U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3139

March 2012 Printed on recycled paper

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

Groundwater Quality in the San Fernando–San 

Gabriel Groundwater 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. The San Fernando 

and San Gabriel groundwater basins 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 aquifers

with concentrations in the three specified categories. Values

on piechart may not equal 100 due to rounding of percentages. 

58 33

9

39

43

18

Los Angeles River

Los Angeles Aqueduct

Los Angeles River

San Gabriel River

Santa Ana River

Rio Hondo

SAN

BERNARDINO

CO

LOS ANGELES CO

ORANGE CO Pacific Ocean

San Gabriel Mountains

Elysian

Hills Merced

Hills

Puente

Hills

Whittier

Narrows

Repetto

Hills Santa Monica Mountains

Santa Susana

Mountains

Azusa

Burbank

Los Angeles

Northridge

Pasadena

San Fernando

West Covina

Alhambra

0 5 10 Miles

0 5 10 Kilometers

EXPLANATION

San Fernando

Groundwater

 basin boundary

San Gabriel 

STUDY AREA

Raymond

groundwater

basin

San Gabriel

Valley

groundwater

basin

San Fernando

Valley

groundwater

basin

The San Fernando–San Gabriel Study Unit

The San Fernando–San Gabriel study unit is approximately 460 square miles and 

consists of the San Fernando Valley, Raymond, and San Gabriel Valley groundwater basins 

(California Department of Water Resources, 2003). The study unit has hot, dry summers 

and cool, moist winters. Average annual rainfall ranges from 17 to 21 inches over the three 

basins. The study areas are drained by the Los Angeles, San Gabriel, and Rio Hondo rivers 

and their tributaries (Land and Belitz, 2008).

The San Fernando and San Gabriel Valleys are sedimentologically diverse basins situated within the Transverse Ranges of southern California. These structurally complex basins 

formed as a result of the dextral slip of the San Andreas Fault system in the late TertiaryQuaternary (Tinsley, 2001). The valleys have been filled, up to 6,400 feet, with marine and 

terrestrial sediments of Pleistocene through Holocene age that overlie crystaline basement. 

The alluvium and underlying marine deposits contain freshwater used for supply. The primary aquifers in the study unit are defined as those parts of the aquifers corresponding to the 

perforated intervals of wells listed in the California Department of Public Health (CDPH) 

database. Public-supply wells are typically drilled to depths of 400 to 785 feet, consist of 

solid casing from the land surface to a depth of about 157 to 300 feet, and 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. 

Groundwater movement is generally 

from the northern 

parts of the valleys 

south towards the 

Pacific Ocean.

Land use 

in the study unit 

is approximately 

83 percent (%) 

urban, 16% natural, 

and 1% agricultural. 

The urban landscape consists of residential, commercial, and industrial areas. Natural lands 

are mostly shrublands and grasslands. The largest urban areas are Los Angeles, San Fernando, Northridge, Burbank, Pasadena, Alhambra, Azusa, and West Covina.

Recharge to the groundwater basins occurs from the infiltration of runoff and imported 

water at spreading basins, infiltration of precipitation and irrigation, and infiltration of 

streamflow from the major rivers and their tributaries. Streamflow is a combination of runoff 

from the surrounding mountains, imported water, industrial discharges, and treated wastewater effluent. The primary sources of discharge are water pumped for municipal supply and 

for irrigation.

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 the inorganic constituents 

can be affected by natural processes as 

well as by human activities. In the San 

Fernando–San Gabriel study unit, one or 

more inorganic constituents were present 

at high concentrations in about 9% of the 

primary aquifers and at moderate concentrations in about 33%.

Man-made organic constituents are 

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

constituents 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 18% of the primary 

aquifers and at moderate concentrations 

in about 43%
 

 

Trace and 

minor elements

Inorganics with

non-health

benchmarks

Nutrients

Radioactive

constituents

NDMA

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENTS

86

12

99

27

9

65

8 5

87

Perchlorate

12

11

77

18

79

3

1

2

RESULTS: Groundwater Quality in the San Fernando–San Gabriel 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 San Fernando–San Gabriel study unit, trace and minor elements were present at high concentrations in about 2% of the primary aquifers. Chromium was the trace element 

detected at high concentration, and fluoride was the minor element detected at high 

concentration. Aluminum, lead, and vanadium also occurred at high concentrations, 

but in less than 1% of the primary aquifers.

