Environmental Engineering Reference
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(Na
+
+ K
+
) in pre- and post-monsoon seasons indicating that silicate weathering is
the dominant process for cations (Fig.
3b
).
Most of the samples of pre-monsoon and post-monsoon falling above the equi-
line due to excess SO
4
2−
+HCO
3
−
over Ca
2+
+ Mg
2+
indicate ion exchange process
dominance.Therefore,theexcesspositivechargeofSO
4
2−
andHCO
3
−
must be bal-
anced by alkalies (Na
+
+ K
+
).TheplotofCa
2+
+ Mg
2+
versusSO
4
2−
+HCO
3
−
will be
close to the 1:1 line if the dissolution of calcite, dolomite and gypsum are the domi-
nant reactions in a system (Anupam et al.
2012
). Na
+
+ K
+
plottedagainstCl
−
+SO
4
2−
(Fig.
3c
) shows that majority of samples fall below the 1:1 trend line indicating that
these ions have a common source. Most of the groundwater samples fall towards
Na
+
+ K
+
indicating the release of Na
+
from rock weathering (Meybeck
1987
). In
arid regions, the Na
+
−Cl
−
relationship is generally used to determine the mecha-
nism of salinity acquisition and saline intrusions (Ravikumar et al.
2010
). The dis-
solution of halite is an important source for Na
+
andCl
−
in groundwater, along with
soil salts, anthropogenic activities, poor drainage and agricultural activities (Raju
2012b
). A good correlation exists between Na
+
−Cl
−
in pre-monsoon (
r
= 0.96) and
in post-monsoon (
r
= 0.95) (Table
4
). When the Na
+
concentration is plotted against
thatofCl
−
, most of the water samples lie slightly below the 1:1 trend line towards
to Na
+
(Fig.
3d
). The high Na
+
/Cl
−
ratios in the study area are probably due to sili-
cate weathering, ion-exchange process or water-rock interactions (Adomako et al.
2011
). High Na
+
/Cl
−
(>1) ratio is used to account for silicate weathering contribu-
tion of Na
+
(Meybeck
1987
).
The dissolved Ca
2+
-HCO
3
−
in the water resulting from calcite weathering by
carbonic acid would be 1:2 whereas from dolomite weathering by carbonic acid
would be 1:4 (Stallard and Edmond
1983
).TheplotofCa
2+
versusHCO
3
−
(Fig.
3e
)
showsthatmostofthedatafallabovethe1:1linetowardsHCO
3
−
.ThelowCa
2+
/
HCO
3
−
ratio (<1) of both the seasons in the study area may be due to either Ca
depletion by cation exchange or HCO
3
−
enrichment. In the study area the Ca
2+
/
HCO
3
−
ratio varies between 0.50 and 0.89 with an average of 0.65 in pre- and 0.53-
1.16 with an average of 0.68 in post-monsoon. If sulphuric acid is responsible for
carbonateweatheringthenCa
2+
-SO
4
2−
is almost 1:1 for calcite and 1:2 for dolomite
(Stallard and Edmond
1983
).TheplotofSO
4
2−
versusCashowsthatmostofthe
datafallabovethe1:1linetowardsCa
2+
(Fig.
3f
). This indicates carbonic acid con-
tribution towards carbonate or silicate dissolution in the study area. Correlation
studyshowsthatgoodcorrelationexistsbetweenCa
2+
andHCO
3
−
(
r
= 0.85) in pre-
and (
r
= 0.52) in post-monsoon, which supports the carbonate and/or silicate weath-
ering process for the contribution of Ca
2+
. Ca
2+
and Mg
2+
has positive correlation
(
r
= 0.28) in pre- but negative correlation (
r
= −0.38) in post-monsoon depicting that
dolomiteisprovidingCa
2+
and Mg
2+
in groundwater in pre-monsoon but not in post-
monsoon.IfgypsumisthemajorsourceforCa
2+
in groundwater it will dissociate
Ca
2+
andSO
4
2−
in equal concentration, there is positive and good correlation between
Ca
2+
and SO
4
2−
(
r
= 0.51) in pre-monsoon and poor correlation (
r
= 0.02) in post-
monsoonseason,indicatinggypsumdissolutionassourceforCa
2+
in groundwater
in pre-monsoon season. It has been observed that Ca
2+
, Na
+
, K
+
and Cl
−
contents
exhibit mutual positive correlation and correlate strongly with TDS in both pre- and
post-monsoon (Table
4
).
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