Occurrence and Temporal Variability of

Methyl tert-Butyl Ether (MTBE) and

Other Volatile Organic Compounds in

Select Sources of Drinking Water:

Results of the Focused Survey

Water-Resources Investigations Report 02-4084

U.S. Department of the Interior

U.S. Geological Survey

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

NATIONAL SYNTHESIS ON VOLATILE ORGANIC COMPOUNDS

Prepared in cooperation with the Metropolitan Water District of Southern California,

Oregon Health & Science University, and the

American Water Works Association Research Foundation
U.S. Department of the Interior

U.S. Geological Survey

Occurrence and Temporal Variability of

Methyl tert-Butyl Ether (MTBE) and

Other Volatile Organic Compounds in

Select Sources of Drinking Water:

Results of the Focused Survey

By Gregory C. Delzer and Tamara Ivahnenko

Water-Resources Investigations Report 02-4084

Prepared in cooperation with the Metropolitan Water District of Southern California,

Oregon Health & Science University, and the

American Water Works Association Research Foundation
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute endorsement by the U.S. Government.

Rapid City, South Dakota: 2003

For additional information write to:

District Chief

U.S. Geological Survey

1608 Mt. View Road

Rapid City, SD 57702

Copies of this report can be purchased from:

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Foreword III

FOREWORD

The U.S. Geological Survey (USGS) is 

committed to providing the Nation with accurate and 

timely scientific information that helps enhance and 

protect the overall quality of life and that facilitates 

effective management of water, biological, energy, and 

mineral resources (http://www.usgs.gov/). Information 

on the quality of the Nation’s water resources is 

critical to assuring the long-term availability of water 

that is safe for drinking and recreation and suitable for 

industry, irrigation, and habitat for fish and wildlife. 

Population growth and increasing demands for 

multiple water uses make water availability, now 

measured in terms of quantity and quality, even more 

essential to the long-term sustainability of our 

communities and ecosystems.

The USGS implemented the National WaterQuality Assessment (NAWQA) Program in 1991 to 

support national, regional, and local information needs 

and decisions related to water-quality management 

and policy (http://water.usgs.gov/nawqa). Shaped by 

and coordinated with ongoing efforts of other Federal, 

State, and local agencies, the NAWQA Program is 

designed to answer: What is the condition of our 

Nation’s streams and ground water? How are the 

conditions changing over time? How do natural 

features and human activities affect the quality of 

streams and ground water, and where are those effects 

most pronounced? By combining information on 

water chemistry, physical characteristics, stream 

habitat, and aquatic life, the NAWQA Program aims to 

provide science-based insights for current and 

emerging water issues and priorities. 

From 1991-2001, the NAWQA Program 

completed interdisciplinary assessments in 51 of the 

Nation’s major river basins and aquifer systems, 

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

nawqa/studyu.html). Baseline conditions were 

established for comparison to future assessments, and 

long-term monitoring was initiated in many of the 

basins. During the next decade, 42 of the 51 Study 

Units will be reassessed so that 10 years of 

comparable monitoring data will be available to 

determine trends at many of the Nation’s streams and 

aquifers. The next 10 years of study also will fill in 

critical gaps in characterizing water-quality 

conditions, enhance understanding of factors that 

affect water quality, and establish links between 

sources of contaminants, the transport of those 

contaminants through the hydrologic system, and the 

potential effects of contaminants on humans and 

aquatic ecosystems.

The USGS aims to disseminate credible, timely, 

and relevant science information to inform practical 

and effective water-resource management and 

strategies that protect and restore water quality. We 

hope this NAWQA publication will provide you with 

insights and information to meet your needs, and will 

foster increased citizen awareness and involvement in 

the protection and restoration of our Nation’s waters. 

The USGS recognizes that a national 

assessment by a single program cannot address all 

water-resource issues of interest. External 

coordination at all levels is critical for a fully 

integrated understanding of watersheds and for costeffective management, regulation, and conservation of 

our Nation’s water resources. The NAWQA Program, 

therefore, depends on advice and information from 

other agencies—Federal, State, interstate, Tribal, and 

local—as well as nongovernmental organizations, 

industry, academia, and other stakeholder groups. 

Your assistance and suggestions are greatly 

appreciated.

Robert M. Hirsch

Associate Director for Water

Contents V

CONTENTS

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

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

Purpose and Scope....................................................................................................................................................... 6

Acknowledgments ....................................................................................................................................................... 6

Study Design.......................................................................................................................................................................... 7

Site Selection ............................................................................................................................................................... 7

Sample Collection........................................................................................................................................................ 9

Occurrence and Temporal Variability of Volatile Organic Compounds ................................................................................ 10

Occurrence and Comparison to the Random Survey................................................................................................... 10

Temporal Variability .................................................................................................................................................... 19

Ground Water..................................................................................................................................................... 20

Reservoirs and Lakes......................................................................................................................................... 22

Rivers and Streams ............................................................................................................................................ 23

Anthropogenic Factors Associated with MTBE, Gasoline Oxygenates, and Trihalomethanes ............................................ 25

MTBE and Gasoline Oxygenates ................................................................................................................................ 25

Trihalomethanes .......................................................................................................................................................... 28

Summary and Conclusions .................................................................................................................................................... 29

References ............................................................................................................................................................................. 31

Appendixes ............................................................................................................................................................................ 33

A. Number of community water system sources sampled per State .......................................................................... 35

B. Detailed review of field quality-control data ......................................................................................................... 39

C. Data acceptance criteria using quality-control data ............................................................................................... 53

D. Summary of VOC concentrations in the Focused Survey ..................................................................................... 61

FIGURES

1. Map showing methyl tert-butyl ether (MTBE) high-use areas and location of source waters

sampled in the Focused Survey................................................................................................................................... 8

2-8. Graphs showing:

2. Detection frequency at or above the minimum reporting level of individual volatile organic

compounds in source waters sampled in the Focused Survey versus the Random Survey................................ 11

3. Detection frequency at or above the minimum reporting level of volatile organic compound

subgroups in community water system source waters sampled in the Focused Survey

and the Random Survey ..................................................................................................................................... 15

4. Detection frequency at or above the minimum reporting level of any volatile organic

compound (VOC) and subgroup of VOCs in ground-water sources sampled in the

Focused Survey and Random Survey by size of community water system....................................................... 17

5. Detection frequency at or above the minimum reporting level of any volatile organic

compound (VOC) and subgroup of VOCs in surface-water sources sampled in the

Focused Survey and Random Survey by size of community water system....................................................... 18

6. Number of surface-water samples collected by source type and season ........................................................... 19

7. Detection frequency at or above the minimum reporting level of any volatile organic

compound (VOC) and by subgroup of VOCs in community water system source

waters by source type......................................................................................................................................... 20

8. Comparison of the detection frequency of at least one volatile organic compound (VOC)

at or above the method detection limit by subgroup of VOCs in the first and second

sampling of ground-water source water............................................................................................................. 21
VI Contents

FIGURES—Continued

9. Boxplots of detected concentrations of volatile organic compound subgroups in ground-water

source water collected in the first and second samples .............................................................................................. 21

10. Graph showing detection frequency of at least one volatile organic compound (VOC) at or above

the method detection limit by subgroup of VOCs in reservoir and lake source waters by season............................. 22

11. Boxplots of detected concentrations of volatile organic compound subgroups in reservoir

and lake source waters by season............................................................................................................................... 23

12. Graph showing detection frequency of at least one volatile organic compound (VOC) at or above

the method detection limit by subgroup of VOCs in river and stream source waters by season ............................... 24

13. Boxplots of detected concentrations of volatile organic compound subgroups in river

and stream source waters by season........................................................................................................................... 24

14. Boxplots of detected concentrations of methyl tert-butyl ether (MTBE) in reservoir and

lake source waters by location in MTBE low- and high-use areas ............................................................................ 26

15. Boxplots of detected concentrations of methyl tert-butyl ether (MTBE) in river and stream

source waters by location in MTBE low- and high-use areas.................................................................................... 27

16. Graph showing average sum of gasoline oxygenate concentrations versus watercraft-use factor............................. 27

17. Boxplots of average concentrations of gasoline oxygenates by watercraft-use quartiles .......................................... 28

B1. Graph showing concentration of methyl tert-butyl ether in field duplicate and source-water samples..................... 47

TABLES

1. Volatile organic compounds (VOCs) analyzed as part of the Focused Survey and related

information by subgroups of VOCs........................................................................................................................ 4

2. Summary of number of source waters sampled, frequency of sample collection, and number

of samples collected during the Focused Survey.................................................................................................... 9

3. Summary of the frequency and number of quality-control samples collected during the Focused Survey ........... 9

4. Summary of detection frequencies of any volatile organic compound (VOC), any subgroup of VOCs,

and individual VOCs in source waters sampled in the Focused and Random Surveys.......................................... 12

5. Distribution of source waters sampled in the Focused and Random Surveys by source-type and size

of community water system based on population served ....................................................................................... 16

A1. Number of Community Water System source waters sampled that were suspected of having

concentrations of methyl tert-butyl ether, by State and source-size category ........................................................ 36

A2. Number of Community Water System source waters sampled with known concentrations of

methyl tert-butyl ether, by State and source-size category..................................................................................... 37

A3. Number of Community Water System source waters sampled as control sites where methyl

tert-butyl ether would likely not be present, by State and source-size category .................................................... 37

B1. Summary of volatile organic compounds detected in field-equipment blanks, field-reagent blanks,

and trip-reagent blanks ........................................................................................................................................... 41

B2. Relation between concentrations of volatile organic compounds in 292 companion field blank

samples and trip blank samples .............................................................................................................................. 48

B3. Relation between concentrations of volatile organic compounds in 27 companion field equipment

blank samples and field reagent blank samples...................................................................................................... 49

B4. Relation between concentrations of volatile organic compounds in 27 companion field equipment

blank samples and trip reagent blank samples........................................................................................................ 50

B5. Statistical comparison of concentrations of volatile organic compounds in source-water and

duplicate samples.................................................................................................................................................... 51

C1. Schematic diagram for adjusting the frequency of detection of volatile organic compounds in

source-water samples to reflect possible random sample contamination............................................................... 54

C2. Number of source-water samples that meet specified conditions at the method detection limit,

with respect to the possibility of random contamination........................................................................................ 56

C3. Detection frequency of volatile organic compounds detected at or above the method detection

level in source water before and after applying the quality-control screening process.......................................... 58

C4. Schematic diagram for adjusting the frequency of detection of methyl tert-butyl ether in

source-water samples to reflect possible random sample contamination............................................................... 60

D1. Statistical summary of volatile organic compounds in source-water samples in the Focused Survey................... 62
Abstract 1

Occurrence and Temporal Variability of Methyl

tert-Butyl Ether (MTBE) and Other Volatile Organic 

Compounds in Select Sources of Drinking 

Water: Results of the Focused Survey

By Gregory C. Delzer and Tamara Ivahnenko

ABSTRACT

The large-scale use of the gasoline oxygenate methyl tert-butyl ether (MTBE), and its high 

solubility, low soil adsorption, and low biodegradability, has resulted in its detection in ground water 

and surface water in many places throughout the 

United States. Studies by numerous researchers, 

as well as many State and local environmental 

agencies, have discovered high levels of MTBE in 

soils and ground water at leaking underground 

gasoline-storage-tank sites and frequent occurrence of low to intermediate levels of MTBE in 

reservoirs used for both public water supply and 

recreational boating.

In response to these findings, the American 

Water Works Association Research Foundation 

sponsored an investigation of MTBE and other 

volatile organic compounds (VOCs) in the 

Nation’s sources of drinking water. The goal of the 

investigation was to provide additional information on the frequency of occurrence, concentration, and temporal variability of MTBE and other 

VOCs in source water used by community water 

systems (CWSs). The investigation was completed 

in two stages: (1) reviews of available literature 

and (2) the collection of new data. Two surveys 

were associated with the collection of new data. 

The first, termed the Random Survey, employed a 

statistically stratified design for sampling source 

water from 954 randomly selected CWSs. The 

second, which is the focus of this report, is termed 

the Focused Survey, which included samples collected from 134 CWS source waters, including 

ground water, reservoirs, lakes, rivers, and 

streams, that were suspected or known to contain 

MTBE. The general intent of the Focused Survey 

was to compare results with the Random Survey 

and provide an improved understanding of the 

occurrence, concentration, temporal variability, 

and anthropogenic factors associated with frequently detected VOCs.

 Each sample collected was analyzed for 66 

VOCs, including MTBE and three other ether gasoline oxygenates (hereafter termed gasoline oxygenates). As part of the Focused Survey, 451 

source-water samples and 744 field qualitycontrol (QC) samples were collected from 78 

ground-water, 39 reservoir and (or) lake, and 17 

river and (or) stream source waters at fixed intervals for a period of 1 year.

Using a common assessment level of 

0.2 μg/L (micrograms per liter) (2.0 μg/L for 

methyl ethyl ketone), 37 of the 66 VOCs analyzed 

were detected in both surveys. However, VOCs, 

especially MTBE and other gasoline oxygenates, 

were detected more frequently in the Focused 

Survey than in the Random Survey. MTBE was 

detected in 55.5 percent of the CWSs sampled in 

the Focused Survey and in 8.7 percent of those 

sampled in the Random Survey. Little difference 

in occurrence, however, was observed for trihalomethanes (THMs), which were detected in 

16.4 and 14.8 percent of Focused Survey and 
2 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Random Survey CWSs, respectively. This may 

indicate a pervasive occurrence of THMs in 

several source-water types, regardless of CWS size 

or geographic location.

Using data at or above the method detection 

limit to assess temporal variability and anthropogenic factors associated with frequent detection of 

select VOCs, concentrations in the Focused 

Survey in ground-water, reservoir, and river source 

waters were typically less than 1 μg/L. Also, at a 

95-percent confidence interval, no statistically 

significant differences were observed in comparing concentrations in the first and second groundwater sample. A weak seasonal pattern was 

observed in samples collected from reservoirs and 

lakes where gasoline oxygenates and other gasoline compounds were detected more frequently 

during spring and summer, presumedly a result of 

increased use of motorized watercraft during these 

seasons. In contrast, seasonal patterns were not 

observed in samples collected from rivers and 

streams. The lack of seasonal differences in river 

and stream source waters sampled may indicate a 

common and continuous source of contamination.

The most frequently detected VOC (MTBE) 

and the two most frequently occurring subgroups 

of VOCs (gasoline oxygenates and THMs) 

detected in CWS source waters were further characterized to evaluate some anthropogenic factors 

that may better explain their frequent occurrence. 

Gasoline oxygenates were detected in 73.9 percent 

of all CWSs sampled. The concentration of gasoline oxygenates was slightly correlated with watercraft use on reservoirs inside MTBE high-use 

areas (r2=0.3783) but not outside of these areas 

(r2=0.0242). In general, the concentration of gasoline oxygenates increased as watercraft use 

increased. THMs were detected in 47.8 percent of 

the CWSs supplied by surface water. The frequent 

occurrence of THMs in reservoir source waters 

was determined to be an artifact of disinfection and 

the recycling of chlorinated water to these reservoirs. All CWSs with frequent occurrence of 

THMs served by a reservoir indicated that chlorine 

was added to waters for various reasons and that 

the chlorinated water was then released back to, 

or upstream of, the reservoir or lake that was 

sampled. 

INTRODUCTION

The gasoline oxygenate methyl tert-butyl ether 

(MTBE) has been added to gasoline to enhance octane 

levels since the late 1970’s, and in 1990, the Clean Air 

Act (CAA) Amendments mandated seasonal or yearround use of oxygenated compounds (oxygenates) in 

gasoline in specific parts of the United States. Oxygenates are added to gasoline to increase the oxygen 

content, which enhances combustion and decreases 

vehicular carbon monoxide emissions. Oxygenates 

also reduce the need for benzene and other ozoneforming, aromatic compounds in gasoline. MTBE is 

the most commonly used oxygenate, followed by 

ethanol. 

Oxygenates are added to gasoline during the 

winter months in areas where winter concentrations of 

carbon monoxide exceed established air-quality standards. This gasoline is called oxygenated (OXY) gasoline, which contains oxygenates at 2.7 percent by 

weight (15 percent by volume for MTBE) (U.S. Environmental Protection Agency, 1999b). In select areas, 

oxygenates are added to gasoline year round to abate 

ozone pollution during the summer months and carbon 

monoxide pollution during the winter months (U.S. 

Environmental Protection Agency, 1999a). This gasoline is called reformulated gasoline (RFG), which contains oxygenates at 2 percent by weight (11 percent by 

volume for MTBE). 

The implementation of the CAA Amendments 

resulted in a substantial increase in the production and 

use of MTBE during the 1990’s. For example, MTBE 

went from the 39th highest produced organic chemical 

in the United States in 1970 to the fourth highest in 

1998 (Johnson and others, 2000). During that period, 

the aggregate production of MTBE was 60 million 

metric tons. In 1998, 39.7 million liters per day were 

used in the United States, 40 percent of which was used 

in California alone. Most of MTBE use is associated 

with RFG. MTBE is used in only about 3 percent of 

OXY gasoline, whereas MTBE is used in about 

85 percent of all RFG (Wigglesworth, 1999).

MTBE is a possible human carcinogen. The U.S. 

Environmental Protection Agency (USEPA) has issued 

a taste and odor drinking water advisory of concentrations in the range of 20 to 40 μg/L (micrograms per 

liter) to avert unpleasant taste and odor effects in 

drinking water causing it to become non-potable (U.S. 

Environmental Protection Agency, 1997). The chemical properties of MTBE, such as high solubility in 

water, low soil-sorption properties, and low biodegradability in ground water may result in contamination of 
Introduction 3

some private and public source waters, and some cities 

have lost substantial amounts of drinking-water supplies to MTBE contamination. For example, in Santa 

Monica, California, 75 percent of the drinking-water 

wells are now unusable due to MTBE; in South Lake 

Tahoe, California, one-third of the city's 34 drinkingwater wells have been shut down because of MTBE 

contamination; in Windham, Maine, wells have been 

taken offline; and Los Angeles, San Francisco, Santa 

Clara Valley, and Sacramento in California all have 

wells affected by MTBE (Bourelle, 1998; State of 

Maine, 1998; City of Santa Monica, 1999; California 

Department of Health Services, 2001). La Crosse, 

Kansas, also has had wells affected by MTBE; however, officials there have taken steps to remediate the 

problem and to treat the water rather than to remove the 

wells from service (Hattan, 2000).

Although isolated instances of MTBE contamination have been observed, the overall extent of MTBE 

occurrence in the Nation's drinking-water supplies has 

not been fully evaluated. The Interagency Assessment 

of Oxygenated Fuels (Zogorski and others, 1997) 

attempted to address the national occurrence of MTBE 

in drinking-water supplies, but was unable to do so due 

to insufficient data. As a result, additional assessments 

were recommended.

One such assessment, the focus of this report, 

was sponsored by the American Water Works Association Research Foundation (AWWARF), an organization 

that sponsors numerous research studies for the benefit 

of the drinking-water community. This assessment of 

MTBE in the Nation's drinking-water supplies was 

completed by the Metropolitan Water District of 

Southern California (MWDSC), the U.S. Geological 

Survey (USGS) National Water-Quality Assessment 

(NAWQA) Program, and the Oregon Health & Science 

University (OHSU). This assessment studied not only 

MTBE but also 65 other volatile organic compounds 

(VOCs), including other ether gasoline oxygenates 

(hereafter termed gasoline oxygenates) in drinkingwater supplies (table 1), and tert-butyl alcohol 

(TBA)—a degradation by-product of MTBE. However, 

TBA results are not presented herein.

Drinking-water supplies studied were specifically termed source waters. Source water is defined by 

the American Water Works Association as “the supply 

of water for a water utility [that] is usually treated 

before distribution to consumers.” A source water can 

be a river, brook, stream, lake, reservoir, impoundment, 

spring, or aquifer from which a supply of water is 

obtained. As such, the assessment did not study any 

waters that are within a distribution line or after treatment processes have been applied to the water.

The complete list of compounds included in this 

assessment also included 13 VOCs that are on the 

USEPA’s Contaminant Candidate List (CCL) (U.S. 

