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Using H- and O-isotopic data for estimating the relative contributions of rainy and dry season precipitation to groundwater: example from Cheju Island, Korea
Journal of Hydrology 222 (1999) 65–74
Using H- and O-isotopic data for estimating the relative contributions of rainy and dry season precipitation to groundwater: example from Cheju Island, Korea Kwang-Sik Lee a,*, D.B. Wenner b, Insung Lee c a
Korea Basic Science Institute, Isotope Research Group, Eoeun-dong 52, Yusung-ku, Taejeon 305-333, South Korea b Department of Geology, University of Georgia, Athens, GA 30602, USA c Department of Geological Science, Seoul National University, Seoul 151-742, South Korea Received 8 December 1998; received in revised form 26 May 1999; accepted 30 June 1999
Abstract A comparison of deuterium excess or d-values of precipitation and groundwater at Cheju Island, Korea, indicates that, unlike in many temperate climates, precipitation during the whole year contributes to groundwater recharge. This in turn suggests that evapotranspiration effects are minimal, consistent with the fact that the island contains highly permeable volcanic rocks overlain by thin soils. This hypothesis is contrary to current water budget models that ascribe a significant role to evapotranspiration processes. Three coastal springs, and several streams and ponds throughout the island have enriched d 18O and d D values that plot off the local meteoric water line and have relatively high Cl concentrations. This relationship is suggestive of seawater mixing for the coastal springs and evaporation for the streams and ponds. This database thus provides a useful means for evaluation of groundwater resources of the island. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Deuterium excess; Precipitation; Groundwater; Cheju Island; Korea
1. Introduction This investigation is an outgrowth of another study (Lee et al., in preparation) of the oxygen and hydrogen isotopic composition of precipitation at Cheju Island, Korea. This study revealed that summer d , 110‰ and winter precipitation d . 115‰ had distinct deuterium excess values, a pattern that could potentially provide a means for evaluating the relative importance of summer and winter recharge to groundwater. It is with this background that a study was initiated of selected surface and groundwaters. * Corresponding author. Fax: 182-42-865-3419. E-mail address: [email protected] (K.-S. Lee)
Cheju Island, located at the southern tip of the Korean Peninsula (Fig. 1(A)), outcrops over an aerial extent of approximately 1825 km 2, with a length of 74 km and a width of 32 km. It is predominantly composed of highly permeable volcanic rocks with thin soils. The island is gently sloping along its periphery and rises in the center to a height of 1950 m (Mt. Halla) (see Fig. 1(B)). Most habitation and development exist along the outer edge of the island. At the present time, groundwater supplies almost all of the drinking water needs of the island. Extensive groundwater development, initiated in the early 1970s, has produced a number of environmental problems whose severity has increased over time.
0022-1694/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-169 4(99)00099-2
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Fig. 1. Map of Northeast Asia showing the location of (A) Cheju Island and (B) the topography and sampling locations.
These problems include decline of groundwater levels, salinization of groundwater in coastal areas and degradation of groundwater quality (Hahn et al., 1997). Numerous wells on the island have been contaminated by seawater, sewage, animal wastes and chemical fertilizers. The present study focuses on measuring the D/H and 18O/ 16O compositions, along with the chloride concentration from selected samples, of various surface and groundwaters acquired from streams, ponds, springs and wells throughout the island. In this study, we have attempted to quantify the relative contribution of rainy and dry season precipitation to groundwater recharge and the extent of evaporation of surface water by comparing the isotopic composition of groundwater with precipitation data. Further, it was of interest to measure the altitude effect of precipitation from the isotopic composition of springs collected at different elevations. Finally, we were able to evaluate the cause of salinization process observed in some surface and groundwaters by using a combination of isotopic and chloride data.
To date, a limited number of groundwater samples have been analyzed for H- and O-isotopic compositions from Cheju Island. Davis et al. (1970) reported oxygen and hydrogen isotopic and tritium data and concluded that altitude effect of precipitation is not preserved in groundwaters. Based on both stable isotopic data and Cl/HCO3 ratios, Ahn et al. (1992) suggested that groundwater in the eastern part of the island is significantly contaminated by seawater, but not in the western part of the island. We address both of these conclusions in this study.
