The Mojave Basin Shallow Aquifer Study Unit

The Mojave Basin Shallow Aquifer study unit (Mojave) covers approximately 

4,680 square kilometers (2,908 square miles) in the western part of the Mojave Desert 

in San Bernardino County. The study unit consists of groundwater basins along the 

Mojave River, the El Mirage Valley groundwater basin, and part of the Harper Valley 

groundwater basin. The Mojave study unit was divided into two study areas—the 

floodplain study area along the Mojave River and the regional study area surrounding 

and underlying the floodplain study area. Aquifers in the floodplain study area consist 

of coarse granitic river-channel and floodplain alluvium deposited by the Mojave 

River. Aquifers in the 

regional study area 

consist of alluvium 

derived from older 

stream deposits, locally 

derived alluvial fans, 

playa lake deposits, 

and fractured bedrock 

(Stamos and others, 

2001; Groover and 

Izbicki, 2019).

This study 

examined the quality 

of groundwater 

resources used for 

domestic drinkingwater supply. Previous 

studies of groundwater resources used for public drinking-water supply in the Mojave 

Desert have observed elevated concentrations of some constituents, primarily trace 

elements, in some wells (Dawson and Belitz, 2012; Metzger and others, 2015). 

Domestic wells in the study unit typically are drilled to depths of 27–186 meters 

(Groover and others, 2019), which is shallower than the depths of public-supply wells in 

the same area (typically 90–300 meters deep; Dawson and Belitz, 2012). Water levels in 

domestic wells in the study unit typically are 9–140 meters below land surface (Groover 

and others, 2019).

This study was designed to provide a statistically representative assessment 

of the quality of groundwater resources used for domestic drinking water. A total 

of 48 domestic wells were sampled between January and May 2018 (Groover and 

others, 2019). Eleven additional wells were sampled to evaluate processes affecting 

groundwater quality, but these wells are not included in this assessment.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2019–3033

May 2019

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20193033

Overview of Water Quality

GAMA’s Priority Basin Project 

evaluates the quality of untreated 

groundwater. For context, concentrations 

measured in groundwater are 

compared to benchmarks established 

for drinking-water quality, such as 

maximum contaminant levels (MCL). 

A concentration above a benchmark 

is defined as high. Benchmarks and 

definitions of moderate and low 

concentrations are discussed on page 3.

Many inorganic constituents 

naturally occur in groundwater. 

The concentrations of the inorganic 

constituents can be affected by natural 

processes as well as human activities. 

In the Mojave study unit, one or more 

inorganic constituents were present at 

high concentrations in about 44 percent 

of the groundwater resources used for 

domestic drinking water.

Organic constituents are found in 

products used in the home, business, 

industry, and agriculture and can enter 

the environment through normal usage, 

spills, or improper disposal. Organic 

constituents were not present at moderate 

or high concentrations in the groundwater 

resources used for domestic drinking 

water in the Mojave study unit.

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

Groundwater Quality in Shallow Aquifers in the 

Western Mojave Desert, 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 (PBP) of the GAMA Program provides a 

comprehensive assessment of the State’s groundwater quality and increases public access to 

groundwater-quality information. One GAMA-PBP study unit is in the western part of the 

Mojave Desert, where shallow aquifers provide drinking water for many rural households.

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Values are a percentage of the area of the groundwater 

resources used for domestic drinking water with 

concentrations in the three specified categories.

Organic

constituents

Inorganic

constituents

44

19

37

100

117°30' 117° 116°30'

35°

34°30'

Base from U.S. Geological Survery and other federal and state 

digital data, various scales; Lambert Conformal Conic 

projection, standard parallels are 34.0333 and 35.4667; 

