J. Environ. Eng. Manage., 19(4), 185-191 (2009)
185
USING STABLE ISOTOPES FOR ASSESSING THE HYDROLOGIC
CHARACTERISTICS AND SOURCES OF GROUNDWATER RECHARGE
Hsin-Fu Yeh,1 Cheng-Haw Lee,1,* Kuo-Chin Hsu,1 Po-Hsun Chang1 and Chung-Ho Wang2
1
Department of Resources Engineering
National Cheng Kung University
Tainan 701, Taiwan
2
Institute of Earth Sciences
Academia Sinica
Taipei 115, Taiwan
Key Words: Oxygen and hydrogen isotopes, groundwater, recharge
ABSTRACT
The stable isotopes of oxygen and hydrogen were used as tracers to determine the seasonal
contributions of precipitation and river water to the groundwater in Chih-Pen and Jin-Lun Creek
basins. The correlations of stable water isotopes were analyzed to assess the sources of groundwater
recharge and hydrological variations. Using mass balance analysis for the oxygen and hydrogen
isotopic compositions, the groundwater recharge percentages of every recharge source were
evaluated. Results show that for the Chih-Pen Creek basin, 79% of the groundwater is recharged
from river water of the mountain watershed and 21% is from the rainfall on the basin. For the JinLun basin, about 78% of the groundwater is recharged from the river water of the mountain
watershed and 22% is from the rainfall on the basin. The isotopic characteristics of precipitation
indicate that wet seasons are relatively depleted compared to dry seasons, due to the amount of the
precipitation. The isotopic ranges of the river water are relatively smaller and depleted than those of
precipitation. This indicates that the river water mainly came from upstream precipitation. The
oxygen and hydrogen isotopic compositions of the groundwater show a significant shift in the
meteoric-line of the Taitung area. It was estimated that the groundwater is a mixture of river water and
precipitation, and that the effect of river water recharge is greater than the infiltration of precipitation.
INTRODUCTION
Environmental isotopes are routinely used in
geochemical and hydro-geological investigations.
Oxygen and hydrogen isotopes of water are widely
used as tracers to understand hydro-geological processes such as precipitation, groundwater recharge,
groundwater-surface water interactions, and basin hydrology [1-3]. The combination of stable isotopes of
oxygen and hydrogen is constant in normaltemperature groundwater. The stable isotopes of oxygen and hydrogen can be used as conservative
groundwater tracers because values of isotopes remain
constant as long as there are no phase changes or fractionation along the flow-path. The stable isotopes of
oxygen and hydrogen maintain almost the same combination as it of the meteoric water, which means it
records the status of the initial formed meteoric water,
and is a permanent natural tracer [2]. Accordingly, af*Corresponding author
Email: [email protected]
ter collecting the information of meteoric water and
stable isotopes of oxygen and hydrogen in groundwater in a database, and analyzing the hydro-geological
structure and the groundwater flow in the target area,
we can define the status of mixed groundwater recharge areas and different recharge water sources.
Moreover, studying stable isotopes of oxygen and hydrogen can help us identify different groundwater recharge zones [4]. In Taiwan, stable isotope analyses of
water mass were performed in the early 1980s [5-8].
These studies provided fingerprints of the hydrological cycle, such as the identification of groundwater recharge areas and seawater intrusion signals. This information is used for groundwater resource management and numerical modeling [9,10].
The government of Taiwan has invested significant labor and financial resources to survey five main
groundwater areas (Cho-Shui River alluvial fan, Pingtung Plain, Chia-Nan Plain, Lan-Yang Plain, and
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J. Environ. Eng. Manage., 19(4), 185-191 (2009)
Hsin-Chu and Miaoli Region) to construct a database
of hydrogeology and groundwater. Additionally,
groundwater monitoring stations have been established, but only on the western plain of Taiwan. Very
limited information, such as precipitation, river flux,
hydro-geological properties, groundwater consumption, and groundwater recharge, is available for the
eastern mountain area of Taiwan. The assessment and
planning of groundwater resources is particularly difficult for the mountain area of Taiwan, which is the
main source of groundwater recharge. Groundwater
resources in the eastern mountainous region of Taiwan
are becoming increasingly insufficient due to recent
economic development. The declining groundwater
level in the mountain area has raised the alarm of decreasing groundwater resources [11]. The purpose of
this study is to use oxygen and hydrogen isotopes as
natural tracers to identify the possible sources of
groundwater in the Chih-Pen and Jin-Lun Creek basins. The results provide useful information about hydrological processes such as the interaction of precipitation, river water, and groundwater.
