/glossary.html). This definition 

is more practical and allows both manmade constituents and those with geologic sources in water to be defined as contaminants. 

However, it does not define what “adverse” means, and what may be adverse in one way might be beneficial in another. In this circular, 

a contaminant is defined as any physical, chemical, biological, or radiological substance or matter in groundwater that is manmade or 

that impairs the use of water for its intended purpose. Impairment is determined by comparing a measured concentration to benchmarks 

or guidelines. By this definition, all manmade compounds, such as pesticides and volatile organic compounds, are contaminants 

because they do not occur naturally in groundwater. If a constituent with a geologic source, such as arsenic, occurs in drinking water at 

a concentration above its human-health benchmark, it also is considered a contaminant. 

Chapter 5

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation in Southwest Study Basins 41
Figure 5–1. Concentrations in drinking-water wells of contaminants from geologic sources exceeded human-health benchmarks 

more frequently than those of contaminants from human-related sources in all of the Southwest study basins except for the eastern 

San Joaquin Valley, California. Human-related contaminants (nitrate, volatile organic compounds [VOCs], and pesticide compounds) 

exceeded benchmarks most frequently in this area of intense agricultural land use where shallow contaminated groundwater is moving 

deeper in the aquifer system (see chapter 6 for information on these contaminants in San Joaquin Valley groundwater).

NEVADA

CALIFORNIA

ARIZONA

NEW MEXICO

COLORADO

UTAH

EXPLANATION 

Principal Aquifers

 Basin and Range basin-fill aquifers 

 California Coastal Basin aquifers

 Central Valley aquifer system

 Rio Grande aquifer system

Sampled well

NOTE: Radon is represented 

in this figure using the 

proposed alternative 

Maximum Contaminant Level 

of 4,000 picocuries per liter.

N

0

20

40

1

1

2

3

4

5

6 7 8 9 10

2

3

4

10

9

7

8

5

6

0

20

40

0

20

40

0

Nitrate

VOC or

pesticide

Contaminant from

a geologic source

20

40

0

20

40

0

20

40

0

20

40

0

20

40

0

20

40

0

20

40

0

20

40

Percentage of 

drinking-water-well 

samples containing a 

contaminant at a 

concentration 

exceeding its humanhealth benchmark

Basins in

Northern

Coast Ranges

area

Truckee

Meadows

and Spanish

Springs,

Eagle, and

Carson Valleys

Great Salt

Lake Basins,

including Salt

Lake Valley

Las Vegas

Valley

Sacramento

Valley

San Joaquin

Valley

Santa

Ana

Basin

West

Salt

River

Valley

Upper

Santa Cruz

Basin and

Sierra Vista

Subbasin

Rio Grande

Valley 

and Middle

Rio Grande

Basin

Percentage of drinking-water-well samples containing a contaminant at a concentration exceeding its human-health benchmark

42 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Concentrations of constituents measured for this assessment 

were compared to human-health benchmarks to place study 

findings in the context of human health. The benchmarks are 

threshold concentrations in water above which the concentration 

of a contaminant in drinking water could adversely affect human 

health. Human-health benchmarks were available for about 

two-thirds of the 287 constituents and properties measured for 

the Principal Aquifer assessments (see appendix 2, table A2–1, 

for list of constituents and properties analyzed). Two types 

of human-health benchmarks were used: U.S. Environmental 

Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) 

(http://water.epa.gov/drink/contaminants/index.cfm) and U.S. 

Geological Survey (USGS) Health-Based Screening Levels (HBSLs). 

MCLs are legally enforceable drinking-water standards that specify 

the maximum permissible level of a constituent in water that is 

delivered to any user of a public water system(36) (values used in 

this report were current as of February 2012). Although MCLs are 

used to regulate the quality of drinking water only from publicsupply sources, they also are useful for evaluating the quality of 

water from domestic and monitoring wells. An MCL was available 

for 53 of the constituents measured. For some constituents for 

which an MCL has not been established, the USGS, in collaboration with the USEPA and others, developed non-enforceable 

HBSLs by using standard USEPA methods for establishing 

drinking-water guidelines and current toxicity information(59–61)

(values used in this report were current as of February 2012; see 

http://water.usgs.gov/nawqa/HBSL ). An HBSL was available for 

135 of the constituents measured. Radon has neither an MCL nor 

an HBSL, but two MCLs have been proposed. Copper and lead 

have USEPA action levels rather than an MCL.

In addition to human-health benchmarks, non-healthbased guidelines—Secondary Maximum Contaminant Levels 

Human-health benchmarks and other guidelines used in this assessment

(SMCLs)—were available for some of the constituents measured 

in this assessment. The SMCLs are non-enforceable guidelines 

for concentrations of “nuisance” constituents in drinking water 

that can cause unwanted cosmetic effects, such as skin or tooth 

discoloration; aesthetic effects, such as unpleasant taste, odor, or 

color; or technical effects, such as corrosion or sedimentation of 

plumbing or reduced effectiveness of water treatment.(23)

Concentrations greater than one-tenth of a human-health 

benchmark were used in this assessment to indicate which 

contaminants occurred, either individually or as mixtures, at 

concentrations that approach those of potential concern for 

human health, and to identify contaminants that might warrant 

additional monitoring and study. The criterion of one-tenth of a 

benchmark is consistent with various State and Federal practices 

for reporting contaminant occurrence in groundwater and for 

identifying contaminants of potential human-health concern (for 

example, see U.S. Environmental Protection Agency;(62) New 

Jersey Department of Environmental Protection(63)).

Screening-level assessments, such as this one, provide 

perspective on the potential relevance of detected contaminants 

to human health and can help in planning future studies.(60) They 

are not designed to evaluate specific effects of contaminants 

on human health and are not a substitute for comprehensive 

risk assessments. It is important to note that occurrence of a 

contaminant at a concentration greater than its benchmark does 

not mean that adverse effects are certain to occur, because the 

benchmarks are conservative (protective) and source-water 

samples were collected prior to any treatment or blending that 

could alter contaminant concentrations in finished drinking water. 

There are water treatment options, such as charcoal filtration, 

that can be used to lower the concentration of the contaminant to 

below the benchmark before the water is consumed.

Chapter 5

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation in Southwest Study Basins 43
Arsenic and Uranium—Constituents From Geologic Sources in Southwest 

Groundwater That Most Commonly Exceeded Human-Health Benchmarks

Contaminants from geologic sources can accumulate in groundwater to concentrations 

of potential concern to human health, even in areas unaffected by human-related activities. 

This type of contamination occurs when geochemical conditions favor dissolution or release 

of these constituents from the solid aquifer materials. Of the contaminants, other than radon, 

with geologic sources that are listed in table 5–1, only concentrations of arsenic and uranium 

exceeded human-health benchmarks in more than 5 percent of samples collected from Southwest drinking-water wells.

Arsenic has been recognized for centuries as a toxic element and is of concern because 

elevated concentrations can contribute to a wide variety of adverse health effects, including 

skin damage, circulatory problems, and several types of cancer.(20) Arsenic concentrations 

exceeded the MCL of 10 μg/L in 16 percent of Southwest drinking-water wells, ranging from 

39 percent of samples from the Rio Grande aquifer system to less than 3 percent of samples 

from California Coastal Basin aquifers.

Uranium is a radioactive element that, at elevated concentrations, can contribute to kidney 

damage and several types of cancer. Uranium concentrations exceeded the MCL of 30 μg/L 

in 7 percent of Southwest drinking-water wells, ranging from 22 percent of samples from 

the Central Valley aquifer system to less than 1 percent of samples from California Coastal 

Basin aquifers.

Radon is a naturally occurring, radioactive gas that is part of the decay series of uranium. 

There is no human-health benchmark for radon, but the USEPA has proposed both an MCL for 

radon in public water systems of 300 picocuries per liter (pCi/L) and an alternative MCL of 

4,000 pCi/L. The regulation of radon concentrations is complicated because its primary hazard 

is through inhalation of indoor air in living spaces rather than from ingestion of drinking water. 

The lower proposed MCL for radon applies to states and public water systems that do not 

develop programs to address health risks from radon in indoor air; the higher proposed alternative MCL applies to States and public water systems that have established such programs.(21, 22)

Although radon was frequently measured at concentrations greater than the proposed MCL of 

300 pCi/L in Southwest drinking-water wells (86 percent of samples), concentrations exceeded 

the proposed alternative MCL of 4,000 pCi/L in only two samples.

As

33

75

U

92

238

Rn

86

222

Photograph by Jack Haskell

44 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Consolidated rocks in mountains, such as Camelback Mountain in Phoenix, Arizona, are a geologic 

source of chemical constituents to basin-fill aquifers. 
Fertilizers, a potential source 

of nitrate to groundwater, are 

used to maximize crop yield in 

the Southwest. Here, fertilizer 

is added to irrigation water in 

Yuma, Arizona.

Nitrate and Human-Related Organic Compounds

Nitrate is a nutrient that has human-related sources, such as fertilizer and wastewater, 

but that also occurs naturally in parts of the Southwest. Elevated nitrate concentrations in 

drinking water can cause methemoglobinemia (“blue-baby” syndrome) in infants. Nitrate 

concentrations greater than the MCL (10 mg/L as nitrogen [N]) were measured in 11 percent of 

Southwest drinking-water wells, ranging from 26 percent of samples from the Central Valley 

aquifer system to no exceedances in samples from the Rio Grande aquifer system.

Four of the more than 200 human-related organic compounds analyzed—the fumigant 

dibromochloropropane (DBCP), the insecticide dieldrin, and the solvents perchloroethene 

(PCE) and trichloroethene (TCE)—were measured at concentrations greater than humanhealth benchmarks in Southwest domestic and public-supply wells. All of the exceedances 

for these compounds except one were in samples from the Central Valley aquifer system and 

California Coastal Basin aquifers. An increased risk of cancer is associated with long-term 

exposure to water with DBCP, dieldrin, PCE, and TCE at concentrations above human-health 

benchmarks. At least one VOC, pesticide, or pesticide degradate was measured, mostly at low 

concentrations, in 57 percent of samples from drinking-water wells. The occurrence of these 

human-related organic compounds at low concentrations in groundwater is of concern because 

it indicates that many basin-fill aquifers in the Southwest are vulnerable to contamination from 

human activities and have the potential to be affected by higher concentrations of these humanrelated contaminants in the future.

Chapter 5

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation in Southwest Study Basins 45

At least one VOC or 

pesticide compound was 

measured, mostly at 

low concentrations, in 

57 percent of samples from 

drinking-water wells. 

Photograph by Jeff Vanuga, Natural Resources Conservation Service, U.S. Department of Agriculture
Secondary Standards for Drinking Water and 

Guidelines for Irrigation Water

The USEPA has established Secondary Maximum 

Contaminant Levels (SMCLs) (rather than MCLs) for several 

“nuisance” constituents that, at elevated concentrations, can 

make water look, smell, or taste unpleasant, stain plumbing, or 

cause other undesirable effects (table 5–2). These secondary 

drinking-water standards are designed to help public suppliers 

provide water that is acceptable to consumers, and adherence 

to them is voluntary (see sidebar, Human-health benchmarks 

and other guidelines used in this assessment, p. 43).

Dissolved solids in water are primarily made up of the 

major ions calcium, sodium, bicarbonate, sulfate, and chloride. 

The SMCL for dissolved solids is 500 mg/L and is designated 

to avoid a salty taste, color, staining, or greater costs because 

of mineral deposits in plumbing for consumers of public water 

supplies.(23) Dissolved-solids concentrations were greater than 

the SMCL in 26 percent of Southwest drinking-water wells, 

ranging from 32 percent of samples from Basin and Range 

basin-fill aquifers to 19 percent of samples from California 

Coastal Basin aquifers and the Rio Grande aquifer system.

Dissolved-solids concentration is a measure of 

salinity—the salinity of irrigation water can adversely affect 

crop yield because elevated concentrations of salts in water or 

soil reduce the ability of crops to extract water from the soil. 

Concentrations between 450–2,000 mg/L in irrigation water 

can cause slight to moderate restrictions in its use on crops, 

and concentrations greater than 2,000 mg/L can severely limit 

use.(24) Eleven percent of samples from 45 irrigation wells 

had a dissolved-solids concentration greater than 2,000 mg/L 

compared to 7 percent of samples from all 1,079 wells 

assessed in NAWQA Program studies in the Southwest.

Perchlorate: An emerging contaminant of 

concern in Southwest groundwater

Perchlorate is a chemical that contains chlorine and 

oxygen. Perchlorate occurs naturally in arid regions where 

it can accumulate in the unsaturated zone as a result of 

atmospheric deposition and evaporative concentration.(64) 

It also is contained in some nitrate deposits in Chile that are 

mined and exported for use in the United States as fertilizer. 

A strong oxidizer, perchlorate is manufactured to produce 

rocket fuel, fireworks, and explosives. Perchlorate is highly 

soluble, mobile, and persistent in groundwater.(65) In humans, 

perchlorate interferes with iodide uptake into the thyroid gland 

and has a human-health benchmark of 15 μg/L because of its 

potential adverse effect on pregnant women. Under natural 

conditions, the probability of perchlorate concentrations 

greater than 1 μg/L in deep groundwater in the Southwest 

is unlikely.(66) Predicted concentrations greater than 1 μg/L 

typically are influenced by human-related sources or excess 

irrigation water that transports naturally deposited perchlorate 

to the aquifer. For example, perchlorate concentrations in 

groundwater are elevated in several areas in California and 

near Las Vegas, Nevada, where the chemical is associated 

with industries that manufacture, use, or store rocket fuel.(67)

Table 5–2. Secondary Maximum Contaminant Levels (SMCLs) in drinking water are recommended for some constituents. 

[mg/L, milligrams per liter; μg/L, micrograms per liter]

Constituent

Secondary 

Maximum 

Contaminant 

Level

Percentage of samples with concentrations that exceeded the Secondary Maximum Contaminant Level

All aquifers in Southwest 

Principal Aquifers group

Basin and Range 

basin-fill aquifers

Central Valley 

aquifer system

California Coastal 

Basin aquifers

Rio Grande 

aquifer system

Dissolved solids 500 mg/L 26 32 25 19 19

Manganese 50 μg/L 8.3 3.0 8.5 15 15

Chloride 250 mg/L 5.3 10 1.3 1.8 3.0

Sulfate 250 mg/L 5.1 7.8 2.0 1.8 9.0

Fluoride 2 mg/L 3.7 7.8 0.7 0 3.0

Iron 300 μg/L 3.7 1.7 2.6 10 3.0

46 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 5–2. Concentrations of arsenic, nitrate, uranium, and dibromochloropropane (DBCP) 

exceeded human-health benchmarks about two to six times more frequently in drinking-water wells 

in Southwest basin-fill aquifers than in drinking-water wells in Principal Aquifers nationwide.

Constituents of Concern in Southwest Principal Aquifers—

A National Context 

Concentrations of arsenic, nitrate, uranium, and DBCP exceeded human-health benchmarks about two to six times more frequently in drinking-water wells sampled in Southwest 

basin-fill aquifers than in Principal Aquifers nationwide (fig. 5–2). Nitrate concentrations were 

greater than the MCL in 11 percent of Southwest drinking-water wells compared to 5 percent 

of drinking-water wells nationwide. Dissolved-solids concentrations exceeded the SMCL in 

26 percent of Southwest drinking-water wells compared to 16 percent nationwide. The environmental and hydrogeologic settings of the region have a large influence on the concentrations of 

these contaminants (see chapters 4 and 6). In contrast, concentrations of manganese and radon 

exceeded human-health benchmarks (the higher proposed MCL for radon) less frequently in 

samples from Southwest drinking-water wells than in samples nationwide. Water-quality data 

from the Southwest basin-fill aquifers in a national context are available in appendix 3.

Dissolved solids

Arsenic

Nitrate

Uranium

DBCP

Manganese

Fluoride

Molybdenum

Dieldrin

Radon*

Perchloroethene

Trichloroethene

0 5 10 15 20 25

Percentage of samples from drinking-water wells with concentrations 

greater than human-health benchmarks or a secondary standard

Southwest Principal Aquifers dataset

EXPLANATION

National dataset

*Proposed alternative Maximum Contaminant

 Level of 4,000 picocuries per liter

Samples from drinking-water wells with concentrations greater than 

human-health benchmarks or a secondary standard

Chapter 5

Chapter 5: Quality of Groundwater Used for Drinking and Irrigation in Southwest Study Basins 47
Figure 5–3. Concentrations of several constituents in shallow groundwater and in water 

from deeper, drinking-water wells exceeded human-health benchmarks. The human-related 

contaminants nitrate, dibromochloropropane, and dieldrin are or were used at land surface and 

have moved deeper into basin-fill aquifers. For example, nitrate exceeded its Maximum Contaminant 

Level in almost 25 percent of shallow monitoring wells in agricultural areas across the Southwest 

compared to 18 percent from domestic wells and 4 percent from public-supply wells.

Shallow Groundwater Quality Affects Future Drinking-Water Resources

Monitoring wells installed in urban 

and agricultural areas are used to sample 

groundwater that is shallower and younger 

than water pumped from domestic and 

public-supply wells. Several contaminants 

of concern in deeper groundwater used for 

drinking water also exceeded human-health 

benchmarks in shallow monitoring wells 

(fig. 5–3). Artificial recharge and pumping 

can cause shallow groundwater and any 

contaminants it contains to move further 

down into the aquifer. This applies to 

the human-related contaminants nitrate, 

DBCP, and dieldrin that are used at land 

surface, but constituents with geologic 

sources—arsenic and uranium—can be 

present naturally at elevated concentrations 

at greater aquifer depths. Surprisingly, 

uranium concentrations in the Central Valley 

aquifer system exceeded the MCL in one in 

five shallow monitoring wells in agricultural 

areas and one in four domestic wells, with 

evidence that elevated concentrations are 

moving deeper into the aquifer system. See chapter 6 for more information on the processes 

that affect the movement of uranium—and other constituents—from shallow groundwater 

affected by human activities and processes at the land surface to deeper groundwater used for 

drinking-water supply.

Water is reused for irrigation in the Phoenix area 

of Arizona and many other parts of the Southwest 

to conserve limited supplies of groundwater and 

surface water. Irrigation affects the quality of 

groundwater in the Southwest by moving artificial 

recharge—and any contaminants it contains—into 

the aquifer.

Samples with concentrations greater than human-health benchmarks

0 5 10 15 20 25 30

EXPLANATION

Dieldrin 

Dibromochloropropane (DBCP)

Nitrate 

Uranium 

Arsenic 

Urban monitoring wells

Agricultural monitoring wells

Domestic wells

Public-supply wells

Direction of generally 

increasing well depth

Percentage of groundwater samples with concentrations

greater than human-health benchmarks

48 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Arsenic, uranium, nitrate, dissolved solids, VOCs, and pesticides—these 

are the contaminants that are most likely to limit the use of groundwater now or in the future. Where do they occur, and what sources or 

processes control their concentrations? Answering these questions for selected 

basin-fill aquifers can help to identify areas across the Southwest that might be 

especially vulnerable to contamination and to estimate concentrations in areas 

where no data are available.

Chapter 6

Chapter 6: Understanding Where and Why 

Contaminants Occur in Southwest 

Basin-Fill Aquifers

The heavily irrigated Coachella Valley in southern California 

extends to the Salton Sea, a topographically low area that 

naturally accumulates minerals in groundwater through 

evapotranspiration.

Photograph by Chris Austin, www.aquafornia.com

This chapter describes 

the sources of and 

factors that affect 

arsenic, uranium, nitrate, 

dissolved solids, VOCs, 

and pesticides in Southwest basin-fill aquifers, 

with focus on the intermediate to deeper parts of 

the aquifers that supply 

water for drinking and 

irrigation purposes.
circ1358 ch06fig1

A. Geologic sources and natural factors that affect groundwater quality

B. Human-related sources and factors that affect groundwater quality

Rocks, sediments, and soils in 

recharge areas are a source of 

some contaminants that can 

react with water recharging the 

aquifer

Geochemical processes can cause 

some contaminants to adsorb to or be 

released from aquifer sediments 

causing concentrations to increase 

or decrease in groundwater

Faults can facilitate vertical 

movement of mineralized 

groundwater from deep 

and (or) geothermal 

sources to basin-fill 

aquifers

Evapotranspiration of precipitation 

and streamflow in recharge areas 

and of shallow groundwater in 

discharge areas can concentrate 

solutes in groundwater

Confining layers and upward 

hydraulic gradients can 

impede downward movement 

of some contaminants to 

depths in an aquifer used for 

drinking water 

High rates of artificial recharge and 

pumping enhance movement of some 

contaminants to deeper parts of the 

aquifer

Evapotranspiration of artificial 

recharge increases concentrations of some contaminants

Preferential flow along wells can 

result in upward or downward 

movement of contaminants

Runoff, leaking sewer lines and 

septic tanks, treated wastewater, 

and point sources in urban areas 

can contribute contaminants to 

groundwater 

Excess irrigation water 

and wastewater in 

agricultural areas can 

contribute contaminants to 

groundwater 

Engineered recharge through 

impoundments, spreading facilities, or 

injection wells can contribute 

contaminants to groundwater

Modified from Bexfield and others(19)

Figure 6–1. Several common geologic and human-related sources and factors influence water quality in Southwest basin-fill 

aquifers. A, Geologic sources and natural processes can result in high concentrations of constituents in groundwater. For example, 

evapotranspiration can increase concentrations of arsenic, uranium, nitrate, and dissolved solids in groundwater. B, Urban and 

agricultural activities can release nitrate, dissolved solids, volatile organic compounds, and pesticides to groundwater and move them 

to greater depths in an aquifer.

50 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Arsenic occurs throughout the Southwest from geologic sources such as volcanic rocks (basalt, left) and geothermal water (discharge 

from a geothermal powerplant in Dixie Valley, Nevada, right).

Photographs: Left, Michael Rosen; right, Doug Maurer, USGS

Much of the information presented in this section is from the 

report “Predicted nitrate and arsenic concentrations in basinfill aquifers of the southwestern United States” (available at 

http://pubs.usgs.gov/sir/2012/5065/ ).

