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 Borrego Valley, Central Desert, and 

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

2008–2010: California GAMA Priority Basin Project

Scientific Investigations Report 2014–5001

Photo placement
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: Joshua Tree and public supply well, Yucca Valley, California. (Photograph taken by 

Dara Goldrath, U.S. Geological Survey)

Back cover: Borrego Valley at sunset, California. (Photograph taken by Michael Solt, U.S. Geological Survey) 

Sampling priority

Study area

Low-use basins

Priority basins

Areas that are outside CDWR-defined

 groundwater basins
Status of Groundwater Quality in the 

Borrego Valley, Central Desert, and 

Low-Use Basins of the Mojave and 

Sonoran Deserts Study Unit, 2008–2010: 

California GAMA Priority Basin Project

By Mary C. Parsons, Tracy Connell Hancock, Justin T. Kulongoski, and 

Kenneth Belitz

A product of the California Groundwater Ambient Monitoring and Assessment 

(GAMA) Program

Prepared in cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2014–5001

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

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

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Suggested citation:

Parsons, M.C., Hancock, T.C., Kulongoski, J.T., and Belitz, Kenneth, 2014, Status of groundwater quality in the Borrego 

Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, 2008–2010—California 

GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2014–5001, 88 p., http://dx.doi.

org/10.3133/sir20145001.

ISBN 978-1-4113-3776-3

ISSN 2328-031X (print)

ISSN 2328-0328 (online) 
iii

Contents

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

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

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

Hydrogeologic Setting of the CLUB Study Unit.........................................................................................4

Borrego Valley Study Area................................................................................................................20

Central Desert Study Area ................................................................................................................20

Low-Use Basins of the Mojave and Sonoran Deserts Study Area ............................................21

Methods ........................................................................................................................................................22

Relative-Concentrations and Water-Quality Benchmarks ..........................................................22

Datasets Used for Status Assessment............................................................................................23

U.S. Geological Survey Grid Data ...........................................................................................23

California Department of Public Health Grid Data ...............................................................25

Additional Data Used for Spatially Weighted Calculation ..................................................27

Selection of Constituents for Additional Evaluation .....................................................................27

Calculation of Aquifer-Scale Proportions.......................................................................................28

Potential Explanatory Factors ...................................................................................................................29

Land Use...............................................................................................................................................29

Well Depth and Depth to Top-of-Perforation .................................................................................30

Groundwater Age................................................................................................................................30

Geochemical Condition......................................................................................................................30

Status of Water Quality...............................................................................................................................33

Inorganic Constituents.......................................................................................................................33

Trace and Minor Elements ......................................................................................................51

Uranium and Radioactive Constituents..................................................................................51

Nutrients......................................................................................................................................51

Inorganic Constituents with Aesthetic Benchmarks...........................................................52

Organic Constituents .........................................................................................................................52

Volatile Organic Compounds (VOCs) ......................................................................................52

Pesticides....................................................................................................................................58

Special-Interest Constituents...........................................................................................................58

Summary .......................................................................................................................................................58

Acknowledgments.......................................................................................................................................59

References....................................................................................................................................................59

Appendix A. Ancillary Datasets.................................................................................................................65

Appendix B. Use of Data from the CDPH Database...............................................................................75

Appendix C. Estimation of Aquifer-Scale Proportions...........................................................................78

Appendix D. Calculating Total Dissolved Solids .....................................................................................85

Appendix E. Comparison of Data from the CDPH and USGS-GAMA Program..................................85
iv

Figures

1. Map showing location of the Borrego Valley, Central Desert, and Low-Use Basins of 

the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project, 

and the California hydrogeologic provinces ............................................................................3

2. Maps showing geographic features and locations of study area grid cells, USGS 

grid and understanding wells, and CDPH wells, Borrego, Central Desert, and 

Low-Use Basins of the Desert study unit, California GAMA Priority Basin Project, 

December 2008–March 2010 ......................................................................................................6

3. Geologic map of the Borrego Valley, Central Desert, and Low-Use Basins of the 

Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.........17

4. Ternary diagrams showing percentages of urban, agricultural, and natural land use 

in the study unit and study areas, and the 500-meter radius surrounding each USGSgrid well in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave 

and Sonoran Deserts study unit, California GAMA Priority Basin Project .......................18

5. Map showing land use and the locations of USGS-grid and understanding wells in 

the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran 

Deserts study unit, California GAMA Priority Basin Project ...............................................19

6. Box plots showing well depths, depths to top-of-perforations, and perforation 

lengths for grid and understanding wells, Borrego Valley, Central Desert, and 

Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA 

Priority Basin Project, December 2008 to March 2010.........................................................31

7. Box plots showing relation of groundwater age classification to depth to top-ofperforations and to well depth, and bar chart showing age classification in relation 

to well depth classification for USGS-grid wells and USGS-understanding wells, 

Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran 

Deserts study unit, California GAMA Priority Basin Project, December 2008–

March 2010 ..................................................................................................................................32

8. Graph showing maximum relative-concentrations of constituents detected in grid 

wells, by constituent class, Borrego Valley, Central Desert, and Low-Use Basins of 

the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project, 

December 2008–March 2010 ....................................................................................................36

9. Graphs showing relative-concentrations of inorganic constituents in samples from 

grid wells with health-based or aesthetic benchmarks categorized as high, medium, 

or low in grid wells, Borrego, Central Desert, and Low-Use Basins of the Mojave 

and Sonoran Deserts study unit, California GAMA Priority Basin Project, 

December 2008–March 2010 ....................................................................................................37

10. Maps showing relative-concentrations of selected inorganic constituents for 

the USGS grid wells and CDPH wells (data from the period December 3, 2005–

December 1, 2008), Borrego, Central Desert, and Low-Use Basins of the Mojave 

and Sonoran Deserts study unit, California GAMA Priority Basin Project: arsenic, 

boron, fluoride, molybdenum, uranium, gross alpha radioactivity, nitrate, 

total dissolved solids, chloride, iron, and manganese .........................................................39

11. Graph showing detection frequency and maximum relative-concentration of 

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

Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts 

study unit, California GAMA Priority Basin Project ..............................................................53

12. Graphs showing detection frequency and relative-concentrations of selected

organic and special-interest constituents in grid wells in the Borrego Valley, 

Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study 

unit, California GAMA Priority Basin Project.........................................................................54
v

13. Maps showing concentrations of selected organic and special-interest constituents 

in the USGS grid wells and CDPH wells (data from the period December 3, 2005–

December 1, 2008), Borrego, Central Desert, and Low-Use Basins of the Desert 

study unit, California GAMA Priority Basin Project: tetrachloroethene (PCE), 

chloroform, and perchlorate.....................................................................................................55

Figures—Continued

Tables

1. Number of wells sampled by the U.S. Geological Survey for the fast and slow sampling 

schedules, and number of constituents sampled in each constituent class for the 

Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran 

Deserts study unit, California GAMA Priority Basin Project, December 2008 to 

March 2010 ..................................................................................................................................24

2. Benchmark type and value for inorganic constituents and number of grid cells with 

U.S. Geological Survey–GAMA data and California Department of Public Health data 

for each constituent, Borrego Valley, Central Desert, and Low-Use Basins of the 

Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.........25

3. Benchmark type and value for organic and special-interest constituents 

selected for additional evaluation, Borrego Valley, Central Desert, and Low-Use 

Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority 

Basin Project...............................................................................................................................27

4. Constituents reported at concentrations greater than benchmark values in the 

California Department of Public Health database between February 2, 1980, and 

December 3, 2005, Borrego Valley, Central Desert, and Low-Use Basins of the 

Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.........28

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

constituents with high relative-concentrations during December 3, 2005–December 1, 

2008, from the California Department of Public Health (CDPH) database, constituents 

with moderate or high relative-concentrations in samples collected from USGSgrid wells (December 2008–March 2010), or organic constituents with detection 

frequencies of greater than 10 percent in the USGS-grid wells (December 2008–

March 2010), Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and 

Sonoran Deserts study unit, California GAMA Priority Basin Project...............................34

6. Numbers of constituents analyzed and detected by the U.S. Geological Survey 

by constituent class and benchmark type, Borrego Valley, Central Desert, and 

Low-Use Basins of the Desert study unit, California GAMA Priority Basin Project, 

December 2008 to March 2010 .................................................................................................35

7A. Aquifer-scale proportions for inorganic constituent classes, Borrego Valley, 

Central Desert, and Low-Use Basins of the Desert study unit, California GAMA 

Priority Basin Project, December 2008–March 2010 ............................................................50

7B. Aquifer-scale proportions for organic constituent classes, Borrego Valley, Central 

Desert, and Low-Use Basins of the Desert study unit, California GAMA Priority 

Basin Project, December 2008–March 2010 ..........................................................................50
vi

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square 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 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). Concentrations or activities for 

radioactive constituents in water are given in picocuries per liter (pCi/L), percent modern 

carbon (pmc), or tritium units (TU).

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).

Abbreviations and Acronyms

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

BV Borrego Valley study area

CD Central Desert study area 

CLUB Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran 

Deserts study unit

GAMA Groundwater Ambient Monitoring and Assessment Program
vii

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

HBSL Health-based screening level

LSD land-surface datum

LUB Low-Use Basins study area

MCL maximum contaminant level 

MCL-CA California Department of Public Health maximum contaminant level 

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

NL-CA California Department of Public Health notification level 

pmc percent modern carbon

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

SC specific conductance

SMCL secondary maximum contaminant level 

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 U.S. Environmental Protection Agency treatment technique 

TU tritium unit

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

SWRCB State Water Resources Control Board (California)

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

Selected Chemical Names

NDMA N-nitrosodimethylamine

TDS total dissolved solids

THM trihalomethane

VOC volatile organic compound

Abbreviations and Acronyms—Continued
viii
Abstract

Groundwater quality in the approximately 963-squaremile Borrego Valley, Central Desert, and Low-Use Basins of 

the Mojave and Sonoran Deserts 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 San Bernardino, 

Riverside, San Diego, and Imperial 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 Borrego Valley, Central Desert, and LowUse Basins of the Mojave and Sonoran Deserts study was 

designed to provide a spatially unbiased assessment of the 

quality of untreated (raw) groundwater in the primary aquifer 

system. The assessment is based on water-quality and ancillary 

data collected by the U.S. Geological Survey from 52 wells 

(49 grid wells and 3 understanding wells) and on waterquality data from the California Department of Public Health 

database. The primary aquifer system was defined by the depth 

intervals of the wells listed in the California Department of 

Public Health database for the Borrego Valley, Central Desert, 

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

unit. The quality of groundwater in the primary aquifer system 

may be different from that in the shallower or deeper waterbearing zones; shallow groundwater may be more vulnerable 

to surficial contamination. 

This study assesses the status of the current quality of the 

groundwater resource by using data from samples analyzed for 

volatile organic compounds (VOCs), pesticides, and naturally 

occurring inorganic constituents, such as major ions and trace 

elements. This status assessment is intended to characterize 

the quality of groundwater resources in the primary aquifer 

system of the Borrego Valley, Central Desert, and Low-Use 

Basins of the Mojave and Sonoran Deserts 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 nonregulatory benchmarks for drinking-water quality. A relativeconcentration greater than 1.0 indicates a concentration 

greater than a benchmark, and a relative-concentration less 

than or equal to 1.0 indicates a concentration equal to or 

less than a benchmark. Relative-concentrations of organic 

constituents and special-interest constituents [perchlorate and 

N-nitrosodimethylamine (NDMA)] were classified as high

(relative-concentration greater than 1.0), moderate (relativeconcentration greater than 0.1 and less than or equal to 1.0), or 

low (relative-concentration less than or equal to 0.1). Relativeconcentrations of inorganic constituents were classified as high

(relative-concentration greater than 1.0), moderate (relativeconcentration greater than 0.5 and less than or equal to 1.0), or 

low (relative-concentration less than or equal to 0.5).

Aquifer-scale proportion was used as the primary 

metric in the status assessment for evaluating regional-scale 

groundwater quality. High aquifer-scale proportion is defined 

as the percentage of the area of the primary aquifer system 

with a high relative-concentration for a particular constituent 

or class of constituents; this percentage is based on an areal 

rather than a volumetric basis. Moderate and low aquifer-scale 

proportions were defined as the percentages of the primary 

aquifer system with moderate and low relative-concentrations, 

respectively, of a constituent or class of constituents. 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 to each other 

(within 90-percent confidence intervals) in the study unit.

Inorganic constituents (one or more) with health-based 

benchmarks were detected at high relative-concentrations 

in 48 percent of the primary aquifer system and at moderate 

relative-concentrations in 26 percent of the primary aquifer 

system. The high aquifer-scale proportion of inorganic 

constituents primarily reflected high aquifer-scale proportions 

Status of Groundwater Quality in the Borrego Valley, 

Central Desert, and Low-Use Basins of the Mojave and 

Sonoran Deserts Study Unit, 2008–2010: California GAMA 

Priority Basin Project

By Mary C. Parsons, Tracy Connell Hancock, Justin T. Kulongoski, and Kenneth Belitz
2 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

of fluoride (27 percent), arsenic (18 percent), molybdenum 

(16 percent), boron (10 percent), uranium (5.6 percent), gross 

alpha radioactivity (9.7 percent), and nitrate (2.7 percent). The 

inorganic constituents with secondary maximum contaminant 

levels (SMCLs) were detected at high relative-concentrations 

in 13 percent of the primary aquifer system and at moderate 

relative-concentrations in 39 percent. The high aquifer-scale 

proportion for SMCL constituents reflected high aquiferscale proportions of total dissolved solids (TDS, 11 percent), 

manganese (2.8 percent), and chloride (2.8 percent). 

Organic constituents were not detected at high relativeconcentrations in the primary aquifer system, and were present 

at moderate relative-concentrations in 5.0 percent, and at low 

relative-concentrations or were not detected in 95 percent of 

the primary aquifer system. Of the 148 organic constituents 

analyzed, 12 constituents were detected. Two organic 

constituents, chloroform and tetrachloroethene (PCE), were 

detected in more than 10 percent of samples, but were detected 

mostly at low relative-concentrations.

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, 

2013, website at http://www.waterboards.ca.gov/gama/). 

The statewide GAMA Program currently consists of four 

projects: the (1) GAMA Priority Basin Project, conducted by 

the USGS (U.S. Geological Survey, 2013, website at http://

ca.water.usgs.gov/gama/); (2) the GAMA Domestic Well 

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

Studies, conducted by LLNL; and (4) the online database 

GeoTracker GAMA, conducted by the SWRCB. On a 

statewide basis, the Priority Basin Project focused primarily 

on the portion of the groundwater resource represented by the 

depth intervals of wells listed in the California Department 

of Public Health database (the primary aquifer system), and 

the SWRCB Domestic Well Project generally focused on the 

shallow aquifer systems. The primary aquifer system 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 contaminants, such as volatile 

organic compounds (VOCs) and nitrate, may be lower in wells 

screened in the primary aquifer system than in shallower 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). The GAMA Priority Basin Project was initiated 

in response to the Groundwater Quality Monitoring Act of 

2001 to assess and monitor the quality of groundwater in 

California (State of California, 2001b). The GAMA Priority 

Basin Project is a comprehensive assessment of statewide 

groundwater quality designed to improve understanding 

and identification of 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 in California were considered in this statewide 

assessment of groundwater quality. Belitz and others (2003) 

partitioned the State into 10 hydrogeologic provinces, 

each with distinctive hydrologic, geologic, and climatic 

characteristics (fig. 1). These hydrogeologic provinces include 

groundwater basins designated by the CDWR (California 

Department of Water Resources, 2003a). Groundwater basins 

generally consist of relatively permeable, unconsolidated 

deposits of alluvial origin. Eighty percent of California’s 

approximately 16,000 public-supply wells are in designated 

groundwater basins. Groundwater basins were prioritized for 

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

with secondary consideration given to municipal groundwater 

use, agricultural pumping, the number of historically leaking 

underground fuel tanks, and the number of 1-square-mile 

sections having registered pesticide applications (Belitz and 

others, 2003). Of the 472 basins designated by the CDWR, 

116 priority basins, containing approximately 95 percent of 

the CDPH wells located in basins, were identified. The 116 

priority basins, a subset of the remaining 356 low-use basins, 

and additional areas outside defined groundwater basins were 

grouped into 35 study units, which include approximately 

95 percent of public-supply wells in California. The Borrego 

Valley, Central Desert, and Low-Use Basins of the Mojave and 

Sonoran Deserts study unit contains 4 priority groundwater 

basins and 43 low-use basins in the Desert hydrogeologic 

province.
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

BORREGO VALLEY, CENTRAL DESERT,

AND THE LOW-USE BASINS

OF THE MOJAVE AND SONORAN

DESERTS STUDY UNIT

(figure 2)

San Diego

Drainages

Bakersfield

San

Francisco

Sacramento

OREGON

NEVADA

MEXICO

ARIZONA

Redding

Los Angeles

San Diego

P a 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

IP027935_Figure 01

Figure 1. Location of the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran 

Deserts study unit, California GAMA Priority Basin Project, and the California hydrogeologic provinces.
4 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Purpose and Scope

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

description: description of the hydrogeologic setting of the 

Borrego Valley, Central Desert, and Low-Use Basins of the 

Mojave and Sonoran Deserts study unit (fig. 1), hereinafter 

referred to as the CLUB study unit, (2) status assessment: 

assessment of the status of the current quality of groundwater 

in the primary aquifer system in the CLUB study unit, and 

(3) compilation of ancillary data: compilation of data for 

selected potential explanatory factors that may be useful for 

understanding relations between water quality and the human 

and natural factors that may affect water quality.

Water-quality data for samples collected by the USGSGAMA Program in the CLUB study unit and details of sample 

collection, analysis, and quality-assurance procedures for the 

CLUB study unit are reported by Mathany and others (2012). 

Utilizing those same data, this report describes methods used 

in designing the sampling network, identifying CDPH data 

for use in the status assessment, estimating aquifer-scale 

proportions for categories of relative-concentrations, analyzing 

ancillary datasets, classifying groundwater age, and assessing 

the status of groundwater quality by statistical and graphical 

approaches.

The status assessment includes analyses of waterquality data for 52 wells, 49 of which were selected for 

spatial coverage of 1 well per grid cell (hereinafter referred 

to as USGS-grid wells) across the CLUB study unit. All of 

the USGS-grid wells were production wells. Samples were 

collected for analysis of anthropogenic constituents, such 

as VOCs and pesticides, and naturally occurring inorganic 

constituents, such as major ions and trace elements. Waterquality 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 wells 

and selected CDPH wells was considered to be representative 

of the primary aquifer system in the CLUB study unit; the 

primary aquifer system is defined by the depth intervals of 

the wells listed in the CDPH database for the CLUB study 

unit. GAMA status assessments are designed to provide a 

statistically robust characterization of groundwater quality 

in the primary aquifer system at the basin-scale (Belitz and 

others, 2003). The statistically robust design also allows basins 

to be compared and results to be synthesized on regional and 

statewide scales.

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 are intended to characterize the 

quality of untreated groundwater resources in the primary 

aquifer system within the study unit, not the 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. Regulatory benchmarks 

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

to untreated groundwater.

Hydrogeologic Setting of the CLUB 

Study Unit

The CLUB study unit covers approximately 963 square 

miles (mi2) in San Bernardino, Riverside, San Diego, and 

Imperial Counties in southern California. The study unit lies 

within the Desert hydrogeologic province (fig. 1) (Belitz 

and others, 2003) and contains 47 groundwater basins. The 

groundwater basins cover approximately 12,103 mi2, although 

wells are not found in much of this area. For the purpose 

of this study, these 47 groundwater basins and subbasins 

were grouped into 3 study areas based primarily on location: 

the Borrego Valley study area located within the Borrego 

and Lower Borrego Valleys, the Central Desert study area 

located within the Southern California high desert, and the 

Low-Use Basins of the Mojave and Sonoran Deserts study 

area (fig. 2A). As part of the Priority Basin Project, untreated 

groundwater samples were collected from 52 wells (49 grid 

wells and 3 understanding wells) in the CLUB study unit 

from December 2, 2008, to March 4, 2010 (Mathany and 

others, 2012).

The primary aquifer system in the study unit consists of 

Quaternary alluvium of Pleistocene to Holocene age, and to 

a lesser extent, Tertiary alluvium primarily found underlying 

the Quaternary-age alluvial deposits (fig. 3). These alluvial 

deposits are composed largely of unconsolidated to semiconsolidated gravels, sands, silts, and clays. The thickness of 

water-bearing formations in the central and northern parts of 

the study unit averages 2,000 feet (ft) (California Department 
Hydrogeologic Setting of the CLUB Study Unit 5

of Water Resources, 2004c, 2004g–r, 2004u–bb); whereas, in 

the southern portion of the study area, the thickness averages 

about 850 ft (California Department of Water Resources, 

2004s, 2004t, 2004cc–ss).

Water resources for public drinking-water supply include 

groundwater from local public-supply wells, in addition to 

delivered water from neighboring basins. The All American 

and Coachella Canals deliver Colorado River water to the 

Imperial Valley for use in irrigation. The primary aquifer 

system targeted by this study includes groundwater-bearing 

zones in which public-supply wells (CDPH database) are 

completed. These wells vary in depth from 36 to 1,950 ft 

below land-surface datum (LSD), depending on their location 

and depth of the alluvium. Groundwater in the alluvium flows 

under a natural hydraulic gradient that generally conforms to 

the surface topography. 

Faults are abundant throughout the study unit and may 

act as hydrologic barriers to groundwater flow (Mendez and 

Christensen, 1997; California Department of Water Resources, 

2004s, 2004t, 2004cc–ss). Many faults in the study unit and 

throughout the Low-Use Basins of the Mojave and Sonoran 

Deserts study area are part of the larger San Jacinto and San 

Andreas Fault Zones (fig. 3), which are among the most 

seismically active zones in California. The northern border of 

the study unit is bounded by the Garlock Fault Zone. In the 

Borrego Valley study area, the eastern boundary of the study 

area is defined by the Coyote Creek and Superstition Mountain 

Faults (fig. 2B), which are part of the larger San Jacinto 

Fault Zone. The Central Desert (CD) study area contains the 

Emerson Fault, Surprise Spring Fault, and Johnson Valley 

Fault Zone (fig. 2C). 

