Physicochemical Parameters
Table 2 lists the results of the measurement of physiochemical parameters and microbial analysis in the 30 groundwater samples taken from the Sulur region during the post- and pre-monsoon periods. The pH of groundwater samples collected during post- and pre-monsoon ranged from 7.11 to 8.20 and 6.90 to 7.90, respectively (Table 2), illustrating the slightly alkaline nature of the groundwater. The values of pH in all groundwater samples fell within the desirable range 6.5–8.5 of pH for drinking water set by the WHO (2017).
Table 2 Analytical data representing important physical and chemical parameters detected in the groundwater samples of post- and pre-monsoon seasons The total dissolved solids (TDS) levels ranged from 482 to 3022 mg/l and 338 to 2237 mg/l for post- and pre-monsoon seasons, respectively (Table 1). The elevated TDS values observed in samples collected in the post-monsoon season (n = 8,17) declined in samples collected in the pre-monsoon season at the same locations, which may have been due to the reduced anthropogenic inputs resulted by COVID-19 lockdown.
The order of the levels of major ions in the post- and pre-monsoon samples were Na+ > Ca2+ > K+ > Mg2+ for cations and HCO3− > Cl- > SO
2-4
> NO
-3
N > F- for anions (Table 2). The minimum and maximum values of calcium and magnesium are depicted in Table 2. The average Na+ concentration varied slightly among post- and pre-monsoon samples (Table 2), which is probably because Na+ mainly comes from geogenic sources. The consumption of high levels of Na+ can cause high blood pressure, hyperosmolarity, arteriosclerosis, and oedema (Aravinthasamy et al. 2020a; Karunanidhi et al. 2020a, b, c; Sarath Prasanth et al. 2012). We speculate that percolation of potassium applied in agricultural fertilizers through the soil was responsible for the increase in the concentration of K+ in the post-monsoon period.
The recommended limit for HCO
-3
concentrations is < 600 mg/l (WHO 2017). Higher amounts of HCO
-3
were found pre-monsoon samples when compare with post-monsoon samples, which are probably due to ion-exchange and dilution by rainfall infiltration (Karunanidhi et al. 2020c). The foremost source of Cl- in groundwater is through percolation from municipal, domestic, and industrial wastes (Subba Rao et al. 2017; 2019). The Cl- concentration exceeded the prescribed limit at one location (sample no. 14) in pre-monsoon. This may be due to contamination with domestic waste, coupled with a decline in groundwater levels that increased the concentration of this solute in groundwater.
The application of nitrogen fertilizers and gypsum may be important sources of oxyanions in the region. The probable sources for SO
2-4
in the study area are fertilizers, bur also may include textile effluents and domestic wastes (Marghade et al. 2019a, 2019b; 2020). However, sulphate ions were within the permissible limits in both the seasons. The NO
-3
levels in groundwater in the study area ranged from 23 to 180 mg/l and 10 to 117 mg/l in samples from the post- and pre-monsoon periods, respectively (Table 2). High concentrations of NO
-3
in drinking water can cause methemoglobinemia in infants (Arya et al. 2020; Karunanidhi et al. 2020e; Marghade et al. 2019a, b, 2011). The concentrations of NO
-3
in groundwater that were greater than the ingestion threshold of 45 mg/l set by the WHO (2011) occurred in 76.7% (23) and 43.33% (13) of the samples collected from the post- and pre-monsoon periods, respectively. The spatial variation maps for NO
-3
in samples collected in the post- and pre-monsoon periods are shown in Figs. 2a and 2b, respectively. Figure 2a indicates that a 24.84 km2 area in the study region had groundwater that was safe for drinking and a 558.16 km2 area had groundwater at risk for drinking purposes in the samples collected in the post-monsoon period. However, spatial analysis from the pre-monsoon samples indicated an improvement, as the NO
-3
safe zone increased to 302.74 km2 and the risk zone declined to 280.26 km2, respectively. Infiltration of contaminated water during the monsoon period probably contributed to high NO
-3
levels in the January 2020 samples. The drastic decrease in nitrate levels in the pre-monsoon samples relative to the post-monsoon samples is likely due to the cessation of industrial discharges during the COVID-19 lockdown, particularly due to the shutdown of textile industries where NO
-3
-bearing dyes are used. Reduced nitrate also could be due to a reduction in agriculture activities in the region.