As

33

75

Almost one-half of water 

samples from publicsupply wells and onequarter from domestic 

wells sampled as part 

of studies in the Rio 

Grande aquifer system 

contained arsenic at 

a concentration that 

exceeded the Maximum 

Contaminant Level.

Arsenic

Some geochemical conditions can cause arsenic in basin-fill sediments to dissolve into 

groundwater. As a result, groundwater in Southwest basin-fill aquifers commonly has arsenic 

concentrations that exceed the MCL of 10 μg/L. Many factors affect arsenic concentrations in 

groundwater—source rock type, geochemical conditions, the amount of time that groundwater 

is in contact with arsenic-containing rocks and sediment, the amount of groundwater moving 

through the aquifer, and evaporative concentration.

Arsenic is a nonmetallic trace element that commonly is present in rocks in the Southwest. 

Drinking water is the primary means by which people are exposed to arsenic, and long-term 

exposure is associated with many illnesses including skin, bladder, and lung cancers; immunological impairments; and cardiovascular disease. In 2001, the USEPA lowered the MCL for 

arsenic in drinking water from 50 to 10 μg/L.(25) Groundwater used for public supply in areas 

where the MCL is exceeded must be treated or mixed with water that contains lower arsenic 

concentrations to bring levels below the drinking-water standard. Many public suppliers have 

installed costly facilities to treat contaminated groundwater (see sidebar, The cost of arsenic 

contamination, p. 54).

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 51
Figure 6–2. Arsenic concentrations in wells ranged from less than 1 to more than 100 μg/L. 

Although median concentrations in the monitoring, domestic, and public-supply wells were less than 

the Maximum Contaminant Level (MCL), many samples had concentrations up to 10 times higher than 

the MCL.

Arsenic concentrations exceeded the MCL in about 16 percent of drinking-water wells 

sampled as part of studies of the Southwest Principal Aquifers (table 5–1). The MCL for 

arsenic was more frequently exceeded in domestic and public-supply wells in the Rio Grande 

aquifer system and Basin and Range basin-fill aquifers than in the Central Valley aquifer 

system or California Coastal Basin aquifers (table 6–1). Samples from almost one-half of 

the public-supply wells and one-quarter of domestic wells in the Rio Grande aquifer system 

exceeded the MCL for arsenic. Elevated concentrations—two or more times higher than the 

MCL—were measured in shallow groundwater sampled from monitoring wells, in addition to 

deeper groundwater sampled from drinking-water wells in the Southwest (fig. 6–2). Overall, 

arsenic concentrations exceeded the MCL in 19 percent of the 1,018 wells sampled as part of 

the NAWQA Program studies in the Southwest. This amount is consistent with a larger USGS 

dataset for the region (fig. 6–3) that was used to simulate arsenic concentrations.

Table 6–1. Percentage of samples with an arsenic concentration that exceeded the MCL of 10 μg/L. See appendix 2, table A2–2, for 

the number of wells sampled.

Type of well

All aquifers 

in Southwest 

Principal Aquifers 

group

Basin and Range 

basin-fill 

aquifers

Central Valley 

aquifer system

California Coastal 

Basin aquifers

Rio Grande 

aquifer system

Urban land-use monitoring wells 28 42 15 20 11

Agricultural land-use monitoring wells 15 51 10 Not sampled 3.3

Domestic wells 14 21 8.1 0 24

Public-supply wells 19 24 0 3.2 48

All sampled wells 19 30 8.8 6.3 17

10.5 10 100 1,000

Arsenic concentration, in micrograms per liter

Public-supply wells

28%

15%

14%

19%

Domestic wells

Agricultural monitoring wells

Urban monitoring wells Direction of

generally

increasing

well depth

MCL for arsenic is 10 μg/L.

Percentage values are the percentage of samples with an 

arsenic concentration that exceeded the MCL.

See sidebar, Boxplots, p. 53

52 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 6–3. During 1980–2009, USGS studies in the Southwest were conducted to measure arsenic in a total of 4,162 wells that tap 

basin-fill aquifers. About 19 percent of arsenic concentrations in these wells were greater than the MCL of 10 μg/L, consistent with 

data from this study (table 6–1). There are few or no arsenic data for groundwater in large areas in the Southwest, mostly because 

there are few people in these areas and, therefore, few wells.

Boxplots

Boxplots are used to illustrate how results are distributed within a group. The “box” ranges 

from the 25th to the 75th percentile and represents 50 percent of the data. The horizontal line in 

the middle of the box is the median value—one-half of the values in the group are greater than 

the median and one-half are less. 

Percentiles describe the percentage of values in a group that are less than the given value: 

25 percent of the values in a group are less than the 25th percentile; 75 percent of the values in 

a group are less than the 75th percentile. The median is also the 50th percentile.

If, for example, the 75th percentile for the measured concentration of a contaminant in a 

group of wells is equal to the human-health benchmark for that contaminant, then 75 percent, or 

three-fourths, of the wells have a concentration of that contaminant less than the benchmark, 

and 25 percent, or one-fourth, have a concentration greater than the benchmark.

The “whiskers” (vertical lines) in these figures extend to the 10th and 90th percentiles; box 

and whiskers together represent 80 percent of the data. Values greater than the 90th or less 

than the 10th percentile are shown as individual points (outliers).

EXPLANATION

25th percentile

10th percentile

75th percentile

50th percentile

 (median)

90th percentile

Outlier

Outlier

50 percent of data

fall within the box 

80 percent of data fall 

 between endpoints of whiskers 

Southwest Principal Aquifers

study area boundary

Pacif ic

Ocean

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Modified from Anning and others (28)

N

Los Angeles

Phoenix

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

San Francisco

<1.0

1.0 to <5.0

5.0 to ≤10.0

>10.0

 EXPLANATION

Measured arsenic concentration in basin-fill aquifers

Southwest

Principal Aquifers

Measured arsenic 

 concentration in 

 basin-fill aquifers,

 in micrograms per liter

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 53
Volcanic rocks, an arid 

to semiarid climate, and 

closed or constricted basins 

contribute to elevated 

concentrations of arsenic in 

Southwest groundwater.

The cost of arsenic contamination

The USEPA estimated in 2001 that the annual cost to 

reduce arsenic concentrations to below the MCL would range 

from $0.86 to $32 per household for customers of large public 

water systems (more than 10,000 people) to $165 to $327 per 

household for very small systems (25–500 people).(25) Water 

supplies in the Southwest are limited and often naturally 

contain arsenic concentrations high enough to require 

treatment. According to fiscal year 2010 statistics,(68) 274 public 

water systems in California, Nevada, Utah, Arizona, and New 

Mexico—mostly small systems that lack a large customer base 

to pay for water treatment—had a water source containing 

arsenic concentrations that exceeded the MCL for arsenic.

In Fallon, Nevada, groundwater supplying about 

8,400 residents and a nearby Naval Air Station consistently 

contains arsenic concentrations exceeding the MCL. In 2004 a 

treatment facility was installed for arsenic removal at a cost of 

$19 million.(69) Upgrades to water systems throughout Nevada 

to remove arsenic have been estimated to cost many millions of 

dollars.(70)

The arsenic treatment system for Fallon, Nevada, adds 

dissolved iron to the water. The iron reacts with the 

dissolved arsenic to form particles that then are filtered out 

of the water.

Photograph by Jeff Ross

Rock Type, Climate, and Location Within a Basin Affect Arsenic 

Concentrations in Groundwater

The Southwest contains many areas with volcanic rocks, an arid to semiarid climate, and 

closed or constricted basins—all factors that contribute to elevated arsenic concentrations in 

groundwater. Arsenic commonly is found in Southwest basin-fill aquifers in areas with volcanic 

and crystalline rocks (see extent of volcanic rocks in figure 3–2), geothermal water, and (or) 

sulfide minerals associated with these geologic settings.(26) Low precipitation rates and high 

evapotranspiration rates result in low groundwater recharge rates to many Southwest basins. 

Arsenic remaining after water evaporates accumulates in aquifers in closed or constricted 

basins with low recharge rates. In contrast, basins with higher recharge rates have more 

groundwater to flush arsenic from the aquifer.

Arsenic concentrations increase as groundwater moves from recharge areas near mountain 

fronts through arsenic-bearing sediment in the basin toward discharge areas in the basin 

lowlands.(27) The long flow paths typical of Southwest aquifers contribute to elevated arsenic 

concentrations measured in basin lowlands because the water is in contact with the aquifer 

sediment for long periods of time, allowing important geochemical reactions to occur. The area 

where groundwater is located along a flow path in a basin can be used to infer groundwater 

age and geochemical conditions, which influence concentrations of dissolved arsenic and other 

constituents with geologic sources in an aquifer. In general, as groundwater moves along a flow 

path, pH increases and the concentration of dissolved oxygen decreases.

The interaction of arsenic with metal oxides, particularly iron oxide, is one of the most 

important processes by which groundwater can become enriched or depleted in arsenic. When 

geochemical conditions favor arsenic release (desorption) from oxide surfaces, groundwater 

concentrations are higher than when geochemical conditions favor arsenic attachment 

(adsorption) to oxide surfaces. Adsorption and desorption of arsenic are controlled largely 

by the concentration of dissolved oxygen in groundwater and by pH. Generally, if the pH of 

the groundwater is less than 8, arsenic adsorbs to iron oxides, removing it from groundwater 

(fig. 6–4). If the pH is greater than 8, arsenic tends to desorb from the iron oxides and dissolves 

into the groundwater. Under anoxic conditions, iron oxides dissolve and release iron and 

arsenic into the groundwater regardless of pH.

54 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 6–4. More arsenic typically is dissolved in groundwater from Southwest basin-fill 

aquifers under oxic conditions at a pH greater than 8 than under anoxic conditions at a pH 

less than 8. The pH and redox condition of the groundwater affect whether arsenic adsorbs to 

iron oxides on the aquifer sediment or is released into groundwater.

Arsenic can attach to iron oxide (reddish-brown color visible in photographs) that is present in cracks in rocks (left) and that coats 

basin-fill sediment (right). The pH and redox conditions in an aquifer determine if the arsenic remains attached to the iron oxide 

or dissolves into the groundwater. Recharged oxygenated irrigation water and groundwater pumping commonly cause the water 

table to fluctuate, which can release arsenic from basin-fill sediment into the groundwater.(32)

Photographs: Left, Michael Rosen; right, Susan Thiros, USGS

circ1358 ch06fig4

Oxic Anoxic

Concentration, in micrograms per liter

pH <8 pH >8

701 wells 44 wells

204 wells 23 wells

Detection limit

Detection limit

Maximum

Contaminant

Level

Maximum

Contaminant 

Level

0.1

10

1

100

1,000

0.1

10

1

100

1,000

6.0 4.0

2.3

12.5

EXPLANATION 

90th percentile

75th percentile

Median

25th percentile

10th percentile

Outlier

2.3

Data are from samples 

collected for NAWQA studies 

from all types of wells.

Arsenic concentrations in groundwater

See sidebar, Boxplots, p. 53

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 55
Arsenic concentrations greater than 20 μg/L—twice the 

current MCL—have been measured in groundwater throughout 

the Middle Rio Grande Basin, New Mexico. Groundwater is 

the main source of drinking water for many communities in the 

basin. To deliver drinking water that does not contain arsenic 

concentrations above the MCL, some public water suppliers 

use arsenic treatment systems, blend high-arsenic groundwater with low-arsenic water, or both. Groundwater was the 

primary source of drinking water for Albuquerque residents 

until late 2008 when low groundwater levels caused by 

pumping prompted the city to import and treat surface water; 

since that time, groundwater continues to be a secondary 

source of drinking water.

Arsenic in the Middle Rio Grande Basin originates 

predominately from one of two geologic sources, depending on 

the location within the basin:

•	 Groundwater inflow from the Jemez Mountains to the 

north. High arsenic concentrations in recharge water 

from the Jemez Mountains result from contact between 

water and volcanic rocks rich in arsenic. This source of 

groundwater recharge affects the aquifer primarily in the 

northwestern part of the Middle Rio Grande Basin.

Residents of the Albuquerque area, New Mexico, depend on water from the Rio Grande and from the basin-fill aquifer. Groundwater 

west and north of the city contains elevated concentrations of arsenic.

Middle 

Rio Grande

Basin

Photograph by Johanna Siegmann Photography, www.johannasiegmann.com

•	 Deep mineralized groundwater. 

Groundwater circulating at 

depth in the Middle Rio Grande 

Basin has high concentrations 

of arsenic, chloride, and other 

elements. This deep groundwater comes into contact with 

shallower groundwater where it flows upward, primarily 

along fault zones throughout the basin. Evidence of this 

mixing of shallow and deep groundwater includes locally 

high dissolved-solids concentrations (exceeding 5,000 mg/L), 

hot water temperatures (exceeding 125 degrees Fahrenheit 

[°F]), and the occurrence of arsenite (the reduced form of 

arsenic) in groundwater at relatively shallow depths.

Arsenic concentrations in groundwater from the Middle Rio 

Grande Basin also are affected by pH-controlled desorption from 

iron oxides that coat the aquifer sediment. In some areas where 

the groundwater was oxygenated with a pH higher than 8.5, 

arsenic concentrations exceeded 20 μg/L. The arsenic dissolved in 

this groundwater was present primarily as arsenate (the oxidized 

form of arsenic).

The Middle Rio Grande Basin—An example of the influence of geology, 

water source, and geochemical conditions on arsenic concentrations 

in groundwater

56 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Chapter 6

The highest concentrations of arsenic in groundwater in the Middle Rio Grande Basin are associated with volcanic 

rocks in the northern part of the basin and with fault zones through which deep, mineralized groundwater rises. 

Arsenic concentrations in groundwater generally are lower along the eastern and western margins of the basin, 

where low-arsenic recharge water enters the aquifer, than near the center of the basin, where groundwater has 

been in contact with arsenic-bearing sediment for a long time.

Circ1358 ch06 sidebar

Arroyo Tonque

Arroyo Chico

Rio

Puerco

Rio

Salado

Jemez

River

Santa Fe River

Creek

Galisteo

Rio

Grande

Tijeras

Arroyo

Grande Rio

Rio

Puerco

Abo Arroyo

Rio Salado

Jemez River

R io San Jose

EXPLANATION

Interpolated arsenic 

concentration in 

groundwater, in 

micrograms per liter

Groundwater 

sampling site

<3

3 to <5

5 to 20

>20

Jemez

Mountains Española Basin

Albuquerque

Sandia Mountains

Mountains

Manzanita

Sierra Lucero

Los Pinos

Mountains

Ladron

Peak

Manzano Mountains

San Juan Basin

Albuquerque

Estancia Basin

municipal boundary

Middle Rio Grande

Basin boundary

Modified from Plummer and others(78)

Arsenic concentration in Middle Rio Grande Basin groundwater

N

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 57
Figure 6–5. Arsenic concentrations in Southwest basins with volcanic and crystalline rocks 

in the adjacent mountains typically increase with distance along the groundwater flow path 

because of geochemical reactions between the groundwater and basin-fill sediments derived from 

the mountains. Aquifers in these areas that have relatively low recharge rates (less than about 

2 inches per year) tend to have longer groundwater residence times and higher predicted arsenic 

concentrations than those that have higher rates of recharge.

Estimating Arsenic Concentrations in Areas Where No Data Are Available

The relation between arsenic concentrations in groundwater and rock type, climate, and 

location within a basin (for example, basin lowlands) was explored using a statistical model.(28)

The model provides estimates of arsenic concentrations in areas where no measurements are 

available (fig. 6–3).

The statistical model found that, in a basin surrounded by volcanic and crystalline rocks, 

arsenic concentrations are greater in areas with less recharge to the aquifer than in areas with 

more recharge (fig. 6–5). Areas with relatively low groundwater recharge rates tend to have a 

longer groundwater residence time resulting in less flushing of arsenic and other constituents 

from the aquifer than areas with higher rates of recharge. As groundwater moves along the flow 

circ1358 ch06fig5

Basins with 

low recharge

Basins with

high recharge

48%

57%

16%

65%

7%

16%

33%

Basin margin Middle basin

Water table

Basin lowlands

Groundwater flow path

Volcanic and

crystalline rocks

in mountains

EXPLANATION

Modified from Anning and others (28)

Percentage of basin area with

 estimated arsenic concentration

 in groundwater ≥10 micrograms

 per liter

NV

WY

OR ID

CA

AZ NM

UT CO

Estimated recharge to 

basin from contributing 

areas, in inches per year

0 to 2.0

2.1 to 26

Low (0 to 2)

High (>2)

58 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 6–6. Almost half (43 percent) of the areal extent of Southwest basins likely has arsenic concentrations in groundwater 

that equal or exceed the MCL of 10 μg/L, on the basis of a statistical model. Proximity to volcanic rocks, an arid climate, and lack of 

groundwater drainage from a basin contribute to high arsenic concentrations. If used for public supply, groundwater in areas with 

these conditions likely would require treatment to decrease arsenic concentrations to levels acceptable for drinking.

path from recharge areas near basin margins to discharge areas in basin lowlands, arsenic 

concentrations were likely to increase. Arsenic concentrations greater than the MCL are 

more likely to occur in basin lowlands than in other parts of the basin because of a longer 

residence time in the aquifer and geochemical reactions between the groundwater and arsenicbearing sediment.

The model estimates that 43 percent of the area with basin-fill aquifers has arsenic 

concentrations equal to or greater than 10 μg/L (fig. 6–6). This percentage is higher than might 

be expected on the basis of measured concentrations because of the inclusion of large areas 

with arsenic-bearing rocks and constricted groundwater flow. Many of the basins with these 

conditions are sparsely populated with relatively little groundwater pumping (fig. 4–5) so there 

are few data on concentrations, primarily because there are so few wells to sample.

circ1358 ch06fig6

Los Angeles

Phoenix

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

San Francisco

Modified from Anning and others(28)

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pacif ic

Ocean

Southwest Principal Aquifers

study area boundary

 EXPLANATION

Estimated arsenic concentrations 

 in basin-fill aquifers, in 

 micrograms per liter

<1.0

 1.0 to <5.0

 5.0 to <10.0

≥10.0

Area not modeled

Estimated arsenic concentration in basin-fill aquifers

N

Southwest

Principal Aquifers

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 59
Uranium

Uranium concentrations in Southwest basin-fill aquifers are high where uranium-rich rocks 

and geochemical conditions that promote the release of uranium from those rocks are present. 

Concentrations exceeded the MCL of 30 μg/L in about 7 percent of samples from drinkingwater wells. Recharge of excess irrigation water and pumping from deeper parts of the aquifer 

can cause downward movement of shallow groundwater enriched in uranium to parts of the 

aquifer used for drinking-water supply.

Uranium is a radioactive trace element present in many rocks and basin-fill sediments in 

the Southwest. The USEPA MCL for uranium is 30 μg/L, largely because of the potential for 

kidney damage that could arise from long-term exposure through drinking water.(29)

Uranium concentrations exceeded the MCL in about 7 percent of drinking-water wells 

sampled as part of the studies in the Southwest (table 5–1). Domestic wells with uranium 

concentrations that exceeded the MCL were primarily in the agricultural eastern Central Valley, 

California, where 24 percent of the wells had concentrations greater than the MCL (table 6–2). 

About one in four samples 

from domestic wells in the 

eastern San Joaquin Valley, 

California, contained 

uranium at a concentration 

that exceeded the Maximum 

Contaminant Level.

U

92

238

Photographs: Left, David Berger; right, Tim Rowe; bottom, Michael Rosen, USGS

60 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Granitic rock, like that at the top of Mount Whitney in the Sierra Nevada (upper left), commonly contains uranium. As the granitic rock 

erodes, the fragments are transported by streams and rivers to lower altitudes (sediment-laden water in Ward Creek near Lake Tahoe, 

upper right). Over geologic time, this eroded material accumulates in the downstream basin to form the basin-fill aquifer (Eagle Valley, 

Nevada, bottom). As groundwater moves through the basin-fill aquifer, it dissolves some of the constituents—including uranium—

in the sediment.
Figure 6–7. Median concentrations of uranium are higher in shallow parts of Southwest basin-fill 

aquifers than in deeper parts, which are tapped by domestic and public-supply wells. Nonetheless, 

concentrations of uranium exceeded the MCL in 12 percent of domestic wells, which are not required 

to be monitored or treated.

Shallow groundwater in Southwest basins generally has higher concentrations of uranium than 

water in deeper parts of the aquifer (fig. 6–7). The potential for movement of uranium from 

shallow to deeper parts of the aquifer where groundwater is used for public supply is of concern 

in the eastern San Joaquin Valley, because uranium occurs in the sediments and human activities have increased the downward movement of shallow groundwater (see sidebar, Humaninduced changes to the Central Valley aquifer system, California, p. 36). About 14 percent of 

uranium concentrations in 1,307 wells sampled by the USGS during 1980–2009—78 percent of 

these were sampled as part of NAWQA studies—were greater than the MCL (fig. 6–8).

10.5 10 100 1,000 2,000

Uranium concentration, in micrograms per liter

Public-supply wells

18%

15%

12%

1.2%

Domestic wells

Agricultural monitoring wells

Urban monitoring wells Direction of

generally

increasing

well depth

MCL for uranium is 30 μg/L.

Percentage values are the percentage of samples 

with a uranium concentration that exceeded the MCL.

See sidebar, Boxplots, p. 53

Chapter 6

Table 6–2. Percentage of samples with a uranium concentration that exceeded the MCL of 30 μg/L. See appendix 2, table A2–2, for 

the number of wells sampled.

Type of well

All aquifers 

in Southwest 

Principal Aquifers 

group

Basin and Range 

basin-fill 

aquifers

Central Valley 

aquifer system

California Coastal 

Basin aquifers

Rio Grande 

aquifer system

Urban land-use monitoring wells 18 19 0 48 9.1

Agricultural land-use monitoring wells 15 31 19 Not sampled 6.6

Domestic wells 12 3.1 24 0 4.0

Public-supply wells 1.2 2.0 0 1.1 0

All sampled wells 11 9.9 18 8.9 5.4

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 61
Figure 6–8. Data on uranium concentrations in Southwest groundwater are sparse compared to those for arsenic. Concentrations of 

uranium in groundwater are elevated in the southern part of the San Joaquin Valley in California and in the Carson Desert in Nevada. 

