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 as part of the National Water-Quality Assessment Program of the U.S. Geological Survey between 1996 and 2002. 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 up to 30 monitoring wells in each study area. The median well depth was 10 meters. Groundwater was recharged after 1950 in 95 percent of the samples with age dates. Samples were analyzed for 53 volatile organic compounds (VOCs). Concentrations ranged from about 0.001 to 1000 micrograms per liter, with less than 1 percent of the samples exceeding a Maximum Contamination Limit or Dinking Water Advisory set by the U.S. Environmental Protection Agency. At least one VOC was detected in 88 percent of the samples and at least two VOCs were detected in 69 percent of the samples. Chloroform, toluene, and tetrachloroethylene were the three most frequently detected VOCs.

Various methods of analysis were used to evaluate the fate, transport and sources of the 14 most frequently detected VOCs. Fugacity models showed that the halogenated and nonhalogenated VOCs could be distinguished based on their environmental behavior. To evaluate sources, land use within 500 meters of the sampled well was compared with detections of the 14 VOCs. This analysis showed that point-source contamination was not the likely source of these VOCs, but nonpoint source contamination (such as leakage from sewers) was a potential source. Dissolved oxygen concentration was the strongest explanatory variable for detections and concentrations of VOCs. Halogenated VOCs (for example, chloroform) were positively correlated and nonhalgenated VOCs (for example, toluene) were negatively correlated with oxygen content. Degradation of halogenated VOCs was supported by the increased detection frequency of some potential degradation products in anoxic water. An estimated natural recharge index was the next most important explanatory variable, demonstrating that VOCs in shallow groundwater are dependent on the rate of transport through the unsaturated zone.