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Environ. Sci. Technol. 2007, 41, 74-81

Chlorinated Solvents in Groundwater of the United States MICHAEL J. MORAN,* JOHN S. ZOGORSKI, AND PAUL J. SQUILLACE U.S. Geological Survey, 1608 Mountain View Road, Rapid City, South Dakota 57702

Four chlorinated solventssmethylene chloride, perchloroethene (PCE), 1,1,1-trichloroethane, and trichloroethene (TCE)swere analyzed in samples of groundwater taken throughout the conterminous United States by the U.S. Geological Survey. The samples were collected between 1985 and 2002 from more than 5,000 wells. Of 55 volatile organic compounds (VOCs) analyzed in groundwater samples, solvents were among the most frequently detected. Mixtures of solvents in groundwater were common and may be the result of common usage of solvents or degradation of one solvent to another. Relative to other VOCs with Maximum Contaminant Levels (MCLs), PCE and TCE ranked high in terms of the frequencies of concentrations greater than or near MCLs. The probability of occurrence of solvents in groundwater was associated with dissolved oxygen content of groundwater, sources such as urban land use and population density, and hydraulic properties of the aquifer. The results reinforce the importance of understanding the redox conditions of aquifers and the hydraulic properties of the saturated and vadose zones in determining the intrinsic susceptibility of groundwater to contamination by solvents. The results also reinforce the importance of controlling sources of solvents to groundwater.

Introduction Chlorinated solvents are used in a variety of commercial and industrial applications and can be found in a variety of household and consumer products (1). Four solvents were selected for analysis in this studysmethylene chloride (also known as dichloromethane), perchloroethene (PCE), 1,1,1trichloroethane (TCA), and trichloroethene (TCE)sbecause of their long histories of use, their large production and usage relative to other solvents, and their relatively common occurrence in groundwater resources (2). Large quantities of these four solvents have been, and continue to be, used by many commercial and industrial sectors in the United States. For example, as of 2000 PCE was the cleaning solvent for nearly all of the approximately 30,000 dry cleaners and launderers in the United States (3). Solvents are frequently released to the environment. According to the U.S. Environmental Protection Agency (USEPA) Toxic Release Inventory, during 1998-2001 total on- and off-site releases of methylene chloride, PCE, TCA, and TCE averaged about 33 million pounds, 4 million pounds, 0.5 million pounds, and 11 million pounds, respectively (4). Furthermore, methylene chloride, PCE, and TCE are among * Corresponding author phone: (605) 394-3244; fax: (605) 3554523; e-mail: [email protected]. 74

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29 of the chemicals, metals, and other substances most commonly found at USEPA Superfund sites (5). The four solvents examined here are some of the most commonly identified organic chemicals in groundwater (2, 6). Once released to the environment, solvents have a tendency for widespread groundwater contamination due to their unique combination of physical and chemical properties. For example, relative to some other commonly used volatile organic compounds (VOCs) solvents have high vapor pressures, high solubilities, low organic partitioning coefficients, and low viscosities and interfacial tensions. Solvents have been associated with both acute and chronic human-health problems including liver damage and possible kidney effects, spontaneous abortions, reduced fertility, cancer, and childhood leukemia. Because of potential health effects, the USEPA has set Maximum Contaminant Levels (MCLs) for some solvents in drinking water at very low concentrations (7). The purpose of this paper is to examine the nature and extent of solvent contamination in groundwater in the United States at a national scale and to link this water-quality information with natural (hydrogeologic) and human (anthropogenic) factors that might control or influence the occurrence of solvents in groundwater. The occurrence and distribution of the four solvents are described by detection frequencies, ranges of concentrations, and areal patterns of detection. Concentrations of solvents in groundwater were compared to MCLs to determine the quality of groundwater relative to standards applied to drinking water for human consumption. Associations between the occurrence of solvents in groundwater and hydrogeologic and anthropogenic variables were examined using logistic regression. This study represents a first-time assessment of the occurrence of these commonly used solvents in groundwater resources of the entire United States and utilizes low-level analytical methods for determining concentrations. In contrast, this study does not represent an assessment of solvent occurrence at point-source releases such as landfills, leaking underground storage tanks, or Superfund sites. Instead, this study investigates the occurrence of solvents in aquifers and shallow groundwater at a large scale and provides a general characterization of water-quality conditions across the Nation as a whole. The national scale and low-level analytical methods used here make this study unique in identifying the patterns of occurrence of solvents in groundwater and in identifying trends in the occurrence and concentrations of solvents, especially in groundwater that may be used now or in the future for human consumption.

