Stable Isotopes and Volatile Organic 

Compounds Along Seven Ground-Water Flow 

Paths in Divergent and Convergent Flow 

Systems, Southern California, 2000

By Barbara J. Milby Dawson, Kenneth Belitz, Michael Land, and Wesley R. Danskin

Sacramento, California

2003

Water-Resources Investigations Report 03-4059

U.S. GEOLOGICAL SURVEY

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

Prepared in cooperation with the

CALIFORNIA STATE WATER RESOURCES CONTROL BOARD 1016-40
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall – Suite 2012

6000 J Street

Sacramento, California 95819-6129

http://ca.water.usgs.gov

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286
Contents iii

CONTENTS

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

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

Study Area................................................................................................................................................... 2

Study Design ............................................................................................................................................... 4

Field Methods ............................................................................................................................................. 4

Laboratory Methods.................................................................................................................................... 5

Quality Control............................................................................................................................................ 5

Data Analysis Methods ............................................................................................................................... 6

Accessing Data............................................................................................................................................ 7

Acknowledgements..................................................................................................................................... 7

Central Basin, Los Angeles County ..................................................................................................................... 8

Hydrogeology.............................................................................................................................................. 8

Stable Isotopes ........................................................................................................................................... 15

Volatile Organic Compounds...................................................................................................................... 15

VOC Score ......................................................................................................................................... 15

Chloroform......................................................................................................................................... 21

MTBE................................................................................................................................................. 21

TCE .................................................................................................................................................... 21

PCE .................................................................................................................................................... 28

CFC-11............................................................................................................................................... 28

Main Basin, Orange County................................................................................................................................. 33

Hydrogeology.............................................................................................................................................. 33

Stable Isotopes ........................................................................................................................................... 37

Volatile organic compounds ....................................................................................................................... 37

VOC Score ......................................................................................................................................... 37

Chloroform......................................................................................................................................... 45

MTBE................................................................................................................................................. 48

TCE .................................................................................................................................................... 48

PCE .................................................................................................................................................... 53

CFC-11............................................................................................................................................... 53

Bunker Hill Basin, San Bernardino County......................................................................................................... 58

Hydrogeology.............................................................................................................................................. 58

Stable Isotopes ............................................................................................................................................ 65

Volatile organic compounds ....................................................................................................................... 65

VOC Score ......................................................................................................................................... 65

Chloroform ....................................................................................................................................... 65

MTBE................................................................................................................................................. 71

TCE .................................................................................................................................................... 71

PCE .................................................................................................................................................... 71

CFC-11............................................................................................................................................... 75

Summary and Conclusions................................................................................................................................... 77

References Cited .................................................................................................................................................. 78
iv Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

FIGURES

Figure 1. Map showing study area, basin, and well locations in Southern California...................................... 3

Figure 2. Map showing locations of the western and eastern ground-water flow paths, wells samples, 

and major hydrologic features of the Central Basin, Los Angeles County, California ..................... 9

Figure 3. Map showing ground-water level contours in the Central Los Angeles Basin, 

California, spring 2000 ...................................................................................................................... 10

Figure 4. Graph showing relation between delta oxygen-18 and delta deuterium in ground water 

along flow paths in the Central Basin, Los Angeles County, California........................................... 16

Figure 5. Graphs showing VOC score versus distance along the western and eastern flow paths, 

Central Basin, Los Angeles County, California, 2000 ...................................................................... 19

Figure 6. Graphs showing VOC score in wells along the western and eastern flow paths, 

Central Basin, Los Angeles County, California, 2000 ...................................................................... 20

Figure 7. Graphs showing trichloromethane (chloroform) concentration versus distance along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 22

Figure 8. Graphs showing trichloromethane (chloroform) concentrations in wells along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 23

Figure 9. Graphs showing methyl tert-butyl ether (MTBE) concentration versus distance along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 24

Figure 10. Graphs showing methyl tert-butyl ether (MTBE) concentrations in wells along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 25

Figure 11. Graphs showing trichloroethene (TCE) concentration versus distance along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 26

Figure 12. Graphs showing trichloroethene (TCE) concentrations in wells along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 27

Figure 13. Graphs showing tetrachloroethene (PCE) concentration versus distance along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 29

Figure 14. Graphs showing tetrachloroethene (PCE) concentration versus distance along the 

western and eastern flow paths in the Central Basin, Los Angeles County, California, 2000 .......... 30

Figure 15. Graphs showing trichlorofluoromethane (CFC-11) concentration versus distance along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 31

Figure 16. Graphs showing trichlorofluoromethane (CFC-11) concentrations in wells along the 

western and eastern flow paths, Central Basin, Los Angeles County, California, 2000 ................... 32

Figure 17. Map showing geology, locations of wells sampled, and major hydrologic features of the 

Main Basin, Orange County, California ............................................................................................ 34

Figure 18. Map showing ground-water level contours in the Central Los Angeles Basin, 

California, spring 2000 ...................................................................................................................... 35

Figure 19. Graph showing relation between delta oxygen-18 and delta deuterium in ground water 

along flow paths in the Main Basin, Orange County, California ...................................................... 39

Figure 20. Graphs showing VOC score versus distance along the Santa Ana River west, Santa Ana River 

east, and Santiago flow paths, Main Basin, Orange County, California, 2000 ................................. 43

Figure 21. Graphs showing VOC scores in wells along the Santa Ana River west, Santa Ana River east, 

and Santiago flow paths, Main Basin, Orange County, California, 2000.......................................... 44

Figure 22. Graphs showing trichloromethane (chloroform) concentration versus distance along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 46

Figure 23. Graphs showing trichloromethane (chloroform) concentrations in wells along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 47
Figures v

Figure 24. Graphs showing methyl tert-butyl ether (MTBE) concentration versus distance along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 49

Figure 25. Graphs showing methyl tert-butyl ether (MTBE) concentrations in wells along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 50

Figure 26. Graphs showing trichloroethene (TCE) concentration versus distance along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 51

Figure 27. Graphs showing trichloroethene (TCE) concentrations in wells along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 52

Figure 28. Graphs showing tetrachloroethene (PCE) concentration versus distance along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 54

Figure 29. Graphs showing tetrachloroethene (PCE) concentrations in wells along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 55

Figure 30. Graphs showing trichlorofluoromethane (CFC-11) concentrations in wells along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 56

Figure 31. Graphs showing trichlorofluoromethane (CFC-11) concentrations in wells along the 

Santa Ana River west, Santa Ana River east, and Santiago flow paths, Main Basin, 

Orange County, California, 2000....................................................................................................... 57

Figure 32. Map showing locations of wells sampled and major hydrologic features of the 

Bunker Hill Basin, San Bernardino County, California .................................................................... 59

Figure 33. Map showing ground-water level contours and well locations in the 

Bunker Hill Basin, San Bernardino County, California .................................................................... 60

Figure 34. Map showing well locations and distance measurement along flow paths in the 

Bunker Hill Basin, San Bernardino County, California .................................................................... 62

Figure 35. Graph showing relation between delta oxygen-18 and delta deuterium in ground water 

along flow paths, Bunker Hill Basin, San Bernardino County, California........................................ 66

Figure 36. Graphs showing VOC score versus distance and in wells along flow paths, 

Bunker Hill Basin, San Bernardino County, California, 2000 .......................................................... 69

Figure 37. Graphs showing trichloromethane (chloroform) concentration versus distance and 

in wells along flow paths in the Bunker Hill Basin, California, 2000............................................... 70

Figure 38. Graphs showing methyl tert-butyl ether (MTBE) concentrations versus distance and in 

wells along flow paths, Bunker Hill Basin, San Bernardino County, California, 2000 .................... 72

Figure 39. Graphs showing trichloroethene (TCE) concentrations versus distance and in wells 

along flow paths, Bunker Hill Basin, San Bernardino County, California, 2000.............................. 73

Figure 40. Graphs showing tetrachloroethene (PCE) concentrations versus distance and in wells 

along flow paths, Bunker Hill Basin, San Bernardino County, California, 2000.............................. 74

Figure 41. Graph showing trichlorofluoromethane (CFC-11) concentrations versus distance and 

in wells along flow paths, Bunker Hill Basin, California, 2000........................................................ 76
vi Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

TABLES

Table 1. Wells sampled in Central Basin, Los Angeles County, California, 2000.......................................... 11

Table 2. Statistical differences between the western and eastern ground-water flow paths, 

Central Basin, Los Angeles County, California, 2000 ...................................................................... 13

Table 3. Statistical correlations along ground-water flow paths in the Central Basin, 

Los Angeles County, California, 2000 .............................................................................................. 14

Table 4. Volatile organic compounds detected in 68 ground-water samples from selected wells in the 

Central Basin, Los Angeles County, California, 2000 ...................................................................... 17

Table 5. Detection frequencies of volatile organic compounds in the 68 wells in the Central Basin, 

Los Angeles County, California, 2000 .............................................................................................. 18

Table 6. Wells sampled in the Main Basin, Orange County, California, 2000 ............................................... 36

Table 7. Statistical differences between ground-water flow paths in the Main Basin, 

Orange County, California, 2000....................................................................................................... 38

Table 8. Statistical correlations along ground-water flow paths in the Main Basin, Orange County, 

California, 2000 ................................................................................................................................. 40

Table 9. Volatile organic compounds detected in 51 ground-water samples from selected wells in the 

Main Basin, Orange County, California, 2000 .................................................................................. 41

Table 10. Comparison of detection frequencies of volatile organic compounds detected in 51 wells 

sampled in the Main Basin, Orange County, California, 2000.......................................................... 42

Table 11. Wells sampled in the Bunker Hill Basin, San Bernardino County, California, 2000........................ 61

Table 12. Statistical differences between the Newmark and Santa Ana River ground-water flow paths, 

Bunker Hill Basin, San Bernardino County, California, 2000 .......................................................... 63

Table 13. Statistical correlations along ground-water flow paths in the Bunker Hill Basin, 

San Bernardino County, California, 2000 ......................................................................................... 64

Table 14. Volatile organic compounds detected in 26 ground-water samples from selected wells in the 

Bunker Hill Basin, San Bernardino County, California, 2000 .......................................................... 67

Table 15. Detection frequencies of volatile organic compounds detected in 26 wells sampled in the 

Bunker Hill Basin, San Bernardino County, California, 2000 .......................................................... 68
Abstract 1

Stable Isotopes and Volatile Organic Compounds Along 

Seven Ground-Water Flow Paths in Divergent and 

Convergent Flow Systems, Southern California, 2000

By Barbara J. Milby Dawson, Kenneth Belitz, Michael Land, and Wesley R. Danskin

ABSTRACT

Ground water is a major source of drinking 

water in southern California. In an effort to 

understand factors influencing the susceptibility of 

ground water tapped by public supply wells, the 

U.S. Geological Survey has undertaken studies in 

cooperation with the California State Water 

Resources Control Board. The vertical and lateral 

distribution of stable isotopes (deuterium and 

oxygen-18) and volatile organic compounds 

(VOC) were examined along seven ground-water 

flow paths in three urban ground-water basins in 

southern California: Central Basin in Los Angeles 

County, Main Basin in Orange County, and 

Bunker Hill Basin in San Bernardino County. 

Forty-seven monitoring wells and 100 public 

supply wells were sampled.

The results of this study suggest that the 

direction of flow and perhaps the degree of 

confinement in an aquifer system are important 

controls on the distribution of VOCs. Groundwater flow in the Central and Main Basins in the 

southern California coastal plain is characterized 

as radially divergent, with ground-water flow 

directions moving outward from focused areas of 

recharge in the unconfined part of the aquifer 

system toward dispersed areas of discharge in the 

more confined part. In these basins, there is a 

volume of water containing VOCs that extends out 

into a volume of water containing no VOCs. This 

pattern suggests that radially divergent flow 

systems disperse VOCs in distal areas. The overall 

pattern also suggests that ground water in the 

pressure area is generally insulated from 

compounds introduced at land surface. These two 

factors—dispersion of VOCs due to divergence of 

flow and insulation from land-surface inputs—

suggest that the susceptibility of public supply 

wells to surface contamination decreases with 

distance in radially divergent, well confined 

ground-water flow system.

In the inland Bunker Hill Basin, groundwater flow is characterized as radially convergent; 

ground-water flow directions move inward from 

dispersed recharge areas in the unconfined part of 

the aquifer system, toward an area of focused 

discharge in the more confined part. The number 

of VOCs increased and the concentrations of 

individual VOCs increased, or remained the same, 

with increasing travel distance. Methyl tert-butyl 

ether was detected only in wells in the confined 

part of the aquifer system, suggesting that the 

confining units present in the distal part of the 

Bunker Hill Basin do not prevent VOCs from 

reaching ground water. These results suggest that 

VOCs in the Bunker Hill Basin are collected and 

concentrated as ground water moves downgradient 

because of radial convergenence of flow. They also 

suggest that ground water in the Bunker Hill Basin 

has an increasing opportunity to pick up VOCs 

introduced at land surface as it moves along a flow 

path. Some of the downgradient increase in VOC 

occurrence and concentration may be due to 

pumping that selectively removes cleaner ground 

water, thus leaving ground water containing more 

VOCs in the aquifer. These two factors—

collection of VOCs due to convergence of flow 

and increasing opportunity to collect surficial 

contaminants perhaps due to a relative absence of 
2 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

confinement—suggest that the susceptibility of 

public supply wells to surface contamination 

increases with distance in radially convergent 

ground-water flow systems, particularly those that 

are unconfined.

INTRODUCTION

Ground water is a major source of drinking water 

in southern California. Some of the areas of heaviest 

ground-water use are in intensively urbanized areas: 

the Los Angeles–Orange County Coastal Plain and the 

San Bernardino Valley. The ground water is intensively 

managed in these areas: recharge has been engineered 

to increase the amount of water entering aquifers that 

supply drinking water and that have been historically 

depleted by pumping. While this engineering was 

undertaken to increase the available ground water and 

maintain ground-water levels, there is some concern 

about the effects on the quality of ground water. 

Compounds associated with urban land use that are 

known health risks, such as volatile organic compounds 

(VOC), are present in drinking water supplies, in some 

cases, over the drinking-water standards of the federal 

and state environmental protection agencies. Currently, 

no statewide program evaluates ground-water quality 

between the shallow, contaminated ground water near 

land surface (monitored by shallow monitoring wells) 

and the deeper zones tapped by public supply wells 

(monitored by California Department of Health 

Services). 

In response to concerns about the ground-water 

quality, the California State Water Resources Control 

Board (SWRCB) has implemented the California 

Aquifer Susceptibility (CAS) assessment to determine 

the factors influencing the susceptibility of ground 

water tapped by public supply wells to contamination. 

CAS studies evaluate the distribution and occurrence of 

VOCs, deuterium, oxygen-18, tritium, helium-3, and 

helium-4 in aquifers used for public supply, and relate 

that distribution to ground-water flow. CAS is part of 

the Ambient Groundwater Monitoring and Assessment 

(GAMA) Program, a multiagency program that is 

developing a comprehensive ambient ground-water 

monitoring plan. Under the GAMA Program, the 

U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program is 

collaborating with SWRCB, the California Department 

of Health Services, the California Department of Water 

Resources, and the Lawrence Livermore National 

Laboratory to implement the CAS assessment. 

This report examines the vertical and lateral 

distribution of VOCs and stable isotopes along selected 

ground-water flow paths from ground-water recharge 

areas to public supply wells in three urban groundwater basins in southern California. Tritium and helium 

isotope data were collected and analyzed during this 

study, but were not used in this analysis and are beyond 

the scope of this report. These data were interpreted 

and published by Hudson and others (2002). Relatively 

high numbers or concentrations of VOCs are 

considered indications of increased susceptibility of the 

ground water to contamination from land-use activities. 

These compounds are used in this study as tracers of 

ground water that has recharged since VOCs began to 

be used in large amounts during the 1940s. Shelton and 

others (2001) CAS study on the use of VOCs as tracers 

of ground-water movement in the southern California 

coastal plain found VOCs in aquifers over most of the 

coastal plain and determined that VOC occurrence is 

due primarily to lateral transport rather than vertical 

migration from the land surface. 

Study Area

Three southern California ground-water basins 

were selected to assess VOC occurrence along groundwater flow paths: the Central Basin in Los Angeles 

County, the Main Basin in Orange County, and the 

Bunker Hill Basin in San Bernardino County (fig. 1). 

The Central and Main Basins are located in the 

southern California coastal plain, and Bunker Hill 

Basin is located in the inland Santa Ana Valley. 
Introduction 3

34° 15'

33° 45'

117°

117° 118 30' ° 118° 30'

Bunker Hill Basin

map area

Main Basin map area

Central Basin

map area

Central Basin eastern flow path public supply Wells

Central Basin western flow path monitoring wells

Central Basin eastern flow path monitoring wells

Central Basin western slow sath sublic supply wells

Main Basin Santa Ana River west flow path public supply wells

Main Basin Santa Ana River east flow path public supply wells

Main Basin Santiago Flow path public supply wells

Bunker Hill Basin Santa Ana River flow path public supply wells

Bunker Hill Basin Santa Ana River flow path monitoring wells

Bunker Hill Basin Newmark flow path monitoring wells

Bunker Hill Basin Newmark flow path public supply wells

Explanantion

San

Bernardino

Mountains

San Gabriel

Mountains

San Gabriel

Valley Inland

Santa Ana

Basin

Southern California coastal plain

Los Angeles

County

Unconsolidated Sediments

Consolidated Deposits

0

0

50 Miles

50 Kilometers

California

Study area

Orange County

San Bernardino

County

Santa Ana

Mountains

Figure 1. Study area, basin, and well locations in Southern California.
4 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

The aquifers in the southern California coastal 

plain consist of Recent to Pleistocene unconsolidated 

alluvial and marine sediments. Ground-water flow in 

the coastal plain moves from the forebay (also referred 

to in this report as proximal) areas into the pressure 

(also referred to as distal) areas. The ground-water flow 

systems in the coastal plain are divergent; ground water 

moves radially away from a focused point of recharge 

(proximal) into the rest of the basin (distal). The 

change from forebay to pressure area is based on the 

relative abundance of finer-grained deposits and is not 

well defined. Aquifers in the forebay areas are 

generally unconfined and more connected to overlying 

land use and each other than in the pressure areas; clay 

and silt lenses generally do not significantly impede 

vertical ground-water flow. The aquifers in the pressure 

area consist of sands and gravels separated by relatively 

continuous layers of silt and clay that act as confining 

layers. The pressure area covers roughly 60 percent of 

the area in the Central Basin and in the Main Basin. 

Ground-water pumping is generally from about 50 to 

300 m (meter) below land surface (California 

Department of Water Resources, 1961; Herndon and 

others, 1997).

The valley-fill aquifers in the inland Santa Ana 

Basin consist of alluvial fan and stream deposits eroded 

from the surrounding mountains. These deposits 

consist of sands and gravels with layers of silt and clay. 

Faults in the sediment act as barriers to ground-water 

flow in places, and divide the valley into several 

ground-water basins. The flow paths selected for this 

study are located in the Bunker Hill Basin, which is 

bounded by faults in the northeastern part of the Santa 

Ana Valley. The ground-water flow system in the 

Bunker Hill Basin is convergent; ground water moves 

radially away from dispersed recharge areas (proximal) 

toward an area of focused discharge (distal). The 

aquifer system in the Bunker Hill Basin is divided into 

upper, middle, and lower aquifers (Dutcher and Garrett, 

1963). According to Dutcher and Garrett (1963, p. 63), 

the upper aquifer is confined in the distal area of the 

flow paths by a near-surface confining member, which 

“is discontinuous; it may be absent, thinner, or locally 

semipermeable in the vicinity of Warm Creek,” and is 

mapped by Duell and Shroeder (1989, fig. 3) as cut by 

river channel deposits that are highly permeable. In the 

distal area, the confining units in the aquifer system are 

described by Duell and Schroeder (1989, p. 9) as “clay 

layers that act as leaky confining beds.” The confined 

(pressure) area covers roughly 20 percent of Bunker 

Hill Basin (Dutcher and Garrett, 1963; Duell and 

Schroeder, 1989). 

Study Design

Ground-water flow paths originating near 

engineered recharge facilities were chosen on the basis 

of available ground-water-level data and the location of 

existing USGS monitoring wells in the selected 

ground-water basins (fig. 1). Along the flow paths in 

the Central and Bunker Hill Basins, 47 USGS 

monitoring wells at 12 sites were chosen for sampling. 

One hundred public supply wells were selected in all 

three basins from the wells sampled and described in 

Shelton and others (2001) and Hamlin and others 

(2002). Public supply wells were originally selected for 

those studies using a grid-based, random method 

(Scott, 1990) in each basin. In this report, well numbers 

along each flow path are ordered from upgradient to 

downgradient; monitoring wells at each site were given 

the same well number with a decimal number ordered 

from shallowest to deepest (for example, the shallowest 

monitoring well at the fifth well site along a flow path 

was well number 5.1). Ground-water samples collected 

from the USGS monitoring wells and public supply 

wells were analyzed for the stable isotopes deuterium 

(

2H) and oxygen-18 (18O), and for 86 VOCs. Stable 

isotope, VOC, well depth, and distance data were 

analyzed using graphical and statistical methods to 

determine spatial distribution and trends, and to 

compare VOC occurrence in three different groundwater flow systems. 

