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

In Ghana, artisanal and small-scale mining (ASM) and smallholder vegetable farming occupy the same landscapes, and in many communities the same households combine both livelihoods, yet no systematic account has previously quantified how much potentially toxic element (PTE) contamination travels from mine waste into food. This review addresses that gap for the pre-harvest and post-harvest stages of the value chain for the four vegetables that define Ghanaian diets: tomato (Solanum lycopersicum), pepper (Capsicum annuum), garden egg (Solanum aethiopicum), and onion (Allium cepa). Adhering to PRISMA 2020 guidelines, three databases (PubMed, ScienceDirect, Google Scholar) were searched for peer-reviewed studies published January 2020-January 2025 reporting lead (Pb), cadmium (Cd), chromium (Cr), mercury (Hg), and arsenic (As) concentrations in the four target vegetables from Ghana's mining-affected regions. Soil-to-plant transfer factors (TF), water-to-plant bioconcentration factors (BCF, where irrigation-water data were reported), and non-carcinogenic health risk indices (hazard quotient, HQ; hazard index, HI) were extracted from 45 eligible peer-reviewed studies across 11 mining districts, supplemented by one grey-literature technical report used conservatively for site-level data unavailable elsewhere. Study quality was appraised using a four-criterion framework; inter-rater agreement was strong (Cohen's κ = 0.87). This is a narrative synthesis with descriptive, sample-size-weighted aggregation rather than a formal inverse-variance meta-analysis. Cadmium showed the highest soil-to-plant transfer of the five PTEs in tomato and garden egg (TF > 1.0), though a transfer ratio above unity describes a concentration relationship rather than, by itself, proof of an active-uptake mechanism. Chromium's soil-to-plant transfer was comparatively restricted (TF < 0.50) even where soil concentrations exceeded 300 mg/kg, but this cannot be interpreted as low toxicological risk because almost none of the reviewed studies distinguished Cr (III) from the far more toxic Cr (VI). Across the four regions studied, vegetable lead, arsenic and mercury concentrations exceeded WHO/FAO Codex limits at several sites, most markedly in Ashanti and Western Region samples (up to 200-fold for arsenic and 400-fold for mercury at the single highest-contamination sites reported); comparable regional breakdowns for Eastern and Central Region were more limited in the available literature. Cumulative HI exceeded 1.0 in every mining-adjacent community for which it could be calculated, indicating elevated non-carcinogenic risk for regular vegetable consumers. Ghana's vegetable value chain is a plausible but unevenly quantified PTE exposure pathway in mining-dependent communities; the magnitude of risk varies by metal, crop, location and the exposure assumptions used. The evidence supports targeted monitoring, source-specific risk assessment, improved post-harvest handling, and site-specific rather than uniformly prescribed agricultural buffer zones. Chromium and arsenic speciation, cumulative and synergistic risk modelling, and longitudinal human biomonitoring are identified as the priority research needs.