Experimental and Methods Solvent and Ancillary Data. Data on the occurrence of four solvents were collected or compiled by the U.S. Geological Survey’s (USGS) National Water-Quality Assessment (NAWQA) Program. The NAWQA Program was designed to describe occurrence of contaminants and trends in the quality of the Nation’s groundwater and surface water resources (8). In this study, data from three types of groundwater surveys completed by the NAWQA Program were used. These surveys can be summarized as follows 1. Aquifer Surveys. These provide a broad overview of groundwater quality within each NAWQA Study Unit. This was accomplished by sampling large areal and depth dimensions of aquifers considered locally and regionally as important sources of drinking-water supply (8). More than 10.1021/es061553y CCC: $37.00

 2007 American Chemical Society Published on Web 12/01/2006

50 individual NAWQA aquifer surveys around the country sampled groundwater and analyzed the samples for solvents. 2. Shallow Groundwater Surveys in Urban Areas. These were designed to determine the quality of recently recharged (generally less than 10 years old) shallow groundwater generally underlying large metropolitan areas (8). For some surveys, at least 75% of a 500-meter radial area around the well was required to be within an area of land use classified as “new” residential/commercial (9). Twenty individual NAWQA surveys sampled shallow groundwater beneath urban areas and analyzed the samples for solvents. 3. Shallow Groundwater Surveys in Agricultural Areas. These were designed to define the quality of recently recharged shallow groundwater underlying areas of intensive agricultural land use (8). At least 75% of a 500-meter radial area around the well was required to be within an area of land use classified as agricultural. Four individual NAWQA surveys sampled shallow groundwater beneath agricultural areas and analyzed the samples for solvents. The characterization of water quality in each of the three surveys generally was achieved by sampling 20-30 spatially distributed, randomly selected wells throughout each aquifer or shallow groundwater area of interest. For aquifer surveys, existing domestic wells were selected because their distribution best fit the study objective of assessing the groundwater quality of aquifers using randomly selected and spatially distributed sampling points (8). Sampling of other well types, such as public, monitoring, and other, was also performed in aquifer studies if domestic wells were not available or if sampling of these well types met the objectives of the aquifer surveys. In the case of domestic wells, samples were taken prior to any treatment or before storage in a holding or pressure tank. In the case of public wells, samples were taken prior to treatment and distribution. For shallow groundwater surveys, many monitoring wells were installed by NAWQA to meet the criteria for sampling the uppermost part of the groundwater system using lowcapacity or observation wells. In situations where new wells could not be installed, existing wells were sampled. Groundwater samples were collected from a total of 3,883 wells in NAWQA aquifer or shallow groundwater surveys around the country. The total number of samples of groundwater available for each individual solvent was as follows: methylene chloride, 3,877; PCE, 3,811; TCA, 3,883; and TCE, 3,879. NAWQA sampling in aquifer surveys did not cover all areas of the United States. Some local, State, and Federal agencies have collected data on VOCs in aquifer surveys that have design characteristics and data-collection objectives similar to those of NAWQA and in some cases these aquifers do not overlap with NAWQA sampling. These are referred to as retrospective data and were considered to be similar enough in design to the NAWQA surveys to augment them and provide a broader national coverage of groundwater quality. Retrospective data were used to supplement NAWQA aquifer survey data only if they met specific criteria in terms of monitoring objectives, design, well construction, methods of sample collection, laboratory analysis, and quality control (10). The NAWQA Program has compiled some of these retrospective data, with information from 1,185 wells included in this study. Additional details on the design of the data compilation and characteristics of the retrospective data set can be found elsewhere (10, 11). The number of samples of groundwater available from retrospective data for each solvent was as follows: methylene chloride, 1,177; PCE, 1,185; TCA, 1,185; and TCE, 1,185. Thus, the total number of samples of groundwater analyzed for each individual solvent, including both aquifer surveys and shallow groundwater

