An Investigation into the Depth and Rate of Weathering on Gold Tailings Dam Surfaces as Key Information for Long Term AMD Risk Assessment
Report No. 1347/1/05
November 2005



The oxidation of sulphide minerals in gold Fine Residue Deposits (FRDs) of the Witwatersrand and its potential effect on water resources is arguably one of the major regional strategic environmental issues for the gold mining industry and the South African government.

Seepage from these FRDs tends to be alkaline to neutral during the operational phase, but can become acidic in the post operational phase as Acid Mine Drainage (AMD) processes become significant when the piezometric surface subsides.

International research and field observations have shown that an oxidised outer cap forms on FRDs, below which limited oxidation takes place. Enough acidity may be produced in this outer cap to consume all available neutralisation potential within an FRD body, however, as oxidation slows, the risk associated with AMD decreases.

The key question is thus how the oxidised zone will develop over time and how much acidity will be generated relative to available neutralisation potential in the operational phase of an FRD. This will determine the likelihood that a particular FRD will start generating acidic seepage.


A top down approach was used to evaluate practical methods for evaluating the acidification risk associated with Witwatersrand FRD’s. The study was guided by three study objectives, including:
  1. To understand the relationship between the depth and rate of weathering and the potential mass of acidity at gold FRDs through empirical analysis.
  2. Develop procedures to conduct rapid accurate AMD risk assessments of gold FRDs.
  3. Evaluate mitigation or risk reduction measures by making use of the AMD risk assessment procedure and provide industry guidelines.

Characteristics of FRD’s

Acid Mine Drainage (AMD) originates due to the combined effects of flow or transport processes, and geochemical processes. Transport processes include transport of liquids and gases through the mineral matrix of a tailings deposit and geochemical processes include all mineral reactions and interactions between gases, liquids and solids within a deposit.

The parameters relating to each of these processes are determined by the physico-chemical nature of the tailings material in question. It was the purpose of this study to discuss the transport and geochemical processes as they pertain specifically to gold FRDs of the Witwatersrand, as these deposits are relatively well constrained in character and composition.

The nature of FRD varies throughout its lifecycle and depends amongst other factors on the method by which it was constructed, its mineralogy, climate, particle size distributions local topography and geology, age, dimensions and geochemical character. Available data indicate that the tailings materials of the Witwatersrand do not vary greatly in terms of their physico-chemical characteristics. Only two depostitional methods were recorded in the literature, i.e. paddock systems and cycloned tailings deposits.

Witwatersrand FRDs are typically very large deposits. They are typically greater than 1km in length and breadth and up to 60m high. Mineralogy is dominated by quartz (70% and 90%), phylosilicates (mainly sericite) (10% to 30%) and minor amounts of primary minerals (1% to 2%), including pyrite and calcite, which are key minerals in controlling AMD.

Some particle size fractionation may occur by hydraulic separation, thus resulting in differential permeabilities within these deposits. This effect is enhanced in cycloned deposits, where the wall sections are constructed with thickener underflow. The effect is expected to be relatively limited in Witwatersrand tailings, as the particle size distribution tends to fall within a relatively narrow range, when compared to tailings from some other mineral deposits.

The moisture content within an FRD or between different FRDs may be highly variable. Factors determining moisture content include climatic conditions and physical properties of the FRD on a macro and micro scale. Fine-grained layers, which may form capillary breaks are important in FRDs, as they cause local variation in moisture contents and can significantly affect the flow of gases. Existing measurements on the study site indicated that moisture tends to vary between 20 and 40% by volume in the material. Along with porosity, the moisture content plays a major role in determining the diffusion coefficient within a waste body, as it has been determined that the diffusion coefficient through waste is proportional to the air filled porosity. This is largely a result of the fact that the diffusion coefficient of oxygen in water is four orders of magnitude slower than the diffusion coefficient of oxygen through air.


Water and gas transport are the main processes affecting AMD and pollution mobilisation at gold FRDs. These processes are expected to be rate limiting in that flow processes determine the rate at which contaminants are mobilised from the tailings material into the receiving environment, whereas gas transport processes limit the rate at which AMD processes take place within the tailings deposit.

The flow of water determines how much of the contaminant load is mobilised and also determines how fast and in which direction it will move. Additionally, it also determines the moisture content within the tailings body at any one position in time and space and thus influences the oxidation rate by determining the fraction of air-filled pore space available for air to move through.

In fine tailings material, where oxygen diffusion is the major gas transport mechanism related to pyrite oxidation, gas transport rates are essential in determining the rate at which sulphides can be oxidised. This amounts to a direct measurement of the rate at which contaminants (acidity and toxic elements) are generated.
The rate of contaminant generation, along with the rate of contaminant transport will determine the concentration and volume of seepage from the FRD’s and thus the environmental impact.

