chapter 3 hydrochemistry and sources of dissolved...

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CHAPTER 3

HYDROCHEMISTRY AND SOURCES OF DISSOLVED SOLIDS

IN GROUNDWATERS OF SEMI-CONFINED 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 Pulido-Bosch, 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 vis-à-vis sea level, type of recharge,

groundwater flow patterns and hydrodynamic conditions, groundwater-seawater

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 semi-confined aquifer (Figure 3.1) and 40 groundwater

52

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 semi-confined 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, Ca2+

and Mg2+

were determined titrimetrically using standard EDTA. Na+

and K+ were determined by flame photometry. Cl

- was estimated by standard AgNO3

53

titration method. HCO3- was determined by titration with HCl. SO4

2- was determined

by UV spectrophotometer. NO3 - was determined using Cadmium column reduction

method. TDS and TH were estimated using the following formulae (Raghunath,

2007):

TDS (mg/L) = Ca2+

+ Mg2+

+ Na+ + K

++0.49 HCO3

- + SO4

2- + Cl

- + NO3

- (all ions in

mg/l)

TH (mg/l) = Sum of milliequivalent/L of Ca2+

and Mg2+

× 50

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.

Physico-chemical characteristics of groundwater samples (n=70) of the semi-

confined 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 3.11. The following

pages of this chapter provide separately the details on the hydrochemistry and sources

of dissolved solids in groundwaters of semi-confined 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 semi-confined aquifer extends upto the southern coastline of the

Caspian sea, hence it is necessary to check and distinguish groundwater types present

in the semi-confined aquifer.

Hydrochemical data of the groundwater in the semi-confined 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-/(CO3

2-+HCO3

-) 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),

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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-/(CO3

2-+

HCO3-) 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/non-saline 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

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>0.5. 3 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 NO3-

(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

semi-confined aquifer.

Salinewater zone, starting from eastern part of the coast line (northeastern

corner of the semi-confined aquifer) extends towards the center of the aquifer along

NE-SW direction for a distance of more than 40 km. Apart from the continuous

salinewater body, the semi-confined 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 semi-confined aquifer was created entirely

by seawater intrusion or also by another source (upconing of salinewater/brine

underlying the semi-confined 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

3.4.1 Physico-chemical characteristics

Physico-chemical characteristics of 50 samples of fresh groundwater are

provided in Table 3.1 Fresh groundwater, in terms of meq/L, is characterized by

Ca2+

>Na2+

>Mg2+

>K+

and HCO3->Cl

->SO4

2->NO3

-. Average contribution of

individual cation to total cations is: 59.18% Ca2+

, 22.26% Na+, 17.18% Mg

2+, and

0.96% K+. On an average, anions are made up of 70.09% HCO3

-, 21.12% Cl

-, 8.23%

SO42-

and 0.54% NO3-. 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 (HCO3

-+ SO4

2-) (7.23 meq/L) is

higher than that of (Cl-+ NO3

-) (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.

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Table 3.1 Physico-chemical characteristics of fresh groundwater samples (n=50)

BW A* pH EC TH TDS Ca2- Mg2+ Na+ K+ HCO3- SO42- Cl- NO3

-

No.

1 12 7.2 1048 405 681 140.27 13.37 48.28 4.30 518.65 24.00 42.54 2.42 2 125 7.8 618 270 396 72.14 21.87 16.09 2.35 323.39 4.80 21.27 0.31

3 119 7.8 645 275 432 76.15 20.66 18.39 2.35 329.49 4.80 24.82 0.62

4 10 7.7 874 350 568 116.23 14.58 36.78 3.13 366.10 62.40 42.54 0.31

5 99 8.2 488 220 313 50.10 23.09 9.20 1.56 250.17 4.80 17.73 0.62

6 100 7.4 1048 425 681 142.28 17.01 39.08 4.30 463.73 38.40 63.81 3.22

7 22 7.6 681 315 436 92.18 20.66 9.20 2.35 360.00 4.80 21.27 2.98

8 104 7.8 713 230 463 66.13 15.80 55.18 2.74 329.49 9.60 46.09 1.43

9 11 7.1 1022 355 664 124.24 10.94 66.67 4.30 469.83 9.60 74.45 0.68

10 17 7.6 889 405 578 122.24 24.31 13.79 3.13 469.83 9.60 24.82 4.71

11 89 7.6 689 160 441 42.08 13.37 82.76 2.35 323.39 4.80 46.09 0.93

12 159 7.5 682 305 438 88.17 20.66 13.79 2.35 311.19 28.80 31.91 1.98

13 10 7.3 1153 420 749 140.27 17.01 66.67 4.30 518.65 9.60 92.18 5.70

14 11 7.5 861 385 560 132.26 13.37 18.39 3.13 317.29 72.00 60.27 4.53

15 86 7.7 938 425 610 150.29 12.15 16.09 3.52 427.12 28.80 53.18 6.70

16 134 7.8 894 385 581 126.25 17.01 25.29 3.13 427.12 9.60 53.18 2.79

17 9 7.8 905 400 588 122.24 23.09 20.69 3.52 396.61 62.40 35.45 3.53

18 122 7.6 763 250 496 60.12 24.31 57.47 2.74 244.07 62.40 74.45 0.93

19 133 7.6 779 340 498 94.18 25.52 20.69 2.74 323.39 52.80 42.54 0.19

20 174 7.6 551 195 354 60.12 10.94 34.48 1.95 237.97 9.60 42.54 0.99

21 10 7.8 926 365 602 130.25 9.72 41.38 3.52 366.10 52.80 67.36 0.50

22 108 7.6 973 365 632 120.23 15.80 52.88 3.52 353.90 19.20 116.99 2.60

25 129 7.7 902 210 586 64.12 12.15 108.05 3.52 280.68 28.80 127.63 0.68

26 13 7.0 1326 435 875 152.30 13.37 98.86 4.30 518.65 86.40 95.72 0.31

27 11 7.4 1120 435 728 142.28 19.44 52.88 4.30 323.39 163.20 81.54 2.48

29 10 7.1 1289 510 824 140.27 38.89 57.47 4.30 457.63 120.00 95.72 0.00

31 22 7.1 756 335 491 100.20 20.66 16.09 2.74 360.00 14.40 39.00 2.11

34 119 7.5 1058 265 688 78.15 17.01 114.95 4.30 329.49 19.20 159.54 0.99

35 11 7.5 895 405 582 102.20 36.46 16.09 3.13 335.59 91.20 46.09 1.43

36 110 7.5 682 310 436 104.20 12.15 11.49 2.35 329.49 9.60 35.45 2.98

37 12 7.5 773 340 502 96.19 24.31 18.39 2.74 323.39 43.20 46.09 12.4

0 38 146 7.7 1081 415 703 96.19 42.53 52.88 4.30 347.80 9.60 166.63 1.12

40 6 6.8 1423 485 939 162.32 19.44 98.86 4.30 665.09 9.60 138.27 3.91

41 82 6.9 1384 525 913 132.26 47.39 71.27 4.30 610.17 57.60 85.09 2.91

43 10 7.1 1250 445 825 152.30 15.80 78.17 4.30 366.10 177.60 92.18 7.87

47 15 7.0 1087 450 706 150.29 18.23 36.78 4.30 482.04 67.20 46.09 11.90 48 102 7.4 926 355 602 112.22 18.23 45.98 3.52 427.12 9.60 63.81 1.30

50 9 7.6 1217 350 803 120.23 12.15 114.95 4.30 463.73 28.80 134.72 0.43

51 10 7.0 1254 500 828 166.32 20.66 52.88 4.30 549.15 43.20 85.09 0.81

54 9 6.9 1043 440 678 148.29 17.01 32.19 4.30 408.81 76.80 67.36 13.83 55 97 7.7 765 240 497 80.16 9.72 62.07 2.74 317.29 19.20 63.81 1.80

57 86 7.8 787 125 512 40.08 6.08 119.55 2.74 311.19 4.80 85.09 0.68

59 96 7.4 1002 420 651 146.28 13.37 34.48 3.52 445.43 9.60 81.54 6.70

60 44 7.4 883 360 574 88.17 34.03 34.48 3.13 402.71 28.80 49.63 4.09

61 83 7.4 1055 285 686 86.17 17.01 105.75 4.30 488.14 19.20 67.36 0.00

63 95 7.6 968 390 629 128.25 17.01 39.08 3.52 408.81 4.80 92.18 4.28

64 10 7.8 802 320 521 88.17 24.31 34.48 2.74 341.70 9.60 70.91 2.42

65 79 7.5 1092 400 710 140.27 12.15 62.07 4.30 494.24 9.60 85.09 0.00

68 78 7.3 1036 390 673 136.27 12.15 52.88 4.30 451.53 9.60 88.63 0.00

69 45 7.6 951 385 618 100.20 32.81 39.08 3.52 366.10 57.60 74.45 3.41

Av. 7.4 1018 374 664 115.44 20.87 57.77 3.71 420.22 35.48 83.70 3.71

A*= Depth of the bore wells below water table level (m). All ions, TH and TDS are in mg/l; EC:

uS/cm.