Many of these uranium concentrations are in samples from wells less than 50 feet deep that are affected by evaporative concentration 

and recharge of excess irrigation water.

circ1358 ch06fig8

EXPLANATION

Measured uranium concentration

 in basin-fill aquifers, in 

 micrograms per liter

<3

3 to <15

15 to ≤30

>30

Measured uranium concentration in basin-fill aquifers

Southwest Principal Aquifers

study area boundary

Pacif ic

Ocean

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Carson

Desert

San Joaquin Valley

N

Los Angeles

Phoenix

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

San Francisco

(Data from the USGS National 

Water Information System)

Southwest

Principal Aquifers

62 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Uranium can be an indicator of other radionuclides in groundwater

Uranium concentrations in crystalline rock aquifers in the northeastern United States indicate the 

possible presence of three other radionuclides with human-health concerns: radon, radium, and gross 

alpha radioactivity.(71) Uranium, which is relatively easy to analyze, could prove to be a useful indicator 

of radon, radium, and gross alpha radioactivity in Southwest groundwater, where there are few 

measurements of these radionuclides. Better coverage of uranium concentrations in basin-fill aquifers 

would therefore address multiple potential human-health concerns, in addition to providing a better 

understanding of the major factors that affect uranium in the region’s groundwater. Granite, an igneous 

rock, can be enriched 

in uranium.
Because uranium bonds 

strongly with bicarbonate, 

it is not likely to sorb to 

aquifer sediments and tends 

to be dissolved in Southwest 

groundwater.

Figure 6–9. In the Southwest, high uranium concentrations 

generally occur in oxic groundwater that has a relatively high 

alkalinity (bicarbonate is a large component of alkalinity) because 

uranium forms soluble complexes with bicarbonate.

Rock Type and Geochemistry Affect Uranium Concentrations in 

Groundwater

The main source of uranium in groundwater in Southwest basins is the weathering 

of metamorphic and intrusive rocks (fig. 3–2), especially granitic rocks, and the sediment 

derived from these rocks. Large exposures of granitic rocks are less widely distributed than are 

volcanic rocks in the Southwest, so there are fewer areas where high uranium concentrations in 

groundwater are a problem compared to arsenic. Evaporative concentration in low-lying areas 

of constricted and closed basins combined with the transport of evaporite salts through the 

soil by irrigation water, such as in the southern San Joaquin Valley in California,(30) can cause 

uranium concentrations in shallow groundwater to increase.

Redox conditions, pH, and the presence of dissolved carbonate species (mostly carbonate 

and bicarbonate) influence uranium concentrations in groundwater. In sand and gravel aquifers 

across the United States, uranium tends to occur in groundwater that is oxic and has a pH equal 

to or greater than 7.(31) High concentrations of bicarbonate increase the solubility of uranium 

in oxygenated groundwater.(32) Uranium bonds strongly (complexes) with bicarbonate; as a 

result, the uranium is less likely to sorb to aquifer sediments. This tendency for uranium to be 

dissolved in Southwest groundwater is apparent in the relation between uranium concentrations and alkalinity, a measure of the capacity of water to neutralize or buffer acids (bicarbonate is an important buffer) (fig. 6–9). When high concentrations of carbonate species are 

not present in oxic groundwater and pH is near neutral, uranium does not remain in solution 

but sorbs to metal oxides, organic material, and clays in the aquifer sediments.(33) In anoxic, 

strongly reducing groundwater (see sidebar, How do redox reactions work?, p. 31), uranium 

also is removed from solution, not through adsorption but through the precipitation of stable 

uranium-forming minerals, which results in low uranium concentrations in groundwater.

Minerals can be transported 

by water to basin-fill deposits 

where they can accumulate.

Alkalinity, in milligrams per liter as calcium carbonate

Uranium concentrations in groundwater

0 100 200 300 400 500 600 700 800

0.1

1

10

100

Concentrations of uranium less than 1 microgram per liter

Concentration, in micrograms per liter

are considered below detection for this dataset

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 63
Irrigation Has Caused Sediments to Release Uranium Into Shallow 

Groundwater 

Uranium sorbed to sediment can be released to groundwater by excess irrigation water, 

as has occurred in the southern Carson Desert in Nevada(18, 34) and the eastern San Joaquin 

Valley in California.(32) Groundwater with elevated concentrations of uranium is widespread 

in the eastern San Joaquin Valley. About 19 percent of wells (65 of 350) sampled as part of the 

USGS and California Ground Water Ambient Monitoring and Assessment (GAMA) studies 

had uranium concentrations that exceeded the MCL of 30 μg/L. Concentrations were highest 

in monitoring wells tapping shallow parts of the aquifer (32 percent exceeded the MCL) 

and decreased with depth. Uranium concentrations exceeding the MCL were measured in 

25 percent of the domestic wells sampled.(32) In contrast, the MCL for uranium was exceeded in 

less than 3 percent of domestic wells sampled by the NAWQA Program nationally.(35)

Uranium concentrations in shallow groundwater in the eastern San Joaquin Valley are 

high because of the presence of elevated concentrations of calcium and bicarbonate. Although 

calcium and bicarbonate occur naturally in groundwater, their concentrations have increased 

since the 1850s mainly as a result of agriculture. Plant root respiration and microbial oxidation of organic matter cause calcium carbonate minerals to dissolve into irrigation water that 

infiltrates into soils. This process leads to increases in both calcium and bicarbonate concentrations in the recharge water that reaches the uranium-bearing aquifer sediments. Uranium sorbed 

to aquifer sediments bonds with the calcium and bicarbonate and is released into shallow 

groundwater, which is then transported to deeper parts of the aquifer by pumping (fig. 6–10). 

Because the developed part of the aquifer system in the eastern San Joaquin Valley is largely 

oxic, uranium remains dissolved in groundwater and, therefore, poses a threat to the long-term 

sustainability of the aquifer as a source of drinking water.(32)

Figure 6–10. Pumping groundwater from deeper parts of the aquifer in the eastern San 

Joaquin Valley, California, pulls shallow groundwater that can contain relatively high uranium 

concentrations downward and toward the long screened intervals of pumping wells. The high 

uranium concentrations in shallow groundwater result from mobilization of uranium in the sediment 

by bonding with dissolved bicarbonate ions. 

Irrigation well

Domestic well

High bicarbonate 

concentrations in 

shallow groundwater

Low bicarbonate

concentrations in 

deeper groundwater

Public-supply well

Uranium

released

from

sediments

Silt and clay

Unconsolidated deposits

Floodplain

deposits

Modified from Jurgens and others;(32) Ayotte and others(31)

64 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Nitrate

Nitrate concentrations in most Southwest basin-fill aquifers are low relative to the MCL of 

10 mg/L as nitrogen because of the relatively small amount of land development. In agricultural and urban areas, however, nitrate concentrations are above background levels, and in 

some areas concentrations in groundwater exceed the MCL. Nitrate concentrations are likely 

to increase in agricultural and urban areas as nitrate from past fertilizer applications or other 

sources reaches the water table and makes its way deeper into the aquifer.

The old adage “too much of a good thing” holds true for nitrate. Nitrate and other forms of 

nitrogen are essential nutrients for human health and the growth of plants and animals. At high 

concentrations, however, nitrate can pose human-health and ecological risks. The USEPA has 

set a MCL of 10 mg/L as nitrogen (N) for nitrate in drinking water to protect against “blue-baby 

syndrome.”(36)

Nitrate is a relatively common groundwater contaminant in Southwest study basins, and 

concentrations exceeded the MCL in about 11 percent of drinking-water wells sampled as part 

of the studies in the Southwest (table 5–1). This percentage was much higher for domestic 

Fertilizers are a source of nitrate. Irrigation of agricultural land (irrigated lettuce field near Yuma, 

Arizona, on left) and urban and suburban land (fertilizer application to lawn in Orem, Utah, on right) 

moves some of the nitrate in fertilized soils down to the water table.

Photograph by Jeff Vanuga, Natural Resources Conservation Service, U.S. Department of Agriculture

Chapter 6

Photograph by Alan M. Cressler, USGS

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 65

Much of the information presented in this section is from

the report “Predicted nitrate and arsenic concentrations

in basin-fill aquifers of the southwestern United 

States” (available at http://pubs.usgs.gov/sir/2012/5065/).
wells in some areas—nitrate concentrations exceeded the MCL in 29 percent of domestic wells 

in the Central Valley aquifer system, which includes the predominantly agricultural eastern 

San Joaquin Valley (table 6–3). In contrast, nitrate was measured at concentrations greater than 

the MCL in only 4 percent of domestic wells sampled nationally.(35) The similarity between 

nitrate concentrations in shallow groundwater samples from monitoring wells in agricultural 

settings and deeper groundwater from domestic wells (fig. 6–11) likely reflects the effects of 

agriculture on groundwater quality. About 10 percent of nitrate concentrations in 5,787 wells 

in the Southwest sampled by the USGS during 1980–2009 (1,070 wells sampled as part of the 

NAWQA Program) were greater than the MCL (fig. 6–12).

Table 6–3. Percentage of samples with a nitrate concentration that exceeded the MCL of 10 mg/L as nitrogen. See appendix 2, 

table A2–3, for the number of wells sampled.

Type of well

All aquifers 

in Southwest 

Principal 

Aquifers group

Basin and Range 

basin-fill 

aquifers

Central 

Valley aquifer 

system

California Coastal 

Basin aquifers

Rio Grande 

aquifer system

Urban land-use monitoring wells 6.3 7.1 0 21 0

Agricultural land-use monitoring wells 25 22 20 Not sampled 28

Domestic wells 18 11 29 0 0

Public-supply wells 3.6 2.0 0 7.5 0

All sampled wells 14 11 22 7.7 14

Nitrate concentrations 

exceeded the Maximum 

Contaminant Level in 

18 percent of samples from 

domestic wells and in less 

than 4 percent of samples 

from public-supply wells.

Figure 6–11. Median nitrate concentrations were highest in domestic wells, which mostly 

were located in agricultural areas, and in monitoring wells installed in agricultural areas. Deeper 

groundwater sampled from public-supply wells, which typically are in urban areas, had the lowest 

median concentration and the fewest exceedances of the MCL of 10 mg/L as N.

0.10.05 1 10 100

Nitrate concentration, in milligrams per liter as nitrogen

Public-supply wells

6.3%

25%

18%

3.6%

Domestic wells

Agricultural monitoring wells

Urban monitoring wells

Direction of

generally

increasing

well depth

MCL for nitrate is 10 mg/L 

as N. Percentage values are 

the percentage of samples 

with a nitrate concentration 

that exceeded the MCL.

See sidebar, Boxplots, p. 53

66 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 6–12. Nitrate concentrations measured in wells that tap basin-fill aquifers in the Southwest mostly represent nitrate in those 

parts of the groundwater system used for water supply. Areas where nitrate concentrations in wells exceeded the MCL of 10 mg/L as 

N include the West Salt River Valley in Arizona, the Santa Ana Basin and San Joaquin Valley in California, and the San Luis Valley in 

Colorado. 

Photograph from www.shutterstock.com

The cost of nitrate contamination

In California’s San Joaquin Valley, intensive agricultural fertilizer use 

has resulted in widespread contamination of groundwater with nitrate, 

including, in some places, groundwater that is pumped for public supply. 

Many small communities cannot afford to supply water that meets the 

nitrate drinking-water standard,(72) and some residents with access only to 

contaminated groundwater are forced to purchase bottled or vended water 

for drinking.

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pacif ic

Ocean

Phoenix

Southwest Principal Aquifers

study area boundary

San

Luis

Valley

West Salt

River Valley

Santa Ana Basin

San Joaquin Valley

Modified from Anning and others(28)

Measured nitrate concentration in basin-fill aquifers

N

Southwest

Principal Aquifers

Los Angeles

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

San Francisco

EXPLANATION

<0.50

0.50 to <1.0

1.0 to <2.0

2.0 to <5.0

5.0 to ≤10.0

>10.0

Measured nitrate concentration 

 in basin-fill aquifers, in 

 milligrams per liter as nitrogen

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 67
Human Activities Can Combine With Natural Factors to Increase Nitrate 

Concentrations in Groundwater

Human-related sources of nitrate to Southwest groundwater include fertilizers and 

manure applied to irrigated fields and turf areas, wastewater from concentrated animal feeding 

operations, wastewater from septic or sewer systems, and treated urban wastewater applied to 

irrigated areas.(19, 37) Groundwater is especially vulnerable to nitrate contamination where the 

water table is shallow and nitrate sources are plentiful, although shallow groundwater typically 

is not used as a source of drinking water in Southwest basins. Nitrate concentrations were 

greater than 10 mg/L as N in 25 percent of shallow agricultural monitoring wells sampled in 

Southwest agricultural areas (fig. 6–11)—agricultural areas are associated with relatively large 

amounts of fertilizer and (or) manure applications compared to other types of land cover.

Some natural hydrogeologic and geochemical 

factors combine with human-related sources and 

activities to increase the vulnerability of basin-fill 

aquifers to nitrate contamination (fig. 6–1). The 

arid climate leads to evaporative concentration, 

and coarse-grained soils promote rapid infiltration. 

The presence of dissolved oxygen in groundwater 

in most areas prevents denitrification (the transformation of nitrate to nitrogen gas) (fig. 6–13).

Wells that pump water from below laterally 

extensive layers of clay, which impede downward 

groundwater movement, are less likely than other 

wells to be affected by nitrate contamination from 

the land surface. An illustration of this is in the 

southwestern part of the West Salt River Valley, 

Arizona. In this agricultural area, the median 

nitrate concentration in wells screened (open to 

the aquifer) above an areally extensive clay layer 

was 19 mg/L as N—almost twice the MCL—and 

in wells screened below the clay layer the median 

was 2 mg/L as N.(38) If, however, a well penetrates 

such a clay layer and is screened both above and 

below the clay layer, the well can be a conduit for 

vertical groundwater flow and cause mixing of 

waters with low and high nitrate concentrations (see 

sidebar, Wells can “short circuit” groundwater flow 

paths and increase contaminant movement, p. 37).

Nitrate Concentrations in Deeper Parts of Basin-Fill Aquifers Can Increase 

Given Sufficient Time

Concern about nitrate contamination arises where the shallow part of an aquifer is hydrologically linked to deeper parts that are, or could be, used for drinking water. The movement of 

nitrate from sources at the land surface to shallow parts of basin-fill aquifers and subsequently 

to greater depths can take many years, even decades. Because of this long travel time, nitrate 

concentrations measured in deep groundwater does not reflect the full effect of past and present 

human activities.

For example, nitrate concentrations in shallow, intermediate, and deeper parts of the 

basin-fill aquifer in the eastern San Joaquin Valley have increased (fig. 6–14A).(39) This trend 

is attributed to increases in fertilizer and manure application rates and, therefore, in nitrate 

concentrations in excess irrigation water that recharges the aquifer. Nitrate concentrations are 

highest at shallow aquifer depths and are higher at intermediate depths, where domestic wells 

Groundwater is especially 

vulnerable to nitrate 

contamination where the 

water table is shallow 

and nitrate sources are 

plentiful.

Figure 6–13. Nitrate concentrations commonly are lower in anoxic groundwater 

than in oxic groundwater because denitrification—the transformation of nitrate to 

harmless nitrogen gas—occurs only under 

anoxic conditions. Most groundwater in 

Southwest basin-fill aquifers is oxic.

Granular fertilizer containing 

nitrogen supplies nutrients to 

lawns where the fertilizer is 

applied.

Concentration, in milligrams per 

liter as nitrogen

Detection limit = 

0.03 mg/L as N

Data from all well types sampled by NAWQA

Maximum 

Contaminant Level

0.1

0.01

10

1

100

0.04

2.7

Groundwater redox condition

Anoxic Oxic

Nitrate concentration

See sidebar, Boxplots, p. 53

68 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
typically are screened, than at greater depths, where publicsupply wells are screened (fig. 6–14B). Concentrations in 

deeper parts of the aquifer likely will be higher in the future 

as nitrate-rich groundwater continues to move downward. 

Because of the travel time required for groundwater to move 

from the water table to deeper parts of the aquifer, current 

nitrate concentrations in water from public-supply wells 

reflect the effects of management practices that occurred 

40 to 50 years ago, and current management practices will be 

reflected in concentrations measured in the future.

In another example, increasing nitrate concentrations 

in groundwater have been measured in the Carson Valley, 

Nevada (fig. 6–15). Of 27 monitoring wells sampled during 

1985–2001, 15 had an upward trend in nitrate concentration, 3 had a downward trend, and 9 wells had no trend.(40)

Most of the wells with upward trends in nitrate concentration 

were located in areas of septic tank use, which has increased 

in the Carson Valley over the past 40 years.

Figure 6–14. A, Nitrate concentrations in groundwater in the eastern San Joaquin Valley, California, 

have increased, largely because more nitrogen-containing fertilizer is being applied. B, Elevated 

nitrate concentrations in shallow parts of the aquifer reflect recent rates of fertilizer application. 

Because of the time it takes for groundwater to flow from shallow to deeper parts of the aquifer, 

nitrate concentrations at greater depths are lower, reflecting past rates of fertilizer application.

circ1358 ch06fig14

MCL

Public-supply wells

Domestic wells

Monitoring wells

15

10

5

0

10

5

0

5

0

1993–95 2001–02

Median concentration, in milligrams per 

liter as nitrogen

Nitrate concentrations in groundwater

A B Well in 

urban area

Wells in 

agricultural area

Monitoring well

Domestic well

Public-supply well

Water

table

100

0

200

Depth below land surface, in feet

300

>10

5–10

<5

Concentration, in milligrams per liter as nitrogen

Direction of groundwater flow

Modified from Dubrovsky and others(42)

Chapter 6

Figure 6–15. In the Carson Valley, Nevada, nitrate from the land 

surface has moved downward, which has caused nitrate concentrations in groundwater to increase. The higher concentrations 

largely are a result of contamination from nitrate-rich septictank leachate.

circ1358 ch06fig15

0

1

2

3

4

5

1985 1988 1991 1994

Year

1997 2000

Concentration, in milligrams per 

liter as nitrogen

Well 1 

Well 2 

Well 3 

Modified from Rosen(40)

Nitrate concentration

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 69
Where Are High Nitrate Concentrations in Basin-Fill Aquifers Located in 

the Southwest?

The Southwest is a large area, and nitrate concentrations have not been measured in many 

parts of its numerous basin-fill aquifers. One way to estimate nitrate concentrations in areas 

where no measurements are available and to gain a better understanding of concentrations 

across the region is to use a statistical model. One such statistical model is based on the relation 

between measured nitrate concentrations in groundwater (fig. 6–12) and spatially distributed 

information about nitrate sources, climate, soil, land use, and water use.(28)

Figure 6–16 A statistical model used measured nitrate concentrations in groundwater and spatially distributed information about 

nitrate sources, climate, soil, land use, and water use to estimate nitrate concentrations in parts of basin-fill aquifers where data were 

not available. Only 2.4 percent of the areal extent of Southwest basin-fill aquifers was estimated to have nitrate concentrations equal to 

or greater than the MCL of 10 mg/L as N.

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

Southwest Principal Aquifers

study area boundary

UTAH COLORADO

Pacif ic

Ocean

Modified from Anning and others(28)

N

Los Angeles

Phoenix

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

San Francisco

<0.50

0.50 to <1.0

1.0 to <2.0

2.0 to <5.0

5.0 to 10.0

≥10.0

Estimated nitrate concentration in basin-fill aquifers

Southwest

Principal Aquifers

Estimated nitrate concentration 

 in basin-fill aquifers, in milligrams 

 per liter as nitrogen

EXPLANATION

70 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 6–17. Estimated concentrations of nitrate are higher in Southwest basin-fill aquifers where the overlying land is 

substantially developed for agricultural or urban uses than in areas with minimal land development.

Nitrate concentrations equal to or greater than the MCL of 10 mg/L as N were estimated 

by the model for only 2.4 percent of the extent of basin-fill aquifers in the Southwest 

(fig. 6–16). This percentage is small because large areas with undeveloped rangeland 

are included in the statistical analysis. Areas where there is the most demand for potable 

groundwater—agricultural and urban land—are most likely to have groundwater with elevated 

concentrations of nitrate (fig. 6–17).

Denitrification results in decreasing nitrate concentrations in groundwater. Nitrate 

concentrations are likely to be lower in groundwater from basin lowlands, especially those 

with wetland areas where anoxic conditions exist and denitrification commonly occurs, than in 

groundwater from near the basin margin where dissolved oxygen is present (fig. 6–17).

<0.50

0.50 to <5.0

5.0 to 10.0

≥10.0 

Percentage of basin area with

 estimated nitrate concentrations in 

 groundwater, in milligrams per liter

 as nitrogen

EXPLANATION

UNDEVELOPED AREAS

DEVELOPED AREAS

Areas with substantial

(50 percent or more) 

agricultural and urban 

land development* 

Areas with minimal (less 

than 5 percent) land 

development*

BASIN MARGIN MIDDLE BASIN

Non-wetland areas

BASIN LOWLANDS

Wetland areas

* This summary excludes areas with 

 Sonoran Desert biotic communities. 

10%

47% 27%

16%

40%

30%

18%

12%

61%

17%

9%

13%

93%

2%

1%4%

31%

69%

54%

45%

1%

74%

25%

1%

97%

1% 2%

Water table

Groundwater flow path

Modified from Anning and others(28)

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 71
Groundwater in the Southwest Has a Wide 

Range in Background Nitrate Concentrations 

Nitrate in some Southwest groundwater can come from 

natural sources and processes. Precipitation in the Southwest 

contains concentrations of nitrate plus ammonium dissolved 

in rain that range from about 0.50 to 2.0 mg/L as N.(41) The 

nitrogen that is not taken up by plants or stored in soils and 

the underlying unsaturated zone is carried in recharge water 

down into the basin-fill aquifers. For much of the region, 

nitrate concentrations measured in groundwater are less than 

2.0 mg/L as N, and in many areas concentrations are less 

than 0.50 mg/L as N (fig. 6–12).(28) These concentrations of 

nitrate in Southwest groundwater are similar to the national 

background concentration of about 1.0 mg/L as N estimated 

for areas with minimal effects from human activities.(42)

Soils and plants in some areas of the Southwest 

contribute nitrate to groundwater. The atmosphere contains 

mostly nitrogen gas, which is converted to nitrate by bacteria 

in the soil and in the roots of some desert plants (legumes). 

