Analysis of Selected Potentially Harmful Metal Elements in Soils and Vegetables in Gold Mining Region: Case Study Evaluated in Kenya, Africa

Abstract

This study aimed to assess heavy metal and associated trace element contamination in soils and vegetables from artisanal gold mining areas in Migori County, Kenya. Soil concentrations were markedly elevated, with Pb (15.4–706 mg/kg), Cd (0.14–6.07 mg/kg), Ni (0.2–33.4 mg/kg), Cr (11.9–119.3 mg/kg), As (0.1–37.4 mg/kg), Zn (38–1454 mg/kg), Se (0.1–0.8 mg/kg), and Hg (0.51–1830 mg/kg) all exceeding international guideline values. Corresponding vegetable concentrations were as follows: Pb (0.17–71.3 mg/kg), Ni (0.2–111 mg/kg), Cr (2.4–244 mg/kg), As (1.2–399 mg/kg), Hg (0.22–35 mg/kg), Zn (11.2–67.4 mg/kg), and Se (0.1–5.7 mg/kg). Brassica oleracea var. capitata (cabbage) exhibited the highest uptake, while Amaranthus hybridus (smooth pigweed) showed the lowest. Estimated daily intake (EDI) values for Pb, Ni, Cr, As, Zn, and Hg exceeded FAO/WHO limits, with hazard quotients (HQ) > 1 for all metals and hazard index (HI) values between 15.6 and 30.4, indicating significant non-carcinogenic and carcinogenic risks. These findings highlight severe contamination linked to geological background and mining activity, underscoring the urgent need for regular monitoring and mitigation to protect food safety and public health.

1. Introduction

Leafy vegetables are a major component of diets in Sub-Saharan Africa, providing affordable and accessible sources of vitamins, minerals, and dietary fiber. However, they are also known to accumulate toxic elements when grown in contaminated soils or exposed to atmospheric deposition. This makes vegetables an important pathway for human exposure to potentially harmful elements (PHEs), especially in areas affected by mining activities [1,2,3,4]. Globally, vegetables are described as essential components of a balanced diet since they form a crucial source of nutraceuticals in everyday human life [1]. Thus, ingestion of vegetables as food provides a fast and cheap mode through which adequate sources of minerals, vitamins, and fiber necessary for proper metabolism to the body [2] can be provided. Some vegetables comprise significant amounts of proteins, carbohydrates, and fats necessary for a healthy, balanced diet.

Although cereals such as millet, maize, and wheat, together with animal protein sources like beef and mutton, constitute major dietary staples across Africa, vegetables remain an indispensable component of daily meals, particularly in rural and peri-urban households. Dietary surveys have demonstrated that vegetables contribute substantially to micronutrient intake and are frequently consumed alongside cereal-based staples. For instance, household food consumption surveys in Kenya reported average vegetable intakes of 250–320 g/day, particularly in rural populations. Similarly, studies in Nigeria and Ghana indicate that vegetables such as leafy greens and tomatoes are regularly consumed and represent a significant source of dietary vitamins and minerals. These findings underscore the nutritional importance of vegetables in African diets, justifying their consideration in dietary exposure assessments [3,4,5,6].

Heavy metals and trace elements such as lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As) (a metalloid), and chromium (Cr) (light metal) are among the most hazardous environmental contaminants because of their persistence, bioaccumulation, and toxicity. Their concentrations in soils can be elevated through natural processes such as weathering, but, in mining regions, anthropogenic inputs are the dominant source. Artisanal and small-scale gold mining (ASGM) in particular is widely practiced in East Africa and often involves the use of mercury in gold amalgamation. Studies from Tanzania, Ghana, and Zimbabwe have reported dangerously high concentrations of Pb, As, and Hg in soils and crops near mining sites, posing both carcinogenic and non-carcinogenic health risks [5,6,7]. Several authors have identified human activities such as logging, agricultural production, vehicular pollution, chemical usage, and fertilizer application as some of the causes of elevated heavy metal levels in the environment [6,7,8]. Chronic exposure to heavy metals has serious implications for human health. Lead has been linked to neurodevelopmental disorders, impaired growth, and organ damage. Mercury is a potent neurotoxin that can affect children and unborn babies even at low concentrations. Arsenic is a well-established carcinogen associated with cancers of the skin, bladder, and gastrointestinal tract, while cadmium accumulates in the kidneys and bones, causing chronic disease. Such health risks are intensified in populations relying heavily on locally grown vegetables as daily food staples [9,10,11,12]. These heavy metals have been reported as significant pollutants in the environment globally, predominantly in regions with increased anthropogenic activities such as mining [13] and intense agricultural activities. Cr, Cu, and Zn have been linked to non-cancer development risks [14,15] such as liver disease, headaches, and neurologic involvement when each exceeds the recommended maximum safe limit [16,17]. In addition, Pb has been reported to decrease some essential elements such as Zn in the body. This has been linked to decreased immune abilities, fetal growth restriction, impaired psychological and social abilities, infirmities linked to malnourishment, and elevated occurrence of cancers of the upper gastrointestinal section, among many other human health disorders [13,18]. Thus, the presence of heavy metals in any environmental media poses a great health threat to humans, animals, and plants [12] by compromising the quality of plant productivity, which, in turn, affects food security.

Heavy metals (HMs) are naturally occurring components of the natural environment, but their geochemical dynamics and biochemical balance have been disrupted by their indiscriminate application for human purposes. Edible vegetable varieties that are readily available in the area under study are particularly susceptible to heavy metal contamination from the soils and airborne load brought on by gold mining operations. Although several African studies have examined heavy metal contamination in mining regions, there is limited data from Kenya. In Migori County, artisanal gold mining is a dominant livelihood activity, yet little is known about how local soils and vegetables are affected by mining-related pollution and what risks this poses to consumers. To address this gap, the present study (i) quantified concentrations of Hg, As, Cd, Cr, Pb, Ni, Se, and Zn in soils and edible vegetables from artisanal mining areas of Migori, (ii) calculated soil–plant transfer coefficients, and (iii) assessed human health risks using estimated daily intake (EDI), hazard quotient (HQ), and hazard index (HI).

2. Materials and Methods

2.1. Study Area Setting

This study was carried out in Migori gold mines in Migori County, Western Kenya. This is one of the 47 counties in Kenya, with an approximate area of 2596 km² and a population of 1,152,164, and is bordered to the west by Lake Victoria and to the south by the Republic of Tanzania (Figure 1). Figure 2 shows the study area, which is located within a geologically metal-rich zone within the Lake Victoria Basin. Lake Victoria is approximately 68,800 km² and is the world’s second-largest freshwater body, forming a catchment area of approximately 84,000 km². This catchment area is shared by three riparian nations, namely, Kenya, Uganda, and Tanzania. The main human economic activities comprise fishing, subsistence agriculture, and gold mining, mainly artisanal. About 90% of the population within the study area depends on gold mining both directly and indirectly for their livelihood.

