San Jacinto basin

Scientific Investigations Report 2005-5032

North

Occurrence and Distribution of Volatile Organic Compounds and Pesticides

in Ground Water in Relation to Hydrogeologic Characteristics and Land Use

in the Santa Ana Basin, Southern California

In cooperation with the

California State Water Resources Control Board

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

National Water-Quality Assessment Program

Inland basin Coastal basin
Cover Photographs

Clockwise from upper left:

Recharge facilities on Santa Ana

River. Photo courtesy of Orange 

County Water District.

Urban development, Yorba Linda

area. Photo: Phil Contreras, USGS

Dairy farm with San Gabriel 

Mountains in background.

Photo: Phil Contreras, USGS

Looking east toward the San Jacinto

Mountains from Hemet. Photo: Phil

Contreras, USGS

Recharge facilities on the San Jacinto

River. Photo: Barbara Dawson, USGS

Urban development overlooking the

Santa Ana River near Yorba Linda.

Photo: Phil Contreras, USGS

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123456

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6
Occurrence and Distribution of Volatile 

Organic Compounds and Pesticides in 

Ground Water in Relation to Hydrogeologic 

Characteristics and Land Use in the Santa 

Ana Basin, Southern California

By Scott N. Hamlin, Kenneth Belitz, and Tyler Johnson

In cooperation with the California State Water Resources Control Board

Scientific Investigations Report 2005-5032

U.S. Department of the Interior

U.S. Geological Survey

National Water-Quality Assessment Program
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2005

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Hamlin, S.N., Belitz, Kenneth, and Johnson, Tyler, 2005, Occurrence and distribution of volatile organic compounds 

and pesticides in ground water in relation to hydrogeologic characteristics and land use in the Sana Ana Basin, 

Southern California: U.S. Geological Survey Scientific Investigations Report 2005-5032, 40 p. 
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Description of Study Area ...................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Acknowledgments................................................................................................................................4

Approach ........................................................................................................................................................4

Description and Comparison of Explanatory Factors in the Ground-Water Basins ...........................6

Occurrence and Distribution of VOCs and Pesticides in the San Jacinto Basin .............................15

Detection Frequencies of Tritium, VOCs, and Pesticides.............................................................15

Relation to Ground-Water Age and Depth to the Top of the Well Screen.................................17

Relation to Engineered Recharge in the Hemet Area ..................................................................20

Relation to Land Use and Population Density................................................................................22

Occurrence and Distribution of VOCs and Pesticides in the Inland Basin ........................................23

Detection Frequencies of Tritium, VOCs, and Pesticides.............................................................23

Relation to Ground-Water Age and Depth to the Top of the Well Screen.................................24

Relation to Land Use and Population Density................................................................................24

Occurrence and Distribution of VOCs and Pesticides in the Coastal Basin .....................................26

Detection Frequencies of Tritium, VOCs, and Pesticides.............................................................26

Relation to Ground-Water Age and Depth to the Top of the Well Screen.................................27

Relation to Hydrogeologic Setting ..................................................................................................29

Relation to Proximity to Engineered Recharge Facilities Along the Santa Ana River.............30

Relation to Land Use and Population Density................................................................................31

Summary and Conclusions ........................................................................................................................31

References....................................................................................................................................................33

Appendix : Classification of Wells in the San Jacinto, Inland, and Coastal Basins .........................36

Figures

Figure 1. Map showing location of the study area and the distribution of alluvial 

deposits in the Santa Ana Basin, southern California............................................................3

Figure 2. Map showing location of the San Jacinto Basin and wells within this 

subbasin of the Santa Ana Basin, southern California...........................................................7

Figure 3. Map showing location of the Inland Basin and of wells within this subbasin 

of the Santa Ana Basin, southern California............................................................................8

Figure 4. Map showing location of the Coastal Basin and of wells within this subbasin 

of the Santa Ana Basin, southern California............................................................................9

Figure 5. Graph showing depth to the top of the well screen grouped by subunit 

surveys for the three subbasins of the Santa Ana Basin, southern California ................10

Figure 6. Graphs showing depth to the top of the well screen grouped by unconfined 

and confined areas of the Coastal Basin within the Santa Ana Basin, 

southern California .....................................................................................................................11

Figure 7. Graphs showing land use in a 500-meter radius of wells sampled, grouped 

by subunit surveys for the three subbasins of the Santa Ana Basin, 

southern California .....................................................................................................................11
iv

Figure 8. Graphs showing percentage of urban land use in a 500-meter radius of 

wells sampled, grouped by unconfined and confined areas in the Coastal 

Basin of the Santa Ana Basin, southern California...............................................................12

Figure 9. Graphs showing population density grouped by subunit surveys for the 

three subbasins of the Santa Ana Basin, southern California............................................12

Figure 10. Graphs showing detection frequencies for tritium at or above 1 pCi/L and 

for volatile organic compounds and pesticides above and below the 

laboratory reporting limit for the three subbasins of the Santa Ana 

Basin, southern California.........................................................................................................13

Figure 11. Graphs showing detection frequencies for volatile organic compounds 

by individual compound for the three subbasins of the Santa Ana Basin, 

southern California .....................................................................................................................14

Figure 12. Graphs showing number of volatile organic compounds detected 

and number of pesticides detected in the San Jacinto, Inland, and Coastal 

Basins within the Santa Ana Basin, southern California.....................................................15

Figure 13. Graphs showing detection frequencies for pesticide compounds by 

individual compound above and below the laboratory reporting limit for the 

three subbasins of the Santa Ana Basin, southern California............................................16

Figure 14. Graphs showing detection frequencies for tritium, volatile organic compounds

 (VOCs), and pesticides; number of VOCs detected, and number of pesticides 

detected grouped by depth to the top of the well screen in the San Jacinto 

Basin of the Santa Ana Basin, southern California .............................................................17

Figure 15. Maps showing water-level contours and depths to the tops of well 

screens, and distribution of tritium, VOCs, and pesticides in the Hemet area 

of the San Jacinto Basin of the Santa Ana Basin, southern California ............................19

Figure 16. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides; number of VOCs detected, and number of 

pesticides detected by depth to the top of the well screen in the Hemet 

area of the San Jacinto Basin of the Santa Ana Basin, southern California....................20

Figure 17. Graphs showing detection frequencies for tritium, volatile organic compounds 

(VOCs), and pesticides; number of VOCs detected per well, and number of 

pesticides detected by proximity to engineered recharge facilities in the 

Hemet area of the San Jacinto Basin of the Santa Ana Basin, southern California.......21

Figure 18. Graphs showing number of compounds detected in the San Jacinto Basin 

of the Santa Ana Basin, southern California..........................................................................22

Figure 19. Graphs showing detection frequencies for tritium, volatile organic compounds

 (VOCs), and pesticides; number of VOCs detected, and number of pesticides 

detected grouped by depth to the top of the well screen in the Inland 

Basin of the Santa Ana Basin, southern California...............................................................23

Figure 20. Graphs showing number of volatile organic compounds (VOCs) and

pesticides detected in agricultural and urban areas of the Inland Basin 

of the Santa Ana Basin, southern California..........................................................................25

Figure 21. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides, and number of VOCs and pesticides 

detected by depth to the top of the well screen in the Coastal Basin 

of the Santa Ana Basin, southern California..........................................................................26

Figure 22. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides, and number of VOCs and pesticides 

detected in unconfined and confined areas of the Coastal Basin 

of the Santa Ana Basin, southern California .........................................................................27
v

Appendix

Table A1. San Jacinto Basin explanatory factors for the occurrence and distribution 

of volatile organic compounds (VOCs) and pesticides within the 

Santa Ana Basin, southern California .....................................................................................36

Table A2. Inland Basin explanatory factors for the occurrence of compounds within the 

Santa Ana Basin, southern California .....................................................................................37

Table A3. Coastal Basin explanatory factors for occurrence of compounds within the 

Coastal Basin of the Santa Ana Basin, southern California....................................................38

Figure 23. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides in unconfined areas, and number 

of VOCs and pesticides detected by depth to the top of the well screen 

in the Coastal Basin of the Santa Ana Basin, southern California.....................................27

Figure 24. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides in confined areas and number of VOCs 

and pesticides detected by depth to the top of the well screen in the 

Coastal Basin of the Santa Ana Basin, southern California................................................28

Figure 25. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides, and number of VOCs and pesticides 

detected by position relative to engineered recharge facilities in the 

Coastal Basin of the Santa Ana Basin, southern California................................................28

Figure 26. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides in unconfined areas, and number 

of VOCs and pesticides detected grouped by position relative to 

engineered recharge facilities in the Coastal Basin of the Santa Ana 

Basin, southern California ........................................................................................................29

Figure 27. Graphs showing detection frequencies for tritium, volatile organic 

compounds (VOCs), and pesticides in confined areas, and number of 

VOCs and pesticides detected by position relative to engineered 

recharge facilities in the Coastal Basin of the Santa Ana Basin,

 southern California ....................................................................................................................30

Tables

Table 1. Statistical comparison of depth to the top of well screen, percent urban 

land use, percent agricultural land use, number of volatile organic 

compounds (VOCs) detected per well, and number of pesticides detected 

per well for the San Jacinto, Inland, and Coastal Basins, within the Santa 

Ana Basin, southern California.................................................................................................10

Table 2. Detection frequencies for volatile organic compounds (VOCs) and pesticides

in younger and older water in the San Jacinto, Inland, and Coastal Basins, 

within the Santa Ana Basin, southern California. .................................................................15

Table 3. Statistical tests for difference between groups of data (Wilcoxon test) and 

for correlation (Kendall’s test) of potential explanatory factors with the 

number of VOCs detected and number of pesticides detected in the 

San Jacinto, Inland, and Coastal Basins, within the Santa Ana Basin, 

southern California .....................................................................................................................18
vi

Multiply By To obtain

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3/yr)

foot (ft) 0.3048 meter (m)

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2) 

Multiply By To obtain

kilometer (km) 0.62137 mile (mi)

square kilometer (km2) 0.3861 square mile (mi2)

meter (m) 3.28084 feet (ft)

In the text, Inch/Pound units are used, except for the NAWQA contributory area near wells 

which was defined by a 500-meter radius.

Vertical Datum

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations

CAS California Aquifer Susceptibility 

CFC-11 (Freon 11) trichlorofluoromethane

CFC-12 (Freon 12) dichlorofluoromethane

CFC-113 (Freon 113) 1,1,2-trichlorofluoroethane

DCE 1,1-dichloroethene

LRL laboratory reporting limit

MTBE methyl tert-butyl ether

NAWQA National Water-Quality Assessment (Program)

NWQL National Water Quality Laboratory

OCCAS Orange County California Aquifer Susceptibility (program)

PCE tetrachloroethene (or tetrachloroethylene)

pCi/L picocuries per liter

SAR Santa Ana River

SUS subunit survey

Conversion Factors, Vertical Datum, and Abbreviations
vii

TCA 1,1,1-trichloroethane 

TCE trichloroethene (or trichloroethylene)

TDS total dissolved solids

TIN total inorganic nitrogen

VOC volatile organic compound

Organizations

EMWD Eastern Municipal Water District

OCWD Orange County Water District

SAWPA Santa Ana Watershed Project Authority

USGS U.S. Geological Survey

 
Abstract

This report presents an evaluation of the occurrence and 

distribution of VOCs and pesticides in the Santa Ana groundwater basins in relation to two types of explanatory factors: 

hydrogeologic characteristics and land use. The Santa Ana 

Basin is subdivided into the San Jacinto, the Inland, and the 

Coastal ground-water basins. Most wells sampled were deep 

and used for public supply. Data from regional studies were 

used to evaluate the occurrence and distribution of pesticides 

and volatile organic compounds (VOCs) in relation to hydrogeologic characteristics and land uses that could potentially 

explain variations between basins. Additional data from special studies (flow path and aquifer susceptibility) were used to 

evaluate potential factors affecting water quality for individual 

basins. The hydrogeologic characteristics evaluated in this 

report were hydrogeologic setting, ground-water age, depth 

to the top of the well screen (top of well perforations), and 

proximity to engineered recharge facilities. Urban land use, 

agricultural land use, and population density were characterized within a 500-meter radius of sampled wells and at the 

basin scale. 

Aquifers in the San Jacinto Basin are generally unconfined, and major land-use categories are urban (33 percent), 

agricultural (37 percent), and undeveloped (25 percent). 

Recharge is primarily from the overlying landscape, but 

engineered recharge is locally important in the Hemet area. 

VOCs and pesticides were detected more frequently in 

younger ground water (less than 50 years old) than in older 

ground water, and more frequently in shallower wells than 

deeper wells; the numbers of VOCs and pesticides detected 

also were significantly higher in the younger ground water and 

in the shallower wells. In the Hemet area of the San Jacinto 

Basin, VOCs and pesticides were detected more frequently in 

wells proximal to engineered recharge facilities than in distal 

wells. These patterns illustrate the importance of proximity to 

sources of recharge in relation to the occurrence and distribution of VOCs and pesticides in ground water. 

Aquifers in the Inland Basin also are generally unconfined, and the major land-use category is urban (58 percent), 

with lesser amounts of agricultural (13 percent) and undeveloped (28 percent) land. Recharge is from engineered facilities 

that utilize local runoff and imported water and from vertical 

infiltration. VOCs and pesticides were detected more frequently in younger ground water than in older ground water, 

and more frequently in shallower wells than deeper wells. 

The number of VOCs detected per well also was significantly 

higher in the younger ground water and in the shallower wells. 

Several solvent plumes extending between 5 and 10 kilometers 

illustrate the large distances that contaminants travel in basins 

with intensive use of ground water.

Aquifers in the Coastal Basin, in contrast to the other 

basins, are generally confined. Land use in the basin is largely 

urban (80 percent), with lesser amounts of agricultural 

(7 percent) and undeveloped (12 percent) land. Recharge is 

primarily from engineered facilities that utilize water diverted 

from the Santa Ana River and imported water. Consequently, 

VOCs and pesticides were detected more frequently in wells 

proximal to engineered recharge facilities than in distal wells. 

These compounds were also detected more frequently in the 

unconfined area than in the confined area of the basin. In the 

confined area, the numbers of VOCs and pesticides detected 

per well were not significantly different in wells with shallower and deeper screens. This distribution reflects the dominance of lateral flow and insulation from overlying land use in 

the confined aquifers of the Coastal Basin. 

In the unconfined area of the Coastal Basin, the numbers 

of VOCs and pesticides detected per well were significantly 

higher in shallower wells than in deeper wells. VOC and 

pesticide detections were not statistically correlated to urban 

land use, agricultural land use, or population density near the 

sampled wells, in contrast to national findings. 

Occurrence and Distribution of Volatile Organic 

Compounds and Pesticides in Ground Water in Relation to 

Hydrogeologic Characteristics and Land Use in the Santa 

Ana Basin, Southern California

By Scott N. Hamlin, Kenneth Belitz, and Tyler Johnson
Nationally, positive correlations have been observed 

between the percentage of urban land use near sampled wells 

(500 meters) and pesticide and VOC detection frequencies 

in shallow ground water. Similarly, national studies have 

shown correlation between the percentage of agricultural land 

use near sampled wells and pesticide detection frequency. 

In contrast, pesticide and VOC detections in the Santa Ana 

ground-water basins were not positively correlated with urban 

land use, agricultural land use, or population density near the 

sampled wells. These contrasts with national findings could be 

due to differences in the types of wells sampled in the Santa 

Ana ground-water basins: deep, production wells compared 

with shallow, domestic wells in the national evaluation. The 

capture zones for these deep wells are complex (asymmetrical) 

and commonly extend beyond 500 meters, more than 

10 kilometers in some cases as illustrated by solvent plumes. 

