Within the United States, contaminated sites are present at a wide range of facilities and
operations spanning simple to complex activities over small to large areas. These sites include
gasoline stations, drycleaners, manufacturing plants, energy generation facilities, mining
operations, disposal facilities, and nuclear and military installations. Soil and groundwater
continue to be the most prevalent contaminated media. The most frequent contaminants of
concern (COCs) are volatile organic compounds (VOCs), which are present at a majority of
sites. At many sites, semivolatile organic compounds (SVOCs) and metals are also prevalent.
The Agency for Toxic Substances and Disease Registry (ATSDR) has identified 275 hazardous
substances most commonly found at facilities on the U.S. NPL (ATSDR, 2009). Table 1.1
provides a list of 20 chemicals (or chemical groups), along with their prevalence and priority as
COCs at NPL sites in the United States. Some of these same substances are also pervasive at
thousands of non-NPL sites.