THE EFFECT OF THE CHEMICAL PROPERTIES OF TAILINGS AND WATER APPLICATION ON THE ESTABLISHMENT OF A VEGETATIVE COVER ON GOLD TAILINGS DAMS
Report No 899/1/04
The core economy of South Africa has developed around the mining industry but the finite quantity of an ore body places a limit on the life of a mine. As the viability of mines begins to decline, mine closure becomes more and more a reality, as do the liabilities associated with mine closure. One of the liabilities is the rehabilitation of areas disturbed by mining or associated activities. Regulatory standards and guidelines for the rehabilitation of tailings dams are becoming increasingly prescriptive as regards procedural and technical requirements. Sustainability is a closure requirement, however, no guidelines for the assessment of the sustainability and/or rehabilitation success are provided by mining legislation in South Africa.
Although the successful rehabilitation of gold tailings dams goes back as far as the early 1960's there is still uncertainty with regards to the sustainability of the vegetation cover established using different rehabilitation methods. Uncertainty with regard to the sustainability of re-vegetated systems stems from:
Objectives of the study
The primary objective of this study was:
A literature survey had been conducted with regard to- the properties of gold tailings, the environmental impacts associated with tailings dams, rehabilitation methods, methods to evaluate rehabilitation projects, sustainability of vegetated areas and regulations regarding closure.
The following rehabilitation methods most commonly used in South Africa for re-vegetating tailings dams, were selected for evaluation:
Plots on the south-facing slope of the old President Steyn number 6 tailings dam at Welkom, were vegetated in accordance with the method applicable to a particular rehabilitation method. For each of the methods the following was recorded:
The following were monitored for each method over the period November 1998 to March 2002:
To assess the "tailings quality" and the "vegetation quality" obtained in this study it was decided to compare them with the "tailings quality" and "vegetation quality" of tailings dams rehabilitated in the past, using different methods, which currently sustain a good vegetation cover.
The following tailings dams were selected representing the different methods of vegetation establishment on gold tailings dams:
Discussion and conclusions
Criteria used to evaluate the sustainability of a vegetation cover include, diversity of species, dynamic stability, and productivity. The productivity of plants depends largely on climate and the fertility of the growing medium. Tailings fertility is related to:
The dynamic nature of re-vegetated systems requires data not only on the initial conditions but also over time to monitor the system's evolution.Tailings quality
The amelioration of tailings, in accordance to the different rehabilitation methods, has changed the following chemical attributes for the 0-30cm layer significantly, pH, K, Mg, Ca, Al, CEC, and P. Liming had the largest effect on the chemical attributes of the tailings as it caused an increase in pH, Ca, and Mg. The increase in pH in turn caused Al to precipitate and the CEC to increase. The measured active and reserve acidity (H+ + Al3+) before amelioration indicate that 3 tons lime per ha was needed to neutralise it. But the results obtained indicate that almost 13 tons of lime per hectare was needed to neutralise the active and reserve acidity. This large discrepancy can be explained by the presence of iron sulphate minerals in the tailings that also contribute to reserve acidity. The standard agricultural laboratory tests to determine reserve acidity must be modified for material containing iron sulphate minerals to be more applicable for rehabilitation purposes.
After amelioration, a comparison between the leaching methods indicates that none of the chemical attributes differs significantly, while a comparison between the chemical amelioration methods indicate that only the EC and K differ significantly. When the leaching methods are compared with the chemical amelioration methods the chemical attributes that differ significantly were pH, Al, and CEC. However, all methods had after amelioration produced a growing medium that is suitable to sustain a vegetation cover. Six weeks after amelioration none of the chemical attributes within the 30-60 cm layer had changed significantly for any of the rehabilitation methods.
The application of compost had increased the water holding capacity of the 0-30 cm layer significantly, and had an additional effect on the CEC. The compost had however, no effect on the EC of the tailings, which is in contradiction with laboratory studies. For this study it was found that the EC of the saturated paste was not a good indicator of the salt content of the tailings. This could be explained by the formation of the ion pair CaSO40, which is important in solutions containing more than 1000 mg/l sulphate, (on average more than 2000 mg/l sulphate for this specific tailings) as more than half the dissolved calcium could be present in the form of the CaSO40 ion pair. This neutral ion pair had no influence on the electrical conductivity of the saturated paste, and the net result of the ion pair is that electrical conductivity measurements underestimate the ion concentration and total dissolved solids.
The phosphorus status of the tailings after amelioration is on the high side if compared against the phosphorus level of the adjacent natural veld. Potassium did not increase in relation to applied fertilisers because it either was leached from the tailings or become fixed through the precipitation of jarosite.
The change in chemical attributes in the period after amelioration to the end of the study period was very similar between the different rehabilitation methods although the magnitude of change differed. At the end of the study period the only chemical attribute for the 0-30 cm layer that differed significantly between the leaching methods was Ca. Between the chemical amelioration methods the drip irrigation method differs in EC, Ca, and Cl from the other irrigation methods. This difference can be explained in terms of the amount of possible leaching that could take place, between the different irrigation systems. The chemical attributes that differ at the end of the study period between the leaching and chemical amelioration methods were mainly EC, Ca, and Cl. The chemical amelioration methods had at the end of the study period, on average, a higher salt content compared to the leaching methods.
