Water Chemistry
Physical parameters:
The pH of the groundwater in the study area ranges from 6.21 to 8.38 for all the four different seasons. The lowest pH is detected in NEM, and highest pH in PRM (Table 2). Similarly highest value of EC is noted in NEM lowest in PRM. However, the EC of the samples are higher in few samples irrespective of seasons. Based on the classification given by Saxena et al., (2003) the EC value of the study area, the groundwater has been divided into three classes (1) fresh (<1,500 μS/cm), (2) brackish (1,500–3,000 μS/cm), and (3) saline >3,000 μS/cm. The number of samples falling within each category for four different seasons are noted in Table 3. The trend as observed in table 3 values fresh category of samples has a increasing value from POM, to NEM, SWM and PRM which may be attributed to the infrequent rainfall during PRM with respect to other seasons.
Considering the spatial distribution of EC of the groundwater samples (Fig 3),the higher values are noted in the northeast and a minor representations is observed in the southern region in all seasons. Northeast and southern part of the study area is covered with sedimentary rock units (Fig 1). The percolation of the domestic sewage infiltration along the flood plain of the Vellar River in the northeastern part may be expected to resulting in higher EC in this part. The EC was observed to be highest adjoining the river decreases with distance from the river increases. Ranjana and Champa Naverathna (2011) reported similar results in groundwaters of urban areas of Sri Lanka. Similarly, the lowest EC is noticed on the western side of the area irrespective of all the four seasons which is mainly covered with hard rock units (Fig 1). Hard rocks with fractured and weathered outcrops have a quick response to rainfall. During recharge, the significant ion concentration either dilutes or the dissolve chemical compounds found along the flow path (Vetrimurugan 2014,Panda et al. 2020). Rainfall recharge is considered to be as one of the leading controls on the seasonal fluctuation of EC. Recharge reduces the ionic concentration of groundwater whereas leaching, dissolution, and evaporation tend to increase ionic strength in general (Chidambaram et al. 2019).
The highest TDS is observed in NEM, and lowest values are noted in PRM. The TDS value of the study area has been categorized in to four categories based on the classification scheme given by Carrol (1962) and provided in Table 4. There are a few numbers of samples are observed in brackish water quality category for all four seasons and during NEM its highest (Table 4). However most of the samples are within fresh category
Chemical parameters:
The lowest concentration of Ca is observed in PRM whereas the highest concentration is noted in NEM (Table 2). Presence of Calcium in drinking water is reported to cause cardiovascular diseases, nervous system defects, and cancer. Calcium content in water gives it hardness which may create scales in pipes, encrustation of utensils and difficulty in soap lathering (Chidambaram et al 2010). NEM has the highest Mg concentration, while PRM has the lowest. The chief cation observed in the groundwater of the study area is sodium (Table 2) and is highest during PRM, whereas the lowest concentration is noted in POM season indicating the contribution from weathering of Na feldspar from the underlying rock types along with the dissolution and anthropogenic process (Vasudevan et al 2020). The highest concentration of K is noted in NEM, and lowest values are noted in PRM. The elevated level of K in groundwater is considered to be due to ion exchange or weathering of Fissile hornblende-biotite gneiss and Charnockite (Adithya 2016).