Environmental Protection Agency, 1998). The CCL is 

a list of contaminants not currently regulated by a 

National Primary Drinking Water Standard. Additional data on these compounds are needed before a 

regulatory determination can be made. This national 

assessment of MTBE and other VOCs began in 

September 1998.

The national assessment of MTBE and other 

VOCs in source-water supplies was accomplished by 

a two-phase approach: (1) reviews of available literature, and (2) the collection of new drinking-waterquality data. The literature review was conducted in 

two phases. One review focused on MTBE taste and 

odor threshold concentrations and their relevance to 

aesthetic effects and possible water-treatment requirements. The second review concentrated on the occurrence of MTBE in public and domestic drinking-water 

supplies reported by national, regional, or statewide 

assessments (Delzer and Ivahnenko, 2003). 

Two surveys were associated with the collection 

of new drinking-water data. The first survey, termed 

the Random Source-Water Survey (hereafter termed 

Random Survey), employed a statistically stratified 

design for sampling source water from 954 community water systems (CWSs) randomly selected on the 

basis of the total number of systems within five population served-size categories, source-water type, and 

the total number of people served by each of the 

source-size categories used within the United States. 

Findings of the Random Survey are reported elsewhere (Rowe and others, 2001; Grady, 2003). The 

second survey, which is the focus of this report, is 

termed the Focused Source-Water Survey (hereafter 

termed Focused Survey). The Focused Survey 

included samples collected from CWS source waters 

that were selected on the basis of factors that appear to 

be or are known to be related to the frequent detection 

of MTBE in water. The Focused Survey provides an 

improved understanding of the occurrence, concentration, temporal variability, and anthropogenic factors 

associated with the detection of MTBE and other frequently identified VOCs. It is important to note, for 

both the Random Survey and the Focused Survey, that 

CWSs were promised anonymity in presenting results. 

Specific information on the design of the Random and 

Focused Surveys is detailed in Ivahnenko and others 

(2001). 
4 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table 1. Volatile organic compounds (VOCs) analyzed as part of the Focused Survey and related information by subgroups 

of VOCs

[IUPAC, International Union of Pure and Applied Chemistry; --, none; CCL, compound is listed on the U.S. Environmental Protection Agency’s Contaminant Candidate List]

Compound name (used

throughout report)

Chemical 

abstract service 

number

IUPAC name Other common names Chemical 

formula

Gasoline Oxygenates

tert-Amyl methyl ether (TAME) 994-05-8 2-Methoxy-2-methylbutane -- C6H14O

Diisopropyl ether (DIPE) 108-20-3 2,2′ -oxybis[propane] -- C6H14O

Ethyl tert-butyl ether (ETBE) 637-92-3 2-Ethoxy-2-methylpropane -- C6H14O

Methyl tert-butyl ether (MTBE)CCL 1634-04-4 2-Methoxy-2-methylpropane -- C5H12O

Other Gasoline Compounds

Benzene 71-43-2 Benzene -- C6H6

n-Butylbenzene 104-51-8 n-Butylbenzene 1-Phenylbutane C10H14

sec-Butylbenzene 135-98-8 (1-Methylpropyl)benzene -- C10H14

tert-Butylbenzene 98-06-6 (1,1-Dimethylethyl)benzene -- C10H14

Ethylbenzene 100-41-4 Ethylbenzene Phenylethane C8H10

NaphthaleneCCL 91-20-3 Naphthalene -- C10H8

Toluene 108-88-3 Methylbenzene -- C7H8

1,3,5-Trimethylbenzene 108-67-8 1,3,5-Trimethylbenzene Mesitylene C9H12

m,p-Xylene 108-38-3/

106-42-3

1,3-Dimethylbenzene and

1,4-Dimethylbenzene 

m-Xylene, p-Xylene C8H10

o-Xylene 95-47-6 1,2-Dimethylbenzene -- C8H10

Trihalomethanes 

Bromodichloromethane 75-27-4 Bromodichloromethane Dichlorobromomethane CHBrCl2

Bromoform 75-25-2 Tribromomethane -- CHBr3

Chlorodibromomethane 124-48-1 Chlorodibromomethane Dibromochloromethane CHBr2Cl

Chloroform 67-66-3 Trichloromethane -- CHCl3

Solvents

BromobenzeneCCL 108-86-1 Bromobenzene Phenyl bromide C6H5Br

Carbon tetrachloride 56-23-5 Tetrachloromethane -- CCl4

Chlorobenzene 108-90-7 Chlorobenzene Monochlorobenzene C6H5Cl

Chloroethane 75-00-3 Chloroethane Ethyl chloride C2H5Cl

2-Chlorotoluene 95-49-8 1-Chloro-2-methylbenzene o-chlorotoluene C7H7Cl

4-Chlorotoluene 106-43-4 1-Chloro-4-methylbenzene p-chlorotoluene C7H7Cl

Dibromomethane 74-95-3 Dibromomethane Methylene bromide CH2Br2

1,2-Dichlorobenzene 95-50-1 1,2-Dichlorobenzene o-Dichlorobenzene C6H4Cl2

1,3-Dichlorobenzene 541-73-1 1,3-Dichlorobenzene m-dichlorobenzene C6H4Cl2

1,1-DichloroethaneCCL 75-34-3 1,1-Dichloroethane Ethylidene chloride C2H4Cl2
Introduction 5

Solvents—Continued

1,2-Dichloroethane 107-06-2 1,2-Dichloroethane Ethylene dichloride C2H4Cl2

1,1-Dichloroethene 75-35-4 1,1-Dichloroethene Vinylidene chloride C2H2Cl2

cis-1,2-Dichloroethene 156-59-2 cis-1,2-Dichloroethene (Z)-1,2-dichloroethene C2H2Cl2

trans-1,2-Dichloroethene 156-60-5 trans-1,2-Dichloroethene (E)-1,2-dichloroethene C2H2Cl2

1,2-Dichloropropane 78-87-5 1,2-Dichloropropane Propylene dichloride C3H6Cl2

Hexachloroethane 67-72-1 1,1,1,2,2,2-Hexachloroethane Carbon hexachloride C2Cl6

Methylene chloride 75-09-2 Dichloromethane -- CH2Cl2

Methyl ethyl ketone (MEK) 78-93-3 2-Butanone -- C4H8O

n-Propylbenzene 103-65-1 n-Propylbenzene 1-Phenylpropane C9H12

1,1,1,2-Tetrachloroethane 630-20-6 1,1,1,2-Tetrachloroethane -- C2H2Cl4

1,1,2,2-TetrachloroethaneCCL 79-34-5 1,1,2,2-Tetrachloroethane -- C2H2Cl4

Tetrachloroethene 127-18-4 Tetrachloroethene Perchloroethene, PCE C2Cl4

1,2,4-Trichlorobenzene 120-82-1 1,2,4-Trichlorobenzene -- C6H3Cl3

1,1,1-Trichloroethane 71-55-6 1,1,1-Trichloroethane Methyl chloroform C2H3Cl3

1,1,2-Trichloroethane 79-00-5 1,1,2-Trichloroethane Vinyl trichloride C2H3Cl3

Trichloroethene (TCE) 79-01-6 Trichloroethene Trichloroethylene C2HCl3

1,2,3-Trichloropropane 96-18-4 1,2,3-Trichloropropane Allyl trichloride C3H5Cl3

Organic Synthesis

Acrylonitrile 107-13-1 2-Propenenitrile -- C3H3N

Bromochloromethane 74-97-5 Bromochloromethane Methylene chlorobromide CH2BrCl

1,3-DichloropropaneCCL 142-28-9 1,3-Dichloropropane Trimethylene dichloride C3H6Cl2

2,2-DichloropropaneCCL 594-20-7 2,2-Dichloropropane -- C3H6Cl2

1,1-DichloropropeneCCL 563-58-6 1,1-Dichloropropene -- C3H4Cl2

HexachlorobutadieneCCL 87-68-3 1,1,2,3,4,4-Hexachloro-1,3-butadiene -- C4Cl6

Isopropylbenzene 98-82-8 (1-Methylethyl)benzene Cumene C9H12

p-IsopropyltolueneCCL 99-87-6 1-Isopropyl-4-methylbenzene -- C10H14

Styrene 100-42-5 Ethenylbenzene Vinyl benzene C8H8

1,2,3-Trichlorobenzene 87-61-6 1,2,3-Trichlorobenzene -- C6H3Cl3

1,2,4-TrimethylbenzeneCCL 95-63-6 1,2,4-Trimethylbenzene Pseudocumene C9H12

Vinyl bromide 593-60-2 Bromoethene -- C2H3Br

Vinyl chloride 75-01-4 Chloroethene -- C2H3Cl

Table 1. Volatile organic compounds (VOCs) analyzed as part of the Focused Survey and related information by subgroups 

of VOCs—Continued

[IUPAC, International Union of Pure and Applied Chemistry; --, none; CCL, compound is listed on the U.S. Environmental Protection Agency’s Contaminant Candidate List]

Compound name (used

throughout report)

Chemical 

abstract service 

number

IUPAC name Other common names Chemical 

formula
6 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Fumigants

BromomethaneCCL 74-83-9 Bromomethane Methyl bromide CH3Br

1,4-Dichlorobenzene 106-46-7 1,4-Dichlorobenzene p-Dichlorobenzene C6H4Cl2

cis-1,3-DichloropropeneCCL,1 10061-01-5 cis-1,3-Dichloropropene (Z)-1,3-Dichloropropene C3H4Cl2

trans-1,3-DichloropropeneCCL,1 10061-02-6 trans-1,3-Dichloropropene (E)-1,3-Dichloropropene C3H4Cl2

Refrigerants

Chloromethane 74-87-3 Chloromethane Methyl chloride CH3Cl

Dichlorodifluoromethane 75-71-8 Dichlorodifluoromethane CFC 12, Freon 12 CCl2F2

Trichlorofluoromethane 75-69-4 Trichlorofluoromethane CFC 11, Freon 11 CCl3F

1,1,2-Trichloro-1,2,2-trifluoroethane76-13-1 1,1,2-Trichloro-1,2,2-trifluoroethane CFC 113, Freon 113 C2Cl3F3

1

 The CCL lists only 1,3-dichloropropene. The two isomers, cis- and trans-, are measured separately in the analytical method used in this study.

Table 1. Volatile organic compounds (VOCs) analyzed as part of the Focused Survey and related information by subgroups 

of VOCs—Continued

[IUPAC, International Union of Pure and Applied Chemistry; --, none; CCL, compound is listed on the U.S. Environmental Protection Agency’s Contaminant Candidate List]

Compound name (used

throughout report)

Chemical 

abstract service 

number

IUPAC name Other common names Chemical 

formula

Purpose and Scope

This report describes the occurrence and temporal variability of MTBE and other VOCs in CWS 

source water suspected or known to contain MTBE as 

part of the Focused Survey. In addition, comparisons 

between results of the Focused and Random Surveys on 

the occurrence of MTBE and other VOCs are summarized. This report also analyzes anthropogenic factors 

associated with the frequent occurrence of MTBE, 

other gasoline oxygenates, and trihalomethanes 

(THMs).

The 451 samples were collected at fixed intervals 

from 78 ground-water, 39 reservoir and (or) lake, and 

17 river and (or) stream source waters in selected areas 

of the Nation. In addition, 744 field quality-control 

(QC) samples were collected, including trip-reagent 

blanks, field-reagent blanks, field-equipment blanks, 

and duplicate samples. All data underwent a thorough 

quality-control review process to accept or reject 

source-water data for inclusion in the Focused Survey. 

The resulting source-water data set included VOC concentrations greater than the method detection limit 

(MDL). However, an assessment level equal to the minimum reporting level (MRL) was used for comparing 

Focused Survey and Random Survey results.

Temporal variability of subgroups of VOCs was 

characterized for CWSs supplied by surface water by 

examining the frequency of occurrence and detected 

concentrations by season of the year. VOC subgroups 

include gasoline oxygenates, other gasoline compounds (excluding gasoline oxygenates), THMs, solvents, organic synthesis compounds, fumigants, and 

refrigerants.

Anthropogenic factors and their relation to 

concentrations of MTBE and two frequently occurring 

subgroups of VOCs—gasoline oxygenates and 

THMs—were examined. These factors include MTBE 

high-use areas, watercraft density, and ancillary information obtained from CWS personnel. These factors 

were characterized to determine their potential significance with respect to MTBE, gasoline oxygenates, and 

THM contamination.

Acknowledgments

The authors would like to thank the AWWARF 

and Kenan Ozekin for their support and oversight of 

this study. Also, the AWWARF project advisory committee, consisting of Homer C. Emery of the San 

Antonio Water System, David Lipsky of the New York 
Study Design 7

City Department of Environmental Protection, and 

Howard Weinberg of the University of North Carolina, 

are recognized for their technical advice. Importantly, 

this assessment would not have been possible without 

the cooperation and assistance of the owners, operators, 

and staff of the CWSs that allowed access to sample 

their source waters. 

The cooperation and assistance of the 31 USGS 

District offices that collected samples and worked 

directly with CWSs is greatly appreciated. All chemical 

analyses for volatile organic compounds in sourcewater samples were conducted at the MWDSC laboratory in LaVerne, California, under the direction of Bart 

Koch and Melissa Dale. The authors would like to 

thank John Zogorski, Stephen Grady, Rick Clawges, 

and Barbara Rowe of the USGS, Bart Koch of the 

MWDSC, and Paul Tratnyek of the Oregon Health & 

Science University for their technical questions, comments, and suggestions during the design and implementation of this study.

Bart Koch of the MWDSC and Ryan Thompson 

and Stephen Grady of the USGS are thanked for providing thorough technical review and insight to this 

report. Finally, Janet Carter, Ella Decker, Connie Ross, 

and Chester Zenone of the USGS provided valuable 

editorial review and assistance in completing this 

report.

STUDY DESIGN

The Focused Survey was designed to sample 

source waters suspected or known to be contaminated 

with MTBE. An equal number of samples was intended 

to be collected from ground-water, reservoir and (or) 

lake, and river and (or) stream source waters. Furthermore, a fixed-interval sampling was followed in order 

to better understand temporal variability. Groundwater, reservoir and (or) lake, and river and (or) stream 

source waters were sampled biannually, quarterly, and 

eight times per year for one year, respectively. Finally, 

a rigorous field QC sampling regime and data interpretation process was followed in order to ensure that the 

samples collected were representative of the source 

water being sampled. Data from these field QC samples 

were used to accept or reject source-water data from the 

analyses presented in this report.

Site Selection

The original design of the Focused Survey 

called for 480 samples to be collected at fixed intervals 

from 140 CWS source waters. Specifically, 80 

ground-water, 40 reservoir and (or) lake, and 20 river 

and (or) stream source waters were to be sampled. 

CWS source waters were selected for sampling if 

MTBE was suspected or known to occur in the source 

water. Source waters suspected to be contaminated 

with MTBE included those in highly urbanized, high 

population-density settings, usually in MTBE highuse areas. Occasionally, a source water was suspected 

to be contaminated with MTBE for other reasons. For 

example, reservoirs near MTBE high-use areas that 

allowed the use of motorized watercraft were suspected to be contaminated with MTBE. Source waters 

known to contain MTBE were identified on the basis 

of recently completed reports (Delzer and Ivahnenko, 

2003) and discussions with local, State, or other 

Federal agency personnel. In addition, source waters 

analyzed as part of the Random Survey of this study in 

which concentrations of MTBE were greater than 

0.5 μg/L also were selected for sampling. Further 

information on site selection and the design of the 

Focused Survey is presented in Ivahnenko and others 

(2001).

The final selection resulted in 134 CWS source 

waters sampled. The geographic distribution of the 

134 source waters sampled in the Focused Survey with 

respect to the location of MTBE high-use areas, is 

shown in figure 1. MTBE high-use areas were defined 

as areas within the RFG Program that use MTBE in 

gasoline (U.S. Environmental Protection Agency, 

1999a). Areas designated as “high use” had a median 

content of MTBE in gasoline greater than 9 percent by 

volume in at least one year or season from 1995 

through 1999. Medians of MTBE content by volume 

in gasoline were determined from yearly data from the 

Reformulated Gasoline Survey (Stuart Romanow, 

U.S. Environmental Protection Agency, written 

commun., 1999) and seasonal (winter and summer) 

data from Motor Gasoline surveys conducted by the 

National Institute for Petroleum and Energy Research 

(Cheryl Dickson, TRW Systems & Information Technology Group, written commun., 2001). All areas currently (2002) in the Federal RFG Program were 

considered MTBE high-use areas, with the exception 

of the Chicago and Milwaukee metropolitan areas, 

which use ethanol in gasoline to meet Federal oxygenate requirements. Two areas that were previously 
8 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Figure 1. Methyl tert-butyl ether (MTBE) high-use areas and location of source waters sampled in the Focused Survey.

250 500 MILES

0 250 500 KILOMETERS

0

Base modified from U.S. Geological Survey

digital data 1:200,000, 1994

Albers Equal Area Conic projection

Standard Parallels: 29o30', 45o30'

Central Meridan: 96o W

AREAS WHERE HIGH-VOLUME MTBE

 USE HAS OCCURRED

GROUND-WATER SAMPLING SITE

SURFACE-WATER SAMPLING SITE

EXPLANATION

in the Federal RFG Program also were considered 

MTBE high-use areas for this analysis. Areas of southern Maine voluntarily entered (opted into) the RFG 

Program in 1995 and opted out in 1999. Phoenix, Arizona, voluntarily entered the RFG Program in 1997 and 

opted out in June 1998. Both of these areas have shown 

significant MTBE use (greater than 9 percent by volume) during the period 1995-99.

Two other areas also were considered MTBE 

high-use areas for this analysis. All areas in the State of 

California, including those outside Federal RFG 

Program areas, are considered to be MTBE high-use 

areas because of documented statewide MTBE highuse in state wintertime oxygenate and (or) California 

Cleaner Gasoline requirements (California Environmental Protection Agency, 1997; California Senate, 

1998). Additionally, Yuma and Mohave Counties in 

Arizona are considered MTBE high-use areas because 

they likely receive gasoline containing high volumes of 

MTBE from California distribution sources (Arizona 

Department of Environmental Quality, 1999).

Seventy-seven source waters selected were suspected to contain MTBE. Of these, about 50 percent 

were ground-water source waters and 50 percent were 

surface-water source waters. Fifty-seven source waters 

selected were known to contain MTBE. Of these, 

67 percent were ground-water source waters and 

33 percent were surface-water source waters. In addition to the 134 CWS sources sampled, four source 

waters (two wells, one reservoir, and one river) were 

selected as control sites where MTBE was not suspected to be present. A summary of source waters 

sampled in each State by size of CWS is presented in 

Appendix A. Excluding the control sites, the 78 

ground-water, 39 reservoir and (or) lake, and 17 river 

and (or) stream source waters were selected for participation in the Focused Survey.
Study Design 9

Sample Collection

Sample collection and analysis began in 

September 1999 and was completed in April 2001. 

Samples were collected by USGS personnel using 

USGS protocols prior to any water treatment. Groundwater samples were collected from water-supply wells 

as described by Koterba and others (1995). Surfacewater samples were collected as close to the intake as 

was reasonable and safe within 5 feet of the surface 

using a VOC hand-sampler as described by Shelton 

(1997) and Halde and others (1998).

The 451 samples were collected and analyzed 

from the 134 source waters selected—160 groundwater samples, 155 reservoir and (or) lake samples, 

and 136 river and (or) stream samples (table 2). The 

scheduled number of samples was collected from most 

of the sites. However, 2 samples collected from reservoirs were lost in shipment, 5 (instead of 4) samples 

were collected from one reservoir, and 3 (instead of 2) 

samples were collected from three wells over the 

sampling period.

The 744 field QC samples were collected as part 

of the Focused Survey (table 3). One hundred percent 

of source-water samples were collected in association 

with a trip-reagent blank (TRB). TRB samples were 

prepared at the MWDSC laboratory by filling sampling 

vials with VOC-free water and sending them to the field 

office that would conduct the sampling. TRB samples 

were never opened to the atmosphere and traveled with 

all other samples from the field site to the laboratory for 

analysis. TRB samples were intended to document if 

VOC contamination of the source-water samples could 

Table 2. Summary of number of source waters sampled, 

frequency of sample collection, and number of samples 

collected during the Focused Survey

[CWS, community water system; --, not applicable]

Source-water

type

Number of 

CWS

sources

sampled

Frequency

of sample

collection

(number

per year)

Total

number of

samples

collected

Ground water 78 2 160

Reservoir and (or) lake 39 4 155

River and (or) stream 17 8 136

Total 134 -- 451

have occurred in transit and, thus, were analyzed only 

if VOCs were detected in the corresponding sourcewater sample. As such, 304 TRB samples were 

analyzed.