surveys, was as follows: methylene chloride, 5,054; PCE, 4,996; TCA, 5,068; and TCE, 5,064. A variety of ancillary data were used in the relational analyses to represent various hydrogeologic and anthropogenic processes that could control or influence the sources, transport, or fate of solvents in groundwater. The ancillary variables used in the logistic regression analyses are shown in Table 1 of the Supporting Information. Ancillary data were available in a variety of geospatial formats including polygon data, grid data, and point data. Averaging of ancillary data was necessary since it was not possible to obtain a direct value for each ancillary variable at each well. For polygon data, the value of an ancillary variable at a well was computed using area-weighted averages of 500-meter or 1-kilometer buffers. For grid and point data, the value of an ancillary variable at a well was computed as averages of linear interpolations of the values of the nearest grid centers or points. Sampling and Analytical Methods. The goal of this study was to provide a national-scale assessment of groundwater quality without regard to temporal variations. Samples of groundwater were collected during 1985-2002 and represent the general water quality at the sampling point during this time period. Although some wells were re-sampled for analysis of temporal changes in water quality, only the first sample collected by the NAWQA Program from each well was included in this study. Field collection of groundwater samples by the NAWQA Program followed prescribed, consistent protocols including the collection of quality-control samples (12). Quality-control samples were analyzed to determine if systematic contamination of groundwater samples was indicated. Groundwater samples suspected of having systematic contamination were excluded from this study. Additional information about the quality-control techniques and samples used in this study is presented elsewhere (13). All groundwater samples collected for NAWQA studies were analyzed at the USGS National Water Quality Laboratory (NWQL) in Denver, Colorado, using gas chromatography/ mass spectrometry (GC/MS) (14, 15). Samples were analyzed for as many as 86 different VOCs (14); however, only 55 VOCs were included in NAWQA’s national assessment of VOCs as compounds of interest (16). The four solvents examined in this study are among these 55 VOCs. Samples analyzed by the NWQL prior to April 1996 had a uniform laboratory reporting level for most VOCs, including the four solvents, of 0.2 micrograms per liter (µg/L). Implementation of a new low-level analytical method after this date resulted in lower laboratory reporting levels for many VOCs including the four solvents (15). However, the laboratory reporting levels for most VOCs were different from one another and also varied as method changes were implemented or new instrumentation was used (17). Although the specific laboratory reporting levels for each of the four solvents analyzed by the low-level method were not identical, the median laboratory reporting level for the four solvents was about 0.02 µg/L. Statistical Methods. Information on the techniques used to compute detection frequencies, determine assessment levels, and apply statistical tests can be found elsewhere (1820). Because of differences in the laboratory reporting levels for each solvent, when comparing detection frequencies or median concentrations between solvents a common concentration level was used. In most cases, the concentration level used was 0.2 µg/L. However, in cases where it was important to examine the occurrence of solvents at low concentrations, a concentration level of 0.02 µg/L was used. Logistic regression was used to determine relations or associations between the occurrence of solvents in groundwater and various hydrogeologic and anthropogenic variVOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Locations of wells sampled for four solvents and locations of wells where one or more solvents were detected, using all available concentrations. ables. Systat 11 software package was used to perform the analyses. The relational analyses were performed to determine factors associated with the occurrence of solvents. This identification may further understanding of the sources and pathways of solvents to groundwater and the vulnerability of aquifers to solvent contamination. The relational analyses were not performed to reveal any novel or unanticipated sources and physical processes or to provide definitive identification of sources, transport mechanisms, or fate processes of solvents in any specific parcel of groundwater. In addition, the specificity of the relational analyses was limited by the generally poor quality and low resolution of the available ancillary data.

Results Detection Frequencies. Using all of the available concentration data, one or more of the four solvents were detected in 17% (881 of 5,068) of the samples of groundwater. The detection frequency of each individual solvent was as follows: PCE, 11%; TCA, 7%; TCE, 5%; and methylene chloride, 3%. Relative to 55 VOCs, the four solvents ranked in the top ten with respect to detection frequencies in groundwater, except for methylene chloride (Table 2 in Supporting Information). At lower concentration levels, the detection frequency of each solvent increased. The distribution of wells in which one or more solvents were detected was spread throughout the conterminous United States (Figure 1). However, the detections of solvents appeared to occur most frequently in the Northeast region and several other locations such as Iowa and California. The detection frequency for each solvent was higher in shallow groundwater under urban areas and lower in shallow groundwater under agricultural areas compared to mixed land-use areas (Figure 2). The pattern of detection frequencies of individual solvents was similar in shallow groundwater under urban areas and 76

