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Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial aquifers of southwest Punjab
Author’s Accepted Manuscript Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial aquifers of southwest Punjab Keesari Tirumalesh, Diana A. Sharma, Madhuri S. Rishi, Diksha Pant, H.V. Mohokar, Ajay Kumar Jaryal, U.K. Sinha www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(17)30275-0 http://dx.doi.org/10.1016/j.apradiso.2017.07.022 ARI7974
To appear in: Applied Radiation and Isotopes Received date: 10 March 2017 Revised date: 6 July 2017 Accepted date: 6 July 2017 Cite this article as: Keesari Tirumalesh, Diana A. Sharma, Madhuri S. Rishi, Diksha Pant, H.V. Mohokar, Ajay Kumar Jaryal and U.K. Sinha, Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial aquifers of southwest Punjab, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.07.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial
2
aquifers of southwest Punjab
3
Tirumalesh Ka*, Diana A. Sb, Madhuri S. Rb, Diksha Pa, Mohokar H.Va, Jaryal A.Ka, Sinha U. Ka a
4 5 6
b
Isotope and Radiation Application Division, Bhabha Atomic Research Centre, Trombay, Mumbai
Department of Environment Studies, Panjab University, Chandigarh
7 8
*
9
Abstract
Corresponding author. [email protected]
10
Groundwater samples collected from the alluvial aquifers of southwest Punjab, both shallow and
11
deep zones were measured for environmental tritium (3H) and stable isotopes of water (2H and
12
18
13
wide variation in isotopic signature (δ18O: -11.3 to -5.0 ‰) reflecting multiple sources of
14
recharge. The average isotopic signature of shallow groundwaters (δ18O: -6.73 ± 1.03 ‰) is
15
similar to that of local precipitation (-6.98 ± 1.66 ‰) indicating local precipitation contributes to
16
a large extent compared to other sources. Other sources have isotopically distinct signatures due
17
to either high altitude recharge (canal sources) or evaporative enrichment (irrigation return flow).
18
Deep groundwater shows relatively depleted isotopic signature (δ18O: -8.6‰) and doesn’t show
19
any evaporation as compared to shallow zone indicating recharge from precipitation occurring at
20
relatively higher altitudes. Environmental tritium indicates that both shallow (3H: 5 – 10 T.U.)
21
and deeper zone (3H: 1.5 – 2.5 T.U.) groundwaters are modern. In general the inter-aquifer
22
connections seem to be unlikely excepting a few places. Environmental isotope data suggests
23
that shallow groundwater is dynamic, local and prone to changes in land use patterns while deep
24
zone water is derived from distant sources, less dynamic and not impacted by surface
25
manifestations. A conceptual groundwater flow diagram is presented.
26
Key words:
27
Environmental isotopes; groundwater recharge; aquifer dynamics, conceptual flow model
O) to evaluate the source of recharge and aquifer dynamics. The shallow groundwater shows
1
28 29
1. Introduction
30 31
Understanding the recharge characteristics of alluvial aquifers is an important aspect in
32
evaluating their vulnerability to anthropogenic contamination and also to changing climatic
33
conditions. Understanding the groundwater flow in human impacted alluvial aquifers has become
34
a challenging task for hydrogeologists. Obtaining clear understanding of recharge sources and
35
their dynamics is critical to many problems such as declining water levels, contaminant transport
36
and nuclear waste repository safety assessment, etc. Declining groundwater levels are a global
37
concern since most of the freshwater needs for a large part of the human population are met by
38
shallow aquifers (UNESCO, 2009). Studies have shown that human interventions have
39
significantly impacted groundwater levels (Van Loon et al., 2016) through pumping for drinking-
40
water supplies (Willis, 1998), irrigation (Amelung et al., 1999; Foster et al., 2004; Konikow and
41
Kendy, 2005; Wada et al., 2012) and industrial use (Hayashi et al., 2009).
42
Environmental tracers can provide reliable datasets which can be used to understand
43
groundwater recharge sources and their dynamics over a wide range of spatial and temporal
44
scales. Water table fluctuations and lysimeters provide estimates of local recharge over a few
45
days to a few years, environmental tritium (3H) and chlorofluorocarbons (CFCs) in groundwater
46
typically constrain recharge rates over years to decades, while
47
recharge on longer timescales and over larger areas (Bouhlassa and Aiachi, 2002, Scanlon et al.,
48
2002). Because of its half life of 12.32 years, 3H is a potential candidate for dating groundwater
49
recharged over the last 50 to 100 years. 3H is part of the water molecule and its abundance in
50
groundwater isolated from the atmosphere is only affected by radioactive decay and not by
51
reactions between water and aquifer matrix. Due to the production of 3H during atmospheric
52
nuclear tests the 3H input function in rainfall has a distinct peak in the 1950s–1960s. This “bomb
53
3
54
1991). Since the input function of 3H is not constant, accurate dating of groundwater by single
55
measurement of 3H concentrations could not be achieved. However time series measurements of
36
Cl and
14
C constrain average
H pulse” has been utilised to trace the flow of water recharged during this period (Fritz et al.,
2
56
3
H can yield quantitative age determination. Most commonly 3H presence in groundwater is
57
interpreted as modern recharge.
58
Stable isotopes ( δ18O and δ2H ), on the other hand, are commonly used to identify linkages
59
between the surface water and groundwater systems, and have widely been recognized as useful
60
tracers in providing insights into water movements (Kendall and Mac Donnell, 1998). Isotopes
61
are powerful integrative recorders of key processes like, evaporation, transpiration, recycling and
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mixing (Tarki et al., 2016). Recharge by direct precipitation, runoff, lakes, snow and glaciers can
63
be differentiated by their characteristic stable isotopic signatures (Jasechko et al., 2013;
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McDonnell et al., 1990). Various other hydrologic processes that can modify the isotopic
65
compositions in groundwater are mixing with different source waters (Lambs, 2004), enrichment
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in heavier isotopes owing to evapotranspiration (Bouragba et al., 2011; Simpson and
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Herczeg,1991; Telmer and Veizer, 2000), isotopic fractionation during rainfall (Taylor et al.,
68
2002) and enrichment in oxygen isotope during water rock interaction in geothermal fields
69
(Panichi and Gonfiantini, 1977).
70
Both shallow and deep groundwater is used as a dominant source of water supply for irrigation
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as well as domestic needs in Punjab state, which is an agriculture dominant state (CGWB, 2013;
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Sharma et al., 2016a). The decline in rainfall of about 40 -50 % has been reported for the last two
73
decades (PHRED, 2014). In addition to the decline in rainfall, the surface water resources are
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also fully utilized, which led to extra burden on groundwater resources to meet the increasing
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demand for irrigation and domestic needs. Southwest Punjab is facing problems like water-
76
logging, salt water encroachment, groundwater pollution and salinity which are the consequences
77
of intensive irrigation (Chopra and Krishan, 2014; Kochhar et al., 2007; Sharma et al. 2016a,b,c).
