CHAPTER 3 HYDROCHEMISTRY AND SOURCES OF DISSOLVED SOLIDS IN GROUNDWATERS OF SEMI-CONFINED AND UNCONFINED AQUIFERS

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1 CHAPTER 3 HYDROCHEMISTRY AND SOURCES OF DISSOLVED SOLIDS IN GROUNDWATERS OF SEMICONFINED AND UNCONFINED AQUIFERS 3.1 INTRODUCTION In several coastal aquifers, fresh groundwater is subjected to various degrees of salinization, the latter being the widespread form of water contamination (Richter and Kreitler, 1993). Salinization of coastal groundwater is attributed to various sources, which include: sea water, relict connate saline (sea) water and Cl salts bearing sedimentary rocks of marine origin in aquifers, brine/saline groundwater underlying fresh water aquifers upconed along pumping wells as a consequence of excessive extraction of fresh groundwater, irrigation return flow and other anthropogenic inputs (Calvache and PulidoBosch, 1997; Gimenez and Morell, 1997; Petalas and Diamantis, 1999; Zhang and Dai, 2001; Cardona et al., 2004; Petalas et al., 2009). In plan view, the salinized groundwater bodies of coastal aquifers exhibit diversified patterns and their evolution is controlled by regional and local factors, which include ground and water table elevations visàvis sea level, type of recharge, groundwater flow patterns and hydrodynamic conditions, groundwaterseawater interface, end member sources of saline water, geological set up and mineralogical composition of the aquifer sediments, faults/fractures traversing aquifers, and patterns and intensity of groundwater extraction (e.g., Oetting et al., 1996). For the purposes of groundwater flow modeling and simulation of sea water intrusion, geochemical investigations were taken up to gain knowledge on the processes of mixing of seawater with fresh groundwater by understanding the types of water present, chemical facies, ion exchange reactions and sources of dissolved solids in the groundwater of the study area. 3.2 METHODOLOGY To understand the hydrochemistry and sources of solutes in the groundwaters of the present study it is necessary to know the hydrochemical composition of the groundwater. For this purpose, during October 2011, 70 groundwater samples from 70 bore wells drilled in semiconfined aquifer (Figure 3.1) and 40 groundwater 51

2 samples from 40 bore wells drilled in unconfined aquifer (Figure 3.2) were collected (in duplicate) in precleaned polypropylene sampling bottles (1 liter capacity). Before collecting the water samples, water was pumped out from bore wells for about 10 minutes to remove stagnant water. Water was filtered through 0.45 μm Millipore membrane filters to separate suspended particles. Water samples meant for cation analyses were acidified with HNO3 to decrease the ph. Electrical Figure 3.1 Plan view of the semiconfined aquifer with locations of 70 bore wells (sampling locations) conductivity (EC μs/cm) and ph were measured in the field using precalibrated portable conductivity and ph meters. Chemical analysis of the water samples was carried out at MRWA, Sari city, Mazandaran Province, North Iran, following the analytical procedures suggested by the American Public Health Association (APHA, 1995). During the chemical analysis, Ca 2+ and Mg 2+ were determined titrimetrically using standard EDTA. Na + and K + were determined by flame photometry. Cl was estimated by standard AgNO 3 52

3 titration method. HCO 3 was determined by titration with HCl. SO 2 4 was determined by UV spectrophotometer. NO 3 was determined using Cadmium column reduction method. TDS and TH were estimated using the following formulae (Raghunath, 2007): TDS (mg/l) = Ca 2+ + Mg 2+ + Na + + K HCO 3 + SO 4 + Cl + NO 3 (all ions in mg/l) TH (mg/l) = Sum of milliequivalent/l of Ca 2+ and Mg Charge balance errors (CBC) were calculated using the following formula (Freeze and Cherry 1979) and were found within the permissible limit of ±10%. CBC = (ΣZmc Σ Zma) / (ΣZmc + Σ Zma)] 100 Where Z is the ionic valence, mc is the molarity of cation species and ma is the molarity of anion species. Physicochemical characteristics of groundwater samples (n=70) of the semiconfined aquifer are provided in table 3.1 and 3.6 and the same of the groundwater samples (n= 40) from the unconfined aquifer are given in table The following pages of this chapter provide separately the details on the hydrochemistry and sources of dissolved solids in groundwaters of semiconfined aquifer and unconfined aquifer. SEMI CONFINED AQUIFER 3.3 GROUNDWATER TYPES While dealing with hydrological studies on groundwaters of coastal aquifers extending upto to the boundary of coast lines, it is mandatory to check the presence or otherwise of saline water bodies (pockets) in aquifers. In the study area the northern boundary of the semiconfined aquifer extends upto the southern coastline of the Caspian sea, hence it is necessary to check and distinguish groundwater types present in the semiconfined aquifer. Hydrochemical data of the groundwater in the semiconfined aquifer of the study area (Table 3.1, 3.11) reveals the presence of variously salinized water bodies. In the present study, the values of EC, TDS and Cl /(CO 2 3 +HCO 3 ) molar ratio (Simpson s ratio) have been considered to demarcate the variously salinized groundwater pockets in the freshwater aquifer. According to EC based classification of Saxena et al. (2003), the groundwater from 53 bore wells (EC <1500 µs/cm), 13 bore wells (EC = 1500 to 3000 µs/cm) and 4 bore wells (EC >3000 µs/cm), 53

