U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report

Low-Level Volatile Organic Compounds in Active

Public Supply Wells as Ground-Water Tracers in the 

Los Angeles Physiographic Basin, California, 2000

By Jennifer L. Shelton, Karen R. Burow, Kenneth Belitz, Neil M. Dubrovsky, 

Michael Land, and JoAnn Gronberg

Sacramento, California

2001

01-4188

5025-29

Prepared in cooperation with the

CALIFORNIA STATE WATER RESOURCES CONTROL BOARD

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

from:

U.S. Geological Survey

Water Resources Division

Placer Hall, Suite 2012

6000 J Street

Sacramento, California 95819-6129
Robert M. Hirsch

Associate Director for Water

FOREWORD

The U.S. Geological Survey (USGS) is committed to 

serve the Nation with accurate and timely scientific information that helps enhance and protect the overall quality of 

life, and facilitates effective management of water, biological, energy, and mineral resources. (http://www.usgs.gov/). 

Information on the quality of the Nation’s water resources is 

of critical interest to the USGS because it is so integrally 

linked to the long-term availability of water that is clean and 

safe for drinking and recreation and that is suitable for 

industry, irrigation, and habitat for fish and wildlife. Escalating population growth and increasing demands for the 

multiple water uses make water availability, now measured 

in terms of quantity and quality, even more critical to the 

long-term sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality 

Assessment (NAWQA) Program to support national, 

regional, and local information needs and decisions related 

to water-quality management and policy. (http://

water.usgs.gov/nawqa). Shaped by and coordinated with 

ongoing efforts of other Federal, State, and local agencies, 

the NAWQA Program is designed to answer: What is the 

condition of our Nation’s streams and ground water? How 

are the conditions changing over time? How do natural features and human activities affect the quality of streams and 

ground water, and where are those effects most pronounced? By combining information on water chemistry, 

physical characteristics, stream habitat, and aquatic life, the 

NAWQA Program aims to provide science-based insights 

for current and emerging water issues and priorities. 

NAWQA results can contribute to informed decisions that 

result in practical and effective water-resource management 

and strategies that protect and restore water quality.

Since 1991, the NAWQA Program has implemented 

interdisciplinary assessments in more than 50 of the 

Nation’s most important river basins and aquifers, referred 

to as Study Units. (http://water.usgs.gov/nawqa/

nawqamap.html). Collectively, these Study Units account 

for more than 60 percent of the overall water use and population served by public water supply, and are representative 

of the Nation’s major hydrologic landscapes, priority ecological resources, and agricultural, urban, and natural 

sources of contamination. 

Each assessment is guided by a nationally consistent 

study design and methods of sampling and analysis. The 

assessments thereby build local knowledge about 

quality issues and trends in a particular stream or 

while providing an understanding of how and wh

quality varies regionally and nationally. The cons

multi-scale approach helps to determine if certain 

water-quality issues are isolated or pervasive, and 

direct comparisons of how human activities and n

cesses affect water quality and ecological health 

Nation’s diverse geographic and environmental s

Comprehensive assessments on pesticides, nutrie

tile organic compounds, trace metals, and aquatic 

are developed at the national scale through comp

analysis of the Study-Unit findings. (http://water.

nawqa/natsyn.html). 

The USGS places high value on the commun

and dissemination of credible, timely, and relevan

so that the most recent and available knowledge a

resources can be applied in management and poli

sions. We hope this NAWQA publication will pr

the needed insights and information to meet your 

thereby foster increased awareness and involvem

protection and restoration of our Nation’s waters

The NAWQA Program recognizes that a nati

assessment by a single program cannot address a

resource issues of interest. External coordination 

els is critical for a fully integrated understanding 

sheds and for cost-effective management, regulat

conservation of our Nation’s water resources. The 

therefore, depends extensively on the advice, coo

and information from other Federal, State, interst

and local agencies, non-government organization

academia, and other stakeholder groups. The assi

suggestions of all are greatly appreciated.
Contents 

CONTENTS

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Hydrogeologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Coastal Los Angeles Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Coastal Santa Ana Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Study Design and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Well Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Sample Collection and Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Quality-Control Data for VOCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Active Public Supply Wells Frequently Contain Low Concentrations of VOCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

VOC Distribution Indicates Enhanced Recharge and Ground-Water Pumping are Primary Controls on 

Ground-Water Flow System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Stable Isotope Data Relate Source of Recharge Water to VOC Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Assessment of Relation Between VOC Distribution and Potential Sources of VOCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Surface Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Population Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Leaking Underground Fuel Tank Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Hydrogeologically Vulnerable Areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

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

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

FIGURES

1. Major hydrogeologic features of the Los Angeles physiographic basin, California, showing 

study area and simplified cross-section showing the direction of ground-water flow . . . . . . . . . . . . . . . . . . . 

2. Public supply wells identified as candidates for sampling and resulting well network, Los Angeles 

physiographic basin, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

3. The sampling well network and the number of detections of volatile organic compounds . . . . . . . . . . . . . . . 

above the laboratory reporting limit, Los Angeles physiographic basin, California, . . . . . . . . . . . . . . . . . . . . 

4. Scatter plot of the number of detections of volatile organic compounds in samples from wells 

in the Coastal Los Angeles and Coastal Santa Ana forebay areas, California, as a function of distance 

from recharge facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

5. Wells showing data for the most frequently detected volatile organic compound in each of four primary use 

categories in the Los Angeles physiographic basin, California for fuel oxygenate (MTBE), 

solvent (TCE), refrigerant (CFC-11), and the disinfection byproduct (chloroform). . . . . . . . . . . . . . . . . . . . . 

 6. Boxplot of the number of detections of volatile organic compounds above the laboratory reporting limit 

in samples from wells grouped by depth categories in the forebay areas of the Los Angeles physiographic 

basin, California, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

7. Scatter plot of the delta oxygen-18 versus delta deuterium for samples from active public supply wells in 

the Los Angeles physiographic basin, California, showing hypothetical mixing lines for sources of recharge 

water to wells, and regression lines for samples with no detections and with detections above the LRL . . . . 
vi Contents

8. Histogram of the frequency of MTBE detections in samples from all wells in the Los Angeles 

physiographic basin, in samples from wells in the Coastal Los Angeles Basin, and from wells in the 

Coastal Santa Ana Basin and the density of leaking underground fuel tanks within a 

500-meter radius around the wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

9. Map showing the number of detections of volatile organic compounds in samples from active 

public supply wells in the Los Angeles physiographic basin, California, overlain by the 

California State Water Resources Control Board’s hydrogeologically vulnerable area . . . . . . . . . . . . 

TABLES

1. Summary of well construction information for 178 active public supply wells sampled in the 

Los Angeles physiographic basin, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

2. Volatile organic compounds detected in samples from 178 active public supply wells in the 

Los Angeles physiographic basin, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

3. Volatile organic compounds analyzed for but not detected in samples from 178 active public 

supply wells in the Los Angeles physiographic basin, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

4. Volatile organic compounds detected in 7 percent or more of the samples from 178 active public supply 

wells in the Los Angeles physiographic basin, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

5. Comparison of the six most frequently detected volatile organic compounds in ground-water samples fro

the Los Angeles physiographic basin, California, and from urban areas across the United States . . . . . . . 

6. Volatile organic compounds detected in samples from the Santa Ana River above the uppermost 

recharge facility in the Coastal Santa Ana Basin and from 178 active public supply wells in the 

Los Angeles physiographic basin, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
Contents 

CONVERSION FACTORS, VERTICAL DATUM, ACRONYMS, AND ABBREVIATIONS

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = 1.8(°C) + 32

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both

the United States and Canada, formerly called Sea Level Datum of 1929.

Acronyms and Abbreviations

δ, delta

μg/L, microgram per liter

acre-ft, acre-foot

mL, milliliter

ft, foot

CAS, California Aquifer Susceptibility

CFC-11 (Freon 11), trichlorofluoromethane

CFC-113 (Freon 113), 1,1,2-trichloro-1,2,2-trifluoroethane

D, deuterium

DCE, 1,1-dichloroethene

DHS, California Department of Health Services

GAMA, Ambient Ground Water Monitoring and Assessment Program

GWSI, Ground Water Site Inventory

HCl, hydrochloric acid

LLNL, Lawrence Livermore National Laboratory

LRL, laboratory reporting limit

LUFT, leaking underground fuel tank

LUST, leaking underground storage tank

MCL, maximum contaminant level

MTBE, methyl tert-butyl ether

NAWQA, National Water-Quality Assessment (Program)

NWQL, National Water Quality Laboratory

18O, Oxygen-18

OCWD, Orange County Water District

PCE, tetrachloroethene (or tetrachloroethylene)

SWRCB, California State Water Resources Control Board

TCA, 1,1,1-trichloroethane

TCE, trichloroethene (or trichloroethylene)

USGS, U.S. Geological Survey

UST, underground storage tank

VOC, volatile organic compound

WRD, Water Replenishment District of Southern California

Multiply By To obtain

kilogram (kg) 2.205 pound avoirdupois (lb avdb)

kilometer (km) 0.6214 mile (mi)

cubic kilometer (km3) 0.0008107 acre-foot (acre-ft)

meter (m) 3.281 foot (ft)

square kilometer (km2

) 0.3861 square mile (mi2)
Abstract 1

Low-Level Volatile Organic Compounds in Active

Public Supply Wells as Ground-Water Tracers in the 

Los Angeles Physiographic Basin, California, 2000

by Jennifer L. Shelton, Karen R. Burow, Kenneth Belitz, Neil M. Dubrovsky, 

Michael Land, and JoAnn M. Gronberg

ABSTRACT

Data were collected to evaluate the use of 

low-level volatile organic compounds (VOC) to 

assess the vulnerability of public supply wells in 

the Los Angeles physiographic basin. Samples of 

untreated ground water from 178 active public 

supply wells in the Los Angeles physiographic 

basin show that VOCs were detected in 61 percent 

of the ground-water samples; most of these detections were low, with only 29 percent above 1 μg/L 

(microgram per liter). Thirty-nine of the 86 VOCs 

analyzed were detected in at least one sample, and 

11 VOCs were detected in 7 percent or more of the 

samples. The six most frequently detected VOCs 

were trichloromethane (chloroform) (46 percent); 

trichloroethene (TCE) (28 percent); tetrachloroethene (PCE) (19 percent); methyl tert-butyl ether 

(MTBE) (14 percent); 1,1-dichloroethane 

(11 percent); and 1,1,1-trichloroethane (TCA) 

(11 percent). These VOCs were also the most frequently detected VOCs in ground water representative of a wide range of hydrologically conditions 

in urban areas nationwide. Only two VOCs (TCE 

and PCE) exceeded state and federal primary 

maximum contaminant levels (MCL) for drinking 

water in a total of seven samples. Because samples 

were collected prior to water treatment, sample 

concentrations do not represent the concentrations 

entering the drinking-water system.

Ground water containing VOCs may be 

considered to be a tracer of postindustrial-aged 

water—water that was recharged after the onset of 

intense urban development. The overall distribution of VOC detections is related to the hydrological and the engineered recharge facilities in the 

Coastal Los Angeles Basin and the Coastal Santa 

Ana Basin that comprise the Los Angeles physiographic basin. Most of the ground-water recharge 

occurs at engineered recharge facilities in the generally coarse-grained northeastern parts of the 

study area (forebay areas). Ground-water recharge 

from the land surface is minimal in the southwestern part of the basins, distal from the recharge 

facilities, where clay layers impede the vertical 

migration of ground water (pressure areas).

VOCs are not uniformly distributed over the 

study area. Most of the wells with multiple VOC 

detections, which also have the highest concentrations, are in the forebay areas and are clustered 

proximal to the recharge facilities. In addition, the 

number of VOC detections and VOC concentrations decrease beyond about 10–15 kilometers 

from the recharge facilities. The distribution of 

individual VOCs is also related to their history of 

use. MTBE traces ground water recharged during 

about the last decade and is detected almost exclusively in the forebay areas. Chloroform, which has 

been used since the 1920s, is more widely 

distributed and is detected at the greatest distances 

from the recharge facilities.

