Accelerated mining activities have increased water contamination with potentially toxic elements (PTEs) and their associated human health risk in developing countries. The current study investigated the distribution of PTEs, their potential sources and health risk assessment in both ground and surface water sources in mining and non-mining areas of Khyber Pakhtunkhwa, Pakistan. Water samples (n = 150) were taken from selected sites and were analyzed for six PTEs (Ni, Cr, Zn, Cu, Pb and Mn). Among PTEs, Cr showed a high mean concentration (497) μg L−1, followed by Zn (414) μg L−1 in the mining area, while Zn showed the lowest mean value (4.44) μg L−1 in non-mining areas. Elevated concentrations of Ni, Cr and a moderate level of Pb in ground and surface water of Mohmand District exceeded the permissible limits set by WHO. Multivariate statistical analyses showed that the pollution sources of PTEs were mainly from mafic-ultramafic rocks, acid mine drainage, open dumping of mine wastes and mine tailings. The hazard quotient (HQ) was the highest for children relative to that for adults, but not higher than the USEPA limits. The hazard index (HI) for ingestions of all selected PTEs was lower than the threshold value (HIing < 1), except for Mohmand District, which showed a value of HI >1 in mining areas through ingestion. Moreover, the carcinogenic risk (CR) values exceeded the threshold limits for Ni and Cr set by the USEPA (1.0E-04–1.0E-06). In order to protect the drinking water sources of the study areas from further contamination, management techniques and policy for mining operations need to be implemented.

  • Elevated Ni, Cr and Pb levels exceeded the WHO permissible limits in ground and surface water.

  • Mafic-ultramafic rocks, acid mine drainage, mine wastes and mine tailings are the main primary factors influencing the distribution of PTEs and their contamination.

  • Health risk was relatively high for children than for adults in mining and non-mining areas.

  • Management policies and monitoring strategies need to be implemented for mining operations.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Water is one of the essential natural resources for human life and development and an important part of the biological system. Less than 3% of the total water resources are freshwater resources, and only less than 1% is suitable for human use (Saleh et al. 2019). In sustaining aquatic and terrestrial life, freshwater resources play an unavoidable role and are directly linked to drinking, agriculture and aquaculture (Pilotto et al. 2019). Water resources could be vulnerable to contamination of potentially toxic elements (PTEs) in the environment (Ciazela et al. 2018). In freshwater ecosystems, PTEs have become one of the most toxic chemicals due to their persistence in nature (Strungaru et al. 2018). PTEs are released to the environment by anthropogenic activities such as mining, smelting, metallurgical and industrial beneficiation processes, excavation and transportation, leading to further contamination of the surrounding environment (Li et al. 2014; Nawab et al. 2017; Zhang et al. 2017; Oyebamiji et al. 2018; Kumar et al. 2019; Nawab et al. 2019; Santana et al. 2020). Natural processes also result in the occurrence of PTEs due to complex physiochemical reactions by weathering of parent rocks, oxidation, mineral dissolution and migration of acid mine drainage containing high contents of PTEs that could severely affect and deteriorate the geochemical surrounding environment and reach the groundwater by erosion or the leaching process (Kefeni et al. 2017; Yang et al. 2017; Nawab et al. 2018c). Also, natural geochemical changes caused by mining operations could alter and affect the surface and groundwater quality over a long period of time on a regional basis, even after ceasing mining activities (Baeten et al. 2018). Freshwater resources are mostly susceptible to the direct impacts of mining on the environment (Santana et al. 2020).

Mining operations are considered as the most influential anthropogenic activities that could damage the natural habitat, degrade land resources and result in soil and water contamination with PTEs (Shifaw 2018). For instance, mine tailings are exposed to the agriculture lands of non-mining areas, resulting in serious pollution by dispersion and mobilization of PTEs (Zhu et al. 2018). As a result, PTEs such as Pb, Cd, As, Cu and Zn could discharge to rivers due to surface runoff by wastewater or excess of rainwater, leading to a contamination of waterbodies and the aquatic ecosystem (Mohammadnejad et al. 2018). In spite of these natural processes, improper treatment of abandoned old mines as well as mine tailings may cause more PTE pollution in adjacent (non-mining) regions of agricultural soils, surface water and groundwater (Queiroz et al. 2018; Sun et al. 2018; Wang et al. 2019c). Numerous studies have been conducted on PTE contamination in mining and adjacent non-mining surrounding areas (Wang et al. 2019a, 2019d; Santana et al. 2020). Previous studies of abandoned mines and tailing ponds showed the high contents of PTEs in mining areas and led to contamination of the local soil and rivers water due to the mobilization of mining wastes (Shen et al. 2019). Due to these factors, mining is perceived to be one of the human activities with the biggest detrimental effects on the environment (Paraguassú et al. 2019). Therefore, a comparative study is needed to conduct the assessment of PTE distribution in mining and non-mining areas, by identifying their potential sources that contribute to the contamination of surface and groundwater sources.

Groundwater pollution has been recognized as an actual route to transfer pollutants to surface water sources (Adyasari et al. 2018; Xiao et al. 2019). Pollutants such as PTEs can migrate more readily through water sources and drastically decrease the consistency of water in rivers, reservoirs and groundwater (Northey et al. 2016). Elevated concentrations of PTEs could deteriorate water quality and pose significant public health risks due to their toxicity, persistence and bio-accumulative nature (Muhammad et al. 2011; Alves et al. 2018; Yang et al. 2018) and adversely impact human health (Rehman et al. 2018). For instance, a high concentration of toxic PTEs in surface and groundwater may have unforeseeable adverse effects on people of all age classes, particularly in children (Yang et al. 2013). Further, it is well-known that long-term exposure to toxic PTEs can result in adverse effects on the nervous, immune and endocrine systems (Li et al. 2014) and lead to cancer or disability in both children and adults (Patlolla et al. 2012; Wu et al. 2018). Also, other health problems such as stomach and heart diseases, hypertension and anorexia can occur (Qian et al. 2020). Chronic PTE toxicity has adverse effects on human health, such as lung disease, renal failure, and bone fracture and may lead to hypertension, fertility and hormonal, immune, liver function and endocrine system deficiencies (Yuanan et al. 2020). To understand this kind of threat, it is important to evaluate the surface and groundwater quality by means of PTE distribution in disaster-prone mining and non-mining areas and their associated health risks.

In Pakistan, most of the mining sites are located in high mountainous and rural regions where people are illiterate, poor and mostly unaware of the health impacts of mining. The open dumping practices of mine wastes are common in Pakistan, leading to an impact on the agricultural soils by surface runoff, erosion and landslides (Nawab et al. 2018a). Major developments took place in recent times in the evaluation of the threat and the effect of mine-impacted water and even in the remediation technological tools to minimize the burden of point source pollution of mining water on groundwater and surface water (Liu & Li 2019). Significant environmental evaluation of water resources is still required as water contamination of PTEs contributes to an over-exploitation of groundwater and surface water. Local inhabitants have been using drinking water from both (surface and groundwater) sources in Khyber Pakhtunkhwa (KPK) newly merged districts of Pakistan. In the previous administrative system, the newly merged districts were termed agencies and had separate laws and regulations. For this reason, the quality and exposure assessment of contaminated surface water and groundwater are important before its utilization in mining and non-mining regions of the three newly merged districts (Mohmand, Bajaur and Khyber) in KPK province. Therefore, the primary objectives of the present study were as follows: (i) to investigate the PTE distribution in surface and groundwater of mining and non-mining areas; (ii) to identify the potential sources of PTEs by multivariate statistical methods and (iii) to evaluate the health risk assessment of PTEs for inhabitants of the study area. Our findings would provide a solid framework for further decision-making to take appropriate steps to manage contamination in order to protect the quality of drinking water and to prevent public health hazards resulting from drinking water sources.

Study area description

The study area of Mohmand, Khyber and Bajaur Districts is located in KPK province of Pakistan. Mohmand District is located in Peshawar division at a latitude of 34° 22′ 20″ N and a longitude 71° 27′ 26″ E, and the region is geographically made up of mountainous ranges with rocky hills and scattered along the sides of the Kabul River. The Lower Mohmand region is very fertile, while the Upper area is generally less productive. Many farmlands are rain-fed with adequate rainfall. Mohmand chromite reserves are primarily located within the harzburgite and dunite system containing metagabbro minerals, which extend through Dargai across Skhakot to Mohmand District as a linear chain of approximately 60 km in width and 2–6 km in length. Almost all of the dunite sandstones are barren, but chromites mostly occur in the lowest layer in areas and bands that are well-known in the region. The entire ultramafic complex is sporadically crossed by a thin pyroxenite dyke. In some locations, tremolite veins and talc-carbonate and quartz are common (Uppal 1972; Rafiq et al. 1984).

