Occurrence and Implications of Selected 

Chlorinated Solvents in Ground Water and 

Source Water in the United States and in 

Drinking Water in 12 Northeast and 

Mid-Atlantic States, 1993–2002

Scientific Investigations Report 2005–5268

U.S. Department of the Interior

U.S. Geological Survey

NATIONAL WATER-QUALITY ASSESSMENT PROGRAM

NATIONAL SYNTHESIS ON VOLATILE ORGANIC COMPOUNDS
Occurrence and Implications of Selected 

Chlorinated Solvents 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

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005–5268
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

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

endorsement by the U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Moran, M.J., 2006, Occurrence and implications of selected chlorinated solvents in ground water and source water in 

the United States and in drinking water in 12 Northeast and Mid-Atlantic States, 1993–2002: U.S. Geological Survey 

Scientific Investigations Report 2005–5268, 70 p.
 iii

Foreword

The U.S. Geological Survey (USGS) is committed to serve the Nation with accurate and timely scientific information that helps enhance and protect the overall quality of life, and facilitates effective management of water, biological, energy, and mineral resources. Information on the quality 

of the Nation’s water resources is of critical interest to the USGS because it is so integrally linked 

to the long-term availability of water that is clean and safe for drinking and recreation and that is 

suitable for industry, irrigation, and habitat for fish and wildlife. Escalating population growth and 

increasing demands for the multiple water uses make water availability, now measured in terms 

of quantity and quality, even more critical to the long-term sustainability of our communities and 

ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program to support 

national, regional, and local information needs and decisions related to water-quality management and policy. 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. NAWQA results can contribute to informed decisions that result in 

practical and effective water-resource management and strategies that protect and restore water 

quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 

of the Nation’s most important river basins and aquifers, referred to as Study Units. Collectively, 

these Study Units account for more than 60 percent of the overall water use and population served 

by public water supply, and are representative of the Nation’s major hydrologic landscapes, priority ecological resources, and agricultural, urban, and natural sources of contamination. 

Each assessment is guided by a nationally consistent study design and methods of sampling and 

analysis. The assessments thereby build local know-ledge about water-quality issues and trends 

in a particular stream or aquifer while providing an understanding of how and why water quality 

varies regionally and nationally. The consistent, multi-scale approach helps to determine if certain 

types of water-quality issues are isolated or pervasive, and allows direct comparisons of how 

human activities and natural processes affect water quality and ecological health in the Nation’s 

diverse geographic and environmental settings. Comprehensive assessments on pesticides, nutrients, volatile organic compounds, trace metals, and aquatic ecology are developed at the national 

scale through comparative analysis of the Study-Unit findings. 

The USGS places high value on the communication and dissemination of credible, timely, and relevant science so that the most recent and available knowledge about water resources can be 

applied in management and policy decisions. We hope this NAWQA publication will provide you 

the needed insights and information to meet your needs, and thereby foster increased awareness 

and involvement in the protection and restoration of our Nation’s waters. 
iv 

The NAWQA Program 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 cost-effective management, regulation, and conservation of our Nation’s water resources. The Program, therefore, depends extensively on the 

advice, cooperation, and information from other Federal, State, interstate, Tribal, and local agencies, non-government organizations, industry, academia, and other stakeholder groups. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water
 v

Contents

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

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

Physical and Chemical Properties and Environmental Behavior of Solvents . . . . . . . . . . . . .2

Transformation of Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

Production and Usage of Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Methylene Chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Perchloroethene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

1,1,1-Trichloroethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Trichloroethene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Health Effects of Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Methylene Chloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

Perchloroethene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

1,1,1-Trichloroethane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

Trichloroethene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Potential Sources of Solvents to Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Anthropogenic Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

Natural Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Data and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Solvent Data Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Ground-Water Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Source-Water Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Drinking-Water Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Computational Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Statistical Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Occurrence of Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

Occurrence and Distribution of Solvents in Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . .18

Occurrence and Distribution of Solvents in Source Water . . . . . . . . . . . . . . . . . . . . . . . . . . .26

Occurrence and Distribution of Solvents in Drinking Water . . . . . . . . . . . . . . . . . . . . . . . . . .28

Concentrations of Solvents in Ground Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Concentrations of Solvents in Source Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Concentrations of Solvents in Drinking Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Comparisons of Solvent Occurrence and Concentrations Between Ground Water, 

Source Water, and Drinking Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

Associations of Solvents with Hydrogeologic and Anthropogenic Variables . . . . . . . . . . . . . . . . .44

Variables Associated With One or More Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

Variables Associated With Individual Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

Summary of Significantly Associated Explanatory Variables . . . . . . . . . . . . . . . . . . . . . . . . . .47

Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53
vi 

Appendixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59

1. VOCs and their U.S. Environmental Protection Agency Maximum Contaminant 

Levels (U.S. Environmental Protection Agency, 2004a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

2. Ranking of 55 VOCs relative to detection frequency in ground water at an 

assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3. Ranking of 52 VOCs relative to detection frequency in source water at an 

assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4. Ranking of 51 VOCs relative to detection frequency in drinking water at an 

assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

5. Ranking of median concentration of 47 VOCs that had quantifiable concentrations 

in ground water, at an assessment level of 0.02 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

6. Ranking of frequency of exceedance of Maximum Contaminant Levels for VOCs 

that had quantifiable concentrations in ground water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7. Ranking of median concentration of 40 VOCs that had quantifiable concentrations 

in source water, at an assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

8. Ranking of frequency of exceedance of Maximum Contaminant Levels for VOCs 

that had quantifiable concentrations in source water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

9. Ranking of median concentration of 44 VOCs that had quantifiable concentrations 

in drinking water, at an assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

10. Ranking of frequency of exceedance of Maximum Contaminant Levels for VOCs 

that had quantifiable concentrations in drinking water . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Figures

1. Molecular diagram showing biotransformation of perchloroethene to 

trichloroethene through reductive dechlorination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2–6. Graphs showing:

2. Production of four solvents in the United States from 1960–1998 . . . . . . . . . . . . . . . . 7

3. Detection frequencies of four solvents in ground water . . . . . . . . . . . . . . . . . . . . . . 21

4. Detection frequencies of four solvents in ground water from National 

Water-Quality Assessment studies at an assessment level of 0.2 μg/L . . . . . . . . . . 22

5. Detection frequencies of four solvents in ground water from National 

Water-Quality Assessment studies at an assessment level of 0.02 μg/L . . . . . . . . . 22

6. Detection frequency of four solvents by individual study type at an 

assessment level of 0.02 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

7–12. Maps showing locations where:

7. Ground water was sampled for four solvents and where one or more 

solvents were detected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

8. Ground water was sampled for methylene chloride and where 

methylene chloride was detected. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

9. Ground water was sampled for PCE and where PCE was detected. . . . . . . . . . . . . 25

10. Ground water was sampled for TCA and where TCA was detected . . . . . . . . . . . . 25

11. Ground water was sampled for TCE and where TCE was detected . . . . . . . . . . . . . 26

12. Source water was sampled for four solvents and where one or more 

solvents were detected at an assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . 28
 vii

13–17. Maps showing locations of community water systems where:

13. Drinking water was sampled and where one or more solvents were 

detected at an assessment level of 0.2 μg/L. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

14. Drinking water was sampled and where methylene chloride was 

detected at an assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

15. Drinking water was sampled and where PCE was detected at an 

assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

16. Drinking water was sampled and where TCA was detected at an 

assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

17. Drinking water was sampled and where TCE was detected at an 

assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

18. Quantile plots of concentrations of four solvents in ground water at an 

assessment level of 0.02 microgram per liter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

19. Quantile plots of concentrations of four solvents in source water at an 

assessment level of 0.2 microgram per liter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

20. Quantile plots of concentrations of four solvents in drinking water at an 

assessment level of 0.2 microgram per liter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

22–29. Graphs showing:

21. Detection frequencies of one or more solvents in ground water, source 

water, and drinking water in 12 Northeast and Mid-Atlantic States 

at an assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

22. Detection frequencies of individual solvents in ground water, source 

water, and drinking water in 12 Northeast and Mid-Atlantic States 

at an assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

23. Detection frequencies of individual solvents in ground water and source

water nationwide at an assessment level of 0.2 μg/L . . . . . . . . . . . . . . . . . . . . . . . . . 39

24. Quantiles of concentrations of methylene chloride in ground water, source 

water, and drinking water in 12 Northeast and Mid-Atlantic States at an 

assessment level of 0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

25. Quantiles of concentrations of PCE in ground water, source water, and 

drinking water in 12 Northeast and Mid-Atlantic States at an 

assessment level of 0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

26. Quantiles of concentrations of TCA in ground water, source water, and

drinking water in 12 Northeast and Mid-Atlantic States at an 

assessment level of 0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

27. Quantiles of concentrations of TCE in ground water, source water, and

drinking water in 12 Northeast and Mid-Atlantic States at an 

assessment level of 0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

28. Frequencies of concentrations greater than the MCLs by individual 

solvents in ground water, source water, and drinking water in 

12 Northeast and Mid-Atlantic States at an assessment level 

of 0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

29. Frequencies of concentrations greater than the MCLs in ground water 

from National Water-Quality Assessment studies by individual 

solvents and land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

30. Schematic of movement of solvents through an aquifer from a concentrated 

discharge release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
viii 

Tables

1. Selected information for the four solvents examined in this report. . . . . . . . . . . . . . . . . . . 3

2. Selected physical and chemical properties of the four solvents examined 

in this report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Possible transformation products of the four solvents examined in this report . . . . . . . . 6

4. Number of ground-water samples with concentration results for solvents by 

study type and well type for National Water-Quality Assessment data . . . . . . . . . . . . . . 13

5. Number of ground-water samples with concentration results for solvents by 

well type for retrospective data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6. Median and range of laboratory reporting levels for solvents for each 

data source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

7. Median and range of laboratory reporting levels for solvents analyzed using 

the National Water Quality Laboratory’s low-level method. . . . . . . . . . . . . . . . . . . . . . . . . 16

8. Hydrogeologic and anthropogenic variables that were used in the logistic 

regression analyses of solvents in ground water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

9. Detection frequencies and concentrations of 11 solvent mixtures in ground 

water nationwide at assessment levels of 0.2 microgram per liter and 

0.02 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

10. Detection frequencies of 11 solvent mixtures in source water nationwide at 

an assessment level of 0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

11. Number of exceedances of MCLs for each solvent in ground water nationwide 

by well type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

12. Results of Pearson’s chi-square tests comparing detection frequencies 

of individual solvents between data sets at an assessment level of 

0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

13. Ranking of occurrence of solvents among a suite of VOCs in ground water, 

source water nationwide and drinking water in 12 Northeast and 

Mid-Atlantic States based on detection frequencies at an assessment 

level of 0.2 microgram per liter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

14. Results of Pearson’s chi-square tests comparing nationwide detection 

frequencies of individual solvents by land use at an assessment level 

of 0.2 microgram per liter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

15. Results of Kolmogorov-Smirnov tests comparing the distributions of 

concentrations of solvents in ground water, source water, and 

drinking water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

16. Rank of frequency of concentrations of solvents greater than their Maximum 

Contaminant Levels (MCLs) compared to other volatile organic 

compounds (VOCs) nationwide in ground water and source water, 

and in drinking water in 12 Northeast and Mid-Atlantic States . . . . . . . . . . . . . . . . . . . . . 42

17. Results of Pearson’s chi-square tests comparing frequencies of 

concentrations greater than the Maximum Contaminant Levels 

for individual solvents between data sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

18. Results of Pearson’s chi-square tests comparing frequencies of 

concentrations greater than the Maximum Contaminant Levels for 

individual solvents in ground water between land uses . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

19. Hydrogeologic and anthropogenic variables that were associated with the 

probability of occurrence of one or more solvents or individual solvents in 

ground water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

20. Rank sum and number of solvents associated with each hydrogeologic and 

anthropogenic variable in ground water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

21. Ranking of type of variable associated with each solvent in ground water 

based on significant associations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
 ix

Acronyms Used in This Report:

AWWARF American Water Works Association Research Foundation

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act

CWS Community water system

GC/MS Gas chromatography/mass spectrometry

MCL Maximum Contaminant Level

MTBE Methyl tert-butyl ether

MWDSC Metropolitan Water District of Southern California

NAWQA National Water-Quality Assessment

NWQL National Water Quality Laboratory

PCE Perchloroethene

RCRA Resource Conservation and Recovery Act

SDWA Safe Drinking Water Act

TCA 1,1,1-Trichloroethane

TCE Trichloroethene

THMs Trihalomethanes

TRI Toxics Release Inventory

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

UST Underground storage tank

VOC Volatile organic compound
Occurrence and Implications of Selected Chlorinated 

Solvents 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

Abstract

The U.S. Geological Survey has collected or compiled 

data on select chlorinated solvents in samples of ground water, 

source water, and drinking water. The water samples were 

collected during 1993–2002. Data on solvents in ground water 

and source water were available for the conterminous United 

States, Alaska, and Hawaii. Data on solvents in drinking water 

only were available for 12 Northeast and Mid-Atlantic States. 

The occurrence and distribution of four solvents were examined—methlyene chloride, 1,1,1-trichloroethane, tetrachloroethene, and trichloroethene. 

Out of 51 to 55 volatile organic compounds (VOCs) 

analyzed in samples, solvents were among the most frequently 

detected VOCs in all data sets. When data from ground water 

and source water were confined to 12 Northeast and MidAtlantic States, the detection frequencies of solvents were 

similar among all three data sets. Although low concentrations 

of solvents commonly were detected in ground water, some 

solvents had higher median quantified concentrations than other 

VOCs. Relative to other VOCs, solvents were ranked high in all 

data sets in terms of the frequency of concentrations higher than 

U.S. Environmental Protection Agency Maximum Contaminant Levels. Mixtures were a common mode of occurrence of 

solvents.

The probability of occurrence of solvents was most 

strongly related to the dissolved-oxygen content in ground 

water, with solvents having a higher probability of occurrence 

in conditions of relatively high dissolved oxygen compared to 

conditions of relatively low dissolved oxygen. The probability 

of occurrence of solvents in ground water also was strongly 

associated with urban land use and population density and with 

variables that represented the transport of solvents through the 

soil zone to ground water.

It is important for ground-water resource managers to 

understand the contamination potential posed by solvents, especially in resources that are critical as drinking-water supplies. 

Low-level analytical methods are most useful for determining 

the complete environmental distribution of solvents in ground 

water. To protect ground-water resources, it is important for 

ground-water managers to (1) delineate the redox conditions of 

ground water in the aquifer in order to predict the potential fate 

of solvents, (2) determine and control the sources, or potential 

sources, of solvents to ground water, and (3) establish the 

susceptibility of aquifers by fully ascertaining the hydraulic 

properties of the saturated and vadose zones.

Introduction

Chlorinated solvents are volatile organic compounds 

(VOCs) that contain chlorine. In general, chlorinated solvents 

have low water solubilities and high volatilities and densities 

relative to other VOCs. They are used in a variety of commercial, industrial, manufacturing, and domestic applications. 

Chlorinated solvents (hereafter referred to as solvents) are used 

in the aerospace and electronics industries; dry cleaning; manufacture of foam; paint removal/stripping; manufacture of pharmaceuticals; metal cleaning and degreasing; and wood manufacturing (Halogenated Solvents Industry Alliance, 2004a). 

Solvents also can be found in a variety of household consumer 

products including drain, oven, and pipe cleaners; shoe polish; 

household degreasers; typewriter correction fluid; deodorizers; 

leather dyes; photographic supplies; tar remover; waxes; and 

pesticides (U.S. Environmental Protection Agency, 1980).

Production of solvents began in the United States in the 

early 20th century. Carbon tetrachloride was the first solvent 

produced in the United States and was the main solvent used for 

the first half of the 20th century. Production of trichloroethene 

(TCE) and perchloroethene (PCE) began in the 1920s (Pankow 

and Cherry, 1996). After World War II, production and usage of 

these two solvents in industry increased markedly, and they 

became the most commonly used solvents. 

Two other commonly used solvents during this time period 

were methylene chloride and 1,1,1-trichloroethane (TCA). The 

production of methylene chloride, PCE, TCA, and TCE 

generally peaked in the 1970s and has been declining since then 

due mostly to the human-health and environmental concerns 
2 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

associated with these compounds (Pankow and Cherry, 1996). 

For example, TCE has been linked to potential human-health 

effects including anemia, arthritis, cancer, birth defects, and 

damage to the liver, kidneys, immune system, and nervous 

system. As another example, TCA is being phased out for most 

uses because of its ozone depletion potential in the upper atmosphere. Under the Montreal Protocol, production of TCA for 

emissive uses was phased out at the end of 1995 in Europe and 

at the end of 1996 in the United States, Japan, and other industrialized countries (Halogenated Solvents Industry Alliance, 

1994).

Although production has been declining recently, large 

quantities of these solvents continue to be produced and used by 

many commercial and industrial sectors of society. Chemicals 

that have large production volumes and wide usage are highly 

susceptible to release to the environment. Once released to the 

environment, solvents have a tendency for widespread groundwater contamination due to their unique combination of physical and chemical properties. Indeed, the four solvents examined here are some of the most commonly identified organic 

chemicals in ground water (Pankow and Cherry, 1996; 

Squillace and others, 2002).

The potential sources of solvents to the environment are 

numerous. For example, methylene chloride, PCE, and TCE are 

among 29 of the chemicals, metals, and other substances most 

commonly found at U.S. Environmental Protection Agency 

(USEPA) Superfund sites (U.S. Environmental Protection 

Agency, 2005a). Considerable quantities of solvents are 

released to the environment according to information in the 

USEPA’s Toxics Release Inventory (TRI) database. The TRI 

provides information on the release of toxic chemicals in the 

United States from various manufacturing, commercial, and 

industrial processes. According to the TRI, during 1998–2001, 

total on- and off-site releases of methylene chloride, PCE, TCE, 

and TCA averaged about 33 million pounds, 4 million pounds, 

11 million pounds, and 0.5 million pounds, respectively (U.S. 

Environmental Protection Agency, 2003). Despite a decline in 

production due to increased regulations, PCE is still the solvent 

of choice for 85 to 90 percent of the approximately 30,000 dry 

cleaners and launderers in the United States (Doherty, 2000). 

Releases of PCE can occur at many points during dry-cleaning 

operations including air emissions, spills, inadequate storage, 

and drain disposal of spent PCE (U.S. Environmental Protection Agency, 1995).

As mentioned, solvents have been associated with both 

acute and chronic human-health problems. Some are suspected 

human carcinogens, and USEPA has set Maximum Contaminant Levels (MCLs) for solvents in drinking water at very low 

concentrations. Many of the solvents have water solubilities 

that are high relative to their MCLs. This means that even small 

spills of some solvents can result in substantial ground-water 

contamination problems with respect to human health.

The purpose of this report is to present information on the 

occurrence and implications of four solvents in ground water, 

source water, and drinking water. The data for this report were 

collected or compiled by the U.S. Geological Survey’s (USGS) 

National Water-Quality Assessment (NAWQA) Program and 

other agencies. The water samples were collected during 1993–

2002. 

Four solvents were selected for analyses in this report—

methylene chloride (also known as dichloromethane), PCE, 

TCA, and TCE. Many organic compounds are classified as 

solvents but only these four were selected because of their long 

histories of use and their large production and usage relative to 

other solvents.

The occurrence and distribution of these solvents were 

determined by detection frequencies, ranges of concentrations, 

and areal patterns of detection. Concentrations of the solvents 

were compared to USEPA drinking-water standards. Occurrence, distribution, and concentration information is given for 

three data sets: (1) ground water on a national scale, (2) source 

water (derived from ground water) on a national scale, and 

(3) drinking water from 12 Northeast and Mid-Atlantic States.

Potential anthropogenic sources of solvents to ground 

water are identified and the most likely sources are highlighted. 

Hydrogeologic and anthropogenic factors associated with the 

occurrence of solvents in ground water are identified. The identification of factors associated with the occurrence of solvents 

may aid in understanding the sources and pathways of these 

chemicals to ground water and the susceptibility of aquifers to 

contamination.

Background

The four solvents examined in this report—methylene 

chloride, PCE, TCA, and TCE—are members of the family of 

aliphatic halogenated hydrocarbons. These compounds are 

produced in substantial quantities, have a variety of industrial, 

commercial, and domestic uses, and their release to the environment has resulted in widespread contamination of ground water. 

The presence of solvents in ground water, especially ground 

water used as a source of drinking water, can pose a potential 

threat to human health due to the solvent’s mobility, longevity, 

and toxicity. Table 1 presents information for the four solvents 

examined in this report, including chemical abstract numbers, 

molecular formulas, some common synonyms and abbreviations, and the abbreviation used in this report for each 

compound.

Physical and Chemical Properties and 

Environmental Behavior of Solvents

Table 2 lists selected physical and chemical properties of 

the four solvents. From table 2, some general observations can 

be made. First, the densities of all four solvents are greater 

than 1. This means that the pure liquid phase of each solvent has 

a density greater than water. If pure liquid phases of these 

solvents are released to the environment, they could penetrate 

the water table, move down through the saturated zone, and 
Background 3

collect in pools on top of less permeable layers. Once pooled in 

such a manner, the absolute removal rates from the aquifer are 

usually low because the solvent pool presents a very low cross 

section relative to ground-water flow in the aquifer (Johnson 

and Pankow, 1992). In this way, the pure phase of a solvent 

could become a long-term source of low concentrations in an 

aquifer and could be very difficult to remediate.

Organic chemicals that have vapor pressures greater than 

about 10 Pascals are considered to be volatile. All of the four 

solvents have vapor pressures much higher than this value 

(table 2) and generally higher than other commonly used VOCs 

like gasoline hydrocarbons. This means that the pure liquid 

phase of these compounds can volatilize when spilled onto a 

surface or exposed to the atmosphere. Although they can readily 

volatilize, some mass of a solvent can be transported to the 

water table following a release. The solvent can be transported 

to the water table by gaseous diffusion, downward movement of 

solvent dissolved in recharge water, or by gravity movement of 

pure solvent liquid through the vadose zone. 

In environmental systems, water solubility is one of the 

most important chemical properties that determines the occurrence of organic chemicals in ground water. The aqueous solubilities of the four solvents are generally higher than those of 

other common VOCs like gasoline hydrocarbons. For example, 

the aqueous solubilities of three common gasoline components 

benzene, ethylbenzene, and toluene are approximately 1,787, 

175, and 565 milligrams per liter (mg/L) at 25 degrees Celsius 

(°C) (Mackay and others, 1993). The solubility of benzene is 

higher than the solubility of any of the solvents except for 

methylene chloride. However, the solubilities of all four 

solvents except perchloroethene are higher than either toluene 

or ethylbenzene. 

Table 1. Selected information for the four solvents examined in this report.

[C, carbon; Cl, chlorine; H, hydrogen; --, none]

Compound

name

Chemical Abstract 

Services Registry Number

Chemical

formula Common synonyms and abbreviations

Abbreviation

used in this 

report

Methylene chloride 75-09-2 CH2Cl2 Dichloromethane

Perchloroethene 127-18-4 Cl2C=CCl2 Tetrachloroethene, perchloroethylene, 

tetrachloroethylene, PERC, PCE

1,1,1-Trichloroethane 71-55-6 CH3CCl3 Methyl chloroform, MC, TCA

Trichloroethene 79-01-6 HClC=CCl2 Trichloroethylene, TCE

Table 2. Selected physical and chemical properties of the four solvents examined in this report.

[g/cm3, grams per cubic centimeter; °C, degrees Celsius; Pa, Pascals; mg/L, milligrams per liter; m3, cubic meters]

Compound

name

Molecular

weight1

Density2

(g/cm3 at 

25°C)

Vapor

pressure2

(Pa at 25°C)

Aqueous

solubility2

(mg/L at 25°C)

Organic 

carbon

partitioning 

coefficient

2

(log Koc)

Henry’s Law 

constant

2

at 25°C

(Pa-m3/

mole)

Half-life in

ground water

3

(in days)

Methylene chloride 84.94 1.33 57,365 18,680 1.20 319 35

Perchloroethene 165.83 1.62 2,462 150 2.56 1,628 540

1,1,1-Trichloroethane 133.41 1.34 16,180 1,312 2.18 1,638 343

Trichloroethene 131.39 1.46 9,680 1,100 2.03 958 987

1From Mackay and others, 1993.

2

Average of values from Mackay and others, 1993.

3

Average from Howard and others, 1991.

--

PCE

TCA

TCE
4 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Although the solubilities of the four solvents generally are 

higher than other commonly used VOCs, their absolute solubilities are low. Nonetheless, the aqueous solubilities of all the 

solvents examined in this report are much higher than their 

human-health benchmarks for drinking water. This means that 

even small spills of solvents have the potential to cause contamination of ground water at concentrations that could be harmful 

to human health.