Downward migration of VOCs from the land 

surface may be a viable process for VOCs to reach 

aquifers in parts of the forebay areas, but there is 

little indication that the same process is active in 
2 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

the pressure area. The lack of contrast in the number of VOC detections between wells of different 

depths over most of the study area suggests that the 

downward migration from the land surface is not a 

dominant pathway for VOCs to travel to the capture zones of public supply wells. Isolated occurrences of multiple VOC detections and high 

concentrations of VOCs in individual wells may 

indicate rapid vertical transport from a localized 

source. 

Stable isotope data indicate that ground 

water containing VOCs is a mixture of local precipitation and runoff with water that is isotopically 

lighter (more negative) than the local sources. The 

isotopically lighter water could either be Colorado 

River water or State Water Project water, both of 

which are imported to the basin and used as a 

source of recharge to the ground-water flow system. The stable isotope data support the interpretation that VOCs in ground water are associated with 

the engineered recharge facilities.

Two of the most frequently detected VOCs 

in ground water, MTBE and chloroform, were 

detected more frequently and at higher concentrations in the Santa Ana River than in ground-water 

samples from active public supply wells, suggesting that the Santa Ana River may be a source of 

these compounds. In contrast, the maximum concentrations and frequency of detection of TCE and 

PCE are much higher in the ground-water samples 

than in the Santa Ana River. The VOCs detected in 

surface water sampled in the present time are only 

an approximation of past surface-water quality, 

which likely varied as a complex function of 

urbanization, water-quality regulations, and climate; and current surface-water quality alone can 

not explain VOC occurrence in ground water. 

Correlations between VOC occurrence and 

sources of VOCs on the land surface do not explain 

VOC occurrence in this study. The overall lack of 

correlation between these surficial features and the 

spatial distribution of VOCs in active public supply 

wells in this study indicates that vertical migration 

of contaminants is not a dominant process outside 

the susceptible forebay areas. Hydrogeology and 

the acceleration of the lateral rate of ground-water 

flow produced by engineered recharge and groundwater pumping in the Coastal Los Angeles Basin 

and the Coastal Santa Ana Basin appear to be the 

dominant factors in controlling the distribution of 

VOCs in active public supply wells in these coastal 

basins. Overall, the data show that active public supply wells in the forebay areas are more vulnerable to 

contamination than those in the pressure area in the 

Los Angeles physiographic basin study area.

INTRODUCTION 

During 1995, 14.5 billion gallons per day of 

ground water were extracted in California for a variety 

of uses, including 2.7 billion gallons per day for public 

supply used primarily for domestic purposes (Solley 

and others, 1998). Ground water supplies an estimated 

30 to 60 percent of California’s drinking water (Solley 

and others, 1998; Association of California Water 

Agencies, 2001), about half of which is used in the 

heavily urbanized parts of the southern California 

region (William E. Templin, U.S. Geological Survey, 

written commun., 2000). The quality of drinking-water 

resources is potentially susceptible to contamination 

from point- and nonpoint-source contaminants released 

into the urban environment. One specific class of chemicals—volatile organic compounds (VOC)—have a 

wide variety of uses and may be released into the urban 

environment during their production, distribution, storage, and use. Of particular concern is the long-term 

affect of the gasoline additive methyl tert-butyl ether 

(MTBE) from point sources, such as leaking underground storage tanks and spills. 

In addition to the high number of potential contaminant sources, the potential susceptibility of these 

drinking-water resources is also heightened by the 

intense engineering of ground-water recharge. In general, the ground-water systems in southern California 

have been engineered to greatly increase the amount of 

water recharged (often focused in small areas) and the 

ground-water pumpage in public supply wells to provide water for a growing population. The vertical rate of 

ground-water movement also has been increased by 

the presence of abandoned or improperly constructed 

wells in some areas. An acceleration of the rate of 

ground-water recharge and movement increases the 

susceptibility of a public supply well to contamination 

by decreasing the travel time between potential contaminant sources and the well. There is a compelling need 

to understand the factors that control the occurrence, 

distribution, and transport of contaminants in urban 

ground-water systems and the degree of susceptibility 
Hydrogeologic Setting 3

of drinking-water resources in contrasting hydrogeologic settings. 

In response to concern for future ground-water 

quality, the California State Water Resources Control 

Board (SWRCB) has implemented the California 

Aquifer Susceptibility (CAS) assessment to determine 

the water quality and susceptibility of ground water 

that serves as a source for public water supplies to 

potentially contaminating activities. CAS is part of the 

Ambient Groundwater Monitoring and Assessment 

(GAMA) Program that employs ground-water age dating techniques and low-level analyses for VOCs. The 

GAMA Program was developed by the SWRCB as a 

result of a mandate by the California State Legislature 

(California Legislative Analyst’s Office, 1999) to 

develop a comprehensive ambient ground-water monitoring plan after public supply wells were closed 

because of the detection of MTBE and various industrial solvents. Under the GAMA Program, the U.S. 

Geological Survey (USGS) National Water-Quality 

Assessment (NAWQA) Program is collaborating with 

the SWRCB, the California Department of Health 

Services (DHS), the California Department of Water 

Resources, and the Lawrence Livermore National Laboratory (LLNL) to implement the CAS assessment. 

This study was part of the pilot investigation of 

the feasibility of developing standard methodology for 

screening public supply wells to assess the groundwater age and source of recharge. Data were collected 

from a network of wells in the Los Angeles physiographic basin to evaluate the use of ground-water age 

(using tritium–helium analysis) and low-level 

concentrations of VOCs as indicators of the potential 

susceptibility of public supply wells and, by inference, 

specific aquifers to potential contamination. Age dating techniques used during this study provide information on the presence of young water (less than 50 years 

since recharged) in the well, and analysis of low concentrations of VOCs may provide an early warning of 

potential VOC contamination moving toward a public 

supply well. For example, the presence of young 

ground water in a public supply well indicates that the 

producing zone of the aquifer may be expected to be 

more susceptible to potential contamination because of 

the short travel time of water from the land surface to a 

well. Low-level analysis for VOCs such as MTBE will 

also allow water managers to identify trends in groundwater quality in their region and respond before concentrations exceed water-quality criteria. The data 

were evaluated in combination with the locations of 

potential contaminant sources and existing knowledge 

of the hydrogeology to determine the dominant factors 

that control vulnerability of the ground-water resource 

in the Los Angeles physiographic basin.

This report presents an analysis of the distribution and occurrence of VOCs in active public supply 

wells sampled prior to water treatment in the Los 

Angeles physiographic basin, and relates the distribution of VOCs to the hydrogeologic framework and the 

potential contaminant sources to assess the susceptibility of active public supply wells. During August 

through November 2000, the USGS collected 

untreated ground-water samples from a network of 178 

active public supply wells. Samples were analyzed for 

a suite of 86 VOCs and stable isotopes. Stable isotope 

ratios of the ground water assisted in identifying the 

sources of recharge water contributed to the wells. 

Samples were collected for tritium/helium-3 and 

shipped to the LLNL for analysis to determine groundwater age; results of the analyses will be presented in a 

subsequent report. The authors thank the many water 

purveyors in the study area who allowed USGS 

personnel access to their wells for sampling. 

HYDROGEOLOGIC SETTING

The study area covers the Los Angeles physiographic basin (fig. 1A), a 2,700 km2 northwest-trending 

alluviated lowland plain, which is bounded on the 

north, east, and southeast by mountains and hills, and 

is sometimes referred to as the coastal plain (Yerkes 

and others, 1965). Ground-water flow is largely controlled by engineered recharge along the San Gabriel, 

Public supply well (photograph by 

Scott Hamlin, U.S. Geological Survey).
4 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

the Rio Hondo, and the Santa Ana Rivers, and by 

ground-water pumping from the many hundreds of 

wells distributed across the area. The direction of 

ground-water flow is primarily lateral and radial from 

the recharge facilities toward the coast (fig. 1B). Along 

the coast near the seawater intrusion barriers, the 

direction of ground-water flow is inland owing to 

pumping.

The Los Angeles physiographic basin is subdivided into two basins determined by the sources of 

recharge water—the Coastal Los Angeles Basin and 

the Coastal Santa Ana Basin. These basins have 

historically been delineated by the Los Angeles 

County–Orange County line (fig. 1A) (Poland and 

others, 1956; California Department of Water 

Resources, 1961). Although the hydrogeologic setting 

is similar between the Coastal Los Angeles Basin and 

the Coastal Santa Ana Basin, each of these basins has 

distinct radial flow paths originating from the engineered recharge facilities. Each basin can be further 

subdivided into two areas—forebay and pressure 

areas—based on the texture and permeability of aquifer materials (Eckis, 1934; California Department of 

Water Resources, 1961). The forebay areas are in the 

northeastern part of the Los Angeles physiographic 

basin along and adjacent to the rivers (fig. 1A). The 

sediments in the forebay areas consist of unconsolidated, interbedded sands and gravels with occasional 

lenses of silt and clay material. The relatively small 

amount of fine-grained sediments in the forebay areas 

are laterally discontinuous and generally do not 

impede the vertical movement of ground water. The 

forebay areas are the primary areas in which groundwater recharge occurs. The pressure area extends from 

the southwestern edge of the forebay to the Pacific 

Ocean; it occupies most of the land area in the basin 

and is characterized by sand, gravel, and thick lenses 

of silt and clay. The laterally extensive and thick lenses 

of fine-grained sediments in the pressure area can 

impede vertical movement of ground water.

Coastal Los Angeles Basin

The hydrogeology of the Coastal Los Angeles 

Basin was characterized using the extensive amount of 

hydrologic and geologic data collected as part of an 

ongoing study between the USGS and the Water 

Replenishment District of Southern California (WRD). 

The simplified cross section shown in figure 1B 

illustrates the regional flow path in the study area. 

The Coastal Los Angeles Basin occupies the 

northern half of the study area (fig. 1A). The 

fresh-water-bearing deposits are up to 900-m thick and 

consist of gravel, sand, silt, and clay of fluvial and 

marine origin. Depth to water ranges from about 100 m 

to near land surface adjacent to the ground-water 

recharge facilities (Water Replenishment District of 

Southern California, 2001). 

The Coastal Los Angeles Basin is composed of 

several smaller basins, the two largest of which are the 

Central Basin and West Coast Basin. The Central and 

West Coast Basins are delineated by the Newport–

Inglewood Uplift (fig. 1A), the Pacific Ocean, and by 

surrounding hills. Ground water flows primarily 

laterally and radially from the recharge facilities in the 

Central Basin and eastward from the Pacific Ocean in 

the West Coast Basin (Water Replenishment District of 

Southern California, 2001). The primary sources of 

engineered recharge water differs in these basins: the 

Central Basin receives recharge water from the groundwater recharge facilities in the forebay areas, and the 

West Coast Basin receives recharge water from injection wells that form the seawater intrusion barriers 

(fig. 1A) (Water Replenishment District of Southern 

California, accessed June 13, 2001). 

The forebay areas in the Coastal Los Angeles 

Basin are in the Central Basin and can be subdivided 

into two areas: the Los Angeles forebay and the Montebello forebay (fig. 1A). The Los Angeles forebay is 

located along and adjacent to the Los Angeles River as 

it flows from the San Fernando Valley. Intense urban 

development and lining of the Los Angeles River has 

greatly limited the amount of natural ground-water 

recharge in this western forebay area. The Montebello 

forebay is located along and adjacent to the Rio Hondo 

and the San Gabriel Rivers downgradient from the 

Whittier Narrows Flood Basin in the San Gabriel 

Valley (fig. 1A). Recharge facilities in the Montebello 

forebay receive a combination of water from the San 

Gabriel Valley, treated wastewater, and imported water, 

and serve as the primary source of recharge to the 

Coastal Los Angeles Basin. The large Whittier 

Narrows Flood Basin just north of the study area boundary in the San Gabriel Valley is not actively managed for 

ground-water recharge purposes. However, the flood 

basin receives storm-water runoff from the San Gabriel 

Valley, which can recharge the aquifer. Additional 

recharge occurs at a series of injection wells operated to 

restrict seawater intrusion along parts of the coast. 

