Water is crucial for our survival. It is used in our bodies for a number of purposes:
- 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
|Pollutant||General effect||Effect on biota||Effect on water supplies||Sources: natural||Sources: result of human activity|
|Organic (biodegradable wastes)||Increased oxygen demand; food provided for organisms lower down in food chain||Tolerated in moderate quantities if release not too quick, serious if dissolved oxygen (DO) drops too quickly||Increased need of treatment||Run-off and seepage through soil||Domestic sewage, food processing, animal wastes|
|Plant nutrients||Excessive plant growth||Demand on DO||Increased need of treatment||Natural degradative processes||Animal wastes, fertilisers, detergents, industrial wastes|
|Toxic chemicals (e.g. heavy metals, pesticides, phenols, PCBs)||Toxic to humans, animals and plants||Could be lethal||Increased need of treatment or control||Rare||Detergents, pesticides, tanneries, pharmaceuticals, wool scouring, refineries|
|Endocrine disruptors||Alteration of ecology||May adversely affect health and reproduction of humans and animals||Can be present in water sold in plastic bottles||Fusarium species of fungus||Chemical manufacture, intensive farming|
|Acids/alkalis||Lowering/raising of pH; acids can dissolve heavy metals||Only narrow range of pH tolerable for most plants and animals; heavy metals toxic||Corrosion||Naturally acid or alkaline rock||Battery, steel, chemical and textile manufacturing; coal mining|
|Suspended solids||Reduction in light penetration (increased turbidity), blanketing, introduction of colour||Photosynthesis reduced; blanketing of benthic plants and animals; obstruction of gills of fish||Obstruction of filters; increased need of treatment||Soil erosion, storms, floods||Pulp mills, quarrying, any building or development work involving ground disturbance|
|Immiscible liquids||Formation of a layer at the water surface that could prevent O2/CO2 interchange||Reduced DO; insect breeding affected||Interference with treatment processes||Unlikely||Oil-related activity|
|Heat||Decrease in DO; increase in metabolic rate of aquatic organisms||Possible reduced breeding or growth of aquatic organisms||None||Unlikely||Power plants, steel mills|
|Taste-, odour- and colour-forming compounds||Taste, malodour, colour||Tainting of fish||Increased need of treatment||Peat||Chemical manufacture or processing|
|Microorganisms||Pathogenic to humans||None||Increased need of treatment||Animal excrement||Contamination from human wastes|