Another explanation for the contrasts with national findings, 

particularly in the San Jacinto Basin, is that current land use 

may not reflect land use at the time of recharge. An additional 

explanation for the lack of agreement with national findings 

in the Coastal Basin is the predominance of confined aquifers 

that are insulated from overlying land use. 

Introduction

Description of Study Area

The Santa Ana Basin is located in southern California 

between Los Angeles and San Diego (fig. 1). The 2,700-mi2

watershed is home to nearly 5 million people, and the population is expected to increase by more than 50 percent by the 

year 2020. During the same period, water demand is expected 

to increase by somewhat less than 50 percent (Santa Ana 

Watershed Project Authority [SAWPA], 1998). The Santa Ana 

Basin includes parts of Orange, San Bernardino, Riverside, 

and Los Angeles Counties. Population density for the entire 

study area is 1,500 people per mi2; excluding areas too steep 

to develop, the population density is about 3,000 people per 

mi2. In the city of Santa Ana, the population density is as high 

as 20,000 people per mi2 (California Department of Finance, 

accessed March 3, 2005).

The Santa Ana River is the largest stream system in 

southern California, beginning in the San Bernardino Mountains (which reach altitudes exceeding 10,000 ft above sea 

level [NAVD, 1988]) and flowing more than 100 mi to the 

Pacific Ocean near Huntington Beach. The climate is Mediterranean with hot, dry summers and cool, wet winters. Average 

annual rainfall ranges from 12 in. in the coastal plain and 

18 in. in the inland valley to 40 in. in the San Bernardino 

Mountains. 

Ground water is the main source of supply in the watershed, providing about two-thirds of the total water used (about 

1.2 million acre-ft/yr). Imported water from northern California and the Colorado River accounts for about one-quarter of 

the total consumptive demand. Local surface water provides 

the remaining supply. Urban water use (63 percent) exceeds 

agricultural water use (28 percent of total use) in the study 

area (Hamlin and others, 1999). 

The Santa Ana Basin can be subdivided into three primary ground-water basins: the San Jacinto Basin, the Inland 

Basin, and the Coastal Basin (fig.1). Relatively impervious 

hills and mountains bound water-bearing deposits in the 

alluvium-filled basins. Urban and agricultural land uses occur 

primarily in the alluvium-filled valleys and the coastal plain. 

Land use in the watershed is about 35 percent urban; 10 percent agricultural; and 55 percent open space, primarily steep 

mountain slopes (Belitz and others, 2004). 

Numerous studies of ground-water quality have been 

conducted in the Santa Ana Basin. The Santa Ana Watershed 

Project Authority (SAWPA) was formed to protect water 

quality in the basin. To help local agencies manage water 

resources in the basin, SAWPA has produced a report that 

summarizes water use, water quality, water-quality issues, 

and the water budget (Santa Ana Watershed Project Authority, 1998). SAWPA has identified two primary water-quality 

issues: high concentrations of total dissolved solids (TDS) and 

total inorganic nitrogen (TIN). A task force was formed to 

study and address these issues and has produced a report that 

describes the distribution, variation, and management alternatives for TDS and TIN in surface and ground water (Wildermuth Environmental, 2000). Ground-water quality in the Santa 

Ana Basin is discussed in relation to drinking water standards 

in a report completed for the U.S. Geological Survey (USGS) 

National Water-Quality Assessment Program (NAWQA) 

(Hamlin, 2002). 

2 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
San Andreas

Fault

San Jacinto Fault

COASTAL BASIN

INLAND BASIN

SAN JACINTO BASIN

Santa Ana Mts

Prado Res

San Jacinto Mts

Lake

Elsinore

Lake

Matthews Lake

Perris

San Bernardino Mts

Cucamonga

Cr

San Gabriel Mts

San Bernardino

County

Riverside County

Los Angeles County

Chino

Creek

San Bernardino

Irvine

Santa Ana

Chino Hills

Santa Ana

San Jacinto

River

River

Orange County

EXPLANATION

Alluvium

Quaternary

Terrace deposits

Quaternary

Basin boundary

Subbasin boundary

Recharge facility

San Jacinto

Creek

San

Timoteo

Riverside

Pacific

Ocean

0 10 20 MILES

0 10 20 KILOMETERS

Los Angeles

San Diego

San

Francisco

Santa Ana

Basin

California

Pacific Ocean

118 00’ 45' 30' 15' 117 00’ 116 45’

34

15’

34

00’

33

45’

Huntington Beach oil field

Big Bear Lake

Geology derived from California Division of Mines and

Geology, 1965 and 1981, and Southern California Aerial

Mapping Project, 1999

Base from U.S. Geological Survey digital

elevation model, 1:24,000, 1999

Albers Equal-Area Conic Projection

Standard parallels 29 30' and 45 30',

central meridian -117 15"

Figure 1. Location of the study area and the distribution of alluvial deposits in the Santa Ana Basin, southern California.

Introduction 3
Many previous studies address water-quality issues 

specific to the individual ground-water basins. Several USGS 

studies have been done in cooperation with the Eastern 

Municipal Water District (EMWD) to describe geohydrology 

and water quality in the San Jacinto Basin (Burton and others, 

1996; Kaehler and others, 1998; Kaehler and Belitz, 2003). 

The USGS has also completed studies in the Inland Basin in 

cooperation with local water agencies to investigate nitrate and 

volatile organic compound (VOC) contamination of ground 

water, to evaluate ground-water chemistry and recharge, and to 

optimize ground-water use and pumping (Klein and Bradford 

1980; Duell and Schroeder, 1989; Danskin and Freckleton, 

1992; Rees and others, 1994; Woolfenden and Kadhim, 

1997; Izbicki and others, 1998). The Orange County Water 

District (OCWD) manages and monitors ground water in the 

Coastal Basin. The OCWD has published several reports that 

describe VOC plumes, water quality, and geohydrology in 

the Coastal Basin (Herndon and Reilly, 1989; Carlson and 

others, 1991; Herndon and Goodrich, 1991; Herndon and 

others, 1997). General water quality in the Coastal Basin is 

described in reports by the USGS: a description of geohydrology and characterization of seawater intrusion (Poland, 1959) 

and a description of aquifers and evaluation of ground-water 

quality in the Irvine subbasin (Singer, 1973). The USGS 

has also evaluated the occurrence of VOCs and pesticides in 

ground water to help assess the potential, or susceptibility, for 

contamination of ground water in the Coastal Basin (Shelton 

and others, 2001), and in flow paths in the Coastal and Inland 

Basins (Dawson and others, 2003).

Purpose and Scope

The purpose of this report is to evaluate the occurrence 

and distribution of VOCs and pesticides in the Santa Ana 

ground-water basins in relation to two types of explanatory 

factors: hydrogeologic characteristics and land use. This 

evaluation was based on chemical analyses of water samples 

primarily from deep, public-supply wells for water-quality studies done from 1999 to 2001 as part of the USGS’s 

NAWQA program (Hamlin and others, 2002). The hydrogeologic characteristics include setting (unconfined versus 

confined), ground-water age (based on tritium concentration), 

depth to well screen (top of well perforations), and proximity 

to engineered (or artificial) recharge facilities. 

A particular land use may be a source of VOCs and/or 

pesticides. Land use was evaluated at two scales: the entire 

basin (basin scale) and within 500 m of the sampled wells 

(well scale). Evaluation of land use within 500 m of sampled 

wells was consistent with the approach generally used in 

NAWQA studies (Koterba, 1998). Land-use factors evaluated 

at each scale were the percentage of urban land, the percentage of agricultural land, and the population density. Land use 

can be considered as a surrogate for sources of contamination. The size (and shape) of the area around a well that may 

contribute contaminants presumably ranges between the well 

scale (within 500 m) and the basin scale (tens of thousands 

of meters). It is beyond the scope of the present study to use 

ground-water flow modeling to assess the size and shape of 

contributing areas for individual wells sampled. 

Acknowledgments

The authors thank the many water purveyors and companies that allowed the USGS to sample their wells: Agri 

Empire, Cucamonga County Water District, Diamond Brothers, East Orange Water Company, East Valley Water District, 

Elsinore Valley Municipal Water District, Fontana Water 

Company, Irvine Ranch Water District, Jurupa Community 

Services District, Lake Hemet Municipal Water District, Mesa 

Consolidated Water District, Serrano Water District, Southern 

California Water Company, The Irvine Company, University of California at Riverside, West San Bernardino County 

Water District, Yucaipa Valley Water District; and the cities 

of Anaheim, Chino Hills, Colton, Corona, Fountain Valley, 

Garden Grove, Hemet, Huntington Beach, Newport Beach, 

Nuevo, Ontario, Orange, Pomona, Redlands, Riverside, San 

Bernardino, San Jacinto, Santa Ana, Tustin, and Westminster. 

We also thank the water districts that coordinated sampling efforts within their boundaries: Orange County Water 

District in the Coastal Basin and Eastern Municipal Water 

District in the San Jacinto Basin. Without their cooperation, 

this study would not have been possible.

Approach 

Data from two different types of studies were used in 

this report (Hamlin and others, 2002). Subunit surveys (SUS) 

utilized randomized, spatially distributed sampling networks 

in each of the ground-water basins, and thus provided an unbiased assessment of the resource. Data from the SUS studies 

were used to assess water quality within basins and to compare 

water quality between basins. Data from special studies (flow 

path and aquifer susceptibility) were used, along with the SUS 

data, to evaluate explanatory factors affecting water quality. 

Most of the wells sampled for the SUS and special studies are 

deep and are used for public supply.

The San Jacinto, Inland, and Coastal Basins were 

described and compared with respect to hydrogeologic characteristics (degree of aquifer confinement, depth to the top of 

well perforations, and proximity to engineered recharge), land 

use (urban and agricultural), and population density. These 

factors were evaluated and compared at the basin scale. In 

addition, land-use factors and population density were evaluated at the well scale.

4 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
Whether an aquifer is unconfined or confined can affect 

the distribution of anthropogenic compounds in ground water. 

In most cases, VOCs and pesticides are expected to occur 

more frequently in unconfined aquifers than in confined aquifers (Squillace and others, 2002), and the basins are qualitatively compared with respect to degree of aquifer confinement. 

Whether a well screen is shallow or deep can be an 

important explanatory factor in the distribution of anthropogenic compounds. Wells are classified as “shallow” or 

as “deep” relative to the median value for depth to the top 

perforation. In unconfined aquifers, shallow wells are generally closer to sources of recharge and to overlying sources 

of contamination than are deep wells. As a consequence, 

VOCs and pesticides are expected to occur more frequently 

in shallow wells (Squillace and others, 2002). Depths of wells 

sampled for the SUS studies were compared for the groundwater basins.

Proximity to engineered recharge facilities can be an 

important explanatory factor in the Coastal Basin and in the 

Hemet area of the San Jacinto Basin. Wells near recharge 

facilities were classified as “proximal” and more distant wells 

were classified as “distal.” Engineered recharge of aquifers 

can introduce trace amounts of VOCs and pesticides into 

ground water (Shelton and others, 2001; Dawson and others, 

2003). Anthropogenic compounds may be widely distributed 

in ground water as recharged water moves through the flow 

system. The relative amount of engineered recharge (groundwater use) was compared for the ground-water basins.

Water from shallow wells and wells proximal to recharge 

facilities is generally younger than water from deep wells and 

wells distal from recharge facilities. In this study, the presence 

of tritium at concentrations at or above 1 pCi/L was taken as 

an indicator of “young” water recharged since the early 1950s; 

other samples were defined as “old.” Concentrations of tritium 

higher than 1 pCi/L were produced during atmospheric nuclear 

tests that began in the early 1950s; tritium concentrations 

have been decreasing since atmospheric tests were banned in 

1963. In the comparison of ground-water basins, the relative 

amount of young water was used as an indicator of engineered 

recharge.

Urban and agricultural land uses are potential explanatory 

factors because they indicate whether or not anthropogenic 

compounds have been used. Population density reflects urban 

land use and can also be an explanatory factor. The data were 

compiled from the Southern California Association of Governments database (1993) and included various urban classifications, agricultural land, and vacant land (Southern California 

Association of Governments, accessed March 3, 2005). Land 

use and population density were evaluated at the basin scale 

and the well scale.

At the well scale, land use and population density were 

classified within a 500-m (1,640-ft) radius around each well 

sampled using procedures recommended by the NAWQA 

program (Koterba, 1998). In this report, land use and population density within the 500-m radius are classified as “near” 

the well. On a national basis, evaluation of NAWQA data 

indicated a positive correlation between VOC occurrence in 

ground water and urban land use; pesticide occurrence was 

associated with both agricultural and urban land uses (Squillace and others, 1999; Squillace and others, 2002). These 

national results were based on an analysis of land use within 

a 500-m (1,640-ft) radius of shallow, domestic wells. The 

national evaluation also found that water quality from the 

domestic wells can be different from public supply wells for 

several reasons, including generally shallower depth and lower 

pumping rates. Public supply wells can have high pumping 

rates and large capture zones, which can increase the number 

of potential contamination sources (Stackleberg and others, 

2001).

The current study evaluates the relation of VOC and pesticide occurrence to land use at the basin scale and at the well 

scale; the actual scale of the capture zones for the sampled 

wells lies somewhere between the two. An important component of the current study was to evaluate whether land use 

and population density within a 500-m (1,640-ft) radius of the 

deep wells sampled is an explanatory factor for the occurrence 

and distribution of VOCs and pesticides. 

Water quality was evaluated primarily in terms of detection frequency and number of compounds detected per well. 

Detection frequencies were used to qualitatively determine 

whether or not the potential explanatory factors control the 

occurrence and distribution of VOCs and pesticides. The 

number of compounds detected per well was evaluated with 

nonparametric statistical tests to quantitatively determine 

whether or not the potential explanatory factors can predict the 

distribution of VOCs and pesticides. 

Two nonparametric statistical tests were used to quantitatively evaluate the significance of the potential explanatory 

factors (Helsel and Hirsch, 1992). The Wilcoxon test (twosided test) was used to assess the significance of differences 

between two groups of wells. If the Wilcoxon test indicated a 

significant difference between groups of data, Kendall’s test 

was used to evaluate potential correlation between explanatory variables and response variables (detections of VOCs and 

pesticides). Probability values (p) less than 0.05 (95-percent 

confidence level) are taken as a strong correlation (Savoca 

and others, 2000). P values between 0.05 and 0.10 (90-percent 

confidence level) indicate a weaker correlation.

Approach 5
All detections of VOCs and pesticides, whether above or 

below laboratory reporting limits (LRLs), were used in calculating detection frequencies and in nonparametric statistical 

tests. The USGS National Water Quality Laboratory (NWQL) 

uses LRLs for reporting nondetections, but is able to quantify 

detections of compounds below the LRL (Childress and others, 1999). If a water sample has a compound at a concentration above the LRL, the NWQL has a high degree of confidence that that the compound will be detected in that sample 

under routine operating conditions. If a water sample has a 

concentration lower than the LRL, the NWQL does not have a 

high degree of confidence that that compound will be routinely 

detected. To reflect that uncertainty, the NWQL reports nondetections as less-than values rather than reporting them as zeros. 

Under some conditions, the NWQL can quantify detections 

below the LRL; in those cases, the NWQL reports detections 

as estimated values.

Description and Comparison of 

Explanatory Factors in the GroundWater Basins

The Santa Ana Basin includes three ground-water basins: 

the San Jacinto, Inland, and Coastal Basins. In this section, 

the hydrogeology of each of the basins is briefly described. 

In addition, the basins are compared with one another with 

respect to selected potential explanatory factors. Selected 

explanatory factors are listed by ground-water basin and compiled in the appendix.