Injection wells receive a combination of highly treated 

wastewater and imported water. 

Replenishment and management of the groundwater resources necessary to offset historic overdraft 

began during the early 1960s by the WRD, although 
Hydrogeologic Setting 5

Figure 1. Major hydrogeologic features of the Los Angeles physiographic basin, California, (A) Study area and (B) Simplified cross-section 

showing the direction of ground-water flow.
6 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

artificial recharge of ground water has taken place in 

the Los Angeles area since about 1920 (Peterson, 

1961; Water Replenishment District of Southern California, 2001).The primary sources of recharge are 

from engineered ground-water recharge, which 

includes water that is spread at the recharge facilities 

and injected into wells that form seawater intrusion 

barriers. For the 2000 water year (October 1999 

through September 2000), about 0.227 km3

 (184,300 

acre-ft) of water was recharged. About 77 percent 

(0.174 km3 or 141,700 acre-ft) of the total was from 

engineered ground-water recharge (Water Replenishment District of Southern California, 2001): about 

0.139 km3 (113,000 acre-ft) at recharge facilities in the 

forebay area, and about 0.035 km3 (28,700 acre-ft) at 

injection wells that form the seawater intrusion barriers. Other sources of recharge include underflow from 

adjacent basins, recharge along the margins of mountains and hills, and precipitation.

Outflow or ground-water production is from 

ground-water pumping primarily from the pressure 

area. About 0.310 km3

 (251, 525 acre-ft) of ground 

water was pumped during the 2000 water year, most of 

which was from the Central Basin (79 percent), and 

the remainder was from the West Coast Basin. The 

main production zones generally range from 120 to 

370 m (400 to 1,200 ft) below land surface (Michael 

Land, U.S. Geological Survey, unpublished data, 

2000). 

Coastal Santa Ana Basin

The hydrogeology of the Coastal Santa Ana 

Basin was characterized using data collected by the 

Orange County Water District (OCWD) and the 

USGS’s Santa Ana River Basin study team of the 

NAWQA Program. Similar to the flow path in the 

Coastal Los Angeles Basin, ground-water studies indicate that regional ground-water flow in the Santa Ana 

River Basin extends laterally and radially outward 

from the ground-water recharge facilities toward the 

coast (Orange County Water District, 1997). 

The Coastal Santa Ana Basin occupies the 

southern half of the study area. The thickness of freshwater-bearing deposits is as much as 1,200 m (Orange 

County Water District, 1996). These deposits consist 

of alluvium derived from the mountains to the east and 

southeast and from marine deposits. Similar to the 

Coastal Los Angeles Basin, depth to water ranges from 

about 100 m below land surface to near land surface 

along the rivers, creeks, and ground-water recharge 

facilities (Orange County Water District, 1996).

The forebay area occupies about 130 km2

 along 

and adjacent to the Santa Ana River after it leaves the 

Santa Ana Mountains (fig. 1A). Recharge facilities in 

the forebay area receive a combination of treated 

wastewater, Santa Ana River water, and imported 

water, providing the primary source of recharge to the 

ground-water basin. 

Since the formation of OCWD in 1933, groundwater recharge and pumpage from public supply wells 

have been intensively managed to offset historic 

ground-water overdraft (Orange County Water District, 

accessed June 13, 2001). In 1999, 0.342 km3

 (278,000 

acre-ft) was recharged primarily at facilities along the 

Santa Ana River. Lesser amounts of recharge to the 

aquifer system occur along Santiago Creek from natural infiltration along the margins of the hills that surround the coastal plain and from vertical leakage 

through the confining layers. In 1999, about 0.438 km3

(356,000 acre-ft) was pumped from the aquifer system 

primarily in the confined pressure area of the basin. 

The upper aquifer zone is generally from 90 to 450 m 

(300 to 1,500 ft) below land surface, with most of the 

pumpage from 150 to 300 m (500 to 1,000 ft) below 

land surface. In the confined pressure area, the upper 

aquifer zone is overlain by 90 to 150 m (300 to 500 ft) 

of deposits that consist primarily of silts and clays, 

which typically impede vertical movement of groundwater (Orange County Water District, 1996). 

STUDY DESIGN AND METHODS

Design of this study is based on established protocols and methods developed for the NAWQA 

Program (Leahy and others, 1990; Gilliom and others, 

1995). Equally important was an understanding of the 

hydrogeology of the Coastal Los Angeles Basin and 

the Coastal Santa Ana Basin developed by prior 

research (Orange County Water District, 1997; U.S. 

Geological Survey, 1998a, 2000a; Hamlin and others, 

1999; Water Replenishment District of Southern California, 2001), which made it possible to incorporate 

the hydrogeologic and geochemical framework of each 

aquifer system into the study design. 

Well Selection

A network of 178 wells were selected for sampling by using a grid-based random sampling approach 

(Scott, 1990), and by selecting wells screened in the 
Study Design and Methods 7

dominant producing zones tapped by most public 

supply wells. Candidate wells were identified using a 

systematic inventory of data provided by the WRD, 

OCWD, USGS Ground Water Site Inventory (GWSI) 

database, DHS, and the LLNL. The inventory produced a total of 1,691 candidate public supply wells, 

of which active public supply wells were preferentially 

selected when well status was identified (fig. 2). Public 

supply wells that were inactive, abandoned, or 

destroyed—some of which may contain constituents in 

violation of drinking water standards—were not 

selected. The active public supply wells are representative of the ground-water resources currently used for 

drinking water. 

The unconsolidated deposits in the study area 

were partitioned into a grid of 100 equal-area cells 

(20 km2

). Wells were then randomly chosen to be representative of the well depths and screened intervals in 

each cell to obtain a statistical representation of the 

ground-water quality captured by most active public 

supply wells. Candidate wells occupied 94 of these 

cells. Each well in a cell was randomly assigned a rank 

based on the number of wells in an individual cell. 

In addition, each well was assigned a priority code 

based on the information on well construction, state 

well number, and well owner. Well construction information (well depth or depth below land surface, and 

top and bottom of screened intervals reported in depth 

below land surface) was identified for 685 of the candidate wells. When possible, well owners also were 

contacted to obtain construction information for wells 

with a high rank. 

The aquifer depth tapped by the public supply 

wells that have well screened-interval information 

ranged from 4 m (the most shallow of the top of 

screened intervals) to 667 m (the deepest of the bottom 

of the screened intervals). Wells in individual cells 

were separated into three depth categories based on 

well depth and top and bottom of screened interval: 

shallow wells (well depth less than or equal to 152 m), 

deep wells (top of screened interval greater than 122 m 

and well depth greater than 152 m), and composite 

wells (top of screened interval less than or equal to 

122 m and well depth greater than 152 m) that were 

screened in both the shallow and deep parts of the 

aquifer. Two wells from each cell were prioritized for 

selection according to the highest rank and screened 

interval depths that were most representative of the 

shallow and deep producing zones where possible 

(table 1). Wells were selected for sampling in 84 of the 

Figure 2. Public supply wells identified as candidates for sampling and resulting well network, Los Angeles physiographic basin, California
8 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

100 equal-area cells. When there were no public supply wells in a cell or access could not be obtained, 

wells in adjacent cells were identified for sampling to 

maintain a spatially distributed network in the study 

area. After all possible wells were identified in candidate cells, 40 additional wells were selected from those 

where permission to sample was obtained. The resulting network contains 176 active public supply wells, 1 

irrigation well, and 1 industrial well. Because the two 

irrigation/industrial wells are screened in the same part 

of the aquifer tapped by active public supply wells, no 

further distinction will be made between them and the 

176 active public supply wells in this report. This network provides a representative sampling of the 1,691 

public supply wells in the study area (fig. 2).

Sample Collection and Analysis

Samples were collected from 178 active public 

supply wells by USGS personnel from August through 

November 2000 and analyzed for VOCs, stable isotopes, and tritium/helium-3. Sample equipment, collection, and cleaning procedures were used to collect a 

representative sample from the aquifer prior to water 

treatment, to minimize airborne contamination and 

volatilization of VOCs during sample collection, and 

to minimize carry-over contamination between wells. 

In general, standard USGS and NAWQA protocols 

were followed (Koterba and others, 1995; U.S. 

Geological Survey, 1998b).

Samples were collected prior to water treatment 

and after a minimum of three casing volumes were 

extracted, which help ensure that a representative 

sample was obtained from the aquifer. Active public 

supply wells that are continuously pumped required no 

additional pumping prior to sampling the wells, but 

several public supply wells in standby status were 

pumped to evacuate a minimum of three casing 

volumes prior to sample collection. Three types of 

sampling lines were used to adapt the access ports 

available for sampling and to minimize airborne contamination and volatilization of VOCs during sample 

collection. Most of the samples (148 of the 178) were 

collected using sampling lines made of either copper or 

high-purity Tygon tubing, and the remaining 30 samples were collected using sampling lines made of 

Teflon. All three sampling lines were cleaned prior to 

use following a procedure similar to methods used by 

the NAWQA Program (Koterba and others, 1995), 

except that the copper lines were baked at 100°C for 

about 4 hours to remove any volatiles. Tygon sampling 

lines were used only once and discarded to avoid 

cross-contamination between sampling sites. 

Prior to VOC sample collection, the sampling 

line was placed at the base of the 40-mL sample vials 

until overflow to displace the air from the vial. Unfiltered water then was collected in vials, preserved with 

hydrochloric acid (HCl) that has been analyzed for 

impurities, capped without headspace, and chilled to 

4°C. All VOC samples were sent to the USGS National 

Water Quality Laboratory (NWQL) and analyzed for 

86 compounds using purge and trap capillary-column 

gas chromatography/mass spectrometry (Connor and 

others, 1998). Unfiltered water collected in 60-mL bottles were sent to the USGS isotope laboratory in 

Reston, Virginia, and analyzed for stable isotopes using 

hydrogen equilibration (Coplen and others, 1991). 

Tritium/helium-3 samples were sent to the LLNL for 

analysis.

Laboratory reporting limits (LRL) for the 86 

VOCs analyzed represent the minimum concentration 

that can be identified, measured, and reported with 

99 percent confidence (Connor and others, 1998; 

Childress and others, 1999). However, some VOC concentrations were reported below the LRL when specific 

analytical identification criteria were met, and these 

Table 1. Summary of well construction information for 178 active public supply wells sampled in the Los Angeles physiographic basin,

California

[Depth category: shallow, well depth less than or equal to 152 meters (m); composite, top of screened interval less than or equal to 122 m and

well depth greater than 152 m; deep, top of screened interval greater than 122 m and well depth greater than 152 m. Well depths and

screened interval depths are in meters below land surface. No., number; *, screened interval data not available for 1 well] 

Depth 

category

No. of

wells

Top of

screened intervals

Bottom of

screened intervals

Screened

interval lengths Well depths

Range Median Range Median Range Median Range Median

Shallow 47 12–130 72 43–149 108 3–117 30 43–152 112

Composite 64 24–139* 93* 93–472* 204* 5–440* 139* 154–472 220

Deep 67 123–324 154 149–512 299 8–328 122 153–512 311

All wells 178 12–324* 107* 43–512* 200* 3–440* 89* 43–512 210
Study Design and Methods 9

concentrations are qualified as estimated (“E” remark 

code). The NWQL collects quality-control data on a 

continuing basis to establish LRLs, and reevaluates the 

LRLs annually. The LRL controls the probability of 

falsely reporting a nondetection. The chance of falsely 

reporting a nondetection for a sample that contained a 

VOC at a concentration equal to or greater than the 

LRL is less than 1 percent (Childress and others, 

1999). The chance of falsely reporting a detection for a 

sample that does not contain a VOC is much less than 

1 percent. 

Throughout this report, two types of analytical 

results are reported: detection of a VOC above the 

LRL and detection of a VOC below the LRL. The LRL 

is different for every VOC because it is dependent on 

chemical behavior and, hence, laboratory qualitycontrol data. Therefore, VOC detections were screened 

at the LRL for comparisons of the number of VOCs 

detected between sites. Alternately, all detections were 

reported for comparisons of the detection or concentration of a single VOC between sites instead of only 

reporting detections above the LRL. Thus, the effect of 

falsely reporting a detection or nondetection on the 

data interpretation is minimized, and the amount of 

information obtained from the data is maximized. 