HCO3- is one of the principal ions in the groundwater and concentration of HCO3- is highest in NEM and lowest in PRM (Table 2). The HCO3- in the groundwater of this region may have resulted from carbon dioxide of the atmosphere, soils and by suspension of carbonate rocks. Charnockite and Fissile Hornblende Biotite Gneiss rocks are predominant rock types in the study area which could be a possible source of HCO3- through the process of weathering (Devaraj et al 2018, Thivya 2019). Highest chloride concentration is noted during NEM and the lowest in PRM. The higher chloride concentration in the study area is may be from the wastage produced due to domestic activities. The SO42- concentration is highest in PRM and lowest in NEM (Table 2). The higher concentration is may be due to intensive anthropogenic activities. In minerals such as gypsum and marcasite, the amount of dissolved Sulphate ion varies (Anandhan, 2005). Other anthropogenic sources like bacterial fixation, fertilizer effect, tannery is also considered to be the major source for sulphate ion in the groundwater (Chidambaram et al., 2012). Similarly, the highest concentration of nitrate was higher in PRM season compared to other seasons. The intensive agricultural activities and domestic sewage may result in higher concentration of nitrate in the groundwater (Panda et al. 2018). Higher concentration of H4SiO4 was noted in PRM indicating rock dissolution in alkaline environment. Studies show that excess intake of silica usually results in silicosis (Cherry et al., 1997, 1998) It can also lead to enduring heart and lung disease. Parameters such as seasonal flux of precipitation, mechanical and chemical properties of bedrock, temperature and water table conditions determine the quantity of silica dissolved in groundwater (Dobrzynski, 2005). In general, all the parameters including physical and hydrochemical show higher concentration, exceeding to WHO standard for drinking purpose. Thus, it is recommended to assess the appropriateness of the groundwater characteristic of this region for the sustainable management of their source.
Water quality Index
For the groundwater samples in the study area, eleven water quality criteria were chosen to determine the water quality index. The factors were chosen for their respective importance in determining water quality for human consumption as well as their close proximity to data.. The guidelines are used to set the criteria according to the WHO (2011) standard for drinking water purpose. Each of the 11 parameters were assigned with a weight (wi) based on its relative significance with respect to overall drinking water quality which is considered to be between the integer 1 to 5 ( Table 5). Because of the significance of nitrate and TDS value in water quality evaluation, a maximum weight of 5 has been assigned, similarly pH, EC, SO42 were assigned with 4, HCO3, Cl with 3, Ca2+, Na+, K+ with 2 and Mg with 1. The lowest weight is assigned to Mg because it might not be dangerous on its own consumption (Vasanthavigar et al. 2010, Thivya et al.2014; Thilagavthi et al. 2016).
In the following step, relative weights (Wi) were studied using the equation 1.
n
Wi = wi/ ∑ wi (1)
i=1
Where,
Wi, wi and n: relative weight, Each parameters weight and number of parameters respectively.
To determine a quality rating for each parameter, its concentration was divided by the corresponding water quality requirements (WHO, 2011) and multiplied by 100 (Eq. 2).
qi = (Ci/Si) × 100, (2)
qi, Ci and Si: quality rating, Chemical parameter’s concentration of each water sample (mg/l)
and WHO standard limit for each parameter respectively.
To determine the WQI, the SI has to be calculated (Eq 3). Final WQI will be calculated by summing up the SI values of the each sample .
SIi= Wi × qi (3)
WQI = ∑ SIi, (4)
SIi, qi and n: the sub-index of ith parameter, rating based on the concentration of ith parameter and n is the number of parameters respectively.
Considering the groundwater samples of PRM, 7% of the total samples are within excellent category, 76% of the samples are within moderate quality, and 17% of the samples are within bad quality. In SWM, 74% of the samples are in the good category, 19% of the samples are in poor category, and the remaining are within very poor category. When comparing the samples of both NEM and POM to other two seasons 50% of samples are in the poor category, with a few samples falling into the extremely poor and unsuitable category (Table 6). These samples are unfit for drinking purpose. Spatial distributions of WQI (Fig 4) for four seasons show predominance of excellent and good category water in the study area. The water quality index of PRM and NEM ranges from 42.16 (Kulukanatham) to 375.06 (Karapadi) respectively. A small patch of un-suitable category of water is noted in the north-eastern part of the study field during POM (Fig 5) may be due to ion leaching, over-exploitation, direct discharge of domestic and industrial effluents along the Vellar River.
Electrical Conductivity vs Water Quality Index
The relationship between WQI and EC is plotted (Fig 4). It is interesting to note that EC values increases with the increase of WQI which may be an indication of water pollution. There are two groups of the samples represented in the plot (Fig 4). One with high EC and high WQI and the other with low EC and low WQI. But in general, there exist a linear relationship between EC and WQI, reflecting the fact that they are directly related and the increase is mainly due to the anthropogenic influence (Thivya et al., 2013).
