Physicochemical parameters of agricultural soil
The results of the physicochemical analysis of the study area shown in Table 1. The values of the agricultural soil samples from all the sites ranged in a narrow interval from 7.7 to 8.44, with an average of 7.99. The electrical conductivity (EC) found ranged from 1.52 to 4.94 with a mean of 3.2 dS m−1, while the organic matter content (OMC) oscillated from 2.06 to 3.34 and a mean of 2.8%. As observed in Table 1, the soil samples collected from the northern sites displayed the highest EC value, and south sites had the lower value. However, the OMC value in the soil samples was higher in both the northern and southern sites, comparing to places from the center of the Communities of El Bordo and El Lampotal. Analysis of the soil textural the relative percentages of sand, loam, and clay was in the range (37-65%) for sand, (22-38%) for loam, and (13-39%) for clay. According to the USDA, the region samples showed a certain degree of homogeneity with a predominance of clay sandy loam, sandy loam, and clay loam. In general, the pH of the soil analyzed was close to 8.0 at all sites. In this regard, Merry et al. (1986) stated that increasing soil pH decreased Pb concentrations in vegetable crops; since Pb is relatively immobile and As very slowly leaches through soils (Hood 2006). In this context, Alam et al. (2003) suggest that the relatively neutral soil pH (7.6–8.5) As will be immobile in the local soil profile. Therefore, the alkaline range of soil (>8.0) restricts the mobilization of PTE, thus reducing their uptake and transference from soil to crops (Cheng 2007; Sharma et al. 2007). The EC value indicates soil salinity. Horneck et al. (2011) reported that soil with EC values less than 1 mS cm−1 is suitable for crop production. Although, values of EC showed a normal range (1.52) to slightly saline (4.94), and vegetables did not show symptoms by salinity.
As concentration in agricultural soil
The analysis of PTE concentration in agricultural soil is shown in Table 1. Toxic elements were detected in all samples, and the concentrations were in the following order: Pb>As. In this study, all soil samples contained As with a concentration higher than several reference values established for agricultural soils; this indicates that the soils analyzed are contaminated. The concentration of As in soil from all the sites ranging from 39.02 at 165.00 mg/kg−1, an average of 109.22 mg/kg−1 did exceed the critical level (>20 mg/kg−1), Mexican standard, for agricultural soil and residential is far superior (NOM-147-SEMARNAT/SSAI-2004). Likewise, the maximum limit (ML) for agricultural soil of 20.0 mg kg−1 as recommended by the European Community (Rahman et al. 2007). In addition, levels exceeded different guidelines for agricultural, residential, and commercial land uses U.S. EPA (1993). Also, the concentrations are higher and surpassed environmental critical limit concentrations of the WHO permissible limit for As in agricultural soils is 0.5 mg/kg−1 (WHO 2004). Accordingly, total As represents a potential source of possible risk through bioaccumulation and consumption of vegetables because the concentration of As in soil in the area is higher than the reported global average of 10.0 mg kg−1 (Das et al. 2004). Norton et al. (2013) found SW field survey soil in horticultural produce grown in the impacted mining region the average As concentration was 110.3 mg kg−1.
In Mexico, previous studies found a range of As concentrations similar to those reported in this work. For example, Santos–Santos et al. (2006) and Gonzalez et al. (2012) showed that As concentration all soil valley of the agricultural region of Guadalupe, Zacatecas, average 109 mg/kg−1. However, Mendoza-Amezquita et al. (2006) found that As concentration contaminated soils in the old mining region of Guanajuato, Mexico, ranged from 21-36 ppm. Although, differ significantly from the concentrations reported in other studies from an old mining area in Zimapan, Hidalgo, Mexico, 2,550-14,600 ppm (Ortega-Larrocea et al. 2010). Furthermore, in the present study, As concentrations were significantly higher than those found in other studies in agricultural soils around and near sites with mining activities (Baig and Kazi 2012; Rahama et al. 2013; Alam et al. 2016; Bui et al. 2016). Consequently, these results corroborate other findings of waste mining, which reported elevated levels of PTE in the farmland around of mining area (Zhuang et al. 2009a).
