# 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

# Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r a n d So ur ce Wate r i n the U n ited States
a n d i n D r i n ki n g Wate r i n 12 No r theast
a n d Mid-Atla n tic States, 199 3 –2002

By Michael J. Moran

U.S. Depa r tme n t of the I n te r io r U.S. Geological S ur vey 

# S cientific Investigat ions Report 2005–5268

## U.S. Depa r tme n t of the I n te r io r Gale A. Norton, Secretary

U.S. Geological S ur vey P. Patrick Leahy, Acting Director

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

For sale by U. S . Geological S urvey, Information S ervices Box 25286, Denver Federal Center Denver, CO 80225
For more information about the U S G S and its products: Telephone: 1-888-A S K-U S G S World Wide Web: ht tp://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 material s contained within this report.

Suggested citation: Moran, M.J., 2006, Occurrence and implic ations of selected chlorinated solvents in ground water and source water in the United S tates and in drinking water in 12 Northeast and Mid-Atlantic S tates, 1993–2002: U. S . Geological S urvey S cientific Investigations Report 2005–5268, 70 p.iii



# Fo r ewo r d

The U. S . Geological S urvey (U S G S ) is committed to serve the Nation with accurate and timely sci

entific information that helps enhance and protect the overall quality of life, and facilitates effec

tive management of water, biological, energy, a nd mineral resources. Information on the quality of the Nation’s water resources is of critical interest to the U S G S 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 U S G S implemented the National Water-Quality Assessment (NAWQA) Program to support national, regional, and local information needs and decisions related to water-quality manage

ment and policy. S haped by and coordinated with ongoing efforts of other Federal, S tate, 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.

S ince 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 S tudy Units. Collectively, these S tudy 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, prior

ity ecological resources, and agricultural, ur ban, 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. Co mprehensive assessments on pesticides, nutri

ents, volatile organic compounds, trace metals, and aquatic ecology are developed at the national scale through comparative analysis of the S tudy-Unit findings.

The U S G S places high value on the communication and dissemination of credible, timely, and rel

evant 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 as sessment by a single program cannot address all water-resource issues of interest. External coordination at all levels is critical for a fully inte

grated understanding of watersheds and for cost-effective management, regulation, and conser

vation of our Nation’s water resources. The Program, therefore, depends extensively on the advice, cooperation, and information from other Federal, S tate, interstate, Tribal, and local agen

cies, non-government organizations, industry, ac ademia, and other stakeholder groups. The assis

tance and suggestions of all are greatly appreciated.

Robert M. Hirsch Associate Director for Waterv



# Co n te n ts

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 Anthr opogenic 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53vi

Appendixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59 1. VOCs and their U.S. Environmental Protection Agency Maximum Contaminant Levels (U.S. Environmental Protecti on 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


# Fig ur es

1. Molecular diagram showing biotransformation of perchloroethene to trichloroethene through reductive dechlorination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2–6. Graphs showing: 2. Production of four solv ents 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 . . . . . . . . . . . . . . . . . . . . 28vii

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50viii



# 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 so lvents 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 8ix



# Ac r o n yms Used i n This Repo r t:

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

# Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r a n d So ur ce Wate r i n the U n ited States a n d i n D r i n ki n g Wate r i n 12 No r theast
a n d Mid-Atla n tic States, 199 3 –2002

By Michael J. Moran



# Abst r act

The U.S. Geological Survey has collected or compiled data on select chlorinated solven ts 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 availabl e 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 exam

ined—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 waterand 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 de tected in ground water, some solvents had higher median quanti fied concentratio ns 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 Contami

nant 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 pote ntial posed by solvents, espe

cially in resources th at are critical as dr inking-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 th e 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.



# I n t r od u ctio n

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 ar e used in a variety of commer

cial, industrial, manufacturing, and domestic applications. Chlorinated solvents (hereafter referred to as solvents) are used in the aerospace and electronics industries; dry cleaning; manu

facture of foam; paint removal/stripping; manufacture of phar

maceuticals; metal cleaning and degreasing; and wood manu

facturing (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 20 th century. Carbon tetrachloride was the first solvent produced in the United States an d was the main solvent used for the first half of the 20 th 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 comm only 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 an d has been declining since then due mostly to the human-health and environmental concerns2 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

associated with these compounds (Pankow and Cherry, 1996). For example, TCE has been linked to potential human-health effects including anemia, arthri tis, 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 depl etion potential in the upper atmo

sphere. 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 indus

trialized 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 en vironment. Once released to theenvironment, solvents have a tendency for widespread groundwater contamination due to th eir unique combination of phys

ical and chemical properties. Indeed, the four solvents exam

ined here are some of the mo st commonly identified organic chemicals in ground water (Pankow and Cherry, 1996; Squillace and others, 2002). The potential sources of solv ents 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 acco rding to information in the USEPA’s Toxics Release Inventory (TRI) database. The TRI provides information on the rele ase of toxic ch emicals 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 Protec

tion 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 Contami

nant Levels (MCLs) for solvents in drinking water at very low concentrations. Many of the solvents have water solubilities that are high relative to their MC Ls. 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 samp les 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 thes e four were selected because of their long histories of use and their large production and usage relative to other solvents. The occurrence and distributio n of these solvents were determined by detection frequenci es, ranges of concentrations, and areal patterns of detection. Concentrations of the solvents were compared to USEPA drin king-water standards. Occur

rence, 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 groun d water are identified. The iden

tification 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.


# Backg r o un d

The four solvents examined in this report—methylene chloride, PCE, TCA, and TCE—ar e members of the family of aliphatic halogenated hydrocarbons. These compounds are produced in substantial quantities, have a variety of industrial, commercial, and domestic uses, an d their release to the environ

ment has resulted in widespread contamination of ground water. The presence of solvents in gr ound water, especially ground water used as a source of drinki ng 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, incl uding chemical abstract numbers, molecular formulas, some common synonyms and abbrevia

tions, and the abbreviation used in this report for each compound.


## Physical a n d Chemical P r ope r ties a n d E n vi r o n me n tal Behavio r of Solve n ts

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 liq uid phase of each solvent has a density greater than water. If pure liquid phases of these solvents are released to the en vironment, they could penetrate the water table, move down through the saturated zone, andBackg r o un d 3

collect in pools on top of less permeable layers. Once pooled in such a manner, the absolute remo val 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 atmosphe re. Although th ey can readily volatilize, some mass of a solv ent 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 occur

rence of organic chemicals in ground water. The aqueous solu

bilities 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 tolu ene 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]

Compo un d

n ame Chemical Abst r act Se r vices Regist r y N u mbe r Chemical fo r m u la Commo n sy n o n yms a n d abb r eviatio n s Abb r eviatio n
u sed i n this

r epo r t
Methylene chloride 75-09-2 CH 2 Cl 2 Dichloromethane

Perchloroethene 127-18-4 Cl 2 C=CCl 2 Tetrachloroethene, perchloroethylene, tetrachloroethylene, PERC, PCE 1,1,1-Trichloroethane 71-55-6 CH 3 CCl 3 Methyl chloroform, MC, TCA

Trichloroethene 79-01-6 HClC=CCl 2 Trichloroethylene, TCE

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

[g/cm 3 , grams per cubic centimeter; °C, degrees Celsiu s; Pa, Pascals; mg/L, milligrams per liter; m 3 , cubic meters]

Compo un d

n ame Molec u la r weight 1 De n sity 2 (g/cm 3 at

25°C) Vapo r p r ess ur e 2 (Pa at 25°C) Aq u eo u s sol u bility 2 (mg/L at 25°C) O r ga n ic
ca r bo n pa r titio n i n g coefficie n t 2

(log Koc) He nr y’s Law
co n sta n t 2

at 25°C (Pa-m 3 / mole) Half-life i n g r o un d wate r 3 (i n 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

1 From Mackay and others, 1993.

2 Average of values from Mackay and others, 1993.
3 Average from Howard and others, 1991. -- PCE

TCA

TCE4 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Although the solubilities of the four solvents generally are higher than other commonly used VOCs, their absolute solubil

ities are low. Nonetheless, the aqueous solubilities of all the solvents examined in this repor t are much higher than their human-health benchmarks for drinking water. This means that even small spills of solvents ha ve the potential to cause contam

ination of ground water at concentrations that could be harmful to human health. Chemicals with high organic partitioning coefficients will adsorb strongly to soil and ro ck material. The organic parti

tioning coefficients of the solv ents 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 orga nic carbon partitioning coeffi

cients for both the solvents and common gasoline components are high enough to cause some retardation of their movement in the subsurface relative to th e 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 parti

tioning for organic chemicals in the environment. For chemicals with Henry’s Law constants greater than about 1 Pascal-cubic meter per mole (Pa-m 3 /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 viscos

ities 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 prov ide a source for long-term, low concentration contamin ation of ground water. Low interfacial tension also contributes to low retention capacities of soils for solvents.