VOC concentrations are reported to the same number 

of significant digits as the NWQL reports for the LRL. 

The LRLs changed (reduced by about half) for 

nine VOCs after 33 of the 178 wells were sampled; 

only four of these VOCs were detected (trichloromethane; 1,1-dichloroethane; dichloromethane; and 

chloromethane). Almost all of the samples with concentrations of these VOCs between the two LRLs were 

collected after the LRLs were changed or lowered. 

Therefore, the LRL that was active when the sample 

was analyzed was used to determine the number of 

VOCs detected above the LRL. 

Quality-Control Data for VOCs

Quality-control samples were collected and analyzed to qualify the interpretation of environmental 

data (samples from wells) and to identify the sources 

and magnitude of bias and variability in the data associated with sample collection, processing, transportation, and laboratory analysis. Blank samples (blanks), 

consisting of organic-free water (blank water) certified 

by the NWQL to be free of VOCs, were analyzed by 

the same methods used for environmental samples. 

Four different types of blanks were collected: field 

(14 samples); source-solution (18 samples); equipment 

(2 samples); and trip (11 samples). Field blanks measure the overall bias of the environmental data, and the 

other types of blanks measure specific sources of bias 

associated with the blank water and with various 

aspects of sampling, processing, transportation, and 

laboratory analysis. 

Five VOCs (methylbenzene, chloromethane, 

ethylbenzene, trichloromethane, and dichloromethane) 

that were detected in environmental samples were also 

detected below the LRL in field blanks. Trichloromethane and dichloromethane were each detected in 

one field blank; however, these detections could be 

attributed to blank water contamination because they 

were detected in source-solution blanks at higher concentrations than environmental samples and thus do not 

affect the interpretation of the environmental data. 

Methylbenzene was detected in 1 equipment blank and 

in 7 of the 11 trip blanks at similar concentrations as in 

the three field blanks, possibly indicating that the 

source of contamination may be from transportation or 

shipment of samples. Chloromethane was detected in a 

source-solution blank and in one lot of HCl preservative (reported in the Certificate of Analysis from the 

NWQL) used during 1 week of sampling. Consequently, chloromethane was detected in seven environmental samples at a concentration similar to that in the 

two field blanks, indicating that the preservative may 

be the source of contamination for the low-level detections. Ethylbenzene also was detected in one trip blank 

at a concentration similar to that in one field blank, 

indicating that the source of contamination may be 

from transportation or shipment of samples. 

VOC concentrations detected in environmental 

samples were censored at or below the maximum concentration detected in a field blank sample and, subsequently, were not reported as a detection. This 

censoring reduced the number of environmental detections of methylbenzene from 9 to 0, of ethylbenzene 

from 1 to 0, and of chloromethane from 13 to 8. VOCs 

were not detected more frequently in any of the different types of sampling lines in either blank or 

environmental samples. 

Overall, the data show that VOC sample collection, processing, and field-cleaning procedures were 

successful in minimizing environmental sample contamination or carryover. Further, on the basis of the trip 

and source-solution blank data, most of the detections 

in the field blanks could be attributed to contamination 

during transportation or preservation.

The NWQL also routinely analyzes internal 

quality-control samples to monitor analytical method 
10 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

performance. Results of quality-control data were 

evaluated from laboratory samples processed by 

NWQL at the same time as the samples submitted 

from this study, and from long-term historical data on 

method precision and recoveries. It is beyond the 

scope of this report to present all of the qualitycontrol data collected and analyzed by the NWQL, 

but short- and long-term quality-control data is 

available for the NWQL (U.S. Geological Survey, 

2000b).

ACTIVE PUBLIC SUPPLY WELLS FREQUENTLY 

CONTAIN LOW CONCENTRATIONS OF VOCS 

Ground-water samples from 108 (61 percent) 

of the 178 wells contained one or more VOCs at 

concentrations above the LRL (fig. 3). Most of these 

detections were at low-levels, with only 29 percent 

above 1 μg/L. Six or more VOCs were detected at 

concentrations above the LRL in samples from 

7 percent of the wells. Most of the wells with the highest number of VOC detections also contained the 

highest concentrations of individual compounds. 

Thirty-nine of the 86 VOCs analyzed were 

detected: 29 were detected at concentrations above the 

LRL, and 10 were only detected at concentrations 

below the LRL and are reported as estimated. This 

report will focus on VOCs detected at concentrations 

above the LRL because the LRL is based on qualitycontrol data, which, as noted earlier, greatly reduces 

the probability of falsely reporting a nondetection. 

However, the 11 most frequently detected VOCs are 

the same regardless of whether the number of VOC 

detections above the LRL or the total number of VOC 

detections are counted (table 2). Forty-seven of the 86 

VOCs analyzed were not detected (table 3). 

Eleven VOCs were detected at concentrations 

above the LRL in 7 percent or more of the samples 

(table 4), and 6 VOCs were detected at concentrations 

above the LRL in 11 percent or more of the samples. 

Trichloromethane (chloroform), a disinfection byproduct and the most frequently detected VOC, had concentrations above the LRL in 82 (46 percent) of the 

samples. The solvents trichloroethene (TCE); tetrachloroethene (PCE); 1,1-dichloroethane; and 1,1,1-

trichloroethane (TCA) were detected at concentrations 

above the LRL in 50 (28 percent), 34 (19 percent), 20 

(11 percent), and 20 (11 percent) of the samples, 

respectively. The fuel oxygenate MTBE was detected 

at concentrations above the LRL in 25 (14 percent) of 

Figure 3. The sampling well network and the number of detections of volatile organic compounds above the laboratory reporting limit, Los 

Angeles physiographic basin, California. VOC, volatile organic compound; LRL, laboratory reporting limit.
Active Public Supply Wells Frequently Contain Low Concentrations of VOCs 11

the samples. The other VOCs detected in 7 percent or 

more of the samples include the disinfection byproduct 

bromodichloromethane; the refrigerants trichlorofluoromethane (CFC-11 or Freon 11) and 1,1,2-

trichloro-1,2,2-trifluoroethane (CFC-113 or Freon 

113); 1,1-dichloroethene (DCE), which is used in the 

synthesis of organic chemicals; and the solvent 

cis-1, 2- dichloroethene. 

This group of frequently detected VOCs is 

similar to those found in ground water in urban areas 

representative of a wide range of hydrogeologic conditions across the United States (Squillace and others, 

1999). This group also is similar to VOCs detected in 

drinking water in the Northeast and mid-Atlantic 

regions of the United States (Grady and Casey, 2001), 

and of ground-water samples from monitoring and 

public supply wells in southern New Jersey (Stackelberg and others, 2000). The six VOCs detected most 

frequently in this study were also the most frequently 

detected VOCs in a study of urban areas across the 

Table 2. Volatile organic compounds detected in samples from 178 active public supply wells in the Los Angeles physiographic basin,

California

[LRL, laboratory reporting limit; 2 LRLs used during sampling period. μg/L, microgram per liter] 

Compound name

 (common name)

LRL

(μg/L)

Number of 

detections

above LRL

Number of 

detections 

below LRL

Total

number of

detections

Trichloromethane (Chloroform) 0.052*, 0.024 82 20 102

Trichloroethene (TCE) 0.038 50 19 69

Tetrachloroethene (PCE) 0.10 34 28 62

Methyl tert-butyl ether (MTBE) 0.17 25 18 43

1,1-Dichloroethane 0.066*, 0.035 20 14 34

1,1,1-Trichloroethane (TCA) 0.032 20 20 40

Bromodichloromethane 0.048 18 5 23

Trichlorofluoromethane (CFC-11) 0.09 16 20 36

1,1,2-Trichloro-1,2,2-trifluoroethane (CFC-113) 0.060 16 4 20

cis-1,2-Dichloroethene 0.038 13 11 24

1,1-Dichloroethene (DCE) 0.044 12 10 22

Chlorodibromomethane 0.18 5 1 6

1,2-Dichloroethane 0.13 4 0 4

Tribromomethane (Bromoform) 0.06 4 0 4

Tetrachloromethane (Carbon tetrachloride) 0.06 3 5 8

Carbon disulfide 0.07 2 5 7

trans-1,2-Dichloroethene 0.032 2 1 3

Bromochloromethane 0.044 2 0 2

Dichloromethane (Methylene chloride) 0.38*, 0.16 1 1 2

1,2,3,4-Tetramethylbenzene 0.23 1 0 1

1,2,3,5-Tetramethylbenzene 0.20 1 0 1

2-Ethyltoluene 0.06 1 0 1

1,2,3-Trimethylbenzene 0.12 1 0 1

1,2,4-Trimethylbenzene 0.056 1 0 1

1,3,5-Trimethylbenzene 0.044 1 0 1

Chlorobenzene 0.028 1 0 1

n-Propylbenzene 0.042 1 0 1

sec-Butylbenzene 0.032 1 0 1

Diisopropyl ether 0.10 1 0 1

Dichlorodifluoromethane (CFC-12) 0.27 0 9 9

Chloromethane (Methyl chloride) 0.5*, 0.25 0 8 8

Isopropylbenzene 0.032 0 3 3

1,4-Dichlorobenzene 0.05 0 2 2

Benzene 0.035 0 2 2

n-Butylbenzene 0.19 0 1 1

p-Isopropyltoluene 0.07 0 1 1

Tetrahydrofuran 2.2 0 1 1

1,3 and 1,4-Dimethylbenzene (meta and para-Xylene) 0.06 0 1 1
12 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

Table 4. Volatile organic compounds detected in 7 percent or more of the samples from 178 active public supply wells in the Los Angeles

physiographic basin, California

[E, estimated; LRL, laboratory reporting limit; No., number; *, 2 LRLs used during sampling period. μg/L, microgram per liter]

Compound name

 (common name)

LRL

(μg/L)

Percentage

of samples 

with a 

detection 

above the 

LRL

No. of 

detections 

above the

LRL

Detected concentrations (μg/L)

Primary use

Minimum Median Maximum 

Trichloromethane (Chloroform) 0.052*, 0.024 46 82 E 0.011 0.060 11 Disinfection byproduct

Trichloroethene (TCE) 0.038 28 50 E 0.0089 0.095 69 Solvent 

Tetrachloroethene (PCE, PERC) 0.10 19 34 E 0.0099 0.14 11 Solvent 

Methyl tert-butyl ether (MTBE) 0.17 14 25 E 0.052 0.18 0.85 Fuel oxygenate

1,1-Dichloroethane 0.066*, 0.035 11 20 E 0.012 0.046 0.50 Solvent 

1,1,1-Trichloroethane (TCA) 0.032 11 20 E 0.0063 0.032 0.33 Solvent 

Bromodichloromethane 0.048 10 18 E 0.019 0.069 5.5 Disinfection byproduct

Trichlorofluoromethane (CFC-11) 0.09 9 16 E 0.01 0.06 8 Refrigerant

1,1,2-Trichloro-1,2,2-trifluoroethane 

(CFC-113)

0.060 9 16 E 0.014 0.13 2.5 Refrigerant

cis-1,2-Dichloroethene 0.038 7 13 E 0.016 0.040 1.4 Solvent 

1,1-Dichloroethene (DCE) 0.044 7 12 E 0.012 0.047 3.3 Organic synthesis

Table 3. Volatile organic compounds analyzed for but not detected in samples from 178 active public supply wells in the Los Angeles

physiographic basin, California

[LRL, laboratory reporting limit; *, 2 LRLs used during sampling period. μg/L, microgram per liter]

Compound (common name) LRL (μg/L) Compound (common name) LRL (μg/L)