The rocks found in the sampling sites within Migori County are Archaean in age, estimated to be around 2.8 billion years old, and are known as the Migori granite–greenstone complex. The volcanic rocks display a bimodal mafic–felsic composition, with felsic rocks being more prevalent. The mafic rocks are primarily tholeiitic, while the felsic rocks are calc-alkaline and rich in potassium, particularly in the form of high-potassium dacites. These dacites are the main volcanic rocks in the area and form a chemical continuum with many granites. The basalts and calc-alkaline felsic rocks were originally erupted in a submarine setting, while the younger high-potassium dacites were deposited in a subaerial environment. This mixture of tholeiitic and calc-alkaline rocks is typical of volcanic arc regions, where the presence of high-potassium volcanic rocks indicates the existence of continental crust. The host rocks include metabasalt, banded ironstone, shales, and andesites [13].

The soil in the Migori gold mine areas of Kenya is predominantly lateritic. It is typically red or reddish-brown due to the high iron oxide content. These areas are characterized by weathering processes that have leached out more soluble materials, leaving behind the more resistant minerals such as iron and aluminum oxides. The presence of lateritic soils often indicates underlying mineral deposits, including gold, which is why these areas have been the focus of mining activities. Additionally, the soils can have varying textures, ranging from gravelly to clayey, depending on the local geological conditions and the degree of weathering [13].

2.2. Sampling Design and Procedures

All equipment was pre-soaked in a 1:1 volume mix of concentrated nitric acid (65%) and sulfuric acid (30%) solutions prior to use. Edible portions of four vegetable species—smooth pigweed, collard greens, cabbage, and black nightshade—were harvested from the same farms where soil samples were collected. For each species, three replicate samples were collected from different plants within each farm to avoid repeated harvesting from the same specimen. The vegetables were sliced into small pieces, oven-dried at 105 °C to a constant weight, ground into a fine powder using a mortar and pestle, sieved, and packed separately in sterile polyethylene bags for storage.

A total of 30 soil samples were collected at depths of 0–20 cm within the artisanal gold mining region from three different sampling zones, each comprising ten georeferenced sampling points recorded using a global positioning system (GPS). At each sampling point, four subsamples were randomly taken from the four corners of a 20 m × 20 m square, in triplicate, following IGCP 259 guidelines [20]. The triplicate subsamples were homogenized to form a composite sample, which was quartered on a clean plastic sheet. All soil samples were packed in sterile polyethylene bags and transported to the laboratory for analysis.

In the laboratory, each soil sample was air-dried, ground, and sieved through a 0.5 mm plastic sieve before being stored in sterile brown bags at –4 °C until digestion. Hydrofluoric (HF) acid was used for soil digestion to enable complete dissolution of silicate-rich matrices. In contrast, vegetable samples, which were primarily organic, were digested using nitric and sulfuric acids, which are sufficient for complete mineralization of plant tissues. Certified reference materials (CRMs) were analyzed alongside samples to ensure QA/QC. Recovery rates for the metals of interest ranged between 85% and 110%, which falls within acceptable analytical limits for ICP-MS.

2.3. Preparation and Analysis of Samples

For all soil samples, approximately 0.25 g was digested with 5 mL of 65% nitric acid (HNO₃), 10 mL of 40% hydrofluoric acid (HF), and 4 mL of 70% perchloric acid (HClO₄) at 100 °C. This digestion method provides total elemental concentrations rather than bioavailable or mobile fractions. The resultant solution was then cooled to room temperature (24 °C) and filtered and distilled water was added to make up a dilution of 50 mL. Similarly, thirty samples of edible parts of commonly consumed vegetables, including collard greens, cabbage, and black nightshade, collected in the study area were thoroughly rinsed with deionized water to remove adhering soil and dust particles before drying, ensuring that the measured trace element concentrations reflected plant uptake rather than external contamination. The samples were then homogenized and ground. About 2 g of each vegetable sample was digested using HNO₃ and HClO₄ in portions of 5:1 at 180 °C. The digested vegetable samples were then filtered and distilled water was added to achieve a volume of 50 mL.

Following preparation, the samples were properly packaged, tagged, and delivered to Bureau Veritas Laboratories Company in Canada. Using Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) at Bueritas Labs in Vancouver, Canada, the concentrations of heavy metals and associated trace elements, such as As, Cd, Cr, Pb, Ni, Se, Hg, and Zn, were determined in soil and vegetable samples. The total amount of heavy metals found in the vegetables and soils is expressed as mg/kg for uniformity.

2.4. Calibration Process

The calibration of the ICP-MS instrument was carried out using multi-element standard solutions obtained from certified suppliers. Calibration curves were constructed by analyzing a series of standard solutions at varying concentrations, typically ranging from 0.1 µg/L to 100 µg/L, depending on the element of interest. Each standard solution was prepared in a matrix-matched diluent to ensure accuracy. The calibration process included at least five concentration points to ensure a robust and linear calibration curve. The correlation coefficient (µ) for all calibration curves was maintained above 0.999 to confirm linearity.

2.5. Certified Reference Materials (CRMs) and Quality Control Measures

This study employed certified reference materials (CRMs) to validate the accuracy of the measurements. For this study, CRMs such as NIST SRM 1643f (Trace Elements in Water) and ERM^®-CC141 (Trace Elements in River Water) were used. These reference materials were analyzed at regular intervals to ensure that the instrument maintained accuracy throughout the analytical process. For each metal, blank reagents and a standardized solution of the metals to be studied were prepared from the stock solutions. The accuracy and precision were checked by repetitive analysis against the standard materials, with a confidence limit of over 95%. Quality control measures included the analysis of procedural blanks, duplicate samples, and spiked recovery tests. The procedural blanks helped identify any potential contamination during sample preparation, while duplicate samples ensured consistency. Spiked recovery tests involved adding known concentrations of target elements to samples, with recoveries expected within 85–115% to confirm method accuracy.

2.6. Instrument Efficiency and Drift Monitoring

Instrument efficiency was regularly evaluated by monitoring internal standard performance. Internal standards, such as Scandium (Sc), Germanium (Ge), and Yttrium (Y), were added to all samples and standards at a fixed concentration. Variations in internal standard signal intensities beyond ±10% indicated potential matrix effects or drift, which were corrected in real-time. To monitor instrumental drift, quality control samples were analyzed after every 10 samples. These QC samples consisted of mid-level calibration standards treated as unknowns. Consistency in their measured concentrations indicated stable instrument performance over time, ensuring the reliability of the acquired data.

