U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2014–5173

Prepared in cooperation with the California State Water Resources Control Board

A product of the California Groundwater Ambient Monitoring and Assessment (GAMA) Program

Geochemical Conditions and the Occurrence of Selected Trace Elements 

in Groundwater Basins Used for Public Drinking-Water Supply, Desert and 

Basin and Range Hydrogeologic Provinces, 2006–11: California GAMA

Priority Basin Project

Photo placement
Sampling priority

Study area

Low-use basins

Priority basins

Areas that are outside CDWR-defined

 groundwater basins

Front Cover Map: Groundwater basins categorized by sampling priority. Location of groundwater basin 

boundaries from California Department of Water Resources (CDWR, 2003).

Cover photographs: 

Front cover: Sampling an observation well, Alabama Hills near Lone Pine, California, 2006. (Photograph by 

Michael Wright, U.S. Geological Survey.)

Back cover: Joshua Tree and public supply well, Yucca Valley, California. (Photograph by Dara Goldrath, 

U.S. Geological Survey.)
Geochemical Conditions and the Occurrence 

of Selected Trace Elements in Groundwater 

Basins Used for Public Drinking-Water 

Supply, Desert and Basin and Range 

Hydrogeologic Provinces, 2006–11: 

California GAMA Priority Basin Project

By Michael T. Wright, Miranda S. Fram, and Kenneth Belitz

A Product of the California Groundwater Ambient Monitoring and Assessment 

(GAMA) Program

Prepared in cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2014–5173

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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

Suggested citation:

Wright, M.T., Fram, M.S., and Belitz, Kenneth, 2015, Geochemical conditions and the occurrence of selected trace 

elements in groundwater basins used for public drinking-water supply, Desert and Basin and Range hydrogeologic 

provinces, 2006–11—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations 

Report 2014–5173, 48 p., http://dx.doi.org/10.3133/sir20145173.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978-1-4113-3869-2
iii

Contents

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

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

Previous GAMA Work in the Desert and Basin and Range Hydrogeologic Provinces.............3

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

Description of the Study Unit and Hydrogeology .....................................................................................4

Description of Wells Sampled .....................................................................................................................6

Methods...........................................................................................................................................................6

Sample Collection and Analysis.........................................................................................................6

Groundwater Age Determination.......................................................................................................8

Redox Classification.............................................................................................................................8

Aqueous Speciation and Saturation State .......................................................................................8

Statistical Analysis ...............................................................................................................................9

Geochemical Conditions of the Study Unit................................................................................................9

Groundwater Age..................................................................................................................................9

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

Major Ion Content...............................................................................................................................16

Occurrence of Selected Trace Elements.................................................................................................18

Sources of Trace Elements to Groundwater..................................................................................18

Relation to Groundwater Age ...........................................................................................................18

Relation to Redox and pH ..................................................................................................................20

Arsenic.........................................................................................................................................20

Vanadium, Molybdenum, and Fluoride...................................................................................22

Uranium .......................................................................................................................................22

Boron and Strontium .................................................................................................................23

Relation to Major Ion and Phosphate Content...............................................................................23

Effects of Complexation and Mineral Precipitation on the Distribution of 

Molybdenum and Fluoride ..........................................................................................23

Effects of Complexation and Mineral Precipitation on the Distribution of Uranium ......24

Relation of Total Dissolved Solids and Trace Elements................................................................27

Conclusions...................................................................................................................................................27

Acknowledgments.......................................................................................................................................28

References....................................................................................................................................................28
iv

Figures

1. Map showing geographic features, boundaries, and well locations of the Desert 

and Basin and Range hydrogeologic provinces and the nine individual study areas, 

California GAMA Priority Basin Project....................................................................................5

2. Graph showing depths to the uppermost and lowermost well openings for wells 

sampled in each study area, 2006–11, Desert and Basin and Range study unit, 

California GAMA Priority Basin Project....................................................................................7

3. Map showing distribution of groundwater ages in the Desert and Basin 

and Range study unit, California GAMA Priority Basin Project, and graph 

showing groundwater age categories for the overall study unit, for each 

study area, and for the categories of ≤ 212 and >212 feet median 

depth to the top of the uppermost interval .............................................................................10

4. Map showing distribution of redox classes in the Desert and Basin and Range 

study unit, California GAMA Priority Basin Project, and graph showing Redox 

classes organized by study unit, study area, and depth to the top of the 

uppermost interval......................................................................................................................12

5. Map showing distribution of pH values in the Desert and Basin and Range 

study unit, California GAMA Priority Basin Project, and graph showing pH 

values organized by study unit, study area, and median depth to the top of the 

uppermost interval......................................................................................................................14

6. Piper diagrams showing major ion content of samples categorized by study 

area in the Desert and Basin and Range study unit: Borrego Valley, Central 

Desert, Coachella Valley, Colorado River, and Low-Use Basins, and 

Owens Valley, Indian Wells Valley, Antelope Valley, and Mojave......................................17

7. Graphs showing relations of trace element concentrations to (uncorrected) 

groundwater age, Desert and Basin and Range study unit, California GAMA 

Priority Basin Project.................................................................................................................19

8. Graphs showing distribution of selected trace element concentrations with 

respect to redox classification and pH, Desert and Basin and Range study unit, 

California GAMA Priority Basin Project..................................................................................21

9. Graphs showing factors affecting the solubility of fluoride in groundwater: 

Relation of fluoride concentrations to saturation indices of fluorapatite, relation 

of pH to the saturation indices of fluorapatite and hydroxylapatite, distribution of 

calcium in samples with regard to pH, and distribution of phosphate in samples 

with regard to pH, Desert and Basin and Range study unit, California GAMA 

Priority Basin Project.................................................................................................................25

10. Graphs showing factors affecting the solubility of uranium in groundwater: 

Relation of the percentage of total uranium that exists as Ca2UO2(CO3)

3

 and uranium 

concentrations in samples, and relation of pH to the saturation index of calcite, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project..........26
v

Tables

1. Identification and construction information for wells sampled in the Desert 

and Basin and Range hydrogeologic provinces, California GAMA Priority Basin 

Project, 2006–11 ..........................................................................................................................35

2. Fluoride, molybdenum, and uranium reactions added to the minteqV4.dat database 

that is distributed with PHREEQC version 2 computer program.........................................38

3. Geochemical conditions determined from the analyses of samples collected at 

well sites in the each of the nine study areas, Desert and Basin and Range study unit, 

California GAMA Priority Basin Project, 2006–11..................................................................39

4. Results of non-parametric analysis of correlations between selected potential 

explanatory variables, Desert and Basin and Range study unit, California GAMA 

Priority Basin Project.................................................................................................................45

5. Summary concentration statistics for trace elements and minor ions presented in 

this report, California GAMA Priority Basin Project .............................................................46

6. Results of non-parametric analysis of correlations between water-quality 

constituents and potential explanatory factors, Desert and Basin and Range 

hydrogeologic provinces, California GAMA Priority Basin Project ...................................46

7. Aqueous speciation predicted by speciation modeling (PHREEQC) in groundwater 

and summary statistics for trace element concentrations, Desert and Basin and 

Range hydrogeologic provinces, California GAMA Priority Basin Project.......................47

Conversion Factors and Datum

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2)

Flow rate

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

SI to Inch/Pound

Multiply By To obtain

Volume

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

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

liter (L) 61.02 cubic inch (in3)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

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

°F=(1.8×°C)+32.

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

°C=(°F–32)/1.8.

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

Abbreviations and Acronyms

bls below land surface 

BP before present

DBR Desert and Basin and Range (hydrogeologic provinces and study unit)

GAMA Groundwater Ambient Monitoring and Assessment Program (USGS)

HAL-US U.S. Environmental Protection Agency lifetime health advisory level 

I ionic strength

MCL-CA California Department of Public Health maximum contaminant level 

MCL-US U.S. Environmental Protection Agency maximum contaminant level 

min minute

NL-CA California Department of Public Health notification level 

PBP Priority Basin Project (USGS GAMA)

pmc percent modern carbon

SI saturation indices

SM speciation modeling 

μm micrometer

> greater than

< less than

≤ less than or equal to 

≥ greater than or equal to

Conversion Factors and Datum—Continued
vii

Organizations

CDPH California Department of Public Health (Department of Health Services prior to 

July 1, 2007)

CDPR California Department of Pesticide Regulation

CDWR California Department of Water Resources

LLNL Lawrence Livermore National Laboratory

NWQL National Water Quality Laboratory (USGS)

SWRCB State Water Resources Control Board (California)

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Selected chemical symbols or abbreviations

3

H tritium

14C carbon-14

As arsenic

B boron

F fluoride

Fe iron

Mo molybdenum

S sulfur

Sr strontium

U uranium

V vanadium

TDS total dissolved solids
viii
Geochemical Conditions and the Occurrence of Selected 

Trace Elements in Groundwater Basins Used for Public 

Drinking-Water Supply, Desert and Basin and Range 

Hydrogeologic Provinces, 2006–11: California GAMA 

Priority Basin Project

By Michael T. Wright, Miranda S. Fram, and Kenneth Belitz

Abstract

The geochemical conditions, occurrence of selected

trace elements, and processes controlling the occurrence of

selected trace elements in groundwater were investigated in

groundwater basins of the Desert and Basin and Range (DBR)

hydrogeologic provinces in southeastern California as part of

the Priority Basin Project (PBP) of the Groundwater Ambient

Monitoring and Assessment (GAMA) Program. The GAMA

PBP is designed to provide an assessment of the quality of

untreated (raw) groundwater in the aquifer systems that are

used for public drinking-water supply. The GAMA PBP is

being conducted by the California State Water Resources

Control Board in collaboration with the U.S. Geological

Survey and the Lawrence Livermore National Laboratory.

The DBR hydrogeologic provinces consist of 141 defined

groundwater basins separated by mountain ranges, faults,

and other features. This report presents analyses of data

collected from nine study areas within the DBR hydrogeologic

provinces: Antelope Valley, Borrego Valley, the Central Desert

area, Coachella Valley, Colorado River, Indian Wells Valley,

Low-Use Basins of the Mojave and Sonoran Deserts, the

Mojave, and Owens Valley. Collectively, these nine study

areas are referred to as the DBR study unit. The study unit

covers approximately 7,000 square miles and includes the

63 groundwater basins in the DBR hydrogeologic provinces in

which groundwater is used for public drinking-water supply.

The vast majority of the 223 wells sampled for this study

were long-screened production wells used primarily for public

supply.

Uncorrected carbon-14 (

14C) groundwater ages for

samples collected in the DBR study unit ranged from less than

(<) 100 to 33,700 years before present (BP). Sixty-six percent

of sample ages were greater than (>) 100 years BP, and

40 percent were >3,800 years BP. Samples collected from wells

located adjacent to mountain-front recharge areas or major

surface-water features generally had younger groundwater

ages than did samples collected from wells located away

from mountain fronts or towards the distal ends of basin

groundwater flow paths. Most groundwater sampled in the

DBR study unit had alkaline pH: 89 percent of sample pH

values ranged from 7.1 to 9.8, with 37 percent greater than or

equal to (≥) 7.9. Groundwater age was significantly correlated

(positively) with pH, likely because silicate weathering is a

primary control on groundwater pH and is a slow process.

The oxidation-reduction (redox) condition of the groundwater

sampled in the DBR study unit was predominantly oxic

(71 percent), except in the Colorado River study area where

organic-rich fluvial aquifers provide the electron donors

necessary to support iron-reducing (anoxic-Fe) redox

processes. The cation type of 78 percent of the samples was

either sodium- or mixed-type, and the anion type of 83 percent

of the samples was either bicarbonate- or mixed-type. Sodiumtype groundwaters generally were older and more alkaline

than calcium-type groundwaters, consistent with the change

in water chemistry expected from cation exchange between

groundwater and aquifer sediments over long periods of time.

Because of the correlation with young groundwater, calciumtype groundwater was predominantly from wells located

adjacent to mountain-front recharge areas.

Arsenic (As), boron (B), fluoride (F), molybdenum

(Mo), strontium (Sr), uranium (U), and vanadium (V) were

selected for assessment in this study because they occurred at

concentrations greater than California Department of Public

Health or U.S. Environmental Protection Agency regulatory

or non-regulatory drinking-water-quality benchmarks in

more than 2 percent of the 223 samples collected in the

DBR study unit. As and F were detected most commonly

(18 and 13 percent, respectively) at concentrations above

associated water-quality benchmarks and Sr and V least

frequently (both at 3 percent). Given that 14C groundwater

ages are predominantly >100 years BP, land use in the study

unit is primarily undeveloped 

anthropogenic sources, such as volatile organic compounds,
2 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

were infrequently detected, high concentrations of these trace

elements in groundwater were most likely the result of natural

factors and not anthropogenic factors.

As, F, Mo, and V concentrations showed significant

positive correlations to groundwater age and to pH. This

relation is partly due to the sources of trace elements likely

being the weathering of primary minerals, such as silicate

minerals, which is a slow process that takes place over

hundreds to thousands of years. This relation also reflects

the positive correlation between groundwater age and pH.

Geochemical modeling predicted that the dominant species

of As, Mo, and V in solution were oxyanions (HAsO4

2– ,

MoO4

2–, and H2

VO4–), which are likely to be mobile in alkaline

groundwater because mineral surfaces composing aquifer

matrices have a predominantly negative surface charge

under alkaline conditions. F also exists predominantly as a

negatively charged ion (F–). At pH values >7.5, saturation

indices generated by the geochemical modeling program

PHREEQC indicated that F solubility may be somewhat

limited by the precipitation of the mineral fluorapatite

[Ca5(PO4)

3

F]. Speciation modeling of As in anoxic-Fe

groundwater (iron-reducing conditions) showed that samples

were supersaturated with orpiment (As2S3), indicating

that mineral precipitation may be responsible for low As

concentrations observed in reducing groundwater.

In contrast, U concentrations showed significant

negative correlations to groundwater age and to pH. Higher

U concentrations generally occurred in samples for which

geochemical modeling indicated that the uncharged ternary

complex Ca2UO2(CO3)3 was the dominant aqueous U species.

This uncharged complex is not attracted to the charged surfaces

of minerals and thus increases U solubility. Formation of

Ca2UO2(CO3)3 was greater in younger groundwaters because

calcium and uranium concentrations generally were lower in

older groundwaters, likely due to cation-exchange processes

and precipitation of the mineral calcite as groundwater pH

increased. Co-precipitation of U with the calcite (CaCO3) may

remove U from the aqueous phase. Saturation indices indicated

that the anoxic-Fe groundwaters from the Colorado River study

area were supersaturated with the mineral uraninite (UO2),

suggesting that UO2 precipitation may be responsible for the

low concentrations of U observed in these samples.

Concentrations of strontium, which exists primarily in

a cationic form (Sr2+), were not significantly correlated with

either groundwater age or pH. Strontium concentrations

showed a strong positive correlation with total dissolved solids

(TDS). Dissolved constituents, such as Sr, that interact with

mineral surfaces through outer-sphere complexation become

increasingly soluble with increasing TDS concentrations of

groundwater. Boron concentrations also showed a significant

positive correlation with TDS, indicating the B may interact

to a large degree with mineral surfaces through outer-sphere

complexation.

Introduction

To assess the quality of ambient groundwater in aquifers

used for drinking-water supply, and to establish a baseline

groundwater-quality monitoring program, the California State

Water Resources Control Board (SWRCB), in collaboration

with the U.S. Geological Survey (USGS) and Lawrence

Livermore National Laboratory (LLNL), implemented the

Groundwater Ambient Monitoring and Assessment (GAMA)

Program (California State Water Resources Control Board,

2003, website at http://www.waterboards.ca.gov/gama/).

The statewide GAMA Program currently consists of four

projects: the (1) GAMA Priority Basin Project (PBP),

conducted by the USGS (U.S. Geological Survey, 2013, website

at http://ca.water.usgs.gov/gama/); (2) the GAMA Domestic

Well Project, conducted by the SWRCB; (3) the GAMA Special

Studies, conducted by LLNL; and (4) the GeoTracker GAMA

publicly available database, maintained by the SWRCB.

The SWRCB initiated the GAMA Program in 2000 in

response to Legislative mandates (State of California, 1999,

2001a, Supplemental Report of the 1999 Budget Act 1999–00

Fiscal Year). The GAMA PBP was initiated in response to

the Groundwater Quality Monitoring Act of 2001 (State of

California, 2001b, Sections 10780–10782.3 of the California

Water Code, Assembly Bill 599) to assess and monitor

the quality of groundwater in California. The GAMA PBP

is a comprehensive assessment of statewide groundwater

quality, designed to help better understand and identify risks

to groundwater resources and to increase the availability of

information about groundwater quality to the public. For the

PBP, the USGS, in collaboration with the SWRCB, developed

a monitoring plan to assess groundwater basins through

direct sampling of groundwater and other statistically reliable

sampling approaches (Belitz and others, 2003; California State

Water Resources Control Board, 2003). Additional partners

in the GAMA PBP include the California Department of

Public Health (CDPH), the California Department of Pesticide

Regulation (CDPR), the California Department of Water

Resources (CDWR), and local water agencies and well owners

(Kulongoski and Belitz, 2004). Note that as of July 1, 2014,

the Drinking Water Program administrated by the CDPH was

transferred to the SWRCB.

The range of hydrologic, geologic, and climatic

conditions that exist in California were considered in this

statewide assessment of groundwater quality. Belitz and

others (2003) partitioned the State into 10 hydrogeologic

provinces, each with distinctive hydrologic, geologic, and

climatic characteristics. These hydrogeologic provinces

include groundwater basins and subbasins designated by the

CDWR (California Department of Water Resources, 2003).

