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Evaluation of Heavy Metal Pollution and Physico-Chemical Parameters in Agricultural Soils of Bouly, Burkina-Faso

Yalgado Zakaria Sawadogo, Telado Luc Bambara*, Moumouni Derra, Inoussa Zongo, Karim Kaboré, François Zougmoré

Laboratory of Materials and Environment, Physics Department, University Joseph Ki-Zerbo, Ouagadougou, Burkina Faso

Abstract: This study evaluates the physicochemical characteristics as well as the presence of heavy metals in agricultural soils located near an industrial gold mine in Burkina Faso. To do this, two soil pollution indices were used, namely the geoaccumulation index (Igeo) and the enrichment factor (EF). A total of thirteen (13) soil samples were collected and analysed in the laboratory at the Bureau of Mines and Geology of Burkina Faso (BUMIGEB) using atomic absorption spectrometry (AAS) to determine the concentrations of Hg, As, Pb, Cr, Zn, Cu, Ni and Al. The physicochemical parameters were analysed at the National Soils Office (BUNASOL). The results indicate that the studied soils have high acidity, marked salinity, silty texture, as well as a satisfactory C/N ratio. Concentrations of Hg, As, Cu and Cr were found to be higher than average continental crust values. In addition, the concentrations of Hg and As exceed South African standards, while Hg also exceeds the standards set by WHO and FAO. The use of the Igeo and FE indices made it possible to demonstrate that the studied area was highly enriched in mercury, significantly enriched in arsenic and weakly to moderately enriched in copper, which suggests an influence of human activities. Arsenic seems to come mainly from mining activities, while mercury comes from both mining and agricultural activities.

Keywords: heavy metals, concentration, geoaccumulation, pollution, enrichment factor, physico-chemical parameters

Received October 19, 2023; Revised November 21, 2023; Accepted November 27, 2023

Citation: Yalgado Zakaria Sawadogo, Telado Luc Bambara, Moumouni Derra, Inoussa Zongo, Karim Kaboré, François Zougmoré, Evaluation of Heavy Metal Pollution and Physico-Chemical Parameters in Agricultural Soils of Bouly, Burkina-Faso. Journal of Materials Physics and Chemistry, 11 (2). 38-47. January 2023. doi:10.12691/jmpc-11-2-2

1. Introduction

Soil preservation is fundamental to ensure food security and ecological balance. Soil, as an essential component of the terrestrial ecosystem, is vulnerable to human activities such as agriculture (irrigation, use of pesticides), mining and industrial activities, as well as urbanization and road transport [1-8].

Mining, although it can contribute to the economic development of a region, has negative consequences for the environment and the health of local populations [8]. Globally, it is recognised as a major source of environmental pollution [9,10]. Soil pollution by heavy metals is of particular concern. Heavy metals, due to their non-biodegradable nature, high toxicity and high mobility, are easily absorbed and accumulated by crops [11-14] and end up contaminating the food chain, leading to human health risks [15-17].

The Kongoussi region, located in the Bam Province in north-central Burkina Faso, is home to an industrial gold mine that plays a vital role in the local and national economy. However, if mining activity is not managed responsibly, it can have adverse effects on the environment, especially on agricultural soils near the mine [18,19]. Pollution by heavy metals (chromium, nickel, copper, zinc, arsenic, mercury and lead, …) is one of the main environmental concerns in such regions, because these contaminants accumulate in soils and crops, putting endangering the food security of local communities and threatening the biodiversity of ecosystems [10,15] [20-22].

The objective of this study was therefore to contribute to the assessment of heavy metal pollution and physico-chemical parameters in agricultural soils near the Kongoussi industrial mine. By emphasizing the importance of preserving the environment and the health of local populations, this assessment aims to provide solid scientific data to better understand the potential impact of mining activity on terrestrial ecosystems and mainly on agricultural soils.

