Lakes are one of the planet’s most important freshwater resources providing water for domestic, agricultural and industrial uses for much of the world’s population. We are all familiar with the image of Earth as the “blue planet” when seen from space. It gives the impression that water is plentiful and indeed it is. However, pictures can be deceiving. Freshwater only accounts for 2% of all the Earth’s water. But even that percentage is deceiving because 99% of all surface freshwater is locked away in continental ice. Freshwater resources are vital for meeting basic human needs and inadequate protection of the quality and the supply of freshwater can set important limits to sustainable development.

Freshwater pollution can be divided into two main categories: non-persistent and persistent. Non-persistent pollutants are degradable; they can be broken down by chemical reactions or by natural bacteria into simple, non-polluting substances such as carbon dioxide and nitrogen. Organic waste is an example of a non-persistent pollutant. The breakdown of organic waste can lead to low oxygen levels and ‘eutrophication’ (a process of nutrient enrichment), but the damage is reversible. Organic waste may also contain microorganisms which are the waterborne agents of diseases such as cholera, typhoid, and dysentery. Sources of non-persistent pollution include domestic sewage, fertilisers, some household cleaners, and some industrial wastes.

Freshwater acidification is another environmental problem. Natural acidification of freshwater environments has been taking place since the last Ice Age. However, the recent rapid acidification of many of lakes throughout the world can not be attributed to natural causes, but instead to the effects of acidic pollution from the burning of fossil fuels by mankind. Lakes and streams that are generally regarded as acidified contain very nutrient-poor water.

Groundwater is water found in the tiny spaces between soil particles or in cracks in bedrock, much like the water in a sponge. The underground areas of soil or rock where substantial quantities of water are found are called ‘aquifers’. These aquifers are the source of wells and springs. It is the top of the water in these aquifers that form the ‘water table’.

Groundwater can also be also polluted by the inflow of agricultural pesticides. Sewage from poorly maintained septic tanks and leachate from older landfill sites are also causes for concern. Because groundwater flows so slowly, contaminants are not carried away and diluted as rapidly as they are in rivers or oceans. It also purifies itself very slowly since the microbes that normally break down organic pollutants require oxygen, and groundwater is cut off from the atmosphere. For these reasons, polluted groundwater may remain contaminated for centuries.