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. Gross alpha radioactivity occurred at high levels in less 

than 1% of the primary aquifers. Radium had high concentration in the CDPH database in less than 1% of the primary aquifers, but this high value was not the most 

recent data used in this assessment.

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 applying fertilizer to 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 9% of the primary aquifers, and at moderate concentrations in about 27% of the primary aquifers.

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

dissolution of minerals in soils. Anoxic conditions (very low amounts of dissolved 

oxygen) in groundwater may result in release of the naturally occurring elements 

manganese and iron from minerals into groundwater. 

In the San Fernando–San Gabriel study unit, iron was present at high concentrations in about 3% of the primary aquifers. TDS was present at high concentrations 

(greater than the upper limit) in less than 1% of the primary aquifers. About 18% of 

the primary aquifers had moderate TDS concentrations (between the recommended 

and upper limits). 

 Perchlorate and N-Nitrosodimethylamine (NDMA) 

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

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, 

and other products, may be present in some fertilizers, and also occurs naturally 

at low concentrations in groundwater. Perchlorate was detected at high concentrations in about 11% of the primary aquifers, and at moderate concentrations in 12%. 

NDMA, a semi-volatile organic chemical, is of special interest in California because 

it has recently been detected in drinking-water supplies. NDMA was present at high 

concentrations in about 5% of the primary aquifers, and at moderate concentrations 

in about 8%.
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

Solvents

Other volatile

organic

compounds

ORGANIC CONSTITUENTS

Herbicides

Trihalomethanes

40

18

42

91

3 6

6

94

3

97

RESULTS: Groundwater Quality in the San Fernando–San Gabriel 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. VOCs and pesticides detected at these low concentrations can be used to 

help trace water from the landscape into the aquifer system.

Volatile Organic Compounds with Human-Health Benchmarks

VOCs, which include solvents, organic synthesis reagents, and trihalomethanes, 

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 San Fernando–San Gabriel study unit, solvents were present at high concentrations in about 18% of the primary aquifers. The solvents detected at high concentrations 

were trichloroethene (TCE) (14.8%), perchloroethene (PCE) (11.2%), carbon tetrachloride (6.5%), 1,2-dichloroethane (1.9%), cis-1,2-dichloroethene (1.0%), and 1-dichloroethane (0.5%). Solvents were present at moderate concentrations in about 40% of the 

primary aquifers.

Trihalomethanes may form during municipal water purification and enter groundwater by the infiltration of landscape irrigation water. Trihalomethanes were not 

detected at high concentrations, and were detected at moderate concentrations in about 

3% of the primary aquifers.

Within the classification of “other VOCs,” the organic synthesis reagent 

1,1-dichloroethene was detected at high concentration in about 3% of the primary aquifers, and at moderate concentrations in about 6%. Refrigerants and gasoline additives, 

also included in the class “other VOCs”, were detected at moderate concentrations in 

less than 1% of the primary aquifers (Land and others, 2012).

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. In the San Fernando–San Gabriel study 

unit, herbicides were not detected at high concentrations in the primary aquifers, but the 

herbicide atrazine was detected at moderate concentrations in about 6% of the primary 

aquifers. Insecticides were not detected at 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 when available. Nonregulatory 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, HAL) were used 

when Federal or California regulatory benchmarks were not available.

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 are generally less prevalent and have smaller concentrations relative to benchmarks than inorganic constituents. 

