CHAPTER 6 GROUNDWATER HYDROCHEMISTRY AND...
Transcript of CHAPTER 6 GROUNDWATER HYDROCHEMISTRY AND...
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CHAPTER 6
GROUNDWATER HYDROCHEMISTRY
AND HYDROCHEMICAL PROCESSES
6.1 GENERAL
Groundwater contains a wide range of dissolved solids and contain
small amount of dissolved organic matter and gases. Groundwater, which is
always in motion through aquifers and it interacts with the aquifer material in
the subsurface environment. During this movement groundwater may
dissolve, transport and deposit mineral matter. These changes are mainly
based on the surface and subsurface environment. The ionic composition of
groundwater is controlled by the chemical composition of rain, composition
of infiltrating surface water, properties of soil and rock in which the
groundwater moves, contact time and contact surface between groundwater
and geological material along its flow path, rate of geochemical
(oxidation/reduction ion exchange, dissolution, evaporation, precipitation)
process and microbiological process. Generally, the chemical quality of
groundwater depends, to a large extent, on the host of rock constituting the
aquifers (Eriksson and Khunakassen, 1966). Geologically the Tondiar basin is
underlain by rocks of Archean age consisting of granites, gneiss and
charnockites. Hydrogeochemical studies of groundwater were carried out to
determine the groundwater nature in the Tondiar River Basin, Southern India.
Groundwater samples were collected from September 2005 to November
2006, from 45 wells located in the study area. These samples were analysed
for concentration of major ions, trace elements and nutrients. In this chapter
the interpretation made from the study of major ions are discussed. About
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four hundred groundwater samples of the study area were collected and
analysed for EC, pH, Ca2+, Mg2+, Na+, K+, HCO3-, CO3
2-, Cl- and SO42-.
6.2 PHYSICAL PARAMETERS OF GROUNDWATER
The most common physical parameters were measured in the field
at the time of sampling are EC, pH, Eh provides useful preliminary
information of the area. The groundwater is generally colourless, odourless
and taste it varies according to locations. The spatial distribution of the
groundwater pH during July 2006 is given in Figure 6.1 and it varies from 6.5
to 8.3, with a mean value of 6.9. pH of water is a very important indication of
its quality, which is controlled by the amount of dissolved Carbondioxide,
carbonates and bicarbonates. Addition of salts to water may cause rapid rise
in pH. The CaCo3 increases the pH of water making it alkaline. Ghandour et
al reported (1985) pH decreases with increasing salinity. The pH values of the
groundwater samples are within permissible limit (BIS, 2003) in this area.
The central part of the study area has relatively high pH. In general the
groundwater is alkaline in nature. The EC of groundwater of the study area
ranges from 625 to 4688 μS/cm, with the mean value of 1958 μS/cm. The
spatial variation of EC (μS/cm) in the months of March 2006 and November
2006 are given in the Figure 6.2 and 6.3. Groundwater of the well situated in
Pennagar, Desur, Konagampattu, Rettani, Pelampattu have high EC value.
The groundwaters in these locations are slightly saline in nature and this is
due to the bedrock formation, agricultural activities and local pollution occurs
as isolated patches of this area. There is not much difference in the EC value
between March 2006 and November 2006. During the monsoon period the EC
is slightly reduced due to the rainfall recharge. The minimum EC value is
found in the Northeastern part of the study area (Korrakottai) and northern
part of the basin (Kottupakkam). The redox potential indicates the oxidation
and reduction process in the groundwater. The redox potential (Eh) generally
varies from 26mv to 207mv as shown in Figure 6.4.
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0
100
200
300
400
500
600
Conc
entra
tion
(mg/
l)
Na K Ca Mg Cl HCO3 CO3 SO4
MeanMinimumMaximum
6.3 MAJOR ION CHEMISTRY
The study of major ion concentration of the groundwater of this
area will provide information about the hydrochemical status of an aquifer.
