Spatial variations and health risk assessment of heavy metal levels in groundwater of Qatar

The present work’s objective is to give a comprehensive overview of the quality of groundwater in Qatar in terms of heavy metals content as well as investigating the cause and effect of the elevation in their levels above the WHO/US-EPA standards. The scope of the study included (1) physical and chemical analysis of 82 groundwater samples collected from various locations around Qatar, (2) development of ArcGIS maps depicting the variations in the levels, (3) assessment of the human health risks associated with the existing levels using three of the most used models which are: Hazard index (HI), Nemerow comprehensive pollution index (NCPI) and Incremental Lifetime Cancer Risk (ILCR). There is no extensive study ever reported to assess the health risks linked with the consumption of groundwater characterized with such heavy metals levels in Qatar. The chronic daily intake (CDI) of the investigated heavy metals (Ag, Mn, Cr, V, Mo and Sr) through ingestion and dermal pathways had a range of 1.4 × 10–5–6.7 × 10–1 mg/kg/day while the NCPI’s range was reported at 0–4.39. Moreover, the HI and ILCR were found to have a range of 0–3.2 and 5.6 × 10–4–5.5 × 10–2, respectively. The assessment of health risks, conducted in the present work, could be beneficial in building the baseline of heavy metals levels in groundwater in Qatar. This will also help in the determination of any future contamination of groundwater.


Selection and collection of groundwater samples
In this study, 82 groundwater samples were collected from various parts around Qatar as shown in Fig. 1.The locations of the samples were carefully selected to cover the whole map of Qatar as well as give a representation of the various factors which may affect the quality of the collected samples such as groundwater well application, basin type, etc.
Prior to sample collection and in order to collect representative samples, the groundwater pump was turned on for 15 min in order to make sure that the collected samples are the well water and not the stagnant water kept in the pipes.Moreover, in the present work, for each groundwater well, three samples were collected and analyzed, and the average value was reported.
The limitation of the present study was in the selection of well distributed sampling points throughout the map of the country.Initially, we distributed the 82 groundwater samples were selected evenly to cover all the areas in Qatar.However, replacement sampling wells had to be selected in the vicinity of the sampling areas due to: (a) some of the selected wells dried up and the wells as well as the farms did not exist anymore and (b) some of the wells belong to industrial or private areas that did not want to participate in the study.

Measurement of physical and chemical parameters
The physical measurements such as pH and TDS were evaluated in the field using HI-9829 multiparameter (Hanna Instruments, Woonsocket, Rhode Island, USA).The electrical conductivity (EC) was used to determine the total dissolved solids (TDS) in the collected groundwater samples using Eqs.(1) and ( 2).These equations were obtained from Schlumberger Water Services report that investigated the correlation between electric conductivity and total dissolved solids for Qatari groundwater between 1971 and 2009 7 .
The alkalinity of the groundwater samples (in mg CaCO 3 /L) was determined using APHA 2320B method 37 in which groundwater samples were titrated with sulfuric acid (with a normality of 0.1 N) in the presence of sodium carbonate (Na 2 CO 3 ).Equation (3) was used to estimate the alkalinity 37 : where A is the volume of standard acid used which is sulfuric acid; N is the normality of sulfuric acid and V is the volume of the sample in ml.
The heavy metals analyzed in this work are: Lead (Pb), Iron (Fe), Copper (Cu), Nickel (Ni), Manganese (Mn), Zinc (Zn), Aluminum (Al), Mercury (Hg), Cadmium (Cd), Barium (Ba), Antimony (Sb), Arsenic (As), Chromium (Cr), Selenium (Se), Beryllium (Be) and Silver (Ag).The evaluation of the level of heavy metals in groundwater was carried out using 5800 ICP OES inductively coupled plasma-optical emission spectrometry (Agilent Technologies, Santa Clara, California, USA).The calibration curves were generated by dilution of the 1000 ppm Agilent Technologies standard stock calibration solution.The measurement of the parameters was repeated three times and the range as well as the mean values were included in this work.The mean values as well as the standard deviations were calculated using Microsoft Excel.
The correlation between the physicochemical parameters in the present work was analyzed by the determination of Pearson correlation coefficient between any two parameters.Equation ( 4) was used to estimate the Pearson correlation coefficient (r) 38 : where x i and y i represent the value of the x-variable and y-variables, respectively in any sample and x stands for mean value of the x-variable and y stands for the mean value of the y-variable.

