Identification of major sources controlling groundwater chemistry from a hard rock terrain — A case study from Mettur taluk, Salem district, Tamil Nadu, India
ABSTRACT The study area Mettur forms an important industrial town situated NW of Salem district. The geology of the area is mainly
composed of Archean crystalline metamorphic complexes. To identify the major process activated for controlling the groundwater
chemistry an attempt has been made by collecting a total of 46 groundwater samples for two different seasons, viz., pre-monsoon
and post-monsoon. The groundwater chemistry is dominated by silicate weathering and (Na + Mg) and (Cl + SO4) accounts of about 90% of cations and anions. The contribution of (Ca + Mg) and (Na + K) to total cations and HCO3 indicates the domination of silicate weathering as major sources for cations. The plot for Na to Cl indicates higher Cl in
both seasons, derived from Anthropogenic (human) sources from fertilizer, road salt, human and animal waste, and industrial
applications, minor representations of Na also indicates source from weathering of silicate-bearing minerals. The plot for
Na/Cl to EC indicates Na released from silicate weathering process which is also supported by higher HCO3 values in both the seasons. Ion exchange process is also activated in the study area which is indicated by shifting to right
in plot for Ca + Mg to SO4 + HCO3. The plot of Na-Cl to Ca + Mg-HCO3-SO4 confirms that Ca, Mg and Na concentrations in groundwater are derived from aquifer materials. Thermodynamic plot indicates
that groundwater is in equilibrium with kaolinite, muscovite and chlorite minerals. Saturation index of silicate and carbonate
minerals indicate oversaturation during pre-monsoon and undersaturation during post-monsoon, conforming dissolution and dilution
process. In general, water chemistry is guided by complex weathering process, ion exchange along with influence of Cl ions
from anthropogenic impact.
- SourceAvailable from: minersoc.org
Article: Solutions, Minerals and equilibria01/1966;
- [show abstract] [hide abstract]
ABSTRACT: Various geochemical processes occur when groundwater from a fractured-bedrock aquifer discharges through clay-rich debris-flow deposits. Such colluvial debris-flow deposits occur in the lower part of valleys in catchments of the Yass River on the Southern Tablelands of New South Wales, Australia, and are commonly severely affected by dryland salinity. Groundwater chemistry was evaluated from 25 deep bores and 58 shallow piezometers. Groundwater in the fractured-bedrock aquifer is dominated by Mg and SO4 ions. Reactions within the colluvium include ion exchange of calcium for sodium in basal clays; dissolution of sodium chloride from dust-derived salt particles in the clay-rich colluvium; and microbiological reactions. Bivariate plots of sodium-versus-chloride and sodium-versus-calcium indicate that cation-exchange reactions occur in the colluvium. On the basis of hydrochemical modeling, it is concluded that the observed chemistry in the colluvial aquifer cannot be accounted for by evaporative concentration. Rsum: Diffrents processus gochimiques se produisent lorsque l'eau souterraine d'un aquifre fractur de socle s'coule au travers de dpts dtritiques riches en argile. De telles formations dtritiques colluviales occupent l'aval des valles des bassins versants de la rivire Yass dans les Tablelands du Sud (Nouvelles-Galles du Sud, Australie) et sont en gnral fortement touchs par la salinisation en zone aride. Le chimisme de l'eau souterraine a t tudi partir de 25 forages profonds et de 58 pizometres peu profonds. L'eau souterraine d'un aquifre fractur de socle est marque par les ions Mg et SO4. Les ractions dans les colluvions mettent en jeu des changes de calcium avec le sodium des argiles de base, la dissolution du chlorure de sodium provenant de particules de poussire produite par le sel dans les colluvions argileuses, et des ractions microbiologiques. Des corrlations sodium - chlorure et sodium - calcium montrent qu'il se produit des ractions d'change de cations dans les colluvions. Le recours une modlisation hydrochimique conduit conclure que le chimisme observ dans l'aquifre colluvial ne peut pas tre expliqu par une reconcentration par vaporation. Resumen: Diferentes procesos geoqumicos tienen lugar cuando las aguas subterrneas procedentes de un acufero fracturado descargan a travs de depsitos con alto contenido en arcillas procedentes de flujos de derrubios. Estos depsitos de derrubios coluviales aparecen en la parte