Assessment of hydrogeochemistry and the quality of groundwater in 24-Parganas districts, West Bengal

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Environmental Earth Sciences ISSN 1866-6280 Environ Earth SciDOI 10.1007/s12665-014-3431-2

Assessment of hydrogeochemistry and thequality of groundwater in 24-Parganasdistricts, West Bengal

Neha Singh, Ravi Prakash Singh, VikasKamal, Ratan Sen & Saumitra Mukherjee

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ORIGINAL ARTICLE

Assessment of hydrogeochemistry and the quality of groundwaterin 24-Parganas districts, West Bengal

Neha Singh • Ravi Prakash Singh • Vikas Kamal •

Ratan Sen • Saumitra Mukherjee

Received: 26 November 2013 / Accepted: 10 June 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract Hydrogeochemistry of an area helps in under-

standing the geological processes which control the

chemistry of water and play an important role in deter-

mining the suitability of groundwater for various purposes.

In the present study, an attempt has been made to under-

stand the geological processes controlling the quality of

water in a part of North and South 24-Parganas districts of

West Bengal. 39 representative groundwater samples were

collected from the study area and physico-chemical

parameters were analyzed for all the samples. Schoeller

and Durov diagram were used to understand the hydro-

chemical nature of water. Results obtained from water

chemistry were used in the interpretation of controlling

processes using different conventional graphs, and deter-

mining the quality of groundwater. Silicate weathering and

ion exchange are the dominant processes controlling the

chemistry of groundwater in the study area, where calcium

and magnesium in the water are replaced by the sodium

and potassium in the minerals from the host rock as chloro-

alkaline indices are negative at most of the places. Satu-

ration index was calculated to understand the mineralogy

of the subsurface. The groundwater is oversaturated with

iron containing minerals like Fe(OH)3, goethite, and

hematite, while undersaturated with anhydrite and gypsum.

The groundwater suitability was determined by calculating

water quality index for drinking purpose; while SAR, and

residual sodium carbonate indices for the agricultural

purpose. The groundwater in the study area is not suitable

for drinking, but can be used for other household use and in

irrigation for agriculture.

Keywords Hydrochemical facies � Silicate weathering �Ion exchange � Chloro-alkaline indices � Saturation index

Introduction

Groundwater is the major source of freshwater in many

parts of the world for meeting the requirements of domestic

and agricultural purposes. Approximately one-third of the

world’s population depend on groundwater for drinking

purpose (UNEP 1999). Due to the rapid growth of popu-

lation, urbanization and accelerated pace of industrializa-

tion, and due to inadequate supply of surface water, there

has been a tremendous increase in the demand of the

groundwater resources as fresh water. Geology of an area,

the degree of chemical weathering of various rock types,

anthropogenic factors affect the chemistry of groundwater

(Giridharan et al. 2008), and the quality of groundwater is

altered by an increase in concentration of chemical species

with its movement below the ground surface (Freeze and

cherry 1979; Kortatsi 2007).

Urban growth has led to overexploitation of ground-

water (Jameel and Sirajudeen 2006) adversely affecting its

quality and quantity. According to Central Ground Water

Board, out of 432 km3/year fresh groundwater resources in

India, 396 km3 is estimated to be utilizable. Due to the

varied utilization of land, there is a lot of variation in the

quality of groundwater from place to place. A number of

studies (Durvey et al. 1997; Dasgupta and Purohit 2001;

Khurshid et al. 2002; Subba Rao 2006) have been carried

out in different parts of India to determine the groundwater

quality with respect to drinking and irrigation purposes. An

understanding about the chemical quality of groundwater is

essential in determining its usefulness for domestic,

industrial and agricultural purposes (Mukherjee et al.

N. Singh � R. P. Singh � V. Kamal � R. Sen � S. Mukherjee (&)

School of Environmental Sciences, Jawaharlal Nehru University,

New Delhi 110067, India

e-mail: [email protected]

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DOI 10.1007/s12665-014-3431-2

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2005). The chemistry of groundwater depends on the

minerals present in geological formations and is also con-

trolled by many interrelated processes, and thus can be

used to understand the hydrogeological processes, the

mechanisms controlling the quality of groundwater (Zuane

1990).

West Bengal lies within the Ganga–Brahmaputra delta

basin and is one of the states, which has high contamina-

tion of arsenic in groundwater (Mukherjee et al. 2008).