1,1,1,2-Tetrachloroethane 0.03 Bromoethene 0.10

1,1,2,2-Tetrachloroethane 0.09 Bromomethane 0.26

1,1,2-Trichloroethane 0.06 Chloroethane 0.12

1,1-Dichloropropene 0.026 Chloroethene 0.11

1,2,3-Trichlorobenzene 0.27 cis-1,3-Dichloropropene 0.09

1,2,3-Trichloropropane 0.16 Dibromomethane 0.05

1,2,4-Trichlorobenzene 0.19 Diethyl ether 0.17

1,2-Dibromo-3-chloropropane 0.21 Ethylbenzene 0.03

1,2-Dibromoethane 0.036 Ethyl methacrylate 0.18

1,2-Dichlorobenzene 0.031*, 0.048 Ethyl tert-butyl ether (ETBE) 0.054

1,2-Dichloropropane 0.029*, 0.068 Hexachlorobutadiene 0.14

1,2-Dimethylbenzene (ortho-Xylene) 0.038 Hexachloroethane 0.19

1,3-Dichlorobenzene 0.03*, 0.054 Iodomethane (Methyl iodide) 0.12

1,3-Dichloropropane 0.12 Methyl acrylate 1.4

2,2-Dichloropropane 0.05 Methyl acrylonitrile 0.6

2-Butanone 1.6 Methylbenzene (Toluene) 0.05

2-Chlorotoluene 0.026*, 0.042 Methyl methacrylate 0.35

2-Hexanone 0.7 Naphthalene 0.25

2-Propenenitrile 1.2 Styrene 0.042

3-Chloro-1-propene 0.07*, 0.2 tert-Amyl methyl ether (TAME) 0.11

4-Chlorotoluene 0.06 tert-Butylbenzene 0.06

4-Methyl-2-pentanone 0.37 trans-1,3-Dichloropropene 0.09

Acetone 7 trans-1,4-Dichloro-2-butene 0.7

Bromobenzene 0.036
VOC Distribution Indicates Enhanced Recharge and Ground-Water Pumping Are Primary Controls on Ground-Water Flow System 13

tion of 69 μg/L, and tetrachloroethene (PCE) concentrations exceeded the MCL (5 μg/L) in five samples 

with a maximum concentration of 11 μg/L. Water purveyors were aware of these MCL exceedances in the 

ground water from their routine monitoring, and the 

water was already being treated to remove these compounds. In this study, samples were collected prior to 

water treatment and, thus, sample concentrations do 

not represent the concentrations entering the drinkingwater system. More than half of the remaining samples 

with TCE and PCE detections contained concentrations between 0.5 and 5 μg/L.

VOC DISTRIBUTION INDICATES ENHANCED 

RECHARGE AND GROUND-WATER PUMPING 

ARE PRIMARY CONTROLS ON GROUND-WATER

FLOW SYSTEM

As a simple approximation, the ground-water 

flow system can be conceptualized as a radial wedge or 

pie-slice with the apex at the recharge facilities in the 

forebay and flow paths extending outward toward the 

coast. Within the wedge is water bearing the fingerprint 

of urbanization—that is, water containing VOCs. 

Outside the wedge is native ground water. VOCs might 

be introduced in the coarse-grained forebay areas 

either at the recharge facilities or in other sources of 

recharge that encounter point or nonpoint contaminant 

sources. This radial pattern of flow is reflected in the 

distribution of VOCs (fig. 3), which act as tracers of 

postindustrial-aged water (water recharged since the 

onset of intense urban development). Inspection of 

water-level contour maps (Orange County Water 

District, 1996) also indicates flow originating at the 

engineered recharge facilities and radiating outward. 

However, because of changes in the source and quality 

of recharge water over time, ground-water quality in 

the flow system is not the same as the quality of water 

presently recharged at the forebay areas and the 

recharge facilities. This radial wedge concept of the 

ground-water flow system, however, does not describe 

the flow system west of the Newport–Ingelwood Uplift 

in the West Coast Basin of the Coastal Los Angeles 

Basin. The Newport–Inglewood Uplift separates the 

Central Basin from the West Coast Basin and impedes 

lateral ground-water flow, thus the West Coast Basin 

receives more water from the seawater intrusion 

Table 5. Comparison of the six most frequently detected volatile

organic compounds in ground-water samples from the Los Angeles

physiographic basin, California, and from urban areas across the

United States

Compound name

(common name)

Percentage of

detections censored

0.2 micrograms per liter

Urban areas 

across the 

United States

(Squillace and 

others, 1999)

Los Angeles 

physiographic

basin, 

California

Trichloromethane (Chloroform) 26 10

Tetrachloroethene (PCE) 17 15

Methyl tert-butyl ether (MTBE) 17 11

Trichloroethene (TCE) 12 14

1,1,1-Trichloroethane (TCA) 10 1

1,1-Dichloroethane 6 1

Recharge facility in the Coastal Santa Ana Basin (photograph by 

Carmen Burton, U.S. Geological Survey).

United States (table 5), when both data sets are censored at a reporting limit of 0.20 μg/L (Squillace and 

others, 1999).

Most VOCs were detected at low concentrations, but maximum concentrations varied greatly 

(table 4). The three most frequently detected VOCs 

also had the highest maximum concentrations. Eight 

of the 11 most frequently detected VOCs had maximum concentrations greater than 1 μg/L in a total of 

25 samples. Two VOCs exceeded the state and federal 

primary maximum contaminant level (MCL) for 

drinking water (U.S. Environmental Protection 

Agency, 1996) in a total of seven samples. Trichloroethene (TCE) concentrations exceeded the MCL 

(5 μg/L) in four samples with a maximum concentra-
14 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

Figure 4. Scatter plot of the number of detections of volatile organic 

compounds in samples from wells in the Coastal Los Angeles and 

Coastal Santa Ana forebay areas, California, as a function of 

distance from recharge facilities. VOC, volatile organic compound; 

LRL, laboratory reporting limit; <, less than. 

barriers than from recharge in the forebay areas 

(Water Replenishment District of Southern California, 

2001).

The spatial distribution of the VOCs detected 

above the LRL can be quantified in terms of the distance between the recharge facility and the location of 

the well. This distance represents distance along the 

radial flow path and is a surrogate for the time of 

travel. The distance from the recharge facilities is negatively correlated with the number of VOCs detected 

above the LRL in the Central Basin of the Coastal Los 

Angeles Basin and in the Coastal Santa Ana Basin 

(p < 0.001 and p = 0.002, respectively; Spearman’s 

rank correlation) (fig. 4). This analysis shows samples 

with the most VOC detections are from wells located 

within 15 km of the recharge facilities, with a few 

exceptions. The wells with VOCs detected above the 

LRL almost form a line at approximately 10–15 km of 

the recharge facilities beyond which few or no VOCs 

were detected (fig. 3). Thus, the low-level concentrations of VOCs seem to act as a reasonable groundwater tracer for postindustrial-aged water and indicate 

the distance water has traveled from where it was 

recharged in the forebay areas. 

The most notable exceptions to this generalization are the four wells in which more than five VOCs 

were detected outside the wedge of postindustrial-aged 

water: one is in the southern part of the Coastal Santa 

Ana Basin, one is in the West Coast Basin of the Coastal 

Los Angeles Basin, and two are in the extreme northwestern corner of the Coastal Los Angeles Basin (fig. 

3). The VOCs in wells located in the older unconsolidated deposits and outside the radial flow paths are 

probably not derived from the water recharged in the 

forebays. The injection wells that make up the seawater 

intrusion barriers contribute to water recharged in areas 

along the coast and may be a potential low-level source 

of VOCs to public supply wells in those areas.

Although the recharge facilities along the Rio 

Hondo, the San Gabriel, and the Santa Ana Rivers are 

the major sources of recharge, they are not the only 

sources. The Los Angeles River, prior to being lined 

with concrete, and Santiago Creek are also sources of 

recharge water. In addition, there are a variety of potential point and nonpoint sources of VOCs in urban areas. 

The coarse-grained sediments and discontinuous silt 

and clay lenses in the forebay areas also allow vertical 

migration of precipitation, infiltration from excess landscape watering, storm-water runoff, chemical spills, 

leaky underground fuel tanks, and leaky sewer and 

water distribution lines to recharge the aquifer. Age 

dating from the tritium/helium-3 analyses may assist in 

the interpretation of potential sources of recharge water 

and VOCs.

The use of VOCs as tracers of postindustrial-aged 

ground water may be further examined by evaluating 

the most frequently detected VOCs with reference to 

when these VOCs were introduced into the environment. Figure 5 shows the distribution of sample concentrations of four of the most frequently detected VOC in 

each of four primary use categories (table 4). The spatial 

distribution of the fuel oxygenate MTBE, which has the 

most recent history of use, is shown in figure 5A. MTBE 

was used to enhance octane levels in gasoline during the 

late 1970s (Zogorski and others, 1996), and the use of 

MTBE increased significantly during the early 1990s 

after fuel oxygenates were mandated by the Clean Air 

Act amendments to reduce atmospheric concentrations 

of carbon monoxide. In 1996, about 10.6 billion liters 

(2.8 billion gallons) of MTBE were produced in the 

United States, over 95 percent of the gasoline used in 

California was blended with MTBE, and the annual production capacity of MTBE in California was about 719 

million liters (190 million gallons) (Gomez and others, 

1998). MTBE has the most localized distribution of 

detections with all but two of the detections at concentrations above the LRL in wells close to the recharge 
VOC Distribution Indicates Enhanced Recharge and Ground-Water Pumping Are Primary Controls on Ground-Water Flow System 15

facilities in the forebay (fig. 5A). This nearly exclusive 

occurrence of MTBE close to the area of recharge is 

consistent with the relatively recent introduction of 

MTBE as a fuel additive, the concept of the radial-flow 

system, and the use of VOCs as ground-water tracers 

for postindustrial water. 

Most of the MTBE detections and the highest 

MTBE concentrations are found in samples from wells 

close to the recharge facilities and the Rio Hondo or 

the San Gabriel Rivers in the Montebello forebay area 

in the Coastal Los Angeles Basin. The reason for this 

higher frequency of occurrence and concentration in 

the Coastal Los Angeles Basin than in the Coastal 

Santa Ana Basin is unknown. Perhaps it is due to 

inflow from adjacent basins or the greater amount of 

surface area of recharge facilities or unlined reaches of 

rivers in the Montebello forebay than in the Coastal 

Santa Ana Basin. Approximately 3.4 km2

 (850 acres) 

of recharge facilities or unlined reaches of the Rio 

Hondo and the San Gabriel Rivers are in the Montebello forebay area. The rapid infiltration of large volumes of surface water potentially could provide a more 

rapid way for MTBE to be transported to the aquifer. 

TCE has been in use longer than MTBE and is 

more widely distributed. TCE has been produced in 

large quantities since the 1960s (Stackelberg and others, 2000) and is considered to be a high production 

volume chemical in that over 450,000 kg (1 million 

pounds) are produced annually in the United States 

(Environmental Defense, 2001a). In contrast with 

MTBE, TCE is detected farther from the recharge 

facilities and at about the same frequency in the forebay areas as it is detected in the pressure areas 

(fig. 5B). Because TCE has been in production longer 

than MTBE, TCE has traveled with recharge water a 

greater lateral distance from the forebay. Similar to 

MTBE, TCE is detected more frequently and occurs at 

higher concentrations in the Coastal Los Angeles 

Basin than in the Coastal Santa Ana Basin. However, 

unlike MTBE, some of the highest concentrations of 

TCE are isolated and are not in proximity to the 

recharge facilities (fig. 5B); these detections may be 

due to rapid vertical migration from a localized source 

of contamination.

The spatial distribution of the most frequently 

detected refrigerant, CFC-11, is shown in figure 5C. 

CFC-11 was frequently used in air conditioners and 

coolants during the 1970s and is also considered a 

high production volume chemical (Environmental 

Defense, 2001b). In July 1992, producers of CFC-11 

were required to gradually reduce production, and 

production was to be completely phased out by January 1, 2000 (U.S. Environmental Protection Agency, 

1999). This reduction may have decreased the amount 

of chlorofluorocarbon that enters the ground-water system and explain the lack of detections in the forebay 

areas and the higher frequency of detections in the 

pressure areas. Most of the detections of CFC-11 

above the LRL (0.09 μg/L) are from wells in the pressure areas. 

