A £6m project aimed at detoxifying an Eden river polluted by mining activity has got underway.

The River Nent is thought to be one of the most polluted major waterways in England as a result of 19th-century industrial activity.

Run-off water from the former Nanthead mine near Alston has entered and contaminated the river - bringing with it high concentrations of harmful metals such as silver, cadmium, lead and zinc.

These deposits are toxic to the Nent's ecosystem - in particular, to fish and the insects they feed on.

One of the main sources of water pollution in the area is found at the Haggs adit, an abandoned mine water drainage tunnel at Nentsberry, near Alston.

The Coal Authority and Environment Agency have developed the Nent Haggs mine water treatment scheme to tackle this unwanted by-product of the area's industrial legacy - with three tonnes of zinc alone thought to be discharged by the Haggs adit into the river each year.

A planning application for the treatment scheme to be built on land between Blagill and Nentsberry was granted planning permission by the county council last June.

The first stage of new work to treat the river is now underway and is expected to last for around two years.

Phase one involves the construction of a pumping station near Nentsberry, and is expected to last around two years.

Phase two will see the creation of three treatment ponds containing limestone, straw and wood chippings and a new wetland at nearby West Foreshield.

It is hoped that these treatment processes will remove the mining metal deposits from the water, which can then return to the river.

A spokesman for the Department for Environment, Food and Rural Affairs said of the new project: "Although almost all the mines had been closed by the early 20th Century, they have since flooded and polluted water continues to pour out of the ground and will do so for hundreds of years without government action.

"The effect on water quality and aquatic life can be seen for 60km along the River South Tyne, with the metals ultimately accumulating in the River Tyne estuary sediments.

"This will help deliver economic and environmental benefits for many years to come."