Introduction

The quality of coastal aquifers is controlled by the variations of hydrogeochemical processes like salt water intrusion, geogenic process (weathering, ion exchange and rock-water interaction), and anthropogenic activities (agriculture, industry and urbanization). Table 1 explains the various controls of hydrogeochemical processes in the coastal aquifers worldwide. Every part of the country has its own causative factors for the variation of groundwater quality. So, it is important to determine the chemistry of groundwater quality for its sustainable development (Subba Rao et al. 2012). The area chosen for this study is in a such complex environment, occupied by the river Uppanar, Perumal lake, industries, and agricultural field.

Table 1 List of studies carried out worldwide on coastal aquifers
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Groundwater is the only dependable water resource for human consumption, as well as for drinking, irrigation, domestic, and industrial uses for many people around the world. Rapid depletion of groundwater supplies due to continuous populace growing and industrial development threatens the quality of water in many aquifers. Many researchers have stated that rapid urbanization along the coastline aquifers openly or secondarily affects the groundwater quality and quantity (Boukari et al. 1996; Lambrakis 1998; Zilberbrand et al. 2001; Foppen 2002; Faye et al. 2004). The process of assessing the suitability of groundwater for various purposes requires a proper thought on the hydrochemistry of an area. Further, it is important to understand how water–rock interaction (weathering) or any type of anthropogenic impact affects groundwater quality (Todd 1980; Kelley 1940). Much of the groundwater in coastline aquifers in India is affected from seawater intrusion due to over-extraction. However, hydrogeochemical patterns are complex because of the input from different water sources (Subba Rao 2006; Sanford and Pope 2010). Generally saline water intrusion in coastal regions occurs naturally, but it can be aggravated due to the exploitation of coastal aquifers as water sources (Martinez and Bocanegra 2002; Ozler 2003). In addition to the natural salinity of groundwater, anthropogenic activities contribute significantly to the deterioration of water quality (Egun 2010). Thus, the main objective of the study was to identify the factor controls the groundwater chemistry in the coastal region.

Study area

The study area is situated in a coastal region of Cuddalore district, Tamil Nadu, India, which lies between north latitudes 11°44′34″ and 11°29′59″ and east longitudes 79°35′06″ and 78°46′11″ (Fig. 1). The region is chiefly composed of Tertiary and Quaternary formations. Tertiary formation includes sandstone, clay, and grit sand, whereas Quaternary formation mostly covered by fluvial sediments of silt, clay, fine- to coarse-grained sand, and laterite. Tertiary formation gently dips towards east–southwest direction and it occurs at the depth of 100–457 m, and Quaternary formation occurs at the depth less than 30 mbgl (Arogyasami 1967; Sundaram 1979). In some locations, sandstones intervene with clay lenses that underlie the alluvial sand up to a depth of 50 m bgl. Soils in these regions are of oxysol type with the thickness greater than 3 m (Subramaniam 1969).

Fig. 1
figure 1

Sampling location map of the study area

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The average annual rainfall in these regions for the period 2003–2012 was about 138.70 mm. Uppanar is the major river in this region and it has developed a thick cover of alluvium with mixtures of sand, silt, and clay with the thickness varying from 10 to 15 m (Jeevanandam et al. 2006). It originates from the southern part of perumal Eri. It generally flows from south to north side and discharges into the Bay of Bengal. Subdentritic type of drainage pattern is noted in this region. The major occupation in the area is agriculture, with more than 60% of the population engaged in this activity. The study region lies in the coastline of less surface water resources. The level of groundwater development in this region is 89%, which is noted in the semi-critical stage (CGWB 2009), resulting in anthropogenic contamination by seawater intrusion. All the industries of SIPCOT (Small Industries Promotion Corporation of Tamil Nadu) are situated near the NE part of coastal region in the study area. The industries surrounding this region have continuously discharging chemicals like sulfur, lime, pesticides and it leads to the subsurface pollution in the northeastern side of the region (Sankaran et al. 2009).

