Rain or shine, drought or flood, the water table beneath London has been rising for 30 years - by up to three metres a year. It already poses a threat to the London Underground, and in the next century it could start to move entire buildings.
The reason is that people take far less water from the aquifer beneath the centre of the capital than they used to. The water table is returning towards its original, natural level much nearer to the surface.
A committee chaired by London Underground has been monitoring and planning research into the threat for four years. It includes representatives of property and insurance associations, Thames Water and British Telecom, which has deep-level cable ducts at risk.
Mike Gellatley, infrastructure systems engineer for London Underground, said the group was poised to send a position statement to ministers. "We think the time has come for a single pumping strategy headed by one accountable body, perhaps the Environment Agency." About 30 million litres a day would have to be removed.
The Tube network, first in the firing line, has had to put increasing effort into monitoring the inflow of water and pumping it from its tunnels. Worst affected is the southern end of the Northern Line towards Tooting Bec, where the water table has almost reached the surface.
Water tables are rising under several other large European cities too, including Birmingham.
As the water rises it saturates the clay sitting on top of the chalk aquifer. That may alter the way in which it supports large buildings constructed on top of piles sunk deep into the ground.
Vinn Robinson, principal hydrogeologist with the Thames region of the Government's Environment Agency, advocates a detailed study to see how buildings in one small area of central London might be affected.
"At this stage, I don't think many buildings are likely to settle," he said. "But it would only take damage to a few to justify the expense of a pumping strategy to stop the groundwater rising further."
Thames Water will come under pressure to use this pumped out water for public supply, helping solve worsening water shortfalls at the same time.
But Mr Robinson conceded this was not economically attractive. Exploiting the rising water would require several pumping stations, and extensive treatment to make it drinkable, in return for meeting only a tiny proportion of the capital's daily needs.
There used to be hundreds of boreholes and wells in the square mile of the City of London alone. Manufacturers and businesses such as laundries and breweries pumped huge quantities of water up from the underlying rocks and it was common for large office buildings such as the Bank of England to have their own supply. The water table was as low as 90m below ground level in central London in the 1960s as a result.
It was deepest below the West End, with the water table tending to rise the further one moved out from the centre. In the outermost suburbs, it had only been drawn down a few metres. By the 1960s, however, the heavy water users had moved away and businesses and housing left in the centre relied more and more on piped supplies.Reuse content