Field-reagent blank (FRB) samples also were 

collected in association with 100 percent of sourcewater samples during most of the study. However, 

FRB samples were collected only in association with 

about 15 percent of source-water samples during the 

first three months of sampling. The 321 FRB samples 

were prepared by pouring VOC-free water into a sampling vial at the field site and were sent to the laboratory for analyses. These blank samples were intended 

to document all aspects of contamination including 

sample collection, processing, shipment, and analyses.

Field-equipment blank (FEB) samples and 

duplicate source-water samples were prepared in association with 15 percent of source-water samples collected. Fifty FEB samples were collected at surfacewater sites as described by Shelton (1997) by passing 

VOC-free water through the USGS VOC hand sampler and filling the vials accordingly. FEB samples 

were not collected at ground-water sites because the 

supply wells sampled had dedicated, submersible 

pumps. Finally, 69 duplicate samples were collected 

and analyzed. Ground-water duplicates were collected 

sequentially, and surface-water duplicates were 

collected concurrently. 

Table 3. Summary of the frequency and number of 

quality-control samples collected during the Focused 

Survey

[--, not applicable]

Quality-control

sample type

Frequency of 

quality-control

sample collection

(percent)

Number of

quality-control

samples

analyzed

Trip-reagent blank 100 304

Field-reagent blank 1100 321

Field-equipment blank 15 50

Duplicate source-water 

samples

15 69

Total -- 744

1

Sample frequency was 15 percent during the first three months of 

sample collection and 100 percent thereafter.
10 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Review of the QC data indicates no systematic 

contamination of samples. Detected VOC concentrations in blank samples typically were less than 

0.2 μg/L, and very little variability was associated with 

the sample collection, transportation, and analytical 

processes for the majority of VOCs included in this 

study. A detailed review of these QC data is presented 

in Appendix B.

OCCURRENCE AND TEMPORAL VARIABILITY 

OF VOLATILE ORGANIC COMPOUNDS

The results of the chemical analyses of sourcewater samples collected for the Focused Survey were 

evaluated for the occurrence and temporal variability of 

individual VOCs and subgroups of VOCs (table 1). 

VOC subgroups examined include gasoline oxygenates, other gasoline related compounds (excluding the 

oxygenates), THMs, solvents, organic synthesis compounds, fumigants, and refrigerants. All source-water 

data used in these analyses have undergone an extensive QC screening procedure. This systematic procedure compares source-water data with respective FRB 

samples to ultimately accept or reject the source-water 

data. Thus, only those source-water data that met dataquality criteria were accepted and included in these 

analyses. Details of this screening procedure are 

presented in Appendix C.

Detection frequencies resulting from the QC 

screening process for each VOC detected in the 

Focused Survey also are presented in Appendix C. The 

detection frequencies in Appendix C are for all 467 

samples collected from the three source-water types 

(includes data from four control sites). It is important to 

note, however, that the analytical method reports all 

detections above the MDL. Thus, the detection frequencies in Appendix C were calculated using data that 

were not censored to a statistically determined MRL. 

The MDL concentrations typically were less than 

0.05 μg/L and ranged from 0.014 μg/L for 1,1,2-

trichloroethane to 0.645 μg/L for methyl ethyl ketone 

(MEK) (Ivahnenko and others, 2001). The MRL was 

0.2 μg/L for all compounds except MEK, which had a 

MRL of 2.0 μg/L.

Concentrations of VOCs detected in the Focused 

Survey generally were less than 1 μg/L (Appendix D); 

however, the concentrations of benzene, tetrachloroethene, and trichloroethene in some samples exceeded 

the USEPA Maximum Contaminant Levels (MCLs) set 

for those compounds. It is important to note that MCLs 

are established for finished drinking water and not for 

source water. MCLs for benzene, tetrachloroethene, 

and trichloroethene have been set at 5 μg/L for each 

compound (U.S. Environmental Protection Agency, 

2001). Benzene concentrations exceeded the MCL in 

4 samples, tetrachloroethene in 13 samples, and 

trichloroethene in 7 samples. Concentrations of MTBE 

detected in the Focused Survey exceeded the lower 

limit of the USEPA Drinking Water Advisory of 20 to 

40 μg/L (U.S. Environmental Protection Agency, 

1997) in 12 samples. All samples in which concentrations exceeded MCLs or the Drinking Water Advisory 

for MTBE were collected from ground-water source 

waters. Of the 4 control sites—2 wells, 1 reservoir, 

1 river—that were selected because MTBE was not 

suspected to be detected, MTBE was detected only in 

1 ground-water sample at a concentration of 0.05 μg/L. 

Control site data are not included in table 4.

Occurrence and Comparison to the

Random Survey

One objective of the Random Survey was to provide a statistically representative assessment of the 

quality of the source water of the Nation’s public 

drinking-water supplies with respect to the occurrence 

of MTBE and other VOCs. This objective differs markedly from that of the Focused Survey, which intentionally sampled source waters suspected or known to be 

contaminated with MTBE. Some comparisons between 

the two surveys, however, may augment conclusions 

drawn from both surveys.

Results from the Random Survey (Rowe and 

others, 2001; Grady, 2003) were based on the analyses 

of source water obtained from 954 randomly selected 

CWSs sampled between May 3, 1999, and October 23, 

2000. The Random Survey data underwent the same 

QC screening process that was applied to data from the 

Focused Survey. VOC concentrations detected during 

the Random Survey, however, were only reported at or 

above the MRL. As such, any meaningful comparisons 

between the two surveys had to be conducted with a 

common assessment level. Thus, the Focused Survey 

data were also censored at the MRL in order to make 

comparisons with the Random Survey.

Additional adjustments to Focused Survey data 

were needed in order to compare findings between the 

two surveys. Results from the Random Survey were 

commonly presented such that reservoir, lake, river, 

and stream source waters were combined into one 
Occurrence and Temporal Variability of Volatile Organic Compounds 11

surface-water category. Thus, all Focused Survey 

surface-water data were combined into one category. 

Finally, in the Random Survey one sample was collected from each CWS source water, whereas in the 

Focused Survey multiple samples were collected from 

the same CWS source water. Because concentrations 

may change from one sample to the next at the same 

site, one sample was randomly selected from each 

source water sampled as part of the Focused Survey 

and used for comparing results with the Random 

Survey.

As expected, most VOCs were detected more 

frequently in the Focused Survey than in the Random 

Survey (fig. 2). The Focused Survey sampled source 

waters suspected or known to contain MTBE whereas 

the Random Survey did not. Thus, MTBE was detected 

much more frequently at sites sampled in the Focused 

Survey than in the Random Survey. MTBE was 

detected in 60 and 49.1 percent of CWSs supplied by 

ground and surface water, respectively, in the Focused 

Survey and in 5.4 and 13.8 percent of CWSs supplied 

by ground and surface water, respectively, in the 

Random Survey. MTBE was detected in 55.5 percent 

of the 134 CWSs sampled in the Focused Survey and in 

8.7 percent of 954 CWSs sampled in the Random 

Survey (table 4).

In addition to MTBE, numerous other VOCs 

were detected more frequently in the Focused Survey 

than in the Random Survey (fig. 2). This may be 

explained by population density in areas sampled in the 

Focused Survey. High-population density, which was 

one factor associated with the selection of a site to be 

sampled as part of the Focused Survey, has been shown 

to be related to increased VOC detection frequencies 

(Reiser and O’Brien, 1998; Squillace and others, 1999; 

Bush and others, 2000; Lopes and Furlong, 2001). Similarly, results from the Random Survey concluded that 

the occurrence of VOCs was greatest in the largest 

ground-water CWSs serving populations greater than 

50,000 people and medium-sized surface-water CWSs 

serving populations of 3,001 to 10,000 people.

Individual VOC detection frequencies in CWSs 

sampled as part of the Focused and Random Surveys 

are presented in table 4. Detection frequencies are presented for data at or above the MRL for both surveys 

and also at or above the MDL for the Focused Survey. 

Detection frequencies are lower when using data at or 

above the MRL because detections of VOCs at or 

above the MDL, yet below the MRL, were not included 

in the calculation (Appendix C).

Figure 2. Detection frequency at or above the minimum reporting level of individual volatile organic

compounds in source waters sampled in the Focused Survey versus the Random Survey.

0

10

20

30

40

50

60

70

0 5 10 15 20

FREQUENCY OF DETECTION IN THE RANDOM SURVEY, IN PERCENT 

FREQUENCY OF DETECTION IN THE FOCUSED SURVEY,

IN PERCENT

1:1 Line

Ground water

Surface water

Methyl tert-butyl ether
12 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table 4. Summary of detection frequencies of any volatile organic compound (VOC), any subgroup of VOCs, and individual 

VOCs in source waters sampled in the Focused and Random Surveys

[VOC, volatile organic compound; CWS, community water system; n, number of samples]

Volatile organic

compound subgroup

and individual

compound name

Focused Survey Random Survey

Detection frequency at or above

the MDL (percent)

Detection frequency at or above

the MRL (percent)

Detection frequency at or above

the MRL (percent)

All CWSs

(n=134)

Ground

water

(n=78)

Surface

water

(n=56)

All CWSs

(n=134)

Ground

water

(n=78)

Surface

water

(n=56)

All CWSs

(n=954)

Ground

water

(n=579)

Surface

water

(n=375)

Any VOC 92.5 92.3 92.9 61.2 66.7 53.6 26.9 25.2 29.6

Gasoline Oxygenates

tert-Amyl methyl ether 20.3 20.8 19.6 4.5 6.5 1.8 0.2 0.4 0.0

Diisopropyl ether 6.0 10.3 0.0 3.0 5.1 0.0 0.2 0.4 0.0

Ethyl tert-butyl ether 1.5 2.6 0.0 0.7 1.3 0.0 0.1 0.2 0.0

Methyl tert-butyl ether 77.3 68.0 90.6 55.5 60.0 49.1 8.7 5.4 13.8

Any gasoline oxygenate 73.9 65.4 85.7 53.7 59.0 46.4 8.5 5.3 13.3

Other Gasoline Compounds

Benzene 22.0 15.8 30.4 7.6 6.6 8.9 0.3 0.4 0.3

n-Butylbenzene 1.5 1.3 1.8 0.0 0.0 0.0 0.1 0.2 0.0

sec-Butylbenzene 1.5 2.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0

tert-Butylbenzene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Ethylbenzene 4.8 4.1 6.0 1.6 1.4 2.0 0.5 0.5 0.5

Naphthalene 9.9 10.5 9.1 2.3 3.9 0.0 0.2 0.2 0.3

Toluene 18.3 9.4 31.1 5.5 1.6 11.1 1.1 0.5 1.9

1,3,5-Trimethylbenzene 4.5 1.3 8.9 2.2 1.3 3.6 0.0 0.0 0.0

m,p-Xylene 8.6 5.3 13.5 3.1 1.3 5.8 0.8 0.7 0.8

o-Xylene 14.2 10.8 18.9 3.9 1.4 7.5 0.6 0.7 0.5

Any gasoline compound 35.8 29.5 44.6 9.0 7.7 10.7 2.2 1.9 2.7

Trihalomethanes

Bromoform 4.5 5.1 3.6 2.2 3.8 0.0 3.4 4.7 1.3

Bromodichloromethane 20.9 15.4 28.6 2.2 2.6 1.8 6.4 5.9 7.2

Chlorodibromomethane 12.7 10.3 16.1 1.5 2.6 0.0 5.5 6.1 4.5

Chloroform 48.4 55.6 38.9 17.5 23.6 9.3 12.7 11.8 14.0

Any trihalomethane 47.8 53.8 39.3 16.4 21.8 8.9 14.8 15.2 14.0

Solvents

Bromobenzene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Carbon tetrachloride 2.2 3.8 0.0 1.5 2.6 0.0 0.7 0.9 0.5

Chlorobenzene 3.7 5.1 1.8 1.5 2.6 0.0 0.1 0.2 0.0

Chloroethane 0.7 1.3 0.0 0.0 0.0 0.0 0.1 0.2 0.0 

2-Chlorotoluene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

4-Chlorotoluene 0.7 1.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Occurrence and Temporal Variability of Volatile Organic Compounds 13

Solvents—Continued

Dibromomethane 0.7 1.3 0.0 0.7 1.3 0.0 0.2 0.4 0.0

1,2-Dichlorobenzene 1.5 2.6 0.0 0.7 1.3 0.0 0.1 0.2 0.0

1,3-Dichlorobenzene 2.2 3.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1,1-Dichloroethane 15.9 26.3 1.8 5.3 9.2 0.0 1.2 1.4 0.0

1,2-Dichloroethane 11.9 20.5 0.0 4.5 7.7 0.0 0.3 0.5 0.0

1,1-Dichloroethene 6.0 10.3 0.0 3.7 6.4 0.0 0.8 1.4 0.0

cis-1,2-Dichloroethene 12.8 22.1 0.0 6.8 11.7 0.0 1.3 1.9 0.3

trans-1,2-Dichloroethene 4.5 7.7 0.0 2.2 3.8 0.0 0.0 0.0 0.0

1,2-Dichloropropane 3.7 6.4 0.0 0.7 1.3 0.0 0.1 0.2 0.0

Hexachloroethane 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Methylene chloride 0.8 1.4 0.0 0.8 1.4 0.0 0.2 0.2 0.3

Methyl ethyl ketone 0.8 1.3 0.0 0.0 0.0 0.0 0.5 0.4 0.8

n-Propylbenzene 3.0 1.3 5.4 0.8 1.3 0.0 0.1 0.0 0.3 

1,1,1,2-Tetrachloroethane 1.5 2.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Tetrachloroethene 21.1 36.4 0.0 13.5 23.4 0.0 2.8 4.2 0.8

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

1,2,4-Trichlorobenzene 3.0 2.6 3.6 0.0 0.0 0.0 0.0 0.0 0.0

1,1,1-Trichloroethane 14.2 24.4 0.0 7.5 12.8 0.0 1.0 1.7 0.0

1,1,2-Trichloroethane 1.5 2.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

Trichloroethene 17.3 29.9 0.0 10.5 18.2 0.0 2.3 3.3 0.8

1,2,3-Trichloropropane 3.8 3.8 3.7 0.0 0.0 0.0 0.1 0.2 0.0

Any solvent 41.8 61.5 14.3 21.6 37.2 0.0 6.3 8.8 2.4

Organic Synthesis

Acrylonitrile 0.7 1.3 0.0 0.7 1.3 0.0 0.0 0.0 0.0

Bromochloromethane 3.7 5.1 1.8 0.0 0.0 0.0 0.0 0.0 0.0

1,3-Dichloropropane 3.0 2.6 3.6 0.7 1.3 0.0 0.0 0.0 0.0

2,2-Dichloropropane 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1,1-Dichloropropene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Hexachlorobutadiene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Isopropylbenzene 3.8 5.2 1.8 0.8 1.3 0.0 0.2 0.2 0.3

p-Isopropyltoluene 0.7 0.0 1.8 0.0 0.0 0.0 0.0 0.0 0.0

Table 4. Summary of detection frequencies of any volatile organic compound (VOC), any subgroup of VOCs, and individual 

VOCs in source waters sampled in the Focused and Random Surveys—Continued

[VOC, volatile organic compound; CWS, community water system; n, number of samples]

Volatile organic

compound subgroup

and individual

compound name

Focused Survey Random Survey

Detection frequency at or above

the MDL (percent)

Detection frequency at or above

the MRL (percent)

Detection frequency at or above

the MRL (percent)

All CWSs

(n=134)

Ground

water

(n=78)

Surface

water

(n=56)

All CWSs

(n=134)

Ground

water

(n=78)

Surface

water

(n=56)

All CWSs

(n=954)

Ground

water

(n=579)

Surface

water

(n=375)
14 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Organic Synthesis—Continued

Styrene 16.3 21.3 9.3 3.8 6.6 0.0 0.5 0.4 0.8

1,2,4-Trimethylbenzene 7.6 1.3 16.7 1.5 1.3 1.9 0.4 0.2 0.8

1,2,3-Trichlorobenzene 5.2 6.4 3.6 0.0 0.0 0.0 0.0 0.0 0.0

Vinyl bromide 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Vinyl chloride 2.2 3.8 0.0 2.2 3.8 0.0 0.1 0.2 0.0

Any organic synthesis 32.8 33.3 32.1 7.5 11.5 1.8 1.0 0.7 1.6

Fumigants

Bromomethane 3.0 3.8 1.8 0.0 0.0 0.0 0.2 0.2 0.3

1,4-Dichlorobenzene 4.5 6.5 1.8 0.8 1.3 0.0 0.3 0.2 0.5

cis-1,3-Dichloropropene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 

trans-1,3-Dichloropropene 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Any fumigant 7.5 10.3 3.6 0.7 1.3 0.0 0.5 0.4 0.8

Refrigerants

Chloromethane 3.0 3.8 1.8 0.7 1.3 0.0 0.3 0.5 0.0

Dichlorodifluoromethane 0.7 1.3 0.0 0.7 1.3 0.0 0.4 0.7 0.0

1,1,2-Trichloro1,2,2-trifluoroethane0.7 1.3 0.0 0.7 1.3 0.0 0.1 0.2 0.0 

Trichlorofluoromethane 2.2 3.8 0.0 2.2 3.8 0.0 0.5 0.9 0.0 

Any refrigerant 5.2 7.7 1.8 3.0 5.1 0.0 1.0 1.7 0.0

Table 4. Summary of detection frequencies of any volatile organic compound (VOC), any subgroup of VOCs, and individual 

VOCs in source waters sampled in the Focused and Random Surveys—Continued

[VOC, volatile organic compound; CWS, community water system; n, number of samples]

Volatile organic

compound subgroup

and individual

compound name

Focused Survey Random Survey

Detection frequency at or above

the MDL (percent)

Detection frequency at or above

the MRL (percent)

Detection frequency at or above

the MRL (percent)

All CWSs

(n=134)

Ground

water

(n=78)

Surface

water

(n=56)

All CWSs

(n=134)

Ground

water

(n=78)

Surface

water

(n=56)

All CWSs

(n=954)

Ground

water

(n=579)

Surface

water

(n=375)

When considering data above the MDL and 

MRL as part of the Focused Survey, about 93 and 

61 percent of all source waters sampled contained at 

least one of the 66 VOCs analyzed, respectively. This 

compares to about 27 percent detected in source waters 

sampled as part of the Random Survey. Although detection frequencies were less in the Random Survey than 

in the Focused Survey, a similar number of VOCs were 

detected in both surveys. Fifty-eight and 41 VOCs were 

detected at least one time in the Focused Survey above 

the MDL and MRL, respectively, whereas, 42 were 

detected at levels above the MRL in the Random 

Survey. Thirty-seven of the 41 VOCs detected in the 

Focused Survey also were detected in the Random 

Survey. Again, only the detection frequencies above the 

MRL should be used for comparison to the Random 

Survey. A summary of detected concentrations of 

VOCs collected as part of the Focused Survey is presented in Appendix D. Additional discussion of the 

occurrence of VOCs in the Focused Survey above 

the MDL is included in the next section—Temporal 

Variability.

Each subgroup of VOCs was detected at or above 

the MRL at a higher frequency in the Focused Survey 

than in the Random Survey (table 4, fig. 3). The largest 

difference in detection frequencies was found in the 

gasoline oxygenate subgroup, wherein these compounds were detected in 53.7 percent of CWSs sampled 
Occurrence and Temporal Variability of Volatile Organic Compounds 15

in the Focused Survey but in only 8.5 percent of CWSs 

sampled in the Random Survey. This difference is due 

to the intentional selection of sites with suspected or 

known occurrence of MTBE in the Focused Survey. A 

slightly higher detection frequency for gasoline 

oxygenates was found in ground-water source waters 

(59.0 percent) than in surface-water source waters 

(46.4 percent) in the Focused Survey. This pattern was 

reversed, however, for samples collected in the 

Random Survey, where gasoline oxygenates were 

detected in 5.3 and 13.3 percent of ground and surface 

water, respectively. MTBE was the most frequently 

detected gasoline oxygenate in source waters sampled 

in the Focused and Random Surveys, occurring in 55.5 

and 8.7 percent, respectively.

