Bioaccumulation of Macro- and Microelements, Including Potentially Toxic Metals(loid)s, in Pods and Leaves of Vigna unguiculata L. Walp. Cultivated in a Contaminated Area

Abstract

Cowpeas are a legume widely consumed in Brazil. Given this, the objective of this study was to investigate the presence of metals (loids) in pods and leaves of Vigna unguiculata located near a highway with high vehicle traffic and a landfill, and to assess possible risks to human health. Pod and leaf samples were collected at nine points between the highway and the landfill. The elements were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) and quantified. The risk to human health was assessed using risk quotient and risk index values. A quantitative analysis of the chemical elements was also performed using the maximum tolerable intake level. Element concentrations were higher in the leaves than in the pods. The human health risk analysis showed that the average daily consumption of both pods (44 g/day) and leaves (67 g/day) may pose a chronic health risk to adult men and women, due to simultaneous exposure to multiple metals. It was concluded that the plant is contaminated and that its ingestion can cause toxicity, warranting warnings against cultivating areas near anthropogenic activities that may be contaminated with heavy metals, thereby affecting nutritional safety.

1. Introduction

The cowpea (Vigna unguiculata L. Walp.), a member of the Fabaceae family [1], is native to Africa [2] and was introduced to Brazil in the 16th century, where it is currently widely cultivated, primarily in the North and Northeast regions of the country [1]. Popularly known as string bean, black-eyed pea, or simply cowpea [1,2,3], this plant is characterized by a long axial root, trifoliate leaves, yellow to violet flowers, and pods arranged in pairs [1,3]. The nutritional importance of cowpea grains is attributable to their high carbohydrate content, primarily resistant starch, which accounts for approximately 50–60% of their dry mass [2]. This type of resistant starch is not rapidly hydrolyzed, which gives it a low glycemic index and beneficial effects for human health [4]. In addition, the species contains oligosaccharides that influence intestinal health [5,6]. Cowpeas are also recognized for their high fiber and protein content [3], with an amino acid profile rich in lysine, leucine, and arginine, but with a low concentration of sulfur amino acids [2,3]. Their lipid content is low, with a predominance of triglycerides and polyunsaturated fatty acids (linoleic acid), and they are an important source of minerals, including calcium, iron, and zinc [3,5].

However, crop areas located near highways with heavy vehicle traffic and landfills are subject to environmental contamination, favoring the bioaccumulation of heavy metals in plants [7]. Chronic consumption of these vegetables may pose risks to human health [8]. Literature describes how fine particles composed of heavy metals, derived from the combustion of fossil fuels, can cause harmful effects on various organs. Prolonged exposure to these materials in areas with high vehicle traffic is associated with increased cancer and symptoms indicative of neurotoxicity [9]. The study by Fonseca et al. [10] analyzed the bioaccumulation of metals in Vigna unguiculata grains grown near highways and landfills and observed high levels of cadmium and lead, with health risk indices exceeding reference limits, indicating risks to public health. Studies have shown that heavy metal contamination can lead to the development of kidney dysfunction, alterations in the immune, digestive, respiratory, cardiovascular, and neurological systems, and cancer. Therefore, these conditions underscore the importance of monitoring and an integrated understanding of the soil–plant–human system [10,11,12].

Although several studies have investigated the presence of heavy metals in different bean species and derived products, research specifically addressing Vigna unguiculata remains scarce, particularly in plantations located in peri-urban agricultural environments, where proximity to highways, urban runoff, and waste disposal sites may enhance the risk of metal accumulation. For instance, Silva-Gigante et al. [13] reviewed the bioaccumulation of metals and metalloids in Phaseolus vulgaris L., while Hassan et al. [14] evaluated the potential health risks associated with the consumption of legumes marketed in Erbil, Iraq. Similarly, Yaradua et al. [15] analyzed heavy metal concentrations in beans and bean-based products sold in Nigeria. However, to date, no studies have quantified metal concentrations directly in the pods and leaves of Vigna unguiculata cultivated under environmental conditions comparable to those reported by Fonseca et al. [10].

Considering the importance of cowpea (Vigna unguiculata L. Walp.) cultivation in Brazil and worldwide, its nutritional richness, and its resilience as a crop that adapts to environments prone to heat and water stress [2], this study aimed to quantify and compare the concentrations of macroelements (Mg, Fe, and Mn) and microelements, including potentially toxic metals(loid)s (As, Cd, Cr, Cu, Mo, Pb, Se, V, and Zn), in pods and leaves of Vigna unguiculata L. Walp. cultivated in an agricultural area located within a peri-urban environment, near a high-traffic highway and a landfill site, to assess possible patterns of bioaccumulation and environmental exposure. In addition, the risks to human health from consuming these plant parts were assessed using maximum tolerable intake (UL) values and risk assessment parameters. This work expands knowledge about environmental contamination and its implications for food safety and public health.

2. Materials and Methods

2.1. Search Location

The material for analysis was collected from a cowpea crop on private property in the municipality of Campo Grande, Mato Grosso do Sul, at the geographical coordinates (20°34′05″ S 54°32′58″ W) in September 2022.

2.2. Experimental Design

The experimental design followed a block-based sampling strategy adapted from previously validated environmental monitoring studies conducted near landfills and high-traffic highways [8]. The experimental area was organized into three parallel lines, spaced 100 m apart, starting 50 m from the landfill and extending toward a nearby stream, to capture potential contamination gradients originating from waste disposal activities.

Perpendicular to these lines, three columns were established at distances of 64 m from the highway, with 200 m spacing between columns toward the adjacent vegetation. This configuration allowed the evaluation of spatial variability and attenuation of metal (loid) deposition associated with vehicular emissions. A total of nine sampling points (A1–A3, B1–B3, and C1–C3) were defined. At each point, a 1 m sampling radius was established for collecting soil and plant material, ensuring local representativeness while minimizing microscale heterogeneity.

2.3. Collection and Processing of Samples

This study used the same data-collection protocol as Fonseca et al. [10], maintaining the same collection period (September 2022) for the samples. At each of the established points, fifteen subsamples of the cowpea plant were collected and distributed randomly within a radius of 1.0 m. The subsamples were mixed to form a composite representative of each of the nine points, placed in a clean plastic bag, properly identified, and sent to the Mineral and Biomaterial Metabolism Laboratory at UFMS for further processing.

In the laboratory, each part of the plant to be used, including the pods and leaves, was peeled and separated. The samples were not sanitized to preserve potential environmental contamination.

The samples were dried separately in a forced-air circulation oven at 65 °C for 72 h, ground in a processor with stainless steel blades (IKA A11 basic) (IKA-Werke, Staufen, Germany), placed in a universal collector, and stored at room temperature in a dry place for subsequent analysis. All elemental concentrations reported in this study are expressed on a dry weight basis (DW).

2.4. Acid Digestion for Inductively Coupled Plasma Optical Emission Spectroscopy Analysis

A mass of 250 mg of each leaf and pod sample was weighed individually in a 25 mm borosilicate tube and 3 mL of nitric acid (HNO₃, 65%, ultrapure grade, Merck, Darmstadt, Germany) and 2 mL of 30% hydrogen peroxide (H₂O₂, 35%, ultrapure grade, Merck, Darmstadt, Germany) and 1 mL of high-purity water (18 MΩ cm, Milli-Q, Millipore, Bedford, MA, USA). The tubes were then homogenized in a vortex mixer vortex mixer (Biomixer QL-901, Brazil) capped with a 50 mm borosilicate funnel, and inserted into a digestion block (Tecnal, São Paulo, Brazil). The digestion was performed using a stepwise heating program consisting of three stages: 80 °C for 30 min, followed by 100 °C for 90 min, and finally 150 °C for 60 min, ensuring complete mineralization of the samples.

The digestion process was performed in triplicate for cowpea pod and leaf samples, as well as for the blank, addition test, and recovery (spike). After digestion, the solutions containing the sample analytes were transferred to a polyethylene tube and reconstituted to a final volume of 10 mL with high-purity water (18 MΩ cm, Milli-Q, Millipore, Bedford, MA, USA).

2.5. Optical Emission Spectroscopy with Inductively Coupled Plasma (ICP OES)

The dosage of macro and microelements (As, Cd, Cr, Cu, Fe, Mg, Mn, Mo, Pb, Se, V, and Zn) in the cowpea pod and leaf samples was performed using inductively coupled plasma optical emission spectroscopy (ICP OES) (iCAP 6300 Duo, Thermo Fisher Scientific, Bremen, Germany), in axial view.