Unlike the spatial distribution of MTBE and 

TCE, CFC-11 is more frequently detected and occurs 

at higher concentrations in the Coastal Santa Ana 

Basin than in the Coastal Los Angeles Basin (fig. 5C). 

The second most frequently detected refrigerant, 

CFC-113, has a similar pattern and spatial distribution. 

The reasons for the increased frequency of refrigerant 

detections in the Coastal Santa Ana Basin are 

unknown, but could potentially result from localized 

sources.

The distribution of chloroform (fig. 5D) is similar to the overall distribution of VOCs. Chloroform, 

often produced as a byproduct of the disinfection of 

drinking water, has been produced since the 1920s for 

many uses, including manufacturing of pharmaceuticals, dry cleaning, fire extinguishers, fumigants, and as 

an anesthetic (Stackelberg and others, 2000; U.S. 

Department of Health and Human Services, 2001). 

Most of the chloroform detections are above the LRL 

(0.024 μg/L) and are found in almost equal numbers in 

both the Coastal Los Angeles and the Coastal Santa 

Ana Basins. Sources of chloroform that would result in 

a wide distribution of this disinfection byproduct 

include chlorination of drinking water, leaks in drinking water distribution systems, lawn irrigation, and 

recharge of large volumes of treated wastewater and 

State Water Project water at recharge facilities. Chloroform detections close to the coast may be related to 

injection of treated wastewater into the wells that form 

the seawater intrusion barriers. 

Degradation along ground-water flow paths 

could also result in lower concentrations of VOCs at 

greater distances from where ground water is 

recharged. Assuming the spatial distribution of VOCs 

is dominated by lateral flow from the forebay, then the 

current extent of any specific VOC is minimal because 

loss of VOC mass from sorption and degradation along 

the ground-water flow path have not been accounted 

for in this study.

An alternative to the radial flow concept of the 

flow system is that the VOC distribution could be 
16 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

Figure 5. Wells showing data for the most frequently detected volatile organic compound in each of four primary use categories in the Los

Angeles physiographic basin, California for (A) fuel oxygenate (MTBE), (B) solvent (TCE), (C) refrigerant (CFC-11), and (D) the disinfection 

byproduct (chloroform). Detected concentrations are divided into three ranges: less than or equal to the 50th percentile, greater than the 

50th percentile and less than or equal to the 90th percentile, and greater than the 90th percentile. MTBE, methyl tert-butyl ether; TCE, 

trichloroethene, CFC-11, trichlorofluoromethane; μg/L, microgram per liter.
VOC Distribution Indicates Enhanced Recharge and Ground-Water Pumping Are Primary Controls on Ground-Water Flow System 17

Figure 5.—Continued.
18 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

controlled by vertical movement of ground water in 

proximity to a well. Given that VOCs are likely to 

enter the ground-water system at the water table, a reasonable expectation would be that the frequency and 

magnitude of VOC concentrations would decrease 

with increased depth of a public supply well. However, 

this relation was not observed in the data for the entire 

study area. The number of VOC detections above the 

LRL was not significantly different (p = 0.72; KruskalWallis test) among samples from wells grouped into 

three depth categories—shallow, composite, and 

deep—and located throughout the entire study area. 

Conversely, the number of VOC detections above the 

LRL is significantly different (p = 0.03; KruskalWallis test) among samples from wells grouped by the 

three depth categories for the 61 wells in the forebay 

areas (fig. 6), and the median of the number of VOC 

detections above the LRL is higher in the shallow 

wells (4) than the deep wells (1). The number of VOC 

detections above the LRL is not significantly different 

(p = 0.27; Kruskal-Wallis test) among samples from 

wells grouped by the three depth categories for the 

117 wells in the pressure area. There was no difference 

in well depth between the forebay and pressure area. 

These results indicate that the greater thickness of clay 

layers and other fine-grained sediment in the pressure 

area possibly impede the downward migration of 

VOCs. 

The differences in the number of detections of 

VOCs above the LRL among the three depth categories 

in the forebay areas is due solely to contrasts in VOC 

detections among the 31 wells in the forebay of the 

Coastal Santa Ana Basin (p = 0.01, Kruskal–Wallis 

test); there are no significant differences in the number 

of detections of VOCs above the LRL among depth 

categories in the forebay in the Coastal Los Angeles 

Basin (p = 0.88; Kruskal-Wallis test). The contrast 

in the number of VOC detections with depth in the 

Coastal Santa Ana Basin forebay indicates that there 

could be a vertical component of transport affecting 

the distribution in this area. Rapid vertical transport 

could result from several mechanisms; for example, 

a leaking well casing, inter-borehole flow, or local 

high vertical gradients from pumping. More data 

is necessary to determine the significance of these 

mechanisms and to distinguish the effects of lateral 

or vertical movement of ground water in the 

forebay.

The data indicate that VOC detections above the 

LRL are not uniformly distributed over the study area 

Figure 6. Boxplot of the number of detections of volatile organic compounds above the laboratory reporting limit in samples from wells 

grouped by depth categories in the forebay areas of the Los Angeles physiographic basin, California. VOC, volatile organic compound; LRL, 

laboratory reporting limit.
Stable Isotope Data Relate Source of Recharge Water to VOC Occurrence 19

(fig. 3). Most of the wells with multiple VOC detections, which also have the highest concentrations, are 

in the forebay areas and clustered proximal to the 

recharge areas. In addition, there appears to be a distance of about 10–15 km from the recharge facilities 

beyond which the number of VOC detections above 

the LRL (figs. 3 and 4) and VOC concentrations (fig. 

5) decrease. The pattern of VOC distribution is related 

to the history of use. For example, chloroform has the 

longest history of use and is the most widely distributed when compared with MTBE, which has the most 

recent history of use and is detected at concentrations 

above the LRL almost exclusively in the forebay areas. 

This distribution and the history of introduction of the 

most frequently detected VOCs into the urban landscape indicate that engineered recharge and groundwater pumping are the dominant controls on the flow 

system; VOCs travel along radial flow paths to a distance that is indicative of the age of the recharge water 

or the time in which the VOC was introduced into the 

environment. 

The data also suggests that downward vertical 

migration of VOCs from the water table may be a viable process for contaminating aquifers in the forebay 

areas but there is little evidence that the same process 

is active and affecting the occurrence of VOCs in many 

wells in the pressure areas. The significant difference 

in the number of VOC detections above the LRL 

between different depth categories in the forebay areas 

suggests that vertical downward migration of VOCs is 

a potential pathway to the capture zone of public supply wells in the Coastal Santa Ana forebay. The lack of 

contrast in the number of VOC detections between 

well depth categories basinwide and the nonuniform 

distribution of VOCs basinwide suggests that vertical 

migration is not the dominant pathway. The data are 

insufficient, however, to exclude the possibility that (1) 

VOCs traveling vertically from near-surface have not 

yet reached the wells or (2) VOCs are present but not 

detectable in the uppermost fraction of the well 

screens because of the mixing over the large lengths of 

the screened intervals. Additional data along the 

ground-water flow paths collected from discrete zones 

in the aquifer at various depths may aid in assessing 

the transport of VOCs to public supply wells. 

STABLE ISOTOPE DATA RELATE SOURCE OF 

RECHARGE WATER TO VOC OCCURRENCE

The presence of recharge from precipitation; 

urban runoff; Santiago Creek; and the Santa Ana, the 

San Gabriel, and the Los Angeles Rivers can be partly 

discerned from analysis of stable isotope ratios in 

ground- and surface-water samples. Oxygen-18 (18O) 

and deuterium (D) are naturally occurring stable isotopes of oxygen and hydrogen, respectively. The abundance of these isotopes in water is generally expressed 

as a ratio in delta (δ) notation, and on a global scale 

the relation between δ18O and δD is linear, plotting 

along the global meteoric water line (Craig, 1961). 

Deviations from the global meteoric water line can 

occur as a consequence of several hydrologic 

processes, including evaporation and mixing. 

Figure 7 shows plots of the isotopic composition 

of water samples from active public supply wells in the 

Los Angeles physiographic basin. The samples are differentiated on the basis of the occurrence of VOCs: 

samples with no detections, samples with one or more 

detections above the LRL, samples with detections 

only below the LRL. Also shown in both figures is the 

global meteoric water line. In general, the groundwater samples plot along and to the right of (below) the 

global meteoric water line.

Figure 7A illustrates hypothetical mixing lines 

for mixtures of imported Colorado River water and 

southern California surface water. The isotopic composition of Colorado River water plots to the right of 

(below) the global meteoric water line owing to evaporation. The isotopic composition of southern California 

surface water plots over a range of values, but is consistently heavier than Colorado River water. For the 

purposes of illustration, three hypothetical southern 

California surface water end-members are considered 

that may represent the source of the ground-water sampled in the active public supply wells: one that is isotopically light (more negative), one that is isotopically 

heavy (less negative), and one that is intermediate. 

Each of the mixing lines illustrates the plotting positions of water samples that represent varying amounts 

of the Colorado River end-member and one of the 

southern California end-members. A sample with a 

relatively high proportion of Colorado River water 

would plot relatively close to the Colorado River endmember. A sample with a relatively low proportion of 

Colorado River water would plot relatively close to 

the global meteoric water line. Overall, mixing of 

imported Colorado River water with southern 

California surface water results in a shift in the isotopic 

composition away from the global meteoric water line. 

Along any particular mixing line, the shift is downward and to the left towards the Colorado River endmember. For a group of samples obtained from a set of 
20 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

Figure 7. Scatter plot of the delta oxygen-18 versus delta deuterium for samples from active public supply wells in the Los Angeles 

physiographic basin, California, showing (A) hypothetical mixing lines for sources of recharge water to wells, and (B) regression lines for 

samples with no detections and with detections above the LRL. VOC, volatile organic compound; LRL, laboratory reporting limit. 
Assessment of Relation Between VOC Distribution and Potential Sources of VOCs 21

mixing lines, however, the net effect is for the set of 

samples to plot to the right of the global meteoric 

water line. 

The second of the two isotope plots (figure 7B) 

includes two regression lines: one for the samples with 

no VOC detections (R2

 = 0.87), and one for the samples with VOC detections above the LRL (R2

 = 0.82). 

The regression line for the samples with no VOC 

detections plots nearly coincident with the global 

meteoric water line, indicating that the source of the 

water in these wells is southern California precipitation and runoff. Nearly all of the wells with no VOC 

detections at concentrations above the LRL are located 

in the pressure areas, away from the forebay areas 

where most of the ground-water recharge occurs 

(fig. 3); thus, samples from wells in the pressure areas 

likely represent predominantly older ground water 

than samples from wells in the forebay areas. The 

ground-water samples from wells in the pressure areas 

with no VOC detections at concentrations above the 

LRL were likely from water recharged prior to the 

onset of intense urban development and from largescaled engineered recharge and ground-water pumping. This source of water would likely be from natural 

inputs to the ground-water systems, so it would have 

originated from precipitation and runoff.

The regression line for the samples with VOC 

detections above the LRL plots below, and at an angle 

to, the global meteoric water line, suggesting that the 

water in these wells is a mixture of local sources and 

water that is isotopically lighter (more negative) than 

the local sources. The isotopically lighter water could 

either be Colorado River water or State Water Project 

water, both of which are imported to the basin and 

used as a source of recharge to the ground-water flow 

system. Overall, the regression lines suggest that the 

occurrence of VOCs in ground water is associated with 

the engineered recharge of imported surface water, 

which occurs in the forebay areas.

ASSESSMENT OF RELATION BETWEEN 

VOC DISTRIBUTION AND POTENTIAL 

SOURCES OF VOCS

VOCs are widely used in urban areas and may 

be transported to ground water through urban runoff, 

atmospheric deposition and infiltration, or from other 

surface and below-ground sources. Potential sources 

of VOCs include household products, leaking fuel 

storage facilities, accidental spills, gasoline stations, 

vehicle exhaust, and industries that use VOCs as part 

of manufacturing, processing, or cleaning. Lopes and 

Bender (1998) determined that VOCs on urban land 

surfaces (contributing to urban runoff) and in urban air 

are important sources of VOCs in drinking water. Previous studies have shown that potential sources in 

proximity to wells increase the likelihood that VOCs 

will be detected in samples of well water. Such findings indicate a component of vertical flow exists, 

accounting for the movement of VOCs from sources 

near the well to the screened interval or capture zone of 

the well. In this study, relations between the occurrence of potential sources near wells generally were 

not significantly correlated with the occurrence of 

VOCs in well samples. This finding further indicates 

that the primary factor governing the distribution of 

VOCs is lateral transport of VOCs from the recharge 

facilities and other sources in the forebay areas.