Figure 2 shows the location of the San Jacinto Basin. The 

aquifers in this basin are generally unconfined and consist 

of a series of interconnected alluvium-filled valleys bounded 

by steep-sided bedrock mountains and hills. The thickness of 

deposits in these valleys typically ranges from 200 to 

1,000 ft (Eastern Municipal Water District, 2002). Collectively, alluvium covers about one-half of the total area in the 

subunit. Prior to development, recharge to the local aquifers 

was from infiltration of mountain streams, primarily the San 

Jacinto River. Presently, sources of recharge include irrigation 

return flows, engineered (artificial) recharge facilities (fig. 2), 

and infiltration from streams. Water levels in sampled wells 

ranged from 90 to 540 ft below land surface, with an average 

depth of 230 ft. The basin is virtually closed; ground-water 

discharge occurs primarily by ground-water pumping.

Figure 3 shows the location of the Inland Basin. The 

aquifers of this basin generally are unconfined and comprise 

several subbasins filled with alluvial deposits eroded from the 

surrounding mountains. The thickness of these deposits ranges 

from less than 200 to more than 1,000 ft (Dutcher and Garrett, 

1963). Recharge to the basin varies seasonally and is largely 

from infiltration of runoff from the San Gabriel and San Bernardino Mountains. Much of the runoff is diverted into stormdetention basins, which also operate as ground-water recharge 

facilities (fig. 3). Surface water imported from northern 

California and the Colorado River is also used to recharge the 

ground-water basin. Incidental recharge occurs from landscape 

irrigation. Water levels in sampled wells ranged from 

20 to 560 ft below land surface, with an average depth of 

200 ft. Ground-water discharge occurs primarily by pumping 

for public supply (fig. 3). Several VOC plumes in the basin 

define ground-water flow paths. 

The Coastal Basin is subdivided into the Main and Irvine 

Basins (fig. 4; Singer, 1973). The Main Basin was divided, 

on the basis of relative abundance of shallow clay layers, into 

unconfined (forebay) and confined (pressure) areas (California 

Department of Public Works, 1934). The unconfined area is 

small (28 percent) compared with the confined area 

(72 percent) (fig. 4). Within this basin, the thickness of 

freshwater-bearing deposits is as great as 4,000 ft (California 

Department of Water Resources, 1967). Almost all recharge 

in the basin is from engineered facilities located within and 

along the Santa Ana River and along Santiago Creek (Herndon 

and others, 1997). Ground-water discharge occurs primarily 

by pumping for public supply. Water levels in sampled wells 

ranged from 20 to 230 ft below land surface, with an average 

depth of 100 ft.

VOCs and pesticides were detected more frequently in 

younger ground water (recharged since the early 1950s) than 

in older ground water. In the three ground-water basins, VOCs 

were detected in 88 percent of the younger samples and in 

36 percent of the older samples. Pesticides were detected in 

72 percent of the younger samples and in 14 percent of the 

older samples.

Data from wells sampled for the three subunit surveys 

were used to compare one basin with another. Depth to the 

top of the well screen can be an important explanatory factor 

for the occurrence of VOCs and pesticides, particularly in 

unconfined aquifers. The median depth to the top of the well 

screen is 270 ft in the San Jacinto Basin, 240 ft in the Inland 

Basin, and 388 ft in the Coastal Basin (fig. 5). The depths to 

the tops of the well screens are significantly deeper in the 

Coastal Basin than in either the San Jacinto Basin or Inland 

Basin (table 1; fig. 5). Total well depths range from 328 to 

1720 ft in the San Jacinto Basin, from 225 to 1,180 ft in the 

Inland Basin, and from 98 to 1,550 ft in the Coastal Basin. 

Median total depths of sampled wells are 696 ft in the San 

Jacinto Basin, 585 ft in the Inland Basin, and 910 ft in the 

Coastal Basin. 

6 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
34

00'

33

45'

117 15' 117 00' 116 45'

Base from U.S. Geological Survey digital

elevation model, 1:24,000, 1999 Albers

Equal-Area Conic Projection Standard parallels

29 30' and 45 30', central meridian -117 15’

Hemet

San

Jacinto

Riverside County

Orange

County

Bautista

Creek

San Jacinto Mts

Santa Ana Mts

River

San Jacinto

EXPLANATION

Alluvium

Quaternary

Basin boundary

Subbasin boundary

Lake Elsinore

Lake Perris

Lake Hemet

15 215

215

10

10

74

San Jacinto Subunit study

San Jacinto California Aquifer

Susceptibility study

Wells Used for sampling

Recharge Facility

10 15 MILES

10

5

5 15 KILOMETERS

Geology derived from California Division of

Mines and Geology, 1965 and 1981, and

Southern California Aerial Mapping Project,

1999

INLAND BASIN

SAN JACINTO BASIN

Lake Matthews

Canyon Lake

0

0

Figure 2. Location of the San Jacinto Basin and wells within this subbasin of the Santa Ana Basin, southern California.

Description and Comparison of Explanatory Factors in the Ground-Water Basins 7
Santa Ana Mts

San

M

Gabriel

ts

Prado

Res

San Jacinto Mts

San Bernardino Mts

Cucamonga

Creek

Chino Creek

Creek

San Bernardino

County

Santa

San

Jacinto

River

Ana

River

San Timoteo

Lake

Matthews

Chino

Ontario

Riverside

San Bernardino

San Jacinto

Hemet

5 15

15

215

215

215

10

10

405

60

60

91

91

Riverside County

Los Angeles

County

Orange

County

COASTAL BASIN

INLAND BASIN

SAN JACINTO BASIN

Lake Perris

10 20 MILES

0 10

0

20 KILOMETERS

Big Bear Lake

Geology derived from California Division of

Mines and Geology, 1965 , and Southern

California Aerial Mapping Project, 1999

Alluvium

EXPLANATION

Inland Basin subunit survey

Inland Basin flow-path study

Wells Used for sampling

Recharge facility

Volatile organic

compound plume

Basin boundary

Subbasin boundary

Quaternary

117 45' 30' 15' 117 00' 116 45'

34

15'

34

00'

33

45'

Base from U.S. Geological Survey digital

elevation model, 1:24,000, 1999 Albers

Equal-Area Conic Projection Standard parallels

29 30' and 45 30', central meridian -117 15’

VOC plumes from Wildermuth Environmental,

Inc., 1999, Optimum Basin Management

Program Phase 1 Report, Prepared for the

Chino Basin Watermaster, August 1999,

various paging.

Figure 3. Location of the Inland Basin and of wells within this subbasin of the Santa Ana Basin, southern California.

8 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
Orange County California

Aquifer Susceptibility

Coastal Subunit Survey

Coastal Flow-Path study

Recharge facilities

Santa Ana River (SAR)

flowpath and

direction of flow

Coastal Land-Use study

Wells Used for sampling

EXPLANATION

Alluvium Quaternary

Terrace deposits

Quaternary

Basin boundary

Boundary between confined and unconfined areas

Volatile organic

compound plume

0 5

0 5

10 MILES

10 KILOMETERS

Geology derived from California Division of

Mines and Geology, 1981, obtained from

Orange County Water District

Los Angeles

County

Orange County

Orange County

5

5

405

91

91

Santa

Ana

River

Creek

Santiago

Prado Res

Santa Ana

Irvine

Santa Ana Mts

Chino Hills

Huntington

Beach

oil field

Pacific

Ocean

San Bernardino County

Riverside

County

Irvin Basin

Boundary

PRESSURE AREA

CONFINED

FOREBAY AREA

UNCONFINED

Basin

Irvine

Main

Basin

55

33

45’

34

00’

118 00’ 45’ 117 30’

Base from U.S. Geological Survey digital

elevation model, 1:24,000, 1999 Albers

Equal-Area Conic Projection Standard parallels

29 30' and 45 30', central meridian -117 15’

INLAND BASIN

COASTAL BASIN

Figure 4. Location of the Coastal Basin and of wells within this subbasin of the Santa Ana Basin, southern California.

Description and Comparison of Explanatory Factors in the Ground-Water Basins 9
DEPTH TO TOP

OF WELL SCREEN, IN FEET

0

200

400

600

800

1,000

Inland Coastal

BASINS

San

Jacinto

Median

75th percentile 90th percentile

Outlier

Number of wells

sampled

EXPLANATION

25th percentile 10th percentile

(20) (29) (20)

(29)

Figure 5. Depth to the top of the well screen grouped by subunit surveys for 

the three subbasins of the Santa Ana Basin, southern California.

Table 1. Statistical comparison of depth to the top of well screen, percent urban land use, percent agricultural land use, number of 

volatile organic compounds (VOCs) detected per well, and number of pesticides detected per well for the San Jacinto, Inland, and 

Coastal Basins, within the Santa Ana Basin, southern California.

[p value, probability value; Wilcoxon p value less than or equal to 0.10 indicates a significant difference between basins (green shading); ft, feet; mi2, square 

mile]

Explanatory factors and 

Basins compared by Wilcoxon test 

(p values):

response variables San Jacinto and Inland San Jacinto and Coastal Inland and Coastal

Depth to top of well screen, ft 0.38 0.05 0.006

 below land surface

Percent urban land use1 0.37 0.08 0.29

(within 500-meter radius of well)

Percent agricultural land use 0.29 0.01 0.08

(within 500-meter radius of well)

Percent undeveloped land 0.99 0.001 0.001

(within 500-meter radius of well)

Population density, people per mi2 0.31 0.000 0.0001

(within 500-meter radius of well)

Number of VOCs detected 0.005 0.13 0.19

(per well)

Number of pesticides detected 0.0006 0.11 0.0001

(per well)

1

Urban land use includes residental, commercial, industrial, military, and transportation.

10 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
Within the Coastal Basin, the depths to the tops of the 

well screens in the unconfined and confined areas are not 

significantly different (p = 0.80, Wilcoxon test; fig. 6A). Similarly, when the comparison between unconfined and confined 

areas is restricted to wells in the Santa Ana River flow path, 

depths to well screens are not statistically different (p = 0.67, 

Wilcoxon test; fig. 6B). 

Nationally, land use is an important explanatory factor 

for the occurrence of VOCs and pesticides (Squillace and 

others, 2002). At the basin scale, the percentage of urban land 

was highest in the Coastal Basin (80 percent), followed by the 

Inland Basin (58 percent), and the San Jacinto Basin

 (33 percent). Within each basin, the median value for urban 

land use near the wells (within a 500-m radius of the wellhead) was greater than the percentage in the basins as a whole. 

The median value for the wells was highest for the Coastal 

Basin (90 percent) and was about the same for the Inland and 

San Jacinto Basins (75 percent) (fig. 7A; table 1). Development in the ground-water basins has followed a general trend 

of increasing urbanization of agricultural and undeveloped 

land.

A B

200

0

400

600

800

200

0

400

600

800

Unconfined areaConfined areaUnconfined areaConfined area

Median

75th percentile 90th percentile

EXPLANATION

25th percentile 10th percentile

Unconfined

area

Confined area

DEPTH TO TOP

OF WELL SCREEN,

IN FEET

DEPTH TO TOP

0F WELL SCREEN,

IN FEET

All wells

Wells proximal to

engineered recharge

facilities

(37) (58) (11) (30)

(37) Number of wells sampled

Outlier

URBAN LAND USE,

IN PERCENT 100

80

60

40

20

0

Median

75th percentile 90th percentile

Outlier

Number of wells

sampled

EXPLANATION

25th percentile 10th percentile

A

B

AGRICULTURAL LAND USE,

IN PERCENT 100

80

60

40

20

0

C

UNDEVELOPED LAND USE,

IN PERCENT 100

80

60

40

20

0

Inland Coastal

BASINS

San Jacinto

Inland Coastal

BASINS

San Jacinto

Inland Coastal

BASINS

San

Jacinto

(23) (29) (20)

(23) (29) (20)

(29)

(29)

(23) (20)

Figure 7. Land use in a 500-meter radius of wells sampled, 

grouped by subunit surveys for the three subbasins of the Santa 

Ana Basin, southern California. A, Percent urban use. B, Percent 

agriculture use. C, Percent undeveloped.

Figure 6. Depth to the top of the well screen grouped by unconfined and confined areas of the Coastal Basin within the Santa 

Ana Basin, southern California. A, all wells. B, wells proximal to 

engineered (artificial) recharge facilities.

Description and Comparison of Explanatory Factors in the Ground-Water Basins 11
Urban land use was further evaluated in the Coastal Basin 

in relation to hydrogeologic setting. For all wells sampled 

(fig. 8A), the percentage of urban land use was not significantly different in the unconfined and confined areas of the 

basin (p = 0.77, Wilcoxon test). Similarly, for wells proximal 

to recharge facilities (fig. 8B), urban land use was not significantly different in unconfined and confined areas (p = 0.80, 

Wilcoxon test). 

Nationally, population density is an important explanatory factor for the occurrence of VOCs and pesticides (Squillace and others, 1999; Squillace and others, 2002). Population 

density in the year 2000 was highest in the Coastal Basin 

(3,160 people per km2), intermediate in the Inland Basin 

(1,620 people per km2), and lowest in the San Jacinto Basin 

(560 people per km2) (fig. 9A). The median population density 

near the sampled wells is lower than that for the basins as a 

whole and shows a similar pattern (fig. 9B); the median population density near the sampled wells is 350 people per km2

for the San Jacinto Basin, 795 people per km2 for the Inland 

Basin, and 2,110 people per km2 for the Coastal Basin. The 

population density in the Coastal Basin is significantly higher 

than that of the San Jacinto and Inland Basins; the population 

density near sampled wells is not significantly different than 

that of the San Jacinto and Inland Basins (table 1).

During the past 30 years, the Coastal Basin has been the 

most densely populated, and the San Jacinto Basin has been 

the least densely populated (fig. 9A). In addition, the 1970 

population density in the Coastal Basin (1,797 people per km2) 

is higher than the 2000 population density in either the San 

Jacinto or the Inland basins. Historical variation in population density indicates the duration and relative amount of 

urban land use, and may be used to evaluate the occurrence of 

anthropogenic compounds.

A B

URBAN LAND USE

IN 500-M RADIUS OF

WELL, IN PERCENT

URBAN LAND USE

IN 500-M RADIUS OF

WELL, IN PERCENT

100

80

60

40

20

0

100

80

60

40

20

0

Unconfined areaConfined areaUnconfined areaConfined area

Median

75th percentile 90th percentile

EXPLANATION

25th percentile 10th percentile

Unconfined

area

Confined area

(37) (58)

(11) (30)

All wells

Wells proximal to

engineered recharge

facilities

(37) Number of wells sampled

Outlier

Figure 8. Percentage of urban land use in a 500-meter radius of 

wells sampled, grouped by unconfined and confined areas in the 

Coastal Basin of the Santa Ana Basin, southern California. A, all 

wells. B, wells proximal to engineered (artificial) recharge facilities.

A B

8,000

6,000

4,000

2,000

3,000

5,000

7,000

1,000

0

Median

75th percentile 90th percentile

Outlier

Number of wells

sampled

EXPLANATION EXPLANATION

25th percentile 10th percentile

Inland Coastal

YEAR BASINS

San

Jacinto

(23) (29) (20)

(29)

0

1,000

2,000

3,000

4,000

P

1970 1980 1990 2000 OPULATION DENSITY, PEOPLE

PER SQUARE KILOMETER

San Jacinto Basin

Inland Basin

Coastal Basin

1990 POPULATION DENSITY

IN 500-METER RADIUS

OF SA

MPLED WELL

Figure 9. Population density grouped by subunit 

surveys for the three subbasins of the Santa Ana 

Basin, southern California. 

A, Population density per 

square kilometer. B, Population density for 1990 

within a 500-meter radius 

of sampled wells.

