An investigation into the problems associated with revegetating chrysotile tailings

Abstract

The present investigation was initiated because of the limited success achieved in the past at attempts to revegetate chrysotile tailings with their associated inherent environmental risk factors. The current set of experiments and surveys, as with several previous studies, were conducted at Msauli chrysotile mine, Mpumalanga Province, South Africa. All soil and leaf samples were collected, and the total percentage canopy cover, species frequency, percentage cover per species, percentage alive biomass, and phenology per species quantified. This was done in nine randomly selected 1m2 areas, on both the upper (steeper) and lower (less steep) slopes of previously revegetated slopes, with comparable samples from areas within the native vegetation serving as controis. Nine composite (consisting on average of three 500g samples) replicate soil samples per slope gradient, as well as leaf samples (ca. 5g of dried above ground leaf material) representative of the species composition, were sampled in the same plots where the plant surveys were conducted. The same procedure was followed, both in areas where the tailings were removed, or not prior to reyegetation, in areas that can qualitatively be described as: well revegetated, poorly revegetated and areas with no plant cover, on both a north and south facing slopes, respectively. Although having been independently assayed, the soil and leaf analysis data per slope gradient were pooled as no statistically significant differences could be detected. Three data sets of the same number of composite replicate samples (comparable native vegetation served as control) and descriptive vegetation data were also collected on the top level surface of two chrysotile tailings dumps that had prior to revegetation been covered with 300mm and 500mm of topsoil, respectively. As with the above, the soil and leaf analysis data were once again pooled because of the absence of any statistically significant differences. The die-back phenomenon observed at Msauli can probably primarily be ascribed to an inadequate supply of Ca, of which the low Ca concentration per se is both responsible for the imbalances in terms of N metabolism (N02- accumulation) and limited root development. Also, the low Ca concentrations encouraged the uptake of Fe to potentially toxic levels, as was reflected by the high foliar Fe concentrations of the plants on the revegetated areas. The potentially toxic B concentrations in both the soil and plants sampled on the revegetated areas necessitates an increase in the Ca concentrations, which will also limit the stress response of anthocyan production. In this regard it is interesting to note that the incorrect use of CaSO4 to lower the high pH values assoduted with chrisotile tailings probably suppressed the uptake of K, a situation worsened by the high Mg concentrations. The plant symptomatic response of general chlorosis, as well as the rapid development of brown necrotic lesions along the leaf margins, are probably the consequence of MgCl absorption. The generally low micronutrient and heavy metal concentrations typifying the revegetated areas can probably be ascribed to the low solubility of their hydroxides at the alkaline pH values of these areas. Since no statistically significant differences either in terms of nutritional status or vegetative variables monitored could be observed in the areas where top soil layers of different thickness were used, this would not seem to be one of the primary factors determining revegetation success. Because of the poor correlation found to exist in terms of species composition between the native vegetation and that typifying the revegetated areas it would seem futile to use any other seed in revegetation attempts than that already adapted to the serpentine site specific conditions.