U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3001

April 2014 Printed on recycled paper

U.S. Geological Survey and the California State Water Resources Control Board

Groundwater Quality in the Borrego Valley, Central 

Desert, and Low-Use Basins of the Mojave and 

Sonoran Deserts, California 

Groundwater provides more than 40 percent of California’s drinking water. To protect this vital resource, 

the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. 

The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State’s 

untreated groundwater quality and increases public access to groundwater-quality information. Selected 

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

Deserts constitute one of the study units being evaluated.

Organic

constituents

Inorganic

constituents

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Values are a percentage of the area of the primary aquifer system

with concentrations in the three specified categories. Values on

pie chart may not equal 100 due to rounding of percentages.

95

48

26

26

5

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

Antelope, Coachella, Colorado River,

Mojave River Basins

sac14-0517_Figure 01

Salton

Sea

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

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Approximate

boundary

Mojave

Desert

Sonoran

Desert

The Borrego Valley, Central Desert, and Low-Use 

Basins of the Mojave and Sonoran Deserts Study Unit

The Mojave and Sonoran Deserts include 57 groundwater basins (California Department of Water 

Resources, 2003). Basins in the Antelope, Mojave River, Coachella, and Colorado River Valleys were discussed by Dawson and Belitz (2012). The remaining 47 basins compose the Borrego Valley, Central Desert, 

and Low-Use Basins of the Mojave and Sonoran Deserts study unit (CLUB study unit) and were grouped into 

three study areas. Basins containing few or no public-supply wells are defined as low-use basins. Publicsupply wells are sparse in the 12,103-square-mile area of the CLUB study unit, and this study focused on the 

963-square-mile area that is within about 2 miles of a public-supply well. 

 The climate in the Mojave Desert is characterized as arid high desert, with hot, dry summers and cool 

winters with limited rainfall. 

The Sonoran Desert has 

an arid subtropical climate 

characterized by long, hot 

summers, mild winters, and 

summer and winter rainy 

seasons. Annual precipitation generally ranges from 

0 to 10 inches. Several 

creeks, washes, and ephemeral streams drain the study 

unit, flowing into lakes and 

sinks. 

This study evaluated water quality in the 

part of the aquifer system 

used for public supply, the 

primary aquifer system. 

The primary aquifer system 

is defined as those parts of 

the aquifer corresponding 

to the perforated intervals 

of wells listed in the California Department of Public 

Health (CDPH) database. 

Public-supply wells in the 

study unit typically are drilled to depths between 350 and 600 feet, consist of solid casing from the land surface to a depth of about 200 to 400 feet, and are perforated below the solid casing. Water quality in the shallower and deeper parts of the aquifer system can differ from that in the primary aquifer system. The primary 

aquifer system consists of unconfined and confined Pleistocene- to Holocene-age Quaternary alluvium, and to 

a lesser extent, Tertiary alluvium and sedimentary deposits.

Land use in the CLUB study unit is approximately 91 percent (%) natural, 6% urban, and 3% agricultural. 

Natural lands are mostly shrubland and bare rock or sediment, with a small percentage of grassland and forest. 

Sources of groundwater recharge include runoff from mountains surrounding the basins and infiltration 

of imported surface water and groundwater used for irrigation. The primary sources of groundwater discharge 

are water pumped for irrigation and for public supply, natural discharge to streams, and evapotranspiration.

Overview of Water Quality

GAMA’s Priority Basin Project evaluates the quality of untreated groundwater 

relative to human-health and aestheticbased benchmarks established for drinkingwater quality. Benchmarks and definitions 

of high, moderate, and low concentrations 

are discussed in the inset box on page 3.

Inorganic constituents in groundwater 

commonly are derived from natural sources 

and processes but also can be affected by 

human activities. In the CLUB study unit, 

one or more inorganic constituents were 

present at high concentrations in about 48% 

of the primary aquifer system and at moderate concentrations in about 26%.

Human-made organic constituents 

can be found in products used in the home, 

business, industry, and agriculture, and 

can enter the environment through normal usage, spills, or improper disposal. In 

this study unit, organic constituents were 

not present at high concentrations in the 

primary aquifer system, and one or more 

organic constituents were present at moderate concentrations in about 5%.
 

 

Trace and

minor elements 

Total

dissolved

solids 

Nutrients

Uranium and

radioactive

constituents

Iron or

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENT

42

23

35

68

20

12

94

3 3

39

50

11

95

3 2

32

68

RESULTS: Groundwater Quality in the Borrego Valley, Central Desert, and 

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

Inorganic Constituents with Health-Based Benchmarks

Trace and minor elements are naturally present in the minerals in rocks and soils, 

and in the water that comes into contact with those materials. In the CLUB study 

unit, trace and minor elements were detected at high concentrations in about 42% 

of the primary aquifer system, and at moderate concentrations in about 23% of the 

system. Fluoride, arsenic, molybdenum, boron, and vanadium were the elements that 

most frequently occurred at high and moderate concentrations. Chromium also was 

detected at high concentrations, but in less than 1% of the primary aquifer system.