Bajaur District is situated in Malakand division at 34° 41′ 0′ N, 71° 30′ 0′ E in the northern region of KPK province (Figure 1). Regionally, it is situated east of the Kunar Valley in Afghanistan and shares borders with Malakand District in the south-east, Dir district in the north-east and Mohmand District in the south. It has a surface area of 1,290 km2. About 40% of the district's territory is protected by desolate mountains and 60% by broad valleys. Crop farming, small-scale enterprise and skilled employment are the primary forms of jobs. The region has simple-to-medium igneous minerals, which are pyroxenite, goethite, pegmatite, norite, hypersthene, granite and diorite. The middle section of Bajaur consists of Kamila amphibolites, diorites and a few tiny flecks of granodiorites, whereas the southern part is inhabited by the Main Mantle Thrust (MMT) region. The MMT region consists of volcanic, ultramafic, granite, marble, gneiss, salt, calcareous shale, and quartzite (Ullah et al. 2017).
Figure 1

Location map of the study area showing the sampling districts.

Figure 1

Location map of the study area showing the sampling districts.

Close modal

Khyber District is located at 34.02° N latitude and 71.28° E longitude with a total elevation of 1,070 m and an area of 2.567 km2. The temperature ranges from 25 °C (77 °F) to over 40 °C (104 °F) in summer and from 4 to 18.35 °C in winter. Its boundaries are Peshawar to the east, Afghanistan to the west, Orakzai to the south and Kurram to the southwest. It has an area of 2,576 km2 with a population of 8,45,309. Sedimentary rocks at the Khyber District contain granite tiles, granite, dolomite, sand, barite, malachite, graphite and quartz. Mullagori marble is among the largest deposits in the study field, and soapstone is the second largest element in the area. The region has a very small manufacturing base, and oil mills, tobacco factories, steel plants and marble factories in Barra, Shakas Jamrud and Mullagori are the main industries in the district.

Sample collection

A total (n = 150) of 150 drinking water samples were collected from Mohmand, Bajaur and Khyber (50 each from these districts) in January 2018 from the surface water and groundwater sources. All the water samples were collected from different sources such as water pumps, bore wells, dug wells, tap water, hand pumps and spring water. Sampling sites were chosen on the basis of mining operations in the surrounding areas and non-mining areas. All the water samples were collected in pre-washed high density 500 ml polyethylene bottles, containing 10% nitric acid (HNO3) solutions. Before water sampling, water from water pumps, bore wells and tap water was allowed to continuously run for a few minutes according to the procedure adopted by Khan et al. (2013). The geographical location of the samples was recorded using GPS coordinates. Water samples were acidified with HNO3 to pH <2 to minimize microbial growth, precipitation and solubility of PTEs in container walls. While sampling, every bottle was labeled and then immediately transported to the laboratory and stored at 4 °C for further analysis (Ullah et al. 2019).

Analytical procedure

The water samples were analyzed for selected PTEs such as Ni, Cr, Zn, Cu, Pb and Mn by using a graphite furnace atomic absorption spectrophotometer (PerkinElmer, USA, ASS-PEA-700) through the standard conditions. The physical parameter of pH was measured using a digital pH meter (Model C93, Turnhout, Belgium). The acids and reagents used in the study were 99.9% analytical grade with spectroscopic purity (Merck Darmstadt, Germany). For the study of PTEs, the samples were transformed to a pH standard of <2 with conc. HNO3 as a preservative for further analysis. The standard solutions of PTEs were prepared by diluting 1,000 mg L−1 standard certified solution (Fluka Kamica, Buchs, Switzerland).

Data precision and accuracy

Atomic absorption spectrometry (AAS) was calibrated to a validated level from Fluka Kamica (Buchs, Switzerland) with a dilution of 1,000 mg L−1 of deionized (DI) water after each 10 samples. Each sample was determined in triplicate under normal optimum AAS conditions with error >0.999. The findings of the AAS were verified by examining standard blanks and replicating them as unknown samples at intervals of every 10 samples. Reproducibility and recovery of these findings was detected at confidence levels of 93 ± 6 and 91 ± 5, overall. Mean results were used for the analysis of the data. Analytical grade reagents (Merck, Germany) were used to ensure quality data, and glassware and new plastic items were washed with 10% HNO3 solution with water and dried in the oven.

Health risk assessment

Oral absorption and dermal contact are known to be the main pathways (over 90%) of human exposure to PTEs (Ullah et al. 2019). The chronic and cancer risks associated with the ingestion of PTEs in drinking water sources were analyzed. The persistent vulnerability was measured by exposure risk assessment. The average daily intake (ADI) showed the PTE consumption and was calculated by ingestion and dermal contact using the given equations. Exposure can be measured by multiplying the PTE concentration by the duration of the contact. The average daily dose (ADD) (mg/kg/day) reflected the average dosage over the duration of treatment.

Average daily intake

The calculation of ADI through ingestion for selected PTEs was calculated by using the following equation (Long et al. 2021):
formula
(1)
where CS is the concentration of selected PTEs in the sample, IR is the ingestion rate, ED is the exposure duration, EF is the exposure frequency and BW is the body weight of children and adults given (Supplemental material, Table S1). AT is the average time for both adults and children.
The ADI dermal values calculations were performed using the following equation, adopted by Ngo et al. (2021):
formula
(2)
where CS is the concentration of selected PTEs in the sample, SA is the surface area of the skin exposed to PTEs, Kp is the permeability coefficient, ET is the exposure time, CF is the conversion factor and ABS is the dermal absorption factor (USDOE 2011).

Hazard quotient

Hazard quotient (HQ) indicates non-carcinogenic risk of PTEs via ingestion and dermal contact and was calculated by using Equation (3) as the ratio of ADI by the reference dose (RfD) (mg/kg/day), followed by Nawab et al. (2018a):
formula
(3)
where RfD is the oral reference doses of PTEs as shown in Supplemental material, Table S2. HQ > 1 means that the metal has a potential non-carcinogenic health risk in the assessment system (Qiao et al. 2020).
HQderm is non-carcinogenic risk, calculated by using the following equation:
formula
(4)
where ADI is the average daily intake via dermal contact and RfD is the dermal reference dose provided in Supplemental material, Table S2. Reference doses for dermal absorption have been calculated by multiplying the water ingestion reference doses with gastrointestinal absorption (GIABS) variables as indicated by USDOE-RAIS (Equation (12)) (USDOE 2011).

The hazard index

The health index (HI) was calculated using Equation (5), adopted from Nyambura et al. (2020):
formula
(5)
where HQ ing/derm of the selected PTEs were calculated to find HI values. HI > 1 shows that the non-carcinogenic risk due to a particular route of exposure or chemical is assumed to be insignificant.

Cancer risk

Cancer risk through the consumption of selected PTEs in drinking water was calculated for oral ingestion and dermal contact (USEPA 2005).
formula
(6)
where CSFing are slope factors of Ni, Cr and Pb as shown in Supplemental material, Table S2.
formula
(7)
where CSFderm are dermal factors as given in Supplemental material, Table S2. However, the CSF values for dermal absorption are calculated by multiplying the ADI values with CSF ingestion and divided by GIABS factors suggested by USDOE (2011).

Statistical analysis

Analytical tools such as MS-Excel 2019 have been used for statistical analysis of mean, range and standard deviation of PTEs. The principal component analysis (PCA) was used for PTE source identification and Pearson's correlation analysis was used for metal correlation, employing OriginPro (2018 version) and SPSS (17) version. The geostatistical analysis and spatial distribution maps were identified by using ArcGIS 10.1.

