Hydrochemical characteristic of coastal aquifer from ... - Springer Link

Environ Monit Assess (2011) 175:531–550 DOI 10.1007/s10661-010-1549-6

Hydrochemical characteristic of coastal aquifer from Tuticorin, Tamil Nadu, India Nepal C. Mondal · Vijay P. Singh · Somvir Singh · Vijay S. Singh

Received: 21 December 2009 / Accepted: 26 May 2010 / Published online: 12 June 2010 © Springer Science+Business Media B.V. 2010

Abstract This article deals with a systematic hydrochemical study carried out in coastal aquifers, Tuticorin, Tamil Nadu, to assess groundwater quality. A total of 29 groundwater samples were collected and analyzed. Results showed that total dissolved solids (TDS), sodium (Na+ ), magnesium (Mg2+ ), chloride (Cl− ), and sulfate (SO42− ) significantly damaged groundwater systems. The degree of salinization due to seawater mixing in a well or a given area could be indicated by an increase in nearly all major cations and anions. Toxic elements (i.e., Pb and As) were higher than the maximum permissible limits of drinking water. Cross plot of HCO3− /Cl− (molar ratios) versus TDS indicated that about 62% of the analyzed samples were saline. Factor analysis

showed that groundwaters, affected by seawater intrusion/industrial activity, were separated from the clusters. An attempt was made to identify the hydrochemical processes that accompany current intrusion of seawater using ionic changes. It was estimated that the mixing rate of seawater intrusion was about 5.81% during April 2007. An index, called ‘Seawater Mixing Index’ (SMI), was also adopted and its value was SMI > 1.18 with EC > 3,000 μS/cm about 62% of the sampled waters, were saline. Further, a few trace elements (i.e., Sr, B, and Li) were used as indicators for responding to the change in fresh to saline groundwater environments in coastal aquifers. Keywords Groundwater · Coastal aquifers · Seawater intrusion · Seawater mixing index · Ionic change · Tamil Nadu · India

Introduction N. C. Mondal (B) · S. Singh · V. S. Singh National Geophysical Research Institute (Council of Scientific & Industrial Research), Uppal Road, Hyderabad 500 606, India e-mail: [email protected] N. C. Mondal · V. P. Singh Department of Biological & Agricultural Engineering and Department of Civil & Environmental Engineering, Texas A & M University, College Station, TX 77843, USA

Seawater intrusion is one of the most common problems in almost all coastal aquifers around the globe (Park et al. 2005; Batayneh 2006; Sherif et al. 2006; Mondal et al. 2010a). It takes place when saline water displaces or mixes with freshwater in aquifers (Todd 1953). This phenomenon can be attributed to a variety of conditions like gentle coastal hydraulic gradients, tidal and estuarine activity, sea level rises, low infiltration,

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excessive withdrawal, and local hydrogeological conditions (Sarma and Krishnaiah 1976; Sarma et al. 1982; Prasad et al. 1983; Longe et al. 1987; Diamantis and Petalas 1989; Rajmohan et al. 1997, 2000; Nowroozi et al. 1999; Barrett et al. 2002; Saxena et al. 2004; Melloul and Collin 2006; Lee and Song 2007; Sherif and Kacimov 2007; Mondal et al. 2008; Kim et al. 2009). However, natural hazards, such as tidal waves and tsunami engulfing the coastal regions, result in the percolation of seawater into shallow and unconfined aquifers (Todd 1953; Villholth et al. 2006). Thus, one of the most common methods for assessing seawater intrusion through an aquifer in coastal belts is a periodic analysis of groundwater chemistry (Todd 1980; Sukhija et al. 1996; Saxena et al. 2003; Beddows et al. 2007; Sarwade et al. 2007; Kim et al. 2009). When seawater intrusion is a main cause of high salinity, groundwater generally exhibits high concentrations not only in total dissolved solids (TDS) but also in major cations and anions (Richter and Kreitler 1993) as well accumulation of selective trace elements (Saxena et al. 2004; Mondal et al. 2010b). The study area, located in the eastern cost of Southern India, is not immune from the effect of seawater intrusion in presence of the Bay of

Fig. 1 Location map representing geology and sampling points of the study area

Environ Monit Assess (2011) 175:531–550

Bengal. So far no study has been done to examine hydrochemical behavior in this costal area. Therefore, the objective of this study is to report the general status of groundwater quality and salinization on shallow groundwaters. In order to achieve the above objective, the following tasks are to be carried out: (1) analysis and interpretation of hydrochemical datasets to discriminate the effect of different hydrochemical processes; (2) principal component analysis (PCA) for separation of groundwater samples from natural hydrochemical behavior; (3) calculation of ionic deviations for better understanding of hydrochemical processes; (4) development of an index, called seawater mixing index (SMI), for evaluating the relative degree of seawater mixing, and (5) examination of a few sensitive trace elements for responding to the change in fresh to saline groundwater environment.