Groundwater basins and subbasins throughout California

generally consist of relatively permeable, unconsolidated
Introduction 3

deposits of alluvial origin. Groundwater basins were prioritized

for sampling on the basis of the number of public-supply wells,

with secondary consideration given to municipal groundwater

use, agricultural pumping, the number of historically leaking

underground fuel tanks, and registered pesticide applications

(Belitz and others, 2003). Of the 472 CDWR-designated

basins, 116 contain approximately 95 percent of the publicsupply wells in basins and were defined as priority basins.

These 116 priority basins, a subset of the remaining low-use

basins, and additional areas outside defined groundwater basins

were grouped into 35 study units. The 6 study units located

within the boundaries of the Desert and the Basin and Range

(DBR) hydrogeologic provinces include 15 priority basins and

37 low-use basins and collectively form the Desert and Basin

and Range (DBR) study unit.

Previous GAMA Work in the Desert and Basin 

and Range Hydrogeologic Provinces

USGS Data Series reports have been published for each

of the six study units located within the DBR hydrogeologic

provinces: Owens and Indian Wells Valleys (Densmore

and others, 2009), Coachella Valley (Goldrath and others,

2009), Mojave (Mathany and Belitz, 2009), Antelope Valley

(Schmitt and others, 2009), Colorado River (Goldrath and

others, 2010), and Borrego Valley, Central Desert, and LowUse Basins of the Mojave and Sonoran Deserts (Mathany

and others, 2012). In addition to reporting all of the data

collected for the study, Data Series reports also describe

the hydrogeologic setting, study design, sample collection

procedures, and results of quality-assurance and qualitycontrol analyses. A total of 497 wells were sampled for the

6 study units; of these, samples from 223 wells were analyzed

for the inorganic constituents presented in this report.

Two USGS Scientific Investigations Reports were

published (Dawson and Belitz, 2012; Parsons and others,

2014) to describe the current status of groundwater quality

in the six study units. GAMA status assessments are

designed to provide a statistically robust characterization

of groundwater quality in the primary aquifer system at the

basin-scale (Belitz and others, 2003). The primary aquifer

systems in the six study units are defined as that part of the

aquifer corresponding to the depth of the open intervals of

public-supply wells listed in the California Department of

Public Health’s water-quality database. The status assessment 

was also designed to identify the constituents and classes

of constituents that may be of concern for drinking-water

quality because of their high concentrations and (or) their

prevalence. Concentrations of constituents were compared to

concentrations of CDPH and U.S. Environmental Protection

Agency (USEPA) regulatory and non-regulatory benchmarks

used for drinking-water quality. Concentrations above the

comparison benchmark were defined as “high.” GAMA

status assessments included calculating the areal proportion

of the primary aquifer system having high concentrations

of an individual constituent or of one or more constituents

in a constituent class. Constituent classes assessed for the

GAMA PBP included volatile organic compounds, pesticides,

trace elements, nutrients, and radioactive constituents. In

all six DBR study units, trace elements were present at high

concentrations in a greater proportion of the primary aquifer

system than any other constituent class. Trace elements with

health-based drinking-water-quality benchmarks that were

most frequently detected at high concentrations were arsenic

(As), boron (B), fluoride (F), molybdenum (Mo), strontium

(Sr), uranium (U), and vanadium (V) (Dawson and Belitz,

2012; Parsons and others, 2014).

Purpose and Scope

The purpose of this report is to (1) identify the

geochemical conditions present in groundwater sampled in

the six study areas within the DBR study unit, (2) examine the

distribution of arsenic, boron, fluoride (hereinafter classified

as a trace element), molybdenum, strontium, uranium, and

vanadium with respect to varying geochemical conditions

of redox, pH, and major ion content and groundwater age,

and (3) identify the processes that control the concentrations

or solubility of these trace elements as they relate to the

geochemical conditions of groundwater.

A rigorous identification of trace element sources

in groundwater is beyond the scope of this report, and

only a brief discussion of sources is provided. The DBR

hydrogeologic provinces are geologically complex, and the

mineralogical and geochemical compositions of the sediments

composing the fill of the groundwater basins investigated

in this study likely vary considerably across the provinces.

This heterogeneity in the potential sources of trace elements

to groundwaters may obscure identification of processes

controlling the distribution of trace elements in groundwater.

However, the large number (n = 223) of samples analyzed

in this report, coupled with the spatial distribution of wells

sampled, should help to mitigate uncertainty attributable

to heterogeneity in source material. The groundwater wells

sampled in this study were primarily production wells with

long open intervals (table 1). Samples collected from wells

with long open intervals can be a mixture of groundwater

from different aquifer zones that have different geochemical

conditions. Therefore, the geochemical conditions, trace

element distribution, and processes controlling trace element

distribution reported here should be considered a composite of

what is occurring in the primary aquifer systems sampled in

the DBR hydrogeologic provinces.
4 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Description of the Study Unit and 

Hydrogeology

The DBR hydrogeologic provinces cover approximately

45,000 square miles (mi2) in southeastern California (fig. 1).

The DBR provinces are bounded on the east by the Colorado

River, on the northeast by the State of Nevada, on the south by

Mexico, and on the west by crystalline rocks of five mountain

ranges. From north to south, these mountain ranges are the

Sierra Nevada, the San Gabriel Mountains, the San Bernardino

Mountains, the San Jacinto Mountains, and Peninsular

Ranges of Southern California. The boundary between the

Desert and the Basin and Range hydrogeologic provinces is

defined by groundwater basin boundaries that approximate the

Garlock Fault.

The DBR study unit was defined as the 63 groundwater

basins within the DBR hydrogeologic provinces that contained

wells listed in the CDPH’s database of public-supply wells as

of 2003 (Belitz and others, 2003). The DBR study unit covers

approximately 7,000 mi2 and was divided into nine study areas

for this report: Owens Valley, Indian Wells Valley, Antelope

Valley, Mojave, Central Desert, Low-Use Basins, Colorado

River, Coachella Valley, and Borrego Valley (fig. 1). The

following paragraphs consist of brief descriptions of each of

the nine study areas in the DBR study unit. More complete

descriptions can be found in Densmore and others (2009)

(Owens and Indian Wells Valleys), Goldrath and others (2009)

(Coachella Valley), Mathany and Belitz (2009) (Mojave River

Valley), Schmitt and others (2009) (Antelope Valley), Goldrath

and others (2010) (Colorado River basins), and Mathany and

others (2012) (Borrego, Central Desert, and Low-Use Basins).

The northern Owens Valley is located in the Great Basin

Desert, while the southern Owens Valley, Indian Wells Valley,

Antelope Valley, Mojave, Central Desert, and the northern

parts of the Low-Use Basins and Colorado River study areas

are located in the Mojave Desert (Duell, 1987; Berenbrock

and Martin, 1991; Putnam and Smith, 1995, p. 4; Izbicki and

others, 2000). The Coachella Valley, Borrego Valley, and

southern parts of the Low-Use Basins and Colorado River

study areas are located in the Sonoran Desert, in the part

locally known as the Colorado Desert (Phillips and Comus,

2000, p. 14). The climate in the DBR study unit varies from

semi-arid to hyper-arid, with average precipitation ranging

from approximately 8 inches per year in the Great Basin

Desert to less than 1 inch per year in the Sonoran Desert

(PRISM Group, Oregon State University, 2007). Because it is

located at higher elevations than the other deserts, the Great

Basin Desert has relatively cold winters. Precipitation in the

Great Basin Desert falls infrequently throughout the year,

with snow in winter and rain in summer. The Mojave Desert

has hot summers and cold winters. Precipitation here falls

mostly in the winter, though summer thunderstorms provide

moisture east of the 117o W longitude (Hereford and others,

2004). The Colorado Desert is located at lower elevations

in southern California and has hot, dry summers and mild

winters. Most of the precipitation occurs in the winter in the

form of rain. Except for the Colorado River study area, all the

study areas are closed groundwater basins. Stream and river

flow originates in the surrounding mountains or along valley

edges and discharges into dry lake beds or the Salton Sea. The

major rivers that drain internally are the Owens and Mojave

Rivers, which drain into Owens Lake and Soda Dry Lake,

respectively, and the Whitewater, New, and Alamo Rivers,

which drain into the Salton Sea.

The DBR study unit is predominantly composed of

undeveloped, natural landscape (Dawson and Belitz, 2012;

Parsons and others, 2014). Areas of intensive agricultural

operations are made possible in part by the conveyance of

surface water from the Colorado River and northern California

through a network of canals. The largest urban centers in the

DBR study unit are located along the mountain fronts of the

San Gabriel, San Jacinto, and San Bernardino Mountains and

the Peninsular Ranges of Southern California.

Groundwater basins in the DBR hydrogeologic provinces

are underlain by granitic, metamorphic, sedimentary, and

volcanic rocks (Hollet and others, 1991; Berenbrock and

Schroeder, 1994; Danskin, 1998; Stonestrom and Harrill,

2007). The basins primarily are filled with sediments derived

from erosion of the surrounding mountains, in particular the

Sierra Nevada, San Gabriel, San Bernardino, San Jacinto, and

Southern California Peninsular mountain ranges that form

the western boundary of the provinces. These high-relief

mountains are composed mainly of Mesozoic granite and

associated metamorphic rocks (Jennings and others, 1977). In

the eastern parts of the provinces, the geology is much more

varied, with groundwater basins that are bounded by relatively

low-relief mountains composed of Quaternary, Tertiary, and

Mesozoic volcanic rock, Mesozoic granitic rock, and Paleozoic

marine sedimentary and metasedimentary rocks, among others.

Groundwater recharge in the DBR hydrologic provinces

primarily comes from mountain-front recharge and infiltration

of surface water from rivers and ephemeral streams as they

flow from the mountains towards the centers of the basins

(Tyley, 1974; Berenbrock and Schroeder, 1994; Danskin,

1998; Leighton and Phillips, 2003; California Department of

Water Resources, 2004; U.S. Geological Survey, 2009). The

exception is in the northern and central area of the Colorado

River study area where groundwater recharge comes primarily

from the Colorado River (Metzger and Loeltz, 1973). In

some basins, irrigation return flows, engineered recharge

of surface water imported from northern California and the

Colorado River, and leakage from surface-water canals,

wastewater treatment plants, and septic systems are sources

of groundwater recharge. Historically, groundwater discharge

occurred through evapotranspiration and underflow to dry lake

beds. Groundwater pumping is now the primary mechanism

of groundwater discharge in many of the basins (Metzger

and Loeltz, 1973; Tyley, 1974; Mitten and others, 1988;

Berenbrock and Schroeder, 1994; Danskin, 1998; Nishikawa

and others, 2001, 2003, 2005; Stamos and others, 2001;

California Department of Water Resources, 2003).
Description of the Study Unit and Hydrogeology 5

Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping,

Aerogrid, IGN, IGP, and the GIS User Community

IP037705_Figure 01

Pacific Ocean

120° 116°

38°

34°

S i e r r a N e v a d a

MEXICO

U.S.

ARIZONA

NEVADA

CALIFORNIA

San Bernardino Mts

San Jacinto Mts

San Gabriel Mts

Peninsular Ranges

EXPLANATION

Provinces

Mojave

 Desert

Study Areas

Antelope Valley

Borrego Valley

Central Desert

Coachella Valley

Colorado River

Indian Wells Valley

Low-Use Basins

Mojave

Owens Valley

Sampled

 wells

Rivers

Playa

0 50 100 Miles

0 50 100 Kilometers

Provinces from Belitz and others, 2003

Alamo River

Whitewater River

Mojave

Owens River

New

River

Colorado River

Salton Sea

Owens

Dry

Lake

Soda

Dry

Lake

Desert

Basin and Range

 River

Sonoran

 Desert

Garlock fault zone

Basin

and

Range

Klamath

Mountains

Desert

Cascades

and Modoc

Plateau

Southern 

Coast

Ranges

Central Valley

Sierra Nevada

Northern

Coast

Ranges

San Diego

Drainages

Transverse Ranges and

selected Peninsular Ranges

OREGON

NEVADA

MEXICOARIZONA

Pacific Ocean

Figure 1. Geographic features, boundaries, and well locations of the Desert and Basin and Range hydrogeologic provinces and the 

nine individual study areas, California GAMA Priority Basin Project.
6 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Description of Wells Sampled

Groundwater samples were collected from

223 production and monitoring wells in the DBR study unit

(table 1). Eighty-seven percent of the wells sampled were

classified as production wells and 13 percent as monitoring

wells. The majority of production wells were being used for

public drinking-water supply (70 percent), while smaller

percentages of the production wells sampled were being used

for irrigation (13 percent), domestic supply (11 percent), or

commercial, industrial, or recreational purposes (6 percent).

The production wells that were not public-supply wells

generally had openings in the same depth interval as nearby

public-supply wells. The monitoring wells generally had

openings in shallower parts of the aquifer system. An

additional 82 wells were sampled by the GAMA-PBP in these

study areas, but were not included in this study because data

for trace elements were not collected for those wells.

Differences in well depths between study areas were

characterized by comparing the median depths to the tops and

bottoms of the perforated intervals for each study area (fig. 2).

The median depth to the top of the uppermost opening for

wells sampled in the DBR study unit ranged from 118 feet

(ft) below land surface (bls) in the Owens Valley study area to

430 ft bls in the Mojave study area. The median depth to the

lowermost well opening ranged from 226 ft bls in the Owens

Valley study area to 652 ft bls in the Coachella study area. The

median length of open intervals ranged from 54 ft in the Owens

Valley study area to 306 ft in the Coachella Valley study area.

Methods

Sample Collection and Analysis

Prior to sampling, each well was pumped continuously

to purge at least three casing-volumes of water from the well

(U.S. Geological Survey, 2006). Sampling points were located

as close to the well head as possible and were upstream

from water-storage tanks and well-head treatment systems.

Most samples were collected inside an enclosed chamber

in a mobile laboratory and connected to the well head by a

10–50-ft length of Teflon® tubing, and some were collected at

the well head using a 1-ft length of Teflon® tubing (Lane and

others, 2003).

Field measurements of dissolved oxygen, temperature,

pH, and specific conductance were collected using a

flow-through chamber fitted with a multi-probe meter

that simultaneously measures all parameters. Field

measurements were made in accordance with protocols

in the U.S. Geological Survey National Field Manual

(U.S. Geological Survey, variously dated). Field

measurements were recorded at 5-minute (min) intervals

for at least 30 min, and when these values remained stable

for 20 min, samples were collected. For a subset of samples

collected in the DBR study unit, if a sulfide odor was detected,

then a sulfide measurement was done onsite using a portable

colorimeter (Eaton and Franson, 2005).

Detailed sampling protocols are described in Koterba

and others (1995) and the U.S. Geological Survey’s National

Field Manual (U.S. Geological Survey, variously dated);

therefore, only brief descriptions are given here. Before

sample collection, all polyethylene sample bottles were

rinsed twice with deionized water and then rinsed once with

groundwater. Samples were filtered using a 0.45-micrometer

(μm) pore-size capsule filter that was rinsed with 2 liters (L)

of deionized water and then rinsed with 1 L of groundwater

prior to sample collection. Trace element and major cation

samples were filtered and preserved to pH <2 with certified,

traceable 7.5-Normal (N) nitric acid. Major anions, total

dissolved solids (TDS; residue on evaporation), and nutrient

samples were filtered, but not preserved with acid; nutrient

samples were stored in brown polyethylene bottles. Stable

isotopes of carbon in dissolved inorganic carbon and

carbon-14 abundance samples were filtered and bottom-filled

into 500-milliliter (mL) glass bottles that first were overfilled

with three bottle volumes of groundwater. These samples

had no headspace and were sealed with conical caps to avoid

atmospheric contamination. All samples were stored and

shipped cold after collection.

Trace element, major ion, and nutrient samples were

analyzed at the USGS National Water Quality Laboratory

(NWQL) in Denver, Colorado, and stable isotopes of carbon in

dissolved inorganic carbon and carbon-14 abundance samples

were analyzed at the Woods Hole Oceanographic Institution,

National Ocean Sciences Accelerator Mass Spectrometry

Facility in Woods Hole, Massachusetts (MA-WHAMS) under

a contract with the NWQL. Iron speciation samples were

analyzed at the USGS National Research Program Trace Metal

Laboratory in Boulder, Colorado.

Data used in this study are available from the six

USGS Data Series reports referenced in the section

“Previous GAMA Work in the Desert and Basin and Range

Hydrogeologic Provinces,” from the USGS National Water

Information System online database (U.S. Geological Survey,

http://waterdata.usgs.gov/ca/nwis/), and from the SWRCB’s

GeoTracker GAMA online database (California State Water

Resources Control Board, 2009, website at https://geotracker.

waterboards.ca.gov/gama).
Methods 7

Antelope

Valley

Borrego

Valley

Central

Desert

Coachella

Valley

Colorado

River

Indian Wells

Valley

Low-Use

Basins

Mojave Owens

Valley

Study Area

1,000

1,200

1,400

800

600

400

200

0

Depth, in feet below land surface

15

15

3

3

5

5

24

24

17

14

15

15

22

21

17

16

43

42

IP037705_Figure 02_well depths

Top of the

uppermost

opening

Bottom of the

lowermost

opening

26 Number of wells

 with data

75th percentile

Median

25th percentile

Data point that is

 outside of 1.5 × IQR

 of the 25th percentile

Data point that is

 outside of 1.5 × IQR

 of the 75th percentile

EXPLANATION

Nearest data point

 that is within 1.5 × IQR

 of the 75th percentile

Nearest data point

 that is within 1.5 × IQR

 of the 25th percentile

Interquartile

range (IQR)

Figure 2. Depths to the uppermost and lowermost well openings for wells sampled in each study area, 2006–11, Desert and Basin and 

Range study unit, California GAMA Priority Basin Project.
8 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Groundwater Age Determination

Carbon-14 (

14C) chronology was used to determine

the approximate groundwater ages of the samples collected

in this study. Correlating age with concentrations of trace

elements in groundwater can be useful in helping to determine

geochemical processes that affect trace element solubility. The

age dating method relies on evaluating the radiocarbon (

14C)

content of dissolved inorganic carbon species in groundwater. 14C is formed in the atmosphere by the interaction of cosmicray neutrons with atmospheric gases, is incorporated into

carbon dioxide (CO2), and then is assimilated into living

organisms by uptake of CO2 in photosynthesis. Decay of

organic material and root respiration releases CO2 in soils and

in the unsaturated zone, and this CO2 dissolves in groundwater

during recharge. The 14C age (residence time) is calculated

based on the decrease in 14C activity as a result of radioactive

decay with time since groundwater recharge, relative to an

assumed initial 14C concentration using the following equation

(Clark and Fritz, 1997):

t

t

A

A

meas

o

= − ×

1

2

ln 2

ln( )

(1)

where Ameas is the measured 14C activity in groundwater in

percent modern carbon (pmc),

t

1⁄2 is the half-life of 14C, 5,730 years,

A0 is the initial 14C activity (84 pmc for this

study), and

t is the 14C age.

