Environmental Engineering Reference
In-Depth Information
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FIGURE 14.2
Percentages of impaired lakes and streams in the United States by type and
cause of impairment (data from the U.S. Environmental Protection Agency for 1994).
eral concepts of toxicology, causes and effects of pollution by inorganic
and organic contaminants, and thermal pollution. Mitigating solutions are
also discussed. Nutrient pollution has had a strong influence on aquatic
systems; however, this will be discussed in Chapter 17.
BASIC TOXICOLOGY
Exposure to toxins can either come in large pulses over a short period
of time
(acute)
or in low doses over long periods of time
(chronic).
Re-
sponses can be
lethal, sublethal
(not causing death), or
cumulative
(a re-
sponse to numerous events). Studies of toxicity include accounting for vari-
ability in responses of organisms. Thus, the
lethal dose,
the amount ingested
that causes death, is labeled with a subscript that indicates the percentage
of animals killed (e.g., LD
50
is the lethal dose for 50% of the animals
tested). Organisms can also be exposed to toxins through the water, in-
cluding absorption across cell membranes, gills, and skin. Thus, toxicolo-
gists also report lethal concentrations (e.g., LC
50
). Effective concentration
(EC) is the concentration that causes some effect other than death; a
subscript is also used to denote the percentage showing the effect (Mason,
1996).
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