Gibbs plot for evaluation of geochemical control on groundwater
The samples of all four different sampling periods fall within weathering and evaporation zone. However, the samples of cation plot show a dominance of weathering than that anion plot (Fig 6) Majority of the samples regardless of season fall outside of the plot which signifies the anthropogenic impact in the study area. The concept of the diagram is based on the TDS and ion ratios as mentioned in the figure 6, but however samples which are outside of the plot and the samples which are having higher TDS but are within the evaporation zone may also be due to anthropogenic sources other than the natural sources like rain, weathering, and evaporation. Thus, it can be inferred that evaporation and wethering plays a significant role in variation of groundwater chemistry of the region nalong with anthropogenic activities.
Geochemical classification
Majority of the samples are within the field 1 to 4 and a few representations of samples are also noted in field 5 irrespective of seasons (Fig 7). The samples which are within Field 2 are of the Na-Cl type, Field 3 of Ca-Na-HCO3 type and Field 4 of Ca-Mg-Cl type, and Field 1 of Ca-HCO3 and Field 5 of Ca-Cl type (Fig 7). The Na-Cl type dominates because ions are removed from the solution by adsorption or precipitation (Chidambaram et al., 2007). (Thilagavathi and colleagues, 2012, Vetrimurugan 2013). The migration of samples from the mixed Ca-Mg-Cl facies to the Na-Cl facies could be attributable to seawater effect in the sedimentary area, as well as the extended residence period of shallow groundwater (Prasanna et al., 2012), where Na exceeds Ca and Mg, Cl exceeds HCO3 and SO4 (Prasanna et al., 2012). (Prasanna et al. 2008). A saline environment is indicated by the presence of Na-Cl water in the discharge zone (Prasanna et al., 2009). The high Cl concentration could be the result of saline soil residues seeping into the groundwater system, which is frequent in dry and semi-arid areas (Zaheeruddin and Khurshid 2004).
The evolution of groundwater chemistry represented by the samples in field 1, 4 and 5 are Ca-HCO3, Ca-Mg-Cl, and Ca-Cl type. The Ca-HCO3 facies represent the recharge process of the region (Thilagavathi et al., 2011). Anions are considered to change from HCO3 to Cl from recharge to discharge due to ion dissolution in the pathway or precipitation and removal of HCO3 from the aqueous system (Tirumalesh et al., 2007). Most of the samples fall into field 4, indicating Ca-Mg-Cl type (Fig 7), with a few samples falling into Ca-HCO3 and Na-Cl types, indicating secondary precipitation action. In general, alkali exceeds alkali earth, indicating that strong acid controls the chemistry of water.
Correlation Matrix
In PRM season, good correlation observed between Mg-Cl, Mg-EC, Mg-TDS, Na-Cl, Na-HCO3, Cl-TDS, HCO3-SO4, HCO3-EC, HCO3-TDS, SO4-EC, SO4-TDS and EC-TDS (Table 7). All the variables have a negative and poor correlation with pH (Table 7), which can be explained by acidic media's higher aggressiveness against soil and host rocks, which increases the concentrations of the other ions. Due to the high agricultural and industrial activity i.e., fertilizers and/ or industrial wastes are Predominant in the region, which release all these ions into the aquifer increasing its concentration. TDS and EC (R= 0.991) have a strong positive correlation, indicating the impact of all dissolved constituent ions in the water increase. In SWM, good correlation exists between Ca-Mg, Na, Cl, SO4EC, TDS, Mg-Na, Cl, SO4, EC, TDS, Na-Cl, SO4, EC, TDS, Cl-HCO3, SO4, EC, TDS, HCO3-EC, TDS, SO4-EC, TDS, EC-TDS indicating the dominance of weathering and leaching processes. Cl-has strong associations with Na, Ca, Mg, and K, indicating the predominance of secondary salt leaching. HCO3-has a strong correlation with Ca, Mg, Na, and K, suggesting the presence of dissolved solids due to chemical weathering. The high correlation with Na may be due to the higher Na content in the source material, which causes water to