Authors: Mulligan, DR; Doley, D; Baumgartl, T; Lynch, KM

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Mulligan, DR, Doley, D, Baumgartl, T & Lynch, KM 2008, 'The Role of Vegetation in Mine Waste Cover Systems with Particular Reference to Australian Mine Rehabilitation', in AB Fourie, M Tibbett, I Weiersbye & P Dye (eds), Proceedings of the Third International Seminar on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 727-738.

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Abstract:
Vegetation can potentially assist in limiting the ingress of water into rock dumps or tailings storage facilities, an attractive potential when the geochemical characteristics of such waste materials indicate that water ingress should be minimal in order to reduce long-term impacts and liabilities. However, the presence and/or necessity for vegetation can present potential conflicts for cover design and management as the cover system objectives are often to minimize the amount of cover material required and to establish a low-cost but effective means of removing water from above the waste to reduce the likelihood of deep drainage. Due to the physical nature and heterogeneity of particle sizes and sorting characteristics of many cover materials used, the distribution patterns of water infiltration into constructed profiles may be very irregular, with penetration to depth (and thus potentially the underlying waste material or impervious capping layer) a high probability in zones of higher hydraulic conductivity. This irregularity of water distribution will also result in heterogeneous patterns of vegetation distribution and growth, especially in low rainfall environments. Although actual evapo-transpiration can account for up to 90% of annual rainfall at some locations, vegetation is unlikely to dry cover material completely or prevent water percolation through the root zone during high intensity precipitation events, especially if vegetation distribution and infiltration patterns are irregular. In humid environments, a seasonal water table is likely to develop, making lateral water discharge from within the cover essential, and the integrity of any impervious capping layer (if present over the waste as a part of the design) capable of preventing penetration by plant roots. In semi-arid environments, a thick cover of benign rock may prevent infiltrating water from ever reaching the impervious capping layer, provided plant roots can penetrate the wetting zone and extract all of the water. Trees have many shallow ephemeral roots and fewer roots that may penetrate more than 20 m but they require permanent water for survival. Grasses on the other hand may have a root biomass that fluctuates more between seasons and their dense root systems may be more effective than those of woody plants in removing water from the surface horizons. Detailed site physical and plant physiological data enable soil and plant water balance and plant growth models to predict temporal variations in vegetation cover, but site heterogeneity requires the use of two- or three-dimensional models. There is also the need to increase the capacity to model and quantify the contribution of vegetation to the hydrological processes occurring in cover systems. This paper reviews the information that provided the background and context for a current major research program involving three Australian universities, Canadian collaborators and nine mining company sponsors. Among other goals, this project seeks to increase the understanding of the role of vegetation in cover performance, and the extent of variation in the function and performance of covers over time due to cover construction design, climate, soil physical and chemical changes, and the likely effects of vegetation changes. All sources of variability must be considered in cover design, and if site and species characteristics are well understood, the extent of variation can be indicated clearly and unreal expectations concerning the role and impact of a vegetative cover can be avoided. The Role of Vegetation in Mine Waste Cover Systems with Particular Reference to D.R. Mulligan et al. Australian Mine Rehabilitation

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