The ranges of concentration of major ions in groundwater of the study area
are given in Figure 6.5. The concentrations of dissolved major cations and
anions in the groundwater vary both regionally and seasonally. The general
order of dominance of cations is Na+>Ca2+>Mg2+>K+ and for anions is HCO3-
>Cl->SO42->CO3
-. Thus Na+ and HCO3- are the dominant ions present in
groundwater of this area.
Figure 6.5 Range of concentration of major
6.4 SEASONAL VARIATION OF MAJOR IONS
In general groundwater quality will change due to the variation in
rainfall recharge, exploitation of groundwater, variation in land use, irrigation
return flow, geochemical reaction and geological formation. These factors
play a major role in seasonal variation of ionic composition in groundwater in
this area. The composition of the infiltrating rain water depends on the
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frequency of rainfall, soil environment, agriculture pattern and thickness of
vadose zone (Scheytt 1997). The monthly variation of the major ions
concentration of groundwater of three representative wells of this area
illustrated in the Figures 6.6, 6.7 and 6.8 along with the rainfall and
groundwater level. The EC and the concentration of major ions of
groundwater of this area vary significantly with respect to time. In the study
area rainfall recharge occurs generally from the month of October to January
and comparatively high evaporation occurs from the month of March to May.
These two factors play a major role for the seasonal variation of major ions.
Monthly variation in major ion chemistry of the wells located in hard rock
formation respond to rainfall more quickly due to the intensive of weathering
and fracturing of hard rocks. The comparison between rainfall and water
level indicates the rise in water level when the monthly rainfall exceeds
300mm. The rise in groundwater level in this area during the Northeast
monsoon has resulted in decrease in ionic concentrations due to the dilution.
The recharge process reduces the ionic concentration of groundwater.
However, during the non-monsoon period, increase in major ion
concentration is observed, due to the lowering of water level and the
evaporation process. The concentration of the most of the major ions follows
the water level fluctuation pattern. Similar results were observed in the hard
rock aquifers of Guntur district, Andhra Pradesh, southern India (Subba
2005). However, in a few wells there is a slight increase in ionic
concentration with rise in water level due to local pollution (animal solid
waste storage and human waste) and due the dissolution of precipitated salts.
In the hard rock area the water table generally fluctuates within the weathered
and fractured rock zone in semi-confined conditions. Major ion concentration
increases due to dissolution of precipitates present along flow path during
recharge. Thus the seasonal variations of the study area is mainly controlled
by the recharge process and strongly influenced by the bedrock geology, but
may also be attributed to the impact of agricultural pollution.
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20
40
60
80
100
SO4
(mg/
l)
Figure 6.7 Monthly variation of rainfall and concentration of major
ions in Chendur (well no.40)
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6.5 SPATIAL VARIATION OF MAJOR IONS
The major ion concentrations of groundwater of the study area vary
spatially according to groundwater recharge due to variation in amount of
precipitation, irrigation return flow, agricultural activities and geochemical
reactions of the formation. Sodium, among the alkalis, is a predominant
chemical constituent of the natural water. The sodium ion is the dominant
cation (Figure 6.9) of the study area and it varies from 10 to 457 mg/l.
Maximum concentration of sodium ion is found along the northwestern and
southern part of the study area. Potassium range from 1 to 159 mg/l and it
varies from season to season. The occurrence of potassium is found less in
nature and therefore, it is found at lower concentrations than sodium.
Maximum concentration of the potassium is observed in the well nos 16 and
29. High concentration of sodium and relatively low concentration of
potassium in the groundwater might be due to the weathering of silicates. This
type of low proportion of potassium and high sodium has been reported by
few researchers (Mohan et al 2000).
Calcium is a common and widespread element and it is distributed
widely in soils and rocks. Calcium is the second dominant cation in the
groundwater of this region and it ranges from 35 to 218 mg/l (Figure 6.10).
Usually the groundwater in the hard rock regions has the higher concentration
of calcium. The maximum concentration of calcium is found in the north
western and south western parts of this area. Magnesium concentration ranges
from 10 to 67 mg/l with the mean value of 28 mg/l (Figure 6.11). There is not
much variation in the concentration of the magnesium ion of the groundwater
samples. Magnesium content is generally controlled by the presence of CO2.
