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CHAPTER 5

GROUNDWATER CHEMISTRY AND QUALITY

ASSESSMENT

5.1 INTRODUCTION

Study of chemistry of groundwater is important as it will help to

understand the various chemical processes that may take place in an aquifer

system, as explained earlier groundwater samples were collected from 45

monitoring wells and analysed from March 2008 to January 2010. Collection

and analysis of groundwater samples from the study area indicate that the pH

of the groundwater ranges from 6.9 to 9.3 with an average of 7.5. Thus the

groundwater of this area is slightly alkaline in nature. The EC of the

groundwater samples ranges from 375 µS/cm to 5030 µS/cm with an average

of 1040 µS/cm. The Eh ranged from -93mv to 880mv an average 176 mv.

Reasonably high Eh values indicate that the groundwater is oxidized state

(Figure 5.1).

Figure 5.1 Minimum, maximum and average values of EC, pH and Eh

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5.2 MAJOR ION CHEMISTRY

The variation in the major ion concentration of groundwater of this

area is shown in the form of box plots (also called box-and-whisker plots)

(Figure 5.2). The line across the box represents the median, whereas the

bottom and top of the box show the locations of the first and third quartiles.

The whiskers are the lines that extend from the bottom and top of the box to

the lowest and highest concentrations of groundwater of the study area.

Figure 5.2 Box whisker diagram showing the variation of major ions in

groundwater

5.2.1 Groundwater types

The general order of dominance of cations in the groundwater of the

study area is Ca2+

>Na+ > Mg

2+ > K

+ while that for anions it is Cl

- >HCO3

- >

SO4-2

. Piper trilinear diagram (1944) was used to identify the groundwater

types. In this diagram (Figure 5.3), points of groundwater samples of the

study area gets plotted in the central diamond shaped field. The diamond

shaped field is divided into six major groundwater types such as CaHCO3,

NaCl, mixed CaNaHCO3, mixed CaMgCl, CaCl and NaHCO3 . The dominant

groundwater types of this area are Ca-HCO3, Na-Cl, Ca-Na-HCO3 and

Ca-Mg-Cl (Rajesh et al 2012). In general chemical composition of the

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groundwater is mainly influenced by the composition of recharge of

rainwater, geological and hydrogeological variation within the aquifer. The

seasonal and spatial variations in the groundwater chemistry of this area are

discussed in the following sections.

Figure 5.3 Piper diagram showing the groundwater types of the study

area

5.3 TEMPORAL VARIATION

Seasonal variation in groundwater chemistry of an area is

essentially due to variations in groundwater recharge, land use, pumping, well

lithology and geochemical reactions. In general rainfall recharge is a major

source for variation in groundwater chemistry. In the study area rainfall

recharge occurs during the monsoon period from July to September and

evaporation is very high in April to June when the temperature rises to 44° C.

In the study area the concentration of major ions of groundwater, water level

and rainfall vary significantly with respect to time (Figure 5.4 and 5.5). In

some of the wells the groundwater level decreases the ionic concentration

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increase. On the other hand in some cases when the groundwater level lowers

the ionic concentration of groundwater decrease. Similar results were

observed in the Arani Kortlai basin (Elango 1992), Rajmohan and Elango

(2006) and also observed in the hard rock region of Nalgonda District (Rajesh

et al 2012). There are two types of seasonal variations in groundwater ionic

concentrations are found in this study area (Figure 5.4 and 5.5).

The first type of variation occurs during the rainfall recharge as the

concentration of ions decreases in groundwater and the water level increases

(Figure 5.4). The decreasing concentrations of the major ions are due to the

dilution of groundwater by recharge of rainfall in the study area. In the second

type of variation, the concentration of ions in groundwater increases with

decreasing water level in some of the wells (Figure 5.5). During dry periods,

when the groundwater level decrease, the major ion concentration is increases

which indicate the processes of evaporation. Further, during the beginning of

monsoonal rainfall recharge, the recharging water dissolves the salts

deposited during the preceding dry months in the soil zone and takes them to

the groundwater, which increases the major ions in groundwater. If the

rainfall continues for more times, the concentration of major ion is decrease in

groundwater. As it is arid dry land where irrigation is practices use both

surface and groundwater, the water used for irrigation undergoes evaporation

leading to increase the concentration of ions. This evaporation enriched

irrigated water enters the groundwater zone as recharge, which is pumped

again for irrigation. Thus pumping of groundwater for irrigation and its

evaporation from the irrigated area leads to increase in concentration of salts

in the soil zone (Rajesh et al 2011). Thus the seasonal variation is mainly

controlled by the recharge processes apart from the other geochemical

processes which are discussed in the later sections.

