Introduction

Groundwater is the main source of water used for drinking, domestic, industrial, and agricultural purposes in regions where surface water is scarce (Amiri et al. 2015b; Delgado et al. 2010). Groundwater geochemical evolution is controlled by both natural processes and human impacts. Generally in natural systems, geochemical processes of groundwater quality and major components are controlled by natural processes such as mineral weathering, ion exchange, and water-rock interactions, and occurring of these processes is slow (Li et al. 2013a). However, specific hydrogeochemical processes may occur in different hydrogeologic settings. In recharge areas, dissolution of minerals (including carbonates and silicates) dominates (Sung et al. 2012). Precipitation of secondary minerals prevails in discharge areas (Edmunds et al. 2006).

In order to assess hydrogeochemical processes and geochemical evolution in the complex system at basin scale, many methods, including hydrogeochemical diagrams (such as Piper third-line diagrams), multivariate statistical analysis, water-rock interaction simulation, geochemical modeling, and mineral phase equilibrium calculations have been intensively used (Sikdar et al. 2001; Li et al. 2010; Alfy 2012; Li et al. 2013b, c; Wu et al. 2014; Li et al. 2014; Devic et al. 2014; Amiri et al. 2015a), as well as major components, stable isotopes, trace elements, and redox indicators (Barbecot et al. 2000; Edmunds et al. 2002; Li et al. 2013a, c; Rademacher et al. 2001).

Groundwater is not only the major source of drinking water and agriculture in Iran, especially in semiarid and arid regions, but also supplies more than 50 % of public water utilities and accounts for almost all the water supply to rural households (Mahdavi 2007). In such areas, the quality of groundwater is equally important to its quantity owing to the suitability of water for various purposes (Alaya et al. 2013). Despite the importance of groundwater in Iran, quality assessment of groundwater has received little attention and efforts to use hydrochemical data to solve particular problems are less or nonexistent. Therefore, hydrochemical analysis of the groundwater has become a high priority concern.

This study was carried out in Hamadan province, western Iran. Groundwater is the major source of water for domestic, industrial, and agricultural uses. So far, some researches were reported that in western Iran (e.g., Hamadan province), there exist a variety of environmental problems, such as nitrate contamination and salinization of groundwater (Jalali 2005a, 2007). On the other hand, the studies carried out in this area are focused on some local areas (e.g., Jalali 2009, 2011).

By considering the important role of groundwater in production of agricultural products and drinking purpose, a comprehensive study was needed to evaluate the water chemistry in all aquifers of Hamadan province. Therefore, we attempted to determine the hydrogeochemical nature of the groundwater, identify the geochemical processes responsible for the present status of groundwater quality, and assess the suitability of groundwater for drinking and irrigational uses.

Materials and methods

Study area

Hamadan province with an area of about 19,546 km2 is situated between longitudes of 47° 45′ E and 49° 29′ E and latitudes of 34° 0′ N and 35° 42′ N, in western Iran (Fig. 1). Hamadan is in the vicinity of the Alvand Mountain and has a cold, mountainous climate, with snowy winters. Hamadan province lies in a temperate mountainous region to the east of Zagros. The vast plains of the north and northeast of the province are influenced by strong winds, which almost last throughout the year. The temperature may drop below −30 °C on the coldest days. The area has a semiarid climate with precipitation during the rainy season, from June to October with a long time average of 234.7 mm per year.