The primary source of carbonate and bicarbonate ions in groundwater is the
dissolved carbondioxide in rainwater (Karnath, 1989). Bicarbonate (Figure
6.12) values ranges from 192 to 665 mg/l and is the dominant anions of the
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study area. Higher concentration of bicarbonate in the study area might be due
to the weathering of silicate rocks and bicarbonate in present I the infiltrating
rainwater. Almost the entire area has high concentration of the bicarbonate.
The alkalinity of the water in this area is caused by dissolved bicarbonate
salts. The concentration of bicarbonate ions decrease slightly after the
monsoon. Carbonate concentration in groundwater of the study area ranges
from 0 to 53 mg/l. Maximum concentration of the carbonate is observed in the
well no: 24. In this well carbonate is present during the monsoon season
when there is flow in the river and during the rest of the period the carbonate
is absent. Thus carbonate is usually present only during the rainy season in
two wells of this area.
Figure 6.13 shows chloride concentration ranges from 26 to 899
mg/l. Chloride is considered as a strong acid compared to other ions. The
maximum concentration is found in the well nos 6 and 33 in northwestern and
south western part of the study area. The chloride concentration of
groundwater of the wells located in gneiss rock formations is higher than that
of the wells located in the Charnockite rock formations. Sulphate is widely
distributed in reduced form in both metamorphic and sedimentary rocks as a
metallic Sulphide through it is not a major constituent of the earth’s outer
crust. Sulphate concentration in groundwater of this area ranges from 10 to
400 mg/l. The well no 37 has the higher Sulphate concentration which is
located in the southern part of the area. Sulphate concentration in natural
water is less than chloride and the same is observed in the groundwater of the
study area also. Sulphate concentration in this area is influenced by the
agriculture patterns, since the man-made chemical fertilisers are used in this
area. In general the regional variations of all major ions behave more or less
in a similar manner. Thus the higher concentration is observed in the
Northwest and Southwest part of the study area. Low concentration is
observed in the northern part and northeastern of the study area. In most of
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May 2006
the months, as there is no flow in the Tondiar River, the concentration of ions
in groundwater is high. The tanks can store water only for 1 or 2 months.
Hence the ionic concentration is high most of the months, except during the
month of November to January. During the month of October to November
there is heavy rain which dilutes the groundwater by the recharge process.
Hence, the concentration of certain ions decreases and there is also increase of
certain ions like potassium and nitrate due to the applications of fertilizers.
Figure 6.9 Spatial distribution of Sodium (mg/l) concentration of
groundwater May 2006
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May 2006
Figure 6.10 Spatial distribution of Calcium (mg/l) concentration of
groundwater May 2006
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Figure 6.11 Spatial distribution of Magnesium (mg/l) concentration of
groundwater May 2006
May 2006
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Figure 6.12 Spatial distribution of Bicarbonate (mg/l) concentration of
groundwater (May 2006)
May 2006
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Figure 6.13 Spatial distribution of Chloride (mg/l) concentration of
groundwater May 2006
May 2006
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6.6 VARIATION RATIO BETWEEN HCO3/Cl
The Figure 6.15 shows the rational variation in the ratio between
chloride and bicarbonate. This variation generally follows the direction of
groundwater flow. Uphari and Toth (1989) observed the groundwater evolves
from bicarbonate dominate facies in the recharge area to Chloride dominate
facies in the discharge area. Similar results were observed in the study area.
When the groundwater flows towards the discharge area (Tondiar River
during summer season) the younger water gets enriched in Chloride. During
the groundwater flow, the groundwater becomes more mineralized as it
dissolves more aquifer material. This was clearly revealed by the regional
variation in HCO3/Cl ratio of groundwater. The ratio decreases towards
southern part and clearly shows recharge area and groundwater flow towards.
The Bicarbonate may be derived from the soil zone CO2 and at the
time of weathering of the parent materials (Hudson 1997, Mohan et al 2000).
