Pandora Mine - Management of Mine Contamination: Consultation on Options (Shortlist)

Overview

NRW is carrying out an appraisal of surface water management options to alleviate the contamination associated with Pandora Mine. This project is part of a drive to improve water quality of rivers and catchments under the Water Framework Directive (WFD). We are currently considering different options.

As part of this process, our engineering and environmental consultancy Binnies LTD, have consulted with stakeholders on behalf of NRW to get feedback on a potential ‘long list’ of options. This feedback has now informed the selection of a ‘shortlist’ of options and will further inform the design of any agreed surface water management interventions at Pandora Mine. The shortlist of options is presented here, along with a summary of why this shortlist has been selected.

Background:
Pandora Mine was an operational zinc and lead mine from the 1840s until 1920. A little of its legacy can be seen at the surface, notably the spoil heaps (the large piles of stone near Caban Pandora and the black soil to the south of the road known as ‘dressing floor’ area) and remnants of old mine buildings. Below the surface, underground workings are still mostly open, any water that enters the mine outflows from the Pontifex Adit near Llyn Geirionydd.

Site Location:
Pandora Mine is south east of Llyn Geirionydd, near Trefriw, in Snowdonia National Park.

Mining Heritage and Ecology:
The Pandora Mine sits within an area of high archaeological and ecological sensitivity. Many features of archaeological and ecological interest exist across the site relating to the mine and its associated spoil heaps. These include ecological features of interest within the Mwyngloddiau Fforest Gwydir Special Area of Conservation (SAC), the Pandora Reservoirs Site of Special Scientific Interest (SSSI) and Mwyngloddiau A Chreigiau Gwydyr SSSI. In particular a number of rare plants are present that often grow on the metal rich mine spoil heaps, especially lichens and bryophytes forming ‘Calaminarian Grassland’ habitat which grows on the exposed ‘dressing floor’ spoil heaps (the southern area of the mine where rock was processed).

An Ecological Assessment,  Archaeology/Heritage Assessment and Geotechnical Ground Investigation of the site have been undertaken. Details of these are available upon request.

Remediation Driver:
Water enters the mine through the various shafts and washes through the spoil heaps. The mine and spoil heaps retain lead and zinc content, even though mining work has long ceased. Water leaving the mine, or that passes through spoil heaps, becomes contaminated with lead and zinc at levels that are harmful to the environment before flowing into Llyn Geirionydd and downstream river network (Afon Crafnant and Afon Conwy). The elevated metals contribute to the Afon Crafnant being classified under WFD as ‘moderate quality’.

In addition to this, a high flow of water through the mine could lead to local collapses within the mine. This would lead to pressures building up until the blockage is suddenly overcome, releasing a high volume of contamination into the water network, referred to as a ‘mine blow out’ and causing severe environmental issues downstream.

The proposed scheme will look at various options to manage the surface water across the site to try and reduce the amount of water that enters the mine and flows through the spoil heaps. This will help reduce the amount of lead and zinc contamination washing into Llyn Geirionydd and will improve the water quality in the Afon Crafnant and Afon Conwy downstream. It will also be the first step to reducing the risk of the mine suffering a ‘blow out’.

Benefits of Surface Water Management and Remediation:

• Reduction in the amount of contaminated water and contaminated sediment entering into downstream lakes and watercourses.
• Downstream water bodies will be more likely to achieve good ecological status under the European Water Framework Directive (WFD). Although there are pressures on these waterbodies from other mine water discharges these are being considered separately by NRW and collectively they can improve downstream water quality.
• There is potential to develop an educational resource at the site in line with the Well-being of Future Generations Act. For example, information boards within the Llyn Geirionydd car park area, explaining the remediation and ongoing site management, the mining heritage landscape and archaeology, ecology and biodiversity.
• Potential for additional wellbeing benefits in the form of access improvements, including provision of a new water access ramp at Llyn Geirionydd.

Considering the Options:
The results of recent studies have been integrated into the options appraisal in a holistic approach to safeguard sensitive features. The scheme will also be refined following a Geo-Environmental site investigation and will take into account feedback received as part of this consultation.

Given the complexity of the site it is likely that no single engineering intervention will be suitable across the area and a combination of interventions will be required.

A longlist options workshop was held by Binnies and NRW in August 2021 to review potential interventions and to derive a working shortlist of interventions to form a preliminary strategy for the Pandora site.

The current shortlist of options, comprising 6 options or combinations thereof, are detailed within Figure 1 (below, downloadable using the arrow symbol) and described below. Further Options Sketches providing detail of the shortlisted options taken forward are included on the following pages. Reasons for discounting other options from the shortlist to be taken forward are also described below.

• Option 1 Do Nothing – We always consider the option of taking no interventions as a comparison to the other options. Agreed that this is not acceptable. However, option should remain as a baseline to rate other options against. 
   