A second large difference was found in the 

solvent subgroup, again with the larger detection 

frequency occurring in the Focused Survey. In both surveys, most of the solvent detections were in groundwater source waters (table 4). This may be due to solvents degrading slower in ground water than in surface 

water. The difference in detection frequency between 

the two surveys (fig. 3) may be due, in part, to intentionally sampling ground-water sites as part of the 

Focused Survey that were considered more vulnerable 

than other sites. That is, preference was given to sampling ground-water source waters withdrawn from 

shallow, unconsolidated, unconfined aquifers in urban 

areas serving large populations. The solvents detected 

most frequently in both surveys were tetrachloroethene 

and trichloroethene.

Interestingly, THMs were detected in the 

Focused and Random Surveys at approximately similar 

frequencies—16.4 and 14.8 percent of CWSs, respectively. Chloroform was the most frequently detected 

THM in both surveys. However, chloroform was 

detected more frequently in ground-water source 

waters in the Focused Survey (23.6 percent) than in the 

Random Survey (11.8 percent). Similar THM detection 

frequencies in both surveys indicates a pervasive occurrence of THMs in several source-water types regardless 

of CWS size or location.

Other gasoline compounds, organic synthesis 

compounds, fumigants, and refrigerants all were 

detected at or above the MRL in less than 10 percent of 

CWSs sampled in each survey. Although the gasoline 

oxygenate MTBE was detected frequently in the 

Focused Survey, VOCs in the other gasoline compound 

subgroup were detected in only 9 percent of CWS 

source waters. This may be due to these compounds

Figure 3. Detection frequency at or above the minimum reporting level of volatile organic compound

subgroups in community water system source waters sampled in the Focused Survey and the

Random Survey.

0

10

20

30

40

50

60

FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Solvents Trihalomethanes

Other

gasoline

compounds

Organic

synthesis

Refrigerants Fumigants

VOC SUBGROUP

Focused Survey

Random Survey
16 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

degrading and (or) volatilizing more rapidly in ground 

and surface water in comparison to MTBE. Organic 

synthesis compounds also were detected more frequently in the Focused Survey (7.5 percent) than in the 

Random Survey (1.0 percent). The majority of detections in the Focused Survey were found in groundwater source waters (11.5 percent) rather than in 

surface-water source waters (1.8 percent). Again, this 

may be due to the Focused Survey’s sampling urban 

ground-water source waters in shallow, unconsolidated, 

unconfined aquifers that may be susceptible to contamination. Very little difference in detection frequencies 

between either survey was observed for fumigants and 

refrigerants.

CWSs were subdivided into five size categories 

based on population served: (1) very small (serves 25 

to 500 people); (2) small (serves 501 to 3,300 people); 

(3) medium (serves 3,301 to 10,000 people); (4) large 

(serves 10,001 to 50,000 people); and (5) very large 

(serves more than 50,000 people). Table 5 lists the relative percent of CWSs per size of system sampled in the 

Focused and Random Surveys. A summary of source 

waters sampled per State by size of CWS is presented 

in Appendix A.

A fairly equal distribution of ground-water CWS 

sizes was sampled in the Focused Survey—from 10.4 

to 26 percent of each size category—whereas the 

majority of ground-water source waters sampled 

(50.4 percent) in the Random Survey were very small 

systems. The abundance of very small ground-water 

source waters in the Random Survey is due to the relative abundance of very small ground-water supplied 

CWSs on a national basis and reflects the random 

selection process used in the Random Survey. A disproportionate number of very large CWSs supplied by surface water were sampled (85.7 percent) in the Focused 

Survey. Similarly, the majority of CWSs supplied by 

surface water sampled (81.9 percent) as part of the 

Random Survey were large or very large CWSs. The 

abundance of large and very large surface-water 

systems sampled as part of the Random Survey was due 

to the cumulative population served, which was one 

weighting factor used in the design of that survey 

(Ivahnenko and others, 2001).

The highest VOC detection frequency in the 

Random Survey for each subgroup of VOCs (except 

THMs) was found in the very large CWSs supplied by 

ground water (fig. 4). THMs were detected slightly 

more often in the very small CWSs than in the very 

large CWSs. This likely reflects the fact that many 

more of the larger systems have source waters within 

high-population density, urban areas than do the 

smaller CWSs. This pattern was not evident in the 

Focused Survey for gasoline oxygenates, other gasoline compounds, solvents, and refrigerants (fig. 4). This 

is likely due to the fact that the source waters selected 

were considered more vulnerable to VOC contamination and MTBE was suspected or known to occur in the 

source, regardless of CWS size.

In the Random Survey, gasoline oxygenates and 

other gasoline compounds were detected more frequently in the very large CWSs supplied by surface 

water than in other size categories (fig. 5). This pattern 

was generally observed in the Focused Survey. 

Although the Focused and Random Surveys each sampled disproportionately more large and very large 

CWSs than other size categories, gasoline oxygenates, 

other gasoline compounds, solvents, THMs, and 

organic synthesis compounds were detected in virtually 

every size category sampled in the Random Survey. 

This was not observed in the Focused Survey. Gasoline 

oxygenates primarily were detected in large and very 

large CWSs; however, solvents, THMs, and organic 

synthesis compounds were not detected at all or only 

detected in a few sizes of CWSs. The reason for this 

difference is not known.

Table 5. Distribution of source waters sampled in the 

Focused and Random Surveys by source-type and size of 

community water system based on population served

Community water 

system size (based 

on population 

served)

System

Focused Random

Percent of ground-water systems sampled

Very small 10.4 50.4

Small 16.8 20.9

Medium 20.8 8.6

Large 26.0 10.9

Very large 26.0 9.2

Total 100.0 100.0

Percent of surface-water systems sampled

Very small 0.0 3.2

Small 7.1 6.9

Medium 1.8 8.0

Large 5.4 21.0

Very large 85.7 60.9

Total 100.0 100.0
Occurrence and Temporal Variability of Volatile Organic Compounds 17

Figure 4. Detection frequency at or above the minimum reporting level of any volatile organic

compound (VOC) and subgroup of VOCs in ground-water sources sampled in the Focused

Survey and Random Survey by size of community water system. FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Solvents TrihalomethanesOther

gasoline

compounds

Organic

synthesis

Refrigerants Fumigants

Focused Survey

0

10

20

30

40

50

60

70

80

Any VOC

FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Solvents TrihalomethanesOther

gasoline

compounds

Organic

synthesis

Refrigerants Fumigants

Random Survey

0

10

20

30

40

50

60

70

80

Any VOC

VOC SUBGROUP

VOC SUBGROUP

Very small (25 to 500)

Small (501 to 3,300)

Medium (3,301 to 10,000)

Large (10,001 to 50,000)

Very large (more than 50,000)

Very small (25 to 500)

Small (501 to 3,300)

Medium (3,301 to 10,000)

Large (10,001 to 50,000)

Very large (more than 50,000)
18 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Figure 5. Detection frequency at or above the minimum reporting level of any volatile organic

compound (VOC) and subgroup of VOCs in surface-water sources sampled in the Focused

Survey and Random Survey by size of community water system. FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Solvents TrihalomethanesGasoline

compounds

Organic

synthesis

Refrigerants Fumigants

Focused Survey

0

10

20

30

40

50

60

Any VOC

FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Solvents TrihalomethanesGasoline

compounds

Organic

synthesis

Refrigerants Fumigants

Random Survey

Any VOC

0

10

20

30

40

50

60

VOC SUBGROUP

VOC SUBGROUP

Very small (25 to 500)

Small (501 to 3,300)

Medium (3,301 to 10,000)

Large (10,001 to 50,000)

Very large (more than 50,000)

Very small (25 to 500)

Small (501 to 3,300)

Medium (3,301 to 10,000)

Large (10,001 to 50,000)

Very large (more than 50,000)
Occurrence and Temporal Variability of Volatile Organic Compounds 19

Temporal Variability

Samples were collected for one year at fixed 

intervals during the Focused Survey. Ground-water 

source waters were sampled biannually, reservoir and 

(or) lake source waters were sampled quarterly, and 

river and (or) stream source waters were sampled eight 

times per year (table 2). The intent of the fixed-interval 

sampling was to obtain sufficient data to evaluate temporal variability of VOCs in each source-water type. To 

do so, samples were collected more frequently in surface-water source waters than in ground-water source 

waters to account for likely changes in water quality 

caused by temperature and other seasonal affects.

The ground-water sampling design did not specifically distribute samples seasonally as was done for 

surface-water sampling. Ground-water samples were 

collected approximately six months apart and only two 

samples were collected from each well. Because of the 

slow movement of ground water, the elapsed time 

between samples was presumed insufficient to allow 

for significant changes in ground-water quality to 

occur. Thus, analyses of temporal variability in groundwater source waters in this report compare only the first 

sample to the second sample and do not evaluate 

seasonal patterns. In contrast, the analyses of temporal 

variability in surface-water source waters do evaluate 

seasonal patterns.

The four seasons considered in the temporal 

analyses were defined as spring (April-June), summer 

(July-September), fall (October-December), and winter 

(January-March). The number of reservoir and (or) 

lake source-water samples collected were distributed 

fairly evenly among the different seasons (fig. 6). The 

number of reservoir and lake samples collected ranged 

from 37 during winter to 46 in the spring. However, 

more samples were collected from river and (or) stream 

source waters during winter than in other seasons; 

about 30 river and (or) stream samples were collected 

during the spring, summer, and fall, and 52 samples 

were collected during the winter. Generally, this may 

result in an increase in VOC occurrence in rivers and 

streams during the winter months due to higher air-towater partitioning and decreased volatility of VOCs at 

winter temperatures.

Temporal analyses were performed to examine 

seasonal and concentration differences in those subgroups of VOCs that occur most frequently in CWS 

source water. It is important to note that all data above 

the MDL were used in the temporal analyses to maximize the number of VOC detections that are available 

for analyses. A summary of concentrations detected in 

source water is presented in Appendix D.

Figure 6. Number of surface-water samples collected by source type and season.

0

10

20

30

40

50

60

70

80

90

100

NUMBER OF SAMPLES COLLECTED

Spring Summer Fall Winter

SEASON

Reservoirs and (or) lakes

Rivers and (or) streams

Total
20 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Analyses of the data from the 134 CWS source 

waters previously used for comparing the Focused 

Survey to the Random Survey (table 4) showed that at 

least 1 of the 66 VOCs analyzed was detected in more 

than 90 percent of the source waters sampled as part of 

the Focused Survey (fig. 7). Each of the 17 CWSs supplied by rivers or streams contained at least one VOC, 

a result of the detection of a gasoline oxygenate in each 

of the 17 CWS river and (or) stream source waters. In 

addition, gasoline oxygenates, other gasoline compounds, THMs, and organic synthesis compounds were 

detected more frequently in CWSs supplied by river 

source waters than in any other source water sampled. 

This may be due to the presence of point sources 

upstream from the sampling site that provide a fairly 

continuous source of contamination.

Gasoline oxygenates, other gasoline compounds, 

THMs, and organic synthesis compounds were 

detected in more than 25 percent of CWSs served by 

ground-water, reservoir and (or) lake, and river and (or) 

stream source waters (fig. 7). Therefore, these four subgroups of VOCs were used in subsequent temporal 

analyses. In addition, solvents were evaluated only in 

ground-water source waters because they were not 

detected frequently in surface-water source waters. 

Fumigants and refrigerants were not analyzed for temporal variability due to their infrequent detection.

Ground Water

Analyses of data from all 78 ground-water 

source waters sampled biannually revealed that detection frequencies of VOC subgroups did not change significantly from the first sample collected to the second 

sample (fig. 8). Similarly, no statistically significant 

differences in detected concentrations of VOCs were 

found at the 95-percent confidence level using the 

Wilcoxon rank sum test (Helsel and Hirsch, 1992) 

(fig. 9). Because samples were collected approximately 

6 months apart, it is presumed that an insufficient 

amount of time passed to show any significant changes 

in ground-water quality.

Although these subgroups of VOCs occur frequently in the source waters sampled, the concentrations detected were small, typically less than 1 μg/L. 

However, some higher concentrations were 

observed—maximum concentrations of MTBE, tetrachloroethene, and trichloroethene in ground-water 

source waters were 924, 290, and 165 μg/L, respectively.

Figure 7. Detection frequency at or above the minimum reporting level of any volatile organic

compound (VOC) and by subgroup of VOCs in community water system source waters by source

type.

0

20

40

60

80

100

120

FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Any VOC Solvents TrihalomethanesOther

gasoline

compounds

Organic

synthesis

Fumigants Refrigerants

VOC SUBGROUP

All source types

Ground water

Reservoirs and (or) lakes

Rivers and (or) streams
Occurrence and Temporal Variability of Volatile Organic Compounds 21

Figure 8. Comparison of the detection frequency of at least one volatile organic compound (VOC)

at or above the method detection limit by subgroup of VOCs in the first and second sampling of

ground-water source water.

Figure 9. Detected concentrations of volatile organic compound subgroups in ground-water source water collected in

the first and second samples.

0

10

20

30

40

50

60

70

80

FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Other gasoline Trihalomethanes Solvents

compounds

Organic synthesis

VOC SUBGROUP

First sample

Second sample

0.01

0.1

1

10

100

1,000

10,000

CONCENTRATIONS, IN MICROGRAMS PER LITER

Gasoline

oxygenates

Other gasoline Trihalomethanes Solvents

compounds

Organic

synthesis

50 52 22 19 45 41 50 47 28 23

x

x

x

x

x

o

x x

x

x

x

x

30 Number of values 

EXPLANATION

x

x

o

75th percentile

Median

25th percentile

Lower outside

Upper detached

A

AA AA AA AA AA

Populations that share

 the same letter are

 not significantly dif-

 ferent at a 95-percent

 confidence interval

Upper outside

First sample

Second sample

VOC SUBGROUP
22 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Reservoirs and Lakes

A slight seasonal pattern was evident in the 

occurrence of gasoline oxygenates and other gasoline 

compounds (fig. 10) in reservoir and lake source 

waters. Analyses of data from all 39 reservoirs and (or) 

lakes sampled quarterly revealed that both of these subgroups of VOCs were detected more frequently in the 

spring and summer than in the fall and winter. However, these compounds were still fairly prevalent during 

the fall and winter, when they were detected in at least 

27 percent of samples. Thirty-three of the 39 reservoirs 

sampled allow motorized watercraft. Thus, the greater 

detection of gasoline-related compounds in the spring 

and summer is presumed to be related to the increased 

use of motorized watercraft during these seasons. The 

continued detection of these compounds during fall and 

winter may be due, in part, to the continued use of 

watercraft containing reformulated gasoline during 

these seasons in areas with warmer climates, and the 

chemical characteristics of MTBE, which allow it to 

persist longer in water in comparison to other VOCs.

Trihalomethanes and organic synthesis compounds were detected in about 30 percent of samples 

collected throughout the year with no significant trend 

identified. The lack of any substantial, seasonal 

variability in THMs and organic synthesis compounds 

indicates the possibility of a common and continuous 

source of contamination for these compounds. 

Concentrations of VOCs detected in reservoir 

and lake source waters were typically less than 1 μg/L,

and very few statistically significant differences in 

seasonal concentrations were identified (fig. 11). 

Although gasoline oxygenates and other gasoline compounds were detected more frequently in the spring and 

summer than in fall and winter, statistically significant 

differences in detected median concentrations of VOCs 

typically were not identified by the Wilcoxon rank sum 

test at an alpha of 0.05. There were no statistically significant differences between seasonal concentrations of 

gasoline oxygenates and THMs. Statistically significant differences were found for other gasoline compounds and organic synthesis compounds. For these 

VOC subgroups, winter and (or) fall concentrations 

were significantly less than spring and (or) summer 

concentrations. 

Figure 10. Detection frequency of at least one volatile organic compound (VOC) at or above the

method detection limit by subgroup of VOCs in reservoir and lake source waters by season.

0

20

10

30

40

50

60

70

80

90

100

FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Other gasoline Trihalomethanes

compounds

Organic synthesis

VOC SUBGROUP

Spring

Summer

Fall

Winter
Occurrence and Temporal Variability of Volatile Organic Compounds 23

Figure 11. Detected concentrations of volatile organic compound subgroups in reservoir and lake source waters by

season.

0.01

0.1

1

10

100

CONCENTRATIONS, IN MICROGRAMS PER LITER

Gasoline

oxygenates

Other gasoline Trihalomethanes

compounds

Organic

synthesis

20 Number of values 

EXPLANATION

x

o

75th percentile

Median

25th percentile

Upper detached

A

A A A A AB B AC C A A A A A A B AB

Populations that share

 the same letter are

 not significantly dif-

 ferent at a 95-percent

 confidence interval

Upper outside

Spring

Summer

Fall

Winter

VOC SUBGROUP

43 46 31 29 69 58 20 27 18 21 23 29 16 15 8 11

x

x

x

o

Rivers and Streams

No seasonal pattern was evident for any of the 

more frequently occurring subgroups of VOCs in river 

and stream source waters (fig. 12). The detection of 

gasoline oxygenates was found to drop slightly from 

spring to winter; however, these VOCs were found 

to be consistently present in the source waters sampled—in at least 78 percent of the samples. This may 

be indicative of a common source of gasoline oxygenates to the source waters sampled. Other gasoline compounds, THMs, and organic synthesis compounds also 

were detected at similar frequencies throughout the 

year. Other gasoline compounds and THMs were 

detected in more than 50 percent of all samples, and 

organic synthesis compounds were detected in more 

than 20 percent of all samples collected throughout the 

year. Again, the absence of seasonal differences for 

these subgroups of compounds may indicate an 

unknown, but common and continuous source of 

contamination.

Concentrations of the VOCs (subgroups) most 

frequently detected in river and stream source waters 

were typically less than 1 μg/L, and few statistically 

significant differences in concentrations were noticed 

on a seasonal basis (fig. 13). Although these subgroups 

of VOCs were found frequently year round (fig. 12), 

the only statistically significant differences in the concentrations of VOCs were within the gasoline oxygenate subgroup, which were found at slightly greater 

concentrations during the summer than during the 

winter season.
24 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Figure 12. Detection frequency of at least one volatile organic compound (VOC) at or above the

method detection limit by subgroup of VOCs in river and stream source waters by season.

Figure 13. Detected concentrations of volatile organic compound subgroups in river and stream source waters by season.

0

20

10

30

40

50

60

70

80

90

100

FREQUENCY OF DETECTION, IN PERCENT

Gasoline

oxygenates

Other gasoline Trihalomethanes

compounds

Organic synthesis

VOC SUBGROUP

Spring

Summer

Fall

Winter

0.01

0.1

1

10

100

CONCENTRATIONS, IN MICROGRAMS PER LITER

Gasoline

oxygenates

Other gasoline Trihalomethanes

compounds

Organic

synthesis

20 Number of values 

EXPLANATION

x

o

75th percentile

Median

25th percentile

Upper detached

A

AB B BC AC A A A A A A A A A A A A

Populations that share

 the same letter are

 not significantly dif-

 ferent at a 95-percent

 confidence interval

Upper outside

Spring

Summer

Fall

Winter

x

x

x

xx

x

x

x

x

xx x

x

x

x

x

o

o

x

x

VOC SUBGROUP

35 29 28 48 34 30 38 46 28 33 26 80 14 6 14 9
Anthropogenic Factors Associated with MTBE, Gasoline Oxygenates, and Trihalomethanes 25

ANTHROPOGENIC FACTORS ASSOCIATED 

WITH MTBE, GASOLINE OXYGENATES,

AND TRIHALOMETHANES

Few temporal patterns were found when examining occurrence and concentrations of VOC subgroups 

in source waters where MTBE was suspected or known 

to occur. One apparent trend was found in reservoirs 

and lakes, where gasoline oxygenates and other gasoline compounds were detected more frequently during 

the spring and summer. This was likely associated with 

the increased use of motorized watercraft on the reservoirs during these seasons. However, no significant 

statistical differences were noticed when examining 

MTBE concentrations alone on a seasonal basis. 

