From tailings to soil: long-term effects of amendments on progress and trajectory of soil formation and in situ remediation in bauxite residue

Abstract

Bauxite residue is an alkaline, saline-sodic byproduct of the Bayer process for alumina production. In situ remediation of bauxite residue is a cost-effective management strategy that transforms the residue into a soil-like medium; however, the effects of applied amendments on trajectories and progress of soil formation over the medium-long term are unclear. Here, we investigated how in situ remediation amendments guided the progress and trajectories of soil formation in bauxite residue over 20 years. Chemical and mineralogical properties of samples from a field site in Corpus Christi, Texas, were analyzed to compare the effects of three different amendments (sewage sludge, yard waste, and topsoil) on progress and trajectories of soil formation in bauxite residue. Bauxite residue was deposited ca. 40 years prior to sampling; amendments were applied at varying frequencies for 20 years prior to sampling. Sewage sludge was the most effective amendment for decreasing pH, EC, and total alkalinity of bauxite residue and increasing plant nutrients and exchangeable cations. Overall soil development is in an early stage; however, incipient soil horizons were identified from differences in chemical and mineralogical properties with depth. Although amendment type influenced the progress of soil formation, it did not appear to influence the overall trajectory of soil formation processes. Soil derived from the bauxite residue parent material is likely to progress from a spolic Technosol to a technic Cambisol and finally either a Ferralsol or Luvisol depending on organic matter dynamics. Long-term impacts of amendments were consistent with those observed in previous short-term studies, with some outcomes improving further over the long term (e.g., Na+ leaching) and were realized to a depth enabling maintenance of a vegetation cover. These outcomes provide strong support for the use of in situ remediation as an alternative to soil capping that enables transformation of bauxite residue to a soil capable of supporting a self-sustaining ecosystem.