occurrence data. Multivariate logistic regres- sion analyses were used to determine relations between probability of VOC occurrence rela- tive to hydrogeologic and anthropogenic vari- ables. Information on this statistical approach and variables (Moran et al. 2006) is available in Appendix 1 of the Supplemental Material (available online at http://www.ehponline.org/ members/2007/10253/suppl.pdf). The signiﬁ- cance of logistic regression analyses was tested using various statistical criteria (Helsel and Hirsch 1992). We computed standardized coefficients to compare slope coefficients directly between one another (Menard 2002). Screening-level assessment. Of the 42 VOCs detected in domestic well samples, 35 have established human-health bench- marks. VOC concentrations for 27 regulated compounds were compared to their MCLs, and concentrations for 8 unregulated VOCs were compared to their HBSLs. VOC con- centrations of potential human-health con- cern in domestic well samples were deﬁned as those concentrations greater than MCLs or HBSLs. We identiﬁed VOC concentrations within one order of magnitude of MCLs or HBSLs as compounds that may warrant addi- tional monitoring to analyze trends in occur- rence and to provide an early indication of concentrations approaching human-health benchmarks (Toccalino and Norman 2006). State and federal agencies use a variety of thresholds (typically one-tenth or one-half of a human-health benchmark) to identify contaminants that may warrant additional monitoring (U.S. EPA 2000, 2002) or for related purposes, such as ranking the suscepti- bility of wells to contamination (New Jersey Department of Environmental Protection 2003, 2004) and identifying contaminants of potential human-health concern (U.S. EPA 1993). Using concentrations within one order of magnitude of a human-health benchmark to identify compounds that may warrant additional monitoring is therefore consistent with various state and federal practices. Results VOC occurrence. We detected one or more VOCs in 65% of the domestic well samples. Of the 55 VOCs monitored, 42 compounds were detected. Detection frequencies were > 10% for chloroform (25.6%), toluene (17.9%), 1,2,4-trimethylbenzene (1,2,4-TMB; 15.2%), and perchloroethene (PCE; 11%) (Table 1). Nineteen VOCs had detection fre- quencies between 1 and 10%, and 18 VOCs had detection frequencies > 0.1 and < 1% (Table 1). The 15 frequently detected com- pounds represent all VOC groups except fumigants. These VOCs have widespread applications and multiple uses, and most compounds were detected throughout the conterminous United States and in Alaska. Compound Chloroform Toluene 1,2,4-TMB PCE Chloromethane 1,1,1-TCA Methylene chloride MTBE Dichlorodiﬂuoromethane TCE Benzene Bromodichloromethane m- and p-Xyleneb 1,1-Dichloroethane Styrene 1,4-Dichlorobenzene Trichloroﬂuoromethane Bromoform 1,1-Dichloroethene Chlorobenzene Carbon tetrachloride Dibromochloromethane cis-1,2-Dichloroethene 1,2-Dichloropropane Isopropylbenzene o-Xylene Ethylbenzene tert-Amyl methyl ether 1,3-Dichlorobenzene Trichlorotriﬂuoroethane Chloroethane 1,2-Dichlorobenzene Diisopropyl ether DBCP Naphthalene n-Propylbenzene n-Butylbenzene 1,2-Dichloroethane 1,2,3-Trichloropropane 1,1,2-Trichloroethane Vinyl chloride Abbreviations: DBCP, dibromochloropropane; MTBE, methyl tert-butyl ether; Org syn, organic synthesis compound; PCE, perchloroethene; 1,1,1-TCA, 1,1,1-trichloroethane; TCE, trichloroethene; TMB, 1,2,4-trimethylbenzene. VOCs are listed in order of decreasing detection frequency. aVOCs not detected: acrolein, acrylonitrile, bromomethane, trans-1,2-dichloroethene, cis-1,3-dichloropropene, trans-1,3- dichloropropene, ethyl tert-butyl ether, ethylene dibromide, hexachlorobutadiene, hexachloroethane, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, and vinyl bromide. bConsidered as 2 of the 55 compounds included in this assessment. Environmental Health Perspectives • VOLUME 115 | NUMBER 11 | November 2007 1541 VOCs in drinking water from domestic wells VOC concentrations generally were low; of the sampled wells, 91% had total VOC con- centrations ≤ 1 µg/L and about 1% had total VOC concentrations > 10 µg/L. Of the well samples, 31% had a single VOC detection and 34% had VOC mixtures, which we deﬁned as two or more VOCs that are present in a domestic well sample. The 10 most frequently detected VOCs each occurred more frequently in samples with mixtures than as single VOCs in samples (Supplemental Material, Table 3, available online at http://www.ehponline.org/members/ 2007/10253/suppl.pdf). The median concen- trations for these VOCs generally were greater when detected in a mixture than alone. Of the 10 frequently detected VOCs, methyl tert- butyl ether (MTBE) had the greatest median concentration both alone and in mixtures. Median concentrations generally were greatest for gasoline oxygenates and refrigerants, and median concentrations were lowest for fumi- gants and gasoline hydrocarbons. We detected as many as 24 individual VOCs in one sample; however, the most fre- quently occurring mixtures were composed o f t w o o r t h r e e u n i q u e c o m p o u n d s . Chloroform was a common constituent in the four most frequently detected mixtures, co-occurring in two-compound mixtures with, in decreasing order of detection fre- quency, 1,1,1-trichloroethane (1,1,1-TCA), PCE, toluene, and MTBE (Supplemental Material, Table 4, available online at http:// www.ehponline.org/members/2007/10253/ suppl.pdf). The co-occurrence of chloroform Table 1. Detection frequencies of volatile organic compounds (VOCs)a in 1,208 domestic well samples analyzed with the USGS’s low-level analytical method and reported with no censoring of data. No. of well samples No. of detections Detection frequency (%) VOC group THM Gasoline hydrocarbon Gasoline hydrocarbon Solvent Solvent Solvent Solvent Gasoline oxygenate Refrigerant Solvent Gasoline hydrocarbon THM Gasoline hydrocarbon Solvent Gasoline hydrocarbon Fumigant Refrigerant THM Org syn Solvent Solvent THM Solvent Fumigant Gasoline hydrocarbon Gasoline hydrocarbon Gasoline hydrocarbon Gasoline oxygenate Solvent Refrigerant Solvent Solvent Gasoline oxygenate Fumigant Gasoline hydrocarbon Solvent Gasoline hydrocarbon Solvent Fumigant Solvent Org syn 1,207 1,203 1,190 1,179 1,207 1,208 1,207 1,208 1,208 1,207 1,208 1,207 1,206 1,207 1,202 1,208 1,208 1,206 1,207 1,208 1,207 1,207 1,207 1,207 1,208 1,205 1,208 1,206 1,208 1,207 1,207 1,208 1,096 1,208 1,208 1,208 1,208 1,208 1,208 1,208 1,208 309 215 181 130 117 103 74 67 43 41 37 34 28 27 26 23 23 22 19 16 13 13 11 9 9 8 7 6 6 6 4 4 4 3 3 3 2 2 2 1 1 25.6 17.9 15.2 11.0 9.7 8.5 6.1 5.5 3.6 3.4 3.1 2.8 2.3 2.2 2.2 1.9 1.9 1.8 1.6 1.3 1.1 1.1 0.91 0.75 0.75 0.66 0.58 0.50 0.50 0.50 0.33 0.33 0.36 0.25 0.25 0.25 0.17 0.17 0.17 0.083 0.083