Bicarbonate may also derive from the dissolution of Carbonates and Silicates
present in the study area. The soil zone consists of roots, decay matter,
organic matter which in turn combines with the rainwater/infiltrating water to
form Bicarbonates by the following reactions
CO2 + H2O H2CO3 (6.1)
H2CO3 H + + HCO3- (6.2)
The source of high concentration of Bicarbonates may also be
derived from the dissolution of soil CO2 during the percolation of irrigation as
well as rain water and also silicates present in this area.
Chloride is considered as a strong acid compared to other ions.
However, the chloride concentration is comparatively higher in a few wells
located in the gneiss formation. In general the chloride concentration is low in
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0
1
2
3
4
5
6
7
8
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44Well No.
HC
O3/
Cl (
meq
/l)
Wells in Gneiss rock formations
Wells in Charnockite rock formations
the wells in the charnockite areas. Chloride concentration of water samples
indicates that the possible sources may be due to the irrigation return flow and
rainfall recharge. The HCO3/Cl (meq/l) molar ratio is generally less than 4 in
groundwater of hard rocks; however in few wells in the hard rock formation is
higher (Figure 6.15)
Figure 6.15 HCO3/Cl ratio groundwater samples
6.7 HDROCHEMICAL FACIES OF GROUNDWATER
The geochemical nature of groundwater can be understood by
plotting the concentrations of major cations and anions in the piper trilinear
diagram. The trilinear diagram of Piper (1953) is very useful in bringing out
the chemical relationship in groundwater. This is useful to understand the
total chemical character of groundwater samples in terms of cations and
anions pairs. The study area is most dominant cations is sodium and the most
dominant anions is Bicarbonate. Four major hydrochemical facies have been
identified from the Piper diagram (Figure 6.16) based on the major ion
chemistry of groundwater of this area. They are:
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i) CaHCO3 Type
ii) NaCl Type
iii) Mixed CaMgCl Type
iv) Mixed CaNaHCO3 Type
Figure 6.16 Piper diagram for classifying groundwater types
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The CaHCO3 (Carbonate hardness) type and NaCl (primary
salinity) type of water are the dominate type of water in the study area. The
above type of water might have been derived from the groundwater recharge,
irrigation return flow and ion exchange process. There is no difference in the
distribution of various hydrochemical facies between the groundwater of
occurring in gneiss and charnockite formation. Schoeller’s (1965) diagram
(Figure.6.17) also shows that the groundwaters in the study area have similar
composition irrespective of geological formation. This Figure also indicates
the similarity in the chemical ratios of concentrations, which indicate that the
groundwater flow makes the groundwater is more or less similar in nature.
Figure 6.17 Scholler diagram for groundwater samples (January 2006)
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6.8 HDROCHEMISTRY AND LAND USE
Most part of the study area is under intensive cultivation, human
settlement and the other part is isolated hillocks and forest mostly in the
northern part of the study area. Hence, the land use variation is reflected in the
hydrochemistry of this area as in other place. (Cain et al. 1989). Variation of
groundwater quality in an area is a function of physical and chemical
parameters that are greatly influenced by geological formations and
anthropogenic activities. At several sites the spatial variation between the land
use and hydrochemistry was observed. Well nos. 9 and 13 is the well contains
the mostly fresh water in the study area, as it is located near the hillocks in the
northwestern part of the study area. As there is no human settlement around
this well, and the soil is deep red soil and there is very less agricultural
cultivation and mostly dry crops cultivation and also somewhat close to the
forest area The wells located very close to the human settlement, saline soil
and also to the intensive agriculture cultivation have very high salinity. In few
wells the aquifer it self in saline nature and this is due the geological
formation and the nature of soil is saltiest
As almost entire area is being intensively cultivated and irrigated
and it is importance of understand its effect on hydrochemistry. This area has
been subjected to the application of excessive inorganic fertilisers for almost
two decades. In the case of agricultural areas, these activities may generate
great quantities the ionic concentration of potassium and nitrate of some wells
are generally higher located in agricultural fields than the domestic wells.
Generally domestic wells are constructed in this study area inside the Tanks
and away from the human settlement.
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6.9 HYDROGEOCHEMICAL PROCESS
Hydrogeochemical process occurring within groundwater zone by
interaction with aquifer minerals result in the chemical nature of water.