Human activity on land surface and in the subsurface can 

affect groundwater quality. Land-use data commonly are used 

to identify potential effects of such activity because each type 

of land use typically is associated with specific activities and 

certain changes in water quality. Land use in the study unit is 

92 percent natural, 2.7 percent agricultural, and 5.7 percent 

urban, according to classifications from USGS National Land 

Cover Data (Vogelmann and others, 2001; Price and others, 

2003) (figs. 4A, 5; appendix A). Most of the agricultural 

and urban land is in the valleys. Natural lands are mostly 

shrubland and bare rock or sediment, with a small percentage 

of grassland and forest. Agricultural land is used primarily 

for pasture and for cultivation of hay and small grains, with 

a small percentage for vineyards. Urban areas are mostly 

residential and include the towns of Baker, Borrego Springs, 

California City, Desert Center, El Centro, Joshua Tree, and 

Landers, and the Twentynine Palms and Fort Irwin military 

bases (fig. 2A). The percentages of land in selected land-use 

categories were calculated for the CLUB study unit, for each 

study area, and for the cumulative area within a 1,640-ft [500-

meter (m)] radius around each well (fig. 4). Figure 5 shows 

the CLUB study unit USGS-grid wells displayed on a landuse classification map that is based on satellite imagery (see 

appendix A for details). 
6 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figures 2A–K. Geographic features and locations of study area grid cells, U.S. Geological Survey (USGS) grid and understanding 

wells, and California Department of Public Health (CDPH) wells, Borrego, Central Desert, and Low-Use Basins of the Desert study unit, 

California GAMA Priority Basin Project, December 2008–March 2010.

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

EXPLANATION

Streams

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins 

of the Mojave and

Sonoran Deserts

Counties

IP027935_Figure 02a

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua

Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

San Diego

Los

Angeles

Pacific Ocean

MEXICO

Lake

Dry lake

USGS-grid well

USGS-understanding well

CDPH Wells

Grid cell

A

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Approximate boundary of

 the Sonoran Desert

Approximate boundary of

 the Mojave Desert

Area shown on

figure 2B

Area shown

on figure 2C

Area shown

on figure 2D

Area shown

on figure 2F

Area shown

on figure 2G

Area shown on figure 2H

Area shown

on figure 2I

Area shown

on figure 2J

Area shown

on figure 2K

Area shown

on figure 2E
Hydrogeologic Setting of the CLUB Study Unit 7

Salton Sea

Clark

Lake

(Dry)

?

Coyote Creek

San Felipe Creek

Caririzo Creek

86

78

0 5 10 MILES

0 5 10 KILOMETERS

SAN DIEGO CO

IMPERIAL CO

Ocotillo

Wells

Borrego

Springs

BV-07

BV-06

BV-05

BVU-01

BV-04

BV-03

BV-02

BV-01

BV-02 USGS-grid well and identifier

Borrego Valley

 grid cell

Borrego Valley

 groundwater basin

USGS-understanding well and identifier

California Department of Public Health

 (CDPH) well

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

EXPLANATION

BVU-01

Desert Lodge

Fault-Dashed where

 approximate, dotted

 where concealed

Boundary of Anza-Borrego

 Desert State Park

San Ysidro Mts

Vallecito Mts

Coyote Mts

Superstition Mtn

Unnamed Wash

Santa Rosa Mts

Fish

Creek

 Mts

Superstition Mountain Fault

Coyote Creek Fault

IP027935_Fig02b_BorregoV.ai

Borrego

Sink

B

Figures 2A–K. —Continued.
8 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figures 2A–K. —Continued.

IP027935_Figure 02c

Deadman

Lake

(Dry)

Mesquite

Lake

(Dry)

Emerson

Lake

(Dry)

U.S. Marine Corps 

Air/Ground Combat Center

(USMCAGCC)

Pipes

Pipes

Big

Little

Morongo

Morongo

Creek 

Creek 

Wash

Wash

62

247

CD-15

CD-14

CD-13

CD-12

CD-11

CD-10

CD-08

CD-07

CD-06

CD-04 CD-05

CD-03

CD-09

CD-02

CD-01

Landers

Joshua

Tree

Yucca

Valley

Morongo Valley

Twentynine Palms

0 5 10 MILES

0 5 10 KILOMETERS

CD-02

USGS-grid well and identifier

Central Desert grid cell

Central Desert

 groundwater basin

California Department of Public Health

 (CDPH) well

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

Melville

Lake

(Dry)

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

SAN

BERNARDINO

CO

RIVERSIDE

CO

San Bernardino Mts

Lower

Morongo

Canyon

Little San Bernardino Mts

Pinto Mts

Copper Mt

Hidalgo Mt

Iron Ridge

Joshua Tree

National Park

Surprise Spring Fault

Mountain Fault

Pinto

Emerson Fault

Johnson Valley Fault Zone

Morongo

Valley Fault

C
Hydrogeologic Setting of the CLUB Study Unit 9

Rogers

Lake

(Dry)

Koehn

Lake

(Dry)

Rosamond

Lake (Dry)

Los Angeles 

Aqueduct

58

14

LUB-02

USGS-grid well and identifier

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

California Department of Public Health

 (CDPH) well

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

KERN CO

SAN

BERNARDINO

CO

Mojave

Cantil

California City

LUB-11

LUB-04

LUB-03

LUB-02

LUB-01

D

T e h a c h a p i M t s

Rand Mts

El Paso Mts

Chuckwalla Mt

Garlock Fault Zone

Garlock Fault Zone

IP027935_Fig02d_LUBWest.ai

Figures 2A–K. —Continued.
10 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figures 2A–K. —Continued.

Searles

Lake

(Dry)

Harper

Lake

(Dry)

Coyote

Lake

(Dry)

Superior

Lake

(Dry)

15

LUB-07

LUBU-02 USGS-understanding well

 and identifier

USGS-grid well and identifier

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

?

SAN

BERNARDINO

CO

E

Goldstone

LUBU-02

LUB-08

LUB-07

LUB-06

Fort Irwin

G r a n i t e M t s

California Department of Public Health

 (CDPH) well

Garlock Fault Zone

Garlock Fault Zone Panamint Mts

IP027935_Fig02e_LUBWest2.ai
Hydrogeologic Setting of the CLUB Study Unit 11

Figures 2A–K. —Continued.

Soda

Lake

(Dry)

Bristol

Lake

(Dry)

Ivanpah

Lake

(Dry)

Silver

Lake

(Dry)

15

40

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

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

SAN

BERNARDINO

CO

Baker

Midway

Ludlow

Chambless LUB-27

LUB-26

LUB-25

LUB-09

LUB-24

LUB-10

F

NEVADA

Cady Mts

Soda Mts

Clark Mts

LUB-24

California Department of Public Health

 (CDPH) well

Rand Mts

Mojave River

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

USGS-grid well 

IP027935_Fig06f_LUBEast.ai
12 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figures 2A–K. —Continued.

Lucerne

Lake

(Dry)

247

247

18

18

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

0 5 10 MILES

0 5 10 KILOMETERS

SAN

BERNARDINO

CO

LUB-13

LUB-12

LUB-05

Lucerne Valley

Big Bear Lake

Baldwin Lake

G

LUB-05

USGS-grid well and identifier

California Department of Public Health

 (CDPH) well

San Bernardino Mts

Graniter Mts

Ord Mts

Bighorn Mts

H e l e n d a l e F a u l t Z o n e

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

IP027935_Fig02g_LUBMiddle.ai
Hydrogeologic Setting of the CLUB Study Unit 13

Figures 2A–K. —Continued.

Dale

Lake

(Dry)

Mesquite

Lake

(Dry)

Deadman

Lake

(Dry)

62

Twentynine Palms

34°

15’

34°

0 5 10 MILES

0 5 10 KILOMETERS

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

H

SAN

BERNARDINO

CO

RIVERSIDE CO

LUBU-01

LUBU-01

California Department of Public Health

 (CDPH) well

P i n t o M t s

USGS-understanding well

 and identifier

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

IP027935_Fig02h_LUBMiddle3.ai

B u l l i o n M t s

116° 115°45’
14 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figures 2A–K. —Continued.

Aqueduct

Colorado

Palen

Lake

(Dry)

Ford

Lake

(Dry)

10

111

62

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

?

?

?

?

?

?

SAN

BERNARDINO

CO

RIVERSIDE CO

IMPERIAL CO

Salton Sea

34°

15’

34°

33°

45’

33°

30’

LUB-23

LUB-22

LUB-21

Desert Center

I

LUB-21 Grid cell shown on

 figure 2H

California Department of Public Health

 (CDPH) well

C h u c k w a l l a M t s

C o t t o n w o o d M t s

C o x c o m b M t s

USGS-grid well and identifier

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

IP027935_Fig02i_LUBMiddle2.ai

115°45’ 115°30’ 115°15’ 115° 114°45’
Hydrogeologic Setting of the CLUB Study Unit 15

Figures 2A–K. —Continued.

8

86

94

78

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

0 5 10 MILES

0 5 10 KILOMETERS

SAN DIEGO CO

IMPERIAL CO

Salton Sea

Jacumba

Ocotillo

Ocotillo Wells

LUB-20

LUB-19

LUB-18

LUB-15

LUB-14

J

MEXICO

LUB-15

California Department of Public Health

 (CDPH) well

Fish

Creek

 Mts

San Ysidro

 Mts

Vallecito Mts

Laguna Mts

Coyote Mts

Jacumba Mts

San Felipe

San Felipe Creek

Caririzo Creek

Vallecitos Creek

Creek

Superstition Mountain Fault

Coyote Creek

Fault

USGS-grid well and identifier

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

IP027935_Fig02j_LUBSouth2.ai

116°30’

33°

32°

45’

116°15’ 116°
16 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figures 2A–K. —Continued.

8

86

111

78

MEXICO

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Fault-Dashed where

 approximate, dotted

 where concealed

Shaded relief derived from U.S. Geological Survey

National Elevation Dataset, 2006. 

Albers Equal Area Conic Projection

IMPERIAL CO

LUB-17

LUB-16

Imperial

El Centro

Date City

K

ARIZONA

Cargo Muchacho Mts

Sand Hills

C h o c o l a t e M t s

New River

Alamo River

Coachella Canal

East Highline Canal

Colorado

River

All American Canal

LUB-16

California Department of Public Health

 (CDPH) well

IP027935_Fig02k_LUBSouth.ai

USGS-grid well and identifier

Low-Use Basins

 grid cell

Low-Use Basins

 groundwater basin

115°30’

33°

15’

32°

45’

33°

115°15’ 115° 114°45’
Hydrogeologic Setting of the CLUB Study Unit 17

Figure 3. Geologic map of the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, 

California GAMA Priority Basin Project.

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua

Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

Lake Streams Fault

Dry lake Counties

IP027935_Figure 03

Salton

Sea

ARIZONA

NEVADA

Pacific Ocean

MEXICO

USGS-grid well

USGS-understanding

 well

GEOLOGIC CLASS

Metamorphic

Plutonic

Sedimentary

Volcanic

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

S an A n d r e as F au l t

S an J aci n t o F au l t Z o n e

San Diego

Los

Angeles
18 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

B

20

40

60

80

100

0

PERCENT URBAN

100 80 60 40 20 0

PERCENT AGRICULTURE

100

80

60

40

20

0

PERCENT NATURAL

BORREGO VALLEY, CENTRAL DESERT, AND THE

LOW-USE BASINS OF THE MOJAVE AND

SONORAN DESERTS STUDY UNIT

Borrego Valley

Central Desert

Low-Use Basins

STUDY AREA

UNDERSTANDING WELLS

A

20

40

60

80

100

0

PERCENT URBAN

100 80 60 40 20 0

PERCENT AGRICULTURE

100

80

60

40

20

0

PERCENT NATURAL

Borrego Valley

Central Desert

Low-Use Basins

BORREGO VALLEY, CENTRAL DESERT, AND THE

LOW-USE BASINS OF THE MOJAVE AND

SONORAN DESERTS STUDY UNIT

Data point:

Land use

based on

satellite

imagery

Data point:

Land use

based on a

500-meter-radius

area around

grid wells

STUDY AREA

STUDY UNIT

IP026177_Figure 04ab

Figure 4. Percentages of urban, agricultural, and natural land use in (A) the study unit and study areas, and (B) the 500-meter radius 

surrounding each USGS-grid well in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study 

unit, California GAMA Priority Basin Project.
Hydrogeologic Setting of the CLUB Study Unit 19

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

Streams

LAND-USE CLASSIFICATION

Urban

Agricultural

Natural

STUDY AREA

Counties

IP027935_Figure 05

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua

Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

USGS-grid wells

USGS-understanding well

Lake

Dry lake

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Figure 5. Land use and the locations of USGS-grid and understanding wells in the Borrego Valley, Central Desert, and Low-Use 

Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.
20 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Borrego Valley Study Area

The Borrego Valley (hereinafter referred to as BV) 

study area is located in San Diego County about 80 miles 

(mi) east of San Diego (fig. 2A). The BV study area covers 

approximately 87 mi2 and consists of the Borrego Valley and 

Lower Borrego Valley groundwater basins. Land cover in the 

BV study area is mostly natural and is composed of shrubland 

and bare rock or sediment with some forest (fig. 5). There are 

some small residential communities in the towns of Borrego 

Springs, Desert Lodge, and Ocotillo Wells (fig. 2B).

The study area is bounded to the west by the San Ysidro, 

Vallecito, Fish Creek, and Coyote Mountains; to the north by 

the Santa Rosa Mountains; to the east by the Coyote Creek 

and Superstition Mountain Faults, which are part of the larger 

San Jacinto Fault Zone; and to the south by a large unnamed 

wash that stretches from the Coyote Mountains northeast to 

Superstition Mountain (fig. 2B). Altitudes in the BV study 

area range from approximately 30 ft below LSD in the lower 

Borrego Valley to over 4,500 ft above LSD in the San Ysidro 

Mountains, located just southwest of the town of Borrego 

Springs. 

The climate in the BV study area is classified as 

arid desert, with hot, dry summers and cool winters 

(PRISM Climate Group, Oregon State University, 2010; 

U.S. Department of Commerce, 2010). Average annual 

precipitation in the BV study area is less than 7 in. and is fairly 

evenly distributed throughout the year (U.S. Department of 

Commerce, 2010). The BV study area is drained by Coyote 

and San Felipe Creeks and by the Borrego Sink, which is a 

large surface drainage feature in the northern part of the BV 

study area and is a major collection point for surficial runoff 

in the study area (fig. 2B). Sources of groundwater recharge 

include percolation of rainfall and river and stream infiltration. 

The primary aquifer in the BV study area is the upper 

aquifer of a three-tiered, alluvium-filled aquifer system 

that is underlain by crystalline bedrock. The upper aquifer 

is composed of alluvial, playa, eolian, and fan deposits of 

Pleistocene to Holocene age. The middle and lower aquifers 

consist of continental deposits of moderately consolidated 

gravel, sand, and boulders of Pleistocene age (California 

Department of Water Resources, 2004a). 

Infiltration of surface runoff from the surrounding 

mountains into Coyote Creek, San Felipe Creek, and 

Borrego Sink is the primary source of recharge. Seepage 

from small, intermittent streams and washes originating 

from the mountains west and north of the study area is a 

secondary source of recharge (California Department of Water 

Resources, 2004a). The general groundwater-flow direction in 

the BV study area is southeast from the western and northern 

sides of the study area toward the center, then southward, 

following the topography of the Borrego and Lower Borrego 

Valleys. The San Jacinto Fault Zone, containing the two 

major northwest-trending faults (Superstition Mountain and 

Coyote Creek), acts as barrier to groundwater flow (California 

Department of Water Resources, 2004a) (fig. 2B).

Central Desert Study Area

The Central Desert study area (hereinafter referred to as 

CD) is located in San Bernardino County about 30 mi north 

of Palm Springs, within the Southern California high desert 

(fig. 2A). The CD study area covers 156 mi2 and consists of 

six CDWR-defined basins and subbasins. Land cover in the 

CD study area is mostly natural and is composed of shrubland 

and bare rock or sediment with some forest. Small residential 

communities are located in the towns of Landers, Morongo 

Valley, and Yucca Valley and the Joshua Tree National Park 

and Twentynine Palms military base (fig. 2C).

The CD study area is bounded to the west by the uplands 

of the San Bernardino Mountains; to the north by the Emerson 

Fault, Johnson Valley Fault Zone, and Iron Ridge; to the 

east by the Surprise Spring Fault and the Hidalgo, Copper, 

and Pinto Mountains; and to the south by Lower Morongo 

Canyon, the Little San Bernardino Mountains, and the uplands 

areas of the Joshua Tree National Park (fig. 2C). Elevations in 

the CD study area range from approximately 2,300 ft above 

LSD near the often dry bed of Emerson Lake, to over 5,000 ft 

above LSD in the Little San Bernardino Mountains, located 

south of the town of Yucca Valley. The southernmost reaches 

of the study area are drained by the Big and Little Morongo 

Creeks. A large surface drainage feature named Pipes Wash 

drains the northern and western portions of the CD study area 

to Emerson Lake. Ephemeral streams drain the eastern parts 

of the CD study area towards Emerson and Deadman Lakes, 

which are often dry beds (California Department of Water 

Resources, 2003b, 2004b–f). 

The climate in the CD study area is classified as arid 

desert, with hot, dry summers and cool winters (PRISM 

Climate Group, 2010; U.S. Department of Commerce, 2010). 

Average annual precipitation in the CD study area ranges from 

6 in. around the Yucca Valley to 10 in. in the Morongo Valley, 

and precipitation is fairly evenly distributed throughout the 

year (U.S. Department of Commerce, 2010). Groundwater 

recharge comes from infiltration of runoff from surrounding 

mountains and hills into Little and Big Morongo Creeks, Pipes 

Wash, and Emerson Lake; seepage from ephemeral streams; 

infiltration and percolation of precipitation; and infiltration 

and percolation from percolation ponds and septic tanks 

(U.S. Geological Survey, 2009).

The primary aquifer system in the CD study area consists 

of unconsolidated to partly consolidated continental deposits 

of Miocene to Quaternary age. These unconfined waterbearing formations consist of interbedded conglomerates, 

gravels, sands, and silts deposited into alluvial fan systems. 

To a lesser extent, groundwater in the CD study area is found 

in deposits of clays, sandy clays, distal silts, and dune sand 

(California Department of Water Resources, 2003b, 2004b–f). 
Hydrogeologic Setting of the CLUB Study Unit 21

Groundwater flows from the southern, western, and 

eastern parts of the study area, towards the center, then 

northward to the Emerson Lake area. In the southwesternmost part of the CD study area, groundwater flows through 

Lower Morongo Canyon, and then exits the study area. 

The east-west–trending Pinto Mountain fault is the most 

influential hydrologic barrier to groundwater flow in the 

study area, with water-table differences up to 125 ft across 

the fault. Additionally, the Emerson, Surprise Spring, and 

Morongo Valley Faults and the Johnson Valley Fault Zone 

act as hydrologic barriers to groundwater flow (Mendez and 

Christensen, 1997; California Department of Water Resources, 

2003b, 2004b–f; U.S. Geological Survey, 2009; fig. 2C).

Low-Use Basins of the Mojave and Sonoran 

Deserts Study Area

The Low-Use Basins of the Mojave and Sonoran Deserts 

(hereinafter referred to as Low-Use Basins, or LUB) study 

area extends over a large area in southeastern California 

that encompasses parts of Imperial, Kern, Riverside, San 

Bernadino, and San Diego Counties (fig. 2A). The LUB study 

area covers approximately 720 mi2 and contains 40 CDWRdefined basins and subbasins (California Department of Water 

Resources, 2004c, 2004g–ss). Similar to the other study 

areas, land use in the LUB study area is mostly natural and is 

composed of shrubland and bare rock or sediment with some 

grassland and forest (fig. 5). There are some small residential 

communities in the towns of Baker, California City, Desert 

Center, El Centro, Ludlow, and Midway and in Twentynine 

Palms and Fort Irwin military bases.

The LUB study area is bounded to the west by the 

Laguna, San Bernardino, and Little San Bernardino 

Mountains; to the north by the Garlock Fault Zone and the 

Tehachapi and Panamint Mountains; to the east by the State 

of Nevada and the Colorado River; and to the south by the 

international border with Mexico (California Department of 

Water Resources, 2004c, 2004g–ss; figs. 2D–K). Elevations 

in the northern and central parts of the LUB study area range 

from approximately 600 ft above LSD at Bristol Lake, to over 

7,900 ft above LSD in the Clark Mountains located northeast 

of the city of Baker. Elevations in the southern part of the 

LUB study area range from approximately 230 ft below LSD 

in the basins surrounding the Salton Sea to approximately 

9,700 ft above LSD in the Santa Rosa Mountains near the 

Borrego Valley. The major surface drainage features of LUB 

study area are the Salton Sea, the Mojave and Colorado 

Rivers, and numerous ephemeral streams, creeks, and washes 

that flow towards dry lake beds located inside or adjacent to 

the groundwater basins and (or) subbasins (figs. 2D–K). The 

Salton Sea is fed by the New, Alamo, and Whitewater Rivers, 

Salt and San Felipe Creeks, and unnamed ephemeral streams. 