Development of Arc-GIS maps
The development of GIS maps was conducted using ArcGIS Desktop 10.8.1 software (version: 10.8.1.14362)which was developed by Environmental Systems Research Institute (https:// www.esri.com) in which the map of Qatar was captured from Google Earth.The interpolation method followed in the present work was inverse distance weighted (IDW) in which the cell values were determined via a group of sample points that were weighted linearly.Moreover, the surface which is interpolated must be a variable that is location-dependent while the weight must be dependent or a function of inverse distance.

Spatial analysis of pollution in groundwater
In the present work, the evaluation of pollution of groundwater by heavy metals was carried out using Nemerow comprehensive pollution index (NCPI).The main two parameters which are used to assess the pollution of any system with contaminants are pollution index (I i ) and NCPI.I i is used to express the pollution index of groundwater with a particular element i whereas NCPI is used to estimate the total pollution created by the presence of all the studied elements.Equations (1) and (2) were used to determine the pollution index (I i ) and NCPI 39 : where C i and R i stand for the concentration and reference level of element i, respectively in mg/L.I max and I avg refer to the average and maximum value of the pollution index corresponding to element i.The classification of pollution with an element i according to the I i value as well as the total pollution by heavy metals according to their NCPI value is listed in Table 1. (3) (5) Table 1.Classification of pollution level in groundwater 39 .www.nature.com/scientificreports/

Evaluation of the non-carcinogenic human health risks
The estimation of the human health risk due to the exposure to heavy metals from water was covered by the United States Environmental Protection Agency (US EPA) reports in 1989 40 and 2004 41 .The human health risk was attributed to take place as a result of 2 pathways which are the ingestion and dermal exposure.The total non-carcinogenic exposure can be figured out by estimating chronic daily intake (CDI) of heavy metals in mg/kg/day.The risk factor resulting from the exposure to non-carcinogenic ingestion and dermal doses can be obtained by determining the hazard quotients (HQ ingestion and HQ dermal ).Hazard Quotient is the ratio of the potential exposure level to a particular species to a reference dose level at which there will be no detrimental effects (RfD) 40 .The summation of the individual hazard quotients for various species and exposure pathways is called Hazard Index (HI).
The estimation of the health risks on human by heavy metals was conducted using Eqs.(7-11) which were obtained from US EPA reports 40,41 .The chronic daily intake (mg/kg/day) as well as the hazard quotients (HQ) received through ingestion and dermal exposures can be obtained using Eqs.(7-11) 41-44 : where ED is the duration of exposure of 70 years; EF is the exposure frequency of 350 days; IR is the ingestion rate of drinking water of 2 L/day; AT is the average time for non-carcinogens of 25,550 days; BW is the average body weight of 70 kg; ET is the exposure time of 0.2 h/day; SA is the skin area that is exposed which is 18,000 cm 2 ; Kp is the dermal permeability constant which has a value of 0.001 cm/h for Ba, B, Cu, Mn, Sr and V 41 and 0.002 cm/h for Cr and 0.006 cm/h for Mo 41,44 ; AB f is the absorption factor corresponding to dermal exposure which is 0.001; C f is the conversion factor between L and cm 3 which accounts for 1 L = 1000 cm 3 ; RfD is the reference dose for the investigated elements which is 70, 200, 3, 40, 20, 5, 1, 600 and 1 µg/kg/day for the oral ingestion pathway of Ba, B, Cr, Cu, Mn, Mo, Ag, Sr and V, respectively and 14, 180, 0.06, 12, 0.8, 1.9, 3, 120 and 0.01 µg/kg/day for the dermal pathway [44][45][46] .Hazard Index (HI) can be obtained by the summation of all of the individual hazard quotients due to the exposure to heavy metals through dermal and ingestion pathways.

Evaluation of the carcinogenic human health risks
The potential of cancer risks due to the exposure of public to groundwater containing carcinogenic heavy metals can be estimated using the US EPA's Incremental Lifetime Cancer Risk (ILCR) equation 42 : where CSF is the cancer slope factor (mg/kg/day) and CDI is the chronic daily intake of carcinogenic heavy metals through the lifetime consumption of groundwater (mg/kg/day).
The CSR can be determined from the risk generated when 1 mg of a particular contaminant is consumed per 1 kg of human body weight for a lifetime.In other words, the ILCR is used to show the probability of the potential health risks developed from the lifetime consumption of groundwater at the existing concentration of carcinogenic heavy metals.The ILCR value must be within the range of 1 × 10 -6 -1 × 10 -4 in order to be considered acceptable by regulatory bodies.