Also, it is consistent with the previous fact that highlights that historic metal ore mining is considered one of the most important sources of soil contamination (Dudka and Adriano 1997). Similarly, results confirm comparable As values than those of literature reported in vegetables (Cao et al. 2009; Liu et al. 2010; Chang et al. 2014; Tasrina et al. 2015). However, this study’s As levels observed in horticultural crops soils were lower than those found in contaminated agricultural soils (Filippi et al. 2004; Chakraborti et al. 2013).
HERE GOES TABLE 1
Pb concentration in agricultural soil
Likewise, Pb concentrations of agricultural soils in this study were low in most of the soil samples from four Communities, except at El Bordo and La Era (Table 1). These levels show that contamination by Pb was not very extensive. However, the maximum and minimum Pb concentrations were 1206 and 25 mg/kg−1, respectively, and the mean concentration was 355.4 mg/kg−1. Besides, not exceed the acceptable level for Pb in the agricultural soil of Mexico (400 mg/kg−1) (NOM-147-SEMARNAT/SSAI-2004). Nevertheless, this value is comparable to the ML Pb concentration in soils for agricultural purposes, 375 mg/kg−1 (OECD 1993). However, it exceeds standard value ÖNORM L 1075 (Austrian standard L 1075) 100 mg/kg−1 defined as the limit for agricultural and grassland soils (ÖNORM 1990). Also, exceed 200 considered ecological risks but is below 700 that represent health risks (Ministry of Environment of Finland 2007; Tóth et al. 2016). Likewise, Kabata-Pendias (2011) reported that the range and the mean world content of Pb in soil are 3–90 and 27 mg kg−1, respectively. Also, that to name a contaminated soil Pb this must exceed 100 mg/kg−1.
In this regard, the concentration of Pb was two times higher than the permitted standards in El Bordo (1201.4 mg/kg−1), and La Era (1205.8 mg/kg−1) samples exceeded the maximum limits of 400 mg/kg (WHO 1993). Besides, no evidence of phytotoxicity was observed from the levels of Pb in the sites studied. Therefore, the soil is unsuitable for agricultural use. In Mexico, previous studies different of Pb concentrations were reported in diverse crops soils of Valle of the Mezquital, Mexico (22.86 mg/kg−1) (Prieto-Garcia et al. 2007); in agricultural soils near mining regions of Guadalupe, Zacatecas, Mexico (100 and 400 mg/kg−1), respectively (Santos–Santos et al. 2006; Gonzalez et al. 2012); Yaqui and Mayo agricultural valleys, Sonora, Mexico (10-56 mg/kg−1) (Meza-Montenegro et al. 2012) and rural community in Fresnillo, Zacatecas, Mexico (4940 mg/kg−1) (Salas and Vega 2016). On the other hand, in this study, Pb concentrations were significantly higher than those reported in contaminated soils (Sharma et al. 2007; Saint-Laurent et al. 2010; Gałuszka et al. 2015; Kumar and Maiti 2015). Thereby, results corroborate other studies on mining activities, which also reported that elevated Pb levels in soil were ubiquitous in the vicinities of mines (Zhuang et al. 2009b; Luo et al. 2011). Likewise, the levels of Pb in vegetables reported in this study were comparable to those found by Codling et al. (2015), Eissa and Negim (2018); Ćwieląg-Drabek et al. (2020). However, in this study, Pb concentrations were significantly lower than those found in agricultural soil near mining areas (Koleli and Halisdemir 2005; Gisbert et al. 2006; Girisha and Ragavendra 2009; Chu et al. 2019).