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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 semi-confined 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 Chloro-alkaline indices of fresh groundwater samples

Bw.

No.

Gibb's Gibb's CAI-1 CAI-2 Bw.

No.

Gibb's Gibb's CAI-1

CAI-2

Ratio 1 Ratio 2 Ratio 1 Ratio 2

1 0.27 0.08 -0.84 -0.10 29 0.31 0.17 0.03 0.01 2 0.20 0.06 -0.27 -0.03 31 0.16 0.10 0.30 0.04 3 0.21 0.07 -0.23 -0.03 34 0.60 0.33 -0.14 -0.06 4 0.26 0.10 -0.40 -0.06 35 0.16 0.12 0.40 0.06 5 0.18 0.07 0.12 0.01 36 0.12 0.10 0.44 0.07 6 0.23 0.12 -0.01 -0.01 37 0.18 0.12 0.33 0.06 7 0.11 0.06 0.23 0.02 38 0.37 0.32 0.49 0.22 8 0.47 0.12 -0.90 -0.17 40 0.39 0.17 -0.13 -0.03 9 0.36 0.14 -0.43 -0.09 41 0.36 0.12 -0.34 -0.06 10 0.12 0.05 0.03 0.01 43 0.35 0.20 -0.35 -0.07 11 0.67 0.12 -1.82 -0.35 47 0.21 0.09 -0.32 -0.04 12 0.15 0.09 0.27 0.04 48 0.31 0.13 -0.16 -0.03 13 0.34 0.15 -0.16 -0.04 50 0.50 0.23 -0.34 -0.11 14 0.14 0.16 0.48 0.10 51 0.26 0.13 0.00 0.00 15 0.12 0.11 0.47 0.08 54 0.20 0.14 0.21 0.04 16 0.18 0.11 0.21 0.04 55 0.45 0.17 -0.54 -0.13 17 0.17 0.08 0.01 0.01 57 0.75 0.21 -1.20 -0.38 18 0.50 0.23 -0.22 -0.06 59 0.21 0.15 0.31 0.07 19 0.20 0.12 0.19 0.03 60 0.30 0.11 -0.13 -0.02 20 0.38 0.15 -0.29 -0.07 61 0.56 0.12 -1.48 -0.27 21 0.26 0.16 0.01 0.01 63 0.25 0.18 0.31 0.09 22 0.32 0.25 0.28 0.10 64 0.30 0.17 0.22 0.05 25 0.64 0.31 -0.33 -0.14 65 0.32 0.15 -0.17 -0.04 26 0.40 0.16 -0.63 -0.13 68 0.30 0.16 0.04 0.01 27 0.29 0.20 -0.05 -0.01 69 0.30 0.17 0.15 0.03

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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

3.4.3 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 (HCO3-)

exceeds strong acids (Cl- and SO4

2-). 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 facies-wise, 41 samples are calcium

type, 3 samples are sodium type and in 6 samples none of the cation dominates.

Anion facies-wise, 49 samples are bicarbonate type and in one sample none of the

anions dominates. Hydrochemical facies-wise, 45 samples belong to Ca-Mg-HCO3

type, 3 samples, to mixed Ca-Na-HCO3 type, and 2 samples, to Ca-Mg-Cl- type.

Hydrochemical data of the fresh groundwater samples fed to Aquachem software

programme revealed the presence of only two sub-hydrochemical facies: Ca2+

- HCO3-

(n=45) and Na+- HCO3

- (n=5).

1

10

100

1000

10000

100000

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

To

tal

dis

olv

ed s

oli

ds

(TD

S)

Na++K+/Na++K++Ca2+ (mg/L)

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Figure 3.4 Piper triliner diagram showing the hydrochemical characteristics of fresh

groundwater (•) and saline groundwater (+) samples of the semi-confined aquifer

3.4.4 Ion exchange reactions

Ion exchange reactions involved by groundwater samples can be deciphered

based on the values of their Chloro-Alkaline Indices (CAI). CAI-1 and CAI-2 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

(Ca1-x Mgx) CO3 + H++Na2- clays → 2Na+HCO3+(Ca1-x Mgx) – clays

Reverse cation – anion exchange reaction

2Na+HCO3-+(Ca1-xMgx) – clays →(Ca1-x Mgx) CO3+H

++Na2-clays

Among 50 fresh groundwater samples of the semi-confined aquifer, 23

samples yielded positive values of CAI (Table 3.2). This data suggest that part of the

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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 Ca2+

(and/or Mg2+

) of the fresh groundwater, thereby enriching

the groundwater with Na+ (and/or K

+) and HCO3

-, at the expense of its Ca

2+ (and/or

Mg2+

) 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 cation-anion 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 Mg2+

) of the clays of the aquifer matrix, thereby enriching the groundwater

with Ca2+

(and/or Mg2+

) and hydrogen ion and depleting its Na+ (and/or K

+) and

HCO3- contents.

3.4.5 Inter-elemental 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 inter-

elemental correlation (Singh et al., 2011). Correlation analysis was carried out

between 12 variables of the physico-chemical 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, Ca2+

shows strong positive

correlation with HCO3- (r

2 = 0.75) and K

+ (r

2 = 0.73) and moderately weak positive

correlation with SO42-

(r2

= 0.44). Mg2+

is weakly correlated with SO42-

(r2

= 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 HCO3

- (r

2=0.27). K

+ exhibits

strong positive correlation with HCO3- (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

SO42-

(r2 = 0.44).

Strong positive correlation between (1) Ca2+

and HCO3-, (2) K

+ and HCO3

-,

(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

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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.

The observed weak positive correlation of SO42-

with Ca2+

and Mg2+

suggest

three other possible additional sources of these ions: (1) sulfuric acid - aided

weathering of dolomite, (2) dissolution of gypsum and Mg-sulfate, and (3) residual

connate salinewater.

Table 3.3 Spearman’s Rank Correlation coefficient matrices of variables of the physico-

chemical data of the fresh groundwater samples (n=50)

EC PH TDS TH Ca2+

Mg2+

Na+

K+

HCO3-

SO42-

EC 1.00

PH -0.36 1.00

TDS 1.00 -0.36 1.00

TH 0.61 -0.36 0.60 1.00

Ca2+ 0.44 -0.35 0.43 0.66 1.00

Mg2+ 0.41 -0.09 0.41 0.56 -0.18 1.00

Na+ 0.80 -0.11 0.80 0.12 0.06 0.18 1.00

K+ 0.89 -0.13 0.89 0.55 0.50 0.34 0.68 1.00

HCO3- 0.56 -0.35 0.56 0.40 0.23 0.25 0.34 0.56 1.00

SO42- 0.27 -0.19 0.27 0.55 0.32 0.29 0.00 0.24 -0.03 1.00

Cl- 0.81 -0.13 0.81 0.25 0.19 0.23 0.86 0.71 0.19 0.02

NO3- 0.37 0.03 0.36 0.16 0.22 -0.01 0.34 0.38 0.28 0.14

3.4.6 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 physico-

chemical parameters (pH, EC, TH, TDS, Ca2+

, Mg2+

, Na+, K

+, HCO3

-, SO4

2-,Cl

- and

NO3-) 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, Ca2+

, K+, HCO3

- and modest positive loading of

NO3-. Factor 2 accounts for 24.59% of total variance with high positive loadings of

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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 Mg2+

and SO42-

. Positive

loadings of the factor 1 (viz., Ca2+

, K+, HCO3

- and NO3

-) 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., Mg2+

and SO42-

) suggest two possible sources for acquisition of

these ions: (1) Sulfuric acid-aided 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 physico-chemical data of the fresh groundwater samples

(n=50)

Rotated Matrix of Variables

Variables Principal Components

Factor 1 Factor 2 Factor 3

PH -.775 -.214 -.140

EC .755 .601 .249

TH .859 .010 .446

TDS .753 .603 .245

Ca2+

.941 .029 .107

Mg2+

-.029 -.045 .878

Na+ .030 .927 -.195

K+ .739 .574 .139

HCO3- .816 .290 -.047

SO42-

.307 .047 .626

Cl- .177 .866 .095

NO3- .595 -.391 .011

Eigen values 5.053 2.951 1.583

% of Variance 42.106 24.590 13.194

Cumulative % 42.106 66.696 79.890

3.4.7 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

63

possibly constitutes another additional but minor source for Na+, K

+, Cl

-, Ca

2+, Mg

2+,

HCO3- and SO4

2- 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 CO2 and oxidation of pyrite. The proportion of HCO3- and SO4

2- 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 HCO3-/(HCO3

-+SO4

2-) equivalent ratio (C-ratio, Brown et

al., 1996) of the groundwater samples. The values of C-ratio 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 Ca2+

, Mg2+

, K+ and HCO3

- ions. If the SO4

2- 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

Ca2+

and Mg2+

and significant source of SO42-

. The other minor source of SO42-

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 Ca2+

/ HCO3-, Ca

2+ / SO4

2-, Mg

2+ / HCO3

- and Mg

2+ /

SO42-

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 semi-confined aquifer the values of

Ca2+

/ HCO3- molar ratio vary from 0.39 to 1.34 (av. = 0.85); Ca

2+ / SO4

2- molar ratio,

from 2.05 to 46 (av. = 18.20); Mg2+

/ HCO3- molar ratio, from 0.12 to 0.54 (av. =

0.26); and Mg2+

/ SO42-

molar ratio, from 0.35 to 19.00 (av. = 5.22).

