VOCs in Shallow Groundwater in New Residential/Commercial Areas of the United States

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 μg/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.