North American Datum of 1983

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Grid well

Study area

Regional aquifer

Floodplain aquifer

Study-unit boundary

Barstow

Victorville
Inorganic Constituents with Human-Health Benchmarks

Trace elements are naturally present in the minerals of rocks and sediment that 

compose groundwater aquifers and can dissolve into the groundwater that comes in 

contact with those materials. About 38 percent of the groundwater resources used 

for domestic drinking water in the Mojave study unit had high concentrations of 

one or more trace elements and about 31 percent had moderate concentrations. Five 

elements were present at concentrations above benchmarks: arsenic (29 percent), 

fluoride (15 percent), hexavalent chromium (4 percent), molybdenum (12 percent), and 

strontium (2 percent). Boron and vanadium were present at moderate concentrations 

in 8 percent of groundwater. Hexavalent chromium was present at moderate 

concentrations in 25 percent of groundwater. About 6 percent of groundwater resources 

used for domestic drinking water in the Mojave study unit had hexavalent chromium 

concentrations above the former California MCL of 10 ppb, compared to 4 percent of 

groundwater resources used for public drinking water statewide (Izbicki and others, 

2015). Groundwater in the Mojave study unit typically had a neutral to alkaline pH. 

Wells with acidic pH values (6 percent of groundwater) had lower concentrations of 

hexavalent chromium than neutral to alkaline wells, similar to the correlation between 

chromium and pH statewide (Izbicki and others, 2015; Groover and others, 2019).

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 uranium and thorium isotopes that are naturally present in aquifer materials. 

Uranium concentrations and 72-hour adjusted gross alpha particle activities were 

above benchmarks in about 6 percent of the groundwater resources used for domestic 

drinking water.

Nutrients, including nitrate, are naturally present at low concentrations in 

groundwater, and high concentrations generally result from human activities. Common 

sources of nutrients include fertilizer applied to crops and landscaping, seepage from 

septic systems, and human and animal waste. Nitrate was present at high concentrations 

in about 4 percent of the groundwater resources used for domestic drinking water in the 

Mojave study unit.

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 can create nuisance problems, such as staining and scaling. The benchmarks 

used for these constituents are non-regulatory secondary maximum contaminant 

level benchmarks.

Total dissolved solids (TDS) concentration is a measure of the salinity of 

the groundwater. All water naturally contains TDS as a result of the weathering 

and dissolution of minerals in rocks and sediment. The state of California has a 

recommended and an upper limit for TDS in drinking water. Total dissolved solids 

were present at high concentrations (greater than the upper limit) in about 10 percent 

and at moderate concentrations (between the upper and recommended limits) in about 

25 percent of the groundwater resources used for domestic drinking water.

Anoxic conditions defined as having a dissolved oxygen concentration less 

than 0.5 ppm can result in the release of manganese, iron, and other associated trace 

elements from minerals into groundwater. Most groundwater in the Mojave study 

unit had dissolved oxygen concentrations greater than 0.5 ppm. Iron was present at 

concentrations above benchmarks set for aesthetic concerns in about 4 percent of the 

groundwater resources used for domestic drinking water. Manganese was not present at 

high concentrations.

RESULTS: Groundwater Quality in Shallow Aquifers in the Western Mojave Desert, California

INORGANIC CONSTITUENTS

31 38

31

6

13

81

90

65

10

25

96

4

6 4

Trace

elements

Nitrate

Total

dissolved

solids

Manganese

or iron

Uranium

and

radioactivity
Organic Constituents with Human-Health Benchmarks

The Priority Basin Project used laboratory methods that can detect concentrations 

of volatile organic compounds (VOCs) that are far below human-health benchmarks. 

The VOCs detected at these very low concentrations can be used to help trace 

movement of water from the land surface into the aquifer system.

Many household, commercial, and industrial products, including solvents, 

gasoline components, and refrigerants, contain VOCs. No VOCs were detected at high 

or moderate concentrations in the Mojave study unit, and VOCs were detected at low 

concentrations in 23 percent of the groundwater resources used for domestic drinking 

water. The most common VOCs detected were disinfection by-products (15 percent), 

followed by solvents (10 percent).

Microbial Indicators

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

Microbial indicator constituents are used to evaluate the potential for fecal 

contamination of water sources. In the Mojave study unit, total coliforms were detected 

in 15 percent of the groundwater resources used for domestic drinking water and 

Escherichia coli (E. coli) were not detected. Total coliforms are naturally present in soils 

and digestive tracts of animals, whereas E. coli specifically indicate contamination with 

animal (or human) fecal waste (California State Water Resources Control Board, 2016).