## T r a n sfo r matio n of Solve n ts

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 biodeg

radation 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 excep

tions, the half-lives of the gaso line hydrocarbons generally are much smaller than the half-lives of the solvents. This means that the solvents transform very slowly relative to gasoline hydro

carbons 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 re duction 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 th e reaction are rarely met, and thus, microbes mediate most transformations of solvents (Pankow and Cherry, 1996). Bio tic processes for the transfor

mation 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 fo r organisms that use an electron acceptor other than oxygen (nitrate respiration); (3) through their use as a growth substr ate in acetogenic fermentation; (4) through co-metabolism; and (5) through their use as an electron acceptor under hypoxic conditions (reductive dechlori

nation) (Beek, 2001). Halogenated aliphatic organic compounds are strong oxidants, and thus anaerobic biodegrada

tion, anaerobic co-m etabolism, or their use as electron accep

tors are the most co mmon 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 elect ron 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 TC E through reductive dechlorina

tion using a reductive dehalogenase enzyme. Eventually, reduc

tive dechlorination can transform PCE to et hane by sequential removal of chlorine atoms (Swindoll and Troy, 1996).Hydrogen Chloride Hydrogen Chlorine

Carbon

Perchloroethene Trichloroethene Backg r o un d 5

Fig ur e 1. Biotransformation of perchloroethene to trichloroethene through reductive dechlorination.

Another possible avenue for transformation of solvents is through their use as electron donor s. 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 sulfat e-reducing and methanogenic bacteria that us e sulfate and carbon dioxide as electron acceptors (Chu and Alva rez-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 accepto r 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 occursthrough co-metabolism (Bielefeldt and others, 1995; MunkataMarr and others, 1996). Co-metabolism is the process whereby solvents are fortuitously degr aded as microbes consume other organic substrates. Although the so lvents are transformed in the respiration process, the microb es do not derive any direct benefit from the transformati ons (Swindoll and Troy, 1996). Only a small number of organic compounds have been identi

fied as serving as suitable prim ary 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 chlori

nated molecules. However, the specific transformation products formed during reductive dechlorina tion are a result of both the mechanism of degradation and the type of electron acceptors that are available (Swindoll an d Troy, 1996; Nyer and Duffin, 1997).



## P r od u ctio n a n d Usage of Solve n ts

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 va lues do not neces sarily indicate usage of solvents in the United States because substantial amounts of some solvents are exported. In addition, production may not accurately indicate usag e 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 wi dely as a chemical solvent and replaced more flammabl e 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 fabri

cated metal parts, in the pro duction of thermoplastics, in nonflammable adhesive formulations for industrial applica

tions, and as an extraction so lvent (Halogenated Solvents Industry Alliance, 2003). The larg est 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 (Haloge

nated Solvents Industry Alliance, 2003). Methylene chloride also can be found in a wide vari ety 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 us e 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 k ilograms (Halogenated Solvents Industry Alliance, 2003). In 2000, approximately 15 million kilograms of methylene chloride were imported and approxi

mately 57 million kilograms were exported (U.S. Department of Health and Human Services, 2005).



## Perchloroethene
In the late 1940s, PCE began replacing petroleum deriva

tives 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 work

place 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-tr ichoroethane; TCE, trichoroethene; CO 2 , carbon dioxide; CO, carbon monoxide]

Pa r e n t compo un d Possible t r a n sfo r matio n p r od u ct i n wate r Refe r e n ces

Methylene chloride chloromethane methane formaldehyde formic acid hydrogen chloride methanol CO 2 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 CO 2 Parsons and others, 1985; Fathepure and ot hers, 1987; van der Meer and others, 1992 Parsons and others, 1985; va n Eekert and Schraa, 2001 Parsons and others, 1985; va n der Meer and others, 1992 Swindoll and Troy, 1996; van Eekert and Schraa, 2001 Parsons and others, 1985; va n 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 CO 2 Parsons and others, 1985; Criddle and Mc Carty, 1991; Sempri ni 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 CO 2 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, 1 986; 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; Alva rez-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 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r**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**  Backg r o un d 7

Fig ur e 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 che mical intermediate in the manufacture of several hydrofluor ocarbons 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 de greasing, 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 treat

ments, water repellents, automo tive cleaners, silicone lubri

cants, 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 regula

tions (U.S. Department of Health and Human Services, 2005; Halogenated Solvents Industry Alliance, 2005). However, in recent years the demand for PCE has increased sl ightly from about 112 million kilograms in 1996 to about 125 million kilo

grams 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 fo r carbon tetrachloride. TCA is used mainly for vapor degreasin g and cold cleaning of fabri

cated metal parts and other materials. TCA also is used in fluo

ropolymer synthesis, as a solvent in adhesive and aerosol formulations, for the production of certain coatings and inks, for a variety of textile applications, for cl eaning motion picture film, and for dry cleaning leathe r and suede garments (Haloge

nated Solvents Industry Alliance, 1994). TCA can also be found8 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

in a variety of comm ercial 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 prod

ucts containing or made with TCA. Pursuant to the Clean Air Act, the USEPA accelerated a ph ase-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 kilo

grams of TCA produced in 1997 and 1998 (fig. 2). Under the Clean Air Act, TCA may be produ ced 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 Solven ts Industry Alliance, 2001). TCE is especially valuable as a metal degreaser because of its cleaning properties, low flammabilit y, 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 ch emical 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 cleaner s (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 Solve n ts

Solvents have been associated with both acute and chronic human-health problems. Acute health problems are those that reach a critical point in a relativ ely 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 diff erent depending on the route of exposure. There are numerous expos ure 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 solven ts, not all data indicate a nega

tive 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 po pulation. 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 inhalati on of methylene chloride in humans include decreased visual, auditory, and psychomotor functions and effects on the cen tral 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 centr al 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 demonstrat ed that methylene chloride crosses the placental barrier, an d 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 exposur e (Agency for Toxic Substances and Disease Registry, 2000). According to the U.S. Department of Health and Human Services (2005) in it s eleventh annual report on carcinogens, methylene chloride is reasonab ly anticipated to be a human carcinogen based on sufficient ev idence of carcinogenicity in experimental animals. The USEP A has set an MCL in drinkingBackg r o un d 9

water of 5 μg/L for methylene chloride to be protective of both cancer and noncancer h ealth effects. The USEPA also classifies methylene chloride as a group B2 carcinogen (probable human carcinogen), indicating that su fficient evidence in animals exists to classify it as a carci nogen but evidence in humans is lacking (U.S. Environmental Pr otection 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 inhalati on of PCE in humans include irritation of the upper respir atory 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 unconscious

ness. However, mice exposed acute ly to PCE from oral routes indicated low toxicity. The ma jor noncancer effects from chronic inhalation exposure to PCE in humans include neuro

logical effects, such as se nsory symptoms and headaches, impairments in cognitive and motor neurobehavioral func

tioning, color vision decremen ts, 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 occu

pationally exposed to inhalation of PCE. However, no defini

tive conclusions can be made becau se of the limi tations of the studies (Agency for Toxic Subs tances and Disease Registry, 1997a). In one study of residents exposed to drinking water contaminated with PCE and othe r solvents, there was an indica

tion that birth defects were asso ciated with exposure. However, no firm conclusions can be drawn from this study due to multiple chemical exposures a nd problems with the analysis (Agency for Toxic Substances and Disease Registry, 1997a). According to the U.S. Depart ment 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 stud ies 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 in cidence 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 associa

tion 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 Prot ection 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 contin uum between probable human carcinogen (B2) and possible human carcinogen (C). The Inter

national 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-h ealth effects of PCE but the study will not be completed until 2006 or later (U.S. Environ

mental 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), lighth eadedness, impaired balance, dizziness, nausea, vomiting, diarrhea, loss of consciousness, decreased blood pressure, and at axia. 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 expo sure. Most studies have not reported adverse effects from chronic oral or inhalation expo

sure 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 an d exposure of mothers or fathers to TCA. Animal studies have not reported develop

mental 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. Envi

ronmental 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 th at the chemical is not classifi

able as to its human carcinogenicity. The USEPA is currently reassessing the human-health effe cts of TCA but the study will not be completed until 2006 or later (U.S. Environmental Protection Agency, 2005b).10 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r


## Trichloroethene
Acute inhalation of TCE in humans primarily produces effects on the central nervous sy stems with symptoms such as sleepiness, fatigue, headache, confusion, and feelings of euphoria. Effects on the liver, ki dneys, 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, head

ache, sleepiness, nausea, confus ion, blurred vision, facial numbness, and weakness (Agency for Toxic Substances and Disease Registry, 1997b). Effects to the liver, kidneys, and immune and endocrine systems al so have been seen in humans occupationally exposed to inhalation of TCE or from contami

nated drinking water (U.S. Environmental Protection Agency, 2001). Some studies have indicated reproductive or develop

mental 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 inci dence of miscarriages. The pres

ence 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 s hown 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. Depart ment 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 su fficient evidence of carcinoge

nicity in experimental animals. An analysis of available epide

miological studies reports TCE ex posure to be associated with several types of cancers in huma ns, 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 report ed 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 bo th 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 cu rrently 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). 