Chemicals with high organic partitioning coefficients will 

adsorb strongly to soil and rock material. The organic partitioning coefficients of the solvents are relatively low (table 2) 

and similar to those of the gasoline hydrocarbons. This means 

that sorption to organic matter and clay in soils and aquifers will 

not substantially retard the movement of solvents through the 

subsurface. However, the organic carbon partitioning coefficients for both the solvents and common gasoline components 

are high enough to cause some retardation of their movement in 

the subsurface relative to the velocity of ground water.

The Henry’s Law constant is the equilibrium concentration 

of a dissolved organic chemical in solution relative to the partial 

pressure of the chemical in air above the solution. At relatively 

low concentrations, this value represents the air-water partitioning for organic chemicals in the environment. For chemicals 

with Henry’s Law constants greater than about 1 Pascal-cubic 

meter per mole (Pa-m3/mole), partitioning from water to air is 

important. For all the solvents, Henry’s Law constants are 

substantially higher than this level (table 2) indicating that 

partitioning from water to air can be an important process in 

their environmental behavior. However, once in the saturated 

zone, partitioning of solvent mass across the capillary fringe has 

been shown to be very slow (McCarthy and Johnson, 1992).

Solvents have relatively low viscosities and low interfacial 

tensions compared to other commonly used VOCs. Low viscosities mean that pure liquid phases of solvents can move rapidly 

down through the vadose zone by gravity (Pankow and Cherry, 

1996). Low interfacial tensions between solvents and water 

allow solvents to easily enter into small fractures and pore 

spaces in the subsurface and provide a source for long-term, low 

concentration contamination of ground water. Low interfacial 

tension also contributes to low retention capacities of soils for 

solvents.

Transformation of Solvents

In ground water, solvents can be transformed through 

abiotic and biotic processes (Leisinger, 1996). The half-lives of 

the solvents in ground water are presented in table 2. These 

half-lives are based on scientific judgment from various biodegradation studies and screening test data and represent the total 

transformation rates of the compounds whether by biotic or 

abiotic reactions (Howard and others, 1991). With a few exceptions, the half-lives of the gasoline hydrocarbons generally are 

much smaller than the half-lives of the solvents. This means that 

the solvents transform very slowly relative to gasoline hydrocarbons and that they could persist for long periods of time in 

certain ground-water systems.

For the solvents, three types of abiotic transformation 

processes are important—hydrolysis, reduction, and photolysis. 

Hydrolysis involves the reaction of a solvent with the hydroxyl 

ion of water and is highly dependent on pH. Hydrolysis usually 

produces an alcohol but in some cases non-alcohol products are 

formed. Transformation of solvents by hydrolysis has been 

documented in various laboratory and field studies (Dilling and 

others, 1975; Pearson and McConell, 1975; Kollig, 1993; 

Washington, 1995). 

For solvents, reduction generally involves hydrogenolysis 

where chlorine is replaced by hydrogen. Higher chlorination of 

the molecule tends to produce higher potential energy release 

during reduction. Although most solvents have sufficient Gibbs 

free energy to undergo abiotic reduction reactions, the potential 

is rarely met except under very reducing conditions and not 

without mediation by bacteria (Pankow and Cherry, 1996). 

Photolysis occurs when light strikes a chemical and 

converts it to a new molecule through electron distribution. In 

this situation, the chemical is usually oxidized. Photolysis of 

solvents has been demonstrated for PCE and TCE (Pearson and 

McConell, 1975; Haag and others, 1996).

Although the free energy of many solvents is high enough 

to allow abiotic reactions like hydrogenolysis to occur, the 

conditions necessary to undergo the reaction are rarely met, and 

thus, microbes mediate most transformations of solvents 

(Pankow and Cherry, 1996). Biotic processes for the transformation of solvents can be either aerobic or anaerobic and occur 

by five possible pathways: (1) through their use as a sole carbon 

and energy source for growth of aerobic bacteria; ( 2) through 

their use as a growth substrate for organisms that use an electron 

acceptor other than oxygen (nitrate respiration); (3) through 

their use as a growth substrate in acetogenic fermentation; 

(4) through co-metabolism; and (5) through their use as an 

electron acceptor under hypoxic conditions (reductive dechlorination) (Beek, 2001). Halogenated aliphatic organic 

compounds are strong oxidants, and thus anaerobic biodegradation, anaerobic co-metabolism, or their use as electron acceptors are the most common means of their transformation (Beek, 

2001).

Reductive dechlorination is an important biodegradation 

process for the transformation of solvents (Fathepure and 

others, 1987; Nyer and Duffin, 1997). Reductive dechlorination 

requires that microbes have access to a substrate that serves as 

a source of carbon and energy and as an electron donor 

(Montoux and others, 1996). During reductive dechlorination, 

the solvent is used as an electron acceptor, and chlorine atoms 

are sequentially removed and replaced with hydrogen. An 

example of reductive dechlorination is shown in figure 1. In this 

case, PCE is transformed to TCE through reductive dechlorination using a reductive dehalogenase enzyme. Eventually, reductive dechlorination can transform PCE to ethane by sequential 

removal of chlorine atoms (Swindoll and Troy, 1996). 
Hydrogen 

 Chloride

Hydrogen 

Chlorine 

Carbon 

Perchloroethene Trichloroethene

Background 5

Figure 1. Biotransformation of perchloroethene to trichloroethene through reductive dechlorination.

Another possible avenue for transformation of solvents is 

through their use as electron donors. The solvent is used as a 

primary substrate that supplies carbon and energy for microbes. 

This process usually occurs anaerobically, and microbes known 

to anaerobically degrade solvents include sulfate-reducing and 

methanogenic bacteria that use sulfate and carbon dioxide as 

electron acceptors (Chu and Alvarez-Cohen, 1995; van Eekert 

and Schraa, 2001). In some cases microbes can utilize less 

chlorinated compounds (such as chloroethene) aerobically with 

oxygen as an electron acceptor and the compound as the 

primary substrate (Davis and Carpenter, 1990). Although this 

process has been demonstrated in the laboratory for smaller 

molecules, the importance of this metabolic pathway for larger 

molecules is uncertain.

Aerobic biodegradation of solvents generally occurs 

through co-metabolism (Bielefeldt and others, 1995; MunkataMarr and others, 1996). Co-metabolism is the process whereby 

solvents are fortuitously degraded as microbes consume other 

organic substrates. Although the solvents are transformed in the 

respiration process, the microbes do not derive any direct 

benefit from the transformations (Swindoll and Troy, 1996). 

Only a small number of organic compounds have been identified as serving as suitable primary substrates in which the cometabolism of solvents can occur.

Some potential transformation products of solvents in 

water are shown in table 3. In some cases, especially for more 

highly chlorinated molecules, the degradation of a solvent 

results in a metabolite that also is a solvent. This usually occurs 

during reductive dechlorination that proceeds with a sequential 

dechlorination of highly chlorinated molecules to less chlorinated molecules. However, the specific transformation products 

formed during reductive dechlorination are a result of both the 

mechanism of degradation and the type of electron acceptors 

that are available (Swindoll and Troy, 1996; Nyer and Duffin, 

1997).

Production and Usage of Solvents

 Figure 2 shows the production of the four solvents from 

1960 to 1998, in millions of kilograms. For the most part, 

production of each of the solvents has decreased since about 

1970 or 1980. Figure 2 only shows production of these solvents 

in the United States, and these values do not necessarily indicate 

usage of solvents in the United States because substantial 

amounts of some solvents are exported. In addition, production 

may not accurately indicate usage because substantial amounts 

of solvents are recycled in some commercial/industrial 

processes. For example, most dry-cleaning equipment recycles 

95 to 99 percent of the PCE used in the dry-cleaning process 

(Doherty, 2000).

Methylene Chloride

Methylene chloride is used widely as a chemical solvent 

and replaced more flammable solvents more than 60 years ago. 

It is commonly used in paint removers and industrial adhesive 

formulas. It also is used in the production of flexible urethane 

foams, pharmaceutical products, as a cleaning agent for fabricated metal parts, in the production of thermoplastics, in 

nonflammable adhesive formulations for industrial applications, and as an extraction solvent (Halogenated Solvents 

Industry Alliance, 2003). The largest use of methylene chloride 

is in paint-removal formulations. Methylene chloride also is 

commonly used as an extractant in the recovery and purification 

of a wide variety of materials including oils, fats, and waxes, 

and the decaffeination of coffee and tea, oleoresin extraction 

from a variety of spices, and for the extraction of hops (Halogenated Solvents Industry Alliance, 2003). Methylene chloride 

also can be found in a wide variety of commercial and consumer 

products such as spot removers, wood floor and panel cleaners, 

contact cement, super glues, spray adhesives, adhesive 

removers, silicone lubricants, specialized electronic cleaners, 

wood stains, varnishes and finishes, paint thinners, paint 

removers, aerosol spray paints, primers, aerosol rust removers, 

outdoor water repellents, glass frosting/artificial snow, spray 

lubricant for cars, transmission cleaners, battery terminal 

protectors, brake quieter/cleaner, and gasket removers (U.S. 

Department of Health and Human Services, 2005).

Since 1985, the production of methylene chloride has 

declined, in part, due to the increase in the manufacture of 

solvent systems that do not use methylene chloride (U.S. 

Department of Health and Human Services, 2005). In the 

United States in 2000, the demand for methylene chloride was 

estimated at about 91 million kilograms (Halogenated Solvents 

Industry Alliance, 2003). In 2000, approximately 15 million 

kilograms of methylene chloride were imported and approximately 57 million kilograms were exported (U.S. Department 

of Health and Human Services, 2005).

Perchloroethene

In the late 1940s, PCE began replacing petroleum derivatives as the main solvent used in dry cleaning (Halogenated 

Solvents Industry Alliance, 2005). By the early 1960s, PCE had 

become the most widely used dry-cleaning solvent in the United 

States (State Coalition for Remediation of Drycleaners, 2002). 

However, during the 1990s its use in dry cleaning declined in 

order to meet more stringent government regulations for workplace exposure (U.S. Department of Health and Human 

Services, 2005). Nonetheless, as of 2002, PCE was still used as 

the primary cleaning solvent in about 85 percent of professional 

dry cleaners (Sinsheimer and others, 2002). 
Table 3. Possible transformation products of the four solvents examined in this report.

[PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene; CO2, carbon dioxide; CO, carbon monoxide]

Parent compound Possible transformation

product in water

References

Methylene chloride chloromethane

methane

formaldehyde

formic acid

hydrogen chloride

methanol

CO2

CO

Swindoll and Troy, 1996

Swindoll and Troy, 1996

Dilling and others, 1975; Belkin, 1992; Kollig, 1993; Washington, 1995

Dilling and others, 1975

Dilling and others, 1975; Kollig, 1993

Dilling and others, 1975

Swindoll and Troy, 1996

Dilling and others, 1975

PCE TCE 

trans-1,2-dichloroethene

cis-1,2-dichloroethene

1,1-dichloroethene

vinyl chloride

1,2-dichloroethane

chloroethane

ethene

ethane

trichloroacetic acid

hydrochloric acid

CO2

Parsons and others, 1985; Fathepure and others, 1987; van der Meer and others, 1992

Parsons and others, 1985; van Eekert and Schraa, 2001

Parsons and others, 1985; van der Meer and others, 1992

Swindoll and Troy, 1996; van Eekert and Schraa, 2001

Parsons and others, 1985; van der Meer and others, 1992

Swindoll and Troy, 1996

Swindoll and Troy, 1996

Swindoll and Troy, 1996; van Eekert and Schraa, 2001

van der Meer and others, 1992

Dilling and others, 1975

Dilling and others, 1975

Swindoll and Troy, 1996

TCA 1,1-dichloroethane

1,1-dichloroethene

chloroethane

chloroethanol

ethane

formaldehyde

acetic acid

hydrochloric acid

CO2

Parsons and others, 1985; Criddle and McCarty, 1991; Semprini and others, 1992

Dilling and others, 1975; Kollig, 1993; McNab and Narasimhan, 1994

Semprini and others, 1992

Swindoll and Troy, 1996

Swindoll and Troy, 1996

Swindoll and Troy, 1996

Criddle and McCarty, 1991; Semprini and others, 1992; Washington, 1995

Washington, 1995

Swindoll and Troy, 1996

TCE trans-1,2-dichloroethene

cis-1,2-dichloroethene

1,1-dichloroethene

vinyl chloride

1,2-dichloroethane

trichloromethane

chloroethane

ethene

ethane

trichloroethene oxide

dichloroacetic acid

hydrogen chloride

hydrochloric acid

cis-dihydrodiol

3-methylcatechol

2,2,2-trichloroacetaldehyde

glyoxylic acid

formic acid

CO2

CO

Kloepfer and others, 1985; Barrio-Lage and others, 1986; Wilson and others, 1986

Barrio-Lage and others, 1986; Barrio-Lage and others, 1987

Fliermans and others, 1988

Barrio-Lage and others, 1986; Wilson and others, 1986

Swindoll and Troy, 1996

Fliermans and others, 1988

Swindoll and Troy, 1996

Swindoll and Troy, 1996

Swindoll and Troy, 1996

Belkin, 1992

Dilling and others, 1975; Alvarez-Cohen and McCarty, 1991

Dilling and others, 1975

Fliermans and others, 1988

Nelson and others, 1988

Nelson and others, 1988

Alvarez-Cohen and McCarty, 1991

Alvarez-Cohen and McCarty, 1991

Alvarez-Cohen and McCarty, 1991

Swindoll and Troy, 1996

Alvarez-Cohen and McCarty, 1991

6 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water
0

50

100

150

200

250

300

350

400

1960 1965 1970 1975 1980 1985 1990 1995 2000

YEAR

PRODUCTION, IN MILLION OF KILOGRAMS

Methylene chloride

Perchloroethene

1,1,1-Trichloroethane

Trichloroethene

Background 7

Figure 2. Production of four solvents in the United States from 1960–1998 (Pankow and Cherry, 1996; Chemical Manufacturers 

Association, 1997, 1998).

In 1998, the majority of PCE used in the United States 

(about 50 percent) was used as a chemical intermediate in the 

manufacture of several hydrofluorocarbons that are alternatives 

to chlorofluorocarbon refrigerants. About 25 percent was used 

in dry cleaning and textile processing. The remainder was used 

in automotive aerosols, metal cleaning and degreasing, and 

miscellaneous uses (Halogenated Solvents Industry Alliance, 

2005). PCE can be found in a variety of commercial and 

consumer products such as insulating fluid and cooling gas in 

electrical transformers, paint removers, printing inks, art 

supplies, adhesive formulations, paper coatings, leather treatments, water repellents, automotive cleaners, silicone lubricants, spot removers, as a pesticide intermediate, and as an 

extractant for pharmaceuticals (U.S. Department of Health 

and Human Services, 2005; Halogenated Solvents Industry 

Alliance, 2005).

Production of PCE in the United States declined more than 

60 percent from 1980 to 1993 (fig. 2) because of increased 

solvent recycling and increased occupational exposure regulations (U.S. Department of Health and Human Services, 2005; 

Halogenated Solvents Industry Alliance, 2005). However, in 

recent years the demand for PCE has increased slightly from 

about 112 million kilograms in 1996 to about 125 million kilograms in 1999 (fig. 2). According to the U.S Department of 

Health and Human Services (2005), three companies in the 

United States produced PCE as of 2005, with a combined 

capacity of about 160 million kilograms. From 1998 to 2001, 

the United States imported about 16 million kilograms of PCE 

per year and exported about 27 million kilograms per year (U.S. 

Department of Health and Human Services, 2005).

1,1,1-Trichloroethane

Beginning in the mid-1950s, TCA was used as a cold 

cleaning solvent substitute for carbon tetrachloride. TCA is 

used mainly for vapor degreasing and cold cleaning of fabricated metal parts and other materials. TCA also is used in fluoropolymer synthesis, as a solvent in adhesive and aerosol 

formulations, for the production of certain coatings and inks, for 

a variety of textile applications, for cleaning motion picture 

film, and for dry cleaning leather and suede garments (Halogenated Solvents Industry Alliance, 1994). TCA can also be found 
8 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

in a variety of commercial and consumer products such as 

aerosol formulations, adhesives, protective surface coatings, 

cutting oils, and printing inks (U.S. Environmental Protection 

Agency, 2000a).

Because of concerns about ozone depletion, the USEPA 

has restricted future production of TCA in the United States. 

Under the Montreal Protocol on Substances That Deplete the 

Ozone Layer, as amended in June 1990 and November 1992, 

and under Title VI of the Clean Air Act, the use of TCA is being 

phased out (Halogenated Solvents Industry Alliance, 1994). 

Under Title VI of the Clean Air Act, restrictions on the uses of 

TCA were imposed and labeling was required for certain products containing or made with TCA. Pursuant to the Clean Air 

Act, the USEPA accelerated a phase-out in the production and 

import of TCA allowing only essential uses of the compound 

after 1996 (U.S. Environmental Protection Agency, 1993). 

Although use of TCA is being phased out, sources indicate that 

relatively large quantities of TCA were still being produced in 

the United States after this date, with 151 and 147 million kilograms of TCA produced in 1997 and 1998 (fig. 2). Under the 

Clean Air Act, TCA may be produced domestically for export 

(Agency for Toxic Substances and Disease Registry, 2004).

Trichloroethene

TCE is used mainly as a chemical intermediate in the 

production of hydroflurocarbon refrigerants and as a metal 

cleaning agent (Halogenated Solvents Industry Alliance, 2001). 

TCE is especially valuable as a metal degreaser because of its 

cleaning properties, low flammability, and lack of a measurable 

flashpoint. Four main industries use TCE in vapor or cold 

degreasing operations—furniture and fixtures, fabricated metal 

products, electrical and electronic equipment, and transport 

equipment (U.S. Department of Health and Human Services, 

2005). TCE also is used as a chemical process intermediate in 

fluorochemical and polyvinyl chloride production. In 1999, 

more than 50 percent of TCE was used as a chemical intermediate, 42 percent was used in metal degreasing and cleaning, 

and the remainder was used for miscellaneous purposes 

(Halogenated Solvents Industry Alliance, 2001). TCE also can 

be found in a variety of commercial and consumer products 

such as adhesives, lubricants, paints, varnishes, paint strippers, 

pesticides, and cold metal cleaners (U.S. Department of Health 

and Human Services, 2005).

Production of TCE declined by almost 80 percent from 

1970 to 1990 (fig. 2). This decline is most likely due to the 

human-health concerns associated with the use of TCE. In 

1998, U.S. demand for TCE was about 78 million kilograms, of 

which about 7 million kilograms were imported. In 1998, about 

38 million kilograms of TCE were exported (Halogenated 

Solvents Industry Alliance, 2001).

Health Effects of Solvents

Solvents have been associated with both acute and chronic 

human-health problems. Acute health problems are those that 

reach a critical point in a relatively short period of time. Chronic 

health problems are those that occur over a lengthy period of 

time and include both cancer and noncancer effects. The health 

effects of solvents can be different depending on the route of 

exposure. There are numerous exposure routes for solvents in 

the human body. For solvents in ground water, exposure occurs 

mainly through drinking water containing solvents and through 

breathing solvents that have been transferred from water to air. 

Solvents can get into air by volatilization from contaminated 

drinking water in showers or taps.

Although a large amount of data indicates negative health 

effects from exposure to solvents, not all data indicate a negative health effect. In 1992, concentrations of as much as 

212 micrograms per liter (μg/L) of TCE and 180 μg/L of PCE 

were found in the drinking water of two villages in Finland. 

Even though the time of exposure may have been as long as 

several decades for some people, no increased incidence rates of 

total cancer, liver cancer, non-Hodgkin’s lymphoma, 

Hodgkin’s disease, multiple myeloma, or leukemia were 

observed in the exposed population. The contaminants 

appeared to be metabolized by exposed individuals and 

excreted as dichloroacetic or trichloroacetic acid (Vartiainen 

and others, 1993).

Methylene Chloride

The effects of acute inhalation of methylene chloride in 

humans include decreased visual, auditory, and psychomotor 

functions and effects on the central nervous system, but these 

effects are reversible once exposure ceases. Tests involving 

acute exposure of animals to methylene chloride have shown 

moderate acute toxicity from oral exposure (Agency for Toxic 

Substances and Disease Registry, 2000). The major effects 

from chronic inhalation exposure to methylene chloride in 

humans are effects on the central nervous system, headaches, 

dizziness, nausea, and memory loss. Animal studies indicate 

that the inhalation of methylene chloride causes effects on the 

liver, kidney, central nervous system, and cardiovascular 

system (Agency for Toxic Substances and Disease Registry, 

2000).

Animal studies have demonstrated that methylene chloride 

crosses the placental barrier, and minor skeletal variations and 

lowered fetal body weights have been noted. No studies have 

indicated developmental or reproductive effects in humans 

from inhalation or oral exposure (Agency for Toxic Substances 

and Disease Registry, 2000).

According to the U.S. Department of Health and Human 

Services (2005) in its eleventh annual report on carcinogens, 

methylene chloride is reasonably anticipated to be a human 

carcinogen based on sufficient evidence of carcinogenicity in 

experimental animals. The USEPA has set an MCL in drinking 
Background 9

water of 5 μg/L for methylene chloride to be protective of both 

cancer and noncancer health effects. The USEPA also classifies 

methylene chloride as a group B2 carcinogen (probable human 

carcinogen), indicating that sufficient evidence in animals 

exists to classify it as a carcinogen but evidence in humans is 

lacking (U.S. Environmental Protection Agency, 2004a). The 

USEPA is currently reassessing the human-health effects of 

methylene chloride but the study will not be completed until 

2006 or later (U.S. Environmental Protection Agency, 2005b).

Perchloroethene

The effects of acute inhalation of PCE in humans include 

irritation of the upper respiratory tract and eyes, kidney 

dysfunction, and at lower concentrations, neurological effects 

such as reversible mood and behavioral changes, impairment of 

coordination, dizziness, headache, sleepiness, and unconsciousness. However, mice exposed acutely to PCE from oral routes 

indicated low toxicity. The major noncancer effects from 

chronic inhalation exposure to PCE in humans include neurological effects, such as sensory symptoms and headaches, 

impairments in cognitive and motor neurobehavioral functioning, color vision decrements, cardiac arrhythmia, liver 

damage, and possible kidney effects (Agency for Toxic 

Substances and Disease Registry, 1997a).

Some adverse reproductive effects, such as spontaneous 

abortions, menstrual disorders, altered sperm structure, and 

reduced fertility, have been reported in studies of workers occupationally exposed to inhalation of PCE. However, no definitive conclusions can be made because of the limitations of the 

studies (Agency for Toxic Substances and Disease Registry, 

1997a). In one study of residents exposed to drinking water 

contaminated with PCE and other solvents, there was an indication that birth defects were associated with exposure. However, 

no firm conclusions can be drawn from this study due to 

multiple chemical exposures and problems with the analysis 

(Agency for Toxic Substances and Disease Registry, 1997a).

According to the U.S. Department of Health and Human 

Services (2005) in its eleventh annual report on carcinogens, 

PCE is reasonably anticipated to be a human carcinogen based 

on sufficient evidence of carcinogenicity in experimental 

animals. Epidemiological studies of dry-cleaning workers 

exposed to PCE and other solvents indicate an increased risk for 

a variety of cancers. However, these studies were complicated 

by confounding factors such as lifestyles of the participants 

(Agency for Toxic Substances and Disease Registry, 1997a). In 

animal studies, an increased incidence of liver tumors in mice 

from inhalation and gavage exposure to PCE has been reported. 

An increased rate of kidney and mononuclear cell leukemia in 

rats through inhalation exposure also has been reported (U.S. 

Environmental Protection Agency, 1988, 2000b; Agency for 

Toxic Substances and Disease Registry, 1997a). One study 

involving humans reported that there was a potential association between drinking water contaminated with PCE and an 

increased risk of childhood leukemia. However, the statistical 

significance of the incidence of leukemia has not been resolved 

(Agency for Toxic Substances and Disease Registry, 1997a).

The USEPA has set an MCL in drinking water of 5 μg/L 

for PCE to be protective of both cancer and noncancer health 

effects (U.S. Environmental Protection Agency, 2004a). As of 

2004, the USEPA does not have a classification for the 

carcinogenicity of PCE (U.S. Environmental Protection 

Agency, 2004a). However, the USEPA’s Science Advisory 

Board placed PCE on a continuum between probable human 

carcinogen (B2) and possible human carcinogen (C). The International Agency for Research on Cancer also has classified 

PCE as a probable human carcinogen (Agency for Toxic 

Substances and Disease Registry, 1997a). The USEPA is 

currently reassessing the human-health effects of PCE but the 

study will not be completed until 2006 or later (U.S. Environmental Protection Agency, 2005b).