Results physical and chemical parameters of groundwater
Figure S1 shows the investigated groundwater wells in Qatar.Table 2 shows the measured physical parameters of the investigated groundwater samples in comparison with WHO standards and Qatar Electricity and Water Corporation (Kahramaa) drinking water standards 47 .The local drinking water standards reported by Kahramaa, which owns and operates the distribution and transmission of electricity and water in Qatar, was issued in 2014.Figure 2 shows the variation in the pH, and TDS across the map of Qatar.As seen, the pH range and average of the studied samples was found to be 6.03 ± 0.235-7.92± 0.22 and 7.4 ± 0.48, respectively which was observed to fall within the WHO and Qatari drinking water standards.This was observed to be in good agreement with the work published by Shomar 36 .
On the other hand, the TDS values had a range of 703.3 ± 146-31,710 ± 715 mg/L.The United States Salinity Laboratory (USSL) issued a handbook on the categorization of irrigation water 48,49 .Table 3 depicts the categorization of irrigation water according to their EC and TDS.As seen, the salinity level in the analyzed groundwater was found to be high.In fact, about 5% of the samples had salinity levels between 500 and 1500 mg/L which was ( 7) www.nature.com/scientificreports/ reported by USSL to damage plants with low salinity tolerance.On the other hand, about 95% of the samples were found to have a hazard class that is very high with consequent damage to high salinity tolerant plants.Moreover, the average TDS value was 5454 ± 4664 mg/L which is way greater than 1500 mg/L limit which describes water with very high salinity.
The spatial distribution analysis of TDS in Qatar conducted by Shomar showed that the northern parts of Qatar are characterized with more brackish groundwater compared with south 35 .This can be attributed to the fact that up to 10% of the estimated annual rainfall of 25 million cubic meters in Qatar occurs on the northern part whereas up to 5% only takes place on the southern parts 50 .
In the present work, the determination of the major cations and anions was carried using ion chromatography.Table 4 shows the level of Potassium (K + ), Sodium (Na + ), Lithium (Li + ), Magnesium (Mg 2+ ), Calcium  Table 3. Categorization of saline water according to their EC and TDS as reported by USSL 48,49 .) in 82 groundwater samples in Qatar.Moreover, Fig. 3 shows ArcGIS maps depicting the spatial variations in the level of major cations and anions in Qatar.The level of chloride, bromide, sulphate, sodium, magnesium and calcium in the present study were found to exceed Qatar drinking and crop irrigation standards as presented in Table 4.The sulphate level in the groundwater ranged between 2.22 and 3680 mg/L with a mean of 1383 ± 901.34 mg/L.While the recommended sulphate level in drinking water should not exceed 50 mg/L, it was observed that 76 locations (or 93% of the studied samples) had sulphate levels above that.Similarly, the Qatari crop irrigation limit for sulphate should be below 400 mg/L 7 .It was found that 67 locations (82% of the studied samples) exceeded the Qatari crop irrigation limit.The elevation in the level of sodium and chloride in the studied samples above the drinking standards showed 78 and 81 exceedances, respectively.Likewise, the elevation in the level of calcium in the studied samples above the drinking standards of 80 mg/L showed 82 exceedances or 100% of the analyzed samples.It was worth pointing out that the bromide level in 7 locations  www.nature.com/scientificreports/ in the present study exceeded Kahramaa's drinking water limit of 0.1 mg/L.Boron (B) is a trace element that leaches out from natural rocks due to its well solubility in water and can be found in some water bodies such as groundwater and seawater.Boron is key for osteogenesis and bone development; however, boron can be harmful to humans above safe levels.The Qatari drinking water guidelines for boron should not exceed 1 mg/L.Boron level as high as 4.1 mg/L was observed in the present study which was greater than the allowable limit set by WHO of 2.4 mg/L and Kahramaa of 1 mg/L in drinking water.About 29% of the studied groundwater samples showed boron levels that are greater than the WHO drinking water limit of 2.4 mg/L 51 whereas 80% showed boron levels that exceeded Kahramaa drinking water standards.Boron at high levels can cause toxicity to plants and may be responsible for several male fertility problems.The presence of boron in groundwater could be attributed to the desorption of boron from mineral rocks by infiltration rainwater as well as the infiltration of wastewater which is characterized with elevated boron levels from detergents and soaps 33,52 .The boron level in groundwater was found to agree with the level reported by Ahmad 33 with a range of 0.3-3.8mg/L and a mean of 1.8 mg/L.Moreover, it was also found to agree well with the boron level reported by Schlumberger during their analysis of groundwater in Qatar with a range of 0.25-5.63mg/L and an average of 2.06 mg/L 7 .The removal of boron and other major contaminants can be achieved by using various treatment techniques such as membrane filtration or adsorption [53][54][55][56][57][58][59][60][61][62][63][64][65] .For instance, the preparation of novel sustainable adsorbents from natural materials, such as banana peel 62 , olive stones 63 and pistachio shell 65 as well as carbon nanotubes 64 for the removal of various pollutants in water was covered in literature.The developed adsorbents such as activated carbon and biochar performed well in the removal of PAH as well as pharmaceuticals (ciprofloxacin and diclofenac) from wastewater.