2. Hydrogeology and climate Cheju Island was formed by Cenozoic volcanism and is composed mainly of basalts, andesites and trachytes. The volcanic rocks are generally good aquifers because they are vesicular and have numerous joints, fractures and fissures. The best aquifers include pyroclastic deposits and clinkers interbedded between lava flows (Hahn et al., 1997). Sedimentary units, the
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Fig. 2. Monthly average (A) temperature and (B) precipitation amount during the 74-year period (1923–1996) at Cheju Island.
Seogipo and Sehwari formations, are confined to subsurface portions of the island. These sedimentary formations consist of reworked volcanic detritus and volcanic ashes deposited on the old sea bottom and serve as confining units because of their low permeability (Davis et al., 1970; Hahn et al., 1997). Despite the heavy rainfall in summer, there is little sustained streamflow on the island. Torrential runoff occurs during and right after large rain events, but this water immediately infiltrates into the ground due to high permeability of the volcanic rocks. Small streams fed by springs in the mid- and high-altitude areas flow locally and then disappear rapidly into the ground. Along the southern coast, a few major streams fed by coastal springs flow directly into the sea (Davis et al., 1970; Hahn et al., 1997). According to Hahn et al. (1997), the groundwater bodies on the island can be divided into three groups: high-level, basal and parabasal. The high-level groundwaters are perched and occur in the mid- and high-altitude areas. The basal groundwater is distributed widely along the coast. The parabasal groundwater is freshwater whose lower boundary is not in direct contact with seawater but occurs on imperme-
able or less permeable sub-surface sedimentary units. Springs on the island are classified as high-level and coastal (including sub-sea springs), depending upon their location. High-level springs originate from the high-level groundwater sources and the coastal springs from the basal groundwater. Climatically, the island is temperate and thus shows four distinct seasons with moderately hot summers and cold winters. Storm fronts reaching the island differ with the season. In winter, winds commonly originate from the north–northeast resulting in a wide range of temperatures typical of northern continental conditions. In summer, winds commonly come from south–southeast resulting in high temperatures and humidity. Temperatures recorded over a 74-year period from 1923 to 1996 show a typical cyclic variation between 5.0 and 26.28C with an annual mean temperature of about 158C (Fig. 2(A)). Rainfall is abundant; at Seongsan, on the east coast of the island, the 74-year average annual precipitation amount is 1467 mm. However, rainfall varies considerably from place to place, being the highest on the southern coast. High temperatures and humidity from the northern Pacific air mass usually results in a
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Table 1 Oxygen and hydrogen isotopic compositions, d-values and chloride concentrations of surface and groundwaters at Cheju Island Sample Sea CH3 CH8 CH10 CH12 CH20 CH27 CH33 Stream CH14 CH15 CH18 CH19 CH21 CH23 CH24 CH26 CH28 CH30 CH36 CH37 CH41 CH42 CH46 CH59 Pond CH31 CH35 CH61 Spring CH7 CH11 CH13 CH25 CH39 CH40 CH43 CH45 CH62 Well CH1 CH5 CH6 CH9 CH29 CH32 CH47 CH52 CH55 CH57 CH58 a
Cl 2 (ppm)
d 18O (‰)
d D (‰)
20.6 20.5 20.5 20.8 21.6 20.9 21.7
22 23 26 26 212 25 29
2.6 0.6 22.4 0.6 0.8 1.7 4.6
15 372 – 16 444 – 13 480 – 13 053
0.4 20.1 23.8 27.2 27.0 26.6 26.6 26.5 26.8 20.7 26.8 27.8 28.6 28.1 26.3 25.6
29 212 226 243 244 243 243 243 244 210 246 250 255 253 238 239
212.0 211.1 3.7 14.3 12.1 10.0 10.0 8.9 9.7 24.2 8.5 12.5 13.8 11.8 12.5 5.9
5 7 117 – – – – – – 38 – – – – – 3
24.7 23.1 1.2
233 227 27
4.7 22.1 216.1
78 22 39
24.1 22.6 22.8 26.8 28.5 28.8 29.1 28.3 26.8
224 217 219 241 255 256 261 252 244
9.1 4.1 4.0 13.2 12.9 14.3 11.5 15.0 10.9
9214 10 250 10 158 11 – 3 – – –
27.2 27.0 26.6 26.6 27.1 27.1 27.4 27.8 27.2 26.5 27.3
244 242 241 240 244 245 247 249 244 241 244
13.5 14.1 11.5 12.0 13.2 12.1 12.4 12.6 13.1 10.8 14.3
– – – – – – – – 8 – –
d dD28d 18 O:
d a (‰)
Altitude (m)
,0 ,0 ,0 120 1020 1431 1685 1080 153
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Fig. 3. Plot of d D versus d 18O in water samples in comparison to groundwaters reported by Davis et al. (1970). LMWL represents local meteoric water line from data of Lee et al. (in preparation).
