Study area and sample collection

The study was conducted in the industrialized city of Solapur district, which is located in the Indian state of Maharashtra. Handlooms and power-looms are the main industries in the area.^26,41 The city experiences a tropical monsoon-type climate.⁴²

Soil sampling was carried out at 6 locations around a 10 km radius of the industrial hub for 2 seasons viz. winter (January) and summer (April) in the year 2017 to understand soil quality (Figure 1). After removing the surface vegetation cover, visible roots, plant litter, soil fauna, stones, plastic materials and artificial matter, soil samples were collected using stainless steel auger soil sampler at a depth of 50, 150, and 300 cm and mixed thoroughly and analyzed as a single unit sample. The land area near the root zones and foundations of civil constructions were avoided. Samples were analyzed for physical parameters [texture, bulk density (%) and porosity (%)] and heavy metals content (As, Cd, Cr⁶⁺, Cu, Hg, Mn, Ni, Pb, and Zn) (mg/kg).

Figure 1.

Study area and sampling locations.

An adequate amount (1 kg) of soil samples was collected from each site. The samples were packed in a waterproof zip lock pouch bag and transported to the laboratory for testing.

Frequently consumed vegetables and fruits (Table 1) by the human population grown in the agricultural land within the study area were collected from farms located in Chandrahal (SQ-1) (Figure 1) for investigating the concentration of As, Cd, Hg, and Pb.

Table 1.

Names of vegetables and Fruits analyzed.

Sample preparation and analysis

Vegetables and fruit samples were washed with distilled water and then with deionized water to remove various airborne and soil contaminants. Then the samples were air-dried, crushed, and stored at room temperature in the laboratory. For analysis, 1 to 2 g of samples were digested with the repeated infusion of conc. HNO₃, conc. HCl, and 30% H₂O₂ along with heating as indicated in EPA method 3050B⁴³ and FSSAI guidelines.⁴⁴ After cooling, the solution was diluted by adding deionized water to 100 ml and then centrifuged. The obtained solutions were analyzed for heavy metals (Pb, Cd, As, Hg) using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) Agilent 7500. Laboratory equipment and utensils were washed with soap and water, then rinsed with 1.5% v/v HNO₃ and distilled water after every use to prevent any contamination.

Human health risk assessment

Metal Pollution Index (MPI)

The overall heavy metal concentration (Pb, Cd, As, and Hg) contained in vegetable and fruit samples were estimated by Metal Pollution Index (MPI). This index was calculated using the equation⁴⁵:

Where Cf_n = concentration of metal in nth sample.

Daily intake of metal (DIM) and Health Risk Index (HRI)

To calculate the human exposure dose, the daily intake of metal was calculated by using the equation^6,46:

Where, C_metal = Concentration of heavy metals in vegetables (mg/kg), D_{food intake} = Daily intake of vegetables (average consumption = 0.280 kg/person/day),⁴⁷ B_{average weight} = Average body weight “56 kg/person.”⁴⁸

The ratio of estimated test vegetables and fruits exposure to oral reference dose was used to estimate the health risk index.⁴⁹ The health risk index is also known as Hazard Quotient (HQ). It indicates potential hazards to human health. The index was calculated using the equation:

The oral reference dose (RfD) of As, Cd, Hg, and Pb are 0.0003,⁵⁰ 0.001,⁵⁰ 0.01, and 0.004 mg/kg/day,⁵¹ respectively.

Hazard Index (HI)

To evaluate the potential risk to human health through more than one heavy metal, the hazard index (HI) has been developed.⁵² The hazard index was calculated using the equation:

It is assumed that magnitude of the adverse effect will be proportional to the sum of multiple metal exposure. It also assumes similar working mechanisms that linearly affect the target organ.⁵³ When the value of HI > 1, there is a greater possibility of non-carcinogenic health effects, and the probability increases with a rising value of HI.^3,54 The hazard index of the toxic elements (As, Cd, Hg, and Pb) has been calculated in the present study.

Soil parameters

The results of all the specified parameters at each location were then compiled for both seasons and are tabulated in Table 2. The soil samples were observed to be neutral to slightly basic in nature with pH ranging from 7.64 to 8.0 for both seasons. The soil samples were mostly sandy clay and sandy clay loam in texture. Sand percentage varied between 55.0%and 67.5%, silt content was between 1.5%and 5.7% and clay content was between 29%and 42.5%. The bulk density varied between 1.36%and 1.50% and porosity was between 42%and 47% for both seasons.

Table 2.

Soil characteristics and heavy metal concentration.

The values for the Pb for all locations vary between 4.1 and 16 mg/kg, Cu ranges from 2.0 to 14 mg/kg, Zn from 6.0 to 28 mg/kg, and Mn ranges between 1.1 and 2.0 during both seasons (Table 2). Heavy metal concentrations were found to be below the permissible levels set by Indian⁵⁵ and European standards⁵⁶ (Table 3). The mean concentration of heavy metals in soil shows the trend: Zn > Cu > Pb > Mn.

Table 3.

Standard levels of selected heavy metals (mg/kg) in soil, fruits, and vegetables.

Soil pH plays a critical role in determining solubility, mobility, and final bioavailability of metals.^57–59 Numerous studies have found a negative correlation between soil pH and heavy metal mobility and availability to plants. For example, in the case of Cd, Pb, and Zn, decreasing soil pH resulted in substantial increases in heavy metal desorption from soil components and dissolution in soil solution.^60,61 Reduced soil pH increases heavy metal mobility and bioavailability,^62–64 promoting heavy metal absorption by plants and posing a health risk.⁶⁵ Also, the particle size of soil has been shown to have a significant impact on heavy metal concentrations in soils.^66,67 Due to greater surface areas, the finer particles show higher concentrations of heavy metals.^68,69 In addition, it was reported that in sandy clay soil, soil pH appeared to be the most important predictor of Cr, Pb, and Zn solubility and mobility.⁷⁰ In comparison to other sampling sites with sandy clay loam soil, Chandrahal (SQ-1) and Kumthe (SQ-5) with sandy clay soil show higher Pb content (Table 2). Additionally, soil pH varied very slightly across all locations, and no discernible variation in the quantities of selected heavy metals were observed.

Heavy metals in fruits and vegetables

Heavy metal contamination was found in nearly all samples examined. Some items contain a higher proportion of one metal compared to the others. This can be explained by differential absorptions of heavy metal from soil and subsequent translocation within the plant.^6,71,72 Table 4 represents the concentration of heavy metals in fruit and vegetable samples. Among them, sugarcane has the highest Pb accumulation (1.741 mg/kg) while garlic has the highest Cd accumulation (0.37 mg/kg). Sugarcane has preferential absorption for Pb; hence it is used to reduce the bioavailability of Pb in soil.⁷³ Garlic has a higher mean concentration of Cd and Pb than other fruits and vegetables, suggesting it has the potential to hyper-accumulate Cd and Pb. Similar observations about garlic were reported earlier and this principle is used for phytoremediation of soil.⁷⁴

Table 4.

Concentration of heavy metals, MPI, DIM, HRI and HI of different fruits and vegetables.

In the majority of fruits and vegetables, heavy metal concentrations were found to be within permissible levels set by Indian⁵⁵ and WHO/FAO standards.⁷⁵ The level of Cd in garlic (0.37 mg/kg) exceeds the WHO/FAO standard (0.2 mg/kg) but was within the Indian threshold (1.5 mg/kg). While in garlic (0.123 mg/kg), okra (0.17 mg/kg), fenugreek (0.235 mg/kg), sugarcane (0.035 mg/kg), tamarind (0.147 mg/kg), and sorghum (0.356 mg/kg) Hg concentration exceeds WHO/FAO standard (0.03 mg/kg).

It was found that selected heavy metal concentration in fruits and vegetables was not proportionate to the native soil. For example, Cd levels in soil samples were found to be below detectable levels, but Cd levels in garlic were found to be considerable. This might be related to garlic’s tendency to hyper-accumulate Cd from the soil,⁷⁴ as well as other probable sources of Cd exposure, such as wastewater irrigation.⁶

Fruits showed lower metal accumulation compared to vegetables. Figure 2 represents the mean concentration of metals in fruits and vegetables. Similar observations were reported widely suggesting vegetables have a higher heavy metal accumulation capacity than fruits.^36,76–78 There are several reasons given to explain this phenomenon. Firstly, leafy vegetables have higher growth and transpiration rates than fruits. It facilitates increased heavy metal absorption through roots and subsequent translocation from roots to leaves. Secondly, in fruit plants, large quantities of heavy metals absorbed are stored in other organs, especially leaves than the edible part. Thirdly, leaves have a higher surface area, which makes vegetables more vulnerable to physical contamination by rainwater and dust from soil.^76,79

Figure 2.

Level of heavy metals in fruits and vegetables.

Human health risk assessment

MPI varied from 0.002414736 (Chickpea) to 0.12531021 mg/kg (Garlic). Figure 3 represents the MPI of different fruit and vegetable samples examined. A higher value of MPI signifies a greater concentration of heavy metal pollution. If vegetables and fruits with higher MPI values are consumed in greater quantities, it can lead to higher heavy metal accumulation in the body. In our study, among different vegetables, garlic showed the highest value of MPI followed by potato (Table 4). As compared to vegetables, fruits showed a lower metal pollution index which is explained by the lower metal accumulative capacity of fruits compared to vegetables.^76,79

Figure 3.

Metal Pollution Index (MPI) in different vegetables and fruits analyzed.

HRI of Pb, As, Cd, and Hg were determined for both vegetable and fruit samples (Table 4). In our study, HRI of more than 1 was noted in garlic (HRI Cd = 1.85) and sugarcane (HRI Pb = 2.176). HI varied from 0.048416667 (Cucumber) to 2.99566667 (Garlic). Figure 4 represents the HI of various vegetables and fruits examined. For reference, an index of more than 1 is considered not safe for human health (USEPA, 2002). The hazard index gives an overall estimation of adverse health effects. A higher hazard index means more magnitude of health problems.^3,54 HI of garlic is mainly determined by Cd followed by Pb whereas, in the case of sugarcane and potato, HI is determined by Pb (Figure 4). The result showed HI of Garlic > Sugar cane > Potato. Hence these vegetables cause more human health risks compared to others.

Figure 4.

Hazard Index of toxic elements (Hg, As, Cd, Pb) for different vegetables.

The study only included the human health risk assessment of heavy metals (Pb, Cd, As, and Hg) through contaminated fruit and vegetable consumption. There are other ways of heavy metal exposure such as inhalation, dermal contact and ingestion of contaminated soil (for children). All these factors may increase the overall heavy metal intake but were not considered in this study.