Introduction

Heavy metal(loid)s (HM) are hazardous contaminants owing to their persistent, bio-accumulative, and toxic behavior in the environment (Dhandhayuthapani, 2022; Ustaoğlu et al., 2022). Among HM, certain metals like copper (Cu), iron (Fe), and zinc (Zn) in a specific amount are necessary for the health of human beings (Assunção et al., 2022; Weyh et al., 2022). However, their higher concentration may cause toxicity in humans and other living beings. HM including cadmium (Cd), cobalt (Co), chromium (Cr), lead (Pb), mercury (Hg), and arsenic (As) are toxic even in tiny quantities and cause various health problems (Gu et al., 2022; Jayamurali et al., 2021). For example, hypertension, headache, nausea, fever, renal failure, and bone fractures are all symptoms of Cd poisoning (Baghaie & Fereydoni, 2019; Rehman et al., 2018; Yu et al., 2021), while Cr poisoning causes stomach pain, vomiting, nausea, prostration, liver, and kidney disorders (Doabi et al., 2018). Polycythemia, abnormal thyroid arteries, respiratory problems, and overproduction of red blood cells are all adverse symptoms of Co (Doabi et al., 2018; Packer, 2016).

Soil is an important component of the environment for agriculture production and human survival (Drobnik et al., 2018, Rajendran et al., 2022, Tokatli & Islam, 2022). The HM enters the soil via natural sources such as disintegrating as well as erosions of ore deposits and bed-rocks (Muhammad & Ullah, 2022, Ullah & Muhammad, 2020) and anthropogenic including mining, metallurgy, wastewater irrigation, traffic, industrial emission, and agrochemical application (Bakht et al., 2022; Cruzado-Tafur et al., 2021; Guadie et al., 2021; Rajendran et al., 2022). These HM are neutralized to a specific level by the soil via its buffering capacity (Kicińska et al., 2022; Stigliani, 1996); however, if that level exceeded the critical load, then the soil becomes a source rather than a sink (Batjes & Bridges, 1993, Fontes & Gomes, 2003). These HM accumulate in the growing food, contaminate the food chain and cause threats to ecological and human life (Bawwab et al., 2022; Rubio-Gracia et al., 2022; Tokatli, 2019).

Heavy metal(loid)s had threatened the ecological environment and human life via their high mobility, poor degradation ability, and toxicity in the environment (Yanez et al., 2002). Recently, the pollution quantification factors (Hao et al., 2022), geospatial distribution (Mandal et al., 2022), ecological (Ren et al., 2022; Varol et al., 2022; Yüksel et al., 2022), and health risks assessment (Aguilera et al., 2021) techniques have been used for the formulation of environmental protection policies. HM contaminations and potential risks of soil have been sporadically studied in Pakistan (Jadoon et al., 2020a; Saddique et al., 2018). However, there is a lack of comprehensive study of HM contaminations in soil, pollution factors, and risk assessments. Therefore, it is imperative to investigate HM contamination levels at a national level for pollution factors, associated risks, and distribution patterns. HM concentrations published in the last three decades were collected and evaluated for the pollution factors using single and integrated indices to present a comprehensive environmental picture.

Study area description

Pakistan is located in the South Asian subcontinent (23° to 37.5° N of latitudes & 61° to 78° E of longitudes). It shares a border with China in the north, in the south with the Arabian Sea, India in the east, and in the west with Iran and Afghanistan (Fig. 1). The country holds a diverse type of topography varying prominently from region to region (Shahzad et al., 2021). The series of Himalaya-Karakoram-Hindu Kush is acknowledged as the natural water reservoir and vital for life sustenance of South Asia having an enormous number of glaciers, lakes, rivers, and the resultant tributaries (Archer & Fowler, 2004). Toward its North and North-West, mountains with highest altitudes are located which remain too cold during winter, while in summer (April–September), they remain very pleasant. Moreover, the plain areas (of the Indus-Valley) remain very hot during summer and persist cold during winter. The coastal areas mostly remain temperate (Karim et al., 2021). The annual average rainfall fluctuates from 13 cm in the lower parts of the Indus plains to 89 cm in the Himalayan areas.

