Source of Bath's hot springs may be just a stone's throw away

Subterranean detective work around the city of Bath suggests that its hot spring waters come from rocks near its boundaries rather than as thought from the distant Mendip Hills – a finding that could have important implications for future development.

A team of geologists believes that, rather than filtering through the limestone from the Mendips 20 miles away, the heated waters may well up from a source a mile from the city centre.

Roughly 1.3 million litres (about 0.3 million gallons) of water heated to between 45C (113F) and 47C rise at the spa in the city every day. The hot waters have been known of since Roman times, when the city's baths were built.

"In fact, the water has surely been rising there for thousands of years," said Rhodri Samuel, the co-ordinator on the Bath Spa project, which aims to rebuild and understand the springs. "But nobody has ever clearly established the origin of the natural spring that gives rise to it."

Geologists had thought that the water came from the Mendips. Their theory, put forward decades ago, suggests that rain water falling on the hills is filtered through the ancient limestone and seeps down into the Earth's crust where it is warmed, before rising through fault lines.

But renewed examination by the Bath Spa project, which shares a £7.78m grant from the Millennium Commission to revitalise the spa, turned up another theory: that the waters are gathered much closer to the spring.

Mr Samuel said: "The survey data shows that Bath sits on a sort of limestone plateau, which falls away very sharply about six miles to the south-west. Our geological adviser said that it is highly unlikely that the faults in the limestone would actually allow the waters to rise up there."

That could rule out the Mendips as the water's source. "The important thing about knowing where the spring waters really originate is that we have to be able to protect the source," Mr Samuel said. "We would have to examine what sort of developments are allowed, such as deep boring around the city."

There is already concern that deep quarrying in the Mendips could disturb the source – if the hills are indeed the source of the waters.

Clive McCann, a professor of geophysics at Reading University, who has been helping with the research, said: "The decision about quarrying is very much bound up in politics, but [the new theory] could affect how that is seen."

The renovated spa will open in October next year, and public bathing will be made possible for the first time since 1978, when the National Health Service closed its treatment centre in the city, which had used the baths.

Earlier attempts to restore the spas have been stalled by the cost of restoration and worries over the purity and the source of the water – which is reckoned to take 10,000 years to filter through the rock before it wells up.