Surface Water

The limited amount of data on current VOC concentrations in surface water that recharge the forebay 

area show that some of the most frequently detected 

VOCs in ground-water samples were also detected in 

surface-water samples. Sources of surface water entering the aquifer system in the Los Angeles physiographic basin are precipitation; Coast Range runoff; 

water from the Los Angeles, the Rio Hondo, the San 

Gabriel, and the Santa Ana Rivers; water from Santiago Creek; stormwater runoff; and imported and 

treated wastewater spread at the recharge facilities in 

the forebay areas and injected into wells along the seawater intrusion barriers. The predominant sources, 

however, are from imported and treated wastewater 

spread at the recharge facilities along the Rio Hondo, 

the San Gabriel, and the Santa Ana Rivers. The sources 

and the amounts of the different sources of water used 

Santa Ana River surface-water site (photograph by 

Carmen Burton, U.S. Geological Survey).
22 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

at the recharge facilities, and the quality of those 

recharge waters, have changed over time; however, the 

presence of VOCs in surface water currently used at 

the recharge facilities provides some information on 

potential sources of VOCs in ground water. As part of 

the NAWQA Program, the Santa Ana NAWQA River 

Basin study team obtained surface-water quality data 

for various sites along the Santa Ana River, the primary surface-water component of ground water 

recharge in the Coastal Santa Ana Basin part of the 

flow system. Surface-water quality data from the Santa 

Ana River presented in this section is from a site 

located upstream of the uppermost recharge facility in 

the Coastal Santa Ana Basin (fig. 1). VOCs reported in 

the 40 surface-water samples from this site were collected from November 1998 through December 2000. 

Two VOCs were detected more frequently and at 

higher concentrations in the surface-water samples 

than in the ground-water samples (table 6). The 

median concentrations of all detections of MTBE and 

chloroform were higher in the surface-water samples 

(0.24 and 0.36 μg/L, respectively) than in the groundwater samples (0.18 and 0.060 μg/L, respectively). The 

maximum MTBE concentration is also higher in the 

surface-water samples (1.2 μg/L) than in the groundwater samples (0.85 μg/L). These data indicate that the 

Santa Ana River may be a potential source of these 

VOCs in ground water. 

In contrast, current VOC occurrence in surface 

water cannot account for the high concentration and 

frequency of detection of other VOCs in ground water. 

The maximum concentrations of TCE and PCE in 

surface-water samples were over 100 times lower (estimated 0.040 and 0.080 μg/L, respectively) than in the 

ground-water samples (69 and 11 μg/L, respectively) 

(table 6). These VOCs were also detected about twice 

as often in the ground-water samples than in the 

surface-water samples. Furthermore, CFC-11, the most 

frequently detected refrigerant in ground-water samples, was not detected in any of the surface-water samples. These are all indications that surface water alone 

cannot explain the concentrations of VOCs detected in 

the ground water.

Extrapolation of these comparisons must be made 

with caution for several reasons. The Santa Ana River 

water is only one component of the water recharged at 

the Coastal Santa Ana Basin recharge facilities, and, 

therefore, concentrations observed in the surface-water 

samples are not necessarily representative of all of the 

water directly recharged to the ground-water system. 

No similar VOC analyses exist for surface water 

recharged in the Coastal Los Angeles Basin. The VOCs 

detected in surface water sampled today are only an 

approximation of past surface-water quality, which 

likely varied as a complex function of urbanization, regulation, and climate, among other factors. Nonetheless, 

the occurrence of VOCs in air and surface water in the 

study area is similar to VOC occurrence reported in 

other urban areas in the United States (Delzer and others, 1996; Lopes and Bender, 1998; Bender and others, 

2000). Although the surface-water data are limited, the 

high concentrations of some VOCs in the surface water 

suggest that additional work is needed to evaluate the 

potential for surface water to contribute VOCs to 

ground water in this area.

Population Density

Because the number of potential VOC sources 

generally increases with the number of people living in 

an area, and population data is readily available, population density has been used to evaluate the aggregate 

Table 6. Volatile organic compounds detected in samples from the Santa Ana River above the uppermost recharge facility in the Coastal

Santa Ana Basin and from 178 active public supply wells in the Los Angeles physiographic basin, California

[E, estimated; ND, not detected; GW, Los Angeles physiographic basin active public supply wells; SW, Santa Ana River above spreading

diversion below Imperial Highway near Anaheim. μg/L, microgram per liter]

Compound name

 (common name)

Percent of samples

with a detection

Detected concentrations (μg/L)

Median Maximum

SW GW SW GW SW GW

Methyl tert-butyl ether (MTBE) 92 24 0.24 0.18 1.2 0.85

Trichloromethane (Chloroform) 100 57 0.36 0.060 0.86 11

Trichloroethene (TCE) 18 39 E 0.014 0.095 E 0.040 69

Tetrachloroethene (PCE) 15 35 E 0.010 0.14 E 0.080 11

Trichlorofluoromethane (CFC-11) ND 20 ND 0.06 ND 8
Assessment of Relation Between VOC Distribution and Potential Sources of VOCs 23

effect of urban activities on VOC occurrence in ground 

water used for drinking water (Maine Bureau of 

Health, 1998; Squillace and others, 1999). In the current study, however, population density was only 

weakly related to the occurrence of VOCs. Population 

density for each well was determined by locating the 

well on a 1-km2

 grid derived from the 1990 census 

block data (Price and Clawges, 1999). The population 

density ranged from 89 to 8,742 people per square 

kilometer in the Coastal Los Angeles Basin and from 

48 to 8,773 people per square kilometer in the Coastal 

Santa Ana Basin. Although population density is not 

evenly distributed across the study area, most of the 

wells sampled (65 percent) are located in the areas 

with 2,000 to 5,000 people per square kilometer. 

The population density was significantly different at wells where at least one VOC was detected 

above the LRL than at wells where VOCs were not 

detected, but only in the Coastal Los Angeles Basin 

(p = 0.048; Mann–Whitney test). Conversely, population density was not significantly correlated to the total 

number of detections of VOCs in either the Coastal 

Los Angeles Basin or the Coastal Santa Ana Basin 

(p = 0.55 and p = 0.76, respectively; Spearman's rank 

correlation). The weak relation between the population 

density and the occurrence of VOCs may be due to the 

narrow range in population density within each area. 

Previous studies (Maine Bureau of Health, 1998; 

Squillace and others, 1999) that found significant relations between VOC occurrence and population density 

made comparisons across regions with large differences in population density. For example, in a statewide assessment of MTBE and other gasoline 

compounds in Maine drinking water, MTBE was 

detected more frequently in areas with a population 

density above 70 people per square kilometer (Maine 

Bureau of Health, 1998).

Squillace and others (1999) determined that 

VOCs were more likely to be detected in urban landuse settings classified as more than 386 people per 

square kilometer, than in rural land-use settings. Squillace and others (1999) used a logistic regression model 

of population density to estimate the probability of 

detecting VOCs in ambient ground water. The population density in the Coastal Los Angeles Basin and the 

Coastal Santa Ana Basin is at or above the high end of 

the range used to validate the model, but comparison 

between the data from the current study and the modeled probabilities reveals that the data from this study 

area fall below the expected probability of detection of 

a VOC above 0.2 μg/L. Because of the overall high 

density in population in the Coastal Los Angeles Basin 

and Coastal Santa Ana Basin, the influence of overlying sources may reach a threshold value beyond which 

the impact does not increase. In this study area, the 

acceleration of the lateral rate of ground-water flow 

produced by engineered recharge and ground-water 

pumping may be more important in controlling the 

occurrence of VOCs than the proximity of the well to 

local overlying sources of VOCs. 

In addition to the narrow range of population 

density in the Coastal Los Angeles Basin and the 

Coastal Santa Ana Basin compared with the state of 

Maine (Maine Bureau of Health, 1998) and the nationwide study (Squillace and others, 1999), depth to water 

and consequently well depths may affect whether population density can be correlated with VOC occurrence. The public supply wells in this study likely are 

deeper than in Maine because ground water is generally shallower in the coarse-grained aquifers of the 

northeastern United States. Median well depth for this 

study (210 m) (table 1) was more than five times 

greater than the median well depth for all rural and 

urban wells that reported construction information (40 

m) in the national study (Squillace and others, 1999).

Leaking Underground Fuel Tank Density

Because MTBE was widely used as a fuel oxygenate, leaking underground fuel tank (LUFT) sites are 

a potential source of MTBE and other gasoline compounds such as benzene, toluene, ethylbenzene, and 

total xylene (BTEX) in ground water used for drinking 

water (Moran and others, 1999; Johnson and others, 

2000). A previous study found MTBE in shallow 

ground water at 75 percent of leaking underground 

storage tank (LUST) sites in California (Happel and 

others, 1998). The current study, however, did not find 

Photograph by Phil Contreras (U.S. Geological Survey).
24 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

any relation between the density of LUFT sites and the 

occurrence and concentration of MTBE or the number 

of detections of VOCs above the LRL.

The most current LUFT data were obtained in 

electronic files from the State Water Resources Control 

Board (Amy Tong, California State Water Resources 

Control Board, written commun., 2001). LUFT density at each well was calculated by determining the 

density falling within a circle centered on the well. 

Circles of 100, 500, and 1,000 m radius were used. For 

comparison, a 500-m radius is approximately equal to 

the California Department of Health Services (DHS) 

minimum radius of 1,500 ft for porous media aquifers 

for the 10-year time-of-travel zone as specified in the 

DHS Drinking Water Source Assessment and Protection Program (California Department of Health Services, 2001). The LUFT densities calculated for each 

radius were not significantly different between sites 

where MTBE was detected and sites with no MTBE 

detections. Further, the frequency of detection of 

MTBE in samples from all wells in the study area and 

from subsets of wells in the Coastal Los Angeles Basin 

and the Coastal Santa Ana Basin was not significantly 

correlated to LUFT density at either the 100, 500 (fig. 

8), or 1,000-m radius. This data demonstrates that 

LUFT density is not a good predictor of the occurrence 

of VOCs in active public supply wells in this area. 

In contrast to these findings, Squillace and 

Moran (2000) determined that the density of above and 

underground storage tanks (UST) was an effective 

variable in explaining the occurrence of MTBE in 

ambient ground water in the Northeast and midAtlantic regions. Univariately, or in combination with 

other variables, LUST density also was found to be 

significantly related to MTBE occurrence in certain 

situations. LUST density, however, was dropped out of 

the final multivariate logistic regression model through 

a stepwise elimination procedure, and ultimately was 

not a significant variable in estimating the probability 

of detecting MTBE in the final model (Michael Moran, 

U.S. Geological Survey, written commun., 2001).

Similar to the population density, the LUFT 

density does not vary widely across the Coastal Los 

Angeles Basin and the Coastal Santa Ana Basin. 

Conversely, there is a wide range of well depths (43 to 

512 m) in this study (table 1). Differences in well 

depth or type may affect whether UST or LUFT densities are an effective variable in explaining the occurrence or frequency of detection of MTBE in ground 

water. The active public supply wells sampled as part 

of this study are likely deeper than the combination of 

shallow monitoring wells, private wells, and municipal 

wells sampled in the Northeast and mid-Atlantic regions 

as part of the Squillace and Moran (2000) study.