Though much work has been carried out focusing the

arsenic contamination and its effect on the human health in

the study area, very limited studies have been carried out to

determine undergoing processes controlling the quality of

groundwater, hydrogeochemistry of the area and the suit-

ability of groundwater for different purposes. Groundwater

quality assessment was done in the South 24-Parganas of

West Bengal to find the suitability of groundwater for

drinking and agricultural purposes (Mukherjee et al. 2005).

In the present study, the assessment of hydrogeochemistry

of the study area has been carried out to determine the

processes controlling the groundwater chemistry. Though

the area comes under the arsenic contaminated districts of

West Bengal, an assessment of groundwater quality for

drinking purpose using water quality index, and for agri-

cultural purpose using indices like sodium adsorption ratio,

sodium percentage, residual sodium carbonate, Kelley’s

ratio, magnesium hazard, and the permeability index has

been carried out.

Study area and methods

Study area

The study area is a part of North 24-Parganas and South

24-Parganas districts of West Bengal (Fig. 1). The area lies

within the Bengal basin, a large asymmetrical pericratonic

basin in the north eastern part of India. The area has a hot

and humid climate receiving adequate rainfall from north

east and south west monsoon. The region has levees, del-

taic plains and swamp areas as major geomorphological

features. In the study area, intermediate and lower aquifers

are reported to be located at shallower depth in North

24-Parganas, while at greater depth in South 24-Parganas

Fig. 1 Study area

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(Chakraborti et al. 1996). The shallow and intermediate

aquifers are found to be arsenic enriched.

In North 24-Parganas the river is slow and meandering

and the region is a part of mature delta plain, while South

24-Parganas falls in an active delta region where the for-

mation of the delta is a continuous process. Mineral

assemblage of kyanite–garnet–staurolite–biotite–tourma-

line–chlorite–hornblende–epidote in the subsurface sedi-

ments of the 24-Parganas represents highly metamorphosed

rocks. Groundwater in the intermediate aquifer contains

calcium, magnesium, bicarbonate as the major ions with

elevated concentration of iron, phosphate and arsenic

(PHED 1991; Das et al. 1995). Elangovan and Chalakh

(2006) reported the presence of major quantities of quartz

and feldspar in the XRD analysis of the sediments.

North 24-Parganas is the most populous district of West

Bengal. It is the second most and South 24-Parganas is the

sixth most populous district in India. As agriculture is the

dominating sector in both the districts, net irrigated area

reported to be 200.56 thousand ha in North 24-Parganas

and 115.73 thousand ha in South 24-Parganas. As

groundwater is the main source of freshwater supply, the

district depends on groundwater for drinking as well as

agricultural purposes. A huge amount of groundwater is

required for irrigation as rice is the main agricultural

product in this region. Thus, determining the suitability of

groundwater for drinking and agriculture purpose has been

carried out in the present study.

Sampling and sample analysis

Based on the surface manifestations of hydrogeomorpho-

logical features as inferred from satellite imagery, total 39

groundwater samples were collected from North 24-Parg-

anas and northern part of South 24-Parganas in the month

of February, 2012. The locations of the sampling sites were

recorded using Garmin GPS. Polypropylene bottles (Tar-

son: 250 and 125 ml) were acid washed, rinsed with dis-

tilled water and dried before using them for sampling

purpose. These dried bottles were carried to field in the

sampling bag, and were used during the collection of water

samples from each site. Groundwater samples were col-

lected from the hand pumps that were in working condi-

tions and were in use by the local residents. Water was

discarded for 20–25 strokes to minimize the impacts of iron

pipe, and then the bottles were rinsed two to three times

with groundwater to be sampled. Samples collected in

125 ml bottle were acidified with HNO3 to bring the pH \2

for cation and heavy metal analysis, and samples of 250 ml

bottle were preserved without acidification (Rina et al.

2013; Chetia et al. 2011). Field blank was also collected to

evaluate the contamination in the sample container. pH and

EC were measured onsite using respective electrode

(Hanna). The samples were brought to the laboratory in ice

containing Styrofoam boxes, vacuum filtered using

0.45 lm Millipore filter paper and stored at 4 �C for further

analysis.

The physical and chemical parameters were determined

by following the standard protocol given by the American

Public Health Association (APHA 2005). Bicarbonate and

chloride were analyzed using titration method, while

phosphate, sulphate, nitrate and fluoride were analyzed

using double beam UV–Vis spectrophotometer (Perkin

Elmer) following APHA. Sodium, potassium and calcium

were analyzed on Flame photometer, while other cations

were analyzed on Thermo Scientific Atomic Adsorption

Spectrophotometer (AAS). Arsenic was analyzed on AAS

using a hydride generator. Analytical accuracy of mea-

surement of ions was estimated by calculating normalized

charge balance index.

