The chemical characteristics of acid mine drainage with particular reference to sources, distribution and remediation: The Witwatersrand Basin, South Africa as a case study
Introduction
The metropolis of Johannesburg, South Africa’s largest city and financial centre, owes its origin to the Au-bearing rocks of the Witwatersrand Basin (Fig. 1). The basin – roughly oval in shape, approximately 350 km in a north east to south west direction and stretching over 150 km across – hosts seven major goldfields.
The Au-bearing conglomerate mined in the Witwatersrand Basin has a typical mineralogical composition of (Feather and Koen, 1975):
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Quartz (70–90%);
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Phyllosilicates (10–30%), consisting mainly of sericite, KAl2(AlSi3O10)(OH)2;
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Accessory and minor minerals (1–5%). More than 70 minor minerals have been identified in the reefs, including base metal sulfides and U-bearing minerals, but the most common is pyrite (FeS2) with an abundance of 3–5%.
Gold mining started in the basin 1886 with Au being extracted from coarsely crushed ore using Hg amalgam. The tailings were deposited in large dumps, known as sand dumps. As mining became deeper and unoxidised, pyritic ore was encountered, the process became inefficient, and was replaced by cyanide extraction in 1915. This required finer milling, and the tailings were piped to disposal sites, known as slimes dumps. Cyanide extraction is highly selective for Au and Ag, and other minor minerals are unaffected by the process and remain in the tailings. Efficient and safe cyanide extraction requires a high pH (generally around 10), and this is obtained by the addition of lime to the cyanide solution. The tailings produced are therefore generally alkaline.
Secondary mining operations (reworking of old dumps containing Au of economic value) in this region have left a legacy of pollution in the form of acid mine drainage (AMD). The oxidation of pyrite in the tailings can be described by the reaction (Singer and Stumm, 1970):The buffering capacity of the tailings is usually insufficient to neutralise the acid, and acidification occurs. Oxidation reactions involving other minor minerals release dissolved U, As, Cu, Ni, Pb, Co and Zn. The products of these reactions are transported downwards by percolating acidified water into the underlying aquifers (Blowes et al., 1998). Upon discharge to nearby surface water bodies, Fe(II) is oxidised to Fe(III) (Reaction (B)). The stability of Fe(III) depends on the pH. At pH lower than 3.5, the Fe(III) continues in solution and acts as another oxidising agent of pyrite (Reaction (C)). However, at pH greater than 3.5, the Fe(III) precipitates as a Fe(OH)3 (Reaction (D)) (Singer and Stumm, 1970, Stumm and Morgan, 1996):The precipitation of Fe(OH)3 is acid generating and buffers the pH (between 2.5 and 3.5) of the AMD (Espana et al., 2005). This process has two opposing consequences: (1) the acidity increases the mobility and toxicity of metals, and (2) the Fe(OH)3 precipitation produces co-precipitation and adsorption of metals in solution. The latter is considered the more prevalent process (Webster et al., 1994, McGregor et al., 1998).
Witman and Förstner (1976) first drew attention to the enrichment of contaminant metals in streams draining the Central Rand Goldfield and Marsden (1986) reported that there were elevated concentrations of cyanide and trace metals such as Cd, Co, Cu, Fe, Ni, Mn, Pb, Ra, U and Zn in surface waters in the vicinity of Au mine tailings dumps. A number of studies have since been carried out to assess the pollution caused by previous mining operations on the Witwatersrand. Jones et al. (1988) noted a steady rise in the metal loads in rivers draining the Witwatersrand, much of which they attributed to mining activities. A study of groundwater seeping from disused tailings dumps found that groundwater with low pH and elevated metal concentrations was emerging at surface and entering streams on the Central Rand (Naicker et al., 2003). Even decades after decommissioning and clearing, significant loads of salts, metals and in some cases radionuclides remain beneath reclaimed tailings dumps (Rosner and van Schalkwyk, 2000, Rosner et al., 2001), and while most of those metals are not present in labile forms, they would in the long run be leached into the groundwater. Considerable work has also been done on the contribution of mining to U pollution in the region (Winde et al., 2004, Winde and Sandham, 2004, Tutu, 2006).
A study by Davidson (2003) showed that water quality in the Witwatersrand drainage network deteriorates in the proximity of mine tailings and improves in the distal regions. The general dramatic downstream improvement in water quality has been attributed to the presence of large tracts of Malmani dolomite (CaMg(CO3)2) traversed by the Klipspruit and Natalspruit and the existence of extensive wetlands in the area. The wetlands serve as a sink for pollution where polluting metals are trapped in sediments and peatlands (McCarthy and Venter, 2006). Roychoudhury and Starke (2006) assessed the impact of mining on sediment quality in the Blesbokspruit in the East Rand area. They found that most trace metals in the sediments were preferentially associated with the carbonate and Fe–Mn oxide fractions, the former originating largely from the dolomite system.
Most studies have relied on widely scattered sampling sites, and as such do not reveal the full impact of the environmental problem that has arisen from past mining activities. For this reason, in this study sampling was concentrated within the immediate mining area in the Central Rand Goldfield, so as to obtain a more comprehensive insight into the local effects of mining activities on water quality. Moreover, the seasonality of pollution load was also investigated in the study.
Section snippets
Study area and distribution of mining-related pollution in the Witwatersrand Goldfields
The geology of the Central Rand has been well documented (Mellor, 1917, Pretorius, 1964). Over the century or so of active mining, some 1.3 billion tonnes of ore have been extracted (Handley, 2004), and the tailings deposited in dumps along the outcrop zone (Fig. 2).
The climate of the region is temperate, with a short mild winter and a warm to hot summer. The rainfall occurs predominantly in summer (October–March) as intense thunderstorms accounting for most of the rainfall. Annual precipitation
Sampling procedure
Water samples were collected in April 2002 and August 2003, at the ends of the wet and dry seasons, respectively. The water samples were collected in polypropylene (PP) bottles according to commonly accepted sampling procedures (Mugo and Orians, 1993, Hermond and Fechner-Levy, 2000; http://water.usgs.gov/owq/FieldManual/chapter1). Sampling comprised the collection of three samples at each site. The PP bottles were washed with HNO3 for 12 h before sampling.
Surface water samples were taken from
Spatial distribution of pollution
The study involved the analysis of water in the immediate environments of past mining activity, as discussed in Sections 1 and 3.1. Mapping of pollution distribution in the drainage network and associated water bodies was done based on the analytical results for surface water samples collected at the end of the wet season (April 2002, see Table 1a) and is presented below.
Conclusions
The purpose of this study has been to characterise the chemistry of water affected by AMD in the mining areas of Johannesburg, to examine the spatial distribution of polluted water, and to examine its seasonality. Surface water quality was found to be lowest in the immediate vicinity of dumps and dump retreatment operations, but water quality was found to improve downstream of these areas. This improvement is unrelated to the presence of carbonate rocks. Lake water in the area is relatively
Acknowledgement
We thank the National Research Foundation (NRF) for financial assistance.
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