Abbey Consols Metal Mine Remediation Design by WSP
Shortlisted for Brownfield Awards Category 2: Best Scientific/Technical/Digital Advance or Innovation
Abbey Consols Metal Mine Remediation Design
Background information
Impacts of abandoned metal mines on our water environment have been an unresolved long-term problem in several parts of the UK, Europe and worldwide. The remediation of historical mine sites is complex and expensive, such that remediation plans often fail to demonstrate an acceptable cost versus benefit ratio (i.e. costs becoming disproportionate). Complexity also increases where mining heritage and ecological constraints limit remediation options and where significant metal loading is released via discharges from flooded mine workings requiring treatment prior to discharge.
Natural Resources Wales (NRW), supported by Welsh Government, has implemented the Welsh Metal Mine Programme. This aims to minimise the impacts of historical metal mine sites in Wales (approx. 1300 known mine sites), which currently cause failure to achieve ‘Good’ EU Water Framework Directive (WFD) status of approximately 700 km of Welsh rivers.
As part of the WFD monitoring programme, NRW identified the Abbey Consols former lead and zinc mine site, located in Ceredigion, Mid Wales, as the main source of zinc pollution in the River Teifi causing failure to achieve ‘Good’ WFD status for many kilometres downstream of the site. Following a prioritisation process, NRW identified Abbey Consols as the priority site for remediation in the Metal Mines Programme - the first site in Wales to achieve full remediation including a mine water treatment solution. The site was not only prioritised because of its clear impact on the River Teifi (designated SSSI/SAC), it was also an ideal site to develop and test remediation options for both typical metal sources, mine waste and mine water discharges.
Figure 1 Current condition of the former Abbey Consols mine processing area (looking south towards the River Teifi along the tree line)
Active mining at Abbey Consols ceased approximately 100 years ago but mining history of the area goes back to at least the 12th century – being closely linked to the nearby Strata Florida Abbey, one of the most important places for Welsh history.
The site is relatively well defined and still contained within the former mine processing area. A buried adit leads north into the hillside towards a network of historical mine workings and serves as a discharge route for mine water from the partially flooded workings. Over recent decades the site has been subject to landowner activities such as local quarrying adjacent to the adit entrance and construction of a racetrack, which includes a section across the former mine processing area. This processing area contains spoil heaps and buried filter beds, that provides habitats for rare lower plants (lichens and bryophytes).
In addition to satisfying technical remediation targets, NRW projects are also delivered in line with the Wellbeing of Future Generation (Wales) Act (2015) which aims to generate additional benefits to the local communities and Welsh society.
Key challenges
The key challenges of mitigating the impacts on the water environment from historical mining sites typically comprise:
∙ The definition and breaking of pollutant linkages in what is often a complex interaction between groundwater, mine water and surface water;
∙ The need to develop an innovative cost-effective mine water treatment solution and integration of wider remediation benefits to overcome a common issue of
disproportionately high remediation costs; and
∙ The need to meet objectives for sustainable and carbon neutral solutions.
Methodology
To overcome the challenges for these type of projects WSP applied a science-based, innovation focused approach which has included the involvement of research establishments such as Cardiff University and specialist contractors. A detailed ground investigation in early 2019 provided a wealth of information that allowed a more targeted approach to research on metal release behaviour of the mine waste and to evaluate innovative mine water treatment options. Hydrochemical fingerprinting and geochemical modelling using PHREEQC software, were used to confirm the conceptual understanding of the pollutant linkages to ensure that remediation focused trials and remediation design can be undertaken. Laboratory trials were carried out to test metal load release from a low permeability cap of the mine waste including a storm event flushing effect and treatability of the mine waters emanating from the buried adit. Environmental opportunities have been mapped and integrated in the multidisciplinary design of the remediation measures.
Laboratory trials for treating the mine water with sodium carbonate in late 2019 showed promising results and were further investigated through two stages of field trials in 2020. Raising the pH of the mine water together with offering a carbonate source led to the precipitation of metal carbonate, which appeared to be particular efficient for zinc removal, the main contaminant of concern on this site. In addition to treatment, studies were also carried out to assess the sustainability of the potential re-use of the mine water treatment waste (predominantly zinc carbonate) and to avoid the disposal of waste to landfill. The design of the mine water treatment also required the consideration of the historical appearance of the site, environmental opportunities, the landowner’s business interests, and the opportunity to minimise energy needs by designing a largely gravity-run system supplemented by solar power.
Outcome of the study
As part of metal loading assessments, it was estimated that on average approximately 1,200 kg per year of dissolved zinc was released from the site into the River Teifi resulting in WFD failure for at least 11 km and a measurable impact for over 40 km. Metal loadings released from the site vary substantially according to meteorological conditions - being highest under very wet conditions, and lowest under dry conditions. This suggests that reducing the zinc loading from the site by a minimum of 70% (and possibly as high as close to 100%) could be achievable if mine waste is separated from contact with water and an efficient mine water treatment solution can be identified.
