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Heavy Metal Concentration in Market Gardening Soils and Risk of Food Chain Contamination in the Central Region of Burkina Faso

Moumouni Derra, Luc Telado Bambara*, Yalgado Zakaria Sawadogo, Karim Kaboré, Ousmane Cissé, François Zougmoré

Physics and chemical Department, Institute of Sciences and Technology, “Ecole Normale Supérieure”, Burkina Faso

Abstract: Market gardening is an important activity from a socio-economic and health perspective. Very often practiced in the dry season near water reservoirs (dams, channels, wells, etc.), it contributes enormously to reducing the unemployment rate and improving the nutritional quality of populations. However, the lack of prospective study of market gardening areas is the cause of contamination of products (leaves, vegetables, fruits) by trace metal elements (TME) or heavy metals. Thus, the objective of this study was to reassure ourselves of the heavy metal quality of market garden products in order to contribute to the preservation of population health. Thus, eighteen (18) samples, including six (6) of soil (0-20 cm), six (6) of roots, three (3) of leaves and three (3) of fruits, were analyzed using the ICP-MS to characterize and quantify eight (8) elements: As, Cr, Hg, Ni, Mn, Pb, Se, and Zn. The Pollution Load Index and Geoaccumulation Index show that the soil S5 was polluted with Cr. Also, soils S2, S3 and S5 were slightly contaminated with arsenic (As) with Igeo of 0.51, 0.60 and 0.08 respectively; S1 and S6 in mercury (Hg) and S4 in Zinc (Zn). As for the correlation between soil-root concentrations and the calculation of transfer coefficients (CT) between roots-leaves or roots-fruits, we note that it is solanum licopersicum (tomato) which absorbs the most elements such as As, Mn and Zn with coefficients ranging from 0.96 to 23.4. Also, brassica oleracea (cabbage) appears to accumulate Mn with a transfer coefficient of 4.28.

Keywords: heavy metals, pollution load index, geoaccumulation index, transfer

Received December 05, 2023; Revised January 08, 2024; Accepted January 15, 2024

Citation: Moumouni Derra, Luc Telado Bambara, Yalgado Zakaria Sawadogo, Karim Kaboré, Ousmane Cissé, François Zougmoré, Heavy Metal Concentration in Market Gardening Soils and Risk of Food Chain Contamination in the Central Region of Burkina Faso. American Journal of Environmental Protection, 12 (1). 1-8. January 2024. doi:10.12691/env-12-1-1

1. Introduction

Market gardening is an important activity in the central region of Burkina Faso. It is practiced in the dry season around dams, channels, wells, etc. Market gardening contributes enormously to reducing the unemployment rate and improving the nutritional quality of populations [1]. Indeed, the products (vegetables, fruits, leaves, etc.) resulting from this activity contain trace elements and vitamins which are naturally necessary for the proper functioning of the human body [2]. However, the lack of prospective study of market gardening areas, which may initially be contaminated, can cause contamination of products by trace metal elements (TME) or heavy metals [3,4,5]. Previous studies have revealed that the contamination of market garden areas by trace metal elements can be natural or anthropogenic [6,7,8] to the point of constituting a threat to the health of consumers via market garden products [9]. Indeed, the quality of the environment (soil, water, air) of market gardening considerably impacts that of the cultivated plants [10,11,12,13,14]. Thus, the objective of our work was to evaluate the concentrations of heavy metals in soils, roots, leaves, to quantify soil pollution and to evaluate the transfer from soil to agricultural products. The study also aimed to determine the transfer coefficients of ETM transfer between roots-leaves and roots-fruits. Eighteen (18) samples, including six (6) of soil (0-20 cm), six (6) of roots, three (3) of leaves and three (3) of fruits (or vegetables), were analyzed using ICP-MS to characterize and quantify eight (8) elements: As, Cr, Hg, Ni, Mn, Pb, Se, and Zn. Then, the determination of the Pollution Indices or Pollution Load Index (PLI) and the Geoaccumulation index (Igeo) allowed us to assess the quality of the soil samples. The establishment of the correlation between the soil-root concentrations and the calculation of the transfer coefficients (TC) between roots-leaves or roots-fruits, allowed us to assess the capacity of each plant to absorb each of the eight (08) metallic trace elements.

2. Materials and Methods

2.1. Presentation of the Study Area

The sites designated for this study contribute enormously to the production of market garden products in the Center Region of Burkina Faso. The geographical positions of the market gardening sites where the samples were taken are shown in Figure 1.