Low
The studies reported that the adsorbent can be potentially used for large scale application 65 .
The piper diagram plotted for 10 groundwater samples is shown in Fig. 4. The cations triangle in the present work was observed to be dominated with sodium and potassium type while the anions triangle was dominated by chloride and sulfate types.The plotting of the investigated groundwater samples on the diamond-shaped diagram in Fig. 4 showed that the groundwater in Qatar is dominated by sodium-potassium-chloride-sulfate as well as calcium-magnesium-chloride-sulfate types.In other words, groundwater in Qatar was found to mainly exist in 2 areas which are: (1) Ca-Mg and Cl-SO 4 as well as (2) Na-K and Cl-SO 4 .This was observed to be in good agreement with the characterization and development of piper diagrams for groundwater in Qatar 7,33 which reported the groundwater to vary from sodium to calcium type and chloride to sulphate type.It was also reported that most of the groundwater in Qatar is dominated by sulfate and chloride anions.
The spatial distribution analysis of the major cations and anions in Qatar depicted in Fig. 3 shows that northern parts have relatively lower concentrations of these elements.This could be confirmed by observing the levels of Fluoride, Chloride, Nitrite, Bromide, Sulfate, Nitrate, Potassium, Magnesium, Calcium and Boron which are concentrated in the southern part of the country.This can be attributed to the fact that up to 10% of the estimated annual rainfall of 25 million cubic meters in Qatar occurs on the northern part whereas up to 5% only takes place on the southern parts 50 .www.nature.com/scientificreports/ The measurement of the heavy metals content in the groundwater was conducted using ICP-OES.Figure 5 depicts the level of some of the heavy metals in the groundwater compared with Qatari, WHO or US EPA standards in ppb (a) and ppm (b).The determination of the levels of heavy metals in the present work showed that all the sample results fall within the acceptable limits set by WHO and Qatari standards except for molybdenum and strontium which were found to be higher than the WHO and Qatari Standards.Moreover, Fig. 6 depicts the spatial variations in the level of heavy metals in groundwater of Qatar using ArcGIS maps.In the present work, with the exception of few outliers, the elevation in the heavy metals levels in the groundwater in the present study do not indicate the existence of severe health threat 7 .The main concern associated with groundwater in Qatar is related to the presence of sodium and high salinity levels.
The level of Mo in the present study was observed to have a range of 1-319.5 ppb and an average of 37.9 ppb which was found to agree with the level reported by Ahmad (Range: 7.8-294 ppb and average: 53.9 ppb) 33 .Moreover, the analysis reported by Schlumberger in 2009 showed similar levels and attributed the elevation in molybdenum due to the presence of evaporite conditions (high temperature accompanied with low precipitation rate) in Qatar 7 .The spatial distribution analysis of the elevation in molybdenum level showed their occurrence in the center areas of Qatar.The reason behind the elevation in the level of molybdenum could be attributed to the enrichment of these metals by the existence of hydrogeochemical environment.This also could be attributed to the contamination from the well itself as a result of the corrosion in the pump or well casing 7 .It is worth pointing out that the elevation in the molybdenum level in the groundwater was few orders of magnitude higher than normal levels which indicates the anthropogenic sources of this elevation 7 .Moreover, Kuiper investigated the level of Mo in groundwater of Qatar 66 and reported a range of 2.7-103 ppb and average: 26.9 ppb 35 .The reason behind the elevation in the Mo level in groundwater can be attributed to several factors such as (1) longer groundwater residence time, (2) abundance of sulfide and iron oxides minerals, (3) neutral to alkaline groundwater pH and (4) existence of reducing conditions in the aquifer 66,67 .The presence of saline groundwater was reported to enrich Mo levels in groundwater as a result of the impediment in the sorption of Mo in soils that are characterized with arid and alkaline conditions 66,[68][69][70][71] .As seen in the present work, the geological and chemical properties of the groundwater in Qatar was observed to agree with the factors discussed above in terms of soil salinity, alkalinity and pH.The existence of dolomite, gypsum and limestone formations in Qatar will give rise to the elevation in Mo level following the demineralization of geological formations.
The level of lead in the groundwater of Qatar was observed to be below the detection level of 1 µg/L which is definitely below the WHO drinking water standard of 10 µg/L.This was found to agree with the work published by Ahmad 33 which reported the lead level in the groundwater of Qatar to be below the detection level.
The level of strontium in the present work had a range of 16.39-24,347.58and an average of 11,516.73± 4802.95 µg/L.The reported levels are not considered high as there are no WHO guidelines/limit on the allowable level of strontium in drinking water.A recommended level of 4 mg/L 7,72 , followed in the present study, was reported by US EPA and was also adopted by the local distributor of water in Qatar 73 .Moreover, the range of the reported levels were found to agree with the levels found in Ahmad's work who investigated the levels of Sr in groundwater of Qatar and reported similar levels.For instance, the Sr level in groundwater of Qatar was reported to have a range of 3534-20,273 and an average of 13,226 µg/L 33 .The Sr level in groundwater was attributed by Ahmad to occur due to the evaporite deposits 33 .Moreover, the level of Sr in groundwater of Qatar was reported by Shomar 35 who analyzed the level of Sr in groundwater as well as soil and reported a ratio of Ca:Sr in soil to be correlated with the ratio in groundwater.The increase in the salinity of the soil was found to increase the level of Sr in soil due to the characteristic high level of Cl − and SO 4 in saline soil which is also rich in Sr 35 .
Table 5 shows the Pearson correlation coefficients between the investigated parameters in the present work.Normally, Pearson correlation coefficient values vary between − 1 and 1.A Pearson correlation coefficient of − 1 indicates a completely negative linear correlation between the two variables whereas a value of 1 indicates a completely positive correlation.Moreover, a value of 0 designates a no correlation between the investigated variables 74 .Furthermore, a Pearson correlation coefficient < 0.3 designates a weak correlation whereas values between 0.3 and 0.7 have moderate correlations and correlations having values ≥ 0.7 are considered strongly correlated 75 .As seen in Table 5, pH showed an intermediate correlation with F − with a value of − 0.32 which Figure 5.The average level of heavy metals in the groundwater samples compared with Qatari, WHO or US EPA standards in ppb (a) and ppm (b).*LOR: limit of reporting, which is 0.05 ppb for Hg, 1 ppb for Pb, 0.5 ppb for Mn, 1 ppb for Ni, 1 ppb for Cu, 0.05 ppm for Fe, 0.05 ppm for Zn, 0.05 ppm for Al, 0.5 ppb for Cd, 0.1 ppm for Sb, 0.05 ppm for As, 0.1 ppm for Se, 0.5 ppb for Be, 1 ppb for Ag, 0.1 ppm for TI, and 0.1 ppm for U.
indicates that pH and F − are inversely proportional to each other.An inverse relationship between pH and F − was reported by Sivasankar 76 and Umarani 77 in Tamilnadu (India).Moreover, the correlation coefficients between TDS and Cl − showed a strong correlation with a Pearson correlation coefficient of 0.94.On the other hand, TDS was moderately correlated with F − , SO 4 2− , NO 3 − and Ca 2+ , Na + , Mg 2+ and K + with a correlation coefficient range of 0.32-0.64.This was found to agree with the values reported by Sakram 78 who reported a moderate correlation between the TDS and F − , Ca 2+ , Na + , Mg 2+ and K + .Furthermore, Ca 2+ showed a strong correlation with Cl − and SO 4 2− with a correlation coefficient of 0.71 and 0.82, respectively.Likewise, Mg 2+ and Na + were strongly correlated with a correlation coefficient of 0.90.
On the other hand, Sr has also a moderate correlation with TDS, Na + , Cl − , F − , SO 4 2− , Mg 2+ , Ca 2+ and K + as confirmed by the Pearson correlation coefficients which were between 0.3 and 0.7.These correlations were also confirmed by Ahmad 33 who reported the Pearson correlation coefficients of Sr with TDS, Na + , K + , Ca 2+ Mg 2+ , F − , Cl − and SO 4 2− in groundwater of Qatar to be 0.58, 0.46, 0.45, 0.65 and 0.68, 0.59, 0.49 and 0.67 respectively, which were found to be in the moderately-correlated zone.The level of NO 3 − had a intermediate correlation with SO 4 2− with a Pearson correlation coefficient of 0.47.This was found to agree with the correlation reported by Burhan 79 with a Pearson correlation coefficient < 0.7.
On the other hand, the correlation between Mo and SO 4 2− in the present work was observed to be moderately correlated with a Pearson correlation coefficient of 0.33.This was also reported by Tasneem who analyzed the hydrochemistry of the groundwater in Wadi Al Arab aquifer, which is located on the northwestern side of Jordan 80 and found a moderate correlation between Mo and SO 4 2− in groundwater.

