Case study:River Nent: Abandoned Metal Mines
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Project overview
| Status | In progress |
|---|---|
| Project web site | |
| Themes | Environmental flows and water resources, Habitat and biodiversity, Monitoring, Water quality |
| Country | England |
| Main contact forename | Hugh |
| Main contact surname | Potter |
| Main contact user ID | |
| Contact organisation | Environment Agency |
| Contact organisation web site | |
| Partner organisations | The Coal Authority (UK) |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
The EU Water Framework Directive (WFD) sets out a legislative framework for the analysis, planning and management of water bodies. In England it is delivered through River Basin Management Plans (RBMP), which describe waterbody conditions and objectives for their improvement.
The River Nent is ranked as the most impacted catchment in the Northumbrian River Basin District and the 2nd worst in England with respect to mine water related pollution. The River Nent waterbody is assessed in the current RBMP (2015) as failing to achieve the required WFD objective of ‘Good Potential’. For the River Nent to achieve the required objective, the levels of zinc, cadmium and lead need to be reduced, together with other measures to improve its quality.
There have been lead mines in the Nent valley since Roman times, and over 90 mine entries have been recorded throughout the area. As technology advanced in the 1700s and 1800s and longer tunnels and drainage levels could be constructed, the centres of activity became concentrated around Nenthead and Nentsberry. The Nent Force drainage Level was constructed in the 1800s and runs over 7 miles from the Nenthead mines and discharges to the River Nent at Alston. By the 1920s mining had ceased, but during the second world war spoil heaps were reworked for lead and zinc, leaving behind large tailings dams downstream of Nenthead village. In 1998 these dams were capped and stabilised by Cumbria County Council.
The history and extent of mining here has left large areas of contaminated spoil with little vegetation cover; as a result, spoil is being eroded by high rainfall and washed into the River Nent. Two sites in the catchment are listed on the MINING WASTE DIRECTIVE INVENTORY since they cause serious pollution. Many of the spoil heaps are protected as Scheduled Ancient Monuments for industrial heritage and/or Sites of Special Scientific Interest (SSSI), as the high metal levels allow rare calaminarian grasslands to flourish.
The mining industry has left a huge underground network of mine workings. The workings drain different parts of the Nent catchment and also act as a preferential pathway for groundwater to reach the surface. These drainage pathways allow the water to come into contact with the mineral veins that were mined and with mine waste stored within the mine workings. This causes the water to become rich in metals like zinc, cadmium and lead. The water from the mine workings discharges at several locations throughout the Nent valley. There are four known main discharges that contribute significant amounts of metals to the River Nent: two at Nenthead, (Caplecleugh and Rampgill adits), one at Nentsberry (Haggs adit) and the Nent Force Level at Alston.
The many inputs of metals into the River Nent cause average concentrations at Alston to exceed Environmental Quality Standards (EQS) by up to 57 times (57 times for zinc, 28 times for cadmium and 3 times for lead according to the EA catchment characterisation report). Due to this high level of pollution, very little invertebrate life can live in a 10km+ stretch of the river. The River Nent joins the River South Tyne at Alston, bringing the pollution with it and causing the River South Tyne to also fail EQS for zinc and cadmium for 60km downstream.
Investigations by Newcastle University indicate that the River Nent contributes over 50% of the total metal load (lead and zinc) reaching the South Tyne every year. Reducing the impacts from mines in the Nent valley would improve the water quality of the River Nent and all of the downstream waterbodies, and would be a significant step in achieving EQS in the South Tyne.
The Environment Agency has teamed up with other partners to look at ways to reduce the polluting effects of the mines on the River Nent, with funding from Defra. We're working with the Coal Authority and the North Pennines Area of Outstanding Natural Beauty partnership (AONB) to find ways of removing metals from mine water discharges at three of the four main discharges. We're also working with the Tyne Rivers Trust on ways to reduce the amount of metal-rich sediments that get washed into the river – for example willow planting. Without any action, these mines will continue to pollute the Rivers Nent and South Tyne for centuries to come.
There are so many point and diffuse sources of zinc, cadmium and lead in the area that we're unlikely to be able to achieve EQS in the River Nent itself. However, we hope to be able to significantly improve water quality and therefore achieve good status in around 60 km of the River South Tyne. Other benefits of tackling these sources of pollution include improving habitats for aquatic life, increasing biodiversity and protecting Scheduled Ancient Monuments.
Treatment schemes will be required to clean up all these discharges and the spoil heaps, and we’re investigating the best ways of doing this. We expect the first scheme will be to treat the Haggs discharge. The treatment scheme is being designed to remove at least 90% of zinc and cadmium from the mine water – making a significant improvement to water quality in the Nent at Alston and also the South Tyne.
What technology is available?
One treatment technology that is available is a Vertical Flow Pond (VFP) and a wetland, similar to the scheme at Force Crag mine in Cumbria, the first treatment scheme of its type in the UK. The shape and size of the Nent schemes will be different, and is likely to look like a series of large ponds, along with a wetland containing reeds and other plants.
The VFPs work by using a combination of compost, limestone and woodchip to encourage microbial activity to bind the dissolved metals as sulphides, trapping them in the compost mixture. The water is then passed through a wetland to re-oxygenate the water before finally being discharged to the River Nent.
Monitoring surveys and results
Significant water quality monitoring of the River Nent and the point source discharges from the abandoned metal mines has been carried out by the Environment Agency in recent years.
The water quality results from 3 of the main discharges are summarised below. As the concentrations of metals and rate of flow is variable in discharges like these, the figures below are presented as minimum to maximum ranges taken from the EA catchment characterisation report.
Caplecleugh • Zinc:2700 - 8810 kg/yr • Lead: 0 – 10 kg/yr • Cadmium: 0 - 10 kg/yr
Rampgill • Zinc: 170 - 400 kg/yr • Lead: 0 – 2kg/yr • Cadmium: 0.24 - 0.76 kg/yr
Haggs • Zinc: 2000 – 5100 kg/yr • Lead: 0 – 1 kg/yr • Cadmium: 2 – 3.3 kg/yr
River Nent at Alston • Zinc: 2300 - 35000 kg/yr • Lead: 20 – 600 kg/yr • Cadmium: 10 - 90 kg /yr
Lessons learnt
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