U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2011–3111

December 2011 Printed on recycled paper

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

Groundwater Quality in the San Diego Drainages 

Hydrogeologic Province, California 

More than 40 percent of California’s drinking water is from groundwater. 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 Diego Drainages 

Hydrogeologic Province (hereinafter referred to as San Diego) 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.

3 3

94 32

18

50

Santa

Margarita

River

San Luis Rey River

San Diego River

Sweetwater

River

SAN DIEGO COUNTY

RIVERSIDE COUNTY

Desert Province

Transverse Ranges and selected

Peninsular Ranges Province

UNITED STATES

MEXICO

ORANGE

COUNTY

Dana Point

San Diego

PENINSULAR 

Temecula

RANGES

Oceanside PACIFIC OCEAN

EXPLANATION

Temecula Valley study area

Warner Valley study area

Alluvial Basins study area

Hard Rock study area

Study unit boundary

0 10 20 MILES

0 10 20 KILOMETERS

 The San Diego Study Unit

The San Diego study unit is approximately 3,900 square miles and consists of the 

Temecula Valley, Warner Valley, and 12 other alluvial basins (California Department of Water 

Resources, 2003). The study unit also consists of all areas outside defined groundwater basins 

that are within 3 kilometers of a public-supply well. The study unit was separated, based 

primarily on hydrogeologic settings, into four study areas: Temecula Valley, Warner Valley, 

Alluvial Basins, and Hard Rock (Wright and others, 2005). The sampling density for the Hard 

Rock study area, which consists of areas outside of groundwater basins, was much lower than 

for the other study areas. Consequently, aquifer proportions for the Hard Rock study area 

are not used to calculate 

the aquifer proportions 

shown by the pie charts. An 

assessment of groundwater 

quality for the Hard Rock 

study area can be found in 

Wright and Belitz, 2011.

The temperatures in the 

coastal part of the study unit 

are mild with dry summers, 

moist winters, and an average annual rainfall of about 

10 inches. The temperatures 

in the mountainous eastern 

part of the study unit are 

cooler than in the coastal 

part, with an annual precipitation of about 45 inches that 

occurs mostly in the winter. 

The primary aquifers consist 

of Quaternary-age alluvium 

and weathered bedrock in 

the Temecula Valley, Warner 

Valley, and Alluvial Basins study areas, whereas in the Hard Rock study area the primary aquifers consist mainly of fractured and decomposed granite of Mesozoic age. The primary aquifers 

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 

typically are drilled to depths between 200 and 700 feet, consist of solid casing from the land 

surface to a depth of about 60 to 170 feet, and are perforated, or consist of an open hole, below 

the solid casing. Water quality in the shallow and deep parts of the aquifer system may differ 

from water quality in the primary aquifers.

Municipal water use accounts for approximately 70 percent of water used in the study 

unit; the majority of the remainder is used for agriculture, industry, and commerce. Groundwater accounts for approximately 8 percent of the municipal supply, and surface water, the majority of which is imported, accounts for the rest. Recharge to groundwater occurs through streamchannel infiltration from rivers and their tributaries, infiltration in engineered recharge basins, 

and infiltration of water from precipitation and irrigation. The primary source of discharge is 

water pumped from wells.

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 alluvial-filled 

basins of the San Diego study unit, one or 

more inorganic constituents were present at 

high concentrations in 18 percent of the primary aquifers and at moderate concentrations in 32 percent of the primary aquifers.

Organic constituents are in products used in the home, business, industry, 

and agriculture. Organic constituents can 

enter the environment through normal 

usage, spills, or improper disposal. In the 

alluvium-filled basins of the study unit, 

one or more organic constituents were 

present at high concentrations in 3 percent 

of the primary aquifers and at moderate concentrations in 3 percent of the 

primary aquifers. 
 s hh

Trace and 

minor elements

Total

dissolved

solids 

Nutrients

Radioactive

constituents

Iron or

manganese

Perchlorate

INORGANIC CONSTITUENTS

SPECIAL-INTEREST CONSTITUENTS

~63

~36

58 28

14

83

3 14

90

7 3

<1

31

14

55

83

14

3

RESULTS: Groundwater Quality in the San Diego 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 alluvialfilled basins in the San Diego study unit (Temecula Valley, Warner Valley, and 

Alluvial Basins), one or more trace elements were present at high concentrations in 

about 14 percent of the primary aquifers. Vanadium, arsenic, and boron were detected 

at high concentrations in at least 4 percent of the primary aquifers. Antimony was 

detected at high concenetrations in about 3 percent of the primary aquifers.

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 in minerals in the 

sediments of the aquifer. Radioactivity from the radioactive constituents occurred at 

high concentrations in 3 percent of the primary aquifers, and at moderate concentrations in 14 percent of the primary aquifers. Gross-alpha radioactivity was detected at 

high concentrations in 3 percent of the primary aquifers.

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. Septic systems and livestock, 

when in concentrated numbers, also produce nitrogenous waste that can leach into 

groundwater. Nutrients, in the form of nitrate, were present at high concentrations in 

3 percent of the primary aquifers, and at moderate concentrations in about 7 percent 

of the primary aquifers.

Inorganic Constituents with Non-Health Benchmarks

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

Some constituents, such as total dissolved solids (TDS), affect the aesthetic 

properties of water, such as taste, color, and odor. Other constituents, such as iron and 

manganese, can create nuisance problems, such as scaling and staining.

The State of California has a recommended an upper limit for TDS in drinking 

water. In the alluvial-fill basins of the San Diego study unit, TDS was present at high 

concentrations (greater than the upper limit) in 14 percent of the primary aquifers, at 

moderate concentrations (between the recommended and upper limit) in 31 percent 

of the primary aquifers, and at low concentrations (less than the recommended limit) 

in 55 percent of the primary aquifers.