Most of the LUB study area is in the Mojave Desert; 

the southern part is in the Sonoran Desert (fig. 2A). The 

climate in the Mojave Desert is classified as high arid desert, 

with hot, dry summers and cool winters (PRISM Climate 

Group, Oregon State University, 2010; U.S. Department 

of Commerce, 2010). Rainfall in the Mojave Desert occurs 

mainly in winter months from storms originating in the 

Pacific Ocean, and average annual precipitation is less than 

6 in. (PRISM Climate Group, Oregon State University, 2010; 

Western Regional Climate Center, 2010). The climate in 

the Sonoran Desert is classified as subtropical desert and is 

generally hotter than in the Mojave Desert. Average annual 

precipitation is approximately 3.5 in., and the rainfall pattern 

is bimodal, with winter rain from storms originating in the 

Pacific Ocean and summer rain derived from the North 

American Monsoon, which is drawn northward through 

Mexico from the Pacific Ocean and (or) the Gulf of Mexico 

in the months of August through October (Western Regional 

Climate Center, 2010). 

The primary aquifer system in the LUB study area mostly 

consists of alluvium of Quaternary age and, to a lesser extent, 

alluvium of Tertiary age that underlies the alluvial deposits 

of Quaternary age. Both the Quaternary and Tertiary deposits 

are composed largely of unconsolidated to semi-consolidated 

gravels, sands, silts, and clays. The thickness of waterbearing 

formations averages 2,000 ft in the central and northern parts 

of the study area and averages 850 ft in the southern part of 

the study area (California Department of Water Resources, 

2004s, 2004t, 2004cc–ss). 

Groundwater recharge in the region comes from a 

variety of sources: infiltration of runoff from the surrounding 

mountains into alluvial fan deposits; direct percolation of 

precipitation; seepage from ephemeral rivers, streams, washes, 

and unlined canals; recharge of imported surface water 

used for agricultural irrigation; and subsurface inflow (from 

non-alluvial geologic units that bound the alluvial basins) 

(California Department of Water Resources, 2004s, 2004t, 

2004cc–ss).

The general direction of groundwater flow in the 

LUB study area is from the surrounding mountains and 

hills towards rivers, creeks, and streams that discharge into 

lakes, such as the Salton Sea, and to various dry lake beds 

located inside or adjacent to the basins and (or) subbasins. 

Nearly every basin and subbasin contains faults. Parts of the 

San Jacinto and San Andreas Fault Zones in the study area 

act as hydrologic barriers, although it is unknown whether 

other faults act as barriers to groundwater flow (California 

Department of Water Resources, 2004c, 2004g–r, 2004u–bb; 

fig. 3). To a lesser extent, clay deposits and bedrock act as 

barriers to groundwater flow. Also, north of the Twentynine 

Palms military base, an anticline restricts groundwater flow 

to the south within three groundwater basins and subbasins 

(California Department of Water Resources, 2004w–y). 
22 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Methods 

The status assessment provides a spatially unbiased 

assessment of groundwater quality in the primary aquifer 

system of the CLUB study unit. This section describes the 

methods used in this study to (1) define groundwater quality, 

(2) assemble the datasets used for the status assessment, 

(3) determine which constituents warrant additional 

evaluation, and (4) calculate aquifer-scale proportions. 

Methods used for compilation of data on potential explanatory 

factors are described in appendix A.

In this study, groundwater-quality data are presented as 

relative-concentrations, the concentrations of constituents 

measured in groundwater relative to regulatory and nonregulatory benchmarks used to evaluate drinking-water 

quality. Constituents were selected for additional evaluation 

in the assessment based on objective criteria defined in terms 

of relative-concentrations. Groundwater-quality data collected 

by the U.S. Geological Survey for the GAMA Priority Basin 

Project (USGS-GAMA) and data compiled in the CDPH 

database are used in the status assessment. Two statistical 

methods based on spatially unbiased equal-area grids are used 

to calculate aquifer-scale proportions of low, moderate, or 

high relative-concentrations: (1) the “grid-based” method uses 

one value per grid cell to represent groundwater quality, and 

(2) the “spatially weighted” method uses many values per grid 

cell (Belitz and others, 2010).

The CDPH database contains historical records from 

more than 25,000 wells, necessitating targeted retrievals to 

effectively access relevant water-quality data. For example, 

for the area representing the CLUB study unit, the historical 

CDPH database contains more than 103,506 records from 

202 wells. The CDPH data were used in three ways in the 

status assessment: (1) to fill in gaps in the USGS data for 

the grid-based calculations of aquifer-scale proportions, 

(2) to aid in selecting constituents for additional evaluation 

in the assessment, and (3) to provide the majority of the data 

used in the spatially weighted calculations of aquifer-scale 

proportions.

Relative-Concentrations and Water-Quality 

Benchmarks

Concentrations of constituents are presented as relativeconcentrations in the status assessment:

Relative-concentration Sample concentration

Benchmark conce = ntration

Regulatory and non-regulatory benchmarks apply to treated 

water that is served to the consumer, not to untreated 

groundwater. However, to provide some context for the 

results, concentrations of constituents measured in the 

untreated groundwater were compared with benchmarks 

established by the U.S. Environmental Protection Agency 

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

Agency, 2006; California Department of Public Health, 

2008a,b). Relative-concentrations less than 1 (< 1.0) 

indicate sample concentrations less than the benchmark, 

and relative-concentrations greater than 1 (> 1.0) indicate 

sample concentrations greater than the benchmark. The use 

of relative-concentrations also permits comparisons on a 

single scale among constituents that can be present at a wide 

range of concentrations. Relative-concentrations can only be 

computed for constituents with water-quality benchmarks; 

therefore, constituents without water-quality benchmarks are 

not included in this status assessment.

The benchmarks used for each constituent were selected 

in the following order of priority:

1. Regulatory, health-based CDPH and USEPA maximum 

contaminant levels (MCL-CA and MCL-US), action 

levels (AL-US), and treatment technique levels (TT-US).

2. Non-regulatory, aesthetic-based CDPH and USEPA 

secondary maximum contaminant levels (SMCL-CA and 

SMCL-US). For constituents with both recommended and 

upper SMCL-CA levels, the values for the upper levels 

were used. 

3. Non-regulatory, health-based CDPH notification levels 

(NL-CA), USEPA lifetime health advisory levels 

(HAL-US), and USEPA risk-specific doses at a risk factor 

of 10–5 (for a risk of 1:100,000) (RSD5-US).

Note that for constituents with multiple types of benchmarks, 

this hierarchy may not result in selection of the benchmark with 

the lowest concentration. Additional information on the types of 

benchmarks and listings of the benchmarks for all constituents 

analyzed is provided by Mathany and others (2012).
Methods 23

Toccalino and others (2004), Toccalino and Norman 

(2006), and Rowe and others (2007) previously used the 

ratio of the measured sample concentration to the benchmark 

concentration [either USEPA MCLs or health-based screening 

levels (HBSLs)], and defined this ratio as the benchmark 

quotient. HBSLs were not used in this report because they are 

not currently used as benchmarks by California drinking-water 

regulatory agencies. Because different water-quality benchmarks 

may be used to calculate relative-concentrations than to 

calculate benchmark quotients, the values of these ratios may 

not be the same for all constituents (Fram and Belitz, 2012).

For ease of discussion, relative-concentrations of 

constituents were classified into low, moderate, and high

categories:

Category

Relative-concentrations 

for organic and specialinterest constituents

Relative-concentrations 

for inorganic 

constituents

High > 1 > 1

Moderate > 0.1 and ≤ 1 > 0.5 and ≤ 1

Low ≤ 0.1 ≤ 0.5

The boundary between “moderate” and “low” relativeconcentrations was set at 0.1 for organic and special-interest 

constituents for consistency with other studies and reporting 

requirements (U.S. Environmental Protection Agency, 1998; 

Toccalino and others, 2004). For inorganic constituents, 

the boundary between “moderate” and “low” relativeconcentrations was set at 0.5. The primary reason for using 

a higher threshold was to focus attention on the inorganic 

constituents of most immediate concern (Fram and Belitz, 

2012). Most inorganic constituents are naturally occurring 

and tend to be more prevalent than organic constituents in 

groundwater. Although more complex classifications could be 

devised based upon the properties and sources of individual 

constituents, use of a single moderate/low boundary value 

for each of the two major groups of constituents provided a 

consistent objective criterion for distinguishing constituents 

occurring at moderate rather than low concentrations.

Datasets Used for Status Assessment

U.S. Geological Survey Grid Data

The primary data used for the grid-based calculations of 

aquifer-scale proportions of relative-concentrations were data 

from wells sampled by USGS-GAMA. Detailed descriptions 

of the methods used to identify wells for sampling are given 

in Mathany and others (2012). Briefly, the three study areas 

each were divided into equal-area grid cells, and the objective 

was to sample one public-supply well in each cell. CDPH 

wells were not evenly distributed within the study areas, so 

to minimize the number of cells without any public-supply 

wells, only the parts of the study areas near CDPH wells were 

included in the gridded area. A 1.86-mi [3-kilometer (km)] 

radius circle was drawn around each CDPH well, and the 

collective area encompassed by the circles in each study area 

was divided into grid cells (Scott, 1990). One CDPH well 

was randomly selected for sampling in each cell, and if a cell 

had no accessible CDPH wells, then an appropriate well was 

selected by door-to-door canvassing. The BV study area was 

divided into nine equal-area grid cells, each approximately 

9.7 mi2, and seven wells were sampled inside this grid network 

(fig. 2B; table A1). The CD study area was divided into 17 

equal-area grid cells, each approximately 9.2 mi2, and 15 wells 

were sampled inside this grid network (fig. 2C; table A1). The 

LUB study area was divided into 36 equal-area grid cells, each 

approximately 20 mi2, and 27 wells were sampled inside this 

grid network (figs. 2D–K; table A1). 

The CLUB study unit contained 62 total grid cells, and 

the USGS sampled wells in 49 of those cells (USGS-grid 

wells). Of the 49 USGS-grid wells, 40 were listed in the 

CDPH database. The nine wells that were not listed in the 

CDPH database are perforated at depths similar to the depths 

of CDPH wells in their respective cells. USGS-grid wells 

were named with an alphanumeric GAMA ID consisting of a 

prefix identifying the study area and a number indicating the 

order of sample collection (fig. B1B; table A1). The following 

prefixes were used to indicate study areas: BV, Borrego Valley 

study area; CD, Central Desert study area; and LUB, Low-Use 

Basins of the Mojave and Sonoran Deserts study area. 

Samples collected from USGS-grid wells were analyzed 

for 165 to 212 constituents (table 1), of which 113 had 

benchmarks for use in calculating relative-concentrations. 

Dissolved oxygen, temperature, pH, specific conductance, 

major ions, VOCs, pesticides, noble gases, and perchlorate were 

analyzed in samples from all of the USGS wells. Trace elements, 

nutrients, hexavalent chromium, radioactive constituents, 

selected isotopes, and N-nitrosodimethylamine (NDMA) were 

analyzed in samples from a subset of the wells. The collection, 

analysis, and quality-control data for the analyte classes listed in 

table 1 are described by Mathany and others (2012). 
24 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Table 1. Number of wells sampled by the U.S. Geological Survey for the fast and slow sampling schedules, and number of constituents 

sampled in each constituent class for the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study 

unit, California GAMA Priority Basin Project, December 2008 to March 2010.

[ns, not sampled]

Sampling schedule

Fast Slow

Well summary Number of wells

Number of grid wells sampled 26 23

Number of understanding wells sampled 2 1

Constituent classes Number of constituents

Inorganic constituents

Specific conductance 1 1

Trace elements ns 1 23

Nutrients ns 1 5

Major ions, alkalinity, total dissolved solids (TDS) ns 1 12

Hexavalent chromium (chromium-VI) 2 ns 1

Uranium and radioactive constituents

Uranium ns 1 1

Radon-222 ns 1

Radium ns 1

Gross alpha and gross beta particle activities 3 ns 2

Organic constituents 4

Volatile organic compounds (VOCs) 5 85 85

Pesticides and pesticide degradates 63 63

Special-interest constituents

Perchlorate 1 1

N-Nitrosodimethylamine (NDMA) ns 1

Geochemical and age-dating tracers

Dissolved oxygen, temperature, and pH 3 3

δ2

H and δ18O of water 2 2

Carbon-14 and δ13C of dissolved carbonates 2 2

Tritium 6 1 1

Noble gases (He, Ne, Ar, Kr, Xe), 3

He/4He, and tritium 7 7 7

Total 165 212

1

 These constituents were not analyzed in samples from wells on the fast schedule in the Borrego Valley and Central Desert study areas, but were analyzed in 

samples from wells on the fast schedule in the Low-Use Basins of the Mojave and Sonoran Deserts study area.

2

 Hexavalent chromium analysis was added to the slow sampling schedule starting October 2009.

3

 Both gross alpha and gross beta particle activities were measured after 72-hour and 30-day holding times; the 72-hour results are used in this report.

4

 Fourteen pharmaceutical compounds were analyzed in samples from wells on the slow schedule, and results are discussed in Fram and Belitz (2011).

5

 Includes nine constituents classified as fumigants or fumigant synthesis byproducts.

6

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

7

Analyzed at Lawrence Livermore National Laboratory, Livermore, California.
Methods 25

California Department of Public Health Grid Data

The three study areas were divided into 62 grid cells, of 

which 13 cells did not have a USGS-grid well. The CDPH 

database was queried to provide the missing data for inorganic 

constituents for these cells. CDPH wells with data for the most 

recent 3 years available at the time of sampling (December 3, 

2005, to December 1, 2008) were considered. If a well had 

more than one analysis for a constituent in the 3-year interval, 

then the most recent data were selected.

The procedures used to identify suitable data from CDPH 

wells are described in appendix B. Briefly, the first choice 

was to use CDPH data from the same well sampled by the 

USGS (USGS-grid well). In this case, “DG” was added to the 

GAMA ID to signify that the data were from a well sampled 

by the USGS and that the data were supplemented from the 

CDPH database (fig. B1A; table A1). If the DG well did not 

have all of the needed data, then a second well in the cell was 

randomly selected from the subset of CDPH wells with data, 

and a new identification with “DPH” was assigned to that 

well (fig. B1A; table A1). The combination of the USGS-grid 

wells and the CDPH-grid wells produced a grid-well network 

covering 51 of the 62 grid cells in the CLUB study unit 

(table A1). No accessible wells or data were available for the 

remaining 11 cells.

The CDPH database generally did not contain data for 

all of the 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 database is the only statewide database of groundwaterchemistry data available for comprehensive analysis. 

Table 2. Benchmark type and value for inorganic constituents and number of grid cells with U.S. Geological Survey–GAMA data and 

California Department of Public Health data for each constituent, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and 

Sonoran Deserts study unit, California GAMA Priority Basin Project.

[Benchmark type: Regulatory, health-based benchmarks: MCL-US, USEPA maximum contaminant level; Prop MCL-US, proposed USEPA maximum 

contaminant level; AL-US, USEPA action level; MCL-CA, CDPH maximum contaminant level. Non-regulatory, health-based benchmarks: HAL-US, USEPA 

lifetime health advisory level; NL-CA, CDPH notification level. Non-regulatory, aesthetic/technical-based benchmarks: SMCL-CA, CDPH secondary 

maximum contaminant level. Benchmark types and values as of October 2013. Benchmark units: μg/L, micrograms per liter; mg/L, milligrams per liter; 

μS/cm, microsiemens per centimeter; pCi/L, picocuries per liter. Other abbreviations: USGS, U.S. Geological Survey; GAMA, Groundwater Ambient 

Monitoring and Assessment Program; USEPA, U.S. Environmental Protection Agency; CDPH, California Department of Public Health]

Constituent Benchmark type 1 Benchmark 

value Units

Number of grid cells 

with USGS-GAMA 

data

Number of grid 

cells with CDPH 

data

Selected for 

additional 

evaluation

Trace and minor elements with health-based benchmarks

Aluminum MCL-CA 1,000 μg/L 39 5 no

Antimony MCL-US 6 μg/L 39 5 no

Arsenic MCL-US 10 μg/L 39 7 yes

Barium MCL-CA 1,000 μg/L 39 5 no

Beryllium MCL-US 4 μg/L 39 5 no

Boron NL-CA 1,000 μg/L 39 5 yes

Cadmium MCL-US 5 μg/L 39 5 no

Chromium MCL-CA 50 μg/L 39 6 yes

Copper AL-US 1,300 μg/L 39 5 no

Fluoride MCL-CA 2 mg/L 39 7 yes

Lead AL-US 15 μg/L 39 5 no

Molybdenum HAL-US 40 μg/L 39 0 yes

Nickel MCL-CA 100 μg/L 39 5 no

Selenium MCL-US 50 μg/L 39 5 no

Strontium HAL-US 4,000 μg/L 39 0 no

Thallium MCL-US 2 μg/L 39 5 no

Vanadium NL-CA 50 μg/L 39 5 yes
26 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Constituent Benchmark type 1 Benchmark 

value Units

Number of grid cells 

with USGS-GAMA 

data

Number of grid 

cells with CDPH 

data

Selected for 

additional 

evaluation

Uranium and radioactive constituents

Uranium MCL-US 30 μg/L 39 4 yes

Gross alpha radioactivity MCL-US 15 pCi/L 23 14 yes

Gross beta radioactivity MCL-CA 50 pCi/L 23 2 no

Radium MCL-US 5 pCi/L 23 15 no

Radon-222 Prop. MCL-US 4,000 pCi/L 22 0 no

Nutrients

Ammonia, as nitrogen HAL-US 2

 24.7 mg/L 39 0 no

Nitrate, as nitrogen MCL-US 10 mg/L 39 9 yes

Nitrite, as nitrogen 3 MCL-US 1 mg/L 39 6 no

Inorganic constituents with SMCL benchmarks

Chloride SMCL-CA 500 mg/L 39 5 yes

Sulfate SMCL-CA 500 mg/L 39 5 yes

Total dissolved solids (TDS) SMCL-CA 1,000 mg/L 4

 48 2 yes

Iron SMCL-CA 300 μg/L 39 5 yes

Manganese SMCL-CA 50 μg/L 39 5 yes

Silver SMCL-CA 100 μg/L 39 5 no

Zinc SMCL-US 5,000 μg/L 39 5 no

1

 Maximum contaminant level benchmarks are listed as MCL-US when the MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists.

2

 HAL-US benchmark is 30 mg/L for ammonia, as ammonia.

3

 Concentrations of nitrate, as nitrate, reported in the CDPH database are converted to concentrations of nitrate, as nitrogen, for comparison with USGSGAMA data.

4

 Total dissolved solids (TDS) were measured directly or calculated from specific conductance (see appendix D).

Table 2. Benchmark type and value for inorganic constituents and number of grid cells with U.S. Geological Survey–GAMA data and 

California Department of Public Health data for each constituent, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and 

Sonoran Deserts study unit, California GAMA Priority Basin Project.—Continued

[Benchmark type: Regulatory, health-based benchmarks: MCL-US, USEPA maximum contaminant level; Prop MCL-US, proposed USEPA maximum 

contaminant level; AL-US, USEPA action level; MCL-CA, CDPH maximum contaminant level,. Non-regulatory, health-based benchmarks: HAL-US, USEPA 

lifetime health advisory level; NL-CA, CDPH notification level. Non-regulatory, aesthetic/technical-based benchmarks: SMCL-CA, CDPH secondary 

maximum contaminant level. Benchmark types and values as of October 2013. Benchmark units: μg/L, micrograms per liter; mg/L, milligrams per liter; 

μS/cm, microsiemens per centimeter; pCi/L, picocuries per liter. Other Abbreviations: USGS, U.S. Geological Survey; GAMA, Groundwater Ambient 

Monitoring and Assessment Program; USEPA, U.S. Environmental Protection Agency; CDPH, California Department of Public Health.]

pesticide degradates at each of the 49 grid wells sampled by the 

USGS in the CLUB study unit (table 1). In addition, method 

detection limits for USGS-GAMA analyses typically were one 

to two orders of magnitude lower than the reporting levels for 

analyses compiled by the CDPH (Fram and Belitz, 2012).

CDPH data were not used in calculations of grid-based 

aquifer-scale proportions for VOCs, pesticides, or specialinterest constituents because a larger number of VOCs and 

pesticide compounds are analyzed for by the USGS-GAMA 

Program than are available from the CDPH database. USGSGAMA collected data for 85 VOCs plus 63 pesticides and 
Methods 27

Additional Data Used for Spatially Weighted 

Calculation

The spatially weighted aquifer-scale proportions of 

relative-concentrations were calculated with data from the 

USGS-grid wells, from additional wells sampled by USGSGAMA, and from all wells in the CDPH database with waterquality data collected during the 3-year interval December 3, 

2005–December 1, 2008. For wells with USGS and CDPH 

data for inorganic or radioactive constituents, only the USGS 

data were used. 

Three additional, non-randomized wells were sampled by 

the USGS to increase the sampling density in the BV and LUB 

study areas to better understand specific groundwater-quality 

issues. These “USGS-understanding” wells were numbered 

with prefixes modified from those used for the USGSgrid wells (BVU-01, LUBU-01, and LUBU-02) (fig. B1B; 

table A1). 

Selection of Constituents for Additional 

Evaluation

Of the 113 constituents with benchmarks that were 

analyzed in samples from CLUB study unit wells, only 

a subset of these constituents is discussed in this report. 

Three criteria were used to select constituents for additional 

evaluation:

1. Constituents present at high or moderate relativeconcentrations in the CDPH database within the 3-year 

period (December 3, 2005–December 1, 2008);

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

3. Organic constituents with detection frequencies of greater 

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

study unit.

These criteria identified 7 organic and special-interest 

constituents and 14 inorganic constituents for additional 

evaluation in the status assessment (tables 2, 3). An additional 

8 organic constituents and 32 inorganic constituents were 

detected by USGS-GAMA, but were not selected for 

additional evaluation in the status assessment because either 

benchmarks were not established, or detection was at low 

relative-concentrations. A complete list of the constituents 

investigated by USGS-GAMA in the CLUB study unit can be 

found in Mathany and others (2012).