The above data reveal that in all groundwater samples the average values of

Ca2+

/ HCO3-, Ca

2+ / SO4

2-, Mg

2+ / HCO3

- and Mg

2+ / SO4

2- 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

64

carbonate mineral weathering, but also from other sources (e.g., aluminosilicate

weathering, reverse cation-anion exchange reaction).

Weathering of aluminosilicates is one among the major sources of Ca2+

and

Mg2+

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 cation-anion exchange reaction during water-rock

interaction. These reactions provided substantial ionic load of alkaline earths to the

fresh groundwater. The values of Ca2+

/(Ca2+

+ SO42-

) molar ratio of the fresh

groundwater samples are > 0.5 and this data rules out the possibility of derivation of

Ca2+

and SO42-

through dissolution of gypsum. Mg2+

shows weak correlation with

SO42-

(r2 = 0.20) and suggests the remote possibility of minor contribution of Mg

2+

and SO42-

from dissolution of Mg sulfate.

Table 3.5 Values of the molar ratios of dissolved ions (viz., Ca2+

/ HCO3-, Ca

2+ / SO4

2-, Mg

2+ /

HCO3- and Mg

2+/ SO4

2- ) 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 H2CO3

Ca2+

/HCO3-= 0.50

Dolomite H2CO3

Ca2+

/HCO3-= 0.25

Calcite H2SO4

Ca2+

/SO42-

= 1.00

Dolomite H2SO4

Ca2+

/SO42-

= 0.50

Dolomite H2CO3

Mg2+

/SO42-

= 0.25

Dolomite H2SO4

Mg2+

/SO42-

= 0.50

Carbonates* H2CO3

Ca2+

/HCO3-= from 0.25 to 0.50

Carbonates* H2SO4

Ca2+

/SO42-

= from 0.50 to 1.00

Carbonates* H2CO3

Mg2+

/ HCO3-= from 0 to 0.25

Carbonates* H2SO4

Mg2+

/SO42-

= 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 (Ca2+

+Mg2+

)/HCO3-equivalent ratio mark the upper limit of

bicarbonate input from weathering of carbonates (Stallard and Edmond, 1983; Richter

and Kreitler, 1993). On the bivariate (Ca2+

+ Mg2+

) versus HCO3- equivalent diagram

majority of the fresh groundwater samples plot above 1:1 line indicating that (Ca2+

+

Mg2+

) content is in excess of alkalinity (Diagram is not provided). On the bivariate

(Ca2+

+ Mg2+

) versus (HCO3-+SO4

2-) equivalent diagram, out of 50 groundwater

65

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

(HCO3-+SO4

2-) content is higher than that of (Ca

2++Mg

2+) and in 23 samples (Ca

2+ +

Mg2+

) dominates over (HCO3-+SO4

2-). The excess positive charge of (Ca

2++Mg

2+) in

23 samples is balanced by Cl- and / or NO3

- (Cerling et al., 1989; Fisher and

Mullican, 1997). On the bivariate (Ca2+

+ Mg2+

) versus TZ+ equivalent diagram all

samples of the fresh groundwater (n = 50) plot below 1: 1 equiline, the departure

being more pronounced at high TZ+ 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 (Ca2+

+

Mg2+

) / (Na+ + K

+) equivalent ratio of the fresh groundwater samples is 3.23. 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 cation-anion exchange reaction. This data

indicates that prior to reverse cation-anion 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

66

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 Na-bearing 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 NO3-

(av = 0.05

meq/L). NO3- 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 NH4, industrial effluents, human and animal waste and biocombustion

(Prospero and Savoie, 1989; Carling and Hammar, 1995; Min et al., 2003).

67

Figure 3.5 Bivariate (Ca2+

+Mg2+

) versus (HCO3- + SO4

2-) (a) and bivariate

(Ca2+

+Mg2+

) versus Tz+ (b) equivalent diagram of the fresh groundwater samples

(n = 50)

Figure 3.6 Bivariate Na+ versus Cl

- equivalent diagram

of the fresh groundwater samples (n = 50)

3.5 SALINE GROUNDWATER


Physico-chemical 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

->HCO3

->SO4

2->NO3

-.

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% HCO3

-, 6.10% SO4

2- and

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

Ca

2++

Mg

2+

HCO3- + SO4

2-

a

0

5

10

15

20

0 5 10 15 20

Ca

2++

Mg

2+

Tz +

b

0

2

4

6

8

0 2 4 6 8

Na +

Cl-

68

0.22% NO3-. Average meq/L content of (Na

+ + K

+) (13.46 meq/L) is higher than that

of (Ca2+

+ Mg2+

) (8.91 meq/L) and the average meq/L content of (Cl-+NO3

-) (13.85

meq/L) is higher than that of (HCO3-+SO4

2-) (8.44 meq/L). Average total

concentration of ions is 44.66 meq/L, which is 2.40 times higher than that of the fresh

groundwater (av. = 18.54 meq/L).

Table 3.6 Physico-chemical characteristics of the saline groundwater samples (n=20)

Bw A* pH EC TH TDS Ca2- Mg2+ Na+ K+ HCO3- SO4

2- Cl- NO3-

No.

23 25 7.4 1514 415 995 122.24 26.74 154.03 4.30 463.73 14.40 251.71 0.00

24 106 7.6 1426 290 941 70.14 27.95 188.52 4.30 61.02 4.80 457.34 0.12

28 77 8.1 1519 255 1003 62.12 24.31 227.60 4.30 341.70 9.60 326.16 0.81

30 73 8.4 1371 225 905 62.12 17.01 206.91 4.30 67.12 4.80 432.52 0.00

32 126 7.4 1994 615 1336 178.35 41.32 170.12 4.30 286.78 14.40 521.15 0.62

33 149 8.0 1521 180 1004 48.09 14.58 262.08 4.30 384.41 14.40 297.80 0.50

39 130 7.9 1260 170 832 52.10 9.72 206.91 4.30 274.58 72.00 226.90 0.19

42 111 7.8 2520 405 1587 126.25 21.87 388.53 4.69 195.25 14.40 762.23 0.12

44 6 7.5 3130 675 2097 190.37 48.61 404.62 5.08 549.15 134.40 684.24 17.1

1 45 9 7.8 2130 460 1427 108.21 46.18 273.58 4.69 463.73 14.40 467.98 0.93

46 6 7.5 4040 875 2707 210.41 85.07 519.57 5.47 579.66 43.20 1052.95 8.31

49 63 7.3 2730 480 1829 150.29 25.52 400.02 4.69 335.59 9.60 755.14 0.50

52 8 7.7 1715 355 1149 118.23 14.58 227.60 4.30 360.00 100.80 319.07 0.31

53 10 7.6 1811 115 1213 38.07 4.86 358.64 4.30 634.58 9.60 255.26 0.50

56 15 7.4 2460 665 1648 194.38 43.75 255.19 4.69 762.71 235.20 248.17 0.99

58 8 7.2 4310 515 2888 134.26 43.75 747.17 5.47 738.31 19.20 1074.22 0.81

62 8 7.6 2440 255 1635 64.12 23.09 439.10 4.69 640.68 120.00 397.07 0.43

66 10 7.6 1956 690 1311 204.40 43.75 126.44 4.30 244.07 436.80 219.81 24.61 67 9 8.3 3270 665 2191 150.29 70.48 441.40 5.08 433.22 9.60 893.41 0.99

70 20 7.8 1817 600 1217 152.30 53.47 135.64 4.30 817.63 19.20 141.81 0.50

Av. 7.6 2641 520 1761 141.66 40.38 362.88 4.75 519.58 89.72 559.33 4.32

A*= Depth of the bore wells below water table level (m). All ions, TH and TDS are in mg/l; EC:uS/cm.


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 SO4

2-) exceed weak acid (HCO3

-) and in the remaining 4

samples weak acid exceeds strong acids. In 15 samples non-carbonate 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 facies-wise,

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,

69

15 samples belong to Na+

- Cl- type, 3 samples to, Ca

2+ - Mg

2+- HCO3

- type, one

sample, to mixed Ca2+

- Na+- HCO3

- type and one sample, to mixed Ca

2+ - Mg

2+- Cl

-

type. Aquachem software program revealed the presence of 5 sub-hydrochemical

facies in the saline groundwater: Na+ - Cl

- (n = 14), Na

+ - HCO3

- (n = 3), Ca

2+- HCO3

-

(n = 1), Ca2+

- Cl- (n = 1) and Ca

2+- SO4

2- (n = 1).