baja de valles, en la zona de captacin del Ro Yass, en la Meseta Sur de Nueva Gales del Sur, Australia, y normalmente se encuentran fuertemente afectadas por salinizacin. Del estudio de la qumica de las aguas subterrneas en 25 sondeos profundos y 58 piezmetros superficiales, se deduce que los iones dominantes en el acufero fracturado son Mg y SO4. Las reacciones con los depsitos coluviales incluyen intercambio inico de calcio por sodio en las arcillas basales, disolucin de cloruro sdico procedente de partculas de sal en la zona coluvial rica en arcilla, y reacciones microbiolgicas. Los diagramas de sodio-cloruro y sodio-calcio indican que las reacciones de intercambio de cationes tienen lugar en el coluvin. Basndose en un modelo hidroqumico, se concluye que la qumica observada en el acufero coluvial no puede deberse a concentracin por evaporacin.Hydrogeology Journal 03/1997; 5(4):71-88. · 1.68 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: On the basis of analytical chemical data for numerous rain, river, lake, and ocean samples, the three major mechanisms controlling world surface water chemistry can be defined as atmospheric precipitation, rock dominance, and the evaporation-crystallization process.Science 01/1971; 170(3962):1088-90. · 31.03 Impact Factor
Identification of major sources controlling groundwater
chemistry from a hard rock terrain – A case study from
Mettur taluk, Salem district, Tamil Nadu, India
K Srinivasamoorthy∗, S Chidambaram, M V Prasanna, M Vasanthavihar,
John Peter and P Anandhan
Department of Earth Sciences, Annamalai University 608 002, Tamil Nadu, India.
The study area Mettur forms an important industrial town situated NW of Salem district. The
geology of the area is mainly composed of Archean crystalline metamorphic complexes. To iden-
tify the major process activated for controlling the groundwater chemistry an attempt has been
made by collecting a total of 46 groundwater samples for two different seasons, viz., pre-monsoon
and post-monsoon. The groundwater chemistry is dominated by silicate weathering and (Na + Mg)
and (Cl + SO4) accounts of about 90% of cations and anions. The contribution of (Ca + Mg) and
(Na + K) to total cations and HCO3 indicates the domination of silicate weathering as major
sources for cations. The plot for Na to Cl indicates higher Cl in both seasons, derived from
Anthropogenic (human) sources from fertilizer, road salt, human and animal waste, and industrial
applications, minor representations of Na also indicates source from weathering of silicate-bearing
minerals. The plot for Na/Cl to EC indicates Na released from silicate weathering process which
is also supported by higher HCO3values in both the seasons. Ion exchange process is also acti-
vated in the study area which is indicated by shifting to right in plot for Ca + Mg to SO4+ HCO3.
The plot of Na − Cl to Ca + Mg − HCO3− SO4confirms that Ca, Mg and Na concentrations in
groundwater are derived from aquifer materials. Thermodynamic plot indicates that groundwa-
ter is in equilibrium with kaolinite, muscovite and chlorite minerals. Saturation index of silicate
and carbonate minerals indicate oversaturation during pre-monsoon and undersaturation during
post-monsoon, conforming dissolution and dilution process. In general, water chemistry is guided
by complex weathering process, ion exchange along with influence of Cl ions from anthropogenic
Groundwater contains minerals carried in solution,
the type and concentration of which depends upon
several factors like soluble products of rock weath-
ering and decomposition in addition to external
polluting agencies and changes in space and time.
As a result of chemical and biochemical interac-
tion between groundwater and contaminants from
urban, industrial and agricultural activities along
with geological materials through which it flows,
it contains a wide variety of dissolved inorganic
chemical constituents in various concentrations.
The character of groundwater in different aquifers
over space and time proved to be an important
technique in solving different geochemical prob-
lems (e.g., Chebotarev 1955; Hem 1959; Back et al
1966; Gibbs 1970; Srinivasamoorthy et al 2005).
Calculation of mineral saturation index and ther-
modynamic equilibrium studies were initiated by
Garrels and Christ (1965) to decipher the possi-
ble reactant and product minerals and indication
Keywords. Groundwater; residence time; ion exchange; thermodynamics; saturation index.
J. Earth Syst. Sci. 117, No. 1, February 2008, pp. 49–58
© Printed in India.