4 respectively belongs to (1) fresh, (2) brackish, and (3) salinewater categories. According to TDS based salinity classification of Rabinove et al. (1958), the groundwater from 54 bore wells (TDS <1000 mg/l), and 16 bore wells (TDS = 1000 to 3000 mg/l) belongs respectively to (1) fresh and (2) slightly salinewater categories. According to Simpson s 5 fold classification based on the values of the Cl /(CO HCO 3 ) molar ratio, 54, 8, 5 and 3 bore wells in the study area are yielding, respectively (1) fresh (ratio = <0.5), (2) slightly contaminated (ratio = 0.5 to 1.3), (3) moderately contaminated (ratio = 1.3 to 2.5), and (4) highly contaminated (ratio = 2.5 to 15.5) water categories(todd, 1953). If the above 3 parameters (viz., EC, TDS and Simpson s ratio) are taken together into consideration, then the groundwater drawn Figure 3.2 Plan view of unconfined aquifer showing the locations of 40 bore wells (sampling locations) from 50 bore wells can be classified as fresh/nonsaline type and the same pumped from the remaining 20 bore wells can be categorized as variously salinized (contaminated) water type (hence forth will be referred to as salinewater type). It is to be noted here that even though all salinewater samples (n=20) have Simpson s ratio 54

5 > samples among them have EC values <1500 µs/cm and TDS content <1000 mg/l. In another sample only TDS content is <1000 mg/l. Groundwater of the present study contains low concentrations of NO 3 (av. = 0.5 meq/l), hence, the observed contamination has to be attributed essentially to seawater influx. Figure 1.2 shows the areal distribution of fresh groundwater and variously salinized groundwater of the semiconfined aquifer. Salinewater zone, starting from eastern part of the coast line (northeastern corner of the semiconfined aquifer) extends towards the center of the aquifer along NESW direction for a distance of more than 40 km. Apart from the continuous salinewater body, the semiconfined aquifer also possesses an isolated salinewater pocket in continuation of the above mentioned salinewater body (Figure 1.2). It is not clear whether the salinewater bodies of the semiconfined aquifer was created entirely by seawater intrusion or also by another source (upconing of salinewater/brine underlying the semiconfined aquifer along pumping bore wells as a result of excessive extraction of the groundwater). The NE SW trend of the salinewater body is at an angle to the groundwater s south to north flow direction. It is not clear whether the seawater intrusion was caused due to excessive pumping of groundwater and/or due to the presence of a NE SW trending highly permeable zone (e.g., normal fault) at the site, now occupied by the salinewater body. 3.4 FRESH GROUNDWATER Physicochemical characteristics Physicochemical characteristics of 50 samples of fresh groundwater are provided in Table 3.1 Fresh groundwater, in terms of meq/l, is characterized by Ca 2+ >Na 2+ >Mg 2+ >K + and HCO 3 >Cl >SO 2 4 >NO 3. Average contribution of individual cation to total cations is: 59.18% Ca 2+, 22.26% Na +, 17.18% Mg 2+, and 0.96% K +. On an average, anions are made up of 70.09% HCO 3, 21.12% Cl, 8.23% SO 2 4 and 0.54% NO 3. Average content of (Ca 2+ + Mg 2+ ) (7.11 meq/l) is higher than that of (Na + +K + ) (2.20 meq/l) and average content of (HCO 3 + SO 2 4 ) (7.23 meq/l) is higher than that of (Cl + NO 3 ) (2.00 meq/l). Fresh groundwater contains low concentration of total ions (av.=18.54 meq/l), but it has significant amounts of Na + (av.=2.11 meq/l) and Cl (av.=1.95 meq/l) which together, on an average constitutes 21.89% of the total dissolved ions. 55

6 Table 3.1 Physicochemical characteristics of fresh groundwater samples (n=50) BW A * ph EC TH TDS Ca 2 Mg 2+ Na + K + HCO 3 SO4 2 Cl NO 3 No Av A*= Depth of the bore wells below water table level (m). All ions, TH and TDS are in mg/l; EC: us/cm. 56

7 3.4.2 Natural mechanisms controlling the hydrochemistry Plots of Gibbs ratios of groundwater samples in Gibbs diagrams (Gibbs, 1970) can provide information on the relative importance of three major natural mechanisms controlling water chemistry: (1) atmospheric precipitation, (2) mineral weathering, and (3) evaporation and fractional crystallization. On the bivariate TDS versus Gibbs ratio [weight ratio of (Na + +K + )/(Na + +K + +Ca 2+ )] diagram (Gibbs diagram), the fresh groundwater samples plot essentially in rock dominance field but very close to the boundary line between rock dominance and evaporation dominance. Only a few samples plot in the evaporation dominance field (Table 3.2; Figure 3.3). This data suggest that the hydrochemistry of the groundwater of the semiconfined aquifer is controlled mainly by chemical interaction between aquifer rocks and groundwater, and to some extent, by evaporative concentration processes, the latter operating essentially near the coastal region, where the aquifer is unconfined and water table is at shallow levels. Table 3.2 Gibbs ratio and Chloroalkaline indices of fresh groundwater samples Bw. Gibb's Gibb's CAI1 CAI2 Bw. Gibb's Gibb's CAI1 CAI2 Ratio 1 Ratio 2 Ratio 1 Ratio