3.5.3 Ion-exchange reactions

In the fresh groundwater-seawater mixing zone of coastal aquifer the resulting

groundwater invariably involves a non-conservative mixing of freshwater from

aquifer and saltwater from sea. When seawater intrudes fresh groundwater of an

aquifer, direct cation and reverse cation-anion 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 Ca2+

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 Mg2+

and K+ are also adsorbed on exchanger in addition to

Na+ (Appelo, 1994). In fresh groundwater-seawater mixing zone, cation exchange

reactions may take place not only between Ca2+

/ Na+ and Mg

2+/ Na

+ but also between

Ca2+

/ 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 HCO3

- and

depletion of Ca2+

(and/or Mg2+

). Remaining 9 saline groundwater samples exhibit

negative values of CAI and indicate the involvement of reverse cation-anion exchange

reaction. In these saline groundwater samples, reverse cation-anion reaction increased

the concentration of Ca2+

(and/or Mg2+

) and decreased the concentration of HCO3-

and Na+ (and/or K

+).

70

Table 3.7 Gibbs ratio and Chloro-alkaline indices of the saline groundwater samples (n=20)

Bw.

No.


No.

Gibb's Gibb's CAI-1

CAI-2


23 0.56 0.35 0.04 0.01 46 0.71 0.64 0.23 0.17

24 0.73 0.88 0.36 0.33 49 0.73 0.69 0.18 0.14

28 0.79 0.49 -0.09 -0.05 52 0.66 0.47 -0.11 -0.06

30 0.77 0.87 0.25 0.23 53 0.91 0.29 -1.18 -0.48

32 0.49 0.65 0.49 0.36 56 0.57 0.25 -0.60 -0.17

33 0.85 0.44 -0.37 -0.21 58 0.85 0.59 -0.08 -0.05

39 0.80 0.45 -0.42 -0.22 62 0.87 0.38 -0.72 -0.33

42 0.76 0.80 0.21 0.18 66 0.39 0.47 0.10 0.03

44 0.68 0.55 0.08 0.05 67 0.75 0.67 0.23 0.18

45 0.72 0.50 0.09 0.06 70 0.48 0.15 -0.50 -0.11


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 HCO3

- (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 HCO3- (r

2 = 0.48) and SO4

2- (r

2 = 0.31). Mg

2+ shows

strong positive correlation with Ca2+

(r2 = 0.80), good positive correlation with K

+

(r2

= 0.61) and weak positive correlation with HCO3- (r

2 = 0.38), Cl

- (r

2 = 0.33) and

SO42-

(r2 = 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 SO4

2- (r

2 = 0.49) and

weak positive correlation with HCO3- (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 Ca2+

, Mg2+

, SO42-

and HCO3- clearly

project the brine / seawater signature. Weak positive correlation of Ca2+

and Mg2+

with HCO3- and SO4

2- 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 HCO3- with Na

+ (r

2 = 0.39) and K

+ (r

2 = 0.48)

possibly also suggests the weathering of alkali feldspars.

71

Table 3.8 Spearman’s Rank Correlation coefficient matrices of variables of the physico-

chemical data of the saline groundwater samples (n=20)

EC PH TDS TH Ca2+ Mg2+ Na+ K+ HCO3

- SO42- Cl- NO3

-

EC 1.00

PH -0.45 1.00

TDS 1.00 -0.47 1.00

TH 0.69 -0.44 0.70 1.00

Ca2+ 0.65 -0.51 0.66 0.97 1.00

Mg2+ 0.62 -0.26 0.63 0.89 0.80 1.00

Na+ 0.75 -0.15 0.75 0.13 0.06 0.18 1.00

K+ 0.91 -0.21 0.91 0.59 0.53 0.61 0.87 1.00

HCO3-

0.47 -0.33 0.51 0.27 0.23 0.38 0.39 0.48 1.00

SO42-

0.31 -0.30 0.33 0.45 0.49 0.24 0.04 0.31 0.42 1.00

Cl- 0.67 -0.19 0.64 0.30 0.23 0.33 0.74 0.75 -0.07 -0.27 1.00

NO3-

0.29 -0.15 0.32 0.62 0.53 0.58 0.04 0.35 0.23 0.23 0.08 1.00

3.5.5 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+,

Mg2+

, Cl-, HCO3

-, SO4

2- and NO3

-. 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,

Ca2+

, SO42-

and NO3- and moderate positive loading of Mg

2+. Positively loaded

variables of the factor 2 may be attributed to the input of Ca2+

, Mg2+

and SO42-

ions

from either mineral dissolution / weathering and/or seawater. Positive loading of NO3-

indicate nitrate input from anthropogenic sources. Factor 3 accounts for 14.51% of the

total variance with strong loading of HCO3-. The latter indicates alkalinity of the

saline groundwater. The alkaline signature is attributed essentially to rock weathering.

High positive loadings of Ca2+

, Mg2+

, HCO3-, SO4

2-, Na

+, Cl

+, K

+ and NO3

-

of the saline groundwater in the fresh groundwater – seawater mixing zone clearly

72

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 Ca2+

,

Mg2+

, Na+, K

+ and

HCO3

- in the groundwater of the fresh groundwater – seawater

mixing zone.

Table 3.9 Factor analysis of the physico-chemical data of the saline groundwater samples

(n=20)

Rotated Matrix of Variables

Variables Principal Components

Factor 1 Factor 2 Factor 3

PH -.132 -.214 -.777

EC .917 .214 .313

TH .511 .782 .157

TDS .912 .222 .324

Ca2+

.383 .826 .221

Mg2+

.648 .589 .026

Na+ .861 -.218 .305

K+ .937 .151 .226

HCO3- .252 .026 .814

SO42-

-.300 .775 .213

Cl- .966 .008 -.082

NO3- -.022 .845 -.045

Eigen values 5.222 3.168 1.741

% of Variance 43.519 26.396 14.512

Cumulative % 43.519 69.915 84.426

3.5.6 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

73

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 ei (in meq/L) is the concentration of specific ion (i), fsea 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 ei 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 (fsea) is

calculated by weighing up the seawater contribution from the concentration of Cl- of

the sample (eCl- sample), the concentration of Cl

- of the freshwater (eCl

- .fresh), and the

concentration of Cl- of the seawater (eCl

-.sea), where concentration of Cl

- is expressed

in meq/L.

100 × fsea = % 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+ , SO4

2- and Cl

-

and higher concentrations of HCO3-

and Ca2+

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

74

concentration of Ca2+

, Mg2+

, Na

+, Mg

2+ and HCO3

- and lower concentrations of K

+ ,

SO42-

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 3.10. 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. Ca2+

Mg2+

Na+ K

+ HCO3

- SO4

2-

23 -0.651 0.037 1.414 -0.059 1.331 -0.519 0.032

24 -5.098 -0.525 -0.799 -0.153 -5.254 -0.792 0.070

28 1.609 -0.409 3.230 -0.094 -0.726 -0.646 0.046

30 -5.627 -1.346 0.442 -0.142 -5.168 -0.783 0.065

32 -2.340 0.373 -2.731 -0.182 -1.519 -0.614 0.081

33 2.406 -1.120 5.233 -0.081 -0.042 -0.536 0.041

39 -1.203 -1.295 4.091 -0.049 -1.881 0.689 0.028

42 0.191 -1.990 2.490 -0.281 -2.886 -0.698 0.125

44 -2.269 0.457 4.575 -0.236 2.871 1.829 0.111

45 -2.372 0.942 2.713 -0.148 1.352 -0.595 0.072

46 -10.421 2.290 3.031 -0.392 3.575 -0.200 0.177

49 -0.560 -1.668 3.116 -0.278 -0.590 -0.796 0.123

52 -0.541 -1.187 3.355 -0.091 -0.430 1.257 0.045

53 -3.600 -1.785 10.188 -0.062 4.035 -0.621 0.033

56 3.449 1.437 5.814 -0.049 6.131 4.081 0.032

58 -4.968 -1.178 12.553 -0.402 6.186 -0.707 0.181

62 -0.181 -0.734 11.171 -0.116 4.213 1.629 0.059

66 37.446 1.527 0.717 -0.046 -2.385 8.291 0.027

67 9.833 1.595 2.462 -0.330 1.087 -0.844 0.148

70 -2.975 2.574 2.501 -0.011 6.972 -0.381 0.013

Average 0.606 -0.100 3.778 -0.160 0.844 0.452 0.076

Chemical composition (me/L)

Ca2+

Mg2+

Na+ K

+ HCO3

- SO4

2- Cl

-

Caspian sea 44 19.2 100.8 2.6 3.4 2.7 158.8

Fresh water 5.51 1.60 2.11 0.09 6.47 0.76 1.95

f sample indicates the fraction of seawater in the mixed freshwater – seawater samples. f sea=

100, Av. Of f samples = 7.6%.

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

75

Ca2+

are most probably due to direct contact of Na+-rich water (seawater) with Ca

2+-

rich water (fresh groundwater). In the present study e change of Na+ is positive in

majority of salinewater samples. Positive values of e change of Na+ are justified

owing to the fact that the freshwater contains comparatively lower concentration of

Na+. The negative values of e change of Na

+ shown by 2 samples indicate that the

groundwater is probably witnessing simple mixing with seawater.