Field Methods 

Ground-water samples were collected from the 

selected wells using USGS protocols (U.S. Geological 

Survey, 1998) and the protocols described in Shelton 

and others (2001). Monitoring wells were sampled 

using portable, stainless-steel submersible pumps 

attached to Teflon tubing with stainless-steel fittings. 

Some public supply wells were sampled using Teflon 

tubing attached to a sampling point on the well 

discharge pipe as close to the well as possible, using 

brass and stainless-steel fittings; others were sampled 

using silicon or copper tubing and brass faucets 

attached to a sampling point. All equipment was 
Introduction 5

cleaned after each use and stored in plastic bags, except 

the silicon and copper sampling lines, which were 

cleaned before use and then discarded after a single 

sampling. 

Laboratory Methods

Stable isotope samples were analyzed for 

deuterium and oxygen-18 at the USGS Isotope 

Laboratory in Reston, Va., using the methods described 

in Epstein and Mayeda (1953) and Coplen and others 

(1991). Deuterium and oxygen-18 are reported as ratios 

per mill (‰, or per thousand) of 2

H / 1H (δD) and 

18O / 16O (δ18O), respectively, and are referenced to 

Vienna Standard Mean Ocean Water (VSMOW, δD and 

δ18O = 0 ‰). In the discussion of stable isotopes, the 

term “lighter” refers to waters with less deuterium and 

oxygen-18 (more negative per mil values), whereas the 

term “heavier” refers to waters that have more 

deuterium and oxygen-18 (less negative per mil 

values). VOCs were analyzed at the USGS National 

Water Quality Laboratory (NWQL) in Denver, Colo., 

using the methods described in Connor and others 

(1998); VOC concentrations are reported in 

micrograms per liter (μg/L). All chemical constituents 

were determined from unfiltered water samples.

NWQL uses two thresholds for reporting 

analytical results: (1) laboratory reporting level (LRL), 

which is set to minimize the reporting of false 

negatives (not detecting a compound when it is actually 

present in a sample) to less than 1 percent, and 

(2) long-term method detection level (LT-MDL), which 

is set to minimize the reporting of false positives 

(reporting a detection of a compound that is not 

actually in the sample) to less than 1 percent (Childress 

and others, 1999). The LT-MDL is calculated annually 

from data collected throughout the year for selected 

analytical methods; the LRL is set at two times the 

LT-MDL. Detections below the LRL are reported as 

estimated concentrations (designated with an “E” 

before the value in tables and text, and shown on 

graphs as open symbols). For information-rich methods 

(including the VOC method used in this study), 

detections below LT-MDL also are reported as 

E-values. E-values also result for detections outside the 

range of calibration standards, for detections that did 

not pass laboratory quality-control analyses, or from 

diluted samples.

Quality Control

Quality-control (QC) samples were collected to 

evaluate the bias of VOC data introduced during 

sample collection, processing, transportation, and 

laboratory analysis. Blank samples (blanks) of organicfree water were analyzed by the same methods used for 

ground-water samples. Two types of blanks were used 

in this analysis: field and source solution. Twenty-two 

pairs of field and source solution blanks were collected, 

which is 15 percent of the number of ground-water 

samples collected. Field blanks measure the overall 

bias of the water-chemistry data. Source-solution 

blanks measure bias in the field blanks. QC sample 

data collected in 2000 in each of the three groundwater study areas described in this report were used for 

this QC analysis.

Field blanks were collected using specially 

prepared water that was certified to contain less than 

the LRL of the selected constituents. The blank water 

was pumped or poured through clean sampling 

equipment, processed, and analyzed using the same 

methods as for the ground-water samples. Because of 

the variation in VOCs detected in field blanks, the data 

was grouped by basin and by well type for QC 

analyses. The concentrations detected in public supply 

well field blanks were compared to the concentrations 

detected in ground-water samples from the same basin. 

Concentrations detected in monitoring well field blanks 

were compared to concentrations detected in groundwater samples from monitoring wells in the same 

basin.

A constituent was of potential QC concern when 

it was detected in one or more field blanks, in groundwater samples, and when the minimum concentration 

detected in ground-water samples was lower than the 

maximum concentration detected in the field blanks. 

When a VOC was of potential QC concern, the 

concentration detected in the field blank was compared 

with the concentration detected in the associated 

source-solution blank. The VOC was not of QC 

concern in the field blanks when the VOC was also 

detected in the source solution at a similar 

concentration. If the VOC was not detected in the 

source solution, the concentration in the field blank 

then was compared with the concentrations detected in 

ground-water samples collected before that blank 

sample. Detections in the field blank that are the same 

or higher than previous ground-water detections 

indicate carry-over contamination—introduction of a 
6 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

VOC into a new sample from the sample equipment, in 

spite of cleaning, after sampling a well with a detection 

of that VOC. If carry-over contamination was identified 

as the cause of the detection in the field blank, 

subsequent ground-water samples and field blanks also 

were evaluated for evidence of carry-over 

contamination. For ground-water detections 

determined to be from carry-over contamination, a 

less-than LRL value was substituted for the measured 

concentration and counted as a nondetection.

Contamination was assumed to be limited to the 

blank and not to have affected the ground-water 

samples when a detection in a blank was at or above the 

LRL and accompanied by many nondetections in 

ground-water samples obtained prior to and subsequent 

to the field-blank. When detections in a blank could not 

be related to the source solution, to carry-over 

contamination, or to contamination limited to the 

blank, all ground-water samples with concentrations 

lower than the maximum blank value were considered 

contaminated.

On the basis of these criteria, ground-water data 

for the following compounds may have been affected 

by sample contamination in this study: benzene, ethyl 

benzene, methyl benzene (toluene), tetrachloroethene 

(PCE), and trichloroethene (TCE). Detections of 

benzene and ethyl benzene in one ground-water sample 

from the Bunker Hill Basin were determined to be from 

carry-over contamination; the LRLs were substituted in 

the results and counted as nondetections. Toluene was 

detected in blank samples at similar concentrations to 

ground-water samples in all three study areas; 

therefore, toluene detections in ground water are 

summarized, but were not used for detection frequency 

calculations or statistical analyses. Detections of PCE 

and TCE at monitoring well sites in the Central Basin 

were determined to be carry-over contamination from 

high concentrations in previous ground-water samples. 

For these wells, the measured values that were less than 

or equal to the highest blank detection (0.01 μg/L for 

PCE and 0.054 μg/L for TCE) were converted to a 

less-than value at the LRL and counted as 

nondetections.

Data Analysis Methods

Distance along each flow path was measured 

downgradient from the first well on each cross section 

in the Central and Main Basins. In the Bunker Hill 

Basin, the distance downgradient was measured from a 

point of flow path convergence selected on the San 

Jacinto Fault. The depth used for statistical tests was 

the depth to the midpoint of well openings. The terms 

“depth” and “well depth” refer to the depth from land 

surface to the midpoint of well openings. Inferences of 

ground-water age (time since recharge) were made on 

the basis of presence or absence of VOCs and position 

along a flow path. Ground water containing VOCs was 

inferred to have at least partially recharged since the 

1940s. Ground water that was closer to a recharge area 

or shallower in the aquifer system was inferred to be 

younger than ground water that was further from a 

recharge area and deeper in the aquifer system. In the 

Main and Bunker Hill Basins, locations of wells were 

compared to known locations of high TCE 

concentrations (TCE plumes) in ground water that 

resulted from a point-source contamination (Orange 

County Water District, 1991a, fig. 9; Orange County 

Water District, 1991b, fig. 7; Wildermuth 

Environmental, Inc., 2000b, fig. 3). In this report, wells 

in the mapped area of the TCE plumes are considered 

associated with the TCE plumes; well openings may 

not be at the same depth as the TCE plume in the 

ground water.

The four ways used in this investigation to count 

the number of VOC detections are based on the LRLs 

set by NWQL. The first count is a total of all VOC 

detections, regardless of whether the detected 

concentrations were above or below LRL. The second 

count is the number of only those VOC detections 

above LRL; this is the most reliable count of VOC 

detections to use for comparison between wells, as the 

certainty of detecting a VOC at the LRL is 99 percent 

or greater. The third count is the number of VOC 

detections below LRL. The fourth count is a new 

variable defined in this report as the VOC score.

The VOC score is a number with the count of 

VOC detections above LRL put to the left of the 

decimal point, and the count of detections below LRL 

put to the right of the decimal point. For example, a 

VOC score of 3.2 means that 3 VOCs were detected in 

that sample at concentrations above the LRL and 

2 VOCs were detected in that sample at concentrations 

below the LRL. This way of counting gives the number 

of detections above LRL 10 times the weight of 

detections below LRL.

The one-tenth weight given to the below LRL 

detections is subjective. In all cases where the different 

counts of VOC detections were statistically tested, the 
Introduction 7

strength of the tests were nearly the same for the VOC 

score, the number of detections above LRL, and the 

total number of VOCs detected. The number of 

detections below LRL did not correlate with any of the 

variables tested. VOC score is used in this report as a 

method to show and classify all of the VOC detections 

when plotting the data. When detections were censored 

at the LRL, much of the information on VOC 

occurrence and distribution was lost, as roughly half of 

all the VOC detections in the investigation were below 

LRL. In this report, VOC score will be used to analyze 

the number of VOC detections in each well and along 

flow paths.

The VOCs selected for comparison in this report 

were trichloromethane (chloroform), a by-product of 

water chlorination; methyl-tert-butyl-ether (MTBE), a 

gasoline additive; trichloroethene (TCE), a solvent 

widely used in metal degreasing; tetrachloroethene 

(PCE), a solvent widely used in textile industry and dry 

cleaning; and trichlorofluoromethane (CFC-11), a 

refrigerant. These compounds are among the most 

frequently detected VOCs in ground water in the study 

areas (Shelton and others, 2001; Hamlin and others, 

2002).

Spatial patterns in the distribution of VOC 

detections (number and concentration) were identified 

by graphical analysis. Statistical tests were used to 

identify significant monotonic trends and relations in 

the data. Nonparametric tests were used because many 

of the data have nonnormal distributions and many 

outliers. For statistical tests on data with censored 

values (less-than values), three values were substituted 

for the less-than value: zero, the LT-MDL, and half of 

the LT-MDL. In all tests, the results were nearly 

identical. On graphs, detected values are plotted at the 

measured concentrations; nondetections are plotted at 

half of the LT-MDL. The results of statistical tests are 

reported using half of the LT-MDL. For constituents 

with two LT-MDLs, the lower value was raised to the 

higher value. Raised LRLs were discarded and not used 

for data plotting or analysis.

The Spearman rank correlation test was used to 

determine the degree of correlation between two 

variables; the assumption, or the null hypothesis, is that 

there is no correlation (Helsel and Hirsch, 1995). The 

linear correlation coefficient rho (Spearman's rho) was 

computed on the ranks of the data; rho varies between 

–1 and 1, with values closer to –1 indicating an inverse 

correlation, values closer to 1 indicating a positive 

correlation, and values closer to 0 indicating no 

correlation (Helsel and Hirsch, 1995). The rank-sum 

test (also known as the Mann–Whitney or Wilcoxon 

rank-sum test) was used to determine whether one 

group of data had statistically different values from a 

second group; the null hypothesis is that the groups are 

the same (Helsel and Hirsch, 1995).

A significance level of 90 percent was used for 

all statistical tests in this report. For each statistical test, 

the p-value was used to determine the significance of 

the test. The p-value is the probability of the null 

hypothesis being true on the basis of the data used 

(Helsel and Hirsch, 1995). All tests resulting in a pvalue equal to or less than 0.10 were considered to 

significantly show that the null hypothesis was not true. 

The terms “significant” and “correlation” are always 

used in terms of statistical results.

Accessing Data

Users of the data discussed in this report are 

encouraged to access information through the USGS’s 

National Water Information System (NWIS) web page 

(NWISWeb) located at http://water.usgs.gov/nwis/. 

NWISWeb serves as a public interface to the USGS 

NWIS database of site information, real-time, groundwater, surface-water, and water-quality data collected 

from locations throughout the 50 states and elsewhere. 

Data on NWISWeb are updated from the NWIS 

database on a regularly scheduled basis. Users can 

retrieve data by category, geographic area, specific 

location, and parameter field. NWISWeb is able to 

output data tables (in HTML and ASCII tab formats), 

water-level and water-quality graphs, and to develop 

site-selection lists. Additional site and sample 

information not contained in NWIS are kept on file at 

the USGS offices in San Diego and Sacramento, 

California. (Formal requests for specific data should be 

directed to the U.S. Geological Survey, California 

District Office, Hydrologic Data Center, 6000 J Street, 

Sacramento, California, 95819-6129.)

Acknowledgements

The authors wish to thank the many agencies that 

assisted in the completion of this study. The California 

State Water Resources Control Board provided 

cooperative funding for this study. The USGS 

monitoring wells sampled for this study were 
8 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

cooperatively funded by San Bernardino Municipal 

Water District and the Water Replenishment District of 

Southern California. Public supply wells were sampled 

with permission and assistance from the following 

local agencies: city of Anaheim, Bellflower-Somerset 

Mutual Water Company, city of Compton, Compton 

Municipal Water District, city of Downey, East Orange 

County Water District, East Valley Water District, city 

of Fountain Valley, city of Garden Grove, city of 

Huntington Beach, Irvine Ranch Water District, La 

Habra Heights County Water District, city of 

Lakewood, city of Long Beach, city of Lynwood, Mesa 

Consolidated Water District, city of Orange, Orange 

County Environmental Management Agency, Park 

Water Company, Peerless Water Company, city of Pico 

Rivera, city of Redlands, city of Riverside, San 

Bernardino Valley Water Conservation District, city of 

Santa Ana, Serano Water District, Southern California 

Water Company, city of Southgate, city of Westminster, 

and Yorba Linda Water Company. Their cooperation is 

greatly appreciated. 

CENTRAL BASIN, LOS ANGELES COUNTY

Hydrogeology

Two ground-water flow paths were selected in 

the Central Ground Water Basin of the Los Angeles 

Coastal Plain (California Department of Water 

Resources, 1961), hereinafter referred to as the Central 

Basin; both flow paths begin in the Montebello forebay 

area upgradient of the Rio Hondo and San Gabriel 

recharge facilities and end in the Central Basin pressure 

area upgradient of the Newport–Inglewood fault zone 

(fig. 2). The western flow path is 21 km (kilometer) 

long and is located near and west of the Rio Hondo and 

the Los Angeles Rivers. The eastern flow path is 22 km 

long and is located near and west of the San Gabriel 

River. Both flow paths were selected to follow groundwater gradients based on water-level contours (fig. 3) 

(Water Replenishment District of Southern California, 

2000).

Historically, about two thirds of ground-water 

recharge to the Central Basin was from the Rio Hondo 

and the San Gabriel Rivers as they entered the basin at 

the Whittier Narrows from the inland San Gabriel 

Valley to the northeast; minor recharge resulted from 

ground-water flow through the Whittier Narrows 

(California Department of Water Resources, 1961). 

Other sources of recharge include irrigation, local 

precipitation, and runoff from the surrounding hills. 

Ground-water recharge has been engineered since the 

1930s at recharge facilities on the Rio Hondo and San 

Gabriel Rivers southwest of the Whittier Narrows 

(Water Replenishment District of Southern California, 

2001). A variety of water sources have been used 

historically for engineered recharge, including storm 

flow and treated wastewater from the San Gabriel 

Valley and imported water from northern California 

and the Colorado River (Water Replenishment District 

of Southern California, 2001). Nearly all ground-water 

discharge from the basin is by pumping from the 

forebay and pressure areas (Water Replenishment 

District of Southern California, 2001).

Ground-water levels measured in wells tapping 

the main pumping zone range from more than 40 m 

above sea level near the recharge facilities, to more 

than 20 m below sea level near the Newport–Inglewood 

fault zone (fig. 3) (Water Replenishment District of 

Southern California, 2000). The water-level contour 

pattern in most of the Central Basin is semicircular and 

centered on the Whittier Narrows. The ground-water 

flow system in the Central Basin is divergent; ground 

water moves radially away from a common area of 

recharge below the Whittier Narrows into the rest of the 

basin.

The two flow paths for the 68 wells sampled in 

the Central Basin (table 1) begin at a well upgradient of 

the Rio Hondo and the San Gabriel recharge facilities 

(fig. 2). Twenty-five sites—23 are public supply wells 

and 2 are USGS monitoring sites with 4 to 6 individual 

wells of varying depths—were on the western flow 

path; 33 wells were sampled (fig. 2). Twenty-two 

sites—19 are public supply wells and 3 are USGS 

monitoring sites with 5 to 6 individual wells of varying 

depths—were on the eastern flow path; 36 wells were 

sampled. Geologic, hydrologic, and additional waterquality data from the USGS monitoring wells are 

described in Land and others (2002). The wells on the 

western flow path ranged from 49 to 409 m deep, and 

wells on the eastern flow path ranged from 27 to 366 m 

deep. There was no significant difference between well 

depths or distances on the western and eastern flow 

paths (table 2) and no correlation between distance and 

well depth on either Central Basin flow path (table 3).
Central Basin, Los Angeles County 9

Rivers and streams

Newport–Inglewood fault zone

Recharge facilities

Recent unconsolidated sediments

Older recent unconsolidated sediments

Newport-Inglewood Fault Zone

San Gabriel Valley

Whittier

Narrows

Montebello

Forebay

Area

Central Basin

Pressure Area

Rio Hondo River

San Gabriel River

Los Angeles River

Western flow path wells

Eastern flow path wells

Consolidated deposits

EXPLANATION

Los Angeles

Forebay

Area

Western Flow Path

Eastern flow path

Rio Hondo

Recharge

Facility

San Gabriel

Recharge

Facility

34°

118° 118° 118 7' 30" ° 15'

33°

52'

30" LAE 17

Los Angeles River

LAW 1

LAW 2

LAW 3 LAW 4.1-4.6

LAW 5 LAW 6

LAW 7 LAW 8

LAW 9

LAW 10.1-10.4 LAW 11

LAW 12

LAW 13

LAW 14

LAW 15 LAW 16

LAW 17

LAW 18

LAW 19

LAW 20

LAW 21

LAW 22

LAW 23 LAW 24

LAW 25

LAE 1

LAE 2

LAE 3

LAE 4

LAE 5.1 - 5.6

LAE 6

LAE 7 LAE 8

LAE 9

LAE 10

LAE 11

LAE 12.1 - 12.6

LAE 13

LAE 14

LAE 15

LAE 16

LAE 18

LAE 19

LAE 20

LAE 21.1-21.5

LAE 22

Forebay - Pressure area boundary

0

0

5

5

10Miles

10 Kilometers

Figure 2. Locations of the western and eastern ground-water flow paths, wells samples, and major hydrologic features of the Central Basin, Los Angeles 

County, California.
10 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Rivers and streams

Newport–Inglewood fault zone

Recharge Basins

Western flow path wells

Eastern flow path wells

Consolidated deposits

EXPLANATION

Ground-water-level contour, in meters above sea level;

contour interval is 5 meters (Water Replenishment District

of Southern California, 2000)

Newport-Inglewood Fault Zone

San Gabriel Valley

Whittier

Narrows

San Gabriel River

Los Angeles River

Western Flow Path

Eastern Flow Path

Recent unconsolidated sediments

Older recent unconsolidated sediments

Rio Hondo River

0

0

5

5

10Miles

10 Kilometers

–10

–10

–10

34o

118o 118o 118 7' 30" o 15'

33o

52'

30"

–15

Figure 3. Ground-water level contours in the Central Los Angeles Basin, California, spring 2000.
Central Basin, Los Angeles County 11

Table 1. Wells sampled in Central Basin, Los Angeles County, California, 2000.