Results and discussion

The result of groundwater quality data has been given in

Table 1 in the form of minimum, maximum, and mean.

Chemical data of the groundwater samples were plot-

ted on Schoeller and Durov plot to infer hydrogeochem-

ical facies. Schoeller diagram (1955) represents variation

of major cations and anions in the groundwater samples

on a single graph. Diagram (Fig. 2) shows that calcium

and bicarbonate are the dominating ions, while the con-

centration of sodium ? potassium, chloride and sulphate

vary in all the groundwater samples. Durov diagram is

based on percentage of milliequivalent values of major

ions, and provides a better display of hydrochemical types

Table 1 Mean, maximum, and average chemical composition of

groundwater

Parameters Min Max Avg

pH 6.8 8.1 7.27

EC (lS/cm) 540 1,300.0 869.23

Na2? (ppm) 48.5 173.700 85.27

K? (ppm) 1 8.20 3.85

Mg2? (ppm) 56.79 88.61 65.39

Ca2? (ppm) 67.53 139.34 97.83

Fe2? (ppm) 0.00 15.29 2.15

As (ppb) 0.77 69.45 15.39

HCO3-(mg/l) 496.12 997.50 706.22

Cl- (mg/L) 4.74 457.45 112.63

PO43-(mg/L) 12.13 312.32 119.34

NO3- (mg/L) 0.00 18.00 2.36

SO42- (mg/L) 0.01 49.12 9.74

Dissolved silica (mg/L) 29.63 83.29 60.63

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along with the values of TDS and pH. Durov (1949)

described the groundwater having 100–300 mg/L TDS, as

the simple or primary water having bicarbonates of cal-

cium and magnesium. When water of first type is enri-

ched by soluble sulphate and chloride salts of magnesium

and alkali, its mineralization get increased from 500 to

1,000 mg/L. In the Durov diagram for the groundwater

samples, the anions and cations together total 100 %.

TDS value of the groundwater samples in the present

study ranges from 400 to 800 mg/L, which indicates that

the groundwater in the study area is mainly of secondary

or transition type. Durov diagram (Fig. 3) shows that

groundwater have higher concentration of bicarbonate and

chloride anions, while alkali (Na ? K) and alkaline earth

Fig. 2 Schoeller diagram for

water samples

Fig. 3 Durov diagram for water

samples

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metals (Ca ? Mg) are present nearly in equilibrium. It

may be due to the enrichment of bicarbonates of calcium

and magnesium in groundwater with sulphate and chlo-

ride salts of alkali and magnesium. pH of the groundwater

varies from 6.8 to 8.1.

The Schoeller and Durov diagram shows the dominance

of bicarbonate, chloride, alkali and alkaline metals. Thus, it

is essential to understand the hydrogeological processes

which control the groundwater chemistry. The results

obtained from the water analysis were subjected to various

conventional graphs to understand these processes in the

study area.

Hydrogeochemistry

In the scatter diagram of (Ca ? Mg) vs. (HCO3 ? SO4),

samples falling above the equiline (1:1), represent car-

bonate weathering (dissolution of calcite, dolomite, and

gypsum) and reverse ion exchange as dominant processes;

while samples falling below the equiline indicate the sili-

cate weathering and ion exchange within the area (Datta

et al. 1996; Rajmohan and Elango 2004). In the present

study, most of the samples fall below the equiline (Fig. 4)

due to an excess of bicarbonate, which indicate that silicate

weathering and ion exchange are the governing processes,

while some of the samples present above the equiline

indicate that carbonate weathering and reverse ion-

exchange processes also occur at some of the places.

However, in scatter plot of HCO3- vs. Na? (Fig. 5),

increased concentration of HCO3- compared to Na? sug-

gests that silicate weathering is the dominant process in the

study area. Atmospheric CO2 reaches to the ground in the

form of carbonic acid with rainwater, and initiates the

weathering process. Bicarbonate is the major anionic

product of the weathering process, besides the release of

various cations. A general reaction for silicate weathering

can be written as:

ðNaþ; Mg2þ; Ca2þ; KþÞ silicates þ H2CO3

! H4SiO4 þ HCO�3 þ Naþ þ Mg2þ þ Ca2þ þ Kþ þ Clays

whereas calcite dissolution can be represented as:

CaCO3 þ H2CO3 ! Ca2þ þ 2HCO�3

Depending on the availability of dissolved CO2 and

carbonic acid, the carbonate minerals, which are present on

the path, get dissolved during infiltration of rainwater.