In this way, concentrations of As and Pb in the agricultural soil showed heterogeneity and irregular distribution, indicating contamination, not uniform, and a strong influence by historical mining in the selected sites. These findings could have caused variability from layers of soil enriched with metals of different concentrations accumulated in the sediments originated by the historical mining that formed the topography of the agricultural valley. Because due to redistribution of the PTE in farmland during centuries by wind and water erosion processes. Resulting in a variable zone of enrichment and contamination of As in most farmland soils and lower risk of Pb in sites studied. Therefore, these results agree with those reported by Ha et al. (2011), where high PTE concentrations soil has been continuous dispersal downstream from the tailings mining. Renshaw et al. (2006) showed that erosion of contaminated soil is significant in the dispersion of As and metals within drainage basins. According to Nedelescu et al. (2017), a detailed analysis of the distribution of metals in a site requires a higher number of locations and samples to understand the possible patterns of this significant heterogeneity.
The PTE in soils are derived from lithogenic source and various anthropogenic sources. Many different anthropogenic sources of heavy metals contamination affect both agricultural and urban soils (Alloway 2013). For example, geological and geochemical studies have revealed that As is released from both natural and anthropogenic activities (Bundschuh et al. 2011). The natural dispersion of As in the environment is governed by a combination of region and site-specific biogeochemical and hydrological factors (Smedley and Kinniburgh 2002). Likewise, PTE in the soils may be associated with their natural concentration, which tends to be elevated in mining areas (De Gregori et al. 2003). The mining of gold and base metals, frequently associated with sulfide mineralization, has induced or exacerbated localized As contamination (Williams 2001; Garelick et al. 2009). On the other hand, various sources of toxic elements input may also contribute to increasing PTE in agricultural soils (Gupta et al. 2012; Amin et al. 2013); including mineral fertilizers (Nziguheba and Smolders 2008). The PTE concentrations confirming that the rock phosphates are the primary source of these elements in mineral fertilizers (Nziguheba and Smolders 2008; Kratz et al. 2011). In this regard, Molina et al. (2009) found that the long-term use of these P-fertilizers in some agricultural systems may increase the concentration of PTEs in soil.
In Mexico is little published information on the presence of PTE in fertilizers and its impact on agriculture. Also, in the region, there are no studies on the use of fertilizers. However, approximately 30-35% of vegetable producers apply P-bearing fertilizers, N-bearing fertilizers, S-bearing fertilizers and micronutrients. Possibly, although the use of fertilizers is not very widespread in the study region, long-term use of mineral fertilizers to high doses may affect soil and groundwater geochemistry as they are potentially enriched whit PTE and can be contributing to the increased levels of As and other PTE in agricultural soils (Kratz et al. 2016; Papazotos et al. 2019).
As and Pb concentration in vegetables
In 1993, the World Health Organization reduced its recommended criteria for drinking water from 50 to 10 mg of As/L (WHO 1984; 2006). Nevertheless, the WHO evaluates its quality and provides international norms for water quality used as the basis for regulation and standard settings targeted to protect human health. These guidelines are adopted by many countries as national guidelines, even if they are not necessarily enforceable by law (UNEP 2008). Also, the regulations related to As levels in food are complex, which varies in each country and over time (Henke 2009). Likewise, this is reflected in the values established by the Codex Alimentarius for various products (FAO-WHO 2019). Although WHO and Codex Alimentarius are not regulators the power to set standards and enforce them, its recommended criteria regarding As are included in the regulations of various countries (WHO 2006), among them Mexico.
HERE GOES FIGURE 2
The average concentration of total As and Pb (mg kg−1 dw) in the selected vegetables is listed in Table 2. According to their edible parts, vegetables were classified into root vegetables (carrot and garlic) and fruit vegetables (pepper). The results show variable levels of As and Pb in each vegetable. Also showed accumulation factor and agricultural soil degree of contamination. As concentration between root vegetable and fruit vegetable they were similar, fruit vegetable average was higher (95.66 mg kg−1 dw) and root vegetable (92.33 mg kg−1 dw). Whereas Pb concentration between root vegetable and fruit vegetable varied, fruit vegetable average was higher (9.6 mg kg−1). Although in two samples were not detected. While Pb concentration in roots vegetable was 4.8 mg kg−1 but in one sample was not detected. Between A. sativum and C. annuum, find highest As concentration in C. annuum was 111 mg kg−1 site nine, and Pb concentration was the highest D. carota 9.9 mg kg−1 site five.
