U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3037

July 2012 Printed on recycled paper

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

Groundwater Quality in the Upper Santa Ana 

Watershed Study Unit, 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. The Upper Santa Ana Watershed is 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 aquifers with

concentrations in the three specified categories. Values on pie

chart may not equal 100 due to rounding of percentages.

7

82

33

29

38 11

Big Bear Lake

Bautista Creek

91

60

10

15

5

15

15

215

215

10

ORANGE CO

RIVERSIDE CO

LOS

ANGELES

CO

SAN BERNARDINO CO

San Juan Creek

California Aqueduct

Arroyo

Trabuco

Santa Ana River

MWD Upper

Feeder

MWD Lower

Feeder

San Gabriel River

Deep Creek

Colorado River Aqueduct

Lytle Creek

Santa Ana River

Mojave River

Bear Creek

Chino Creek

Perris

Dam Santiago

Dam

UPPER SANTA

ANA VALLEY

SAN

JACINTO

ELSINORE

San Bernardino

Riverside

Lake

Elsinore

Hemet

Railroad

Canyon

Dam

San Jacinto River San Gorgonio River

Prado Dam

Wetlands

Mathews

Dam

Chino

Dairy

Preserve

0 10 20 Miles

0 10 20 Kilometers

San Bernardino Mts

San Gabriel Mts

Santa Ana Mts

Chino Hills

San Jacinto Mts

The Upper Santa Ana Watershed Study Unit

The Upper Santa Ana Watershed study unit covers approximately 1,000 square miles. 

Within this study unit, the San Jacinto and Elsinore groundwater basins each constituted a 

study area, and the nine subbasins of the Upper Santa Ana Valley groundwater basin were 

grouped into four additional study areas based on geography: Bunker Hill/Cajon/RialtoColton; Cucamonga/Chino; Riverside-Arlington/Temescal; and Yucaipa/San Timoteo (Kent 

and Belitz, 2009). 

The study unit has hot, 

dry summers and cool, moist 

winters. Average annual 

rainfall ranges from 10 to 

24 inches in the valleys and 

from 24 to 48 inches in the 

mountains within and surrounding the study unit. 

The Santa Ana River, 

with its headwaters in the 

San Bernardino Mountains, is 

the longest river in Southern 

California and flows southwestward through the Upper 

Santa Ana Valley. Streamflow 

in the Santa Ana River varies 

seasonally; the highest flows 

are generally in the winter 

and spring. In the dry season, 

most streamflow in the Santa 

Ana River consists of tertiary 

treated wastewater (Belitz and 

others, 2004).

Most aquifers of the 

study unit are unconfined, with some confined groundwater in the Bunker Hill and Chino 

Subbasins and the eastern part of the San Jacinto Basin. Groundwater flow in most of the 

study unit is affected by numerous faults cutting the alluvial sediments. The primary aquifers of the Upper Santa Ana Watershed study unit are defined as those parts of the aquifers 

underlying the study unit and corresponding to the perforated intervals of the wells listed in 

the California Department of Public Health (CDPH) database. Public-supply wells typically 

are drilled to depths of 200 to 650 feet, consist of solid casing from the land surface to depths 

of about 175 to 500 feet, and then are perforated below the solid casing. Water quality in the 

primary aquifers may differ from that in the shallower and deeper parts of the aquifer system. 

Land use in the study unit is approximately 44 percent (%) natural, 21% agricultural, 

and 35% urban. Most of the land use adjacent to the study unit is natural and consists of steep 

mountains or hills generally covered by forest or chaparral. 

Recharge to the groundwater system occurs through direct infiltration, return irrigation 

flow, and stream-channel infiltration from the Santa Ana and San Jacinto Rivers and their 

tributaries, and from numerous engineered recharge facilities operating in the study unit. The 

primary sources of discharge are water pumped for municipal supply, evaporation from areas 

with a shallow depth to water, and discharge to streams.

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.

Many inorganic constituents occur 

naturally in groundwater. The concentrations of inorganic constituents can be 

affected by natural processes as well as 

by human activities. In the Upper Santa 

Ana Watershed study unit, one or more 

inorganic constituents were present at 

high concentrations in about 33% of the 

primary aquifers and at moderate concentrations in about 29% of the primary 

aquifers.

Organic constituents can be found 

in products used in the home, business, 

industry, or agriculture, and can enter the 

environment through normal usage, spills, 

or improper disposal. In this study unit, 

one or more organic constituents were 

present at high concentrations in about 7% 

of the primary aquifers and at moderate 

concentrations in about 11%.
 s h

Trace and

minor elements 

Total

dissolved

solids 

Nutrients

(nitrate plus

nitrite)

Uranium and

radioactive

constituents

Iron or

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENT

7

16

77

82

4 14

49

25

25

25

5

70

90

11

53

36

4 6

RESULTS: Groundwater Quality in the Upper Santa Ana Watershed Study Unit

Inorganic Constituents with Human-Health 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 Upper 

Santa Ana Watershed study unit, one or more of the trace elements were present at 

high concentrations in about 7% of the primary aquifers, on an areal basis. Arsenic, 

boron, and molybdenum were the trace elements that most frequently occurred at 

high concentrations. Aluminum, fluoride, lead, uranium, and vanadium also were 

detected at high concentrations, but each of them were detected at high concentrations in less than 2% of the primary aquifers. 