## Pote n tial So ur ces of Solve n ts to G r o un d Wate r
There are numerous potentia l sources of solvents to ground water. Solvents in ground water can have two distinct sources—anthropogenic and natura l. Anthropogenic sources of solvents are defined in this re port 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 assess

ments, a fraction of the concentr ations 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 needle s, leached from them , and transported to ground water through recharge would be considered a natural

source. It may be possible, through isotopic fractionation, to iden

tify the component of solvents in ground water in some areas that may originate from anthr opogenic 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/strip

ping, electronics and pharmaceu tical manufacturing, and aero

space industries. In addition, sm all 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 co ncentrated (point source) or dispersed (nonpoint source) discharges. Potential sources of concentrated discharges includ e leaks from underground or aboveground tanks used for stor ing 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 wasteData a n d 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 transf er processes; land farming of contaminated waste or sludges; evaporative losses from manu

facturing or cleaning pr ocesses; runoff from agricultural areas where solvents have been applied (such as adjuvants in pesti

cides) 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 sout hern Finland near a metal scrap plant. However, the origin of th e 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 spru ce forest in Denmark. In ground water in the same area, chloro form, 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 m easurable 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 proces ses, 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 th an their background atmospheric concentrations. Nonetheless, the water equilibrium concentra

tions 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 a n d 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 Unit s were then aggregated and analyzed on a comprehensive national basis. Data on solvents also were collected from a stud y 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.



## Solve n t 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 consis

tent 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 f actors 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 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

1. Aquifer Studies —These studies were broad assessments of the occurrence and distri bution 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. An other important purpose of these studies was to determine which anthropogenic and natural factors most strongly affect the occurrence of contaminants in these shallo w 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 th e well was required to be within an area of land use classified as new residential/ commercial (Squillace and Price, 1996).
3. Shallow Ground-Water Stud ies 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 agri cultural 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 no t 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 in terest. For aquifer studies, existing wells were sampled to achieve a preliminary assess

ment (Gilliom and others, 1995). Domestic wells were selected preferentially over other types of wells, if available. Domestic wells were chosen because in mo st areas their di stribution 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 instal

lation of new monito ring 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 th e 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. Th e 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 prov ide a decadal assessment of ground-water quality without regard to temporal variations. If more than one environmental samp le 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. Alt hough NAWQA occasionally sampled nested wells in special studies, no data from nested wells were used in this analysis. Thus, each well repr esents 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, con centration 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 st udies 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 spec

trometry (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 concentrationresults for solvents by study type and well type for National WaterQuality Assessment data.

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

Well type Methyle n e chlo r ide 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 a n d Methods 1 3

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 assess

ment of VOCs (Bender and others, 1999). The four solvents examined in this repo rt are among these 55 VOCs. The list of 55 VOCs and the rationale and pr ocedures 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 we re analyzed to determine if systematic contamination of environmental samples was indi

cated. If systematic contaminat ion 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 Fe deral agencies have collected data on VOCs in aquifer studies with the same design character

istics and data-collection objectives as the NAWQA aquifer studies. The NAWQA Program has compiled some of these retrospective data sets. These re trospective 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, labo

ratory 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 retro

spective 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 retrospec

tive 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 retrospectiv e data, and thus , each well repre

sents a distinct geographic site. Only one sample of ground water from each well in the retr ospective 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 assess

ment. Only VOC analyses perfor med by GC/MS in a laboratory certified by the USEPA were included. Details of the require

ments 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 Methyle n e chlo r ide 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 1,18514 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

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 sma ller subset of VOCs. For inclu

sion 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 occur

rence, distribution, and status of solvents in ground water usedas 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, partici

pating 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 s ource 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-wate r 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 th at 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 usin g USEPA approved method 524.2 (Ivahnenko and others, 2001). Each sample from these wells was analyzed for 66 VOCs includ ing 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 wa ter, 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 repr esent 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 solven ts have MCLs, they are moni

tored 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 measur able 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 concentr ation, generally was computed using all samples available for each CWS. Using all detected concentrations avoided bias in selection of specific concentra

tions by CWS. Solvent data were available for a total of 1,682 CWSs. The number of systems with data on each individu al solvent in drinking-water samples was as follows: methylene chloride, 1,680; PCE, 1,681; TCA, 1,678; and TCE, 1,681.Data a n d Methods 15 

## Comp u tatio n al P r oced ur es
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 detec

tion 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 es timate 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 reporti ng levels may not reflect true differences in water quality. In stead, differences in detection frequencies between water-qualit y analytes may simply repre

sent differences in instrument performance, instrument sensi

tivity, equipment operators, or laboratory conditions. The laboratory reporting level is the minimum concentra

tion value at which a laboratory w ill 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 resolu

tion 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. Ho wever, the range of laboratory reporting levels varied for each compound (table 6). This vari

ability was probably due to differences in instrument perfor

mance (which often is compound dependent), instrument sensi

tivity, 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 sensi

tivity, 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 vari

ability 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 concen trations 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 concentra

tions are compared, differences between individual solvents and different data sets can be determined. For comparisons of solvent de tection frequencies between individual solvents in data from ground water and source water, an assessment level of 0.2 μg/L was used. The median labora

tory 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 b ecause 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 concen

trations 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 eff ective 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-trichoroetha ne; TCE, trichoroethene]

Compo un d

n ame Media n labo r ato r y r epo r ti n g 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 Ra n ge i n labo r ato r y r epo r ti n g levels (μg/L)

G r o un d wate r So ur ce wate r D r i n ki n g wate r G r o un d wate r So ur ce wate r D r i n ki n g wate r16 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

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 solv ents 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 detectio n 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 concentrat ion. 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. Implementa

tion 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 leve l 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 labo

ratory reporting level for methyl ene 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 concen

trations 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 consis

tent 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 analy zed 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 indi vidual 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; therefor e, 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 con centrations between solvents and other VOCs also were made using an assessment level. This was done to ensure th at 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 comparis ons of concentra

tions 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]

Compo un d

n ame Media n labo r ato r y
r epo r ti n g level (μg/L) Ra n ge i n labo r ato r y
r epo r ti n g levels (μg/L) Pe r ce n t of q u a n tified

co n ce n t r atio n s

> 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 71Data a n d Methods 17

Mixtures of solvents also we re computed using an assess

ment 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 specifi c 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 mix

ture median concentrations. For source water, an assessment level of 0.2 μg/L was used in computing mixture detection fre

quencies and mixture median con centrations. Because data on solvents in drinking water were summarized by CWS, an anal

ysis of mixtures would not be meaningful and was not attempted. An important point to note is that quantified concentra

tions 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. Th e presence of low concentrationinformation 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 fre quencies, 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 proper

ties. Most of these VOCs also have enforceable drinking-water standards or other important hu man-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 ap plies 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 resp ect to detection frequencies, concentrations, or frequencies of concentrations exceeding the MCL. The ranking of each VOC fo r 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 sour ce 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 thei r meaning. In all cases, nonpara

metric statistical tests were us ed because the data were not assumed to have any particular distribution. For all tests in this report, a level of sta tistical 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 in dependence, with Yates’ conti

nuity correction, was performed. Yates continuity correction was used to give a more conserva tive test of si gnificance. In the case of two independent groups of continuous data, a Wilcoxon rank-sum test, or Mann-Whitney test, was performed. In thecase of matched pairs of continuous data, a Wilcoxon signedrank test was used. To determine a relation between two contin

uous variables, a Spearman correl ation 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 concen

trations of solvents, quantile pl ots were used. Values less than the laboratory reporting level were given a default concentra

tion 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 distribu

tional differences, the nonparametric Kolmogorov-Smirnov test was used. For determining associations between multiple explana

tory 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 asso

ciations between solvent occurr ence and explanatory variables and to determine the strength an d direction of th e associations; the results were not used for predictive purpos es. Explanatory variables were selected that could provide general insight or understanding into the sources, transport processes, or environ

mental mechanisms that influen ce 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 occu rrence of the response variable18 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

in a manner similar to linear regression. The magnitude and sign of the estimated slope coefficien ts 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 β 0 β 1 X 1 ...β i X i + + ( )

1 e β 0 β 1 X 1 ...β i X i + + ( ) + ------------------------------------------------- =



### where P = probability of detecting a solvent; β 0 = the y-intercept; β i = slope coefficient of X i explanatory variables; and X

i = 1 to i explanatory variables.

