U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the California State Water Resources Control Board

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

Status of Groundwater Quality in the

Upper Santa Ana Watershed, November 2006–March 2007:

California GAMA Priority Basin Project

Photo placement

Scientific Investigations Report 2012–5052
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: Urban development overlooking the Santa Ana River near Yorba Linda. 

(Photograph taken by Phil Contreras, U.S. Geological Survey, 1998) 

Back cover: Production well in Riverside County.

(Photograph taken by Mike Judd, U.S. Geological Survey, 2007) 

Sampling priority

Study area

Low-use basins

Priority basins

Areas that are outside CDWR-defined

 groundwater basins
Status of Groundwater Quality in the 

Upper Santa Ana Watershed, 

November 2006–March 2007: 

California GAMA Priority Basin Project

By Robert Kent 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 2012–5052

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

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

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Kent, Robert, and Belitz, Kenneth, 2012, Status of groundwater quality in the Upper Santa Ana Watershed, 

November 2006–March 2007—California GAMA Priority Basin Project: U.S. Geological Survey Scientific 

Investigations Report 2012–5052, 88 p.
iii

Contents

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

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

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

Description of Study Unit..............................................................................................................................4

Land Use.................................................................................................................................................7

Hydrogeologic Setting ........................................................................................................................7

Water Management .............................................................................................................................7

Methods ........................................................................................................................................................10

Relative-Concentrations and Water-Quality Benchmarks ..........................................................10

Design of Sampling Network for Status Assessment...................................................................11

CDPH-Grid Well Selection.................................................................................................................13

Selection of Constituents for Additional Evaluation .....................................................................18

Calculation of Aquifer-Scale Proportions.......................................................................................18

Status of Water Quality ..............................................................................................................................19

Inorganic Constituents.......................................................................................................................19

Trace Elements and Minor Ions ..............................................................................................26

Arsenic................................................................................................................................26

Boron ................................................................................................................................26

Molybdenum......................................................................................................................26

Uranium and Radioactive Constituents..................................................................................26

Gross Alpha Activity.........................................................................................................38

Nutrients......................................................................................................................................38

Nitrate ................................................................................................................................38

Constituents with Secondary Maximum Contaminant Level Benchmarks......................38

Total Dissolved Solids ......................................................................................................38

Iron and Manganese........................................................................................................39

Organic Constituents .........................................................................................................................39

Trihalomethanes.........................................................................................................................45

Solvents.......................................................................................................................................45

PCE and TCE ......................................................................................................................54

Fumigants ...................................................................................................................................54

DBCP ................................................................................................................................54

Other VOCs..................................................................................................................................54

Herbicides...................................................................................................................................55

Insecticides and Fungicides ....................................................................................................55

Special-Interest Constituents...........................................................................................................55

Perchlorate .................................................................................................................................55

Summary .......................................................................................................................................................55

Acknowledgments.......................................................................................................................................56

References....................................................................................................................................................56
iv

Contents—Continued

Appendix A. Use of Data from the California Department of Public Health (CDPH) Database .....63

Appendix B. Comparison of California Department of Public Health and U.S. Geological 

Survey-GAMA Data........................................................................................................................72

Appendix C. Calculation of Aquifer-Scale Proportions ........................................................................75

Appendix D. Ancillary Datasets................................................................................................................76

Figures

Figure 1. Map showing Upper Santa Ana Watershed study unit, California GAMA 

Priority Basin Project, and the California hydrogeologic provinces .................. 3

Figure 2. Map showing geographic and cultural features of the Upper Santa Ana

Watershed study unit, California GAMA Priority Basin Project........................ 5

Figure 3. Map showing study areas of the Upper Santa Ana Watershed study unit,

California GAMA Priority Basin Project................................................... 6

Figure 4. Map showing land use, locations of grid and understanding wells sampled, 

and locations of California Department of Public Health wells used for 

supplemental data on inorganic constituents, Upper Santa Ana Watershed 

study unit, California GAMA Priority Basin Project .................................... 8

Figure 5. Map showing geologic formations of the Upper Santa Ana Watershed study 

unit, California GAMA Priority Basin Project .......................................... 9

Figure 6. Map showing grid cells for each study area, grid and understanding wells

sampled during November 2006–March 2007, grid wells from which data for

inorganic constituents from the California Department of Public Health (CDPH)

database were used, and all CDPH wells in the Upper Santa Ana Watershed

study unit, California GAMA Priority Basin Project .................................... 12

Figure 7. Graph showing maximum relative-concentrations of constituents detected in 

grid wells by constituent class, Upper Santa Ana Watershed study unit, 

California GAMA Priority Basin Project................................................... 23

Figure 8. Graphs showing relative-concentrations of selected trace elements, 

radioactive constituents, nutrients, and major and minor ions in grid wells, 

Upper Santa Ana Watershed study unit, GAMA Priority Basin Project............... 27

Figure 9. Maps showing relative-concentrations for selected inorganic constituents,

arsenic, boron, molybdenum, uranium, vanadium, fluoride, gross alpha

radioactivity, radium, radon, nitrate, chloride, iron, manganese, and total

dissolved solids, Upper Santa Ana Watershed study unit, GAMA Priority 

Basin Project, USGS grid and understanding wells representative of the 

primary aquifer and the most recent analysis during November 30, 2003, to 

December 1, 2006, for CDPH wells......................................................... 31

Figure 10. Graph showing detection frequency and maximum relative-concentration of

organic and special-interest constituents detected in USGS-grid wells in the

Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project... 40
v

Figures—Continued

Figure 11 (A–L). Graphs showing detection frequency and relative-concentrations of 

selected organic and special-interest constituents in grid wells in the Upper 

Santa Ana Watershed study unit, California GAMA Priority Basin Project, 

November 2006–March 2007 ............................................................... 41

Figure 12 (A–P). Maps showing relative-concentrations for selected organic and specialinterest constituents, chloroform, bromodichloromethane, tetrachloroethene, 

trichloroethene, carbon tetrachloride, cis-1,2-dichloroethene, 1,2-dibromo-3-

chloropropane, methyl tert-butyl ether, 1,1-dichloroethene, CFC-11, CFC-12, 

atrazine, simazine, bromacil, diuron, and perchlorate, Upper Santa Ana 

Watershed study unit, GAMA Priority Basin Project.................................... 46

Tables

Table 1. Constituent class and the number of constituents and wells sampled for each

analytical schedule in the Upper Santa Ana Watershed study unit, California

GAMA Priority Basin Project, November 2006–March 2007 ........................... 14

Table 2. Inorganic constituents and associated benchmark information, and the number

of grid wells with U.S. Geological Survey-GAMA data and CDPH data, for each

constituent, Upper Santa Ana Watershed study unit, California GAMA Priority

Basin Project................................................................................. 15

Table 3. Comparison of the number of compounds and median laboratory reporting 

levels or method detection levels by type of constituent for data stored in the 

California Department of Public Health database and data collected for the 

Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project, 

November 2006–March 2007 ............................................................... 16

Table 4. Constituents reported at concentrations greater than benchmarks in the

California Department of Public Health (CDPH) database between July 13, 

1956, and November 30, 2003, Upper Santa Ana Watershed study unit, 

California GAMA Priority Basin Project................................................... 17

Table 5. Aquifer-scale proportions from grid-based and spatially weighted approaches

for constituents with high relative-concentrations during November 30,

2003–December 1, 2006, from the California Department of Public Health 

(CDPH) database, or with moderate or high relative-concentrations in samples 

collected from USGS-grid wells (November 2006–March 2007), Upper Santa 

Ana Watershed study unit, California GAMA Priority Basin Project .................. 20

Table 6. Number of constituents analyzed and detected with associated benchmarks 

in each constituent class, Upper Santa Ana Watershed study unit, California 

GAMA Priority Basin Project, November 2006 to March 2007 ........................ 24

Table 7A. Aquifer-scale proportions for inorganic constituent classes, Upper Santa Ana

Watershed study unit, California GAMA Priority Basin Project........................ 25

Table 7B. Aquifer-scale proportions for organic and special-interest constituent 

classes, Upper Santa Ana Watershed study unit, California GAMA Priority 

Basin Project................................................................................. 25
vi

Conversion Factors, Datums, and Abbreviations and Acronyms

Inch/Foot/Mile 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 foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Radioactivity

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

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.

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

at 25°C).

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

or micrograms per liter (μg/L). One milligram per liter is equivalent to 1 part per million (ppm); 

1 microgram per liter is equivalent to 1 part per billion (ppb); 1 nanogram per liter (ng/L) is 

equivalent to 1 part per trillion (ppt); 1 per mil is equivalent to 1 part per thousand.

Datums

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

(NAVD 88).

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

Abbreviations and Acronyms

AB Assembly Bill (through the California State Assembly)

AL-US U.S. Environmental Protection Agency action level 

GAMA Groundwater Ambient Monitoring and Assessment Program

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

HBSL health-based screening level

LRL laboratory reporting level

LSD land-surface datum

LT-MDL long-term method detection level

MCL-CA California Department of Public Health maximum contaminant level 

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

MDL method detection limit

NL-CA California Department of Public Health notification level 

PSW public-supply wells

RC relative-concentration

RPD relative percent difference

RSD5-US U.S. Environmental Protection Agency risk-specific dose at a risk factor of 10–5

SMCL-CA California Department of Public Health secondary maximum contaminant level 

SMCL-US U.S. Environmental Protection Agency secondary maximum contaminant level 

TEAP terminal electron-acceptor processes

TT-US treatment technique levels

USAW Upper Santa Ana Watershed (study unit)

USAWB identifier prefix for grid wells in the Bunker Hill/Cajon/Rialto-Colton study area

USAWC identifier prefix for grid wells in the Cucamonga/Chino study area

USAWE identifier prefix for grid wells in the Elsinore study area

USAWR identifier prefix for grid wells in the Riverside-Arlington/Temescal study area

USAWU identifier prefix for additional wells sampled for understanding specific 

groundwater-quality issues

USAWY identifier prefix for grid wells in the Yucaipa/San Timoteo study area

Conversion Factors, Datums, and Abbreviations and 

Acronyms—Continued
viii

Selected Chemical Names

DBCP 1,2-dibromo-3-chloropropane

MTBE methyl tert-butyl ether

NDMA N-nitrosodimethylamine

PCE perchloroethene (tetrachloroethene)

1,2,3-TCP 1,2,3-trichloropropane

TCE trichloroethene

TDS total dissolved solids

THM trihalomethane

VOC volatile organic compound

Units of Measure

pmc percent modern carbon

TU tritium unit

> greater than

< less than

≤ less than or equal to

% percent

Conversion Factors, Datums, and Abbreviations and 

Acronyms—Continued

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
Status of Groundwater Quality in the Upper Santa Ana 

Watershed, November 2006–March 2007: California 

GAMA Priority Basin Project

By Robert Kent and Kenneth Belitz

Abstract

Groundwater quality in the approximately 1,000-squaremile (2,590-square-kilometer) Upper Santa Ana Watershed 

(USAW) study unit was investigated as part of the Priority 

Basin Project of the Groundwater Ambient Monitoring and 

Assessment (GAMA) Program. The study unit is located in 

southern California in Riverside and San Bernardino Counties. 

The GAMA Priority Basin Project 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 GAMA USAW study was designed to provide 

a spatially unbiased assessment of untreated groundwater 

quality within the primary aquifer systems in the study unit. 

The primary aquifer systems (hereinafter, primary aquifers) 

are defined as the perforation interval of wells listed in the 

California Department of Public Health (CDPH) database 

for the USAW study unit. The quality of groundwater in 

shallower or deeper water-bearing zones may differ from 

that in the primary aquifers; shallower groundwater may be 

more vulnerable to surficial contamination. The assessment is 

based on water-quality and ancillary data collected by the U.S. 

Geological Survey (USGS) from 90 wells during 

November 2006 through March 2007, and water-quality data 

from the CDPH database. 

The status of the current quality of the groundwater 

resource was assessed based on data from samples analyzed 

for volatile organic compounds (VOCs), pesticides, and 

naturally occurring inorganic constituents, such as major 

ions and trace elements. The status assessment is intended 

to characterize the quality of groundwater resources within 

the primary aquifers of the USAW study unit, not the treated 

drinking water delivered to consumers by water purveyors.

Relative-concentrations (sample concentration divided 

by the health- or aesthetic-based benchmark concentration) 

were used for evaluating groundwater quality for those 

constituents that have Federal or California regulatory or 

non-regulatory benchmarks for drinking-water quality. 

A relative-concentration greater than (>) 1.0 indicates a 

concentration above a benchmark, and a relative-concentration 

less than or equal to (≤) 1.0 indicates a concentration 

equal to or less than a benchmark. Organic and specialinterest constituent relative-concentrations were classified 

as “high” (>1.0), “moderate” (0.1 < relative-concentration 

≤1.0), or “low” (≤0.1). Inorganic constituent relativeconcentrations were classified as “high” (>1.0), “moderate” 

(0.5 < relative‐concentration ≤1.0), or “low” ( ≤0.5).

Aquifer-scale proportion was used as the primary 

metric in the status assessment for evaluating regional-scale 

groundwater quality. Aquifer-scale proportions are defined as 

the percentage of the area of the primary aquifer system with 

concentrations above or below specified thresholds relative 

to regulatory or aesthetic benchmarks. High aquifer-scale 

proportion is defined as the percentage of the area of the 

primary aquifers with a relative-concentration greater than 

1.0 for a particular constituent or class of constituents; 

percentage is based on an areal, rather than a volumetric basis. 

Moderate and low aquifer-scale proportions were defined as 

the percentage of the primary aquifers with moderate and 

low relative-concentrations, respectively. Two statistical 

approaches—grid-based and spatially weighted—were used to 

evaluate aquifer-scale proportions for individual constituents 

and classes of constituents. Grid-based and spatially weighted 

estimates were comparable in the USAW study unit (within 

90-percent confidence intervals).

Inorganic constituents with human-health benchmarks 

had relative-concentrations that were high in 32.9 percent 

of the primary aquifers, moderate in 29.3 percent, and low 

in 37.8 percent. The high aquifer-scale proportion of these 

inorganic constituents primarily reflected high aquifer‐scale 

proportions of nitrate (high relative-concentration 

in 25.3 percent of the aquifer), although seven other 

inorganic constituents with human-health benchmarks 

also were detected at high relative-concentrations in some 

percentage of the aquifer: arsenic, boron, fluoride, gross 

alpha activity, molybdenum, uranium, and vanadium. 
2 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Perchlorate, as a constituent of special interest, was evaluated 

separately from other inorganic constituents, and had 

high relative-concentrations in 11.1 percent, moderate in 

53.3 percent, and low or not detected in 35.6 percent of the 

primary aquifers. In contrast to the inorganic constituents, 

relative-concentrations of organic constituents (one or more) 

were high in 6.7 percent, moderate in 11.1 percent, and low or 

not detected in 82.2 percent of the primary aquifers. 

Of the 237 organic and special-interest constituents 

analyzed for, 39 constituents were detected (21 VOCs, 

13 pesticides, 3 pharmaceuticals, and 2 constituents 

of special interest). All of the detected VOCs had 

health‐based benchmarks, and five of these—

1,1-dichloroethene, 1,2-dibromo-3-chloropropane 

(DBCP), tetrachloroethene (PCE), carbon tetrachloride, 

and trichloroethene (TCE)—were detected in at least 

one sample at a concentration above a benchmark (high 

relative-concentration). Seven of the 13 pesticides had 

health-based benchmarks, and none were detected above 

these benchmarks (no high relative-concentrations). 

Pharmaceuticals do not have health-based benchmarks. 

Thirteen organic constituents were frequently detected 

(detected in at least 10 percent of samples without regard 

to relative-concentrations): bromodichloromethane, 

chloroform, cis-1,2-dichloroethene, 1,1-dichloroethene, 

dichlorodifluoromethane (CFC‐12), methyl tert-butyl ether 

(MTBE), PCE, TCE, trichlorofluoromethane (CFC-11), 

atrazine, bromacil, diuron, and simazine.

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, 

2012, website at http://www.waterboards.ca.gov/gama/). The 

statewide GAMA Program currently consists of three projects: 

(1) the GAMA Priority Basin Project, conducted by the USGS 

(U.S. Geological Survey, 2010, website at http://ca.water.usgs.

gov/gama/); (2) the GAMA Domestic Well Project, conducted 

by the SWRCB; and (3) the GAMA Special Studies, 

conducted by LLNL. On a statewide basis, the GAMA Priority 

Basin Project focused primarily on the deep portion of the 

groundwater resource, and the SWRCB Domestic Well Project 

generally focused on the shallow aquifer systems. The primary 

aquifers may be at less risk of contamination than the shallow 

wells, such as private domestic and environmental monitoring 

wells, which are closer to surficial sources of contamination. 

As a result, concentrations of constituents, such as volatile 

organic compounds (VOCs) and nitrate, in wells screened in 

the deep primary aquifers may be lower than concentrations 

of constituents in shallow wells (Kulongoski and others, 2010; 

Landon and others, 2010).

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 1999–2000 Fiscal Year). The GAMA Priority Basin 

Project was initiated in response to the Groundwater 

Quality Monitoring Act of 2001 (State of California, 2001b, 

Section 10780-10782.3 of the California Water Code, 

Assembly Bill 599) to assess and monitor the quality of 

groundwater in California. The GAMA Priority Basin Project 

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 

Priority Basin Project, 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 Priority Basin Project 

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). 

The range of hydrologic, geologic, and climatic 

conditions that exist in California should be considered in 

an assessment of groundwater quality. Belitz and others 

(2003) partitioned the State into 10 hydrogeologic provinces, 

each with distinctive hydrologic, geologic, and climatic 

characteristics (fig. 1). All of these hydrogeologic provinces 

include groundwater basins and subbasins designated by the 

CDWR (California Department of Water Resources, 2003). 

Groundwater basins generally consist of relatively permeable, 

unconsolidated deposits of alluvial or volcanic origin. Eighty 

percent of California’s approximately 16,000 public-supply 

wells are located in designated groundwater basins. 

Groundwater basins and subbasins 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 historical leaking 

underground fuel tanks, and registered pesticide applications 

(Belitz, and others, 2003). The 116 priority basins and 

additional areas outside defined groundwater basins were 

grouped into 35 study units for the GAMA study. These 35 

study units include approximately 95 percent of public-supply 

wells in California’s groundwater basins. The Upper Santa 

Ana Watershed (USAW) study unit is located in the Transverse 

Ranges and selected Peninsular Ranges hydrogeologic 

province (fig. 1) (Belitz and others, 2003).
Introduction 3

Basin and Range

Central

Valley

Klamath

Mountains

Desert

Cascades and

Modoc Plateau

Transverse Ranges and

selected Peninsular Ranges

Southern Coast

Ranges

Sierra

Nevada

Northern

Coast

Ranges

San Diego

Drainages

Bakersfield

San

Francisco

Sacramento

OREGON

NEVADA

MEXICO

ARIZONA

Redding

Los Angeles

San Diego

PA C I F I C O C E A N

0 100 200 MILES

0 100 200 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

Provinces from Belitz and others, 2003

Upper Santa Ana

Watershed

(USAW)

(figure 2)

IP017194_Figure 01

Figure 1. Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project, and the California 

hydrogeologic provinces.
4 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Purpose and Scope 

The purposes of this report are to provide a (1) study 

unit description: description of the hydrogeologic setting 

of the Upper Santa Ana Watershed GAMA study unit, 

hereinafter referred to as the USAW study unit (fig. 1), 

(2) status assessment: assessment of the status of the current 

(2006–2007) quality of groundwater in the primary aquifers 

in the USAW study unit, and (3) compilation of ancillary 

datasets: compilation of ancillary datasets that might be used 

to help explain the status assessment of groundwater quality in 

the primary aquifers in the USAW study unit. 

The status assessment in this report includes analyses 

of water-quality data for samples from 90 wells (hereinafter 

referred to as USGS-grid wells). The USGS-grid wells 

mostly were public-supply wells listed in the CDPH database, 

but included other wells (irrigation, domestic, monitoring, 

and industrial) with perforation intervals similar to wells 

listed in the CDPH database. Samples were collected from 

USGS-grid wells for analysis of anthropogenic constituents 

such as VOCs and pesticides, as well as naturally occurring 

constituents, such as major ions, nutrients, and trace elements. 

Water-quality data from the CDPH database also were used to 

supplement data collected by USGS for the GAMA Program. 

The resulting set of water-quality data from USGS-grid and 

selected CDPH wells was considered to be representative of 

the primary aquifers in the USAW study unit.

To provide context, the water-quality data discussed in 

this report were compared to California and Federal regulatory 

and non-regulatory benchmarks for drinking water. The 

assessments in this report characterize the quality of untreated 

groundwater resources in the primary aquifers within the study 

unit, not the treated drinking water delivered to consumers 

by water purveyors. This study does not attempt to evaluate 

the quality of water delivered to consumers; after withdrawal 

from the ground, water typically is treated, disinfected, 

and (or) blended with other waters to maintain acceptable 

water quality for consumers. Regulatory benchmarks apply 

to drinking water that is delivered to the consumer, not to 

untreated groundwater.

The appendixes of this report include discussion of 

the methods used to attribute wells and characteristics of 

explanatory factors that may be used in an assessment 

for understanding in future reports. Potential explanatory 

factors examined included land use, well depth, indicators of 

groundwater age, pH, and oxidation-reduction conditions. In 

addition to the 90 grid wells sampled for the status assessment, 

9 additional wells were sampled by USGS for the purpose of 

understanding some known or suspected water-quality issue in 

the study unit (hereinafter referred to as understanding wells). 

Attributes of all grid and understanding wells are presented 

in appendix A.

Water-quality data for samples collected by the USGS 

for the GAMA Program in the USAW study unit and details of 

sample collection, analysis, and quality-assurance procedures 

for the USAW study unit are presented by Kent and Belitz 

(2009). Using the same data, this report describes methods 

used in designing the sampling network, identification of 

CDPH data for use in the status assessment, analysis of 

ancillary datasets, and estimation of aquifer-scale proportions. 

Aquifer-scale proportions are defined as the percentage of 

the area of the primary aquifer system with concentrations 

above or below specified thresholds relative to regulatory or 

aesthetic benchmarks.

Description of Study Unit

The USAW study unit covers approximately 1,000 square 

miles (mi2) in Riverside and San Bernardino Counties and 

has a population of nearly two million people (California 

Department of Finance, 2000). The USAW study unit lies 

within the Transverse Ranges and selected Peninsular 

Ranges hydrogeologic province (fig. 1) and contains three 

groundwater basins—Upper Santa Ana Valley, San Jacinto, 

and Elsinore—considered high priority for assessment by 

Belitz and others (2003) (fig. 2). For the purpose of this study, 

these groundwater basins were grouped into six study areas 

(fig. 3). The Upper Santa Ana Valley Groundwater Basin 

includes nine subbasins defined by the CDWR: Bunker Hill, 

Cajon, Rialto-Colton, Chino, Cucamonga, Yucaipa, San 

Timoteo, Riverside-Arlington, and Temescal. These were 

combined into the following four study areas: Bunker Hill/

Cajon/Rialto-Colton, Cucamonga/Chino, Riverside-Arlington/

Temescal, and Yucaipa/San Timoteo. The fifth and sixth 

study areas were composed of the San Jacinto and Elsinore 

groundwater basins respectively. Mountainous areas, 

consisting mostly of hard rock geology, were excluded from 

this study of the alluvial basins.

The USAW study unit is characterized by prominent 

mountains that rise steeply from the valleys. The San Gabriel 

and the San Bernardino Mountains make up the northern and 

northeastern edges, respectively, of the study unit (fig. 2). 

The San Jacinto Mountains lie on the southeastern edge of 

the study unit. The tallest peaks of each of these three ranges 

exceed 10,000 feet (ft) in elevation. The smaller Santa Ana 

Mountains and Chino Hills form the western edge of the study 

unit, and separate it from the Coastal Santa Ana Basin. The 

lowest elevation in the USAW region is about 500 ft above 

sea level in the area around Prado Dam in Corona, and most 

of the study unit valley floors are less than 2,000 ft above sea 

level. The climate of the USAW region is Mediterranean, with 

hot, dry summers and cool, wet winters. Temperatures range 

from daytime highs of about 90 degrees Fahrenheit (°F) in 

summer to night-time lows of about 40°F in winter (Danskin 

and others, 2006). Average annual precipitation ranges from 

10 to 24 inches in the valleys and from 24 to 48 inches in 

the mountains, where much of it comes in the form of snow 

(Belitz and others, 2004).
Description of Study Unit 5

IP017194_Figure 02 Regional map

Big Bear Lake

Bautista Creek

EXPLANATION

91

60

10

15

5

15

15

215

215

10

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

San Juan Creek

California Aqueduct

Arroyo

Trabuco

Santa Ana River

MWD Upper

Feeder

MWD Lower

Feeder

San Gabriel River

Deep Creek

Colorado River Aqueduct

Lytle Creek

Santa Ana River

Mojave River

Bear Creek

Chino Creek

Lake

Perris

Lake Irvine

San Bernardino

Riverside

Corona

Lake

Elsinore

Hemet

Canyon

Lake

San Jacinto River

San Gorgonio River

Prado

Wetlands

Lake

Mathews

Chino

Dairy

Preserve

0 10 20 MILES

0 10 20 KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Groundwater basins from California Department of Water Resources,

 Bulletin 118 (2003). 

Upper Santa Ana Valley San Jacinto

Water bodies County boundaries

Aqueducts

Wetlands Major roads

Rivers

Elsinore

San Bernardino Mts

San Gabriel Mts

Santa Ana Mts

Chino Hills

San Jacinto Mts

Figure 2. Geographic and cultural features of the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.
6 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

IP017194_Figure 03 Regional map

91

60

10

15

5

15

15

215

215

10

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

San Juan Creek

Arroyo

Trabuco

Santa Ana River

San Gabriel River

Deep Creek

Lytle Creek

Santa Ana River

Mojave River

Bear Creek

Chino Creek

San Bernardino

Riverside

Corona

Lake

Elsinore

San Jacinto River

Hemet

San Gorgonio River

Cucamonga/Chino

Bunker Hill/Cajon/Rialto-Colton

Yucaipa/San Timoteo

San Jacinto

Elsinore

RiversideArlington/Temescal

San Jose Fault

Red Hill Fault

Redlands Fault

Banning Fault

Cherry Valley Fault

San Jacinto Fault

Claremont Fault

Casa LomaFault

Cucumonga Fault

Rialto-Colton Fault

San Andreas Fault

Loma Linda Fault

Chino Fault

Elsinore

Fault

Big Bear Lake

Bautista Creek

Lake

Perris

Lake Irvine

Canyon

Lake

Prado

Wetlands

Lake

Mathews

0 10 20 MILES

0 10 20 KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection Upper Santa Ana Valley

study areas

Bunker Hill/Cajon/ San Jacinto

Rialto-Colton

Cucamonga/

Chino

Yucaipa/

San Timoteo

County

boundary

Fault

Rivers Major roads

Study area

 boundary

Elsinore

Riverside-Arlington/

Temescal

EXPLANATION

Figure 3. Study areas of the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.
Description of Study Unit 7

Land Use

Land use in the study unit is 44 percent natural, 

21 percent agricultural, and 35 percent urban, based on 

classifications from USGS National Land Cover Data 

(Nakagaki and others, 2007) (figs. 4, D1A). Agricultural land 

use in USAW is mostly in the San Jacinto study area and the 

Chino Dairy Preserve portion (fig. 2) of the Cucamonga-Chino 

study area. However, the area of the Chino Dairy Preserve 

is being converted rapidly to an urban area, and most of the 

rest of the Cucamonga-Chino study area is urban (fig. 4). 