Fig. 1
figure 1

Location of the studied sites in the study area (modified after Soil Survery of Pakistan)

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Tectonics and geology of Pakistan

The Pakistan-tectonic has comprised two convergent boundaries: an active collision zone (continent–continent collision) and a lithospheric subduction zone (oceanic) in the northeast and southwest, respectively. These collision boundaries are associated with the Chaman Transform zone, a huge north–south displacement, i.e., strike-slip faults (Kazmi & Jan, 1997). Geologically, Pakistan is comprised of two major basins (sedimentary in origin), namely Indus Basin and Baluchistan Basin which developed across distinct geological periods. These two basins were combined jointly during Cretaceous and Paleocene time along the Chaman fault (Kazmi & Abbasi, 2008; Malkani, 2015). The Indus Basin occupies the eastern part of the country and is situated on the western side of the Indian Plate. The Indus Basin consists of the Precambrian to Cenozoic sediments, whereas Baluchistan occupies the western part of the country and is located on the southern margin of the Eurasian Plate. The active continental margin of Baluchistan represents an arc-trench system west of the Chaman Ornach Transform Fault zone. The exposed rocks (sedimentary) in the Baluchistan basin are of the Cretaceous-Quaternary period (Kadri, 1995; Kazmi & Jan, 1997).

Data management

For this study, scientific publications published over the last 30 years focusing on HM concentrations in the soil of Pakistan were collected (Table 1). The research articles were searched over different search media like Springer, Science Direct, Willey, Mendeley, ResearchGate, Google Scholar, etc. The data were compiled by means of range and average of HM as described in the literature.

Table 1 Heavy metal(loid)s concentrations (mg/kg) in soils of previously reported studies in Pakistan
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Pollution quantification factors

Contamination factor

The contamination factor (CF) values of the selected HM were calculated using Hakanson (1980) Eq. (1), which is the ratio of the concentration of HM to its reference or background values. The background values are 1 mg/kg for (Cd), 29 (Co), 90 (Cr), 50 (Cu), 35,900 (Fe), 750 (Mn), 68 (Ni), 70 (Pb), 175 (Zn), 1.5 (Mo) and 15 (As) (Hakanson, 1980; Ma et al., 2016; Raju et al., 2012) and are classified by Jiao et al. (2015) into five classes (Table 2).

$${\text{CF = Soil concentration in studies/Background}}$$
(1)
Table 2 Parameters along with abbreviations, values, and symbols used for risk assessment
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Geo-accumulation index

Geo-accumulation index (Igeo) is also calculated individually for HM and can be obtained by using Eq. (2). It gives valuable information about the severity and range and abundance of HM contents in the soils of studied sites (Muller, 1969) and their classification is given in Table 2.

$${\text{Igeo }} = \, \log 2\left[ {{\text{Cn}}/ 1.5 x {\text{ Bn}}} \right]$$
(2)

Here, Cn represents the calculated concentration of HM and Bn represents their background concentration (Nazeer et al., 2014):

Potential ecological risk index

The potential ecological risk index (PERI) was introduced in studies (Hakanson, 1980; Håkanson et al., 1988) to assess the contamination level in soil, toxicity based on HM, and the environmental response (Wu et al., 2015). The PERI is calculated by the summation of risk values (Eq. 5). Its basic purpose is to recognize the contaminants variety and to assess threats (ecological) triggered by HM (Soliman et al., 2015) in the studied sites.

$${\text{PERI}} = \sum\limits_{i = 1}^{n} {{\text{ERI}}}$$
(3)
$${\text{ERI}} = {\text{Tr }} \times {\text{CF}}$$
(4)

whereas ERI is the monomial-ecological risk value.

It characterizes the toxicity of particular metals and calculates the potential threats of HM in the studied sites and the environmental sensitivity to the contaminants/pollutants. Hakanson (1980) estimated the toxic response (Tr) values for Cd, Cr, Pb, Zn, Ni, Cu, and As as 30, 2, 5, 1, 5, 5, and 10, respectively. The Tr factor values for Mo, Co, and Mn are 15, 5, and 1, respectively (Lu et al., 2015). The categorization of PERI and ERI is shown in Table 2.

Human health risk assessment

The non-cancerous and cancerous risks (CR) of HM in soil exposure have been calculated for humans of various age groups, i.e., adults and children, using equations, adopted from the studies (Li et al., 2014; Li et al., 2013a; Sun et al., 2013; USEPA, 1989a; Zheng et al., 2013).

The average daily dose (ADD) for exposure measurement was determined for each HM in studied soils. The ADD via ingestion, inhalation, as well as dermal absorption was calculated by the USEPA (1989b) Eqs. (7–9):

$${\text{ADD}}_{{{\text{ing}}}} = \frac{{C*IngR x {\text{EF}}x{\text{ED}}x{\text{C}}F}}{{{\text{BW}}x{\text{AT}}}}$$
(5)
$${\text{ADD}}_{{{\text{inh}}}} = \frac{C*InhR x EFxED}{{BWxATxPEF}}$$
(6)
$${\text{ADD}}_{{{\text{derm}}}} = \frac{C*SAx AFx EVxABSx EFxEDxCF}{{BWxAT}}$$
(7)

All the limits and other estimated parameters used in the computation are based on the USEPA issued data and other relevant literature (Izhar et al., 2016; Li et al., 2013b; Verla et al., 2020).

Non-carcinogenic risk

The non-carcinogenic risk such as hazard quotient (HQ) was calculated by Eq. 10, i.e., dividing the quotient of ADD by the reference dose (RfD) of respective HM (USEPA, 1989b).

$$HQi \, = \, \Sigma \, ADDi \, / \, RfDi$$
(8)

The hazard index (HI) is the summation of HQ (Zheng et al., 2010); for all the sites, it was calculated using Eq. (11) (USEPA, 1989b).

$$HI \, = \, \Sigma \, HQi$$
(9)

According to Zheng et al. (2010), if HI is less than 1, it is considered safe (no significant effects), while if HI is greater than 1, there is a greater probability of non-cancerous effects to exist.

Carcinogenic risk

The carcinogenic risk or CR was calculated by Eq. 12 (USEPA, 1989b).

$$CR \, = \, \Sigma ADDi \, x \, CSF$$
(10)

Here CSF is the carcinogenicity-slope factor value (Table 2).

Statistical analyses

The graphical presentation of published HM concentration data in soil was accomplished utilizing sigma plot ver. 12.5 (San Jose, California, USA), and spatial distribution maps were produced using ArcMap (Version 10.3).

Results and discussion

The HM is the Earth-crust's natural constituent and is unable to be destroyed/degraded (Komijani et al., 2021). HM encounter their path to human beings through ingestion or inhalation or consumption of contaminated food. Long-time exposure to HM may cause both cellular and neurological problems/disorders and can also cause several kinds of cancers in humans (Khalid et al., 2020; Rehman et al., 2020a).

Heavy metal(loid)s

Average HM concentrations in various provinces and federal areas are summarized in Fig. 2a. Among HM concentrations, the highest mean was observed for Fe in Baluchistan province and the lowest for Cd in Sindh province. Results revealed that the Baluchistan province also showed maximum concentrations of Cr, Mn and Ni compared to other provinces. A higher level of HM contaminations in the Baluchistan province soil may be due to the exposed ore deposits in a mineralized zone (Arif, 2000; Bilgrami, 1969). It mostly consists of ophiolites, dunites, serpentinites, chromites, peridotites, and harzburgites cut dolerites as well as redingote dikes (Bilgrami, 1969, Bilgrami & Howie, 1960). Soil originating from mafic/ultramafic rocks may have a higher amount of HM (Muhammad et al., 2019).

Fig. 2
figure 2

Soil average heavy metal(loid)s, a concentrations (mg/kg), b contamination factor, and c geo-accumulation index values, d ecologicl risk index values

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Contamination factor and geo-accumulation index

Among the single indices, the maximum CF was determined for Ni in Baluchistan and the lowest for Fe in Gilgit-Baltistan (Fig. 2b). At an average, soil CF values were in the order as: Baluchistan (2.8) > Punjab (1.95) > Khyber Pakhtunkhwa (1.64) > Gilgit-Baltistan (0.92) > Islamabad (0.89) > Sindh (0.56) (Fig. 2b). Very high CF values were exhibited by Ni (12.1) and Cr (4.07) in Baluchistan; Zn (2.53) and Cu (1.54), in Gilgit-Baltistan; Mo (6.55) and Cd (4.28) in Khyber Pakhtunkhwa; Cd (8.38) and Co (2.91) in Punjab whereas; As (1.08) and Pb (0.94) in Sindh province (Fig. 2b). Very high CF was observed for Ni, Mo, and Cd in Baluchistan, Khyber Pakhtunkhwa, and Punjab provinces, respectively. Similarly, “considerable CF” values are due to Cr in Baluchistan, Islamabad, Khyber Pakhtunkhwa, and Punjab, respectively. While moderate CF values were contributed by Zn and Cu in Gilgit-Baltistan, Islamabad, and Sindh as per the classification described by Jiao et al. (2015) (Table 2). Higher contamination of HM in the soil of the Baluchistan and Khyber Pakhtunkhwa provinces were characterized by the natural enrichment of these metals in the bedrock hosting mafic–ultramafic ore deposits and mining activities (Muhammad et al., 2019; Saddique et al., 2018). However, those of Sindh and Punjab provinces were mainly attributed to anthropogenic activities such as wastewater irrigation (Jamali et al., 2007; Khan et al., 2017).

As for as Igeo is concerned, Ni showed heavy pollution in Sindh, whereas moderate-heavy pollution in Baluchistan, followed by moderate pollution due to Mo in Khyber Pakhtunkhwa and Cd in Punjab (Fig. 2c) as categorized by Sakan et al. (2009). Similarly, mean Igeo values were in the order as: Baluchistan (0.55) > Gilgit-Baltistan (0.18) > Islamabad (0.18) > Sindh (0.61) > Punjab (0.39) > Khyber Pakhtunkhwa (0.33) (Fig. 2c). Higher contamination of HM in study area may be attributed to the human sources and local geology (mafic/ultramafic rocks hosting ophiolites) (Arif & Jan, 1993; Saddique et al., 2018).

Potential ecological risk index

The highest mean ERI values were investigated for Cd and the minimum for Cr (Fig. 2d). Results revealed that average Cd risk values (160 < ERI ≤ 320) pose high risk in the Punjab province only as categorized by Hakanson (1980) (Table 2). The spatial distribution of ERI values showed high and very high risks due to Cd contamination in the Islamabad, Khyber Pakhtunkhwa and Punjab provinces for about 12% of studied sites, while high risk of Mo in the Khyber Pakhtunkhwa only for 2% sites (Fig. 3). High ERI values in soil were assigned to the HM contaminations as a result of the local geology and anthropogenic activities, and their Tr values (Muhammad et al., 2019; Nawab et al., 2016). The ERI values determined were recorded as slightly higher than those by Pan et al. (2016) in populated agricultural soil in China and lower than described by Bai and Zhao (2020).

Fig. 3
figure 3

Ecological risk index values of soil heavy metal(loid)s in the study area

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Human health risk assessment

Non-carcinogenic

The contaminants HM in soil are toxic to living beings and may cause adverse effects (Saddique et al., 2018). Exposure to HM from soil may occur through multiple routes namely inhalation, ingestion, and dermal pathways (Mama et al., 2020, Rehman et al., 2018). Among HM exposure, the Fe was found the highest, while Cd was the lowest via all the three exposure routes (Fig. 4a–f). It was found that through the ingestion pathway, the intake of HM was the highest (Fig. 4a, b), followed by dermal absorption (Fig. 4e, f), and lowest in the inhalation pathway (Fig. 4c, d). It is obvious from the results that the exposures were higher in Baluchistan province compared to the rest of the provinces. The reason for this may be due to bedrock geology and mining activities (Muhammad et al., 2019; Umar et al., 2013). The intake of HM was used for the potential health risk assessment by HI and CR (Chen et al., 2015; Man et al., 2010).

Fig. 4
figure 4

Human exposure to soil heavy metal(loid)s through various routes in the study area: a, c and e ADD adults (mg/kg-day); b, d and f ADD children (mg/kg-day)

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The HQ was computed for the selected HM for adults as well as children via various routes (Fig. 5a–f). Among these routes, ingestion and dermal contact are the major exposure routes for adults and children. Results showed the highest HQ value for children (1.59) due to As in Punjab province via ingestion (Fig. 5b), while the lowest for Zn (2.5E−08) in Punjab adults through inhalation pathway (Fig. 5c). It is obvious from the findings that the highest risk was via ingestion (Fig. 5a, b), followed by dermal (Fig. 5e, f) and the lowest via inhalation (Fig. 5c, d). The HQ values beyond the threshold limit may cause different chronic health problems (Rehman et al., 2018).