The F- concentrations in groundwater is 60% and 43.3% of samples collected in the post- and pre-monsoon periods, respectively, exceeded the safe level of 1.5 mg/l set by the WHO (2011). The key source of F- in groundwater is weathering of fluoride containing minerals (Marghade et al. 2019a, 2019b; Karunanidhi et al. 2019). As the study area is an industrial region, possible anthropogenic sources include alloy, steel cast, and semiconductor industries. Based on the F- level, five different contamination categories were identified as low (< 1 mg/l), moderate (1 to 1.5 mg/l), high (1.5 to 2 mg/l), very high (2 to 2.5 mg/l), and extremely high (> 2.5 mg/l), as defined by Aravinthasamy et al. (2019). As shown in Table 3, F- contamination was low at five locations in the post-monsoon period and at three locations in the pre-monsoon period. Similarly, F- concentrations in the moderate risk category were observed at 7 wells during post-monsoon and at 14 locations during pre-monsoon. High risk concentrations of F− were observed in ten samples and three samples collected during the post- and pre-monsoon periods, respectively. Eight samples of both seasons and two samples of pre-monsoon season represented very high and extremely high-risk categories, respectively (Table 3). None of the samples were within the extremely high category during the post-monsoon period. The spatial distribution maps for fluoride in both monsoon periods are shown in Figs. 3a and 3b, respectively. For data from the post-monsoon samples, the risk zone covered an area of 310.74 km2 and the safe zone area was 272.26 km2. Similarly, data from the pre-monsoon samples indicated considerable change, with an area of 341.07 km2 in the risk zone and an area of 241.93 km2 in the safe zone.
Table 3 Comparison of fluoride levels in groundwater samples of post- and pre- monsoon and their impact on human health Groundwater Types
The water categorization hinges on the ionic composition of groundwater. Examination of a trilinear Piper diagram (Piper, 1944) in Fig. 4 shows that the groundwater in 87.6% and 60% of samples have Na+ + K+ as dominant cations in the post- and pre-monsoon periods, respectively. Approximately 66.7% of the samples collected in both periods had bicarbonate as the dominant anion. The minimization of anthropogenic inputs due to a lockdown period and reverse ion-exchange probably resulted in a drop in the dominancy of Na+ + K+ in 27.6% of the samples collected in the pre-monsoon period. Reverse ion-exchange is evidenced by increased concentration in the pre-monsoon samples.
In the post-monsoon samples, 57% of the samples belonged to the Na- HCO3-Cl water types, whereas 20% belonged to the Na–Cl water type and the remaining samples belonged to the Ca–Mg-HCO3 type, except for only one sample of the Ca–Mg-Cl type. The water types identified for the pre-lockdown (i.e. post-monsoon) period that are rich in sodium and chloride highlighted the impact of anthropogenic inputs on water quality. The anthropogenic inputs such as sequestering agents and water softening processes used within the textile industries, as well as leaching from solid wastes were the important sources in the study area. In addition, the probable reasons for Na+ enhancement in groundwater in the study area particularly during post-monsoon season may be due to geogenic processes (e.g., ion exchange processes and silicate weathering). In the pre-monsoon samples, 43.3% of the samples were found to be of the Ca–Mg-HCO3 type, 30% were of the Ca–Mg-Cl type of water, and the remaining samples were of the Na-HCO3-Cl type of water (Fig. 4). The reversion back of Na–Cl and Na-HCO3-Cl water types in samples from the post-monsoon period into Ca-HCO3 type in samples from the pre-monsoon period illustrates that there was a drastic decrease in groundwater pollution. This improvement in groundwater quality is consistent with extensive reductions in anthropogenic inputs, such as complete shutdown of industries and slightly reduced agricultural activities throughout the lockdown period.
Geogenic Signatures
In general, the hydrochemical characteristics of groundwater account for the quality of groundwater. Various geogenic factors, such as solute aqueous phase interactions and ion exchange processes, that occur in groundwater are largely responsible for the quality of groundwater (Arya et al. 2019). The quality is further altered by climatic factors to some extent (e.g., recharge from increased rainfall) and ongoing anthropogenic factors. The molar ratios and scatter plots among the ions are generally utilized as geochemical characteristics to indicate the influence of geogenic and anthropogenic sources and the influence of land use patterns on the hydrochemistry of the groundwater (Thilagavathi et al. 2015; Marghade et al. 2019a).
