Toxic Metals in Kano Farm Vegetables Exceed Safe Limits, Study Warns

On the outskirts of Kano, one of Nigeria’s largest commercial cities, vegetable farms line the banks of the River Tatsawarki. During the long dry season, when rainfall is scarce, farmers pump water directly from the river to irrigate tomatoes, spinach, lettuce, and onions destined for local markets. But that river also collects untreated wastewater from the Sharada Phase I industrial district, home to tanneries, textile mills, and food processing plants. A new study published in Discover Toxicology has now measured exactly what that water is delivering to the fields, and the results point to a quiet but serious public health problem: cadmium and chromium concentrations in both soil and vegetables that exceed World Health Organization safety limits, with children facing the greatest exposure risk.

The research team, led by Blessing Edogbo of the National Open University of Nigeria together with colleagues at Ahmadu Bello University in Zaria, collected 108 soil samples and 108 vegetable samples from three farms along the river between February and April 2022. The soils proved to be sandy loams with low organic matter, high porosity, and a slightly alkaline pH ranging from 7.30 to 8.20. Electrical conductivity values were low, between 0.22 and 0.55 dS/m, suggesting that soluble salts had largely leached away through repeated irrigation. The team then digested the samples with a nitric, sulfuric, and perchloric acid mixture and quantified three toxic metals—cadmium, chromium, and zinc—using flame atomic absorption spectrophotometry, a technique that measures how much light each metal absorbs at a characteristic wavelength, allowing precise concentration readings down to very low levels.

The numbers were striking. Mean soil concentrations reached 9.32 milligrams per kilogram for cadmium, 78.73 for chromium, and 33.67 for zinc. While zinc remained within the WHO permissible limit of 300 milligrams per kilogram, cadmium exceeded its limit of 3 and chromium its limit of 30. Cadmium levels were broadly similar across all three farms, at roughly 8.5 to 10 milligrams per kilogram, but chromium varied dramatically, from about 47 milligrams per kilogram at one farm to 114 at another, with the highest value significantly exceeding the others. In the vegetables themselves, cadmium ranged from undetectable to 13.50 milligrams per kilogram and chromium from 1.50 to 82.25, with lettuce, onion, and spinach among the worst accumulators. Cadmium in every vegetable type surpassed the FAO/WHO permissible limit of 0.3 milligrams per kilogram.

To translate raw concentrations into a picture of contamination severity, the researchers calculated a battery of pollution indices. The geo-accumulation index placed cadmium in the moderately contaminated category, chromium between uncontaminated and moderately contaminated, and zinc effectively uncontaminated. The enrichment factor for cadmium, at 15.47, signaled a strong industrial fingerprint, since values above two generally indicate human rather than natural inputs. The contamination factor told a similar story: cadmium scored 15.53, classified as very high contamination, chromium 3.03, indicating significant contamination, and zinc just 0.35. Perhaps most telling was the potential ecological risk assessment, in which each metal’s concentration is weighted by its toxicity. Cadmium’s ecological risk factor reached 466, dwarfing chromium’s 6.05, and the total ecological risk index of 472.4 placed the farmland in the considerable ecological threat category.

Where did the metals come from? To answer that question, the team turned to multivariate statistics. Principal component analysis, which compresses many variables into a few underlying axes of variation, revealed that the first three components explained 45.79, 32.08, and 22.13 percent of the variance respectively. Cadmium and zinc loaded together on the same component, suggesting a shared origin, while chromium behaved independently. Hierarchical cluster analysis, which groups variables by similarity, confirmed this split: cadmium and zinc fell into one cluster, chromium into another. The researchers interpret this pattern as evidence that cadmium and zinc derive mainly from anthropogenic sources—industrial effluents, agrochemicals, and urban runoff—whereas chromium reflects a mixture of lithogenic and human contributions, consistent with chromium’s role as a major component of tannery waste and leather manufacturing byproducts discharged into the river.

The study also traced how efficiently each metal moves from soil into plant tissue, a critical step in human exposure. Transfer factors, calculated as the ratio of metal concentration in the vegetable to that in the corresponding soil, varied by crop. Zinc showed the highest overall mobility, reaching 1.20 in lettuce and 1.14 in onion, meaning these crops accumulated more zinc than was present in an equivalent mass of soil. Chromium transferred most efficiently into spinach, at 0.88, while cadmium’s highest transfer factor, 0.74, also occurred in spinach. These differences reflect plant physiology: species differ in root uptake, translocation, and sequestration mechanisms, and soil properties such as pH and texture modulate metal solubility. The alkaline soils of Tatsawarki tend to immobilize metals, yet the sheer magnitude of contamination still pushed plant concentrations above safety thresholds.

