Evaluating the remediation of arsenic-rich tailings at the historic Royal George tin mine, Tasmania, Australia

The aim of this study was to evaluate the remediation of As-rich tailings at the historic Royal George tin mine, northeastern Tasmania. Abandonment and neglect of the Royal George mine have led to a site that is characterised by a severely modified (or lack of) vegetation and the presence of a 36,000 m³ tailings dump. The acid-generating tailings have minor (i.e. >100 ppm) mean concentrations of As, Cu, Mn, Pb, S, Sn and IN, and traces (i.e. <100 ppm) of Ag, Bi, Cd, Co, Sb, Th, Tl, U and Zn. Since 2006, remediation works by statutory authorities have focused on: (a) the construction of gravel armoured drains and drainage diversion channels; {b) the establishment of a fenced-off revegetation trial plot; and (c) the application of crushed limestone and fertiliser pellets to the tailings dump surface. Apart from these minor remediation works, the tailings disposal area has been left to natural processes of surface amelioration and gradual vegetation colonisation.

Today, the vegetation surrounding the site mainly comprises dry eucalypt woodland, whereas the amended tailings remain largely devoid of any vegetation, with only some moist or ponded areas being colonised by the rush Juncus pallidus. Biogeochemical analyses demonstrate that the plant has a pronounced trace metal tolerance and the tendency to exclude environmentally significant elements (AI, As, Co, Cu, Fe, Mn, Pb, Th, Tl, U, W, Zn) from its above-ground biomass. The exclusion of metals from the above-ground plant biomass of J. pallid us reduces the exposure of wildlife and grazing animals to metals and limits the transfer of metals up the food chain. The establishment of a J. pallidus cover on otherwise barren ground also reduces tailings erosion and dust generation as well as water infiltration and associated leaching of contaminants. Such processes reduce the potential exposure of animals and humans to metals and may also open pathways for natural ecological restoration and biodiversification.

Geochemical characterisation of the amended tailings demonstrates that the remediated wastes have alkaline rinse pH as well as positive NAPP values. Laboratory leaching experiments using Dl water as the extraction solution reveal that leaching of limestone-neutralised tailings results in a lower As release compared to non-amended tailings. However, leaching in 0.5 M K₂SO₄, which mimics the presence of sulphates observed in AMD environments, shows a three-fold increase in As release from limestone and fertiliser-amended tailings compared to non-amended tailings. Desorption of As from metal oxide and hydroxide phases at alkaline pH conditions, in addition to the combined presence of phosphates and sulphates could result in the release of As into tailings pore waters and its discharge to surface water seepages and streams. This study demonstrates that neutralisation of As-rich tailings using crushed limestone and fertiliser-amendments, should be pursued with caution. Increased dissolved sulphate concentrations may result in As desorption and mobility into pore and surface waters.