PTE distribution in mining and non-mining areas

The descriptive results of PTE concentrations in ground and surface water of mining and non-mining areas are summarized in Table 1. The mean pH values of groundwater in the Mohmand, Bajaur and Khyber districts of mining areas were 7.27, 7.42, and 7.45, while for surface water they were 7.56, 8.10, and 7.65, respectively. In contrast, the mean pH values of groundwater and surface water were observed to be lower (6.95, 7.10 and 7.20) and (7.35, 7.65 and 7.52) in the Mohmand, Bajaur and Khyber districts of non-mining areas, respectively. All of the observed pH mean values were above 7.00 for all the districts (except 6.95 for Mohmand District) and were found within the permissible limit of WHO (2011). The results showed that both groundwater and surface water ranged from slightly acidic to alkaline. Determination of pH is important due to its special effects on surface and groundwater chemistry, chemical speciation, alkalinity and metal solubility (Selvakumar et al. 2014). pH concentration could be influenced by wastewater discharge and industrial and agricultural runoff (Nawab et al. 2018b). The pH values of groundwater and surface water were found to be in the order Bajaur District > Khyber District > Mohmand District in mining areas, while those of groundwater and surface water were also in the order Bajaur District > Khyber District > Mohmand District in non-mining areas. The concentrations of PTEs in groundwater sources were moderate to high and ranged from 43–177, 56–612, 16–95, 15–81, and 14–78 to 217–326, with mean values of 92.3, 414, 47, 40, 37 and 242 μg L−1 for Ni, Cr, Zn, Cu, Pb and Mn, respectively. High concentrations were observed for Cr, with mean values of 414 and 497 μg L−1 for both ground and surface water, respectively. The lowest mean values were detected for Pb (37.0 and 46.4) μg L−1 in groundwater and surface water, respectively, in Mohmand district. For surface water samples, high concentrations of Ni, Cr, Zn, Cu, Pb and Mn were observed as compared to groundwater, with mean values of 110, 497, 58, 50.2, 46.44 and 179 μg L−1, respectively, in Mohmand District. Among PTEs, elevated concentrations of Ni, Cr and Pb in the ground and surface water of Mohmand District exceeded the standard permissible limits of WHO (2017). These elevated PTE concentration in both water resources could be attributed to mine tailings and acid mine drainage wastes, released from mining sites and deposited to the underlying bedrocks (Taylor 2007), which can contaminate both groundwater and surface water resources. Furthermore, the mining operations are well-known as the main sources of environmental contaminants (Razo et al. 2004). For instance, Nawab et al. (2019) also reported that the concentration of PTEs like Cr, Cd, Cu, Pb and Zn has increased due to mining activities in the past century. Moreover, these PTEs could have originated from the waste tailings left over in the environment for a long period of time by mining activities due to improper treatment and management (Rashed 2010). For Bajaur district, the concentrations of PTEs were lower than those of Mohmand district, with mean values of 12.8, 112, 59.3, 19.9, 10.1 and 19.2 μg L−1 in groundwater sources, while surface water sources had moderate mean concentrations (16.9, 19.4, 19, 20.2, 90 and 15.9) μg L−1 for Ni, Cr, Zn, Cu, Pb and Mn, respectively. Moreover, the mean concentrations of PTEs in Khyber district were 11.2, 19.6, 56.7, 19.4, 10.5 and 18.9 μg L−1 and 12.0, 21.3, 77.2, 21.3, 12.6 and 21.5 μg L−1 for Ni, Cr, Zn, Cu, Pb and Mn in groundwater and surface water, respectively, and were lower than those of the Mohmand and Bajaur districts of mining regions.

Table 1

PTE concentrations (μg L−1) in the water sources of the studied regions (mining and non-mining) and WHO guideline values

pH
Ni
Cr
Zn
Cu
Pb
Mn
LocationStatisticsGroundSurfaceGroundSurfaceGroundSurfaceGroundSurfaceGroundSurfaceGroundSurfaceGroundSurface
Mining 
Mohmand Agency Range 6.60–7.95 6.9–8.20 43–177 51–188 56–612 305–678 16–95 21–98 15–81 21–87 14–78 16–85 217–326 123–234 
Mean 7.27 7.56 92.3 110 414 497 47.0 58.0 40.0 50.2 37.0 46.4 242 179 
Std 0.95 0.93 48.4 49.5 160 118 26.3 25.7 22.3 22.1 22.7 23.1 39.6 39.14 
Bajaur Agency Range 7.20–7.65 7.80–8.40 12–13 12–43 18–225 10–27 50–70 15–23 16–24 12–27 5–15 85–98 14–27 13–18 
Mean 7.42 8.10 12.8 16.9 112 19.4 59.3 19.0 19.9 20.2 10.1 90.4 19.2 15.9 
Std 0.31 0.42 0.5 9.89 90.2 5.54 7.15 2.78 2.61 5.33 3.64 4.5 4.1 1.94 
Khyber Agency Range 6.65–8.25 7.20–8.10 5–19 9–17 15–24 14–2 52–61 71–86 15–24 13–29 10–11 9–17 14–23 17–27 
Mean 7.45 7.65 11.2 12.0 19.6 21.3 56.7 77.2 19.4 21.3 10.5 12.6 18.9 21.5 
Std 1.31 0.63 4.65 2.4 2.8 3.67 3.37 5.01 2.84 5.48 0.37 2.62 2.94 3.15 
Non-Mining 
Mohmand Agency Range 6.20–7.70 6.80–7.90 16–38 23–48 19–56 32–63 119–327 117–339 14–33 16–48 13–24 9–23 140–318 139–371 
Mean 6.95 7.35 25.7 31.5 32.5 44.2 203 225 23.0 32.2 17.0 13.6 227 254 
Std 1.06 0.77 7.39 8.63 12.3 11.0 97.4 70.7 6.38 9.24 3.77 4.78 47.5 77.3 
Bajaur Agency Range 6.50–7.50 7.20–8.10 3–8 2–9 5–17 10–18 2–9 10–27 17–31 9–19 4–10 2–16 3–19 2–24 
Mean 7.10 7.65 5.33 5.78 11.1 14.3 5.11 19.5 23.11 14.1 6.56 8.56 9.78 13.4 
Std 0.70 0.63 1.58 2.63 4.24 2.95 2.57 6.83 4.67 4.19 2.06 4.63 5.95 8.94 
Khyber Agency Range 6.70–7.70 7.15–7.90 2–21 7–21 9–14 10–21 2–8 12–24 2–21 15–22 2–9 2–10 7–21 7–18 
Mean 7.20 7.52 7.87 12.8 11.8 15.3 4.44 18.0 12.3 17.7 5.56 5.90 13.5 13.5 
Std 0.70 0.53 2–21 7–21 9–14 10–21 2–8 12–24 2–21 15–22 2–9 2–10 7–21 7–18 
WHO (2017))   6.60–8.50 70 50 3,000 2,000 10 300 
pH
Ni
Cr
Zn
Cu
Pb
Mn
LocationStatisticsGroundSurfaceGroundSurfaceGroundSurfaceGroundSurfaceGroundSurfaceGroundSurfaceGroundSurface
Mining 
Mohmand Agency Range 6.60–7.95 6.9–8.20 43–177 51–188 56–612 305–678 16–95 21–98 15–81 21–87 14–78 16–85 217–326 123–234 
Mean 7.27 7.56 92.3 110 414 497 47.0 58.0 40.0 50.2 37.0 46.4 242 179 
Std 0.95 0.93 48.4 49.5 160 118 26.3 25.7 22.3 22.1 22.7 23.1 39.6 39.14 
Bajaur Agency Range 7.20–7.65 7.80–8.40 12–13 12–43 18–225 10–27 50–70 15–23 16–24 12–27 5–15 85–98 14–27 13–18 
Mean 7.42 8.10 12.8 16.9 112 19.4 59.3 19.0 19.9 20.2 10.1 90.4 19.2 15.9 
Std 0.31 0.42 0.5 9.89 90.2 5.54 7.15 2.78 2.61 5.33 3.64 4.5 4.1 1.94 
Khyber Agency Range 6.65–8.25 7.20–8.10 5–19 9–17 15–24 14–2 52–61 71–86 15–24 13–29 10–11 9–17 14–23 17–27 
Mean 7.45 7.65 11.2 12.0 19.6 21.3 56.7 77.2 19.4 21.3 10.5 12.6 18.9 21.5 
Std 1.31 0.63 4.65 2.4 2.8 3.67 3.37 5.01 2.84 5.48 0.37 2.62 2.94 3.15 
Non-Mining 
Mohmand Agency Range 6.20–7.70 6.80–7.90 16–38 23–48 19–56 32–63 119–327 117–339 14–33 16–48 13–24 9–23 140–318 139–371 
Mean 6.95 7.35 25.7 31.5 32.5 44.2 203 225 23.0 32.2 17.0 13.6 227 254 
Std 1.06 0.77 7.39 8.63 12.3 11.0 97.4 70.7 6.38 9.24 3.77 4.78 47.5 77.3 
Bajaur Agency Range 6.50–7.50 7.20–8.10 3–8 2–9 5–17 10–18 2–9 10–27 17–31 9–19 4–10 2–16 3–19 2–24 
Mean 7.10 7.65 5.33 5.78 11.1 14.3 5.11 19.5 23.11 14.1 6.56 8.56 9.78 13.4 
Std 0.70 0.63 1.58 2.63 4.24 2.95 2.57 6.83 4.67 4.19 2.06 4.63 5.95 8.94 
Khyber Agency Range 6.70–7.70 7.15–7.90 2–21 7–21 9–14 10–21 2–8 12–24 2–21 15–22 2–9 2–10 7–21 7–18 
Mean 7.20 7.52 7.87 12.8 11.8 15.3 4.44 18.0 12.3 17.7 5.56 5.90 13.5 13.5 
Std 0.70 0.53 2–21 7–21 9–14 10–21 2–8 12–24 2–21 15–22 2–9 2–10 7–21 7–18 
WHO (2017))   6.60–8.50 70 50 3,000 2,000 10 300 

Std, standard deviation.

