Geography Reference
In-Depth Information
population (Peters
et al
. 1998). Effluents from
industrial processes such as canning, milling,
tanning, and manufacture of textiles,
pharmaceuticals and coffee, may also introduce
large quantities of organic wastes into
watercourses (Meybeck
et al.
1989). Point source
domestic and industrial organic waste inputs into
watercourses are biodegraded, with well-known
consequences for the downstream dissolved
oxygen balance, water chemistry and aquatic
organisms (Figure 11.2).
Agricultural practices may also provide large
amounts of organic material in the form of animal
slurry, silage liquor, sewage sludge spread on to
agricultural land and effluents from dairies,
abattoirs and vegetable-processing plants (National
Rivers Authority 1992). Major pollution incidents
involving organic farm wastes are often associated
with poor containment of slurry or silage liquors
and spillage following failure of storage areas.
These wastes can be very damaging if they enter
watercourses because of their very high
biochemical oxygen demand (BOD), which for
silage liquor is typically 60,000 mg l
-1
, compared
with 350 mg l
-1
for untreated human sewage and
<5 mg l
-1
for clean river water
(ibid.)
.
Figure 11.2
Effects of an organic effluent on the quality
and ecology of a river.
Pathogens
It has recently been highlighted that the
microbiological safety of water is in global decline
(Ford and Colwell 1996), and water-related disease
is one of the most important worldwide human
health concerns in terms of morbidity, mortality
and cost. For example, more than half the world
population has suffered diseases resulting from
drinking polluted waters (Barabas 1986) and more
than 10 million deaths occur each year from
water-related diseases. Furthermore, the social cost
of gastro-intenstinal illness not requiring physician
consultation or a period in hospital was estimated
at more than US$19.5 billion in 1985 for the
United States (Garthright
et al
. 1988)
.
Agents of waterborne disease in freshwater
bodies include bacteria such as
Shigella
and
Salmonella,
viruses such as hepatitis A and
enterovirus, protozoans such as
Giardia
and
Entamoeba,
and parasitic worms such as the beef
tapeworm
(Taenia saginata)
and blood flukes
(
Schistosoma
spp.) (Meybeck
et al
. 1989). New
water-transmitted pathogens are continually being
discovered, such as
Helicobacter pylori,
which is now
believed to be the cause of many types of ulcer
(Klein
et al
. 1991). The major source of freshwater
pathogens is faecal contamination from infected
humans, pets, farm animals and wildlife (Geldreich
1997). Sewage discharges often introduce a variety
of pathogens to watercourses, and the occurrence
Source:
After Mellanby 1980.