Site Number USGS Site ID State well number

Distance down 

gradient, in 

kilometers

Depth to middle of 

openings, in meters 

below land surface

LAW 1 340112118033701 002S011W05N004S 0.0 87

LAW 2 335914118062501 002S012W23B004S 5.6 133

LAW 3 335915118062401 002S012W23B008S 5.7 130

LAW 4.1 335829118065206 002S012W26D014S 7.1 46

LAW 4.2 335829118065205 002S012W26D013S 7.1 88

LAW 4.3 335829118065204 002S012W26D012S 7.1 134

LAW 4.4 335829118065203 002S012W26D011S 7.1 219

LAW 4.5 335829118065202 002S012W26D010S 7.1 280

LAW 4.6 335829118065201 002S012W26D009S 7.1 341

LAW 5 335826118164601 002S012W26D007S 7.6 127

LAW 6 335821118070801 002S012W27A005S 7.7 203

LAW 7 335817118070701 002S012W27H001S 7.8 154

LAW 8 335752118080201 002S012W28J006S 9.3 194

LAW 9 335745118081701 002S012W28Q001S 9.6 135

LAW 10.1 335642118103705 003S012W06B008S 13.6 70

LAW 10.2 335642118103704 003S012W06B007S 13.6 175

LAW 10.3 335642118103703 003S012W06B006S 13.6 280

LAW 10.4 335642118103702 003S012W06B005S 13.6 405

LAW 11 335643118105801 003S012W06D004S 14.1 207

LAW 12 335645118105601 003S012W06D002S 14.2 212

LAW 13 335635118110001 003S012W06D001S 14.3 207

LAW 14 335515118112101 003S013W12J001S 16.4 209

LAW 15 335537118115701 003S013W12E004S 16.6 149

LAW 16 335542118124701 003S013W11C001S 17.3 89

LAW 17 335452118121601 003S013W13D001S 17.8 169

LAW 18 335529118130901 003S013W11E001S 18.0 188

LAW 19 335433118125701 003S013W14F015S 19.1 177

LAW 20 335411118132001 003S013W15R001S 19.9 75

LAW 21 335350118132001 003S013W22H002S 20.5 74

LAW 22 335357118143501 003S013W21B001S 21.5 94

LAW 23 335318118134401 003S013W22Q004S 21.5 98

LAW 24 335356118145801 003S013W21C006S 21.9 83

LAW 25 335302118072201 003S013W27G001S 22.0 145

LAE 1 340112118033701 002S011W05N004S 0.0 87

LAE 2 335859118043101 002S011W19F001S 4.3 114

LAE 3 335849118043701 002S011W19M001S 4.7 134

LAE 4 335817118051301 002S012W25G001S 5.9 105

LAE 5.1 335818118051206 002S012W25G008S 5.9 34

LAE 5.2 335818118051205 002S012W25G007S 5.9 75

LAE 5.3 335818118051204 002S012W25G006S 5.9 101

LAE 5.4 335818118051203 002S012W25G005S 5.9 174

LAE 5.5 335818118051202 002S012W25G004S 5.9 256

LAE 5.6 335818118051201 002S012W25G003S 5.9 363

LAE 6 335712118054901 002S012W36M006S 8.1 131

LAE 7 335810118070201 003S012W02H004S 9.6 100
12 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Table 1. Wells sampled in Central Basin, Los Angeles County, California, 2000—Continued

Site Number USGS Site ID State well number

Distance down 

gradient, in 

kilometers

Depth to middle of 

openings, in meters 

below land surface

LAE 8 335609118064001 003S012W02L001S 10.4 186

LAE 9 335612118070101 003S012W03J001S 10.7 136

LAE 10 335529118065501 003S012W11E001S 11.6 149

LAE 11 335532118082401 003S012W09G001S 13.0 152

LAE 12.1 335517118081306 003S012W09J006S 13.2 30

LAE 12.2 335517118081305 003S012W09J005S 13.2 79

LAE 12.3 335517118081304 003S012W09J004S 13.2 116

LAE 12.4 335517118081303 003S012W09J003S 13.2 180

LAE 12.5 335517118081302 003S012W09J002S 13.2 290

LAE 12.6 335517118081301 003S012W09J001S 13.2 360

LAE 13 335440118081101 003S012W16H001S 14.4 170

LAE 14 335330118083901 003S012W21L001S 16.6 78

LAE 15 335209118082001 003S012W33B001S 17.2 190

LAE 16 335313118083101 003S012W28C001S 17.3 44

LAE 17 335208118073801 003S012W34F001S 18.9 322

LAE 18 335209118080101 003S012W33A007S 19.1 190

LAE 19 335301118073401 003S012W27C002S 19.1 255

LAE 20 335108118080301 004S012W04J003S 20.7 100

LAE 21.1 335112118090406 004S012W05H010S 20.9 24

LAE 21.2 335112118090405 004S012W05H009S 20.9 46

LAE 21.3 335112118090404 004S012W05H008S 20.9 88

LAE 21.4 335112118090403 004S012W05H007S 20.9 140

LAE 21.5 335112118090402 004S012W05H006S 20.9 198

LAE 22 335113118090801 004S012W05J001S 21.0 229
Central Basin, Los Angeles County 13

Table 2. Statistical differences between the western and eastern ground-water 

flow paths, Central Basin, Los Angeles County, California, 2000.

[VOC, volatile organic compound] 

1 blue, p = 0.10 to 0.06

2 green, p = 0.05 to 0.02

3 yellow, p = 0.01 to < 0.0001

Variable Rank-sum test p-value

Distance 0.71

Depth to top of openings 0.96

Depth to mid-point of openings 0.47

Depth to bottom of openings 0.23

δ Oxygen-18 0.17

δ Deuterium 0.45

VOC Score 20.03

Number of detections above LRL 20.03

Total number of VOCs detected 10.08

Trichloromethane (Chloroform) 30.004

Tetrachloroethene (PCE) 20.02

Trichloroethene (TCE) 0.30

1,1-Dichloroethane 20.03

Methyl tert-butyl ether (MTBE) 0.49

cis-1,2-Dichloroethylene 20.05

Chloromethane 30.006

1,1,1-Trichloroethane 10.10

Trichlorofluoromethane (CFC-11) 0.89

1,1-Dichloroethylene 0.35

Bromodichloromethane 0.22

Carbon disulfide 0.57

1,4-Dichlorobenzene 0.62

Dichlorodifluoromethane 10.09

trans-1,2-Dichloroethylene 0.98

1,1,2-Trichlorotrifluoroethane 0.98

Diisopropyl ether 0.18

Benzene 0.17

Dichloromethane 0.13

1,2-Dichloroethane 0.35

Dibromochloromethane 0.35

Ethylbenzene 0.31

Isopropylbenzene 0.35

m- and p-Xylene 0.31
14 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Table 3. Statistical correlations along ground-water flow paths in the Central Basin, Los Angeles County, California, 2000.

[n, number of ground-water samples] 

1 blue, p = 0.10 to 0.06 2 green, p = 0.05 to 0.02 3 yellow, p = 0.01 to < 0.0001

Variable

 Western Flow Path, n = 33 Eastern Flow Path, n = 36

Spearman 

p-value

Spearman's

rho

Statistical 

Correlation

Spearman 

p-value

Spearman's

rho

Statistical 

Correlation

Correlations with distance down gradient:

Depth to top of openings 0.23 –0.21 No 0.30 0.18 No

Depth to mid-point of openings 0.47 –0.13 No 0.73 0.06 No

Depth to bottom of openings 0.65 –0.08 No 0.77 –0.05 No

δDeuterium 20.04 20.39 Positive2 20.05 20.34 Positive2

δOxygen-18 0.24 0.22 No 0.70 –0.07 No

VOC Score 10.07 1–0.32 Inverse1 30.0002 3–0.61 Inverse3

Number of detections above LRL 0.11 –0.28 No 30.0002 3–0.62 Inverse3

Total number of VOCs detected 0.13 –0.27 No 30.0005 3–0.59 Inverse3

Trichloromethane (Chloroform) 0.86 –0.03 No 20.04 2–0.35 Inverse2

Methyl tert-butyl ether (MTBE) 3<0.0001 3–0.75 Inverse3 3<0.0001 3–0.84 Inverse3

Trichloroethene (TCE) 0.95 0.01 No 20.02 2–0.39 Inverse2

Tetrachloroethene (PCE) 0.92 0.02 No 30.004 3–0.48 Inverse3

Trichlorofluoromethane (CFC-11) 0.72 0.06 No 0.52 –0.11 No

Correlations with depth to the mid-point of openings:

Distance down gradient 0.47 -0.13 No 0.71 0.06 No

δDeuterium 30.0002 3–0.70 Inverse3 0.51 –0.12 No

δOxygen-18 3<0.0001 3–0.79 Inverse3 0.84 –0.04 No

VOC Score 0.19 0.23 No 0.85 –0.03 No

Number of detections above LRL 0.28 0.19 No 0.91 –0.02 No

Total number of VOCs detected 10.09 10.30 Positive1 0.97 –0.01 No

Trichloromethane (Chloroform) 30.008 30.47 Positive3 0.74 0.06 No

Methyl tert-butyl ether (MTBE) 0.34 –0.17 No 0.83 –0.04 No

Trichloroethene (TCE) 0.88 0.03 No 0.66 –0.07 No

Tetrachloroethene (PCE) 10.07 10.32 Positive1 0.55 –0.10 No

Trichlorofluoromethane (CFC-11) 0.19 0.23 No 0.83 –0.04 No
Central Basin, Los Angeles County 15

Stable Isotopes 

A plot of δ18O in relation to δD for the western 

and eastern Central Basin ground-water flow paths is 

shown in figure 4; ground-water values plot along and 

slightly below the Global Meteoric Water Line (Craig, 

1961). Stable isotope values from both Central Basin 

flow paths were statistically the same (table 2). Along 

both flow paths, δD showed a significant increase with 

distance (table 3). No correlation was found between 

δ18O and distance. Along both flow paths, δD showed a 

significant increase with distance, and no correlation 

was found between δ18O and distance (table 3). 

However, statistical differences were found between 

the flow paths in correlations with depth: along the 

western flow path δD and δ18O showed an inverse 

correlation with depth, while no correlation was found 

on the eastern flow path between depth and δD, or 

between depth and δ18O (table 3). A more detailed 

analysis of stable isotopes in the Central Basin 

indicates a mixture of sources to ground water in the 

Central Basin and differences between the eastern and 

western flow paths (M.T. Land, U.S. Geological 

Survey, written comm., 2002). However, these 

differences are not greater than the overall range of 

values, which is in contrast to the stable isotope values 

along flow paths in the Main and Bunker Hill Basins.

Volatile Organic Compounds

Sixteen VOCs were detected at concentrations 

above LRL in ground water along the Central Basin 

flow paths; an additional 8 VOCs were detected at 

concentrations only below LRL (table 4). Eighty-four 

percent (57 of 68) of the wells sampled in the Central 

Basin had one or more detections of any VOC above 

the LRL. Four percent (3 of 68) of wells had detections 

of VOCs only below LRL, and 15 percent (10 of 68) of 

the wells had no VOC detections.

Three VOCs were detected at concentrations 

above a federal or state maximum contaminant level 

(MCL) (California Department of Health Services, 

2001; U.S. Environmental Protection Agency, 2001): 

PCE, TCE, and cis-1,2-dichloroethylene (table 4). 

Most of the VOCs detections were well below their 

respective MCLs; 62 percent of all detected VOC 

concentrations were less than 1 percent of the MCL.

Chloroform, TCE, and PCE were the most 

frequently detected compounds in the Central Basin. 

The five compounds discussed in this report 

(chloroform, MTBE, TCE, PCE, and CFC-11), were all 

detected above LRL on both flow paths, and are among 

the 10 most frequently detected compounds (table 5).

VOC Score

VOC scores were significantly higher on the 

western flow path than on the eastern flow path (table 2

and fig. 5). VOC score showed a slight statistical 

decrease with distance on the western flow path and 

strong statistical decrease with distance on the eastern 

flow path (fig. 5). The highest VOC scores occurred 

around 15 km on both flow paths. No correlation was 

found on either Central Basin flow path between VOC 

score and well depth (table 3).

Wells with VOC detections are at all depths 

throughout the forebay and most of the pressure area. 

In contrast, wells with no VOC detections (VOC scores 

of 0.0) were deep monitoring wells or the most distal 

wells on the flow paths (fig. 6). This suggests that 

ground-water recharged since the use of VOCs began 

(in the 1940s) has migrated along most Central Basin 

flow paths and mixed with older ground water 

containing no VOCs.

The distribution of VOC scores on the western 

and eastern flow paths suggests that the number of 

VOCs introduced to ground water was higher in the 

past. Wells with the highest VOC score were at middle 

distances (in the proximal pressure area) and at middle 

depths along both flow paths (fig. 6). Low VOC scores 

were detected in shallow wells near the recharge 

facilities on the eastern flow path, in the more distal 

wells on both flow paths, and dispersed throughout 

both flow paths. High VOC scores were in both the 

forebay and pressure areas and in the well upgradient 

of the recharge facilities, suggesting that ground water 

containing multiple VOCs enters the Central Basin 

through the Whittier Narrows.
16 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

–75

–70

–65

–60

–55

–50

–45

–40

–10.5 –10 –9.5 –9 –8.5 –8 –7.5 –7 –6.5 –6

Delta oxygen-18, in per mill

Delta deuterium, in per mill

Global meteoric water

line (Craig, 1961)

Eastern flow path

Western flow path Western median

Eastern median

Explanation

Figure 4. Relation between delta oxygen-18 and delta deuterium in ground water along flow paths in the Central Basin, Los Angeles County, California.
Central Basin, Los Angeles County 17

Table 4. Volatile organic compounds detected in 68 ground-water samples from selected wells in the Central Basin, Los Angeles County, California, 2000.

[LRL, laboratory reporting limit; μg/L, micrograms per liter; MCL, maximum contaminant level; —, no value available or applicable; E -- 

concentration estimated]

1 U.S. Environmental Protection Agency, 2001

2 California Department of Water Resources, 2001

3 Total Trihalomethanes, as of 01/01/02 (U.S. Environmental Protection Agency, 2001) 4 Methyl benzene concentrations may be due to sample contamination during collection and analysis 5 2 LRLs used during the sampling period

Constituent LRL, in μg/L

Number of 

detections at 

or above LRL

Number of 

detections 

below LRL

Minimum 

detection, in 

μg/L

Median 

detection, in 

μg/L

Maximum 

detection, in 

μg/L

MCL, in 

μg/L

Number of 

wells 

exceeding a 

MCL

Total number of VOCs detected — —— 0 4 13 — —

VOC score — —— 0.0 2.5 8.5 — —

Trichloromethane (Chloroform) 0.052, 0.0245 39 7 E0.014 0.080 0.623 801,3 0

Tetrachloroethene (PCE) 0.1 29 17 E0.03 0.150 16.4 51 5

Trichloroethene (TCE) 0.038 35 8 E0.009 0.095 8.780 51 2

1,1-Dichloroethane 0.066, 0.0355 12 10 E0.009 0.053 0.285 52 0

Methyl tert-butyl ether (MTBE) 0.17 22 7 E0.05 0.37 0.85 132 0

cis-1,2-Dichloroethylene 0.038 13 6 E0.016 0.083 8.820 62 1

Chloromethane 0.5, 0.255 0 18 E0.05 E0.07 E0.15 — 0

1,1,1-Trichloroethane 0.032 4 12 E0.005 E0.005 E0.075 2001 0

1,1-Dichloroethylene 0.04 5 8 E0.01 E0.03 0.35 6 0

Trichlorofluoromethane (CFC-11) 0.09 6 6 E0.01 0.09 0.94 1502 0

Bromodichloromethane 0.048 3 4 E0.019 E0.029 0.550 801,3 0

Carbon disulfide 0.07 3 2 E0.006 0.12 0.18 — 0

1,4-Dichlorobenzene 0.05 0 5 E0.01 E0.01 E0.02 52 0

Dichlorodifluoromethane 0.27 0 3 E0.04 E0.11 E0.13 — 0

trans-1,2-Dichloroethylene 0.032 2 0 E0.033 0.190 0.347 1001 0

1,1,2-Trichlorotrifluoroethane 0.06 1 1 E0.02 E0.04 E0.07 1,2002 0

Diisopropyl ether 0.1 1 1 E0.1 E0.1 E0.1 — 0

Benzene 0.035 0 2 E0.008 E0.011 E0.015 12 0

Dichloromethane 0.38, 0.165 0 2 E0.04 E0.04 E0.04 51 0

1,2-Dichloroethane 0.13 1 0 0.15 — — 0.52 0

Dibromochloromethane 0.18 1 0 0.41 — — 801,3 0

Ethylbenzene 0.03 0 1 E0.01 — — 7001 0

Isopropylbenzene 0.032 0 1 E0.010 — — — 0

m- and p-Xylene 0.06 0 1 E0.01 — — 1,7502 0

Methyl benzene4 0.05 0 10 E0.004 E0.01 E0.02 1502 0
18 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000 Table 5. Detection frequencies of volatile organic compounds in the 68 wells in the Central Basin, Los Angeles County, California, 2000. [LRL, laboratory reporting limit; n, number of ground-water samples] Constituent Detection Frequency, in percent All Detections Detections above LRL Detections below LRL Combined n = 68 Western n = 33 Eastern n = 36 Combined n = 68 Western n = 33 Eastern n = 36 Combined n = 68 Western n = 33 Eastern n = 36 Trichloromethane (Chloroform) 68 85 53 56 76 39 12 9 14 Tetrachloroethene (PCE) 68 79 58 43 61 28 25 18 31 Trichloroethene (TCE) 63 70 58 51 58 47 12 12 11 1,1-Dichloroethane 44 36 53 29 18 39 15 18 14 Methyl tert-butyl ether (MTBE) 43 48 39 32 36 31 10 12 8 cis-1,2-Dichloroethylene 28 39 19 18 30 5.6 10 9.1 14 Chloromethane 26 30 22 0.0 0.0 0.0 26 30 22 1,1,1-Trichloroethane 24 36 11 5.9 12 0.0 18 24 11 1,1-Dichloroethylene 19 27 14 7.4 12 2.8 12 15 11 Trichlorofluoromethane (CFC-11) 18 21 14 8.8 9.1 8.3 8.8 12 5.6 Bromodichloromethane 10 15 5.6 4.4 6.1 2.8 5.9 9.1 2.8 Carbon disulfide 7.4 12 2.8 4.4 9.1 0.0 2.9 3.0 2.8 1,4-Dichlorobenzene 7.4 9.1 5.6 0.0 0.0 0.0 7.4 9.1 5.6 Dichlorodifluoromethane 4.4 3.0 5.6 0.0 0.0 0.0 4.4 3.0 5.6 trans-1,2-Dichloroethylene 2.9 3.0 2.8 2.9 3.0 2.8 0.0 0.0 0.0 1,1,2-Trichlorotrifluoroethane 2.9 3.0 2.8 1.5 0.0 2.8 1.5 3.0 0.0 Diisopropyl ether 2.9 0.0 5.6 1.5 0.0 2.8 1.5 0.0 2.8 Benzene 2.9 3.0 2.8 0.0 0.0 0.0 2.9 3.0 2.8 Dichloromethane 2.9 3.0 2.8 0.0 0.0 0.0 2.9 3.0 2.8 1,2-Dichloroethane 1.5 0.0 2.8 1.5 0.0 2.8 0.0 0.0 0.0 Dibromochloromethane 1.5 0.0 2.8 1.5 0.0 2.8 0.0 0.0 0.0 Ethylbenzene 1.5 3.0 0.0 0.0 0.0 0.0 1.5 3.0 0.0 Isopropylbenzene 1.5 0.0 2.8 0.0 0.0 0.0 1.5 0.0 2.8 m- and p-Xylene 1.5 3.0 0.0 0.0 0.0 0.0 1.5 3.0 0.0
Central Basin, Los Angeles County 19

VOC Score

Spearman's rho = –0.32

p = 0.07

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Spearman's rho = –0.62

p = 0.0002

Distance in kilometers

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

0 5 10 15 20 25

Weak inverse correlation

Strong inverse correlation

Monitoring Well

Public Supply Well

Explanation

Color indicates flow path

A

B

Figure 5. VOC score versus distance along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, California, 2000.
20 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Distance, in kilometers

Elevation, in meters above sea level

Land surface

Land surface

–400

–300

–200

–100

0

100

200

300

–400

–300

–200

–100

0

100

200

300

0 5 10 15 20 25

5.0 6.0

4.2

2.3

2.3

7.3

1.1

3.1

5.2

2.1

0.2

7.5

4.3

2.3

4.5

8.5

6.3

5.4

3.4

1.2

1.0

1.0

2.1

5.1

4.2

3.4

4.0

1.3

4.1

4.5

5.2

3.2

5.3

1.1

2.4

2.6

3.0

4.3

4.3

6.2

1.1 2.1

1.1

2.1

1.2

0.4

2.1

Forebay

area

Pressure

area

Forebay

area

Pressure

area

4.3

3.1

4.2

3.1

1.0

Engineered

recharge

Engineered

recharge

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line; shared wells show other

flow path color

VOC score, greater than or equal to 1.0

Explanation

4.3

0.2 VOC score, less than 1.0

2.1

Highest VOC score

2.1

4.2

2.1

3.3

3.5

6.1

VOC score = 0.0; no VOCs detected

A

B

Figure 6. VOC score in wells along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, California, 2000.
Central Basin, Los Angeles County 21

Chloroform

Chloroform was detected in ground water from 

68 percent of the wells (table 5). Chloroform was 

detected more often at higher concentrations along the 

western flow path than the eastern (table 5 and fig. 7). 

Chloroform concentrations did decrease significantly 

downgradient along the eastern flow path, but no 

correlation with distance was found along the western 

flow path (fig. 7). Chloroform concentrations were 

significantly higher in wells with deeper openings 

along the western flow path (table 3). No statistical 

correlation with depth was found on the eastern flow 

path; the highest concentrations were at mid-depth 

(table 3 and fig. 7).

