The quality of shallow groundwater in urban areas was investigated by sampling 518 monitoring wells between 1996 and 2002 as part of the National Water-Quality Assessment Program of the U.S. Geological Survey. Well networks were installed primarily in new residential/commercial areas less than about 30 years old (17 studies) and in small towns (2 studies) by randomly locating as many as 30 monitoring wells in each study area. The median well depth was 10 m. Based on samples with age-date information, almost all groundwater was recharged after 1950. Samples were analyzed for 53 volatile organic compounds (VOCs). Concentrations ranged from about 0.001 to 1000 ??g/L (median 0.04), with less than 1% of the samples exceeding a Maximum Contamination Level or Drinking Water Advisory established by the U.S. Environmental Protection Agency. Using uncensored concentration data, at least one VOC was detected in 88% of the samples, and at least two VOCs were detected in 69% of the samples. Chloroform, toluene, and perchloroethene were the three most frequently detected VOCs. Dissolved oxygen concentration, estimated recharge index, and land-use were significant variables in logistic regression models that explained the presence of the commonly detected VOCs. Dissolved oxygen concentration was the most important explanatory variable in logistic regression models for 6 of the 14 most frequently detected VOCs. Bromodichloromethane, chloroform, and 1,1,1-trichloroethane had a positive correlation with dissolved oxygen; in contrast, dichloroethane, benzene, and toluene had a negative correlation with dissolved oxygen.
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VOCs in shallow groundwater in new residential/commercial areas of the United States