2. Materials and Methods

2.1. Study Area

The soil samples were taken from the village of Bouly. Bouly is a village in the city of Kongoussi, located in the province of Bam in the center-north of Burkina Faso, between the geographical coordinates 13⁰11'520 north and 01⁰31'180 west. Bissa is located about 85 km north of Ouagadougou, the capital of Burkina Faso, and is accessible by Route Nationale 22. Bouly is located just 5 km from Bissa, on the border of the rural communes of Sabce and Mane. The Bouly mine is a single deposit operation. The region's climate is of the Sudano-Sahelian type, with two distinct seasons: a long dry season from November to May, followed by a short rainy season from June to October. The region receives between 571 and 924 mm of precipitation annually, with temperatures ranging from 22°C to 41°C. The soil in the study area is of tropical ferruginous type (BUNASOLS, 1998). The main economic activities in the region are agriculture and animal husbandry.

2.2. Sampling

The soil samples were taken from agricultural soils located near an industrial gold mine. For each agricultural soil, samples were taken at three different points, then the soils were mixed, homogenized and packed in labelled sterile plastic bags to form a sample. This process was repeated on other agricultural soils to constitute thirteen (13) samples.

2.3. Analysis of Samples

The various soil samples were dried in ambient air, crushed and sieved using a certified 2 mm sieve for laboratory analysis. The extraction and determination of metals in the samples were carried out by the laboratory of the Bureau of Mines and Geology of Burkina (BUMIGEB) in Ouagadougou. Mineralization of metallic trace elements (Ni, Cr, Cu, Ni, Pb, Zn, As, Al and Hg) were carried out at controlled temperature (90 ± 5°C water bath) for one hour, using a mixture hydrochloric acid (7.5ml) and nitric acid (2.5ml). The analysis was carried out by atomic absorption spectrometry (AAS), PERKIN ELMER AANALYST 200 model. The results are expressed in mg/kg of dry weight of the soil.

The physicochemical parameters of the soil samples were analysed at the National Soil Office (BUNASOL).

2.4. Estimation of Contamination Intensity

To estimate the intensity of the contamination, indices were used, their principle is based on the comparison of the measured values with reference values such as the average content of elements in the earth's crust.

Figure 1. Location of the study area (Source: https://nordgold.com/operations/production/bissa-and-bouly/)

2.1.1. Geo-accumulation Index

Another criterion used to assess the intensity of metal pollution is the geoaccumulation index, introduced by Muller in 1969 [8,23]. This empirical index compares the measured concentration of a metal to a value considered to be the background geochemical concentration [1,8]. It includes seven classes that determine the degree of pollution (Table 1). The geo-accumulation index covers different ranges of pollution (Table 1), ranging from natural levels synonymous with an absence of pollution to heavily polluted soils [24].

(1)

Where: Cn is the metal concentration, Cb is the geochemical background value. The constant 1.5 is the correction factor that compensates for the natural fluctuations of a given metal while minimizing anthropogenic impacts.

Table 1. Pollution class and intensity according to Igeo

2.1.2. Enrichment Factor

The determination of the metal content in the natural environment can serve as an indicator of contamination, but does not provide information on the nature and intensity of this contamination. In order to differentiate anthropogenic inputs from natural sources and to assess the intensity of contamination, the enrichment factor (EF) has been proposed by researchers such as Lodhaya et al. (2017) and Barbieri et al. (2015) [25,26].

The calculation of the enrichment factor consists in making the ratio the content of a contaminating element in the sample by the concentration of a normalizing element, by comparing this ratio with that found in a reference material. Aluminum is often used as a normalizing element due to its stability and significant presence in clay minerals, which leads to consistent results [25] [27-29]. The upper continental crust (according to Wedepohl, 1995) [30] is chosen as reference material in this studie. The enrichment factor was calculated using equation 2 [26,29] [31,32].

(2)

Where: Cn is the metal concentration in the soil, Cb is the geochemical background value of the metal, Rn is the concentration of the normalizing element in the soil, Rb is the geochemical background value of the same element.

Enrichment factor values was used to define five categories of enrichment as shown in Table 2 [26] [31,32].

Table 2. Class and Level of enrichment according to EF

3. Results and Discussions

3.1. Physico-chemical Parameters

Table 3 presents the results of the average values of the physico-chemical parameters (pH, electrical conductivity (EC), total nitrogen (N), assimilable phosphorus (P) and texture) measured in the soil samples collected at various sampling sites of a gold mining area in Kongoussi.