The spatial distribution of chloroform 

concentrations in ground water is similar to the pattern 

of VOC scores. High concentrations on both Central 

Basin flow paths in the forebay and pressure areas are 

generally surrounded vertically and laterally by lower 

concentrations and nondetections (fig. 8). The highest 

chloroform concentrations were in the proximal 

pressure area at mid-depths at a distance of 14 km 

(fig. 8), surrounded vertically and laterally by lower 

concentrations and nondetections. This indicates that 

more chloroform was reaching ground water in the 

Central Basin in the past than at present.

MTBE

MTBE was detected in 43 percent of the wells 

(table 5). MTBE occurrence was not significantly 

different between the two Central Basin flow paths 

(table 2). On both Central Basin flow paths, the highest 

MTBE concentrations were in the forebay area, with 

concentrations decreasing very significantly with 

increasing distance along the flow paths (figs. 9 and 

10). Although no correlation was found between 

MTBE concentration and depth for all wells (table 3), 

concentrations in wells in the forebay area are 

generally higher in the shallower wells (fig. 10).

The spatial distribution of MTBE concentrations 

is different than for VOC score or chloroform. The 

highest concentrations are in the most upgradient wells 

(fig. 10), and there is a strong inverse correlation 

between concentration and distance (table 3 and fig. 9). 

This suggests that MTBE is currently being introduced 

into ground water in the forebay area. The highest 

concentration is in the well upgradient from the 

engineered recharge facilities (fig. 10), suggesting that 

MTBE is associated with the Whittier Narrows ground 

water, as well as with water being recharged in the 

Central Basin. No MTBE was detected in wells greater 

than 15 km along the western flow path or greater than 

12 km along the eastern flow path (figs. 9 and 10). The 

low concentrations in wells located in the upgradient 

pressure area suggest that MTBE is migrating 

downgradient from the forebay (fig. 10).

TCE

TCE was detected in 63 percent of the wells 

(table 5). Two detections are above the federal and state 

MCL of 5 μg/L (table 4) (California Department of 

Health Services, 2001; U.S. Environmental Protection 

Agency, 2001). TCE occurrence was not significantly 

different between the Central Basin flow paths 

(table 2). No correlation with distance was found along 

the Western flow path (table 3 and fig. 11); high 

concentrations were in both forebay and pressure area 

wells (fig. 12). Concentrations along the eastern flow 

path showed a statistical decrease with distance, with 

more nondetections in the most distal wells. No 

correlation was found on either Central Basin flow path 

between TCE concentration and depth (table 3).

The spatial distribution of TCE concentrations is 

similar to the patterns of VOC score. Along both flow 

paths, high concentrations were detected at middle to 

deeper depths in middle distances, and are surrounded 

vertically and laterally by lower concentrations 

(fig. 12). This suggests that relatively higher 

concentrations entered the basin at some time in the 

past. High concentrations also were in the 

northernmost well, upgradient of the engineered 

recharge facilities, an indication of the TCE 

contamination that continues to enter the Central Basin 

through the Whittier Narrows from known sources in 

the San Gabriel Valley (U.S. Environmental Protection 

Agency, 1998). Concentrations in wells beneath the 

recharge facilities are lower than in wells upgradient or 

downgradient, suggesting that the current recharge 

water is diluting TCE concentrations in older water.

The highest concentrations of TCE were in 

shallow pressure area wells (fig. 12). On the western 

flow path, the highest occurrence was at about 20 km in 

a shallow public supply well. The highest on the 

eastern flow path was at about 13 km in a shallow 

monitoring well in the pressure area. These wells are 

downgradient from much lower concentrations, 

indicating that even in the pressure area, ground water 

is susceptible to contamination from VOC sources.
22 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

0.001

0.01

0.1

1

Chloroform concentration, in micrograms per liter

0.001

0.01

0.1

1

Distance in kilometers

0 5 10 15 20 25

LRL = 0.052 mg/L

LRL = 0.052 mg/L

Spearman's rho = –0.03

p = 0.86

No correlation

Spearman's rho = –0.35

p = 0.04

Moderate inverse correlation

Monitoring Well Detection,

quanitified

Monitoring Well Detection,

estimated

Monitoring Well No Detection,

plotted at one quarter of the

less-than value

Explanation

Public Supply Well Detection,

quanitified

Public Supply Well Detection,

estimated

Public Supply Well No Detection,

plotted at one quarter of the

less-than value

Color indicates flow path

A

B

Figure 7. Trichloromethane (chloroform) concentration versus distance along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, 

California, 2000.
Central Basin, Los Angeles County 23

Distance, in kilometers

Elevation, in meters above sea level

Land surface

–400

–300

–200

–100

0

100

200

300

–400

–300

–200

–100

0

100

200

300

0 5 10 15 20 25

.04 .06

.13

.04

.04

.25

--

--

.08

.11

.07

.06

.08

--

.20

.32

.24

.14

.19

.18

.08

.04

--

--

--

.01

.22

.06

.05

.14

--

.06

.08

.07 --

--

--

.02

.41

.09

--

.04

.04

.62

--

--

--

--

--

--

.02 --

.03

--

.06

.04 .14

.15

.18

.17

.10 .31 .04

.01 .07

.04 -- --

.03

Forebay

area

Pressure

Engineered area

recharge

Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Not detected

Explanation

.22

.02

--

Land surface

Forebay

area

Pressure

area Engineered

recharge

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line; shared wells show other

flow path color

Highest concentration

A

B

Figure 8. Trichloromethane (chloroform) concentrations in wells along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, 

California, 2000.
24 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

0 5 10 15 20

Distance in kilometers

0.01

0.1

1

MTBE concentration, in micrograms per liter

0.01

0.1

1

25

LRL= 0.17 mg/L

LRL= 0.17 mg/L

Spearman's rho = –0.75

p < 0.0001

Very strong inverse correlation

Spearman's rho = –0.84

p < 0.0001

Very strong inverse correlation

Monitoring Well Detection,

quanitified

Monitoring Well Detection,

estimated

Monitoring Well No Detection,

plotted at one quarter of the

less-than value

Explanation

Public Supply Well Detection,

quanitified

Public Supply Well Detection,

estimated

Public Supply Well No Detection,

plotted at one quarter of the

less-than value

Color indicates flow path

A

B

Figure 9. Methyl tert-butyl ether (MTBE) concentration versus distance along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, 

California, 2000.
Central Basin, Los Angeles County 25

Distance, in kilometers

Elevation, in meters above sea level

Land surface

–400

–300

–200

–100

0

100

200

300

–400

–300

–200

–100

0

100

200

300

0 5 10 15 20 25

.69 .39

.59

.48

.45

.18

--

--

.44

.30

.44

.51

.36

--

.14

--

--

.05

.12

.08

.56

.55

.68

.32

.56

.67

.29

.12

.10

--

.37

.18

.16

.18 --

--

--

--

--

--

--

.85

.85

--

--

--

--

--

--

--

-- --

--

--

--

-- --

--

--

--

-- -- --

-- -- ---- --

--

Forebay

area

Pressure

Engineered area

recharge

Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Not detected

Explanation

.29

.10

--

Land surface

Forebay

area

Pressure

Engineered

area

recharge

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line; shared wells show other

flow path color

Highest concentration

A

B

Figure 10. Methyl tert-butyl ether (MTBE) concentrations in wells along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, 

California, 2000.
26 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Distance in kilometers

0.001

0.01

0.1

1

10

TCE concentration, in micrograms per liter

0.001

0.01

0.1

1

10

0 5 10 15 20 25

LRL= 0.038 mg/L

LRL= 0.038 mg/L

Spearman's rho = 0.01

p = 0.95

No correlation

Spearman's rho = –0.39

p = 0.02

Moderate inverse correlation

Monitoring Well Detection,

quanitified

Monitoring Well Detection,

estimated

Monitoring Well No Detection,

plotted at one quarter of the

less-than value

Explanation

Public Supply Well Detection,

quanitified

Public Supply Well Detection,

estimated

Public Supply Well No Detection,

plotted at one quarter of the

less-than value

Color indicates flow path

MCL = 5 mg/L

MCL = 5 mg/L

A

B

Figure 11. Trichloroethene (TCE) concentration versus distance along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, 

California, 2000.
Central Basin, Los Angeles County 27

Pressure

area

Forebay

area

Distance, in kilometers

Elevation, in meters above sea level

Land surface

–400

–300

–200

–100

0

100

200

300

–400

–300

–200

–100

0

100

200

300

0 5 10 15 20 25

.11 .08

--

--

.06

.28

--

--

.02

.18

--

.009

1.7

--

1.2

.09

--

.83

.54

.63

.06

.02

--

--

--

--

.08

.15

.14

.27

.03

.18

.05

.16

.10

5.1

.80

.07

--

.18

--

.72

.72

.04

--

--

--

--

--

--

-- .44

--

.09

.02

-- .02

.10

.04

.08

.03 .04

.04

8.8 1.9

.04 --

--

--

Forebay

area

Pressure

Engineered area

recharge

Concentration, in micrograms per liter, quanitified

Concentration, in micrograms per liter, estimated

Not detected

Explanation

.14

.02

--

5.1 Concentration, in micrograms per liter, above

maximum contaminant level

Land surface

Engineered

recharge

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line; shared wells show other

flow path color

Highest concentration

A

B

Figure 12. Trichloroethene (TCE) concentrations in wells along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, California, 2000.
28 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

PCE

PCE was detected in 68 percent of the wells 

(table 5). Five detections exceeded the federal and state 

MCL of 5 μg/L (table 4) (California Department of 

Health Services, 2001; U.S. Environmental Protection 

Agency, 2001). Concentrations were statistically higher 

on the western flow path (table 2 and fig. 13); no 

correlation with distance was found along the western 

flow path (table 3). PCE concentrations along the 

eastern flow path decreased significantly with distance 

(table 3 and fig. 13). PCE concentrations were 

significantly higher in deeper wells on the western flow 

path; no correlation with depth was found along the 

eastern flow path (table 3).

The spatial distribution of PCE concentrations is 

similar to that of TCE. High concentrations were 

detected in wells at middle depths and distances, and in 

the first well (upgradient of recharge facilities) on the 

flow paths. Low concentrations and nondetections were 

in the most distant wells and in some shallow wells at 

all distances (fig. 14). As with TCE, the pattern 

suggests that PCE continues to enter Central Basin 

through the Whittier Narrows from known sources in 

the San Gabriel Valley (U.S. Environmental Protection 

Agency, 1998), that concentrations are lower than in 

the past, and that current recharge in the Central Basin 

is diluting PCE concentrations.

CFC-11

CFC-11 was detected in 18 percent of the wells 

(table 5). There was no significant difference in 

CFC-11 between the two flow paths (table 2); however, 

the distribution is slightly different (fig. 15). CFC-11 

detections on the western flow path are grouped in the 

mid-distance wells, with higher concentrations in 

deeper wells (fig. 16). On the eastern flow path, 

CFC-11 detections are spread along the flow path, with 

higher concentrations in middle to more distal wells. 

No correlations with distance or depth were found 

(table 3), with the highest concentrations at middle 

distances and depths.

The spatial distribution of CFC-11 

concentrations is similar to VOC score, chloroform, 

TCE, and PCE; however, overall detection frequency is 

lower. The nondetections in shallow wells suggest that 

CFC-11 is not currently reaching ground water at 

detectable concentrations. The pattern suggests that 

higher concentrations were reaching ground water in 

the past than at present.
Central Basin, Los Angeles County 29

Distance in kilometers

PCE concentration, in micrograms per liter

0 5 10 15 20 25

LRL= 0.1 mg/L

LRL= 0.1 mg/L

0.001

0.01

0.1

1

10

100 Spearman's rho = 0.02

p = 0.92

No correlation

Spearman's rho = –0.48

p = 0.004

Strong Inverse Correlation

0.001

0.01

0.1

1

10

100

Monitoring Well Detection,

quanitified

Monitoring Well Detection,

estimated

Monitoring Well No Detection,

plotted at one quarter of the

less-than value

Explanation

Public Supply Well Detection,

quanitified

Public Supply Well Detection,

estimated

Public Supply Well No Detection,

plotted at one quarter of the

less-than value

Color indicates flow path

MCL = 5 mg/L

MCL = 5 mg/L

A

B

Figure 13. Tetrachloroethene (PCE) concentration versus distance along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, 

California, 2000.
30 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Distance, in kilometers

Elevation, in meters above sea level

Land surface

Land surface

–300

–400

–200

–100

0

100

200

300

–400

–300

–200

–100

0

100

200

300

0 5 10 15 20 25

.29 .14

.25

.03

.04

.14

--

--

.12

.15

.04

.04

.22

--

12.1

.79

.34

6.9

2.6

.05

--

--

--

--

16.4

4.4

.40

2.2

.04

.07

.15

.08 .07

.06

--

.06

.59

.12

--

1.2

1.2

.06

--

--

--

--

--

--

-- --

.03

--

.05

.04 .12

2.5

5.6 .04 --

--

--

--

Forebay

area

Pressure

area

Forebay

area

Pressure

area

Engineered

recharge

Engineered

recharge

Concentration, in micrograms per liter, quanitified

Concentration, in micrograms per liter, estimated

Not detected

Explanation

.15

.05

--

16.4 Concentration, in micrograms per liter, above

maximum contaminant level

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line; shared wells show other

flow path color

Highest concentration

5.2

2.3

.64 4.6

.54

.07

1.7

A

B

Figure 14. Tetrachloroethene (PCE) concentration versus distance along the (A) western and (B) eastern flow paths in the Central Basin, Los Angeles County, 

California, 2000.
Central Basin, Los Angeles County 31

Distance in kilometers

0.01

0.1

1

CFC-11 concentration, in micrograms per liter

LRL= 0.09 mg/L

LRL= 0.09 mg/L

0.01

0.1

1

0 5 10 15 20 25

Spearman's rho = 0.06

p = 0.72

No Correlation

Spearman's rho = –0.11

p = 0.52

No Correlation

Monitoring Well Detection,

quanitified

Monitoring Well Detection,

estimated

Monitoring Well No Detection,

plotted at one quarter of the

less-than value

Explanation

Public Supply Well Detection,

quanitified

Public Supply Well Detection,

estimated

Public Supply Well No Detection,

plotted at one quarter of the

less-than value

Color indicates flow path

A

B

Figure 15. Trichlorofluoromethane (CFC-11) concentration versus distance along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles 

County, California, 2000.
32 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

--

Distance, in kilometers

Elevation, in meters above sea level

Land surface

–400

–300

–200

–100

0

100

200

300

–400

–300

–200

–100

0

100

200

300

0 5 10 15 20 25

-- --

--

--

--

--

--

--

--

--

--

.31

--

--

.05

--

--

--

.13

.04

.03

.03

--

--

--

--

--

--

--

--

--

--

-- .94 --

--

--

--

--

--

--

--

--

.55

--

--

--

--

--

--

.23 --

--

--

--

-- --

.22

.01

.05

-- --

-- -- ---- --

--

Forebay

area

Pressure

Engineered area

recharge

Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Not detected

Explanation

.94

.05

--

Land surface

Forebay

area

Pressure

area Engineered

recharge

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line; shared wells show other

flow path color

Highest concentration

A

B

Figure 16. Trichlorofluoromethane (CFC-11) concentrations in wells along the (A) western and (B) eastern flow paths, Central Basin, Los Angeles County, 

California, 2000.
Main Basin, Orange County 33

MAIN BASIN, ORANGE COUNTY

Hydrogeology

Three ground-water flow paths were selected in 

the Main Basin of the Coastal Santa Ana ground-water 

basin. Two flow paths, the Santa Ana River (SAR) west 

and SAR east, begin near the recharge facilities along 

the Santa Ana River in the forebay area and end in the 

pressure area, upgradient of the Newport–Inglewood 

fault zone (fig. 17). The SAR west flow path is west of 

the Santa Ana River, and the SAR east flow path is near 

and east of the Santa Ana River. The third flow path, 

Santiago, begins near the recharge facilities on 

Santiago Creek, is near and south of Santiago Creek, 

and ends on the SAR east flow path.

The ground-water flow system in the Main Basin 

includes two flow systems, one associated with the 

Santa Ana River and the other with Santiago Creek. As 

in the Central Basin, both are divergent flow systems; 

ground water moves radially away from a focused 

point of recharge (proximal) into the rest of the basin 

(distal). In contrast to the Central Basin, there are two 

main points of recharge, one along the Santa Ana River 

and a smaller one along Santiago Creek.

Three-fourths of ground-water recharge takes 

place at engineered recharge facilities in and along the 

Santa Ana River and at the smaller facility on Santiago 

Creek. Other sources of recharge include irrigation 

return, local precipitation, and ground-water inflow 

from other basins (Herndon and others, 1997). 

Engineered recharge facilities, in operation along the 

Santa Ana River since the 1940s, have used stormflow 

and base flow from the inland Santa Ana Basin, 

imported Colorado River water, and imported northern 

California water (Moreland and Singer, 1969; Herndon 

and others, 1997). The Santiago Creek recharge 

facility, in operation for about 10 years, uses local 

mountain runoff from the Santa Ana Mountains and 

excess stormflow diverted from the Santa Ana River 

recharge facilities. Additional engineered recharge is 

from injection wells at the Talbert and Alamitos 

seawater barriers (fig. 17). The Talbert barrier has been 

in operation since 1975 and uses a mixture of treated 

wastewater and deep ground water (Herndon and 

others, 1997; Orange County Water District, 2001). 

The Alamitos barrier has been in operation since 1965 

and uses imported water (Herndon and others, 1997; 

Orange County Water District, 2001). Nearly all 

ground-water discharge in the Main Basin is from 

ground-water pumping in the pressure area.

The Main Basin flow paths were selected to 

approximately follow ground-water gradients based on 

water-level contours (fig. 18) (Wildermuth 

Environmental, Inc., 2000). Well information for the 

51 wells sampled in the Main Basin are given in 

table 6. Wells were selected that tap the upper 

production zone (from 60 to 120 m below land 

surface), the lower production zone (from 150 to 300 m 

below land surface), and both zones.

The SAR west flow path (25 km long) follows 

the middle of the radial flow from the Santa Ana River 

recharge facilities from proximal to distal areas. 

Twenty-three public supply wells were sampled on the 

SAR west flow path (fig. 17 and table 6).

The Santiago flow path (11 km long) follows the 

middle of the radial flow from the Santiago Creek 

recharge facilities from proximal to distal areas. 

Although the Santiago flow path is shorter in length, it 

covers the same parts of that flow system as the SAR 

west flow path (proximal to distal). The shorter length 

reflects the shorter time of operation and lower 

recharge rates of the Santiago Creek recharge facility. 

Ten public supply wells were sampled on the Santiago 

flow path (fig. 17 and table 6).

The SAR east flow path (22 km long), which 

follows the eastern edge of the radial flow from the 

Santa Ana River recharge facilities from proximal to 

distal areas, is also distal to the Santiago flow path. 

Twenty public supply wells were sampled on the SAR 

east flow path (fig. 17 and table 6).

Well distances and depths are statistically similar 

along the SAR west and east flow paths (table 7). The 

wells on the Santiago flow path are more evenly 

distributed along the entire flow path than the wells on 

the SAR flow paths, which have more wells located in 

the downgradient part of the flow paths (fig. 17). Well 

depths are similar between the SAR east and Santiago 

flow paths (table 7). The bottoms of well openings are 

statistically deeper on the SAR west than on the 

Santiago flow path (table 7). There was no correlation 

between well depths and distance downgradient along 

either SAR flow path. The midpoints and bottoms of 

well openings were significantly deeper with distance 

downgradient along the Santiago flow path.
34 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Rivers and streams

Public supply well, Santiago flow path (SANTI)

Public supply well, Santa Ana River west flow path (SARW) Recharge facility

Explanation

Recent unconsolidated sediments

Older recent unconsolidated sediments

Consolidated deposits

Marine sediments

Public supply well, Santa Ana River east flow path (SARE)

Santiago Creek

Santa Ana River

118° 117° 52' 30" 117° 45'

33°

52'

30"

33°

45' Newport–Inglewood Fault Zone

Forebay Area

Pressure

Area

Santa Ana

Mountains

Santa Ana River

Recharge Facilities

Santiago Creek

Recharge

Facilities

SARW 1

SARW 2

SARW 3

SARW 5

SARW 4

SARW 6

SARW 7

SARW 8

SARW 9

SARW 10

SARW 11

SARW 12

SARW 13

SARW 14

SARW 15 SARW 16

SARW 17

SARW 18

SARW 19

SARW 20

SARW 21

SARW 22

SARW 23

SARE 1

SARE 2

SARE 3

SARE 4 SARE 5

SARE 6

SARE 7

SARE 8

SARE 9

SARE 10

SARE 11

SARE 12

SARE 13

SARE 14

SARE 15

SARE 16

SARE 17

SARE 18

SARE 19

SARE 20

SANTI 1

SANTI 2

SANTI 3

SANTI 4

SANTI 5

SANTI 6

SANTI 7

SANTI 8

SANTI 9

SANTI 10

Forebay - Pressure area boundary

0 10 Kilometers

0 10 Miles

Figure 17. Geology, locations of wells sampled, and major hydrologic features of the Main Basin, Orange County, California.
Main Basin, Orange County 35

Public supply well, Santiago flow path (SANTI)

Public supply well, Santa Ana River west flow path (SARW) Recharge facility

Explanation

Recent unconsolidated deposits

Older recent unconsolidated deposits

Consolidated deposits

Marine deposits

Public supply well, Santa Ana River east flow path (SARE)

Ground-water level contours, 1997, in meters above sea level,

contour interval 5 meters (Wildermuth Environmental, Inc., 2000).