become alkaline. PO43 shows significant correlation with K and SO42 indicates fertilizer impact from agricultural practices. In NEM, good correlation exists between Ca-Mg, Na, Cl, SO4, EC, TDS, Mg-Na, Cl, SO4, EC, TDS, Na-K, Cl, SO4, EC, TDS, Cl-SO4, EC, TDS, SO4-EC, TDS, EC-TDS (Table 7). HCO3, Cl, Na, Ca, and Mg are the main ions with strong to moderate correlation with other ions in almost all seasons. This could be due to secondary salts leaching from fissures or hydrophobic areas of the formations (Chidambaram et al., 2009).In POM good correlation exists between Ca-Mg, Na, Cl, SO4, EC, TDS, Mg-Na, Cl, SO4, EC, TDS, Na-K, Cl, SO4, EC, TDS, Cl-SO4, EC, TDS, SO4-EC, TDS, EC-TDS (Table 7). Cl have a strong relationship with Ca, Mg, and Na, suggesting that chemical weathering and secondary salt leaching are prevalent during this season (Prasanna 2008). The lack of correlation between H4SiO4 and other ions suggests that silica has a lesser influence during this season. The lack of a positive association between PO43 and K signifies the influence of anthropogenic activities basically the fertilisers applied in the cultivated land. Cl-has a strong positive correlation with Mg, Ca, and Na, suggesting secondary salt leaching. Thus, the main factor which contributes to the variation in groundwater chemistry of this region is may be weathering, salt leaching and anthropogenic activities.
Factor Analysis
In PRM 4 factors were extracted with 68.09% (Table 8) of Total Data Variance (TDV). Factor 1 have a strong loading (TDV: 27.59%) of EC, TDS, SO42- and with moderate loading of Mg and HCO3. Factor 1's ion association indicates weathering mechanisms as well as secondary salt leaching along the fracture (Chidambaram, 2000). The presence of Ca and Mg ions indicates natural recharge and rock water interaction process (Olobaniyi, 2006). The moderate loading of factor 2 with K, PO4 and HCO3 (TDV: 15.96%), indicates anthropogenic activities like application of fertilizers in agricultural field, and dissolution of HCO3 ion during weathering process. Factor 3 with a TDV of 13.78% represents strong loading of Na and Cl indicating anthropogenic influence. A strong loading of NO3 represented in Factor 4 (TDV: 10.77%), indicating source from fertilizers applied in the agricultural land.
Four factors were extracted with 74.49 % of TDV (Table 8). Ca, Mg, Na, Cl, HCO3, SO42, EC, TDS shows a strong positive loading for factor 1 (TDV: 43.47 %). This association is distinguished by the predominance of secondary salt dissolution into groundwater during the monsoon. Factor 2 (TDV: 11.42 %) shows moderate loading of K, NO3, PO43, and SO42, which suggests anthropogenic impacts from agricultural activities such as fertilizers (Vengosh et al.1995). There is a negative loading of Mg and pH is noted in Factor 3 (TDV: 10.13 %). In Factor 4 (TDV: 9.47%) strong and weak loading of H4SiO4 and NO3is noted, indicating anthropogenic influence and leaching of silicate ions into the aquifers.
The NEM have four major factors that explained 71% of the total variance. The Factor 1 (TDV: 42.65%) with positive loadings of Ca, Mg, Na, Cl, SO42, and EC, suggesting secondary salt leaching into the aquifer (Table 8). Other two factors like factor 2 and 3 does not show any strong loadings of ions. However, the strong positive loadings of H4SiO4 in Factor 4 (TDV: 10.76 %) suggests the influence of silicate weathering,
During POM, four factors were extracted with (Table 8) 76.56% of TDV. Factor 1 show 23.5% of TDV. The positive loading of the Ca2+, Mg2+, Na+, Cl-, HCO3-, and EC in Factor 1 is attributed to the secondary leaching of salts. Factor 2 with of 12.49% of TDV shows positive loading of K and PO43 is mainly due to the anthropogenic stress in groundwater. Factor 3 (TDV: 10.89%) does not show any significant loadings of ions. Factor 4 (TDV: 10.06%) represents strong positive loading of HCO3-. indicating water-/rock interaction and also this factor has a moderate loading of H4SiO4 indicating the leaching of silicate ions into the (Prasanna 2010).