The impact of acid mine drainage on the ecology of the Cradle of Humankind and Krugersdorp Game Reserve on the West Rand

The Witwatersrand Supergroup is the world’s richest goldbearing geological deposit. This approximately 2.8 milliard years old unit is overlain by the 2.71–2.67 milliard years Ventersdorp Supergroup which is in turn overlain by the 2.67–2.46 milliard years Chuniespoort Group of the Transvaal Supergroup. The dolomite-rich Chuniespoort Group also contains a large aquifer which supplies water to farms in southern Gauteng, southeastern North West Province and northern Free State. The megalitres of water in the dolomite flooding the adjacent gold mines in the Witwatersrand Supergroup need to be pumped out constantly to keep the mines operational. The gold mines in the Witwatersrand Supergroup are amongst the deepest mines in the world and those in Gauteng are up to 3 km deep. The mines of the Central Rand, the East Rand and the West Rand, which extend over an area of 100 km, were shut down, one after the other, as the gold ore was depleted to a depth of approximately 3 km and it became uneconomical to continue. The enormous mine void, occupying several million cubic metres, gradually started to fill up with groundwater again, causing the springs in the vicinity of the Witwatersrand mines to flow after more than a century of dewatering. The water flowing from these springs, however, is not clean dolomitic water but acid mine water containing a high concentration of sulphuric acid, sulphate salts and metals, including radioactive heavy metals. The auriferous geological layers constituting the Witwatersrand Supergroup also contain large quantities of iron pyrite (FeS2) which forms sulphuric acid when it comes into contact with water and oxygen. The deterioration of the ecology in the Krugersdorp Game Reserve and the Cradle of Humankind World Heritage Site is a good example of the negative impact acid mine water has on the ecology.

Download Full-text

Hydrogeology of the Sterkfontein Cave System, Cradle of Humankind, South Africa

South African Journal of Geology ◽

10.25131/gssajg.120.3.403 ◽

2017 ◽

Vol 120 (3) ◽

pp. 403-420

Author(s):

P. Hobbs ◽

N. de Meillon

Keyword(s):

Water Resources ◽

Water Level ◽

Mine Water ◽

World Heritage Site ◽

Karst Water ◽

Water Drainage ◽

Cave System ◽

Fossil Site ◽

Cradle Of Humankind ◽

The Impact

Abstract A water level rise of almost 3 m in the space of two years in the Sterkfontein Cave system since late-2009 necessitated the re-routing of the tourist path through the cave to successively higher elevations on three occasions. It also raised concern for a possible association with copious acidic and sulphate-rich mine water drainage from the West Rand Goldfield (a.k.a. Western Basin) starting in early-2010, and the related threat to the UNESCO-inscribed fossil site. Although these circumstances have had little impact on the tourist value of the site, a prognosis of the impact on cave water level and quality is indicated by virtue of its karst setting and palaeontological significance. Historical and recent potentiometric data, together with ancillary hydrogeological and hydrochemical information acquired in the course of a water resources monitoring programme for the broader Cradle of Humankind World Heritage Site, provides new insight into the hydrogeology of the cave system. An improved understanding of the hydrophysical and hydrochemical response of the cave water system sheds light on the location of this system within the water resources environment. It is proposed that the present-day maximum cave water level is constrained to an elevation of ~1440 m above mean sea level. The recent electrical conductivity of 78 mS/m for cave water is 32% greater than the 59 mS/m recorded in mid-2010 and earlier. Similarly, the recent sulphate concentration of 161 mg/L is 178% greater than the 58 mg/L recorded before 2010. Compared to coeval values for ambient karst groundwater represented by the normative Zwartkrans Spring water, the magnitude of the increases in the springwater are similar, viz. 48% (from 84 to 124 mS/m) in salinity and 166% (from 154 to 409 mg/L) in sulphate. Although a distinct mine water impact is evident in both instances, the values indicate a muted impact on the cave water chemistry compared to the springwater. These and other documented observations better inform the threat from various poorer quality water sources to the fossil site in particular, and to the broader karst water resource in general. This contextualises concern for the hydroenvironmental future of Sterkfontein Cave and other nearby fossil sites such as Swartkrans, Rising Star and Bolt’s Farm. The dynamic response of the water resources environment to a variety of hydrological and hydrogeological drivers reinforces the need for monitoring vigilance across a range of disciplines.

Download Full-text