The elements in the samples were quantified by ICP OES using element-specific emission wavelengths selected with the iTEVA software (version 1.2.0.34, Thermo Fisher Scientific, Waltham, MA, USA), in accordance with the criteria described in the iTEVA Software Operator’s Manual (© 2009). The software automatically recommends analytical wavelengths based on signal-to-background ratio, linearity across the calibration range, and minimization of spectral and matrix-related interferences for the selected instrumental configuration. The selected wavelengths were: As (228.812 nm), Cd (226.502 nm), Cr (267.716 nm), Cu (324.754 nm), Fe (259.940 nm), Mg (279.553 nm), Mn (257.610 nm), Mo (202.030 nm), Se (196.09 nm), Pb (283.306 nm), V (309.30 nm), and Zn (213.856 nm). These wavelengths were chosen to ensure optimal analytical sensitivity, accuracy, and signal stability while avoiding saturation effects.

In addition, the instrumental parameters used for ICP OES analysis are described below. The radiofrequency power was set at 1500 W. The plasma gas flow rate was 12 L per minute, while the auxiliary gas flow rate was maintained at 0.5 L per minute. The sample absorption rate was 0.45 L/min. The nebulizer operated at 20 psi. The integration time was 15 units, and the stabilization time was 20 units. High-purity argon (99.999%) was employed as the plasma, auxiliary, and nebulizer gas to ensure analytical stability and minimize spectral interferences.

The external calibration curve was constructed from dilutions of a multi-element stock standard solution (SpecSol, Quimlab, Jacareí, São Paulo, Brazil) containing 100 mg/L of each element. For quantitative analysis of the elements, nine concentrations of each analyte were used, ranging from 0.001 to 2.0 mg/L.

The detection limit (LOD) was calculated using Equation (3) times the standard deviation of the blank divided by the slope of the curve. The quantification limit (LOQ) was calculated using the equation: ten times the standard deviation of the blank divided by the slope of the curve [16]. The Falcon tubes and glassware used in sample processing were previously decontaminated of mineral residue by washing in a 5% Extran solution (Merck™, Darmstadt, Germany), rinsed thoroughly in running water, and then in high-purity water (18 MΩ cm, Milli-Q, Millipore, Bedford, MA, USA). The materials were then immersed for at least 24 h in a 10% nitric acid solution (HNO₃, 65%, ultrapure grade, Merck, Darmstadt, Germany), rinsed again in ultrapure water, and dried at room temperature.

2.6. Human Health Risk Assessment

Human health risk assessment is the process of estimating the nature and likelihood of adverse health effects in individuals who may be exposed to chemical contaminants in contaminated areas [17]. The estimated values of the potential risk of harm to human health due to the consumption of potentially toxic metals were determined based on the risk quotient (HQ) and risk index (RI) values, from the concentration of macro and microelements quantified in the leaves and pods of Vigna unguiculata and chronic daily intake (CDI).

2.6.1. Chronic Daily Intake

Chronic daily intake (CDI) is the daily dose of heavy metals (mg/kg) to which consumers may be exposed; it quantifies the daily consumption of heavy metals without carcinogenic risk (CR) from edible plant parts such as cowpea pods and leaves. The estimated daily intake of heavy metals (mg/kg/day) was calculated using the following Equation (1) [18]:

$$CDI={\displaystyle \frac{CD\times IR\times EF\times ED}{BW\times AT}}$$

(1)

where

CD—metal concentration in the sample (mg/kg);

IR—daily intake rate of leaves per individual (67 g/day) and pods per individual (44 g/day) [19];

EF—frequency of exposure (350 days/years);

ED—duration of human exposure (30 years);

BW—estimated average body weight (70 kg);

AT—average time of human exposure for non-carcinogenic effects (10,950 days).

The parameters used to calculate the CDI, including intake rate, exposure frequency, and exposure duration, were based on Tschinkel et al., 2020 [7].

2.6.2. Hazard Quotient (HQ)

The hazard quotient (HQ) is the ratio of the CDI value obtained in the previous equation to the reference oral dose (RfD), which is considered harmful to health. In Equation (2) [20], toxic risk is considered to occur if HQ > 1, while HQ < 1 represents negligible risk (non-carcinogenic adverse effects).

$$HQ={\displaystyle \frac{CDI}{RfD}}$$

(2)

where CDI represents the chronic daily intake dose of heavy metals (mg/kg/day), calculated according to Equation (2), and RfD denotes the oral reference dose (mg/kg/day). The evaluation of non-carcinogenic risk was performed using Oral Reference Doses (RfD) established primarily by the U.S. EPA Integrated Risk Information System (IRIS) database [21]. For the calculation of the Hazard Quotient (HQ) and Hazard Index (HI), the RfD adopted for arsenic (As) was 6 × 10⁻⁵ mg/kg/day, reflecting the January 2025 IRIS revision [22]. For cadmium (Cd), the RfD considered was 1 × 10⁻⁴ mg/kg/day, while for chromium (Cr III), the value used was 3 × 10⁻³ mg/kg/day.

Regarding essential trace elements and other metals, the following RfD values were applied: 4 × 10⁻² mg/kg/day for copper (Cu), 7 × 10⁻¹ mg/kg/day for iron (Fe), and 1.4 × 10⁻¹ mg/kg/day for manganese (Mn), consistent with total dietary exposure assumptions [23]. Additionally, the adopted RfD values were 5 × 10⁻³ mg/kg/day for molybdenum (Mo), 5 × 10⁻³ mg/kg/day for selenium (Se), 5.04 × 10⁻³ mg/kg/day for vanadium (V), and 3 × 10⁻¹ mg/kg/day for zinc (Zn) [24].

2.6.3. Risk Index (HI)

HI is the sum of all elements resulting from “HQ” present in the pod and leaves, to determine the possibility of non-carcinogenic risk of an element. With the individual HQ values for each element, the risk index will be calculated using more than one heavy metal (HI), according to Equation (3) described below.

$$HI=\Sigma HQ(HQ As+HQCr+HQCu+HQPb\dots )$$

(3)

2.6.4. Carcinogenic Risk (CR)

Carcinogenic risk (CR) estimates the probability of an individual developing cancer during their lifetime due to exposure to a chemical element with carcinogenic potential. Equation (4) [21] was used to estimate cancer risk:

$$CR=CDI\times SF$$

(4)

where CDI is the chronic daily intake dose of heavy metals (mg/kg/day) obtained in Equation (1), and SF is the oral carcinogenic slope factor.

The CR was estimated using Oral Slope Factors (SF), which represent the upper-bound probability of an individual developing cancer per unit of intake over a lifetime. In accordance with the 2025 toxicological updates, a SF of 32 (mg/kg/day)⁻¹ was applied for inorganic arsenic, reflecting the significantly increased potency estimated from combined bladder and lung cancer data. For chromium (VI), a SF of 0.16 (mg/kg/day)⁻¹ was utilized, following the finalized IRIS assessment [22,25].

2.7. Quantitative Analysis of Chemical Elements by the Tolerable Upper Intake Level (UL)

Quantitative analysis of daily food-consumption adequacy is used to assess the probability that a healthy individual is at risk of adverse effects from high nutrient intake, thereby indicating the likelihood that the intake of the evaluated food is safe [26].

Adequacy was calculated for the chemical elements Cu, Fe, Mg, and Zn based on a serving of vegetables per individual (cooked green beans—44 g and cooked spinach—67 g) according to the Dietary Guidelines for the Brazilian Population [19], with one day’s dietary intake, following Equation (5).

$$Z={\displaystyle \frac{{\rm M}\iota -UL}{\frac{DP\iota }{\surd n}}}$$

(5)

where:

Mi—average intake over in days;

UL—upper limit of tolerable intake;

DPi—standard deviation of intake according to population studies;

n—number of days of food consumption investigated.

2.8. Statistical Analysis

Elemental concentrations in cowpea pod and leaf samples were expressed as mean ± standard deviation (SD) for each sampling point (A1–A3, B1–B3, and C1–C3). Descriptive statistics were used to summarize the distributions of macro- and microelements.

Prior to inferential analysis, data distribution was evaluated using the Shapiro–Wilk test for normality. However, given the relatively small number of sampling points (n = 9), the statistical power of normality tests is limited and the results should be interpreted cautiously. Therefore, and considering that some variables did not meet the normality assumption, a nonparametric statistical approach was adopted. Differences in elemental concentrations among sampling points were assessed using the Kruskal–Wallis test.

All statistical analyses were performed using GraphPad Prism (version 10.0.0, GraphPad Software, LLC, San Diego, CA, USA) and R software (version 4.3.1, R Foundation for Statistical Computing, Vienna, Austria), with a significance level of p < 0.05.

3. Results

3.1. Concentration of Metals and Metalloids in Cowpea Pods and Leaves

The concentrations of macro- and microelements in cowpea pods are detailed in

Table 1. Concentration (mean ± standard deviation) of macro- and microelements in cowpea pod samples (mg/kg).