Geochemical processes are very important as they control the composition of
the groundwater in the aquifer system. The geochemical processes are
responsible for the seasonal and regional variation in groundwater quality as
discussed earlier. The geochemical process changes the groundwater quality
during its flow from the recharge area. The geochemical properties of various
groundwater bodies are determined by the chemistry of water in the recharge
area as well as the subsurface formation. The various geochemical processes
that are responsible for the chemical character of the groundwater of this area
are discussed below.
6.9.1 MECHANISMS CONTROLLING GROUNDWATER
QUALITY
Gibbs (1970) proposed a diagram to understand the relationship of the
chemical components of waters and classified the groundwater chemistry
resulting due to three mechanisms as shown in (Figure 6.18). This plot
explains the relationship between water chemistry and aquifer lithology. Such
a relationship, help to identify the factors controlling the groundwater
chemistry. The Fig 6.18 suggests that the chemical weathering of rock-
forming minerals is influencing the groundwater quality. As most of the
points plot in the region of rock water interaction, this is likely to be the
dominant process controlling the groundwater chemistry of this area.
However, some points also fall in the region near the evaporation, indicating
that this process is also responsible for the groundwater chemistry.
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Figure 6.18 Gibbs diagram
Evaporation is the natural process that would increase the
concentration of the ions in groundwater. Thus evaporation is an important
process that increases the concentration of ions, especially during the dry
period and the evaporation increases the water will tend move toward salinity.
This would cause increase in concentration of ions in surface and subsurface
water. When evaporation is a dominant takes place, entire area this would
enrich the concentration ions and increases salinity in soil zone, due to decline
of the groundwater table level Evaporation is a dominant process in the entire
study area as this area fall in semi- arid region, where the ionic concentration
increases with lowering of water level. The presence of such linear
relationship between sodium-to-chloride ratio vs EC is indicative of
concentration by evaporation or evapotranspiration as reported by Jankowski
and Acworth 1997. The plot shows the Na/Cl vs EC (Figure 6.19) would give
a straight line, which would then be an effective indicator of concentrations of
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ions by evaporation or evapotranspiration. Thus, evaporation is an important
process that increases the concentration of ions, during dry period and
groundwater diluted during subsequent monsoon recharge.
Similarly sodium vs chloride plot (Figure 6.20) indicates that most
of samples plot above the fresh water evaporation line. This indicates that
evaporation may not be the major process controlling groundwater quality.
Hence, sodium in the groundwater might have been derived from some other
processes. If halite dissolution is responsible for sodium, Na/Cl molar ratio
should be approximately equal to 1, where as ratio greater than 1 indicates
that Na is released from silicate weathering reaction (Meybeck 1987).
Samples having Na/Cl ratio greater than one (Figure 6.19) indicates excess
sodium, which might have come from silicate weathering. If Silicate
weathering is a probable source of sodium, the water samples would have
HCO3 as the most abundant anions (Rogers 1989).In the present study,
bicarbonate is the dominant anions. Hence silicate weathering may be the
reason for sodium in groundwater. Samples having Na/Cl ratio approximately
less than one indicate the possibility of some other chemical sources.
Figure 6.19 Plot of Na/Cl (meq/l) Vs EC (μS/cm)
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Figure 6.20 Plot of Na(meq/l) Vs Cl (meq/l)
6.9.2 SILICATE WEATHERING PROCESS
Silicate weathering process is an important process that is expected
to control the groundwater chemistry in the hard rock formation. The
groundwater occurring in the hard rock formation generally has high
concentration of major ions due to the weathering of rocks. Groundwater in
the Tondiar river basin comprises of hard rocks and these rocks are highly
weathered and fractured. Silicate weathering is understood by the
relationships between the major ions present in the groundwater. In this area
sodium is the dominant cation next to calcium in the groundwater of the study
area. A relationship between (Ca+Mg) vs HCO3 diagram shown in Figure
6.21 indicates that most of the data points fall above the1:1 equiline, although
few points below the equiline.
