Carcinogenic and Non-Carcinogenic Health Risk Evaluation of Heavy Metals in Water Sources of the Nubian Sandstone Aquifer in the El-Farafra Oasis (Egypt)

Abstract

Expanding anthropogenic activities, globally and in Egypt, have increased concentrations of heavy metals in surface and ground waters. Contamination of drinking water may threaten public health. In the present study, the concentrations of 10 heavy metals were analyzed from natural springs (6) and drilled wells (10) in the Nubian Sandstone Aquifer of the El-Farafra Oasis and the White Desert National Park, Egypt. The average concentrations of heavy metals were in most cases below critical values of the WHO drinking water standard, except for Fe and Mn (average values were 495 and 107 µg·L⁻¹, respectively). There is a surface circulation that develops within limestone (Post-Nubian Aquifer System—PNAS) and feeds the springs, while the water present in the wells (at least for the deeper ones) comes from the ferruginous sandstone (Nubian Sandstone Aquifer System—NSAS). This double circulation could account for the differences in the EC and TDS values (typical of a circulation in limestone-type aquifers for springs) and the Fe and Mn enrichment coming from the ferruginous sandstone of the NSAS. The average chronic daily intake (CDI) values for heavy metals in the study area are listed in decreasing order in the following: Fe > Mn > Zn > Co > Ni > Cr > As > Pb > Co > Cd. The total hazard quotient (HQ_total = HQ_oral + HQ_dermal) and Hazard Index (HI) values calculated for different heavy metals were well below the acceptable limit, indicating no significant non-carcinogenic health risks to the residents of both areas via oral and dermal absorption of drinking water. Furthermore, the results obtained for the total risk to human health showed that oral ingestion is the major pathway. Carcinogenic risk analysis indicated that the Incremental Lifetime Cancer Risk (ILCR) values for Pb, Cd, Ni, and Cr were well below the acceptable limits.

1. Introduction

The pollution of the aquatic environment with organic and inorganic pollutants has become a worldwide problem in recent years as they are indestructible and most of them have toxic effects on organisms. The major organic pollutants are polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs). These pollutants are ubiquitous contaminants in different environments.

During the past decades, the monitoring and assessment of groundwater contamination have received widespread recognition [1,2,3]. Extremely low concentrations of certain heavy metals, such as Fe, Cu, Zn, and Co, may be essential for human health, while concentrations above a specific threshold are mostly harmful [2,4]. However, metals such as Cd, Pb, Hg, Cr, and Ni are carcinogenic and even lethal already at very low levels [5]. In general, heavy metals, carcinogenic and potential toxic elements (PTEs), are harmful to living organisms [6]. Therefore, health risk assessments are carried out to estimate the critical threshold concentrations when heavy metals turn carcinogenic for people [4,7]. Water used for the industrial, domestic, and agricultural sectors, both from surface- and groundwater-dependent ecosystems, is often contaminated with heavy metals [8]. Groundwater-dependent resources are characterized by flow spheres and intricate hydrological formations [9].

Extensive anthropogenic activities may impact groundwater quality; therefore, geochemical and hydrogeological studies are required to assess its quality [10,11]. Due to their toxicity, heavy metals lead to a variety of degenerative health conditions, such as muscular and joint pain, mental and gastrointestinal disorders, immunity weakness, chronic weariness, and eyesight complications [6].

Heavy metals enter water resources via natural and anthropogenic paths [12]. Several heavy metals are natural constituents of the Earth’s crust. Rock weathering and decomposition transfer them to surface and groundwater and expose humans to them [13,14]. However, most heavy metals in water are produced and released by human activities, and metals accumulate in the food chain and vegetation, affecting human health [15]. For example, heavy metals are released from fossil fuel combustion, waste disposal, pesticides, and fertilizers, as well as from atmospheric deposition derived from smelting and mining operations [15].

Health risk evaluation and assessment are important approaches for assessing the impacts of heavy metals, singly and in concert, on drinking water safety [16,17]. The HQ (hazard quotient), HI (Hazard Index), and carcinogenic and non-carcinogenic risk exposure are applied in several parts of the world [2,13,14,18]. With the projected increase in global human population and expected climate change, it is plausible that the domestic water supply will become more reliant on groundwater as a source of drinking water in the future.

The objective of the present study was to evaluate the spatiotemporal distribution and concentration of heavy metals in the groundwater emerging from 16 natural springs and drilled wells from the Nubian Sandstone Aquifer in the El-Farafra Oasis and the White Desert National Park (WDNP) in the Western Desert of Egypt. As a part of the New Valley Project, which began in the 1960s to harness groundwater, nowadays this desert area is involved in the “1.5-million-feddan reclamation project”. Another applicable goal was to assess the carcinogenic and non-carcinogenic health risks of these heavy metals.

This work is the second outcome of our project on the assessment of the hydrogeological setting and chemical quality of the El-Farafra Oasis and the WDNP (Western Desert of Egypt) groundwater resources in relation to human uses. In the first part, we evaluated the groundwater suitability for human drinking and irrigation purposes [19].

