Geochemical controls on the distribution of radio-trace elements in groundwater resources of Shir-Kuh granitoid aquifer, central Iran

Granitoid is a preliminary term used for plutonic rocks which essentially contain quartz and feldspars, tentatively identified as granite, granodiorite, or tonalite (Le Maitre et al. 2005). These rocks have appreciable amounts of quartz, alkali feldspar, and/or plagioclase and other aluminosilicate minerals such as micas. Quartz has very low solubility under existing pH and temperature; however, incongruent dissolution of aluminosilicates releases common cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺) into groundwater (Singhal & Gupta 2010). Very low weatherability of silicate minerals in addition to the low residence time of groundwater mainly through the fissures and fractures result in the total dissolved solids (TDS) of granitic groundwater to be usually less than 300 mg/L. As a result of the silicates weathering, the general groundwater type from granitoids is Ca-HCO₃ and the silica content ranges from 10 to 30 mg/L. High concentrations of trace elements such as Pb, Cr, Fe, and Mn are also possible to detect (Singhal & Gupta 2010). For example, high values of Pb (about 0.19 mg/L) and Cr (around 2.6 mg/L) were reported in acidic granitic groundwater in Ghana (Pelig-Ba et al. 1991). Analysis results of the water samples collected around granitic rocks in Turkey (Yuce et al. 2009) revealed high concentrations of trace elements, including Al, Ba, Fe, and Cr, over the drinking water norms.

High natural values of major and trace elements were observed in water samples collected from boreholes in granitic bedrock aquifer in Norway (Banks et al. 1995a), with some of them, including F, Na, Fe, U, Rn, and Al, exceeding drinking water norms. Banks et al. (1998) compared the hydrochemistry of the granitic island aquifers of Scilly (UK) and Hvaler (Norway) with similar bulk lithologies of peraluminous S-type, U/Th-enriched granites. The results showed dissimilarities in health-related trace elements of Rn, U, and F due to differences in trace elements' composition of granite and fracture fillings, weathering histories and hydrodynamic factors. Assessing the geochemistry of groundwater in structurally deformed granitic terrain in India (Satyanarayanan et al. 2007) revealed significant variations of some trace elements (Fe, Mn, Al, Be, Co, Pb, U, and Zn) in dug and bore wells in relation to differential mineral weathering and dissolution/precipitation reactions along the fractures.

In terms of natural radioactivity, granitic rocks are strongly enriched in U (on average 5 ppm) and Th (on average 15 ppm) (Tzortzis & Tsertos 2004), compared to the Earth's continental crust (average 1.8 and 7.2 ppm for U and Th, respectively) (Mason & Moore 1982). U and Th increase mainly in accessory minerals like orthite or alunite, monazite, zircon, apatite, and sphene which usually concentrate in granites (Mason & Moore 1982). As a result of mineral weathering, the most commonly found radioelements in granitic groundwaters belong to the U and Th family (Örgün et al. 2005). Their concentrations in groundwater depend on the type of mineral sources in the aquifer, chemical composition of water, and residence time of water in contact with the rocks (Freeze & Cherry 1979). The mobility of U in groundwater is strongly redox- and pH-sensitive. Solubility is high over a wide range of pH in oxidizing groundwater systems, where U occurs in its hexavalent (U^+VI) state (Frengstad & Banks 2014). The concerns regarding U in drinking water are linked to its chemotoxicity rather than radiotoxicity (Milvy & Cothern 1990). The guideline values (maximum admissible concentrations) for U in drinking water have now been set at 30 μg/L by both the World Health Organization (WHO 2011) and US Environmental Protection Agency (EPA 2002). In contrast to uranium, Th is highly insoluble under all conditions, existing almost exclusively in an oxidation sate of 4+ (Banks et al. 1995b).

In general, groundwaters coming from granitic rocks are not highly pure. It is widely recognized from several studies (e.g., Bonotto 1989; Gascoyne 1989; Banks et al. 1995a, 1995b; Hagedorn & Hasholt 2004; Örgün et al. 2005; Skeppström & Olofsson 2007; Raju et al. 2012; Ramaiah et al. 2012; Singh et al. 2013; Thivya et al. 2014, 2016; Desbarats et al. 2016) that they may contain minor and trace elements, such as heavy metals (Pb, Zn, Cu, Sb, Hg) and radioelements (U, Th, Rn, Ra) at levels over the allowable standards for drinking water.

The Shir-Kuh granitoid batholith contains important groundwater resources in Yazd province, a semi-arid region in central Iran, where groundwater is mainly utilized by local residents for domestic and agricultural uses. In addition, it is the major recharge zone of the surrounding alluvial plain aquifers. This research aimed to investigate the chemistry of groundwater resources in this important granitic aquifer with the emphasis on heavy metals and natural radioelements.

The elongated northeast–southwest trending batholith of Shir-Kuh granitoid in central Iran (31°23′–31°45′N, 53°50′–54°30′E) covers an area of about 550 km² (Figure 1). To the east, the batholith is covered with lower Cretaceous conglomerate and sandstones of the Sangestan formation which are overlaid by the thick-bedded limestone of the Taft formation. This non-conformity indicates a Middle Jurassic age for the emplacement of the Shir-Kuh pluton (Sheibi et al. 2010). The Shir-Kuh granitoid outcrop consists of three units: granodiorite, mozogranite, and leucogranite (Figure 1). The mozogranites, which range from relatively mafic (cordierite-bearing) to felsic (muscovite-bearing) rocks, are the main ones spread throughout the region. They are composed mainly of plagioclase, quartz, K-feldspar, biotite, muscovite, garnet, cordierite, zircon, and monazite minerals (Sheibi et al. 2010).

From a hydrogeological point of view, the occurrence and the movement of groundwater in the Shir-Kuh granitoid aquifer (SGA) with a hard rock double-porosity characteristic are mainly controlled by fractures and the weathered layer (Jafari 2003). Fractures and other discontinuities mainly control groundwater flow and occurrences in the highland massive (unweathered) granitic rocks, where they form potential passages of groundwater movements. The weathered layer (regolith) is developed on the granitic basement in the valleys intersecting the Shir-Kuh batholith. This is an important source of groundwater for rural water supply and the groundwater is now being used by local villagers via the qanats (horizontal dug wells that drain the aquifer gravitationally) and wells. The recharge into the SGA takes place in topographic highs (average elevation of more than 4,000 m above the mean sea level) by the rain and snow falls. It is discharged by 64 springs and 101 qanats located along the valleys intersecting the granitic mass. Most of them have a discharge flowrate between 1 and 10 L/s (Taheri 2017). There are also 68 private dug wells at rural houses that discharge the weathered layer accumulated in the valley bottom. No piezometers have been installed in this hard rock aquifer. Regarding the surface topography, general groundwater flow direction is from the highlands toward the valleys (Figure 2), where the flow paths are concentrated and finally discharged by the springs and qanats or drained underground into the surrounding alluvial plains (Jafari 2003). The mean annual temperature and precipitation are about 12 °C and 250 mm, respectively. About half of the precipitation is snow, falling entirely during the wet season (November to April). Subsequent melting of the snow and its infiltration through the fractures provides suitable conditions for aquifer recharge in this arid region (Taheri 2017).

A set of more than 50 trace elements including Si, heavy metals, and radioelements (U, Th) was determined using HP Agilent 4500 ICP-MS (USA) at Iranian Zar-Azma Mineral Studies Company. The quality assurance/quality control (QA/QC) was carried out by qualified experts of the laboratory, including the analysis of reagent blanks, analytical duplicates, and certified reference materials.

The average values of EC and pH in the studied groundwater of the SGA are 624 μs/cm and 7.5, respectively. The silica contents vary in a range of 5.2 to 10.6 with an average value of 8.7 mg/L. The steep hydraulic gradient of groundwater which is expected to occur in this hard rock aquifer results in fast movement, thus, lower residence time of the groundwater in contact with granitic rocks. In addition, very low solubility of the quartz and other silicate minerals limit releasing elements from weathering of the granitic rocks into the groundwater.