8 Total disolved solids (TDS) Na + +K + /Na + +K + +Ca 2+ (mg/l) Figure 3.3 Bivariant TDS versus Gibbs ratio [weight ratio of (Na + +K + )/( Na + +K + +Ca 2+ )] showing the natural mechanisms controlling the hydrochemistry of the fresh groundwater of the semi confined aquifer Hydrochemical facies Hydrochemical facies of groundwater can be evaluated by plotting the chemical analysis data on Piper s trilinear diagram (Piper, 1944). On the diamond shaped Piper diagram the fresh groundwater samples (n=50) plot in the fields 1, 2, 3, 5 and 9 (Figure 3.4). Among them, 47 samples contain alkaline earths in excess of alkalis. In 3 samples alkalis exceed alkaline earths. In 48 samples weak acid (HCO 3 ) exceeds strong acids (Cl and SO 2 4 ). In 2 samples strong acids exceed weak acid. In 45 samples carbonate hardness (secondary salinity) exceeds 50 %. In 5 samples none of the cation or anion pairs exceeds 50 %. Cation facieswise, 41 samples are calcium type, 3 samples are sodium type and in 6 samples none of the cation dominates. Anion facieswise, 49 samples are bicarbonate type and in one sample none of the anions dominates. Hydrochemical facieswise, 45 samples belong to CaMgHCO 3 type, 3 samples, to mixed CaNaHCO 3 type, and 2 samples, to CaMgCl type. Hydrochemical data of the fresh groundwater samples fed to Aquachem software programme revealed the presence of only two subhydrochemical facies: Ca 2+ HCO 3 (n=45) and Na + HCO 3 (n=5). 58

9 Figure 3.4 Piper triliner diagram showing the hydrochemical characteristics of fresh groundwater ( ) and saline groundwater (+) samples of the semiconfined aquifer Ion exchange reactions Ion exchange reactions involved by groundwater samples can be deciphered based on the values of their ChloroAlkaline Indices (CAI). CAI1 and CAI2 are calculated according to the following formulae (Schoeller 1977): Ion exchange reactions between groundwater and exchanger (mainly clay minerals) of aquifer matrix, after taking into account of cation and proton exchange, ph variations and carbonate reactions, can be expressed by the following two equations (Cardona et al., 2004). Direct cation exchange reaction (Ca 1x Mg x ) CO 3 + H + +Na 2 clays 2Na+HCO 3 +(Ca 1x Mg x ) clays Reverse cation anion exchange reaction 2Na+HCO 3 +(Ca 1x Mg x ) clays (Ca 1x Mg x ) CO 3 +H + +Na 2clays Among 50 fresh groundwater samples of the semiconfined aquifer, 23 samples yielded positive values of CAI (Table 3.2). This data suggest that part of the 59

10 Na + (and/or K + ) content in 23 fresh groundwater samples is derived from direct cation exchange reaction, during which the Na + (and/or K + ) of clays of the aquifer matrix was exchanged with Ca 2+ (and/or Mg 2+ ) of the fresh groundwater, thereby enriching the groundwater with Na + (and/or K + ) and HCO 3, at the expense of its Ca 2+ (and/or Mg 2+ ) content. Out of the remaining 27 fresh groundwater samples, 26 samples are characterized by negative values of CAI (Table 3.2), which suggest the involvement of reverse cationanion exchange reaction. During the ion exchange reaction, part of the Na + (and/or K + ) content of the fresh groundwater was exchanged with Ca 2+ (and/or Mg 2+ ) of the clays of the aquifer matrix, thereby enriching the groundwater with Ca 2+ (and/or Mg 2+ ) and hydrogen ion and depleting its Na + (and/or K + ) and HCO 3 contents Interelemental relationships Correlation coefficient is commonly used to measure and establish the relationship between two variables. The sources of dissolved solids in groundwater can also be evaluated from the relative abundance of individual ions and their interelemental correlation (Singh et al., 2011). Correlation analysis was carried out between 12 variables of the physicochemical data of 50 fresh groundwater samples, following the methodology of calculation of Spearman s rank correlation coefficient matrices, which is based on the ranking of the data and not their absolute values (Kumar et al., 2006) (Table 3.3). Among the cations, Ca 2+ shows strong positive correlation with HCO 3 (r 2 = 0.75) and K + (r 2 = 0.73) and moderately weak positive correlation with SO 2 4 (r 2 = 0.44). Mg 2+ is weakly correlated with SO 2 4 (r 2 = 0.20). Na + shows strong positive correlation with Cl (r 2 = 0.79), good positive correlation with K + (r 2 = 0.60) and weak positive correlation with HCO 3 (r 2 =0.27). K + exhibits strong positive correlation with HCO 3 (r 2 = 0.75), Ca 2+ (r 2 = 0.73) and Cl (r 2 = 0.72), good positive correlation with Na + (r 2 = 0.60) and weak positive correlation with SO 2 4 (r 2 = 0.44). Strong positive correlation between (1) Ca 2+ and HCO 3, (2) K + and HCO 3, (3) K + and Na +, and (4) Ca 2+ and K + suggest carbonic acid aided dissolution/weathering of carbonates and aluminosilicate minerals (e.g., augite, calcic plagioclase, bytownite, labradorite and alkali feldspar). Strong correlation between 60