Further concentration of HCO3- in ground water is also

increased by silicate weathering as can be inferred in the

present study from the scatter plot of HCO3 vs. Na (La-

kshmanan et al. 2003). This graph shows the excess of

bicarbonate in groundwater in comparison to sodium,

which means that the excess bicarbonate must be balanced

by other cations. The scatter plot for Ca ? Mg vs.

HCO3 ? CO3 (Fig. 6) suggests that an excess of bicar-

bonate in the water is balanced by Ca ? Mg, as nearly all

samples fall above the equiline (1:1).

The scatter plot of Ca ? Mg vs. total cations (Fig. 7)

shows an increasing contribution of alkalies to major ions

which suggest that silicate weathering plays an important

role within the study area (Subba Rao 2008) as all the

Fig. 4 Scatter plot for Ca ? Mg vs. SO4 ? HCO3

Fig. 5 Scatter plot for HCO3 vs. Na

Fig. 6 Scatter plot for Ca ? Mg vs. HCO3 ? CO3

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samples fall below the equiline. In the scatter plot of

Na ? K vs. total cations (Fig. 8), all the samples are

present below the equiline and above the Na ? K/0.50 TC

line, which indicates that silicate weathering is more sig-

nificant and is the main source of cations (Stallard and

Edmond 1983). Increase in alkalies with the simultaneous

increase of Cl- ? SO4 (Fig. 9) indicates a common source

of these ions, resulting from dissolution of soil salts (Sarin

et al. 1989; Datta and Tyagi 1996). Excess of Na over K is

observed due to the greater resistance of K to chemical

weathering (Subba Rao 2008).

Besides weathering and dissolution of minerals, ion

exchange and adsorption of cations play an important role

in controlling the ion ratios in groundwater. The scatter

plot of Ca ? Mg vs. HCO3 ? SO4 suggests that ion

exchange is prevalent throughout the study area. In Na vs.

Ca scatter diagram (Fig. 10) most of the samples fall below

the equiline showing increased concentration of Ca com-

pared to Na. This further strengthens the support for ion-

exchange process in the study area. Ion-exchange process

may be responsible for higher concentration of Na in

groundwater. Most of the samples have a Na/Cl ratio

around or above 1 (Table 2) indicating that ion exchange is

prevalent throughout the study area (Kumar et al. 2006).

Ion exchange can lead to replacement of Ca in groundwater

from Na adhered on clay particles.

Changes in chemical composition of groundwater take

place during its travel in the subsurface (Sastri 1994). The

chloro-alkaline indices CAI-1 and CAI-2 indicate the ion

exchange between the groundwater and its host environ-

ment during a residence time or travel (Schoeller 1965,

1967, 1977). The chloro- alkaline indices used in evalua-

tion are calculated using the formulae:

Chloro Alkaline Indices ðCAI-1Þ ¼ ½Cl�ðNaþ KÞ�=Cl

Chloro Alkaline Indices ðCAI-2Þ¼ ½Cl�ðNaþ KÞ�=ðSO4 þ HCO3 þ CO3 þ NO3Þ

When there is cation exchange of sodium and potas-

sium from water with magnesium and calcium in the host

rock, the exchange is known as direct, when indices are

positive. If the indices are negative, the exchange is

reversed and indirect indicating chloro-alkaline disequi-

librium (Schoeller 1967; Kumar et al. 2007). Figure 11

shows that most of the samples have negative values for

both indices, which indicate an indirect base exchange

reaction, which means that Ca2? and Mg2? in the water

are exchanged with Na? and K? in the minerals from the

host rock. This explains the reason for the abundance of

alkalies in the groundwater over alkaline earth elements.

Cation exchange is possible when exchange sites such as

Fig. 7 Scatter plot for Ca ? Mg vs. TC Fig. 8 Scatter plot for Na ? K vs. total cations

Fig. 9 Scatter plot for Na ? K vs. Cl ? SO4

Fig. 10 Scatter plot for Na vs. Ca

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clay minerals are present. Clay minerals are the product

of silicate weathering (Acheampong and Hess 1998; Yi-

dana et al. 2008) thus enhancing the ion-exchange process

in the study area. The chloro-alkaline index values of the

groundwater samples from the study area indicate a base

exchange reaction (chloro-alkaline disequilibrium) exist-

ing in the majority of samples (33 samples). These sam-

ples may be referred as base exchange softened water,

where the alkaline earth had been exchanged for Na?