Pollution assessment
Table 6 shows the statistical analysis of the pollution index of as well as NCPI for the investigated 82 groundwater samples.The estimation of the pollution index and NCPI helps categorize the degree of pollution in the www.nature.com/scientificreports/groundwater by comparing the I i and NCPI values with the values published in US EPA's report.The average I i values in the present work were found to range from 0.04 to 2.84 whereas the maximum values ranged between 0.58 and 6.09.According to the average I i values in the table, degree of pollution in the groundwater was observed to increase in this order: Mn < Cr < Ag < V < Mo < Sr.As per the classification of pollution in Table 1, there is no pollution in terms of Mn, Ag, Cr, V and Mo; however, the average pollution index value shows that there is a moderate pollution in terms of the Sr content in the groundwater.The level of Sr in groundwater was found to be higher than the US EPA's recommended level in drinking water of 4 mg/L despite the absence of any recommended Sr level in WHO's drinking water guidelines.The estimation of the pollution index in the present work was based on the US EPA's recommended level in drinking water (4 mg/L) 7,72 .This was also seen in the pollution index values corresponding to the average level as well as the maximum level of Sr which were 2.84 and 6.09, respectively.According to Table 6, the pollution indices corresponding to the average and maximum levels of Sr categorize the groundwater in Qatar into moderate and high pollution, respectively.Also, in terms of Sr level, about 19% of the samples showed light pollution and 28% showed moderate pollution whereas 46% showed high pollution.In terms of Vanadium, despite showing a no pollution category based on the average I i value, the maximum Ii value corresponding to Vanadium (Ii = 0.74) was found to be in the slight pollution category based on a safe limit of 100 µg/L suggested by Dr. Meisch 46,81 .Moreover, despite the fact that the average Mo level in groundwater fall in the no pollution category, the highest level of Mo in groundwater showed a pollution index of 4.56 which falls in the high pollution category based on a US-EPA level of 70 µg/L 82 .About 14% of the samples have either slight or light pollution while 5% of the samples showed moderate or high Mo pollution.About 82% of the samples showed no Mo pollution.The spatial variations of the level of Mo and Sr in Qatar can be depicted in Arc GIS maps in Fig. 7.
The Nemerow Index values in the present work was found to range from 0.01 to 4.39 with an average of 2.08.The average NCPI value categorizes the pollution in groundwater into moderate pollution with the main pollution source coming from Sr with a pollution index of 2.88.About 7% of the samples showed an NCPI in the slight pollution category whereas 30% were in the light pollution category.About 42% of the samples showed NCPI in moderate pollution and 16% in high pollution.Figure 7 shows the spatial variation in the level of NCPI across the map of Qatar.
The main findings of this work were observed to agree with the work reported in literature.For example, the evaluation of the heavy metal pollution in Huaibei Plain farmlands (Xiaoxian County) in China was performed by Zhang 28 .The study analyzed the levels of Ni, Cr, Cd, Cu and Zn in 69 groundwater samples and health impacts on public.The work showed that the NCPI had a range of 0.25-9.3with an average of 1.9.Moreover, the  23 .The study showed that 31.9% of the samples had an NCPI range of 1-4.2 while 43% had NCPI values > 4.2.It was also reported that there is a serious pollution in the Jinghui canal, and the groundwater is not suitable for drinking or irrigation.Similarly, Mamat 29 also analyzed the pollution of Ibinur Lake Basin in China with Pb, Se, As, Mn, Zn, Cu, Cd and Cr by analyzing 75 groundwater samples.The study revealed that the NCPI of the studied groundwater samples ranged between 0.1 and 3. Likewise, Singh 27 studied the pollution of groundwater in Haridwar district in India and found out that 84% of the samples were slightly polluted while the pollution index had a range of 0.99-4.32.The main findings of the present work in comparison with the results reported in literature is presented in Table 7.

