Mining Metals Are Slipping Into Ghana’s Vegetables, Landmark Review Finds

In the gold-rich landscapes of southern Ghana, two economies share the same soil. Artisanal and small-scale mining, known locally as galamsey, carves mercury-laced pits into farmland, while smallholder farmers grow the tomatoes, peppers, garden eggs and onions that anchor the national diet. In many villages, the same households do both. A new systematic review now offers the most complete accounting yet of how much of that mining legacy ends up on the dinner plate, and the numbers are sobering.

The review, published in Environmental Monitoring and Assessment by researchers at Ghana’s CSIR-Crops Research Institute and partner institutions, followed PRISMA 2020 guidelines and combed three databases for peer-reviewed studies published between January 2020 and January 2025. The team focused on five potentially toxic elements, lead, cadmium, chromium, mercury and arsenic, in the four vegetables that define Ghanaian cuisine. From an initial sweep of the literature, 45 eligible studies spanning 11 mining districts made the cut, supplemented cautiously by a single grey-literature technical report used only for site-level data unavailable elsewhere. Study quality was scored against a four-criterion framework, and agreement between the researchers doing the appraisals was strong, with a Cohen’s kappa of 0.87, a statistic that indicates the independent assessors rated the same studies in near-identical ways.

Because the underlying studies varied widely in design and sampling intensity, the authors chose a narrative synthesis with descriptive, sample-size-weighted aggregation rather than a formal inverse-variance meta-analysis. That methodological honesty matters: the review is designed to map where the evidence is strong and where it is thin, not to overstate precision. Even so, the patterns that emerge across the pooled data are difficult to dismiss.

The standout finding concerns cadmium. This heavy metal, which accumulates in kidneys and bones over years of chronic exposure, showed the highest soil-to-plant transfer of all five elements in tomato and garden egg, with transfer factors exceeding 1.0. A transfer factor above unity means the concentration in the edible plant tissue exceeds that in the soil on a mass basis. The authors are careful to note that such a ratio describes a concentration relationship rather than, by itself, proof of an active-uptake mechanism, but the practical implication is the same: cadmium moves readily from mining-affected soils into two of Ghana’s most consumed vegetables.

Chromium tells a different story, and a subtler one. Its soil-to-plant transfer was comparatively restricted, staying below 0.50 even at sites where soil concentrations climbed past 300 milligrams per kilogram. A casual reading might suggest chromium is the least worrying of the five. The review warns against that conclusion. Almost none of the studies examined distinguished between chromium(III), a relatively immobile and less toxic form, and chromium(VI), a carcinogenic oxidized species classified by the International Agency for Research on Cancer as a group 1 human carcinogen. Without speciation data, a low transfer factor for total chromium says little about the actual toxicological risk, because the small fraction that does enter plants, or the fraction that people inhale from dust, could be dominated by the dangerous form.

The regional picture is stark. Across the four regions covered by the evidence, vegetable concentrations of lead, arsenic and mercury exceeded the WHO/FAO Codex limits at several sites, with the worst exceedances concentrated in Ashanti and Western Region samples. At the single most contaminated sites reported in the literature, arsenic levels in vegetables reached up to 200 times the Codex threshold, and mercury up to 400 times. Those extremes come from the highest-contamination locations and should not be read as typical of every farm in every district, but they illustrate how severe localized contamination can become where mining waste and irrigation water intersect. Comparable regional breakdowns for Eastern and Central Region were more limited in the available literature, a gap the review flags explicitly.

To translate contamination into health terms, the researchers extracted hazard quotients and hazard indices, standard non-carcinogenic risk metrics that compare estimated intake against reference doses, drawing on the United States Environmental Protection Agency’s Integrated Risk Information System toxicity values. The result was consistent: the cumulative hazard index exceeded 1.0 in every mining-adjacent community for which it could be calculated. An index above 1.0 signals that regular consumers of locally grown vegetables face elevated non-carcinogenic risk, meaning combined exposures across the five metals surpass the thresholds regulators consider safe for chronic intake. Because the index aggregates across elements, a community can cross the line even when no single metal alone would trigger concern, which is precisely why mixture exposure is the more realistic framing for people eating these vegetables daily.

The review also extends its lens beyond the farm gate, examining the post-harvest stages of the value chain where contamination can be introduced or amplified. Handling, washing and market conditions all shape the final dose a consumer receives, and the evidence base for these stages is even patchier than for soil-to-plant transfer. Related work cited in the review, including studies on vegetable washing as a risk-reduction measure in urban agriculture, suggests simple interventions can lower exposure, but the review stops short of endorsing any single fix without site-specific data. The authors call instead for improved post-harvest handling practices grounded in local measurement rather than assumption.

What distinguishes this review is its refusal to flatten a complicated picture into a single alarm. The magnitude of risk, the authors conclude, varies by metal, by crop, by location and by the exposure assumptions each underlying study adopted. Cadmium’s mobility in tomato and garden egg does not imply the same behavior in onion. A 400-fold mercury exceedance at one site does not describe the neighboring valley. This unevenness is itself the central finding: Ghana’s vegetable value chain is a plausible and, in places, severe exposure pathway, but it is unevenly quantified, and policy built on averages will misallocate resources. The evidence supports targeted monitoring of the worst sites, source-specific risk assessment that traces which mining operations feed which fields, and site-specific rather than uniformly prescribed agricultural buffer zones between mines and cropland.

The research agenda that follows is equally concrete. Chromium and arsenic speciation studies top the list, since total-element measurements cannot distinguish benign forms from carcinogenic ones. Cumulative and synergistic risk modelling comes next, capturing the reality that consumers absorb mixtures, not single metals. And longitudinal human biomonitoring, tracking metal burdens in residents of mining-adjacent communities over time, would finally connect the environmental measurements to actual health outcomes. For now, the review delivers a clear message to Ghanaian regulators and consumers alike: the vegetables grown beside the gold pits carry a measurable mining signature, the risk is real but uneven, and the first step toward managing it is knowing precisely where, and in what chemical form, the danger lies.

Subject of Research: Mining-related potentially toxic element contamination of Ghana's vegetable value chain and associated human health risks

Article Title: Potentially toxic element contamination in Ghana’s vegetable value chain: a systematic review of mining-related soil-to-food transfer and human health risk

Article References: 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(10), Article 1116. https://doi.org/10.1007/s10661-026-15934-6

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15934-6

Keywords: Ghana, artisanal and small-scale mining, cadmium, arsenic, mercury, lead, chromium speciation, vegetable contamination, soil-to-plant transfer, hazard index, food safety, systematic review