11 Na + K + Cl may suggest the presence of minor amounts of either residual connate water and/or Cl salts in the Quaternary sediments of the aquifer. 2 The observed weak positive correlation of SO 4 with Ca 2+ and Mg 2+ suggest three other possible additional sources of these ions: (1) sulfuric acid aided weathering of dolomite, (2) dissolution of gypsum and Mgsulfate, and (3) residual connate salinewater. Table 3.3 Spearman s Rank Correlation coefficient matrices of variables of the physicochemical data of the fresh groundwater samples (n=50) EC 1.00 EC PH TDS TH Ca 2+ Mg 2+ Na + K + HCO 3 PH TDS TH Ca Mg Na K HCO SO Cl NO SO Factor analysis Hydrochemical data of 50 fresh groundwater samples were subjected to Principle Component Analysis (PCA) using SPSS and following the procedure provided by Sharma (1996) and Mondal et al. (2010). Factor analysis of the physicochemical parameters (ph, EC, TH, TDS, Ca 2+, Mg 2+, Na +, K +, HCO 3, SO 2 4,Cl and NO 3 ) was carried out to quantify the contributions from various possible sources, including natural chemical weathering, residual connate salinewater/ dissolution of Cl salts and anthropogenic inputs, to the chemical composition of the fresh groundwater. Table 3.4 summarizes the results of the PCA. For the present study, factors with eigenvalues higher than 1 are considered. Following this procedure, 3 independent factors were extracted, which accounted for 79.89% of the total variance of the original data set. Factor 1 accounts for 42.10% of the total variance with high positive loadings of EC, TH, TDS, Ca 2+, K +, HCO 3 and modest positive loading of NO 3. Factor 2 accounts for 24.59% of total variance with high positive loadings of 61

12 EC, TDS, Na + and Cl and moderate positive loading of K +. Factor 3 accounts for 13.19% of total variance with strong positive loadings of Mg 2+ and SO 2 4. Positive loadings of the factor 1 (viz., Ca 2+, K +, HCO 3 and NO 3 ) indicate essentially carbonate and silicate mineral weathering and anthropogenic nitrite inputs. Positive loadings of the Factor 2 (viz., Na +, K + and Cl ) suggest the possible presence of residual connate salinewater or Cl salts in the Quaternary sediments of the aquifer. Positive loadings of the Factor 3 (viz., Mg 2+ and SO 2 4 ) suggest two possible sources for acquisition of these ions: (1) Sulfuric acidaided dissolution of dolomite, and/or dissolution of Mg sulfate mineral of the aquifer matrix and (2) residual connate saline groundwater. Table 3.4 Factor analysis of the physicochemical data of the fresh groundwater samples (n=50) Rotated Matrix of Variables Variables Principal Components Factor 1 Factor 2 Factor 3 PH EC TH TDS Ca Mg Na K HCO 3 SO Cl NO Eigen values % of Variance Cumulative % Sources of dissolved solids It is well known that mineral dissolution/weathering, ion exchange processes and inputs from atmospheric, soil and anthropogenic sources are the major solute acquisition mechanisms controlling the concentration of chemical constituents in groundwater (Berner and Berner, 1987). In the study area, considerable amounts of residual connate salinewater in the Quaternary sediments of the freshwater aquifer 62

13 possibly constitutes another additional but minor source for Na +, K +, Cl, Ca 2+, Mg 2+, HCO 3 and SO 2 4 ions. During chemical weathering of carbonates, silicates and aluminosilicates, bulk of the protons are provided by two reactions involving dissolution of atmospheric and soil sourced CO 2 and oxidation of pyrite. The proportion of HCO 3 and SO 2 4 in groundwater reflects the relative abundance of the two sources of protons during chemical weathering (Singh et al., 2007). The relative importance of the two major proton producing reactions (viz., carbonation and sulfide oxidation) can be evaluated on the basis of the values of HCO 3 /(HCO 3 +SO 2 4 ) equivalent ratio (Cratio, Brown et al., 1996) of the groundwater samples. The values of Cratio of fresh groundwater samples vary from 0.61 to 0.99 (av. = 0.90). These values suggest that carbonic acid aided weathering of carbonates and aluminosilicates has played major role in the acquisition of Ca 2+, Mg 2+, K + 2 and HCO 3 ions. If the SO 4 content in the fresh groundwater is derived partly from oxidation of pyrite, then sulfuric acid aided weathering of carbonates can be considered as another additional but minor source of Ca 2+ and Mg 2+ and significant source of SO 2 4. The other minor source of SO 2 4 in the fresh groundwater has to be attributed to residual connate salinewater fraction of the freshwater aquifer. In groundwaters dissolved weathering products of calcite, dolomite and carbonates (mixture of calcite and dolomite ranging from 0 to 100% of each mineral) can be quantified based on the weathering agent involved. Table 3.5 provides the values of the molar ratios of Ca 2+ / HCO 3, Ca 2+ / SO 2 4, Mg 2+ / HCO 3 and Mg 2+ / 2 SO 4 of the dissolved ionic load derived from weathering of calcite, dolomite and carbonates by two different weathering agents, viz., carbonic acid and sulfuric acid. In the fresh groundwater samples of the semiconfined aquifer the values of Ca 2+ / HCO 3 molar ratio vary from 0.39 to 1.34 (av. = 0.85); Ca 2+ / SO 2 4 molar ratio, from 2.05 to 46 (av. = 18.20); Mg 2+ / HCO 3 molar ratio, from 0.12 to 0.54 (av. = 0.26); and Mg 2+ / SO 2 4 molar ratio, from 0.35 to (av. = 5.22). The above data reveal that in all groundwater samples the average values of Ca 2+ / HCO 3, Ca 2+ / SO 2 4, Mg 2+ / HCO 3 and Mg 2+ / SO 2 4 molar ratios are higher than the theoretically deduced values of the same molar ratios of the dissolved ionic load derived from weathering of calcite, dolomite and carbonates (mixture of calcite and dolomite) (Table 3.5). The obtained values of the above molar ratios of the fresh groundwater samples indicate that alkaline earths were derived not only from 63

