U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3036

January 2013 Printed on recycled paper

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

Groundwater Quality in the Mojave Area, 

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 groundwater quality and increases public access to groundwater-quality information. Four 

groundwater basins along the Mojave River make up one of the study areas being evaluated. 

The Mojave Study Area Overview of Water Quality

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

However, for context, benchmarks established for drinking-water quality are used 

for comparison. Benchmarks and definitions of high, moderate, and low concentrations are discussed in the inset box on 

page 3. The USGS sampled 52 wells for 

this assessment; data from the California 

Department of Public Health database were 

used to supplement USGS data.

Many inorganic constituents occur 

naturally in groundwater. The concentrations of the inorganic constituents can be 

affected by natural processes as well as by 

human activities. In the Mojave study area, 

one or more inorganic constituents were 

present at high concentrations in 28% of the 

primary aquifers and at moderate concentrations in 36%.

Organic constituents are present in 

products used in the home, business, industry, and agriculture. Organic constituents 

can enter the groundwater system through 

normal usage, spills, or improper disposal. 

In the Mojave study area, organic constituents were present at high concentrations in 

2% of the primary aquifers and at moderate 

concentrations in 6%.

The Mojave study area is approximately 1,500 square miles (3,885 square 

kilometers) and includes four contiguous groundwater basins: Upper, Middle, and 

Lower Mojave River Groundwater Basins, and the El Mirage Valley (California 

Department of Water Resources, 2003). The Mojave study area has an arid climate, and 

is part of the Mojave Desert. Average annual rainfall is about 6 inches (15 centimeters). 

Land use in the study area is approximately 82 percent (%) natural (mostly shrubland), 

4% agricultural, and 14% urban. The primary crops are pasture and hay. The largest 

urban areas are the cities of Victorville, Hesperia, and Apple Valley (2010 populations 

of 116,000, 90,000 and 69,000, respectively).

Groundwater in these basins is used for public and domestic water supply and for 

irrigation. The main water-bearing units are gravel, sand, silt, and clay derived from 

surrounding mountains. The primary aquifers in the Mojave study area are defined as 

those parts of the aquifers corresponding to the perforated intervals of wells listed in the 

California Department of Public Health database. Public-supply wells in the Mojave 

study area are completed to depths between 200 and 600 feet (18 to 61 meters),

consist of solid casing from the land surface to a depth of 130 to 420 feet (40 to 

128 meters), and are screened or perforated below the solid casing. Recharge to the 

groundwater system is primarily runoff from the mountains to the south, mostly through 

the Mojave River channel. The primary sources of discharge are pumping wells and 

evapotranspiration.

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Pie charts illustrate the proportion of the primary 

aquifers, on an areal basis with concentrations in 

three specified categories.

36

28 36

Organic

constituents

Inorganic

constituents

Number analyzed: 27 Number analyzed: 79

6

2

92

117°45’ 117°30’ 117°15’

35°

34°45’

34°30’

34°15’

117° 116°45’ 116°30’

EXPLANATION

Lake

Dry lake bed

Urban area

Alluvium

Hard rock

Study area 

boundary

KERN COUNTY LOS ANGELES COUNTY

SAN BERNARDINO COUNTY

0 5 10 20 MILES

1050 20 KILOMETERS

Mojave River Mojave River

Mojave RiverMojave River

Upper

Mojave

Groundwater 

Basin

Middle Mojave

Groundwater Basin

Lower Mojave

Groundwater Basin

El Mirage

Groundwater

Basin Hesperia

Victorville Apple

Valley
RESULTS: Groundwater Quality in the Mojave Study Area

Inorganic Constituents with Human-Health Benchmarks

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

water that comes into contact with those materials. In the Mojave study area, trace elements with human-health benchmarks were present at high concentrations in 33% of the 

primary aquifers on an areal basis, and at moderate concentrations in 22%. Of the 17 

trace elements with human-health benchmarks analyzed in this study, 6 were detected 

at high concentrations: arsenic, boron, fluoride, molybdenum, strontium, and vanadium. 