2. Materials and Methods

2.1. Study Area

El-Farafra Oasis (Figure 1), situated between 26°00″–27°30″ N and 26°30″–29°00″ E, constitutes the smallest oasis dug out of the limestone plateau that forms the central region of Western Egypt [20,21]. This natural geological depression is located about 650 km SW of Cairo and covers an area of ~10,000 km² [22]. The climatic regime of the El-Farafra Oasis is marked by a hyper-arid desert climate with an average annual air temperature of about 22 °C and a precipitation of less than 10 mm [21,23]. The El-Farafra Oasis is dotted with a prominent geographical attraction known as the White Desert National Park (Sahara El-Beyda). The Park is located about 45 km N of the oasis.

The study area is characterized by the presence of an enormous chalky carbonate and white creamy layer rock formation that has been subjected to erosion via atmospheric agents and sandstorms for millions of years [20]. The Nubian Sandstone Aquifer System (NSAS), the world’s major non-renewable groundwater resource and the only existing water supply in this Saharan area, is the source of groundwater used in the El-Farafra Oasis and the WDNP [19,23,24]. The early Paleozoic to Cretaceous sandstone deposits that make up the NSAS are wedge-shaped, with the narrow edge of the wedge protruding in the highlands of Sudan and Chad to the south. The aquifer in Egypt is located beneath impermeable upper Cretaceous–Eocene rocks and is 1–2 km deep [25]. There is little groundwater recharge currently in much of the NSAS, which was replenished thousands to millions of years ago during pluvial periods [23,24]. Due to excessive usage of the world’s largest aquifer, the NSAS, oases have already been abandoned and their natural springs have been dewatered. This will eventually cause oases to disappear, with fundamental consequences on nature and people alike [23]. It has been shown that there is a direct correlation between the growing number of drilled wells and the sharp decline in naturally flowing springs (such as in Egypt’s Western Desert oases), and this link is expected to worsen because of the absence or disregard of sustainable exploitation strategies [19,24].

2.2. Samples Collection and Analysis

Water samples for heavy metal assessment were collected from 16 springs and drilled wells (

Table 1. GPS data of the springs and drilled wells investigated in the present study.

Samples No.	Name	Latitude	Longitude	Elevation	Temp.	pH	E.C.	T.D.S.
		(N)	(E)	m (a.s.l.)	°C		µS·cm−1	mg·L−1
S1	Ain El-Balad (Qasr El-Farafra)	27°03′24.6″	27°57′49.3″	117 ± 5.7	36.3	6.74	430	291
S2	Bir Sitta (Qasr El-Farafra)	27°04′49.6″	27°55′11.6″	74.2 ± 8.1	37	6.19	390	254
S3	Ain Bishwa (Qasr El-Farafra)	27°04′53.8″	27°58′10.3″	83 ± 3.9	23.8	7.78	1220	864
S4	Ain El-Hateyya (Qasr El-Farafra	27°04′54.2″	27°57′17.4″	82.1 ± 6.4	26.3	6.60	1290	901
S5	Bir Gelaw (Qasr El-Farafra)	27°00′46.8″	27°58′34.8″	107.7 ± 3.4	33.7	6.33	320	227
S6	Bir Felao (Qasr El-Farafra)	27°00′58.2″	27°55′41.1″	100.6 ± 5	38.4	6.85	330	227
S7	Bir at El-Saad Company for Land Reclamation (Sahl Baraka)	26°58′13.7″	28°14′31.3″	103 ± 3.8	34.6	6.15	530	322
S8	Bir 10 at Qarawein Company for Land Reclamation (Qarawein)	27°05′46.2″	28°31′44.7″	98 ± 4.1	34.4	6.20	650	419
S9	Bir 7 (Lewa Soubah village)	27°04′18.7″	27°52′47.3″	63.7 ± 3.5	39	6.19	320	216
S10	Bir 4 (El-Nahda village)	27°08′35.6″	27°55′23.3″	54.8 ± 7.6	40.6	6.31	300	200
S11	Bir 150 (El-Nahda village)	27°09′00.5″	27°56′28.3″	58.2 ± 3.7	42	6.20	280	157
S12	Bir Kamel (El-Nahda village)	27°08′14.3″	27°56′21.4″	59.1 ± 3.7	39	6.20	230	143
S13	Bir Abd El-Azem (Esha Abd El-Rahman village)	27°07′12.5″	27°57′31.6″	63.7 ± 3.9	36.6	6.19	300	177
S14	Ain Khadra, also called “Ain El-Wadi” (the White Desert National Park)	27°22′15″	28°13′08.8″	31.3 ± 4.3	24	7.25	330	200
S15	Ain Maqfi, also called “Ain Abu Hawas” (the White Desert National Park)	27°24′54.9″	28°20′50.6″	39.3 ± 5.4	24.1	7.09	440	260
S16	Ain El-Serw (the White Desert National Park)	27°22′11.6″	28°20′43.3″	72.7 ± 4.3	29.3	6.86	520	292