Estimated slope coefficients wi th positive signs indicate an increase in the probability of detecting a solvent with an increase in the explanatory vari able, whereas estimated coeffi

cients with negative signs indi cate a decrease in the probability of detecting a solvent with an increase in the explanatory vari

able. Uncorrected estimated slope coefficients do not give an accurate assessment of the streng th of an association because the units of each variable, especial ly continuous variables, have large differences in magnitude and variance. Th erefore, stan

dardized coefficients were com puted in order to compare the slope coefficients directly be tween one another. The standard

ized coefficients indicate how many standard deviations of change in the dependent variable are associated with one stan

dard deviation of change in th e explanatory variable (Menard, 2002). Standardized coefficien ts were computed following Menard (2002). Explanatory variables were entered into the logistic regres

sion manually in a step-wise manner, and the regression was analyzed for significance at each step. For the overall regres

sion, 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 addi

tional coefficient was added, the Wald statistic of the coefficient was used to determine signifi cance. 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 signifi cantly associated with the prob

ability 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 iden

tical to the observed responses. Therefore, a higher p -value for this test indicates a better co rrespondence between the observed and predicted responses. A variety of explanatory data were used in the logistic regression analyses. These data represented various hydrogeo

logic and anthropogenic factors th at 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 variab le 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. 

# Occ urr e n ce of Solve n ts

The occurrence of solvents wa s 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 distri bution 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 occurren ce and concentrations of solvents among all three data sets.



## Occ urr e n ce a n d Dist r ib u tio n of Solve n ts i n G r o un d Wate r
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 so lvent 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 detec

tion 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 chlo

ride, 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 excep tion 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 meth ylene 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 indi

vidual 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/cm 3 , grams per cubic centimeter; ft, feet; in., inch ; °C, degrees Celsius; μS/cm, microsiemens per centimeter; km, kilometer; km 2 , 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 Assessm ent Program; USEPA, U.S. Envi ronmental Protection Agency; CIE SIN, Consortium for International Earth Science Information Network; TRI, Toxic Release Inventory; CERCLA, Comprehensive Environmental Response , Compensation, and Liability Act; RCRA, Resource Conservation and Recove ry Act; S, source; T, transport; F, fate]

Va r iable (type) U n its Desc r iptio n Codi n g So ur ce 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/cm 3 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 wate r 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) μ S/cm average specific conductance of water in the well continuous QWDATA Occ urr e n ce of Solve n ts 19

0–100
0–100
0–100

0–100

0.02–0.69

0–100

0–1
0–100Table 8. Hydrogeologic and anthropogenic variables that were used in the logistic regression analyses of solvents in ground water.—Continued

Va r iable (type) U n its Desc r iptio n Codi n g So ur ce Anthropogenic variables Population (S) persons/km 2 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 wate r 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 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

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 Number of samples**  **2,297**

**1,1,1-Trichloroethane Perchloroethene Trichloroethene Methylene chloride**  Occ urr e n ce of Solve n ts 21

Fig ur e 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 det ection 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 detec

tion frequencies highest for methylene chloride followed by TCE, PCE, and TCA. The source of PCE in studies in urban areas and in aquifer studies proba bly is releases of the solvent from industrial and commercial facilities where it is used, like dry cleaners. The source of met hylene chloride in agricultural areas could be the transformatio n 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 detecti on frequency of each individual solvent, with the exception of me thylene chloride, was higher in studies in urban areas than in aquifer studies (fig. 5). The detec

tion frequencies of PCE and TCA were higher in studies in agri

cultural areas than in aquifer studies. The source of low concen

trations 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 easte rn 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 i ndividual 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 agricul

tural 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)**  **Number of samples 832** 

**Shallow ground-water underlying agricultural areas 832** 

**847 847** 

**844 3,449** 

**3,497**

**3,498**

**3,487**

**715 720**  **723 723** 

**1,1,1-Trichloroethane Perchloroethene Trichloroethene Methylene chloride**  22 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 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 1,1,1-Trichloroethane**  **Perchloroethene Trichloroethene Methylene chloride Shallow ground-water underlying urban areas**  **Aquifer studies (mixed land use) Number of samples**  **543**

**Shallow ground-water underlying agricultural areas**

Fig ur e 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** Occ urr e n ce of Solve n ts 2 3

Fig ur e 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 con centrated in the Northeast region and several other locations such as eastern Iowa and California. The distribution of wells sa mpled 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, respectivel y. Without applying an assess

ment level, the detections of each solvent were spread throughout the conterminous United States. Detections of methylene chloride (fig. 8) appear to be concentr ated in the Great Lakes area and eastern Iowa whereas detections of PCE (fig. 9) appear to be concentrated in the North east region and in eastern Iowa. Detections of TCA (fig. 10) and TCE (fig. 11) appear to be concentrated in th e Northeast Region. One or more of the four solv ents 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 assess

ment 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 frequen

cies 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 24 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983

Fig ur e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 Occ urr e n ce of Solve n ts 25

Fig ur e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983
Fig ur e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 26 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 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 assessm ent 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 sour ces for both solvents (Haloge

nated 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).



## Occ urr e n ce a n d Dist r ib u tio n of Solve n ts i n So ur ce Wate r

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 rankin g of detection frequencies of solvents in source water was as follows: PCE, 6; TCE, 7; TCA, 10; and methyl ene 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 occu rrence 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 s ource 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 solven ts in source water were distrib

uted throughout the country. B ecause of the small number of detections for each individual so lvent, their geographic distri

butions 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 majo r 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]

Solve n t mixt ur e Assessme n t level of 0.2 μg/L Assessme n t level of 0.02 μg/L

Detectio n f r eq u e n cy i n g r o un d wate r (pe r ce n t)

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 Occ urr e n ce of Solve n ts 27

Media n
co n ce n t r atio n of each solve n t

i n mixt ur e

(μg/L) Detectio n f r eq u e n cy i n g r o un d wate r (pe r ce n t) Media n
co n ce n t r atio n of each solve n t

i n mixt ur e

(μg/L) De n omi n ato r is all samples i n which solve n ts
we r e a n alyzed De n omi n ato r is samples i n which o n e o r
mo r e solve n ts
we r e detected De n omi n ato r is all samples i n which solve n ts
we r e a n alyzed De n omi n ato r is samples i n which o n e o r
mo r e solve n ts
we r e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 28 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 12. Locations where source water from ground water was sampled for methylene chloride, perchloroethene, 1,1,1-trichloroethane, or trichl oroethene 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 mi xture 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.