The study area The study area (latitudes, 8.77◦ –8.85◦ N and longitudes, 78.04◦ –78.17◦ E) lies in the east coastal belt, which is the west of Tuticorin town in the state of Tamil Nadu, Southern India (see Fig. 1). It


covers an area of about 112 km2 and is drained by a network of streams oriented in the northwest– southeast direction, which are of ephemeral in nature. Topographic elevation varies from 27 m (amsl) to a few meters (amsl) near Tuticorin town and slopes from west to east. The slope is gentle in the western and the central part and nearly flat in the eastern part. The topography of the study area shows that the groundwater trough developed toward the northwestern part of the Steralite Industrial (India) Ltd (SIIL) premises (see Fig. 2). The area receives rainfall during the northeast monsoon season, which is active during the months of October, November, and December. The long-term average annual rainfall of Tuticorin town is 568 mm (IMD data). Daily rainfall of the year 2006 recorded at the SIIL rain gauge station indicated that rainfall was above normal with significant high intensity daily rainfall events as compared to previous years since 2000. The area experiences semi-arid tropical climatic conditions; and falls in east coast plains and hills regions agro-climatic zone as classified by Indian Council of Agricultural Research (ICAR, India). The land is utilized for cultivation of cotton, maize and some medicinal plants. Some of the land is fallow and some is barren with vegetation such as thorny shrubs with thin cover of dry grass and palms.

Fig. 2 Water level contours (in m, amsl) and its flow direction (April 2007)

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Geological and hydrogeological characteristics Gneisses, charnockites, and quartzites of Archaean age, calcareous sandstone and shell limestone of tertiary age, and alluvium of recent age underlie the area (see Fig. 1). The Archaean groups of formations are crystalline and metamorphic, and finely foliated with a general NW–SE trend described by Balasubramanaian et al. (1993). The formations, including quartzites as ridges in the western part, are weathered, jointed, and fractured. Recent to sub-recent sand occupies coastal areas. It consists of coarse and calcareous grits sandstone and shell limestone. The area is covered with black soils in the western part (around the SIIL plant), red soil (sandy loam to sandy soil) in the central part and alluvial sandy soils in the eastern part. The maximum soil thickness is about 3 m (Sankul Techno 2002; Singh et al. 2006). The sandy soils originated from sandstones and these have low soil moisture retentivity. The alluvium soils are wind-blown sands and shells constitute beach sand and coastal dunes, which have very low soil moisture retentivity. There are a large number of open and bore wells tapping shallow phreatic aquifers and fractured aquifer systems. The wells are being used for domestic and irrigation purposes. The depth

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of open wells ranges from 7–12 m and bore wells from a few meters to a maximum depth of 70 m Singh et al. (2006). Most of the wells are less than 20 m in depth (Table 1). In tertiary and alluvial areas, the sandy zone is the main aquifer system. The static water level during the pre-monsoon period varies from 1.8 to 14.45 m (bgl) and in postmonsoon period it is from 0.90 to 12.86 m (bgl). The shallow groundwater had been recorded in the SIIL premises and towards the coastal area. The water level contour maps for April 2007 had been prepared with the aid of SURFER v.8.0 (2002), Golden Software, Inc., using the kriging method (see Fig. 2). This figure shows that the

groundwater flow direction in the extreme northern, western, and southwestern parts of the area was in the NW–SE direction. The groundwater crest developed in and around the SIIL plant. Groundwater flow was in all directions from the SIIL premises. On the coastal side, it was towards sea except for the northern side of the coast and sometimes it had gone below the mean sea level (bmsl) due to the salt pans activities. Groundwater mounds and troughs were also visible in a few locations due to local disturbances. Groundwater was highly saline, having an electrical conductivity as high as 18,605 μS/cm/ at 25◦ C (Mondal et al. 2009).

