Groundwater Chemistry for drinking purpose
The physico-chemical parameters and its concentration in the collected groundwater samples have been evaluated to determine the suitability of groundwater for both drinking and agricultural purpose. The nature of some major ions (Ca, Mg, Na, K, Cl, F, SO4, NO3 and HCO3) and other important physico-chemical parameters such as pH, EC, TDS and total hardness were analyzed and discussed below. The analytical results of all the analyzed parameters (n=34) have been given in Table 1. The pH of water provides vital information in many types of geochemical equilibrium or solubility calculations (Hem, 1985). According to BIS (1993) and WHO (2004) standards, the limit of pH value for drinking water is specified as 6.5 to 8.5. The obtained value of pH shows around 7.9 to 8.2 (Fig. 2) which is permitted for drinking purpose but slightly basic in nature. This nature of water may be due to the dissolution of carbonates predominantly in the HCO3 form (Adams et al., 2001). The value of electrical conductivity may be an approximate index of the total content of dissolved substance (salt) in water and also depends on the temperature as well as the type and concentration of ions present in the water (Hem, 1985). The EC in the studied samples varied from 530 to 3330 µS/cm during the pre-monsoon and 540 to 2320 µS/cm during the post monsoon (Fig. 2). The EC can be classified into three types as type I (EC <1500 µS/cm; Salt enrichment is low), type II (EC 1500-3000 µS/cm; Salt enrichment is medium) and type III (EC >3000 µS/cm; Salt enrichment is high) (Sarath Prasanth et al., 2012). Based on this classification, about 59% and 76% of samples from the pre-monsoon and post-monsoon falls under type I indicating less dissolution of salts. The remaining samples from the pre-monsoon classified under type II (29%) and type III (12%). During the post-monsoon, no samples fall under the type III of EC showing less or medium dissolution due to increased recharge of water from the monsoonal rains.
The TDS values are considered as important for determining the usage of water because groundwater with high TDS values is not suitable for both irrigation and drinking purpose (Sarath Prasanth et al., 2012). According to Freeze and Cherry (1979), water with TDS values <1000 mg/l are considered as freshwater type and those above the given values belong to brackish water type. Therefore, by considering this classification 59% (pre-monsoon) and 76% (post-monsoon) are freshwater type and the rest falls under brackish water type (Fig. 2). Based on Davis and De Weist (1966), water with TDS values >3000 mg/l are not even suitable for irrigation purpose. Thus, the analytical results suggest that all the samples are suitable for irrigation purpose in both the seasons. Higher concentration of TDS in the groundwater samples is due to the leaching of salts and also due to the domestic sewage. The total hardness (TH) values of groundwater samples imply 41% from the pre-monsoon and 59% from the post-monsoon were under the BIS standards. Sawyer and McCarty (1967) classified the TH of groundwater into four categories such as soft (TH > 75 mg/l), moderately hard (75-150 mg/l), hard (150-300 mg/l) and very hard (>300 mg/l). Based on the above said classification, no samples show soft and moderately hard water. About 41% and 59% in the pre-monsoon has hard and very hard water respectively. Similarly, 64% and remaining 36% in the post-monsoon show hard and very hard water respectively (Fig.2). The analytical results indicate that the water in the study area is hard to very hard and this hardness of groundwater is due to the presence of alkaline earths such as calcium and magnesium (Sarath Prasanth et al., 2012).