## Occ urr e n ce a n d Dist r ib u tio n of Solve n ts i n D r i n ki n g Wate r

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 de tection 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 North

east 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, perchl oroethene; TCE, trichoroethene; TCA, 1,1,1-trichoroethane]

Detectio n f r eq u e n cy i n so ur ce wate r (pe r ce n t) Media n
co n ce n t r atio n of each solve n t

i n mixt ur e

(μg/L) De n omi n ato r is all samples i n which solve n ts
we r e a n alyzed De n omi n ato r is samples i n which o n e o r
mo r e solve n ts
we r e 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

0 0 NA TCA Methylene chloride

PCE

0 0 NA TCE Methylene chloride

TCE

0 0 NA TCA Methylene chloride

PCE

0 0 NA TCE

TCA Methylene chloride Occ urr e n ce of Solve n ts 29

Solve n t mixt ur eEXPLANATION CWS sampled with no detection of one or more solvents CWS sampled with detection of one or more solvents

Base modified from U.S. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 **0 125 250 MILES**

**0 125 250 KILOMETERS** 3 0 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 1 3

. Locations of community water systems (CWSs) that were sampled for methylene chloride, perchloroethene, 1,1,1-trichloroethane, or trichlor oethene 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 Occ urr e n ce of Solve n ts 3 1

Fig ur e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 **0 125 250 MILES**

**0 125 250 KILOMETERS** 3 2 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 **0 125 250 MILES**

**0 125 250 KILOMETERS** Occ urr e n ce of Solve n ts 33

Fig ur e 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. Geolo g ical S u r v e y di g ital data, 1 : 2,000,000, 1990 Albers Eq u al-Area P rojection North American Dat u m of 1983 **0 125 250 MILES**

**0 125 250 KILOMETERS** 3 4 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 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.Occ urr e n ce of Solve n ts 3 5


## Co n ce n t r atio n s of Solve n ts i n G r o un d Wate r

At an assessment level of 0.02 μg/L, the four solvents were ranked as follows with respect to the median quantified concen

trations 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 concen

tration of 0.02 μg/L or greater. The solvents do not have the largest concentrations among th e 47 VOCs with quantified concentrations and in stead 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 quan

tiles 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 concentra

tions 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 differen ce in the environmental distri

bution 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 great er than MCLs in ground water nationwide was as follows: PCE, 1; TCE, 3; and methylene chloride, 9 (Appendix 6). Unlike median qu antified concentra

tions, the frequencies of concentr ations 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 concentrat ions 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; meth

ylene chloride, 0.06 percent; and TCA, 0 percent. The numbers of samples in which concentrati ons 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 concentra

tions 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 ty pes of wells. For methylene chlo

ride, 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**

**0.1 1 10 100 0.01**  **0.001 0.0001**  **0.95 1.00**  **FRACTION OF DATA**

**Trichloroethene 1,1,1-Trichloroethane**  **Perchloroethene Methylene chloride** 

Fig ur e 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 rela tively high because there were few samples with these mixtures , and the concen trations 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 asse ssment 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 Solve n t

PCE TCE Methyle n e chlo r ide

Public
Domestic Monitoring
Other
Total 3 42
5 60
15 6 1
14 4 0
37 20 33 6 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r


## Co n ce n t r atio n s of Solve n ts i n So ur ce Wate r

The four solvents were ranked as follows with respect to the median quantified concentr ations 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 concen

tration. The median quantified concentrations of solvents in source water were as follows: TCA, 4.4 μg/L; methylene chlo

ride, 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 ch loride 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 high est 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**

**0.1 1 10 100 0.01**  **0.001 0.0001**  **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**

Fig ur e 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 concentratio ns 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: PC E, 6; TCE, 5; TCA, 0; and methylene chloride, 0. For PCE, the locations of public wells from which samples had concentr ations 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 concentra tions greater than the MCL

were as follows: New York, 2; Io wa, 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 solv ents 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).



## Co n ce n t r atio n s of Solve n ts i n D r i n ki n g Wate r
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 quanti

fied VOC concentrations for each CWS was the median of all quantified VOC concentrations fr om 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 ch loride; however, the concentra

tions of TCA, PCE, and TCE were nearly equal. The quantified concentration for each CWS wa s 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**

**0.1 1 10 100 0.01**  **0.001 0.0001**  **0.995**

**0.990 1.000**  **FRACTION OF DATA**

**Trichloroethene 1,1,1-Trichloroethane**  **Perchloroethene Methylene chloride**  Occ urr e n ce of Solve n ts 3 7

Fig ur e 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 concen

trations of solvents were available. TCA concentrations were not greater than the MCL in any samples from any CWS. The frequencies of concentrat ions 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; meth

ylene chloride, 0.2 percent; and TCA, 0 percent. The numbers of CWSs in which concentrati ons 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 concentra

tions greater than the MCL occurred in New York, Pennsyl

vania, and Massachusetts. For TCE, the vast majority of concentrations greater than th e 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.


## Compa r iso n s of Solve n t Occ urr e n ce a n d Co n ce n t r atio n s Betwee n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r
The occurrence of any one or mo re 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 thedrinking-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 so lvents 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 exam

ined nationwide, the detection frequencies of one or more solvents were about 7 percent for ground water and about 6 percent for source water. The di fferences in nationwide detec

tion frequencies of one or more solvents between ground water and source water were not signif icant at the 95-percent confi

dence 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

Fig ur e 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.3 8 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

The detection frequencies of each individual solvent in ground water, source water, and dr inking water are illustrated in figure 22. The detection frequencies were computed using an assessment level of 0.2 μg/L. Because the dr inking-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 compar isons 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 freque ncies 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; TC E, trichoroethene]

Compa r iso n p -val u e
Methyle n e

chlo r ide 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**  **Number of samples 957** 

**Drinking water**

**957 938** 

**954**

**957**

**98 93**  **95**

**96**

**1,678**

**1,681**

**1,680 1,681** 

Fig ur e 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.Occ urr e n ce of Solve n ts 3 9

The detection frequencies of individual solvents nation

wide 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 an d 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-percen t 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 ra nks 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 frequen

cies at the 95-percent confidence interval were significant in every land-use comparison for both PCE and TCE. No signifi

cant differences were found between detection frequencies by land use for methylene chloride . For TCA, differences between detection frequencies were signi ficant in every land comparison except mixed land use compared to agricultural land use.

Table 1 3

. 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 Solve n t r a n ki n g
Methyle n e

chlo r ide PCE TCA TCE

Ground water

Source water Drinking water

R a nk s um

**0 1**  **2 3**  **4 5** **DETECTION FREQUENCY, IN PERCENT Ground water**  **Source water Number of samples**  **4,996**

**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

Fig ur e 2 3

. 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]

Compa r iso n p -val u e
Methyle n e chlo r ide PCE TCA TCE

Mixed 1 land use and urban 2 land use 0.5497 0 0 0.0002
Mixed 1 land use and agricultural 3 land use .9491 0 .0559 .0001 Agricultural 3 land use and urban 2 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.
3 Data from NAWQA shallow ground-water studies in agricultural areas. 40 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

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 concen

trations 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 compar

isons with drinking water more equitable. For drinking water, the quantified concen tration 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 c oncentrations of each solvent in ground water, source water, and drinking water in the 12 North

east and Mid-Atlantic States. N one of the comparisons of the distributions of the concentratio ns were significantly different at the 95-percent confidence interval.

**CONCENTRATION, IN MICROGRAM PER LITER 0.97**  **0.98**

**0.1 1 10 100 0.01**  **0.001 0.0001**  **0.99 1.00**  **FRACTION OF DATA**

**Drinking water Ground water** 

Fig ur e 24. Quantiles of concentrations of methylene chloridein 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**

**0.1 1 10 100 0.01**  **0.001 0.0001**  **0.95 1.00**  **FRACTION OF DATA**

**Drinking water Source water**  **Ground water**

Fig ur e 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**

**0.1 1 10 0.01**  **0.001 0.0001**  **0.95 1.00**  **FRACTION OF DATA**

**Drinking water Source water**  **Ground water** Occ urr e n ce of Solve n ts 41

Fig ur e 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 0.1 1 10 100**  **0.01 0.001**  **0.0001 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**

Fig ur e 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-At lantic States. PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Compa r iso n p -val u e
Methyle n e

chlo r ide 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 great er than the MCLs relative to other VOCs. Table 16 also includes a rank sum for each solvent indicating the relative frequency of concen trations greater than MCLs for each solvent. The rankings fo r 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 concen

trations greater than the MCL were PCE and TCE. They were followed by methylene chloride and TCA, which had concen

trations 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 c oncentration of each solvent in each CWS was selected when multiple con centrations 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 N u mbe r of VOCs that exceeded

a n MCL Solve n t exceeda n ce f r eq u e n cy r a n ki n g Methyle n e

chlo r ide PCE TCA TCE

Ground water 11 9 1 1 12 3
Source water 5 1 6 1 1 6 2 Drinking water 12 6 1 1 13 2
R a nk s um 21 3 31 7

1 Frequency of exceedance of MCLs was 0 percent. 42 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

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 frequen

cies 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-tr ichoroethane; TCE, trichoroethene]

Compa r iso n p -val u e Methyle n e

chlo r ide 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 Number of samples**  **938**

**Drinking water**

**938**

**954**

**957 957**  **100 101**  **101**

**101 1,681** 

**1,681**

**1,680 1,678** 
**1,1,1-Trichloroethane Perchloroethene Trichloroethene Methylene chloride**  1

.5496

.322

Fig ur e 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.Occ urr e n ce of Solve n ts 4 3

