Environmental Engineering Reference
In-Depth Information
PCBs are not very soluble, have high sorptive potential, and low vapor pressures. This
combination of physical properties largely determines their environmental distribution,
as they are only present at high concentrations in water or air when sorbed to particulate
matter. Sinks where PCBs are frequently detected include soil near release points and sedi-
ments in rivers, streams, and lakes (USEPA 2009b). PCBs bioaccumulate in the leaves and
above-ground parts of plants and food crops, and in aquatic organisms and fish where
PCBs are present in sediments (Section 8.3.4). As a result, humans and other organisms
ingesting impacted plant material, food crops, or organisms containing PCBs may bioac-
cumulate PCBs in their body tissues (ATSDR 2001a).
8.4.4 SVOCs
SVOCs are much less volatile than VOCs, but notable exceptions here are the amine com-
pounds that exist as a gas at room temperature and standard pressure (ATSDR 2002a).
In terms of solubility, phthalates and phenols do not readily dissolve in water, whereas
amines and esters may dissolve, become mobile, and reach groundwater (ATDSR 1999c,
2002a,b,c,d).
When released into the environment, SVOCs are commonly detected in soil because they
have high sorptive potentials. Many SVOCs, including pentachlorophenol, are degraded
by microorganisms under favorable conditions and are also susceptible to photolysis and
hydrolysis (ATSDR 2001, 2002a). Amines and esters degrade in minutes when exposed to
direct sunlight (ATSDR 1999).
8.4.5 Heavy Metals
Heavy metals are released directly to air, water, and soil. In most cases, these contaminants
do not remain in the atmosphere for long periods of time because they have high specific
gravity and become deposited onto the land surface shortly after being emitted. Mercury,
however, has been detected as far away as 50 mi from its source after being released into the
atmosphere (USEPA 1997). Lead is also considered a common air pollutant (USEPA 2008a).
Major sources of lead include metal melting facilities, battery manufacturing, and
leaded gasoline and fuels. The good news is there has been a 92% decrease in atmospheric
lead concentrations over the period from 1980 through 2008. Contributing to the observed
decrease has been the removal of lead from gasoline and fuels, increased efficiency in air
pollution control equipment, and the regulation of lead emissions sources (USEPA 2008a).
The solubility of heavy metals in water is very low, except for some varieties of arsenic
and chromium VI at a neutral pH. Due to their inability to form a solution, the preferred
sinks for heavy metals are soil and sediments. Many metals undergo some transformation
such as oxidation after being released into the environment but are not destroyed and
remain in the environment (ATSDR 1999a,b, 2003, 2004a, 2005b,c, 2007a,b,c, 2008a,b). Due
to their low solubility and high specific gravities, they tend to remain near release points.
This is why increased concentrations of heavy metals are present in the near surface soil of
urban areas as a result of releases from anthropogenic sources (Murray et al. 2004).
Some heavy metals such as mercury accumulate in sediment of lakes, rivers, and
streams. Mercury may undergo a transformation through a process known as methylation
after being released into the environment, typically when it reaches a surface water body.
The process of methylation transforms elemental mercury to methyl mercury (CH
3
Hg).
Methyl mercury is the most toxic form of mercury and has the potential to bioaccumulate
in aquatic organisms, including fish (USGS 2000).
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