Lead was present at moderate concentrations.

Radioactivity is the release of energy or energetic particles during structural 

changes in the nucleus of an atom. Most of the radioactivity in groundwater comes 

from decay of naturally occurring isotopes of uranium and thorium that are present in 

minerals in the aquifer. In the Mojave study area, radioactive constituents were detected 

at concentrations above benchmarks in 2% of the primary aquifers, and at moderate 

concentrations in 15%. Six radioactive constituents were analyzed; of these, gross alpha 

radioactivity was detected at concentrations above benchmarks.

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

groundwater. High and moderate concentrations generally occur as a result of human 

activities, such as fertilizer application, livestock waste, or septic-system seepage. Of 

the three nutrients with health-based benchmarks analyzed, one, nitrate, was detected at 

concentrations above benchmarks.

Inorganic Constituents with Non-Health Benchmarks

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

Other inorganic constituents, such as total dissolved solids and manganese, affect 

the aesthetic properties of water, such as taste, color, or odor. In the Mojave study area, 

these constituents were present at high concentrations in 9% of the primary aquifers, 

and at moderate concentrations in 14%. Of the seven constituents with non-healthbased benchmarks that were analyzed, five were detected at concentrations above 

benchmarks: chloride, iron, manganese, sulfate, and total dissolved solids.

Special Interest: Perchlorate

Perchlorate is an inorganic constituent which has been regulated in California 

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

may be present in some fertilizers, and also occurs naturally at low concentrations 

in groundwater. In the Mojave study area, perchlorate was found at moderate 

concentrations in 22% of the primary aquifers.

Other

inorganic

constituents

Perchlorate

33 45

22

83

98

77

22

9

14

78

2 15

2

Trace 

elements

Radioactive 

constituents

Nutrients

INORGANIC CONSTITUENTS
RESULTS: Groundwater Quality in the Mojave Study Area

Organic Constituents with Human-Health Benchmarks

The Priority Basin Project uses laboratory methods that can detect the presence 

of volatile organic compounds (VOCs) and pesticides at very low concentrations, 

far below human-health benchmarks. VOCs and pesticides detected at these low 

concentrations can be used to help trace water from the land surface into the aquifer 

system.

Volatile Organic Compounds

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

products and can form during the disinfection of water supplies; they are characterized 

by their tendency to volatilize into the air. Of 56 VOCs with health-based benchmarks 

analyzed in the Mojave study area, 3 VOCs (1,2-dichloroethane, tetrachloroethene, and 

carbon tetrachloride) were detected at moderate concentrations.

Pesticides

Pesticides are used on lawns, in gardens, around buildings, along roads, and in 

agriculture to help control unwanted vegetation (weeds), insects, fungi, and other pests. 

Of the 20 pesticides with health-based benchmarks that were analyzed, one, dieldrin, 

was detected at a moderate concentration in one well. The use of dieldrin was banned in 

the United States in 1987.

Special Interest: N-Nitrosodimethylamine (NDMA)

NDMA is an organic constituent that is an industrial and water disinfection 

byproduct. NDMA was detected at a high concentration in one well, equating to 5% of 

the primary aquifers in the Mojave study area. Because fewer samples were collected 

for NDMA than for other organic constituents, the proportion of the primary aquifers 

with high concentrations of one or more organic constituents (page 1, “Overview of 

Water Quality”) is less than the proportion for NDMA.

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, or exposed to the 

atmosphere before being delivered to consumers. Federal and California regulatory 

benchmarks for protecting human health (Maximum Contaminant Level, MCL) are 

used when available. Otherwise, nonregulatory benchmarks for protecting human 

health (Notification Level, NL, and Lifetime Health Advisory, HAL) and nonregulatory 

benchmarks for protecting aesthetic properties such as taste and odor (Secondary 

Maximum Contaminant Level, SMCL) are 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, concentrations are moderate

if they are greater than one-half of a benchmark. For organic and special-interest constituents, concentrations are considered moderate

if they are greater than one-tenth of a benchmark; this lower threshold was used because organic constituents are generally less 

prevalent and have smaller concentrations relative to benchmarks than inorganic constituents. Low concentrations include nondetections and values less than moderate concentrations. Methods for evaluating water quality are discussed by Milby Dawson and 

Belitz (2012).