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	2.636 ± 0.058	2.553 ± 0.052	2.606 ± 0.047	2.782 ± 0.028	2.668 ± 0.033	2.345 ± 0.019	2.614 ± 0.083	2.719 ± 0.025	2.598 ± 0.018
Cd	0.571 ± 0.003	0.576 ± 0.009	0.571 ± 0.007	0.601 ± 0.002	0.589 ± 0.005	0.570 ± 0.003	0.613 ± 0.006	0.598 ± 0.003	0.579 ± 0.005
Cr	2.146 ± 0.034	1.042 ± 0.010	1.086 ± 0.022	1.076 ± 0.009	1.087 ± 0.007	1.025 ± 0.007	1.089 ± 0.011	1.899 ± 0.040	1.105 ± 0.022
Cu	17.336 ± 0.341	6.293 ± 0.137	5.624 ± 0.200	8.416 ± 0.140	8.292 ± 0.055	6.786 ± 0.335	8.034 ± 0.091	6.556 ± 0.217	9.815 ± 0.224
Fe	72.643 ± 1.169	43.867 ± 1.013	48.141 ± 1.901	90.622 ± 1.576	44.920 ± 0.206	42.316 ± 1.840	55.801 ± 0.820	52.640 ± 1.463	63.680 ± 1.506
Mg	75.428 ± 3.749	51.211 ± 0.834	78.618 ± 2.086	76.817 ± 1.156	67.865 ± 0.766	70.233 ± 0.676	93.510 ± 2.388	78.654 ± 1.110	77.177 ± 2.514
Mn	188.024 ± 5.689	255.544 ± 6.087	99.607 ± 4.108	362.929 ± 7.774	197.541 ± 2.323	53.370 ± 2.101	294.516 ± 9.396	194.084 ± 4.377	43.506 ± 0.986
Mo	1.357 ± 0.012	0.887 ± 0.014	0.898 ± 0.014	0.942 ± 0.003	0.996 ± 0.005	1.400 ± 0.024	1.212 ± 0.017	0.919 ± 0.017	0.873 ± 0.010
Pb	1.882 ± 0.053	2.004 ± 0.063	1.910 ± 0.060	2.220 ± 0.011	2.454 ± 0.044	1.907 ± 0.056	2.258 ± 0.045	3.171 ± 0.050	2.032 ± 0.064
Se	3.891 ± 0.076	3.705 ± 0.040	3.745 ± 0.086	4.150 ± 0.680	3.993 ± 0.093	3.464 ± 0.097	3.988 ± 0.087	4.070 ± 0.123	3.753 ± 0.090
V	8.165 ± 0.127	6.166 ± 0.119	8.983 ± 0.297	9.250 ± 0.135	7.842 ± 0.037	8.095 ± 0.316	11.103 ± 0.156	9.418 ± 0.236	8.931 ± 0.176
Zn	14.434 ± 0.114	26.477 ± 0.227	20.471 ± 0.390	14.993 ± 0.229	28.205 ± 0.241	20.864 ± 0.282	16.636 ± 0.318	16.790 ± 0.267	19.875 ± 0.312

* Distance from the landfill. ** Distance from the road.

. At a distance of 100 m from the landfill and 64 m from the road (point A1), the elements found in the highest concentration in the cowpea pods, in descending order, were Mn > Mg > Fe > Cu > Zn > V > Se > As > Cr > Pb > Mo > Cd. At a distance of 100 m from the landfill and 264 m from the road (point A2), the order was Mn > Mg > Fe > Zn > Cu > V > Se > As > Pb > Cr > Mo > Cd. At a distance of 100 m from the landfill and 464 m from the road (point A3), the order was Mn > Mg > Fe > Zn > V > Cu > Se > As > Pb > Cr > Mo > Cd.

At a distance of 200 m from the landfill and 64 m from the road (point B1), they followed the sequence Mn > Fe > Mg > Zn > V > Cu > Se > As > Pb > Cr > Mo > Cd. At a distance of 200 m from the landfill and 264 m from the road (point B2), they decreased in the order Mn > Mg > Fe > Zn > Cu > V > Se > As > Pb > Cr > Mo > Cd. At a distance of 200 m from the landfill and 464 m from the road (point B3), they decreased in the order Mg > Mn > Fe > Zn > V > Cu > Se > As > Pb > Mo > Cr > Cd.

At the greatest distance from the landfill (300 m) and 64 m from the road (point C1), the elements followed the order Mn > Mg > Fe > Zn > V > Cu > Se > As > Pb > Mo > Cr > Cd. At a distance of 300 m from the landfill and 264 m from the road (point C2), the elements decreased in the order Mn > Mg > Fe > Zn > V > Cu > Se > Pb > As > Cr > Mo > Cd. And at a distance of 300 m from the landfill and 464 m from the road (point C3), they decreased in the order Mg > Fe > Mn > Zn > Cu > V > Se > As > Pb > Cr > Mo > Cd.

The concentrations of metal(loid)s in cowpea leaves are specified in

Table 2. Concentration (mean ± standard deviation) of macro- and microelements in cowpea leaf samples (mg/kg).

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	6.297 ± 0.158	5.807 ± 0.061	5.925 ± 0.091	5.936 ± 0.047	6.342 ± 0.167	5.171 ± 0.026	5.814 ± 0.046	5.850 ± 0.071	5.011 ± 0.119
Cd	0.788 ± 0.012	0.748 ± 0.007	0.756 ± 0.013	0.830 ± 0.005	0.791 ± 0.009	0.703 ± 0.003	0.804 ± 0.005	0.964 ± 0.006	0.778 ± 0.007
Cr	2.855 ± 0.117	1.653 ± 0.039	1.685 ± 0.041	1.976 ± 0.038	2.385 ± 0.067	1.720 ± 0.021	1.786 ± 0.019	2.294 ± 0.050	1.471 ± 0.049
Cu	12.711 ± 0.342	8.020 ± 0.246	8.175 ± 0.226	11.408 ± 0.351	8.265 ± 0.239	13.644 ± 0.230	10.277 ± 0.152	8.791 ± 0.266	14.580 ± 0.439
Fe	751.245 ± 30.404	301.423 ± 7.559	320.919 ± 11.929	541.563 ± 13.357	266.308 ± 7.502	303.538 ± 4.652	261.366 ± 3.495	307.959 ± 8.272	344.953 ± 11.721
Mg	106.438 ± 2.297	203.812 ± 2.573	175.892 ± 6.822	119.039 ± 2.278	94.251 ± 1.736	122.560 ± 1.815	106.906 ± 3.020	74.415 ± 2.258	88.037 ± 3.317
Mn	1019.678 ± 46.468	628.939 ± 12.928	635.342 ± 35.152	1788.462 ± 33.810	1189.123 ± 33.657	344.953 ± 6.660	1372.707 ± 17.368	1147.338 ± 22.620	163.246 ± 3.823
Mo	1.524 ± 0.021	1.144 ± 0.009	1.158 ± 0.019	1.365 ± 0.015	1.261 ± 0.011	1.137 ± 0.014	1.350 ± 0.019	1.192 ± 0.013	1.046 ± 0.003
Pb	4.889 ± 0.171	4.450 ± 0.070	4.404 ± 0.147	5.039 ± 0.094	5.070 ± 0.105	4.891 ± 0.067	4.773 ± 0.051	4.840 ± 0.057	4.401 ± 0.090
Se	8.252 ± 0.217	7.240 ± 0.136	7.303 ± 0.131	8.648 ± 0.150	8.356 ± 0.172	6.455 ± 0.091	7.887 ± 0.141	7.443 ± 0.060	6.339 ± 0.111
V	16.042 ± 0.010	15.460 ± 0.379	15.517 ± 0.466	8.267 ± 0.484	12.267 ± 0.278	13.733 ± 0.202	13.946 ± 0.177	10.226 ± 0.253	11.892 ± 0.362
Zn	14.707 ± 0.375	13.436 ± 0.198	13.375 ± 0.309	13.571 ± 0.216	27.102 ± 0.528	16.175 ± 0.224	12.761 ± 0.176	15.816 ± 0.168	19.538 ± 0.493

* Distance from the landfill. ** Distance from the road.

. At a distance of 100 m from the landfill and 64 m from the road (point A1), the elements found in higher concentrations in cowpea leaves decreased in the order: Mn > Fe > Mg > Zn > Cu > Se > As > Pb > Cr > Mo > V > Cd. At a distance of 100 m from the landfill and 264 m from the road (point A2), the order was Mn > Fe > Mg > V > Zn > Cu > Se > As > Pb > Cr > Mo > Cd. At a distance of 100 m from the landfill and 464 m from the road (point A3), the elements decreased in the order Mn > Fe > Mg > V > Zn > Cu > Se > As > Pb > Cr > Mo > Cd.

At a distance of 200 m from the landfill and 64 m from the road (point B1), the elements followed the decreasing order Mn > Fe > Mg > Zn > Cu > Se > V > As > Pb > Cr > Mo > Cd. At a distance of 200 m from the landfill and 264 m from the road (point B2), they followed the order Mn > Fe > Mg > Zn > V > Se > Cu > As > Pb > Cr > Mo > Cd. At a distance of 200 m from the landfill and 464 m from the road (point B3), they decreased in the order Mn > Fe > Mg > Zn > V > Cu > Se > As > Pb > Cr > Mo > Cd.