For groundwater underlying biotic communities where 

legumes are common in the Sonoran Desert in southeastern 

California and southwestern Arizona, nitrate concentrations 

are commonly measured (fig. 6–12) and estimated to be 

(figs. 6–16 and 6–18) 5.0 mg/L as N 

and as high as 10.0 mg/L as N in some 

places, including areas with no agriculture 

or urbanization. In fact, background 

concentrations in Sonoran Desert groundwater 

can exceed concentrations in other parts 

of the Southwest and the Nation that are 

elevated as a result of human activities. A 

substantial amount of nitrate has accumulated 

in the unsaturated zone beneath desert soils 

in parts of California, Nevada, Arizona, and 

New Mexico—if development of these lands 

results in additional recharge that transports 

the nitrate to the aquifer, it could pose a 

concern for groundwater contamination.(43)

Figure 6–18. Desert legumes, such as mesquite, ironwood, smoke, and palo 

verde (shown in photograph) trees, convert nitrogen gas in the atmosphere to 

nitrate, which they release into the soil. These plants often grow along desert 

washes (dry streambeds). When it rains, some of the water that runs into the 

washes becomes groundwater recharge that moves the nitrate from the soil down 

to the aquifer. Legumes are common in Sonoran Desert biotic communities, and 

nitrate concentrations are estimated to be higher in the underlying groundwater 

in these communities (darker shades in pie diagrams) than in other biotic 

communities in the Southwest.

15%

3%

30% 44%

8%

Area underlying Sonoran Desert biotic communities 

with estimated nitrate concentration in groundwater

Estimated nitrate concentrations in Sonoran Desert groundwater 

49%

13%

31%

1% 6%

Area underlying all other biotic communities 

with estimated nitrate concentration in groundwater

Sonoran

Desert

Sonoran

Desert

<0.50

0.50 to <1.0

1.0 to <2.0

2.0 to <5.0

5.0 to <10.0

Nitrate concentration in groundwater, 

 in milligrams per liter as nitrogen

EXPLANATION

Modified from Anning and others(28)

72 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Dissolved Solids

One of every four drinking-water wells sampled in the 

Southwest contained dissolved solids at a concentration that 

exceeded the SMCL of 500 mg/L, which can give water an 

unpleasant taste. Elevated dissolved-solids concentrations 

in groundwater are the result of natural processes, such 

as dissolution and evaporative concentration, and humanrelated sources and alterations of groundwater flow systems. 

Artificial recharge and well pumping in several basins 

have caused dissolved solids to move from shallow parts of 

aquifers to deeper parts used for water supply. Because of 

its effect on the availability of potable and irrigation water, 

elevated dissolved-solids concentrations are an important 

water-quality concern in many areas of the Southwest.

High dissolved-solids concentrations (see sidebar, 

What are dissolved solids?, this page) in water used for 

domestic, public, industrial, and agricultural supply can 

cause numerous problems. These problems can include an 

objectionable taste, higher water-treatment costs, greater use 

of detergents and soaps because of more water hardness, 

precipitation of minerals in plumbing, staining, corrosion of 

metallic surfaces, reduced equipment lifespan, and restricted 

use for irrigation. The USEPA has established a nonenforceable secondary drinking-water standard (SMCL) for dissolved 

solids of 500 mg/L because of concerns related to aesthetic 

effects rather than human-health effects. Elevated concentrations limit which crops will grow or result in decreased 

crop yields.

The word “salinity” describes the amount of salt in a water sample and, 

because salts make up the bulk of dissolved solids in water, the terms 

“salinity” and “dissolved solids” commonly are used interchangeably in 

the Southwest.

[mg/L, milligrams per liter]

Terms commonly used to describe ranges of dissolved-solids 

concentrations in water

Fresh Less than 1,000 mg/L

Brackish 1,000–10,000 mg/L

Slightly saline 1,000–3,000 mg/L

Moderately saline 3,000–10,000 mg/L 

Very saline 10,000–35,000 mg/L*

Brine Greater than 35,000 mg/L

*

*Ocean water is about 35,000 mg/L.

CConcentration ranges from Alley(73) and the National Ground Water 

Association.(82)

What are dissolved solids?

Dissolved-solids concentration is a measure 

of all substances, both inorganic and organic, 

dissolved in water. Major ions (calcium, magnesium, sodium, potassium, bicarbonate, sulfate, 

chloride, nitrate, and silica) typically make up 

most of the dissolved solids in water. Dissolvedsolids concentration is reported in milligrams per 

liter (mg/L) and represents the mass of dry solids 

that remain after all the water in a 1-liter sample 

is evaporated at a temperature of 180 degrees 

Celsius (°C). Potable water generally is considered to have less than 1,000 mg/L dissolved solids. 

Water with greater than 1,000 mg/L dissolved 

solids is used for domestic supply in areas where 

water of lower dissolved-solids content is unavailable, but water containing more than 3,000 mg/L 

generally is too salty to drink.(73)

Much of the information presented in this section is from the report

”Dissolved solids in basin-fill aquifers and streams in the southwestern 

United States” (available at http://pubs.usgs.gov/sir/2006/5315/).

Photograph by Bert Duet, USGS

Agricultural or urban development of an arid basin, such as 

the West Salt River Valley near Phoenix, Arizona, often results 

in dissolved solids being added to surface water before the 

water recharges the underlying basin-fill aquifer.

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 73
Dissolved-solids concentrations exceeded the SMCL of 500 mg/L in 26 percent of 

drinking-water wells sampled as part of the studies in the Southwest (table 5–2). Of the four 

Southwest Principal Aquifers, Basin and Range basin-fill aquifers had the highest percentage 

of samples from all wells with concentrations greater than 500 mg/L (table 6–4). Higher 

dissolved-solids concentrations typically were measured in shallow groundwater in agricultural 

and urban areas than in deeper groundwater used for drinking (fig. 6–19). Most groundwater in 

Southwest basin-fill aquifers has less than 1,000 mg/L dissolved solids, but concentrations can 

exceed 10,000 mg/L in some areas (fig. 6–20).

Figure 6–19. Dissolved-solids concentrations generally are higher in shallow parts of Southwest basinfill aquifers than in deeper parts, which are the source of groundwater commonly used for drinking.

Table 6–4. Percentage of samples with a dissolved-solids concentration that exceeded the SMCL of 500 mg/L. See appendix 2, 

table A2–2, for the number of wells sampled.

Type of well

All aquifers 

in Southwest 

Principal Aquifers 

group

Basin and Range 

basin-fill 

aquifers

Central Valley 

aquifer system

California Coastal 

Basin aquifers

Rio Grande 

aquifer system

Urban land-use monitoring wells 64 66 35 96 59

Agricultural land-use monitoring wells 45 50 42 Not sampled 45

Domestic wells 31 37 27 11 40

Public-supply wells 19 26 6.7 20 7.1

All sampled wells 40 47 30 34 40

Dissolved-solids 

concentrations exceeded 

the Secondary Maximum 

Contaminant Level in about 

one-quarter of drinkingwater wells sampled.

0 1,000 2,000 3,000 4,000 5,000

Dissolved-solids concentration, in milligrams per liter

Public-supply wells

Domestic wells

Agricultural monitoring wells

Urban monitoring wells

19% 

31% 

45% 

64% 

Secondary Maximum Contaminant Level (SMCL) for dissolved 

solids is 500 mg/L. Percentage values are the percentage of samples 

that exceeded the SMCL. 

Direction of

generally

increasing

well depth

NOTE: Twelve samples with

concentrations greater than

5,000 mg/L are not shown. 

See sidebar, Boxplots, p. 53

74 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Figure 6–20. Dissolved-solids concentrations measured in Southwest basin-fill aquifers typically are highest (more than 10,000 mg/L) 

in topographically low areas of closed basins, such as the Great Salt Lake Desert in western Utah and the Salton Sea area in southern 

California. As part of USGS studies in the Southwest, dissolved solids were measured in more than 21,000 wells that tap basin-fill 

aquifers. More than half of these groundwater samples had dissolved-solids concentrations greater than the SMCL of 500 mg/L.

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pacif ic

Ocean

Southwest Principal Aquifers

study area boundary

Great

Salt Lake

Desert

Salton

Sea

N

Measured dissolved-solids concentration in basin-fill aquifers

Southwest

Principal Aquifers

 EXPLANATION

Measured dissolved-solids 

 concentration in basin-fill 

 aquifers, in milligrams per liter

 ≤500

 501 to 1,000

 1,001 to 3,000

 3,001 to 10,000

 >10,000

 (Data from the USGS National 

Water Information System)

Carson

Desert

Los Angeles

Phoenix

Sacramento

San Diego

Tucson

Salt Lake City 

Las Vegas

Albuquerque

Reno

San Francisco

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 75
Natural Factors Combine With Land and Water Development to Increase 

Dissolved Solids in Groundwater

High concentrations of dissolved solids in groundwater occur naturally in many Southwest 

basins as a result of minerals dissolving from easily weathered rocks in the surrounding mountains and from sediments within the alluvial basins. Some types of rocks are more soluble than 

others, resulting in greater concentrations of dissolved solids in streams and groundwater. Given 

similar climate, land cover, and geomorphology, sedimentary rocks—including shale, carbonate 

rocks, and evaporites—contribute about 20 times more dissolved solids to streams than do 

crystalline rocks (such as granite).(44) Groundwater also receives more dissolved solids from 

sedimentary rocks and sediment derived from these rocks than from crystalline rocks.

Dissolved-solids concentrations tend to increase naturally as groundwater moves through 

the aquifer from basin margins to lower lying discharge areas (see figure 3–3) because 

of mineral dissolution. The upward movement of mineralized groundwater of deep and 

(or) geothermal origin along faults is another geologic source of elevated dissolved-solids 

concentrations to basin-fill aquifers (fig. 6–1A). Evaporative concentration can cause dissolved 

solids to accumulate in the groundwater and soils over time in discharge areas, particularly 

in constricted or closed basins, such as near the Great Salt Lake in Utah. Concentrations can 

become so high that the groundwater is unsuitable for drinking, irrigation, or other uses.

The same processes—dissolution and evapotranspiration—that concentrate dissolved 

solids in water that naturally recharges aquifers also affect recharge from excess agricultural 

and urban irrigation and other human activities. Substantial amounts of dissolved solids have 

accumulated in heavily irrigated agricultural areas, such as in parts of the Gila River Basin in 

southwestern Arizona, as a result of evapotranspiration. The highest dissolved-solids accumulation rate in the Southwest occurs in the Salton Sea area in southern California—a closed basin 

where water imported for irrigation undergoes evapotranspiration.(44)

Low precipitation rates and temperatures that reach more than 120 °F result in high potential 

evapotranspiration rates, such as in this area in Death Valley, California. Evapotranspiration of 

surface water and shallow groundwater in basin lowlands leaves behind dissolved solids and 

other constituents that accumulate in the basin and the underlying groundwater.

Photograph by Phil Stoffer, USGS

76 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Irrigation water often is applied to areas in the Southwest where little or no natural 

recharge has occurred and, as a result, can flush dissolved solids, which have accumulated 

in the unsaturated zone over thousands of years, down to the water table. The two highest 

concentrations (65,000 and 28,500 mg/L) in shallow groundwater sampled as part of these 

studies in the Southwest are from the Carson Desert in Nevada. Recharge from irrigation since 

the early 1900s in this area has raised the shallow water table, dissolving solids concentrated in 

the soil.(18)

Detergents, water softeners, fertilizers, road salt, and animal and human waste contribute 

dissolved solids to groundwater. In arid basins with little natural recharge, septic-tank leachate, 

which contains elevated concentrations of dissolved solids and nitrate, can make up a large 

fraction of groundwater recharge. As population in these basins grows, the amount of groundwater recharge from septic tanks increases unless a sewer system is constructed. Over a 16-year 

period (1985–2001) in Carson Valley, Nevada, dissolved-solids concentrations increased in 

more than half of 27 monitoring wells sampled—concentrations were higher and increases 

were larger in areas where there was a greater density of septic tanks.(40) Most of the wells with 

upward trends in dissolved solids are near basin margins where residential developments using 

septic tanks are underlain by permeable basin-fill deposits. If it takes a long time for water to 

move through the unsaturated zone to the water table, elevated dissolved-solids concentrations 

in the aquifer from septic-tank leachate could continue into the future even if that source of 

recharge is removed.

Clay layers and upward hydraulic gradients help to protect the deeper parts of basin-fill 

aquifers from recharge water with high dissolved-solids concentrations, as illustrated in 

an irrigated agricultural area in the West Salt River Valley, Arizona. High dissolved-solids 

concentrations in shallow groundwater (median greater than 3,000 mg/L) result from excess 

irrigation water that recharges the shallow part of the aquifer, where it is commonly pumped 

out and reused. The clay layer prevents the downward movement of this more mineralized 

water to deeper parts of the aquifer where the median dissolved-solids concentration is about 

750 mg/L.(38) Wells that short circuit clay layers can allow high concentrations in recharge 

water to reach deeper parts of the aquifer (see sidebar, Wells can “short circuit” groundwater 

flow paths and increase contaminant movement, p. 37). Where human modifications to natural 

flow systems are more substantial and (or) clay layers are not present, high dissolved-solids 

concentrations from recharge water can more easily reach greater aquifer depths (see sidebars, 

Dissolved solids have increased in groundwater used for public supply in Salt Lake Valley, 

Utah, p. 78, and Groundwater in California’s Santa Ana Basin is managed for both quantity and 

quality, p. 80).

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 77

Vertical flow through a 

well can short circuit 

protective clay layers and 

allow shallow groundwater 

with high dissolved-solids 

concentrations to enter 

deeper parts of an aquifer.

The Central Arizona Project 

imports Colorado River water 

to several Arizona basins for 

municipal and agricultural use.
Dissolved-solids concentrations have increased in groundwater 

used for public supply in Salt Lake Valley, Utah

Dissolved-solids concentrations in deeper parts of the basin-fill aquifer used for public 

supply in Salt Lake Valley, Utah, typically are less than 500 mg/L, but there have been substantial 

increases in concentrations in some areas (see sidebar figures). Increasing concentrations of 

dissolved solids in water from some public-supply wells likely are related to pumping, which can 

cause shallow groundwater to move downward by reversing the hydraulic gradient in areas where 

it is naturally upward.(74) Shallow groundwater also can move deeper into an aquifer along well 

bores, which can serve as short-circuit pathways through intervening clay layers. Because of 

these factors, dissolved-solids concentrations are likely to increase over time in water from wells 

tapping deeper parts of basin-fill aquifers, raising concerns about the potential long-term effects 

on groundwater suitability as a drinking-water supply.

A, Dissolved-solids concentrations have increased in water from a public-supply 

well in Salt Lake Valley, Utah. Concentrations in water from a nearby well, which 

accesses deeper parts of the aquifer, have not changed during a similar time 

period. B, Dissolved-solids concentrations in water from two wells used for 

public supply in the northeastern part of the valley have increased in the past 

30 years. A potential source of the dissolved solids in this area is recharge from 

urban runoff that contains sodium chloride (salt) used to deice roads.(75)

200

600

1,000

1930 1940 1950 1960 1970 1980 1990 2000 2010

400

600

800

Dissolved-solids concentration, in milligrams per liter

Well open from 335 to 384 feet

Well open from 395 to 945 feet

Flowing well open at 130 feet

Well and replacement well

open from 225 to 470 feet 

A

B

 EXPLANATION

Dissolved-solids concentration 

 in groundwater, in milligrams per liter

 <250

 250 to <500

 500 to <1,000

 1,000 to 2,000

 

circ1358 ch06 Sidebar

Salt Lake

Valley

78 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Chapter 6

The spatial distribution of dissolved solids in the deeper part of the basin-fill aquifer used 

for public supply in Salt Lake Valley, Utah, has changed over time. A larger area now has 

concentrations of dissolved solids greater than the SMCL of 500 mg/L (yellow areas), 

particularly in the northeastern part of the valley.

Salt Lake City and Salt Lake Valley, Utah, looking southeast.

Photograph by Alan M. Cressler, USGS

1988–1992

Dissolved solids in groundwater in part of Salt Lake Valley, Utah, for two time periods

1998–2002

Jordan Riv er

Jordan Riv e

S A L T L A K E V A L L E Y

r

S A L T L A K E V A L L E Y

Salt Lake City Salt Lake City

N

Dissolved-solids concentration,

 in milligrams per liter

 <250

 250 to <500

 500 to <1,000

 1,000 to 2,000

Boundary of basin-fill deposits 

 in Salt Lake Valley

Well used to measure dissolved

 solids in part of aquifer used for

 public supply

EXPLANATION

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 79
Dissolved-solids concentrations in basin-fill aquifers of the Santa Ana Basin, California, range from less than 250 mg/L 

where the aquifers receive recharge from mountainous areas to greater than 3,000 mg/L in areas where evapotranspiration, 

weathering of soluble marine rocks, and intrusion of ocean water caused by well pumping occurs. Engineered recharge 

facilities are used to replenish the aquifer, some of which recharge water with higher dissolved-solids concentrations than 

the mountain runoff.

Groundwater is a critical resource in the Santa Ana Basin, supplying about 1 billion 

gallons per day or two-thirds of the water used by the 5 million people who live there.(76) In 

parts of the basin, however, concentrations of dissolved solids in groundwater are greater than 

1,000 mg/L (see figure below). More than 100 years of irrigated agriculture and more recent 

human-related sources of dissolved solids, combined with evaporative concentration and 

other natural sources, contribute to these high concentrations. Many aspects of the hydrologic 

system now are managed—through artificial recharge, desalination, high-level wastewater 

treatment, and saltwater intrusion barrier wells—to ensure that there is enough good-quality 

groundwater to supply the population’s water needs.

Groundwater in California’s Santa Ana Basin is managed for both 

quantity and quality

Santa Ana

Basin

 EXPLANATION

Dissolved-solids concentration,

 in milligrams per liter

 <250

 250 to <500

 500 to <1,000

 1,000 to <3,000

 3,000 to 25,000

Engineered recharge facility

Wastewater-treatment plant

Approximate direction 

 of groundwater flow

Inland Basin

Dissolved solids in groundwater and engineered recharge facilities in the Santa Ana Basin, California

San Jacinto Basin

Coastal Basin

Santa

Ana

River

N

Data from Wildermuth Environmental, Inc., appendix 8(80)

80 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Chapter 6

Photograph from Western Municipal Water District

Groundwater in parts of the Santa Ana Basin in southern California is treated to reduce 

dissolved-solids concentrations so that the groundwater can be used for water supply and to 

improve the quality of groundwater that discharges into the Santa Ana River.

Much of the runoff from the mountains that rise steeply along the northern sides of the Santa Ana 

Basin naturally recharges the basin-fill aquifers. To increase the volume of groundwater recharge in the 

San Jacinto and Inland Basin parts of the Santa Ana Basin, engineered recharge facilities are used to 

move stormwater runoff from within the basin and treated municipal wastewater—sources that typically 

have higher dissolved-solids concentrations than the mountain recharge—to the aquifers.

In the Coastal Basin (the furthest downgradient part of the Santa Ana Basin), engineered groundwater 

recharge facilities capture much of the flow in the Santa Ana River and use it to recharge the basin-fill 

aquifer. As the river enters the Coastal Basin from the Inland Basin, its flow includes treated municipal 

wastewater, intermittent stormwater runoff from agricultural and urban land, and discharge of groundwater that has been affected by human activities.(76) Because the Santa Ana River is the source of almost 

all of the recharge to the aquifer in the Coastal Basin, sources of dissolved solids to the river water must 

be managed so that groundwater supplies in the Coastal Basin remain potable. For example, wells and 

desalination plants in the downstream part of the Inland Basin are used to reduce the volume of groundwater with high dissolved-solids concentrations that discharges to the river; these wells also provide 

potable water to residents of the Inland Basin (see sidebar photograph). Recently (since 2008), water 

from a state-of-the art advanced wastewater-treatment facility in the Coastal Basin, which reduces total 

dissolved solids in the effluent to less than 100 mg/L, also is being used to recharge the aquifer at facilities 

near the Santa Ana River. This facility is the largest water purification project of its kind in the world and 

cost $480 million to build.(77) A $143 million expansion at the facility is scheduled for completion in 2014.

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 81
Potential Actions That Can Provide Acceptable Concentrations of 

Dissolved Solids in Groundwater Used for Drinking Water

Concentrations of dissolved solids in the deep parts of basin-fill aquifers in Southwest 

basins with land and water development are likely to increase as a result of increased groundwater flow rates, mixing within aquifers, and loading of the aquifers with dissolved solids. 

Some of the options available to water suppliers to provide water with acceptable concentrations of dissolved solids are:

• Blending with other water that contains lower concentrations of dissolved solids, 

if available;

• Treatment to remove dissolved solids (desalination, typically with reverse osmosis, 

is required);

• Seeking alternative sources of water that are suitable for drinking, such as 

imported water;

• Sealing off well openings in zones of poor quality water; and

• Deepening existing wells or drilling additional wells in areas unaffected by 

elevated concentrations.

All of these actions have associated costs. The purchase and installation of a household 

reverse osmosis under-the-sink system can cost a homeowner a few hundred to a few thousand 

dollars. Construction of a water-treatment facility capable of treating several million gallons per 

day can cost tens of millions of dollars, and there are additional, ongoing costs for energy and 

maintenance and for disposal of the residual brine. Because water supplies in the Southwest 

are limited, it can be more cost effective to prevent degradation of potable water supplies than 

to seek new sources of water. In Salt Lake Valley, Utah, the State engineer has implemented 

a groundwater-management plan that places limits on pumping to prevent water-quality 

degradation caused by the movement of groundwater with elevated dissolved-solids concentrations. In southern California, residents are being informed about increases in dissolved 

solids caused by water softeners and the effect it has on their recycled (treated wastewater) 

water supply.(45)

82 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Volatile Organic Compounds and Pesticides

Volatile organic compounds (VOCs), pesticides, or pesticide degradates were detected in 

57 percent of drinking-water wells tapping Southwest basin-fill aquifers, but concentrations rarely approached levels of concern for human health. The frequent presence of these 

contaminants at low concentrations beneath agricultural and urban settings indicates that 

groundwater in many areas of the Southwest is vulnerable to contamination caused by human 

activities. Several organic compounds are persistent under environmental conditions common 

in basin-fill aquifers, and the contaminants likely will be transported through the aquifer 

system for many years.