Water flow processes

The hydraulic conductivity in Witwatersrand gold FRDs has been found to be highly anisotropic (the ratio of horizontal permeability vs. vertical permeability is significantly different), largely due to compaction of the sediment column in a water-saturated environment. Hydraulic conductivity in the horizontal plane is considerably higher than in the vertical plane. The anisotropy is caused mostly by the stratification within the sediment and shrinkage cracks. Anisotropy coefficients of between 5 and 10 are considered normal, but values up to 200 have been reported (Rösner et al. (2001).

During the operational phase, seepage from an FRD in the vertical direction is largely dependant on the permeability of the material at the base of the FRD and the presence of subsurface drains. Where the base material is less permeable in the vertical direction than the tailings material, seepage will move horizontally towards the walls of the FRD. The results of this movement are observed in toe seepage. When tailings material is less permeable than the underlying material, seepage will tend to move downward, through the foundation into the local groundwater system.

Both saturated and unsaturated flow processes occur within an FRD, especially during the operational phase. In the post closure phase, unsaturated flow processes become progressively more dominant as the piezometric surface subsides. Future recharge then occurs under unsaturated conditions. Previous modelling studies found that recharge would occur at a rate of between 1 and 6% of MAP after decommissioning.

Flow under saturated conditions can be represented by a simple form of Darcy’s equation and is applicable below the piezometric surface within an FRD. This zone extends from the edges of the pool along the hydraulic gradient towards the edges of the FRD. If the piezometric surface intersects the wall of the FRD, toe seepage occurs. Morin and Hutt (1997) indicated that flow in the walls of an FRD is likely to be more rapid than near the pool. This is because of the coarser grain sizes and the shorter flow path length. Anisotropy can also be expected to play a part. In the post closure phase, it has been shown that the piezometric surface subsides at approximately 0.5m per year, which is an indication of the saturated hydraulic conductivity. The saturated hydraulic conductivities of Witwatersrand tailings have been determined to be between 10-6 and 10-8 ms-1.

Unsaturated flow also obeys Darcy’s law, however, it is more complex to calculate, because it is dependent on the degree of moisture saturation which typically varies over time. This is because large pores drain more quickly than small pores and water filled pores conduct water more easily than air filled pores. Empirically, unsaturated hydraulic conductivity is a non-linear function of volumetric water content and is also dependent on hydraulic head. It is commonly expressed as a function of soil suction (K(θ), pore water pressure (K(Uw) and/or effective pore radius (K(R). The effect of hystersis has resulted in the preferential use of the moisture retention (soil suction) curve, as the differences between wetting and drying curves are less than for pore water pressure. A large number of empirical models have been developed to make use of these parameters.

Gas transport processes

Gas transport is generally described as occurring either by advective or diffusive flow. In tailings materials it has been shown that advective flow, including barometric pumping (movement of air due to changes in atmospheric pressure), does not contribute significantly to oxygen transport, due to the fine-grained and hence relatively impermeable nature of tailings material. Where preferential pathways exist in the near surface environment, a limited amount of advective flow may occur, however, this is not expected to significantly affect the overall oxygen transport rate within an FRD. Throughout the literature surveyed, diffusion was recognised as the major mechanism for oxygen transport through an FRD, and also serves as the basis upon which oxidation models for fine tailings are based.
Diffusion in tailings is driven by the concentration gradient between the tailings surface and the matrix, caused by weathering reactions that consume oxygen, e.g. the oxidation of pyrite. Oxidation is usually confined to a few metres below the surface as a result of oxygen transport limitations due to diffusion, with oxygen concentrations becoming progressively lower with depth.
Oxygen is the most significant gas in determining reactions in tailings material. The ability to determine the oxygen diffusion rate through an FRD over time will thus enable long-term prediction of AMD potential. Along with knowledge of water flow, calculation and/or modelling of pyrite oxidation rates provides the tools for determining pollutant concentrations within and around a tailings deposit over time.
Gas diffusion is mathematically described by Fick’s law. Previous studies have showed that the limiting parameters controlling pyrite oxidation through diffusion are the diffusion coefficient and the kinetic rate constant. As the diffusion depth increases, oxygen diffusion becomes a more dominant parameter than the inherent pyrite reaction rate in determining oxygen flux.

Investigations into diffusion controlled pyrite oxidation in FRDs internationally has led to the conclusion that determinations that are made on bulk tailings tend to overestimate reaction rates. This is caused by local variation in moisture saturation levels within the tailings profile, due to the presence of more saturated fine-grained layers and less saturated coarser grained layers.