The Yucaipa-San Timoteo and Elsinore study areas have 

predominantly natural land cover (fig. 4). Natural lands are 

mostly steep areas that are difficult to develop, or forests on 

the edges of the study unit. Most of the land use adjacent 

to the USAW study unit is natural and consists of steep 

mountains or hills generally covered by forest or chaparral. 

Hydrogeologic Setting 

Aquifers of the Upper Santa Ana Valley Groundwater 

Basin are generally unconfined and consist of alluvial 

deposits eroded from the surrounding mountains filling 

several subbasins (fig. 5) (Hamlin and others, 2005; 

California Department of Water Resources, 2004a,b,c,d,e,f,g; 

2006a,b,c,d). The thicknesses of these alluvial deposits 

range from less than 200 ft to more than 1,000 ft (Dutcher 

and Garrett, 1963). Faults play an important role in the 

groundwater flow system. The San Andreas Fault, which 

lies along the base of the San Bernardino Mountains, and 

other faults, which lie along the base of the San Gabriel 

Mountains and Chino Hills, bound the groundwater basin on 

three sides (fig. 3) (Hamlin and others, 2002). Other faults, 

such as the Rialto-Colton Fault, divide the Upper Santa Ana 

Valley Groundwater Basin into its subbasins. These interior 

faults locally influence groundwater flow and control the 

location of groundwater discharge (Woolfenden and Kadhim, 

1997; Izbicki and others, 1998; Hamlin and others, 2002). 

Groundwater flow in the San Bernardino area of the Upper 

Santa Ana Valley, known as the Bunker Hill groundwater 

subbasin, is characterized by flow paths that originate along 

the mountain front and converge to a focused discharge area in 

San Bernardino near the convergence of the Santa Ana River 

and the San Jacinto Fault (Wildermuth Environmental, Inc., 

2000; Dawson and others, 2003).

Groundwater flow in aquifers of the Elsinore Basin is 

also affected by several faults cutting alluvial and lacustrine 

sediments (fig. 5) (California Department of Water Resources, 

2006c). Flood-plain deposits in the interior of the valley 

typically reach a thickness of about 200 ft, while the principal 

water-bearing unit of the basin beneath Lake Elsinore reaches 

a maximum thickness of 2,200 ft (California Department 

of Water Resources, Southern District, 1981; California 

Department of Water Resources, 2006c).

Aquifers of the San Jacinto Groundwater Basin are 

generally unconfined and consist of a series of interconnected 

alluvium-filled valleys bounded by steep-sided bedrock 

mountains and hills (fig. 5) (Hamlin and others, 2005). 

However, some confined conditions occur in the eastern part 

of the basin (California Department of Water Resources, 

2006d). The deposits in the San Jacinto Basin valleys typically 

are 200 to 1,000 ft thick (Eastern Municipal Water District, 

2002), but may exceed 5,000 ft in the eastern part of the 

basin between the Casa Loma and Claremont Faults (fig. 3) 

(California Department of Water Resources, 2006d).

Water Management

Two stipulated judgments broadly adjudicated water 

rights in the Santa Ana Basin in 1969 (Orange County Water 

District v. City of Chino, Superior Court No. 117628; and 

Western Municipal Water District of Riverside County v. East 

San Bernardino County Water District, Superior Court No. 

78426). Currently, the Santa Ana Watermaster compiles 

hydrologic and water-quality data in annual reports, and 

the Santa Ana Watershed Project Authority, a Joint Powers 

Authority, classified as a Special District (government 

agency), plans and builds facilities to protect the water quality 

of the Santa Ana River Watershed. In addition, the Chino 

Basin was separately adjudicated in 1978; the Chino Basin 

Watermaster was directed to establish a comprehensive basin 

management program there (Miller and others, 2007). Partly 

as a result of these cases, water managers in the USAW study 

unit have been proactive in their response to declines in the 

quantity and quality of groundwater. 
8 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Big Bear Lake

Bautista Creek

EXPLANATION

91

60

10

15

5

15

15

215

215

10

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

San Juan Creek

Arroyo

Trabuco

Santa Ana River

San Gabriel River

Deep Creek

Lytle Creek

Santa Ana River

Mojave River

Bear Creek

Chino Creek

San Bernardino

Riverside

Lake

Elsinore

San Jacinto River

Hemet

San Gorgonio River

Lake

Perris

Lake Irvine

Canyon

Lake

Prado

Wetlands

Lake

Mathews

0 10 20 MILES

0 10 20 KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Land use from USGS National Land Cover Dataset

 (Nakagaki and others, 2007)

LAND-USE CLASSIFICATION WELLS—

Urban

Agricultural

Natural

County

 boundaries

Major roads

Rivers Study area

 boundary

USGS-grid well

CDPH-grid well

USGS-understanding well

IP017194_Figure 04 Figure 4. Land use, locations of grid and understanding wells sampled, and locations of California Department of Public Health 

wells used for supplemental data on inorganic constituents, Upper Santa Ana Watershed study unit, California GAMA Priority Basin 

Project. 
Description of Study Unit 9

EXPLANATION

Study unit and area boundary

Geologic unit

?

Fault—Dashed where approximately

 located, dotted where concealed,

Plio-Pleistocene sediment Franciscan complex queried where uncertain

Quaternary alluvium

Quaternary other sediment

Quarternary, mafic volcanics Pyroclastic volcanics

Tertiary, mafic volcanics

Tertiary nonmarine sediment

Tertiary marine sediment Granitic rocks

Metamorphic other

Metasediment

Ultramafic/mafic Water bodies

Wetlands

Rivers IP017194_Figure 05 Geology

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

San Juan Creek

Arroyo

Trabuco

Santa Ana River

San Gabriel River

Deep Creek

Lytle Creek

Santa Ana River

Mojave River

California Aqueduct

Bear Creek

Chino Creek

San Bernardino

Riverside

Corona

Lake

Elsinore

San Jacinto River

Hemet

San Gorgonio River

Cucamonga/Chino

Bunker Hill/Cajon/Rialto-Colton

Yucaipa/San Timoteo

San Jacinto

Elsinore

RiversideArlington/Temescal

San Jose Fault

Red Hill Fault

Redlands Fault

Banning Fault

Cherry Valley Fault

San Jacinto Fault

Claremont Fault

Casa LomaFault

Cucumonga Fault

Rialto-Colton Fault

San Andreas Fault

Loma Linda Fault

Chino Fault

Elsinore

Fault

Big Bear Lake

Bautista Creek

Lake

Perris

Lake Irvine

Canyon

Lake

Prado

Wetlands

Lake

Mathews

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

0 10 20 MILES

0 10 20 KILOMETERS

Figure 5. Geologic formations of the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.
10 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Methods 

Methods used for the GAMA Priority Basin Project 

were selected to achieve the following objectives: (1) design 

a sampling plan suitable for statistical analysis, (2) combine 

CDPH data with data collected in 2006–07 by the USGS for 

assessing water quality, (3) evaluate proportions of the primary 

aquifers having high, moderate, and low concentrations for 

constituent classes and individual constituents for additional 

evaluation, (4) select constituents for additional evaluation, 

and (5) compile and classify relevant ancillary data, so that 

relations of potential explanatory factors to water quality 

might be identified and discussed in future reports. 

This study was designed to provide a spatially unbiased 

assessment of untreated groundwater quality within the 

primary aquifer systems. The primary metric for defining 

groundwater quality in this study was relative-concentration, 

which compares groundwater chemistry to regulatory and 

non-regulatory benchmarks used to evaluate drinking-water 

quality. All constituents with benchmarks were included in the 

status assessment. Constituents were selected for additional 

evaluation in the assessment on the basis of objective 

criteria by using their measured relative-concentrations. 

Groundwater-quality data collected by the USGS for the 

GAMA Program and data compiled in the CDPH database 

were used in the status assessment. Two statistical approaches 

based on spatially unbiased grids with equal-area cells 

within each study area were used to calculate aquifer-scale 

proportions of the three relative-concentration categories. 

The status assessment included two primary 

steps. First, water-quality data were normalized to their 

respective water-quality benchmarks by calculating the 

relative-concentrations of constituents (Toccalino and others, 

2004; Toccalino and Norman, 2006). Second, aquifer-scale 

proportions were determined for categories of “high,” 

“moderate,” and “low” based on the spatial aggregation of 

the relative-concentrations using two approaches: (1) grid 

based, and (2) spatially weighted. The grid-based approach 

uses one well per cell to represent groundwater quality, and 

water-quality data are from wells sampled by the USGS, 

augmented with data from selected wells in the CDPH 

database. The spatially weighted approach uses data for wells 

sampled by the USGS and all wells in the CDPH database, and 

weights the relative-concentration category (high, moderate, 

low) of each well such that each grid cell contributes equally 

to represent groundwater quality. The influence (weight) 

of each well’s relative-concentration category is reduced 

in proportion to the number of wells in its cell. In turn, the 

influence of each cell to determine aquifer proportion in the 

study unit is reduced in proportion to the number of cells in 

the study unit. Results for the two approaches were compared, 

and results from the preferred approach were used to select 

constituents for additional evaluation.

Relative-Concentrations and 

Water-Quality Benchmarks

Concentrations of water-quality constituents are 

presented as relative-concentrations in the status assessment, 

where

Sample concentration Relative-concentration = . Water-quality benchmark concentration

Relative-concentrations provide a means to relate 

concentrations of constituents in groundwater samples 

to water-quality benchmarks. Relative-concentrations 

less than 1.0 indicate sample concentrations less than the 

benchmark; relative-concentrations greater than 1.0 indicate 

sample concentrations greater than the benchmark. The 

use of relative-concentrations also normalizes a wide range 

of concentrations for different constituents to a common 

scale relative to benchmarks. Toccalino and others (2004), 

Toccalino and Norman (2006), and Rowe and others (2007) 

previously used the ratio of measured concentration to a 

benchmark [either maximum contaminant levels (MCLs) or 

Health-Based Screening Levels (HBSLs)] and defined this 

ratio as the benchmark quotient. Relative-concentrations 

used in this report are equivalent to the benchmark quotient 

reported by Toccalino and others (2004) for constituents 

that have water-quality benchmarks. HBSLs were not 

used in this report because they are not currently used as 

benchmarks by California drinking-water regulatory agencies. 

Relative-concentrations were only computed for compounds 

with water-quality benchmarks. About half of the constituents 

analyzed for in the USAW study unit have benchmarks.

Regulatory and non-regulatory water-quality 

benchmarks apply to water that is served to the consumer, 

not to untreated groundwater. However, to provide context 

for the water-quality results, concentrations of constituents 

measured in the untreated groundwater were compared with 

regulatory and non-regulatory human-health-based waterquality benchmarks established by the U.S. Environmental 

Protection Agency (USEPA) and CDPH (U.S. Environmental 

Protection Agency, 2006; California Department of Public 

Health, 2008a). The human-health benchmarks used include 

MCLs, notification levels (NLs), health advisory levels 

(HALs), action levels (ALs), and risk-specific dose (1 in 

100,000 lifetime risk of cancer; RSD5-US). Non-regulatory 

benchmarks set for aesthetic concerns, secondary maximum 

contaminant levels defined by CDPH and USEPA (SMCL-CA 

and SMCL-US, respectively), also were used. If a constituent 

had multiple types of benchmarks, the benchmark used for 

calculation of relative-concentration was selected according 

to the following order of priority: regulatory human-health 

(MCL and AL), non-regulatory aesthetic (SMCL), and 
Methods 11

non-regulatory human-health (NL-CA, HAL-US, and 

RSD5-US, in that order). For the regulatory human-health 

benchmarks, Federal benchmark levels were used, unless the 

California levels were lower. California public health goals 

were not used in this report. Additional information on the 

types of benchmarks and the benchmarks for all constituents 

analyzed are provided by Kent and Belitz (2009). 

Relative-concentrations were classified into high, 

moderate, and low categories. 

Category

Relative-concentrations for 

organic and special-interest 

constituents

Relative-concentrations 

for inorganic constituents

High > 1 > 1

Moderate > 0.1 and < 1 > 0.5 and < 1

Low < 0.1 < 0.5

A relative-concentration of 0.1 was used as a 

threshold between low and moderate values of organic 

and special-interest constituents compared with a 

relative-concentration of 0.5 for inorganic constituents. A 

larger threshold value was used for inorganic constituents 

because naturally occurring inorganic constituents tend to 

be more prevalent than organic constituents in California 

groundwater (Landon and others, 2010). Also, the USEPA 

has used a relative-concentration of 0.1 of the regulatory 

benchmark as a threshold concentration at or above which 

the USEPA wants to be informed of a pesticide’s presence in 

surface water or groundwater (U.S. Environmental Protection 

Agency, 1998). In contrast, inorganic constituents typically 

occur naturally at concentrations that could be greater than 

0.1 of regulatory benchmarks; consequently, it would be 

difficult to identify the highest-priority inorganic constituents 

that may have elevated concentrations above background 

levels if a relative-concentration of 0.1 were used as the 

threshold between moderate and low relative-concentrations. 

Therefore, a relative-concentration of 0.5 was used as a 

threshold between low and moderate values of inorganic 

constituents, rather than 0.1 as was used for the organic and 

special-interest constituents.

Design of Sampling Network for 

Status Assessment

 The wells selected for sampling by the USGS in this 

study were selected to provide a statistically unbiased, 

spatially distributed set of wells for the assessment of the 

quality of groundwater in the primary aquifers. Water-quality 

data from the USGS-grid wells were augmented with data 

from selected wells from the CDPH database (CDPH-grid 

wells, discussed further in next section) to obtain more 

complete grid coverage, including constituents that were not 

analyzed for at every USGS-grid well. These data were used 

to assess proportions of the primary aquifer system having 

high, moderate, and low relative-concentrations. 

The primary data used for the grid-based calculations of 

aquifer-scale proportions were data from wells sampled by 

the GAMA Priority Basin Project. Detailed descriptions of 

the methods used to identify wells for sampling are given in 

Kent and Belitz (2009). Briefly, each study area was divided 

into equal-area grid cells, and in each cell, one well was 

randomly selected to represent the cell (figs. 6, A1) (Scott, 

1990). Wells were selected from the population of wells in 

statewide databases maintained by the CDPH and the USGS. 

Water-quality data from the USGS-grid wells were combined 

with data from selected wells from the CDPH database 

(CDPH-grid wells) to provide better spatial grid coverage 

(fig. A2), including data for constituents not analyzed 

for at every USGS-grid well, to assess proportions of the 

primary aquifers having high, moderate, and low relativeconcentrations. In addition, nine understanding wells were 

selected for sampling by the USGS to increase the density in 

several areas to address specific groundwater-quality issues in 

the study unit. 

The USGS-grid wells were selected by using a 

randomized grid-based method (Scott, 1990). The network of 

grid wells was selected by first plotting the location of wells 

listed in the statewide databases maintained by the CDPH and 

USGS on a regional map of the six study areas. Five of the six 

study areas were divided into grids of equal-area cells (10 mi2; 

~26 km2)—the Bunker Hill/Cajon/Rialto-Colton (USAWB) 

study area (20 grid cells), the Yucaipa/San Timoteo (USAWY) 

study area (12 grid cells), the Riverside-Arlington/Temescal 

(USAWR) study area (13 grid cells), the Cucamonga/Chino 

(USAWC) study area (27 grid cells), and the San Jacinto 

(USAWS) study area (31 grid cells) (fig. 6). One public-supply 

well per 10-mi2 grid cell was randomly chosen to be sampled 

(Kent and Belitz, 2009). The relatively small (approximately 

40 mi2) Elsinore groundwater basin (USAWE) has an uneven 

distribution of available wells to sample and was not divided 

into cells; instead, four wells (approximately one well per 

10 mi2) that were spread throughout the basin were chosen 

for sampling to represent four “equivalent cells.” The varied 

shapes of the equal-area grid cells were drawn by objectively 

using the method reported by Scott (1990) and were influenced 

by the irregular shapes of the boundaries of the study areas 

(figs. 6, A1, A2). Geographic features near the edges of some 

study areas caused some grid cells to be divided into more 

than one section to achieve a 10-mi2 area for these cells. If a 

grid cell contained more than one public-supply well, each 

well was randomly assigned a rank. The highest ranking well 

that met basic sampling criteria (for example, sampling point 

located upstream of treatment, capability to pump for several 

hours, and available well construction information), and for 
12 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

IP017194_Figure 06 Well map

Big Bear Lake

Bautista Creek

EXPLANATION

91

60

10

15

5

15

15

215

215

10

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

San Juan Creek

Arroyo

Trabuco

Santa Ana River

San Gabriel River

Deep Creek

Lytle Creek

Santa Ana River

Mojave River

Bear Creek

Chino Creek

Lake

Perris

Lake Irvine

Lake

Elsinore

Canyon

Lake

San Jacinto River

San Gorgonio River

Prado

Wetlands

Lake

Mathews

0 10 20 MILES

0 10 20 KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Major roads

Rivers

Grid cell

WELLS

USGS-grid well

CDPH-grid well

USGS-understanding well

CDPH-other well

Upper Santa Ana Valley

study areas

Bunker Hill/Cajon/ San Jacinto

Rialto-Colton

Cucamonga/

Chino

Yucaipa/

San Timoteo

Study area

 boundary

Elsinore

Riverside-Arlington/

Temescal

Hemet

Riverside

San Bernardino

Corona

Figure 6. Grid cells for each study area, grid and understanding wells sampled during November 2006–March 2007, grid wells from 

which data for inorganic constituents from the California Department of Public Health (CDPH) database were used, and all CDPH 

wells in the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.
Methods 13

which permission to sample could be obtained, was selected. 

If a grid cell did not contain accessible public-supply wells, 

then irrigation, monitoring, domestic, or other types of wells 

were considered. The USGS-grid wells were sampled by 

the USGS for the Priority Basins Project, but are owned by 

other organizations or individuals. Grid wells in the USAW 

study unit were labeled with prefixes that vary by study area 

(USAWB-, USAWC-, USAWE-, USAWR-, USAWS-, or 

USAWY-) (table A1). 

USGS-grid wells were sampled in 19 of the 20 grid 

cells in the Bunker Hill/Cajon/Rialto-Colton study area, 

9 of the 12 grid cells in the Yucaipa/San Timoteo study area, 

12 of the 13 grid cells in the Riverside-Arlington/Temescal 

study area, 25 of the 27 cells in the Cucamonga/Chino study 

area, and 21 of the 31 grid cells in the San Jacinto study 

area (fig. 6). Seventeen grid cells were not sampled because 

they either had no wells, or permission to sample was not 

granted. As previously stated, four wells were sampled in 

the Elsinore basin study area, and these are considered grid 

wells representing the “equivalent cells” for the purposes 

of statistical analyses. The 90 USGS-grid wells sampled 

included 73 public supply, 8 irrigation, 5 desalter (wells 

that extract groundwater with high salinity for treatment), 

1 monitoring, 1 domestic, 1 industrial, and 1 recreation well 

(used exclusively to maintain water hazards at a golf course). 

The seventeen grid wells that were not public supply wells 

had screened intervals at depths similar to those of the public 

supply grid wells.

The grid wells in USAW were sampled by using a 

tiered analytical approach (Kent and Belitz, 2009). All wells 

were sampled for a standard set of constituents, including 

water-quality indicators (field parameters), VOCs, pesticides, 

perchlorate, pharmaceutical compounds, noble gases, and the 

stable isotopes of hydrogen and oxygen in water (table 1). The 

standard set of constituents was termed the “fast” schedule. 

Forty-one grid wells and 1 understanding well were sampled 

on the fast schedule. Wells on the “intermediate” schedule 

were sampled for all of the constituents on the fast schedule, 

plus inorganic constituents and selected hydrologic tracers. 

Sixteen grid wells and 2 understanding wells were sampled 

for the intermediate schedule. The “topical” schedule was 

the same as the intermediate schedule, except that it did 

not include pesticide or pharmaceutical compounds. One 

grid well and six understanding wells were sampled on 

the topical schedule (the grid well was designated as an 

understanding well at the time that it was sampled). Wells on 

the “slow” schedule were sampled for all of the constituents 

on the intermediate schedule, plus dissolved organic carbon, 

1,4-dioxane, N-nitrosodimethylamine (NDMA), low-level 

1,2,3-trichloropropane (1,2,3-TCP), the species of arsenic, 

chromium, and iron, an additional analysis for tritium, and 

radioactive and microbial constituents (table 1). Thirty-two 

grid wells were sampled on the slow schedule. The collection, 

analysis, and quality-assurance of the samples are described 

by Kent and Belitz (2009). 

CDPH-Grid Well Selection

Samples to be analyzed for inorganic constituents 

were collected by the USGS from 49 of the 107 grid cells. 

The CDPH database was used to provide data for inorganic 

constituents for the cells that lacked these data (table 2). In 

this way, at least some inorganic data were obtained for an 

additional 34 grid cells. For 29 of these 34 cells, the inorganic 

data were obtained for the same grid well sampled by the 

USGS. In these cases, the grid well identifier contains “DG” 

(fig. A2; table A1). CDPH inorganic data for three of the 

additional grid cells were obtained for cells sampled by the 

USGS, but at wells other than the ones sampled by the USGS. 

In these cases, the grid well identifier contains “DPH” (fig. A2; 

table A1). In addition, CDPH inorganic data were obtained 

for two grid cells that were not sampled by the USGS. In 

these two cases, the grid well identifier also contains “DPH” 

(USAWC-DPH-1 and USAWY-DPH-1; fig. A2; table A1). 

No CDPH inorganic data were available for nine grid cells 

sampled by the USGS for organic constituents. Finally, 15 of 

the 107 grid cells had no wells available for sampling and no 

wells with data in the CDPH database (table A1). 

The CDPH database generally did not contain data 

for all missing inorganic constituents at every CDPH grid 

well; therefore, the number of wells used for the grid-based 

assessment differed for various inorganic constituents 

(table 2). Although other organizations also collect 

water-quality data, the CDPH data is the only statewide 

database of groundwater-chemistry data available for 

comprehensive analysis. All other CDPH wells with data from 

the current period (November 30, 2003, through December 1, 

2006) not selected to be CDPH-grid wells are referred to 

as “CDPH-other” wells. Data from these “CDPH-other” 

wells were used to calculate raw detection frequencies of 

water-quality constituents. 
14 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Table 1. Constituent class and the number of constituents and wells sampled for each analytical schedule in the Upper Santa Ana 

Watershed study unit, California GAMA Priority Basin Project, November 2006–March 2007.

[1,2,3-TCP, 1,2,3-trichloropropane; NDMA, N-nitrosodimethylamine; TDS, total dissolved solids]

Analytical schedule

Fast Intermediate Topical Slow

Well summary Number of wells

Total number of wells 42 18 7 32

Number of grid wells sampled 41 16 1 32

Number of understanding wells sampled 1 2 6 0

Analyte Groups Number of constituents

Field measurements: specific conductance, pH, dissolved oxygen, and temperature 4 4 4 4

Volatile organic compounds (VOCs) 1 84 84 84 84

Pesticides and pesticide degradates 83 83 83

Polar pesticides and degradates 2 52 52 52

Perchlorate 1 1 1 1

Noble gases 3 6 6 6 6

Stable isotopes of hydrogen and oxygen of water 2 2 2 2

Pharmaceutical compounds 14 14 14

Alkalinity4 1 1 1

Major and minor ions, silica, TDS, and trace elements 37 37 37

Dissolved gases (carbon dioxide, argon, methane, nitrogen, oxygen) 5 3 3 3

Carbon isotopes 2 2 2

Isotopes of nitrogen and oxygen in nitrate 2 2 2

Isotopes of nitrogen gas 1 1 1

Nutrients 5 5 5

Dissolved organic carbon 1

1,4-Dioxane, NDMA, and low-level 1,2,3-TCP 6 3

Arsenic, chromium, and iron species 7 6

Tritium8 1

Radon, uranium, and radium isotopes 4

Gross alpha and beta radioactivity 2

Microbial constituents 4

Total: 246 298 149 318

1

 Includes nine constituents classified as fumigants.

2

 Excludes six constituents in common with first group of “pesticides and pesticide degradates,” as well as caffeine, counted with pharmaceutical compounds 

below.

3

 Analyzed at Lawrence Livermore National Laboratory (LLNL). Samples were also submitted to LLNL for tritium analysis, but LLNL tritium results are not 

available.

4

 Lab alkalinities only for intermediate and topical samples. Both field and lab alkalinities for slow samples.

5

Argon analyzed in all samples as a noble gas. Dissolved oxygen measured in all samples as a field parameter.

6 1,2,3-TCP analyzed with a method reporting level of 0.005 microgram per liter (μg/L) here was also on the USGS VOC analytical schedule with a laboratory 

reporting level of 0.12 μg/L.

7

 Total dissolved arsenic, chromium, and iron results are counted with trace elements above.

8

 Analyzed at U.S. Geological Survey Tritium Laboratory, Menlo Park, California.
Methods 15

Table 2. Inorganic constituents and associated benchmark information, and the number of grid wells with U.S. 

Geological Survey-GAMA data and CDPH data, for each constituent, Upper Santa Ana Watershed study unit, 

California GAMA Priority Basin Project.

[CDPH, California Department of Public Health; MCL-CA, CDPH maximum contaminant level; MCL-US, U.S. Environmental 

Protection Agency maximum contaminant level; SMCL, secondary maximum contaminant level; NL, notification level; AL, action level, 

HAL, lifetime health advisory level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; USGS, U.S. 

Geological Survey]

Constituent Benchmark 

type

Benchmark 

value Units

Number of grid 

wells with 

USGS-GAMA 

data

Number of grid 

wells with 

CDPH data

Nutrient

Ammonia, as nitrogen HAL-US 30 mg/L 49 0

Nitrate plus nitrite, as nitrogen MCL-US 10 mg/L 49 34

Nitrite, as nitrogen MCL-US 1 mg/L 49 30

Trace element

Aluminum MCL-CA 1,000 μg/L 49 30

Antimony MCL-US 6 μg/L 49 30

Arsenic MCL-US 10 μg/L 49 30

Barium MCL-CA 1,000 μg/L 49 30

Beryllium MCL-US 4 μg/L 49 30

Boron NL-CA 1,000 μg/L 49 18

Cadmium MCL-US 5 μg/L 49 30

Chromium MCL-CA 50 μg/L 49 30

Copper AL-US 1,300 μg/L 49 32

Iron SMCL-CA 300 μg/L 49 32

Lead AL-US 15 μg/L 49 29

Manganese SMCL-CA 50 μg/L 49 32

Mercury MCL-US 2 μg/L 32 31

Molybdenum HAL-US 40 μg/L 49 0

Nickel MCL-CA 100 μg/L 49 30

Selenium MCL-US 50 μg/L 49 30

Silver SMCL-CA 100 μg/L 49 30

Strontium HAL-US 4,000 μg/L 49 0

Thallium MCL-US 2 μg/L 49 30

Vanadium NL-CA 50 μg/L 49 13

Zinc SMCL-US 5,000 μg/L 49 32

Minor ion

Fluoride MCL-CA 2 mg/L 49 30

Major ion

Chloride SMCL-CA 500 mg/L 49 32

Sulfate SMCL-CA 500 mg/L 49 32

Total dissolved solids (TDS) SMCL-CA 1,000 mg/L 49 32

Radioactive

Gross alpha radioactivity, 72-hour count MCL-US 15 pCi/L 32 22

Gross beta radioactivity, 72-hour count MCL-CA 50 pCi/L 32 0

Radium-226 + Radium-228 MCL-US 5 pCi/L 32 11

Radon-222 MCL-US 1 4,000 pCi/L 32 4

Uranium MCL-US 20 pCi/L 49 12

1Proposed.
16 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Samples from all 90 USGS-grid wells were analyzed for 

VOCs and perchlorate. Samples from 89 of these USGS-grid 

wells were analyzed for pesticide compounds (only 

USAWC-24 lacks USGS pesticide data). VOC and perchlorate 

data are lacking for 27 out of 107 total grid cells; pesticide 

data are lacking for 28 grid cells. The USGS-grid-well data 

for VOCs, perchlorate, and pesticides have lower reporting 

levels than are available from the CDPH database (table 3); 

therefore, CDPH data for these constituents were not used to 

supplement USGS-grid-well data for the status assessment. 