1,1,1-Trichloroethane

The effects of acute inhalation of TCA in humans include 

mild hepatic effects, mild motor impairment (for example, 

increased reaction time), lightheadedness, impaired balance, 

dizziness, nausea, vomiting, diarrhea, loss of consciousness, 

decreased blood pressure, and ataxia. Cardiac arrhythmia and 

respiratory arrest may result from the depression of the central 

nervous system (Agency for Toxic Substances and Disease 

Registry, 2004). Tests involving acute exposure of rats, mice, 

rabbits, and guinea pigs have demonstrated that TCA has a low 

acute toxicity from oral exposure. Most studies have not 

reported adverse effects from chronic oral or inhalation exposure to low levels of TCA in humans or animals (Agency for 

Toxic Substances and Disease Registry, 2004). Some liver 

damage and neurological effects have been observed in rodents 

chronically exposed to TCA by inhalation (U.S. Environmental 

Protection Agency, 2000a). 

Epidemiologic studies have found no relation between 

adverse pregnancy outcomes and exposure of mothers or 

fathers to TCA. Animal studies have not reported developmental or reproductive effects from exposure to TCA (Agency 

for Toxic Substances and Disease Registry, 2004).

Information is not available on the carcinogenic effects of 

TCA in humans. Two animal studies have not demonstrated 

carcinogenicity from oral or inhalation exposure to TCA; 

however, the data are considered to be inadequate (U.S. Environmental Protection Agency, 2000a). The USEPA has set an 

MCL in drinking water of 200 μg/L for TCA to be protective of 

both cancer and noncancer health effects. The USEPA classifies 

TCA as a group D carcinogen based on no reported human data 

and inadequate animal data (U.S. Environmental Protection 

Agency, 2004a). This indicates that the chemical is not classifiable as to its human carcinogenicity. The USEPA is currently 

reassessing the human-health effects of TCA but the study will 

not be completed until 2006 or later (U.S. Environmental 

Protection Agency, 2005b).
10 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Trichloroethene

Acute inhalation of TCE in humans primarily produces 

effects on the central nervous systems with symptoms such as 

sleepiness, fatigue, headache, confusion, and feelings of 

euphoria. Effects on the liver, kidneys, gastrointestinal system, 

and skin also have been noted. Tests involving acute exposure 

of rats and mice have shown TCE to have moderate toxicity 

from oral exposure. The major noncancer effects from chronic 

inhalation exposure to TCE in humans include dizziness, headache, sleepiness, nausea, confusion, blurred vision, facial 

numbness, and weakness (Agency for Toxic Substances and 

Disease Registry, 1997b). Effects to the liver, kidneys, and 

immune and endocrine systems also have been seen in humans 

occupationally exposed to inhalation of TCE or from contaminated drinking water (U.S. Environmental Protection Agency, 

2001). 

Some studies have indicated reproductive or developmental effects from exposure to TCE (U.S. Environmental 

Protection Agency, 2000c). Studies of women occupationally 

and non-occupationally exposed to inhalation of TCE have 

reported increases in the incidence of miscarriages. The presence of other chemicals, however, limits the ability to draw 

conclusions specific to TCE. Studies involving animals have 

indicated developmental effects from exposure to TCE and its 

metabolites (U.S. Environmental Protection Agency, 2000c). 

Also, several studies have shown associations between the 

occurrence of TCE and PCE in drinking water and small birth 

weights in babies (Sonnenfeld and others, 2001; Rodenbeck and 

others, 2000).

According to the U.S. Department of Health and Human 

Services (2005) in its eleventh annual report on carcinogens, 

TCE is reasonably anticipated to be a human carcinogen based 

on limited human studies and sufficient evidence of carcinogenicity in experimental animals. An analysis of available epidemiological studies reports TCE exposure to be associated with 

several types of cancers in humans, especially kidney, liver, 

cervix, and lymphatic system (U.S. Environmental Protection 

Agency, 2001). These results are supported by recent molecular 

epidemiology studies showing specific renal cell mutations 

found primarily in renal cell carcinoma patients exposed to 

TCE. Animal studies have reported increases in lung, liver, 

kidney, and testicular tumors and lymphoma from inhalation 

and oral exposures to TCE in rats and mice (Agency for Toxic 

Substances and Disease Registry, 1997b).

The USEPA has set an MCL in drinking water of 5 μg/L 

for TCE to be protective of both cancer and noncancer health 

effects. The USEPA classifies TCE as a group B2 carcinogen 

(probable human carcinogen), indicating that sufficient 

evidence in animals exists to classify it as a carcinogen but 

evidence in humans is lacking (U.S. Environmental Protection 

Agency, 2004a). The USEPA currently is reassessing the 

human-health effects of TCE but the study will not be 

completed until 2006 or later (U.S. Environmental Protection 

Agency, 2005b).

Potential Sources of Solvents to Ground Water

There are numerous potential sources of solvents to 

ground water. Solvents in ground water can have two distinct 

sources—anthropogenic and natural. Anthropogenic sources of 

solvents are defined in this report as sources that ultimately 

have a human origin. For example, PCE released from a leaking 

underground storage tank (UST) and transported to ground 

water by gravitational flow would be considered to be an 

anthropogenic source of PCE. Although anthropogenic sources 

generally are the focus of most ground-water quality assessments, a fraction of the concentrations of solvents in ground 

water in some areas may have natural origins. Natural sources 

of solvents are defined in this report as sources that do not have 

a human origin. For example, methylene chloride that was 

formed in pine needles, leached from them, and transported to 

ground water through recharge would be considered a natural 

source.

It may be possible, through isotopic fractionation, to identify the component of solvents in ground water in some areas 

that may originate from anthropogenic sources compared to 

natural sources. However, separating the anthropogenic and 

natural components of solvent concentrations in ground water 

has not yet been attempted and would likely be difficult to 

achieve. Nonetheless, it is still useful to examine some of the 

possible anthropogenic and natural sources of solvents to 

ground water.

Anthropogenic Sources

A wide variety of anthropogenic sources can contribute 

solvents to ground water. Some commercial and industrial 

activities use solvents in large quantities and could be direct 

sources of solvents to ground water. These activities include 

chemical manufacturing, cold cleaning, metal degreasing/

cleaning, dry cleaning, textile production, paint removal/stripping, electronics and pharmaceutical manufacturing, and aerospace industries. In addition, small businesses in nearly every 

community use solvents in many activities including dry 

cleaning, machine cleaning, photographic processing, and 

printing.

Solvents can be released to the environment in many 

different ways and in either concentrated (point source) or 

dispersed (nonpoint source) discharges. Potential sources of 

concentrated discharges include leaks from underground or 

aboveground tanks used for storing solvents or hazardous waste 

containing solvents; leaks from chemicals drums or other 

smaller storage containers; leaks from distribution pipelines; 

spills at bulk loading or unloading facilities; spills from truck or 

rail transport; intentional disposal of waste materials; leaks 

from municipal landfills, settling ponds, or lagoons; leaks from 

hazardous waste storage, treatment, or disposal facilities; and 

large consumer releases of solvent-containing products or 

wastes. Disposal of some solvent-containing waste may have 

occurred in the past in underground injection wells. This waste 
Data and Methods 11

could then act as a concentrated source of solvents to ground 

water. However, underground disposal of waste containing any 

of the four solvents examined in this report was prohibited in 

1990 (U.S. Environmental Protection Agency, 2002).

Potential sources of dispersed discharges include leaks or 

releases from domestic septic systems; leaks from domestic 

waste disposal facilities; evaporative losses from storage tanks, 

transport pipelines, or transfer processes; land farming of 

contaminated waste or sludges; evaporative losses from manufacturing or cleaning processes; runoff from agricultural areas 

where solvents have been applied (such as adjuvants in pesticides) or deposited from air; releases from dry wells or drainage 

tiles; partitioning from air and wet deposition; and small 

consumer releases of solvent-containing products or wastes.

Natural Sources

Few natural sources have been identified for the four 

solvents examined here. Sinkkonen and others (1995) found 

concentrations of methylene chloride and PCE in samples of 

pine needles from an area in southern Finland near a metal scrap 

plant. However, the origin of the solvents was not clear. It is 

possible that the solvents were bio-accumulated by the trees 

from contaminated air or water at the site. In this case, the origin 

of the solvents would be anthropogenic and not natural. On the 

other hand, some or all of the concentrations could have been 

produced naturally by the trees.

Laturnus and others (2000) found TCE, PCE, and TCA in 

ambient air and soil air in a spruce forest in Denmark. In ground 

water in the same area, chloroform, PCE, and TCE frequently 

were detected. However, only the concentrations of chloroform 

were presumed to have a possible natural source. The authors 

concluded that the concentrations of PCE and TCE in ground 

water and TCA in air were consistent with global background 

air concentrations from anthropogenic sources.

Both TCE and PCE are produced by several temperate and 

subtropical species of marine microalgae (Abrahamsson and 

others, 1995). It has been concluded that the emission of these 

compounds by these algae is of such a magnitude that it should 

be considered in a global atmospheric chlorine budget. 

However, it is not clear if production of these compounds by 

marine algae has resulted in measurable concentrations of TCE 

or PCE in terrestrial ground water, although ground water in 

proximity to marine environments might be affected.

Haloalkanes like methylene chloride sometimes can be 

trapped in rocks. When the rocks are crushed during mining 

operations or weathering processes, small concentrations of 

some of these compounds can be released (Isidorov and others, 

1993). Methylene chloride, TCE, and PCE also have been 

detected in gas emissions from volcanoes around the world 

(Jordan and others, 2000). In some cases, the concentrations of 

these solvents were higher than their background atmospheric 

concentrations. Nonetheless, the water equilibrium concentrations were still quite low being on the order of 10-4 to 10-1 μg/L.

Thus, some measurable concentrations of these solvents in 

ground water could occur in the vicinity of volcanic emissions; 

however, the concentrations in ground water likely would 

decrease quickly away from the volcanic source.

Data and Methods

A variety of data sources, analysis methods, and statistical 

tests were used in this analysis of solvents. Much of the data 

used in this analysis came from the USGS NAWQA Program. 

Local water-quality assessments that included analyses for 

solvents formed the basis of the NAWQA Program and these 

areas were known as Study Units. Data on solvents as collected 

or compiled by the Study Units were then aggregated and 

analyzed on a comprehensive national basis. Data on solvents 

also were collected from a study of source water for community 

water systems (CWSs) located throughout the United States. 

Finally, data were compiled on solvents in drinking water from 

CWSs in 12 Northeast and Mid-Atlantic States.

Solvent Data Sets

The concentration data for solvents came from three 

sources: (1) the NAWQA Program that collected or compiled 

data from ground water throughout the United States from 

1993–2002, (2) a collaborative effort of the NAWQA Program 

and other organizations that sampled ground water used by 

CWSs as a source of drinking water (prior to treatment, if any) 

throughout the United States from 1999–2000, and (3) the 

NAWQA Program that compiled data on drinking water from 

CWSs in 12 Northeast and Mid-Atlantic States that were 

sampled from 1993–1998. For ease of understanding, the data 

from the NAWQA studies (source 1) will be referred to as 

ground water; the data from the source-water survey (source 2) 

will be referred to as source water; and the data from the 

drinking-water survey (source 3) will be referred to as drinking 

water.

Ground-Water Data

Data on solvents in ground water came from the NAWQA 

Program. The primary goals of NAWQA during its first decade 

of sampling (Cycle 1) were (1) to provide a nationally consistent description of current water-quality conditions for a large 

part of the Nation’s water resources; (2) to define long-term 

trends in water quality; and (3) to identify, describe, and 

explain, as possible, the major factors that affect water-quality 

conditions and trends.

Major elements of the design of ground-water quality 

sampling in Cycle 1 were aquifer and shallow ground-water 

studies (Gilliom and others, 1995). NAWQA conducted three 

types of studies in Cycle 1 that can be summarized as follows:
12 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

1. Aquifer Studies—These studies were broad assessments 

of the occurrence and distribution of contaminants in 

aquifers used for drinking-water supply. Originally 

called Study-Unit Surveys (Gilliom and others, 1995) 

and also called Major Aquifer Surveys, these studies 

sampled large areal and depth dimensions of the aquifer 

of interest. The potential sampling area was the extent of 

the aquifer, regardless of land use. Some existing 

network-based water-quality data, collected by other 

agencies, also were compiled. These data collected by 

other agencies are referred to as retrospective data and 

are considered to be very similar to NAWQA aquifer 

studies and have the same design characteristics and 

data-collection objectives (Lapham and Tadayon, 1996).

2. Shallow Ground-Water Studies in Urban Areas—These 

studies were assessments of the occurrence and 

distribution of contaminants in shallow ground water 

under large (population greater than 250,000) 

metropolitan areas. Originally classified as Land-Use 

Studies (Gilliom and others, 1995), the purpose of these 

studies was to define the quality of shallow ground water 

in urban land-use areas. Another important purpose of 

these studies was to determine which anthropogenic and 

natural factors most strongly affect the occurrence of 

contaminants in these shallow aquifers. In some cases, 

the focus was on recently recharged ground water in 

shallow aquifers underlying new residential/commercial 

regions (Squillace and Price, 1996). For studies in new 

residential/commercial areas, at least 75 percent of a 500-

meter (m) radial area around the well was required to be 

within an area of land use classified as new residential/

commercial (Squillace and Price, 1996).

3. Shallow Ground-Water Studies in Agricultural 

Areas—These studies were assessments of the 

occurrence and distribution of contaminants in shallow 

ground water underlying agricultural areas. Originally 

classified as Land-Use Studies (Gilliom and others, 

1995), the purpose of these studies was to define groundwater quality in recharge areas of shallow aquifers 

underlying a specific agricultural setting and to 

determine which anthropogenic and natural factors most 

strongly affect the occurrence of contaminants in these 

shallow aquifers (Gilliom and others, 1995).

Data from all three studies are included in this report as 

ground water. In some cases, results are given for ground water 

as a whole and sometimes by study. Identification of sampling 

sites in aquifer studies did not focus on a specific land use 

around the well. Thus, ground water sampled in the aquifer 

studies underlies a mixture of urban, agricultural, and other land 

uses and is hereafter referred to as a mixed land-use setting.

The characterization of water quality in each of the three 

studies generally was achieved by sampling 20 to 30 spatially 

distributed, randomly selected wells throughout each aquifer or 

shallow ground-water area of interest. For aquifer studies, 

existing wells were sampled to achieve a preliminary assessment (Gilliom and others, 1995). Domestic wells were selected 

preferentially over other types of wells, if available. Domestic 

wells were chosen because in most areas their distribution best 

fit the study objective of assessing the ground-water quality of 

aquifers using randomly selected and spatially distributed 

sampling points. Also, existing wells were preferred over installation of new monitoring wells because of the high costs 

involved in installing thousands of wells.

For shallow ground-water studies, however, many wells 

were installed by the NAWQA Program in order to meet the 

criteria for sampling the uppermost part of the ground-water 

system using low capacity or observation wells. In situations 

where new wells could not be installed, existing wells were 

sampled in shallow ground-water studies. In general, the wells 

sampled by NAWQA were not located in proximity to known 

point-source releases of solvents.

Ground-water samples collected from a total of 3,883 

wells in NAWQA aquifer or shallow ground-water studies were 

analyzed for one or more of the four solvents. The number of 

samples of ground water available for each solvent was as 

follows: methylene chloride, 3,877; PCE , 3,811; TCA, 3,883; 

and TCE, 3,879.

For ground water sampled by NAWQA, only one sample 

from each well was analyzed. The sample analyzed represented 

the first environmental sample collected from each well. 

Although some wells were re-sampled for analysis of trends in 

water quality, only one sample was included in this report 

because the goal was to provide a decadal assessment of 

ground-water quality without regard to temporal variations. If 

more than one environmental sample from a well was available, 

the sample collected most recently was selected. NAWQA 

sampled many different types of wells in its studies of VOCs in 

ground water. Although NAWQA occasionally sampled nested 

wells in special studies, no data from nested wells were used in 

this analysis. Thus, each well represents a distinct geographic 

site.

Because only one sample from each well was analyzed, the 

term sample, as used here, is synonymous with well. For 

various reasons, concentration information is not available for 

every solvent in every sample. The numbers of samples that 

have concentration results for each solvent in each NAWQA 

study type, by well type, are given in table 4.

Discussion of USGS field and laboratory procedures 

including sampling and handling, and laboratory analytical 

methods can be found in Moran and others (in press). All 

samples collected for NAWQA studies were analyzed at the 

USGS National Water Quality Laboratory (NWQL) near 

Denver, Colorado. Ground-water samples analyzed by the 

NWQL were quantified using gas chromatography/mass spectrometry (GC/MS). Methods of analysis used by the NWQL are 

described in Rose and Schroeder (1995) and Connor and others 

(1998). 
Table 4. Number of ground-water samples with concentration 

results for solvents by study type and well type for National WaterQuality Assessment data.

[PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Well type Methylene

chloride PCE TCA TCE

Aquifer studies

Public 321 320 321 321

Domestic 1,620 1,591 1,621 1,620

Monitoring 135 119 136 136

Other 234 234 235 235

Total 2,310 2,264 2,313 2,312

Shallow ground-water studies in urban areas

Public 7 7 7 7

Domestic 16 16 16 16

Monitoring 765 752 767 767

Other 56 57 57 57

Total 844 832 847 847

Shallow ground-water studies in agricultural areas

Public 1 1 1 1

Domestic 247 247 247 247

Monitoring 438 430 438 435

Other 37 37 37 37

Total 723 715 723 720

Grand total 3,877 3,811 3,883 3,879

Data and Methods 13

Samples collected by NAWQA were analyzed for as many 

as 86 different VOCs (Connor and others, 1998); however, only 

55 of these VOCs were included in NAWQA’s national assessment of VOCs (Bender and others, 1999). The four solvents 

examined in this report are among these 55 VOCs. The list of 

55 VOCs and the rationale and procedures for their selection 

can be found in Bender and others (1999).

A variety of quality control (QC) samples were collected 

during NAWQA sampling. The types of QC samples collected 

and their purpose are outlined in Moran and others (in press). 

The results of the QC samples were analyzed to determine if 

systematic contamination of environmental samples was indicated. If systematic contamination of environmental samples 

was indicated, environmental samples associated with 

suspected contamination were not used in this analysis.

NAWQA sampling did not cover all areas of the United 

States. Some local, State, and Federal agencies have collected 

data on VOCs in aquifer studies with the same design characteristics and data-collection objectives as the NAWQA aquifer 

studies. The NAWQA Program has compiled some of these 

retrospective data sets. These retrospective data are considered 

to be similar enough in design to the NAWQA aquifer studies 

to augment them and provide a broader national coverage of 

ground-water quality. 

Data on VOCs in ground water from more than 16,000 

wells from about 50 ambient monitoring programs or other 

studies have been compiled as part of this retrospective effort. 

Retrospective data were used to supplement NAWQA data only 

if they met specific criteria in terms of monitoring objectives, 

design, well construction, methods of sample collection, laboratory analysis, and quality control (Lapham and Tadayon, 

1996). Details of the design of the compilation effort and the 

sampling and analytical criteria required for selection of retrospective data can be found in Lapham and Tadayon (1996). 

Retrospective data from 1,185 wells have been included in 

this report to supplement the NAWQA aquifer study data. The 

number of samples of ground water available from retrospective data for each solvent was as follows: methylene chloride, 

1,177; PCE, 1,185; TCA, 1,185; and TCE, 1,185. Adding in the 

data from wells sampled by NAWQA, the total number of 

samples of ground water analyzed for each solvent was as 

follows: methylene chloride, 5,054; PCE, 4,996; TCA, 5,068; 

and TCE, 5,064.

Like NAWQA data, no samples were collected from 

nested wells in the retrospective data, and thus, each well represents a distinct geographic site. Only one sample of ground 

water from each well in the retrospective data was included in 

this report, and thus, the term sample is synonymous with well. 

If more than one sample from a well was available, the most 

recent sample was selected. Because of variations in design and 

data-collection objectives, all of the four solvents were not 

analyzed in all ground-water samples in the retrospective data 

set. The numbers of samples in the retrospective data that have 

concentration results for each solvent, by well type, are given in 

table 5.

For retrospective data, careful selection procedures were 

used to ensure their quality for inclusion in a national assessment. Only VOC analyses performed by GC/MS in a laboratory 

certified by the USEPA were included. Details of the requirements of the retrospective data for inclusion in the assessment 

have been published elsewhere (Lapham and Tadayon, 1996; 

Lapham and others, 2000).

Table 5. Number of ground-water samples with concentration 

results for solvents by well type for retrospective data.

[PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Well type Methylene

chloride PCE TCA TCE

Public 192 192 192 192

Domestic 515 517 517 517

Monitoring 194 199 199 199

Other 276 277 277 277

Total 1,177 1,185 1,185 1,185
14 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Retrospective data compiled by NAWQA were analyzed 

for as many as 55 of the VOCs included in NAWQA’s national 

assessment (Bender and others, 1999). However, in many cases 

samples were analyzed for a smaller subset of VOCs. For inclusion into the data used in this report, samples from retrospective 

data were required to be analyzed for at least 17 VOCs.

Source-Water Data

Data from another study were used to determine the occurrence, distribution, and status of solvents in ground water used 

as a source of drinking water for CWSs. This national sourcewater 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 & Sciences University, participating CWSs, and the USGS. The main purpose of this survey 

was to determine the occurrence and distribution of the gasoline 

oxygenate methyl tert-butyl ether (MTBE) in both ground and 

surface water that served as source water for CWSs. However, 

samples were analyzed for a suite of VOCs to determine their 

occurrence in source water for CWSs.

Two phases of the source-water survey, with respect to 

ground water, were accomplished: (1) a random survey of 579 

CWSs with ground-water sources throughout the country, and 

(2) a focused survey of 79 CWSs with ground-water solvent 

sources throughout the country that were considered susceptible 

to MTBE contamination (Clawges and others, 2001). In this 

analysis, only data from the random survey were included in 

order to avoid potential bias in the distribution of solvent 

sources. The selection of CWSs sampled in the random survey 

was statistically based and random and was stratified by 

population-served size category and total number of people 

served (Ivahnenko and others, 2001). More details on the design 

of the random survey were presented by Ivahnenko and others 

(2001). Results of the random survey were presented by Grady 

(2003). 

Detailed explanations of the field and laboratory methods 

used in the source-water survey are described in Koch and 

others (2003). Samples of source water were analyzed for 

VOCs at the MWDSC laboratory in LaVerne, California. VOC 

analyses were performed using USEPA approved method 524.2 

(Ivahnenko and others, 2001). Each sample from these wells 

was analyzed for 66 VOCs including each of the four solvents 

examined in this report.

All source-water wells were public-supply wells. Samples 

were collected at the wellhead, or as close to the wellhead as 

possible and before any treatment or blending. No samples were 

collected from nested wells in the source-water data, and thus, 

each well represents a distinct geographic site. Like NAWQA 

and retrospective data, only one sample of ground water from 

each well in the source-water data was included in this report, 

and thus, the term sample is synonymous with well. If more 

than one sample from a well was available, the most recent 

sample was selected. The number of samples of source water 

available for each solvent was as follows: methylene chloride, 

577; PCE, 577; TCA, 578; and TCE, 577.

Drinking-Water Data

Data on solvents in drinking water were compiled by the 

USGS from State drinking-water agencies in 12 Northeast and 

Mid-Atlantic States. 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 two regions. 

These two regions were selected because they are generally 

highly populated, have a long-term history of urbanization, and 

are areas with high use of public-water supply (Grady and 

Casey, 2001; Moran and others, 2001). A random subset of all 

CWSs in the two regions was selected using a statistical 

approach that stratified selection by State, source of water, and 

number of people served by the utility (Grady and Casey, 2001).

The drinking-water data represented CWSs with water 

supplied exclusively by surface water, supplied exclusively by 

ground water, or supplied by a combination of surface and 

ground water. In this report, only data from CWSs supplied 

exclusively by ground water were included. Data on solvents 

were available for a total of 1,682 CWSs supplied exclusively 

by ground water.

The drinking-water data represent samples collected to 

meet the monitoring requirements of the Safe Drinking Water 

Act (SDWA). The samples were collected from 1993–1998 and 

were analyzed by a variety of State and private laboratories. 

Most analyses for SDWA compliance use USEPA methods 

502.2 or 524.2 for determining VOC occurrence (Grady and 

Casey, 2001). In most cases, analyses for 21 regulated and 21 

unregulated VOCs are determined routinely in drinking-water 

samples analyzed for SDWA compliance (Grady and Casey, 

2001). Because the four solvents have MCLs, they are monitored in public drinking water regulated under the SDWA.