A0 refers to the 14C activity of groundwater entering

the saturated zone and is generally less than 100 pmc under

natural conditions because of reactions between infiltrating

groundwater and carbonate minerals in the unsaturated

zone that contain older carbon (and hence, have lower 14C

activities). A0 under natural conditions cannot generally be

directly measured because atmospheric testing of nuclear

weapons increased 14C production, raising the 14C activity of

groundwater recharged since 1952 above natural levels. The

initial 14C activity used in this study is based on a previous

study that examined the ages of groundwaters in the western

Mojave Desert (Izbicki and Michel, 2004). In that study,

A0 was empirically defined as the highest Ameas in tritium

“dead” groundwater samples. However, calculated 14C ages

presented here are still considered “uncorrected” because

they have not been adjusted to account for dilution by older

CO2 produced by oxidation of sedimentary organic carbon.

Therefore, groundwater ages presented in this report should

not be thought of as absolute ages, but rather as approximate

ages, which were instructive for making comparisons of

groundwater geochemistry in samples collected at local

and regional scales. Solely for the purpose of discussing

distribution in the DBR study unit, groundwater ages were

categorized into four groups based on Jenks Natural Breaks

(Jenks and Caspall, 1971). A fifth category, <100 years before

present (BP), was added to show where modern groundwater

recharge is occurring in the DBR study unit. The age

categories are as follows: <100 years BP; 100–3,800 years

BP; 3,801–11,000 years BP; 11,001–20,800 years BP; and

28,801–33,700 years BP.

Redox Classification

Geochemical conditions investigated as potential

explanatory variables in this report include oxidationreduction (redox) characteristics. Microorganisms can affect

the redox conditions of groundwater by utilizing terminal

electron acceptors during the degradation of organic carbon.

A terminal electron acceptor is a compound that receives

or accepts an electron during oxidation of a carbon source.

The order of terminal electron acceptor utilization is O2 >

NO3

– > Mn4+ > Fe3+ > SO42– > CO2 (Drever, 1997). With the

successive utilization and subsequent depletion of terminal

electron acceptors, the redox condition of groundwater

progresses from oxidizing (positive Eh values) to reducing

(negative Eh values). Oxidation-reduction (redox) conditions

affect the mobility of many inorganic constituents, and thus

redox class was considered an important potential explanatory

factor.

Classification of redox conditions was done using a

modification of the framework of McMahon and Chapelle

(2008) and an automated Excel® workbook developed by

Jurgens and others (2009). Groundwater samples were classified

as oxic, anoxic, anoxic-Fe, or mixed based on dissolved redox

indicator species. Samples were classified as oxic if dissolved

O2 > 0.5 milligram per liter (mg/L) and total dissolved Fe

≤ 100 micrograms per liter (μg/L); anoxic if O2 ≤ 0.5 mg/L and

total dissolved Fe ≤ 100 μg/L; anoxic-Fe if O2 ≤ 0.5 mg/L and

total dissolved Fe >100 μg/L; and mixed if O2 > 0.5 mg/L and

total dissolved Fe > 100 μg/L. These classifications were used

because all redox-sensitive elements presented in this report,

except for V, are thermodynamically predicted to be reduced

from their more oxidized states to their more reduced states at

redox potentials low enough for iron reduction to occur (Turner

and others, 1981; Wang, 2007). Samples with a mixed redox

classification (O2 > 0.5 mg/L and total dissolved Fe >100 μg/L)

are identified, but are not used in analysis of the occurrence of

trace elements in groundwater because of the ambiguity in the

dominant redox processes occurring.

Aqueous Speciation and Saturation State

The distribution of aqueous species in groundwater and

the saturation state of groundwater relative to a set of minerals

and amorphous solid phases were calculated for each sample in

order to assess whether mineral precipitation and electrostatic
Geochemical Conditions of the Study Unit 9

interactions between aqueous species and mineral surfaces may

affect the distribution of trace elements. PHREEQC version 2

(Parkhurst and Appelo, 1999), a computer program for

simulating chemical reactions in natural or polluted water, was

used to calculate aqueous species and saturation indices for the

chemical constituents presented in this report. Thermodynamic

data contained in the minteq.v4 database that is distributed

with PHREEQC version 2 were used for these calculations

(U.S. Environmental Protection Agency, 1998). In addition

to the trace elements discussed in this report, the constituents

and water-quality parameters used for modeling were: Ca2+,

Mg2+, Na+, K+, HCO3

–

, CO3

2–, Cl–

, H2S, SO4

2– , NH4+

, NO3

2–,

PO4

3–, Fe2+, Fe3+, dissolved O2

, pH, and temperature. Additional

thermodynamic data were added to the minteq.v4 database

for F, Mo, and U aqueous species and solid phases. These

supplemental data are presented in table 2.

Statistical Analysis

Nonparametric statistical methods were used to test the

significance of correlations between water-quality variables

and potential explanatory factors. Nonparametric statistics are

robust techniques that generally are not affected by outliers and

do not require that the data follow any particular distribution

(Helsel and Hirsch, 2002). The significance level (p) used to

test hypotheses for this report was compared to a threshold

value (α) of 5 percent (α = 0.05) to evaluate whether the

relation was significant (p ≤ α). Correlations were investigated

by using Spearman’s method to calculate the rank-order

correlation coefficient (ρ) between continuous variables. The

values of ρ can range from +1.0 (perfect positive correlation)

to 0.0 (no correlation) to –1.0 (perfect negative correlation).

The Kruskal-Wallis Analysis of Variance on Ranks was the

statistical test used to compare three of the more independent

populations (data groups or categories) to determine whether

one population contains larger values than the others (Helsel

and Hirsch, 2002). If a significant difference was found among

the groups, a multiple comparison test (Dunn’s Test) was used

to determine exactly which groups are different.

Geochemical Conditions of the 

Study Unit

Groundwater Age

Uncorrected 14C groundwater ages for samples collected

in the DBR study unit ranged from <100 to 33,600 years BP

(table 3). Thirty-four percent of the samples were classified as

having ages <100 years BP and are considered in this study

as being recharged during modern times. The majority of

groundwater sampled in the DBR study unit appears to have

been recharged before modern times. Thirty-five percent of the

samples were classified as having ages of 100 to 11,000 years

BP and 21 percent as having ages >11,000 years BP (fig. 3).

The areal distribution of groundwater ages in the DBR

study unit generally followed the expected groundwater flow

patterns of groundwater recharge that occurs along mountain

fronts and major surface-water drainages flowing towards

discharge areas of basins that are often demarcated by the

presence of playas and terminal lakes. The Colorado River

study area had the highest percentage of young groundwater

ages with 54 percent of the samples classified as <100 years

BP (fig. 3B). Most of the wells sampled were relatively

shallow wells (fig. 2) tapping fluvial aquifers that were

recharged either directly or indirectly (through irrigation

practices) by the Colorado River (Metzger and Loeltz, 1973;

Olmsted and others, 1973; California Department of Water

Resources, 2003). In the Owens Valley study area, 49 percent

of the wells were classified as having ages <100 years BP

(fig. 3B). Samples of relatively young groundwater in the

Owens Valley study area were primarily collected from

shallow wells (fig. 2) located in the western part of the

groundwater basin that abuts the Sierra Nevada (fig. 3A). In

contrast, the oldest groundwater in the study area came from

wells located at the terminal end of the basin flow path at

Owens Dry Lake. The Sierra Nevada is the major source of

groundwater recharge because 60 to 80 percent of the annual

precipitation in the drainage basin falls in these mountains

and flows towards the discharge area at Owens Dry Lake

(Danskin, 1998). Groundwater age distribution also followed

the general groundwater flow patterns in the Coachella

Valley study area where groundwater ages were the youngest

(<100 years BP) in samples collected from wells near the San

Bernardino and San Jacinto mountain-front recharge areas

and oldest (>3,800 years BP) in samples collected from wells

located towards the basin discharge area near the Salton Sea

(fig. 3A). In the Mojave study area, several samples classified

as having ages <100 years BP were collected from wells

located away from mountain-front recharge areas and adjacent

to the Mojave River (fig. 3A). These wells are tapping the

flood plain aquifer of the Mojave River, which is a major

and relatively constant source of groundwater recharge in the

western Mojave Desert (Lines, 1996).

The Low-Use Basins and the Indian Wells Valley study

areas had the oldest groundwater ages in the DBR study

unit, with 47 percent of groundwater samples having ages

>11,000 years BP (fig. 3B). The Low-Use Basins study area

is primarily composed of groundwater basins located away

from mountain-front recharge areas that are also not bisected

by major surface-water drainages. In the Indian Wells study

area, groundwater ages generally increased from west to east

across the basin (fig. 3A). The oldest groundwater samples

(>28,800 years BP) were collected from deep wells located

close to historical discharge areas of the groundwater basin.
10 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping,

Aerogrid, IGN, IGP, and the GIS User Community

IP037705_Figure 03a

Pacific Ocean

120° 116°

38°

34°

Sierra Nevada

MEXICO

U.S.

ARIZONA

NEVADA

CALIFORNIA

San Bernardino Mts San Gabriel Mts

Peninsular Ranges

EXPLANATION

Study Areas

Antelope Valley

Borrego Valley

Central Desert

Coachella Valley

Colorado River

Indian Wells Valley

Low-Use Basins

Mojave

Owens Valley

Rivers

Playa

0 50 100 Miles

0 50 100 Kilometers

Provinces from Belitz and others, 2003

Alamo River

Mojave

Owens River

New

River

Colorado River

Salton Sea

Owens

Dry

Lake

Soda

Dry

Lake

A

Groundwater age,

 in years 

<100

100 to 3,800

3,801 to 11,000

11,001 to 20,800

20,801 to 33,600

Whitewater River

 River

San Jacinto Mts

Provinces

Desert

Basin and Range

Figure 3. (A) Distribution of groundwater ages in the Desert and Basin and Range study unit, California GAMA Priority Basin Project, 

and (B) groundwater age categories for the overall study unit, for each study area, and for the categories of ≤ 212 and >212 feet median 

depth to the top of the uppermost interval. Groundwater ages categorized for purposes of discussion. Categories based on Jenks 

Natural Breaks.
Geochemical Conditions of the Study Unit 11

0

10

20

30

40

50

60

Study

unit

Study area

Antelope

Valley

Borrego

Valley 

Central

Desert 

Coachella

Valley

Colorado

River

Indian Wells

Valley

Low-Use

Basins

Mojave Owens

Valley

Well depth

< 212 feet

Well depth

> 212 feet

Percentage of samples

B

< 100

100 to 3,800

3,801 to 11,000

11,001 to 20,800

20,801 to 33,600

EXPLANATION

Groundwater-age categories,

 in years before present

Figure 3. —Continued

Redox and pH

Seventy-one percent of the samples collected in the 

DBR study unit were classified as oxic, 16 percent as anoxic, 

7 percent as mixed, and 6 percent as anoxic-Fe (iron-reducing) 

(table 3; fig. 4). Samples were predominantly classified as 

oxic in each of the study areas, with the exception of the 

Colorado River study area where anoxic-Fe samples were 

detected just as frequently as oxic samples. A recent study 

conducted by McMahon and Chapelle (2008) shows that oxic 

redox conditions are predominating in arid areas across the 

southwestern United States. In arid areas, the predominance 

of oxic conditions has been attributed to groundwater bearing 

deposits that are generally lacking the electron donors 

necessary (for example, organic carbon) to support anoxic 

redox processes (Winograd and Robertson, 1982; McMahon 

and others, 2004). In the Colorado River study area, anoxic-Fe 

and anoxic conditions are likely predominant because samples 

were often collected from wells tapping flood plain aquifers 

that are composed of organically rich fluvial deposits. These 

organic-matter–rich deposits provide the electron donors 

necessary to deplete dissolved O2 in groundwater, which 

promotes anoxic-Fe and anoxic redox conditions.

Redox conditions and pH showed significant relations to 

groundwater age. Anoxic samples had the oldest groundwater 

ages, and anoxic-Fe samples had the youngest (table 4). 

Seventy-four percent of the anoxic samples that have age 

data had ages >3,800 years BP, and 51 percent had ages 

>11,000 years BP. Groundwater near the Owens Dry Lake and 

the Salton Sea, the distal ends of the general groundwater flow 

systems in the Owens and Coachella Valleys, respectively, 

generally were anoxic (fig. 4A) and old (fig. 3A). 

IP037705_Figure 03b
12 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping,

Aerogrid, IGN, IGP, and the GIS User Community

Pacific Ocean

120° 116°

38°

34°

Sierra Nevada

MEXICO

U.S.

ARIZONA

NEVADA

CALIFORNIA

San Bernardino Mts San Gabriel Mts

Peninsular Ranges

EXPLANATION

Study Areas

Antelope Valley

Borrego Valley

Central Desert

Coachella Valley

Colorado River

Indian Wells Valley

Low-Use Basins

Mojave

Owens Valley

Rivers

Playa

0 50 100 Miles

0 50 100 Kilometers

Provinces from Belitz and others, 2003

Alamo River

Owens River

New

River

Colorado River

Salton Sea

Owens

Dry

Lake

Soda

Dry

Lake

A

Redox class

Oxic

Anoxic

Anoxic-Fe

Mixed

Whitewater River

Mojave

 River

San Jacinto Mts

Provinces

Desert

Basin and Range

Figure IP037705_Figure 04a 4. (A) Distribution of redox classes in the Desert and Basin and Range study unit, California GAMA Priority Basin Project, and 

(B) Redox classes organized by study unit, study area, and depth to the top of the uppermost interval. Classes based on McMahon and 

Chapelle (2008).
Geochemical Conditions of the Study Unit 13

The occurrence of old, anoxic groundwater at the 

distal ends of groundwater flow systems likely reflects that 

groundwater has been in the subsurface for thousands of years 

where redox processes have had time to remove dissolved O2

from groundwater. Eighty percent of anoxic-Fe samples had 

groundwater ages of <100 years BP. The reason anoxic-Fe 

samples have the youngest ages is because anoxic-Fe samples 

were almost exclusively collected from wells in the Colorado 

River study area where organic-rich fluvial groundwater basins 

are rapidly recharged by the Colorado River.

Groundwater samples collected in the DBR study unit 

have pH values ranging from 6.4 to 9.8 (table 3; fig. 5). The 

pH of groundwater was categorized by using Jenks Natural 

Breaks (Jenks and Caspall, 1971) for the purpose of discussing 

the distribution of pH values in the study unit. Eighty-nine 

percent of the samples have pH values ≥7.1, and 37 percent 

have pH values ≥7.9 (fig. 5B). Sample pH is positively 

correlated to groundwater age and well depth (table 4). The 

correlation of pH with well depth is likely due to the fact that 

deeper wells are generally tapping older groundwater. The 

correlation between pH and groundwater age is likely due to 

pH being primarily controlled by the dissolution of silicate 

minerals. For example, the incongruent dissolution of the 

silicate mineral albite to form kaolinite is represented by the 

following reaction:

2 9 NaAlSi H O +

3 8 O H+ + 2 3 CO 2 + → H

(2) Al S2 2i O5 ( ) OH + + 4 2 4 Na + H S4 4 iO + HCO −

3

This reaction consumes carbonic acid (dissolved CO2

) and 

H+

, thus increasing the pH of groundwater. Because the 

weathering of silicate minerals takes place over long time 

periods (Appelo and Postma, 2005), pH values generally 

increase with increasing groundwater age.

0

10

20

30

40

50

100

90

80

70

60

Study

unit

Study area

Antelope

Valley

Borrego

Valley 

Central

Desert 

Coachella

Valley

Colorado

River

Indian Wells

Valley

Low-Use

Basins

Mojave Owens

Valley

Well depth

< 212 feet

Well depth

> 212 feet

Percentage of samples

B

Oxic

Anoxic

Anoxic-Fe

EXPLANATION

Redox class

IP037705_Figure 04b

Mixed

Figure 4. —Continued
14 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Source: Esri, DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping,

Aerogrid, IGN, IGP, and the GIS User Community

IP037705_Figure 05a

Pacific Ocean

120° 116°

38°

34°

Sierra Nevada

MEXICO

U.S.