The ionic ratios calculated for Na+/(Na++Cl−) lie in the range of 0.42 to 0.80 and 0.12 to 0.80 for samples collected in the post- and pre-monsoon seasons, respectively. A value of Na+/(Na++Cl−) greater than 0.5 indicates the contributions of sources other than the halite minerals, such as plagioclase mineral weathering, ion exchange reactions between the aqueous phase and mineral phase, and anthropogenic inputs (Naderi et al. 2020; Singh et al. 2018). In 73% of samples collected in the post-monsoon, the concentrations of Na+ were greater than Cl−, highlighting the role of the natural softening process (Eq. 7), albite (plagioclase) weathering (Eq. 8), and possible inputs from textile industries:
$${\text{Ca}}^{ 2+ } + {\text{ Na - Clay}} \to {\text{Na}}^{ + } + {\text{ Ca - Clay}}$$
(7)
$$\begin{aligned} 2 {\text{Na}}^{ + } \left[ {{\text{AlSi}}_{ 3} {\text{O}}_{ 8} + {\text{ 2CO}}_{ 2} } \right]^{ - } + {\text{ 11H}}_{ 2} {\text{O}} \to 2 {\text{Na}}^{ + } + 2 {\text{HCO}}_{ 3}^{ - } {\text{ + Al}}_{ 2} {\text{Si}}_{ 2} {\text{O}}_{ 5} ( {\text{OH)}}_{ 4} {\text{ + Mg}}^{{ 2 { + }}} {\text{ + 4H}}_{ 4} {\text{SiO}}_{ 4} \hfill \\ \quad \quad {\text{Albite}} \hfill \\ \end{aligned}$$
(8)
The plot of Na+ versus Cl− (Fig. 5a) shows that the salinity increased in groundwater samples collected during post-monsoon; 83% of the samples fell below the 1:1 line, indicating the effect of ion exchange processes and silicate weathering, consistent with predictions from the Na+/(Na++Cl−) ratio. This may be due to recharging of aquifers through rain water which accelerates the kinetics of albite weathering and natural softening through ion exchange processes (i.e., Equation 7) (Subramani et al. 2013). Conversely, in the pre-monsoon samples, 66% of the samples were above the 1:1 line in the Na+ versus Cl− plot. This anomalous rise in Cl– concentration during pre-monsoon period is probably due to enhanced weathering of amphiboles and biotite minerals along with accelerated reverse ion exchange (Eq. 9) associated with the dry climate. It could also be attributed to evaporation of soil pore water, which is associated with an increase in chloride concentration, and displacement of this soil water into groundwater. This is further supported by the Na+/Cl− versus EC plot (Fig. 5b), in which 73% of the samples had the molar ratio greater than one (Na+/Cl− > 1), consistent with a process of albite weathering and natural softening from ion exchange processes, along with high anthropogenic inputs (Subramani et al. 2009). Conversely, during the post-lockdown period, 77% of the groundwater samples fell below the Na+/Cl− ratio of 1 (Fig. 5b), consistent with biotite weathering and reverse ion exchange processes.
$$2 {\text{Na}}^{ + } + {\text{ Ca}}^{ 2+ } {\text{ - Clay}} \to {\text{Ca}}^{ 2+ } + {\text{ Na - Clay}}$$
(9)
Approximately 57% of the samples fell under the 1:1 line on the Ca2+ + Mg2+ versus HCO3− + SO42− plot (Fig. 5c) in samples collected during the post-monsoon period, signifying albite (silicate) and carbonate weathering (Marghade et al. 2019a, 2019b; Hussin et al. 2016). The wide deviation of sampling points from the 1:1 line in the lockdown sampling period (i.e., pre-monsoon) indicates that there was less dissolution of carbonates and cation ion exchange, which facilitate reverse ion exchange process. This finding is further supported by the Ca2+ + Mg2+ versus HCO3− plot (Fig. 5d) and Ca2+ versus Na+ (Fig. 5e). The Ca2+ and Na+ bivariate plot indicates that ion exchange/silicate weathering is an important process nullifies the probability of weathering carbonates in post-monsoon seasons. Conversely, the scattering of points above the 1:1 line supports the influence of the process of carbonate weathering along with reverse ion exchange. Approximately 70% of sampling point on the plot of Ca2+ - Mg2+ - SO4 2− - HCO3− versus Na+-Cl− (Fig. 5f), in post- monsoon samples fell in the range favoring cation exchange processes, whereas 66% of the samples from the pre-monsoon samples fell in the range favoring reverse ion exchange processes.
Higher fluoride concentrations in groundwater of geogenic origin are commonly associated with granitic gneisses, syenites, granodiorites, quartz-monzonites, and hornblende-biotite-gneisses (Edmunds and Smedley 2005; Karunanidhi et al. 2020a, b, c, d; Mukherjee et al. 2019). Fluoride is commonly present in minerals, such as fluorapatite [Ca5(PO4)3 OH,F], topaz [Al2(SiO4)F2](CaF2), cryolite (Na3AlF6), villiaumite (NaF), amphiboles, and micas (Adimalla and Qian 2019; Mukherjee et al. 2019). Because the groundwater of the study region is predominantly alkaline, it favors the dissolution of fluoride bearing minerals in rocks. The Eq. 10 for biotite weathering indicates that OH− radicals in the groundwater with higher pH values are exchanged with F−, resulting in fluoride enriched groundwater.
