Fly ash as a permeable cap for tailings management: pedogenesis in bauxite residue tailings

Purpose: Closure of tailings facilities typically involves either a ‘cap and store’ or ‘direct revegetation’ approach. Both have been used in the management of bauxite residue (alumina refining tailings), with mixed results. This study evaluated the merit of an intermediate approach, using a permeable cap, and examined the pedogenic trajectory of the Technosol. Materials and methods: Chemical, mineralogical and physical properties of samples from a Brazilian bauxite residue deposit, which had been capped with fly ash 14 years prior and supported a vegetation cover, were compared to evaluate soil formation and pedogenic trajectory of the developing Technosol according to the World Reference Base for Soil Resources. Samples were collected at three points along an elevation gradient, and from 0 to 150 cm below the surface. Results and discussion: Rainfall leaching was identified as the most important pedogenic process occurring in the tailings, lowering salinity and pH. The Technosol classification was poorly suited to describe the soil materials within the study site because two wastes (fly ash and bauxite residue) were co-disposed in discrete layers. Conclusions: The permeability of the fly ash cap is key to soil development in these tailings: it provides a suitable medium for plant growth whilst still allowing contact between the tailings and the surrounding environment. The introduction of a novel prefix qualifier, ordic, would enable more accurate description of layered Technosols. The Technosol at this site is likely to develop towards an Andosol or Ferralsol.