In this context, all edible parts of species vegetables analyzed in this work showed concentrations of As far exceeded maximum permissible level (MPL) of Latin American countries. For example, the maximum level allowed by Chilean legislation for cereals, legumes, and leguminous plants (1 µg g−1 of ww) and the food group “other solid products” (Muñoz et al. 2002). The limit established by Argentine Food Code (1 mg/kg) (CAA 2020). The MPL for inorganic contaminants in food proposed by the Brazilian Ministry of Health (limit: 1 mg kg−1) (MS 2013). Likewise, the limit of 1.0 mg kg−1 (ww) for the classification of the concentrations for edible plants in Mexico (Osuna-Martínez et al. 2021). Also, results on the analyzed concentrations of As in vegetable were significantly higher than the maximum level set in EU legislation, as well as by Codex Alimentarius-FAO/WHO standards for cereals and vegetables (FAO/WHO 2011). Likewise, these results far exceeded the MPL of Europe permissible limit for As in fodder crops is generally between 2 and 4 mg of As per kg (moisture content 12%) (Gulz et al. 2005); for food plants, the statutory limit for total As in edible fruits and vegetables is set up at 1 mg As kg−1 fresh weight exists, e.g., in Spain (BOE 1978; Lario et al. 2002) and other European countries (Feldmann et al. 2000). Australian and New Zealand Food Standards Code (FSANZ 2019) includes to MPL level in cereal grains and milled cereal products of 1.0 mg kg−1 total As. The China food safety quality standard for a maximum of contaminants in foods (GB 2762-2012), exceed the Chinese standards for total As 0.5 mg kg−1 (f.w.) in vegetables (MHPRC 2013). The Centre for Food Safety (2018) of the Hong Kong Government has established an MPL of 1.4 mg kg−1 (ww) total As in all foods. Based on these results, the As levels detected in vegetables in this study were significantly higher than those found in vegetables grown in areas highly contaminated with As in India, Northern Vietnam, China and Bangladesh (Bhattacharya et al. 2010; Rahman et al. 2013; Bui et al. 2016; Li et al. 2017).
On the other hand, all edible vegetal parts analyzed in this work showed Pb levels with ranges from 2.06 – 9.82 mg/kg−1; these dates were higher than those recommended (standard level of 0.1 mg/kg in root and tuber vegetables) by European Union (2006); the Food and Agricultural Organization (FAO)/World Health Organization (WHO) CODEX (2011); the Chinese Ministry of Health (CMH 2005); Chilean standards for food as 0.5 µg/g (Gonzalez 1998) and the maximum limit established (0.1 mg/kg) by MERCOSUR (2014). In Mexico, although there is an official standard NOM-117-SSa1-1994, it does not mention the permissible limit take as reference to the standards established by the Codex Alimentarius and the European Union. Other countries with serious Pb contamination problems and with levels lower than those reported in this study are Bangladesh (Alam et al. 2003; Islam et al. 2016), China (Zhuang et al. (2009b), Egypt (Eissa and Negim 2018), and Romania (Lăcătuşu and Lăcătuşu 2008; Roba et al. 2016; Harmanescu et al. 2011). Concerning the absorption capacity, Kabata-Pendias (2011) highlighted that forms of the anthropogenic trace metals, availability to plants are higher than those of natural origin. In this work, differences observed in As and Pb concentrations could be due to the variable uptake capacity of the crops and the accumulation of toxic elements (Pandey and Pandey 2009).
As accumulation in vegetables
The As concentration in vegetable species is shown in Table 2. In general terms, As concentration in vegetables recorded similar concentration from the nine sampled sites belonging the communities El Bordo, El Lampotal, La Era, and Santa Rita (89 ± 9.7, 91 ± 8.7, 90 ± 9.1, 105 ± 12.5 mg kg−1 dw, respectively). The As concentration average of vegetables was 93.44 mg/kg−1 value, which was higher than the maximum limit established by international standards (1.0 mg/kg−1), permitted in many countries, including Mexico (Osuna-Martínez et al. 2021). The highest As concentrations were found in C. annum and A. sativum (111 ± 11.97 and 100 ± 13.04, respectively, site nine from Community La Era). The communities of Santa Rita, El Bordo, and Lampotal registered similar concentrations, with an average of 90 ± 9.09 mg kg−1 dw, in all sites.