2.7. Coefficient of Soil–Plant Transfer in Percent

The enrichment factor or the soil–plant transfer coefficient gave the relative variations in bioavailability of heavy metals to plants from the environment or soil media. Using equation A [21], the soil transfer coefficient was calculated as a ratio of a heavy metal in the vegetable (dry weight) studied to the total heavy metal content analyzed in the soil.

TC = C__vegetable X 100/C__soil

(1)

TC represents the percentage of the calculated transfer coefficient; C__vegtable is the concentration of heavy metals in vegetable fleshy parts, expressed as mg/100 g; and C__soil is the concentration of metals in soil in mg/100 g of dry soil.

2.8. Estimated Daily Intake (EDI) of Heavy Metals Studied

Using formula B [22], the estimated daily intake (EDI) of heavy metals was calculated as the product of mean daily vegetable consumption per person, the proportion of vegetable dry weight, and the average heavy metal concentration per dry weight vegetable. In equation B, EDI is expressed as mg/individual/day, Av. _consumption refers to the average daily intake of edible vegetables (g/day), and % DW__vegetable represents the proportion of vegetable dry weight, calculated as (% DW = [(100 − % moisture)/100]). C__{heavy metal} denotes the mean heavy metal concentration in dry weight vegetables (mg/g) and 0.001 is a unit conversion factor. The Estimated Daily Intake (EDI) of heavy metals was calculated in both mg/day and mg/kg body weight/day to align with international risk assessment standards. For consistency, an adult reference body weight of 60 kg, as recommended by the USEPA (2011) and WHO (2010), was adopted throughout this study. Daily vegetable consumption was assumed to be 300 g/day, based on dietary survey data from Sub-Saharan Africa indicating a range of 250–350 g/day in rural populations, where vegetables constitute a major part of meals. However, it is acknowledged that vegetable intake varies widely across African countries. For example, FAO (2022) reports average intakes of 69 g/day in Equatorial Guinea and 196 g/day in Tunisia. We therefore recognize that our estimate represents an upper-bound scenario intended to provide a conservative assessment of potential dietary exposure to heavy metals from vegetables [22,23,24]. Table S1 (Supplementary File) presents tolerable daily intake values for selected metals based on international standards [25,26,27,28,29,30].

EDI = (Av_{. consumption}) × (DW__vegetable) × (C__{heavy metal}) × 0.001

(2)

2.9. Hazard Quotient (HQ)

This study discusses the hazard quotient (HQ), which estimates the likelihood and severity of adverse effects from exposure to elements or chemicals, using a value either below or above 1 (calculated HQ value = <1>). It represents the ratio of exposure to a reference dose, such as the estimated daily intake (EDI). An HQ of <1 indicates no expected health risks, whereas an HQ of >1 suggests potential health risks [31]. As shown in equation C, HQ is calculated by dividing the estimated dose by the reference dose [32]. The EDI reflects the average daily vegetable consumption (mg/kg/day), while the reference dose (RfD) indicates the daily exposure level considered safe throughout a lifetime, varying for different metals (e.g., Zn: 0.3 mg/kg/day, Pb: 0.004 mg/kg/day, Cu: 0.04 mg/kg/day) (Table S1). Carcinogenic potency slopes oral (CPSo), provided by the USEPA [33] and detailed in Table S2 (Supplementary File), represent the lifetime cancer risk associated with exposure to specific contaminants by relating intake to cancer probability [33,34].

HQ = EDI/RfD

2.10. Hazard Index (HI)

When multiple pollutants are present, their effects are cumulative. The hazard index (HI) is used to estimate the overall risk from such combined exposure. An HI value greater than 1 indicates significant health risks from toxic substances in food. HI is calculated by summing the individual hazard quotients (HQs) of each pollutant, such as lead (Pb) and zinc (Zn), reflecting the total combined effect. Equation D [35] represents the HI, which is a common method in risk assessment to estimate non-carcinogenic health risks from exposure to multiple toxicants (e.g., heavy metals).

$$\mathrm{H}\mathrm{I}={\sum }_{\mathrm{i}=1}^4\mathrm{H}\mathrm{Q}\left[\begin{array}{cc}\mathrm{E}\mathrm{D}\mathrm{I}\mathrm{P}\mathrm{b}\mathrm{X}& \mathrm{C}\mathrm{h}\mathrm{m}\mathrm{P}\mathrm{b}+\mathrm{E}\mathrm{D}\mathrm{I}\mathrm{Z}\mathrm{n}\mathrm{X}\mathrm{C}\mathrm{h}\mathrm{m}\mathrm{Z}\mathrm{n}\\ \mathrm{R}\mathrm{f}\mathrm{D}& \mathrm{R}\mathrm{f}\mathrm{D}\end{array}\right]+\mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{s}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{i}\mathrm{s}\mathrm{e}\mathrm{x}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{e}\mathrm{d}\mathrm{t}\mathrm{o}$$

3. Data Analysis

All data was analyzed using IBM SPSS Statistics for Windows, Version 21. For all measurements, the W test [36] was also used to test the log-normal distributions of the results. To satisfy the criterion of normality prior to the statistical method, all nonparametric data was log-transformed using the equation x′ = log (x + 1) [37]. All reported heavy metal and trace element concentration data from analyzed samples of soils and vegetables collected from sampling sites are presented as geometric means (GMs). Descriptive data analysis was also carried out and the means obtained were weighted using Tukey’s post hoc test at a significance level of 5% [38]. Correlation tests were conducted to obtain the relationship between metals.

4. Results and Discussion

4.1. Heavy Metal Concentration in Soil

Zinc had the greatest concentrations of heavy metals in agricultural soil samples, followed by Cd, Pb, Cr, Ni, As, and Se in that order. Pb concentrations in soil samples ranged from 15.44 to 705.98 mg/kg, Cd concentrations ranged from 0.14 to 6.07 mg/kg, Ni concentrations ranged from 0.2 to 33.4 mg/kg, and Cr concentrations ranged from 11.9 to 119.3 mg/kg. Concentrations ranged from 0.1 to 37.4 mg/kg. Zn concentrations ranged from 38.0 to 1454.3 mg/kg and Se concentrations ranged from 0.1 to 0.8 mg/kg for dry soil. Mercury concentrations ranged from 0.51 to 1830 mg/kg in the soil samples analyzed. According to FAO/WHO (1999), Hg concentration in unpolluted agricultural soils is 0.05–0.08 mg/kg. The amounts of heavy metals and trace elements found in the majority of the soil samples analyzed were above the allowable standard limits in agricultural soil set by various international bodies (

Table 1. Observed means of heavy metal and trace element levels in soil in ppm (mg/kg) (n = 28).