The overall results showed that Cr had high concentrations in ground and surface water sources of mining areas, followed by Zn and Mn. This high Cr concentration could be attributed to chromite ore deposits and ultramafic rocks in the region that transferred from old mine tailings to the groundwater system through leaching from the surface waterbodies (Dhakate & Singh 2008). Mafic and ultramafic rock deposits and mining wastes are the primary source of high levels of PTE release to the environment (Nawab et al. 2015). Furthermore, the concentrations of Pb in all three agencies were exceeded; also, Ni and Cr concentrations in Mohmand District were above the permissible limits of WHO (2017) in both water resources of mining areas. The high enrichment of Pb in the water sources of the mining areas could have resulted from possible geogenic mafic and ultramafic rocks that have already been reported by several researchers (Khan et al. 2018; Rashid et al. 2018; Liu & Li 2019). On the other hand, elevated Ni concentrations and high occurrence of other PTEs in the water resources could be related to mining and regional industrial activities, smelting, mafic and ultramafic rocks and drainage discharges as reported elsewhere (Xia et al. 2018; Zhang et al. 2018). Moreover, Ni could mainly originate from the leaching of ultramafic rocks in drinking water sources (Aleksandra & Urszula 2008). Overall, the mean concentrations of PTEs in the mining areas of Mohmand, Bajaur and Khyber districts were found in decreasing order of Cr > Mn > Ni > Zn > Cu > Pb, Cr > Pb > Zn > Cu > Mn > Ni and Zn > Cr > Cu > Mn > Ni > Pb in all the groundwater and surface water sources, respectively. The results of the present study were found to be in agreement with the previous studies conducted in the mining areas of Hunan province, China (Gong et al. 2014), in Dabaoshan of Guangdong province, China (Wang et al. 2019b), in Palma, Spain (Rodellas et al. 2014), in Brajrajnagar of Jharsuguda district, India (Sahoo & Khaoash 2020), in Taojian, China (Chen et al. 2019), in Maracas at Port of Spain (Santana et al. 2020) and in Mantaro, Peru (Custodio et al. 2020), respectively, as shown in Supplemental material, Table S3.

In non-mining areas, there is a moderate variation observed in the mean concentrations of PTEs, as presented in Table 1. The results showed that the mean concentrations of Mn were high (227 μg L−1), followed by Zn (203 μg L−1) in groundwater. Mn also showed a high mean concentration (254 μg L−1), followed by Zn (225 μg L−1) in the surface water sources of Mohmand district. For Bajaur and Khyber districts, the concentrations of PTEs were lower than those of Mohmand district, with high mean values of Cu (23.1 μg L−1) and Mn (18 μg L−1) in surface water and a low mean value of (5.11 μg L−1) for Zn in groundwater and (4.44 μg L−1) for Zn in surface water, respectively. The high enrichment levels of Cr, Ni and Pb (except Zn) in the present study were found to be lower than those in previous study of the non-mining site of agriculture soils near the mining-impacted northern areas of Pakistan (Nawab et al. 2016). The mean concentrations of all PTEs were low in non-mining areas, as compared to those in the mining areas, and were found within acceptable limits, except that Pb had exceeded the standard limit of WHO (2017) in both water sources in Mohmand district. Furthermore, the studied mean concentrations of PTEs in the non-mining areas of Mohmand, Bajaur and Khyber districts were observed in decreasing order of Zn > Mn > Cr > Cu > Ni > Pb, Zn > Cu > Cr > Mn > Pb > Ni and Cu > Zn > Mn > Cr > Ni > Pb in both water sources, respectively. PTE concentrations in the present study were lower than those in the previous studies reported by Qin et al. (2014) and Huang et al. (2013). In addition, the Cr, Zn, Cu, Mn had comparatively high concentrations among PTEs in all three districts of non-mining areas. However, the elevated values of PTEs in both water sources of non-mining areas could be attributed to mafic and ultramafic rocks and open dumping of chromite mining wastes that could be dispersed through runoff by rainfall and wind erosion and thereby accumulated in surrounding areas (Nawab et al. 2015). In addition, numerous studies showed high PTE concentrations in the surrounding mining areas and ore deposits as well as in terrestrial ecosystems (Rashed 2010).

Correlation matrix of PTEs in surface water and groundwater

Pearson's correlation coefficient values of PTEs in surface water and groundwater parameters for mining and non-mining sites are given in Table 2. The correlation coefficient values support the PCA results in the present study. A significant positive correlation has been observed in the groundwater and surface water samples of PTEs to obtain the relevant information on common sources. The significant correlation positive values were observed for pH and Pb (0.74), Cr and Ni (r = 0.98), Cu and Ni (r = 0.99), Cu and Cr (r = 0.97), Mn and Ni (r = 0.94), Mn and Cr (r = 0.93), and Mn and Cu (r = 0.90), and negative correlations were obtained for pH and Zn (−0.57), and Pb and Zn (r = −0.86) in the study area of the mining site, indicating a common origin of PTEs. High metal concentrations and their strong correlations (r > 0.50) in the groundwater and surface water of mining areas indicate the high anthropogenic and geochemical natural sources.

Table 2

Pearson's correlation matrix of selected PTE concentrations in mining and non-mining areas

pHNiCrZnCuPbMn
Mining 
pH 1       
Ni −0.35 1      
Cr −0.45 0.98 1     
Zn − 0.57 −0.03 0.03 1    
Cu 0.32 0.99 0.97 0.04 1   
Pb 0.74 0.22 0.12 − 0.86 0.19 1  
Mn −0.49 0.94 0.93 −0.03 0.90 0.14 
Non-mining 
pH 1       
Ni −0.15 1      
Cr −0.10 0.97 1     
Zn −0.24 0.96 0.97 1    
Cu −0.30 0.78 0.82 0.78 1   
Pb −0.31 0.84 0.85 0.93 0.62 1  
Mn −0.28 0.96 0.97 0.99 0.78 0.92 1 
pHNiCrZnCuPbMn
Mining 
pH 1       
Ni −0.35 1      
Cr −0.45 0.98 1     
Zn − 0.57 −0.03 0.03 1    
Cu 0.32 0.99 0.97 0.04 1   
Pb 0.74 0.22 0.12 − 0.86 0.19 1  
Mn −0.49 0.94 0.93 −0.03 0.90 0.14 
Non-mining 
pH 1       
Ni −0.15 1      
Cr −0.10 0.97 1     
Zn −0.24 0.96 0.97 1    
Cu −0.30 0.78 0.82 0.78 1   
Pb −0.31 0.84 0.85 0.93 0.62 1  
Mn −0.28 0.96 0.97 0.99 0.78 0.92 1 

Note: Bold values show that correlation is significant at P < 0.01 level (two-tailed).

In contrast, the significant positive correlation pairs were dominantly observed in non-mining areas for all the PTEs as follows: Cr and Ni (r = 0.97), Zn and Ni (r = 0.96), Zn and Cr (r = 0.97), Cu and Ni (r = 0.78), Cu and Cr (r = 0.82), Cr and Zn (r = 0.78), Pb and Ni (r = 0.84), Pb and Cr (r = 0.85), Pb and Zn (r = 0.93), Pb and Cu (r = 0.62), Mn and Ni (r = 0.96), Mn and Cr (r = 0.97), Mn and Zn (r = 0.99), Mn and Cu (r = 0.78) and Mn and Pb (r = 0.92), suggesting the common origin in non-mining areas. The results of these strong positive correlations demonstrate that surface water and groundwater variables were influenced by the common geogenic or anthropogenic sources in the study area, which could have resulted from mafic and ultramafic rock deposits and mining wastes, releasing high levels of PTEs to the environment (Nawab et al. 2015). Also, open dumping and mobilization of mining wastes in abandoned mines and tailing ponds could be another reason, leading to contamination of the surrounding local soils, surface river water and groundwater with PTEs (Shen et al. 2019). The results of these correlation analyses of major ions in the present study were similar to those in the previous studies, conducted by Wang et al. (2019b) and Santana et al. (2020).

Source identification of PTEs

PCA is an important technique used to describe and identify the pollution sources via the dimension reduction method (Kannel et al. 2007). The PCA results of six target PTE observations of mining and non-mining agencies are listed in Table 3. Overall, three loading factors (F1 and F2) were obtained with eigenvalues of (>1) for the surface water and groundwater of mining and non-mining agencies. The two principal components F1 and F2 described as 93.3 and 92.5% of total variance with the eigenvalues of 9.60 and 3.54 for mining and non-mining areas, respectively, are shown in Figure 2. The first two significant factors were observed for mining and non-mining areas, as presented in Figure 3. The positive loading factors indicate that the groundwater and surface water samples are influenced by the presence of the water parameters. Contrarily, the negative loading factors suggest that the groundwater and surface water quality are not influenced by the water parameters.
Table 3

Overall loading factors of PTEs in surface water and groundwater in mining and non-mining regions

PTEsMining
Non-mining
F1F2F1F2
pH − 0.51 0.77 − 0.29 0.95 
Ni 0.99 0.03 0.97 −0.01 
Cr 0.98 0.12 0.98 0.04 
Zn −0.06 0.96 0.99 −0.09 
Cu 0.98 0.08 0.83 0.82 
Pb 0.25 − 0.93 0.91 − 0.52 
Mn 0.95 0.07 0.99 −0.08 
Eigenvalue 4.10 2.44 5.50 1.10 
Variance (%) 58.5 34.8 78.5 14.0 
Cumulative (%) 58.5 93.3 78.5 92.5 
PTEsMining
Non-mining
F1F2F1F2
pH − 0.51 0.77 − 0.29 0.95 
Ni 0.99 0.03 0.97 −0.01 
Cr 0.98 0.12 0.98 0.04 
Zn −0.06 0.96 0.99 −0.09 
Cu 0.98 0.08 0.83 0.82 
Pb 0.25 − 0.93 0.91 − 0.52 
Mn 0.95 0.07 0.99 −0.08 
Eigenvalue 4.10 2.44 5.50 1.10 
Variance (%) 58.5 34.8 78.5 14.0 
Cumulative (%) 58.5 93.3 78.5 92.5 

Values in bold are significant in PCA.