SANTI Flow Path

118° 117° 52' 30" 117° 45'

33°

52'

30"

33°

45'

Mapped area of trichloroethene plume in ground water,

(Orange County Water District, 1991a and 1991b)

SARW Flow Path

SARE Flow Path

0 10 Kilometers

0 10 Miles

Figure 18. Ground-water level contours in the Central Los Angeles Basin, California, spring 2000.
36 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Table 6. Wells sampled in the Main Basin, Orange County, California, 2000.

Site Number USGS Site ID State well number Distance down 

gradient, in kiloeters

Depth to middle of 

openings, in meters 

below land surface

SARW 1 335118117481401 004S009W03H002S 0.0 37

SARW 2 335148117494001 003S009W33K005S 2.3 88

SARW 3 335145117494101 003S009W33K006S 2.3 76

SARW 4 335148117510101 003S009W32L001S 4.4 265

SARW 5 335004117533901 004S010W11Q002S 9.5 105

SARW 6 334932117532401 004S010W14H003S 9.9 324

SARW 7 334903117535201 004S010W23B002S 11.0 101

SARW 8 334949117544301 004S010W15B005S 11.1 102

SARW 9 334833117555501 004S010W21L001S 14.1 124

SARW 10 334710117552101 004S010W34E002S 15.0 230

SARW 11 334745117564501 004S010W29K001S 15.9 261

SARW 12 334825117573101 004S010W20M001S 16.0 263

SARW 13 334704117561101 004S010W33F002S 16.2 200

SARW 14 334659117561201 004S010W33F004S 16.3 56

SARW 15 334747117582701 004S010W30E002S 17.9 256

SARW 16 334744117594601 004S011W26J002S 19.4 80

SARW 17 334601117582401 005S011W01H002S 20.1 194

SARW 18 334719118003901 004S011W26N001S 20.9 194

SARW 19 334613118005901 005S011W03C001S 22.7 134

SARW 20 334500118003001 005S011W10J004S 23.7 106

SARW 21 334552118020201 005S011W04L001S 24.3 188

SARW 22 334501118013001 005S011W09J002S 24.8 174

SARW 23 334439118011801 005S011W15D003S 25.1 85

SARE 1 335118117481401 004S009W03H002S 0.0 37

SARE 2 334958117514401 004S009W07P001S 5.9 105

SARE 3 334911117510601 004S009W17N001S 7.1 226

SARE 4 334826117514101 004S009W19K001S 8.6 222

SARE 5 334826117521401 004S010W24J002S 8.8 219

SARE 6 334822117521401 004S010W24J001S 9.0 246

SARE 7 334753117525901 004S010W25F001S 10.2 105

SARE 8 334716117525301 004S010W36C002S 11.2 213

SARE 9 334608117525601 005S010W01E002S 13.2 134

SARE 10 334609117530401 005S010W01E003S 13.2 283

SARE 11 334603117533301 005S010W02K002S 13.7 153

SARE 12 334538117541901 005S010W03R003S 14.8 291

SARE 13 334405117534701 005S010W14L001S 17.3 279

SARE 14 334350117553201 005S010W21B004S 18.5 176

SARE 15 334338117553901 005S010W21B003S 18.9 135

SARE 16 334231117534001 005S010W26L001S 20.0 283

SARE 17 334246117545001 005S010W27C002S 20.1 185

SARE 18 334253117555001 005S010W28C009S 20.4 219

SARE 19 334147117542001 005S010W34H010S 21.6 135

SARE 20 334143117545401 005S010W34F003S 22.0 142

SANTI 1 334833117490301 004S009W22M001S 0.0 128

SANTI 2 334806117492001 004S009W28A001S 1.0 181
Main Basin, Orange County 37

Table 6. Wells sampled in the Main Basin, Orange County, California, 2000—Continued

Site Number USGS Site ID State well number Distance down 

gradient, in kiloeters

Depth to middle of 

openings, in meters 

below land surface

SANTI 3 334722117491901 004S009W28R002S 1.8 118

SANTI 4 334707117502601 004S009W32B004S 3.4 200

SANTI 5 334631117504101 004S009W32P002S 4.5 254

SANTI 6 334635117512401 004S009W31R001S 5.2 187

SANTI 7 334631117513001 004S009W31Q001S 5.3 234

SANTI 8 334501117524001 005S010W12L003S 8.7 191

SANTI 9 334443117523401 005S010W13B009S 8.9 221

SANTI 10 334405117534701 005S010W14L001S 11.0 279

Stable Isotopes 

A plot of δ18O in relation to δD for the three 

Main Basin ground-water flow paths is shown on 

figure 19. Most of the ground-water values plot along 

and slightly below the Global Meteoric Water Line 

(Craig, 1961). Stable isotope values from both SAR 

flow paths were statistically identical, indicating a 

similar source of ground water. The two lighter values 

on the SAR west flow path may indicate the mixing 

with lighter sources of recharge, such as water from the 

inland Santa Ana Basin, northern California aqueduct 

water, and Colorado river water. Stable isotope values 

were significantly heavier on the Santiago flow path 

(table 7), reflecting a greater proportion of ground 

water from local precipitation. Except for δD on the 

SAR East flow path, δ18O and δD were inversely 

correlated with depth on the Main Basin flow paths 

(table 8).

Volatile organic compounds

Of the 86 VOCs analyzed for in this study, 

9 VOCs were detected at concentrations at or above 

LRL in ground water along the Main Basin flow paths; 

an additional 8 VOCs were detected below LRL 

(table 9). Fifty-one percent (26 of 51) of wells sampled 

contained one or more VOCs above LRL; an additional 

31 percent (16 of 51) of the wells had one or more 

VOCs only below LRL. Twenty-two percent (11 or 51) 

of the wells sampled had no VOC detections. All VOCs 

detected along the three Main Basin flow paths were 

below federal and state MCLs (table 9) (California 

Department of Health Services, 2001; 

U.S. Environmental Protection Agency, 2001).

The five compounds discussed in this report 

(chloroform, MTBE, TCE, PCE, and CFC-11) were all 

detected above LRL in the Main Basin and are among 

the seven compounds most frequently detected 

(table 10). Chloroform, MTBE, and TCE were detected 

on all three flow paths. PCE and CFC-11 were not 

detected on the Santiago flow path (table 10).

VOC Score

VOC score was statistically highest along the 

SAR west flow path (table 7). VOC scores on the SAR 

east flow path were significantly lower than on the 

SAR west flow path, but statistically similar to those on 

the Santiago flow path (table 7 and fig. 20).

There is an inverse correlation between distance 

and VOC score on all three flow paths (table 8). The 

distal wells have the lowest number of VOC detections 

(figs. 20 and 21). A transition from high to low VOC 

scores (detections mostly above LRL to only detections 

below LRL) occurs on the SAR west flow path at about 

18 km downgradient in the pressure area (fig. 20A). 

This transition occurs on the SAR east line slightly 

closer to the SAR recharge area (14 km) (fig. 20B). 

There is a monotonic decrease in VOC score along the 

Santiago flow path, with the most VOCs detected in the 

well closest to the Santiago Creek recharge facilities 

(fig. 21C). These correlations and patterns indicate that 

VOCs were introduced in the forebay either with 

engineered recharge water or from the land surface.
38 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Table 7. Statistical differences between ground-water flow paths in the Main Basin, Orange County, California, 2000.

[LRL, laboratory reporting limit; SARW , Santa Ana River West; SARE, Santa Ana River East] 

1 blue, p = 0.10 to 0.06

2 green, p = 0.05 to 0.02

3 yellow, p = 0.01 to < 0.0001

Variable

SARW vs SARE SARW vs Santiago SARE vs Santiago

Rank-sum test p-value Rank-sum test p-value Rank-sum test p-value

Distance 0.44 30.001 30.0007

Depth to top of openings 0.38 0.67 0.48

Depth to mid-point of openings 0.13 0.14 0.83

Depth to bottom of openings 0.13 10.06 0.52

δDeuterium 0.59 30.0001 30.0004

δOxygen-18 0.88 30.009 30.008

VOC Score 20.05 10.06 0.74

Number of detections above LRL 20.04 10.07 1.00

Total number of VOCs detected 20.02 20.05 0.60

Trichloromethane (Chloroform) 20.05 0.19 0.98

1,1,1-Trichloroethane 20.04 30.004 0.21

1,1,2-Trichlorotrifluoroethane 30.002 30.0009 0.18

Methyl tert-butyl ether (MTBE) 0.37 0.20 0.61

Trichlorofluoromethane (CFC-11) 0.18 20.02 0.33

Trichloroethene (TCE) 10.07 10.08 0.77

Tetrachloroethene (PCE) 1.00 0.58 0.33

Bromodichloromethane 0.28 0.40 0.97

1,1-Dichloroethylene 0.84 0.58 0.61

Dichlorodifluoromethane 0.72 0.62 0.38

1,1-Dichloroethane 20.04 20.05 0.89

Chloromethane 0.10 20.03 0.51

Dibromochloromethane 0.49 0.14 0.12

cis-1,2-Dichloroethylene 0.49 0.60 0.97

1,2,4-Trimethylbenzene 0.87 0.37 0.33

Tetrahydrofuran 0.95 0.88 0.87
Main Basin, Orange County 39

–75

–70

–65

–60

–55

–50

–45

–40

–10.5 –10 –9.5 –9 –8.5 –8 –7.5 –7 –6.5 –6

Delta oxygen-18, in per mill

Delta deuterium, in per mill

Global meteoric water line (Craig, 1961)

Santa Ana River West flow path

Santa Ana River East flow path

Explanation

Santiago flow path

Santa Ana River West median

Santa Ana River East median

Santiago median

Figure 19. Relation between delta oxygen-18 and delta deuterium in ground water along flow paths in the Main Basin, Orange County, California.
40 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Table 8. Statistical correlations along ground-water flow paths in the Main Basin, Orange County, California, 2000.

[SARW, Santa Ana River West; SARE, Santa Ana River East; n, number of samples; nd, not detected] 

1 blue, p = 0.10 to 0.06 2 green, p = 0.05 to 0.02 3 yellow, p = 0.01 to < 0.0001

Variable

SARW n = 22 SARE n = 20 Santiago n = 10

Spearman 

p-value

Spearman's 

rho

Statistical 

Correlation

Spearman 

p-value

Spearman's 

rho

Statistical 

Correlation

Spearman 

p-value

Spearman's 

rho

Statistical 

Correlation

Correlations with distance down gradient:

Depth to top of openings 0.51 0.14 No 0.97 0.009 No 0.94 –0.02 No

Depth to mid-point of openings 0.62 0.11 No 0.59 0.12 No 20.03 20.73 Positve2

Depth to bottom of openings 0.95 0.01 No 0.73 0.08 No 20.02 20.78 Positve2

δDeuterium 0.86 –0.04 No 0.72 0.08 No 10.09 1–0.56 Inverse1

δOxygen-18 30.006 3–0.59 Inverse3 20.02 2–0.52 Inverse2 20.04 2–0.70 Inverse2

VOC Score 30.006 3–0.59 Inverse3 10.08 1–0.40 Inverse1 20.05 2–0.64 Inverse2

Number of detections above LRL 30.006 3–0.58 Inverse3 10.10 1–0.38 Inverse1 20.04 2–0.69 Inverse2

Total number of VOCs detected 30.008 3–0.56 Inverse3 0.13 –0.35 No 10.08 1–0.57 Inverse1

Trichloromethane (Chloroform) 30.01 3–0.55 Inverse3 0.17 –0.33 No 20.02 2–0.79 Inverse2

Methyl tert-butyl ether (MTBE) 30.003 3–0.65 Inverse3 0.20 –0.30 No 10.06 1–0.62 Inverse1

Trichloroethene (TCE) 30.002 3–0.67 Inverse3 0.50 –0.16 No 0.11 –0.52 No

Tetrachloroethene (PCE) 30.0006 3–0.73 Inverse3 10.10 1–0.39 Inverse1 nd nd nd

Trichlorofluoromethane (CFC-11) 20.02 2–0.52 Inverse2 0.60 –0.12 No nd nd nd

Correlations with depth to the mid-point of openings:

Distance down gradient 0.62 0.11 No 0.59 0.12 No 20.03 20.73 Positve2

δDeuterium 30.003 3–0.63 Inverse3 0.39 –0.20 No 10.09 1–0.56 Inverse1

δOxygen-18 30.004 3–0.62 Inverse3 20.03 2–0.51 Inverse2 20.02 2–0.76 Inverse2

VOC Score 0.51 0.14 No 0.13 –0.35 No 0.13 –0.50 No

Number of detections above LRL 0.49 0.15 No 0.20 –0.29 No 10.07 1–0.60 Inverse1

Total number of VOCs detected 0.63 0.10 No 10.07 1–0.41 Inverse1 0.18 –0.44 No

Trichloromethane (Chloroform) 0.32 0.22 No 0.25 –0.28 No 10.06 1–0.62 Inverse1

Methyl tert-butyl ether (MTBE) 0.19 –0.28 No 0.56 –0.14 No 20.04 2–0.68 Inverse2

Trichloroethene (TCE) 10.09 1–0.36 Inverse1 10.10 1–0.40 Inverse1 0.22 –0.41 No

Tetrachloroethene (PCE) 0.15 –0.30 No 10.10 1–0.39 Inverse1 nd nd nd

Trichlorofluoromethane (CFC-11) 0.16 0.31 No 30.01 3–0.60 Inverse3 nd nd nd
Main Basin, Orange County 41

Table 9. Volatile organic compounds detected in 51 ground-water samples from selected wells in the Main Basin, Orange County, California, 2000.

[LRL, laboratory reporting limit; μg/L, micrograms per liter; MCL, maximum contaminant level; —, no value available or applicable; E, concentration 

estimated] 

1 U.S. Environmental Protection Agency, 2001

2 California Department of Water Resources, 2001

3 Total Trihalomethanes, as of 01/01/02 (U.S. Environmental Protection Agency, 2001) 4 Methyl benzene concentrations may be due to sample contamination during collection and analysis 5 2 LRLs used during the sampling period

Constituent LRL, in μg/L

Number of 

detections at or 

above LRL

Number of 

detections below 

LRL

Minimum 

detection, in 

μg/L

Maximum 

detection, in 

μg/L

MCL, in 

μg/L

Number of 

wells 

exceeding a 

MCL

Total number of VOCs detected — —— 0 9 — —

VOC Score — —— 0.0 6.1 — —

Trichloromethane (Chloroform) 50.052, 0.024 22 11 E0.011 1.200 1,380 0

1,1,1-Trichloroethane 0.032 12 5 E0.004 0.334 1200 0

1,1,2-Trichlorotrifluoroethane 0.06 11 4 E0.01 2.54 21,200 0

Methyl tert-butyl ether (MTBE) 0.17 3 10 E0.06 0.23 213 0

Trichlorofluoromethane (CFC-11) 0.09 5 7 E0.02 7.98 2150 0

Trichloroethene (TCE) 0.038 6 5 E0.011 0.531 15 0

Tetrachloroethene (PCE) 0.1 2 5 E0.02 0.3 15 0

Bromodichloromethane 0.048 4 1 E0.019 0.167 1,380 0

1,1-Dichloroethylene 0.04 2 3 E0.02 0.11 6 0

Dichlorodifluoromethane 0.27 0 4 E0.05 E0.12 — —

1,1-Dichloroethane 50.066, 0.035 0 3 E0.014 E0.019 25 0

Chloromethane 50.5, 0.25 0 2 E0.04 E0.04 — —

Dibromochloromethane 0.18 0 1 E0.02 E0.02 1,380 0

cis-1,2-Dichloroethylene 0.038 0 1 E0.017 E0.017 26 0

1,2,4-Trimethylbenzene 0.056 0 1 E0.029 E0.029 — —

Tetrahydrofuran 2.2 0 1 E1.3 E1.3 — —

Methyl benzene4 0.05 0 4 E0.005 E0.02 2150 0
42 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000 Table 10. Comparison of detection frequencies of volatile organic compounds detected in 51 wells sampled in the Main Basin, Orange County, California, 2000. [LRL, laboratory reporting limit; SARW, Santa Ana River West; SARE, Santa Ana River East] Constituent Detection Frequency, in percent All detections Detections at or above LRL Detections below LRL Com- bined n = 51 SARW n = 22 SARE n = 20 Santi- ago n = 10 Com- bined n = 51 SAR W n = 22 SARE n = 20 Santi- ago n = 10 Com- bined n = 51 SAR W n = 22 SARE n = 20 Santi- ago n = 10 Trichloromethane (Chloroform) 65 73 55 60 43 55 30 40 22 18 25 20 1,1,1-Trichloroethane 33 50 25 10 24 41 15 0.0 10 9.1 10 10 1,1,2-Trichlorotrifluoroethane 29 59 10 0.0 22 45 5.0 0.0 7.8 14 5.0 0.0 Methyl tert-butyl ether (MTBE) 25 36 15 20 6 14 0.0 0.0 20 23 15 20 Trichlorofluoromethane (CFC-11) 24 41 15 0.0 10 14 10 0.0 14 27 5.0 0.0 Trichloroethene (TCE) 22 41 5.0 10 12 18 5.0 10 10 23 0.0 0.0 Tetrachloroethene (PCE) 12 23 5.0 0.0 3.9 9.1 0.0 0.0 9.8 18 5.0 0.0 Bromodichloromethane 10 18 0.0 10 7.8 14 0.0 10 2.0 4.5 0.0 0.0 1,1-Dichloroethylene 10 14 10 0.0 3.9 0.0 10 0.0 5.9 14 0.0 0.0 Dichlorodifluoromethane 7.8 14 0.0 10 0.0 0.0 0.0 0.0 7.8 14 0.0 10 1,1-Dichloroethane 5.9 14 0.0 0.0 0.0 0.0 0.0 0.0 5.9 14 0.0 0.0 Chloromethane 3.9 4.5 5.0 0.0 0.0 0.0 0.0 0.0 3.9 4.5 5.0 0.0 Dibromochloromethane 2.0 0.0 0.0 10 0.0 0.0 0.0 0.0 2.0 0.0 0.0 10 cis-1,2-Dichloroethylene 2.0 0.0 0.0 10 0.0 0.0 0.0 0.0 2.0 0.0 0.0 10 1,2,4-Trimethylbenzene 2.0 4.5 0.0 0.0 0.0 0.0 0.0 0.0 2.0 4.5 0.0 0.0 Tetrahydrofuran 2.0 4.5 0.0 0.0 0.0 0.0 0.0 0.0 2.0 4.5 0.0 0.0
Main Basin, Orange County 43

VOC Score

Distance in kilometers

Spearman's rho = –0.64

p = 0.05

Moderate inverse correlation

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Spearman's rho = –0.40

p = 0.08

Weak inverse correlation

Spearman's rho = –0.59

p = 0.006

Strong inverse correlation

0 5 10 15 20 25 30

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

A

B

C

Figure 20. VOC score versus distance along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, Main Basin, Orange County, 

California, 2000.
44 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000 Elevation, in meters above sea level

Distance, in kilometers

–500

–400

–300

–200

–100

0

100

200

0 5 10 15 20 25 30

–500

–400

–300

–200

–100

0

100

200

–500

–400

–300

–200

–100

0

100

200

Land surface

Land surface

Land surface

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

0.1

4.1 1.3

4.5

4.5

1.0

4.1 4.4

5.4

3.0

5.2

3.6

3.0

4.1

2.2

0.1

0.2

0.2

0.1

0.1

1.2

1.2

1.0

1.0

6.1

0.2

3.1

0.2

1.0 0.1

0.1

1.0

0.1

3.3

0.1

1.3

0.1

0.1

1.0

Explanation

Well openings, showing top and

bottom elevations; color indicates

flow path; shared wells show other

flow path color

VOC Score, greater than or equal

to 1.0

VOC Score, less than 1.0

4.3

0.1

Highest concentration

VOC score = 0.0; no VOCs

detected

A

B

C

Figure 21. VOC scores in wells along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, Main Basin, Orange County, 

California, 2000.
Main Basin, Orange County 45

The spatial distribution of VOC scores on the 

SAR west flow path shows three groups of relatively 

high numbers of VOCs: group I shallow wells and a 

deep well in the proximal forebay, group II shallow 

wells that tap the upper aquifer in the distal forebayproximal pressure area, and group III deeper wells that 

tap the lower aquifer in the pressure area (fig. 21A). 