Geochemical processes

Mineral reactions

Witwatersrand gold tailings are constrained within a relatively narrow compositional range. Minerals that contribute to the Acid Potential (AP) are present in relatively low concentrations, however, AP is generally higher than the Neutralisation Potential (NP).
Acid Potential is predominantly ascribed to pyrite, as it is the most dominant sulphide mineral, with other sulphides occurring less frequently. Pyrite may react along different evolutionary paths, depending on the pH and redox conditions in which the reaction takes place.

Neutralisation Potential is ascribed primarily to carbonates of Ca and Mg. Silicates may also contribute significantly to the NP in a tailings deposit, depending on their inherent neutralisation potential and reactivity. Some silicate minerals are capable of neutralising acidity, especially where reaction rates are slow.

The formation of secondary minerals was also shown to be of major importance as the reaction stoichiometries of neutralisation reactions are critically dependent on mineral reactions that occur. Formation of ferrihydrite after pyrite oxidation, for example, results in double the requirement in neutralisation potential to prevent the reaction solution from acidifying.

Mineral reaction kinetics

Mineral reaction kinetics focus mostly on the oxidation of pyrite. The shrinking core model has commonly been used to calculate oxidation rates of pyrite grains over time. An oxidised layer, which forms through reaction of pyrite has been measured in the laboratory and calibrated

empirically to have a secondary diffusion coefficient in the region of approximately 7.8x10-12 m2s-1

Bacteria such as Thiobacillus ferrooxidans play an important part in determining oxidation rates of pyrite, however, in a diffusion controlled environment this has a more limited effect. It has been determined that under conditions of less than approximately 8% oxygen these bacteria do not contribute significantly to pyrite oxidation reactions.


Fieldwork for this research concentrated on supplementing field data that were collected for Anglogold Limited by Pulles, Howard and de Lange in 2002. The approach that was followed, was to generate additional data and to discover relevant field related variables that would be of interest in constructing an oxygen diffusion based model for assessing acidification risk. Parameters that were determined in the field and laboratory included sample descriptions, particle sizes, X-Ray diffraction mineralogical analyses (XRD) and shake flask tests. Additional parameters, such as ABA analyses, moisture contents, particle size distributions, porosities and sulphur concentrations were obtained from the literature.

Observations and data.

Surface features were found to be abundant. Shallow mud cracks, the growth of plants and changes in particle size distributions were observed across the surface of the deposit. The presence of mud cracks and plants is expected to result in some variation in permeability across the surface of the FRD through macropore spaces. In the case of particle size distributions, variation is expected between conductivities in the wall and beach sections of the deposit.

Subsurface features included the presence of a leached zone directly below the surface, a hardpan horizon below this and an oxidation front, where iron stained sediments contacted grey, apparently unoxidised tailings.

The leached horizon is expected to be the result of rainfall leaching of oxidised products. The hardpan horizon is a result of mobilised iron from pyrite oxidation forming secondary minerals. Evidence of variable oxygen concentrations and preferential flow paths was noted by the presence of mottling and accentuated staining respectively.

The phenomenon of the oxidation front was researched and it was found that this transition is related to the presence of oxidised Fe and that it does not necessarily define the point of zero oxygen concentration. Redox conditions were found to be a function of the Fe2+/Fe3+ concentration. The rapid transition, colour transition that is normally observed, along with occasional evidence of pyrite formation in cracks below the transition zone, indicates that the colour transition is likely to represent the position of the transition from oxic to anoxic conditions.

The presence of moist, fine-grained layers was observed. These layers are known to retard oxygen diffusion and were found to occur at different levels within an oxidation profile. Few moist layers were found in the top two meters of the tailings profiles investigated, probably as a result of desiccation during the long preceding dry season.

A number of yellow stained strata were observed in tailings auger cuttings that were collected. These were ascribed to syn-depositional oxidation. Due to rapid oxidation of near surface pyrite grains, such layers form rapidly during any drying cycle experienced across an FRD.

A significant finding that was observed from hydrometer particle size distributions, was that a significant proportion of analyses indicated that vertical variability in particle sizes is at least as significant as horizontal variability. This is important with respect to hydraulic conductivity and moisture retention, two key parameters in modelling impacts from tailings dams.


A conceptual model was developed to represent the generation of acidity from pyrite oxidisation within a typical Witwatersrand FRD. According to the model, wall sections of a tailings dam will undergo rapid acidification, as a result of horizontal influx of oxygen across the profile. The bulk of an FRD will, however, undergo oxidation through vertical ingress of oxygen by diffusion through the tailings profile. A large store of unreacted material, with associated NP occurs below the oxidation zone and potentially provides a means to delay the onset of acidic seepage from the base of the FRD for an extended period.
Use was made of the dual diffusion PYROX model to develop envelopes for oxidation rates of typical Witwatersrand FRD’s. Making use of the PYROX modelling results, a spreadsheet engineering type model was developed to model the likelihood of acidification from an FRD due to AMD process. The purpose of the probabilistic spreadsheet model (Tailings Acidification Prediction or TAP model) that was constructed as part of this project, is to make use of this concept to predict the likely rate of acidification of seepage at the base of an FRD. The model consists of two components, i.e. an oxygen diffusion component to predict trends in pyrite oxidation rates and a neutralisation component to predict the rate of consumption of NP in the FRD.