50K Srinivasamoorthy et al
Figure 1. Location, geology and sampling locations of the study area.
of the equilibrium state of groundwater and the
surrounding materials. Nandakumar and Murthy
(1997) attempted for groundwater quality varia-
tion in shallow basaltic aquifers and identified Base
Exchange reaction in canal command areas. The
study area gains its own importance by its loca-
tion in a hard rock terrain, frequent failure in mon-
soon, highly industrialized and urbanized zone all
of which contributes for a proper appraisal of the
chemical quality of groundwater in the study area.
2. Study area
The study area (figure 1) occupies northwestern
part of Salem district in Tamil Nadu and lies
between north latitudes 11◦30?and 12◦00?and east
longitudes 77◦45?and 78◦00?with a total extent
of 777.15km2. The average rainfall of the study
area is about 804.3mm which is meager when
compared with the district average of 1019mm
(TWAD 2002). The geology is composed of gran-
ites like charnockites, basic granulites and mag-
netite quartzite. Peninsular gneiss and calc gneiss
of metasedimentary group are also exposed in
the study area. Ultramafic and basic intrusives
like pyroxenite, peridotite, anorthosite and dolerite
dykes along pegmatoidal granites are exposed all
around the study area. Hills, undulating plains,
plateaus, pediments and bazadas are the mixture
of geomorphic units. Northern and southwestern
parts of the study area include hill ranges and high
lands where eastern and southeastern parts are low
lying due to the flow of river Cauvery. The main
drainage of the study area is by river Cauvery
which flows from north to south with a sub-dentric
drainage pattern. Groundwater in the study area
occurs in weathered portions of rocks along joints
and fractures. Groundwater flow was noted along
NS and NW direction which coincides with the
topography of the study area (Srinivasamoorthy
2005). Weathered layer thickness in gneissic terrain
ranges from 2.2 to 50m. In charnockite weathered
layer, thickness was between 5.8 and 55m. Ground-
water occurs under semi-confined conditions in the
alluvium along the river course. Water table fluctu-
ation in the study area ranges from 0.2m to 13.5m
BGL (Below Ground Level).
Groundwater chemistry from a hard rock terrain51
Table 1. Chemical composition of groundwater during pre-monsoon season (all values in meq/I except pH, EC, H4SIO4 and TDS).
52K Srinivasamoorthy et al
Table 2. Chemical composition of groundwater during post-monsoon season (all values in meq/I except pH, EC, H4SIO4 and TDS).
Groundwater chemistry from a hard rock terrain 53
Figure 2. Gibbs plot for pre- and post-monsoons.
3. Materials and methods
A total of 46 representative groundwater samples
were collected (figure 1) for two different seasons,
viz., pre-monsoon (June) and the post-monsoon
(December) along with a maximum representation
covering the entire study area. The samples were
filtered using 0.45µm Millipore filters and immedi-
ately stored in polyethylene bottles and analyzed
for major and minor cations and anions using stan-
dard procedures (see APHA 1995). pH and EC
were measured in situ by pH conductivity meter
model CONSORT C425. HCO3, Cl, Ca and Mg
were analyzed using titration. F was analyzed using
Orion fluoride ion electrode model (94-09, 96-09).
SO4, PO4, H4SIO4was determined by digital spec-
trophotometer model GSS 700A (Electronic Cor-
poration of India). Na and K were determined by
using flame photometer (Systronics mk-1/mk-III).
The analytical precision for the measurements of
ions was determined by calculating the ionic bal-
ance error that varies by about 5–10%. TDS/EC
ratio is 0.50/1.0 (with excess of anions in water).
4. Results and discussion
Groundwater in general was alkaline in nature
andhigher EC values were noted during the
pre-monsoon season. HCO3in the study area was
higher in pre-monsoon season due to weathering
of silicates. Cl was higher in post-monsoon sea-
son indicating leaching from upper soil layers due
to industrial and domestic activities and dry cli-
mates (Herman Bouwer 1978). SO4was higher in
pre-monsoon season indicating breaking of organic
substances from topsoil/water, leachable sulphate
preset in fertilizer and other human influences
(Miller 1979; Craig and Andeson 1979). Fluo-
ride and NO3 was higher in pre-monsoon indi-
cating leaching of fluoride rich rocks and organic
substances from weathered soil (tables 1 and 2).
