From tailings to soil?

Talitha Santini                                                                                                                                    School of Geography and Earth Sciences, McMaster University

Figure 1 Vegetation cover at sewage sludge treated area.

‘Cap and store’ approaches to tailings management are increasingly being replaced by in situ remediation initiatives, in which plant cover is established directly in the tailings material. In situ remediation involves changing tailings properties (e.g. pH, EC) from very low (pH 1-3) or very high (pH 9-13, EC >4 mS/cm) values to plant-tolerable values, by way of applied treatments and weathering. Pedogenesis is implicit in in situ remediation strategies because establishment of plant cover is usually the end goal. Identification of processes and thresholds controlling soil formation on tailings deposits is essential to achieving rapid in situ remediation and establishment of plant cover. In the case of bauxite residue, the major barriers to plant growth are the high pH and salinity of fresh residue, followed by lack of biological activity, organic matter and available plant macro- and micronutrients, and a massive, apedal structure which causes waterlogging and limits root penetration.

Figure 2 Vegetation cover at sewage sludge + clay soil treated area, showing development of crumb structure in bauxite residue mud.

As part of my PhD project investigating soil formation in bauxite residue deposits, I sampled four bauxite residue field sites (Linden, Guyana; Corpus Christi, Texas; Bauxite, Arkansas; and Sao Luis, Brazil) to investigate the potential influence of initial residue properties, age, applied treatments (e.g. sewage sludge, pyritic mine spoil, green waste), and climate upon the weathering trajectories and pedogenetic processes occurring within the residue deposits. Chemical (pH, EC, titratable alkalinity, extractable/exchangeable plant nutrients, total elements), mineralogical (XRD, amorphous Fe/Al/Si/Mn oxides) and physical (particle size) analyses were performed to evaluate the extent of soil formation under the various treatments at each site.

Figure 3 Development of crumb to blocky structure under sewage sludge treatment.

At the 2011 Western Australian Soils Conference, I presented results from the Texas field site, which was closed in 1967 and had sewage sludge, sewage sludge + tree/lawn waste, sewage sludge + clay soil, dredge spoil and shale-rich topsoil applied as treatments during the last 20 years. A good vegetation cover is present over most of the site (Figure 1), with development of a crumb-like to blocky structure near the surface under

Figure 4 Patch of gravel developed from untreated barren mud

some treatments (Figure 2; Figure 3) and gravelly concretions under others (Figure 4). Significant differences in a variety of soil properties such as pH, EC, alkalinity, organic C, total N, and cation exchange capacity with depth are indicative of horizon development and support visual observations of soil formation. Sewage sludge appears to be a key ingredient in reducing pH and building up carbon and nitrogen. Differences in the chemical and mineralogical properties of the bauxite residue in response to applied treatments suggests that thresholds and feedback loops in soil formation can be targeted to accelerate in situ remediation of bauxite residue and other tailings materials. Such targeted approaches would minimise time and money required for successful remediation, and ensure a satisfactory outcome for industry and stakeholders.

Acknowledgements: Financial support for this work was provided by Alcoa of Australia and BHP Billiton Worsley Alumina, a Minerals and Energy Research Institute of Western Australia scholarship, and a Geoffrey Kennedy UWA Postgraduate Research Travel Award.

Talitha Santini is currently continuing her research in bauxite residue remediation in a postdoctoral capacity at McMaster University, Canada, in collaboration with the University of Western Australia, where she completed her PhD research.

This article was first published in Profile, the newsletter of Soil Science Australia, Issue 168, March 2012, and is republished here with the permission of the Author and Soil Science  Australia


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