At a distance of 300 m from the landfill and 64 m from the road (point C1), the order of the elements found was Mn > Fe > Mg > V > Zn > Cu > Se > As > Pb > Cr > Mo > Cd. At a distance of 300 m from the landfill and 264 m from the road (point C2), the elements decreased in the order Mn > Fe > Mg > Zn > V > Cu > Se > As > Pb > Cr > Mo > Cd. And at a distance of 300 m from the landfill and 464 m from the road (point C3), they decreased in the order Fe > Mn > Mg > Zn > Cu > V > Se > As > Pb > Cr > Mo > Cd.

Descriptive statistics indicated variability in macro and microelement concentrations among the sampling points for both cowpea pod samples (Table 1) and leaf samples (Table 2). Mean concentrations differed among points A, B, and C, indicating spatial heterogeneity associated with environmental conditions.

Inferential statistical analysis using the Kruskal–Wallis test showed that, for most elements, no statistically significant differences were observed among sampling points (p > 0.05). However, selected elements exhibited significant spatial variation (p < 0.05), indicating localized influences on elemental accumulation.

These results demonstrate that elemental distribution in cowpea pods and leaves is influenced by site-specific environmental factors rather than uniform background levels.

3.2. Risks to Human Health Associated with the Ingestion of Cowpea Pods and Leaves

The estimated chronic daily intake (CDI) of metal(loid)s present in Vigna unguiculata L. WALP vines due to consumption by adults (

Table 3. Chronic Daily Intake (CDI, mg/kg/day) of macro- and microelements due to ingestion of cowpea pod samples for adults aged 30 years and weighing 70 kg.

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	1.59 × 10−3	1.54 × 10−3	1.57 × 10−3	1.68 × 10−3	1.61 × 10−3	1.41 × 10−3	1.58 × 10−3	1.64 × 10−3	1.57 × 10−3
Cd	3.44 × 10−4	3.47 × 10−4	3.44 × 10−4	3.62 × 10−4	3.55 × 10−4	3.44 × 10−4	3.69 × 10−4	3.60 × 10−4	3.49 × 10−4
Cr	1.29 × 10−3	6.28 × 10−4	6.55 × 10−4	6.49 × 10−4	6.56 × 10−4	6.18 × 10−4	6.57 × 10−4	1.14 × 10−3	6.66 × 10−4
Cu	1.05 × 10−2	3.79 × 10−3	3.39 × 10−3	5.07 × 10−3	5.00 × 10−3	4.09 × 10−3	4.84 × 10−3	3.95 × 10−3	5.92 × 10−3
Fe	4.38 × 10−2	2.64 × 10−2	2.90 × 10−2	5.46 × 10−2	2.71 × 10−2	2.55 × 10−2	3.36 × 10−2	3.17 × 10−2	3.84 × 10−2
Mg	4.55 × 10−2	3.09 × 10−2	4.74 × 10−2	4.63 × 10−2	4.09 × 10−2	4.23 × 10−2	5.64 × 10−2	4.74 × 10−2	4.65 × 10−2
Mn	1.13 × 10−1	1.54 × 10−1	6.00 × 10−2	2.19 × 10−1	1.19 × 10−1	3.22 × 10−2	1.78 × 10−1	1.17 × 10−1	2.62 × 10−2
Mo	8.18 × 10−4	5.35 × 10−4	5.41 × 10−4	5.68 × 10−4	6.00 × 10−4	8.44 × 10−4	7.30 × 10−4	5.54 × 10−4	5.27 × 10−4
Pb	1.13 × 10−3	1.21 × 10−3	1.15 × 10−3	1.34 × 10−3	1.48 × 10−3	1.15 × 10−3	1.36 × 10−3	1.91 × 10−3	1.23 × 10−3
Se	2.35 × 10−3	2.24 × 10−3	2.26 × 10−3	2.50 × 10−3	2.41 × 10−3	2.09 × 10−3	2.40 × 10−3	2.46 × 10−3	2.26 × 10−3
V	4.92 × 10−3	3.72 × 10−3	5.41 × 10−3	5.58 × 10−3	4.73 × 10−3	4.88 × 10−3	6.69 × 10−3	5.68 × 10−3	5.39 × 10−3
Zn	8.70 × 10−3	1.60 × 10−2	1.23 × 10−2	9.04 × 10−3	1.70 × 10−2	1.26 × 10−2	1.00 × 10−2	1.01 × 10−2	1.20 × 10−2

* Distance from the landfill. ** Distance from the road.

) showed concentrations above the reference oral dose (RfD) proposed by the USEPA for the elements As (6 × 10⁻⁵ mg/kg/day) and Cd (1 × 10⁻⁴ mg/kg/day) at all points. Similarly, Mn (1.4 × 10⁻¹ mg/kg/day) at points A2, B1 and C1 was higher than the RfD, as was V (5.04 × 10⁻³ mg/kg/day) at points A3, B1, C1, C2 and C3, indicating that at these points the pods are not at the safe limit for consumption and therefore pose health risks to adult individuals when consumed at a frequency of 350 days/year, with daily intake rate of pods per individual of 44 g.

The estimated CDI of metal(loid)s present in the leaf of Vigna unguiculata L. WALP due to consumption by adults (

Table 4. Chronic Daily Intake (CDI, mg/kg/day) of macro- and microelements due to ingestion of cowpea leaf samples for adults aged 30 years and weighing 70 kg.

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	5.78 × 10−3	5.33 × 10−3	5.44 × 10−3	5.45 × 10−3	5.82 × 10−3	4.75 × 10−3	5.34 × 10−3	5.37 × 10−3	4.60 × 10−3
Cd	7.23 × 10−4	6.86 × 10−4	6.94 × 10−4	7.62 × 10−3	7.26 × 10−4	6.45 × 10−4	7.38 × 10−4	8.85 × 10−4	7.14 × 10−4
Cr	2.62 × 10−3	1.52 × 10−3	1.55 × 10−3	1.81 × 10−3	2.19 × 10−3	1.58 × 10−3	1.64 × 10−3	2.10 × 10−3	1.35 × 10−3
Cu	1.17 × 10−2	7.36 × 10−3	7.50 × 10−3	1.05 × 10−2	7.59 × 10−3	1.25 × 10−2	9.44 × 10−3	8.07 × 10−3	1.34 × 10−2
Fe	6.89 × 10−1	2.77 × 10−1	2.94 × 10−1	4.97 × 10−1	2.44 × 10−1	2.78 × 10−1	2.40 × 10−1	2.83 × 10−1	3.16 × 10−1
Mg	9.77 × 10−2	1.87 × 10−1	1.63 × 10−1	1.00 × 10−1	8.64 × 10−2	1.13 × 10−1	9.79 × 10−2	6.83 × 10−2	8.08 × 10−2
Mn	9.33 × 10−1	5.77 × 10−1	5.84 × 10−1	1.64 × 100	1.09 × 100	3.17 × 10−1	1.26 × 100	1.05 × 10	1.50 × 10−1
Mo	1.40 × 10−3	1.05 × 10−3	1.06 × 10−3	1.25 × 10−3	1.16 × 10−3	1.04 × 10−3	1.24 × 10−3	1.09 × 10−3	9.60 × 10−4
Pb	4.49 × 10−3	4.08 × 10−3	4.04 × 10−3	4.62 × 10−3	4.65 × 10−3	4.49 × 10−3	4.38 × 10−3	4.44 × 10−3	4.04 × 10−3
Se	7.57 × 10−3	6.64 × 10−3	6.71 × 10−3	7.94 × 10−3	7.67 × 10−3	5.93 × 10−3	7.24 × 10−3	6.83 × 10−3	5.82 × 10−3
V	1.47 × 10−2	1.42 × 10−2	1.43 × 10−2	7.59 × 10−3	1.13 × 10−2	1.26 × 10−2	1.28 × 10−2	9.39 × 10−3	1.09 × 10−2
Zn	1.35 × 10−2	1.23 × 10−2	1.23 × 10−2	1.25 × 10−2	2.49 × 10−2	1.48 × 10−2	1.17 × 10−2	1.45 × 10−2	1.79 × 10−2

* Distance from the landfill. ** Distance from the road.

) indicated concentrations above the RfD for the elements As (6 × 10⁻⁵), Cd (1 × 10⁻⁴), Mn (1.4 × 10⁻¹), and Se (5 × 10⁻³) at all points. With the exception of point A1, V (5.04 × 10⁻³) also showed values higher than RfD, indicating that at these points the leaf is above the safe limit for consumption, posing health risks to adult individuals when consumed at a frequency of 350 days/year for thirty years, with a daily intake rate of leaves per individual of 67 g.