Figure 1. Geographic positions of the sites

2.2. Sample Preparation

In total, six (06) soil samples in the 0-20 cm profile and twelve (12) plant samples were taken. These are three plants with edible leaves and three others with fruits (or vegetables). For plants with edible leaves, these are lactuca sativa L (lettuce), brassica oleracea (cabbage) and amaranthus sp (thorny amaranth). As for fruit plants, they are: alium cepa (onion), solanum licopersicum (tomato) and solanum melongena L (eggplant). For each of the six (06) plants, the contents of heavy metals were determined in the roots (R) and in the leaves (Fe) or fruits (Fr). Kept in plastic packaging during collection, to avoid any external contamination, each sample was dried away from the sun and reduced to powder in the laboratory. Then, mineralized and dosed at the Bureau of Mines and Geology of Burkina (BU.MI.GE.B) according to the protocol in force. The conversion of metal concentrations from mg/l to mg/kg was done according to formula (1) [15].

[picture] | (1)

2.3. Tools for Assessing Soil Quality

To reassure ourselves of the quality of the soil samples, the Pollution Load Index (PLI) and the Geoaccumulation Indices (Igeo) were calculated using the following equations (2) [16,17] and (3) [18,19,20]:

[picture] | (2)

With: : Contamination Factor of the metal i considered.

For PLI < 1, the soil is unpolluted; PLI = 1, the quality of soil is pollution reference level and PLI > 1, the soil is polluted [21,22].

And,

[picture] | (3)

With: Cn= Concentration of element n in the sample; Bn= Concentration of element n in the geochemical background; 1.5 is exaggeration factor of the geochemical background whose function is to take into account the natural fluctuations of the geochemical background.

The Geoaccumulation Index not only provides an idea of the degree of soil pollution but also clarifies the level of contribution of each metal to the pollution of the site. Table 1 below gives the six (06) classes making it possible to assess the degree of pollution of a soil according to the values of the Geoaccumulation Index [23,24,25].

Table 1. The assessment classes according to the value of the Geoaccumulation Index (Igeo)

2.4. Absorption Capacity of Trace Metal Elements by Plants

To assess the capacity of plants to accumulate heavy metals in leaves and fruits, the Transfer Coefficients (TC) was determined using equation (4). Table 2 allows to identify the nature of the plant according to the value of this coefficient.

[picture] | (4)

Table 2. Indicative values of the transfer coefficient (TC)

3. Results and Discussions

3.1. Soil Quality According to Pollution Load Index (PLI) Values

The concentrations of heavy metals in the six (6) soil samples, at the 0-20 cm horizon, made it possible to detrmine the PLI which are represented in Figure 2.

Figure 2. PLI value in the 0-20 cm soil profile

S1 to S6 designate the six soil samples assayed. In Figure 1, we observe that only sample S5 has a PLI>1 compared to a minimum of 0.14 for sample S6. Thus, soil S5 is the only one which is slightly polluted with heavy metals because its PLI of 1.10 is close to 1 which is the reference value. The soil samples are loaded with these eight metallic trace elements in the following order: S5>S2>S1>S3>S4>S6.

3.2. Soil Quality According to the Values of the Geoaccumulation Indices (Igeo)

The values of the Geoaccumulation Indices are recorded in table 3 where Igeoi is the Geoaccumulation Index of the soil Si.

Table 3. Igeo values

The values below zero (0) of Igeo reveal that despite the presence of elements such as manganese (Mn), lead (Pb) and selenium (Se) in the sites studied, their contents are not as high to the point to pose a threat to soil functions. However, soils S2, S3 and S5 are slightly contaminated with arsenic (As) with Igeo of 0.51, 0.60 and 0.08 respectively; S1 and S6 in mercury (Hg) with 0.34 and 0.17 like Igéo and S4 in Zinc (Zn) with 0.27 like Igéo. Also, S1 and S2 are respectively contaminated moderately in Zinc (Zn) and mercury (Hg) with Igeo of 1.29 and 1.36. Moderate to heavy contaminations in zinc (Zn) and nickel (Ni) are respectively noted at the level of S2 (Igeo=2.12) and S3 (Igeo=2.15). Sample S5 is the most polluted in chromium (Cr) with an Igeo of around 4.11. Thus, the values of Geoaccumulation Indices confirm the pollution of the S5 soil revealed more by the PLI. Better, the Geoaccumulation Index shows that chromium (Cr) is the major contributor to this pollution with its Igeo which is the highest.

