U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3096

September 2012 Printed on recycled paper

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

Groundwater Quality in the Coastal Los Angeles 

Basin, 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 Coastal Los Angeles Basin constitutes 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 aquifer system

with concentrations in the three specified categories. 

26

6

68

4 13

83

Santa 

Monica

Santa Ana

Los Angeles

Huntingon

Beach

Long

Beach

Anaheim

Central

Basin

West Coast 

Basin

Hollywood Basin

Santa

Monica

Basin

Orange County

Coastal Plain

Los Angeles River

San Gabriel River

Rio Hondo 

Santa Ana River

LOS ANGELES CO

ORANGE CO

SAN BERNARDINO CO

Pacific Ocean

Santiago Ck

Forebay area

Pressure area

118°20’ 117°40’

34°

33°

40’

118°

0

0 5 10 MILES

5 10 KILOMETERS

West Coast Basin

Barrier Project

Alamitos Gap

Barrier Project

Talbert

Barrier Project

Dominguez Gap

Barrier Project

Rio Hondo

Recharge Facility

San Gabriel River

Recharge Facility

Santa Ana River

Recharge Facility

Santiago Creek

Recharge Facility

San Pedro

Bay

Santa

Monica

Bay Newport-Inglewood Fault Zone

The Coastal Los Angeles Basin Study Unit

The Coastal Los Angeles Basin study unit is approximately 860 square miles and consists 

of the Santa Monica, Hollywood, West Coast, Central, and Orange County Coastal Plain 

groundwater basins (California Department of Water Resources, 2003). The basins are bounded 

in part by faults, including the Newport-Inglewood fault zone, and are filled with Holocene-, 

Pleistocene-, and Pliocene-age marine and alluvial sediments. The Central Basin and Orange 

County Coastal Plain are divided into a forebay zone on the northeast and a pressure zone in the 

center and southwest. The forebays consist of unconsolidated coarser sediment, and the pressure zones are 

characterized 

by lenses of 

coarser sediment divided 

into confined 

to semi-confined aquifers 

by lenses of 

finer sediments. The 

primary aquifer system in 

the study unit 

is defined as 

those parts 

of the aquifer 

system corresponding to 

the perforated 

intervals of 

wells listed 

in the California Department of Public Health (CDPH) database of public-supply wells. The 

majority of public-supply wells are drilled to depths of 510 to 1,145 feet, consist of solid casing 

from the land surface to a depth of about 300 to 510 feet, and are perforated below the solid 

casing. Water quality in the primary aquifer system may differ from that in the shallower and 

deeper parts of the aquifer systems.

The study area has hot, dry summers and cool, moist winters. Average annual rainfall is 

about 15 inches. The Los Angeles, San Gabriel, and Santa Ana Rivers and the Rio Hondo, with 

headwaters in the mountains to the north and east of the study unit, are the primary streams 

traversing the study unit. Land use in the study unit is about 86 percent (%) urban, 12% natural, 

and 2% agricultural. Natural lands include coastal ecological preserves. The urban landscape 

consists of residential, commercial, and industrial areas, and the large urban centers include the 

cities of Los Angeles, Long Beach, Anaheim, Santa Ana, and Huntington Beach.

Recharge to the Central Basin and Orange County Coastal Plain occurs primarily by engineered recharge of stormwater, imported water, and reclaimed water along the upper reaches 

of the San Gabriel and Santa Ana Rivers and the Rio Hondo. Recharge to the West Coast Basin 

occurs primarily by injection of imported water and reclaimed water into wells of the seawater 

intrusion barrier and by underflow from the Central Basin. Groundwater flow directions are 

controlled by the engineered recharge and by groundwater pumping from the many hundreds of 

wells distributed across the region (Shelton and others, 2001; Dawson and others, 2003). 

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 blue 

section at the bottom of 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 Coastal Los 

Angeles Basin study unit, one or more 

inorganic constituents were present at high 

concentrations in about 6% of the primary 

aquifer system and at moderate concentrations in about 26%.