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Average depth to groundwater ranges from 5 to 10 m below ground level. Moreover, rise in
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water level in southern part and decline in northern part of the district has been observed
80
(CGWB, 2013). In addition, groundwater quality is also impacted by effluents emerging from
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thermal and other industrial activities located in the study area, such as, Guru Nanak Dev
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Thermal Plant, Guru Har Gobind Thermal Plant, fertilizer plants, Bathinda chemicals and
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refineries (CGWB, 2013). Uranium contamination in drinking water is widely noted in southwest
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part of the Punjab state (Madhuririshi et al., 2017). Not many studies have investigated the
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recharge conditions of the groundwater present in both shallow as well as deeper horizons which 3
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is essential to understand the quality degradation and also to plan the remedial measures to
87
control further damage.
88
This study employs isotope techniques: environmental tritium (3H) along with stable isotopes of
89
water (δ2H and δ18O), to provide insights into various water bearing zones present in subsurface
90
and the present-day groundwater recharge conditions in two districts of southwest Punjab state
91
(viz., Mansa and Bathinda), which are facing severe problems of water logging, pesticide and U
92
contamination. Based on the isotope inferences a conceptual groundwater flow is prepared for
93
this region.
94
2. Materials and Methods
95
2.1 Study area
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The study area falls in the south western region of Punjab State in India covering an area of
97
about 5000 km2. The geographical coordinates are; longitudes 7430 and 7550 E and latitudes
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29 20 and 30 30 N (Fig. 1). The study area is underlain by an alluvial complex of fluviatile
99
origin deposited by the Indus River system and geologically it forms a part of Indo-Gangetic
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alluvium of Quaternary period. The Quaternary alluvium has been deposited on semi-
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consolidated Tertiary rocks, which are underlain by a thick sequence of Vindhyan halite and
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evaporites. The climate of this region is semi-arid (400-500 mm/a) and mostly influenced by the
103
Western Himalayas in the north and the Thar Desert in the south and southwest. The southwest
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monsoon (June to September) accounts for the majority of the total annual rainfall, with the rest
105
originating from thunder storms and western disturbances. The western disturbance is a non-
106
monsoonal and extratropical storm originating in the Mediterranean region that brings sudden
107
winter rain to the northwestern parts of the Indian subcontinent.
108
This region is one of the highly cultivated regions of India with irrigation predominantly
109
provided by groundwater extraction. The mean rate of groundwater decline of 4 cm / year
110
(Rodell et al., 2009). The groundwater abstraction is widespread and prolific due to easy
111
accessibility of water potential zones. Both shallow and deep groundwater is being exploited for
112
growing agricultural and domestic needs (CGWB, 2011). Regional scale modelling studies have
113
shown that groundwater levels have been falling for the last two decades due to intense
114
groundwater abstraction (Cheema et al., 2014). Understanding the recharge processes and the 4
115
groundwater dynamics is critical to assess the vulnerability of groundwater resources towards
116
anthropogenic contamination as well as resilience to over-exploitation.
117 118
The sediments typically consist of fine to medium grained sand. Kankar and sand with admixture
119
of clay constitutes the aquifer system of this area (CGWB, 2013; Singh et al., 2011). The top
120
aquifer ranges from 40 to 56 m followed by a thick clay bed of thickness 15 - 35 m and beneath
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this a granular zone exists extending up to a depth of 300 m (CGWB, 2013). The general slope
122
of the water table is towards SW from North, NE, East and SE. Based on the groundwater draft,
123
the study area falls in critical category and based on the agro-climatic zonation, it falls under
124
western plain zone characterized by semi-arid (dry to sub-moist) climate zone. A modified fence
125
diagram, retraced and trimmed to the study area is shown in Fig. 1 depicting the nature of
126
sediments in the sediments and their vertical distribution (CGWB 2007; Sharma et al., 2016a).
5
127
Fig. 1 a) Study area and sampling locations (HP- Hand Pump, TW-Tube Well and BW-Bore
128
well) and b) subsurface cross section of southwest Punjab (modified from CGWB 2011)
129
2.2 Methodology
130
Groundwater (shallow 29 nos. and deep 10 nos.) and canal water samples (2 nos) were collected
131
from the study area covering shallow and deep wells. Both domestic hand pumps as well as
132
agricultural wells (bore wells and tube wells) were sampled so that proper spatial representation
133
is achieved. Hand pumps and shallow tube wells tap groundwater up to a depth of 50 m bgl
134
which constitute shallow groundwater system, while bore wells and deep tube wells with depths
135
more than 50 m bgl represent deep groundwater system. The sample locations are shown in Fig.
136
1.
137
A total of 36 samples were measured for environmental tritium. For environmental tritium, 1 liter
138
of water sample is collected and measured using liquid scintillation counter (LKB Wallac, Model
139
No.1215) preceded by electrolytic enrichment. Measurement of environmental tritium is carried
140
out in four steps, i) distillation, ii) electrolytic enrichment, iii) neutralization and iv) counting
141
using low background liquid scintillation counter (Jovana Nikolov et al., 2013; Nair, 1983). 250
142
ml of the pre-distilled water is electrolytically enriched using an electrolytic cell having mild
143
steel cathode and stainless steel anode, with sodium peroxide as electrolyte. The sample after
144
enrichment is subjected to distillation after adding lead chloride to neutralize the media. The
145
distillate is mixed with organic scintillator (Optiphase Hisafe-3) in a 20-ml capacity scintillation
146
vial. The vials are counted for the beta radiation of the enriched sample in 10 cycles of 50
147
minutes each (a total of 500 minutes) to achieve better statistics. Tritium concentration is
148
expressed in tritium unit (TU), where 1 TU corresponds to one tritium atom per 10 18 protium
149
atoms (i.e. 1TU = 3.2 pCi L-1 or 0.118 Bq/l). The precision of tritium measurements is 0.5 TU
150
(2 criterion).
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A total of 41 samples were collected for stable isotope measurement. For stable isotopes (δ2H
152
and δ18O) analysis, water samples were collected in 50 ml airtight high density polyethylene
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bottles and measured by continuous flow Isotope Ratio Mass Spectrometer (Isoprime 100). For
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δ2H analysis, 1 ml of the water sample was equilibrated with H2 in presence of Pt–coated Hokko
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beads catalyst at 50oC for 90 minutes and the gas was introduced into the mass spectrometer. The 6
156
δ18O of the sample was measured by equilibrating 1 ml of water with CO2 gas at 50oC for 8
157
hours and the equilibrated gas was introduced into the mass spectrometer. The results are
158
reported in δ-notation and expressed in units of parts per thousand (denoted as ‰). The δ values
159
are calculated using (Coplen, 1996):
160
(‰)
161
where R denotes the ratio of heavy to light isotope (e.g. 2H/1H or 18O/16O) and Rx and Rs are the
162
ratios in the sample and standard respectively. The precision of measurement for 2H is ± 0.5 ‰
163
and for 18O is ± 0.1 ‰ (2).
164
3. Results and Discussion
165
3.1 Stable isotopes
166
Groundwater samples show a wide variation in stable isotopic contents, -11.3 to -5.1 ‰ for δ18O
167
and -41.7 to -80.1 ‰ for δ2H. Histograms of the stable isotopes data are shown in Fig. 2 a & b.
