In many sub-Saharan countries where poverty and an abundance of natural resources seem to coexist, it comes as no surprise that legal and illegal artisanal mining is on the rise as the poor seek a source of income. However, while relatively lucrative, because of unsophisticated extraction techniques, small-scale gold miners release about 1,350 tons of mercury into water bodies annually. Ghana, the second greatest producer of gold in Africa, releases close to five tons of mercury per year. Nartey et al. (2011) measured the mercury levels up and downstream rivers, streams, boreholes and, sediments during rainy and dry seasons in the Birim North District of Ghana where small-scale mining is common. The samples were then compared to the Environmental Protection Agency’s (EPA) mercury guideline value for sediments and World Health Organization’s (WHO) mercury guideline limit for drinking water. All the samples showed a significant increase in mercury during the wet season while the samples downstream also showed a significant increase in mercury. The researchers found that the mercury levels in all but one water sample did not exceed the WHO’s guideline limit. However, all the sediment samples exceeded the EPA’s guideline value. Though the mercury levels in the water samples were within WHO standards, the sediment levels and the fluctuations in mercury levels between the wet and dry seasons were causes of concern. —Monkgogi Otlhogile
Nartey, V K.,Klake, R K., Hayford, E K., Doamekpor, L K., Appoh, R K., 2011. Assessment of mercury pollution in rivers and streams around artisanal gold mining areas of the Birim North District of Ghana. Journal of Environmental Protection 2, 1227–1239.
Nartey et al.picked six rivers and streams—Pra, Nwi, Suten, Nyanoma, Nkwasua and Tainsu—which were strategically placed within range of small-scale mining activity. The scientists then choose sites up and downstream from the mining activity to obtain river, stream, and sediment samples. These samples were taken from the middle of each water body. They also took water samples from six boreholes which were considered severely affected by mining activity. Over a 12 month period—during which the district experienced two rainy seasons—the researchers collected 300 ml of water and 30 g of sediment from each one their designated sites. The water samples were treated with stabilizers and spectroscopy was used to measure the amount of light absorbed by the water samples. The absorbance values of the water samples were used to ascertain the total mercury concentrations of each sample site. The sediment samples were dried, ground, heat treated, and diluted with water. The solutions were analysed using an atomic absorption spectrometer which can determine the concentration of an element such as mercury in a solid state such the sediment.
All of the sample sites downstream from the mining activity showed higher concentrations of mercury than the upstream sites because of the flow of the water and mercury. The mercury concentrations of the water samples taken from the rivers and streams during the wet season were significantly lower than the samples taken during the dry season. The authors suggest that this could have been influenced by increased mining during dry seasons, though they did not observe a significant increase in small-scale mining during the 12 month period. They also suggest that the evaporation of surface water that occurs during the dry season may have caused an increase in the mercury concentrations. Although there were significant differences in the concentration of the upstream and downstream samples, none of the river and stream samples exceeded the WHO’s guideline limit of 1µg/L except for the Tainsu downstream sample taken during the wet season with a concentration of 1.343 µg/L. However, as a result of previous samples taken near the River Pra which showed higher concentrations of mercury, the authors had reason to believe that either the samples had been mishandled and miscalculated, or had been affected by the operational status of the mining sites.
Unlike the river and stream samples, the boreholes samples did not show a significant difference in mercury levels between the wet and dry seasons because unlike surface water, the underground water of boreholes is not immediately diluted by rain water. None of the borehole samples exceeded the WHO’s guideline limit even though the Nyafoman borehole had a moderately high total mercury concentration of 0.619 µg/L. Much like the other water samples, previous samples led the researchers to believe that their collection methods and the operational status of the mining sites near the boreholes may have had an effect on the significantly lower mercury levels they obtained.
Though the authors concede that high mercury concentrations in sediment have no direct human impact, they suggest that the sediment’s role in aquatic ecosystems is a cause of concern. Not only can sediment concentrations act as water quality indicators, but sediment pollution also affects the food chains we take part in through fishing. Because of the previously mentioned reasons, the total mercury concentrations of the sediment samples during the wet season were significantly lower than the samples taken during the dry season. The minimal river mixing during the dry season, which keeps the mercury in the sediment, also plays a role in the fluctuation of mercury levels. In the sediment samples, all but one of the total mercury concentrations were above the EPA guideline value of 0.2 mg/kg. The Tainsu sediment had the highest total mercury concentration at 1.881 mg/kg during the dry season. A comparison of data between previous samples and the study’s sediment samples showed the study samples to have higher total mercury concentrations. This suggests that the direct dumping of mining waste into water bodies may be causing the settling of mercury in the sediment.