12 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
The percentage of industrial and military land use was 

highest for the Coastal Basin (13 percent), intermediate for the 

Inland Basin (6 percent), and lowest for the San Jacinto Basin 

(3 percent). In contrast, the median percentage of industrial 

and military land use near the sampled wells was highest for 

the Inland Basin (14 percent), followed by the San Jacinto 

Basin (3 percent), and the Coastal Basin (2 percent). Numerous solvent plumes associated with industrial and military 

facilities have been documented in the Inland Basin (Wildermuth Environmental, 1999; Hamlin and others, 2002; Dawson 

and others, 2003).

The percentage of agricultural land use was highest in the 

San Jacinto Basin (39 percent), followed by the Inland Basin 

(13 percent), and the Coastal Basin (7 percent). The median 

percentage of agricultural land use near the sampled wells 

was lower than that for the basins as a whole and was highest 

for the San Jacinto Basin (6 percent), followed by the Inland 

(1 percent) and Coastal (0 percent) basins (fig. 7B; table 1). 

Agricultural land use is inversely proportional to the degree 

of urbanization in the basins (fig. 7A,B). Prior to urbanization, 

agricultural land use was important in the Inland and Coastal 

Basins.

The percentage of undeveloped land is highest in the 

Inland Basin (28 percent), followed closely by the San Jacinto 

Basin (25 percent), and is lowest in the Coastal Basin 

(12 percent). Median values are lower near sampled wells than 

in the basins as a whole. The median values near the sampled 

wells (fig. 7C; table 1) are identical for the San Jacinto and 

Inland Basins (6 percent). The median percentage near wells 

sampled in the Coastal Basin is zero percent; only two of the 

20 wells sampled had any undeveloped land (10 and 

51 percent) within a 500-m radius. 

Figure 10. Detection frequencies for tritium at or above 1 pCi/L and for volatile organic compounds (VOCs) and pesticides above and 

below the laboratory reporting limit (LRL) for the three subbasins of the Santa Ana Basin, southern California. A, San Jacinto Basin. 

B, Inland Basin. C, Coastal Basin.

Description and Comparison of Explanatory Factors in the Ground-Water Basins 13



&IGURE

         

$%4%#4)/.&2%15%.#9).0%2#%.4

4RITIUM

6/#S

0ESTICIDES

4RITIUM

6/#S

0ESTICIDES

4RITIUM

6/#S

0ESTICIDES

)NLAND

!TORABOVEP#I,

#OASTAL

3AN

*ACINTO

"ELOW,2,

!BOVE,2,

"ELOW,2,

!BOVE,2,

4RITIUM

6/#S

%80,!.!4)/.

0ESTICIDES

!

"

#
A B C

Inland Basin

Chloroform

PCE

TCE

DCE

cis-1,2-Dichloroethene

Bromodichloromethane

TCA

1,1-Dichloroethane

CFC-12

Carbon Tetrachloride

MTBE

1,2-Dichloromethane

1,4-Dichlorobenzene

CFC-11

CFC-113

DBCP

Bromoform

Chlorodibromomethane

o-Dichlorobenzene

Dibromomethane

Bromochlromethane

Chlorobenzene

Methylene Chloride

Toluene

Percent below LRL

EXPLANATION

Percent above LRL

PERCENT DETECTION

0 20 40 60 80 100

Any VOC

Coastal Basin 0 20 40 60 80 100

PERCENT DETECTION

Chloroform

MTBE

CFC-11

TCA

Bromodichloromethane

TCE

CFC-12

cis-1,2-Dichloroethylene

DCE

1,2,4-Trimethylbenzene

Carbon Disulfide

Chlorodibromomethane

PCE

1, 1-Dichloroethane

Methylene Chloride

Ethyl Ether

trans-1,2-Dichloroethene

CFC-113

Any VOC

Chloroform

Any VOC

PCE

Bromodichloromethane

TCE

Bromoform

1,1-Dichloroethane

DCE

Toluene

1,2-Dichloropropane

Carbon Tetrachloride

CFC-11

Chlorodibromomethane

San Jacinto Basin PERCENT DETECTION

0 20 40 60 80 100

Figure 11. Detection frequencies for volatile organic compounds (VOCs) by individual compound for the three subbasins of the Santa 

Ana Basin, southern California. A, San Jacinto Basin. B, Inland Basin. C, Coastal Basin.

14 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
To ensure that the potential explanatory factors were 

not statistically related, Kendall’s test was used to evaluate 

the potential correlation of depth to the top of the well screen 

with land use. For depth to the top of the well screen in the 

San Jacinto Basin, p values were 0.77 for urban land use and 

0.53 for agricultural land use. For depth to the top of the well 

screen in the Inland Basin, p values were 0.97 for urban land 

use and 0.68 for agricultural land use. For depth to the top of 

the well screen in the Coastal Basin, p values were 0.80 for 

urban and 0.26 for agricultural land use. These results indicate 

that the depth to the top of the well screen and land use are not 

correlated.

Occurrence and Distribution of VOCs 

and Pesticides in the San Jacinto 

Basin 

Detection Frequencies of Tritium, VOCs, and 

Pesticides

Tritium was the most frequently detected anthropogenic 

compound in the San Jacinto Basin; pesticides were least frequently detected (fig. 10). Tritium was detected in 78 percent 

of the wells sampled in the basin, indicating that the ground 

water was mostly young (recharged during the past 50 years). 

VOCs were detected in 70 percent of the wells sampled, which 

may reflect mostly urban land use near the wells. Pesticides 

were detected in 48 percent of the wells sampled. In some 

cases, pesticide detections may reflect agricultural land that 

has been recently urbanized, a common trend in the San 

Jacinto Basin. 

In the San Jacinto Basin, the most commonly detected 

VOCs (more than 20 percent of the sampled wells) were 

the disinfection byproducts chloroform and bromodichloromethane and the solvent PCE (fig. 11A). Recharge of ground 

water by chlorinated water, particularly in the Hemet area, is 

a potential source of disinfection byproducts in the sampled 

wells. The number of VOCs detected per well was significantly lower in the San Jacinto Basin than in the Inland Basin, 

but not different from that for the Coastal Basin (fig. 12A; 

table 1). The lower number of VOCs detected per well may 

reflect the relatively low percentage of urban land use (33 percent, low population density (fig. 2), and the absence of large 

solvent plumes in the areas of the San Jacinto Basin that were 

sampled (fig. 2). 

Inland Coastal

BASINS

BASINS

NUMBER OF

VOCs DETECTED,

PER WELL 16

12

8

4

0

NUMBER OF

PESTICIDES DETECTED,

PER WELL 12

10

8

6

4

0

2

Median

75th percentile 90th percentile

Outlier

EXPLANATION

25th percentile 10th percentile

A

B

San Jacinto

San Jacinto Inland Coastal

(10) Number of wells sampled

(23)

(20)

(29)

(23)

(20)

(28)

Figure 12. (A) number of volatile organic compounds (VOCs) 

detected and (B) number of pesticides detected in the San 

Jacinto, Inland, and Coastal Basins within the Santa Ana Basin, 

southern California.

Table 2. Detection frequencies for volatile organic compounds 

(VOCs) and pesticides in younger and older ground water in the 

San Jacinto, Inland, and Coastal Basins, within the Santa Ana 

Basin, southern California.

[Younger ground water (recharged since the 1950s) is defined by a tritium 

concentration at or above 1 pCi/L]

Basin

VOCs Pesticides

Younger Older Younger Older

San Jacinto 83 20 72 20

Inland 92 33 88 33

Coastal 90 50 36 0

All basins 88 36 72 14

Occurrence and Distribution of VOCs and Pesticides in the San Jacinto Basin 15
A B C

Inland Basin PERCENT DETECTION

0 20 40 60 80 100

Percent below LRL

EXPLANATION

Percent above LRL

San Jacinto Basin 0 20 40 60 80 100

PERCENT DETECTION

Coastal Basin PERCENT DETECTION

0 20 40 60 80 100

3(4-chlorophenly)-1-

methyl urea

Any pesticide

Deethyldeisopropylatrazine

Deethyldeisopropylatrazine

Any pesticide

Deethyldeisopropylatrazine

3(4-chlorophenly)-1-

methyl urea

Deethyldeisopropylatrazine

Deethylatrazine

Atrazine

Simazine

Tebuthiuron

Prometon

Hydroxyatrazine

Diuron

Sulfometruron Methyl

Bromacil

Metalxyl

Deethylatrazine

Atrazine

Simazine

Bromacil

Diuron

Prometon

Tebuthiuron

Diphenamid

Methylparathion

Prometon

Hydroxyatrazine

Bromoxynil

Dinoseb

Norflurazon

Any pesticide

3(4-chlorophenly)-1-

methyl urea

Atrazine

Simazine

Deethylatrazine

Diuron

Molinate

Bromocil

Prometon

Propanil

Imazethapyr

Tebuthiuron

Dinoseb

Deethyldeisopropyl- atrazine

Deisopropylatrazine

Figure 13. Detection frequencies for pesticide compounds by individual compound above and below the laboratory reporting limit (LRL) 

for the three subbasins of the Santa Ana Basin, southern California. A, San Jacinto Basin. B, Inland Basin. C, Coastal Basin. 

16 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
The most commonly detected pesticides were the herbicides atrazine and simazine and the atrazine degradation 

products deethylatrazine and deisopropylatrazine 

(fig. 13A). Atrazine was detected more frequently than its 

daughter product deethylatrazine in the San Jacinto Basin, in 

contrast to detections in the other ground-water basins, indicating a shorter residence time in ground water. 

Pesticides were detected less frequently in the San Jacinto 

Basin than in the Inland Basin (fig. 10), even though both 

basins are unconfined and the San Jacinto Basin has a higher 

percentage of agricultural land use. In addition, the number of 

pesticides detected per well was significantly lower in the San 

Jacinto Basin than in the Inland Basin (table 1; fig. 12B). The 

lower detection frequency, and the lower number of pesticides 

detected per well, may reflect the lower use of ground water 

(less pumping and engineered recharge) and consequently the 

lower flow (and transport) rates for the San Jacinto Basin than 

for the Inland Basin. Pesticides were detected more frequently 

in the San Jacinto Basin than in the Coastal Basin (fig. 10); the 

higher detection frequency is consistent with the basin being 

unconfined and having more agricultural land use. In addition, the median number of pesticides detected per well in the 

San Jacinto Basin was higher than that for the Coastal Basin 

(table 1; fig. 12B); however, the difference was not statistically 

significant (table 1). The pesticide data for the Coastal Basin 

were not suitable for basin-to-basin comparison; several wells 

located near recharge facilities had outliers with more than 

four detections per well (fig. 12B).

Relation to Ground-Water Age and Depth to the 

Top of the Well Screen

In the San Jacinto Basin, VOCs and pesticides were 

detected more frequently in younger ground water than older 

ground water (table 2). VOCs were detected in 83 percent of 

the younger samples and in 20 percent of the older samples. 

Similarly, pesticides were detected in 72 percent of the 

younger samples, and in 20 percent of the older samples. The 

higher detection frequencies in younger ground water suggest that these compounds have been introduced to the aquifer 

system since the early 1950s. The presence of VOCs and 

pesticides in older ground water may reflect the use of these 

compounds prior to the early 1950s. Alternatively, these compounds may have been introduced directly into older ground 

water from surface or shallow sources via well-bore leakage.

In the San Jacinto Basin, tritium was detected more 

frequently in shallower wells than in deeper wells (fig. 14A). 

Tritium was detected in all (100 percent) of the shallower 

wells and 70 percent of the deeper wells. The more frequent 

detection of tritium in shallower wells suggests that recharge 

to the ground-water flow system is primarily from above.

VOC detection frequency was higher in shallower wells 

(90 percent) than in deeper wells (40 percent; fig. 14A). In 

addition, the number of VOCs detected per well was significantly higher in shallower wells than in deep wells (fig. 14B; 

table 3), and inversely correlated to well-screen depth 

(table 3). 

A B C

San Jacinto Basin

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

<270 Feet

Depth to top of well screen

>270 Feet

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION EXPLANATION

25th percentile 10th percentile

NUMBER OF

VOCs DETECTED,

PER WELL 7

6

5

4

3

2

1

0 <270 FEET >270 FEET

DEPTH

NUMBER OF

PESTICIDES DETECTED

PER WELL 7

6

5

4

3

2

1

0 <270 FEET >270 FEET

DEPTH

(10) (10) (10)

(10)

(10)

Figure 14. (A) detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides; (B) number of VOCs detected, and 

(C) number of pesticides detected grouped by depth to the top of the well screen in the San Jacinto Basin. 

Occurrence and Distribution of VOCs and Pesticides in the San Jacinto Basin 17
Table 3. Statistical tests for difference between groups of data (Wilcoxon test) and for correlation (Kendall’s test) of potential 

explanatory factors with the number of VOCs detected and number of pesticides detected in the San Jacinto, Inland, and Coastal 

Basins, within the Santa Ana Basin, southern California.

[Kendall’s test was only done when Wilcoxon test indicated significant difference between groups of data; p value, probability value; p values less than or equal 

to 0.10 indicate significant difference or correlation (green shading); —, not calculated; proximal, near recharge facilities]

Ground-water basin Compound 

category Explanatory variable Wilcoxon test p value

Kendall’s test

p value Kendall’s tau

San Jacinto VOCs Screen depth 0.01 0.04 −0.33

Pesticides Screen depth 0.001 0.02 −0.36

VOCs Percent urban land use 0.85 — —

Pesticides Percent urban land use 0.04 0.09 −0.25

VOCs Percent agricultural land use 0.41 — —

Pesticides Percent agricultural land use 0.5 — —

VOCs Population density (by median) 0.27 — —

Pesticides Population density (by median) 0.04 0.01 −0.36

San Jacinto, Hemet area VOCs Screen depth (by median) 0.1 0.07 −0.28

Pesticides Screen depth (by median) 0.04 0.04 −0.44

VOCs Proximity to recharge 0.68 — —

Pesticides Proximity to recharge 0.07 — —

Inland VOCs Screen depth (by median) 0.19 — —

Pesticides Screen depth (by median) 0.65 — —

VOCs Screen depth (by <150, >350) 0.01 0.1 −0.3

Pesticides Screen depth (by <150, >350) 0.2 — —

VOCs Percent urban land use 0.5 — —

Pesticides Percent urban land use 0.46 — —

VOCs Percent agricultural land use 0.68 — —

Pesticides Percent agricultural land use 0.46 — —

VOCs Population density (by median) 0.66 — —

Pesticides Population density (by median) 1 — —

Coastal VOCs Unconfined vs confined areas 0.05 — —

Pesticides Unconfined vs confined areas 0.02 — —

VOCs Proximity to recharge facilities 0 — —

Pesticides Proximity to recharge facilities 0.07 — —

VOCs Percent urban land use 0.86 — —

Pesticides Percent urban land use 0.32 — —

VOCs Population density (by median) 0.59

Pesticides Population density (by median) 0.49

Coastal, proximal VOCs Unconfined vs confined areas 0.08 — —

Pesticides Unconfined vs confined areas 0.0006 — —

Coastal, unconfined VOCs Screen depth 0.01 0.03 −0.26

Pesticides Screen depth 0.03 0.2 −0.24

VOCs Proximity to recharge facilities 0.01 — —

Pesticides Proximity to recharge facilities 0.01 — —

VOCs Percent urban land use 0.6 — —

Pesticides Percent urban land use 1 — —

VOCx Population density (by median) 0.69 — —

Pesticides Population density (by median) 0.44 — —

Coastal, confined VOCs Screen depth 0.19 — —

Pesticides Screen depth 0.17 — —

VOCs Proximity to recharge facilities 0.01 — —

Pesticides Proximity to recharge facilities 0.27 — —

18 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
San Jacinto

River

Bautista

Creek

Bautista

Creek

Bautista

Creek

Bautista

Creek

1,200

1,250

1,350

1,300

1,400

1,425

1,325

1,50

1,4

0

40

1,700

1,400

1,400

1,250

1,600

1,350

1,300

1,500

1,700

1,400

1,350 1,60

1,325 0

Hemet

San Jacinto

A

1,500

San Jacinto Mts

San Jacinto

River

1,200

1,250

1,350

1,300

1,400

1,425

1,325

1,50

1,4

0

40

1,700

1,400

1,400

1,250

1,600

1,350

1,300

1,500

1,700

1,400

1,350 1,60

1,325 0

Hemet

San Jacinto

C

San Jacinto Mts

San Jacinto Mts

San Jacinto

River

1,200

1,250

1,350

1,300

1,400

1,425

1,325

1,440

1,700

1,400

1,400

1,250

1,600

1,350

1,300

1,500

1,700

1,400

1,350 1,60

1,325 0

Hemet

San Jacinto

San Jacinto

River

1,200

1,250

1,350

1,300

1,400

1,425

1,325

1,50

1,4

0

40

1,700

1,400

1,400

1,250

1,600

1,350

1,300

1,500

1,700

1,400

1,350 1,60

1,325 0

Hemet

San Jacinto

San Jacinto Mts

B

D

780

465

NN

470

470

TD650

438 TD328

364

288

600

322

56

150

125

340

TD603

240 300

350

136

450

400

EXPLANATION

Alluvium—Quaternary

Basin boundary

Fault

Direction of ground-water flow

Depth to top of well screen — To top

of well perforation. In feet below land

surface

Total well depth (TD) — In feet below

land surface Recharge facility

Proximal well —No detections

Distal well —No detections

W Fault block well —No detections ater-level contour, 1997 —Shows

altitude of water level. Contour

interval variable. In feet above sea

level, NGVD 88

Well type

10 MILES

10 KILOMETERS

5

5

Geology derived from California Division of

Mines and Geology, 1965 and 1981, and

Southern California Aerial Mapping Project, 1999

0

0

Pesticide detections in well type indicated. (Open symbol

indicates no detections)