The Cuddalore sandstone and unconsolidated Quaternary alluvium form the potential and principal aquifer systems in this region and it covers up to a depth of about 100 m bgl (CGWB 1997). Comparison of geoelectrical logging data (CGWB 1997) with depth-wise resistivity values at certain sites in the study area indicate discontinuous, unconfined and confined to semi confined aquifers constitute the major aquifer system in the study area.

Methodology

A total of 64 groundwater samples were collected from the study area during March 2011. The sample bottles are then labeled, sealed, and brought to the laboratory and conserved by adopting standard procedures by APHA (2005). Ca, Mg, Cl, and HCO3 were measured by titration and SO4, NO3, PO4, and H4SiO4 were obtained by using Digital Spectrophotometer (ModelGS5 700A). Na and K were measured by flame photometer (Systronics mk-1/mk-III). The accuracy of chemical analysis was checked by computing the cation–anion balance and it is noted to be less than ± 5 to 10% (Domenico and Schwartz 1990). Oxygen (δ18O) and deuterium (δD) isotopes in the groundwater samples were measured with Isotopic Ratio Mass Spectrophotometer (FinniganDeltaplusXp, Thermo Electron Corporation, Bermen, Germany). The standard deviation of the measurements is ± 1.72‰ for oxygen and ± 0.8‰ for hydrogen. The measurements of samples are periodically checked and calibrated with international isotope water standards recommended by IAEA (V-SMOW, GSIP and SLAP). The results are reported in terms of δ units (Permil deviation of the isotope ratio from the international standard V-Smow), with δ being defined by

$$\delta \, = \, \left[ {\left( {R_{\text{sample}} - \, R_{\text{SMOW}} } \right)/ \, \left( {R_{\text{SMOW}} } \right)} \right] \, \times \, 10^{3} ,$$

where R = D/H or 18O/16O.

General parameters

pH varies from 5.2 to 9.8 with an average 7.1, indicating slightly acidic to alkaline nature. EC values ranged from 928 μs/cm to 13,975 μs/cm with an average of 4850 μs/cm (Table 2). The spatial distribution of electrical conductivity (EC) values for groundwater samples is shown in Fig. 2. Higher EC values (> 5250 and 4250–5250 µs/cm) occur in the north eastern and south-western region of the study area which lies along the coastline, with an aerial extent of about 116 sq km. EC values range from 3000 to 4250 µs/cm in the central part of the region along the Uppanar river mouth, covering an area of about 104 sq km. EC values from 2250 to 3000 µs/cm cover an area of about 95 sq km in the north western and south-eastern part of the study area followed by the ranges from 1750 to 2250 µs/cm, covering a region of about 63 sq km in north western part. EC values (< 1750 µs/cm) cover an area of about 34 sq km in western part of the study area. Overall higher values were noted in the eastern part of the study area indicating the leaching of secondary salts and anthropogenic impact by agricultural activities and aquaculture apart from sea water intrusion. This may also be due to anthropogenic impacts from an industrial estate SIPCOT (Small Industries Promotion Corporation of Tamil Nadu), which is located close to the study area. The long-term extraction of this coastal aquifer leads to the impact of the effluents generated by them.

Table 2 Chemical concentrations of groundwater samples collected in the study area (all values in mg/L and except EC in µS/cm and pH)
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Fig. 2
figure 2

Spatial distribution of electrical conductivity (EC) in the study area

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Geochemical processes

Piper plot

Groundwater chemical data were processed on a Piper diagram (Piper 1944) to govern the compositional trends in groundwater, and this is shown in Fig. 3. This diagram indicates that samples plot in a range of hydrochemical facies, but the dominant facies observed are of Ca–Mg–SO4–Cl, Ca–HCO3, and Na–Cl types. Groundwater in the study area indicates a pattern of hydrogeochemical trends, which starts from a Ca–Mg–SO4–Cl type via Na–HCO3 type to Na–Cl type; or from Ca–HCO3− type directly to Na–Cl type (Fig. 5). The path of mixing/migration of the groundwater composition (evolutionary path) with the seawater/end solutions can be obtained (Varma et al. 1996) from the Piper plot.