Multiple samples of drinking water from each CWS were 

available for the analysis period of 1993–1998. In this analysis, 

the occurrence of solvents was reported by CWS and not by 

sample because the number of samples available by CWS was 

highly variable. This variability can significantly skew the 

detection frequencies. Therefore, the data on drinking water 

were summarized by system. A CWS was counted as having a 

detection of a solvent if a measurable concentration of a solvent 

was reported in any one or more water samples for that CWS. 

However, concentration information on solvents in drinking 

water, such as median concentration, generally was computed 

using all samples available for each CWS. Using all detected 

concentrations avoided bias in selection of specific concentrations by CWS.

Solvent data were available for a total of 1,682 CWSs. The 

number of systems with data on each individual solvent in 

drinking-water samples was as follows: methylene chloride, 

1,680; PCE, 1,681; TCA, 1,678; and TCE, 1,681. 
Data and Methods 15

Computational Procedures

The detection frequency of solvents in ground water was 

computed in several ways. For each solvent in ground water, 

and for one or more solvents in ground water, uncensored detection frequencies were computed as the number of wells with a 

detection of the solvent, or one or more solvents, divided by the 

number of wells in which the solvent, or one or more solvents, 

was analyzed. Although the laboratory reporting levels of the 

individual solvents were variable, these uncensored detection 

frequencies represent the best estimate of the overall occurrence 

of each solvent in ground water. 

It has been shown that the detection frequency computed 

for a VOC is strongly dependent on the laboratory reporting 

level for the analyte (Lapham and others, 2000). Comparisons 

of detection frequencies between water-quality analytes that 

have different laboratory reporting levels may not reflect true 

differences in water quality. Instead, differences in detection 

frequencies between water-quality analytes may simply represent differences in instrument performance, instrument sensitivity, equipment operators, or laboratory conditions.

The laboratory reporting level is the minimum concentration value at which a laboratory will report the concentration of 

an analyte in water within a specified level of confidence. 

Although the laboratory reporting levels of individual solvents 

were variable, a good measure of the overall analytical resolution of each solvent is the median reporting level. Table 6 shows 

the median laboratory reporting level for each solvent in ground 

water as well as in source water and drinking water. The range 

in reporting levels for each solvent for each source of data also 

is shown in table 6.

For ground water, the median laboratory reporting levels 

of each solvent were the same. However, the range of laboratory 

reporting levels varied for each compound (table 6). This variability was probably due to differences in instrument performance (which often is compound dependent), instrument sensitivity, equipment operators, and laboratory conditions. 

For source water, the median laboratory reporting levels of 

each solvent were different from one another. Also, the range of 

laboratory reporting levels varied within each compound 

(table 6). Like ground-water data, this variability was probably 

due to differences in instrument performance, instrument sensitivity, equipment operators, and laboratory conditions. 

For drinking water, the median laboratory reporting levels 

of each solvent were the same. However, like ground- and 

source-water data, the range of laboratory reporting levels 

varied within each compound (table 6). In this case, the variability was probably due, in part, to factors such as differences 

in the laboratory procedures and reporting levels for the various 

State and private laboratories that performed the analyses. 

Although the laboratory reporting levels for solvents in ground 

water, source water, and drinking water differed and were quite 

variable, it was possible to identify and isolate several uniform 

reporting levels common to all three data sets.

When comparing among solvents or among data sources, 

detection frequencies and concentrations were computed using 

a uniform assessment level. An assessment level is a fixed 

concentration for comparing water-quality results for analytes 

that have variable laboratory reporting levels. By equalizing the 

concentration at which detection frequencies and concentrations are compared, differences between individual solvents 

and different data sets can be determined.

For comparisons of solvent detection frequencies between 

individual solvents in data from ground water and source water, 

an assessment level of 0.2 μg/L was used. The median laboratory reporting levels for all four solvents in ground water were 

0.2 μg/L (table 6). Although the median laboratory reporting 

levels for solvents in source water were less than 0.2 μg/L, all 

detected concentrations of VOCs in source water were assigned 

a minimum reporting level of 0.2 μg/L. This minimum 

reporting level was necessary because contamination of field 

quality-control samples in the source water data was common 

at concentrations less than 0.2 μg/L (David Bender, U.S. 

Geological Survey, written commun., 2005). Detected concentrations less than 0.2 μg/L in source water had low confidence 

of representing true environmental information. Thus, solvent 

data for source water had an effective laboratory censoring level 

of 0.2 μg/L.

Table 6. Median and range of laboratory reporting levels for solvents for each data source.

[μg/L, micrograms per liter; PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Compound

name

Median laboratory reporting level (μg/L)

Methylene chloride 0.2 0.042 0.5 0.02–16 0.03–0.18 0.031–3

PCE .2 .098 .5 0.02–0.5 0.05–0.19 0.037–4

TCA .2 .09 .5 0.02–0.5 0.05–0.19 0.044–3

TCE .2 .068 .5 0.02–0.5 0.04–0.19 0.03–3

Range in laboratory reporting levels (μg/L)

Ground

water

Source

water

Drinking

water

Ground

water

Source

water

Drinking

water
16 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

The median laboratory reporting levels for all four 

solvents in drinking water were 0.5 μg/L (table 6). Although 

this is higher than 0.2 μg/L, the range in the reporting levels of 

drinking water extended below 0.2 μg/L (table 6). Additionally, 

the detection frequencies of solvents in drinking water were not 

different between assessment levels of 0.5 μg/L and 0.2 μg/L. 

Therefore, for consistency with ground water and source water, 

an assessment level of 0.2 μg/L was used for comparisons of 

detection frequencies between individual solvents in drinking 

water.

For comparisons of detection frequencies of solvents 

among all three data sources, an assessment level of 0.2 μg/L 

was used. This assessment level was selected because detection 

frequencies of individual solvents in all three data sources were 

computed at this concentration, and the detection frequencies 

were assumed to be accurate at this concentration. Using an 

assessment level of 0.2 μg/L, rather than a lower assessment 

level, also allowed for results obtained in this report to be 

compared to data from other work with higher reporting levels. 

One additional assessment level for ground water was 

identified. Prior to April 1996, the laboratory reporting level 

used by the NWQL for most VOCs was 0.2 μg/L. Implementation of a new low-level analytical method after this date resulted 

in lower laboratory reporting levels for many compounds 

(Oblinger Childress and others, 1999). However, many 

compounds have laboratory reporting levels that vary as 

method changes are implemented or new instrumentation is 

used (Oblinger Childress and others, 1999). 

Table 7 presents the median and range in laboratory 

reporting levels for the solvents in ground water analyzed by the 

NWQL using the new low-level method. For the low-level 

method, the laboratory reporting level is defined as a level that 

confines the rate of false positives and false negatives to less 

than 1 percent (Oblinger Childress and others, 1999). Unlike 

the composite ground-water data, the median and ranges in 

laboratory reporting levels for the solvents in this data subset 

are lower and more variable (table 7). The higher median laboratory reporting level for methylene chloride compared to the 

other solvents (table 7) is a result of methylene chloride being a 

frequent laboratory contaminant during much of the analysis 

period. This required a higher reporting level for methylene 

chloride. In addition, the new low-level reporting conventions 

used by the NWQL allowed for quantification of VOC concentrations below the laboratory reporting levels. A review of the 

reporting conventions for the new low-level method can be 

found in Moran and others (in press).

The minimum laboratory reporting level for all four 

solvents analyzed with the new low-level method was near 

0.02 μg/L, and a large percentage of detected concentrations for 

each solvent were greater than or equal to 0.02 μg/L (table 7). 

Thus, an additional assessment level of 0.02 μg/L was applied 

to solvent concentrations determined using the new low-level 

method. This assessment level for low-level analyses is consistent with methods used in a national assessment of VOCs in the 

Nation’s aquifers by NAWQA (Moran and others, in press). 

The low-level analyses only were performed for a subset of all 

NAWQA-sampled wells analyzed after April 1996. The 

number of samples that had the low-level analyses for one or 

more solvents was 2,333. The number of samples that had 

low-level analyses for each individual solvent was as follows: 

methylene chloride, 2,331; PCE, 2,297; TCA, 2,333; TCE, 

2,332.

To accurately compute detection frequency at a specific 

assessment level, non-detect values above the assessment level 

should be removed from the data set (Lapham and others, 

2000). However, this can cause a substantial loss of data. For 

the analyses performed here, an unacceptable loss of data would 

occur using this method; therefore, a simplified approach to 

computing detection frequencies was used. Non-detect values 

above the assessment level were not removed from the analysis 

and were considered to be the same as non-detects below the 

assessment level. Using this approach did not result in detection 

frequencies significantly different from the approach where 

non-detects are removed. A detailed review of the assessment 

level concept and its application can be found in Lapham and 

others (2000).

Comparisons of detected concentrations between solvents 

and other VOCs also were made using an assessment level. This 

was done to ensure that comparisons were equitable between 

VOCs with variable laboratory reporting levels. For NAWQA 

data, comparisons of concentrations were made only with the 

low-level analyses, and with an assessment level of 0.02 μg/L. 

The low-level data were used because they were believed to 

more fully represent the range in environmental concentrations. 

For data on source water and drinking water, comparisons of 

concentrations between solvents and other VOCs were made at 

an assessment level of 0.2 μg/L. For comparisons of concentrations among all three data sets, an assessment level of 0.2 μg/L 

was used.

Table 7. Median and range of laboratory reporting levels for 

solvents analyzed using the National Water Quality Laboratory’s 

low-level method.

[μg/L, micrograms per liter; >, greater than or equal to; PCE, perchloroethene; 

TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Compound

name

Median

laboratory 

reporting level

(μg/L)

Range in

laboratory 

reporting levels

(μg/L)

Percent of 

quantified 

concentrations

>0.02 μg/L

Methylene 

chloride

0.38 0.06–7.57 73

PCE .1 0.027–0.5 61

TCA .032 0.032–0.5 57

TCE .038 0.038–0.5 71
Data and Methods 17

Mixtures of solvents also were computed using an assessment level. The detection frequencies of mixtures of solvents 

were determined by dividing the number of times that two or 

more unique combinations of solvents were detected in a 

sample by the number of times all solvents in the mixture were 

analyzed together, at a specific assessment level. The total 

number of unique combinations of two or more solvents 

detected in a sample is given by the equation (Sheskin, 1997):

i!

x!( ) i x – ! ----------------------

x = 2

i

∑

where

x = number of VOCs in mixture, and

i = number of detected VOCs.

For ground water, assessment levels of 0.2 μg/L and 0.02 μg/L 

were used in computing mixture detection frequencies and mixture median concentrations. For source water, an assessment 

level of 0.2 μg/L was used in computing mixture detection frequencies and mixture median concentrations. Because data on 

solvents in drinking water were summarized by CWS, an analysis of mixtures would not be meaningful and was not 

attempted.

An important point to note is that quantified concentrations of solvents less than 0.02 μg/L exist in the ground-water 

data set, although they generally were not considered when 

computing detection frequencies or examining concentrations 

at either assessment level. The presence of low concentration 

information on VOCs was a result of the unique, informationrich nature of the analytical method used by the NWQL. Details 

of the information-rich methodology used by the NWQL can be 

found in Connor and others (1998) and Oblinger Childress and 

others (1999).

When examining detection frequencies, concentrations, or 

frequencies of concentrations exceeding the MCL, the four 

solvents often were compared to the 55 VOCs that were 

included in NAWQA’s national assessment of VOCs (Bender 

and others, 1999). These VOCs were chosen through a rigorous 

identification and selection process and represent compounds 

that have important physical, chemical, and biological properties. Most of these VOCs also have enforceable drinking-water 

standards or other important human-health effects (Bender and 

others, 1999). Only 24 of the 55 examined VOCs have MCLs. 

The VOCs with MCLs and their MCL values are given in 

Appendix 1. The MCL for trihalomethanes (THMs) applies to 

the sum of all four THM compounds in a sample—chloroform, 

bromoform, bromodichloromethane, and dibromochloromethane. The MCL for xylenes applies to the sum of all three 

xylenes isomers in a sample—o-xylene, m-xylene, and 

p-xylene.

Comparisons of solvents to the 55 VOCs were limited to 

the ranking of each VOC with respect to detection frequencies, 

concentrations, or frequencies of concentrations exceeding the 

MCL. The ranking of each VOC for each of these statistics in 

each data source is given in Appendixes 2– 10. For ground 

water, most samples were analyzed for all of the 55 VOCs. 

Samples in the retrospective data sometimes were analyzed for 

fewer than the 55 VOCs, but each sample was required to be 

analyzed for a minimum of 17 of these compounds to be 

included in this report. For source water, samples were analyzed 

for only 52 of the 55 VOCs. For drinking water, samples were 

analyzed for only 51 of the 55 VOCs.

Statistical Tests

A variety of statistical tests were performed to analyze the 

data on solvents in ground water, source water, and drinking 

water to better understand their meaning. In all cases, nonparametric statistical tests were used because the data were not 

assumed to have any particular distribution. For all tests in this 

report, a level of statistical significance (α) of 0.05 was used, 

which is equivalent to a 95-percent confidence interval. 

Detailed information on the statistical tests used in this report 

can be found in Helsel and Hirsch (1992). 

In the case of two or more categorical variables, a 

Pearson’s chi-square test of independence, with Yates’ continuity correction, was performed. Yates continuity correction 

was used to give a more conservative test of significance. In the 

case of two independent groups of continuous data, a Wilcoxon 

rank-sum test, or Mann-Whitney test, was performed. In the 

case of matched pairs of continuous data, a Wilcoxon signedrank test was used. To determine a relation between two continuous variables, a Spearman correlation test was performed. For 

sample sizes <20, Kendall’s tau test was performed to measure 

the strength of a monotonic relation between two variables. 

For determining differences in the distributions of concentrations of solvents, quantile plots were used. Values less than 

the laboratory reporting level were given a default concentration value for plotting of 10-4. Quantile plots allow for the 

comparison of the distribution of all concentrations of two or 

more solvents. To test the statistical significance of distributional differences, the nonparametric Kolmogorov-Smirnov test 

was used.

For determining associations between multiple explanatory variables and the probability of occurrence of solvents in 

ground water, multivariate logistic regression analyses were 

used. The regression results only were used to determine associations between solvent occurrence and explanatory variables 

and to determine the strength and direction of the associations; 

the results were not used for predictive purposes. Explanatory 

variables were selected that could provide general insight or 

understanding into the sources, transport processes, or environmental mechanisms that influence or affect the occurrence of 

solvents. 

In logistic regression, the response or dependent variable is 

the occurrence, or non-occurrence, of one or more solvents or 

individual solvents. In its simplest form, the response variable 

is binary coded as 0 or 1, with 0 indicating a non-occurrence and 

1 indicating an occurrence. The explanatory variables are then 

related to the probability of occurrence of the response variable 
18 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

in a manner similar to linear regression. The magnitude and sign 

of the estimated slope coefficients determine the strength and 

direction of the association of explanatory variables with the 

probability of detecting solvents in water according to the 

following equation:

P e

β0 β1X1 ...βiXi ( ) + +

1 e

β0 β1X1 ...βiXi ( ) + +

+

= -------------------------------------------------

where

P = probability of detecting a solvent;

β0 = the 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 a solvent with an 

increase in the explanatory variable, whereas estimated coefficients with negative signs indicate a decrease in the probability 

of detecting a solvent with an increase in the explanatory variable. Uncorrected estimated slope coefficients do not give an 

accurate assessment of the strength of an association because 

the units of each variable, especially continuous variables, have 

large differences in magnitude and variance. Therefore, standardized coefficients were computed in order to compare the 

slope coefficients directly between one another. 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). Standardized coefficients were computed following 

Menard (2002).

Explanatory variables were entered into the logistic regression manually in a step-wise manner, and the regression was 

analyzed for significance at each step. For the overall regression, if the likelihood ratio of the model produced a p-value of 

<0.05, all explanatory variables were considered significantly 

associated with the probability of occurrence of a solvent. The 

significance of nested logistic regression models was tested 

using the partial likelihood ratio test. For cases where one additional coefficient was added, the Wald statistic of the coefficient 

was used to determine significance. 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 a solvent.

An additional test was performed to evaluate the logistic 

regression analyses. The Hosmer-Lemeshow test was used to 

assess how well the observed binary responses were predicted 

by the model equation (Hosmer and Lemeshow, 1989). The null 

hypothesis for this test is that the predicted responses are identical to the observed responses. Therefore, a higher p-value for 

this test indicates a better correspondence between the observed 

and predicted responses.

A variety of explanatory data were used in the logistic 

regression analyses. These data represented various hydrogeologic and anthropogenic factors that could control or influence 

the sources, transport, or fate of solvents in ground water. 

Table 8 lists the hydrogeologic and anthropogenic variables 

that were used in the logistic regression analyses. Included in 

this table are the units for each variable, a description of each 

variable, the coding of each variable in the regression analyses, 

and the source of the data for each variable. In the logistic 

regression analyses, density, percent, polygon, and point data 

were averaged for a 500-m radius around each well. In some 

cases, density data also were estimated for each well as an 

average for a 1-kilometer (km) radius around each well.

Occurrence of Solvents

The occurrence of solvents was examined in seven main 

sections: (1) the occurrence and distribution of solvents in 

ground water, (2) the occurrence and distribution of solvents in 

source water, (3) the occurrence and distribution of solvents in 

drinking water, (4) the concentrations of solvents in ground 

water, (5) the concentrations of solvents in source water, (6) the 

concentrations of solvents in drinking water, and 

(7) comparisons of the occurrence and concentrations of 

solvents among all three data sets.

Occurrence and Distribution of Solvents in

Ground Water

Without applying an assessment level, one or more of the 

four solvents were detected in 881 of 5,068 samples of ground 

water, or 17 percent. Without applying an assessment level, the 

detection frequencies of each solvent were as follows: PCE, 

11 percent; TCA, 7 percent; TCE, 5 percent; and methylene 

chloride, 3 percent. 

At assessment levels of 0.2 μg/L and 0.02 μg/L, the detection frequencies of one or more solvents in ground water were 

7 percent and 18 percent, respectively. Relative to the 55 VOCs 

included in NAWQA’s national assessment of VOCs, the 

ranking of detection frequencies of solvents in ground water 

were as follows: PCE, 3; TCE , 4; TCA, 5; and methylene chloride, 13 (Appendix 2). The assessment level used was 0.2 μg/L, 

and 43 of the 55 VOCs were detected in one or more samples at 

0.2 μg/L. The detection frequencies of the solvents were higher 

than most other VOCs and were in the top 5 most frequently 

detected VOCs, with the exception of methylene chloride.

The detection frequencies of each solvent in ground water, 

at an assessment level of 0.2 μg/L, are shown in figure 3. At this 

assessment level, PCE had the highest detection frequency 

followed by TCE, TCA, and methylene chloride. The detection 

frequencies of each solvent in ground water at an assessment 

level of 0.02 μg/L also are shown in figure 3. For each individual solvent, the detection frequency of solvents in ground 

water was higher at an assessment level of 0.02 μg/L than at 

an assessment level of 0.2 μg/L. At an assessment level of 

0.02 μg/L, the detection frequency of PCE was highest, 

followed by TCA, TCE, and methylene chloride.
Table 8. Hydrogeologic and anthropogenic variables that were used in the logistic regression analyses of solvents in 

ground water.

[mm/yr, millimeters per year; in/hr, inches per hour; g/cm3, grams per cubic centimeter; ft, feet; in., inch; °C, degrees Celsius; μS/cm, microsiemens per 

centimeter; km, kilometer; km2, square kilometer; yr, year; USGS, U.S. Geological Survey; GWSI, USGS Ground-Water Site Inventory; QWDATA, USGS 

Water-Quality Database; NAWQA, USGS National Water-Quality Assessment Program; USEPA, U.S. Environmental Protection Agency; CIESIN, Consortium 

for International Earth Science Information Network; TRI, Toxic Release Inventory; CERCLA, Comprehensive Environmental Response, Compensation, and 

Liability Act; RCRA, Resource Conservation and Recovery Act; S, source; T, transport; F, fate]

Variable (type) Units Description Coding Source

Hydrogeologic variables

Recharge (T) mm/yr average recharge continuous Wolock (2003)

Soil permeability (T) in/hr average soil permeability by soil unit continuous Wolock (1997)

Soil sand (T) percent average soil sand content by soil unit Wolock (1997)

Soil silt (T) percent average soil silt content by soil unit Wolock (1997)

Soil clay (T) percent average soil clay content by soil unit Wolock (1997)

Soil organic matter (T) percent average organic matter content by soil 

unit

Wolock (1997)

Soil bulk density (T) g/cm3 average bulk density of soil by soil 

unit

continuous Wolock (1997)

Soil erodibility (T) unitless soil erosion rate per soil erosion unit Wolock (1997)

Vertical soil permeability (T) in/hr average vertical soil permeability 

by soil unit

continuous Wolock (1997)

Hydric soils (T) percent average percent hydric soils by soil 

unit

Wolock (1997)

Available water capacity (T) fraction average water fraction by soil unit Wolock (1997)

Slope of land surface (T) percent average slope of land surface by

soil unit

Wolock (1997)

Seasonal water level (T) ft average depth to seasonally high 

water table by soil unit

continuous Wolock (1997)

Soil thickness (T) in. average depth to bedrock by soil unit continuous Wolock (1997)

Precipitation (T) cm average estimated annual 

precipitation

continuous U.S. Department of Commerce 

(1995)

Temperature (T) °C average estimated annual temperature continuous U.S. Department of Commerce 

(1995)

Well casing diameter (T) ft average casing diameter by well continuous GWSI

Well depth (T) ft total depth of well continuous GWSI

Water level (T) ft average water level by well GWSI

Depth to top of screened 

interval (T)

ft depth to the top of the screened 

interval in the well

continuous GWSI

Aquifer confinement (T) unitless confining condition of the aquifer 1 - unconfined; 

0.5 - mixed; 

0 - confined

GWSI

Temperature of ground water 

(F)

°C average temperature of water in 

the well

continuous GWSI

Dissolved oxygen (F) mg/L average dissolved oxygen content 

of water in the well

continuous QWDATA

Specific conductance (F) μS/cm average specific conductance of 

water in the well

continuous QWDATA

Occurrence of Solvents 19

0–100

0–100

0–100

0–100

0.02–0.69

0–100

0–1

0–100
Table 8. Hydrogeologic and anthropogenic variables that were used in the logistic regression analyses of solvents in 

ground water.—Continued

Variable (type) Units Description Coding Source

Anthropogenic variables

Population (S) persons/km2 block group population continuous Bureau of the Census (1991); 

CIESIN (1995); Dobson and 

others (2000)

Septic system sewerage (S) percent block group percent of households Bureau of the Census (1991)

Public sewerage (S) percent block group percent of households Bureau of the Census (1991)

Other sewerage (S) percent block group percent of households Bureau of the Census (1991)

Public water supply usage (S) percent block group percent of households Bureau of the Census (1991)

Domestic water supply usage 

(S)

percent block group percent of households Bureau of the Census (1991)

Households with drilled wells 

(S)

percent block group percent of households Bureau of the Census (1991)

Households with dug wells 

(S)

percent block group percent of households Bureau of the Census (1991)

Households with other water 

service (S)

percent block group percent of households Bureau of the Census (1991)

Year of home construction (S) yr block group median age of home 

construction

Bureau of the Census (1991)

Leaking gasoline underground 

storage tanks (S)

count number of sites within 1 km of well continuous Vista Information Solutions (1999)

Leaking dry-cleaning 

underground storage tanks 

(S)

count number of sites within 1 km of well continuous Vista Information Solutions (1999)

Gasoline underground storage 

tanks (S)

count number of sites within 1 km of well continuous Vista Information Solutions (1999)

Dry-cleaning underground 

storage tanks (S)

count number of sites within 1 km of well continuous Vista Information Solutions (1999)

Septic systems count number of sites within 1 km of well continuous Bureau of the Census (1991)

Urban land use (S) percent average land use around well USGS (1999); Wright and others 

(1995); Vogelmann and others 

(2001); C.V. Price, USGS, 

written commun. (2004)

Agricultural land use (S) percent average land use around well USGS (1999); Wright and others 

(1995); Vogelmann and others 

(2001); C.V. Price, USGS, 

written commun. (2004)

Undeveloped land use (S) percent average land use around well USGS (1999); Wright and others 

(1995); Vogelmann and others 

(2001); C.V. Price, USGS, 

written commun. (2004)

Roads (S) percent average percent road area around well Bureau of the Census (1991)

TRI sites (S) count number of sites within 1 km of well continuous USEPA (2004b)

CERCLA sites (S) count number of sites within 1 km of well continuous USEPA (2004b)

RCRA sites (S) count number of sites within 1 km of well continuous USEPA (2004b)

20 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

0–100

0–100

0–100

0–100

0–100

0–100

0–100

0–100

0–100

0–100

0–100

0–100

0–100
0

1

2

3

4

5

6

7

8

9

10

11

12

DETECTION FREQUENCY, IN PERCENT

2,297

2,332

2,331

2,333

4,996

5,068

5,064

5,054

Assessment level of 0.02 microgram per liter

Assessment level of 0.2 microgram per liter

2,297 Number of samples

Perchloroethene Trichloroethene Methylene chloride 1,1,1-Trichloroethane

Occurrence of Solvents 21

Figure 3. Detection frequencies of four solvents in ground water.

