Understanding The Hydrogeology Of A Bauxite Residue Disposal Area

Worsley Alumina Pty Ltd (WAPL) has bauxite mining and refining operations in the Darling Range, near Collie, Western Australia. Bauxite ore refining residue (bauxite residue) is a waste product from the refining of bauxite ore to alumina.

Bauxite residue is a by-product of the Bayer process in which bauxite ore is physically and chemically transformed to extract alumina, leaving a viscous slurry by-product. The bauxite residue is dewatered to give a slurry of approximately 62-64% solids. The solids consist of combined sand and fine silt to clay sized fractions, of which iron (oxy) hydroxides are the dominant mineral. Before and after drainage, the residue remains alkaline, sodic and saline. Consistent with residue chemistry, the drainage liquor is alkaline, saline and contains humic substances.

Environmental legislation demands safe containment of the bauxite residue, which in this case is achieved by deposition into large dams, known as bauxite residue disposal areas (BRDAs), situated within the refinery lease. Worsley Alumina is legally bound to develop a decommissioning plan for their BRDAs. One BRDA in particular (BRDA1) is approaching decommissioning stage. A sound knowledge of the hydrogeology of this BRDA is essential for the development of a satisfactory decommissioning plan.

When BRDA1 is closed and rehabilitated, it will be capped with between 300 to 500 mm of soil and re-vegetated. An interception drain and capillary break will underlie the soil cap, so that it will be non-infiltrating (rather than “impermeable”).