(HCO3- [ Ca2? ? Mg2?) (Table 2). In the rest of the

samples at six sites, the values are positive, indicating

chloro-alkaline equilibrium, in which the Na? ions have

been exchanged for the alkaline earths (Ca2? ?

Mg2? [ HCO3-); these can be referred as a base

exchange hardened water (Handa 1969). In the present

study, a majority of the samples had HCO3- concentra-

tion higher than alkaline earths, indicating base exchange

softened water.

Table 2 Na/Cl, CAI-1, CAI-2

values for all groundwater

samples

S. no. Na/Cl CAI1 CAI2 Ca ? Mg/HCO3

1 0.61 0.30 0.27 1.19 Ca2?? Mg2? > HCO3-

2 0.75 0.14 0.11 1.13 Ca2?? Mg2? > HCO3-

3 0.52 0.40 0.59 1.34 Ca2?? Mg2? > HCO3-

4 1.82 -1.12 -0.32 0.96 HCO3- > Ca2?? Mg2?

5 0.62 0.27 0.24 1.05 Ca2?? Mg2? > HCO3-

6 0.64 0.24 0.17 0.87 HCO3- > Ca2?? Mg2?

7 1.32 -0.60 -0.15 0.71 HCO3- > Ca2?? Mg2?

8 1.91 -1.31 -0.25 0.78 HCO3- > Ca2?? Mg2?

9 6.09 -6.53 -0.33 0.89 HCO3- > Ca2?? Mg2?

10 2.98 -2.75 -0.37 1.12 Ca2?? Mg2? > HCO3-

11 1.25 -0.49 -0.16 0.96 HCO3- > Ca2?? Mg2?

12 1.22 -0.53 -0.13 0.83 HCO3- > Ca2?? Mg2?

13 1.14 -0.38 -0.14 0.97 HCO3- > Ca2?? Mg2?

14 2.05 -1.56 -0.21 0.81 HCO3- > Ca2?? Mg2?

15 3.97 -4.23 -0.25 0.88 HCO3- > Ca2?? Mg2?

16 4.54 -4.82 -0.23 0.85 HCO3- > Ca2?? Mg2?

17 1.25 -0.67 -0.12 1.04 Ca2?? Mg2? > HCO3-

18 1.09 -0.44 -0.08 1.00 Ca2?? Mg2? > HCO3-

19 1.58 -0.99 -0.20 0.95 HCO3- > Ca2?? Mg2?

20 15.79 -21.38 -0.22 0.76 HCO3- > Ca2?? Mg2?

21 3.42 -3.81 -0.19 0.81 HCO3- > Ca2?? Mg2?

22 2.54 -2.31 -0.19 0.84 HCO3- > Ca2?? Mg2?

23 1.94 -1.70 -0.16 0.89 HCO3- > Ca2?? Mg2?

24 1.44 -0.99 -0.12 0.86 HCO3- > Ca2?? Mg2?

25 1.30 -0.69 -0.11 0.82 HCO3- > Ca2?? Mg2?

26 0.42 0.51 0.57 1.15 Ca2?? Mg2? > HCO3-

27 1.54 -0.94 -0.15 0.86 HCO3- > Ca2?? Mg2?

28 1.48 -1.00 -0.14 0.87 HCO3- > Ca2?? Mg2?

29 2.34 -2.01 -0.33 1.25 Ca2?? Mg2? > HCO3-

30 3.35 -3.33 -0.21 0.73 HCO3- > Ca2?? Mg2?

31 2.40 -2.04 -0.21 0.75 HCO3- > Ca2?? Mg2?

32 3.76 -4.15 -0.18 0.70 HCO3- > Ca2?? Mg2?

33 1.69 -1.11 -0.21 0.78 HCO3- > Ca2?? Mg2?

34 1.05 -0.16 -0.07 0.74 HCO3- > Ca2?? Mg2?

35 1.23 -0.42 -0.15 0.75 HCO3- > Ca2?? Mg2?

36 1.27 -0.63 -0.09 0.67 HCO3- > Ca2?? Mg2?

37 1.30 -0.61 -0.15 0.99 HCO3- > Ca2?? Mg2?

38 6.63 -7.92 -0.28 0.91 HCO3- > Ca2?? Mg2?

