A few times a week, a group of volunteers dotted along the Yazor Brook, which rises in a rural catchment and flows into the River Wye at Hereford, brave the brambles and muddy banks to take samples of the gently flowing water.

They conduct the tests on at least four sites along the brook and upload their results online. They are now among more than 200 citizen scientists who regularly test the River Wye from its source in the Cambrian mountains to the Severn estuary, compared with the sporadic testing by regulators.

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Three years ago, campaigners sounded the alarm over the decline of the River Wye in England and Wales. They warned that phosphate-rich runoff from intensive poultry farms in the supermarket supply chain was sullying the Wye’s waters and devastating the ecosystem with the spread of thick algae blooms.

Natural Resources Wales blamed sunny weather for the proliferation of algae blooms. It said there was no evidence of a link between river pollution and intensive poultry units.

Furious campaigners were already counting the many millions of chickens being housed across the Wye catchments in the supermarket supply chain. The Brecon & Radnor branch of the Campaign for the Protection of Rural Wales mapped more than 760 chicken sheds containing more than 20 million chickens.

Vast quantities of manure were being spread across farmland and spilling into watercourses across the Wye catchment. In the face of a lack of effective action by the regulator, anglers, conservationists and local residents started to test the water.