Radioactivity is the emission 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 

aquifer sediment. Gross alpha radioactivity, uranium, or radium-228 occurred at high 

levels in about 4% of the primary aquifers, and at moderate levels in about 14%.

Nutrients, such as nitrate and nitrite, can be naturally present at low concentrations in groundwater. High and moderate concentrations generally occur as a result 

of human activities, such as applying fertilizer to crops. Livestock, when in concentrated numbers, and septic systems also produce nitrogenous waste that can leach 

into groundwater. Nitrate plus nitrite was present at high concentrations in about 

25% of the primary aquifers, and also at moderate concentrations in about 25% of the 

primary aquifers.

Inorganic Constituents with Non-Health Benchmarks

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

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

color, and odor, or can create nuisance problems, such as staining and scaling. The 

State of California has a recommended limit 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 Upper Santa Ana Watershed study unit, TDS was present at high concentrations (greater than the upper limit) in about 5% of the primary aquifers. About 

25% of the primary aquifers had moderate TDS concentrations (between the recommended and upper limit). Iron and manganese are naturally occurring elements, but 

anoxic conditions (low amounts of dissolved oxygen) in groundwater can result in 

release of iron and manganese from minerals into groundwater. Iron or manganese, 

or both, were present at high concentrations in about 4% of the primary aquifers. Iron 

or manganese, or both, were present at moderate concentrations in about 6% of the 

primary aquifers and at low concentrations in about 90%.

Perchlorate 

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

Perchlorate, an inorganic constituent, is of special interest in California because 

it has recently been found in drinking-water supplies. Perchlorate in groundwater is 

monitored by the California Department of Public Health (http://www.cdph.ca.gov). 

It is an ingredient in rocket fuel, fireworks, safety flares, and other products, may be 

present in some fertilizers, and also occurs naturally at low concentrations in groundwater. Perchlorate was present at high concentrations in about 11% of the primary 

aquifers, and at moderate concentrations in about 53% of the primary aquifers.
High concentrations

Moderate concentrations

Low concentrations

EX

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

concentrations in the three specified categories. Values on pie

chart may not equal 100 due to rounding of percentages.

CONSTITUENT CONCENTRATIONS

Fumigants

Solvents

Organic

synthesis

reagents

ORGANIC CONSTITUENTS

96

Herbicides

Trihalomethanes

and other VOCs 100

96

1 3

4

99

1

88

3 9

RESULTS: Groundwater Quality in the Upper Santa Ana Watershed Study Unit

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect low concentrations of volatile organic compounds (VOCs) and pesticides, far below human-health 

benchmarks. The presence of 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.

Solvents are used for a number of purposes, including manufacturing and cleaning. 

In the Upper Santa Ana Watershed study unit, solvents were present at high concentrations in about 3% of the primary aquifers. The solvents found at high concentrations 

were tetrachloroethene (PCE), trichloroethene (TCE), and carbon tetrachloride. Solvents were present at moderate concentrations in about 9% of the primary aquifers, and 

were low or not detected in about 88%.

Organic synthesis reagents are compounds used in the manufacture or preparation 

of organic compounds. One organic synthesis reagent, 1,1-dichloroethene, was found at 

high concentrations in about 1% of the primary aquifers, and at moderate concentrations 

in about 3%.

Other VOCs include gasoline additives, trihalomethanes, and refrigerants. No 

gasoline additives, trihalomethanes, or refrigerants were detected at high or moderate 

concentrations in the Upper Santa Ana Watershed study unit. 

Pesticides with Human-Health Benchmarks

Pesticides are applied to crops, gardens, lawns, around buildings, and along roads 

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

Upper Santa Ana Watershed study unit, insecticides were only detected at low concentrations in the primary aquifers. However, 1,2-dibromo-3-chloropropane (DBCP), a 

fumigant that was used to control crop damage from nematode worms prior to 1977, 

was found at high concentrations in about 4% of the primary aquifers. Herbicides were 

not detected at high concentrations in the primary aquifers, but the herbicide atrazine 

was detected at moderate concentrations in about 1% of the primary aquifers. All other 

pesticides, when detected, were at low 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 

(Secondary Maximum Contaminant Level, SMCL), such as taste and odor, and nonregulatory benchmarks for protecting human health (Notification Level, NL, and Lifetime 