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mixed land-use areas (aquifer surveys) with PCE having the highest detection frequencies (Figure 2). The source of PCE in these areas may be releases from industrial and commercial facilities where it is used, such as dry cleaners. Non-point sources of PCE may also contribute to groundwater contamination in urban areas (21). The pattern of solvent detections was different in shallow groundwater under agricultural areas, with methylene chloride having the highest detection frequency. The source of methylene chloride in agricultural areas could be the transformation of carbon tetrachloride, which was used as a fumigant in grain storage bins from about 1933 to 1972 (22). Figure 3 shows the distribution of detection frequencies of each solvent by each individual NAWQA survey. With the exception of methylene chloride, the median detection frequencies of solvents were higher in shallow groundwater beneath urban areas than in either agricultural areas or mixed land-use areas (aquifer surveys). Because only four shallow groundwater studies in agricultural areas were analyzed for solvents, the median detection frequencies for the agricultural areas may be biased and were considered to be preliminary estimates. In agricultural areas, the source of low concentrations of some solvents may be their use as active or inert ingredients in pesticide formulations (23). In previous studies, mixtures of solvents have been shown to occur in groundwater (24). In this study, mixtures were a common mode of occurrence among the four solvents in groundwater. About 30% of samples with detections of any of the four solvents contained mixtures of two or more solvents. However, when examined relative to the total number of samples of groundwater the detection frequencies of any solvent mixture did not exceed 3%. Eleven unique mixtures of the four solvents are possible. The detection frequencies of each solvent mixture in groundwater are shown in Table 3 of the Supporting Information. The combination of PCE and TCE was the most

FIGURE 2. Detection frequencies of four solvents in groundwater by land use, at or above a concentration of 0.2 µg/L.

FIGURE 3. Detection frequencies of four solvents in groundwater by individual NAWQA survey, at or above a concentration of 0.02 µg/L. frequently occurring mixture. The frequent occurrence of the mixture of PCE and TCE may be the result of the transformation of PCE to TCE through reductive dechlorination or it may due to the common usage of these solvents as metal degreasers. Concentrations. The four solvents ranked in the middle of 55 VOCs with respect to the median quantified concentrations in groundwater (Table 4 in Supporting Information). The median quantified concentrations of each solvent were well below MCLs and ranges in concentrations were the following: TCE, 0.02-230 µg/L; PCE, 0.02-4800 µg/L; TCA, 0.02-120 µg/L; and methylene chloride, 0.02-100 µg/L. The MCLs for the solvents are methylene chloride 5 µg/L; PCE 5 µg/L; TCA 200 µg/L; TCE 5 µg/L (7). Relative to 24 VOCs that have MCLs, three of the four solvents ranked in the top ten in terms of frequencies of concentrations greater than MCLs (Table 5 in Supporting Information). PCE ranked number 1 while TCE ranked number 3. TCA did not exceed the MCL in samples of groundwater. Overall, the actual

frequencies of concentrations greater than MCLs for each solvent were relatively low. The majority of samples with concentrations of PCE greater than the MCL were from monitoring wells or wells classified for various other types of uses (29 of 37). The majority of samples with concentrations of TCE greater than the MCL were from monitoring or domestic wells (12 of 20). Two samples from public wells and one sample from a monitoring well had concentrations of methylene chloride that exceeded the MCL. Figure 4 is a graph showing the distribution of each VOC with an MCL with respect to the fraction of samples with detections (x-axis) and the fraction of concentrations that were less than the MCL but greater than one-tenth the MCL (y-axis). The purpose of this figure is to illustrate those VOCs with MCLs that may be a concern in groundwater because they are frequently occurring and because they have a large fraction of concentrations close to, but not exceeding, the MCL. Only 17 VOCs are shown in Figure 4. Seven other VOCs VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Fraction of samples with detections versus fraction of concentrations less than or equal to the MCL but greater than one-tenth of the MCL. had too few concentrations to compute a meaningful fraction of concentrations. The four solvents examined in this study are bolded for ease of identification. With the exception of TCA, solvents appear in the upper right-hand quadrant of the graph indicating that they have both a relatively high overall fraction of samples with detections and a relatively high fraction of concentrations close to the MCL. Relational Analyses. Table 1 shows the results of the relational analyses. Variables with standardized coefficients greater than 0.1 (absolute value) were considered to be strongly associated with the probability of occurrence of solvents and are presented in bold. Table 2 summarizes the results of the relational analyses. A rank was given to each variable relative to the strength of the association with each solvent. These ranks were then summed to give a rank sum. Lower rank sums indicate variables more frequently and strongly associated with the probability of occurrence of solvents in groundwater. An overall association ranking was given to each variable based on the rank sum of strength from the individual solvent associations and the number of solvents associated. Variables with strong overall association had 3 solvents associated and relatively low rank sums; variables with moderate overall association had 2 solvents associated and relatively moderate rank sums; variables with weak overall association had 1 solvent associated and relatively high rank sums. Dissolved-oxygen content, urban land use around the well, and population density around the well were the variables most strongly and frequently associated with the probability of occurrence of solvents in groundwater. These variables represent the fate and surrogates for the sources of solvents. In every case, an increase in any of these three variables resulted in an increase in the probability of occurrence of solvents. The variables that were moderately associated with the probability of occurrence of solvents included sand content of the soil, depth to the top of the screened interval, and the 78