The results of the major ions analysis in groundwater samples from the SGA are plotted on a Piper diagram (Figure 3), indicating the dominant water type as Ca-HCO₃, basically due to dissolution of the calcite from the rock matrix or as fracture mineralization and also weathering of the silicate minerals (Edmunds & Shand 2008) such as plagioclase feldspar (Na_0.8Ca_0.2Al_1.4Si_2.6O₈) found in granitic rocks of Shir-Kuh (Khalili 1997). The results of mass balance studies (Jafari 2003) confirm the role of both the calcite dissolution and silicate weathering in chemical evolution of the groundwaters from SGA (Table 2). Calcite presents in minor amounts (1–5%) in Shir-Kuh granitic rocks (Khalili 1997). However, the dissolution rate of calcite is typically six orders of magnitude more than the aluminosilicate minerals under neutral pH conditions (Mast et al. 1990). It highlights the importance of calcite dissolution compared with silicate weathering in supplying calcium and bicarbonate ions into the groundwater resources of SGA (Table 2). In addition, dissolution of halite and gypsum as secondary minerals filling fractures of the granitic rocks, was also introduced as the origin of Cl and SO₄ (Table 2) as they are not significant constituents in silicate rocks (Freeze & Cherry 1979). The process is likely the main cause of a little shift of the water samples towards the Cl and SO₄ corners in the Piper diagram (Figure 3). It must be mentioned that the higher solubility and availability of halite in comparison with gypsum explains Cl concentrations more than SO₄ in groundwater from the SGA. The observed changes in cation concentrations along the groundwater flow path (Ca reduction with increase in Mg and Na) are also attributed to the cation exchange process, as proved in previous studies (Jafari 2003).

In conclusion, PCA in supporting the mass balance studies (Table 2) confirms the dissolution of fracture-filling sediments and weathering of the calcite and biotite as the main processes enhancing major ions in the SGA.

The mean values of the major ions in groundwater resources of the SGA are presented in a Schoeller diagram (Figure 4) in comparison with other granitic waters around the world. As is observed, SO₄ ion varies a great deal in granitic waters. The overall increases of the ions concentration and resulting higher salinity values of the groundwaters in the SGA highlights the impact of climatic conditions on the groundwater quality. The mean rate of annual evaporation (E) in the study area is about 1,370 mm, which is at least five times above the average precipitation (250 mm/year), resulting in more salinity of the groundwater resources.

Igneous rock weathering is a possible natural (geogenic) source of the trace elements, especially heavy metals in groundwater (Singhal & Gupta 2010). The occurrence of these elements may limit the groundwater for drinking uses due to the possible effects on health.

More than 50 trace elements in groundwater samples were investigated. However, the elements U, Th, Ba, Sr, Sn, W, Cu, and Zn with concentrations above the detection limit of the instrument were determined (Table 4). The box-and-whisker plots for these elements are presented in Figure 5. They reveal normal distribution of the elements, as approximately 95% of the data lie between the inner fences (whisker). The mean (or median) values arranged in descending order are as follows: Sr > Ba > Sn > W > Cu > U > Zn > Th.

The Shir-Kuh granitoid with the S-type nature of magma was derived from the local anatexis of a metasedimentary source which is characterized by enrichment in large ions lithophile elements such as Rb, U, Th, and K (Sheibi et al. 2010). No distinctive minerals were identified in the granitic rocks as the primary source of the above-mentioned elements having high concentrations in the groundwater samples. Nevertheless, the similarity in chemistry causes entry of the trace elements into the crystal lattices of the primary minerals. For example, Sr is a fairly common element replacing Ca and K in granitic igneous rocks minerals (Hem 1985). Therefore, the occurrence of trace elements in groundwater resources is expected as the primary minerals are weathered. Regarding the mass balance studies, weathering of the primary silicate mineral of biotite is introduced as the possible source of the heavy metals in groundwater resources of the SGA.

The spatial distribution of the trace elements in the SGA is shown in Figures 6 and 7. In general, higher concentrations, especially for U and Th (Figure 7), are observed in the northwest, where the felsic granitic rocks with expected higher enrichment in heavy metals are more relatively dominant. From a hydrogeological viewpoint, the longest groundwater flow path from highland recharge areas to the discharge points at the valley bottom (Sanij valley) occurs in this area. Consequently, higher weathering rate of the granitic rocks in relation to the longer residence time of the groundwater in contact with the aquifer materials could be the other reason for the enhanced trace elements in groundwater samples located downstream of the flow path or discharge zones (Figure 2). The Zn concentration is clearly increased in the samples adjacent to the limestone units on the Shir-Kuh granitoid in the east. The existence of the typical carbonate-hosted Zn deposit of Darreh-Zanjir (Maghfouri et al. 2017) in cretaceous sediments of the Taft formation in the east of the study area, suggests the carbonate dissolution as the main possible source of Zn in the groundwater samples.

