Evaluating the benefits of water covers as a rehabilitation strategy in temperate climates: a Tasmanian perspective

Environmentally safe disposal of reactive sulphidic tailings is one of the major management challenges facing the mining industry. Consequently, there are a broad range of tailings rehabilitation strategies suitable for a variety of climates. One option is placement of tailings under a water cover to reduce oxygen influx. However, for sites in Australia that lie in a cool temperate climate, the suitability of engineered water covers is not well documented. The Old Tailings Dam (OTD) located at the Savage River Mine, Tasmania provides a unique opportunity to evaluate water covers as a rehabilitation strategy. The OTD extends roughly 1500 m northeast to southwest and 170 m east to west at its widest point (Hassell, 2005). The northern end is submerged under water, as the OTD was constructed on a natural gradient. Therefore, up to 0.2 km² are permanently submerged under water (maximum depth of 10 m) with some seasonal variation. In contrast, the southern portion in contrast has been exposed since 1982. Tailings from 14 trenches and 11 sediment cores were collected across the OTD. Selected samples were subjected to geochemical testwork and also used in mineralogical studies whereby a modified sulphide alteration index was developed. Results revealed a complex, heterogeneous sediment architecture with ten facies observed. From this, four zones were defined (A, unsaturated and coarse-grained, B- unsaturated and fine-grained, C- intermediate saturation and D- saturated). Geochemically, Zone D was identified as the lowest risk, with Zones A to C classified as having a high to extreme acid forming nature, which concurred with bulk mineralogy evaluations showing relatively high quantities of unoxidised pyritic tailings (5 to 9 wt. %). Sulphide alteration index assessments allowed for oxidation to be observed on a micro-scale, and indicated that a minimum water cover depth of approximately 1 to 1.5 m would be sufficient to significantly retard oxidation. Ultimately, a phased rehabilitation strategy which requires geotechnical earthworks and installation of flow-through reactors in combination with a water cover should be developed to successfully reduce AMD from this site.