Wells-screen depths (Total well depth - TD, given for well

lacking screen depth)

Pesticides

Tritium concentration at or above 1 pCi/L. (Open symbol

indicates no detections)

Tritium

VOC detections in well type indicated. (Open symbol indicates

no detections)

VOCs

1,300

603

TD603

33

47'

30”

33

45’

33

42'

30”

33

47'

30”

33

45’

33

42'

30”

33

47'

30”

33

45’

33

42'

30”

117 00' 116 55' 116 50'

33

47'

30”

33

45’

33

42'

30”

117 00' 116 55' 116 50'

117 00' 116 55' 116 50' 117 00' 116 55' 116 50'

Base from U.S. Geological Survey digital

elevation model, 1:24,000, 1999 Albers

Equal-Area Conic Projection Standard parallels

29 30' and 45 30', central meridian -117 15’ Water-level contours from Wildermuth

Environmental, Inc., 1999

Figure 15. (A) Water-level contours and depths to the tops of well screens, and distribution of (B) tritium, (C) VOCs, and (D) pesticides in 

the Hemet area of the San Jacinto Basin of the Santa Ana Basin, southern California. 

Occurrence and Distribution of VOCs and Pesticides in the San Jacinto Basin 19
Pesticides, like VOCS, were more frequently detected in 

shallower wells (70 percent) than in deeper wells (30 percent; 

fig. 14A). In addition, the number of pesticides detected per 

well was significantly higher in shallower wells than in deeper 

wells, and inversely correlated to well-screen depth (table 3; 

fig. 14C). 

The inverse correlation of VOC and pesticide occurrence 

with depth to the top of the well screen indicates that shallower wells tend to be more susceptible to contamination than 

deeper wells in the San Jacinto Basin. This correlation could 

be due to the proximity of the well screen to the sources of 

these compounds at the land surface or due to the presence 

of these compounds in younger recharge. In addition, the less 

frequent occurrence of VOCs and pesticides in deeper wells 

could reflect degradation of these compounds during transport 

through the aquifer system.

Relation to Engineered Recharge in the Hemet 

Area 

The distribution of VOCs and pesticides in ground water 

in the Hemet area of the San Jacinto Basin reflects proximity 

to sources of recharge (fig. 15). These sources include natural and engineered infiltration of water along the San Jacinto 

River, and natural infiltration along Bautista Creek. Recharge 

facilities located in and along the San Jacinto River take 

advantage of coarse soils and the availability of water. Water 

used for recharge may include water imported from northern 

California and from the Colorado River, as well as reclaimed 

wastewater (Williams and others, 1993). In the Hemet area, 

ground-water flow is generally from the San Jacinto River 

toward the city of Hemet where there is a large depression in 

water levels due to municipal pumping (fig. 15A; Rees and 

others, 1994). 

In the Hemet area, tritium was detected more frequently 

in shallower wells (70 percent) than in deeper wells 

(44 percent; fig. 16A). The more frequent detection of tritium in shallower wells suggests recharge by younger water, 

primarily from above. However, the shallower wells are also 

located closer to engineered recharge sources than are the 

deeper wells (p = 0.04; Wilcoxon test).

Tritium was detected more frequently in water from 

wells near sources of engineered recharge (90 percent) than 

in wells farther from recharge sources (30 percent; fig. 17A). 

Based on detection frequency, proximity to sources of engineered recharge appears to be a more important factor than 

well-screen depth for explaining the presence or absence of 

younger ground water in the Hemet area.

Tritium was detected in two wells located relatively far 

from sources of recharge (triangle symbols, fig. 15B). These 

detections may reflect preferential flow along faults because 

they are moderately deep (fig. 15A) and not likely affected by 

sources near the wellhead. Each well is located along a fault 

that is crossed by Bautista Creek, which may be a source of 

recharge.

A B C

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

<350 Feet

Hemet area of San Jacinto Basin

Depth to top of well screen

>350 Feet Median

75th percentile 90th percentile

Outlier

EXPLANATION EXPLANATION

25th percentile 10th percentile

NUMBER OF

VOCs DETECTED,

PER WELL

5

4

3

2

1

0

5

4

3

2

1

0 <350 FEET >350 FEET

DEPTH

NUMBER OF

PESTICIDES DETECTED,

PER WELL

<350 FEET >350 FEET

DEPTH

0%

(10) (9) (4) (5)

(10) Number of wells sampled

Figure 16. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides; (B) number of VOCs detected, 

and (C) number of pesticides detected by depth to the top of the well screen in the Hemet area of the San Jacinto Basin of the Santa 

Ana Basin, southern California. 

20 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
VOCs were detected more frequently in shallower wells 

(100 percent) than in deeper wells (67 percent; fig. 16A) in 

the Hemet area. In addition, the number of VOCs detected per 

well (fig. 16B) was significantly higher in shallow wells than 

in deeper wells, and was inversely correlated with screen depth 

(table 3). Although VOCs were detected more frequently in 

wells near engineered recharge sources (100 percent) than 

in distal wells (70 percent; fig. 17A), the number of VOCs 

detected per well was not statistically different between the 

two areas (table 3). Based on detection frequency, screen 

depth and proximity to sources of recharge are about equal 

in their ability to explain VOC occurrence in the Hemet area. 

Based on the number of VOCs detected per well, however, 

screen depth is the more important explanatory factor.

In contrast to other areas of the Santa Ana Basin, VOCs 

occur over a more widespread area than tritium in the Hemet 

area (fig. 15C). Of the 11 wells without detections of tritium 

in the Hemet area, 8 of them contained at least one VOC. The 

more widespread distribution of VOCs in this area could be 

due to the use of VOCs prior to the early 1950s or could be 

due to the introduction of VOCs into older ground water as a 

result of recharge from the overlying landscape. For example, 

chloroform, widely used since the 1920s, was detected in four 

wells and could have entered the ground-water flow system 

prior to the introduction of tritium. In contrast, TCE has been 

widely used only since the 1960s (Stackelberg and others, 

2000) and was detected in one sample of older ground water. 

Other VOCs detected in older ground water were 1,2-dichloropropane (2 wells), cis-1,2-DCE (1 well), PCE (1 well), 

and tetrahydrofuran (1 well); the production and use of these 

compounds postdates the generation of anthropogenic tritium 

in the early 1950s. One additional VOC was detected in older 

ground water: carbon disulfide, which can occur naturally or 

as a product of organic synthesis. The lack of tritium detections and the widespread occurrence of VOCs used only since 

the 1960s in older ground water indicates that VOCs migrate 

downward from the land surface into older ground water in the 

Hemet area.

Pesticides were detected more frequently in shallower 

wells (60 percent) than in deeper wells (no detections; 

fig. 16A). In addition, the number of pesticides detected per 

well was significantly higher in the shallower wells and was 

inversely correlated with screen depth (table 3; fig. 16C). 

These results indicate that well-screen depth is an explanatory 

factor for the occurrence of pesticides in the Hemet area.

Pesticides were detected in all the wells located near 

sources of recharge and in none of the distal wells (fig. 17A). 

In addition, the number of pesticides detected per well was 

significantly higher in the wells near sources of recharge than 

in distal wells (fig. 17C; table 3). These results indicate that 

proximity to sources of recharge is an explanatory factor for 

the occurrence of pesticides in the Hemet area.

A B C

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

Near Far

Proximity to recharge

PROXIMITY TO RECHARGE

Median

75th percentile 90th percentile

Outlier

EXPLANATION EXPLANATION

25th percentile 10th percentile

NUMBER OF

VOCs DETECTED,

PER WELL

5

4

3

2

1

0

5

4

3

2

1

0

Near Far

PROXIMITY TO RECHARGE

Near Far

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Hemet area of San Jacinto Basin

0%

(10) (10) (6) (4)

(10) Number of wells sampled

Figure 17. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides; (B) number of VOCs detected per 

well, and (C) number of pesticides detected by proximity to engineered recharge facilities in the Hemet area of the San Jacinto Basin of 

the Santa Ana Basin, southern California. 

Occurrence and Distribution of VOCs and Pesticides in the San Jacinto Basin 21
Relation to Land Use and Population Density

The San Jacinto Basin as a whole is the least urbanized 

(33 percent) of the Santa Ana ground-water basins and has the 

highest percentage of land used for agricultural purposes 

(37 percent). However, the median percentage of urbanized 

land within a 500-m (1,640-ft) radius of the sampled wells—

75 percent— (fig. 7A) is relatively high because public-supply 

wells tend to be located in urbanized areas in proximity to 

highest demand. The median percentage of agricultural land 

use near the sampled wells in the San Jacinto Basin (6 percent) 

was higher than that for the Inland (1 percent) and Coastal 

(0 percent) Basins, but was much less than that for the basin as 

a whole (37 percent). Compared with the other ground-water 

basins, the San Jacinto Basin has had the least amount, and 

shortest history, of urban development. Urban development in 

the basins is mirrored by growth in population density 

(fig. 9A).

VOC detection frequencies in the San Jacinto Basin were 

lower for wells classified as more urban than for wells classified as less urban, 67 percent as compared with 73 percent. In 

addition, the number of VOCs detected per well was not statistically different between the two groups (fig. 18A; table 3). 

The absence of correlation between urban land use and VOC 

occurrence contrasts with national findings (Squillace and 

others, 2002). This contrast may reflect the relatively recent 

urbanization of agricultural land; the time since land-use conversion may be an additional factor to consider in the evaluation of VOC occurrence. The contrast with national findings 

may also reflect the relatively large depth of wells sampled in 

the San Jacinto Basin, which generally have large contributing 

areas influenced by land use beyond a 500-m radius. 

VOC detection frequency was lower in wells classified as 

more agricultural (55 percent) than in wells classified as less 

agricultural (91 percent). This result is consistent with national 

findings (Squillace and others, 2002). However, the number of 

VOCs detected per well was not statistically different between 

the two groups (table 3; fig. 18B). These results may reflect 

the long history of agricultural land use; agricultural land use 

has had a relatively widespread impact on ground-water quality in the basin over a relatively long period of time. 

The relationship between pesticide occurrence and land 

use within 500 m of wells sampled in the San Jacinto Basin 

contrasts with national findings. Nationally, pesticide occurrence directly correlated with urban and agricultural land 

use (Squillace and others, 2002). In the San Jacinto Basin, 

pesticide detection frequency was lower in wells classified as 

more urban (42 percent) than in wells classified as less urban 

(82 percent). In addition, the number of pesticides detected 

per well was significantly higher in the wells classified as less 

urban (table 3; fig. 18C), and was inversely correlated with 

percentage urbanization (table 3; Kendall’s test). The inverse 

correlation is the opposite of national findings (Squillace and 

others, 2002). In addition, pesticide detection frequencies were 

the same for wells classified as more and less agricultural 

(58 percent). Similarly, the number of pesticides detected per 

well was not statistically different between the two groups 

(table 3; fig. 18D). These results indicate that current land use 

near the sampled wells is not an important factor for explaining pesticide occurrence in the San Jacinto Basin. 

A B C D

Median

75th percentile 90th percentile

EXPLANATION

25th percentile 10th percentile

Less — Less than median value

More — More than median value

NUMBER OF

VOCs DETECTED,

PER WELL

5

6

7

4

3

2

1

0

5

6

7

4

3

2

1

0

More

urban

San Jacinto Basin

Less

urban

5

6

7

4

3

2

1

0

NUMBER OF

PESTICIDES DETECTED,

PER WELL

More

urban

Less

urban

NUMBER OF

PESTICIDES DETECTED,

PER WELL

More

agricultural

Less

agricultural

NUMBER OF

VOCs DETECTED,

PER WELL

More

agricultural

Less

agricultural

5

6

7

4

3

2

1

0

(12) (11) (11) (12) (12) (11) (11) (12)

(10) Number of wells sampled

Outlier

Figure 18. Number of compounds detected in the San Jacinto Basin of the Santa Ana Basin, southern California. A, Number of VOCs 

detected per well for urban use. B, Number of VOCs detected for agricultural land use. C, Number of pesticides detected for urban use. 

D, Number of pesticides detected for agricultural land use.

22 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
The number of VOCs detected per well was not significantly different when wells were classified by higher and 

lower population density (table 3). This result contrasts with 

national findings (Squillace and others, 2002). In addition, the 

number of pesticides detected per well was significantly lower 

for wells in areas classified as having higher population density than in areas of lower population density, and the number 

of pesticides was negatively correlated to population density 

near the sampled wells (table 3). These results contrast with 

national findings (Squillace and others, 2002). The contrasts 

may reflect contribution of pesticides from agricultural land 

that has been recently urbanized. 

Occurrence and Distribution of VOCs 

and Pesticides in the Inland Basin

Detection Frequencies of Tritium, VOCs, and 

Pesticides

Tritium was detected somewhat more frequently

 (90 percent) than VOCs (86 percent) or pesticides

(83 percent) in the Inland Basin (fig. 10). The detection 

frequencies of tritium, VOCs, and pesticides were higher in 

the Inland Basin than the corresponding frequencies in the 

San Jacinto Basin (fig. 10). The higher detection frequencies reflect the more intense utilization of the ground-water 

resource (pumping and engineered recharge) and longer history of urbanization in the Inland Basin than in the San Jacinto 

Basin. At the basin scale, the percentage of urbanization 

in the Inland Basin has been higher than in the San Jacinto 

Basin and therefore may be a useful surrogate for the intense 

utilization of the resource. In contrast, the percentage of land 

use around the sampled wells was not significantly different 

in these basins. The detection frequencies of tritium, VOCs, 

and pesticides were also higher in the Inland Basin than the 

corresponding frequencies in the Coastal Basin (fig. 10). The 

higher detection frequencies probably reflect the generally 

unconfined conditions in the Inland Basin as compared to the 

more widespread confined conditions in the Coastal Basin. 