Although one or two VOCs typically dominate the 

occurrence within VOC subgroups (table 4), the 

increased number of potential detections in subgroups 

should be beneficial in detecting significant trends not 

otherwise noticed when examining individual VOCs. 

As such, the most frequently detected VOC (MTBE) 

and the two most frequently occurring subgroups of 

VOCs (gasoline oxygenates and THMs) in the Focused 

Survey were further characterized to evaluate anthropogenic factors that may better explain and (or) to understand their frequent occurrence.

The two most frequently occurring subgroups of 

VOCs in the Focused Survey were gasoline oxygenates 

and THMs; these subgroups were detected in 73.9 and 

47.8 percent of CWS source waters sampled, respectively (table 4). In CWS source waters, MTBE was the 

most frequently detected gasoline oxygenate 

(77.3 percent) followed by TAME (20.3 percent). Chloroform and bromodichloromethane were the two most 

frequently detected THMs, occurring in 48.4 and 

20.9 percent of CWS source waters, respectively. 

 Anthropogenic factors associated with gasoline 

oxygenates were evaluated on the basis of MTBE highuse areas for CWSs served by ground-water, reservoir 

and (or) lake, and river and (or) stream source waters, 

and on watercraft use on reservoirs and lakes. Anthropogenic factors associated with THMs detected in 

reservoir source waters were evaluated on the basis of 

ancillary data provided by CWS personnel. All analyses were performed using data at or above the MDL.

MTBE and Gasoline Oxygenates

Detected concentrations of MTBE in groundwater source waters inside and outside of MTBE highuse areas were compared. Fifty-five wells were within 

MTBE high-use areas and 23 were outside of these 

areas. MTBE was detected more frequently in MTBE 

low-use areas than in MTBE high-use areas. When 

both samples collected from individual wells were 

considered, MTBE was detected in 73.9 percent (34 of 

46) of ground-water samples collected in MTBE lowuse areas and in 62.7 percent (69 of 110) of groundwater samples collected in MTBE high-use areas. The 

differences in occurrence and concentration may be 

explained, in part, by how these sites were selected. 

Thirty-eight CWS wells were selected because of the 

suspected occurrence of MTBE and 37 were selected 

because of a known occurrence (Appendix A). However, 21 of the 37 sites where MTBE was known to 

occur were not in areas of MTBE high use. This comparison is then skewed toward those sites outside of 

MTBE high-use areas and is merely a result of intentionally selecting wells with a known occurrence of 

MTBE. For this reason, statistical comparisons were 

not completed.

Concentrations of MTBE in reservoir and (or) 

lake source waters inside and outside of MTBE highuse areas were compared seasonally (fig. 14). Twentyfive reservoir and (or) lake sources were inside MTBE 

high-use areas and 14 were outside of these areas. 

MTBE was detected more frequently and concentrations were greater in MTBE high-use areas than in 

MTBE low-use areas. This is likely due, in part, to the 

much larger percentage of MTBE volume/volume 

present in gasoline in MTBE high-use areas than in 

other areas. However, because as few as four detections of MTBE were found in reservoir and (or) lake 

source waters in MTBE low-areas, statistical comparisons were not completed.

Concentrations of MTBE in river and (or) 

stream source waters inside and outside of MTBE 

high-use areas also were compared seasonally 

(fig. 15). Eleven of the river and (or) stream intakes 

were inside MTBE high-use areas and six were outside of these areas. Similar to reservoir and (or) lake 

source waters, MTBE was detected more frequently 

and concentrations were greater in MTBE high-use 

areas than in MTBE low-use areas. However, because 

as few as three detections of MTBE were found in 

river and (or) stream sources in MTBE low-use areas, 

statistical comparisons were not completed.
26 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Figure 14. Detected concentrations of methyl tert-butyl ether (MTBE) in reservoir and lake source waters by location

in MTBE low- and high-use areas.

0.01

0.1

1

10

100

CONCENTRATIONS, IN MICROGRAMS PER LITER

Spring Summer Fall Winter

4 20 12 20 7 14 4 17

30 Number of values 

EXPLANATION

x

x

75th percentile

Median

25th percentile

Lower outside

Upper outside

MTBE low-use area

MTBE high-use area

x

x

SEASON

Gasoline oxygenates were analyzed in relation to 

watercraft use as reported in an ancillary data form 

completed by personnel at each CWS (Ivahnenko and 

others, 2001). The average number of watercraft per 

reservoir per year was divided by the surface area (in 

acres) of each respective reservoir to obtain a watercraft-use factor (number of boats per acre per year). In 

order to characterize the effect of watercraft on the 

occurrence and concentration of gasoline oxygenates, 

the sum of the average detected concentration of all 

gasoline oxygenates for each reservoir was plotted 

relative to the watercraft-use factor in MTBE high- and 

low-use areas (fig. 16). 

Linear regression analyses indicated a weak positive trend associated with the average gasoline oxygenate concentration and watercraft use in MTBE highuse areas. The relation between the average gasoline 

oxygenate concentration and watercraft use in reservoirs inside MTBE high-use areas resulted in an r2 of 

0.3783. However, no relation was observed for reservoirs in MTBE low-use areas (r2=0.0242). In general, 

as watercraft use increased in areas of MTBE high-use, 

the average concentration of gasoline oxygenates also 

increased.

All watercraft-use values (inside and outside of 

MTBE high-use areas) were then divided into four 

quartile categories: (1) 0 to 25th percentile; (2) 26th to 

50th percentile; (3) 51st to 75th percentile; and (4) 76th 

to 100th percentile. The average gasoline oxygenate 

concentrations were then plotted per quartile category 

(fig. 17). Again, the average concentration of gasoline 

oxygenates increased as watercraft use increased.
Anthropogenic Factors Associated with MTBE, Gasoline Oxygenates, and Trihalomethanes 27

Figure 15. Detected concentrations of methyl tert-butyl ether (MTBE) in river and stream source waters by location in

MTBE low- and high-use areas.

Figure 16. Average sum of gasoline oxygenate concentrations versus watercraft-use factor.

0.01

0.1

1

10

CONCENTRATIONS, IN MICROGRAMS PER LITER

Spring Summer Fall Winter

4 21 5 18 3 17 4 23

30 Number of values 

EXPLANATION

x

75th percentile

Median

25th percentile

Lower detached

Upper outside

MTBE low-use area

MTBE high-use area

o o

x

SEASON

0

1

2

3

4

5

6

7

8

0 5 10 15 20 25 30 35 40 45

WATERCRAFT USE FACTOR, IN NUMBER OF BOATS PER ACRE PER YEAR 

CONCENTRATION, IN MICROGRAMS PER LITER

r2 = 0.3783

r2 = 0.0242

MTBE high-use area

MTBE low-use area 

Linear (MTBE high-use area)

Linear (MTBE low-use area )
28 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Figure 17. Average concentrations of gasoline oxygenates by watercraft-use quartiles.

0.01

0.1

1

10

CONCENTRATIONS, IN MICROGRAMS PER LITER

Quartile 1 Quartile 2 Quartile 3 Quartile 4

9 10 9 9

10 Number of values 

EXPLANATION

WATERCRAFT USE

75th percentile

Median

25th percentile

A

A AB BC C

Populations that share

 the same letter are

 not significantly dif-

 ferent at a 95-percent

 confidence interval

Trihalomethanes

Trihalomethanes were detected in 11 of the 

reservoirs and (or) lakes sampled in the Focused 

Survey. The most frequently detected THM was chloroform, which was found in 10 of the 39 reservoirs and 

(or) lakes (25.6 percent), followed by bromodichloromethane, which was found in 7 of the 39 reservoirs 

and (or) lakes (17.9 percent). Seven reservoirs and (or) 

lakes serving CWSs contained detectable concentrations of at least two THMs. THMs commonly are 

referred to as disinfection by-products because they can 

occur in drinking water that has been treated with chlorine. Because samples in the Focused Survey were collected directly from the source water—not finished 

drinking water—each of these seven CWSs were contacted to better understand the occurrence of these compounds, and each provided input to the occurrence and 

likely source of THMs in their source water.

CWS personnel indicated that THMs were 

present in their source water as disinfection by-products. Three of the CWSs return filter backwash to the 

source-water reservoir; one disinfects water from other 

sources with chlorine and then releases the disinfected 

water to the reservoir; one CWS adds chlorine upstream 

of the source reservoir for algae control; one CWS has 

a powerplant that withdraws water from the reservoir, 

disinfects the water with chlorine for algal control, uses 

the water for cooling purposes, and then releases the 

water back to the reservoir. Lastly, one CWS indicated 

that the source of THMs in the source water could be 

due to a wastewater treatment facility discharging 

upstream of the reservoir.

THMs were detected in 11 of the 17 rivers and 

(or) streams sampled in the Focused Survey. Chloroform was the most frequently detected THM, occurring 

in 11 of the 17 rivers and (or) streams (64.7 percent). 

Nine of these 11 rivers and (or) streams contained at 

least two THMs.
Summary and Conclusions 29

The CWSs that obtain their source water from the 

rivers and (or) streams with more than one detectable 

concentration of any THM were not contacted for additional information to better understand the occurrence 

of the THMs. It is presumed, however, that the sources 

of these compounds may include point-source discharges upstream from the CWS intakes. Discharges 

could include those from wastewater treatment plants, 

power plants, and industrial discharge. Nonpoint 

sources, such as runoff of treated irrigation water from 

lawns, inflow of ground water containing THMs, and 

combined sewer overflows that are chlorinated prior to 

discharge, also could explain the occurrence of these 

compounds.

SUMMARY AND CONCLUSIONS

The U.S. Geological Survey National WaterQuality Assessment Program in cooperation with the 

Metropolitan Water District of Southern California 

(MWDSC) and the Oregon Health & Science University completed an investigation of the Nation’s 

drinking-water supplies. The study was sponsored by 

the American Water Works Association Research 

Foundation to provide information on the occurrence, 

concentration, and temporal variability of methyl tertbutyl ether (MTBE) and other volatile organic compounds (VOCs) in source waters used by community 

water systems (CWSs). 

The national investigation was completed in two 

stages: (1) reviews of available literature, and (2) the 

collection of new drinking-water data. Two surveys 

were completed for the collection of new drinkingwater data. The first, termed the Random Survey, 

employed a statistically stratified design for sampling 

source waters from 954 randomly selected CWSs based 

on the total number of systems within five populationserved size categories, source-water type, and the total 

number of people served by each of the source-size 

categories used within the United States. The second 

survey, which is the focus of this report, is termed the 

Focused Survey, which included samples collected 

from CWS source waters that are suspected or known to 

contain MTBE. The intent of the Focused Survey was 

to provide an improved understanding of the occurrence, concentration, temporal variability, and some 

anthropogenic factors associated with the frequent 

occurrence of MTBE and other frequently identified 

VOCs.

Each sample collected was analyzed for 66 

VOCs, including MTBE and three other ether gasoline 

oxygenates (hereafter termed gasoline oxygenates), at 

the MWDSC laboratory. The 451 samples were collected at fixed intervals for one year from 78 groundwater, 39 reservoir and (or) lake, and 17 river and (or) 

stream source waters in select areas of the Nation. 

Ground-water source waters were sampled biannually, 

reservoirs and lakes were sampled quarterly, and 

rivers and streams were sampled eight times per year. 

In addition, 744 field quality-control (QC) samples, 

including trip-reagent blanks, field-reagent blanks, 

field-equipment blanks, and duplicate samples were 

collected. All data underwent an exhaustive QC 

review process to accept or reject source-water data 

for inclusion in the Focused Survey’s findings.

Occurrence data from the Focused Survey were 

compared to results obtained from the Random Survey 

using a common assessment level of 0.2 μg/L. Fortyone and 42 individual VOCs were detected in the 

Focused Survey and Random Survey, respectively, 37 

of which were detected in both surveys. However, the 

detection frequency of many VOCs was greater in 

samples collected as part of the Focused Survey than 

in the Random Survey. Because source waters suspected or known to contain MTBE were sampled in 

the Focused Survey, MTBE was detected in 

55.5 percent of the 134 CWSs sampled in the Focused 

Survey, whereas MTBE was detected in only 

8.7 percent of 954 CWSs sampled in the Random 

Survey. The increased detection frequency of other 

VOCs in the Focused Survey may be explained by 

high-population density, which was one factor associated with the selection of source waters to be sampled 

as part of the Focused Survey.

The 66 VOCs analyzed were subdivided into 

seven subgroups of VOCs, based mainly on the primary use of the compounds. Each subgroup of VOCs 

was found more frequently in the Focused Survey than 

in the Random Survey. The gasoline oxygenate subgroup, which includes MTBE, was found most frequently and was detected in 53.7 percent of CWSs in 

the Focused Survey and in 8.5 percent of CWSs sampled in the Random Survey. Again, this difference is 

due largely to the selection of sites with suspected or 

known occurrence of MTBE. A large difference in 

occurrence also was observed in the solvent subgroup; 

solvents were detected in 21.6 percent and 6.3 percent 

of CWSs sampled in the Focused and Random Survey, 

respectively. The two most frequently detected solvents in both surveys were tetrachloroethene and 
30 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

trichloroethene; however, these compounds were 

mainly detected only in CWSs supplied by ground 

water. Little difference in occurrence was observed for 

the THM subgroup; these compounds were detected in 

16.4 percent of Focused Survey and 14.8 Random 

Survey CWSs, respectively. This may indicate a pervasive occurrence of THMs in source waters, regardless 

of CWS size or geographic location.

In contrast to results of the Random Survey, the 

occurrence of subgroups of VOCs does not appear to be 

related to the size (based on population served) of 

CWSs in the Focused Survey supplied by ground water. 

In the Focused Survey, each subgroup of VOCs was 

detected at similar frequencies in each CWS size category. However, there were some differences in CWSs 

supplied by surface water in that the gasoline oxygenate and other gasoline compound subgroups were 

predominantly detected in the Focused Survey, whereas 

solvents, THMs, and organic synthesis compounds 

were detected more frequently in several sizes of CWSs 

sampled in the Random Survey.

Only two samples were collected from each 

ground-water source water included in the Focused 

Survey; thus, temporal variations on a seasonal basis 

could not be identified. However, comparisons were 

made between the first sample and the second sample 

collected from 78 CWSs. No apparent differences were 

observed for any subgroup of VOCs between the first 

and second ground-water source water samples analyzed. It is presumed that an insufficient amount of time 

passed between the collection of the first and second 

ground-water sample, typically 6 months, to observe 

any significant changes in ground-water quality. The 

occurrence of each subgroup of VOCs was virtually 

identical between the two samples, and no statistically 

significant differences at an alpha of 0.05 were noticed 

in the detected VOC concentrations. Detected concentrations were small, typically less than 1 μg/L. However, the maximum concentrations of MTBE, 

tetrachloroethene, and trichloroethene in ground-water 

source waters were 924, 290, and 165 μg/L, respectively.

On the basis of data at or above the method 

detection limit, concentrations of VOCs detected in the 

Focused Survey in reservoir and (or) lake source waters 

were typically small (less than 1 μg/L). Furthermore, 

very few statistically significant differences in detected 

concentrations were noticed on a seasonal basis among 

subgroups of VOCs. However, a slight seasonal pattern 

was evident for the occurrence of gasoline oxygenates 

and other gasoline compounds in the 39 reservoirs and 

lakes sampled. Both of these subgroups of VOCs were 

detected more frequently during spring and summer 

than in fall and winter. Thirty-three of the 39 reservoir 

and (or) lakes sampled allow motorized watercraft. 

Thus, the greater occurrence of these compounds 

during spring and summer is presumed to be related to 

the increased use of watercraft during these seasons. 

No seasonal pattern was observed for the THMs or 

organic synthesis subgroups. This indicates the possibility of a common and continuous source of contamination for these VOCs.

As in ground-water and reservoir and (or) lake 

source waters, concentrations of VOCs in river and (or) 

stream source waters typically were small (less than 

1 μg/L), and very few statistically significant differences in detected concentrations were observed on a 

seasonal basis among subgroups of VOCs. Similarly, a 

seasonal pattern was not evident for the detection frequency of any subgroup of VOCs in the 17 river and 

(or) stream source waters sampled. Gasoline oxygenates, other gasoline compounds, THMs, and organic 

synthesis compounds were detected at similar frequencies within their respective subgroups year-round. The 

lack of seasonal differences for these subgroups of 

compounds may indicate a common and continuous 

source of contamination upstream of the intake location.

The most frequently detected VOC (MTBE) and 

the two most frequently occurring subgroups of VOCs 

(gasoline oxygenates and THMs) detected in CWS 

source waters sampled in the Focused Survey were further characterized to evaluate some anthropogenic factors that may better explain their frequent occurrence. 

MTBE, gasoline oxygenates, and THMs were detected 

in 77.3, 73.9, and 47.8 percent of CWS source waters 

sampled, respectively.

Fifty-five of 78 wells, 25 of 39 reservoirs and 

(or) lakes, and 11 of 17 rivers and streams were inside 

MTBE high-use areas. Concentrations of MTBE in reservoir and (or) lake and river and (or) stream source 

waters inside and outside of MTBE high-use areas 

were also compared seasonally. MTBE was detected 

more frequently and concentrations were greater in 

MTBE high-use areas than in MTBE low-use areas. 

However, because as few as three detections of MTBE 

were found in surface-water source waters in MTBE 

low-use areas, statistical comparisons were not completed.
References 31

The concentration of gasoline oxygenates in 

reservoirs was slightly correlated with watercraft use 

(boats/year/acre) on reservoirs inside MTBE high-use 

areas (r2

=0.3783). As watercraft use increased, the 

average concentration of gasoline oxygenates also 

increased. No relation between gasoline oxygenates in 

reservoirs and watercraft use was observed in reservoirs 

outside of MTBE high-use areas (r2

=0.0242).

Trihalomethanes were detected in 11 of the 

reservoirs and (or) lakes sampled. Seven of these reservoirs contained detectable concentrations of more than 

one THM. Attempts were made to contact each of the 

seven CWSs with multiple THM detections. Each CWS 

provided additional information on reasons why THMs 

may occur in their source water. In all cases, the presence of THMs were the result of the addition of chlorine to waters that ultimately recycled back to, or 

upstream of, the reservoir that was sampled.

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on methyl tertiary butyl ether (MTBE), October 1, 1999: 

Arizona Department of Environmental Quality, 

accessed March 1, 2002, at URL 

http://www.adeq.state.az.us/environ/waste/ust/ 

mtbe.html

Bourelle, Andy, 1998, MTBE—A real concern for South 

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http://www.tahoe.com/tribune/stories.8.28.98/news/ 

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Hornig, C.E., and Lanning-Rush, Jennifer, 2000, Water 

quality in south-central Texas, 1996-98: U.S. Geological Survey Circular 1212, 32 p.

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California drinking water: accessed December 6, 2000, 

at URL http://www.dhs.cahwnet.gov/ps/ddwem/ 

chemicals/MTBE/mtbeindex.htm

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(methyl tertiary butyl ether), Briefing paper prepared by 

the California Environmental Protection Agency, 

April 24, 1997 (updated September 3, 1998): accessed 

February 28, 2002, at URL 

http://www.arb.ca.gov/cbg/oxy/oxy.htm

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http://www.sen.ca.gov/ftp/sen/sor/environ/98mtbe.htm

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http://epwm.santamonica.org/epwm/ 

watquality/1999report/waterweb/news05.htm

Delzer, G.C., and Ivahnenko, Tamara, 2003, A review of 

literature for methyl tert-butyl ether in sources of 

drinking water in the United States: U.S. Geological 

Survey Open-File Report 01-322, 19 p.

Grady, S.J., 2003, A national survey of methyl tert-butyl 

ether and other volatile organic compounds in 

drinking-water sources—Results of the random 

survey: U.S. Geological Survey Water-Resources 

Investigations Report 02-4079, 85 p.

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design and analytical results used to evaluate a 

surface-water point sampler for volatile organic 

compounds: U.S. Geological Survey Open-File 

Report 98-651, 31 p.

Hattan, Greg, 2000, MTBE treatment systems in Kansas: 

Association of State and Territorial Solid Waste 

Management Officials, MTBE and Fuel Oxygenates 

Workgroup Newsletter, v. 3, no. 1, 31 p.