The CDPH database contains more than 1.5 million 

historical water-quality results from more than 1,000 wells 

in the USAW study unit, necessitating targeted retrievals to 

effectively access water-quality data. CDPH data were used 

with USGS-grid data to identify constituents in the USAW 

study unit with concentrations greater than water-quality 

benchmarks at any time during the period of record. Data 

were retrieved from the CDPH database for samples from 

all wells located within the USAW study unit for the full 

period of record (July 13, 1956, to December 1, 2006). 

Concentrations of constituents were identified as “historically 

high” (table 4) if they had high relative-concentrations at any 

time before November 30, 2003, and during the period of 

record, but did not have high relative-concentrations in the 

most recent 3-year period of CDPH data (November 30, 2003, 

through December 31, 2006, hereinafter referred to as current 

period) or in the USGS-grid data. These “historically high” 

constituents do not reflect current conditions on which the 

status assessment is based. 

Table 3. Comparison of the number of compounds and median laboratory reporting levels or method detection levels 

by type of constituent for data stored in the California Department of Public Health database and data collected for the 

Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project, November 2006–March 2007.

[CDPH, California Department of Public Health; MDL, method detection limit; LRL, laboratory reporting level; mg/L, milligrams per liter; 

μg/L, micrograms per liter; pCi/L, picocuries per liter; SSMDC, sample-specific minimum detectable concentration; nc, not collected]

Constituent type

CDPH GAMA

Number of 

compounds

Median

MDL

Number of 

compounds

Median 

LRL

Median 

units

Volatile organic compounds 78 0.5 84 0.07 μg/L

Pesticides plus degradates 154 1 135 0.03 μg/L

Nutrients, major and minor ions 16 0.4 17 0.03 mg/L

Trace elements 20 10 24 0.11 μg/L

Radioactive constituents (SSMDC) 1 8 1 9 1 pCi/L

Perchlorate 1 4 1 0.5 μg/L

N-Nitrosodimethylamine (NDMA) 1 21.9 1 0.002 μg/L

Pharmaceutical compounds (MDL) nc nc 14 0.04 μg/L

1

 Value reported in GAMA column of median LRL is a median SSMDC for eight radioactive constituents collected and analyzed by GAMA, 

and excludes uranium, which had an LRL expressed as 0.04 μg/L with no SSMDC.

2

 Two detects reported by CDPH were at levels of 0.018 μg/L and 0.006 μg/L, implying a lower detection capability than given here.
Methods 17

Table 4. Constituents reported at concentrations greater than benchmarks in the California Department of Public Health (CDPH) 

database between July 13, 1956, and November 30, 2003, Upper Santa Ana Watershed study unit, California GAMA Priority Basin 

Project.

[high, concentration above human-heath benchmark; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH 

maximum contaminant level; NL-CA, CDPH notification level; HAL-US, U.S. Environmental Protection Agency Lifetime Health Advisory; AL-US, U.S. 

Environmental Protection Agency action level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter]

Constituent Benchmark 

type

Benchmark 

value Units

Date of most 

recent high 

value 

Number of 

historically 

high wells

Number of 

wells with 

analysis

Trace elements

Antimony MCL-US 6 μg/L 10-03-02 3 647

Cadmium MCL-US 5 μg/L 11-02-95 8 726

Copper AL-US 1,300 μg/L 07-17-97 1 732

Mercury MCL-US 2 μg/L 06-19-98 6 725

Thallium MCL-US 2 μg/L 04-08-03 1 647

Nutrient

Nitrite (as nitrogen) MCL-US 1 mg/L 06-12-01 1 650

Radioactive constituent

Radium-226 MCL-US 5 pCi/L 01-26-00 3 170

Trihalomethanes

Total trihalomethanes MCL-US1 80 μg/L 04-14-98 1 743

Solvents

1,1-Dichloroethane MCL-CA 5 μg/L 07-02-96 2 769

cis-1,2-Dichloroethene MCL-CA 6 μg/L 05-17-91 1 733

Dichloromethane (methylene chloride) MCL-US 5 μg/L 12-08-89 5 769

1,1,2-Trichloroethane MCL-US 5 μg/L 06-13-86 1 769

Organic synthesis

Chloromethane HAL-US 30 μg/L 12-03-99 1 769

Vinyl chloride MCL-CA 0.5 μg/L 06-11-86 2 769

Fumigants

1,2-Dichloropropane MCL-US 5 μg/L 09-28-98 4 763

1,2-Dibromoethane (EDB) MCL-US 0.05 μg/L 12-19-96 3 712

Gasoline oxygenate degradate

tert-Butyl alcohol NL-CA 12 μg/L 04-27-01 1 553

Pesticides

Heptachlor MCL-CA 0.01 μg/L 11-20-89 3 627

Potential wastewater indicator

Bis(2-ethylhexyl)phthalate MCL-CA 4 μg/L 03-31-92 7 571

Constituent of special interest

N-Nitrosodimethylamine (NDMA) NL-CA 0.01 μg/L 08-01-01 1 86

1

 The MCL-US benchmark for trihalomethanes is the sum of chloroform, bromoform, bromodichloromethane, and dibromochloromethane.
18 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Selection of Constituents for 

Additional Evaluation

More than 300 constituents were analyzed in samples 

from the USAW study unit wells; however, only a subset of 

these constituents is discussed in this report. Three criteria 

were used to select constituents for additional evaluation:

1. Constituents that were present at high or moderate 

relative-concentrations in the CDPH database within the 

3-year interval (November 30, 2003–December 1, 2006);

2. Constituents present at high or moderate relativeconcentrations in the USGS-grid wells or understanding 

wells; or

3. Organic constituents with detection frequencies of greater 

than 10 percent in the USGS-grid-well dataset for the 

study unit.

Constituents that were selected for additional evaluation 

and that were present at high relative-concentrations in 

greater than 2 percent of the primary aquifers are discussed 

in sections of this report under individual headings named 

for these constituents. Constituents that were selected 

for additional evaluation, but that were present at high 

relative-concentrations in less than 2 percent of the primary 

aquifers, including organic constituents detected at any 

concentration in more than 10 percent of the primary aquifers, 

are not given individual headings. These are discussed in 

sections identified by constituent class. 

Calculation of Aquifer-Scale Proportions

Aquifer-scale proportions are defined as the percentage of 

the area (rather than the volume) of the primary aquifer system 

with concentrations greater or less than specified thresholds 

relative to regulatory or aesthetic water-quality benchmarks. 

Two statistical approaches were selected to evaluate the 

proportions of the primary aquifers (Belitz and others, 2010) 

in the USAW study unit with high, moderate, or low 

relative-concentrations of constituents relative to benchmarks: 

• Grid-based: One value per grid cell, from either 

a USGS-grid or CDPH-grid well, was used 

to represent the primary aquifer system. The 

proportion of the primary aquifer system with high 

relative-concentrations was calculated by dividing 

the number of grid cells represented by a high 

relative-concentration for a particular constituent 

by the total number of grid cells with data for that 

constituent (appendix C). Proportions of moderate 

and low relative-concentrations were calculated 

similarly. Confidence intervals for grid-based 

detection frequencies of high relative-concentrations 

were computed by using the Jeffreys interval for the 

binomial distribution (Brown and others, 2001). The 

grid-based estimate is spatially unbiased. However, 

the grid-based approach may not identify constituents 

that are present at high relative-concentrations in small 

areas of the primary aquifers. 

• Spatially weighted: All available data from the 

following sources were used to calculate the 

aquifer-scale proportions—all CDPH wells in 

the study unit (most recent analyses that pass the 

quality-control tests from each well with data for that 

constituent during the current period, November 30, 

2003, to December 1, 2006), USGS-grid wells, 

and understanding wells with perforation depth 

intervals representative of the primary aquifer system. 

For the spatially weighted approach, proportions 

were computed on a cell-by-cell basis (Isaaks and 

Srivastava, 1989) rather than as an average of all 

wells. The proportion of high relative-concentrations 

for each constituent for the primary aquifers was 

computed by (1) determining the proportion of wells 

with high relative-concentrations in each grid cell; 

and (2) averaging together the grid-cell proportions 

computed in step (1) (appendix C). Similar procedures 

were used to calculate the proportions of moderate 

and low relative-concentrations of constituents. The 

resulting proportions are spatially unbiased (Isaaks and 

Srivastava, 1989). 

In addition, for each constituent, the detection frequencies 

of high and moderate relative-concentrations for individual 

constituents were calculated by using the same dataset as used 

for the spatially weighted calculations. However, these “raw” 

detection frequencies are not spatially unbiased because the 

wells in the CDPH database are not uniformly distributed 

throughout the USAW study unit (fig. 6). Consequently, high 

relative-concentrations in wells clustered in a particular area 

representing a small part of the primary aquifers could be 

given a disproportionately high weight compared to spatially 

unbiased methods. Raw detection frequencies are provided 

for reference, but were not used to assess aquifer-scale 

proportions (appendix C).

 Aquifer-scale proportions discussed in this report 

primarily were estimated by using the grid-based approach, 

and secondarily by using the spatially weighted approach. 

The grid-based aquifer-scale proportions were used unless the 

spatially weighted proportions were significantly different. 

Significantly different results were defined as follows:

1. If the aquifer-scale proportion for the high category was 

zero using the grid-based approach, and non-zero using 

the spatially weighted approach, then the result from the 

spatially weighted approach was used. This situation can 

arise when the concentration of a constituent is high in a 

small fraction of the primary aquifers.
Status of Water Quality 19

2. If the grid-based aquifer-scale proportion for the high 

category was non-zero, then the 90 percent confidence 

interval (based on the Jeffreys interval for the binomial 

distribution; Brown and others, 2001) was used to 

evaluate the difference. If the spatially weighted 

proportion was within the 90 percent confidence 

interval, then the grid-based proportion was used. If the 

spatially weighted proportion was outside the 90 percent 

confidence interval, then the spatially weighted proportion 

was used.

Aquifer-scale proportions for the moderate and low 

categories were determined primarily from the grid-based 

estimates because, for some constituents, the reporting levels 

for analyses in CDPH were too high to distinguish between 

moderate and low relative-concentrations. 

Aquifer-scale proportions of high relative-concentrations 

also were determined for classes of constituents. The classes 

of organic constituents for which aquifer-scale proportions 

were calculated include trihalomethanes, solvents, fumigants, 

other VOCs, and herbicides. The classes of inorganic 

constituents with human-health benchmarks for which 

aquifer-scale proportions were calculated include trace 

elements, radioactive constituents, and nutrients. There are 

two classes of inorganic constituents with aesthetic, rather than 

human-health benchmarks, for which aquifer-scale proportions 

were calculated: salinity indicators (TDS, chloride, sulfate) 

and trace elements (iron, zinc, manganese, silver).

Status of Water Quality 

The status assessment was designed to identify 

the constituents or classes of constituents most likely 

to be of water-quality concern because of their high 

relative-concentrations or their prevalence. USGS sample 

analyses, plus additional data from the CDPH database, 

were included in the assessment of groundwater quality for 

the USAW study unit. The spatially distributed, randomized 

approach to grid-well selection and data analysis yields 

a view of groundwater quality in which all areas of the 

primary aquifers are weighted equally; regions with a high 

density of groundwater use or with high density of potential 

contaminants were not preferentially sampled (Belitz and 

others, 2010). 

The following discussion of the status assessment

results is divided into three parts—inorganic, organic, 

and special-interest constituents. The assessment begins 

with a survey of how many constituents were detected at 

any concentration compared to the number analyzed and 

a graphical summary of the relative-concentrations of 

constituents detected in the grid wells. Results are presented 

for the subset of constituents that met criteria for selection for 

additional evaluation based on concentration, or for organic 

constituents, prevalence. 

The aquifer-scale proportions calculated using the 

spatially weighted approach were within the 90 percent 

confidence intervals for their respective grid-based aquifer 

high proportions for all 36 constituents listed in table 5, 

providing evidence that the grid-based and spatially weighted 

approaches yield statistically equivalent results. The maximum 

relative-concentration (sample concentration divided by the 

benchmark concentration) for each constituent is shown 

in figure 7.

Twelve inorganic constituents were detected at high 

relative-concentrations in one or more grid wells. These were 

arsenic, boron, chloride, fluoride, gross alpha radioactivity, 

iron, manganese, molybdenum, nitrate, total dissolved 

solids, uranium, and vanadium (fig. 7; table 5). Five organic 

constituents were detected at high relative-concentrations 

in one or more grid wells. These were carbon tetrachloride, 

1,2-dibromo-3-chloropropane (DBCP), 1,1-dichloroethene 

(1,1-DCE), tetrachloroethene (PCE), and trichloroethene 

(TCE). Perchlorate, an inorganic special-interest constituent, 

also was detected at high relative-concentrations in grid 

wells. Constituents that were detected at moderate (but not 

high) relative-concentrations in at least one grid well sample 

included two VOCs, 1,1- and 1,2-dichloroethane, the herbicide 

atrazine, the major ion sulfate, and the radioactive constituents 

adjusted gross alpha activity, radon-222, and radium 

(226+228) (fig. 7).

Inorganic Constituents

Inorganic constituents generally occur naturally 

in groundwater, although their concentrations may be 

influenced by human activities (Ayotte and others, 2011). 

Forty-eight out of the 49 inorganic constituents analyzed 

by the GAMA Priority Basin Project were detected in 

samples from the USAW study unit; beryllium was the only 

inorganic constituent not detected (perchlorate is an inorganic 

constituent but is discussed separately as a constituent of 

special interest). Thirty-four of these 48 detected inorganic 

constituents had human-health or aesthetic benchmarks 

(tables 2, 6). The 14 inorganic constituents without 

benchmarks included four major ions (calcium, magnesium, 

potassium, sodium), two minor ions (bromide, iodide), three 

trace elements (cobalt, lithium, tungsten), two nutrient species 

(total nitrogen, phosphate), two radioactive constituents (gross 

alpha and beta 30-day counts), and silica. 
20 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project Table 5. Aquifer-scale proportions from grid-based and spatially weighted approaches for constituents with high relative-concentrations during November 30, 2003– December 1, 2006, from the California Department of Public Health (CDPH) database, or with moderate or high relative-concentrations in samples collected from USGS-grid wells (November 2006–March 2007), Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project. [Grid-based aquifer proportions for organic constituents are based on samples collected by the U.S. Geological Survey from 90 grid wells during November 2006–March 2007. Spatially weighted aquifer proportions are based on CDPH data from the period December 1, 2003–November 30, 2006, in combination with grid well and understanding well data. High; concentrations greater than benchmark; moderate, concentrations less than benchmark and greater than or equal to 0.1 (for organic constituents) or 0.5 (for inorganic constituents) of benchmark; low, concentrations less than 0.1 (for organic constituents) or 0.5 (for inorganic constituents) of benchmark; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level; SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; THM, trihalomethane] Constituent Benchmark type Benchmark value Units Raw detection frequency Spatially weighted aquifer proportions Grid-based aquifer proportions 90 percent confidence interval for grid-based high proportion 1 Grid well detection frequency (organic and special- interest constituents only) Number of wells Percent moderate (percent) Percent high (percent) Number of cells Proportion moderate (percent) Proportion high (percent) Number of cells Proportion moderate (percent) Proportion high (percent) Lower limit (percent) Upper limit (percent) Trace elements and minor ions Aluminum MCL-CA 1,000 μg/L 554 0.5 0.2 81 0.5 0.1 79 0.0 0.0 0.0 2.0 Arsenic MCL-US 10 μg/L 556 4.1 3.4 81 2.8 5.1 79 2.5 5.1 2.3 10.8 Boron NL-CA 1,000 μg/L 401 3.0 1.7 76 5.8 3.0 67 6.0 3.0 1.0 8.6 Fluoride MCL-CA 2 mg/L 563 2.8 1.2 82 3.1 2.0 79 5.1 1.3 0.3 5.5 Lead AL-US 15 μg/L 536 0.2 0.2 78 0.3 1.3 78 0.0 0.0 0.0 2.1 Molybdenum HAL-US 40 μg/L 2 49 4.1 4.1 2 49 4.1 4.1 2 49 4.1 4.1 1.4 11.6 Vanadium NL-CA 50 μg/L 338 6.2 2.4 74 8.2 2.4 62 9.7 1.6 0.4 6.9 Trace elements - SMCL Iron SMCL-CA 300 μg/L 576 2.8 3.0 82 2.7 3.7 81 3.7 2.5 0.8 7.2 Manganese SMCL-US 50 μg/L 576 3.1 2.8 82 4.5 4.4 81 3.7 2.5 0.8 7.2 Radioactive constituents Adjusted gross alpha activity 3 MCL-US 15 3 pCi/L 435 2.5 0.2 74 4.0 0.3 54 3.7 0.0 0.0 3.5 Gross alpha activity MCL-US 15 3 pCi/L 435 8.9 3.7 74 12 5.6 54 11.0 5.6 2.0 12 Uranium MCL-US 20 pCi/L 249 8.0 4.0 68 9.6 0.9 61 8.2 1.6 0.4 7.0 Radium-226 + Radium-228 MCL-US 5 pCi/L 177 0.6 0.0 46 2.2 0.0 51 2.0 0.0 0.0 3.7 4Radon-222 MCL-US 4,000 pCi/L 113 0.9 0.0 43 2.3 0.0 36 2.8 0.0 0.0 4.4 Nutrients Nitrate, as nitrogen MCL-US 10 mg/L 618 25.4 18.3 85 23.4 21.8 83 25.3 25.3 18.3 33.8 Major ions and total dissolved solids Chloride SMCL-CA 500 mg/L 571 0.9 0.4 81 3.6 1.2 81 2.5 1.2 0.3 5.3 Sulfate SMCL-CA 500 mg/L 571 1.4 0.0 81 1.5 0.0 81 1.2 0.0 0.0 2.0 Total dissolved solids (TDS) SMCL-CA 1,000 mg/L 576 13.5 2.8 81 21.9 4.7 81 24.7 4.9 2.2 10.6 
Status of Water Quality 21 Constituent Benchmark type Benchmark value Units Raw detection frequency Spatially weighted aquifer proportions Grid-based aquifer proportions 90 percent confidence interval for grid-based high proportion 1 Grid well detection frequency (organic and special- interest constituents only) Number of wells Percent moderate (percent) Percent high (percent) Number of cells Proportion moderate (percent) Proportion high (percent) Number of cells Proportion moderate (percent) Proportion high (percent) Lower limit (percent) Upper limit (percent) Constituent of special interest Perchlorate MCL-CA 6 μg/L 448 21.9 17.6 92 33.1 12.7 90 53.3 11.1 6.8 17.7 67 Solvents 1,1-Dichloroethane MCL-CA 5 μg/L 601 0.2 0.0 92 1.1 0.0 90 1.1 0.0 0.0 1.5 9 1,2-Dichloroethane MCL-CA 0.5 μg/L 601 0.2 0.0 92 0.1 0.0 90 1.1 0.0 0.0 1.5 1 cis-1,2-Dichloroethene MCL-CA 6 μg/L 601 0.3 0.0 92 0.2 0.0 90 0.0 0.0 0.0 1.5 11 Carbon tetrachloride MCL-CA 0.5 μg/L 601 0.2 0.2 92 0.1 1.1 90 1.1 1.1 0.2 4.3 4 Tetrachloroethene (PCE) MCL-US 5 μg/L 603 7.6 2.0 92 6.1 2.9 90 4.4 3.3 1.2 7.6 39 Trichloroethene (TCE) MCL-US 5 μg/L 605 10.1 3.5 92 6.5 2.4 90 4.4 2.2 0.6 6.0 26 Trihalomethanes (THM) Chloroform MCL-US 5 80 μg/L 600 0.0 0.0 92 0.0 0.0 90 0.0 0.0 0.0 1.5 68 Bromodichloromethane MCL-US 5 80 μg/L 598 0.0 0.0 92 0.0 0.0 90 0.0 0.0 0.0 1.5 20 Fumigant 1,2-Dibromo-3-chloropropane (DBCP) MCL-US 0.2 μg/L 574 5.4 4.0 92 4.1 3.9 90 0.0 4.4 1.9 9.1 4 Other VOCs Benzene MCL-US 1 μg/L 601 0.0 0.2 92 0.0 0.3 90 0.0 0.0 0.0 1.5 0 Dichlorodifluoromethane (CFC-12) NL-CA 1,000 μg/L 587 0.0 0.0 92 0.0 0.0 90 0.0 0.0 0.0 1.5 10 1,1-Dichloroethene MCL-CA 6 μg/L 602 2.7 0.8 92 3.9 0.4 90 3.3 1.1 0.2 4.3 12 Methyl tert-butyl ether (MTBE) MCL-CA 13 μg/L 607 0.0 0.0 92 0.0 0.0 90 0.0 0.0 0.0 1.5 11 Trichlorofluoromethane (CFC-11) MCL-CA 150 μg/L 601 0.0 0.0 92 0.0 0.0 90 0.0 0.0 0.0 1.5 11 Table 5. Aquifer-scale proportions from grid-based and spatially weighted approaches for constituents with high relative-concentrations during November 30, 2003– December 1, 2006, from the California Department of Public Health (CDPH) database, or with moderate or high relative-concentrations in samples collected from USGS-grid wells (November 2006–March 2007), Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.—Continued [Grid-based aquifer proportions for organic constituents are based on samples collected by the U.S. Geological Survey from 90 grid wells during November 2006–March 2007. Spatially weighted aquifer proportions are based on CDPH data from the period December 1, 2003–November 30, 2006, in combination with grid well and understanding well data. High; concentrations greater than benchmark; moderate, concentrations less than benchmark and greater than or equal to 0.1 (for organic constituents) or 0.5 (for inorganic constituents) of benchmark; low, concentrations less than 0.1 (for organic constituents) or 0.5 (for inorganic constituents) of benchmark; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level; SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; THM, trihalomethane]
22 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project Constituent Benchmark type Benchmark value Units Raw detection frequency Spatially weighted aquifer proportions Grid-based aquifer proportions 90 percent confidence interval for grid-based high proportion 1 Grid well detection frequency (organic and special- interest constituents only) Number of wells Percent moderate (percent) Percent high (percent) Number of cells Proportion moderate (percent) Proportion high (percent) Number of cells Proportion moderate (percent) Proportion high (percent) Lower limit (percent) Upper limit (percent) Herbicide Atrazine MCL-CA 1 μg/L 522 0.4 0.2 90 0.1 0.1 87 1.1 0.0 0.0 1.5 62 Bromacil MCL-US 70 μg/L 321 0.0 0.0 91 0.0 0.0 89 0.0 0.0 0.0 1.5 19 Diuron HAL-US 10 μg/L 261 0.4 0.0 90 0.3 0.0 89 0.0 0.0 0.0 1.5 12 Simazine MCL-CA 4 μg/L 552 0.0 0.0 90 0.0 0.0 87 0.0 0.0 0.0 1.5 54 

1 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001).2 Only GAMA data were available for molybdenum during the study period.3 Adjusted gross alpha activity is equal to gross alpha activity minus uranium activity. Results are presented for both adjusted and unadjusted gross alpha activity. The MCL-US applies to adjusted gross alpha 

activity. 

4 Proposed.5 The MCL-US threshold for trihalomethanes is the sum of chloroform, bromoform, bromodichloromethane, and dibromochloromethane. 

Table 5. Aquifer-scale proportions from grid-based and spatially weighted approaches for constituents with high relative-concentrations during November 30, 2003–December 1, 2006, from the California Department of Public Health (CDPH) database, or with moderate or high relative-concentrations in samples collected from USGS-grid wells (November 2006–March 2007), Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.—Continued[Grid-based aquifer proportions for organic constituents are based on samples collected by the U.S. Geological Survey from 90 grid wells during November 2006–March 2007. Spatially weighted aquifer proportions are based on CDPH data from the period December 1, 2003–November 30, 2006, in combination with grid well and understanding well data. High; concentrations greater than benchmark; moderate, concentrations less than benchmark and greater than or equal to 0.1 (for organic constituents) or 0.5 (for inorganic constituents) of benchmark; low, concentrations less than 0.1 (for organic constituents) or 0.5 (for inorganic constituents) of benchmark; MCL-US, U.S. Environmental Protection Agency maximum contaminant level; MCL-CA, CDPH maximum contaminant level; NL-CA, CDPH notification level; SMCL-CA, CDPH secondary maximum contaminant level; SMCL-US, U.S. Environmental Protection Agency secondary maximum contaminant level; mg/L, milligrams per liter; μg/L, micrograms per liter; pCi/L, picocuries per liter; THM, trihalomethane]
Status of Water Quality 23

0.001

0.01

0.1

1

10

Silver Name and center of symbol is the maximum relative-concentration for that constituent-

 Unless indicated by following location line: 

EXPLANATION

Moderate

Low

Moderate

Low

Moderate

High High

Organic and specialinterest constituentsInorganic

constituents

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

Volatile

organic

compounds

Fumigants Pesticides Special

interest

Nutrients Trace

elements

Major and

minor ions

and trace

elementsSMCL

Uranium and

radioactive

constituents

Number of 

compounds

detected

< 0.001

1 0 3 0 0 0 0 0

IP017194_Figure 07

Abbreviations: TDS, total dissolved solids

Simazine

Perchlorate

Tetrachloroethene (PCE)

cis-1,2-Dichloroethene

trans-1,2-Dichloroethene

CFC-12

CFC-113

CFC-11

Carbon tetrachloride

Bromodichloromethane

Dibromochloromethane

Dichloromethane

1,1,1-Trichloroethane

Bromoform

Chloroform

Methyl tert-butyl ether

Bromacil

Atrazine

Diuron

Nitrite

Ammonia

Nitrate

Manganese

Iron

TDS

Zinc

Sulfate

Fluoride Chloride

Molybdenum

Boron

Copper

Radon-222

Unadjusted

gross alpha

Gross beta

Tritium

Radium226 + 228

Vanadium

Cadmium

Chromium

Lead

Barium

Uranium

Antimony

Selenium

Thallium

Arsenic

Mercury

Aluminum

Nickel

Strontium

Trichloroethene (TCE)

1,1-Dichloroethene

1,1-Dichloroethane

1,2-Dichloroethane

1,2,3-Trichloropropane

1,2-Dichloropropane

1,2-Dibromo-3-chloropropane

(DBCP)

Silver

Figure 7. Maximum relative-concentrations of constituents detected in grid wells by 

constituent class, Upper Santa Ana Watershed study unit, California GAMA Priority Basin 

Project.
24 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project Table 6. Number of constituents analyzed and detected with associated benchmarks in each constituent class, Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project, November 2006 to March 2007. [VOC, volatile organic compound; NWQL, National Water Quality Laboratory; USEPA, U.S. Environmental Protection Agency; CDPH, California Department of Public Health; MCL, USEPA or CDPH maximum contaminant level; HAL, USEPA lifetime health advisory; NL, CDPH notification level; RSD5, USEPA risk-specific dose at 10–5; AL, USEPA action level; SMCL, USEPA or CDPH secondary maximum contaminant level; TDS, total dissolved solids; TT-US, USEPA treatment technique—a required process intended to reduce the level of contamination in drinking water] Benchmark type Organic constituent class Special interest Organic and special interest Volatile organic compounds Pesticides and degradates Polar pesticides and degradates Pharmaceutical compounds Number of constituents Number of constituents Number of constituents Number of constituents Number of constituents Number of constituents Analyzed Detected Analyzed Detected Analyzed Detected Analyzed Detected Analyzed Detected Analyzed Detected MCL 48 23 33 20 6 2 8 0 0 0 1 1 HAL 33 5 7 0 16 3 9 1 0 0 1 1 NL 15 1 13 1 0 0 0 0 0 0 2 0 RSD5 7 1 4 0 2 0 1 1 0 0 0 0 None 134 9 27 0 59 4 34 2 14 3 0 0 Total: 237 39 84 21 83 9 52 4 14 3 4 2 Inorganic constituent class Benchmark type Sum of inorganic Major and minor ions, silica, and TDS Nutrients Trace elements Radioactive Microbial Number of constituents Number of constituents Number of constituents Number of constituents Number of constituents Number of constituents Analyzed Detected Analyzed Detected Analyzed Detected Analyzed Detected Analyzed Detected Analyzed Detected MCL 21 20 1 1 2 2 11 10 7 7 1 1 HAL 3 3 0 0 1 1 2 2 0 0 0 0 NL 2 2 0 0 0 0 2 2 0 0 0 0 AL 2 2 0 0 0 0 2 2 0 0 0 0 SMCL 7 7 3 3 0 0 4 4 0 0 0 0 TT-US 0 0 0 0 0 0 0 0 0 0 3 0 None 14 14 9 9 2 2 3 3 0 0 0 0 Total: 49 48 13 13 5 5 24 23 7 7 4 1 Organic, inorganic, and microbial constituents combined1

: 290 88

1 This total (290) differs from the total number of constituents listed on table 1 (318) because it excludes 5 field measurements, 9 dissolved gases, 7 isotopes, 6 oxidation/reduction species of 

metals, and dissolved organic carbon; all of which were detected, but none of which have health-based thresholds.
Status of Water Quality 25

Thirteen inorganic constituents had high or moderate 

relative-concentrations in greater than 2 percent of the 

grid-based aquifer proportions (table 5). Eight of these thirteen 

inorganic constituents had high relative-concentrations 

in greater than 2 percent of the primary aquifers, and are 

discussed under their own individual headings. These are 

the trace elements arsenic, boron, molybdenum, iron, and 

manganese; the radioactive constituent gross alpha activity; 

Table 7A. Aquifer-scale proportions for inorganic constituent classes, Upper Santa Ana 

Watershed study unit, California GAMA Priority Basin Project.

