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 1Department of Resources Engineering

National Cheng Kung University Tainan 701, Taiwan

2Institute 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 Jin-Lun 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.

*Corresponding author Email: leech@mail.ncku.edu.tw

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 proc-esses such as precipitation, groundwater recharge, groundwater-surface water interactions, and basin hy-drology [1-3]. The combination of stable isotopes of oxygen and hydrogen is constant in normal-temperature groundwater. The stable isotopes of oxy-gen and hydrogen can be used as conservative groundwater tracers because values of isotopes remain constant as long as there are no phase changes or frac-tionation along the flow-path. The stable isotopes of oxygen and hydrogen maintain almost the same com-bination 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-

ter collecting the information of meteoric water and stable isotopes of oxygen and hydrogen in groundwa-ter 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 re-charge areas and different recharge water sources. Moreover, studying stable isotopes of oxygen and hy-drogen can help us identify different groundwater re-charge zones [4]. In Taiwan, stable isotope analyses of water mass were performed in the early 1980s [5-8]. These studies provided fingerprints of the hydrologi-cal cycle, such as the identification of groundwater re-charge areas and seawater intrusion signals. This in-formation is used for groundwater resource manage-ment and numerical modeling [9,10].

The government of Taiwan has invested signifi-cant labor and financial resources to survey five main groundwater areas (Cho-Shui River alluvial fan, Ping-tung Plain, Chia-Nan Plain, Lan-Yang Plain, and

186 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 estab-lished, but only on the western plain of Taiwan. Very limited information, such as precipitation, river flux, hydro-geological properties, groundwater consump-tion, and groundwater recharge, is available for the eastern mountain area of Taiwan. The assessment and planning of groundwater resources is particularly dif-ficult 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 de-creasing 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 ba-sins. The results provide useful information about hy-drological processes such as the interaction of precipi-tation, 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 ar-eas 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 av-erage 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 win-ter. Therefore, the wet and dry seasons are very dis-tinct 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 mini-mum flow occurs during January and February [11].

SAMPLING AND ANALYTICAL METHOD

Precipitation, river water, and groundwater sam-

ples were collected for oxygen and hydrogen isotopic analyses between January and December 2007. Sam-pling was carried out during both wet and dry periods. The sampling locations are shown in Fig. 1. River wa-

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 col-lected on rainy days. Stable oxygen isotopic composi-tions 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 af-ter 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 normal-ized on a scale on which the δ18O and δD of Standard Light Antarctic Precipitation are -55.5 and -428‰, re-spectively. The precisions for δ18O and δD were 0.1 and 1.5‰, respectively.

RESULTS AND DISCUSSION

1. Mass-balance of Isotopic Compositions of

Groundwater The meteoric 18O-D signal is important for un-

derstanding 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. There-fore, deviations of the groundwater isotopic ratio from that of precipitation are expected. The transfer func-tion from precipitation to groundwater must be under-stood for groundwater provenance studies. The trans-fer function also provides basic information about the mechanisms of recharge [2]. In this study, geothermal water was collected in the Jin-Lun Creek basin be-cause there is no groundwater well. Studies of geo-thermal areas throughout the world show that geo-thermal water is usually enriched in 18O relative to the source waters [2,14] while generally maintaining the

Jin-Lun Creek

Yeh et al.: Isotopes for Assessing Groundwater Source 187

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 18O [15]. Therefore, the stable isotopic compositions of groundwater samples were collected from ground-water and the geothermal well. The oxygen and hy-drogen isotopes were used to analyze the isotopic characteristics in the Chih-Pen and Jin-Lun Creek ba-sins, respectively.

The mean values of oxygen isotopic composi-tions 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 wa-ter for Jin-Lun Creek were -60.9‰ (ranging from -48.2 to -68.4‰). Generally, the stable isotopic com-positions 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 pre-cipitation 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‰, respec-tively. The mean values of hydrogen isotopic compo-sitions of dry and wet seasons for the Jin-Lun Creek were 4.9 and 22.5‰, respectively. The ratio of pre-cipitation for dry and wet seasons was 0.18:0.82 from 1981 to 2006 (according to the Central Weather Bu-reau). The weighted average δ18O of precipitation was -6.1‰ in the Chih-Pen Creek basin. The weighted av-erage δD of precipitation was -17.6‰ in the Jin-Lun Creek basin.

