Result from descriptive statistics
The water quality parameters results are given in Table 1 (pre-monsoon) and Table 2 (post-monsoon). Descriptive statistics results of chemical analysis for pre- and post-monsoon seasons are presented in Tables 3 and 4, respectively. The pH value of groundwater over study area ranges from 7.90 to 8.70 in pre-monsoon and 7.60 to 8.20 during post-monsoon seasons. It may be noted that all well’s pH value falls under WHO (1993) limit except well no. 6, 17 and 18 during pre-monsoon (Fig. 2a). The pH value of water indicates whether the water is acidic or alkaline. It is controlled by carbon-dioxide, carbonate and bicarbonate equilibrium. The combination of CO2 with water form carbonic acid affects the pH of the water (Rawat et al. 2012, 2013). If the pH is not within the prescribed limits, it damages mucous membrane present in eyes, nose, mouth, abdomen, anus, etc. Tables 1 and 2 show that pattern of EC in groundwater is not smooth (Fig. 2b). It varies from 705 to 2650 μS/cm in pre-monsoon whereas during post-monsoon season, it ranges from 685 to 3400 μS/cm. EC is a measure of salinity hazard. EC in water is due to ionization of dissolved inorganic solids. EC is a measure of water capacity to convey electric current (Rawat et al. 2012, 2013). It is used to estimate the amount of dissolved solids. It increases as the amount of dissolved mineral (ions) increases. It is used as a basic index to select the suitability of water for agricultural purposes. The concentrations of major constituents Ca2+ and Mg2+ show wide variation but their mean values for the pre-monsoon season are 71.79 and 34.79 mg/l, respectively (Table 2; Fig. 3). During post-monsoon season, the average concentration of Ca2+ and Mg2+ lies between 66.58 mg/l and 32.42 mg/l, respectively (Table 4; Fig. 3). Finally, the Ca2+ and Mg2+, which are in high variability in the ground waters of the study area, ranges from 16 to 200 and 4 to 78 mg/l, respectively during the pre-monsoon while 28 to 176 and 8 to 126 mg/l, respectively in post-monsoon (Tables 2, 3; Fig. 2c, d). Ca2+ occurs in water naturally and it cannot be found alone in nature (Rawat and Tripathi 2015). Mg2+ is the most abundant elements in the natural surface and groundwater. Magnesium is washed from rocks and subsequently ends up in water. Magnesium has many different purposes and consequently may end up in water in many different ways. Chemical industries add magnesium to plastics and other materials as a fire protection measure or as filler (Naik et al. 2007). Similarly Cl− and SO4
2− ranges from 100 to 560 and 20 to 380 mg/l during pre-monsoon, respectively whereas during post-monsoon they vary from 12 to 780 and 22 to 298 mg/l respectively. The distribution of Cl− and SO4
2− in ground water of study area is shown in Fig. 2e, f for pre- and post-monsoon. High concentration of Cl− may be injurious to some people suffering from diseases of the heart and kidney, taste, indigestion, corrosion and palatability are affected (Rawat and Tripathi 2015). Figure 2f, shows good variability in distribution pattern of SO4
2− but under the WHO (1993) limit. Hardness of water is primarily due to the result of long-term interaction of water and the geological formations (Hem 1989). The TH concentration of the groundwater in the study area ranges from 128.90 to 778.80 mg/l during pre-monsoon period and from 131 to 956.60 mg/l during post-monsoon period. The permissible limit of TH in drinking water is 600 mg/l (WHO 1993). Results show that all well are under the permissible limit (Fig. 2g) except well no. 8 (during pre-monsoon) and 16 (during post-monsoon). TDS of all samples values ranges from minimum 460 to 2400 mg/l during pre-monsoon period. It is reported that all well are under the permissible limit of TDS except four well (well no. 1, 8, 10 and 18; Fig. 2h) and from 445 to 2210 mg/l during post-monsoon season (and well no. 1, 10, 12, 16 and 18 are exceed the WHO limit; Fig. 2h).
