Environmental Engineering Reference
In-Depth Information
From the cationic and anionic triangular files of Piper diagram, it is observed that
56 %, 33 % and 11 % of groundwater samples fall into the Na
+
, no-dominant and
Ca
2+
fields in cation facies of pre-monsoon, respectively whereas 56 %, 33 % and
11 % of groundwater samples fall into the no-dominant, Na
+
andCa
2+
fields in cat-
ionfaciesofpost-monsoon,respectively.Conversely,89%and11%ofgroundwa-
ter samples fall into the HCO
3
−
and Cl
−
fields in anion facies for both pre- and
post-monsoon, respectively. The diamond shaped field between the two triangles is
used to represent the composition of water with respect to both cations and anions.
The points of both the cations and anions are plotted on the appropriate triangle
diagrams. The positions of the points are projected parallel to the magnesium and
sulphate axes until they intersect in the centre field. The plot of chemical data on
diamond shaped trilinear diagram (Fig.
2
) reveals that majority of groundwater
samples fall in the fields of 2, 3 and 5 suggesting that alkalies exceed alkaline earths,
weak acids exceed strong acids and the ions representing carbonate hardness (sec-
ondary alkalinity) exceed 50 %, respectively in the pre-monsoon, whereas in the
post-monsoon season majority of samples fall in the fields of 1, 3 and 5 suggesting
that alkaline earths exceed alkalies, weak acids exceed strong acids and the ions
representing carbonate hardness (secondary alkalinity) exceed 50 %, respectively.
From the data plots, it is apparent that the total hydrochemistry is dominated by
equal sharing of alkalies and alkaline earths and weak acids in the pre-monsoon, and
alkaline earths and weak acids in the post-monsoon season. However, some of the
groundwater samples having high chloride concentration falls in 1 and 4 fields indicat-
ing alkaline earths exceed alkalies and strong acids exceed weak acids in pre-monsoon,
whereas in the post-monsoon season samples fall in 2 and 4 fields indicating alkalies
exceed alkaline earths and strong acids exceed weak acids. Some samples also fall in
the field 9 indicating mixed water having no one cation-anion pair exceeding 50 %.
Ca
2+
/Mg
2+
ratioisusedtoknowthesourceofCa
2+
in groundwater of the study
area.IfCa
2+
/Mg
2+
ratioisunity,therewouldbedolomitedissolutionreleasingCa
2+
and Mg
2+
ions in equal ratio. If ratio is towards higher side, calcite dissolution domi-
nates over dolomite dissolution in releasing the calcium ion in groundwater. If ratio
exceeds 2, then silicate weathering provides Ca
2+
and Mg
2+
in the aquifer system
(Katzetal.
1998
). The ratio varies between 1.55-6.76 with an average of 4.85 in
pre- and 1.54-2.85 with an average of 2.11 in post-monsoon indicating silicate
weatheringasdominantsourcefortheCa
2+
in the aquifer system of the study area.
CorrelationstudyshowsthatthereisapoorpositivecorrelationbetweenCa
2+
and
Mg
2+
(
r
= 0.28) in pre- and (
r
= −0.32) in post-monsoon (Table
4
) indicating that the
majorsourceofCa
2+
and Mg
2+
may not be from the carbonate weathering in ground
waters. A good correlation between Ca
2+
-HCO
3
−
(
r
=0.85 in pre- and
r
= 0.52 in
post-monsoon) and Mg
2+
-HCO
3
−
(
r
= 0.14 in pre- and
r
= −0.27 in post-monsoon)
indicatessilicateweathering.TherecouldbealsogypsumdissolutionforCa
2+
avail-
abilityinthegroundwater.IfgypsumisthemajorsourceofCa
2+
in groundwater it
will dissociate Ca
2+
and SO
4
2−
in equal concentration. There is positive and good
correlation between Ca
2+
-SO
4
2−
(
r
=0.51 in pre- and
r
= 0.02 in post-monsoon)
(Table
4
) indicating gypsum dissolution as source of Ca in groundwater in pre-
monsoon but not in the post-monsoon.
Search WWH ::
Custom Search