$${\text{KMg}}_{ 3} \left( {{\text{AlSi}}_{ 3} {\text{O}}_{ 10} } \right){\text{ F}}_{ 2} + {\text{ 2OH}}^{ - } \to {\text{KMg}}_{ 3} \left( {{\text{AlSi}}_{ 3} {\text{O}}_{ 10} } \right) \, \left( {\text{OH}} \right)_{ 2} + {\text{ 2F}}^{ - }$$
(10)
Anthropogenic Indicators
The correlations between different ions and with TDS are used as anthropogenic indicators to show the impact of fertilizers, discharges of wastewater effluents, and leaching from municipal solid wastes. A correlation coefficient (r) value below 0.3 specifies the correlation as “weak,” an r- value of 0.3 to 0.7 identifies the correlation as “moderate,” and an r- value greater than 0.7 identifies the correlation as “strong” (Emenike et al. 2018). In samples collected during the post- monsoon period, TDS was positively correlated with Na+ (r = 0.89) and K+ (r = 0.95), indicating a higher contribution of pollution from anthropogenic activities for these cations (Fig. 6). Also, correlations between Cl− and TDS (r = 0.85) and EC (r = 0.86) indicate Cl− enrichment due to anthropogenic activities (Ahada and Suther 2018; Marghade et al. 2011). TDS was positively correlated with SO42− (0.63), and NO3− (r = 0.34), consistent with anthropogenic inputs due to industries, domestic waste, and leaching from irrigation. The infiltration of rainwater, coupled with fertilizer usage in agricultural areas, leaching of industrial and municipal wastes and discharges of domestic sewage into surface waters are important sources of groundwater contamination.
In pre-monsoon samples (i.e., during lockdown), the strong positive correlation between TDS and K+ (r = 0.78) and Na+ (r = 0.77) indicates physiochemical reactions, such as ion exchange and oxidation – reduction in the aquifer system (Udayalaxami et al. 2010) (Fig. 7). The weak positive correlation of NO3− with Cl− (r = 0.29) indicates that nitrate pollution of groundwater was mostly affected by surface activities, such as domestic and industrial wastes dumped on the surface (Venkatesan et al. 2020).
Saturation Index
The saturation index (SI) based on mineral solid phases such as gypsum (CaSO4.2H2O), fluorite (CaF2), calcite (CaCO3), halite (NaCl), anhydrite (CaSO4), dolomite (CaMg(CO3)2), and aragonite (CaCO3) was computed using the USGS software, PHREEQC (version 3.2.0) to evaluate the impact of climatic and solubility conditions on the groundwater environment (Table 4). The source for the high SO42− and Ca2+ concentrations in the groundwater is mainly anhydrite dissolution, as indicated by the negative SI values in samples collected in the post- and pre-monsoon periods (i.e., −2.2239 to −1.219 and −2.7214 to −1.1813). The negative values in the post- (−1.987 to −0.983) and pre-monsoon samples (−2.484 to −0.944) indicate the dissolution effect of gypsum in the groundwater system (Table 4). The SI value for dolomite (CaMg (CO3)2) and calcite (CaCO3) in post-monsoon samples varies from −0.9905 to 2.2836 and −0.3172 to 1.1057. The SI dolomite positive values in 63% of samples indicate calcium precipitation (Li et al. 2018). The positive saturation index for calcite signifies precipitation of calcite in 70% and 90% of the samples collected during the post- and pre-monsoon periods, respectively, reflect Na+ enhancement in groundwater. The negative SI values of halite (NaCl) in all samples collected during the post- and pre-monsoon periods highlight the role of dissolution of halite minerals, leading to Na+ enrichment of groundwater.
Table 4 Saturation index (SI) of various mineral phases indicating dissolution and precipitation mechanisms occurred in the groundwater environment during post- and pre-monsoon periods Health Risk Assessment for Fluoride
In the Sulur region, fluoride is present in groundwater collected from many locations at concentrations that are greater than the allowable limits for drinking water set by the WHO. Hence, it is vital to conduct the health risk survey by calculating hazard quotient (HQ) for fluoride based on these regulatory limits. HQ values for noncarcinogenic risks from ingestion of contaminated drinking water and exposure through dermal pathways and total hazard index (THI) values were calculated for children, adult women, and adult men (Table 5). Table 5 presents minimum, maximum, and average values of HQ and THI for the above categories. The values indicate that dermal contact is much lower in terms of hazards from exposure to fluoride than the hazards of ingestion of fluoride in drinking water.
Table 5 Noncarcinogenic health risks of fluoride based on ingestion and dermal pathways The THI values for ingestion of fluoride ranged from 0.074 to 1.696 (mean 1.127) for children, 0.065 to 1.491 (mean 0.991) for adult women, and 0.061 to 1.399 (mean 0.929) for adult men. These THI results indicate that 73%, 60%, and 50% of the samples were higher than the permissible limits for health risks from ingestion of fluoride by children, women, and men, respectively. Based on this study, children are more at risk than the adult women and men. The elevated noncarcinogenic risk for children is mainly due to the lesser body weights (BW) of children compared with adult women and men (i.e., in Eq. 2 and 3, BW is in denominator).