The mean and standard deviations (σ) were used to compare the deviation of water quality parameters from WHO standards (WHO 1993). Whenever mean exceeds the permissible limit fixed by WHO, it is concluded that those particular places are all contaminated with respect to the water quality parameters. From Table 5 and 6 (Fig. 3), mean value of pH [pre 8.35 or 8.35 × 100 and post 7.99 or 7.99 × 100 (multiplied by 100 for graphical representation (Fig. 3) as other parameter were above 100)] and TH (pre 322.11 and post 299.37) for study area under permissible limit (8.5 or 8.5 × 100 for pH and 600 mg/l for TH) of WHO (1993), Ca2+ and Mg2+ are also under the prescribed permissible limit as 75 and 50 mg/l during pre- and post-monsoon. It may be noted that mean value of each parameters are under the WHO permissible limit except limit of EC during post-monsoon (Fig. 3). Elevated concentration of water quality parameters in drinking water have been identified in groundwater of 18 bore wells in Chennai district of Tamil Nadu. These bore wells namely: Besant Nagar, Chindatripet, Kodungaiyur, Egmore, K.K. Nagar, Kolathur, Kotturpuram, Koyambedu, Litte Mount, Pulianthope, Royapettah, Royapuram, T. Nagar, Thirumangalam, Tondiarpet, Velachery, Villivakkam, and Virugampakkam were reported high concentration of analyzed parameters.
Result from correlation coefficient matrix
Correlation matrix of various variables are presented in the Table 3 and shown in Fig. 4. Each table shows the degree of a linear association between two of the parameters (Rawat and Tripathi 2015), as measured by the simple correlation coefficient (r). The correlation among parameters in the pre- and post-monsoon seasons has shown approximately, an anomalous trend. Strong to good correlations among the various water quality parameters has been observed. Strong correlation of TDS and TH with Ca2+, Mg2+ and SO4
2− indicates that all of them have originated from the same source. It shows that TDS and TH are highly contributed by Ca2+, Mg2+, Cl−, and SO4
2− during pre-monsoon monitoring period. From Table 6 Cl− does not play any role in the formation of TDS and TH (due to low r = 0.05Cl-TDS, 0.07Cl-TH). Table 3 also shows a moderate to poor degree of correlation of pH with Ca2+ and Cl− during both monsoon periods. However, pH was found to be significantly correlated (Table 6; Fig. 4) with Mg2+, SO4
2−, TDS and TH during post-monsoon season indicating the presence of + and − ions (which is evidence of strong pH formation) from Mg2+ and SO4
2− during post-monsoon. EC is highly correlated with SO4
2− during both monsoon period. EC and Ca2+ shows moderate correlation (r
pre = 0.54 and r
post = 0.58), conforming that conductivity increases as the concentration of dissolved calcium ions increases. TH exhibits strong and good correlations with Ca2+ (r
pre = 0.88 and r
post = 0.52) and Mg2+ (r
pre = 0.81 and r
post = 0.91) in both the seasons indicating that hardness of groundwater in the study area is mainly due to the salts like CaCO3 and MgCO3.
Multivariate statistical analysis
The PCA results along with factor loading values and percentage of variance for 18 bore wells are presented in Table 7. An Eigen value gives a measure of the significance of the factor (Kaiser 1960; Dalton and Upchruch 1978; Cattle 1966), the factor with highest Eigen values are the most significant. Eigen values of 1.0 or greater are considered significant. Factor loading is classified as strong, moderate and weak corresponding to absolute loading values of >0.75, 0.75–0.50 and 0.50–0.30, respectively (Liu et al. 2003) Two factors or PCs explained 73.10 and 83.50 % of the total variances for pre- and post-monsoon, respectively, which was adequate to give a good initiative of the data structure.
During pre-monsoon season (Table 7; Fig. 5a), variance of Factor1 during pre-monsoon season is 59.15 %. It is reported that Eigen value with 4.732 and variance of 59.15 % has high loadings on Ca2+, Mg2+, SO4
2−, Cl−, TDS, and TH, moderate loadings on EC and low loadings on pH.