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Figure 5.4 temporal variations in rainfall, groundwater level and major

ion concentration

Figure 5.5 Temporal variation in rainfall, groundwater level and major

ion concentration

Well NO: 4

Well No: 2

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5.3.1 Groundwater chemistry across the dykes

Dykes are usually expected to act as barriers for groundwater flow

but in this area they are not functioning as a barrier due to the high intensity

of weathering. The groundwater level of open wells located on both sides of

an dykes was compared and it was found that these intrusive rocks do not play

a significant role as a barrier as discussed in section 4.4. In order to verify this

fact with the help of groundwater chemistry, Schoeller diagram of pairs of

wells located on either side of the dykes were prepared and as example the

one for the month of September 2009 is shown in the Figure 5.5a this diagram

represent that the concentration of all ions in the wells located either side of

the dykes are more or less similar. This conforms that the dykes are not acting

as barriers for groundwater flow

Figure 5.5a Comparison of groundwater chemistry (September 2009) of

wells located across the dykes

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5.4 SPATIAL VARIATION

The study of spatial variation in major ion concentration of the

groundwater of this area will give information about the hydrochemical nature

of an aquifer. The concentration of sodium in groundwater (Figure 5.6) is

relatively high in the southeastern part of the area and it is low in the central

part of the study area. The potassium concentration is high in the south

eastern part and it is low in the north and southwestern part of the area

(Figure 5.7). Calcium concentration in groundwater is high in southeastern

part of the area, where as it is relatively low in groundwater in the

southwestern part of the area (Figure 5.8).

Figure 5.6 spatial variations in concentration of Na (mg/l) in

groundwater during November 2009

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Figure 5.7 Spatial variations in concentration of K (mg/l) in

groundwater during November 2009

Figure 5.8 Spatial variation in concentration of Ca (mg/l) in

groundwater during November 2009

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Spatial variation of magnesium concentration of groundwater is high in

southeastern part of the area and it is low in the southwestern and central part

of the area (Figure 5.9). The chloride concentration (Figure 5.10) in groundwater

is high in the southwestern and low in the central part of the area.

Figure 5.9 Spatial variation in concentration of of Mg (mg/l) in

groundwater during November 2009

Figure 5.10 Spatial variation in concentration of Cl (mg/l) in

groundwater during November 2009

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The bicarbonate concentration of groundwater is high in the

northwestern part of the area (Figure 5.11), and it is relatively low in the

southeastern part of the study area. Carbonate concentration is very low and it

has no significant variation in the groundwater of the study area.

Figure 5.11 Spatial variation in concentration of HCO3 (mg/l) in

groundwater during November 2009

In general the spatial variations of all major ions are more are less

similar but the pattern of bicarbonate variation is of slightly different. High

concentration of most of the ions in the south eastern part of the area is

mainly due to the occurrence of shale intercalation with quartzite. Low

concentrations of major ions are observed in the central part and

south-western part of the study area due to comparatively higher rainfall

recharge. Spatial variation of major ions also controlled by hydro

geochemical processes of groundwater as discussed detail in chapter 6.

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5.5 QUALITY ASSESMENT

5.5.1 Drinking water quality

The groundwater chemistry was used to determine the suitability of

groundwater in the study area for drinking purpose by comparing with the

standard guideline values as suggested by the BIS (1991, 2003) Table 5.1.

The table represents the most desirable limits and maximum allowable limits

of various parameters. The pH, Mg, SO4 and HCO3 are within the permissible

limits. The TDS, F and NO3 exceed the permissible limits as shown in

Table 5.1.