Fig. 3
figure 3

Geochemical classification of groundwater in the study area

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The effect of mixing of groundwater and its behavior on the Piper plot is well explained by Karmegam et al. (2011) and Singaraja et al. (2012). The observed pattern of compositional change suggests that cation exchange reactions are taking place when saltwater intrusion occurs (Jeen et al. 2001; Richter and Kreitler 1993). Most of the groundwater samples indicated a chemical composition dominated by sodium and chloride ions, which usually specifies a strong seawater influence (Pulido-Leboeuf 2004). The Ca–HCO3 type of water probably signifies the partly flushed water or confined saline water source, in which HCO3 is higher than the alkaline earth cations (Ca + Mg) in equal concentrations (Mercado 1985). This is more prominent in Alluvial aquifer due to frequent flushing or recharge of groundwater. These excess HCO3 ions release the alkali ions (usually Na+) which enter into the groundwater by ion exchange processes (Jeen et al. 2001), whereas it results in the domination of Na+ in groundwater.

Chadda’s hydrogeochemical process evaluation

Generally, anionic composition of groundwater changes systematically from bicarbonate dominance to chloride dominance as groundwater flows from a recharge zone to a discharge area (Appelo and Postma 1996). The factors that influence the change of chemical composition of groundwater can be investigated graphically by plotting the difference in milli equivalent concentrations between alkaline earths (calcium + magnesium) and alkali metal elements (sodium + potassium) on the x-axis and the difference in milli-equivalent concentrations between weak acids (bicarbonate) and strong acids (chloride + sulfate) on the y axis (Fig. 4). Samples falling in Field 2 (Ca–Mg–Cl) are prominent between western side of the region and the coastal region possibly represents ion exchange reactions or an hydrochemical evolutionary path from Ca–Mg–Cl type water to Na–Cl mixed sea water, whereas Ca–Mg–Cl represents the reveres ion exchange processes. Samples represent Na–Cl type in Field 3 indicates the sea water mixing, which are mostly constrained in the coastal region and recharge characteristics was noted in Field 1.

Fig. 4
figure 4

Chadda’s plot for groundwater samples to determine the chief chemical processes

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Relationship of Cl−/HCO3 − ratio

The Cl−/HCO3− ionic ratios were also studied to characterize the origin of salinity in groundwater. Ionic ratios of groundwaters are often used to assess the seawater intrusion in coastal regions (Sanchez-Martos et al. 1999; Kim et al. 2003). As previously noted in this study, TDS is an often a good surrogate for Cl− and thus the ratio is the good indicator of salinization due to the seawater encroachment (Nwankwoala and Judom 2011). The ratios of Cl/HCO3 in the study area range between 0.28 and 13.47 and had a strong positive direct relation with Cl− concentrations (Fig. 5). It shows the mixing of fresh groundwater with saline water. Based on Cl/HCO3 ratios groundwater can be classified into unaffected (< 0.5), slightly or moderately affected (0.6–6.6), and strongly affected (> 6.6) by salinization process. (Revelle 1941; Todd 1959). On the basis of the ratio of Cl−/HCO3−, 84.3% of the groundwaters were considered to be strongly influenced by the saline water, and 9.3% were slightly or moderately affected by industrial activities. Similar observations were made by Demirel (2004) to categorize the saline water intrusion by Chloride enrichment in Mersin-Kazanli coastal region of Turkey. A combined effect of seawater and urban wastewaters may also be a reason for the poor quality of groundwater, which is influenced by topographic low close to the coastal area (Subba Rao et al. 2001). It was also interesting to note that 6.2% of the samples unaffected by salinization was located near Perumal Lake in the study area, which may be due to the dilution effect from the surface water recharged into the aquifer or lesser extraction of groundwater due to the increased usage of surface water.

