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Effects of pollutants on the aquatic environment
Effects of pollutants on the aquatic environment

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Water is crucial for our survival. It is used in our bodies for a number of purposes:

  • cooling
  • as a waste disposal medium
  • as a conductor for nerve impulses
  • as a component in the digestion of food
  • as a solvent in which chemical reactions take place.

The hydrological cycle is the continuous cycling of water between land, water surfaces and the sea. Pollutants entering a river can be washed away to sea, or degraded by microorganisms present in the river. Excess pollution in a river can damage the plant and animal life present in the river by reducing the oxygen content of the water.

Point sources of pollution are discharge points where pollutants collected by a network of pipes or channels are released. Diffuse sources, on the other hand, are characterised by multiple discharge points that cannot be located exactly. Point sources can be easily controlled, while diffuse sources pose great difficulty in terms of collection and control.

Lakes are much more prone than rivers to pollution as they do not have the flushing effect of rivers. They also do not have the dilution effect of large bodies of water such as the sea. Eutrophication can be a particular problem for lakes.

The major sources of water pollution are:

  • discharges from sewage works, often containing industrial wastes
  • discharges from manufacturing and industrial plants, including mines
  • discharges from animal rearing, fish farming and agriculture
  • seepage from domestic and industrial landfill sites
  • urban surface water run-off.

Different pollutants affect the aquatic environment in different ways. While at low concentrations many pollutants (e.g. organic materials, N and P) may be beneficial, at high levels they can adversely affect the ecology of the system. Excess nitrate can be particularly harmful to babies.

Many of the toxic pollutants in effluents are synthetic, and therefore do not easily biodegrade naturally.

The effects of physical pollution on the ecology of a river system can be complex, affecting the feeding and breeding habits of the different species.

Biological pollutants can spread disease through water, and also disrupt the ecology.

The measurement and control of water quality is therefore of crucial importance in the interests of public health and the maintenance of the environment.

Table 4 gives a summary of the effects of the different pollutants discussed in this unit.

Table 4  Classification of pollutants
PollutantGeneral effectEffect on biotaEffect on water suppliesSources: naturalSources: result of human activity
Organic (biodegradable wastes)Increased oxygen demand; food provided for organisms lower down in food chainTolerated in moderate quantities if release not too quick, serious if dissolved oxygen (DO) drops too quicklyIncreased need of treatmentRun-off and seepage through soilDomestic sewage, food processing, animal wastes
Plant nutrientsExcessive plant growthDemand on DOIncreased need of treatmentNatural degradative processesAnimal wastes, fertilisers, detergents, industrial wastes
Toxic chemicals (e.g. heavy metals, pesticides, phenols, PCBs)Toxic to humans, animals and plantsCould be lethalIncreased need of treatment or controlRareDetergents, pesticides, tanneries, pharmaceuticals, wool scouring, refineries
Endocrine disruptorsAlteration of ecologyMay adversely affect health and reproduction of humans and animalsCan be present in water sold in plastic bottlesFusarium species of fungusChemical manufacture, intensive farming
Acids/alkalisLowering/raising of pH; acids can dissolve heavy metalsOnly narrow range of pH tolerable for most plants and animals; heavy metals toxicCorrosionNaturally acid or alkaline rockBattery, steel, chemical and textile manufacturing; coal mining
Suspended solidsReduction in light penetration (increased turbidity), blanketing, introduction of colourPhotosynthesis reduced; blanketing of benthic plants and animals; obstruction of gills of fishObstruction of filters; increased need of treatmentSoil erosion, storms, floodsPulp mills, quarrying, any building or development work involving ground disturbance
Immiscible liquidsFormation of a layer at the water surface that could prevent O2/CO2 interchangeReduced DO; insect breeding affectedInterference with treatment processesUnlikelyOil-related activity
HeatDecrease in DO; increase in metabolic rate of aquatic organismsPossible reduced breeding or growth of aquatic organismsNoneUnlikelyPower plants, steel mills
Taste-, odour- and colour-forming compoundsTaste, malodour, colourTainting of fishIncreased need of treatmentPeatChemical manufacture or processing
MicroorganismsPathogenic to humansNoneIncreased need of treatmentAnimal excrementContamination from human wastes