39 2.15 -1.77 -0.24 1.18 Ca2?? Mg2? > HCO3-

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Geochemical modelling

Saturation index of minerals was calculated by the USGS

geochemical code PHREEQC 3.1.2 to evaluate the degree

of equilibrium between water and the respective mineral.

Calculation of saturation indices helps in predicting the

reactive mineralogy of the subsurface from groundwater

data (Deutsch 1997). Saturation index is the logarithm of

the ratio of ion activity product (IAP) to the mineral

equilibrium constant (Ksp).

SI ¼ logIAP

Ksp

� �

A positive saturation index (SI [ 0) indicates that the

groundwater is oversaturated with respect to a particular

mineral and hence is incapable of dissolving more of the

mineral. Such index values reflect groundwater discharging

from an aquifer containing ample amounts of mineral with

sufficient resident time. A negative saturation index

(SI \ 0) indicates that the groundwater is undersaturated

with respect to particular mineral and reflect insufficient

amount of mineral for solution or short residence time.

Neutral SI indicates that the groundwater is in equilibrium

with the particular mineral phase.

Variation of the saturation index for different minerals is

shown in Fig. 12. The calculated saturation index values of

minerals by PHREEQC demonstrate that the groundwater

in the area is supersaturated with iron containing minerals

like Fe(OH)3, goethite, and hematite. The water is also

saturated with the calcite, chalcedony, dolomite, quartz.

Sepiolite, siderite, chrysotile and talc are oversaturated at

some of the places and undersaturated at some sites. The

groundwater is undersaturated with anhydrite and gypsum.

Oversaturation of hematite, goethite and Fe(OH)3 in the

study area indicates the abundance of iron minerals in the

aquifer. Groundwater gets sufficient residence time to be in

contact with these minerals and their weathering has

resulted in abundance of iron in the groundwater. Calcite,

dolomite and siderite are the carbonate minerals, while

chalcedony, quartz, sepiolite, chrysotile are the silicate

minerals. Sepiolite is a clay mineral formed of complex

magnesium silicate and is frequently found either with

gypsum or dolomite (Leguey et al. 2010). Groundwater is

undersaturated with anhydrite and gypsum, and less con-

centration of sulphate in the groundwater explains the less

dissolution of these minerals. The high concentration of

calcium in groundwater may be attributed to the dissolution

of calcite and dolomite as carbonate weathering also occurs

at some of the places (Fig. 2). Talc is hydrated magnesium

silicate which can be formed from either reaction between

dolomite and silica or the metamorphism of magnesium

minerals in the presence of carbon dioxide and water.

Oversaturation of talc in groundwater may be the result of

either process in the aquifer.

Mineral assemblage of the subsurface sediment is mainly

silicate group. Biotite is a phyllosilicate mineral containing

potassium, magnesium and iron with the silicate group.

Tourmaline is a silicate mineral with elements like iron,

magnesium, sodium, potassium, calcium. Garnets are a group

of silicate minerals in which silicate group is balanced by the

divalent cations like calcium, magnesium, and iron, and tri-

valent cations like aluminium, iron, and chromium. Staurolite

contains iron as the divalent cation in its silicate structure.

Hornblende is a mixture of calcium–iron–magnesium sili-

cate, aluminium–iron–magnesium silicate and iron–magne-

sium silicate. Epidote is also a silicate mineral with calcium,

aluminium and iron. Aquifer of the study area is composed of

mainly silicate minerals, thus the silicate weathering plays an

important role in the study area. The dissolution of the sili-

cate minerals present in the aquifer determines the chemistry

of groundwater and is responsible for increased concentration

of cations and bicarbonate in the area. Singh et al. (2013)

have tried to relate the elevated concentration of arsenic with

the process of silicate weathering and ion exchange. Goethite

forms through the weathering of various iron rich minerals in

the zone of oxidation within the soil. Oversaturation of

groundwater with goethite indicates that the mineral may

have formed in the subsurface because of the aerated zone

within the aquifer created by the fluctuation in the water

table. Arsenic was reported to be adsorbed to iron hydroxide

Fig. 11 Scatter plot for CAI-1 vs. CAI-2

Fig. 12 Plot of saturation indices (SI) of minerals

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coated sand grain margins and to clay minerals (Acharyya

et al. 1999) and on the surface of grains coated with Fe

oxyhydroxides (Nickson et al. 2000). Saturation index indi-

cates the presence of iron minerals and iron oxyhydroxides

(FeOOH, goethite) in abundance in the aquifer. Thus,

weathering of these minerals helps in the release of arsenic

into the groundwater resulting in the elevated concentration

of arsenic in the groundwater.