Fig. 5
figure 5

Relationship between Cl−/HCO3− vs Cl− concentration in the study area

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Log pCO2

Log pCO2 values for samples were calculated to determine the recharge processes (Prasanna et al. 2009; Raymahashay 1986). Log pCO2 value ranges from − 5.34 to − 0.05 (Fig. 6). The distribution of the ionic strength (IS) values in the samples varied from < 0.006 to > 0.032. There is a linear relationship between IS and Log pCO2. The increase of IS values reflects the long residence time of the saline water or due to the recharge of evaporated saline water (zone 2). Low ionic strength and lower Log pCO2 values were noted in most of the samples (zone 1), which likely indicates inflow of fresh water to the aquifer (Prasanna et al. 2007). The lesser Log pCO2 values with higher ionic strength indicate closed system (Chidambaram et al. 2011). Hence it is understood that three basic environments are prevailing in the study area: frequent flushing region/open system, may be shallow groundwaters; groundwaters of the closed environment with long residence time and sea water intrusion regions. Samples are more prominently represented in zone1 and zone 3.

Fig. 6
figure 6

Relationship of log pCO2 to ionic strength in the study area

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Table 3 shows an extensive research on groundwater quality and its chemistry in the coastal aquifers at different parts of the world and compared with the present study area. Results from this study show that the mean value of Ca, Mg, Na, K, HCO3 and SO4 is comparatively lower than the other parts of the world except few locations. Interestingly, EC and TDS mean values are higher compared to other places except at Tuticorin, India. The mean value of pH is almost same as in other places. Overall, it indicates the ionic concentration in the coastal aquifers of present study is comparatively lower than in the other parts of the coastal aquifers worldwide.

Table 3 Comparison of hydrochemical parameters of the present study with other areas around the world (all average values in mg/L except pH, temperature (°C) and EC (µs/cm)
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Environmental isotopes

The local meteoric water line (LMWL) for Tamilnadu state exhibits an equation of δD = 7.89 δ18O ± 10.38 (Chidambaram et al. 2009), which is closer to the global meteoric water line (GMWL) derived by Rozanki et al. (1993). The slight variations are due to the different climatic influences which include air temperature, secondary evaporation, variation in precipitation, and moisture (Clark and Fritz 1997). The rain water isotope data for the region nearby the study area were taken from Chidambaram et al. (2009). It exhibits an LMWL (Fig. 7) of

$$\delta {\text{D}} = \, 7.6\;\delta^{18} {\text{O}} + \, 3.2.$$
Fig. 7
figure 7

Plot for δ18O versus δD of groundwater samples

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The results of δD and δ18O analyses show that isotopic compositions of groundwater range from − 7.7 to − 2.1‰ for δ18O and from − 55.6 to − 18.5‰ for δD. The comparison of the stable isotopes of the rain water with the groundwater samples show that most of the samples (Group A) fall in line with the LMWL indicating the recharge from the local rainfall recharge. There are few samples (Group B) indicative of evaporative enrichment/sea water intrusion. It is also observed that the heavier isotopes are more in five samples which may be due to the precipitation near the shore or that of saline water intrusion (Prasanna et al. 2009). The origin of salinization in ground water is generally distinguished by using δD–δ18O relationship, because the stable isotope fractionation is mostly related to temperature (Kendall and McDonnel 1998; Huddart et al. 1999). Group B samples also show that there is an end member of isotopic composition indicating sea water intrusion. It is evident that there is a clear mixing noted in between meteoritic samples and the sea water end member.

Relationship between δ18O vs EC and Cl

The relationship of the stable isotopes with the groundwater quality in this region was studied by plotting δ18O, δD, EC, and Cl−. Figures 8 and 9 show the connections amongst δ18O and EC, and Cl−, respectively. The comparison of δD and EC shows that most of the samples (Zone I) fall in recharge region; the six samples (Zone II) fall in other sources of the region and five samples (Zone III) are indicative of seawater intrusion. So it indicates that during the mixing of fresh groundwater and highly mineralized brine, the water quality and δ18O value rise with the quantity of the mixing waters. In zone III, it is evident that δ18O increases with a linear logarithm relationship with Cl− and EC in the mixing process (Dutkiewicz et al. 2000).