Fig. 1
figure 1

a Digital elevation map and water sample points, b geology map, and c lithostratigraphic column of Hamadan province

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Geological and hydrogeological setting

Based on structural geology, Hamadan province is located at interface between several structural zones. Southwestern margin of Hamadan province is a portion of high Zagros Mountains, divided from other parts by Borujerd-Morvarid young active faults. Main lithology of this province includes Jurassic-Cretaceous Mountain forming carbonates. The carbonate sheets have been thrusted on each other due to activity of embracing thrusts; therefore, southwestern Hamadan structural characteristics are of thrust faults features. Most parts of Hamadan province belong to northwestern portion of Sanandaj-Sirjan tectono-sedimentary zone which form NW-SE trending heights around Hamadan. These regions are mainly made of metamorphic rocks of two ages: Paleozoic to early Mesozoic and Mesozoic to early Cenozoic. Northeastern parts of Hamadan are plain like regions, which are covered by Quaternary alluvial sediments. Lower cretaceous carbonatic, volcanic rocks specially Oligo-Miocene limestones (Qom formation) outcrops in southern Razan with NW-SE trend.

From hydrogeological points of view, Hamadan province can be divided into eight major aquifers including Asad Abad, Toyserkan, Nahavand, Malayer, Hamadan-Bahar, Razan, Ghahavand, and Kabudrahang. Generally, total groundwater discharge from all aquifers in Hamadan province is about 2381.6 million cubic meters (MCM).

The Asad Abad aquifer extends over an area about 295 km2. The specific storage in this convertible confined-unconfined aquifer is 0.05. This main lithological unit of this area is limestone formation covering the northern, western, and southern parts. Toward the east and northeast, the aquifer is influenced by the presence of low permeability schist. The thickness of aquifer ranges from 400 m in central part to 20 m in northwest. Groundwater flow down from north and east parts to southern parts of aquifer. The mean annual discharge from this aquifer is about 291.5 MCM per year.

The unconfined Toyserkan aquifer with an area of around 198 km2, belong to northwestern portion of Sanandaj-Sirjan zone which form NW-SE trend. The specific storage in Toyserkan aquifer is 0.06. Toyserkan aquifer is bounded by limestone units in the south and west. In the northeast and northwest of the aquifer, igneous and metamorphic rocks make the main units. Groundwater flow direction is, in general, from northeast to the southwest. The mean annual discharge from Toyserkan aquifer is about 113.1 MCM.

The unconfined Nahavand aquifer has an area about 644 km2 is a portion of high Zagros Mountains, divided from other parts by Borujerd-Morvarid young active faults. The specific storage in this aquifer is 0.05. Main lithology of this area includes Jurassic-Cretaceous Mountain forming carbonates and metamorphic rocks. The aquifer has a mean thickness of 100 m and groundwater flow from south to the north. The mean annual groundwater discharge from this aquifer is near 475.2 MCM.

The unconfined Malayer aquifer extends over an area about 520 km2 and is mainly surrounded by metamorphic (phyllite) and igneous rocks. The specific storage in this aquifer is 0.04. In this area, limestone unit has a small outcrop in the southeast of the area which form NW-SE trend (Fig. 1a). Groundwater flow direction has a SE-NW trend. The mean annual discharge from this aquifer is about 290.8 MCM.

The unconfined Hamadan-Bahar aquifer with an area of around 468 km2 and specific storage equal to 0.047 belongs to northwestern portion of Sanandaj-Sirjan tectono-sedimentary zone which form NW-SE trending heights around Hamadan. These regions are mainly made of metamorphic rocks of two ages: Paleozoic to early Mesozoic and Mesozoic to early Cenozoic. Northeastern parts of Hamadan are plain-like regions, which are covered by Quaternary alluvial sediments. Lower cretaceous carbonatic, volcanic rocks specially Oligo-Miocene limestones (Qom formation) outcrops in southern Razan with NW-SE trend. The mean annual groundwater discharge from Hamadan-Bahar aquifer is about 475.2 MCM.

The unconfined Razan and Ghahavand aquifer with an area about 1709 km2 and is mainly bounded by metamorphic (phyllite), igneous rocks (andesite), Tuff and shale. The specific storage in this aquifer is 0.045. Agriculture is the dominated land use and comprises mostly slope farmland, orchard, field crops, sparse forestland, and residential land. The mean annual discharge from Razan and Ghahavand aquifer is about 486 MCM.