The minimum, maximum, and average values of U in groundwater samples are 1.1, 13.6, and 6.9 μg/L, respectively. The accessory minerals of zircon and monazite enclosed in biotite minerals of the Shir-Kuh granitoid (Sheibi et al. 2010) are potentially considered as the main source of U ion in the groundwater from the SGA. These heavy minerals accumulate U and other incompatible elements from magma in the later stages of crystallization (Porcelli & Swarzenski 2003). The direct relationship between U and SiO₂ (Figure 8(a)) confirms the major role of silicate mineral weathering in enhancement of U in groundwater resources. The U concentration also increases with the rise in EC (Figure 8(b)), which may be attributed to the increase in the residence time of water in contact with the rock minerals.

The geochemical behavior and distribution of U in groundwater which is controlled by redox processes, pH, and partial pressure of CO₂ (Langmuir 1997) is more complex. In summary, uranium is rather soluble in oxidizing and particularly acidic conditions (Banks et al. 1995b). Complexation with hydroxide, carbonate, and phosphate results in elevated solubility as well (Drever 1988). Estimation of redox potential of groundwater in the SGA has been made from iron (Fe) values. Total Fe concentrations in groundwater from the SGA are less than 0.01 ppm (below detection limit of the ICP-MS) and the groundwaters lie in the narrow pH range of 7.1 to 7.9. At these pH values, Fe²⁺ (soluble form of Fe) becomes stable at low Eh (<0) values (Fetter & Fetter 1999). Consequently, the low Fe concentrations suggest that the SGA groundwaters are more oxidizing than this value. Oxidizing conditions are generally the direct result of dissolved oxygen in the SGA groundwaters. In addition, abundance of the bicarbonate ions resulting from calcite dissolution and biotite weathering are extremely important in mobilization of U in the SGA, as they greatly increase the solubility of U-bearing minerals, facilitate the oxidation of U(IV), and also limit the adsorption of uranium in oxidized waters (Langmuir 1997). The concurrent increase of U and pH (Figure 8(c)) confirms the major impacts of calcite dissolution on complexation of U with carbonate and resulting solubility enhancement in groundwater from the SGA. In pH ranges of the SGA groundwaters (7.1–7.9) the most prevalent stable species of U are expected to be UO2(CO3)₂²⁻ and UO2(CO3)₃⁴⁻ complexes (Fetter & Fetter 1999). Carbonate ions are mostly supplied from calcite dissolution (Table 2), which consumes carbon dioxide and then enhances groundwater pH. The processes explain the direct relationship of U and pH observed in Figure 8(c). This relation also may reflect kinetic factors (Frengstad & Banks 2014), for example, lowest uranium concentrations occur in low pH groundwaters with a low degree of water–rock interaction and low residence time, and vice versa.

The concentration of Th in the groundwaters from SGA varied in a range of 3.9–5.2 μg/L, averaging as 4.3 μg/L. The lower mobility of thorium as compared with uranium (Langmuir 1997) explains its lower concentration in the groundwater resources. The direct relationship between Th and U concentrations (Figure 8(d)) suggests their occurrence from the same origin of rock weathering process. However, the lower solubility of Th in oxidized conditions (tetravalent state) tends to retain thorium in restitute mineral (Hem 1985), decreasing its concentration in the groundwater resources.

