Geogenic Sources of Arsenic and Fluoride in Groundwater: Examples from the Zagros Basin, the Kurdistan Region of Iraq

Abstract

Groundwater is one of the crucial water resources for domestic, agriculture and other purposes in the Kurdistan Region of Iraq, which is counted as a semiarid region with seasonal precipitation in winter. The geogenic source of arsenic and fluoride in groundwater has been studied in the Kurdistan Region of Iraq, which is a part of the Zagros Basin, using the hydrogeochemical method. The analysis results showed that the concentrations of arsenic and fluoride range from 0.19 to 7.8 µg/L and from 0.01 to 2.1 mg/L, respectively. The hydrogeochemical characteristics of the groundwater in the studied area were connected to the fluoride F⁻ and arsenic As concentrations for understanding their sources and behavior. The hydrogeochemical relations between F and As indicate geogenic sources and relatively simple aquifer conditions. Some samples may indicate the presence of contamination sources in addition to geogenic sources. Considering the WHO guidelines, the concentrations of As in most of the samples do not exceed the WHO limit, but the F in some samples shows a higher concentration than the WHO limit, indicating a serious risk of fluorosis in some spots. Connecting the changes in F concentrations to depth and aquifer types, a higher F concentration is associated with an intergranular aquifer and decreases in a karst aquifer. The speciation of F⁻ and As is controlled by pH and redox conditions. Adsorption, cation exchange, and the dissolution of carbonate minerals with the possible dissolution of fluorite are the most dominant geochemical processes that control the concentrations of As and F⁻ in groundwater. The principal sources of F⁻ and As in the study area seem to be geogenic.

1. Introduction

Geogenic arsenic (As) and fluoride (F⁻) are observed worldwide in groundwater [1,2] and groundwater also represents the main source of their intake [3]. A high As concentration in drinking and irrigation water usually enters the food chain and causes health problems [4]. Arsenic can be derived from anthropogenic and geogenic sources, but the latter causes larger groundwater contamination. As is released from the oxidation of sulfide minerals in sedimentary rocks (mostly shale with mean values of 28 ppm), which is higher than those of igneous and metamorphic rocks, by the reductive dissolution of ferric oxyhydroxides, and is also a component of thermal waters [5,6]. Geogenic sources of As are arsenic minerals such as arsenopyrite, orpiment, realgar, claudetite, arsenolite, pentoxide, scorodite, and arsenopalledenite. Alluvial deposits rich in organic matter driving reductive dissolution of ferric oxyhydroxides with adsorbed As are major sources of As in sedimentary formations in countries such as Bangladesh and West Bengal in India [5]. Besides this, industrial waste, coal combustion, oil, cement, phosphate, fertilizers, mine tailing, smelting, ore processing, metal extraction, metal purification, chemicals, glass, leather, textiles, alkalis, petroleum refineries, acid mines, alloys, pigments, insecticides, herbicides, fungicides, and catalysts contribute to arsenic contamination of the groundwater, soil, and air [7].

Fluorine is essential for human health and low F⁻ concentrations in water possibly cause dental decay [8], while high F⁻ concentrations in drinking water have unfavorable health effects, including dental fluorosis (occurrence of yellow-brown spots in the teeth), skeletal fluorosis, and neurological problems resulting from drinking such water for long periods of time [9,10]. Regarding rock-forming minerals, fluorine is only an essential component in fluorite (CaF₂) and topaz (Al₂SiO₄ (F, OH)₂), but it is also found in accessory minerals such as cryolite (Na₃AlF₆) and apatite (Ca₅(PO₄)₃ (F, Cl₂). Minerals such as phlogopite, biotite, epidote, and amphibole (tremolite and hornblende) also have F. Ultramafic rocks have less than 100 ppm of F, while granitic rocks have F in the order of 800 ppm [11]. Fluoride is released into the groundwater mainly through water–rock interaction by various fluoride-bearing minerals. Fluorite (CaF₂) is the principal mineral of fluorine occurring in nature and is commonly found as an accessory in granitic gneiss [12,13]. Fluorine is also abundant in other rock-forming minerals such as apatite, micas, amphiboles, and clay minerals [14,15,16,17,18,19].