The health risk calculations are where the findings become most alarming. The estimated daily intake of metals, computed for adults of 70 kilograms and children of 20 kilograms consuming 65 grams of vegetables per day, revealed that children absorb proportionally far more of the contaminant load. For children, estimated daily intakes of cadmium ranged from 0.25 to 1.58 milligrams per kilogram of body weight, chromium from 3.76 to 11.49, and zinc from 5.26 to 13.24—several times higher than the corresponding adult figures. The target hazard quotient, a ratio comparing exposure to a reference dose below which adverse effects are not expected, exceeded the safety threshold of one for cadmium in nearly all vegetables. For adults, cadmium target hazard quotients ranged from 0.80 to 5.11; for children, they spanned 2.80 to 17.90, with spinach and onion the riskiest items on the plate.

The health risk index, which aggregates these exposures, reinforced the same conclusion. In adults, values ranged from 0.92 for lettuce to 5.18 for spinach, while in children they climbed from 3.22 to 18.14 across the same crops. Any value above one signals potential harm, and every vegetable except lettuce in adults crossed that line. Cadmium is a particularly insidious contaminant: it accumulates in the kidneys and other organs over years of low-level exposure and has been linked to chronic kidney disease, hypertension, diabetes, and atherosclerosis. Chromium, especially in its hexavalent form common in tannery waste, can cause skin ulceration through dermal contact and is associated with respiratory and carcinogenic risks. The study’s authors note that children are more vulnerable not only because they eat more food per unit of body weight but also because their developing bodies absorb metals more readily.

The findings sit within a broader pattern documented across Nigeria and beyond. Previous work in the Challawa industrial area of Kano State reported cadmium levels in vegetables well above permissible limits, and studies in Bangladesh, India, Ethiopia, and Ecuador have found similar contamination in crops irrigated with tannery effluent or wastewater. What distinguishes the new research is its comprehensive approach: by combining pollution indices, source-tracing statistics, and formal health risk assessment in a single framework, it connects the industrial discharge in the river to the dinner plate with an unbroken chain of evidence. The authors also acknowledge limitations—the sampling covered only the dry season, a single soil depth, and farms near industry without distant control sites—leaving room for follow-up studies on seasonal variation and root, stem, and leaf partitioning of metals.

The researchers argue that the solution requires more than monitoring. They call for regulatory enforcement against untreated effluent discharge, community education, and remediation strategies such as phytoremediation, in which specific plants extract or stabilize contaminants in soil. Given that irrigation farming in Kano is driven by necessity in a region with limited rainfall, simply telling farmers to stop using river water is not a realistic option. The study instead offers policymakers a data-driven baseline for intervention: treat industrial wastewater before it reaches the river, test produce entering urban markets, and prioritize protecting children, whose exposure estimates suggest they bear the heaviest burden of a contamination problem they did nothing to create. For now, the vegetables of Tatsawarki remain on local tables, carrying with them an invisible load of industrial legacy.

Subject of Research: Toxic metal contamination of soils and vegetables irrigated with industrial wastewater in Kano State, Nigeria, and associated human health risks

Article Title: Health risk analysis of toxic metals in soils and vegetables from Tatsawarki Farms in Kano State, Nigeria

Article References: Edogbo, B., Adamu, S. U., Saliu, O. A., Nasir, S., Sada, S. A., Ayodeji, I. O., Tijjani, H., Ebhodaghe, F., Yisa, A., Aluwong, T., Okolocha, E., & Uchendu, C. (2025). Health risk analysis of toxic metals in soils and vegetables from Tatsawarki Farms in Kano State, Nigeria. Discover Toxicology, 2(1), Article 17. https://doi.org/10.1007/s44339-025-00033-x

Image Credits: AI Generated

DOI: 10.1007/s44339-025-00033-x

Keywords: heavy metals, cadmium, chromium, zinc, soil contamination, vegetables, food safety, wastewater irrigation, health risk assessment, Nigeria, Kano, pollution indices