Table 1
Analytical results (minimum, maximum, average, standard deviation) of different parameters in the study area
|
Pre-monsoon
|
Post-monsoon
|
Physico-chemical Parameters
|
Minimum
|
Maximum
|
Average with Standard Deviation
|
Minimum
|
Maximum
|
Average with Standard Deviation
|
pH
|
7.9
|
8.2
|
8.09 ± 0.11
|
8.0
|
8.2
|
8.06 ± 0.08
|
EC (µS/cm)
|
530
|
3330
|
1598.82 ± 936.7
|
540
|
2320
|
1175.29 ± 608.85
|
TDS (mg/l)
|
282
|
1943
|
923.35 ± 566.87
|
304
|
1390
|
664.94 ± 364.97
|
TH
|
170
|
750
|
388.24 ± 184.48
|
165
|
580
|
285.59 ± 124.27
|
Ca (mg/l)
|
12
|
116
|
56.35 ± 33.18
|
16
|
92
|
41.06 ± 18.55
|
Mg (mg/l)
|
20.66
|
116.64
|
60.11 ± 31.94
|
18.23
|
94.77
|
44.45 ± 23.07
|
Na (mg/l)
|
23
|
451
|
179.47 ± 145.88
|
48
|
304
|
137.53 ± 87.47
|
K (mg/l)
|
3
|
113
|
29.41 ± 40.57
|
1
|
60
|
10.24 ± 14.47
|
Cl (mg/l)
|
46
|
780
|
257.59 ± 231.06
|
50
|
496
|
194.94 ± 141.25
|
F (mg/l)
|
0.27
|
1.22
|
0.77 ± 0.29
|
0.14
|
1.04
|
0.52 ± 0.27
|
SO4 (mg/l)
|
21
|
259
|
99.53 ± 74.44
|
23
|
125
|
53.88 ± 32.86
|
HCO3 (mg/l)
|
158.46
|
597.8
|
343.91 ± 125.1
|
103.97
|
414.8
|
271.21 ± 99.85
|
Among the cations, concentrations of Ca, Mg, Na and K (Fig.3) shows an average of 56.35, 60.11, 179.47 and 29.41 mg/l during the pre-monsoon and 41.06, 44.45,137.53 and 10.24 mg/l during the post-monsoon. The average concentration of Ca comes under the desirable limit (75 mg/l) and the maximum concentration was under the maximum permissible limit (200 mg/l) during both the monsoons. Mg with maximum concentration of 116.64 mg/l during the pre-monsoon exceeds the maximum permissible limit (100 mg/l) suggested by BSI (1993). But comes under the maximum permissible limit (150 mg/l) of WHO (2004). The samples from the post-monsoon were under the maximum permissible limit of BSI (1993) at all the locations. The concentrations of Ca and Mg in the groundwater was mostly derived from leaching of carbonate minerals such as Calcite and Dolomite (Magesh et al., 2012). The elevated Ca content can cause ailments in humans and it was also not useful for domestic uses as it causes scaling (Sarath Prasanth et al., 2012). Na the most important cation, it ranks sixth among the elements in the order of elements in order od abundance in natural waters. In freshwater, Na is generally present in lower concentrations than Ca snd Mg. The concentration of Na varied from 23 to 451 mg/l and 48 to 304 mg/l during the pre and post-monsoon respectively (Fig.3). The maximum permissible limit of Na is 200 mg/l and it reveals that a few samples in both the monsoon exceeding the permissible limit of WHO. Groundwater with high Na content is not suitable for agricultural purpose and also causes increased blood pressure in humans (Sarath Prasanth et al., 2012). Potassium, a naturally occurring element in the earths crust as well as in groundwater has lower concentrations than Na because of the slower weathering rate of K-bearing rocks than those of Na-bearing rocks. The concentration of K seldom reached 20 mg/l but its maximum permissible limit is 12 mg/l. The analytical concentration of K in the study area shows elevated concentrations in most of the locations. This may be due to the use of fertilizer in agriculture fields as well as the discharge from domestic wastes (Saha et al., 2019).
The Chloride (Cl) concentration of groundwater can be attributed naturally due to the presence of chlorides from rocks, evaporates and juvenile water or by the contamination of groundwater through industrial waste and domestic sewargs (Saha et al., 2019). In general, the concentration of Cl in groundwater will be less than 30 mg/l (Nag, 2009). The Cl concentration in the studied samples ranges from 46 to 780 mg/l in pre-monsoon and 50-496 mg/l during the post-monsoon (Fig.4). The higher value of chloride content in most of the groundwater samples is an indication of high salinity in the study area (Saha et al., 2019). The elevated concentration of Cl in groundwater samples also indicates the influence of surface contamination sources such as agricultural activities and domestic wastewaters in the study region.
The concentration of F in the studies water shows values between 0.27 and 1.22 mg/l during the pre-monsoon and 0.14 to 1.04 mg/l during the post-monsoon. This value ranges falls within the standard values mentioned by WHO (2004) suggesting that the water is not in contact with fluoride bearing minerals and is suitable for drinking purpose (Krishnakumar et al., 2011).
The SO4 concentration during the pre-monsoon ranges from 21 to 259 mg/l suggesting that some of the samples exceeds the maximum permissible limit suggested by WHO (2004). During the post-monsoon, all the samples (23-125 mg/l) come within the WHO suggested permissible limit (150 mg/l) with an average of 53.88 mg/l. The occurrence of SO4 in groundwater can be resulted from the oxidation of sulfur from the surrounding igneous rocks in the region (Saha et al., 2019). The low concentration of SO4 indicates that the study area was less affected by anthropogenic sources especially industries (Mostafa et al., 2017). The possible sources of Bicarbonate are the presence of organic matter in the aquifers and weathering of silicate minerals (Gastmans et al., 2010).