) during the period of 9–11 April 2015 from the Nubian Sandstone Aquifer in the El-Farafra Oasis and the WDNP in pre-rinsed and de-contaminated polyethylene bottles with 10% nitric acid and then double-distilled water [26]. Polyethylene sampling bottles were properly labeled and returned to the laboratory for heavy metal determination. For the trace element analysis, 10 mL aliquots were transferred to 12 mL ultra-clean LDPE vials and acidified with ultra-pure HNO₃ to obtain 2% solutions (v/v); sample preparation was carried out using clean room procedures (class 1000 clean room) to avoid external contamination. The concentrations of 25 trace elements (Li, Be, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Sr, Ag, Cd, Ba, Pb, Bi, and U) were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-QMS; Agilent 7500ce, Santa Clara, CA, USA). The instrument is equipped with a standard peristaltic pump, a Micro Mist concentric nebulizer, a Peltier-cooled spray chamber, Plasma Forward Power, the Shield Torch System, and a Collision/Reaction Cell system. An autosampler (ASX 520, Cetac Technologies, Omaha, NE, USA) was employed to introduce the analytes into the plasma of the ICP-MS. Sample introduction was performed with a Micro Mist concentric nebulizer and Scott-type quartz glass spray chamber. The external calibration curve method was used for the quantification of the analytes. For trace element determination, five standard solutions (concentration range from 0.09 to 23.8 ng/g) were prepared by diluting a multi-elemental standard solution (ULTRA Scientific ICUS-1208, Santa Clara, CA, USA). For crustal elements, single calibration standard solutions were prepared to cover, for each element (Ca, Mg, Na, K, Fe, Al, Ba, Ti; Merck, Darmstadt, Germany, 1000 pmm), the concentration ranges present in real samples. The intensities of standard solutions were fitted using a linear regression, and the y-axis intercept at zero concentration, which was assumed to represent an average blank of the standards, was subtracted for calibration. Detection limits were calculated as three times the standard deviation of the blanks and are appropriate for the determination of all the elements in the analytical range of real samples. Accuracy was evaluated analyzing two Reference Materials provided by Environmental Canada (TMRAIN-95: rainwater; SLRS: spring water), certified for 21 and 22 trace elements, respectively. Our data agreed very well with certified values; the recovery ratio ranged between 88 and 126%.

2.3. Statistical Analyses

One-way ANOVAs on two groups (equivalent to two independent-sample t tests) were carried out using Past (free software for scientific data analysis, version 4.17) to test for equal means of selected parameters and factors (temperature, conductivity, manganese, iron, total dissolved solids) in the drilled wells and springs studied.

2.4. Human Health Risk Evaluation

2.4.1. Non-Carcinogenic Health Hazard

Risk evaluation and assessment involve the study of the possibility of occurrence of a given plausible amount of harmful health consequences over a given period [27]. The health risk evaluation of each heavy metal is based on the assessment of the risk level and is categorized as a non-carcinogenic or carcinogenic health hazard [28]. The hazard quotient (HQ), Hazard Index (HI), and Incremental Lifetime Cancer Risk (ILCR) were employed to estimate the potential heavy metal contamination and possible carcinogenic and non-carcinogenic health hazards via oral and dermal exposure to heavy metals in the groundwater from the Nubian Sandstone Aquifer in the El-Farafra Oasis and the WDNP. The target population group in this was the adults.

$${\mathit{CDI}}_{\mathit{oral}} (\mathrm{mg}·{\mathrm{kg}}^{-1}{\mathrm{day}}^{-1})=\frac{\mathrm{Chm}\times \mathrm{DI}\times \mathrm{ABS}\times \mathrm{EF}\times \mathrm{EP}}{\mathrm{BW}\times \mathrm{AT}}$$

(1)

$${\mathit{CDI}}_{\mathit{dermal}} (\mathrm{mg}·{\mathrm{kg}}^{-1}{\mathrm{day}}^{-1})=\frac{\mathrm{Cw}\times \mathrm{SA}\times \mathrm{Kp}\times \mathrm{ABS}\times \mathrm{ET}\times \mathrm{EF}\times \mathrm{EP}\times \mathrm{CF}}{\mathrm{BW}\times \mathrm{AT}}$$

(2)

The above given equations (Equations (1) and (2)) adopted by the United States Environmental Protection Agency (USEPA) were used to determine the chronic daily intake (CDI) via oral and dermal exposure routes, respectively [28,29].

In the equations, Cw is the heavy metal concentration in the water sample, SA (cm²) is the amount of skin area accessible for contact, Kp is the permeability coefficient in cm/h, ABS is a unitless dermal absorption factor, DI is the average intake of water in the area in L·day⁻¹, ET is the exposure time (hour/event), EF characterizes the annual exposure frequency (days·year⁻¹), ED is the exposure time in years, CF is the conversion factor (L·cm⁻³), BW is the body weight (kg/person), and AT is the average time (days). For estimation and calculation of the chronic daily intake via oral and dermal absorption, the input assumptions and their corresponding values are given in

Table 2. Input parameters for calculation of exposure and risk assessment though oral and dermal routes.

		Values
Parameter	Unit	Ingestion	Dermal Absorption	References
Heavy Metal Concentration (Cw)	mg·L−1	−	−	[14]
Daily Average Intake (DI)	L·day−1	2.2		[29]
Skin Surface Area (SA)	cm2	−	18,000	[29]
Permeability Coefficient (Kp)	cm·h−1		Fe, Cd, Cu, Mn, Co & Cr = 0.001,	[29]
			Ni = 0.0002, Zn= 0.0006, Pb = 0.0001
Exposure Time (ET)	h/event	−	0.58	[29]
Exposure Frequency (EF)	days·year−1	365	350	[29]
Exposure Duration (ED)	year	71.8	30	[29,30,31]
Conversion Factor (CF)	L·cm−3	−	0.001	[30,31,32]
Average Body Weight (ABW)	kg	70	70	[29,32]
Absorption Factor (ABS)		0.001	0.001	[29,33]
Average Time (AT)	day	25,550	25550	[29]

.