VOCs and pesticides are released to the environment by human activities. VOCs include 

many common chemicals—solvents, gasoline components, refrigerants—that are used for a 

variety of purposes in industry, commerce, households, and occasionally agriculture. Pesticides 

are chemicals used in agricultural and urban settings to control weeds, insects, and other 

pests. The occurrence of VOCs and pesticides in Southwest basin-fill aquifers (appendix 2, 

table A2–3) demonstrates that these aquifers are vulnerable to contamination from the 

land surface.

Human-health benchmarks (see sidebar, Human-health benchmarks and other guidelines 

used in this assessment, p. 43) have been established for several VOCs and pesticides in 

drinking water. Many potential human-health effects are related to exposure to VOCs or 

pesticides, including increased risk of cancer and effects on the nervous, immune, circulatory, 

and reproductive systems. Most human-health benchmarks for individual chemicals are higher 

than 0.2 μg/L, or part per billion, whereas laboratory methods used by the NAWQA Program 

typically can detect VOCs and pesticides in water at concentrations as much as 100 times 

smaller. Groundwater commonly contains more than one chemical, but little is known about 

the potential health effects of most mixtures of contaminants in drinking water—it is possible 

that the toxicity of the mixture may be greater than that of any single VOC or pesticide 

compound present.

Chapter 6

Volatile organic compounds 

and pesticides are manmade 

chemicals used at the 

land surface that can be 

transported to groundwater. 

A potential source is illustrated 

by corroded barrels that might 

have stored chemicals.

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 83 Photograph by Richard Thornton, www.shutterstock.com
VOCs Were Detected Primarily in Groundwater From Urban Settings, 

Typically at Concentrations Substantially Below Human-Health 

Benchmarks

Although at least one VOC was detected in water from 43 percent of all wells sampled 

in Southwest basin-fill aquifers, concentrations generally were low. VOCs were detected 

most commonly in shallow groundwater in urban settings and in samples from public-supply 

wells, which tend to be located in urban areas (fig. 6–21). Most VOCs are associated primarily 

with urban sources—for example, chlorinated water, gas stations, dry cleaners, metals shops, 

and many industrial processes. Only 10 of the more than 80 VOCs included in the analysis 

of groundwater samples accounted for most detections, especially among drinking-water 

(domestic and public-supply) wells (fig. 6–21). In particular, only the trihalomethanes 

chloroform and bromodichloromethane (see sidebar, Common VOCs and their uses, p. 86) 

and the solvents perchloroethene (PCE) and trichloroethene (TCE) were detected in more than 

10 percent of public-supply wells. PCE and TCE each were detected in two drinking-water 

wells at a concentration that exceeded their MCLs, but concentrations exceeded one-tenth 

of the MCL in fewer than 2 percent of drinking-water wells (table 5–1). Concentrations of 

the fumigant dibromochloropropane (DBCP) exceeded its MCL in 16 domestic wells in the 

eastern San Joaquin Valley, California. The history of the use and movement of DBCP through 

the aquifer is described in more detail later in this section. Mixtures of two or more VOCs 

were detected in 22 percent of drinking-water wells, and mixtures of four or more VOCs were 

detected in 4.7 percent of drinking-water wells.

Chloroform, a trihalomethane, was the VOC most commonly detected in Southwest 

basin-fill aquifers and in groundwater sampled nationwide as part of the NAWQA Program.(16) 

A common source of trihalomethanes to groundwater is chlorinated water used to irrigate urban 

landscaping (including lawns and parks) as well as leaks from drinking-water distribution and 

sewer pipes (fig. 6–1B). The number of detections of chloroform in shallow urban groundwater 

in the Southwest increased with the number of households connected to public sewers.(37) 

Households connected to public sewers commonly also are connected to chlorinated public 

water-supply systems.

Releases of PCE, TCE, and MTBE (methyl tert-butyl ether—a gasoline oxygenate) 

typically occur where solvents or gasoline have been stored, spilled, or improperly 

disposed. In shallow urban groundwater in the Southwest, PCE, TCE, and MTBE were 

commonly detected in areas with industrial land use.(37)

VOCs detected in publicsupply wells reflect the 

chemicals used in urban 

areas.

Gasoline contains VOCs that can be released to groundwater from leaking storage tanks at 

facilities where vehicles are fueled. VOCs present in common household, commercial, and 

industrial products can reach groundwater as a result of spills or improper disposal.

Photograph on right by Joel Beamer

84 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Chapter 6

Figure 6–21. VOCs were detected more frequently in shallow groundwater in urban areas and in public-supply 

wells, which tend to be located in urban areas, than in shallow groundwater in agricultural areas or in domestic wells, 

which tend to be located in rural areas. These detections reflect the more common use of VOCs in urban areas. Note 

that because the lowest detectable concentration differs among compounds, detection frequency is shown for any 

concentration and for a common assessment level of 0.2 μg/L.

Groundwater used

for water supply,

mixed land uses

Shallow groundwater,

urban areas

Public-supply wells

Shallow groundwater, Domestic wells

agricultural areas*

0 20 40 60 80 100

Percentage of samples

in which compound was detected

0 20 40 60 80 100

Percentage of samples

in which compound was detected

*Includes domestic wells sampled as 

part of agricultural land-use studies 

in the San Joaquin Valley, California.

Trichloroethene (TCE)

Bromodichloromethane

Methyl tert-butyl ether (MTBE)

1,1,1-Trichloroethane

Chloromethane

Chloroform

Perchloroethene (PCE)

Bromodichloromethane

Trichloroethene (TCE)

Methyl tert-butyl ether (MTBE)

1,1,1-Trichloroethane

Trichlorofluoromethane (CFC-11)

Dibromochloropropane (DBCP)

1,2-Dichloropropane (DCP)

1,2-Dichloropropane (DCP)

Chloromethane

Dibromochloropropane (DBCP) 2

2

Chloroform

Perchloroethene (PCE) 2

Bromodichloromethane

Trichloroethene (TCE) 2

Methyl tert-butyl ether (MTBE)

1,1,1-Trichloroethane

Trichlorofluoromethane (CFC-11)

Trichlorofluoromethane (CFC-11)

Dibromochloropropane (DBCP) 16

1,2-Dichloropropane (DCP)

EXPLANATION

Any >0.2

Concentration of VOCs,

 in micrograms per liter

Agricultural area

Urban area

Mixed land use

Domestic well

Public-supply well

Number of exceedances of 

human-health benchmarks 

listed for drinking-water 

wells

Perchloroethene (PCE)

Chloroform

Chloromethane

Detection frequency of VOCs in groundwater

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 85
 The VOCs most commonly detected in Southwest groundwater have diverse uses.

VOCs most commonly detected Category of VOCs Source or uses

Chloroform, bromodichloromethane Trihalomethanes

Commonly produced during chlorination of 

drinking water and wastewater through the 

reaction of chlorine and organic material.

Perchloroethene (PCE), trichlorethene (TCE), 

1,1,1-trichloroethane, chloromethane Solvents

Used for many industrial, commercial, and 

household applications, including manufacturing and degreasing.

Methyl tert-butyl ether (MTBE) Gasoline oxygenates Added to gasoline to improve combustion and 

reduce harmful motor vehicle emissions.

Trichlorofluoromethane (CFC–11) Refrigerants Used as a heat transport medium in refrigerators 

and air cooling systems.

Dibromochloropropane (DBCP), 

1,2-dichloropropane (DCP) Fumigants

Used to destroy, repel, or control organisms such 

as insects, bacteria, or rodents, and are also 

considered pesticides.

Common VOCs and their uses

The VOCs most commonly detected in Southwest 

groundwater (fig. 6–21) fall into five categories: trihalomethanes, 

solvents, gasoline oxygenates, refrigerants, and fumigants as 

shown in the table below. Many VOCs are included in more than 

one category. For example, chloroform is a trihalomethane and a 

solvent. Under the right environmental conditions, some VOCs can 

degrade to form other VOCs (degradates)—PCE, for example, can 

degrade to form TCE. More details on the uses of individual VOCs 

can be found in appendix 4 of Zogorski and others.(16)

Photograph by Jay Cedarberg, USGS

Farm in Salt Lake Valley, Utah, with above-ground 

storage tanks.

86 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Pesticide Compounds Detected in Groundwater From Agricultural and 

Urban Settings Were Typically at Concentrations Substantially Below 

Human-Health Benchmarks

Although at least one pesticide compound (parent or degradate) was detected in 

water from nearly half (45 percent) of all wells sampled in Southwest basin-fill aquifers, 

concentrations generally were low. As a group, pesticide compounds were detected with 

similar frequencies in shallow monitoring wells, domestic wells, and public-supply wells in 

agricultural and urban areas, reflecting the use of pesticides in both agricultural and urban 

areas. Because of their pattern of use, however, some individual compounds were more 

commonly detected in one or the other setting (fig. 6–22). Most detections in groundwater, 

especially in drinking-water wells, were for just a few pesticide compounds. In particular, 

only four pesticide compounds—simazine, atrazine, and the atrazine degradates deethylatrazine and deisopropyl atrazine—were detected in more than 10 percent of domestic 

and public-supply wells (fig. 6–22). Dieldrin is an insecticide that was used in the past for 

agricultural and urban uses, but has been banned because of risks to human health. Although 

not among the most commonly detected pesticide compounds, dieldrin was the only pesticide 

compound detected in drinking-water wells at a concentration that exceeded its humanhealth benchmark of 0.002 μg/L (5 of 560 public-supply and domestic wells) (table 5–1). 

Figure 6–22. Pesticide compounds were detected in similar percentages of samples of shallow groundwater in 

agricultural and urban areas and in domestic and public-supply wells, reflecting use of pesticides in both agricultural and 

urban areas. Note that because the lowest detectable concentration differs among compounds, detection frequency is 

shown for any concentration and for a common assessment level of 0.1 μg/L.

0 20 40 60 80 100

Percentage of samples

in which compound was detected

0 20 40 60 80 100

Percentage of samples

in which compound was detected

*Deisopropyl atrazine

*Deethylatrazine

Simazine

Prometon

Diuron

*Hydroxyatrazine

*N-(4-chlorophenyl)-N’-methylurea

Atrazine

*Deisopropyl atrazine

*Deethylatrazine

Simazine

Prometon

Diuron

*Hydroxyatrazine

*N-(4-chlorophenyl)-N’-methylurea

Atrazine

*Deisopropyl atrazine

*Deethylatrazine

Simazine

Prometon

Diuron

*Hydroxyatrazine

*N-(4-chlorophenyl)-N’-methylurea

Atrazine Groundwater used for

water supply,

mixed land uses

Shallow groundwater,

urban areas Public-supply wells

Shallow groundwater, Domestic wells

agricultural areas**

*Pesticide degradates

**Includes domestic wells sampled as 

part of agricultural land-use studies in 

the San Joaquin Valley, California.

EXPLANATION

Any > 0.1

Concentration of pesticide 

 compounds, in micrograms 

 per liter

Agricultural area

Urban area

Mixed land use

Domestic well

Public-supply well

Detection frequency of pesticide compounds in groundwater

Chapter 6

Pesticides were detected 

with similar frequencies 

in shallow groundwater 

in urban and agricultural 

areas and in domestic 

and public-supply wells, 

which reflects pesticide 

use in both urban and 

agricultural settings.

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 87
Mixtures of two or more pesticide compounds were detected in 28 percent of drinking-water 

wells, and mixtures of four or more pesticide compounds were detected in 10 percent of 

drinking-water wells. Toxicity data for mixtures of pesticide compounds are seldom available. Information on the occurrence and composition of mixtures can help to characterize the 

potential exposure of humans, aquatic life, and wildlife to mixtures and make it possible to 

prioritize mixtures that may occur in groundwater for further study.(46)

The herbicide atrazine (see sidebar, Common pesticides and their uses, p. 89) was the most 

commonly detected pesticide in Southwest basin-fill aquifers and also in groundwater sampled 

nationwide as part of the NAWQA Program.(46) In the Southwest, the atrazine degradates 

deethylatrazine and deisopropyl atrazine were detected even more commonly in groundwater 

than their parent compound. Although atrazine was detected in groundwater in both agricultural 

and nonagricultural areas in several study basins, presumably because of its application for 

weed control in both types of settings, the herbicide and its degradates deethylatrazine and 

hydroxyatrazine were detected more often in shallow groundwater in urban settings than in 

agricultural settings (fig. 6–22). The use of atrazine in agricultural areas is not as common 

in the Southwest as it is in other parts of the United States, especially in the Corn Belt of the 

Midwest where application rates are high.(47)

Similar to atrazine, broad patterns in the use of the herbicides simazine and prometon 

affected their detection frequencies. Simazine commonly is used for both agricultural and 

nonagricultural purposes (such as weed control near buildings and along roads) and was 

detected about as commonly in shallow urban groundwater as in shallow agricultural groundwater. Prometon, in contrast, is used by homeowners to control weeds and is not used on crops. 

It was detected about four times more commonly in shallow urban groundwater than in shallow 

agricultural groundwater (fig. 6–22).

Pesticides are used on crops in agricultural settings (lettuce crop in Yuma, Arizona, left) and also on 

golf courses, yards, and other areas in urban settings (top).

88 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009

Photographs: Left, Jeff Vanuga, Natural Resource Conservation Service, U.S. Department of Agriculture; top right, Lynn Betts, Natural Resource 

Conservation Service, U.S. Department of Agriculture
Herbicides and herbicide degradates were detected more commonly in Southwest groundwater than were other types 

of pesticides.

Pesticide compounds most 

commonly detected

Category of pesticide 

compounds Source or uses

Atrazine Herbicide

Used to control unwanted plants in mostly agricultural 

areas. Also used for nonagricultural purposes, such 

as to control roadside vegetation. 

Diuron Herbicide Used to control unwanted plants in urban areas, such 

as along roadsides.

Prometon Herbicide Used by homeowners to control weeds.

Simazine Herbicide Used to control unwanted plants in agricultural and 

urban areas.

Deethylatrazine, deisopropyl atrazine, and 

hydroxyatrazine Herbicide degradates Degradates of atrazine.

N-(4-chlorophenyl)-N'-methylurea Herbicide degradate Degradate of monuron, an herbicide used in 

urban areas.

Common pesticides and their uses

Herbicides and their degradates were detected more commonly in basin-fill aquifers than were other types of pesticides. 

Degradates are compounds formed by the degradation (breakdown) of a “parent” compound. In groundwater, the concentration of 

a degradate sometimes exceeds the concentration of the parent compound, and some degradates are more toxic than their parent 

compound. Degradates were analyzed as part of the NAWQA Program for only a relatively small number of pesticides. Details on the 

use and properties of individual pesticide compounds can be found in appendixes 1 and 2 of Gilliom and others.(46)

Chapter 6

Photograph by Tyler Olson, www.shutterstock.com

Pesticides are sprayed on some agricultural fields to control unwanted plants or pests.

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 89
Environmental Conditions Affect the Persistence of VOCs and Pesticides

Many organic compounds degrade to some degree within the soil zone before they 

reach the groundwater. Some compounds volatilize into air or adhere to soil particles or 

organic matter, further decreasing the amount of the compound that reaches the water table. 

Such compounds are less likely to be detected in groundwater than are more persistent and 

less volatile or soluble compounds. In some instances, persistence can be more important to 

detection than the volume of a compound used (fig. 6–23).

Much of the groundwater in Southwest basin-fill aquifers contains dissolved oxygen 

(is oxic), which affects the persistence of many organic compounds in groundwater (see section 

Geochemical Conditions Are Important Controls on Contaminant Transport in chapter 4). 

Some VOCs, such as chloroform and PCE, degrade less readily under oxic conditions than 

under anoxic conditions.(16) In shallow groundwater in urban areas—the setting in which 

chloroform(37) was most commonly detected (fig. 6–24)—chloroform was detected about eight 

times more frequently in samples of oxic groundwater than in samples of anoxic groundwater. 

Other VOCs, however, degrade relatively rapidly under oxic conditions. For example, the 

gasoline compounds benzene and toluene are used intensively and widely on a national 

scale,(16) but were detected in less than 4 percent of Southwest wells.

Figure 6–23. Although metolachlor use in agricultural 

areas of the Southwest (1992–2002) was several 

times higher than atrazine use, the two herbicides 

were detected in a similar percentage of shallow, 

agricultural monitoring wells. This apparent 

contradiction is because metolachlor breaks down 

much more rapidly in soil and water than does atrazine. 

The greater persistence of atrazine in soil and water 

corresponds with its being detected about 10 times 

more frequently than metolachlor in deeper domestic 

and public-supply wells.

Figure 6–24. In wells in 

urban areas, chloroform was 

more frequently detected in 

shallow groundwater with 

oxic conditions than in shallow 

groundwater with anoxic 

conditions, because chloroform 

does not degrade readily in 

oxic groundwater.

circ1358 ch06fig23

0

5

10

15

20

25

Shallow agricultural

monitoring wells

Drinking-water

wells

Atrazine

Metolachlor

Atrazine and metolachlor in 

Southwest groundwater

Detection frequency, in percent

Metolachlor

Atrazine

0

20

40

60

80

Chloroform in shallow 

groundwater in 

urban setting

Anoxic

Groundwater redox category

Oxic

Detection frequency, in percent

90 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Public-supply wells, such as this one being sampled in Salt Lake Valley, Utah, can pump large 

volumes of water from the basin-fill aquifer.

Land and water development in Southwest basins 

increases the vulnerability 

of deep groundwater to 

contamination by VOCs 

and pesticides.

VOCs and Pesticides in Public-Supply Wells Indicate Aquifer Vulnerability 

to Contamination

Land and water development in Southwest basins increases the vulnerability of deep 

groundwater to contamination by VOCs and pesticides (fig. 6–1B). Activities associated with 

agricultural and urban land uses introduce potential sources of VOCs and pesticides. Increased 

recharge from irrigation enhances the transport of these compounds to the water table and 

through the aquifer. The downward migration to depths used for drinking-water supply can be 

further enhanced by water-level declines that result from the pumping of deep groundwater by 

water-supply wells and (or) by preferential movement of groundwater directly down wells from 

shallow to deeper depths (see sidebar, Wells can “short circuit” groundwater flow paths and 

increase contaminant movement, p. 37).

The combined effect of more contaminant sources, increased recharge, groundwater 

pumping, and preferential flow on downward migration of VOCs and pesticides increases 

with more development. In highly developed basins, these stresses eventually can counteract 

the protective effects of naturally upward groundwater flow and (or) confining layers, thereby 

increasing the vulnerability of deeper parts of aquifers to contamination. Because changes to 

the quality of shallow groundwater generally are evident more quickly, detections of these 

compounds at shallow depths provide an early indication that, over time, the deeper groundwater used for drinking-water supply might be affected.

Chapter 6

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 91
Detections of compounds in young groundwater reflect the local 

history of chemical use in the eastern San Joaquin Valley, California

Intensive farming in the eastern San Joaquin 

Valley is coupled with rapid population growth. 

The occurrence of DBCP at concentrations 

exceeding its MCL of 0.2 μg/L in domestic wells 

is a human-health concern in this area. Elevated 

concentrations of nitrate in groundwater also are 

of concern, as described in the nitrate section of 

this chapter.

Local groundwater quality reflects patterns 

in the use of the soil fumigant DBCP and the 

herbicide simazine in this area over the past 

several decades.(39) DBCP use began in the 

1950s for control of nematodes, primarily in 

orchards and vineyards. Its use increased until 

1977, when agricultural use of DBCP was banned 

because of human-health concerns. The ban is 

reflected in the differences in concentrations 

in groundwater from domestic and monitoring 

wells. DBCP was detected more commonly and 

at higher concentrations in groundwater from 

domestic wells—water that typically recharged 

around 1980—than in monitoring wells, which 

The pesticides dibromochloropropane (DBCP) and 

simazine historically have 

been applied to orchards and 

vineyards in the eastern San 

Joaquin Valley, California, and 

have been detected in local 

groundwater.

Photographs by Chris Austin, www.aquafornia.com

Patterns in concentrations of the pesticides A, dibromochloropropane (DBCP) and B, simazine with depth in the aquifer 

beneath agricultural settings in the eastern San Joaquin Valley, California, reflect the history of local chemical use. 

For DBCP, the decrease in concentrations in groundwater recharged between about 1980 and 1990 reflects a 1977 ban 

on its use. For simazine, the increase in concentrations in groundwater recharged over the same time period reflects 

increasing use of this pesticide.

0 0.2 0.4 0.6 0 0.04 0.08

1950

Increasing

groundwater age

Increasing depth 

below water table

1960

1970

1980

1990

0.12

A. DBCP B. Simazine

Approximate year

of recharge

Typical midpoint

of open intervals

Median concentration, in 

micrograms per liter

Median concentration, in 

micrograms per liter

0

100

200

300

400

Depth below 

water table, 

in feet Public-supply wells

Monitoring wells

Domestic wells

Sidebar circ1358ch06

Modified from Burow and others(39)

are shallower and therefore typically tap 

groundwater that recharged around 1990. 

Concentrations were low in public-supply wells, 

which typically are deeper than either domestic 

wells or monitoring wells and therefore tap water 

that recharged before DBCP was in use.

Simazine use for agricultural and nonagricultural weed control also began in the late 

1950s, and simazine still is commonly used. 

The increase in simazine concentrations in 

groundwater over time reflects its increasing 

use. Concentrations are highest in shallow 

groundwater that recharged in about 1990 and 

decrease with depth.

DBCP continues to be detected in groundwater decades after its agricultural use was 

banned. Because of the persistence of DBCP 

and simazine in groundwater and the downward 

movement of groundwater in the eastern 

San Joaquin Valley, concentrations of these 

pesticides in water from public-supply wells are 

likely to increase.(39)

92 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Chapter 6

Of 31 public-supply wells 

sampled in Salt Lake Valley, 

Utah, at least one VOC or 

pesticide was detected in 

almost all of the wells that 

pumped a substantial fraction 

of “young” groundwater 

(recharged since about 1950). 

