Water in every phase of the hydrological cycle, from precipitation through terrestrial surface and groundwater systems to the marine environment, has a quality dimension that can be described by reference to numerous physical, chemical and biological properties, and is controlled by a myriad of natural factors and human influences. Water quality is of fundamental importance in the provision of potable supplies to sustain human life and in the health of aquatic ecosystems. It also significantly affects a wide range of human uses of water in industry, agriculture, transport and recreation. At the same time, these uses and other human activities, directly or indirectly, provide manifold sources of water contamination. Where the consequences or side-effects of human scientific, industrial and social habits result in conditions within the water environment that are harmful or unpleasant to life, the term ‘water pollution’ is used (Sweeting 1994). Acute water quality problems, however, may also arise from natural climatic or geological conditions.
Problems of freshwater pollution, on which the present chapter focuses, have a long history and have changed in character as world population has grown and human technological capability has increased and become more complex. Local contamination of the aquatic environment has been recognised for at least two millennia, and in some countries legal means were taken to prevent water pollution as early as medieval times. In the UK, for example, laws were passed in the thirteenth century to prohibit washing the products of charcoal burning in the River Thames (ibid.). Water pollution, and its deleterious consequences for human and ecosystem health, has accelerated since the nineteenth century with the increasing urbanisation and industrialisation of human society and intensification of agriculture to support an ever-growing population (UNEP/WHO 1988). One of the first documented examples of the inimical effects of bad water quality concerned the outbreaks of cholera in London, which were traced by John Snow in 1854 to the gross pollution of the River Thames by raw sewage. Problems of faecal contamination of rivers used for public water supply in developed economies were subsequently largely solved by the invention of sand filtration and the use of chlorination. The sequence of problem occurrence and perception followed by the application of control measures is one that has been repeated, especially during the last fifty years (Figure 11.1). Over this period, increases in public awareness of pollution, in the ability to develop remedial measures and in the political will to implement strategies to control water contamination have to a certain extent paralleled the rapid emergence of a succession of water quality problems.
A conceptual model of water pollution occurrence and control has been proposed by Meybeck et al. (1989) using the example of the history of domestic sewage contamination in Western Europe over the last two centuries (Box 11.1). This model can also be applied to other types of pollution and to countries that have different patterns of economic development. In the latter