168
Additional set of sub-figures showing the different distributions in the shallow and deep wells is
169
also provided along with the histograms of δ2H and δ18O. From the figures it can be noticed that
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the spread in the δ2H values in groundwater follows log normal distribution with a mean value of
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-57.6 ‰ for δ2H (Fig. 2a) while δ18O variation is random. The mean value obtained from the log
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normal distribution of δ18O is -7.89 ‰ (Fig. 2b). The isotope distributions among the shallow
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and deep wells doesn’t show any systematic distribution. But, the range of isotope values of deep
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wells fall within the range of shallow wells for both δ2H and δ18O indicating isotopically there is
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no significant difference between shallow and deep groundwater.
176
The spatial distribution of the δ18O content of both the shallow and deep groundwaters is shown
177
in the contour diagram (Fig. 3). This demonstrates that although the isotope values of the shallow
178
and deep aquifers overlap, the shallow aquifer accounts for the majority of the variability in the
179
entire dataset. High depletion in the δ18O content (about -11 ‰) is noticed in the extreme western
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region of the study area, while highest enrichment (about -3.5 ‰) is observed in the central part
181
of the study area (Fig. 3). This clearly indicates the influence of geomorphological features like,
182
topography, presence of sand dunes, canals, wetlands and surface water bodies, on the recharge
Rx 1 1000 Rs
(1)
7
183
conditions of the aquifer. Since the isotopic values of deep wells don’t not vary much and also
184
fall within the range of shallow wells, it can be stated that the the observed regional trends (e.g.
185
western, central) are representative of the overall groundwater system. However, more data is
186
needed to further refine the isotope distributions in shallow and deep aquifers.
187 8
188
Fig. 2 Histogram of stable isotope data of groundwater a) δ2H and b) δ18O
189
Locations falling on the western border of the study area where canals are present showed a
190
depleted isotopic composition in groundwater, while locations near irrigated sites showed
191
enriched values.
192 193
Fig. 3 Spatial distribution of δ18O in groundwater
194
In addition to δ18O fluctuations, the δ2H and δ18O are compared with the Global Meteoric Water
195
Line (GMWL: δ2H = 8 × δ18O + 10), which serves as a reference line to determine the deviations
196
of the relation between δ18O and δ2H in groundwater samples (Craig, 1961). The offset of 10 is
197
determined by kinetic isotope fractionation that occurs during non-equilibrium processes such as
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evaporation (Craig, 1961). The slope of the equation represents the degree of evaporation in the
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falling raindrop or the surface waters before recharging the groundwater. A slope less than 8 in
200
groundwater samples indicates evaporation effect (Clark and Fritz, 1997). A local meteoric water
201
line (LMWL) constructed for Punjab rainwater by Rao et al. (2017) was also used to interpret the
202
isotope results obtained in this study. 9
203
A plot of δ2H versus δ18O composition of the groundwater samples is shown in Fig. 4a. The
204
average value of δ2H (-49 ±13.3 ‰) and δ18O ( -6.98 ±1.66 ‰) for rainwater samples ( n= 14) is
205
also shown in the figure. Majority of the groundwater samples fall on the LMWL indicating
206
precipitation as dominant source of recharge for both shallow and deep zone groundwaters. It is
207
observed that canal samples show depleted isotopic content (δ18O: -10 ‰) and fall along with
208
some groundwater samples. The Group (a) represents influence of canal on the shallow
209
groundwaters. Since the canal water is originated from the higher elevations the isotopic
210
composition is depleted compared to rest of the samples of this area. An isotopic composition of
211
δ18O: -12.1 to -11.5‰ in canal water is reported previous researchers (Rai et al., 2014). Depleted
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isotopic composition in springs and canal waters originating from higher altitudes is reported by
213
other researchers as well (Shivanna et al. 2008; Rao et al., 2017). It is reported that the canal
214
seepages contribute to groundwater in the study area (Tripathi et al., 2016). The deep
215
groundwater shows intermediate isotopic values between -9.2 and -7.1 ‰, which is a much
216
smaller range compared to the shallow groundwater samples. The histogram of the isotope data
217
shows a narrow distribution of δ18O with a mean of -8.6‰ ± 0.71 (Fig. 4b), indicating their
218
recharge from depleted sources, like precipitation occurring at higher elevations. The isotopic
219
composition of the rainwater occurring at higher elevations typically range from -14 to -11 ‰
220
(Sharma et al., 2016).
221 10
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Fig. 4 a) δ2H versus δ18O values of groundwater, box plot of b) deep zone (n = 10) and c)
223
shallow groundwaters (n= 29). The standard deviation for rainwater composition is 1.66 for δ18O
224
and 13.3 for δ2H.
225
The majority of the shallow groundwater falls in a separate cluster (group c) with enriched
226
isotopic composition (δ18O: -7.5 to -4.6‰) except a few samples influenced by canal recharge.
227
The isotope data shows a wide distribution compared to deep zone with an average value of the
228
δ18O: -6.7 ± 1.1‰ as shown by the histogram (Fig. 4c). This suggests the shallow groundwater
229
receives water from less depleted sources such as irrigation return flow or/and local precipitation.
230
The recharge for shallow groundwaters seems to differ from that of deep groundwater. The best
231
fit line to isotopic data of the shallow groundwater (excluding those impacted by canals) shows a
232
slope of 5.3 indicating evaporative enrichment. The average value of shallow groundwaters is
233
δ18O: -6.7‰ which is closely resembles precipitation water with δ18O: -6.98 ‰ indicating the
234
dominant recharge to shallow zone is low elevation precipitation while the deep zone doesn’t
235
receive local rainwater towards recharge. The average value for precipitation in Bist Doab region
236
was found to be δ18O: -6.5 ‰ (Rao et al., 2017), which is similar to our findings. Precipitational
237
recharge to shallow aquifers is also reported in other parts of Punjab by researchers (Rai et al.,
238
2014; Rao et al., 2017).
11
239
Fig.5 Vertical distribution of δ18O in groundwater
240
The subsurface cross section depicted in Fig. 1b corroborates the findings that vertical recharge
241
to deep zone is limited by the presence of thick clay lenses between shallow and deep aquifers.
242
This observation is important as it confirms that the shallow groundwater receive a major portion
243
of its recharge from local precipitation while other sources contribute to a lesser extent.
244
Vertical distribution of stable isotope variation (δ18O) in groundwaters is shown in Fig. 5. Three
245
major sources of recharge can be envisaged from the figure. Groundwater samples falling in the
246
western parts are influenced by canal recharge. A similar pattern is seen in Fig. 4 (group (a)
247
samples). Another group represents shallow groundwater, which have major recharge
248
contribution from local precipitation as well as irrigation return flow. These samples are
249
relatively enriched in stable isotopic composition. A similar behaviour is observed for group (c)
250
samples in Fig. 4. The moderate enrichment (-8.7 to -8.0 ‰ for δ18O) in groundwater can be
251
attributed to recharge by local precipitation while enriched isotopic composition (-8.5 to -5.1 ‰
252
for δ18O) could be due to recharge by evaporated sources such as wetlands, irrigation return flow
253
and industrial wastes. Another major source of recharge is found to be precipitation occurring at
254
higher elevations, i.e. distant recharge. Deep groundwater samples typically show a depleted
255
isotopic composition compared to the shallow groundwater which indicates that the source of
256
recharge could be from elevated parts of eastern reaches. The isotopic variation in deep
257
groundwater is also narrow (-9.6 to -8.5 ‰ for δ18O), indicating that the groundwater is not
258
significantly influenced by the multiple sources like irrigation return flows, canal, wetland and
259
rainfall. Similar observation is noted in Fig. 4 (for group (b) samples). Since the deep
260
groundwater data do not show a similar distribution in the isotope data as shown in the case of
261
shallow groundwater, it can be stated that the possibility of groundwater interconnection between
262
shallow and deep zones is less likely at most of the studied locations.
