are shown in Figure 2. Samples with VOC concentrations greater than MCLs generally were localized within regions or states (Supplemental Material, Table 6, available online at http://www.ehponline.org/mem- bers/2007/10253/suppl.pdf) and generally were detected in highly populated areas, such as New Jersey, or were potentially associated with a particular VOC use, such as the his- torical application of DBCP on crops in the Central Valley of California (Burow et al. 1999). Samples with ≥ 1 VOC concentra- tion within one order of magnitude of an MCL were distributed throughout the con- terminous United States, and samples with VOC concentrations less than one order of an MCL or HBSL were distributed through- out the conterminous United States and in Alaska (Figure 2). We also examined the 15 VOCs that had concentrations greater than and/or within one order of magnitude of MCLs in the 2,401 domestic well samples relative to their overall detection frequencies. The solvents methylene chloride, PCE, and TCE were among the most frequently detected compounds, and compared with other VOCs, had a large per- centage of samples with concentrations greater than or within one order of magnitude of an MCL (Supplemental Material, Figure 2, avail- able online at http://www.ehponline.org/ members/2007/10253/suppl.pdf). These sol- vents are important VOCs to consider for source control and monitoring programs at local, state, and national levels. Discussion Scope of NAWQA study and relevance of ﬁnd- ings. Several groundwater studies have focused on select VOCs or VOC groups. A study by the State of Maine (1998) focused on MTBE, benzene, toluene, ethylbenzene, and xylene (State of Maine 1998). Since the 1980s the Wisconsin Department of Natural Resources (2003) has tested for VOCs in water from domestic wells that were known or suspected to be vulnerable to VOC contamination. In a NAWQA study, Moran et al. (2002) reported preliminary VOC occurrence information on 55 VOCs based on 1,926 domestic well sam- ples using a reporting level of 0.2 µg/L. The present study is the ﬁrst national investigation to report the occurrence of a large number of VOCs in ambient groundwater from domes- tic well samples analyzed using low-level ana- lytical methods and to assess the potential human-health relevance of individual VOC concentrations that have established human- health benchmarks. About two-thirds of the NAWQA wells sampled throughout the United States since 1996 contained low-level VOC concentra- tions, indicating that VOC contamination of drinking water supplied by domestic wells VOCs in drinking water from domestic wells may be more prevalent than previously reported by monitoring programs that used analytical methods with higher reporting lev- els, such as those described by Squillace et al. (1999) and the State of Maine (1998). As analytical reporting levels decrease, the detec- tion frequency of VOCs increases; however, this does not necessarily indicate differences in water quality from a human-health per- spective, but may reﬂect the greater resolving power of analytical instruments. Mixtures were a common mode of VOC occurrence in domestic well samples; however, chemical regulation and toxicologic research have historically focused on individual chemi- cals rather than on mixtures (Monosson 2005). Furthermore, few data are available on the potential human-health effects resulting from long-term exposure to multiple com- pounds at low concentrations. The present study identified frequently detected VOC mixtures, and in addition, a NAWQA study now in progress is assessing multicontaminant occurrence of nitrate, radon, pesticides, VOCs, and trace elements (including arsenic and uranium) from domestic well samples throughout the United States. Identiﬁcation of commonly occurring mixtures may aid the Table 3. Six regulated VOCs were detected at concentrations greater than U.S. EPA MCLs in domestic well samples, ﬁve of which are regulated carcinogens in drinking water, and most may adversely affect the liver. Compound DBCP 1,1-Dichloroethene 1,2-Dichloropropane Ethylene dibromide PCE TCE VOC group Fumigant Org syn Fumigant Fumigant Solvent Solvent MCL (µg/L) 0.2 7 5 0.05 5 5 No. of sampled wells 1,962 2,400 2,400 2,085 2,371 2,400 No. of samples with conc > MCL Carcinogena 14 1 3 3 5 6 Yes No Yes Yes Yes Yes Potential health effects from exposure to conc > MCLb Reproductive problems; increased risk of cancer Liver problems Increased risk of cancer Problems with liver, stomach, reproductive system, or kidneys; increased risk of cancer Liver problems; increased risk of cancer Liver problems; increased risk of cancer Abbreviations: conc, concentration; Org syn, organic synthesis compound. aConsidered a probable or likely human carcinogen by the U.S. EPA (2006b). bData from U.S. EPA (2003). Carbon tetrachloride Dibromochloromethanea Benzene Bromoforma Chloroforma DBCP 1,2-Dichloroethane 1,1-Dichloroethene 1,2-Dichloropropane Ethylene dibromide Methylene chloride 1,1,1-TCA PCE TCE Vinyl chloride 0.001 0.1 1 10 100 1,000 Concentration (μg/L) VOC concentration for a sampleb U.S. EPA MCL Concentration one order of magnitude below U.S. EPA MCL Figure 1. Distribution of 15 VOCs with concentrations greater than and/or within one order of magnitude of U.S. EPA MCLs in 2,401 domestic well samples. aThe U.S. EPA MCL of 80 µg/L is the sum of the concentration of four THMs. bMultiple samples with equal concentrations appear as a single symbol. Environmental Health Perspectives • VOLUME 115 | NUMBER 11 | November 2007 1543