Biogeochemistry of mercury in an impacted gold mining tropical aquatic system: The Pra River basin in southwestern Ghana.

摘要

Artisanal gold mining (AGM) with metallic mercury (Hg0) has a long history in Ghana. It is believed to be over 2000 years old. Today, AGM in Ghana has escalated into a new dimension consuming about half of the country, thanks to its legalization by the government in 1989. This study investigated contamination levels and fate of mercury (Hg) in one of the 3 major river systems draining the gold-rich southwestern portion of Ghana, where gold is extracted primarily by the Hg amalgamation technique. Samples of water, sediment, soil and biota (i.e., human hair and fish) were collected from locations within the Pra River basin during the rainy and dry seasons of 2002 and 2003. Collected samples were: (1) analyzed for different Hg species and other trace metals (e.g. As, Pb, V, Cr, Ni, and Zn); and (2) used in controlled laboratory experiments to assess the fate of Hg introduced into the Pra River system by gold mining activities. In the aqueous phase, the average total-Hg (THg) concentration remained nearly constant regardless of the season (∼161 ng Hg L-1), while the average methyl-Hg (MeHg) levels increased from 0.42 to 2.37 ng as Hg L-1 from the dry to the wet season. THg and MeHg concentrations in soil and sediment spanned a broad range of values and increased from the dry to the wet season. On the other hand, AGM seemed to have a little impact on ambient levels of other trace metals. Overall, Hg levels and speciation in the studied aquatic system were controlled by the wet/dry cycles, which drive the hydrology and dictate differences in flow regimes versus seasons.; Laboratory experiments assessing the fate of Hg introduced into the Pra River system by AGM focused primarily on the ability of both soil and sediment to produce and accumulate MeHg, the most toxic and readily bio-available Hg species in natural systems. These experiments led to the following conclusions: (1) soil hydration plays a key role in MeHg production in soils. Therefore, the rainy season would correspond to the time of high MeHg production in soils. Produced MeHg would then enter both surface- and ground-water by surface runoff and infiltration, respectively; (2) sediments in the Pra river basin do have a high potential to produce and accumulate MeHg, hence the potential to funnel Hg into the food chain; and (3) solar radiation is likely to exacerbate the production of Hg released in aqueous systems by AGM operations by making Hg previously bound to recalcitrant high molecular weight organic matter more accessible to methylating microbes. (Abstract shortened by UMI.)