The detection frequencies of individual solvents by 

NAWQA study type and at an assessment level of 0.2 μg/L are 

illustrated in figure 4. At an assessment level of 0.2 μg/L, one 

or more solvents were detected in samples from 218 of 3,498 

wells (6 percent) in aquifer studies, 120 of 847 wells 

(14 percent) in shallow ground-water studies in urban areas, 

and 12 of 723 wells (2 percent) in shallow ground-water studies 

in agricultural areas. The detection frequency of each solvent 

was higher in urban areas and lower in agricultural areas 

compared to aquifer studies (fig. 4).

The pattern of detection frequencies of solvents was 

similar in shallow ground-water studies in urban areas and 

aquifer studies with detection frequencies highest for PCE, 

followed by TCA, TCE, and methylene chloride (fig. 4). 

However, the pattern of solvent detections was different in 

shallow ground-water studies in agricultural areas with detection frequencies highest for methylene chloride followed by 

TCE, PCE, and TCA. The source of PCE in studies in urban 

areas and in aquifer studies probably is releases of the solvent 

from industrial and commercial facilities where it is used, like 

dry cleaners. The source of methylene chloride in agricultural 

areas could be the transformation of carbon tetrachloride, which 

was used as a fumigant in grain storage bins (U.S. Department 

of Agriculture, 2005).

At an assessment level of 0.02 μg/L, one or more solvents 

were detected in samples from 248 of 1,687 wells (15 percent) 

in aquifer studies, 159 of 548 wells (29 percent) in studies in 

urban areas, and 19 of 98 wells (19 percent) in studies in 

agricultural areas. The detection frequency of each individual 

solvent, with the exception of methylene chloride, was higher in 

studies in urban areas than in aquifer studies (fig. 5). The detection frequencies of PCE and TCA were higher in studies in agricultural areas than in aquifer studies. The source of low concentrations of PCE and TCA in studies in agricultural areas is not 

clear but these compounds may be active or inert ingredients in 

pesticide formulations (Grady and Mullaney, 1998). Few 

samples from studies in agricultural areas were available for 

analyses of solvents at 0.02 μg/L. The samples were located in 

only a few areas including eastern Iowa, eastern Tennessee, the 

Puget Sound area of Washington, and parts of New Jersey.

The distribution of detection frequencies of each solvent 

by each individual NAWQA study at an assessment level of 

0.02 μg/L is shown in figure 6. The detection frequencies were 

computed for each individual study and grouped into study 

types (urban areas, agricultural areas, and aquifer studies). The 

detection frequencies of individual solvents by individual study 

were highly variable. In most cases, the median detection 

frequency of solvents by individual study was higher in urban 

areas than in either agricultural areas or aquifer studies. The 

number of aquifer studies sampled was 57, and the number of 

shallow ground-water studies in urban areas sampled was 20. 

However, only four shallow ground-water studies in agricultural areas were analyzed for solvents using the low-level 

methods; therefore, the median detection frequencies for the 

agricultural areas, as shown in figure 6, are only estimates.
0

1

2

3

4

5

6

7

8

9

10

DETECTION FREQUENCY, IN PERCENT

Shallow ground-water underlying urban areas

Aquifer studies (mixed land use)

832 Number of samples

Shallow ground-water underlying agricultural areas

832

847

847

844

3,449

3,497

3,498

3,487

715 720

723 723

Perchloroethene Trichloroethene Methylene chloride 1,1,1-Trichloroethane

22 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 4. Detection frequencies of four solvents in ground water from National Water-Quality Assessment studies at an 

assessment level of 0.2 microgram per liter.

0

2

4

6

8

10

12

14

16

18

DETECTION FREQUENCY, IN PERCENT

543

548

548

548

1,685

1,656

1,687

1,686

98

98

98

98

Perchloroethene Trichloroethene Methylene chloride 1,1,1-Trichloroethane

Shallow ground-water underlying urban areas

Aquifer studies (mixed land use)

543 Number of samples

Shallow ground-water underlying agricultural areas

Figure 5. Detection frequencies of four solvents in ground water from National Water-Quality Assessment studies at an 

assessment level of 0.02 microgram per liter.
0

10

20

30

40

50

60

70

80

90

100

DETECTION FREQUENCY, IN PERCENT

Shallow ground water

underlying agricultural areas 

Aquifer studies Shallow ground water

underlying urban areas

Median

Methylene chloride

Perchloroethene

1,1,1-Trichloroethane

Trichloroethene

Occurrence of Solvents 23

Figure 6. Detection frequency of four solvents by individual study type at an assessment level of 0.02 microgram 

per liter.

The distribution of wells sampled for ground water for any 

of the four solvents and the distribution of wells where one or 

more solvents were detected is shown in figure 7. Without 

applying an assessment level, the detections of solvents were 

spread throughout the conterminous United States. Detections 

of solvents seemed to be concentrated in the Northeast region 

and several other locations such as eastern Iowa and California.

The distribution of wells sampled for each of the four 

solvents and the distribution of wells where each solvent was 

detected are shown in figures 8–11 for methylene chloride, 

PCE, TCA, and TCE, respectively. Without applying an assessment level, the detections of each solvent were spread throughout the conterminous United States. Detections of methylene 

chloride (fig. 8) appear to be concentrated in the Great Lakes 

area and eastern Iowa whereas detections of PCE (fig. 9) appear 

to be concentrated in the Northeast region and in eastern Iowa. 

Detections of TCA (fig. 10) and TCE (fig. 11) appear to be 

concentrated in the Northeast Region.

One or more of the four solvents examined in this report 

dominated mixtures of the 55 VOCs in ground water from 

aquifer studies. At an assessment level of 0.2 μg/L, 9 of the 10 

most frequently occurring mixtures of VOCs in aquifer studies 

contained one or more of the four solvents (Zogorski and others, 

in press). The most frequently occurring mixture in aquifer 

studies was PCE-TCE, occurring in about 1.5 percent of 

samples (Zogorski and others, in press).

Mixtures were a common mode of occurrence among the 

four solvents in samples of ground water. At an assessment 

level of 0.2 μg/L, 30 percent of detections of any of the four 

solvents were mixtures of two or more solvents. At an assessment level of 0.02 μg/L, 27 percent of detections of any of the 

four solvents were mixtures of two or more solvents. However, 

when examined relative to the total number of samples of 

ground water, the detection frequency of mixtures was only 

2 percent at an assessment level of 0.2 μg/L, and 5 percent at an 

assessment level of 0.02 μg/L.

Eleven unique mixtures of the four solvents are possible. 

The detection frequencies of each potential mixture in all 

ground-water samples are shown in table 9. Detection frequencies are given in table 9 at assessment levels of 0.2 and 

0.02 μg/L for all samples and for a subset of samples with 

detections of one or more solvents.
Oahu

EXPLANATION

Well sampled with no detection of a solvent

Well sampled with detections of one or more solvents

Alaska

Hawaii

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data,

1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

24 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 7. Locations where ground water was sampled for methylene chloride, perchloroethene, 1,1,1-trichloroethane, or trichloroethene and where one or more solvents were detected using no assessment level.

Oahu

EXPLANATION

Well sampled with no detection of methylene chloride

Well sampled with detection of methylene chloride

Alaska

Hawaii

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data,

1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

Figure 8. Locations where ground water was sampled for methylene chloride and where methylene 

chloride was detected using no assessment level.
Oahu

EXPLANATION

Well sampled with no detection of PCE

Well sampled with detection of PCE

Alaska

Hawaii

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data,

1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

Occurrence of Solvents 25

Figure 9. Locations where ground water was sampled for perchloroethene (PCE) and where PCE 

was detected using no assessment level.

Oahu

EXPLANATION

Well sampled with no detection of TCA

Well sampled with detection of TCA

Alaska

Hawaii

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data,

1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

Figure 10. Locations where ground water was sampled for 1,1,1-trichloroethane (TCA) and where 

TCA was detected using no assessment level.
Oahu

EXPLANATION

Well sampled with no detection of TCE

Well sampled with detection of TCE

Alaska

Hawaii

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data,

1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

26 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 11. Locations where ground water was sampled for trichloroethene (TCE) and where 

TCE was detected using no assessment level.

The combination of PCE and TCE was the most frequently 

occurring mixture at both assessment levels. At an assessment 

level of 0.02 μg/L, the mixture of PCE and TCA occurred as 

frequently as PCE and TCE. This may be due to the common 

usage of both PCE and TCA in metal degreasing and dry 

cleaning, and thus, common sources for both solvents (Halogenated Solvents Industry Alliance, 1994, 2005). In general, the 

most frequently occurring solvent mixtures were the same at 

assessment levels of 0.2 and 0.02 μg/L (table 9).

Occurrence and Distribution of Solvents in

Source Water

At an assessment level of 0.2 μg/L, one or more solvents 

were detected in 36 of 579 samples of source water, or 

6 percent. The detection frequencies of individual solvents were 

as follows: PCE, 4 percent; TCE, 3 percent; TCA, 2 percent; 

and methylene chloride, 0.2 percent (one sample). 

The detection frequencies of the individual solvents in 

source water were higher than most other VOCs and were in the 

top 10 most frequently detected VOCs, with the exception of 

methylene chloride. Relative to 52 VOCs analyzed for in the 

source-water samples, the ranking of detection frequencies of 

solvents in source water was as follows: PCE, 6; TCE, 7; TCA, 

10; and methylene chloride, 28 (Appendix 3). The assessment 

level used was 0.2 μg/L, and 40 of the 52 VOCs were detected 

in one or more samples. 

The pattern of occurrence and detection frequencies of 

individual solvents in source water was very similar to that of 

ground water at an assessment level of 0.2 μg/L. This is 

expected as the quality of source water from ground water 

should be nearly the same as that of the ground-water resource 

itself.

Figure 12 shows the distribution of detections of one or 

more of the four solvents in source water throughout the United 

States. The detections of solvents in source water were distributed throughout the country. Because of the small number of 

detections for each individual solvent, their geographic distributions are not shown in this report.

Unlike ground water, solvents did not dominate mixtures 

involving VOCs in source water. Of the five most frequently 

occurring VOC mixtures in public well samples, of which the 

source-water survey was a major component, none contained 

one or more of the four solvents (Zogorski and others, in press). 

In public well samples, the most frequently occurring mixtures 

were various combinations of trihalomethanes (Zogorski and 

others, in press). This suggests different sources of VOCs to 

public wells than to ground water from aquifer studies.
Table 9. Detection frequencies and concentrations of 11 solvent mixtures in ground water nationwide at assessment levels of 

0.2 microgram per liter and 0.02 microgram per liter.

[μg/L, micrograms per liter; PCE, perchloroethene; TCE, trichoroethene; TCA, 1,1,1-trichoroethane]

Solvent

mixture

Assessment level of 0.2 μg/L Assessment level of 0.02 μg/L

Detection frequency in ground water

(percent)

PCE

TCE

PCE

TCA

TCE

TCA

PCE

TCE

TCA

PCE

Methylene chloride

TCE

Methylene chloride

TCA

Methylene chloride

PCE

TCE

Methylene chloride

PCE

TCA

Methylene chloride

TCE

TCA

Methylene chloride

PCE

TCE

TCA

Methylene chloride

Occurrence of Solvents 27

Median

concentration

of each solvent

in mixture

(μg/L)

Detection frequency in ground water

(percent) Median

concentration

of each solvent

in mixture

(μg/L)

Denominator is

all samples in

which solvents 

were analyzed

Denominator is 

samples in

which one or

more solvents

were detected

Denominator is

all samples in

which solvents 

were analyzed

Denominator is 

samples in

which one or

more solvents

were detected

1.5 21

2.0

2.6 14

0.17

1.0 0.14

0.9 13

1.1

2.6 14

0.07

0.4 0.07

0.8 11

1.5

1.3 7.0

0.14

0.8 0.06

0.5 7.7

2.3

1.1 5.9

0.15

2.0 0.15

0.8 0.06

0.2 2.6

0.7

0.6 3.3

0.16

1.7 0.09

0.2 2.3

1.3

0.2 1.2

0.15

1.8 0.06

0.1 1.7

0.7

0.3 1.9

0.13

0.6 0.04

0.1 2.0

0.9

0.2 1.2

0.32

1.4 0.15

1.9 0.06

0.1 1.1

2.4

0.1 0.7

0.39

0.8 0.08

25 0.05

0.1 1.4

1.1

0.1 0.7

1.13

0.8 0.08

0.7 0.05

0.1 1.1

2.4

0.1 0.7

0.39

4.3 1.13

0.8 0.08

25 0.05
Oahu

EXPLANATION

Well sampled with no detection of one or more solvents

Well sampled with detection of one or more solvents

Alaska

Hawaii

0 200 400 MILES

0 200 400 KILOMETERS

Base modified from U.S. Geological Survey digital data,

1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

28 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 12. Locations where source water from ground water was sampled for methylene chloride, 

perchloroethene, 1,1,1-trichloroethane, or trichloroethene and where one or more solvents were 

detected at an assessment level of 0.2 microgram per liter.

Of the 36 samples of source water with one or more 

solvents, 15 contained two or more solvents. This is 42 percent 

of the samples with one or more solvents and about 3 percent of 

all samples of source water that were analyzed. The detection 

frequencies of each potential mixture in source water are shown 

in table 10. Only four mixtures of solvents were detected in 

source water at an assessment level of 0.2 μg/L. The mixture of 

PCE and TCE was most frequently occurring (table 10). No 

mixtures with methylene chloride occurred because this solvent 

only was detected once and not with any other solvent.

Occurrence and Distribution of Solvents in

Drinking Water

At an assessment level of 0.2 μg/L, one or more of the four 

solvents were detected in samples of drinking water from 177 

of 1,682 CWSs, or 11 percent, in 12 Northeast and Mid-Atlantic 

States. The detection frequencies of individual solvents were as 

follows: TCA, 5 percent; PCE, 4; percent; TCE, 4 percent; and 

methylene chloride, 3 percent. Because the drinking-water data 

consisted of multiple samples of solvents from each CWS, a 

CWS was counted as having a detection of a solvent if any 

sample in the CWS contained a detection of the solvent at the 

specified assessment level.

The detection frequencies of the individual solvents, with 

the exception of methylene chloride, in drinking water were 

higher than most other VOCs and were in the top 10 most 

frequently detected VOCs. Relative to 51 VOCs, the ranking of 

detection frequencies of solvents in drinking water was as 

follows: TCA, 6; PCE, 7; TCE, 8; and methylene chloride, 11 

(Appendix 4). The assessment level used was 0.2 μg/L, and 44 

of the 51 VOCs were detected in one or more samples at this 

assessment level. 

Figure 13 shows the distribution of drinking water from 

CWSs sampled for solvents and the distribution of CWSs in 

which one or more solvents were detected. The data on solvents 

in drinking water from CWSs were only available for 12 Northeast and Mid-Atlantic States. The majority of solvent detections 

appeared to be in the more heavily populated areas of the 

regions along the coast.

The distribution of drinking water from CWSs where each 

of the four solvents was analyzed and the distribution of CWSs 

where each solvent was detected are shown in figures 14–17 for 

methylene chloride, PCE, TCA, and TCE, respectively. For the 

most part, no geographic pattern of occurrence was obvious in 

the distributions of detections of individual solvents. Because 

the occurrence of solvents in drinking water was reported by 

CWS and not by sample, no meaningful analysis of mixtures of 

solvents in drinking water could be performed.
Table 10. Detection frequencies of 11 solvent mixtures in source water nationwide at an 

assessment level of 0.2 microgram per liter.

[NA, not applicable; μg/L, microgram per liter; PCE, perchloroethene; TCE, trichoroethene; TCA, 1,1,1-trichoroethane]

Detection frequency in source water (percent)

Median

concentration

of each solvent

in mixture

(μg/L)

Denominator is

all samples in

which solvents

were analyzed

Denominator is 

samples in

which one or

more solvents

were detected

PCE

2.1 33

4.6

TCE 2.7

TCE

0.9 14

4.2

TCA 7.6

PCE

0.7 11

2.8

TCA 3.9

PCE

0.5 8.3

4.8

TCE 8.7

TCA 7.6

PCE

0 0 NA

Methylene chloride

TCE

0 0 NA

Methylene chloride

TCA

0 0 NA

Methylene chloride

PCE

TCA 0 0 NA

Methylene chloride

PCE

TCE 0 0 NA

Methylene chloride

TCE

TCA 0 0 NA

Methylene chloride

PCE

0 0 NA

TCE

TCA

Methylene chloride

Occurrence of Solvents 29

Solvent mixture
EXPLANATION

CWS sampled with no detection of one or more solvents

CWS sampled with detection of one or more solvents

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

0 125 250 MILES

0 125 250 KILOMETERS

30 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 13. Locations of community water systems (CWSs) that were sampled for methylene chloride, 

perchloroethene, 1,1,1-trichloroethane, or trichloroethene and where one or more solvents were detected 

at an assessment level of 0.2 microgram per liter.
EXPLANATION

CWS sampled with no detection of methylene chloride

CWS sampled with detection of methylene chloride

0 125 250 MILES

0 125 250 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

Occurrence of Solvents 31

Figure 14. Locations of community water systems (CWSs) that were sampled for methylene chloride and where 

methylene chloride was detected at an assessment level of 0.2 microgram per liter.
EXPLANATION

CWS sampled with no detection of PCE

CWS sampled with detection of PCE

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

0 125 250 MILES

0 125 250 KILOMETERS

32 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 15. Locations of community water systems (CWSs) that were sampled for perchloroethene (PCE) 

and where PCE was detected at an assessment level of 0.2 microgram per liter.
EXPLANATION

CWS sampled with no detection of TCA

CWS sampled with detection of TCA

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

0 125 250 MILES

0 125 250 KILOMETERS

Occurrence of Solvents 33

Figure 16. Locations of community water systems (CWSs) that were sampled for 1,1,1-trichloroethane (TCA) 

and where TCA was detected at an assessment level of 0.2 microgram per liter.
EXPLANATION

CWS sampled with no detection of TCE

CWS sampled with detection of TCE

Base modified from U.S. Geological Survey digital data, 1:2,000,000, 1990

Albers Equal-Area Projection

North American Datum of 1983

0 125 250 MILES

0 125 250 KILOMETERS

34 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 17. Locations of community water systems (CWSs) that were sampled for trichloroethene (TCE) 

and where TCE was detected at an assessment level of 0.2 microgram per liter.
Occurrence of Solvents 35

Concentrations of Solvents in Ground Water

At an assessment level of 0.02 μg/L, the four solvents were 

ranked as follows with respect to the median quantified concentrations of 55 VOCs in ground water nationwide: TCE, 12; 

PCE, 20; TCA, 27; and methylene chloride, 30 (Appendix 5). 

Forty-seven of the 55 VOCs had at least one quantified concentration of 0.02 μg/L or greater. The solvents do not have the 

largest concentrations among the 47 VOCs with quantified 

concentrations and instead tend to appear near the middle of 

the rankings. The median quantified concentrations of solvents 

in ground water were as follows: TCE, 0.12 μg/L; PCE, 

0.07 μg/L; TCA, 0.06 μg/L; and methylene chloride, 0.05 μg/L. 

These values were well below the USEPA MCLs (Appendix 1).

A more complete way to examine the concentrations of 

solvents in ground water relative to one another is shown in 

figure 18. The symbols in the graph in figure 18 illustrate quantiles of concentrations of 0.02 μg/L or greater for each solvent. 

All concentrations less than 0.02 μg/L were assigned a default 

concentration value of 10-4 for plotting.

At an assessment level of 0.02 μg/L, concentrations of 

PCE were highest followed by TCE, then TCA and finally 

methylene chloride (fig. 18). It is interesting to note in figure 18 

that, although concentrations of TCE were higher than TCA 

overall, the concentrations of TCA were higher than concentrations of TCE less than about 0.08 μg/L. Because the median 

laboratory reporting levels for these two compounds using the 

NWQL low-level method were very similar (table 6), the 

change in the concentration distributions between TCE and 

TCA may reflect a true difference in the environmental distribution of concentrations of these two solvents.

Relative to 24 VOCs measured in ground water that have 

MCLs, the ranking of the four solvents with respect to the 

frequency of concentrations greater than MCLs in ground water 

nationwide was as follows: PCE, 1; TCE, 3; and methylene 

chloride, 9 (Appendix 6). Unlike median quantified concentrations, the frequencies of concentrations greater than MCLs for 

solvents were higher than most other VOCs, with the exception 

of TCA, which did not exceed the MCL in samples of ground 

water.

The frequencies of concentrations of each of the four 

solvents in ground water that were greater than their MCLs 

were as follows: PCE, 0.7 percent; TCE, 0.4 percent; methylene chloride, 0.06 percent; and TCA, 0 percent. The numbers 

of samples in which concentrations were greater than the MCL 

for each solvent were as follows: PCE, 37; TCE, 20; methylene 

chloride, 3; and TCA, 0. Table 11 lists the number of concentrations greater than the MCL for each solvent in ground water by 

well type. For PCE, the majority of samples with concentrations 

greater than the MCL were in monitoring or other types of 

wells. For TCE, equal numbers of samples with concentrations 

greater than the MCL occurred in monitoring or domestic wells 

compared to public or other types of wells. For methylene chloride, two samples from public wells and one sample from a 

monitoring well had concentrations that were greater than the 

MCL.

CONCENTRATION, IN MICROGRAM PER LITER

0.85

0.90

0.0001 0.001 0.01 0.1 1 10 100

0.95

1.00

FRACTION OF DATA

Trichloroethene

1,1,1-Trichloroethane

Perchloroethene

Methylene chloride

Figure 18. Quantiles of concentrations of each of four solvents in 

ground water at an assessment level of 0.02 microgram per liter.

The median concentrations of solvents in mixtures in 

ground water are given in table 9. Most concentrations were 

less than about 2 μg/L. Several concentrations of methylene 

chloride in mixtures were relatively high because there were 

few samples with these mixtures, and the concentrations of 

methylene chloride were high in the samples containing these 

mixtures. No trend is apparent in the median concentration of 

solvents in a mixture compared to the frequency of occurrence 

of the mixture. Also, the median concentration of solvents in 

mixtures at an assessment level of 0.02 μg/L was about one 

order of magnitude less than the median concentration of 

solvents in mixtures at an assessment level of 0.2 μg/L (table 9).

Table 11. Number of exceedances of MCLs for each solvent in 

ground water nationwide by well type.

[PCE, perchloroethene; TCE, trichoroethene]

Well type

Solvent

PCE TCE Methylene 

chloride

Public

Domestic

Monitoring

Other

Total

3 42

5 60

15 6 1

14 4 0

37 20 3
36 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Concentrations of Solvents in Source Water

The four solvents were ranked as follows with respect to 

the median quantified concentrations of 52 VOCs in source 

water nationwide: TCA, 3; methylene chloride, 8; TCE, 10; 

and PCE, 16 (Appendix 7). The assessment level used was 

0.2 μg/L, and 40 of the 55 VOCs had at least one quantified 

concentration of 0.2 μg/L or greater. Unlike the ground-water 

samples, solvents in source-water samples were in the upper 

one-half of the rankings with respect to the median of quantified 

concentrations of 40 VOCs with at least one quantified concentration. The median quantified concentrations of solvents in 

source water were as follows: TCA, 4.4 μg/L; methylene chloride, 1.7 μg/L; TCE, 1.5 μg/L; and PCE, 0.9 μg/L. Although 

these median values are less than MCLs (Appendix 1), the 

median values for methylene chloride and TCE are only about 

three times lower.

Figure 19 shows the quantiles of concentrations of each 

solvent in source water at an assessment level of 0.2 μg/L. 

Values less than 0.2 μg/L were assigned a default value of 10-4

for plotting. Based on the quantiles, the concentrations of PCE 

in source water generally were highest followed by TCE, TCA, 

and methylene chloride. However, between about 0.5 and 

1.0 μg/L, the concentrations of TCE were higher than the 

concentrations of PCE. The highest concentration of a solvent 

in source water was for TCE.

Relative to 23 VOCs measured in source water that have 

MCLs, the rankings of the four solvents with respect to the 

frequency with which concentrations were greater than their

0.90

0.93

0.92

0.91

0.0001 0.001 0.01 0.1 1 10 100

0.99

0.98

0.97

0.96

0.95

0.94

1.00

FRACTION OF DATA

CONCENTRATION, IN MICROGRAM PER LITER

Trichloroethene

1,1,1-Trichloroethane

Perchloroethene

Methylene chloride

Figure 19. Quantiles of concentrations of each of four solvents in 

source water at an assessment level of 0.2 microgram per liter.

