Occurrence and Implications of Methyl tert-Butyl

Ether and Gasoline Hydrocarbons in Ground Water

and Source Water in the United States and in

Drinking Water in 12 Northeast and Mid-Atlantic

States, 1993-2002

Water-Resources Investigations Report 03-4200

U.S. Department of the Interior

U.S. Geological Survey

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

NATIONAL SYNTHESIS ON VOLATILE ORGANIC COMPOUNDS
U.S. Department of the Interior

U.S. Geological Survey

Occurrence and Implications of Methyl tert-Butyl

Ether and Gasoline Hydrocarbons in Ground Water

and Source Water in the United States and in

Drinking Water in 12 Northeast and Mid-Atlantic

States, 1993-2002

By Michael J. Moran, John S. Zogorski, and Paul J. Squillace

Water-Resources Investigations Report 03-4200
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: 2004

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:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286
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 ........................................................................................................................................................................... 1

Purpose and Scope....................................................................................................................................................... 2

Data and Methods........................................................................................................................................................ 3

Assessment Level Computations....................................................................................................................... 5

Statistical Methods ............................................................................................................................................ 5

Occurrence of MTBE and Gasoline Hydrocarbons............................................................................................................... 6

MTBE .......................................................................................................................................................................... 6

NAWQA Studies................................................................................................................................................ 7

Concentrations................................................................................................................................................... 8

Gasoline Hydrocarbons ............................................................................................................................................... 10

Benzene ............................................................................................................................................................. 11

Other Ethers................................................................................................................................................................. 11

Factors Affecting Occurrence...................................................................................................................................... 11

Sources of MTBE .............................................................................................................................................. 12

Use of MTBE in Gasoline as a Surrogate for Environmental Input.................................................................. 12

MTBE and Dissolved Oxygen in Ground Water............................................................................................... 16

Other Associations with MTBE in Ground Water............................................................................................. 17

Implications Involving MTBE and Gasoline Hydrocarbons................................................................................................. 19

MTBE in Drinking Water ............................................................................................................................................ 19

MTBE Occurrence by Land Use and Well Type ......................................................................................................... 20

MTBE Concentrations and Human Health.................................................................................................................. 20

Gasoline Hydrocarbon Occurrence ............................................................................................................................. 21

Associations with MTBE Occurrence ......................................................................................................................... 22

Summary................................................................................................................................................................................ 23

References ............................................................................................................................................................................. 24
VI Contents

FIGURES

1. Map showing locations of samples of ground water, source water, and drinking water that were

analyzed for MTBE and the locations of samples with detections of MTBE using no assessment level ............... 7

2-9. Graphs showing:

2. Detection frequencies of MTBE or any gasoline hydrocarbon in samples of ground water,

source water, and drinking water using an assessment level of 0.2 microgram per liter................................. 8

3. Detection frequencies of MTBE in samples from various NAWQA occurrence studies

using no assessment level ................................................................................................................................ 9

4. Detection frequencies of MTBE in samples from various well types (NAWQA studies)

using no assessment level ................................................................................................................................ 9

5. MTBE concentrations in samples of ground water, source water, and drinking water

using an assessment level of 0.2 microgram per liter...................................................................................... 10

6. Detection frequencies of any gasoline hydrocarbon in samples of ground water for selected

concentration ranges of MTBE using an assessment level of 0.2 microgram per liter ................................... 13

7. Detection frequencies of MTBE in samples of ground water, source water, and drinking water

for areas of high and low MTBE use using an assessment level of 0.2 microgram per liter .......................... 14

8. Detection frequencies of MTBE in samples of ground water for areas of urban and non-urban

land use and by areas of high and low MTBE use using no assessment level ................................................ 15

9. Comparison of the distributions of concentrations of MTBE in ground water in areas of urban

land use for areas of high and low MTBE use using no assessment level ...................................................... 16

TABLES

1. Anthropogenic and hydrogeologic variables that were used in the logistic regression analysis............................ 18

2. Anthropogenic and hydrogeologic variables that were significantly associated with the

probability of detecting MTBE in ground water .................................................................................................... 19
Introduction 1

Occurrence and Implications of Methyl tert-Butyl

Ether and Gasoline Hydrocarbons in Ground Water

and Source Water in the United States and in

Drinking Water in 12 Northeast and Mid-Atlantic

States, 1993-2002

By Michael J. Moran, John S. Zogorski, and Paul J. Squillace

ABSTRACT

The occurrence and implications of methyl 

tert-butyl ether (MTBE) and gasoline hydrocarbons were examined in three surveys of water 

quality conducted by the U.S. Geological 

Survey—one national-scale survey of ground 

water, one national-scale survey of source water 

from ground water, and one regional-scale survey 

of drinking water from ground water. The overall 

detection frequency of MTBE in all three surveys 

was similar to the detection frequencies of some 

other volatile organic compounds (VOCs) that 

have much longer production and use histories in 

the United States. The detection frequency of 

MTBE was higher in drinking water and lower in 

source water and ground water. However, when 

the data for ground water and source water were 

limited to the same geographic extent as drinkingwater data, the detection frequencies of MTBE 

were comparable to the detection frequency of 

MTBE in drinking water. In all three surveys, the 

detection frequency of any gasoline hydrocarbon 

was less than the detection frequency of MTBE. 

No concentration of MTBE in source water 

exceeded the lower limit of U.S. Environmental 

Protection Agency’s Drinking-Water Advisory of 

20 μg/L (micrograms per liter). One concentration 

of MTBE in ground water exceeded 20 μg/L, and 

0.9 percent of drinking-water samples exceeded 

20 μg/L.

The overall detection frequency of MTBE 

relative to other widely used VOCs indicates that 

MTBE is an important concern with respect to 

ground-water management. The probability of 

detecting MTBE was strongly associated with 

population density, use of MTBE in gasoline, and 

recharge, and weakly associated with density of 

leaking underground storage tanks, soil permeability, and aquifer consolidation. Only concentrations 

of MTBE above 0.5 μg/L were associated with dissolved oxygen. Ground water underlying areas 

with high population density, ground water underlying areas where MTBE is used as a gasoline oxygenate, and ground water underlying areas with 

high recharge has a greater probability of MTBE 

contamination. Ground water from public-supply 

wells and shallow ground water underlying urban 

land-use areas has a greater probability of MTBE 

contamination compared to ground water from 

domestic wells and ground water underlying rural 

land-use areas.

INTRODUCTION

The Clean Air Act (CAA) Amendments of 1990 

specified that oxygen must be added to gasoline in 

areas where air-quality standards have not been 
2 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

attained (U.S. Environmental Protection Agency, 

1990). Oxygenates are compounds that contain 

oxygen and are added to gasoline in order to meet the 

requirements of the CAA Amendments. Since the late 

1980s, oxygenates have been used in gasoline in areas 

regulated under the Reformulated Gasoline (RFG) and 

Oxygenated Fuel (Oxyfuel) Programs of the CAA 

Amendments. As of 1998, approximately 30 percent of 

all gasoline in the United States contained oxygenates 

for compliance with RFG requirements and an additional 4 percent contained oxygenates for compliance 

with Oxyfuel requirements (U.S. Environmental 

Protection Agency, 1998). 

Although the CAA Amendments did not specify 

which oxygenate must be added to gasoline to achieve 

oxygen requirements, the alkyl ether methyl tert-butyl 

ether (MTBE) has been, and continues to be, the most 

commonly used oxygenate. As of 1998, MTBE was 

used in more than 80 percent of oxygenated gasoline. 

Recent information indicates that MTBE is the most 

frequently used gasoline oxygenate in RFG areas (U.S. 

Environmental Protection Agency, 2003a), whereas 

ethanol is the most frequently used gasoline oxygenate 

in Oxyfuel areas (U.S. Environmental Protection 

Agency, 2003b). In addition to its use as a gasoline oxygenate, MTBE also has been widely used to enhance 

octane in gasoline (U.S. Environmental Protection 

Agency, 1998).

Since about 1991, large volumes of MTBE have 

been produced every year in the United States and used 

as a gasoline oxygenate. In 2001, production of MTBE 

in the United States was over 12 billion liters (Department of Energy, 2002). From 1993 to 1998, MTBE was 

the second most-produced organic chemical in the 

United States (U.S. Environmental Protection Agency, 

1998). 

A chemical with such large production and widespread use has a substantial potential of release to the 

environment. Once released to the environment, MTBE 

can be transported to ground-water resources through a 

variety of mechanisms. Once in ground water, MTBE 

concentrations can adversely affect the quality of water 

from aquifers that are used to supply water for private 

household wells or public water-supply systems.

MTBE has been detected in ground water in the 

United States and other countries such as Germany 

(Klinger and others, 2002; Moran, Clawges, and 

Zogorski, 2002; Moran, Lapham, and others, 2002). 

MTBE also has been found in some reservoirs and 

rivers that are used as public water supplies (Grady, 

2003). Recent data show the presence of MTBE in 

ground water used as a source for drinking water 

(Grady, 2003) and in drinking water in the United 

States (Grady and Casey, 2001; Williams and others, 

2002). 

Although the magnitude of the problem is not 

known, contamination of ground water used as a source 

for drinking water already has resulted in the closure of 

many drinking-water wells around the country 

including both public and private wells. MTBE in 

drinking water may have possible human health consequences including carcinogenic potential and detrimental reproductive and developmental effects 

(Hartley and others, 1999). The U.S. Environmental 

Protection Agency (1997) has issued a drinking-water 

advisory suggesting that MTBE concentrations be less 

than 20 to 40 μg/L in drinking water (micrograms per 

liter).

Research has shown that detections of MTBE in 

ground water are more frequent in areas where MTBE 

is used as a gasoline oxygenate, such as RFG or Oxyfuel areas, in comparison to areas where it is not used 

as a gasoline oxygenate (Moran, Clawges, and 

Zogorski, 2002). However, MTBE also has been 

detected in ground water in many other areas of the 

Nation outside of the RFG and Oxyfuel areas. The geographically widespread detection of MTBE indicates 

that ground water in many areas may be at potential risk 

of MTBE contamination (Moran, Clawges, and 

Zogorski, 2002).

