Introduction

The contamination of environment with potentially toxic elements (PTEs) is a great global environmental concern due to their toxic nature for all living organisms. These PTEs are highly persistent in nature and easily bio-accumulated in ecological resources such as plants and animals, and subsequently find their way to reach into the human body (Antoniadis et al. 2017b; Ishtiaq et al. 2018; Sun et al. 2016). These PTEs are released from both natural (weathering and erosion of bedrocks and ore deposits) and anthropogenic (mining, agricultural, and industrial) activities (Antoniadis et al. 2017a; Shah et al. 2012) and eventually contaminate the water sources (Khan et al. 2018; Tripathee et al. 2016). The contamination of water with PTEs not only impairs its quality but also poses threats to the exposed population (Khan et al. 2016). Ingestion of PTE contaminated water (Kumar et al. 2016) and food (Riaz et al. 2018) are the major pathways for human exposure. Among PTEs, iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn) are essential elements and required for normal human body functions. However, these essential elements may cause health hazards if exceeded their safe threshold limits of exposure (Ullah et al. 2017). Other PTEs including lead (Pb), cadmium (Cd), nickel (Ni), cobalt (Co), arsenic (As), and mercury (Hg) are extremely hazardous even in minute concentrations (Saddique et al. 2018; Shakoor et al. 2017). Low-dose exposure to PTEs in non-occupational environment could become a serious problem, especially to the most sensitive subgroups of population, including pregnant women and children (Rodríguez-Barranco et al. 2013).

The PTEs, after their intake via different pathways, are partially metabolized by human body and eliminate them in saliva, sweat, urine, and feces (Gil et al. 2011; Omokhodion and Crockford 1991). Therefore, various studies have focused on human blood, urine, nails, and scalp hair samples as biomarkers of PTE exposure (Molina-Villalba et al. 2015; Sheng et al. 2016; Song and Li 2015). Each of these human biomarkers has advantages and limitations over others. For example, PTEs concentrations in blood and urine reflect recent exposure, while hair levels reflect the past exposure, providing an average of the growth over period (weeks to years) depending on length of hair, easy collection, transport, and storage (Bermejo-Barrera et al. 1998; Molina-Villalba et al. 2015; Rahman et al. 2015).

In this study, we quantified the concentrations of PTEs including Mn, Fe, Co, Ni, Cu, Pb, and Cd in drinking water samples obtained from different sources present in southern Khyber Pakhtunkhwa, Pakistan. The potential health risk of these PTEs was also evaluated using the hazard index (HI) via drinking water consumption. In addition, the selected PTE concentrations were measured in human biomarkers including blood (plasma and RBCs), urine, nails, and scalp hair collected from the residents within the study area. Furthermore, this study also tested the correlations between PTEs concentrations in drinking water and human biomarkers.

Materials and methods

Study area

The study area, comprised of districts Dera Ismail Khan (DI Khan), Bannu, Karak, Lakki Marwat and Tank in Southern Khyber Pakhtunkhwa Province (Pakistan), has a total population of 4.95 million (Fig. 1). The selected area has hot summer (48.9 °C) and cold winter (2.8 °C) and mostly drained and irrigated by the rivers Indus, Kuram, and Gambila. The people within the study area are mainly associated with the agriculture sector, and the major crops include wheat, maize, rice, sugarcane, grams, barley, and vegetables.

Fig. 1
figure 1

Location map along with sample sites and PTE concentration (μg/l) distribution maps in the study area

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Water sampling and preparation

Water samples (n = 190) were collected from various drinking water sources with replicates (n = from 5 to 18) from tube wells, bore wells, dug wells, hand pumps, springs, and streams/ponds throughout the selected districts (n = 5), as mentioned earlier. Before collection of each sample, the water from tube wells, bore wells, and hand pumps were allowed to flow for 5–10 min, and then, the bottle (250 ml) was washed three times and filled with sampling water. Each sample was filtered and 400 μl of concentrated nitric acid (65% HNO3, Suprapur, Merck, Germany) was added, marked, and transferred to the laboratory of the Department of Environmental Sciences, University of Peshawar, and stored in the freezer (− 20 °C) according to procedure adopted from Kippler et al. (2016). Latitude and longitude coordinates were also noted for each sampling point using Global positioning system (GPS).