14 carbonate mineral weathering, but also from other sources (e.g., aluminosilicate weathering, reverse cationanion exchange reaction). Weathering of aluminosilicates is one among the major sources of Ca 2+ and Mg 2+ in the fresh groundwater of the study area. Solution products of silicate weathering are difficult to quantify owing to the fact that the weathering of silicates incongruently generates solid phases (mostly clays) along with dissolved species (Singh et al., 2007). As mentioned earlier, 26 out of 50 fresh groundwater samples indicate the involvement of reverse cationanion exchange reaction during waterrock interaction. These reactions provided substantial ionic load of alkaline earths to the fresh groundwater. The values of Ca 2+ /(Ca 2+ + SO 2 4 ) molar ratio of the fresh groundwater samples are > 0.5 and this data rules out the possibility of derivation of Ca 2+ and SO 2 4 through dissolution of gypsum. Mg 2+ shows weak correlation with SO 2 4 (r 2 = 0.20) and suggests the remote possibility of minor contribution of Mg 2+ and SO 2 4 from dissolution of Mg sulfate. Table 3.5 Values of the molar ratios of dissolved ions (viz., Ca 2+ / HCO 3, Ca 2+ / SO 2 4, Mg 2+ / HCO 3 and Mg 2+ 2 / SO 4 ) derived from weathering of calcite, dolomite and carbonates (mixture of calcite and dolomite in proportions ranging from 0 to 100% of each mineral) Mineral(s) Weathering agent Values of cation/anion molar ratio Calcite H 2 CO 3 Ca 2+ /HCO 3 = 0.50 Dolomite H 2 CO 3 Ca 2+ /HCO 3 = 0.25 Calcite H 2 SO 4 Ca 2+ /SO 2 4 = 1.00 Dolomite H 2 SO 4 Ca 2+ /SO 2 4 = 0.50 Dolomite H 2 CO 3 Mg 2+ /SO 2 4 = 0.25 Dolomite H 2 SO 4 Mg 2+ /SO 2 4 = 0.50 Carbonates* H 2 CO 3 Ca 2+ /HCO 3 = from 0.25 to 0.50 Carbonates* H 2 SO 4 Ca 2+ /SO 2 4 = from 0.50 to 1.00 Carbonates* H 2 CO 3 Mg 2+ / HCO 3 = from 0 to 0.25 Carbonates* H 2 SO 4 Mg 2+ /SO 2 4 = from 0 to 0.50 Carbonates*: Mixture of calcite and dolomite in proportions ranging from 0 to 100% of each mineral. Data mainly from Sarin et al. (1989) The values of (Ca 2+ +Mg 2+ )/HCO 3 equivalent ratio mark the upper limit of bicarbonate input from weathering of carbonates (Stallard and Edmond, 1983; Richter and Kreitler, 1993). On the bivariate (Ca 2+ + Mg 2+ ) versus HCO 3 equivalent diagram majority of the fresh groundwater samples plot above 1:1 line indicating that (Ca 2+ + Mg 2+ ) content is in excess of alkalinity (Diagram is not provided). On the bivariate (Ca 2+ + Mg 2+ ) versus (HCO 3 +SO 2 4 ) equivalent diagram, out of 50 groundwater 64

15 samples 23 samples plot above 1:1 equiline, 23 samples, below 1:1 equiline and 4 samples on 1:1 equiline. (Figure 3.5a).This diagram suggests that in 23 samples (HCO 3 +SO 2 4 ) content is higher than that of (Ca 2+ +Mg 2+ ) and in 23 samples (Ca 2+ + Mg 2+ ) dominates over (HCO 3 +SO 2 4 ). The excess positive charge of (Ca 2+ +Mg 2+ ) in 23 samples is balanced by Cl and / or NO 3 (Cerling et al., 1989; Fisher and Mullican, 1997). On the bivariate (Ca 2+ + Mg 2+ ) versus T + Z equivalent diagram all samples of the fresh groundwater (n = 50) plot below 1: 1 equiline, the departure being more pronounced at high T + Z concentration (Figure 3.5b). This feature suggests increasing contribution of Na + and K + with increasing dissolved solids. The Na + and K + content in ground waters is generally attributed to one or more of the following sources: (1) rain water, (2) dissolution of Cl salts, (3) weathering of alkali feldspars, (4) input from cation exchange reactions, (5) seawater spray and (6) anthropogenic inputs. In the fresh groundwater samples, Na + and K +, on an average, contribute 23.62% of the total cations. Average value of (Ca 2+ + Mg 2+ ) / (Na + + K + ) equivalent ratio of the fresh groundwater samples is This data suggests that the chemical composition of the fresh groundwater is significantly influenced by weathering of alkali feldspars. The values of Na + / Cl molar ratio of 50 fresh groundwater samples vary from 0.47 to 2.42, out of which the values of Na + / Cl molar ratio of 26 samples are >1; 24 samples, <1; one sample, equals one. This data is graphically shown in bilateral Na + versus Cl equivalent diagram (Figure 3.6). Values of Na + / Cl molar ratio >1 indicate derivation of Na + from weathering of alkali feldspars (Stallard and Edmond, 1983; Meybeck, 1987). Hence, bulk of the Na + content in 26 out of 50 fresh groundwater samples is the product of alkali feldspar weathering. These samples (barring one) are characterized by negative values of CAI, thus implying the involvement of reverse cationanion exchange reaction. This data indicates that prior to reverse cationanion exchange reactions, the fresh groundwater representing these samples (n=25) had still higher concentration of Na + (± K + ). Groundwater samples (n=24), having the values of Na + / Cl molar ratio <1, exhibit positive CAI values. This data indicate derivation of Na + from clays of the aquifer matrix. The Na content in the remaining single groundwater sample, which has the value of Na + / Cl molar ratio = 1, possibly derived from dissolution of halite or rain water. Among the 50 fresh groundwater samples, the values of Na + /Cl molar ratio of 8 samples are very close to 1 (range = 0.90 to 1.10). These samples may 65