[SMCL, secondary maximum contaminant level; values are grid based]

Constituent class

Aquifer proportion

Low

(percent)

Moderate

(percent)

High

(percent)

Inorganics with human-health benchmarks

Trace elements and minor ions 77.9 15.6 6.5

Uranium and radioactive constituents 1 81.7 14.1 4.2

Nutrients 49.4 25.3 25.3

Any inorganic with human-health benchmarks 1 37.8 29.3 32.9

Inorganics with aesthetic benchmarks (SMCLs)

Salinity indicators (SMCL) 2 75.0 20.0 5.0

Manganese and (or) iron (SMCL) 90.0 6.3 3.8

Any inorganic with an SMCL 62.5 28.8 8.8

1

 Aquifer-scale proportions for the classes uranium and radioactive constituents and any inorganic constituents 

with health-based benchmarks were calculated using unadjusted gross alpha activity. If adjusted gross alpha activity 

had been used instead, the high and moderate aquifer-scale proportions would be 1.4% and 11% respectively, 

for uranium and radioactive constituents, and 31% and 30%, respectively, for any inorganic constituents with 

health-based benchmarks.

2

 Salinity indicators consist of total dissolved solids, chloride, and sulfate.

the nutrient nitrate (as nitrogen); and total dissolved solids 

(TDS) (table 5). Inorganic constituents with human-health 

benchmarks, as a group (nutrients, trace elements and 

minor ions, and radioactive constituents), had high aquifer 

proportions in 32.9 percent, moderate proportions in 29.3 

percent, and low proportions (including non-detections) in 

37.8 percent of the primary aquifers (table 7A). 

Table 7B. Aquifer-scale proportions for organic and special-interest constituent classes, Upper Santa Ana 

Watershed study unit, California GAMA Priority Basin Project.

[VOCs, volatile organic compounds; values are grid based]

Constituent class

Aquifer proportion

Not 

detected

Low 

(percent)

Moderate 

(percent)

High 

(percent)

Organics with human-health benchmarks

Solvents 55.6 32.2 8.9 3.3

Trihalomethanes 32.2 67.8 0.0 0.0

Fumigants 95.6 0.0 0.0 4.4

Other VOCs 71.1 24.4 3.3 1.1

Any VOC 23.3 61.1 8.9 6.7

Herbicides 31.5 67.4 1.1 0.0

Any organic with human-health benchmarks 16.7 65.6 11.1 6.7

Constituent of special interest

Perchlorate 33.3 2.2 53.3 11.1
26 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Trace Elements and Minor Ions 

Trace elements and minor ions, as a class, had high 

relative-concentrations (for one or more constituents) in 

6.5 percent of the primary aquifers, moderate values in 

15.6 percent, and low values in 77.9 percent (table 7A). 

Among trace elements and minor ions, only arsenic had high 

relative-concentrations in greater than 5 percent of the primary 

aquifers (5.1 percent). Three trace elements with health-based 

benchmarks—arsenic, boron, and molybdenum—had high 

relative-concentrations (grid-based) in greater than 2 percent 

of the primary aquifers and are discussed under individual 

headings below. An additional trace element, vanadium, and 

the minor ion fluoride, both with health-based benchmarks, 

had high relative-concentrations (grid-based) in less than 2 

percent of the primary aquifer (fig. 8A, table 5). 

Figure 8 shows relative-concentrations in grid wells 

for the 14 inorganic constituents that met the criteria for 

discussion in this status assessment. Figure 9 is a set of 

maps showing the distribution and concentrations by 

relative-concentration category (low, medium, or high) for 

these constituents in USGS-grid wells and CDPH wells from 

November 30, 2003, to December 1, 2006. 

Fluoride was detected at a high relative-concentration 

in 1.3 percent of the primary aquifers, and at a moderate 

relative-concentration in 5.1 percent (table 5; fig. 8A). High 

and moderate relative-concentrations were detected in a 

few wells of all study areas except for the Elsinore study 

area (fig. 9F). 

Vanadium was detected at a high relative-concentration 

in 1.6 percent of the primary aquifers, and was detected 

at moderate relative-concentration in 9.7 percent (table 5; 

fig. 8A). Including all wells, high relative-concentrations 

of vanadium were detected in all study areas except 

for the Riverside-Arlington/Temescal study area. The 

Riverside-Arlington/Temescal study area did have wells with 

moderate relative-concentrations of vanadium (fig. 9E).

Arsenic

Arsenic had high relative-concentrations in 5.1 percent 

of wells and moderate values in 2.5 percent (table 5; fig. 8A). 

High relative-concentrations of arsenic occurred in a few wells 

located in all study areas except for the Yucaipa/San Timoteo 

study area, where relative-concentrations were moderate and 

low (fig. 9A).

Boron

Boron was detected at high relative-concentrations 

in 3.0 percent of the primary aquifers, and at moderate 

relative-concentrations in 6.0 percent of the primary aquifers 

(table 5; fig. 8A). High and moderate relative-concentrations 

of boron were detected in the southwestern areas of the 

Cucamonga/Chino and Riverside-Arlington/Temescal 

study areas, and in the central part of the San Jacinto study 

area (fig. 9B). 

Molybdenum

Molybdenum data were limited to the 49 GAMA 

grid wells that were sampled for trace elements because 

CDPH does not collect data for molybdenum. High 

relative-concentrations of molybdenum were detected in 

2 of the 49 GAMA grid wells (4.1 percent of the primary 

aquifers), with that same proportion (4.1 percent) of moderate 

relative-concentrations of molybdenum in the primary aquifers 

(table 5; fig. 8A). High and moderate relative-concentrations 

were measured in the Elsinore study area (fig. 9C). A moderate 

relative-concentration of molybdenum also was detected in the 

San Jacinto study area.

Uranium and Radioactive Constituents

Concentrations of uranium and radioactive constituents 

generally are low in the USAW study unit with a few 

exceptions. Radium (combined 226 and 228) and radon-222 

had moderate relative-concentrations in 2.0 and 2.8 percent 

of the primary aquifers, respectively (table 5; fig. 8B). 

The single moderate relative-concentration of radium was 

detected in the San Jacinto study area in a CDPH-grid well 

(fig. 9H). A moderate relative-concentration of radon-222 was 

detected in the Elsinore study area in a USGS GAMA-grid 

well (fig. 9I). Relative-concentrations for radon-222 were 

calculated by using the higher of two proposed MCLs for 

this constituent—4,000 picocuries per liter (pCi/L), a level 

which assumes that the State or local water agency has an 

approved multimedia mitigation program to address radon in 

indoor air (U.S. Environmental Protection Agency, 1999). If 

the determination of relative-concentrations had been based 

on the lower proposed MCL of 300 pCi/L (applicable in 

the absence of such a program), 58 percent of USAW grid 

wells that were sampled for radon-222 would have had high 

relative-concentrations. However, the number of cells with 

radon-222 data is small. Therefore, the calculated aquifer 

proportions may not be representative of the study unit for 

this constituent.

Uranium was detected at a high relative-concentration 

in 1.6 percent of the primary aquifers, and at moderate 

relative-concentrations in 8.2 percent of the primary aquifers 

(table 5; fig. 8B). High relative concentrations of uranium 

were detected in the Bunker Hill/Cajon/Rialto-Colton study 

area. Moderate relative-concentrations of uranium were found 

in all study areas except for the San Jacinto and Elsinore 

study areas and were concentrated in the Riverside-Arlington/

Temescal study area (fig. 9D). 
Status of Water Quality 27

Arsenic Boron Molybdenum Vanadium Fluoride

High

Moderate

Low

Bunker Hill/Cajon/

Rialto-Colton

Yucaipa/San Timoteo

Riverside-Arlington/Temescal

Elsinore

Cucamonga/Chino San Jacinto

IP017194_Figure 08a

0.001

0.01

0.1

1

10

RELATIVE-CONCENTRATION, DIMENSIONLESS

EXPLANATION

TRACE ELEMENTS

A

STUDY AREA

Figure 8. Relative-concentrations of (A) selected trace elements, (B) radioactive constituents, 

(C) nutrients, and (D) major and minor ions in grid wells (USGS and CDPH), Upper Santa Ana Watershed 

study unit, GAMA Priority Basin Project.
28 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Unadjusted gross alpha

radioactivity

Radium-226 and -228 Radon Uranium

High

Moderate

Low

EXPLANATION

Bunker Hill/Cajon/

Rialto-Colton

Yucaipa/San Timoteo

Riverside-Arlington/Temescal

Elsinore

Cucamonga/Chino

San Jacinto

IP017194_Figure 08b

0.001

0.01

0.1

1

10

RELATIVE-CONCENTRATION, DIMENSIONLESS

RADIOACTIVE CONSTITUENTS

B

STUDY AREA

Figure 8.—Continued.
Status of Water Quality 29

Figure 8.—Continued.

Nitrate

High

Mod.

Low

NUTRIENT THAT HAS A HEALTH-BASED THRESHOLD

(MAXIMUM CONTAMINANT LEVEL) 

Bunker Hill/Cajon/

Rialto-Colton

Yucaipa/San Timoteo

Riverside-Arlington/Temescal

Elsinore

Cucamonga/Chino

San Jacinto

IP017194_Figure 08c

0.001

0.01

0.1

1

10

RELATIVE-CONCENTRATION, DIMENSIONLESS

C

STUDY AREA

EXPLANATION
30 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Figure 8.—Continued.

Chloride Total dissolved

solids

Iron Manganese

High

Moderate

Low

Bunker Hill/Cajon/

Rialto-Colton

Yucaipa/San Timoteo

Riverside-Arlington/Temescal

Elsinore

Cucamonga/Chino San Jacinto

IP017194_Figure 08d

0.001

0.01

0.1

1

10

RELATIVE-CONCENTRATION, DIMENSIONLESS

MAJOR AND MINOR IONS THAT HAVE SECONDARY MAXIMUM CONTAMINANT LEVELS

D

STUDY AREA

EXPLANATION
Status of Water Quality 31

IP017194_Figure 09AB

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

A EXPLANATION

B

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

USGS

well 

ARSENIC,

IN MICROGRAMS PER LITER

0.12 to 5.0

Not detected

5.1 to 10.0

10.1 to 71.0 10.1 to 95.8

0.4 to 5.0

5.1 to 10.0

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

USGS

well 

BORON,

IN MICROGRAMS PER LITER

7 to 500

501 to 1,000

1,001 to 4,200 1,001 to 1,460 

300 to 500

501 to 1,000

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

Figure 9. Relative-concentrations for selected inorganic constituents, (A) arsenic, (B) boron, (C) molybdenum, (D) uranium, (E) 

vanadium, (F) fluoride, (G) gross alpha radioactivity, (H) radium, (I) radon, (J) nitrate, (K) chloride, (L) iron, (M) manganese, and (N) 

total dissolved solids (TDS), Upper Santa Ana Watershed study unit, GAMA Priority Basin Project, USGS grid and understanding 

wells representative of the primary aquifer and the most recent analysis during November 30, 2003, to December 1, 2006, for CDPH 

wells. 
32 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

IP017194_Figure 09CD

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

C EXPLANATION

D

USGS

well 

MOLYBDENUM,

IN MICROGRAMS PER LITER

1 to 20

21 to 40

41 to 52 

Measured

concentration

Relativeconcentration

Low

Moderate

High

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

USGS

well 

URANIUM,

IN MICROGRAMS PER LITER

0.04 to 15.0

15.1 to 30.0

30.1 to 61.4 

0.3 to 15.0

15.1 to 30.0

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

Figure 9.—Continued.
Status of Water Quality 33

IP017194_Figure 09EF

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

VANADIUM,

IN MICROGRAMS PER LITER

0.1 to 25

26 to 50

51 to 100 51 to 110 

3.0 to 25

26 to 50

CDPH

well

Not detected

E

F

Measured

concentration

Relativeconcentration

Low

Moderate

High

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

USGS

well 

FLUORIDE,

IN MILLIGRAMS PER LITER

0.10 to 1.0

1.1 to 1.5

2.1 to 3.1

0.10 to 1.0

1.1 to 2.0

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

Figure 9.—Continued.
34 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

IP017194_Figure 09GH

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

G EXPLANATION

H

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

USGS

well 

RADIUM,

IN PICOCURIES PER LITER

0.4 to 0.9

Not detected

0.05 to 2.5

2.6 to 3.2

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

USGS

well 

UNADJUSTED GROSS ALPHA RADIOACTIVITY,

IN PICOCURIES PER LITER

1.0 to 7.5

Not detected

7.51 to 15.0

15.1 to 58 15.1 to 17

0.05 to 7.5

7.51 to 15.0

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

Figure 9.—Continued.
Status of Water Quality 35

IP017194_Figure 09IJ

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION I

J

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

USGS

well 

NITRATE AS NITROGEN,

IN MILLIGRAMS PER LITER

0.1 to 5.0

Not detected

5.1 to 10.0

10.1 to 90.4 10.1 to 34.8 

0.1 to 5.0

5.1 to 10.0

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

USGS

well 

RADON,

IN PICOCURIES PER LITER

130 to 2,000

2,001 to 2,230

100 to 1,350

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

Figure 9.—Continued.
36 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

IP017194_Figure 09KL

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

K EXPLANATION

L

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

USGS

well 

CHLORIDE,

IN MILLIGRAMS PER LITER

3 to 250

251 to 500

501 to 1,100 501 to 539

2 to 250

251 to 500

CDPH

well

Measured

concentration

Relativeconcentration

Low

Moderate

High

USGS

well 

IRON,

IN MICROGRAMS PER LITER

6 to 150

Not detected

301 to 13,000 301 to 441

20 to 150

151 to 300

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

Figure 9.—Continued.
Status of Water Quality 37

IP017194_Figure 09MN

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

M EXPLANATION

N

USGS

well 

TOTAL DISSOLVED SOLIDS,

IN MILLIGRAMS PER LITER

164 to 500

501 to 1,000

1,001 to 3,000 1,001 to 1,160 

120 to 500

501 to 1,000

CDPH

well

Measured

concentration

Relativeconcentration

Low

Moderate

High

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

USGS

well 

MANGANESE,

IN MICROGRAMS PER LITER

0.2 to 25

Not detected

26 to 50

51 to 940 51 to 275

1.1 to 25

26 to 50

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

Figure 9.—Continued.
38 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Gross Alpha Activity

Gross alpha activity had high relative-concentrations 

in 5.6 percent of the primary aquifers, and moderate 

relative-concentrations in 11 percent of the primary aquifers. 

These results are for unadjusted gross alpha activity. 

The MCL-US (15 pCi/L) for gross alpha activity applies 

to adjusted gross alpha activity, which is equal to the 

measured gross alpha activity minus uranium activity (U.S. 

Environmental Protection Agency, 2000). Data collected 

by USGS-GAMA and data compiled in the CDPH database 

are reported as gross alpha activity without correction for 

uranium activity. Gross alpha is used a screening tool to 

determine whether additional radioactive constituents must 

be analyzed (California Department of Public Health, 2012). 

For regulatory purposes, analysis of uranium is only required 

if gross alpha activity is greater than 15 pCi/L; therefore, the 

CDPH database contains more data for gross alpha activity 

than for uranium. As a result, it is not always possible to 

calculate adjusted gross alpha activity. For this reason, gross 

alpha data without correction for uranium are the primary data 

used in the status assessments made by USGS-GAMA for 

the Priority Basin Project. Examination of data from samples 

having USGS-GAMA data for uranium and gross alpha 

indicated that, in the absence of data for uranium, uncorrected 

gross alpha data likely provide a more accurate estimate of 

the aquifer-scale proportions for uranium and radioactive 

constituents as a class, than do adjusted gross alpha data 

(Miranda Fram, USGS California Water Science Center, 

written commun., 2012). All of the groundwater with high or 

moderate relative-concentrations of gross alpha activity also 

had high or moderate relative-concentrations of uranium.

USGS-GAMA reports data for gross alpha activity 

counted 72 hours and 30 days after sample collection. 

Regulatory sampling for gross alpha activity permits use of 

quarterly composite samples (U.S. Environmental Protection 

Agency, 2000; California Department of Public Health, 2012); 

thus, the USGS-GAMA gross alpha 30-day count data may 

be more appropriate to use when combining USGS-GAMA 

and CDPH datasets. Gross alpha activity in a groundwater 

sample may change with time after sample collection due 

to radioactive decay and ingrowth (activity may increase or 

decrease depending on sample composition and holding time) 

(Arndt, 2010).

Most data for uranium in the CDPH database are reported 

as activities in units of picocuries per liter, and the majority 

of uranium data gathered by USGS-GAMA are reported as 

concentrations in units of micrograms per liter. The factor used 

to convert uranium mass concentration to uranium activity 

depends on the isotopic composition of the uranium (U.S. 

Environmental Protection Agency, 2000). This report uses a 

conversion factor of 0.79. 

Nutrients

Three of the four species of nitrogen analyzed for in this 

study have health-based thresholds—ammonia, nitrite, and 

nitrate. One of these, nitrite plus nitrate (hereinafter referred 

to as nitrate), was frequently detected at concentrations 

above its MCL. All detections of the other nitrogen species 

with health-based thresholds—nitrite and ammonia—were 

at low relative-concentrations. The only phosphorus species 

measured, orthophosphate, has no drinking-water aesthetic or 

regulatory benchmark.

Nitrate

Nitrate was detected at high relative-concentrations in 

25.3 percent of the primary aquifers, with this same proportion 

of primary aquifers having moderate relative-concentrations 

(table 5; fig. 8C). Thus, just over half of USAW grid wells 

had high or moderate relative-concentrations of nitrate. High 

relative-concentrations of nitrate were detected in all USAW 

study areas except for the Elsinore study area (fig. 9J). High 

and moderate relative-concentrations were most prevalent 

in the Cucamonga/Chino and Riverside-Arlington/Temescal 

study areas. 

Constituents with Secondary Maximum 

Contaminant Level Benchmarks

CDPH has established non-enforceable benchmarks, 

secondary maximum contaminant levels (SMCL-CA), which 

are based on aesthetic properties rather than on human-health 

concerns for selected constituents. For total dissolved solids 

(TDS) and the major ions chloride and sulfate, CDPH 

defines a “recommended” and an “upper” SMCL-CA. The 

“upper” SMCL-CA benchmarks were used for computing 

relative-concentrations for this report. CDPH defines a single 

benchmark for iron and a single benchmark for manganese.

Chloride was detected at a high relative-concentration in 

one grid well (1.2 percent of the primary aquifers), and was 

detected at moderate relative-concentrations in 2.5 percent 

of the primary aquifers (table 5; fig. 8D). High and moderate 

relative-concentrations of chloride were detected only in the 

San Jacinto study area (fig. 9K). 

Total Dissolved Solids

Relative-concentrations of total dissolved solids 

(TDS) were high in 4.9 percent of the primary aquifers, and 

moderate in 24.7 percent (table 5; fig. 8D). High and moderate 

relative-concentrations of TDS were most prevalent in the 

San Jacinto, Riverside-Arlington/Temescal, and Cucamonga/

Chino study areas. Relative-concentrations of TDS in the 

Bunker Hill/Cajon/Rialto-Colton, Yucaipa/San Timoteo, and 

Elsinore study areas were mostly low (fig. 9N).
Status of Water Quality 39

Iron and Manganese

Two trace elements with SMCL-CAs, iron and 

manganese, were each detected at high relative-concentrations 

in 2.5 percent of the primary aquifers, and at moderate 

relative-concentrations in 3.7 percent (table 5; fig. 8D). High 

or moderate relative-concentrations of iron and manganese 

were observed in all of the study areas with one exception. 

The Yucaipa/San Timoteo study area had no grid wells with 

high relative-concentrations of iron and no grid wells with 

high or moderate relative-concentrations of manganese 

(figs. 8D, 9L, and 9M).

Organic Constituents 

In this report, organic compounds are organized 

by constituent class, including four classes of volatile 

organic compounds (VOCs), two classes of pesticides, 

and pharmaceuticals. VOCs may be present in paints, 

solvents, fuels, refrigerants, or disinfected water, and are 

characterized by their tendency to evaporate. VOCs are 

classified here as trihalomethanes, solvents, fumigants, or 

other VOCs (including gasoline additives and refrigerants). 

Pesticides are used to control weeds (herbicides), insects 

(insecticides), or fungi (fungicides) in agricultural, urban, 

and suburban settings. Pesticides are classified here as 

herbicides or insecticides and fungicides. Once released into 

the environment, pesticides are transformed, over time, by a 

variety of chemical, photochemical, and biologically mediated 

reactions into other compounds, which are referred to in this 

report as degradates (Gilliom and others, 2006). 

In contrast to the nearly ubiquitous inorganic 

constituents, only 39 of the 237 organic and special-interest 

constituents analyzed were detected at concentrations greater 

than their respective long-term method detection limits 

(dissolved organic carbon is not included in this discussion). 

These compounds included 21 VOCs, 13 pesticides and 

pesticide degradates, 3 pharmaceuticals, and 2 special-interest 

constituents (perchlorate and 1,2,3-trichloropropane). Most of 

the organic and special-interest constituents detected (30 of 

the 39) have human-health benchmarks (table 6). Of the 

nine detected organic compounds lacking benchmarks, three 

are herbicide degradates, and three are pharmaceuticals; the 

other three are the fungicide, metalaxyl, and the herbicides, 

norflurazon and pendimethalin. Two of the three herbicide 

degradates that were detected, 2-chloro-4-isopropylamino-6-

amino-s-triazine (deethylatrazine) and 2-chloro-6-ethylamino4-amino-s-triazine (deisopropyl-atrazine), are degradates of 

atrazine and other triazine herbicides (Gilliom and others, 

2006). Atrazine and another triazine herbicide, simazine, 

were each frequently detected (detected in at least 10 percent 

of samples), and both have health-based benchmarks. The 

third herbicide degradate detected, 3,4-dichloroaniline, is a 

degradate of diuron, which was frequently detected and has a 

health-based benchmark. Diuron is among the most heavily 

used herbicides in the Santa Ana Basin (Kent and others, 

2005). Three pharmaceutical compounds were detected at 

concentrations greater than or equal to method detection limits 

in one or more samples from the USAW study unit. Fram and 

Belitz (2011) present all results for pharmaceutical compounds 

in groundwater samples collected for 28 of the GAMA Priority 

Basin Project study units (studies that took place during 

May 2004 through March 2010). The three pharmaceuticals 

that were detected in USAW groundwater samples were 

acetaminophen (analgesic), carbamazepine (antiepileptic), 

and caffeine. 

An additional 13 pesticide compounds and 2 VOCs were 

reported as detections by the NWQL in USAW samples at 

concentrations less than their long-term method detection 

limits (LT-MDL) (Kent and Belitz, 2009). For purposes of this 

study, these compounds are not considered detections because 

detections with concentrations less than the LT-MDL have 

greater than a 1 percent probability of being falsely-positive 

detections (Childress and others, 1999). 

Among all organic constituents with human-health 

benchmarks, the proportion of USAW primary aquifers 

with high concentrations was 6.7 percent (table 7B), which 

reflects the high relative-concentrations of the solvents 

PCE (3.3 percent) and TCE (2.2 percent), and the fumigant 

1,2-dibromo-3-chloropropane (DBCP) (4.4 percent) (table 5). 

The proportion of the aquifers having moderate relativeconcentrations of organic constituents with human-health 

benchmarks was 11.1 percent (table 7B).

Organic constituents were detected in 83 percent of the 

primary aquifers. Fifteen organic compounds had maximum 

relative-concentrations that were high or moderate in more 

than 2 percent of the primary aquifers, detection frequencies 

greater than 10 percent, or both. Five organic compounds—the 

solvents carbon tetrachloride, PCE, and TCE, the fumigant 

DBCP, and 1,1-dichloroethene (1,1-DCE, organic synthesis)—

were detected at maximum concentrations greater than their 

health-based thresholds (maximum relative-concentrations 

greater than 1, or “high”) (figs. 7, 10). Three of these five—

1,1-DCE, TCE, and PCE—also had detection frequencies 

greater than or equal to 10 percent (table 5; fig. 10). Ten 

additional organic compounds with health-based thresholds 

were detected in at least 10 percent of the primary aquifers, 

although all but one (atrazine) were detected only at low 

maximum relative-concentrations. These 10 compounds 

were the THMs chloroform and bromodichloromethane, the 

solvent cis-1,2-dichloroethene (cis-1,2-DCE), the gasoline 

oxygenate methyl tert-butyl ether (MTBE), the refrigerants 

trichlorofluoromethane (CFC-11) and dichlorodifluoromethane 

(CFC-12), and the herbicides atrazine, bromacil, diuron, 

and simazine (fig. 11). Atrazine was detected at a moderate 

relative-concentration in one grid well.

The detection frequency of one or more VOCs in the 

90 grid wells was 77 percent. Of the 21 VOCs detected, 

14 were detected only at low relative-concentrations (fig. 10). 

