Monitoring rehabilitation success on Namakwa Sands heavy minerals mining operations, Namaqualand, South Africa

[摘要] Anglo American Corporation's Namakwa Sands heavy minerals mining and beneficiation operation hasbeen strip-mining a heavy mineral deposit, rich in the commercially valuable minerals ilmenite, rutile andzircon, since September 1994. The mine is located in the vicinity of Brand-se-Baai on the west coast ofSouth Africa, approximately 385 km north of Cape Town. Strip-mining causes total destruction of naturalecosystems through the removal of vegetation and soil in the area where mining is being undertaken.Namakwa Sands has been rehabilitating mined out areas as the mining front moves forward. Due to thedifficulty of rehabilitating mined out areas as a result of harsh environmental factors, Namakwa Sands hasinitiated various research projects to gain an understanding of the baseline conditions and ecosystemfunction in order to increase plant cover and biodiversity on post-mined areas. This on-going researchand the development of rehabilitation and mining techniques have resulted in the implementation of fourrehabilitation techniques varying in investment of topsoil replacement, seeding and plant translocation.This study assesses the success and effectiveness of these techniques in terms of various vegetationand soil parameters. In addition, those parameters that are considered useful for monitoring are identified.This study indicated that topsoil replacement and plant translocation facilitate the return of similarity,species richness, species diversity and vegetation cover to post-mined areas. The rehabilitation site thathad the greatest amount of biological input (topsoil replacement and plant translocation) appeared to bethe most successful technique in facilitating vegetation recovery similar to reference sites. In comparison,the site that had the least amount of biological input performed the worst and requires adaptivemanagement, e.g. reseeding and / or plant translocation. Namakwa Sands should continue to replacetopsoil in all future rehabilitation efforts and, when possible (e.g. after sufficient winter rain), continue totranslocate species in multi-species clumps.In terms of species selected for translocation, Othonna cylindrica, Ruschia versicolor and Lampranthussuavissimus should be considered for future large-scale translocation projects. Zygophyllum morgsanaappears to be more difficult to re-establish under the current climatic conditions (below average rainfall).The long-term viability of rehabilitated Z. morgsana populations needs to be determined beforeconsidering this species for any future large-scale translocation purposes. No translocated Asparagusspp. individuals survived and should therefore not be considered for any further translocation purposes.The grass Ehrharta calycina, which is dominant in the site seeded, should continue to be considered forfuture seeding.Species and functional diversity appear to be the most limiting factors within all the rehabilitation sites andNamakwa Sands will not be able to meet their long-term objective of small-stock farming if diversity andthe number of palatable species do not increase significantly. Adaptive management should seriously beconsidered in order to speed up this process. Alternatively, an appropriate grazing strategy, which isrelated to the Tetragonia fruticosa dominated vegetation within rehabilitation sites, would need to bedetermined and adopted.More time is needed to ameliorate the rehabilitated soil profiles to the same level as in reference sites,especially with regard to carbon, pH and sodium levels. In order to increase organic matter withinrehabilitation areas, Namakwa Sands should consider creating clumps with cleared vegetation from themining front.Since the long-term rehabilitation goal has not been achieved, Namakwa Sands will need to continue tomonitor plant and soil changes until it has been achieved. The objectives of the current rehabilitationprogramme are limited and Namakwa Sands should develop additional objectives relating to the structureand function of the natural vegetation. This will give a better indication of whether rehabilitation sites areprogressing towards the desired end point and if adaptive management is required. In addition, thecurrent monitoring programme (vegetation survey) implemented at Namakwa Sands could be improvedby increasing the vegetation parameters to be monitored. It is recommended that the following vegetationparameters be monitored as part of the long-term monitoring programme: species composition andsimilarity, species richness, species diversity, vegetation cover, species dominance, vertical structure andfunctional diversity of the vegetation (clumps and inter-clumps). It is also recommended that carbon, pHand sodium of soil profiles be monitored as part of the long-term monitoring programme. Theseparameters should not be seen as exhaustive as this study only considered various vegetationparameters and soil chemistry between rehabilitation and reference sites. The results of other studies onthe fauna, mycorrhiza, insects, etc. should also be taken into consideration and the monitoringparameters expanded accordingly.

[发布机构] Stellenbosch University