The high VOC score in each group is about the same. 

This pattern suggests that that the number of VOCs 

reaching ground water has remained about the same for 

a period of time. Farther along the flow path in the 

pressure area, VOC scores are all below 1.0, indicating 

older water that probably recharged before the 1940s. 

In contrast to the Central Basin flow paths, the most 

distal wells on the SAR west and east flow paths 

contain VOCs at low concentrations, with 

nondetections located just upgradient of these low 

VOC scores. These low VOC scores at the distal end of 

the flow paths may be showing the influence of 

injection wells near the coast.

VOC scores on the SAR east flow path have a 

different distribution than VOC scores on the SAR west 

flow path. On the SAR east flow path, only one group 

of high VOC scores was found in the shallow wells in 

the distal forebay-proximal pressure area (fig. 21B). 

This area corresponds to group II on the SAR west flow 

path in shallow wells between 9 and 15 km (fig. 21A). 

Group I on the SAR west flow path may not have been 

sampled on the SAR east flow path because no wells 

are at the same distance (between 2 and 5 km). There is 

no corresponding group III on the SAR east flow path; 

deeper wells at that distance (15 to 19 km) have low or 

no VOC scores (fig. 21B). The group of VOC scores of 

0.0 in the pressure area (16 to 21 km) on the SAR east 

flow path includes the well at the end of the Santiago 

flow path. This 0.0 group may be showing the 

influence of the Santiago flow system on the SAR 

ground water chemistry.

The pattern of VOC scores along the Santiago 

flow path is different from the SAR flow paths. On the 

Santiago flow path, the highest number of VOCs was 

detected in the well closest to the engineered recharge 

facilities, and the most distal wells contained no VOCs 

detectable (fig. 21C). This pattern suggests that more 

VOCs are currently reaching ground water along the 

Santiago flow path than in the past, which may reflect 

the shorter operation time of the Santiago Creek 

recharge facilities compared to the Santa Ana River 

recharge facilities.

Chloroform

Chloroform was detected in 65 percent of the 

wells (table 10). It was the most frequently detected 

compound on all three flow paths, for all detections, 

and for detections above LRL. Chloroform 

concentrations were statistically higher on the SAR 

west flow path than on the SAR east and Santiago flow 

paths (figs. 22 and 23). There was no statistical 

difference in chloroform concentrations between the 

Santiago flow path and the SAR west or east flow paths 

(table 7).

There were inverse correlations between 

chloroform concentration and distance along the SAR 

west and Santiago flow paths, with more nondetections 

and lower concentrations in the distal wells. No 

correlation was found along the SAR east flow path 

(table 8 and fig. 22B). The highest chloroform 

concentrations on the SAR West flow path occurred at 

a distance of about 16 km in the pressure area, and no 

detections above LRL occurred more than 18 km 

downgradient (fig. 22A). On the Santiago flow path, 

chloroform concentrations decreased monotonically 

with distance, and no chloroform was detected at more 

than 6 km (fig. 22C). Chloroform concentrations at 

concentrations below LRL were all along the SAR east 

flow path, with the exception of wells between 15 and 

21 km, which had no detectable chloroform (figs. 22B

and 23B).

There was no correlation between chloroform 

concentration and depth on the SAR west or east flow 

paths (table 8). However, along the SAR west flow 

path, the highest concentrations were in deeper wells in 

the pressure area (fig. 23A). There was a slight decrease 

in chloroform concentration with increasing depth 

along the Santiago flow path, which may be due to the 

correlation between distance and well depth along the 

Santiago flow path (table 8).
46 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

0.001

0.01

0.1

1

10

0 5 10 15 20 25 30

Distance in kilometers

Santiago

Spearman's rho = –0.79

p = 0.02

Moderate Inverse

Correlation

Chloroform concentration, in micrograms per liter

Santa Ana River East

Spearman's rho = –0.33

p = 0.17

No Correlation

Spearman's rho = –0.55

p = 0.01

Strong Inverse

Correlation

Santa Ana River West

LRL = 0.052 mg/L

0.001

0.01

0.1

1

10

0.001

0.01

0.1

1

10

Public Supply Well Detection,

quanitified

Public Supply Well Detection,

estimated

Public Supply Well Non-Detect,

plotted at one quarter of the

less-than value

Explanation

Color indicates flow path

LRL = 0.052 mg/L

LRL = 0.052 mg/L

A

B

C

Figure 22. Trichloromethane (chloroform) concentration versus distance along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow 

paths, Main Basin, Orange County, California, 2000.
Main Basin, Orange County 47

Elevation, in meters above sea level

Distance, in kilometers

–500

–400

–300

–200

–100

0

100

200

0 5 10 15 20 25 30

–500

–400

–300

–200

–100

0

100

200

–500

–400

–300

–200

–100

0

100

200

Land surface

Land surface

Land surface

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

.12

.12

.34

.90

1.2

.10

.12

.02

.01

-- --

--

.02

.05

-- .02

.01 --

--

.02

.08

.02

--

--

--

.20

.08

.15

.02

.01

.02

--

-- -- --

--

.04

.02

.04

.07

.02 -- --

--

--

.03 .02

.05

.03 .10

--

.05

--

Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Nondetect

Explanation

.12

.02

--

Well openings, showing top and

bottom elevations; color indicates

flow path; shared wells show other

flow path color

Highest concentration

A

B

C

Figure 23. Trichloromethane (chloroform) concentrations in wells along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, 

Main Basin, Orange County, California, 2000.
48 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

The distribution of chloroform concentrations on 

the SAR west flow path (fig. 23A) shows one group of 

high concentrations in deeper wells in the pressure 

area, suggesting that current concentrations of 

chloroform reaching the ground water are lower than in 

the past. Low concentrations were in the forebay and 

pressure areas. Most nondetections were in the pressure 

area, mixed with low concentrations.

Chloroform concentrations in wells on the SAR 

east flow path are all below 0.08 and most are below 

the LRL (fig. 22B). At the same distance and depth that 

high concentrations were on the SAR west flow path, 

nondetections were on the SAR east flow path 

(figs. 22A and B). As with VOC score, this may be 

showing the diluting influence of the Santiago flow 

system. The most distal wells on both SAR flow paths 

contain low concentrations, and wells upgradient, had 

nondetections, perhaps showing an influence from the 

coastal injection wells (figs. 23A and B).

On the Santiago flow path (fig. 23C), the highest 

chloroform concentration was in the most upgradient 

well in the forebay area of the Santiago flow path. As 

with the VOC scores, this suggests that chloroform is 

currently reaching ground water in the proximal part of 

the flow path.

MTBE

MTBE was detected in 25 percent of the wells 

(table 10). It was detected most frequently on the SAR 

west flow path, and was only detected below LRL on 

the Santiago and SAR east flow paths. MTBE 

concentrations were not significantly different between 

any Main Basin flow path (table 7).

MTBE concentrations were inversely correlated 

with distance along the SAR west and Santiago flow 

paths, with the highest MTBE concentrations found 

proximal to the recharge facilities (figs. 24 and 25). On 

the SAR east flow path, MTBE was detected at low 

concentrations in three wells in the proximal part of the 

flow path. No MTBE was detected in wells more than 

17 km along the SAR west flow path, more than 11 km 

along the SAR east flow path, or more than 2 km along 

the Santiago flow path (fig. 25). No correlation was 

found on either Santa Ana River flow path between 

MTBE concentration and depth. MTBE concentration 

did decrease with depth to the midpoint of well 

openings along the Santiago flow path; as with 

chloroform, this correlation may be due to the 

correlation between distance and well depth on the 

Santiago flow path (table 8).

The spatial distribution of MTBE on the SAR 

flow paths is similar to VOC score on each flow path. 

However, the highest concentration on the SAR west 

flow path was in the proximal forebay area, and the 

distal wells on both flow paths contained no MTBE 

(fig. 25A). The highest concentration on the SAR east 

flow path was in a shallow well in the proximal 

pressure area (fig. 25B). On the Santiago flow path, the 

two detections of MTBE were in shallow wells in the 

forebay (fig. 25C). Detections are mixed with 

nondetections on all three flow paths. The inverse 

correlations with distance and the predominance of 

detections in the forebay areas of all the flow paths 

indicate that MTBE is currently reaching ground water 

in the Main Basin.

TCE

TCE was detected in 22 percent of the wells 

(table 10). TCE was detected most frequently on the 

SAR west flow path, and only detected once on the 

SAR east and Santiago flow paths (figs. 26 and 27). 

There is an inverse correlation between TCE 

concentration and distance along the SAR west flow 

path, with the highest concentrations between 9 and 

11 km, and no detections more than 17 km (figs. 26A

and 27A). The two highest concentrations were 

detected in shallow wells associated with mapped TCE 

plumes (fig. 27A). Low concentrations were detected in 

wells near the Santa Ana River recharge facilities. TCE 

concentration decreased significantly with depth along 

the SAR west flow path (table 8). The one detection of 

TCE on the SAR east flow path occurred at about 

10 km, just outside of a mapped TCE plume (fig. 27B) 

(Orange County Water District, 1991a, fig. 9; Orange 

County Water District, 1991b, fig. 7). The one 

detection on the Santiago flow path was in the most 

upgradient well, near the Santiago Creek recharge 

facility (fig. 27C).
Main Basin, Orange County 49

Spearman's rho = –0.62

p = 0.06

Weak inverse correlation

MTBE concentration, in micrograms per liter

Spearman's rho = –0.30

p = 0.20

No correlation

Spearman's rho = –0.65

p = 0.003

Strong inverse correlation

0.01

0.1

1

0.01

0.1

1

0 5 10 15 20 25 30

Distance in kilometers

0.01

0.1

1

LRL = 0.17 mg/L

LRL = 0.17 mg/L

LRL = 0.17 mg/L

Public supply well

Detection, quanitified

Public supply well

Detection, estimated

Public supply well

non-detect, plotted at

one quarter of the lessthan value

Explanation

Color indicates flow path

A

B

C

Figure 24. Methyl tert-butyl ether (MTBE) concentration versus distance along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow 

paths, Main Basin, Orange County, California, 2000.
50 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000 Elevation, in meters above sea level

Distance, in kilometers

–500

–400

–300

–200

–100

0

100

200

0 5 10 15 20 25 30

–500

–400

–300

–200

–100

0

100

200

–500

–400

–300

–200

–100

0

100

200

Land surface

Land surface

Land surface

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

.21

.12

.10

.06

-- --

--

.15

--

--

-- --

--

--

--

-- -- --

.08

.16

-- --

--

.23 .08

.12

--

.18

--

--

--

--

--

--

--

--

--

--

-- --

--

--

--

-- --

--

.09 .07

--

--

--

Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Nondetect

Explanation

.18

.08

--

Well openings, showing top and

bottom elevations; color indicates

flow path; shared wells show other

flow path color

Highest concentration

A

B

C

Figure 25. Methyl tert-butyl ether (MTBE) concentrations in wells along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, 

Main Basin, Orange County, California, 2000.
Main Basin, Orange County 51

Spearman's rho = –0.52

p = 0.11

No correlation

TCE concentration, in micrograms per liter

LRL = 0.038 mg/L

Spearman's rho = –0.16

p = 0.50

No correlation

LRL = 0.038 mg/L

Spearman's rho = -0.67

p = 0.002

Strong inverse correlation

0.01

0.1

1

0.001

0.01

0.1

1

0.001

0.01

0.1

1

0.001

0 5 10 15 20 25 30

Distance in kilometers

LRL = 0.038 mg/L

Public supply well detection,

quanitified

Public supply well detection,

estimated

Public supply well non-detect,

plotted at one quarter of the

less-than value

Explanation

Color indicates flow path

A

B

C

Figure 26. Trichloroethene (TCE) concentration versus distance along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, 

Main Basin, Orange County, California, 2000.
52 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Well located inside mapped area

of TCE plume

Elevation, in meters above sea level

Distance, in kilometers

–500

–400

–300

–200

–100

0

100

200

0 5 10 15 20 25 30

–500

–400

–300

–200

–100

0

100

200

–500

–400

–300

–200

–100

0

100

200

Land surface

Land surface

Land surface

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

.45

.06

.01

-- --

--

--

--

-- --

--

--

--

-- -- --

--

.22

-- --

--

.02

.53

--

.06

--

--

--

--

--

--

--

--

--

-- --

--

--

--

-- --

--

.05

--

--

--

--

--

--

.01 Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Non-detect

Explanation

.53

.02

--

Well openings, showing top and

bottom elevations; color indicates

flow path; shared wells show other

flow path color

.02

.02

Highest concentration

--

A

B

C

Figure 27. Trichloroethene (TCE) concentrations in wells along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, Main 

Basin, Orange County, California, 2000.
Main Basin, Orange County 53

The patterns of TCE occurrence along the Main 

Basin flow paths suggest that higher TCE 

concentrations in ground water are associated with 

contaminant plumes. However, low concentrations 

were detected upgradient of the mapped TCE plumes 

and in deeper wells, suggesting another source of TCE 

to ground water in the Main Basin. Despite the 

presence of a TCE plume along the SAR east flow 

path, there was only one detection. No TCE was 

detected in distal wells on the SAR west, SAR east, or 

Santiago flow paths (fig. 27).

PCE

PCE was detected in 12 percent of the wells 

(table 10). It was detected most frequently on the SAR 

west flow path, with the highest concentrations in wells 

near the Santa Ana River recharge facilities. On the 

SAR east flow path, PCE was detected at a low 

concentration in the most upgradient well, and in no 

wells on the Santiago flow path (figs. 28 and 29). An 

inverse correlation was found between PCE 

concentration and distance along the SAR west flow 

path; all detections occurred at less than 12 km, with 

the highest concentrations at less than 5 km (fig. 29A). 

No correlation between PCE concentration and depth 

was found on the SAR west flow path (table 8).

PCE concentrations in the Main Basin have 

similar distributions to MTBE. The highest measured 

concentrations on the SAR west flow path were in 

proximal wells, and the only detection on the SAR east 

flow path was in the most proximal well. No detections 

were in the pressure area or on the Santiago flow path. 

This pattern suggests that a recent source of PCE is 

currently reaching ground water on the SAR flow paths 

in the forebay, but not on the Santiago flow path.

CFC-11

CFC-11 was detected in 24 percent of the wells 

(table 10). Detection frequency was higher on the SAR 

west flow path than on the SAR east flow path; CFC-11 

was not detected on the Santiago flow path (table 10

and figs. 30 and 31). There was an inverse correlation 

between CFC-11 concentration and distance along the 

SAR west flow path (table 8), with the highest 

concentration occurring at 11 km (fig. 30A). No 

correlation was found between concentration and 

distance on the SAR east flow path, although the 

highest concentration also occurred near 11 km in a 

deeper well (fig. 30B). CFC-11 concentrations did 

decrease significantly with depth on the SAR east flow 

path, but showed no correlation with depth on the SAR 

west flow path (table 8).

The pattern of CFC-11 detections on both SAR 

flow paths is similar to that of the VOC score. 

Detections occur in middle distance wells (figs. 31A

and B), suggesting that higher concentrations of 

CFC-11 were reaching ground water at some time in 

the past. The nondetections in most of the proximal 

wells on all three flow paths suggests that CFC-11 is 

not currently reaching ground water at detectable 

concentrations.
54 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

0.01

0.1

1

Distance in kilometers

LRL = 0.1 mg/L

PCEconcentration, in micrograms per liter

LRL = 0.1 mg/L

Spearman's rho = –0.09

p = 0.68

No correlation

Spearman's rho = –0.73

p = 0.0006

Strong inverse correlation

0.01

0.1

1

0.01

0.1

1

0 5 10 15 20 25 30

LRL = 0.1 mg/L

No detections

Public supply well

detection, quanitified

Public supply well

detection, estimated

Public supply well

Non-detect, plotted at

one quarter of the

less-than value

Explanation

Color indicates flow path

A

B

C

Figure 28. Tetrachloroethene (PCE) concentration versus distance along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, 

Main Basin, Orange County, California, 2000.
Main Basin, Orange County 55

Elevation, in meters above sea level

Distance, in kilometers

–500

–400

–300

–200

–100

0

100

200

0 5 10 15 20 25 30

–500

–400

–300

–200

–100

0

100

200

–500

–400

–300

–200

–100

0

100

200

Land surface

Land surface

Land surface

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

.18

.04

--

-- --

--

--

--

-- --

--

--

--

-- -- --

-- --

--

.25 .02

.02

-- --

--

--

--

--

--

--

--

--

-- --

--

--

--

-- --

--

--

--

--

--

--

--

.03

Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Not detected

Explanation

.25

.02

--

Well openings, showing top and

bottom elevations; color indicates

flow path; shared wells show other

flow path color

--

--

.03

--

--

Highest concentration

--

A

B

C

Figure 29. Tetrachloroethene (PCE) concentrations in wells along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow paths, Main 

Basin, Orange County, California, 2000.
56 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

0.01

0.1

1

10

Distance in kilometers

LRL = 0.09 mg/L

CFC-11 concentration, in micrograms per liter

Spearman's rho = –0.17

p = 0.47

No correlation

LRL = 0.09 mg/L

Spearman's rho = –0.52

p = 0.02

Moderate inverse

correlation

LRL = 0.09 mg/L

0.01

0.1

1

10

0 5 10 15 20 25 30

0.01

0.1

1

10

No detections

Public supply well

detection, quanitified

Public supply well

detection, estimated

Public supply well

non-detect, plotted at

one quarter of the

less-than value

Explanation

Color indicates flow path

A

B

C

Figure 30. Trichlorofluoromethane (CFC-11) concentrations in wells along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow 

paths, Main Basin, Orange County, California, 2000.
Main Basin, Orange County 57

Elevation, in meters above sea level

Distance, in kilometers

–500

–400

–300

–200

–100

0

100

200

0 5 10 15 20 25 30

–500

–400

–300

–200

–100

0

100

200

–500

–400

–300

–200

–100

0

100

200

Land surface

Land surface

Land surface

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

Engineered

recharge

Forebay

area

Pressure

area

.05

.05

--

-- --

--

--

--

-- --

--

--

--

-- -- --

-- --

--

-- --

.03

.16 --

--

.09

.04

.04

--

--

--

--

8.0 .15

1.4

--

--

-- --

--

--

--

--

--

--

--

-- Concentration, in micrograms

per liter, quanitified

Concentration, in micrograms

per liter, estimated

Not detected

Explanation

1.4

.02

--

Well openings, showing top and

bottom elevations; color indicates

flow path; shared wells show other

flow path color

--

.04

--

--

--

.02

Highest concentration

A

B

C

Figure 31. Trichlorofluoromethane (CFC-11) concentrations in wells along the (A) Santa Ana River west, (B) Santa Ana River east, and (C) Santiago flow 

paths, Main Basin, Orange County, California, 2000.
58 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

BUNKER HILL BASIN, SAN BERNARDINO 

COUNTY

Hydrogeology

Two ground-water flow paths were selected in 

the Bunker Hill Basin of the inland Santa Ana groundwater basin (fig. 32). Both paths begin near engineered 

recharge facilities located along the San Bernardino 

Mountain front, and end near the San Jacinto Fault. The 

SAR flow path is located along of the Santa Ana River, 

and the Newmark flow path is located along the Warm 

and the East Twin creeks. The ground-water flow 

system in the Bunker Hill Basin is radially convergent. 

Ground water recharges at several points along the 

upgradient edges of the basin (proximal area) and 

moves to a focused area of discharge near the San 

Jacinto Fault (distal area). The distal parts of the flow 

paths are confined (fig. 32). However, Dutcher and 

Garrett (1963, p. 28 and 63) describe the uppermost 

confining unit near land surface as discontinuous and 

ranging from semiconfining to confining. Duell and 

Schroeder (1989, p. 9) describe the confining units at 

depth in the aquifer system as poorly permeable and 

leaky.

Recharge to the flow system varies seasonally 

and is largely from infiltration of runoff from streams 

exiting the San Gabriel and San Bernardino Mountains 

(fig. 32). Much of the runoff is diverted to engineered 

recharge facilities along the mountain front, in or 

adjacent to stream channels. These facilities have been 

in operation since the early 1900s using runoff from the 

San Bernardino Mountains to the north and the San 

Gabriel Mountains to the west (Moreland, 1972), and 

imported water from northern California in the 1970s 

and 1980s (Hardt and Freckleton, 1987). The flow 

paths are influenced by recharge facilities on the East 

Twin Creek, Santa Ana River, Mill Creek, and, to a 

lesser extent, Warm Creek and City Creek. Groundwater discharge is primarily by pumping.

The Bunker Hill Basin flow paths were selected 

to approximately follow ground-water gradients based 

on water-level contours (fig. 33) (Wildermuth 

Environmental, Inc., 2000). Depths to water range from 

tens of meters near the flanks of mountains to near land 

surface along rivers and in wetland areas in the distal 

area of the flow paths. Well information for wells 

sampled in the Bunker Hill Basin is given in table 11. 