Human biomarkers sampling and preparation

The biomarkers including blood, urine, nails, and scalp hair were used to estimate the internal dose of PTEs exposure (Davis et al. 2017; Rahman et al. 2015). Biomarkers’ samples (n = 60) including mid-stream spot urine (early morning time), blood, nails, and scalp hair were collected from individuals in the study area. This study was performed after taking the approval from the ethical committee. Adult participants and parents of children for this study were asked for their consents before taking the samples from them.

Blood and urine

Fasting blood and mid-stream spot urine samples were collected in K2 EDTA tubes (5 ml) using new plastic bottles (100 ml). Blood samples were centrifuged and separated to plasma and red blood cells (RBCs). After collection, samples were properly marked, acidified with HNO3 (65% Suprapur, Merck, Germany) and stored in cooling blocks (− 20 °C) and transported to laboratory for further analyses.

Blood (5 ml) and urine (25 ml) samples were put in 25 and 100 ml polypropylene tubes and mixed with 1 ml of 1% HNO3 (65% Suprapur, Merck, Germany) and kept overnight as the procedure adopted from Ettinger et al. (2017).

Nails and scalp hair

Nails and scalp hair samples were collected using stainless steel nail clippers and scissors and stored in zip lock polyethylene bags, labeled and transported to the laboratory. Nails and scalp hair were washed, clean, and dried according to procedure adopted from the Gault et al. (2008).

Nails and scalp hair were digested according to the method adopted from Rahman et al. (2015). Briefly, (100-mg) samples were mixed with 2 ml of concentrated HNO3 (65% Suprapur, Merck, Germany) and 1 ml of H2O2 in polypropylene tubes and kept overnight. Samples were treated for 15 min each at 70 and 115 °C in the microwave accelerated reaction system (CEM-Mars.V.194A05). After cooling, samples were diluted with 1% of HNO3.

Acidified water and digested blood, urine, nails, and scalp hair samples were analyzed within 2-month period using an inductively coupled plasma mass spectrometer (ICP-MS) in the Institute of Urban Environment, Chinese Academy of Sciences, 1799, Jimmie Road, Xiamen 361021 China.

Precision and accuracy

Before reading the actual water and biomarkers samples, a mix of standard was run on the ICP-MS. After 10 samples, another separate standard was run to check the accuracy of machine and method. The accuracies of PTEs measurements were verified with standard reference materials of urine (Clinchek-control; RECIPE, Munich, Germany) and human hair (GBW09101, Shanghai Institute of Nuclear Research Academia Sinica, China). During analyses, each sample was measured in triplicate and the mean values were used for result interpretation. Recoveries of PTEs were found in the confidence level of 94 ± 8% for standard reference materials and 95 ± 6% for sample triplicate. All glassware and new plastic wares were washed with 2% HNO3 (65% Suprapur, Merck, Germany) and deionized water.

Risk assessment

Basic information like drinking water consumption and source, age, gender, body weight, exposure frequency, and health data were collected during the field survey. Respondents (male and female) were included both children (1–14 years) and adults (15–65 years). In the study area, water is pumped or collected from various sources and stored in plastic or cemented container. More than 98% of population uses this fresh or stored water without any treatment for drinking and other domestic purposes. Therefore, fresh or raw water was used to calculate the risk. Oral intake of PTEs through contaminated water is the major pathway for human exposure (Khan et al. 2018). Risk assessment was calculated through exposure such as average daily intake (ADI) and risk index, e.g., hazard quotient (HQ) and HI.

Average daily intake of PTEs through drinking water consumption was calculated using the equation adopted from the US EPA (1998).

$$ \mathrm{ADI}=\left(\mathrm{C}\times \mathrm{IR}\times \mathrm{EF}\times \mathrm{ED}\right)/\left(\mathrm{BW}\times \mathrm{AT}\right) $$
(1)

where C, IR, EF, ED, BW, and AT are the PTEs concentrations in drinking water (mg/l), ingestion rate (2 l/day), frequency of exposure (365 days/year), exposure duration (30 years), body weight (children 30.6 kg (< 14 years age) and adults 72 kg (> 15 years age), and average time, i.e., 365 days/year × ED for non-carcinogens, respectively (Muhammad et al. 2010).