STUDY AREA
The study areas, the Chih-Pen and Jin-Lun Creek
basins, are located in the southeast of Taiwan. The areas of the Chih-Pen and Jin-Lun Creek basins are
about 198 and 123 km2, respectively. The length of
the river for Chih-Pen Creek is about 39.3 km, lying
between longitudes 12°05´-121°50´ E and latitudes
22°35´-22°45´ N. The length of the river for Jin-Lun
Creek is about 26.4 km, between longitudes 120°43´120°57´ E and latitudes 22°28´-22°34´ N. Figure 1
shows the geographical locations of both basins. The
research region belongs to the tropical marine climate,
with a mean annual temperature of 24.5 °C and an average annual precipitation of 1,800 mm (1971-2006).
During the summer, southwest monsoons occur and
typhoons bring heavy rainfall. The northeast monsoon
brings water vapor from the Pacific Ocean during the
winter. Because water vapor is blocked by the Central
Mountains of Taiwan, there is little rainfall in the winter. Therefore, the wet and dry seasons are very distinct in this region. The wet season is from May to
October, and the dry season is from November to
April. Evapotranspiration is approximately 750 mm y-1.
The maximum streamflow on annual hydrographs
occurs during August and September, and the minimum flow occurs during January and February [11].
SAMPLING AND ANALYTICAL METHOD
Precipitation, river water, and groundwater samples were collected for oxygen and hydrogen isotopic
analyses between January and December 2007. Sampling was carried out during both wet and dry periods.
The sampling locations are shown in Fig. 1. River wa-
Jin-Lun Creek
Fig. 1. The location of the study region. Sampling sites
of precipitation (circles), river water (squares),
and groundwater (triangles) samples.
ter and groundwater were sampled once per month
during the study period, and precipitation was collected on rainy days. Stable oxygen isotopic compositions were analyzed using the CO2-H2O equilibration
method [12]. The equilibrated CO2 gas was measured
by a VG SIRA 10 isotope ratio mass spectrometer.
The hydrogen isotopic compositions were determined
on a VG MM602D isotope ratio mass spectrometer after water was reduced to H2 using zinc shots made by
Biogeochemical Laboratory of Indiana University [13].
All isotopic ratio results are reported as the δ-notation
(‰) relative to the international VSMOW (Vienna
Standard Mean Ocean Water) standard and normalized on a scale on which the δ18O and δD of Standard
Light Antarctic Precipitation are -55.5 and -428‰, respectively. The precisions for δ18O and δD were 0.1
and 1.5‰, respectively.
RESULTS AND DISCUSSION
1. Mass-balance
Groundwater
of
Isotopic
Compositions
of
The meteoric 18O-D signal is important for understanding the groundwater recharge. The isotopic
composition of groundwater equals to the average
weighted values of recharge sources such as annual
composition of precipitation and river water. Therefore, deviations of the groundwater isotopic ratio from
that of precipitation are expected. The transfer function from precipitation to groundwater must be understood for groundwater provenance studies. The transfer function also provides basic information about the
mechanisms of recharge [2]. In this study, geothermal
water was collected in the Jin-Lun Creek basin because there is no groundwater well. Studies of geothermal areas throughout the world show that geothermal water is usually enriched in 18O relative to the
source waters [2,14] while generally maintaining the
Yeh et al.: Isotopes for Assessing Groundwater Source
original δD ratios. This ‘‘18O-shift’’ is caused by the
high-temperature exchange of 18O between the host
rock and groundwater, enriching the groundwater in
18
O [15]. Therefore, the stable isotopic compositions
of groundwater samples were collected from groundwater and the geothermal well. The oxygen and hydrogen isotopes were used to analyze the isotopic
characteristics in the Chih-Pen and Jin-Lun Creek basins, respectively.