	Pb	Cd	Ni	Cr	As	Zn	Se	Hg
MDL	0.01	0.01	0.10	0.50	0.1	0.10	0.10	0.01
Observed mean in soil samples	171.08	0.86	15.50	39.10	5.90	313.50	0.20	1370
Permissible limits for metals in soil (mean values)	29	0.6	34	84	11	60	0.4	0.1
Permissible limits for heavy metals in plants (mg/kg)	2	0.02	10	1.30		0.60	0.000786–0.005714 (RDI)	0.5
MDL RDI	Minimum Detection Limit Recommended Daily Intake

). As a result, these data imply that the investigated soil samples were heavily polluted by the aforementioned heavy metals. The elevated Zn and Pb levels detected might be related to the parental material of soils [13,19] at the study location as well as artisanal gold mining operations. The Migori granite–greenstone complex is made up of 2.8-billion-year-old Precambrian rocks. Along the Migori River Valley, doleritic dykes intrude on these rocks. Gold mineralization may be found in quartz veins that cut through mafic volcanic rocks in the system [13]. The concentrations and distribution of heavy metals such as Pb and Zn in the soil samples analyzed may be linked to the source material and soil formation processes. Several studies have found correlations between heavy metal background levels and soil elemental composition [39,40,41], indicating that these variables play essential roles in heavy metal dynamics, particularly concentrations in agricultural soils within mining areas. Gold mining activities release Pb and Zn into the environment, which find their way into soils and waterways, enhancing their concentration in soils. As a consequence of these findings, it was determined that the soil in the research region was significantly contaminated by the seven metals established in the soil samples analyzed.

Elevated concentrations of Pb, Cd, Ni, Cr, As, and Zn at the study sites likely reflect both natural background levels and inputs associated with gold ores. Similar patterns have been reported elsewhere. For example, Xiao et al. [42] investigated soils in Tongguan, Shaanxi, China, and found high Pb, Cd, Cu, and As contents from anthropogenic activities, while Cr and Zn were primarily geogenic, and Ni was derived from both natural and human sources. Gold deposits in greenstone belts are commonly associated with As, and, in some cases, Hg; the latter is often introduced through the widespread use of mercury in amalgamation processes. Comparable findings were reported in India, where Chakraborti et al. [43] observed elevated As concentrations in soils near gold mines, compared with lower levels at more distant sites. Selenium is an essential trace element, meaning that humans need a small amount for health, but it is toxic in large doses. Because of this dual nature, the WHO has established a recommended daily intake (RDI) rather than a permissible limit in plants. Plants grown in selenium-rich soil can have high concentrations, while those in selenium-deficient areas may have very low concentrations. For adults, the RDI for selenium is approximately 0.000786 mg/kg, with a safe upper limit of 0.005714 mg/kg [25,26,27,28,29,30]. It is important to note that the applied HNO₃/HClO₄/HF digestion method provides total elemental concentrations, whereas FAO–WHO guideline values are typically based on bioavailable or mobile fractions. Direct comparison must therefore be approached with caution since total concentrations may overestimate the environmentally or biologically available fractions, a distinction emphasized to prevent misinterpretation.

4.2. Heavy Metal and Trace Element Levels in Edible Leafy Vegetables

The metals and trace elements selected for this study were chosen due to their high toxicity, persistence in the environment, and bioaccumulative potential. These elements are released into soils and aquatic systems through both natural processes—such as rock weathering and volcanic activity—and anthropogenic activities, including mining, industrial discharge, and agricultural practices.

Analysis of metal concentrations for the four leafy vegetables revealed substantial deviations from the maximum permissible limits established by FAO/WHO (Table S3, Supplementary File). Lead (Pb) concentrations ranged from 0.17 to 71.28 mg/kg, while cadmium (Cd) levels varied between 0.01 and 0.26 mg/kg. Nickel (Ni) showed a broad range of 0.2 to 111 mg/kg and chromium (Cr) concentrations spanned from 2.4 to 243.7 mg/kg. Arsenic (As) exhibited the highest values among the analyzed contaminants, ranging from 1.2 to 398.6 mg/kg. Mercury (Hg) concentrations were between 0.22 and 35 mg/kg (dry weight). Essential elements such as zinc (Zn) and selenium (Se) were also present, ranging from 11.2 to 67.4 mg/kg and 0.1 to 5.7 mg/kg, respectively.

Species-specific differences were evident. Smooth pigweed accumulated the highest levels of Cd, Cr, As, and Zn; black nightshade showed elevated Pb and Ni uptake; and cabbage contained the highest concentrations of Se among the vegetables analyzed. With the exception of Cd, all measured metals exceeded the maximum permissible levels (

Table 2. The mean heavy metal and trace element levels (ppm) and the percentage of moisture content in the four vegetables (n = 30).

Vegetable Name	Moisture Content (%)	Pb	Cd	Ni	Cr	As	Zn	Se	Hg
MDL		0.01	0.01	0.1	0.1	0.1	0.1	0.1	0.01
Smooth pigweed	85.89	16.59	0.074	21.6	38.53	75.2	49.6	0.69	18.8
Collard greens	78.23	16.29	0.065	21.5	35.4	72.7	46.1	0.74	22.9
Cabbage	82.13	16.10	0.069	20.9	36.73	74.8	48.5	0.76	26.6
Black nightshade	76.34	16.87	0.071	21.9	35.73	71.5	47.1	0.67	16.3
Mean level in the vegetables		16.625	0.0685	21.55	34.83	71.3	46.55	0.705	21.15
WHOMPL		2.0	0.02	10	1.3	0.1	0.6	-	-
Metal trend in vegetables in descending order				As > Zn > Cr >Ni >> Hg > Pb > Se > Cd

MDL: Minimum detectable limits. MPL: Maximum permissible limits. WHO/FAO (2007) Expert committee on food additives. Cambridge: Cambridge University Press.

), highlighting a potential risk of dietary exposure through consumption of these vegetables.

There was no significant difference between the means of each pollutant in the four different types of edible vegetables at a significance level of p > 0.05.

Pb, Cd, Ni, Cr, and As, among the heavy metals and trace elements investigated, were reported to show high concentrations in vegetables, similar to what was observed in the soil samples. However, the essential Se content of the vegetables was low, reflecting what was found in the soil. As a result, the heavy metals found in the vegetables can be linked to the soil’s composition. In a study on artisanal gold mines in China, researchers [44] established high levels of Pb, above permissible levels, that were associated with gold mining activities. Arsenic is a toxic element even in the lowest concentrations. The high levels of As in the four different types of vegetables investigated are consistent with findings from studies using vegetable samples collected near Korea’s decommissioned Songcheon Au–Ag mine [45], which found considerably high amounts of As in the vegetables analyzed.