The chemical constraints that the tailings of the experimental site imposed on vegetation establishment were not very severe because the elementary amelioration method that was neither a leaching method nor a chemical amelioration method was just as successful for establishing vegetation and there were no extreme differences in chemical attributes relative to the other rehabilitation methods.
A comparison between the chemical attributes of tailings dams rehabilitated in the past with that of this study indicate that the chemical conditions of the experimental site had at present a better overall fertility. The only negative elements were Na and Cl, which were higher for the experimental site. If the fertility can be maintained then all the rehabilitation methods were successful in turning the tailings into a suitable growing medium to sustain a good vegetation cover. However, it is foreseen that the tailings of the chemical amelioration plots will become more saline in future although the pH will remain more or less neutral. On the other hand the tailings of the leaching methods will become more acidic but the salinity would be relatively constant.
Cone penetration studies conducted during the study period indicate that plant roots can exploit the upper 60 cm of the tailings once the chemical constraints that limit root development were removed.Vegetation quality
The initial seeding involved the sowing of a mixture of 16 species for the leaching methods and the sowing of a mixture of 8 species for the chemical amelioration methods (five species were common to both methods). At the end of the study period 8 of the sown species remained of a total number of 39 species that have been identified on the site at the end of the study period. However, the three dominant plant species, in order of dominance, at the end of the study period were Dactylis glomerata, Medicago sativa, and Eragrostis curvula for the leaching methods and Medicago sativa, Eragrostis curvula and Dactylis glomerata for the chemical amelioration methods.
The results obtained from tailings dams rehabilitated in the past reveals that in the early stages of succession (up to 5 years after re-vegetation) the plant community is dominated by Cloris gayana, at a later stage Eragrostis curvula become the dominant species, to be follow by a Hyparrhenia hirta dominated community which can be regarded as the secondary succession climax on tailings dams. In this study Chloris gayana dominate only in the first year after re-vegetation. The fast disappearance of Chloris gayana was attributed to the fact that the experimental site was located on the south facing slopes of the tailings dam (Chloris gayana still dominated the north facing perimeter wall of the experimental plots at the end of the study period). Although Eragrostis curvula was not the dominant specie at the end of the study period, this specie had steadily increased over the study period to be the subdominant specie for all chemical ameliorated methods. At the end of the study period the Hyparrhenia hirta occurring on the experimental plots were those established by hand planting.
It is anticipated that in the near future the vegetation will have to adapt to changes in the chemical conditions of the tailings, which is expected to become more saline for the chemical ameliorated plots while the leaching plots will become more acidic. The neutral pH and higher salinity conditions for the chemical amelioration plots is more compatible with chemical conditions occurring in soils of dry climates and therefore natural colonising of the plots by grass species from the surrounding veld is foreseen to be favoured towards the chemically amelioration plots.
At this stage (three years after rehabilitation) no conclusive statement can be made regarding the sustainability of the vegetation cover for any of the rehabilitation methods. However, results from tailings dams rehabilitated according to the different methods in the past indicate that stable plant communities can be obtained. The leaching method had the longest track record with stable Hyperhenia hirta communities in excess of 40 years old on certain tailings dams.Water needed for the different rehabilitation methods
Water is used in rehabilitation methods to establish vegetation and in the leaching methods also for reclamation (leaching salts and acids out of the root zone). In this study the water inputs in terms of rainfall and water applied through irrigation were recorded. The exact amounts of water that leached through the root zone could only be estimated roughly.
Where a micro-jet irrigation system was used, about twice as much water was used in the leaching method compared to the chemical amelioration method. The amount of water applied for a chemical amelioration method, under sprinkler irrigation, was about the same as that of a leaching method under a micro-jet irrigation system. The least amount of water was applied under a drip irrigation system, i.e. three times less than for a leaching method under micro-jet irrigation.
The experimental site was unfortunately on a tailings dam where the conditions were favourable for the establishment of vegetation. Furthermore the experimental site was on a south-facing slope, which is wetter than a north, west, or east, facing slope. The beneficial role of water in terms of vegetation establishment or as a reclamation agent was therefore not highlighted in this study.
Further studies and recommendations
During this study some of the problems that become apparent were:
The problem experienced over the years is that once vegetation has been established the mining companies viewed it as a walk away solution and see little value in maintenance and monitoring as part of the rehabilitation process. Hopefully this attitude will change, as the application for closure of a tailings dam has to be substantiated with data to prove that the rehabilitation measures taken are effective.
Future research needs to address biological productivity/diversity, and tenure systems. Tenure systems (land management) should be evaluated and tested against the five pillars of sustainability, namely, productivity/services, security/resilience/risk, protection, viability, and acceptability.
Recommendations with regard to future research are: