Enrichment, sources, and distributions of toxic elements in the farming land's topsoil near a heavily industrialized area of central Bangladesh, and associated risks assessment

Abstract

Toxic element accumulation in the surrounding soils of the advanced industry- and agriculture-oriented areas may lead to severe environmental degradation and harmful impact on inhabitants. This work examined the concentration of some concerned toxic elements (Cr, Pb, Cd, Cu, As, and Ni) in the representative topsoil from 10 industrially contaminated sites in central Bangladesh (Narayanganj district) using an Inductively Coupled Plasma Mass Spectrometer concerning the probable ecological and human health risks. The mean concentrations (mg/kg) of the elements were found in the order of Ni (58.1 ± 11.8) > Pb (34.1 ± 14.3) > Cr (32.1 ± 6.77) > Cu (14.5 ± 3.30) > Cd (2.74 ± 1.08) > As (1.49 ± 0.43). The findings pointed out that diversified manmade events enhanced the intensities of elemental contamination through the studied sites. Source analysis showed that Cr, Pb, As, and Cd may originate from industrial wastewater and agricultural activities, whereas Cu and Ni came from natural sources. The geo-accumulation index level for Cd (1.70–3.39) was determined as grade 3 (moderately to strongly polluted), the enrichment factor score for Cd (13.9) fell in the very severe enhanced category (cluster 5), and the highest contamination factor value was found for Cd (15.7). The contamination degree values for all the tested elements signify a moderate to severe contamination grade; conversely, pollution load index levels depicted the nonexistence of elemental pollution. The assessment revealed serious Cd pollution in agricultural soils and moderate to significant potential ecological risk for the rest of the examined toxic elements. Furthermore, hazard index values exceeded the safe exposure levels, indicating that there was potential non-carcinogenic risk in the soils for children and adults. Ingestion exposure had much higher carcinogenic risk values than inhalation and cutaneous exposure, and children are exposed to considerable carcinogenic hazards. Therefore, it is suggested that the harmful practices that expose this farming soil to contaminants should be stopped immediately and effective environment-friendly techniques of waste management and effluent treatment should be employed in the study area.

Graphical abstract

Highlights

• •Enrichment of toxic elements (TEs) in industry-impacted farming soils was assessed.
• •TEs concentrations exceed the standard limit and follow the order Ni > Pb > Cr > Cu > Cd > As.
• •Origin of TEs is both anthropogenic (Cr, Pb, As, and Cd) and natural (Cu and Ni).
• •Severe Cd pollution and moderate ecological risk of TE in farming soils were observed.
• •TEs impose both non-carcinogenic (children & adults) and carcinogenic (children) risks.

1. Introduction

Soil is the earth's permeable surface, frequently referred to as a crucial constituent of the ecosystem and traditionally regarded as a part of the environment prone to heavy metal accumulation [1]. Because of its widespread distribution, noxiousness, and imperishable tendency, trace metal poisoning of soil is a massive conservational issue throughout the biosphere [2]. Farming soil contamination by toxic substances close to manufacturing zones is a major distress in both emerging and industrialized countries, with current ecological repercussions [3,4]. Furthermore, with the accelerated rise of industrialization, urbanization, and increased reliance on herbicides and pesticides, this contamination is becoming more significant [5,6].

Heavy metals are ubiquitous in nature, and they harm natural balance and people's health by accumulating in various organs of the body, such as the renal, skeleton, and hepatic, through vegetable consumption, direct cutaneous interaction, or ingestion [7,8]. Inhalation of chronic low-level soil metal ingestion also poses a major health danger to humans. Many metals are found in soil consequently of a both regular and unnatural action. Heavy metal concentrations in soils are primarily determined by the kind and chemistry of the parent materials from which the soils are produced [9,10]. Human endeavors might extensively intensify metallic concentrations in topsoil. Excess agrochemicals, wastewater watering, air exposure, manufacturing, and other unnatural events are the main manmade sources of heavy metals in cultivated soils [11]. Due to the increasing usage of agrochemicals and inorganic fertilizers, modern farming performances have resulted in agricultural pollution, resulting in the ecosystem and environmental damage [12].

There has been growing emphasis in Bangladesh caused by severe pollution generated by increasing industrialization and the threat of elemental contamination caused by the rapid farmland congestion, engineering expansion, and activity from industries [13,14]. Trace elements can be found in soils in industrial regions as a result of a variety of activities, causing physical, chemical, and biological changes that have a considerable detrimental impact on land productivity [15,16]. The concentration of hazardous materials has been observed in many research conducted in Bangladeshi industrial and agricultural soils [1,11,17].

To measure the eco-toxicological perils of noxious components in topsoil, a variety of indices such as enrichment factor (EF), geo-accumulation index (I_geo), and contamination factor (CF) have been extensively applied [18,19]. The CF, EF, and I_geo of discrete noxious elements in topsoil are intended with their entire amount and dirt excellence guide worth [20]. The pollution load index (PLI) and potential ecological risk index (PER) have also been established to determine the cumulative risk of poisonous metals in soil [21]. The PLI relates the extent of heavy metals to the baseline level, which aids in determining whether harmful metals have accumulated in soil [22,23]. The intense accumulated concentration of toxic elements such as Cr, Pb, Cd, Cu, As, and Ni in the soil can lead to a severe threat to human health through long time exposure. These may cause genetic disorders, cognitive and temperamental decline, less reproductivity, delayed development, nephrotic syndrome, heart disease, liver and kidney damage, tumors, and neurotoxicity [5,24,25]. Cr and Ni can cause several pulmonary diseases, such as emphysema, lung inflammation, fibrosis, and tumors while Cd is poisonous to the kidneys, bones, and cardiovascular system. Thus, the assessment of the health risks of toxic elements is crucial. A potential human health risk assessment includes several approaches, including hazard identification, dose-response analysis, exposure assessment, and risk characterization [5,24,25]. The chronic daily intake (CDI), target hazard quotient (THQ), and hazard index (HI) also were employed for toxic elements to assess the carcinogenic and non-carcinogenic health risks through their multiple exposure routes, including ingestion, inhalation, and skin contact. However, the study into the impending healthiness perils associated with weighty metal effluence in industrial agricultural soil is critical.

In Bangladesh, the Narayanganj is an urban and industrialized region having extensive agricultural activities near the industrial area. This region is distinguished for agronomic output, and it supplies a significant amount of pastoral goods to the rest of the country [26]. However, the industrial growth in this area makes it highly vulnerable to pollution during the previous decade. Garments, leather manufacturing, wrapping diligences, coloring, bar furnaces, metallic garages, dry cell manufacturing, clothing industries, agrochemicals manufacturing, and other foodstuff manufacture diligences are all examples of industrial units lying in this region. It's a heavily industrialized area in Bangladesh that's thought to be extensively contaminated with toxic elements. Particularly, the agricultural land's soil in this area is suspected to be highly contaminated with toxic elements due to the embracing of untreated or partially treated industrial discharge. To understand the contamination level of toxic elements and the possible risks posed by these contaminants in soils from agricultural farms of this region, it is critical to quantify the level of toxic elements pollution in those soils. Even though numerous studies [[27], [28], [29], [30]] have been accompanied to assess biological jeopardy owing to heavy metals contamination of soil in urban and industrialized areas around the world, no systematical comprehensive scientific research on toxic elements in soil and their antagonistic possessions on the ecosystem and public health has been conducted to date, particularly in the industrial area of Narayanganj, Bangladesh. Hence, the purposes of the study were to (i) measure the concentrations of some potentially toxic elements (Cr, Pb, Cd, Cu, As, and Ni) in farming soils, (ii) identify a relationship between toxic element's abundances and their potential sources, (iii) evaluate the contamination level of toxic elements in soils using pollution indices, and (iv) assess the prospective ecological and human health risks of toxic elements in the surrounding agricultural soils impacted by industrial discharge. Moreover, the spatial distribution of hazardous components in agricultural soils was explored in this study in order to monitor the extent of contamination and their potential impact on environmental health. Understanding the likely causes, ecological effects, and health effects of metal toxicity in extensively industrialized soils is crucial for developing effective management and conservation strategies for this terrain. The findings of this study will give decision-makers vital knowledge for achieving the Sustainable Development Goals (SDGs-2030) in this ecosystem by lowering pollution levels and reestablishing terrestrial ecology.

2. Materials and methods

2.1. Description of the study area and sampling sites

The study area of this research was located at the Chashara industrialized zones of the Narayanganj district of central Bangladesh (Fig. 1). The region is roughly 16 km apart from the megacity Dhaka, the capital of the country, along the southeast side and has a population of about 2 million. The climate in this region is described as pleasant in both wet and dry seasons with a yearly rainfall of 2047 mm and an annual temperature of 30 °C [31]. The region is a business and industrial hub, particularly for the jute trade and processing factories as well as enormous levels of textile mills. The study region is mainly encompassed by agricultural lands and several industries, and the soils in the surrounding agricultural lands are becoming increasingly contaminated due to mostly unplanned industrial activities. The situation has gotten so severe that this land is ecologically and physiochemically degraded as a result of the indiscriminate dumping of household, urban, and industrial waste, and the failure of the authorities to safeguard the ecological health of the agricultural lands. We chose this location so that we could assess the potential effects of toxic elements poisoning on both the ecosystem and public health.

Fig. 1.

Map of the study area showing all the sampling sites in the Narayanganj district, Bangladesh.

From the study location, 10 sampling points were carefully selected regarding the expected soil quality and degree of contamination. Site-1 (23°62′35.29″N; 90°48′28.25″E) was surrounded by the garments industry and some paddy fields where the industrial activities discharged directly their untreated or partially treated effluents at these facilities of open lands. Site-2 (23°62′31.77″N; 90°47′73.32″E) was situated close to the Chashara Knit garments and the leading pollution sources of this site mainly include the runoff of industrial effluents. Site-3 (23°62′31.36″N; 90°47′57.23″E) was located just beside the Chashara Knit garments where the discharge from these garments was the core pollution source. Site-4 (23°55′65.67″N; 89°43′90.29″E) was situated in very close proximity to a fertilizer industry in which the industrial wastewater from this industry fell into the nearby agricultural land through the municipal drainage system. Site-5 (23°61′95.35″N; 90°47′93.81″E) was dominated by cement, textile, and pesticide industries; therefore, the industrial discharge along with domestic and municipal wastewater were the major pollution sources in this sampling site. Site-6 (23°59′71.33″N; 90°49′44.47″E) was rigorously influenced by heavy gauge engineerings such as paper-tissue production and pliable trade estate. Municipal discharges were also found to be a significant source of pollution at this site. Site-7 (23°62′68.42″N; 90°46′49.47″E) was situated beside the apparel company where wastes were directly deposited on the open lands. Site-8 (23°62′37.96″N; 90°45′68.47″E) was considered from the residential area which was nearly far from industrial areas, but being a source of pollution by the discharge of domestic wastes. Site-9 (23°61′25.87″N; 90°46′54.72″E) was beside the knitwear and garments factories of Narayanganj Sadar Upazila, and the factories in this area discharged their effluents directly into the open lands. Lastly, Site-10 (23°61′43.57″N; 90°46′70.71″E) was surrounded by food and beverage industries along with cement factories which altogether discharged their wastes into the surrounding agricultural lands and thus contaminate the soil.

2.2. Sample collection and processing

During the dry season (mid-January 2021), 30 soil samples were collected from the selected 10 sampling sites. The dry season is the optimum time to detect elemental contamination because of the reduced rainfall [32]. From every 10 sites, 3 samples were collected; each of the 3 samples was of the composite sample of six randomly taken sub-samples of the cultivated top soils (0–15 cm depth). After the collection, all samples were transported to the laboratory and air-dried for two weeks at room temperature. The dry samples were then crushed in a porcelain mortar with a pestle, homogenized, and sieved through a 2 mm nylon sieve earlier being frozen in an airtight Ziploc bag until chemical analysis [1,11].

2.3. Soil samples digestion, analysis, and quality control

For chemical analysis, about 0.5 g of powdered soil samples were digested using a 15 mL mixture of ultra-pure HClO₄ and HNO₃ (1:2) in a 100 mL glass beaker (Pyrex, Germany) at around 130 °C in a hot plate. When the solution volume approaches about 2–3 mL in a beaker, the digestion of the sample was continued with further addition of 5 mL of the acid mixture until the solution appears to have a clear or light color. Then, digested samples were filtered using filter paper (Whatman no. 41) after cooling and the final volume of the solution was made 20 mL using deionized water. Subsequently, the samples were then subjected to the analysis for elements viz., Cr, Pb, Cd, Cu, As, and Ni using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, Agilent 7500i, USA).

During analysis, the calibration curves were generated through the measurements of the prepared working standards from the mixed certified reference materials (CRM) of the elements purchased from Fluka analytical, Sigma-Aldrich, Germany. The concentration of the individual element was then determined against the separately constructed calibration curve for elements. The standard and samples were prepared using deionized water with conductivity less than 0.2 μS/cm and the acids used for sample preparations were of analytical grades (Merck, Germany). Prior to use, all glassware was washed with 10% HNO₃ solution followed by deionized water. All samples and standards were analyzed three times and the mean values of the individual element's concentrations with relative standard deviation (RSD) of less than 5% were taken into consideration. During measurement in ICP-MS, the accuracy of the analytical results was ensured through the measurement of the CRM whereas the good precision of the results was confirmed through the replicate measurements (RSD < 5%) of the CRM as well as the experimental samples. The results of certified and measured values were Cr (39.1 ± 2.8, 39.5 ± 2.18), Ni (21.8 ± 2.5, 21.9 ± 0.51), Cu (23.1 ± 3.1, 23.3 ± 2.16), As (8.6 ± 1.0, 8.53 ± 0.37), Cd (0.34 ± 0.02, 0.33 ± 0.05), and Pb (31.3 ± 1.1, 31.7 ± 3.11) which showed recovery in the range of 97–101%. The certified and measured values indicated the good precision of the instrumental analysis.

2.4. Pollution level and ecological risk assessment

The geo-accumulation index (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}})$, enrichment factor ($\mathrm{E}\mathrm{F})$, contamination factor $\left(\mathrm{C}\mathrm{F}\right)$, contamination degree ($\mathrm{C}\mathrm{D})$, pollution load index ($\mathrm{P}\mathrm{L}\mathrm{I})$, and potential ecological risk ($\mathrm{P}\mathrm{E}\mathrm{R})$ indices were utilized to evaluate the extent of soil contamination and the associated ecological risk in the study area. The salient features of these indices are described in Table 1.

Table 1.

Description of the pollution evaluation indices and ecological risk assessment indices used in this study.

Index and formula	Descriptions	Standards	References
Geo-accumulation index (${\mathbf{I}}_{\mathbf{g}\mathbf{e}\mathbf{o}})$:	${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$, broadly used for the assessment of metal contamination in soils by comparing the measured concentrations to background concentrations.	Class 0 (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$ ≤ 0): uncontaminated, Class 1 (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$ = 0–1): uncontaminated to moderately contaminated, Class 2 (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$ = 1–2): moderately contaminated, Class 3 (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$ = 2–3): moderately to strongly contaminated, Class 4 (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$ = 3–4): strongly contaminated, Class 5 (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$ = 4–5): strongly to extremely contaminated, and Class 6 (${\mathrm{I}}_{\mathrm{g}\mathrm{e}\mathrm{o}}$ ≥ 5): extremely contaminated.	[1,6,33,34]
${\mathbf{I}}_{\mathbf{g}\mathbf{e}\mathbf{o}}={\mathbf{log}}_{\mathbf{2}}\left({\mathbf{C}}_{\mathbf{n}}/\mathbf{1.5}\times {\mathbf{B}}_{\mathbf{n}}\right)$	${\mathrm{C}}_{\mathrm{n}}$ is the measured concentration of examined metal (n), ${\mathrm{B}}_{\mathrm{n}}$ is the geochemical background concentration of that corresponding metal (n). Factor 1.5 is applied for the probable deviations in background values because of the lithological effect.
Enrichment factor ($\mathbf{E}\mathbf{F})$:	EF is a significant tool in assessing the degree of anthropogenic heavy metal contamination.	$\mathrm{E}\mathrm{F}$ < 1 (no enrichment), $\mathrm{E}\mathrm{F}$ < 3 (minor enrichment), $\mathrm{E}\mathrm{F}$ = 3–5 (moderate enrichment), $\mathrm{E}\mathrm{F}$ = 5–10 (moderately severe enrichment), $\mathrm{E}\mathrm{F}$ = 10–25 (severe enrichment), $\mathrm{E}\mathrm{F}$ = 25–50 (very severe enrichment), and $\mathrm{E}\mathrm{F}$ > 50 (extremely severe enrichment).	[1,11,35]
$\mathbf{E}\mathbf{F}=\frac{\left(\raisebox{1ex}{${\mathbf{C}}_{\mathbf{M}}$}\!\left/ \!\raisebox{-1ex}{${\mathbf{C}}_{\mathbf{A}\mathbf{l}}$}\right.\right)\mathbf{S}\mathbf{a}\mathbf{m}\mathbf{p}\mathbf{l}\mathbf{e}}{\left(\raisebox{1ex}{${\mathbf{C}}_{\mathbf{M}}$}\!\left/ \!\raisebox{-1ex}{${\mathbf{C}}_{\mathbf{A}\mathbf{l}}$}\right.\right)\mathbf{B}\mathbf{a}\mathbf{c}\mathbf{k}\mathbf{g}\mathbf{r}\mathbf{o}\mathbf{u}\mathbf{n}\mathbf{d}}$	$({\mathrm{C}}_{\mathrm{M}}$ and ${\mathrm{C}}_{\mathrm{A}\mathrm{l}}$)Sample is the average concentration of the examined metal in the soil sample, $({\mathrm{C}}_{\mathrm{M}}$ and ${\mathrm{C}}_{\mathrm{A}\mathrm{l}}$)Background is the background concentration used as the reference element, and Al is used as the reference element in this study due to its geochemistry.
Contamination factor$\left(\mathbf{C}\mathbf{F}\right)$and Contamination degree ($\mathbf{C}\mathbf{D})$:$\mathbf{C}\mathbf{F}=\left({\mathbf{C}}_{\mathbf{m}}\right)/\left({\mathbf{B}}_{\mathbf{m}}\right)$	$\mathrm{C}\mathrm{F}$ evaluates surface soil layer contamination.	$\mathrm{C}\mathrm{F}$ < 1: low, 1 ≤ $\mathrm{C}\mathrm{F}$ < 3: moderate, 3 ≤ $\mathrm{C}\mathrm{F}$ < 6: considerable, and $\mathrm{C}\mathrm{F}$ ≥ 6: very high.	[14,[36], [37], [38]]
$\mathbf{C}\mathbf{D}=\sum _{\mathbf{i}=\mathbf{1}}^{\mathbf{n}}\mathbf{C}\mathbf{F}$	${\mathrm{C}}_{\mathrm{m}}$ is the measured concentration of heavy metal in soil, ${\mathrm{B}}_{\mathrm{m}}$ is the preindustrial background concentration of the concerned metal, $CD$ was computed by the sum of the six heavy metals in soils of the study area.	$\mathrm{C}\mathrm{D}$ < 6: low, 6 ≤ $\mathrm{C}\mathrm{D}$ < 12: moderate, 12 ≤ $\mathrm{C}\mathrm{D}$ < 24: considerable, and $\mathrm{C}\mathrm{D}$ ≥ 24: very high.
Pollution load index ($\mathbf{P}\mathbf{L}\mathbf{I})$:	$\mathrm{P}\mathrm{L}\mathrm{I}$ evaluates mutual pollution weight at divergent locations through the dissimilar metals in soils and provided an evaluation of the inclusive toxicity grade of each single sampling location. $\mathrm{P}\mathrm{L}\mathrm{I}$ values of 0, 1, and above 1 mean perfection, the existence of only baseline levels of pollutants, and progressive deterioration of site quality, respectively.	PLI >1 specifies that pollution exists, conversely, PLI< 1 designate that there is a nonexistence of metal pollution.	[1,[37], [38], [39]]
$\mathbf{P}\mathbf{L}\mathbf{I}=\sqrt[\mathbf{n}]{\left({\mathbf{C}\mathbf{F}}_{\mathbf{1}}\times {\mathbf{C}\mathbf{F}}_{\mathbf{2}}\times {\mathbf{C}\mathbf{F}}_{\mathbf{3}}\times \dots .{\mathbf{C}\mathbf{F}}_{\mathbf{n}}\right)}$
Potential ecological risk ($\mathbf{P}\mathbf{E}\mathbf{R})$:${\mathbf{C}}_{\mathbf{f}}^{\mathbf{i}}={\mathbf{C}}_{\mathbf{m}}^{\mathbf{i}}/{\mathbf{C}}_{\mathbf{n}}^{\mathbf{i}}$	${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}\text{,}$ indicates the sensitivity of the biotic community to the toxic elements and exemplifies the $\mathrm{P}\mathrm{E}\mathrm{R}$ caused by the overall contamination. ${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$ is the potential ecological risk coefficient of a single metal, ${\mathrm{C}}_{\mathrm{f}}^{\mathrm{i}}$ is the accumulating coefficient of metal (i), ${\mathrm{T}}_{\mathrm{f}}^{\mathrm{i}}$ is the toxic-response factor of metal (i), ${\mathrm{C}}_{\mathrm{m}}^{\mathrm{i}}$ is the value of heavy metal concentration in the soil samples, and ${\mathrm{C}}_{\mathrm{n}}^{\mathrm{i}}$ is the background value of heavy metals in soils. Toxic-response factors for Pb, Cu, Cr, Cd, Ni, and As were considered 5, 5, 2, 30, 4, and 10, respectively.	${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$ < 40 or PER < 150: low, 40 ≤ ${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$ < 80 or 150 ≤ PER < 300: moderate, 80 ≤ ${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$ < 160 or 300 ≤ PER < 600: considerable, and 160 ≤ ${\mathrm{E}}_{\mathrm{r}}^{\mathrm{i}}$ < 320 or 600 ≤ PER: very high ecological risk for the soils.	[11,14,36,40]
${\mathbf{E}}_{\mathbf{r}}^{\mathbf{i}}={\mathbf{C}}_{\mathbf{f}}^{\mathbf{i}}\times {\mathbf{T}}_{\mathbf{f}}^{\mathbf{i}}$
$\mathbf{P}\mathbf{E}\mathbf{R}=\sum _{\mathbf{i}=\mathbf{1}}^{\mathbf{n}}{\mathbf{E}}_{\mathbf{r}}^{\mathbf{i}}$

2.5. Human health risk assessment

2.5.1. Non-carcinogenic risk

The USEPA recognized carcinogenic and non-carcinogenic risk models to estimate the harmful effects of heavy metals on human health [41,42]. The risk produced by heavy metals in soils can be estimated through numerous pathways of exposure, for instance, hazard identification, dose-response measurement, exposure pathways, and risk characterization [41,43]. The average daily dose (ADD) (mg/kg/day) of a pollutant via direct ingestion (ADD_ing), inhalation (ADD_inh), and absorption exposure pathways could be estimated using equations (1), (2), (3) [41,42,44].

$$\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{i}\mathrm{n}\mathrm{g}=Csoil\times \frac{ingR\times EF\times ED}{BW\times ATnoncarc}\times {10}^{-6}$$ (1)

$$\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{i}\mathrm{n}\mathrm{h}=Csoil\times \frac{inhR\times EF\times ED}{PEF\times BW\times ATnoncarc}\times {10}^{-6}$$ (2)

$$\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{m}=Csoil\times \frac{SL\times SA\times ABS\times EF\times ED}{BW\times ATnoncarc}\times {10}^{-6}$$ (3)

The values of all parameters used in equations (1), (2), (3) are given in Supplementary Table S1. A hazard quotient (HQ), as formulated in equation (4), is the ratio of the potential exposure to a substance and the level at which no adverse effects are expected, where HQ is for a single substance and ΣHQ is for multiple substances.

$$\mathrm{H}\mathrm{Q}=\frac{\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{i}\mathrm{n}\mathrm{g}+\mathrm{i}\mathrm{n}\mathrm{h}+\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{m}}{\mathrm{R}\mathrm{f}\mathrm{D}}$$ (4)

where RfD stands for reference dose and its values are available in Supplementary Table S1. The hazard index (HI), as formulated in equation (5), is a measure of the combined exposure pathway of ingestion, inhalation, and dermal contact doses to non-carcinogenic risk [42].

$$\mathrm{H}\mathrm{I}=\sum HQ\mathrm{i}\mathrm{n}\mathrm{g}+\mathrm{i}\mathrm{n}\mathrm{h}+\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{m}$$ (5)

If HI < 1, this indicates that there is no significant risk of non-carcinogenic effects. If HI > 1, it indicates the probability of adverse health effects with the increase in HI values [41].

2.5.2. Carcinogenic risk (CR)

CR can be used to calculate an individual's lifelong cancer risk. It is calculated by multiplying the cancer slope factor (SF) (mg/kg-day) to the lifetime average daily dose (LADD) (mg/kg-day) linked with ingestion, inhalation, and dermal contact exposed for children and adults using equation (6).

$$\mathrm{C}\mathrm{R}=\mathrm{L}\mathrm{A}\mathrm{D}\mathrm{D}\times \mathrm{S}\mathrm{F}$$ (6)

The following equations (7)–(9) are applicable to determine the LADDs [42,45,46].

$$\mathrm{L}\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{i}\mathrm{n}\mathrm{g}=Csoil\times \frac{ingR\times EF\times ED}{BW\times AT}\times {10}^{-6}$$ (7)

$$\mathrm{L}\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{i}\mathrm{n}\mathrm{h}=Csoil\times \frac{inhR\times EF\times ED}{PEF\times BW\times AT}\times {10}^{-6}$$ (8)

$$\mathrm{L}\mathrm{A}\mathrm{D}\mathrm{D}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{m}=Csoil\times \frac{SL\times SA\times ABS\times EF\times ED}{BW\times AT}\times {10}^{-6}$$ (9)

The SF is associated with the ingestion of Pb, inhalation of Pb, Cd, Ni, and Cr, and dermal contact of Pb. For dermal exposure, SF is not available, but it can be found by multiplying the oral cancer slope by a gastrointestinal absorption factor [43]. All health risks are cumulative [41,47], therefore, the total cancer risk (lifetime carcinogenic risk) was calculated by summing the individual cancer risk in equation (10) [44].

$$\mathrm{T}\mathrm{C}\mathrm{R}=\sum \mathrm{C}\mathrm{R}=\mathrm{C}\mathrm{R}\mathrm{i}\mathrm{n}\mathrm{g}+\mathrm{C}\mathrm{R}\mathrm{i}\mathrm{n}\mathrm{h}+\mathrm{C}\mathrm{R}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{m}$$ (10)

The soil's carcinogenic threats to human health can be regarded as insignificant (CR < 10⁻⁶), significantly tolerable risk (CR = 10⁻⁶ to 10⁻⁴), and a high risk of developing cancer in humans (CR > 10⁻⁴) [44].

2.6. Statistical analyses

Various statistical techniques, such as Pearson's correlation coefficient analysis (CCA) and principal component analysis (PCA) were used in this study to reveal the relationships between the elements studied as well as to identify their plausible sources in agricultural soil using IBM SPSS Statistics 19 [48]. The CCA allows for the evaluation of the strength of element-element connections [13,39]. ArcGIS software was used to create the geographic distribution maps of elements while Microsoft Excel 2013 was used for the rest of the calculations.

3. Results and discussions

3.1. Occurrence and abundance of toxic elements in agricultural soils

Table 2 summarizes the descriptive statistics of the data on the concentrations of the detected elements (Cr, Pb, Cd, Cu, As, and Ni) in agricultural soils obtained from the industrial region of Bangladesh's Narayanganj district. The abundance of Cr, Pb, Cd, Cu, As, and Ni in farming soils ranged from 21.9 to 42.1; 9.89 to 48.6; 1.46 to 4.71; 9.38 to 19.4; 1.01 to 2.17, and 42.9–78.4 mg/kg, respectively. The average amount (mg/kg) of explored ingredients declined with the subsequent downward direction Ni (58.1 ± 11.8) > Pb (34.1 ± 14.3) > Cr (32.1 ± 6.77) > Cu (14.5 ± 3.30) > Cd (2.74 ± 1.08) > As (1.49 ± 0.43) and the findings exposed those quantities were very assorted throughout the inspected state. Disposal of intrinsic waste, home debris, textiles waste, and manufacturing of wastewater together with large transportation loads and construction projects in the Narayanganj area could all be contributing to the diverse variety of elemental contents [14,49]. Moreover, Ni, Pb, and Cr notably exhibit elevated levels than that of other elements which are associated with galvanizing and melting operations in Narayanganj City's manufacturing expenses. A decreased degree of production discharge is most likely associated with a lower quantity of certain potential toxic chemicals in farming soils [50].

Table 2.

Descriptive statistics of toxic element concentration (mg/kg) in agricultural soils collected from Narayanganj, Bangladesh.

Elements	Mean	SD (±)	Min.	Max.	VC (%)	Skewness	Kurtosis	ASV-World	PTE-MPC	SI-TRV
Cr	32.1	6.77	21.9	42.1	21.1	0.227	−1.245	90	200	0.2
Pb	34.1	14.3	9.89	48.6	41.8	−0.767	−1.022	20	300	100
Cd	2.74	1.08	1.46	4.71	39.6	0.710	−0.425	0.3	0.3	10
Cu	14.5	3.30	9.38	19.4	22.8	0.273	−0.825	45	100	32
As	1.49	0.43	1.01	2.17	29.1	0.582	−1.225	13	20	0.25
Ni	58.1	11.8	42.9	78.4	20.3	0.614	−0.914	68	50	100

Note: SD = Standard Deviation, VC = Variance of Coefficient, ASV = World Average Shale Value (background value) by Turekin and Wedephol [53], PTE-MPC = maximum permissible concentrations of potentially toxic elements for agricultural soils of China by CEPA [54], SI-TRV = soil invertebrate toxicity reference values by USEPA [52].

The variance of coefficient (VC) is a valuable variable for introducing dangerous elemental exposure. The higher and lower VC figures respectively suggest that the element excrements are induced unnaturally and natively [51]. VC scores of Cr, Pb, Cd, Cu, As, and Ni in soils obtained from farming lands were perceived as 21.1, 41.8, 39.6, 22.8, 29.1, and 20.3%, respectively (Table 2), and the findings pointed out that diversified anthropogenic events enhanced the intensities of elemental contamination of Pb and Cd all through the studied sites. The soil invertebrate toxicity reference values (SI-TRV) are being used to examine if toxic elements in tillage soils are harmful to soil-dwelling invertebrates [52]. The inquiry findings found that Cr levels were significantly advanced than the USEPA's approved SI-TRV limits. However, in terms of the ecological hazard imposed by elements in the assessed area's cultivated soils, Pb, Cd, Cu, As, and Ni intensities were lower than the SI-TRV, and soil samples characterize an ecological concern. Moreover, the investigation revealed that the average content of Pb and Cd was much greater than the individual ASV-World merits when evaluated to the background levels proposed by Turekin and Wedephol [53]. In contrast, Pb, Cr, Cu, As, and Ni contents were drastically lower than the associated ASV-World levels. Furthermore, except for Cd, the current investigation discovered that Pb, Cu, Cr, As, and Ni concentrations in farmed soils were much below the appropriate PTE-MPC approved by CEPA [54]. Elevated levels of Cd and Pb in tillage soils are caused by abandoned batteries, automobile and manufacturing exhaust emissions, organometallic manufacture, wastewater, compost, chemical application, and so on [14,55].

Supplementary Table S2 compares toxic element concentrations in agricultural soils in Narayanganj City to those in other cities throughout the world, as well as other standards/guidelines. The fact that toxic elements are distributed differently in dissimilar towns could be attributed to the fact that each city may have its own unique set of elemental affinities [39]. Toxic element concentrations in agricultural soils in industrially growing cities, such as Kushtia-Jhinaidah are notably lower than those found in the current study, even though Cu and As concentrations are combatively advanced. Except for Cu and As, most elemental contents in Tangail City's tillage soils are substantially lower than in the present study. The level of toxic elements in the present study is significantly lower than those observed in farming soils in Dhaka, a heavily industrialized city. The Noakhali region of Bangladesh is located on the coast and has the lowest concentration of toxic elements (excluding Ni, Cu, and As) in cultivated soils compared to the present study. The concentrations of Cr, Cu, As, Cd, and Pb in soils in the north-Bengal district of Bogra are consistently elevated, although Ni extent is typically decreased. Concentrations of elements in cultivated soils are shown to be greater for Cu and Pb in Guandong, higher for Pb in Murcia and Kayseri, and higher for all metals in Maharastra than those in the present investigation. Overall, average concentrations of Ni and Cd were found to be significantly greater than the Dutch Soil Quality Standard (target value) and Canadian Environmental Quality Guidelines, whereas the average concentrations of all other elements were encountered to be less than that of the Dutch Soil Quality Standard (intervention value) and the Australian Guideline for Soil Quality. The diversified proportions of toxic substances could be linked to particular actions [50]. On the other hand, the agricultural soil of Dhaka followed by Maharastra and Bogra is more polluted by toxic elements than the soils of other considered areas throughout the globe. The concentrations of Ni, Cu, and Cr appear to have the biggest magnitude of all the toxic elements explored in all districts, presumably resulting from manufacturing advancements such as metallurgical, plating, compound, and energy burning in industrial districts [39,51].

3.2. Spatial distribution of toxic elements in agricultural soils

Fig. 2 depicts the spatial distribution maps of the evaluated toxic elements. Because of notable influences such as congested transportation, exhaust fumes, farming usage of agrochemicals, and atmospheric deposition, the agricultural soils of the exploration region are consistently disturbed by numerous elements [14,56]. In this study, the highest mean concentration of Cr (42.1 mg/kg) was observed at Site-5, and the lowest Cr concentration (21.9 mg/kg) was found at Site-4. Because Cr salt is utilized in the textiles, raw textile polluted water can be discharged to croplands in the examined areas [57]. Cr has a deleterious effect on plant development, interfering with certain key biochemical activities [58]. The highest Pb content (48.6 mg/kg) was identified at Site-5, while the lowest amount (9.89 mg/kg) was noted at Site-1 in the current test. Pb levels are elevated in Site-5 soils as a result of metallic factories leaking Pb into the immersive area, as well as other manmade influences [1,59]. Sites-4 and Sites-3 had the highest (4.71 mg/kg) and lowest (1.46 mg/kg) Cd quantities, respectively. Manufacturing activity, metal treatment, meteorological dispersion, and Cd-coated items could all contribute to increasing Cd levels in topsoil [60]. Site-3 had the largest expanse of Cu (19.4 mg/kg) whereas Site-10 had the least extent. Increased Cu levels from Cu-enriched effluents are highly toxic to plants and certain microbes are harmed by this Cu level [1,58]. Site-2 had the greatest proportion of As (2.17 mg/kg) whereas Site-3 had the smallest (1.01 mg/kg). Arsenic in cultivated fields can come from both natural and manmade sources, such as irrigation with arsenic-contaminated underground water and excessive usage of As-enhanced agrochemicals [6,61]. Site-8 had the maximum Ni level (78.4 mg/kg) whereas Site-3 had the minimum Ni content (42.9 mg/kg). This finding revealed increased Ni contents as a consequence of rapid metallurgy and electroplating operations or unintentional leakages of Ni-inclosing products [1].

Fig. 2.

Spatial distribution of toxic element concentrations in agricultural soils of Narayanganj, Bangladesh.

As per the findings, the largest accumulation of toxic elements is found at Site-5, followed by Site-2, Site-6, Site-7, and Site-8, presumably attributable to regional pollution bases [50]. Such geographical attributes could promote particulate deposition of toxic elements in Site-5 topsoil. Then again, agricultural soils in the study sites were extremely polluted by the examined (Cr, Pb, Cd, Cu, As, and Ni) toxic elements. Additional causes of elemental excrements in these study areas include traffic situations, landscape usage, manufacturing trends, landforms, and density of population [39].

3.3. Identification of the sources of toxic elements in agricultural soils

Evaluating toxic element's origins can help elucidate their dispersion. The connection and incidence of toxic elements could be investigated using Pearson's correlation analysis and Principal components analysis (PCA) [1,39,50]. Pearson's correlation analysis was used to understand the relationships between toxic elements and to determine the key aspects influencing elemental contamination transport and dispersal [60]. Supplementary Table S3 shows the results of Pearson's correlation matrix for the analyzed toxic elements in soils. Positive correlations were found between Cr and Pb, and Cr and As, although negative correlations were found between Pb and Cd. The positive association between the elemental pairs indicates that the elements were interconnected and may have come from identical sources in the study area [50]. Other correlations between soil components were not found to be noteworthy.

The PCA with perpetual facts was used to accurately assess the element scores determined in soil samples (Fig. 3). Depending on eigenvalues greater than 1, three principal components were demonstrated, accounting for 79.3% of the systemic variance. PC-1 accounted for 39.5% of the variance as shown in Fig. 3, and the eigenvalue was 2.37, implying a modest favorable connection in Cr (0.255) and Pb (0.255) that is presumably from a likely cause such as lithogenic, as also shown in correlation test. Because of the weaker beneficial connection with Cr and Pb, PC-1 might be classified as a natural cause [44]. The PC-2 had a variance of 23.5% and an eigenvalue of 1.41, showing that As (0.461) and Cu (0.514) have a significant positive relationship. The biogeochemical reliance of As and Cu from pedogenic genesis is established, just as it is in the iron series [44], according to PC-2. Extracellular release, for instance, animal feces is believed to cause substantial growth in Cu concentrations in the studied area's topsoil [50]. The PC-3 had a variance of 16.3% and an eigenvalue of 1.00, indicating a weaker favorable association for Cd (0.099) and Ni (0.920). In the study area, the artificial inputs of Cd and Ni revealed a mix of lithogenic and human activities [14]). In ecological risk analysis, Cd is by far the greatest harmful chemical of contamination, posing considerable ecological complications. The indiscriminate use of nourishments (phosphate) is to blame for the global dissemination of Cd [1]. Generally, it shows that human impacts in the exploration expanse territory such as textiles, various manufacturing units, localized incinerations, and compost and chemical treatments have a massive effect on Cr, Pb, As, Cu, Cd, and Ni [39,44]. As per the assessment, there seems to be a significant difference between the surface soils in the investigated area.

Fig. 3.

Principal component analysis (PCA) of toxic elements in agricultural soil collected from Narayanganj industrial area of Bangladesh.

3.4. Pollution level and ecological risk assessment for agricultural soil

Toxic element contamination levels in agricultural soil in the Narayanganj district were assessed using the geo-accumulation index (I_geo), and the obtained I_geo scores for the elements tested are provided in Supplementary Fig. S1. Average I_geo scores of the analyzed elements exhibited a declining direction of Cd (2.51) > Pb (0.02) > Ni (−0.84) > Cr (−2.10) > Cu (−2.26) > As (−3.77). Based on the Muller (1981) classifications [33], I_geo merits for Cd were determined as grade 3 (moderately to strongly polluted), for Pb, it was determined as grade 1 (unpolluted to moderately polluted), and for Ni, Cr, Cu, and As, it was determined as grade 0 (unpolluted). Results also exposed that I_geo scores were found highest for Cd (3.39) at Site-4 whereas the lowest I_geo was found for As (−4.27) at Site-3. The study depicted that the farming soil samples were strongly contaminated (I_geo = 2.00–3.00) through Cd contamination among the analyzed elements. The greatest I_geo level of Cd across the study sites of the Narayanganj industrial region could be attributable to the combustion of fossil fuels, the use of phosphate-based agrochemicals, sewage effluent additives, and manufacturing exoneration [14,44,50].

The enrichment factor (EF) is a useful metric for estimating pollution levels in contaminated sites [39]. In the present study, Cd had the largest EF average of 13.9, and dropped in the succeeding direction of Pb (2.61) > Ni (1.31) > Cr (0.544) > Cu (0.491) > As (0.175) (Supplementary Fig. S2). Average EF scores for Cu, As, and Cr appertain to cluster 1 or no enhanced; for Ni, cluster 2 and 3; and Pb was defined as a minor enhanced in cluster 2 possibly because of the presence of banging automobile garages, dense traffic, insect killer and fertilizer industry, scuffle supplies, and motor vehicle workspaces [6]. On the contrary, EF scores for Cd fall into very severe enhanced (cluster 5). The study's findings demonstrated that most of the soils were significantly contaminated with Cd. The leading reasons for this are the potential consequences of manufacturing and battery-powered plants in the study regions [60].

Table 3 shows the contamination factor (CF) for all the analyzed toxic elements. Average CF scores for the tested elements moved in a downward direction of Cd (9.13: very high) > Pb (1.71: moderate) > Ni (0.855: low) > Cr (0.356: low) > Cu (0.322: low) > As (0.114: low) and the study quantified that farmed soil of the investigated areas was disposed of with toxic elements. The maximum CF value was found for Cd (15.7) at Site-4 which might be due to unnatural causes (manufacturing discharges and fertilizer application) and the minimum CF value was found for As (0.078) at Site-3. Most of the sampling sites were exposed to moderate to low contamination in cultivated soil, representing a potential risk to the adjacent ecosystems [19]. The degree of contamination (CD) is employed to define the entire pollution intensities of toxic elements in tillage soils [62]. Table 3 shows the CD scores intended in farming soil samples. At Site-4, the highest CD value was found to be 17.8, followed by Site-9, Site-2, and Site-8. On the other hand, Site-3 has the lowest CD value of 8.32. The CD scores for all experienced metals in all studied sites extended from 6.00 to 18.0, signifying a moderate to a significant amount of contamination that ultimately might damage ecological stability [1]. According to the analysis, the present level of toxic element contamination in surface soils can be traced back to both biogeochemical and man-made sources [6,17]. The PLI (Pollution load index) is a quantitative representation of toxic element exposure in a particular sample that is proportional to the number of times toxic element levels in the soil exceed geochemical background quantities [60]. The dignified merits of the PLI in the farming soil samples are illuminated in Table 3. The PLI merits of all the tested toxic elements diverged from 0.589 (Site-1) to 0.888 (Site-2). The results depicted the nonexistence of elemental pollution in soils in terms of PLI but the growing number of expanded industries may cause advanced declinations of soils and as a result, PLI value will be increased [63].

Table 3.

Contamination factor (CF), contamination degree (CD), and pollution load index (PLI) of toxic elements in soil collected from Narayanganj district, Bangladesh.

Site	Contamination factor (CF)						CD		PLI
	Cr	Pb	Cd	Cu	As	Ni	Value	Status	Value	Status
Site-1	0.300	0.495	8.27	0.348	0.091	1.08	10.6	Moderate	0.589	Absence
Site-2	0.450	1.34	12.1	0.428	0.167	0.931	15.5	Considerable	0.888	Absence
Site-3	0.351	1.97	4.87	0.430	0.078	0.630	8.32	Moderate	0.643	Absence
Site-4	0.243	0.687	15.7	0.266	0.088	0.779	17.8	Considerable	0.603	Absence
Site-5	0.468	2.43	7.27	0.301	0.135	0.768	11.4	Moderate	0.798	Absence
Site-6	0.396	2.34	4.90	0.378	0.163	0.721	8.90	Moderate	0.766	Absence
Site-7	0.427	2.16	7.73	0.277	0.082	0.991	11.7	Moderate	0.736	Absence
Site-8	0.301	2.24	9.00	0.268	0.112	1.15	13.1	Considerable	0.771	Absence
Site-9	0.309	1.23	13.7	0.315	0.133	0.778	16.5	Considerable	0.744	Absence
Site-10	0.315	2.18	7.73	0.208	0.095	0.714	11.2	Moderate	0.650	Absence
Mean	0.356	1.71	9.13	0.322	0.114	0.855	12.5	Considerable	0.589	Absence
Min.	0.243	0.495	4.87	0.208	0.078	0.630	8.32	Moderate	0.888	Absence
Max.	0.468	2.43	15.7	0.430	0.167	1.15	17.8	Considerable	0.643	Absence

Table 4 outlines the computed potential ecological risk factors (Eⁱ_r) of toxic elements, which were used to estimate the potential ecological risk (PER) levels in agricultural soil samples. Cd had the greatest Eⁱ_r score of 471 in agricultural soil at Site-4 in the Narayanganj region. Elevated concentrations of Cd might be related to manufacturing wastewater disposal and the widespread use of agrochemicals (phosphate fertilizers) in the studied area's tillage soils [1,14]. The mean Eⁱ_r scores for Cr, Pb, Cd, Cu, As, and Ni were 0.712, 8.54, 274, 1.61, 1.14, and 5.13, respectively. The Eⁱ_r scores for dignified elemental contamination were positioned in the succeeding direction of Cd > Pb > Cr > Ni > Cu > As. The PER merits fluctuated from 163.2 to 481.8. The soils of Site-2, Site-4, and Site-9 have PER >300, demonstrating considerable ecological risk from toxic elements pollution. Soils in the other sites pose a moderate ecological risk for toxic elements that have been explored. The results of the study exposed the merits of PER specifying that cultivated soils of the study area pretense moderate to considerable potential ecological risk. This could be owing to the impact of irregular events at the local scale such as a massive amount of manufacturing facilities and the excessive consumption of agrochemicals [64].

Table 4.

Potential ecological risk factors (Eⁱ_r) and potential ecological risk (PER) of toxic elements in soil collected from Narayanganj district, Bangladesh.

Site	Potential ecological risk factors (Eir)						PER
	Cr	Pb	Cd	Cu	As	Ni	Value	Status
Site-1	0.599	2.47	248	1.74	0.908	6.48	260.2	Moderate
Site-2	0.900	6.72	364	2.14	1.67	5.59	381.0	Considerable
Site-3	0.703	9.82	146	2.15	0.777	3.78	163.2	Moderate
Site-4	0.486	3.43	471	1.33	0.885	4.67	481.8	Considerable
Site-5	0.936	12.2	218	1.50	1.35	4.61	238.5	Moderate
Site-6	0.792	11.7	147	1.89	1.63	4.33	167.3	Moderate
Site-7	0.854	10.8	232	1.38	0.815	5.95	251.8	Moderate
Site-8	0.602	11.2	270	1.34	1.12	6.92	291.1	Moderate
Site-9	0.618	6.15	412	1.57	1.33	4.67	426.3	Considerable
Site-10	0.631	10.9	232	1.04	0.954	4.28	249.7	Moderate
Mean	0.712	8.54	274	1.61	1.14	5.13	291.1	Moderate
Min.	0.486	2.47	146	1.04	0.777	3.78	163.2	Moderate
Max.	0.936	12.2	471	2.15	1.67	6.92	481.8	Considerable

3.5. Evaluation of human health risk

The calculated hazard quotient (HQ) and hazard index (HI) of toxic elements of the non-carcinogenic hazard exposure for both children and adults in the studied Narayanganj district region via ingestion, inhalation, and dermal contact are presented in Table 5. In summary, the average exposure route of toxic elements for children and adults was decreasing in the order of HQ_ingestion < HQ_dermal < HQ_inhalation [46]. In the present study, oral exposure is higher than dermal as well as inhalation. Additionally, similar studies were found around the world where for children HQ (via all routes of exposures to studied elements) shows higher than adults [41,65,66]. The non-carcinogenic risk (HQ) of toxic elements were decreasing in the order of Cr > Pb > As > Cd > Ni > Cu for both adult and children. The HQ_{ing + inh + derm} for all the elements was >1 for children and adults, signifying that non-carcinogenic risks were found in the study area. HI was a single-element summation of hazard quotients of three exposure pathways (ingestion, inhalation, and dermal contact). Besides, integrated HI values (ΣHI) were the combined determination of all toxic elements which is more prominent to find an adverse effect [42]. However, the HI and ΣHI results for all toxic elements were higher than the safe limit (HI > 1), signifying non-carcinogenic risk in agricultural soil. Additionally, the ΣHI values for all elements for both adults and children crossed the exposure limit which might adversely affect human health [46]. As a result, non-carcinogenic illnesses, and chronic disorders such as lack of appetite, nausea, and headache might be adversely affected human health [[67], [68], [69]].