The CDPH database also was used to identify 

constituents with high relative-concentrations historically, 

but not currently. The historical period was defined as from 

the earliest record maintained in the CDPH database to 

December 3, 2005 (February 2, 1980, through December 3, 

2005). Constituent concentrations may be historically 

high, but not currently high, because of improvement of 

groundwater quality with time or abandonment of wells 

with high concentrations. Historically high concentrations 

of constituents that do not otherwise meet the criteria 

for additional evaluation in the status assessment are not 

considered representative of potential groundwater-quality 

concerns in the study unit from 2005 to 2008. For the CLUB 

study unit, 10 constituents had historically high relativeconcentrations (table 4). Seven of the constituents reported 

at high concentrations only during the historical period were 

reported at high concentrations in only 1 well.

Table 3. Benchmark type and value for organic and specialinterest constituents selected for additional evaluation, Borrego 

Valley, Central Desert, and Low-Use Basins of the Mojave 

and Sonoran Deserts study unit, California GAMA Priority 

Basin Project.

[Benchmark type: MCL-US, USEPA maximum contaminant level; 

MCL-CA, CDPH maximum contaminant level; RSD5-US, USEPA 

risk-specific dose at a factor of 10–5. Benchmark types and values as of 

October 2013. Benchmark units: μg/L, micrograms per liter. Other 

abbreviations: GAMA, Groundwater Ambient Monitoring and Assessment 

Program; USEPA, U.S. Environmental Protection Agency; CDPH, California 

Department of Public Health]

Constituent and 

constituent class

Benchmark 

type 1

Benchmark 

value Units

Volatile organic compounds (VOCs)

Benzene MCL-CA 1 μg/L

Tetrachloroethene (PCE) MCL-US 5 μg/L

Bromoform MCL-US 2 80 μg/L

Chloroform MCL-US 2 80 μg/L

Dibromochloromethane MCL-US 2 80 μg/L

Pesticides

Dieldrin RSD5-US 0.02 μg/L

Special interest

Perchlorate MCL-CA 6 μg/L

1

 Maximum contaminant level benchmarks are listed as MCL-US when the 

MCL-US and MCL-CA are identical, and as MCL-CA when the MCL-CA is 

lower than the MCL-US or no MCL-US exists.

2

 The MCL-US for trihalomethanes applies to the sum of the concentrations 

of chloroform, bromodichloromethane, dibromochloromethane, and 

bromoform.
28 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Table 4. Constituents reported at concentrations greater than benchmark values in the California Department of Public Health 

database between February 2, 1980, and December 3, 2005, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and 

Sonoran Deserts study unit, California GAMA Priority Basin Project.

[High concentrations are concentrations greater than the benchmark value. Benchmark type: MCL-US, USEPA maximum contaminant level; AL-US, USEPA 

action level SMCL-CA, CDPH secondary maximum contaminant level; RSD5-US, USEPA risk-specific does at a factor of 10–5. Benchmark types and values 

as of October 2013. Benchmark units: μg/L, micrograms per liter; mg/L, milligrams per liter; pCi/L, picocuries per liter. Other Abbreviations: USEPA, U.S. 

Environmental Protection Agency; CDPH, California Department of Public Health]

Constituent Benchmark 

type

Benchmark 

value

Benchmark 

units

Number of wells 

with analysis

Number of wells with a 

high concentration

Date of most 

recent high value 

Inorganic constituents

Antimony MCL-US 6 μg/L 151 2 1/14/2005

Lead AL-US 15 μg/L 165 5 8/16/2005

Selenium MCL-US 50 μg/L 165 1 3/23/1987

Radium-226 MCL-US 1

 5 pCi/L 25 1 9/2/1987

Radium-228 MCL-US 1

 5 pCi/L 40 1 1/22/2003

Sulfate SMCL-CA 500 mg/L 168 2 4/26/2004

Organic constituents

Total trihalomethanes MCL-US 2

 80 μg/L 170 1 8/18/2005

1,2-Dibromoethane (EDB) MCL-US 0.05 μg/L 108 1 1/23/1990

Aldicarb sulfone MCL-US 3 μg/L 54 1 6/4/2003

Aldrin RSD5-US 0.02 μg/L 81 1 6/4/2003

1

 The MCL-US for radium applies to the sum of the activities of radium-226 and radium-228.

2

 The MCL-US for trihalomethanes applies to the sum of the concentrations of chloroform, bromodichloromethane, dibromochloromethane, and bromoform.

Calculation of Aquifer-Scale Proportions

The status assessment is intended to characterize the 

quality of groundwater resources in the primary aquifer system 

of the CLUB study unit. The primary aquifer system is defined 

by the depth intervals over which wells listed in the CDPH 

database are perforated; these wells primarily are classified as 

municipal and community public-supply wells. The use of the 

term “primary aquifer system” 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 are domestic 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 proportions of the 

primary aquifer system in the CLUB study unit having high, 

moderate, and low relative-concentrations of constituents 

(Belitz and others, 2010). For ease of discussion, these 

proportions are referred to as “high,” “moderate,” and “low” 

aquifer-scale proportions. Calculations of aquifer-scale 

proportions were made for individual constituents meeting 

the criteria for additional evaluation in the status assessment, 

as well as for classes of constituents. The classes consisted 

of groups of related individual constituents. For constituents 

with human-health benchmarks, the classes included trace 

and minor elements, uranium and radioactive constituents, 

nutrients, trihalomethanes, and insecticides. For constituents 

with aesthetic-based benchmarks, the classes included salinity 

indicators and manganese and (or) iron. 

The grid-based calculation uses the grid-well dataset 

assembled from the USGS-grid and CDPH-grid wells. For 

each constituent, the high aquifer-scale proportion was 

calculated by dividing the number of cells with concentrations 

greater than the benchmark by the total number of grid cells 

with data for that constituent in each of the study areas. The 

proportion for each study area is calculated individually 

because grid-cell sizes are not uniform across the study 

areas. The proportion for the study unit is then determined by 

calculating the area-weighted sum. Moderate and low aquiferscale proportions were calculated similarly. A more detailed 

discussion of the calculation used for aquifer-scale proportion 

is located in appendix C. Confidence intervals for the high 

aquifer-scale proportions were computed using the Jeffreys 

interval for the binomial distribution (Brown and others, 2001; 

Belitz and others, 2010). The grid-based estimate is spatially 

unbiased. However, the grid-based approach may not detect 

constituents that are present at high concentrations in small 

proportions of the primary aquifer system. The confidence 

intervals provide a range that is likely (with 90-percent 

confidence) to contain the true high aquifer-scale proportion. 

For calculation of aquifer-scale proportions for classes of 
Potential Explanatory Factors 29

constituents, cells were considered high if values for any of 

the constituents in that class were high. Cells were considered 

moderate if values for any of the constituents were moderate, 

but no values were high.

The spatially weighted calculation uses the dataset 

assembled from all CDPH and USGS-GAMA wells. For each 

constituent, the high aquifer-scale proportion was calculated 

by computing the proportion of wells with high values in each 

cell and then averaging the proportions for all of the cells 

(Belitz and others, 2010). As with the grid-based calculation, 

the spatially weighted calculations were made separately for 

the three study areas, and then the results were combined on 

an area-weighted basis. The moderate aquifer-scale proportion 

was calculated similarly. Confidence intervals for spatially 

weighted detection frequencies of high concentrations are 

not presented in this report. For calculation of aquifer-scale 

proportions for classes of constituents, values for wells were 

considered high if the values for any of the constituents in that 

class were high. Values for wells were considered moderate if 

the values for any of the constituents were moderate, but no 

values for wells were high.

In addition, for each constituent, the raw detection 

frequencies of high and moderate values for individual 

constituents were calculated by using the same dataset as was 

used for the spatially weighted calculations. However, raw 

detection frequencies are not spatially unbiased because the 

wells in the CDPH database are not uniformly distributed 

throughout the CLUB study unit (fig. 2). For example, if a 

constituent were present at high concentrations in a small 

region of the aquifer with a high density of wells, the raw 

detection frequency of high values would be greater than the 

true high aquifer-scale proportion. Raw detection frequencies 

are provided for reference, but were not used to assess aquiferscale proportions (see appendix C for details of statistical 

methods). Raw detection frequencies were not area-weighted.

The grid-based high aquifer-scale proportions were used 

to represent proportions in the primary aquifer system unless 

the spatially weighted proportions were significantly different 

from the grid-based values. Significantly different results were 

defined as follows:

• If the grid-based high aquifer-scale proportion was 

zero and the spatially weighted proportion was greater 

than zero, then the spatially weighted result was used. 

This situation can happen when the concentration of 

a constituent is high in a small fraction of the primary 

aquifer system.

• If the grid-based high aquifer-scale proportion was 

greater than zero and the spatially weighted proportion 

was outside the 90-percent confidence interval (based 

on the Jeffreys interval for the binomial distribution), 

then the spatially weighted proportion was used.

The grid-based moderate and low proportions were used 

in most cases because the reporting levels for many organic 

constituents and some inorganic constituents in the CDPH 

database were higher than the threshold between moderate and 

low categories (Fram and Belitz, 2012). However, if the gridbased moderate proportion was zero and the spatially weighted 

proportion was greater than zero, then the spatially weighted 

value was used as a minimum estimate for the moderate 

proportion.

Potential Explanatory Factors 

Brief descriptions of potential explanatory factors, 

including land use, physical characteristics of the wells, 

indicators of groundwater age, and geochemical conditions 

of the aquifer, are given in this section. Data sources 

and methodology used for assigning values for potential 

explanatory factors are described in appendix A. 

Land Use

Land use was described by three land-use types: urban, 

agricultural, and natural (appendix A). Percentages of the 

three types were calculated for the study unit and study areas, 

and for areas within a radius of 500 meters (m) (1,640 ft), 

hereinafter referred to as 500-m buffers, around wells 

(Johnson and Belitz, 2009). Land classified as natural made up 

the greatest percentage (92 percent) of the total land area in the 

study unit (taking into account the entire area of the study unit, 

and not just the area around grid wells), whereas urban land 

use was only 5.7 percent of the area, and agricultural land use 

was 2.7 percent of the study unit area (figs. 4A, 5). Likewise, 

land use within the 500-m buffers around USGS-grid wells in 

the CLUB study unit was mostly natural (88 percent), urban 

land use accounted for 11 percent, and agricultural land use 

accounted for 1.4 percent (fig. 4A). Most of the agricultural 

land is in the Imperial Valley south of the Salton Sea. Natural 

lands are mostly shrubland and bare rock or sediment, with 

a small percentage of grassland and forest. The primary use 

of agricultural land is for pasture, hay, and small grains, with 

a small percentage for vineyards. Urban areas are mostly 

residential and include towns such as Baker, Borrego Springs, 

California City, Chambless, Desert Lodge, Landers, Ludlow, 

Midway, Morongo Valley, and Yucca Valley; Joshua Tree and 

Mojave National Parks; and Twentynine Palms and Fort Irwin 

military bases (figs. 2A–K). The CD study area has more urban 

land use (19 percent of that study area) than the BV and LUB 

study areas have (5.0 percent and 2.7 percent, respectively; 

fig. 4A). The CD study area contains the towns and areas 

with larger populations such as Landers, Morongo Valley, 

Twentynine Palms, and Yucca Valley and the area of Joshua 

Tree National Park (fig. 2C).
30 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Well Depth and Depth to Top-of-Perforation

Well construction information was available for 42 of 

the 49 grid wells sampled by the USGS in the CLUB study 

unit. Depths of grid wells ranged from 36 to 1,200 ft below 

LSD; the median was 490 ft (fig. 6; table A3). The depths to 

top-of-perforations ranged from 0 to 690 ft below LSD, with 

a median of 220 ft. The perforation lengths were as much as 

565 ft, with a median of 190 ft. Well construction information 

also was available for three understanding wells. The 

understanding wells have ranges in well depth and perforation 

length and a median value for depth to top-of-perforation 

(fig. 6; table A3) similar to those of the grid wells. The median 

well depth was 400 ft, and the median depth to top-ofperforation was 162 ft. 

Groundwater Age

Groundwater samples were assigned age classifications 

on the basis of the tritium and carbon-14 content of the 

samples (appendix A). Age classifications were assigned to 

49 USGS-grid and 3 understanding well samples; 6 were 

classified as modern (recharged after 1952), 5 were mixed, and 

41 were pre-modern (table A4). 

Comparisons of well depths and depths to top-ofperforations with groundwater samples classified as modern, 

mixed, and pre-modern ages did not show significant 

differences (figs. 7A,B). However, other GAMA studies with 

well construction information for wells with groundwater 

samples classified as modern reported that median 

groundwater ages increased for deeper top-of-perforations and 

well depths (Kulongoski and others, 2010). 

Geochemical Condition

An abridged classification of oxidation-reduction 

(redox) conditions adapted from the framework presented by 

McMahon and Chapelle (2008) was used to classify redox 

conditions for the 49 wells sampled by the USGS-GAMA 

Priority Basin Project and is given in appendix A (table A5). 

Groundwater was oxic in 92 percent of the wells and anoxic in 

8 percent of the wells.
Potential Explanatory Factors 31

Figure 6. Well depths, depths to top-of-perforations, and perforation lengths for grid and understanding wells, Borrego Valley, 

Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project, 

December 2008 to March 2010.

Well

depth

Top-ofperforationsPerforation

length

Well

depth

Top-ofperforationsPerforation

length

Grid wells

42

Understanding wells

32 31 3 3 3

1,000

1,200

800

600

400

200

0

Depth below land surface or length, in feet

32 Number of wells with data

75th percentile

Median

25th percentile

Data point that is outside of

1.5×IQR of the 25th percentile

Nearest data point that is within

1.5×IQR of the 75th percentile

Nearest data point that is within

1.5×IQR of the 25th percentile

Interquartile

range (IQR)

EXPLANATION

IP027935_Figure 06_box plots
32 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 7. Relation of groundwater age classification to (A) depth to top-of-perforations and to (B) well depth, and (C) age classification 

in relation to well depth classification for USGS-grid wells and USGS-understanding wells, Borrego Valley, Central Desert, and Low-Use 

Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project, December 2008–March 2010.

Modern

Mixed

Pre-modern

NUMBER OF WELLS

0 5 10 15 20 25

2 4 30 0

200

400

Depth to top-of-perforations,

in feet below land-surface datum

600

800

1,200

1,000

GROUNDWATER AGE CLASSIFICATION

GROUNDWATER AGE

 CLASSIFICATION

EXPLANATION

A

B

C

Modern Mixed Pre-modern

4 5 36 0

200

400

Well depth,

in feet below land-surface datum

600

800

1,200

1,000

GROUNDWATER AGE CLASSIFICATION

Modern Mixed Pre-modern

Depth of perforations below land-surface datum

4 Number of wells with data

75th percentile

Median

25th percentile

Data point that is outside of

1.5×IQR of the 25th percentile

Data point that is outside of

1.5×IQR of the 75th percentile

Nearest data point that is within

1.5×IQR of the 75th percentile

Nearest data point that is within

1.5×IQR of the 25th percentile

Interquartile

range (IQR)

EXPLANATION

Entire perforation

interval is within

200 feet of land

surface

Entire perforation

interval is equal to

or greater than

200 feet of land

surface

Top of perforation

interval less than

200 feet;

Bottom of perforation

interval equal to or

greater than 200 feet

IP027935_Figure 07abc
Status of Water Quality 33

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. More than 

10,000 individual analytical results were included in the 

assessment of groundwater quality for the CLUB study 

unit. The status assessment applies only to constituents 

with regulatory or non-regulatory health-based or aesthetic/

technical-based benchmarks established by the USEPA or the 

CDPH (as of 2012). 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 

aquifer system are weighted equally; regions with a high 

density of groundwater use or with high density of potential 

contaminants were not preferentially sampled or represented 

(Belitz and others, 2010). 

The following discussion of the status assessment 

results is divided into inorganic, organic, and specialinterest constituents. The assessment begins with a survey 

of how many constituents were detected compared to the 

number analyzed and a graphical summary of the relativeconcentrations of constituents detected in the grid wells. 

Results are presented for those constituents that met criteria 

for selection for additional evaluation based on concentration, 

or for organic constituents, prevalence. 

The aquifer-scale proportions calculated by using the 

spatially weighted approach were within the 90-percent 

confidence intervals for their respective grid-based aquifer 

high proportions for all constituents listed in table 5, providing 

evidence that the grid-based approach yields results that are 

statistically equivalent to those calculated using the spatially 

weighted approach. 

Inorganic Constituents

Inorganic constituents generally occur naturally in 

groundwater, although their concentrations may be influenced 

by human activities as well as by natural factors. All 47 

inorganic constituents analyzed by the USGS-GAMA 

were detected in the CLUB study unit; 26 of the 47 had 

regulatory or non-regulatory health-based benchmarks, 8 

had non-regulatory aesthetic-based benchmarks, and 13 

had no established benchmarks (table 6). Twelve inorganic 

constituents—arsenic, boron, fluoride, molybdenum, 

vanadium, manganese, chloride, sulfate, TDS, gross alpha 

radioactivity, uranium, and nitrate—met the selection criterion 

of having maximum relative-concentrations greater than 0.5 

(moderate or high) in the grid-well dataset (fig. 8; table 5). 

The relative-concentrations of these 12 inorganic constituents 

detected in grid wells from the 3 study areas are shown in 

figure 9. Two other constituents, chromium and iron, were 

detected at high concentrations in CDPH wells between 

December 3, 2005, and December 1, 2008 (table 5), but not in 

the grid wells. 

Inorganic constituents with health-based benchmarks 

(trace and minor elements, uranium and radioactive 

constituents, and nutrients), as a class, had high relativeconcentrations in 48 percent of the primary aquifer system and 

moderate relative concentrations in 26 percent (table 7A). At 

least one inorganic constituent with a health-based benchmark 

was detected at a high relative-concentration in each of the 

study areas. Inorganic constituents with aesthetic-based SMCL 

benchmarks, as a class, had high relative-concentrations in 

13 percent of the primary aquifer system and moderate relativeconcentrations in 39 percent (table 7A). The spatial distributions 

of the eleven inorganic constituents that were present at 

high relative-concentrations in greater than 2 percent of the 

primary aquifer system (table 5) are shown in figures 10A–K. 

These maps show inorganic constituent data for USGS-grid 

and USGS-understanding wells from December 2008 to 

March 2010 and for CDPH-grid wells and other CDPH wells 

from December 3, 2005, to December 1, 2008.
34 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit Table 5. Aquifer-scale proportions from grid-based and spatially weighted approaches for (1) constituents with high relative-concentrations during December 3, 2005– December 1, 2008, from the California Department of Public Health (CDPH) database, (2) constituents with moderate or high relative-concentrations in samples collected from USGS-grid wells (December 2008–March 2010), or (3) organic constituents with detection frequencies of greater than 10 percent in the USGS-grid wells (December 2008– March 2010), Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.

[Relative-concentration categories: high; concentrations of inorganic or organic or special-interest constituents greater than water-quality benchmark; moderate, concentrations of inorganic constituents greater than or equal to 0.5 of benchmark but less than benchmark or concentrations of organic or special-interest constituents greater than or equal to 0.1 of benchmark but less than benchmark; low, concentrations of inorganic constituents less than 0.5 of benchmark or concentrations of organic or special-interest constituents less than 0.1 of benchmark or not detected. Benchmark types and values for constituents listed in tables 2 and 3]

Constituent

Raw detection 

frequency 1

Spatially weighted 

aquifer-scale proportion 1

Grid-based 

aquifer-scale proportion

90-percent confidence interval 

for grid-based high proportion 2

Number 

of wells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of cells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of wells

Moderate 

values 

(percent)

High 

values 

(percent)

Lower limit 

(percent)

Upper limit 

(percent)

Trace and minor elements

Arsenic 110 14 18 46 19 19 46 23 18 8.3 26Boron 90 10 5.6 44 15 11 44 17 10 4.0 19Chromium 104 0 1.0 45 0 0.4 45 0 0 0 3.0Fluoride 111 14 20 46 15 28 46 22 27 15 44Molybdenum 42 9.5 17 40 8.5 16 40 10 16 7.8 27Vanadium 87 12 3.4 44 12 2.3 44 13 1.5 0.3 9.9

Radioactive constituents

Gross alpha activity 3 90 22 8.9 39 23 8.5 38 24 9.7 4.6 21

Uranium 80 16 6.3 45 6.7 4.4 44 4.3 5.6 2.6 16

Nutrients

Nitrate 131 4.6 2.3 48 6.5 2.2 48 2.7 2.7 0.4 7.9

Inorganic constituents with SMCLs

Iron 107 1.9 3.7 45 1.9 2.9 44 0 0 0 3.0Manganese 107 0 2.8 45 0 3.1 44 0 2.8 0.4 8.5Chloride 107 1.9 0.9 44 4.2 2.8 44 4.2 2.8 0.4 8.5Sulfate 107 10 0 44 18 0 44 14 0 0 3.0Total dissolved solids (TDS) 111 31 4.5 45 37 11.8 49 41 11.0 4.8 19

Volatile organic compounds (VOCs)

Benzene 94 1.5 0 49 0.9 0 49 0 0 0 2.7

Tetrachloroethene (PCE) 94 1.5 0 49 1.4 0 49 0 0 0 2.7Bromoform 94 3.0 0 49 1.4 0 49 0 0 0 2.7Chloroform 94 2.1 0 49 1.2 0 49 0 0 0 2.7Dibromochloromethane 94 3.0 0 49 1.4 0 49 0 0 0 2.7

Pesticides

Dieldrin 68 1.5 0 48 1.4 0 47 2.8 0 0 2.8

Special interest

Perchlorate 145 8.0 0 49 24 0 49 32 0 0 2.7

1 Based on the most recent data for each CDPH well during the period December 3, 2005—December 1, 2008, combined with GAMA grid and understanding well data.2 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001).3 Gross alpha activities were not adjusted for uranium activity. The MCL-US for gross alpha activity applies to adjusted gross alpha activity.
Status of Water Quality 35

Table 6. Numbers of constituents analyzed and detected by the 

U.S. Geological Survey by constituent class and benchmark type, 

Borrego Valley, Central Desert, and Low-Use Basins of the Desert 

study unit, California GAMA Priority Basin Project, December 2008 

to March 2010.