Table. 5.1 Groundwater samples of the study area exceeding the

permissible limits suggested by BIS for drinking purposes

Parameters

BIS (1991,2003) Percentage of samples

exceeding permissible limits

N=450 Permissible limits

pH 9.2 NIL

TDS (mg/l) 1500 2

Ca (mg/l) 200 4

Mg (mg/l) 100 NIL

Cl (mg/l) 1000 NIL

SO4 (mg/l) 400 NIL

HCO3 (mg/l) 600 NIL

TH (mg/l) 600 1

F(mg/l) 1.5 25

NO3 (mg/l) 100 13

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0102030405060708090

100

Fresh water Brackish

water

Saline water Brine water

< 1000 1000-10000 10000-100000 >100000

Ranges

Perc

en

tag

e Percentage of

samples

5.5.1.1 Total Dissolved Solids (TDS)

To determine the suitability of groundwater for any purpose, it is

essential to classify the groundwater depending upon their hydrochemical

properties based on the TDS values (Catroll 1962; Freeze and Cherry 1979).

In the study area, out of about 400 groundwater samples analyzed, 17%

percentage of samples falls under brackish water. Figure 5.12 shows the

percentage of samples in different classes based on TDS. This figure indicate

more number of groundwater samples had less amount of soluble salts in

groundwater which can be used for drinking. Comparatively high TDS in

groundwater was found in the southwest of the study area. This is due to the

presence of shale formation in this part of the area.

Figure 5.12 Percentage of groundwater samples in different ranges of

TDS (mg/l)

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5.5.1.2 Total Hardness

The groundwater classification is based on Total Hardness (TH).

Majority of samples fall under hard and very hard category as given in this

Figure 5.13. TH of the groundwater was calculated using the formula given

below (5.1) (Sawyer et al 2003).

TH (as CaCO3) mg/l = (Ca + Mg) meq/l x 50 (5.1)

The hardness values range from 75 mg/l to 1926 mg/l with an

average value of 245 mg/l. The maximum allowable limit of TH for drinking

purpose is 600 mg/l and the most desirable limit is 200 mg/l as per the BIS

standard. Groundwater exceeding the limit of 300 mg/l is considered to be

very hard (Sawyer and McMcartly 1967). The 10% of groundwater samples

falls under moderately hard, 61% of falls in hard and remaining 29% of

samples are very hard. The graph (Figure 5.13) shows that the concentration

of TH as per the BIS (1991, 2003). Moderately hard to hard category present

in the central part of area. The southeastern part of the study area having very

hard category (Figure 5.14, 5.15). The CaCO3% of 26 soil samples collected

from this area ranges from 0.4% to 27.3%. They also observed preferential

formation of calcite in the chromusters while dolomite occurs only in the

Rhodustalfs.

The soils in the Nalgonda region are of Ustrothents and Rhodustalfs

types (Gajbhiye and Mandal 2000). Hence, it is reasonable to assume that

these minerals are reactive in groundwater environment and they can control

ion concentration (Rajesh et al 2012).

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0102030405060708090

100

Soft Moderately

Hard

Hard Very Hard

<75 74 - 150 150 - 300 > 300Ranges

Perc

en

tag

e

Percentage of

samples

Figure 5.13 Percentage of groundwater samples in different ranges of

total hardness (mg/l)

Figure 5.14 Spatial variation of total hardness (mg/l) of groundwater

during November 2008

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Figure 5.15 Spatial variation of total hardness (mg/l) of groundwater

during November 2009

5.5.2 Irrigation Water Quality

5.5.2.1 Salinity and Alkalinity Hazard

Salinity is the total amount of inorganic solid material dissolved in

any natural water, and water salinization relates to an increase in TDS and

overall chemical content of water. Electrical conductivity of groundwater of

this region ranges from 144 to 5030 µS/cm with an average of 1032 µS/cm

during the study period Quality of water to be used for irrigation based on

electrical conductivity (Ragunath 1987) is given in Table 5.2 and is found that

23% of samples fall in good, 61% of samples falls in permissible limit and

16% of samples exceed the permissible limits. The spatial variation of EC of

groundwater indicates that the groundwater is good to permissible category in

the central part of the study area. In the southeastern part of the study area the

groundwater is unsuitable (Figures 5.16 and 5.17) for irrigation. Excess

amount of dissolved ion such as sodium, bicarbonate and carbonate in

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irrigation water will affect the agricultural soil physically and chemically,

thus will decrease the productivity of crops.

Table 5.2 Irrigation water quality based on electrical conductivity

EC (µS/cm) Water Class

Percentage of samples

N=450

< 250 Excellent NIL

250 - 750 Good 23

750 - 2000 Permissible 61

2000 - 3000 Doubtful 1

> 3000 Unsuitable 15

Figure 5.16 Spatial variation of EC (µS/cm) of groundwater during

November 2008

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Figure 5.17 Spatial variation of EC (µS/cm) of groundwater during

November 2009

Excess salinity will reduce the osmotic activity of plants and this

interferes with the adsorption of water and nutrients from the soil (Saleh et al

1999).