Purpose and Scope

The purpose of this report is to describe the 

occurrence and implications of MTBE and gasoline 

hydrocarbons in ground water, source water, and 

drinking water from data collected or compiled by the 

U.S. Geological Survey’s (USGS) National WaterQuality Assessment (NAWQA) Program and cooperators. In this report, gasoline hydrocarbons refers to a 

select group of volatile organic compounds (VOCs) 

whose primary use is believed to be in gasoline and 

includes the BTEX (benzene, toluene, ethylbenzene, 

and xylenes) compounds as well as naphthalene, 

n-butylbenzene, iso-propylbenzene, styrene, and 1,2,4-

trimethylbenzene. Although these VOCs are components of gasoline, they do have other domestic, com-
Introduction 3

mercial, and industrial uses, and they may have other 

sources to ground water in addition to gasoline.

This report provides an overview for the entire 

Nation of the occurrence of MTBE in ground water and 

ground water that is withdrawn for human consumption 

prior to any treatment (hereafter referred to as source 

water). The occurrence of MTBE in drinking water in 

12 Northeast and Mid-Atlantic States also is examined. 

The focus of this report is on the occurrence and concentrations of MTBE and gasoline hydrocarbons in 

these media, and on the occurrence of MTBE in areas 

where MTBE content in gasoline is used as a surrogate 

for MTBE input to the environment. The implications 

of the occurrence of MTBE and gasoline hydrocarbons 

in these media also are described. This report also 

describes other factors that are associated with the 

occurrence of MTBE in ground water and that may aid 

in understanding the sources and pathways of MTBE to 

ground water and the vulnerability of aquifers to MTBE 

contamination.

Data and Methods

Extensive data sets were used for analyses in this 

report. Data were analyzed from three sources: (1) the 

NAWQA Program that sampled ground water 

throughout the United States from 1993-2002 as a part 

of occurrence studies (ground-water survey); (2) a collaborative effort of the NAWQA Program and other 

organizations in a national survey that sampled ground 

water used as a source of drinking water throughout the 

United States from 1999-2000 (source-water survey); 

and (3) the NAWQA Program that compiled data on 

drinking water from community water systems (CWSs) 

in 12 Northeast and Mid-Atlantic States that were sampled from 1993-1998 (drinking-water survey). 

As part of NAWQA occurrence studies, personnel collected samples of ground water between 1993 

and 2002. These samples were analyzed for MTBE 

and other VOCs at the USGS National Water-Quality 

Laboratory (NWQL). The ground-water occurrence 

studies of NAWQA included two main components: 

(1) major aquifer surveys of water quality in one or 

more aquifers underlying each NAWQA study area, 

and (2) land-use studies assessing the quality of ground 

water underlying agricultural and urban land-use areas 

(Gilliom and others, 1995). In the major aquifer surveys, NAWQA personnel sampled large areal and depth 

dimensions of a principal aquifer that constitutes an 

important ground-water resource within the study 

area. In the land-use studies, NAWQA personnel sampled shallow ground water in areas of predominant 

agricultural or urban land use (Gilliom and others, 

1995). Sampling in the land-use studies was designed 

to target recently recharged ground water generally 

less than 10 years old.

The design for each of the NAWQA occurrence 

studies was network-based consisting of the selection 

of about 30 wells (sampling sites) in an unbiased, 

random, equal-area distribution throughout the study 

area. In general, the wells sampled by NAWQA were 

not located in proximity to known point source 

releases. As of January 2003, data on MTBE and 

other VOCs were available for 3,964 wells sampled in 

the NAWQA occurrence studies. These data constitute 

the ground-water survey.

Personnel from CWSs throughout the United 

States collected samples of ground water used as a 

source of drinking water from 1999 to 2000 as part of 

a national source-water survey. This survey was a collaborative effort between the American Water Works 

Association Research Foundation (AWWARF), the 

Metropolitan Water District of Southern California 

(MWDSC), the Oregon Health and Sciences University, participating CWSs, and the USGS. The purpose 

of this survey was to assess the magnitude and spatial 

extent of MTBE contamination of source water 

whether derived from surface water or ground water. 

In this report, only source water from ground water 

was considered. Two phases of the national sourcewater survey, with respect to ground water, were 

accomplished: (1) a random survey of 579 groundwater sources throughout the country, and (2) a 

focused survey of 78 ground-water sources 

throughout the country that were known to have 

MTBE contamination or were considered susceptible 

to MTBE contamination (Clawges and others, 2001). 

Detailed results of the random and focused surveys 

can be found in Grady (2003) and Delzer and Ivahnenko (2003), respectively. In this report, only data 

from the random survey were used in order to avoid 

potential bias in the occurrence of MTBE.

The selection of CWSs sampled in the random 

survey was statistically based and was stratified by 

population-served size category, source of water 

(ground water or surface water), and total number of 

people served (Ivahnenko and others, 2001). For 
4 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

ground water, all samples were collected prior to treatment and as close as possible to the wellhead. The 

MWDSC laboratory analyzed the samples of source 

water for MTBE and other VOCs. Data on MTBE were 

available from a total of 571 samples of ground water. 

These data constitute the source-water survey.

Data on MTBE and other VOCs in drinking 

water were compiled by the USGS from various agencies in 12 Northeast and Mid-Atlantic States. Unlike 

the previous ground-water and source-water surveys, 

the drinking-water survey was not national in scope and 

only covered these regions. The purpose of this survey 

was to describe the occurrence and distribution of 

MTBE and other VOCs in public drinking water supplied by CWSs in these regions. These regions were 

selected because they generally are densely populated, 

have a long-term history of urbanization, and are areas 

with high use of public-water supply and high use of 

MTBE in gasoline (Grady and Casey, 2001; Moran and 

others, 2001). In addition, large parts of these regions 

are regulated under the RFG Program. A random subset 

of all CWSs in the regions was selected using a statistical approach that stratified selection by State, source 

of water, and number of people served (Grady and 

Casey, 2001).

The data from the 12 Northeast and Mid-Atlantic 

States represent drinking water that was sampled to 

meet the requirements of the Safe Drinking Water Act 

(SDWA). The samples were collected from 1993-98 

and were analyzed by a variety of State and private 

laboratories. The data on drinking water were summarized by system and do not reflect occurrence by 

source. A CWS was counted as having a detection of 

MTBE if a measurable concentration of MTBE was 

reported in one or more water samples associated with 

any source or sample location for that CWS. In this 

report, only samples of drinking water obtained from 

ground-water sources were used for analysis. Data on 

MTBE and other VOCs were available for a total of 985 

CWSs with ground-water sources. These data constitute the drinking-water survey (Grady and Casey, 

2001).

Laboratory reporting levels for MTBE and gasoline hydrocarbans varied between and within the 

surveys. For ground water collected by NAWQA, 

variability in the laboratory reporting level for MTBE 

primarily was the result of the introduction of new, 

information-rich methods and a change in the policy of 

NWQL in 1996 concerning the determination of 

laboratory reporting levels (Rose and Schroeder, 1995; 

Connor and others, 1998; Oblinger Childress and 

others, 1999). Laboratory reporting levels for MTBE in 

ground-water samples ranged from 0.1 to 1.66 μg/L 

with a median of 0.17 μg/L. Laboratory reporting levels 

for gasoline hydrocarbons ranged from 0.03 to 5 μg/L 

with a median of 0.1 μg/L. Laboratory reporting levels 

greater than 0.2 μg/L may be the result of dilution of 

environmental samples or laboratory censoring of data 

after review of laboratory quality-control (QC) information.

Field QC data collected by the NAWQA 

Program for the occurrence studies indicated that 

systematic contamination of environmental samples by 

MTBE did not occur as a result of collection, processing, handling, and shipping procedures (G.C. 

Delzer, U.S. Geological Survey, oral commun., 2002). 

Detections of some gasoline hydrocarbons in field 

blanks indicated that some systematic contamination 

by these compounds was suspected (D.A. Bender, U.S. 

Geological Survey, oral commun., 2002). Environmental samples that were associated with field blanks 

that had suspected systematic contamination were not 

used in this analysis.

For source-water samples analyzed by MWDSC, 

the laboratory reporting level for MTBE was constant 

at 0.078 μg/L. However, the laboratory reporting level 

for the gasoline hydrocarbon compounds differed from 

this level, ranging from 0.044 to 0.13 μg/L with a 

median of 0.066 μg/L. A review of field QC data indicated systematic low-concentration (<0.2 μg/L) contamination of field and trip blanks by various VOCs. As 

a result, all source-water environmental samples were 

censored at a level of 0.2 μg/L for all VOCs to ensure 

that reported concentrations represent environmental 

concentrations of VOCs and not systematic contamination.

For drinking-water samples, the laboratory 

reporting level for MTBE ranged from 0.1 to 10 μg/L 

with a median of 0.5 μg/L. The laboratory reporting 

level for gasoline hydrocarbons also was variable 

ranging from 0.017 to 1.5 μg/L with a median of 

0.245 μg/L. The variability in reporting levels in this 

data set primarily was a result of different censoring 

criteria and instrument performances of the various 

State and private laboratories that performed the analyses. For inclusion of drinking-water data in this 

analysis, collection of field QC data was required to 

ensure that a minimum standard of data quality was 
Introduction 5

maintained. It also was required that QC data be used to 

evaluate and, if necessary, to censor drinking-water data 

to ensure that reported concentrations represent environmental concentrations of VOCs and not systematic 

contamination (Grady and Casey, 1999). 

In all three surveys, each well, source, or sampling site was allowed to have only one set of environmental data representing a time snapshot of water 

quality. In most cases, the primary environmental 

sample taken from the well was analyzed. For NAWQA 

data, the primary environmental sample represents the 

first environmental sample taken from the well. If more 

than one set of environmental water-quality results was 

available, the most recent set was selected. Therefore, 

each well, source, or sampling site had only one set of 

water-quality results. Consequently, the term “sample” 

is used in this report to indicate the results from a single 

well, source, or sampling site.

