A freshwater lake that attracts more than 30,000 swimmers a year is under threat of closure from an Environment Agency (EA) plan to reduce flooding that will channel in polluted river water, according to campaigners.

Almost 50,000 people have signed a petition calling on the EA and Surrey county council to reroute the flood channel away from the lake, which is a site of nature conservation. The EA and Surrey council say the 50-metre wide channel will go ahead, bisecting the lake and feeding river flood water into its centre.

Ferris Meadow Lake hosts an annual 24-hour charity swim for Level Water, raising money to provide access to swimming for disabled children across the UK. Bathing water quality is regularly tested and is excellent.

Emma Pattinson, whose family have owned the lake since the 1960s, opened it up for swimming in 2010. By 2024 the lake had hosted a quarter of a million swims, as swimming in nature grew in popularity, with research showing how beneficial it is to mental health.

“People love swimming here because they are surrounded by nature, and because the water is healthy and supports wildlife and a varied ecosystem. It is a massive boost to people’s mental health,” said Pattinson. “Our water quality is always excellent. Surfers Against Sewage have tested the water quality in the River Thames next door to the lake and these tests shows high levels of enterococci and coliform bacteria.

“If this channel is cut through the middle of our lake it will bring in sewage pollution but it also contaminate the water with metals and PFAS chemicals. We are talking about a lake, not a flowing body of water, so the pollution will just sit there.

“I would just not be happy running the lake as a swimming location in that case because I won’t be able to account for the water quality.”

Pattinson said digging into the lake would also disrupt the delicate ecosystems that have built over decades. Her family have owned the lake since it was a gravel pit, and won nature awards for its restoration when it was filled in. The lake is now a site of nature conservation.

Pattinson said she has been consulting with the EA since the plan was first dicussed in 2009 and by 2014 it was clear they wanted to channel flood water into the lake. “They really treated us as insignificant the whole way along,” she said. “They dismissed us. When they saw how many people swim here, they were a bit shocked and then they saw the size of the petition and they agreed to do a second consultation on the chosen route of the channel.

“That has just closed but they are still insisting that the channel has to come through the lake. We believe there are other options which can avoid the lake, but the EA is fixed on this because it has been so long in the planning and they are reluctant to look at this again.”

The River Thames scheme has been designed with two channels to divert flood water and help protect 11,000 homes. It involves digging two new river channels to divert floodwater, one of which, the Spelthorne channel, will bisect Ferris Meadow Lake.

The UK has very few inland bathing water areas compared with other European countries.

An EA spokesperson said: “The River Thames scheme represents a landmark opportunity to manage flood risk while enhancing the environment for generations to come. Through integrating green infrastructure, we are committed to creating a sustainable solution that benefits both people and nature.”