Aquatic Mercury Pollution from Artisanal and Small-Scale Gold Mining in Sub-Saharan Africa: Status, Impacts, and Interventions

By inergency On Mar 2, 2024

1. Introduction

Mercury (Hg) pollution and toxicity remain an environmentally relevant global concern across the different biospheric matrices due to its ubiquitous and non-degradable nature [1]. Studies have identified the impact of natural and anthropogenic sources of Hg on the functioning of ecosystems and biological processes [2]. Chemically, Hg occurs in three major forms: elemental or metallic (Hg0), organic (OrgHg), predominantly methyl mercury (MeHg), and inorganic (IHg), mainly as mercuric chloride (HgCl₂) [3,4]. These different forms are cumulatively called total mercury (THg) [5,6]. Hg, a non-essential element, is liquid at ambient temperature and highly volatilises into the atmosphere. Atmospheric Hg can be transported and deposited in terrestrial and aquatic environments [1,7,8]. MeHg is among the top 10 highly toxic contaminants that negatively impact aquatic biota, accounting for up to 50% of THg pollution [9,10].

In the aquatic ecosystem, under favourable environmental conditions, microbially-mediated biogeochemical (e.g., sulphate-reducing bacteria) and abiotic processes (e.g., transmethylation) transform the IHg into the most bioavailable and toxic MeHg [11,12,13]. Mercury methylation in aquatic environments results in the bioaccumulation and biomagnification of MeHg, which increases the toxicity risk to higher trophic-level biota [14,15] and humans [4].

The fourth Global Mercury Assessment report [16] catalogued the trends of Hg emissions from key sectors in 2015 (Figure 1). The contribution of atmospheric THg increased by 450% to 4400 tonnes/yr, and almost half of this (2500 tonnes/yr) came mainly from anthropogenic emissions from industrial and artisanal mining and mineral processing, energy production, and losses from Hg-based products and processes. Approximately 600 tonnes entered the freshwater ecosystems, at least 30% of which were conveyed to the marine environment. Natural Hg, releases from volcanic processes, soils, and vegetation burning, accounted for 2100 tonnes of Hg in the atmosphere, while the oceans contributed 250% of Hg (3400 tonnes) in emissions. In the terrestrial environment, natural sources, including organic and mineral soils, retained 750,000 tonnes/yr. The oceanic Hg cycle retained about 2600 tonnes/yr in surface waters, 310,000 tonnes/yr (37%) of the land, and atmospheric THg deposits in the intermediate and deep waters. Net losses of THg from the marine environment contributed about 45–55% of THg to the atmosphere, both transported to land and freshwater or redeposited in the oceans.

Amotaey and Baawain [17], in a global review of the impact of metal pollution on aquatic biota, observed Hg bioaccumulation in fish muscles, eggs, ovaries, and zooplankton biomass. La Colla et al. [14] reiterated the significant health risk of THg accumulation in fish muscles along an anthropogenically impacted southeast Argentina’s Bahía Blanca estuarine ecosystem. Aquatic MeHg concentrations and biomagnification above acceptable limits in fish communities (732–918 ng/g), frogs (1.075 ng/g), and riparian spiders (347–1140 ng/g) in tropical highland aquatic systems in southwest Colombia have also been reported [18,19,20]. In addition, Lino et al. [21] observed that artisanal and small-scale gold mining (ASGM) and deforestation were the primary sources of THg and MeHg in Brazil’s Tapajós River basin. In the Gambia River flowing through the Kedougou region in eastern Senegal, pollution of stream sediments was in the magnitude of 2–6 mg/kg THg [22] but recently reduced to 1.16 mg/kg THg and 3.2 ng/g MeHg [23].

