Assessment of heavy metal contamination in soils around Krugersdorp mining area, Johannesburg, South Africa.

Abstract

The mining industry remains a driving force of the South African economy, making a significant contribution to economic activity, job creation and foreign exchange earnings. However, in the process of mining and mineral production, millions of tons of waste are produced making this sector the potential source of air, soil and water pollution. There are more than 270 waste dumps known as tailing dams in the Witwatersrand Basin alone, covering 400 square kilometres in surface area (Oelofse et al., 2007, Rosner, 1999). Poor management of these tailing dams and waste rock dumps lead to uncontrolled release of acid mine drainage that in some cases cause soil degradation and water contamination around these sites. The aim of this study was to assess pollution levels of As, Cd, Cu, Cr, Pb, Hg, Ni and Zn in soils from the mining area of Krugersdorp and to compare them with the South African and international standards. The geo-accumulation index (Igeo), the single ecological risk factor (Er), and potential risk index were calculated to assess the degree of contamination by these metals in soils. Thirty one soil samples were collected from eleven different sites around the Krugersdorp area. Twenty three were collected from the Krugersdorp Game Reserve, four from private farmland, two from Mintails Gold Mine and two from Rand Uranium Mine. All collected samples were properly marked and identified by their sampling locations using a Global Positioning System (GPS). Samples were acid digested using microwave oven according to U.S EPA 3052B protocols and analysed for arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), mercury (Hg), nickel (Ni) and zinc (Zn) with Inductively Coupled Plasma – Optical Emission Spectrometry technique (ICP-OES). The concentration levels of heavy metals in soils were in the range of 4.1-155.5 mg/kg for As, 0.1-0.7 mg/kg for Cd, 13.4-119.1 mg/kg for Cu, 54.0-147.0 mg/kg for Cr, 0.02-1.36 mg/kg for Hg, 24.1-168.9 mg/kg for Ni, 5.5-82.5 mg/kg for Pb, and 12.5-641.0 mg/kg for Zn. Three sampled sites (KS7, KS8 and KS10) recorded high contamination levels by heavy metal compared with the relevant South African standard and other international standrads. The geo-accumulation index assessment revealed that the soils vary between uncontaminated to the extreme pollution soil category. The ecological risk factor (Er ) mean values of As, Cd, Cr, Pb and Zn revealed low ecological risk to soils, while values for Cu and Hg indicated moderate ecological risk. Ni values indicate a considerable ecological risk. The potential ecological risk index (RI) values of three sites (KS1, KS5, and KS9) indicated low ecological risk. RI values for KS3, KS6 and KS11showded that these sites posed moderate ecological risk. Considerable ecological risk was displayed by sites KS2, KS4 and KS8, while RI values of site KS7 and 10 displayed very high ecological risk. These results indicate that the anthropogenic mining activities have polluted the area with heavy metals. As a result a series of recommendation are forwarded in order to rehabilitate the quality of the soils affected.