ARIZONA

NEVADA

CALIFORNIA

San Bernardino Mts San Gabriel Mts

Peninsular Ranges

EXPLANATION

Study Areas

Antelope Valley

Borrego Valley

Central Desert

Coachella Valley

Colorado River

Indian Wells Valley

Low-Use Basins

Mojave

Owens Valley

Rivers

Playa

0 50 100 Miles

0 50 100 Kilometers

Provinces from Belitz and others, 2003

Alamo River

Owens River

New

River

Colorado River

Salton Sea

Owens

Dry

Lake

Soda

Dry

Lake

A

pH, in standard

 units

6.4 to 7.0

7.1 to 7.8

7.9 to 8.6

8.7 to 9.8

Whitewater River

Mojave

 River

San Jacinto Mts

Provinces

Desert

Basin and Range

Figure 5. (A) Distribution of pH values in the Desert and Basin and Range study unit, California GAMA Priority Basin Project, and (B) pH 

values organized by study unit, study area, and median depth to the top of the uppermost interval. pH values categorized for purposes of 

discussion. Categories based on Jenks Natural Breaks.
Geochemical Conditions of the Study Unit 15

The significant positive correlation with groundwater 

age means the distribution of pH values in the study unit 

was similar to the distribution of groundwater ages. Samples 

collected near mountain-front recharge areas tended to be 

acidic or near neutral, and samples collected away from 

mountain-front recharge areas or near the distal end of 

groundwater flow paths tended to be more alkaline. Several 

samples collected from wells in the Mojave study area that 

are adjacent to the San Bernardino Mountains had pH values 

>8.7, where near-neutral pH values could reasonably be 

expected. The tops of the uppermost openings for these wells 

were deeper (365–850 ft bls) than the tops of the uppermost 

openings for other wells nearby that had lower pH values, and 

the groundwater in the wells that had pH values >8.7 also was 

older than the groundwater from the other nearby wells, which 

may explain the high pH values.

Seventy-five percent (18/23) of the samples with pH 

values ≤7.0 were collected from wells located in the western 

part of the Owens Valley study area (fig. 5A). Nearly all of 

these samples were also oxic (fig. 4A) and had ages <100 years 

BP (fig. 3A). These wells are located close to the Sierra Nevada 

mountain front and likely tap groundwater at the proximal 

end of flow paths, where the minimal dissolution of aquifer 

minerals has likely occurred and pH values of groundwater are 

likely closest to the initial pH values of precipitation. Annual 

data collected from several sites in the central Sierra Nevada 

showed that pH values of precipitation ranged from 5.1 to 5.9 

(National Atmospheric Deposition Program, 2014).

The evolution of groundwater pH from near-neutral 

values to alkaline values is particularly evident in the 

distribution of sample pH values collected in the Coachella 

Valley study area. Groundwater pH values increased from 

≤7.8 near the major mountain-front recharge areas of the 

groundwater basin to ≥8.7 near the discharge area of the 

groundwater basin close to the Salton Sea where groundwater 

ages are tens of thousands of years old (figs. 3A, 5A).

0

10

20

30

40

50

80

70

60

Study

unit

Study area

Antelope

Valley

Borrego

Valley 

Central

Desert 

Coachella

Valley

Colorado

River

Indian Wells

Valley

Low-Use

Basins

Mojave Owens

Valley

Well depth

< 212 feet

Well depth

> 212 feet

Percentage of samples

B

6.4 to 7.0 

7.1 to 7.8 

7.9 to 8.6 

8.7 to 9.8 

EXPLANATION

pH, in standard units

IP037705_Figure 05b

Figure 5. —Continued
16 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Major Ion Content

Groundwater samples are classified by major ion

content based on whether or not an ion composes greater than

50 percent of the total cations or anions, on a milliequivalent

basis, in a sample; if no one ion is greater than 50 percent of

the total, then the groundwater was classified as mixed. With

respect to cations, 46 percent of the samples were classified

as sodium-type, 32 percent as mixed-cation, and 22 percent as

calcium-type (table 3; fig. 6). Samples classified as sodiumtype were significantly older, had higher pH values and TDS

concentrations, and were from wells with greater depths to

the top of the open interval compared to samples classified

as calcium-type (table 4). The dominance of sodium over

calcium in older groundwater may be due to the replacement

of calcium ions in groundwater by sodium ions associated

with mineral surfaces through cation-exchange processes over

time (Drever, 1997). Samples with a mixed-cation-type had

significantly younger groundwater ages than sodium-type

samples and significantly older groundwater than calciumtype samples. These relations may be due to the evolution of

groundwater as it moves through the aquifer system, evolving

from a younger, calcium-type to an older, sodium-type. With

respect to anions, 49 percent of the samples were classified as

bicarbonate-type, 34 percent were classified as mixed-anion,

10 percent were classified as sulfate-type, and 7 percent were

classified as chloride-type (table 3; fig. 6). Samples classified

as chloride-type had significantly older groundwater ages than

all other types, and samples classified as bicarbonate-type had

significantly lower TDS than all other types (table 4). Eleven

different combined cation-anion classes were present, and

80 percent of the samples were classified in the following

five classes: sodium or mixed-cation with mixed-anion, and

calcium or sodium or mixed-cation with bicarbonate.

Calcium-type samples predominantly came from

wells located next to mountain-front recharge areas.

Seventy-six percent of calcium-type samples were from wells

located in either the Owens Valley (46 percent) or Coachella

Valley (30 percent) study areas. The majority of these wells

were located in the western part of the groundwater basins that

abut major mountain-front recharge areas where groundwater

ages were the youngest. Sodium-type samples were more

spatially distributed across the DBR study unit than were the

calcium-type samples, but areal patterns were still discernable.

The Low-Use Basins and Indian Wells Valley study areas had

the highest percentage of samples classified as sodium-type,

with 70 and 62 percent of their samples classified as such,

respectively. Sodium-type samples in the Low-Use Basins and

Indian Wells Valley study areas primarily came from wells

located away from mountain-front recharge areas where wells

are tapping some of the oldest groundwater in the DBR study

unit (fig. 3A). In the Owens and Coachella Valley study areas,

sodium-type samples only came from wells located near the

discharge areas of Owens Dry Lake and the Salton Sea. A

similar pattern was observed in the Mojave study area where

the majority of samples collected at the terminal end of the

Mojave River (fig. 1) were sodium-type.

Bicarbonate-type samples came from wells located

in every study area except for the Colorado River and the

Borrego Valley study areas (table 3). The Owens Valley and

Coachella Valley study areas had the largest percentage of

bicarbonate-type samples. Forty-four percent of bicarbonatetype samples came from wells in the Owens Valley, and

16 percent came from wells in the Coachella Valley.

Bicarbonate-type samples came from wells located throughout

the Owens Valley, while in the Coachella Valley, the majority

of bicarbonate-type samples came from wells located in

the northern and western parts of the basin. The majority of

sulfate-type samples came from wells located in study areas

that are south of the Mojave study area. The largest percentage

(36 percent) of sulfate-type samples came from wells located

in the Coachella Valley; most of these wells were located

in the eastern and northeastern parts of the basin. Chloridetype samples came from areas in the DBR study that had the

oldest groundwater ages. Forty-four percent of chloride-type

samples came from wells located in the Low-Use Basins that

were tapping groundwater that ranged in age from 7,400 to

27,400 years BP.
Geochemical Conditions of the Study Unit 17

Percent

Calcium

 100 

 80 

 60 

 40 

 20  0 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Fluoride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Carbonate plus Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

Percent

EXPLANATION

Borrego Valley

Central Desert

Coachella Valley

Colorado River

Low-Use Basins

Percent

Percent

Calcium

 100 

 80 

 60 

 40 

 20 

 0 

 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Fluoride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride

 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

Percent

Percent

EXPLANATION

Antelope Valley

Indian Wells Valley

Mojave

Owens Valley

IP037705_Figure 06ab_piper

A

B

Figure 6. Major ion content of samples categorized by study area in the Desert and Basin and Range study unit: (A) Borrego Valley, 

Central Desert, Coachella Valley, Colorado River, and Low-Use Basins, and (B) Owens Valley, Indian Wells Valley, Antelope Valley, 

and Mojave.
18 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Occurrence of Selected 

Trace Elements

Summary statistics for the concentrations of selected

trace elements are shown in table 5. Concentrations ranged

over several orders of magnitude, from less than laboratory

reporting levels to more than 50 times the associated waterquality benchmarks. B and As had the widest range in

sample concentration, and F and U had the narrowest. As

and F were most frequently detected at concentrations above

associated water-quality benchmarks (18 and 13 percent,

respectively), and Sr and V were the least frequently

detected at concentrations above associated water-quality

benchmarks (both at 3 percent). B, Mo, and U were detected at

concentrations above their respective benchmarks in 11, 7, and

4 percent of samples, respectively. A robust statistical analysis

of the occurrence of these constituents in groundwater of the

DBR study unit can be found in Dawson and Belitz (2013) and

Parsons and others (2014).

Sources of Trace Elements to Groundwater

A rigorous analysis of the sources of trace elements in

groundwater sampled in the DBR study unit is beyond the scope

of this report. Insight to possible sources of trace elements,

however, may be obtained by determining if groundwater

is mainly influenced by geologic factors, anthropogenic

inputs, or both. Given that urban and agricultural land use

is relatively sparse in the DBR study unit, volatile organic

compounds (which can indicate anthropogenic influence)

were rarely detected, nitrate concentrations were rarely

greater than associated water-quality benchmarks (Dawson

and Belitz, 2013; Parsons and others, 2014), and the majority

of groundwater ages are >100 years BP, it can reasonably be

assumed that geologic, not anthropogenic, sources are the

main contributors of trace elements to groundwater. Geologic

sources of the constituents presented in this report may include

the dissolution of trace element minerals such as arsenopyrite,

fluorite, or powellite, or minerals that host trace elements such

as the silicate minerals feldspar, mica, and amphibole (Reimann

and de Caritat, 1998). Trace elements adsorbed to mineral

surfaces that compose aquifer matrices may also be a source

to groundwater. Adsorbed trace elements can be mobilized

due to exchange reactions and dissolution of mineral coatings

that can occur under reducing redox conditions or because of

variations in groundwater pH. In addition, the distribution of

trace elements in groundwater can be affected by heterogeneity

in the source material.

Relation to Groundwater Age

Groundwater geochemistry evolves over time in large

part because of the interaction with the minerals composing

aquifer matrices. Generally, in groundwater unaffected by

anthropogenic inputs, the longer the water has been in an

aquifer, the greater the concentrations of dissolved solids.

This is because the dissolution of trace-element–containing

minerals, such as silicate minerals, in the natural environment

is a slow process that takes place over hundreds to thousands

of years (Appelo and Postma, 2005). Therefore, it can

reasonably be expected that many trace element concentrations

in groundwater will generally increase with increasing

groundwater age.

All constituent concentrations, with the exception

of Sr, were significantly correlated to groundwater age

(table 6; fig. 7). As, B, F, Mo, and V concentrations had

significant positive correlations with groundwater age, with

the correlations for As, F, and B being the most significant.

The majority of As, B, and F detections with concentrations

above their respective water-quality benchmarks were

in samples having groundwater ages >10,000 years BP.

In contrast, U concentrations have a significant negative

correlation to groundwater age. Of the six samples with

groundwater age data and U concentrations above the

USEPA maximum contaminant level (MCL-US) of 30 μg/L,

five had groundwater ages <100 years BP. The fact that U

concentrations tend to be higher in younger groundwater

indicates that other factors, such as the presence of

complexing ions, are more important in contributing U

to groundwater than long dissolution times of minerals

composing aquifer matrices.

The solubility of trace elements in groundwater is

largely controlled by adsorption/desorption reactions at the

particle-water interface and (or) the precipitation/dissolution

of secondary mineral phases. These reactions are in turn

controlled by the geochemical conditions of the aquifer

system. Geochemical conditions that affect trace element

solubility in groundwater include (1) oxidation-reduction

(redox) conditions, (2) pH, and (3) major ion content.

Evaporative effects can also increase the concentration of trace

elements in groundwater. An analysis of the stable isotopes

deuterium and oxygen-18 (data not shown) indicates that

evaporative effects may be important in a limited number of

cases, but is not a dominant process in the relatively deeper

aquifer systems sampled in the DBR study unit. The effects

that redox conditions, pH, and major ion content have on

the distribution of trace elements in the DBR study unit are

discussed in the following sections.
Occurrence of Selected Trace Elements 19

IP037705_Figure 07

0.01

0.1

1

10

100

1,000

1

10

100

1,000

10,000

100,000

<100 1,000 10,000 <100 1,000 10,000

Concentration, 

in micrograms per liter

Carbon-14 age, in years before present 

Carbon-14 age, in years before present 

Arsenic (MCL) Boron (NL)

0.01

0.1

1

10

100

<100 1,000 10,000

Fluoride (MCL)

10

0.1

1

100

1,000

<100 1,000 10,000

Molybdenum (HAL)

<100 1,000 10,000

Strontium (MCL)

1

10

100

1,000

10,000

100,000

<100 1,000 10,000

Uranium (MCL)

0.01

0.1

1

10

100

<100 1,000 10,000

Vanadium (NL)

10

0.1

1

100

1,000

Carbon-14 age, in years before present 

Concentration, 

in micrograms per liter

Concentration, 

in micrograms per liter

Concentration, 

in milligrams per liter

Measured value for constituent

U.S. Environmental Protection Agency

 maximum contaminant level (MCL),

 or health advisory level (HAL)

California Department of Public Health

 maximum contaminant level (MCL),

 or notification level (NL)

Linear fit for constituent

EXPLANATION

Figure 7. Relations of trace element concentrations to (uncorrected) groundwater age, Desert and Basin and Range study unit, 

California GAMA Priority Basin Project.
20 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Relation to Redox and pH

The effects of redox can account for substantial

variability in the concentrations of trace elements in

groundwater. The solubility of trace elements may be sensitive

to redox conditions in groundwater because (1) some trace

elements can exist in multiple oxidation states in natural

waters, and the aqueous species formed by the element in

these different oxidation states may have different adsorption

and ion complexation properties; (2) redox conditions of

the groundwater can affect the adsorbing surface of aquifer

matrices; and (3) redox conditions can control the formation

of secondary mineral phases. Variation in pH may have

an effect on the adsorption of many trace elements by

altering the variable surface charge of minerals such as iron

oxyhydroxides. Some redox-sensitive trace elements, such

as U, are more mobile under relatively acidic pH and oxic

conditions (Fox and others, 2006), while oxyanion-forming

trace elements, such as As, are generally more mobile under

alkaline pH conditions (Smedley and Kinniburgh, 2002;

Stollenwerk, 2003).

Anoxic-Fe samples only accounted for a small percentage

(fig. 4B) of the total samples for which redox classification

was possible (table 3). Of the 12 samples classified as

anoxic-Fe, 10 samples were from the Colorado River study

area. Samples classified as anoxic-Fe account for 36 percent

of the total samples collected in the Colorado River study

area. Therefore, while anoxic-Fe groundwater conditions are

not important at the study-unit scale, they are important in

describing trace element distribution in the Colorado River

study area.

Figure 8 shows the distribution of the selected trace

element concentrations with respect to redox classification and

pH. The seven trace elements were put into four groups on the

basis of correlations between trace element concentrations,

redox class, and pH (table 6): (1) positively correlated with pH

and greater in anoxic conditions than in oxic conditions—As;

(2) positively correlated with pH and no significant difference

between oxic and anoxic—Mo, F, V; (3) negatively correlated

with pH and greater in oxic conditions—U; (4) no correlation

with pH—B, Sr.

Arsenic

Arsenic concentrations were significantly higher in

samples collected from wells tapping anoxic groundwater

than in wells tapping oxic groundwater (table 6). In part,

concentrations of As may be higher in anoxic rather than oxic

samples because anoxic redox conditions tend to occur in

relatively old groundwater where the dissolution of primary

aquifer minerals has likely occurred to a greater extent due

to the longer period of interaction between groundwater and

minerals that compose aquifer matrices. Furthermore, anoxic

samples tend to be more alkaline (higher pH), which could

increase the mobility of As in the aqueous phase. In oxic and

anoxic redox conditions, speciation modeling (SM) predicts

that As exists in a fully oxidized state (As5+) primarily as the

oxyanion HAsO4

2– (table 7). In alkaline conditions, HAsO42– is

expected to be more mobile in groundwater as the variable

surface charge on aquifer minerals becomes increasingly

negative as pH increases, thus inhibiting the adsorption and

(or) promoting the desorption of anionic species (Stumm,

1992). Reductive dissolution of secondary minerals, such as

Fe-oxides, that can release adsorbed trace elements like As

to the aqueous phase is not likely an important process under

anoxic conditions as defined in this study. This is because

reduction potentials are likely not low enough for oxide

reduction to occur as is indicated by the low concentrations of

Fe in anoxic samples.

Arsenic concentrations were lowest in anoxic-Fe

groundwater (table 6; fig. 8). This was unexpected because

dissolution of iron oxyhydroxides under iron-reducing

conditions would normally release As into solution (Smedley

and Kinniburgh, 2002; Welch and others, 2006; Izbicki

and others, 2008). Geochemical modeling indicates that

precipitation of As-bearing sulfide minerals may explain the

low As concentrations in anoxic-Fe groundwater from the

Colorado River study area.