Figure 2

Overall loading factors of (a) mining and (b) non-mining areas of water sources in KPK.

Figure 2

Overall loading factors of (a) mining and (b) non-mining areas of water sources in KPK.

Close modal
Figure 3

First two significant factors of (a) mining and (b) non-mining areas of water sources in KPK.

Figure 3

First two significant factors of (a) mining and (b) non-mining areas of water sources in KPK.

Close modal

The first factor F1 described the 58.5 and 78.5% of variance with eigenvalues of 4.10 and 5.50, respectively, for mining and non-mining sites. F1 showed the high positive loadings of significant correlation coefficient (r) values of Ni (r = 0.99), Cr (r = 0.98), Cu (r = 0.98) and Mn (r = 0.95) for mining sites. However, F1 had strong positive loadings of significant correlation coefficient (r) values of all PTEs like Ni (r = 0.97), Cr (r = 0.98), Zn (r = 0.99), Cu (r = 0.83), Pb (r = 0.91) and Mn (r = 0.99) for non-mining areas. Hence, F1 contributed to the strong loadings of PTEs for the surface water and groundwater of mining and non-mining sites in PCA results, suggesting the origin of natural and anthropogenic sources. Moreover, the strong loadings of the aforementioned PTEs are attributed to their high concentration in the study area. The elevated PTE concentration in both water resources could be attributed to mine tailings and acid mine drainage wastes, released from mining sites and deposited to the underlying bedrocks (Taylor 2007). Thus, F1 showed the mixed sources of anthropogenic and natural sources in the study area.

Factor 2 (F2) accounted for 34.8 and 14.0% of variability with eigenvalues of 2.44 and 1.10. The significant correlation coefficient (r) values of surface water and groundwater variables were Zn (r = 0.96) and a negative loading of Pb (r = −0.93) for mining site. However, the significant positive correlation coefficient (r) value of F2 was Cu (r = 0.82) and also a negative loading of Pb (r = −0.52) for the surface water and groundwater of non-mining areas. The high loadings of PTEs demonstrate the origin of open dumping of mine-wastes, resulting in contamination of the environment by surface runoff, erosion and landslides (Nawab et al. 2018a). Hence, F2 is assumed to show the mixed sources of both natural geogenic and anthropogenic sources in mining and non-mining sites.

ADI dose

The ADI ingestion and dermal contact values of selected PTEs for both adults and children are summarized in Supplemental material, Table S4. The respective ADI values of PTEs via consumption of ground and surface water in mining and non-mining areas were calculated, according to USEPA (2011) adopted values. The results showed that the ADI values of all the groundwater and surface water were lower for adults and children in Khyber district than in Mohmand and Bajaur districts. Cr had the highest ADI ingestion value (4.97E-05) for children, via groundwater consumption. However, Pb had the lowest ADI values (4.44E-06 and 1.15E-06) for both children and adults, via groundwater consumption in Mohmand District, respectively. A similar trend of high ADI of Cr value (1.35E-05) was observed for children, while a low ADI value of Pb (3.16E-07) was observed for adults in the groundwater of Bajaur district, respectively. All the other calculated PTEs had intermediate ADI ingestion values for both adults and children in Mohmand and Bajaur districts. For Khyber district, Zn had high ADI values (6.81E-06 and 1.76E-06) for children and adults in groundwater, respectively. The high ADI levels of Cr and Zn could be attributed to their high concentration levels in ground and surface water of mining areas. Higher ADIs of Cr and Zn values may contribute to a number of problems in the exposed human population. Based on the drinking water quality in mining areas, the ADI ingestion values for Mohmand, Bajaur and Khyber districts were observed in decreasing order of Cr > Mn > Ni > Zn > Pb > Cu, Cr > Pb > Zn > Cu > Mn > Ni and Zn > Cr > Cu > Mn > Ni > Pb in mining areas via both surface water and groundwater consumption for adults and children, respectively.

The results of the ADI ingestion values of non-mining water sources varied and were lower than those of mining water sources, as shown in Supplemental material, Table S5. The lowest ADI ingestion values of Pb (4.22E-07 and 5.27E-07) were calculated for adults, while Mn exhibited the highest ADI values (3.06E-05 and 2.73E-05) for children via surface and groundwater consumption, respectively. For Bajaur district, the high ADI ingestion values of Cu (2.77E-06) were recorded for children in groundwater, while Zn had the lowest ADI value (1.58E-07) for adults in groundwater, respectively. Similar results were observed for Khyber district via groundwater and surface water consumption. Comparatively, the ADI ingestion values of PTEs were found in the order of Mn > Zn > Cr > Cu > Ni > Pb, Cu > Cr > Mn > Pb > Zn > Ni and Zn > Cu > Mn > Cr > Ni > Pb in non-mining areas of three agencies, respectively. As a result, mining areas via water source consumption showed higher ADI ingestion values as compared to non-mining areas for children and adults. Children showed high ADI ingestion values in both mining and non-mining areas due to their high consumption rate and low body weight. However, the ADI ingestion values of all PTEs were observed to be within safe limits (<1). In addition, several ADI intake levels of PTEs are significantly similar to their respective reference dosage limits, suggesting whether the use of these polluted drinking water supplies tends to have an effect on the life span of both adults and children.

Among the source classification of water in mining areas, Bajaur District showed the lowest ADI dermal value 4.81E-08 of Pb in groundwater for adults. A high ADI dermal value of Cr (2.35E-05) was observed for children in the surface water of mining areas in Mohmand district, as listed in Supplemental material, Table S4. Likewise, Zn was observed with ADI dermal values (1.61E-05 and 2.19E-05) for adults in the groundwater and surface water of Khyber district, respectively. Based on the ADI dermal value calculation, it was observed that the children were more exposed to PTEs as compared to adults. Similarly, Custodio et al. (2020) also reported high ADI values for children in drinking water sources due to elevated PTE concentration levels resulting from mining activities. For the non-mining areas of Mohmand district, Pb was observed for a low ADI dermal value (8.02E-08) for adults, via surface water consumption, whereas the highest ADI dermal value (1.80E-04) was observed for Zn through surface water consumption for children (Supplemental material, Table S5). Furthermore, the highest Zn ADI dermal value (1.56E-05), followed by Cu (1.08E-05), was recorded for children in Bajaur district through groundwater and surface water consumption, respectively. However, the lowest Pb ADI values of 3.09E-08 and 2.62E-08 were observed for adults through groundwater in Bajaur and Khyber districts, respectively. Overall, the ADI dermal values were frequently low among all agencies for adults and children in non-mining areas, although high ADI dermal contact values for children were recorded, followed by adults. Also, the consumption of PTE-contaminated drinking water and dermal contact could be high enough to require steps to be taken to reduce adverse health effects on the exposed public (Rajeshkumar et al. 2018).

Non-carcinogenic health risk

HQs of ingestion and dermal contact were calculated for individual PTEs in three agencies of mining and non-mining areas, presented in Tables 4 and 5. The results showed that all the calculated HQs of individual PTEs were less than 1, suggesting that intake of PTEs via ingestion of water does not pose a potential health hazard. The intake of PTEs in children was higher than that in adults, resulting in comparatively high HQ values. The lowest HQ ingestion (1.88E-05) was recorded for Zn via groundwater consumption, while the highest value 6.33E-04 was observed for Mn through surface water consumption for children in Mohmand district of mining areas. A low HQ value was observed for Zn (7.60E-06) through surface water consumption for children, and a high value was recorded for Ni (7.70E-05) via groundwater intake of Bajaur district, and similar results were observed for Khyber district. The HQ values are dependent on toxicity, RfD values and metal concentrations. The HQ ingestion values of PTEs for Mohmand, Bajaur and Khyber districts were observed in the order of; Mn > Ni > Cu > Cr > Pb > Zn, Ni > Mn > Cu > Cr > Pb > Zn and Mn > Ni > Cu > Cr > Pb > Zn in mining areas via the surface water and groundwater consumption for adults and children, respectively. In Mohmand district of non-mining areas, a high HQ value was calculated for Mn (6.65E-04), while the lowest value was recorded for Cr (6.73E-07) via groundwater and surface water consumption for children and adults, respectively. Similar results of HQs were found for both Bajaur and Khyber districts, as given in Table 5. The HQ indices for all PTEs were (<1) and do not indicate any risk to the local population, according to USEPA (2005). However, the HQ indices of Cu, Mn, Ni, Pb and Zn metals tend to be higher than those reported in a drinking water study reported by Kavcar et al. (2009) and in groundwater and surface water by Lim et al. (2008).