The hazard quotient (HQ) for each heavy metal was assessed by employing the ratio of calculated daily intake (CDI, mg·kg⁻¹·day⁻¹) of the metal ingested with contaminated drinking water to the reference oral dose (RfD) through oral and dermal absorption for the population. Summation of all the HQs provides an estimation of the potential health risks or HI. The HI caused by contaminated water is calculated by Equations (3) and (4), respectively.

$${\mathit{HQ}}_{\mathit{oral}}=\frac{CDI oral}{RfD oral}$$

(3)

$${\mathit{HQ}}_{\mathit{dermal}}=\frac{CDI dermal}{RfD dermal}$$

(4)

The CDI and RfD are expressed as mg·kg⁻¹·day⁻¹. The reference dose (RfD) and cancer slope factor (CSF) for various heavy metals are given in

Table 3. Reference dose (RfD) and cancer slope factor (CSF) for various heavy metals.

Element *	UTIL (Upper Tolerable Intake Level) (mg·day−1·person−1)	RfDoral(mg·kg−1·day−1)	RfDdermal (mg·kg−1·day−1)	CSF (mg·kg−1·day−1)	References
Cr	NE	3.00 × 10−3	0.015	41
Mn	11	0.014	0.014	−	* [34] * [35] * [36] * [37] * [38] * [39]
Fe	45	7.00 × 10−1	7.00 × 10−1	−
Co	NE	2.00 × 10−2	5.70 × 10−6
Ni	1	2.00 × 10−2	5.60 × 10−3	0.84
Cu	10	3.700 × 10−2	2.40 × 10−2	−
Zn	40	3.00 × 10−1	7.50 × 10−2	−
As	<0.14	3.00 × 10−4	3.00 × 10−4	−
Cd	0.064	5.00 × 10−4	5.00 × 10−4	6.1
Pb	0.24	3.60 × 10−3	3.60 × 10−3	8.5

.

The HI gives the total possible non-carcinogenic health risks caused by several heavy metals present in a drinking water sample. It was calculated using USEPA guidelines for oral and dermal absorption for area residents using Equations (5) and (6), respectively.

$$\begin{gathered} {\mathit{H}\mathit{I}}_{\mathit{Oral}} ={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{HQ}}_{ \mathrm{oral}}={\mathrm{HQ}}_{\mathrm{oral}}={\mathrm{HQ}}_{\mathrm{cu}}+{\mathrm{HQ}}_{\mathrm{fe}}={\mathrm{HQ}}_{\mathrm{mn}} \\ +{\mathrm{HQ}}_{\mathrm{zn}}+{\mathrm{HQ}}_{\mathrm{cr}}+{\mathrm{HQ}}_{\mathrm{pb}}+{\mathrm{HQ}}_{\mathrm{as}}+{\mathrm{HQ}}_{\mathrm{ni}}+{\mathrm{HQ}}_{\mathrm{co}}+{\mathrm{HQ}}_{\mathrm{cd}} \end{gathered}$$

(5)

$$\begin{gathered} {\mathit{H}\mathit{I}}_{\mathit{dermal}} ={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{HQ}}_{ \mathrm{dermal}}={\mathrm{HQ}}_{\mathrm{cu}}+{\mathrm{HQ}}_{\mathrm{fe}}+{\mathrm{HQ}}_{\mathrm{mn}} \\ +{\mathrm{HQ}}_{\mathrm{zn}}+{\mathrm{HQ}}_{\mathrm{cr}}+{\mathrm{HQ}}_{\mathrm{pb}}+{\mathrm{HQ}}_{\mathrm{as}}+{\mathrm{HQ}}_{\mathrm{ni}}+{\mathrm{HQ}}_{\mathrm{co}}+{\mathrm{HQ}}_{\mathrm{cd}} \end{gathered}$$

(6)

The calculated HI values were compared with the global standard values to find out the possibility of non-carcinogenic health impacts on the population of the area when HI > 1 and no non-carcinogenic health impacts when HI < 1 [37].

2.4.2. Carcinogenic Health Risk Assessment

The possibility of occurrence of carcinogenic health risk in response to the exposure to a certain dose of a heavy metal in a drinking water sample can be calculated using the ILCR [3,14,38]. The ILCR refers to the incremental probability of a person developing any kind of cancer over a lifetime owing to the twenty-four-hour per day exposure to a certain dose of heavy metal for 70 years [39]. Equation (7) is used for the calculation of lifetime cancer risk.

$$\mathbf{ILCR}=\mathrm{CDI}\times \mathrm{CSF}$$

(7)

where CDI is the chronic daily intake (mg·kg⁻¹·day⁻¹) and CSF is the cancer slope factor and refers to the risk generated by the lifetime mean amount of one mg·kg⁻¹·day⁻¹ of carcinogenic chemical. The prescribed limit for the ILCR is 10⁻⁶ for a single carcinogenic element and <10⁻⁴ for multi-element carcinogens [40].

3. Results

The concentrations of 10 heavy metals, temperature, conductivity, and total dissolved solids values for groundwater samples collected from 16 springs and drilled wells of the Nubian Sandstone Aquifer in the El-Farafra Oasis and the WDNP are reported in

Table 4. Heavy metal concentration (µg·L⁻¹), T (°C), CE (µS·cm⁻¹), and TDS (mg·L⁻¹) from springs and drilled wells in the Nubian Sandstone Aquifer of the El-Farafra Oasis and the White Desert National Park, Egypt.