In the sequence of reduction reactions that occur as

reduction potential decreases, As is thermodynamically

predicted to be reduced from As5+ to As3+ at lower reduction

potentials than the potentials at which reduction of iron

from Fe3+ to Fe2+ begins (Turner and others, 1981). Because

reduction of sulfur from SO4

2– to HS– also begins at lower

reduction potentials than the potentials at which reduction of

iron from Fe3+ to Fe2+ begins (McMahon and Chapelle, 2008),

SM for As in anoxic-Fe samples requires data for SO4

2– and

HS– concentrations (Parkhurst and Appelo, 1999). A sulfide

odor was detected in 7 of 12 anoxic-Fe samples collected;

sulfide concentrations were determined for 3 of these samples

(table 3). Dissolved arsenic species in samples with a sulfide

odor were modeled using a sulfide concentration of 50 μg/L,

which is half the lowest concentration of the three sulfide

samples analyzed. Speciation modeling predicted that the

uncharged H3AsO3 molecule would be the dominant species

in sulfidic groundwater. Because an uncharged As species

would not be attracted to charged mineral surfaces, thereby

decreasing adsorption and increasing As concentrations in

groundwater, As concentrations in the anoxic-Fe samples are

likely affected by processes other than adsorption-desorption

reactions. Calculated saturation indices (SI) for As-bearing

minerals indicated that low As concentrations in anoxic-Fe

samples may be partly due to precipitation of As-bearing

minerals. SI calculations showed that anoxic-Fe samples are

supersaturated with orpiment (As2S3; SI = –1.0 to 4.3, median

= 1.2) and pyrite (FeS2; SI = 4.9 to 6.3, median = 5.2). Arsenic

can co-precipitate with FeS2 forming arsenopyrite (FeAsS)

and other pyrite analogs (Lowers and others, 2007). Therefore,

the low As concentrations observed in anoxic-Fe groundwater

may be due in part to the precipitation of As-containing

minerals.
Occurrence of Selected Trace Elements 21

IP037705_Figure 08

0.01

0.1

1

10

100

1,000

1

10

100

1,000

10,000

100,000

Concentration, 

in micrograms per liter

pH, in standard units

pH, in standard units

Arsenic (MCL) Boron (NL)

6 7 8 9 10 6 7 8 9 10

Oxic 

Anoxic 

Anoxic-Fe 

pH, in standard units

Concentration, 

in micrograms per liter

Concentration, 

in micrograms per liter

Concentration, 

in milligrams per liter

0.01

0.1

1

10

100

Fluoride (MCL)

6 7 8 9 10

10

0.1

1

100

1,000

Molybdenum (HAL)

6 7 8 9 10

Strontium (MCL)

1

10

100

1,000

10,000

100,000

6 7 8 9 10

Uranium (MCL)

0.01

0.1

1

10

100

6 7 8 9 10

Vanadium (NL)

10

0.1

1

100

1,000

6 7 8 9 10

U.S. Environmental Protection Agency

 maximum contaminant level (MCL),

 or health advisory level (HAL)

California Department of Public Health

 maximum contaminant level (MCL),

 or notification level (NL)

EXPLANATION

Redox class

Figure 8. Distribution of selected trace element concentrations with respect to redox classification and pH, Desert and Basin and 

Range study unit, California GAMA Priority Basin Project.
22 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Low concentrations of As in anoxic-Fe groundwater

could also be due to lack of source material and (or) the

flushing of groundwater with surface water that has relatively

low concentrations of As.

Previous investigations have indicated two primary

mechanisms for elevated arsenic concentrations in

groundwater (Welch and others, 2000; Stollenwerk, 2003;

Welch and others, 2006). One is the release of arsenic

resulting from reductive dissolution of iron or manganese

oxyhydroxides, and the other is desorption of arsenic from

mineral surfaces under oxic, alkaline conditions. The results

from this study indicate that the latter mechanism is more

important in the DBR study unit.

Vanadium, Molybdenum, and Fluoride

V concentrations in oxic and anoxic groundwater were

not significantly different; however, V concentrations in

anoxic-Fe groundwater were significantly lower than in either

oxic or anoxic groundwater (table 6). In oxic and anoxic

redox conditions, SM predicts that V is in a fully oxidized

state (V5+) primarily as the oxyanion H2VO4

– (table 7). In this

form, V is expected to be more mobile in alkaline conditions

than in neutral or acidic conditions because repulsive forces

from negatively charged mineral surfaces decrease adsorption

of negatively charged species. Within the DBR study unit,

V concentrations showed a significant positive correlation

to pH (table 6), indicating that V is more mobile in oxic and

alkaline groundwater. The positive correlation between V

concentrations and oxic and alkaline groundwater conditions

observed in this study was also observed in a study that

examined the distribution of V concentrations in groundwater

throughout the State of California (Wright and Belitz, 2010).

The distribution of V concentrations in anoxic-Fe

samples indicates that V is sparingly soluble when reduction

potentials are low enough for Fe reduction to occur. Vanadium

concentrations in anoxic-Fe samples did not exceed 1.8 μg/L,

and 92 percent of the samples had concentrations <0.8 μg/L.

Speciation modeling predicts that soluble V exists as the

uncharged V(OH)3 molecule in anoxic-Fe redox conditions

(table 7). If V did exist as an uncharged, aqueous species,

concentrations would be higher in anoxic-Fe groundwater

because adsorption to charged mineral surfaces would be

mitigated. A laboratory study conducted by Wanty and

Goldhaber (1992) showed that H2S is capable of reducing

V4+ to V3+ and that V3+ subsequently precipitates as a solid

oxyhydroxide. Hydrogen sulfide detected in anoxic-Fe

samples occurred at lower concentrations than those used in

the laboratory experiments. Nevertheless, the detection of H2S

indicates that the precipitation of V3+ oxyhydroxide may be a

viable mechanism for the removal of aqueous V in anoxic-Fe

groundwater. Additionally, a study by Wright and others

(2014) indicated that in anoxic-Fe groundwater, V3+ may be

removed from the aqueous phase by co-precipitating with Fe3+

to form mixed (Fe,V)-oxide minerals.

Like As and V, Mo was predicated to exist primarily

in an oxyanionic form. Unlike As and V, Mo concentrations

were not significantly correlated to redox conditions

(table 6). Speciation modeling predicts that molybdenum

exists primarily in an uncomplexed form as the oxyanion

MoO4

2– (table 7). The prediction that Mo exists primarily

as an oxyanion is in agreement with the variations of

Mo concentrations observed with respect to the pH of

groundwater. Molybdenum concentrations generally increased

with increasing high pH values, although the correlation was

weaker than for that of As and V.

F concentrations were not significantly associated with

any redox conditions. This is likely due to the fact the F is

not a redox sensitive element, and therefore its speciation,

and consequently adsorption and precipitation behavior in

groundwater, is not directly affected by the redox conditions

of groundwater. F was predicted by SM to exist primarily as a

negatively charged species (table 7). As a negatively charged

species, F– would be repelled from negatively charged mineral

surface, thus increasing its solubility in groundwater. This

hypothesis is supported by the significant positive correlation

of F concentrations with pH.

Uranium

Uranium concentrations are significantly higher in oxic

groundwater than in either anoxic or anoxic-Fe groundwater

(fig. 8; table 6). Uranium may be less soluble in anoxic-Fe

redox conditions because the predominant form of U in

reducing conditions is U4+, which has low solubility and

tends to precipitate as uraninite (Langmuir, 1997). If sulfide

concentrations of 50 μg/L are assumed for anoxic-Fe samples,

SM predicts that uraninite (UO2) would be supersaturated (SI

= 0.7 to 3.5, median = 1.6) in anoxic-Fe samples. Therefore,

under anoxic-Fe groundwater conditions, U concentrations

may be controlled by mineral precipitation.

In oxic and alkaline conditions, the predominant

oxidation state of U in groundwater is 6+, and concentrations

are primarily controlled by adsorption/desorption reactions

(Hsi and Langmuir, 1985; Davis and Curtis, 2003). U6+ will

form the uranyl (UO2

+2) cation in the absence of complexing

ions, such as HCO3

–

. However, complexing ions are generally

abundant in groundwater, and samples collected in the DBR

study unit are predicted to exist as an uncharged ternary

complex Ca2UO2(CO3)3 (table 7). The results of previous

studies have indicated that this uncharged ternary complex

is responsible for increased U mobility in groundwater

(Bernhard and others, 2001; Fox and others, 2006; Jurgens

and others, 2010). Uranium concentrations in samples from

the DBR study unit generally decreased with increasing pH

values (fig. 8) indicating that reactions other than adsorption/

desorption are controlling the solubility of U in solution at

alkaline pH values. Further discussions on the controls on U

solubility are presented later in this report.
Occurrence of Selected Trace Elements 23

Boron and Strontium

B was predicted by SM to only exist in a single

oxidation state in groundwater sampled in the DBR study

unit (table 7). Boron concentrations were significantly higher

in samples collected from wells tapping anoxic rather than

oxic groundwater (table 6). The positive correlation with

anoxic redox conditions for B may be due in part to the

fact the anoxic samples generally had older groundwater

ages than oxic samples (table 4). SM predicts that B exists

predominantly as an uncharged molecule, H3BO3, at pH values

<9.2 and predominantly as an anion, H2BO3

–

, at pH values

>9.2. A study by Goldberg and Glaubig (1986) showed that

B adsorption was greatest between pH values of 8.5 and 10.

However, B concentrations in samples collected in the DBR

study unit were not significantly correlated with pH values

(table 6).

Sr was also predicted to only exist in a single oxidation

state in groundwater sampled in the DBR study unit.

Concentrations of Sr were higher in anoxic-Fe groundwater

than in oxic or anoxic groundwater (table 6). Because

Sr was predicted to exist in only one oxidation state, the

association between anoxic-Fe groundwater and higher Sr

concentrations likely reflects processes other than oxidationreduction–related processes. Sr concentrations showed

a strong positive correlation with TDS concentrations

(table 6), and TDS concentrations were significantly greater

in anoxic-Fe groundwater (table 4). The relation of TDS and

Sr concentrations is discussed more in the following section.

Although Sr concentrations were not correlated with pH over

the full range of pH values (fig. 8; table 6), Sr concentrations

showed a negative correlation with pH for pH values ≥8.0

(Spearman’s test, p-value <0.001, rho = –0.48). SM predicted

that Sr predominantly exists as the cation Sr2+ (table 7). The

negatively charged mineral surfaces that predominate at

alkaline pH values would favor adsorption of cationic species

as pH increased, thereby decreasing the aqueous concentration

of Sr. Additionally, Sr may also be removed from the aqueous

phase by co-precipitating with calcite and (or) aragonite

(Banner, 1995; Reimann and de Caritat, 1998).

Relation to Major Ion and Phosphate Content

Complexation reactions with major ions and phosphate

can play an important role in influencing the solubility of trace

elements in groundwater (Violante and others, 2007). Trace

elements that exist primarily as oxyanionic or anionic species

(table 7) can form complexes with alkali earth metals, and

trace elements that exist as oxycationic or cationic species

can form complexes with inorganic ligands. Complexes of

cationic species with organic ligands, such as oxalate, are

also important, but are not addressed in this report because

organic ligand content in samples were not determined as

part of this study. Trace element complexes with major ions

and phosphate can increase or decrease solubility by altering

adsorption/desorption characteristics or by inducing the

precipitation of mineral phases. For example, the anionic

species of Mo (MoO4

2–) can form uncharged complexes with

the alkali earth metals Ca2+ and Mg2+ (CaMoO4 and MgMoO4)

that may increase the solubility of Mo in groundwater

(Essington, 1992). However, if the geochemical conditions

are right and minerals are supersaturated in groundwater,

then Mo and Ca complexes can lead to the precipitation of

the mineral powellite (CaMoO4), thereby removing Mo from

the aqueous phase (Conlan and others, 2012). Boron also has

been shown to form complexes with Ca and Mg [CaB(OH)4

+

and Mg B(OH)4

+

], which may be important at alkaline pH

values (Mattigod and others, 1985; Goldberg and Suarez,

2011). In groundwater, the cationic species of U (UO2

2+)

forms complexes with the inorganic ligand HCO3

– , which can

increase the solubility of U in groundwater (Fox and others,

2006; Jurgens and others, 2010). Conversely, complexes of

UO2

2+ with phosphate (PO43–) can decrease the solubility of U

at acidic to near-neutral pH values (Payne and others, 1998).

Effects of Complexation and Mineral 

Precipitation on the Distribution of 

Molybdenum and Fluoride

Speciation modeling predicted that MoO4

2– and F– are the

only oxyanionic/anionic species to form complexes with alkali

earth metals in groundwater sampled in the DBR study unit

(table 7). Mo forms aqueous complexes with Na (NaMoO4

–

),

Ca (CaMoO4), and Mg (MgMoO4), although the uncomplexed

form of Mo is the predominant species in the majority of

samples. The negatively charged NaMoO4

– complexes rarely

consist of more than 20 percent of the total Mo in solution

(table 7) and are therefore not to be expected to significantly

affect the distribution of Mo concentrations in groundwater.

The uncharged species, CaMoO4 and MgMoO4, are more

frequently predicted to occur in samples collected in the DBR

study unit. These uncharged species would be expected to

be more mobile in groundwater because they would not be

attracted to the surface charge of minerals. The distribution

of Mo concentrations, however, shows no significant increase

with increasing percentage of CaMoO4 and MgMoO4

species in samples (Spearman’s rho = 0.004, p-value = 0.95).

Saturation indices obtained from speciation modeling indicate

that the mineral powellite (CaMoO4) is undersaturated in all

samples and thus is not indicated as an important factor in

controlling the solubility of Mo.

Aqueous complexes of F with Ca and Mg (CaF+ and

MgF+) were predicted to only occur to a small extent and are

thus not likely an important factor controlling F concentrations

in groundwater (table 7). Calculation of SIs indicated the

precipitation of the mineral fluorapatite [Ca5(PO4)3F] may

be a factor in controlling F solubility (fig. 9A). Saturation

indices for fluorapatite ranged from –8.3 to 6.1 with a median

of 0.4. Conversely, fluorite (CaF2), a mineral that has been

attributed to controlling F solubility in groundwater in other
24 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

hydrogeologic settings (Jacks and others, 2005; Chae and

others, 2007), was undersaturated in most samples (SI range =

–4.0 to 0.4, median = –1.8).

The precipitation of fluorapatite occurs by the following

reaction (Ball and Nordstrom, 1991):

5 3 3 2

4

2

5 4 3 Ca HPO F Ca PO F H s

+ − − +

( ) + + → + ( ) . (3)

According to the law of mass action, when the reaction is at

or near equilibrium, fluorapatite precipitation is proportional

to the concentrations of Ca2+, PO4

3–, and F– in groundwater

and inversely proportional to the H+ concentration. From a pH

of 6.4 to 7.3, fluorapatite is predominantly undersaturated in

samples (fig. 9B). At low pH values, high concentrations of

H+ coupled with low concentrations of F and Ca (figs. 8, 9C)

favor the dissolution of fluorapatite. However, F solubility at

this pH range may be limited by adsorption reactions because

mineral surfaces will have a relatively high density of positive

charge. Above a pH of approximately 7.3, F solubility may

be controlled by mineral precipitation. Saturation index

calculations predicted that groundwater in the DBR study unit

with pH greater than about 7.3 becomes increasingly more

supersaturated with fluorapatite (fig. 9B), which approximately

corresponds with peak concentrations of Ca2+ and PO4

3–

(figs. 9C, D); the precipitation of fluorapatite would decrease

F concentrations in groundwater. However, at alkaline pH

values, Ca2+ can be removed from solution through cationexchange reactions and precipitation of carbonate minerals

such as calcite (CaCO3), and phosphate may be removed from

solution through hydroxylapatite [Ca5(PO4)3OH] precipitation,

which is predicted to be favored over fluorapatite precipitation

at increasingly higher pH values (fig. 9B). Precipitation

of calcite and hydroxylapatite would decrease Ca and P

concentrations and thereby increase the amount of F that

could remain in solution without causing precipitation of

fluorapatite. This process may partially be responsible for the

positive correlation between pH and F.

Slow-reaction kinetics may provide a barrier to

the precipitation of calcium phosphate minerals such as

fluorapatite and hydroxylapatite. However, laboratory studies

have shown that poorly crystalline hydroxylapatite will

precipitate in relatively short time periods in a simplified

system such as Ca(OH)2–H3PO4–H2O (Zawacki and others,

1990; Borkiewicz and others, 2010). Wright and others (2011)

showed that poorly crystalline hydroxylapatite formed in a

synthetic groundwater within a week at 25 degrees Celsius

(°C). Phosphate concentrations in that experiment were three

orders of magnitude greater than the phosphate concentrations

of the groundwater sampled in this study. Nevertheless, these

studies indicate that the precipitation of fluorapatite and

hydroxylapatite may be a viable control of F solubility in

groundwater in the DBR study unit.

Effects of Complexation and Mineral 

Precipitation on the Distribution of Uranium

In oxic conditions, and in the absence of complexing

ions, U is thermodynamically predicted to exist as the UO2

2+

oxycation. However, in groundwater, Ca and HCO3

–

/CO3

2–

will complex with dissolved U (Bernhard and others, 2001;

Dong and others, 2005). Speciation modeling predicted that

U predominantly exists as the ternary complex Ca2UO2(CO3)3

in samples collected in the DBR study unit (table 7). Because

it is an uncharged molecule, the formation of this ternary

complex increases the solubility of U in groundwater because

it is not attracted to charged particle surfaces. The increased

solubility of U due to the formation of the Ca2UO2(CO3)3

ternary complex is supported by the significant positive

correlation between the percentage of total U that is predicted

to exist as Ca2UO2(CO3)3 and the total U concentration in

samples (Spearman’s rho = 0.28, p-value = 0.00). Except for

one sample, U does not occur at concentrations greater than

one-half the MCL-US of 30 μg/L if the predicted percentage

of Ca2UO2(CO3)3 is not greater than 70 percent (fig. 10A).