Radioactivity is the release of energy or energetic particles during spontaneous decay of unstable atoms. Most of the radioactivity in groundwater comes from 

the decay of naturally occurring isotopes of uranium and thorium in minerals in the 

sediments of the aquifer. Uranium and (or) gross alpha radioactivity occurred at high 

levels in 12% of the primary aquifer system and at moderate levels in about 20% of 

the system.

Nutrients, such as nitrate and nitrite, are naturally present at low concentrations 

in groundwater. Elevated concentrations generally occur as a result of human activities, such as applying commercial fertilizer or animal waste to land used for crops. 

Livestock, when in concentrated numbers, and septic systems also produce nitrogenous waste that can leach into groundwater. Nitrate was present at high concentrations in about 3% of the primary aquifer system and at moderate concentrations in 

about 3%.

Inorganic Constituents with Non-Health Benchmarks

(Not included in water-quality overview charts shown on the front page) 

Some constituents affect the aesthetic properties of water, such as taste, color, 

and odor, or may create nuisance problems, such as staining and scaling. The State 

of California has a recommended and an upper limit for total dissolved solids (TDS) 

in drinking water. Groundwater naturally contains TDS as a result of the weathering 

and dissolution of minerals in soils. In the CLUB study unit, TDS was present at high 

concentrations in about 11% of the primary aquifer system and at moderate concentrations (between the recommended and upper limit) in 39%.

Anoxic conditions (low amounts of dissolved oxygen) in groundwater may result 

in release of the naturally occurring elements manganese and iron from minerals into 

groundwater. Iron, or manganese, or both were present at high concentrations in 

about 3% of the primary aquifer system.

Constituent of Special Interest: Perchlorate 

(Not included in water-quality overview charts shown on the front page)

Perchlorate is an inorganic constituent that has been regulated in California 

drinking water since 2007. It is an ingredient in rocket fuel, fireworks, safety flares 

and other products, may be present in some fertilizers, and also occurs at low concentrations under natural conditions in groundwater. Perchlorate was not detected at 

high concentrations, but was detected at moderate concentrations in about 32% of the 

primary aquifer system. 
High

Moderate

Low or not detected

EX

AMPLE Values are a percentage of the area of the primary aquifer system

with concentrations in the three specified categories. Values

on pie chart may not equal 100 due to rounding of percentages.

CONSTITUENT CONCENTRATIONS

Solvents

Other

VOCs

ORGANIC CONSTITUENTS

Herbicides and

insecticides

(including

fumigants)

Trihalomethanes

Gasoline

hydrocarbons

99

1

99

1

99

1

2

98

100

RESULTS: Groundwater Quality in the Borrego Valley, Central Desert, and 

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

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect volatile organic 

compounds (VOCs) and pesticides at low concentrations far below human-health 

benchmarks. VOCs and pesticides detected at these low concentrations can be used to 

trace water from the landscape into the aquifer system.

Volatile Organic Compounds with Human-Health Benchmarks

VOCs are in many household, commercial, industrial, and agricultural products 

and are characterized by their tendency to volatilize (evaporate) into the air.

Gasoline hydrocarbons are a component of gasoline and may be used as additives to make gasoline burn more efficiently, improve emissions quality, and clean 

engine parts. In the CLUB study unit, gasoline hydrocarbons were not detected at high 

concentrations and were present at moderate concentrations in about 1% of the primary 

aquifer system.

Solvents are used for a number of purposes, including manufacturing and cleaning. Solvents were not detected at high concentrations and were present at moderate 

concentrations in about 1% of the primary aquifer system.

Trihalomethanes may form during disinfection of drinking-water supplies and 

may enter groundwater by the infiltration of landscape irrigation water or leakage from 

distribution systems. Trihalomethanes were not detected at high concentrations and 

were present at moderate concentrations in about 2% of the primary aquifer system.

Other volatile organic compounds, including organic synthesis reagents, were not 

detected at either high or moderate concentrations.

Pesticides with Human-Health Benchmarks

Pesticides, which include herbicides, fumigants, and insecticides, are applied to 

crops, gardens, lawns, around buildings, and along roads to help control unwanted vegetation (weeds), insects, fungi, and other pests. Insecticides were not detected at high 

concentrations in the primary aquifer system and were detected at moderate concentrations in about 1% of the primary aquifer system. Herbicides and fumigants were not 

detected at either high or moderate concentrations.

BENCHMARKS FOR EVALUATING GROUNDWATER QUALITY

GAMA’s Priority Basin Project uses benchmarks established for drinking water to 

provide context for evaluating the quality of untreated groundwater. After withdrawal, 

groundwater may be disinfected, filtered, mixed, and exposed to the atmosphere before 

being delivered to consumers. Federal and California regulatory benchmarks for protecting human health (Maximum Contaminant Level, MCL) were used for the evaluation 

when available. Otherwise, non-regulatory benchmarks for protecting aesthetic properties, such as taste and odor (Secondary Maximum Contaminant Level, SMCL), and nonregulatory benchmarks for protecting human health (Notification Level, NL, and lifetime 

Health Advisory Level, HAL) were used.