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Figure 6.21 Plot of (Ca+Mg) Vs HCO3 plot indicating silicate weathering
It suggests that an excess of alkalinity of the waters have been
balanced alkalies. The excess of alkaline earth elements (Ca + Mg) over
HCO3 in samples reflect an extra source of calcium and magnesium ions. It
might have been balanced by Cl- and SO42-or supplied by silicate weathering.
The ratio will be close to unity, if the dissolution of calcite, dolomite and
gypsum is the dominant reaction in aquifer system. The data points toward the
Y axis (Figure 6.21) indicate high concentration of Ca+Mg over HCO3 which
is mainly balanced by ion exchange process .The (Ca+Mg) vs Total cations
(TZ) shows that the data lie far below the theroritical line (1:1) as shown in
(Figure 6.22) depicating an increasing contribution of alkalies to the major
ions. But a few groundwater samples of wells located in the hard rock regions
have higher concentration of (Na +K) than (Ca+Mg).The Na+K vs Total
cations scatter diagram (Fig. 6.23) of the study area shows sample points
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falling both along and above the Na+K = 0.5 Total cations. This suggests that
the cations in the groundwater might have been derived from silicate
weathering. Datta and Tyagi (1996) observed that the contribution of cations
may be derived from silicate weathering when Na+K = 0.5 Total cations. The
slightly lower concentration of (Na+K) is likely to be caused by Ca/Na
exchange process, which might have reduced the amount of Na in the
groundwater. Since this region comprise of composite gneiss and charnockite,
weathering of silicates might be the possible source of ions. Weathering of
silicate rocks resulting in high Na and K has also been reported in the hard
rock regions Naini Industrial area, Uttar Pradesh by Mohan eta al (2000).
Silicate dissolution is the probable source of Na in the study area
because water that derives solutes primarily by silicate weathering has high
HCO3, which is the most abundant anion in this area (Equiline 1:1). A Na/Cl
ratio approximately equal to 1 is usually attributed to halite dissolution.,
where as ratio greater than 1 is Na is released from silicate weathering A
molar ratio Na/Cl ratio >1 (Figure 6.24) is due to Na is released due to silicate
weathering reactions while the molar ratio of Na/Cl <1 are due to the halite
dissolution (Meybeck 1987).In the study area the molar ratio of Na/Cl of the
groundwater samples generally ranges from 0.04 to 4.72 with an average of
1.10. Figure 6.24 shows that the value of Cl as a function of Na in the
groundwater. The dissolution of halite in water release equal concentrations
of Na and Cl into the solution and the figure 6.24 the data point s are clustered
around the equiline 1:1 This indicates that silicate weathering is the source of
sodium.
2NaAlSi3O8 +2H2CO3 +9H2O =› Al2Si2O5 (OH)4 +2Na+ +4H4SiO4 +2HCO3
(6.3)
(Albite) (Silicate weathering) (Kaolinite)
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Thus it is observed that a silicate weathering is an important
process occurring in the study area.
Figure 6.22 Plot of (Ca+Mg) Vs Total Cations indicating silicate
weathering
Figure 6.23 Plot of (Na+K) Vs Total Cation indicating silicate weathering
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Figure 6.24 Plot of Na Vs Cl plots explaining the mixing process
6.9.3 ION EXCHANGE PROCESS
Under certain conditions, the ions attracted to a solid surface may
be exchanged for other ions in aqueous solution. This process is known as ion
exchange process, but in some natural soil (Clay) cation exchange is dominant
and the clay has high percentage of colloidal sized particle.
A plot of Na vs Cl concentration of groundwater of the study area
with 1:1 line is given in Figure 6.25. The sample points fall above and below
the 1:1 line. The sample points plotting below the 1:1 line indicate the
depletion of sodium with respect to chloride. Similarly the sample points
plotting above the 1:1 line indicate the increase of sodium with respect to
chloride. Both the process shows the evidence of cation exchange process
(Jankowski and Acworth 1997; Salama 1993). Excess of Ca and Mg in
groundwater may be due to the exchange of Na in water by Ca and Mg in clay
particle. The cation exchange process is explained by the following reaction
Ca+2Na (exchanged) ↔2Na+ + Ca (exchanged) (6.4)
Mg2+ +2Na (exchanged) ↔2Na+ +Mg (exchange) (6.5)
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Where (exchanged) denotes the cation exchanged on water or soil.