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 concen

trations of individual solvents greater than the MCLs nation

wide 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 th e 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 ar eas. The concentrations of TCA were not greater than the MCL in any land-use area, and the concentrations of methylene ch loride greater than the MCL were less than 1 percent in mixed and agricultural land-use areas. The frequency of concentr ations 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]

Compa r iso n p -val u e Methyle n e

chlo r ide PCE TCA TCE

Mixed 1 land use

and urban 2 land

use 0.8439 0.0085 1 0.8211

Mixed 1 land use

and agricultural 3 land use .9814 .0558 1 .1374

Agricultural 3 land

use and urban 2 land use .9382 .0013 1 .1789

1 Data from National Water-Quality Assessment (NAWQA) aquifer studies.

2 Data from NAWQA shallow ground-water studies in urban areas.
3 Data from NAWQA shallow ground-wat er 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) Number of samples**  **3,449**

**Shallow ground-water underlying agricultural areas**

**3,449**

**3,497**

**3,498 3,487**  **844 832** 

**847**

**847 715**  **720 723** 

**723**
**1,1,1-Trichloroethane Perchloroethene Trichloroethene Methylene chloride** 
Fig ur e 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 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r 

# Associatio n s of Solve n ts with Hyd r ogeologic a n d A n th r opoge n ic Va r iables

The results of the logistic re gression analyses for one or more of the four solvents and individual solv ents are summa

rized in table 19. Table 19 lists the variables that were signifi

cantly associated with one or more solvents or individual solvents, the coefficient for each association in the logistic regression equation, the standa rdized coefficient, and the Hosmer-Lemeshow statistic p -value. Standardi zed coefficients greater than 0.1 were considered to be strongly associated with the probability of occu rrence of solvents a nd are highlighted in gray in table 19.



## Va r iables Associated With O n e o r Mo r e Solve n ts

The probability of detecting one or more of the four solvents was most strongly associ ated 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 down

ward movement of contaminants like solvents to ground water. However, in the vadose zone at low pressure head or water content, the unsaturated hydrau lic 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 pr operty, 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 popula

tion density in proximity to the wells (table 19). The probability of detecting one or more solven ts 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 th ere is an increase in multiple potential sources of solvents, su ch as commerci al, 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-met abolism), and therefore a higher concen

tration 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 sc reened 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 de pth to the top of the screened interval increases, the time requi red for a solvent to reach the screened interval by movement through the vadose and satu

rated zones also increases, provided there is no short cutting of traveltime to the screened inte rval (Moran and others, 2002). This increased time allows for increased attenuation of solvent concentrations through natural lo ss processes such as biodegra

dation, 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 fo r clean up. CERCLA sites may be sources of solvents to ground wate r if solvents were released at the site. Likewise, septic system s could be a source of solvents to ground water because many ty pes of household products contain solvents and it is likely that these are disposed of in septic systems. However, the nu mber 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, Comprehens ive Environmental Response, Compensation, and L iability Act; PCE, perchloroethene; TCA, 1,1,1-trichoroethane; TCE, trichoroethene]

Solve n t Associated va r iables (g r ay shadi n g i n dicates a weak 1 associatio n ) Type of

va r iable U n sta n da r dized coefficie n t i n logistic

r eg r essio n eq u atio n Sta n da r dized coefficie n t Hosme r Lemeshow

statistic p

-val u e

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. Associatio n s of Solve n ts with Hyd r ogeologic a n d A n th r opoge n ic Va r iables 45

0.714

0.093

0.160

0.174

0.13346 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r


## Va r iables Associated With I n divid u al Solve n ts

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 in creased. This variable repre

sents sources of methylene chlori de 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 cont ent 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 mecha

nisms 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 variab le for source of solvents to ground water from domestic ac tivities and possibly from private septic systems, with older homes having a greater like

lihood of contributing solvents to ground water. The probability of detecting PCE was most strongly asso

ciated with urban land use (table 19). The probability of detecting PCE increased as per cent urban land use around the wells increased. This variable re presents sources of PCE to the environment and is an aggregat e surrogate variable. The proba

bility of detecting PCE also was strongly associated with sand content of the soil, dissolved o xygen, 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 suscep tibility of a soil to detachment and transport by water (Natur al Resources Conservation Service, 2005). Some of the most important properties that affect soil erodibility are texture, organic matter content, struc

ture size and class, and saturated hydraulic conductivity of the soil (Natural Resources Conser vation Service, 2005). Soil erod

ibility factors are measured by applying a series of simulated rainstorms on freshly tilled plots and computed using soil char

acteristics 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 ox ic ground water, less biotrans

formation of PCE occurs by reductive dechlorination, and therefore, a higher concentrati on 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 nu mber 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 ge nerate, 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 rel eased 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 vari ables 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 surro

gate 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, th e probability of detecting a contaminant transported through the vadose zone also should increase. The probability of detecting TCA in ground water was weakly asso

ciated 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 diamet er of the well increased. The average diameter of the well cas ing is a surrogate for average discharge from the well. Larger diameter wells generally have larger pumping capaci ties. An examination of NAWQA data indicated a good correlation betw een average discharge from a well and average casing diamet er (Spearman’s rho 0.398; p = 0). Wells with higher pumping rates usually obtain water from larger volumes of an aquifer co mpared to wells with smaller pumping rates. Larger volumes of aquifer contribution to a wellAssociatio n s of Solve n ts with Hyd r ogeologic a n d A n th r opoge n ic Va r iables 47

generally are obtained from a larg er geographical area that can provide a greater number of potential sources of TCE to ground water.



## S u mma r y of Sig n ifica n tly Associated Expla n ato r y Va r iables

Table 20 is a summary of explan atory variables that were significantly associated with th e 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 wi th the variable, and the type of variable—source, transport, or fate. The rank sum was computed by ranking each variab le associated with each indi

vidual solvent according to the st andardized 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 sign ificantly associated with each solvent. These numbers were th en normalized relative to the total number of variable types available in the logistic regres

sion analyses and then ranked for each solvent. Lower ranks indicate a variable type that wa s 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 vari

ables 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 indi

vidual solvents that could be attributed to the fate of solvents in ground water. Dissolved oxygen was associated with the prob

ability of occurrence of one or more solvents and three indi

vidual solvents, and fate was the most important type of explan

atory variable for every indivi dual solvent except methylene chloride (table 21). In every case where dissolved oxygen was related to the probability of occurrence of solvents, the proba

bility of occurrence increased with increasing dissolved-oxygen content (table 19). This indicates that increased rates of degra

dation of solvents are occurring in conditions of low dissolved oxygen. Reductive transformatio n reactions are the most impor

tant for solvents in ground water (Beek, 2001). Thus, it is presumed that solvents are pref erentially degraded in conditions of relatively low dissolved oxygen in ground water and conse

quently are detected more freq uently 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, Comprehens ive Environmental Response, Compensation, and L iability Act]

Expla n ato r y va r iable Ra n k s u m N u mbe r of solve n ts associated with va r iable Type of

va r iable Associatio n st r e n gth

Dissolved oxygen 12 3 Fate Strong Urban land use 13 3 Source 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 SourceTable 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]

Va r iable type Ra n ki n g of sig n ifica n t associatio n s
Methyle n e

chlo r ide PCE TCA TCE

Source Transport

Fate 48 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

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 individua l solvent. In every case where either or both of these two variables were related to the proba

bility of occurrence of solven ts, 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 represen t aggregate surr ogate variables for input of solvents to groun d 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 solv ents 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 vari

ables 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 repre

sent sources and transport of solvents to ground water. Trans

port and sources variables were less important than fate in deter

mining the probability of o ccurrence 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 indi rectly affect the transport of solvents to ground water. In every case where soil sand content was associated with the probabi lity of occurrence of solvents, the probability of o ccurrence decreased wi th 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 conduc

tivity 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 so il 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 fo r 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 prob

ability 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 increas es provided there is no short cutting of traveltime to the aquifer. This increased time allows for increased attenuation of so lvent concentrations through natural loss processes such as biodegradation, sorption, disper

sion, 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 thes e variables were related to the probability of occurrence of solvents, the probability of occur

rence increased with increasing nu mber of sites near the wells (table 19). Unlike urban land use and population density, these terms may represent specific so urces of solvents to ground water or they could be surrogate s for indirect sources. The legis

lation that established CERCLA and RCRA sites was designed to protect ground water at hazard ous waste sites and to clean up uncontrolled hazardous wast e sites (U.S. Environmental Protection Agency, 2005a). Since the enactment of the legisla

tion that established these sites more than 20 years ago, data from thousands of hazardous wa ste 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 hazard ous waste sites in the United States indicated that four of th e top five most frequently identi

fied 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 chemi cals 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 vari ables, with one exception, represent the source or transpor t 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 probab ility of occurrence of solvents, the probability of o ccurrence increased w ith increasing number 1 222