16 possibly also suggest minor contribution of Na + to the fresh groundwater from atmospheric sources/halite dissolution. In the fresh groundwater the average concentration of Na + (2.11 meq/l) is significantly higher than that of K + (0.09 meq/l). Very low concentration of K + may be due to insignificant content of potash feldspar in the aquifer, high resistance to weathering of potash feldspar in comparison with Nabearing silicates and preferential cation exchange of Na + of clay minerals with alkaline earths of the fresh ground water. In groundwaters Cl content is generally attributed to one or more of the following sources: (1) rain water, (2) chloride from seawater trapped in sediments, (3) solution of halite and other Cl salts, (4) evaporative concentration of chloride contributed by rain or snow, (5) solution and downward seepage of dry fallout from atmosphere, and (6) marine aerosols. In the fresh groundwater samples, the Na + content varies from 0.6 to 5.2 meq/l and Cl content, from 0.5 to 4.7 meq/l. This data may suggest contribution of Cl from meteoric water and residual connate salinewater in sediments of the aquifer. Average Na + /Cl molar ratio (1.07) of the fresh groundwater is higher than that of marine aerosols (Zhang et al., 1995) and suggests absence or insignificant contribution of Cl from marine aerosols. Fresh groundwater samples contain low concentration of NO 3 (av = 0.05 meq/l). NO 3 has no lithological source (Jeong, 2001), hence its presence in fresh groundwater samples has to be related to nitrogenous fertilizers, nitrification of organic N and NH 4, industrial effluents, human and animal waste and biocombustion (Prospero and Savoie, 1989; Carling and Hammar, 1995; Min et al., 2003). 66

17 14 12 a 20 b Ca 2+ + Mg Ca 2+ + Mg HCO 3 + SO 4 2 Figure 3.5 Bivariate (Ca 2+ +Mg 2+ ) versus (HCO 3 + SO 4 2 ) (a) and bivariate (Ca 2+ +Mg 2+ ) versus Tz + (b) equivalent diagram of the fresh groundwater samples (n = 50) Tz + 6 Na Cl Figure 3.6 Bivariate Na + versus Cl equivalent diagram of the fresh groundwater samples (n = 50) 3.5 SALINE GROUNDWATER Physicochemical characteristics Physicochemical characteristics of 20 saline groundwater samples are provided in Table 3.6. In the saline groundwater the order of relative abundance of ions, in terms of meq/l, are: Na + >Ca 2+ >Mg 2+ >K + and Cl >HCO 3 >SO 2 4 >NO 3. Cations, on an average, are composed of 59.63% Na +, 27.17% Ca 2+, 12.65% Mg 2+ and 0.53 K +. Anions are made up of 61.91% Cl, 31.76% HCO 2 3, 6.10% SO 4 and 67

18 0.22% NO 3. Average meq/l content of (Na + + K + ) (13.46 meq/l) is higher than that of (Ca 2+ + Mg 2+ ) (8.91 meq/l) and the average meq/l content of (Cl +NO 3 ) (13.85 meq/l) is higher than that of (HCO 3 +SO 2 4 ) (8.44 meq/l). Average total concentration of ions is meq/l, which is 2.40 times higher than that of the fresh groundwater (av. = meq/l). Table 3.6 Physicochemical characteristics of the saline groundwater samples (n=20) Bw A * ph EC TH TDS Ca 2 Mg 2+ Na + K + HCO 3 2 SO 4 Cl NO 3 No Av A*= Depth of the bore wells below water table level (m). All ions, TH and TDS are in mg/l; EC:uS/cm Hydrochemical facies Hydrochemical characteristics of the saline groundwater are evaluated based on the region of plots of the hydrochemical data of the saline groundwater samples on Piper s diagram (Piper, 1944). On the Piper s diagram 20 saline groundwater samples plot in the fields 1, 2, 3, 4, 5 and 7 (Figure 3.4). Among them, 16 samples contain alkalis in excess of alkaline earths. In 4 samples alkaline earths exceed alkalis. In 16 samples strong acids (Cl and SO 2 4 ) exceed weak acid (HCO 3 ) and in the remaining 4 samples weak acid exceeds strong acids. In 15 samples noncarbonate alkali (primary salinity) exceeds 50%. In 3 samples carbonate hardness (secondary salinity) exceeds 50%. In 2 samples none of the cation or anion pairs exceed 50%. Cation facieswise, 15 samples are sodium type, 4 samples are no dominate type and 1 sample is calcium type. Anion facies wise, 15 samples are chloride type, 3 samples are bicarbonate type and 2 samples are no dominate type. Among the 20 saline groundwater samples, 68