Health Advisory, 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, concentrations are moderate

if they are greater than one-half of a benchmark. For organic and special-interest constituents, 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 Kent and Belitz (2012).
Tetrachloroethene

(PCE)

Trichloroethene

(TCE)

1,1-Dichloroethene

(1,1-DCE)

1,2-Dibromo-3-chloropropane

(DBCP)

Carbon tetrachloride

Nitrate

0 10 20 30 40 50 60 70 80 90 100

DETECTION FREQUENCY, IN PERCENT

CONCENTRATION CATEGORY

OF COMPOUND NAMED

High

Moderate

Low (excludes

 nondetections) 

Perchlorate

Anthropogenic Constituents in the Upper Santa Ana Watershed 

Study Unit

In California, high concentrations of organic constituents introduced by human 

(anthropogenic) activity generally occur in less than 1% of the aquifer system (Belitz 

and Landon, 2010). However, in the Upper Santa Ana Watershed and nearby San 

Gabriel and San Fernando Valleys, high concentrations of organic constituents (greater 

than health-based thresholds) are present in nearly 7% of the primary aquifers (Kent and 

Belitz, 2012). Solvents, including tetrachloroethene (PCE), trichloroethene (TCE), and 

carbon tetrachloride, were detected at high concentrations in about 3% of the primary 

aquifers. 1,2-Dibromo-3-chloropropane (DBCP), a fumigant, was detected at high 

concentrations in about 4% of the primary aquifers. 

Nitrate and perchlorate are inorganic chemicals with natural and anthropogenic 

sources, but in the Upper Santa Ana Watershed, both enter groundwater mostly through 

human activities (Belitz and others, 2004). Anthropogenic sources of nitrate include 

agriculture (fertilizer and livestock 

waste), effluent 

from wastewater 

treatment plants, 

and septic systems 

(Belitz and others, 

2004). Nitrate was 

present at high concentrations in about 

25% of the primary 

aquifers in the 

Upper Santa Ana 

Watershed study 

unit. Nationwide, 

anthropogenic 

sources of perchlorate in groundwater include residues from road flares, explosives, and 

fireworks, along with accidental releases from perchlorate manufacturing. In addition 

to these potential sources, perchlorate was incidentally introduced to the Upper Santa 

Ana Watershed in the early to mid-1900s from nitrate fertilizer that was imported for 

use in citrus groves (Santa Ana Regional Water Quality Control Board, 2004). Perchlorate was detected at high concentrations in about 11% of the primary aquifers in the 

present study. High concentrations of nitrate and perchlorate are more prevalent in the 

Upper Santa Ana Watershed than in other California study units.

SELECTED REFERENCES

Belitz, Kenneth, Hamlin, S.N., Burton, C.A., Kent, Robert, Fay, R.G., and Johnson, Tyler, 2004, Water quality 

in the Santa Ana Basin, California, 1999–2001: U.S. Geological Survey Circular 1238, 37 p. (Also available at http://pubs.usgs.gov/circ/2004/1238/.)

Belitz, Kenneth, and Landon, M.K., 2010, Groundwater quality in the Central Eastside San Joaquin Valley, 

California: U.S. Geological Survey Fact Sheet 2010-3001, 4 p. (Also available at http://pubs.usgs.gov/

fs/2010/3001/.)

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

Resources Bulletin 118, 246 p. (Also available at http://www.water.ca.gov/groundwater/bulletin118/

update2003.cfm.) 

Kent, Robert, and Belitz, Kenneth, 2009, Ground-water quality data in the Upper Santa Ana Watershed Study 

Unit, November 2006–March 2007—Results from the California GAMA Program: U.S. Geological Survey 

Data Series 404, 116 p. (Also available at http://pubs.usgs.gov/ds/404.) 

Kent, Robert, and Belitz, Kenneth, 2012, Status of groundwater quality in the Upper Santa Ana Watershed, 

November 2006–March 2007—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2012-5052, 88 p. (Also available at http://pubs.usgs.gov/sir/2012/5052/.)

Santa Ana Regional Water Quality Control Board, 2004, Senate Select Committee on Perchlorate Contamination—Status of cleanup efforts in the Inland Empire, February 27, 2004, presentation by 

Gerard J. Thibeault, Executive Officer California Regional Water Quality Control Board, Santa Ana 

Region, accessed December 8, 2010, at http://www.waterboards.ca.gov/rwqcb8/water_issues/programs/

perchlorate/docs/2004_feb_27_senate_hearing.pdf.

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 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 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 waterquality, hydrologic, geographic, and 

other data to help assess the factors that 

affect water quality. In the Upper Santa 

Ana Watershed study unit, data were 

collected by the PBP in 2006–2007 

and from the CDPH database for 

2003–2006. 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

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/

water_issues/programs/gama

By Robert Kent and Kenneth Belitz