The mean values of oxygen isotopic compositions of groundwater for the Chih-Pen Creek were 9.6‰ (ranging from -9.1 to -10.5‰). The mean values
of hydrogen isotopic compositions of geothermal water for Jin-Lun Creek were -60.9‰ (ranging from 48.2 to -68.4‰). Generally, the stable isotopic compositions of precipitation decrease with increasing
rainfall amount (the so-called amount effect); the
amount effect is pronounced in tropical regions [16].
The stable isotopic compositions of precipitation
weighted average values were obtained using the precipitation of dry and wet seasons. The mean values of
oxygen isotopic compositions of dry and wet seasons
for the Chih-Pen Creek were -4.1 and -6.5‰, respectively. The mean values of hydrogen isotopic compositions of dry and wet seasons for the Jin-Lun Creek
were 4.9 and 22.5‰, respectively. The ratio of precipitation for dry and wet seasons was 0.18:0.82 from
1981 to 2006 (according to the Central Weather Bureau). The weighted average δ18O of precipitation was
-6.1‰ in the Chih-Pen Creek basin. The weighted average δD of precipitation was -17.6‰ in the Jin-Lun
Creek basin.
In any region with even minor relief, topographic precipitation will occur as a vapor mass rises
over the landscape and cools adiabatically (by expansion), driving rainfall. At higher altitudes where the
average temperature is low, precipitation will be isotopically depleted. The depletion of 18O varies between about -0.15 and -0.5‰ per 100 m rise in altitude, with a corresponding decrease of about -1 to 4‰ for δD. This altitude effect (also called the elevation effect) is useful in hydro-geological studies, as it
distinguishes groundwater recharged at high altitudes
from those recharged at low altitude [17,18].
In this study, 42 samples of river water were collected in the wet season and 65 samples of river water
were collected in the dry season. The regression results for δD and δ18O of river water with respect to the
altitude in dry and wet seasons are shown in Figs. 2a2d. δD and δ18O become more negative with increasing altitude. Decrease rates of -0.2 ‰ in δ18O and -2‰
in δD per 100 m in altitude were obtained from the
slope regression equation. These results are similar to
those of other studies [19]. The resulting linear regressions for dry and wet seasons are described by the following equations:
δD = -0.029H – 61.72 (wet season)
(1)
187
Fig. 2. Regression lines (a) δD and altitude for the wet
season; (b) δD and altitude for the dry season; (c)
δ18O and altitude for the wet season; (d) δ18O and
altitude for the dry season.
δD = -0.012H – 64.11 (dry season)
(2)
18
(3)
18
(4)
δ O = -0.0032H – 9.21 (wet season)
δ O = -0.0016H – 9.76 (dry season)
where H is the elevation (m). The altitudes of 471 and
256 m (above sea level, asl) were the highest sampling
locations of river water in the Chih-Pen and the JinLun Creek basin, respectively. The area above the
highest sampling location of river water is considered
as the recharge area of the mountain watershed. The
values of oxygen isotopic compositions of river water
for dry and wet seasons in 471 m asl of the Chih-Pen
Creek were -10.4 and -10.5‰, respectively. The ratio
of stream flow for dry and wet seasons was 0.14:0.86
from 1980 to 2005 (according to the Water Resources
J. Environ. Eng. Manage., 19(4), 185-191 (2009)
188
Agency). The river water weighted average value for
δ18O was -10.5‰ in 471 m asl of the Chih-Pen Creek.
The values of hydrogen isotopic compositions of river
water for dry and wet seasons in 256 m asl of the JinLun Creek were -69.3 and -73.9‰, respectively. The
ratio of stream flow for dry and wet seasons was
0.18:0.82 from 2002 to 2004 (according to the Water
Resources Agency). The river water weighted average
value for δD was -73.1‰ in 256 m asl of the Jin-Lun
Creek.