263
3.2 Environmental tritium
264
Tritium is produced in the atmosphere by cosmic ray neutron interaction with nitrogen and
265
becomes a part of the water molecule in stratosphere. Tritium is also released into the
266
atmosphere due to various anthropogenic reasons. Anthropogenic tritium was released into the
267
atmosphere mostly during the testing of nuclear weapons that started in 1951 and continued till 12
268
the 1963. Two basic assumptions are made for dating groundwater using tritium that: 1) 3H-free
269
(i.e., below normal detection limits) groundwater contains only precipitation water from the "pre-
270
bomb period" and is, thus, >50 yr old; and 2) 3H-rich (up to 100 TU) water contains much
271
precipitation water from the 1960s (Dieter, 1993). The use of 3H dating of groundwaters has
272
been based primarily on models of the long-term trend of 3H content in hydrological systems
273
(Moser and Rauert, 1980).
274
Environmental tritium values in the study range from 4.5 to 9.8 TU with a mean of 6.5 TU. A
275
histogram of tritium distribution in groundwater samples is shown in Fig. 6. This range coincides
276
with that of other inland study sites, viz., Hyderabad, Telangana and Bist Doab, Punjab
277
(Unnikrishnan et al. 2013; Rao et al., 2017) in northeast part of Punjab. The tritium values also
278
suggest that groundwater is of modern origin and less than 50 years old.
279
Fig. 6 Histogram of environmental tritium data of groundwater
280
The vertical distribution of tritium concentration in groundwater of this region is shown in Fig. 7.
281
The data show that groundwater show wide variation in tritium content in shallow zone,
282
reflecting both fast and slow recharge to shallow groundwater. The subsurface lithology of the
283
area suggests presence of clay zones of variable thickness at many places (CGWB, 2013),
284
therefore it is possible that vertical percolation of rainwater or irrigation return flow is not
285
uniform throughout entire study area. Presence of thick clay zones retard the vertical percolation
286
leading to low tritium values while sandy zone allows easy vertical percolation leading to higher 13
287
tritium concentrations of groundwater. In the case of deep groundwater, tritium content is
288
relatively low (1 - 5 TU). Tritium levels as low as < 2 TU were noticed in deep zones of
289
northeast Punjab (Rao et al., 2107). Low tritium in deep zone groundwater again indicates that
290
the groundwater is older and recharged by single source compared to shallow zone where
291
multiple sources contribute to recharge. A few deep aquifer samples containing 3H close to 5 TU
292
indicate groundwater receiving vertical percolation due to poor well construction or due to
293
absence of clay zone at those sites. In general, for deep zones, regional groundwater flows
294
originating from higher elevations such as mountain fronts acts as recharge source. However
295
groundwater in both the zones is modern and represents active hydrological condition. There is
296
no component of recharge from “bomb- era” in these groundwater samples. But, the groundwater
297
residence time in shallow zone is lesser compared to deep zone. From stable isotope and
298
environmental tritium trends it can be understood that significant inter-connection between
299
shallow and deep zone groundwater is not observed.
300 301 302
Fig. 7 Vertical distribution of 3H in groundwater, dotted arrow is eye-guide to data points 3.3 Conceptualisation of groundwater flow 14
303
Major aquifer system of the study area is more complex, comprising different recharge sources,
304
the inference from the topographic elevation map shows that the slope is trending towards
305
southwest. Hence, the groundwater flow is also believed to follow the slope of the region, i.e. NE
306
to SW. The formations of this regions are typically made up of alternating layers of sand and
307
clay. Presence of kankar and sandy clay with varied thickness both laterally and vertically render
308
this geometry of the aquifers highly variable. Application of environmental tracers such as stable
309
isotopes (2H and 18O) provide a regional picture of groundwater recharge characteristics while
310
radio isotope tritium provides the insights into the dynamics of groundwater. From the
311
environmental isotope trends a conceptual groundwater flow model relating the recharge sources
312
and dynamics in shallow and deep groundwater is prepared and depicted in Fig. 8. The major
313
recharge processes and sources area as follows:
314
I.
Primary source is the rainwater recharge process. Most of the recharge to shallow and
315
deep zones happen from rainwater, whether direct infiltration or through regional flows.
316
The groundwater transit times are shorter in shallow zone compared to deep zone.
317
II.
Secondary source consists of evaporated surface waters such as irrigation return flows
318
and other surface water bodies. The canal influence is seen at selected locations. These
319
sources mainly act on the shallow zone.
320
Fig. 8 Systematic diagram showing the groundwater origin and recharge processes.
321 15
322
III.
On a broader scale the aquifer – aquifer interconnection is not observed, which is also
323
evident form the subsurface geology of this region. Clay layers of thickness up to 30 m
324
are present in this region rendering the deep aquifer semi-confined to confined in nature.
325
However, a few places belonging to deep zone have contribution from shallow zone. A
326
targeted research is necessary to understand the aquifer interconnections in this region
327
and to identify potential contamination threats to the deeper aquifer to both man-made
328
and natural perturbations.
329
4. Summary
330
Identifying groundwater recharge sources and their dynamics is critical to water authorities and
331
municipalities for implementing better management practices for preservation and enhancement
332
of aquifer recharge. From the environmental isotope data of the groundwater samples collected
333
from both shallow and deep zones of southwest Punjab, it can be inferred that shallow zones
334
have direct contact with the rainfall recharge as well as irrigation return flow whereas the deeper
335
zones are fed predominantly by regional groundwater flows originating from higher elevations.
336
Canal contribution is observed in the shallow groundwater in some places. In general, shallow
337
and deep aquifers are not interconnected. However, tritium data suggest that some level of inter-
338
connection is present in a few locations, and therefore deep groundwater in these locations may
339
be more susceptible to anthropogenic impacts. The results of our study show that monitoring
340
water isotopes and/or tritium concentrations in the deep aquifer for increased variability could be
341
a powerful tool for early identification of changes in water source for the deep aquifer.
342
Acknowledgement
343
Authors sincerely acknowledge the constant support and encouragement by Shri K.S.S.Sarma,
344
Head, Isotope and Radiation Application Division and Dr. B.S. Tomar, Director, Radiochemistry
345
and Isotope Group, Bhabha Atomic Research Centre, Mumbai. The authors would like to
346
acknowledge the BRNS, DAE for providing the financial support (letter no. 35/14/11/2014-
347
BRNS-193). Authors thank the review for his valuable suggestions.
348
16
349
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465 466
Highlights:
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The environmental isotopic study of southwest Punjab groundwaters is presented.
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Sources contributing to shallow and deep zone groundwater recharge are delineated.
469
Impact of irrigation return flow and canal water is limited to shallower depths at few
470
places.
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Hydraulic interconnection between shallow and deep zones is remote.
