Water supplies in England and Wales are set to become more polluted as a long-term result of the continuing drought, a hydrogeologist predicted this week.

A third of our drinking water comes from groundwater aquifers that accumulate water as it seeps through the ground. The rest comes from upland storage, such as lakes and rivers. Last summer's drought affected upland areas such as Yorkshire and Cornwall that get their water from lakes and rivers.

But after a dry winter, acquifers are now low in water too, some 30 per cent below their usual level. This means that water supplies in places such as the south-east of England are threatened as well if there is little rain this summer.

But hydrogeologists meeting at the Applied Geoscience Conference held by the Geological Society at Warwick University this week looked further ahead and voiced worries about what would happen when it started raining again.

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They fear that pollutants, which would normally be gradually washed through the soil by rain, will have built up. When it rains again high concentrations of these chemicals will drip into the aquifers.

Nick Robins, hydrogeologist with the British Geological Survey, said: "If you have a long dry period or a period of no recharge all the pollutants are now sitting in the soil."

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Mr Robins said that pollutants include nutrient run-off from farmland, pesticides, organic solvents, salt from roads and chemicals leaching from landfill sites.

The problem had been measured with nitrates in the past. But he said: "In terms of organics we have no idea what kind of effect it will have."

The problem of polluted water reaching aquifers is a long-term one: it can take up to 50 years for rain falling on land to reach the aquifer.