Common Contaminants in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 230

of contaminants may produce enhanced or diminished effects. Common mixtures include
gasoline, often containing more than 250 individual chemical compounds, industrial
wastes, and a malfunctioning sewage treatment plant, typically discharging chemical and
biological contaminants (USEPA 2008a).
The following sections briefly describe the major contaminant categories encountered
in urban areas. For each contaminant category, specific contaminants, their common uses,
physical attributes, and toxicities are presented.
7.3 Volatile Organic Compounds
VOCs are organic compounds that generally volatilize or evaporate readily under normal
atmospheric pressure and temperatures (USGS 2006; USEPA 2008a). Table 7.1 is a list of the
common VOCs and includes those sought out and analyzed when conducting a subsur-
face environmental investigation (USEPA 2008b). Included in Table 7.1 is the CAS registry
number for each chemical compound. The CAS registry is maintained by the American
Chemical Society and contains over 50 million disclosed substances, each with a unique
numerical identifier and common use (American Chemical Society 2009).
VOCs have a high vapor pressure (evaporate quickly), low-to-medium solubility, and
low molecular weight (USGS 2006). Most VOCs are considered toxic or harmful to humans
and other organisms (USEPA 2009b). In addition, many VOCs are flammable and must be
handled with extreme care.
The use of VOCs in the United States increased tenfold from 1945 to 1985 (USGS
2006). Some VOCs have had, and continue to have, very heavy usage. An example is
gasoline, which contains numerous VOC compounds and its production and use con-
tinues to increase. Table 7.1 demonstrates many common uses of VOCs. As noted, many
VOCs are flammable and are components of gasoline and other fuels such as diesel
fuel, kerosene, and fuel oil. In addition, VOCs are commonly used as solvents, ingre-
dients in paints, paint thinners, in the manufacturing process of pharmaceuticals, for
caffeine extraction, nail polish removers, dry cleaning chemicals, mothballs, pesticides
and fumigants, adhesives, refrigerants, and as a by-product of chlorination for water
purification.
For hydrogeological purposes, VOCs are separated into two general categories—
DNAPLs and LNAPLs (USGS 2009a,b). An additional category of VOCs are the trihalo-
methanes (THMs). Each is discussed separately in the following sections.
7.3.1 Light Nonaqueous Phase Liquids
LNAPLs are liquids lighter than water that do not mix or dissolve in water readily.
Common LNAPL compounds include the VOCs: benzene, ethyl benzene, toluene, and
xylenes. These compounds are also often symbolized by the acronym BTEX. BTEX com-
pounds are common components of gasoline and are often used as indicator analytes
when evaluating whether there has been a release of gasoline or other similar fuels to the
environment (USEPA 1989b; USGS 2009b). LNAPLs also include other compounds lighter
than water such as PAHs, and are discussed in the next section.