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number of Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) and Resource Conservation and Recovery Act (RCRA) sites within a 1-kilometer radius of the wells. These variables represent the transport and potential sources of solvents. The remaining associated variables, with one exception, were only weakly and infrequently associated with the probability of occurrence of solvents in groundwater. Although the number of septic systems within a 500-meter radius of the wells was only weakly associated, this variable was significant in regressions for PCE, TCA, and one or more solvents. Septic systems likely represent a source of solvents to groundwater. In every case, an increase in the number of septic systems near the well resulted in an increase in the probability of occurrence of solvents.

Discussion In samples of groundwater nationwide, PCE, TCA, and TCE were in the top 10 in terms of detection frequency relative to 55 VOCs analyzed in groundwater. In a recent USGS report on the occurrence of VOCs in groundwater from aquifers across the United States, solvents (including a total of 18 VOCs classified as solvents) were the second most frequently detected VOC group (24). Consequently, determining the occurrence of solvents in groundwater resources is important, especially for groundwater resources that are used as drinking-water supplies. The occurrence of individual solvents increased at lower concentration levels. Relatively low concentrations (above 0.02 µg/L) of solvents occurred more frequently in groundwater from all 3 types of NAWQA surveys compared to relatively high concentrations (above 0.2 µg/L). Use of analytical methods with low-level detection limits for solvents may be important for determining the sources of solvents, as a potential early warning regarding the vulnerability of an aquifer, and for determining trends and patterns in occurrence of solvents in aquifers.

TABLE 1. Hydrogeologic and Anthropogenic Variables Associated with the Probability of Occurrence of One or More Solvents and Individual Solvents in Groundwater type of variable

coefficient in logistic regression equation

standardized coefficient

One or More Solvents transport source fate source transport source source

-0.026 1.544 0.134 0.303 -0.001 0.917 0.004

-0.22 0.21 0.16 0.12 -0.06 0.05 0.05

Methylene Chloride source transport transport source source

0.614 3.675 -0.016 -0.847 -0.018

0.11 0.06 -0.06 -0.05 -0.03

source transport fate transport source transport source

2.165 -0.028 0.137 -4.575 0.027 -0.001 0.003

0.28 -0.22 0.16 -0.15 0.06 -0.05 0.04

fate source source transport transport source source

0.230 1.461 0.456 -0.003 0.003 1.043 0.003

0.20 0.14 0.13 -0.12 0.11 0.04 0.03

source fate source source transport

1.139 0.121 0.035 0.871 0.033

0.26 0.08 0.04 0.03 0.03

associated variables (bold type indicates a strong association)a sand content of soil urban land use dissolved oxygen population density depth to top of screened interval CERCLA sites within 1-km radius septic systems within 500-km radius population density bulk density of soil sand content of soil urban land use median year of home construction

PCE urban land use sand content of soil dissolved oxygen soil erodability RCRA sites within 1-km radius depth to top of screened interval septic systems within 500-m radius TCA dissolved oxygen urban land use population density depth to top of screened interval recharge CERCLA sites within 1-km radius septic systems within 500-m radius TCE population density dissolved oxygen RCRA sites within 1-km radius CERCLA sites within 1-km radius casing diameter

a Variables strongly associated with the occurrence of solvents are those with standardized coefficients greater than 0.1 (absolute value); m, meter; km, kilometer; RCRA, Resource Conservation and Recovery Act; CERCLA, Comprehensive Environmental Response, Compensation, and Liability Act.