Sites	Cr	Mn	Fe	Co	Ni	Cu	Zn	As	Cd	Pb	T	CE	TDS
Drilled wells
S2	0.119372	177.2177	545.5475	0.026926	0.286247	0.11249	14.15531	0.103312	0.001771	0.009463	37	390	254
S5	0.159037	103.2946	90.1865	0.02282	0.129873	0.382748	10.73514	0.138157	0.003134	0.01907	33.7	320	227
S6	0.125202	229.8951	1679.592	0.027751	0.248687	0.854854	6.741019	0.098807	0.001226	0.090233	38.4	330	227
S7	0.129763	87.02292	245.137	0.016241	0.134799	0.071018	5.539477	0.088233	0.001226	0.00338	34.6	530	322
S8	0.146351	130.0505	1377.297	0.045411	0.153779	0.124949	8.305639	0.182359	0.002862	0.139771	34.4	650	419
S9	0.103199	220.8075	697.2878	0.023991	0.271787	0.149543	8.339093	0.065525	0.002315	0.002274	39	320	216
S10	0.180237	205.5416	441.0293	0.017329	0.161211	0.114995	9.021342	0.07159	0.001498	0.004076	40.6	300	200
S11	0.088197	166.1444	1602.556	0.014526	0.10241	0.077896	7.320414	0.05755	0.003134	0.007558	42	280	157
S12	0.125882	125.8638	534.9328	0.018039	0.119695	0.616044	9.478833	0.072629	0.001771	0.013662	39	230	143
S13	0.095393	149.6548	236.9866	0.018682	0.122119	0.095456	8.477634	0.075057	0.000817	0.005715	36.6	300	177
Springs
S1	0.110907	37.13324	242.2767	0.046912	0.274857	0.078209	9.453508	0.100885	0.002452	0.012761	36.3	430	291
S3	0.417823	5.549176	88.5735	0.080239	0.49829	0.711812	2.220363	0.328666	0.004495	0.027879	23.8	1220	864
S4	0.3819	4.786005	60.77042	0.101349	1.538941	5.788521	4.928506	0.43216	0.005177	0.054179	26.3	1290	901
S14	0.107467	4.634081	29.42385	0.025508	0.125834	0.121665	7.550634	0.091699	0.001226	0.006083	24	330	200
S15	0.119226	8.042004	23.45446	0.021633	0.114364	0.114475	7.323754	0.07783	0.001908	0.004711	24.1	440	260
S16	0.13674	49.73614	29.94921	0.025805	0.143926	0.172211	10.87517	0.084417	0.001771	0.004998	29.3	520	292

. The descriptive statistics for the 10 heavy metals concentrations, along with the drinking water standards, are summarized in

Table 5. Descriptive statistics for heavy metals from 16 springs and drilled wells of the Nubian Sandstone Aquifer (El-Farafra Oasis), Egypt, and drinking water standards.

Metals	Heavy Metal Concentrations (µg·L−1)				Drinking Water Standards (µg·L−1)
	Mean	Min	Max	SD (±)	[41]
Cr	0.16	0.09	0.41	0.09	50
Mn	106.6	4.6	229.89	81.0	100
Fe	495.3	23.4	1679.59	566.7	300
Co	0.03	0.014	0.101	0.02	2000
Ni	0.28	0.10	1.54	0.35	20
Cu	0.6	0.07	5.79	1.4	50
Zn	8.1	2.2	14.15	2.7	5000
As	0.13	0.057	0.43	0.10	10
Cd	0.002	0.00081	0.0052	0.001	3
Pb	0.025	0.0023	0.14	0.04	10

. The concentration of chromium (Cr) in the study area was observed between 0.088197 and 0.417823 µg·L⁻¹ at S11 and S3, respectively. The manganese (Mn) concentration varied from a minimum of 4.634 to 229.8 µg·L⁻¹ at S14 and S6, respectively. Iron (Fe) concentration ranged from a minimum of 23.45 to a maximum of 1679.02 µg·L⁻¹ at S15 and S11, respectively. Cobalt (Co) concentration ranged from a minimum of 0.014526 µg·L⁻¹ to a maximum of 0.101349 µg·L⁻¹ at S11 and S3, respectively. The concentration of nickel (Ni) in the study area was observed between 0.10241 and 1.53 µg·L⁻¹ at S15 and S4, respectively. The copper (Cu) concentration varied from a minimum of 0.071018 µg·L⁻¹ to 5.7 µg·L⁻¹ at S7 and S4, respectively. Zinc (Zn) concentration ranged from a minimum of 2.220 µg·L⁻¹ to a maximum of 14.15 µg·L⁻¹ at S3 and S2. Arsenic (As) concentration varied from a minimum of 0.05755 µg·L⁻¹ to a maximum of 0.432 µg·L⁻¹ at S11 and S4, respectively. The concentration of cadmium (Cd) in the study area under investigation was observed between 0.000817 and 0.005177 µg·L⁻¹ at S13 and S4, respectively. Lead (Pb) concentration varied from a minimum of 0.002274 µg·L⁻¹ to a maximum of 0.1397 µg·L⁻¹ at S9 and S8, respectively.

The statistically significant differences presented in

Table 6. Minimum, maximum, and average values for selected parameters and factors (temperature, conductivity, manganese, iron, total dissolved solids) in the drilled wells and springs studied. The last column reports the results of one-way ANOVAs on two groups carried out to show that the average values in springs and wells are not equal.