Unexpectedly, some young, 

contaminated groundwater 

occurred in wells in discharge 

areas with extensive clay 

layers, indicating that these 

areas also are vulnerable to 

groundwater contamination 

from human activities.

The presence of young groundwater (recharged since about 

1950) and VOCs and pesticides introduced by human activities 

is related in Salt Lake Valley, Utah. At least one VOC or pesticide 

compound was detected in 19 of 20 public-supply wells that produced 

dominantly young groundwater or a mixture of young and old water. 

Even in discharge areas of the valley where groundwater would be 

expected to be old, some public-supply wells that pump water from 

beneath relatively extensive clay layers, which confine and impede 

vertical groundwater flow and protect groundwater quality, produced 

a component of young groundwater. Short circuiting between open 

VOCs and pesticides related to human activities detected in young groundwater from 

public-supply wells in Salt Lake Valley, Utah

intervals in a well or reversals in the upward gradient caused by 

pumping could allow young groundwater at shallow depths to move 

down past the clay layers. In contrast to detection frequencies in 

young groundwater, a VOC or a pesticide compound was detected in 

only 3 of 11 public-supply wells that produced dominantly old water, 

and all of the concentrations were low. These results demonstrate 

that human activities in the Salt Lake Valley have caused young, 

contaminated groundwater to move past protective clay layers 

to deeper parts of the aquifer used for public supply and that clay 

layers are not always effective at protecting groundwater quality.

Sidebar Circ1358 ch06fig6

T R A V E R S E M O U N T A I N S

OQUIRRH MOUNTAINS

W A S A T C H RANGE

River Jordan

Tailings ponds

Great

Salt

Lake

 

Primary recharge area—Few clay layers; unconfined aquifer conditions

Secondary recharge area—Some relatively discontinuous clay layers; confined aquifer conditions

Discharge area—Relatively extensive, continuous clay layers; confined aquifer conditions

Boundary of basin-fill sediments

Public-supply well, old groundwater

Public-supply well, dominantly young groundwater or a mixture of young and old groundwater

VOC or pesticide compound detected

Data from Thiros and Manning (74)

EXPLANATION

Wells producing

dominantly

young water

or a mixture

of young

and old water

Wells producing

dominantly

old water

VOC and pesticide compounds in Salt Lake Valley, Utah, groundwater

VOC or pesticide

compound detected

No VOC or pesticide

compound detected

95%

73%

5%

27%

N

Chapter 6: Understanding Where and Why Contaminants Occur in Southwest Basin-Fill Aquifers 93

References Cited

1. Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995,

Design of the National Water-Quality Assessment

Program—Occurrence and distribution of water-quality

conditions: U.S. Geological Survey Circular 1112, 33 p.,

accesed August 14, 2013, at http://pubs.usgs.gov/circ/

circ1112/.

2. Hopple, J.A., Delzer, G.C., and Kingsbury, J.A., 2009,

Anthropogenic organic compounds in source water of

selected community water systems that use groundwater,

2002–05: U.S. Geological Survey Scientific Investigations Report 2009–5200, 74 p., accessed August 12,

2013, at http://pubs.usgs.gov/sir/2009/5200/.

3. Eberts, S.M., Thomas, M.A., and Jagucki, M.L., 2013,

The quality of our Nation’s waters—Factors affecting

public-supply-well vulnerability to contamination—

Understanding observed water quality and anticipating

future water quality: U.S. Geological Survey Circular

1385, 120 p. (Also available at http://pubs.usgs.gov/

circ/1385/.)

4. McKinney, T.S., and Anning, D.W., 2009, Geospatial data to support analysis of water-quality conditions in basin-fill aquifers in the southwestern United

States: U.S. Geological Survey Scientific Investigations Report 2008–5239, 16 p. (Also available at

http://pubs.usgs.gov/sir/2008/5239/.)

5. Woodhouse, C.A., Meko, D.M., MacDonald, G.M.,

Stahle, D.W., and Cook, E.R., 2010, A 1,200-year

perspective of 21st century drought in southwestern

North America: Proceedings of the National Academy of

Sciences of the United States of America, v. 107, no. 50,

p. 21283–21288.

6. Stonestrom, D.A., and Harrill, J.R., 2007, Ground-water

recharge in the arid and semiarid southwestern United

States—Climatic and geologic framework, in Stonestrom, D.A., Constantz, Jim, Ferré, T.P.A., and Leake,

S.A., eds., Ground-water recharge in the arid and semiarid southwestern United States: U.S. Geological Survey

Professional Paper, 1703–A, p. 1–27. (Also available at

http://pubs.usgs.gov/pp/pp1703/a/.)

7. Oak Ridge National Laboratory, 2005, LandScanTM

global population database: Oak Ridge National Laboratory Web site, accessed May 17, 2010, at http://www.

ornl.gov/landscan/.

8. U.S. Census Bureau, 2005, Interim state population

projections—Table 1—Ranking of census 2000 and

projected 2030 state population and change: U.S. Census

Bureau Web site, accessed April 12, 2012, at http://www.

census.gov/population/projections/data/state/.

9. Thiros, S.A., Bexfield, L.M., Anning, D.W., and Huntington, J.M., eds., 2010, Conceptual understanding and

groundwater quality of selected basin-fill aquifers in

the southwestern United States: U.S. Geological Survey

Professional Paper 1781, 288 p. (Also available at

http://pubs.usgs.gov/pp/1781/.)

10. Howard, J.B., 1992, Hohokam legacy—Desert canals:

Pueblo Grande Museum Profiles no. 12, 5 p., accessed

August 28, 2013, at http://waterhistory.org/histories/

hohokam2/hohokam2.pdf.

11. Great Valley Center, 2009, The state of the great

Central Valley of California—Assessing the region

via indicators—The economy (3d ed.): Modesto,

Calif., Great Valley Center, 40 p., accessed April 12,

2012, at http://www.greatvalley.org/wp-content/

uploads/2012/01/3rd_2009_Economy.pdf.

12. U.S. Department of Agriculture, 2009, Census of agriculture, 2007 census volume 1, chapter 2—County level

data, Arizona: U.S. Department of Agriculture Web site,

accessed October 29, 2012, at http://www.agcensus.

usda.gov/Publications/2007/Full_Report/Volume_1,_

Chapter_2_County_Level/Arizona/.

13. Maupin, M.A., and Barber, N.L., 2005, Estimated withdrawals from Principal Aquifers in the United States,

2000: U.S. Geological Survey Circular 1279, 46 p. (Also

available at http://pubs.usgs.gov/circ/2005/1279/.)

14. Anning, D.W., and Konieczki, A.D., 2005, Classification of hydrogeologic areas and hydrogeologic flow

systems in the Basin and Range Physiographic Province, southwestern United States: U.S. Geological

Survey Professional Paper 1702, 37 p. (Also available at

http://pubs.usgs.gov/pp/2005/pp1702/.)

15. McMahon, P.B., Cowdery, T.K., Chapelle, F.H., and

Jurgens, B.C., 2009, Redox conditions in selected

Principal Aquifers of the United States: U.S. Geological Survey Fact Sheet 2009–3041, 6 p. (Also available

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

16. Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara,

Lapham, W.W., Moran, M.J., Rowe, B.L., Squillace, P.J.,

and Toccalino, P.L., 2006, The quality of our Nation’s

waters—Volatile organic compounds in the Nation’s

ground water and drinking-water supply wells: U.S.

Geological Survey Circular 1292, 101 p. (Also available

at http://pubs.usgs.gov/circ/circ1292/.)

17. Seiler, R.L., and Allander, K.K., 1993, Water-level

changes and direction of ground-water flow in the shallow aquifer, Fallon area, Churchill County, Nevada:

U.S. Geological Survey Water-Resources Investigations

Report 93–4118, 74 p.

18. Welch, A.H., and Lico, M.S., 1998, Factors controlling

As and U in shallow ground water, southern Carson

Desert, Nevada: Applied Geochemistry, v. 13, no. 4,

p. 521–539.

References

References Cited 95
19. Bexfield, L.M., Thiros, S.A., Anning, D.W., Huntington, J.M., and McKinney, T.S., 2011, Effects of natural

and human factors on groundwater quality of basin-fill

aquifers in the southwestern United States—Conceptual

models for selected contaminants: U.S. Geological Survey Scientific Investigations Report 2011–5020, 90 p.

(Also available at http://pubs.usgs.gov/sir/2011/5020/.)

20. National Research Council, 2001, Arsenic in drinking water—2001 update: Washington, D.C., National

Academy Press, 225 p.

21. U.S. Environmental Protection Agency, 1999, Proposed radon in drinking water rule: U.S. Environmental

Protection Agency Technical Fact Sheet 815–F–99–006,

6 p., accessed February 8, 2012, at http://www.epa.gov/

ogwdw000/radon/fact10.pdf.

22. U.S. Environmental Protection Agency, 2009, Proposed

radon in drinking water rule: U.S. Environmental Protection Agency Web site, accessed February 24, 2010, at

http://www.epa.gov/safewater/radon/proposal.html.

23. U.S. Environmental Protection Agency, 2012, Secondary drinking water regulations—Guidance for nuisance

chemicals: U.S. Environmental Protection Agency

816–F–10–079, accessed April 24, 2013 http://water.

epa.gov/drink/contaminants/secondarystandards.cfm.

24. Ayers, R.S., and Wescot, D.W., 1994, Water quality for

agriculture: Food and Agriculture Organization Irrigation and Drainage Paper 29, 174 p., accessed June 15,

2012, at http://www.fao.org/DOCREP/003/T0234E/

T0234E00.HTM.

25. U.S. Environmental Protection Agency, 2001, Drinking

water standard for arsenic: U.S. Environmental Protection Agency Fact Sheet EPA 815–F–00–015, accessed

December 22, 2010, at http://water.epa.gov/lawsregs/

rulesregs/sdwa/arsenic/regulations_factsheet.cfm.

26. Welch, A.H., Westjohn, D.B., Helsel, D.R., and Wanty,

R.B., 2000, Arsenic in ground water of the United

States—Occurrence and geochemistry: Ground Water,

v. 38, no. 4, p. 589–604.

27. Robertson, F.N., 1989, Arsenic in groundwater under

oxidizing conditions, southwest United States: Environmental Geochemistry and Health, v. 11, nos. 3–4,

p. 171–185.

28. Anning, D.W., Paul, A.P., McKinney, T.S.,

Huntington, J.M., Bexfield, L.M., and Thiros, S.A.,

2012, Predicted nitrate and arsenic concentrations in

basin-fill aquifers of the southwestern United States:

U.S. Geological Survey Scientific Investigations

Report 2012–5065, 78 p., 17 app., (Also available at

http://pubs.usgs.gov/sir/2012/5065/.)

29. U.S. Environmental Protection Agency, 2010, Basic

information about the radionuclides rule: U.S. Environmental Protection Agency Web site, accessed March 22,

2011, at http://water.epa.gov/lawsregs/rulesregs/sdwa/

radionuclides/basicinformation.cfm.

30. Fujii, Roger, and Swain, W.C., 1995, Areal distribution

of selected trace elements, salinity, and major ions in

shallow ground water, Tulare Basin, southern San Joaquin Valley, California: U.S. Geological Survey WaterResources Investigations Report 95–4048, 67 p., 1 pl.

31. 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

Scienti ic Investigations Report 2011–5059, 115 p. (Also

available at http://pubs.usgs.gov/sir/2011/5059/.)

32. Jurgens, B.C., Fram, M.S., Belitz, Kenneth, Burow,

K.R., and Landon, M.K., 2010, Effects of groundwater

development on uranium, Central Valley, California,

USA: Ground Water, v. 48, no. 6, p. 913–928.

33. Wanty, R.B., and Nordstrom, D.K., 1993, Natural radionuclides, in Alley, W.M., ed., Regional ground-water

quality: New York, Van Nostrand Reinhold, chap. 17,

p. 423–441.

34. Thomas, J.M., Welch, A.H., Lico, M.S., Hughes, J.L.,

and Whitney, Rita, 1993, Radionuclides in ground

water of the Carson River Basin, western Nevada and

eastern California, U.S.A.: Applied Geochemistry, v. 8,

p. 447–471.

35. DeSimone, L.A., 2009, Quality of water from domestic wells in Principal Aquifers of the United States,

1991–2004: U.S. Geological Survey Scientific Investigations Report 2008–5227, 127 p. (Also available at

http://pubs.usgs.gov/sir/2008/5227/.)

36. U.S. Environmental Protection Agency, 2012, National

primary drinking water regulations—Drinking water

contaminants: U.S. Environmental Protection Agency

Web site, accessed May 25, 2012 http://water.epa.gov/

drink/contaminants/index.cfm.

37. Paul, A.P., Seiler, R.L., Rowe, T.G., and Rosen, M.R.,

2007, Effects of agriculture and urbanization on quality

of shallow ground water in the arid to semiarid western United States, 1993–2004: U.S. Geological Survey

Scientific Investigations Report 2007–5179, 56 p.

38. Edmonds, R.J., and Gellenbeck, D.J., 2002, Groundwater quality in the West Salt River Valley, Arizona,

1996–98—Relations to hydrogeology, water use, and

land use: U.S. Geological Survey Water Resources

Investigations Report 01–4126, 58 p., accessed

July 5, 2012, at http://az.water.usgs.gov/pubs/WRIR01-

4126intro.html.

96 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
39. Burow, K.R., Shelton, J.L., and Dubrovsky, N.M., 2008,

Regional nitrate and pesticide trends in ground water

in the eastern San Joaquin Valley, California: Journal

of Environmental Quality, v. 37, September–October

[supplement], p. S249–S263, doi:10.2134/jeq2007.0061.

40. Rosen, M.R., 2003, Trends in nitrate and dissolvedsolids concentrations in ground water, Carson Valley,

Douglas County, Nevada, 1985–2001: U.S. Geological

Survey Water Resources Investigations Report 03–4152,

6 p. (Also available at http://pubs.usgs.gov/wri/

wri034152/.)

41. National Atmospheric Deposition Program, 2010,

National trends network data: National Atmospheric

Deposition Program Web site, accessed July 5, 2012, at

http://nadp.sws.uiuc.edu/data/.

42. Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg,

J.M., Hamilton, P.A., Hitt, K.J., Mueller, D.K., Munn,

M.D., Nolan, B.T., Puckett, L.J., Rupert, M.G., Short,

T.M., Spahr, N.E., Sprague, L.A., and Wilber, W.G.,

2010, The quality of our Nation’s waters—Nutrients

in the Nation’s streams and groundwater, 1992–2004:

U.S. Geological Survey Circular 1350, 174 p., accessed

March 14, 2012, at http://pubs.usgs.gov/circ/1350/.

43. Walvoord, M.A., Phillips, F.M., Stonestrom, D.A.,

Evans, R.D., Hartsough, P.C., Newman, B.D., and

Striegl, R.G., 2003, A reservoir of nitrate beneath desert

soils: Science, v. 302, no. 5647, p. 1021–1024.

44. Anning, D.W., Bauch, N.J., Gerner, S.J., Flynn, M.E.,

Hamlin, S.N., Moore, S.J., Schaefer, D.H., Anderholm, S.K., and Spangler, L.E., 2007, Dissolved solids

in basin-fill aquifers and streams in the southwestern

United States: U.S. Geological Survey Scientific Investigations Report 2006–5315, 168 p. (Also available at

http://pubs.usgs.gov/sir/2006/5315/.)

45. Southern California Salinity Coalition, 2009, Salinity and water softeners: Fountain Valley, Calif.,

Southern California Salinity Coalition Salinity

Fact Sheet, 2 p., accessed February 25, 2011, at

http://www.socalsalinity.org/.

46. Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton,

P.A., Martin, J.D., Nakagaki, Naomi, Nowell, L.H.,

Scott, J.C., Stackelberg, P.E., Thelin, G.P., and Wolock,

D.M., 2006, The quality of our Nation’s waters—Pesticides in the Nation’s streams and ground water, 1992–

2001: U.S. Geological Survey Circular 1291, 172 p.

(Also available at http://pubs.usgs.gov/circ/2005/1291/.)

47. U.S. Geological Survey, 2010, Pesticide use maps for

1992, 1997, and 2002: U.S. Geological Survey Web site,

accessed June 10, 2013, at http://water.usgs.gov/nawqa/

pnsp/usage/maps/.

48. U.S. Environmental Protection Agency, 2009, Water

on tap—What you need to know: U.S. Environmental

Protection Agency Office of Water EPA 816–K–09–002,

34 p., accessed June 14, 2011, at http://water.epa.gov/

drink/guide/upload/book_waterontap_full.pdf.

49. Welch, A.H., Bright, D.J., and Knochenmus, L.A.,

eds., 2007, Water resources of the Basin and Range

carbonate-rock aquifer system, White Pine County,

Nevada, and adjacent areas in Nevada and Utah: U.S.

Geological Survey Scienti ic Investigations Report

2007–5261, 96 p. (Also available at http://pubs.usgs.

gov/sir/2007/5261/.)

50. Heilweil, V.M., and Brooks, L.E., eds., 2011, Conceptual

model of the Great Basin carbonate and alluvial aquifer

system: U.S. Geological Survey Scienti ic Investigations

Report, 191 p. (Also available at http://pubs.usgs.gov/

sir/2010/5193/.)

51. Schaefer, D.H., Thiros, S.A., and Rosen, M.R., 2011,

Ground-water quality in the carbonate-rock aquifer of

the Great Basin, Nevada and Utah, 2003: U.S. Geological Survey Scienti ic Investigations Report 2005–5232,

version 1.1, 41 p. (Also available at http://pubs.usgs.

gov/sir/2005/5232/.)

52. Maupin, M.A., and Arnold, T.L., 2010, Estimates

for self-supplied domestic withdrawals and population served for selected Principal Aquifers, calendar

year 2005: U.S. Geological Survey Open-File Report

2010–1223, 10 p. (Also available at http://pubs.usgs.

gov/of/2010/1223/.)

53. Carpenter, Everett, 1915, Ground water in southeastern

Nevada: U.S. Geological Survey Water-Supply Paper

365, 86 p., accessed August 16, 2013, at http://pubs.

er.usgs.gov/publication/wsp365.

54. Harrill, J.R., 1976, Pumping and ground-water storage depletion in Las Vegas Valley, Nevada, 1955–74:

Nevada Department of Conservation and Natural

Resources Bulletin 44, 70 p.

55. Wildermuth Environmental, Inc., 2007, 2007 CBWM

groundwater model documentation and evaluation of the

Peace II project description: Wildermuth Environmental,

Inc., Chino Basin Watermaster, accessed June 24, 2013,

at http://rmp.wildermuthenvironmental.com/supportingdocs.html.

56. Cone, Tracy, 1997, The vanishing valley: San Jose

Mercury News West Magazine, June 29, p. 9–15.

57. Landon, M.K., Jurgens, B.C., Katz, B.G., Eberts, S.M.,

Burow, K.R., and Crandall, C.A., 2009, Depth-dependent sampling to identify short-circuit pathways to public-supply wells in multiple aquifer settings in the United

States: Hydrogeology Journal, v. 18, no. 3, p. 577–593,

accessed July 9, 2014, at http://oh.water.usgs.gov/tanc/

pubs/landon_DOI10.1007s10040-009-0531-2.pdf.

References

References Cited 97
58. Jurgens, B.C., Burow, K.R., Dalgish, B.A., and Shelton,

J.L., 2008, Hydrogeology, water chemistry, and factors

affecting the transport of contaminants in the zone of

contribution of a public-supply well in Modesto, eastern

San Joaquin Valley, California: U.S. Geological Survey

Scientific Investigations Report 2008–5156, 78 p. (Also

available at http://pubs.usgs.gov/sir/2008/5156/.)

59. Toccalino, P.L., Norman, J.E., Phillips, R.H., Kauffman, L.J., Stackelberg, P.E., Nowell, L.H., Krietzman,

S.J., and Post, G.B., 2004, Application of health-based

screening levels to ground-water quality data in a statescale pilot effort: U.S. Geological Survey Scientific

Investigations Report 2004–5174, 64 p. (Also available

at http://pubs.usgs.gov/sir/2004/5174/.)

60. Toccalino, P.L., 2007, Development and application of

health-based screening levels for use in water-quality

assessments: U.S. Geological Survey Scientific Investigations Report 2007–5106, 12 p. (Also available at

http://pubs.usgs.gov/sir/2007/5106/.)

61. Toccalino, P.L., Norman, J.E., Booth, N.L., Thompson,

J.L., and Zogorski, J.S., 2012, Health-based screening

levels—Benchmarks for evaluating water-quality data:

U.S. Geological Survey Web site, accessed April 24,

2013 http://water.usgs.gov/nawqa/HBSL/.

62. U.S. Environmental Protection Agency, 1994, Evaluating and identifying contaminants of concern for human

health: U.S. Environmental Protection Agency Region 8

Superfund Technical Guidance RA–03, 10 p., accessed

September 23, 2013, at http://www2.epa.gov/sites/

production/files/documents/r8_ra03-cocs.pdf.

63. New Jersey Department of Environmental Protection,

2003, Susceptibility of source water to community

water-supply wells in New Jersey to contamination by

volatile organic compounds: Trenton, N.J., New Jersey

Department of Environmental Protection, 21 p.,

accessed September 23, 2013, at http://www.state.nj.us/

dep/swap/reports/gw_voc.pdf.

64. Rao, Balaji, Anderson, T.A., Orris, G.J., Rainwater,

K.A., Rajagopalan, Srinath, Sandvig, R.M., Scanlon,

B.R., Stonestrom, D.A., Walvoord, M.A., and Jackson,

W.A., 2007, Widespread natural perchlorate in unsaturated zones of the southwest United States: Environmental Science and Technology, v. 41, no. 13, p. 4522–4528.

65. U.S. Environmental Protection Agency, 2009, Emerging contaminant—Perchlorate: U.S. Environmental

Protection Agency Office of Solid Waste and Emergency

Response Fact Sheet EPA 505–F–09–005, 4 p., accessed

January 28, 2010, at http://www.clu-in.org/download/

contaminantfocus/epa505f09005.pdf.