Benchmark type

Number of constituents

Analyzed Detected 

Volatile organic compounds (VOCs)

Regulatory health-based 33 6

Non-regulatory health-based 25 0

No benchmark 27 0

Total: 85 6

Pesticides

Regulatory health-based 3 2

Non-regulatory health-based 17 2

No benchmark 43 2

Total: 63 6

Special interest

Regulatory health-based 1 1

Non-regulatory health-based 1 0

No benchmark 0 0

Total: 2 1

Inorganic

Regulatory health-based 21 21

Non-regulatory health-based 5 5

Non-regulatory aesthetic-based 8 8

No benchmark 13 13

Total: 47 47

Geochemical and age-dating tracers

Regulatory 2 2

Non-regulatory 0 0

No benchmark 13 13

Total: 15 15

All constituents

Regulatory health-based 60 32

Non-regulatory health-based 48 7

Non-regulatory aesthetic-based 8 8

No benchmark 96 28

Total: 212 75
36 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 8. Maximum relative-concentrations of constituents detected in grid wells, by constituent 

class, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts 

study unit, California GAMA Priority Basin Project, December 2008–March 2010.

Moderate

Low

Moderate

Low

High High

Organic and specialinterest constituentsInorganic

constituents

Maximum relative-concentration, dimensionless

Volatile

organic

compounds

Pesticides Special

interest

Nutrients Trace

elements

Major and

minor ions

and trace

elementsSMCL

Uranium and

radioactive

constituents

0.01

0.001

0.1

1

10

100

Simazine

Perchlorate

Tetrachloroethene

(PCE)

Trichloroethene (TCE)

Chloroform

Bromoform

Benzene

Atrazine

Dieldrin

Nitrite-N

Ammonia-N

Nitrate-N TDS

Sulfate

Iron

Manganese

Chloride

Molybdenum

Fluoride

Selenium

Boron

Lead

Zinc

Copper

Gross alpha radioactivity

Gross beta radioactivity

Radon-222

Radium-228

Radium-226

Vanadium

Uranium

Chromium

Antimony

Beryllium

Arsenic

Aluminum

Thallium

Barium

Nickel

Strontium

Cadmium

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

 Unless indicated by following location line: (data for 39 to 50 grid wells) 

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

 Unless indicated by following location line: (data for 22 to 38 grid wells) 

EXPLANATION

IP027935_Figure 08

Radon-222

Abbreviations: N is nitrogen, TDS is total dissolved solids

Number of 1 1 0 0 0 0 0

compounds

detected

< 0.001
Status of Water Quality 37

Figure 9. Relative-concentrations of inorganic constituents in samples from grid wells with 

(A) health-based or (B) aesthetic benchmarks categorized as high, medium, or low in grid wells, 

Borrego, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, 

California GAMA Priority Basin Project, December 2008–March 2010.

0.001

0.01

0.1

1

10

100

RELATIVE-CONCENTRATION, DIMENSIONLESS

Arsenic Boron Fluoride Molybdenum Uranium Vanadium

A

IP026177_Figure 09a

Nitrate as

Nitrogen

Gross alpha

radioactivity

CONSTITUENTS THAT HAVE HEALTH-BASED THRESHOLDS

(MAXIMUM CONTAMINANT LEVEL, HEALTH

ADVISORY LEVEL, NOTIFICATION LEVEL) 

RELATIVE-CONCENTRATION

High

Moderate

Low

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts
38 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 9. —Continued

IP027935_Figure 09b

0.001

0.01

0.1

1

10

B

Chloride Manganese Sulfate Total

dissolved

solids

CONSTITUENTS THAT HAVE SECONDARY

MAXIMUM CONTAMINANT LEVELS

RELATIVE-CONCENTRATION, DIMENSIONLESS

RELATIVE-CONCENTRATION

High

Moderate

Low

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts
Status of Water Quality 39

Figure 10. Relative-concentrations of selected inorganic constituents for the U.S. Geological Survey (USGS) grid wells and 

California Department of Public Health (CDPH) wells (data from the period December 3, 2005–December 1, 2008), Borrego, Central 

Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project: (A) arsenic, 

(B) boron, (C) fluoride, (D) molybdenum, (E) uranium, (F) gross alpha radioactivity, (G) nitrate, (H) total dissolved solids, (I) chloride, 

(J) iron, and (K) manganese.

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

IP027935_Figure 10a

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

A

ARSENIC

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in micrograms

per liter

Relativeconcentration

Low

Moderate

High

< 5

>5 to < 10

>10
40 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

IP027935_Figure 10b

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

B

BORON

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in micrograms

per liter

Relativeconcentration

Low

Moderate

High

< 500

>500 to < 1,000

>1,000
Status of Water Quality 41

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

IP027935_Figure 10c

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

C

FLUORIDE

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in milligrams

per liter

Relativeconcentration

Low

Moderate

High

< 1

>1 to < 2

>2
42 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

IP027935_Figure 10d

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

D

MOLYBDENUM

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS-grid

well 

USGSunderstanding

well 

Concentration,

in micrograms

per liter

Relativeconcentration

Low

Moderate

High

< 20

>20 to < 40

>40
Status of Water Quality 43

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

IP027935_Figure 10e

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

E

URANIUM

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in micrograms

per liter

Relativeconcentration

Low

Moderate

High

< 15

>15 to < 30

>30
44 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

IP027935_Figure 10f

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

F

GROSS ALPHA RADIOACTIVITY (72-HOUR COUNT)

STUDY AREA

Borrego Valley

Central Desert

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in picocuries

per liter

Relativeconcentration

Low

Moderate

High

< 7.5

>7.5 to < 15

>15
Status of Water Quality 45

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

IP027935_Figure 10g

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

G

NITRATE, AS NITROGEN

LAND-USE

CLASSIFICATION

Urban

Agricultural

Natural

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in milligrams

per liter

Relativeconcentration

Low

Moderate

High

< 5

>5 to < 10

>10
46 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

IP027935_Figure 10h

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

H

TOTAL DISSOLVED SOLIDS (TDS)

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in milligrams

per liter

Relativeconcentration

Low

Moderate

High

< 500

>500 to < 1,000

>1,000

 
Status of Water Quality 47

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

IP027935_Figure 10i

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

I

CHLORIDE

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

Concentration,

in milligrams

per liter

Relativeconcentration

Low

Moderate

High

< 250

>250 to < 500

>500
48 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

Streams

STUDY AREA

Borrego Valley

Central Desert

Counties

IP027935_Figure 10j

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

Lake

Dry lake

J

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Low-Use Basins

of the Mojave and

Sonoran Deserts

San Diego

Los

Angeles

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

IRON

Concentration,

in micrograms

per liter

Relativeconcentration

Low

Moderate

High

< 150

>150 to < 300

>300

Figure 10. —Continued
Status of Water Quality 49

Figure 10. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

Streams

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

Counties

IP027935_Figure 10k

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

Lake

Dry lake

K

USGS- and

CDPH-grid

well 

USGSunderstanding

well 

CDPHother 

well 

MANGANESE

Concentration,

in micrograms

per liter

Relativeconcentration

Low

Moderate

High

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

< 25

>25 to < 50

>50
50 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Table 7A. Aquifer-scale proportions for inorganic constituent classes, Borrego Valley, Central Desert, and Low-Use Basins of the 

Desert study unit, California GAMA Priority Basin Project, December 2008–March 2010.

[Relative-concentration categories: high; concentrations of constituents greater than water-quality benchmark; moderate, concentrations of constituents greater 

than or equal to 0.5 of benchmark but less than benchmark; low, concentrations of constituents less than 0.5 of benchmark. SMCL, secondary maximum 

contaminant level]

Constituent class

Aquifer-scale proportion

Low relativeconcentration (percent)Moderate relativeconcentration (percent)High relativeconcentration (percent)

Inorganic constituents with health-based benchmarks

Trace and minor elements 35 23 42

Uranium and radioactive constituents 1 68 20 12

Nutrients 95 2.7 2.7

Total for inorganic constituents with health-based benchmarks 1 26 26 48

Inorganic constituents with SMCL benchmarks

Salinity Indicators 2 50 39 11

Manganese and (or) iron 95 3

 1.9 2.8

Total for inorganic constituents with aesthetic benchmarks 48 39 13

1

 Aquifer-scale proportions for the classes uranium and radioactive constituents and all inorganic constituents with health-based benchmarks were calculated 

using unadjusted gross alpha activity.

2

 Salinity indicators are chloride, sulfate, and total dissolved solids.

3

 Spatially weighted result. Grid-based result was 0 percent.

Table 7B. Aquifer-scale proportions for organic constituent classes, Borrego Valley, Central Desert, and Low-Use Basins of the Desert 

study unit, California GAMA Priority Basin Project, December 2008–March 2010.

[Relative-concentration categories: high; concentrations of constituents greater than water-quality benchmark; moderate, concentrations of constituents greater 

than or equal to 0.1 of benchmark but less than benchmark; low, concentrations of constituents less than 0.1 of benchmark]

Constituent class

Aquifer-scale proportion

Low relative-concentration (percent) Moderate relativeconcentration 

(percent) 1

High relativeconcentration 

(percent) Not detected Detected low 2

Gasoline hydrocarbons 97 1.9 0.9 0

Solvents 82 16 1.4 0

Trihalomethanes 71 27 2.2 0

Any volatile organic compound (VOC) 61 37 2.2 0

Insecticides 99 0 1.4 0

Any pesticide 90 8.2 1.4 0

Total for organic constituents with health-based benchmarks 60 35 5.0 0

1

 Proportions for moderate relative-concentrations were calculated using the spatially weighted approach.

2

 Proportions for low relative-concentrations were calculated by subtracting the spatially weighted moderate proportion from the area-weighted detection 

frequency in the USGS-grid wells.
Status of Water Quality 51

Trace and Minor Elements 

Trace and minor elements, as a class, had high relativeconcentrations (for one or more constituents) in 42 percent of 

the primary aquifer system, moderate relative-concentrations 

in 23 percent, and low relative-concentrations in 35 percent 

(table 7A). High relative-concentrations of trace and minor 

elements result from the high relative-concentrations of 

fluoride (27 percent), arsenic (18 percent), molybdenum 

(16 percent), boron (10 percent), and vanadium (1.5 percent) 

(table 5). One trace element, chromium, had spatially 

weighted high relative-concentrations in 0.4 percent of the 

aquifer area, which was within the 90-percent confidence 

interval for this element for the grid-based approach (table 5). 

The spatially weighted approach includes data from a larger 

number of wells than the grid-based approach, and therefore 

is more sensitive to detecting constituents present at very low 

proportions of the primary aquifer system.

The trace elements antimony, lead, and selenium had 

high relative-concentrations in at least one well reported in 

the CDPH database before December 3, 2005 (table 4), but 

not during the current period of study (December 3, 2005–

December 1, 2008); these high values represent historical 

values rather than current values.

Arsenic was detected at high relative-concentrations in 

18 percent of the primary aquifer system, and at moderate 

relative-concentrations in 23 percent (table 5). It was detected 

at high relative-concentrations in the CD and LUB study areas 

(figs. 9A, 10A). The highest relative-concentration was in 

the CD study area (fig. 9A). In the LUB study area, high and 

moderate relative-concentrations were distributed throughout 

the area (fig. 10A). 

Boron was detected at high relative-concentrations in 

10 percent of the primary aquifer system, and at moderate 

relative-concentrations in 17 percent (table 5). High and 

moderate relative-concentrations of boron occurred only in the 

LUB study area (figs. 9A, 10B). 

Fluoride, a minor ion, was detected at high relativeconcentrations in 27 percent of the primary aquifer system 

and at moderate relative-concentrations in 22 percent 

(table 5). Fluoride was detected at high and moderate relativeconcentrations in all three study areas (figs. 9A, 10C). 

Molybdenum was detected at high relative-concentrations 

in 16 percent of the primary aquifer system and at moderate 

relative-concentrations in 10 percent (table 5). It was detected 

at high relative-concentrations in the CD and LUB study areas 

(figs. 9A, 10D). 

Uranium and Radioactive Constituents

Uranium and radioactive constituents had high relativeconcentrations for one or more constituents in 12 percent of 

the primary aquifer system, moderate relative-concentrations 

in 20 percent, and low relative-concentrations in 68 percent 

(table 7). The high and moderate relative-concentrations 

reflect detections of uranium and gross alpha radioactivity 

(table 5). The other radioactive constituents, such as radon222, radium-226, and radium-228, were detected at low 

concentrations throughout the CLUB study unit. In addition, 

radium-226 and radium-228 were detected at high relativeconcentrations in at least one well reported in the CDPH 

database before December 3, 2005, but not during the current 

period of study (table 4); these high values represent historical 

values rather than current values.

Uranium was detected at high relative-concentrations 

in 5.6 percent of the primary aquifer system and at moderate 

relative-concentrations in 4.3 percent (table 5). It was detected 

at high relative-concentrations in the CD and LUB study 

areas and at moderate relative-concentrations in all three 

study areas (figs. 9A, 10E). Uranium was detected at high 

relative-concentration in a grid well sampled by the CDPH 

during December 3, 2005, to December 1, 2008; however, 

this result was not the most recent value from the CDPH 

database representing that well and was not used in the status 

assessment.

Unadjusted gross alpha radioactivity was detected at 

high relative-concentrations in 9.7 percent of the primary 

aquifer system and at moderate relative-concentrations in 

24 percent (table 5). It was detected at high and moderate 

relative-concentrations in grid wells in all three study areas 

(figs. 9A, 10F). Of the 14 wells with moderate or high relativeconcentrations of gross alpha radioactivity, only 3 wells also 

had moderate or high relative-concentrations of uranium. 

Gross alpha radioactivity was detected at a high relativeconcentration in one well by the CDPH during December 3, 

2005, to December 1, 2008; however, this result was not the 

most recent value from the CDPH database representing that 

particular well. 

Nutrients

Nutrients, as a class, were detected at high relativeconcentrations in 2.7 percent of the primary aquifer system 

and at moderate relative-concentrations in 2.7 percent 

(table 7A) resulting from the detection of nitrate (table 5). 

Ammonia and nitrite were detected only at low concentrations. 

Nitrate was detected at high relative-concentrations in all three 

study areas and at moderate relative-concentrations in the CD 

and LUB study areas (figs. 9A, 10G). 
52 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Inorganic Constituents with Aesthetic 

Benchmarks

For selected constituents, the CDPH has established 

SMCL-CA benchmarks, which are non-enforceable 

benchmarks based on aesthetic properties rather than on 

health-based concerns. For TDS and the major ions chloride 

and sulfate, the CDPH defines a “recommended” and an 

“upper” SMCL-CA. The “upper” SMCL-CA benchmarks 

were used for computing relative-concentrations in this 

report. Inorganic constituents with SMCLs had high relativeconcentrations in 13 percent of the primary aquifer system and 

moderate relative-concentrations in 39 percent of the primary 

aquifer system (table 7A).

TDS were detected at high relative-concentrations in 

11 percent of the primary aquifer system and at moderate 

relative-concentrations in 41 percent (table 5). TDS were 

detected at moderate relative-concentrations in all study areas 

of the CLUB study unit, and at high relative-concentrations in 

the LUB study area (figs. 9B, 10H). 

Chloride was detected at high relative-concentrations 

in 2.8 percent of the primary aquifer system and at moderate 

relative-concentrations in 4.2 percent (table 5). All samples 

having high or moderate relative-concentrations of chloride 

also had high or moderate relative-concentrations of TDS. 

Chloride had high and moderate relative-concentrations only 

in the LUB study area (figs. 9B, 10I). 

Iron had spatially weighted high relative-concentrations 

in 2.9 percent of the primary system and spatially weighted 

moderate relative-concentrations in 1.9 percent (table 5). High 

relative-concentrations of iron were observed in all three study 

areas (fig. 10J). Iron was not detected at high or moderate 

relative-concentrations in the grid wells (fig. 8).

Manganese had high relative-concentrations in 

2.8 percent of the primary aquifer system (table 5). High 

relative-concentrations of manganese were detected in the BV 

and LUB study areas (figs. 9B, 10K). 

Organic Constituents 

The organic constituents are organized into two classes: 

VOCs and pesticides. VOCs may be in paints, solvents, fuels, 

and refrigerants; VOCs also can be byproducts of water 

disinfection and are characterized by their volatile nature or 

tendency to evaporate. Pesticides are used to control weeds, 

insects, or fungi in agricultural, urban, and suburban settings.

Organic constituents, as a class, were not detected at 

high relative-concentrations in the primary aquifer system 

and were detected at moderate relative-concentrations in 

5.0 percent of the primary aquifer system (table 7B). The 

trihalomethane chloroform and the solvent tetrachloroethene 

(PCE) were detected in more than 10 percent of the primary 

aquifer system (figs. 11, 12). The spatial distributions of the 

detections of PCE and chloroform are illustrated on maps 

showing data for USGS-grid and USGS-understanding wells 

from December 2008 to March 2010 and for CDPH-grid wells 

and other wells from December 3, 2005, to December 1, 2008 

(figs. 13A,B). Of the 148 organic constituents analyzed for 

in the CLUB study unit, 12 were detected (table 6). Of these 

12 organic constituents detected, 10 had regulatory or nonregulatory health-based benchmarks (fig. 11; table 6). The 

individual constituents that were not detected and the wells 

that were sampled in the study unit are listed in Mathany and 

others (2012).

Volatile Organic Compounds (VOCs)

VOCs were not detected at high relative-concentrations 

in the primary aquifer system and were detected at moderate 

relative-concentrations in 2.2 percent (table 7B). The areaweighted detection frequency of VOCs (one or more) in the 

CLUB primary aquifer system was 39 percent. Six VOCs 

with health-based benchmarks were detected in USGSgrid wells at low relative-concentrations only (fig. 11). The 

solvent tetrachloroethene (PCE) and the gasoline hydrocarbon 

benzene were reported at moderate relative-concentrations in 

one well each in the CDPH database between December 3, 

2005, and December 1, 2008, and the trihalomethanes 

chloroform, dibromochloromethane, and (or) bromoform were 

reported at moderate relative-concentrations in five CDPH 

wells. PCE and the trihalomethane chloroform were the only 

VOCs with area-weighted detection frequencies greater than 

10 percent in the primary aquifer system (figs. 11–13). Both 

are among the most commonly detected VOCs in groundwater 

nationally (Zogorski and others, 2006).

Solvents are used for various industrial, commercial, 

and domestic purposes. The solvent PCE had a spatially 

weighted moderate aquifer-scale proportion of 1.4 percent 

(tables 5, 7B). Gasoline hydrocarbons are used to increase the 

efficiency of combustion of fuels. The hydrocarbon benzene 

had a spatially weighted moderate aquifer-scale proportion of 

0.9 percent (tables 5, 7B).

Water used for drinking water in domestic and municipal 

systems commonly is disinfected with chlorine solutions 

(bleach). As a side effect of disinfection, the chlorine reacts 

with organic matter and dissolved bromide to produce 

trihalomethanes (THMs) and other disinfection byproducts. 

Chloroform, dibromochloromethane, and bromoform had 

spatially weighted moderate aquifer-scale proportions of 1.2, 

1.4, and 1.4 percent, respectively (table 5), and the moderate 

aquifer-scale proportion for THMs as a class was 2.2 percent 

(table 7B).
Status of Water Quality 53

Figure 11. Detection frequency and maximum relative-concentration of organic and special-interest constituents detected in USGSgrid wells in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA 

Priority Basin Project.

Moderate

Low

High

RELATIVE-CONCENTRATION, DIMENSIONLESS 0.001

0.0001

0.00001

0.01

0.1

1

10

EXPLANATION

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

 Unless indicated by following location line: (49 grid wells sampled) 

1 10 100

DETECTION FREQUENCY AT ANY CONCENTRATION, IN PERCENT

IP027935_Figure 11

Simazine

Atrazine

Dieldrin

Bromoform

Metolachlor

Bromodichloromethane

Chloroform

Perchlorate

Trichloroethene (TCE)

Benzene

Tetrachloroethene (PCE)
54 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 12. (A) Detection frequency and (B) relative-concentrations of selected organic and special-interest constituents in grid wells 

in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority 

Basin Project.

10% detection line

STUDY UNIT

DETECTION FREQUENCY,

IN PERCENT STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

A B

RELATIVE-CONCENTRATION

High

Moderate

Low

NUMBER OF

USGS-GRID WELLS

WITH DETECTION OF

THE CONSTITUENT

14

23

4

1

0

0

3

9

1

0

5

1

0.0001 0.001 0.01 0.1

RELATIVE-CONCENTRATION, DIMENSIONLESS

0 20 40 60 80 100 1 10

Chloroform

Tetrachloroethene

(PCE)

Dieldrin

Perchlorate
Status of Water Quality 55

Figure 13. Concentrations of selected organic and special-interest constituents in the U.S. Geological Survey (USGS) grid wells 

and California Department of Public Health (CDPH) wells (data from the period December 3, 2005–December 1, 2008), Borrego, 

Central Desert, and Low-Use Basins of the Desert study unit, California GAMA Priority Basin Project: (A) tetrachloroethene (PCE), 

(B) chloroform, and (C) perchlorate.