Hazard quotient values through PTEs exposure were calculated by dividing the ADI values over the RfD of respective element as equation adopted from the US EPA (1998).

$$ \mathrm{HQ}=\mathrm{ADI}/\mathrm{RfD} $$
(2)

where oral toxicity reference dose values (RfD) of PTEs such as Pb, Zn, Co, Ni, Cu, Fe, and Mn were used as 0.036, 0.3, 0.03, 0.02, 0.037, 0.7, and 0.14 mg/kg/day, respectively (USEPA 2005). Exposed population is assumed to be safe if HQ < 1 (Muhammad et al. 2011).

$$ \mathrm{HI}=\Sigma\ \mathrm{HQ} $$
(3)

where HI is health index and sum of HQ for all selected PTEs.

Mapping for PTEs

Latitude and longitude of each sampling site was marked with GPS and used for the PTEs concentrations and distribution maps using ArcGIS.

Statistical analysis

Statistical analyses such as Pearson’s correlation of data and graphical presentation were performed using Sigma plot 12.5 and SPSS 21 (SPSS Inc., Chicago, IL, USA).

Results and discussion

Potentially toxic elements concentrations in drinking water

The concentrations and distribution of PTEs in drinking water sources of selected five districts of Khyber Pakhtunkhwa were summarized (Table 1, Fig. 1). The concentrations of PTEs showed great variations in sources and district/location of drinking water. The studied PTEs showed higher concentrations in shallow-water sources (hand pumps) as compared to deep water sources (tube wells) of the study area except Bannu, Lakki Marwat, and Tank districts and surface water sources (ponds) of Karak (Table 1). Higher contamination levels of shallow-water sources (hand pumps) were consistent with the results reported by Khan et al. (2018) for drinking water. Hand pumps water stayed close to earth surface; therefore, higher PTEs concentrations in shallow water could be attributed to ongoing surface agriculture, domestic, and mining activities (Khan et al. 2018). However, PTEs concentrations were highest in the deep tube well water sources (Table 1). Higher Fe, Zn, and Pb contaminations in the tube well water of district DI Khan could be attributed to PTEs enrichment in bedrocks and rusting of old plumbing pipes. Among PTEs, the highest (1405 μg/l) mean concentration was observed for Zn and the lowest not determined (ND) for Co (Table 1). Other PTEs including Mn, Fe, Ni, Cu, and Pb concentrations were observed between the two extremes. The concentrations of PTEs including Mn, Fe, Co, Ni, Cu, Zn, and Pb were observed within the safe drinking water guidelines of respective elements set by WHO (2011). However, these limits were surpassed by Mn and Fe in hand pump’s water of district Tank, Fe in hand pump’s water of Bannu, DI Khan, Tank and Lakki Marwat (bore well also), and Pb in bore wells of district DI Khan. The results revealed that selected PTEs concentrations were observed lower than those reported by Begum et al. (2015) for drinking water in Swat (northern Pakistan) along the mafic and ultramafic rocks which act as a source of these elements in water.

Table 1 Potentially toxic elements concentrations (μg/l) in various sources of drinking water present of the study area
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Risk assessment

Human exposure to the PTEs could occur through main routes, e.g., ingestion or intake of contaminated drinking water (Muhammad et al. 2011) and food (Khan et al. 2013). Hence, in order to understand that how much of PTEs were transferred into the human body, it will be very essential to investigate the ADI of these elements ingested via drinking water.

Table 2 summarizes the values of ADI through PTEs consumption in various drinking water sources of five districts of Khyber Pakhtunkhwa. The ADI values revealed great variation in different drinking water sources and locations. The majority of shallow drinking water sources (hand pumps) were observed with higher ADI values for PTEs consumption as compared to deep (tube wells) sources. Higher ADI values through consumption of shallow-water were due to their higher contamination levels. The DI Khan district showed higher ADI values for majority of PTEs consumption as compared to other districts (Table 2).