The As concentration soil samples average of 109.22 mg/kg−1, which did exceed the critical level (22 mg/kg−1) Mexican Standard for agricultural soil (NOM-147-SEMARNAT/SSAI-2004) and permissible limit for As in agricultural soil 0.5 mg/kg according to WHO (2004). These results indicated that vegetables accumulate As. Therefore, this implied that the vegetables that are grown in these soils absorb more As.
HERE GOES TABLE 2
Plant As concentration tends to increase with increasing soil As and then stabilize at some maximal value at higher concentrations in soil (Tasrina et al. 2015). Thereby, McBride et al. (2015) indicated that vegetable As concentration increase with increasing soil total As. The As uptake by vegetables is related to its concentration in the respective soil (Ramirez-Andreotta et al. 2013; Khan et al. 2018). Elevated concentrations of As, were higher than the FAO/WHO permissible limits, shower this element toxic might represent risk by consumption these vegetable species.
A. sativum: In the present study, the As concentration observed in garlic bulbs was 87 mg/kg, and all vegetables were the lowest concentration. These results showed that all vegetable samples exceeded the maximum concentration of As recommended by EU (2006) and WHO/FAO (2011). Additionally, these results revealed that As concentration was higher than those found in previous studies on As in garlic bulbs. For example, Muñoz et al. (2002) in Northern Chile (0.238 µg g); Bhattacharya et al. (2010) in West Bengal, India (0.126 mg/kg−1); Alam et al. (2016) in India (0.25 mg/kg−1) and Rehman et al. (2016) in Pakistan (0.04 mg/kg−1) and Aguilar et al. (2018) central Chile under the limit of detection. D. carota: In study, average concentration As observed in carrots was 93.4 mg/kg−1. These results showed that it exceeded the maximum As concentration recommended by EU (2006) and CODEX (2011). Thereby, As concentration was higher than those found in previous studies on As in carrots. For example, Muñoz et al (2002) in Northern Chile (0.132 ± 0.001 µg g); Garcia-Rico et al. (2012) in Mexico (0.041 mg kg−1); Rahaman et al. (2013) in West Bengal, India (0.235 ± 0.004 mg/kg−1); Tasrina et al. (2015) in Bangladesh (<0.1 mg/kg−1) and Zhou et al. (2016) in China (0.188 ± 0.030 mg/kg). C. annuum: According to the results obtained average As concentration observed in pepper fruits was 95.6 mg/kg−1. This result showed that exceed the maximum concentration of As recommended by the EU (2006) and CODEX (2011). This value indicates a comparatively higher value than other studies on As in fruits pepper reported by Castro-Larragoitia et al. (1997) in San Luis Potosí, Mexico (3.0 mg/kg); Roychowdhury et al. (2003) in West Bengal, India (68.24 µg/kg); Prieto-García et al. (2005) in Zimapán mining areas, Mexico (6.26 mg/kg−1); Ramirez-Andreotta et al. (2013) in Arizona, USA Solanaceae (bell pepper, green chili, jalapeno, and tomato) range from 0.00132–0.0100 mg kg−1. Similarly, in this work, As concentrations between root vegetables and fruit vegetables were comparable. However, this result differs from those reported by Cao and Ma (2004) suggested that direct soil contact by root vegetables leads to higher concentrations than leafy vegetables, which must translocate As from roots to shoots. Therefore, As concentrations in garlic and carrot roots tissues and pepper fruits were positively proportional to As levels in soil.