Fluoride concentration in groundwater is influenced by a number of factors, such as temperature, pH, DIC, the presence or absence of complexing or precipitating ions and colloids [20], the solubility of fluorine-bearing minerals, the anion exchange capacity of aquifer materials, the dimensions and types of geological formations that water flows through, and the length of time that the water is in touch with a specific formation [21]. The concentration of F⁻ is generally related to the flow systems and is controlled by the water–rock interaction and the residence times in aquifers [10]. The fluorine concentration in metamorphic rocks is between 100 and 5000 ppm in regional metamorphic and contact metamorphic rocks [22]. Moreover, fluoride enters the groundwater through soil contamination as a result of fertilizers and phosphate pesticide application [23]. High F⁻ concentrations in groundwater are typically linked to Na-HCO₃ waters with low Ca²⁺ concentrations [24,25]. In high Ca²⁺ concentration waters, the F⁻ concentration is controlled by fluorite, CaF₂, and precipitation [26]. High F⁻ concentrations are typical for low Ca²⁺ groundwater, e.g., at the Ethiopian Rift [27].

Long-term exposure to arsenic and fluoride, mainly through drinking water intake, can lead to arsenicosis and fluorosis [3]. The initial symptoms of arsenicosis are skin lesions (hyperkeratosis), and the long-term consumption of As-rich water leads to cancer of lungs and bladder [28]. The primary health problems caused by excessive fluoride are dental fluorosis, skeletal fluorosis, and the deformation of bones in children and adults [29]. Fluorosis has the most significant impact on growing teeth, and children under seven years old are particularly vulnerable [30]. There is also evidence that the adverse health effects of fluoride are enhanced by the lack of Ca, vitamins, and protein in the diet [31,32].

The purpose of this study was (1) to review As and F occurrences in Iraqi Kurdistan with a focus on geogenic sources and (2) to determine the factors responsible for their release and mobility control in groundwater.

2. Study Area

The area of interest is located in the Kurdistan Region, north of Iraq and north-west of the Zagros Basin. It is divided into two zones. The first zone is in the Sulaimani governorate, and the second is in Erbil governorate.

• Sulaimani Area (Sulaimani city, Tanjero, Arbat, Tasluja, Bazian, Piramagroon, Makook)

This zone is located mainly in the Sulaimani governorate, Ranya, Halabja, and Sulaimani districts in the foothills of the Zagros Mountains, Kurdistan Region, Iraq (Figure 1). The Sulaimani city, Tanjero, Arbat, and Tasluja zones are parts of Tanjero Basin. It is situated within latitudes 35°50′–36°30′ north and longitudes 45°52′–45°05′ east in the elevation range from 700 m to >2000 m above sea level (a.s.l.).

• Erbil Area (Koysinjaq, Shiwashok, Shaqlawa, Harrir)

This zone is located mainly in the Erbil governorate, Erbil, Koysinjaq, and Shaqlawa-Harrir districts in the Kurdistan Region, Iraq (Figure 1). The Koysinjaq and Shiwashok zone is part of the Koysinjaq-Surdash basin and the Shaqlawa-Harrir Basin is located in the northeast of Erbil city. The Shaqlawa and Harrir Basin is divided into three small basins: Tawska, Hiran, and Harash. The area is situated within latitudes 35°50′–36°45′ north and longitudes 44°04′–44°57′ east in the elevation range from 400 m to >1000 m above sea level (a.s.l.).

The climate in the study area is semiarid, with a distinct rainy period in winter and a dry period in summer when temperatures reach more than 40 °C. The average annual temperature is 21 °C. The mean annual precipitation in the Sulaimani area is 669 mm, and most pre-precipitation falls in January and February when most of the aquifer recharge occurs [33]. Meanwhile, Meteorological data obtained from the ground meteorological station in Salahaddin district/Erbil Governorate (Pirmam meteorological station) for the period between 1992 and 2012 show that the annual precipitation is about 589 mm in Erbil zone [34].

2.1. Geologic Setting

From a geological viewpoint, the study area is divided into two provinces: Sulaimani and Erbil. The geological units that cropped out in the area are presented in Figure 2 and

Table 1. Geology and stratigraphy [24,35,36].

