Threat of London’s rising groundwater result of hsitorical over abstraction
Thames Water's project manager for the London groundwater abstraction programme has stated that over abstraction from the 19th century through to the 1940s has created the current problem.
“Our problem isn’t that the groundwater is rising, but that it was kept low for so long,” Stuart Shurlock of Thames Water told an audience of water and environment professionals on 20 October. Shurlock pointed out that the period during which industry in London abstracted heavily from the aquifer under London was the same period during which deep tunnels and the first tall buildings with deep foundations were built.
From 1970, groundwater abstraction declined rapidly and the aquifer began recovering. Currently, groundwater under London is rising by 2m a year and now threatens many of the tunnels and buildings built when groundwater levels were lower.
The first study into the risk posed by rising groundwater was undertaken in 1989. A joint effort, under the name Gardit, to deal with the problem was begun in 1993 by London Underground, the National Rivers Authority and Thames Water. Further studies by LU followed and Gardit membership was expanded to include infrastructure owners like BT and BR as well as the City Corporation and representatives of the property and insurance industries.
“The previous Government didn’t attempt to monitor the situation,” said Shurlock. A meeting with Deputy Prime Minister John Prescott in November 1997 was the start of the current Government’s involvement.
Gardit’s 5-stage strategy to avert widescale damage from rising groundwater was launched in March this year. “The overall simplest and cheapest way to control the problem is to abstract water,” Shurlock stated, referring to the strategy’s focus on bringing old boreholes back into operation and drilling new ones.
Thames Water has re-opened several boreholes and has drilled new ones on sites it owns (see related story). Next, it will need to find appropriate sites on land it does not yet own for further drilling. Shurlock stated that the costs of drilling and pumping are small compared to the challenges of treating the water. Where to treat the groundwater, whether by transporting it to existing water treatment plants or by building new treatment plants, has not yet been fully decided.
Blending groundwater with the water Londoners are used to – water coming from the River Thames – will also be a challenge. The two waters have chemical differences and, thus, do not taste the same. Thames Water estimates that about 70% of the groundwater it abstracts will be provided to consumers as potable water.
Gardit’s strategy includes provision for private, commercial boreholes to be drilled. These boreholes could produce water that, after treatment, could be distributed to residential developments or industry. The Metropolitan Water Company, founded in 1997, is seeking to develop such commercial boreholes.