Assessment Level Computations

Data analysis was complicated by the variations 

in laboratory reporting levels between and within the 

surveys. In order to accurately compare detection frequencies of MTBE and gasoline hydrocarbons between 

different data sets that have varying reporting levels, it 

was necessary to apply a uniform censoring level that is 

referred to as an assessment level in this report. If a 

water-quality constituent has reporting levels that vary 

between data sets, comparisons of detection frequencies between the data sets may not reflect true differences in water quality. Instead, they may simply reflect 

differences in instrument sensitivity, analytical 

methods, or censoring levels between the different 

laboratories. 

For comparing the detection frequencies of 

water-quality constituents with varying laboratory 

reporting levels, an assessment level should be applied 

before detection frequencies are computed. The application of an assessment level allows for the most accurate comparison of detection frequencies between data 

sets with different laboratory reporting levels.

In this report, an assessment level of 0.2 μg/L 

was applied whenever MTBE detection frequencies 

were compared between the three surveys. However, if 

comparisons of detection frequencies were made 

between subsets of a single survey, or detection frequency was examined wholly within a single survey, no 

assessment level was applied. To execute the assessment level, analyses with a detectable concentration 

less than 0.2 μg/L were converted to non-detections. 

Detection frequency was then computed as the 

number of samples that had a detected concentration 

of MTBE greater than or equal to 0.2 μg/L divided by 

the number of samples with MTBE analyses times 

100. 

For comparisons of MTBE concentrations 

between the three surveys, an assessment level of 

0.5 μg/L was applied to data for ground water and 

source water to make the data most comparable to the 

drinking-water data. This level was selected because 

the median laboratory reporting level for data from the 

drinking-water survey is 0.5 μg/L and there are no 

concentrations below that level in this data set.

Statistical Methods

To better understand the significance of the 

results and the hydrologic processes that have caused 

them, statistical tests were performed to evaluate and 

compare data. The statistical significance level (α) 

used in this report for all tests was 0.05. The results of 

some statistical tests are shown graphically as letter 

symbols in the figures. If two groups of data share the 

same letter symbol, the null hypothesis was not 

rejected for that comparison. If two groups of data do 

not share the same letter symbol, the null hypothesis 

was rejected for that comparison.

Contingency table tests, using Pearson’s chisquare test of independence, were performed to determine if detection frequencies between one or more 

surveys, or detection frequencies between subsets of 

one survey, were independent. Contingency table tests 

were used because detection frequencies are nominal, 

categorical variables. The null hypothesis states that 

the row variables are independent from the column 

variables. If the null hypothesis is rejected, the row 

variables are dependent on the column variables and 

the detection frequencies were considered dependent 

on the survey.

The nonparametric Mann-Whitney test (Helsel 

and Hirsch, 1992) was used to compare two independent groups of data such as the population densities 

around wells between areas of high and low MTBE 

use in gasoline. The null hypothesis states that the 

values in one group are not higher than the values in 

the other group. If the null hypothesis is rejected, the 

population densities around one group of wells were 

higher than another.
6 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

The nonparametric Kolmogorov-Smirnov test 

(Helsel and Hirsch, 1992) was used to compare 

MTBE concentrations between surveys in order to 

determine if the distributions of concentrations were 

significantly different. The null hypothesis states that 

the distributions are not significantly different. If the 

null hypothesis was rejected, the distributions were 

considered significantly different. If the distributions 

of concentrations were significantly different, the 

concentrations of one distribution was higher or lower 

than the other.

For determining associations between multiple 

independent variables and MTBE occurrence, a multivariate logistic regression analysis was used (Helsel 

and Hirsch, 1992). For water-quality analyses, the 

explanatory variables selected for analysis are often 

anthropogenic or hydrogeologic parameters that can 

provide insight and understanding into the sources, 

transport processes, or environmental mechanisms that 

affect contamination. Logistic regression often is used 

to predict the probability of occurrence of a contaminant as it relates to various explanatory variables. 

However, in this case the regression results were not 

intended to provide a capability for predicting the probability of detecting MTBE. Instead they were used 

to determine associations between MTBE occurrence 

and explanatory variables and to determine the strength 

and direction of the associations.

For the overall regression equation, if the overall 

likelihood produced a p-value of <0.05, all explanatory 

variables were considered significantly associated with 

the probability of MTBE occurrence. The significance 

of nested models was tested using the partial likelihood 

ratio test. For cases where one additional coefficient 

was added, the Wald statistic p-value was used to determine the significance of the slope coefficient. If the 

Wald statistic p-value of the slope coefficient was less 

than 0.05 and the upper and lower bounds of the odds 

ratio did not include 1, the additional variable was considered significantly associated with the probability of 

occurrence of MTBE. Non-nested regression analyses 

were tested using Akaike’s Information Criteria (Helsel 

and Hirsch, 1992). Standardized coefficients were used 

to compare the strength of a relation between the 

dependent variable and different explanatory variables. 

The method for computing standardized slope coefficients followed Menard (2002). 

OCCURRENCE OF MTBE AND GASOLINE 

HYDROCARBONS

The geographic distribution of samples analyzed 

for MTBE in all three surveys is shown in figure 1. The 

circles in figure 1 represent the locations where samples were collected. The open circles show the location 

of samples that did not have a detected concentration of 

MTBE, whereas the filled circles show the location of 

samples that had a detected concentration of MTBE. In 

all three surveys combined, 420 samples had detected 

concentrations of MTBE using no assessment level. 

This corresponds to an overall detection frequency of 

7.6 percent in a total of 5,520 samples. Samples with a 

detected concentration of MTBE are most intensively 

represented in the Northeast region of the country 

(fig. 1).

For gasoline hydrocarbons, the overall detection frequency was 16 percent using no assessment 

level. One or more gasoline hydrocarbons were 

detected in 931 of 5,818 total samples. Samples with 

a detected concentration of one or more gasoline 

hydrocarbons are less intensively represented in the 

Northeast region of the country compared to MTBE.

MTBE

The detection frequency of MTBE in each of the 

three surveys is illustrated in figure 2. Data for ground 

water and source water were analyzed at an assessment 

level of 0.2 μg/L to make them comparable to data for 

drinking water. For ground-water data, the detection 

frequency is only for MTBE analyses in the major 

aquifer surveys because this is the best representation 

of the quality of the ground-water resource. The bold 

letters in figure 2 graphically display the results of 

chi-square tests of independence comparing 

MTBE detection frequencies between the three 

surveys. 

The detection frequency of MTBE was lowest in 

ground water at 2.9 percent and highest in drinking 

water at 9 percent (fig. 2). The detection frequency of 

MTBE in source water was 5.4 percent. The detection 

frequencies of MTBE were dependent on the survey for 

all comparisons (p <0.05).
Occurrence of MTBE and Gasoline Hydrocarbons 7

Data from the ground-water and source-water 

surveys also were examined in only the 12 Northeast 

and Mid-Atlantic States and at an assessment level of 

0.5 μg/L to make them comparable to the drinkingwater data. For ground water, the detection frequency 

of MTBE in these regions was 6.6 percent, whereas the 

detection frequency of MTBE in source water from 

these regions was 11.3 percent. These detection frequencies were similar to the detection frequency of 

MTBE in drinking water. In this analysis, the detection 

frequencies of MTBE were not dependent on the 

survey for all comparisons (p <0.05).

NAWQA Studies

As indicated previously, NAWQA conducts both 

major aquifer surveys and land-use studies. The landuse studies examine shallow ground water in areas of 

predominant agricultural or urban land use. Figure 3 

illustrates the detection frequencies of MTBE in all 

studies combined, major aquifer surveys, and land-use 

studies using no assessment level. The detection frequency of MTBE was highest in urban land-use studies 

at 13 percent and lowest in agricultural land-use studies 

at 1.2 percent. The detection frequency of MTBE in the 

major aquifer surveys was intermediate at 2.8 percent. 

For all of NAWQA studies combined, the detection 

frequency of MTBE was 4.7 percent. 

For various studies of ground-water quality, the 

NAWQA program samples many different types of 

wells. Figure 4 illustrates the detection frequencies of 

MTBE in four different types of wells sampled by the 

NAWQA Program using no assessment level. Although 

domestic-supply wells are most frequently sampled by 

NAWQA for MTBE, the detection frequency of MTBE 

is highest in monitoring wells.

Figure 1. Locations of samples of ground water, source water, and drinking water that were analyzed for MTBE and

the locations of samples with detections of MTBE using no assessment level.

EXPLANATION

SAMPLE WITH MTBE DETECTION (420)

SAMPLE WITHOUT MTBE DETECTION (5,100)

Oahu, Hawaii

Alaska

0 200 400 MILES

0 200 400 KILOMETERS

Base from U.S. Geological Survey, digital data, 1:2,000,000, 1994

Albers Equal-Area Conic projection

Standard Parallels: 29o30', 45o30' Central Meridan: 96oW
8 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

Figure 2. Detection frequencies of MTBE or any gasoline hydrocarbon in samples of ground water, source water

and drinking water using an assessment level of 0.2 microgram per liter.

0

1

2

3

4

5

6

7

8

9

10

11

Ground water

(United States)

Source water

(United States)

Drinking water

(Northeast

United States)

DETECTION FREQUENCY, IN PERCENT

A A

B B

C

A 2,364 2,365

571

579

1,271

 985

MTBE

GASOLINE HYDROCARBONS

2,364 NUMBER OF SAMPLES

COMPARISON BETWEEN MTBE

 OCCURRENCE—Samples that 

 share a letter are not significantly

 different at the 95-percent confidence

 interval

A

COMPARISON BETWEEN GASOLINE

 HYDROCARBON OCCURRENCE—

 Samples that share a letter are not

 significantly different at the 95-percent

 confidence interval

A

EXPLANATION

Concentrations

Concentrations of MTBE in each of the three 

surveys are illustrated in figure 5. In this figure, the data 

from the ground-water survey include only MTBE concentrations from the major aquifer surveys. An assessment level of 0.2 μg/L was used to make ground-water 

data comparable to source-water and drinking-water 

data. However, in ground water many detections of 

MTBE were below 0.2 μg/L. Of all the detections of 

MTBE in ground water, 38 percent (113 of 300) were 

less than 0.2 μg/L. One reason for the numerous lowlevel detections in ground water was the informationrich reporting conventions employed by the NWQL for 

analysis of VOCs in water after 1996. The informationrich method used for analyzing VOCs allows for 

reporting of concentrations below both the laboratory 

reporting level and the long-term method detection 

limit (Oblinger Childress and others, 1999). It has been 

demonstrated that as the level of detection of a VOC is 

lowered, the frequency of detection in ground water 

increases (Moran, Lapham, and others, 2002).