Stratigraphic Unit	Age	Lithology	Location
Recent alluvial deposits and River Terraces	Quaternary	Clastic sediments, clay, sand, pebbles, and boulders	Sulaimani, Koysinjaq
Bai Hassan and Muqdadiya Formation	Pliocene	Sandstone, siltstone, and conglomerate	Sulaimani, Koysinjaq
Injana Formation	Late Miocene	Claystone, siltstone, and sandstone	Sulaimani, Koysinjaq
Fatha Formation	Middle Miocene	Alternation of sandstone, claystone, limestone, gypsum, rock salt, and anhydrite	Sulaimani, Koysinjaq,
Pila Spi Formation	Middle-Late Eocene	Bituminous, chalky, and crystalline limestone with chert nodules	Sulaimani, Koysinjaq
Sinjar, Khurmala and Gercus Formation	Early Eocene	Red mudstone, sandstone, shale, and conglomerate	Sulaimani, Koysinjaq
Kolosh Formation	Paleocene	Clastic, shale, limestone, marl, and mudstone	Koysinjaq, Shaqlawa, Harrir, Erbil Area
Tanjero and Shiranish Formations	Late Cretaceous	Blue marl overlaying limestone and marly limestone	Sulaimani, Koysinjaq, Makook
Aqra-Bekhme, Kometan and Qamchuqa Formations	Late Cretaceous	Bituminous dolomitic limestone and massive organic limestone	Sulaimani, Koysinjaq, Makook
Sarki and Sehkaniyan Formation, Naokelekan and Barsarin Formation and Chia Gara Formation	Jurassic	Limestone, dark dolomite, bituminous limestone, marl, and shale	Sulaimani, Koysinjaq and Makook

. The age of the units ranges from Early Cretaceous to Quaternary. The oldest unit of the Lower Cretaceous age is the Qamchuqa Formation, predominantly composed of carbonates. The Upper Cretaceous formations are represented by carbonates of the Aqra-Bekhme Formation and impure carbonate rocks of the Shiranish and Tanjero Formations. Alternation of flysch and carbonates of the Kolosh, Khurmala, Gercus, and Pila Spi Formations overlie the Cretaceous rocks.

2.2. Hydrogeology

Four aquifer systems are recognized in the study area: karst, fissured karst, Tanjero, and intergranular aquifers. The rocks of the Tanjero Formation often form an aquitard and are composed of alternating marly limestone, sandstone, marl, and occasionally conglomerate beds, but they are highly deformed and fractured (especially in the Sulaimani city area), which allows the formation of secondary porosity [37]. Where permeability is increased due to secondary porosity, the Tanjero Formation becomes a medium-depth to deep aquifer (25–120 m) that yields a good quantity of water in the sandstone and marly limestone facies. The majority of the wells in the study area are drilled within the Tanjero porous aquifer. The shallow Quaternary intergranular aquifer (0–25 m depth) is represented by shallow terraces and recent alluvium deposits, which are composed of unconsolidated gravel and sand that yield a good quantity of water [38]. Another part of the intergranular aquifer is represented by the Bai Hassan and Muqdadiya Formations. Typical characteristics of these aquifers are the inter-bedded layers of fine, medium, and coarse-grained textures and variations in permeability from one site to another within the same aquifer horizon. Karst aquifers are represented by the Bekhme and Qamchuqa Formations and are anisotropic and heterogeneous, with very high permeability and transmissivity [39] and a conduit flow regime containing large groundwater reserves [40]. Wells in these formations have high specific yields and the drawdowns in pumping wells are very small [39]. Most freshwater in the Shaqlawa and Harrir basins is produced from this aquifer [34]. Fissured karst aquifers are represented by the Kometan and Pila Spi Formations containing medium to large groundwater reserves. The aquifer is characterized by high permeability and transmissivity and a turbulent flow regime [39]. Both karst and fissured karst are developed in limestone, dolomites, marly limestones, and dolomitic limestones. The other formations, such as Injana, Fatha, and Kolosh, contain a limited amount of groundwater because they are composed mainly of impermeable layers such as claystone and marls (mostly forming aquiclude) and partially alternate with permeable fractured limestone, which, in turn, results in low permeability and a limited amount of groundwater.