monsoonal rainy season lasting from June to September. This period accounts for about 65% of the total annual rainfall.
3. Sampling and analytical methods Samples from streams, ponds, springs, wells and the sea adjacent to the island were collected for oxygen and hydrogen isotope measurements and chloride analysis in September 1996. These data are presented in Table 1. Sample locations are shown in Fig. 1(B). Water samples for oxygen isotopic analysis were prepared by H2O–CO2 equilibration (Epstein and Mayeda, 1953). About 2 ml of each water sample was equilibrated with tank CO2 gas at 258C. The CO2 gas was then extracted and cryogenically purified. For deuterium analysis, metallic zinc was used to produce hydrogen gas (Coleman et al., 1982). The oxygen and hydrogen isotopic compositions of the samples were determined using a VG Prism II stable isotope ratio mass spectrometer at the Korea Basic Science Institute (KBSI). The analytical reproducibility is ^ 0.1‰ for d 18O and ^ 1‰ for d D. All oxygen and hydrogen
isotopic analyses are reported in the usual d notation relative to the V-SMOW, in which d R=RV2SMOW 211000, where R represents either the 18O/ 16O or the D/H ratio of the sample and standard, respectively. Chloride concentrations were determined using an ion chromatograph at the Seoul National University.
4. Results and discussion A completed oxygen and hydrogen isotopic study of precipitation was carried out at Seongsan on the east coast of Cheju Island (Lee et al., in preparation). During the study period (May 1995–May 1997), the isotopic composition of precipitation is quite variable, with d 18O ranging from 213.5 to 21.0‰ and d D from 2105 to 120‰ (see Fig. 3). No seasonal variation (temperature effect) was recognized in the oxygen and hydrogen isotopic data. However, the values of deuterium excess (d-values) for summer precipitation d , 110‰ were clearly distinct from winter precipitation d ., 115‰: This same pattern appears to exist in a number of locations
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throughout Northeast Asia (IAEA, 1992). Accordingly, two different local meteoric water regression lines (LMWLs) were drawn to describe the isotopic d D 7:94 ^ data for different seasons: 0:20d 18 O 1 8:84 ^ 1:47 for rainy season precipitation (June–September) and d D 6:92 ^ 0:42d 18 O 1 16:60 ^ 2:26 for dry season precipitation (October–May). The slope and intercept of the regression line for rainy season precipitation are virtually identical to the global meteoric water line (GMWL) defined by Craig (1961). Lee et al. (in preparation) explained such seasonal isotopic differences as reflecting different air masses affecting the island during different seasons: cold and dry continental Siberian air masses in winter and hot and humid maritime North Pacific air masses in summer. The stable oxygen and hydrogen isotopes of groundwater are generally considered to be transported conservatively in the absence of high temperature water–rock interaction and significant evaporation, whereas most of chemical constituents are potentially reactive and do not mix conservatively in the hydrological cycle. Only a few halogen series elements such as chloride and bromide are known to be chemically unreactive, that is, once in solution, they are not easily removed by processes other than precipitation at very late evaporation stages (Gat, 1981; Richter and Kreitler, 1993). Therefore, stable isotope and chloride data of groundwaters can provide important information about their origin, source, recharge area and salinization for groundwater resource evaluation. The results of the d D and d 18O analyses are plotted in Fig. 3. It can be seen from this figure that waters fall into two major groupings: (1) the majority of samples, obtained from springs, wells and some streams, that lie close to but slightly above the rainy season local meteoric water line (LMWL); and (2) samples that lie below the rainy season LMWL and are isotopically enriched. Each of these two groupings are discussed separately below. 4.1. Samples lying close to the rainy season LMWL All of the groundwater samples obtained from wells and springs plot close to the LMWL for rainy season precipitation. A regression line drawn through these data is: d D 7:45d 18 O18:75 n 17; r2 0:96;