Table 4

HQ and HI of PTEs for adults and children in mining areas

Mining
HQ by ingestion in adults
HQ by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 1.43E-04 1.71E-04 1.99E-05 2.62E-05 1.74E-05 1.87E-05 Ni 1.09E-04 1.30E-04 1.51E-05 2.00E-05 1.33E-05 1.42E-05 
Cr 8.56E-06 1.03E-05 2.33E-06 4.03E-07 4.06E-07 4.41E-07 Cr 1.51E-03 1.81E-03 4.09E-04 7.08E-05 7.13E-05 7.75E-05 
Zn 4.86E-06 5.99E-06 6.13E-06 1.96E-06 5.86E-06 7.98E-06 Zn 4.44E-05 5.48E-05 5.60E-05 1.79E-05 5.36E-05 7.29E-05 
Cu 3.10E-05 3.89E-05 1.55E-05 1.57E-05 1.51E-05 1.65E-05 Cu 4.72E-05 5.93E-05 2.35E-05 2.39E-05 2.30E-05 2.51E-05 
Pb 3.28E-05 4.11E-05 9.02E-06 8.01E-05 9.33E-06 1.12E-05 Pb 4.99E-06 6.27E-06 1.37E-06 1.22E-05 1.42E-06 1.71E-06 
Mn 1.63E-04 1.21E-04 1.29E-05 1.07E-05 1.28E-05 1.45E-05 Mn 6.22E-03 4.60E-03 4.93E-04 4.08E-04 4.86E-04 5.53E-04 
HI 3.84E-04 3.88E-04 6.58E-05 1.35E-04 6.09E-05 6.93E-05 HI 7.94E-03 6.66E-03 9.99E-04 5.53E-04 6.48E-04 7.44E-04 
HQ by ingestion in children
HQ by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 5.54E-04 6.61E-04 7.70E-05 1.02E-04 6.75E-05 7.22E-05 Ni 1.08E-03 1.29E-03 1.50E-04 1.98E-04 1.31E-04 1.40E-04 
Cr 3.32E-05 3.98E-05 9.01E-06 1.56E-06 1.57E-06 1.71E-06 Cr 1.49E-02 1.79E-02 4.04E-03 7.00E-04 7.04E-04 7.66E-04 
Zn 1.88E-05 2.32E-05 2.37E-05 7.60E-06 2.27E-05 3.09E-05 Zn 1.25E-05 1.55E-05 1.58E-05 5.07E-06 1.51E-05 2.06E-05 
Cu 1.20E-04 1.51E-04 5.98E-05 6.07E-05 5.84E-05 6.39E-05 Cu 4.72E-04 5.93E-04 2.35E-04 2.39E-04 2.30E-04 2.51E-04 
Pb 1.27E-04 1.59E-04 3.49E-05 3.10E-04 3.61E-05 4.34E-05 Pb 1.41E-05 1.77E-05 3.88E-06 3.45E-05 4.01E-06 4.82E-06 
Mn 6.33E-04 4.68E-04 5.01E-05 4.15E-05 4.94E-05 5.62E-05 Mn 6.15E-03 4.55E-03 4.87E-04 4.04E-04 4.80E-04 5.46E-04 
HI 1.49E-03 1.50E-03 2.55E-04 5.23E-04 2.36E-04 2.68E-04 HI 2.26E-02 2.43E-02 4.94E-03 1.58E-03 1.56E-03 1.73E-03 
Mining
HQ by ingestion in adults
HQ by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 1.43E-04 1.71E-04 1.99E-05 2.62E-05 1.74E-05 1.87E-05 Ni 1.09E-04 1.30E-04 1.51E-05 2.00E-05 1.33E-05 1.42E-05 
Cr 8.56E-06 1.03E-05 2.33E-06 4.03E-07 4.06E-07 4.41E-07 Cr 1.51E-03 1.81E-03 4.09E-04 7.08E-05 7.13E-05 7.75E-05 
Zn 4.86E-06 5.99E-06 6.13E-06 1.96E-06 5.86E-06 7.98E-06 Zn 4.44E-05 5.48E-05 5.60E-05 1.79E-05 5.36E-05 7.29E-05 
Cu 3.10E-05 3.89E-05 1.55E-05 1.57E-05 1.51E-05 1.65E-05 Cu 4.72E-05 5.93E-05 2.35E-05 2.39E-05 2.30E-05 2.51E-05 
Pb 3.28E-05 4.11E-05 9.02E-06 8.01E-05 9.33E-06 1.12E-05 Pb 4.99E-06 6.27E-06 1.37E-06 1.22E-05 1.42E-06 1.71E-06 
Mn 1.63E-04 1.21E-04 1.29E-05 1.07E-05 1.28E-05 1.45E-05 Mn 6.22E-03 4.60E-03 4.93E-04 4.08E-04 4.86E-04 5.53E-04 
HI 3.84E-04 3.88E-04 6.58E-05 1.35E-04 6.09E-05 6.93E-05 HI 7.94E-03 6.66E-03 9.99E-04 5.53E-04 6.48E-04 7.44E-04 
HQ by ingestion in children
HQ by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 5.54E-04 6.61E-04 7.70E-05 1.02E-04 6.75E-05 7.22E-05 Ni 1.08E-03 1.29E-03 1.50E-04 1.98E-04 1.31E-04 1.40E-04 
Cr 3.32E-05 3.98E-05 9.01E-06 1.56E-06 1.57E-06 1.71E-06 Cr 1.49E-02 1.79E-02 4.04E-03 7.00E-04 7.04E-04 7.66E-04 
Zn 1.88E-05 2.32E-05 2.37E-05 7.60E-06 2.27E-05 3.09E-05 Zn 1.25E-05 1.55E-05 1.58E-05 5.07E-06 1.51E-05 2.06E-05 
Cu 1.20E-04 1.51E-04 5.98E-05 6.07E-05 5.84E-05 6.39E-05 Cu 4.72E-04 5.93E-04 2.35E-04 2.39E-04 2.30E-04 2.51E-04 
Pb 1.27E-04 1.59E-04 3.49E-05 3.10E-04 3.61E-05 4.34E-05 Pb 1.41E-05 1.77E-05 3.88E-06 3.45E-05 4.01E-06 4.82E-06 
Mn 6.33E-04 4.68E-04 5.01E-05 4.15E-05 4.94E-05 5.62E-05 Mn 6.15E-03 4.55E-03 4.87E-04 4.04E-04 4.80E-04 5.46E-04 
HI 1.49E-03 1.50E-03 2.55E-04 5.23E-04 2.36E-04 2.68E-04 HI 2.26E-02 2.43E-02 4.94E-03 1.58E-03 1.56E-03 1.73E-03 
Table 5