66. Fram, M.S., and Belitz, Kenneth, 2011, Probability of

detecting perchlorate under natural conditions in deep

groundwater in California and the southwestern United

States: Environmental Science and Technology, v.

45, issue 4, p. 1271–1277, accessed May 22, 2012, at

http://pubs.acs.org/doi/full/10.1021/es103103p.

67. California State Water Resources Control Board, 2010,

Perchlorate: California State Water Resources Control

Board Groundwater Information Sheet, 11 p., accessed

April 12, 2011, at http://www.waterboards.ca.gov/

water_issues/programs/gama/docs/coc_perchlorate.pdf.

68. U.S. Environmental Protection Agency, 2011, Fiscal

year 2010 drinking water and ground water statistics: U.S. Environmental Protection Agency Office

of Ground Water and Drinking Water EPA 817–K–

11–001, accessed May 29, 2012, at http://water.epa.

gov/scitech/datait/databases/drink/sdwisfed/upload/

new_Fiscal-Year-2010-Drinking-Water-andGround-Water-Statistics.pdf.

69. White, E.L., 2012, Fallon, NV—Pooling resources to

construct arsenic treatment facility: U.S. Environmental

Protection Agency Arsenic in Drinking Water Compliance Success Stories, 5 p., accessed February 29, 2012,

at http://www.epa.gov/ogwdw/arsenic/casestudies/pdfs/

casestudy_fallon.pdf.

70. Walker, Mark, and Montecinos, Crystel, 2001, Arsenic in drinking water—Issues associated with revising

the standard: Reno, University of Nevada Cooperative

Extension Fact Sheet 01–08, 4 p., accessed July 15,

2014, at http://www.unce.unr.edu/publications/ iles/

ag/2001/fs0108.pdf.

71. Flanagan, S.M., Ayotte, J.D., and Robinson, G.R., Jr.,

2012, Quality of water from crystalline rock aquifers in

New England, New Jersey, and New York, 1995–2007:

U.S. Geological Survey Scienti ic Investigations Report

2011–5220, 104 p. (Also available at http://pubs.usgs.

gov/sir/2011/5220/.)

72. Harter, Thomas, Lund, J.R., Darby, Jeannie, Fogg, G.E.,

Howitt, Richard, Jessoe, K.K., Pettygrove, G.S., Quinn,

J.F., Viers, J.H., and others, 2012, Addressing nitrate in

California’s drinking water, with a focus on Tulare Lake

Basin and Salinas Valley groundwater: Davis, Calif.,

University of California, Davis, 78 p., accessed July 3,

2012, at http://groundwaternitrate.ucdavis.edu/.

73. Alley, W.M., 2003, Desalination of ground water—Earth

science perspectives: U.S. Geological Survey Fact Sheet

075–03, 4 p. (Also available at http://pubs.usgs.gov/fs/

fs075-03/pdf/AlleyFS.pdf.)

98 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
74. Thiros, S.A., and Manning, A.H., 2004, Quality and

sources of ground water used for public supply in Salt

Lake Valley, Salt Lake County, Utah, 2001: U.S. Geological Survey Water-Resources Investigations Report

03–4325, 95 p. (Also available at http://pubs.water.usgs.

gov/wri034325/.)

75. Thiros, S.A., and Spangler, L.E., 2010, Decadal-scale

changes in dissolved-solids concentrations in groundwater used for public supply, Salt Lake Valley, Utah: U.S.

Geological Survey Fact Sheet 2010–3073, 6 p. (Also

available at http://pubs.usgs.gov/fs/2010/3073/.)

76. Belitz, Kenneth, Hamlin, S.N., Burton, C.A., Kent,

Robert, Fay, R.G., and Johnson, Tyler, 2004, Water quality in the Santa Ana Basin, California, 1999–2001: U.S.

Geological Survey Circular 1238, 38 p. (Also available

at http://pubs.usgs.gov/circ/2004/1238/.)

77. Groundwater Replenishment System, 2013, Groundwater replenishment system: Fountain Valley, Calif.,

Groundwater Replenishment System Fact Sheet, 24 p.,

accessed May 2, 2013, at http://www.gwrsystem.com/

images/stories/AboutGWRS/GWRS%20Technical%20

Brochure.pdf.

78. Plummer, L.N., Bexfield, L.M., Anderholm, S.K., Sanford, W.E., and Busenberg, Eurybiades, 2004, Geochemical characterization of ground-water flow in the Santa

Fe Group aquifer system, Middle Rio Grande Basin,

New Mexico: U.S. Geological Survey Water-Resources

Investigations Report 03–4131, 395 p. (Also available at

http://pubs.usgs.gov/wri/wri034131/.)

79. U.S. Geological Survey, 2004, Estimated use of water

in the United States—County-level data for 2000: U.S.

Geological Survey Web site, accessed June 10, 2013, at

http://water.usgs.gov/watuse/data/2000/index.html.

80. Wildermuth Environmental, Inc., 2008, Recomputation

of ambient water quality in the Santa Ana Watershed

for the period 1987 to 2006: Wildermuth Environmental, Inc. Final Technical Memorandum [prepared for

the Basin Monitoring Program Task Force; variously

paged].

81. Faunt, C.C., Hanson, R.T., and Belitz, Kenneth, 2009,

Introduction, overview of hydrology, and textural model

of California’s Central Valley, in Faunt, C.C., ed.,

Groundwater availability of the Central Valley Aquifer,

California: U.S. Geological Survey Professional Paper

1766, p. 1–57. (Also available at http://pubs.usgs.gov/

pp/1766/.)

82. National Ground Water Association, 2010, Brackish

groundwater: Westerville, Ohio, National Ground Water

Association Information Brief, 4 p., accessed May 15,

2012, at http://www.ngwa.org/Media-Center/briefs/

Documents/Brackish_water_info_brief_2010.pdf.

83. Homer, C.H., Fry, J.A., and Barnes, C.A., 2012, The

National Land Cover Database: U.S. Geological

Survey Fact Sheet 2012–3020, 4 p. (Also available at

http://pubs.usgs.gov/fs/2012/3020/fs2012-3020.pdf.)

84. Desimone, L.A., McMahon, P.B., and Rosen, M.R.,

2014, The quality of our Nation's waters—Water quality

in Principal Aquifers of the United States, 1991–2010:

U.S. Geological Survey Circular 1360, 151 p. (Also

available at http://dx.doi.org/10.3133/cir1360.)

References

References Cited 99
Glossary

A

adsorption Process in which ions or 

molecules dissolved in groundwater become 

attached to the surfaces of solid materials, 

such as sediment or rock particles in aquifers, 

either temporarily or permanently.

alluvial fan A low, outspread, relatively flat 

to gently sloping mass of loose rock material, 

shaped like an open fan or a segment of a 

cone, deposited by a stream (especially in a 

semiarid region) at the place where the rock 

mass issues from a narrow mountain valley on 

a plain or broad valley.

anoxic Water with no dissolved oxygen 

or a very low concentration (less than 

0.5 milligram per liter) of dissolved oxygen.

aquifer A geologic formation, group 

of formations, or part of a formation that 

contains a sufficient amount of saturated 

permeable material (for example, soil, sand, 

gravel and (or) rock) to yield substantial 

quantities of water to wells and springs.

artesian Referring to confined groundwater. 

If the potentiometric surface in a confined 

aquifer is higher than the land surface, water 

discharges spontaneously from a well tapping 

the aquifer; this is called a “flowing artesian 

well.”

artificial recharge Replenishment of an 

aquifer through human effort, for example, 

spreading water, recharge wells, or ditches; 

recharge through human activities that occurs 

at a rate greater than that of naturally occurring activities; the water artificially recharging 

an aquifer.

B

background concentration A concentration 

of a substance in a particular environment that 

corresponds to minimal influence by human 

(anthropogenic) sources or activities.

base flow Groundwater seepage into a 

stream or river. The continual contribution 

of groundwater to streams and rivers is an 

important source of streamflow between rain 

events.

biotic community A group of organisms that 

exist together because the organisms possess 

an integrated system (food chain or food web) 

of energy transfer operating through several 

different feeding or trophic levels.

C

closed basin An enclosed area having no 

drainage outlet, from which water escapes 

only by evaporation, as in an arid region.

common assessment level A single concentration threshold used to establish an equal 

basis for comparing detection frequencies 

among multiple chemicals. Use of a common 

assessment level avoids biases in detection 

frequencies caused by one compound having 

a lower detection level than another. Also 

sometimes referred to as a “common detection 

level.”

complexation A chemical process that 

combines simple ions into a larger ion or by 

which an ion adheres to a charged surface.

confined aquifer (artesian aquifer) An 

aquifer in which the groundwater is bounded 

between layers of relatively impermeable 

material, such as clay or dense rock. When 

tapped by a well, water in a confined aquifer 

is forced up, sometimes above the land 

surface, by pressure within the aquifer.

confining layer Geologic material with little 

or no permeability or hydraulic conductivity. 

Water does not pass through this layer or the 

rate of movement is extremely slow.

consolidated rock Tightly bound geologic 

formation composed of sandstone, limestone, 

granite, or other rock.

constituent A chemical or biological 

substance in water, sediment, or biota that 

can be measured by an analytical (laboratory) 

method.

100 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
contaminant For the purposes of this report, 

any manmade compound at any concentra-

tion, or any constituent with a geologic source 

measured at a concentration exceeding the 

designated human-health benchmark. D

degradate A compound formed by the 

transformation of a parent compound, 

typically an organic contaminant or another 

degradate, by chemical, photochemical, or 

biological reactions.

denitrification The bacterial reduction of 

dissolved nitrate to nitrogen gas. Denitrifica-

tion is the primary process by which nitrate 

can be eliminated naturally in groundwater.

desorption The release of a sorbed mate-

rial from the solid to which it was sorbed. 

Opposite process of adsorption.

discharge The rate of flow of surface 

water or groundwater past a given point at a 

given moment, expressed as volume per unit 

of time. Also, the outflow from an aquifer, 

spring, or well or up through a streambed.

dissolution The process of dissolving a 

solid (mineral) into a homogeneous solution 

(water). Dissolution reactions result in the 

addition of ions to water as minerals react 

with water. Common dissolution reactions 

include dissolution of carbonate rock (lime-

stone or dolomite) and incongruent dissolu-

tion of silicate minerals (feldspar) by carbonic 

acid (H

2

CO

3

).

domestic well A privately owned well that 

typically serves one home and supplies water 

for human consumption and other homeowner 

uses. E

evaporative concentration Process in which 

high rates of evaporation lead to enrichment 

of a constituent by removing water and 

leaving the constituent in the remaining water.

evapotranspiration Loss of water from 

soil by evaporation and plant transpiration 

combined. F

flow path The route or pathway of water 

flowing through the hydrologic system. Typi-

cally refers to subsurface (groundwater) flow.

G

groundwater recharge The infiltration 

of water to the saturated zone. Also refers 

to water that reaches the water table by 

infiltration of precipitation or irrigation water 

through the unsaturated zone or by seepage 

of water from surface-water bodies, such as 

streams and lakes.

groundwater residence time The average 

amount of time it takes for groundwater 

to move from the point where it enters the 

aquifer to a specific point of discharge, such 

as a well or stream. H

Health-Based Screening Level (HBSL) An 

estimate of concentration (for a noncar-

cinogen) or concentration range (for a 

carcinogen) in water that (1) may be of 

potential human-health concern, (2) can 

be used as a threshold value against which 

measured concentrations of contaminants 

in ambient groundwater samples can be 

compared, and (3) is consistent with U.S. 

Environmental Protection Agency Office of 

Water methodologies.

human-health benchmark A threshold 

concentration above which the concentration 

of a contaminant in drinking water could have 

adverse effects on human health. Treatment or 

other measures can be used before the water 

is consumed to lower the concentration of the 

contaminant below the benchmark.

hydraulic gradient In an aquifer, the rate of 

change of total head (water-level altitude in 

a well) per unit of distance of flow at a given 

point and in a given direction. Water will flow 

from higher hydraulic head to lower hydraulic 

head. I

igneous rock Rock that solidified from 

molten (melted) or partly molten material. 

Granite is an example of an igneous rock.

intrinsic susceptibility A measure of the 

ease with which a contaminant in water enters 

and moves through an aquifer; a characteristic 

of the aquifer and overlying material and 

hydrologic conditions independent of the 

chemical characteristics of the contaminant 

and its sources.

Glossary

Glossary 101
L

land-use study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program to assess the 

effects of a specific land-use type (generally 

agricultural or urban) on groundwater quality, 

in most cases by sampling groundwater from 

monitoring wells that tap water from or near 

the water table.

legume A member of the large plant 

family Leguminosae, many of which harbor 

nitrogen-fixing bacteria on their roots, and 

many of which contain edible parts. Beans, 

alfalfa, and mesquite are examples of 

legumes.

M

major-aquifer study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program that involves 

sampling of water at 20 to 30 wells (primarily 

domestic wells) that withdraw water from 

major aquifers. The major-aquifer studies 

represent a mix of land uses and target water 

that is used for drinking-water supply.

Maximum Contaminant Level (MCL)

Maximum permissible level of a contaminant 

in water that is delivered to any user of a 

public water system. MCLs are enforceable 

standards established by the U.S. Environmental Protection Agency.

metamorphic rock Rock derived from 

mineralogical, chemical, or structural changes 

to preexisting rocks in response to marked 

changes in temperature, pressure, shearing 

stress, and chemical environment, generally 

at depth in the Earth’s crust. Gneiss and schist 

are examples of metamorphic rocks.

monitoring well A well used to measure 

water quality or groundwater levels continuously or periodically. Not typically used as a 

source of drinking water. Sometimes referred 

to as an “observation well.”

O

open basin A basin where, under natural 

conditions, water is drained by a throughflowing stream (the basin is topographically 

open) or by subsurface outflow, either through 

basin fill or consolidated rock.

oxic Water with a concentration of 

dissolved oxygen greater than or equal to 

0.5 milligram per liter.

P

permeability A measure of the relative ease 

with which a porous or fractured medium 

can transmit groundwater. Rock formations 

that transmit fluids readily are described as 

permeable.

pH A measure of the acidity (pH less than 7) 

or alkalinity (pH greater than 7) of a solution; 

a pH of 7 is neutral. Formally defined as the 

logarithm of the reciprocal of the hydrogen 

ion concentration (activity) of a solution.

point source A stationary location or 

fixed facility from which contaminants are 

discharged, for example, a pipe, ditch, ship, 

ore pit, or factory smokestack.

potential evapotranspiration The theoretical 

maximum loss of water that would occur 

from a given area by evapotranspiration if 

unlimited water were available to the root 

system of plants.

predevelopment The time prior to substantial groundwater development by humans or 

effects of agricultural, urban, suburban, or 

other human-related land uses.

Principal Aquifer A regionally extensive 

aquifer or aquifer system that has the potential 

to be used as a source of potable water. A 

Principal Aquifer can be composed of one or 

more major aquifers.

public-supply well A privately or publicly 

owned well that provides water for public 

use to (1) a community water system, 

(2) a transient noncommunity water system, 

such as a campground, or (3) a nontransient, 

noncommunity system, such as a school.

R

recently recharged groundwater Groundwater that was recharged after 1952, as 

indicated by tritium concentrations greater 

than 0.5 tritium unit

reduction/oxidation (redox) Chemical 

reactions that involve the transfer of electrons 

from one chemical species to another, 

resulting in a change in the valence state of 

the species. Redox processes in groundwater 

often are microbially facilitated.

S

saturated The condition in which all the 

pores (voids, interstices) within a material are 

filled with a liquid, typically water.

102 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Secondary Maximum Contaminant Level 

(SMCL) Guidelines set by the U.S. Environ-

mental Protection Agency for concentrations 

of "nuisance" constituents in drinking water 

that may cause unwanted effects, such as 

unpleasant taste, color, or odor; discoloration 

of skin or teeth; or corrosion or staining of 

plumbing fixtures. Public drinking-water 

systems are recommended but not required to 

comply with these guidelines.

sedimentary rocks Rocks composed of 

particles derived from the erosion or weath-

ering of preexisting rocks or from chemical 

precipitation from water. Sandstone and 

limestone are examples of sedimentary rocks.

susceptibility See intrinsic susceptibility. U

unconfined aquifer An aquifer that has a 

water table; an aquifer containing unconfined 

groundwater.

unconsolidated deposit Deposit of loosely 

bound sediment that typically fills topographi-

cally low areas.

unsaturated zone A subsurface zone 

containing both water and air. The unsaturated 

zone is limited above by the land surface and 

below by the water table. V

volatile organic compound (VOC) An 

organic chemical that has a high vapor 

pressure relative to its water solubility. VOCs 

include components of gasoline, fuel oils, 

lubricants, organic solvents, fumigants, some 

inert ingredients in pesticides, and some 

byproducts of chlorine disinfection.

vulnerability The tendency or likelihood for 

contaminants to reach a specified position in 

the groundwater system after introduction at 

some location above the uppermost aquifer. 

The vulnerability of a groundwater resource 

to contamination depends both on the intrinsic 

susceptibility of the resource and on the 

locations and types of human and geologic 

sources of contaminants, locations of wells, 

and the characteristics of the contaminant(s). W

water table The upper surface of the 

saturated zone below which all voids (spaces) 

are filled with water.

Glossary

Glossary 103
Acknowledgments

Special thanks to the following individuals and organizations for their 

contributions:

NAWQA personnel in the Southwest Principal Aquifers study area for their contributions 

of data and research results.

NAWQA’s many partners in the governmental and non-governmental sectors who have 

helped guide scientific efforts and ensure that NAWQA information meets the needs of local, 

State, Tribal, regional, and national stakeholders.

USGS reviewers:

• Devin Galloway

• Lisa Stillings

• Karen Burow

• Wayne Lapham

• Barbara Mahler

• Leslie DeSimone

• Dennis Wentz

• William Wilbur

External reviewers:

• Bart Faris, New Mexico Environment Department

• Jan Stepek, California State Water Resources Control Board, Division of Water Quality

• Doug Towne, Arizona Department of Environmental Quality

104 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Appendix 1. Study Components of the Southwest Principal Aquifers WaterQuality Assessment

How does land use affect groundwater quality? How does water quality change as it moves through an 

aquifer? What is the quality of the drinking-water resource?

NAWQA groundwater assessments include different types of studies, specifically designed to answer questions 

such as these. 

•	 Land-use studies were conducted to sample water from water-table monitoring wells installed in urban and agricultural areas to 

assess the effects of these land uses on the quality of the underlying groundwater. Although not usually used for drinking, this water 

supplies recharge to the deeper aquifer system. 

•	 Flow-path studies were conducted to investigate how water quality changes as it moves along a groundwater flow path. Samples were 

collected from wells installed along a groundwater flow path (http://pubs.usgs.gov/circ/circ1112/flowpath.html).

•	 Major-aquifer studies provide a broad overview of the quality of the deeper aquifer system used for drinking-water supply. Most of the 

wells sampled were domestic wells that were distributed across a large area in a mixture of land uses. 

•	 Vulnerability or contaminant-transport studies were designed to assess the vulnerability of water delivered by public-supply wells 

to contamination from natural and manmade contaminants. Samples were collected from multiple depths in public-supply wells to 

determine where and how contaminants from different sources enter the wells and how natural processes and human activities affect 

water quality (http://oh.water.usgs.gov/tanc/NAWQATANC.htm).

•	 Source-water quality assessment studies were conducted to sample water from public-supply wells to understand 

the occurrence of unregulated manmade chemicals in the groundwater resources that serve large numbers of 

people (http://water.usgs.gov/nawqa/swqa/).

Results of these studies were reinforced by locating some of the studies within the boundaries of larger studies. For example, the recharge 

areas for flow-path studies were located within the boundaries of the land-use studies to provide information on the quality of the recharge. In 

turn, the boundaries of the land-use studies were located within the boundaries of the major-aquifer studies to provide information on how the 

quality of the recharge affects that of the deeper groundwater.

For each study, water was sampled from a network of a few to as many as 30 wells. Data from these studies were available for the 

Principal Aquifer assessments.

Appendixes

circ1358 appendix 1 fig 1

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal Basin

Middle Rio Grande Basin

Sacramento Valley

San Joaquiin Valley

Southwest Principal Aquifers

study area boundary

NEVADA

OREGON

IDAHO

CALIFORNIA

ARIZONA NEW MEXICO

UTAH COLORADO

Pac if i c Oc ean

 EXPLANATION

Principal Aquifers

 Basin and Range basin-fill aquifers 

 California Coastal Basin aquifers

 Central Valley aquifer system

 Rio Grande aquifer system

Map number of study component

Study basin 

4,5

1,2,3

6

7,8

9,10,11

12–19

20,21

21,23

21,22,23

24,25

27,28

26

29

30,31

32

33

36,37,38

34,35

39

41,43,44

42

40

29

Southwest

Principal Aquifers

N

Appendixes 105
Table A1–1. Study components of the Southwest Principal Aquifers water-quality assessment.—Continued

[F, field parameters including water temperature, specific conductance, pH, alkalinity, turbidity, and dissolved oxygen; M, major ions; N, nutrients; D, dissolved organic carbon; 

other than tritium; MB, microbiological; AC, other anthropogenic organic compounds; ps, public-supply wells; mon, monitoring wells; dom, domestic wells: irr, irrigation wells; 

Map 

number Study component Study location Study purpose 

1 Major-aquifer study Coastal Basin of the Santa Ana Basin, California Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

2 Land-use study Coastal Basin of the Santa Ana Basin, California Characterize the quality of recently recharged water in an urban setting. 

3 ¶Flow-path study Coastal Basin of the Santa Ana Basin, California Characterize the spatial and vertical variations in water quality along a 

groundwater flow path.

4 Major-aquifer study Inland Basin of the Santa Ana Basin, California Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

5 ¶Flow-path study Bunker Hill subbasin, Inland Basin of the Santa Ana 

Basin, California

Characterize the spatial and vertical variations in water quality along a 

groundwater flow path.

6 Major-aquifer study San Jacinto Basin of the Santa Ana Basin, California Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

7 Major-aquifer study Inland Basins in Northern Coast Ranges area, California Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

8 Major-aquifer study Coastal Basins in Northern Coast Ranges area, 

California

Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

9 Major-aquifer study Southeastern Sacramento Valley of Central Valley, 

California

Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

10 Land-use study Sacramento Valley of Central Valley, California Characterize the quality of recently recharged water in a rice 

agricultural setting.