Wells on two different flow paths were selected in the 

Bunker Hill Basin. The Newmark flow path (6 km 

long) follows the radial flow that originates near the 

East Twin Creek recharge facilities (fig. 32). Ten 

monitoring wells and 1 public supply well were 

sampled on the Newmark flow path. The SAR flow 

path (16 km long) follows the radial flow that 

originates near the Santa Ana River and Mill Creek 

recharge facilities. Ten monitoring wells and 5 public 

supply wells were sampled on the SAR flow path.

Distance downgradient along the Bunker Hill 

flow paths was measured relative to a point of 

convergence on the San Jacinto Fault (fig. 34). The 

convergence point is roughly 16 km from the recharge 

area on the SAR flow path, but is only about 11 km 

from the recharge area on the Newmark flow path. For 

consistency with distance measurements in the coastal 

plain flow paths, well distances increase in the 

downgradient direction. Consequently, in Bunker Hill, 

a well at a distance of 10 km on the SAR flow path is 

10 km from the recharge area, but on the Newmark 

flow path a well at 10 km is only about 5 km from the 

recharge area.

The Newmark path has flow lines running north 

to south, and the Santa Ana River path has flow lines 

running east to west. The distributions of wells on the 

Bunker Hill flow paths differ. Wells on the Newmark 

path are located in the middle and downgradient parts 

of that flow path, with no wells in the upgradient area. 

Wells on the SAR flow path are located upgradient and 

downgradient, with no wells in the middle part of that 

flow path. For this investigation, these two flow paths 

were analyzed as one, increasing the coverage of wells 

along the flow paths and the number of wells available 

for analysis. To justify the combining of the two flow 

paths, statistical comparisons were made along the 

lower parts of both flow paths (distances greater than 

11 km) where the distribution of wells was the same. 

No statistical difference was found between the flow 

paths, except in stable isotopes and benzene occurrence 

(table 12). Depths to the tops and midpoints of the 

wells are similar between both flow paths; the bottoms 

of well openings are significantly deeper on the SAR 

flow path (table 12). There was no correlation between 

well depths and distance downgradient along the 

Bunker Hill flow paths (table 13).
Bunker Hill Basin, San Bernardino County 59

117° 7' 30" 117° 15'

34°

7'

30"

34°

3'

45"

San Jacinto Fault

San Andreas Fault

Santa Ana River

Bunker Hill Basin

Rivers and streams

Faults

Recent unconsolidated sediments

Consolidated deposits

Leaky confined area (Hardt and

Freckleton, 1987)

Historic Marsh area (Hardt and

Freckleton, 1987)

BH 1.1-1.2

BH 2

BH 3

BH 4

BH 5.1-5.4

Monitoring well, Newmark flow path

Monitoring well, Santa Ana River flow path

Public supply well, Santa Ana River flow path

Public supply well, Newmark flow path

Recharge facility

BH 8

BH 9

BH 11.1-11.4

BH 6.1-6.3

BH 7.1-7.3

BH 12.1-12.4

BH 10

BH 2

BH 10

BH 1.1-1.2

BH 12.1-12.4

Explanation

San Bernardino Mountains

East Twin Creek

Warm Creek

City Creek

Mill Creek

Warm Creek

Lytle Creek

Santa Ana River

Newmark Flow Path

Santa Ana River Flow Path

Sand Creek

0 5 Kilometers

0 5 Miles

Figure 32. Locations of wells sampled and major hydrologic features of the Bunker Hill Basin, San Bernardino County, California.
60 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

117° 117 7' 30" ° 15'

34°

7'

30"

34°

3'

45"

San Jacinto Fault

San Andreas Fault

Bunker Hill Basin

Faults

Monitoring well, Newmark flow path

Monitoring well, Santa Ana River flow path

Public supply well, Santa Ana River flow path

Public supply well, Newmark flow path

Recharge facility

Ground-water level contour, in meters above sea level,

contour interval 25 meters (Wildermuth Environmental, Inc., 2000).

Explanation

Approximate boundary between flow paths

Santa Ana River Flow Path

San Bernardino Mountains

Recent Unconsolidated Sediments

Consolidated Deposits

BH 2

BH 3

BH 4

BH 8

BH 9

BH 2

BH 1.1-1.2

BH 5.1-5.4

BH 11.1-11.4

BH 1.1-1.2

BH 10

BH 10

Newmark Flow Path

BH 6.1-6.3

BH 7.1-7.3

BH 12.1-12.4

BH 12.1-12.4

Mapped area of trichloroethene

plume in ground water, (Wildermuth

Environmental Inc., 2000b)

0 5 Kilometers

0 5 Miles

Figure 33. Ground-water level contours and well locations in the Bunker Hill Basin, San Bernardino County, California.
Bunker Hill Basin, San Bernardino County 61

Table 11. Wells sampled in the Bunker Hill Basin, San Bernardino County, California, 2000.

Site Number USGS Site ID State well number

Distance down 

gradient, in kilometers

Depth to middle of openings, in meters below 

land surface

BH 1.1 340541117074402 001S002W07Q002S 0.4 155

BH 1.2 340541117074401 001S002W07Q001S 0.4 238

BH 2 340426117092701 001S003W23A005S 3.0 119

BH 3 340623117104001 001S003W03Q003S 4.4 244

BH 4 340505117101901 001S003W14E001S 4.6 70

BH 5.1 340503117104105 001S003W15K005S 5.1 152

BH 5.2 340503117104104 001S003W15K004S 5.1 232

BH 5.3 340503117104103 001S003W15K003S 5.1 287

BH 5.4 340503117104102 001S003W15K002S 5.1 120

BH 6.1 340707117162708 001S004W02D008S 10.0 55

BH 6.2 340707117162707 001S004W02D007S 10.0 113

BH 6.3 340707117162706 001S004W02D006S 10.0 160

BH 7.1 340615117170904 001S004W10B004S 11.9 34

BH 7.2 340615117170903 001S004W10B003S 11.9 94

BH 7.3 340615117170902 001S004W10B002S 11.9 210

BH 8 340423117155201 001S004W23H001S 13.2 48

BH 9 340438117162301 001S004W23C003S 14.3 191

BH 10 340437117170301 001S004W22B006S 14.5 107

BH 11.1 340408117165304 001S004W22J004S 14.7 11

BH 11.2 340408117165303 001S004W22J003S 14.7 76

BH 11.3 340408117165302 001S004W22J002S 14.7 162

BH 11.4 340408117165301 001S004W22J001S 14.7 259

BH 12.1 340439117173907 001S004W22D007S 15.0 8

BH 12.2 340439117173902 001S004W22D002S 15.0 55

BH 12.3 340439117173905 001S004W22D005S 15.0 172

BH 12.4 340439117173904 001S004W22D004S 15.0 199
62 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

117° 117 7' 30" ° 15'

34°

7'

30"

34°

3'

45"

San Jacinto Fault

San Andreas Fault

Bunker Hill Basin

Faults

BH 1.1

BH 1.2

BH 2

BH 3

BH 4 BH 5.1

BH 5.2

BH 5.3

BH 5.4

Monitoring well, Newmark flow path

Monitoring well, Santa Ana River flow path

Public supply well, Santa Ana River flow path

Public supply well, Newmark flow path

Recharge facility

BH 9

BH 11.1

BH 11.2

BH 11.3

BH 11.4

BH 7.1

BH 7.2

BH 7.3

BH 12.1

BH 12.2

BH 12.3

BH 12.4

BH 2

BH 10

BH 1.1

BH 12.1

0 15 14 13 12 10 11 9 8 765 4321

15 14 13 12 10 11 9

8

7 6

0

16

5

2

5

Distance Scale, in kilometers

Recent Unconsolidated Sediments

Consolidated Deposits

15 14 13 12

Explanation

0 5 Kilometers

0 5 Miles

BH 10

BH 8

BH 6.1

BH 6.2

BH 6.3

Figure 34. Well locations and distance measurement along flow paths in the Bunker Hill Basin, San Bernardino County, California.
Bunker Hill Basin, San Bernardino County 63

Table 12. Statistical differences between the Newmark and Santa Ana River ground-water flow paths, Bunker 

Hill Basin, San Bernardino County, California, 2000.

[LRL, laboratory reporting limit; VOC, Volatile Organic Compounds] 

1 blue, p = 0.10 to 0.06 2 green, p = 0.05 to 0.02 3 yellow, p = 0.01 to < 0.0001

Variable

All Wells Wells from 11 to 16 kilometers

Rank-sum p-value Rank-sum p-value

Distance 20.03 0.84

Depth to top of openings 0.30 1.00

Depth to mid-point of openings 0.15 0.95

Depth to bottom of openings 10.09 0.70

δ Oxygen-18 30.0005 20.05

δ Deuterium 30.0004 20.03

VOC Score 0.12 0.70

Number of detections above LRL 0.10 0.69

Total number of VOCs detected 0.19 1.00

Trichlorofluoromethane (CFC-11) 0.74 0.89

Tetrachloroethene (PCE) 0.87 0.69

Trichloromethane (Chloroform) 0.53 0.30

Trichloroethene (TCE) 0.90 0.33

Methyl tert-butyl ether (MTBE) 20.02 0.15

Benzene 20.03 10.08

1,2-Dimethylbenzene 30.01 0.24

1,4-Dimethylbenzene 30.01 0.24

cis-1,2-Dichloroethene 0.77 0.81

Bromodichloromethane 0.77 0.24

1,1-Dichloroethene 0.90 0.93

Ethylbenzene 20.04 0.24

1,2-Dichloropropane 0.14 0.53

Carbon Disulfide 0.84 0.42

Tribromomethane 0.40 0.24

Chlorodibromomethane 0.35 0.24

Dichlorodifluoromethane 0.40 0.79

2-Ethyltoluene 10.10 0.24

1,2,4-Trimethylbenzene 20.04 0.24

Isopropylbenzene 0.10 0.24

n-Propylbenzene 0.10 0.24

Chloromethane 0.36 0.37

1,1-Dichloroethane 0.64 0.71

1,1,1-Trichloroethane 0.78 1.00

1,2-Dichlorobenzene 10.02 0.13

Dibromomethane 0.10 0.24

Napthalene 0.28 0.47

1,2,3,4-Tetramethylbenzene 0.28 0.47

sec-Butylbenzene 0.28 0.47

1,1,1,2-Tetrachloroethane 0.91 0.47

Chlorobenzene 1.00 1.00

1,4-Dichlorobenzene 0.18 0.20

1,2,3-Trimethylbenzene 0.28 0.47

1,3,5-Trimethylbenzene 0.10 0.24

Bromochloromethane 0.10 0.24

n-Butylbenzene 1.00 1.00

p-Isopropyltoluene 0.18 0.20
64 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Table 13. Statistical correlations along ground-water flow paths in the Bunker Hill Basin, San 

Bernardino County, California, 2000.

[VOC, Volatile Organic Compounds]

1 blue, p = 0.10 to 0.06 2 green, p = 0.05 to 0.02 3 yellow, p = 0.01 to < 0.0001

Variable Spearman 

p-value

Spearman's 

rho

Statistical 

Correlation

Correlations with distance down gradient:

Depth to top of openings 0.19 0.26 No

Depth to mid-point of openings 0.13 0.30 No

Depth to bottom of openings 0.12 0.32 No

δ Oxygen-18 30.01 30.54 Positive3

δ Deuterium 30.01 30.53 Positive3

VOC Score 10.06 10.37 Positive1

Number of detections above LRL 10.06 10.37 Positive1

Total number of VOCs detected 0.12 0.31 No

Trichloromethane (Chloroform) 0.81 –0.05 No

Methyl tert-butyl ether (MTBE) 20.04 20.40 Positive2

Trichloroethene (TCE) 0.40 0.17 No

Tetrachloroethene (PCE) 0.19 0.26 No

Trichlorofluoromethane (CFC-11) 30.008 30.53 Positive3

Correlations with depth to the mid-point of openings:

Distance down gradient 0.13 0.30 No

δ Oxygen-18 10.10 1–0.35 Inverse1

δ Deuterium 10.07 1–0.39 Inverse1

VOC Score 10.09 1–0.34 Inverse1

Number of detections above LRL 20.05 2–0.40 Inverse2

Total number of VOCs detected 0.36 –0.18 No

Trichloromethane (Chloroform) 0.64 –0.09 No

Methyl tert-butyl ether (MTBE) 0.34 –0.19 No

Trichloroethene (TCE) 20.05 2–0.40 Inverse2

Tetrachloroethene (PCE) 0.69 –0.08 No

Trichlorofluoromethane (CFC-11) 0.31 –0.20 No
Bunker Hill Basin, San Bernardino County 65

Stable Isotopes

A plot of δ18O in relation to δD measured in 

wells along the SAR and Newmark flow paths is shown 

in figure 35. Ground-water values from all wells plot 

along the Global Meteoric Water Line (Craig, 1961). 

Delta oxygen-18 and δD are significantly lighter along 

the SAR flow path than along the Newmark flow path 

(fig. 35). This difference may be related to the elevation 

of the drainage basins of the creeks that recharge the 

aquifer system. Elevations of the San Bernardino 

Mountains increase from west to east, suggesting that 

recharge water to the SAR flow path may originate 

from a higher elevation than recharge water to the 

Newmark flow path, and would, therefore, have a 

lighter isotopic signature.

Volatile organic compounds

Twenty-seven VOCs were detected above LRL 

in ground water in the Bunker Hill Basin; an additional 

11 VOCs were detected only below LRL (table 14). 

Sixty-five percent (17 of 26) of the wells sampled 

contained one or more VOCs above LRL; an additional 

15 percent (4 of 26) of the wells had one or more VOCs 

only below LRL. Nineteen percent (5 of 26) of the 

wells sampled had no VOC detections. All but two of 

the VOC detections—trichloroethene and benzene—

along Bunker Hill Basin flow paths were below federal 

or state MCLs (table 14) (California Department of 

Health Services, 2001; U.S. Environmental Protection 

Agency, 2001 ). Of VOCs that had MCLs, 78 percent 

of all detections were less than 1 percent of their MCL 

concentrations. The 5 compounds discussed in this 

report (chloroform, MTBE, TCE, PCE, and CFC-11) 

were detected on both flow paths and are the most 

frequently detected VOCs in the Bunker Hill Basin 

(table 15).

VOC Score

VOC score showed a positive correlation with 

distance downgradient, increasing monotonically along 

the flow paths from proximal to distal areas (fig. 36A). 

VOC scores were statistically higher in the shallower 

wells (table 13), although high numbers of VOCs were 

detected at all well depths (fig. 36B).

The spatial distribution of VOC scores on the 

Bunker Hill flow paths is the opposite of the general 

patterns found in the Central and Main Basins. The 

higher numbers of VOCs detected in more distal wells 

suggest that either (1) the number of VOCs currently 

reaching ground water in the proximal part of the flow 

paths is much lower than in the past, or (2) VOCs are 

being contributed to ground water all along the flow 

paths. If the VOCs detected in deep wells in the distal 

part of the flow path were introduced at land surface in 

the distal area, then the confining units are not 

insulating the ground water from compounds 

introduced at land surface in the Bunker Hill Basin. 

Therefore, as ground water flows downgradient, it is 

continually picking up VOCs. It is also possible that 

pumping wells are selectively removing cleaner ground 

water, thus increasing the amount of VOCs in the 

remaining ground water in the distal parts of the flow 

system.

Chloroform 

Chloroform was detected in 27 percent of the 

wells (table 15). Unlike the coastal basins, chloroform 

was not the most frequently detected VOC. The lower 

detection frequency may reflect the mountain source of 

recharge to the Bunker Hill Basin; mountain recharge 

contains relatively little water that has previously been 

used, such as treated wastewater that could contain 

disinfection by-products such as chloroform. No 

correlation was found between chloroform 

concentration and distance, or between chloroform 

concentration and depth, on either flow path (table 13).

The spatial distribution of chloroform is similar 

to VOC score. Concentrations of chloroform were 

similar in the proximal and distal areas, with slightly 

more detections and higher concentrations detected in 

distal wells (fig. 37A). The highest concentration was 

detected in the distal part of the flow paths at mid-depth 

(fig. 37B), whereas the highest VOC score was detected 

in a shallow well. Nondetections were in the proximal 

and distal parts of the flow paths.
66 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Delta oxygen –8, in per mill

Delta deuterium, in per mill

Global Meteoric Water Line (Craig, 1961)

Newmark flow path

Santa Ana River flow path Santa Ana River median

Newmark median

Explanation

–75

–70

–65

–60

–55

–50

–45

–40

–10.5 –10 –9.5 –9 –8.5 –8 –7.5 –7 –6.5 –6

Figure 35. Relation between delta oxygen-18 and delta deuterium in ground water along flow paths, Bunker Hill Basin, San Bernardino County, California.
Bunker Hill Basin, San Bernardino County 67

Table 14. Volatile organic compounds detected in 26 ground-water samples from selected wells in the Bunker Hill Basin, San Bernardino County, California, 

2000.

[LRL, laboratory reporting limit; MCL, maximum contaminant level; —, no value available or applicable; E, concentration estimated] 

1 U.S. Environmental Protection Agency, 2001.

2 California Department of Water Resources, 2001.

3 Total Trihalomethanes, as of 01/01/02 ,U.S. Environmental Protection Agency, 2001. 4 Total Xylenes, California Department of Water Resources, 2001. 5 Methyl benzene concentrations may be due to sample contamination during collection and analysis. 6 2 LRLs used during the sampling period.

Constituent LRL, in μg/L

Number of 

detections at 

or above LRL

Number of 

detections 

below LRL

Minimum 

detection, 

in μg/L

Maximum 

detection, 

in μg/L

MCL, 

in μg/L

Number of 

wells exceeding a MCL

Total number of VOCs detected — —— 0 16 — —

VOC Score — —— 0.0 12.0 — —

Trichlorofluoromethane (CFC-11) 0.09 7 3 0.02 2.53 2150 0

Tetrachloroethene (PCE) 0.1 3 4 0.0163 0.532 15 0

Trichloromethane (Chloroform) 60.052, 0.024 5 2 0.021 0.543 1,380 0

Trichloroethene (TCE) 0.038 5 2 0.009 10.6 15 1

Methyl tert-butyl ether (MTBE) 0.17 2 3 0.07 4.35 213 0

Benzene 0.021 3 1 0.007 25.8 21 1

1,2-Dimethylbenzene 0.038 3 1 0.032 0.133 41,750 0

1,4-Dimethylbenzene 0.06 3 1 0.05 0.99 41,750 0

cis-1,2-Dichloroethene 0.038 3 0 0.074 0.242 26 0

Bromodichloromethane 0.048 2 1 0.046 0.502 1,380 0

1,1-Dichloroethene 0.04 2 1 0.01 0.06 26 0

Ethylbenzene 0.03 2 1 0.01 0.44 1700 0

1,2-Dichloropropane 0.029 1 2 0.014 0.059 15 0

Carbon Disulfide 0.07 1 2 0.02 0.73 — —

Tribromomethane 0.06 2 0 0.09 0.72 1,380 0

Chlorodibromomethane 0.18 1 1 0.08 0.67 1,380 0

Dichlorodifluoromethane 0.27 1 1 0.15 0.28 — —

2-Ethyltoluene 0.06 1 1 0.05 1.86 — —

1,2,4-Trimethylbenzene 0.056 1 1 0.016 0.154 — —

Isopropylbenzene 0.032 1 1 0.022 26.100 — —

n-Propylbenzene 0.042 1 1 0.040 38.000 — —

Chloromethane 60.5, 0.25 0 2 0.04 0.04 — —

1,1-Dichloroethane 60.066, 0.035 0 2 0.020 0.029 25 0

1,1,1-Trichloroethane 0.032 0 2 0.009 0.020 1200 0

1,2-Dichlorobenzene 0.048 0 2 0.024 0.036 1600 0

Dibromomethane 0.05 1 0 0.05 0.05 — —

Napthalene 0.25 1 0 21.9 21.9 — —

1,2,3,4-Tetramethylbenzene 0.23 1 0 6.93 6.93 — —

sec-Butylbenzene 0.032 1 0 5.5 5.5 — —

1,1,1,2-Tetrachloroethane 0.06 1 0 0.19 0.19 21,200 0

Chlorobenzene 0.028 0 1 0.004 0.004 1100 0

1,4-Dichlorobenzene 0.05 0 1 0.01 0.01 25 0

1,2,3-Trimethylbenzene 0.12 0 1 0.04 0.04 — —

1,3,5-Trimethylbenzene 0.044 0 1 0.038 0.038 — —

Bromochloromethane 0.044 0 1 0.023 0.023 — —

n-Butylbenzene 0.19 0 1 0.04 0.04 — —

p-Isopropyltoluene 0.07 0 1 0.02 0.02 — —

Methyl benzene5 0.05 4 4 0.01 0.89 2150 0
68 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Table 15. Detection frequencies of volatile organic compounds detected in 26 wells sampled in the Bunker Hill Basin, San Bernardino County, 

California, 2000.

