Rowe et al. with a VOC from another group (e.g., PCE) may result from wide distribution of these compounds that spatially overlap. VOC mix- tures, such as PCE and 1,1,1-TCA, may co-occur because the physical and chemical properties of these compounds could result in similar environmental behavior. Mixtures may also result from VOCs sharing the same source, such as toluene and 1,2,4-TMB in gasoline products. Mixtures such as PCE and trichloroethene (TCE) may result from the degradation of the parent compound (PCE) to the by-product (TCE). Relational analyses. Results of logistic regression analyses for frequently detected VOCs and associated variables are summarized in Table 2. VOCs that had a variable with a standardized coefﬁcient ≥ 0 indicate that as the variable increases, the probability of detecting that compound increases; conversely, VOCs that had a variable with a standardized coefﬁ- cient < 0 indicate that as the variable increases, the probability of detecting that compound decreases. If a standardized coefficient of a variable for a VOC was ≥ 0.1 (absolute value), the variable was considered to be strongly asso- ciated with the probability of a compound’s occurrence; if a standardized coefﬁcient of a variable for a VOC was < 0.1 (absolute value), the variable was considered to be weakly asso- ciated with the probability of the compound’s occurrence. Based on the strength of variable associations and the frequency of VOC detec- tions, the ﬁve variables in order of decreasing importance were a) dissolved-oxygen content; b) precipitation; c) the number of Resource Conservation and Recovery Act [RCRA (U.S. EPA 2006c)] sites (that generate, transport, store, or dispose of hazardous waste materials) within a 1-km radius of the well; d) aquifer type; and e) water temperature. The hydrogeologic variable we found to be most strongly and frequently associated with VOC occurrence in domestic well water was dissolved-oxygen content, which is a key factor in biodegradation. The probability of detecting chloroform, MTBE, 1,1,1-TCA, and toluene increased with increasing dissolved-oxygen content (Table 2), whereas the probability of detecting chloromethane decreased with increasing dissolved-oxygen content. The probability of detecting MTBE increased with increasing precipitation, as expected. Precipitation is the driving force for recharge and the transport of VOCs from land surface to the water table. The probabil- ity of detecting MTBE decreased with increasing depth to the top of the well’s screened interval. Depth to the top of the screened interval is believed to be roughly equivalent to the depth to the top of the aquifer (Moran et al. 2006). Increased depth to the screened interval allows more travel time from the MTBE source to the aquifer, Table 2. Alphabetized listing of frequently detected VOCs and statistical summary of hydrogeologic and anthropogenic variables associated with the probability of occurrence of the compound in domestic well samples.a No. of samples in analyses 842 1,182 1,184 1,168 969 1,155 1,184 1,183 1,165 1,178 Associated variablesb Dissolved-oxygen content Depth to water level Dissolved-oxygen content Recharge Water temperature Dissolved-oxygen content Depth of well Permeability of soil Aquifer type Dissolved-oxygen content Depth to top of screened interval LUST sites within 1 km of wellc Precipitation Water temperature Dissolved-oxygen content RCRA sites within 1 km of wellc Dissolved-oxygen content RCRA sites within 1 km of wellc Precipitation RCRA sites within 1 km of wellc Agricultural land use Precipitation Aquifer type Dissolved-oxygen content Type of variable Fate Transport Fate Transport Fate Fate Transport Transport Transport Fate Transport Source Transport Fate Fate Source Fate Source Transport Source Source Transport Transport Fate Coefﬁcient in logistic regression equation 1.142 –0.005 –0.990 –0.002 –0.087 –0.536 –0.003 0.104 –0.943 1.291 –0.008 1.907 0.073 –0.184 0.384 0.158 2.592 0.307 0.026 0.196 0.004 –0.046 –0.600 0.657 Standardized coefﬁicient 0.20 –0.18 –0.15 –0.07 –0.06 –0.08 –0.13 0.09 –0.07 0.10 –0.15 0.10 0.17 –0.13 0.09 0.16 0.30 0.15 0.09 0.10 0.05 –0.05 –0.09 0.11 Compound Chloroform Chloromethane Dichlorodiﬂuoromethane Methylene chloride MTBE PCE 1,1,1-TCA TCE 1,2,4-TMB Toluene 1542 which may allow for increased attenuation of MTBE concentrations through natural loss processes such as biodegradation, sorption, dispersion, and volatilization. Furthermore, the probability of detecting MTBE decreased with increasing water temperature, which is believed to be related to the biologic activity necessary for transformation of the compound (Moran et al. 2006). The anthropogenic variable most strongly and frequently associated with probability of VOC occurrence was the number of RCRA sites within a 1-km radius of the well. The probability of detecting three solvents—PCE, TCE, and 1,1,1-TCA—increased with increasing number of sites near the wells. The probability of detecting the oxygenate MTBE increased with increasing number of leaking underground storage tank sites within a 1-km radius of the well. Comparison of VOC concentrations to human-health benchmarks. One or more VOC concentrations were greater than a human-health benchmark in 1.2% of the domestic well samples. Six VOCs had con- centrations greater than MCLs: dibromo- chloropropane (DBCP), 1,2-dichloropropane, ethylene dibromide (fumigants); PCE and TCE (solvents); and 1,1-dichloroethene (organic synthesis compound) (Table 3 and Figure 1). With the exception of ethylene dibromide, these VOCs also had concentra- tions less than but within one order of magni- tude of their MCL in samples. One or more VOCs were detected within one order of magnitude of their MCL in 2.1% of the sam- ples and include nine regulated VOCs: ben- zene (gasoline hydrocarbon); vinyl chloride (organic synthesis compound); carbon tetra- chloride, 1,2-dichloroethane, methylene chloride, and 1,1,1-TCA (solvents); and bromoform, chloroform, and dibromo- chloromethane (THMs) [Figure 1 and Supplemental Material, Table 5 (available online at http://www.ehponline.org/members/ 2007/10253/suppl.pdf)]. No VOC concentra- tions were greater than or within one order of magnitude of HBSLs. Seven VOCs detected in samples but without established MCLs or HBSLs were, in order of decreasing detection frequency, MTBE, 1,1-dichloroethane, 1,2,4-TMB, chloroethane, tert-amyl methyl ether, diisopropyl ether, and n-propylbenzene. VOCs with concentrations of potential human-health concern commonly occurred in mixtures. Of the 32 VOC concentrations greater than MCLs, 23 were in samples con- taining VOC mixtures. Of the 67 VOC con- centrations within one order of magnitude of MCLs, 37 were in samples containing VOC mixtures. Sampling locations and spatial distribu- tions of VOC concentrations in the United States relative to human-health benchmarks LUST, leaking underground storage tank. aMoran et al. (2006) provide a thorough discussion of logistic regression analysis used in this study, including the statisti- cal approach, ancillary variables, and sources of data. bVariables strongly associated with the occurrence of VOCs are those with absolute values of standardized coefﬁcients of ≥ 0.1. cNumber of sites within a 1-km radius of well. VOLUME 115 | NUMBER 11 | November 2007 • Environmental Health Perspectives