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Revegetation of Heavy Metal Contaminated Mine Dumps Using Locally Serpentine-Adapted Grassland Species

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Revegetation of Heavy Metal Contaminated Mine Dumps Using Locally Serpentine-Adapted Grassland Species View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector South African Journal of Botany 2004, 70(5): 784–789 Copyright © NISC Pty Ltd Printed in South Africa — All rights reserved SOUTH AFRICAN JOURNAL OF BOTANY ISSN 0254–6299 Revegetation of heavy metal contaminated mine dumps using locally serpentine-adapted grassland species T Morgenthal, M Maboeta and L van Rensburg* School for Environmental Sciences and Development, North-West University (Potchefstroom Campus), Private Bag X6001, Potchefstroom 2520, South Africa * Corresponding author, e-mail: [email protected] Received 8 September 2003, accepted in revised form 3 June 2004 Experimental studies with conventional rehabilitation and natural soil. Typical of serpentine soils, the methods have proven unsuccessful to rehabilitate exchangeable magnesium is abnormally high (9 003.67 chrysotile tailings. This is largely due to the ± 1 375.81mg kg–1) and calcium is nearly devoid unfavourable macro-element ratios and elevated heavy (5.4–8.5mg kg–1). The nutrient status of the topsoil on metal concentrations of chrysotile tailings and associ- the rehabilitation trials compared favourably with the ated natural soil. This paper investigates the rehabilita- natural soil samples and could sustain a locally adapt- tion of ameliorated chrysotile tailings using locally ed grass cover. The experimental site was initially adapted ecotypes and available soil. The experimental colonised by Melinis repens and other pioneer grasses. plots were established in 1995 and were evaluated dur- The successional trend portrayed by the DCA predicts ing 1997, 2002 and 2003 by estimating the crown cover that with proper amelioration and topsoil cover, vegeta- in a 0.5m x 0.1m quadrat. This study reports on the tion would develop towards grasslands consisting pre- change in vegetation composition during the eight-year dominantly of the perennial grasses Hyperthelia disso- period as well as the nutrient status of the topsoil of luta, Cymbopogon excavatus, Themeda triandra and rehabilitated areas in comparison to chrysotile tailings Heteropogon contortus. Introduction Chrysotile is a fibrous mineral consisting of thousands of var. rogersiana, have been shown to be nickel hyper-accu- parallel bundles of highly stretchable fibrils. The fibrous mulators and are indicators of serpentine conditions in nature of chrysotile and asbestos in general makes it a South Africa (Cambell-Young and Balkwill 2000). health hazard (asbestosis). All asbestos waste generated The properties of the soils on serpentine resulted in the from mining is therefore required to be properly covered and development of edaphically adapted species capable of revegetated to ensure no further exposure to people in sur- growing on these infertile soils (Specht et al. 2001). The veg- rounding communities. Soil derived from serpentineferous etation composition growing on ultramafic areas is therefore rock has, due to its mineral composition, characteristically frequently characteristically different from the surrounding low calcium content but excessively high magnesium levels vegetation (Robinson et al. 1997, Specht et al. 2001) and (Brooks 1987). Heavy metal phyto-toxicity associated with must be adapted to the adverse soil chemical limitations vegetation on serpentine derived soils has been widely (Kruckeberg 1954, Kruckeberg 1967). Chrysotile waste has reported (Brooks 1987, Mengel and Kirkby 1987). specific soil physical limitations and cements into an imper- Particularly elevated concentrations of nickel, chromium and meable material and is prone to crust formation (Briers et al. cobalt are known to occur (Proctor and Woodell 1975). The 1988, Moore and Zimmermann 1977). Van Rensburg and interaction with other heavy metals and microelements such Pistorius (1998) have dealt with the soil chemical limitation as iron (Proctor and Woodell 1975), the influence of soil pH associated with revegetating chrysotile at Msauli in detail. on heavy metal solubility (Williamson et al. 1982) and organ- Until recently Chrysotile asbestos has been extensively ic matter content (Briers et al. 1988) makes nickel availabil- mined in southern Africa. Msauli Mine was operational until ity a complex process. Proctor and McDowan (1976) found 2001 and asbestos is still being mined in Zimbabwe. that high magnesium concentrations could ameliorate the Associated with these mines are large spoil dumps consist- toxic effect of high nickel availability. Local studies suggest ing of serpentine waste material, since chrysotile invariably that plants growing on serpentine soils have the capacity to occurs as part of serpentine rock (Windisch 1978). Briers et accumulate high levels of nickel (Morrey et al. 1989). al. (1988) and Van Wyk (1994) have noted that, although the Specifically two species, Berkeya coddii and B. rehmannii mines are situated in a high rainfall sub-tropical climate, nat- South African Journal of Botany 