HQ and HI indices of PTEs for adults and children in non-mining areas

Non-mining
HQ by ingestion in adults
HQ by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 4.00E-05 4.88E-05 8.27E-06 8.96E-06 1.22E-05 2.00E-05 Ni 3.04E-05 3.72E-05 6.29E-06 6.82E-06 9.28E-06 1.52E-05 
Cr 6.73E-07 9.14E-07 2.29E-07 2.96E-07 2.44E-07 3.18E-07 Cr 1.18E-04 1.61E-04 4.03E-05 5.20E-05 4.28E-05 5.58E-05 
Zn 2.10E-05 2.33E-05 5.28E-07 2.02E-06 4.59E-07 1.86E-06 Zn 1.92E-04 2.12E-04 4.82E-06 1.84E-05 4.19E-06 1.70E-05 
Cu 1.78E-05 2.50E-05 1.79E-05 1.09E-05 9.56E-06 1.37E-05 Cu 2.71E-05 3.80E-05 2.73E-05 1.67E-05 1.46E-05 2.09E-05 
Pb 1.51E-05 1.20E-05 5.81E-06 7.58E-06 4.92E-06 5.23E-06 Pb 2.29E-06 1.83E-06 8.84E-07 1.15E-06 7.49E-07 7.96E-07 
Mn 1.53E-04 1.72E-04 6.59E-06 9.06E-06 9.06E-06 9.16E-06 Mn 5.80E-03 6.50E-03 2.49E-04 3.43E-04 3.43E-04 3.47E-04 
HI 2.48E-04 2.82E-04 3.93E-05 3.88E-05 3.64E-05 5.03E-05 HI 6.17E-03 6.95E-03 3.29E-04 4.38E-04 4.15E-04 4.56E-04 
HQ by ingestion in children
HQ by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 1.55E-04 1.89E-04 3.20E-05 3.47E-05 4.72E-05 7.73E-05 Ni 3.01E-04 3.68E-04 6.23E-05 6.75E-05 9.18E-05 1.50E-04 
Cr 2.60E-06 3.54E-06 8.88E-07 1.15E-06 9.44E-07 1.23E-06 Cr 1.17E-03 1.59E-03 3.99E-04 5.15E-04 4.24E-04 5.52E-04 
Zn 8.14E-05 9.00E-05 2.04E-06 7.82E-06 1.78E-06 7.20E-06 Zn 5.43E-04 6.00E-04 1.36E-05 5.21E-05 1.19E-05 4.80E-05 
Cu 6.90E-05 9.67E-05 6.93E-05 4.23E-05 3.70E-05 5.31E-05 Cu 2.69E-04 3.76E-04 2.70E-04 1.65E-04 1.44E-04 2.07E-04 
Pb 5.83E-05 4.66E-05 2.25E-05 2.93E-05 1.90E-05 2.02E-05 Pb 6.46E-06 5.17E-06 2.49E-06 3.25E-06 2.11E-06 2.24E-06 
Mn 5.93E-04 6.65E-04 2.55E-05 3.51E-05 3.51E-05 3.54E-05 Mn 5.74E-02 6.43E-02 2.47E-03 3.39E-03 3.39E-03 3.43E-03 
HI 9.59E-04 1.09E-03 1.52E-04 1.50E-04 1.41E-04 1.95E-04 HI 5.96E-02 6.73E-02 3.22E-03 4.20E-03 4.07E-03 4.39E-03 
Non-mining
HQ by ingestion in adults
HQ by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 4.00E-05 4.88E-05 8.27E-06 8.96E-06 1.22E-05 2.00E-05 Ni 3.04E-05 3.72E-05 6.29E-06 6.82E-06 9.28E-06 1.52E-05 
Cr 6.73E-07 9.14E-07 2.29E-07 2.96E-07 2.44E-07 3.18E-07 Cr 1.18E-04 1.61E-04 4.03E-05 5.20E-05 4.28E-05 5.58E-05 
Zn 2.10E-05 2.33E-05 5.28E-07 2.02E-06 4.59E-07 1.86E-06 Zn 1.92E-04 2.12E-04 4.82E-06 1.84E-05 4.19E-06 1.70E-05 
Cu 1.78E-05 2.50E-05 1.79E-05 1.09E-05 9.56E-06 1.37E-05 Cu 2.71E-05 3.80E-05 2.73E-05 1.67E-05 1.46E-05 2.09E-05 
Pb 1.51E-05 1.20E-05 5.81E-06 7.58E-06 4.92E-06 5.23E-06 Pb 2.29E-06 1.83E-06 8.84E-07 1.15E-06 7.49E-07 7.96E-07 
Mn 1.53E-04 1.72E-04 6.59E-06 9.06E-06 9.06E-06 9.16E-06 Mn 5.80E-03 6.50E-03 2.49E-04 3.43E-04 3.43E-04 3.47E-04 
HI 2.48E-04 2.82E-04 3.93E-05 3.88E-05 3.64E-05 5.03E-05 HI 6.17E-03 6.95E-03 3.29E-04 4.38E-04 4.15E-04 4.56E-04 
HQ by ingestion in children
HQ by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 1.55E-04 1.89E-04 3.20E-05 3.47E-05 4.72E-05 7.73E-05 Ni 3.01E-04 3.68E-04 6.23E-05 6.75E-05 9.18E-05 1.50E-04 
Cr 2.60E-06 3.54E-06 8.88E-07 1.15E-06 9.44E-07 1.23E-06 Cr 1.17E-03 1.59E-03 3.99E-04 5.15E-04 4.24E-04 5.52E-04 
Zn 8.14E-05 9.00E-05 2.04E-06 7.82E-06 1.78E-06 7.20E-06 Zn 5.43E-04 6.00E-04 1.36E-05 5.21E-05 1.19E-05 4.80E-05 
Cu 6.90E-05 9.67E-05 6.93E-05 4.23E-05 3.70E-05 5.31E-05 Cu 2.69E-04 3.76E-04 2.70E-04 1.65E-04 1.44E-04 2.07E-04 
Pb 5.83E-05 4.66E-05 2.25E-05 2.93E-05 1.90E-05 2.02E-05 Pb 6.46E-06 5.17E-06 2.49E-06 3.25E-06 2.11E-06 2.24E-06 
Mn 5.93E-04 6.65E-04 2.55E-05 3.51E-05 3.51E-05 3.54E-05 Mn 5.74E-02 6.43E-02 2.47E-03 3.39E-03 3.39E-03 3.43E-03 
HI 9.59E-04 1.09E-03 1.52E-04 1.50E-04 1.41E-04 1.95E-04 HI 5.96E-02 6.73E-02 3.22E-03 4.20E-03 4.07E-03 4.39E-03 

The lowest HQ dermal value 1.77E-05 was observed for Pb by children from surface water in Mohmand district in mining areas. However, Mn showed the high HQ dermal values through surface water for Bajaur and Khyber districts. The HQ dermal values in non-mining areas of Mohmand, Bajaur and Khyber districts are presented in Table 4. In Mohmand district, Pb had a low HQ value (6.46E-06) in non-mining areas for children, while a maximum value of 1.17E-03 was recorded for Cr in children in groundwater. Similarly, Cr and Mn had the highest HQ dermal values for children in both Bajaur and Khyber districts of non-mining areas. The results revealed that children showed higher HQ values than adults due to their low body weight and vulnerability. The HQ values were found to be lower than 1, as compared to USEPA (2005) threshold values. All the HQs of dermal contact of PTEs were found in the same order of HQ ingestion values via surface water and groundwater consumption in the three agencies of mining and non-mining areas. In addition, the HI values for both adults and children exposed to all PTEs via ingestion and dermal contact indicated low non-carcinogenic risks (Tables 4 and 5). However, the total HI values in both mining and non-mining areas of Mohmand district were found to be higher for children, indicating that there may be potential non-carcinogenic risk via ingestion continually. Finally, more analyses related to the analyzed HQ, HI and CR support this study by showing that intake or contact with water polluted with toxic PTEs presents a risk to human health.

Carcinogenic risk

The carcinogenic risk (CR) associated with selected PTEs (Ni, Cr and Pb) was calculated via ingestion and dermal contact for both adults and children in mining and non-mining areas. Based on the ingestion exposure, the CR ingestion values for Ni were found to be lower (8.92E-07) in Bajaur district via surface water consumption, as listed in Table 6. The highest CR ingestion value of Cr (2.98E-05) in groundwater and the lowest CR value of Pb (9.75E-09) in surface water were recorded for children and adults in Mohmand District of mining areas, respectively. The highest CR ingestion value of 3.02E-07 in Bajaur district was found for Cr through surface water and the lowest (2.68E-09) was found for Pb through groundwater for adults, respectively. Similarly, the CR ingestion values were found to be higher for Cr and lower for Pb, respectively, through ground and surface water consumption in the mining areas of Khyber district. Moreover, the non-mining data show a relatively low CR as compared to mining area water sources. The CR ingestion highest value of 4.48E-07 for Ni was observed in Bajaur district through surface water for children, whereas the lowest value of 8.73E-09 was also found for Pb through surface water, respectively. Furthermore, Cr had a high CR ingestion value of 2.65E-06 via surface water for children, while Pb had the lowest values for children and adults in all agencies of non-mining areas. However, the CR values of PTEs were found in the order of Ni > Cr > Pb through surface and groundwater consumption in all three agencies of mining areas. A CR value of 1.0E-6 (one person per million) is considered to be CR for both adults and children, according to USEPA (2011), while values lower than 1.0E-6 are negligible, indicating no CRs. CR values of Cr and Ni were found to be higher than the standard limit of 1.0E-6 in mining areas for children. The high CR of Ni and Cr and its exposure to high toxicity could be potentially harmful in their early stages of growth and may affect the immune, digestive, reproductive and nervous systems of children (Peek et al. 2018). Thus, a high approximate CR of Cr and Ni via surface and groundwater consumption in mining regions could pose CR to children and require high enough treatment of water sources to minimize the adverse health effects on the exposed public in the study area. According to WHO (2017), children are more prone to health hazards because of high drinking water consumption, ingestion of more calories and breathing of more air in comparison with adults. The results of the present study were found consistent with related previous studies of drinking water sources conducted in mining areas (Bhattacharya et al. 2012; Ewusi et al. 2017; Dorleku et al. 2018).

