Water samples for analysis of volatile organic compounds (VOCs) were collected from 118 of the 169 wells used in this assessment; at least 1 compound was present in water from 26 of the 118 wells (Daly and Lindsey, 1996; Lindsey, Breen, and Daly, 1997). Analyses for 60 VOCs at detection levels ranging from 0.2 to 1.0 µg/L revealed the presence of 23 compounds. These compounds are present in commonly used industrial solvents and degreasers or are components of gasoline.
None of the concentrations of the VOCs detected in samples from wells used as drinking-water supplies exceeded the MCLs or Lifetime Health Advisory Levels established by the USEPA. Methyl tert-butyl ether, a gasoline additive, was the most commonly detected compound. Concentrations of methyl tert-butyl ether, detected in 11 of the 118 wells, ranged from 0.2 to 51µg/L. The 51µg/L of methyl tert-butyl ether, detected in a monitoring well, exceeded the lower limit of the compound's Lifetime Health Advisory Level. Chloroform was the second most commonly detected compound. Chloroform is a byproduct of using chlorine as a disinfectant. Chloroform also is present in septic-system effluent, and it has industrial uses. The highest chloroform concentration detected in a water sample was 61µg/L.
The presence of VOCs in limestone aquifers in the Great Valley near Harrisburg, Pa., is affected by land use as illustrated by the graph below. VOCs were detected more frequently in the urban area (subunit 4) than in the agricultural area (subunit 3). Within the urban area, analyses of samples from wells, springs, and a spring-fed stream indicate that contaminated ground water flows from springs into the streams.
The frequency of detections of VOCs in urban areas is likely to be a result of the numerous urban sources of VOCs, including spills, leaks from underground tanks, improper disposal, atmospheric deposition, runoff from pavement, and leaking sewerlines.
Underground storage tanks that contain gasoline and other fuels can be a source of VOCs in ground water. The drill rig shown above is used to install monitoring wells so the property owner can determine whether leakage or contamination has occurred.
In the rural areas in the Appalachian Mountain subunits, no VOCs were detected in well water (see graph below). The low population densities in rural areas and fewer sources of VOCs are likely explanations for the lack of detections of VOCs. In rural areas, leaking storage tanks, septic systems, improper disposal, and atmospheric deposition are potential sources of VOCs.
Numerous sources of VOCs in urban areas are a likely explanation for the high frequency of detections in the urban subunit. In the urban area, ground water contaminated with VOCs flows from springs that feed coldwater streams known for their trout populations.