Nine VOCs had a detection frequency of at least 10 percent in 

the grid wells: chloroform, PCE, TCE, bromodichloromethane, 

1,1-DCE, cis-1,2-DCE, MTBE, CFC-11, and CFC-12 

(figs. 10, 11; table 5). 
40 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Moderate

Low

Moderate

High

Diuron

Tebuthiuron

DCPA (Dacthal)

Prometon

Prometon

1,2,3-Trichloropropane (1,2,3-TCP)

Carbon

tetrachloride

Bromoform

CFC-113

Bromodichloromethane

Bromacil

Simazine

Chloroform

Chloroform

Atrazine

1,1-Dichloroethene

Perchlorate

Perchlorate

Trichloroethene

(TCE) Tetrachloroethene

(PCE)

1,1-Dichloroethane

Methyl tert-butyl ether

(MTBE)

trans-1,2-Dichloroethene

1,2-Dichloroethane

cis-1,2-Dichloroethene

Dibromochloromethane

Dichloromethane

1,2-Dichloropropane

(1,2-DCP)

1,1,1-Trichloroethane (TCA)

CFC-11

CFC-12

1,2-Dichlorobenzene

1,2-Dibromo-3-chloropropane

(DBCP)

IP017194_Figure 10

EXPLANATION

Name and center of symbol is the maximum relative concentration for that constituent-

 Unless indicated by following location line: 

1 10 100

0.0001

0.00001

0.001

0.01

0.1

1

10

MAXIMUM RELATIVE-CONCENTRATION, DIMENSIONLESS

DETECTION FREQUENCY AT ANY CONCENTRATION, IN PERCENT

Volatile organic compound

Herbicide

Special-interest constituent

Figure 10. Detection frequency and maximum relative-concentration of organic and special-interest 

constituents detected in USGS-grid wells in the Upper Santa Ana Watershed study unit, California GAMA 

Priority Basin Project.
Status of Water Quality 41

Chloroform

Total

Trihalomethanes

Bromodichloromethane

STUDY AREA

Bunker Hill/Cajon/Rialto-Colton

Cucamonga/Chino

Elsinore

Riverside-Arlington/Temescal

San Jacinto

Yucaipa/San Timoteo

12

19

3

12

4

12

12

19

3

12

12

2

5

1

6

1

3

4

RELATIVE-CONCENTRATION, DIMENSIONLESS

NUMBER OF

WELLS CONTAINING

THE CONSTITUENT

0.0001 0.001 0.01 0.1 1 10

ENTIRE STUDY UNIT

DETECTION FREQUENCY,

IN PERCENT

0 10080604020

10% detection

IP017194_Figure 11ab Trihalomethanes

A B

Figure 11 (A–L). Detection frequency (bar charts) and relative-concentrations (dot plots) of selected organic and 

special-interest constituents in grid wells in the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project, 

November 2006–March 2007.
42 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Trichloroethene

(TCE)

Tetrachloroethene

(PCE)

cis-1,2-Dichloroethene

STUDY AREA

Carbon

Tetrachloride

Bunker Hill/Cajon/Rialto-Colton

Cucamonga/Chino

Elsinore

Riverside-Arlington/Temescal

San Jacinto

Yucaipa/San Timoteo

RELATIVE-CONCENTRATION, DIMENSIONLESS

NUMBER OF

WELLS CONTAINING

THE CONSTITUENT

0.0001 0.001 0.01 0.1 1 10

ENTIRE STUDY UNIT

DETECTION FREQUENCY,

IN PERCENT

0 5040302010

10% detection

IP017194_Figure 11cd Solvents

C D

10

9

4

6

9

6

7

4

4

2

1

3

2

2

2

2

1

Figure 11.—Continued.
Status of Water Quality 43

1,2-Dibromo3-chloropropane

(DBCP)

STUDY AREA

Bunker Hill/Cajon/Rialto-Colton

Cucamonga/Chino

Elsinore

Riverside-Arlington/Temescal

San Jacinto

Yucaipa/San Timoteo

1,1-Dichloroethene

Methyl tertbutyl ether

(MTBE)

CFC-11

CFC-12

* Some relative-concentrations were less than 0.0001

RELATIVE-CONCENTRATION, DIMENSIONLESS

NUMBER OF

WELLS CONTAINING

THE CONSTITUENT

0.0001 0.001 0.01 0.1 1 10

ENTIRE STUDY UNIT

DETECTION FREQUENCY,

IN PERCENT

0 5040302010

10% detection

IP017194_Figure 11efgh Fumigants/Other VOCs

E F

RELATIVE-CONCENTRATION, DIMENSIONLESS

0.0001 0.001 0.01 0.1 1 10

ENTIRE STUDY UNIT

DETECTION FREQUENCY,

IN PERCENT

0 5040302010

10% detection

G H

1

1

2

3

3

2

1

2

6

1

1

2

4

1

3

2

1

3*

4*

2

Figure 11.—Continued.
44 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

STUDY AREA

Bunker Hill/Cajon/RiaIto-Colton

Cucamonga/Chino

Elsinore

Riverside-Arlington/Temescal

San Jacinto

Yucaipa/San Timoteo

Perchlorate

Simazine

Atrazine

Bromacil

Diuron

12

19

12

13

4

12

13

5

10

13

3

10

7

3

12

9

5

6

1

1

4

6

1

4

4

1

NUMBER OF

WELLS CONTAINING

THE CONSTITUENT

IP017194_Figure 11efgh Fumigants/Other VOCs

RELATIVE-CONCENTRATION, DIMENSIONLESS

0.0001 0.001 0.01 0.1 1 10

ENTIRE STUDY UNIT

DETECTION FREQUENCY,

IN PERCENT

0 10080604020

10% detection

K L

RELATIVE-CONCENTRATION, DIMENSIONLESS

0.0001 0.001 0.01 0.1 1 10

ENTIRE STUDY UNIT

DETECTION FREQUENCY,

IN PERCENT

0 10080604020

10% detection

I J

Figure 11.—Continued.
Status of Water Quality 45

One or more pesticides or pesticide degradates were 

detected in 68 percent of the primary aquifers. Of the 

135 pesticides and pesticide degradates analyzed, 13 were 

detected. Seven of the detected pesticides or pesticide 

degradates were parent compounds with benchmarks, three 

were parent compounds without a benchmark, and three 

were degradates without benchmarks. All concentrations 

of pesticides with human-health benchmarks were below 

the benchmarks. One pesticide, atrazine, was detected at a 

moderate relative-concentration in one sample (fig. 11J). 

All of the other pesticide compounds were detected only 

at low relative-concentrations. Four herbicides—atrazine, 

simazine, bromacil, and diuron—and two herbicide 

degradates—2-chloro-4-isopropylamino-6-amino-s-triazine 

(deethylatrazine) and 1,4-dichloroaniline—were detected 

in more than 10 percent of the primary aquifers. The four 

herbicide parent compounds are discussed in further detail 

below. Discussion of the frequently occurring herbicide 

degradates is beyond the scope of this study. The individual 

constituents that were not detected and the wells sampled in 

the USAW study unit are listed in Kent and Belitz (2009).

Trihalomethanes

Water used for drinking water and other household uses 

in domestic and municipal systems commonly is disinfected 

with some form of chlorine disinfectant. As a side effect to 

disinfecting the water, the chlorine reacts with organic matter 

to produce trihalomethanes (THMs) and other chlorinated 

and/or brominated disinfection byproducts. Potential 

urban sources of THMs include recharge from landscape 

irrigation with disinfected water, leakage from distribution 

or sewer systems, and industrial and commercial sources 

(Ivahnenko and Barbash, 2004). The four chlorinated and/

or brominated THMs (chloroform, bromodichloromethane, 

dibromochloromethane, and bromoform) were detected at 

low relative-concentrations. Two of these—chloroform and 

bromodichloromethane—were frequently detected (detected in 

at least 10 percent of samples).

Chloroform was detected in 68 percent of primary 

aquifers (figs. 10, 11A). Chloroform was detected throughout 

the USAW study unit (fig. 12A). The study area with the 

lowest detection frequency was Yucaipa/San Timoteo at 

44 percent (Kent and Belitz, 2009). In contrast, chloroform 

was detected in all (100 percent) of the grid wells in the 

Riverside-Arlington/Temescal study area. In general, the 

analytical methods used by the CDPH to analyze for organic 

compounds have higher detection limits than those used by 

the NWQL; therefore, non-detections are more common in 

CDPH wells for chloroform, as well as for other organic 

compounds (fig. 12).

Bromodichloromethane was another THM frequently 

detected at low relative-concentrations in USAW grid wells 

(figs. 10, 11). Bromodichloromethane was detected throughout 

the USAW study unit, with a detection frequency of about 

20 percent in the study unit (Kent and Belitz, 2009). However, 

in contrast to the 100 percent detection frequency for 

chloroform in the Riverside-Arlington/Temescal study area, 

bromodichloromethane was detected in only 1 of the 12 grid 

wells (8 percent) in the Riverside-Arlington/Temescal study 

area (fig. 12B). 

The health-based threshold for chlorinated/brominated 

THMs is applied to the summed concentration of the four 

compounds; sometimes referred to as “total trihalomethanes” 

for regulatory purposes. Similar to concentrations of the 

individual THMs, concentrations of the summed compounds 

were low during the current study period. Chloroform was 

the predominant compound making up total THMs and was 

present in all samples where a THM was detected. One well in 

the CDPH database for the USAW study unit had a historically 

high relative-concentration for total THMs in a sample 

collected before December 2003 (table 4). 

Solvents

Solvents are a class of VOCs used for a variety of 

industrial, commercial, and domestic purposes. Of the 

29 solvents analyzed for in this study, 10 were detected 

(table 3A of Kent and Belitz, 2009). Four of these met 

selection criteria for further discussion here, either because 

they were frequently detected (detected in more than 

10 percent of the grid wells), or because they were detected 

at a high or moderate relative-concentrations in more than 

2 percent of the primary aquifers. The solvent compounds that 

were frequently detected in USAW grid wells were PCE, TCE, 

and cis-1,2-dichloroethene. The solvent carbon tetrachloride, 

though not frequently detected, had high and moderate 

grid-based aquifer proportions of 1.1 percent each (table 5). 

Solvents, as a group, had a high aquifer-scale proportion 

of 3.3 percent, and a moderate aquifer-scale proportion of 

8.9 percent (table 7B).

Carbon tetrachloride (tetrachloromethane) was detected 

in three USAW grid wells, with high, moderate, and low 

relative-concentrations each occurring in one well. The 

moderate relative-concentration occurred in a well in the 

Bunker Hill/Cajon/Rialto-Colton study area, and the high and 

low relative-concentrations occurred in the San Jacinto study 

area (figs. 11, 12E). Cis-1,2-dichloroethene was detected at 

low relative-concentrations in 11 percent of USAW grid wells, 

and was detected in all of the USAW study areas except for 

Elsinore (figs. 11, 12F). This solvent, frequently detected in 

USAW grid wells, had no high values during the current study 

period, but did have a historically high value in the CDPH 

database from the period prior to November 30, 2003 (table 4). 

Other solvents with no high values during the current period of 

study, but with historically high values in the CDPH database, 

were 1,1-dichloroethane, dichloromethane (methylene 

chloride), and 1,1,2-trichloroethane (table 4).
46 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

IP017194_Figure 12ab

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

CHLOROFORM,

IN MICROGRAMS PER LITER

0.02 to 5.13

Not detected

1.1 to 7.3

CDPH

well

Not detected

A

B

Measured

concentration

Relativeconcentration

Low

USGS

well 

BROMODICHLOROMETHANE,

IN MICROGRAMS PER LITER

0.02 to 0.51

Not detected

1.1 to 2.1

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

Figure 12 (A–P). Relative-concentrations for selected organic and special-interest constituents, (A) chloroform, 

(B) bromodichloromethane, (C) tetrachloroethene, (D) trichloroethene, (E) carbon tetrachloride, (F) cis-1,2-dichloroethene, 

(G) 1,2-dibromo-3-chloropropane, (H) methyl tert-butyl ether, (I) 1,1-dichloroethene, (J) CFC-11, (K) CFC-12, (L) atrazine, 

(M) simazine, (N) bromacil, (O) diuron, and (P) perchlorate, Upper Santa Ana Watershed study unit, GAMA Priority Basin Project.
Status of Water Quality 47

IP017194_Figure 12cd

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

TETRACHLOROETHENE (PCE),

IN MICROGRAMS PER LITER

0.01 to 0.50

Not detected

0.51 to 5.00

5.01 to 43 5.01 to 12.4

0.51 to 5.00

CDPH

well

Not detected

C

D

Measured

concentration

Relativeconcentration

Low

Moderate

High

USGS

well 

TRICHLOROETHENE (TCE),

IN MICROGRAMS PER LITER

0.01 to 0.50

Not detected

0.51 to 5.00

5.01 to 238 5.01 to 21.7 

0.51 to 5.00

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

Figure 12.—Continued.
48 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

IP017194_Figure 12ef

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

CARBON TETRACHLORIDE,

IN MICROGRAMS PER LITER

0.4 to 0.05

Not detected

0.06 to 0.5

0.6 to 0.9 

CDPH

well

Not detected

E

F

Measured

concentration

Relativeconcentration

Low

Moderate

High

USGS

well 

cis-1,2-DICHLOROETHENE,

IN MICROGRAMS PER LITER

0.01 to 0.31

Not detected

0.5 to 0.6

0.7 to 1.5

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

Figure 12.—Continued.
Status of Water Quality 49

IP017194_Figure 12gh

0

0 5 10 15

15105 MILES

KILOMETERS

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

1,2-DIBROMO-3-CHLOROPROPANE,

IN MICROGRAMS PER LITER

Not detected

0.21 to 1.47 0.25 to 0.52 

0.01 to 0.02

0.021 to 0.20

CDPH

well

Not detected

G

H

Measured

concentration

Relativeconcentration

Low

Moderate

High

USGS

well 

METHYL tert -BUTYL ETHER,

IN MICROGRAMS PER LITER

0.05 to 0.61

Not detected

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

Moderate

High

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

Figure 12.—Continued.
50 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Figure 12.—Continued.

IP017194_Figure 12ij

0

0 5 10 15

15105 Miles

Kilometers

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

USGS

well 

CFC-11,

IN MICROGRAMS PER LITER

CDPH

well

Not detected

J

Measured

concentration

Relativeconcentration

Low

Moderate

High

0.04 to 2.16

Not detected

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

1,1-DICHLOROETHENE,

IN MICROGRAMS PER LITER

0.01 to 0.60

Not detected

0.61 to 6.00

6.01 to 41 6.01 to 6.80

0.61 to 6.00

CDPH

well

Not detected

I

Measured

concentration

Relativeconcentration

Low

Moderate

High

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural
Status of Water Quality 51

Figure 12.—Continued.

IP017194_Figure 12kl

0

0 5 10 15

15105 Miles

Kilometers

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

ATRAZINE,

IN MICROGRAMS PER LITER

Not detected

2.6

CDPH

well

Not detected

L

Measured

concentration

Relativeconcentration

Low

Moderate

High

0.004 to 0.100

0.11

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

K

USGS

well 

CFC-12,

IN MICROGRAMS PER LITER

CDPH

well

Measured

concentration

Relativeconcentration

Low

Moderate

High

0.50 to 5.00

Not detected

0.07 to 1.58

Not detected
52 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Figure 12.—Continued.

IP017194_Figure 12mn

0

0 5 10 15

15105 Miles

Kilometers

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

M

USGS

well 

SIMAZINE,

IN MICROGRAMS PER LITER

0.003 to 0.139

Not detected

CDPH

well

Not detected

Measured

concentration

Relativeconcentration

Low

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

BROMACIL,

IN MICROGRAMS PER LITER

Not detected

CDPH

well

Not detected

N

Measured

concentration

Relativeconcentration

Low 0.02 to 1.99

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural
Status of Water Quality 53

Figure 12.—Continued.

IP017194_Figure 12op

0

0 5 10 15

15105 Miles

Kilometers

33°

50’

34°

10’

O

USGS

well 

DIURON,

IN MICROGRAMS PER LITER

0.02 to 0.23

Not detected

CDPH

well

Not detected

1.50

Measured

concentration

Relativeconcentration

Low

Moderate

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

33°

50’

34°

10’

Study area and

 unit boundary

EXPLANATION

USGS

well 

PERCHLORATE,

IN MICROGRAMS PER LITER

0.5 to 0.60

Not detected

0.61 to 6.00

6.01 to 130 6.01 to 11

4.1 to 6.00

CDPH

well

Not detected

P

Measured

concentration

Relativeconcentration

Low

Moderate

High

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural
54 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

PCE and TCE

PCE and TCE were detected in all USAW study 

areas except Elsinore, and PCE was detected at a high 

relative-concentration in one grid well in each of the following 

study areas: Cucamonga/Chino, Riverside-Arlington/

Temescal, and San Jacinto (figs. 11, 12C). The grid wells 

with high relative-concentrations of PCE in the Cucamonga/

Chino and Riverside-Arlington/Temescal study areas also 

had high relative-concentrations of TCE. The San Jacinto 

grid well with a high relative-concentration of PCE had a 

moderate relative-concentration of TCE (fig. 12D). High and 

moderate relative-concentrations of PCE and TCE in CDPH 

wells generally were detected in the same areas where they 

were detected in USAW grid wells, with a few exceptions. 

For example, high and moderate relative-concentrations of 

TCE were detected in CDPH wells in the southern portion of 

the Cucamonga/Chino study area. TCE was not detected in 

USAW grid wells in this area. 

Fumigants 

Fumigants are used as agricultural or household 

pesticides. Of the nine fumigants (table 3A, Kent and Belitz, 

2009) sampled for in this study, two—1,2-dichloropropane 

and 1,2 dibromo-3-chloropropane (DBCP)—were detected in 

USAW grid wells. 

DBCP

DBCP was detected in four USGS grid wells, and all four 

detections were at concentrations above the USEPA MCL. The 

four detections at high relative-concentrations resulted in a 

grid-based high aquifer-scale proportion of 4.4 percent for this 

compound (table 5). Two of the USGS grid-well detections 

occurred in wells located in the Riverside-Arlington/Temescal 

study area. The other two USGS grid-well detections occurred 

in one well each of the Cucamonga/Chino and Bunker Hill/

Cajon/Rialto-Colton study areas (figs. 11, 12G). Detections of 

DBCP in CDPH wells were in these same general locations. 

The analytical method used by CDPH had a lower detection 

limit than that used by the USGS, allowing detections of 

DBCP at low and moderate relative-concentrations for CDPH 

wells in the study unit. The spatially weighted moderate 

aquifer proportion was 4.1 percent; the spatially weighted 

high aquifer proportion of 3.9 percent was similar to the 

grid-based aquifer-scale proportion of 4.4 percent and 

within the 90 percent confidence interval of the grid-based 

estimate (table 5). Fumigants that have been historically, 

though not currently, high in the USAW study unit include 

the previously mentioned 1,2-dichloropropane and 

1,2-dibromoethane (table 4).

Other VOCs

In this report, the 43 compounds under the category 

“other VOCs” (any VOC other than a THM, solvent, 

or fumigant) include fuel components, fire retardants, 

refrigerants, and compounds used in the synthesis of organic 

chemicals. Four compounds in the category “other VOCs” met 

the selection criteria for further discussion: the fuel component 

MTBE, the organic synthesis compound 1,1-DCE, and the 

refrigerants CFC-11 and CFC-12.

MTBE is a compound that was used to oxygenate 

gasoline until its use was discontinued in California after 

December 2003 (Rausser and others, 2004). MTBE was 

detected at low relative-concentrations in 11 percent of USAW 

grid wells (table 5) and was detected in all of the USAW study 

areas except for San Jacinto and Elsinore (figs. 11, 12H). 

While all current and historical detections of MTBE in the 

USAW study unit have been at low relative-concentrations, a 

similar compound—tert-butyl alcohol, classified as a gasoline 

oxygenate degradate—was detected in a well at a high 

relative-concentration prior to November 2003 (table 4).

The organic synthesis compound 1,1-DCE was detected 

in 12 percent of the primary aquifers. Relative-concentrations 

of 1,1-DCE were high in 1.1 percent and moderate in 

3.3 percent of the primary aquifers (table 5). 1,1-DCE was 

detected in grid wells of all USAW study areas except for the 

Elsinore study area (fig. 12I). The single detection at a high 

relative-concentration occurred in the Cucamonga/Chino study 

area, and moderate relative-concentrations were detected in 

grid wells of the Cucamonga/Chino, Riverside-Arlington/

Temescal, and Yucaipa/San Timoteo study areas (figs. 11, 12I). 

A few additional detections of 1,1-DCE at high and moderate 

relative-concentrations occurred in CDPH wells that were 

not grid wells (fig.12I). Two additional organic synthesis 

compounds, chloromethane and vinyl chloride, had high 

relative-concentrations in at least one well in the CDPH data 

before November 2003 (table 4). Neither of these compounds 

was detected in USAW grid wells.

Two refrigerant compounds, both chlorofluorocarbons, 

were detected at low relative-concentrations in at least 

10 percent of the grid wells. CFC-11 was detected in 

11 percent of USAW grid wells (table 5) and was detected 

in all of the study areas except for the Yucaipa/San Timoteo 

and the Cucamonga Chino study areas (figs. 11, 12J). 

Dichlorodifluoromethane (CFC-12) was detected in 10 percent 

(9 of 90) of the grid wells (table 5) and was detected in all 

of the study areas except for the Yucaipa/San Timoteo and 

Elsinore study areas (figs. 11, 12K).
Summary 55

Herbicides

Four herbicides—atrazine, simazine, bromacil, and 

diuron—were detected in 10 percent or more of the grid wells 

which, for this report, defines “frequently detected.” At least 

one herbicide was detected in 68 percent of the 90 grid wells. 

The largest relative-concentration for a herbicide was 0.11 

(moderate) for atrazine. Atrazine was also the herbicide most 

frequently detected in USAW grid wells (62 percent), followed 

closely by simazine (54 percent) (table 5; fig. 11); these two 

herbicides were frequently detected in all of the USAW study 

areas (fig. 12L, 12M). Detections of bromacil and diuron were 

at low relative-concentrations (fig. 11). Bromacil was detected 

in all of the USAW study areas except for the Yucaipa/San 

Timoteo study area (figs. 11, 12N). Diuron was detected in 

all of the USAW study areas except for the Elsinore study 

area (figs. 11, 12O). Hamlin and others (2005) reported 

concentrations and detection frequencies for herbicides 

similar to those in this study in samples collected from 1999 

to 2001 as part of a USGS National Water-Quality Assessment 

Program study carried out in the Santa Ana Basin. Atrazine 

was detected at a high relative concentration in one CDPH 

well in the Yucaipa\San Timoteo study area (fig. 12L). 

Insecticides and Fungicides

None of the 35 insecticides or 21 insecticide degradates 

analyzed for in this study were detected (Kent and Belitz, 

2009). Relative concentrations cannot be calculated for 

metalaxyl because it does not have a benchmark. A fungicide, 

metalaxyl, was detected at low concentrations in two grid 

wells (Kent and Belitz, 2009). There were no detections of the 

other seven fungicides analyzed for in this study, nor of the 

single fungicide degradate, 3,4-dichloroaniline. Historically, 

the only pesticide with high relative-concentrations in the 

CDPH data before November 2003 was the insecticide 

heptachlor (table 4).

Special-Interest Constituents

Constituents of special interest analyzed for in the 

USAW study unit were 1,4-dioxane, N-nitrosodimethylamine 

(NDMA), 1,2,3-trichloropropane (1,2,3-TCP), and 

perchlorate. These constituents were selected because they 

recently have been detected in, or are considered to have 

the potential to reach, drinking-water supplies (California 

Department of Public Health, 2008b,c,d). NDMA is a highly 

toxic byproduct of the chlorination of wastewater (Bradley 

and others, 2005), and prior to April 1976, it was produced as 

a component of rocket fuel (Agency for Toxic Substances and 

Disease Registry, 1989). 1,4-Dioxane is a compound used as 

a stabilizer for chlorinated solvents (Tilman, 2009). NDMA 

and 1,4-dioxane were not detected in any samples (Kent 

and Belitz, 2009). 

1,2,3-TCP is sometimes classified as a solvent (Bender 

and others, 1999), or as a fumigant (Oki and Giambelluca, 

1987; Zebarth and others, 1998; Zogorski and others, 2006; 

Landon and others, 2010), reflecting spatial variations in 

its predominant use. The classification of 1,2,3-TCP is of 

little importance here because its occurrence did not meet 

the selection criteria for additional evaluation in this report. 

1,2,3-TCP was detected at low relative-concentrations in 

9.4 percent of the grid wells.

Perchlorate

Most perchlorate found in groundwater has been 

attributed to its use as an oxidizer in solid propellants for 

rockets, fireworks, and other explosives (Orris and others, 

2003). Perchlorate also has natural sources, such as Chilean 

caliche, is used as a nitrate fertilizer (Urbansky and others, 

2001), and it can be present at low concentrations in 

groundwater under natural conditions (Fram and Belitz, 2011). 

Perchlorate was detected in 67 percent of the grid wells, had 

a high aquifer proportion of 11.1 percent, and had a moderate 

aquifer proportion of 53.3 percent (table 5). Perchlorate 

was detected at high and moderate relative-concentrations 

in all USAW study areas except for the Elsinore study area; 

however, high relative-concentrations were most prevalent in 

the Riverside-Arlington/Temescal study area (figs. 11, 12P). 

The Yucaipa/San Timoteo study area had moderate, but no 

high relative-concentrations in grid wells.

Summary 

Groundwater quality in the approximately 1,000-squaremile (2,590-square-kilometer) Upper Santa Ana Watershed 

(USAW) study unit was investigated as part of the Priority 

Basin Project of the Groundwater Ambient Monitoring and 

Assessment (GAMA) Program. Samples were collected 

during November 2006 through March 2007 from 99 wells in 

6 study areas: Bunker Hill/Cajon/Rialto-Colton, Cucamonga/

Chino, Riverside-Arlington/Temescal, Yucaipa/San Timoteo, 

San Jacinto, and Elsinore. The GAMA Priority Basin Project 

is designed to provide a statistically robust characterization 

of untreated-groundwater quality in the primary aquifers at 

the basin-scale. Ninety wells were randomly selected within 

spatially distributed grid cells across the USAW study unit 

to assess the quality of the groundwater. An additional nine 

wells were sampled for the purposes of better understanding 

the relation of water quality to explanatory factors. 

Samples from USGS-grid wells were analyzed for up to 

318 constituents. CDPH inorganic data from the prior 3-year 

period (November 30, 2003, to December 1, 2006) were used 

to complement USGS data and provide additional information 

about groundwater quality. 
56 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Relative-concentrations (sample concentration divided 

by the health- or aesthetic-based benchmark concentration) 

were used for evaluating groundwater quality for those 

constituents that have Federal and (or) California regulatory or 

non-regulatory benchmarks for drinking-water quality.

Aquifer-scale proportion was used as the primary 

metric for evaluating regional-scale groundwater quality. 

High aquifer-scale proportion is defined as the percentage 

of the primary aquifers with relative-concentration greater 

than 1.0 for a particular constituent or class of constituents; 

proportion is based on an areal rather than a volumetric basis. 