Helsel, D.R., and Hirsch, R.M., 1992, Statistical methods in 

water resources: New York, Elsevier, 529 p.

Ivahnenko, Tamara, Grady, S.J., and Delzer, G.C., 2001, 

Design of a national survey of methyl tert-butyl ether 

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Zogorski, J.S., 2000, MTBE—To what extent will past 

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Ground-water data collection protocols and procedures for the National Water-Quality Assessment 

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Environmental Toxicology and Chemistry, v. 20, 

no. 4, p. 727-737.

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seasonal variability of volatile organic compounds in 

seven New Jersey streams: U.S. Geological Survey 

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APPENDIXES

Appendix A - Number of community water

system sources sampled per State
36 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table A1. Number of community water system source waters sampled that were suspected of having concentrations of 

methyl tert-butyl ether, by State and source-size category

[GW, ground water; SW, surface water; VSM, very small; SM, small; MED, medium; LRG, large; VLRG, very large]

State1

Source-size category

GW- Total

VSM

GWSMGWMEDGWLRGGWVLRGSWVSMSWSMSWMEDSWLRGSWVLRG

CA 0 0 2 4 6 0 0 0 1 6 19

CO 0 0 0 0 0 0 0 0 0 1 1

CT 0 0 0 2 1 0 1 0 0 0 4

DE 2 2 3 1 1 0 0 0 0 2 11

GA 0 0 0 1 0 0 0 0 0 2 3

IN 0 0 0 0 0 0 0 0 0 1 1

KY 0 0 0 2 0 0 0 0 0 1 3

LA 0 0 0 0 0 0 0 0 0 1 1

MA 0 0 0 0 0 0 0 0 0 2 2

MD 0 0 0 0 0 0 0 0 0 1 1

MS 0 0 0 0 0 0 0 0 0 1 1

NJ 0 0 0 0 0 0 0 0 0 1 1

NV 0 0 0 0 0 0 0 0 0 1 1

NY 0 0 0 0 4 0 0 0 0 0 4

OK 0 0 0 0 0 0 0 0 0 2 2

OR 0 0 1 3 0 0 1 1 0 0 6

PA 0 0 0 0 0 0 0 0 0 3 3

RI 0 0 0 0 0 0 0 0 0 1 1

TX 0 1 0 0 2 0 0 0 0 3 6

VA 0 0 0 0 1 0 0 0 0 3 4

VT 0 0 0 0 0 0 0 0 0 1 1

WV 0 0 2 0 0 0 0 0 0 0 2

Total 2 3 8 13 15 0 2 1 1 33 78

1

Postal Service State abbreviation.
Appendix A 37

Table A2. Number of community water system source waters sampled with known concentrations of methyl tert-butyl ether, 

by State and source-size category

[GW, ground water; SW, surface water; VSM, very small; SM, small; MED, medium; LRG, large; VLRG, very large]

State1

Source-size category

GW- Total

VSM

GWSMGWMEDGWLRGGWVLRGSWVSMSWSMSWMEDSWLRGSWVLRG

AZ 0 0 0 1 0 0 0 0 0 1 2

CA 0 0 0 0 0 0 1 0 0 8 9

CT 1 0 0 0 0 0 0 0 0 0 1

FL 0 0 0 0 2 0 0 0 0 1 3

IA 0 2 0 0 0 0 0 0 0 0 2

IL 0 2 2 0 0 0 0 0 0 0 4

KY 1 0 0 0 0 0 0 0 0 0 1

KS 0 2 1 1 0 0 0 0 0 0 4

MA 1 0 0 1 0 0 0 0 0 0 2

MD 0 1 1 1 0 0 0 0 0 0 3

ME 0 1 2 1 0 0 0 0 0 0 4

NC 0 0 1 0 0 0 0 0 0 1 2

NH 0 0 1 1 0 0 0 0 1 1 4

NJ 0 0 0 2 2 0 0 0 0 0 4

NY 0 0 0 0 1 0 0 0 0 0 1

OH 0 1 0 0 0 0 0 0 0 0 1

OR 2 0 0 0 0 0 0 0 0 0 2

PA 1 0 0 0 0 0 0 0 0 1 2

TN 0 0 0 0 0 0 0 0 1 0 1

TX 0 0 0 0 0 0 1 0 0 2 3

VA 0 1 0 0 0 0 0 0 0 0 1

Total 6 10 8 8 5 0 2 0 2 15 56

1

Postal Service State abbreviation.

Table A3. Number of community water system source waters sampled as control sites where methyl tert-butyl ether would 

likely not be present, by State and source-size category

[GW, ground water; SW, surface water; VSM, very small; SM, small; MED, medium; LRG, large; VLRG, very large]

State1

Source-size category

Total GWVSM

GWSMGWMEDGWLRGGWVLRGSWVSMSWSMSWMEDSWLRGSWVLRG

SD 0 0 0 0 0 0 0 0 0 1 1

MN 0 0 0 1 1 0 0 0 0 1 3

1Postal Service State abbreviation.

Appendix B - Detailed review of field quality-control data
40 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

SUMMARY OF QUALITY-CONTROL SAMPLES

A review of the field quality-control (QC) data 

collected in the Focused Survey has been conducted 

with parallel protocols as those of the Random Survey. 

Field-reagent blank (FRB) and trip-reagent blank 

(TRB) samples were collected in both surveys; however, duplicate samples and field-equipment blank 

(FEB) samples also were collected in the Focused 

Survey. FEB samples were collected only for surfacewater samples.

As in the Random Survey, FRB samples were 

used to measure contamination from the environment 

around the sampling site and during sample shipment. 

In the Focused Survey, a bottle containing VOC-free 

water was provided to U.S. Geological Survey (USGS) 

field personnel with each sample kit. FRB samples 

were prepared by directly filling VOC 40 mL (milliliters) vials labeled “Field Blank” at each sampling site 

with VOC-free water immediately prior to or following 

the collection of the source-water sample. USGS field 

personnel preserved the FRB sample with a 1:1 hydrochloric acid solution to a pH of 2. It is important to note 

that FRB samples are used extensively to accept or 

reject detected concentrations of volatile organic compounds (VOCs) in source-water samples in the following appendix (Appendix C). TRB samples, 

however, were prepared at the laboratory by filling 

vials with VOC-free water. TRB samples were placed 

in each sample kit and traveled along with the sourcewater samples back to the Metropolitan Water District 

of Southern California (MWDSC) laboratory. TRB 

samples were never opened and were intended to measure any contamination associated with samples during 

shipment. Because ground-water samples were collected from permanently installed pumps, FEB samples 

were collected only for surface-water source waters. 

FEBs were intended to represent all contamination 

associated with the surface-water sampling process, 

sampling equipment, processing, transportation, and 

laboratory handling/analysis. FEB samples were collected by passing VOC-free water through the USGS 

VOC hand sampler and filling the vials accordingly. 

FEB sample collection procedures for the VOC hand 

sampler are described by Shelton (1997). Field duplicate source-water samples were collected at about 

15 percent of the surface- and ground-water sites to 

characterize variability associated with sampling, processing, transportation, and analysis. Ground-water 

duplicates were collected sequentially, and surfacewater duplicates were collected concurrently with their 

respective source-water sample.

Field-Reagent Blank Samples

In addition to data for 467 (451 source-water 

samples plus 16 from control sites) source-water 

samples analyzed as part of the Focused Survey, the 

MWDSC laboratory reported VOC analysis for 321 

FRB samples. For the first three months of sampling, 

FRB samples were not collected with 100 percent of 

the source-water samples. Subsequently, a FRB accompanied all source-water samples. Forty-six VOCs were 

detected in one or more of the 321 FRB samples 

(table B1) with a concentration range of 0.03 to 

17.0 μg/L for all VOCs detected. Detection frequencies 

ranged from 0.31 percent for several VOCs to 

75.4 percent for toluene.

As in the Random Survey, toluene and methylene 

chloride were the most frequently detected compounds, 

reported in nearly two-thirds of the FRB samples. 

Seven other VOCs, including MTBE, ethylbenzene, 

m,p-xylene, o-xylene, chloroform, styrene, and 1,2,4-

trimethylbenzene were reported in 10 percent of the 

FRB samples (table B1). Up to 67 percent of the detections in the FRBs were less than 0.2 μg/L.

Trip-Reagent Blank Samples

TRB samples were sent with 100 percent of the 

source-water samples; however, TRB samples were 

analyzed only when a VOC was detected in the sourcewater sample. The MWDSC laboratory analyzed 304 

TRB samples. There were 163 source-water samples 

with no VOCs detected in the source-water sample, 

thus the accompanying TRB sample was not analyzed.

Concentrations of the detected VOCs in the TRB 

samples ranged from 0.03 to 2.56 μg/L (table B1). 

Detection frequencies ranged from 0.3 percent for several VOCs to 58.9 percent for toluene. Thirty-five of 

the 66 VOC compounds analyzed were detected in the 

304 TRB samples. 

As with the FRB samples in both the Focused 

and Random Surveys, methylene chloride 

(58.0 percent) and toluene (58.9 percent) were the most 

frequently detected compounds, reported in nearly 

two-thirds of the TRB samples. Similarly, six (MTBE, 

ethylbenzene, m,p-xylene, o-xylene, styrene, and 1,2,4-

trimethylbenzene) of the seven VOCs detected in FRB 

samples also were detected in TRB samples. Up to 

54.7 percent of the detected concentrations in the TRBs 

were less than 0.2 μg/L.
Appendix B 41

Table B1. Summary of volatile organic compounds detected in field-equipment blanks, field-reagent blanks, and trip-reagent 

blanks

[FEB, field-equipment blank; FRB, field-reagent blank; TRB, trip-reagent blank; ND, not detected; --, not applicable]

Volatile organic compound Type of

blank

Number of

samples

Number of

detections

Range in

concentration

(micrograms per liter)

Detection frequency

(percent)

Gasoline Oxygenates

tert-Amyl methyl ether (TAME) FRB 321 7 0.04-0.42 2.2

TRB 304 4 0.04-0.05 1.3

FEB 50 ND -- --

Diisopropyl ether (DIPE) FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Ethyl tert-butyl ether (ETBE) FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Methyl tert-butyl ether (MTBE) FRB 321 143 0.04-17.02 44.6

TRB 304 153 0.04-1.62 50.3

FEB 50 9 0.04-1.74 18.0

Other Gasoline Compounds

Benzene FRB 321 11 0.03-9.46 3.4

TRB 304 5 0.03-0.08 1.6

FEB 50 2 0.05-2.01 4.0

n-Butylbenzene FRB 321 1 0.07 0.3

TRB 304 ND -- --

FEB 50 1 0.07 2.0

sec-Butylbenzene FRB 321 1 0.05 0.3

TRB 304 ND -- --

FEB 50 ND -- --

tert-Butylbenzene FRB 321 1 0.04 0.3

TRB 304 ND -- --

FEB 50 ND -- --

Ethylbenzene FRB 321 125 0.04-0.79 38.9

TRB 304 50 0.04-0.3 16.4

FEB 50 9 0.04-0.88 18.0

Naphthalene FRB 321 22 0.06-0.15 6.9

TRB 304 15 0.06-0.13 4.9

FEB 50 7 0.06-0.38 14.0

Toluene FRB 321 242 0.03-6.95 75.4

TRB 304 179 0.03-0.62 58.9

FEB 50 17 0.03-6.01 34.0
42 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Other Gasoline Compounds—Continued

1,3,5-Trimethylbenzene FRB 321 2 0.05-0.18 0.6

TRB 304 ND -- --

FEB 50 2 0.03-0.29 4.0

m,p-Xylene FRB 321 123 0.07-1.49 38.3

TRB 304 57 0.07-0.76 18.8

FEB 50 7 0.07-3.73 14.0

o-Xylene FRB 321 103 0.03-0.88 32.1

TRB 304 37 0.03-0.23 12.2

FEB 50 6 0.03-1.68 12.0

Trihalomethanes

Bromodichloromethane FRB 321 2 0.02-0.03 0.6

TRB 304 ND -- --

FEB 50 ND -- --

Bromoform FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Chlorodibromomethane FRB 321 2 0.02-0.03 0.6

TRB 304 ND -- --

FEB 50 ND -- --

Chloroform FRB 321 45 0.03-5.7 14.0

TRB 304 13 0.03-0.06 4.3

FEB 50 4 0.03-0.18 8.0

Solvents

Bromobenzene FRB 321 1 0.03 0.3

TRB 304 1 0.03 0.3

FEB 50 ND -- --

Carbon tetrachloride FRB 321 1 0.28 0.3

TRB 304 ND -- --

FEB 50 ND -- --

Chlorobenzene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Chloroethane FRB 321 2 0.34-0.84 0.6

TRB 304 ND -- --

FEB 50 ND -- --

Table B1. Summary of volatile organic compounds detected in field-equipment blanks, field-reagent blanks, and trip-reagent 

blanks—Continued

[FEB, field-equipment blank; FRB, field-reagent blank; TRB, trip-reagent blank; ND, not detected; --, not applicable]

Volatile organic compound Type of

blank

Number of

samples

Number of

detections

Range in

concentration

(micrograms per liter)

Detection frequency

(percent)
Appendix B 43

Solvents—Continued

2-Chlorotoluene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 1 0.14 2.0

4-Chlorotoluene FRB 321 1 0.03 0.3

TRB 304 ND -- --

FEB 50 1 0.04 2.0

Dibromomethane FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

1,2-Dichlorobenzene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

1,3-Dichlorobenzene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

1,1-Dichloroethane FRB 321 3 0.04-0.08 0.9

TRB 304 1 0.04 0.3

FEB 50 ND -- --

1,2-Dichloroethane FRB 321 1 0.32 0.3

TRB 304 ND -- --

FEB 50 ND -- --

1,1-Dichloroethene FRB 321 1 0.1 0.3

TRB 304 ND -- --

FEB 50 ND -- --

cis-1,2-Dichloroethene FRB 321 3 0.03-0.09 0.9

TRB 304 ND -- --

FEB 50 ND -- --

trans-1,2-Dichloroethene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

1,2-Dichloropropane FRB 321 1 0.03 0.3

TRB 304 ND -- --

FEB 50 ND -- --

Table B1. Summary of volatile organic compounds detected in field-equipment blanks, field-reagent blanks, and trip-reagent 

blanks—Continued

[FEB, field-equipment blank; FRB, field-reagent blank; TRB, trip-reagent blank; ND, not detected; --, not applicable]

Volatile organic compound Type of

blank

Number of

samples

Number of

detections

Range in

concentration

(micrograms per liter)

Detection frequency

(percent)
44 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Solvents—Continued

Hexachloroethane FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Methylene chloride FRB 318 191 0.03-1.18 60.1

TRB 300 174 0.03-0.71 58.0

FEB 50 31 0.03-0.54 62.0

Methyl ethyl ketone FRB 321 13 0.8-2.87 4.0

TRB 304 11 0.71-1.09 3.6

FEB 50 1 0.85 2.0

n-Propylbenzene FRB 321 20 0.05-0.26 6.2

TRB 304 8 0.05-0.11 2.6

FEB 50 2 0.14-0.17 4.0

1,1,1,2-Tetrachloroethane FRB 321 1 0.02 0.3

TRB 304 ND -- --

FEB 50 ND -- --

1,1,2,2-Tetrachloroethane FRB 321 1 0.1 0.3

TRB 304 ND -- --

FEB 50 ND -- --

Tetrachloroethene FRB 321 6 0.05-1.1 1.9

TRB 304 1 0.05 0.3

FEB 50 ND -- --

1,2,4-Trichlorobenzene FRB 321 10 0.05-0.13 3.1

TRB 304 1 0.05 0.3

FEB 50 4 0.05-0.14 8.0

1,1,1-Trichloroethane FRB 321 1 0.4 0.3

TRB 304 ND -- --

FEB 50 ND -- --

1,1,2-Trichloroethane FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Trichloroethene FRB 321 1 0.25 0.3

TRB 304 ND -- --

FEB 50 ND -- --

Table B1. Summary of volatile organic compounds detected in field-equipment blanks, field-reagent blanks, and trip-reagent 

blanks—Continued

[FEB, field-equipment blank; FRB, field-reagent blank; TRB, trip-reagent blank; ND, not detected; --, not applicable]

Volatile organic compound Type of

blank

Number of

samples

Number of

detections

Range in

concentration

(micrograms per liter)

Detection frequency

(percent)
Appendix B 45

Solvents—Continued

1,2,3-Trichloropropane FRB 321 7 0.03-0.1 2.2

TRB 304 ND -- --

FEB 50 4 0.03-0.09 8.0

Organic Synthesis

Acrylonitrile FRB 321 1 0.31 0.3

TRB 304 ND -- --

FEB 50 ND -- --

Bromochloromethane FRB 321 1 0.04 0.3

TRB 304 ND -- --

FEB 50 ND -- --

1,3-Dichloropropane FRB 321 3 0.04-0.15 0.9

TRB 304 6 0.03-0.23 2.0

FEB 50 1 0.03 2.0

2,2-Dichloropropane FRB 321 1 0.07 0.3

TRB 304 1 0.29 0.3

FEB 50 ND -- --

1,1-Dichloropropene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Hexachlorobutadiene FRB 321 1 0.11 0.3

TRB 304 ND -- --

FEB 50 ND -- --

Isopropylbenzene FRB 321 29 0.04-0.34 9.0

TRB 304 9 0.04-0.12 2.6

FEB 50 2 0.04-0.31 4.0

p-Isopropyltoluene FRB 321 ND -- --

TRB 304 8 0.04-0.1 3.0

FEB 50 ND -- --

Styrene FRB 321 230 0.03-8.61 71.7

TRB 304 154 0.03-2.56 50.7

FEB 50 20 0.03-0.87 40.0

1,2,3-Trichlorobenzene FRB 321 11 0.05-0.11 3.4

TRB 304 7 0.05-0.1 2.3

FEB 50 3 0.06-0.1 6.0

Table B1. Summary of volatile organic compounds detected in field-equipment blanks, field-reagent blanks, and trip-reagent 

blanks—Continued

[FEB, field-equipment blank; FRB, field-reagent blank; TRB, trip-reagent blank; ND, not detected; --, not applicable]

Volatile organic compound Type of

blank

Number of

samples

Number of

detections

Range in

concentration

(micrograms per liter)

Detection frequency

(percent)
46 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Organic Synthesis—Continued

1,2,4-Trimethylbenzene FRB 321 49 0.03-0.58 15.3

TRB 304 31 0.03-0.06 10.2

FEB 50 6 0.03-1.34 12.0

Vinyl bromide FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Vinyl chloride FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Fumigants

Bromomethane FRB 319 6 0.1-0.18 1.9

TRB 299 5 0.09-0.13 1.7

FEB 50 ND -- --

1,4-Dichlorobenzene FRB 321 4 0.04-0.11 1.3

TRB 304 1 0.06 0.3

FEB 50 ND -- --

cis-1,3-Dichloropropene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

trans-1,3-Dichloropropene FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Refrigerants

Chloromethane FRB 321 8 0.13-1.53 2.5

TRB 304 ND -- --

FEB 50 2 0.13-0.18 4.0

Dichlorodifluoromethane FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Trichlorofluoromethane FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

1,1,2-Trichloro-1,2,2-trifluoroethane FRB 321 ND -- --

TRB 304 ND -- --

FEB 50 ND -- --

Table B1. Summary of volatile organic compounds detected in field-equipment blanks, field-reagent blanks, and trip-reagent 

blanks—Continued

[FEB, field-equipment blank; FRB, field-reagent blank; TRB, trip-reagent blank; ND, not detected; --, not applicable]

Volatile organic compound Type of

blank

Number of

samples

Number of

detections

Range in

concentration

(micrograms per liter)

Detection frequency

(percent)
Appendix B 47

FIELD-EQUIPMENT BLANK SAMPLES

FEB samples were collected only for equipment 

used to collect samples at surface-water sites by 

passing VOC-free water through the USGS VOC hand 

sampler and filling the vials accordingly. Fifty FEB 

samples were collected. Twenty-three VOCs were 

reported in one or more of the FEB samples. Concentrations of these VOCs ranged from 0.03 to 6.01 μg/L. 