Human-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 about 4% of the primary 

aquifer system and at moderate concentrations in about 13%. 
 s h

Trace and

minor elements 

Total

dissolved

solids 

Nutrients

Uranium and

radioactive

constituents

Iron or

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENT

47 51

66

19

15

2 6

92

78

20 2

96

2 2

2

<1

35

64

RESULTS: Groundwater Quality in the Coastal Los Angeles Basin 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 Coastal Los Angeles Basin study unit, trace elements were detected at high concentrations in about 2% 

of the primary aquifer system, and at moderate concentrations in about 6%. Arsenic 

was the trace element that most frequently occurred at high and moderate concentrations. 

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. Uranium and gross alpha radioactivity occurred at high levels in 

about 2% of the primary aquifer system, and at moderate levels in about 20%.

Nutrients are naturally present at low concentrations in groundwater, and high 

and moderate concentrations generally occur as a result of 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 2% of the primary aquifer system, and at moderate concentrations in 

about 2%.

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 groundwater naturally contains TDS as a result of the weathering and 

dissolution of minerals in sediments. Anoxic conditions in groundwater (low amounts 

of dissolved oxygen) may result in release of manganese and iron from minerals into 

groundwater.

In the Coastal Los Angeles Basin study unit, TDS concentrations were high 

(greater than the upper limit) in about 2% of the primary aquifer system, and moderate (between the recommended and upper limits) in about 47%. Iron or manganese (or 

both) were present at high concentrations in about 19% of the primary aquifer system, 

and at moderate concentrations in about 15%.

Special-Interest Constituents 

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

Perchlorate is an inorganic constituent that has been regulated in California drinking water since 2007. It is an ingredient in rocket fuel, fireworks, and other products, 

may be present in some fertilizers, and occurs at low concentrations under natural 

conditions in groundwater. Perchlorate was detected at high concentrations in <1% of 

the primary aquifer system and at moderate concentrations in about 35%. 

1,4-Dioxane is used as a stabilizer for solvents, and N-nitrosodimethylamine 

(NDMA) is an ingredient in rocket fuel and may be formed as a byproduct of disinfection treatment. 1,4-Dioxane was detected at high and moderate concentrations in the 

primary aquifer system, and NDMA was detected at moderate concentrations; however, the number of samples was not great enough to provide a statistically representative estimate of proportions of the primary aquifer system with high or moderate 

concentrations.
High concentrations

Moderate concentrations

Low concentrations or

not detected

EX

AMPLE Values are a percentage of the area of the primary aquifer system

with concentrations in the three specified categories.

CONSTITUENT CONCENTRATIONS

Solvents

Gasoline

additives

ORGANIC CONSTITUENTS

Herbicides

Trihalomethanes

and other VOCs

98

2

4 11

85

99

<1

94

6

RESULTS: Groundwater Quality in the Coastal Los Angeles Basin 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 Coastal Los Angeles Basin study unit, solvents were present at high concentrations in about 4% of the primary aquifer system, and at moderate concentrations in 

11%. The solvents detected at high concentrations were tetrachloroethene (PCE), trichloroethene (TCE), 1,1-dichloroethene, 1,2-dichloroethane, and carbon tetrachloride. 

Trihalomethanes can form during disinfection of water supplies and may enter 

groundwater by the infiltration of landscape irrigation water or leakage of distribution 

systems. Trihalomethanes were not detected at high concentrations and were present at 

moderate concentrations in about 2% of the primary aquifer system.

Gasoline additives increase the efficiency of fuel combustion. Gasoline additives 

were not detected at high concentrations and were present at moderate concentrations in 

<1% of the primary aquifer system.

Other VOCs, including organic synthesis reagents and refrigerants, were not 

detected at moderate or high concentrations.

