Abingdon reservoir: Your questions answered

Thames Water has launched its public consultation into plans to build a reservoir to supply 15 million people

The construction of a new reservoir - one of the largest in the UK - has sparked interest and concern among local residents.

Thames Water's so-called South East Strategic Reservoir Option will be created near Abingdon, Oxfordshire, to cope with increasing demand. It says it will provide water for 15 million people.

An 11-week consultation about the controversial project, which has an estimated cost of between £5.5bn and £7.5bn, runs until 13 January. The water company is hoping to apply for planning consent from the government in autumn 2026.

Leonie Dubois, head of engagement at Thames Water, sat down with BBC Radio Oxford to answer your questions on the project.

Why is the reservoir so big?

The reservoir is expected to be 4.5 sq miles (7 sq km), roughly the same size as Gatwick airport.

LD: It's because the challenge we're facing is so enormous. It's all about water security for the whole of the south of England, so that is customers for Thames Water, Southern Water, and Affinity Water.

We need it because our water resources are very much under pressure. We know through climate change, population growth and our need to have less impact on the environment that we need to secure more water for the future.

If Thames Water fixed its leaking pipes would we still need a reservoir?

The company has faced repeated calls to fix leaks and modernise outdated infrastructure.

LD: We need to work on both. The first 80% of our plan is about making sure we're fixing leaks which we know we need to do. There's also leaks on customers' pipes.

It's also about us using less water as a society… but we know that's not enough. For the last 20% of the plan we need to invest in new projects like this reservoir to make sure we've got enough water for the future.

Leonie Dubois from Thames Water says the reservoir will be a place that local people can enjoy visiting

Can Thames Water successfully deliver this project considering its recent history?

The water operator was fined £122.7m this year for breaching rules over sewage spills and shareholder payouts, the biggest ever issued by the water industry regulator Ofwat.

LD: I can understand that people feel the way they do about Thames Water at the moment, but we just delivered the Thames Tideway Tunnel and it was delivered on time, on budget, and is making a huge difference to the River Thames.

The same model is going to be used for the reservoir. Private investment is sought up front. This is a private company that funds the reservoir construction, and that's paid back by customers over a very, very long period of time, over 100 years, so it's a separate company that will deliver the construction.

Is there a risk of the reservoir flooding?

LD: Safety is at the heart of reservoir design… the reservoirs in the UK are one of the heaviest legislative practices in the world so we have to follow very strict rules to design this reservoir.

The water company is hoping to apply for planning consent from the government in autumn 2026

What about the quarries - can they not be repurposed as reservoirs instead?

Tony, a caller from Witney, noted that there were quarries across the Thames Valley that had filled with water and become lakes. He wondered if they could be of use.

LD: We looked at over 2,000 different options when we were looking at the best way to secure people's water for the future, and time and time again the reservoir came out as the one that was the best value for our customers.

It's ideally placed at the location because of what's in the ground. The clay we dig out and use to make the embankments, and then we can connect back into the River Thames and take water when it's got plenty in it during the wetter months, and then we'll put the water back in the drier months when the environment is under stress.

It's a really good suggestion… we're always looking for ideas from customers. We put community engagement at the heart of what we're doing.

Will other counties benefit more from the reservoir than Oxfordshire?

The reservoir is expected to supply water to customers across the south east.

LD: Oxfordshire will benefit from it. There will be local water provided from the reservoir, but there is also this huge, amazing new place being created... it's a big environmental experiment, where we're putting in place these news wildlife corridors that over time will mature and become really exciting from a biodiversity point of view, and that's all locally-based that people can enjoy.