Table 1 Well inventory of the study area Sample codes

Village names

Type of well

Depth of well (m, bgl)

Dimensions (m)

MSL (m) of MP

MP (m)

Water level (in m)

W-1 W-2 W-3 W-4 W-5 W-6 W-7 W-8 W-9 W-10 W-11 W-12 W-13 W-14 W-15 W-16 W-17 W-18 W-19 W-20 W-21 W-22 W-23 W-24 W-25 W-26 W-27 W-28 W-29

Swaminatham (S) Nayinapuram Kumargiri Terku Virapadiyapuram SIIL-II(N,PZ-11) Pandarampatti SIIL-XII(W,PZ-6) SIIL-VII(W,PZ-2) Vadakka Silukkanpatti A. Shunmughapura Kailashpuram Ayynaduppu Madathur (N) Milavittam Pandarampatti (W) Sankararapperi (E) Ramanathampuram Pudur Pandiyapuram Davishpuram Rajapallam Arokayipuram Mappali Urani Silverpuram Chinakannupur SBI Colony Seetapuram Nagar Levinjpuram PNT Colony (S) Periyanayakapuram

DW TW TW TW BW BW BW BW TW DW DW DW DW DW TW DW DW DW DW DW DW DW DW DW DW DW DW DW DW

12.00 54.86 70.00 70.00 18.28 19.01 18.28 18.28 60.00 8.60 6.30 10.66 9.00 12.40 33.52 10.00 11.50 8.40 6.88 5.83 5.96 5.50 9.00 6.75 4.09 2.96 6.25 3.20 4.58

6.7 × 7.0 0.165 0.165 0.165 0.102 0.165 0.102 0.102 6.5 5.1 × 5.3 3.3 × 3.4 4.3 × 6.25 2.6 × 3.7 4.1 × 4.3 0.165 7.3 × 4.8 4.2 × 4.0 5.0 × 6.2 1.55 1.65 1.85 1.0 6.7 0.97 1.0 1.0 3.4 1.0 6.65 × 7.25

21.925 16.470 26.625 23.885 17.235 12.340 19.625 17.955 26.790 10.945 9.320 9.660 11.220 12.320 11.460 9.050 11.415 8.735 3.325 2.675 3.455 4.890 7.900 4.905 5.155 2.875 4.845 4.050 5.800

0.00 0.15 0.46 0.43 0.35 0.33 1.07 0.20 0.60 0.95 1.00 0.40 0.73 0.75 0.46 0.90 0.60 0.60 0.73 0.94 0.86 1.00 0.87 0.80 0.62 0.20 0.74 0.38 1.38

4.90 11.34 14.25 13.50 5.39 7.75 4.10 1.80 11.02 6.11 4.29 5.20 6.90 8.54 7.68 6.53 8.00 6.05 6.00 4.10 4.82 4.69 7.95 3.00 2.78 1.00 4.55 2.00 2.26

DW dug well, TW tube well, BW bore well, bgl below ground level, MSL mean sea level (in m, amsl), MP measuring point and water level measurement was taken on April 2007


Materials and methods

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and the exact longitudes and latitudes of sampling points.

Sample collection and analysis For the evaluation of groundwater quality, 29 groundwater samples were collected during April 2007 from dug wells, tube wells and bore wells distributed throughout the area, which were under use at 0.5 m below the water table, and were pumped for more than 5 min. Methods of collection and analysis of water samples that were followed are essentially the same as given by Brown et al. (1983) and APHA (1985). Samples were collected in 1-l-capacity polythene bottles. Prior to collection, the bottles were thoroughly washed with diluted HNO3 acid, and then with distilled water in the laboratory before filling bottles with samples. Each bottle was rinsed to avoid any possible contamination in bottling and every other precautionary measure was taken. WTW portable EC and pH meters measured hydrophysical parameters, such as pH and EC, on site. Sodium (Na+ ) and potassium (K+ ) were determined by atomic absorption spectrophotometer (AAS). Total hardness (TH) as CaCO3 , calcium (Ca2+ ), bicarbonate (HCO3− ), and chloride (Cl− ) were analyzed by volumetric methods. Magnesium (Mg2+ ) was calculated from TH and Ca2+ contents. Sulfate (SO42− ) was estimated by the spectrophotometric technique, whereas nitrate (NO3− ) was determined by ion chromatography and fluoride (F− ) by fluoride meter. All concentrations are expressed in milligrams per liter (mg/l), except pH and EC. Analytical precision for measurements of cations and anions, indicated by the ionic balance error (IBE), was computed on the basis of ions expressed in me/l. The value of IBE was observed to be within a limit of ±5% (Mandel and Shiftan 1980; Domenico and Schwartz 1990). The selective trace elements (i.e., Sr, B, Li, Mn, Cr, As, and Pb) were also analyzed using inductively coupled plasma mass spectrometry (Balaram and Rao 2004). The precise locations of sampling points (as shown in Fig. 1), were determined in the field through the development of GARMIN 12 Channel Instrument, based on the principles of Global Positioning System (GPS),

