Mining results in the production of waste rock and tailings material. Depending on geology and production techniques, these wastes can be sulfidic or contain elevated levels of metals. When these hazardous waste forms are exposed to oxygen and water, chemical reactions with minerals like pyrite may cause hazardous drainage, contaminating adjacent ecosystems. Australian legislation requires mining companies to ensure that these impacts are prevented, during as well as after mine closure.
In arid and semi-arid climates the use of evapotranspiration (ET) covers is often proposed as a technique to mitigate hazardous drainage from waste material. This technique encapsulates the waste and intercepts precipitation. After precipitation ceases, moisture will be lost through evaporation and plant transpiration. Cover material, design and thickness vary between sites and climates.
ET covers are a widely applied concept but several important questions have not been addressed thoroughly in the past. While a large proportion of mining operations in Australia are situated in semi-arid and arid regions, the northern part of the continent experiences strong wet-dry seasonality. This thesis examines the influence of rainfall distribution on ET cover systems and therefore the suitability of areas with distinct wet-dry seasonality for this technique.
In addition, while well-graded and fine-textured material is commonly suggested for such cover systems, the usage of benign waste rock for cover construction has increased in recent years in Australia. The coarse and very diverse nature of waste rock material may introduce small-scale spatial differences in hydraulic properties. The extent of this heterogeneity on cover performance is investigated in this research.
Lastly, the ET cover technique relies on water loss from the covering material through evaporation and transpiration, which can be summed as evapotranspiration. However, knowledge about actual evapotranspiration rates is limited to date.
The effects of climate, material properties and plant water use were investigated at two field sites as well as in a controlled column trial.
The column trial demonstrated that cumulative actual evaporation (aEV) from waste rock material may be modelled successfully to within 5 % of the actual value for varying waste rock material compositions (16 % and 35 % of material smaller than 2 mm) and varying surfaces (waste rock surface and topsoil surface).
Field trials were conducted in semi-arid north-west Queensland (QLD) (Xstrata Zinc, Mt Isa) and semi-arid western New South Wales (NSW) (New Gold Inc, Cobar). Both sites had similar long term annual rainfall of approximately 400 mm but different precipitation patterns. In NSW, approximately uniform precipitation occurred every month and daily rain seldom exceeded 50 mm. In this climate, precipitation was successfully retained with the ET cover. In QLD, rainfall was concentrated in the three-month wet season and rainfall events could reach daily rainfall intensities of almost 100 mm. For the duration of this project, the annual precipitation exceeded long term average values. This climatic situation led to repeated water ingress into hazardous wastes.
In addition, the ET cover trials at Mt Isa demonstrated the pronounced influence of coarse waste rock on cover performance. On this site, two cover designs with three replicates were trialled. Cover material was comprised of over 80 % of particles larger than 2 mm. Measurements showed widely differing hydraulic properties for all six plots, resulting in highly diverse seepage rates. The coarseness of the material resulted in very high saturated hydraulic conductivities, reaching 884 cm day-1. The research showed that the hydraulic heterogeneity of cover systems must be addressed when modelling cover performance in order to gain higher accuracy of model outcomes.
Evapotranspiration measurements were conducted on two species on the cover trials in NSW. Results showed that ET rates were dependant on species and plant size, with ET values between 4 mm day-1 (Senna artemisioides) and 3 mm day-1 (Sclerolaena birchii). In addition, it was observed that ET was determined through climatic demands in the mornings, while for the majority of the day the moisture availability in the cover material determined ET rates. This held true for bare soil evaporation as well, which was lower than ET (0.8 – 0.7 mm day-1). Furthermore, the research pointed out the importance of evaporation on ET cover systems, which is due to the relative low vegetation cover in semi-arid regions.
In conclusion, this thesis has shown that ET covers may offer a practicable technique for minimising rainfall percolation into hazardous mine waste in semi-arid areas with uniform rainfall distribution. However, this is unlikely to be achieved with traditional ET covers in areas with wet-dry seasonality. In addition, benign waste rock with a large proportion of rocks and boulders introduces an element of uncertainty into cover systems, negatively impacts on cover performance and complicates long-term performance modelling. Vegetation establishment positively influences cover performance, but its role for extracting water from an ET cover in water-limited climates is relatively small due to the generally low plant coverage.