SM predicted that the U-PO4

3– complex, UO2

(HPO4)2

2–,

was present in groundwater sampled in the DBR study

unit. On a study-unit scale, UO2(HPO4)2

2– complexes were

predicted to be the dominant U species in only 8 percent of

samples. On a study-area scale, the UO2(HPO4)2

2– complex

was fairly prevalent in the Owens Valley study area and was

the dominant U species in 32 percent of samples. Previous

studies have shown that under acidic to neutral pH conditions,

negatively charged U-PO4

3– complexes strongly adsorb to iron

oxide surfaces (Payne and others, 1998; Cheng and others,

2004). The pH values of samples from the Owens Valley study

area, where UO2(HPO4)2

2– was dominant, ranged from 6.4 to

8.4 with 14 of 17 measurements being ≤7.0. Samples that were

predicted to have the UO2(HPO4)2

2– complex as the dominant

U species in solution had significantly lower U concentrations

than samples where UO2(HPO4)2

2– was not the dominant

species in solution (Wilcoxon Rank Sum, p = 0.05). This

result indicates that the UO2(HPO4)2

2– complex decreases the

solubilty of U in groundwater sampled in the Owens Valley

study area.
Occurrence of Selected Trace Elements 25

0 2 4 6 8 10 12 14

Fluoride, in milligrams per liter

–10

–8

–6

–4

–2

0

2

4

6

8

10

Fluorapatite saturation index Saturation index

Supersaturated

Undersaturated

Equilibrium

A

6 7 8 9 10

pH, in standard units

–10

–6

–2

2

6

10

–10

–6

–2

2

6

10

Fluorapatite

Hydroxlapatite

Equilibrium

Supersaturated

Undersaturated

Supersaturated

Equilibrium

Undersaturated

6 7 8 9 10

pH, in standard units

0

75

150

225

300

Calcium, in milligrams per liter

C

6 7 8 9 10

pH, in standard units

0.01

0.1

1

10

Phosphate, in milligrams per liter

D

IP037705_Figure 09abcd

B

Figure 9. Factors affecting the solubility of fluoride in groundwater: (A) Relation of fluoride concentrations to saturation indices of 

fluorapatite, (B) relation of pH to the saturation indices of fluorapatite and hydroxylapatite, (C) distribution of calcium in samples with 

regard to pH, and (D) distribution of phosphate in samples with regard to pH, Desert and Basin and Range study unit, California GAMA 

Priority Basin Project. Trend lines for figs. 9B–D are Lowess smoothing curves.
26 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

Uranium concentration, in micrograms per liter 

Percentage of total uranium as Ca2UO2(CO3)3 

A

B

6 7 8 9 10

pH, in standard units

–3

–2

–1

0

1

Calcite saturation index

Equilibrium

Supersaturated

Undersaturated

0.5 × U.S. MCL

IP037705_Figure 10ab

Figure 10. Factors affecting the solubility of uranium in groundwater: (A) Relation of the percentage of total uranium that exists as 

Ca2

UO2(CO3)

3

 and uranium concentrations in samples, and (B) relation of pH to the saturation index of calcite, Desert and Basin and 

Range study unit, California GAMA Priority Basin Project. Trend line is Lowess smoothing curve.

Uranium concentrations showed a negative correlation

with pH (fig. 8; table 6). At increasingly alkaline pH values,

adsorption processes are not likely an important solubility

limit on U in groundwater because Ca2UO2(CO3)3 and

UO2(CO3)3

4– are predicted to be the predominant U species at

pH values ≥8. SI calculations indicate that U minerals are well

undersaturated in all samples and thus are not likely a control

on U solubility. However, the co-precipitation of U with

CaCO3 may be a viable mechanism to explain decreases in U

concentrations with increasing pH. The co-precipitation of U6+

with CaCO3 has been confirmed in laboratory studies (Meece

and Benninger, 1993; Reeder and others, 2001), and U has

been identified as a component of naturally occurring calcite

minerals (Kelly and others, 2003, 2006). CaCO3 is generally

undersaturated in solution at acidic to neutral pH values

(fig. 10B). At pH values >7.0, SIs show that samples are

predominantly in equilibrium with CaCO3 and trend towards

increasingly more supersaturated conditions with increasing

pH. This indicates that CaCO3 may be precipitating to a

greater extent as the pH of groundwater becomes increasingly

more alkaline. The co-precipitation of U with CaCO3 would

be in agreement with the variation of U concentrations with

respect to pH values observed for samples collected in the

DBR study unit.
Conclusions 27

Relation of Total Dissolved Solids and 

Trace Elements

As, B, F, Mo, and Sr concentrations had significant

positive correlations with TDS concentrations (table 6).

Part of the reason for this correlation may be the dissolution

of minerals and amorphous phases that compose aquifer

matrices; dissolution of minerals releases trace elements to

the aqueous phase while simultaneously increasing the TDS

concentration of groundwater. However, as discussed in

preceding sections, the trace elements presented in this report

interact with the surfaces of aquifer matrices and thus do not

behave conservatively. Therefore, correlation of trace element

concentrations with TDS also may be caused by the effects

that the ionic strength of groundwater has on adsorption/

desorption reactions due to complexation reactions and

competitive ion effects.

Ionic strength is related to the electrolyte concentration in

solution and is a measure of the interaction of ions in solution

(Sparks, 1995). Ionic strength (I) can be estimated from TDS

concentrations by the following equation (Wang and others,

2005):

I T = × ( )( ) DS − 2 5 10 5 . . (4)

This relation shows that ionic strength increases with

increasing TDS. Ionic strength is used as an indirect

measurement to determine the type of adsorption reactions

(inner-sphere versus outer-sphere) that occur with trace

elements at the mineral–water interface (Violante and others,

2007). An increase in trace element concentration with

increasing ionic strength or TDS indicates that a trace element

interacts with mineral surfaces by way of relatively weak

outer-sphere coordination (McBride, 1997). Conversely, trace

elements whose concentrations are not, or are only slightly,

affected by ionic strength are thought to interact with mineral

surfaces through inner-sphere coordination.

Results from this study indicate that Sr, a constituent

known to interact with particle surfaces through outer-sphere

complexes (Chen and others, 1998), has a significant positive

correlation to TDS (table 6). B concentrations also showed a

significant positive correlation to TDS in samples collected

from the DBR study unit. B adsorption can occur by way of

both outer- and inner-sphere coordination (Peak and others,

2003; Goldberg, 2005), which may explain the lower strength

of the statistical correlation between B and TDS as compared

to that of Sr and TDS. Conversely As and V show weak or

no significant correlation with TDS concentrations. Both of

these trace elements, in their most oxidized form, have been

shown to interact with mineral surfaces through inner-sphere

coordination (Foster, 2003; Peacock and Sherman, 2004) and

therefore would not be expected to be greatly affected by

variations in TDS concentrations in groundwater.

Conclusions

The geochemical conditions of the groundwater sampled

in the DBR study unit are predominantly oxic and alkaline and

are coupled with generally older groundwater ages. The major

ion classification (water type) was primarily sodium-type with

respect to cations and bicarbonate-type with respect to anions.

Young groundwater generally came from wells near mountainfront recharge areas and was characterized by calcium-type

waters with near-neutral pH values. Older groundwater

generally came from wells located away from mountain-front

recharge areas and was characterized by sodium-type water

with alkaline pH values. The dominance of sodium over

calcium in older groundwater may be due to cation-exchange

processes where the replacement of sodium ions adsorbed to

mineral surfaces by dissolved calcium ions occurs over time.

The correlation of alkaline pH values with old groundwater

ages may be due to pH being primarily controlled by the

dissolution of silicate minerals, which is a process that takes

place over long periods of time. Oxic redox conditions were

predominant throughout the DBR study unit, likely because

the aquifer systems are low in electron-rich organic matter

needed to support redox processes that promote reducing

groundwater conditions.

Concentrations for the majority of trace elements (As,

Mo, V, and F) examined in this study had significant positive

correlations to groundwater age and pH. The correlation with

groundwater age is likely due in part to the long time periods

required for the dissolution of trace-element-containing

silicate minerals to occur. The correlation with groundwater

age is also likely due to the fact the older groundwater is

generally alkaline in pH. Because As, Mo, V, and F are

predicted to primarily exist as uncomplexed oxyanions

and anions (HAsO4

2–, MoO42–, H2

VO4

–

, and F–), these trace

elements should be more soluble in alkaline groundwater

where the variable surface charge of minerals is predominantly

negative. In addition to adsorption/desorption reactions, F

solubility may also be limited due to the precipitation of the

mineral fluorapatite, which saturation indices indicate may

be important at near neutral to slightly alkaline pH values. B

concentrations also had a significant positive correlation to

groundwater age. However, unlike As, F, Mo, and V, B exists

primarily as the uncharged species H3BO3 and thus adsorption/

desorption reactions are not likely as an important factor

controlling B solubility; this hypothesis is supported by the

lack of a significant correlation between B concentrations and

pH.

U concentrations had a significant negative correlation

with groundwater age and pH, which is likely due to

a combination of processes. One process could be that

contributing sources of U are relatively labile minerals,

such as U-substituted carbonate minerals, which dissolve

relatively quickly. Another, perhaps more important process,

is the mobilization of adsorbed U from mineral surfaces by
28 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

HCO3

– and the subsequent formation of the uncharged ternary

complex Ca2UO2(CO3)3. At increasingly more alkaline pH

values, calcite becomes supersaturated in solution, indicating

that U may be co-precipitating with calcite. The formation

of an uncharged U complex at acidic to slightly alkaline pH

values and the co-precipitation of U with calcite at more

alkaline pH values is consistent with the correlations of

groundwater age, pH, and U concentrations observed in

this study.

Sr concentrations showed a strong correlation with TDS,

but no significant correlation with either groundwater age or

pH. The relation of Sr concentrations with TDS may be due to

the effect that competitive ions have on dissolved constituents,

such as Sr, that interact with the mineral surface through outersphere complexation reactions. Although Sr concentrations

were not correlated with pH over the full range of pH values,

Sr concentrations showed a negative correlation with pH for

values ≥8.0. This correlation indicates that the adsorption

of the Sr2+ on negatively charged mineral surfaces and (or)

removal from the aqueous phase by co-precipitation with

carbonate minerals are limiting Sr concentrations in alkaline

groundwater. B concentrations were also strongly correlated

with TDS, indicating that competitive ion effects may be

an important factor in controlling B solubility in the DBR

study unit.

Oxic redox conditions in groundwater were predominant

throughout the DBR study unit, and thus anoxic-Fe redox

conditions were only an important factor in the Colorado River

study area. Under anoxic-Fe conditions, saturation indices

indicate that mineral precipitation of orpiment and pyrite may

be limiting As solubility and that uraninite precipitation may

be limiting U solubility. Saturation indices did not indicate the

precipitation of V-containing minerals. However, the relatively

low concentrations of V in anoxic-Fe conditions suggest that V

may be precipitating as an insoluble phase which is consistent

with the findings of previous studies.

Acknowledgments

The authors thank the following cooperators for their

support: the California State Water Resources Control

Board, Lawrence Livermore National Laboratory, California

Department of Public Health, and California Department of

Water Resources. We especially thank the cooperating well

owners and water purveyors for their generosity in allowing

the U.S. Geological Survey to collect samples from their

wells. The authors would also like to thank Bryant Jurgens

of the U.S. Geological Survey for his helpful discussions on

uranium chemistry and Larry Schneider of the U.S. Geological

Survey for the preparation of the figures. Funding for this

work was provided by State of California bonds authorized by

Proposition 50 and administered by the State Water Board.

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Tables 35 Tables

Table 1. Identification and construction information for wells sampled in the Desert and Basin and Range hydrogeologic provinces, 

California GAMA Priority Basin Project, 2006–11.

[ft, foot; bls, below land surface; na, not available]

GAMA 

identification

Well 

type

Depth 

of well 

(ft bls)

Bottom of 

lowermost 

opening 

(ft bls)

Top of 

uppermost 

opening 

(ft bls)

Screen 

length 

(ft bls)

Antelope Valley

ANT-02 Production 400 400 190 210

ANT-04 Production 765 765 350 415

ANT-07 Production 527 527 260 267

ANT-09 Production 213 213 93 120

ANT-12 Production 1,200 na na na

ANT-15 Production 680 na na na

ANT-17 Production 552 552 192 360

ANT-26 Production 320 300 160 140

ANT-34 Production 606 606 96 510

ANT-40 Production 843 833 583 250

ANT-43 Production 550 540 210 330

ANT-45 Production na na na na

ANT-46 Production 475 475 250 225

ANT-47 Production 1,100 1,100 115 985

ANT-48 Production na na na na

ANT-49 Production 801 801 500 301

ANT-51 Production 328 328 188 140

ANT-52 Production 300 300 200 100

ANT-U Production 700 693 235 458

Borrego Valley

BV-03 Production 350 350 150 200

BV-04 Production 630 630 420 210

BV-07 Production 95 na na na

BVU-01 Production 392 372 162 210

Central Desert

CD-01 Production na na na na

CD-03 Production na na na na

CD-04 Production 350 345 195 150

CD-07 Production 740 720 470 250

CD-09 Production 1,115 1,115 550 565

CD-11 Production na na na na

CD-12 Production 550 na na na

CD-13 Production 260 247 147 100

CD-14 Production 600 580 390 190

Coachella Valley

COA-01 Production 1,200 1,200 900 300

COA-02 Production na 480 230 250

COA-03 Production na na na na

COA-04 Production 1,070 1,060 500 560

GAMA 

identification

Well 

type

Depth 

of well 

(ft bls)

Bottom of 

lowermost 

opening 

(ft bls)

Top of 

uppermost 

opening 

(ft bls)

Screen 

length 

(ft bls)

Coachella Valley—Continued

COA-05 Production 1,090 1,090 710 380

COA-06 Production 654 654 203 451

COA-07 Production 240 na na na

COA-08 Production 342 342 258 84

COA-09 Production na na na na

COA-10 Production na na na na

COA-11 Production na na na na

COA-12 Production 525 525 445 80

COA-13 Production 890 870 510 360

COA-14 Production 820 820 420 400

COA-15 Production 400 380 180 200

COA-16 Production 650 650 300 350

COA-17 Production na na na na

COA-18 Production 790 790 280 510

COA-19 Production na na na na

COAU-01 Production 96 43 12 31

COAU-02 Production 600 600 288 312

COAU-03 Production na na na na

COAU-04 Production 400 400 280 120

COAU-05 Production 909 906 306 600

COAU-06 Production 1,130 1,110 580 530

COAU-07 Production 553 553 225 328

COAU-08 Production 730 726 476 250

COAU-09 Production na na na na

COAU-10 Production 1,070 1,050 410 640

COAU-11 Production 400 400 220 180

COAU-12 Production 160 155 55 100

COAU-13 Production na na na na

COAU-14 Production 550 530 330 200

COAU-15 Production 700 700 400 300

COAU-16 Production na na na na

Colorado River

COLOR-01 Production na na na na

COLOR-02 Production na na na na

COLOR-03 Production 512 497 437 60

COLOR-04 Production 90 90 30 60

COLOR-05 Production 610 590 380 210

COLOR-06 Production 505 na 421 na

COLOR-07 Production 438 438 20 418
36 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Table 1. Identification and construction information for wells sampled in the Desert and Basin and Range hydrogeologic provinces, 

California GAMA Priority Basin Project, 2006–11.—Continued

[ft, foot; bls, below land surface; na, not available]

GAMA 

identification

Well 

type

Depth 

of well 

(ft bls)

Bottom of 

lowermost 

opening 

(ft bls)

Top of 

uppermost 

opening 

(ft bls)

Screen 

length 

(ft bls)

Colorado River—Continued

COLOR-08 Production 500 480 140 340

COLOR-09 Production na na na na

COLOR-10 Production 1,000 na na na

COLOR-11 Production na na na na

COLOR-12 Production 420 420 300 120

COLOR-13 Production 205 na na na

COLOR-14 Production na na na na

COLOR-15 Production na na na na

COLOR-16 Production 600 486 378 108

COLOR-17 Production 210 200 80 120

COLOR-18 Production 600 580 150 430

COLOR-19 Production 143 140 118 22

COLOR-20 Production 700 na na na

COLORU-01 Production 492 492 292 200

COLORU-02 Production 454 na 436 na

COLORU-03 Production 335 na 310 na

COLORU-04 Production 241 160 54 106

COLORU-05 Production 105 82 38 44

COLORU-06 Production 500 480 160 320

COLORU-07 Production 130 na na na

COLORU-08 Production na na na na

Indian Wells Valley

OIW-01 Production 850 830 320 510

OIW-02 Production 480 470 310 160

OIW-03 Production na na na na

OIW-04 Production 400 400 320 80

OIW-05 Production 1,020 1,000 560 440

OIW-06 Production 232 181 135 46

OIW-07 Production 620 600 260 340

OIW-08 Production 190 na na na

OIW-09 Production na na na na

OIW-10 Production 260 260 200 60

OIW-11 Monitoring 77 77 75 2

OIW-12 Monitoring 200 na 100 na

OIW-13 Monitoring 70 70 60 10

OIWU-01 Production 1,220 1,200 600 600

OIWU-02 Production 730 730 430 300

OIWU-03 Production 920 900 600 300

OIWU-04 Monitoring 690 690 232 458

GAMA 

identification

Well 

type

Depth 

of well 

(ft bls)

Bottom of 

lowermost 

opening 

(ft bls)

Top of 

uppermost 

opening 

(ft bls)

Screen 

length 

(ft bls)