The correlation matrix is one of the important tests that show the connections between two independent parameters and the most familiar one is the Pearson correlation matrix (Bahar and Reza, 2010). The correlation matrix for the analyzed physico-chemical parameters of groundwater samples from the Dindigul Taluk are shown in Table 2 and 3. Both the Table 2 and 3 shows that F is very less correlation (r<0.3) with all the other parameters and negative correlation (r= -0.3) with K. This shows that F is not associated with the other elements. Also, EC shows very good correlation (r>0.7) with all other parameters except pH, K and F during both the pre and post-monsoon mentioning that EC is associated with the concentration of various ions. The Ca, Mg and Na shows very good correlation (r>0.6) with EC, TDS and TH indicating that these physico-chemical parameters of groundwater is associated with the presence of alkaline earths such as sodium, calcium and magnesium. Very less correlation of pH with HCO3 (r=0.2 and 0.08) indicates that the role of carbonate is very low in controlling the pH during both the pre and post-monsoon. Very high correlation of TH and TDS (r>0.8, pre-monsoon & r>0.9, post-monsoon) indicates dissolution process of rock forming minerals in water.
Table 2
Pearson correlation matrix (r) of analyzed physico-chemical parameters during the pre-monsoon
|
pH
|
EC
|
TDS
|
TH
|
Ca
|
Mg
|
Na
|
K
|
Cl
|
F
|
SO4
|
HCO3
|
pH
|
1
|
|
|
|
|
|
|
|
|
|
|
|
EC
|
0.35
|
1
|
|
|
|
|
|
|
|
|
|
|
TDS
|
0.35
|
0.996
|
1
|
|
|
|
|
|
|
|
|
|
TH
|
0.22
|
0.87
|
0.85
|
1
|
|
|
|
|
|
|
|
|
Ca
|
0.04
|
0.70
|
0.72
|
0.77
|
1
|
|
|
|
|
|
|
|
Mg
|
0.29
|
0.78
|
0.74
|
0.92
|
0.45
|
1
|
|
|
|
|
|
|
Na
|
0.39
|
0.96
|
0.95
|
0.71
|
0.52
|
0.67
|
1
|
|
|
|
|
|
K
|
0.06
|
0.30
|
0.38
|
0.15
|
0.50
|
-0.12
|
0.22
|
1
|
|
|
|
|
Cl
|
0.38
|
0.96
|
0.95
|
0.83
|
0.61
|
0.78
|
0.96
|
0.12
|
1
|
|
|
|
F
|
0.42
|
-0.02
|
-0.05
|
-0.09
|
-0.15
|
-0.03
|
0.06
|
-0.40
|
0.13
|
1
|
|
|
SO4
|
0.39
|
0.94
|
0.94
|
0.74
|
0.55
|
0.69
|
0.94
|
0.27
|
0.94
|
0.14
|
1
|
|
HCO3
|
0.20
|
0.69
|
0.69
|
0.61
|
0.53
|
0.53
|
0.62
|
0.45
|
0.50
|
-0.38
|
0.51
|
1
|
Table 3
Pearson correlation matrix (r) of analyzed physico-chemical parameters during the post-monsoon
|
pH
|
EC
|
TDS
|
TH
|
Ca
|
Mg
|
Na
|
K
|
Cl
|
F
|
SO4
|
HCO3
|
pH
|
1
|
|
|
|
|
|
|
|
|
|
|
|
EC
|
0.26
|
1
|
|
|
|
|
|
|
|
|
|
|
TDS
|
0.26
|
1.00
|
1
|
|
|
|
|
|
|
|
|
|
TH
|
0.26
|
0.92
|
0.92
|
1
|
|
|
|
|
|
|
|
|
Ca
|
0.56
|
0.67
|
0.67
|
0.74
|
1
|
|
|
|
|
|
|
|
Mg
|
0.07
|
0.88
|
0.87
|
0.95
|
0.49
|
1
|
|
|
|
|
|
|
Na
|
0.24
|
0.97
|
0.97
|
0.82
|
0.58
|
0.79
|
1
|
|
|
|
|
|
K
|
0.05
|
0.68
|
0.70
|
0.53
|
0.36
|
0.52
|
0.65
|
1
|
|
|
|
|
Cl
|
0.27
|
0.96
|
0.96
|
0.88
|
0.63
|
0.84
|
0.95
|
0.56
|
1
|
|
|
|
F
|
0.12
|
0.20
|
0.16
|
0.14
|
0.11
|
0.13
|
0.28
|
-0.39
|
0.29
|
1
|
|
|
SO4
|
0.30
|
0.96
|
0.97
|
0.86
|
0.72
|
0.77
|
0.93
|
0.78
|
0.91
|
-0.003
|
1
|
|
HCO3
|
0.08
|
0.78
|
0.76
|
0.70
|
0.46
|
0.70
|
0.76
|
0.53
|
0.62
|
0.27
|
0.69
|
1
|