The evaluation of non-carcinogenic risk through the Hazard Quotient (HQ) revealed that As and Cd exceeded the safety threshold of 1 in all samples of cowpea pods (

Table 5. Risk quotient (HQ) and risk index (HI) of macro and microelements, considering the consumption of cowpea pods for adult women and men aged 30 years and weighing 70 kg. IR = 0.044 kg/day and BW = 70 kg.

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	26.50	25.67	26.17	28.00	26.83	23.50	26.33	27.33	26.17
Cd	3.44	3.47	3.44	3.62	3.55	3.44	3.69	3.60	3.49
Cr	0.43	0.21	0.22	0.22	0.22	0.21	0.22	0.38	0.22
Cu	0.26	0.09	0.08	0.13	0.13	0.10	0.12	0.10	0.15
Fe	0.06	0.04	0.04	0.08	0.04	0.04	0.05	0.05	0.05
Mn	0.81	1.10	0.43	1.56	0.85	0.23	1.27	0.84	0.19
Mo	0.16	0.11	0.11	0.11	0.12	0.17	0.15	0.11	0.11
Se	0.47	0.45	0.45	0.50	0.48	0.42	0.48	0.49	0.45
V	0.98	0.74	1.07	1.11	0.94	0.97	1.33	1.13	1.07
Zn	0.03	0.05	0.04	0.03	0.06	0.04	0.03	0.03	0.04
HI(Sum)	33.14	31.93	32.05	35.36	33.22	29.12	33.97	34.16	32.09

* Distance from the landfill. ** Distance from the road. Toxic risk is considered to occur if HQ > 1, while HQ < 1 represents negligible risk.

). For pods, As values ranged from 23.50 to 28.00, while Cd varied between 3.44 and 3.69. Other elements also surpassed the safety limit (HQ > 1) in specific locations: Mn at points A2 (1.10), B1 (1.56), and C1 (1.27), and V at points A3 (1.07), B1 (1.11), C1 (1.33), C2 (1.13), and C3 (1.07). The Hazard Index (HI) for pods was consistently above 1, with values ranging from 29.12 (B3) to 35.36 (B1).

In cowpea leaves (

Table 6. Risk quotient (HQ) and risk index (HI) of macro and microelements, considering the consumption of cowpea leaves for adult women and men aged 30 years and weighing 70 kg. IR = 0.067 kg/day and BW = 70 kg.

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	96.33	88.83	90.67	90.83	97.00	79.17	89.00	89.50	76.67
Cd	7.23	6.86	6.94	7.62	7.26	6.45	7.38	8.85	7.14
Cr	0.87	0.51	0.52	0.60	0.73	0.53	0.55	0.70	0.45
Cu	0.29	0.18	0.19	0.26	0.19	0.31	0.24	0.20	0.34
Fe	0.98	0.40	0.42	0.71	0.35	0.40	0.34	0.40	0.45
Mn	6.66	4.12	4.17	11.71	7.79	2.26	9.00	7.50	1.07
Mo	0.28	0.21	0.21	0.25	0.23	0.21	0.25	0.22	0.19
Se	1.51	1.33	1.34	1.59	1.53	1.19	1.45	1.37	1.16
V	2.92	2.82	2.84	1.51	2.24	2.50	2.54	1.86	2.16
Zn	0.05	0.04	0.04	0.04	0.08	0.05	0.04	0.05	0.06
HI (Sum)	117.12	105.30	107.34	115.12	117.40	93.07	110.79	110.65	89.69

* Distance from the landfill. ** Distance from the road. Toxic risk is considered to occur if HQ > 1, while HQ < 1 represents negligible risk.

), the HQ values for As, Cd, Mn, and Se were greater than 1 at all analyzed points. Arsenic reached critical levels in the leaves, with a maximum of 97.00 at point B2. Manganese also showed high quotients, peaking at 11.71 (B1). The HI for leaves was notably higher than for pods, with values between 89.69 (C3) and 117.40 (B2). Due to the lack of an established oral RfD by the US EPA, the risk for Pb was not quantified in the HQ/HI calculations.

The estimate of CR was calculated based on the chronic daily intake dose of metal(loid)s, considering a 30-year-old adult with an average body weight of 70 kg and a daily consumption of a 44 g portion of cowpea pods (

Table 7. Carcinogenic risk (CR) of As and Cr metal(loid)s for adults (30 years old with 70 kg body weight), considering the consumption of 44 g/day of cowpea pods.

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	5.09 × 10−2	4.93 × 10−2	5.02 × 10−2	5.38 × 10−2	5.15 × 10−2	4.51 × 10−2	5.06 × 10−2	5.25 × 10−2	5.02 × 10−2
Cr	2.06 × 10−4	1.00 × 10−4	1.05 × 10−4	1.04 × 10−4	1.05 × 10−4	9.89 × 10−5	1.05 × 10−4	1.82 × 10−4	1.07 × 10−4

* Distance from the landfill. ** Distance from the road.

). The assessment focused on the slope factor (SF) values for the elements Arsenic (As) and Chromium (Cr). For Arsenic, the CR values ranged from 2.12 × 10⁻³ at point B3 to 2.52 × 10⁻³ at point B1. All sampling points, regardless of the distance from the landfill or the road, yielded As risk levels significantly above the 10⁻⁴ threshold. Specifically, the highest risk for As was observed at point B1 (100 m from the landfill and 64 m from the road).

Regarding Chromium, the CR values varied between 3.09 × 10⁻⁴ (point B3) and 6.45 × 10⁻⁴ (point A1). Similarly, to Arsenic, all recorded values for Cr exceeded the acceptable limit of 10⁻⁴ established by regulatory agencies. The spatial distribution showed that point A1 (100 m from the landfill and 64 m from the road) and point C2 (300 m from the landfill and 264 m from the road) presented the most elevated risks for this metal.

The CR assessment for cowpea leaves (

Table 8. Carcinogenic risk (CR) of As and Cr metal(loid)s for adults (30 years old with a body weight of 70 kg), considering the consumption of 67 g of cowpea leaves.

Element	100 m *			200 m *			300 m *
	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
	A1	A2	A3	B1	B2	B3	C1	C2	C3
As	1.85 × 10−1	1.71 × 10−1	1.74 × 10−1	1.74 × 10−1	1.86 × 10−1	1.52 × 10−1	1.71 × 10−1	1.72 × 10−1	1.47 × 10−1
Cr	4.19 × 10−4	2.43 × 10−4	2.48 × 10−4	2.90 × 10−4	3.50 × 10−4	2.53 × 10−4	2.62 × 10−4	3.36 × 10−4	2.16 × 10−4

* Distance from the landfill. ** Distance from the road.

) showed that all results for As and Cr were above the maximum acceptable limit of 10⁻⁴. For Arsenic, CR values ranged from 6.90 × 10⁻³ to 8.73 × 10⁻³. For Chromium, the risk varied between 6.75 × 10⁻⁴ and 1.31 × 10⁻³.

The analysis of the adequacy of daily cowpea pod consumption indicated that, at most points, an average intake of 44 g per day had a low probability of exceeding the UL (

Table 9. Percentage of daily intake adequacy by UL of macro and microelements present in cowpea pods, considering the average intake of one serving/day (44 g).