Streets et al. [24] globally observed a 1.4 Tg cumulative release of Hg between the year 1510 and 2010 period, of which 23% was atmospheric and 77% dissipated into aquatic and terrestrial environments. Furthermore, the authors noted that North America and Europe contributed a cumulative 413 Gg and 427 Gg in this period. With the increased ASGM activities in Africa and the Middle East, about 72% of cumulative atmospheric Hg0 was released [25]. Additionally, ASGM was the largest source of global Hg emissions (775 Mg) in 2015. The aquatic and terrestrial environments that accumulated over 83% of the atmospheric THg were Africa, the Middle East, and Oceania, which contributed 77%, while up to 89% came from South America. Streets et al. [25] further documented an increase in annual Hg emissions from 2188 Mg to 2390 Mg (+1.8%) between 2010 and 2015, where emissions increased in Eastern Africa (>4%), South Asia (>4.6%), and Central America (>5.4%).

In 2011, the ASGM industry employed over 6 million miners globally, who extracted 380–450 tonnes of gold, and by 2018, over 44.67 million people worked in the ASGM sector [16,26]. In sub-Saharan Africa, approximately 1.322 million people work in the ASGM sector, where 27,200 kg of gold was produced in 2014 [16,27]. According to UNEP [16], global Hg emission sources ranged from 40 kg in the ferrous metal industries to 52 kg from biomass burning, 162 kg from waste products, 313 kg from non-ferrous metal production, and 838 kg from ASGM. In 2015, ASGM contributed nearly 1220 tonnes of Hg to global water and terrestrial ecosystems from mining activities in sub-Saharan Africa (8%), East and Southeast Asia (36%), and South America (53%) [16]. The increased use of Hg in ASGM has contaminated aquatic ecosystems. Mercury transforms into highly toxic MeHg, which bioaccumulates in aquatic biota, threatening aquatic productivity and the health of aquatic resource users [28,29]. Furthermore, the ASGM communities risk exposure to inhalation of gaseous Hg, ingestion of contaminated water and food, and dermal contact, resulting in health problems, including neurological disorders [30,31]. For example, communities near ASGM sites in countries like Indonesia [32,33,34] and Ghana [35,36,37] have reported both environmental impacts and health issues associated with mercury exposure.

Additionally, Hg used in ASGM enters the global Hg cycle following atmospheric release during gold amalgams, contributing to long-range transport and deposition thus impacting ecosystems globally [38,39]. In aquatic ecosystems, Hg bioaccumulation threatens aquatic biodiversity, particularly in ASGM-intensive areas [40]. For instance, studies in the Amazon basin reported high Hg levels in fish, impacting aquatic ecosystems and human health [41,42]. Although ASGM is often a source of livelihood for marginalised communities, it poses environmental and health risks associated with Hg use, which further undermines the health and productivity of ASGM communities.

In October 2013, the UNEP-led Minamata Convention on Mercury, enforced in August 2017, targeted, among other interventions, the elimination of Hg use in ASGM [43]. Parties to the Convention were obliged to conduct enabling assessments under the Minamata Initial Assessments (MIA) and generate country profiles on Hg status [43]. According to Anan and Toda [44], analysis of country priorities from MIA reports shows that most African countries prioritised three key areas for reduction of Hg release into the environment (Article 8), including Hg waste management, eliminating Hg in products such as cosmetics and dental amalgam (Article 4), and adopting Hg-free gold processing in the ASGM industry (Article 7) (Figure 2). The identified priority area later formed the basis for developing National Action Plans (NAP) to enforce the Minamanta Convention. Furthermore, Anan and Toda [44] identified 16 SSA countries that had prioritised the phasing out of Hg-added products and the safe handling of Hg wastes to reduce environmental contamination. Similarly, 14 country MIA reports highlighted total elimination or reduction of Hg use in ASGM. Waste incineration, open burning of mercury wastes, and uncontrolled waste damping release approximately 132,776kg (23.2%) and 229,681 kg (40.1%) Hg, respectively. The application, use, and disposal of Hg-added products and dental amalgam contribute 54,521 kg (10%) of Hg, whereas the ASGM sector, dominated by the use of Hg amalgamation during gold production accounts for 156,350 kg (27.3%) of Hg releases. Cumulatively, the implementation of these action areas would reduce approximately 573,328 kg of Africa’s contribution to global Hg emissions [44].