High, moderate, and low concentrations are defined relative to benchmarks

Concentrations are considered high if they are greater than a benchmark. For inorganic constituents (except perchlorate), concentrations are moderate if they are greater than one-half of a benchmark. For organic and 

special-interest constituents (including perchlorate), concentrations are moderate if they are greater than one-tenth of a benchmark; 

this lower threshold was used because organic constituents generally are less prevalent and have smaller concentrations relative to 

benchmarks than inorganic constituents. Low includes nondetections and values less than moderate concentrations. Methods for evaluating water quality are discussed in Parsons and others (2014).
0

10

20

30

40

50

60

Percentage of primary aquifer system 

Low-Use Basins

Central Desert

Borrego Valley

Arsenic Boron Fluoride Molybdenum Vanadium

HIGH MODERATE

HIGH MODERATE

HIGH MODERATE

29% 

14% 

57% 

Trace and minor elements

Borrego Valley Central Desert

Relative concentration

Low-Use Basins

18% 

27% 

55% 

High Moderate Low

48% 

23% 

29% 

Trace and Minor Element Concentrations in the CLUB Study Unit

In the CLUB study unit, trace and minor elements were present at high concentrations in 42% of the primary aquifer system. The proportion of the primary aquifer system 

having high concentrations was greatest in the Low-Use Basins study area (48%) and 

lower in the Borrego Valley (29%) and Central Desert (18%) study areas. For comparison, 

the proportion of the primary aquifer system having high concentrations in the four other 

Mojave and Sonoran Desert areas ranged from 32% to 40% (Dawson and Belitz, 2012). 

The main contributors to high and moderate concentrations of trace and minor elements in 

the CLUB and other Mojave and Sonoran Desert study units were arsenic, boron, fluoride, 

molybdenum, and vanadium; however, different elements were important in different 

areas (Dawson and Belitz, 2012; Parsons and others, 2014). All five elements were present at high or moderate concentrations in the Low-Use Basins study area, whereas, three 

were present at high or moderate concentrations in the Borrego Valley study area (arsenic, 

fluoride, and vanadium), and three were present in the Central Desert study area (arsenic, 

fluoride, and molybdenum).

SELECTED REFERENCES

California Department of Water Resources, 2003, California’s groundwater: California Department of Water 

Resources Bulletin 118, 246 p. Available at http://www.water.ca.gov/groundwater/bulletin118/update2003.

cfm. 

Dawson, B.J.M., and Belitz, Kenneth, 2012, Status of groundwater quality in the California Desert Region, 

2006-2008—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations 

Report 2012-5040, 110 p. Available at http://pubs.usgs.gov/sir/2012/5040/.

Mathany, T.M., Wright, M.T., Beuttel, B.S., and Belitz, Kenneth, 2012, Groundwater-quality data in the 

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

2008–2010—Results from the California GAMA Program: U.S. Geological Survey Data Series 659, 100 p. 

Available at http://pubs.usgs.gov/ds/659/.

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. Available at http://pubs.usgs.gov/sir/2014/5001/. 

Priority Basin Assessments 

GAMA’s Priority Basin Project 

(PBP) assesses water quality in that part 

of the aquifer system used for drinking 

water, primarily public supply. Water 

quality in shallower and deeper parts may 

differ from that in the primary aquifer 

system. GAMA’s Domestic Well Project 

assesses water quality in the shallower 

parts of the aquifer system. Ongoing 

assessments are being conducted in more 

than 120 basins throughout California. 

The PBP assessments are based on a 

comparison of constituent concentrations 

in untreated groundwater with benchmarks established for the protection of 

human health and for aesthetic concerns. 

The PBP does not evaluate the quality of 

drinking water delivered to consumers. 

The PBP uses two scientific 

approaches for assessing groundwater 

quality. The first approach uses a network 

of wells to statistically assess the status 

of groundwater quality. The second 

approach combines water-quality, hydrologic, geographic, and other data to help 

assess the factors that affect water quality. 

In the Borrego Valley, Central Desert, 

and Low-Use Basins of the Mojave 

and Sonoran Deserts study unit, data 

were collected by the PBP from 2008 to 

2010 and from the CDPH database for 

2005–2008. The PBP includes chemical 

analyses not generally available as part 

of regulatory compliance monitoring, 

including measurements at concentrations 

much lower than human-health benchmarks and measurement of constituents 

that can be used to trace the sources and 

movement of groundwater.

For more information

Technical reports and hydrologic 

data collected for the GAMA Program 

may be obtained from

GAMA Project Chief

U.S. Geological Survey

California Water Science Center

6000 J Street, Placer Hall

Sacramento, CA 95819

Telephone number: (916) 278-3100

WEB: http://ca.water.usgs.gov/gama

GAMA Program Unit Chief

State Water Resources Control Board

Division of Water Quality

PO Box 2231, Sacramento, CA 95812

Telephone number: (916) 341-5779

WEB: http://www.waterboards.ca.gov/gama

By Mary C. Parsons and Kenneth Belitz

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143001