Hazard index values in various provinces and federal areas of Pakistan were summarized in Fig. 6a, b. On average, the highest HI (1.61) value was found for the As in Punjab for children and the lowermost of Zn (0.0003) for adults. Furthermore, As (0.02–1.61) was followed by Cr (0.01–1.11), Ni (0.001–0.59), Pb (0.03–0.51), Fe (0.01–0.26), Cd (0.004–0.13), Co (0.001–0.06), Mo (0.002–0.03), Cu (0.0003–0.01), and Zn (0.0003–0.01). The higher HI values found in Punjab Province compared to the rest of the provinces may be attributed to higher contamination from various industrial and agricultural activities, mining, weathering, and erosion of parent material, i.e., sedimentary as well as mafic and ultramafic rocks (Shahid et al., 2018). It (higher HI) may also be attributed to the increased level of HM intake, their severity of toxicity, and their lower values of RfD (Khan et al., 2013). Higher HI values of As and Cr may result in chronic/long-lasting health issues. Non-cancerous exposure to Cr and As may cause diabetes, cancer, tumor of respiratory and kidney problems (Rehman et al., 2018). Similarly, if the concentration of Ni and Pb further increased, chronic risks due to these two may also be probable in the population.

Fig. 5
figure 5

Non-carcinogenic such as hazard quotient (HQ) of heavy metal(loid)s in the soil to human age groups: a, c and e HQ adults (unitless); b, d and f HQ children (unitless)

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Cancerous risk

The carcinogenic elements like Cd, Co, Cr, Ni, As, and Pb may produce CR in the exposed population. Moreover, these HM are known to cause CR through multiple exposure routes like ingestion, inhalation, and dermal absorption of soil particles (Hamad et al., 2014; Nickens et al., 2010). The highest (4.04E−07) CR was observed due to Co for children in Punjab (Fig. 7b), while the lowest (2.74E−11) via Pb in Baluchistan for adults (Fig. 7a). The rest of the HM were found between these two extremes (2.74E−11 to 4.04E−07) as exhibited in Fig. 7a, b. Results showed that Co in Punjab, Ni in Baluchistan, and Gilgit-Baltistan Provinces were found to cause higher CR. Results revealed that CR values for the children were higher as compared to adults in all the provinces and for studied HM. This higher CR in children than adults may be ascribed to more exposure due to their hand-to-mouth activities and passing more time outdoor during playing and carelessness during other similar activities (Wang et al., 2010). From the literature, it was found that CR results were determined higher than those recorded by Ferreira-Baptista and De Miguel (2005). CR values for adults and children in all the provinces were found within the tolerable range (10–4–10–6) according to USEPA (2002a) and Hu et al. (2012). Although the CR values in the aforesaid areas were found close to the limits. If these issues are not specifically addressed in the future, they will produce adverse effects on the human population.

Fig. 6
figure 6

Hazard index from soil heavy metal(loid)s to human age groups: a adults; b children

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Fig. 7
figure 7

Cancer or carcinogenic risk via consumption of carcinogenic heavy metal(loid)s to human age groups: a adults; b children

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Conclusion

The current study described the HM distribution, ecological as well as human health risk assessment via soil exposure in all the provinces of Pakistan. It was found that the Baluchistan soil showed higher HM pollution that was attributed by higher CF, Igeo, and ERI. Among the single indices, moderate CF in soils was found for As and Co in Punjab, Zn in Gilgit-Baltistan, Cd in Baluchistan, and Ni in Khyber Pakhtunkhwa; the Igeo pattern was observed in the sequence of Sindh > Baluchistan > Punjab > Khyber Pakhtunkhwa > Gilgit-Baltistan > Islamabad. Whereas, integrated index such as ERI ascribed high ecological average risk in the Punjab province due to Cd. Higher HM contaminations in the soil of the Baluchistan province may be attributed to local geological input and anthropogenic activities. The HQ was found maximum (1.59) for children due to As via ingestion, whereas the minimum risk was due to Zn through the inhalation pathway in the Punjab province. This study concluded that a higher concentration of HM may produce cancerous and non-cancerous health problems in humans. In comparison with adults, the children group is more prone to HM intake; thus, they may easily be affected adversely. The non-cancerous risk from exposure to As (1.61) in soil was higher than other HM for both adults and children. Similarly, CR values (both for adults as well as children) were determined within the threshold limit and may produce chronic health risks in the future if not monitored and regularized properly. Therefore, the current study recommends the periodic evaluation of HM in the soil of Pakistan generally and that of Baluchistan, Gilgit-Baltistan, and Sindh provinces particularly, where anthropogenic activities are frequently occurring in connection to abundantly available mineral ore deposits (like Fe, chromite, and gold deposits). Further studies are needed to know HM concentrations and to prioritize the strategies for remediation of toxic pollutants to be undergone that will contribute to ecological and environmental sustainability.