Table 3. Statistical quantities of physico-chemical parameters

3.1.1. pH

The average pH of the soil samples studied was 5.96, with a minimum value of 5.5 and a maximum value of 6.4. According to the pH scale of Chesworth et al., (2008) [33], this average classifies the soil as moderately acidic [34]. However, analysis of the samples reveals that 15.38% of them were strongly acidic, 30.76% were weakly acidic and 53.8% were moderately acidic. This relative acidity can be attributed to acid mine drainage that comes from the oxidation of sulphide minerals contained in the tailings [35,36]. Another possible cause is the high metal load of the studied area [37]. Soil pH is considered to be one of the most important factors determining the concentration of metals in soil solution, their mobility and availability to plants [38]. Thus, a low (acidic) pH promotes the solubilization of metals, thereby increasing their mobility and bioavailability [9,14] [39,40]. For pH varying between 5 and 6, about 90% of lead (Pb) becomes mobile in the form of Pb²⁺ ion [40], and the average pH value of the soils in this study was 5.93 . Discussing the relatively high mobility of heavy metals as a function of pH, Fuller concluded that in acidic soils (pH=4.2-6.6), the elements Cd, Ni, and Zn are highly mobile, while Cr is moderately mobile [41]. This pH range includes the values observed in this study, suggesting similar conclusions for Ni, Zn and Cr. Batjes, in 1995, considered that for pH below 5.5, Cu, Zn, Fe, Co and Mn may be excessively available, while for pH above 7 their availability decreases [42]. The pH range (5.5 to 6.4) of this study therefore suggests that Cu and Zn are moderately available. According to Mohamed, in 2017, in an acid medium, the mobility of Hg, Zn, Ni, Co, Mn and Cd is very high, while the mobility of Pb, Cr, Al and Cu is medium [43]. Since the soils in this study contain Hg, Zn, Ni, Cr, Pb, and Al, the pH will promote their mobility. Some authors claim that many crops grow best when the soil pH is in the range of 6 to 7.5, although some crops prefer acidic or alkaline soils [44]. Others, like Helyar et al. (1990), indicate that on most soils, plants can achieve near maximum growth rates in the pH range of 5 to 7 [45]. Thus, the pH range obtained in this study (5.5 to 6, 4) could promote the growth of plants and crops. The results of this study were compared with those of other similar studies carried out in different regions. For example, in soils near former gold mine tailings, Ngole-Jeme and Fantke (2017) [35] found acidic pH values ranging from 2 to 6.23. pHs ranging from extremely acidic (1.90) to slightly alkaline (7.4) have been reported at Korê, in soils near an Au-Ag mine [36]. Favas et al., reported pH ranging from 3.3 to 5.2, 3.5 to 6.3, 4.0 to 6.7 and 3.6 to 6.4, respectively in soils mines of Sarzedas, Gatas, Domingos and Barbadalhos [46]. Contrary to our results, other authors have observed alkaline pHs. For example, El-Hachimi et al. (2014) [47] measured a maximum pH of 8.9 and a minimum of 7.6 for all the soils located near the abandoned mines of Aouli-Mibladen and Zeida in eastern Morocco. Rafiei et al. (2020) [48] reported pH varying between 7.15 and 7.75 in a study conducted in Iran.

3.1.2. Electrical Conductivity

Electrical conductivity (EC) has been reported as a good indicator of soil salinity. Long before Shirokova et al. (2000) [49] then Alonge et al. (2018) [50], Richards (1954) [51] described the intensity of soil salinity as a function of electrical conductivity. The EC values of the studied samples varied between 13 mS/cm and 20 mS/cm, suggesting that the samples ranged from very saline to extremely saline. The average value of EC was 16.89 mS/cm, exceeded the salinity limit of agricultural soils according to FAO (2006) [52].

Soil EC is defined as the ability of soil water to carry electrical current [53. In general, the higher the concentration of ions in the soil, the greater the electrical conductivity. This range of high values suggests that considerable amounts of ions were present in the soil [54]. However, EC does not indicate the specific ions that might be present, it is simply a good indicator of the presence of salts such as sodium, potassium, chloride or sulphate [14].