Moderate and low aquifer-scale proportions were defined as 

the percentage of the primary aquifers with moderate and 

low relative-concentrations, respectively. Two statistical 

approaches, grid-based and spatially weighted, were used to 

evaluate aquifer-scale proportions for individual constituents 

and classes of constituents. Grid-based and spatially weighted 

estimates were comparable in the USAW study unit (within 

90 percent confidence intervals). However, the spatially 

weighted approach was superior to the grid-based proportion 

when the relative concentration of a constituent was high in a 

small fraction of the aquifer.

Inorganic constituents with human-health benchmarks 

were detected at high relative-concentrations in 32.9 percent 

of the primary aquifers, moderate in 29.3 percent, and low in 

37.8 percent. The high aquifer-scale proportion of inorganic 

constituents primarily reflected high aquifer-scale proportions 

of nitrate plus nitrite (25.3 percent), gross alpha activity 

(5.6 percent), arsenic (5.1 percent), molybdenum (4.1 percent), 

and boron (3.0 percent). 

Inorganic constituents with aesthetic benchmarks and 

secondary maximum contaminant levels (SMCLs) were 

detected at high relative-concentrations in 8.8 percent of the 

primary aquifers, and at moderate relative-concentrations in 

28.8 percent. Total dissolved solids (TDS) was the inorganic 

constituent with an SMCL that most frequently had high 

relative-concentrations (4.9 percent).

Of the 84 volatile organic compounds (VOCs) analyzed, 

21 were detected (this excludes 1,2,3-trichloropropane, 

considered separately as a constituent of special interest). 

All 21 of the VOCs detected had human-health benchmarks, 

and 5 were detected at high relative-concentration in at 

least one sample. These were the solvents tetrachloroethene, 

trichloroethene, and carbon tetrachloride, the fumigant 

1,2-dibromo-3-chloropropane, and the organic synthesis 

compound 1,1-dichloroethene. The two isomers of 

dichloroethane (1,1- and 1,2-) each had moderate 

relative-concentrations in 1.1 percent of the primary aquifers. 

The remaining VOCs that were detected were detected at only 

low relative-concentrations. Six of these were detected in 

10 percent or more of the grid wells. These were the solvent 

cis-1,2-dichloroethene, the trihalomethanes chloroform and 

bromodichloromethane, the refrigerants CFC-11 and CFC-12, 

and the gasoline oxygenate methyl tert-butyl ether.

Of the 135 pesticide compounds analyzed, 13 were 

detected. Seven of these 13 had human-health benchmarks. 

Pesticides did not have high relative-concentrations in any 

proportion of the primary aquifer system. The pesticide, 

atrazine, had a moderate relative-concentration in 1.1 percent 

of the grid wells, and was detected in 62 percent of the 

wells. Three pesticides, simazine, bromacil, and diuron, 

were detected at low relative-concentrations in 54 percent, 

19 percent, and 12 percent of the grid wells, respectively. 

Four constituents of special interest were analyzed for in 

the USAW study unit: 1,4-dioxane, N-nitrosodimethylamine 

(NDMA), 1,2,3-trichloropropane (1,2,3-TCP), and perchlorate. 

NDMA and 1,4-dioxane were not detected in any samples, and 

1,2,3-TCP was detected only at low relative-concentrations 

in 9.4 percent of the grid wells. In contrast, perchlorate was 

detected in 67 percent of the primary aquifers. Perchlorate 

was detected at high relative-concentrations in 11.1 percent of 

the primary aquifers, and at moderate relative-concentrations 

in 53.3 percent.

Acknowledgments

The authors thank the following cooperators for their 

support: the California State Water Resources Control Board, 

Lawrence Livermore National Laboratory, the California 

Department of Public Health, and the California Department 

of Water Resources. We especially thank the cooperating well 

owners and water purveyors for their cooperation in allowing 

the USGS to collect samples from their wells. 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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Appendix A 63

California requires regular sampling of public-supply 

wells under Title 22. Historical data derived from these 

samples are available from the CDPH database. Assembly 

Bill 599 directs the Groundwater Ambient Monitoring and 

Assessment (GAMA) Program to use existing data and collect 

new data as needed. The GAMA Priority Basin Project uses 

these monitoring data along with newly collected data to 

characterize the water quality of the primary aquifers. The 

CDPH database provided additional water-quality data for the 

grid-based and spatially weighted approaches to estimating 

aquifer-scale proportions for a wide range of constituents. 

CDPH data were not used to supplement USGS-grid-well 

data for VOCs, pesticides, or perchlorate because reporting 

levels for these constituents in the CDPH database generally 

were not low enough to differentiate between “low” and 

“moderate” relative-concentrations.

Of the 107 grid cells in the study unit (including the 

4 “equivalent cells” in the Elsinore Basin), 90 cells had 

USGS-grid data for organic constituents. Of these 90 cells, 

49 also had USGS-grid data for inorganic constituents. Of 

the 107 grid cells, 17 did not have USGS-grid data for any 

constituents because no well was sampled (two of these—

cell 1 for both the Cucamonga/Chino and the Yucaipa/San 

Timoteo study areas—did have inorganic data from wells 

sampled by the CDPH) (table A1). Three approaches were 

used to select CDPH inorganic constituent data for each 

grid cell where the USGS did not sample for inorganic 

constituents. The first step was to select CDPH data for the 

USGS-grid well (well sampled by the USGS for organic 

constituents, but not for inorganic constituents), provided 

that the CDPH data met quality-control criteria to minimize 

the selection of poor-quality data. Cation-anion balance was 

used as the quality-control assessment metric for selecting 

chemical analyses for a CDPH-grid well. Because water is 

electrically neutral, the total positive charge on dissolved 

cation species in a water sample must equal the total negative 

charge on dissolved anion species. The cation/anion imbalance 

commonly is used as a quality-control check for water sample 

analysis (Hem, 1992). Cation-anion balance was calculated 

as the difference between the total cations and total anions 

divided by the sum, expressed as a percentage:

An imbalance, or percent difference, of greater than or 

equal to 10 percent indicates uncertainty in the quality of the 

data. The most recent CDPH data from USGS-grid wells were 

evaluated to determine whether the CDPH data had a cation/

anion imbalance of less than 10 percent. If so, the CDPH 

inorganic data from the well were selected for use as grid well 

data for inorganic constituents. It was assumed that analyses 

with high-quality major ion data also had high-quality data 

for trace elements, nutrients, and radiochemical constituents. 

This step resulted in the selection of inorganic data from 

CDPH at 28 wells that were also USGS-grid wells. For 

identification purposes, data from the CDPH for these grid 

wells were assigned GAMA identification numbers equivalent 

to the GAMA USGS-grid well but with DG inserted between 

the study area prefix and sequence number (for example, 

CDPH-grid well USAWB-DG-03 is the same well as 

USGS-grid well USAWB-03, table A1).

If the first step did not yield CDPH inorganic data for a 

grid cell, the second step was to search the CDPH database to 

identify the highest randomly ranked well within that cell with 

a cation/anion imbalance of less than 10 percent. This step 

resulted in the selection of CDPH-grid wells for five grid cells, 

with CDPH inorganic data from a well not sampled by USGS 

as the grid well for that cell. These five CDPH-grid wells did 

not coincide with their cell’s respective USGS-grid well. To 

identify these new CDPH-grid wells, a well ID was created 

that added DPH after the study area prefix and then added the 

grid-cell number for the study area (for example, CDPH-grid 

well USAWB-DPH-17).

Appendix A. Use of Data from the California Department of Public Health 

(CDPH) Database

Percent difference *100,

where

is the sum of the concentrations of 

calcium, magnesium, sodium, and

potassium in milliequivalents per

liter (meq/L), and

cations anions

cations anions

cations

anions

  − =  

+  

∑ ∑

∑ ∑

∑

is the sum of the concentrations of

chloride, sulfate, fluoride, nitrate and

bicarbonate in meq/L.

∑
64 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

If no CDPH well had data with a cation/anion 

balance of less than 10 percent, the third step was to select 

the highest ranked well in the CDPH database that had 

any of the needed inorganic data. This step resulted in 

selection of one USGS-grid well where CDPH data were 

used for nitrate plus nitrite only. Because the well was a 

USGS‐grid well, a well ID was created that added DG to the 

GAMA ID (USAWB-DG-09). 

The result of these steps was one grid well per cell 

having data from either the USGS database or the CDPH 

database, or a combination of data from both sources. 

Inorganic data were collected from 49 of the 90 USGS-grid 

wells (fig. A1). Inorganic data from 34 CDPH-grid wells 

in the CDPH database were used to supplement these data 

(fig. A2). Nitrate plus nitrite values were available for all 34 

of these wells (table 2). As a result of combining data from 

these 34 CDPH-grid wells with USGS-grid well inorganic data 

(49 wells), at least some inorganic data were available for 83 

of the 107 grid cells. 

Estimates of aquifer-scale proportion for constituents 

made on the basis of a smaller number of wells have a larger 

error associated with the 90 percent confidence intervals 

(based on the Jeffreys interval for the binomial distribution, 

Brown and others, 2001). Analysis of the combined 

datasets to evaluate the occurrence of high or moderate 

relative-concentrations for inorganic constituents was not 

affected by differences in reporting levels between USGS-grid 

and CDPH-grid data because concentrations above one-half of 

water-quality benchmarks (relative-concentration >0.5) were 

generally substantially higher than the highest reporting levels. 

Comparisons between USGS-grid and CDPH-grid data are 

described in appendix B.
Appendix A 65 Table A1. Grid cell number, USGS-grid wells sampled, and CDPH-grid well identification number if CDPH data were used to supplement grid cell inorganic data, land-use categories, well construction information, and the relative elevation as the normalized position along a flowpath for wells sampled November 2006–March 2007 for the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project Upper Santa Ana Watershed study unit. [CDPH, California Department of Public Health; USAWB, Bunker Hill/Cajon/Rialto-Colton study area well; USAWC, Cucamonga/Chino study area well; USAWE, Elsinore study area well; USAWR, Riverside- Arlington/Temescal study area well; USAWS, San Jacinto study area well; USAWY, Yucaipa/San Timoteo study area well; ft, feet; m, meter; LSD, land surface datum; na, not available; ns, cell not sampled (no data available); USGS, U.S. Geological Survey; USAWB-##, naming convention for USGS-grid well; USAWU-##, naming convention for USGS understanding well; USAWB-DG-##, naming convention for CDPH data from a USGS-grid well; USAWB-DPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public-supply well; DOM, domestic well; IND, industrial well; IRR, irrigation well; DES, desalter well; MON, monitoring well; REC, recreation use well (to fill ponds)] Grid cell number USGS GAMA well identification number CDPH-grid well identification number Well type Land-use categories Construction information Relative elevation as the normalized position along flowpath (dimensionless) (value in parentheses is for separate well with CDPH data used to supplement GAMA data) Agricultural land use within 500 m of the well (percent) Natural land use within 500 m of the well (percent) Urban land use within 500 m of the well (percent) Well depth (ft below LSD) Top of perforations (ft below LSD) Bottom of perforations (ft below LSD) Length from top of uppermost perforated interval to bottom of perforations (ft below LSD) Bunker Hill/Cajon/Rialto-Colton study area grid wells 1 USAWB-14 USGS data PSW 0 22 78 1,060 300 1,040 740 0.627 2 USAWB-02 USGS data PSW 0 96 4 234 61 230 169 0.924 3 USAWB-03 USAWB-DG-03 PSW 0 94 6 195 45 175 130 1.231 4 USAWB-09 USAWB-DG-09 1 PSW 0 79 21 126 52 126 74 1.749 5 ns ns ns ns ns ns ns ns ns ns ns 6 USAWB-05 USAWB-DG-05 PSW 0 94 6 50 21 48 27 1.050 7 USAWB-06 USAWB-DG-06 PSW 0 63 37 400 177 400 223 0.772 8 USAWB-04 USGS data PSW 0 72 28 900 464 884 420 0.629 9 USAWB-08 USGS data PSW 0 22 78 580 126 560 434 0.471 10 USAWB-18 USGS data PSW 1 13 86 534 244 534 290 0.413 11 USAWB-01 USAWB-DG-01 PSW 0 45 55 1,067 497 1,047 550 0.395 12 USAWB-07 USGS data PSW 0 36 64 654 240 462 222 0.347 13 USAWB-16 USGS data PSW 0 22 78 963 888 951 63 0.366 14 USAWB-10 USGS data PSW 0 0 100 708 490 680 190 0.437 15 USAWB-11 USGS data PSW 0 5 95 606 378 569 191 0.512 16 USAWB-17 USAWB-DG-17 PSW 0 5 95 421 173 415 242 0.449 17 USAWB-12 USAWB-DPH-17 PSW 6 65 29 950 500 930 430 0.569 (0.494) 18 USAWB-15 USAWB-DG-15 PSW 16 65 19 1,020 600 1,000 400 0.563 19 USAWB-19 USAWB-DG-19 IRR 18 20 62 na na na na 0.861 20 USAWB-13 USAWB-DG-13 PSW 4 17 79 474 150 442 292 0.556 Cucamonga/Chino study area grid wells 1 no USGS well USAWC-DPH-1 PSW na na na na na na na 0.086 2 USAWC-23 USGS data PSW 52 20 28 1,180 440 1,180 740 0.096 3 USAWC-05 USAWC-DG-05 PSW 1 17 82 800 300 775 475 0.222 4 USAWC-04 USGS data PSW 0 18 82 560 317 535 218 0.249 5 USAWC-06 USAWC-DG-06 PSW 0 0 100 903 472 849 377 0.349 6 USAWC-16 USAWC-DG-16 PSW 17 16 67 491 230 460 230 0.915 7 USAWC-18 USAWC-DG-18 PSW 0 10 90 764 322 702 380 0.651
66 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project Grid cell number USGS GAMA well identification number CDPH-grid well identification number Well type Land-use categories Construction information Relative elevation as the normalized position along flowpath (dimensionless) (value in parentheses is for separate well with CDPH data used to supplement GAMA data) Agricultural land use within 500 m of the well (percent) Natural land use within 500 m of the well (percent) Urban land use within 500 m of the well (percent) Well depth (ft below LSD) Top of perforations (ft below LSD) Bottom of perforations (ft below LSD) Length from top of uppermost perforated interval to bottom of perforations (ft below LSD) Cucamonga/Chino study area grid wells—Continued

8 USAWC-14 USGS data PSW 0 10 90 1,028 415 1,007 592 0.317

9 USAWC-24 USGS data MON 21 38 41 300 280 300 20 0.237

10 USAWC-10 USAWC-DPH-10 PSW 18 39 43 520 290 500 210 0.090 (0.103)

11 USAWC-09 USAWC-DG-09 PSW 0 69 31 na na na na 0.075

12 USAWC-12 USGS data PSW 80 14 6 na na na na 0.096

13 USAWC-25 USGS data IRR 26 36 38 435 230 430 200 0.191

14 USAWC-15 USAWC-DG-15 PSW 64 13 23 1,028 415 1,007 592 0.281

15 USAWC-17 USGS data PSW 26 19 55 900 370 885 515 0.333

16 USAWC-21 USGS data PSW 12 5 83 1,110 400 1,090 690 0.772

17 USAWC-19 USAWC-DG-19 PSW 0 100 0 Spring Spring Spring na 2.148

18 ns ns ns ns ns ns ns ns ns ns ns19 USAWC-13 USAWC-DG-13 PSW 36 23 40 na na na na 0.322

20 USAWC-08 USGS data PSW 21 32 47 370 210 370 160 0.197

21 USAWC-11 USGS data PSW 13 18 69 350 200 350 150 0.173

22 USAWC-07 USAWC-DG-07 PSW 0 25 75 218 113 213 100 0.121

23 USAWC-22 no CDPH data PSW 0 10 90 27,31 na na na 0.361

24 USAWC-01 USGS data PSW 1 20 78 795 na na na 0.397

25 USAWC-02 USGS data PSW 3 17 80 950 na na na 0.461

26 USAWC-20 USGS data PSW 4 20 76 918 na na na 0.512

27 USAWC-03 USAWC-DG-03 PSW 0 10 90 1,040 500 1,030 530 0.623

Elsinore study area grid wells

1 USAWE-01 USGS data PSW 20 65 15 1,430 420 1,410 990 0.525

3 USAWE-02 USGS data PSW 0 98 2 1,720 380 1,700 1,320 0.512

3 USAWE-03 USGS data PSW 20 13 67 980 570 980 410 0.556

4 USAWE-04 USGS data PSW 21 55 24 740 300 730 430 0.518

Table A1. Grid cell number, USGS-grid wells sampled, and CDPH-grid well identification number if CDPH data were used to supplement grid cell inorganic data, land-use categories, well construction information, and the relative elevation as the normalized position along a flowpath for wells sampled November 2006–March 2007 for the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project Upper Santa Ana Watershed study unit.—Continued[CDPH, California Department of Public Health; USAWB, Bunker Hill/Cajon/Rialto-Colton study area well; USAWC, Cucamonga/Chino study area well; USAWE, Elsinore study area well; USAWR, Riverside-Arlington/Temescal study area well; USAWS, San Jacinto study area well; USAWY, Yucaipa/San Timoteo study area well; ft, feet; m, meter; LSD, land surface datum; na, not available; ns, cell not sampled (no data available); USGS, U.S. Geological Survey; USAWB-##, naming convention for USGS-grid well; USAWU-##, naming convention for USGS understanding well; USAWB-DG-##, naming convention for CDPH data from a USGS-grid well; USAWB-DPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; DOM, domestic well; IND, industrial well; IRR, irrigation well; DES, desalter well; MON, monitoring well; REC, recreation use well (to fill ponds)]
Appendix A 67 Grid cell number USGS GAMA well identification number CDPH-grid well identification number Well type Land-use categories Construction information Relative elevation as the normalized position along flowpath (dimensionless) (value in parentheses is for separate well with CDPH data used to supplement GAMA data) Agricultural land use within 500 m of the well (percent) Natural land use within 500 m of the well (percent) Urban land use within 500 m of the well (percent) Well depth (ft below LSD) Top of perforations (ft below LSD) Bottom of perforations (ft below LSD) Length from top of uppermost perforated interval to bottom of perforations (ft below LSD) Riverside/Arlington-Temescal study area grid wells 1 ns ns ns ns ns ns ns ns ns ns ns 2 USAWR-12 USGS data PSW 0 6 94 500 200 250 50 0.163 3 USAWR-10 USGS data PSW 0 13 87 220 108 204 96 0.102 4 USAWR-03 USGS data PSW 49 20 32 183 24 170 146 0.183 5 USAWR-06 USGS data IRR 0 17 83 190 120 180 60 0.214 6 USAWR-07 USGS data DES 8 11 81 170 80 150 70 0.149 7 USAWR-04 USAWR-DPH-7 PSW 0 70 30 89 28 89 61 0.208 (0.120) 8 USAWR-09 USGS data IRR 0 17 83 346 60 346 286 0.222 9 USAWR-01 USAWR-DG-01 PSW 33 27 40 na na na na 0.262 10 USAWR-11 USGS data PSW 1 57 42 644 250 634 384 0.407 11 USAWR-05 USGS data DOM 13 46 41 192 na na na 0.254 12 USAWR-08 USGS data PSW 0 28 72 402 40 380 340 0.275 13 USAWR-02 USAWR-DG-02 PSW 5 67 28 613 284 600 316 0.224 San Jacinto study area grid wells 1 ns ns ns ns ns ns ns ns ns ns ns 2 USAWS-12 USGS data PSW 7 73 20 760 240 740 500 0.184 3 USAWS-04 no CDPH data IND 0 91 9 150 50 150 100 0.256 4 USAWS-01 USAWS-DG-01 PSW 6 22 72 225 100 205 105 0.369 5 USAWS-08 USGS data PSW 16 9 76 428 170 420 250 0.602 6 ns ns ns ns ns ns ns ns ns ns ns 7 USAWS-03 no CDPH data IRR 72 26 2 160 55 140 85 0.207 8 USAWS-09 USAWS-DG-09 DES 68 31 1 370 140 350 210 0.162 9 USAWS-10 no CDPH data DES 16 75 9 360 90 320 230 0.090 10 USAWS-11 no CDPH data DES 61 30 9 380 80 340 260 0.114 11 ns ns ns ns ns ns ns ns ns ns ns 12 USAWS-05 USGS data PSW 26 15 59 625 100 620 520 0.129 13 USAWS-13 no CDPH data DES 74 19 8 410 230 390 160 0.105 14 ns ns ns ns ns ns ns ns ns ns ns 15 ns ns ns ns ns ns ns ns ns ns ns Table A1. Grid cell number, USGS-grid wells sampled, and CDPH-grid well identification number if CDPH data were used to supplement grid cell inorganic data, land-use categories, well construction information, and the relative elevation as the normalized position along a flowpath for wells sampled November 2006–March 2007 for the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project Upper Santa Ana Watershed study unit.—Continued [CDPH, California Department of Public Health; USAWB, Bunker Hill/Cajon/Rialto-Colton study area well; USAWC, Cucamonga/Chino study area well; USAWE, Elsinore study area well; USAWR, Riverside-Arlington/Temescal study area well; USAWS, San Jacinto study area well; USAWY, Yucaipa/San Timoteo study area well; ft, feet; m, meter; LSD, land surface datum; na, not available; ns, cell not sampled (no data available); USGS, U.S. Geological Survey; USAWB-##, naming convention for USGS-grid well; USAWU-##, naming convention for USGS understanding well; USAWB-DG-##, naming convention for CDPH data from a USGS-grid well; USAWB-DPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; DOM, domestic well; IND, industrial well; IRR, irrigation well; DES, desalter well; MON, monitoring well; REC, recreation use well (to fill ponds)]
68 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project Grid cell number USGS GAMA well identification number CDPH-grid well identification number Well type Land-use categories Construction information Relative elevation as the normalized position along flowpath (dimensionless) (value in parentheses is for separate well with CDPH data used to supplement GAMA data) Agricultural land use within 500 m of the well (percent) Natural land use within 500 m of the well (percent) Urban land use within 500 m of the well (percent) Well depth (ft below LSD) Top of perforations (ft below LSD) Bottom of perforations (ft below LSD) Length from top of uppermost perforated interval to bottom of perforations (ft below LSD) San Jacinto study area grid wells—Continued 16 ns ns ns ns ns ns ns ns ns ns ns 17 ns ns ns ns ns ns ns ns ns ns ns 18 USAWS-16 USGS data PSW 50 41 9 na na na na 0.156 19 USAWS-02 USGS data PSW 48 22 30 518 104 518 414 0.208 20 ns ns ns ns ns ns ns ns ns ns ns 21 USAWS-07 no CDPH data REC 55 27 18 300 na na na 0.119 22 USAWS-15 USGS data PSW 5 4 91 328 na na na 0.423 23 ns ns ns ns ns ns ns ns ns ns ns 24 USAWS-17 no CDPH data IRR 51 45 4 na na na na 0.254 25 USAWS-19 no CDPH data IRR 76 22 3 na na na na 0.184 26 USAWS-21 USGS data IRR 67 32 1 900 440 900 460 0.223 27 USAWS-06 USGS data PSW 43 14 43 650 200 650 450 0.336 28 USAWS-14 USGS data PSW 55 13 32 1,550 470 1,530 1,060 0.471 29 USAWS-18 USGS data PSW 0 24 75 714 324 714 390 0.473 30 USAWS-20 USGS data PSW 12 15 73 696 300 676 376 0.412 31 ns ns ns ns ns ns ns ns ns ns ns Yucaipa/San Timoteo study area grid wells

1 no USGS well USAWY-DPH-1 1 PSW na na na na na na na 0.981

2 ns ns ns ns ns ns ns ns ns ns ns3 USAWY-03 USAWY-DG-03 PSW 19 17 64 600 150 576 426 1.084

4 USAWY-04 USGS data PSW 0 10 90 1,710 705 1,690 985 1.077

5 USAWY-02 USAWY-DG-02 PSW 8 44 48 596 290 584 294 1.423

6 USAWY-05 USGS data PSW 0 17 83 585 320 585 265 1.322

7 USAWY-06 USGS data PSW 11 45 45 790 150 340 190 1.321

8 USAWY-01 USAWY-DG-01 PSW 4 75 21 350 300 315 15 1.357

9 ns ns ns ns ns ns ns ns ns ns ns

10 USAWY-09 USAWY-DG-09 PSW 2 4 94 946 320 694 374 1.452

11 USAWY-08 USAWY-DG-08 PSW 9 33 58 444 58 434 376 1.784

12 USAWY-07 USAWY-DG-07 PSW 17 63 20 314 164 314 150 1.618

Table A1. Grid cell number, USGS-grid wells sampled, and CDPH-grid well identification number if CDPH data were used to supplement grid cell inorganic data, land-use categories, well construction information, and the relative elevation as the normalized position along a flowpath for wells sampled November 2006–March 2007 for the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project Upper Santa Ana Watershed study unit.—Continued[CDPH, California Department of Public Health; USAWB, Bunker Hill/Cajon/Rialto-Colton study area well; USAWC, Cucamonga/Chino study area well; USAWE, Elsinore study area well; USAWR, Riverside-Arlington/Temescal study area well; USAWS, San Jacinto study area well; USAWY, Yucaipa/San Timoteo study area well; ft, feet; m, meter; LSD, land surface datum; na, not available; ns, cell not sampled (no data available); USGS, U.S. Geological Survey; USAWB-##, naming convention for USGS-grid well; USAWU-##, naming convention for USGS understanding well; USAWB-DG-##, naming convention for CDPH data from a USGS-grid well; USAWB-DPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; DOM, domestic well; IND, industrial well; IRR, irrigation well; DES, desalter well; MON, monitoring well; REC, recreation use well (to fill ponds)]
Appendix A 69 Grid cell number USGS GAMA well identification number CDPH-grid well identification number Well type Land-use categories Construction information Relative elevation as the normalized position along flowpath (dimensionless) (value in parentheses is for separate well with CDPH data used to supplement GAMA data) Agricultural land use within 500 m of the well (percent) Natural land use within 500 m of the well (percent) Urban land use within 500 m of the well (percent) Well depth (ft below LSD) Top of perforations (ft below LSD) Bottom of perforations (ft below LSD) Length from top of uppermost perforated interval to bottom of perforations (ft below LSD) USGS-understanding wells RAT-09 USAWU-01 USGS data PSW 0 32 68 170 50 170 120 0.198 YUTI-04 USAWU-02 USGS data PSW 10 17 72 1,100 390 1,100 710 1.091 off grid USAWU-03 no inorganic data PSW 0 46 54 220 180 220 40 2.973 RAT-10 USAWU-04 USGS data PSW 1 23 75 540 320 520 200 0.386 BUNCO-09 USAWU-05 USGS data PSW 0 1 99 na 310 787 na 0.542 CUCHI-04 USAWU-06 USGS data PSW 0 24 76 800 295 784 489 0.280 CUCHI-20 USAWU-07 USGS data PSW 39 25 36 980 400 980 580 0.304 CUCHI-02 USAWU-08 USGS data PSW3 53 20 27 na na na na 0.097 CUCHI-14 USAWU-09 USGS data MON 7 10 83 310 29 310 281 0.211 1 Inorganic data limited to nitrate plus nitrite from CDPH (no USGS inorganic data). 2 Well depth determined from hole depth reported on driller’s log. 3 Water from this well was not being served to the public at the time of this study. Table A1. Grid cell number, USGS-grid wells sampled, and CDPH-grid well identification number if CDPH data were used to supplement grid cell inorganic data, land-use categories, well construction information, and the relative elevation as the normalized position along a flowpath for wells sampled November 2006–March 2007 for the Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project Upper Santa Ana Watershed study unit.—Continued [CDPH, California Department of Public Health; USAWB, Bunker Hill/Cajon/Rialto-Colton study area well; USAWC, Cucamonga/Chino study area well; USAWE, Elsinore study area well; USAWR, Riverside-Arlington/Temescal study area well; USAWS, San Jacinto study area well; USAWY, Yucaipa/San Timoteo study area well; ft, feet; m, meter; LSD, land surface datum; na, not available; ns, cell not sampled (no data available); USGS, U.S. Geological Survey; USAWB-##, naming convention for USGS-grid well; USAWU-##, naming convention for USGS understanding well; USAWB-DG-##, naming convention for CDPH data from a USGS-grid well; USAWB-DPH-##, naming convention for CDPH-data from a CDPH-grid well; PSW, public supply well; DOM, domestic well; IND, industrial well; IRR, irrigation well; DES, desalter well; MON, monitoring well; REC, recreation use well (to fill ponds)]
70 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection EXPLANATION