Acidification of seepage is considered to be a major impact, relative to neutral drainage, as a result primarily of the potential for metal leaching to the receiving environment, but also as a result of acidity being harmful in itself.

Actual impacts will be a function of flow processes, coupled with the rate of contaminant generation within an FRD and the sensitivity of the receiving environment. Receptor impacts, however, will have to be evaluated on a case-by-case basis.

PYROX modelling

The PYROX model developed at the University of Waterloo is based on a double diffusion model developed by Davis and Ritchy (1986) and has been used extensively in international tailings investigations. The model was used to perform sensitivity analyses on relevant variables on the control of pyrite oxidation rates. Parameters were tested within the known compositional range of Witwatersrand tailings, in order to assist in developing a simplified empirical model, which could be used for assessing the likely time span for generation of acidic seepage from the base of a Witwatersrand gold FRD.

Typical to conservative values of sulphur content, particle size, moisture content, porosity, temperature, secondary diffusion coefficient and the thickness of the tailings profile were used as a baseline for determining sensitivities of other parameters.

It was found that within the ranges of these parameters within a Witwatersrand FRD, the absolute values of pyrite oxidation rates vary as expected. It was also, however, found that the rate of change of the oxidation rate over time fell within a relatively narrow range of fitted functions, especially when viewed over the long term. These findings were used to construct the TAP model as a function of static diffusion rates at initial compositions.

The TAP model was constructed from the point of view that the impact due acidification of seepage from the base of an FRD is dependent on the time span over which it is expected to occur. Consideration of the likely lifespan of suggested mitigation measures is a major consideration in this respect, as well as the likelihood of major impacts occurring after an extended period of weathering, when contaminant generation rates are likely to be much lower.

The model makes use of humidity cell secondary oxidation rates, porosity, the degree of moisture saturation, pyrite concentrations, the depth of a tailings profile and the NP:AP ratio of the tailings to predict a time to acidification of seepage.

Sensitivity analyses that were run for the model, indicated that most FRDs are likely to take more than 1000 years before acidic seepage will be generated from their bases. Exceptions were found to occur for FRDs with low sulphide concentrations and for facilities of low height. These findings are ascribed to the deeper penetration of oxygen in a low sulphide scenario, where oxygen consumption occurs more slowly and the relatively large proportion of oxidised tailings relative to unoxidised tailings in a shallow profile.


Modelling and field observations indicate that weathering in the top of the tailings profile in a typical Witwatersrand FRD is rapid for approximately 10 years after decommissioning and that an oxidised horizon of up to 5m deep will form during this time. As discussed above, variations in physico-chemical parameters can modify these figures to form shallower or deeper profiles. Modelling results show that pyrite oxidation is only likely to consume available NP after approximately 1000 years in tailings of typical composition.

Considering the particle size distributions of tailings, it is likely that engineered surface covers will not significantly reduce oxidation rates on an FRD, except in the short period after decommissioning. The value of a cover may be realised in maintaining higher moisture levels and in reducing recharge through an FRD, however.


Field measurements

Field measurements need to concentrate on providing calibration data for the TAP model. Relevant measurements, which will assist in this regard include:
Sampling should be conducted such that oxidation reactions are prevented, through exclusion of air and if possible the inclusion of inert gases in samples.

Potential mitigation options

A number of potentially promising mitigation options were identified on the basis of the conceptual model. The various options need to be subjected to technical and financial feasibility assessments before they can be considered as guidelines, however. A list of identified measures is included below:
The suggested measures need to be considered in the context of available opportunities throughout the lifecycle of an FRD, for instance, a liner must be installed prior to construction for maximum efficiency, whereas a cover can be constructed only after decommissioning of the facility.


The research has shown that the depth and rate of weathering in Witwatersrand FRDs is highly dependent on physico-chemical properties, however, it was shown that facilities of typical composition and morphology should take more than 1000 years, before acidic seepage is released to the receiving environment below the base of the facility.

The objectives for the study were partially achieved, in so far as it was found that accurate modelling of oxidation rates is relatively complex and data intensive. Providing a general model is therefore difficult due to the various site-specific variations in parameters affecting AP and NP realization on an FRD. The recommended mitigation measures need to be subjected to technical and financial feasibility assessments before they can be accepted as best practice measures.

It is thus recommended that further work should be conducted in evaluating these mitigation measures. Other recommendations include: