Caplecleugh mine water treatment scheme: reducing pollution from historical metal mining
Published 5 July 2021
Applies to England
© Crown copyright 2021
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Cleaning up water pollution from abandoned metal mines is identified as a priority for the Department for Environment Food & Rural Affairs in the government’s 25 Year Environment Plan and to deliver the statutory Northumbria River Basin Management Plan.
1. Background to the Caplecleugh Level
The Caplecleugh Level is part of the Nenthead mining complex, described by Historic England as “the most intact mining landscape within the North Pennines”. Although mining activity in the area dates back to at least the 12th century, the main period of activity was from the 17th to 19th centuries.
Construction of the tunnel from which the Caplecleugh mine water discharges started in the early 19th century, and today still drains groundwater from a large area of mine workings. More information on the history of the Caplecleugh Level and the Nenthead mines can be found by searching for “Caplecleugh Low Level” on the internet.
2. Pollution of rivers by metal mines in the Tyne catchment
Although most mining activity had stopped by the mid-20th century, more than 150km of rivers in the Tyne catchment are still being polluted by cadmium, lead and zinc. These metals harm fish and other river life, and end up being deposited in sediments in the Tyne estuary. The Water and Abandoned Metal Mines Programme aims to reduce the current level of pollution by installing a series of measures to stop metals entering rivers, and limit the impacts from metals already in the river system.
3. Pollution caused by the Caplecleugh Level mine water
On average, about 3 tonnes of zinc enter the River Nent each year from the Caplecleugh Level as well as a smaller amount of cadmium. The flows emerging from the adit vary, increasing after rain, but never drying up. The average flow is around 12 litres per second (l/s) and is usually between 8 and 33 l/s. The concentration of metals in the mine water also varies with flows, with higher concentrations when there is less mine water being discharged.
Monitoring of the River Nent and the Caplecleugh mine water by the Environment Agency shows that the most severe pollution (the highest metal concentrations) in the river also depends on rainfall. In dry summers, there may be very little dilution of the mine water. Zinc concentrations just downstream of the Caplecleugh Level can be more than 200 times the Environmental Quality Standard (EQS) which is used by Government to assess whether rivers are polluted. This causes severe impacts on river wildlife like river flies and fish.
Together with other sources of metals such as the Haggs and Rampgill discharges and diffuse mine wastes, the Caplecleugh mine water means that about 60km of the River Nent and the River South Tyne are polluted. The objective in the statutory Northumbria River Basin Management Plan is to clean up all rivers and groundwaters by 2027.
The Caplecleugh Level
4. Benefits of cleaning up the Caplecleugh Level mine water
The treatment scheme will capture more than 90% of the metals in the mine water before they can pollute the river. This will improve water quality in 60km of the River Nent and River South Tyne, particularly at the lowest river flows when the pollution is currently most severe. The impacts on river wildlife will be reduced. Up to 3 tonnes of zinc and cadmium will be captured each year and no longer be deposited in sediments in the Tyne estuary.
5. Treatment scheme size
Ideally, all of the Caplecleugh Level mine water would be treated to stop the pollution by metals. However, the size of the treatment scheme depends on the volume of water being treated. As the mine water flow increases, so does the cost of construction and the amount of land needed to build the scheme.
The Environment Agency has calculated that the most environmental benefit is obtained by treating about 10 l/s. This would mean that in drier conditions, the scheme would be able to treat all of the mine water. Since this is when there is least dilution by cleaner river water, a scheme of this size would substantially decrease the most severe river pollution. When there is more than 10 l/s mine water discharging from the adit, only some of it would be treated and the rest would enter the river as happens now. However, higher mine water flows happen when rainfall means that there is more water in the river, so the pollution is less severe and there is less harm to river wildlife.
6. Treatment scheme technology: passive removal of metals
The proposed scheme works by capturing the mine water near the Caplecleugh Level, and pumping it through a small buried pipeline to the treatment ponds. These open-water ponds contain a layer of natural material such as limestone, straw, compost and woodchips. Natural reactions in this layer remove the metals so they can no longer pollute the river. The water will then go through a constructed wetland before being put back into a pipeline and discharged to the river near the Caplecleugh Level.
This is the same technology as used in the Nent Haggs scheme which is being built from 2020 to 2022. This is similar to the Force Crag mine water treatment scheme, which was built for the WAMM Programme in 2014 at an environmentally sensitive location in the Lake District National Park. The design of the Nent Haggs and Caplecleugh treatment schemes has been adapted from the Force Crag scheme based on how that scheme has performed since 2014, and because the Nent schemes are less remote. Similar schemes have been operating successfully in North America for many years.
View details of the Force Crag mine water treatment scheme
7. Treatment scheme location
The location proposed for the treatment scheme has been selected following several years of investigations and discussions with the public and other stakeholders. Several sites were discounted after assessment of technical and practical issues, including proximity to properties and costs. A summary report explaining how we selected this site is being prepared.
The proposed location is on the former Nenthead mine site, adjacent to the Handsome Mea reservoir.