London's water is mainly drawn from, and returned to, the Thames (and River Lee). Few tap water tests fail environmental standards for chemical and biological pollution. Private water supplies may need closer inspection. New investment is needed to reduce sporadic Cryptosporidium infections, and to replace delivery pipes, which lose 25% of supplied water. More public information on sewage disposal is needed.

Thames Water Plc supplies London from surface water, principally from the Rivers Thames and Lee. (A small amount of water is drawn from deep sources, including private wells.) Drinking water is distributed through 31,000 km of mains. London's water is relatively protected from contamination with heavy metals or pesticides, and from major infections such as cholera or hepatitis.

In 1998, three of 1065 tap samples (0.3%) failed the lead standard:the Drinking Water Inspectorate advised the householders of the problem. Sixteen samples (0.6%) failed the iron standard. Nitrites were present in 2.3% of samples, representing pollution of ground water by fertilizers. Pesticides were found in less than 0.01% of samples, reflecting recent investment in removing pesticides from drinking water. (Drinking Water Inspectorate, 1998) The health effects of pollution at these levels is unknown but likely to be very small.

In 1998, samples showed contamination with faecal coliforms in 4 of 96 treatment works, 2 of 372 service reservoirs. There were bacteriological failures at consumers' taps in 1.7% of samples taken. Most of these were considered due to the condition of the tap, but the Drinking Water Inspectorate took action at one treatment works and two service reservoirs. (Drinking Water Inspectorate, 1998).

However, reports from environmental health departments to the Public Health Laboratory Service indicate for London around 200 episodes each year of diarrhoeal disease caused by Cryptosporidium species (see table). Only occasionally are these clearly clustered demonstrating a source (there were two clusters investigated in North Thames in 1998, Stanwell-Smith 1999). These episodic infections are thought to be due to contamination not detected by current water surveillance (Bouchier 1998). In response, the Government have introduced new requirements for monitoring water at treatment plants and for new filtration measures.

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The safety of water disinfection

Potential health effects

Monitoring of levels of chlorination byproducts in drinking water

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The Drinking Water Inspectorate monitors water sampling from household taps and water installations undertaken by Thames Water Plc. The water company provides information to local authorities and the Health Protection Agency about the sampling.

Published data on water quality have shown improvements over recent years, suggesting a programme continuous quality improvement. However, Thames Water Plc do not provide open access to information. In particular, we were not able to identify information on leakage and overflows of sewage, an issue which needs further public health attention. Demand for water is increasing, and the public has been concerned by water restrictions during some summers in the 1990s (due also to low rainfall in the previous winter). However, 25% of water supplied by Thames Water is lost through leaking pipes, and the Environment Agency recommends that Thames Water replace pipes rather than seek to increase reservoir capacity. It notes that this will be 'a sizeable challenge sincereplacement of pipes / channels and repairs can cause significant disruption to the capital' (Environment Agency 1999).

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Monitoring of water quality in London's rivers and canals is undertaken by the Environment Agency that regularly grades water quality on a six point scale. Compared with the average for England and Wales and for the Thames Region, London's rivers tend to be of poorer quality on this scale.