Nenthead Mine Water Treatment Scheme - How we selected the site

Overview

This section explains how we selected the Nenthead mine site as our preferred location for the mine water treatment ponds. This follows a lengthy assessment process where a large number of sites were considered. 

We consider several criteria when identifying a potential site for a treatment scheme. These can include engineering factors as well as the proximity to residential properties and the ecological or archaeological sensitivity of sites. For the Nenthead site, we have changed the criteria used to assess potential locations as the project has developed, and particularly in response to feedback from members of the public. 

The details below explains our approach and how this has evolved since our first feasibility study was carried out in 2013. 

2013: April 

Consultants carried out a desk-based review of potential locations up to 1.5km downstream of the Caplecleugh and Rampgill Levels and within 0.5km of the River Nent. 

Only engineering factors were considered with no assessment of site suitability with regard to visual impact, proximity to residential properties or land ownership: 

• distance to site from mine water discharges, 
• shape and size of the site,  
• ease of access by existing roads or tracks, 
• potential pipeline routes, 
• whether the mine water would need to be pumped, 
• topography of site, 
• ecological, heritage and flood designations, 
• historical use of the land and contamination risk. 

A scoring system was applied based on: 

• 3 = preferable 
• 2 = acceptable 
• 1 = unknown or marginal 
• 0 = unacceptable 

The total score for each site was calculated. A score of 0 for any criteria meant the site was deemed unfeasible. 

Three potential sites were chosen for further investigation (sites 1, 4 and 5). Another three sites which required mine water to be pumped were recommended as reserve sites if the first three sites were not available (sites 2, 8 and 9).  

Figure 1. Potential sites in 2013 assessment

2013: December 

The highest scoring site, site 5, was reviewed in more detail. It was noted that substantial earthworks would be required and that the site was overlooked by several nearby properties. The main recommendation was to carry out a ground investigation.  

A second site, site 13 (Nenthead Mines Heritage Centre car park), that was not included in the initial desk-based assessment was also reviewed. It was concluded that this site was too small for further assessment. 

2015: 

The Nenthead project was then awarded to a different consultant following a competitive tender which led to a second review of potential sites before any ground investigations were carried out. This consultant carried out the following tasks: 

• Review of mining hydrogeology to improve the “conceptual model” of how water moves through the mine workings before being discharged from the Caplecleugh and Rampgill Levels. 

• Review of potential mine water treatment technologies: 

            Sixteen treatment techniques were evaluated and four options
            shortlisted for further investigation:  
                - Sulphate reducing bioreactors installed in tanks
                - Sulphate reducing bioreactors installed in lagoons (“vertical flow
                   ponds”)
               - Liquid bioreactant reactors installed in tanks
               - Chemical sulphide precipitation 

Our expert consultants proposed that using vertical flow ponds was the preferred technology since “this treatment method is relatively well established and proven in the UK” and the whole life costs were comparable to the alternatives. 

• New desk-based review of potential treatment sites: 
  
  The thirteen sites considered in 2013 were re-evaluated plus the Wright Bros bus depot and the Brewery Shaft. The slightly wider set of twelve criteria were: 
          - Distance from mine water discharges 
          - Land area 
          - Access from public roads
          - Potential pipeline route
          - If mine water can be delivered by gravity or needs pumping 
          - Topography of the site for earthworks 
          - Heritage impacts
          - Ecological constraints 
          - Land use including potential contamination
          - Flood risk 
          - Visual impacts on residential properties
          - Planning constraints 

Each site was assigned a score from 1 to 3 for each criterion. The heritage category was permitted a score of zero for particularly sensitive sites. The maximum total score was 36. 

Sites 2 and 3 were discounted after initial discussions with the landowners.  

Site 4 (Policeman’s Bank tailings dam): following a ground investigation, it was concluded that the site was not suitable to build a mine water treatment scheme.  

By early 2016, the Nenthead project was put on hold to focus on the Nent Haggs scheme. 

2016: autumn 

In October and November 2016, we began a new site selection process for both the Nenthead (Caplecleugh) and Nent Haggs mine water treatment schemes.  

The decision to do this was made after discussion with the public and other stakeholders alongside learning from the Nent Haggs project. We wanted to make the site selection criteria more inclusive of the views of the local community.  

Two public engagement events were held to ask the public what criteria they would like included alongside our technical and engineering considerations.  

A revised set of criteria were proposed to and accepted by the local community. These were: 

• Access to the potential treatment site (closest boundary)
• Closeness to housing and businesses (from site boundary to nearest building)
• Current and previous land use – ease of construction
• Degree of slope
• Distance from the mine water discharge point to the potential treatment site
• Ecological constraints
• Flood risk
• Protected site designations – natural and historical
• Pumping costs and carbon footprint 
• Route of the pipeline to transfer the mine water 
• Site size – layout, landscaping and planting 
• Visual impact 

For each category, a site could score from 1 to 5. The criteria were applied in two phases: 

• Phase 1: initial assessment using only distance from site, access from roads, pumping costs, closeness to housing and businesses, and site size. 
• Phase 2: the remaining criteria were applied to calculate a total score. 