UNCONFINED AQUIFER

3.6 GROUNDWATER TYPES

For better understanding of the hydrochemical features of groundwater of any

given coastal aquifer it is necessary to evaluate its hydrogeological setting. In the

study area, the northern boundary of the unconfined aquifer, at its western and eastern

ends, is at a distance of, respectively 2.5 and 16 km away from the Caspian sea coast

line. This geographical setup may not provide access to lateral intrusion of Caspian

sea water. However, the large semi-confined aquifer underlying the unconfined

aquifer witnessed significant intrusion of Caspian sea water. In the semi-confined

aquifer the salinized water zone, starting from the eastern part of the coast line (ie.,

north eastern boundary of the semi-confined aquifer), extends towards the center of

the semi-confined aquifer (Figure 1.2). Excessive extraction of groundwater from

bore wells located above the saline water zone may lead to upconing of saline water

along bore wells of the unconfined aquifer and salinization of the fresh groundwater.

Further, in the northern half of the unconfined aquifer the water table is at 0 to ~15 m

below msl. The above hydrological setting indicates the possible presence of saline

water pockets / zones in the unconfined aquifer.

Fresh and saline groundwaters can be distinguished based on the values of

their EC (µS/cm) and TDS content. According to EC-based classification of Saxena et

al. (2003), groundwaters having EC values (a) <1500 µS/cm and (b) between 1500 to

3000 µS/cm belongs, respectively to (a) fresh and (b) brackish water categories.

According to TDS-based classification of Rabinove et al. (1958) groundwaters having

TDS content (a) <1000 mg/l and (b) between 1000 and 3000 mg/l belongs,

respectively to (a) fresh and (b) slightly saline water categories.

In the unconfined aquifer the groundwater from 36 bore wells is characterized

by EC values <1500 µS/cm and TDS content <1000 mg/l (Table 3.11). Hence, the

76

groundwater from these 36 bore wells belongs to fresh water category. EC values and

TDS content of the groundwater from the remaining 4 bore wells range, respectively

from 1564 to 2750 µS/cm and from 1032 to 1843 mg/l (Table 3.11). This data

indicate that the groundwater from these 4 bore wells belongs to brackish

(classification of Saxena et al., 2003) or slightly saline (Classification of Robinove et

al., 1958) water category. Hence forth, the groundwater from these 4 bore wells will

be referred to as brackish groundwater.

3.7 FRESH GROUNDWATER


Physico-chemical characteristics of 36 fresh groundwater samples are

provided in Table 3.11. Fresh groundwater, in terms of meq/L and on average, is

characterized by Ca2+

>Na+>Mg

2+>K

+ and HCO3

->Cl

->SO4

2->NO3

-. Average

contribution of individual cation to total cations is: 54.62% Ca2+

, 26.29% Na+,

18.14% Mg2+

and 0.93% K+. On an average, anions are made up of 68.19% HCO3

-,

23.19% Cl-, 8.32% SO4

2- and 0.28% NO3

-. Average content of (Ca

2++Mg

2+) (7.86

meq/L) is higher than that of (Na++K

+) (2.94 meq/L) and average content of (HCO3

-

+SO42-

) (8.18 meq/L) is higher than that of (Cl-+NO3

-) (2.56meq/L). Average total

ionic concentration in the fresh groundwater equals 21.50 meq/L. The fresh

groundwater contains significant concentrations of Na+ (av. = 2.84 meq/L) and Cl

-

(av. = 2.48 meq/L), which together constitute 24.74% of the total ions. Na+ and Cl

- are

the second most abundant ions among cations and anions.

3.7.2 Degree of evolution

As mentioned earlier, in groundwaters the general evolutionary sequence for

dominant anion is HCO3- to SO4

2- to Cl

- which correlates sequentially with increasing

distance along flow path and an increasing residence time or age of water

(Chebotarev, 1955). In the fresh groundwater of the unconfined aquifer, HCO3-

constitutes the dominant anion among anions and this feature is similar to that of

semi-confined aquifer. Groundwater containing predominantly bicarbonate ion is

considered as the one, which witnessed minimum residence time and is generally

present in areas of active groundwater circulation.

77

Table 3.11 Physico-chemical characteristics of fresh groundwater samples (n=36) and

brackish groundwater samples (n=4)

B.W. pH EC TH TDS Ca2- Mg2+ Na+ K+ HCO3- SO42- Cl- NO3

-

Fresh groundwater

1 7.80 1001 320 651 74.14 32.81 80.46 3.52 366.10 9.60 127.63 5.21

2 7.70 874 350 568 116.23 14.58 36.78 3.13 366.10 62.40 42.54 0.31

3 7.40 892 405 580 136.27 15.80 16.09 3.13 402.71 62.40 28.36 2.91

4 7.50 1009 410 656 140.27 14.58 39.08 4.30 433.22 57.60 53.18 3.53

5 7.70 1121 420 729 146.28 13.37 59.77 4.30 451.53 81.60 67.36 2.79

6 7.10 1342 490 886 156.30 24.31 78.17 4.30 579.66 86.40 67.36 6.01

7 7.20 926 400 602 86.17 44.96 25.29 3.52 469.83 9.60 39.00 0.31

8 7.50 974 430 623 148.29 14.58 22.99 3.52 451.53 28.80 53.18 2.00

9 7.20 1225 315 809 84.16 25.52 131.04 4.30 567.46 9.60 88.63 0.19

10 7.50 942 390 612 94.18 37.67 34.48 3.52 439.32 33.60 46.09 0.12

11 7.20 1007 425 655 94.18 46.18 34.48 3.52 512.54 14.40 42.54 0.31

12 7.00 1426 540 941 174.34 25.52 73.57 4.30 335.59 120.00 212.72 0.93

13 7.10 1048 400 681 140.27 12.15 29.89 4.30 512.54 9.60 56.72 4.53

14 7.50 766 345 468 80.16 35.24 20.69 2.74 311.19 57.60 39.00 0.99

15 7.50 771 350 501 82.16 35.24 20.69 2.74 280.68 67.20 53.18 1.12

16 7.10 984 340 640 120.23 9.72 66.67 3.52 366.10 43.20 95.72 0.62

17 7.40 1061 415 690 140.27 15.80 48.28 4.30 475.93 9.60 85.09 0.50

18 7.30 952 315 619 106.21 12.15 71.27 3.52 384.41 4.80 102.81 0.62

19 7.20 1043 375 678 98.19 31.60 62.07 4.30 390.51 62.40 88.63 2.00

20 7.40 797 350 518 112.22 17.01 18.39 2.74 390.51 9.60 39.00 3.53

21 7.20 988 380 642 128.25 14.58 48.28 3.52 378.31 52.80 81.54 0.50

23 7.30 1096 405 712 142.28 12.15 64.37 4.30 439.32 33.60 99.27 0.43

24 7.20 1493 425 985 120.23 30.38 142.54 4.30 604.07 19.20 155.99 4.59

25 7.20 1263 330 834 112.22 12.15 133.34 4.30 518.65 9.60 131.18 3.78

26 7.10 1329 500 877 140.27 36.46 68.97 4.30 555.26 57.60 95.72 5.33

28 7.70 766 215 498 70.14 9.72 73.57 2.74 360.00 19.20 39.00 1.43

29 7.60 1153 340 749 106.21 18.23 101.16 4.30 433.22 48.00 113.45 7.38

30 7.10 1362 480 899 160.31 19.44 87.36 4.30 573.56 62.40 95.72 1.18

31 7.30 1340 505 886 138.27 38.89 71.27 4.30 445.43 115.20 124.08 2.00

32 7.50 1398 335 923 110.21 14.58 160.93 4.30 427.12 28.80 216.26 2.91

33 7.50 1191 450 774 110.21 42.53 62.07 4.30 439.32 86.40 95.72 2.00

35 7.30 1048 385 682 116.23 23.09 57.47 4.30 457.63 14.40 85.09 2.00

39 7.10 1274 480 842 110.21 49.83 66.67 4.30 506.44 86.40 85.09 1.50

36 7.40 1141 290 742 90.18 15.80 124.14 4.30 457.63 9.60 124.08 2.00

37 7.30 965 365 627 128.25 10.94 50.58 3.52 408.81 9.60 88.63 2.00

39 7.10 1274 480 842 110.21 49.83 66.67 4.30 506.44 86.40 85.09 1.50

Av. 7.3 1090 393 712 117.34 24.37 65.27 3.87 444.41 43.87 87.35 2.20

Brackish groundwater

22 7.10 1564 485 1032 108.21 52.26 131.04 4.30 463.73 168.00 152.45 2.00

27 7.10 1700 530 1122 142.28 42.53 140.24 4.30 622.37 110.40 148.90 7.01

38 7.80 1847 475 1237 114.22 46.18 200.01 4.30 646.78 19.20 255.26 3.53

40 7.30 2750 1040 1843 228.44 114.23 147.13 4.69 353.90 14.40 748.05 2.00

Av. 7.30 1965 633 1309 148.29 63.80 154.61 4.40 521.70 78.00 326.17 3.64

TDS, TH, and ions in mg/l. EC= µS/cm.

