Introduction
Tomato, pepper, garden egg and onion sit at the heart of almost every Ghanaian kitchen. Light soup depends on them. So do stews, shito and the everyday sauces eaten across the country, from Tamale to Takoradi. Because these four vegetables reach so many homes so often, any contaminant carried within them reaches the same homes with the same regularity.
Across the gold-bearing belts of the Ashanti, Western, Eastern and Central Regions, artisanal and small-scale mining, known locally as galamsey, now operates on land that was once farmed, and in many places still is. Pits are dug beside vegetable beds. Streams that carry mine sediment are used to irrigate crops. In some communities the same household mines in the dry season and plants vegetables when the rains return. Concerns about galamsey have long centred on muddy rivers and lost forest cover, yet far less attention has been given to what travels from disturbed soil into the food sold in local markets.
Our study, published in Environmental Monitoring and Assessment, set out to determine how far five potentially toxic elements (lead, cadmium, chromium, mercury and arsenic) move from mining-affected soil and irrigation water into tomato, pepper, garden egg and onion, and to estimate the resulting non-cancer health risk for consumers in Ghana. To answer this, a systematic review guided by the PRISMA 2020 statement (Page et al., 2021) was carried out, drawing on peer-reviewed studies from PubMed, ScienceDirect and Google Scholar published between January 2020 and January 2025, and the extracted data were synthesised through transfer factors, bioconcentration factors, hazard quotients and hazard indices.
Why this question mattered to us
At the CSIR-Crops Research Institute, horticulture is not an abstract field. Vegetable farmers are our closest partners, and their crops feed both their households and the urban markets that depend on them. Much of my own fieldwork has taken place in mining-affected farming communities in the Ashanti Region, where the pressure from galamsey on farmland can be seen at first hand.
Individual studies on heavy metals in Ghanaian vegetables were already available. They were scattered, though. Each focused on one district, one crop or one metal, and their methods differed widely. What was missing was a single, careful synthesis that could tell farmers, regulators and health authorities where the evidence was strong, where it was thin, and where the risk to consumers appeared greatest.
How the evidence was gathered
After screening, 45 studies from 11 mining districts satisfied our inclusion criteria. One technical report was also retained, but only for site information that the peer-reviewed literature did not provide. Three kinds of evidence were then extracted. Transfer factors describe how readily a metal passes from soil into the edible plant. Bioconcentration factors capture movement from irrigation water into the crop, wherever water had been sampled. Hazard quotients and hazard indices estimate the likelihood of non-cancer health effects for people who eat these vegetables regularly.
Study quality was scored independently by two reviewers using a four-point framework. We agreed closely (Cohen's kappa = 0.87). Because the studies varied so much in design and reporting, we chose a narrative synthesis supported by sample-size-weighted summaries rather than a formal meta-analysis, which the data could not honestly support.
What we found
Cadmium was the clearest signal. In tomato and garden egg, it showed the highest soil-to-plant transfer of the five elements, with transfer factors above 1.0. Put simply, the concentration in the vegetable could equal or exceed that in the soil beneath it. This is a statement about concentration ratios, not by itself proof of the uptake mechanism, and we have been careful to present it that way.
Chromium behaved differently. Transfer into the plants stayed low (transfer factors below 0.5), even where soil levels exceeded 300 mg/kg. That result should not be read as reassurance. Almost none of the reviewed studies distinguished trivalent chromium from hexavalent chromium, the far more toxic form, so a low transfer of total chromium could still conceal a genuine hazard.
Lead, arsenic and mercury exceeded the WHO/FAO Codex limits for vegetables at several sites, most strikingly in the Ashanti and Western Regions. At the single most contaminated sites reported, arsenic reached up to 200 times the limit and mercury up to 400 times. Coverage of the Eastern and Central Regions was more limited.
For public health, one finding stood above the rest. In every mining-adjacent community where the cumulative hazard index could be calculated, it exceeded 1.0. People who regularly consume vegetables from these places therefore face an elevated non-cancer health risk.
The difficulties behind the numbers
Bringing 45 studies together proved harder than expected. Units differed from paper to paper. Key details, such as paired soil and water data or the chemical form of chromium, were often missing, and regional coverage was uneven. Several judgements had to be made about what the evidence could bear, and those limits are reported openly in the paper. The risk is real. Its size, though, varies with the metal, the crop, the location and the exposure assumptions used.
What the evidence asks of us
Three practical steps follow from this review. Monitoring of vegetables, soil and irrigation water should be targeted at mining-affected districts rather than spread evenly across the country. Post-harvest washing and handling can reduce contamination on the surface of produce before it reaches markets, although they cannot remove metals already taken up into plant tissue. Finally, agricultural buffer zones around mining sites should be set from local contamination data instead of a single fixed distance.
Three research gaps also deserve attention. Chromium and arsenic need to be measured by chemical form. The combined effects of several metals acting together remain poorly modelled. And long-term human biomonitoring in affected communities is needed, so that what is found in the food can be linked to what is found in people.
Looking ahead
This work belongs to a team. I am grateful to my co-authors, Michael Kwabena Osei, Harry Okyere and James Korang, and to colleagues at the CSIR-Crops Research Institute and the CSIR College of Science and Technology for their guidance throughout.
Galamsey is usually discussed in terms of rivers and forests. Our hope is that this review places the food on Ghanaian plates firmly within that conversation. Researchers, regulators and farmer organisations working on food safety, mining or soil remediation in West Africa are warmly invited to comment below or get in touch. Much remains to be done, and it is better done together.
Read the paper
Bonsu, E. O., Osei, M. K., Okyere, H., & Korang, J. (2026). Potentially toxic element contamination in Ghana's vegetable value chain: A systematic review of mining-related soil-to-food transfer and human health risk. Environmental Monitoring and Assessment, 198, 1116. https://doi.org/10.1007/s10661-026-15934-6
Reference
Page, M. J., McKenzie, J. E., Bossuyt, P. M., Boutron, I., Hoffmann, T. C., Mulrow, C. D., et al. (2021). The PRISMA 2020 statement: An updated guideline for reporting systematic reviews. BMJ, 372, n71. https://doi.org/10.1136/bmj.n71
Image credits
Header image: "Bridge over destroyed river in Assin Kushea, Ghana", photograph by Goldenwabbit, Wikimedia Commons (Wiki Loves Earth 2024), licensed under CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/). Used unaltered.
In-text image: "Land disturbances at ASGM site in Western Region, Wassa West District, Ghana", from Rajaee M. et al. (2015), Int. J. Environ. Res. Public Health 12(8): 8971–9011, via Wikimedia Commons, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Used unaltered.