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A conceptual groundwater flow model for southwest Punjab is presented.
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PII: DOI: Reference:
S0969-8043(17)30275-0 http://dx.doi.org/10.1016/j.apradiso.2017.07.022 ARI7974
To appear in: Applied Radiation and Isotopes Received date: 10 March 2017 Revised date: 6 July 2017 Accepted date: 6 July 2017 Cite this article as: Keesari Tirumalesh, Diana A. Sharma, Madhuri S. Rishi, Diksha Pant, H.V. Mohokar, Ajay Kumar Jaryal and U.K. Sinha, Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial aquifers of southwest Punjab, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.07.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Isotope investigation on groundwater recharge and dynamics in shallow and deep alluvial
2
aquifers of southwest Punjab
3
Tirumalesh Ka*, Diana A. Sb, Madhuri S. Rb, Diksha Pa, Mohokar H.Va, Jaryal A.Ka, Sinha U. Ka a
4 5 6
b
Isotope and Radiation Application Division, Bhabha Atomic Research Centre, Trombay, Mumbai
Department of Environment Studies, Panjab University, Chandigarh
7 8
*
9
Abstract
Corresponding author. [email protected]
10
Groundwater samples collected from the alluvial aquifers of southwest Punjab, both shallow and
11
deep zones were measured for environmental tritium (3H) and stable isotopes of water (2H and
12
18
13
wide variation in isotopic signature (δ18O: -11.3 to -5.0 ‰) reflecting multiple sources of
14
recharge. The average isotopic signature of shallow groundwaters (δ18O: -6.73 ± 1.03 ‰) is
15
similar to that of local precipitation (-6.98 ± 1.66 ‰) indicating local precipitation contributes to
16
a large extent compared to other sources. Other sources have isotopically distinct signatures due
17
to either high altitude recharge (canal sources) or evaporative enrichment (irrigation return flow).
18
Deep groundwater shows relatively depleted isotopic signature (δ18O: -8.6‰) and doesn’t show
19
any evaporation as compared to shallow zone indicating recharge from precipitation occurring at
20
relatively higher altitudes. Environmental tritium indicates that both shallow (3H: 5 – 10 T.U.)
21
and deeper zone (3H: 1.5 – 2.5 T.U.) groundwaters are modern. In general the inter-aquifer
22
connections seem to be unlikely excepting a few places. Environmental isotope data suggests
23
that shallow groundwater is dynamic, local and prone to changes in land use patterns while deep
24
zone water is derived from distant sources, less dynamic and not impacted by surface
25
manifestations. A conceptual groundwater flow diagram is presented.
26
Key words:
27
Environmental isotopes; groundwater recharge; aquifer dynamics, conceptual flow model
O) to evaluate the source of recharge and aquifer dynamics. The shallow groundwater shows
1
28 29
1. Introduction
30 31
Understanding the recharge characteristics of alluvial aquifers is an important aspect in
32
evaluating their vulnerability to anthropogenic contamination and also to changing climatic
33
conditions. Understanding the groundwater flow in human impacted alluvial aquifers has become
34
a challenging task for hydrogeologists. Obtaining clear understanding of recharge sources and
35
their dynamics is critical to many problems such as declining water levels, contaminant transport
36
and nuclear waste repository safety assessment, etc. Declining groundwater levels are a global
37
concern since most of the freshwater needs for a large part of the human population are met by
38
shallow aquifers (UNESCO, 2009). Studies have shown that human interventions have
39
significantly impacted groundwater levels (Van Loon et al., 2016) through pumping for drinking-
40
water supplies (Willis, 1998), irrigation (Amelung et al., 1999; Foster et al., 2004; Konikow and
41
Kendy, 2005; Wada et al., 2012) and industrial use (Hayashi et al., 2009).
42
Environmental tracers can provide reliable datasets which can be used to understand
43
groundwater recharge sources and their dynamics over a wide range of spatial and temporal
44
scales. Water table fluctuations and lysimeters provide estimates of local recharge over a few
45
days to a few years, environmental tritium (3H) and chlorofluorocarbons (CFCs) in groundwater
46
typically constrain recharge rates over years to decades, while
47
recharge on longer timescales and over larger areas (Bouhlassa and Aiachi, 2002, Scanlon et al.,
48
2002). Because of its half life of 12.32 years, 3H is a potential candidate for dating groundwater
49
recharged over the last 50 to 100 years. 3H is part of the water molecule and its abundance in
50
groundwater isolated from the atmosphere is only affected by radioactive decay and not by
51
reactions between water and aquifer matrix. Due to the production of 3H during atmospheric
52
nuclear tests the 3H input function in rainfall has a distinct peak in the 1950s–1960s. This “bomb
53
3
54
1991). Since the input function of 3H is not constant, accurate dating of groundwater by single
55
measurement of 3H concentrations could not be achieved. However time series measurements of
36
Cl and
14
C constrain average
H pulse” has been utilised to trace the flow of water recharged during this period (Fritz et al.,
2
56
3
H can yield quantitative age determination. Most commonly 3H presence in groundwater is
57
interpreted as modern recharge.
58
Stable isotopes ( δ18O and δ2H ), on the other hand, are commonly used to identify linkages
59
between the surface water and groundwater systems, and have widely been recognized as useful
60
tracers in providing insights into water movements (Kendall and Mac Donnell, 1998). Isotopes
61
are powerful integrative recorders of key processes like, evaporation, transpiration, recycling and
62
mixing (Tarki et al., 2016). Recharge by direct precipitation, runoff, lakes, snow and glaciers can
63
be differentiated by their characteristic stable isotopic signatures (Jasechko et al., 2013;
64
McDonnell et al., 1990). Various other hydrologic processes that can modify the isotopic
65
compositions in groundwater are mixing with different source waters (Lambs, 2004), enrichment
66
in heavier isotopes owing to evapotranspiration (Bouragba et al., 2011; Simpson and
67
Herczeg,1991; Telmer and Veizer, 2000), isotopic fractionation during rainfall (Taylor et al.,
68
2002) and enrichment in oxygen isotope during water rock interaction in geothermal fields
69
(Panichi and Gonfiantini, 1977).
70
Both shallow and deep groundwater is used as a dominant source of water supply for irrigation
71
as well as domestic needs in Punjab state, which is an agriculture dominant state (CGWB, 2013;
72
Sharma et al., 2016a). The decline in rainfall of about 40 -50 % has been reported for the last two
73
decades (PHRED, 2014). In addition to the decline in rainfall, the surface water resources are
74
also fully utilized, which led to extra burden on groundwater resources to meet the increasing
75
demand for irrigation and domestic needs. Southwest Punjab is facing problems like water-
76
logging, salt water encroachment, groundwater pollution and salinity which are the consequences
77
of intensive irrigation (Chopra and Krishan, 2014; Kochhar et al., 2007; Sharma et al. 2016a,b,c).
78
Average depth to groundwater ranges from 5 to 10 m below ground level. Moreover, rise in
79
water level in southern part and decline in northern part of the district has been observed
80
(CGWB, 2013). In addition, groundwater quality is also impacted by effluents emerging from
81
thermal and other industrial activities located in the study area, such as, Guru Nanak Dev
82
Thermal Plant, Guru Har Gobind Thermal Plant, fertilizer plants, Bathinda chemicals and
83
refineries (CGWB, 2013). Uranium contamination in drinking water is widely noted in southwest
84
part of the Punjab state (Madhuririshi et al., 2017). Not many studies have investigated the
85
recharge conditions of the groundwater present in both shallow as well as deeper horizons which 3
86
is essential to understand the quality degradation and also to plan the remedial measures to
87
control further damage.
