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Long term plans to lower London water table

RISING GROUNDWATER levels under London which threaten tube lines and deep basements will be halted by a long term programme of borehole abstraction, a London meeting was told this week.

The General Aquifer Research, Development and Investigation Team - an unofficial body made up of water companies, infrastructure owners and insurers - presented its strategy to local authorities and property owners at the Guildhall in the City of London.

Water levels in the chalk aquifer under the capital are recovering from a century of industrial extraction. Levels are now 50m higher than in 1970 and rising at around 2m a year.

Most under threat in the short term is London Underground. Resaturation of the London Clay is likely to pose more problems than flooding of deep tunnels because it will reduce bearing capacity and increase subsidence.

GARDIT claimed the abstraction of 70Ml of water a day from the aquifer by 2004 is needed to halt the rise. This would involve abstraction from both existing Thames Water boreholes and new boreholes throughout London.

A significant number of these are expected to be commercial ventures, with Thames Water and the Metropolitan Water Company looking for customers to buy water for use in air conditioning systems or for watering parks.

But Thames Water project manager Stuart Shurlock warned that the cumulative effect of the boreholes on ground water levels is currently impossible to predict.

'Our rule of thumb is to aim for 1990 levels over most of London and current levels under the City,' he said.

'But until we develop a reliable computer model of the aquifer and test it with actual bores we can't say what the long term levels in any particular area will be.'

This model is now under development as a joint venture between Thames Water and the Environment Agency.

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