Pilot study on the separation and physical characterisation of lateritic material from the Ranger project area


Internal Report 579
Saynor MJ & Harford A
Supervising Scientist Division
Department of Sustainability, Environment, Water, Population and Communities

About the report

The trial landform is a major project being undertaken by Energy Resources of Australia (ERA) to test the effectiveness of the company's proposed design and revegetation strategies for the final mine landform. This initiative provides an opportunity to measure erosion rates, to determine the composition of sediment being eroded from test areas constructed with different substrates, and to test the capacity of computer landform evolution models to predict rates of erosion through time. The final landform could potentially be a significant source of fine particulates to Magela Creek. Consequently, there is a need to characterise the nature of the particulate matter that could be eroded from the landform, and to investigate its possible impact on aquatic biota downstream of the rehabilitated site, in the context of developing closure criteria and an appropriate performance monitoring framework.

The Pit 3 'lateritic' materials used in this study were nominated by ERA as being representative of the material that was to be used in the construction of the trial landform. In this context 'laterite' is the generic term used on site to describe material excavated from the upper weathered horizon of the pit. The depth of weathering can extend to 50m in some places. 'Laterite' is a very friable material containing substantial fine-grained minerals produced by the weathering process. This is distinct from the correct geomorphic use of the term 'laterite' to describe the upper cemented (ferricrete) horizon of the soil profile.

This report describes the development of the most suitable method for separating the finer particulate material, in this case the <63 μm (0.063 mm) fraction from bulk lateritic material collected from Pit 3 at the Ranger mine. This is the fraction of most interest for assessing the effects of fine suspended sediment on aquatic biota. The separation method will subsequently be used to produce a stock of the fines fraction for use in the project 'The direct effects of suspended sediment on tropical freshwater biota'; SG2008/0179). The process of wet sieving was determined to be the best method, and full details of the procedure/protocol are given in Appendix 2.