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

IP027935_Figure 13a

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

A

TETRACHLOROETHENE (PCE)

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

USGSgrid

well 

USGSunderstanding

well 

Measured

concentration,

in micrograms

per liter

Relativeconcentration

Not detected (<RL)

Low

Moderate

< 0.02

>0.02 to < 0.5

>0.5 to < 5

< 0.5

>0.5 to < 5

Streams

Counties

Lake

Dry lake

CDPHwell 
56 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure 13. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

IP027935_Figure 13b

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

B

CHLOROFORM

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

Not detected (<RL)

Low

Moderate

<0.01

>0.01 to < 8

<0.5

>0.5 to < 5

Streams

Counties

Lake

Dry lake

USGSgrid

well 

USGSunderstanding

well 

Measured

concentration,

in micrograms

per liter

RelativeconcentrationCDPHwell 
Status of Water Quality 57

Figure 13. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

IP027935_Figure 13c

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro

Pacific Ocean

MEXICO

C

PERCHLORATE

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

Streams

Counties

Lake

Dry lake

USGSgrid

well 

USGSunderstanding

well 

Measured

concentration,

in micrograms

per liter

Relativeconcentration

Not detected (<RL)

Low

Moderate

<0.1

>0.1 to <0.6

>0.6 to <6
58 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Pesticides

Pesticides were not detected at high relativeconcentrations in the primary aquifer system, and were 

detected at moderate relative-concentrations in 1.4 percent 

(table 7B). The area-weighted detection frequency of 

pesticides (one or more) in the primary aquifer system was 

9.6 percent (table 7B). Three herbicides were detected in 

USGS-grid wells at low relative-concentrations, and the 

insecticide dieldrin was detected at moderate relativeconcentrations (fig. 11). No individual pesticide constituent 

had an area-weighted detection frequency greater than 

10 percent.

No detections of pesticides were reported in the CDPH 

database from December 3, 2005, to December 1, 2008. High 

values for the insecticide aldrin and the degradate aldicarb 

sulfone were recorded in the CDPH database for the period 

before December 3, 2005 (table 4), but not during the current 

period of study. Aldrin is an organochlorine insecticide 

that was widely used until the 1970s to treat seed and soil. 

Aldicarb sulfone is a degradate of the carbamate insecticide 

and pesticide aldicarb.

Special-Interest Constituents

The special-interest constituents analyzed 

for in the CLUB study unit were perchlorate and 

N-nitrosodimethylamine (NDMA). When the USGS began 

sampling for the GAMA Priority Basin Project in 2004, these 

constituents were selected as special-interest constituents 

because they recently had been detected in drinking-water 

supplies or were considered to have the potential to reach 

drinking-water supplies (California Department of Public 

Health, 2008a,b). An MCL-CA benchmark for perchlorate was 

promulgated in 2007. Perchlorate was not detected at high 

relative-concentrations in the primary aquifer system and was 

detected at moderate relative-concentrations in 32 percent of 

the primary aquifer system (table 5). Perchlorate was detected 

at moderate or low relative-concentrations in 84 percent of 

the primary aquifer system (figs. 11, 12). Moderate relativeconcentrations of perchlorate were detected in the BV and 

LUB study areas, and low relative-concentrations were 

detected in all three study areas (fig. 13C). NDMA, a semivolatile organic chemical, was not detected in the grid wells 

sampled (Mathany and others, 2012).

Summary 

Groundwater quality in the approximately 963-squaremile Borrego Valley, Central Desert, and Low-Use Basins of 

the Mojave and Sonoran Deserts (CLUB) study unit, which 

includes the Borrego Valley (BV), Central Desert (CD), 

and Low-Use Basins of the Mojave and Sonoran Desert 

(LUB) study areas, was investigated as part of the Priority 

Basin Project of the Groundwater Ambient Monitoring 

and Assessment (GAMA) Program. The CLUB study was 

designed to provide a spatially unbiased characterization of 

untreated groundwater quality in the primary aquifer system 

within this study unit. The assessment is based on waterquality and ancillary data collected during 2008–2010 by 

the U.S. Geological Survey (USGS) from 51 wells and on 

water-quality data from the California Department of Public 

Health (CDPH) database (for the period December 3, 2005–

December 1, 2008). 

The status of the current quality of the groundwater 

resource was assessed by evaluating 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 characterizes 

the quality of groundwater resources in the primary aquifer 

system of the BV, CD, and LUB study areas, 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 and (or) California regulatory or nonregulatory 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 aquifer system 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 aquifer system 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 CLUB study 

unit (within 90-percent confidence intervals). The spatially 

weighted approach is more likely to reflect concentrations of 

a constituent that is high in a small fraction of the primary 

aquifer system.
References 59

Relative-concentrations of one or more inorganic 

constituents with health-based benchmarks were high in 

48 percent of the primary aquifer system, moderate in 

26 percent, and low (or not detected) in 26 percent. The 

high aquifer-scale proportion of inorganic constituents with 

health-based benchmarks primarily reflected high aquiferscale proportions of fluoride (27 percent), arsenic (18 percent), 

molybdenum (16 percent), boron (10 percent), uranium 

(5.6 percent), gross alpha activity (9.7 percent), and nitrate 

(2.7 percent). Three inorganic constituents with secondary 

maximum contaminant levels, total dissolved solids, 

manganese, and chloride, had high relative-concentrations in 

11 percent, 2.8 percent, and 2.8 percent of the primary aquifer 

system, respectively. 

Relative-concentrations of one or more organic 

constituents were high in 0 percent, moderate in 5.0 percent, 

and low in 35 percent of the primary aquifer system. 

Organic constituents were not detected in 60 percent of the 

primary aquifer system. Of the 148 organic constituents 

analyzed, 12 constituents were detected. Chloroform and 

tetrachloroethene (PCE) were the only organic constituents 

detected in more than 10 percent of the primary aquifer 

system. 

Acknowledgments

The authors thank the following cooperators for their 

support: the California State Water Resources Control 

Board, Lawrence Livermore National Laboratory, California 

Department of Public Health, and California Department of 

Water Resources. We especially thank the cooperating well 

owners and water purveyors for their generosity in allowing 

the 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 California Water Board.

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California, California Environmental Protection Agency, 

State Water Resources Control Board, Division of Water 

Quality.

California State Water Resources Control Board, 2010, 

GAMA—Groundwater Ambient Monitoring and 

Assessment Program, accessed September 9, 2010, at http://

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Appendix A. Ancillary Datasets 65

Well identification numbers are presented in table A1. 

Land-use classifications and percentages of each classification, 

well construction information, groundwater age data and 

classifications, and geochemical conditions are listed in 

tables A2–A5. 

Land-Use Classification

Land use was classified by using an enhanced version 

of the satellite-derived [98-ft (30-m) pixel resolution] USGS 

Enhanced 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. The imagery 

was classified into 25 land-cover classifications (Nakagaki 

and Wolock, 2005). These 25 land-cover classifications were 

aggregated into three principal land-use classes—urban, 

agricultural, and natural. Average land use (proportions of 

urban, agricultural, and natural) for the study unit, for each 

study area, and for areas within a radius of 1,640 ft (500 m) 

surrounding each well (table A2) were calculated using 

ArcGIS (version 9.2) (Johnson and Belitz, 2009). 

Well Construction Information

Well construction data primarily were obtained from 

driller’s logs filed with CDWR. Other sources of well 

construction data were ancillary records from well owners, 

and the USGS National Water Information System database. 

Well construction data are not available in the CDPH database. 

Well identification verification procedures are described by 

Mathany and others (2012). Well depths, depths to the tops 

and bottoms of the perforated intervals, and lengths of the 

perforated intervals for wells are listed in table A3.

Groundwater Age Classification

Groundwater dating techniques provide a measure of 

the time since the groundwater was last in contact with the 

atmosphere (residence time in the aquifer). The techniques 

used in this report to estimate groundwater residence times or 

‘age’ were those based on tritium (for example: Tolstikhin and 

Kamenskiy, 1969; Torgersen and others, 1979) and carbon-14 

activities (for example: Vogel and Ehhalt, 1963; Plummer and 

others, 1993). 

Appendix A. Ancillary Datasets

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; Kulongoski and others, 2010; Landon and others, 2010). 

For samples collected for the CLUB study unit in 2008–2010, 

tritium values greater than a threshold of 1.0 TU were defined 

as indicating the presence of groundwater recharged since 

1952 (Kulongoski and others, 2010; Landon and others, 

2010). Water recharged since 1952 is defined as “modern” 

groundwater. 

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, and by above-ground 

nuclear explosions. 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 99 pmc is 

assumed for this study, with estimated errors on calculated 

groundwater ages up to ±20 percent. Calculated 14C ages 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 pmc can be >100 in 

groundwater samples containing a significant component 

of water recharged after 1952 because the definition of 

pmc is based on 14C activity of carbon in the absence of 14C 

contributed by above-ground nuclear explosions (Clark and 

Fritz, 1997). For the CLUB study unit, 14C activity <90 pmc 

was defined as indicating the presence of groundwater 

recharged before the modern era (Kulongoski and others, 

2010; Landon and others, 2010). Water recharged before the 

modern era is defined as “pre-modern” groundwater. 
66 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Tritium and 14C data and age classifications are reported 

in table A4. Because of uncertainties in age distributions, 

particularly the uncertainties caused by mixing of waters of 

different ages in wells with long screened or open intervals 

and high withdrawal rates, the uncorrected 14C ages were not 

specifically used for quantifying the relation between age 

and water quality in this report. While more sophisticated 

lumped parameter models for analyzing age distributions that 

incorporate mixing are available (Cook and Böhlke, 2000), 

use of these alternative models to understand age mixtures was 

not needed for the assessments in this report. Classification 

into modern, mixed, and pre-modern categories was sufficient 

to provide an appropriate and useful characterization for the 

purposes of examining groundwater quality.

For the CLUB study unit, groundwater ages were 

classified as follows: 

Classification Tritium (TU) 14C (pmc)

Modern ≥1.0 ≥ 90

Pre-modern < 1.0 < 90 or no data

Mixed ≥ 1.0 < 90

Mixed < 1.0 ≥ 90

Classification of Geochemical Condition

Geochemical conditions investigated as potential 

explanatory variables in this report include oxidationreduction characteristics (table A5). 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 

dominates at a particular time and aquifer location (Chapelle 

and others, 1995; Chapelle, 2001). The predominant TEAPs 

are oxygen-reduction, nitrate-reduction, manganese-reduction, 

iron-reduction, sulfate-reduction, and methanogenesis. The 

presence of redox-sensitive chemical species suggesting 

more than one TEAP may indicate mixed waters from 

different redox zones upgradient of the well, a well screened 

across more than one redox zone, or spatial heterogeneity in 

microbial activity in the aquifer.

Redox conditions were assigned to each sample using a 

modified version of the classification scheme of McMahon 

and Chapelle (2008) and Jurgens and others (2009) (table A5). 

Samples with DO > 0.5 mg/L were classified as oxic, and 

samples with DO ≤ 0.5 mg/L were classified as anoxic. The 

anoxic samples were further classified according to the TEAPs 

inferred from data for nitrate, manganese, and iron. Data for 

these constituents were obtained from USGS-GAMA where 

available and from the CDPH database (“DG” CDPH-grid 

wells). Inorganic constituent data were not available for all 

oxic samples. 
Appendix A. Ancillary Datasets 67

USGSGAMA well 

identification 

number

Grid cell 

number

Grid 

supplemented by 

CDPH data from 

same well

Grid 

supplemented by 

CDPH data from 

different well

Borrego Valley study area grid wells

BV-01 6 — BV-DPH-01

BV-02 2 BV-DG-02 BV-DPH-02

BV-03 3 — —

BV-04 1 — —

BV-05 1 5 BV-DG-05 —

BV-06 8 BV-DG-06 BV-DPH-06

BV-07 9 — —

Central Desert study area grid wells

CD-01 5 — —

CD-02 4 — —

CD-03 3 — —

CD-04 15 — —

CD-05 16 — CD-DPH-05

CD-06 2 CD-DG-06 CD-DPH-06

CD-07 10 — —

CD-08 14 CD-DG-08 —

CD-09 8 — —

CD-10 6 CD-DG-10 —

CD-11 1 — —

CD-12 9 — —

CD-13 17 — —

CD-14 12 — —

CD-15 13 — —

Low-Use Basins study area grid wells

LUB-01 1 — —

LUB-02 2 LUB-DG-02 —

LUB-03 5 — —

LUB-04 3 LUB-DG-04 —

USGSGAMA well 

identification 

number

Grid cell 

number

Grid 

supplemented by 

CDPH data from 

same well

Grid 

supplemented by 

CDPH data from 

different well

Low-Use Basins study area grid wells—Continued

LUB-05 12 LUB-DG-05 LUB-DPH-05

LUB-06 9 LUB-DG-06 —

LUB-07 10 — —

LUB-08 8 LUB-DG-08 —

LUB-09 18 LUB-DG-09 --

LUB-10 17 — —

LUB-11 6 LUB-DG-11 —

LUB-12 16 LUB-DG-12 —

LUB-13 11 — —

LUB-14 14 — —

LUB-15 24 LUB-DG-15 —

LUB-16 36 — —

LUB-17 34 — —

LUB-18 13 — —

LUB-19 15 — —

LUB-20 25 — —

LUB-21 26 — LUB-DPH-21

LUB-22 32 — LUB-DPH-22

LUB-23 23 — —

LUB-24 20 — —

LUB-25 30 LUB-DG-25 —

LUB-26 29 — —

LUB-27 21 — —

— 22 — LUB-DPH-37

— 27 — LUB-DPH-38

USGS-understanding wells

BVU-01 5 — —

LUBU-01 15 — —

LUBU-02 9 — —

1

 Data from USGS-understanding well BVU-01 were used to supplement 

the grid-well data in this cell.

Table A1. USGS-GAMA well identification numbers and grid cell numbers for well data used in the Borrego Valley, Central Desert, and 

Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.

[A USGS-GAMA well identification number indicates the use of USGS data from the grid well; a CDPH-GAMA well identification number with ‘DG’ indicates 

the use of CDPH inorganic data from the grid well; a CDPH-GAMA well identification number with ‘DPH’ indicates the use of CDPH data from a different 

well. BV, Borrego Valley study area well; CD, Central Desert study area well; LUB, Low-Use Basin of the Mojave and Sonoran Deserts study area well; BVU 

or LUBU, understanding well; —, no wells sampled or selected]
68 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Table A2. Well type, percent land use by category, land-use classification, septic density, leaking underground fuel tank density, grid 

cell number, and USGS-GAMA well identification number for GAMA well data and CDPH-grid well data used in the Borrego Valley, 

Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.

[USGS-GAMA well identification number: BV, Borrego Valley study area well; CD, Central Desert study area well; LUB, Low-Use Basins of the 

Mojave and Sonoran Deserts study area well. Well types: PROD, production well; UN, unused well. Other abbreviations: USGS, U.S. Geological Survey; 

CDPH, California Department of Public Health; LUFT, leaking (or formerly leaking) underground fuel tank; na, not available; tanks/km2, number of tanks per 

square kilometer]

USGS-GAMA well 

identification number Well type Land use (in percent) 1 Land-use classification

Septic density 

(tanks/km2

) 2

LUFT density 

(tanks/km2

) 3 Agricultural Natural Urban 

Borrego Valley study area grid wells

BV-01 PROD 0 100 0 Natural 2.6 0.006

BV-02 PROD 0 97 3 Natural 0.5 0.004

BV-03 PROD 0 100 0 Natural 0.5 0.004

BV-04 PROD 0 100 0 Natural 2.6 0.004

BV-05 PROD 0 100 0 Natural 2.6 0.061

BV-06 PROD 0 100 0 Natural 0.2 0.001

BV-07 PROD 0 100 0 Natural 2.0 0.006

Central Desert study area grid wells

CD-01 PROD 0 33 67 Urban 2.5 0.003

CD-02 PROD 0 36 64 Urban 1.5 0.010

CD-03 PROD 0 100 0 Natural 1.5 0.004

CD-04 PROD 0 100 0 Natural 11.3 0.008

CD-05 PROD 0 82 18 Natural 73.2 0.008

CD-06 PROD 0 60 40 Natural 12.9 0.004

CD-07 PROD 0 94 6 Natural 9.4 0.002

CD-08 PROD 0 91 9 Natural 5.3 0.002

CD-09 PROD 0 63 63 Natural 71.1 0.010

CD-10 PROD 0 100 0 Natural 2.5 0.003

CD-11 PROD 0 50 50 Natural 21.2 0.003

CD-12 PROD 0 100 0 Natural 10.4 0.002

CD-13 PROD 0 93 7 Natural 9.8 0.001

CD-14 PROD 0 100 0 Natural 0.0 0.001

CD-15 PROD 0 100 0 Natural 0.0 0.002

Low-Use Basins study area grid wells

LUB-01 PROD 3 96 1 Natural 7.4 0.029

LUB-02 UN 0 98 2 Natural 59.1 0.004

LUB-03 PROD 3 88 10 Natural 0.4 0.002

LUB-04 PROD 15 84 0 Natural 2.4 0.004

LUB-05 PROD 0 100 0 Natural 7.6 0.004

LUB-06 PROD 0 74 26 Natural 0.1 0.003

LUB-07 PROD 0 100 0 Natural 0.1 0.003

LUB-08 PROD 0 100 0 Natural 0.1 0.003

LUB-09 PROD 0 81 19 Natural 0.0 0.001

LUB-10 PROD 0 100 0 Natural 0.3 0.003

LUB-11 PROD 0 100 0 Natural 0.4 0.002

LUB-12 PROD 0 86 14 Natural 1.5 0.003

LUB-13 PROD 1 69 29 Natural 5.6 0.005
Appendix A. Ancillary Datasets 69

USGS-GAMA well 

identification number Well type Land use (in percent) 1 Land-use classification

Septic density 

(tanks/km2

) 2

LUFT density 

(tanks/km2

) 3 Agricultural Natural Urban 

Low-Use Basins study area grid wells—Continued

LUB-14 PROD 0 100 0 Natural 0.2 0.005

LUB-15 PROD 0 57 43 Natural 8.8 0.003

LUB-16 PROD 0 91 9 Natural 0.0 0.003

LUB-17 PROD 0 100 0 Natural 0.0 0.001

LUB-18 PROD 0 72 28 Natural 4.6 0.099

LUB-19 PROD 0 100 0 Natural 0.2 0.001

LUB-20 PROD 0 96 4 Natural 0.1 0.003

LUB-21 PROD 46 43 12 Mixed 0.0 0.001

LUB-22 PROD 0 98 2 Natural 0.1 0.001

LUB-23 PROD 0 100 0 Natural 0.0 0.001

LUB-24 PROD 0 87 13 Natural 0.0 0.001

LUB-25 PROD 0 98 2 Natural 0.0 0.003

LUB-26 PROD 0 100 0 Natural 0.0 0.001

LUB-27 PROD 0 90 10 Natural 0.0 0.000

CDPH-grid wells

BV-DPH-01 PROD 0 96 4 Natural na na

CD-DPH-05 PROD 0 75 25 Natural na na

CD-DPH-06 PROD 1 56 43 Natural na na

LUB-DPH-05 PROD 0 84 16 Natural na na

LUB-DPH-21 PROD 18 61 21 Natural na na

LUB-DPH-22 PROD 0 100 0 Natural na na

LUB-DPH-37 PROD 0 99 1 Natural na na

LUB-DPH-38 PROD 0 100 0 Natural na na

USGS-understanding wells

BVU-01 PROD 0 96 4 Natural 2.6 0.061

LUBU-01 PROD 3 76 24 Natural 63.1 0.003

LUBU-02 UN 0 100 0 Natural 0.1 0.000

1

 Land-use percentages within 500-meter radius of well site (Johnson and Belitz, 2009).

2

 Septic tank density within 500-meter radius of well site, based on 1990 U.S. Census data.

3

 LUFT density within 500-meter radius of well site, based on GEIMS LUFT database (California Environmental Protection Agency, 2001).

Table A2. Well type, percent land use by category, land-use classification, septic density, leaking underground fuel tank density, grid 

cell number and USGS-GAMA well identification number for GAMA well data and CDPH-grid well data used in the Borrego Valley, 

Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.—Continued

[USGS-GAMA well identification number: BV, Borrego Valley study area well; CD, Central Desert study area well; LUB, Low-Use Basins of the 

Mojave and Sonoran Deserts study area well. Well types: PROD, production well; UN, unused well. Other abbreviations: USGS, U.S. Geological Survey; 

CDPH, California Department of Public Health; LUFT, leaking (or formerly leaking) underground fuel tank; na, not available; tanks/km2, number of tanks per 

square kilometer]
70 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Table A3. Well construction information and aridity index for wells used in the Borrego Valley, Central Desert, and Low-Use Basins of 

the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.

[USGS-GAMA well identification number: BV, Borrego Valley study area well; CD, Central Desert study area well; LUB, Low-Use Basins study area well; 

BVU or LUBU, understanding well. Other abbreviations: USGS, U.S. Geological Survey; na, not available; ft, feet; LSD, land-surface datum]

USGS-GAMA 

well 

identification 

number

Construction information

Aridity index 

(dimensionless) 1

Well 

depth 

(ft below LSD)

Top of 

perforations 

(ft below LSD)

Bottom of 

perforations 

(ft below LSD)

Length from top of top perforated 

interval to bottom of well 

(ft)

Borrego Valley study area grid wells

BV-01 na na na na 0.08

BV-02 490 na na na 0.09

BV-03 350 150 350 200 0.11

BV-04 630 420 630 210 0.09

BV-05 580 248 568 320 0.08

BV-06 391 252 285 33 0.08

BV-07 95 na na na 0.08

Central Desert study area grid wells

CD-01 na na na na 0.11

CD-02 358 na na na 0.14

CD-03 na na na na 0.18

CD-04 350 195 345 150 0.10

CD-05 430 220 420 200 0.09

CD-06 na na na na 0.18

CD-07 740 470 720 250 0.13

CD-08 425 na na na 0.10

CD-09 1,115 550 1,115 565 0.16

CD-10 604 220 580 360 0.13

CD-11 na na na na 0.19

CD-12 550 na na na 0.14

CD-13 260 147 247 100 0.07

CD-14 240 na na na 0.08

CD-15 600 390 580 190 0.08

Low-Use Basins study area grid wells

LUB-01 381 205 381 176 0.10

LUB-02 543 na na na 0.10

LUB-03 456 na na na 0.09

LUB-04 na na na na 0.10

LUB-05 490 322 480 158 0.12

LUB-06 490 220 475 255 0.08

LUB-07 660 160 440 280 0.08

LUB-08 800 300 780 480 0.08

LUB-09 224 184 224 40 0.07

LUB-10 300 190 300 110 0.06

LUB-11 600 452 590 138 0.11

LUB-12 840 650 800 150 0.10

LUB-13 500 100 500 400 0.10

LUB-14 240 na na na 0.19
Appendix A. Ancillary Datasets 71

USGS-GAMA 

well 

identification 

number

Construction information

Aridity index 

(dimensionless) 1

Well 

depth 

(ft below LSD)

Top of 

perforations 

(ft below LSD)

Bottom of 

perforations 

(ft below LSD)

Length from top of top perforated 

interval to bottom of well 

(ft)

Low-Use Basins study area grid wells—Continued

LUB-15 312 212 312 100 0.06

LUB-16 105 0 105 105 0.06

LUB-17 584 484 584 100 0.06

LUB-18 36 15 36 21 0.26

LUB-19 na na na na 0.07

LUB-20 100 70 na na 0.06

LUB-21 400 300 400 100 0.06

LUB-22 1,200 690 1,190 500 0.07

LUB-23 403 200 400 200 0.10

LUB-24 788 235 na 553 0.13

LUB-25 866 534 864 330 0.11

LUB-26 711 na na na 0.15

LUB-27 400 140 400 260 0.07

USGS-understanding wells

BVU-01 392 162 372 210 0.08

LUBU-01 1,010 350 940 590 0.07

LUBU-02 400 140 380 240 0.09

1

 Aridity index is average annual precipitation divided by average annual evapotranspiration.