Analysis of the samples shows that 76.92% of them were classified as extremely saline, which would be favourable only to very salt-tolerant crops [50,51] [55].

3.1.3. Texture

All soil samples studied were composed of clay, silt and sand. The clay percentages in the samples ranged from 9.8% to 33.33%, with an average of 17.91%. The silt contents fluctuated between 25.49% and 47.09%, with an average of 35.29%. As for the sand contents, they fluctuated between 35.29% and 60.78%, with an average of 47.58%. According to the USDA classification [56], the soils had a silty structure. This structure gives them good drainage and low retention of heavy metals compared to clay soils [57].

3.1.4. Organic Carbon, Total Nitrogen, C/N Ratio and Available Phosphate

The organic carbon contained in the samples taken varied between 0.31% and 0.72%, with an average of 0.54%. Eighty-five percent (85%) of the samples contain a moderate level of carbon, as their contents are greater than 0.5% (Fernande-Delgado et al., 2018). According to Landon (1991) [58], nitrogen (N) values in tropical soils are considered low below 0.2%, medium between 0.2% and 0.5%, and high above 0.5% [59]. In this study, the total nitrogen contained in all samples was less than 1%, suggesting a very low level. The C/N ratio is an indicator of soil fertility and the balance between carbon and nitrogen. In this study, the C/N ratio varied between 7.08 and 13.89, with a mean of 10.56. This average ratio suggests good quality organic matter.

The available phosphorus contents varied between 0.62 g/kg and 24.8 g/kg, with an average of 7.45 g/kg.

3.2. Concentration of Heavy Metals

The minimum, maximum and average concentration of heavy metals (Cr, Ni, Cu, Zn, As, Hg, Pb and Al) in the soil samples are recorded in Table 4 and the concentration histogram is shown in Figure 2.

Table 4. Statistical quantities of heavy metal concentrations in soils

Figure 2. Histogram of heavy metal concentrations in soils samples

Chromium (Cr): Chromium concentrations in the soil samples analysed ranged from 2.8 mg/kg to 93.5 mg/kg, with an average of 42.1 mg/kg (Table 4). The highest concentration was obtained in sample F5 and the lowest in sample F11 (Figure 2). The average Cr concentration of this study (42.1 mg/kg) was higher than the upper continental crust (UCC) value set at 35 mg/kg according to Wedepohl (1995)[30]. The guide values set by the FAO/WHO, French legislation (NF U44 04410) and South Africa are respectively 100 mg/kg [60] ,150 mg/kg [22,61] and 46 mg/kg [62]. All samples had Cr concentration below these guide values. Analysis of the samples shows that only two samples had levels above the level of 70 mg/kg which is the normal value for uncontaminated soils [63]. Cr concentrations higher or lower than the average of this study have been reported by other researchers. In Malaysia, the highest average Cr concentration obtained by Diami et al. (2016) [64] in surface soils associated with iron ore mining was 11.07 mg/kg. In the same year, Olobatoke et al. (2016) [65] measured in the soils and in the tailings pond near a former gold mine in South Africa, an average concentration of 173.39 mg/kg. In China, Pan et al. (2023) [66] measured an average Cr concentration of 54.58 mg/kg in soils from a lead-zinc mining watershed.

Nickel (Ni): The average value of the nickel concentration in the soils samples from Bouly was 11.4 mg/kg. It was lower than the continental crust average set at 16.8 mg/kg according to Wedepohl (1995) [30], as well as the standards established by FAO/WHO, South Africa and France (NF-U4404411). The Ni concentrations in the samples varied between 6.8 mg/kg (sample F8) and 16.1 mg/kg (sample F12). All these values were below the normal value for uncontaminated soils set at 50 mg/kg by Bowen in 1979 [63], which suggests that the soils in the sampling area were not contaminated with Ni. These concentrations were also lower than the result of Wang et al. (2021) [67], who obtained an average value of 48 mg/kg. In soils around the Kapan mining area in Armenia, Gevorgyan et al in (2017) [68] obtained an average concentration of 67.35 mg/kg.