The general dominance of anion was in the order
of Cl > HCO3> SO4> NO3> PO4. For cations
Na was higher in pre-monsoon season indicating
weathering from plagioclase bearing rocks. K was
lesser in both the seasons indicating its lower geo-
chemical mobility. Ca and Mg were higher in pre-
monsoon indicating the weathering from primary
mineral sources. The general dominance of cations
was in the order of Na > Ca > Mg > K. H4SIO4
was higher indicating the existence of alkaline envi-
ronment and abundance of silicate minerals in the
The results from the water analysis were used
as a tool to identify the process and mecha-
nisms affecting the chemistry of groundwater from
the study area. Gibbs (1970) plot was used to
54K Srinivasamoorthy et al
Figure 3.(A–D) Plot for relationship of ions during pre- and post-monsoon seasons.
determine the mechanism controlling the water
chemistry (figure 2). Samples from both seasons
fall in rock dominance zone suggesting precip-
itation induced chemical weathering along with
dissolution of rock forming minerals.
The (Ca + Mg) versus TZ+plot (figure 3A) for
both seasons lie far below equiline with average
equilibrium ratio of 0.40 to 0.35 indicating that
alkalis are enriched twice to thrice the amount of
Ca and Mg due to leaching from silicate weath-
ering which is dominant in the aquifer materials
of the study area. In the plot for (Ca + Mg) ver-
sus HCO3, the data point irrespective of seasons
fall away from equiline 1:1 to 2:1, indicating pre-
dominance of alkali earth by silicate weather-
ing over bicarbonate. Minor representations are
also noted in bicarbonate zone due to the reac-
tion of the feldspar minerals with carbonic acid
in the presence of water, which releases HCO3
(Elango et al 2003) (figure 3B). The plot for
(Na + K) versus TZ+(figure 3C) indicates weath-
ering process of both alkali and alkali earth from
feldspars alongwith additional sources like contri-
bution from alkali/saline soil and residence time
are major contributing sources for ions in ground-
water irrespective of seasons. The plot for Na
versus Cl (figure 3D) shows that, majority of
samples for both the seasons fall near Cl and Na
zones indicating Na derived from weathering from
silicate bearing minerals. Since Cl is abundant
in both the seasons and due to rare Cl bearing
minerals in silicate terrain, it might have derived
from Anthropogenic (human) sources of chloride
include fertilizer, road salt, human and animal
waste, and industrial applications. These sources
can result in significant concentrations of chloride
in groundwater because chloride is readily trans-
ported through the soil (Stallard and Edmond
1983). This is well evidenced from Cl levels of the
study area which is a major industrial town of
In the plot for Na/Cl versus EC (figure 4A, U
and V) the Na/Cl shows a decreasing trend with
increasing EC along with higher Na/Cl ratio, indi-
cates Na released from silicate weathering process.
This is also supported by higher HCO3 values in
groundwater due to reaction of feldspar minerals
with carbonic acid might be one of the reasons for
increase in EC (Jankowski and Acworth 1997) in
both the seasons.
The plot for Ca + Mg versus SO4+ HCO3 is
a major indicator to identify ion exchange pro-
cess activated in the study area. If ion exchange
is the process, the points shift to right side of the
Groundwater chemistry from a hard rock terrain 55
X – post-monsoon and (C) Na–Cl vs. Ca + Mg–HCO3–SO4; Y – pre-monsoon, Z – post-monsoon.
(A) Na/Cl vs. EC; U – pre-monsoon, V – post monsoon. (B) Ca + Mg vs. SO4+ HCO3; W – pre-monsoon,
plot due to excess SO4+ HCO3. If reverse ions
exchange is the process, points shift left due to
excess Ca + Mg. In pre-monsoon, the point clusters
around the equiline 1:1 and falls in SO4+ HCO3
indicating the ion exchange process which may
be due to the excess bicarbonate (figure 4B,
W and X). In post-monsoon season, the plot
points Ca + Mg indicating excess calcium and
56K Srinivasamoorthy et al
Figure 5.Thermodynamic stability plot for (A) Ca, (B) Mg system, (C) Na and (D) K system.
magnesium in groundwater exchanged with sodium
from aquifer materials. The plot of Na − Cl versus
Ca + Mg − HCO3− SO4confirms that Ca, Mg and
Na concentrations in the groundwater is derived
from aquifer materials (figure 4C, Y and Z).
