Volatile Organic Compounds in the Nation's Ground Water and Drinking-Water Supply Wells: Supporting Information

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Chapter 1—Major Findings and Conclusions

This national assessment of 55 volatile organic compounds (VOCs) in ground water gives emphasis to the occurrence of VOCs in aquifers that are used as an important supply of drinking water. In contrast to the monitoring of VOC contamination of ground water at point-source release sites, such as landfills and leaking underground storage tanks (LUSTs), our investigations of aquifers are designed as large-scale resource assessments that provide a general characterization of water-­quality conditions. Nearly all of the aquifers included in this assessment have been identified as regionally extensive aquifers or aquifer systems.(2) The assessment of ground water (Chapter 3) included analyses of about 3,500 water samples collected during 1985–2001 from various types of wells, representing almost 100 different aquifer studies. This is the first national assessment of the occurrence of a large number of VOCs with different uses, and the assessment addresses key questions about VOCs in aquifers. The assessment also provides a foundation for subsequent decadal assessments of the U.S. Geological Survey (USGS) National Water-­Quality Assessment (NAWQA) Program to ascertain long-term trends of VOC occurrence in these aquifers. The occurrence of VOCs in samples collected from drinking-water supply wells, specifically domestic and public wells, also is included (and discussed separately from aquifer studies) in this assessment (­Chapter 4), ­recognizing that various agencies, organizations, decision makers, and others have different interests and information needs. Occurrence findings are compared between domestic and public wells to distinguish the separate issues for these well types related to supply, environmental setting, and sources of VOCs. For this purpose, the occurrence of 55 VOCs is based on analyses of samples collected at the well head, and before any treatment or blending, from about 2,400 domestic wells and about 1,100 public wells. Findings from domestic well samples update earlier USGS studies and provide improved national coverage of sampled wells. As such, this assessment provides important information on VOC occurrence for domestic well samples that may be useful to public health agencies. Findings for public well samples constitute the most current understanding of the occurrence of a large number of VOCs in untreated ground water used by public water systems (PWSs) across the Nation. Our assessment of public well water complements compliance monitoring by water utilities that typically focus on drinking water delivered to the public. Major findings that may be most relevant to the management and monitoring of the Nation’s ground water and drinking-water supply wells are emphasized in the following discussion. Additional information is included in subsequent chapters of this report and at a supporting Web site (http://water.usgs.gov/nawqa/vocs/national_assessment).

VOCs were detected in many aquifers across the Nation. Almost 20 percent of the water samples from aquifers contained one or more of the 55 VOCs, at an assessment level of 0.2 microgram per liter (µg/L). This detection frequency increased to slightly more than 50 percent for the subset of samples analyzed with a low-level analytical method and for which an order-of-magnitude lower assessment level (0.02 µg/L) was applied. VOCs were detected in 90 of 98 aquifer studies completed across the Nation, with most of the largest detection frequencies in California, Nevada, Florida, and the New England and Mid-Atlantic States. Trihalomethanes (THMs), which may originate as chlorination by-products, and solvents were the most frequently detected VOC groups. Furthermore, detections of THMs and solvents and some individual compounds were geographically widespread; however, a few compounds, such as methyl tert-butyl ether (MTBE), ethylene dibromide (EDB), and dibromochloropropane (DBCP), had regional or local occurrence patterns. The widespread occurrence of VOCs indicates the ubiquitous nature of VOC sources and the vulnerability of many of the Nation’s aquifers to low-level VOC contamination. The findings for VOCs indicate that other compounds with widespread sources and similar behavior and fate properties also may be occurring. (See p. 16, 18, 20, and 21.)

CONCLUSIONS

Many VOCs were detected, but typically at low concentrations. In water samples from aquifers, the concentrations of each VOC and the total concentration of all VOCs analyzed generally were low (defined in this report as concentrations less than 1 µg/L). For example, 90 percent of the total VOC concentrations in samples were less than 1 µg/L. Forty-two of the 55 VOCs were detected in one or more samples at an assessment level of 0.2 µg/L. Furthermore, VOCs in each of the seven VOC groups considered in this assessment were detected in the samples; these groups included fumigants, gasoline hydrocarbons, gasoline oxygenates, organic synthesis compounds, refrigerants, solvents, and THMs. The finding that most VOC concentrations in ground water are less than 1 µg/L is important because many previous monitoring programs did not use low-level analytical methods and therefore would not have detected such contamination. (See p. 16, 17, 23, and Appendixes 6 and 7.)