In terms of correlations, U is highly correlated with Ba (R = 0.66), Th (R = 0.61), and Sr (R = 0.58) at a significance level of 0.05. This suggests the same origin for these elements, likely from weathering of silicate minerals such as biotite. The idea was further examined by hierarchical agglomerative cluster analysis (HACA) of the trace elements, excluding the extreme outlier values observed in box-and-whisker plots (Figure 5). HACA is the most common method to classify variables into clusters, by starting with the most similar pair of objects and forming higher clusters step by step. The results of HACA are illustrated by a dendrogram that provides a visual summary of the clustering process (Wang et al. 2013). In this study, HACA was performed using IBM SPSS Statistics software (Version 21) for classification of trace elements into different groups, and the achievement of HACA was through the between-group linkage method using squared Euclidean distances. Based on the resulting dendrogram (Figure 9), the elements are differentiated in three clusters. U and Th are grouped with Sr, Ba, and Cu in cluster 1. The elements Sr and Ba in this group, with chemical similarities to Mg (released from biotite weathering as previously mentioned), are considered as the key for introduction of the biotite as the main source of these elements in groundwater resources. It must be stated that all these elements generally migrate in carbonate forms (Fetter & Fetter 1999) and the carbonate ions in the SGA groundwaters also mainly come from carbonate dissolution. The element Zn is individually situated in cluster 2, probably due to the different origin from dissolution of carbonates. The Sn–W pair in cluster 3 may be linked to continental collision-related Sn–W mineralizations observed in peraluminous S-type granitoids of the middle Jurassic period like Shir-Kuh (Jazi et al. 2013). The idea merits more research to confirm the details.

The maximum admissible concentrations of Ba, Cu, Zn, and U in drinking water have been set at 700, 1,000, 3,000 and 15 μg/L, respectively, by the Institute of Standards and Industrial Research of Iran (ISIRI 2010). There are no legislated maximum permissible values for Sr, Sn, W, and Th in Iran. Regarding the measured values of the trace elements in the SGA (Table 4), the maximum concentrations of most elements are too far below the ISIRI permissible limits in drinking water. The only health concerns may relate to U, as the maximum observed values are closer to the ISIRI maximum permissible standards. There is a general agreement that the chemotoxicity rather than the radiotoxicity related to U in drinking water becomes a concern in the range of 10–30 μg/L, as the prolonged consumption of U-rich waters may lead to renal injuries (Frengstad & Banks 2014). Current drinking-water standards for U in different countries display a range of 2–30 (Frengstad & Banks 2014). The strictest limits were established by Germany, where the maximum values are defined as 2 μg/L for the preparation of baby food and 10 μg/L for others. Regarding these limits, 80% of the water resources are unusable for baby food and about 33%, including those located in the Sanij valley at the northwest of the Shir-Kuh granitoid mass, are not suitable for drinking. Based on the maximum standard limit of 30 μg/L, recommended by both the WHO (2011) and EPA (2002), there are no drinking water health concerns over U in groundwater resources from the SGA at the present time. However, an increase in groundwater's EC from a mean value of 516 μS/cm in 2003 (Jafari 2003) to 624 μs/cm in 2016 demands a continuous quality monitoring program, especially on the U values in drinking groundwater resources.

Geochemistry of groundwater in the SGA has evolved to Ca-HCO₃⁻ type, mainly as a result of the calcite dissolution and biotite weathering. The processes, in combination with dissolution of fracture fillings increase water salinity, releasing the trace elements of Sr, Ba, Sn, W, Cu, U, Zn, and Th (arranged in decreasing order of abundance) in the groundwater resources as well. Except for Zn, the trace elements are spatially enhanced in the northwest of the study area, where the felsic rocks with expected higher enrichment in heavy metals are more relatively dominant and the longest groundwater flow path with a resulting increase in water–rock interactions also occurs. Elevated values of Zn are observed in the east, where limestone units overlaid on the granitic rocks with typical carbonate-hosted Zn deposits are found. Weathering of the biotite with the enclosed accessory minerals of zircon and monazite are considered as the potential source of radioelements (U and Th) in the groundwater resources of the SGA. The oxidizing nature of the groundwater, inferred from the absence of Fe in water samples, the major impact of calcite dissolution on mobilization of U, and the degree of water–rock interactions are the main factors controlling U in the groundwater resources from the SGA. The maximum concentrations are currently less than the Iranian drinking guidelines. However, concentration enhancement is expected in the future, as the salinity of the groundwater resources are being temporally increased. The lower solubility of Th in oxidized conditions explains its low concentration in the SGA groundwater.

The cooperation of the Shahrood University of Technology is greatly acknowledged.