• Option 2 – Line the existing channels that interact with the ‘dressing floor’ spoil tip to prevent water becoming contaminated through contact with spoil and prevent contaminated spoil being washed downstream. Option in shortlist as potential to add value if executed effectively. Extent of channel lining will need to be confirmed, sketch shows current understanding.
   
• Option 3 – Culvert flow from former pond area in east of ‘dressing floor’ area under road. Roadside ditch improvements utilising new council ditch and creating new ditch where necessary. Similar result to Option 2, preventing contamination of water in contact with spoil and reducing erosion of spoil and washing into downstream watercourses. As with option 2 carried to shortlist. Look at the best way to undertake option.
   
• Option 4 – Install cover system over the ‘dressing floor’ spoil heaps (e.g. bentonite cap with soil cover system). However, this may have adverse impact on the heritage and ecology of this area of spoil heaps. Option discounted as would destroy the rare flora. General feedback was that no one was in favour of covering over the heaps.
   
• Option 5 – Cap the Great Shaft – Concrete cap at rock head, with side grouting as required to make a low permeability seal. Backfilled with natural soils to ground level. Prevents water entering the mine system at this point and enhances the safety of the area. Option discounted. However, the option scores highly in the appraisal scoring. The diversion of water options are preferred, but if these prove unfeasible then capping the shaft may become a shortlisted option for further consideration. 
   
• Option 6 – Divert water from the Great Shaft by reshaping the ground locally and forming a channel away from shaft. This will stop/significantly reduce water entering the mine system at this point. Option carried through to shortlist. Currently exploring diversion paths in more detail. This option is preferable to option (5) as is considered simpler engineering challenge with reduced costs and carbon with the same benefits.
   
• Option 7 – Pipe water through from the eastern spoil extent down through the site to avoid mine spoil heap and Caban Pandora. This will prevent the water interacting with any mine spoil and entering mine Shafts. Option carried through to shortlist. Generally, this constitutes the easiest way to keep water away from the mine spoil and mine shafts. Could potentially be coupled with a hydroelectric scheme, which theoretically would meet Welsh government objectives, but it is acknowledged that obtaining consent can sometimes be difficult and lengthy. A ‘blank’ offtake could be installed to accommodate a hydroelectric scheme in the future. 
   
• Option 8 – Reroute flow so that water from higher in the catchment runs to the north of the Pandora mining site. Would stop the water from entering the mine area. Some reprofiling of the hillside would be required to ensure that the water flows in the desired direction. This option would require the reinstatement of the currently disused reservoir. Option discounted. Doesn’t provide good cost/benefit as the reservoir would be quickly filled in any rainfall event. Without the reservoir storage element, there are preferable diversion routes for the water rather than the longer loop formed by this option. This option was also generally unsupported by the consulted stakeholders.
   
• Option 9 – Channel the water from Pandora Reservoirs through an engineered channel with tunnel or pipeline as required. This would divert the main source of water from the mining area away from the contamination. Option carried through to shortlist. This diversion option follows the existing topography and watercourses most closely. Some culverting/channels/pipe sections may be required to maintain existing access, but predominantly in open channel following existing watercourses and ditches. This is an attractive option with the benefit of moving water away from the mine spoil, great shaft and properties.
   
• Option 10 – Rewetting upland areas through blocking of streams with woody debris. This will increase water retention higher in the catchment and reduce surface water runoff by increasing infiltration to soil and groundwater. This option could act as a flood mitigation measure and a carbon sink while increasing the biodiversity of the area. Option discounted. Limited space on site to implement this, coupled with the risk that increased infiltration will lead to additional water entering the mine.  It was agreed that this approach would not work and could have a negative impact on the catchment.
   
• Option 11 – Reinstate historic leat that runs along the south of the mine area. This option is the similar in principle to Option 7 but carries water in a different direction entering Llyn Geirionydd from the southwest. Option discounted. Additional information has been reviewed showing that the line this option follows is not a historic leat, but the rail track used to transport material to the dressing floor area. As such, there is no leat to reinstate. The topography does not favour this option over the other diversion options.
   
• Option 12 – Divert water as it reaches the eastern point of the mine site down through topography to the northwest of the mine site. Avoids the mine spoil heap and Caban Pandora. Option discounted. Due to the potential to increase flood risk at downstream properties this option was felt to be unsuitable – other options to divert water were felt to provide the same benefits without the increased flood risk.

* Documents relating to the Longlist Consultation can be found below. 

Why your views matter

The initial phase of consultation has informed the selection of 'shortlist' options as detailed above. We are now asking for stakeholders' views on each of the shortlisted options, and we welcome and encourage your feedback. 

The findings will be used to identify the preferred option, as well as to inform and assist with the design of any agreed surface water management interventions at Pandora Mine.

Please note that the feedback response period is open for 2 weeks until the 12th November 2021. We cannot guarantee that responses received after this date will be incorporated in the next phase of optioneering and final design selection, although all efforts will be made to do so.