CONCLUSION

Some VOCs were detected more frequently than others. Although 42 VOCs were detected in aquifer samples, only 15 occurred in about 1 percent or more of the samples. The most frequently detected VOCs include 7 solvents, 4 THMs, 2 refrigerants, 1 gasoline oxygenate, and 1 gasoline hydrocarbon. The THM chloroform was the most frequently detected compound, and its source is attributed, in part, to the recycling of chlorinated waters to aquifers. The solvent perchloro-ethene (PCE) and the gasoline oxygenate MTBE were the second and third most frequently detected compounds, respectively. Overall, the 15 most frequently detected compounds comprise a large fraction of the low-level VOC contamination and provide a logical focus for future monitoring of aquifers and for follow-up studies to ­better understand their sources and pathways to aquifers. (See p. 22 and Appendix 6.)

VOCs found in about 1 percent or more of aquifer samples, at an assessment level of 0.2 µg/L (compounds listed by decreasing detection frequency)
Compound name VOC group
Chloroform trihalomethane
Perchloroethene solvent
Methyl tert-butyl ether gasoline oxygenate
Trichloroethene solvent
Toluene gasoline hydrocarbon
Dichlorodifluoromethane refrigerant
1,1,1-Trichloroethane solvent
Chloromethane solvent
Bromodichloromethane trihalomethane
Trichlorofluoromethane refrigerant
Bromoform trihalomethane
Dibromochloromethane trihalomethane
trans-1,2-Dichloroethene solvent
Methylene chloride solvent
1,1-Dichloroethane solvent

CONCLUSIONS

Explaining VOC contamination in aquifers is complex—VOC occurrence is determined not only by sources but also by natural and anthropogenic factors that affect the transport and fate of VOCs in aquifers. The complexity of explaining VOC contamination in aquifers was affirmed in this assessment through statistical models for 10 frequently detected compounds. Factors describing the source, transport, and fate of VOCs were all important in explaining the national occurrence of these VOCs. For example, the occurrence of PCE was statistically associated with the percentage of urban land use and density of septic systems near sampled wells (source factors), depth to top of well screen (transport factor), and presence of dissolved oxygen (fate factor). National-scale statistical analyses provide important insights about the factors that are strongly associated with the detection of specific VOCs, and this information may benefit many local aquifer investigations in selecting compound- and aquifer-specific information to be considered. Continued efforts to reduce or eliminate low-level VOC contamination will require enhanced knowledge of sources of contamination and aquifer characteristics. (See p. 24 and 25.)

Factors most commonly associated with VOCs in aquifers

CONCLUSIONS

Despite the short period of its extensive use, MTBE was one of the most frequently detected VOCs. As noted previously, MTBE was the third most frequently detected VOC in aquifers. MTBE production peaked in the 1990s with the majority of it used voluntarily by refineries for the Nation’s Reformulated Gasoline (RFG) Program. Concentrations of MTBE in aquifer samples were rarely of concern relative to the U.S. Environmental Protection Agency’s (USEPA) drinking-water advisory based on taste and odor; however, MTBE concentrations in ground water were detected more frequently in RFG Program areas than in other areas. The relatively frequent detection of MTBE in aquifers was not an anticipated outcome at the commencement of NAWQA’s assessment because of MTBE’s short and recent use. A period of only a decade or less was required for the detection of MTBE in some of the Nation’s aquifers. MTBE findings demonstrate how quickly some anthropogenic chemicals, especially those that are mobile and persistent like MTBE, may reach aquifers that are especially susceptible to land-surface or atmospheric contamination. (See p. 22, 50–53.)

CONCLUSIONS

Some VOCs were not detected in aquifer samples. Thirteen of the VOCs included in this national assessment were not detected in any aquifer samples at a concentration of 0.2 µg/L or larger. The 13 compounds include 5 VOCs predominantly used in organic synthesis, 4 solvents, 2 fumigants, 1 gasoline hydrocarbon, and 1 gasoline oxygenate. The specific reason(s) why each of these compounds was not detected has not been ascertained; however, their lack of occurrence likely is attributed to one or more of the following factors: (1) limited use in industry, commerce, and household products; (2) small releases to water and land; (3) most use occurs in controlled industrial processes or in organic synthesis; (4) the compound degrades quickly to other compounds in the environment; and (5) insufficient time has elapsed to allow the compound to reach wells sampled in this assessment. (See Appendix 6.)

VOCs not detected in aquifer samples, at an assessment level of 0.2 g/L (compounds listed by VOC group)
Compound name VOC group
Acrolein organic synthesis compound
Acrylonitrile organic synthesis compound
Hexachlorobutadiene organic synthesis compound
1,2,3-Trichlorobenzene organic synthesis compound
Vinyl bromide organic synthesis compound
1,3-Dichlorobenzene solvent
Hexachloroethane solvent
1,2,4-Trichlorobenzene solvent
1,1,2-Trichloroethane solvent
cis -Dichloropropene fumigant
trans -Dichloropropene fumigant
Styrene gasoline hydrocarbon
Ethyl tert -butyl ether gasoline oxygenate

