Rowe et al. U.S. EPA and others in prioritizing future toxicologic and risk-assessment research. Results of the relational analyses of VOCs in samples with variables reinforce the importance of identifying VOC sources and assessing the vulnerability of drinking-water supplies to VOCs. Identification of hydro- geologic and anthropogenic variables associ- ated with the occurrence of an individual VOC, when coupled with the knowledge of the behavior and fate of the compound, can aid in understanding the vulnerability of aquifers to VOC contamination. We found VOC concentrations to be generally lower than human-health bench- marks; concentrations of six VOCs in 1.2% of the samples were of potential human- health concern because their concentrations were greater than MCLs. With the exception of 1,1-dichloroethene, all of these compounds are regulated as carcinogens in drinking water (U.S. EPA 2006b). The potential health effects of these six VOCs with concentrations greater than MCLs are summarized in Table 3. Ingestion of water containing VOC concentrations greater than MCLs or HBSLs does not necessarily indicate that adverse human-health effects will occur. Human- health benchmarks are conservative (protec- tive) because they incorporate safety factors to account for uncertainty in toxicity informa- tion and are based on lifetime exposure. Additionally, all samples in this study were collected at the wellhead before treatment. Treatment of water from domestic wells may reduce VOC concentrations at the tap (Toccalino et al. 2006). Nine additional VOCs had concentra- tions less than but within one order of magni- tude of MCLs (Supplemental Material, Table 5, available online at http://www. ehponline.org/members/2007/10253/suppl. pdf). These ﬁndings indicate that 15 VOCs may warrant inclusion in regional and national low-concentration, trends-monitor- ing programs. The potential human-health relevance for the 7 VOCs that were detected in samples but do not have established MCLs or HBSLs cannot be evaluated at this time. However, 1,2,4-TMB, MTBE, and 1,1,-DCA are presently included on the U.S. EPA’s Contaminant Candidate List and are priori- tized for monitoring and data collection for public water systems (U.S. EPA 2006a). Potential human exposure to VOCs by ingestion of drinking water from domestic wells. The effective protection and manage- ment of the quality of drinking water supplied by domestic wells are shared responsibilities among well owners, governmental agencies, research organizations, and academic insti- tutions. Proactive domestic well owners can properly maintain wells, routinely test drinking water, and implement manage- ment practices around the home to reduce VOC contamination. Although drinking Hawaii 1544 Alaska Domestic well samples ≥ 1 VOC concentration > MCL ≥ 1 VOC concentration less than but within one order of magnitude of an MCL ≥ 1 VOC detection with concentration less than one order of magnitude of an MCL or HBSL No VOC detected Figure 2. Locations of domestic wells sampled for VOCs and locations of measured VOC concentrations relative to human-health benchmarks, including U.S. EPA MCLs for regulated VOCs and to HBSLs for unregulated VOCs. water from domestic wells is not regulated by federal standards, some homeowners are improving their drinking-water quality by using granular activated-carbon ﬁltering sys- tems and ﬁlters mounted on taps and refrig- erator dispensers. Reverse osmosis also is used to treat drinking water. Domestic well own- ers may rely on water supplied by jugs with ﬁlters or on bottled water for consumption. At the federal level, bottled water is regulated as a packaged food product and is governed by the U.S. Food and Drug Administration by Standards of Quality (U.S. EPA 2004). A survey of 373 households was con- ducted by Probe Research Inc. (2005) to characterize drinking water consumed by domestic well users in the United States. An estimated 52% of the surveyed households indicated that drinking water was used straight from the tap with no treatment. If the statistics from the Probe Research Inc. drinking-water survey are representative of drinking water ingested by members of households throughout the United States, > 22 million people are drinking untreated domestic well water straight from the tap. Furthermore, 65% of the domestic well sam- ples analyzed with NAWQA’s low-level method contained one or more VOCs. Applying this VOC detection frequency to the statistics provided by Probe Research Inc. (2005) indicates that > 14 million people throughout the conterminous United States and Alaska may be exposed to low VOC con- centrations in their drinking water. In addi- tion, 1.2% of the domestic wells sampled in the NAWQA assessment contained at least one VOC concentration greater than its MCL, indicating that as many as 300,000 domestic well users may be potentially ingest- ing drinking water that contains one or more VOCs at concentrations greater than MCLs. The number of domestic well users exposed to VOCs may be underestimated using data from Probe Research Inc. (2005). Calculations based on the Probe Research Inc. (2005) survey indicate that nearly 9 mil- lion additional domestic well users rely on household-water treatment systems for removal of VOCs from drinking water; how- ever, the effectiveness of VOC removal by in-house water-treatment systems is undeter- mined. Evaluating the effective removal of VOCs by in-house treatment methods from water supplied by domestic wells would necessitate routine monitoring, which presently is not required. Most states and some local agencies pro- vide guidance to well owners through web sites and printed materials. Guidance varies by state; however, information provided may include guidelines for well construction, proper well maintenance, preventative meas- ures for contaminants, recommendations for VOLUME 115 | NUMBER 11 | November 2007 • Environmental Health Perspectives