Contains the full lesson along with a supporting toolkit, including teachers’ notes.
A reliable supply of water is required for healthy ecosystems and a functioning society and so anything interfering with its quality or quantity is a cause for concern. We need water to drink but we also need water for washing, sanitation, agriculture, industry, power generation and recreation. History shows that as societies evolve and urbanisation increases, the fulfilment of immediate human needs often occurs to the detriment of the environment. Eventually people realise that they must live in harmony with theenvironment and they begin to take steps to protect it. The growth of the global human population is leading to increasing development and urbanisation, thus adding urgency to the task of maintaining and improving the quality of water resources.
Many people on Earth rely ongroundwater for their needs. Without due care such water may be become contaminated by animal and human sewage or by run-off from agricultural land that has been heavily fertilised. The effects of such contamination depend on several factors, including the nature and amount of pollutant, the soilpermeability and the movement of water underground. Surface waters such as lakes and rivers may be similarly affected.
Fertiliser and animal waste generally contain nitrates andphosphates. These are essential for the growth of algae and plants but when they occur in high concentrations they enrich the water and cause rapid growth of phytoplankton which decreases light penetration through the water column and reducesphotosynthesis, particularly at lower levels.
As a result the oxygen concentration in the deeper water falls below the level that many aquatic animals need to survive. When the algae eventually die parts of the bottom of the lake or stream may be covered with decaying organic matter, and this also leads to oxygen deficiency. Such eutrophication in large bodies of water may not only kill fish but may indeed endanger people who use the water for recreation, for their livelihood or as a source of water for domestic use.
Sewage is likely to contain human pathogens and so special care is required to ensure that it does not make its way into water untreated.