Detection frequencies (table B1) ranged from 2 percent 

for several VOCs to 62 percent for methylene chloride.

Duplicate Samples

In addition to the FEBs, FRBs, and TRBs, duplicate samples were collected sequentially at groundwater sampling sites and concurrently at surface-water 

sampling sites. Duplicate samples were processed in 

the same manner as the source-water sample and provide an indication of sampling and analytical reproducibility. Sixty-nine source-water samples were collected 

with field duplicates. Detected concentrations of 

MTBE in field duplicate samples and corresponding 

source-water samples are shown in figure B1. Most of 

the points fall directly on or near the 1:1 line with an r2

value of 0.999, indicating very little variation in the 

concentrations of duplicates and source-water samples.

STATISTICAL COMPARISON OF QUALITYCONTROL SAMPLES

The low-level (less than 0.2 μg/L) contamination 

evident in the field quality-control data required further 

analysis to determine the level of uncertainty associated with VOC detections. Similarities between 

detected concentrations in different blank types may 

indicate a common source of contamination, whereas 

differences may indicate more of a random source. The 

statistical analyses described below include comparisons between FRB, TRB, and FEB samples. In addition, duplicate source-water samples are further 

analyzed.

Figure B1. Concentration of methyl tert-butyl ether in field duplicate and source-water samples.

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

CONCENTRATION IN SOURCE-WATER SAMPLES, IN MICROGRAMS PER LITER

CONCENTRATION IN DUPLICATE SAMPLES,

IN MICROGRAMS PER LITER
48 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Field-Reagent Blank Samples Versus TripReagent Blank Samples

A review of the FRB and TRB sample data was 

completed to determine if there was a relation between 

the frequency and concentration of VOC detections in 

these samples for sites where both quality-control samples were collected. Of the 304 TRB samples collected, 

292 had a companion FRB sample.

 A Wilcoxon signed-rank test (Helsel and Hirsch, 

1992), was used to statistically determine the relation 

between concentrations in FRB and TRB samples. 

VOCs detected in either blank type were used in the 

statistical analysis. Results for all 292 pairs of samples 

indicated that the concentrations of seven of the VOCs 

detected in both blanks were statistically different at 

the 95-percent confidence level (table B2). These seven 

VOCs include chloromethane, methylene chloride, 

MTBE, n-propylbenzene, styrene, TAME, and toluene. 

Concentrations of six of these VOCs were found to be 

statistically greater in the FRB samples than in the TRB 

samples. This may indicate that the detections of VOCs 

might be due to greater exposure of the source water for 

the FRB samples to conditions in the field than the 

TRB samples, which were unopened during field operations. Concentrations of chloromethane were found to 

be statistically greater in the TRB samples than in the 

FRB samples, but this compound was detected in only 

16 source-water samples.

Table B2. Relation between concentrations of volatile organic compounds in 292 companion field-blank samples and tripblank samples

[Statistically different results are the 95-percent confidence interval are given in boldface; Ho, null hypothesis; FRB, field-reagent blank; TRB, trip-reagent 

blank; <, less than; >, greater than]

Volatile organic

compound

Number of 

detections in 

FRB samples

Number of 

detections in 

TRB samples

Null hypothesis tested and p-value

Ho: FRB=TRB1 Ho: FRB<TRB2 Ho: FRB>TRB2

tert-Amyl methyl ether (TAME) 5 3 0.0457 0.0228 0.9773

Benzene 8 4 0.5394 0.7310 0.2697

Bromomethane 6 5 0.7679 0.3839 0.6169

Chloroform 37 12 0.4377 0.7815 0.2189

Chloromethane 8 0 0.0347 0.9827 0.0174

1,4-Dichlorobenzene 4 1 0.3190 0.0904 0.8413

1,1-Dichloroethane 3 1 0.9989 0.5021 0.4993

Ethylbenzene 115 46 0.3452 0.1726 0.8276

Isopropylbenzene 23 7 0.5298 0.2649 0.7356

p-Isopropyltoluene 0 8 0.1577 0.0788 0.9215

Methyl ethyl ketone 11 10 0.2224 0.1112 0.8891

Methylene chloride 172 166 0.0427 0.0214 0.9787

Methyl tert-butyl ether (MTBE) 131 145 <0.0001 <0.0001 1.0000

Naphthalene 19 13 0.5981 0.7016 0.2990

n-Propylbenzene 16 7 0.0605 0.0303 0.9699

Styrene 213 148 <0.0001 <0.0001 1.0000

Toluene 222 171 <0.0001 <0.0001 1.0000

1,2,3-Trichloropropane 5 0 0.4215 0.7901 0.2108

1,2,4-Trimethylbenzene 42 29 0.8231 0.4116 0.5890

m,p-Xylene 112 53 0.2678 0.1339 0.8663

o-Xylene 91 32 0.4475 0.2238 0.7765

1Two-sided p-values <0.1 significant at 95-percent confidence level.

2

One-sided p-values <0.05 significant at 95-percent confidence level.
Appendix B 49

Field-Reagent Blank Samples Versus FieldEquipment Blank Samples

Concentrations of VOCs were statistically compared using the Wilcoxon signed-rank test for FEBs 

and FRBs in 27 paired sets of blank samples. As in the 

test comparing FRB samples to TRB samples, VOCs 

detected in either blank type were used in the statistical 

analysis. Concentrations of MTBE and styrene were 

found to be statistically different at the 95-percent 

confidence level (table B3). Concentrations of MTBE 

were determined to be statistically higher in FEB samples than in FRB samples. Conversely, concentrations 

of styrene were found to be statistically higher at the 

95-percent confidence level in the FRB samples than in 

the FEB samples. Concentration differences between 

FRB samples and FEB samples for all other compounds analyzed were not found to be statistically 

different.

Trip-Reagent Blank Samples Versus FieldEquipment Blank Samples

Concentrations of FEB samples and TRB samples were also compared for 27 sets of paired blanks. 

For all compounds analyzed, none were statistically 

different, nor were any concentrations of compounds 

statistically higher or lower for any of the blanks 

(table B4).

Duplicate Samples

A Wilcoxon signed-rank test for the VOCs was 

used to test whether there were significant differences 

between the samples and the duplicates. Results of the 

test indicated that for the 69 pairs of source-water samples and duplicates, the concentrations of three compounds (naphthalene, trichloroethene, and 1,3,5-

trimethylbenzene) were significantly different at the 

95-percent confidence level (table B5).

Table B3. Relation between concentrations of volatile organic compounds in 27 companion field-equipment blank samples 

and field-reagent blank samples

[Statistically different results at the 95-percent confidence interval are given in boldface; Ho, null hypothesis; FEB, field-equipment blank; FRB, fieldreagent blank; <, less than; >, greater than]

Volatile organic

compounds

Number of

detections in FEB 

samples

Number of

detections in FRB 

samples

Null hypothesis tested and p-value

Ho: FEB=FRB1 Ho: FEB<FRB2 Ho: FEB>FRB2

Benzene 1 0 0.3356 0.1678 0.8501

Chloroform 2 5 0.1920 0.0960 0.9100

Chloromethane 2 1 0.3356 0.8501 0.1678

Ethylbenzene 3 12 0.9675 0.5270 0.4838

Isopropylbenzene 1 0 0.3356 0.8501 0.1678

Methylene chloride 16 17 0.9796 0.4898 0.5204

Methyl tert-butyl ether (MTBE) 4 9 0.0155 0.0078 0.9928

Naphthalene 4 0 0.9844 0.4922 0.5234

n-Propylbenzene 1 0 0.3356 0.1678 0.8501

Styrene 9 20 0.0013 0.9994 0.0006

Toluene 8 19 0.9802 0.5198 0.4901

1,2,4-Trichlorobenzene 2 0 1.0000 0.5000 0.5213

1,2,3-Trichloropropane 2 2 0.3356 0.8501 0.1678

1,2,4-Trimethylbenzene 4 5 0.5790 0.2895 0.7255

m,p-Xylene 2 12 0.2093 0.1047 0.8367

o-Xylene 2 8 0.1267 0.0633 0.9402

1

Two-sided p-values <0.1 significant at 95-percent confidence level.

2

One-sided p-values <0.05 significant at 95-percent confidence level.
50 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table B4. Relation between concentrations of volatile organic compounds in 27 companion field-equipment blank samples 

and trip-reagent blank samples

[Ho, null hypothesis; FEB, field-equipment blank; TRB, trip-reagent blank; <, less than; >, greater than]

Volatile organic

compound

Number of

detections in FEB 

samples

Number of

detections in

TRB samples

Null hypothesis tested and p-value

Ho: FEB=TRB1 Ho: FEB<TRB2 Ho: FEB>TRB2

Benzene 1 0 0.3356 0.1678 0.8501

Chloroform 2 2 0.9844 0.5234 0.4922

Chloromethane 2 0 0.1654 0.9252 0.0827

Ethylbenzene 3 6 0.4346 0.2173 0.7923 

Isopropylbenzene 1 1 1.0000 0.5213 0.5000

p-Isopropyltoluene 0 2 0.1654 0.0827 0.9252

Methylene chloride 15 19 0.2060 0.1030 0.9014

Methyl tert-butyl ether (MTBE) 4 13 0.5280 0.2640 0.7442

Naphthalene 4 0 0.9687 0.4844 0.5312

n-Propylbenzene 1 0 0.3356 0.1678 0.8501

Styrene 10 15 0.1375 0.9345 0.0688

Toluene 8 14 0.7010 0.3505 0.6590

1,2,3-Trichlorobenzene 3 0 0.6097 0.3049 0.7105

1,2,3-Trichloropropane 2 0 0.1654 0.9252 0.0827

1,2,4-Trimethylbenzene 4 6 0.6097 0.7105 0.3049

m,p-Xylene 2 7 0.1548 0.0774 0.9268

o-Xylene 2 3 1.0000 0.5000 0.5156

1

Two-sided p-values <0.1 significant at 95-percent confidence level. 2

One-sided p-values <0.05 significant at 95-percent confidence level.
Appendix B 51

Table B5. Statistical comparison of concentrations of 

volatile organic compounds in source-water and duplicate 

samples

[Statistically different results at the 95-percent confidence interval are given 

in boldface; Ho, null hypothesis; ENV, source-water sample; DUP, duplicate sample]

Volatile organic compound

Null hypothesis tested 

and p-value1

Ho: ENV=DUP

tert-Amyl methyl ether (TAME) 0.9772

Benzene 0.5970

Bromodichloromethane 0.7101

Bromomethane 0.1604

Carbon tetrachloride 1.0000

Chlorodibromomethane 0.9706

Chloroform 0.3490

Chloromethane 0.3174

1,1-Dichloroethane 0.9820

1,2-Dichloroethane 0.9940

cis-1,2-Dichloroethene 0.2406

1,3-Dichloropropane 0.1604

Ethylbenzene 0.4321

Isopropylbenzene 0.3245

Methylene chloride 0.5811

Methyl tert-butyl ether (MTBE) 0.5230

Naphthalene 0.0620

n-Propylbenzene 0.5754

Styrene 0.5923

1,1,1-Trichloroethane 0.9820

Tetrachloroethene 0.9883

Toluene 0.4417

Trichloroethene 0.0354

1,2,4-Trimethylbenzene 0.6894

1,3,5-Trimethybenzene 0.0848

m,p-Xylene 1.0000

o-Xylene 0.8334

1

Two-sided p-values <0.1 significant at 95-percent confidence level 

shown in bold.

Appendix C - Data acceptance criteria

using quality-control data
54 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

SOURCE-WATER DATA ACCEPTANCE

CRITERIA

Two objectives of the Focused Survey include: 

(1) to determine the temporal variability, and 

(2) characterize sampling sites that had frequent occurrence of MTBE and other VOCs in source water. In 

order to accomplish these two objectives, data analyses 

will involve the use of concentration values to determine temporal trends as well as a presence/absence 

approach for site characterizations. The strength of 

these types of comparisons relies on the amount of data 

(detections) available for data analyses. That is, if a 

compound is determined to be present, it may be used 

in statistical tests to determine if the VOC is associated 

with some environmental risk factor. However, if the 

concentration of a compound detected below the minimum reporting level (MRL) is censored, low-level 

temporal trends and explanatory factors could be 

missed. It is important to note that data below the MRL 

can only be used in these analyses if the QC screening 

process supports those activities.

Source-Water Data Screening Process

As with the Random Survey, an analysis of 

source-water data and field QC data can identify potential contamination of individual VOCs in source-water 

samples. Subsequently, the frequency of detection can 

be adjusted to account for possible contamination. The 

source-water samples were subdivided into seven conditions that relate to the level of uncertainty associated 

with source-water VOC detections. These conditions 

are described in table C1.

Table C1. Schematic diagram for adjusting the frequency of detection of volatile organic compounds in

source-water samples to reflect possible random sample contamination

[VOCs, volatile organic compounds; <, less than; >=, greater than or equal to]

Condition Possibility of random contamination of 

source-water samples

Condition A:

Source water = no VOCs detected

Field blank = no VOCs detected

None

Condition B:

Source water = no VOCs detected

Field blank = VOCs detected

None

Condition C:

Source water = no VOCs detected

Field blank = not analyzed

None

Condition D:

Source water = VOCs detected

Field blank = no VOCs detected

None

Condition E1:

Source water = VOCs detected

Field blank = same VOCs detected, however, source water 

concentrations >= 5 times the field blank concentrations

None

Condition E2:

Source water = VOCs detected

Field blank = same VOCs detected, however, source water

concentrations < 5 times the field blank concentrations

Random contamination suspected

Condition F:

Source water = VOCs detected

Field blank = not analyzed

Undetermined—Possibility of random sample 

contamination
Appendix C 55

Conditions A, B, C, and D all indicate that 

random sample contamination of source-water samples 

above the method detection limit (MDL) is not likely. 

As presented in table C2, the majority of source-water 

samples had no detections (conditions A, B, and C), 

thus, random contamination was not measurable. For 

example, many source-water samples did not have a 

detected VOC in the source-water sample or FRB 

sample (condition A). There also were many sourcewater samples with no VOC detections, but with detections in the corresponding FRB samples 

(condition B)—methylene chloride, styrene, and toluene were detected in 179, 118, and 111 FRB samples, 

respectively, but were not detected in the corresponding 

source-water samples. Concentrations for methylene 

chloride, styrene, and toluene in FRB samples ranged 

from 0.03 μg/L for all three compounds to 1.18, 8.61, 

and 6.95 μg/L, respectively. This may be an indication 

that the VOC-free water used in the collection of these 

FRBs was contaminated with those compounds when 

the FRB sample was collected. Condition C VOCs 

were not detected in source-water samples, thus they 

are not considered contaminated and, as stated in the 

protocol, the FRB samples were not analyzed unless 

compounds were detected in the source-water samples. 

Also, chloroform and MTBE were detected in 146 and 

134 source-water samples, respectively, but with no 

detections in the corresponding FRB samples 

(condition D). This further indicates contamination 

associated with those compounds under that specific 

condition is unlikely. 

The validity of only two categories of data 

remain in question: (1) the samples that have the same 

VOCs in the source-water sample as well as the associated FRB sample (conditions E1 and E2); and (2) those 

source-water samples with VOC detections that do not 

have an associated FRB sample (condition F). Samples 

classified into category E1 and E2 have detections of 

the same VOCs in both the source-water sample and 

associated FRB sample (table C2). Samples classified 

as E1 have VOC concentrations 5 times greater than the 

concentrations in the accompanying FRB sample. Samples classified as E2 have VOC concentrations less than 

5 times the concentrations in the accompanying FRB 

sample. The source-water samples that were classified 

as E1 were judged to have essentially no possibility that 

the source-water concentration was an artifact of 

random contamination. These samples were few, except 

for MTBE, which had 82 samples in this category. In 

addition, toluene and ethylbenzene had 18 and 3 samples in condition E1, respectively (table C2).

For samples with VOCs at concentrations less 

than five times the FRB sample concentration there 

was a clear possibility that the source-water detection 

was due to random contamination, especially when 

concentrations approached a 1:1 ratio for one or more 

compound. Accordingly, 487 source-water VOC 

detections that fall under condition E2 (223 of these 

detections were due to styrene and toluene) were censored from the source-water data and not included in 

the analyses of VOCs subsequently reported for 

source water. As in the QC analysis for the Random 

Survey, this is a conservative approach to reporting 

VOC occurrence data, and it is recognized that the 

detection of the same VOC in both the FRB and 

source-water samples does not definitively indicate 

that random contamination of the source-water sample 

has occurred, but indicates that it may have occurred 

or cannot be ruled out.

Source-water samples classified as condition F 

had a detected VOC in the source-water sample but an 

associated FRB sample was not available. This condition is considered to have some uncertainty associated 

with the detection, however, as in the Random Survey, 

these concentrations were not censored from the 

source-water data. Thus, only E2 samples were censored from the data set and are not included in any 

analyses subsequently reported for source water.

Volatile Organic Compound Detection 

Frequencies

The total number of samples collected, the original detection frequency before adjusting for E2 conditions, and the adjusted detection frequency after 

adjusting for E2 conditions for data at the MDL are 

listed in table C3. As in the Random Survey analyses, 

the adjusted detection frequency is equal to the total 

number of times a VOC was detected minus the 

number of condition E2 samples divided by the total 

number of samples minus the number of condition E2 

samples. Table C4 illustrates an example of calculating the adjusted detection frequency for MTBE at 

the MDL.

