In this study, the vegetables samples were collected in different agricultural areas at Loumbila market garden. Distance of eighteen (18) kilometers from Ouagadougou capital city of Burkina, Loumbila market garden is expanding around the dam. The dam is located at a longitude of 01˚24'07.4 West and a latitude of 12˚29'35.8 North with the water capacity of 42.2 million cubic meter. It is used by market gardeners to irrigate the plants [9] .

Loumbila’s market gardening areas have a much-diversified production of vegetables, namely onion, tomato, okra, zucchini, African eggplant, eggplant, pepper, bell pepper, lettuce, cabbage, carrot, green bean, and potato. Market garden products from Loumbila can be found in most markets in the city of Ouagadougou or exported to neighboring countries. These reasons led to the choice of the Loumbila market gardening areas to carry out this study, by choosing the most consumed vegetables in the city of Ouagadougou.

During this study, a vegetables samples collection campaign was carried out in the Loumbila market gardening area located downstream from the dam. The following materials were used for sample collection:

➢ A pickaxe to dig up the chosen plant species;

➢ A ceramic knife to separate leaves and fruits lengthwise to ensure consistency in analyzes. The ceramic knife prevents any external contamination;

➢ The plastic seals to wash the samples of the plants taken;

➢ Distilled water for rinsing the leaves and fruits;

➢ A cooler containing ice for storing samples.

Sampling was carried out at regular intervals and over the entire plot for each type of sample. The different plants were sampled by separating the different plant tissues (roots, leaves, stems and fruits) of each plant using a ceramic knife to avoid contamination. To measure an average concentration, at the plot level, the samples were mixed by family and constitute an aliquot. Plant samples for analysis of heavy metal were placed in plastic bags. These samples were then stored at −20˚C in the BUMIGEB Analysis Laboratory. African eggplant, cabbage, okra, eggplant, corchorus olitorius leaf, amaranthus hybridus leaf, lettuce, spinach, tomato and chili were collected during the campaign.

The vegetables were washed up with tap water thoroughly to remove the attached dust particles, soil, unicellular algae, etc. Then they were washed with distilled water and finally with deionized water. The washed vegetables were dried at room temperature to remove surface water. The vegetables were immediately kept in desiccators to avoid further evaporation of moisture from the materials. After that the vegetables were chopped into small pieces and were oven dried. Then the vegetables were crushed into fine powder using a porcelain mortar and pestle. The resulting powder was kept in air tight polythene packet at room temperature before being taken to the laboratory for digestion and metals analysis.

The vegetable sample (1 g) was weighed into a 100 ml volumetric flask and concentrated acids of 10.0 mL of concentrated sulfuric acid were added to each sample. The samples obtained after adding concentrated acid were gently on a hot plate, stirring occasionally until the powder completely dissolved in the solution (about 10 to 15 minutes). Then 10 ml of distilled water was added and the whole was heated gently for a few minutes (5 to 10 mins). Finally, the solution was left to stand for it to settle well and the filtrate was taken for analysis.

A model AANALYST 200 flame atomic absorption spectrometer from PERKIN ELMER was used for copper (Cu), chromium (Cr), zinc (Zn), lead (Pb), nickel (Ni), and cadmium (Cd) analysis.

The final concentration of each metal in the plant was calculated using the following formula:

C   final ( mg / kg ) = Cmetal ∗ dilutionfactor ∗ nominalvolume sampleweight ( g ) (1)

To assess the health risks associated with the consumption of the plants studied, the estimated daily intake, target hazard quotient and the hazard index were calculated from the concentrations obtained.

Table 3 shows the concentrations of heavy metals in carrot, onion leaves, onion, lettuce, green bean, and bell pepper from LOUMBILA’s market gardening areas.

Copper is an essential micronutrient which functions as a biocatalyst required for body pigmentation in addition to iron, maintain a healthy central nervous system, prevents anaemia and interrelated with the functions of zinc and iron in the body [21] . In excess in the body copper can be the origin of organic diseases such as gastroenteritis with nausea and intestinal irritations [22] . The Cu level in studies vegetables from Loumbila market garden ranged between 16.06 mg/kg in green bean and 19.81 mg/kg in Onion. The Cu levels in the vegetables presented in this study were lower than WHO/FAO suggested safe limits of 40 mg/kg.

Nickel is essential for growth and reproduction in livestock and man, but could be carcinogenic (nose, lung) in high amount in the body. Nickel can also cause diseases such as allergies, dermatitis, eczema, asthma, congenital malformations, headaches, dizziness, and lack of sleep [22] [23] [24] . In this study, the concentrations of nickel in the vegetables from the Loumbila market garden ranged between 5.09 mg/kg in lettuce and 11.16 mg/kg in Onion leaves.