19 15 samples belong to Na + Cl type, 3 samples to, Ca 2+ Mg 2+ HCO 3 type, one sample, to mixed Ca 2+ Na + HCO 3 type and one sample, to mixed Ca 2+ Mg 2+ Cl type. Aquachem software program revealed the presence of 5 subhydrochemical facies in the saline groundwater: Na + Cl (n = 14), Na + HCO 3 (n = 3), Ca 2+ HCO 3 (n = 1), Ca 2+ Cl (n = 1) and Ca 2+ SO 2 4 (n = 1) Ionexchange reactions In the fresh groundwaterseawater mixing zone of coastal aquifer the resulting groundwater invariably involves a nonconservative mixing of freshwater from aquifer and saltwater from sea. When seawater intrudes fresh groundwater of an aquifer, direct cation and reverse cationanion exchange reactions take place between seawater and clay fraction of aquifer matrix, and as a result, composition of groundwater changes along its flow path (Nadler et al., 1980; Magaritz and Luzier, 1985; Appelo and Postma, 1996). During ion exchange reactions Ca 2+ exhibits comparatively higher affinity to clays (exchanger) of aquifer matrix than other cations (Na +, K + and Mg 2+ ). However, under equilibrium conditions with seawater, appreciable amounts of Mg 2+ and K + are also adsorbed on exchanger in addition to Na + (Appelo, 1994). In fresh groundwaterseawater mixing zone, cation exchange reactions may take place not only between Ca 2+ / Na + and Mg 2+ / Na + but also between Ca 2+ / K + and Ca 2+ / Mg 2+ (e.g., Aquia aquifer, Maryland, USA., Appelo, 1994). In the present study, 11 out of 20 saline groundwater samples are characterized by positive values of CAI (Table 3.7) and indicate the derivation of part of their Na + (and/or K + ) content through direct cation exchange reaction of Na + (and/or K + ) of clay fraction of the aquifer matrix with Ca 2+ (and/or Mg 2+ ) of the groundwater, thereby resulting in the enrichment of Na + (and /or K + ) and HCO 3 and depletion of Ca 2+ (and/or Mg 2+ ). Remaining 9 saline groundwater samples exhibit negative values of CAI and indicate the involvement of reverse cationanion exchange reaction. In these saline groundwater samples, reverse cationanion reaction increased the concentration of Ca 2+ (and/or Mg 2+ ) and decreased the concentration of HCO 3 and Na + (and/or K + ). 69

20 Table 3.7 Gibbs ratio and Chloroalkaline indices of the saline groundwater samples (n=20) Bw. No. Gibb's Gibb's CAI1 CAI2 Bw. Gibb's Gibb's CAI1 CAI2 Ratio 1 Ratio 2 No. Ratio 1 Ratio Interelemental relationships Spearman s rank correlation matrices were calculated between variables of 20 saline groundwater samples and the obtained correlation matrices are presented in Table 3.8. Among the cations, Na + exhibits strong positive correlation with K + (r 2 = 0.87) and Cl (r 2 = 0.74), and weak positive correlation with HCO 3 (r 2 = 0.39). K + exhibits strong positive correlation with Na + (r 2 = 0.87) and Cl (r 2 = 0.75), good to moderately good positive correlation with Mg + (r 2 = 0.61) and Ca 2+ (r 2 = 0.53) and weak positive correlation with HCO 3 (r 2 = 0.48) and SO 2 4 (r 2 = 0.31). Mg 2+ shows strong positive correlation with Ca 2+ (r 2 = 0.80), good positive correlation with K + (r 2 = 0.61) and weak positive correlation with HCO 3 (r 2 = 0.38), Cl (r 2 = 0.33) and SO 2 4 (r 2 = 0.24). Ca 2+ shows strong positive correlation with Mg 2+ (r 2 = 0.80), good to moderately good positive correlation with K + (r 2 = 0.53) and SO 2 4 (r 2 = 0.49) and weak positive correlation with HCO 3 (r 2 = 0.23) and Cl (r 2 = 0.23). The observed strong correlation between Na + K + Cl ions and their moderately good to weak correlation with Ca 2+, Mg 2+ 2, SO 4 and HCO 3 clearly project the brine / seawater signature. Weak positive correlation of Ca 2+ and Mg 2+ with HCO 3 and SO 2 4 may possibly indicate two additional sources of these ions: (1) weathering/dissolution of aluminosilicates and carbonates, and (2) dissolution of Mg sulfate. Weak positive correlation of HCO 3 with Na + (r 2 = 0.39) and K + (r 2 = 0.48) possibly also suggests the weathering of alkali feldspars. 70

