percent. The following conclusions were drawn in the Zogorski et al. study: (1) public wells are more vulnerable to contamination than private domestic wells, (2) public wells had higher concentrations of VOCs (50 percent of public wells had total VOC concentrations > 1 μg/L), and (3) public wells were more likely to have mixtures of VOCs than domestic wells. These effects were attributed, by the authors, to the larger withdrawal rates and closer proximity to urban areas of many public supply wells.
Further analysis of domestic wells by DeSimone et al. (2009) showed that organic contaminants were detected in 60 percent of 2,100 sampled wells. Wells were sampled in 48 states in parts of 30 regionally extensive aquifers used for water supply. Aquifers were randomly selected for sampling and there was no prior knowledge of contamination. Seventeen VOCs were detected in more than 1 percent of wells at concentrations greater than 0.02 μg/L (see Figure 2-4 below, VOCs are in black). TCE was detected above the maximum comtaminant level (MCL) of 5 μg/L in 0.1 percent of wells. PCE was detected above the MCL of 5 μg/L in 0.05 percent of wells.
Rowe et al. (2007) compiled data for 2,400 domestic wells sampled from 1985 until 2002. Sixty-five percent of domestic wells had a VOC detection 0.02 μg/L or greater (31 percent had a single VOC, 34 percent had more than one VOC). The top five VOCs detected were chloroform (25.6 percent), toluene (17.9 percent), 1,2,4-trimethylbenzene (15.2 percent), PCE (11 percent), and chloromethane (9.7 percent). PCE, TCE, and chloromethane were the compounds with the largest fraction of samples at 0.1 × MCL or greater. The presence of a LUST site within 1 km of the sampled well strongly correlated with MTBE detections, and the presence of an RCRA site (as determined by the EPA Envirofacts database) within 1 km of the well strongly correlated with the detections of PCE, TCE, and 1,1,1-TCA.
Toccalino and Hopple (2010) and Toccalino et al. (2010) focused on 932 public supply wells across the United States. The public wells sampled in this study represent less than 1 percent of all groundwater that feeds the nation’s public water systems. The samples, however, were widely distributed nationally and were randomly selected to represent typical aquifer conditions. Overall, 60 percent of public wells contained one or more VOCs at a concentration of ≥ 0.02 μg/L, and 35 percent of public wells contained one or more VOCs at a concentration of ≥ 0.2 μg/L. The percentages are higher than those reported by Zogorski et al. (2006), but this study focused on a larger suite of VOCs (85 vs. 55 compounds). Overall detection frequencies for individual compounds included 23 percent for PCE, 15 percent for TCE, 14 percent for MTBE, and 12 percent for 1,1,1-TCA (see Figure 2-5). PCE and TCE exceeded the MCL in approximately 1 percent of the public wells sampled. About 70 percent of VOC detections were from sand and gravel aquifers. Public wells in sand and gravel aquifers more often