Nickel content in vegetables was found to be lower than the estimated maximum guideline set by United State Food and Drug Administration of 70 - 80 mg/g. These variations of Ni concentration in various vegetables might be due to the pollutants in soil, air, water, factory wastages, sewerages etc. Nickel is reported to be a common cause of allergic contact dermatitis [21] .

In human body, Zn is essential for normal growth, mental ability, immune system, reproduction and healthy function of the heart [21] . The level of Zn in the studies vegetables from Loumbila market garden ranged between 42.95 mg/kg and 73.37 mg/kg with the highest recorded in onion and the least in carrot. All concentrations of Zn obtained in this study, were higher than the permissible levels by FAO/WHO in vegetables of 20 mg/kg. In the study done by Bambara et al. (2015), the concentration of Zn in the vegetables from Loumbila market garden ranged between 0.26 mg/kg and 4.52 mg/kg with the highest recorded in tomato [25] . Consumption of vegetables from Loumbila market garden could cause nausea, vomiting, loss of appetite, abdominal cramps, diarrhea, headaches and inhibition of copper absorption which sometimes produce copper deficiency and associated anemia [21] .

The concentrations of Cr in the studies vegetables from Loumbila market garden ranged between 1.3 mg/kg in carrot to 5.89 mg/kg in green bean. Chromium levels in onion leaves, onion, lettuce, green bean and bell pepper was higher than the safe limits (2.3 mg/kg). In a study done by Bambara et al., (2015), the concentration of tomato from Loumbila market garden was higher than the safe limits [25] . These high concentrations of Cr might be accumulated in the vegetables due to the contamination of soil, wastewater or fertilizer [3] [4] [26] . Cr is a trace element necessary for carbohydrate, fat and cholesterol metabolism and important for many hormones and enzyme activity in a certain concentration (up to 200 mg/day) but chronic exposure to Cr may damage liver and kidney and causes organic diseases such as cancers (lungs, nose, stomach, intestine) and dermatitis [21] [22] .

The highest Pb concentration in vegetables obtained from Loumbila market garden was recorded in carrot (27.02 mg/kg) and the least in bell pepper (1.73 mg/kg). All concentrations of Pb obtained in this study, were higher than the permissible levels by FAO/WHO in vegetables of 0.3 mg/kg. The values have been previously reported in leafy vegetables which include 8.194 mg/kg from Baskuy and 5.307 mg/kg from Boulbi by Derra et al. (2018) for lettuce [27] . The high levels of lead in all vegetable’s samples may probably be attributed to pollutants in irrigation water, farm soil, fertilizer or due to pollution from the highways traffic and industrial sites located around the sampled locations [3] [4] [26] . Pb is highly toxic heavy element and its intake via vegetable consumption can cause effect on liver, kidney, vascular and immune system [21] . The consumption of vegetable from Loumbila market garden can cause also neurological and psychiatric effects such as depressions leading to suicide, lack of attention, impairment of visual intelligence and motor functions, memory impairment, learning difficulties, states of fatigue, restlessness, aggressiveness, psychoses, hallucinations, peripheral polyneuropathy, encephalopathy, and lead poisoning [23] .

Figure 1 presents the histograms of the concentrations of copper, nickel, zinc, chromium and lead in carrot, onion leaves, onion, lettuce, green bean and pepper.

The results of this study show that carrot, onion leaves, onion, lettuce, green bean and bell pepper accumulated more zinc.

The carrot accumulated more lead than the other metals studied and its lead concentration is above the limit. Therefore carrot consumers could be exposed to haematological and cardiovascular effects (hypertension), kidney damage [23] , and liver disease [22] .

Onion leaves are more nickel accumulating. Onion was the most copper accumulator among the plants studied. Hence a high consumption of onions and these leaves can lead to an accumulation of copper and nickel in the body.

The green bean was the plant that accumulated the most chromium and zinc. The concentrations of both storage metals in green bean were above the limit. The consumption of green beans can lead to cancers (lungs, nose, stomach, intestine) and dermatitis [22] also leads to liver and kidney problems [21] .

Figure 2 presents the average concentrations of copper, nickel, zinc, chromium and lead in vegetables and gives a comparison between the average concentrations and the FAO/WHO limit values.

The average values of copper and nickel in the studied vegetables were below the FAO/WHO limits. The average concentration of zinc was more than three (3) times greater than the FAO/WHO limit and that of lead is more than thirteen nine (39) times greater. This allows us to say that the consumption of carrot, onion leaves, onion, lettuce, green bean, and bell pepper from Loumbila market garden can be the cause of health problems linked to lead and zinc. The high concentrations of lead and zinc led to an assessment of the health risks associated with the consumption of carrot, onion leaves, onion, lettuce, green bean, and bell pepper.