88
This study employs isotope techniques: environmental tritium (3H) along with stable isotopes of
89
water (δ2H and δ18O), to provide insights into various water bearing zones present in subsurface
90
and the present-day groundwater recharge conditions in two districts of southwest Punjab state
91
(viz., Mansa and Bathinda), which are facing severe problems of water logging, pesticide and U
92
contamination. Based on the isotope inferences a conceptual groundwater flow is prepared for
93
this region.
94
2. Materials and Methods
95
2.1 Study area
96
The study area falls in the south western region of Punjab State in India covering an area of
97
about 5000 km2. The geographical coordinates are; longitudes 7430 and 7550 E and latitudes
98
29 20 and 30 30 N (Fig. 1). The study area is underlain by an alluvial complex of fluviatile
99
origin deposited by the Indus River system and geologically it forms a part of Indo-Gangetic
100
alluvium of Quaternary period. The Quaternary alluvium has been deposited on semi-
101
consolidated Tertiary rocks, which are underlain by a thick sequence of Vindhyan halite and
102
evaporites. The climate of this region is semi-arid (400-500 mm/a) and mostly influenced by the
103
Western Himalayas in the north and the Thar Desert in the south and southwest. The southwest
104
monsoon (June to September) accounts for the majority of the total annual rainfall, with the rest
105
originating from thunder storms and western disturbances. The western disturbance is a non-
106
monsoonal and extratropical storm originating in the Mediterranean region that brings sudden
107
winter rain to the northwestern parts of the Indian subcontinent.
108
This region is one of the highly cultivated regions of India with irrigation predominantly
109
provided by groundwater extraction. The mean rate of groundwater decline of 4 cm / year
110
(Rodell et al., 2009). The groundwater abstraction is widespread and prolific due to easy
111
accessibility of water potential zones. Both shallow and deep groundwater is being exploited for
112
growing agricultural and domestic needs (CGWB, 2011). Regional scale modelling studies have
113
shown that groundwater levels have been falling for the last two decades due to intense
114
groundwater abstraction (Cheema et al., 2014). Understanding the recharge processes and the 4
115
groundwater dynamics is critical to assess the vulnerability of groundwater resources towards
116
anthropogenic contamination as well as resilience to over-exploitation.
117 118
The sediments typically consist of fine to medium grained sand. Kankar and sand with admixture
119
of clay constitutes the aquifer system of this area (CGWB, 2013; Singh et al., 2011). The top
120
aquifer ranges from 40 to 56 m followed by a thick clay bed of thickness 15 - 35 m and beneath
121
this a granular zone exists extending up to a depth of 300 m (CGWB, 2013). The general slope
122
of the water table is towards SW from North, NE, East and SE. Based on the groundwater draft,
123
the study area falls in critical category and based on the agro-climatic zonation, it falls under
124
western plain zone characterized by semi-arid (dry to sub-moist) climate zone. A modified fence
125
diagram, retraced and trimmed to the study area is shown in Fig. 1 depicting the nature of
126
sediments in the sediments and their vertical distribution (CGWB 2007; Sharma et al., 2016a).
5
127
Fig. 1 a) Study area and sampling locations (HP- Hand Pump, TW-Tube Well and BW-Bore
128
well) and b) subsurface cross section of southwest Punjab (modified from CGWB 2011)
129
2.2 Methodology
130
Groundwater (shallow 29 nos. and deep 10 nos.) and canal water samples (2 nos) were collected
131
from the study area covering shallow and deep wells. Both domestic hand pumps as well as
132
agricultural wells (bore wells and tube wells) were sampled so that proper spatial representation
133
is achieved. Hand pumps and shallow tube wells tap groundwater up to a depth of 50 m bgl
134
which constitute shallow groundwater system, while bore wells and deep tube wells with depths
135
more than 50 m bgl represent deep groundwater system. The sample locations are shown in Fig.
136
1.
137
A total of 36 samples were measured for environmental tritium. For environmental tritium, 1 liter
138
of water sample is collected and measured using liquid scintillation counter (LKB Wallac, Model
139
No.1215) preceded by electrolytic enrichment. Measurement of environmental tritium is carried
140
out in four steps, i) distillation, ii) electrolytic enrichment, iii) neutralization and iv) counting
141
using low background liquid scintillation counter (Jovana Nikolov et al., 2013; Nair, 1983). 250
142
ml of the pre-distilled water is electrolytically enriched using an electrolytic cell having mild
143
steel cathode and stainless steel anode, with sodium peroxide as electrolyte. The sample after
144
enrichment is subjected to distillation after adding lead chloride to neutralize the media. The
145
distillate is mixed with organic scintillator (Optiphase Hisafe-3) in a 20-ml capacity scintillation
146
vial. The vials are counted for the beta radiation of the enriched sample in 10 cycles of 50
147
minutes each (a total of 500 minutes) to achieve better statistics. Tritium concentration is
148
expressed in tritium unit (TU), where 1 TU corresponds to one tritium atom per 10 18 protium
149
atoms (i.e. 1TU = 3.2 pCi L-1 or 0.118 Bq/l). The precision of tritium measurements is 0.5 TU
150
(2 criterion).
151
A total of 41 samples were collected for stable isotope measurement. For stable isotopes (δ2H
152
and δ18O) analysis, water samples were collected in 50 ml airtight high density polyethylene
153
bottles and measured by continuous flow Isotope Ratio Mass Spectrometer (Isoprime 100). For
154
δ2H analysis, 1 ml of the water sample was equilibrated with H2 in presence of Pt–coated Hokko
155
beads catalyst at 50oC for 90 minutes and the gas was introduced into the mass spectrometer. The 6
156
δ18O of the sample was measured by equilibrating 1 ml of water with CO2 gas at 50oC for 8
157
hours and the equilibrated gas was introduced into the mass spectrometer. The results are
158
reported in δ-notation and expressed in units of parts per thousand (denoted as ‰). The δ values
159
are calculated using (Coplen, 1996):
160
(‰)
161
where R denotes the ratio of heavy to light isotope (e.g. 2H/1H or 18O/16O) and Rx and Rs are the
162
ratios in the sample and standard respectively. The precision of measurement for 2H is ± 0.5 ‰
163
and for 18O is ± 0.1 ‰ (2).
164
3. Results and Discussion
165
3.1 Stable isotopes
166
Groundwater samples show a wide variation in stable isotopic contents, -11.3 to -5.1 ‰ for δ18O
167
and -41.7 to -80.1 ‰ for δ2H. Histograms of the stable isotopes data are shown in Fig. 2 a & b.
