Environmental Engineering Reference
In-Depth Information
geological environment where the contaminants are released, and (4) the presence, type,
and distribution of microorganisms (Howard et al. 1997; USEPA 1998b; McKone and Enoch
2002; USGS 2006a).
Degradation rates also vary by media. In general, organic compounds in the atmosphere,
including VOCs, degrade more quickly than the same organic compounds released and
migrating to subsurface soil and groundwater (USEPA 1998a,b).
VOCs not degrading very easily include many of the chlorinated solvents or DNAPL
compounds (USEPA 1996a). Many chlorinated VOCs including PCE, TCE, 1,1,1-TCA, DCE,
and vinyl chloride are considered very persistent in the environment once released and
can remain present for decades (USEPA 1996a). Other VOCs, such as benzene, toluene,
ethyl benzene, and xylenes, are not typically as persistent in the environment and have
been known to biodegrade in a few months to years if conditions are favorable (Rogers
1995; USEPA 1996a).
The VOC MTBE is persistent in the environment. Its relatively high solubility in water
and low sorptive properties compared to other common VOC contaminants has resulted in
significant MTBE-contaminated groundwater supplies at many urban locations through-
out the United States (USGS 2006a; USEPA 2009a).
The trihalomethane VOCs include chloroform, bromoform, bromodichloromethane,
and dibromochloromethane. These compounds have been detected in the groundwater of
many U.S. aquifers (USGS 2006a). Trihalomethanes have high relative vapor pressures and
commonly evaporate quickly when in contact with the atmosphere. Therefore, exposure
to trihalomethanes is of special concern during showering and washing. They degrade by
photolysis when exposed to direct sunlight and can also be degraded by microorganisms
(ATSDR 1997a, 2005a).
8.4.2 PAHs
PAHs are LNAPL compounds, and being lighter than water they float on surface water
and groundwater. They do not readily dissolve in water and have low vapor pressures
compared to most VOCs. PAHs are common constituents of automobile exhaust, espe-
cially from diesel fuel (USGS 2006a), and are common air contaminants in urban areas. In
addition, PAHs have a high sorptive affinity and can therefore attach to particulate matter
(ATSDR 1996).
Many PAH compounds biodegrade under favorable conditions. The half-life of PAHs is
shortest in the atmosphere due to photochemical degradation and lasts just a few days or
weeks (ATSDR 1996). The half-life of PAH compounds in soil and groundwater is longer
and may last for several years or decades. When released to soil, PAHs tend to migrate
more slowly than VOCs because of their higher molecular weight and sorption to soils
with high organic content.
8.4.3 PCBs
Once in the environment, PCBs do not readily degrade—they remain in the environment
for long periods of time and often cycle between air water and soil. PCBs can be carried
long distances attached to particulate matter and have been detected in snow and sea
water far away from any known point of release (USEPA 2009b). This transport capability
is confirmed by their worldwide detection. The lighter the PCB compound (fewer number
of chlorine atoms in its structure; Section 7.5), the farther it can be transported from its
release point.
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