Sodium concentration plays a major role in evaluating the

groundwater quality for irrigation because sodium causes an increase in the

hardness of soil as well as a reduction in its permeability (Tijani 1994). The

sodium or alkali hazard during the use of water for irrigation is expressed by

determining the sodium adsorption ratio (SAR) and it can be calculated by

equation (5.2) (Karanth 1987)

SAR=Na/ (Ca + Mg) 1/2

/2 (5.2)

where the concentrations are represented in meq/l

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The SAR values ranges from 1 to 30 with an average value of 3.

Most of the groundwater samples of the study area fall in the low sodium

class (S1). This implies that no alkali hazard is anticipated to the crops in this

region. If the SAR values is greater than 6 to 9, it suggest that degree to which

irrigation water tends to enter into cation exchange reaction in soil. Sodium

replacing adsorbed calcium and magnesium is a risk as it causes damage to

the soil structure, as the soil will become more compact and impervious.

The analytical data plotted on the US salinity diagram (Richards

1954) suggest that groundwater samples (Figure 5.18) falls in the domain

C2S1, C3S1, indicating water of medium-high salinity and low sodium,

which can be utilized for irrigation in all types of soil with slight cause of

exchangeable sodium. Some of the samples fall in the domain C4S1

indicating high salinity and low alkalinity hazard. This water is desirable for

plants having good salt tolerance and its restrains its suitability for irrigation,

especially in soils with poor drainage. A few samples fall in the C3S2

domain. The groundwater of the study area in general falls into the categories

of good to moderate.

The sodium percentage (Na %) is calculated using the equation (5.3)

given below:

Na % = (Na + K) X 100/ (Ca + Mg + Na + K) (5.3)

where all the concentrations are expressed in meq/l

Most of groundwater samples (Table 5.3) fall in the good to

permissible category based on Na% (Ragunath 1987). The Wilcox (1955)

Diagram pertaining to sodium percentage and total concentration

(Figure 5.19) show that most of the groundwater samples fall in the domain of

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excellent to good except for a few samples falling in the domain of good to

permissible, doubtful and unsuitable zones.

Figure 5.18 USSL classification of groundwater for irrigation purpose

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Figure 5.19 Suitability of groundwater for irrigation purpose by Wilcox

diagram

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Table 5.3 Suitability of groundwater for irrigation based on sodium

percentage

% Na Water Class

Percentage of samples

N=450

< 20 Excellent NIL

20 - 40 Good 25

40 - 60 Permissible 38

60 - 80 Doubtful 20

> 80 Unsuitable 17

5.5.2.2 Residual Sodium Carbonate

The excess of carbonate and bicarbonate in groundwater over the

sum of calcium and magnesium also influences the suitability of groundwater

for irrigation. This is denoted as residual sodium carbonate (RSC), which is

calculated by equation 5.4 as follows (Ragunath 1987)

RSC= (HCO3 + CO3) – (Ca +Mg) (5.4)

Where the concentration as represented as meq/l

The classification of irrigation water based on the RSC values is

presented in Table 5.4. 70% of groundwater samples fall in good, 10% of

samples fall in doubtful and the remaining 20% of samples falls under

unsuitable category for irrigation.

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Table 5.4 Quality of groundwater based on residual sodium carbonate

RSC Water Class

Percentage of samples

N=450

<1.25 Good 70

1.25 - 2.5 Doubtful 10

> 2.5 Unsuitable 20

5.5.2.3 Permeability Index

The soil permeability is affected by long term use of water for

irrigation over an area. It is influenced by sodium, calcium, magnesium and

bicarbonate content of the soil and water. Doneen (1964) has developed a

graph based on the permeability index (PI) and total salt concentration for

classification of irrigation water. Permeability index is defined as equation 5.5

below:

3HCO

PI Na x100(Ca Mg Na)

(5.5)

Where the concentrations are represented as meq/l

In this classification the water used for irrigation is classified in to

Class I, Class II and Class III to find out suitability for irrigation purpose. The

groundwater of the study area (Figure 5.20) fall in class I which indicate the

water is good for irrigation purpose.

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Figure 5.20 Suitability of groundwater for irrigation purpose based on

permeability index