Copper (Cu): The average value of copper in the studies samples was 33.9 mg/kg. This value was higher than the UCC value (14.3 mg/kg according to [30]) and the South African standard (16 mg/kg according [62]). However, this average remains significantly lower than the regulatory value of 100 mg/kg defined by the French Association for Standardization under standard NFU 44-041[22] and by the FAO/WHO [60]. The lowest and highest values of Cu concentrations were found in samples F13 (22 mg/kg) and F10 (49.7 mg/kg), respectively. About 61.5% of the samples had concentrations above the normal value of 30 mg/kg for uncontaminated soils [63], suggesting that they are contaminated with Cu. To make comparisons, we looked at the results of other researchers in other regions. For example, in the vicinity of a coal mine in China, Linhua and Songbao in 2019 [69] obtained an average Cu concentration of 15.5 mg/kg, which is significantly lower than that of this study. On the other hand, Bokar et al. (2020) [70] obtained an average of 91.3 mg/kg near a gold mine in Mali, which is significantly higher than the Cu average concentration of this study.

Zinc (Zn): The average zinc concentration obtained in this study was 42.9 mg/kg. It was lower than the average value for the continental crust (52 mg/k) [30], and clearly lower than the standards of South Africa (200 mg/kg) and FAO/WHO (300 mg/kg) [60,62]. The maximum value of zinc concentration recorded in sample F12 was 56.3 mg/kg, while the minimum value found in sample F2 was 37.1 mg/kg. The maximum value obtained is lower than the limit value of 90 mg/kg for uncontaminated soils [63], which shows that the soils in the sampling area were not contaminated by Zn. In Korea, the average concentration of Zn near the mine at Goseong was 249.17 mg/kg [71] and that was significantly higher than the average concentration of this study. However, in South Africa, Confidence Muzerengi (2017) obtained a value of 26 mg/kg [72] for Zn in soils near the New Union gold mine in Limpopo province, which is significantly lower than that of this study.

Arsenic (As): The average concentration of arsenic (As) in the soils samples from Bouly was 16.2 mg/kg. This value was considerably higher than the continental crust average (2 mg/kg) [30] and the South African standard set at 5.6 mg/kg [62]. Arsenic levels in the studied samples varied between 4.6 mg/kg and 61.5 mg/kg (Table 4). These concentrations were respectively recorded in samples F8 and F12 (Figure 2). About 86.4% of the samples had concentrations above the global soil average (6.83 mg/kg according to [73]). About 15.38% of the soil samples had As concentrations between 29 mg/kg (target value according to Deutsch) and 55 mg/kg (action value according to Deutsch), which suggests that they were contaminated [74,75]. About 23% of the soil samples exceeded the phytotoxicity limit set at 20 mg/kg for As in soils, suggesting that they are potentially toxic to plants. In general, plants growing in natural soil contain low levels of arsenic (<3.6 mg/kg according to [76]. At higher concentrations, arsenic is toxic for most plants [77]. The results of this study were compared with those of other researchers in other countries. For example, in Côte d'Ivoire, average arsenic concentrations in industrial gold mining areas ranged from 294.38 ± 195.40 μg/g at Afema to 7.38 ± 4.39 μg/g at Bonikro[10]. The arsenic concentration obtained in this study was significantly higher than that obtained in the soils around the Bonikro gold mine and lower than that obtained in the soils around the Afema gold mine. In Chile, Reyes et al. (2020) [39] in his study on mining waste showed that the arsenic concentration was 17 mg/kg, which was relatively close to the concentration obtained in this study.