MCLs in source water nationwide was as follows: PCE, 1; TCE, 

2; TCA, 6; and methylene chloride, 6 (Appendix 8). Neither 

TCA nor methylene chloride had concentrations greater than 

the MCL in any samples of source water. In source water, only 

five VOCs had concentrations greater than MCLs, and two of 

them were solvents with PCE ranking first and TCE ranking 

second.

The frequencies of concentrations that were greater than 

their MCLs for each of the four solvents in source water were 

as follows: PCE, 1.1 percent; TCE, 0.9 percent; TCA, 

0 percent; and methylene chloride, 0 percent. The numbers of 

samples in which concentrations were greater than the MCL for 

each solvent were as follows: PCE, 6; TCE, 5; TCA, 0; and 

methylene chloride, 0. For PCE, the locations of public wells 

from which samples had concentrations greater than the MCL 

were as follows: California, 2; New York, 2; Ohio, 1; and 

Pennsylvania, 1. For TCE, the locations of public wells from 

which samples had concentrations greater than the MCL 

were as follows: New York, 2; Iowa, 1; California, 1; and 

Pennsylvania, 1. 

The concentrations of solvents in mixtures in source water 

are given in table 10. In contrast to ground-water samples, most 

concentrations of solvents in mixtures in source-water samples 

were greater than 2 μg/L. No trend was apparent in the median 

concentration of solvents in a mixture compared to the 

frequency of occurrence of the mixture (table 10).

Concentrations of Solvents in Drinking Water

The four solvents were ranked as follows with respect to 

the median quantified concentrations of 51 VOCs in drinking 

water: TCE, 8; PCE, 11; TCA, 25; and methylene chloride, 31 

(Appendix 9). Because the drinking-water data consisted of 

multiple samples from each CWS, the median value of quantified VOC concentrations for each CWS was the median of all 

quantified VOC concentrations from that CWS. The assessment 

level used was 0.2 μg/L, and 44 of the 55 VOCs had at least one 

quantified concentration of 0.2 μg/L or greater. With respect to 

median quantified concentrations, solvents in drinking water 

ranked in the upper one-half of the 49 VOCs with at least one 

quantified concentration, with the exception of TCA and 

methylene chloride. The median quantified concentrations of 

solvents in source water were as follows: TCE, 1.5 μg/L; PCE, 

1.4 μg/L; TCA, 1 μg/L; and methylene chloride, 1 μg/L. 

Although these median values were less than MCLs, the median 

values for PCE and TCE were only about three times lower.

Figure 20 shows the quantiles of concentrations of each 

solvent in drinking water at an assessment level of 0.2 μg/L. 

Concentrations of TCA in drinking water were highest followed 

by PCE, TCE, and methylene chloride; however, the concentrations of TCA, PCE, and TCE were nearly equal. The quantified 

concentration for each CWS was the median of quantified 

concentrations if multiple quantified concentrations from a 

CWS were available. Values less than 0.2 μg/L were given a 

default concentration value of 10-4 for plotting.
CONCENTRATION, IN MICROGRAM PER LITER

0.980

0.985

0.0001 0.001 0.01 0.1 1 10 100

0.995

0.990

1.000

FRACTION OF DATA

Trichloroethene

1,1,1-Trichloroethane

Perchloroethene

Methylene chloride

Occurrence of Solvents 37

Figure 20. Quantiles of concentrations of each of four solvents in 

drinking water at an assessment level of 0.2 microgram per liter.

Relative to the 24 VOCs measured in drinking water that 

have MCLs, the ranking of the four solvents with respect to the 

frequency with which concentrations were greater than their 

MCLs in drinking water was as follows: PCE, 1; TCE, 2; 

methylene chloride, 6; and TCA, 13 (Appendix 10). In this case, 

the highest concentration from each CWS was selected for 

comparison to MCLs if multiple samples of quantified concentrations of solvents were available. TCA concentrations were 

not greater than the MCL in any samples from any CWS.

The frequencies of concentrations of each of the four 

solvents in drinking water that were greater than their MCLs 

were as follows: PCE, 1.6 percent; TCE, 1.2 percent; methylene chloride, 0.2 percent; and TCA, 0 percent. The numbers 

of CWSs in which concentrations were greater than the MCL 

for each solvent were as follows: PCE, 26; TCE, 20; methylene 

chloride, 4; and TCA, 0. For PCE, the majority of concentrations greater than the MCL occurred in New York, Pennsylvania, and Massachusetts. For TCE, the vast majority of 

concentrations greater than the MCL occurred in New York. 

For methylene chloride, the locations of CWSs from which 

samples had concentrations greater than the MCL were as 

follows: Massachusetts, 2; New York, 1; and New Jersey, 1.

Comparisons of Solvent Occurrence and 

Concentrations Between Ground Water, 

Source Water, and Drinking Water

The occurrence of any one or more of the four solvents in 

ground water, source water, and drinking water is illustrated in 

figure 21. The detection frequencies shown in figure 21 were 

computed using an assessment level of 0.2 μg/L. Because the 

drinking-water data were limited to 12 Northeast and MidAtlantic States, the data for ground water and source water also 

were constrained to these areas to make the comparisons of 

detection frequencies with drinking water most equitable. 

The detection frequency of one or more solvents was 

higher in source water at about 13 percent and lower in both 

ground water and drinking water at about 11 percent. However, 

the detection frequencies of solvents in each data set were 

similar and vary by less than 2 percent. The differences in 

detection frequencies between any of the data sets were not 

significant at the 95-percent confidence interval. When examined nationwide, the detection frequencies of one or more 

solvents were about 7 percent for ground water and about 

6 percent for source water. The differences in nationwide detection frequencies of one or more solvents between ground water 

and source water were not significant at the 95-percent confidence interval.

0

2

4

6

8

10

12

14

16

18

Ground water Source water Drinking water

DETECTION FREQUENCY, IN PERCENT

A

A

A

101

1,682

957

Number of samples

Populations that share the same letter are

 not signifcantly different at a 95-percent

 confidence interval

957

A

Figure 21. Detection frequencies of one or more solvents in 

ground water, source water, and drinking water in 12 Northeast 

and Mid-Atlantic States at an assessment level of 0.2 microgram 

per liter.
38 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

The detection frequencies of each individual solvent in 

ground water, source water, and drinking water are illustrated in 

figure 22. The detection frequencies were computed using an 

assessment level of 0.2 μg/L. Because the drinking-water data 

were limited to 12 Northeast and Mid-Atlantic States, the data 

for ground water and source water also were constrained to 

these areas to make the comparisons of detection frequencies 

with drinking water most equitable.

The detection frequency of TCA was highest in ground 

water followed by PCE, TCE, and methylene chloride. In 

source water, the detection frequency of PCE was highest, 

followed by TCE, TCA, and methylene chloride. In drinking 

water, the detection frequency pattern of individual solvents 

followed that of ground water with TCA being the highest 

followed by PCE, TCE, and methylene chloride.

The results of Pearson’s chi-square tests of independence 

between data sets for detection frequencies of individual 

solvents in 12 Northeast and Mid-Atlantic States are shown in 

table 12. The only difference in detection frequencies at the 

95-percent confidence interval that was significant was the 

difference in detection frequencies of methylene chloride 

between ground water and drinking water. The detection 

frequency of methylene chloride was higher in drinking water 

compared to ground water.

Table 12. Results of Pearson’s chi-square tests comparing 

detection frequencies of individual solvents between data sets 

at an assessment level of 0.2 microgram per liter.

[Bolded p-value indicates significance at α = 0.05. Data limited to 12 

Northeast and Mid-Atlantic States. PCE, perchloroethene; TCA, 1,1,1-

trichoroethane; TCE, trichoroethene]

Comparison

p-value

Methylene 

chloride PCE TCA TCE

Ground water and 

source water

0.6387 0.4427 0.6213 0.4185

Source water and 

drinking water

.1901 .1015 .8737 .2416

Ground water and 

drinking water

.0073 .0872 .367 .5366

0

1

2

3

4

5

6

7

8

9

1,1,1-Trichloroethane Perchloroethene Trichloroethene Methylene chloride

DETECTION FREQUENCY, IN PERCENT

Ground water

Source water

957 Number of samples

Drinking water

957

938

954

957

98

93

95

96

1,678

1,681

1,680

1,681

Figure 22. Detection frequencies of individual solvents in ground water, source water, and drinking water in 12 Northeast 

and Mid-Atlantic States at an assessment level of 0.2 microgram per liter.
Occurrence of Solvents 39

The detection frequencies of individual solvents nationwide and at an assessment level of 0.2 μg/L are shown in 

figure 23. Only data from ground water and source water were 

available nationwide. The detection frequencies of TCE and 

methylene chloride had the same pattern nationwide as 

compared to the 12 Northeast and Mid-Atlantic States, with 

TCE having a higher detection frequency in source water and 

methylene chloride having a higher detection frequency in 

ground water. Detection frequencies were not significantly 

different between ground water and source water for any 

solvent at the 95-percent confidence interval.

Table 13 lists the nationwide detection frequency ranking 

of each solvent relative to a suite of 51 to 55 VOCs analyzed in 

each data source. A sum of the ranks also is given in this table. 

The assessment level for computing detection frequencies was 

0.2 μg/L. Based on a sum of the ranks of each solvent by data 

set, PCE is the most frequently occurring of the four solvents in 

ground water, source water, or drinking water followed by TCE, 

TCA, and methylene chloride.

The nationwide detection frequencies of individual 

solvents also were examined in ground water by land use 

(figs. 4 and 5). Table 14 gives the results of Pearson’s chisquare tests of independence for comparisons of the nationwide 

detection frequencies of individual solvents by land use at an 

assessment level of 0.2 μg/L. Mixed land use refers to areas 

sampled in the aquifer studies. Differences in detection frequencies at the 95-percent confidence interval were significant in 

every land-use comparison for both PCE and TCE. No significant differences were found between detection frequencies by 

land use for methylene chloride. For TCA, differences between 

detection frequencies were significant in every land comparison 

except mixed land use compared to agricultural land use.

Table 13. Ranking of occurrence of solvents among a suite of 

VOCs in ground water, source water nationwide and drinking 

water in 12 Northeast and Mid-Atlantic States based on detection 

frequencies at an assessment level of 0.2 microgram per liter.

[PCE, perchloroethene; TCA, 1,1,1-trichoroethane;; TCE, trichoroethene]

Data set

Solvent ranking

Methylene 

chloride PCE TCA TCE

Ground water

Source water

Drinking water

Rank sum

0

1

2

3

4

5

DETECTION FREQUENCY, IN PERCENT

Ground water

Source water

4,996 Number of samples

4,996

5,064

577

577

577

5,054

5,068

578

1,1,1-Trichloroethane Perchloroethene Trichloroethene Methylene chloride

13 3 5 4

28 6 10 7

11 7 6 8

52 16 21 19

Figure 23. Detection frequencies of individual solvents in ground water and source water, nationwide at an assessment level of 

0.2 microgram per liter.
Table 14. Results of Pearson’s chi-square tests comparing nationwide detection frequencies of individual solvents by 

land use at an assessment level of 0.2 microgram per liter.

[Bolded p-value indicates significance at α = 0.05. PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Comparison

p-value

Methylene chloride PCE TCA TCE

Mixed1 land use and urban2 land use 0.5497 0 0 0.0002

Mixed1 land use and agricultural3 land use .9491 0 .0559 .0001

Agricultural3 land use and urban2 land use .6565 0 0 0

1

Data from National Water-Quality Assessment (NAWQA) aquifer studies.

2

Data from NAWQA shallow ground-water studies in urban areas.

3Data from NAWQA shallow ground-water studies in agricultural areas.

40 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Quantiles of concentrations of methylene chloride, PCE, 

TCA, and TCE in ground water, source water, and drinking 

water are illustrated in figures 24–27, respectively. All concentrations were compared at an assessment level of 0.2 μg/L. The 

data for ground water and source water were limited to 12 

Northeast and Mid-Atlantic States in order to make the comparisons with drinking water more equitable. For drinking water, 

the quantified concentration for each CWS was the median 

quantified concentration by CWS when multiple quantified 

concentrations were available.

Table 15 gives the results of Kolmogorov-Smirnov tests 

comparing the distributions of concentrations of each solvent in 

ground water, source water, and drinking water in the 12 Northeast and Mid-Atlantic States. None of the comparisons of the 

distributions of the concentrations were significantly different 

at the 95-percent confidence interval.

CONCENTRATION, IN MICROGRAM PER LITER

0.97

0.98

0.0001 0.001 0.01 0.1 1 10 100

0.99

1.00

FRACTION OF DATA

Drinking water

Ground water

Figure 24. Quantiles of concentrations of methylene chloride 

in ground water and drinking water in 12 Northeast and MidAtlantic States at an assessment level of 0.2 microgram per liter.

CONCENTRATION, IN MICROGRAM PER LITER

0.85

0.90

0.0001 0.001 0.01 0.1 1 10 100

0.95

1.00

FRACTION OF DATA

Drinking water

Source water

Ground water

Figure 25. Quantiles of concentrations of perchloroethene in 

ground water, source water, and drinking water in 12 Northeast 

and Mid-Atlantic States at an assessment level of 0.2 microgram 

per liter.
CONCENTRATION, IN MICROGRAM PER LITER

0.85

0.90

0.0001 0.001 0.01 0.1 1 10

0.95

1.00

FRACTION OF DATA

Drinking water

Source water

Ground water

Occurrence of Solvents 41

Figure 26. Quantiles of concentrations of 1,1,1-trichloroethane in 

ground water, source water, and drinking water in 12 Northeast 

and Mid-Atlantic States at an assessment level of 0.2 microgram 

per liter.

CONCENTRATION, IN MICROGRAM PER LITER

0.0001 0.001 0.01 0.1 1 10 100

FRACTION OF DATA

0.90

0.93

0.92

0.91

0.99

0.98

0.97

0.96

0.95

0.94

1.00

Drinking water

Source water

Ground water

Figure 27. Quantiles of concentrations of trichloroethene in 

ground water, source water, and drinking water in 12 Northeast 

and Mid-Atlantic States at an assessment level of 0.2 microgram 

per liter.

Table 15. Results of Kolmogorov-Smirnov tests comparing the 

distributions of concentrations of solvents in ground water, source 

water, and drinking water.

[Data limited to 12 Northeast and Mid-Atlantic States. PCE, perchloroethene; 

TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Comparison

p-value

Methylene 

chloride PCE TCA TCE

Ground water and 

source water

1 1 1 0.998

Source water and 

drinking water

1 1 1 1

Ground water and 

drinking water

1 .19 .266 .618

Table 16 lists the rank of each solvent in ground water, 

source water, and drinking water with respect to the frequency 

of concentrations greater than the MCLs relative to other VOCs. 

Table 16 also includes a rank sum for each solvent indicating 

the relative frequency of concentrations greater than MCLs for 

each solvent. The rankings for ground water and source water 

were for all data nationwide while the rankings for drinking 

water were for CWSs in 12 Northeast and Mid-Atlantic States. 

As indicated by their rank sum, in ground water, source water, 

and drinking water, the VOCs that most frequently had concentrations greater than the MCL were PCE and TCE. They were 

followed by methylene chloride and TCA, which had concentrations greater than the MCLs much less frequently (table 16).

The frequency of concentrations greater than the MCLs for 

each solvent in each data set is shown in figure 28. The data for 

ground water and source water were limited to 12 Northeast and 

Mid-Atlantic States in order to make the comparisons to 

drinking water more equitable. For drinking-water data, the 

highest quantified concentration of each solvent in each CWS 

was selected when multiple concentrations were available. The 

results of Pearson’s chi-square tests of independence comparing 

frequencies of concentrations greater than the MCLs for each 

individual solvent between data sets are given in table 17.
Table 16. Rank of frequency of concentrations of solvents 

greater than their Maximum Contaminant Levels (MCLs) compared 

to other volatile organic compounds (VOCs) nationwide in ground 

water and source water, and in drinking water in 12 Northeast and 

Mid-Atlantic States.

[PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Data set

Number of 

VOCs that 

exceeded

an MCL

Solvent exceedance 

frequency ranking

Methylene 

chloride PCE TCA TCE

Ground water 11 9 1 112 3

Source water 5 16 1 16 2

Drinking water 12 6 1 113 2

Rank sum 21 3 31 7

1Frequency of exceedance of MCLs was 0 percent.

42 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

For both ground water and drinking water, the frequencies 

of concentrations greater than the MCL were highest for PCE 

followed by TCE. However, the frequencies of concentrations 

greater than the MCL for PCE in both ground water or drinking 

water were fairly low at 1.6 percent in drinking water and 

1.2 percent in ground water. In source water, the frequencies of 

concentrations greater than the MCL for PCE and TCE were 

highest and were identical at 3 percent. The concentrations of 

TCA were not greater than the MCL in any samples from any 

data set, and methylene chloride was greater than the MCL only 

in drinking water and only in four CWSs. None of the frequencies of concentrations greater than the MCL of any solvent in 

any of the comparisons between data sets were significantly 

different (table 17).

Table 17. Results of Pearson’s chi-square tests comparing 

frequencies of concentrations greater than the Maximum 

Contaminant Levels for individual solvents between data sets.

[Bolded p-value indicates significance at α = 0.05). Data limited to 12 

Northeast and Mid-Atlantic States. 

PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Comparison

p-value

Methylene 

chloride PCE TCA TCE

Ground water and 

source water

0.3008 1 0.0957

Source water and 

drinking water

.4881 1 .2775

Ground water and 

drinking water

.5452 1 .3597

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

EXCEEDANCE FREQUENCY, IN PERCENT

Ground water

Source water

938 Number of samples

Drinking water

938

954

957 957 100

101 101

101

1,681

1,681

1,680

1,678

Perchloroethene Trichloroethene Methylene chloride 1,1,1-Trichloroethane

1

.5496

.322

Figure 28. Frequencies of concentrations greater than the Maximum Contaminant Levels by individual solvents in ground water, 

source water, and drinking water in 12 Northeast and Mid-Atlantic States at an assessment level of 0.2 microgram per liter.
Occurrence of Solvents 43

The frequencies of concentrations of individual solvents 

greater than the MCL also were examined in ground water 

nationwide by land use (fig. 29). The results of Pearson’s chisquare tests of independence comparing frequencies of concentrations of individual solvents greater than the MCLs nationwide between land uses are given in table 18.

PCE concentrations were greater than the MCL most 

frequently in urban and mixed land-use areas. The frequency of 

concentrations greater than the MCL for TCE were nearly the 

same in urban and mixed land-use areas and lower than PCE. 

No concentrations of PCE, TCA, or TCE were greater than the 

MCL in agricultural land-use areas. The concentrations of TCA 

were not greater than the MCL in any land-use area, and the 

concentrations of methylene chloride greater than the MCL 

were less than 1 percent in mixed and agricultural land-use 

areas. The frequency of concentrations greater than the MCL 

for PCE were higher in urban land-use areas than in either 

mixed or agricultural land-use areas, and each difference was 

statistically significant (table 18). No other comparisons of 

frequencies of concentrations greater than the MCL were 

significantly different.

Table 18. Results of Pearson’s chi-square tests comparing 

frequencies of concentrations greater than the Maximum 

Contaminant Levels for individual solvents in ground water 

between land uses.

[Bolded p-value indicates significance at α = 0.05). PCE, perchloroethene; 

TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Comparison

p-value

Methylene 

chloride PCE TCA TCE

Mixed1land use 

and urban2 land 

use

0.8439 0.0085 1 0.8211

Mixed1land use 

and agricultural3

land use

.9814 .0558 1 .1374

Agricultural3 land 

use and urban2

land use

.9382 .0013 1 .1789

1Data from National Water-Quality Assessment (NAWQA) aquifer 

studies.

2Data from NAWQA shallow ground-water studies in urban areas.

3Data from NAWQA shallow ground-water studies in agricultural areas.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

EXCEEDANCE FREQUENCY, IN PERCENT

Shallow ground-water underlying urban areas

Aquifer studies (mixed land use)

3,449 Number of samples

Shallow ground-water underlying agricultural areas

3,449

3,497

3,498 3,487 844

832

847

715 720 847

723

723

Perchloroethene Trichloroethene Methylene chloride 1,1,1-Trichloroethane

Figure 29. Frequencies of concentrations greater than the Maximum Contaminant Levels in ground water from National 

Water-Quality Assessment studies by individual solvent and land use.
44 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Associations of Solvents with 

Hydrogeologic and Anthropogenic 

Variables

The results of the logistic regression analyses for one or 

more of the four solvents and individual solvents are summarized in table 19. Table 19 lists the variables that were significantly associated with one or more solvents or individual 

solvents, the coefficient for each association in the logistic 

regression equation, the standardized coefficient, and the 

Hosmer-Lemeshow statistic p-value. Standardized coefficients 

greater than 0.1 were considered to be strongly associated with 

the probability of occurrence of solvents and are highlighted in 

gray in table 19.

Variables Associated With One or More 

Solvents

The probability of detecting one or more of the four 

solvents was most strongly associated with sand content of soil 

(table 19). The probability of detecting one or more solvents 

increased as the sand content of the soil decreased. It might be 

assumed that a decrease in sand content of the soil would 

decrease the permeability of the soil and thereby limit the downward movement of contaminants like solvents to ground water. 

However, in the vadose zone at low pressure head or water 

content, the unsaturated hydraulic conductivity of a finetextured soil may be greater than that of a coarse soil (Stephens, 

1996). 

Over the range of pressure heads and water contents likely 

to be encountered in the vadose zone, the unsaturated hydraulic 

conductivity of a soil can vary greatly, and the variability is 

even greater between soils of different textures (Stephens, 

1996). In addition, soil texture properties like sand content can 

be used to estimate soil hydraulic properties indicating that a 

relation exists between texture and hydraulic characteristics 

(Schaap and Leij, 1998). It is likely that sand content of soil is 

a surrogate for another soil property, such as unsaturated 

hydraulic conductivity of the soil zone, that would be expected 

to strongly influence the occurrence of solvents in ground 

water.

The probability of detecting one or more solvents also was 

strongly associated with the percent of urban land and population density in proximity to the wells (table 19). The probability 

of detecting one or more solvents increased as either percent 

urban land use or population density increased. Urban land use 

and population density represent sources of solvents to ground 

water and are aggregate surrogate variables. Although they do 

not represent specific sources of solvents to ground water, as 

both of these variables increase there is an increase in multiple 

potential sources of solvents, such as commercial, industrial, 

and transportation activities.

The probability of detecting one or more solvents also was 

strongly associated with dissolved-oxygen content of ground 

water (table 19). The probability of detecting one or more 

solvents increased as dissolved oxygen of ground-water content 

increased. Dissolved oxygen is a key factor in processes that 

determine the fate of solvents in ground water, primarily 

biodegradation. Solvents are known to biodegrade under anoxic 

conditions. In oxic ground water, less biodegradation occurs 

(other than by co-metabolism), and therefore a higher concentration of solvents can remain in solution. Consequently, the 

probability of detecting solvents should be higher in oxic 

ground water compared to anoxic ground water. In this case, 

oxic and anoxic conditions in ground water are simply defined 

as conditions of relatively high and relatively low dissolved 

oxygen.

The probability of detecting one or more solvents was 

weakly associated with depth to the top of the screened interval 

in the wells, the number of Comprehensive Environmental 

Response, Compensation, and Liability Act (CERCLA) sites 

within 1 km of the wells, and the number of septic systems 

within 1 km of the wells (table 19). The probability of detecting 

one or more solvents decreased as depth to the top of the 

screened interval increased. As depth to the top of the screened 

interval increases, the time required for a solvent to reach the 

screened interval by movement through the vadose and saturated zones also increases, provided there is no short cutting of 

traveltime to the screened interval (Moran and others, 2002). 

This increased time allows for increased attenuation of solvent 

concentrations through natural loss processes such as biodegradation, sorption, dispersion, and volatilization.

The probability of detecting one or more solvents 

increased as either the number of CERCLA sites or the number 

of septic systems in proximity to the wells increased (table 19). 

These two terms represent sources of solvents to ground water. 

CERCLA sites are facilities that have experienced releases of 

toxic or hazardous substances and that the Federal government 

has indicated are a priority for clean up. CERCLA sites may be 

sources of solvents to ground water if solvents were released at 

the site. Likewise, septic systems could be a source of solvents 

to ground water because many types of household products 

contain solvents and it is likely that these are disposed of in 

septic systems. However, the number of septic systems within 

1 km of the wells was the weakest variable associated with the 

probability of occurrence of solvents in ground water, and thus, 

septic systems may not be the primary source for solvents in 

ground water but may be only a contributing source in some 

cases (table 19). 
Table 19. Hydrogeologic and anthropogenic variables that were associated with the probability of occurrence of one or more solvents 

or individual solvents in ground water.