Heavy metal and trace element contents in vegetables may vary owing to differences in absorption capability and mobility inside the vegetables. The means of heavy metals and trace elements showed that the metals exceeded the WHO/FAO recommended limit of pollutants in over 70% of the vegetable samples analyzed. Zinc is required for a range of biological activities in the human body, but its elevated concentration in vegetables can be harmful to human wellbeing. In the current study, the mean concentration of Zn in the different vegetables was 46.55 mg/kg. Other studies have connected excessive zinc consumption to headaches, diarrhea, loss of appetite, nausea, and vomiting. Elevated zinc consumption can influence lipoprotein metabolism, particularly cholesterol, high-density lipoprotein, and low-density lipoprotein levels. Furthermore, excessive zinc consumption has been associated with a weaker immune system and may interact with a variety of medications, including penicillamine, quinolone, and tetracycline antibiotics, as well as diuretics such as thiazides [44]. The elevated concentration of heavy metals in the vegetables may be attributable to the farms’ proximity to gold mining operations. Various heavy metals found together with the gold, such as Hg, Pb, Zn, As, and Cd, are discharged into the environment during gold mining and panning activities. The metals are subsequently absorbed by plants through the soil throughout their growth and development, raising the metal concentrations in the plants [13]. Similar observations have been reported from other mining-impacted environments. For instance, Clidemia sericea, native to gold-mining soils in Colombia, exhibited high bioconcentrations and translocations of Hg, Pb, and Cd. In urban Ethiopian contexts, vegetables grown in wastewater-irrigated plots have also shown elevated Cd and Pb levels, often surpassing safe limits. In Yunnan, China, leafy vegetables—e.g., Malabar spinach—accumulated cadmium when cultivated in mining-impacted soils. Field surveys in polymetallic mining zones have identified plant species capable of phytoextraction and phytostabilization of arsenic and cadmium, while desert-adapted plants in mining areas demonstrate physiological and anatomical resilience to heavy metal stress. Moreover, well-known metallophytes such as Athyrium yokoscense and Thlaspi caerulescens further attest to the capacity of certain plants to tolerate—and even thrive in—heavy metal-rich substrates [4,7,13,35].

4.3. Coefficients of Soil–Plant Transfer

The soil–plant transfer coefficient is a critical indicator of human exposure to heavy metals through the food chain as it reflects the extent to which contaminants are translocated from soils into edible plant tissues [13]. In this study, arsenic (As) exhibited the highest average transfer coefficients across all four vegetables, followed by Ni, Cr, Se, Zn, Pb, and Cd (

Table 3. Soil–vegetable transfer coefficients of heavy metals and trace elements. The values are percentages (%).

Vegetable Name	Pb	Cd	Ni	Cr	As	Zn	Se	Hg	Efficacy
Smooth pigweed	9.70	8.6	139.4	98.54	1274.6	15.8	34.5	8.71	186.05
Collard greens	9.52	7.56	138.7	90.54	1232.2	14.7	37.0	8.31	218.6
Cabbage	9.41	8.03	134.8	93.94	1367.8	15.5	38.0	8.85	239.6
Black nightshade	9.86	8.26	141.3	91.38	1211.9	15.0	33.5	9.41	215.9
Average	9.6	8.11	138.6	93.6	1271.6	15.25	35.8	8.51

). Notably, black nightshade showed pronounced uptake of Pb and Ni, while smooth pigweed accumulated greater amounts of Cd, Cr, As, and Zn. Cabbage demonstrated particularly high transfer of As, whereas collard greens consistently displayed the lowest transfer efficiencies for Cd, Cr, and Zn.

The ranking of transferability (As > Ni > Cr > Se > Zn > Pb > Cd) suggests that As is more readily mobilized and bioavailable in the studied soils than the other metals. This enhanced mobility is likely influenced by soil properties such as pH, organic matter content, and cation exchange capacity (CEC). The mobility of As and Se in soils is largely governed by pH and redox conditions, with maximum anion mobility typically occurring under neutral to alkaline conditions (pH: 8–9), rather than in acidic soils. In addition, while cation exchange capacity (CEC) predominantly influences the behavior of cations, it has a limited direct impact on the mobility of anions such as arsenate (AsO₄³⁻) and selenate (SeO₄²⁻). The relative uptake efficiency among vegetables (cabbage > collard greens > black nightshade > smooth pigweed) further underscores species-specific differences in absorption potential, likely mediated by root morphology, metal transporters, and physiological tolerance to contaminants. These findings highlight the dual influence of soil chemistry and crop-specific traits on metal transfer, underscoring the complexity of dietary exposure risks in mining-impacted agricultural systems.

The soil–plant transfer coefficient reflects not just total soil contamination but the complex interplay of metal bioavailability, chemical speciation, soil chemistry, and species-specific plant physiology. In our study, transfer coefficients were strongly associated with soil metal concentrations, indicating that elevated contamination directly drives uptake, especially for more mobile elements like As and Ni, which are less strongly retained in soils [45,46]. This pattern is consistent with findings from artisanal mining regions in Tanzania, where elevated As and Hg in soils were transferred into cassava leaves at levels significantly above safe thresholds. Such contamination aligns with persistent artisanal gold mining impacts in our study area, which are known to disrupt food chains and heighten public health risks [13].

Interestingly, this study’s transfer dynamic diverges from those reported in earlier research, wherein smooth pigweed accumulated the highest levels of a broader suite of metals (As, Cd, Hg, Pb, Zn) [47]. Instead, our data instead show smooth pigweed predominantly accumulating Cd, Cr, and Zn in edible tissues. This discrepancy underscores variability in soil-to-plant transfer across different mining-affected environments and highlights how localized soil chemistry, metal contamination profiles, and plant-specific uptake capacities can shape exposure pathways.

4.4. Estimated Daily Intakes (EDIs) of Heavy Metals and Trace Elements

The estimation of heavy metal exposure levels is critical in assessing an individual’s health risks [4,5]. There are several pathways of exposure to people, but the food chain is by far the most critical. The heavy metal and trace element EDIs (

Table 4. Estimated daily intake of heavy metals and trace elements (mg/person/day) in adults.

Vegetable Name	Calculated % DW of Vegetables	Pb	Cd	Ni	Cr	As	Zn	Se	Hg
Smooth pigweed	0.1411	7.6	0.034	9.91	17.67	34.48	22.75	0.32	5.1
Collard greens	0.2177	11.53	0.046	15.21	25.5	51.43	32.51	0.52	6.22
Cabbage	0.1787	9.35	0.040	12.14	21.33	43.44	28.17	0.44	7.12
Black nightshade	0.2366	12.97	0.055	16.84	27.47	54.98	37.22	0.34	4.1
Average (mg/kg/day)		10.36	0.044	13.56	22.99	46.08	30.16	0.41	5.63
Tolerable daily intake for adults (µg/kg bw/day) except Zn (mg/kg/bw/day)		0.1–3 [47]	1 [47]	0.03–0.13 [47]	0.02–3 [47]	2.8 [47]	0.3–1.0 [47]	0.9 [47]	1 μg/kg bw/wk [28]
Bw = body weight = 60 kgs. This study used 60 kg to calculate the EDI.
Vegetables: 325 g/day of vegetables.