Principal component analysis Principal component analysis (PCA) yields common and similar relationships, respectively, between measured hydrochemical variables by revealing multivariate patterns that may helps classify the original data. It can effectively reduce numerous hydrochemical data into a few major factors whose eigen values in a correlation matrix exceed one (Reyment and Joreskog 1993). The geochemical interpretation of factors yields insights into the main processes that may govern the distribution of hydrochemical variables. The first step was to standardize the raw data. This standardization tends to increase the influence of variables whose variance is small, and reduce the impact of variables whose variance is large. Furthermore, the standardization procedure eliminates the effect of using different units of measurement, and renders data dimensionless. Then, the data were transformed into factors. The variables for factor analysis were pH, EC, TDS, TH, Ca2+ , Mg2+ , Na+ , K+ , Cl− , SO42− , HCO3− , NO3− , and F− . Factor analysis involves three important stages (Gupta et al. 2005). Determination of the correlation matrix, which contains various interrelated hydrochemical parameters, provides vital information about various geochemical variables. Eigen values and eigenvectors of the correlation matrix are extracted and the least important among them are then discarded. The number of variables retained in the factors or communalities is obtained by squaring the elements in the factor matrix and summing the total within each variable. The magnitude of communalities depends on the number of factors retained. Varimax rotation was then adopted (Sharma 1996). Lower eigen values do not help much in the interpretation of data and hence lower eigen values are not considered for factor extraction. In general, factors showing eigen values > 1 are used for interpretation of results, and in the present work, also, factor extraction had been done with the

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minimum acceptable eigen value > 1 (Harman 1960; Kaiser 1960). Ionic changes Ionic deviations were calculated to better understand the hydrogeochemical processes that take place in coastal aquifers. Calculation of ionic deviations () corresponds to a comparison of measured concentration of each constituent to its theoretical concentration of a known theoretical freshwater–seawater mixture calculated from the Cl− concentration of the sample (Fidelibus 2003) as: Ci = Ci,sample − Ci,mixed

(1)

where Ci is the ionic deviation of ion i; Ci,sample is the measured concentration of ion (i) in the sample, and Ci,mixed is the theoretical concentration of the ion (i) for the theoretical (conservative) mixture of freshwater–seawater. The theoretical mixture concentrations were calculated taking into account seawater contribution ( fsea ) based on chloride contents in the sample (CCl,sample ), the freshwater Cl− concentration (CCl,f ) and the seawater Cl− concentration (CCl,sea ): CCl,sea − CCl, f (2) fsea = CCl,sample − CCl, f This allows calculation of seawater contents (%) in each sample of the study area. This seawater contribution was then used to calculate the theoretical concentration of each ion: Ci,mix = fsea × Ci,sea + (1 − fsea ) × Ci, f

(3)

These calculations take into account that Cl− is a conservative tracer (Tellam 1995). In fact, Cl− is not usually removed from the aquifer system due to its high solubility (Appelo and Postma 1993). The only inputs are either from the aquifer matrix salts or from a salinization sources like seawater intrusion/other. Seawater mixing index For quantitative estimation of the relative degree of seawater mixing in certain water, we adopted a parameter called SMI. This parameter was

based on the concentration of four major ionic constituents in seawater (i.e., Na+ , Mg2+ , Cl− , and SO42− ), which are abundant in seawater, as follows: SMI = a ×

CMg CNa CCl CSO4 +b × +c × +d × TNa TMg TCl TSO4 (4)

where constants a, b , c, and d denote the relative concentration proportion of Na+ , Mg2+ , Cl− , and SO42− (a = 0.31, b = 0.04, c = 0.57, and d = 0.08) in seawater, respectively; C is the calculated concentration of groundwater samples in mg/l; and T represents the calculated regional threshold values of selected ions in groundwater samples, which can be estimated from the interpretation of cumulative probability curves.

Results and discussion General hydrochemistry Table 2 summarizes basic statistics of hydrochemical parameters for the analyzed groundwater samples (N = 29) from the study area. Comparison of hydrochemical data with drinking water standards (WHO 1984) showed that about 82.8% (N = 24), 75.9% (N = 22), 75.9% (N = 22), and 65.5% (N = 19) of the samples exceeded the guideline values for total dissolved solids (TDS, 500 mg/l), magnesium (Mg2+ , 30 mg/l), chloride (Cl− , 200 mg/l) and sodium (Na+ , 200 mg/l), respectively. This indicated a significant water quality deterioration in the study area. The degree of salinization due to seawater mixing in a well or a given area could be indicated by an increase in total dissolved solids and possibly an increase in nearly all major cations and anions. Very wide ranges and high standard deviations were easily recognized for most parameters (see Table 2). In particular, the TDS value had a wide range between 126 and 12,310 mg/l (mean, 3,610.6 mg/l). The ranges of Na+ , Cl− , and SO42− ions were also wide; 6 to 2,443 mg/l (mean, 630.0 mg/l), 7 to 5, 786 mg/l (mean, 1,111 mg/l), and 5 to 4, 224 mg/l (mean, 806.7 mg/l), respectively. Such wide ranges of solute concentrations