Pesticides with Human-Health Benchmarks

Pesticides, including herbicides, insecticides, and fumigants, are applied to crops, 

gardens, lawns, around buildings, and along roads to help control unwanted vegetation 

(weeds), insects, fungi, and other pests. In the Coastal Los Angeles Basin study unit, 

herbicides were not detected at high concentrations in the primary aquifer system, but 

were present at moderate concentrations in about 6%. Insecticides and fumigants 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) are used when available. Otherwise, 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) 

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 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 by Goldrath and others (2012).
Santa 

Monica

Santa Ana

Los Angeles

Huntingon

Beach

Long

Beach

Anaheim

Central

Basin

West Coast 

Basin

Hollywood

Basin

Santa

Monica

Basin

Orange County

Coastal Plain

Los Angeles River

San Gabriel River

Rio Hondo 

Santa Ana River

LOS ANGELES CO

ORANGE CO

SAN BERNARDINO CO

Recharge 

facilities 

Pacific Ocean

Recharge 

facilities 

Santiago Ck

118°20’ 117°40’

34°

33°

40’

118°

0

0 5 10 MILES

5 10 KILOMETERS

RELATIVE-CONCENTRATION

OF SOLVENTS

CDPH Well USGS Well

No detect

Low

Moderate

High

San Pedro

Bay

Santa

Monica

Bay

Factors that Affect Groundwater Quality

In the Coastal Los Angeles Basin study unit, solvents were detected at high 

concentrations in about 4% of the primary aquifer system and at moderate concentrations in about 11%. Solvents were detected in 42% of the grid wells sampled (Goldrath 

and others, 2012). PCE and TCE were the most frequently detected solvents. The map 

below shows the distribution of solvents in groundwater. Groundwater with high and 

moderate concentrations of solvents is most prevalent in the central and eastern parts 

of the Central Basin. Groundwater in the Central Basin moves radially and laterally 

from current and historical recharge areas along the river channels in the forebay areas 

(Shelton and others, 2001; Dawson and others, 2003). The occurrence of solvents in 

the groundwater is not correlated with current overlying land use and human activities. 

Rather, the solvents are from legacy industrial uses, and it has taken decades for the 

groundwater to move from the areas of recharge to the areas of the basin from which it 

is now being pumped (Shelton and others, 2001; Dawson and others, 2003). The discovery of solvents in water supplies in the 1980s prompted development of Federal MCLs 

for solvents and cleanup efforts (U.S. Environmental Protection Agency, 2012). Agencies responsible for stewardship of groundwater resources in the basin have extensive 

treatment programs to remove solvents from drinking-water supplies before delivery to 

consumers (Water Replenishment District of Southern California, 2012). 

SELECTED REFERENCES

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

Resources Bulletin 118, 246 p. 

Dawson, B.J.M., Belitz, Kenneth, Land, Michael, and Danskin, W.R., 2003, Stable isotopes and volatile 

organic compounds along seven ground-water flow paths in divergent and convergent flow systems, 

Southern California, 2000: U.S. Geological Survey Water-Resources Investigations Report 03-4059, 79 p.

Goldrath, Dara, Fram, M.S., Land, Michael, and Belitz, Kenneth, 2012, Status of groundwater quality in the 

Coastal Los Angeles Basin, 2006—California GAMA Priority Basin Project: U.S. Geological Survey 

Scientific Investigations Report 2012-5048, 62 p. (Also available at http://pubs.usgs.gov/sir/2012/5048/.) 

Shelton, J.L., Burow, K.R., Belitz, Kenneth, Dubrovsky, N.M., Land, Michael, and Gronberg, J.A., 2001, 

Low-level volatile organic compounds in active public supply wells as ground-water tracers in the Los 

Angeles physiographic basin, California, 2000: U.S. Geological Survey Water-Resources Investigations 

Report 01-4188, 29 p.

U.S. Environmental Protection Agency, 2012, Safe Drinking Water Act (SDWA), accessed March 2012 at 

http://water.epa.gov/lawsregs/rulesregs/sdwa/.

Water Replenishment District of Southern California, accessed March 2012 at http://www.wrd.org. 

Priority Basin Assessments 

GAMA’s Priority Basin Project 

(PBP) assesses water quality in the part 

of the aquifer systems used for drinking 

water, primarily public supply. Water 

quality in shallower and deeper parts 

may differ from that in the primary 

aquifer system. GAMA’s Domestic 

Well Project assesses water quality in 

the shallower parts of the aquifer systems. 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 Coastal Los 

Angeles Basin study unit, data were 

collected by the PBP in 2006 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 Miranda S. Fram and Kenneth Belitz