Evaluation of the non-carcinogenic human health risk
The estimation of the non-carcinogenic human risk to human health was performed using Eqs.(7-11).In the present work, the chronic daily intake (CDI) as well as the hazard quotient and hazard indices for the investigated heavy metals (Ag, Mn, Cr, V, Mo and Sr) in all of the studied groundwater samples was calculated.Moreover, Table 8 lists the min, max as well as the average values corresponding to CDI Total and HQ.It was observed that the total chronic daily intake (through ingestion and dermal pathways) for all the studied metals was found to have a range of 1.4 × 10 -5 -6.7 × 10 -1 mg/kg/day.The chronic daily intake through ingestion pathway was observed to be more pronounced in comparison with the dermal pathway.For instance, the CDI ingestion is more than 500,000 times the CDI dermal of Sr and V and more than 270,000 the value of CDI dermal corresponding to Cr.This can be attributed to the low absorption rate at the skin compared to the high absorption rate through the gastrointestinal tract or digestive system which increases the toxicity of ingested substances.The protection mechanism accompanied by the smaller surface area of the skin will also be responsible for the reduction of the CDI dermal compared with CDI ingestion and hence, ingestion is the major pathway as confirmed by the CDI ingestion and CDI dermal values in the present work.According to the minimum values of the total chronic daily intake of heavy metals, CDI Total was found to range between 1.4 × 10 -5 and 4.5 × 10 -4 and increase in this order: Cr = Mn < Mo = V = Ag < Sr.Furthermore, according to the maximum values of the total chronic daily intake of heavy metals, CDI Total had a range of 7.9 × 10 -4 -6.7 × 10 -1 mg/kg/day and were observed to increase in this order: Mn < Cr < Ag < V < Mo < Sr.The maximum CDI Total value encountered corresponded to Sr which had a value of 6.7 × 10 -1 mg/kg/day.According to the average values, it was found that total chronic daily intake of heavy metals followed this order: Mn < Cr < Ag < V < Mo < Sr.Moreover, the average CDI Total had ranged between 4.9 × 10 -5 and 3.1 × 10 -1 mg/kg/day.The average hazard quotients of all the studied heavy metals presented in Table 8 were found to range between 2.5 × 10 -3 and 7.4 × 10 -1 and increase in this order: Mn < Cr < Ag < Mo < Sr < V. On the other hand, the maximum HQ ranged between 3.9 × 10 -2 and 1.89 and followed the order: Mn < Cr = V < Sr < Mo < Ag.The total hazard quotient value at the minimum trace metals levels was observed to range between 7.5 × 10 -4 and 2.7 × 10 -2 .Likewise, the total hazard quotient at the maximum level of trace metals had a range of 3.9 × 10 -2 and 1.75.The order of the HQ Total values at the minimum levels was Mn < Sr < Cr < Mo < Ag = V while the order at the maximum levels was Mn < Cr = V < Sr < Mo < Ag.Despite the absence of any mention of vanadium metal in Kahramaa, WHO or US EPA drinking water standards, the evaluation of the non-carcinogenic human health risk associated with vanadium was based on the reference dose (RfD) for ingestion and dermal exposure recommended by US EPA and reported in literature [44][45][46] .The reference doses for oral ingestion (1 µg/kg/day) and dermal exposure (0.01 µg/kg/day) were used in estimating non-carcinogenic human health risk using Eqs.(9 and 10).For instance, the estimation of the health hazards in associated with vanadium in the groundwater water in lower Yellow River in China at a maximum vanadium level of 58 µg/L was reported to have an HQ ingestion and HQ dermal of 0.24 and 0.04 as well as an HQ of 0.29 45 .This was found to be in good agreement with the estimated HQ due to vanadium in the present work which had a maximum HQ value of 0.45.
The estimation of the hazard index (HI) for all the studied heavy metals was found to have a range of 0-3.2 whereas the average was 1.6 which is greater than 1.According to US EPA, HI values greater than 1 was reported www.nature.com/scientificreports/

