One of the UK's largest chicken producers and a water company will be in the High Court on Monday accused of polluting the rivers Wye, Lugg and Usk.
More than 4,500 people who live or work near the rivers along the Welsh-English border have signed up take part in what's being seen as a landmark case against Avara Foods and Welsh Water.
Their lawyers say it's the biggest case ever brought in the UK over environmental pollution in terms of the number of claimants and its geographical spread.
Avara, which dominates chicken farming in the area, and Welsh Water, have respectively called the claims "misconceived" and "misguided".
The River Wye is one of the UK's longest and most celebrated rivers. But in recent years those living nearby have complained that it regularly turns green in the summer and has become smelly and slimy.