78

3.7.3 Natural mechanisms controlling the hydrochemistry

On the bivariate TDS versus Gibb’s ratio-1 [weight ratio of

(Na++K

+)/(Na

++K

++Ca

2+)] diagram 36 fresh groundwater samples plot essentially in

rock dominance field but very close to the boundary line between rock dominance and

evaporation dominance fields. Only a few samples plot in the evaporation dominance

field (Table 3.12; Figure 3.7). This data suggest that the hydrochemistry of the fresh

groundwater is controlled mainly by chemical interaction between groundwater and

aquifer rocks and to some extent, by evaporative concentration.

Table 3.12 Gibbs ratio, Chloro-alkaline indices of fresh groundwater samples (n=36) and

brackish groundwater samples (n=4)

Bw.

No.


No.

Gibb's Gibb's CAI-1

CAI-2


Fresh groundwater 21 0.29 0.18 0.05 0.01 1 0.53 0.26 0.00 0.00 23 0.33 0.18 -0.04 -0.01

2 0.26 0.10 -0.40 -0.06 24 0.55 0.21 -0.43 -0.13

3 0.12 0.07 0.03 0.00 25 0.55 0.20 -0.60 -0.18

4 0.24 0.11 -0.21 -0.03 26 0.34 0.15 -0.15 -0.03

5 0.30 0.13 -0.43 -0.07 28 0.52 0.10 -1.97 -0.29

6 0.35 0.10 -0.85 -0.12 29 0.50 0.21 -0.41 -0.11

7 0.25 0.08 -0.08 -0.01 30 0.36 0.14 -0.45 -0.09

8 0.15 0.11 0.27 0.04 31 0.35 0.22 0.08 0.02

9 0.62 0.14 -1.32 -0.28 32 0.60 0.34 -0.17 -0.07

10 0.29 0.09 -0.22 -0.03 33 0.38 0.18 -0.04 -0.01

11 0.29 0.08 -0.33 -0.04 34 0.47 0.25 0.09 0.03

12 0.31 0.39 0.45 0.19 35 0.35 0.16 -0.09 -0.02

13 0.20 0.10 0.12 0.02 36 0.59 0.21 -0.57 -0.18

14 0.23 0.11 0.12 0.02 37 0.30 0.18 0.08 0.02

15 0.22 0.16 0.35 0.07 39 0.39 0.14 -0.25 -0.05

16 0.37 0.21 -0.11 -0.03 Brackish groundwater

17 0.27 0.15 0.08 0.02 22 0.56 0.25 -0.35 -0.10

18 0.41 0.21 -0.10 -0.03 27 0.50 0.19 -0.48 -0.12

19 0.40 0.18 -0.12 -0.03 38 0.64 0.28 -0.22 -0.09

20 0.16 0.09 0.21 0.03 40 0.40 0.68 0.69 0.54

79

Figure 3.7 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 samples (n = 36)


On the Piper’s diamond shaped diagram (Piper, 1944) the hydrochemical data

of 36 fresh groundwater samples plot in the fields 1, 3, 4, 5, 6 and 9 (Figure 3.8). This

data indicate that in all samples alkaline earths exceed alkalis earths. In 31 samples

weak acid (HCO3-) exceeds strong acids (Cl

- and SO4

2-) and in 5 samples strong acids

exceed weak acid. In 31 samples carbonate hardness (secondary salinity) exceeds

50% and in one sample non-carbonate hardness (secondary salinity) exceed 50%. In

4 samples none of the cation or anion pair exceeds 50%. Cation facies-wise, 17

samples are calcium type, 2 samples are magnesium type and 17 samples are no

dominant type. Anion facies – wise 32 samples are bicarbonate type, 2 samples are

chloride type and 2 samples are no dominant type. Hydrochemical facies – wise, 31

samples belong to CaMgHCO3- type, one sample, to CaMgSO4

2- type and 4 samples,

to mixed type.

1

10

100

1000

10000

100000

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Tota

l d

isolv

ed s

oli

ds

(TD

S)

Na++K+/Na++K++Ca2+ (mg/L)

80

Figure 3.8 Piper trilinear diagram showing the hydrochemical characteristics of fresh

groundwater samples (n = 36)

3.7.5 Factor analysis

Factor analysis of the physico – chemical data of 36 fresh groundwater

samples was carried out using SPSS and following the procedure provided by Sharma

(1996) and Mondal et al. (2010). Factor analysis was carried out to identify the ionic

contributions from various sources to the chemical composition of the fresh

groundwater.

For the present study, factors with eigenvalues higher than 1 are considered.

Following this procedure, 4 independent factors were extracted, which accounted for

82.98% of the total variance of the original data set. Factor 1 accounts for 31.41% of

the total variance with high positive loadings of EC, TDS, Na+, K

+ and Cl

-. Factor 2

accounts for 20.18% of total variance with high positive loadings of TH, Ca2+

and

SO42-

. Factor 3 accounts for 19.83% of total variance with high positive loading of

HCO3- and moderate positive loading of K

+. Factor 4 accounts for 11.49% of the total

variance with high positive loading of Mg2+

(Table 3.13).

Positive loadings of the factors 2, 3 and 4 (ie., Ca2+

, Mg2+

, K+, HCO3

- and

SO42-

) suggest derivation of these ions essentially from mineral weathering. Positive

81

loadings of Factor 1 (ie., Na+, K

+ and Cl

-) clearly indicate the signature of either

dissolved Cl- salts and/or

Table 3.13 Factor analysis of the physico-chemical data of the fresh groundwater samples

(n=36)

Rotated Matrix of variables

Variables Principle Components

Factor 1 Factor2 Factor3 Factor4

PH -.046 -.273 -.680 .006

EC .833 .312 .447 .022

TH .112 .807 .485 .208

TDS .831 .308 .452 .020

Ca2+

.084 .736 .381 -.514

Mg2+

.042 .134 .160 .950

Na+ .914 -.298 .081 -.099

K+ .695 .214 .485 -.069

HCO3- .325 -.118 .896 -.004

SO42-

.095 .882 -.128 .167

Cl- .915 .131 -.127 -.043

NO3- .302 -.037 .192 -.352

Eigen values 3.770 2.422 2.387 1.379

% of Variance 31.417 20.183 19.893 11.491

Cumulative % 31.417 51.600 71.493 82.984

relict connate saline water component of the fresh groundwater. It was mentioned in

section 2.6, chapter 2 that the Quaternary sediments of the two – tier aquifer system

were under Caspian sea water prior to retreat of the coast line of the Caspian sea to

the present geographical position. Hence, the presumption that the positive loadings

of Na+, K

+ and Cl

- with Factor 1 is related to the relict saline water component of the

unconfined aquifer sediments finds support from the reported sub-recent evolutionary

event witnessed by the Mazandaran Province. It is to be noted that the factor analysis

data provided the indication of the presence of relict connate saline water component

in the fresh groundwater of the present study, which under conventional interpretation

of hydrochemical data may not be possible to recognize.

82


The sources of dissolved solids in groundwater can be evaluated from the

relative abundance of individual ions and their inter-elemental relationships (Singh et

al., 2011). During the present study Spearman’s rank correlation coefficient matrices

of 12 variables of the physico – chemical data of the 36 fresh groundwater samples

were calculated follwing the methodology provided by Kumar et al. (2006) and the

obtained results are shown in table 3.14. Among the cations, Ca2+

shows moderate to

weak positive correlation with K+

(r2

= 0.41), SO42-

(r2

= 0.36) and HCO3- (r

2 = 0.30).

Mg2+

shows weak positive correlation with SO42-

(r2

= 0.27) and HCO3- (r

2 = 0.22).

Na+ shows strong positive correlation with Cl

- (r

2 = 0.82) and K

+ (r

2 = 0.60) and weak

positive correlation with HCO3- (r

2 = 0.30). K

+ shows positive correlation with Cl

- (r

2

= 0.64), HCO3- (r

2 = 0.62) and Na

+ (r

2 = 0.60) and moderate positive correlation with

Ca2+

(r2

= 0.41).

Positive correlation between Na+ – Cl

-, Na

+ –K

+ and K

+ – Cl

- clearly indicate

the signature of the residual connate saline water admixed with the fresh groundwater.

Positive correlation between Ca2+

and K+, between HCO3

- and SO4

2-, positive

correlation of Mg2+

with HCO3- and SO4

2-, and positive correlation of Na

+ with HCO3

-

and strong positive correlation of K+ with HCO3

-, together indicate significant

contribution of dissolved solids derived from dissolution of carbonates and

weathering of aluminosilicates, including alkali feldspars.