) reveal that the Pb that can be consumed through these vegetables is above the WHO/FAO-recommended maximum tolerated daily intake of 0.21 mg/kg/person per day, while Cd and Se are within acceptable standard levels. Similarly, Figure 3 shows the EDIs contributed by the four analyzed vegetables.

Except for Cd and Se, all of the heavy metals and trace elements examined (Pb, Cd, Ni, Cr, As, Hg, and Zn) had average EDI levels that were above the recommended dietary allowance intakes for humans set by the FAO/WHO. The high levels of Pb, Cd, Ni, Cr, As, Se, Hg, and Zn may be attributable to increased uptake of the metals from the geological background [47] as well as artisanal mining activities, which are associated with elevated levels of potentially harmful elements in the environment [13], as detected in this study area.

Lead (Pb) toxicity affects multiple human organs, including the liver, kidneys, lungs, and spleen, leading to severe biochemical disorders. High daily intake of Pb in vegetables has been reported in Iran and linked to endemic esophageal cancer [48]. In this study, the estimated daily intake (EDI) of Pb was highest in black nightshade, followed by collard greens, cabbage, and smooth pigweed.

Cadmium (Cd) is a carcinogenic heavy metal with no biological function in humans and is toxic even at low concentrations. Chronic Cd exposure is associated with renal disease, osteoporosis, cardiovascular disease, cancer, and metabolic disorders such as insulin resistance and diabetes [49,50]. In this study, Cd had the lowest EDI among the analyzed vegetables, averaging 0.044 mg/person/day and contributing 0.013% of total heavy metal intake. The EDI ranking for Cd followed the same order as that for Pb: black nightshade > collard greens > cabbage > smooth pigweed.

Nickel-induced mitochondrial destruction results from altered mitochondrial membrane potential, reduced ATP levels, and DNA damage [51]. Excessive or prolonged exposure to nickel can lead to the overproduction of free radicals, including reactive oxygen and nitrogen species [52]. High Ni concentrations have been associated with serious health risks, such as pulmonary embolisms, respiratory failure, and an increased risk of cancer, with nickel oxide, nickel sulfide, and soluble nickel compounds classified as carcinogenic [53]. Additionally, nickel exposure can cause intestinal structural and functional damage, leading to disorders like enteritis [51]. Workers in the nickel industry face a higher risk of lung and nasal cancer due to occupational inhalation exposure compared to those in non-Ni-related environments [54,55]. In this study, the estimated daily intake (EDI) of Ni in the analyzed samples followed the order black nightshade > collard greens > cabbage > smooth pigweed.

Chromium compounds act as respiratory allergens and can trigger pulmonary hypersensitivity when inhaled. Regular exposure to Cr(VI) complexes increases the risk of lung, nasal, and sinus cancers [ref]. Additionally, Cr(VI) exposure can lead to severe dermatitis and painless skin ulceration [56]. Based on evidence from regulatory bodies and recent studies, occupational exposure to Cr has been confirmed to have carcinogenic effects, contributing to lung, nasal, and sinus cancers, as well as stomach and laryngeal carcinomas in exposed individuals [57]. In this study, the estimated daily intake (EDI) of Cr followed the descending order black nightshade > collard greens > cabbage > smooth pigweed.

Arsenic (As) is the 20th most abundant element globally and the 33rd on the periodic table [58]. Elemental forms such as arsenite and arsenate pose environmental and health hazards. Human exposure to As occurs through industrial activities like smelting and microelectronics, as well as through contaminated water, soil, and food due to its presence in wood preservatives, herbicides, insecticides, fungicides, and paints [59]. In the study area, As is linked to parental rock formations and is released into the environment through mining activities and weathering [60]. Arsenic induces reactive oxygen species (ROS), including superoxide (O₂•⁻) and singlet oxygen (¹O₂), which contribute to carcinogenesis through epigenetic modifications, oxidative DNA damage, and ROS-mediated pathways [61,62]. The EDI of As in the analyzed samples followed the order black nightshade > collard greens > cabbage > smooth pigweed.

Zinc (Zn) is an essential mineral necessary for healthy growth, gene regulation, and enzyme activity, including metalloproteinases, carbonic anhydrase, and Cu-Zn superoxide dismutase [63]. However, excessive Zn intake can interfere with Cu and Fe absorption, cause gastrointestinal issues, and disrupt physiological processes [64]. Zn deficiency may result from inadequate dietary intake, poor absorption, increased excretion, or genetic metabolic disorders [64]. In this study, the estimated daily intake (EDI) of Zn was 30.16 mg/person/day, accounting for over 30% of total heavy metal intake from vegetables. The EDI ranking for Zn followed the order black nightshade > collard greens > cabbage > smooth pigweed.

Selenium (Se) is an essential micronutrient with antioxidant properties, protecting against various diseases when consumed in appropriate amounts [65]. Dietary Se has been shown to reduce liver toxicity in vitamin E-deficient rats, while studies indicate that it prevents exudative diathesis in vitamin E-deficient chicks [65]. However, Se levels outside the optimal range have been linked to infertility and other health disorders [66]. The metabolism of different dietary Se sources in humans remains unclear, leading to inconsistencies in dietary recommendations and, in some cases, severe consequences, including death [67]. In this study, the Se concentration in the analyzed vegetables followed the order collard greens > cabbage > black nightshade > smooth pigweed.

Mercury (Hg) is highly toxic and bioaccumulates in living organisms, posing health risks even at low concentrations. Once absorbed, Hg binds strongly to thiol residues in proteins, making it difficult to eliminate via the kidneys. Intracellular Hg can also disrupt sulfur-containing cofactors, enzymes, and hormones, leading to various health disorders in humans and animals. Additionally, Hg accumulation can cause severe neurological impairments in children and adults, particularly affecting unborn babies if maternal MeHg levels are high. The average Hg intake observed in this study (5.63 μg/kg bw/wk) exceeded the recommended limit of 1 μg/kg bw/wk. The observed average mercury (Hg) intake of 5.63 μg/kg bw/wk, exceeding the recommended limit of 1 μg/kg bw/wk, may be attributed to the contamination of the soil with mercury in the mining regions. In areas impacted by gold mining, mercury released into the environment can accumulate in the soil, where it may be absorbed by plants. Vegetables grown in contaminated soil can take up mercury through their roots, leading to higher concentrations in edible parts of the plants. This bioaccumulation process in soils contaminated by mining activities could significantly contribute to elevated mercury intake in local populations, especially in areas where vegetables form a substantial part of the diet. The elevated concentrations of Pb, Ni, Cr, As, Zn, Hg, and Se in commonly consumed vegetables highlight potential public health risks in the study area.