From the above statistics, it remains clear that the environmental presence and use of Hg in the ASGM sector and industrial products exposes the biosphere to toxicity from Hg interaction with air, land, and water and consumption of Hg-contaminated food, moreso in SSA aquatic systems [16,44]. To address the Hg intoxication problem, there is a need to establish the present status, environmental impacts, and existing interventions to address this global and continental problem, particularly aquatic Hg pollution. The ASGM sector remains a critical contributor to aquatic Hg in SSA [5,45,46]. However, few studies have attempted to establish the magnitude and impact of aquatic Hg pollution from the SSA perspective but have not entirely focused on the contribution of the ASGM sector [47,48,49]. Other studies have reviewed the impacts of Hg contamination of aquatic environments from ASGM at the country level, including in South Africa [50], the Lake Victoria basin, Eastern Africa [51], and Ghana [35]. However, regional comparative studies on Hg aquatic pollution and related environmental impacts and interventions across and from sub-regional ASGM activities across SSA that provide regional interventions are missing. Consequently, the review focuses on (1) establishing the current status of aquatic Hg pollution, (2) exploring the environmental impacts of Hg pollution on aquatic ecosystems in sub-Saharan Africa, and (3) highlighting proposed interventions and identifying gaps for further research on the management of aquatic Hg pollution from ASGM in SSA.

4. Socioeconomic, Environmental, and Human Health Impacts of ASGM in SSA

According to Hilson [124,125], ASGM is a significant economic driver in rural SSA, providing direct employment to more than 10 million miners in rural communities. For instance, in Ghana and Tanzania, more than 1 million people are directly employed in the ASGM sector, and at least 0.2 million of the ASGM community in Mali are women [124]. Furthermore, ASGM is a source of capital development beyond direct employment by diversifying livelihoods and micro-economies in rural SSA mining communities, thereby providing indirect employment to over 4 million people [126,127]. Therefore, across the sub-regions of SSA, the socioeconomic contribution of the ASGM sector to the rural transformation of the socioeconomic status and livelihoods of millions of people cannot be underestimated [128].

However, the largely informal organisation of the ASGM sector in SSA competes against the socioeconomic gains among mining communities across the region. Hilson et al. [129] noted that the formalisation of ASGM is a difficult undertaking regardless of the locality of the mining landscape and described the situation as a created and perpetual informality in the ASGM mining space. The biggest drawback to this is the imbalanced working conditions, such as the intensive labour demands involved with gold processing, higher prostitution levels, and abuse of drugs such as narcotics [124]. Competing interests between rural communities and ASGM entrepreneurs have frequently led to gang-related violence, thefts, and related vices. For instance, Mkodzongi [130] reported a dramatic increase in gang-related violence in the Shurugwi mining area of Zimbabwe’s Midlands Province, with cases of robbery of cash, gold, and ore from miners and businesses. In addition, most ASGM entities operate under hazardous conditions, impacting the health and well-being of the predominantly poor rural communities [127,131].

ASGM miners are continuously exposed to gaseous Hg during the processing of gold by Au-Hg amalgam, which involves heating to vaporise the Hg. Hg is also released into aquatic systems during amalgamation processes and washed off during rain and flood events, becoming an important route for human exposure, primarily to MeHg (Figure 9). MeHg can bioaccumulate up the food chain, resulting in human exposure risks. Hence, communities within the ASGM regions that use Hg for gold processing may have a higher exposure through fish consumption, intoxication through inhalation, and physical contact, causing increased disease burden from elevated MeHg burden. For instance, in 2016, Steckling et al. [132] reported health impacts on 1.22–2.39 million miners globally from exposure to IHg. A critical factor determining the level of human exposure to MeHg is the interaction with the aquatic medium and the occurrence of IHg, which is usually associated with the aquatic environments synonymous with ASGM in SSA [133,134].

