Tracking metal pollution through sub-Saharan African ecosystems

In regions of sub-Saharan Africa with artisanal and small-scale gold mining (ASGM), pollution is seldom as straightforward as a numbers game. Over years of environmental monitoring, measurement and regulatory assessment have largely focused on the total metal content in various substances such as soils, rivers, plants, and human blood. But an expanding body of environmental chemistry research says this approach fails to capture the essential details of metal transformation and pathways. These details, say researchers, can be addressed only via stable-isotope analysis.

“Mercury isotope signatures give us information on the predominant sources or pathways of mercury delivery to ecosystems,” says Jeroen Sonke, research director of the National Center for Scientific Research and geochemist at the Géosciences Environnement Toulouse. “Concentrations alone do not.”

That distinction, between how much mercury is present and what form it takes, is becoming central to a growing shift in environmental chemistry research. Across African mining regions, scientists are increasingly using isotope analysis to trace the hidden chemical pathways through which mercury transforms and spreads through ecosystems.

Mercury and Africa’s informal gold mines

Mercury pollution remains one of the defining environmental problems associated with ASGM. Across many parts of Africa, mercury is still widely used to extract gold because it is cheap, accessible, and technically simple to use.

In many informal mining operations, mercury is mixed directly with crushed ore to form a mercury-gold amalgam. During processing, large amounts of mercury escape into rivers, soils, and the atmosphere. When miners heat the amalgam to separate the gold, toxic mercury vapor is released into the air, exposing miners and surrounding communities to serious health risks.

“It has been estimated that 2 g of mercury are discharged into the environment for every gram of gold mined,” says Alseno Mosai, a lecturer and researcher in the Department of Chemistry at the University of Pretoria.

Once released, mercury enters highly dynamic environments where chemistry, climate, and biology interact in complex ways. Some mercury evaporates into the atmosphere, where it can travel long distances before returning to land or water through rainfall and dust deposition. Other mercury enters rivers and wetlands, where microorganisms can convert it into methylmercury, a highly toxic form that accumulates in fish and biomagnifies through food webs.

This process allows mercury concentrations to increase progressively as the mercury moves up the food chain, eventually reaching humans through fish consumption and contaminated crops.

Mining legacies further complicate the picture. According to Mosai, abandoned and historical mining sites in South Africa continue to generate contamination through acid mine drainage, which forms when sulfide-rich rocks such as pyrite and arsenopyrite oxidize after exposure to air and water.

“The dissolved metals in acid mine drainage can move long distances within the soil profile and travel to rivers and groundwater,” Mosai says.

Wind-blown dust from mine tailings can also disperse contaminants into nearby communities, extending exposure pathways far beyond active mining sites.

Why mercury concentration measurements fall short

For decades, environmental monitoring has relied heavily on measuring total mercury concentrations to estimate contamination risk. Nevertheless, researchers increasingly argue that this approach oversimplifies how mercury behaves in real ecosystems.

“Evidence has shown that a concentration measure alone does not fully reflect metal pollution and related health risks,” says Eucharia Oluchi Nwaichi, professor of environmental biochemistry at the University of Port Harcourt. “The misunderstanding that total concentration equals total risk remains common.”

The toxicity of mercury depends strongly on its chemical form, mobility, and biological availability. Understanding its chemical forms requires a branch of environmental chemistry known as speciation analysis, which distinguishes and quantifies individual mercury species, including elemental mercury, inorganic mercury, and methylmercury (MeHg). Because these species can transform through oxidation-reduction reactions, microbial activity, and photochemical processes, researchers apply specialized preservation techniques during sampling, transport, and storage to minimize chemical alteration prior to laboratory analysis.

Environmental factors such as pH, redox conditions, microbial activity, oxygen availability, temperature, and organic-matter content strongly influence these transformations once mercury enters natural systems. As a result, ecosystems are increasingly understood not as static reservoirs of contamination but as chemically dynamic environments in which mercury continuously cycles between air, water, sediments, and biota through processes of dissolution, adsorption, complexation, and biological uptake.

Following mercury’s isotopic fingerprints

One of the most important tools enabling this shift in understanding is stable-isotope analysis, which exploits how isotopes of different masses separate in predictable ways during chemical reactions. By measuring isotope ratios, scientists can reconstruct where mercury originated and what transformations it underwent while moving through ecosystems.

Bridget A. Bergquist and Joel D. Blum established the foundations of this field in 2007 with a study showing that mercury isotopes fractionate during photochemical reactions in aquatic environments. This finding provided scientists with a new way to trace mercury transformations through environmental systems and effectively transformed mercury from a simple pollutant into a tracer of environmental processes (Science, DOI: 10.1126/science.1148050).

“Usually, we can get information on sources in areas where ASGM exists,” as well as information on the exposure pathways, says Bergquist, who is now a professor in the Department of Earth Sciences at the University of Toronto.

Since their adoption in environmental monitoring, mercury isotope signatures have become increasingly valuable in tracing how mercury moves through interconnected environmental systems. Researchers use isotope data to distinguish mercury emitted directly from ASGM from mercury transported through the atmosphere over long distances, to track exchanges between river water and sediments, and to identify pathways through which mercury enters food webs and ultimately reaches human populations.

In ASGM-impacted river catchments, isotope data have revealed how mercury continuously cycles between sediments, water columns, and biological systems rather than remaining fixed at mining sites.

