5 Unit summary

• Freshwater is a finite and limited resource on Earth and, increasingly, much of it is polluted, by both pathogenic microbes and chemical contaminants.

• Human demand for freshwater is increasing; in particular, water is required to irrigate crops to feed the rapidly expanding human population.

• Water cycles globally, through the oceans, the atmosphere and freshwater river systems. At certain points in the cycle, water is purified, both naturally and by treatment plants.

• Freshwater is very unevenly distributed in the world, such that a large proportion of the world's human population has insufficient water for growing crops, for drinking and for sanitation.

• Climate change is altering the global distribution of water, causing droughts in some regions, flooding in others.

• The chemical and physical properties of water are such that, over the range of temperatures that occur on Earth, it supports a rich diversity of plants and animals.

• An enormous variety of chemical compounds, produced by human activities, pollute natural water bodies, causing both acute and chronic pollution.

• Evidence for the effect of chemical pollution is provided by ecotoxicology, the study of the impact of xenobiotic chemicals on wildlife in natural situations.

• As chemical pollutants pass through natural food chains, bioaccumulation causes high levels to build up at certain points, e.g. in the fat reserves of predatory fish and birds. As a result, these animals and their offspring can be exposed to a very high dose.

• DDT is an effective insecticide that is toxic to wildlife, but is also a vital means for combating malaria.

• Levels of mercury compounds in the environment are increasing; they are a threat to wildlife and to people who eat a lot of fish, and especially to their children.

• Levels of nitrogen compounds in the environment are increasing very rapidly. These are toxic to humans at high levels but, more importantly, at lower levels they cause widespread environmental changes, especially eutrophication of water.

• A large number of xenobiotic chemicals, called endocrine disruptors, cause major disruption to the reproductive development of freshwater animals; their possible effects on humans are uncertain.

Water is a natural resource that is vital to human health. It is also a resource that is undergoing a major crisis; its capacity to support plant and animal life is rapidly being destroyed by human activities. The message of this unit is that human health and the health of the natural environment are intimately linked to one another.