High loading: it suggests that the quality of groundwater is mainly controlled by high loading parameters. The high SO4
2− is related to the long-history of evaporation process, the high value of Mg2+ suggests pollution from application of magnesium sulphate fertilizers (anthropogenic pollution factor) to agricultural lands (Rawat and Tripathi 2015; Singh et al. 2015) and high loading on TS and TDS accounts for low mixing of overlying soft soil after percolation due to monsoon runoff pollution. Therefore, two facts are responsible for high loading on these elements, lowering of groundwater table (during pre-monsoon) and anthropogenic pollution due to direct discharge of sewage into water body without treatment (Gautam et al. 2013). During the summer season, without recharge, lowering of groundwater table occurs since high concentration Ca2+, Cl−, and TDS were recorded. The other contributing process is suggested as anthropogenic pollution due to presence of Mg2+ and SO4
2− (leach of magnesium sulphate fertilizers, Hounslow 1995; Hem 1989; Singh et al. 2015).
Moderate loading: EC
Low loading: the negative loading of pH on Factor1 confirms that the concentration of pH in the groundwater does not contribute significantly to Ca2+, Mg2+, SO4
2−, Cl−, TDS, and TH values during pre-monsoon
Factor2 having Eigen value with 1.115 and 13.94 % of the variance, has high loadings of pH and moderate loadings on EC and low loadings on Ca2+, Mg2+, SO4
2−, Cl−, TDS, and TH.
High loading: if pH high (high loadings of pH in Factor2) positive loading which indicate ground water are mostly polluted by discharge of waste water as a regular source in the study area. Owing to degradation of organic material pH Increases due to formation of acids, therefore this factor termed as degradation factor.
Moderate loading: EC represents solubility of minerals.
Low loading: presence of Ca2+, Mg2+, SO4
2−, Cl−, TDS, and TH indicates mild pollution.
During post-monsoon season (Table 7; Fig. 5b), the Factor1 is very strongly correlated with EC, TH and TDS, and strongly correlated with Cl− and Mg2+. It is moderately correlated with Ca2+ and Ca2+, Mg2+, SO4
2−, Cl−, TDS which explain 69.28 % of variability in the dataset. This may be due to recharge effect of rain water since Ca2+and SO4
2− show moderately positive correlation when compared with pre-monsoon seasons.
Factor2 accounts for 14.22 % of variability and include strong positive loading of EC and low loading of pH, by the change in pH of the water, most likely due to constant water inflow. It is due to sudden influx of fresh rain water in the study area.
In the both the season, Factor2 of pH was exhibiting more (in this study each water quality parameter with a strong Eigen value (>75 %) was considered to be significant parameters equally contributing and strong factor loadings in water quality variations for two seasons) strong positive association.
A screen plot (Fig. 5a, b) shows the eigen values sorted from large to small as a function of the principal component number. After the 2 PC (Fig. 5a, b), starting the elbow the downward curve. Other components weight, their eigen values and variances are summarized in Table 7. Loading of varimax rotated matrix for two factor model are shown in Table 7. Evidently, the first factor is usually more correlated with the variables than the second factor (Kaiser 1960; Dalton and Upchruch 1978; Cattle 1966; Singh et al. 2013b, 2015). This is to be expected because these factors are extracted successively, each one accounting for as much of the remaining variance as possible.
According to Liu et al. (2003), total variance (Table 7) has strong positive loadings on Cl−, Ca2+, Mg2+ TH and TDS, weak loading on EC during pre-monsoon season. Whereas, strong positive loading on EC and weak loading on Mg2+, Ca2+, Cl−, SO4
2−, TH and TDS during post-monsoon season are the major solutes in groundwater. EC is positively correlated with the concentration of ions, which can thus be indirectly calculated from EC. Therefore, EC can be regarded as a water salinization index. The association of EC, Cl−, Mg2+, SO4
2−, TH and TDS reflects the influence of sea water intrusion on pollution of groundwater and can thus be termed “the sea water salinization factor”.