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Table 2 Statistical parameters of major cations and anions, and a few selected trace elements of groundwater in Tuticorin, Tamil Nadu, India Parameters

Minimum

Maximum

Mean

SD

WHO guideline value (1984)

pH EC TDS TH Ca2+ Mg2+ Na+ K+ Cl− SO42− HCO3− NO3− F− Sr B Li Mn Cr As Pb

7.1 200 126 72 10 8 6 2 7 5 98 1 0.21 273.9 259.1 4.78 14.76 21.64 5.32 73.66

8.6 18,605 12,310 4,880 832 681 2,443 179 5,786 4,224 854 63 7.23 54,454.4 8,982.4 149.95 1,406.52 77.2 99.5 564.1

7.8 5,438.2 3,610.6 1,392.3 266.4 169.7 630.0 43.6 1,111 806.7 326.7 20.2 1.48 11,035.3 1,978.5 38.01 139.82 40.71 32.35 193.46

0.3 4,588.9 3,069.5 1,280.4 235.6 170.9 633.9 54.1 1,354.7 947.1 152.7 16.1 1.83 12,707.4 2,107.1 33.53 298.67 16.73 21.65 116.65

6.5–8.5 750 500 100 75 30 200 100 200 200 200 45 1.5 70 300 – 100 50 10 100

pH:-log10 H+ at 25◦ C; EC in μS/cm, All Ions in mg/l; Sr, B, Li, Mn, Cr, As and Pb in μg/l; samples were collected on April 2007 SD standard deviation; TH total hardness as CaCO3

suggested that multiple sources and/or complex hydrochemical processes acted to generate the chemical composition in groundwater. The very high nitrate concentrations were observed in well 5 (= 63 mg/l) and well 6 (= 54 mg/l). This also reflects a considerable degradation of groundwaters due to anthropogenic contamination. The TDS of groundwater samples had been contoured, as shown in Fig. 3a. This figure clearly shows that a higher value of TDS > 3000 mg/l was observed in the middle part covering more than 50% of the area, in particular, at wells W6, -13, -14, -16, -17, -19, -21, -22, -24, -27, and SIIL premises. Similarly, wells W-12 and -29 in the southern part and well W-1 in the northwestern part were also affected with higher TDS values. This may indicate the possibility of a high rate of ingression/intrusion in the middle, southern and northwestern parts of the study area, where a high rate of withdrawal of groundwater was also observed. It is noticed that the central and eastern parts are comparatively more populated, have larger industries and also have a large num-

ber of dug, tube, and bore wells (Singh et al. 2006). However, saturation was different and favorable for fresh groundwater aquifers in wells W-2, -9, -11, -18, and -28, which are close to Korampallam tank, canal, and recharge area (Mondal et al. 2009). The TDS in groundwater was observed as low as 126 mg/l in the village of Kailashpuram (W-11), which indicated the availability of fresh groundwater. Results of trace elements reveal that Sr, B, Li, Mn, Cr, As, and Pb varied from 273.9 to 54,454.4, 259.1 to 8,982.4, 4.78 to 149.95, 14.76 to 1,406.52, 21.64 to 77.2, 5.32 to 99.5, and 73.66 to 564.1 μg/l, respectively. The mean value of toxic element Pb was 1.93 times more than the maximum permissible limits of drinking water (WHO 1984) as well as the maximum value (564.1 μg/l) obtained at Ramanathapuram (W-17). Mean Pb concentrations in freshwater and rivers are 1.0 and 3.0 μg/l, respectively (Ward 1995). A contour map of the Pb distribution was prepared and is shown in Fig. 3b. Its occurrence in groundwater may be attributed to the release of lead adsorption