Groundwater suitability

Groundwater chemistry can be utilized to evaluate water

quality for drinking, agricultural and industrial purposes

(Subba Rao 2006; Edmunds et al. 2002). Water quality

index has been used to assess the suitability of groundwater

for drinking purpose and due to abundance of sodium and

bicarbonate in the area indices like SAR and residual

sodium bicarbonate have been used to determine its suit-

ability for agricultural purposes.

Water quality index

Water quality index is an important tool which provides

information about the composite influence of individual

water quality parameters on the overall quality of water for

most of the domestic purposes (Mitra and Member 1998).

The water quality index is calculated to determine water

quality and its suitability for drinking purpose (Tiwari and

Mishra 1985; Naik and Purohit 2001; Avvannavar and

Shrihari 2008). WQI helps in reducing a large number of

physico-chemical variables into a single number which

expresses the overall water quality at certain locations

(Yogendra and Puttaiah 2008). WQI generates a score by

integrating complex data set and helps in understanding

water quality issues and evaluating water quality trends

(Boyacioglu 2007). In the present study, WHO recom-

mended standard for drinking water has been used and

weighted index methods developed by Tiwari and Mishra

(1985); Asadi et al. (2007) have been followed to deter-

mine the suitability of groundwater for drinking purposes.

In this study, the water quality index has been calculated

upon the basis of 14 parameters. In the first step, each

parameter has been assigned a weight (wi) between 1 and 5

according to its relative importance in determining the

overall quality of groundwater in the study area (Table 3).

In the second step, relative weight (Wi) is calculated using

the following equation:

Wi ¼ wi=Xn

i¼1

wi

where, Wi is the relative weight, wi is the weight of each

parameter, n is the number of parameters

In the third step, quality rating scale (qi) for each

parameter is calculated by:

qi ¼ ðCi=SiÞ � 100

where, qi is the quality rating, Ci is the concentration of

each chemical parameter, Si is WHO standard

Water quality index is calculated by determining SI of

each chemical parameter as per the following equation:

SIi ¼ Wi � qi

WQI ¼X

SIi

where, SI is sub-index of ith parameter.

WQI is water quality index

In the present study, maximum weight was given to arsenic

as the study area has the problem of arsenic contamination

and prolonged drinking of contaminated water may dete-

riorate the human health. Iron has been given the weigh-

tage of 4 as the higher concentration of iron in groundwater

worsen the taste and the quality for drinking purpose;

minimum weight was given to nitrate and sulphate as these

are present in very low concentration within the WHO limit

in the study area. Most of the water samples (66.7 %) fall

under the poor category, some samples (17.9 %) fall under

very poor and few samples (12.8 %) come under water

unsuitable for drinking purposes (Table 4).

The quality of water is degraded in the study area due to

higher concentration of arsenic and iron. Nearly all the

samples are unsuitable for the drinking purpose. Calcula-

tion of water quality index may be used to demarcate the

Table 3 Relative weight of chemical parameters

Chemical

parameters

WHO

Standard

Weight

(wi)

Relative

weight (Wi)

pH 8.5 3 0.081

EC 300 3 0.081

TDS 1,000 4 0.108

Na (mg/l) 200 3 0.081

K (mg/l) 12 1 0.027

Ca (mg/l) 75 3 0.081

Mg (mg/l) 50 2 0.054

Zinc (mg/l) 5 2 0.054

Iron (mg/l) 0.3 4 0.108

Arsenic (lg/l) 10 5 0.135

Sulphate (mg/l) 250 1 0.027

Nitrate (mg/l) 50 1 0.027

Chloride (mg/l) 250 3 0.081

Bicarbonate (mg/l) 120 2 0.054

R wi = 37 R Wi = 1

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hand pumps suitable or unsuitable for drinking purpose

taking water chemistry into account.

SAR and sodicity index

SAR is a measure of alkali/sodium hazards to crops and is

an important parameter to determine the suitability of

groundwater for irrigation (Subramani et al. 2005) as it is

directly related to adsorption of sodium by soil. Karanth

(1987) defined SAR as:

SAR ¼ Na=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðCaþMgÞ=2

pwhere, all ionic concentrations are expressed in meq/l.

Groundwater having higher SAR values is generally not

suitable for irrigation. Higher deposition of sodium may

deteriorate the soil characteristics by reducing soil per-

meability and thus inhibiting the supply of water needed

for the crops. SAR is an important parameter to measure

the sodium hazard for ground water having high bicar-

bonate concentration. The SAR values range from 0.94 to

3.15. According to Richards (1954) classification, all the

samples belong to an excellent category. Replacement of

calcium and magnesium with sodium causes the damage to

soil structure by making them compact and impervious.