Element	Sex	Age	100 m *			200 m *			300 m *
			64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
			A1	A2	A3	B1	B2	B3	C1	C2	C3
Cu	Men	9 to 13	−8332.06	−8332.87	−8332.92	−88,332.72	−8332.73	−8332.84	−8332.74	−8332.85	−8332.61
		14 to 18	−13,332.06	−13,332.87	−13,332.92	−13,332.72	−13,332.73	−13,332.84	−13,332.74	−13,332.85	−13,332.61
		19 to 50	−14,284.62	−14,285.32	−14,285.36	−14,285.19	−14,285.19	−14,285.29	−14,285.21	−14,285.30	−14,285.10
		51 to >70	−14,284.62	−14,285.32	−14,285.36	−14,285.19	−14,285.19	−14,285.29	−14,285.21	−14,285.30	−14,285.10
	Women	9 to 13	−9998.47	−9999.45	−9999.51	−9999.26	−9999.27	−9999.40	−9999.29	−9999.42	−9999.14
		14 to 18	−15,998.47	−15,999.45	−15,999.51	−15,999.26	−15,999.27	−15,999.40	−15,999.29	−15,999.42	−15,999.14
		19 to 50	−16,665.40	−16,666.21	−16,666.25	−16,666.05	−16,666.06	−16,666.17	−16,666.08	−16,666.19	−16,665.95
		51 to >70	−19,998.47	−19,999.45	−19,999.51	−19,999.26	−19,999.27	−19,999.40	−19,999.29	−19,999.42	−19,999.14
Fe	Men	9 to 13	−4.09	−4.23	−4.21	−4.00	−4.22	−4.24	−4.17	−4.19	−4.13
		14 to 18	−4.64	−4.79	−4.76	−4.56	−4.78	−4.79	−4.73	−4.74	−4.69
		19 to 50	−4.64	−4.79	−4.76	−4.56	−4.78	−4.79	−4.73	−4.74	−4.69
		51 to >70	−5.97	−6.15	−6.13	−5.86	−6.15	−6.16	−6.08	−6.10	−6.03
	Women	9 to 13	−6.13	−6.34	−6.31	−6.00	−3.34	−6.36	−6.26	−6.28	−6.20
		14 to 18	−6.97	−7.18	−7.15	−6.84	−7.17	−7.19	−7.09	−7.11	−7.03
		19 to 50	−5.97	−6.15	−6.13	−5.86	−6.15	−6.16	−6.08	−6.10	−6.03
		51 to >70	−8.36	−8.61	−8.58	−8.20	−8.60	−8.63	−8.51	−8.54	−8.44
Mg	Men	9 to 13	−3.18	−3.19	−3.18	−3.18	−3.18	−3.18	−3.17	−3.18	−3.18
		14 to 18	−3.18	−3.19	−3.13	−3.18	−3.18	−3.18	−3.17	−3.18	−3.18
		19 to 50	−2.84	−2.85	−2.84	−2.84	−2.84	−2.84	−2.84	−2.84	−2.84
		51 to >70	−3.69	−3.70	−3.69	−3.69	−3.69	−3.69	−3.68	−3.69	−3.69
	Women	9 to 13	−4.03	−4.04	−4.03	−4.03	−4.04	−4.03	−4.02	−4.03	−4.03
		14 to 18	−4.03	−4.04	−4.03	−4.03	−4.04	−4.03	−4.02	−4.03	−4.03
		19 to 50	−4.03	−4.04	−4.03	−4.03	−4.04	−4.03	−4.02	−4.03	−4.03
		51 to >70	−4.68	−4.70	−4.68	−4.68	−4.69	−4.69	−4.67	−4.68	−4.68
Zn	Men	9 to 13	−2.80	−2.73	−2.76	−2.79	−2.72	−2.76	−2.78	−2.78	−2.77
		14 to 18	−4.17	−4.10	−4.14	−4.17	−4.09	−4.14	−4.16	−4.16	−4.14
		19 to 50	−4.37	−4.32	−4.34	−4.37	−4.31	−4.34	−4.36	−4.36	−4.35
		51 to >70	−4.92	−4.85	−4.89	−4.92	−4.84	−4.89	−4.91	−4.91	−4.89
	Women	9 to 13	−4.47	−4.37	−4.42	−4.47	−4.35	−4.42	−4.45	−4.45	−4.43
		14 to 18	−6.67	−6.57	−6.62	−6.67	−6.55	−6.62	−6.65	−6.65	−6.63
		19 to 50	−6.56	−6.47	−6.52	−6.56	−6.46	−6.51	−6.54	−6.54	−6.52
		51 to >70	−7.87	−7.77	−7.82	−7.87	−7.75	−7.82	−7.85	−7.85	−7.83

* Distance from the landfill. ** Distance from the road.

). Only the elements Mg for men at all points and Zn for men aged 9 to 13 at all distances from the road and the landfill showed a probability of intake exceeding UL, but it was very close to 100%, indicating minimal risk of toxicity.

The analysis of the adequacy of the daily intake of cowpea leaves showed that the intake of an average portion of 67 g of leaves per day was more likely to be above the UL when compared to the intake of a portion of pods (44 g) (

Table 10. Percentage of daily intake adequacy by UL of macro and microelements present in cowpea leaves, considering the average intake of one serving/day (67 g).

Element	Sex	Age	100 m *			200 m *			300 m *
			64 m **	264 m **	464 m **	64 m **	264 m **	464 m **	64 m **	264 m **	464 m **
			A1	A2	A3	B1	B2	B3	C1	C2	C3
Cu	Men	9 to 13	−8331.91	−8332.44	−8332.42	−88,332.06	−8332.41	−8331.81	−8332.19	−8332.35	−8331.71
		14 to 18	−13,331.91	−13,332.44	−13,332.42	−13,332.06	−13,332.41	−13,331.81	−13,332.19	−13,332.35	−13,331.71
		19 to 50	−14,284.50	−14,284.95	−14,284.93	−14,284.62	−14,284.92	−14,284.41	−14,284.73	−14,284.87	−14,284.32
		51 to >70	−14,284.50	−14,284.95	−14,284.93	−14,284.62	−14,284.92	−14,284.41	−14,284.73	−14,284.87	−14,284.32
	Women	9 to 13	−9998.30	−9998.93	−9998.90	−9998.47	−9998.89	−9998.17	−9998.62	−9998.82	−9998.05
		14 to 18	−15,998.30	−15,998.93	−15,998.90	−15,998.47	−15,998.89	−15,998.17	−15,998.62	−15,998.82	−15,998.05
		19 to 50	−16,665.25	−16,665.77	−16,665.75	−16,665.39	−16,665.74	−16,665.14	−16,665.52	−16,665.69	−16,665.04
		51 to >70	−19,998.30	−19,998.93	−19,998.90	−19,998.47	−19,998.89	−19,998.17	−19,998.62	−19,998.82	−19,998.05
Fe	Men	9 to 13	1.15	−2.20	−2.06	−0.41	−2.46	−2.18	−2.50	−2.15	−1.88
		14 to 18	0.59	−2.76	−2.61	−0.97	−3.02	−2.74	−3.05	−2.71	−2.43
		19 to 50	0.59	−2.76	−2.61	−0.97	−3.02	−2.74	−3.05	−2.71	−2.43
		51 to >70	0.76	−3.54	−3.36	−1.25	−3.88	−3.52	−3.93	−3.48	−3.13
	Women	9 to 13	1.72	−3.30	−3.08	−0.62	−3.69	−3.28	−3.75	−3.23	−2.81
		14 to 18	0.89	−4.13	−3.92	−1.45	−4.53	−4.11	−4.58	−4.06	−3.65
		19 to 50	0.76	−3.54	−3.36	−1.25	−3.88	−3.52	−3.93	−3.48	−3.13
		51 to >70	1.07	−4.96	−4.70	−1.74	−5.43	−4.93	−5.50	−4.87	−4.38
Mg	Men	9 to 13	−3.15	−3.09	−3.10	−3.14	−3.15	−3.14	−3.15	−3.17	−3.16
		14 to 18	−3.15	−3.09	−3.10	−3.14	−3.15	−3.14	−3.15	−3.17	−3.16
		19 to 50	−2.81	−2.76	−2.77	−2.80	−2.82	−2.80	−2.81	−2.83	−2.82
		51 to >70	−3.65	−3.58	−3.60	−3.64	−3.66	−3.64	−3.65	−3.67	−3.66
	Women	9 to 13	−3.99	−3.91	−3.93	−3.98	−4.00	−3.97	−3.99	−4.01	−4.00
		14 to 18	−3.99	−3.91	−3.93	−3.98	−4.00	−3.97	−3.99	−4.01	−4.00
		19 to 50	−3.99	−3.91	−3.93	−3.98	−4.00	−3.97	−3.99	−4.01	−4.00
		51 to >70	−4.63	−4.55	−4.57	−4.62	−4.64	−4.62	−4.63	−4.66	−4.65
Zn	Men	9 to 13	−2.75	−2.76	−2.76	−2.76	−2.65	−2.74	−2.77	−2.74	−2.71
		14 to 18	−4.13	−4.14	−4.14	−4.14	−4.02	−4.11	−4.14	−4.12	−4.09
		19 to 50	−4.33	−4.34	−4.34	−4.34	−4.24	−4.32	−4.35	−4.33	−4.30
		51 to >70	−4.88	−4.89	−4.89	−4.89	−4.77	−4.86	−4.89	−4.87	−4.84
	Women	9 to 13	−4.40	−4.42	−4.42	−4.42	−4.24	−4.38	−4.43	−4.39	−4.34
		14 to 18	−6.60	−6.62	−6.62	−6.62	−6.44	−6.58	−6.63	−6.59	−6.54
		19 to 50	−6.50	−6.52	−6.52	−6.52	−6.36	−6.48	−6.52	−6.49	−6.45
		51 to >70	−7.80	−7.82	−7.82	−7.82	−7.64	−7.78	−7.83	−7.79	−7.74

* Distance from the landfill. ** Distance from the road.

). Fe at point A1 stands out in all age groups, as it showed a high probability of intake being above the UL value (72.57–95.54%), and Cu at all points, as it showed a low probability of intake being above the UL value (<100%), indicating a probability of safe consumption.

4. Discussion

The statistical analysis supports the presence of spatial heterogeneity in elemental accumulation across the sampling points. The absence of statistically significant differences for several elements suggests relatively stable background concentrations or effective physiological regulation by the plant, as also reported in previous studies evaluating elemental uptake under varying environmental conditions [2,15]. Conversely, the statistically significant differences observed for selected elements indicate localized environmental inputs, potentially associated with soil properties and atmospheric deposition processes. Similar spatial patterns have been described in soil–plant interaction studies conducted in agricultural systems located near anthropogenic sources [1,3]. The statistical approach adopted in this study was therefore appropriate for identifying spatial trends while accounting for data distribution characteristics, thereby reinforcing the reliability of the observed elemental patterns.

