Building Effective Soils on Rehabilitated Bauxite Mines.
Geoffrey Kew1, Robert Gilkes1, and John M. Koch2. (1) University of Western Australia, 35 Stirling Highway, Crawley, WA, Australia, (2) Alcoa World Alumina Australia, PO Box 172, Pinjarra, WA, Australia
Lateritic bauxite profiles in the Darling Range of Western Australia have formed from granitic and doleritic rocks. Mining by Alcoa World Alumina Australia (Alcoa) removes bauxitic materials, leaving underlying regolith materials that may be dense, hard, structureless and inhospitable to plant growth. The rehabilitation of these materials aims to create a functioning soil profile including a deep friable rootzone enabling optimum plant growth. A self-sustaining forest ecosystem with no soil impediments and which is similar to the unmined forest can be established through the use of advanced ripping strategies. Initial rehabilitation in the 1960's involved topsoil return to the mine floor with no ripping and resulted in shallow rootzones, restricted growth and wind throw of trees. During the past 40 years increased bulldozer size has allowed progressively greater ripping depths and the attachment of wings to the ripping tine has improved fracturing of dense regolith materials. As a result of this technology forest productivity measured as tree basal area in 13 year old rehabilitated forest is higher than in the unmined forest. The current ripping strategy involves pre-ripping parallel lines to a depth of 150cm and spacing of 150cm to generate subsoil structure, next the soil surface is leveled and coated with layers of retained overburden (effective A2 horizon) and topsoil (effective A1 horizon) materials to simulate original horizonation. A multi-tine implement is next used to rip this assemblage of soil materials to a depth of 80cm following the contours of the landscaped mine pit (contour ripping) and is a recent replacement for a winged contour ripping tine used at 150cm depth. Contour ripping by either tine arrangement relieves compaction created by heavy vehicles during return of overburden and topsoil materials. This abstract outlines a classification system for soils constructed after bauxite mining, quantifies the impact of ripping on soil properties and relates growth of the re-established forest to regolith properties including plant available waterholding capacity. The morphological classification system describes zones of regolith material using field based determinations of hand texture, structure, colour and coarse fragment content. Regolith materials are placed into eight classes, with three classes being the dominant materials. They are quartz rich (Zm) loamy sands to sandy clays, clay rich (Zp) silty loams to silty clay and red-brown iron oxide rich (Zh) materials. The impact of ripping on constructed soils depends on regolith material properties, moisture content, ripping implement and working depth. In the unripped state quartz rich (Zm) material is characterized by a dense lenticular structure, with few, very fine (< 1mm) sized roots, while clay rich (Zp) material is characterized by an angular blocky structure with common very fine (<1 mm) to medium (2-5 mm) diameter roots growing along ped faces. Ripping should target quartz rich (Zm) materials to promote root growth in constructed soils. Pre-ripping of mine floor material with a straight tine to a depth of 150cm in conjunction with winged contour ripping to 150cm of constructed soils results in 50% of the profile to approximately 150cm depth or more being fractured, including 25% of pre-ripped mine floor material. The shallow working depth of <100cm using a winged pre-ripping tine fractures most materials. Pre-ripping creates artificial channels for water movement and root growth with minimal lateral fracturing of material. Field observations indicate that the consistency of depth for winged tine contour ripping is maintained following a pre-ripping operation. The average plant available waterholding capacity (pF 2 to 4.2) for quartz rich (Zm), clay rich (Zp) and iron oxide cemented materials is 0.12, 0.16 and 0.10 m3/m3 respectively so that growth of re-established forest will be more successful when Zp material is present and can be readily exploited by roots. Some regolith materials remaining after bauxite mining are hard and poorly structured due to compaction during mining or to inherent properties of the lateritic residuum, such materials are recognized by the classification scheme and therefore can be managed appropriately. Building effective soil profiles on rehabilitated bauxite mines that will enable sufficient root colonization requires the identification of materials and application of appropriate deep ripping operations.