The unconfined Kabudrahang aquifer which has an area about 1350 km2 and specific storage equal to 0.04 is characterized by the predominance of metamorphic rocks of both sedimentary and magmatic origins. The metamorphic rocks constitute an assemblage of high to low methamorphic grade rocks from sedimentary sequences that have been affected by more- or less-developed tectono-metamorphic events. Basement rocks and minerals in the studied area consist of granites, schist, plagioclase, dolomite, and limestone (Fig. 1c). The mean annual groundwater discharge from this aquifer is near 443 MCM.

Sampling and analyses

In this study, groundwater from 225 regional monitoring wells was sampled from eight aquifers and analyzed seasonally during 2013–2014. The study was conducted for wet and dry seasons separately. In this study, seven chemical parameters including calcium (Ca2+), sodium (Na+), potassium (K+), magnesium (Mg2+), bicarbonate (HCO3 −), sulfate (SO4 2−), chloride (Cl−), and four physical parameters including total dissolved solid (TDS), pH (hydrogen ion concentration), TH (Total hardness), and electrical conductivity (EC) were used to evaluate the groundwater quality. The samples were collected after 10 min of pumping and stored in polyethylene bottles. Immediately after sampling, pH, EC, and TDS were measured in the field using a multiparameter portable meter.

Water samples collected in the field were analyzed in the laboratory for cations (Ca2+, Mg2+, Na+, and K+) and anions (HCO3 −, SO4 2−, and Cl−) using the standard methods. Ca2+ and Mg2+ were determined titrimetrically using standard EDTA. Chloride was determined by the standard AgNO3 titration method. Bicarbonate was determined by titration with HCl. Sodium and K+ were measured by flame photometry and SO4 2− by spectrophotometric turbidimetry (APHA 2005).

The quality assurance and quality control (QA/QC) was assured through careful standardization, procedural blank measurement (including field blanks, instrument blank, trip blank, etc.) spiking, and duplicating the samples. The ionic charge balance of each sample was within ± ~5 %.

The data were statistically analyzed and a correlation matrix was run using the SPSS 17 software package. The inferential statistics can make judgments of the probability that the difference between groups is a dependable one or independent of each other. The AquaChem 3.7 program was used for hydrochemical assessment of water samples.

The origin and chemical behavior of the groundwater were assessed based on the ionic relationships. These comprise the relationships of TDS vs. Cl−/[Cl−+ HCO3 −], TDS vs. Na+/[Na++ Ca2+], [Ca2+/Ca2++Cl−] vs. EC, [Mg2++ Ca2+] vs. [SO4 2−+HCO3 −], [CO3 2−+ HCO3 −]-[Cl−+ SO4 2−] vs. [Ca2++Mg2+]-[Na++K+] (Chadha diagram), and [Ca2++Mg2+]-[SO4 2−+ HCO3 −] vs. Na-Cl.

The correlations between major cations were studied to clarify which elements contribute to the groundwater composition. In addition, the saturation indices of groundwater samples were calculated using the geochemical model PHREEQC (Parkhurst and Appelo 1999).

The saturation state of the water samples is defined by the saturation index (SI) (Appelo and Postma, 2005) and was calculated based on the activities of free ions in solution. The saturation index (SI) is calculated by comparing the chemical activities of the dissolved ions of the mineral (ion activity product, IAP) with their solubility product (Ksp). In equation form,

$$ SI= \log \left(IAP/{\mathrm{K}}_{sp}\right) $$
(1)

Some minerals react fast upon contact with water, mostly minerals, such as gypsum, halite, and carbonate, which would meet the equilibrium in a short residence time. SI values equal to 0 indicate equilibrium between the mineral and the solution while positive SI values show supersaturation and negative SI values reflect undersaturation. SI values can be expressed in numerous ways, such as to describe the dissolution or precipitation of minerals (Esmaeili-Vardanjani et al. 2015).

Results and discussion

Chemical characteristics of water

Minimum, maximum, and mean values of different physical and chemical parameters in groundwater samples collected in wet and dry seasons are given in Table 1. The mean concentrations of Ca2+ in the wet and dry seasons are 89.62 and 90.90 mg/l, respectively, which are presumably derived from calcium-rich minerals (e.g., carbonate minerals). As shown in Fig. 1c, limestone formations are distributed almost throughout the study area. The mean concentrations of Na+ and K+ in groundwater are 84.43 and 86.26 mg/l, and 1.23 and 1.24 mg/l for wet and dry seasons, respectively. Based on variety of rock formations (i.e., volcanic, metamorphic and sedimentary) in this area and role of water-rock interaction in the change of water chemistry (Fig. 2), the major source of Mg2+ in the groundwater may be due to dissolution of Mg-bearing rocks (e.g., dolomitic formations). The average concentrations of Mg2+ and HCO3 − ions found in the groundwater samples in the study area are 36.97 and 38.61 mg/l, and 326.74 and 334.63 mg/l for wet and dry seasons, respectively. The average values of sulfate in the wet and dry seasons are 142.65 and 144.96 mg/l, respectively. The concentration of Cl− ranges from 7.03 to 1125.60 and 7.03 to 1085.73 mg/l in the wet and dry seasons, respectively. In the wet season, pH values vary from 6.25 to 8.25, while in the dry season, it varies from 6.20 to 8.0. This indicates that most groundwater samples have alkaline nature. The sampling sites with slightly acidic water (pH < 7) are located in the east of study area (Ghahavand aquifer). The most important economic activity of the area is agriculture. Therefore, anthropogenic pollution such as oxidation of ammonium originated from fertilizers can lead to increase in water acidity. The EC values vary from 235.5 to 6740 μS/cm in wet season and from 327 to 7110 μS/cm in dry season (Table 1). In this region, cation dominance order is Ca2+ > Na+ > Mg2+ > K+, while the anion dominance order is SO4 2− > HCO3 − > Cl−. No seasonal variation was noticed in the ionic dominance pattern.

Table 1 Statistical summary of the physical and chemical parameters of groundwater samples in Hamadan province
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Fig. 2
figure 2

Mechanisms governing groundwater chemistry a TDS vs. Na+/[Na++Ca2+], b TDS vs. Cl−/[Cl−+HCO3 −] (circle = wet season; diamond = dry season)

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Pearson correlation analysis is a useful tool in hydrogeochemical studies since the produced correlation matrix can indicate associations between individual parameters, thus demonstrating the overall coherence of the data set and revealing the links between individual variables and various controlling factors (Wang and Jiao 2012).

If the correlation coefficient (r) is greater than 0.7, two parameters are considered to be strongly correlated, whereas if the r value is between 0.5 and 0.7, it indicates a moderate correlation at a significance level p < 0.05 (Guey-Shin et al. 2011). In the present study, the statistical software package SPSS 17.0 for Windows was used to calculate Pearson correlation coefficients, with the obtained results listed in Table 2 (bottom and top triangles indicating wet and dry, respectively). As seen from Table 2, all major cations and anions with the exception of HCO3 − have a strong correlation (r > 0.9) with TDS. Calcium and Mg2+ presented a positive correlation (r = 0.96), indicating a common source (limestone, dolomite, and metamorphose carbonate rocks). Correlation between Ca2+ and HCO3 − (slightly weak) in comparison with correlation between Ca2+ and SO4 2− (strong) shows the latter ions in groundwater are mainly provided by the dissolution of gypsum. There is a strong correlation between Na+ and Cl−, indicating that dissolution of chloride salts (Qom formation) can be referred as the main source of salinity, because the groundwater level descend to a minimum depth of about 10 m. So, evaporation is not the main reconcentration process. EC exhibits an excellent correlation with TDS (r = 0.99) during wet and dry seasons. This occurs because TDS depends on dissolved ion concentration. Groundwater chemistry depends on a number of factors, such as general geology, degree of chemical weathering of the various rock types, quality of recharge water, and inputs from sources other than water-rock interaction (Sarath Prasanth et al. 2012).

Table 2 Pearson correlation coefficients of hydrochemical indices (a) wet season and (b) dry season
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The functional sources of dissolved ions can be broadly assessed by plotting the samples, according to the variation in the ratio of Na+/[Na++Ca2+] and Cl−/[Cl−+HCO3 −] as a function of TDS (Gibbs 1970). A Gibbs plot (Fig. 2a, b) of data indicates that groundwater chemistry in this area may have been mainly governed by rock weathering. However, rock-weathered materials derived from the underlying rocks control the groundwater chemistry of water samples.

From Fig. 2a, some of samples fell near the evaporation-precipitation zone. As it mentioned in previous section, depth to groundwater level in all aquifers is more than 10 m and groundwater is almost impossible to evaporate. But in the past, groundwater depth might be small and evaporation had significant influences on groundwater chemistry (Fig. 2b). It should be noted that some areas are covered by evaporite and quite soluble sediments such as gypsum (Qom formation). Therefore, like the evaporation process, dissolution of such sediments may remain the similar changes on groundwater chemistry.

For further classification of the analyzed waters, we set a van Wirdum’s diagram (van Wirdum 1980), the ionic ratio (IR) vs. the in meq/l (Fig. 3).

$$ IR=\left[{Ca}^{2+}\right]/\left[{Ca}^{2+}+{Cl}^{-}\right] $$
(2)
Fig. 3
figure 3

The van Wirdum’s diagram for Hamadan groundwater samples (At atmospheric, Li lithotrophic, Th thalassotrophic groundwater)

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The EC can be seen as a measure of the salinity, the IR as a measure of the prevalence of calcium among the cations and chloride among the anions. This prevalence is a result of geochemical processes that occurred within the aquifer. Based on the position in the van Wirdum’s diagram, the groundwater can be classified as atmospheric, At (rainwater); lithotrophic, Li (calcium-rich freshwater); thalassotrophic, Th (seawater); or anything in between. The most of samples are distributed around the lithotrophic water type. However, a few of the samples from the Ghahavad aquifer have higher chloride content. As it can be seen in Fig. 3, location of samples plotted near Th zone, both of dry and wet samples have the same location and the sources of salinity cannot be from evaporation, because in wet season in area with cold climate, evaporation from water table is negligible. However, the main sources of salinity provided by dissolution of saline sediments (Qom formation) around the Ghahavand aquifer.

Hydrochemical facies

Groundwater samples of the study area have been plotted on Chadha’s diagram (1999). In this scheme, the difference in milliequivalent (epm) percent between alkaline earth (calcium + magnesium) expressed as percentage reacting value is plotted on the x-axis and the difference in milliequivalent (epm) percentage between weak acid anions (carbonate + bicarbonate) and strong acid anions (chloride, sulfate, and nitrate) is plotted on the y-axis.

The milliequivalent percentage difference between alkaline earth and alkalies and between weak acidic anions and strong acidic anions is plotted on one of the four possible subfields of the diagram. In both wet and dry seasons 80 % samples are Ca-Mg-HCO3 type, 13.72 % samples are Ca-Mg-Cl-SO4 type, and 7.84 % samples are Na-K-Cl-SO4 (Fig. 4).

Fig. 4
figure 4

Classification of groundwater samples on Chadha’s scheme (circle = wet season; diamond = dry season)

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In general, there are three dominant facies, including sulfate, bicarbonate, and chloride. Using results of AquaChem program, the spatial distribution of general water types in the study area have been shown in Fig. 5. This figure generally shows that Hamadan-Bahar, Kabudrahang, and Ghahavand aquifers (central aquifers) have chloride water type and other aquifers (west, south, and northern aquifers) have bicarbonate water type.

Fig. 5
figure 5

Map showing water types in the study area (a wet season, b dry season)

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Saturation indices

The saturation index (SI) values of minerals were calculated using the PHREEQC software to evaluate the degree of equilibrium between water and the respective mineral and to understand the geochemical processes governing hydrochemistry of groundwater in the area. Figure 6 shows SI indices of water samples relative to anhydrite, aragonite, calcite, dolomite, and gypsum. As it shows, groundwater samples are under saturation relative to anhydrite and gypsum (SI < 0) and it can be expected that negative SI values can increase the potential of sulfate compositions dissolution (e.g., anhydrite and gypsum) and lead to an increase in concentration of calcium and sulfate ions along groundwater flow direction. So, it should be expected that the water type change from bicarbonate to sulfate (Amiri et al. 2014). The SI values relative to aragonite, calcite, and dolomite are increasingly supersaturated (SI > 0). It is compatible with the origin of groundwater from limestone and dolomite formations which make highland areas through the study domain. No seasonal variation was noticed in the ionic dominance pattern.

Fig. 6
figure 6

Saturation indices of minerals related to groundwater samples

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Ion exchange reactions

Another significant factor affecting the constituents of groundwater is ion exchange which has become an important topic for hydrogeologists since it regulates the transport of pollutant chemicals in both aquifers and soils (Appelo and Postma 2005). Ion exchange involves the replacement of ions adsorbed on the surface of the fine-grained materials of the aquifers by ions that are in the solutions (Kazemi and Mohammadi 2012).

Ion exchange and industrial and/or agricultural contamination are likely responsible for increase in sodium in a gneissic terrain (Guo and Wang 2004). High concentration of Na+ with respect to Cl− or depletion of Na+ with respect to Cl− is the evidence of cation exchange reactions (Salama 1993; Rajmohan and Elango 2004). In the normal ion exchange reaction, Ca2+ is retained in the aquifer material and Na+ is released to water. The excess Na+ generated by ion exchange reaction is not balanced by Cl− but by alkalinity or SO4 2−. Similarly, in the reverse ion exchange, Na+ is retained by aquifer materials and Ca2+ is released to water.

To investigate the importance of ion-exchange processes in groundwater chemistry, we have examined the relationship between the concentration of (Na+-Cl−) against (Ca2++Mg2+-SO4 2− - HCO3 −) (Fig. 7a). The product of (Na+-Cl−) represents excess sodium, that is, sodium coming from the sources other than halite dissolution, assuming that all chloride is derived from halite.

Fig. 7
figure 7

Plot of a (Ca2++Mg2+)-(SO4 2−+HCO3 −) versus (Na++K+)-Cl− and b Mg2++Ca2+ versus SO4 2−+HCO3 − (diamond = wet season; circle = dry season)

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Also, the product of (Ca2++Mg2+-SO4 2−HCO3 −) represents the calcium and/or magnesium coming from sources other than gypsum and carbonate dissolution. In the absence of these reactions, all data should plot close to the origin (McLean et al. 2000). If these processes are significant composition governing processes, the relation between these two parameters should be linear with a slope of −1. As shown, almost all data plot for both wet and dry seasons have a linear correlation, with a slope of about −0.99 for wet and dry seasons. Figure 7 b shows the amount of Ca2++Mg2+ gained or lost relative to that provided by calcite, dolomite, and gypsum. When HCO3 −+SO4 2− is low (less than 5 meq/l) and the samples plot on 1:1 line, dissolution of calcite and dolomite is the major process influencing water chemistry but when HCO3 −+SO4 2− is more than 5 meq/l, in addition to calcite and dolomite, dissolution of gypsum is also likely (Kalantary et al. 2007). For all samples of the Hamadan province, HCO3 −+SO4 2− is >5 meq/l, indicating that gypsum dissolution is likely to occur. The plot for Ca2++Mg2+ versus SO4 2−+HCO3 − is another major indicator to identify ion exchange process activated in the study area. If ion exchange is the process, the points shift to the right side of the plot due to excess SO4 2− + HCO3 −. If reverse ion exchange is the process, points shift left due to excess Ca2+ + Mg2+. The plot of the Ca2+ + Mg2+ versus SO4 2− + HCO3 − (Fig. 7b) shows that most of groundwater samples in study area, are located around and below the 1:1 line, indicating ion exchange.