which is sub-parallel to the rainy season LMWL with a slope of 7.94. If stream samples lying on or above the rainy season LMWL are included, the regression has a slightly different slope and intercept of: d D 6:95d 18 O14:54 n 28; r 2 0:93: Despite the limited number of samples examined and the one period of sampling, it is noteworthy that our data are very similar to those reported by Davis et al. (1970) as shown in Fig. 3. Their samples were collected during the spring and fall of 1966, with nine locations sampled quarterly in 1967–68. It can thus be concluded that the observations reported here are generally applicable over an extended time period beyond just our one period of sampling. The displacement of the isotopic composition of this group of samples above the rainy season LMWL can be determined from the d-values listed in Table 1. Among all of the samples examined in this study, the best assessment of the isotopic composition of groundwaters on the island can be obtained from the 17 well and spring samples shown in Fig. 3. The mean d-value among these 17 groundwater samples (springs and wells) is 112:8 ^ 1:2‰; which as Fig. 3 shows, is slightly displaced above the LMWL for rainy season precipitation (,110‰). For comparison, the weighted average dvalue for rainy season (June through September) precipitation is 19.2‰ and for dry season (October through May) precipitation is 120‰ (Lee et al., in preparation). Using this mean d-value, the relative contributions of rainy and dry season precipitation to the groundwater recharge can be calculated using a mass-balance equation: dGroundwater XdRainy
season 1 12XdDry season
where X and 12X are the fraction of rainy and dry season precipitation, respectively. Thus it appears, based on their d-values, that groundwaters are composed on average of approximately 67% rainy season precipitation and 33% dry season precipitation. It is noteworthy that this proportion is nearly the same on average as the proportion of the monsoonal summer and dry season precipitation observed at Seongsan over a 74-year period from 1923 to 1996. As Fig. 2(B) indicates, the bulk of rainfall at Seongsan, about 65%, occurs during the period June through September. If this same balance exists
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Fig. 4. d 18O and d D values of selected springs as a function of altitude.
throughout the island as a whole, then it can be inferred that a substantial amount of summer precipitation on the island must recharge the groundwater system. If this were not the case, and evapotranspiration was an important process in controlling groundwater recharge, then one would expect that a greater proportion of winter precipitation would recharge groundwater, and therefore, the groundwater isotopic data would show a greater proportion of winter precipitation (a more enriched d-value). Evapotranspiration processes are most important in summer when plants are actively growing, and tend to be less important in winter when plants are dormant. Such a hypothesis, if true, is consistent with the fact that the island contains highly permeable volcanic rocks overlain by thin soils. Such an environment would permit rapid infiltration, even during times of the year when transpiration is active. This hypothesis is at odds with the study of Hahn et al. (1997) who estimate from a water budget model that approximately 37% of the water input onto the island is lost to evapotranspiration. This situation contrasts to environments where residual soils are thick and infiltration is relatively slow (Wenner et al., 1991; Clark and Fritz, 1997; Winograd
et al., 1998). For example, in the Piedmont of southeast US, thick soils (typically greater than 10 m) develop in situ over igneous and metamorphic bedrock (Wenner et al., 1991). This area is heavily vegetated, and transpiration is dominant in summer. Wenner et al. (1991) observed that only winter rainfall was able to recharge groundwater, since the effects of evapotranspiration from late spring to early fall prevented rainfall from percolating through the soil to any significant depth. Thus groundwaters in this area are dominated by winter rainfall because of the discriminating effects of transpiration during other parts of the year. It is apparent that samples, mainly from springs and small streams from the center of the island have the most depleted d D and d 18O values, reflecting the altitude effect. This effect, commonly observed in precipitation in many locations, can be used for identifying potential sources of recharge to groundwater (e.g. Leontiadis et al., 1996; Clark and Fritz, 1997). A quantitative value of this altitude effect can be determined from springs sampled from six locations at different elevations. Fig. 4 shows that a distinct correlation exists between the d 18O and d D values and elevation. This correlation defines an altitude
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Fig. 5. Plot of chloride content versus d 18O in water samples.