Figure 6.25 plot of Cl Vs Na indicating ion exchange process
Figure 6.26 Relations between Ca+Mg and SO4+HCO3
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Figure 6.27 Relations between Ca+Mg-HCO3-SO4 and Na-Cl
The plot of SO4+HCO3 vs Ca+Mg (Figure 6.26) shows that the
most of the groundwater samples from hard rock formation are clustered
around the 1:1 line, if the dissolution of calcite, dolomite and gypsum are
dominant reaction in a system. Excess of calcium and magnesium in
groundwater of hard rock formation may be due to the exchange of sodium in
water by calcium and magnesium in clay material
The plot of Na-Cl vs Ca+Mg-HCO3-SO4 (Figure 6.27) also help to
identify the ion exchange process in the aquifer system. Na-Cl (meq/l)
represents the amount of Na gained or lost relative to that provided the halite
dissolution, whereas Ca +Mg-HCO3 (meq/l) represents the amount of Ca and
Mg gained or lost relative to that provided by gypsum, calcite and dolomite
dissolution. If ion exchange is a significant composition-controlling process,
the relation between should be linear slope (Jankowski et al 1997). In the
study area, the groundwater samples plotted in the plot have a slope of -0.31,
which indicates certain extent of reverse ion exchange. This confirms that Ca,
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Mg and Na concentration are interrelated to reverse ion exchange process.
Further, to discriminate which ion (Ca or Mg) controls the hydrochemical
reactions, two indices of Base Exchange (IBE), namely the chloroalkaline
indices (CAI1 and CAI2) where estimated and presented below.
The ion exchange between the groundwater and its host
environment during residence or travel can be understood by studying the
chloro-alkaline indices. To know the direction of exchange during the path of
groundwater through the aquifer, Schoeller (1965) suggested 2 chloro-
alkaline indices CAI1 and CaI2 (May 2006) to indicate the exchange of ions
between groundwater and its host environment. The ion exchange and reverse
ion exchange was confirmed using chloro-alkaline indices.
CAI1 = Cl-(Na+K)/Cl (6.6)
CAI2 = Cl-(Na+K)/SO4 + HCO3 + CO3 + NO3 (6.7)
(All values are measured in meq/l)
When there is an ion exchange between Na or K in groundwater
with Mg or Ca in the aquifer material (rock/weathered layer), both of the
indices are positive, indicating ion exchange of sodium in groundwater with
calcium or magnesium in the weathered material. While in reverse exchange
both indices are negative when there is an exchange of Mg or Ca in the waters
with Na and K in the rocks. The chloroalkaline indices (CAI1 and CAI2) are
used to evaluate the event of base-exchange process during rock water
interaction. In the study area, the value of these indices varies between
positive and negative values (Figure 6.28, 6.29). There is no systematic
seasonal variation in the values of indices. So the ion exchange reactions
seem to occur in both the directions depending on the season, groundwater
flow path, mixing of water and evaporation process. Thus the cation exchange
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process is one of the important geochemical processes that control the
groundwater chemistry of the area.
Figure 6.28 Variation in Chloro-alkaline indices (CAI) in all the wells
indicates indicating the Ion exchange process
Figure 6.29 Schoeller Classification of Groundwater
May 2006
Positive areas
Negative areas
LEGEND
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6.9.4 SULPHATE REDUCTION PROCES
The concentrations of sulphate in groundwater are very low through
out the study area. They might have come from the dissolution of gypsum as
in equation 6.8 because there is no acid rain or pyrite source in this area,
which can supply sulphate to the groundwater.
H2O + CaSO4 . 2H2O => Ca2+ + SO42- + 3H2O (6.8)
Very low SO4/Cl ratios (low concentration of SO4) (Figure 6.30)
suggest that sulphate is being depleted, possibly by sulphate reduction (Lavitt
et al. 1997). Earlier, Datta and Tyagi (1996) had observed that groundwater
with high Cl and low SO4 probably indicates reduction. Thus, the low
sulphate concentration in the groundwater of this area may be due to sulphate
reduction and perhaps lack of natural sources in the area.