Groundwater of the basin is recharged from rain
that falls on the basin and river water drained from
mountain watersheds. In basin water budget studies, it
is important to assess the proportion of precipitation
and river water of the mountain that actually recharges
groundwater. The stable isotopic composition of
groundwater is determined by oxygen and hydrogen
isotopic compositions and recharge percentages of
concerned sources. Using mass balance analysis for
the oxygen and hydrogen isotopic compositions, the
groundwater recharge percentages of every recharge
source can be evaluated. In this study, mixing between
two distinct recharge sources can be quantified by a
simple linear algebra equation:
C(VA + VB ) = AVA + BVB
VA
VB
C=A
+B
= A(1- X) + BX
VA + VB
VA + VB
(5)
where A is the precipitation stable isotope value of the
basin; B is the river water stable isotope value of the
mountain watershed; C is the groundwater stable isotope value of the basin; VA is the amount of precipitation; VB is the amount of river water; X is the recharge
proportion of river water; and (1-X) is the recharge
proportion of precipitation.
Based on stable isotopic characteristics, the results show that 79% of the groundwater in the ChihPen Creek basin is derived from river water of the
mountain watershed and 21% is from the rain that
falls on the basin. About 78% of the groundwater is
recharged from river water of the mountain watershed
and 22% is from the rain that falls on the basin in the
Jin-Lun Creek basin. This indicates that the groundwater of the two basins is mainly recharged from river
water of the mountain watershed, primarily due to the
abundant precipitation in the mountain area.
2. Isotopic Compositions of Precipitation
In the Chih-Pen Creek basin, the mean δ18O and
δD for the wet season were -6.5 and -42.7‰, respectively, and those for the dry season were -4.1 and 18.3‰, respectively. In the Jin-Lun Creek basin, the
mean δ18O and δD for the wet season were -4.0 and 22.5‰, respectively, and those for the dry season
were -1.6 and 4.9‰, respectively. More depleted isotopic compositions are found in the wet season than in
the dry season. Generally, the stable isotopic composi-
Fig. 3. A comparison of seasonal charges of δ18O, δD,
precipitation, evapotranspiration, and temperature
in the Chih-Pen Creek basin.
tions of precipitation decrease with decreasing temperature and with increasing rainfall amount [16].
This study shows that the stable isotope compositions
of precipitation decrease with increasing rainfall
amount and air temperature, because the amount effect of precipitation is pronounced. Figure 3 clearly
shows amount effect, while the temperature effect is
not significant. Yurtsever and Gat [16] pointed out
that the temperature effect is generally pronounced in
high-latitude continental regions, whereas the amount
effect is pronounced in tropical regions. The hydrogen
and oxygen isotopic compositions of precipitation collected from eastern Taiwan are shown in Fig. 4. The
regression line that represents the local meteoric water
line (LMWL) is:
δD = 8.4 δ18O+14.2
(6)
This LMWL is similar to the global meteoric water
line defined by Craig [15]. In the process of oceanic
water evaporation becoming inland rainfall, a sequence of isotope fractionation causes the variation of
the composition in isotopes of oxygen and hydrogen
in continental meteoric water. Since the fractionation
is based on the equilibrium processes of the isotopes
of evaporation and condensation, there is a specific
rule that governs the distributions of isotopes of oxygen and hydrogen in rainfall. Figure 5a shows the isotopes data of precipitation distributed along the
LMWL, indicating that the stable isotopes of precipitation do not have significant evaporation.
3. Isotopic Compositions of River Water
River water has three major components based
Yeh et al.: Isotopes for Assessing Groundwater Source
189
40
20
LMWL: δD = 8.4 δ18O + 14.2
0
-20
-40
-60
-80
-100
-120
-140
-160
-180
-200
-25
-20
-15
-10
-5
0
5
δ18O (‰)
Fig. 4. The local meteoric water line (LWML) is
established as δD = 8.4 δ18O + 14.2 using data of
local precipitation.
on the speed of appearance after rainfall: surface runoff, interflow, and groundwater (base-flow). The difference between arrival times for interflow and surface runoff is in the order of hours, so they are both
from recent storms and have similar isotopic compositions. Therefore, from the point of view of isotopic
composition, river water can be considered as being
composed of groundwater and runoff, including surface runoff and interflow [20]. In the Chih-Pen Creek
basin, the mean δ18O and δD for the wet season were 9.5 and -66.0‰, respectively, and those for the dry
season were -9.9 and -65.2‰, respectively. In the JinLun Creek basin, the mean δ18O and δD for the wet
season were -9.2 and -61.9‰, respectively, and those
for the dry season were -9.3 and -62.5‰, respectively.
