Assessment of Technosol formation and in situ remediation in capped alkaline tailings

• Soil development was investigated in capped bauxite residue tailings.
• Organic matter accumulated over time in caps.
• Only shallow changes in pH and EC were observed within tailings.
• Current Technosol classification is poorly suited to describe layered technic soils.
• Introduction of novel prefix qualifier, ordic, would improve Technosol classification.

Technosols formed from mineral processing wastes such as tailings are of growing importance worldwide due to the increasing demand for mineral and energy resources. The construction and nature of tailings and capping materials in tailing storage facilities are likely to influence soil formation rates; however, these have not been evaluated to date. This study used bauxite residue (alumina refining tailings) as a model material to compare soil development and in situ remediation with reference to initial residue composition, landscape position, and cap type. Soil development in the capped tailings deposits was limited to accumulation of organic matter in the upper 0–2 cm of capping materials, and shallow (< 10 cm deep) decreases in pH and salinity within the underlying tailing layers. Landscape position had little effect on soil development due to the formation of technic hard material through calcite cementation in sintered bauxite residues, which inhibited weathering. Weak structure development, likely due to volume shrinkage through desiccation, was observed in unsintered bauxite residue. Leaching of acidic pore waters from pyritic mine spoil caps aided in pH neutralization in unsintered bauxite residues.The current Technosol classification was adequate for description of the soil materials within the study site; however, the introduction of a novel prefix qualifier, Ordic, would enable more accurate description of multilayered Technosols. Unsintered bauxite residue under a pyritic mine spoil cap was predicted to move from a Spolic Technosol to a Haplic Cambisol based on variations in structure and colour within the capping. The lack of observable weathering precluded identification of a likely pedogenic trajectory in sintered bauxite residue. Accelerating natural processes of soil development and in situ remediation may be achieved by application of chemically reactive caps; however, these should not preclude atmospheric gas exchange, leaching, or interaction with biota as pedogenesis will be retarded.