“The rate of dissolution and oxidation determines dispersal and ecological impacts,” Sonke says.

Sonke also explains that “as a redox-sensitive metal, mercury interacts strongly with mineral surfaces. . . . The more clay minerals, iron oxides, and organic matter you have in sediments, the more mercury becomes bound.” That binding changes the mobility and biological availability of mercury, but it doesn’t eliminate the long-term environmental risk.

Mercury in the atmosphere

One of the most important insights from isotope research is that mercury pollution does not remain local after release. In ASGM operations, large quantities of elemental mercury are emitted directly into the atmosphere during the heating of mercury-gold amalgams, used to recover gold. In addition, mercury can volatilize from contaminated soils, river sediments, wetlands, and surface waters, while previously deposited atmospheric mercury can be reemitted back into the air through natural processes such as photochemical reduction and surface exchange.

Once in the atmosphere, mercury can remain airborne for months because of its relatively long atmospheric lifetime, allowing it to undergo repeated oxidation-reduction cycles and long-range transport. During this time, it can be carried thousands of kilometers across continents and oceans before its removal from the atmosphere through wet deposition (rainfall and snowfall) or dry deposition onto land and water surfaces. This transport effectively integrates emissions from African mining regions into the global mercury cycle, linking local sources to hemispheric and global atmospheric reservoirs.

Mercury isotope research has demonstrated that atmospheric photochemical and redox reactions generate distinctive mass-dependent and mass-independent isotopic fractionation signatures, which can be used to trace mercury transformation pathways in the atmosphere. A key advance in this field came from a 2021 Environmental Science and Technology study, which showed that atmospheric oxidation and reduction processes produce characteristic isotopic signatures that can serve as tracers of mercury cycling in the atmosphere (DOI: 10.1021/acs.est.1c02568).

Building on this foundation, Song et al. demonstrated that these isotope signatures can be used to constrain key atmospheric redox mechanisms and reduce uncertainty in the global mercury budget, particularly highlighting the role of oxidants such as bromine in shaping atmospheric mercury chemistry (Environ. Sci. Technol. 2024, DOI: 10.1021/acs.est.4c02600).

Field-based evidence further supports these interpretations: measurements at the Mauna Loa Observatory showed that mercury isotope compositions in the free troposphere reflect ongoing oxidation, transport, and mixing processes that govern long-range atmospheric mercury cycling (Ecotoxicol. Environ. Saf. 2024, DOI: 10.1016/j.ecoenv.2024.116993).

“Because [mercury] is an element that is dispersed globally, it travels in the atmosphere quite extensively,” Sonke says.

Together, these studies demonstrate that atmospheric mercury is not only transported over long distances but also continuously transformed during transport. This dual role of movement and chemical reactivity makes mercury isotopes a powerful tool for linking emissions from mining regions to global atmospheric processes.

Studies conducted in ASGM landscapes have also shown that mercury deposited from the atmosphere can adhere directly to plant surfaces and enter crops. This finding contradicts traditional assumptions that crops absorb mercury primarily through contaminated soils.

In fact, “in some agricultural communities near mining zones, isotope signatures have suggested that mercury uptake in crops is dominated by deposition from the atmosphere,” Nwaichi says. “This means we must move away from the idea of soil-only contamination as a way of intervening.”

The finding could have major implications for public health strategies in agricultural communities located near mining zones. Even where soil mercury levels appear relatively low, airborne contamination may still expose farming populations through food systems.

Africa as a natural laboratory for isotope analysis

Researchers say ecosystems can be considered complex reaction vessels through which toxic metals move and change form. In particular, mercury isotope compositions can be used to trace these hidden transformation processes and relate emissions to exposures via air, water, sediments, crops, and biota.

Despite the rapid developments in the field, analytical challenges still exist, including the prohibitive cost of some equipment and the inconsistent methodologies used across different laboratories.

Nevertheless, some researchers say that mercury isotope chemistry is already revolutionizing their understanding of mercury contamination in African mining regions. “Nigeria is not only a case study; it is a model-generating laboratory,” Nwaichi says.

The combination of informal mining practices, tropical rainfall, atmospheric transport, intense biological activity, seasonal flooding, and rapidly changing land use creates highly reactive environmental conditions rarely replicated in laboratory experiments.

In these systems, mercury does not remain chemically stable long enough to be measured as a static contaminant. Instead, it continuously changes form as it cycles through air, water, sediments, microorganisms, plants, animals, and human communities.

At the same time, advances in environmental analytical chemistry are improving scientists’ ability to study these transformations more accurately. Improved preservation and speciation techniques reduce chemical alteration during sampling and storage, helping researchers analyze mercury in forms that more closely reflect original environmental conditions and providing a clearer picture of mercury’s movement through ecosystems. “There are multiple routes of mercury exposure through air, water, soil, and food systems,” Nwaichi says.

Still, major technical challenges remain before isotope analysis becomes routine in environmental monitoring. Scientists continue to face problems involving analytical variability, interlaboratory inconsistency, and the high costs associated with isotope measurements.

Not everyone agrees, but for Sonke, the conclusion is clear: “It is indeed time,” he says, “for mercury isotope analysis to become a routine monitoring parameter.”

Mkhululi Chimoio is a freelance writer and an investigative and solutions journalist based in Gauteng, South Africa.