2004, 70: 784–789 785 ural establishment on these tailings is totally non-existent kept to a minimum. Harvested seed from the same area was even after 35 years of abandonment. Rehabilitation of the used as seed supplement. No commercial seeds were used tailings and developing a sustainable growth medium to for revegetation. The topsoil was scraped from sections of allow a regenerating vegetation cover is of major concern. semi-natural grassland. The topsoil layer was fertilised with –1 –1 –1 Due to the problems associated with revegetating chrysotile a 100kg ha CaNO3, 50kg ha KNO3, 175kg ha 2:3:2 and a number of experimental studies to ameliorate and estab- 4 000kg ha–1 well matured kraal manure, which was mixed lish self-sustaining vegetation cover have been attempted. into the soil. As a precautionary measure, to improve the Initial trials conducted by Briers et al. (1988) and Van Wyk alkalinity of the soil, 300kg ha–1 flower of sulphur was added. (1994) before 1990 already indicated that without proper The vegetation was sampled during the growing season in amelioration and the covering of a suitable topsoil layer no 1997 and again in 2002 and 2003. The vegetation cover was vegetation establishment would occur. Van Wyk (1994) also estimated using a 0.5m x 1m quadrat. Biomass was also cut concluded that the addition of organic material is also critical in 2002 and sorted into grass and forb biomass, dried at to bind excessive micro-elements. 50°C for 48h and weighed. The aim of this paper was to investigate the establishment Ammonium acetate (Thomas 1982) and 1:2 water extrac- of vegetation on chemically treated chrysotile tailings reveg- tion analyses (Sonnevelt and Van den Ende 1971, Rhoads etated with seed and a soil seed bank, which were harvest- 1996) were conducted on samples collected from three ed from the surrounding area. The objective of the study was chrysotile, three natural soil samples and three topsoil sam- to determine whether these seeds harvested from the sur- ples from the experimental plots. Soil analyses were con- rounding topsoil are successfully able to establish a viable ducted at the soil laboratory of the Institute of Soil Climate vegetation cover. A positive result will be used in the devel- and Water. The natural soil samples were taken on three dif- opment of a viable method for revegetating chrysotile tailings. ferent localities on the mine premises from soils of the Glenrosa soil form (Soil classification work group 1991). Materials and Methods Changes in the vegetation composition were compared by a Detrended Correspondence Analysis (DCA) using Study area CANOCO for Windows (Ter Braak and Šmilauer 1998). Msauli Chrysotile Mine is situated in the Mpumalanga Results and Discussion Province, South Africa near the Swaziland border and geo- logically is part of the Onverwacht Group (Viljoen and Nutrient status Reimold 1999). Generally chrysotile in South Africa is either from emplaced ultra-basic intrusions or from the thermal The results from the soil chemical analyses are summarised metamorphism by diabase sills of chemically suitable zones. in Table 1. Typical of serpentine soils associated with The chrysotile of the Barberton area occurs in serpentinites chrysotile and ultramafic geology, the exchangeable magne- associated with emplaced ultra-basic intrusive (Windisch sium is abnormally high (Chrysotile: 9 003.67 ± 1 375.81mg 1978). The tailing dumps mostly consist of serpentine kg–1; Soil: 904.00 ± 73.08mg kg–1) and calcium is deficient chrysotile, which according to Meyer (1980) is the same as (5.4–8.5mg kg–1). The exchangeable magnesium of the top- the mother rock since the separation of the fibres from the soil and especially the natural soil samples decreased due rock is by mechanical method. X-ray analysis also confirmed to leaching and weathering (Table 1). In contrast, analysis of that 80% of the tailings consist of chrysotile. seven non-serpentine soils, collected from a variety of soil The surrounding natural grassland had a nearly closed types on the Mpumalanga Highveld (Ermelo, Witbank, cover and the general composition was similarly found on Bethal areas), varying in clay contents between 6.5–55%, shallow rocky soils. A vegetation survey of the natural areas had ammonium acetate extractable calcium values of of the mine property conducted during 2003 obtained 344 300–800mg kg–1 and ammonium acetate extractable Mg val- species and concluded that the grasslands is similar to that ues of 32.5–325.5mg kg–1. In all samples the exchangeable described by Stalmans et al. (1999) on higher hills within the magnesium was less than 50% of the exchangeable calci- Komati River valley in the Songimvelo Game Reserve. A um. The calcium:magnesium ratio is therefore totally the number of serpentine endemic species have been reported opposite when considering serpentine soils and chrysotile for the grasslands associated with serpentine soils in the waste to ‘normal’ soils. Plant species growing on serpentine Barberton area (Cambell-Young and Balkwill 2000). substrates such as at Msauli must be physiologically adapt- ed to cope with the Ca:Mg imbalances and calcium defi- Experimental procedures ciency.
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