2.3. Material and Methods

Ninety-two wells within the Sulaimani area and seventy-eight wells and springs in the Erbil area were sampled in this study from three different aquifers: intergranular, fissured, and karst. Therefore, the total number of samples was 170. The water samples were collected in a sterilized polyethene bottle. The samples selected for analysis were filtered with 0.2 μm cellulose acetate filters and then acidified with an ultra-pure nitric acid (30% HNO₃). All water samples were stored in a cool box at 4 °C until analysis in the laboratory. Physical parameters such as temperature, pH, electrical conductivity (EC), and redox potential (Eh) were measured in the field and corrected with respect to the H₂ electrode. Chemical parameters for the samples were determined in the laboratories of Technische Universität Bergakademie Freiberg, Germany, Sulaimanyiah Environmental Protection Office, Iraq, and Charles University in Prague, Czech Republic, according to [41]. Cations and trace elements were examined with the ICP-MS technique at the analytical laboratories of Charles University in Prague, and the laboratories of the Hydrogeology Department, Technical University Bergakademie Freiberg, Germany. The analytical error of the individual solution analyses was below 2%. Anion concentrations were determined by HPLC, Dionex ICS. Ferrous iron was determined by potassium dichromate titration. Alkalinity values were determined by HCl titration with the Gran plot to determine the endpoint. Major cations and anions were determined with ion chromatography (Metrohm-Compact IC Pro). Trace elements were analyzed with ICP-MS (Thermo Scientific-XSERIES 2). Total inorganic carbon (TIC) was measured with an Elementar Liqui-TOC. As was measured with ICP-MS (Thermo Scientific-XSERIES 2) and F with ion chromatography (Metrohm-Compact IC Pro). The raw data were subject to statistical tests using SPSS [42]. The ion balance and saturation indices of minerals and molar concentrations were calculated by PHREEQC code [43], using the WATEQ4F and PHREEQC databases. Location maps and spatial distribution maps were made using ArcGIS 10.7.1.

3. Results and Discussion

3.1. Groundwater Chemistry

3.1.1. Sulaimani Area

The statistical parameters of the water chemistry for each zone in the studied areas are given in

Table 2. Field and laboratory parameter statistics for the groundwater samples.

Area	Zone	Depth	Aquifer	Statistics	Tem.	pH	pE	EC	TDS	Ca2+	Mg2+	K+	Na+	HCO3−	SO42−	Cl−	NO3−	F−	As
Sulaimani area	Zone 1 Sulaimani Tanjero	45–145	Intergranular, Karst, Fissured	Min.	20.1	6.8	5	NA	NA	6.9	1	0.4	20.7	214.5	28.2	13.8	2.4	0.1	0.0002
				Max.	25.5	8.8	6.2	NA	NA	138	29.8	3.9	222	424	144	110.0	25.9	1.65	0.0078
				Mean	22.1	7.5	5.6	NA	NA	68.3	15.0	1.4	100.1	329.1	82.9	51.9	7	0.34	0.0020
	Zone 2 Sulaimani Bazian	6–190	Intergranular, Karst, Fissured	Min.	5.8	5.8	NA	16	1.6	8	0.2	0.9	25.6	NA	46	2.5	0.1	NA	NA
				Max.	9.3	9.3	NA	1177	80	68	2.9	164.9	145	NA	220	50.5	1.7	NA	NA
				Mean	7.1	7.1	NA	360	23.1	37.8	1.2	31.8	212.2	NA	205.8	30.3	0.3	NA	NA
	Zone 3 Sarwchawa-Diyana	Springs	Karst	Min.	9.5	7.3	21.7	310	NA	42.7	10.7	0.3	0.9	172.2	10	1.4	2.6	0.04	0.0002
				Max.	17.4	7.8	23.5	602	NA	76.1	21.6	1.8	4.4	340.4	70.4	8.3	21	0.13	0.0003
				Mean	13.4	7.5	22.7	460	NA	58.2	17.5	0.7	1.8	253.3	20.3	2.7	6.3	0.07	0.0003
	Zone 4 Sulaimani-Warmawa	71–450	Intergranular, Karst, Fissured	Min.	21.8	7.0	NA	328	NA	6.1	2.3	0.3	2.7	140	7.4	1.9	0.2	0.2	0.0001
				Max.	26.8	8.8	NA	2460	NA	113	40.4	2.7	480	320	200.7	462.4	36	0.54	0.0043
				Mean	24.3	7.7	NA	663	NA	51.2	11.9	0.9	55.6	215.3	47.3	43.4	15.4	0.23	0.0011
Erbil area	Zone 5 Shiwashok	80–210	Intergranular, Karst, Fissured	Min.	7.3	NA	NA	500	11.7	6.3	1.2	25.7	198	59	23.3	4.6	0.3	NA	0.0002
				Max.	8.5	NA	NA	1193	164	77.7	19.2	320.2	293	790.7	223	209.3	1.3	NA	0.0012
				Mean	7.8	NA	NA	793	72.3	32	5.1	117.4	237.5	253.8	91.3	49.3	0.5	NA	0.0005
	Zone 6 Koysinjaq	96–281	Intergranular, Karst, Fissured	Min.	24.1	7.3	9.6	462	300.3	64.2	7.1	0.5	0.9	258.7	14	4.7	1.4	0.04	0.0002
				Max.	24.9	7.6	12.7	751	488.2	89.4	30.7	1.2	9.5	382.7	31.7	12.2	8.7	0.64	0.0012
				Mean	24.5	7.4	11.0	572	372	75.2	23.5	0.7	2.9	322	20	6.4	4.5	0.14	0.0005
	Zone 7 Shaqlawa–Harrir	68–239, Springs	Intergranular, Karst, Fissured	Min.	21.2	7.2	6.4	297	193	4	4.1	0.3	1.1	168.9	10.6	4.3	0.1	0.04	0.0002
				Max.	22.5	8.3	13.2	1355	880.8	118	74.7	8.3	117.6	532.1	272.2	114.1	41.5	2.12	0.0020
				Mean	21.8	7.6	10.1	612	397.6	67.1	27.3	1.2	16.5	322.7	38.8	15.5	10.1	0.33	0.0007