SAR indicates the degree to which irrigation water initiates

cation-exchange reactions in soil. The groundwater in the

study area is perfectly suitable for irrigation as all the

collected samples are in excellent category having SAR

values \10.

Sodicity index calculated using SAR (Ravikumar et al.

2011) was also used in the classification of groundwater

samples considering water up to class 2 as suitable for

irrigation. All the samples belong to class 0 (Fig. 13) based

upon the sodicity index except for one having a SAR value

of 3.15, which belongs to class I. Thus, on the basis of SAR

and sodicity index, groundwater can be used for irrigation.

Residual sodium carbonate

To determine the hazardous effect of carbonate and

bicarbonate on the quality of water for agricultural pur-

poses, evaluation of residual sodium carbonate is

important. RSC considers the excess concentration of

bicarbonate and carbonate over the sum of calcium and

magnesium. RSC has been calculated by following equa-

tion (Ragunath 1987):

RSC ¼ ðCO2�3 þ HCO�3 Þ � (Ca2þ þ Mg2þÞ

where, all ionic concentrations are expressed in meq/l

(Eaton 1950).

According to the classification scheme for RSC (Rich-

ards 1954), 44 % of water samples fall under the good

category having RSC below 1.25 meq/l, 31 % samples

have RSC values between 1.25 and 2.5 meq/l and are

considered as doubtful, and rest 25 % having RSC above

2.5 meq/l are unsuitable for irrigation. The high value of

RSC in water may lead to increase in the adsorption of

sodium in the soil (Eaton 1950). In groundwater having a

higher concentration of bicarbonate, calcium and magne-

sium has the tendency to precipitate, and it also causes the

dissolution of organic matter into the soil.

Conclusion

Schoeller and Durov plot for the groundwater sample show

that the groundwater is dominated by alkali, alkaline earth

metals, bicarbonate and chloride. The chemistry of the

analyzed groundwater samples was used to understand the

hydrogeochemistry of the area by plotting various con-

ventional graphs. The mineral assemblage of the subsur-

face in the aquifer is of mainly silicate group. The chemical

composition of groundwater in the study area is greatly

influenced by silicate weathering and ion-exchange pro-

cesses, which control the concentration of calcium, sodium,

magnesium, potassium and bicarbonate in the groundwater.

Clays, the by-product of silicate weathering, have more

surface area and exchange sites thus facilitating more ion-

exchange process. A Na/Cl ratio of 1 and above for most of

the samples indicates prevalence of ion exchange, where

calcium in groundwater can be replaced by sodium on clay

particles, which is further confirmed by the negative values

Fig. 13 Sodicity index of the groundwater samples

Table 4 Water quality classification based on WQI value

Range Type of water Percent of

samples

\50 Excellent water –

50–100.1 Good water 2.6

100–200.1 Poor water 66.7

200–300.1 Very poor water 17.9

[300 Water unsuitable for drinking purposes 12.8

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of chloro-alkaline indices. Saturation indices calculated

using PHREEQC show that groundwater is oversaturated

with the iron bearing minerals. Presence of goethite also

indicates that the fluctuation in water table may have cre-

ated the oxidation zone in the aquifer. Silicate weathering

results in dissolution of arsenic from the surface of the

grains coated with iron hydroxide and iron oxyhydroxide

into the groundwater. Water quality index calculated to

find the suitability of groundwater for drinking purpose

shows that most of the samples fall under the poor to very

poor category. Taking account of all the parameters in the

study, groundwater was found to be unsuitable for drink-

ing. The groundwater in the study area contains higher

concentration of sodium and bicarbonate, thus suitability of

groundwater in terms of SAR and residual sodium car-

bonate was calculated for irrigation purpose. Groundwater

is suitable for irrigation in terms of SAR, but due to excess

of bicarbonate some of the samples come under the

doubtful category. Due to abundance of silicate minerals in

the subsurface sediments of the aquifer, groundwater

chemistry is controlled by silicate weathering and ion-

exchange processes and due to higher concentration of

arsenic and iron, the groundwater is unsuitable for drinking

purpose.

Acknowledgments Financial support as Senior Research fellowship

provided by the University Grant Commission is duly acknowledged.

The author is also thankful to Jawaharlal Nehru University for pro-

viding various research facilities.

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