	Parameters		Statistics
	Drilled Wells	Springs
	T (°C)
Min	33.7	23.8
Max	42.0	36.3
Average	37.5	27.3	F = 29.22, p = 0.00009275
	E.C. (µS·cm−1)
Min	230.0	330.0
Max	650.0	1290.0
Average	365.0	705.0	F = 5.638, p = 0.03241
	Mn (µg·L−1)
Min	87.0	4.6
Max	229.9	49.7
Average	159.5	18.3	F = 44.23, p = 0.00001096
	Fe (µg·L−1)
Min	90.2	23.4
Max	1679.6	242.3
Average	744.0	79.0	F = 7.371, p = 0.01676
	T.D.S. (mg·L−1)
Min	143.0	200.0
Max	419.0	901.0
Average	234.2	468.0	F = 4.922, p = 0.04356

between the average values of the selected parameters and factors (temperature, conductivity, manganese, iron, total dissolved solids) in the drilled wells and springs studied highlight the following: there is a marked difference (almost an order of magnitude) in the average Mn and Fe content between spring waters and well waters, and there is a clear difference between the average EC and TDS values between springs and wells (the average value is double in springs).

The mean, maximum and minimum values of CDI and total CDI values for adults via oral and dermal absorption pathways in the area under investigation are summarized in

Table 7. Chronic daily intake (CDI) values for heavy metals through oral and dermal routes.

Metals	CDIoral			CDIdermal			CDItotal
	Mean	Min	Max	Mean	Min	Max	Mean	Min	Max
Cr	4.59339 × 10−9	2.99807 × 10−9	1.31316 × 10−8	9.18183 × 10−12	5.41 × 10−12	2.56 × 10−11	2.30129 × 10−9	5.41 × 10−12	1.31 × 10−8
Mn	3.57951 × 10−6	1.45643 × 10−7	7.22527 × 10−6	6.14853 × 10−9	2.84 × 10−10	1.41 × 10−8	1.79283 × 10−6	2.84 × 10−10	7.23 × 10−6
Fe	1.41904 × 10−5	7.3714 × 10−7	5.27872 × 10−5	2.85727 × 10−8	1.44 × 10−9	1.03 × 10−7	7.10949 × 10−6	1.44 × 10−9	5.28 × 10−5
Co	9.28348 × 10−10	5.10437 × 10−10	2.52181 × 10−9	7.68902 × 10−13	3.56 × 10−13	2.48 × 10−12	4.64558 × 10−10	3.56 × 10−13	2.52 × 10−9
Ni	6.4279 × 10−9	3.59431 × 10−9	1.56605 × 10−8	3.19208 × 10−12	1.26 × 10−12	1.89 × 10−11	3.21554 × 10−9	1.26 × 10−12	1.57 × 10−8
Cu	8.0244 × 10−9	2.23198 × 10−9	2.68668 × 10−8	3.45644 × 10−11	4.35 × 10−12	3.55 × 10−10	4.02948 × 10−9	4.35 × 10−12	2.69 × 10−8
Zn	2.76915 × 10−7	6.97828 × 10−8	4.44881 × 10−7	2.82228 × 10−10	8.17 × 10−11	5.21 × 10−10	1.38599 × 10−7	8.17 × 10−11	4.45 × 10−7
As	3.51562 × 10−9	2.05935 × 10−9	1.03295 × 10−8	7.4591 × 10−12	3.53 × 10−12	2.65 × 10−11	1.76154 × 10−9	3.53 × 10−12	1.03 × 10−8
Cd	6.32119 × 10−11	2.56645 × 10−11	1.4127 × 10−10	1.3261 × 10−13	5.01 × 10−14	3.17 × 10−13	3.16723 × 10−11	5.01 × 10−14	1.41 × 10−10
Pb	7.33402 × 10−10	7.14574 × 10−11	4.28267 × 10−9	1.46311 × 10−13	1.39 × 10−14	8.57 × 10−13	3.66774 × 10−10	1.39 × 10−14	4.28 × 10−9

and Figure 2 and Figure 3. Furthermore, the mean, minimum, and maximum levels of hazard quotient (HQ) and total hazard quotient (HQ) for adults through oral and dermal absorption pathways are given in

Table 8. HQ_oral, HQ_dermal, and HQ_total values for non-carcinogenic human health hazards posed by heavy metals in the study area via dermal and oral exposure routes.