3.3. Correlation Between Concentrations: Soil-Roots, Roots-Leaves and Roots-Fruits

The concentrations of heavy metals in the soil, root, leaf and fruit samples were represented on the histograms. These figures help to better understand not only the correlation of concentrations between soil-roots but also and above all between roots-aerial parts (leaves or fruits). In the following lines, the notation Si represents the soil support of the plant of Ri roots and Fei leaves or Fri fruits.

3.3.1. Case of amaranthus sp: Between soil-roots and roots-leaves

Table 4 presents the concentrations of heavy metals in the cultivation soil, roots and leaves of amaranthus sp.

Table 4. Concentrations of TMEs (µg/kg) in soil, roots and leaves of amaranthus sp

S1 represents the soil sample which constitutes the support for the amaranthus sp, R1 the sample of its roots and Fe1 that of its leaves. We notice that R1 contains more Cr than the other elements with a total absence of Se.

Figure 3 shows the histograms of the concentrations in the growing soil and the roots of Amaranthus sp, and Figure 4 gives the histograms of heavy metals in the roots and leaves. These two figures make it possible to correlate the concentrations of heavy metals in the growing soil, roots and leaves of Amaranthus sp.

Figure 3. Correlation of ETM contents between soil-roots of amaranthus sp

Figure 4. Correlation of ETM contents between roots-leaves of amaranthus sp

There is a total absence of elements such as Cr, Hg, Mn, Ni, and Se in the roots and leaves of amaranthus sp which are nevertheless contained in the R1 soil. We can therefore think that this plant does not absorb these elements. However, the absence of Se in the roots and leaves of amaranthus sp could be explained by its absence in the soil. As for Pb, it simply managed to cross the soil and end up at the roots with a rate of 6.87%; and, only As and Zn passed from the R1 soil to the leaves with respective rates of 5.76% and 3.28%. Which shows that of all the elements measured, it is As which is most accumulated by amaranthus sp. Also, although the concentrations of different metals in the leaves do not exceed the recommended thresholds, the presence of As in the leaves of amaranthus sp, even tiny, constitutes a problem when we know that it can accumulate in the body and be responsible for certain diseases [26]. Also, Hg and Pb in the soil can contaminate market gardeners through inhalation or ingestion.

3.3.2. Case of Lactuca Sativa L: Between Soil-Roots and Roots-Leaves

Table 5 shows the concentrations of heavy metals in the growing soil, roots and leaves of lactuca sativa L.

Table 5. Concentrations of TMEs (µg/kg) in soil, roots and leaves of lactuca sativa L

S2 represents the soil sample which constitutes the support of lactuca sativa L, R2 the sample of its roots and Fe2 that of its leaves. In soil R2, a total absence of Ni, Pb and Se is observed. It contains more Zn than other elements. Also, the concentrations of metals in the soil R2 and the leaves of lactuca sativa L are below the recommended standards.

Figure 5 shows the histograms of the concentrations in the growing soil and the roots of lactuca sativa L, and Figure 6 gives the histograms of heavy metals in the roots and leaves. These two figures make it possible to correlate the concentrations of heavy metals in the growing soil, roots and leaves of lactuca sativa L.

Figure 5. Correlation of ETM contents between soil and lactuca sativa roots

Figure 6. Correlation of ETM contents between the roots and leaves of lactuca sativa

There is a total absence of Ni, Pb and Se in the R2 soil. This can be explained by the fact that no trace of these elements is observed at the roots and leaves of lactuca sativa L. The Cr and Hg found in the soil are absent in the roots or leaves of the plant. Thus, we can say that lactuca sativa L has little or no capacity to absorb these elements. Also, although As and Zn have slightly high concentrations in the S2 soil, their levels in the leaves remain quite low; we have 1.1 µg/kg and 1.4 µg/kg respectively for As and Zn at leaf level compared to 20.015 µg/kg and 53.635 µg/kg at ground level. Thus, we have respectively 5.21% of As and 7.64% of Zn which passed from the soil R2 to the Fe2 leaves of lactuca sativa L. However, the presence of As in the leaves constitutes a threat to health consumers when we know that it is a non-essential element for the human body. Also, the Hg present in the soil can contaminate market gardeners by inhalation.

3.3.3. Case of Brassica Oleracea: Between Soil-Roots and Roots-Leaves

Table 6 shows the concentrations of heavy metals in the growing soil, roots and leaves of brassica oleracea.

Table 6. Concentrations of TMEs (µg/kg) in soil, roots and leaves of brassica oleracea

S3 represents the soil sample which constitutes the support of the brassica oleracea and R3, Fe3 respectively those of its roots and its leaves. All the elements measured are found in the S3 soil sample with the As concentration of up to 53.485 µg/kg which is the highest of all soils. All the same, no ETM has a concentration in the soil and leaves of brassica oleracea that is above the recommended standard.