Table 5.

Non-carcinogenic risk of HQs and HIs of the toxic elements in the soil in children to adults.

Risk parameters	Cr	Pb	Cd	Cu	As	Ni
RfDing	3.00E-03	3.50E-03	1.00E-03	4.00E-02	3.00E-04	2.00E-02
RfDinh	2.86E-05	3.52E-03	1.00E-03	4.02E-02	3.01E-04	2.06E-02
RfDderm	6.00E-05	5.25E-04	1.00E-05	1.20E-02	1.23E-04	5.40E-03
HQing (Child)	1.37E-01	1.25E-01	3.51E-02	4.63E-03	6.34E-02	3.72E-02
HQing (Adult)	1.46E-02	1.34E-02	3.75E-03	4.96E-04	6.79E-03	3.98E-03
HQinh (Child)	4.00E-04	3.46E-06	9.78E-07	1.29E-07	1.76E-06	1.01E-06
HQinh (Adult)	2.25E-04	1.95E-06	5.51E-07	7.24E-08	9.93E-07	5.67E-07
HQderm (Child)	1.91E-02	2.33E-03	9.81E-03	4.32E-05	4.33E-04	3.85E-04
HQderm (Adult)	2.92E-02	3.56E-03	1.50E-02	6.60E-05	6.61E-04	5.89E-04
HI (Child)	1.56E-01	1.27E-01	4.49E-02	4.68E-03	6.39E-02	3.76E-02
∑HI (Child)	4.34E-01
HI (Adult)	4.41E-02	1.69E-02	1.87E-02	5.62E-04	7.45E-03	4.57E-03
∑HI (Adult)	9.23E-02

3.6. Carcinogenic risk assessment

The CR (carcinogenic risk) for Cr, Pb, Cd, and Ni was assessed through ingestion, inhalation, and dermal mode based on slope factor availability (Table 6). The CR for Cr, Pb, Cu, and As was assessed using ingestion and dermal exposure, while the CR for Cr, Pb, Cd, As, and Ni was calculated via inhalation mode (Table 6). The toxic elements exposure for dermal contact is quite a lot of times lesser than ingestion and inhalation of soil exposure [43]. The slope factor and soil exposures have been used to determine the human health risk assessment [41,45,67]. The CR values of ingestion (Cr, Pb, Cu, As) and dermal exposure (Cu) in children and adults of all carcinogenic toxic elements were found within potentially significant risk limits (10⁻⁶ to 10⁻⁴) indicating a potential risk for the development of cancer in humans. In addition, CR values of toxic elements for ingestion and dermal exposure were alluringly progressive carcinogenic risk than inhalation. As per this study, children have access to considerable carcinogenic threats as a result of toxic elements exposure, which is now a real concern.

Table 6.

Carcinogenic risks of the toxic elements in the soil in children to adults.

Carcinogenic risk assessment
		Children	Adult
Toxic elements	Slope factor	CR for ingestion
Cr	5.00E-01	2.05E-04	2.20E-05
Pb	8.50E-03	3.71E-06	3.98E-07
Cu	1.70E+00	3.15E-04	3.37E-05
As	1.50E+00	2.86E-05	3.06E-06
		CR for inhalation
Cr	4.10E-01	4.69E-09	2.64E-09
Pb	4.20E-02	5.12E-10	2.88E-10
Cd	6.30E+00	6.16E-09	3.47E-09
As	1.51E+01	8.02E-09	4.51E-09
Ni	8.40E-01	1.74E-08	9.81E-09
		CR for dermal
Cr	5.00E-01	5.74E-07	8.76E-07
Pb	8.50E-06	1.04E-11	1.59E-11
Cu	4.25E+01	2.20E-05	3.37E-05
As	3.66E+00	1.95E-07	2.98E-07
ΣTCR		5.75E-04	9.40E-05

4. Conclusions

In this work, the contamination level, distribution, sources, and risk assessment of six concerned toxic elements (Cr, Pb, Cd, Cu, As, and Ni) in soils of the agricultural land near the industrial areas of central Bangladesh (Narayanganj district) were explored. The findings of this study revealed that the mean concentrations of the elements in the selected 10 sampling sites were higher than the Dutch standard and Canadian quality guideline values. Most of the soil samples were substantially contaminated with Cd, as evidenced by the contamination factor, geo-accumulation index, and potential ecological risk. The spatial distribution of toxic elements throughout the sampling sites indicated a range of concentrations from extremely low to very high. Correlation and principal component analysis demonstrated that the elements such as Cr and Pb more likely originated from a lithogenic or natural source. Cd, Cu, As, and Ni in the studied soil may be originated from mixed sources of lithogenic/natural and artificial inputs by a human. Ecological risk analysis showed that the agricultural soils of the study area embraced moderate to considerable ecological risk. The hazard quotient (HQ > 1) and hazard index (HI > 1) of the tested toxic elements revealed significant non-carcinogenic risks for both children and adults in the study area. The carcinogenic risk of ingestion and dermal exposure to the elements also imply a potential cancer risk in both children and adults. Thus, long-term exposure to these hazardous components is feared to constitute both potential ecological and human health risks in the Narayanganj district of Bangladesh's industrial area.

Therefore, regular monitoring of the elemental contamination level in the agricultural soil of the study area is recommended to protect the ecology and the surrounding environment. For a sustainable environment, effective preventive actions should be ensured immediately to reduce the current elemental pollution in the studied region. The findings of our study also advised that (i) industrial discharge adherence to the agricultural lands should be well regulated and (ii) public awareness and education about the origins and harmful environmental consequences of toxic elements should be enhanced. This study considers only six elements which sometimes may not clearly depict the overall elemental pollution scenario in the study area. Thus, further research should be conducted considering some additional toxic elements particularly Hg, Se, Mn, Fe, Co, and Zn due to their toxicity and common persistence in industrial discharge.

Author contribution statement

Fahmida Najnin Moni, Md. Samir Ahmed Miazi, Rifat Shahid Shammi, Md. Eusuf Sarker, and Md. Mehedi Hasan Khan: Performed the experiments; Wrote the paper.

Md. Humayun Kabir: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.

Md. Sirajul Islam: Contributed reagents, materials, analysis tools or data.

Md. Shafiqul Islam and Md. Shakir Ahammed: Analyzed and interpreted the data.

Md. Abu Bakar Siddique and Tapos Kormoker: Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Data availability statement

Data included in article/supp. material/referenced in article.

Additional information

Supplementary content related to this article has been published online at [URL].

Declaration of interest's statement

The authors declare no competing interests.

Appendix A. Supplementary data

The following is the Supplementary data to this article.