Seasonal distribution and speciation of mercury in a gold mining area, north-west province, South Africa

Abstract

The seasonal speciation of mercury (Hg) was determined in water, soil, and sediments from watersheds located in the North-West province of South Africa. The study area is known to have a long history of mining activities which also include the recovery of gold from old tailings. Both inorganic (IHg) and methyl mercury (MHg) were detected at high concentrations (up to 8480 μg IHg kg⁻¹ and 13 μg MHg kg⁻¹) in surface sediments during dry season. A considerable remobilization of Hg from bottom sediments was observed in water from dry to wet season as well as the migration of Hg away from pollution sources due to seasonal influences. Hg in sediments mostly has been speciated as Hg⁰. Enhancement of Hg methylation occurred mainly in deeper sediments at regions corresponding to the lowest redox potential, higher pH, and enrichment of IHg. Hg values in borehole waters were very high (up to 3310 ng L⁻¹) suggesting a serious contamination of the site groundwater which needs to be addressed urgently in order to minimize further impact that affects the Vaal River and other water systems located nearby.

Keywords:

• mercury
• speciation
• methylation
• gold mining
• tailings
• seasonality

Acknowledgments

The authors thank the following: National Research Foundation (NRF, South Africa), Technology and Human Resources for Industry Programme (THRIP, South Africa) for the financial support, AngloGold Ashanti for general information and assistance during sampling, Centre National de la Recherche Scientifique (CNRS, France), and Laboratory of Analytical, Bio-Inorganic and Environmental Chemistry (LCABIE-IPREM, France) for allowing us to carry out part of this study in their facility.