Spatial variability of hydraulic properties on a bauxite residue disposal area in Western Australia

Authors: Gwenzi, W; Hinz, C; Phillips, I; Mullins, IJ

DOI https://doi.org/10.36487/ACG_repo/908_39

Cite As:
Gwenzi, W, Hinz, C, Phillips, I & Mullins, IJ 2009, 'Spatial variability of hydraulic properties on a bauxite residue disposal area in Western Australia', in AB Fourie & M Tibbett (eds), Mine Closure 2009: Proceedings of the Fourth International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 505-513, https://doi.org/10.36487/ACG_repo/908_39

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Abstract:
Engineered cover systems based on the store-release concept are commonly used to restrict drainage into buried reactive rock wastes. In principle, store-release covers are designed to store infiltrating water, and release it slowly through evapotranspiration. A typical store-release cover should have a top layer with high water holding capacity overlying a low permeability base layer designed to act as a hydraulic barrier. The capacity of a cover system to support vegetation, which is considered a key component, depends on soil moisture availability. Hydraulic properties, in particular soil moisture retention and hydraulic conductivity, control soil moisture storage, water flux and plant available soil moisture. Accordingly, hydraulic properties are key inputs in water flow, soil-plant-atmosphere and biogeochemical models. The main objective of this study was to investigate whether a rehabilitated bauxite residue disposal area (RDA) constructed from residue sand can act as a store-release cover. To achieve this objective, Philip-Dunne and Guelph permeameters, and a constant head laboratory method were used to measure saturated hydraulic conductivity (Ks). Geostatistics was used to evaluate Ks spatial variability. Results indicate that Ks was very high (> 100 cm hr-1), but showed low spatial variability (coefficient of variation: < 45%). In general, Ks decreased from 209–400 cm hr-1 in the top 90–115 cm hr-1 at 300 cm depth. The decline was negatively correlated (r2 = 0.7) to mean dry soil bulk density (dry soil bulk density = mass of dry soil/total volume), which increased from 1313.8 kg m-3 at 15 cm to approximately 1500 kg m-3 at 300 cm depth. Correlation analysis showed that Ks for residue sand can be estimated from a simple equation based on dry soil bulk density. On the basis of the high Ks, rapid vertical movement of water is expected during rainfall events. In addition, the low water-holding capacity could lead to severe vegetation water stress particularly in the dry season. In conclusion, a cover system constructed from residue sand cannot act as a store-release cover system. It is therefore recommended that, materials with high Ks should not be used to construct a store- release cover system. To meet the objectives of a store-release cover, a redesign of the bauxite RDA is highly warranted.

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