4.5. Hazard Quotient

The results indicate that the calculated hazard quotient (HQ) values for all potentially harmful metals in the vegetable samples exceeded 1 (

Table 5. Individual metal hazard quotients (HQs) for the vegetable samples investigated.

Vegetables Sampled	Pb	Pb HQ	Cd	Cd HQ	Ni	Ni HQ	Cr	Cr HQ	As	AS HQ	Zn	Zn HQ	Se	Se HQ
RfD mg/kg/day	0.004		0.001		0.91		1.5		0.014		0.3		0.005
Smooth pigweed	7.6	1900	0.034	34	9.91	10.9	17.67	11.8	34.48	2462	22.75	75	0.32	64
Collard greens	11.53	1882	0.046	46	15.21	16.7	25.5	17	51.43	3673	32.51	108	0.52	104
Cabbage	9.35	2337.5	0.040	40	12.14	13.3	21.33	14.2	43.44	3102	28.17	93.9	0.44	88
Black nightshade	12.97	3242.5	0.055	55	16.84	18.5	27.47	18.3	54.98	3927	37.22	124	0.34	68
Average HQ for metals		2340.5		43.6		14.9		15.3		3291		100.2		81

). An HQ value greater than 1 signifies that exposure to the corresponding metal may pose significant health risks to humans. The consistently elevated HQ values observed for all the metals in the studied vegetables suggest considerable carcinogenic health risks for consumers, as also highlighted in previous studies [42,68].

Similar findings have been reported in other mining-impacted regions, such as the Baia Mare mining area in Romania, where Cd and Pb HQ values in vegetables were above permissible limits, underscoring the potential for serious public health complications [68]. Comparable trends have also been documented in African artisanal and small-scale gold mining zones. For instance, studies in Tanzania and Ghana reported HQ values for Pb and As in leafy vegetables ranging between 2 and 5 and Cd values of 1.5–3, far exceeding safe thresholds. Likewise, in Zimbabwe, HQ values above 4 have been recorded in mining belts, reinforcing the widespread risk of dietary exposure in such environments [5,6,7]. These outcomes emphasize the urgent public health threat to local communities from the ingestion of vegetables contaminated with specific heavy metals and trace elements. Therefore, regular monitoring of heavy metal and trace element concentrations in both soils and foodstuffs is critical to enhance food safety and mitigate exposure risks for populations residing in mining-affected regions.

The unusually high HQ values for Pb and As in this study were verified through repeated calculations against the RfD standards, confirming that the results reflect the actual concentrations present in the vegetables. Particularly alarming were the values in black nightshade and cabbage, which suggest a heightened risk of chronic health complications. Comparable observations have been reported in other African countries, where Pb and As accumulation in leafy vegetables poses a persistent dietary risk in mining belts. By contrast, studies from non-mining agricultural areas in Kenya and South Africa generally reported HQ values ranging between 0.2 and 0.8 for most metals, well below the risk threshold of 1, reflecting safer dietary exposures in uncontaminated regions. This stark contrast highlights the disproportionate burden of heavy metal exposure faced by communities in artisanal gold mining environments.

4.6. Hazard Index (HI) Analysis

When all metals are considered together, their combined toxic effects amplify the overall health risk. Therefore, calculating the hazard index (HI), which integrates the hazard quotients (HQs) of multiple metals, is a critical approach for assessing the cumulative non-carcinogenic impacts of dietary exposure [35].

Table 6. Potential risk of heavy metals and trace elements consumed through vegetables.

Vegetables Sampled	Pb HQ	Cd HQ	Ni HQ	Cr HQ	AS HQ	Zn HQ	Se HQ	Total HQ for Each Vegetable (HI)
Smooth pigweed	1.900	3.4	1.09	1.18	2.46	7.5	6.4	23.932
Collard greens	1.882	4.6	1.67	1.7	3.67	1.08	1.04	15.642
Cabbage	2.337	4.0	1.33	1.42	3.10	9.39	8.8	30.377
Black nightshade	3.242	5.5	1.85	1.83	3.93	1.24	6.8	22.265

presents the estimated cumulative health risks associated with heavy metals and trace elements in each vegetable analyzed. The HI values followed the order cabbage > smooth pigweed > black nightshade > collard greens (Figure 4). Specifically, the highest HI was observed in cabbage (30.377), followed by smooth pigweed (23.932), black nightshade (22.265), and collard greens (15.642). Since an HI value exceeding 1 signifies a non-acceptable risk, these results indicate a serious health concern [35,69].

The HI values obtained in this study (15.642–30.377) were far above the threshold of 1, highlighting that consumption of vegetables cultivated in the Migori gold-mining area may result in significant cumulative exposure to toxic metals. Comparatively, studies in other African countries have reported similarly elevated HI values near mining sites. For example, in Tanzania, HI values in leafy vegetables ranged between 12.4 and 24.6, while, in Ghana, values varied from 13.2 to 27.8, particularly for cabbage and lettuce grown near gold mining belts. In Zimbabwe, HI values of 14.8–29.3 were recorded in common leafy vegetables, whereas, in Nigeria, ranges of 11.7–25.5 were observed for amaranth and spinach cultivated near mining and industrial areas.

Taken together, these findings show that the HI values in Migori (15.642–30.377) are within, but in the upper range of, those reported across Africa, underscoring that cabbage and smooth pigweed are consistently among the highest contributors to cumulative risk. Notably, the HI values in Migori were slightly higher than the averages reported in Tanzania (12.4–24.6) and Ghana (13.2–27.8), suggesting that residents in Migori may face an even greater level of dietary exposure compared to other African mining regions. The consistently elevated HI values observed in African mining-impacted regions emphasize that communities consuming vegetables grown in such soils are exposed to significant health hazards. From a health standpoint, this study clearly indicates that both carcinogenic and non-carcinogenic risks exist in Migori, warranting urgent monitoring and intervention to reduce dietary exposure to heavy metals.

4.7. Correlation Analysis of Heavy Metals and Trace Elements Studied in Vegetables

The Pearson correlation analysis revealed significant associations among heavy metal and trace element concentrations in vegetables from the mining-impacted area. Positive correlations were observed between several metal pairs, including Se–Ni, Cr–Ni, Se–Zn, Ni–Zn, Ni–As, Zn–Cd, Se–Cd, and Se–Pb, while negative correlations were noted for Zn–Pb and Se–Pb (

Table 7. The Pearson correlation matrix for the heavy metals and trace elements analyzed in vegetables.