Table 6

CR by ingestion and dermal contact in mining areas for adults and children

Mining
CR by ingestion in adults
CR by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 4.86E-06 5.81E-06 6.76E-07 8.92E-07 5.93E-07 6.34E-07 Ni 7.41E-06 8.85E-06 1.03E-06 1.36E-06 9.03E-07 9.66E-07 
Cr 6.42E-06 7.71E-06 1.75E-06 3.02E-07 3.04E-07 3.31E-07 Cr 9.79E-06 1.17E-05 2.66E-06 4.6E-07 4.63E-07 5.04E-07 
Pb 9.75E-09 1.22E-08 2.68E-09 2.38E-08 2.78E-09 3.33E-09 Pb 1.49E-09 1.86E-09 4.09E-10 3.63E-09 4.23E-10 5.08E-10 
CR by ingestion in children
CR by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 1.88E-05 2.25E-05 2.62E-06 3.45E-06 2.30E-06 2.45E-06 Ni 7.32E-05 8.74E-05 1.02E-05 1.34E-05 8.93E-06 9.55E-06 
Cr 2.49E-05 2.98E-05 6.76E-06 1.17E-06 1.18E-06 1.28E-06 Cr 9.67E-05 1.16E-04 2.63E-05 4.55E-06 4.58E-06 4.98E-06 
Pb 3.77E-08 4.74E-08 1.04E-08 9.23E-08 1.07E-08 1.29E-08 Pb 4.19E-09 5.26E-09 1.15E-09 1.03E-08 1.19E-09 1.43E-09 
Mining
CR by ingestion in adults
CR by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 4.86E-06 5.81E-06 6.76E-07 8.92E-07 5.93E-07 6.34E-07 Ni 7.41E-06 8.85E-06 1.03E-06 1.36E-06 9.03E-07 9.66E-07 
Cr 6.42E-06 7.71E-06 1.75E-06 3.02E-07 3.04E-07 3.31E-07 Cr 9.79E-06 1.17E-05 2.66E-06 4.6E-07 4.63E-07 5.04E-07 
Pb 9.75E-09 1.22E-08 2.68E-09 2.38E-08 2.78E-09 3.33E-09 Pb 1.49E-09 1.86E-09 4.09E-10 3.63E-09 4.23E-10 5.08E-10 
CR by ingestion in children
CR by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 1.88E-05 2.25E-05 2.62E-06 3.45E-06 2.30E-06 2.45E-06 Ni 7.32E-05 8.74E-05 1.02E-05 1.34E-05 8.93E-06 9.55E-06 
Cr 2.49E-05 2.98E-05 6.76E-06 1.17E-06 1.18E-06 1.28E-06 Cr 9.67E-05 1.16E-04 2.63E-05 4.55E-06 4.58E-06 4.98E-06 
Pb 3.77E-08 4.74E-08 1.04E-08 9.23E-08 1.07E-08 1.29E-08 Pb 4.19E-09 5.26E-09 1.15E-09 1.03E-08 1.19E-09 1.43E-09 

All the calculated CR dermal values of selected PTEs were low for all the districts of mining areas, as listed in Table 7. Cr had a high CR dermal value 1.16E-04 among other PTEs for children in Mohmand District, while Pb showed the lowest CR value of 5.08E-10 for adults in the surface water of Khyber district. Similarly, Ni showed a high CR dermal value of 1.03E-05 in Mohmand district, whereas Pb had a low CR dermal value of 9.67E-10 in the surface water of Bajaur district for children in non-mining areas. Overall, Cr and Ni showed relatively high CR dermal values for children in the surface water of Mohmand district, as compared to Bajaur and Khyber districts (Table 7). In children, increased ingestion and dermal contact of these PTEs led to adverse effects on the development of the intestinal system, kidney disorders and lung function in the early stages of growth (Plum et al. 2010). As a result, the CR values of PTEs were found in the order of Ni > Cr > Pb through surface and groundwater in all three agencies of non-mining areas. The CR dermal contact results were also below the permissible standard (1 × 10−4) suggested by the USEPA (2011), indicating that CR could be appropriate for both adults and children in the study area by dermal contact, except that Ni in the study area exceeded the threshold limits (1.0E-04) set by the USEPA (2011). The present study proposed that effective purification enhancement systems should be introduced to protect the health of people in the study area, particularly in Mohmand district.

Table 7

CR by ingestion and dermal contact in non-mining areas for adults and children

Non-mining
CR by ingestion in adults
CR by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 5.59E-07 6.84E-07 1.16E-07 1.25E-07 1.71E-07 2.80E-07 Ni 8.52E-07 1.04E-06 1.76E-07 1.91E-07 2.6E-07 4.26E-07 
Cr 5.05E-07 6.85E-07 1.72E-07 2.22E-07 1.83E-07 2.38E-07 Cr 7.68E-07 1.05E-06 2.62E-07 3.38E-07 2.78E-07 3.63E-07 
Pb 4.48E-09 3.58E-09 1.73E-09 2.25E-09 1.46E-09 1.55E-09 Pb 6.82E-10 5.46E-10 2.63E-10 3.43E-10 2.23E-10 2.37E-10 
CR by ingestion in children
CR by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 2.17E-06 2.65E-06 4.48E-07 4.85E-07 6.61E-07 1.08E-06 Ni 8.43E-06 1.03E-05 1.74E-06 1.89E-06 2.57E-06 4.21E-06 
Cr 1.95E-06 2.65E-06 6.66E-07 8.60E-07 7.08E-07 9.22E-07 Cr 7.6E-06 1.04E-05 2.59E-06 3.35E-06 2.76E-06 3.59E-06 
Pb 1.73E-08 1.39E-08 6.69E-09 8.73E-09 5.67E-09 6.02E-09 Pb 1.92E-09 1.54E-09 7.41E-10 9.67E-10 6.28E-10 6.67E-10 
Non-mining
CR by ingestion in adults
CR by dermal contact in adults
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 5.59E-07 6.84E-07 1.16E-07 1.25E-07 1.71E-07 2.80E-07 Ni 8.52E-07 1.04E-06 1.76E-07 1.91E-07 2.6E-07 4.26E-07 
Cr 5.05E-07 6.85E-07 1.72E-07 2.22E-07 1.83E-07 2.38E-07 Cr 7.68E-07 1.05E-06 2.62E-07 3.38E-07 2.78E-07 3.63E-07 
Pb 4.48E-09 3.58E-09 1.73E-09 2.25E-09 1.46E-09 1.55E-09 Pb 6.82E-10 5.46E-10 2.63E-10 3.43E-10 2.23E-10 2.37E-10 
CR by ingestion in children
CR by dermal contact in children
Mohmand
Bajaur
Khyber
Mohmand
Bajaur
Khyber
Ground WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface WaterGround WaterSurface Water
Ni 2.17E-06 2.65E-06 4.48E-07 4.85E-07 6.61E-07 1.08E-06 Ni 8.43E-06 1.03E-05 1.74E-06 1.89E-06 2.57E-06 4.21E-06 
Cr 1.95E-06 2.65E-06 6.66E-07 8.60E-07 7.08E-07 9.22E-07 Cr 7.6E-06 1.04E-05 2.59E-06 3.35E-06 2.76E-06 3.59E-06 
Pb 1.73E-08 1.39E-08 6.69E-09 8.73E-09 5.67E-09 6.02E-09 Pb 1.92E-09 1.54E-09 7.41E-10 9.67E-10 6.28E-10 6.67E-10 

The distribution levels of PTEs were investigated in the surface and groundwater sources of mining and non-mining areas of the newly merged districts (Mohmand, Bajaur and Khyber) in Pakistan. In this study, the PTE concentrations varied in mining and non-mining water sources. Among PTEs, Cr showed the highest concentrations, followed by Zn and Mn in surface water and groundwater sources, while Pb showed the lowest concentration in the surface water sources of non-mining areas. The concentrations of Ni, Cr and Pb in the ground and surface water of Mohmand district exceeded the permissible limits of WHO. The Pearson's correlation matrix and PCA results showed that the pollution sources of PTEs mainly originated from common geogenic sources of mafic-ultramafic rocks, acid mine drainage released to the regions by erosion, leaching and surface runoff. Anthropogenic sources such as open dumping of mine wastes and mine tailings in the study area highly contributed to water contamination. Drinking water consumption was the primary route of metal exposure for Mohmand District, followed by a dermal contact route. For both target classes, the daily intake of PTEs from water consumption was at least 4–10 times higher than dermal interaction. In the case of dermal exposure, the non-carcinogenic risk and CR threshold for PTEs indicates no health risk hazard for both adults and children. However, the risk assessment revealed that there is a non-carcinogenic risk to children in Mohmand district by ingestion exposure. Risk assessment of exposure to PTEs has shown that the CR from drinking water use ingestion in mining areas is relatively high, in comparison with the protection standard of USEPA risk; thus, people in this study area may be at greater risk and serious attention needs to be paid to this area. Exposure assessments performed by children resulted in more carcinogenic and non-carcinogenic risks through ingestion and residents of Mohmand district were more exposed to Ni and Cr. More research work is required to reduce the levels of PTEs in the drinking water sources of these regions. In addition, appropriate management measures for mine waste must be set in place to protect the local population and reduce public health threats. Further research on the dynamics of other PTEs in mining and non-mining regions should also be undertaken to determine long-term health risks.

This study was financially supported by the Pakistan Science Foundation under the National Sciences Linkages Program Project No. PSF/NSLP/KP-AWKUM (827). The authors are thankful to the reviewers, lab members and lab technicians for their efforts in improving the quality of the paper and in analyzing PTEs.

Z.I.B., M.I., and S.A.K. prepared the methodology and wrote the original draft. J.N., J.G., and Z.U. performed software analysis, validated the study, and analyzed the results. S.K. and S.A.B. did the formal analysis, reviewed, and investigated the study. I.M., Z.U.D., and A.K. did the formal analysis, prepared the methodology, and performed data curation. All the authors have carefully read and approved the final manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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