On a pale-blue Sunday morning on the foreshore of the River Thames, 40 people are searching for what others have tried to lose. Dotted across a 250-metre stretch under Hammersmith Bridge, they are combing through layers of silt, seashells and dirty driftwood to find one of the river’s most stubborn problems: wet wipes.

“People come and walk here and it looks just like mud, at a distance,” says the organiser, Dr Felicity Rhodes. “And then you get closer in and see.”

The dig forms part a citizen science project that has tracked sanitary litter on the banks of the Thames for nearly a decade. Set up by the environmental charity Thames21, volunteers are recruited to help map out the amount of wipes that are flushed down the toilet and washed up along the river. Findings are added to a database that quantifies plastic pollution on the banks of the Thames.