Scilly Water Supply

There’s plenty of rainfall in the Isles of Scilly but much of it soaks into the ground leaving only a few small rivers.

Four of the five inhabited islands have freshwater ponds

and wells have been used to extract water from the ground.

However, the population of the islands is often massively increased by the arrival of tourists, particularly from mainland Britain and passing cruise ships.  At the peak of a dry summer up to 600 cubic metres of water per day can be needed.

This is obtained by a combination of extracting water from the ground via boreholes and purifying seawater via a desalination plant.

For desalination, seawater is taken into tanks, and then pumped across a membrane, which allows water molecules to pass through and returns salt and other impurities to the sea.

And here is where it happens, quietly and automatically, producing 400 cubic metres per day since the plant was opened in 2013.

The resulting water is cleaner than the water obtained from the ground, since the latter has high levels of nitrates from chemicals used in farming.

What actually comes out of the taps is a mixture of the two sources: the desalination water is mixed with the water from the boreholes so that the nitrates are diluted to well below the level deemed to be safe.

Thames and Severn Canal

In 1789 the Thames and Severn Canal was completed, allowing cargo to be transported from Gloucester, through the countryside, to London, buy boat. Via a series of locks, it climbed the western side of the Cotswolds to Sapperton, where it entered what was then the longest tunnel in Britain, at 3½ km.

This emerged from the hillside near the source of the Thames.

The canal then descended, via more locks to meet the Thames at Inglesham, near Lechlade…

…from where the river was navigable to London.

To pass through the Cotswolds in either direction, boats had to use a series of locks, to reach the highest level of the canal.

This lock is in Shropshire but illustrates the point. Coming from the lower level, the boat enters the lock, the gates are closed and then water is allowed in from the higher level to fill the lock and raise the boat. Each time the lock is used, water moves down to the lower level. With this happening at both ends of the tunnel section, this top part of the canal needed a water supply.

Water was taken from the River Churn at Cirencester to feed the canal but this alone wasn’t enough. The solution was found in the area near the source of the Thames, where the meadow was described as having “numerous little fountains” – presumably lots of springs. A borehole was dug and a wind pump was used to transfer several tons of water per minute to the canal.

It’s a controversial point, but this is likely to have lowered the level of the water in the limestone and contributed to the drying up of the spring at the Thames source.

But water wasn’t just leaving the canal via the locks. Next Friday we will be looking more closely at Sapperton tunnel and what was happening to the water inside.

Water from the Cotswolds

Much of the rainwater falling in the Cotswolds disappears into the ground, soaking into the permeable limestone rock. It may eventually emerge lower down the hillside at a spring…

…or maybe not, as in here at what was once the source of the Thames.

Water that has trickled through limestone usually comes out pretty clean and carrying various dissolved minerals, particularly calcium. This usually results in a pleasant taste and gives the water health benefits, calcium being needed for strong bones. Thus it is excellent as drinking water.

But you don’t have to live in the Cotswolds to taste it for yourself. Several companies sell spring water that is bottled at its Cotswold source. These enterprises vary in size, with one company having a licence to remove 73 million litres of water per year from its borehole drilled into the limestone rock.

And then there are the water supply companies. They need to source enough water to keep it flowing down the pipes direct into our homes. Thames Water supplies 15 million customers. It gets around 40% of its water from groundwater and some of that comes from the Cotswolds.

Taking water from the ground is licenced so that the reserves are not over exploited but there are quite a few licences in the area near the source of the Thames as you can see here, represented by the stars on the map.

So if the water is taken directly out of the rock and at a faster rate than rainwater puts it back, the spring may dry up.

The river may eventually appear from a different spring at a lower level. Compare this…

…with this.

But in the area of the source of the Thames, water was not just needed for drinking. A major project needed a water supply and it turned out to need much more than was anticipated. Find out all about it next Friday.