Elsevier

Journal of Geochemical Exploration

Volume 134, November 2013, Pages 111-119
Journal of Geochemical Exploration

The impact of post gold mining on mercury pollution in the West Rand region, Gauteng, South Africa

https://doi.org/10.1016/j.gexplo.2013.08.010Get rights and content

Highlights

  • Hg pollution was studied within an old closed gold mine area and its surroundings.

  • Hg2+ and CH3Hg+ were analyzed in water and soil samples.

  • Hg concentrations were in general far beyond reported threshold levels.

  • Hg released in the area has a significant impact at the neighboring game reserve.

  • Distribution and methylation of Hg were explained using different models.

Abstract

The impact of historical use of mercury (Hg) for the extraction of gold was assessed in watersheds from an abandoned mine in Randfonetin, a town located at 45 km west of Johannesburg (South Africa), which has a long history of gold mining, and its surroundings. The majority of analyzed waters at the site were acidic with pH ranging between 2.9 and 5.0. Elevated total Hg concentrations (HgTOT) were measured in waters (up to 223 ng Hg L 1; mean: 52 ng Hg L 1) as well as in surface sediments (up to 2580 μg Hg kg 1; mean: 970 μg Hg kg 1) from the old mine site. The Krugersdorp Game Reserve watersheds which are directly downstream from the mine drainage appear to become impacted by the pollution from historic gold mines. The high HgTOT measured in a borehole (223 ng Hg L 1) suggests a groundwater contamination in the area.

Hg transported downstream from the mine site is also susceptible to methylation. Relatively high CH3Hg+ (MHg) values were measured in waters (up to 2 ng MHg L 1) and surface sediments (up to 10 μg MHg kg 1) with significant enrichment in bulk sediments. The methylation of Hg seems to be controlled by factors such as the inorganic Hg content, redox potential and the organic matter content.

Sequential extraction procedures together with geochemical modeling have demonstrated the predominance of nonsoluble Hg species (91–95%) in studied soils and the existence of a number of Hg species in watersheds. Diverse mechanisms were proposed to explain both vertical and horizontal transport of Hg at the site.

Introduction

Soon after the discovery of gold in South Africa, in the late nineteenth century, number of mines have developed in the West Rand area located about 50 km west of Johannesburg. The gold-bearing reefs contain different types of minerals such as native gold, uranium oxides and sulfide minerals with pyrite being the most abundant (Naicker et al., 2003). Most of the mines have produced uranium together with gold. Some of these mines have closed down whereas, others have changed owners and, currently, diverse mining operations as well as the reclamation of old sand and slimes dams are taking place in the region.

It was reported, a decade ago, that acidic water began decanting out of a disused mine on Randfontein Estates in the West Rand, at about 42 km south-west of Johannesburg (Lang, 2007) and concern has grown in recent years that acidic water flooding abandoned mine tunnels under Johannesburg will soon spill over into the water table of the surrounding Witwatersrand basin, threatening the health of millions of residents (Feuilherade, 2010). Metals associated with mines located in this area have been reported to travel hundreds of kilometers and impact downstream ecosystems (Tutu, 2012).

It is known that the gold ores in the Witwatersrand basin contain substantial amount of Hg (Erasmus et al., 1982) and also that gold extraction in the mines has been initially processed using Hg amalgamation (McCarthy and Venter, 2006). Unfortunately, information on the ecological impact of the gold exploitation in terms of Hg pollution in the area is practically nonexistent.

Speciation analysis of Hg is also known to contribute in understanding the Hg behavior in a given environmental system. That is why the present study focused on the assessment of the long-term post-operative legacy of gold mining in terms of Hg pollution and fate in the Randfontein area. This study was also an initial step toward determining Hg sources and distribution to this environmental compartment.

Section snippets

Site description

The speciation and fate of Hg were investigated during the dry season in decanting shafts and boreholes, and also in the receiving creek and wetlands. The creek drains within an old gold mining area located in Randfontein near Krugersdorp and receives both acid mine drainage (AMD) discharged from subsurface mine tunnels and runoff from surrounding surface mine waste piles (slimes dam). This creek also drains to the Krugersdorp Game Reserve wetlands. The game reserve is separated from the mining

Materials and methods

All vessels used for sampling and sample preparation were acid cleaned following the cleaning protocol, adapted after Monperrus et al. (2005). Waters were collected as duplicate samples into acid-washed and conditioned borisilicate bottles with PTFE-lined caps, according to commonly accepted sampling procedures (e.g. Quevauviller, 2001, Stoichev et al., 2006, USEPA, 2007). The recommended “clean hands–dirty hands” procedure was used during sampling in order to discriminate the contamination

Mercury in waters

All analyzed waters from the Randfontein site were acidic with pH ranging between 2.9 and 5.0 (Table 1), except for samples W7 to W10 collected from the game reserve which showed higher pH values (i.e. between 6.9 and 7.8).

It has to be mentioned that at the boundary between the mining site and the game reserve, the pH is increased by the company that owns the mine in order to precipitate soluble metals and therefore, minimize the contamination in the water draining to the reserve.

HgTOT measured

Conclusion

The present study has demonstrated that Hg released from closed mining operation in the Randfontein area has a significant impact at the neighboring game reserve. The Krugersdorp Game Reserve watersheds are directly downstream from the mine drainage and appear to become impacted by the pollution from historic gold mines.

On the basis of collected data, it can be inferred that the primary source of Hg to the mine creek is the weathering of mine waste materials from the old slimes dam located few

Acknowledgments

The authors thank CNRS-LCABIE-IPREM (Pau, France) for allowing to carry out part of this study in their laboratory and the Technology and Human Resources Programme (THRIP, South Africa) for the financial support.

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