Thames Water’s trial of a proposed drought-resilience scheme reportedly caused levels of the persistent pollutant PFOS to exceed the legal surface-water limit by up to 13 times. The findings raise questions about whether the £350m-plus project can protect the River Thames while supplying extra water during drought, despite the company’s public assurances that it would not harm the environment.

The plan would take up to 75 million litres a day from the Thames at Teddington and replace it with treated wastewater from Mogden via a new tunnel. Pilot results recorded PFOS at 6.3 and 8.7 nanograms per litre, against a legal limit of 0.65, and also identified high iron levels; Thames Water said compliance would be operationally difficult and costly. The project’s expected cost has risen above £500m, while the company reportedly leaks 571 million litres of water daily and carries about £20bn of debt.