Water is not the same everywhere because it comes from different sources, such as:

• rivers
• reservoirs, lakes and streams
• boreholes and wells.

To a varying extent, these sources contain different impurities which need to be removed. The water is always treated by disinfection, often by filtration and, sometimes, by more complex processes. There are about 1,600 treatment works.

After treatment, drinking water is distributed through a network of pipes about 315,000 km long and some 5,000 tanks called service reservoirs or water towers. For monitoring purposes, each company's network is divided into water supply zones serving not more than 50,000 people. There are nearly 2,500 zones in all.

The Government has set 55 standards for drinking water. Most of these come from an obligatory European Community Directive but some UK standards are more stringent. A few are based directly on World Health Organisation guidelines. The standards generally include wide safety margins.

There are standards for:

• bacteria
• chemicals such as nitrate and pesticides
• the way water looks and tastes.

Water companies must, by law, take water samples and test them to check that quality standards are being met.

They test for:

• bacteria at every treatment works and service reservoir and also at representative consumers' taps in each water supply zone; and
• chemicals and other measures at representative consumers' taps in each water supply zone.

Each water company has to keep a public record of all their test results. In 1995, in England and Wales, nearly 3.2 million tests were carried out - about 80% of them on samples taken from consumers' taps.

The Drinking Water Inspectorate checks whether the water companies carry out quality checks properly. We are a team of professionals with wide-ranging experience and expert knowledge on all aspects of water supply. This includes chemistry, microbiology and engineering. On health matters, we are advised by the Government's Chief Medical Officer.

We inspect each water company every year to check that:

• sampling procedures are satisfactory
• samples are tested by trained staff using accurate methods
• the right numbers of tests are carried out
• the correct results are entered in the public record
• the appropriate water treatment processes are used
• the treatment processes and the water distribution system are operated and maintained satisfactorily.

We also examine each company's water quality test results for each treatment works, service reservoir and water supply zone and check them against the standards.

The outcome of all these checks is given for each water company in our Annual Reports. This overview gives you the overall outcome of the water quality tests carried out in 1995 in England and Wales.

The quality of drinking water can be shown in two ways. The best, because it takes into account all the results in a whole year, is the total of all tests that meet the standards. However, we also check water quality in each supply zone. We treat a zone as not meeting the standards if just one test during a year fails. This is because we can require the company to carry out improvement works for the zone. So although figures relating to zones can give a misleadingly poor picture of drinking water quality, they do show how quality has got better as a result of completing improvement work.