Figure 6.30 Plot of SO4 Vs Cl indicates sulphate reduction
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6.10 GROUNDWATER REDOX POTENTIAL
Oxidation/reduction (Redox) reaction potential of groundwater (Eh)
plays an important role in the geochemical processes that occur in
groundwater. Redox is defined as the transfer of the electrons. Redox
reactions are enormously important in aqueous environmental geochemistry
Eh measurements are useful in identifying the redox zones as its value
decreases with increases in residence time (Champ et al 1979) and reported
that Eh values above 300mV indicate that sulphate would be stable in this
area and it is a recharge area. High Eh values indicate the regions of good
recharge and low Eh values are the regions of less recharge or discharge.
Figure 6.31 shows the relation between Eh and pH were used to determine the
groundwater conditions of the study area. The groundwater samples of the
study area have high Eh value more than 557mV in the well no.34 and the
lowest Eh value of the groundwater measured is 65mV at the sampling
borehole no.36, but this borehole will give during the monsoon period of
September 2006 as -65mV and the colour of the groundwater is yellow in
colour throughout the sampling period. Eh values <100mV suggest that the
redox conditions are low enough for sulphate reduction to occur (Champ et al
1979).Oxidation and reduction of sulphate and iron is a common process in an
aquifer system. The pH vs Eh diagram shows (Figure 6.31) that how Eh in
groundwater is governed in the upper range by oxidation of water to O2 and
lower range by reduction of hydrogen ions to H2.The groundwater samples of
the study area fall under ferrous (Fe2+) i.e reduction state and Fe(OH)2 i.e
oxidation states.
Fe3+ + e- = Fe 2+ reduction state (6.9)
Fe(OH)3 + e- + H+ = Fe(OH)2 + H2O Oxidation state (6.10)
From Figure 6.31 it is concluded that both oxidation and reduction states are
taking places in the study area.
107
Figure 6.31 pH-Eh diagram (May 2006)
6.11 MIXING OF SURFACE AND GROUNDWATER
River, tanks/lakes and ponds are the important sources of the
surface water resource in this area. In order to determine the interaction
between surface water and groundwater, water samples were collected from
surface water bodies located in the study area. These samples were analysed
for the chemical constituents. The chemical composition of the groundwater
along the Tondiar River is similar to the river water during the monsoon
period (Table 6.1). The results confirm the mixing between surface water and
groundwater.
Similar the comparison of groundwater quality of pond and well
was made in (Table 6.2) is similar. This indicates the effect of recharge from
the ponds located in this region results in groundwater with low total
dissolved solids. The storage in pond will be there generally for four months
from the onset of monsoon.
108
Table 6.1 Comparisons of the groundwater and River water quality
(November 2005)
S.No EC Na K Ca Mg Cl HCO3 So4
Well No.24 883 65 2.5 44 19 70 281 50
River water 738 69 4 31 22 99 210 85
All values in mg/l except EC (μS/cm)
Table 6.2 Comparisons of the Pond and Tank water quality
(January 2006)
S.No EC Na K Ca Mg Cl HCO3 CO3 So4
Pond water
(Korrakottai)
490 41 3.4 37 18 46 302 9 14
Well no 19 486 24 2 42 12 31 353 0 15
All values in mg/l except EC (μS/cm)
The mixing process of various water is responsible for the chemical
composition of groundwater in this region. The plot of Na and Cl
concentration of groundwater indicate that the points fall on or near the 1: 1
indicating the dominance of mixing processes (Figure 6.24). The groundwater
samples of the study area falls as a single group along mixing line. This
shows that mixing of freshwater recharge with the existing water is the major
mechanism taking place in the study area. Some of the samples in this area
have high chloride concentration, which may be restricted flow.
Thus in general, the groundwater chemistry of this area is
controlled by mixing water, evaporation, rock water interaction and ion
exchange process.