In environmental discussions, mercury contamination in crops is rarely emphasized. However, a recent study that compared fields only 500 meters from an artisanal and small-scale gold mining (ASGM) site with those located 8 kilometers away found that mercury concentrations in leaves and grains were roughly 10-50 times higher at the nearer farm. What stands out is that the atmosphere, rather than the soil, served as the primary route through which mercury entered plant tissues and, in turn, local diets.1
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How Airborne Mercury Reaches Fields
Airborne mercury travels much further than expected. Vapors released during artisanal and small-scale gold mining rises quickly, then drifts with regional winds before settling on vegetation many kilometers away.
In Nigeria’s gold-mining corridor, researchers recorded gaseous mercury concentrations of nearly 1200 ng/m³, which is significantly above the global background of roughly 1–2 ng/m³.2 A farm just 500 meters from the processing site had soil mercury levels much higher than those at the control field, but the leaves provided the clearest indication of what was happening.
Foliage carried the highest mercury loads, with isotope signatures indicating direct uptake from the air rather than from roots. Negative mass-dependent fractionation values in maize, cassava, and peanut leaves matched what is expected when stomata absorb gaseous elemental mercury during photosynthesis.2 Root-to-shoot transfer contributed less than 7 percent of what accumulated above ground.
Where the Mercury Ends Up in Crops
Most of the mercury sits in the foliage. Leaves capture dry deposition efficiently; in the study, cassava leaves reached several hundred micrograms per kilogram, while tubers and kernels remained at much lower levels.2 Surface deposits cling to waxy cuticles, and internalized mercury stays locked in leaf tissue once taken up.
Animals grazing on contaminated foliage bring another step into the chain, and households using those leaves in stews or as side dishes take in small amounts each day. These doses look minor on paper, but over months, they matter. Children may absorb slightly more relative to their body weight, and adults can accumulate mercury slowly in nerves and soft tissues.
Methylmercury, the more harmful organic form, was detected at very low proportions in Nigeria’s dry soils, accounting for less than one percent of the total.2 However, in wetter rice-growing systems elsewhere, conversion could be higher.
Communities Living Closest to the Problem
Villages near mining sites encounter this issue first. Farmers do not usually link crop quality with airborne metals. The pattern of contamination also varies; one field may show low levels while another, only a short distance away, shows more surface deposition.
Crops grown near mining areas can take up mercury at levels approaching dietary relevance, even when daily intake limits are not exceeded.1 People rely on these crops as a primary food source, so whatever settles on the leaves eventually enters their diets. Overall, exposure builds gradually, shaped by regular meals rather than a single event.
Why Artisanal Mining Drives the Pattern
Artisanal and small-scale gold mining (ASGM) is one of the largest sources of mercury driven by human activity. Miners apply liquid mercury to crushed ore, heat the mixture to separate the gold, and release vapor into the open air. This vapor, mostly elemental Hg(0), can remain airborne long enough to move across surrounding landscapes. Coal combustion adds to emissions in some regions, but ASGM is the dominant source in many areas where crop contamination has been documented.1
The Nigerian isotope study supports this link. Mercury in the air settled directly onto nearby fields, and the isotope signatures in leaves matched vapor-phase inputs rather than older, soil-based contamination.2 Evidence of this kind shifts policy discussions toward the need for air-focused controls.
Efforts to Reduce Emissions
The Minamata Convention encourages a shift away from mercury use in mining and supports alternatives such as gravity separation or borax-assisted gold recovery. Pilot projects in places like Ghana and parts of East Africa show that borax can maintain gold yields while reducing mercury use. Enforcement, however, varies. Many ASGM sites operate informally and lack the necessary equipment to capture vapors or manage tailings.
Industrial controls follow a different path. In countries that regulate mercury emissions from coal-fired power plants, deposition around those facilities has declined. These local gains are helpful, but they do not counter the increase in emissions from ASGM in other regions.
Biological and Technical Interventions
Some responses focus less on preventing emissions and more on reducing the amount of mercury that crops absorb.
- Phytoremediation: Certain non-food plants can absorb mercury from soil, so they are sometimes used in rotation or along field edges. Their role is limited to soil-based mercury, but they can help where soil levels are elevated.
- Microbial treatments: Researchers are testing bacteria that can change mercury into forms that plants absorb less readily. Early laboratory and small-plot trials show lower grain mercury levels when seeds are coated with these microbes.
- Low-cost monitoring tools: Simple passive air samplers allow communities to identify local hotspots without specialized equipment, which helps farmers decide which areas may need different management.
These measures differ in practicality and impact, but together they provide options for gradually reducing exposure.
How Farmers Are Adapting
Farm practices also play a role. Washing leafy vegetables can remove some of the mercury that settles on the surface. Choosing crop varieties that take up less mercury through their leaves also makes a difference, particularly for families who grow most of their own food. Some farmers adjust their rotation near mining areas by planting crops that are less sensitive to airborne inputs. Guidance on cooking methods or which parts of the plant to use can help reduce exposure without major changes to daily meals where extension services are available.
These steps do not remove the problem entirely, but they can lower the amount that reaches the plate.
Monitoring, Data Gaps, and Policy Questions
Stable-isotope analysis used in the Biogeosciences study shows how effective the method can be, as negative ε202Hg values in crops clearly point to an atmospheric source.2 Yet only a limited number of laboratories in Africa and South Asia can run these analyses regularly.
Mapping mining activity brings its own challenges. Rather than treating crop contamination as a separate or later consideration, it should be included more directly in mercury assessments.
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Remote-sensing tools can identify new pits and processing areas, but local reporting varies, and some sites operate only during certain parts of the year.
Weather patterns are another factor. Shifts in rainfall or wind can alter deposition patterns, meaning some seasons may lead to higher exposure than others.
Looking Ahead
Several research groups are examining options such as field barriers, improved emission hoods at processing sites, and crop varieties bred to reduce stomatal uptake, though these remain in early development. It remains uncertain how readily such measures can be applied in rural mining regions.
What current evidence does show is that airborne mercury reaches food crops more easily than previously assumed, with communities closest to artisanal mining sites experiencing the effects first.
Progress will likely depend on a combination of steps, including lowering mercury use in mining, providing practical alternatives for miners, strengthening air monitoring, and helping farmers adopt measures that reduce their intake.
Findings from the Nigerian study offer a useful starting point for this work.1, 2
The movement of mercury may be challenging to notice in daily life, but the data describing its impacts are becoming increasingly clear.
References and Further Reading
- European Geosciences Union. (2025) Invisible poison: Airborne mercury from gold mining is contaminating African food crops, new study warns. [Online] Available at: https://www.egu.eu/news/1510/invisible-poison-airborne-mercury-from-gold-mining-is-contaminating-african-food-crops-new-study-warns/
- Eboigbe, E. O., Veerasamy, N., Odukoya, A. M., Anene, N. C., Sonke, J. E., Sagisaka Méndez, S., & McLagan, D. S. (2025). Mercury contamination in staple crops impacted by artisanal and small-scale gold mining (ASGM): Stable Hg isotopes demonstrate dominance of atmospheric uptake pathway for Hg in crops. Biogeosciences, 22(18), 5591–5605. https://bg.copernicus.org/articles/22/5591/2025/
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