Pb accumulation in vegetables
A. sativum: The found mean concentration for Pb in garlic was 3.0 mg/kg−1. The MPL Pb in bulb vegetables is 0.1 mg/kg (FAO/WHO 2014). Therefore, this value was higher than those found in previous studies (Song et al. 2009). Also, confirm comparable Pb values than those of literature reported in garlic (Guerra et al. 2012; Rehman et al. 2016; Roba et al. 2016). However, the results of this study were lower than those founded by Türkdogan et al. (2003), Maleki and Zarasvand (2008) and Senila (2014). D. carota: The mean concentration found of Pb to carrot was 5.0 mg/kg−1. The permissible limit of Pb in carrots is 0.1 mg/kg (EU 2006; FAO/WHO 2014). Therefore, this value exceeded MPL in carrots (Knapp et al. 2013; Islam et al. 2016; Rehman et al. 2016; Shaheen et al. 2016; Zhou et al. 2016). Also, they were comparable with the found by Banerjee et al. (2010) and Pančevski et al. (2014). However, concentration was lower than the studied by Senila (2014). C. annuum: The found mean concentration for Pb in pepper fruits was 9.6 mg/kg−1. These findings indicate that Pb levels were higher than those reported in other research Antonious and Kochhar (2009), Guerra et al. (2012), Islam and Hoque (2014), Mirecki et al. (2015), Islam et al. (2016), and Antoine et al. (2017). However, concentration was comparable with Ahmad and Goni (2010).
Regarding pH value, it was close to 8.0. These results corroborate Merry et al. (1986), who report that increasing soil pH decreased Pb concentrations in the plants, an effect that was more remarkable in highly contaminated soils. In this way, Singh et al. (2010) affirmed that Pb concentration varied among vegetables reflect the difference in their uptake capabilities and their further translocation to the edible portion of the plants. Thus, the content of Pb in various plant organs can vary with plant species (Sharma and Dubey 2005). Therefore, our results agree with Alexander et al. (2006) and McBride (2013) who highlight the general observation on the barrier to the translocation of Pb from the stem to the fruits in highly contaminated soil. In particular, C. annum samples where Pb concentration no was detected. Also, our results agree with Cai et al. (2020), who found C. annuum has Pb tolerance ability indicating that it can effectively absorb Pb in soil.
In general terms, according to Manzoor et al. (2018) presence of PTE in vegetables might be due to the higher uptake from soil. The uptake of As and Pb in plants is regulated by chemical speciation, biogeochemical characteristics, other physic-chemical parameters of the soil, microbial activity by mycorrhization in soils and plant factors (Davies 1995; Feleafel and Mirdad 2013; Abbas et al. 2018). Thereby, apart from PET content in the soil, the physic-chemical properties of soil also affect the degree of contamination of vegetable crops (Singh and Kumar 2006). The spatial difference in the contamination Pb of As and Pb in plants was possibly due to different levels of contamination and the previously mentioned in the literature (parameters of soil, biogeochemical, microbial, and plant factors). Therefore, the plants absorbed As and Pb from the contaminated soil and possibly, those deposited in plants exposed to dust from the polluted environment (Kachenko and Singh 2006; Singh and Kumar 2006).
Pollution load index
The use of a pollution index indicates whether soil quality is suitable for agricultural use and whether vegetables grown can be consumed eaten safely. Therefore, should further assess the soil of agricultural use to determines the environmental risk caused by contaminated soils (Li et al. 2006). The environmental quality index of the As and Pb in the vegetables is presented (Table 2). The pollution index to As followed a similar trend in all soils. The use of a PLI indicates whether soil quality is suitable for agricultural use and whether vegetables can be consumed safely.
The pollution index to As followed a similar trend in all soils. The calculated Pi values for all soil samples of As and two soil samples of Pb were higher than 1, confirming that these sample soils were highly polluted. On the other hand, the PLI of the rest of the soil samples of Pb were relatively low than 1, indicating that Pb did not pollute the vegetables. However, the PLI values for As were lower than those reported by Khan et al. (2017b). Therefore, soil samples were classified as polluted and indicated that the studied area is highly contaminated (Li et al. 2006). It also showed that the contamination index by As is high and persists a greater possibility of causing health and food safety problems (Khan et al. 2017a). According to the high PLI for As, farmers should not grow agricultural foods such as edible vegetables in this area. Based on these findings, we can assure that soil quality is not suitable for agricultural use and, vegetables harvested in this area can not be consumed safely (Li et al. 2006).
