A £2 million engineering project at a “nationally important” archaeological site is cutting pollution in the North Pennines to help the environment.

Mining took place for 200 years at Carrshield, but the wastes left behind after processing lead and zinc ores were being washed into the River West Allen and damaging the environment.

The site was on Historic England’s register of at risk scheduled monuments because the 19th century dry stone wall to protect the river was at significant risk of suddenly collapsing, which could have caused 1,000 tonnes of lead, zinc and cadmium to severely pollute the river.

The Coal Authority and Environment Agency conducted several investigations to decide how to cost-effectively stop the river being polluted, while preserving the unique industrial heritage at the site.

The long-term solution involved re-profiling the mine wastes and installing a waterproof capping layer and perimeter drains to stop rainfall washing metals out of the wastes.

Damaged sections of the walls were rebuilt using concrete blocks and faced with local stone, while a trap was installed in the river downstream of Carrshield to capture wastes containing high concentrations of metals.

Nick Cox, the Coal Authority’s metal mine programme lead said: “This is a nationally important archaeological reminder of Britain’s industrial past, which is set within a very challenging environment.

“Thanks to the planning and management of the scheme, it has proved that a scheduled monument site can be sensitively but robustly repaired.”

Located within the North Pennines Area of Outstanding Natural Beauty, the site includes plants and wildlife which serve an important part of the biodiversity of the area.