Impacts of artisanal and large scale gold mining on tropical rivers in West Africa: A case study from the Brong Ahafo Region of Ghana
Date of Award
Doctor of Philosophy
School of Science
Associate Professor Mark Lund
Dr Eddie van Etten
Mining communities in more than 70 developing countries, mostly in the tropical regions, still practise artisanal and small scale gold mining (ASGM). ASGM commonly operates along rivers and streams for easy access to process water and as receptacles for mine water discharges. A largely unregulated industry, ASGM employs rudimentary mining and processing methods including the use of mercury amalgamation, and is often found near to larger scale and modern mining (LSM) operations.
The substantial use of mercury by ASGM has drawn the attention of agencies and researchers but so has its persistent economic role in providing much needed rural employment. Mercury toxicity to human and environmental health has attracted much of researches, however ASGM impacts on riverine ecology, particularly at biota community levels, remains understudied.
This study investigated the impacts of ASGM on the ecology of the Surow River and that of an LSM (the Ahafo mine) on the Subri River between February 2013 and April 2014. Both the Surow and Subri rivers are ephemeral tributaries of the Tano River, in Brong Ahafo, Ghana. The Ahafo mine, currently operated by Newmont Ghana Gold Limited (NGGL), has been operating on the Subri River catchment since December 2006, whilst ASGM started operations along the Surow River in 2007. Major ASGM operations ceased in May 2013 although smaller operators and processors remained.
Specifically, the study aimed to determine whether and how ASGM and LSM impacted the respective river’s water and sediment quality, macroinvertebrate and microbial (Archaea and Bacteria) community structures and resulted in mercury biomagnification in fish. ASGM impact on river water and sediment quality was determined using a reversed BACI (Before/After and Control/Impact pairing) experimental design, whilst that of LSM used a conventional BACI design. Impacts on macroinvertebrate community structure were determined by comparing multiple control and impact sites sampled multiple times. The sequencing of 16S rRNA of Archaea and Bacteria was based on a one-off sampling and comparison between multiple control and impact sites on both rivers. The biomagnification of mercury in fish was tested via analysis of correlations between mercury concentrations in fish tissue and fish trophic level, fish length (proxy for age) and fish weight.
The study demonstrated that gold mining, regardless of size and methods, significantly impacts the river ecosystems studied. Sediment particulates and minerals naturally available in the rock formation but exposed to the environment by mining activities were the most significant pollutants in the affected riverine ecosystems. The study area is in the tropic and experiences intensive rainfall. This, results in excess water which may come into contact with exposed rocks and wastes in the mining areas that eventually runs into or is discharged into the Surow and Subri rivers. Changes in the sediment and water quality due to mining were reflected in the macroinvertebrate communities of both rivers, while the sediment microbial communities tended to respond to differences in water quality. The study, however, strongly indicated that the types, magnitudes and effects of the environmental impacts of ASGM were different from that of LSM. The use, or the lack of, environmental management systems to mitigate impacts appeared to be the most important differentiating factor. The study also witnessed significant improvements in both water and sediment quality in the Surow River with the cessation of major ASGM in the area. Mercury, which was used in the ASGM (in relatively small quantities in Ghana compared to other countries) was detected in the Surow River sediment (despite naturally low concentrations of Hg in the local soils), but was largely undetectable in the waters. However, it posed health risks to humans and biota. This study found mercury biomagnified in fish from both the Surow and Subri rivers as well as the Tano River, indicating the presence of mercury in the rivers. The source of Hg, however, could not be clearly established but may have been from artisanal amalgamation processes and from smelting.
Although mercury remains a concern in ASGM impacted rivers, it is not the only contaminant of concern. Sedimentation and particulate bound elements such as Al, As, Cu, Fe, Hg, and Pb were the main river pollutants resulting from ASGM operations. Elevated concentrations of metals in the turbid water due to the lack of sediment controls exceeded the Ghanaian and US EPA standards for the protection of aquatic life as well as that of Ghanaian raw water to be processed as drinking water. ASGM also significantly elevated the concentrations of salt ions and sulfate in river water particularly due to discharges of water from mine dewatering. During the active ASGM period, concentrations of Cu, Cr, Hg and Ni in the Surow River’s sediment exceeded the threshold effect level / TEL, lowest effect level / LEL, Australian effect low range /ERL and threshold effect level for Hylella azteca 28-day test or TEL HA28. Increased sediment load and decreased sediment quality in the Surow River were reflected in the macroinvertebrate community structure that was dominated by sediment-tolerant taxa but with only a few pollutant-sensitive taxa including Ephemeroptera and Trichoptera families.
In the Subri River affected by the Ahafo gold mine, the impacts of mining were ameliorated by sediment control measures applied by the mine. The sediment control measures on the Subri River included the use of environmental control dams (ECD), one on a major tributary stream to the river, the other on a minor tributary. The ECDs reduced not only turbidity and total suspended solids, but also electrical conductivity, concentrations of most salt ions, nitrates and sulfate, and most metals both as total and dissolved forms from in the mine water being discharged into the environment. The improved water quality in downstream Subri River compared to that of the mine site and upstream was also reflected in the sediment quality, which had lower concentrations of most pollutants than that of the Surow River. The mine affected area in downstream Subri River also had more sensitive taxa including Ephemeroptera families than the Surow River. Nevertheless, mine discharge in downstream Subri appeared to alter the ecosystem compared to upstream control sections. Cessation of mine discharges at closure could see downstream sections of the river return to conditions more consistent with upstream.
The exploratory microbial community study, a relatively novel study in the region, showed that the composition and diversity of the Archaea and Bacteria communities found in the Surow and Subri Rivers were comparable to those found in other studies including in the temperate regions. We also observed that microbial composition spatial variability within was greater than between rivers and that the variability was unrelated to riverine sediment chemistry but significantly related to water chemistry, particularly turbidity and concentrations of sulfate, Fe and FRP. The study also demonstrated a shift in sediment microbial community composition due to mine dewatering, particularly in the Surow River reaches affected by ASGM dewatering discharges. Given the one-off sampling nature of the sediment microbiology study, however, further study with repeated sampling regime is recommended. Sedimentation at ASGM sites dramatically altered the river morphology and biota. Further, metals carried by the sediments were deposited along the river downstream during the dry season and remobilised during the rainy seasons. The use of simple small scale ECD equivalents would substantially sediment based pollution. Discharges from mine dewatering from ASGM activities increased conductivity of the river and under full scale operations would have been problematic for biota and water quality. Although discharges from the LSM were of higher quality, they were also in high quantity and substantially altered downstream water quality and biota. Although these changes resulted in increased sensitive taxa, the long-term sustainability of these discharges is unknown. This study demonstrated that impact assessment of ASGM or LSM on rivers should not be limited to the physical and chemical properties of water and sediment, but also include its riverine biota. This study supports the use of macroinvertebrate and potentially microbes as indicators of impact of ASGM and mining in tropical rivers. Moreover, an understanding of the ecological impacts of mining large and small can assist in the
Macdonald, K. F. (2016). Impacts of artisanal and large scale gold mining on tropical rivers in West Africa: A case study from the Brong Ahafo Region of Ghana. https://ro.ecu.edu.au/theses/1932