[km, kilometer; RCRA, Resource Conservation and Recovery Act; CERCLA, Comprehensive Environmental Response, Compensation, and Liability Act; PCE, 

perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Solvent

Associated variables

(gray shading indicates a

weak1 association)

Type of 

variable

Unstandardized 

coefficient in logistic 

regression equation

Standardized 

coefficient

HosmerLemeshow 

statistic

p-value

One or more solvents Sand content of soil Transport -0.026 -0.22

Urban land use Source 1.544 0.21

Dissolved oxygen Fate 0.134 0.16

Population density Source 0.303 0.12

Depth to top of screened interval Transport -0.001 -0.06

CERCLA sites within 1 km Source 0.917 0.05

Septic systems within 1 km Source 0.004 0.05

Methylene chloride Population density Source 0.614 0.11

Bulk density of soil Transport 3.675 0.06

Sand content of soil Transport -0.016 -0.06

Urban land use Source -0.847 -0.05

Median year of home construction Source -0.018 -0.03

PCE Urban land use Source 2.165 0.28

Sand content of soil Transport -0.028 -0.22

Dissolved oxygen Fate 0.137 0.16

Soil erodibility Transport -4.575 -0.15

RCRA sites within 1 km Source 0.027 0.06

Depth to top of screened interval Transport -0.001 -0.05

Septic systems within 1 km Source 0.003 0.04

TCA Dissolved oxygen Fate 0.230 0.20

Urban land use Source 1.461 0.14

Population density Source 0.456 0.13

Depth to top of screened interval recharge Transport -0.003 -0.12

Recharge Transport 0.003 0.11

CERCLA sites within 1 km Source 1.043 0.04

Septic systems within 1 km Source 0.003 0.03

TCE Population density Source 1.139 0.26

Dissolved oxygen Fate 0.121 0.08

RCRA sites within 1 km Source 0.035 0.04

CERCLA sites within 1 km Source 0.871 0.03

Casing diameter Transport 0.033 0.03

1

Weak associations are those with a standardized coefficient of less than 0.1.

Associations of Solvents with Hydrogeologic and Anthropogenic Variables 45

0.714

0.093

0.160

0.174

0.133
46 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Variables Associated With Individual Solvents

The probability of detecting methylene chloride in ground 

water was strongly associated with population density 

(table 19). As population density increased, the probability of 

detecting methylene chloride increased. This variable represents sources of methylene chloride to the environment and is 

an aggregate surrogate variable. The probability of detecting 

methylene chloride in ground water was weakly associated with 

bulk density of the soil, sand content of the soil, urban land use, 

and median year of home construction within 1 km of the wells 

(table 19). These terms represent source and transport mechanisms in the environment. Both bulk density of the soil and soil 

sand content likely are surrogates for another soil property such 

as saturated or unsaturated hydraulic conductivity of the soil 

zone. In fact, saturated and unsaturated hydraulic conductivity 

can be estimated from soil data such as soil texture and bulk 

density (Schaap and Leij, 1998). Median year of home 

construction is a surrogate variable for source of solvents to 

ground water from domestic activities and possibly from 

private septic systems, with older homes having a greater likelihood of contributing solvents to ground water.

The probability of detecting PCE was most strongly associated with urban land use (table 19). The probability of 

detecting PCE increased as percent urban land use around the 

wells increased. This variable represents sources of PCE to the 

environment and is an aggregate surrogate variable. The probability of detecting PCE also was strongly associated with sand 

content of the soil, dissolved oxygen, and soil erodibility. The 

probability of detecting PCE decreased as both sand content of 

the soil and soil erodibility increased. Sand content of soil has 

been identified as being a possible surrogate for another soil 

property, such as unsaturated zone hydraulic conductivity. 

Soil erodibility also likely is a surrogate variable. Soil 

erodibility is defined as the susceptibility of a soil to detachment 

and transport by water (Natural Resources Conservation 

Service, 2005). Some of the most important properties that 

affect soil erodibility are texture, organic matter content, structure size and class, and saturated hydraulic conductivity of the 

soil (Natural Resources Conservation Service, 2005). Soil erodibility factors are measured by applying a series of simulated 

rainstorms on freshly tilled plots and computed using soil characteristics such as size fraction, organic matter content, and 

permeability. Thus, soil erodibility is a surrogate variable that 

represents the interaction between several soil properties. The 

particular combination of soil properties that the soil erodibility 

term represents is strongly inversely related to the probability of 

detecting PCE in ground water, and thus, soil erodibility may 

represent a property such as saturated vertical hydraulic 

conductivity of the soil zone, which would be expected to 

strongly influence the occurrence of solvents in ground water.

The probability of detecting PCE also increased as 

dissolved-oxygen content of ground water increased (table 19). 

PCE is known to biodegrade through reductive dechlorination 

(Swindoll and Troy, 1996). In oxic ground water, less biotransformation of PCE occurs by reductive dechlorination, and 

therefore, a higher concentration remains in solution. Thus, the 

probability of detecting PCE should be greater in oxic ground 

water compared to anoxic ground water. 

The probability of detecting PCE in ground water also was 

weakly associated with the number of Resource Conservation 

and Recovery Act (RCRA) sites within 1 km of the wells, the 

depth to the top of the screened interval in the wells, and the 

numbers of septic systems within 1 km of the wells (table 19). 

RCRA sites are facilities that generate, transport, store, or 

dispose of hazardous waste, as defined under RCRA, and are 

required to register the amounts and types of waste with the 

USEPA (U.S. Environmental Protection Agency, 2004c). 

RCRA sites may be sources of solvents to ground water if 

solvents were released at the site.

The probability of detecting TCA in ground water was 

strongly associated with dissolved oxygen, urban land use, 

population density, depth to the top of the screened interval, and 

recharge (table 19). Except for depth to the top of the screened 

interval, the probability of detecting TCA in ground water 

increased as each of these variables increased. The probability 

of detecting TCA in ground water decreased as depth to the top 

of the screened interval increased. 

Population density and urban land use are aggregate surrogate variables for sources of TCA to ground water. Dissolved 

oxygen controls the fate of TCA in ground water. TCA has been 

shown to biodegrade under anoxic conditions, and thus, the 

probability of detecting TCA should be greater in oxic ground 

water compared to anoxic ground water (Parsons and others, 

1985; Semprini and others, 1992). Recharge is the transport 

mechanism that moves solvents from the surface or nearsurface areas through the vadose zone to ground water. As 

recharge increases, the probability of detecting a contaminant 

transported through the vadose zone also should increase. The 

probability of detecting TCA in ground water was weakly associated with the number of CERCLA sites within 1 km of the 

wells and the numbers of septic systems within 1 km of the 

wells (table 19).

The probability of detecting TCE in ground water was 

strongly associated with population density (table 19). The 

probability of detecting TCE increased as population density 

increased. Population density is an aggregate surrogate variable 

for sources of TCE to ground water. The probability of 

detecting TCE in ground water was weakly associated with 

dissolved oxygen, the number of RCRA sites within 1 km of the 

wells, the number of CERCLA sites within 1 km of the wells, 

and the average diameter of the well casing (table 19). 

The probability of detecting TCE in ground water 

increased as the average diameter of the well increased. The 

average diameter of the well casing is a surrogate for average 

discharge from the well. Larger diameter wells generally have 

larger pumping capacities. An examination of NAWQA data 

indicated a good correlation between average discharge from a 

well and average casing diameter (Spearman’s rho 0.398; p = 

0). Wells with higher pumping rates usually obtain water from 

larger volumes of an aquifer compared to wells with smaller 

pumping rates. Larger volumes of aquifer contribution to a well 
Associations of Solvents with Hydrogeologic and Anthropogenic Variables 47

generally are obtained from a larger geographical area that can 

provide a greater number of potential sources of TCE to ground 

water.

Summary of Significantly Associated 

Explanatory Variables

Table 20 is a summary of explanatory variables that were 

significantly associated with the probability of occurrence of 

any individual solvent. Included in this table is the explanatory 

variable, the rank sum of the variable, the number of individual 

solvents that were associated with the variable, and the type of 

variable—source, transport, or fate. The rank sum was 

computed by ranking each variable associated with each individual solvent according to the standardized coefficient, with 

higher standardized coefficients given lower ranks. Ties were 

given the same rank. The ranks for each explanatory variable 

were then summed across each solvent for a rank sum. The 

lower rank sums indicate a variable that was consistently more 

strongly associated with solvent occurrence.

Table 21 lists the ranking of each type of variable that was 

associated with the probability of occurrence of individual 

solvents. The ranking was obtained by summing the number of 

times each variable type was significantly associated with each 

solvent. These numbers were then normalized relative to the 

total number of variable types available in the logistic regression analyses and then ranked for each solvent. Lower ranks 

indicate a variable type that was more strongly and frequently 

associated with solvent occurrences.

Dissolved-oxygen content, percent urban land use around 

the well, and population density were the variables most 

strongly and frequently associated with the probability of 

occurrence of solvents in ground water (table 20). These variables represent the sources and fate of solvents.

Dissolved oxygen was the only variable associated with 

the probability of occurrence of one or more solvents or individual solvents that could be attributed to the fate of solvents in 

ground water. Dissolved oxygen was associated with the probability of occurrence of one or more solvents and three individual solvents, and fate was the most important type of explanatory variable for every individual solvent except methylene 

chloride (table 21). In every case where dissolved oxygen was 

related to the probability of occurrence of solvents, the probability of occurrence increased with increasing dissolved-oxygen 

content (table 19). This indicates that increased rates of degradation of solvents are occurring in conditions of low dissolved 

oxygen. Reductive transformation reactions are the most important for solvents in ground water (Beek, 2001). Thus, it is 

presumed that solvents are preferentially degraded in conditions 

of relatively low dissolved oxygen in ground water and consequently are detected more frequently when dissolved-oxygen 

concentrations are relatively high.

Table 20. Rank sum and number of solvents associated with each hydrogeologic and anthropogenic variable in ground water.

[km, kilometer; RCRA, Resource Conservation and Recovery Act; CERCLA, Comprehensive Environmental Response, Compensation, and Liability Act]

Explanatory variable Rank sum

Number of solvents 

associated 

with variable

Type of

variable

Association

strength

Dissolved oxygen 12 3 Fate

Urban land use 13 3 Source Strong

Population density 13 3 Source

Sand content of soil 19 2 Transport

Moderate

Depth to top of screened interval 22 2 Transport

RCRA sites within 1 km 22 2 Source

CERCLA sites within 1 km 24 2 Source

Soil bulk density 24 1 Transport

Weak

Soil erodibility 24 1 Transport

Recharge 25 1 Transport

Septic systems within 1 km 26 2 Source

Casing diameter 27 1 Transport

Median year of home construction 27 1 Source
Table 21. Ranking of type of variable associated with each 

solvent in ground water based on significant associations.

[PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Variable type

Ranking of significant associations

Methylene 

chloride PCE TCA TCE

Source

Transport

Fate

48 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Urban land use and population density represent sources of 

solvents to ground water. At least one of these two variables 

was associated with the probability of occurrence of one or 

more solvents and every individual solvent. In every case where 

either or both of these two variables were related to the probability of occurrence of solvents, the probability of occurrence 

increased with increasing percent urban land use or population 

density (table 19). Unlike specific sources of solvents to ground 

water, these variables represent aggregate surrogate variables 

for input of solvents to ground water because one or more 

events or actions likely have resulted in release of solvents to 

the environment in these areas. For example, urban land use 

may represent releases of solvents from activities such as 

leaking USTs, spills at bulk loading and unloading facilities, 

and runoff from commercial/industrial areas where solvents 

have been deposited from air. Population density and year of 

home construction also probably are aggregate surrogate variables for solvent input to ground water just like urban land use. 

Population density was strongly correlated with urban land use 

(Spearman’s rho 0.73; p = 0), and year of home construction 

also was strongly correlated with urban land use (Spearman’s 

rho 0.131; p = 0).

The variables that were moderately associated with the 

probability of occurrence of solvents, in magnitude and 

frequency, included sand content of the soil, depth to the top of 

the screened interval, and the number of CERCLA and RCRA 

sites within 1 km of the wells (table 20). These variables represent sources and transport of solvents to ground water. Transport and sources variables were less important than fate in determining the probability of occurrence for every individual 

solvent except methylene chloride (table 21).

Sand content of the soil and depth to the top of the screened 

interval are properties that indirectly affect the transport of 

solvents to ground water. In every case where soil sand content 

was associated with the probability of occurrence of solvents, 

the probability of occurrence decreased with increasing soil 

sand content (table 19). Although these results might seem to be 

the opposite of what is expected, the consistency in the results 

indicates that the association is valid. Soil sand content is 

believed to be a surrogate for the unsaturated hydraulic conductivity of the soil zone. As mentioned, in the vadose zone at low 

pressure head or water content, the unsaturated hydraulic 

conductivity of a fine-textured soil may be greater than that of 

a coarse soil (Stephens, 1996). Thus, in some cases, the 

hydraulic conductivity of soil in the vadose zone may vary 

inversely with soil sand content.

Depth to the top of the screened interval is believed to be 

roughly equivalent to the depth to the top of the aquifer and 

represents the distance needed for solvents to move from the 

land surface, or from their point of release in the vadose zone, 

through the vadose zone to ground water. In every case where 

depth to the top of the screened interval was related to the probability of occurrence of solvents, the probability of occurrence 

decreased with increasing depth (table 19). As distance to the 

aquifer from the release point increases, the time required for a 

solvent to reach it also increases provided there is no short 

cutting of traveltime to the aquifer. This increased time allows 

for increased attenuation of solvent concentrations through 

natural loss processes such as biodegradation, sorption, dispersion, and volatilization.

The numbers of CERCLA and RCRA sites within 1 km of 

the wells represent sources of solvents to ground water. In every 

case where either or both of these variables were related to the 

probability of occurrence of solvents, the probability of occurrence increased with increasing number of sites near the wells 

(table 19). Unlike urban land use and population density, these 

terms may represent specific sources of solvents to ground 

water or they could be surrogates for indirect sources. The legislation that established CERCLA and RCRA sites was designed 

to protect ground water at hazardous waste sites and to clean up 

uncontrolled hazardous waste sites (U.S. Environmental 

Protection Agency, 2005a). Since the enactment of the legislation that established these sites more than 20 years ago, data 

from thousands of hazardous waste sites have been gathered 

and analyzed. CERCLA sites that are rated as highest priority 

for cleanup are included on the National Priority List and also 

are called Superfund sites.

An early survey of 183 hazardous waste sites in the United 

States indicated that four of the top five most frequently identified organic contaminants were solvents (methylene chloride, 

TCE, PCE, and 1,2-dichloroethane) (Pankow and Cherry, 

1996). The USEPA indicates that PCE, TCE, and methylene 

chloride are among the chemicals most commonly found at 

Superfund sites (U.S. Environmental Protection Agency, 

2005a). Thus, these sites would be plausible sources of solvents 

to ground water. However, it is not known if the solvents 

detected in ground water in this report specifically had a source 

at these sites. 

The remaining associated variables, with one exception, 

represent the source or transport of solvents and were only 

weakly and infrequently associated with the probability of 

occurrence of solvents in ground water (table 20). The one 

exception was the number of septic systems within 1 km of the 

wells. Although only weakly associated, this variable was 

significant in regressions for PCE, TCA, and one or more 

solvents (table 19). In every case where the number of septic 

systems was related to the probability of occurrence of solvents, 

the probability of occurrence increased with increasing number 

1 222

1 233

3 111
Associations of Solvents with Hydrogeologic and Anthropogenic Variables 49

of septic systems near the wells (table 19). Septic systems likely 

represent direct sources of solvents to ground water.

There are many lines of evidence that septic systems can 

be sources of solvents to ground water. Many types of household products contain solvents, and it is likely that these are 

disposed of in septic systems. At one time, fluids used to clean 

septic tanks were known to contain TCE and methylene chloride. In fact, in 1976 New York State revealed that aquifers on 

Long Island were extensively contaminated by TCE because of 

homeowner use of septic tank cleaning products containing 

TCE (Pankow and Cherry, 1996). 

DeWalle and others (1980) found five VOCs, two of which 

were solvents examined in this report (methylene chloride and 

PCE), in samples of septic tank effluent from a community 

septic tank serving 91 homes in a subdivision of Tacoma, 

Washington. Viraraghavan and Hashem (1986) found methylene chloride, along with two other VOCs, in septage and 

septic tank effluent from a house in Regina, Canada. Ayres 

Associates (1993) found methylene chloride and TCA in 

septage and septic tank effluent beneath the septic system leach 

fields of homes in several Florida counties. 

Septic systems consist of two reactor areas: (1) the septic 

tank that acts as an anaerobic reactor, and (2) the drainage field 

or leach field that acts as an aerobic reactor. Wastewater from 

the house first flows into a holding tank called the septic tank. 

The main purpose of the septic tank is to settle solid particles 

and separate grease from the wastewater. In addition to these 

purposes, the septic tank also acts as an anaerobic reactor and 

begins breakdown of organic material. If solvents from 

domestic products are disposed of in the septic system, some 

reductive biotransformation of them may occur in the septic 

tank. 

Effluent from the septic tank then flows through a series of 

trenches in the soil filled with gravel, called the leach field. In 

this area, further breakdown of organic material occurs by 

aerobic biodegradation. Because solvents primarily biotransform in low dissolved-oxygen conditions, little breakdown of 

solvents will occur once they have been passed through the 

septic tank to the leach field. 

Although solvents may be transformed in the septic tank, 

the residence time of dissolved-organic material in the septic 

tank is relatively short. Because of this relatively short residence time, much of the mass of solvents disposed of in septic 

systems may not be removed. In addition, quick transfer of 

material from the septic tank to the leach field can occur if the 

septic system is improperly designed, installed, or maintained. 

For example, septic tanks require pumping every 3 to 5 years to 

remove solids from the tank. If this is not done, a septic tank can 

fill with solids, and wastewater from the home can be transferred more quickly to the leach field. This “short-circuiting” of 

fluid through the septic tank could allow solvents from 

domestic products to enter the vadose zone with virtually no 

breakdown.

Most of the remaining variables that were weakly associated with the probability of occurrence of solvents were transport variables (table 20). Transport of solvents generally 

involves recharge, permeability, and distance. Recharge, 

precipitation moving from the land surface to the water table, 

transports solvents to ground water through advection. Permeability facilitates, or impedes, the vertical movement of water 

from the surface to ground water and the movement of ground 

water through an aquifer. Distance affects the amount of time 

required for a solvent to reach ground water.

Soil bulk density, soil erodibility, recharge, and casing 

diameter, were weakly and infrequently associated with the 

probability of occurrence of solvents in ground water (table 20). 

Soil bulk density and soil erodibility are believed to be surrogates for the unsaturated and saturated hydraulic conductivities 

of the soil zone. These properties represent the ability and speed 

through which solvents could move through the soil zone to 

ground water, although the specific relations between these 

factors and hydraulic conductivity are not known. Although 

transport processes would seem to be most important for unconfined or semi-confined aquifers, aquifer confinement was not 

found to be significantly related to the occurrence of solvents. 

Transport appears to be important to the probability of occurrence of solvents regardless of the confining conditions of the 

aquifer.

Casing diameter was weakly associated with the probability of occurrence of TCE. Casing diameter is believed to be a 

surrogate for discharge from the well. As discharge from the 

well increases, the well draws from a larger volume of water in 

the aquifer. Drawing from a larger volume of ground water 

increases the chances for including ground water that is contaminated by solvents. In addition, drawing from a larger volume of 

ground water also means drawing from a larger area of recharge 

to the aquifer, which increases the chances for intercepting 

releases of solvents by increasing the zone of influence of the 

well and also increasing the chances for short circuiting transport of solvents through higher permeable zones. These mechanisms could result in an increase in the probability of detecting 

solvents with an increase in casing diameter. However, 

increased discharge from a well could have the opposite effect 

of decreasing the likelihood of detecting a solvent due to dilution effects (Einarson and Mackay, 2001).

Recharge is believed to be the primary mechanism for 

moving solvents from their point of release to ground water, 

although diffusion of gas-phase concentrations of some VOCs 

from the atmosphere to shallow ground water can occur after 

long periods of time (Pankow and others, 1997). Recharge was 

only associated with the probability of occurrence of TCA. The 

reason for the lack of associations of the recharge with other 

solvents is unclear, but large releases of a solvent from a 

concentrated source like a UST can move directly through the 

vadose zone to ground water by gravity. Once in contact with 

ground water, the density of the solvent can cause it to continue 

to sink through ground water until it reaches the base of the 

aquifer. In this situation, the solvent becomes a continual source 

of contamination to ground water without requiring transport by 

recharge. This process is illustrated in figure 30. It is possible 

that many concentrated discharges of solvents behave in this 

manner thereby reducing the relation of solvents to recharge or 

even to their sources.
Recharge

0

50 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

Figure 30. Movement of solvents through an aquifer from a concentrated discharge release.

For TCA and TCE, transport was the least important type 

of variable associated with solvent occurrence (table 21). In the 

case of PCE, source and transport variables were both moderately important to solvent occurrence. For methylene chloride, 

source and transport variables were the most important variable 

types although they were mostly only weakly associated.

Implications

All of the four solvents, with the exception of methylene 

chloride, were in the top 10 most frequently detected VOCs in 

ground water, source water, and drinking water, and between 40 

and 47 VOCs analyzed were detected at an assessment level of 

0.2 μg/L. Thus, out of a large number of VOCs detected, 

solvents were some of the most frequently occurring in ground 

water, source water, and drinking water. Determining the occurrence of solvents in ground-water resources is important, especially for ground-water resources that are used as drinkingwater supplies. Also, the frequent detection of solvents in 

ground water suggests that analyzing for a full suite of solvents 

in ground water is important in obtaining a complete picture of 

environmental occurrence.

In ground water, the detection frequencies of individual 

solvents were higher at an assessment level of 0.02 μg/L than at 

an assessment level of 0.2 μg/L. Thus, low concentrations of 

solvents occur more frequently in ground water than high 

concentrations. Use of analytical methods with low-level detection limits for solvents are necessary to fully ascertain the 

environmental distribution of solvents. Low-level analyses may 

not be important for concentrations of solvents relative to 

MCLs. However, low-level analyses could be important for 

determining the sources of solvents, as a potential early warning 

regarding the vulnerability of an aquifer, and for determining 

trends and patterns in occurrence of solvents in aquifers.

The rankings of detection frequencies of individual 

solvents were identical in ground water and source water 

nationwide at an assessment level of 0.2 μg/L and were as 

follows, from highest to lowest: PCE, TCE, TCA, and methylene chloride. The similarity of the rankings is probably indicative of similarities in the number and strength of sources of 

each solvent and the transport and fate mechanisms affecting 

each between ground water and source water.

Mixtures were a common mode of occurrence for solvents, 

with mixtures occurring in about 30 to 40 percent of groundwater and source-water samples with detections of one or more 

solvents at an assessment level of 0.2 μg/L. For both ground 

water and source water, the mixture of PCE-TCE was the most 

frequently occurring. The frequent occurrence of this mixture 

probably is the result of the transformation of PCE to TCE 

through reductive dechlorination (fig. 1). 

Because reductive dechlorination occurs anaerobically, 

the occurrence of PCE should be higher in ground water with 

relatively high dissolved oxygen compared to ground water 

with relatively low dissolved oxygen. In ground-water samples, 

the occurrence of PCE was 12.6 percent in oxic ground water 

and 6.3 percent in anoxic ground water. In this case, oxic 

ground water was defined as having dissolved oxygen greater 
Implications 51

than or equal to 0.5 mg/L, and anoxic ground water was defined 

as having dissolved oxygen less than 0.5 mg/L. In addition, the 

molar concentration ratios of PCE/TCE were three times higher 

in oxic ground water than in anoxic ground water. These lines 

of evidence support the hypothesis that PCE is being degraded 

to TCE in anoxic ground water. 

Evaluation of mixtures of solvents in ground-water 

resources is important, especially for ground water used for 

drinking-water supplies. The study of chemical mixtures and 

their effect on human health is an important area of research 

because of potential human-health effects of mixtures (Suk and 

others, 2002). For example, exposure to mixtures of TCE, PCE, 

and TCA have been shown, through computer modeling, to 

increase the blood concentration of TCE in humans, and this 

could lead to increased potential health effects from TCE exposure, such as an increased chance for renal tumors (Dobrev and 

others, 2002).