Similar to detection frequencies, the median 

detected MTBE concentration was lowest in ground 

water and highest in drinking water. Using an assessment level of 0.2 μg/L, median concentrations were 

0.67 μg/L in ground water, 0.71 μg/L in source water, 

and 1.8 μg/L in drinking water. For ground water, only 

one sample had a detected concentration above 

20 μg/L, which is the lower limit of the U.S. Environmental Protection Agency’s Drinking-Water Advisory 

for MTBE (U.S. Environmental Protection Agency, 

1997). For source water, no detected MTBE concentration exceeded 20 μg/L. For drinking water, nine samples (0.9 percent) had detected MTBE concentrations 

above 20 μg/L. The samples with concentrations of 

MTBE in drinking water that exceeded 20 μg/L were 

collected in the following states: Connecticut (2), New 

York (3), Rhode Island (1), and Virginia (3).
Occurrence of MTBE and Gasoline Hydrocarbons 9

Figure 3. Detection frequencies of MTBE in samples from various NAWQA occurrence studies using no

assessment level.

Figure 4. Detection frequencies of MTBE in samples from various well types (NAWQA studies) using no

assessment level.

0

2

4

6

8

10

12

14

All studies Major aquifer

surveys

Urban

land-use

studies

Agricultural

land-use

studies

DETECTION FREQUENCY, IN PERCENT

3,964

2,364

754

846

3,964 Number of samples

0

1

2

3

4

5

6

7

8

Public

supply

Domestic

supply

Monitoring Other

DETECTION FREQUENCY, IN PERCENT

346

1,912

237

1,469 346 Number of samples
10 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

Figure 5. MTBE concentrations in samples of ground water, source water, and drinking water using an

assessment level of 0.2 microgram per liter. Statistical analyses were performed at an assessment level

of 0.5 microgram per liter.

0.1 1 10 100 1,000

MTBE CONCENTRATION, IN MICROGRAMS PER LITER

Drinking water

(Northeast

United States)

Source water

(United States)

Ground water

(United States) 

Median

Lower limit of U.S. Environmental

Protection Agency's drinking-water

advisory

189

31

68

A

B

AB

COMPARISON BETWEEN MTBE CONCENTRATIONS—Samples

 that share a letter are not significantly different at the 95-percent

 confidence interval

A

68 NUMBER OF CONCENTRATIONS

EXPLANATION

A statistical comparison was made of the MTBE 

concentrations between each of the three surveys. The 

statistical analysis was performed at an assessment 

level of 0.5 μg/L in order to make the data for ground 

water and source water comparable to the data for 

drinking water. The results are shown by the letter symbols in figure 5. The results of the statistical analyses 

indicate that the distributions of MTBE concentrations 

between ground water and drinking water were significantly different (p <0.05), but the distributions of 

MTBE concentrations between ground water and 

source water and between source water and drinking 

water were not significantly different (p >0.05). 

Quantile-quantile plots indicated that the concentrations of MTBE in drinking water were higher than the 

concentrations of MTBE in either ground water or 

source water.

Gasoline Hydrocarbons

The detection frequency of one or more gasoline 

hydrocarbons in each of the three data sets also is illustrated in figure 2. Data for ground water and source 

water were analyzed at an assessment level of 0.2 μg/L 

to make them comparable to data for drinking water. 

For ground-water data, the detection frequency is only 

for gasoline hydrocarbon analyses in the major aquifer 

surveys. The bold italics letters in figure 2 graphically 

display the results of chi-square tests of independence 

comparing the detection frequencies of gasoline hydrocarbons between the three surveys.

The detection frequency of one or more gasoline 

hydrocarbons was lowest in drinking water and highest 

in source water (fig. 2). For ground water and drinking 

water, the detection frequency of one or more gasoline 
Occurrence of MTBE and Gasoline Hydrocarbons 11

hydrocarbons was less than the detection frequency of 

MTBE. However, the detection frequency of one or 

more gasoline hydrocarbons in source water was 

slightly higher than the detection frequency of MTBE 

(fig. 2). The detection frequencies of one or more gasoline hydrocarbons were not dependent on the survey 

between ground water and drinking water (p >0.05) but 

were dependent on the survey between source water 

and ground water and between source water and 

drinking water (p <0.05). 

Individually, gasoline hydrocarbons were 

detected less frequently than MTBE in all three surveys. In source water, the detection frequencies of 

individual gasoline hydrocarbons were highest. However, individual gasoline hydrocarbons were detected in 

less than 4 percent of source-water samples, with toluene being detected most frequently in 3.5 percent of 

samples. In ground water and drinking water, the detection frequencies of any individual gasoline hydrocarbon 

were even less, with no compound found in more than 

2 percent and 1 percent of samples. Like source water, 

the most frequently detected gasoline hydrocarbon in 

ground water and drinking water was toluene detected 

in 1 percent of samples and in 0.6 percent of samples.

Benzene

An important aspect of the RFG Program is the 

limitation of benzene in reformulated gasoline to less 

than or equal to 1 percent by volume (U.S. Environmental Protection Agency, 1990). Normally gasoline 

contains between 1 and 1.5 percent benzene by volume. 

Although gasoline in the Oxyfuel Program areas is not 

required to have a 1-percent benzene limitation, the 

high content of MTBE required by the Oxyfuel 

Program (15 percent by volume) probably results in the 

displacement of other fuel components such as benzene. This means that gasoline in areas of high MTBE 

use should contain less benzene relative to areas of low 

MTBE use, and detection frequencies and concentrations of benzene should be lower in areas of high 

MTBE use relative to areas of low MTBE use.

An examination of data from ground water indicated that the detection frequency of benzene was 

higher in areas of low MTBE use, at 4.7 percent, relative to the detection frequency of benzene in areas of 

high MTBE use, at 2 percent, when no assessment 

level was applied. The detection frequency of benzene 

also was dependent on MTBE use (p <0.05) but the 

distributions of concentrations of benzene were not 

significantly different between areas of high and low 

MTBE use (p >0.05).

Other Ethers

In addition to MTBE, the NAWQA Program 

analyzed for three other alkyl ethers that have been 

used commercially as gasoline oxygenates (Zogorski 

and others, 1997). These ethers have been used to a 

more limited extent as gasoline oxygenates compared 

to MTBE, and they generally are not used in conjunction with MTBE but as substitutes. These compounds 

were analyzed in both the NAWQA ground-water 

survey and the source-water survey and include tertamyl methyl ether (TAME), diisopropyl ether (DIPE), 

and ethyl tert-butyl ether (ETBE). In ground water at 

an assessment level of 0.2 μg/L, only two of the three 

ethers were detected—TAME and DIPE. The detection frequencies of both of these compounds were less 

than MTBE and they were detected in less than 

1 percent of samples. TAME was detected in 

0.25 percent of samples (6 of 2,382 samples), whereas 

DIPE was detected in 0.19 percent of samples (4 of 

2,077 samples). The fewer number of ground-water 

samples that were analyzed for ethers as compared to 

MTBE resulted from these compounds not being 

analyzed by the NWQL until April 1996, when they 

were added to the analytical method.

In source water, all three ethers were detected at 

least once at an assessment level of 0.2 μg/L. Like 

ground water, the detection of any of these ether compounds in source water was considerably less than the 

detection of MTBE. Both TAME and DIPE were 

detected in only 0.3 percent of source-water samples 

(2 of 579 samples), whereas ETBE was detected in 

only one sample. No analyses of other ether oxygenates were available for data from the drinking-water 

survey.

Factors Affecting Occurrence

A variety of hydrogeologic and anthropogenic 

factors are responsible for the occurrence and concentrations of MTBE in ground water. Identifying the 

sources and basic transport mechanisms of MTBE is 

vital in understanding its occurrence and the associated potential exposure risks. Using various statistical 

techniques, it is possible to identify anthropogenic and 
12 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

hydrogeologic variables that are related to the occurrence of MTBE in ground water. By identifying anthropogenic variables, like MTBE use and land use that are 

related to the occurrence of MTBE, it is possible to 

make inferences about potential sources. By identifying hydrogeologic variables, like recharge and soil 

permeability that are related to the occurrence of 

MTBE, it is possible to make inferences about the 

transport mechanisms and fate of MTBE in ground 

water and the vulnerability of various aquifers to 

MTBE contamination.

Sources of MTBE

Most of the MTBE used in the United States is 

for oxygenation of gasoline. Thus, the primary source 

of most MTBE in the environment is assumed to be 

gasoline. Besides gasoline, used motor lubricating oil, 

home heating oil, and diesel fuel also have been identified as containing MTBE because of mixing with gasoline that contains MTBE (Robbins and others, 1999, 

2000; Baker and others, 2002). Releases of these 

products also could be possible sources of MTBE. 

Some potential non-point sources of MTBE include 

evaporative losses from tanks or pipelines, incomplete 

combustion in engines, urban storm-water runoff from 

areas with small spills, exhaust from motorized watercraft with incomplete combustion, and leaks from 

watercraft tanks. Some potential point sources of 

MTBE to the environment include leaks from large 

domestic or commercial gasoline, diesel fuel, heating 

oil, or waste oil storage tanks and associated piping 

(underground and aboveground), leaks from transport 

pipelines or bulk stations, motor vehicle or truck accidents, overfill spills, and large consumer releases.

In this report, non-point sources are identified as 

widespread and diffuse releases of MTBE and gasoline 

hydrocarbons leading to relatively low concentrations 

in ground water. Point sources are identified as intense 

sources of MTBE and gasoline hydrocarbons leading to 

relatively high concentrations in ground water. 

Although this is assumed, point sources can lead to low 

concentrations of MTBE and gasoline hydrocarbons by 

dilution and dispersion during advective transport.

