Wales has several hundred old abandoned metal mine sites, of which 50 have been identified as presenting a potentially significant point source of heavy metal contamination to otherwise clean rivers.  The sources can be from mine water flowing out of flooded adits and shafts or leachates derived from spoil tips.  The 50 mine sites (Figure 1) are typically remote and often within areas of great landscape (scenic) value.  They have had little disturbance for upwards of 100 years.  As a result, specialized and highly biodiverse habitats have become established.  Furthermore, these sites can be of considerable mineralogical, historic and archaeological interest.  Consequently, finding a solution to the contamination problem that is acceptable to all stakeholders is particularly challenging.

Appropriate Technology

There are essentially two options available to remove inorganic contamination from water:

• Active treatment typically using lime dosing, settlement, sulphate reduction and ion exchange/adsorption
• Passive treatment with aerobic or anaerobic wetland systems.  Aerobic wetland systems are typically the classic reed bed treatment in which the treated water is exposed at the surface (Figure 2).  Anaerobic systems are those in which the treatment process takes place within a substrate under depleted oxygen conditions.  In the latter, the surface vegetation is less important in terms of water treatment, however it can help to bind the substrate.  Local flora can be used to help minimize the visual impact of the facility.

We used the following hierarchy in our ongoing assessment of the 50 plus mine sites, taking into account the best benefit-to-cost-ratio at each site.

• Do nothing.  The natural decline in contamination was studied using historic data and consideration was given to the residual environmental cost.
• Drainage diversions/channel lining.  These solutions prevent surface water from becoming contaminated, increase the dilution effects and reduce metal mobilization.

Figure 1: Wales Mine Locations.

Figure 2: PB’s successful aerobic wetland treatment for abandoned coal mine drainage.

Figure 3: Dylife Mine showing the main spoil tips and seepage/runoff water collecting at base. This level area is the proposed location for a novel anaerobic treatment wetland.

• Passive treatment.  This solution is sustainable and has low operation costs, but can require large areas of flat land.  The technology and treatment processes are also often poorly understood at present.
• Encapsulation/sealing.  This solution limits the interaction between mine waste and recharge, but it can be expensive, and it can result in significant short-term increases in metal mobilization due to the regrading of spoil tips and the physical disturbance during works.  Most significantly, encapsulation typically has a significant ecological, landscape and heritage impact. 
• Active treatment.  This solution is very effective at sites where affected water can be directed to a discrete location, and it has a small footprint.  Operation costs are typically high, as are costs associated with maintenance, supply of active materials, and sludge disposal.  Such facilities also require services such as electricity, which are typically absent at the remote mine sites.
• Reprocessing spoil tips.  New technologies can extract valuable minerals from existing waste materials, but the investment is uneconomic except at very large sites.

Case Study: Dylife Mine

One of the sites studied was Dylife Mine (Figure 3) developed originally by Cobden & Bright, famous Industrial Revolution reformers.  Located approximately 14 km (8.5 miles) southeast of Machynlleth, it was a major lead producer with some copper and zinc.  A detailed analysis of historic and contemporary water quality and flow data indicated multiple sources of contamination from the mine ranging from:

• Possible minor adit resurgences
• Interaction between the existing watercourse and mining waste, particularly associated with a collapsed culvert that results in loss of surface water to the surrounding spoil
• Numerous spring discharges and surface runoff from the downgradient waste heaps. 

We put forward a detailed remedial approach for the Dylife Mine site that was based on analysis of the contaminant load data, a review of available engineering options and a detailed consultation process with the numerous stakeholder groups to identify key constraints. Our two key recommendations were:

• The restoration of the collapsed culvert within the mine site to limit future interaction between the watercourse flowing through the site and the surrounding mining waste;
• The use of a buried dam to redirect the residual shallow groundwater flow, spring discharges and surface runoff from the downgradient spoil heaps towards a new anaerobic wetland facility capable of removing much of the metal load.

The key challenge now facing the project is the detailed design of the treatment wetland. The passive precipitation of zinc is poorly understood currently, and as a result, there are no existing large-scale passive wetlands treating zinc contamination in the UK.  We are currently in partnership with the University of Newcastle to carry out pilot studies into the most appropriate substrate materials and the required wetland size for defined metal inflow loads. 

Given the current technical uncertainties surrounding the treatment process and the stringent constraints associated with this remote, scenic and heavily designated site, it will be some time before the completed wetland is in place at Dylife Mine.  However, with PB's firm recommendations in place with the regulator together with our ongoing research into the principles and processes involved in passive metal precipitation, we are taking clear steps towards the sustainable remediation and reinstatement of viable fisheries in a number of contaminated rivers in Wales.