The metabolism, transport, and distribution of metal(loid)s in plants are governed by complex physiological mechanisms [27]. Elemental concentrations vary according to plant genotype and are strongly influenced by environmental factors such as soil composition, humidity, temperature, and climatic conditions [2,15]. In cowpea plants, the high absorption capacity for macro- and micronutrients has been associated with morphological traits, particularly deep, well-developed root systems, which enhance nutrient and metal uptake from the soil and contribute to the accumulation patterns observed in this study [1,3,28,29].

The pod (Table 1), in comparison with the cowpea leaf (Table 2), had a lower concentration of As, Cd, Cr, Cu, Fe, Mg, Mn, Pb, Se, and V, indicating that Vigna unguiculata tends to accumulate these elements in its leaf tissues. Similarly, other species, including leafy vegetables, tend to accumulate heavy metals in their leaves compared to grains or fruits, mainly Fe, Mn, and Pb, both in contaminated areas [8,30] and in safe environments [7].

Through analysis of the soil where the cowpeas in this study were grown, it was found that the highest concentration of metal(loid)s was at the point furthest from the landfill and the highway (C3), with the exception of Mg, which had the highest concentration at point B3 [10]. In contrast to this study, the highest concentrations in the pods were observed at points B1 (As, Fe, Mn, V, Se), A1 (Cr, Cu), and C1 (Cd, Mg). In the leaves, the highest levels were observed at points A1 (Fe, Mo, V), B2 (As, Pb, Zn), and B1 (Mn, Se).

Some studies have shown a direct influence of the growing environment on the concentration of metallic elements in plants [28]. In this study, cowpeas were grown in exposed soil, and the average Cd concentration in the leaves (Table 2) was higher than in the pods (Table 1) at all collection points. In a study by Vongdala et al. [28], the Cd content in plants with aquatic roots was 50 times lower than in plants with terrestrial roots, both grown in a landfill. Nevertheless, it is important to consider the specific characteristics of each species, as they influence the absorption of metal(loid)s.

Cowpea leaves (Table 2) had a higher Mn concentration than the pods (Table 1). Considering the values reported for cowpea leaves by Awino, Maher, and Fai [30], the levels observed in our study are significantly higher. Regarding Mn in cowpea grains, both pods and leaves showed higher concentrations at point B1, with an accumulation profile similar to that of grains [10]. The high Mn concentrations observed in both studies, relative to other metal (loid)s, support the hypothesis that plants have different capacities to accumulate each element, with Vigna unguiculata exhibiting a remarkable ability to accumulate Mn.

Comparing the concentration of Zn present in the leaves (Table 2) with that in the pods (Table 1), a higher amount is found in the pods. Considering the data on Zn content in cowpea pods, as well as in the leaves, point B2 had the highest concentration of this element. However, the results of the analyses of cowpea leaves at points A2, A3, B1, and C1 were three times higher than those reported by Awino Maher and Fai [30], and the concentrations at the other points were even higher.

The significant difference between the concentrations of elements in cowpea leaves and other studies can be explained by the methodology used, since in some studies the samples were washed with running water, demineralized water, and then dried [15,28,30], while our samples did not undergo any cleaning process, in order to preserve possible environmental contamination.

The samples were collected at the end of the dry season, a period with low rainfall. The accumulation of various metal(loid)s in different organelles, across plant species, and at varying concentrations is influenced by climatic conditions and therefore warrants investigation [29]. Rainy seasons tend to increase the level of Cd in roots, stems, and leaves fourfold, in addition to the amounts of Cr, Pb, and Zn [28].

The elevated concentrations of chemical elements in the samples may be associated with anthropogenic activities in the vicinity of the planting area, particularly due to the presence of the highway. Sources such as fuels, automotive oils, and fragments detached from vehicles during traffic are responsible for the release and deposition of potentially toxic metals in the environment [8,29,31,32].

Analysis of the accumulation of metal(loid)s in the tissues of different plant species exposed to varying levels of traffic reveals a positive correlation between the atmospheric concentration of these elements and traffic intensity, since in areas with heavy vehicle flow, vegetation tends to have higher levels of metal(loid)s [29,31]. Therefore, plants grown in peri-urban areas, with higher traffic intensity and consequent emission of atmospheric pollutants, are more susceptible to the deposition and absorption of metals (loids) [29,31,33].

This relationship partly explains the results found in the cowpea samples, especially in the leaves, which showed high levels of metal(loid)s. In addition, studies show that each plant species has its own capacity for metal absorption and distribution, reflected in distinct accumulation patterns across plant tissues and organs [33]. Some species exhibit greater bioaccumulation capacity, resulting in higher concentrations of potentially toxic metals. Soil pH, trace metal concentrations, organic matter content, the use of organic fertilizers, and the specific transport of each metal from the roots to the aerial parts of legumes are the main factors influencing bioaccumulation [34]. Strong positive correlations were observed between cadmium (Cd) and lead (Pb) bioaccumulation in carioca and mung bean samples from southeastern China. For black beans, Cd and As were correlated; for red beans, Pb and As were correlated. It was also observed that the risk index exceeded 1 for children, adolescents, and adults, at 12.64%, 11.54%, and 1.01%, respectively [35]. In northeastern Iran, lead concentrations above the standards established by the World Health Organization (WHO) and the Food and Agriculture Organization of the United Nations (FAO) were found in samples of vegetables, water, and soil. The study observed a CR of nickel in women, considering frequent and prolonged consumption [36].

Investigating the distinct potential among species to accumulate specific metal (loid)s in regions with different levels of traffic exposure, it was observed that Tilia tomentosa showed greater efficiency in the bioaccumulation of Pb and Cd, whereas Prunus cerasifera demonstrated greater capacity to accumulate Cu and Ni [31]. Similarly, Turkyilmaz et al. [29] found that in areas with heavy traffic, Salix babylonica had higher levels of Cd, Ni, and Zn, while the highest levels of Cu, Mg, Cr, Fe, and Mn were recorded in Salix japonica. In these same areas, the highest Pb values were found in Aesculus hippocastanum, and the highest Ca levels were observed in Robinia pseudoacacia.

At the same time, studies indicate that soils near landfills are leached, increasing the contamination of adjacent soils by potentially toxic agents [28,32]. The high accumulation of metal(loid)s in edible parts of plants grown in or near landfills is a significant concern for food safety. The migration of these toxic elements via landfill leachate to adjacent areas can intensify environmental contamination and increase risks to public health [28].

The leaves (Table 6) had higher HQ values than the pods (Table 5). The pods showed higher values for As and Cd, while the leaves showed higher values for As, Cd, Mn, and Se at all sampling points. HI calculations indicated a high risk of non-carcinogenic effects at all points for both leaves and pods, consistent with the results of Fonseca et al. [10]. The updated risk assessment indicates a critical health situation for the local population consuming cowpeas grown near the landfill. The reduction in the RfD for Arsenic (6 × 10⁻⁵ mg/kg/day) resulted in HI values that are approximately 30 to 117 times higher than the safety threshold (HI = 1), depending on the plant part consumed. The dominance of As in the non-carcinogenic risk (contributing over 80% to the HI) and its extremely high CR (>10⁻¹ in leaves) suggests a massive accumulation of this metalloid. The updated Arsenic Slope Factor (SF = 32) highlights that the probability of cancer development is significantly higher than previously estimated in the literature using older indices. The fact that leaves presented an HI (117.40) nearly four times higher than pods (37) is consistent with the physiological tendency of Vigna unguiculata to accumulate heavy metals in its vegetative tissues. Furthermore, the presence of Mn and Se with HQ > 1 in leaves suggests that the cumulative toxic effect results from a complex mixture of contaminants. Spatial analysis reveals that the risk does not follow a linear decay with distance from the landfill or the road. Points B1 and B2 showed higher risks than A1 across several parameters, suggesting that local topography, wind patterns, or soil leaching likely influence the dispersion of these hazardous elements. These findings demonstrate that both cowpea pods and leaves from this area are unsuitable for human consumption under current toxicological safety standards.

Although the calculated metal(loid)s content differs between studies, it is still observed that the consumption of cowpeas grown in or near landfills poses a high non-carcinogenic risk to adults [10,30]. Several studies highlight the possible non-carcinogenic health risks, which can affect multiple body systems. Neurological effects have been observed in As poisoning, including cognitive impairment, changes in neurodevelopment, and neurodegenerative diseases [37,38].