ORGANIC CONSTITUENTS

Pesticides

Volatile

organic

compounds

NDMA

4

96

2

98

5

95

High

Moderate

EX

Low or not detected

AMPLE Values are a percentage of the area of the primary aquifers 

with concentrations in the three specified categories.

CONSTITUENT CONCENTRATIONS
Trace Element Concentrations Throughout the 

California Desert Region

Trace elements were present at high concentrations in all of the Desert Region 

primary aquifers. In Mojave, they were high in 33% of the primary aquifers, on an 

areal basis; the range of high trace element percentages in the Desert Region primary 

aquifers was 15 to 54% (Milby Dawson and Belitz, 2012). The trace elements with the 

highest percentages in the Mojave study area were arsenic (high concentrations in 21% 

of the primary aquifers), fluoride (11%), and vanadium (8%). Arsenic was detected at 

high concentrations in all of the Desert Region primary aquifers. Fluoride was detected 

at high concentrations in five of the six Desert Region study areas. Vanadium was at 

detected high concentrations in the Mojave, Antelope, and Indian Wells study areas in 

the Desert Region.

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 water quality in the 

primary aquifers. 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 approaches for 

assessing groundwater quality. The first 

approach uses a network of wells to 

provide a statistically based assessment 

of the status of groundwater quality. The 

second approach uses additional wells 

to help assess the factors that affect 

water quality. Both approaches use 

data routinely collected for regulatory 

compliance, as well as data collected 

by the PBP. The PBP includes chemical 

analyses not generally available 

as part of regulatory compliance 

monitoring, including measurements at 

concentrations much lower than humanhealth benchmarks, and measurement of 

constituents that can be used to trace the 

sources and movement of groundwater.

By Barbara J. Milby Dawson and Kenneth Belitz

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

Mathany, T.M., and Belitz, K., 2009, Groundwater quality data in the Mojave study 

unit, 2008—Results from the California GAMA Program: U.S. Geological Survey 

Data Series 440, 69 p., available at http://pubs.usgs.gov/ds/440/

Milby Dawson, B.J., and Belitz, K., 2012, Status of groundwater quality in the 

 California Desert Region, 2006–2008—California GAMA Program Priority Basin 

 Project: U.S. Geological Survey Scientific Investigations Report 2012-5040, 100 p. 

For more information

Technical reports and hydrologic 

data collected for the GAMA Program 

may be obtained from:

GAMA Program Unit

U.S. Geological Survey

California Water Science Center

4165 Spruance Road, Suite 200

San Diego, CA 92101

Telephone number: (619) 225-6100

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

GAMA Program Unit

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/

0 25 50

0 25 50 100 KILOMETERS

100 MILES

Owens

Valley

Indian Wells

Mojave

Coachella

Antelope

NEVADA

ARIZONA

MEXICO

CALIFORNIA

Colorado

River

Desert Region Study Areas

0%

20%

40%

60%

80%

100%

0%

20%

40%

60%

80%

100%

0%

20%

40%

60%

80%

100%

0%

20%

40%

60%

80%

100%

Owens

Indian Wells

Coachella

Colorado River

Antelope

Mojave

High aquifer proportion

EXPLANATION

Moderate aquifer proportion

Trace Elements

Arsenic

Fluoride

Vanadium

Percentage of area of primary aquifers with concentrations in the specified categories

0%

20%

40%

60%

80%

100%

0%

20%

40%

60%

80%

100%

0%

20%

40%

60%

80%

100%

0%

20%

40%

60%

80%

100%

Owens

Indian Wells

Coachella

Colorado River

Antelope

Mojave

High aquifer proportion

EXPLANATION

Moderate aquifer proportion

Trace Elements

Arsenic

Fluoride

Vanadium

Percentage of area of primary aquifers with concentrations in the specified categories