168
Additional set of sub-figures showing the different distributions in the shallow and deep wells is
169
also provided along with the histograms of δ2H and δ18O. From the figures it can be noticed that
170
the spread in the δ2H values in groundwater follows log normal distribution with a mean value of
171
-57.6 ‰ for δ2H (Fig. 2a) while δ18O variation is random. The mean value obtained from the log
172
normal distribution of δ18O is -7.89 ‰ (Fig. 2b). The isotope distributions among the shallow
173
and deep wells doesn’t show any systematic distribution. But, the range of isotope values of deep
174
wells fall within the range of shallow wells for both δ2H and δ18O indicating isotopically there is
175
no significant difference between shallow and deep groundwater.
176
The spatial distribution of the δ18O content of both the shallow and deep groundwaters is shown
177
in the contour diagram (Fig. 3). This demonstrates that although the isotope values of the shallow
178
and deep aquifers overlap, the shallow aquifer accounts for the majority of the variability in the
179
entire dataset. High depletion in the δ18O content (about -11 ‰) is noticed in the extreme western
180
region of the study area, while highest enrichment (about -3.5 ‰) is observed in the central part
181
of the study area (Fig. 3). This clearly indicates the influence of geomorphological features like,
182
topography, presence of sand dunes, canals, wetlands and surface water bodies, on the recharge
Rx 1 1000 Rs
(1)
7
183
conditions of the aquifer. Since the isotopic values of deep wells don’t not vary much and also
184
fall within the range of shallow wells, it can be stated that the the observed regional trends (e.g.
185
western, central) are representative of the overall groundwater system. However, more data is
186
needed to further refine the isotope distributions in shallow and deep aquifers.
187 8
188
Fig. 2 Histogram of stable isotope data of groundwater a) δ2H and b) δ18O
189
Locations falling on the western border of the study area where canals are present showed a
190
depleted isotopic composition in groundwater, while locations near irrigated sites showed
191
enriched values.
192 193
Fig. 3 Spatial distribution of δ18O in groundwater
194
In addition to δ18O fluctuations, the δ2H and δ18O are compared with the Global Meteoric Water
195
Line (GMWL: δ2H = 8 × δ18O + 10), which serves as a reference line to determine the deviations
196
of the relation between δ18O and δ2H in groundwater samples (Craig, 1961). The offset of 10 is
197
determined by kinetic isotope fractionation that occurs during non-equilibrium processes such as
198
evaporation (Craig, 1961). The slope of the equation represents the degree of evaporation in the
199
falling raindrop or the surface waters before recharging the groundwater. A slope less than 8 in
200
groundwater samples indicates evaporation effect (Clark and Fritz, 1997). A local meteoric water
201
line (LMWL) constructed for Punjab rainwater by Rao et al. (2017) was also used to interpret the
202
isotope results obtained in this study. 9
203
A plot of δ2H versus δ18O composition of the groundwater samples is shown in Fig. 4a. The
204
average value of δ2H (-49 ±13.3 ‰) and δ18O ( -6.98 ±1.66 ‰) for rainwater samples ( n= 14) is
205
also shown in the figure. Majority of the groundwater samples fall on the LMWL indicating
206
precipitation as dominant source of recharge for both shallow and deep zone groundwaters. It is
207
observed that canal samples show depleted isotopic content (δ18O: -10 ‰) and fall along with
208
some groundwater samples. The Group (a) represents influence of canal on the shallow
209
groundwaters. Since the canal water is originated from the higher elevations the isotopic
210
composition is depleted compared to rest of the samples of this area. An isotopic composition of
211
δ18O: -12.1 to -11.5‰ in canal water is reported previous researchers (Rai et al., 2014). Depleted
212
isotopic composition in springs and canal waters originating from higher altitudes is reported by
213
other researchers as well (Shivanna et al. 2008; Rao et al., 2017). It is reported that the canal
214
seepages contribute to groundwater in the study area (Tripathi et al., 2016). The deep
215
groundwater shows intermediate isotopic values between -9.2 and -7.1 ‰, which is a much
216
smaller range compared to the shallow groundwater samples. The histogram of the isotope data
217
shows a narrow distribution of δ18O with a mean of -8.6‰ ± 0.71 (Fig. 4b), indicating their
218
recharge from depleted sources, like precipitation occurring at higher elevations. The isotopic
219
composition of the rainwater occurring at higher elevations typically range from -14 to -11 ‰
220
(Sharma et al., 2016).
221 10
222
Fig. 4 a) δ2H versus δ18O values of groundwater, box plot of b) deep zone (n = 10) and c)
223
shallow groundwaters (n= 29). The standard deviation for rainwater composition is 1.66 for δ18O
224
and 13.3 for δ2H.
225
The majority of the shallow groundwater falls in a separate cluster (group c) with enriched
226
isotopic composition (δ18O: -7.5 to -4.6‰) except a few samples influenced by canal recharge.
227
The isotope data shows a wide distribution compared to deep zone with an average value of the
228
δ18O: -6.7 ± 1.1‰ as shown by the histogram (Fig. 4c). This suggests the shallow groundwater
229
receives water from less depleted sources such as irrigation return flow or/and local precipitation.
230
The recharge for shallow groundwaters seems to differ from that of deep groundwater. The best
231
fit line to isotopic data of the shallow groundwater (excluding those impacted by canals) shows a
232
slope of 5.3 indicating evaporative enrichment. The average value of shallow groundwaters is
233
δ18O: -6.7‰ which is closely resembles precipitation water with δ18O: -6.98 ‰ indicating the
234
dominant recharge to shallow zone is low elevation precipitation while the deep zone doesn’t
235
receive local rainwater towards recharge. The average value for precipitation in Bist Doab region
236
was found to be δ18O: -6.5 ‰ (Rao et al., 2017), which is similar to our findings. Precipitational
237
recharge to shallow aquifers is also reported in other parts of Punjab by researchers (Rai et al.,
238
2014; Rao et al., 2017).
11
239
Fig.5 Vertical distribution of δ18O in groundwater
240
The subsurface cross section depicted in Fig. 1b corroborates the findings that vertical recharge
241
to deep zone is limited by the presence of thick clay lenses between shallow and deep aquifers.
242
This observation is important as it confirms that the shallow groundwater receive a major portion
243
of its recharge from local precipitation while other sources contribute to a lesser extent.
244
Vertical distribution of stable isotope variation (δ18O) in groundwaters is shown in Fig. 5. Three
245
major sources of recharge can be envisaged from the figure. Groundwater samples falling in the
246
western parts are influenced by canal recharge. A similar pattern is seen in Fig. 4 (group (a)
247
samples). Another group represents shallow groundwater, which have major recharge
248
contribution from local precipitation as well as irrigation return flow. These samples are
249
relatively enriched in stable isotopic composition. A similar behaviour is observed for group (c)
250
samples in Fig. 4. The moderate enrichment (-8.7 to -8.0 ‰ for δ18O) in groundwater can be
251
attributed to recharge by local precipitation while enriched isotopic composition (-8.5 to -5.1 ‰
252
for δ18O) could be due to recharge by evaporated sources such as wetlands, irrigation return flow
253
and industrial wastes. Another major source of recharge is found to be precipitation occurring at
254
higher elevations, i.e. distant recharge. Deep groundwater samples typically show a depleted
255
isotopic composition compared to the shallow groundwater which indicates that the source of
256
recharge could be from elevated parts of eastern reaches. The isotopic variation in deep
257
groundwater is also narrow (-9.6 to -8.5 ‰ for δ18O), indicating that the groundwater is not
258
significantly influenced by the multiple sources like irrigation return flows, canal, wetland and
259
rainfall. Similar observation is noted in Fig. 4 (for group (b) samples). Since the deep
260
groundwater data do not show a similar distribution in the isotope data as shown in the case of
261
shallow groundwater, it can be stated that the possibility of groundwater interconnection between
262
shallow and deep zones is less likely at most of the studied locations.