Hydrogeologically Vulnerable Areas 

The influence of hydrogeology on VOC occurrence can be illustrated by evaluating the relative susceptibility of aquifers to potential contamination. The 

relative susceptibility—that is, the ease with which a 

contaminant present at the land surface will migrate into 

an aquifer—of the surficial aquifers in California have 

been qualified into two general categories (Jan Stepak, 

California State Water Resources Control Board, written commun., 2000): (1) hydrogeologically most susceptible or hydrogeologically vulnerable areas, and (2) 

other areas. About 10 percent of the study area is designated as hydrogeologically vulnerable. This area is generally coincident with the engineered recharge facilities 

and along the upper reaches of the Los Angeles, the San 

Gabriel, and the Santa Ana Rivers (fig. 9). The data 

from this study indicate that the distinction between 

hydrogeologically vulnerable areas and other areas does 

not explain the occurrence of VOCs, whereas the 

Figure 8. Histogram of the frequency of MTBE detections in samples 

from all wells in the Los Angeles physiographic basin, in samples 

from wells in the Coastal Los Angeles Basin, and from wells in the 

Coastal Santa Ana Basin and the density of leaking underground 

fuel tanks within a 500-meter radius around the wells. LUFT, leaking 

fuel tank; MTBE, methyl tert-butyl ether.
Assessment of Relation Between VOC Distribution and Potential Sources of VOCs 25

delineation of the forebay areas explains differences in 

VOC occurrence and hence may provide a reasonable 

approximation of areas hydrogeologically vulnerable 

to current contamination. 

Thirty-three of the 178 wells sampled were 

located in the hydrogeologically vulnerable areas 

(fig. 9). About 73 percent of these wells had one or 

more detections above the LRL, a frequency of VOC 

detection slightly higher than the samples from wells 

located in the areas designated as other (58 percent); 

however, the number of VOC detections above the 

LRL are not significantly different between the hydrogeologically vulnerable and other areas (p = 0.12; 

Chi-square test). 

Although the high overall frequency of VOC 

detection above the LRL in the hydrogeologically 

vulnerable areas is consistent with this area's classification as most susceptible to potential contamination, 

the frequency of VOC occurrence is the same or 

greater in areas designated as other areas. For example, 

45 percent of the MTBE detections, which represents 

some of the most recent contamination in this area 

(and thus the shortest travel times), were in wells outside of the hydrogeologically vulnerable areas. 

This lack of distinction between hydrogeologically vulnerable and other areas is evidence that the 

acceleration of the lateral rate of ground-water flow 

produced by the engineered recharge and ground-water 

pumping in the Coastal Los Angeles Basin and the 

Coastal Santa Ana Basin is a more important factor in 

the occurrence of VOCs in active public supply wells 

than the surficial features used for mapping vulnerability. This engineered acceleration implies that areas 

downgradient of the recharge facilities are susceptible 

to contamination by any contaminant that will persist 

for decades.

The forebay areas represent a more consistent 

designation of susceptibility in the study area, possibly 

because they more closely reflect the zones where 

ground water is more likely to migrate vertically 

compared with the pressure areas where clay layers are 

more extensive and vertical migration of ground water 

is impeded. Sixty-one wells sampled in this study are 

located within the forebay, and 79 percent have one or 

more detections above the LRL. The number of VOC 

detections above the LRL is significantly different in 

wells in the forebay areas and in close proximity to 

the recharge facilities than in wells in the pressure 

Figure 9. Number of detections of volatile organic compounds in samples from active public supply wells in the Los Angeles physiographic 

basin, California, overlain by the California State Water Resources Control Board’s hydrogeologically vulnerable area. VOC, volatile organic 

compound; LRL, laboratory reporting limit.
26 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

areas at greater distances from the recharge facilities 

(p = 0.0008, Kruskal-Wallis test), and the median 

number of VOC detections above the LRL is higher in 

samples from wells in the forebay areas (3) than in 

samples from wells in the pressure areas (1). This relation is slightly stronger when the West Coast Basin 

wells, which are hydrogeologically isolated from the 

forebay areas or recharge facilities, are removed from 

the data set (p = 0.0005, Kruskal-Wallis test). Only 

one of the wells outside the forebay area had a concentration of MTBE above the LRL, whereas 23 wells 

inside the forebay area had a MTBE concentration 

above the LRL (fig. 5A). Unlike MTBE, the concentrations of VOCs in use for decades, such as CFC-11, 

CFC-113, chloroform, PCE, and TCE, are not different 

between the forebay and the pressure areas, and the 

detection rates are similar. 

The overall lack of correlation between population and LUFT density on the land surface and the 

occurrence and distribution of VOCs in active public 

supply wells in this study suggests that vertical migration of contaminants is not a dominant process outside 

the susceptible forebay areas, as evidenced by the 

higher number of VOC detections and the higher concentrations of MTBE in the forebay area. Although 

VOCs are detected in wells outside the forebay areas, 

these wells are further from the source of recharge and 

reflect a longer time of travel, as evidenced by lower 

MTBE concentrations. While the forebay areas may be 

the most susceptible to current contamination, the 

presence of compounds in use for decades at similar 

concentrations both inside and outside the forebay 

areas indicate that the pressure areas are susceptible to 

downgradient migration of persistent contaminants 

recharged upgradient in the forebay areas. 

SUMMARY AND CONCLUSIONS

Results of sampling untreated ground water 

from 178 active public supply wells in the Coastal Los 

Angeles and Coastal Santa Ana Basins show that volatile organic compounds (VOC) were detected in 61 

percent of the ground-water samples at concentrations 

above the laboratory reporting limits (LRL). Thirtynine of the 86 VOCs analyzed were detected: 29 VOCs 

were detected at concentrations above the LRL, and 10 

VOCs were detected only at concentrations below the 

LRL. The most frequently detected VOC was trichloromethane (chloroform), detected at concentrations 

above the LRL in 82 (46 percent) of the samples. 

Trichloroethene (TCE) and tetrachloroethene (PCE) 

were detected at concentrations above the LRL in 50 

(28 percent) and 34 (19 percent) of the samples, respectively. Methyl tert-butyl ether (MTBE) was detected at 

concentrations above the LRL in 25 (14 percent) of the 

samples. VOCs detected in 7 percent or more of the 

samples also include 1,1,1-trichloroethane (TCA); 

1,1-dichloroethane; cis-1,2-dichloroethene; bromodichloromethane; trichlorofluoromethane (CFC-11 or 

Freon 11); 1,1,2-trichloro-1,2,2-trifluoroethane 

(CFC-113 or Freon 113); and dichloromethane. The six 

most frequently detected VOCs—chloroform, TCE, 

PCE, MTBE, TCA, and 1,1-dichloroethane—were also 

the most frequently detected VOCs in a wide variety of 

hydrogeologic conditions in ground water from urban 

areas across the United States. 

Most VOCs were detected at low-level concentrations, with only 29 percent of the detections above 1 

μg/L (microgram per liter). Two VOCs exceeded the 

state and federal primary maximum contaminant level 

(MCL) for drinking water. TCE concentrations 

exceeded the MCL (5 μg/L) in four samples with a 

maximum concentration of 69 μg/L, and PCE concentrations exceeded the MCL (5 μg/L) in five samples 

with a maximum concentration of 11 μg/L. More than 

half of the remaining samples with TCE and PCE detections contained concentrations between 0.5 and 5 μg/L. 

Because samples were collected prior to water treatment, sample concentrations do not represent the concentrations entering the drinking-water system. 

VOC detections above the LRL are not uniformly 

distributed over the study area. Samples from wells in 

the forebay areas contained significantly more VOCs 

than samples from wells in the pressure area downgradient of the forebay. Samples with the most VOC 

detections generally are from wells located within 

10–15 kilometers of the recharge facilities. 

Ground water containing VOCs may be considered to be a tracer of postindustrial-aged water that 

recharged after the onset of intense urban development. 

The overall distribution of VOC detections is related to 

the hydrogeology and the recharge facilities in the forebay, and the distribution of individual VOCs is related to 

their history of use. For example, chloroform has been 

used since the 1920s, is the most widely distributed, and 

is detected at the greatest distances from the recharge 

facilities. In contrast, MTBE has the most recent history 

of use and was detected almost exclusively in the forebay areas at concentrations above the LRL. 
Summary and Conclusions 27

Downward migration of VOCs from the land 

surface may be a viable process for contaminating 

aquifers in parts of the forebay areas, but there is little 

indication that the same process is active in the pressure area. The number of VOC detections above the 

LRL is significantly higher in samples from shallow 

wells than in samples from deep wells in the forebay 

areas; however, affects of the processes of lateral 

movement and downward vertical movement of 

ground water in the forebay areas cannot be separated. 

Detections are not significantly different when wells 

are grouped by depth categories in the pressure area. 

The lack of contrast in the number of VOC detections 

between wells of different depths in the pressure area 

suggests that downward migration from the land surface is not the dominant pathway for VOCs to travel to 

the capture zones of public supply wells; however, the 

data are not sufficient to evaluate other processes that 

could affect the occurrence of VOCs in public supply 

wells, such as mixing over the long screened intervals. 

Isolated occurrences of multiple VOC detections and 

high concentrations of some VOCs, such as TCE, in 

individual wells may be explained by rapid vertical 

transport from localized sources. 

Stable isotope composition indicates that ground 

water containing VOCs is a mixture of local precipitation and runoff and of water that is isotopically lighter 

(more negative) than the local sources. The isotopically lighter water could either be Colorado River 

water or State Water Project water, both of which are 

imported to the basin and used as a source of groundwater recharge. Nearly all of the wells with no VOC 

detections at concentrations above the LRL are located 

in the pressure area and likely represent predominately 

older ground water than samples from wells in the 

forebay areas. The stable isotope data support the 

interpretation that VOCs in ground water are associated with the recharge facilities in the forebay areas.

Two of the most frequently detected VOCs in 

ground water, MTBE and chloroform, were detected 

more frequently and at higher concentrations in the 

Santa Ana River upstream from a recharge facility than 

in ground-water samples from active public supply 

wells, suggesting that the Santa Ana River may be a 

source of these VOCs in ground water. In contrast, the 

maximum concentrations and frequency of TCE and 

PCE detections were much higher in the ground water 

samples than in the Santa Ana River. However, the 

Santa Ana River water is only one component of the 

water used in the engineered recharge facilities and 

these data are not necessarily representative of past 

surface-water quality. In addition, engineered recharge 

water is likely not the sole source of VOCs in the forebay or the only process by which VOCs can enter the 

ground-water flow system. Point and nonpoint sources 

may enter the flow system in the forebay areas and 

spread radially outward.

The location of VOC sources on the land surface, such as population density and leaking underground fuel tank (LUFT) sites, or mapped 

susceptibility features, do not explain VOC occurrence 

in the study area. Population density was only weakly 

related to the occurrence of VOCs. The density of 

LUFT sites was not related to the occurrence or the frequency of MTBE detection, or the number of detections of VOCs above the LRL. The mapped surficial 

areas designated as hydrogeologically vulnerable areas 

did not encompass the area in which most of the VOCs 

were detected. Rather, the delineation of the forebay 

areas provides a reasonable approximation of areas 

susceptible to current contamination. 

Overall, the data show that active public supply 

wells in the forebay areas are more vulnerable to contamination than those in the pressure area but areas 

downgradient of the recharge facilities are susceptible 

to contamination by any contaminant that will persist 

for decades. The lack of correlation between population density and LUFT density on the land surface and 

the occurrence and distribution of VOCs in active public supply wells in this study indicates that vertical 

migration of contaminants is not a dominant process 

outside the susceptible forebay areas, and the occurrence of VOCs is controlled primarily by the direction 

and rate of movement of water and the hydrogeology. 

The overall occurrence of VOCs is not a function of 

localized contaminant sources or well characteristics, 

and rapid vertical transport, while possible, is not dominant. The acceleration of the lateral rate of groundwater flow produced by engineered recharge and 

ground-water pumping in the Los Angeles physiographic basin appears to be the dominant factor in controlling the distribution of VOCs in active public 

supply wells; thus, low-level VOCs can act as tracers 

of postindustrial-aged water recharged in these 

southern California Coastal Basins.
28 Low-Level VOCs in Active Public Supply Wells as Ground-Water Tracers in the Los Angeles Physiographic Basin, California, 2000

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