U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3034

January 2013 Printed on recycled paper

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

Groundwater Quality in the Colorado River

Basins, 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 Colorado River make up one of the study areas being evaluated. 

The Colorado River Study Area Overview of Water Quality

GAMA’s Priority Basin Project 

evaluates the quality of untreated 

groundwater. However, for context, 

benchmarks established for drinkingwater 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 20 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 Colorado River 

study area, one or more inorganic constituents were present at high concentrations in 

45% of the primary aquifers and at moderate concentrations in 35%.

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 Colorado River study area, 

organic constituents were present at moderate concentrations in 10% of the primary 

aquifers.

The Colorado River study area 

is approximately 884 square miles 

(2,290 square kilometers) and includes 

the Needles, Palo Verde Mesa, Palo 

Verde Valley, and Yuma groundwater 

basins (California Department of Water 

Resources, 2003). The Colorado River 

study area has an arid climate and is 

part of the Sonoran Desert. Average 

annual rainfall is about 3 inches 

(8 centimeters). Land use in the study area 

is approximately 47 percent (%) natural 

(mostly shrubland), 47% agricultural, and 

6% urban. The primary crops are pasture 

and hay. The largest urban area is the city 

of Blythe (2010 population of 21,000).

Groundwater in these basins is used 

for public and domestic water supply 

and for irrigation. The main waterbearing units are gravel, sand, silt, and 

clay deposited by the Colorado River or 

derived from surrounding mountains. 

The primary aquifers in the Colorado 

River 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 Colorado River basins are completed 

to depths between 230 and 460 feet (70 

to 140 meters), consist of solid casing 

from the land surface to a depth of 130 

of 390 feet (39 to 119 meters), and are 

screened or perforated below the solid 

casing. The main source of recharge to the 

groundwater systems in the Needles, Palo Verde Mesa, and Palo Verde Valley basins is 

the Colorado River; in the Yuma basin, the main source of recharge is from subsurface 

flow from the groundwater basins to the west. Groundwater discharge is primarily to 

pumping wells, evapotranspiration, and, locally, to the Colorado River.

114°56’

35°3’

34°2’

33°1’

113°55’

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Colorado

River

Colorado River Aqueduct Colorado River Aqueduct

Palo Verde Valley

Groundwater Basin

Palo Verde Mesa

Groundwater Basin

Yuma

Groundwater Basin

Needles

Groundwater Basin

NEVADA

ARIZONA

EXPLANATION

Lake

Dry lake bed

Urban area

Alluvium

Hard rock

Study area 

boundary

Needles

Blythe

High Moderate

CONSTITUENT CONCENTRATIONS

Low or not detected 

Pie charts illustrate the percentages of the primary 

aquifers, on an areal basis, with concentrations in the 

three specified categories.

45

35

20

Organic

constituents

Inorganic

constituents

Number analyzed: 27 Number analyzed: 79

90

10
RESULTS: Groundwater Quality in the Colorado River 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 Colorado River study area, 

trace elements with human-health benchmarks were present at high concentrations in 

40% of the primary aquifers, on an areal basis, and at moderate concentrations in 30%. 

Of the 17 trace elements with human-health benchmarks analyzed in this study, 5 were 

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

Chromium was detected 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 Colorado River study area, radioactive constituents were 

detected at concentrations above benchmarks in 15% of the primary aquifers, and at 

moderate concentrations in 10%. Six radioactive constituents were analyzed; of these, 

uranium and gross alpha radioactivity were detected at concentrations above benchmarks. Radon was detected at values within one-half of the proposed upper benchmark.

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 

moderate concentrations.

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 Colorado River 

study area, these constituents were present at high concentrations in 65% of the primary 

aquifers, and at moderate concentrations in 25%. Of the seven constituents with nonhealth-based 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 that 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 Colorado River study area, perchlorate was found at moderate 

concentrations in 35% of the primary aquifers.

Other

inorganic

constituents

Perchlorate

40 30

30

75

99

10

35

65

25

65

10

15

Trace 

elements

Radioactive

constituents 

Nutrients

INORGANIC CONSTITUENTS

1
RESULTS: Groundwater Quality in the Colorado River 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 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 are characterized by their tendency to volatilize into the air. Of 56 VOCs 

with health-based benchmarks analyzed in the Colorado River study area, the VOC 

1,2-dichloropropane, a fumigant, was detected at moderate concentrations in 10% of 

the primary aquifers. All other VOCs were detected at low concentrations, or were not 

detected.

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. 

In the Colorado River study area, pesticides were not detected or were only detected 

at low concentrations. Of the 20 pesticides with health-based benchmarks that were 

analyzed, none were detected at high or moderate concentrations in the Colorado River 

primary aquifers.

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).

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

ORGANIC CONSTITUENTS

Pesticides

Volatile

organic

compounds

100

90

10
Trace Element Concentrations Throughout the 

California Desert Region

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

primary aquifers. In the Colorado River basins, they were high in 40% of the primary 

aquifers, on an areal basis; the percentage of area with high trace elements in all of the 

Desert Region primary aquifers ranged from 15 to 54% (Milby Dawson and Belitz, 

2012). The trace elements with the highest percentages in the Colorado River basins 

were fluoride (high concentrations in 20%), strontium (10%), and boron (10%). Fluoride and strontium were detected at high concentrations in most of the primary aquifers 

in the Desert Region (five of six and four of six, respectively). Boron was detected at 

high concentrations in all of the primary aquifers 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/

Goldrath, D.A., Wright, M.T., and Belitz, K., 2010, Groundwater-quality data in the 

Colorado River study unit, 2007—Results from the California GAMA program: U.S. 

Geological Survey Data Series 474, 66 p., available at 

http://pubs.usgs.gov/ds/474/

Milby Dawson, B.J., and Belitz, K., 2010, 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

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100 MILES

Owens

Valley

Indian Wells

Mojave

Colorado River

Coachella

Antelope

NEVADA

ARIZONA

MEXICO

CALIFORNIA

20%

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80%

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100%

Owens

Indian Wells

Coachella

Colorado River

Antelope

Mojave

High aquifer proportion

EXPLANATION

Moderate aquifer proportion

Trace Elements

Fluoride

Strontium

Boron

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

20%

0%

40%

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100%

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100%

Owens

Indian Wells

Coachella

Colorado River

Antelope

Mojave

High aquifer proportion

EXPLANATION

Moderate aquifer proportion

Trace Elements

Fluoride

Strontium

Boron

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

Desert Region Study Areas