263
3.2 Environmental tritium
264
Tritium is produced in the atmosphere by cosmic ray neutron interaction with nitrogen and
265
becomes a part of the water molecule in stratosphere. Tritium is also released into the
266
atmosphere due to various anthropogenic reasons. Anthropogenic tritium was released into the
267
atmosphere mostly during the testing of nuclear weapons that started in 1951 and continued till 12
268
the 1963. Two basic assumptions are made for dating groundwater using tritium that: 1) 3H-free
269
(i.e., below normal detection limits) groundwater contains only precipitation water from the "pre-
270
bomb period" and is, thus, >50 yr old; and 2) 3H-rich (up to 100 TU) water contains much
271
precipitation water from the 1960s (Dieter, 1993). The use of 3H dating of groundwaters has
272
been based primarily on models of the long-term trend of 3H content in hydrological systems
273
(Moser and Rauert, 1980).
274
Environmental tritium values in the study range from 4.5 to 9.8 TU with a mean of 6.5 TU. A
275
histogram of tritium distribution in groundwater samples is shown in Fig. 6. This range coincides
276
with that of other inland study sites, viz., Hyderabad, Telangana and Bist Doab, Punjab
277
(Unnikrishnan et al. 2013; Rao et al., 2017) in northeast part of Punjab. The tritium values also
278
suggest that groundwater is of modern origin and less than 50 years old.
279
Fig. 6 Histogram of environmental tritium data of groundwater
280
The vertical distribution of tritium concentration in groundwater of this region is shown in Fig. 7.
281
The data show that groundwater show wide variation in tritium content in shallow zone,
282
reflecting both fast and slow recharge to shallow groundwater. The subsurface lithology of the
283
area suggests presence of clay zones of variable thickness at many places (CGWB, 2013),
284
therefore it is possible that vertical percolation of rainwater or irrigation return flow is not
285
uniform throughout entire study area. Presence of thick clay zones retard the vertical percolation
286
leading to low tritium values while sandy zone allows easy vertical percolation leading to higher 13
287
tritium concentrations of groundwater. In the case of deep groundwater, tritium content is
288
relatively low (1 - 5 TU). Tritium levels as low as < 2 TU were noticed in deep zones of
289
northeast Punjab (Rao et al., 2107). Low tritium in deep zone groundwater again indicates that
290
the groundwater is older and recharged by single source compared to shallow zone where
291
multiple sources contribute to recharge. A few deep aquifer samples containing 3H close to 5 TU
292
indicate groundwater receiving vertical percolation due to poor well construction or due to
293
absence of clay zone at those sites. In general, for deep zones, regional groundwater flows
294
originating from higher elevations such as mountain fronts acts as recharge source. However
295
groundwater in both the zones is modern and represents active hydrological condition. There is
296
no component of recharge from “bomb- era” in these groundwater samples. But, the groundwater
297
residence time in shallow zone is lesser compared to deep zone. From stable isotope and
298
environmental tritium trends it can be understood that significant inter-connection between
299
shallow and deep zone groundwater is not observed.
300 301 302
Fig. 7 Vertical distribution of 3H in groundwater, dotted arrow is eye-guide to data points 3.3 Conceptualisation of groundwater flow 14
303
Major aquifer system of the study area is more complex, comprising different recharge sources,
304
the inference from the topographic elevation map shows that the slope is trending towards
305
southwest. Hence, the groundwater flow is also believed to follow the slope of the region, i.e. NE
306
to SW. The formations of this regions are typically made up of alternating layers of sand and
307
clay. Presence of kankar and sandy clay with varied thickness both laterally and vertically render
308
this geometry of the aquifers highly variable. Application of environmental tracers such as stable
309
isotopes (2H and 18O) provide a regional picture of groundwater recharge characteristics while
310
radio isotope tritium provides the insights into the dynamics of groundwater. From the
311
environmental isotope trends a conceptual groundwater flow model relating the recharge sources
312
and dynamics in shallow and deep groundwater is prepared and depicted in Fig. 8. The major
313
recharge processes and sources area as follows:
314
I.
Primary source is the rainwater recharge process. Most of the recharge to shallow and
315
deep zones happen from rainwater, whether direct infiltration or through regional flows.
316
The groundwater transit times are shorter in shallow zone compared to deep zone.
317
II.
Secondary source consists of evaporated surface waters such as irrigation return flows
318
and other surface water bodies. The canal influence is seen at selected locations. These
319
sources mainly act on the shallow zone.
320
Fig. 8 Systematic diagram showing the groundwater origin and recharge processes.
321 15
322
III.
On a broader scale the aquifer – aquifer interconnection is not observed, which is also
323
evident form the subsurface geology of this region. Clay layers of thickness up to 30 m
324
are present in this region rendering the deep aquifer semi-confined to confined in nature.
325
However, a few places belonging to deep zone have contribution from shallow zone. A
326
targeted research is necessary to understand the aquifer interconnections in this region
327
and to identify potential contamination threats to the deeper aquifer to both man-made
328
and natural perturbations.
329
4. Summary
330
Identifying groundwater recharge sources and their dynamics is critical to water authorities and
331
municipalities for implementing better management practices for preservation and enhancement
332
of aquifer recharge. From the environmental isotope data of the groundwater samples collected
333
from both shallow and deep zones of southwest Punjab, it can be inferred that shallow zones
334
have direct contact with the rainfall recharge as well as irrigation return flow whereas the deeper
335
zones are fed predominantly by regional groundwater flows originating from higher elevations.
336
Canal contribution is observed in the shallow groundwater in some places. In general, shallow
337
and deep aquifers are not interconnected. However, tritium data suggest that some level of inter-
338
connection is present in a few locations, and therefore deep groundwater in these locations may
339
be more susceptible to anthropogenic impacts. The results of our study show that monitoring
340
water isotopes and/or tritium concentrations in the deep aquifer for increased variability could be
341
a powerful tool for early identification of changes in water source for the deep aquifer.
342
Acknowledgement
343
Authors sincerely acknowledge the constant support and encouragement by Shri K.S.S.Sarma,
344
Head, Isotope and Radiation Application Division and Dr. B.S. Tomar, Director, Radiochemistry
345
and Isotope Group, Bhabha Atomic Research Centre, Mumbai. The authors would like to
346
acknowledge the BRNS, DAE for providing the financial support (letter no. 35/14/11/2014-
347
BRNS-193). Authors thank the review for his valuable suggestions.
348
16
349
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465 466
Highlights:
467
The environmental isotopic study of southwest Punjab groundwaters is presented.
468
Sources contributing to shallow and deep zone groundwater recharge are delineated.
469
Impact of irrigation return flow and canal water is limited to shallower depths at few
470
places.
471
Hydraulic interconnection between shallow and deep zones is remote.
472
A conceptual groundwater flow model for southwest Punjab is presented.
21