Mercury: The maximum mercury (Hg) concentration in the samples taken was 3.5 mg/kg, while the minimum concentration was 1.3 mg/kg. The average obtained was 2.4 mg/kg, which was considerably higher than the average value (0.056 mg/kg) of the continental crust [30]. It greatly exceeded the regulatory value of 1 mg/kg defined by the French Association for Standardization under standard NFU 44-041 [61]. The highest mercury content was determined in the sample F12and the lowest in the sample F9 (Figure 2). About 61.53% of the samples have concentrations above the continental crust average. All mercury concentrations in the samples are between 0.3 mg/kg (target value according to Deutsch) and 10 mg/kg (action value according to Deutsch), suggesting that the soils in the area are contaminated with mercury [74,75]. The China Soil Environmental Quality Standard (GB 15618-1995, 1995) [78] stipulates that the maximum allowable value of Hg to ensure agricultural production and human health is 0.30mg/kg and 1mg/kg respectively, for soils with a pH below 6.5 and a pH above 7.5. It is 1.5 mg/kg in areas with high Hg concentration in source rocks. However, for the soils in this study with a pH below 6.4 and an average concentration of 2.4 mg/kg, and greater than 1.5 mg/kg, Hg poses a threat to agricultural production and poses a danger to people in the area [78].

The average value of this study is compared with other values obtained by similar studies in other countries. In China, Chen et al. (2017) [79] reported an average of 0.12 mg/kg. In Ghana, Petelka et al. (2019) [59] reported average mercury (Hg) concentrations varying between 0.41 and 0.76 mg/kg. In an abandoned mercury mine in the Philippines, Hg concentrations ranging from 0.04 mg/kg to 67.5 mg/kg, with an average of 7.4 mg/kg, were obtained by Samaniego et al. (2019) [80]. The average mercury concentration in our study was higher than those obtained in China and Ghana and that obtained in the Philippines.

Lead (Pb): The highest concentration of lead (Pb) in the samples from Bouly was 20.4 mg/kg, while the lowest was 12.4 mg/kg. The average concentration in the soil samples was 15.17 mg/kg (Table 4). This average was slightly lower than the average (17 mg/kg) for the upper continental crust [30], but well below the standards set by France and the FAO/WHO, which are 100 mg/kg [60,61].

Sample analysis shows that the highest value was found in sample F5, while the smallest was found in F12 (Figure 2). Pb concentrations in soils around the world generally range between 15 mg/kg and 32 mg/kg [73], and the Pb concentrations in this study fall within this range. Some research has obtained lower Pb averages than this study. For example, in Côte d'Ivoire, lead concentrations in sediments collected from industrial gold mining areas ranged from 7.28 ± 3.03 μg/g at Afema to 6.90 ± 1.29 μg/g in Bonikro [10]. The same is true in Vietnam, where N'guyen et al. (2020) [81] obtained an average value of 11.9 mg/kg in the Dak Ripen gold mine. Other authors have found higher concentrations. Thus, in Korea, [82] Kim et al. (2019) reported an average concentration of 93.8 mg/kg in soils near the abandoned Yaro gold mine. A mean value of 255.03 mg/kg, significantly higher than that of this study, was obtained in Iran by [83].

3.3. Estimation of Contamination Intensity

3.3.1. Geoaccumulation Index (Igeo)

Table 5 presents the statistical quantities of the geoaccumulation index and Figure 3 presents the histogram of the Igeos for the soil samples from Bouly.

Table 5. Statistical quantities of soil geoaccumulation index

The negative values of the maximum of the Geoaccumulation index (Igeo) indicate that the soils samples from Bouly were not polluted by nickel (Ni), zinc (Zn), lead (Pb) and aluminum (Al). The average Igeo value for copper (Cu), which is between 0 and 1, suggests that the soil is classified in class 1 (Table 2), that is to say unpolluted. For arsenic, the Igeo varies between 0.63 and 4.36, with an average of 1.98, which suggests that the soils were moderately contaminated. With regard to mercury, the Igeo fluctuates between 3.93 and 5.50, with an average of 4.74, indicating class 6 contamination, and the soils pollution level were between heavily polluted and extremely polluted.

The analysis of Figure 3 reveals that 61.54% of the soil samples containing arsenic were moderately polluted and 15.38% had a level of pollution between moderate and heavy pollution. In addition, 38.46% of soil samples containing mercury were extremely polluted and 53.84% had a pollution level between strong and extreme pollution.

The Geoaccumulation Index showed that mercury and arsenic were the main pollutants in the study area.

Figure 3. Histogram of geoaccumulation index of samples

Figure 4. Histogram of enrichment factors for studied soil

3.3.2. Enrichment Factor (EF)

The statistical quantities relating to the enrichment factor are recorded in Table 6.