Evaluation of the carcinogenic human health risks
The ILCR value generated from the consumption of groundwater at the existing Cr level reported in the present study can be obtained by first estimating the chronic daily intake of Cr which ranged between 1.4 × 10 -5 and 1.4 × 10 -3 mg/kg/day.The ILCR corresponding Cr in the present work was calculated using Eq. ( 12) keeping in mind the cancer slope factor of Cr of 41 kg/day/mg 32,42 .The ILCR was found to have a range of 5.6 × 10 -4 -5.5 × 10 -2 with an average of 2.8 × 10 -2 .According to the US EPA, ILCR values between 1 × 10 -3 and 1 × 10 -1 is considered at moderate risk level as it will create a cancer risk of 1 in 1000 and 1 in 10.The ILCR in this work was observed to agree with the work cited in literature.For instance, the carcinogenic risks linked with elevated heavy metals levels in drinking water in Iran had a ILCR between 5 × 10 -4 and 7.6 × 10 -2 with Cr being the highest contributor with a mean ILCR of 6.5 × 10 -342 .Moreover, the assessment of the heavy metal pollution in 69 groundwater samples in Huaibei Plain farmlands in China showed that 19 samples had ILCR of Cr that was greater than 10 -4 and that 27.5% of samples had reported health risks associated with them 28 .It was also observed that Cr is the main contributor to human health risks.Likewise, the investigation of the carcinogenic health effects due to heavy metals in drinking water in Nigeria 83 showed an ILCR of one groundwater source to be 0.14 for adults and 0.16 for children which require urgent attention.The investigation of the human health effect due to the trace metals in the north plain area in China 45 reported the carcinogenic CDI to have a range of 5.1 × 10 -5 -6.9 × 10 -4 .