Although the Coastal Basin is more urbanized on a basin scale 

and near the sampled wells, detection frequencies were lower 

reflecting the predominance of confined conditions. 

The most commonly detected VOCs in the Inland Basin 

were the disinfection byproducts chloroform and bromodichloromethane and the solvents PCE, TCE, DCE, cis-1,2-dichloroethene, TCA, and 1,1-dichloroethane (fig. 11B). The number 

of VOCs detected per well was significantly higher in the 

Inland Basin than in the San Jacinto Basin, but not different 

from the Coastal Basin (table 1; fig. 12A). The greater number 

of VOCs detected per well in the Inland Basin compared with 

that in the San Jacinto Basin likely reflects a higher percentage of urban land use, for a longer period of time, in the Inland 

Basin than in the San Jacinto Basin. 

A B C

Inland Basin

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

< 150 feet 150-350 feet > 350 feet

EXPLANATION EXPLANATION

NUMBER OF

VOCs DETECTED,

PER WELL 12

10

8

6

4

2

0

4

3

2

1

0 < 150 150-350 > 350

DEPTH, IN FEET

< 150 150-350 > 350

DEPTH, IN FEET

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Depth to top of well screen

(8) Number of wells sampled

(8) (13) (8) (8) (13) (8)

Outlier

Median

75th percentile 90th percentile

25th percentile 10th percentile

Figure 19. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides; (B) number of VOCs detected, 

and (C) number of pesticides detected grouped by depth to the top of the well screen in the Inland Basin of the Santa Ana Basin, 

southern California

Occurrence and Distribution of VOCs and Pesticides in the Inland Basin 23
Solvents were detected more frequently in the Inland 

Basin (75 percent) than in the San Jacinto (35 percent) and 

Coastal (35 percent) Basins (Hamlin and others, 2002). The 

higher detection frequency in the Inland Basin may reflect a 

greater percentage of industrial and military land use near the 

sampled wells; the median value was 14 percent in the Inland 

Basin, but only 3 percent in the San Jacinto Basin and 

2 percent in the Coastal Basin (Appendixes A1−A3). Numerous solvent plumes associated with industrial and military 

facilities have been documented in the Inland Basin (Hamlin 

and others, 2002; Dawson and others, 2003).

The most commonly detected pesticides in the Inland 

Basin were the herbicides atrazine, simazine, bromacil, and 

diuron, and the atrazine degradation products deethylatrazine, 

deethyldeisopropylatrazine, and deisopropylatrazine

(fig. 13B). 

Pesticides were detected more frequently in the Inland 

Basin than in the San Jacinto Basin (fig. 10), even though 

both basins are unconfined and the Inland Basin has a lower 

percentage of agricultural land use. In addition, the number 

of pesticides detected per well was significantly higher in the 

Inland Basin (table 1; fig. 12B). The higher detection frequency and the higher number of pesticides detected per well 

may reflect the higher use of ground water (more pumping 

and engineered recharge) and consequently the higher flow 

(and transport) rates in the Inland Basin than in the San Jacinto 

Basin. Pesticides were also detected more frequently in the 

Inland Basin than in the Coastal Basin (fig. 10). Additionally, 

the number of pesticides detected per well was significantly 

higher in the Inland Basin (table 1; fig. 12B). These results 

are consistent with the higher percentage of agricultural land 

in the Inland Basin than in the Coastal Basin and with the 

confined nature of the Coastal Basin.

Relation to Ground-Water Age and Depth to the 

Top of the Well Screen

In the Inland Basin, VOCs and pesticides were detected 

more frequently in younger water than in older ground water 

(table 2). VOCs were detected in 92 percent of the younger 

samples and in 33 percent of the older samples. Similarly, 

pesticides were detected in 88 percent of the younger samples 

and in 33 percent of the older samples. As in the San Jacinto 

Basin, the higher detection frequencies in younger ground 

suggest that these compounds have generally entered the 

aquifer system since the early 1950s. The detection of VOCs 

and pesticides in older ground water may reflect use prior to 

this period. Alternatively, these compounds may have been 

introduced into older ground water from sources at or near 

land surface via well-bore leakage. 

Tritium was detected more frequently in shallower wells 

than in deeper wells in the Inland Basin, 93 and 86 percent, 

respectively. To better define the distribution of younger water, 

the data were grouped into three depth intervals, rather than 

two, and a greater contrast in detection frequency is seen: 

100 percent in wells with the depth to the top of the screen 

within 150 ft of land surface, 92 percent in wells with the top 

of the screen between 150 and 350 ft, and 75 percent in wells 

with the top of the screen more than 350 ft deep 

(fig. 19A). The trend in detection frequency of tritium with 

depth suggests that recharge to the ground-water flow system 

is primarily from above.

The trend in VOC detection frequency with depth is 

nearly the same as the trend for tritium: 100 percent in the 

shallowest wells, 92 percent in intermediate-depth wells, and 

62 percent in the deepest wells (fig. 19A). In addition, the 

number of VOCs detected per well was significantly higher in 

the shallowest wells than in the deepest wells (table 3; 

fig. 18B). The number of VOCs detected per well was 

inversely correlated with screen depth (table 3). These results 

indicate that VOCs are transported downward from the overlying landscape to aquifers in the Inland Basin.

Pesticide detection frequency also decreases with depth: 

100 percent in the shallowest wells, 85 percent in intermediate-depth wells, and 62 percent in the deepest wells (fig. 19A). 

However, the number of pesticides detected per well was not 

significantly different in the shallowest and the deepest wells 

(table 3; fig. 19C), nor correlated with screen depth (table 3). 

The trend in pesticide detection frequency indicates downward 

transport from the overlying landscape in the Inland Basin, but 

the number of compounds detected per well does not show a 

trend with depth.

In the Inland Basin, in contrast to the other basins, pesticides were detected nearly as frequently as VOCs, in both 

shallow and deep wells. The similar detection frequencies 

probably reflect intense use of the ground-water resource. The 

intense use of ground water has accelerated the movement of 

recharge water that might contain trace amounts of VOCs and 

pesticides through the flow system. Consequently, widespread 

distribution of pesticides may result from high flow rates and 

relatively short residence time in ground water, allowing less 

time for degradation and attenuation. 

Relation to Land Use and Population Density

The Inland Basin as a whole has intermediate percentages of urban (58 percent) and agricultural (13 percent) land 

compared with that in the San Jacinto and Coastal Basins. The 

median percentage of urbanized land within 500 m

(1,640 ft) of the sampled wells was higher than for the basin 

as a whole—75 percent. The amount of urbanization in the 

Inland Basin has been consistently higher than that in the San 

Jacinto Basin and consistently lower than that in the Coastal 

Basin (fig. 9A). The median percentage of agricultural land use 

near the sampled wells (1 percent) is intermediate between the 

San Jacinto (6 percent) and Coastal (0 percent) Basins, and is 

much lower than for the basin as a whole.

24 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
VOC detection frequencies in the Inland Basin were 

lower in wells classified as more urban than in wells classified 

as less urban, 79 percent compared with 93 percent. In addition, the number of VOCs detected per well was not statistically different (table 3) between the two groups (fig. 20A). 

The lack of direct correlation between VOCs and urban land 

use within a 500-m radius of sampled wells contrasts with 

national findings (Squillace and others, 2002). These results 

suggest that ground-water quality in the Inland Basin was 

not significantly affected by urban land use near the sampled 

wells. Several VOC plumes extend from 3 to 6 mi (5 to 10 km) 

in the aquifer system, illustrating the large extent that contaminants have been distributed in ground water (Hamlin and 

others, 2002; Dawson and others, 2003). The sources of these 

VOC plumes, and elevated VOC concentrations in general, are 

associated with military and industrial land uses.

VOC detection frequencies were somewhat lower for 

wells classified as more agricultural than in wells classified as 

less agricultural, 79 percent compared with 87 percent. This 

result is consistent with national findings (Squillace and others, 2002). However, the number of VOCs detected per well 

was not statistically different between the two groups 

(table 3; fig. 20B). This absence of correlation suggests that 

current agricultural land use within a 500-m radius of the well 

is not related to VOC occurrence in the Inland Basin; military, 

industrial, and urban land uses at distances beyond 500 m have 

a broader impact on VOC occurrence.

Pesticide detection frequencies were lower in wells classified as more urban than in wells classified as less urban, 

71 percent compared with 93 percent. This contrasts with 

national findings (Squillace and others, 2002). In addition, the 

number of pesticides detected per well was not statistically different between the two groups (table 3; fig. 20C). The current 

urban land use near the sampled wells is not an explanatory 

factor for pesticide occurrence in the Inland Basin.

Pesticide detection frequencies were higher in wells 

classified as more agricultural than in wells classified as less 

agricultural, 93 percent compared with 73 percent. This is 

consistent with national findings (Squillace and others, 2002).

However, the number of pesticides detected per well was not 

statistically different between the two groups (table 3;

fig. 20D). Although not statistically correlated with agricultural land use, pesticide occurrence may be related to pesticide 

usage in the basin.

A B C D

Median

75th percentile 90th percentile

EXPLANATION

25th percentile 10th percentile

Less — Less than median value

More — More than median value

More

urban

Less

urban

More

agricultural

Less

agricultural

12

14

16

10

8

6

4

2

0

More

agricultural

Less

agricultural

12

14

16

10

8

6

4

2

0

12

14

16

10

8

6

4

2

0

More

urban

Less

urban

12

14

16

10

8

6

4

2

0

(14) Number of wells sampled

Outlier

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Inland Basin

(14) (14) (14) (14) (14) (15) (14) (15)

Figure 20. Number of volatile organic compounds (VOCs) and pesticides detected in agricultural and urban areas of the Inland Basin 

of the Santa Ana Basin, southern California. A, Number of VOCs detected per well for urban use. B, Number of VOCs detected per 

well for agricultural land use. C, Number of pesticides detected per well for urban use. D, Number of pesticides detected per well for 

agricultural land use. 

Occurrence and Distribution of VOCs and Pesticides in the Inland Basin 25
In the Inland Basin, the number of VOCs and pesticides 

detected per well was not significantly different for wells 

classified by higher and lower population density (table 3). 

This is in contrast to national findings (Squillace and others, 

1999; Squillace and others, 2002). Population density near 

the sampled wells is not an explanatory factor for VOC and 

pesticide occurrence in the Inland Basin.

The lack of correlation of land use with VOCs and pesticides could be related to the relatively large depth of wells 

sampled in the Inland Basin and to intense use of ground 

water. The basin has been highly urbanized for a relatively 

long period of time and has a high population density that 

relies primarily on ground water for supply. As a consequence, 

ground-water flow rates have increased substantially, which 

has resulted in widespread occurrence of VOCs and pesticides 

in the basin.

Occurrence and Distribution of VOCs 

and Pesticides in the Coastal Basin 

Detection Frequencies of Tritium, VOCs, and 

Pesticides

The Coastal Basin differs from the San Jacinto and 

Inland Basins in that VOCs were detected more frequently 

than tritium (85 percent as compared with 70 percent; fig. 10). 

However, many of the detections were at concentrations below 

the laboratory reporting limit (LRL; see “Approach” section 

for explanation of LRL); the detection frequency of VOCs 

at concentrations above the LRL was lower than the detection frequency of tritium. In the Coastal Basin, as in the other 

basins, pesticides were detected less frequently (25 percent) 

than tritium or VOCs. 

In the Coastal Basin, the most commonly detected VOCs 

were the disinfection byproduct chloroform, the refrigerants 

CFC-113 and CFC-11, and the gasoline additive MTBE 

(fig. 11C). The number of VOCs detected per well in the 

Coastal Basin was not statistically different from the numbers 

detected per well in the San Jacinto and Inland Basins 

(table 1; fig. 12A). If the analysis is restricted to detections 

above the LRL, then the number of VOCs detected per well 

in the Coastal Basin was significantly lower than the number 

detected per well in the Inland Basin (p = 0.08, Wilcoxon test); 

the number of VOCs detected per well was not statistically 

different from the number detected per well in the San Jacinto 

Basin (p = 0.80, Wilcoxon test).

The most commonly detected pesticide compounds in the 

Coastal Basin were the herbicide atrazine and its degradation 

product deethylatrazine (fig. 13C). Pesticides were detected 

less frequently in the Coastal Basin than in the San Jacinto 

and Inland Basins (fig. 10). The number of pesticides detected 

per well was not significantly different from detections in 

the San Jacinto Basin, but was significantly lower than that 

in the Inland Basin (table 1; fig. 12B). The lower detection 

frequency, and lower number of pesticides detected per well, 

probably reflects the degree of aquifer confinement—mostly 

confined in the Coastal Basin and generally unconfined in the 

San Jacinto and Inland Basins.

A

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

< 388 feet > 388 feet

EXPLANATION

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0 < 388 feet > 388 feet

DEPTH TO TOP OF

WELL SCREEN

< 388 feet > 388 feet

DEPTH TO TOP OF

WELL SCREEN

Degree of confinement

Coastal Basin

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION

25th percentile 10th percentile

(10)

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

(10) (10) (10) (10)

B C

Figure 21. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides, and number of (B) VOCs and (C) 

pesticides detected by depth to the top of the well screen in the Coastal Basin of the Santa Ana Basin, southern California. 

26 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
Relation to Ground-Water Age and Depth to the 

Top of the Well Screen

VOCs and pesticides were detected more frequently 

in younger water (recharged since the early 1950s) than in 

older ground water in the Coastal Basin (table 2). VOCs were 

detected in more than 90 percent of the younger samples, 

but in only 50 percent of the older samples. Pesticides were 

detected in 36 percent of the younger samples, but in none of 

the older samples. 

In the Coastal Basin, tritium was detected less frequently 

in wells with shallower screens than in wells with deeper 

screens (60 percent compared with 80 percent; fig. 21A), in 

contrast to that for wells in the San Jacinto and Inland Basins. 

Tritium was detected less frequently than VOCs in shallower 

wells, also in contrast to that for the other basins, with the 

exception of the Hemet area of the San Jacinto Basin. These 

results likely result from the predominance of lateral transport 

in the mostly confined aquifers in the Coastal Basin; the distribution of tritium was not related to well-screen depth.

A B C

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

Unconfined

area

Confined area

EXPLANATION

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

Unconfined areaConfined areaUnconfined areaConfined area

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION

25th percentile 10th percentile

(10)

(37) (58) (14) (27)

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Degree of confinement

Coastal Basin

Figure 22. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides, and number of (B) VOCs and (C) 

pesticides detected in unconfined and confined areas of the Coastal Basin of the Santa Ana Basin, southern California. 

A

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

< 385 feet > 385 feet

EXPLANATION

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0 < 385 feet > 385 feet

DEPTH TO TOP OF

WELL SCREEN

< 385 feet > 385 feet

DEPTH TO TOP OF

WELL SCREEN

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION

25th percentile 10th percentile

(10)

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Depth to top of well screen

Unconfined area of Coastal Basin

(19) (18) (9) (9)

B C

Figure 23. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides in unconfined areas, and number 

of (B) VOCs and (C) pesticides detected by depth to the top of the well screen in the Coastal Basin of the Santa Ana Basin, southern 

California.

Occurrence and Distribution of VOCs and Pesticides in the Coastal Basin 27
VOC detection frequencies in the Coastal Basin followed 

a pattern similar to that for the other basins: higher in shallower wells than in deeper wells (90 percent compared with 

80 percent; fig. 21A). However, the number of VOCs detected 

per well was not significantly different in the shallower and 

deeper wells (fig. 21B). The lack of correlation suggests that 

well-screen depth is not explanatory for VOC occurrence in 

the Coastal Basin.