In any region with even minor relief, topog-raphic precipitation will occur as a vapor mass rises over the landscape and cools adiabatically (by expan-sion), driving rainfall. At higher altitudes where the average temperature is low, precipitation will be iso-topically depleted. The depletion of 18O varies be-tween about -0.15 and -0.5‰ per 100 m rise in alti-tude, with a corresponding decrease of about -1 to -4‰ for δD. This altitude effect (also called the eleva-tion 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 col-lected in the wet season and 65 samples of river water were collected in the dry season. The regression re-sults for δD and δ18O of river water with respect to the altitude in dry and wet seasons are shown in Figs. 2a-2d. δD and δ18O become more negative with increas-ing 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 regres-sions for dry and wet seasons are described by the fol-lowing equations:

δD = -0.029H – 61.72 (wet season) (1)

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)

δ18O = -0.0032H – 9.21 (wet season) (3)

δ18O = -0.0016H – 9.76 (dry season) (4)

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 Jin-Lun 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

188 J. Environ. Eng. Manage., 19(4), 185-191 (2009)

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 Jin-Lun 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:

A B A B

A B

A B A B

C(V + V ) = AV + BV

V VC = A + B = A(1- X) + BX

V + V V + V

(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 iso-tope value of the basin; VA is the amount of precipita-tion; 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 re-sults show that 79% of the groundwater in the Chih-Pen 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 ground-water 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‰, respec-tively, 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 iso-topic 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 tem-perature 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 ef-fect 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 col-lected 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 se-quence 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 oxy-gen and hydrogen in rainfall. Figure 5a shows the iso-topes data of precipitation distributed along the LMWL, indicating that the stable isotopes of precipi-tation 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

-25 -20 -15 -10 -5 0 5

40

20

0

-20

-40

-60

-80

-100

-120

-140

-160

-180

-200

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 run-off, interflow, and groundwater (base-flow). The dif-ference between arrival times for interflow and sur-face runoff is in the order of hours, so they are both from recent storms and have similar isotopic composi-tions. Therefore, from the point of view of isotopic composition, river water can be considered as being composed of groundwater and runoff, including sur-face 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 Jin-Lun 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 wa-ters 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 wa-ter 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 Chih-

Pen 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, re-spectively. In the Jin-Lun Creek basin, the mean δ18O and δD for the wet season were -8.1 and -64.5‰, re-spectively, 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.

δ18O (‰)

LMWL: δD = 8.4 δ18O + 14.2

190 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 es-timated that the groundwater is a mixture of river wa-ter 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 en-riched in 18O relative to the source waters while main-taining the original δD ratio in the Jin-Lun Creek ba-sin.

CONCLUSIONS

The present study examined the stable isotopic

composition of precipitation, river water, and groundwater in the Chih-Pen and Jin-Lun Creek ba-sins. 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 moun-tain area should be protected to avoid the deterioration of water quantity and quality. This study shows that the stable isotopes compositions of precipitation de-crease with increasing rainfall amount and air tem-perature 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 wa-ter mainly came from upstream precipitation. The alti-tude gradients in the river water were estimated to be 2‰/100 m for δD and 0.2‰/100 m for δ18O. The sea-sonal 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 precipi-tation.

ACKNOWLEDGMENTS

The authors would like to thank the Water Re-

sources Agency of the Ministry of Economic Affairs of the Republic of China, Taiwan for financially sup-porting this research under Contract No. MOEAWRA0950416. We would also like to thank the 10th-branch of the Taiwan Water Corporation, Bin-Mao Junior High School, and Bin-Mao Elementary School for collecting samples through the study period.

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Discussions of this paper may appear in the discus-sion 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