RESULTS: Groundwater Quality in Shallow Aquifers in the Western Mojave Desert, California

Benchmark type and value for selected constituents.

[Benchmark types: CA-MCL, California State Water Resources Control Board Division of Drinking Water (CA) maximum contaminant level; CA-NL, CA notification 

level; CA-RL, CA response level; CA-SMCL, CA secondary maximum contaminant level; EPA-HAL, U.S. Environmental Protection Agency (EPA) lifetime health 

advisory; EPA-MCL, EPA maximum contaminant level; EPA-TT, EPA treatment technique; USGS-HBSL, U.S. Geological Survey health-based screening level. 

Abbreviations: pCi/L, picocuries per liter; ppb, parts per billion or micrograms per liter; ppm, parts per million or milligrams per liter; >, greater than]

Constituent

Benchmark

Constituent

Benchmark

Type Value Type Value

Arsenic EPA-MCL 10 ppb Nitrate, as nitrogen EPA-MCL 10 ppm

Hexavalent chromium (noncancer and cancer)

USGS-HBSL 20 ppb and 4 ppb Molybdenum EPA-HAL 40 ppb

Fluoride CA-MCL 2 ppm Iron CA-SMCL 300 ppb

Total dissolved solids (TDS) 

(upper and recommended)

CA-MCL 1,000 ppm and 500 ppm Manganese CA-SMCL 50 ppb

Strontium EPA-HAL 4,000 ppb Chromium CA-MCL 50 ppb

Vanadium CA-RL and CA-NL 500 ppb and 50 ppb Boron EPA-HAL and CA-NL 6 ppm and 1 ppm

Uranium EPA-MCL 30 ppb Escherichia coli (E. coli) EPA-MCL Repeat detection at a site

Adjusted gross alpha particle 

activity

EPA-MCL 15 pCi/L Total coliforms EPA-TT >5 percent of samples with 

detections per month

ORGANIC CONSTITUENTS

100

15

Detected

Not detected

Different colors are used for this pie

diagram because the benchmarks for

microbial constituents specify repeat

sampling to confirm detections, which

was not done in this study.

85

Volatile 

organic 

compounds

Total

coliforms

Benchmarks for Evaluating Groundwater Quality

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

context for evaluating the quality of groundwater. The quality of drinking water can differ 

from the quality of groundwater because of contact with household plumbing, exposure to 

the atmosphere, or water treatment. The U.S. Environmental Protection Agency (EPA) and 

California State Water Resources Control Board Division of Drinking Water (CA) regulatory 

benchmarks set for the protection of human health (maximum contaminant level) were used 

when available. Otherwise, non-regulatory benchmarks set for the protection of aesthetic 

properties, such as taste and odor (secondary maximum contaminant level, SMCL) and nonregulatory benchmarks set for the protection of human health (health-based screening levels, 

HBSL; response levels, RL; notification levels, NL; and lifetime health advisory levels, HAL) 

were used. Water quality in domestic wells is not regulated in California.

High, Moderate, and Low Concentrations are Defined Relative to Benchmarks

Concentrations are considered high if they are greater than the selected benchmark. For most inorganic constituents, 

concentrations between the benchmark and one-half of the benchmark are moderate. For most organic constituents, concentrations 

between the benchmark and one-tenth of the benchmark are moderate. For constituents with California response and notification 

levels, concentrations between the RL and the NL are moderate. For constituents with health-based screening levels (Toccalino and 

Norman, 2006), concentrations above the upper level are high and concentrations between the upper and lower levels are moderate. 

Low concentrations include non-detections and values less than moderate concentrations. Methods for calculating the percentages 

shown in the pie diagrams are discussed by Dawson and Belitz (2012).

High

EXAMPLE

Moderate

Low or not detected

Values are a percentage of the area of the groundwater 

resources used for domestic drinking water with 

concentrations in the three specified categories.