Metals	HQoral			HQdermal			HQtotal
	Mean	Max	Min	Mean	Max	Min	Mean	Max	Min
Cr	4.48 × 10−6	1.31 × 10−5	2.77 × 10−6	6.50379 × 10−10	1.70727 × 10−9	3.60383 × 10−10	2.2401 × 10−6	6.56665 × 10−6	1.38614 × 10−6
Mn	3.68 × 10−3	7.23 × 10−3	1.46 × 10−4	6.53282 × 10−6	1.40906 × 10−5	2.8403 × 10−7	0.00184434	0.003619682	7.29633 × 10−5
Fe	1.65 × 10−2	5.28 × 10−2	7.37 × 10−4	3.03585 × 10−5	0.000102945	1.43756 × 10−6	0.00824087	0.026445058	0.000369289
Co	2.25 × 10−6	6.30 × 10−6	1.14 × 10−6	1.63392 × 10−7	4.96945 × 10−7	7.12275 × 10−8	1.2053 × 10−6	3.40073 × 10−6	6.06293 × 10−7
Ni	3.11 × 10−5	7.83 × 10−5	1.61 × 10−5	6.28071 × 10−13	3.49349 × 10−12	2.32478 × 10−13	1.5568 × 10−5	3.91514 × 10−5	8.04654 × 10−6
Cu	7.68 × 10−6	2.69 × 10−5	2.23 × 10−6	3.06039 × 10−12	2.95656 × 10−11	3.62731 × 10−13	3.8379 × 10−6	1.34334 × 10−5	1.11599 × 10−6
Zn	4.57 × 10−4	7.41 × 10−4	1.16 × 10−4	4.99779 × 10−12	8.67602 × 10−12	1.3609 × 10−12	0.00022832	0.000370734	5.81524 × 10−5
As	3.41 × 10−6	1.03 × 10−5	1.81 × 10−6	2.64177 × 10−9	8.82925 × 10−9	1.17578 × 10−9	1.7058 × 10−6	5.16917 × 10−6	9.04944 × 10−7
Cd	6.54 × 10−8	1.41 × 10−7	2.57 × 10−8	2.81797 × 10−10	6.34575 × 10−10	1.00101 × 10−10	3.2849 × 10−8	7.09524 × 10−8	1.28823 × 10−8
Pb	7.02 × 10−6	4.28 × 10−5	7.15 × 10−7	3.70131 × 10−13	2.03971 × 10−12	3.31798 × 10−14	3.5121 × 10−6	2.14133 × 10−5	3.57287 × 10−7
HI	2.06 × 10−2	6.05 × 10−2	1.40 × 10−3	3.70583 × 10−5	0.000117174	2.03886 × 10−6	0.01034163	0.030293143	0.000703278

and Figure 4. The carcinogenic risk evaluation and assessment for adults are presented in

Table 9. Incremental Life Cancer Risk (ILCR) values for carcinogenic human health risk through dermal and oral routes to the drinking water samples in the study area for adult residents.

Metals	ILCR
	Mean	Min	Max
Cr	9.43528 × 10−8	2.22 × 10−10	1.239 × 10−15
Ni	2.70106 × 10−9	1.05 × 10−12	4.23 × 10−17
Cd	1.93201 × 10−10	3.05 × 10−13	2.72935 × 10−20
Pb	3.11758 × 10−9	1.18 × 10−13	1.33516 × 10−17
Σ ILCR	1.00365 × 10−7	2.23 × 10−10	1.29468 × 10−15

.

4. Discussion

4.1. Geogenic Origin of the Heavy-Metal Enrichment of the Wells and of the Medium-High Solute Content of the Springs

The important differences we found between the average values of selected parameters and factors (temperature, conductivity, manganese, iron, total dissolved solids) in the drilled wells and springs studied could be explained by the existence of two underground feeding/circulation circuits. This double circulation has been highlighted by several studies [42,43].

The Nubian Sandstone Aquifer is composed of thick, coarse clastic sediments of sandstone and sandy clay interbedded with shale and clay beds. The sandstone beds represent the aquifer horizons. The drilled wells in El-Farafra are partially penetrating the aquifer due to its large thickness, which varies between 2525 and 2675 m, and the total depth of the wells ranges from 350 to 1200 m [43].

There is a surface circulation that develops within limestone (Post-Nubian Aquifer System—PNAS) and feeds the springs, while the water present in the wells (at least for the deeper ones) comes from the ferruginous sandstones (Nubian Sandstone Aquifer System—NSAS) (Figure 5). The two aquifers are separated by the low permeability confining layers of Upper Cretaceous to Lower Tertiary shales [42,43].

This double circulation could account for the differences in the EC and TDS values (typical of a circulation in limestone-type aquifers for springs) and the Fe and Mn enrichment coming from the ferruginous sandstone of the NSAS.

However, connections between the two systems occur locally and are characterized by leakage between sedimentary sequences due to the reduced thickness of the PNAS and NSAS or cross-cutting tectonic structures (deep distensive fault systems trending E–W and NE–SW) [42,43]. These faults have enabled the identification of highly permeable zones that may play a role as a pathway for the up-flow of hydrothermal fluid [44]. As a result of the presence of these faults, it cannot be excluded that the groundwater composition is influenced by the interaction with CO₂ -rich geothermal fluids that rise through the extensional faults.

Overall, these findings are in good agreement with the hypothesis that metal enrichments are primarily of natural origin.

4.2. Health-Risk Assessment

The presence and distribution of heavy metals in the water supply and pathway system may increase the risks to human health through various exposure routes (oral and dermal). Significant variation in the mean values of heavy metals was recorded in the spring water samples with the maximum concentration observed for Fe (1679.592 µg·L⁻¹) and the minimum concentration observed for Pb (0.002274 µg·L⁻¹). The heavy-metal toxicity order based on mean concentrations measured in the spring and drilled-well water samples of the area investigated was Fe > Mn > Zn > Co > Ni > Cr > As > Pb > Co > Cd. The assessment of human health hazards and risks involves the estimation of the type and level of negative health impacts that can develop in humans on exposure to toxic substances (USEPA, [44]).