Very few samples analyzed in the three surveys 

had concentrations of MTBE greater than 20 μg/L. It 

is believed that MTBE concentrations greater than 

20 μg/L in ground water are the result of point source 

releases (Moran and others, 1999). It would be unlikely 

for non-point sources to cause higher concentrations in 

ground water than this level. Certain intense non-point 

sources of MTBE, such as atmospheric concentrations 

around certain parking garages, gas stations, or roadways, could result in MTBE concentrations of as much 

as 20 μg/L in shallow ground water through atmospheric deposition (Squillace and others, 1997). However, unless there is some concentrating mechanism, 

such as bioconcentration by plants, the concentration of 

MTBE from a purely atmospheric source can never 

exceed the equilibrium water concentration without the 

addition of other sources. Therefore, if the concentration of MTBE is greater than the highest likely type of 

non-point source, the source must be a point source. 

Low concentrations of MTBE, on the other hand, could 

have either point or non-point sources.

In general, the occurrence of gasoline hydrocarbons with MTBE in a ground-water sample suggests 

proximity to a gasoline release. However, MTBE did 

not occur frequently with gasoline hydrocarbons in the 

three surveys. In all data sets combined and at an 

assessment level of 0.2 μg/L, MTBE occurred in a total 

of 307 samples and gasoline hydrocarbons occurred 

together with MTBE in only 28 of these samples 

(9 percent). However, when MTBE concentrations 

were divided into categories reflecting relatively low, 

medium, and high concentrations, a pattern in the 

occurrence of gasoline hydrocarbons and MTBE 

together became apparent (fig. 6). For relatively low 

(less than 1 μg/L) and medium (1 μg/L to less than 

20 μg/L) concentrations of MTBE, the occurrence of 

gasoline hydrocarbons at an assessment level of 

0.2 μg/L was less than 10 percent. However, for relatively high (greater than 20 μg/L) MTBE concentrations, the occurrence of any gasoline hydrocarbon was 

substantially higher at 41 percent (fig. 6). Although the 

number of samples represented by high MTBE concentrations in figure 6 is small relative to the other two 

categories, the trend in the data is quite clear. As concentrations of MTBE increase, the occurrence of 

gasoline hydrocarbons with MTBE increases. This 

indicates that samples with higher concentrations of 

MTBE are located in proximity to gasoline releases 

relative to samples with lower concentrations of 

MTBE.

Use of MTBE in Gasoline as a Surrogate for 

Environmental Input

To understand the occurrence of MTBE in the 

environment, it would be ideal to know the exact 
Occurrence of MTBE and Gasoline Hydrocarbons 13

location and amount of all releases of MTBE to the 

environment. At a national scale, such knowledge is 

impossible to identify or ascertain. However, because 

almost all MTBE is used in gasoline, releases of MTBE 

to the environment should be related to the amount of 

MTBE used in gasoline. Use of MTBE in gasoline in 

various areas of the country can be estimated in several 

ways. One way of estimating use is by considering 

areas previously or currently under the RFG Program 

and where MTBE is used as the gasoline oxygenate as 

being areas where high amounts of MTBE are used.

Further refinement and specificity in determining 

MTBE use can be obtained using information from gasoline surveys. Several surveys have been conducted to 

provide information on the physical properties and constituents of gasoline including octane number, specific 

gravity, and volumes of olefins, aromatics, benzene, 

alcohols, and various ether oxygenates. The surveys 

provide information on MTBE content in gasoline, in 

percent by volume, for areas that are included within 

the surveys. The purposes of these surveys are to provide comparative information on gasoline composition 

to companies interested in the physical and chemical 

properties of fuels and to verify that oxygen content 

in gasoline is sufficient to meet RFG and Oxyfuel 

Program requirements. Additional information on 

these surveys can be found in Moran, Clawges, and 

Zogorski (2002).

Samples from the three surveys were placed 

into two groups based on the following categorization 

scheme: (1) relatively high MTBE use, and 

(2) relatively low MTBE use or unknown MTBE use. 

A sample was designated as being from an area of relatively high MTBE use if either: (1) the sample was 

located in an area that was designated for RFG usage 

at any time from 1993-2001 and where MTBE was 

used as the gasoline oxygenate, or (2) the sample was 

located in an area where gasoline surveys indicated a 

long-term average for MTBE content in gasoline, 

from 1990-99, of greater than or equal to 3 percent by 

volume. If either 1 or 2 were not true for a sample, or 

if the MTBE use in the area where the sample was 

located was unknown, the sample was coded as being 

from an area of relatively low MTBE use. Using this 

categorization scheme, each sample was placed into a 

category of high or low MTBE use.

Detection frequencies of MTBE within areas of 

high and low MTBE use for each survey are presented 

in figure 7. Data for ground water and source water 

were analyzed at an assessment level of 0.2 μg/L to 

Figure 6. Detection frequencies of any gasoline hydrocarbon in samples of ground water for selected concentration

ranges of MTBE using an assessment level of 0.2 microgram per liter.

0

10

20

30

40

50

DETECTION FREQUENCY OF GASOLINE

HYDROCARBONS, IN PERCENT

MTBE CONCENTRATION RANGE, IN MICROGRAMS PER LITER

Less than 1 1 to less than 20 20 and greater

Number of samples with

 MTBE detections

159

159

126

22
14 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

make them comparable to data for drinking water. For 

ground-water data, the detection frequency is only for 

MTBE analyses in the major aquifer surveys. The letters above each bar represent the results of chi-square 

tests of independence for each survey between areas of 

high and low MTBE use. The numbers between the 

arrows represent the ratios of detection frequencies 

between areas of high and low MTBE use. 

For each survey the detection frequency of 

MTBE was higher in areas of high MTBE use compared to areas of low MTBE use. For each survey, the 

detection frequencies of MTBE were dependent on 

MTBE use (p < 0.05). The ratios of detection frequencies between areas of high and low MTBE use varied 

from 4.9 for drinking water to 7.5 for ground water. 

This indicates that the detection frequency of MTBE 

was about 5 to 7.5 times higher in areas of relatively 

high MTBE use compared to areas of relatively low 

MTBE use. When comparing between the surveys in 

areas of either high or low MTBE use, the detection 

frequencies of MTBE also were dependent on the 

survey (p <0.05).

Research has shown that the probability of 

detecting MTBE at or above 0.5 μg/L in ground water 

is related to population density (Squillace and Moran, 

2000). An analysis was performed to determine if the 

detection frequency of MTBE is related to population 

density. In order to control for MTBE use, a comparison was made of the detection frequencies of MTBE 

between areas of high and low population density for 

areas of high and low MTBE use. For this analysis, 

only data from ground water were used because the 

data for source water had too few detections to provide 

a meaningful comparison and the data for drinking 

water were too limited in geographic extent to provide 

a good overall national distribution. Population density 

Figure 7. Detection frequencies of MTBE in samples of ground water, source water, and drinking water for

areas of high and low MTBE use using an assessment level of 0.2 microgram per liter.

0

5

10

15

20

25

Ground water

(United States)

Source water

(United States)

Drinking water

(Northeast

United States)

DETECTION FREQUENCY, IN PERCENT

A

464

A

458 A

148

B

1,889

B

527

B

411

7.5

5.0 4.9

HIGH MTBE USE

LOW MTBE USE

464 NUMBER OF SAMPLES

RATIO OF DETECTION FREQUENCIES

 BETWEEN HIGH AND LOW MTBE

 USE AREAS

7.5

COMPARISON BETWEEN MTBE

 OCCURRENCE—Samples that 

 share a letter are not significantly

 different at the 95-percent confidence

 interval

A

EXPLANATION
Occurrence of MTBE and Gasoline Hydrocarbons 15

is related to areas of urban and non-urban land use. 

Areas with population densities less than 50 

people/km2 (square kilometer) can be classified as nonurban, whereas areas with population densities greater 

than 50 people/km2 can be classified as urban (Hitt, 

1994). This population density classification scheme 

was used to place all samples within a land-use 

category of either urban or non-urban.

Detection frequencies of MTBE in ground water 

between urban and non-urban land-use areas and areas 

of high and low MTBE use are presented in figure 8. In 

this analysis, ground-water data from all NAWQA 

occurrence studies were used. No assessment level was 

applied in determining detection frequency in ground 

water in figure 8 because no comparisons were made to 

other data sets. The numbers between the arrows represent the ratios of detection frequencies between urban 

and non-urban areas. When controlling for MTBE use, 

the detection frequency of MTBE in ground water was 

clearly related to population density (fig. 8). The ratios 

of detection frequencies between urban and non-urban 

land-use areas for high and low MTBE use were 

3 and 5. The ratios of detection frequencies between 

MTBE use areas were higher than the ratios of detection frequencies between land-use areas. This suggests 

that, although population density is an important factor 

affecting the occurrence of MTBE, MTBE use in gasoline may be more important.

A comparison was made of the concentrations of 

MTBE in ground water between areas of high and low 

MTBE use. In order to make the comparison equitable, 

the effect of population density on MTBE concentrations was considered. Based on the previous analyses, 

only samples with a population density of greater than 

or equal to 50 people/km2

 were selected. Thus, a comparison of the concentrations of MTBE in urban landuse areas for areas of high and low MTBE use was 

made. Figure 9 is a quantile-quantile plot of MTBE 

Figure 8. Detection frequencies of MTBE in samples of ground water for areas of urban and non-urban

land use and by areas of high and low MTBE use using no assessment level.

0

5

15

10

20

30

25

35

High MTBE use Low MTBE use

DETECTION FREQUENCY, IN PERCENT

3.0

4.9

URBAN AREAS

NON-URBAN AREAS

570 NUMBER OF SAMPLES

570

232

1,023

2,077

RATIO OF DETECTION FREQUENCIES

 BETWEEN URBAN AND NON-URBAN

 AREAS

3.0

EXPLANATION
16 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

concentrations in ground water for urban land-use areas 

and between areas of high and low MTBE use. Because 

the data are solely for ground water, no assessment 

level was applied. Each point in figure 9 represents the 

MTBE concentration corresponding to the same quantile in each data set. The quantiles computed for plotting ranged from 0.01 to 1.0, in increments of 0.01. For 

simplicity, the quantile pairs of non-detections are not 

plotted in figure 9. All data pairs are above the 1:1 line 

indicating that higher concentrations of MTBE occur in 

areas of high MTBE use relative to areas of low MTBE 

use (fig. 9). A statistical test indicated that the distributions of MTBE concentrations were significantly 

different between areas of high and low MTBE use 

(p <0.05).