CONSTITUENT

CONCENTRATIONS
Distribution of Selected Constituents

More inorganic constituents were present at high concentrations above MCLs 

in the regional study area compared to the floodplain study area in the Mojave Basin 

Shallow Aquifer study unit. Arsenic, fluoride, molybdenum, and uranium were the 

constituents most commonly present at concentrations above MCLs in the Mojave 

study unit and were present at high concentrations nearly twice as often in the regional 

study area than in the floodplain study area. Total chromium, vanadium, and nitrate 

also were present at concentrations above benchmarks more often in the regional study 

area. Hexavalent chromium was present at moderate concentrations more often in the 

floodplain study area compared to the regional study area (Groover and others, 2019).

References Cited

California State Water Resources Control Board, 2016, Groundwater information 

sheet—Bacterial indicators: State Water Resources Control Board Division of Water 

Quality GAMA Program, 7 p., http://www.waterboards.ca.gov/gama/docs/coc_

bacteria_indicators.pdf.

Dawson, B.J.M., and Belitz, K., 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., 

https://doi.org/10.3133/sir20125040.

Groover, K.D., and Izbicki, J.A., 2019, Selected trace-elements in alluvium and rocks, 

western Mojave Desert, southern California: Journal of Geochemical Exploration, 

v. 200, p. 234–248, https://doi.org/10.1016/j.gexplo.2018.09.005.

Groover, K.D., Goldrath, D.A., Bennett, G.L., Johnson, T.D., and Watson, E.E., 2019, 

Groundwater-quality data in the Mojave Basin Shallow Aquifer study unit, 2018—

Results from the California GAMA Priority Basin Project: U.S. Geological Survey 

Data Release, https://doi.org/10.5066/P9C7U6DW.

Izbicki, J.A., Wright, M.T., Seymour, W.A., McCleskey, R.B., Fram, M.S., Belitz, K., 

and Esser, B.K., 2015, Cr(VI) occurrence and geochemistry in water from 

public-supply wells in California: Applied Geochemistry, v. 63, p. 203–217, 

https://doi.org/10.1016/j.apgeochem.2015.08.007.

Metzger, L.F., Landon, M.K., House, S.F., and Olsen, L.D., 2015, Mapping selected 

trace elements and major ions, 2000–2012, Mojave River and Morongo Groundwater 

Basins, Southwestern Mojave Desert, San Bernardino County, California: 

U.S. Geological Survey Data Release, https://doi.org/10.5066/F7Q23X95.

Stamos, C.L., Martin, P., Nishikawa, T., and Cox, B.F., 2001, Simulation of 

ground-water flow in the Mojave River Basin, California: U.S. Geological 

Survey Water-Resources Investigations Report 2001–4002, 113 p., 

https://doi.org/10.3133/wri014002.

Toccalino, P.L., and Norman, J.E., 2006, Health-based screening levels to evaluate 

U.S. Geological Survey groundwater quality data: Risk analysis, v. 26, no. 5, 

p. 1339–1348, https://doi.org/10.1111/j.1539-6924.2006.00805.x.

Priority Basin Assessments

GAMA’s Priority Basin Project 

(PBP) assesses water quality in 

groundwater resources used for drinkingwater supply. This study in the western 

part of the Mojave Desert focused on 

groundwater resources used for domestic 

drinking water. Domestic wells typically 

tap shallower parts of aquifer systems 

than public-supply wells, and water 

quality can vary with depth in aquifer 

systems. Ongoing assessments are being 

done in more than 120 basins and areas 

outside of basins throughout California.

The GAMA-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 GAMA-PBP does 

not evaluate the quality of drinking water 

delivered to consumers.

The GAMA-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 Mojave Basin 

Shallow Aquifer study unit, data were 

collected by the GAMA-PBP in 2018. 

The GAMA-PBP includes chemical 

analyses not generally available as part 

of regulatory compliance monitoring, 

including measurements at concentrations 

lower than human-health benchmarks and 

measurements 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’s 

Priority Basin Project 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) 278-3000

https://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

https://www.waterboards.ca.gov/gama

EXPLANATION

Concentration

Moderate

High

Floodplain

study area

Regional

study area

100

Percent of groundwater resources 

at moderate or high concentrations 80

60

40

20

0

Any inorganic constituent

Arsenic

Fluoride

Molybdenum

Uranium

Chromium

Vanadium

Nitrate

Hexavalent

chromium

By Krishangi D. Groover and Dara A. Goldrath