Table 6. statistical quantities of the enrichment factors

Enrichment factor (EF) values vary within the following ranges: 50.31 to 134.05 for Hg; 4.48 to 40.40 for As; 2.6 to 6.3 for Cu; 0.13 to 3.596 for Cr; 0.94 to 2.38 for Pb; 0.98 to 1.9 for Zn and 0.78 to 1.82 for Ni. The average EF values, listed in descending order, are as follows: Hg (72.24) > As (13.81) > Cu (4.15) > Cr (2) > Pb (1.63) > Zn (1.46) > Ni (1.17). These mean values suggest that the sample area is extremely enriched in Hg, considerably enriched in As, moderately enriched in Cu and Cr, and shows an absence to low enrichment in Pb, Zn and Ni.

The variations of the EFs in the soils samples are shown in Figure 4.

Enrichment factors (EF) values less than 2 for Ni and Zn show absence to low enrichment of these metals in all samples. Most of the soil samples (86.61%) were not enriched or weakly enriched in Pb, and the rest of samples show moderate enrichment. Samples were evenly split between moderate Cr enrichment and no/low Cr enrichment. Cu, As and Hg were the most polluting elements in the study area. For copper, 76.0% of the samples show moderate enrichment (2 < EF < 5) and the rest (23.07%) show considerable enrichment (5 < EF < 20). Regarding As, the samples enrichment were distributed between considerable enrichment to extremely enriched: 76.92% is considerably enriched, 15.38% is very enriched and the remainder (7.67%) is extremely enriched. As for mercury (Hg), it is extremely enriched in all the samples (EF > 40). According to Zhang and Liu (2002) [84], values of the enrichment factor (EF) between 0.05 and 1.5 indicate an entirely natural origin of the metal, while values of EF greater than 1.5 suggest that the sources are more likely to be the result of human activities. Based on the average values of EF, it is very likely that the elements Cr, Ni, Zn and Pb come from the natural process, while the presence of the other elements such as As, Cu and Hg is more likely due to human activities. Long-term application of excessive fertilizers, organic amendments and pesticides to agricultural land could increase Hg concentration in soils [85]. The high degree of mercury (Hg) and arsenic (As) contamination is related to the mineralization of gold and its associated minerals such as cinnabar, arsenopyrite, realgar, orpiment and galena [48] The breakdown of arsenic-rich pyrites present in gold mineralization during the industrial mining process has resulted in significant contamination of surface sediments in wetlands near the Afema gold mine [10]. Acid mine drainage resulting from the oxidation of sulphurous minerals (arsenopyrite, pyrite) contained in the tailings is the source of contamination of the soils near the mines [35]. The presence of arsenopyrite associated with gold ore in the study area, as well as the acid characteristics of the soils, suggest that the mine may have contributed to the enrichment of arsenic in the extraction area. Agricultural practices and mining activities could be the source of mercury (Hg) contamination, as pesticides and fertilizers are commonly used by farmers.

4. Conclusion

Heavy metal contamination of agricultural soils has become a serious environmental problem, posing a potential threat to both agricultural production and human health. The objective of this study was to evaluate the physicochemical parameters and heavy metal pollution of agricultural soils near an industrial gold mine, using different soil pollution index approaches (Igeo, EF).

The soils studied turned out to be acidic and very saline, with a loamy texture and a satisfactory C/N ratio. Concentrations of Hg, As, Cu and Cr were higher than average continental crust values. In particular, the concentrations of Hg and As greatly exceeded the South African standards, and that of Hg also exceeded the standards set by the WHO and the FAO. The enrichment factor showed that the study area was extremely enriched in mercury, considerably enriched in arsenic and moderately enriched in copper, indicating an influence of anthropogenic activities. The geoaccumulation index revealed extreme mercury pollution, moderate arsenic pollution, and low to moderate copper pollution. The arsenic appeared to come from mining activities, while the mercury came from both mining and agricultural activities.

Mercury and arsenic were found to be the main contaminants in the study area. It is crucial to take measures to mitigate this pollution, in particular by implementing soil decontamination practices and carrying out regular monitoring of soil and crop quality in agricultural areas near industrial mines.

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