The Principal Component Analysis (PCA) was carried out separately for the two monsoon periods to identify the underlying pattern between the measured hydro-chemical parameters which helps in data interpretation. The factor loadings for the pre-monsoon has four latent factors explaining 74.56% of sampling variance (Table 4). Similarly, the PCA for the post-monsoon has three factor loadings with 86.86% of sampling variance (Table 4). The values that are considered as important due to the higher correlation (values >0.7) are marked in boldface type. The factor loadings of the pre-monsoon indicate positive and good correlation of EC (0.81) and K (0.83) with the first component (PC1) and have 33.21% of sampling variance. The PC2/Factor 2 is determined with high positive loadings of Na (0.72) and Cl (0.85) Unlike PC1 the EC and K are positively but very less correlated. During the post monsoon, more parameters were well loaded in the first component such as TDS (0.99), Mg (0.87), Na (0.96), Cl (0.96), SO4 (0.96), HCO3 (0.77) and EC (0.99). The elevated concentrations of these parameters can be related to the influence of anthropogenic pollution in the study area (Dragon, 2006). The PC3 reflects the local variability of hydro-geochemical conditions of water bearing sediments (Dragon, 2006). With respect to this in both the pre-monsoon and post-monsoon pH (0.72 and 0.86) is well loaded in PC3. The pre-monsoon data were loaded with four components, unlike the post monsoon. Here in PC4, Mg is the only element showing loading >0.5 with this component. This can be related to changes of the hydro-geochemical conditions in the aquifer caused by the influx of pollution (Dragon, 2006).
Table 4
Loadings of physico-chemical parameters on significant principal components of groundwater samples
Factor loadings (varimax normalized) of principal components (marked loadings are > 0.7)
|
Pre-monsoon
|
|
Post-monsoon
|
PC1
|
PC2
|
PC3
|
PC4
|
Parameters
|
PC1
|
PC2
|
PC3
|
0.67
|
-0.47
|
-0.03
|
-0.04
|
TDS
|
1.00
|
-0.06
|
-0.03
|
0.13
|
0.45
|
-0.42
|
-0.45
|
Ca
|
0.70
|
0.05
|
0.55
|
0.01
|
-0.65
|
-0.18
|
0.50
|
Mg
|
0.88
|
0.02
|
-0.26
|
0.47
|
0.72
|
-0.25
|
0.27
|
Na
|
0.96
|
0.03
|
-0.08
|
0.83
|
0.07
|
0.27
|
0.31
|
K
|
0.67
|
-0.65
|
-0.04
|
0.25
|
0.86
|
-0.06
|
0.24
|
Cl
|
0.96
|
0.13
|
-0.04
|
-0.82
|
0.40
|
0.01
|
0.02
|
SO4
|
0.96
|
-0.22
|
0.10
|
-0.76
|
-0.21
|
-0.22
|
0.41
|
HCO3
|
0.77
|
-0.04
|
-0.20
|
0.03
|
0.33
|
0.63
|
0.27
|
F
|
0.21
|
0.89
|
-0.16
|
-0.18
|
0.14
|
0.73
|
-0.14
|
pH
|
0.30
|
0.24
|
0.86
|
0.81
|
0.11
|
-0.25
|
0.14
|
EC
|
1.00
|
-0.02
|
-0.04
|
-0.78
|
0.32
|
-0.18
|
0.24
|
TH
|
0.65
|
0.48
|
-0.18
|
3.99
|
2.54
|
1.41
|
1.01
|
Eigen Values
|
7.61
|
1.58
|
1.23
|
33.22
|
21.16
|
11.77
|
8.41
|
% of variance
|
63.43
|
13.17
|
10.25
|
In order to identify the chemical characteristics of groundwater, the analytical values were plotted in Piper trilinear diagram (Piper, 1944) using AquaChem software. This diagram shows the similarities and differences among the water samples as those with similar qualities will have the tendency to fall in the same group (Ramesh et al., 2014). The Piper diagram consist of three distinct fields; two triangular fields (major cations and anions are plotted) and one diamond shaped field (groundwater characteristics).The diamond shaped field of piper is further divided into six classes for the identification of the salt combinations of water (Sarath Prasanth et al., 2012). In the study area the salt combinations of aquifers are dominated by CaHCO3, Nacl, mixed CaNaHCO3 and mixed CaMgCl facies type (Fig.5).
