THE collapse of an abandoned underground mine is the likely cause of a six-mile stretch of river turning orange, the Environment Agency said.

An investigation was launched on Tuesday morning after a stretch of the river Rhymney became discoloured between Machen and Caerphilly.

The Environment Agency’s officers found that the pollution entered the river from the Porset Brook, near Bedwas House Industrial Estate, affecting 6.5 miles in total.

Tests are being carried out to assess any possible damage to water quality and wildlife, and if samples confirm mine water is the cause, the Coal Authority will be asked to help improve the situation.

However, finding the exact source could prove difficult because underground workings are complex and discharges can occur many miles from the responsible mine as drainage tunnels are dug over long distances.

A spokesman for the Environment Agency said there haven’t yet been any signs of fish in distress.

He added: “We’re confident the pollution comes from mine water. Mine water is often bright orange or brown because of the high levels of dissolved ferrous iron.”