1 233

3 111Associatio n s of Solve n ts with Hyd r ogeologic a n d A n th r opoge n ic Va r iables 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 eviden ce that septic systems can be sources of solvents to ground water. Many types of house

hold 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 chlo

ride. 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 meth

ylene chloride, along with two other VOCs, in septage and septic tank effluent from a house in Regina, Canada. Ayres Associates (1993) found met hylene 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 an aerobic reactor, and (2) the drainage field or leach field that acts as an aerobic reactor. Wa stewater 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 wa stewater. In addition to these purposes, the septic tank also act s as an anaer obic 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 f illed with gravel, called the leach field. In this area, further breakdown of organic material occurs by aerobic biodegradation. Because solvents primarily biotrans

form 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-org anic material in the septic tank is relatively short. Because of this relatively short resi

dence 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 trans

ferred more quickly to the leach fi eld. 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 associ

ated with the probability of o ccurrence of solvents were trans

port variables (table 20). Transport of solvents generally involves recharge, permeabil ity, and distance. Recharge, precipitation movi ng from the land surface to the water table, transports solvents to ground water through advection. Perme

ability facilitates, or impedes, the vertical movement of water from the surface to ground wate r and the movement of ground water through an aquifer. Dist ance 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 infr equently associated with the probability of occurrence of solvents in ground water (table 20). Soil bulk density and soil erodibility are believed to be surro

gates 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 uncon

fined 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 occur

rence of solvents regardless of the confining conditions of the aquifer. Casing diameter was weakly associated with the proba

bility of occurrence of TCE. Casing diameter is believed to be a surrogate for discharge from th e well. As discharge from the well increases, the well draws fr om a larger volume of water in the aquifer. Drawing from a larger volume of ground water increases the chances for includi ng ground water that is contam

inated by solvents. In addition, drawing from a larger volume of ground water also means drawing fr om a larger area of recharge to the aquifer, which increases the chances for intercepting releases of solvents by increasin g the zone of influence of the well and also increasing the ch ances for short circuiting trans

port of solvents through higher permeable zones. These mecha

nisms 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 dilu

tion 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 shallo w ground water can occur after long periods of time (Pankow and others, 1997). Recharge was only associated with the probab ility of occurrence of TCA. The reason for the lack of associati ons 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 relati on of solvents to recharge or even to their sources.Re c ha r ge 0 50 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

Fig ur e 3 0. Movement of solvents through an aquifer from a concentrated discharge release.

For TCA and TCE, transport was the least important type of variable associated w ith solvent occurrence (table 21). In the case of PCE, source and transpor t variables were both moder

ately important to solvent occu rrence. For methylene chloride, source and transport variables were the most important variable types although they were most ly only weakly associated.


# Implicatio n s

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 detect ed 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 occur

rence of solvents in ground-wa ter resources is important, espe

cially 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 detec

tion limits for so lvents are necessary to fully ascertain the environmental distribution of so lvents. Low-level analyses may not be important for concentrat ions of solvents relative to MCLs. However, low-level analyses could be important for determining the sources of solven ts, as a potential early warning regarding the vulnerability of an aquifer, and for determining trends and patterns in occurr ence 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 meth

ylene chloride. The similarity of the rankings is probably indic

ative of similarities in the nu mber and strength of sources of each solvent and the transport a nd 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 freque nt occurrence of this mixture probably is the result of the transformation of PCE to TCE through reductive dechlorination (fig. 1). Because reductive dechlori nation 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 greaterImplicatio n s 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, especi ally 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, exposu re 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 potentia l health effects from TCE expo

sure, 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 concentra

tions 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 great er 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 solv ents 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 solven ts (with the exception of meth

ylene 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 an d 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 concen

trated discharges to the enviro nment. 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 la nd-use areas. The exception to this was the occurrence of methyl ene 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 numb er and strength of sources of each solvent and the transport a nd fate mechanisms affecting each. The lack of a relation between the occurrence of meth

ylene chloride in ground water and land use suggests a different source for this solvent relative to the others. Some concentra

tions 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, TC E, and TCA were higher in ground water than in drinking water. The reason for higher overall concentrations of these so lvents in ground water is not clear. However, the differences in the frequencies of concentra

tions greater than the MCLs for PCE and TCE were not statisti

cally 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 vari

ables. Concentrated releases of solvents, like those from regu

lated hazardous waste facilities, we re associated with the occur

rence of solvents in ground water. However, the strongest sources associated with the occu rrence 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 co ncentrated source of solvents like USTs at dry-cleaning facilities would be related to the prob

ability of occurrence of solvents in ground water. But the number of USTs identified as linked with a dry-cleaning opera

tion 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 in52 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate r

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 solven ts and concentrated sources like regulated hazardous waste f acilities or dry-cleaning opera

tions is that dispersed sources are more important in contrib

uting solvents to ground wate r. 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 fo r the weaker association of solvents with concentrated s ources compared to dispersed sources is the unique behavior of solvents in ground water. As mentioned, releases of pure solv ent from a concentrated source like a UST can move directly through the saturated zone due to the density of these compound s relative to water. Once an impermeable layer is reached, the pure solvent could then move downslope with respect to grav ity 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 origin al source (Pankow and Cherry, 1996). Thus, concentrations of a solvent from a point release could be detected at a substantia l distance from the release site and show no apparent association to it. More research is needed in to the specific sources of solvents to ground water. If con centrated 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 rech arge 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 controllin g the occurrence of this solvent, and its daughter product TCE, in ground water. Understanding of the redox conditions of aquifers is important in fully evalu

ating solvent occurrence. 

# S u mma r y

The U.S. Geological Survey (USGS) National WaterQuality Assessment (NAWQA) Progr am 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 Stat es. Occurrence, distribution, and concentrations of solven ts were examined for four commonly used solvents—met hylene chloride, perchloroethene (PCE), 1,1,1-trichloroeth ane (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 con centrations than other VOCs. Relative to other VOCs, solvents also were ranked high in terms of the frequency of concentrat ions 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 proce sses that control the fate of solvents in ground water with solvents being more persistent in relatively high dissolved-oxygen conditions compared to rela

tively 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 sour ces 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 vari ables 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, espe

cially in resources that are cr itical as drinking-water supplies.Refe r e n ces 5 3

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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jamin, S.P., and Brown, J.F., 1995, Integrating multi-source land use and land cover data: U.S. Geological Survey Open File Report 95–652, 22 p. Zogorski, J.S., Carter, J.M., Ivahnenko, Tamara, Lapham,

W.W., Moran, M.J., Rowe, B.L., Squillace, P.J., and Toccalino, P.T., in press, The quality of our Nation’s waters—Volatile organic compounds in the Nation’s ground water and drinking-water supply wells: U.S. Geological Survey Circular 1292.

# Appe n dixesAppe n dix 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 organi c 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 (TC E ) 5

Meth y lene chlori d e 5 cis -1,2-Dichloroethene 70 Perchloroethene (PC E ) 5 Styrene 100 1,1-Dichloroethene 7 1,2-Dibromoethane .05 1,1,1-Trichloroeth a ne (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 Appe n dixes 61Appe n dix 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 Ra n k VOC Ra n k

Chloroform 1 Vinyl chloride 29 Methyl tert -butyl ether 2 Isopropylbenzene 30 Perchloroethene (PC E ) 3 Chloroethane 31
Trichloroe thene (TC E ) 4 tert -Amyl methyl ether 32 1,1,1-Trichloroeth a ne (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

Meth y lene chlori d e 13 Ethylene dibromide 41 1,2,4-Trimethylbenzene 14 Bromomethane 42 1,2-Dichloropropane 15 Styrene 43