Table A3. Well construction information and aridity index for wells used in the Borrego Valley, Central Desert, and Low-Use Basins of 

the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.—Continued

[USGS-GAMA well identification number: BV, Borrego Valley study area well; CD, Central Desert study area well; LUB, Low-Use Basins study area well; 

BVU or LUBU, understanding well. Other abbreviations: USGS, U.S. Geological Survey; na, not available; ft, feet; LSD, land-surface datum]
72 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

USGS-GAMA well 

identification number

Tritium 

(TU)

Percent 

modern carbon

Age 

classification

Borrego Valley study area grid wells

BV-01 <1 44 Pre-modern

BV-02 1.2 116 Modern

BV-03 <1 102 Mixed

BV-04 <1 81 Pre-modern

BV-05 <1 28 Pre-modern

BV-06 <1 4.5 Pre-modern

BV-07 <1 ns Pre-modern

Central Desert study area grid wells

CD-01 <1 88 Pre-modern

CD-02 <1 88 Pre-modern

CD-03 1.4 78 Mixed

CD-04 <1 90 Mixed

CD-05 <1 82 Pre-modern

CD-06 1.4 96 Modern

CD-07 <1 62 Pre-modern

CD-08 <1 13 Pre-modern

CD-09 2.2 94 Modern

CD-10 <1 93 Mixed

CD-11 1.4 110 Modern

CD-12 <1 27 Pre-modern

CD-13 <1 33 Pre-modern

CD-14 <1 27 Pre-modern

CD-15 <1 57 Pre-modern

Low-Use Basins study area grid wells

LUB-01 <1 47 Pre-modern

LUB-02 <1 22 Pre-modern

LUB-03 <1 52 Pre-modern

LUB-04 <1 20 Pre-modern

USGS-GAMA well 

identification number

Tritium 

(TU)

Percent 

modern carbon

Age 

classification

Low-Use Basins study area grid wells—Continued

LUB-05 <1 62 Pre-modern

LUB-06 <1 23 Pre-modern

LUB-07 <1 6.3 Pre-modern

LUB-08 <1 8.5 Pre-modern

LUB-09 <1 23 Pre-modern

LUB-10 <1 15 Pre-modern

LUB-11 <1 7.0 Pre-modern

LUB-12 <1 26 Pre-modern

LUB-13 <1 16 Pre-modern

LUB-14 <1 58 Pre-modern

LUB-15 <1 33 Pre-modern

LUB-16 5.6 93 Modern

LUB-17 <1 9.8 Pre-modern

LUB-18 <1 92 Mixed

LUB-19 <1 32 Pre-modern

LUB-20 <1 35 Pre-modern

LUB-21 <1 15 Pre-modern

LUB-22 <1 3.2 Pre-modern

LUB-23 <1 56 Pre-modern

LUB-24 <1 4.4 Pre-modern

LUB-25 <1 10 Pre-modern

LUB-26 1.2 103 Modern

LUB-27 <1 11 Pre-modern

USGS-understanding wells

BVU-01 <1 19 Pre-modern

LUBU-01 <1 40 Pre-modern

LUBU-02 <1 19 Pre-modern

Table A4. Tritium, percent modern carbon, and age classification of samples, Borrego Valley, Central Desert, and Low-Use Basins of 

the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.

[modern, recharged after 1952; mixed, modern and pre-modern water; pre-modern, recharged prior to 1952; ns, not sampled; <, less than; TU, tritium units]
Appendix A. Ancillary Datasets 73

Table A5. Oxidation-reduction constituents and redox classification for samples from the Borrego Valley, Central 

Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin 

Project.—Continued 

[Redox classification based on framework of McMahon and Chapelle (2008) and Jurgens and others (2009). Non-detections are reported 

as <RL, where RL is the reporting limit. Chemical abbreviations: DO, dissolved oxygen; NO3-N, nitrate as nitrogen; NO3-red, nitrate 

reducing; Mn, manganese; Mn-red, manganese reducing; Fe, iron. Redox classifications: oxic, DO > 0.5 mg/L; anoxic (suboxic), DO < 

0.5 mg/L, NO3-N < 0.5 mg/L, Mn < 50 μg/L, and Fe < 100 μg/L; anoxic (NO3-red), DO < 0.5 mg/L, NO3-N > 0.5 mg/L, Mn < 50 μg/L, 

and Fe < 100 μg/L; anoxic (Mn-red), DO < 0.5 mg/L, NO3-N < 0.5 mg/L, Mn > 50 μg/L, and Fe < 100 μg/L. Other abbreviations: 

mg/L, milligrams per liter; μg/L, micrograms per liter; redox, oxidation-reduction; >, greater than; <, less than; na, not available]

USGS-GAMA well 

identification 

number

Source of 

inorganic 

data

Dissolved 

oxygen 

(mg/L)

Nitrate, as 

nitrogen 

(mg/L)

Manganese 

(μg/L)

Iron 

(μg/L)

Redox 

classification

Borrego Valley study area

BV-01 na 4.1 na na na Oxic

BV-02 CDPH 7.1 0.43 0.4 <20 Oxic

BV-03 GAMA 6.2 0.28 1.1 15 Oxic

BV-04 GAMA 4.4 2.44 <0.2 <6 Oxic

BV-05 CDPH 1.1 0.38 <20 <100 Oxic

BV-06 CDPH 3.3 0.25 na na Oxic

BV-07 GAMA 2.0 0.99 0.8 7 Oxic

Central Desert study area

CD-01 GAMA 6.2 1.55 <0.2 <6 Oxic

CD-02 na 4.8 na na na Oxic

CD-03 GAMA 6.6 2.03 1.3 18 Oxic

CD-04 GAMA 7.0 1.41 0.2 <6 Oxic

CD-05 na 5.8 1.99 na na Oxic

CD-06 CDPH 6.2 <0.1 <20 <100 Oxic

CD-07 GAMA 4.5 2.62 <0.2 <6 Oxic

CD-08 CDPH 2.4 0.47 na na Oxic

CD-09 GAMA 6.3 2.20 0.8 20 Oxic

CD-10 CDPH 4.4 0.99 <20 <100 Oxic

CD-11 GAMA 7.2 1.24 <0.2 <6 Oxic

CD-12 GAMA 3.3 1.90 <0.2 <6 Oxic

CD-13 GAMA 5.1 1.63 <0.2 <6 Oxic

CD-14 GAMA 1.1 1.19 0.5 <6 Oxic

CD-15 na 5.1 na na na Oxic
74 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Table A5. Oxidation-reduction constituents and redox classification for samples from the Borrego Valley, Central 

Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin 

Project.—Continued 

[Redox classification based on framework of McMahon and Chapelle (2008) and Jurgens and others (2009). Non-detections are reported 

as <RL, where RL is the reporting limit. Chemical abbreviations: DO, dissolved oxygen; NO3-N, nitrate as nitrogen; NO3-red, nitrate 

reducing; Mn, manganese; Mn-red, manganese reducing; Fe, iron. Redox classifications: oxic, DO > 0.5 mg/L; anoxic (suboxic), DO < 

0.5 mg/L, NO3-N < 0.5 mg/L, Mn < 50 μg/L, and Fe < 100 μg/L; anoxic (NO3-red), DO < 0.5 mg/L, NO3-N > 0.5 mg/L, Mn < 50 μg/L, 

and Fe < 100 μg/L; anoxic (Mn-red), DO < 0.5 mg/L, NO3-N < 0.5 mg/L, Mn > 50 μg/L, and Fe < 100 μg/L. Other abbreviations: 

mg/L, milligrams per liter; μg/L, micrograms per liter; redox, oxidation-reduction; >, greater than; <, less than; na, not available]

USGS-GAMA well 

identification 

number

Source of 

inorganic 

data

Dissolved 

oxygen 

(mg/L)

Nitrate, as 

nitrogen 

(mg/L)

Manganese 

(μg/L)

Iron 

(μg/L)

Redox 

classification

Low-Use Basins study area

LUB-01 GAMA 6.1 12.10 1.5 40 Oxic

LUB-02 GAMA 5.5 0.41 <0.2 <6 Oxic

LUB-03 GAMA <0.2 0.09 376 22 Anoxic (Mn-red)

LUB-04 GAMA 4.2 0.38 <0.2 8 Oxic

LUB-05 GAMA 6.8 1.03 1.2 18 Oxic

LUB-06 GAMA 3.4 3.60 0.4 6 Oxic

LUB-07 GAMA 1.9 1.57 0.2 <6 Oxic

LUB-08 GAMA 2.6 4.94 0.6 9 Oxic

LUB-09 GAMA 3.9 5.11 <0.2 <6 Oxic

LUB-10 GAMA 0.2 <0.04 3.7 <6 Anoxic (suboxic)

LUB-11 GAMA 1.2 4.10 0.6 13 Oxic

LUB-12 GAMA 2.3 1.50 5 23 Oxic

LUB-13 GAMA 3.5 0.92 0.3 6 Oxic

LUB-14 GAMA 2.1 0.60 0.6 9 Oxic

LUB-15 GAMA 2.9 2.06 0.5 7 Oxic

LUB-16 GAMA 2.0 0.35 8 <6 Oxic

LUB-17 GAMA 1.9 1.35 0.3 11 Oxic

LUB-18 GAMA 1.2 0.25 8.1 17 Oxic

LUB-19 GAMA 0.4 1.22 <0.2 <6 Anoxic (NO3

-red)

LUB-20 GAMA 3.6 1.71 <0.2 <6 Oxic

LUB-21 GAMA 2.6 4.51 0.7 9 Oxic

LUB-22 GAMA 0.2 <0.04 1.2 7 Anoxic (suboxic)

LUB-23 GAMA 7.6 1.47 2 9 Oxic

LUB-24 GAMA 1.3 0.65 3.5 25 Oxic

LUB-25 GAMA 3.6 3.03 3.9 45 Oxic

LUB-26 GAMA 3.8 4.55 3.7 16 Oxic

LUB-27 GAMA 2.5 2.69 <0.2 <6 Oxic

USGS-understanding wells

BVU-01 GAMA 2 3.02 <0.2 <6 Oxic

LUBU-01 GAMA 2.4 0.33 <0.2 <6 Oxic

LUBU-02 GAMA 2.9 2.48 1.6 16 Oxic
Appendix B. Use of Data from the California Department of Public Health (CDPH) Database 75

For the CLUB study unit, the historical CDPH database 

contains more than 103,506 records distributed across more 

than 202 wells, requiring targeted retrievals to manageably 

use the data to assess water quality. The following paragraphs 

summarize the selection process for wells and data from the 

CDPH database for use in the grid-based status assessment 

(fig. B1). 

The grid-based calculation of aquifer-scale proportion 

uses one value per grid cell. Where USGS data for inorganic 

constituents were not available, additional data to represent 

a cell were selected from the CDPH database. Of the 62 

grid cells in the CLUB study unit, 23 cells had USGS-grid 

wells with the full complement of inorganic constituent data 

collected by USGS-GAMA, 26 cells had USGS-grid wells 

with data for trace elements, major ions, and nutrients but 

were missing data for radioactive constituents, and 13 cells did 

not have USGS-grid wells. The CDPH database was queried 

to provide these missing data for inorganic constituents. 

CDPH wells with data for the most recent 3 years available at 

the time of sampling (December 3, 2005–December 1, 2008) 

were considered. If a well had more than one analysis for a 

constituent in the 3-year interval, then the most recent data 

were selected.

The data in the CDPH database are of unknown quality, 

and the database does not contain data for quality-control 

samples with which to make a comprehensive quality-control 

assessment of the data. Cation-anion imbalance was used as 

a rough quality-control metric. Because water is electrically 

neutral and must have a balance between positive (cations) 

and negative (anions) electrically charged dissolved species, 

the cation/anion imbalance commonly is used as a qualityassurance check for water-sample analysis (Hem, 1985). An 

imbalance of less than 10 percent was defined as indicating 

acceptable quality of the major ion data for the sample. It 

was assumed that if the quality of the major ion data were 

acceptable, then the quality of the data for the other inorganic 

constituents also would be acceptable. In practice, however, 

some wells did not have data for major ion constituents, so the 

cation-anion imbalance could not be evaluated.

The first choice was to select CDPH inorganic data 

for the grid well sampled by the USGS (fig. B1B) for other 

constituents, provided the CDPH data met the cation/anion 

balance criteria. This approach resulted in the selection of 

supplemental inorganic data from the CDPH database for 

six USGS-grid wells. To identify the USGS-grid wells that 

incorporated CDPH inorganic data, a well ID was created that 

added “DG” to the GAMA ID for these wells (for example, 

BV-02 with CDPH data was assigned the well identification 

BV-DG-02; table A1).

If the first step did not yield CDPH inorganic data for 

the USGS-grid well, the second step was to search the CDPH 

database to identify the highest-ranked well with a cation/

anion imbalance less than 10 percent in each grid cell. This 

step resulted in selecting CDPH inorganic data from additional 

CDPH wells that were not USGS-grid wells for five grid cells. 

To identify these new CDPH-grid wells, a well ID was created 

that added “DPH” after the study unit prefix. For cells that 

contained a USGS-grid well, the identification number of the 

CDPH-grid well remained the same as that of the USGS-grid 

well identifier (for example, CDPH-grid well BV-DPH-01; 

table A1). For cells that did not contain a USGS-grid well, the 

CDPH-grid well was given a sequential number starting after 

the last GAMA cell number for the study area (for example, 

CDPH-grid well LUB-DPH-37; table A1). 

If no wells in a grid cell met the cation/anion balance 

criteria or if there were insufficient data to evaluate charge 

balance, the third choice for a CDPH-grid well was to select 

the highest-ranked CDPH well with any of the needed 

inorganic data. These new CDPH-grid wells were labeled 

using the same prefix as the other new CDPH-grid wells. 

The result of these steps was that 51 of the 62 grid cells 

were represented by inorganic constituent data from the USGS 

database, the CDPH database, or both databases. In some 

cases, to achieve one value for each constituent per cell, it 

was necessary to select an additional well in a cell for certain 

constituents; hence, some cells have data from two CDPH 

wells. Inorganic data from the CDPH database were used 

for 22 grid cells. CDPH data were available from the CDPH 

database for gross alpha radioactivity and radium-228 for 14 

and 15 wells, respectively, and for 0 to 9 wells for most other 

inorganic constituents (table 2). Estimates of aquifer-scale 

proportion for constituents based on a smaller number of wells 

are subject to a larger error associated with the 90-percent 

confidence intervals (on the basis of Jeffreys interval for the 

binomial distribution).

Differences in constituent reporting levels associated 

with USGS and CDPH data did not affect analysis of high or 

moderate relative-concentrations because concentrations in 

these categories were substantially higher than the reporting 

levels. Several types of comparisons between USGS-collected 

and CDPH data are described in appendix E.

Appendix B. Use of Data from the California Department of Public Health 

(CDPH) Database
76 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

California Department of Public

 Health (CDPH) well and identifier

Grid cell

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

Streams

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

Counties

IP027935_Figure B01a

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua

Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro Pacific Ocean

MEXICO

Lake

Dry lake

A

LUB-DPH-41

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

BV-DG-06

BV-DPH-06

BV-DG-05 BV-DG-02

BV-DPH-02

CD-DG-08

CD-DG-10

CD-DG-06

LUB-DG-12 LUB-DG-05

LUB-DPH-37

BV-DPH-10

LUB-DG-25

LUB-DG-09

LUB-DG-08

LUB-DG-06

CD-DPH-19

CD-DPH-18

LUB-DG-39

LUB-DG-15

LUB-DG-11

LUB-DG-04

LUB-DG-02

LUB-DPH-38

LUB-DPH-41

LUB-DPH-40

Figure B1. Maps showing identifiers and locations of (A) CDPH-grid wells, and (B) USGS-grid and USGS-understanding wells 

sampled during December 2008–March 2010, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts 

study unit, California GAMA Priority Basin Project.
Appendix B. Use of Data from the California Department of Public Health (CDPH) Database 77

Figure B1. —Continued

118° 117° 116° 115° 114°

35°

34°

33°

Shaded relief derived from U.S. Geological Survey 

National Elevation Dataset, 2006, 

Albers Equal Area Conic Projection

EXPLANATION

Streams

STUDY AREA

Borrego Valley

Central Desert

Low-Use Basins

of the Mojave and

Sonoran Deserts

Counties

IP027935_Figure B01b

Salton

Sea

Mojave River

Homer Wash

Whitewater River

Salt Creek

San Felipe Creek

Piute Wash

Milpitas Wash

Chemehuevi Wash

Carrizo Creek

Arroyo Seco

Little Dixie Wash

Coyote Wash

New River

Alamo River

ARIZONA

NEVADA

SAN BERNARDINO CO

RIVERSIDE CO

IMPERIAL CO SAN DIEGO CO

KERN CO

INYO CO

LOS ANGELES CO

ORANGE CO

Baker

Landers

Goldstone

Joshua

Tree

Desert Center

Lucerne Valley

California City

Borrego Springs

El Centro Pacific Ocean

MEXICO

Lake

Dry lake

B

Grid cell

USGS-grid well and identifier

USGS-understanding well

 and identifier

LUB-16

BVU-01

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

San Diego

Los

Angeles

CD-15

CD-14

CD-13

CD-12

CD-11

CD-10

CD-08

CD-07

CD-06

CD-05

CD-04

CD-03

CD-02

BV-07

BV-06

BV-05

BV-04

BV-03

LUB-27

LUB-26

LUB-25

LUB-24

LUB-23

LUB-22

LUB-21

LUB-20

LUB-19

LUB-18

LUB-17

LUB-16 LUB-15

LUB-14

LUB-13

LUB-12

LUB-11

LUB-10

LUB-08 LUB-09

LUB-07

LUB-06

LUB-05

LUB-04

LUB-03

LUB-02

LUB-01

CD-09

CD-01

BV-02

BV-01

LUBU-02

BVU-01

LUBU-01
78 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

For the grid-based calculations of aquifer-scale 

proportions, one well in each grid cell, a “grid well,” was 

randomly selected to represent the primary aquifer system 

(Belitz and others, 2010). For organic constituents, the one 

value in each grid cell was obtained from samples analyzed by 

USGS-GAMA from 49 grid wells. For inorganic constituents, 

the one value in each grid cell was obtained from samples 

analyzed by USGS-GAMA and data selected from the CDPH 

database (appendix B). The proportion of the primary aquifer 

system 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 each of the study areas (Belitz 

and others, 2010). The proportion for each study area is 

calculated individually because grid cell sizes are not uniform 

across the study areas. The proportion for the study unit is then 

determined by calculating the area-weighted sum by using the 

following equation:

AQP A SU = ∑ QPSA SA F (A1)

where:

AQPSU is the aquifer proportion for the study unit,

AQPSA is the aquifer proportion for a study area, and

FSA is the fraction of the total study unit area 

occupied by the study area.

The FSA for the study areas are: Borrego Valley, 0.09; Central 

Desert, 0.16, and the Low-Use Basins of the Mojave and 

Sonoran Deserts, 0.75. Proportions containing moderate and 

low relative-concentrations were calculated similarly. Aquiferscale proportions for individual constituents for each study 

area are listed in tables C1A–C and for classes of constituents 

in tables C2A–B. Confidence intervals for grid-based aquifer 

proportions were computed using the Jeffreys interval for the 

binomial distribution (Brown and others, 2001). The gridbased estimate is spatially unbiased; however, the grid-based 

approach may not identify constituents that exist at high 

concentrations in small proportions of the primary aquifer 

system.

The spatially weighted approach relied on USGS-grid 

well data collected from December 2, 2008, to March 4, 

2010, and CDPH data collected from December 3, 2005, 

to December 1, 2008 (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. The proportion of high relativeconcentrations for each constituent for each study area was 

computed by (1) calculating the proportion of wells with high 

relative-concentrations in each grid cell and (2) averaging 

together the grid-cell proportions computed in step 1 (Isaaks 

and Srivastava, 1989; Belitz and others, 2010). The spatially 

weighted high proportion for the study unit was calculated 

by summing the area-weighted values for each study area 

in the same manner as was used in the grid-based approach. 

Calculations for individual constituents for each study area 

are listed in tables C1A–C and for classes of constituents 

in tables C2A–B. The resulting proportions are spatially 

unbiased. 

The raw detection frequency was calculated by 

considering all of the available data in the period from 

December 3, 2005, to December 1, 2008, for the CDPH 

wells (the most recent analysis per well for all wells), the 

USGS-grid wells, and USGS-understanding wells. However, 

this approach is spatially biased because the CDPH and 

USGS-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 

aquifer system and could be given a disproportionately high 

(or low) weight compared to that given by spatially unbiased 

approaches. 