To inform the remediation design for the mine waste tips, 8 months of laboratory column tests were undertaken by the specialist subcontractor Geochemic Ltd to establish if a low infiltration cover would sufficiently break the pollutant linkage of infiltrating rainfall. The test showed that metal release could potentially get close to the un-remediated scenario and therefore an impermeable liner and additional groundwater level controls were included in the design. This differs from previous mine waste remediation schemes which often applied low permeability covers. Details of the column testing will be presented at the International Mine Water Association (IMWA) conference in July 2021.
Figure 2 Mine waste column tests showing zinc weekly mass load released from average infiltration rate column (1) and 10% average infiltration rate column (2)
Initial laboratory treatability trials indicated that by raising the pH to a range from 8-9 and adding a carbonate source, encourages the heavy metals (in particular zinc) to precipitate as metal carbonate.
A combination of active dosing and passive settlement/filtration was tested through a series of laboratory and field trials eventually achieving dissolved zinc removal rates of over 95%. Issues related to insufficient settling of the precipitate have been investigated to inform the treatment design. A paper regarding the treatment trial results will be presented at the IMWA conference 2021.
Figure 3: Field treatment trials in March and July 2020
Figure 4: Different filters have been tested on site to address particle
wash-out issues
The dosing with sodium carbonate was found to be a viable, cost-effective and environmentally friendly way to treat the mine water, leaving plenty of opportunities for optimisation via further research. The precipitate was found to contain predominantly zinc carbonate which is used in a
range of industries. Initial contacts with chemical suppliers concluded that the waste product could be of commercial interest rather than being disposed to landfill. This requires higher volumes to be generated, e.g. in combination with other historical mine sites, but shows that the developed treatment solution offers a potential zero waste process. Treatment lagoons and sand filters will be applied with the mine water running by gravity through the system, i.e. the only power demand being for dosing pumps and pH sensors which control dosing. This power demand is to be provided by solar PV resulting in a carbon neutral solution.
Integration of filter bed appearance and a protected mine waste area for metallophyte lower plants into the landscape concept and enhanced access via the public footpath and information board offers more wide-ranging benefits to the community and biodiversity.
Figure 5: Site remediation layout for mine waste and mine water treatment
The developed designs and water treatment option becomes an example for other schemes demonstrating that cost-effective solutions can be found. It accelerates the development of specialist skills and research which will enrich the portfolio of options to target more complex sites.
The project has already inspired a range of research initiatives such as:
∙ it contributed to an Engineering and Physical Science Research Council (EPSRC) - Impact Accelerator award (EP/R51150X/1), where the concept was tested and developed further by researchers at Cardiff University; and
∙ it constitutes one of the main demonstration sites for the METAL-SoLVER project. This mine water treatment project is led by Cardiff University, and its principal aim to develop and test innovative mine water treatment technologies. The programme is funded by the Welsh European Funding Office (WEFO) and will run until late 2022, employing a team of 3 research staff.
Tom Williams (NRW Project Manager):
“Abbey Consols has been identified as one of the highest priority sites, and is set to be Wales’s first large-scale metal mine water treatment system, acting as an exemplar for NRW’s ambitious Metal Mine Programme. To ensure the sustainable management of natural resources, remediation schemes for abandoned mine sites must take account of the benefits and impacts to whole ecosystems and the services they provide, rather than simply meeting numerical water quality standards. The iterative approach taken by WSP to the development of the treatment solution, in collaboration with Cardiff University, has yielded a small-footprint, lower-cost system that avoids the use of harmful chemicals in a Special Area of Conservation and Site of Special Scientific Interest. Extensive collaboration with key stakeholders has produced a surface water management design that is sympathetic to the heritage interest at this historic site, and a novel ‘bryophyte garden’ will protect and enhance habitat for rare metallophyte species. The Abbey Consols project will provide a framework to guide our work at other priority sites within the programme, and its successes to date have helped secure further funding and collaboration to trial the treatment solution on different mine waters. With Abbey Consols as its flagship, NRW’s innovative Metal Mine Programme aided a successful bid for Wales to host the 2021 International Mine Waters Association conference, furthering our ambition to be at the forefront of worldwide-efforts to tackle mining legacies.”
Devin Sapsford (Lead Researcher, Geoenvironmental Research Centre, School of Engineering, Cardiff University):
“As is well known, advancing ideas up the technology readiness levels, from initial concept through to implementation is usually a time-consuming and fraught process. This collaboration with WSP was initiated via a WSP-hosted innovation workshop. From the initial concept, subsequent close engagement allowed for a heavily expedited innovation timeline. Consequently, the initial idea has progressed rapidly though laboratory proof-of-concept through to field scale trialling. This approach to research and innovation was very refreshing and it is satisfying to see research ideas progressed so quickly towards practice. Examples of innovative work with industry which leads to tangible outcomes, such as this project, contribute strongly to key performance indicators for the academic sector. As such, I look forward to the continuing collaboration with WSP and hope that project provides a template for how collaborative working between universities and industry can fast-track research to deliver novel technology to clients.”