538 Fig. 3 Spatial distribution of a TDS (mg/l), b Pb (μg/l) and c Cr (μg/l) in groundwater during April 2007



of marine sediments, from dust transported via atmosphere and continental crust erosion, precipitation and deposition of airborne aerosols, and the availability of lead in global pollution (Hunt and Howard 1994). The lead concentration was above permissible limits in the whole area except at Mappali Urani (W-22), SBI Colony (W-25), Madathur (W-13), and Periyanayakapuram (W-29). The high Pb content in groundwater of this area may have been sourced from either Pb released from death and decay of marine organisms and/or through precipitation. Arsenic concentration in the groundwater is exceedingly high at all locations except PNT Colony (W-28) and Terku Varapuram (W-4). The maximum As value recorded for the area over 9.95 times more than the maximum permissible limit (WHO 1984), and the mean value was also found more than 3.23 times of that. The occurrence of As in natural waters is usually associated with sedimentary rocks of marine origin, weathering of volcanic rocks, fossil fuels, mineral deposits, mining wastes, agricultural use, and irrigation practices (Hunt and Howard 1994). However, the elevated As admitted in groundwater of this area could be from marine sediments and fossil fuels. A high concentration of Sr was observed, indicating that the source could be fossils, found in the sedimentary rock. The Sr concentrations were highly elevated in the central part, varying from 273.9 to 54,454.4 μg/l. Ward (1995) reported the mean concentrations of Sr in freshwater and river water are 70 and 60 μg/l, respectively. Simultaneously, Saxena et al. (2004) have established that Sr content could be linked to various water types. They suggested Sr values of 5,000 μg/l for saline groundwater in coastal aquifers. The obtained Sr values indicated that groundwaters had about 59% and 17% contents of saline and brackish waters, respectively. This suggests the aquifers are under interaction with seawater and that the source of Sr content may be linked solely to marine in origin. Non-metallic (B) and lithium (Li) varied from 259.1 to 8982.2 and 4.78 to 149.95 μg/l, respectively. Chromium (Cr) varied from 21.64 to 77.2 μg/l (see Table 2). It was 50 μg/l in industrial belt and suspected wells affected

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by seawater intrusion (see Fig. 3c). In this area, the Mn concentration ranged between 14.76 and 1406.5 μg/l, while the respective mean value was 139.82 μg/l. This mean value was 1.39 times higher than the WHO (1984) guideline limit. However, a higher concentration of Mn (>70 μg/l) was found, possibly linked with the marine environment and industrial activities. Cross plots and trends analysis The main hydrochemical constitutions of groundwater are Na+ , Ca2+ , Mg2+ , Cl− , HCO3− , and SO42− , which increase the TDS value of groundwater. Cross plots and trend analysis of cations and anions showed fairly good correlations. The correlation matrix of different chemical constituents with total dissolved solids (TDS) showed that a correlation coefficient of 0.89 was observed between TDS and Cl− and Mg2+ ; and 0.87 and 0.84 between TDS and, respectively, K+ and Na+ . It was also observed between Mg2+ vs. Cl− (0.84), Ca2+ vs. Cl− (0.72), SO42− vs. NO3− (0.83), and K+ vs. Cl− (0.71), respectively. It was clear that the groundwater quality in the study area deteriorated mainly due to external sources. Further, it was found that TDS varied with HCO3− up to the TDS level of 1000 mg/l (see Fig. 4a). This showed that a proportionate hydrochemical behavior was observed between HCO3− and TDS in fresh groundwater in wells W-2, -9, -11, -18, -20, and -28. As soon as groundwater became brackish (>1,000 mg/l), the ionic behavior also changed but there were no linear relationships between TDS vs. HCO3− in groundwaters. There was a good relationship between SO42− and TDS for fresh and brackish waters but the concentrations of SO42− drastically changed into two different trends with some linear relationships (see Fig. 4a). In the case of Cl− vs. TDS, the saturation was different and a linear relationship had been observed (see Fig. 4b). It had been seen that the maximum chloride (>1,400 mg/l) was obtained at wells W-16, -17, -19, -21, -22, -24, and -29. This suggested the possibility of hydrochemical processes such as seawater intrusion and that dilution with fresh groundwater had taken place simultaneously at various places in this area. But the trends of

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Fig. 4 Cross plots of a HCO3− and SO4− , b Cl− , c (Na+ + K+ ) and (Ca2+ + Mg2+ ), and d molar ratios of HCO3− /Cl− with TDS