Note: Concentration is in mg/L, temperature = °C, EC = µS/cm, depth = meters, and NA = not available.

, and the complete data set is given in the Supplementary Materials in Table S1. The Piper plot for the groundwater chemistry data is presented in Figure 3. In the Piper plot, the evolution from Ca-HCO₃ groundwater type to Na-HCO₃ groundwater type is evident, indicating favorable conditions for F⁻ enrichment in groundwater [20,44].

The values of electrical conductivity range from 25 to 2460 μS/cm; the average is 558.6 μS/cm. The EC values for Tanjero, karst, and intergranular aquifers are in the range from 25 to 935, 310 to 602, and 538 to 2460 μS/cm, respectively. The pH values in the Sulaimani area are in the range from 5.8 to 9.3 and the average pH value is 7.5. The pH value in the Tanjero aquifer is in the range of 5.8 to 8.6, in the karst aquifer in the range of 7.3 to 7.8, and in the intergranular aquifer ranges from 7.3 to 8.8. The average value of Ca is 52.4 mg/L with concentrations from 1.6 to 138 mg/L. In the Tanjero aquifer, the concentration of Ca is in the range from 1.6 to 138 mg/L, in the karst aquifer is in the range from 42.7 to 76.1, and in the intergranular aquifer is in the range from 6.9 to 73.9 mg/L. Concentrations of Mg range from 1 to 77.7 mg/L, with the average being 25.7 mg/L. The concentrations are from 4.7 to 68 mg/L in the Tanjero aquifer, 10.7 to 21.6 mg/L in the karst aquifer, and 1 to 59 mg/L in the intergranular aquifer. Concentrations of Na are from 0.9 to 480 mg/L, with the average being 33.8 mg/L; in the Tanjero aquifer, the concentration of Na is in the range from 0.9 to 222 mg/L, in the karst aquifer is in the range from 0.9 to 1.8 mg/L, and in the intergranular aquifer from 7 to 480 mg/L. The concentration of K ranges from 0.2 to 19.2 mg/L, and the average is 1.3 mg/L; the concentrations of K for the Tanjero, karst, and intergranular aquifers are in the range of 0.2 to 3.9, 0.3 to 1.84, and 0.5 to 2.3 mg/L, respectively. Concentrations of HCO₃ range from 25.6 to 532.1 mg/L, with an average of 264 mg/L; HCO₃ ranges from 25.6 to 424 mg/L in the Tanjero aquifer, from 172.2 to 340.4 mg/L in the karst aquifer, and 140 to 297.8 mg/L in the intergranular aquifer. The concentration of SO₄ ranges from 7.35 to 790 mg/L with an average of 55.19 mg/L; in the Tanjero aquifer, the SO₄ concentration is in the range of 7.35 to 118 mg/L, in the karst aquifer is from 10 to 70.4 mg/L, and in the intergranular aquifer is from 28.2 to 200.7 mg/L. The concentration of Cl ranges from 1.4 to 462.4 mg/L, the average is 63.9 mg/L, and the concentrations of Cl for the Tanjero, karst, and intergranular aquifers are in the range of 1.9 to 288, 1.4 to 8.3, and 13.8 to 462.6 mg/L, respectively.

