Science of Water : Concepts and Applications

216 The Science of Water: Concepts and Applications
and garbage from city streets became a pressing concern. For example, Moran et al. (1986) point
out that “none too frequently, garbage collectors cleaned the streets and dumped the refuse into the
nearest river.” Halsam (1990) reports that as late as 1887, river keepers gained were employed full
time to remove a constant fl ow of dead animals from a river in London. Moreover, the prevailing
attitude of that day was “I don’t want it anymore, throw it into the river.”
As pointed out, as of the early 1970s any threat to the quality of water destined for use for drinking
and recreation has quickly angered those affected. Fortunately, since the 1970s we have moved
to correct the stream pollution problem. Through scientifi c study and incorporation of wastewater
treatment technology, we have started to restore streams to their natural condition.
Fortunately, we are aided in this effort to restore a stream’s natural water quality by the stream
itself through the phenomenon of self-purifi cation.
A balance of biological organisms is normal for all streams. Clean, healthy streams have certain
characteristics in common. For example, as mentioned, one property of streams is their ability
to dispose of small amounts of pollution. However, if streams receive unusually large amounts of
waste, the stream life will change and attempt to stabilize such pollutants; that is, the biota will
attempt to balance the aquarium. However, if stream biota are not capable of self-purifying, then
the stream may become a lifeless body.
√ Important Point: The self-purifi cation process discussed here relates to the purifi cation of
organic matter only.
SOURCES OF STREAM POLLUTION
Sources of stream pollution are normally classifi ed as point or nonpoint sources. A point source is
a source that discharges effl uents, such as wastewater from sewage treatment and industrial plants.
Simply put, a point source is usually easily identifi ed as “end of the pipe” pollution; that is, it emanates
from a concentrated source or sources. In addition to organic pollution received from the
effl uents of sewage treatment plants, other sources of organic pollution include runoffs and dissolution
of minerals throughout an area that are not from one or more concentrated sources.
Nonconcentrated sources are known as nonpoint sources (see Figure 7.2). NPS pollution, unlike
pollution from industrial and sewage treatment plants, comes from many diffuse sources. NPS pollution
is caused by rainfall or snowmelt moving over and through the ground. As the runoff moves,
it picks up and carries away natural and man-made pollutants, fi nally depositing them into streams,
lakes, wetlands, rivers, coastal waters, and even our underground sources of drinking water. These
pollutants include:
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Excess fertilizers, herbicides, and insecticides from agricultural lands and residential
areas
Oil, grease, and toxic chemicals from urban runoff and energy production
Sediment from improperly managed construction sites, crop and forest lands, and eroding
streambanks
Salt from irrigation practices and acid drainage from abandoned mines
Bacteria and nutrients from livestock, pet wastes, and faulty septic systems
Atmospheric deposition and hydromodifi cation are also sources of NPS pollution (USEPA,
1994).
As mentioned, specifi c examples of nonpoint sources include runoff from agricultural fi elds and
also cleared forest areas, construction sites, and roadways. Of particular interest to environmentalists
in recent years has been agricultural effl uents. A case in point, for example, is farm silage
effl uent, which has been estimated to be more than 200 times as potent (in terms of BOD) as treated
sewage (USEPA, 1994).