Figure 7 shows the histograms of the concentrations in the growing soil and the roots of brassica oleracea, and Figure 8 gives the histograms of heavy metals in the roots and leaves. These two figures make it possible to correlate the concentrations of heavy metals in the growing soil, roots and leaves of brassica oleracea.

Figure 7. Correlation of ETM contents between soil and roots of brassica oleracea

Figure 8. Correlation of ETM contents between the roots and leaves of brassica oleracea

Despite the presence of Cr, Hg, Ni and Se in the soil, their concentrations are zero in the roots and leaves of brassica oleracea. Also, As, Pb and Zn have concentrations that are quite low in the leaves although they are high at the root level. This suggests that brassica oleracea prevents these three elements from migrating to the leaves. Indeed, only 2.82%, 0.14% and 4.38% respectively for As, Pb and Zn passed from roots to leaves. However, the presence of As and Pb in the leaves remains a problem for the health of consumers when we know that, even in small quantities, these ETMs are harmful to the human body [26,27,28]. Also, the presence of Hg in the soil constitutes a danger because it is easily inhaled.

3.3.4. Case of Solanum Licopersicum: Between Soil-Roots and Roots-Fruits

Table 7 presents the concentrations of heavy metals in the growing soil, roots and leaves of solanum licopersicum.

Table 7. Concentrations of TMEs (µg/kg) in soil, roots and fruits of solanum licopersicum

S4 represents the soil sample which constitutes the support of solanum licopersicum, R4 and Fr4 respectively those of its roots and its fruits. It is observed that the Se concentration is zero in the soil, roots and fruits. Also, the concentrations of all the ETMs analyzed do not exceed the recommended standards for soils and plants intended mainly for human consumption.

Figure 9 shows the histograms of the concentrations in the growing soil and the roots of solanum licopersicum, and Figure 10 gives the histograms of heavy metals in the roots and leaves. These two figures make it possible to correlate the concentrations of heavy metals in the growing soil, roots and leaves of solanum licopersicum.

Figure 9. Correlation of ETM contents between soil and roots of solanum licopersicum

Figure 10. Correlation of ETM contents between the roots and fruits of solanum licopersicum

The absence of Se in the roots and fruits can be justified by the fact that it does not exist in the soil. Also, the low concentrations of Cr, Ni and Zn in the soil contrast with the high levels of these metals in fruits. Thus, we can say that the functioning system of solanum licopersicum promotes the absorption of these three elements (Cr, Ni, Zn). Especially for Cr with a value of 63 µg/kg in fruits compared to 0 µg/kg in the roots and 8.07 µg/kg in the soil. Previous studies have already revealed the accumulative nature of solanum licopersicum in Cr [29] even if in this study its concentration is not above the recommended standard which is 2.3 mg/kg [30,31,32] . However, the Hg present in the 0-20 cm soil profile with a value of 1.94 µg/kg is completely absent in the roots and fruits. Also the Pb is blocked at the roots with a value of 0.8 µg/kg compared to 0.9 µg/kg at the soil level, i.e. 88.88% of Pb passed from the soil to the roots of solanum licopersicum and 0% in its fruits. All the same, the presence of As in fruits and Hg in the soil constitutes a concern. As for Cr and Ni, their passage from the soil to the fruits of solanum licopersicum (tomato) is quite remarkable because while the roots do not have it, the fruits contain 63 µg/kg and 144 µg/kg respectively.

3.3.5. Case of Solanum Melongena L: Between Soil-Roots and Roots-Fruits

Table 8 presents the concentrations of heavy metals in the growing soil, roots and leaves of solanum melongena L.

Table 8. Concentrations of TMEs (µg/kg) in soil, roots and fruits of solanum melongena L

Ni and Se are absent in the S5 soil sample of solanum melongena L. Also, the concentrations of ETMs in Fr5 fruits and soil are below the thresholds.

The histograms of the concentrations in the growing soil and the roots of solanum melongena L are presented in Figure 11, and the histograms of heavy metals in the roots and leaves are given in Figure 12. These two figures make it possible to correlate the concentrations of heavy metals in the growing soil, roots and leaves of solanum melongena L.

Figure 11. Correlation of ETM contents between the soil and the roots of solanum melongena L.