[LRL, laboratory reporting limit] 

Constituent

Detection Frequency, in percent

All Detections Detections above LRL Detections below LRL

Trichlorofluoromethane (CFC-11) 38 27 12

Tetrachloroethene (PCE) 31 15 15

Trichloromethane (Chloroform) 27 19 7.7

Trichloroethene (TCE) 27 19 7.7

Methyl tert-butyl ether (MTBE) 19 7.7 12

Benzene 15 12 3.8

1,2-Dimethylbenzene 15 12 3.8

1,4-Dimethylbenzene 15 12 3.8

cis-1,2-Dichloroethene 12 12 0.0

Bromodichloromethane 12 7.7 3.8

1,1-Dichloroethene 12 7.7 3.8

Ethylbenzene 12 7.7 3.8

1,2-Dichloropropane 12 3.8 7.7

Carbon Disulfide 12 3.8 7.7

Tribromomethane 7.7 7.7 0.0

Chlorodibromomethane 7.7 3.8 3.8

Dichlorodifluoromethane 7.7 3.8 3.8

2-Ethyltoluene 7.7 3.8 3.8

1,2,4-Trimethylbenzene 7.7 3.8 3.8

Isopropylbenzene 7.7 3.8 3.8

n-Propylbenzene 7.7 3.8 3.8

Chloromethane 7.7 0.0 7.7

1,1-Dichloroethane 7.7 0.0 7.7

1,1,1-Trichloroethane 7.7 0.0 7.7

1,2-Dichlorobenzene 7.7 0.0 7.7

Dibromomethane 3.8 3.8 0.0

Napthalene 3.8 3.8 0.0

1,2,3,4-Tetramethylbenzene 3.8 3.8 0.0

sec-Butylbenzene 3.8 3.8 0.0

1,1,1,2-Tetrachloroethane 3.8 3.8 0.0

Chlorobenzene 3.8 0.0 3.8

1,4-Dichlorobenzene 3.8 0.0 3.8

1,2,3-Trimethylbenzene 3.8 0.0 3.8

1,3,5-Trimethylbenzene 3.8 0.0 3.8

Bromochloromethane 3.8 0.0 3.8

n-Butylbenzene 3.8 0.0 3.8

p-Isopropyltoluene 3.8 0.0 3.8
Bunker Hill Basin, San Bernardino County 69

Distance in kilometers

VOC Score

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

Monitoring Wells

Newmark

Flow Path

Santa Ana River

Flow Path

Public Supply Wells

Spearman's rho = 0.37

p = 0.06

Positive Correlation

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0

50

100

150

200

250

300

350

400

450

500 Land surface

2.9

3.1

0.2

0.1

6.6

7.9

11.2

0.2

1.3

0.1

0.5

2.7

0.2

0.1

1.2

4.3

0.2 4.3

0.2

1.3

0.1

Elevation, in meters above sea level

VOC Score, greater than or equal to 1.0

VOC Score, less than 1.0

Explanation

4.3

0.1 Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line

Location of engineered recharge facilities

along cross section

increasing confinement on

Santa Ana River Flow Path

increasing confinement on

Newmark Flow Path

Highest concentration

VOC score = 0.0; no VOCs detected

A

B

Figure 36. VOC score (A) versus distance and (B) in wells along flow paths, Bunker Hill Basin, San Bernardino County, California, 2000.
70 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line

Location of engineered recharge facilities

along cross section

Elevation, in meters above sea level

Distance, in kilometers

01234567 8 9 10 11 12 13 14 15 16

0

50

100

150

200

250

300

350

400

450

500

.05

--

.02

--

--

--

.54

--

--

--

--

--

.10

--

--

--

.07

--

--

--

.03

--

--

--

--

.38

Concentration, in micrograms per liter, quanitified

Concentration, in micrograms per liter, estimated

Not detected

Explanation

.38

.03

--

Land surface

Chloroform concentration, in

micrograms per liter

LRL = 0.052 mg/L

0.001

0.01

0.1

1

Spearman's rho = –0.05

p = 0.81

No Correlation

Monitoring Wells

Newmark

Flow Path

Newmark

Flow Path

Santa Ana River

Flow Path

Santa Ana River

Flow Path

Public Supply Wells

Detection, quanitified

Detection, estimated

Non-Detect, plotted at one-quarter

of the less-than value

increasing confinement on

Santa Ana River flow path

increasing confinement on

Newmark flow path

Highest concentration

A

B

Figure 37. Trichloromethane (chloroform) concentration (A) versus distance and (B) in wells along flow paths in the Bunker Hill Basin, California, 2000.
Bunker Hill Basin, San Bernardino County 71

MTBE

MTBE was detected in 19 percent of the wells 

(table 15). Only one low detection was on the SAR 

flow path in the deepest downgradient monitoring well. 

MTBE concentrations increased significantly 

downgradient, with the highest concentration detected 

in a deeper monitoring well at 15 km (fig. 38B). No 

MTBE was detected in wells less than 6 km 

downgradient. No correlation was found on either 

Bunker Hill flow path between MTBE concentration 

and depth (table 13).

The distribution of MTBE concentrations is 

similar to VOC score. MTBE was detected in the distal 

area of the ground-water flow paths, but not in the 

proximal areas (fig. 38B). The highest concentration 

was detected in a deep monitoring well at the distal end 

of the flow paths (fig. 38B). This suggests that MTBE, 

a recently introduced compound, is not currently 

reaching ground water in the proximal recharge areas, 

but is reaching ground water in the distal, confined part 

of the flow path. Because MTBE use in the recharge 

area would not have been higher in the past than it is 

now, it probably was not transported horizontally 

downgradient from the forebay areas, but reached 

ground water vertically from land surface in the distal, 

confined area of Bunker Hill flow paths. This suggests 

that the confining units present in the distal part of the 

Bunker Hill Basin do not prevent VOCs from reaching 

ground water.

TCE

TCE was detected in 27 percent of the wells 

(table 15). No correlation was found between TCE 

concentration and distance along either Bunker Hill 

flow path (table 13). The highest concentration over the 

federal and state MCL (California Department of 

Health Services, 2001; U.S. Environmental Protection 

Agency, 2001) was in the distal area at about 13 km in 

a shallow public supply well. Three of the highest 

concentrations were detected in the public supply wells 

associated with mapped TCE plumes (Wildermuth 

Environmental, Inc., 2000b, fig. 3); one high 

concentration was detected in a public supply well 

downgradient from mapped TCE plumes (fig. 39). 

There was a significant decrease in concentration with 

increasing depth (table 13).

As in the Main Basin, higher TCE concentrations 

are associated with mapped TCE plumes (figs. 33 and 

39B). The distribution of TCE is similar to VOC score 

and MTBE. Higher concentrations were detected in 

distal wells or in wells associated with mapped plumes. 

Low concentrations and nondections were in the 

proximal and distal parts of the flow system in wells 

outside of mapped plumes. The presence of TCE in 

wells outside of mapped plumes suggests that there 

may be other sources of TCE, or that the known 

sources have contributed TCE to areas outside of the 

mapped plumes.

PCE

PCE was detected in 31 percent of the wells 

(table 15). There was a no statistical difference in PCE 

concentration between the two Bunker Hill flow paths 

(table 12). No correlation was found between PCE 

concentration and distance along the either flow path 

(table 13 and fig. 40A). However, PCE concentrations 

increased downgradient, with the highest concentration 

detected at about 14 km in a relatively deep public 

supply well (fig. 40B). No correlation was found 

between PCE concentration and depth (table 13).

The spatial distribution of PCE concentration is 

similar to VOC score. High PCE concentrations were 

detected in distal wells on both flow paths (fig. 40B). 

Low concentrations and nondetections occurred at all 

depths in the proximal and distal areas. These patterns 

suggest that either high concentrations were recharged 

in the past in the proximal part of the flow paths and 

migrated downgradient to the distal area or PCE is 

currently reaching ground water all along the flow 

paths.
72 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000 MTBE concentration, in micrograms per liter

LRL = 0.17 mg/L

Distance in kilometers

0.01

0.1

1

10

Spearman's rho = 0.40

p = 0.04

Moderate Positive Correlation

01234567 8 9 10 11 12 13 14 15 16

0

100

150

200

250

300

350

400

450

500

--

.15

--

.28

--

--

.12

--

4.35

--

0.07

--

--

--

-- --

--

--

--

--

--

--

--

--

--

Elevation, in meters above sea level

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line

Location of engineered recharge facilities

along cross section

Concentration, in micrograms per liter, quanitified

Concentration, in micrograms per liter, estimated

Not detected

Explanation

.38

.03

--

Land surface

Monitoring Wells

Newmark

Flow Path

Newmark

Flow Path

Santa Ana River

Flow Path

Santa Ana River

Flow Path

Public Supply Wells

Detection, quanitified

Detection, estimated

Non-Detect, plotted at one-quarter

of the less-than value

increasing confinement on

Santa Ana River flow path

increasing confinement on

Newmark flow path

Highest concentration

A

B

Figure 38. Methyl tert-butyl ether (MTBE) concentrations (A) versus distance and (B) in wells along flow paths, Bunker Hill Basin, San Bernardino County, 

California, 2000.
Bunker Hill Basin, San Bernardino County 73

Elevation, in meters above sea level

Distance, in kilometers

01234567 8 9 10 11 12 13 14 15 16

0

50

100

150

200

250

300

350

400

450

500 Land surface

.11

--

--

--

--

--

1.0

--

--

--

--

--

.20

--

--

.02

10.6

--

--

--

.009

--

--

--

--

.43

TCE concentration, in micrograms per liter

0.001

0.01

0.1

1

10

100

LRL = 0.038 mg/L

Spearman's rho = 0.17

p = 0.40

No Correlation

MCL = 5 mg/L

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line

Location of engineered recharge facilities

along cross section

Well located inside area of TCE plume

Concentration, in micrograms per liter, quanitified

Concentration, in micrograms per liter, estimated

Not detected

Explanation

.38

.03

--

Monitoring Wells

Newmark

Flow Path

Newmark

Flow Path

Santa Ana River

Flow Path

Santa Ana River

Flow Path

Public Supply Wells

Detection, quanitified

Detection, estimated

Non-Detect, plotted at one-quarter of

the less-than value

Concentration, in micrograms per liter, above

maximum contaminant level

10.6

increasing confinement on

Santa Ana River flow path

increasing confinement on

Newmark flow path

Highest concentration

A

B

Figure 39. Trichloroethene (TCE) concentrations (A) versus distance and (B) in wells along flow paths, Bunker Hill Basin, San Bernardino County, California, 

2000.
74 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000 PCE concentration, in micrograms per liter

Spearman's rho = 0.26

p = 0.19

No Correlation

0.01

0.1

1

LRL = 0.1 mg/L

Distance in kilometers

01234567 8 9 10 11 12 13 14 15 16

0

50

100

150

200

250

300

350

400

450

500 Land surface

-- --

--

.02

--

.37

--

.08

--

--

--

--

--

--

--

--

--

--

-- .02 -- .05 .03 .18

.53

Elevation, in meters above sea level

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line

Location of engineered recharge facilities

along cross section Concentration, in micrograms per liter, quanitified

Concentration, in micrograms per liter, estimated

Not detected

Explanation

.38

.03

--

Monitoring Wells

Newmark

Flow Path

Newmark

Flow Path

Santa Ana River

Flow Path

Santa Ana River

Flow Path

Public Supply Wells

Detection, quanitified

Detection, estimated

Non-Detect, plotted at one-quarter of

the less-than value

increasing confinement on

Santa Ana River flow path

increasing confinement on

Newmark flow path

Highest concentration

A

B

Figure 40. Tetrachloroethene (PCE) concentrations (A) versus distance and (B) in wells along flow paths, Bunker Hill Basin, San Bernardino County, California, 

2000.
Bunker Hill Basin, San Bernardino County 75

CFC-11

CFC-11, detected in 38 percent of the wells, was 

the most frequently detected VOC on the Bunker Hill 

flow paths (table 15). This is in contrast to the five 

coastal plain flow paths, where chloroform was the 

most frequently detected compound (tables 5 and 10). 

No statistical difference in CFC-11 concentration was 

found between the Bunker Hill flow paths (table 12). 

CFC-11 concentrations are significantly correlated 

with distance, with higher concentrations detected 

greater than 14 km downgradient (fig. 41); however, in 

contrast to the other VOCs, the highest concentration 

was detected in the upgradient well closest to the Santa 

Ana River recharge facilities (fig. 41B). There was no 

correlation between CFC-11 concentrations and well 

depths (table 13), with detections in shallow, middle, 

and deep wells.

The spatial distribution of CFC-11 is similar to 

VOC score, with one exception. Unlike the other VOCs 

analyzed in the Bunker Hill Basin, the highest 

concentration of CFC-11 was detected in the most 

upgradient well (fig. 41B). However, other high 

concentrations were detected in the most distal wells 

on the flow paths. In the distal wells, the highest 

concentration was detected in a shallow well. This 

pattern suggests that, in contrast to the coastal plain 

flow paths, CFC-11 is currently reaching ground water 

in more than one location along the Bunker Hill flow 

paths.
76 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000 Elevation, in meters above sea level

Distance, in kilometers

01234567 8 9 10 11 12 13 14 15 16 0

50

100

150

200

250

300

350

400

450

500 Land surface

.02

--

--

--

--

--

.04

.87

.11

--

.16

--

--

--

--

--

--

--

2.5

.17

.07

-- --

--

.14

.12

Well openings, showing top and

bottom elevations; color indicates flow path;

all monitoring wells at a site are joined by a

lighter colored line

Location of engineered recharge facilities

along cross section

Concentration, in micrograms per liter, quantified

Concentration, in micrograms per liter, estimated

Not detected

Explanation

.38

.03

--

Monitoring Wells

Newmark

Flow Path

Newmark

Flow Path

Santa Ana River

Flow Path

Santa Ana River

Flow Path

Public Supply Wells

Detection, quantified

Detection, estimated

Non-Detect, plotted at one-quarter of

the less-than value

CFC-11 concentration, in micrograms per liter

LRL = 0.09 mg/L

Spearman's rho = 0.53

p = 0.008

Strong Positive Correlation

0.01

0.1

1

10

increasing confinement on

Santa Ana River Flow Path

increasing confinement on

Newmark Flow Path

Highest concentration

A

B

Figure 41. Trichlorofluoromethane (CFC-11) concentrations (A) versus distance and (B) in wells along flow paths, Bunker Hill Basin, California, 2000.
Summary and Conclusions 77

SUMMARY AND CONCLUSIONS

Stable isotopes and volatile organic compounds 

(VOC) were examined along ground-water flow paths 

in three southern California basins. Five flow paths are 

located in the southern California coastal plain. 

Ground-water flow systems in the Coastal Plain are 

characterized by areas of focused ground-water 

recharge; ground-water flows radially outward into the 

basins toward dispersed areas of ground-water 

pumping (discharge). Two flow paths are located in the 

Inland Santa Ana Basin, which is characterized by 

dispersed areas of ground-water recharge; groundwater flow converges radially toward an area of 

focused discharge.

As part of this study, a new way of counting 

VOC detections in a sample, the VOC score, was 

developed to include detections of VOCs below the 

laboratory reporting levels (LRL) in data analyses. 

About half of the VOC detections in this study were at 

concentrations below the LRL. The VOC score allows 

all the VOC detections to be used for graphical and 

statistical analyses, and takes into account the 

analytical certainty based on the LRL. The VOC score 

gives more weight to detections above LRL, which 

have less than 1 percent chance of false positive or false 

negative detections, and less weight (one tenth) to 

detections below the LRL, which may have a greater 

than 1 percent chance of false negative detections.

In the Central Basin, Los Angeles County, the 

two flow paths sampled originate at the San Gabriel 

and Rio Hondo recharge facilities in the Montebello 

forebay area. Stable isotopes indicate that ground water 

along both flow paths is from similar sources. The 

patterns of VOC score and concentrations indicate that 

a volume of water that contains VOCs and is 

presumably young (has recharged since the 1930s) 

extends about 20 km into the Central Basin from the 

Montebello forebay and to depths of at least 400 m 

below land surface. In this volume of water, VOC score 

and concentration are variable and may reflect the 

history of VOC inputs and fate in the subsurface 

environment. This water is mixing with presumably 

older ground water that contains no VOCs. The 

patterns also suggest that some VOCs were introduced 

to ground water at higher concentrations in the past 

than at present and that VOCs continue to enter the 

Central Basin through the Whittier Narrows. The 

pattern of MTBE concentrations is different from the 

other VOCs, and suggests that MTBE is currently 

being introduced into ground water in the upgradient 

areas of the Central Basin at higher concentrations than 

in the past.

An important exception to the general pattern is 

the very high concentrations of TCE and PCE in 

shallow wells located in the pressure area. These wells 

are downgradient from wells with much lower 

concentrations. This indicates that even in the pressure 

area, ground-water is susceptible to contamination 

from VOC sources.

In the Main Basin, Orange County, three flow 

paths were sampled. The Santa Ana River (SAR) west 

flow path is located in the radial flow system that 

originates at the SAR recharge facilities. The Santiago 

flow path is located in the radial flow system from the 

Santiago Creek recharge facilities. The SAR east flow 

path originates in the SAR flow system, but also is 

along the convergence of the Santa Ana River and 

Santiago Creek flow systems. Ground water is 

isotopically the same on SAR flow paths, indicating 

similar sources of water. Ground water along the 

Santiago flow path is isotopically different from the 

other two flow paths, indicating a different source of 

ground water on the Santiago flow path.

Although stable isotopes show that the SAR west 

and east flow paths are similar and the Santiago flow 

path is different, the VOCs show a contrasting relation. 

VOC score and VOC concentrations on the SAR west 

flow path are different from both the SAR east and 

Santiago flow paths. VOC occurrences were 

statistically similar between the SAR east and Santiago 

flow paths, suggesting that recharge from Santiago 

Creek is affecting VOC occurrences on the SAR east 

flow path. The patterns of VOC scores, chloroform, 

and TCE along the Santiago flow path are different 

than on the SAR flow paths and suggest that more 

VOCs are currently reaching ground water than in the 

past. These patterns may reflect that the Santiago 

Creek recharge facilities have been operational for a 

shorter period of time than the SAR recharge facilities. 

VOC score and concentrations on the SAR west flow 

path show a pattern similar to the Central Basin flow 

paths. A volume of presumably young water 

containing VOCs extends to a distance around 18 km 

from the SAR recharge facilities and a depth of at least 

300 m, and is mixing with presumably older water 

containing no VOCs.
78 Stable Isotopes and Volatile Organic Compounds Along Seven Ground-Water Flow Paths in Divergent and Convergent Flow Systems, Southern CA, 2000

In the Inland Bunker Hill Basin, two flow paths 

were sampled. In contrast to the Coastal Plain basins, 

ground-water recharge in the Bunker Hill Basin is 

dispersed along the mountain front, and ground-water 

flow converges radially to an area of focused discharge 

along the San Jacinto Fault. The two flow paths 

originate in different sections of the mountain front and 

converge near the San Jacinto Fault. The stable 

isotopes indicate different sources of water for the two 

flow paths; however, VOC occurrence is similar 

between them. In contrast to the patterns found in the 

Central and Main Basins, higher VOC scores and 

concentrations of MTBE, TCE, and PCE in the Bunker 

Hill Basin were detected at the distal end of the flow 

paths in the confined part of the flow system. This 

suggests that either (1) high concentrations of these 

VOCs reached ground water at some time in the past in 

the unconfined, upgradient area of the flow paths and 

have migrated downgradient or (2) VOCs are 

introduced all along the flow paths, even in the 

confined part of the aquifer. The absence of MTBE in 

the proximal part of the flow paths and presence in the 

distal part suggest that the confining units present in the 

distal part of the Bunker Hill Basin do not prevent 

VOCs from reaching ground water. The patterns of 

VOC score and concentration in the Bunker Hill Basin 

indicate a different situation than in the coastal plain. In 

Bunker Hill, a volume of presumably young water 

containing relatively few VOCs is moving outward 

from the recharge areas and mixing with a volume of 

possibly intermediate to older water that contains many 

VOCs.

The susceptibility of public supply wells to land 

surface contamination can be understood, in part, by 

developing an understanding of the regional groundwater flow system. In the southern California coastal 

plain, and perhaps other radially divergent confined 

flow systems, VOC score and concentrations tend to 

decrease with increasing distance. In the inland Santa 

Ana Basin, and perhaps other radially convergent, 

confined flow systems, VOC score and concentrations 

tend to increase with increasing distance. These results 

suggest that distal wells tapping older ground water are 

less susceptible to land surface contamination than 

proximal wells tapping younger ground water in 

radially divergent confined systems. Conversely, distal 

wells tapping older ground-water probably are more 

susceptible than proximal wells tapping younger 

ground water in radially convergent, confined systems. 

These differences in susceptibility are partly controlled 

by the pattern of ground-water flow (radially divergent 

or radially convergent) and perhaps by the degree of 

confinement.

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