11 Land-use study Southeastern Sacramento Valley of Central 

Valley, California

Characterize the quality of recently recharged water in an urban setting. 

12 Major-aquifer study Eastern alluvial fans in San Joaquin Valley of Central 

Valley, California

Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

13 Land-use study Eastern alluvial fans in San Joaquin Valley of Central 

Valley, California

Characterize the quality of recently recharged water in a vineyard 

agricultural setting.

14 Land-use study Eastern alluvial fans in San Joaquin Valley of Central 

Valley, California

Characterize the quality of recently recharged water in an almond 

orchard agricultural setting.

15 Land-use study Eastern alluvial fans in San Joaquin Valley of Central 

Valley, California

Characterize the quality of recently recharged water in corn, alfalfa, and 

vegetable row crop agricultural settings.

16 Large-scale TANC study Modesto area, eastern alluvial fans in San Joaquin 

Valley of Central Valley, California

Broadly characterize water quality in the basin-fill aquifer used for 

public supply.

17 ¶Local-scale TANC 

study

Modesto area, eastern alluvial fans in San Joaquin 

Valley of Central Valley, California

Examine factors resulting in detection of natural and anthropogenic 

contaminants in a public-supply well.

18 ¶Local-scale agricultural 

chemicals study

Near Merced River, eastern alluvial fans in 

San Joaquin Valley of Central Valley, California

Characterize the source, transport, and fate of agricultural chemicals 

along direction of groundwater flow toward the Merced River.

19 ¶Flow-path study Vineyard land-use area near Fresno, eastern alluvial fans 

in San Joaquin Valley of Central Valley, California

Characterize chemical and physical processes along groundwater 

flow paths.

20 Land-use study Truckee Meadows, Nevada Characterize the quality of recently recharged water in an urban setting. 

21 Major-aquifer study Eagle Valley, Truckee Meadows, and Spanish 

Springs Valley, Nevada

Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

22 Land-use study Eagle Valley, Nevada Characterize the quality of recently recharged water in an urban setting. 

23 Large-scale TANC study Eagle Valley and Spanish Springs Valley, Nevada Broadly characterize water quality in the basin-fill aquifer used for 

public supply.

24 Major-aquifer study Carson Valley, Nevada Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

25 Land-use study Carson Valley, Nevada Characterize the quality of recently recharged water in an agricultural 

setting.

106 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
TE, trace elements; P, pesticide compounds; V, volatile organic compounds; R, radon; RA, radium; T, tritium; SI, stable isotopes of oxygen and hydrogen; A, age tracers 

TANC, Contaminant-transport (Transport of Anthopogenic and Natural Contaminants) study; --, no data]

Water-quality parameters Year sampled

Number 

of wells 

sampled

Well types Link to report if available online or reference to report

F, M, N, D, TE, P, V, *R, *T, *SI, 

†RA, †MB

1999

2009

20

20

Mostly ps http://pubs.usgs.gov/wri/wri02-4243/text.html

F, M, N, D, TE, P, V, R, T, SI 2000 26 mon

F, M, N, D, TE, P, V, R, T, SI 2000 23 mon

http://pubs.usgs.gov/wri/wri02-4243/text.html

http://pubs.usgs.gov/wri/wrir034059/

F, M, N, D, TE, P, V, R, T, SI 2000 29 ps http://pubs.usgs.gov/wri/wri02-4243/text.html

F, M, N, D, TE, P, V, R, T, SI 2000 21 20 mon, 1 ps http://pubs.usgs.gov/wri/wrir034059/

F, M, N, D, TE, P, V, R, T, SI 2001 23 Mostly ps http://pubs.usgs.gov/wri/wri02-4243/text.html

F, M, N, D, TE, P, V, R, T, SI, A, 

MB, AC

2009 28 ps, dom, irr http://ca.water.usgs.gov/projects/gama/includes/

GAMA_publications.html

F, M, N, D, TE, P, V, R, T, SI, A, 

MB, AC

2009 30 Mostly ps, dom http://ca.water.usgs.gov/projects/gama/includes/

GAMA_publications.html

F, M, N, D, TE, P, V, *R, T, *SI, 

A, †MB

1996

2008

31

25

Mostly dom http://pubs.usgs.gov/wri/wri014125/

F, M, N, D, TE, P, †V, †T 1997

2006

28

23

mon http://ca.water.usgs.gov/archive/reports/wrir014000/

F, M, N, D, TE, P, V, *R, A 1998

2005–06

19

25

mon http://pubs.usgs.gov/sir/2005/5148/

F, M, N, D, TE, P, V, R 1995

2002

30

30

dom http://ca.water.usgs.gov/sanj/pub/usgs/wrir98-4040a/wrir98-4040a.html

F, M, N, D, TE, P, V, R 1993–94

2001

30

30

mon, dom http://ca.water.usgs.gov/sanj/pub/usgs/wrir97-4284/wrir97-4284.html

F, M, N, D, TE, P, V, R 1994

2001

30

30

mon, dom http://ca.water.usgs.gov/sanj/pub/usgs/wrir97-4284/wrir97-4284.html

F, M, N, D, TE, P, V, R 1995

2002

30

30

mon, dom http://ca.water.usgs.gov/sanj/pub/usgs/wrir97-4284/wrir97-4284.html

F, M, N, D, TE, P, V, AC 2002 15 ps --

F, M, N, D, TE, V, R, RA, T, SI, A 2003–05 24 23 mon, 1 ps

F, M, N, D, P, T, A 2004 12 mon

http://pubs.usgs.gov/sir/2008/5035/

and http://pubs.usgs.gov/sir/2008/5156/

http://pubs.usgs.gov/sir/2007/5009/

F, M, N, D, TE, P, V, R, A, AC 1994–95

2003–04

20 mon http://ca.water.usgs.gov/sanj/pub/usgs/wrir99-4059/wrir99-4059.html

F, M, N, D, TE, P, V, *R 1994

2002

28

12

mon http://pubs.er.usgs.gov/publication/ofr97222

F, M, N, D, TE, P, V, R, †T, †MB 1995

2003

§18

30

Mostly ps http://pubs.er.usgs.gov/publication/ofr97222

F, M, N, D, TE, P, V 2002 19 mon

F, M, N, D, TE, P, V, MB, AC 2002 15 ps

--

http://pubs.usgs.gov/sir/2006/5210/

F, M, N, D, TE, P, V, R 1995 18 Mostly dom, ps http://pubs.er.usgs.gov/publication/wri974259

F, M, N, D, TE, P, V, R 1994 20 mon http://pubs.er.usgs.gov/publication/wri974259

Appendixes

Appendixes 107
Table A1–1. Study components of the Southwest Principal Aquifers water-quality assessment.—Continued

[F, field parameters including water temperature, specific conductance, pH, alkalinity, turbidity, and dissolved oxygen; M, major ions; N, nutrients; D, dissolved organic carbon; 

other than tritium; MB, microbiological; AC, other anthropogenic organic compounds; ps, public-supply wells; mon, monitoring wells; dom, domestic wells: irr, irrigation wells; 

Map 

number Study component Study location Study purpose 

26 Land-use study Carson Desert, Nevada Characterize the quality of recently recharged water in an agricultural 

setting.

27 Major-aquifer study Las Vegas Valley, Nevada Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

28 Land-use study Las Vegas Valley, Nevada Characterize the quality of recently recharged water in an urban setting. 

29 ¶Major-aquifer study Basin and Range carbonate-rock aquifers, Utah 

and Nevada

Broadly characterize water quality in the carbonate-rock aquifers. 

30 Major-aquifer study West Salt River Valley, Arizona Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

31 Land-use study Western part of West Salt River Valley, Arizona Characterize the quality of recently recharged water in an agricultural 

setting.

32 Major-aquifer study Upper Santa Cruz Basin, Arizona Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

33 Major-aquifer study Sierra Vista subbasin, Arizona Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

34 Major-aquifer study Great Salt Lake drainage basin, Utah and Idaho Broadly characterize water quality in unconfined part of basin-fill aquifers near the mountain front used for water supply.

35 Major-aquifer study Great Salt Lake drainage basin, Utah and Idaho Broadly characterize water quality in confined part of basin-fill aquifers 

used for water supply where the vertical hydraulic gradient is downward.

36 ¶Major-aquifer study Salt Lake Valley, Utah Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

37 Land-use study Salt Lake Valley, Utah Characterize the quality of recently recharged water in an urban setting. 

38 Large-scale TANC study Salt Lake Valley, Utah Broadly characterize water quality in the basin-fill aquifer used for 

public supply.

39 Land-use study Alamosa Basin, San Luis Valley, Colorado Characterize the quality of recently recharged water in an agricultural 

setting irrigated with groundwater by overhead, center pivot sprinklers.

40 Major-aquifer study Rio Grande floodplain of the Middle Rio Grande, 

Socorro, San Marcial, Palomas, and Mesilla Basins, 

New Mexico

Broadly characterize water quality in the basin-fill aquifer used for 

water supply.

41 Land-use study Rio Grande floodplain near Albuquerque, Middle Rio 

Grande Basin, New Mexico

Characterize the quality of recently recharged water in an urban setting. 

42 Land-use study Rincon-Hatch area of the Rio Grande floodplain, 

Palomas Basin, New Mexico

Characterize the quality of recently recharged water in an agricultural 

setting flood irrigated with surface water.

43 Large-scale TANC study Northern Middle Rio Grande Basin, New Mexico Broadly characterize water quality in basin-fill aquifer used for public 

supply.

44 ¶Local-scale TANC 

study

Albuquerque area, Middle Rio Grande Basin, 

New Mexico

Examine factors resulting in detection of natural and anthropogenic 

contaminants in a public-supply well.

*Analyzed in samples collected during 1991–2001 only.

†Analyzed in samples collected during 2002–09 only.

¶Samples from study not in dataset described in appendix 2. 

§Wells sampled in Truckee Meadows only.

108 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
TE, trace elements; P, pesticide compounds; V, volatile organic compounds; R, radon; RA, radium; T, tritium; SI, stable isotopes of oxygen and hydrogen; A, age tracers 

TANC, Contaminant-transport (Transport of Anthopogenic and Natural Contaminants) study; --, no data]

Water-quality parameters Year sampled

Number 

of wells 

sampled

Well types Link to report if available online or reference to report

F, M, N, D, TE, P, V, R 1994 10 mon http://pubs.er.usgs.gov/publication/wri974259

F, M, N, D, TE, P, V, R 1995 22

F, M, N, D, TE, P, V, R 1993 32

F, M, N, D, TE, P, V, R, SI, T, MB 2003 18 wells, 

12 springs

Mostly ps, dom http://pubs.er.usgs.gov/publication/wri974259

mon http://pubs.er.usgs.gov/publication/ofr96552

Mostly mon, ps http://pubs.usgs.gov/sir/2005/5232

F, M, N, D, TE, P, V, *R, *T, *SI, 

†A, †MB, †RA

1996–97

2008

35

35

Mostly dom, 

ps, irr 

F, M, N, D, TE, P, V, R, T, SI 1997

1998

9

9

mon

F, M, N, D, TE, P, V, R, T, SI 1998 29 Mostly dom

F, M, N, D, TE, P, V, R, T, SI 1996 19 Mostly dom

http://az.water.usgs.gov/pubs/WRIR01-4126intro.html

http://az.water.usgs.gov/pubs/WRIR01-4126intro.html

http://az.water.usgs.gov/pubs/WRIR00-4117intro.html

http://pubs.er.usgs.gov/publication/wri994056

F, M, N, D, TE, P, V, R, T, SI 1998 26 Mostly dom, 

ps, irr

--

F, M, N, D, TE, P, V, R, T, SI 1998 29 Mostly dom, 

ps, irr

--

F, M, N, D, TE, P, V, T, A, MB, RA 2008 34 Mostly dom, ps --

F, M, N, D, TE, P, V, R, T, SI, A 1999 30 mon

F, M, N, D, TE, P, V, R, T, SI, A 2001 31 ps

F, M, N, D, TE, P, *R, †T, †A, †SI 1993

2007

35

33

mon

http://pubs.water.usgs.gov/wri034028 http://

pubs.water.usgs.gov/wri034325 http://

pubs.er.usgs.gov/publication/wri964144

F, M, N, D, TE, P, V, R 1995 30 Mostly dom http://pubs.er.usgs.gov/publication/wri964249

F, M, N, D, TE, P, V, *R, †SI, †T, 

†A

1993

2006

24

31

mon

F, M, N, D, TE, P, *R, †A, †SI 1994

2006

30

30

mon

http://pubs.er.usgs.gov/publication/wri974067

http://pubs.usgs.gov/wri/wri02-4188/pdf/wrir02-4188.pdf

F, M, N, D, TE, P, V, MB, †AC 2005 39 ps --

F, M, N, D, TE, V, R, RA, T, SI, A 2007–09 25 22 mon, 3 ps --

Appendixes

Appendixes 109
Water-quality properties and constituents measured and a summary of data for 1993–2009, including laboratory 

reporting levels and human-health benchmarks for drinking water, are presented only online. A data summary is available 

for download at http://pubs.usgs.gov/circ/1358/ and includes the tables and figure listed below. The complete data archive is 

included with the report that summarizes groundwater quality for many of the Nation's Principal Aquifers(84) and is available 

at http://pubs.usgs.gov/circ/1360/.

Figure A2–1. A concentration of a constituent exceeded its human-health benchmark in a total of 257 cases in samples 

collected by the NAWQA Program from public-supply and domestic wells in the Southwest Principal Aquifers study area. 

Most of the constituents that occurred at a concentration that exceeded a benchmark were those with a geologic source, but 

some were for nitrate, manmade chemicals, or a combination of constituent types.

Table A2–1. Water-quality properties and constituents analyzed in samples collected during 1993–2009 from the 

Southwest Principal Aquifers, including laboratory reporting levels and human-health benchmarks for drinking water.

Table A2–2. Constituents with geologic sources that exceeded a human-health benchmark or Secondary Maximum 

Contaminant Level in water sampled from at least one drinking-water well completed in Southwest basin-fill aquifers.

Table A2–3. Selected human-related constituents detected in water sampled from wells completed in Southwest basinfill aquifers.

Appendix 2. Water-Quality Properties and Constituents Measured, a Summary

of Data, and Complete Data Archive for 1993–2009

110 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Appendix 3. Water-Quality Data From Southwest Basin-Fill Aquifers in a

National Context

This appendix shows graphical comparisons of chemical concentrations (1991 to 2010) for selected inorganic and organic 

constituents of potential health or aquatic-life concern in selected Principal Aquifers of the United States. For each constituent, 

the concentration data are grouped according to two well types: domestic wells and public-supply wells. For each well type, the 

aquifers also are grouped according to aquifer lithology: basalt and volcanics,** crystalline, carbonate, sandstone, semiconsolidated sand and gravel, glacial unconsolidated sand and gravel, and unconsolidated sand and gravel (nonglacial).

The number of samples for a constituent varies among Principal Aquifers, and data for a particular compound were not plotted 

if there were fewer than 10 samples for a Principal Aquifer. Not all Principal Aquifers for which data are available are shown. 

Note that analytical detection limits vary among the constituents. Boxplots are truncated at common assessment levels and data 

reported as non-detects or analytical values below common assessment levels are not shown. The data used in this appendix and 

boxplots for additional constituents are available at http://pubs.usgs.gov/circ/1360/.

** Note: Two of the Principal Aquifers in this group include limited samples from basin-fill aquifers within the extent of the 

basaltic aquifer.

Appendixes

Principal Aquifer (number of samples)

10th

percentile

25th

percentile

75th

percentile

Median

(50th percentile)

Lithology

Basalt and volcanics** Crystalline Carbonate Sandstone Semiconsolidated

 sand and gravel

Glacial unconsolidated sand and gravel Unconsolidated sand and gravel (nonglacial)

MCL Maximum Contaminant Level

U.S. Environmental Protection Agency

 contaminant levels for drinking water 

(NA) Wells of this type were not 

sampled for this constituent

90th

percentile

Data above

90th percentile

Data below

10th percentile*

SMCL Secondary Maximum Contaminant Level

PMCL Proposed Maximum Contaminant Level

AMCL Proposed Alternative Maximum Contaminant Level

* Data below laboratory reporting limits are not shown.

Appendixes 111
circ1358_appendix3a

MCL = 10 μg/L MCL = 10 μg/L

MCL = 30 μg/L MCL = 30 μg/L

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (313)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (135)

California Coastal Basin (18)

Basin and Range basin fill (129)

Glacial–West (23)

Glacial–West Central (76)

Glacial–Central (187)

Glacial–East (68)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (60)

Mississippi embayment (27)

Valley and Ridge siliciclastic (27)

Early Mesozoic (24)

Denver Basin (75)

Cambrian-Ordovician (69)

Valley and Ridge carbonate (28)

Piedmont and Blue Ridge carbonate (NA)

Upper Floridan (128)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (99)

New England crystalline (107)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (54)

Columbia Plateau basin fill–basalt rock (NA)

(NA)

(12)

(31)

(NA)

(NA)

(42)

(15)

(94)

(101)

(11)

15)

(31)

(49)

(8)

 (5)

(13)

(1)

(1)

(15)

(65)

(NA)

(NA)

(35)

 (5)

 (8)

 (1)

 (25)

(NA)

 (22)

Arsenic in domestic wells Arsenic in public-supply wells

0.1 1 10 100 1,000 0.1 1 10 100 1,000

Principal Aquifer

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (313)

Alluvial–Pleistocene Terrace Deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (135)

California Coastal Basin (18)

Basin and Range basin fill (129)

Glacial–West (24)

Glacial–West Central (93)

Glacial–Central (187)

Glacial–East (68)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (60)

Mississippi embayment (27)

Valley and Ridge siliciclastic (82)

Early Mesozoic (48)

Denver Basin (75)

Cambrian-Ordovician (69)

Valley and Ridge carbonate (94)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (128)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (152)

New England crystalline (113)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (54)

Columbia Plateau basin fill–basalt rock (NA)

Principal Aquifer Uranium in domestic wells

0.1 1 10 100 1,000 10,000

Uranium in public-supply wells

(NA)

(12)

(31)

(NA)

(NA)

(42)

(15)

(94)

(101)

(11)

(15)

(31)

(49)

(8)

(5)

(13)

(1)

(2)

(15)

(65)

(2)

(NA)

(35)

(5)

(8)

(1)

(25)

0.1 1 10 100 1,000 10,000

 (NA)

(22)

Concentration, in micrograms per liter

Concentration, in micrograms per liter

Concentration, in micrograms per liter

Concentration, in micrograms per liter

112 Water Quality in Basin-Fill Aquifers of the Southwest: Arizona, California, Colorado, Nevada, New Mexico, and Utah, 1993–2009
Appendixes

circ1358_appendix3b

MCL = 10 mg/L

MCL = 10 mg/L

(NA)

(12)

(31)

(NA)

(NA)

(42)

(15)

(94)

(101)

(11)

(28)

(37)

(49)

(8)

(35)

(41)

(1)

(2)

(15)

(81)

(4)

(NA)

(35)

(5)

(5)

(1)

(25)

1 10 100 1,000 10,000 100,000 1 10 100 1,000 10,000 100,000

(12)

(34)

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (311)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (138)

California Coastal Basin (18)

Basin and Range basin fill (131)

Glacial–West (54)

Glacial–West Central (115)

Glacial–Central (208)

Glacial–East (98)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (66)

Mississippi embayment (27)

Valley and Ridge siliciclastic (80)

Early Mesozoic (68)

Denver Basin (75)

Cambrian-Ordovician (71)

Valley and Ridge carbonate (117)

Piedmont and Blue Ridge carbonate (29)

 Upper Floridan (128)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (152)

New England crystalline (113)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (103)

Columbia Plateau basin fill–basalt rock (62)

Principal Aquifer

Total dissolved solids in 

domestic wells

Total dissolved solids in 

public-supply wells

Nitrate and nitrite in domestic wells

Nitrate and nitrite in 

public-supply wells

(NA)

(12)

(31)

(NA)

(NA)

(42)

 (15)

(93)

(101)

(11)

(28)

(37)

(49)

(8)

(35)

(41)

(1)

(2)

(15)

(80)

4)

(NA)

(34)

 (5)

(8)

(1)

(25)

1 10 100 0.01 0.1 1 10 100

(19)

(34)

Principal Aquifer

Surficial (30)

Mississippi River Valley alluvial (5)

High Plains (314)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (7)

Rio Grande (25)

Central Valley (137)

California Coastal Basin (18)

Basin and Range basin fill (123)

Glacial–West (55)

Glacial–West Central (96)

Glacial–Central (210)

Glacial–East (97)

Texas coastal uplands (50)

Northern Atlantic Coastal Plain (66)

Mississippi embayment (27)

Valley and Ridge siliciclastic (81)

Early Mesozoic (69)

Denver Basin (75)

Cambrian-Ordovician (71)

Valley and Ridge carbonate (116)

Piedmont and Blue Ridge carbonate (29)

Upper Floridan (127)

Castle Hayne (6)

Piedmont and Blue Ridge crystalline (151)

New England crystalline (113)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (151)

Columbia Plateau basin fill–basalt rock (63)

Concentration, in milligrams per liter

Concentration, in milligrams per liter

Concentration, in milligrams per liter

Concentration, in milligrams per liter

0.01 0.1

Appendixes 113

Prepared by the USGS Science Publishing Network

Pembroke, Raleigh, and Menlo Park Publishing Service Centers

Edited by Kay P. Naugle, Raleigh

Illustrations by David Jones, Menlo Park

and Christine T. Mendelsohn, Pembroke

Layout by Christine T. Mendelsohn

For more information concerning this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive

Reston, VA 20192

http://water.usgs.gov/nawqa/
ISSN 1067-084X (print)

ISSN 2330-5703 (online)

http://dx.doi.org/10.3133/cir1358

Southwest Principal Aquifers

(from west to east)—

California Coastal Basin aquifers, 

Central Valley aquifer system,

Basin and Range basin-fill aquifers,

and Rio Grande aquifer system

Printed on recycled Paper