The elevated As and Pb concentration in farmland are attributed to the geogenic origin and anthropogenic by wastes of historical mining activities. Also, the study does not consider other pollution matrices such as abandoned tailing, the processing company, and new tailing ponds. The tailing ponds or impoundments are excavations of highly variable sizes located within farmland. Local processing companies dig and extract the historical mining waste dragged through the streams. Subsequently, tailings ponds were filled with processed waste have a high level of PTE that seep over time and migrate through the soil profile to the deeper layer. Nowadays, in these new tailing ponds are grow various crops. The new mine ponds are recklessly managed and do not exist mechanisms to oblige the tailings processing company to obey the environmental laws and regulations (González et al. 2012).
Also, new tailings ponds found inside farmland probably represent a risk of contamination of the aquifer and a grave health hazard. In this regard, previous studies have found high levels of As and Pb in groundwater extraction wells that supply Guadalupe municipality (Castro et al. 2003; Gonzalez 2011). Also, heavy metal contaminated crops could aggravate human health risks when consumed along PTE contaminated drinking water (Zavala and Duxbury 2008; Brammer and Ravenscroft 2009). Regarding a survey conducted among the population of the area, 18.48% of the respondents reported at least one household member with dark spots on the hand palms (Arsenicosis symptom) (Gonzalez (2013). The results of this study support the proposal that setting As standards for food is urgent and beneficial in combination with existing As water standards for protecting public health (Peralta-Videa et al. 2009; Meharg and Raab 2010) and fulfill the commitments on agri-food safety standards with WHO.
Soil-vegetable bioaccumulation factor
The bioconcentration factor (BCF) is one of the key components of human exposure to metals through the food chain. Also, it is a criterion to assess global human health concerns (Woldetsadik et al. 2017). According to BCF value of plants can be are characterized as excluders (<1.0), and hyperaccumulators (> 1.0 - 10.0), respectively (Ma et al. 2001). The BCF values of As and Pb to vegetables are presented in Table 2. These values varied between vegetable species and sites. However, the trend of BCF was similar to the order of the As and Pb. The highest BCF value was for As, ranging from 2.33 to 0.64, and the average for all samples was 1.01. This value clearly indicates that the As has a higher capacity for As accumulation in the edible parts in root vegetables (carrot and garlic) and fruit vegetable (pepper) compared with Pb. Likewise, root vegetables show the highest BCF. This suggests that these species pose a higher health risk due to consumption by human beings for a longer time (Alam et al. 2016). The highest BCFs were recorded in A. sativum (2.33), D. carota (1.22), and C. annuum (1.21). These might be due to higher mobility of the As with a natural occurrence in soil (Alam et al. 2003) and its lower retention in the soil than other toxic cations (Zurera et al. 1987). Therefore, a high BCF value indicates a higher accumulation potential of metals in the vegetable (Cui et al. 2004).
Also, these data implicated that vegetables are hyperaccumulators (>1.0), and those with values close to 1.0 like accumulators of As. While the Pb that recorded the lowest values BCF in all vegetables showed a relatively low potential for Pb accumulation in agricultural soil studied. However, reduced uptake of PTE is one of the plant's adaptation strategies to avoid metal toxicity (Baker and Walker 1990). Therefore, these results agree with Bui et al. (2016), who found that soil pH slightly alkaline was very similar across sites; we rule out pH as a significant driver of BCF differences in our study. In addition, the results of this study have revealed that the responses of vegetables to exposure to As and Pb are complex due to the heterogeneous tolerance and the various relationships between the concentration of As and Pb in the soil and the plants (Kabata-Pendais et al. 1993). Based on these findings, results agree with Chang et al. (2014), who highlight that the BCF values of PTE in vegetables from soils impacted by mining are obviously high. Also, it suggested a relatively high potential for PTE accumulation in agricultural soils, possibly due to historical mining activities in the region.