Morphology of Built Soils on Rehabilitated Bauxite Mines.
Geoffrey Kew and Robert Gilkes. Univ of Western Australia, 35 Stirling Hwy, Crawley, WA, Australia
Bauxite mining in the Darling Range of Western Australia removes the upper horizons of lateritic regolith that has formed from granitic and doleritic rocks. The morphology of the postmining soil profile is considerably different from that of the premining profile. During mining the iron oxide cemented lateritic duricrust and portions of the mottled zone are removed, reducing the height of the landsurface by approximately 3 to 6m and exposing quartz and clay rich pedolith and saprolith materials. These materials form the substrate for re-establishment of vegetation after bauxite mining and the considerable variations in water retention and strength affect plant growth. This presentation describes the premining morphology of the lateritic bauxite profile and the morphological changes resulting from mining and the rehabilitation practices used to establish vegetation. Near vertical fissures existing within the lateritic duricrust of the premining profile concentrate water and plant roots creating permanent preferred pathways in the underlying regolith. Extensive lateral plant root growth occurs in the surficial gravelly sand horizon above the lateritic duricrust but is mostly restricted to the pathways within the duricrust. Clay eluviation and quartz sand concentration are enhanced in pathways creating a channel occupied by a bleached (10YR7/2 to 10YR8/1 Munsell moist colour) coarse sand to light sandy clay loam. Rehabilitation practices aim to mimic the premining profile so as to provide a suitable substrate for rehabilitation with native forest species. Artificial channels 10 to 30cm wide are created by ripping exposed regolith to a depth of 150cm with a straight tine with a wing attached at the base. Material within the channel is apedal or weakly structured and consists of quartz rich, clay rich, iron oxide cemented materials and or coarse fragments. After ripping the mine pit is landscaped, which loosens and mixes exposed regolith and then 40 to 50cm of loamy sand (10YR3/4 Munsell moist colour) topsoil containing more than 50% lateritic gravel is returned. A second ripping operation that was carried out after landscaping and topsoil return using a large winged tine to a depth of 150cm has now been superceded by the use of a multitine to 80cm depth. The combination of pre-ripping and large winged tine ripping fractures 50% of the reconstructed profile to a depth of approximately 150cm or more, with 25% of mine floor materials ripped. Fractured regolith is relatively weak having lower values of penetrometer resistance and higher depths of knife penetration. Increased water infiltration due to ripping, creates a zone of lower matric potential (pF 2 or 10 kPa) compared to unripped regolith (pF 4 or 1000 kPa) and this encourages plant root development. Lateral plant root growth occurs in fractured regolith ripped by the second operation and artifical rip channels become colonized by plant roots within a year of rehabilitation. Water infiltration and transmission are no longer concentrated into a very limited number of near vertical pathways as in the premining profile, with contents of soil water and roots being higher in materials within much more abundant rip-line channels. The structure of unripped regolith outside the rip channels is the same as in the premining profile with moderate lenticular pedality in quartz rich regolith, while clay rich regolith has a moderate to strongly angular blocky pedality. The extent to which natural pedogenic processes are enhanced and soil properties modified by fracturing of regolith during ripping has been considered. Dissagregation of weathered polycrystalline quartz grains, redistribution of kaolin, formation of structural bridges and iron oxide cementation of grains are among the processes that are responsible for the extreme hardness of some regolith. The increased porosity produced by ripping the regolith appears to enhance some pedogenic processes. Translocated kaolin within voids produces a new fabric lining the void which contains translocated materials with aligned mineral grains including residual muscovite. The kaolin may also bridge between sand sized mineral grains and may infill micropores between mineral grains adjacent to voids. The kaolin within the void may be enriched with iron oxide, which acts as a cementing agent. These fabric alteration processes result in a total porosity within the plant available water range (pF2 to pF4.2) in unripped quartz and clay rich materials of 27% to 34% compared with values of 24% and 21% for granitic and doleritic saprolite. Detrimental alteration of regolith fabric can occur during ripping. Thin sections have shown that realignment of mineral grains resulting from ripping produces a structureless, dense smeared surface several centimetres thick on the wall of ripping lines which restricts plant root growth into unripped regolith. Based on data for limited replicates the dry unconfined compression strength (UCS) of sandy clay loam quartz rich material increased after ripping had induced smearing and the porosity (near saturation to a matric potential of pF 3.7) increased at all matric potentials. The reverse situation occurred in a silty clay rich material where compression of peds may have created more planes of weakness which reduced dry strength. In summary, the postmining soil profile in rehabilitated lateritic bauxite mines is morphologically distinct from premining profiles and allows more rapid infiltration of water into fractured regolith which accelerates natural pedogenic processes.