Swimmers warned of toxic algae at newly designated Thames bathing site

Toxic algae warning at London's first official Thames bathing site

Swimmers are being warned not to enter the water at the first official bathing site on the River Thames in southwest London after toxic blue-green algae was found there. The Environment Agency has issued a formal notice at the Ham and Kingston site, reporting a bloom of potentially toxic algae. The agency stated: "Bathing is not advised, due to pollution from harmful algae."

Richmond Council has also advised visitors not to swim in or around the spot, keep pets out of the water, and avoid contact with visible algae near the riverbank.

The stretch of the Thames between Half Mile Tree and the YMCA Hawker Centre was officially designated as London's first bathing water site on May 15. It has not yet been classified for water quality; samples are still being taken, with classification expected after the bathing season ends on September 30.

Causes and context

Blue-green algae blooms are rare in moving water but are likely caused by extreme heat, low river flow, and nutrient pollution from sewage waste, as reported by The Guardian. Thames Water's Hogsmill sewage treatment plant is located three miles upstream of the Ham bathing site.

Marlene Lawrence of the swimming group Teddington Bluetits, which applied for the bathing water designation, said: "This is so unusual to get blue-green algae in a moving body of water. It is the first time in nearly six years of swimming here that this has happened. The river is very low-flowing at the moment because of the lack of rain and the high temperatures we are having. With climate change, this is likely to happen more."

The Environment Agency is taking water samples to confirm the extent of the bloom, which is at greater risk of growing in hot, dry weather. An agency spokesperson said they have identified blue-green algae on the lower River Thames including the Ham and Kingston designated bathing water, and that an advice against bathing notice has been activated on the SWIMINFO website. The spokesperson added it is likely that the section of the River Thames between Hampton and Kingston has also been impacted.

The algae discovery coincides with concerns raised by campaigners. An Environment Agency inspection on May 11 found the nearby Hogsmill Sewage Treatment Works in breach of its environmental permit on four counts, including inadequate storm sewage storage, as reported by the Evening Standard. The Guardian also reports that Thames Water has been in breach of its permit conditions at Hogsmill, meaning it has discharged raw sewage in breach of the law. Thames Water had promised to upgrade the works last year, but it was one of more than 100 projects postponed and is now unlikely until 2028. The failure to upgrade is being investigated by Ofwat.

Campaigners' concerns and the water recycling scheme

Ian McNuff of campaign group Save Our Lands and River (SOLAR) said: "It's deeply concerning, but sadly not surprising, that London's new bathing site at Ham and Kingston has been hit by a toxic algae warning just weeks after opening." He added: "Families were promised a safe place to swim and enjoy the river, not a public health gamble."

SOLAR has been campaigning against Thames Water's proposed Teddington direct river abstraction project since 2023. The scheme would replace up to 75 million litres of water a day with treated wastewater during drought. According to The Guardian, the £250m scheme involves abstracting 75 million litres of water a day from the Thames at Teddington when the river is in low flow, and replacing it with the same volume of treated sewage from the nearby Mogden sewage treatment works via a new tunnel. The scheme is in the pre-application stage for a development consent order.

The recycling scheme has been opposed by local Liberal Democrat MP Munira Wilson, the council, and river users. It has been designated a strategic infrastructure project, meaning it can be pushed through over the heads of the local authority.

The Environment Agency rejected a previous attempt to develop the scheme because the treated sewage would raise the temperature of the water during low flow, change the salinity of the river, and adversely affect fish and biodiversity.

Campaigners argue the recycling project will provide only 75 million litres of water a day, a fraction of the 570 million litres Thames Water leaks daily, according to The Guardian. Thames Water is reported to be £20bn in debt.

In response, a Thames Water spokesperson said: "Our proposed Teddington direct river abstraction project would not deteriorate water quality and would be required to meet strict environmental and water quality standards set by the Environment Agency." The spokesperson added: "The new treated water outfall would also be located downstream of the proposed bathing water site, meaning recycled water would flow downstream over Teddington Weir into the tidal Thames."

SOLAR is running a petition demanding all new wastewater treatment infrastructure meets stricter standards as demanded in EU countries, effectively adding a fourth stage of treatment to remove micropollutants.