4.3. Non-Carcinogenic Health Impacts

Human health risk evaluation encompasses the analysis of the nature and magnitude of the adverse health impacts in humans exposed to toxic substances in a contaminated environment [3,14]. Human exposure to heavy metals primarily takes place via pathways of drinking water, food, inhaled aerosols, and dust [44,45,46]. The daily intake of heavy metals is closely correlated with their level of toxicity to human health. The present study considered oral and dermal exposure through drinking water. The primary stage in the non-carcinogenic assessment is the calculation of total chronic daily intake (CDI_total) values (in mg·kg⁻¹·day⁻¹), which were 2.30129 × 10⁻⁹ for Cr, 1.792833 × 10⁻⁶ for Mn, 7.10949 × 10⁻⁶ for Fe, 4.64558 × 10⁻¹⁰ for Co, 3.21554 × 10⁻⁹ for Ni, 4.0294 × 10⁻⁹ for Cu, 1.38599 × 10⁻⁷ for Zn, 1.76154 × 10⁻⁹ for As, 3.6677 × 10⁻¹¹ for Cd, and 3.66774 × 10⁻¹⁰ for Pb (Table 5). Therefore, the average values of the CDI_total of heavy metal concentrations for adults are listed in decreasing order in the following: Fe > Mn > Zn > Cu > Ni > Cr > As > Co > Pb > Cd.

Most of the investigated heavy metals in the present study had HQs well below 1, except for Mn, Fe, and As (Table 6). As a result, the health risk evaluation of Mn, Cu, Zn, Co, Ni, Cr, As, Fe, Pb, and Cd resulted in average HQs falling into acceptable levels of non-carcinogenic harmful health hazards for all 16 water samples. From the calculation of HQs, it can be concluded that the contribution of the 10 metals to non-carcinogenic health impacts followed the order Fe > Mn > Zn > Ni > Cu > Pb > Cr > As > Co > Cd. Furthermore, the HQ calculated for each heavy metal was added up and expressed as the HI (Hazard Index) to determine all possible non-carcinogenic consequences caused by several metals simultaneously [47].

The average values of oral and dermal absorption and total HI (Table 6) were 2.91 × 10⁻², 3.70583 × 10⁻⁵, and 0.010341633, respectively. The results indicate insignificant non-carcinogenic risks to human health as the values are below 1 in the case of all heavy metals.

4.4. Carcinogenic Risk Analysis

Heavy metals such as Cd, Cr, Ni, and Pb are characterized by a higher potential for inducing cancer risk in humans [2,14,48,49]. Therefore, a variety of cancers could develop because of prolonged exposure to low concentrations of these toxic metals. The average CDI values provided in Table 5 were used to determine the total exposure of the residents using Cd, Cr, Ni, and Pb as carcinogens [34]. Table 7 provides the carcinogenic risk evaluation for adults. Cancer slope factor (CSF) values for the various metals used in the calculation of assessment of carcinogenic risk are listed in Table 3. For each heavy metal, an ILCR value < 1 × 10⁻⁶ is regarded as insignificant, meaning the cancer risk can be ignored, while an ILCR value > 1 × 10⁻⁴ is regarded as detrimental, meaning that the cancer risk is significant. The acceptable level for all the heavy metals via all exposure routes is 1 × 10⁻⁵ [49,50]. Of all the heavy metals that we have studied, chromium (Cr) has the highest chance of cancer risk (average ILCR 9.43528 × 10⁻⁸), and cadmium (Cd) has the lowest (average ILCR 1.93201 × 10⁻¹⁰) (Table 9). The findings of the present study indicate that the study area’s drinking water does not pose significant cancer risk to its populace due to cumulative oral and dermal contact with the contaminants.

5. Conclusions

The double circulation (surface circulation within the limestone Post-Nubian Aquifer System feeding the springs, and deep water coming from the ferruginous sandstone of the Nubian Sandstone Aquifer System feeding the wells) was found to account for the EC and TDS values typical of a circulation in limestone-type aquifers for the springs, and the Fe and Mn enrichment of the drilled wells.

The purpose of the present study was primarily to assess the health impacts associated with heavy metal exposure based on water samples collected from 16 springs and drilled wells of the Nubian Sandstone Aquifer in the El-Farafra Oasis and the White Desert National Park (the Western Desert of Egypt). Based on the average concentrations recorded in the drinking water samples of the investigated area, the following order of toxicity was assessed: Fe > Mn > Zn > Co > Ni > Cr > As > Pb > Co > Cd. The calculated averages of the CDI_total concentrations of the heavy metals studied were determined to be in the following decreasing order: Fe > Mn > Zn > Cu > Ni > Cr > As > Co > Pb > Cd. The values obtained for the HQs indicated that the primary route of exposure in humans is the oral ingestion pathway followed by the dermal route. The average values of HI_total, HI_oral, and HI_dermal were found to be 2.91 × 10⁻², 3.70583 × 10⁻⁵, and 0.010341633. Carcinogenic risk assessment based on mean ILCR values showed that chromium (Cr) has the highest chance of provoking cancer (average ILCR 9.43528 × 10⁻⁸) and cadmium (Cd) has the lowest (average ILCR 1.93201 × 10⁻¹⁰). Although the results of our health-risk assessments consistently indicate a low-risk situation where metal enrichment is primarily of geogenic origin, with special reference to medium- and long-term exposure, it might nevertheless be wise to adopt proper protective measures and take necessary actions for reducing the heavy metal contamination of drinking water in rural and urban areas.