2017: long-list of potential sites (March) 

The Coal Authority applied the agreed criteria to assign scores to potential sites within 2.5km of the Nenthead mine water discharges, using desk-based digital mapping software. Sites that were too small, too steep or were covered by statutory conservation or heritage designations were excluded. 

The outcome of the scoring exercise was used to create a long-list of twenty potential sites so that the public and other stakeholders could provide comments. The long-list was shared at a public engagement event in March 2017 and is shown in Figure 2. 

Figure 2 Long list of potential treatment scheme sites (March 2017) 

The full list of scores is shown in the table below, along with details of the scoring method: 

Phase 1: screening with initial five criteria

Phase 2: calculation of total score

Total scores for sites after Phase 1 and Phase 2

2017: shortlist of three preferred sites (May/June)

Feedback from the community and other stakeholders was used to short-list three sites which are shown in Figure 3. The scores for the short-list of three preferred sites are in the table below.

In early summer 2017, we asked the community for feedback on these three sites during face to face engagement events and an online information website.

Figure 3. Short-list of three preferred sites.

Based on feedback from the public, professional stakeholders and further reviews of engineering and costs, by September 2017, we concluded that none of these sites were viable due to complexity and excessive costs.

2017: autumn

Mine water flow needing treatment:

We reviewed the mine water monitoring data and calculated that the most significant environmental improvement comes from treating between 10 and 20 litres per second of the total flows from both mine water discharges. The mine water flows fluctuate with rainfall but at higher flows, there is more dilution in the river so the level of pollution and harm is lower. This meant that less land area was needed to build the treatment scheme than if we had to design for the full flow of more than 30 litres per second.

Modified selection criteria:

We held discussions with stakeholders including Historic England, Natural England, Cumbria County Council and the Nenthead Mines Conservation Society about whether it would be possible to consider building a treatment scheme on the Nenthead mine site, which as a scheduled monument was formerly excluded. We also took account of previous feedback from the public.

As a result of these discussions, we modified the site selection criteria to include:

• Sites located on the Nenthead mine site, a scheduled monument (previously excluded)
• Steeper slopes

The Coal Authority then repeated the desk-based mapping exercise using these modified selection criteria. Eight potential sites were identified including four new sites (see Figure 4).

Figure 4. Potential sites from modified selection criteria and indicative costs (based on pumping elevation)

A simplified scoring system was then applied to these sites based on:

• Distance from the mine water discharge
• Closeness to housing and businesses
• Access from roads
• Indicative costs, including pumping costs based on elevation above discharge

The results from the scoring are shown in this table. For each category, a site could score from 1 to 5.

2018: spring

Based on feedback from the 2017 public engagement events, the project team wanted to only present options to the public that were considered technically realistic and affordable.

Therefore, in early February, five sites were excluded based on:

• Closeness to housing and businesses: site 19
• Access from roads: site 102
• Indicative costs: sites 1, 20 and 103

The Coal Authority’s Engineering Team then carried out an initial feasibility study for sites 23, 100 and 101 to assess the suitability for a mine water treatment scheme. The initial site identification was based on a desk-based indication of potential land area. The feasibility study included site visits to see whether the indicative boundaries were appropriate.

2018: May

The feasibility study concluded that site 23 and site 100 should be rejected due to projected costs, the need to take the pipeline route through Nenthead village, access difficulties and other factors.

Site 101:
During the initial site visit with Nenthead Mines Conservation Society, the possibility of moving the treatment scheme further away from the village to above the Handsome Mea reservoir was discussed. It was agreed that this location would be preferable since it would also be partly outside the scheduled monument area. The feasibility study was carried out based on this new site although we did not change the site number. The new and original site 101 locations are shown in Figure 5.

At the time, we did not re-score the new site 101. However, using the same methodology in 2022, there is no change to the total score as shown in the table below:

Figure 5. Location of original and revised site 101

2019: February

We held a public event to explain that our preferred site was on the former Nenthead mine site adjacent to the Handsome Mea Reservoir as shown on Figure 6. We explained that this was expected to include three open water treatment ponds and a new constructed wetland containing reeds to polish the treated water before putting it back into the River Nent.

Figure 6. Proposed location at revised site 101 and indicative scheme layout (February 2019)

Figure 7. Indicative pipeline route and proposed main access route (February 2019)

2022: November

Since the February 2019 public event, we have continued investigations at the proposed location so that we can develop the design in advance of preparing an application for planning permission. Unfortunately, this has taken much longer than originally expected due to the COVID-19 pandemic.

As we have gathered more information, the design of the scheme has slightly changed although the basic layout remains similar. 

 

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