Table 4 shows the estimated daily intake of heavy metals (Cu, Ni, Zn, Cr and Pb) in carrot, onion leaves, onion, lettuce, green bean, and bell pepper from LOUMBILA.

The EDI of Cu, Ni, Zn, Cr and Pb were calculated according to the concentration of each heavy metal in each vegetable and the respective consumption rates. The EDI and maximum tolerable daily intake (MTDI) of studied metals from consumption of vegetables are shown in Table 4. Total daily intakes of Cu, Ni, Zn, Cr and Pb were 2.17 mg/day, 1.08 mg/day, 8.01 mg/day, 0.49 mg/day, 1.43 mg/day respectively. Daily intakes of Cu and Zn were less than the MTDI. Daily intake of Ni, Cr and Pb were higher than the MTDI, so consumption of carrot, onion leaves, onion, lettuce, green bean, and bell pepper can lead to health problems related to Ni, Cr and Pb. In vegetables samples, the EDI decreased in the following order: Zn > Cu > Pb > Ni > Cr.

Target hazard quotient of heavy metals (Cu, Ni, Zn, Cr and Pb) in each vegetable and the Hazard Index of eggplant, African eggplant, okra, pepper, tomato, cabbage, carrot, onion, lettuce, green bean, bell pepper, spinach, African eggplant leaves and onion leaves from LOUMBILA are given in Table 5.

The target hazard quotient (THQ) of Cu was within the range of 0.1170 to 0.1443. The THQ of Nickel ranged between 0.0742 to 0.1626, with the highest value recorded in onion leaves. Zinc THQ value ranged between 0.0417 to 0.0758. The highest THQ of Zn was observed in green bean. The THQ of Chromium was within the range of 0.1263 to 0.5722 and the highest target hazard quotient was observed in green bean. The THQ of lead ranged between 0.1440 to 2.2498, with the highest value recorded in carrot.

Lead is the metal with the highest THQ values in vegetables. Lead THQ values in carrot (2.2498) and lettuce (1.2856) were greater than one. The consumption of carrots and lettuce can cause lead-related diseases.

Figure 3 shows the hazard index in carrot, onion leaves, onion, lettuce, green bean, and bell pepper from LOUMBILA.

The hazard index (HI) of heavy metals (Cu, Ni, Zn, Cr and Pb) in vegetables from LOUMBILA was between 0.8036 and 2.7064. The highest value of HI was observed in carrot. The HI shows when a population is at risk. In this study, it was observed that the HI values for carrot, onion leaves, onion, lettuce and green bean were greater than (>) one (1) which indicates that there might be a potential health risk to those consuming these vegetables. The HI value for bell pepper was less than (<) one (1).

Lead was the metal that contributes the most to the hazard index of carrots, onion leaves, onions and lettuce. Copper contributes the most to the hazard index of green beans and bell peppers.

Cancer risk (CR) of heavy metals (Cu, Ni, Zn, Cr and Pb) in each vegetable and the Target Cancer Risk (TCR) of carrot, onion leaves, onion, lettuce, green bean, and bell pepper from LOUMBILA are given in Table 6.

The cancer risk value of copper in the plants studied varies between 1.40 × 10⁻³ and 1.73 × 10⁻³. Copper cancer risk values were above the maximum threshold value of 1 × 10⁻⁴ [10] [28] , which indicates that there is a very high risk for consumers to develop cancers related to copper.

The minimum cancer risk value for nickel was 2.52 × 10⁻³, which is a threshold of 1 × 10⁻⁴. So, the consumption of carrots, onion leaves, onions, lettuce, green beans and bell peppers can lead to nickel-related cancers.

The cancer risks of zinc and chromium were above the threshold of 1 × 10⁻⁴, which may lead to the development of cancers related to zinc and chromium for consumers of carrot, onion leaves, onion, lettuce, green bean and bell peppers.

All the cancer risk values for lead were below the 1 × 10⁻⁴ threshold, which shows that the consumption of carrots, onion leaves, onions, lettuce, green beans and bell peppers from LOUMBILA does not present any risk of developing a lead-related cancer.

However, 4 out of 5 (80%) heavy metals in samples of carrot, onion leaves, onion, lettuce, green bean and bell peppers from LOUMBILA, for which the estimated CR values were found to be at origin of a risk of cancer for the population.

The TCR value for Cu, Ni, Zn, Cr, and Pb due to consumption of carrot, onion leaves, onion, lettuce, green bean, and bell peppers from LOUMBILA ranged between 1.09 × 10⁻² and 1.38 × 10⁻². All the plants studied have their TCR for Cu, Ni, Zn, Cr and Pb greater than 1 × 10⁻⁴, which can lead to cancers linked to these metals in consumers.