Rowe et al. is delivered to any user of a public water system. MCLs are set as close as feasible to the maximum level of a contaminant at which no known or anticipated adverse effects on human health would occur over a lifetime, taking into account the best avail- able technology, treatment techniques, cost considerations, expert judgment, and public comments (U.S. EPA 2006f). HBSLs are not legally enforceable drink- ing-water standards or clean-up levels. Rather, HBSLs are nonenforceable benchmark con- centrations in water that, when exceeded, may be of potential human-health concern. HBSLs were developed by the USGS in col- laboration with the U.S. EPA, the New Jersey Department of Environmental Protection (Trenton, NJ), and the Oregon Health & Science University (Portland, OR) using stan- dard U.S. EPA Ofﬁce of Water equations for establishing drinking-water guideline values (lifetime health advisory and cancer risk con- centration values) for the protection of human health, and the most current U.S. EPA peer- reviewed, publicly available human-health toxicity information (Toccalino et al. 2003). HBSLs are based on health effects and do not consider cost or technical limitations (Toccalino 2007; Toccalino and Norman 2006; Toccalino et al. 2003) (Supplemental Material, Table 1, available online at http:// www.ehponline.org/members/2007/10253/ suppl.pdf). Drinking water from domestic wells is not regulated by federal standards and typi- cally does not receive the same level of moni- toring and treatment as drinking water supplied by public water systems (U.S. EPA 2003). Although regulations vary by state, the quality of water from privately owned domes- tic wells is the homeowner’s responsibility. This national assessment of 55 VOCs in drinking water from 2,401 domestic well samples has three primary objectives: a) to examine the occurrence of individual VOCs and VOC mixtures; b) to assess the potential human-health relevance of individual VOCs that have established human-health bench- marks; and c) to link VOC occurrence to hydrogeologic and anthropogenic variables that potentially control or influence the occurrence of VOCs in groundwater. Methods VOC selection. We selected the VOCs in this study on the basis of available informa- tion and the feasibility of laboratory analysis of VOCs by purge and trap gas chromatog- raphy/mass spectrometry (P&T GC/MS) (Bender et al. 1999). Equipment used for P&T GC/MS included the Tekmar 3000 Concentrator and the Tekmar Velocity Concentrator (Teledyne Tekmar, Mason, OH) and the Agilent 6890 GC and Agilent 1540 5973 MSD (Agilent Technologies Inc., Santa Clara, CA). Selection of candidate VOCs involved many criteria, including potential human-health cancer risks and noncancer hazards, toxicity to and bioconcentration in freshwater aquatic organisms, physical proper- ties and occurrence statistics, use or potential use as an oxygenate in gasoline, and potential for atmospheric ozone depletion (Bender et al. 1999). Although many VOCs have multiple uses, each compound was placed in a group representing the predominant use (or origin) of the compound; these include fumigants, gasoline hydrocarbons, gasoline oxygenates, organic synthesis compounds (VOCs used in the formation of other organic compounds), refrigerants, solvents, and trihalomethanes (THMs, disinfection by-products). NAWQA data. Domestic well data include 2,401 samples collected during 1985–2002 and represent > 33 of the nation’s 62 regionally extensive aquifers or aquifer sys- tems (Zogorski et al. 2006). The depth of sampled wells ranged from 6 to 1,500 ft and had a median depth of about 140 ft. The term “sample” represents a distinct geographic site and applies to an environmental domestic well sample collected at the wellhead before household treatment. The characterization of water quality was achieved by sampling 20–30 spatially distributed, randomly selected wells throughout network-based, groundwater studies (Gilliom et al. 1995) including a) large areal and depth dimensions of aquifers con- sidered locally and regionally important (1,621 samples); and b) shallow, recently recharged groundwater samples (247 samples from agricultural land-use areas and 16 sam- ples from urban land-use areas). To supplement NAWQA VOC data, we compiled existing domestic well data collected by federal, state, and local agencies (retrospective data). Retrospective data that met NAWQA design characteristics and data-collection procedures (Lapham and Tadayon 1996) provided information for an additional 517 domestic wells sampled during 1985–1995. Sampling and analytical methods. A sin- gle sample collected at each well represents the water quality. Samples were collected at the wellhead before any treatment or holding time in tanks. Most samples were collected by USGS personnel using data-collection protocol and quality-control procedures (Koterba et al. 1995) and analyzed at the USGS National Water Quality Laboratory using P&T GC/MS. Analytical methods and quality-control samples used in this study have been reported elsewhere (Childress et al. 1999; Connor et al. 1998; Rose and Schroeder 1995). Before April 1996, USGS VOC analytical methodology was similar to U.S. EPA method 524.2, revision 3 (U.S. EPA 1995), using a minimum reporting level (MRL) of 0.2 µg/L. This MRL represents the occurrence of VOCs at a historical reporting value for the USGS and other agencies. An enhanced method for the VOC analysis, implemented in April 1996 by the USGS National Water Quality Laboratory, allowed the reporting of VOC concentrations less than the historical MRLs (Connor et al. 1998). Application of this USGS low-level analytical method to domes- tic well samples resulted in substantially lower reporting levels for many VOCs. Laboratory reporting levels for most VOCs were different from one another, and these levels also varied as method changes were implemented or new instrumentation was used (Supplemental Material, Table 2, available online at http:// www.ehponline.org/members/2007/10253/ suppl.pdf). Reporting of VOC data. The VOC data set was examined in two ways to address the objectives of this study. Analytical results from the total 2,401 domestic wells sampled (1985–2002), including 1,193 samples ana- lyzed at an MRL of 0.2 µg/L and 1,208 sam- ples analyzed with the low-level method, were used to gain a broad perspective on VOC concentrations relative to human- health benchmarks. Using data from all 2,401 domestic well samples maximized the number of VOC concentrations that could be compared to human-health benchmarks, thereby providing a basis for a more compre- hensive evaluation of VOC occurrence data in the context of human health. We report VOC occurrence findings, including detection frequencies, concentra- tions, and spatial distributions, using data from a subset of the samples—that is the 1,208 samples collected and analyzed (1996–2002) with the low-level method. These data provide a relevant assessment of low-level VOC concentrations that are pre- sent in ambient groundwater (untreated water characteristic of the aquifer resources) and best describe VOC concentrations that may be present in drinking water supplied by domestic wells. The low-level analytical method also provides the highest analytical resolution data for multivariate statistical analyses of individual compounds to deter- mine explanatory factors associated with VOC occurrence. Furthermore, as precision of analytical methodologies improves, the USGS low-level VOC data in this study can effectively be used in future comparative analyses for assessment of long-term VOC occurrence trends. Based on these collective objectives, low-level analytical results for VOC occurrence from 1,208 samples are reported with no censoring of data. Statistical analyses. We used non- parametric statistical tests to analyze VOC VOLUME 115 | NUMBER 11 | November 2007 • Environmental Health Perspectives