The detection frequency in source-water samples was adjusted downward for 32 of the 58 detected 

VOCs due to possible random contamination. Only 

the adjusted samples (those not affected by 

condition E2) were used in data analyses presented in 

this report.
56 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table C2. Number of source-water samples that meet specified conditions at the method detection limit, with respect to the 

possibility of random contamination

Possible volatile organic compound 

contaminant

Number of samples per condition1

A B C D E1 E2 F

Acrylonitrile 315 1 145 6 0 0 0

tert-Amyl methyl ether (TAME) 227 4 126 88 0 3 19

Benzene 213 6 125 98 1 4 20

Bromobenzene 319 1 145 2 0 0 0

Bromochloromethane 312 1 144 9 0 0 1

Bromodichloromethane 262 2 141 52 0 6 4

Bromoform 306 0 137 15 0 0 8

Bromomethane 306 5 138 10 0 1 5

n-Butylbenzene 313 1 141 8 0 0 1

sec-Butylbenzene 320 0 145 1 0 1 0

tert-Butylbenzene 320 1 145 1 0 0 0

Carbon tetrachloride 313 1 143 8 0 0 2

Chlorobenzene 311 0 141 10 0 0 4

Chlorodibromomethane 274 1 129 46 0 1 16

Chloroethane 318 0 145 2 0 2 0

Chloroform 136 20 88 146 6 14 57

Chloromethane 306 3 142 8 0 5 3

2-Chlorotoluene 321 0 145 0 0 0 0

4-Chlorotoluene 319 1 145 2 0 0 0

Dibromomethane 319 0 145 3 0 0 0

1,2-Dichlorobenzene 316 0 145 5 0 0 1

1,3-Dichlorobenzene 321 0 145 13 1 0 2

1,4-Dichlorobenzene 309 3 142 9 0 1 3

Dichlorodifluoromethane 320 0 144 2 0 0 1

1,1-Dichloroethane 294 0 130 26 0 2 15

1,2-Dichloroethane 291 1 137 30 0 0 8

1,1-Dichloroethene 307 0 145 13 1 0 2

cis-1,2-Dichloroethene 287 0 138 32 1 2 7

trans-1,2-Dichloroethene 312 0 144 10 0 0 1

1,2-Dichloropropane 313 1 144 8 0 0 1

1,3-Dichloropropane 313 3 141 6 0 0 4

2,2-Dichloropropane 321 1 145 0 0 0 0

1,1-Dichloropropene 321 0 145 0 0 0 0

cis-1,3-Dichloropropene 321 0 145 0 0 0 0

trans-1,3-Dichloropropene 321 0 145 0 0 0 0
Appendix C 57

1 Described in table C1.

Diisopropyl ether (DIPE) 307 0 142 15 0 0 3

Ethylbenzene 176 95 140 19 3 29 5

Ethyl tert-butyl ether (ETBE) 318 0 144 4 0 0 1

Hexachlorobutadiene 320 0 145 1 0 1 0

Hexachloroethane 321 0 145 0 0 0 0

Isopropylbenzene 282 25 144 11 0 4 1

p-Isopropyltoluene 321 0 145 1 0 0 0

Methyl ethyl ketone 302 12 141 7 0 1 4

Methylene chloride 124 179 138 3 1 12 7

Methyl tert-butyl ether (MTBE) 43 17 41 134 82 46 104

Naphthalene 280 10 129 23 0 9 16

n-Propylbenzene 292 16 143 10 0 4 2

Styrene 78 118 102 11 1 114 43

1,1,1,2-Tetrachloroethane 318 1 145 3 0 0 0

1,1,2,2-Tetrachloroethane 321 1 145 0 0 0 0

Tetrachloroethene 266 4 131 50 1 1 14

Toluene 52 111 93 32 18 109 52

1,2,3-Trichlorobenzene 301 8 143 10 0 3 4

1,2,4-Trichlorobenzene 302 8 136 10 0 2 9

1,1,1-Trichloroethane 287 0 141 33 0 1 6

1,1,2-Trichloroethane 317 0 145 5 0 0 0 

Trichloroethene 273 0 128 48 0 1 17

Trichlorofluoromethane 315 0 141 7 0 0 4

1,2,3-Trichloropropane 307 3 139 8 0 4 6

1,1,2-Trichloro-1,2,2-trifluoromethane 320 0 144 2 0 0 1

1,2,4-Trimethylbenzene 239 12 140 36 0 35 5

1,3,5-Trimethylbenzene 298 1 143 22 0 1 2

Vinyl bromide 321 0 145 0 0 0 0

Vinyl chloride 317 0 143 5 0 0 2

m,p-Xylene 168 91 136 31 3 29 9

o-Xylene 176 65 132 42 0 39 13

Table C2. Number of source-water samples that meet specified conditions at the method detection limit, with respect to the 

possibility of random contamination—Continued

Possible volatile organic compound 

contaminant

Number of samples per condition1

A B C D E1 E2 F
58 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table C3. Detection frequency of volatile organic compounds detected at or above the method detection level in source 

water before and after applying the quality-control screening process

[--, not applicable]

Volatile organic compound Number of

samples

Number of

original

detections

Unadjusted

detection

frequency

(percent)

Number of

samples after

screening

process

Number of

detections after

screening

process

Adjusted

detection

frequency

(percent)

Acrylonitrile 467 6 1.3 467 6 1.3

tert-Amyl methyl ether (TAME) 467 110 23.6 464 107 23.1

Benzene 467 123 26.3 463 119 25.7

Bromobenzene 467 2 0.4 467 2 0.4

Bromochloromethane 467 10 2.1 467 10 2.1

Bromodichloromethane 467 62 13.3 461 56 12.1

Bromoform 467 23 4.9 467 23 4.9

Bromomethane 465 16 3.4 464 15 3.2

n-Butylbenzene 467 9 1.5 467 9 1.5

sec-Butylbenzene 467 2 0.4 466 1 0.2

tert-Butylbenzene 467 1 0.2 467 1 0.2

Carbon tetrachloride 467 10 2.1 467 10 2.1

Chlorobenzene 467 14 3.0 467 14 3.0

Chlorodibromomethane 467 63 13.5 466 62 13.3

Chloroethane 467 4 0.9 465 2 0.4

Chloroform 467 223 47.8 453 209 46.1

Chloromethane 467 16 3.4 462 11 2.4

4-Chlorotoluene 467 2 0.4 467 2 0.4

Dibromomethane 467 3 0.6 467 3 0.6

1,2-Dichlorobenzene 467 6 1.3 467 6 1.3

1,3-Dichlrobenzene 467 6 1.3 467 6 1.3

1,4-Dichlorobenzene 467 13 2.8 466 12 2.6

Dichlorodifluoromethane 467 3 0.6 467 3 0.6

1,1-Dichloroethane 467 43 9.2 465 41 8.8

1,2-Dichloroethane 467 38 8.1 467 38 8.1

1,1-Dichloroethene 467 16 3.4 467 16 3.4

cis-1,2-Dichloroethene 467 42 9.0 465 40 8.6

trans-1,2-Dichloroethene 467 11 2.4 467 11 2.4

1,2-Dichloropropane 467 9 1.9 467 9 1.9

1,3-Dichloropropane 464 10 2.2 464 10 2.2

Diisopropyl ether (DIPE) 467 19 4.1 467 19 4.1

Ethylbenzene 467 56 12.0 438 27 6.2

Ethyl tert-butyl ether (ETBE) 467 5 1.1 467 5 1.1

Hexachlorobutadiene 467 2 0.4 466 1 0.2
Appendix C 59

Isopropylbenzene 467 16 3.4 463 12 2.6

p-Isopropyltoluene 467 1 0.2 467 1 0.2

Methyl ethyl ketone 467 12 2.6 466 11 2.4

Methylene chloride 464 23 5.0 452 11 2.4

Methyl tert-butyl ether (MTBE) 467 366 78.4 421 320 76.0

Naphthalene 467 48 10.3 458 39 8.5

n-Propylbenzene 467 16 3.4 463 12 2.6

Styrene 467 169 36.2 353 55 15.6

1,1,1,2-Tetrachloroethane 467 3 0.6 467 3 0.6

1,1,2,2-Tetrachloroethane 467 1 0.2 467 1 0.2

Tetrachloroethene 467 66 14.1 466 65 13.9

Toluene 467 211 45.2 358 102 28.5

1,2,3-Trichlorobenzene 467 17 3.6 464 14 3.0

1,2,4-Trichlorobenzene 467 21 4.5 465 19 4.1

1,1,1-Trichloroethane 467 40 8.6 466 39 8.4

1,1,2-Trichloroethane 467 5 1.1 467 5 1.1

Trichloroethene 467 66 14.1 466 65 13.9

Trichlorofluoromethane 467 11 2.4 467 11 2.4

1,2,3-Trichloropropane 467 18 3.9 463 14 3.0

1,1,2-Trichloro-1,2,2-trifluoroethane467 3 0.6 467 3 0.6

1,2,4-Trimethylbenzene 467 76 16.3 432 41 9.5

1,3,5-Trimethylbenzene 467 25 5.4 466 24 5.2

Vinyl chloride 467 7 1.5 467 7 1.5

m,p-Xylene 467 72 15.4 438 43 9.8

o-Xylene 467 94 20.1 428 55 12.9

Table C3. Detection frequency of volatile organic compounds detected at or above the method detection level in source 

water before and after applying the quality-control screening process—Continued

[--, not applicable]

Volatile organic compound Number of

samples

Number of

original

detections

Unadjusted

detection

frequency

(percent)

Number of

samples after

screening

process

Number of

detections after

screening

process

Adjusted

detection

frequency

(percent)
60 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table C4. Schematic diagram for adjusting the frequency of detection of methyl tert-butyl ether in source-water samples to 

reflect possible random sample contamination1

[VOCs, volatile organic compounds; MTBE, methyl tert-butyl ether; <, less than; >=, greater than or equal to]

Possibility of random contamination

of source-water samples Condition

No random sample contamination Condition A: Source water = no VOCs detected

Field blank = no VOCs detected

For MTBE = 43 samples

No random sample contamination Condition B: Source water = no VOCs detected

Field blank = VOCs detected

For MTBE = 17 samples

No random sample contamination Condition C: Source water = no VOCs detected

Field blank = not analyzed

For MTBE = 41 samples

No random sample contamination Condition D: Source water = VOCs detected

Field blank = no VOCs detected

For MTBE = 134 samples

No random sample contamination Condition E1: Source water = VOCs detected

Field blank = same VOCs detected, and source water concentrations >= 5 times the field blank concentrations

For MTBE = 82 samples

Random contamination suspected Condition E2: Source water = VOCs detected

Field blank = same VOCs detected, and source water concentrations < 5 times the field blank concentrations

For MTBE = 46 samples

Undetermined—Possibility of random sample contamination Condition F: Source water = VOCs detected

Field blank = not analyzed

For MTBE = 104 samples

Frequency of detection, unadjusted = (D+E1+E2+F)/(A+B+C+D+E1+E2+F)

For MTBE = (134+82+46+104)/(43+17+41+134+82+46+104) = 366/467 = 78.4 percent

Frequency of detection, adjusted = (D+E1+F)/(A+B+C+D+E1+F)

For MTBE = (134+82+104)/(43+17+41+134+82+104) = 320/421 = 76.0 percent

Estimated frequency of detection that may be attributed to random contamination of source-water samples = frequency of 

detection, unadjusted - frequency of detection, adjusted

For MTBE = 78.4 percent - 76.0 percent = 2.4 percent

1

Based on all detections greater than or equal to the method detection limit.
Appendix D - Summary of VOC concentrations

in the Focused Survey
62 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table D1. Statistical summary of volatile organic compounds in source-water samples in the Focused Survey

[Values in micrograms per liter; N, number of sources sampled; ND, not detected]

Volatile organic

compounds

All community water systems (N=134) Ground Water (N=78)

Minimum 25th Median 75th MaximumMinimum 25th Median 75th Maximum

Gasoline oxygenates

Diisopropyl ether 0.10 0.14 0.27 0.74 3.93 0.10 0.14 0.27 0.74 3.93

Ethyl tert-butyl ether 0.16 0.17 0.18 0.19 0.20 0.16 0.17 0.18 0.19 0.20

Methyl tert-butyl ether 0.04 0.18 0.68 1.63 924.00 0.04 0.57 0.95 4.73 924.00

tert-Amyl methyl ether 0.03 0.04 0.11 0.18 1.27 0.03 0.06 0.11 0.23 1.27

Other gasoline compounds

1,3,5-Trimethylbenzene 0.03 0.06 0.20 0.33 0.49 0.32 0.32 0.32 0.32 0.32

Benzene 0.03 0.04 0.10 0.30 37.90 0.03 0.09 0.14 0.86 37.90

Ethylbenzene 0.05 0.05 0.07 0.54 2.69 0.05 0.06 0.06 1.38 2.69

m,p-Xylene 0.07 0.08 0.09 0.72 3.26 0.07 0.09 0.10 0.89 3.26

Naphthalene 0.06 0.06 0.12 0.17 0.98 0.06 0.11 0.15 0.21 0.98

n-Butylbenzene 0.05 0.09 0.12 0.16 0.19 0.19 0.19 0.19 0.19 0.19

o-Xylene 0.03 0.03 0.04 0.20 1.16 0.03 0.03 0.04 0.09 0.93

sec-Butylbenzene 0.05 0.06 0.06 0.07 0.07 0.05 0.06 0.06 0.07 0.07

tert-Butylbenzene ND ND ND ND ND ND ND ND ND ND

Toluene 0.03 0.05 0.09 0.32 4.98 0.03 0.03 0.05 0.18 4.42

Trihalomethanes

Bromodichloromethane 0.02 0.02 0.04 0.08 1.27 0.02 0.02 0.03 0.04 1.27

Bromoform 0.03 0.09 0.20 0.28 2.61 0.19 0.21 0.26 0.88 2.61

Chlorodibromomethane 0.02 0.03 0.04 0.07 2.43 0.02 0.03 0.04 0.15 2.43

Chloroform 0.03 0.05 0.12 0.29 5.00 0.03 0.06 0.13 0.30 5.00

Organic synthesis

1,1-Dichloropropene ND ND ND ND ND ND ND ND ND ND

1,2,3-Trichlorobenzene 0.05 0.07 0.08 0.13 0.17 0.05 0.07 0.11 0.14 0.17

1,2,4-Trimethylbenzene 0.03 0.07 0.09 0.15 2.42 2.42 2.42 2.42 2.42 2.42

1,3-Dichloropropane 0.05 0.06 0.10 0.16 0.21 0.05 0.09 0.13 0.17 0.21

2,2-Dichloropropane ND ND ND ND ND ND ND ND ND ND

Acrylonitrile 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

Bromochloromethane 0.04 0.04 0.04 0.05 0.08 0.04 0.04 0.04 0.06 0.08

Hexachlorobutadiene ND ND ND ND ND ND ND ND ND ND

Isopropylbenzene 0.04 0.05 0.05 0.09 0.24 0.04 0.05 0.05 0.10 0.24

p-Isopropyltoluene 0.04 0.04 0.04 0.04 0.04 ND ND ND ND ND

Styrene 0.03 0.04 0.07 0.12 1.10 0.03 0.04 0.08 0.21 1.10

Vinyl bromide ND ND ND ND ND ND ND ND ND ND

Vinyl chloride 0.61 0.86 1.10 1.18 1.25 0.61 0.86 1.10 1.18 1.25

Solvents

1,1,1,2-Tetrachloroethane 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

1,1,1-Trichloroethane 0.06 0.13 0.23 0.67 6.84 0.06 0.13 0.23 0.67 6.84
Appendix D 63

Reservoir (N=39) Rivers (N=17) All surface water (N=56)

Minimum 25th Median 75th MaximumMinimum 25th Median 75th MaximumMinimum 25th Median 75th Maximum

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

0.04 0.10 0.40 1.18 14.00 0.09 0.14 0.20 0.64 1.65 0.04 0.11 0.26 0.85 14.00

0.03 0.05 0.12 0.12 0.35 0.03 0.04 0.05 0.08 0.11 0.03 0.04 0.11 0.12 0.35

0.33 0.37 0.41 0.45 0.49 0.03 0.05 0.06 0.07 0.07 0.03 0.06 0.07 0.33 0.49

0.03 0.04 0.06 0.26 1.83 0.03 0.03 0.05 0.05 0.30 0.03 0.03 0.05 0.22 1.83

0.05 0.06 0.07 0.39 0.70 ND ND ND ND ND 0.05 0.06 0.07 0.39 0.70

0.07 0.07 0.33 0.79 2.63 0.08 0.08 0.08 0.08 0.08 0.07 0.08 0.08 0.72 2.63

0.07 0.09 0.07 0.12 0.16 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.07 0.16

ND ND ND ND ND 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

0.03 0.04 0.15 0.37 1.16 0.03 0.03 0.04 0.08 0.21 0.03 0.03 0.04 0.25 1.16

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

0.10 0.19 0.35 1.99 4.98 0.03 0.05 0.07 0.11 0.70 0.03 0.06 0.10 0.35 4.98

0.02 0.03 0.04 0.09 0.21 0.02 0.03 0.06 0.10 0.16 0.02 0.03 0.06 0.10 0.21

0.06 0.06 0.06 0.06 0.06 0.03 0.03 0.03 0.03 0.03 0.03 0.04 0.05 0.05 0.06

0.02 0.03 0.05 0.10 0.15 0.02 0.03 0.03 0.04 0.07 0.02 0.03 0.03 0.07 0.15

0.03 0.05 0.07 0.08 0.33 0.03 0.05 0.10 0.25 0.90 0.03 0.05 0.08 0.13 0.90

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

0.07 0.07 0.08 0.08 0.07 0.08 0.08 ND ND 0.08 0.07 0.07 0.08 0.08 0.08

0.03 0.06 0.09 0.14 0.99 0.06 0.07 0.08 0.11 0.13 0.03 0.06 0.08 0.13 0.99

0.06 0.08 0.10 0.12 0.14 ND ND ND ND ND 0.06 0.08 0.10 0.12 0.14

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

0.09 0.09 0.09 0.09 0.09 ND ND ND ND ND 0.09 0.09 0.09 0.09 0.09

0.04 0.04 0.04 0.04 0.04 ND ND ND ND ND 0.04 0.04 0.04 0.04 0.04

0.03 0.03 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.03 0.04 0.04 0.04 0.04

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND
64 Occurrence and Temporal Variability of MTBE and Other VOCs in Select Sources of Drinking Water

Table D1. Statistical summary of volatile organic compounds in source-water samples in the Focused Survey—Continued

[Values in micrograms per liter; N, number of sources sampled; ND, not detected]

Volatile organic

compounds

All community water systems (N=134) Ground Water (N=78)

Minimum 25th Median 75th MaximumMinimum 25th Median 75th Maximum

Solvents—Continued

1,1,2,2-Tetrachloroethane ND ND ND ND ND ND ND ND ND ND

1,1,2-Trichloroethane 0.04 0.07 0.10 0.12 0.15 0.04 0.07 0.10 0.12 0.15

1,1-Dichloroethane 0.04 0.05 0.09 0.33 3.85 0.04 0.07 0.10 0.36 3.85

1,1-Dichloroethene 0.10 0.14 0.64 1.53 9.78 0.10 0.14 0.64 1.53 9.78

1,2,3-Trichloropropane 0.03 0.04 0.05 0.08 0.09 0.03 0.04 0.05 0.07 0.08

1,2,4-Trichlorobenzene 0.06 0.06 0.06 0.08 0.14 0.06 0.08 0.10 0.12 0.14

1,2-Dichlorobenzene 0.18 0.19 0.20 0.21 0.22 0.18 0.19 0.20 0.21 0.22

1,2-Dichloroethane 0.03 0.06 0.14 0.39 2.14 0.03 0.06 0.14 0.39 2.14

1,2-Dichloropropane 0.04 0.09 0.11 0.17 1.04 0.04 0.09 0.11 0.17 1.04

1,3-Dichlorobenzene 0.04 0.05 0.05 0.11 0.17 0.04 0.05 0.05 0.11 0.17

2-Chlorotoluene ND ND ND ND ND ND ND ND ND ND

4-Chlorotoluene 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03

Bromobenzene ND ND ND ND ND ND ND ND ND ND

Carbon tetrachloride 0.06 0.41 0.75 0.95 1.15 0.06 0.41 0.75 0.95 1.15

Chlorobenzene 0.05 0.07 0.08 0.56 0.81 0.07 0.08 0.32 0.62 0.81

Chloroethane 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15

cis-1,2-Dichloroethene 0.03 0.06 0.27 1.38 10.35 0.03 0.06 0.27 1.38 10.35

Dibromomethane 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30

Hexachloroethane ND ND ND ND ND ND ND ND ND ND

Methyl ethyl ketone 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84

Methylene chloride 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31 0.31

n-Propylbenzene 0.05 0.06 0.13 0.26 0.47 0.47 0.47 0.47 0.47 0.47

Tetrachloroethene 0.05 0.10 0.39 4.24 290.00 0.05 0.10 0.39 4.24 290.00

trans-1,2-Dichloroethene 0.05 0.07 0.19 0.29 0.45 0.05 0.07 0.19 0.29 0.45

Trichloroethene 0.04 0.17 0.43 2.18 165.00 0.04 0.17 0.43 2.18 165.00

Fumigants

1,4-Dichlorobenzene 0.04 0.04 0.06 0.15 0.46 0.04 0.04 0.05 0.18 0.46

Bromomethane 0.09 0.09 0.09 0.10 0.11 0.09 0.09 0.09 0.10 0.11

cis-1,3-Dichloropropene ND ND ND ND ND ND ND ND ND ND

trans-1,3-Dichloropropene ND ND ND ND ND ND ND ND ND ND

Refrigerants

1,1,2-Trichloro-1,2,2-trifluoroethane2.27 2.27 2.27 2.27 2.27 2.27 2.27 2.27 2.27 2.27

Chloromethane 0.11 0.13 0.15 0.78 2.63 0.13 0.15 0.16 1.40 2.63

Dichlorodifluoromethane 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40

Trichlorofluoromethane 0.23 0.35 0.46 0.48 0.50 0.23 0.35 0.46 0.48 0.50
Appendix D 65

Reservoir (N=39) Rivers (N=17) All surface water (N=56)

Minimum 25th Median 75th MaximumMinimum 25th Median 75th MaximumMinimum 25th Median 75th Maximum

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

0.04 0.05 0.07 0.08 0.09 ND ND ND ND ND 0.04 0.05 0.07 0.08 0.09

0.06 0.06 0.06 0.06 0.06 0.06 0.06 ND ND 0.06 0.06 0.06 0.06 0.06 0.06

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

0.05 0.09 0.12 0.16 0.19 0.06 0.06 0.06 0.06 0.06 0.05 0.06 0.06 0.13 0.19

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

0.06 0.06 0.06 0.06 0.06 ND ND ND ND ND 0.06 0.06 0.06 0.06 0.06

0.09 0.09 0.09 0.09 0.09 ND ND ND ND ND 0.09 0.09 0.09 0.09 0.09

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11 0.11

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND