Pore water metals in Cu, Sn and As tailings at Devon Great Consols, South-West England

Three arrays of MacroRhizone pore water samplers were deployed at the abandoned mine complex of Devon Great Consols (DGC) to investigate water/substrate interactions that lead to potential contamination of the surrounding environment. Three contrasting tailings types are available at DGC – from Sn, As and Cu processing conducted at different stages in the history of the mine complex. The site is therefore ideal to study in detail the ongoing biogeochemical evolution of the spoils as they react with surface waters and the atmosphere. Although designed specifically for use in soils, these tension-assisted pore water samplers have proved effective in providing samples for analysis, from the dry summer of 2014. The results obtained from pore waters differ significantly from those of acid extraction methods used on the tailings, but metal concentrations are in the same range as those of distilled water extractions. Average pore water compositions vary significantly between the tailings types, with Cu tailings being richest in Cu, Al and Mn. Samples from the Sn tailings were richest in Sn but lowest in Mg, Co, Ni and Zn and those from the As tailings were highest in As, Na and Sr but lowest in Cu. These observations suggest that on-going chemistry in the weathering tailings piles influences pore water compositions. SEM-EDX X-ray maps of element distributions within the tailings suggest a widespread association of As with secondary iron oxides in all tailings types. The multi-element pattern of the Cu tailings is sub-parallel to that of samples from a drainage pipe, suggesting significant dilution with meteoric water. This work highlights the potential use of MacroRhizone samplers to collect pore water data on a seasonal basis, and to combine them with detailed mineralogical investigation of the degrading tailings materials in order to constrain the on-going geochemistry.