chloride with TDS were reciprocal with SO42− in the samples which were influenced by the industrial activity and seawater intrusion. Cationic concentrations of groundwater were plotted with TDS, which showed that TDS was directly proportional to (Na+ + K+ ), and (Ca2+ + Mg2+ ) up to the TDS of 2000 mg/l (see Fig. 4c). The sum of (Na+ + K+ ) decreased a little; thereafter it increased linearly, but the sum of (Ca2+ + Mg2+ ) increased without any linear relationship. Sodium and chloride are the dominant ions of seawater, while calcium and bicarbonate are generally the major ions of fresh water (Hem 1989). Thus, high levels of Na+ and Cl− ions in coastal groundwater may indicate a significant effect of seawater mixing (Mondal et al. 2008); while considerable amounts of HCO3− and Ca2+ mainly reflect the contribution from the water–rock interaction. A plot of HCO3− /Cl− versus TDS (see Fig. 4d) showed that the values of HCO3− /Cl− (molar ratios) were 2,000 mg/l) range of 62% analyzed samples, while its slope was steep negative in the low TDS concentration range (20 meq/l) in samples that were highly marked by the seawater signature (see Fig. 8b) in groundwater of the same monitoring wells. The downward trend for Na+ and upward trend for Mg2+ with the increased fraction of seawater showed that groundwater was dominated by seawater as can be observed from the negative value of CNa for Na+ as well as positive value of CMg for Mg2+ In the case of ionic changes of SO42− (see Fig. 8c), it was >25.0 meq/l for groundwater samples (W-5, -6, -7, and -8) collected from the industrial area, where gypsum had been dumped, but calculated seawater fractions of groundwaters of W-16, -17, -19, and -24 monitoring wells were >10%. The CSO4


Fig. 8 Ionic changes (C) for a Na+ , b Mg2+ , and c SO42− of the collected samples

in the both the cases were with approximate 90◦ phase. Intersection of seawater mixing Results of the interpretation of cumulative probability curves for chosen hydrochemical parameters (i.e., Na+ , Mg2+ , Cl− , and SO42− )


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Fig. 9 Distribution of cumulative probability curves for the concentrations of a Na+ , b Mg2+ , c Cl− , and d SO42− in groundwater

are shown in Fig. 9. The intersection points on the cumulative probability plots could be considered as regional threshold values (T) for differentiating the samples for the effect of

Fig. 10 Cross plots of SMIndex with EC and Cl− of groundwater

seawater mixing/anthropogenic pollution in the study area. The approximate values obtained were 90 mg/l for Na+ , 50 mg/l Mg2+ , 248 mg/l for Cl− , and 222 mg/l for SO42− .

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Fig. 11 Cross plots of TDS vs. Sr, B, and Li of groundwater

Table 5 Fresh, brackish, and saline water classification based on TDS, Sr, B and Li (in Tuticorin, Tamil Nadu, India) Water sample

Village name

TDS (mg/l)

Sr (μg/l)

B (μg/l)

Li (μg/l)

W-1 W-2 W-3 W-4 W-5 W-6 W-7 W-8 W-9 W-10 W-11 W-12 W-13 W-14 W-15 W-16 W-17 W-18 W-19 W-20 W-21 W-22 W-23 W-24 W-25 W-26 W-27 W-28 W-29

Swaminatham (S) Nayinapuram Kumargiri Terku Virapadiyapuram SIIL-II(N,PZ-11) Pandarampatti SIIL-XII(W,PZ-6) SIIL-VII(W,PZ-2) Vadakka Silukkanpatti (E) A. Shunmughapura Kailashpuram Ayynaduppu Madathur (N) Milavittam Pandarampatti (W) Sankararapperi(E) Ramanathampuram Pudur Pandiyapuram Davishpuram Rajapallam Arokayipuram Mappali Urani Silverpuram Chinakannupur SBI Colony Seetapuram Nagar Levinjpuram PNT Colony (S) Periyanayakapuram

S F B F S S S B F B F S B S F S S F S F S S B S B B S F S

S F S F S S S S F S F S B S B S S F S F S S S S B B B F S

S F S F S S S S F S F S B S F S S F S S S S S S B S S F B

B F B F S S S S F B F B B B F S S F S F S S S S B S S F S

F fresh water for TDS < 1500 mg/l, Sr < 1600 μg/l, B < 600 μg/l and Li < 14 μg/l; B brackish water for TDS ranges from 1500–3000 mg/l, Sr ranges from 1600–5000 μg/l, B ranges from 600–700 μg/l and Li ranges from 14–28 μg/l; and S saline water for TDS > 3000 mg/l, Sr > 5000 μg/l, B > 700 μg/l and Li > 28 μg/l