Figure 12. Correlation of ETM contents between roots and fruits solanum melongena L

The absence of Ni and Se in the roots and fruits of solanum melongena L can be explained by the fact that their concentrations in the soil are zero. Despite the high concentration of Cr in the R5 soil (1899.59 µg/kg), we observe that the roots and fruits do not contain it. The low contents of Pb and Zn, respectively 1.195 µg/kg and 38.65 µg/kg, contrast with their high contents in the roots, respectively 99.4 µg/kg and 92.7 µg/kg. All the same, the concentrations of Pb and Zn are zero in the fruits. Thus, we can think that solanum melongena filters Pb and Zn from its roots even if the passage of these elements from the soil to the roots is quite remarkable. The quantity of 0.3 µg/kg of As which is passed into the fruits constitutes a health problem. It is the same for the 10.61 µg/kg of Hg in the soil.

3.3.6. Case of Alium Cepa: Between Soil-Roots and Roots-Fruits

Table 9 presents the concentrations of heavy metals in the growing soil, roots and leaves of Alium cepa.

Table 9. Concentrations of ETMs (µg/kg) in soil, roots and fruits of Alium cepa

With the exception of Cr, the other elements measured are present in soil sample R6. However, the concentrations of ETMs in all samples are below the thresholds.

The histograms of the concentrations in the growing soil and the roots of Alium cepa are presented in Figure 13, and the histograms of heavy metals in the roots and leaves are given in Figure 14. These two figures make it possible to correlate the concentrations of heavy metals in the growing soil, roots and leaves of Alium cepa.

The absence of Cr in the roots and fruits of alium cepa can be justified by the fact that its concentration in S6 soil is zero. Also, despite the presence of Hg, Ni and Se in the soil, they are completely absent in the roots and fruits of alium cepa. Only 9.55% of As and 12% of Zn passed from roots to fruits. As before, the presence of As in fruits constitutes a problem. It is the same for the 6.76 µg/kg of Hg observed at ground level.

Figure 13. Correlation of ETM contents between the soil and the roots of Alium cepa

Figure 14. Correlation of ETM contents between the roots and fruits of Alium cepa

3.4. Root-Leaf and Root-Fruit Transfer Coefficient

Table 10. Transfer coefficients of ETM between root-leaf and root-fruit

The transfer coefficients between root-leaves and root-fruit of the six (6) plants are recorded in table 10. These values made it possible to represent the histograms below in order to better appreciate the nature of each plant considered.

Figure 15. Histograms of root-aerial part transfer coefficients (leaves or fruits)

The histograms (figure 15) show that solanum licopersicum (tomato) has the greatest transfer coefficient in As, Mn and Zn with respective values of 0.96; 23.4 and 13.05. Thus, solanum licopersicum can be considered to be the plant which allows more As, Mn and Zn among the six plants considered. Also, this plant can be considered as a very accumulator of Mn and Zn because the transfer coefficients for these elements are well above the reference which is 1.5 and as an indicator plant for As. In addition, brassica oleracea (cabbage) also proves to accumulate Mn because its coefficient of this element is 4.28, or 2.85 times more than normal. Finally, all plants appear to exudate Hg, Pb and Se because in these elements they have a coefficient less than 0.1. The low concentration rate of certain metals in plants can also be explained by competition between metallic trace elements in the soil [33].

4. Conclusion

Cr, Hg, and Se had zero concentrations in the roots of all plants measured. For Hg which is present in soils and absent in roots, leaves and fruits, this may be due to its volatile nature which does not allow it to migrate like other elements in the different parts of plants. Also, the concentrations of Pb in the leaves of brassica oleracea and of As in the six samples of leaves and fruits are below the recommended threshold for consumable plants, i.e. 450 µg/kg [30]. Also, the histograms show that the roots of solanum melongena L (eggplant) absorb more As, Pb, and Zn followed by brassica oleracea (cabbage). However, the presence of As in the leaves and fruits of all plants and of Pb in the leaves of brassica oleracea (cabbage) remains a concern because they play no biological role. And that chronic exposure to As is a risk factor for lung, skin, bladder and kidney cancer. Worse, the presence of As, Hg and Pb, at the 0-20 cm horizon of the soil, remains a concern because market gardeners can, by inhalation or ingestion, be contaminated by these elements which are very toxic to the environment. The low content of elements in the soil samples measured shows that this is a diffuse contamination [34]. Even with levels below the recommended standards, regular monitoring is necessary when we know that the cumulative effect of ETMs in the soil can in the long-term lead to the contamination of market garden products. Also, these elements can through mechanisms contaminate surface and groundwater resources and constitute a health problem.

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