Indian Wells Valley—Continued

OIWU-05 Monitoring na na na na

OIWU-06 Monitoring na na na na

OIWU-07 Monitoring 215 215 na 215

OIWU-08 Monitoring 150 130 55 75

Low-Use Basins

LUB-01 Production 381 381 205 176

LUB-02 Production 543 na na na

LUB-03 Production 456 na na na

LUB-04 Production na na na na

LUB-05 Production 490 480 322 158

LUB-06 Production 490 475 220 255

LUB-07 Production 660 440 160 280

LUB-08 Production 800 780 300 480

LUB-09 Production 224 224 184 40

LUB-10 Production 300 300 190 110

LUB-11 Production 600 590 452 138

LUB-12 Production 840 800 650 150

LUB-13 Production 500 500 100 400

LUB-14 Production 240 na na na

LUB-15 Production 312 312 212 100

LUB-16 Production 105 105 na 105

LUB-17 Production 584 584 484 100

LUB-18 Production 36 36 15 21

LUB-19 Production na na na na

LUB-20 Production 100 na 70 na

LUB-21 Production 400 400 300 100

LUB-22 Production 1,200 1,190 690 500

LUB-23 Production 403 400 200 200

LUB-24 Production 788 na 235 na

LUB-25 Production 866 864 534 330

LUB-26 Production 711 na na na

LUB-27 Production 400 400 140 260

LUBU-01 Production 1,010 940 350 590

LUBU-02 Monitoring 400 380 140 240

LUBU-03 Production na na na na

Mojave

MOJO-12 Production 360 360 110 250

MOJO-13 Production na na na na

MOJO-14 Production 530 505 365 140
Tables 37

Table 1. Identification and construction information for wells sampled in the Desert and Basin and Range hydrogeologic provinces, 

California GAMA Priority Basin Project, 2006–11.—Continued

[ft, foot; bls, below land surface; na, not available]

GAMA 

identification

Well 

type

Depth 

of well 

(ft bls)

Bottom of 

lowermost 

opening 

(ft bls)

Top of 

uppermost 

opening 

(ft bls)

Screen 

length 

(ft bls)

Mojave—Contnued

MOJO-15 Production 720 700 495 205

MOJO-16 Production 1,000 1,000 515 485

MOJO-17 Production 810 810 410 400

MOJO-18 Production na na na na

MOJO-19 Production 420 180 160 20

MOJO-25 Production na na na na

MOJO-27 Production 1,110 1,110 850 260

MOJO-29 Production 480 na na na

MOJO-30 Production 650 630 430 200

MOJO-31 Production 1,130 1,120 660 460

MOJO-35 Production 400 400 160 240

MOJO-36 Production 140 na na na

MOJO-39 Production na na na na

MOJO-41 Production na na na na

MOJO-42 Production 388 na 242 na

MOJO-49 Production 400 390 200 190

MOJOU-01 Production 451 451 251 200

MOJOU-02 Monitoring 790 790 770 20

MOJOU-03 Monitoring 610 610 590 20

MOJOU-04 Monitoring 583 574 534 40

MOJOU-05 Monitoring 750 750 730 20

Owens Valley

OV-01 Production 217 na 70 na

OV-02 Production 126 120 60 60

OV-03 Production 250 250 50 200

OV-04 Production 340 na na na

OV-05 Production na na na na

OV-06 Production 196 196 155 41

OV-07 Production 650 650 290 360

OV-08 Production 800 800 240 560

OV-09 Production na na na na

OV-10 Production na na na na

OV-11 Production 161 161 121 40

OV-12 Production 126 116 96 20

OV-13 Production 114 109 69 40

OV-14 Production 237 220 40 180

OV-15 Monitoring 202 180 150 30

OV-16 Production 185 130 20 110

GAMA 

identification

Well 

type

Depth 

of well 

(ft bls)

Bottom of 

lowermost 

opening 

(ft bls)

Top of 

uppermost 

opening 

(ft bls)

Screen 

length 

(ft bls)

Owens Valley—Continued

OV-17 Monitoring 616 322 47 275

OV-18 Production 642 640 210 430

OV-19 Production na na na na

OV-20 Production na na na na

OV-21 Production 255 240 140 100

OV-22 Production 650 640 300 340

OV-23 Production 188 na na na

OV-24 Production 305 300 120 180

OV-25 Production 390 390 70 320

OV-26 Production 128 120 70 50

OV-27 Production 200 200 100 100

OV-28 Production 150 150 130 20

OV-29 Production 200 200 180 20

OV-30 Production 388 360 143 217

OV-31 Production 240 180 150 30

OV-32 Production 125 108 51 57

OV-33 Monitoring 319 298 258 40

OV-34 Monitoring na 700 660 40

OV-35 Monitoring 290 290 250 40

OV-36 Production na na na na

OV-37 Production 400 390 100 290

OV-38 Production 232 232 80 152

OV-39 Production 272 272 55 217

OV-40 Monitoring 775 774 644 130

OVU-01 Production 590 570 180 390

OVU-02 Production 280 na na na

OVU-03 Monitoring 78 78 58 20

OVU-04 Monitoring 138 138 118 20

OVU-05 Monitoring 47 47 27 20

OVU-06 Monitoring 48 48 28 20

OVU-07 Monitoring 390 370 330 40

OVU-08 Production 600 600 150 450

OVU-09 Monitoring 41 41 21 20

OVU-10 Monitoring 315 315 275 40

OVU-11 Production 91 86 66 20

OVU-12 Monitoring 700 na na na

OVU-13 Monitoring 130 130 110 20
38 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Table 2. Fluoride, molybdenum, and uranium reactions added to the minteqV4.dat database that is distributed with PHREEQC version 2 

computer program (Parkhurst and Appelo, 1999).

[(s)= solid phase. All other constituents are in the aqueous phase]

Reactions Equilibrium constants (log K) Reference

Ca5

(PO4)3F(s) + 3H+ = 5Ca2+ + F– + 3HPO4

2– –17.6 Ball and Nordstrom [1991]

MoO4

2– + H+ = HMoO4– 4.24 Smith and Martell [1976]

MoO4

2– + 2H+ = H2

MoO4

0 6.85 Kaback and Runnells [1980]

MoO4

2– + 3H+ = MoO2

OH+ + H20 7.89 Kaback and Runnells [1980]

MoO4

2– + 4H+ = MoO22+ + 2H2

0 8.34 Kaback and Runnells [1980]

MoO4

2– + e– + 4H+ = MoO2+ + 2H2

0 20.95 Kaback and Runnells [1980]

MoO4

2– + 3e– + 8H+ = Mo3+ + 4H2

0 29.52 Kaback and Runnells [1980]

Na+ + MoO4

2– = NaMoO–

4 1.66 Essington [1990]

K+ + MoO4

2– = KMoO–

4 1.29 Essington [1992]

Mg2+ + MoO4

2– = MgMoO40 3.03 Essington [1992]

Ca2+ + MoO4

2– = CaMoO40 2.57 Essington [1992]

Ca2+ + MoO4

2– = CaMoO4(s) 8.45 Kaback and Runnells [1980]

UO2

2+ + 2Ca2+ + 3HCO3– = Ca2

UO2(CO3)3

0 + 3H+ –0.29 Dong and Brooks [2006]

Table 3. Geochemical conditions determined from the analyses of samples collected at well sites in the each of the nine study areas, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project, 2006–11. 

[<, less than; >, greater than; μg/L, micrograms per liter; na, not available; PMC, percent modern carbon; BP, before present; U, analyzed for but not detected;

M, presence verified but not quantified; nc, not collected; Ca, calcium; Na, sodium; HCO3, bicarbonate; SO4, sulfate; Cl, chloride; mixed, no single cation or

anion constitutes more than half of the cations or anions]

GAMA 

identification 

number

Uncorrected carbon-14 

age, years BP 

(Ao

=84 PMC)1

pH

Sulfide

Redox 

category2

Major ion type3

Presence / 

absence

Concentration

(μg/L) Cations Anions

Antelope Valley

ANT-02 < 100 7.7 U nc Oxic Ca HCO3

ANT-04 5,600 8.0 U nc Oxic Mixed HCO3

ANT-07 4,800 8.0 U nc Oxic Na HCO3

ANT-09 < 100 7.0 U nc Mixed Mixed Mixed

ANT-12 2,200 8.0 U nc Oxic Na Mixed

ANT-15 1,400 7.2 U nc Oxic Mixed HCO3

ANT-17 3,000 8.0 U nc Oxic Mixed HCO3

ANT-26 300 7.8 U nc Anoxic Ca HCO3

ANT-34 na 7.4 U nc Mixed Na Mixed

ANT-40 9,000 7.9 U nc Oxic Na HCO3

ANT-43 1,400 7.9 U nc Oxic Mixed Mixed

ANT-45 10,000 8.3 U nc Oxic Na HCO3

ANT-46 2,800 7.8 U nc Oxic Ca Mixed

ANT-47 6,500 7.9 M nc Oxic Mixed HCO3

ANT-48 500 7.4 U nc Oxic Ca Mixed

ANT-49 2,300 8.2 U nc Oxic Na HCO3

ANT-51 7,900 8.9 U nc Oxic Na HCO3

ANT-52 3,800 8.6 U nc Anoxic Mixed HCO3

ANT-U 4,600 8.2 U nc Oxic Mixed HCO3
Tables 39

Table 3. Geochemical conditions determined from the analyses of samples collected at well sites in the each of the nine study areas, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project, 2006–11.—Continued

[<, less than; >, greater than; μg/L, micrograms per liter; na, not available; PMC, percent modern carbon; BP, before present; U, analyzed for but not detected;

M, presence verified but not quantified; nc, not collected; Ca, calcium; Na, sodium; HCO3, bicarbonate; SO4, sulfate; Cl, chloride; mixed, no single cation or

anion constituent more than half of the cations or anions]

GAMA 

identification 

number

Uncorrected carbon-14 

age, years BP 

(Ao

=84 PMC)1

pH

Sulfide

Redox 

category2

Major ion type3

Presence / 

absence

Concentration

(μg/L) Cations Anions

Borrego Valley

BV-03 < 100 7.4 U nc Oxic Mixed SO4

BV-04 500 7.3 U nc Oxic Na Cl

BV-07 na 8.9 U nc Oxic Na Mixed

BVU-01 12,500 8.3 U nc Oxic Na Mixed

Central Desert

CD-01 < 100 7.8 U nc Oxic Mixed HCO3

CD-03 800 7.6 U nc Oxic Na HCO3

CD-04 < 100 7.9 U nc Oxic Ca HCO3

CD-07 2,800 8.1 U nc Oxic Na HCO3

CD-09 < 100 7.6 U nc Oxic Ca Mixed

CD-11 < 100 7.4 U nc Oxic Mixed HCO3

CD-12 9,500 8.5 U nc Oxic Na HCO3

CD-13 8,000 8.5 U nc Oxic Na Mixed

CD-14 9,600 8.6 U nc Oxic Na Mixed

Coachella Valley

COA-01 < 100 7.6 U nc Oxic Ca HCO3

COA-02 24,900 8.0 U nc Anoxic Na Cl

COA-03 2,900 8.0 U nc Oxic Na SO4

COA-04 < 100 8.1 U nc Oxic Mixed HCO3

COA-05 400 8.0 U nc Oxic Mixed HCO3

COA-06 100 7.7 U nc Anoxic Ca Mixed

COA-07 300 7.4 U nc Oxic Ca SO4

COA-08 3,800 8.0 U nc Oxic Na SO4

COA-09 < 100 7.6 U nc Oxic Ca HCO3

COA-10 9,400 9.2 U nc Anoxic Na HCO3

COA-11 16,500 9.1 U nc Anoxic Na Mixed

COA-12 29,300 8.5 U nc Anoxic Na Mixed

COA-13 < 100 7.7 U nc Oxic Ca HCO3

COA-14 < 100 7.9 U nc Oxic Ca HCO3

COA-15 < 100 7.8 U nc Oxic Mixed Mixed

COA-16 1,500 8.1 U nc Oxic Na SO4

COA-17 16,700 7.5 U nc Oxic Mixed SO4

COA-18 1,400 8.0 U nc Oxic Na Mixed

COA-19 16,900 8.4 U nc Anoxic Na SO4

COAU-01 < 100 7.5 U nc Oxic Ca HCO3

COAU-02 1,300 7.8 nc nc Oxic Mixed Mixed

COAU-03 < 100 7.7 U nc Oxic Ca HCO3

COAU-04 4,800 8.0 U nc Anoxic Na SO4
40 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Table 3. Geochemical conditions determined from the analyses of samples collected at well sites in the each of the nine study areas, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project, 2006–11.—Continued

[<, less than; >, greater than; μg/L, micrograms per liter; na, not available; PMC, percent modern carbon; BP, before present; U, analyzed for but not detected;

M, presence verified but not quantified; nc, not collected; Ca, calcium; Na, sodium; HCO3, bicarbonate; SO4, sulfate; Cl, chloride; mixed, no single cation or

anion constituent more than half of the cations or anions]

GAMA 

identification 

number

Uncorrected carbon-14 

age, years BP 

(Ao

=84 PMC)1

pH

Sulfide

Redox 

category2

Major ion type3

Presence / 

absence

Concentration

(μg/L) Cations Anions

Coachella Valley—Continued

COAU-05 < 100 7.6 U nc Oxic Ca HCO3

COAU-06 1,700 7.7 U nc Oxic Mixed HCO3

COAU-07 < 100 7.3 U nc Oxic Ca HCO3

COAU-08 < 100 7.1 U nc Oxic Ca HCO3

COAU-09 7,000 9.1 U nc Anoxic Na HCO3

COAU-10 200 7.6 U nc Oxic Mixed SO4

COAU-11 1,100 7.7 U nc Oxic Na HCO3

COAU-12 < 100 7.2 U nc Oxic Ca HCO3

COAU-13 < 100 7.9 U nc Oxic Ca HCO3

COAU-14 100 7.8 U nc Oxic Ca Mixed

COAU-15 < 100 7.5 U nc Oxic Mixed Mixed

COAU-16 500 7.9 U nc Oxic Ca HCO3

Colorado River

COLOR-01 < 100 7.5 U nc Oxic Mixed SO4

COLOR-02 < 100 7.6 U nc Oxic Na Mixed

COLOR-03 < 100 7.5 M 250 Anoxic-Fe Na Mixed

COLOR-04 < 100 7.7 M 150 Anoxic-Fe Mixed Mixed

COLOR-05 < 100 7.5 M nc Anoxic Mixed Mixed

COLOR-06 < 100 7.5 M 100 Anoxic-Fe Mixed Mixed

COLOR-07 < 100 7.3 M nc Anoxic-Fe Mixed Mixed

COLOR-08 12,200 8.0 M nc Oxic Na Cl

COLOR-09 2,800 7.8 U nc Anoxic-Fe Na Mixed

COLOR-10 < 100 7.4 U nc Anoxic-Fe Na Mixed

COLOR-11 < 100 7.0 U nc Oxic Na Mixed

COLOR-12 12,200 7.9 U nc Oxic Na Mixed

COLOR-13 6,000 7.6 U nc Oxic Na Mixed

COLOR-14 1,900 7.6 U nc Anoxic Na Mixed

COLOR-15 < 100 7.2 U nc Anoxic-Fe Mixed SO4

COLOR-16 < 100 7.6 M nc Anoxic-Fe Mixed Mixed

COLOR-17 6,700 7.8 U nc Oxic Na Cl

COLOR-18 3,600 8.0 U nc Oxic Na Mixed

COLOR-19 < 100 7.5 U nc Mixed Mixed Mixed

COLOR-20 14,400 8.9 U nc Oxic Na Mixed

COLORU-01 200 7.7 M nc Anoxic Mixed Mixed

COLORU-02 300 7.6 M nc Anoxic-Fe Mixed Mixed

COLORU-03 1,200 7.7 M nc Mixed Na Mixed

COLORU-04 < 100 7.1 U nc Mixed Na SO4

COLORU-05 < 100 7.5 U nc Mixed Mixed SO4
Tables 41

Table 3. Geochemical conditions determined from the analyses of samples collected at well sites in the each of the nine study areas, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project, 2006–11.—Continued

[<, less than; >, greater than; μg/L, micrograms per liter; na, not available; PMC, percent modern carbon; BP, before present; U, analyzed for but not detected;

M, presence verified but not quantified; nc, not collected; Ca, calcium; Na, sodium; HCO3, bicarbonate; SO4, sulfate; Cl, chloride; mixed, no single cation or

anion constituent more than half of the cations or anions]

GAMA 

identification 

number

Uncorrected carbon-14 

age, years BP 

(Ao

=84 PMC)1

pH

Sulfide

Redox 

category2

Major ion type3

Presence / 

absence

Concentration

(μg/L) Cations Anions

Colorado River—Continued

COLORU-06 1,600 7.9 U nc Oxic Na Mixed

COLORU-07 15,900 7.8 U nc Anoxic Na Cl

COLORU-08 < 100 7.5 U nc Anoxic-Fe Mixed SO4

Indian Wells Valley

OIW-01 23,100 9.4 U nc Oxic Na HCO3

OIW-02 6,400 7.9 U nc Oxic Na Mixed

OIW-03 na 9.8 U nc Oxic Na HCO3

OIW-04 na na U nc na Mixed HCO3

OIW-05 2,100 7.4 U nc Oxic Mixed Mixed

OIW-06 na 7.8 U nc Oxic Na Cl

OIW-07 26,500 8.7 M nc Anoxic Na Mixed

OIW-08 14,700 7.2 U nc Oxic Na Mixed

OIW-09 < 100 6.9 U nc Oxic Ca Mixed

OIW-10 na 7.8 U nc Anoxic Na Mixed

OIW-11 2,300 7.7 U nc Anoxic Na SO4

OIW-12 11,600 7.5 U nc Mixed Na Mixed

OIW-13 na 7.5 U nc Oxic Mixed Cl

OIWU-01 10,500 8.1 U nc Oxic Na HCO3

OIWU-02 21,400 9.0 U nc Anoxic Na HCO3

OIWU-03 11,800 8.2 U nc Oxic Na Mixed

OIWU-04 3,400 7.7 U nc Oxic Mixed Mixed

OIWU-05 3,100 7.7 U nc Oxic Mixed Mixed

OIWU-06 28,100 8.0 M nc Anoxic Na Cl

OIWU-07 6,000 7.7 U nc Mixed Mixed Mixed

OIWU-08 na 7.4 U nc Oxic Mixed Mixed

Low-Use Basins

LUB-01 5,100 7.7 U nc Oxic Ca Mixed

LUB-02 11,300 7.9 U nc Oxic Na Mixed

LUB-03 4,200 7.7 U nc Anoxic Mixed HCO3

LUB-04 12,200 7.8 U nc Oxic Na Mixed

LUB-05 2,700 7.8 U nc Oxic Mixed HCO3

LUB-06 11,000 8.2 U nc Oxic Na Mixed

LUB-07 21,800 8.1 U nc Oxic Na Mixed

LUB-08 19,200 7.6 U nc Oxic Na Cl

LUB-09 11,000 7.8 U nc Oxic Na Mixed

LUB-10 14,500 9.2 M 120 Anoxic Na HCO3

LUB-11 20,800 7.7 U nc Oxic Na Mixed

LUB-12 9,900 8.1 U nc Oxic Na Mixed
42 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Table 3. Geochemical conditions determined from the analyses of samples collected at well sites in the each of the nine study areas, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project, 2006–11.—Continued