Low concentrations include non-detections and values less than moderate concentrations. Methods for evaluating water quality are 

discussed by Land and others (2012).
Los Angeles River

San Gabriel River

Rio Hondo

0 5 10 Miles

0 5 10 Kilometers

Raymond

groundwater

basin

San Gabriel

Valley

groundwater

basin

San Fernando

Valley

groundwater

basin

Solvent plume with

concentration above

detection limit

TCE plume with

concentration above

detection limit

EXPLANATION

San Fernando

CDPH Well USGS Well

Groundwater

 basin boundary

Solvent or

 TCE plume

San Gabriel 

STUDY AREA RELATIVE-CONCENTRATION

OF SOLVENTS

No detect

Low

Moderate

High

Factors that Affect Groundwater Quality

In the San Fernando–San Gabriel study unit, solvents were detected at high concentrations in 18% of the primary aquifers, while in most California Priority Basin Project study 

units, solvents were detected at high concentrations in less than 2% of the primary aquifers 

(for example, Kulongoski and Belitz, 2011). Overall, solvents were detected in 86% of the grid 

wells sampled. The number of solvents detected in each well ranged from 1 to 11, and the solvent concentrations ranged from 0.01 to 220 micrograms per liter (μg/L). The median humanhealth regulatory benchmark for the 15 solvents detected is 6 μg/L (Land and others, 2012).

The six solvents that were detected at high concentrations also were frequently detected. 

PCE and TCE were detected in more than 50% of the primary aquifers. 1,1-Dichloroethane, 

carbon tetrachloride, and cis-1,2-dichloroethene were detected in more than 30%, and 

1,1,1-trichloroethane was detected in more than 10% of the grid wells sampled. In addition, 

six solvents were recorded at high concentrations in the CDPH database for the period before 

May 1, 2002, but not during the period of study, May 1, 2002–April 30, 2005 (Land and others, 2012).

The map below shows the distribution of solvents in groundwater and the locations of 

known solvent plumes in groundwater. High concentrations of solvents are generally clustered 

in the southern San Gabriel Valley and the central southern San Fernando Valley. Sources of 

solvents include metal plating, machinery degreasing, and dry cleaning. More than a dozen 

solvent plumes have been documented in the area (U.S. Environmental Protection Agency, 

2009; San Gabriel Basin Water Quality Authority, 2011).

 The discovery of solvents in San Gabriel Valley groundwater in 1980 led to the formation of the San Gabriel Basin Water Quality Authority in 1993 and extensive treatment programs to remove volatile organic compounds from drinking-water supplies (San Gabriel Basin 

Water Quality Authority, 2011).

By Justin T. Kulongoski 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.

Kulongoski, J.T., and Belitz, Kenneth, 2011,Status and understanding of groundwater quality in the Monterey Bay 

and Salinas Valley Basins, 2005—California GAMA Priority Basin Project: U.S. Geological Survey Scientific 

Investigations Report 2011–5058, 84 p. Available at http://pubs.usgs.gov/sir/2011/5058/.

Land, Michael, and Belitz, Kenneth, 2008, Ground-water quality data in the San Fernando–San Gabriel study unit, 

2005—Results from the California GAMA Program: U.S. Geological Survey Data Series 356, 84 p. Available 

at http://pubs.usgs.gov/ds/356/.

Land, Michael, Kulongoski, J.T., and Belitz, Kenneth, 2012, Status of groundwater quality in the San Fernando–

San Gabriel groundwater basins, 2005—California GAMA Priority Basin Project: U.S. Geological Survey 

Scientific Investigations Report 2011-5206, 66 p. Available at http://pubs.usgs.gov/sir/2011/5206.

San Gabriel Basin Water Quality Authority, accessed on March 21, 2011, at http://www.wqa.com.

Tinsley, J.C. III, 2001, Aspects of the quaternary geology of the San Fernando Valley, California, Cordilleran Section—97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists 

(April 9–11, 2001).

U.S. Environmental Protection Agency, 2009, San Fernando Valley superfund sites update, and EPA selects second interim remedy for the North Hollywood operable unit, 12 p. Available at http://yosemite.epa.gov/r9/sfund/

r9sfdocw.nsf/vwsoalphabetic/.

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 water-quality, hydrologic, geographic, and other 

data to help assess the factors that affect 

water quality. In the San Fernando–San 

Gabriel study unit, data were collected 

by the PBP in 2005 and from the CDPH 

database for 2002–2005. The PBP 

includes chemical analyses generally not 

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/

gama