The presence of mixed CaMgCl water type suggest that the groundwater in this region was influenced by the dissolution of carbonate bearing minerals as well as the disposal of domestic sewage water in certain locations. However, most of the samples clustered over the CaMgCl and NaCl water type suggesting the mixing of high salinity water especially from the surface contamination. The dominance of Na type and Cl type water also suggests that the groundwater is experiencing salinity issues especially for both drinking and irrigation purposes (Magesh et al., 2012). Few samples also plotted over the CaNaHCO3 and CaHCO3 water types, expressing mineral dissolution and freshwater recharge over some locations.
In addition with Piper diagram, Gibbs plots (Gibbs, 1970) were also used to gain better insight into hydrochemical processes such as precipitation, rock-water interaction and evaporation dominance on groundwater chemistry in the study area. It is used to identify the relationship between the water composition and aquifer characteristics (Magesh et al., 2012). Fig. 6 displays the relationship between groundwater and aquifer lithology in the study area. The generated Gibbs plot for cation (TDS plotted against (Na+K)/(Na+k+Ca)) show that the samples from both the pre and post-monsoon were plotted over the rock-water interaction zone. Similarly, the plot for anion (TDS plotted against Cl/(Cl+ HCO3)) displays that almost all the samples were plotted in the rock-water interaction zone with few samples at the evaporation zone during the pre-monsoon.
This observation suggests that dissolution of carbonate and silicate minerals are the dominant controlling factor of groundwater chemistry in the study area. Further, few samples from the pre-monsoon falls in the evaporation zone suggests the contamination of groundwater through surface processes such as irrigation return flow and some other anthropogenic impacts like domestic sewage disposal.
Irrigation Suitability
In this study area, agriculture is solely depends on the groundwater due to frequent failure of monsoon. Therefore, irrigation suitability of groundwater for Dindigul taluk is carried out for the pre ans post-monsoons of 2019 using EC, SAR, RSC and Na%. Concentrations of EC and Na are very important in classifying the groundwater for irrigation purpose because the increased soluble salt proportions will affect the suitability of groundwater for irrigation (Nagarajan et al., 2010). According to Richards (1954), with reference to EC the groundwater is classified into four groups for irrigation purpose such as low (EC <250 µS/cm), medium (250-750 µS/cm), high (750-2250 µS/cm) and very high (2250-5000 µS/cm).
The calculated values of SAR in the study area vary between 0.68 to 9.6 (pre-monsoon) and 1.332 to 6.35 (post-monsoon). The SAR values of both the seasons were found within the range of excellent category (<10; low sodium) which is suitable for irrigation purpose. This reveals that the groundwater samples in the study area has low sodium content thus suitable for irrigation in all type of soil (Sarath Prasanth et al., 2012). The USSL (1954) plot indicates that 23.53% of samples falls in the C1S1 (low salinity – low sodium) and 50 % of samples fall under the C2S1 (medium salinity- low sodium) category (Fig.7). This categories show that the water is suitable for irrigation. Remaining 23.53% and 2.94% comes within C2S2 (medium salinity-medium sodium) and C2S3 (medium salinity – high sodium) category. Here, C2S2 is suitable for irrigation but not for all types of soils and the samples in C2S3 seems unsuitable for irrigation purpose.
The RSC values suggest that all the samples are suitable for irrigation purpose based on the classifications of Lloyd and Heathcote (1985). According to the RSC values 94% of samples are suitable and the remaining 6 % samples are marginal. Percentage of Na calculated for the groundwater for both the seasons were plotted against EC in the Wilcox diagram (Fig. 8).
From this diagram it is identified that one sample from the pre-monsoon and 5 samples from post monsoon were excellent to good; 5 (pre-monsoon) and 6 (post-monsoon) samples were good to permissible; 3 samples from each pre-monsoon and post-monsoon were permissible to doubtful; 4 samples from pre-monsoon and 3 samples from post-monsoon were doubtful to unsuitable and two pre-monsoon samples are unsuitable. Overall, the groundwater region is suitable for irrigation except in a few locations showing minimal enrichment of sodium and salinity.