Iron and manganese are naturally occurring elements, and one or both were 

present at high concentrations in 14 percent of the primary aquifers. Iron or manganese (or both) were present at moderate concentrations in 3 percent of the primary 

aquifers, and at low concentrations in about 83 percent of the primary aquifers.

Perchlorate, 1,4-dioxane, and 

N-Nitrosodimethylamine (NDMA) 

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

Perchlorate, 1,4-dioxane, and NDMA are of special interest in California 

because these constituents recently have been detected in groundwater and are considered to have the potential to affect the quality of drinking-water supplies. Their 

presence in groundwater is monitored by the California Department of Public Health 

(http://www.cdph.ca.gov). In the San Diego study unit, perchlorate was detected at 

high concentrations in less than 1 percent of the primary aquifers and at moderate 

concentrations in about 36 percent of the primary aquifers. 1,4-dioxane and NDMA 

were not detected.
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

Trihalomethanes

and other VOCs

Pesticides

Gasoline

components

ORGANIC CONSTITUENTS

100

100

97

3

97

3

RESULTS: Groundwater Quality in the San Diego 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 into the air.

Gasoline components are compounds that constitute gasoline, or are additives, 

such as methyl tert-butyl ether (MTBE). In the alluvium-filled basins of the San Diego 

study unit, gasoline components were present at high concentrations in 3 percent of the 

primary aquifers. The gasoline component detected at high concentration was MTBE. 

No gasoline components were detected at moderate concentrations. 

Solvents are used for a number of purposes, including manufacturing and cleaning. Solvents were not detected at high concentrations, but were detected at moderate 

concentrations in 3 percent of the primary aquifers. The solvent detected at moderate 

concentrations was 1,2-dichloropropane. 

Trihalomethanes may form during municipal water purification and enter groundwater by the infiltration of landscape irrigation water. Trihalomethanes were detected 

only at low concentrations in the primary aquifers.

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

detected either at high or at moderate concentrations.

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 San 

Diego study unit, all pesticides, which include herbicides, insecticides, and fumigants, 

were detected only 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) are 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 non-regulatory 

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 in Wright and Belitz (2011).
VANADIUM, IN MICROGRAMS PER LITER

80

70

0

60

50

40

30

20

10

6.0 6.5 7.0 7.5 8.0 8.5 9.0

Anoxic

Oxic

0.5 x Notification level

Notification level

100 

0 

200 

300 

400 

500 

1,000 

1,100 

1,200 

1,300 

BORON, IN MICROGRAMS PER LITER 

6.0 6.5 7.0 7.5 8.0 8.5 9.0 

pH, IN STANDARD UNITS

High

Moderate

Low

High

Moderate

Low

Factors that Affect Groundwater Quality

The trace elements vanadium (V) and boron (B) were detected at high and moderate concentrations in the San Diego study unit (Wright and Belitz, 2011). High concentrations for these constituents were detected almost exclusively in samples collected in 

the Temecula Valley study area. 

Natural sources of these constituents to groundwater primarily are from the dissolution of rocks. Vanadium is most abundant in mafic rocks, whereas B tends to be most 

abundant in shale and 

schist (Reimann and 

de Caritat, 1998). 

Potential anthropogenic sources of V 

to the environment 

include the burning 

of residual crude oil, 

and the waste streams 

associated with the 

ferrous metallurgy 

industry. Boron is 

found in cleaning 

agents and therefore may be present 

in municipal and 

industrial wastes. 

The human-health 

non-regulatory bench 

marks (notification 

levels, NL) for V and 

B are 50 and 1,000 

micrograms per liter, 

respectively.

The mobility of 

V and B in groundwater is controlled by the pH and (or) reduction-oxidation (redox) 

conditions of the aquifer system. Redox conditions are used to describe whether groundwater has oxidizing (oxic) or reducing (anoxic) conditions. 

Vanadium exists in multiple oxidation states in the environment (Hem, 1985); 

therefore, its molecular structure is affected by both redox and pH. The positive correlation of V concentrations from samples collected in the San Diego study unit to oxic 

and alkaline (high pH) groundwater conditions was statistically significant (Wright and 

Belitz, 2011).

Boron does not exist in multiple oxidation states in the environment; therefore, its 

molecular structure is affected by pH, but not by redox conditions (Hem, 1985). The 

positive correlation of B concentrations from samples collected in the San Diego study 

unit to alkaline groundwater conditions was statistically significant, but the correlation 

to redox conditions was not significant (Wright and Belitz, 2011).

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.

Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural water, third edition: 

U.S. Geological Survey Water-Supply Paper 2254, 213 p. 

Reimann, C., and de Caritat, P., 1998, Chemical elements in the environment: Berlin, Springer-Verlag, 

399 p.

Wright, M.T., Belitz, Kenneth, and Burton, C.A., 2005, California GAMA Program—Groundwater quality 

in the San Diego drainages hydrologic province, California, 2004: U.S. Geological Survey Data Series 

129, 91 p.

Wright, M.T., and Belitz, Kenneth, 2011, Status and understanding of groundwater quality in the San 

Diego Drainages Hydrologic Province, 2004—California GAMA Priority Basin Project: U.S. Geological Survey Scientific Investigations Report 2011-5154, 102 p. Available at http://pubs.usgs.gov/

sir/2011/5154/.

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 shallow and deep parts may 

differ from water quality in the primary 

aquifers. GAMA’s Domestic Well Project assesses water quality in the shallow 

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 for use in the assessment of 

the factors that affect water quality. In 

the San Diego study unit, data were 

collected by the PBP in 2004 and from 

the CDPH database for 2001–2004. 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/

water_issues/programs/gama

By Michael T. Wright and Kenneth Belitz