3.1.2. Erbil Area

In the Erbil area, the values of EC range from 297 to 1355 μS/cm, the average is 596.2 μS/cm, and the EC values for the karstic-fissured, karst, and intergranular aquifers are in the range from 412 to 695, 297 to 1319, and 313 to 1355 μS/cm, respectively. The pH values are in the range from 7.2 to 8.5, and the average pH value is 7.5. The pH value in the karst-fissured aquifer is in the range of 7.2 to 8.5, the pH in the karst aquifer is in the range of 7.2 to 8.1, and in the intergranular aquifer ranges from 7.25 to 8.2. The average value of Ca is 69.5 mg/L with concentrations from 4 to 164 mg/L; in the karstic-fissured aquifer, the concentration of Ca is in the range from 4 to 90.5 mg/L, in the karst aquifer is in the range from 18.9 to 118.2, and in the intergranular aquifer is in the range from 12 to 164 mg/L. Concentrations of Mg are from 4 to 77.7 mg/L, the average is 26.8 mg/L, and is in the range of 4 to 77.7 mg/L for the karstic-fissured aquifer, from 8.7 to 66.3 mg/L in the karst aquifer, and from 7.1 to 74.7 mg/L in the intergranular aquifer. Concentrations of Na are from 0.9 to 320.2 mg/L, the average is 23.3 mg/L, and in the karstic-fissured aquifer, the concentration of Na is in the range from 0.9 to 117.6 mg/L, in the karst aquifer is in the range from 1.4 to 74.9 mg/L, and in the intergranular aquifer is in the range from 1 to 320.2 mg/L. In addition, concentrations of K are from 0.2 to 19.3 mg/L, the average is 1.46 mg/L, and the concentrations of K for the karstic-fissured, karst, and intergranular aquifers are in the range from 0.2 to 8.26, 0.4 to 5.58, and 0.3to 19.2 mg/L, respectively. Concentrations of HCO₃ are from 168.9 to 532.1 mg/L, the average is 313.8 mg/L, and the values are from 195.9 to 410.5 mg/L in the karstic-fissured aquifer, from 168.9 to 500.8 mg/L in the karst aquifer, and from 172.3 to 532.1 mg/L in the intergranular aquifer. Concentrations of SO₄ are from 10.5 to 790.7 mg/L, the average is 56 mg/L, and in the karstic-fissured aquifer, the SO₄ concentration is in the range from 12.4 to 224 mg/L, in the karst aquifer is in the range from 10.5 to 272.18 mg/L, and in the intergranular aquifer is in the range from 13.9 to 790.7 mg/L. Concentrations of Cl are from 4.3 to 233 mg/L, the average is 20.9 mg/L, and the concentrations of Cl for the karstic-fissured, karst, and intergranular aquifers are in the range of 4.3 to 99, 4.5 to 98.3, and 4.71 to 223 mg/L, respectively.

3.2. Statistical Analysis

Correlation between F⁻ and As

In the groundwater of the Sulaimani area, the concentrations of As and F show a significant (R² = 0.736) positive correlation (Figure 4A). This relation may reflect their similar adsorption behavior, i.e., their desorption at higher pH values due to their anionic form [1,11,45]. In the case of high calcium and low As concentrations, a positive correlation with F⁻ may reflect the presence of a contamination source in addition to the geogenic source [19]. The groundwater of the Erbil area shows only a positive, but not significant correlation (R² = 0.37) (Figure 4B).

3.3. Occurrence and Geogenic Sources

3.3.1. Variation of Concentrations with Depth

The concentration of As appears not to be depth-dependent because there is no clear trend of As concentration with changing depth. A low As concentration is linked to the shallower aquifer and roughly increases with depth (Figure 5A). Two distinctive groups are recognized from the As–depth relation: Group A and Group B. This grouping is based on the type of aquifer, in which the karst aquifer shows relatively higher As concentrations (0.2–2 µg/L) with a wide range of depth variation (60–280 m). Group B represents groundwater from an intergranular aquifer with a lower As concentration compared to Group A (0.2–0.7 µg/L). The behavior of F⁻ concentration is completely opposite to the As concentration because a higher F concentration is associated with the intergranular aquifer and decreases in the karst aquifer (Figure 5B).