TABLE 2. Summary of Hydrogeologic and Anthropogenic Variables Associated with the Occurrence of Solvents in Groundwatera

rank sum

number of solvents associated with variable

dissolved oxygen urban land use population density

12 13 13

3 3 3

fate source source

strong

sand content of soil depth to top of screened interval RCRA sites within 1-km radius CERCLA sites within 1-km radius

19 22 22 24

2 2 2 2

transport transport source source

moderate

soil bulk density soil erodability recharge septic systems within 500-m radius casing diameter median year of home construction

24 24 25 26 27 27

1 1 1 2 1 1

transport transport transport source transport source

weak

associated variable

type of variable

overall association

a m, meter; km, kilometer; RCRA, Resource Conservation and Recovery Act; CERCLA, Comprehensive Environmental Response, Compensation, and Liability Act.

Mixtures were a common mode of occurrence of solvents, with mixtures occurring in about 30% of groundwater samples with detections of one or more solvents. Evaluation of mixtures of solvents in groundwater resources is important,

especially for groundwater used for drinking-water supplies, because of potential implications on how risk is assessed (25, 26). For example, exposure to mixtures of TCE, PCE, and TCA have been shown through computer modeling to VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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increase the blood concentration of TCE in humans. This could lead to increased potential health effects from TCE exposure such as an increased chance for renal tumors (27). Occurrence surveys can contribute to toxicity studies by identifying the mixtures of solvents that are most frequently occurring in groundwater used as source of supply for drinking water. Relative to 24 VOCs with MCLs, PCE ranked 1 and TCE ranked 3 in terms of the frequency of concentrations greater than MCLs. Also, relative to 17 VOCs with MCLS, PCE and TCE ranked high in terms of fraction of concentrations close to, but not exceeding, their MCLs. In order to be protective of human health, the sources of these two solvents in aquifers need to be understood, especially in aquifers used to supply drinking water. Understanding of solvent occurrence in groundwater at large scales would be improved by more extensive and higher resolution data on the locations of potential sources of solvents throughout broad regions. For example, if data were available on the release of PCE from underground storage tanks at dry-cleaning facilities, a strong association would be expected between PCE occurrence in groundwater and facilities where PCE was released. In addition, many types of intense sources are not represented in the national-scale ancillary data currently available. Few data sets are available for relatively small releases or spills of solvents that are not required to be reported to Federal or State environmental agencies but could be important sources of solvents in some areas. Understanding of the redox conditions of the aquifer and the hydraulic properties of the saturated and vadose zones are important in evaluating the intrinsic susceptibility of groundwater to contamination by solvents. Although flowpaths and redox conditions are well understood for aquifers at a local scale, solvent occurrence at large scales would be improved by more extensive and higher resolution data on regional-scale flowpaths and redox conditions throughout aquifers.

Supporting Information Available Tables indicating the ancillary variables used in the relational analyses and their sources, rank of detection frequency of 55 VOCs in groundwater, results of the mixture analyses, rank of the median quantified concentrations of 55 VOCs in groundwater, and rank of frequencies of concentrations of 24 VOCs greater than their MCL in groundwater. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited (1) U.S. Environmental Protection Agency. Sources of Toxic Compounds in Household Wastewater; 600/2-80-128; Office of Research and Development: Washington, DC, 1980. (2) Pankow, J. F; Cherry, J. A. Dense Chlorinated Solvents; Waterloo Press: Portland, OR, 1996. (3) Doherty, R. E. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1,1,1trichloroethane in the United States. J. Environ. Forensics 2000, 1, 69-81. (4) U.S. Environmental Protection Agency. 2001 Toxics Release Inventory Public Data Release Report; 260-R-03-001; Office of Environmental Information: Washington, DC, 2003. (5) U.S. Environmental Protection Agency. Common Chemicals Found at Superfund Sites; http://www.epa.gov/superfund/ resources/chemicals.htm (accessed January 2005). (6) Squillace, P. J.; Scott, J. C.; Moran, M. J.; Nolan, B. T.; Kolpin, D. W. VOCs, Pesticides, nitrate, and their mixtures in groundwater used for drinking water in the United States. Environ. Sci. Technol., 2002, 36, 1923-1930. (7) U.S. Environmental Protection Agency. 2004 Edition of the Drinking Water Standards and Health Advisories; 822-R-04-005; Office of Water: Washington, DC, 2004. 80

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Received for review June 30, 2006. Revised manuscript received October 10, 2006. Accepted October 19, 2006. ES061553Y

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