Appendix C. Estimation of Aquifer-Scale Proportions
Appendix C. Estimation of Aquifer-Scale Proportions 79

Table C1A. Aquifer-scale proportions from grid-based and spatially weighted approaches for (1) constituents with high relative-concentrations during December 3, 2005–December 1, 2008, from the California Department of Public Health (CDPH) database, (2) constituents with moderate or high relative-concentrations in samples collected from USGS-grid wells (December 2008–March 2010), or (3) organic constituents with detection frequencies of greater than 10 percent in the USGS-grid wells (December 2008–March 2010) for the Borrego Valley study area, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.[Relative-concentration categories: high; concentrations of inorganic or organic or special-interest constituents greater than water-quality benchmark; moderate, concentrations of inorganic constituents greater than or equal to 0.5 of benchmark but less than benchmark or concentrations of organic or special-interest constituents greater than or equal to 0.1 of benchmark but less than benchmark; low, concentrations of inorganic constituents less than 0.5 of benchmark or concentrations of organic or special-interest constituents less than 0.1 of benchmark or not detected. Benchmark types and values for constituents listed in tables 2 and 3]

Constituent

Raw detection frequency 1 Spatially weighted aquifer-scale proportion 1 Grid-based aquifer-scale proportion 90-percent confidence interval for grid-based high proportion 2

Number 

of 

wells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of 

cells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of 

wells

Moderate 

values 

(percent)

High 

values 

(percent)

Lower 

limit 

(percent)

Upper 

limit 

(percent)

Trace and minor elements

Arsenic 18 11 0 7 18 0 7 14 0 0 19

Boron 14 0 0 6 0 0 6 0 0 0 23

Chromium 18 0 1.0 7 0 0 7 0 0 0 19

Fluoride 18 5.6 5.6 7 4.8 14 7 0 14 2.6 44

Molybdenum 4 0 0 4 0 0 4 0 0 0 34

Vanadium 14 21 21 6 29 25 6 17 17 3.0 50

Radioactive constituents

Gross alpha activity 3 10 20 10 6 25 8.3 6 33 0 0 27

Uranium 11 18 0 6 8 0 6 17 0 0 27

Nutrients

Nitrate 19 0 5.3 7 0 4.8 7 0 0 0 17

Inorganic constituents with SMCLs

Iron 18 0 5.6 7 0 2.4 7 0 0 0 19

Manganese 18 0 5.6 7 0 4.8 7 0 0 0 19

Chloride 17 0 0 7 0 0 6 0 0 0 19

Sulfate 17 12 0 7 19 0 6 17 0 0 19

Total dissolved solids (TDS) 17 41 0 7 45 21 7 43 29 8.8 59

Volatile organic compounds (VOCs)

Benzene 18 0 0 7 0 0 7 0 0 0 17

Tetrachloroethene (PCE) 18 0 0 7 0 0 7 0 0 0 17

Bromoform 18 0 0 7 0 0 7 0 0 0 17
80 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit Constituent Raw detection frequency 1 Spatially weighted aquifer-scale proportion 1 Grid-based aquifer-scale proportion 90-percent confidence interval for grid-based high proportion 2 Number of wells Moderate values (percent) High values (percent) Number of cells Moderate values (percent) High values (percent) Number of wells Moderate values (percent) High values (percent) Lower limit (percent) Upper limit (percent) Volatile organic compounds (VOCs)—Continued Chloroform 18 5.6 0 7 3.6 0 7 0 0 0 17 Dibromochloromethane 18 0 0 7 0 0 7 0 0 0 17 Pesticides Dieldrin 8 0 0 7 0 0 7 0 0 0 17 Special interest Perchlorate 22 4.5 0 7 3.6 0 7 14 0 0 17

1 Based on the most recent data for each CDPH well during the period December 3, 2005–December 1, 2008, combined with GAMA grid and understanding well data.2 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001).3 Gross alpha activities were not adjusted for uranium activity. The MCL-US for gross alpha activity applies to adjusted gross alpha activity.

Table C1A. Aquifer-scale proportions from grid-based and spatially weighted approaches for (1) constituents with high relative-concentrations during December 3, 2005–December 1, 2008, from the California Department of Public Health (CDPH) database, (2) constituents with moderate or high relative-concentrations in samples collected from USGS-grid wells (December 2008–March 2010), or (3) organic constituents with detection frequencies of greater than 10 percent in the USGS-grid wells (December 2008–March 2010) for the Borrego Valley study area, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.—Continued[Relative-concentration categories: high; concentrations of inorganic or organic or special-interest constituents greater than water-quality benchmark; moderate, concentrations of inorganic constituents greater than or equal to 0.5 of benchmark but less than benchmark or concentrations of organic or special-interest constituents greater than or equal to 0.1 of benchmark but less than benchmark; low, concentrations of inorganic constituents less than 0.5 of benchmark or concentrations of organic or special-interest constituents less than 0.1 of benchmark or not detected. Benchmark types and values for constituents listed in tables 2 and 3]
Appendix C. Estimation of Aquifer-Scale Proportions 81

Table C1B. Aquifer-scale proportions from grid-based and spatially weighted approaches for (1) constituents with high relative-concentrations during December 3, 2005–December 1, 2008, from the California Department of Public Health (CDPH) database, (2) constituents with moderate or high relative-concentrations in samples collected from USGS-grid wells (December 2008–March 2010), or (3) organic constituents with detection frequencies of greater than 10 percent in the USGS-grid wells (December 2008–March 2010) for the Central Desert study area, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.[Relative-concentration categories: high, concentrations of inorganic or organic or special-interest constituents greater than water-quality benchmark; moderate, concentrations of inorganic constituents greater than or equal to 0.5 of benchmark but less than benchmark or concentrations of organic or special-interest constituents greater than or equal to 0.1 of benchmark but less than benchmark; low, concentrations of inorganic constituents less than 0.5 of benchmark or concentrations of organic or special-interest constituents less than 0.1 of benchmark or not detected. Benchmark types and values for constituents listed in tables 2 and 3]

Constituent

Raw detection 

frequency 1

Spatially weighted 

aquifer-scale proportion 1

Grid-based 

aquifer-scale proportion

90-percent confidence interval 

for grid-based high proportion 2

Number 

of 

wells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of 

cells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of 

wells

Moderate 

values 

(percent)

High 

values 

(percent)

Lower 

limit 

(percent)

Upper 

limit 

(percent)

Trace and minor elements

Arsenic 35 11 17 11 27 13 11 27 9.1 1.6 31

Boron 34 0 0 11 0 0 11 0 0 0 11

Chromium 32 0 3.1 11 0 2.3 11 0 0 0 11

Fluoride 34 8.8 8.8 11 8.3 14 11 18 9.1 1.3 31

Molybdenum 9 11 11 9 11 11 9 11 11 2.0 36

Vanadium 32 0 0 11 0 0 11 0 0 0 11

Radioactive constituents

Gross alpha activity 3 32 31 16 11 40 17 11 36 18 5.4 42

Uranium 25 36 16 11 25 10 11 9.1 18 5.4 42

Nutrients

Nitrate 40 2.5 2.5 13 3.8 2.6 13 0 0 0 10

Inorganic constituents with SMCLs

Iron 34 0 2.9 11 0 2.3 11 0 0 0 11

Manganese 34 0 0 11 0 0 11 0 0 0 11

Chloride 35 0 0 11 0 0 11 0 0 0 11

Sulfate 35 0 0 11 0 0 11 0 0 0 11

Total dissolved solids (TDS) 36 11 0 11 11 0 15 13 0 0 8.5

Volatile organic compounds (VOCs)

Benzene 26 0 0 15 0 0 15 0 0 0 8.5

Tetrachloroethene (PCE) 26 0 0 15 0 0 15 0 0 0 8.5

Bromoform 26 0 0 15 0 0 15 0 0 0 8.5

Chloroform 26 3.8 0 15 2.2 0 15 0 0 0 8.5

Dibromochloromethane 26 0 0 15 0 0 15 0 0 0 8.5
82 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit Constituent Raw detection frequency 1 Spatially weighted aquifer-scale proportion 1 Grid-based aquifer-scale proportion 90-percent confidence interval for grid-based high proportion 2 Number of wells Moderate values (percent) High values (percent) Number of cells Moderate values (percent) High values (percent) Number of wells Moderate values (percent) High values (percent) Lower limit (percent) Upper limit (percent) Pesticides Dieldrin 22 0 0 14 0 0 13 0 0 0 9.7 Special interest Perchlorate 43 0 0 15 0 0 15 0 0 0 8.5

1 Based on the most recent data for each CDPH well during the period December 3, 2005–December 1, 2008, combined with GAMA grid and understanding well data.2 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001).3 Gross alpha activities were not adjusted for uranium activity. The MCL-US for gross alpha activity applies to adjusted gross alpha activity.

Table C1B. Aquifer-scale proportions from grid-based and spatially weighted approaches for (1) constituents with high relative-concentrations during December 3, 2005–December 1, 2008, from the California Department of Public Health (CDPH) database, (2) constituents with moderate or high relative-concentrations in samples collected from USGS-grid wells (December 2008–March 2010), or (3) organic constituents with detection frequencies of greater than 10 percent in the USGS-grid wells (December 2008–March 2010) for the Central Desert study area, Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.—Continued[Relative-concentration categories: high; concentrations of inorganic or organic or special-interest constituents greater than water-quality benchmark; moderate, concentrations of inorganic constituents greater than or equal to 0.5 of benchmark but less than benchmark or concentrations of organic or special-interest constituents greater than or equal to 0.1 of benchmark but less than benchmark; low, concentrations of inorganic constituents less than 0.5 of benchmark or concentrations of organic or special-interest constituents less than 0.1 of benchmark or not detected. Benchmark types and values for constituents listed in tables 2 and 3]
Appendix C. Estimation of Aquifer-Scale Proportions 83

Constituent

Raw detection frequency 1 Spatially weighted 

aquifer-scale proportion 1

Grid-based 

aquifer-scale proportion

90-percent 

confidence interval 

for grid-based high 

proportion 2

Number 

of wells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of cells

Moderate 

values 

(percent)

High 

values 

(percent)

Number 

of wells

Moderate 

values 

(percent)

High 

values 

(percent)

Lower 

limit 

(percent)

Upper 

limit 

(percent)

Trace and minor elements

Arsenic 57 16 25 28 17 23 28 21 21 11 36

Boron 42 21 12 27 20 15 27 22 15 6.4 29

Chromium 54 0 0 27 0 0 27 0 0 0 4.8

Fluoride 59 19 31 28 18 32 28 25 32 19 48

Molybdenum 29 10 21 27 9.3 19 27 11 19 8.9 33

Vanadium 41 17 0 27 12 0 27 15 0 0 4.8

Radioactive constituents

Gross alpha activity 3 48 17 4.2 22 19 6.8 22 18 9.1 2.7 23

Uranium 44 4.6 2.3 28 2.7 3.6 28 0 3.6 0.6 13

Nutrient

Nitrate 72 6.9 1.4 28 7.9 1.8 28 3.6 3.6 0.6 13

Inorganic constituents with SMCLs

Iron 55 3.6 3.6 27 2.5 3.1 27 0 0 0 4.8

Manganese 55 0 1.8 27 0 3.7 27 0 3.7 0.7 14

Chloride 55 3.6 1.8 27 5.6 3.7 27 7.4 3.7 0.7 14

Sulfate 55 16 0 27 21 0 27 19 0 0 4.8

Total dissolved solids 

(TDS)

58 40 8.6 28 42 13 28 50 11.0 2.2 19

Volatile organic compounds (VOCs)

Benzene 50 2.0 0 27 1.2 0 27 0 0 0 4.8

Tetrachloroethene 

(PCE)

50 2.0 0 27 1.9 0 27 0 0 0 4.8

Bromoform 50 4.0 0 27 1.9 0 27 0 0 0 4.8

Chloroform 50 0 0 27 0 0 27 0 0 0 4.8

Dibromochloromethane 50 4.0 0 27 1.9 0 27 0 0 0 4.8

Pesticide

Dieldrin 37 2.7 0 27 1.9 0 27 3.7 0 0 4.8

Special interest

Perchlorate 80 16 0 27 31 0 27 41 0 0 4.8

1

 Based on the most recent data for each CDPH well during the period December 3, 2005–December 1, 2008, combined with GAMA grid and understanding 

well data.

2

 Based on the Jeffreys interval for the binomial distribution (Brown and others, 2001). 

3

 Gross alpha activities were not adjusted for uranium activity. The MCL-US for gross alpha activity applies to adjusted gross alpha activity.

Table C1C. Aquifer-scale proportions from grid-based and spatially weighted approaches for (1) constituents with high relativeconcentrations during December 3, 2005–December 1, 2008, from the California Department of Public Health (CDPH) database, 

(2) constituents with moderate or high relative-concentrations in samples collected from USGS-grid wells (December 2008–March 2010), 

or (3) organic constituents with detection frequencies of greater than 10 percent in the USGS-grid wells (December 2008–March 2010) 

for the Low-Use Basins of the Mojave and Sonoran Deserts study area, Borrego Valley, Central Desert, and Low-Use Basins of the 

Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project. 

[Relative-concentration categories: high, concentrations of inorganic or organic or special-interest constituents greater than water-quality benchmark; 

moderate, concentrations of inorganic constituents greater than or equal to 0.5 of benchmark but less than benchmark or concentrations of organic or specialinterest constituents greater than or equal to 0.1 of benchmark but less than benchmark; low, concentrations of inorganic constituents less than 0.5 of benchmark 

or concentrations of organic or special-interest constituents less than 0.1 of benchmark or not detected. Benchmark types and values for constituents listed in 

tables 2 and 3]
84 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Constituent 

class and 

study area

Aquifer-scale proportion

Low 

relativeconcentration 

(percent)

Moderate 

relativeconcentration 

(percent)

High 

relativeconcentration 

(percent)

Trace and minor elements 

BV 57 14 29

CD 55 27 18

LUB 29 23 48

Radioactive constituents 1

BV 67 33 2 4.2

CD 41 36 23

LUB 75 14 11

Nutrients

BV 96 0 2 4.8

CD 94 2 3.8 2 2.6

LUB 93 3.6 3.6

All inorganic constituents with health-based benchmarks

BV 43 36 21

CD 34 31 35

LUB 23 24 53

Salinity indicators 3

BV 36 43 21

CD 87 13 0

LUB 43 45 12

Manganese and (or) iron

BV 96 0 2 7.1

CD 98 0 2 2.3

LUB 94 2 2.5 3.7

All inorganic constituent with SMCL benchmarks

BV 43 43 14

CD 85 13 0

LUB 39 45 16

1

 Aquifer-scale proportions for the classes of radioactive constituents 

and all inorganic constituents with health-based benchmarks were 

calculated using unadjusted gross alpha activity.

2

 Spatially weighted result. Grid-based result was zero percent. 

3

 Salinity indicators are chloride, sulfate, and total dissolved solids.

Table C2A. Aquifer-scale proportions for inorganic constituent 

classes in the three study areas, Borrego Valley, Central Desert, 

and Low-Use Basins of the Desert study unit, California GAMA 

Priority Basin Project, December 2008–March 2010.

[Relative-concentration categories: high, concentrations of constituents 

greater than water-quality benchmark; moderate, concentrations of 

constituents greater than or equal to 0.5 of benchmark but less than 

benchmark; low, concentrations of constituents less than 0.5 of benchmark. 

Proportions calculated using the grid-based method unless footnoted. Study 

areas: BV, Borrego Valley; CD, Central Desert; LUB, Low-Use Basins of the 

Mojave and Sonoran Deserts. SMCL, secondary maximum contaminant level]

Constituent 

class and 

study area

Aquifer-scale proportion

Low relativeconcentration (percent)

Moderate 

relativeconcentration 

(percent) 2

High 

relativeconcentration 

(percent)

Not 

detected

Detected 

low 1

Gasoline hydrocarbons

BV 100 0 0 0

CD 100 0 0 0

LUB 96 2.5 1.2 0

Solvents

BV 86 14 0 0

CD 100 0 0 0

LUB 78 20 1.8 0

Trihalomethanes

BV 86 9.5 4.8 0

CD 80 18 2.2 0

LUB 67 32 1.8 0

All volatile organic compounds (VOCs)

BV 71 24 4.8 0

CD 80 18 2.2 0

LUB 56 43 1.8 0

Insecticides 

BV 100 0 0 0

CD 100 0 0 0

LUB 98 0 1.8 0

All pesticides

BV 100 0 0 0

CD 92 7.7 0 0

LUB 89 9.3 1.8 0

All organic constituents with health-based benchmarks

BV 71 24 4.8 0

CD 73 25 2.2 0

LUB 56 39 5.6 0

1

 Proportions for detected at low relative-concentrations were calculated by 

subtracting the spatially weighted moderate proportion from the area-weighted 

detection frequency in the USGS-grid wells. 

2

 Proportions for moderate relative-concentrations were calculated using the 

spatially weighted approach.

Table C2B. Aquifer-scale proportions for organic constituent 

classes in the three study areas, Borrego Valley, Central Desert, 

and Low-Use Basins of the Desert study unit, California GAMA 

Priority Basin Project, December 2008–March 2010.

[Relative-concentration categories: high, concentrations of constituents 

greater than water-quality benchmark; moderate, concentrations of 

constituents greater than or equal to 0.1 of benchmark but less than 

benchmark; low, concentrations of constituents less than 0.1 of benchmark. 

Study areas: BV, Borrego Valley; CD, Central Desert; LUB, Low-Use Basins 

of the Mojave and Sonoran Deserts]
Appendix E. Comparison of Data from the CDPH and USGS-GAMA Program 85

Appendix D. Calculating Total 

Dissolved Solids

Specific conductance, an electrical measure of TDS, 

was measured for 49 grid and 3 understanding wells sampled 

by the USGS, whereas TDS was only measured directly as 

residue on evaporation for 42 of these wells. For wells that had 

no measured TDS, TDS values were calculated from specific 

conductance (SC) values using a linear regression equation 

(TDS = 0.57 × SC + 40.09; coefficient of determination, 

R2 = 0.976). TDS values from the CDPH database were 

combined with USGS measured and calculated TDS values.

Appendix E. Comparison of Data from

the CDPH and USGS-GAMA Program

CDPH and USGS-GAMA data were compared to 

assess the validity of combining data from these different 

sources. Because laboratory reporting levels for most organic 

constituents and trace elements were substantially lower for 

USGS-GAMA data than for CDPH data (Fram and Belitz, 

2012), only concentrations of constituents greater than the 

CDPH data laboratory reporting levels could be compared, 

and as a result, there were insufficient data from which to 

evaluate agreement between CDPH and USGS-GAMA data. 

However, concentrations of inorganic constituents (sodium, 

calcium, magnesium, chloride, sulfate, TDS, and nitrate), 

which generally are prevalent at concentrations greater than 

reporting levels, were compared for each well using data from 

both sources. The USGS and CDPH databases contained data 

for major ions or nitrate for 43 to 48 wells. Wilcoxon signed 

rank tests of paired analyses for these seven constituents 

indicated no significant differences between USGS-GAMA 

and CDPH data for these constituents. Although differences 

between the paired datasets occurred for some wells, most 

sample pairs plotted close to a 1:1 line (fig. E1). These plots 

indicated that the GAMA and CDPH inorganic data were 

comparable.

Major ion data for grid wells with sufficient data (USGS 

and CDPH data) were plotted on a trilinear diagram (Piper, 

1944) along with all CDPH major ion data to determine 

whether the groundwater types in grid wells were similar 

to groundwater types observed historically in the study 

unit. Trilinear diagrams show the relative abundance of 

major cations and anions (on a charge equivalent basis) 

as a percentage of the total respective ion content of the 

water (fig. E2). Trilinear diagrams often are used to define 

groundwater type (Hem, 1985). All cation/anion data in the 

CDPH database with a cation/anion imbalance of less than 

10 percent were retrieved and plotted on the trilinear diagram 

for comparison with USGS- and CDPH-grid well data.

The ranges of water types for USGS-grid wells and 

other wells from the historical CDPH database were similar 

(fig. E2). In most samples, bicarbonate accounted for 20 

to 80 percent of the total anions, and chloride and sulfate 

each accounted for 10 to 60 percent. Sodium and potassium 

accounted for 40 to 100 percent of the cations. Many samples 

are described as calcium-sodium-bicarbonate or sodium–

mixed anion type waters. 

The determination that the range of relative abundance of 

major cations and anions in grid wells is similar to the range of 

those in all CDPH wells indicates that the grid wells represent 

most of the types of water present within the primary aquifer 

system in the CLUB study unit. 
86 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

Figure E1. Graph showing paired inorganic constituent concentrations from wells sampled by the GAMA 

Program from December 3, 2005, to December 1, 2008, Borrego Valley, Central Desert, and Low-Use Basins 

of the Mojave and Sonoran Deserts study unit, California GAMA Priority Basin Project.

0.1 1 10 100 1,000

Concentration, USGS-GAMA data

0.1

1

10

100

1,000

Concentration, CDPH data

1:1 line

Calcium

Magnesium

Sodium

Chloride

Sulfate

Total dissolved solids

Arsenic

Concentration values in milligrams per liter

Concentration values in micrograms per liter
Appendix E. Comparison of Data from the CDPH and USGS-GAMA Program 87

Figure E2. Trilinear diagram showing U.S. Geological Survey (USGS) grid well data and data from all wells in the 

California Department of Public Health (CDPH) database that have a charge imbalance of less than 10 percent, Borrego 

Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit, 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 

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

IP027935_Figure E02 Grid wells_CLUB

Appendix E. Comparison of Data from the 

CDPH and USGS-GAMA Program
88 Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit

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Parsons and others—Status of Groundwater Quality in the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts Study Unit, 

2008–2010: California GAMA Priority Basin Project—SIR 2014–5001

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