Table 3.14 Spearman’s rank correlation coefficient matrices of variables of the fresh

groundwater samples (n=36)

EC PH TDS TH Ca2+ Mg2+ Na+ K+ HCO3

- SO42- Cl- NO3-

EC 1.00

pH -0.42 1.00

TDS 1.00 -0.42 1.00

TH 0.48 -0.36 0.47 1.00

Ca2+ 0.42 -0.39 0.41 0.66 1.00

Mg2+ 0.21 -0.07 0.21 0.44 -0.32 1.00

Na+ 0.73 -0.13 0.74 -0.13 -0.04 -0.05 1.00

K+ 0.89 -0.32 0.89 0.40 0.41 0.11 0.60 1.00

HCO3- 0.61 -0.44 0.60 0.43 0.30 0.22 0.30 0.62 1.00

SO42- 0.24 -0.11 0.23 0.56 0.36 0.27 -0.09 0.13 -0.15 1.00

Cl- 0.74 -0.15 0.74 0.02 0.12 -0.01 0.82 0.64 0.15 -0.08 1.00

NO3- 0.32 0.04 0.32 0.08 0.20 -0.11 0.28 0.28 0.19 0.09 0.20 1.00

83

3.7.7 Ion exchange reactions

Ion exchange reactions involved by the fresh groundwater samples were

evaluated based on their CAI values (Table 3.12). Among 36 samples of the fresh

groundwater 12 samples and 23 samples yielded, respectively positive and negative

values of CAI. This data suggest that part of Na+

(and/or K+) content in 12 fresh

groundwater samples was derived from direct cation exchange reaction, during which

alkalis of clay minerals of the aquifer matrix were exchanged with alkaline earths of

the fresh groundwater, thereby enriching the fresh groundwater with Na+

(and / or K+)

and HCO3- and depleting its Ca

2+ (and/or Mg

2+) content.

23 fresh groundwater samples, which yielded negative values of CAI, indicate

involvement of reverse cation-anion exchange reaction. During the ion exchange

reaction part of the alkali content of the fresh groundwater samples was exchanged

with alkaline earths of the clay minerals of the aquifer matrix, thereby enriching the

fresh groundwater samples with Ca2+

(and/or Mg2+

) and hydrogen and depleting its

Na+(and / or K) and HCO3

- content.

3.7.8 Sources of dissolved solids

Factor analysis and Spearman’s rank correlation coefficient matrices of the

variables of the chemical data of the fresh groundwater samples indicate that the

residual connate saline water of the aquifer sediments constitutes the other significant

source of the Na+, K

+, Cl

-, Ca

2+, Mg

2+, HCO3

- and SO4

2- ions present in the fresh

groundwater of the present study. It is not possible to quantify the contribution of the

ionic load provided by the residual connate saline water to the fresh groundwater for

want of chemical data of the mixing end members, viz., fresh groundwater at

recharging areas and palaeosaline water. Hence, this aspect will not be considered

further. The following paragraphs of this section provide details on the other sources

of Ca2+

, Mg2+

, Na+, HCO3

- and SO4

2- ions in the fresh groundwater.

During chemical weathering of carbonates, silicates and aluminosilicates, bulk

of the protons are provided by two reactions involving dissolution of atmospheric and

soil sourced CO2 and oxidation of pyrite (Table 3.15, Eq. 1 to 5). The proportion of

HCO3- and SO4

2- 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

84

be evaluated on the basis of the values of HCO3- / (HCO3

- + SO4

2-) equivalent ratio

(C-ratio, Brown et al., 1996) of the groundwater samples.

The values of the C- ratio of 36 fresh groundwater samples vary from 0.68 to

0.98 (av. = 0.89). These values suggest that carbonic acid - aided mineral weathering

has played relatively significant role in the acquisition of cations and HCO3-. If part

the SO42-

content of the fresh groundwater in derived from oxidation of pyrite, then

sulfuric acid-aided dissolution of carbonates can also be considered as another source

of Ca2+

and Mg2+

and significant source of SO42-

. Dissolution of carbonates takes

place according to the reactions provided in table 3.15, Eq. 6 to 9.

The values of (Ca2+

+Mg2+

)/HCO3- equivalent ratio marks the upper limit of

bicarbonate input from the weathering of carbonates (Stallard and Edmond, 1983;

Richter and Kreitler, 1993). On the bivariate (Ca2+

+Mg2+

) versus HCO3- equivalent

diagram, majority of the fresh groundwater samples plot above 1:1 equiline indicating

that (Ca2+

+Mg2+

) content is in excess of alkalinity (diagram not provided). On the

bivariate (Ca2+

+Mg2+

) versus (HCO3-+SO4

2-) equivalent diagram, out of 36 fresh

groundwater samples, 20 samples plot below 1:1 equiline and the remaining 16

samples plot above 1:1 equiline (Figure 3.9). This diagram suggests that in 16

samples (Ca2+

+Mg2+

) content is in excess (HCO3-+SO4

2) content. The excess positive

charge of (Ca2+

+Mg2+

) has to be balanced by Cl- and / or NO3

- (Stallard and Edmond,

1983; Cerling et al., 1989).

Figure 3.9 Bivariate (Ca2+

+Mg2+

) versus (HCO3- + SO4

2-) equivalent diagram of the

fresh groundwater samples (n = 36)

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14

Ca

2++

Mg

2+(m

eq/L

)

HCO3- + SO4

2-(meq/L)

85

In the fresh groundwater samples the values of Mg2+

/ (Ca2+

+Mg2+

) molar ratio

are <0.5 and indicate involvement of calcite and dolomite weathering (Hounslow,

1995). In groundwaters, dissolved weathering products of calcite, carbonates

(mixture of calcite and dolomite ranging from 0 to 100% of each mineral) and

dolomite can be quantified based on the weathering agent involved. Table 3.5

provides the values of the molar ratios of Ca2+

/ HCO3-, Ca

2+/ SO4

2-, Mg

2+/HCO3

- and

Mg2+

/ SO42-

of dissolved ionic load derived from dissolution of calcite, dolomite and

carbonates by two different weathering agents (viz., either carbonic acid or sulfuric

acid).

In the fresh groundwater samples, the values of the Ca2+

/HCO3- molar ratio

vary from 0.45 to 1.58 (av. = 0.81), Ca2+

/SO42-

molar ratio, from 2.87 to 53.00 (av. =

13.67), Mg2+

/HCO3- molar ratio, from 0.11 to 0.63 (av. = 0.30) and Mg

2+/ SO4

2-

molar ratio, from 0.84 to 18.50 (av. = 4.59). Barring 23 samples, in which the values

of Mg2+

/HCO3- molar ratio are ≤ 0.25, in all samples the values of the Ca

2+/HCO3

-,

Ca2+

/SO42-

, Mg2+

/HCO3- and Mg

2+/ SO4

2- molar ratios are higher than the

theoretically deduced values of the above molar ratios (Table 3.5). This data suggests

that in all samples, the Ca2+

content and in majority of the samples, the Mg2+

content

are derived not only from dissolution of carbonate minerals, but also from other

sources (e.g., silicate weathering, reverse cation – anion exchange reactions).

As mentioned earlier, solution products of aluminosilicate weathering are

difficult to quantify owing to the fact that aluminosilicates weather incongruently and

generate solid phases (mostly clays) along with dissolved salts (Sarin et al., 1989).

Weathering of feldspars with involvement of CO2 and H2O as the weathering agent

for generation of dissolved alkaline earths can take place according to the reactions

(not perfectly balanced) shown in table 5, Eq. 10 to 14.

In the study area reverse cation-anion reaction between the fresh groundwater

and exchangers of the aquifer matrix played significant role in the acquisition of

alkaline earths. As mentioned earlier, out of 36 fresh groundwater samples 23 samples

involved reverse cation-anion exchange reactions which provided additional load of

Ca2+

(and / or Mg2+

) to 23 groundwater samples. The values of Ca2+

/Ca2+

+SO42-

molar ratio of the fresh groundwater samples (n=36) are >0.5 and this data rules out

the possibility of derivation of Ca2+

through dissolution of gypsum.

86

Table 3.15 Reactions involved during generation of H2CO3 and HCO3- (a); oxidation of

pyrite (b); carbonic acid – and sulfuric acid – aided dissolution of calcite and dolomite (c);

CO2- and H2O- aided weathering of a few minerals of feldspar group (d)

(a) Generation of carbonic acid and bicarbonate

CO2 + H2O → H2CO3 1

H2CO3 → H+ + HCO3

- 2

(b) Oxidation of pyrite

2FeS2 + 7 O2+2H2O → 2 Fe2+ + 4 SO4

2- + 4 H

+ 3

4 Fe2+

+ O2 + 4 H+ → 4 Fe

3+ + 2 H2O 4

FeS2 + 14 Fe3+

+ 8 H2O → 15 Fe2+

+ 2 SO42-

+ 16 H+ 5

(c) Carbonic acid - and sulfuric acid - aided dissolution of calcite and

dolomite

CaCO3 + H2CO3 → Ca2+

+ 2HCO3-

6

CaMg(CO3)2 + 2 H2CO3 → Ca2+

+ Mg2+

+ 4 HCO3- 7

CaCO3 + H2SO4 → Ca2+

+ SO42-

+ H2CO3 8

CaMg(CO3)2 + 2 H2SO4 → Ca

2+ + Mg

2+ + 2 SO4

2- + 2 H2CO3 9

(d) CO2 - and H2O - aided weathering of feldspars

Augite* + 6 CO2 + 5 H2O → Kaolinite* + Ca2+

+ Mg2+

+ Fe2+

+ 6 HCO3- + SiO2 10

Calcic plagioclase* + 2 CO2 + 3 H2O → Kaolinite + Ca2+

+2 HCO3- + SiO2 11

Plagioclase + 1.2 CO2+1.8H2O → 0.6 Kaolinite + 0.8 Na++0.2 Ca

2++1.2 HCO3+

1.6 SiO2

12

8 Bytownite* + 14 CO2 + 2 H2O →7 Kaolinite + 2 Na+ + 6 Ca

2+ + 14 HCO3

- + 14

SiO2

13

4 Labradorite* + 6 CO2 + 9 H2O →= 3Kaolinite + 2 Na+ + 2 Ca

2+ + 6 HCO3

- + 4

SiO2

14

2 Albite* + 2 CO2 + 11 H2O → Kaolinite + 2 Na+ + 2 HCO3

- + 4 SiO2 + 8 H2O 15

2 Alkali feldspar* + 2 CO2 + 3 H2O → Kaolinite + 2 Na+ + 2 K

+ + 2 HCO3

- + 4

SiO2

16

Augite*: CaMgFeAl2SiO3O12; Calcic Plagioclase*: CaAl2Si2O8; Plagioclase*: Na0.8Ca0.2Al1.2Si2.8O8;

Bytownite*: Na0.25Ca0.75Al1.75Si2.25O8; Labradorite*: Na0.5Al1.5Si2.5O8; Albite*: NaAlSi3O8;

Alkali feldespar*: (NaK)AlSi3O8; Kaolinite*: Al2Si2O5(OH)4. Not all equations are perfectly balanced.