Pesticides were detected less frequently in shallower 

wells than in deeper wells (20 percent compared with 

30 percent; fig. 21A), and, like tritium, contrasted with detection frequencies observed in the San Jacinto and Inland Basins 

(figs. 14A and 19A). The number of pesticides detected per 

well was not significantly different in the shallower and deeper 

wells (fig. 21C). These results suggest that well-screen depth is 

not explanatory for pesticide occurrence in the Coastal Basin.

A

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

< 380 feet > 380 feet

EXPLANATION

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0 < 380 feet > 380 feet

DEPTH TO TOP OF

WELL SCREEN

< 380 feet > 350 feet

DEPTH TO TOP OF

WELL SCREEN

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION

25th percentile 10th percentile

(9)

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Confined area of Coastal Basin

Depth to top of well screen

(29) (29) (17) (9)

B C

Figure 24. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides in confined areas and number of 

(B) VOCs and (C) pesticides detected by depth to the top of the well screen in the Coastal Basin of the Santa Ana Basin, southern 

California.

A

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

Proximal Distal

EXPLANATION

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

Proximal Distal

POSITION RELATIVE TO

ENGINEERED RECHARGE FACILITIES

Proximal Distal

POSITION RELATIVE TO

ENGINEERED RECHARGE FACILITIES

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION

25th percentile 10th percentile

(26)

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Coastal Basin

Position relative to engineered recharge facilities

(41) (58) (26) (15)

B C

Figure 25. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides, and number of (B) VOCs and 

(C) pesticides detected by position relative to engineered recharge facilities in the Coastal Basin of the Santa Ana Basin, southern 

California.

28 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
Relation to Hydrogeologic Setting 

The hydrogeologic setting (unconfined versus confined) 

is an important explanatory factor for the distribution of tritium, VOCs, and pesticides in the Coastal Basin. Tritium was 

detected more frequently in the unconfined area (97 percent of 

sampled wells) than in the confined area (59 percent of sampled wells; fig. 22A), indicating that ground water is younger 

in the unconfined area than in the confined area, and flows 

laterally from the unconfined to the confined area (Belitz and 

others, 2004).

VOC detection frequencies are similar to those observed 

for tritium (fig. 22A): higher in the unconfined area than in the 

confined area (84 percent versus 59 percent). In addition, the 

number of VOCs detected per well was significantly higher in 

the unconfined area than in the confined area (table 3; 

fig. 22B). 

Pesticide detection frequencies are also similar to those 

observed for tritium (fig. 22A): higher in the unconfined area 

than in the confined area (57 percent versus 26 percent). And 

like VOCs, the number of pesticides detected per well was 

significantly higher in the unconfined area than in the confined 

area (table 3; fig. 22C). 

The distribution of tritium, VOCs, and pesticides is consistent with the dynamics of the flow system. Ground water 

recharged in the unconfined area since the early 1950s contains trace amounts of VOCs and pesticides and flows laterally 

into the confined area.

In the unconfined area of the Coastal Basin, tritium was 

detected in all (100 percent) of the shallower wells and in 

94 percent of the deeper wells (fig. 23A). The higher detection 

frequency in the shallower wells is consistent with downward 

movement of recharge in the unconfined area.

VOCs were also detected more frequently in shallower 

wells (89 percent) than in deeper wells (79 percent) in the 

unconfined area (fig. 23A). In addition, the number of VOCs 

detected per well was significantly higher in the shallower 

wells than in the deeper wells, and was inversely correlated 

with depth to the top of the well screen (table 3; fig. 23B). 

These results indicate downward movement of ground water in 

the unconfined area of the Coastal Basin.

Pesticides were also detected more frequently in shallower wells (75 percent) than in deeper wells (33 percent) in 

the unconfined area (fig. 23A). In addition, the number of pesticides detected per well was significantly higher in shallower 

wells than in deeper wells (table 3; fig. 23C). These results are 

consistent with downward movement of ground water in the 

unconfined area of the basin. 

In the confined area of the Coastal Basin, tritium was 

detected less frequently in shallower wells than in deeper wells 

(52 versus 69 percent; fig. 24A). VOCs were also detected less 

frequently in shallower wells than in deeper wells: 52 percent 

compared to 66 percent (fig. 23A). However, the number of 

VOCs detected per well was not significantly different in 

shallower and deeper wells (table 3; fig. 24B). The detection 

frequencies are the opposite of what one would expect in a 

system where recharge is from above. Instead, recharge in the 

confined area of the aquifer system occurs primarily by lateral 

flow from the unconfined area. 

A

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

EXPLANATION

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

Proximal Distal

UNCONFINED AREA UNCONFINED AREA

Proximal Distal

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION

25th percentile 10th percentile

(27)

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Unconfined area of Coastal Basin

Proximal Distal

Position relative to engineered recharge facilities

(10) (27) (6) (8)

B C

Figure 26. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides in unconfined areas, and number 

of (B) VOCs and (C) pesticides detected grouped by position relative to engineered recharge facilities in the Coastal Basin of the Santa 

Ana Basin, southern California. 

Occurrence and Distribution of VOCs and Pesticides in the Coastal Basin 29
Pesticides were detected at about the same frequency 

in shallower and deeper wells in the confined area (24 and 

22 percent, respectively; fig. 24A). The number of pesticides 

detected per well was not significantly different in shallower 

and deeper wells (table 3; fig. 24C). The median number of 

pesticides detected in both shallower and deeper wells was 

zero; only 4 of 26 wells sampled for pesticides in the confined 

area had detections (fig. 24C). These results reflect relatively 

low stability and low mobility of pesticides, which are lost 

during transport to, and through, the confined aquifer system.

Relation to Proximity to Engineered Recharge 

Facilities Along the Santa Ana River

Within the Coastal Basin, the ground-water flow system is dominated by high rates of recharge from engineered 

recharge facilities along the Santa Ana River (fig. 4). Tritium 

and VOCs define the “Santa Ana River flow path,” which 

extends more than 12 mi (20 km) from the recharge facilities 

in the unconfined area into the confined area (Dawson and 

others, 2003); the distribution of these compounds reflects a 

relatively long history of intense, focused recharge. In the flow 

path, the importance of the recharge facilities is reflected in 

the distribution of anthropogenic compounds; tritium, VOCs, 

and pesticides occur more frequently in wells that are proximal to the engineered recharge facilities than in wells that are 

more distal (fig. 25A). The numbers of VOCs and pesticides 

detected were significantly higher in the proximal wells than 

in wells further from the recharge facilities (table 3;

fig. 25B,C). These results are consistent with increased 

ground-water flow (and transport of anthropogenic compounds) in response to recharge from engineered facilities. 

If the comparison is restricted to wells in the unconfined 

area, the same relations are observed: higher detection frequencies in the proximal wells than in distal wells (fig. 26A). 

Indeed, tritium, VOCs, and pesticides were detected in all 

(100 percent) of the proximal wells. The numbers of VOCs 

and pesticides detected per well were both significantly higher 

in the proximal wells than in the distal wells sampled in the 

Coastal Basin (table 3; fig. 26B,C). 

If the comparison is restricted to wells in the confined 

area, the same relations are again observed: higher detection 

frequencies in the proximal wells than in the distal wells 

(fig. 27A). The number of VOCs detected per well was significantly higher in the proximal wells than in the distal wells 

(table 3; fig. 27B). However, the number of pesticides detected 

per well was not significantly different for the two groups 

of wells (table 3; fig. 27C). The median number of pesticide 

detections for both groups of wells was zero, reflecting loss 

during transport. 

The more frequent occurrence and greater number of 

VOCs and pesticides in wells located in proximity to the engineered recharge facilities reflects the displacement of native 

ground water by water introduced to the flow system since the 

early 1950s; large-scale recharge of imported water began at 

that time.

A

Tritium VOCs Pesticides

DETECTION FREQUENCY,

IN PERCENT 100

80

60

40

20

0

EXPLANATION

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

Proximal Distal

CONFINED AREA CONFINED AREA

Proximal Distal

Median

75th percentile 90th percentile

Outlier

Number of wells sampled

EXPLANATION

25th percentile 10th percentile

(31)

NUMBER OF

VOCs DETECTED,

PER WELL

NUMBER OF

PESTICIDES DETECTED,

PER WELL

Confined area of Coastal Basin

Proximal Distal

Position relative to engineered recharge facilities

(30) (31) (19) (8)

B C

Figure 27. (A) Detection frequencies for tritium, volatile organic compounds (VOCs), and pesticides in confined areas, and number of 

(B) VOCs and (C) pesticides detected by position relative to engineered recharge facilities in the Coastal Basin of the Santa Ana Basin, 

southern California.

30 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
Relation to Land Use and Population Density

The Coastal Basin is the most highly urbanized of 

the three basins; 80 percent of the land overlying the aquifer system is urban. The basin also has a longer history of 

urbanization, as indicated by historical trends in population 

density, than the other ground-water basins (fig. 9A). However, 

the aquifer system is confined in most of the Coastal Basin; 

therefore overlying land use is not expected to have a significant correlation with the general occurrence of anthropogenic 

compounds. The quality of water used in engineered recharge 

facilities and land use in the unconfined (recharge) area of the 

Coastal Basin are primary factors affecting the occurrence and 

distribution of VOCs and pesticides in the aquifer system.

The numbers of VOCs and pesticides detected per well 

were not significantly different when compared by the percentage of urbanization near the wells sampled in the Coastal 

Basin (table 3). Correlation of VOC and pesticide detections 

with urban land use would be more likely in the unconfined 

area of the basin. However, VOC and pesticide detections 

were not significantly different when grouped by percentage 

of urbanization in the unconfined area (table 3). The absence 

of correlation in both cases can be attributed to several factors: 

(1) uniformly high urban land use near the wells (appendix 

table A3), (2) use of large amounts of imported water (containing VOCs and pesticides) for recharge in the basin (from the 

Inland Basin, the Colorado River, and Northern California), 

and (3) widespread distribution of point sources of VOCs.

Similarly, the number of VOCs and pesticides detected 

per well were not significantly different when compared by 

population density near the wells sampled in the Coastal Basin 

(table 3). In addition, VOC and pesticide detections were not 

significantly different when grouped by population density in 

the unconfined area (table 3). Population density follows similar trends to, and may be considered a surrogate for, urbanization. The absence of correlation of VOCs and pesticides with 

population density near the sampled wells can be attributed to 

the same factors as that for percentage of urbanization. 

Summary and Conclusions 

The Santa Ana Basin can be subdivided into three 

primary ground-water basins: the San Jacinto, Inland, and 

Coastal Basins. Aquifers in the San Jacinto Basin are unconfined and land use in the basin is primarily agricultural 

(37 percent), with lesser amounts of urban (33 percent) and 

undeveloped (25 percent) land use. Population density in the 

basin increased from 188 people per km2 in 1970 to 

558 people per km2 in 2000. Aquifers in the Inland Basin are 

also unconfined and land use in the basin is primarily urban 

(58 percent), with lesser amounts of agricultural (13 percent) and undeveloped land (28 percent). Population density 

in the basin increased from 925 people per km2 in 1970 to 

1,620 people per km2 in 2000. The Coastal Basin includes a 

relatively small unconfined area (28 percent) and a relatively 

large confined area (72 percent). Recharge is primarily from 

engineered facilities in the unconfined area. The Coastal Basin 

is the most urbanized (80 percent) of the three ground-water 

basins. Population density in the basin increased from 

1,800 people per km2 in 1970 to 3,160 people per km2 in 2000. 

Data from two different types of studies were used in 

this report. Subunit surveys (SUS) were designed to obtain a 

statistical characterization of the basins as a whole. Data from 

special studies (flow path and aquifer susceptibility) were 

used, along with SUS data, to evaluate explanatory factors 

affecting water quality within individual basins. The wells 

sampled in these studies were primarily deep production wells, 

mostly used for public supply. The total depths of these wells 

ranged from 98 to 1,720 ft. The depths to the tops of the well 

screens ranged from 26 to 818 ft. Land use near the wells 

(within 500 m) was more urban than land use in the basins as a 

whole; public-supply wells tend to be located in urban areas in 

proximity to highest demand.

The most frequently detected VOCs in the San Jacinto, 

Inland, and Coastal Basins (occurring in more than 20 percent 

of the wells in any one basin) were disinfection byproducts 

(chloroform and bromodichloromethane), solvents (PCE, 

TCE, DCE, cis-1,2-dichloroethene, TCA, and 1,1-dichloroethane), refrigerants (CFC-113 and CFC-11), and the gasoline additive MTBE. The most frequently detected pesticide 

compounds were the herbicides atrazine and simazine and the 

atrazine degradation products deethylatrazine, deisopropylatrazine, and deethyldeisopropylatrazine. 

The occurrence and distribution of VOCs and pesticides 

in the Santa Ana ground-water basins were evaluated in 

relation to two types of explanatory factors: hydrogeologic 

characteristics and land use. Hydrogeologic characteristics 

evaluated were setting (unconfined or confined), ground-water 

age, depth to the top of well perforations, and proximity to 

engineered recharge. Land-use factors evaluated were the percentage of urban and agricultural land and population density. 

Population density can be considered a surrogate for the relative degree of urbanization. 

Summary and Conclusions 31
Anthropogenic compounds were detected more frequently in unconfined aquifers than in confined aquifers in 

the Santa Ana ground-water basins. In the mostly unconfined 

Inland and San Jacinto Basins and in the unconfined part of 

the Coastal Basin, tritium was detected in 90, 78, and 

97 percent of the wells sampled, respectively. In contrast, 

tritium was detected in 59 percent of the wells sampled in 

the confined area of the Coastal Basin. Similarly, VOCs were 

detected in more than 70 percent of the wells sampled in the 

unconfined areas, but in only 59 percent of the wells sampled 

in the confined area. Pesticides were detected in 83 and 

48 percent of the wells sampled in the unconfined Inland and 

San Jacinto Basins, respectively. In the unconfined part of the 

Coastal Basin, pesticides were detected in 57 percent of the 

wells sampled, but in only 26 percent of the wells sampled in 

the confined area. 

VOCs and pesticides were detected more frequently in 

younger ground water (recharged since the early 1950s) than 

older ground water. In the three ground-water basins, VOCs 

were detected in 88 percent of the younger samples and in 

36 percent of the older samples. Pesticides were detected in 

72 percent of the younger samples and in 14 percent of the 

older samples. In addition, more compounds (both VOCs and 

pesticides) were detected in younger samples. Water from 

shallow wells and wells proximal to recharge facilities is 

generally younger than water from deep wells and wells distal 

from recharge facilities. 

VOCs and pesticides were detected more frequently in 

shallower wells than in deeper wells in the unconfined San 

Jacinto and Inland Basins, and in the unconfined area of the 

Coastal Basin. In addition, the number of compounds detected 

per well was inversely correlated with depth to the top of the 

well screen. These correlations reflect the presence of younger 

ground water in shallower wells as compared to deeper wells, 

and proximity to sources of contamination from the overlying 

landscape. VOC and pesticide occurrence were not correlated 

with depth to the top of the well screen in confined aquifers of the Coastal Basin; the lack of correlation reflects the 

dominance of lateral flow and insulation from the overlying 

landscape. 

Proximity to engineered recharge facilities is an important explanatory factor for the distribution of anthropogenic 

compounds in the Coastal Basin and in the Hemet area of the 

San Jacinto Basin. VOCs and pesticides were detected more 

frequently in wells near engineered recharge facilities than in 

more distal wells. In addition, the number of VOCs and pesticides detected per well was significantly higher in proximal 

wells than distal wells in the Coastal Basin. In the Hemet area, 

the number of pesticides detected per well was significantly 

higher in proximal wells. In the Coastal Basin, anthropogenic 

compounds (tritium, VOCs, and pesticides) are present in 

ground water as much as 16 mi (25 km) from the recharge 

facilities, illustrating the large-scale displacement of native 

water by water recharged since the early 1950s.