MTBE and Dissolved Oxygen in Ground Water

An analysis of MTBE occurrence under oxic and 

anoxic ground-water conditions was performed. Oxic 

ground water is defined in this report as ground water 

that contains greater than 0.5 mg/L (milligrams per 

liter) of dissolved oxygen, whereas anoxic ground 

water is defined as ground water that contains less than 

or equal to 0.5 mg/L of dissolved oxygen. Freeze and 

Cherry (1979) indicated that natural ground water is 

considered oxidized if it has a redox potential (pE) 

value above 13.4, which is approximately equal to a 

dissolved-oxygen value of 0.5 mg/L. In this report, 

only samples of ground water were used because 

dissolved-oxygen information was not available for 

either source-water or drinking-water data. In order to 

control for MTBE use, only samples from high MTBE 

use areas were examined.

When no assessment level was applied, the 

detection frequency of MTBE in oxic ground water 

was similar at 24.5 percent to the detection frequency 

of MTBE in anoxic ground water at 22.5 percent, and 

the detection frequencies of MTBE were not dependent on dissolved-oxygen conditions (p >0.05). An 

analysis of MTBE concentrations and dissolved 

oxygen, both as continuous variables and using no 

assessment level, indicated that there was not 

a monotonic relation between the two variables 

(p >0.05). 

Figure 9. Comparison of the distributions of concentrations of MTBE in ground water in

areas of urban land use for areas of high and low MTBE use using no assessment level.

0.01

0.1

1

10

100

1,000

0.01 0.1 1 10 100 1,000

MTBE CONCENTRATION (LOW MTBE USE AREAS),

IN MICROGRAMS PER LITER

MTBE CONCENTRATION (HIGH MTBE USE AREAS),

IN MICROGRAMS PER LITER

1:1 line
Occurrence of MTBE and Gasoline Hydrocarbons 17

However, when only higher concentrations of 

MTBE were considered by applying various assessment levels to the data, the detection frequencies of 

MTBE in oxic and anoxic ground water were different. 

When an assessment level of 0.2 μg/L was applied to 

MTBE concentrations, the detection frequency of 

MTBE in oxic ground water was 14.1 percent, and the 

detection frequency of MTBE in anoxic ground water 

was 18.3 percent; however, the detection frequencies of 

MTBE were not dependent on dissolved-oxygen conditions (p >0.05). When an assessment level of 0.5 μg/L 

was applied to MTBE concentrations, the detection 

frequency of MTBE in oxic ground water was 

7.6 percent, and the detection frequency of MTBE in 

anoxic ground water was 14.8 percent. In this case, the 

detection frequencies of MTBE were dependent on 

dissolved-oxygen conditions (p <0.05). When an 

assessment level of 0.5 μg/L was applied to MTBE concentrations, a significant monotonic relation was found 

between MTBE concentrations and dissolved-oxygen 

concentrations, with the concentration of dissolved 

oxygen decreasing as the concentration of MTBE 

increased (Spearman rho = -0.119; p <0.05). 

Other Associations with MTBE in Ground Water

Associations between MTBE occurrence and 

various hydrogeologic and anthropogenic variables 

were examined using multivariate logistic regression. 

For the logistic regression analyses, only data from 

ground water were used because hydrogeologic and 

anthropogenic information was scarce or not available 

for the source-water and drinking-water surveys. The 

dependent or response variable was the detection of 

MTBE, and no assessment level was applied to the 

MTBE data because no comparison was made to other 

data sets. Table 1 lists the explanatory variables that 

were used for these analyses, the units for each variable, 

the coding of the variable in the logistic regression 

analyses, and the source of the data. Prior to analysis, 

population density was transformed using a natural 

logarithm function to normalize the distribution.

Table 2 summarizes the results of the logistic 

regression analyses for the explanatory variables that 

were significantly associated with MTBE occurrence, 

including the standardized estimated slope coefficients. 

The magnitude and sign of the slope coefficients determines the strength and direction of the association of 

the probability of detecting MTBE with the explanatory variables according to the following equation:

where

P = probability of detecting MTBE,

β0 = y-intercept,

βi = slope coefficient of Xi explanatory 

variables, and

Xi = 1 to i explanatory variables.

Estimated slope coefficients with positive signs 

indicate an increase in the probability of detecting 

MTBE with an increase in the explanatory variable, 

whereas estimated slope coefficients with negative 

signs indicate a decrease in the probability of 

detecting MTBE with an increase in the explanatory 

variable. However, estimated coefficients do not give 

an accurate assessment of the strength of the association because the units of each variable, especially continuous variables, have large differences in magnitude 

and variance. Therefore, standardized slope coefficients were computed in order to compare the coefficients directly between one another (table 2). The 

standardized coefficients indicate how many standard 

deviations of change in the dependent variable are 

associated with one standard deviation of change in 

the explanatory variable (Menard, 2002).

Based on a step-wise modeling approach, the 

explanatory variables found to be significantly associated with the probability of detecting MTBE in ground 

water included population density, use of MTBE in 

gasoline, recharge, aquifer consolidation, soil permeability, and the density of leaking underground gasoline storage tanks (table 2). Population density, use of 

MTBE in gasoline, and the density of leaking underground gasoline storage tanks in the vicinity of the 

sampled well represent anthropogenic influences that 

help to approximate input of MTBE to the environment. 

P = e

(β0 + β1X1 + ...βiXi)

1 + e(β0 + β1X1 + ...βi

Xi)
18 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

Table 1. Anthropogenic and hydrogeologic variables that were used in the logistic regression analysis

[GWSI, U.S. Geological Survey Ground Water Site Inventory database; km2, square kilometers; m, meters]

Explanatory variable Units Coding Source

Anthropogenic Variables

Use of MTBE in gasoline None Binary

(0, low MTBE use;

1, high MTBE use)

U.S. Environmental Protection Agency, 

2003c; TRW Petroleum Technologies, written commun., 1999

Population density People/km2 (interpolated) Continuous U.S. Census Bureau, 2001

Urban land-use Percent within 500-m buffer Continuous Vogelmann and others, 2001

Aboveground storage tanks Tanks/500-m grid cell

(interpolated)

Continuous C.V. Price, U.S. Geological Survey, 

written commun., 2003

Underground storage tanks Tanks/500-m grid cell 

interpolated)

Continuous C.V. Price, U.S. Geological Survey, 

written commun., 2003

Leaking underground storage tanks Tanks/500-m grid cell

(interpolated)

Continuous C.V. Price, U.S. Geological Survey, 

written commun., 2003

Hydrogeologic Variables

Well depth Feet Continuous GWSI

Depth to the top of the screened interval Feet Continuous GWSI

Water level Feet below land surface 

datum

Continuous GWSI

Land surface slope Percent Continuous U.S. Department of Agriculture, 1994

Depth to seasonal high water table in soil Inches Continuous U.S. Department of Agriculture, 1994

Soil permeability Inches/hour Continuous U.S. Department of Agriculture, 1994

Soil organic matter content Percent by weight Continuous U.S. Department of Agriculture, 1994

Soil clay content Percent Continuous U.S. Department of Agriculture, 1994

Aquifer confinement None Categorical GWSI

Aquifer consolidation None Binary

(0, consolidated;

1, unconsolidated)

GWSI

Recharge Millimeters per year Continuous D.M. Wolock, U.S. Geological Survey, 

written commun., 2003

Population density was the anthropogenic variable that was found to be most strongly associated with 

the probability of detecting MTBE as indicated by the 

standardized coefficient for population density in 

table 2. This means that an increase in population density results in the greatest increase in the probability of 

detecting MTBE when controlling for the other significant explanatory variables. Use of MTBE in gasoline 

also was strongly associated with the probability of 

detecting MTBE. When controlling for the other 

significant explanatory variables, the probability of 

detecting MTBE was about 5.8 times higher in high 

MTBE use areas compared to low MTBE use areas. 

Although the density of leaking underground gasoline 

storage tanks in the vicinity of the sampled well also 

was associated with the probability of detecting MTBE 

in ground water, the effect of this variable on the probability of occurrence was much smaller relative to population density and use of MTBE in gasoline (table 2). 

The other variables significantly associated with 

the probability of detecting MTBE in ground water 

included recharge, aquifer consolidation, and soil permeability (table 2). These variables represent hydrogeologic conditions that affect the transport and fate of 

MTBE in the environment. Recharge was the hydrogeologic variable found to be most strongly associated 
Implications Involving MTBE and Gasoline Hydrocarbons 19

with the probability of detecting MTBE in ground 

water. The probability of detecting MTBE in ground 

water increased as recharge increased. Although 

aquifer consolidation and soil permeability also were 

associated with the probability of detecting MTBE in 

ground water, the effect of these variables on the 

probability of occurrence was much smaller relative to 

recharge. The probability of detecting MTBE in ground 

water was higher in aquifers composed of consolidated 

material compared to aquifers composed of unconsolidated material. As soil permeability increased, the 

probability of detecting MTBE in ground water also 

increased.

IMPLICATIONS INVOLVING MTBE AND

GASOLINE HYDROCARBONS

The overall detection frequency of MTBE for all 

three surveys combined was less than 10 percent using 

no assessment level. This means that less than 1 of 10 

samples had a detectable concentration of MTBE. One 

explanation for this relatively low national detection 

frequency may be that large quantities of MTBE have 

been used only relatively recently in oxygenated gasoline and the use of oxygenated gasoline has been highly 

regionalized. Drinking water in the Northeast region of 

the country had the highest frequency of MTBE detections among the three surveys. This region is an area of 

substantial current and former use of oxygenated gasoline, has a long-term history of dense urbanization and 

high population, and may be an area of greater aquifer 

vulnerability. 