3.3.2. Speciation of As and F

According to the Eh–pH diagram, the low observed As concentration corresponds to a neutral pH and moderate to high Eh values (Figure 6A). The HAsO4²⁻ is the dominant species of As in both the Sulaimani and Erbil areas. This may be linked to the affinity of Fe-oxyhydroxide minerals for As (V) in adsorption and co-precipitation reactions [46]. The neutral pH and oxidizing conditions assist in retaining As (V) onto Fe minerals. With the rising pH value above 7.8, the As concentration increases (especially in the Sulaimani samples) and reflects the dependency of pH on desorption [5,47]. In the groundwater of the Sulaimani area, As speciation is controlled by pH, redox conditions, temperature, and the dissolution of calcite and dolomite minerals [48].

Regarding F, there is only one oxidation state, F⁻—i.e., it is Eh-independent and samples with F concentration >1 mg/L are characterized by lower pH values of 6.5–7.5. An increasing pH value results in F release due to the competition with adsorbed OH¯ [49]. The high concentrations of HCO₃⁻ and pH conditions are responsible for F enrichment [20,47]. Notably, F concentrations show a positive correlation with HCO₃⁻ and Cl⁻ concentrations (R² = 0.37) in the groundwater of the Erbil area. A high HCO₃⁻ concentration in groundwater may remove Ca²⁺ and promote CaF₂ dissolution, causing the release of F⁻ [49].

3.3.3. Saturation Indices

There is no clear trend in relation to As and F⁻ concentrations and saturation indices for calcite and dolomite (Figure 7A,B). Some high F⁻ groundwaters are mostly supersaturated with respect to calcite (Figure 7B), and most samples are undersaturated with respect to fluorite (Figure 7C) and dolomite (Figure 7D), which is compatible with the findings of [50]. Fast recharge may lead to the dilution of groundwater and cause low F⁻ concentrations and undersaturation with respect to calcite and dolomite [47]. In addition, undersaturation with respect to fluorite in some samples can be caused by sampling from long screen wells in which the concentrations of F⁻ were diluted [26]. In general, negative saturation indices for fluorite suggest the dissolution of the mineral when present in the solid phase of the studied aquifers.

3.4. Correlation of As and F with Hydrochemical Parameters

The correlation matrix of As and F with hydrochemical parameters is given in

Table 3. Correlation matrix for groundwater parameters: A, Sulaimani area; B, Erbil area.

	pH	EC	Ca2+	Mg2+	K+	Na+	HCO3−	SO42−	Cl−	NO3−	F−	As
pH	1
EC	0.16	1
Ca2+	−0.8	0.1	1
Mg2+	−0.63	0.03	0.76	1
K+	−0.17	0.44	0.46	0.14	1
Na+	0.54	0.79	−0.44	−0.45	0.16	1
HCO3−	−0.39	0.24	0.56	0.5	0.45	−0.07	1
SO42−	0.25	0.77	−0.02	−0.05	0.34	0.8	0.19	1
Cl−	0.31	0.93	−0.12	−0.19	0.28	0.85	−0.05	0.67	1
NO3−	−0.36	−0.22	0.27	0.1	−0.13	−0.42	−0.22	−0.29	−0.21	1
F−	0.27	0.27	−0.31	−0.33	−0.07	0.45	−0.01	0.12	0.38	−0.15	1
As	0.42	0.46	−0.27	−0.29	0.07	0.6	0.08	0.41	0.51	−0.28	0.63	1
A—Sulaimani area
	pH	EC	Ca2+	Mg2+	K+	Na+	HCO3−	SO42−	Cl−	NO3−	F−	As
pH	1
EC	−0.28	1
Ca2+	−0.61	0.56	1
Mg2+	−0.3	0.86	0.52	1
K+	−0.09	0.27	0.1	0.09	1
Na+	0.35	0.48	−0.23	0.29	0.28	1
HCO3−	−0.5	0.75	0.74	0.74	0.09	0.19	1
SO42−	−0.13	0.87	0.49	0.73	0.29	0.59	0.54	1
Cl−	−0.04	0.89	0.36	0.79	0.34	0.66	0.56	0.89	1
NO3−	0.34	−0.07	−0.05	−0.13	0.22	−0.11	−0.36	−0.06	−0.03	1
F−	−0.06	0.65	0.32	0.5	0.32	0.47	0.4	0.74	0.65	0.08	1
As	0.05	0.34	0.36	0.33	0.07	0.16	0.25	0.44	0.37	0.19	0.59	1
B—Erbil area

