Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 299

VOCs are also released directly onto the ground surface through leaks or spills at, or
near the surface. Sources of these leaks include underground storage tanks, service sta-
tions, refineries, and pipelines. Because of these surface and shallow subsurface releases,
VOCs are common groundwater contaminants and have been detected in the groundwa-
ter of numerous aquifers in the United States (USGS 2006a). A study of groundwater in
the United States detected VOCs at a concentration of 0.02 μg/L in >50% of approximately
3500 samples collected from 100 different groundwater aquifers across the country (USGS
2006a). The VOCs detected most often included bromoform, bromodichloromethane, chlo-
roform, chloromethane, 1,1-dichloroethane, dichlorodifluoromethane, methylene chloride,
dibromodichloromethane, methyl-tert-butyl ether (MTBE), trichloroethene (TCE), tetra-
chloroethene (PCE), 1,1,1-trichloroethane (TCA), trans -1,2-dichloroethene (DCE), toluene,
and trichlorofluoromethane.
This same study indicates the vulnerable nature of many aquifers of the United States,
and their location corresponds with many urban areas. In fact, of the 28 major urban areas
listed in Table 2.4, 27 have detectable concentrations of VOCs (only Kansas City is missing).
This extent represents a population of 125.4 million—over 41% of the entire U.S. popula-
tion that are potentially exposed to VOCs in their water supply. Figure 8.19 shows the
locations where VOCs were detected, and Figure 8.20 shows the detection frequency of the
most commonly detected VOCs from the USGS study (USGS 2006a).

Oahu

Explanation
Total concentration, in micrograms

Hawaii

per liter

No detection or less than 0.02
0.02 to less than 0.2
0.2 to less than 1
1 to less than 10
10 or greater

Alaska

FIGURE 8.19
Occurrence of VOCs in groundwater aquifers of the United States. (From United States Geological Survey
(USGS), Volatile organic compounds in nation's ground water and drinking-water supply wells, National
Water-Quality Assessment Program, USGS Circular 1292, Washington, DC, 2006a; United States Geological
Survey (USGS), Pesticides in the nation's streams and groundwater, 1992-2001—A summary, USGS Fact Sheet
2006-3028, Washington, DC, 2006a.)