Table 2. Anthropogenic and hydrogeologic variables that 

were significantly associated with the probability of detecting 

MTBE in ground water

Explanatory variable Estimated

coefficient

Standardized

slope

coefficient

Anthropogenic Variables

Population density 0.367 2.4

Use of MTBE in gasoline 1.765 2.1

Leaking underground storage 

tanks

0.463 .04

Hydrogeologic Variables

Recharge 0.005 .17

Aquifer consolidation -0.582 -.08

Soil permeability 0.043 .05

The overall detection frequency of MTBE for 

all three surveys, at 7.6 percent, was higher than the 

detection frequency of trichloroethene (4.5 percent) 

but lower than the detection frequency of tetrachloroethene (11.9 percent). Both trichloroethene (TCE) and 

tetrachloroethene (PCE) have much longer production 

histories than MTBE and both have been produced 

and used in the United States since 1923 (Pankow and 

Cherry, 1996). MTBE has only been produced substantially since about 1970, although production of 

MTBE in the United States has outpaced both TCE 

and PCE since about 1980 (Johnson and others, 2000). 

Consequently, it is surprising to find the detection frequency of MTBE rivaling or surpassing these VOCs 

that have been produced and used for much longer 

periods of time. This indicates that MTBE contamination is an important concern with respect to groundwater management.

MTBE in Drinking Water

The detection frequency of MTBE was higher 

in drinking water in the Northeast compared to the 

national surveys of source water or ground water. The 

higher detection frequency of MTBE in drinking 

water may be related to: (1) the data on drinking water 

are only for public-supply wells, which generally have 

large pumping capacities; (2) the detection frequencies in drinking water may be biased high because the 

Northeast and Mid-Atlantic regions are areas of high 

MTBE use; and (3) the data on drinking water did not 

reflect occurrence by specific source but instead was 

summarized by system.

Because the drinking-water data were exclusively from public-supply wells, a higher detection 

frequency of MTBE might be expected as publicsupply wells generally have higher pumping rates and 

larger areas contributing recharge to the well than 

other well types such as private-supply wells 

(domestic) and monitoring wells (Moran, Lapham, 

and others, 2002). Stackelberg and others (2000) 

found that the number and total concentrations of 

VOCs per sample were significantly higher in ground 

water from public-supply wells compared to monitoring wells in the surficial Kirkwood-Cohansey 

aquifer system in southern New Jersey. The larger 

volumes of water withdrawn from public-supply wells 

compared to monitoring wells and the subsequently 

larger contributing areas were believed to be responsible for intercepting more ground water flowing from 
20 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

VOC point sources (Stackelberg and others, 2000). 

Bruce and Oelsner (2001) found that closely located 

domestic and public-supply wells did not have similar 

detection frequency of pesticides and public-supply 

wells had a higher detection frequency of pesticides. 

They attributed the higher detection frequency of pesticides in public-supply wells to the higher pumping rate 

and seasonal pumping cycles of these wells that produced more recently recharged water containing more 

anthropogenic compounds.

Another factor that may have contributed to a 

higher detection frequency of MTBE in public-supply 

wells is their location in the Northeast and Mid-Atlantic 

regions where substantial use of oxygenated gasoline 

has occurred and where MTBE was the primary oxygenate. In areas of substantial use of MTBE in gasoline, 

an increased probability of release of MTBE to the 

environment and transport to ground water exists. 

Finally, the data on MTBE in drinking water 

were summarized by system and this may have resulted 

in a bias towards a higher detection frequency because 

many systems had multiple sources with multiple 

samples (Grady and Casey, 2001). In fact, when the 

detection frequency of MTBE for drinking water was 

computed for both ground-water and surface-water 

sources relative to the number of samples, rather than 

the number of systems, it was lower at 6.2 percent compared to 9 percent when summarized by system (Grady 

and Casey, 2001).

When the data for ground water and source water 

were limited in extent to only the 12 Northeast and 

Mid-Atlantic States and an assessment level of 

0.5 μg/L was applied, the detection frequencies of 

MTBE in ground water and source water were comparable to the detection frequency of MTBE in drinking 

water. This indicates that the occurrence of MTBE in 

ground water, source water, and drinking water is similar when controlling for use of MTBE.

MTBE Occurrence by Land Use and Well Type

For ground water, the detection frequency of 

MTBE was highest in the NAWQA urban land-use 

studies compared to the major aquifer surveys or agricultural land-use studies. The higher detection frequency of MTBE in urban land-use studies probably is 

a result of several factors: (1) these are areas of highest 

population density and thus highest gasoline use, 

(2) highly urban regions like the Northeast and parts of 

California have most intense use of MTBE in gasoline, 

and (3) the NAWQA urban land-use studies targeted 

shallow ground water and were designed to sample 

recently recharged ground water generally less than 

10 years old. Thus, shallow ground water underlying 

urban land-use areas is at greater risk of contamination 

by MTBE than ground water underlying rural land-use 

areas. However, it is not known if contamination of 

shallow ground water will reach deeper aquifers used 

for supplying drinking water.

For ground water, the detection frequency of 

MTBE was highest in monitoring wells compared to 

other well types. The higher detection frequency of 

MTBE in monitoring wells is due to several factors: 

(1) monitoring wells were used extensively in the urban 

land-use studies, and (2) many monitoring wells were 

installed by NAWQA for land-use studies and were 

shallow in depth, designed to sample recently 

recharged ground water near the top of the water table. 

Although NAWQA sampled fewer public-supply wells 

compared to domestic wells, the detection frequency of 

MTBE in public-supply wells was higher. As previously mentioned, the detection frequency of many 

VOCs has been shown to be higher in public-supply 

wells compared to domestic wells for several reasons. 

Because public water-supply systems that use ground 

water as a source of drinking water served over 100 

million people as of 2001 (Harrigan-Farrelly, 2002), 

the potential of human exposure to MTBE from 

ground-water contamination is large.

MTBE Concentrations and Human Health

The median detected MTBE concentration in 

ground water and source water was less than 1 μg/L, 

and the median detected MTBE concentration in 

drinking water was less than 2 μg/L. These relatively 

low concentrations indicate that many of the MTBE 

detections could be from non-point sources or that 

point sources have not caused widespread areas of 

high-level concentrations near the samples.

High concentrations of MTBE (>20 μg/L) are 

believed to result from point sources of contamination; however, the NAWQA Program sampling does 

not allow for definitive identification of sources of 

MTBE. When the concentration of MTBE was considered, the occurrence of any gasoline hydrocarbon 

together with MTBE was substantially increased for 

higher concentrations of MTBE. This indicates that 
Implications Involving MTBE and Gasoline Hydrocarbons 21

ground-water samples with higher concentrations of 

MTBE are located in proximity to gasoline releases 

relative to samples with lower concentrations of 

MTBE. Low concentrations of MTBE (<20 μg/L) 

probably are the result of an unknown mix of point 

and non-point sources. Additional research is needed 

to better understand the sources of low concentrations 

of MTBE.

No samples of source water exceeded the lower 

limit of the U.S. Environmental Protection Agency’s 

Drinking-Water Advisory. The incidence of MTBE 

concentrations exceeding the lower limit of the advisory relative to all samples was quite low for both 

ground water and drinking water. Thus, most of the 

ground water and drinking water analyzed in this report 

did not contain MTBE concentrations that would cause 

organoleptic effects.

However, the potential long-term human health 

effects of low-level concentrations of MTBE in 

drinking water are not well understood, and there have 

been different conclusions drawn from the relatively 

few animal studies that have been done (Toccalino, 

2003). Intake of MTBE by gavage has been associated 

with acute and long-term (carcinogenic) health effects 

in laboratory animals, and the validity of some of the 

interpretations drawn from these studies has been questioned (Robinson and others, 1990; Belpoggi and 

others, 1995). In 1997, the U.S. Environmental Protection Agency issued a drinking-water advisory recommending that MTBE concentrations in drinking water 

be below the range of 20 to 40 μg/L to protect consumer 

acceptance (taste and odor) and also to provide a large 

margin of safety from toxic effects (U.S. Environmental 

Protection Agency, 1997). The U.S. Environmental 

Protection Agency is continuing to assess the human 

health effects of MTBE and the implications of setting 

a drinking-water standard (Davis, 2002). To be protective of carcinogenic potential and reproductive and 

developmental effects, maximum levels of MTBE in 

drinking water ranging from 5 to 100 μg/L have been 

suggested by other agencies and individuals (Johnson, 

1998; Hartley and others, 1999). The California Department of Health Services has issued a Maximum Contaminant Level for MTBE in drinking water of 13 μg/L 

(California Department of Health Services, 2002). 

Considering the current level of concern regarding the 

health effects of MTBE and the continuing uncertainty 

of the health effects of MTBE, monitoring of drinking 

water for MTBE is still needed. 

Gasoline Hydrocarbon Occurrence

For ground water and drinking water, the detection frequencies of one or more gasoline hydrocarbons 

or individual gasoline hydrocarbons were less than the 

detection frequency of MTBE. The lower detection 

frequencies of gasoline hydrocarbons compared to 

MTBE are believed to be a result of differences in 

transport and fate properties between the different 

compounds. MTBE has higher water solubility than 

gasoline hydrocarbons. MTBE adsorbs only weakly to 

subsurface solids, whereas gasoline hydrocarbons 

adsorb relatively strongly. As a result, MTBE moves 

at velocities that are similar to the velocities of local 

ground water, whereas the velocities of gasoline 

hydrocarbons are retarded relative to the velocities of 

local ground water (Squillace and others, 1997). In 

addition, the aerobic half-life of MTBE in ground 

water has been estimated to be approximately an order 

of magnitude longer than the average aerobic half-life 

of most gasoline hydrocarbons in ground water 

(Howard and others, 1991). Aerobic half-lives in 

ground water were selected for comparison because 

approximately 70 percent of NAWQA ground-water 

samples had sufficient dissolved oxygen (>0.5 mg/L) 

to be classified as oxic. Finally, the percent volume of 

MTBE in oxygenated gasoline is generally higher 

than the percent volume of the gasoline hydrocarbons 

examined in this report (Canadian Petroleum Products 

Institute, 1994). All of the above properties make it 

more likely that MTBE will be detected in ground 

water if released to the environment compared to 

gasoline compared to gasoline hydrocarbons.

The limitation of benzene content in gasoline in 

the RFG Program may have resulted in lower detection frequencies of benzene in ground water in areas of 

high MTBE use relative to areas of low MTBE use, 

but has not resulted in lower benzene concentrations. 

