Environmental Engineering Reference
In-Depth Information
lead was also used in pipes and solder in water-
distribution systems and in the gasoline additive
tetraethyl lead, (C 2 H 5 ) 4 Pb. Although a substantial
decrease in human exposure to lead has been
achieved by eliminating it from gasoline, there is
still a legacy of lead in paint and pipes of old houses
and in land near heavily used roadways. A range
of adverse health effects result from the accumula-
tion of lead in the bloodstream, including anemia,
kidney damage, elevated blood pressure, and
central nervous system effects, such as mental
retardation. Infants and young children are espe-
cially susceptible to lead poisoning because they
absorb ingested lead more readily than do older
humans. lead is a probable human carcinogen.
Mercury (Hg) is a metal of particular concern in
surface waters, where the biological magnification
of mercury in freshwater food fish is a significant
hazard to human health. A significant amount of
mercury discharged into the environment is first
emitted as an air pollutant, but the most damaging
effects typically occur in lakes after the mercury
moves through the atmosphere, is deposited into
a lake, and then undergoes methylation , which is a
process in which mercury is bound to a carbon
molecule. Methyl mercury is an especially toxic
form of mercury that affects the central nervous
system. Human exposure to methyl mercury
occurs primarily through the consumption of con-
taminated fish and seafood.
tration from agricultural areas. When classified accord-
ing to target species, the most common pesticides may
be broadly defined as herbicides, insecticides, or fungi-
cides, depending on whether they are designed to kill
plants, insects, or fungi, respectively. Pesticides, such as
chlordane and carbofuran, are highly persistent in the
environment, since they do not readily break down in
natural ecosystems and thus tend to accumulate in the
tissue of organisms near the top of the food chain, such
as birds and fish.
2.3.6.2  Volatile Organic Compounds.  An important
group of toxic organic compounds are classified as vola-
tile organic compounds (VOCs), and include substances
of particular concern such as: vinyl chloride, carbon
tetrachloride, dichloroethane, tetrachloroethylene,
and trichloroethylene. Chemicals in this class are
often used as industrial or household solvents and as
ingredients in chemical manufacturing processes. Many
of these volatile organic compounds are suspected
or known hazards to the health of humans and
aquatic ecosystems, and all of these compounds have
chemical and physical properties that allow them to
move freely between the water and air phases of the
environment (rathbun, 1998). The distinguishing char-
acteristics of VOCs are low molecular weight, high
vapor pressure, and low-to-medium water solubility.
Because they tend to evaporate easily, the concentra-
tion of VOCs in surface waters is typically much lower
than that in groundwater. In particular, VOCs are typi-
cally found at µ g/l concentrations in surface waters
while they are found at mg/l concentrations in ground-
waters. VOCs are among the most commonly found
contaminants in groundwater.
Metals of biological concern can also be divided into the
following three groups: light metals (e.g., Na, K, and Ca),
which are normally transported as mobile cations in
aqueous solutions; transitional metals (e.g., Fe, Cu, Co,
and Mn), which are essential at low concentrations but
may be toxic at high concentrations; and metalloids
(e.g., Hg, Pb, Sn, Se, and Ar), which are generally not
required for metabolic activity and are toxic at low
concentrations.
2.3.7 Radionuclides
radionuclides are elements with an unstable atomic
nucleus. When radionuclides undergo radioactive
decay, energy is released that can damage exposed
tissue. The effects of excessive levels of radioactivity
on the human body include developmental problems,
nonhereditary birth defects, genetic defects that might
be inherited by future generations, and various types
of cancer. Most radioactivity in water is associated
with natural causes, but there is also a threat of radio-
nuclide contamination from various industrial and
medical processes. A radioactive atom is unstable and
seeks stability by emitting alpha particles, beta particles,
and/or gamma rays.
The radioactive substances that are of concern as
drinking water contaminants are radium, uranium,
radon, and artificial radionuclides. These substances are
described briefly as follows.
2.3.6 Synthetic Organic Chemicals
Synthetic organic chemical s (SOCs) include pesticides,
PCBs, industrial solvents, petroleum hydrocarbons, sur-
factants, organometallic compounds, and phenols. Many
of these organic substances are hazardous to humans in
relatively small concentrations. Complete toxicity and
hazard information is available for only a small percent-
age of the synthetic chemicals produced by industry and
consumed by society.
2.3.6.1  Pesticides.  Pesticides are frequently found in
ground and surface waters that receive runoff and infil-
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