Table 2 Average daily intake (μg/kg/day) of PTEs through consumption of drinking water from various sources in the study area
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Average daily intakes of PTEs were highest for Zn, followed by Fe, while the lowest for Co (Table 2). Higher ADI values for Zn and Fe were attributed to their higher contamination levels as compared to other PTEs in drinking water sources of the study area. The ADI values for PTEs consumption through same drinking water were observed higher for children (91.8 μg/kg/day) as compared to adults (39.0 μg/kg/day) (Table 2). Higher ADI values through PTEs consumption in drinking water may be attributed to their low body weight. Higher ADI values through PTEs consumption for children were consistent with those reported by Saddique et al. (2018).

The hazards of PTEs on human health through water consumption have been well documented (Muhammad et al. 2011; Tripathee et al. 2016). Potential health risk such as HQ values of exposed population posed by the PTEs were estimated from ADI values. The highest mean HQ values through consumption of drinking water were observed for Zn, followed by Pb in the DI Khan, while the lowest in district Karak (Table 3). Higher HQ value of Zn and Pb could be attributed to their concentrations, toxicity, and respective RfD values (Khan 2013). The HQ values depend upon the concentrations of PTEs, RfD values, and toxicity of each the element. Results of this study revealed that HQ values were within the safe limits (< 1). The HQ values were found lower than those reported by Gul et al. (2015) in the drinking water of district Mardan, Khyber Pakhtunkhwa. The HI values are sum of HQ of all studied PTEs. Our results revealed that the highest HI values were observed through consumption of drinking water from hand pumps in district Tank, followed by tube well water in district DI Khan (Fig. 2). Higher risk values through hand pump water in district Tank were due to higher HQ values of individual PTEs that were attributed to higher contamination level of hand pump’s water. Similarly, the higher HI values of children were attributed to their higher sensitivity and low body weight as compared to adults.

Table 3 Hazard quotient values of PTEs through consumption of drinking water from various sources in the study area
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Fig. 2
figure 2

Hazard index values through consumption of PTEs present in various drinking water sources of the study area, while BT, BB, BH, LT, LB, LH, DT, DB, DH, TT, TB, TH, KT, KB, KH, KD, and KP stands for tube wells, bore wells, hand pumps, dug wells and ponds present in district Bannu, Lakki Marwat, DI Khan, Tank, and Karak, respectively

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Potentially toxic elements in biomarkers

The monitoring of PTEs concentrations in the environmental samples, including water (this study), soil, and food (Rehman et al. 2016) revealed the presence of contamination in the study area. The ingestion of contaminated water could induce PTEs health burden in the exposed human population. The current study on the human body burden of PTEs has included five types of human biomarkers including nails, urine, hair, plasma, and RBCs along with other reported studies (Table 4).

Table 4 The concentrations (μg/g or μg/l) of PTEs quantified in human biomarkers
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Human population, especially children and infants, may be very vulnerable to neurotoxic effects of Mn exposure. Exposure to low concentration of Mn has previously been reported with toxic effects on neurodevelopmental outcomes in children by Riojas-Rodríguez et al. (2010). The concentrations of Mn in hair, nails, urine, plasma, and RBCs ranged from 21.5, 10.9, 9.78, 2.85, and 4.85 μg/g, respectively. The concentrations of Mn in the studied population hair and urine were higher than those reported by Huang et al. (2014) and Wang et al. (2011), while those of nails and blood samples were lower than Samanta et al. (2004) and Zheng et al. (2013). Fe is one of the human essential elements and requires for normal function of hemoglobin, myoglobin, and a number of enzymatic activities. Fe concentrations lower than the required concentration could cause deficiency effects, while at higher concentration characterizes for toxic effects including diarrhea, vomiting, liver, kidney, and blood problems (Muhammad et al. 2011). The concentrations of Fe in the blood of studied population were higher than those reported by Samanta et al. (2004).