(source: Raymahshay 1996; Kim et al. 2004; Singh et al. 2007)

In groundwater Na+ content is derived mainly from one or more of the

following sources: (1) rain water, (2) dissolution of halite, (3) weathering of alkali

feldspars (4) direct cation exchange reactions and (5) anthropogenic inputs. In the

study area, residual connate saline water of the aquifer sediments constitutes another

source of Na+ of the fresh groundwater.

Rain water provides more or less equal amounts of Na+ and Cl

- and dissolution

of halite also releases equal amounts of Na+ and Cl

-. In the fresh groundwater samples

the values of Na+/Cl

- molar ratio in 3 samples equals 1 and in the remaining 33

samples the values of the Na+/Cl

- molar ratio are either less than 1 or higher than 1.

This data is graphically shown in Figure 3.10. The above data indicate that the Na+

87

content in 33 fresh groundwater samples was derived from sources other than rain

water and halite. Only 3 samples indicate derivation of Na+ and Cl

- from rain water

and/or halite.

Figure 3.10 Bivariate Na+ versus Cl

- equivalent diagram of the fresh groundwater samples

(n = 36)

Values of Na+/Cl

- molar ratio and CAI can provide information on two sources

of Na+

(±K+), viz., weathering of alkali feldspars and direct cation exchange reaction.

Values of Na+/Cl

- molar ratio higher than 1 indicate contribution of Na

+ to the

groundwater via weathering of alkali feldspars (Stallard and Edmond, 1983;

Meybeck, 1987). Direct cation exchange reaction (positive CAI value) indicates

exchange of alkaline earths of the groundwater with alkalis of the exchanger of the

aquifer matrix.

Out of 33 samples of the fresh groundwater, 20 samples are characterized by

values of Na+/Cl

- molar ratio higher than 1 (range= 1.07 to 2.91) and negative values

of CAI. In the remaining samples (barring sample No.1, table 2) the values of Na+/Cl

-

molar ratio is less than 1 (range = 0.53 to 0.97) and the values of CAI are positive.

The above data indicate that part of the Na+ content in 20 fresh groundwater samples

was derived from alkali feldspar weathering, according to the reactions shown in table

3.15, Eq. 12 to 16. Fresh ground water representing these samples lost certain

amount of their Na+ (±K

+) content due to the involvement of reverse cation – anion

exchange reaction. In 12 samples of the fresh groundwater part of Na+

(±K+) content

was derived from exchanger of the aquifer matrix via direct cation exchange reaction.

0

2

4

6

8

10

0 2 4 6 8 10

Na +

(meq

/L)

Cl- (meq/L)

88

3.8 BRACKISH GROUNDWATER


Brackish groundwater, in terms of meq/L, is characterized by

Ca2+

>Na+>Mg

2+>K

+ and Cl

->HCO3

->SO4

2->NO3

-. Average contribution of individual

cation to total cations is: 37.97% Ca2+

, 34.53% Na+, 26.92% Mg

2+ and 0.56% K

+. On

an average, anions are made up of 46.79% Cl-, 44.50% HCO3

-, 8.44% SO4

2- and

0.26% NO3-. Average content of (Ca

2+ + Mg

2+) (12.63 meq/L) is higher than that of

(Na+ + K

+) (6.83 meq/L) and the average content of (HCO3

-+SO4

2-) (10.16 meq/L) is

slightly higher than that of (Cl-+NO3

-) (9.03 meq/L). Average total concentration of

ions in the brackish groundwater equals 38.65 meq/L, which is 1.80 times (80.35%)

higher than that of the fresh groundwater. In the brackish groundwater (Na++ Cl

-)

constitutes 46.62% of the total concentration of ions, whereas the same in the fresh

groundwater constitutes 24.74% of its total ions. Both fresh and brackish

groundwaters have similar sequence of abundance of cations. Among anions, fresh

groundwater is dominated by HCO3- whereas the brackish groundwater contains Cl

- as

the dominant anion.


On the Piper’s diagram (Piper, 1944) the hydrochemical data of 4 brackish

groundwater samples plot in the fields 1, 4, 6 and 9 (Diagram not provided). This

data indicate that in all samples alkaline earths exceed alkalis and strong acids (Cl-

and SO42-

) exceed weak acid (HCO3-). In one sample non-carbonate hardness

(secondary salinity) exceeds 50% and in 3 samples none of cation or anion pairs

exceed 50%. Cation-wise, all samples belong to no dominant type and anion-wise,

two samples belong to chloride type and 2 samples, to no dominant type.


Spearman’s rank correlation matrices of the physico-chemical data of 4

brackish groundwater samples were calculated (Table not provided). Among the

cations, Ca2+

shows strong positive correlation with K+ (r

2 = 0.77) and weak positive

correlation with Na+ (r

2 = 0.40). Mg

2+ exhibits strong positive correlation with K

+ (r

2

= 0.77) and Cl- (r

2 = 0.80) and weak positive correlation with Ca

2+ (r

2 = 0.20). Na

+

shows positive correlation with Cl- (r

2 = 0.60) and weak positive correlation with

89

HCO3- (r

2 = 0.40), Ca

2+ (r

2 = 0.40) and K

+ (r

2 = 0.26). K

+ shows strong positive

correlation with Cl- (r

2 = 0.77), Mg

2+- (r

2 = 0.77) and Ca

2+ (r

2 = 0.77) and weak

positive correlation with Na+ (r

2 = 0.26). Positive correlation between Na

+ and HCO3

-

may be related to alkali feldspar weathering. Positive correlations between Na+ and

Cl-, K

+ and Cl

- and weak positive correlation between Na

+ and K

+ indicate signatures

of both residual connate saline water and the upconed saline water admixed with fresh

groundwater leading to the development of the brackish groundwater.

3.8.4 Factor Analysis

Hydrochemical data of 12 variables of the brackish groundwater samples

(n=4) were subjected to Factor analysis. During the present study, Factors with

eigenvalues higher than 1 are considered. Following this procedure, 3 independent

factors were extracted (Table not provided).

Factor 1 accounts for 61.88% of the total variance and exhibits high positive

loadings of EC, TH, TDS, Ca2+

, Mg2+

, K+ and Cl

-. Factor 2 accounts for 23.48% of

total variance and consists of high positive loadings of pH, Na+ and HCO3

-. Factor 3

accounts for 14.63% of the total variance and exhibits high positive loadings of

HCO3- and NO3

-. Positive loadings of Ca

2+ and Mg

2+ with Factor 1 and positive

loadings of HCO3- with Factors 2 and 3 indicate essentially rock weathering. Positive

loadings of K+ and Cl

-with Factor 1 and positive loading of Na

+ with Factor 2 reveal

saline water signature. Positive loading of NO3- with Factor 3 indicates anthropogenic

inputs.

3.8.5 Additional sources of dissolved solids in brackish groundwater

The exact source of the additional dissolved solids for generation of 4 brackish

groundwater pockets near the northern boundary of the unconfined aquifer is not

known. Development of brackish groundwater pockets in the unconfined aquifer may

be related to one among the following two possibilities: (1) Increase in the

concentration of TDS along the south to north flow path of the groundwater through

aquifer as a consequence of weathering of minerals (Freeze and Cherry, 1979) and (2)

upconing of saline water from the saline groundwater zone of the underlying semi-

confined aquifer along the bore wells due to excessive extraction of groundwater from

the unconfined aquifer. Among these two possible modes of development of the

brackish groundwater pockets, the second possibility seems to be more convincing, if

90

the hydrogeological setting of the study area is taken into consideration. In that case,

the mixing of the fresh groundwater and upconed saline groundwater has taken place

under conditions of non-conservative mixing process. The latter finds evidence from

the involvement of ion exchange reactions between mixing waters and clay matrix of

the aquifer.

chapter 3 hydrochemistry and sources of dissolved...

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