Biogeochemical Cycle of Mercury in Wetlands Ecosystem Affected by Gold Mining in a Semi-Arid Area

Wetlands are essential habitats in the environment since they fulfill a variety of ecological functions and support rich biodiversity. One major characteristic of wetlands is their ability to act as chemical sinks. There have been biogeochemical models developed for wetlands in Europe and North America but these systems differ from those found in South Africa, where most wetlands are riverfed and therefore undergo seasonal saturations, with periods of flooding in summer followed by drying during winter. High evapotranspiration rates also concentrate pollutants to very high levels. The nature, level and impact of mercury in the environment have not been extensively studied in South Africa, particularly in areas that have been significantly affected by gold mining activities. Aims of this research were to determine the biogeochemical cycling of mercury in permanent and seasonal aquatic ecosystems (rivers, dams, natural and manmade wetlands) that have been heavily affected by mining activities (acid rock drainage) in semi-arid areas with extreme seasonal changes. Research methodology included seasonal sampling, sequential extractions, determination of mercury speciation and modeling. Results show that natural wetlands in South Africa have a greater capacity to trap mercury than manmade wetlands. Most leachable mercury exists in oxidizable form followed by unleachable residue. Organomercury species are produced in anaerobic deeper levels where organic carbon and reduced sulfur content is high. Evapotranspiration and surface precipitation during the dry season exhibited a strong seasonal relationship to acidity and redox conditions. These research results are supported by a biogeochemical model may be used for identification and implementation of potential mercury remediation measures.