Figure 5b shows a plot of δD versus δ18O for river waters collected in both basins, showing a linear array
close to the LMWL. The isotopic ranges of the river
waters are relatively smaller and more depleted than
those of precipitation. This indicates that the river water mainly comes from upstream precipitation. The
seasonal variation in both basins is not pronounced.
This can be explained by fact that the Chih-Pen and
Jin-Lun rivers are mainly a mixture of interflow and
precipitation.
4. Isotopic Compositions of Groundwater
The mean values of the wet season of the ChihPen Creek basin were -9.5 and -65.7‰ for δ18O and
δD, respectively. In contrast, the mean values for the
dry season were -9.6 and -64.9‰ for δ18O and δD, respectively. In the Jin-Lun Creek basin, the mean δ18O
and δD for the wet season were -8.1 and -64.5‰, respectively, and those for the dry season were -7.8 and
Fig. 5. Plot of δD and δ18O for several types of samples
collected for study (a) precipitation samples; (b)
river water samples; (c) groundwater and
geothermal water samples.
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J. Environ. Eng. Manage., 19(4), 185-191 (2009)
-58.7‰, respectively. In Fig. 5c, δ18O and δD of the
groundwater of the Chih-Pen Creek basin are both
closely distributed along the LMWL line, suggesting
that the watersheds have little evaporation. It was estimated that the groundwater is a mixture of river water and precipitation, and that the effect of the river
water recharge is greater than that of the infiltration.
In addition, the geothermal water was obviously enriched in 18O relative to the source waters while maintaining the original δD ratio in the Jin-Lun Creek basin.
2.
3.
4.
CONCLUSIONS
The present study examined the stable isotopic
composition of precipitation, river water, and
groundwater in the Chih-Pen and Jin-Lun Creek basins. The results show that 79% of the groundwater in
the Chih-Pen Creek basin is derived from river water
of the mountain area and 21% is from the meteoric
water in the plain area. About 78% of the groundwater
is recharged from the river water of the mountain area
and 22% is from the meteoric water in the plain area
in Jin-Lun Creek basin. This indicates that the
groundwater of the two basins is mainly recharged
from river water of mountain watersheds. The mountain area should be protected to avoid the deterioration
of water quantity and quality. This study shows that
the stable isotopes compositions of precipitation decrease with increasing rainfall amount and air temperature because the amount effect of precipitation is
pronounced. The amount effect is clear but there was
no temperature effect. The isotopic ranges of the river
waters are relatively smaller and more depleted than
those of precipitation. This indicates that the river water mainly came from upstream precipitation. The altitude gradients in the river water were estimated to be
2‰/100 m for δD and 0.2‰/100 m for δ18O. The seasonal variation in both basins is not pronounced,
which can be explained by the fact that the river basin
discharge is mainly a mixture of interflow and precipitation.
5.
6.
7.
8.
9.
10.
ACKNOWLEDGMENTS
The authors would like to thank the Water Resources Agency of the Ministry of Economic Affairs
of the Republic of China, Taiwan for financially supporting this research under Contract No.
MOEAWRA0950416. We would also like to thank
the 10th-branch of the Taiwan Water Corporation, BinMao Junior High School, and Bin-Mao Elementary
School for collecting samples through the study period.
11.
REFERENCES
13.
1. Gat, J.R., Oxygen and hydrogen isotopes in the
12.
hydrologic cycle. Annu. Rev. Earth Planet. Sci.,
24, 225-262 (1996).
Clark, I.D. and P. Fritz, Environmental Isotopes in
Hydrology. Lewis Publishers, New York (1997).
Gibson, J.J., T.W.D. Edwards, S.J. Birks, N.A. St
Amour, W.M. Buhay, P. McEachern, B.B. Wolfe
and D.L. Peters, Progress in isotope tracer
hydrology in Canada. Hydrol. Process., 19(1),
303-327 (2005).
Payne, B.R. and Y. Yurtsever, Environmental
Isotopes as a Hydrogeological Tool in Nicaragua.
Isotope Techniques in Groundwater Hydrology.