21 Table 3.8 Spearman s Rank Correlation coefficient matrices of variables of the physicochemical data of the saline groundwater samples (n=20) EC PH TDS TH Ca 2+ Mg 2+ Na + K + HCO 3 SO 4 2 Cl NO 3 EC 1.00 PH TDS TH Ca Mg Na K HCO 3 SO Cl NO Factors Analysis Hydrochemical data of 20 saline groundwater samples were subjected to Principle Component Analysis (PCA) according to the procedure provided by Mondal et al. (2010). The variables used for factor analysis are: ph, EC, TH, Na +, K +, Ca 2+, Mg 2+, Cl, HCO 3, SO 2 4 and NO 3. Table 3.9 summarizes the results of the PCA. During the present study, factors with eigenvalues higher than 1 are taken into account. Following this procedure, 3 independent factors were extracted, which account for 84.42% of the total variance. Factor 1 accounts for 43.51% of the total variance with high positive loadings of EC, TDS, Na +, K +, Cl and moderate positive loading of Mg 2+. High positive loadings of variables of factor 1 represent essentially the influx of ions (Na +, K + and Cl ) during salinization process of the fresh groundwater due to intrusion of seawater. Factor 2 accounts for 26.39% of total variance with strong positive loadings of TH, Ca 2+, SO 4 2 and NO 3 and moderate positive loading of Mg 2+. Positively loaded variables of the factor 2 may be attributed to the input of Ca 2+, Mg 2+ and SO 2 4 ions from either mineral dissolution / weathering and/or seawater. Positive loading of NO 3 indicate nitrate input from anthropogenic sources. Factor 3 accounts for 14.51% of the total variance with strong loading of HCO 3. The latter indicates alkalinity of the saline groundwater. The alkaline signature is attributed essentially to rock weathering. High positive loadings of Ca 2+, Mg 2+, HCO 3, SO 2 4, Na +, Cl +, K + and NO 3 of the saline groundwater in the fresh groundwater seawater mixing zone clearly 71

22 indicate that its chemical composition is controlled by different processes and sources which include inputs from carbonate and silicate weathering, seawater and anthropogenic activities. Direct and reverse ion exchange reactions between seawater and aquifer matrix played an important role in fixing the concentrations of Ca 2+, Mg 2+, Na +, K + and HCO 3 in the groundwater of the fresh groundwater seawater mixing zone. Table 3.9 Factor analysis of the physicochemical data of the saline groundwater samples (n=20) Rotated Matrix of Variables Variables Principal Components Factor 1 Factor 2 Factor 3 PH EC TH TDS Ca Mg Na K HCO 3 SO Cl NO Eigen values % of Variance Cumulative % Seawater fraction in saline groundwater Changes in ionic concentration need to be evaluated to better understand the hydrochemical processes that have taken place in the freshwater aquifer of the study area during fresh groundwater seawater mixing process. Mondal et al. (2010) provided the methodology and explanation for calculating the changes in the ionic concentration of freshwater samples admixed with various proportions of seawater. According to them, the processes during freshwater seawater mixing can be evaluated from the calculation of the expected composition, based on conservative mixing of seawater and freshwater and then comparing the results with the actual chemical composition of the salinized groundwater samples. The seawater 72

23 contribution is used for calculating the concentration of each ion (i) in conservative mixing of seawater and freshwater and the calculations are carried out according to the following formula: where e i (in meq/l) is the concentration of specific ion (i), f sea is the fraction of seawater in mixed freshwater seawater samples, and subscripts mix, sea and fresh indicate the conservative mixture of seawater and freshwater. Any change in concentration (ionic change) i.e., e change, as a result of chemical reaction can be evaluated according to the following equation (Fidelibus, 2003). where e i sample is the actual observed concentration of specific ion in the salinized groundwater sample. The fraction of seawater is normally based on the Cl concentration of the sample. These fraction calculations were made, because Cl is assumed to be conservative tracer (Tellam, 1995) and is not usually removed from the system due to its high solubility (Appelo and Postma, 2005). The seawater fraction (f sea ) is calculated by weighing up the seawater contribution from the concentration of Cl of the sample (e Cl sample), the concentration of Cl of the freshwater (e Cl.fresh ), and the concentration of Cl of the seawater (e Cl.sea ), where concentration of Cl is expressed in meq/l. 100 f sea = % of seawater fraction in the sample. In the present study average chemical composition of 50 fresh groundwater samples of the study area is considered as that of freshwater and the chemical composition of the Caspian seawater as that of seawater for calculation purposes. The chemical composition of the water of Caspian Sea (largest land locked water body on earth) significantly differs from that of average seawater. In comparison with the ionic concentrations of average seawater (Appelo and Postma, 2005), the Caspian seawater is characterized by lower concentrations of Na +, K +, Mg 2+, SO 2 4 and Cl and higher concentrations of HCO 3 and Ca 2+ and significantly lower concentration of TDS. In comparison with the chemical composition of freshwater (Appelo and Postma, 2005), the average fresh groundwater of the present study contains higher 73

24 concentration of Ca 2+, Mg 2+, Na +, Mg 2+ and HCO 3 and lower concentrations of K +, SO 2 4 and Cl. The seawater fraction and changes in ionic concentration in 20 saline groundwater samples are calculated following the above methodology of Mondal et al. (2010) and the obtained results are provided in Table From the table it is seen that the variously salinized groundwater samples contains, on average, 7.6% seawater. The fraction of seawater in all saline groundwater samples (n = 20) are positive (Table 3.10). Table 3.10 Ionic changes (e change) of the selected ions and calculated seawater fraction in individual saline groundwater samples from fresh groundwater seawater mixing zone sample Ion change(me/l) f sample No. Ca 2+ Mg 2+ Na + K + HCO 3 2 SO Average Chemical composition (me/l) Ca 2+ Mg 2+ Na + K + HCO 3 SO 4 2 Caspian sea Fresh water f sample indicates the fraction of seawater in the mixed freshwater seawater samples. f sea= 100, Av. Of f samples = 7.6%. Cl The process of hydrochemical changes in the freshwater seawater mixing zone of coastal aquifers is complex and often displays heterogeneous pattern of studied ions (Mondal et al., 2010). The heterogeneous pattern shown by Na + and 74