Evaluation of groundwater quality

Suitability for drinking and domestic purposes

According to Table 1, the average values of individual parameters of groundwater are within the permissible limits recommended by WHO (2004, 2011) whereas some samples show higher values than permissible limits. Incorporated groundwater suitability map for drinking purposes in the study area is created by combining all the quality parameters, e.g., TDS, TH, pH, Na+, K+, Ca2+, Mg2+, Cl−, and SO4 2− (Fig. 8a, b). For preparing this map, a condition was defined for every sample based on three limitations recommended by WHO (2004, 2011), including most desirable limit, maximum allowable limit, and unsuitable. The condition defined so that if every quality parameter in every sample exceeds cutoff value of each group, the sample will be categorized in most desirable limit, maximum allowable limit, and unsuitable groups, respectively. For example, if Cl−≤200 mg/l, and the same for all parameters, sample locates in most desirable limit group; if Cl−>600 mg/l, sample locates in unsuitable group; and if 200 < Cl−≤600, sample locates in maximum allowable limit group. Based on Fig. 8a, b, eight samples of wet season and three samples of dry season lie in unsuitable quality for drinking. In comparison with geology map (Fig. 1c), it can be referred that these samples are around the metamorphic rocks.

Fig. 8
figure 8

Groundwater suitability map (a wet and b dry) for drinking in the study region based on WHO (2004, 2011). Groundwater quality map (c wet and d dry) for irrigation based on US salinity hazard diagram samples

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Irrigation suitability

In this region, groundwater is extensively used for irrigation. Therefore, the water suitability for irrigational purposes was assessed using sodium percentage (Na%), sodium adsorption ratio (SAR), and permeability index (PI).

Sodium percent

In all natural waters, sodium percentage (Na%) is a common parameter used to assess its suitability for agricultural purposes (Wilcox 1958).

A high sodium percent causes de-flocculation and impairment of the tilth and permeability of soils (Karanth 1987). The sodium percentage (Na%) in the groundwater is calculated by the following equation:

$$ Na\%=Na+\mathrm{K}/\left(Ca+\mathrm{Mg}+Na+\mathrm{K}\right)\times 100 $$
(3)

where the quantities of Ca2+, Mg2+, Na+, and K+ are expressed in meq/l. Wilcox diagram shows that 4.5 % of the groundwater samples fall in the field of unsuitable, 4.9 % of the samples fall in the doubtful to unsuitable field, 2.5 % of the samples fall in the field of permissible to doubtful field, and the rest fall in the field of very good and permissible to good (Fig. 9a).

Fig. 9
figure 9

a Classification of irrigation water quality, with respect to total salt concentration and sodium percent and b US salinity hazard diagram samples (after Richards 1954).

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Sodium adsorption ratio

The sodium or alkali hazard in the use of water for irrigation is determined by the absolute and relative concentration of cations and is expressed in terms of SAR. SAR is a measure of the suitability of water for use in agricultural irrigation, because sodium concentration can reduce the soil permeability and soil structure (Todd 1980), the following equation:

$$ SAR=Na/{\left[\left(Ca+\mathrm{Mg}\right)/2\right]}^{0.5} $$
(4)