International Atomic Energy Agency, Vienna,
Austria, pp.193-201 (1974).
Shieh, Y.N., F.P. Cherng and T.C. Hoering,
Oxygen and hydrogen isotope studies of meteoric
and thermal waters in Taiwan. Memo. Geol. Soc.
China, 5, 127-140 (1983).
Peng, T.R., Environmental Isotopic Study (δ13C,
δD, δ18O, 14C, T) on Meteoric Water and
Groundwaters in I-Lan Area. Ph.D. Dissertation,
Department of Geosciences, National Taiwan
University, Taipei, Taiwan (1995) (in Chinese).
Wang, C.H., C.H. Kuo, T.R. Peng, W.F. Chen,
T.K. Liu, C.J. Chiang, W.C. Liu and J.J. Hung,
Isotope characteristics of Taiwan groundwaters.
Western Pacific Earth Sci., 1(4), 415-428 (2001).
Wang, C.H. and T.R. Peng, Hydrogen and oxygen
isotopic compositions of Taipei precipitation:
1990-1998. Western Pacific Earth Sci., 11(4),
429-442 (2001).
Peng, T.R., C.H. Wang, C.C. Huang, W.C. Chen,
L.Y. Fei and T.C. Lai, Groundwater recharge and
salinization in south Chianan Plain: Hydrogen and
oxygen isotope evidences. J. Chin. Soil Water
Conserv., 33(2), 87-100 (2002).
Kuo, C.H. and C.H. Wang, Implication of
seawater intrusion on the delineation of the
hydrogeologic framework of the shallow Pingtung
Plain aquifer. Western Pacific Earth Sci., 1(4),
459-471 (2001).
Yeh, H.F., P.H. Chang, K.C. Hsu and C.H. Lee,
Assessment of groundwater recharge in Chih-Pen
and Jin-Lun creek basins. Proceedings of 3rd
Conference on Resources Engineering in Taiwan,
pp. 1-15 (2007).
Epstein, S. and T. Mayeda, Variation of 18O
content of waters from natural sources. Geochim.
Cosmochim. Acta, 4(5), 213-224 (1953).
Coleman, M.L., T.J. Shepherd, J.J. Durham, J.E.
Rouse and G.R. Moore, Reduction of water with
zinc for hydrogen isotope analysis. Anal. Chem.,
Yeh et al.: Isotopes for Assessing Groundwater Source
14.
15.
16.
17.
18.
54(6), 993-995 (1982).
Ellis, A.J. and W.A.J. Mahon, Chemistry and
Geothermal Systems. Academic Press, New York
(1977).
Craig, H., Isotopic variations in meteoric waters.
Science, 133(3465), 1702-1703 (1961).
Yurtesever, Y. and J.R. Gat, Atmospheric waters.
In: J.R. Gat, R. Gonfiantini (Eds.). Stable Isotope
Hydrology: Deuterium and Oxygen-18 in the
Water Cycle. International Atomic Energy
Agency Technical Report Series No. 210, pp.103142 (1981).
Wright, W.E., δD and δ18O in Mixed Conifer
Systems in the U.S. Southwest: The Potential of
δ18O in Pinus Ponderosa Tree Rings as a Natural
Environmental Recorder. Ph.D. Dissertation,
Department of Geosciences, University of Arizona,
Tucson, AZ (2001).
Blasch, K.W. and J.R. Bryson, Distinguishing
191
sources of ground water recharge by using δ2H
and δ18O. Ground Water, 45(3), 294-308 (2007).
19. Liu, Y., N. Fan, S. An, X. Bai, F. Liu, Z. Xu, Z.
Wang, and S. Liu, Characteristics of water
isotopes and hydrograph separation during the wet
season in the Heishui river, China. J. Hydrol.,
353(4), 314-321 (2008).
20. Lu, H.Y., T.R. Peng, T.K. Liu, C.H. Wang and
C.C. Huang, Study of stable isotopes for highly
deformed aquifers in the Hsinchu-Miaoli area,
Taiwan. Environ. Geol., 50(7), 885-898 (2006).
Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: January 5, 2009
Revision Received: March 10, 2009
and Accepted: March 22, 2009