. In the samples from the Sulaimani area (Table 3(A)), the correlation coefficients show that the most significant correlations are those between SO₄, Cl, Ca, and Na against electric conductivity (EC) and pH. Some samples have increased concentrations of Na, Cl, and SO₄ as a consequence of the dissolution of halite and gypsum embedded in carbonates [33]. This may also be reflected by the predominance of sulphate and sodium chloride facies in some groundwater samples [51]. These correlations may indicate the geological sources of these components [52]. The correlation matrix for Erbil area samples (Table 3(B)) shows a significant correlation of Mg with SO₄, Cl, and bicarbonate. This may be caused by the dolomite dissolution from the dolomite and dolomitic limestone in the karst aquifers of the Erbil area, as indicated by the presence of hydrochemical facies of Mg-Ca-Na-SO₄-HCO₃ groundwater in the Erbil area [40,53]. There is a negative correlation between Na⁺ and Ca²⁺, suggesting cation exchange. This is further supported by intermediate, but significant, correlation between Na⁺ and F⁻ in both areas, linking high F⁻ to Na⁺-rich groundwater [25,44]. Cation exchange removes Ca²⁺ (Equation (1)) and promotes the dissolution of fluorite (Equation (2)), resulting in Na-HCO₃ groundwater rich in F⁻:

CaCO₃ + CO₂ + H₂O + Na₂-X = 2Na⁺ + 2HCO₃⁻ + Ca²⁺

(1)

CaF₂ = Ca²⁺ + 2F

(2)

3.5. Health and Environmental Impact

The concentrations of As and F range from 0.19 to 7.8 µg/L and 0.01 to 2.1 mg/L, respectively. According to the World Health Organization [54], the permissible limit of As in drinking water is 10 μg/L, and for fluoride, it is 1.5 mg/L. Fluoride levels above 1.5 mg/L in drinking water cause dental fluorosis and skeletal fluorosis in higher concentrations [55]. Considering these guidelines, all groundwater samples are below this guideline for As, but some samples show higher concentrations than the permissible level for F, especially in the Sulaimani area. Arsenic and fluoride are two natural components affecting human health. In this study, both areas are considered to be of no or low risk for arsenicosis, but there is the risk of fluorosis, which was not considered in the area previously. In their investigation, the authors in [56] pointed out that there is no indication of the risk of fluorosis in the groundwater of the area, as they only detect low F⁻ concentrations (less than 0.15 mg/L). However, this investigation did not represent the whole studied area and the different groundwater resources and aquifers, as they took only 22 samples. In the current investigation, the mentioned criteria were taken into consideration, including sample number, frequency, distribution, depth, and aquifer types. Therefore, the risk of fluorosis in these spots in the studied area should be seriously considered. The reason for the observed low concentration of arsenic may be due to its limited geogenic and anthropogenic sources [36,37]. This interpretation also applies to the groundwater of the Erbil area, as indicated by [54].

• Spatial Distribution of As and F

The available groundwater data were used to make a geostatistical interpolation and construction of predictive maps for As and F concentrations in groundwater. The maps for As and F⁻ (Figure 8A,B) indicate vulnerable areas. This can help authorities to become aware of potentially contaminated groundwater, especially close to industrial areas.

The spatial distribution of As and F in the studied area is patchy, and different concentration hotspots are present. This may suggest the existence of local sources [45].

4. Conclusions and Recommendations

4.1. Conclusions

The primary outcome of the current study can be summarized as follows:

Concentrations of F⁻ and As range from 0.01 to 2.1 mg/L and 0.19 to 7.8 µg/L, respectively. Concentrations of F⁻ and As in groundwater are correlated to the hydrogeochemical processes and their similar adsorption behavior due to their anionic speciation. The results of the examined samples suggest that the primary sources of F⁻ and As are geogenic. There is no or low risk for arsenicosis. However, the fluoride in some groundwater samples was found in a higher concentration than the permissible WHO limit, indicating a risk of fluorosis in some areas. The risk of fluorosis should be considered seriously, and the monitoring of F is necessary.

The speciation of As and F⁻ is controlled by pH, redox conditions, and groundwater temperature. Adsorption, desorption, cation exchange, the dissolution of carbonate minerals, and possibly fluorite dissolution are the dominant geochemical processes that govern and control F⁻ and As concentrations.

Future studies should focus on concentrations of geogenic contaminants in other governorates of the Iraqi Kurdistan area and on the identification of factors controlling their concentrations.

4.2. Recommendations

It is recommended to take the risk of fluorosis into consideration in areas with high fluoride concentrations and to study each hotspot separately to distinguish between geogenic and contamination sources.