Metals	Cr	Se	Ni	Zn	As	Cd	Pb	Hg
Cr	1	0.204	0.632 b	0.173	0.092	0.209	0.099	0.342 a
Se		1	0.706 b	0.343 a	0.199	0.405 b	0.384 b	0.214
Ni			1	0.757 b	0.566 b	0.549 b	0.109	0.073
Zn				1	0.489 b	0.559 b	−0.052	0.678 b
As					1	0.179	−0.397 b	0.891 b
Cd						1	0.092	0.431 a
Pb							1	0.979
Hg								1

^a Correlation is significant at the 0.05 level. ^b Correlation is significant at the 0.01 level.

; Table S4). The clustering of certain metals suggests a common origin, possibly linked to artisanal mining activities, or reflects similar physicochemical behaviors influencing their co-mobility and uptake by plants [70,71,72]. Comparable findings have been reported elsewhere, where correlations in metal accumulation indicate shared plant uptake mechanisms or environmental controls on metal availability [73].

Environmental processes, including soil mineralogy, pH, and organic matter content, can modulate metal speciation and interactions, thereby shaping the observed relationships. For example, in southern China, abnormal cadmium (Cd) enrichment associated with rock weathering and mining has been shown to significantly alter plant accumulation patterns [74]. These processes likely contribute to the complex interplay of heavy metals and trace elements in Migori, where multiple exposure pathways converge.

The Kenyan context further strengthens this interpretation. Studies in gold mining regions of Kenya have consistently documented elevated concentrations of potentially harmful elements (PHEs), including Pb, Cd, Zn, Ni, As, Hg, and Cr, in soils, water, and crops, often exceeding maximum permissible limits [75]. These contaminants pose both non-carcinogenic and carcinogenic health risks, with children identified as particularly vulnerable due to higher intake rates relative to body weight [76]. Biomonitoring evidence from hair, nails, and urine samples in these populations has confirmed systemic exposure to multiple metals, directly linking environmental contamination to human health [75,76,77]. Similar to the present findings, hazard indices and bioaccumulation factors in these studies highlight that consumption of locally cultivated vegetables—although nutritionally important—remains a significant pathway of toxic metal exposure [78,79,80].

Comparative evidence from other African mining regions reinforces the broader implications. In Ghana, elevated levels of Pb, As, and Cd in vegetables near small-scale mining areas have been reported, with hazard quotients exceeding safe thresholds, particularly for children. In Nigeria, vegetables irrigated with contaminated water around mining zones accumulated Cr, Pb, and Ni at levels surpassing WHO/FAO standards, presenting heightened dietary risks. Likewise, in Zimbabwe and Tanzania, studies have documented similar patterns of heavy metal co-occurrence in food crops grown near artisanal gold mines, with hazard indices pointing to cumulative exposure risks. These parallels underscore that the co-variation of metals is not unique to Migori but reflects a wider regional pattern where mining-driven contamination creates overlapping and mutually reinforcing risks across environmental and dietary pathways.

Overall, the positive correlations among metals observed in this study point to a shared geochemical source linked to artisanal gold mining, while the negative correlations may indicate competitive uptake processes or differential plant affinities. Together with evidence from other African mining regions, these findings highlight the urgent need for integrated risk management strategies, including routine environmental monitoring, stricter regulation of artisanal mining practices, and dietary interventions aimed at reducing toxic exposures. Future research should focus on disentangling the mechanistic basis of metal interactions in soils and crops, incorporating longitudinal biomonitoring to better predict cumulative health risks in vulnerable populations.

It is important to recognize that the concentrations reported in this study reflect the specific vegetables sampled from agricultural fields within the mining-impacted region. Leaf composition and elemental uptake are known to vary not only among plant species but also with soil type, pH, organic matter content, and the degree of contamination. As such, the values presented here should not be interpreted as generalized reference concentrations for the same crops grown in uncontaminated soils. Instead, they provide a localized snapshot of metal accumulation under the prevailing environmental conditions, consistent with reports that plant–soil interactions and species-specific physiology strongly influence metal uptake and tolerance. The consistently high HI values observed across the vegetables in Migori, particularly cabbage and smooth pigweed, reflect a cumulative exposure risk that exceeds both national and international safety thresholds. Compared with other African mining regions such as Tanzania, Ghana, Zimbabwe, and Nigeria, the HI values in Migori fall within the upper range, suggesting that residents here may face even greater health threats. These findings highlight an urgent need for targeted interventions, including routine monitoring of food safety and stricter environmental controls, to mitigate both carcinogenic and non-carcinogenic health risks.

4.8. Conclusions

This study demonstrates that agricultural soils and vegetables from artisanal mining areas in Migori County contain heavy metal and trace element concentrations far exceeding international safety thresholds. The elevated levels detected in commonly consumed vegetables indicate a clear risk of dietary exposure and associated health impacts for local communities. The findings underscore the urgent need for routine monitoring of both soils and food crops in mining-impacted regions, alongside the enforcement of policies that limit heavy metal and trace element contamination in the food chain. Strengthened public awareness programs are equally critical to reduce exposure risks, particularly for vulnerable populations that rely heavily on these vegetables as dietary staples. While this study provides valuable evidence linking artisanal mining to heavy metal and trace element contamination, further research is warranted to better understand long-term health outcomes, soil–plant transfer dynamics under different agronomic conditions, and mitigation strategies suitable for resource-limited settings. By addressing these gaps, interventions can be better tailored to safeguard food security and public health in mining-affected regions.

5. Study Limitations: Clarification on Soil Classification Limitation

1. It is important to note that this study did not include comprehensive field-based soil profile descriptions required for full classification under the World Reference Base for Soil Resources (WRB), 4th edition. As such, we were unable to assign soils to a specific Reference Soil Group with principal and supplementary qualifiers.

2. The analysis was based solely on surface soil samples and laboratory-determined physicochemical parameters. While these data provide valuable insights into soil quality and contamination levels, they do not permit a complete WRB classification. Future studies are encouraged to incorporate detailed pedological investigations to enable full WRB-based classification.

3. Vegetable and soil samples were collected during a single growing season, and potential seasonal variations in heavy metal and trace element uptake were not captured.

4. While the daily vegetable intake value was assumed to be 300 g/day, based on dietary survey data from Sub-Saharan Africa indicating a range of 250–350 g/day in rural populations where vegetables constitute a major part of meals, was adopted from WHO dietary guidelines to facilitate comparability with international studies, it may not fully reflect Kenyan consumption patterns due to the absence of comprehensive national dietary survey data. Similarly, the assumed adult body weight of 60 kg was derived from regional demographic reports, and variations in local populations may influence risk assessment outcomes.

These limitations highlight the need for future studies incorporating multi-seasonal sampling and locally validated dietary and demographic data to provide more representative exposure estimates.