In ground water, TCE and PCE ranked in the upper onehalf of the total number of detected VOCs in terms of the 

median quantified concentrations at an assessment level of 

0.02 μg/L. In source water, all four solvents ranked in the upper 

one-half of the total number of detected VOCs in terms of the 

median quantified concentrations at an assessment level of 

0.2 μg/L. In drinking water, TCE and PCE ranked in the upper 

one-half of the total number of detected VOCs in terms of the 

median quantified concentrations at an assessment level of 

0.2 μg/L. Thus, in general, the four solvents had higher median 

quantified concentrations relative to other VOCs in ground 

water, source water, and drinking water. 

Relative to the total number of VOCs with MCLs, solvents 

ranked consistently high in terms of the frequency of concentrations greater than MCLs. In fact, PCE and TCE ranked 1 and 2 

for both source water and drinking water. In ground water, PCE 

ranked 1 and TCE ranked 3. Therefore, solvents have a higher 

frequency of concentrations greater than the MCL relative to 

other VOCs in ground water, source water, and drinking water. 

Because of this, solvents appear to be the VOCs that pose the 

greatest threat to human health in ground water and drinking 

water. The concentrations of solvents in aquifers need to be 

understood, especially in aquifers used to supply drinking 

water. In addition, it may be necessary to control the sources of 

solvents to domestic and public wells in order to be protective 

of human health.

When confined to data from 12 Northeast and MidAtlantic States, the detection frequencies of one or more 

solvents and individual solvents (with the exception of methylene chloride) were similar between ground water, source 

water, and drinking water. When examined at a national scale, 

the detection frequencies of one or more solvents and individual 

solvents were similar between ground water and source water, 

although they were lower than the detection frequencies 

computed for the 12 Northeast and Mid-Atlantic States. Thus, 

the occurrence of solvents is relatively uniform with respect to 

the intended end use of ground water. However, the occurrence 

of solvents may not be uniform with respect to human activity 

in the area above ground water. 

One likely reason for the higher detection frequencies of 

solvents in the Northeast and Mid-Atlantic regions of the 

country is that these are areas of higher human population and 

greater human activities compared to the country as a whole. 

Areas with greater population and human activity have more 

potential sources of solvents and a greater number of concentrated discharges to the environment. Sampling and monitoring 

of solvents in ground water above areas of increased levels of 

human activity (for example, areas of higher population 

density) is important.

In general, the detection frequencies of individual solvents 

were higher in ground water beneath urban land-use areas 

compared to ground water in aquifer studies (mixed land-use 

areas) or beneath agricultural land-use areas. The exception to 

this was the occurrence of methylene chloride that showed no 

relation to any type of land use. Urban areas have a greater 

number of sources of solvents to ground water and have more 

concentrated solvent discharges compared to rural areas. The 

ranking of detection frequencies of individual solvents in 

ground water beneath urban land-use areas was as follows: 

PCE, TCE, TCA, and methylene chloride. Again, the rankings 

probably are indicative of the number and strength of sources of 

each solvent and the transport and fate mechanisms affecting 

each. The lack of a relation between the occurrence of methylene chloride in ground water and land use suggests a different 

source for this solvent relative to the others. Some concentrations of methylene chloride may be the result of biodegradation 

of other VOCs like trichloromethane (chloroform) (Matheson 

and Tratnyek, 1994). Sampling of aquifers in areas of variable 

land use is important to most accurately represent water quality 

with respect to solvents.

The concentrations of PCE, TCE, and TCA were higher in 

ground water than in drinking water. The reason for higher 

overall concentrations of these solvents in ground water is not 

clear. However, the differences in the frequencies of concentrations greater than the MCLs for PCE and TCE were not statistically significant in any comparison between ground water, 

source water, and drinking water. Therefore, the distribution of 

concentrations of solvents greater than the MCL was not 

different with respect to the intended use of ground water. 

The probability of occurrence of solvents in ground water 

was associated with a variety of source, transport, and fate variables. Concentrated releases of solvents, like those from regulated hazardous waste facilities, were associated with the occurrence of solvents in ground water. However, the strongest 

sources associated with the occurrence of solvents were urban 

land use and population density. These variables represent 

aggregate surrogate terms for overall human input of solvents to 

the environment. 

It might be expected that a concentrated source of solvents 

like USTs at dry-cleaning facilities would be related to the probability of occurrence of solvents in ground water. But the 

number of USTs identified as linked with a dry-cleaning operation was very small. Only 68 samples (1.3 percent) had one or 

more dry-cleaning USTs in proximity, and only 32 samples 

(0.6 percent) had one or more leaking dry-cleaning USTs in 
52 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water

proximity. The numbers of samples with dry-cleaning USTs in 

proximity were too small to establish an association.

One explanation for the weak or non-existent association 

between the occurrence of solvents and concentrated sources 

like regulated hazardous waste facilities or dry-cleaning operations is that dispersed sources are more important in contributing solvents to ground water. Many potential sources of 

solvents exist, as described previously in this report, and most 

of these sources are dispersed. Thus, variables like population 

density and urban land use better represent these more dispersed 

and low-concentration sources of solvents.

Another possible explanation for the weaker association of 

solvents with concentrated sources compared to dispersed 

sources is the unique behavior of solvents in ground water. As 

mentioned, releases of pure solvent from a concentrated source 

like a UST can move directly through the saturated zone due to 

the density of these compounds relative to water. Once an 

impermeable layer is reached, the pure solvent could then move 

downslope with respect to gravity and migrate some distance 

from the original release site (fig. 30). Although the solvent had 

a concentrated source, movement of the pure phase along the 

impermeable layer has displaced it from the source. In addition, 

lateral spreading of solvents in the saturated zone due to 

geologic heterogeneities also can cause an offset in the location 

of the solvent from its original source (Pankow and Cherry, 

1996). Thus, concentrations of a solvent from a point release 

could be detected at a substantial distance from the release site 

and show no apparent association to it.

More research is needed into the specific sources of 

solvents to ground water. If concentrated sources like regulated 

hazardous waste facilities or dry-cleaning operations are found 

to be the main source of solvents to ground water, then it will be 

important to attempt to control these sources through siting 

regulations and UST protection. If dispersed sources like urban 

land use are found to be the main source of solvents to ground 

water, then it will be important to attempt to control these 

sources through stormwater management and treatment and 

best management practices.

Transport variables that were found to be most strongly 

associated with the occurrence of solvents were soil properties 

related to the hydraulic conductivity of the soil zone. Depth to 

the top of the aquifer and recharge were important for TCA. 

Aquifers with overlying soil zones that have high hydraulic 

conductivity may be more susceptible to contamination by 

solvents. Thus, it is important to understand the susceptibility of 

aquifers by fully ascertaining the hydraulic properties of the 

vadose zone.

The variable associated with fate of solvents in ground 

water was dissolved-oxygen content, with solvents being more 

persistent in relatively high dissolved-oxygen conditions 

compared to relatively low dissolved-oxygen conditions. This 

variable was strongly associated with one or more solvents and 

with PCE and TCA. Reductive dechlorination of PCE may be 

an important factor in controlling the occurrence of this solvent, 

and its daughter product TCE, in ground water. Understanding 

of the redox conditions of aquifers is important in fully evaluating solvent occurrence.

Summary

The U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Program collected or compiled 

data on solvents in water during 1993–2002. The data came 

from three sources: (1) the NAWQA Program that sampled 

ground water throughout the United States or compiled similar 

data, (2) a collaborative effort of the NAWQA Program and 

other organizations that sampled ground water used as a source 

of drinking water throughout the United States, and (3) the 

NAWQA Program that compiled data on drinking water from 

ground water for community water systems (CWSs) in 12 

Northeast and Mid-Atlantic States. Occurrence, distribution, 

and concentrations of solvents were examined for four 

commonly used solvents—methylene chloride, perchloroethene (PCE), 1,1,1-trichloroethane (TCA), and trichloroethene 

(TCE).

Out of 51 to 55 volatile organic compounds (VOCs) 

analyzed in samples, solvents were some of most frequently 

detected VOCs in all data sets. Although low concentrations of 

VOCs commonly were detected in ground water, PCE and TCE 

had higher median detected concentrations than other VOCs. 

Relative to other VOCs, solvents also were ranked high in terms 

of the frequency of concentrations greater than the Maximum 

Contaminant Level in all data sets. Mixtures were a common 

mode of occurrence of solvents. In samples of ground water or 

source water where at least one solvent was detected, multiple 

solvents occurred in 30 to 40 percent. When confined to 12 

Northeast and Mid-Atlantic States, the detection frequencies of 

solvents were similar among data sets. In general, the detection 

frequencies of solvents were higher in ground water underlying 

urban land-use areas compared to ground water underlying 

mixed or agricultural land-use areas.

The probability of occurrence of solvents in ground water 

was most strongly associated with dissolved-oxygen content. 

This variable represents processes that control the fate of 

solvents in ground water with solvents being more persistent in 

relatively high dissolved-oxygen conditions compared to relatively low dissolved-oxygen conditions. 

The probability of occurrence of solvents in ground water 

also was strongly and frequently associated with urban land use 

and population density. Although these dispersed sources were 

more strongly associated with solvent occurrence in ground 

water than were concentrated sources like regulated hazardous 

waste sites, concentrated sources like hazardous waste sites 

could contribute a substantial mass of solvents to ground water 

in proximity to these facilities. The probability of occurrence of 

solvents in ground water also was strongly associated with 

decreasing sand content of the soil and the depth to the top of 

the screened interval. These variables represent properties that 

affect the transport of solvents through the soil zone to ground 

water.

It is important for ground-water resource managers to 

understand the contamination potential posed by solvents, especially in resources that are critical as drinking-water supplies. 
References 53

Low-level analytical methods are most useful for determining 

the complete environmental distribution of solvents in ground 

water. Low-level analyses could be important for determining 

the sources of solvents, as a potential early warning on the 

vulnerability of an aquifer, and for determining trends and 

patterns in occurrence of solvents in aquifers. Evaluation of 

mixtures is important in determining the health implications of 

solvents in ground water, especially ground water used for 

drinking-water supplies. To protect ground-water resources, it 

is important for ground-water managers to (1) delineate the 

redox conditions of ground water in the aquifer in order to 

understand the fate of solvents, (2) determine and control the 

sources, or potential sources, of solvents to ground water, and 

(3) establish the susceptibility of aquifers by fully ascertaining 

the hydraulic properties of the saturated zone and vadose zone.

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Appendixes

Appendix 1. VOCs and their U.S. Environmental Protection Agency Maximum Contaminant Levels 

(U.S. Environmental Protection Agency, 2004a).

[Four solvents examined in this report are bolded. VOC, volatile organic compound; MCL, Maximum Contaminant Level; 

μg/L, microgram per liter]

VOC MCL

(μg/L) VOC MCL(μg/L)

Tetrachloromethane 5 1,2-Dichloropropane 5

1,2-Dichloroethane 5 trans-1,2-Dichloroethene 100

Methylbenzene 1,000 1,2,4-Trichlorobenzene 70

Benzene 5 1,4-Dichlorobenzene 75

Chlorobenzene 100 Chloroethene 2

Ethylbenzene 700 Trichloroethene (TCE) 5

Methylene chloride 5 cis-1,2-Dichloroethene 70

Perchloroethene (PCE) 5 Styrene 100

1,1-Dichloroethene 7 1,2-Dibromoethane .05

1,1,1-Trichloroethane (TCA) 200 Xylenes (mixed isomers) 10,000

1,1,2-Trichloroethane 5 1,2-Dibromo-3-chloropropane .2

1,2-Dichlorobenzene 600 Total trihalomethanes 80

Appendixes 61
Appendix 2. Ranking of 55 VOCs relative to detection frequency in ground water at an assessment level 

of 0.2 μg/L.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Chloroform 1 Vinyl chloride 29

Methyl tert-butyl ether 2 Isopropylbenzene 30

Perchloroethene (PCE) 3 Chloroethane 31

Trichloroethene (TCE) 4 tert-Amyl methyl ether 32

1,1,1-Trichloroethane (TCA) 5 o-Xylene 33

Toluene 6 n-Propylbenzene 34

Dichlorodifluoromethane 7 Chlorobenzene 35

Xylenes (mixed isomers) 8 n-Butylbenzene 36

Bromodichloromethane 9 1,4-Dichlorobenzene 37

1,1-Dichloroethane 10 Trichlorotrifluoroethane 38

Trichlorofluoromethane 11 1,2-Dichlorobenzene 39

Chloromethane 12 Diisopropyl ether 40

Methylene chloride 13 Ethylene dibromide 41

1,2,4-Trimethylbenzene 14 Bromomethane 42

1,2-Dichloropropane 15 Styrene 43

Benzene 16 1,1,2-Trichloroethane 144

cis-1,2-Dichloroethene 17 1,2,3-Trichlorobenzene 144

Bromoform 18 1,2,4-Trichlorobenzene 144

trans-1,2-Dichloroethene 19 1,3-Dichlorobenzene 144

Dibromochloromethane 20 Acrolein 144

1,1-Dichloroethene 21 Acrylonitrile 144

1,2,3-Trichloropropane 22 Ethyl tert-butyl ether 144

Naphthalene 23 Hexachlorobutadiene 144

1,2-Dichloroethane 24 Hexachloroethane 144

Dibromochloropropane 25 Vinyl bromide 144

m- & p-Xylene 26 cis-1,3-Dichloropropene 144

Ethylbenzene 27 trans-1,3-Dichloropropene 144

Carbon tetrachloride 28

1Compound not detected in any of the samples.

62 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water
Appendix 3. Ranking of 52 VOCs relative to detection frequency in source water at an assessment level 

of 0.2 μg/L.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Chloroform 1 Isopropylbenzene 27

Dibromochloromethane 2 Methylene chloride 28

Bromodichloromethane 3 1,2-Dichloropropane 29

Methyl tert-butyl ether 4 1,2,3-Trichloropropane 30

Bromoform 5 1,2-Dichlorobenzene 30

Perchloroethene (PCE) 6 1,4-Dichlorobenzene 30

Trichloroethene (TCE) 7 Bromomethane 30

1,1-Dichloroethane 8 Chlorobenzene 30

cis-1,2-Dichloroethene 8 Chloroethane 30

1,1,1-Trichloroethane (TCA) 10 Ethyl tert-butyl ether 30

1,1-Dichloroethene 11 Naphthalene 30

Carbon tetrachloride 12 Trichlorotrifluoroethane 30

Trichlorofluoromethane 12 Vinyl chloride 30

m- & p-Xylene 14 n-Butylbenzene 130

o-Xylene 15 1,1,2-Trichloroethane 141

Dichlorodifluoromethane 16 1,2,3-Trichlorobenzene 141

Toluene 17 1,2,4-Trichlorobenzene 141

Ethylbenzene 18 1,3-Dichlorobenzene 141

1,2-Dichloroethane 19 Acrylonitrile 141

Chloromethane 19 Hexachlorobutadiene 141

Styrene 21 Hexachloroethane 141

Benzene 22 Vinyl bromide 141

Diisopropyl ether 23 cis-1,3-Dichloropropene 141

Ethylene dibromide 23 n-Propylbenzene 141

tert-Amyl methyl ether 23 trans-1,2-Dichloroethene 141

1,2,4-Trimethylbenzene 26 trans-1,3-Dichloropropene 141

1

Compound not detected in any of the samples.

Appendixes 63
Appendix 4. Ranking of 51 VOCs relative to detection frequency in drinking water at an assessment level 

of 0.2 μg/L.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Chloroform 1 Trichlorofluoromethane 27

Bromodichloromethane 2 1,2-Dichlorobenzene 28

Dibromochloromethane 3 Chloroethane 29

Bromoform 4 Bromomethane 30

Methyl tert-butyl ether 5 1,2,4-Trimethylbenzene 31

1,1,1-Trichloroethane (TCA) 6 n-Butylbenzene 32

Perchloroethene (PCE) 7 Vinyl chloride 33

Trichloroethene (TCE) 8 n-Propylbenzene 34

Toluene 9 1,2,4-Trichlorobenzene 35

Xylenes (mixed isomers) 10 Ethylene dibromide 36

Methylene chloride 11 1,1,2-Trichloroethane 37

Ethylbenzene 12 Chlorobenzene 38

1,2-Dichloroethane 13 Styrene 39

1,1-Dichloroethane 14 1,2,3-Trichloropropane 40

o-Xylene 15 cis-1,3-Dichloropropene 41

cis-1,2-Dichloroethene 16 Hexachlorobutadiene 42

1,1-Dichloroethene 17 Dibromochloropropane 43

Chloromethane 18 Isopropylbenzene 44

Benzene 19 1,2,3-Trichlorobenzene 145

Dichlorodifluoromethane 20 1,3-Dichlorobenzene 145

Carbon tetrachloride 21 Acrolein 145

1,4-Dichlorobenzene 22 Acrylonitrile 145

m- & p-Xylene 23 Hexachloroethane 145

1,2-Dichloropropane 24 Trichlorotrifluoroethane 145

Naphthalene 25 trans-1,3-Dichloropropene 145

trans-1,2-Dichloroethene 26

1

Compound not detected in any of the samples.

64 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water
Appendix 5. Ranking of median concentration of 47 VOCs that had quantifiable concentrations in ground 

water, at an assessment level of 0.02 μg/L.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Naphthalene 1 Dibromochloromethane 25

Dibromochloropropane 2 Chlorobenzene 26

1,2-Dichloroethane 3 1,1,1-Trichloroethane (TCA) 27

1,2,3-Trichloropropane 4 1,4-Dichlorobenzene 28

Vinyl chloride 5 o-Xylene 29

n-Butylbenzene 6 Methylene chloride 30

Bromoform 7 Toluene 31

Methyl tert-butyl ether 8 Benzene 32

Dichlorodifluoromethane 9 Chloroethane 32

1,2-Dichloropropane 10 Styrene 34

tert-Amyl methyl ether 11 1,3-Dichlorobenzene 35

Trichloroethene (TCE) 12 n-Propylbenzene 36

Diisopropyl ether 13 1,2,4-Trimethylbenzene 37

Trichlorotrifluoroethane 14 1,1-Dichloroethene 38

1,2-Dichlorobenzene 15 Ethylbenzene 39

Trichlorofluoromethane 16 Chloromethane 40

Isopropylbenzene 17 1,1,2-Trichloroethane 41

Chloroform 18 m- & p-Xylene 42

cis-1,2-Dichloroethene 19 Hexachlorobutadiene 43

Bromomethane 20 trans-1,2-Dichloroethene 44

Perchloroethene (PCE) 20 Ethyl tert-butyl ether 45

Carbon tetrachloride 22 1,2,3-Trichlorobenzene 46

Bromodichloromethane 23 1,2,4-Trichlorobenzene 46

1,1-Dichloroethane 24

Appendixes 65
Appendix 6. Ranking of frequency of exceedance of Maximum Contaminant Levels for VOCs that had 

quantifiable concentrations in ground water.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Perchloroethene (PCE) 1 1,1,2-Trichloroethane 112

Dibromochloropropane 2 1,2,4-Trichlorobenzene 112

Trichloroethene (TCE) 3 1,2-Dichlorobenzene 112

Benzene 4 1,4-Dichlorobenzene 112

1,2-Dichloropropane 5 Carbon tetrachloride 112

Ethylene dibromide 6 Chlorobenzene 112

1,1-Dichloroethene 7 Ethylbenzene 112

Vinyl chloride 8 Styrene 112

Methylene chloride 9 Toluene 112

cis-1,2-Dichloroethene 10 trans-1,2-Dichloroethene 112

1,2-Dichloroethane 11 Trihalomethanes 112

1,1,1-Trichloroethane (TCA) 112 Xylenes 112

1

Frequency of exceedance was zero.

66 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water
Appendix 7. Ranking of median concentration of 40 VOCs that had quantifiable concentrations in source 

water, at an assessment level of 0.2 μg/L.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Bromomethane 1 Methyl tert-butyl ether 21

Dichlorodifluoromethane 2 Ethylene dibromide 22

1,1,1-Trichloroethane (TCA) 3 Styrene 23

Vinyl chloride 4 1,1-Dichloroethane 24

Chloroethane 5 1,2-Dichloroethane 25

Benzene 6 1,2-Dichloropropane 26

1,1-Dichloroethene 7 Carbon tetrachloride 26

Methylene chloride 8 1,2,4-Trimethylbenzene 28

Chlorobenzene 9 Bromodichloromethane 29

Trichloroethene (TCE) 10 Ethylbenzene 29

cis-1,2-Dichloroethene 11 Chloromethane 31

Toluene 12 o-Xylene 31

Diisopropyl ether 13 Isopropylbenzene 33

Trichlorofluoromethane 14 1,2-Dichlorobenzene 34

Trichlorotrifluoroethane 15 1,2,3-Trichloropropane 35

Perchloroethene (PCE) 16 tert-Amyl methyl ether 36

Bromoform 17 1,4-Dichlorobenzene 37

Dibromochloromethane 18 Ethyl tert-butyl ether 37

m- & p-Xylene 19 Naphthalene 39

Chloroform 20 n-Butylbenzene 40

Appendixes 67
Appendix 8. Ranking of frequency of exceedance of Maximum Contaminant Levels for VOCs that had 

quantifiable concentrations in source water.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Perchloroethene (PCE) 1 Benzene 16

Trichloroethene (TCE) 2 Carbon tetrachloride 16

1,1-Dichloroethene 3 Chlorobenzene 16

Ethylene dibromide 3 Methylene chloride 16

Vinyl chloride 5 Ethylbenzene 16

1,1,1-Trichloroethane (TCA) 16 Styrene 16

1,1,2-Trichloroethane 16 Toluene 16

1,2,4-Trichlorobenzene 16 cis-1,2-Dichloroethene 16

1,2-Dichlorobenzene 16 trans-1,2-Dichloroethene 16

1,2-Dichloroethane 16 Trihalomethanes 16

1,2-Dichloropropane 16 Xylenes 16

1,4-Dichlorobenzene 16

1

Frequency of exceedance was zero.

68 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water
Appendix 9. Ranking of median concentration of 44 VOCs that had quantifiable concentrations in 

drinking water, at an assessment level of 0.2 μg/L.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Xylenes (mixed isomers) 1 Isopropylbenzene 23

Bromomethane 2 Ethylbenzene 24

1,2,4-Trimethylbenzene 3 1,1,1-Trichloroethane (TCA) 25

m- & p-Xylene 4 Bromoform 26

1,2-Dichloropropane 5 Carbon tetrachloride 26

cis-1,2-Dichloroethene 6 Dibromochloromethane 26

Chloroform 7 Dibromochloropropane 26

Trichloroethene (TCE) 8 1,1-Dichloroethane 30

1,2,3-Trichloropropane 9 Methylene chloride 31

n-Propylbenzene 9 1,2-Dichlorobenzene 32

Methyl tert-butyl ether 11 n-Butylbenzene 33

Perchloroethene (PCE) 11 Hexachlorobutadiene 34

Vinyl chloride 13 o-Xylene 34

Benzene 13 1,1-Dichloroethene 36

trans-1,2-Dichloroethene 15 1,2,4-Trichlorobenzene 37

Dichlorodifluoromethane 16 Chloroethane 37

Bromodichloromethane 17 1,4-Dichlorobenzene 39

Styrene 17 1,1,2-Trichloroethane 40

Toluene 17 1,2-Dichloroethane 40

Chloromethane 20 cis-1,3-Dichloropropene 42

Trichlorofluoromethane 21 Chlorobenzene 43

Naphthalene 22 Ethylene dibromide 44

Appendixes 69
Appendix 10. Ranking of frequency of exceedance of Maximum Contaminant Levels for VOCs that had 

quantifiable concentrations in drinking water.

[Four solvents examined in this report are bolded]

VOC Rank VOC Rank

Perchloroethene (PCE) 1 Benzene 113

Trichloroethene (TCE) 2 1,1,1-Trichloroethane (TCA) 113

1,1-Dichloroethene 3 1,1,2-Trichloroethane 113

Trihalomethanes 4 1,2,4-Trichlorobenzene 113

Ethylene dibromide 5 1,2-Dichlorobenzene 113

Methylene chloride 6 1,4-Dichlorobenzene 113

Vinyl chloride 7 Chlorobenzene 113

1,2-Dichloropropane 8 Ethylbenzene 113

cis-1,2-Dichloroethene 9 Styrene 113

Dibromochloropropane 10 Toluene 113

1,2-Dichloroethane 11 trans-1,2-Dichloroethene 113

Carbon tetrachloride 11 Xylenes 113

1

Frequency of exceedance was zero.

70 Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water
This publication is available online at

 http://pubs.water.usgs.gov/sir20055268

Prepared by the National Water-Quality Assessment Program,

 Rapid City, South Dakota.

Edited by Ella M. Decker.

Graphics by Connie J. Ross.

Information regarding the National Water-Quality Assessment Program

 and the National Synthesis on Volatile Organic Compounds is

 available at http://water.usgs.gov/nawqa/ and

 http://sd.water.usgs.gov/nawqa/vocns/
Moran —Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water—SIR 2005–5268

Printed on recycled paper