Cadmium is one of the non-essential PTEs and has well-known for acute and chronic toxicity including kidney problems and potential developmental and other harmful health effects in children (ATSDR 2008). Contaminated food and water account for the major source of Cd exposure. The concentrations of Cd in the hair of studied population were higher than those reported by Huang et al. (2014), while those of urine, nails, and blood were lower than those reposted by Samanta et al. (2004), Sheng et al. (2016), and Molina-Villalba et al. (2015). The Pb stays in contaminated environment and considers as highly toxic element that may result in memory loss and reduced growth in children. A study conducted by Watanabe et al. (2000) observed that over 60% of total Pb intake could be attributed to dietary exposure. Toxic effects of Pb include neurologic and developmental effects in children (ATSDR 2008). The concentrations of Pb in the studied population’s hair, urine, nails, and blood were observed lower than the studies conducted by the Wang et al. (2009) and Wongsasuluk et al. (2017). Zn is also one of the essential elements and needs for normal function of living beings. The deficiency of Zn could lead to poor healing of wounds, muscles’ weakness, and hair loss, while high concentration may cause anemia (Muhammad et al. 2011). The concentrations of Zn in the studied population’s hair, nails, and urine were higher than those reported by the Dongarrà et al. (2011), Samanta et al. (2004), and Sheng et al. (2016), while its concentrations in blood were found lower than reported by Sheng et al. (2016).

Nickel is required in a specific amount for cell membrane metabolism, lipid, and hormone. However, its higher concentration may cause burning and redness of skin, itching, and asthma in human beings (Knight et al. 1997). The concentrations of Ni in the studied population nails were higher than those reported by Sukumar and Subramanian (2007), while that of hair, urine, and blood were lower than Sheng et al. (2016) and Huang et al. (2014). Like Ni, Co is also needed in minute quantity for normal body functions. However, higher concentration of Co may cause polycythemia, over-production of RBCs and abnormal thyroid artery (Robert and Mari 2003). The concentrations of Co in the biomarkers of studied population were found higher than those reported by Sheng et al. (2016). Cu is an essential element; however, its higher intake through drinking water can lead to several health problems (Kidd 2003). The concentrations of Cu in the studied population urine were higher than those reported by Wang et al. (2011), while those of hair, nails, and blood were lower than reported by Wang et al. (2009), Samanta et al. (2004) and Sheng et al. (2016).

Statistical analyses (Pearson correlation of PTEs and human biomarkers)

Table 5 summarizes the findings about of correlation between PTEs concentrations in drinking water and human biomarkers. For example, the concentrations of Cu in drinking water showed significant correlation with that of nails, RBCs, and urine. Similarly, Fe in drinking water showed the correlation with that of plasma and RBCs. The Mn in drinking water showed the correlation with that of plasma only (Table 5). Results of this study have found a correlation between drinking water and nails, plasma, RBCs, and urine for the studied PTEs concentrations such as Mn, Fe, and Cu concentrations. Results of this study suggest that nails could be a good biomarker for Cu, plasma for Mn and Fe, RBCs for Fe and Cu, and urine for Cu only. The correlation between PTEs concentrations in biomarkers including urine, nails, and blood has been previously documented to health burden for the assessment of environmental exposures (Gil et al. 2011). Higher correlations of PTEs concentrations in human biomarkers are consistent with those reported by Molina-Villalba et al. (2015), Sheng et al. (2016), and Xing et al. (2016).

Table 5 Pearson’s correlation matrices for PTEs concentrations in drinking water sources and human biomarkers
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Conclusion

This study concluded that highest levels of selected PTEs contamination were observed for shallow-water (hand pump) in districts Bannu, Lakki Marwat, Tank, and Karak. The mean concentrations of selected PTEs including Mn, Co, Ni, Cu, Zn, and Pb were found within the safe drinking water guidelines of respective elements as set by (WHO). However, Fe mean concentration surpassed these limits in all shallow water sources of the study area except district Karak. These limits are also surpassed by Fe in tube well water and Pb in bore well water of DI Khan. Higher contamination levels of shallow drinking water have led to higher ADI values for Zn, followed by Fe as compared to other PTEs in drinking water. The highest HQ value was observed for Zn, followed by Pb. The intake of contaminated drinking water has led to accumulation of PTEs in human biomarkers which was confirmed by the statistical analyses such as Pearson’s correlation that revealed strong positive correlation.