Thermodynamic plotting of [Na]/H, [K]/H,
[Ca]/H and [Mg]/H, for groundwater from study
area are plotted on the stability diagram as a
function of [H4SiO4]. In plot for Ca the samples
(figure 5A) for both the seasons fall in Kaolinite
field indicating impact of dilution. In plot for mag-
nesium (figure 5B) shift of stability from kaolinite
to chlorite is noted in both the seasons indicating
the formation of new clay minerals due to supply
of excess cations and silica to pre-existing kaolinite
from aquifer materials.
The plot of Na falls in the kaolinite stabil-
ity field irrespective of seasons, indicating Na–
Feldspar will dissolve incongruently to produce
kaolinite and dissolved products (figure 5C). As
dissolution of feldspars continues the value of
Si(OH)4and [Na]/[H] increases and water chem-
istry moves to Na–Montmorllianite during SUM.
The plot of K silicates (figure 5D) indicates
incongruent dissolution of kaolinite and formation
of muscovite. Majority of samples irrespective of
seasons fall in kaolinite and muscovite field. When
IAP of K increases as the result, the ratio of K
increases and pH remains almost constant this vari-
ation result in shift of kaolinite to muscovite field
along with release of H+ions which combines with
HCO3to form CO2.
5. Disequilibrium indices
culated by WATEQ4F geochemical model for
those minerals and other solids stored in the
model data book for which the dissolved con-
stituents are reported in groundwater analy-
sis. Disequilibrium indices log(IAP/KT) were
indiceslog(IAP/KT) was cal-
Groundwater chemistry from a hard rock terrain57
Figure 6.Disequilibrium indices for silicate minerals. (A) pre- and (B) post-monsoon seasons.
Figure 7. Disequilibrium indices for carbonate minerals. (A) pre- and (B) post-monsoon seasons.
calculated to determine, if water is in ther-
modynamic equilibrium log(IAP/KT = 0), over-
saturated log(IAP/KT > 0) or undersaturated
log(IAP/KT < 0) with respect to certain solid
phases (Trusdell and Jones 1973).
The disequilibrium indices of silicate minerals
(figure 6A and B) chalcedony, christobalite and
Quartz were represented from the data bank of
WATEQ4F and studied for dissociation factors in
the study area. The data input was categorized on
the basis of two seasons. The status of groundwa-
ter in pre-monsoon indicates that all the minerals
are in the state of oversaturation to equilibrium.
In post-monsoon, quartz follows the same trend
of oversaturation but chalcedony and christobalite
have neared saturation state indicating the effect
The disequilibrium indices of carbonate miner-
als (figure 7A and B) aragonite, calcite, dolomite
and magnesite were represented from the data
bank of WATEQ4F. The samples in pre-monsoon
with aragonite, calcite, dolomite and magnesite
with minor indications in undersaturation state.
The samples in post-monsoon season follow the
same trend as in pre-monsoon. In general, car-
bonate results show that calcite, dolomite and
magnesite are saturated-to-oversaturated which is
indicated by excess input of Ca and Mg ions from
silicate weathering process.
The groundwater in Mettur region is a unique
example for the impact of weathering, ion exchange
and anthropogenic process controlling water chem-
istry. The chemical composition of groundwater of
the study area is strongly influenced by rock water
interaction, dissolution and deposition of silicates
group of minerals. Weathering of silicate minerals
controls the major ion chemistry of calcium, mag-
nesium, sodium and potassium. Cl was dominant
due to anthropogenic impact (human sources). The
58K Srinivasamoorthy et al
ion exchange and reverse ion exchange control the
water chemistry of the study area. Thermody-
namic plot indicates, groundwater is in equilibrium
with kaolinite, muscovite and chlorite minerals. SI
of minerals indicates oversaturation and undersat-
uration of silicate and carbonate minerals with
respect to pre- and post-monsoon seasons. In gen-
eral, water chemistry is guided by lithological influ-
ences on water chemistry by complex weathering
process, ion exchange along with influence of Cl
ions from anthropogenic impact.
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MS received 1 December 2006; revised 21 July 2007; accepted 6 September 2007