Conclusions
In the present work, a comprehensive overview of the quality of groundwater in Qatar in terms of heavy metals content as well as investigating the cause of effect of the elevation in their levels above the WHO/US-EPA standards was presented.The assessment of the human health risks associated with the existing levels using three of the most used models showed some levels which could be of concern to public health.The chronic daily intake (CDI) of the investigated heavy metals (Ag, Mn, Cr, V, Mo and Sr) through ingestion and dermal pathways had a range of 1.4 × 10 -5 -6.7 × 10 -1 mg/kg/day while the NCPI's range was reported at 0-4.39.Moreover, the HI and ILCR were found to have a range of 0-3.2 and 5.6 × 10 -4 -5.5 × 10 -2 , respectively.The assessment of human health risks of groundwater in Qatar in the present work indicated that further investigation must be conducted in order to fully understand the level of contamination and prepare for remediation measures in order to protect public

( 2 )Figure 1 .
Figure 1.Location of groundwater sampling points selected in the present study.

Figure 2 .
Figure 2. ArcGIS maps depicting the spatial variations in the level of pH (a) and TDS (b).

Figure 4 .
Figure 4. Piper diagram for groundwater in Qatar.

Figure 7 .
Figure 7. ArcGIS maps depicting the spatial variations in the pollution index of molybdenum (a), strontium (b) and vanadium (c) as well as NCPI of the heavy metals (d) in groundwater.

Figure 8 .
Figure 8. ArcGIS maps depicting the spatial variations in the HI due to the heavy metals in groundwater.

Table 2 .
Physical parameters of the analyzed groundwater samples.

Table 4 .
The major anions as well as cations analyzed in the collected groundwater samples compared with Kahramaa and WHO standards.

Table 5 .
Pearson correlation coefficients between the investigated parameters.

Table 6 .
Pollution index and NCPI of heavy metals in groundwater.assessment of the health effects associated with Pb, As and Cr 6+ in groundwater in the Jinghui Canal in China was reported by Zhang