Sampled study areas Sampled Wells

USGS-grid well and identifier

Prefix USAW dropped to conserve space Bunker Hill/Cajon/Rialto-Colton

Cucamonga/Chino

Riverside-Arlington/Temescal

San Jacinto

Yucaipa/San Timoteo

Elsinore

Grid cell

IP017194_Figure A01

USGS-grid well without inorganic

 data and identifier

0 10 20 MILES

0 10 20 KILOMETERS

B-01

B-02

B-03

B-04

B-05

B-06

B-07

B-08

B-09

B-10

B-11

B-12

B-13

B-14

B-15

B-16

B-17

B-18

B-19 C-01

C-02

C-03

C-04

C-05

C-06

C-07

C-08

C-09

C-10

C-11

C-12

C-13

C-15

C-16

C-17

C-18

C-19

C-20

C-21

C-22

C-23

C-24

C-25

E-01

E-02

E-03

E-04

R-01

R-02

R-03

R-04

R-05

R-06

R-07

R-08

R-09

R-10

R-11

R-12

S-01

S-02

S-03

S-04

S-05

S-06

S-07

S-05

S-07

S-08

S-09

S-10

S-11

S-12

S-13

S-14

S-15

S-16

S-17

S-18

S-19

S-20

S-21

Y-01

Y-02

Y-03

Y-04

Y-05

Y-06

Y-07

Y-08

Y-09

C-14

Figure A1. Identifiers and locations of USGS-grid wells sampled during November 2006–March 2007, Upper Santa Ana 

Watershed study unit, GAMA Priority Basin Project.
Appendix A 71

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection EXPLANATION

Sampled study areas Sampled wells

California Department of Public Health

 (CDPH)-grid well and identifier

Prefix USAW dropped to conserve space Bunker Hill/Cajon/Rialto-Colton

Cucamonga/Chino

Riverside-Arlington/Temescal

San Jacinto

Yucaipa/San Timoteo

Elsinore

Grid cell

IP017194_Figure A02

0 10 20 MILES

0 10 20 KILOMETERS

C-DG-05

C-DG-06

C-DG-09

C-DG-07

R-DPH-7

R-DG-01

R-DG-02

S-DG-01

S-DG-09

Y-DPH-1

Y-DG-01

Y-DG-09

Y-DG-09

Y-DG-08

B-DG-03

B-DG-09

B-DG-05

B-DG-06

B-DG-01

B-DG-17

B-DPH-17

B-DG-15

B-DG-19

B-DG-13

C-DG-18

C-DG-15

C-DG-19

C-DG-13

C-DPH-1

C-DG-16

C-DPH-10

C-DG-03

Y-DG-03

Y-DG-02

Y-DG-07

Figure A2. Identifiers and locations of CDPH-grid wells from which data for inorganic constituents from the California 

Department of Public Health database were used to supplement USGS data, Upper Santa Ana Watershed study unit, GAMA 

Priority Basin Project.
72 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

CDPH and USGS-GAMA data were compared to assess 

the validity of using data from these different sources. Because 

USGS laboratory reporting levels (LRL) for most organic 

constituents and trace elements were substantially lower than 

the method detection limits (MDL) used to report CDPH 

data (table 3), it was generally not possible to meaningfully 

compare concentrations of these constituent types in 

individual wells. However, concentrations of major ions and 

nitrate, which generally are prevalent and have concentrations 

substantially above LRLs and MDLs, were compared for 

each well with data from both sources. Comparisons were 

made for wells that were analyzed by USGS-GAMA for 

inorganic constituents and had data within the most recent 

3-year interval in the CDPH database. Forty-one wells had 

major ion and nitrate data in common between the datasets. 

Wilcoxon signed rank tests of paired analyses for ten of these 

constituents (fig. B1) indicated significant differences between 

USGS-GAMA and CDPH data for two of the constituents: 

calcium (p=0.032), and fluoride (p<0.001). While differences 

between the paired datasets occurred for these constituents, 

most sample pairs plotted close to a 1-to-1 line (fig. B1). The 

relative percent difference (RPD) was calculated for each data 

pair. The median RPD was 7.3 percent; 83 percent of the RPD 

values were less than 20 percent. These direct comparisons 

indicated that the GAMA and CDPH data for major ions and 

nitrate were not significantly different. 

Major ion data for grid wells with the requisite analyses 

of cations (sodium, potassium, calcium, and magnesium) 

and anions (sulfate, chloride, carbonate, bicarbonate, nitrite, 

and nitrate) were plotted on Piper diagrams (Piper, 1944) 

with CDPH major ion data to determine whether the grid 

wells represented the range of groundwater types that have 

historically been observed in the study unit. Piper diagrams 

show the relative abundance of major cations and anions (on 

a charge equivalent basis) as a percentage of the total ion 

content of the water (fig. B2). Piper diagrams often are used 

to define groundwater type (Hem, 1992). All CDPH data from 

the period November 30, 2003, to December 1, 2006 (prior 

period) having cation/anion data and a cation/anion imbalance 

of less than 10 percent were retrieved and plotted on these 

Piper diagrams for comparison with grid well data.

Calcium bicarbonate was the dominant water type for the 

57 USGS-GAMA wells (51 percent) and for the 477 CDPH 

wells (62 percent). When no single cation or anion accounts 

for more than 50 percent of its group, the water type is 

described as mixed cation or mixed anion (Hem, 1992). 

Twenty percent of the CDPH wells had a mixed cation water 

type compared to 33 percent of the USGS-GAMA wells. 

Similarly, 16 percent of the CDPH wells had a mixed anion

water type compared to 23 percent of the USGS-GAMA 

wells. Sodium was the dominant cation for the relatively small 

proportion of wells (8 percent) with a dominant cation other 

than calcium or mixed. Chloride was the dominant anion 

for the relatively small proportion of wells (3 percent) with 

a dominant anion other than bicarbonate or mixed, although 

six CDPH wells (1 percent) had sulfate as their dominant 

anion. This similarity of the range of relative abundance of 

major cations and anions in USGS wells to the set of CDPH 

wells indicates that the USGS-grid and understanding wells 

represent most of the diversity of water types present within 

the Upper Santa Ana Watershed study unit.

Appendix B. Comparison of California Department of Public Health and 

U.S. Geological Survey-GAMA Data
Appendix B 73

0.1

0.1 1 10 100 1,000

1

10

100

1,000

Calcium

Chloride

Fluoride

Magnesium

Nitrate

Potassium

Sodium

Sulfate

Total dissolved solids

Alkalinity

1:1 line

CONCENTRATION, USGS-GAMA, IN MILLIGRAMS PER LITER

CONCENTRATION, CDPH DATA, IN MILLIGRAMS PER LITER

IP017194_figB01

Figure B1. Paired inorganic concentrations from wells sampled by the Groundwater Ambient Monitoring and 

Assessment (GAMA) Program, November 2006–March 2007, and prior 3-year California Department of Public 

Health data, Upper Santa Ana Watershed study unit, GAMA Priority Basin Project. 
74 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

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

Percent

EXPLANATION

USGS-grid well

CDPH well

IP017194_Figure B02_ All wells_USAW

USGS-understanding well

Figure B2. Piper diagram for water types in USGS-GAMA wells and wells in the California Department 

of Public Health database, Upper Santa Ana Watershed study unit, GAMA Priority Basin Project.
Appendix C 75

The status assessment is intended to characterize the 

quality of groundwater resources in the primary aquifers 

of the USAW study unit. The primary aquifers are defined 

by the perforated depth intervals of the wells listed in the 

CDPH database. The use of the term “primary aquifers” 

does not imply that there exists a discrete aquifer unit. In 

most groundwater basins, municipal and community supply 

wells generally are perforated at greater depths than domestic 

wells. Thus, because domestic wells are not listed in the 

CDPH database, the primary aquifers generally correspond 

to the portion of the aquifer system tapped by municipal and 

community supply wells. A majority of the wells used in the 

status assessment are listed in the CDPH database, and are 

therefore classified as municipal and community drinkingwater supply wells. However, to the extent that domestic wells 

are perforated over the same depth intervals as the CDPH 

wells, the assessments presented in this report also may be 

applicable to the portions of the aquifer systems used for 

domestic drinking-water supplies.

Two statistical approaches, grid-based and spatially 

weighted, were selected to evaluate the aquifer-scale 

proportions of the area of the primary aquifers in the USAW 

study unit with high, moderate, or low relative-concentrations 

of constituents relative to water-quality benchmarks (Belitz 

and others, 2010). The grid-based and spatially weighted 

estimations of aquifer-scale proportions, based on a spatially 

distributed grid cell network across the USAW study unit, 

are intended to characterize the water quality of the primary 

aquifers, or at depths from which drinking water is usually 

drawn. These approaches assign weights to wells based on a 

single well per cell (grid-based) or the number of wells per 

cells (spatially weighted). 

Raw detection frequencies, derived from the percentage 

of the total number of wells with high or moderate 

relative-concentrations, also were calculated for individual 

constituents, but were not used for estimating aquifer-scale 

proportion because this method creates spatial bias towards 

regions with large numbers of wells.

1. Grid-based. One well in each grid cell, a “grid well,” was 

randomly selected to represent the area of the primary 

aquifers (Belitz and others, 2010). Most grid wells 

sampled for the USAW study unit were USGS-grid wells. 

However, data for all constituents were not available for 

some USGS-grid wells, so additional data for CDPHgrid wells were selected to provide data for grid cells 

with no USGS-grid wells. The relative-concentration 

for each constituent (concentration relative to its 

benchmark) was then evaluated for each grid well. The 

proportion of the primary aquifers (by area) with high 

relative-concentrations was calculated by dividing the 

number of cells with concentrations greater than the 

benchmark (relative-concentration greater than 1) by 

the total number of grid wells in the USAW study unit. 

Proportions containing moderate relative-concentrations 

were calculated similarly. Confidence intervals for 

grid-based aquifer-scale proportions were computed 

using the Jeffreys interval for the binomial distribution 

(Brown and others, 2001). The grid-based estimate is 

spatially unbiased because the cells represented are equal 

areas. However, the grid-based approach may not identify 

constituents that exist at high concentrations in small 

proportions of the primary aquifers.

2. Spatially weighted. The spatially weighted approach 

relied on USGS-grid well data collected from 

November 2006 to March 2007, and CDPH data from 

November 30, 2003, to December 1, 2006 (most recent 

analyses per well for all wells within each grid cell), 

and USGS-understanding public-supply well data. 

However, instead of data from only one well per grid 

cell, the spatially weighted approach uses all wells 

in each cell to calculate the high, moderate, and low 

relative-concentrations for the cell. The high, moderate, 

and low aquifer-scale proportions are then calculated 

from the percentage of cells with high, moderate, or low 

relative-concentrations (Isaaks and Srivastava, 1989). The 

resulting proportions are spatially unbiased (Isaaks and 

Srivastava, 1989), again, because the cells represented are 

equal areas. Confidence intervals for spatially weighted 

estimates of aquifer-scale proportion are not described in 

this report.

The raw detection frequency approach merely is the 

percentage (frequency) of wells within the USAW study 

unit with high relative-concentrations. It was calculated by 

considering all of the available data from November 30, 

2003, to December 1, 2006, for the CDPH well data (the 

most recent analysis per well for all wells), the USGS-grid 

well data, and understanding wells. However, this approach is 

not spatially unbiased because the CDPH and understanding 

wells are not uniformly distributed. Consequently, high 

values (or low values) for wells clustered in a particular 

area represent a small part of the primary aquifers, and 

could be given a disproportionately high (or low) weight 

compared to that given by spatially unbiased approaches. 

Therefore, raw detection frequencies were not used to assess 

aquifer-scale proportions. 

Appendix C. Calculation of Aquifer-Scale Proportions
76 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Land-Use Classification

Land use was classified by using an enhanced version 

of the satellite-derived [98-ft (30-m) pixel resolution] USGS 

National Land Cover Dataset (Nakagaki and others, 2007). 

This dataset has been used in previous national and regional 

studies relating land use to water quality (Gilliom and others, 

2006; Zogorski and others, 2006). The dataset characterizes 

land cover during the early 1990s. One pixel in the dataset 

imagery represents a land area of 9,688 ft2 (900 m2), 

calculated from the pixel of 98 ft (30 m). The imagery was 

classified into 25 land-cover classifications (Nakagaki and 

Wolock, 2005). These 25 land-cover classifications were 

assigned to three general classifications for the purpose 

of general categorization of principal land use: urban, 

agricultural, and natural. 

Land-use statistics for the study unit, study areas, and 

for circles with a radius of 1,640 ft (500 m) around each 

study well were calculated based on these classified datasets 

using the software ArcGIS. The 500-m buffer represents 

a contributing area as defined and evaluated by Johnson 

and Belitz (2009). These are given in table A1. Figure 5

displays the land-use map based on the calculation of land 

use in the study areas from aerial coverage (30-meter pixel = 

900 square meters). 

Average land use around grid wells (radius of 500 m) 

in the USAW study unit was dominated by urban land use 

(51 percent of the area) and natural landscape (33 percent 

of the area), while agricultural land use accounted for 

17 percent of the area (fig. 4). Average land use across 

the study unit was more evenly distributed among these 

three major classifications, with 35 percent urban land use, 

44 percent natural landscape, and 21 percent agricultural 

land use in the study unit. However, it appears that land 

use in the 500-m radius surrounding the sampled grid wells 

represented land use in the study areas reasonably well, 

with one exception (fig. D1A). Grid wells sampled in the 

Yucaipa/San Timoteo study area had, on average, land use 

surrounding them that was about 30 percent more urban, 

and about 20 percent less natural landscape than did the 

study area as a whole (figs. D1A, D4). The higher percentage 

of urbanized land surrounding the grid wells reflects the 

association of public-supply wells with population density. 

The area surrounding grid wells, particularly for the Yucaipa/

San Timoteo study area, may reflect greater urban influence 

than might be expected on the basis of the average land 

use of the study areas. Land-use proportions for each grid 

and understanding well sampled by the USGS are shown in 

figure D1B.

Appendix D. Ancillary Datasets

Figure D1A. Ternary diagram showing proportions of urban, agricultural, and natural land-use categories in each study unit (solid 

symbols), and for the USGS wells sampled (open symbols) for the Upper Santa Ana Watershed study unit, GAMA Priority Basin Project. 

[Land uses were determined from USGS National Land Cover Data from Nakagaki and others (2007).]

UPPER

SANTA ANA WATERSHED

STUDY UNIT

STUDY AREA

UPPER

SANTA ANA WATERSHED

STUDY UNIT

Data point:

Land use

based on

aerial

coverage

Data point:

Land use

based on a

500-meter-radius

area around

grid wells

A

B

2

0

20

40

60

80

100

0

PERCENT URBAN

100 80 60 40 20 0

PERCENT AGRICULTURE

100

100

80

60

40

20

0

PERCENT NATURAL

STUDY AREA

Bunker Hill/Cajon/

Rialto-Colton

Yucaipa/San Timoteo

Riverside-Arlington/Temescal

Elsinore

Cucamonga/Chino

San Jacinto

BkHill/Cj/
Appendix D 77

Figure D1B. Ternary diagram showing proportions of urban, agricultural, and natural land-use categories within a 500-meter radius 

surrounding grid and understanding wells sampled for the Upper Santa Ana Watershed study unit, GAMA Priority Basin Project. [Land 

uses were determined from USGS National Land Cover Data from Nakagaki and others (2007).]

STUDY AREA

UPPER

SANTA ANA WATERSHED

STUDY UNIT

B

Understanding well

20

40

60

80

100

0

PERCENT URBAN

100

100 80 60 40 20 0

PERCENT AGRICULTURE

100 80 60 40 20 0

PERCENT AGRICULTURE

100

80

60

40

20

0

PERCENT NATURAL

0

IP017194_Figure D01ab

Bunker Hill/Cajon/

Rialto-Colton

Yucaipa/San Timoteo

Riverside-Arlington/Temescal

Elsinore

Cucamonga/Chino

San Jacinto

Well Construction Information

Well construction data were derived, in part, from 

drillers’ logs. Other sources of well construction data 

included ancillary records provided by well owners and the 

USGS National Water Information System database. Well 

identification verification procedures are described by Kent 

and Belitz (2009). Well depths and depths to the top- and 

bottom-of-perforations for USGS-grid, understanding, and 

CDPH-grid wells are listed in table A1. USAW wells were 

classified as public-supply wells, irrigation wells, desalter 

wells, monitoring wells, an industrial well, a “recreational” 

well, and a domestic well. Public-supply wells pump the 

groundwater from the aquifer to a distribution system. 

Irrigation wells supply water for agriculture and are generally 

located near fields where crops are grown. Desalter wells 

extract high-salinity groundwater for treatment and subsequent 

use by homes, industry, or agriculture. Monitoring wells tend 

to be short-screened wells installed exclusively for monitoring 

purposes. Domestic wells pump groundwater from the aquifer 

for home use. The grid well classified as “recreational” 

(USAWS-07) is used to maintain water hazards on a golf 

course (table A1). Most USAW grid wells were production 

wells used for public supply. However, the seventeen grid 

wells that were not public-supply wells had screened intervals 

at depths similar to those of the public-supply grid wells.

Understanding wells were selected to better understand 

groundwater quality, including the movement of groundwater 

and changes in chemistry along approximate flow paths. Eight 

of the nine understanding wells were public-supply wells, and 

one (USAWU-09) was a monitoring well (table A1).

Depths of USGS- and CDPH-grid wells varied across 

the study unit. Grid wells had depths ranging from 50 to 

1,720 ft (15 to 524 m) below land surface; the median was 

580 ft (177 m) (fig. D2; table A1). Depths to the tops of the 

perforations ranged from 21 to 888 ft (6 to 271 m), with a 

median of 230 ft (70 m). The perforation length (distance from 

the top to the bottom perforation) was up to 1,320 ft (402 m) 

with a median of 287 ft (87 m). The understanding wells 

have ranges in well depth and depth to top of perforations 

very similar to those of the grid wells. Well construction 

information was available for 80 of the 90 grid wells and 8 of 

the 9 understanding wells sampled in the USAW study unit. 

(table A1). 
78 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Groundwater Age Classification

Groundwater age data and classifications are listed in 

table D1. Groundwater dating techniques indicate the time 

since the groundwater was last in contact with the atmosphere. 

Techniques used to estimate groundwater residence times or 

“age” include those based on tritium (for example, Tolstikhin 

and Kamenskiy, 1969; Torgersen and others, 1979) and 

carbon-14 activities (14C) (for example, Vogel and Ehhalt, 

1963; Plummer and others, 1993).

Tritium (3H) is a short-lived radioactive isotope 

of hydrogen with a half-life of 12.32 years (Lucas and 

Unterweger, 2000). Tritium is produced naturally in the 

atmosphere from the interaction of cosmogenic radiation with 

nitrogen (Craig and Lal, 1961), by above-ground nuclear 

explosions, and by the operation of nuclear reactors. Tritium 

enters the hydrologic cycle following oxidation to tritiated 

water. Tritium values in precipitation under natural conditions 

would be about 3 to 15 TU (Craig and Lal, 1961; Clark and 

Fritz, 1997). Above-ground nuclear explosions resulted in a 

large increase in tritium values in precipitation, beginning in 

about 1952 and peaking in 1963 at values over 1,000 TU in 

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

Well

depth

Top-ofperforationsPerforation

length

Well

depth

Top-ofperforationsPerforation

length

Grid wells Understanding wells

DEPTH BELOW LAND SURFACE OR LENGTH, IN FEET

82 76 76 8 8 8

IP017194_Figure D02

76 Number of wells

 with data

75th percentile

Median

25th percentile

10th percentile

90th percentile

Value greater than

90th percentile

Value less than

10th percentile

EXPLANATION

Figure D2. Construction characteristics for grid and understanding wells, Upper Santa Ana 

Watershed study unit, California GAMA Priority Basin Project.

the northern hemisphere (Michel, 1989). Radioactive decay 

over a period of 50 years would decrease tritium values of 

10 TU to 0.6 TU. 

Previous investigations have used a range of tritium 

values from 0.3 to 1.0 TU as thresholds for indicating presence 

of water that has exchanged with the atmosphere since 

1952 (Michel, 1989; Plummer and others, 1993; Michel and 

Schroeder, 1994; Clark and Fritz, 1997; Manning and others, 

2005). For samples collected for the USAW study unit in 

2006–2007, tritium values greater than a threshold of 1.0 TU 

were defined as indicating presence of groundwater recharged 

since 1952. By using a tritium value of 1.0 TU for the 

threshold in this study, the age classification scheme allows a 

slightly larger fraction of modern groundwater to be classified 

as pre-modern than if a lower threshold were used. A lower 

threshold for tritium would result in fewer samples classified 

as pre-moderrn than mixed, when carbon-14 would suggest 

that they were primarily pre-modern. This higher threshold 

was considered more appropriate for this study because many 

of the wells were production wells with long screens and 

mixing of waters of different ages is likely to occur. Tritium 

activities of the water samples are listed in table D1.
Appendix D 79

Table D1. Summary of groundwater age data and classification into modern, mixed, and pre-modern age categories for 

samples collected during November 2006 through March 2007, Upper Santa Ana Watershed study unit, California, GAMA 

Priority Basin Project.

[oC degrees Celsius; TU, tritium units; 14C, carbon–14; pmc, percent modern carbon; <, less than; blank field, no data]

GAMA_ID Tritium 

activity (TU)

Uncorrected 

carbon-14 age 

(years)

Tritium 

uncertainty 

(TU)

14C 

(pmc)1

14C counting 

uncertainty 

(pmc)

Age classification

USAWB-01 0.00 0.12 Pre-modern

USAWB-02 3.41 <1,000 0.31 98 0.34 Modern

USAWB-03 2.72 0.14 Modern

USAWB-04 2.91 <1,000 0.31 91 0.29 Modern

USAWB-05 2.70 0.16 Modern

USAWB-06 2.92 0.15 Modern

USAWB-07 1.31 1050 0.18 87 0.3 Mixed

USAWB-08 2.69 <1,000 0.31 98 0.31 Modern

USAWB-09 2.65 0.13 Modern

USAWB-10 2.69 <1,000 0.31 101 0.44 Modern

USAWB-11 1.59 <1,000 0.18 96 0.31 Modern

USAWB-12 3.33 0.15 Modern

USAWB-13 3.16 0.16 Modern

USAWB-14 1.69 1,060 0.18 87 0.34 Mixed

USAWB-15 3.50 0.17 Modern

USAWB-16 0.00 1,940 0.35 78 0.32 Pre-modern

USAWB-17 2.43 0.32 Modern

USAWB-18 0.66 1,150 0.13 86 0.35 Pre-modern

USAWB-19 3.33 0.19 Modern

USAWC-01 0.04 1,300 0.15 84 0.35 Pre-modern

USAWC-02 0.50 1,540 0.16 82 0.34 Pre-modern

USAWC-03 1.73 0.16 Mixed

USAWC-04 1.50 <1,000 0.31 90 0.36 Modern

USAWC-05 0.50 0.11 Pre-modern

USAWC-06 3.66 0.20 Modern

USAWC-07 1.62 0.38 Mixed

USAWC-08 0.40 1,020 0.31 87 0.35 Pre-modern

USAWC-09 2.14 0.14 Modern

USAWC-10 0.28 0.10 Pre-modern

USAWC-11 0.68 <1,000 0.31 101 0.39 Mixed

USAWC-12 0.74 <1,000 0.13 110 0.42 Mixed

USAWC-13 0.55 0.11 Pre-modern

USAWC-14 0.31 <1,000 0.31 88 0.35 Pre-modern

USAWC-15 0.20 0.05 Pre-modern

USAWC-16 3.58 0.21 Modern

USAWC-17 1.00 <1,000 0.31 90 0.36 Modern

USAWC-18 0.85 0.12 Pre-modern

USAWC-19 2.87 0.15 Modern

USAWC-20 0.44 1,530 0.12 82 0.34 Pre-modern

USAWC-21 0.40 <1,000 0.31 88 0.35 Pre-modern

USAWC-22 0.39 0.12 Pre-modern

USAWC-23 0.09 <1,000 0.18 88 0.35 Pre-modern

USAWC-24 6.33 <1,000 0.29 96 0.38 Modern

USAWC-25 1.24 1,550 0.32 82 0.35 Mixed

USAWE-01 2.50 2,560 0.31 72 0.4 Mixed

USAWE-02 0.59 0.18 Pre-modern

USAWE-03 1.69 1,,180 0.18 84 0.44 Mixed

USAWE-04 2.69 <1,000 0.31 104 0.37 Modern

USAWR-01 0.00 0.78 Pre-modern

USAWR-02 0.41 0.04 Pre-modern

USAWR-03 1.00 <1,000 0.18 100 0.31 Modern
80 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

GAMA_ID Tritium 

activity (TU)

Uncorrected 

carbon-14 age 

(years)

Tritium 

uncertainty 

(TU)

14C 

(pmc)1

14C counting 

uncertainty 

(pmc)

Age classification

USAWR-04 4.16 0.17 Modern

USAWR-05 1.97 <1,000 0.10 94 0.3 Modern

USAWR-06 3.06 <1,000 0.20 108 0.42 Modern

USAWR-07 3.38 <1,000 0.40 105 0.41 Modern

USAWR-08 2.10 <1,000 0.31 107 0.42 Modern

USAWR-09 0.96 <1,000 0.15 98 0.39 Mixed

USAWR-10 5.92 <1,000 0.30 94 0.37 Modern

USAWR-11 0.68 1,230 0.31 85 0.35 Pre-modern

USAWR-12 1.41 1,310 0.31 84 0.34 Mixed

USAWS-01 1.96 0.13 Modern

USAWS-02 0.68 1,200 0.31 85 0.35 Pre-modern

USAWS-03 5.11 0.22 Modern

USAWS-04 4.93 0.24 Modern

USAWS-05 2.10 1,370 0.18 84 0.34 Mixed

USAWS-06 0.00 4,810 0.31 54 0.25 Pre-modern

USAWS-07 1.52 0.33 Mixed

USAWS-08 1.00 <1,000 0.31 98 0.39 Modern

USAWS-09 4.57 0.22 Modern

USAWS-10 3.19 0.20 Modern

USAWS-11 1.73 0.13 Mixed

USAWS-12 0.90 0.31 Pre-modern

USAWS-13 2.06 0.19 Modern

USAWS-14 0.40 3,590 0.18 63 0.28 Pre-modern

USAWS-15 0.00 2,100 0.18 76 0.32 Pre-modern

USAWS-16 0.23 7,650 0.13 38 0.21 Pre-modern

USAWS-17 1.66 0.14 Mixed

USAWS-18 0.00 2,180 0.18 75 0.32 Pre-modern

USAWS-19 0.59 0.15 Pre-modern

USAWS-20 0.59 <1,000 0.18 89 0.36 Pre-modern

USAWS-21 0.83 6,200 0.08 46 0.24 Pre-modern

USAWY-01 0.10 0.09 Pre-modern

USAWY-02 2.16 0.12 Modern

USAWY-03 0.14 0.04 Pre-modern

USAWY-04 0.00 2,460 0.09 73 0.52 Pre-modern

USAWY-05 0.31 2,030 0.18 77 0.32 Pre-modern

USAWY-06 1.00 <1,000 0.31 92 0.37 Modern

USAWY-07 2.01 0.17 Modern

USAWY-08 1.36 0.15 Mixed

USAWY-09 0.00 0.10 Pre-modern

USAWU-01 2.83 <1,000 0.14 99 0.31 Modern

USAWU-02 0.31 0.05 Pre-modern

USAWU-03 3.89 0.18 Modern

USAWU-04 1.06 1,040 0.08 87 0.35 Mixed

USAWU-05 1.26 1,310 0.13 84 0.34 Mixed

USAWU-06 3.01 2,340 0.20 74 0.31 Mixed

USAWU-07 0.36 1,450 0.10 83 0.34 Pre-modern

USAWU-08 0.22 13,060 0.05 19 0.15 Pre-modern

USAWU-09 3.62 <1,000 0.20 116 0.44 Modern

1

 Carbon-14 (pmc) values here differ slightly from the values reported in table 13 of Kent and Belitz (2009). Here carbon-14 values were 

normalized to a standard carbon-13 of –25 per mil (VPDB) and reported as percent modern. These values were converted to non-normalized 

values using the carbon-13 of the sample and converted to pmc using the calculation procedure described in Plummer and others (2004).