where sodium, calcium, and magnesium are in meq/l. Excessive sodium content relative to the calcium and magnesium may deteriorate the soil characteristics, thereby reducing the soil permeability and inhibiting the supply of water needed for the crops. The SAR is used to predict the sodium hazard of high carbonate waters, especially if they contain no residual alkali. There is a significant relationship between SAR values of irrigation water and the extent to which sodium is adsorbed by the soils. If water used for irrigation is high in sodium and low in calcium, the cation-exchange complex may become saturated with sodium. This can destroy the soil structure due to the dispersion of clay particles. On the basis of US Salinity Laboratory, hazard diagram (Fig. 9b) can be plotted by correlating the sodium absorption ratio and electrical conductivity (Richards 1954). Water classes of most of the water samples are C2-S1, C3-S1, indicating medium to high salinity and low sodium type, and about 6.5 % of the samples are C4-S2 and C4-S3 classes, indicating high to very high salinity and medium to high sodium type. This type of water can be used to irrigate salt tolerant and semitolerant crops under favorable drainage condition. In general, Fig. 9b indicates that the water type in the study area has medium salinity with low sodium content (with the exception of some samples in Ghahavand aquifer which have high sodium content) and it can be used for irrigation on all type of soil. Also, like drinking maps, based on US Salinity Laboratory hazard diagram, a condition was defined so that samples can be categorized in four groups which, including C1S1, C2S2, C3(S1,S2), and C4(S1,S2,S3). Figure 8b, c shows the groundwater suitability maps for irrigation. As it can be found, some samples in Ghahavand and Hamadan aquifers lie in C4(S1, S2, S3) which are near the saline sediments of Qom formation. The solution of halite is the main source of salinity in these samples.

Permeability index

Doneen (1962) evolved a criterion for evaluating the suitability of water for irrigation based on PI. This is calculated using the following equation:

$$ \mathrm{PI}=\left[{\left(Na+\left({HCO}_3\right)\right)}^{0.5}/\left(Ca+\mathrm{Mg}+Na\right)\right]\times 100 $$
(5)

Here, all the values are in meq/l. The PI values >75 indicate the excellent quality of water for irrigation. The PI values between 25 and 75 indicate good quality of water and <25 is unsatisfactory for irrigation (Saleh Al-Amry 2008). Collected samples in this area have PI ranged from 22 to 79 %. With respect to PI values, about 96 % of the groundwater samples in the study area can be designated as class II (25–75 %) implying that the water is good quality for irrigation purposes.

Conclusion

Groundwater chemistry in Hamadan province (including eight aquifers), west of Iran, was analyzed to classify the water samples into different categories for drinking and irrigation purposes. The variations of major ions (K+, Na+, Ca2+, Cl−, SO4 2−, and HCO3 −) was used to recognize the hydrochemical characteristics of the groundwater resources. Based on this study, groundwater chemistry is mainly influenced by rock weathering (water-rock interaction). The cation dominance order is Ca2+ > Na+ > Mg2+ > K+, while the anion dominance order follows SO4 2− > HCO3 − > Cl−. The hydrogeological interactions between static and dynamic components of aquifer such as rock formations and groundwater flow, respectively, can determine the groundwater quantity and quality. The groundwater type is largely characterized as a chloride water type in Hamadan-Bahar, Kabudrahang, and Ghahavand aquifers (central aquifers) which are around the saline sediment of Qom formation, and relatively shifted to a HCO3 − water type in the other aquifers (western, southern, and northern aquifers) which locate near the carbonate sediment and metamorphic rocks. The predominance of Na+ ions in the groundwater is mostly from the cation exchange process, which can be supported from the ionic ratio calculation ((Na+-Cl−) against (Ca2+ + Mg2+-SO4 2−HCO3 −) and (SO4 2 + HCO3 −) vs. (Ca2+ + Mg2+)). These observations have major consequences with respect to SI determination which it shows, groundwater samples are under saturation relative to anhydrite and gypsum (SI < 0) and relative to aragonite, calcite, and dolomite are increasingly supersaturated (SI > 0). Groundwater of this area is suitable for domestic purposes except for a few locations where high EC, pH, and other dissolved ions make it unsafe for drinking. SAR, Na%, and PI values suggest suitability of most water samples for irrigation purposes. The majority of water samples belong to C2S1 and C3S1, indicating medium salinity-low sodium water and high salinity-low sodium, which high SAR, PI, and Na% in few locations restricts its suitability for agricultural activities.