Using these T values, calculated SMI values (from Eq. 4) varied from 0.05 to 22.40. It was 3,000 μS/cm and SMI > 1.18, whereas fresh waters were estimated to be about 21% with EC < 1,500 μS/cm, Cl− < 200 mg/l (WHO 1984) and SMI ≤ 1.18. The SMI values were >8.00 for the groundwater samples collected from the industrial as well as seawater intrusion affected area. Identification of saline zones using trace elements Seawater intrusion is a common phenomenon in coastal aquifers. The study area was not also an exceptional case, because it is located along the Bay of Bengal. The trace elements of groundwater revealed especially good correlations between TDS, and Li (0.89), Sr (0.79), and B (0.71; see Fig. 11). In general, the Sr and B concentrations are low in fresh groundwater (Mondal et al. 2010b), but high in brackish and saline waters (see Table 5). The effects of seawater intrusion were comparatively less in the northern and southwestern parts close to Nayinapuram (W-2), Vadakakka Sulukkanpatti (W-9), Kailashpuram (W-11), and Pudur Pandiyapuram (W-18) villages. Groundwaters in wells W-5, -6, -7, -16, -17, -19, -21, -22, and 24 contained comparatively higher concentrations of Sr, B, as well as Li, and revealed more seawater intrusion/anthropogenic pollution. Freshwater zones were identified by strontium (Sr), boron (B), and lithium (Li), which were compared with TDS values (presented in Table 5). The fresh waters were only 21% of the samples. This indicated

547

the applicability of strontium, boron, lithium as useful parameters for the identification of fresh groundwater resources in coastal aquifers.

Conclusions The hydrochemical characteristic of coastal aquifers in Tuticorin, Tamil Nadu, seems to be influenced by various processes together with seawater mixing, anthropogenic contamination, and water–rock interaction, as indicated by very wide ranges and high standard deviations of most hydrochemical parameters, such as TDS, Cl− , SO42− , Mg2+ , and Na+ exceeding the limit of drinking water standard (WHO 1984). The mean values of toxic elements Pb and As are 1.93 and 3.23 times, respectively, whereas the average Mn concentration is 1.39 times higher than the permissible limit. Thus it indicates that groundwaters are significantly degraded and suffer from extensive salinization due to anthropogenic pollution as well as seawater inputs. In order to expound the causes of groundwater chemistry change, we attempted an integration of multiple methods, such as graphical interpretation, geostatistical analysis, estimation of ionic changes and seawater mixing index for the acquired hydrochemical data. The cross plot of HCO3− /Cl− and TDS shows about 62% analyzed samples are saline with molar ratios HCO3− /Cl− of 2,000 mg/l. It may indicate that groundwater with high TDS concentrations is enriched with Cl− due to seawater intrusion. The correlation coefficients among the chemical constituents with TDS show that the groundwater quality is deteriorated mainly due to external sources. Principal component analysis shows that Mg2+ , K+ , Ca2+ , Cl− , and Na+ have high positive loadings on factor I, and SO42+ on factor II, whereas HCO3− on factor III. These loading factors indicate that three different contributions are involved in the hydrochemical compositions. The cross plots of three component scores indicate that there are two distinct sets of samples, which are separated out from the cluster. One set of samples are W-5, -6, -7, and -8, which principally are affected by industries. The second set of samples (i.e.,

548

W-16, -17, -19, and -24) are influenced by seawater intrusion. The mixing rate of seawater intrusion in average during the sampling period is about 5.81%. The most dominant process that takes place with the freshwater–seawater mixing is the cation exchange which occurs mainly as a direct exchange between Na+ with Ca2+ and Mg2+ . In the case of ionic changes of SO42− , it is >25.0 meq/l for the groundwater samples collected from industrial area, where gypsum had been dumped. The SMI values vary from 1.98 to 22.40 with the TDS range, 1, 692 to 12, 310 mg/l for NaCl-type waters obtained towards the sea coast. It indicates that about 62% of the sampled waters are saline with EC > 3,000 μS/cm and SMI values > 1.18. Further, it is confirmed with the aid of a few trace elements (i.e., Sr, B, and Li), which reveal that there are only 21% fresh groundwater samples in the study area, indicating the applicability of strontium, boron, and lithium as useful parameters for the identification of fresh groundwater resources in coastal aquifers. Thus, principal component analysis, ionic changes, seawater mixing index and sensitive trace elements of groundwater provide confidence in identifying different hydrochemical processes and seawater intrusion in coastal aquifers. The information consequently obtained represents a base for future hydrochemical work that will help in the planning, protection, and decision-making regarding groundwater management in coastal aquifers. Acknowledgements This work was performed, in part, under the BOYSCAST Fellowship of the first author funded by Department of Science & Technology (Government of India), New Delhi (Ref. No. SR/BY/A-05/2008, Date: 16–19th January 2009) and also partially by Steralite Industrial (India) Ltd. (Ref. No. SILL/HSE/GWM/06/07, Date: 8th April 2006), Tuticorin, Tamil Nadu on Groundwater Program. The two anonymous reviewers had suggested their constructive comments to improve the article. The authors are thankful to them.

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