Table D1. Summary of groundwater age data and classification into modern, mixed, and pre-modern age categories for 

samples collected during November 2006 through March 2007, Upper Santa Ana Watershed study unit, California, GAMA 

Priority Basin Project.—Continued

[oC degrees Celsius; TU, tritium units; 14C, carbon–14; pmc, percent modern carbon; <, less than; blank field, no data]
Appendix D 81

Carbon-14 (14C) is a widely used chronometer based 

on the radiocarbon content of organic and inorganic 

carbon. Dissolved inorganic carbon species, carbonic acid, 

bicarbonate, and carbonate typically are used for 14C dating 

of groundwater. 14C is formed in the atmosphere by the 

interaction of cosmic-ray neutrons with nitrogen and, to a 

lesser degree, with oxygen and carbon. 14C is incorporated 

into carbon dioxide and mixed throughout the atmosphere. 

The carbon dioxide enters the hydrologic cycle because it 

dissolves in precipitation and surface water in contact with the 

atmosphere. 14C activity in groundwater, expressed as percent 

modern carbon (pmc), reflects the time since groundwater was 

last exposed to the atmospheric 14C source. 14C has a half-life 

of 5,730 years and can be used to estimate groundwater 

ages ranging from 1,000 to approximately 30,000 years 

before present. 

The 14C age (residence time, presented in years) is 

calculated on the basis of the decrease in 14C activity as 

a result of radioactive decay since groundwater recharge, 

relative to an assumed initial 14C concentration (Clark and 

Fritz, 1997). An average initial 14C activity of 100 pmc is 

assumed for this study, with estimated errors on calculated 

groundwater ages up to ± 20%. Calculated 14C ages (table D1) 

in this study are referred to as “uncorrected” because they have 

not been adjusted to consider exchanges with sedimentary 

sources of carbon (Fontes and Garnier, 1979). Groundwater 

with a 14C activity of >88 pmc is reported as having an 

age of <1,000 years; no attempt is made to refine 14C ages 

<1,000 years. Measured values of percent modern carbon 

can be >100 pmc because the definition of the 14C activity in 

“modern” carbon does not include the excess 14C produced 

in the atmosphere by above-ground nuclear weapons testing. 

For the USAW study unit, 14C activity <90 pmc was defined 

as indicative of presence of groundwater recharged before 

the modern era. The threshold value of 90 pmc was selected 

because all groundwater samples with tritium <1.0 TU also 

had 14C <90 pmc. 14C values in table D1 expressed as percent 

modern carbon differ slightly from the values reported in table 

13 of Kent and Belitz (2009) because the values in table D1

were normalized to a standard carbon-13 (13C) of –25 per mil 

(VPDB) and reported as percent modern carbon. 

In this study, the age distributions of samples are 

classified as pre-modern, modern, and mixed (table D1). 

Groundwater with tritium activity less than 1 tritium unit and 

14C less than 90 pmc is designated as pre-modern, defined as 

having been recharged before 1952. Groundwater with tritium 

activity greater than 1 TU and 14C greater than 90 pmc is 

designated as modern, defined as having been recharged after 

1952. Samples with pre-modern and modern components are 

designated as mixed groundwater, which includes substantial 

fractions of old and young waters. In reality, pre-modern 

groundwater could contain small fractions of modern 

groundwater, and modern groundwater could contain small 

fractions of pre-modern groundwater. Tritium concentrations, 

uncorrected carbon-14 age, and sample age classifications are 

reported in table D1. Although more sophisticated lumped 

parameter models used for analyzing age distributions that 

incorporate mixing are available (for example, Cook and 

Böhlke, 2000), use of these alternative models to characterize 

age mixtures was beyond the scope of this report. Rather, 

classification into modern (recharged after 1952), mixed, and 

pre-modern (recharged before 1952) categories was sufficient 

to provide an appropriate and useful characterization for the 

purposes of examining groundwater quality.

Of the 99 grid and understanding wells sampled by 

the USGS-GAMA Priority Basin Project, groundwater ages 

were classified as modern for 42 wells, mixed for 19 wells 

(evidence of modern and pre-modern groundwater in the same 

sample), and pre-modern for 38 wells (table D1). The areal 

distribution of the age classifications of the wells is shown 

in figure D3.

Relative Elevation of Wells 

The relative elevation of wells within the alluvial valleys 

was an additional factor examined for the understanding of 

water quality in the USAW study unit (table A1). Groundwater 

in alluvium moves under a natural hydraulic gradient that 

conforms in a general way to the surface topography (Faye, 

1973). In the Upper Santa Ana Valley, groundwater movement 

generally follows the path of the Santa Ana River, which 

flows from the eastern edge of the valley westward and 

southward towards the Prado Wetlands and Dam (fig. 2). 

In the San Jacinto Basin, groundwater movement generally 

follows the path of the San Jacinto River, which exits the 

San Jacinto Mountains in the southeastern part of the valley 

and flows westward to Lake Elsinore (fig. 2). Relative 

elevations for wells sampled in the USAW study unit were 

determined separately for these two flow systems in the 

following way. First, the elevations at which the Santa Ana 

and the San Jacinto Rivers enter and exit their respective 

valleys were established as maximum and baseline (minimum) 

elevations for the Upper Santa Ana Valley and the San Jacinto 

groundwater basins. Elevation ranges for the two basins were 

defined as the difference between their maximum and baseline 

land-surface elevations. The appropriate baseline elevation 

was then subtracted from the elevation of each well, and this 

difference was divided by the elevation range for the basin 

where the well was located (wells in the Elsinore groundwater 

basin used the Upper Santa Ana Valley baseline and range). 
82 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

Big Bear Lake

Bautista Creek

EXPLANATION

91

60

10

15

5

15

15

215

215

10

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

San Juan Creek

Arroyo

Trabuco

Santa Ana River

San Gabriel River

Deep Creek

Lytle Creek

Santa Ana River

Mojave River

Bear Creek

Chino Creek

Lake

Perris

Lake Irvine

San Bernardino

Riverside

Lake

Elsinore

Hemet

Canyon

Lake

San Jacinto River

San Gorgonio River

Prado 

Wetlands

Lake

Mathews

0 10 20 Miles

0 10 20 Kilometers

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

Land use from USGS National Land Cover Dataset

 (Nakagaki and others, 2007)

LAND-USE CLASSIFICATION GROUNDWATER AGE CLASSIFICATION

Urban

Agricultural

Natural

County

 boundaries

Major roads

Rivers Study area

 boundary

IP017194_figure D03

Modern

Mixed

Pre-modern

Figure D3. Areal distribution of the age classifications assigned to wells sampled for the Upper Santa Ana Watershed study unit, 

California GAMA Priority Basin Project. 
Appendix D 83

The resulting relative elevation value is dimensionless and 

represents the relative position of the groundwater in each well 

along its flow path. A well that is at the same elevation as the 

river where it enters the valley would have a relative elevation 

of 1, while a well at the elevation of the river where it exits the 

valley would have a relative elevation of 0. Fifteen wells in the 

Upper Santa Ana Valley have relative elevation values that are 

greater than 1 because they are at elevations above that of the 

Santa Ana River where it enters the valley. Most (10) of these 

are in the Yucaipa/San Timoteo study area. Relative elevations 

are listed in table A1. Higher values of relative elevation 

indicate locations in the upgradient or proximal portion of 

the flow system, and lower values indicate locations in the 

downgradient or distal portion of the flow system.

Grid wells selected using a spatially distributed 

randomized design were distributed across the entire range 

of relative elevations in the study unit (table A1). However, 

there are differences in relative elevations among the study 

areas (fig. D4), which reflect the landscape positions of each 

of them. Wells in the San Jacinto study area are evaluated 

separately from the other study areas using baseline elevations 

for the San Jacinto River drainage instead of the Santa Ana 

River drainage. The two highest relative-elevation values are 

for wells which are located in the mountains adjacent to the 

study unit (USAWU-03 and USAWC-19, figs. 6, A1).

 In general, the highest relative-elevation values were 

for grid wells in the Yucaipa/San Timoteo study area with 

a median of 1.34 (dimensionless). Next, the Bunker Hill/

Cajon/Rialto-Colton study area grid wells had a median 

relative elevation of 0.57. Relative elevations for the four 

grid wells in the Elsinore study area varied little, from 0.51 to 

0.56. Grid wells in the Cucamonga/Chino study area had 

a median relative elevation of 0.30. The lowest relativeelevation values were observed for grid wells sampled in the 

Riverside-Arlington/Temescal study area (where the Santa 

Ana River exits the study unit), with a median value of 0.22. 

Grid wells in the San Jacinto study area are located at 

disproportionately low relative elevations. This likely reflects 

the dominant terrain of this groundwater basin, which has 

relatively high elevations only on the edges, along with some 

centrally-located hilly outcrops that were excluded from the 

study area. The median relative elevation for grid wells in the 

San Jacinto study area was 0.21.

Relative-elevation values for the understanding wells 

reflect those of the study areas where they are located. The 

highest value, for understanding wells as well as the entire 

study unit, is for USAWU-03, located in the San Bernardino 

mountains (figs. 6, D4). The relative elevation of another 

understanding well, USAWU-02, plots as an outlier for 

understanding wells (fig. D4), but its relative-elevation value 

is below the median for the Yucaipa/San Timoteo study area 

in which it is located. The other understanding wells are 

located in the Cucamonga/Chino study area (four wells), the 

Riverside-Arlington/Temescal study area (two wells), and 

the Bunker Hill/Cajon/Rialto-Colton study area (one well), 

and have relative-elevation values in the ranges of values for 

those study areas.

Geochemical Conditions

Geochemical conditions investigated as 

potential explanatory variables in this report include 

oxidation-reduction characteristics, dissolved oxygen (DO) 

concentrations, and pH (table D2). Oxidation-reduction 

(redox) conditions influence the mobility of many organic 

and inorganic constituents (McMahon and Chapelle, 2008). 

Along groundwater flow paths, redox conditions commonly 

proceed along a well-documented sequence of terminal 

electron acceptor processes (TEAPs); one TEAP typically 

is predominant at a particular time and aquifer location 

(Chapelle and others, 1995; Chapelle, 2001). The predominant 

TEAPs are oxygen-reduction (oxic), nitrate-reduction, 

manganese-reduction, iron-reduction, sulfate-reduction, and 

methanogenesis. The presence of redox-sensitive chemical 

species indicating more than one TEAP may indicate that 

(1) the well’s discharge includes mixed waters from different 

redox zones upgradient of the well, (2) the well is screened 

across more than one redox zone, or (3) there is spatial 

heterogeneity in microbial activity in the aquifer. In addition, 

different redox couples often are not consistent, indicating 

electrochemical disequilibrium in groundwater (Lindburg 

and Runnells, 1984) complicating the assessments of redox 

conditions. 

In this report, redox conditions were represented in two 

ways: as DO concentration and redox category on the basis of 

the predominant TEAP(s). DO concentrations were measured 

at USGS-grid and USGS-understanding wells (Kent and 

Belitz, 2009), but were not reported in the CDPH database 

(table D2). Redox conditions were classified on the basis of 

DO, nitrate, manganese, iron, and sulfate concentrations using 

the classification scheme of McMahon and Chapelle (2008) 

(table D2). An automated workbook program was used to 

assign the redox classification to each sample (Jurgens and 

others, 2009). For wells without USGS inorganic constituent 

data, the most recent data within the previous 3 years for that 

well in the CDPH database were used.

Groundwater in the USAW study unit is primarily oxic. 

Eighty-two of the 101 groundwater samples from wells 

having data for redox characterization are in the oxic category, 

where the predominant redox process is oxygen reduction. 

Samples from an additional three wells had similarly high 

DO concentrations, but also had dissolved iron concentrations 

greater than 100 micrograms per liter, suggesting mixed 

redox processes. A sample from an additional well had no 

DO data and a dissolved iron concentrations greater than 

100 micrograms per liter, and this sample was also categorized 

as a mixed redox process. Samples from only eight wells had 

anoxic redox conditions. These anoxic wells were distributed 

across all USAW study areas except for the Bunker Hill/

Cajon/Rialto-Colton and Yucaipa/San Timoteo study areas.
84 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

0.0

0.5

1.0

1.5

2.0

2.5

3.0

RELATIVE ELEVATION, DIMENSIONLESS

10 19 4 26 12 9 21

IP017194_Figure D04

76 Number of wells

 with data

75th percentile

Median

25th percentile

10th percentile

90th percentile

Value greater than

90th percentile

Value less than

10th percentile

EXPLANATION

STUDY AREA

Understanding

wells

Bunker Hill/

Cajon/

Rialto-Colton

Yucaipa/

San Timoteo

RiversideArlington/

Temescal

Elsinore Cucamonga/

Chino

San Jacinto*

Figure D4. Relative elevations for grid wells (by study area) and understanding wells, Upper Santa Ana Watershed 

study unit, California GAMA Priority Basin Project. *Wells in the San Jacinto study area were evaluated separately from 

the other study areas and understanding wells because their evaluation uses baseline elevations for the San Jacinto 

River drainage instead of those for the Santa Ana River Drainage.
Appendix D 85 Table D2. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project. [anoxic/suboxic, dissolved oxygen < 0.5 mg/L; indeterminate, insufficient data to determine redox classification; mg/L, milligrams per liter; μg/L, micrograms per liter; oxic, dissolved oxygen greater than or equal to 0.5 mg/L; redox, oxidation-reduction; <, less than; E, estimated; CDPH, California Department of Public Health; O2, oxygen; Fe(III), iron; SO4, sulfate; NO3, nitrate; >, greater than; Mn (IV), manganese] USGS GAMA well identification number CDPH GAMA well identification number 1 pH Oxidizing and reducing constituents Redox category Redox process Dissolved oxygen (mg/L) Nitrate plus nitrite (mg/L) Manganese (μg/L) Iron (μg/L) Sulfate (mg/L) USAWB-01 USAWB-DG-01 7.8 1.2 8.4 <20 <100 63.0 Oxic

O2

USAWB-02 7.3 8.0 1.1 <0.2 <6 21.3 Oxic

O2

USAWB-03 USAWB-DG-03 8.1 4.7 2.1 <20 <100 100 Oxic

O2

USAWB-04 7.9 10.0 2.7 <0.2 <6 16.9 Oxic

O2

USAWB-05 USAWB-DG-05 7.5 9.8 1.8 <20 <100 31.0 Oxic

O2

USAWB-06 USAWB-DG-06 7.8 8.1 5.0 <20 <100 35.0 Oxic

O2

USAWB-07 7.7 5.5 4.9 <0.2 <6 74.2 Oxic

O2

USAWB-08 7.4 8.8 10.5 <0.2 <6 53.0 Oxic

O2

USAWB-09 USAWB-DG-09 7.2 20.2 3.2 Oxic

O2

USAWB-10 7.6 9.6 5.5 0.3 <6 61.7 Oxic

O2

USAWB-11 7.3 8.1 7.5 0.3 5.0 116 Oxic

O2

USAWB-12 7.3 Unknown UnknownUSAWB-13 USAWB-DG-13 7.5 15 <20 <100 62.0 Oxic 2 O2USAWB-14 7.4 11.0 2.3 <0.2 <6 17.2 Oxic

O2

USAWB-15 USAWB-DG-15 7.5 4.5 <20 120.0 28.0 Oxic or mixed 3 Fe(III)/SO4USAWB-16 7.8 4.0 1.6 0.2 4.0 38.2 Oxic

O2

USAWB-17 USAWB-DG-17 7.5 7.9 3.4 <20 <100 20.0 Oxic

O2

USAWB-18 7.7 11.1 6.0 E0.1 <6 30.5 Oxic

O2

USAWB-19 USAWB-DG-19 7.5 7.7 7.0 <20 170 29.0 Oxic or mixed 4 Fe(III)/SO4USAWC-01 8 7.8 5.4 <0.2 <6 16.6 Oxic

O2

USAWC-02 8 7.7 3.0 E0.1 <6 8.4 Oxic

O2

USAWC-03 USAWC-DG-03 7.6 9.1 2.9 <0.2 <20 18.0 Oxic

O2

USAWC-04 7.5 9.1 15 E0.1 <6 85.4 Oxic

O2

USAWC-05 USAWC-DG-05 7.5 10.6 16 <0.2 <20 26.0 Oxic

O2

USAWC-06 USAWC-DG-06 7.4 7.8 2.5 0.3 <20 38.0 Oxic

O2

USAWC-07 USAWC-DG-07 7.6 0.7 0.8 <20 <100 120 Oxic

O2

USAWC-08 7.7 8.1 12 <0.2 6.0 26.8 Oxic

O2

USAWC-09 USAWC-DG-09 8.6 0.2 0.0 22 170 38.0 Anoxic Fe(III)/SO4USAWC-10 8.2 10.6 Oxic

O2

USAWC-11 7.4 6.2 5.4 2.0 6.0 60.4 Oxic

O2

USAWC-12 7.5 4.7 34.8 10 6.0 42.2 Oxic

O2

USAWC-13 USAWC-DG-13 7.6 6.5 1.7 45 380 36.0 Oxic or mixed 4 Fe(III)/SO4USAWC-14 7.5 7.7 6.4 E0.1 <6 18.7 Oxic

O2

USAWC-15 USAWC-DG-15 7.8 5.8 3.8 <20 <100 52.0 Oxic

O2

USAWC-16 USAWC-DG-16 7.9 8.2 3.2 <20 <100 27.0 Oxic

O2

USAWC-17 7.7 9.6 2.4 <0.2 <6 13.7 Oxic

O2
86 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project USGS GAMA well identification number CDPH GAMA well identification number 1 pH Oxidizing and reducing constituents Redox category Redox process Dissolved oxygen (mg/L) Nitrate plus nitrite (mg/L) Manganese (μg/L) Iron (μg/L) Sulfate (mg/L) USAWC-18 USAWC-DG-18 7.9 7.9 12.7 <20 160 42.0 Oxic or mixed 4 Fe(III)/SO4 USAWC-19 USAWC-DG-19 7.7 7.0 0.8 <20 <100 13.0 Oxic

O2

USAWC-20 7.8 7.8 4.1 <0.2 <6 9.5 Oxic

O2

USAWC-21 7.5 10.1 5.9 0.5 3.0 41.5 Oxic

O2

USAWC-22 7.6 7.6 Oxic

O2

USAWC-23 7.7 8.8 8.4 <0.2 3.0 34.6 Oxic

O2

USAWC-24 7.9 2.2 1.3 2.4 8.0 17.9 Oxic

O2

USAWC-25 7.4 5.7 13.9 E0.1 <6 67.6 Oxic

O2

USAWE-01 8 5.6 1.4 0.4 10.0 77.5 Oxic

O2

USAWE-02 9.2 0.3 0.1 3.8 <6 81.4 Anoxic Suboxic

USAWE-03 7.4 1.2 4.5 28 <6 167 Oxic

O2

USAWE-04 7.1 8.8 4.3 0.5 <6 136 Oxic

O2

USAWR-01 USAWR-DG-01 7.4 7.4 11.3 <20 <100 42.0 Oxic

O2

USAWR-02 USAWR-DG-02 8 6.9 5.4 <20 <100 30.0 Oxic

O2

USAWR-03 7.3 6.1 12.9 <0.2 <6 97.8 Oxic

O2

USAWR-04 USAWR-DPH-7 8.5 6.6 5.9 <20 <100 76.0 Oxic

O2

USAWR-05 7.1 2.0 10.4 0.6 <6 80.7 Oxic

O2

USAWR-06 7.4 5.8 19.0 0.2 18.0 180 Oxic

O2

USAWR-07 7.2 3.6 17.9 <0.2 <6 232 Oxic

O2

USAWR-08 7.1 0.2 9.2 E0.1 3.0 101 Anoxic NO3USAWR-09 7.2 6.6 16.2 <0.2 <6 78.2 Oxic

O2

USAWR-10 7.2 2.1 3.9 0.2 3.0 189 Oxic

O2

USAWR-11 7.8 10.8 5.6 0.2 6.0 23.7 Oxic

O2

USAWR-12 7.3 7.9 17.5 0.8 11.0 164 Oxic

O2

USAWS-01 USAWS-DG-01 7.1 7.9 10.2 <20 <100 36.0 Oxic

O2

USAWS-02 7.7 6.4 11.0 0.4 10.0 41.2 Oxic

O2

USAWS-03 7.6 6.1 Oxic

O2

USAWS-04 7 3.8 Oxic

O2

USAWS-05 6.9 9.3 6.6 1.7 34.0 196.0 Oxic

O2

USAWS-06 7.4 0.2 0.1 275 441 3.9 Anoxic Mn(IV) and Fe(III)/

SO4

USAWS-07 6.6 6.3 Oxic

O2

USAWS-08 6.8 9.6 13.7 1.1 10.0 22.9 Oxic

O2

USAWS-09 USAWS-DG-09 7.2 7.5 3.8 <20 <100 55.0 Oxic

O2

USAWS-10 7.7 7.5 Oxic

O2

USAWS-11 7.2 6.6 Oxic

O2

USAWS-12 7.1 10.5 5.4 0.2 4.0 44.2 Oxic

O2

Table D2. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.—Continued[anoxic/suboxic, dissolved oxygen < 0.5 mg/L; indeterminate, insufficient data to determine redox classification; mg/L, milligrams per liter; μg/L, micrograms per liter; oxic, dissolved oxygen greater than or equal to 0.5 mg/L; redox, oxidation-reduction; <, less than; E, estimated; CDPH, California Department of Public Health; O2, oxygen; Fe(III), iron; SO4, sulfate; NO3, nitrate; >, greater than; Mn (IV), 

manganese]
Appendix D 87 Table D2. Concentrations of constituents used to classify oxidation-reduction conditions in groundwater, Upper Santa Ana Watershed study unit, California GAMA Priority Basin Project.—Continued [anoxic/suboxic, dissolved oxygen < 0.5 mg/L; indeterminate, insufficient data to determine redox classification; mg/L, milligrams per liter; μg/L, micrograms per liter; oxic, dissolved oxygen greater than or equal to 0.5 mg/L; redox, oxidation-reduction; <, less than; E, estimated; CDPH, California Department of Public Health; O2, oxygen; Fe(III), iron; SO4, sulfate; NO3, nitrate; >, greater than; Mn (IV), manganese] USGS GAMA well identification number CDPH GAMA well identification number 1 pH Oxidizing and reducing constituents Redox category Redox process Dissolved oxygen (mg/L) Nitrate plus nitrite (mg/L) Manganese (μg/L) Iron (μg/L) Sulfate (mg/L) USAWS-13 7 7.0 Oxic

O2

USAWS-14 8.1 0.2 0.2 32 5.0 43.1 Anoxic Suboxic

USAWS-15 7.5 0.8 3.4 1.5 31.0 188.0 Oxic

O2

USAWS-16 8.2 0.6 0.6 1.6 5.0 152.0 Oxic

O2

USAWS-17 7.9 3.2 Oxic

O2

USAWS-18 7.5 2.5 14.6 5.2 26.0 260.0 Oxic

O2

USAWS-19 9.3 0.2 Anoxic UnknownUSAWS-20 7.5 4.7 7.0 0.4 8.0 119.0 Oxic

O2

USAWS-21 8.7 0.2 <0.1 115 82.0 43.8 Anoxic Mn(IV)USAWY-01 USAWY-DG-01 7.6 8.5 3.5 <20 <100 20.5 Oxic

O2

USAWY-02 USAWY-DG-02 7.8 6.7 3.4 <20 <100 20.0 Oxic

O2

USAWY-03 USAWY-DG-03 7.5 <20 <100 41.0 Oxic 5 O2USAWY-04 7.7 1.5 <0.2 <6 33.4 Oxic 2 O2USAWY-05 7.7 6.2 5.6 0.2 5.0 51.0 Oxic

O2

USAWY-06 7.7 8.8 7.4 E0.1 <6 20.9 Oxic

O2

USAWY-07 USAWY-DG-07 7.2 6.5 5.4 <20 <100 30.0 Oxic

O2

USAWY-08 USAWY-DG-08 7.6 4.8 2.0 <20 <100 36.0 Oxic

O2

USAWY-09 USAWY-DG-09 7.8 10.2 0.7 <20 <100 29.0 Oxic

O2

None USAWB-DPH-17 7.1 7.5 <20 <100 23.0 Oxic 2 O2None USAWC-DPH-1 7.8 12 <20 <100 130.0 Oxic 2 O2None USAWC-DPH-10 7.3 90 <20 <100 69.0 Oxic 2 O2None USAWR-DPH-7 8.5 5.9 <20 <100 76.0 Oxic 2 O2None USAWY-DPH-1 <0.5 Unknown Unknown

USAWU-01 7.4 0.8 4.9 42 <6 79.2 Oxic

O2

USAWU-02 7.6 4.8 4.4 <0.2 <6 33.1 Oxic

O2

USAWU-04 7.7 7.5 7.2 <0.2 8.0 32.8 Oxic

O2

USAWU-05 7.5 8.5 8.6 0.2 8.0 52.8 Oxic

O2

USAWU-06 7.5 9.4 8.6 <0.2 <6 38.3 Oxic

O2

USAWU-07 7.6 7.9 7.1 <0.2 <6 20.7 Oxic

O2

USAWU-08 9.2 0.3 1.2 1.9 7.0 25.1 Anoxic NO3USAWU-09 7.2 6.7 21.8 <0.2 <6 44.2 Oxic

O2

1 Samples with a CDPH GAMA identification number are those for which data were obtained by CDPH.2 Sample has no DO data, and is presumed to be oxic. It could be anoxic (NO3-reducing).3 Sample has no DO data and CDPH Fe > 100.4 Sample has DO > 0.5 (Oxic), but CDPH Fe data may indicate Fe-reducing.5 Sample has no DO data and is presumed to be oxic. It could be anoxic (suboxic or NO3-reducing).
88 Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007: California GAMA Priority Basin Project

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Kent and Belitz—Status of Groundwater Quality in the Upper Santa Ana Watershed, November 2006–March 2007:

California GAMA Priority Basin Project—SIR 2012–5052