Dispersed Alkaline Substrate Treatment Pilot Trials at the Parys Mountain and Cwm Rheidol Metal Mines

Description

Lot 1: Abandoned Metal Mines cause multiple failures of the Water Framework Directive (WFD) standards in Wales, with 1300 mines impacting over 700km of rivers. Cwm Rheidol and Parys Mountain are two out of the top five most polluting abandoned metal mines in Wales; Parys Mountain is unique, complex and the most polluting metal mine in the UK, whilst Cwm Rheidol is a mine complex with two point source discharges. Previously, the only proven options available to NRW that will fit the challenging terrain have included High Density Sludge (HDS) treatment, HDS variants or electrochemical methods to successfully remove metals from water that is discharged from metal mines. These are expensive options (cost disproportionate in relation to clean-up benefits afforded) and unsightly in Special Landscape Areas to roll out at Parys Mountain or across the more typical metal mine deposits encountered across a large programme of mines. There is an opportunity at both Cwm Rheidol and Parys Mountain to trial an innovative passive treatment system in Wales that has already been demonstrated at full-scale in Spain, in order to ensure we deliver value engineering treatment solutions for the public purse while ensuring that all the heritage, habitat and landscape constraints at both the Parys Mountain and Cwm Rheidol sites are adhered to. Dispersed Alkaline Substrate (DAS) is a passive treatment system for acid mine drainage with high metal loading, developed over the last decade (Nieto, J.M et al 2013. Acid Mine Drainage in the Iberian Pyrite Belt. 1 & 2. Nieto, J.M et al 2019 & Martinez, N.M. et al 2019). The high treatment performance of DAS is based solely on inorganic dissolution of calcite (which removes Al, Fe(III), Cu and Pb) and Magnesium Oxide (which removes divalent metals), without reliance on bacterial sulphate reduction (typical for Vertical Flow Ponds that would require a much larger footprint of land ~1 hectare unavailable at Cwm Rheidol). DAS achieves higher acidity removal than other conventional passive treatment systems, as metals accumulate intentionally in DAS. This high acidity removal rate reduces the footprint necessary to implement a full-scale DAS passive treatment system, which is also ideal for these mines with challenging terrain. The trials will also include a tertiary treatment (Witherite/BaCO3) element of the solution that may be required to treat the sulphate that will not be removed by the DAS treatment. This purchase will support with assessing the most appropriate long-term treatment, economically & logistically, for both of these mines.