On a national basis, land use within 500 m of shallow 

wells was an explanatory factor for the occurrence of VOCs 

and pesticides. A national evaluation of NAWQA data found 

that urban land use was positively correlated with VOC and 

pesticide occurrence. Similarly, on a national basis, population 

density showed a positive correlation with VOCs and pesticides, and agricultural land use was positively correlated with 

pesticide occurrence. In contrast, land use within 500 m of 

wells sampled in the Santa Ana ground-water basins was not 

an explanatory factor for the occurrence of VOCs and pesticides.

The absence of correlation between land use near wells 

(urban, agricultural, and population density) and the occurrence of VOCs and pesticides in the Santa Ana ground-water 

basins is due to a number of factors. In the Coastal Basin, most 

of the wells sampled are relatively deep and insulated from the 

overlying landscape by a thick confining layer. In the unconfined Inland and San Jacinto Basins, the absence of correlation is likely due to the large depth of the wells sampled; the 

capture zones for these deep wells likely extend beyond 

500 m. As evidence, there are solvent plumes in the Inland 

Basin that extend up to 10 km from the points of entry into 

the system. Another explanation for the lack of correlation 

between land use and the occurrence of VOCs and pesticides 

is that the current land use commonly differs from land use at 

the time of recharge; in the unconfined San Jacinto and Inland 

basins, population density has increased substantially since 

1970.

Qualitatively, there was a relationship between basinscale land use and basin-scale detection frequencies of VOCs 

and pesticides in the unconfined aquifers of the Santa Ana 

ground-water basins. Urban land uses and population density 

are higher in the Inland Basin than in the San Jacinto Basin, 

and VOCs and pesticides were detected more frequently in the 

Inland Basin. In addition, urbanization in the Inland Basin has 

led to more intense use of ground water and increased transport rates of these compounds.

The distribution of anthropogenic compounds in the 

Santa Ana ground-water basins illustrates the extensive influence of human activities on water quality. These compounds 

are widely distributed in ground water in the Santa Ana Basin, 

both laterally and vertically. Engineered recharge, irrigation 

return flow (both agricultural and landscape), and intensive 

municipal pumping have accelerated the movement of ground 

water, and have widely distributed anthropogenic compounds 

in the aquifer systems. The distribution of anthropogenic compounds, generally in trace amounts, refines our understanding 

of the aquifer systems; this knowledge can be used to better 

manage the ground-water resource.

32 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
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Table A1. San Jacinto Basin explanatory factors for the occurrence and distribution of volatile organic compounds (VOCs) and 

pesticides within the Santa Ana Basin, southern California.

[Proximal wells are near recharge facilities; distal wells are more distant; —, no data; n/a, not applicable; mi2, square mile; m, meter]

Appendix : Classification of Wells in the San Jacinto, Inland, and Coastal Basins

NAWQA ID1

Depth to top 

of well screen 

relative to 

median value

Location of well 

relative to 

engineered 

recharge facilities

in the Hemet area

Percent 

agricultural land 

within 500 m

of well

Percent

urban land 

within 500 m

of well

Percent 

undeveloped land 

within 500 m 

of well

Population density 

within 500 m 

of well, 

people per mi2

SAS-1 Deeper Proximal 0 100 0 1,070

SAS-2 — N/a 0 100 0 1,486

SAS-3 Deeper Distal 9 89 0 1,724

SAS-4 Shallower Proximal 0 100 0 1,568

SAS-5 — N/a 6 80 14 529

SAS-6 Deeper Distal 0 51 0 615

SAS-7 — N/a 15 78 6 1,518

SAS-8 Deeper Proximal 33 67 0 1,848

SAS-9 Deeper N/a 0 87 6 2,277

SAS-10 Deeper N/a 0 0 100 408

SAS-11 Deeper N/a 2 20 74 785

SAS-12 Shallower N/a 81 16 3 331

SAS-13 Shallower N/a 12 62 26 88

SAS-14 Shallower N/a 0 100 0 196

SAS-15 Shallower N/a 74 5 20 85

SAS-16 Shallower N/a 0 80 0 352

SAS-17 Shallower N/a 52 41 6 85

SAS-18 Deeper Distal 8 77 0 93

SAS-19 Shallower Proximal 0 81 19 189

SAS-20 Deeper N/a 43 8 50 12

SAS-21 Shallower Proximal 35 18 47 281

SAS-22 Shallower N/a 75 5 20 310

SAS-23 Deeper Distal 0 100 0 345

SAC-1 Deeper N/a 69 12 19 159

SAC-2 Shallower Distal 11 89 0 886

SAC-3 Deeper Proximal 26 57 14 513

SAC-4 Deeper Distal 32 61 0 690

SAC-5 Deeper Distal 7 48 1 1,068

SAC-6 Shallower Distal 0 100 0 1,930

SAC-7 Deeper Proximal 0 100 0 1,040

SAC-8 Deeper Distal 9 86 4 778

SAC-9 Shallower Distal 42 17 41 363

SAC-10 Shallower Proximal 9 45 45 339

SAC-11 Shallower Proximal 6 48 46 405

1

Wells are described and water-quality data are summarized in a report by Hamlin and others (2002).

36 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
Table A2. Inland Basin explanatory factors for the occurrence of compounds within the Santa Ana Basin, southern California.

[mi2, square mile]

NAWQA ID1

Depth to top of well 

screen relative 

to median value

Percent 

agricultural land 

within 500 meters 

of well

Percent

urban land 

within 500 meters 

of well

Percent 

vacant land 

within 500 meters 

of well

Population density 

within 500 meters of 

well, people per mi2

INS-1 Deeper 1 99 0 1,034

INS-2 Deeper 9 74 18 84

INS-3 Deeper 18 59 21 169

INS-4 Shallower 0 100 0 2,491

INS-5 Shallower 0 82 10 1,531

INS-6 Deeper 5 95 0 1,617

INS-7 Deeper 0 100 0 1,887

INS-8 Shallower 0 55 45 20

INS-9 Deeper 0 99 0 2,150

INS-10 Deeper 2 98 0 1,622

INS-11 Deeper 0 40 43 1,790

INS-12 Deeper 61 37 0 441

INS-13 Shallower 0 90 0 1,826

INS-14 Shallower 40 58 3 539

INS-15 Shallower 0 58 42 435

INS-16 Deeper 0 17 66 48

INS-17 Shallower 4 84 7 1,148

INS-18 Shallower 2 21 62 254

INS-19 Shallower 0 98 2 302

INS-20 Deeper 43 55 1 869

INS-21 Shallower 43 25 32 133

INS-22 Deeper 7 76 17 1,194

INS-23 Shallower 3 34 63 421

INS-24 Shallower 1 94 6 794

INS-25 Deeper 18 71 6 25

INS-26 Shallower 0 97 0 1,965

INS-27 Shallower 0 83 0 93

INS-28 Deeper 14 66 19 306

INS-29 Shallower 0 100 0 3,139

1

Wells are described and water-quality data are summarized in a report by Hamlin and others (2002).

Appendix : Classification of Wells in the San Jacinto, Inland, and Coastal Basins 37
Table A3. Coastal Basin explanatory factors for occurrence of compounds within the Coastal Basin of the Santa Ana Basin, southern California.

[m, meters; mi2, square mile]

NAWQA ID1

Hydrogeologic 

setting

(aquifer type)

Depth to top 

of well screen 

relative to 

median value

Location of well 

relative to 

engineered 

recharge 

facilities

Percent

agricultural 

land 

within 500 m 

of well

Percent 

urban land

within 500 m

of well

Percent 

vacant land 

within 500 m

of well

Population 

density within 

500 m of well, 

people per mi2

COS-1 Confined Shallow Proximal 0 52 0 1,835

COS-2 Confined Deep Proximal 0 96 0 2,993

COS-3 Confined Shallow Proximal 0 90 0 1,498

COS-4 Confined Shallow Distal 0 77 0 3,180

COS-5 Confined Shallow Distal 4 93 0 2,103

COS-6 Unconfined Deep Distal 0 93 0 1,942

COS-7 Unconfined Shallow Distal 0 100 0 2,499

COS-8 Unconfined Shallow Distal 0 100 0 1,510

COS-9 Unconfined Shallow Distal 2 32 51 201

COS-10 Confined Deep Distal 0 100 0 6,106

COS-11 Unconfined Deep Distal 0 100 0 2,711

COS-12 Unconfined Deep Distal 6 69 10 1,881

COS-13 Confined Shallow Distal 32 66 0 1,762

COS-14 Confined Shallow Proximal 29 68 0 2,112

COS-15 Confined Deep Distal 0 84 0 3,367

COS-16 Confined Deep Proximal 0 52 0 1,397

COS-17 Confined Shallow Distal 0 100 0 3,720

COS-18 Unconfined Deep Proximal 0 91 0 2,706

COS-19 Unconfined Deep Proximal 0 62 0 56

COS-20 Confined Deep Proximal 8 92 0 3,021

COF-1 Confined Shallow Proximal 7 93 0 3,065

COF-2 Confined Shallow Proximal 0 98 0 1,834

COF-3 Confined Deep Proximal 0 99 0 3,058

COF-4 Confined Shallow Proximal 1 93 0 2,984

COF-5 Confined Deep Proximal 0 100 0 3,542

COF-6 Confined Shallow Proximal 1 99 0 4,113

COF-7 Confined Deep Proximal 9 91 0 1,955

COF-8 Unconfined Deep Distal 0 100 0 2,711

COF-9 Confined Shallow Proximal 0 99 0 2,263

COF-10 Confined Shallow Proximal 0 96 0 4,256

COF-11 Confined Shallow Proximal 3 96 0 2,263

COF-12 Unconfined Shallow Proximal 6 90 0 919

COF-13 Confined Deep Proximal 0 90 0 3,204

COF-14 Unconfined Shallow Distal 0 100 0 2,593

COF-15 Confined Shallow Distal 6 94 0 3,093 

COF-16 Unconfined Shallow Distal 0 73 16 2,669

COF-17 Confined Shallow Proximal 0 100 0 279

COF-18 Unconfined Shallow Proximal 0 100 0 988

COF-19 Unconfined Shallow Proximal 1 99 0 2,582

See footnote at end of table.

38 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.
NAWQA ID1

Hydrogeologic 

setting

(aquifer type)

Depth to top 

of well screen 

relative to 

median value

Location of well 

relative to 

engineered 

recharge 

facilities

Percent

agricultural 

land 

within 500 m 

of well

Percent 

urban land

within 500 m

of well

Percent 

vacant land 

within 500 m

of well

Population 

density within 

500 m of well, 

people per mi2

COF-20 Confined Shallow Proximal 3 97 0 6,749

COF-21 Unconfined Shallow Proximal 0 78 0 1,018

COF-22 Unconfined Shallow Distal 6 62 0 2,058

COF-23 Confined Shallow Distal 0 47 0 4,779

OCC-1 Confined Deep Proximal 3 94 0 2,578

OCC-2 Confined Deep Proximal 0 97 0 2,711

OCC-3 Confined Deep Proximal 0 96 0 2,993

OCC-4 Confined Deep Distal 0 98 0 1,555

OCC-5 Confined Deep Distal 0 100 0 1,270

OCC-6 Confined Shallow Distal 13 84 0 2,081

OCC-7 Confined Deep Proximal 8 92 0 3,021

OCC-8 Confined Deep Proximal 0 98 0 2,744

OCC-9 Unconfined Shallow Proximal 0 96 0 5,296

OCC-10 Confined Deep Proximal 0 84 0 3,492

OCC-11 Confined Deep Proximal 0 92 0 5,589

OCC-12 Unconfined Deep Distal 0 91 0 3,825

OCC-13 Confined Deep Proximal 1 86 0 4,073

OCC-14 Confined Shallow Distal 4 93 0 2,103

OCC-15 Confined Deep Distal 6 82 0 1,472

OCC-16 Confined Shallow Distal 0 100 0 1,165

OCC-17 Unconfined Deep Proximal 0 91 0 2,706

OCC-18 Confined Shallow Distal 0 92 0 1,165

OCC-19 Unconfined Shallow Proximal 0 99 0 3,431

OCC-20 Unconfined Deep Proximal 0 62 0 56

OCC-21 Confined Shallow Distal 0 98 0 3,516

OCC-22 Confined Shallow Distal 0 92 0 8,807

OCC-23 Confined Deep Distal 0 100 0 6,214

OCC-24 Confined Deep Distal 0 100 0 6,106

OCC-25 Confined Deep Distal 0 100 0 1,540

OCC-26 Unconfined Deep Distal 0 93 0 1,942

OCC-27 Unconfined Shallow Distal 0 100 0 1,380

OCC-28 Confined Deep Distal 2 97 0 4,271

OCC-29 Confined Shallow Distal 0 98 0 2,212

OCC-30 Confined Shallow Proximal 0 95 0 3,236

OCC-31 Unconfined Shallow Distal 4 93 0 823

OCC-32 Unconfined Shallow Distal 5 94 0 726

OCC-33 Confined Shallow Distal 28 72 0 544

OCC-34 Confined Shallow Distal 0 60 40 1,669

OCC-35 Unconfined Deep Distal 0 100 0 3,728

OCC-36 Confined Deep Distal 41 16 44 496

OCC-37 Unconfined Deep Distal 0 100 0 2,543

Table A3. Coastal Basin explanatory factors for occurrence of compounds withint eh Coastal Basin of the Santa Ana Basin, southern California—

Continued.

[m, meters; mi2, square mile]

See footnote at end of table.

Appendix : Classification of Wells in the San Jacinto, Inland, and Coastal Basins 39
NAWQA ID1

Hydrogeologic 

setting

(aquifer type)

Depth to top 

of well screen 

relative to 

median value

Location of well 

relative to 

engineered 

recharge 

facilities

Percent

agricultural 

land 

within 500 m 

of well

Percent 

urban land

within 500 m

of well

Percent 

vacant land 

within 500 m

of well

Population 

density within 

500 m of well, 

people per mi2

OCC-38 Confined Deep Distal 0 100 0 1,600

OCC-39 Unconfined Shallow Distal 0 100 0 2,632

OCC-40 Unconfined Deep Distal 3 94 0 1,421

OCC-41 Unconfined Deep Distal 10 81 0 722

OCC-42 Unconfined Deep Distal 6 85 0 762

OCC-43 Confined Deep Distal 0 96 0 7,682

OCC-44 Confined Shallow Distal 0 98 0 3,704

OCC-45 Confined Shallow Distal 0 99 0 6,402

OCC-46 Unconfined Shallow Distal 0 64 0 3,264

OCC-47 Unconfined Shallow Distal 0 84 0 2,354

OCC-48 Unconfined Deep Distal 0 86 0 2,108

OCC-49 Confined Deep Distal 3 87 0 3,614

OCC-50 Confined Deep Distal 0 100 0 5,863

OCC-51 Confined Deep Distal 0 100 0 5,801

OCC-52 Unconfined Shallow Distal 0 100 0 4,425

OCC-53 Confined Deep Distal 14 86 0 1,548

OCC-54 Unconfined Deep Distal 5 95 0 642

OCC-55 Unconfined Shallow Proximal 0 100 0 1,947

OCC-56 Unconfined Deep Distal 0 100 0 2,019

1

Wells are described and water-quality data are summarized in a report by Hamlin and others (2002).

Table A3. Coastal Basin explanatory factors for occurrence of compounds withint eh Coastal Basin of the Santa Ana Basin, southern California—

Continued.

[m, meters; mi2, square mile]

40 Occurrence, Distribution of Volatile Organic Compounds, Pesticides in Ground Water, Santa Ana Basin, Southern Calif.