[<, less than; >, greater than; μg/L, micrograms per liter; na, not available; PMC, percent modern carbon; BP, before present; U, analyzed for but not detected;

M, presence verified but not quantified; nc, not collected; Ca, calcium; Na, sodium; HCO3, bicarbonate; SO4, sulfate; Cl, chloride; mixed, no single cation or

anion constituent more than half of the cations or anions]

GAMA 

identification 

number

Uncorrected carbon-14 

age, years BP 

(Ao

=84 PMC)1

pH

Sulfide

Redox 

category2

Major ion type3

Presence / 

absence

Concentration

(μg/L) Cations Anions

Low-Use Basins—Continued

LUB-13 13,800 7.9 U nc Oxic Mixed HCO3

LUB-14 3,300 7.3 U nc Oxic Mixed SO4

LUB-15 8,000 8.0 U nc Oxic Na Mixed

LUB-16 < 100 7.6 U nc Oxic Mixed SO4

LUB-17 18,100 7.8 U nc Oxic Na Cl

LUB-18 < 100 6.9 U nc Oxic Na Mixed

LUB-19 8,300 8.4 U nc Anoxic Na Cl

LUB-20 7,400 7.7 U nc Oxic Na Cl

LUB-21 14,400 8.9 U nc Oxic Na Cl

LUB-22 27,400 8.8 U nc Anoxic Na Cl

LUB-23 3,700 7.7 U nc Oxic Mixed HCO3

LUB-24 24,700 7.5 U nc Oxic Ca SO4

LUB-25 17,800 8.2 U nc Oxic Na Cl

LUB-26 < 100 7.6 U nc Oxic Mixed Mixed

LUB-27 17,200 8.0 U nc Oxic Na Mixed

LUBU-01 6,500 8.4 U nc Oxic Na HCO3

LUBU-02 12,500 7.8 U nc Oxic Na HCO3

LUBU-03 < 100 7.5 U nc Oxic Mixed Mixed

Mojave

MOJO-12 < 100 7.3 U nc Oxic Mixed HCO3

MOJO-13 18,500 8.3 U nc Anoxic Mixed SO4

MOJO-14 4,600 9.3 U nc Oxic Na HCO3

MOJO-15 6,400 9.6 U nc Oxic Na HCO3

MOJO-16 2,300 8.6 U nc Oxic Na HCO3

MOJO-17 < 100 7.0 U nc Oxic Ca HCO3

MOJO-18 < 100 7.3 U nc Oxic Ca HCO3

MOJO-19 2,600 8.6 U nc Oxic Na Mixed

MOJO-25 300 7.5 U nc Anoxic Mixed Mixed

MOJO-27 12,700 9.0 U nc Oxic Na HCO3

MOJO-29 9,500 7.6 U nc Oxic Na Cl

MOJO-30 7,100 8.4 U nc Oxic Na SO4

MOJO-31 9,100 8.3 U nc Anoxic Na SO4

MOJO-35 < 100 7.7 U nc Oxic Mixed Mixed

MOJO-36 7,000 8.0 U nc Anoxic Na HCO3

MOJO-39 1,300 8.2 U nc Oxic Na Mixed

MOJO-41 200 7.6 U nc Oxic Na HCO3

MOJO-42 < 100 7.9 U nc Oxic Na HCO3

MOJO-49 na 7.9 U nc Oxic Na SO4
Tables 43

Table 3. Geochemical conditions determined from the analyses of samples collected at well sites in the each of the nine study areas, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project, 2006–11.—Continued

[<, less than; >, greater than; μg/L, micrograms per liter; na, not available; PMC, percent modern carbon; BP, before present; U, analyzed for but not detected;

M, presence verified but not quantified; nc, not collected; Ca, calcium; Na, sodium; HCO3, bicarbonate; SO4, sulfate; Cl, chloride; mixed, no single cation or

anion constituent more than half of the cations or anions]

GAMA 

identification 

number

Uncorrected carbon-14 

age, years BP 

(Ao

=84 PMC)1

pH

Sulfide

Redox 

category2

Major ion type3

Presence / 

absence

Concentration

(μg/L) Cations Anions

Mojave—Continued

MOJOU-01 < 100 7.2 U nc Oxic Ca HCO3

MOJOU-02 30,900 9.4 U nc Anoxic Na HCO3

MOJOU-03 < 100 7.4 U nc Mixed Mixed Mixed

MOJOU-04 12,300 8.4 U nc Oxic Na Mixed

MOJOU-05 31,500 9.7 U nc Anoxic Na HCO3

Owens Valley

OV-01 < 100 7.7 U nc Oxic Ca HCO3

OV-02 < 100 6.5 U nc Oxic Ca HCO3

OV-03 na na nc nc na Mixed Mixed

OV-04 < 100 6.8 U nc Oxic Mixed HCO3

OV-05 < 100 6.6 nc nc Oxic Ca HCO3

OV-06 9,500 8.0 U nc Anoxic Ca HCO3

OV-07 < 100 6.8 U nc Oxic Ca HCO3

OV-08 < 100 6.8 U nc Oxic Ca HCO3

OV-09 < 100 6.9 U nc Oxic Ca HCO3

OV-10 < 100 6.8 U nc Oxic Ca HCO3

OV-11 600 8.0 U nc Oxic Mixed HCO3

OV-12 < 100 6.6 U nc Oxic Ca HCO3

OV-13 1,300 7.0 U nc Oxic Mixed HCO3

OV-14 < 100 7.3 U nc Oxic Mixed HCO3

OV-15 2,700 8.6 U nc Oxic Na HCO3

OV-16 < 100 7.5 U nc Oxic Mixed HCO3

OV-17 7,600 7.7 U nc Oxic Na HCO3

OV-18 1,800 7.9 U nc Oxic Ca HCO3

OV-19 < 100 6.8 U nc Mixed Mixed HCO3

OV-20 na 7.8 U nc na Ca HCO3

OV-21 na 7.5 U nc Oxic Mixed HCO3

OV-22 < 100 7.7 U nc Oxic Mixed HCO3

OV-23 na 7.3 U nc Oxic Ca HCO3

OV-24 < 100 7.3 U nc Oxic Ca HCO3

OV-25 < 100 6.6 U nc Oxic Ca HCO3

OV-26 < 100 6.4 U nc Oxic Ca HCO3

OV-27 800 7.5 U nc Oxic Ca Mixed

OV-28 na 7.7 U nc Oxic Ca Mixed

OV-29 na 7.4 U nc Oxic Ca HCO3

OV-30 6,500 7.6 U nc Oxic Mixed HCO3

OV-31 2,300 7.4 U nc Oxic Mixed HCO3

OV-32 24,600 7.4 U nc Anoxic Mixed HCO3
44 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Table 3. Geochemical conditions determined from the analyses of samples collected at well sites in the each of the nine study areas, 

Desert and Basin and Range study unit, California GAMA Priority Basin Project, 2006–11.—Continued

[<, less than; >, greater than; μg/L, micrograms per liter; na, not available; PMC, percent modern carbon; BP, before present; U, analyzed for but not detected;

M, presence verified but not quantified; nc, not collected; Ca, calcium; Na, sodium; HCO3, bicarbonate; SO4, sulfate; Cl, chloride; mixed, no single cation or

anion constituent more than half of the cations or anions]

GAMA 

identification 

number

Uncorrected carbon-14 

age, years BP 

(Ao

=84 PMC)1

pH

Sulfide

Redox 

category2

Major ion type3

Presence / 

absence

Concentration

(μg/L) Cations Anions

Owens Valley—Continued

OV-33 na 7.3 M nc Anoxic-Fe Mixed HCO3

OV-34 3,100 6.7 U nc Mixed Mixed HCO3

OV-35 13,700 6.4 U nc Mixed Na Mixed

OV-36 600 7.2 U nc Oxic Mixed HCO3

OV-37 < 100 6.9 U nc Oxic Ca HCO3

OV-38 na 6.9 U nc Oxic Ca HCO3

OV-39 na 7.1 U nc Anoxic-Fe Na HCO3

OV-40 33,600 7.2 M nc Mixed Na HCO3

OVU-01 < 100 7.8 U nc Oxic Ca HCO3

OVU-02 9,900 7.5 U nc Mixed Mixed Mixed

OVU-03 12,700 7.7 M nc Mixed Mixed HCO3

OVU-04 < 100 7.7 U nc Oxic Mixed HCO3

OVU-05 < 100 7.0 U nc Oxic Ca HCO3

OVU-06 1,400 7.7 U nc Anoxic Na HCO3

OVU-07 15,200 7.7 M 10 Anoxic Na HCO3

OVU-08 600 9.1 U nc Oxic Na HCO3

OVU-09 < 100 6.9 U nc Oxic Mixed HCO3

OVU-10 13,000 7.8 M 100 Anoxic Mixed HCO3

OVU-11 na 7.5 U nc Oxic Ca HCO3

OVU-12 33,600 8.4 M nc Anoxic Na HCO3

OVU-13 33,400 8.7 U nc Anoxic Na HCO3

1 Carbon-14 age calculated using equation: t = –8267*ln(Ameas

/Ao). Ages rounded to nearest hundred [Ao, Initial percent modern carbon-14 ; Ameas measured

percent modern carbon]

2 Mixed redox conditions are not discussed in this report with respect to distribution in groundwater or in relation to trace element occurrence.

3 Major ion types are classifications based on the major ion composition expressed in units of milliequivalents per liter. Cations: Ca, calcium >50 percent; Na,

the sum of sodium and potassium >50 percent; mixed, Ca, Na, and magnesium each are less than 50 percent of cations. Anions: Cl, chloride >50 percent; HCO3,

the sum of bicarbonate plus carbonate >50 percent; SO4, sulfate >50 percent; mixed, Cl, HCO3, and SO4 each are less than 50 percent of anions.
Tables 45

Table 4. Results of non-parametric analysis of correlations between selected potential explanatory variables, Desert and Basin and 

Range study unit, California GAMA Priority Basin Project.

[Results are shown only for those correlations with a p-value <0.05; Rho (ρ), Spearman’s correlation statistic; a positive rho value indicates positive correlations;

a negative rho value indicates negative correlations. Kruskal-Wallis multiple comparison test shows which categorical variable tends to have higher values of the

tested explanatory variable. For example, anoxic versus oxic tested against groundwater age, anoxic > oxic means that samples categorized as anoxic tended to

have older groundwater ages than samples categorized as oxic; Na, sodium; Ca, calcium; Cl, chloride; SO4, sulfate; HCO3, bicarbonate; —, p-value greater than

or equal to 0.05]

Explanatory 

factor pH

Depth to the 

top of uppermost opening

Total 

dissolved 

solids

Oxidation-reduction 

(redox) class

(oxic, anoxic, anoxic-Fe)

Cation class

(Na, Mixed, Ca)

Anion class

(HCO3

, Cl, SO4, Mixed)

Groundwater age 0.61 0.25 0.21 anoxic > oxic > anoxic-Fe Na > Mixed > Ca Cl > HCO3, SO4, and Mixed

pH 0.39 — anoxic > oxic and anoxic-Fe Na > Ca and Mixed —

Depth to the top of

uppermost opening

— — Na > Ca —

Total dissolved solids anoxic-Fe > anoxic > oxic Na and Mixed > Ca Cl and SO4 > Mixed > HCO3

Table 5. Summary concentration statistics for trace elements and minor ions presented in this report, California GAMA Priority Basin 

Project.

[Benchmarks: MCL-US, USEPA maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; HAL-US,

USEPA lifetime health advisory level. Other abbreviations: USEPA, U.S. Environmental Protection Agency; CDPH, California Department of Public Health;

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

Constituent Units

Concentrations in 223 groundwater samples Benchmark1 Percentage 

of samples 

above 

benchmark

Min 10th 

percentile Median 90th percentile Max Type Concentration

Arsenic μg/L 0.07 0.22 2.15 18.6 513 MCL-US 10 18

Boron μg/L 8.0 17 145.5 1,050 37,100 NL-CA 1,000 11

Fluoride mg/L 0.1 0.19 0.64 2.6 12.6 MCL-CA2 2 13

Molybdenum μg/L 0.4 2.0 6.8 36.4 157 HAL-US 40 7

Strontium μg/L 7.7 104 348 1,660 12,100 HAL-US 4,000 3

Uranium μg/L 0.02 0.37 3.42 15 64.4 MCL-US 30 4

Vanadium μg/L 0.09 0.72 7.45 24.60 190 NL-CA 50 3

1 Benchmark values as of March 2014 (U.S. Environmental Protection Agency, 2009, 2012; California Department of Public Health, 2008, 2010).

2 The MCL-US for fluoride is 4 mg/L.
46 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

Table 6. Results of non-parametric analysis of correlations between water-quality constituents and potential explanatory factors, 

Desert and Basin and Range hydrogeologic provinces, California GAMA Priority Basin Project.

[Results are shown only for those correlations with a p-value < 0.05; rho (ρ), Spearman’s correlation statistic; a positive rho value indicates positive correlations;

a negative rho value indicates negative correlations. Kruskal-Wallis multiple comparison test shows which categorical variable tended to have higher

concentrations of the tested constituent. For example, anoxic versus oxic tested against arsenic, anoxic > oxic means that samples categorized as anoxic tended

to have higher concentrations of arsenic than samples categorized as oxic; Na, sodium; Ca, calcium; Cl, chloride; SO4, sulfate; HCO3, bicarbonate; —, p-value

greater than or equal to 0.05]

Constituent 

Spearman’s Rho (ρ) Kruskal-Wallis multiple comparison test

Uncorrected 

carbon-14 

age

pH

Total 

dissolved 

solids

Depth to the 

top of the 

uppermost 

opening

Oxidation-reduction class

(oxic, anoxic, anoxic-Fe)

Cation class

(Na, mixed, Ca)

Anion class

(HCO3

, Cl, SO4, mixed)

Arsenic 0.66 0.48 0.17 — anoxic > oxic > anoxic-Fe Na > mixed > Ca Cl > HCO3 and SO4

Boron 0.52 — 0.62 — anoxic > oxic Na > mixed > Ca Cl > HCO3, mixed, and

SO4mixed > HCO3

Fluoride 0.56 0.41 0.36 — — Na > mixed > Ca Cl > HCO3

Molybdenum 0.32 0.22 0.29 — — Na > Ca and mixed Cl and SO4 > HCO3

Strontium — — 0.84 — anoxic-Fe > oxic and anoxic Na and mixed > Ca Cl, mixed, and SO4

> HCO3

Uranium –0.26 –0.26 — –0.16 oxic > anoxic and anoxic-Fe Ca > Na —

Vanadium 0.33 0.50 — 0.25 oxic and anoxic > anoxic-Fe Na > Ca and mixed —
Tables 47

Table 7. Aqueous speciation predicted by speciation modeling (PHREEQC) in groundwater and summary statistics for trace element 

concentrations, Desert and Basin and Range hydrogeologic provinces, California GAMA Priority Basin Project.

Aqueous 

species 

Portion of the total concentration in a sample present as the species Number of samples 

containing species1 Minimum 10th percentile Median 90th percentile Maximum

Arsenic

HAsO4

2– 24 61 89 98 99 223

H2

AsO4

– 0 2 11 40 76 216

Boron

H3

BO3 20 80 97 99 100 223

H2

BO3

– 0 0 3 19 79 205

Fluoride

F– 79 91 97 99 100 223

CaF+ 0 0 1 1 2 122

MgF+ 0 0 2 7 18 189

Molybdenum

MoO4

2– 35 52 72 86 93 223

MgMoO4 0 1 10 23 41 212

CaMoO4 0 4 10 16 22 217

NaMoO4

– 1 2 6 19 63 223

Strontium

Sr2+ 68 82 92 97 99 223

SrSO4 0 1 5 15 30 216

SrHCO3

+ 1 1 2 4 21 223

SrCO3 0 0 0 1 16 47

Uranium

Ca2

UO2(CO3)3 0 31 94 98 99 221

CaUO2

(CO3)3

2– 0 1 1 4 7 213

UO2

(CO3)2

2– 0 0 2 6 13 180

UO2

(CO3)3

4– 0 1 1 13 99 206

UO2

CO3 0 0 0 0 5 130

UO2

(HPO4)2

2– 0 0 0 24 100 56

Vanadium

H2

VO4

– 0 28 82 96 99 222

HVO4

2– 0 3 14 56 95 221

V(OH)3 0 0 0 0 100 13

1 Total number of samples is 223.
48 Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply

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U.S. Geological Survey

6000 J Street, Placer Hall

Sacramento, California 95819

http://ca.water.usgs.gov

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Wright and others—Geochemical Conditions and the Occurrence of Selected Trace Elements in Groundwater Basins Used for Public Drinking-Water Supply, 

Desert and Basin and Range Hydrogeologic Provinces, 2006–11: California GAMA Priority Basin Project—SIR 2014–5173

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