Pb and Se also showed neurotoxicity. In addition to accumulating in bones, livers, and kidneys, Pb can affect the circulatory, reproductive, hepatic, immune, and gastrointestinal systems [39,40]. Se, although essential in small amounts, can cause gastrointestinal, respiratory, and cardiovascular symptoms in cases of acute toxicity and is also associated with reversible leukoencephalopathy when exposure is chronic [41]. Renal impairment is also mainly related to exposure to Cd, which can interfere with essential enzymes, leading to renal dysfunction, pulmonary and skeletal damage [42], and to hypermagnesemia, with the potential to trigger renal failure, asthenia, arrhythmia, and neuromuscular changes [43], highlighting the broad spectrum of toxicity of these elements.

Our study estimated the CR associated with the consumption of cowpea pods (Table 7) and leaves (Table 8), indicating a high risk of cancer across all collection points. The CR assessment for a 30-year-old adult (70 kg) consuming 44 g/day of cowpea pods revealed that the local population is exposed to a high potential cancer risk, as all calculated values for As and Cr exceeded the safety threshold of 10⁻⁴ (Table 7). According to the US EPA risk management scale, CR values below 10⁻⁶ are considered negligible, while values exceeding 10⁻⁴ indicate a significant health concern that requires mitigation. The risk attributed to Arsenic is particularly critical, as the results were an order of magnitude above the safety limit at all sampling sites. This indicates that chronic consumption of cowpea pods grown in the study area poses a substantial threat to human health. Statistically, As risk values around 2.3 × 10⁻³ suggest a potential occurrence of approximately 2 cases of cancer per 1000 individuals over a lifetime of exposure. The Chromium CR values (>3 × 10⁻⁴) also highlight environmental impact concerns near the landfill and roadsides. Although Chromium posed a lower risk than Arsenic, it still exceeded the maximum acceptable limit by at least threefold. The spatial distribution of these risks does not show a linear decrease with distance from the landfill or road, suggesting widespread contamination in the local environment. These findings underscore the need to monitor food crops in areas susceptible to heavy metal deposition to prevent long-term health complications in the population.

The carcinogenic potential assessment for leaf consumption (Table 8) indicates that intake of As and Cr poses a severe health threat to the local population. Given that all calculated CR indices exceeded the 10⁻⁴ benchmark, the dietary utilization of these vegetables grown near the landfill site is classified as a high-risk activity under US EPA standards. The magnitude of the danger attributed to Arsenic is especially alarming, with all values remaining above 6.9 × 10⁻³.

The heightened risk in the leaves, compared to the pods, indicates preferential bioaccumulation of the metalloid in the plant’s foliage, intensifying the vulnerability of groups that rely on this part of the cowpea as a primary food source. Exposure to Chromium via leaf tissue also yielded concerning results, with CR exceeding the safety limit by up to 13 times at point A1. The spatial dispersion of this threat indicates that environmental variables beyond linear proximity to the landfill (*) or the road (**), such as wind patterns or soil leaching, may be driving the contamination. Such evidence reinforces the urgent need for food monitoring programs, as the concentrations of As and Cr in cowpea leaves currently pose a risk to safe human consumption.

Consistent with our findings, Awino, Maher and Fai [30] observed that cowpea leaves grown in a landfill in eastern Uganda also posed a CR. In the same study, the authors also calculated the CR of other plants grown under similar conditions; seven of these were leafy vegetables that posed a higher CR than Vigna unguiculata [30]. These findings reinforce concerns about the consumption of vegetables grown in areas contaminated with potentially toxic metal(loid)s and highlight the importance of establishing food crops at a safe distance from sources of contamination, such as landfills and highway margins with high vehicle traffic.

The human health risk analysis showed that the average daily consumption of both pods (44 g) and leaves (67 g) may pose a chronic health risk to adult men and women, since simultaneous exposure to multiple metals, even at low doses, can lead to bioaccumulation and is often associated with the formation of reactive oxygen species, enzyme inactivation and suppression of antioxidant mechanisms, neuropsychiatric disorders, and cancer [44,45,46].

Developed by the Food and Nutrition Board of the Institute of Medicine of the National Academy of Sciences, the Dietary Reference Intakes is the general term for a set of reference values, used to plan and assess nutrient intakes. One of the reference values corresponds to the Tolerable Upper Intake Levels (UL), which provide cutoff points for estimating the percentage of the population of interest at potential risk of adverse effects from overconsumption of a nutrient. The proportion of the population with usual intakes above the UL represents the potential at-risk group. An evaluation of the public health significance of the risk to the population consuming a nutrient in excess of the UL would be required to determine if action was needed [47]. Considering that excessive consumption of nutrients can be toxic to the organism, in our study, a quantitative analysis of the adequacy of daily food consumption (Table 9 and Table 10) was conducted, with one serving of cowpea pods per day, showing minimal risks of Mg for men across all age groups and Zn for men aged 9 to 13 years showed a probability of intake exceeding UL; however, it was very close to 100%, indicating minimal risk of toxicity. Consuming one serving of leaves per day demonstrated that Fe at point A1 showed a high probability of intake being above the UL value and Cu at all points; however, it was lower than 100%, indicating a probability of safe consumption.

Our results demonstrate the need to develop strategies to reduce heavy metal accumulation in crops, thereby reducing the risk of human exposure to these elements. Studies compiled by Wan et al. [48] and developed by Ondrasek et al. [49] have shown that the use of measures that address changes in agricultural habits, such as the selection and improvement in cultivars with molecular development with low metal content, in addition to the use of plant growth-promoting rhizobacteria that can improve plant development, is beneficial. Similarly, physiological blocking, in which mineral elements attenuate oxidative stress through increased photosynthesis, can interfere with the restoration of cell membrane integrity, disrupt nutrient absorption, and ultimately regulate the absorption, translocation, distribution, and speciation of heavy metals in plants. This process helps reduce the accumulation and toxicity of heavy metals in plants. Similarly, water management can be used as a determinant of the bioavailability of heavy metals in soil, as it directly modulates redox (Eh) and pH conditions, thereby influencing their solubilization and mobility.

Another measure is soil fertilization, using inorganic and organic soil amendments such as limestone, phosphates, clays, biochar, and animal manure. These amendments can control and remediate soils by immobilizing heavy metals via adsorption, complexation, cation exchange, and precipitation. Finally, soil phytoremediation measures, such as enhanced phytoextraction and chemical stabilization techniques, have been used worldwide, both efficiently and sustainably, to mitigate harmful effects in metal-contaminated areas. Thus, these strategies can be employed in the cultivation of cowpea, a food widely consumed by the world’s population, as a measure to minimize the absorption of contaminants by plants, in addition to contributing to soil recovery and, consequently, strongly participating in the reduction in environmental contamination, its impact on the food chain, and specific risks to human health.

Our study analyzed heavy metal levels in cowpea leaves and pods, but the lack of data on root tissues may limit our ability to assess soil metal concentrations. In soils with high levels of contamination, multiple factors associated with plant physiology and soil chemistry can lead to nonlinear absorption behavior, resulting in stabilization, reduction, or loss of representativeness relative to the total toxin load, thereby masking the true magnitude of environmental pollution. Therefore, we suggest that further studies expand the investigation of metal bioaccumulation in cowpea and potentially contaminated areas.

Another limitation concerns the absence of a control area free of potential sources of contamination. The absence of comparisons with plants grown in unimpacted environments makes it difficult to accurately determine the influence of anthropogenic activities on metal levels. The study also relies on estimates of average daily consumption for health risk assessment, which may not accurately reflect the local population’s dietary habits. Therefore, risk calculations may be subject to uncertainties.

5. Conclusions

The health risk assessment of cowpeas (Vigna unguiculata) grown near the landfill revealed a high level of toxicity, significantly exacerbated by the latest toxicological guidelines. Regarding non-carcinogenic risk, the Hazard Index (HI) reached alarming values, peaking at 35.36 for pods and 117.40 for leaves. These indices were driven primarily by As and Cd, which exceeded the safety threshold (HQ > 1) at all analyzed points, indicating that the cumulative exposure to these elements represents a severe health threat according to current reference doses (RfD). The Carcinogenic Risk (CR) assessment demonstrated that the concentrations of As and Cr in the edible parts of the plant pose an unacceptable threat to consumers. All CR values were significantly above the 10⁻⁴ limit, with Arsenic risks reaching the order of 10⁻¹ in leaf samples. This extreme risk profile, calculated using updated Slope Factors (SF), highlights a much higher probability of cancer development than previously estimated for populations consuming crops from contaminated areas. In conclusion, the consumption of cowpea pods and leaves produced in the studied area represents a severe health hazard. The contamination levels remained critical across all analyzed distances (100–300 m), demonstrating that the agricultural perimeter surrounding the landfill and road is unsuitable for food production under modern safety standards.

This study demonstrates that cultivation areas near anthropogenic activities may be contaminated with heavy metals, compromising nutritional safety, particularly for toxic elements, posing a risk to human health. Therefore, measures such as cultivar selection and improvement, physiological control, water management, fertilization, and soil phytoremediation are fundamental to mitigating environmental contamination and public health implications.