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17.3 Public health impacts of water pollution

Waterborne infectious diseases are transmitted primarily through contamination of the

water sources with excreta of humans and animals who are either active cases or

carriers of disease. Carriers do not show any signs of disease although they havedisease-causing agents in their body that can be transferred to others; active cases are

people who are displaying visible signs of disease. Use of contaminated water for

drinking or cooking, or contact with contaminated water during washing or bathing,

may result in infection.

he dose or amount ingested that is necessary to cause illness depends on the type of

pathogen. !xposure to a single pathogenic organism does not always result in

infection and disease. "ometimes many pathogens, perhaps several hundred, must be

ingested to cause infection. he minimum infectious dose also varies with the age,

health, nutritional and immunological status of the exposed individual. #nfants and

young children, people who are debilitated, people who are living in unsanitary

conditions, people who are sick and the elderly are at greatest risk of waterborne

diseases.

17.4 Indicators of water pollution

he physical, chemical and biological characteristics of water are changed when the

water is contaminated with different pollutants. Water is colourless, odourless and

tasteless, as you know, but when it is polluted with physical and chemical pollutants

the water may have colour, odour and taste.

Question

#f water is completely clear and has no colour or odour, is it safe to drink$

Answer

%ot necessarily, because it may contain microorganisms or dissolved chemicals that

cannot be seen.

!nd of answer

o know whether water is polluted with specific bacterial contaminants, samples

should be taken and sent to a laboratory for analysis. &s noted in "tudy "ession '(,

E.coliis the standard indicator organism for faecal contamination of water and for the

possible presence of faecal pathogens. )or water intended for drinking, the World

*ealth +rganiation W*+ recommends that E.colimust not be detectable in any

'// ml sample. #n most developing countries like !thiopia the priority is to get from

0bad1 2uality more than ',/// faecal coliforms per '// ml to 0moderate1 2uality

less than '/ faecal coliforms per '// ml. 03ood1 2uality is classed as ero faecal

coliforms per '//ml.

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17.5 Effects of pollution on water sources

17.5.1 Streams and riers

"treams and rivers are not only potential water sources for humans, they are also

important a2uatic habitats for many plants and animals. 4ollution can have a

damaging effect on a2uatic ecosystems see 5ox '6.' as well as potentially on

human health.

!o" 17.1 A#uatic ecos$stems

Ecos$stem was defined in "tudy "ession 7. #t means a community of living

organisms that interact with each other, plus the environment in which they live and

with which they also interact. &n ecosystem therefore includes plants, animals and

microorganisms and their physical environment which may be land, trees, soil, ocean,

freshwater, etc., and also includes the relationships between all these components.&2uatic ecosystems can be categorised as marine ecosystems in the ocean, and

freshwater ecosystems in rivers, lakes and other surface water bodies )igure '6.7.

)igure '6.7 8akes and rivers are important a2uatic ecosystems supporting many

forms of life in and around the water. 4hotos9 4am )urniss

he effect of pollution on streams and rivers depends on the type of pollutant. "ome

substances are acutely toxic to a2uatic plants and animals and will cause dead ones

downstream from the pollutant source in which no living organism is found. +ther

pollutants are health concerns to humans, but have little impact on a2uatic

communities.

+ne of the most common types of freshwater pollutant is biodegradable organic

material. When a high concentration of organic material such as raw sewage human

excreta is discharged into a stream, the levels of dissolved oxygen in the water may

fall so low that the water is completely deoxygenated.

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