The lower detection frequencies of benzene in ground 

water in these areas probably also are reflected in 

source water and drinking water. Because benzene is a 

known human carcinogen (U.S. Environmental Protection Agency, 2000), the lower detection frequencies are a significant change in improving groundwater quality and would be particularly important for 

drinking-water sources. However, at present (2003) 

the data for source water and drinking water do not 

yield similar findings, which indicates that the influence of the RFG Program on the environmental occurrence of benzene is not completely understood and 

that more research in this area is warranted.
22 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

Associations with MTBE Occurrence

When use of MTBE in gasoline was used as surrogate for input of MTBE to the environment, there was 

a significant difference in the detection frequency of 

MTBE between areas of high and low MTBE use. The 

detection frequency of MTBE was between 5 and 7.5 

times higher in areas of high MTBE use compared to 

areas of low MTBE use. Previous work has indicated 

that the detection frequency of MTBE was between 2 

and 5 times higher in areas of high MTBE use compared to areas of low MTBE use (Moran, Clawges, and 

Zogorski, 2002). The reason for the greater difference 

in detection frequencies between areas of high and low 

MTBE use for ground water, as presented in this report, 

is probably related to other factors, such as population 

density. As previously mentioned, the probability of 

detecting MTBE is related to population density. 

Further analysis revealed that, for the ground-water 

survey, population density was higher in the high 

MTBE use relative to the low MTBE use areas and that 

this difference was significant (p <0.05). Thus, the 

higher population density in areas of high MTBE use 

relative to areas of low MTBE use probably accounts 

for the higher detection frequency ratios reported here.

The use of MTBE in oxygenated gasoline has 

resulted in higher concentrations of MTBE in ground 

water. When a comparison was made of the distributions of concentrations of MTBE, higher concentrations were found in areas of high MTBE use compared 

to areas of low MTBE use. Thus, ground water underlying areas of high MTBE use has higher detection 

frequencies and concentrations of MTBE compared to 

areas of low MTBE use.

There was no dependence of the detection frequencies of MTBE on dissolved-oxygen conditions in 

ground water when no assessment level was applied but 

there was a dependence of MTBE detection frequencies on dissolved-oxygen conditions when concentrations of MTBE greater than 0.5 μg/L were considered. 

The lack of a dependence of low concentrations of 

MTBE on dissolved-oxygen conditions in ground 

water seems to support that: (1) biodegradation of low 

MTBE concentrations occurs in both oxic and anoxic 

ground-water conditions; (2) little biodegradation of 

low MTBE concentrations occurs in either condition, 

or (3) biodegradation of low MTBE concentrations 

occurs in oxic and/or anoxic ground-water conditions 

but the transformations have no effect on dissolvedoxygen concentrations. When an assessment level of 

0.5 μg/L was applied to MTBE concentrations, 

detection frequencies of MTBE were dependent on 

dissolved-oxygen conditions in ground water with a 

higher detection frequency in anoxic ground water 

compared to oxic ground water. Also, dissolved oxygen 

was found to be significantly associated with the probability of occurrence of MTBE at or greater than concentrations of 0.5 μg/L when controlling for population 

density, use of MTBE in gasoline, and recharge in a 

logistic regression analysis. These results suggest that 

MTBE biodegradation may be occurring in oxic 

ground-water conditions for higher MTBE concentrations. Biodegradation of MTBE has been shown to 

occur under oxic conditions in both laboratory and field 

studies (Borden and others, 1997; Steffan and others, 

1997; Bradley and others, 1999).

After standardizing the slope coefficients, the 

results of the logistic regression analyses indicated that 

three variables were most important in affecting the 

probability of occurrence of MTBE in ground water: 

population density, use of MTBE in gasoline, and 

recharge. As population density in the vicinity of the 

well increased, the probability of detecting MTBE in 

ground water increased. Likewise, the probability of 

detecting MTBE in ground water was higher in areas 

with high use of MTBE in gasoline compared to areas 

of low use. Areas of higher population density and 

higher use of MTBE in gasoline would be expected to 

be associated with larger numbers of potential point 

and non-point sources of MTBE to ground water. Also, 

as recharge increased the probability of detecting 

MTBE also increased. Higher recharge is likely associated with greater transport of contaminants from the 

surface to ground water. Anthropogenic activities in 

urban areas can lead to recharge rates that are higher 

than natural levels. This, along with increased sources, 

could lead to increased MTBE contamination of 

ground water in urban areas.

The results of the logistic regression analyses 

also indicated that three other variables affect the 

probability of occurrence of MTBE in ground water, 

but these have less significance in influencing MTBE 

occurrence. These three variables were aquifer consolidation, soil permeability, and the density of leaking 

underground gasoline storage tanks. The probability of 

detecting MTBE in ground water was higher in consolidated aquifers compared to unconsolidated aquifers. 

The average linear velocities of ground water through 

fractured consolidated aquifer materials can be quite 

high and higher than through unconsolidated aquifer 
Summary 23

materials (Freeze and Cherry, 1979). Thus, contaminants that enter consolidated aquifers with fracture 

porosity, such as karst limestone or fractured crystalline 

rock, can move quickly through the aquifer system and 

less time is available for attenuation by biodegradation, 

dispersion, and diffusion.

The probability of detecting MTBE in ground 

water increased as soil permeability increased. 

Increased soil permeability likely increases the rate of 

transfer of contaminants from the surface to ground 

water. The probability of detecting MTBE in ground 

water increased with increasing number of leaking 

underground gasoline storage tanks in the vicinity of 

the well. Although leaking underground storage tanks 

might seem to be an important source for MTBE, the 

standardized coefficient for this variable was the lowest 

of all variables analyzed. This result suggests that, 

although they may be responsible for some occurrence 

of MTBE, leaking underground gasoline storage tanks 

may not be the primary source for most MTBE in 

ground water. Another important finding is that the 

probability of detecting MTBE was not significantly 

related to the density of aboveground or underground 

storage tanks in the vicinity of the well. This suggests 

that the storage of gasoline alone is not responsible for 

MTBE occurrence in ground water.

SUMMARY

The occurrence and concentrations of methyl 

tert-butyl ether (MTBE) and gasoline hydrocarbons 

were examined in three surveys—one national-scale 

survey for ground water, one national-scale survey for 

source water, and one regional-scale survey for 

drinking water. The overall detection frequency of 

MTBE for all three surveys combined was less than 

10 percent using no assessment level. The overall 

detection frequency of MTBE in these three surveys 

was similar to the detection frequencies of some other 

volatile organic compounds (VOCs) that have much 

longer production and use histories. The quick arrival 

of MTBE in ground water relative to its production 

history indicates that MTBE is an important concern 

with respect to ground-water management.

The detection frequency of MTBE was higher in 

drinking water and lower in source water and ground 

water. The detection frequency of MTBE was higher in 

drinking water because public-supply wells generally 

have large areas contributing recharge to the well; the 

location of these wells is in the Northeast, which is an 

area of high use of MTBE in gasoline; and drinkingwater results were summarized by system. When the 

data for source water and ground water were limited to 

the same geographic extent as drinking-water data, the 

detection frequencies of MTBE in source water and 

ground water were comparable to the detection frequency of MTBE in drinking water. This indicates that 

the occurrence of MTBE in ground water, source 

water, and drinking water in the Northeast is similar 

when controlling for use of MTBE in gasoline.

The detection frequency of MTBE was higher 

in urban land-use studies compared to other study 

types and was higher in monitoring wells compared to 

other well types. The detection frequency of MTBE in 

monitoring wells was higher because many of these 

wells were sampled in urban land-use studies. The 

higher detection frequency of MTBE in urban landuse areas is probably related to the higher use of gasoline, and the more intense use of MTBE. Shallow 

ground water in urban areas appears to be at greater 

risk of contamination compared to deeper ground 

water in rural areas. However, it is not known if contamination of shallow ground water in urban areas will 

reach deeper aquifers used for drinking-water supply.

The median concentration of MTBE in all three 

surveys was relatively low. No concentration of 

MTBE in source water exceeded the lower limit of the 

U.S. Environmental Protection Agency’s Drinking 

Water Advisory of 20 μg/L. Only one concentration of 

MTBE in ground water exceeded this level, and only 

0.9 percent of drinking-water samples exceeded this 

level. Thus, most of the ground-water and drinkingwater samples analyzed did not contain MTBE in concentrations that would cause organoleptic effects.

In all three surveys, the detection frequency of 

one or more gasoline hydrocarbon was less than the 

detection frequency of MTBE. The lower detection 

frequencies of gasoline hydrocarbons compared to 

MTBE are believed to be the result of differences in 

transport and fate properties with MTBE being more 

likely to be detected in ground water if released to the 

environment. The limitation of benzene content in 

reformulated gasoline appears to have resulted in 

lower detection frequencies of benzene in ground 

water in areas of high MTBE use relative to areas of 

low MTBE use. Because benzene is a known human 

carcinogen, reduction of benzene in ground water is a 

significant improvement in water quality.

There was a significant difference in the detection frequency of MTBE between areas of high and 
24 Occurrence of MTBE in Ground Water, Source Water, and Drinking Water, 1993-2002

low MTBE use in data from all three surveys. The 

detection frequency of MTBE was between 5 and 7.5 

times higher in areas of high MTBE use compared to 

areas of low MTBE use. It is clear that the use of MTBE 

in oxygenated gasoline has resulted in higher detection 

frequencies of MTBE in ground water, source water, 

and drinking water. As of 2000, more than 100 million 

people in the United States lived in areas where MTBE 

was used as a gasoline oxygenate, and 92 percent of 

these people also lived in urban areas. Since about onehalf of the people in the United States rely on ground 

water for drinking water (U.S. Environmental Protection Agency, 1999), as many as 45 million people may 

be at an increased risk of exposure to MTBE from 

ground-water contamination. 

Using a logistic regression analysis, three variables were found to be most important in affecting the 

probability of occurrence of MTBE in ground 

water—population density, use of MTBE in gasoline, 

and recharge. As population density in the vicinity of 

the well increased, the probability of detecting MTBE 

in ground water increased. Likewise, the probability of 

detecting MTBE in ground water was higher in areas of 

high MTBE use in gasoline compared to areas of low 

MTBE use in gasoline. As recharge increased, the 

probability of detecting MTBE in ground water also 

increased.

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