ASGM also exacerbates deforestation and soil erosion in many SSA countries [135,136,137]. The direct impact of ASGM is more focused on tropical countries in the global south, particularly SSA. According to Fisher et al. [138], several factors may increase the potential of exposure to Hg from ASGM, including (1) high concentrations of Hg in soils, tailings, and stream sediments; (2) a lack of employment or alternative livelihoods in many rural areas, which promotes low-level employment occupations such as the ASGM; (3) the availability and affordability of Hg required for gold processing and a lack of government control on Hg importation and unregulated use in the ASGM sector; and (4) the high relative market price of gold, which pushes the demand for Hg as a “quick” extraction method.

Additionally, reservoirs constructed for electricity generation in most river basins of SSA are potential convertors of Hg into toxic MeHg in the aquatic environment. Hg cycling in reservoirs typically increases MeHg production due to the ecological, biogeochemical, and hydrological changes in aquatic environments [139]. For instance, Hall et al. [140] observed elevated MeHg output and bioaccumulation from the deposition of organic matter during flooding that increased microbial decomposition and net MeHg retention in reservoirs in northwestern Ontario, Canada. In the boreal Canadian reservoirs, MeHg levels increased 3 to 6 times post-impoundment flooding and remained high several decades later [141]. In the tropics, reservoirs have been observed to increase MeHg production from Hg-laden surface sediments, which bioaccumulate to harmful levels in organisms [142]. In that study, THg accumulation ranged from 50 to 200 ng/g in reservoirs with a surface area between 80 and 400 km².

6. Conclusions and Future Perspectives

Aquatic Hg pollution is rising due to the rapid expansion and intensification of ASGM in SSA. In general, West Africa reported the highest contribution to aquatic Hg pollution (50.2%), followed by Central Africa (39.6%) and Southern Africa (9.6%), while Eastern Africa contributed below 1%. Contamination of freshwater ecosystems was evident in the ASGM regions, from surface and groundwater, stream sediments, bioaccumulation in aquatic biota, and riparian vegetation. The subsequent environmental health risks to humans from fish consumption, water use, and exploitation of other water resources from Hg-contaminated aquatic ecosystems were also significant.

From the MIA report synthesis, the following gaps need urgent action to enhance the effective management of aquatic Hg pollution from ASGM in SSA: (1) limited documentation and information of mercury availability, quantification, use, and safe disposal; (2) inadequate capacity and resources to monitor and regulate Hg-related ASGM; (3) unregulated and illegal Hg use in the gold mining sector; and (4) poor implementation and low uptake of Hg-free alternative ASGM technologies.

We recommend the following options as a way forward for the management and control of Hg pollution in aquatic ecosystems in SSA: (1) the regulation and reduction of Hg use in ASGM through the development of alternative Hg-free gold mining technologies, including the physical (gravity, magnetic, and flotation), hydrometallurgical (borax, potassium carbonate, silica sand), and pyrometallurgical methods (smelting and roasting); (2) the identification, development, and strengthening of environmental policies and interventions to phase out the importation and use of Hg in the ASGM sector; for instance, the closure of ASGM that use Hg should be accompanied by offering alternative Hg-free livelihoods with an equal or better economic income; and (3) the implementation of health educational programs within the existing health care structures to inform communities within the ASGM regions of the negative health impacts of Hg use.

Implementing and enforcing regulations on the use of mercury in ASGM is essential. Environmental health and public policies should be implemented to balance the socioeconomic benefits and environmental impacts of ASGM in SSA. Governments and international organisations are increasingly recognising the need for stricter regulations. For example, the Minamata Convention aims to regulate and reduce Hg use, particularly in ASGM [43]. Encouraging the adoption of cleaner and more efficient gold extraction technologies is crucial. For example, gravity concentration, cyanide leaching, or other methods that minimise or eliminate the need for Hg can significantly reduce environmental and health impacts [144].

Furthermore, providing training and education to artisanal miners on the hazards of mercury and alternative practices is essential. Educational training would empower ASGM communities to make informed choices about sustainable and environmentally friendly mining practices. There is a need to focus efforts on regulatory measures, community engagement, and the promotion of cleaner technologies to mitigate the adverse effects associated with Hg use in ASGM.