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Paper Number: 15
STABLE
ISOTOPE TECHNIQUE TO DISCERN SOURCE AND ORIGIN OF CONTAMINATION IN
GROUNDWATER OF BANGALORE CITY, KARNATAKA, INDIA
Somashekar,
R.K.1*, Ravikumar, P.1 , Deljo
Davis1, Prakash, K.L.1, Shivanna, K.2#
and Pant, H.J.2
1Department
of Environmental Science, Bangalore University, Bangalore – 560056,
India.
__________________________________________________________________________
It is possible to determine the source and origin / recharge of
ground-water, geochemical processes in the aquifer system and major
locations of recharge zones using data on stable isotopes. Lake (n=9),
sewage (n=19), rain (n=11) and groundwater (n=43) samples were collected
during different seasons of the year 2008 and 2009. Measurements of the
stable isotope (viz., δ18O and δ2H) composition of
sewage water samples along lake, rain and groundwater samples were made
after applying correction factors to ascertain interaction and
interrelationship between groundwater and sewage, rain water and surface
water bodies, lake and groundwater. δ18O values for
groundwater samples varied from (-6.0) to (-2.3) ‰ and (-5.9) to (-1.9)
‰ respectively during pre- and post-monsoon seasons of 2008, with
average values of (-3.83 ‰) and (-3.28 ‰). δ2H values
groundwater samples varied from (-25.4) to (-0.8) ‰ and (-24.2) to
(-1.1) ‰ respectively during pre- and post-monsoon seasons of the year
2008, with mean concentration of (-11.46 ‰) and (-14.26 ‰). Similarly,
in the case of sewage samples, δ18O values were (-3.5) to
(-2.0) ‰ and (-3.3) to (-1.7) ‰ respectively during pre- and
post-monsoon seasons (averages: -2.54 and -2.43 ‰) and δ2H
values were (-14.0) to (-0.2) ‰ and (-22.7) to (-1.4) ‰ for pre- and
post-monsoon seasons (averages: -6.06 and -12.25 ‰). In case of lake
samples, δ18O values were (-3.4) to (-0.0) ‰ and (-2.7) to
(-0.2) ‰ respectively during pre- and post-monsoon seasons (averages:
-1.7 and -1.3 ‰) and δ2H values were (-15.3) to (-0.4) ‰ and
(-15.5) to (-3.3) ‰ for pre- and post-monsoon seasons (averages: -8.5
and -10.8 ‰). In rainwater samples, δ18O values and
δ2H values varied from (-8.36) to (-0.97) ‰ and (-54.38) to
(-4.74) ‰, with their respective mean concentration of (-5.62 ‰) and
(-31.53 ‰).
The linear equation for the pre monsoon (δD=2.11*18O -
7.34) and post monsoon ground water samples (δD=5.08* 18O +
7.97) confirms depleted isotopic signature than the LMWL (Local
Meteoritic Water Line). The aquifer highly appears to be influenced by
the complex system of surface water sources and precipitational events
thereby heavily affecting the isotopic composition of the groundwater.
The groundwater flowing along the different valleys is greatly
influenced by the sewage network as indicated by the clustering of
sewage sample and groundwater isotopic signatures. Moreover, the “sewage
influence signature” correlated with the sewage influences group
extracted from the physico-chemical parameters. The measurement of the
stable isotope composition in lakes corroborated with the rain. Majority
of these isotopic values were distributed on both sides of the GMWL and
LMWL, those found below the LMWL indicate the possible source of
precipitation water and those above the LMWL is from the groundwater
resources. The lakes are in tune with the precipitation as well as the
groundwater and thus, a mixed trend or diffused origin of this water is
noticed. However, most of the pre monsoon sewage samples data converged
around the LMWL confirming the influence of rain water in the sewage
flow, as runoff water while that of post monsoon showed a depleting
trend suggesting the stagnant flow occurring in the sewage channel.
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