Benzene 16 1,1,2-Trichloroethane 1 44

cis -1,2-Dichloroethene 17 1,2,3-Trichlorobenzene 1 44
Bromoform 18 1,2,4-Trichlorobenzene 1 44

trans -1,2-Dichloroethene 19 1,3-Dichlorobenzene 1 44
Dibromochloromethane 20 Acrolein 1 44 1,1-Dichloroethene 21 Acrylonitrile 1 44 1,2,3-Trichloropropane 22 Ethyl tert -butyl ether 1 44 Naphthalene 23 Hexachlorobutadiene 1 44 1,2-Dichloroethane 24 Hexachloroethane 1 44 Dibromochloropropane 25 Vinyl bromide 1 44

m - & p -Xylene 26 cis -1,3-Dichloropropene 1 44 Ethylbenzene 27 trans -1,3-Dichloropropene 1 44
Carbon tetrachloride 28

1 Compound not detected in any of the samples. 62 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate rAppe n dix 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 Ra n k VOC Ra n k

Chloroform 1 Isopropylbenzene 27
Dibromochloromethane 2 Meth y lene chlori d e 28

Bromodichloromethane 3 1,2-Dichloropropane 29 Methyl tert -butyl ether 4 1,2,3-Trichloropropane 30
Bromoform 5 1,2-Dichlorobenzene 30 Perchloroethene (PC E ) 6 1,4-Dichlorobenzene 30
Trichloroe thene (TC E ) 7 Bromomethane 30 1,1-Dichloroethane 8 Chlorobenzene 30

cis -1,2-Dichloroethene 8 Chloroethane 30 1,1,1-Trichloroeth a ne (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 1 30

o -Xylene 15 1,1,2-Trichloroethane 1 41
Dichlorodifluoromethane 16 1,2,3-Trichlorobenzene 1 41
Toluene 17 1,2,4-Trichlorobenzene 1 41 Ethylbenzene 18 1,3-Dichlorobenzene 1 41 1,2-Dichloroethane 19 Acrylonitrile 1 41
Chloromethane 19 Hexachlorobutadiene 1 41 Styrene 21 Hexachloroethane 1 41

Benzene 22 Vinyl bromide 1 41 Diisopropyl ether 23 cis -1,3-Dichloropropene 1 41 Ethylene dibromide 23 n -Propylbenzene 1 41

tert -Amyl methyl ether 23 trans -1,2-Dichloroethene 1 41 1,2,4-Trimethylbenzene 26 trans -1,3-Dichloropropene 1 41

1 Compound not detected in any of the samples. Appe n dixes 6 3Appe n dix 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 Ra n k VOC Ra n k

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-Trichloroeth a ne (TCA) 6 n -Butylbenzene 32 Perchloroethene (PC E ) 7 Vinyl chloride 33 Trichloroethene (TC E ) 8 n -Propylbenzene 34
Toluene 9 1,2,4-Trichlorobenzene 35 Xylenes (mixed isomers) 10 Ethylene dibromide 36

Meth y lene chlori d e 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 1 45

Dichlorodifluoromethane 20 1,3-Dichlorobenzene 1 45
Carbon tetrachloride 21 Acrolein 1 45
1,4-Dichlorobenzene 22 Acrylonitrile 1 45

m - & p -Xylene 23 Hexachloroethane 1 45 1,2-Dichloropropane 24 Trichlorotrifluoroethane 1 45 Naphthalene 25 trans -1,3-Dichloropropene 1 45

trans -1,2-Dichloroethene 26

1 Compound not detected in any of the samples. 64 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate rAppe n dix 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 Ra n k VOC Ra n k
Naphthalene 1 Dibromochloromethane 25 Dibromochloropropane 2 Chlorobenzene 26 1,2-Dichloroethane 3 1,1,1-Trichloroeth a ne (TCA) 27 1,2,3-Trichloropropane 4 1,4-Dichlorobenzene 28 Vinyl chloride 5 o -Xylene 29

n -Butylbenzene 6 Meth y lene chlori d e 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 (TC E ) 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 (PC E ) 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 Appe n dixes 65Appe n dix 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 Ra n k VOC Ra n k
Perchloroethene (PC E ) 1 1,1,2-Trichloroethane 1 12 Dibromochloropropane 2 1,2,4-Trichlorobenzene 1 12 Trichloroethene (TC E ) 3 1,2-Dichlorobenzene 1 12

Benzene 4 1,4-Dichlorobenzene 1 12 1,2-Dichloropropane 5 Carbon tetrachloride 1 12 Ethylene dibromide 6 Chlorobenzene 1 12 1,1-Dichloroethene 7 Ethylbenzene 1 12 Vinyl chloride 8 Styrene 1 12

Meth y lene chlori d e 9 Toluene 1 12

cis -1,2-Dichloroethene 10 trans -1,2-Dichloroethene 1 12 1,2-Dichloroethane 11 Trihalomethanes 1 12 1,1,1-Trichloroeth a ne (TCA) 1 12 Xylenes 1 12

1 Frequency of exceedance was zero. 66 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate rAppe n dix 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 Ra n k VOC Ra n k

Bromomethane 1 Methyl tert -butyl ether 21
Dichlorodifluoromethane 2 Ethylene dibromide 22 1,1,1-Trichloroeth a ne (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

Meth y lene chlori d e 8 1,2,4-Trimethylbenzene 28
Chlorobenzene 9 Bromodichloromethane 29 Trichloroethene (TC E ) 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 (PC E ) 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 Appe n dixes 67Appe n dix 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 Ra n k VOC Ra n k
Perchloroethene (PC E ) 1 Benzene 1 6 Trichloroethene (TC E ) 2 Carbon tetrachloride 1 6 1,1-Dichloroethene 3 Chlorobenzene 1 6 Ethylene dibromide 3 Meth y lene chlori d e 1 6 Vinyl chloride 5 Ethylbenzene 1 6 1,1,1-Trichloroeth a ne (TCA) 1 6 Styrene 1 6
1,1,2-Trichloroethane 1 6 Toluene 1 6 1,2,4-Trichlorobenzene 1 6 cis -1,2-Dichloroethene 1 6 1,2-Dichlorobenzene 1 6 trans -1,2-Dichloroethene 1 6
1,2-Dichloroethane 1 6 Trihalomethanes 1 6 1,2-Dichloropropane 1 6 Xylenes 1 6 1,4-Dichlorobenzene 1 6

1 Frequency of exceedance was zero. 68 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate rAppe n dix 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 Ra n k VOC Ra n k
Xylenes (mixed isomers) 1 Isopropylbenzene 23
Bromomethane 2 Ethylbenzene 24 1,2,4-Trimethylbenzene 3 1,1,1-Trichloroeth a ne (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 (TC E ) 8 1,1-Dichloroethane 30 1,2,3-Trichloropropane 9 Meth y lene chlori d e 31

n -Propylbenzene 9 1,2-Dichlorobenzene 32 Methyl tert -butyl ether 11 n -Butylbenzene 33 Perchloroethene (PC E ) 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 Appe n dixes 69Appe n dix 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 Ra n k VOC Ra n k
Perchloroethene (PC E ) 1 Benzene 1 13 Trichloroethene (TC E ) 2 1,1,1-Trichloroeth a ne (TCA) 1 13
1,1-Dichloroethene 3 1,1,2-Trichloroethane 1 13
Trihalomethanes 4 1,2,4-Trichlorobenzene 1 13 Ethylene dibromide 5 1,2-Dichlorobenzene 1 13

Meth y lene chlori d e 6 1,4-Dichlorobenzene 1 13 Vinyl chloride 7 Chlorobenzene 1 13 1,2-Dichloropropane 8 Ethylbenzene 1 13

cis -1,2-Dichloroethene 9 Styrene 1 13 Dibromochloropropane 10 Toluene 1 13 1,2-Dichloroethane 11 trans -1,2-Dichloroethene 1 13
Carbon tetrachloride 11 Xylenes 1 13

1 Frequency of exceedance was zero. 70 Occ urr e n ce a n d Implicatio n s of Selected Chlo r i n ated Solve n ts i n G r o un d Wate r , So ur ce Wate r , a n d D r i n ki n g Wate rThis publi c ation is available online at http://pubs.wate r .usgs.gov/si r 20055268 P r epa r ed by the National Wate r -Quality Assessment P r og r am, Rapid C ity, S outh Dakota. Edited by Ella M. De c ke r

. Gr aphi c s by C onnie J. Ross.

Info r mation r ega r ding the National Wate r -Quality Assessment P r og r am and the National S ynthesis on Volatile O r gani c C ompounds is available at http://wate r .usgs.gov/nawqa/ and http://sd.wate r .usgs.gov/nawqa/vo c ns/Moran —Occurrence and Implications of Selected Chlorinated Solvents in Ground Water, Source Water, and Drinking Water—SIR 2005–5268

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