Fig. 8
figure 8

Plot for δ18O versus EC for groundwater samples

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Fig. 9
figure 9

Plot for δ18O versus Cl for groundwater samples

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δ18O vs Cl diagram (Fig. 9) show similar trend to that of δ18O vs EC. Relationship between δ18O and Cl indicates the mixing trend with rainfall recharged fresh water and the sea water in Zone III samples and it is confirmed by the Cl–δ18O relationship (Kim et al. 2003). In comparison with other groundwater samples, the sample numbers 1, 7, 18, 30, and 28 are enriched and noted to deviate significantly from the LMWL (Fig. 7), and also they have elevated Cl− contents (Fig. 9). These five samples which fall along the line have slope 4.01, which is lower than LMWL (7.6), and it is recognized by evaporation and/or mixing of groundwater with seawater.

Correlation analyses

Cl−–Ca2+ and Mg2+–Ca2+ ions are positively correlated (> 0.5), which indicates the salinization near the industrial area due to the enrichment of some salts (CaCl2, MgCl2, and Na2SO4) including halite and gypsum. Then less significant correlation (0.3–0.5) was noted between Cl−–Na+, Mg2+; HCO3−–Ca2+, Mg2+; PO42−–K+, SO42−. Generally, almost all ions have a positive correlation with Cl−, especially Ca2+ and Mg2+ (Table 4), revealing the derivation from similar source of salt waters (Kim et al. 2003). This correlation also signifies dissolution and leaching of secondary salts, with a significant correlation of HCO3− with Ca2+, Mg2+, PO42−, K+, and SO42−. Hence there may be interplay of the saline water intrusion and the leaching of the salts. NO3−, PO42−, SO42−, and H4SiO4 are poorly correlated with other ions, indicating the individual complex processes (Briz-kishore and Murali 1992).

Table 4 Correlation matrix for groundwater the chemical constitution present in groundwater of the study area
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Factor analysis

Five factors were extracted with 75.29% of total data variability (TDV). Factor 1 represents Na+, Ca2+, Mg2+, and Cl− (Table 5), which indicates the prevalence of reverse ion exchange induced by seawater intrusion (Kim et al. 2003). Factor 2 was characterized by K+, PO4−, and SO42− showing that anthropogenic signature comes from fertilizers. Factor 3 represented by Na+ and Cl− indicates the saline water intrusion to the aquifer increases the concentration of these ions. Factor 4 characterized by H4SiO4 indicate silicate weathering. The study area is principally consisting of alumino silicates, which undertake weathering and discharge silica into the solution. Poor representation of Ca2+ and HCO3− in this factor is due to the enrichment of silica by the precipitation processes of carbonate minerals from the system. Factor 5 is heavily loaded by NO3− indicating the impact of nitrate fertilizer. Nitrate is also probably the most wide-spread contaminant in groundwater which originates from both urban as well as agricultural activities (Nkotagu 1996; Stigter et al. 1998; Zilberbrand et al. 2001; Perez et al. 2003; Min et al. 2003). Hence, it is inferred that the reverse ion exchange, anthropogenic influence, and the sea water intrusion play a major role in determining the groundwater chemistry of the region.

Table 5 Factor analyses for the chemical composition of groundwater in the study area
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Conclusions

Groundwater is an important source of drinking water for many people around the world. In the present study, the relative concentrations of cations and anions occur in the order of Na+> Ca2+> Mg2+> K+ and Cl− > HCO3− > H4SiO4 > SO42− > NO3− > PO4−. The dominant hydrochemical facies of groundwater in the study area is Ca–Mg–SO4–Cl, Ca–HCO3, and Na–Cl types indicating the geochemical variation along the groundwater flow path. The variation of log pCO2 and ionic strength reveals three different types of water: frequent flushing, sea water ingression, and long residence time of water in the aquifer system. δD and EC of the groundwater samples show that most of the samples fall in recharge region; six samples fall in other sources of the region and five samples are indicative for seawater intrusion. The Cl−/(Cl− + HCO3−) ratios and statistical analysis reveal the fact that sea water ingression, silicate weathering, and ion exchange process govern the geochemistry of coastal groundwater in the study area.