Quality characteristics of groundwater
The summary results of groundwater of our study are shown in Table 1. The water temperature ranged from 22.5-27.1 °C during the study period. The pH of water varied from 6.4-8.0 and indicated that the waters were slightly acidic to slightly alkaline (Table 1). The COD of the study water ranged from 1.2-3.9 mgO2/L. The alkalinity and hardness values varied from 12-81 mg/L and 40-226 mg/L, respectively. We found significant variability in EC values, with the values ranging from 131-686 ?S/cm.
The amount of TDS varied from 64-343 mg/L with an average of 160 mg/L (Table 1).
The concentration of K was comparatively low than that of Na in all waters (Table 1), because K minerals have low ability in migration (Nikanorov and Brazhnikova, 2012) and are resistant to decomposition (Pradhan and Pirasteh, 2011).
However, Na+ ranged from 5-20 mg/L with a mean of 9.60 mg/L. Na+ usually comes from the dissolution of evaporites and silicates (Li et al.
, 2013a). Ca2+ and Mg2+ are the results of carbonate dissolution in general. In the study area, the concentrations of Ca2+ and Mg2+ varied from 5.61-37.68 and 5.83-32.08 mg/L, respectively. The average concentrations of Cu2+, Zn2+, Fe3+, As3+ and Mn2+were 0.03 mg/L, 0.048 mg/L, 0.474 mg/L, 0.027 mg/L and 0.298 mg/L, respectively.
The concentrations of CO32- were negligible compared to HCO3- . The sources of CO32- and HCO3- are the dissolution of carbonate rocks resulting in eventual precipitation of CO2 (Nikanorov and Brazhnikova, 2012). Sedimentary rocks and chloride salts are the sources of Cl- (Pradhan and Pirasteh, 2011). The dominant natural sources of SO42- include rock weathering and biochemical processes (Herojeet et al., 2013). Weathering of muscovite, biotite, fluorite and fluoroapatite is the main cause of fluoride in groundwater. However, the concentrations of HCO3-, Cl-, SO42-, NO3-, PO43-, F-, B and SiO2 ranged from 49-207 mg/L, 4-53 mg/L, 0.18-11.72 mg/L, 1.25-7.21 mg/L, 0.003-0.019 mg/L, 0.13-0.52 mg/L, 0.028-0.096 mg/L and 5-33 mg/L, respectively. Fertilizer application can alter the major ion concentrations in groundwater in the study area.
Mechanisms controlling groundwater chemistry
Relations among the different anions and cations are able to explain the mechanisms that control groundwater chemistry. It can be explained by the following headings.
In carbonate weathering, the molar ratio of Ca2+ + Mg2+: HCO3- (Fig. 2f) had greater than unity suggesting the dominancy of carbonate weathering and the source of high HCO3-. The possible cause is the dissolution of gypsum, anorthite, and calcium montmorillonite that will release Ca2+ into groundwater but not HCO3-, inducing the excess of Ca2+ over HCO3-. In addition, cation exchange may also increase the concentration of Ca2+ in groundwater.
The formation of CaCO3 can decrease Ca2+ concentration with a proportional increase of Na+. Consequently, the Mg2+: Ca2+ ratio was greater than 1 (Fig. 2e). With elevated Na+ ions, Mg2+ is dominant over Ca2+ in the increased clay-rich soil (Yousaf et al., 1987). Dominance of Na+ + Mg2+ over Na+ + Ca2+ in groundwater can have greater Na+ hazard (Yousaf et al., 1987).
The HCO3-: Na+ >1 indicates carbonate weathering while its lower value suggests silicate weathering (Krishna Kumar et al., 2009). In this study, the ratios of HCO3- + CO32-: Ca2+ (Fig. 2i) and HCO3- + CO32-: Mg2+ (Fig. 2j) were close to the unity reflecting the dominancy of Ca and Mg-containing minerals. However, the influences of Na-containing minerals are less because the ratio of HCO3- + CO32-: Na+ (Fig. 2d) were far below the unity. In most cases the ratios of Ca2+ + Mg2+: total cations were close to unity (Fig. 2b). The ratios of Na+ + K+: total cations indicate lower concentrations of these two cations over Ca2+ and Mg2+ (Fig. 2a). The Ca2+ + Mg2+ vs HCO3- + SO42- relations of most samples were approaching unity (Fig. 2g) reflecting dominancy of carbonate weathering (Datta and Tyagi, 1996).
The ratio of Na+: total cations (TC) indicate the levels of silicate weathering process (Lakshmanan et al., 2003). Groundwater samples had Na+: TC< 0.25 (Fig. 2k) indicating that there was less silicate weathering. The ratio of Na++ K+: TC was 0.25 (Fig. 2a). This implies that silicate weathering is less responsible to the supply of cations in groundwater. When carbonate and silicate minerals are the major contributors for groundwater chemistry, the HCO3-: TC value would be one (Kim, 2003). This study did not correspond well to the above-mentioned mechanism (Fig. 2h). The plotting of Ca2++Mg2+ versus HCO3- (Fig. 2f) further infers that the groundwater did not have an excess of HCO3-. This HCO3- was not balanced by Na+ (Fig. 2d), as the silicate weathering was not a prime mechanism to release the Na+ and HCO3- into the groundwater. The ratio of Na+: Cl- in most samples was lower than unity (Fig. 2c). The dissolution of NaCl generates a 1:1 ratio of Na+: Cl- while the release of Na+ from silicate weathering produces a wide ratio (Li et al., 2016b).
Salinity and saline water intrusions are related to Na-Cl relationship (Jalali, 2007). The equations of Cl-/?anions > 0.8 and Na+/ (Na++Cl-) < 0.5 suggest seawater intrusion into groundwater (Hounslow, 1995). In this study, the average value of Cl-/?anions ratio was 0.26 and also Na+/ (Na++Cl-) ratio was 0.42 (Table 1). A significant correlation (r =0.56) exists between Na+ and Cl- suggesting that they might be originating from the same sources (Table 3). The cation exchange process may also increase Na+ concentration in addition to the dissolution of halite (Wayland et al., 2003). Fig. 2c shows the deviations of the expected Na+: Cl- (1:1) relation indicating that a fraction of Na is associated with another anion. In the study area, the ratio of Na+/Cl- < 1 meaning another source is contributing chloride to the groundwater.