CONCLUSION

Although VOCs were detected frequently in samples from domestic and public wells, only a small percentage of samples had VOC concentrations of potential human-health concern. One or more VOCs were detected in about 14 and 26 percent of domestic and public well samples, respectively, at an assessment level of 0.2 µg/L. However, only about 1 to 2 percent of domestic and public well samples had concentrations of potential human-health concern (defined in this report as concentrations greater than a USEPA Maximum Contaminant Level (MCL) or concentrations greater than a Health-Based Screening Level (HBSL) for compounds without an MCL). Eight compounds were detected at concentrations of potential concern, and three of these compounds occurred in both domestic and public well samples. Most of the concentrations of potential concern were attributed to the fumigant DBCP (in domestic well samples only) and the solvents PCE and trichloroethene (TCE) in samples from both well types. Because NAWQA’s assessment is based on samples collected at the wellhead, it is unknown if those domestic and public well samples with concentrations of potential concern actually result in concentrations greater than MCLs in drinking water. (See p. 30–35.)

VOCs found at concentration(s) of potential human-health concern (compounds listed by decreasing number of concentrations of potential concern).
Compound name VOC group Domestic wells Public wells
Trichloroethene solvent X X
Dibromochloropropane fumigant X
Perchloroethene solvent X X
1,1-Dichloroethene organic synthesis compound X X
1,2-Dichloropropane fumigant X
Ethylene dibromide fumigant X
Methylene chloride solvent X
Vinyl chloride organic synthesis compound X

CONCLUSIONS

Additional VOCs may warrant inclusion in a low-concentration, trends-monitoring program. Nine VOCs that did not occur at concentrations of potential concern in samples from domestic and/or public wells were detected at concentrations below but within a factor of 10 of an MCL. The 9 compounds include 4 solvents, 4 THMs, and 1 gasoline hydrocarbon. These 9 VOCs, plus the 8 compounds with concentrations of potential concern, are important compounds to consider including in a low-concentration, trends-monitoring program, such as the NAWQA Program. Such programs seek to identify compounds in domestic and public well samples before concentrations reach levels of potential concern. Also noteworthy is the finding that the solvents PCE and TCE had, relative to other VOCs, a large number of concentrations in both domestic and public well samples below but within a factor of 10 of their MCLs. (See p. 32, 34, and Appendixes 9 and 11.)

Additional VOCs that may warrant inclusion in a low-concentration, trends-monitoring program (compounds listed by VOC group)
Compound name VOC group
Benzene gasoline hydrocarbon
Carbon tetrachloride solvent
1,2-Dichloroethane solvent
cis -1,2-Dichloroethene solvent
1,1,1-Trichloroethane solvent
Bromodichloromethane trihalomethane
Bromoform trihalomethane
Chloroform trihalomethane
Dibromochloromethane trihalomethane

CONCLUSIONS

In general, public wells are more vulnerable to low-level VOC contamination than are domestic wells. The detection frequencies of nearly all of the most frequently detected compounds and mixtures of VOCs were larger in samples from public wells than from domestic wells, at an assessment level of 0.2 µg/L. Mixtures of 2 or more of the 55 VOCs were found in about 13 percent of the public well samples—more than three times more frequently than in domestic well samples—and the likelihood of detecting a mixture of VOCs in public well samples was about the same as detecting a single compound. Furthermore, 10 of the 15 most frequently detected VOCs in public well samples were either THMs or solvents, and all but one of the most common VOC mixtures included THMs. The larger detection frequencies in public well samples than in domestic well samples is attributed, in part, to the larger withdrawal rates of public wells and their proximity to developed areas. The larger pumping rates may increase the capture and movement of VOC contamination to public wells. The proximity of ­public wells to developed areas increases the likelihood of VOC sources. (See p. 36–41.)

CONCLUSIONS

Water that has been chlorinated or exposed to household products containing chlorine is an important source of chloroform and possibly other compounds in ground water supplying domestic and public wells. Chloro-form was the most frequently detected VOC in domestic and public well samples. The chloroform detected in ground water may have potential sources associated with its use as a solvent and an extractant, and as an intermediate product in organic synthesis. Also, chloroform and other THMs are by-products of the chlorination of drinking waters and wastewaters, and the disinfection of domestic and public wells. These compounds also may be present in the effluent of septic systems from the use of household products containing chlorine, such as bleach. Furthermore, artificial recharge of chlorinated water containing THMs and potentially other compounds is becoming more common, especially in western States due to, in part, the limited supply of drinking water. The chlorination of water to control water-borne diseases has been a common practice in the United States for nearly a century. This long-term use has allowed ample time for the recharge of waters containing THMs to reach many of the sampled wells. Once introduced to ground water, chloroform and other THMs may persist and move long distances in some aquifers. The relative detection frequencies of the THMs in well samples, and the common occurrence of mixtures of THMs in public well samples, indicate that waters with a history of chlorination and that contain these compounds have reached some of the sampled wells. (See p. 42–45.)

CONCLUSIONS

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