effect of 20.15‰ for d 18O r2 0:98 and 21.14‰ for d D r2 0:95 per 100 m, contrary to Davis et al.’s (1970) observations in which they did not see an altitude effect. The value of the altitude effect (20.15‰ for d 18O) is similar to that observed by Bartarya et al. (1995) from the Kumaun Himalaya region (20.14‰ for d 18 O),(but somewhat smaller than that observed by Leontiadis et al. (1996) for the Eastern Macedonia (20.44‰ for d 18O) and Northern Greece (20.21‰ for d 18O). This almost certainly reflects differences in local climate and topography. The reported overall range of values for the altitude effect in d 18O and d D for precipitation varies from 20.15 to 20.5‰ for d 18 O and from 21 to 24‰ for d D per 100 m rise in altitude (Yurtsever and Gat, 1981; Clark and Fritz, 1997). Our results fall in the lower part of this range. 4.2. Isotopically enriched samples lying below the rainy season LMWL These samples are from three freshwater springs lying adjacent to the sea and five streams and three ponds that, because of their setting, were potentially affected by evaporation. The three coastal springs,
which occur very close to sea level, are distinctly enriched in their stable isotopic compositions. These springs could be easily affected by the tidal effect of seawater because of their setting. Fig. 3 clearly shows a mixing relationship between groundwater and seawater. Such a mixing trend can be confirmed on the Cl– d 18O diagram shown in Fig. 5 (Shivanna et al., 1993). Here, two seawater samples (CH20 and CH33) show relatively depleted isotopic compositions compared to other seawaters (Fig. 3). This is presumably due to dilution by runoff of fresh waters from the island. This mixing is confirmed by the lower chloride contents of these samples (Table 1 and Fig. 5). Some of the pond and stream samples are also greatly enriched in their oxygen and hydrogen isotopic composition and deviate significantly from the rainy season LMWL (Fig. 3). They appear to have undergone varying degrees of evaporation and define an evaporation line with a slope of 4:81 ^ 0:34 n 8; r2 0:97; significantly less than that of local precipitation. When a water body loses water through evaporation, the slope of the d D– d 18O relation becomes smaller than eight, typically 3–6, because evaporation is a non-equilibrium process that enriches D less than 18O in the remaining water
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(Craig, 1961; Clark and Fritz, 1997; Burns and McDonnell, 1998). A spring sample (CH40) from near the center of the island has a low chloride concentration (3 ppm). This indicates that the chloride content of rainwater is generally low and the enhanced chloride contents observed in some streams, ponds and coastal springs is due to the secondary processes including evaporation, contamination by animal waste and mixing with seawater, as noted previously. A groundwater (CH55, 8 ppm) and a coastal spring (CH25, 11 ppm) are slightly enriched in chloride relative to the highlevel spring (CH40). If the enrichment of chloride in groundwater and surface waters resulted from only the evaporation, the chloride content of samples should be correlated with the amount of change in their oxygen isotopic composition (reflecting the degree of evaporation). On the Cl– d 18O plot (Fig. 5), the ponds and some streams show a relatively scattered pattern. It seems likely that such a pattern results from the combination of at least two processes: evaporation and contamination by waste water. With the present data, it is difficult to distinguish between these two options. Although the present chloride data for groundwaters are not enough to clearly identify the cause of salinization, we can evaluate these possibilities by using other parameters. Ahn et al. (1992) reported that the oxygen and hydrogen isotopic composition of groundwaters in the eastern part of the island are generally more enriched than those in the western part. They attributed this to contamination by seawater based on their Cl/HCO3 ratios. The present isotopic and chloride data of coastal springs sampled very close to sea level in the eastern part of the island in part support their conclusion that groundwater in the eastern part was contaminated by seawater. However, Hahn et al. (1997) report from data obtained from the Public Health and Environmental Research Institute (1992) that the chloride content is high even in the western coastal areas and its content decreases soon after rainfall. They argue that the main source of chloride is seawater. The conclusion of Ahn et al. (1992) based on oxygen and hydrogen isotopic data is not compatible with that of Hahn et al. (1997) based on the chloride data. With the present data, it is difficult to verify if groundwater in the eastern part of the island is more contaminated by seawater than that in
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the western part. Clearly, further study on salinization process of groundwaters at the island as well as surface waters is required.
5. Summary Groundwater is the most important freshwater resource at Cheju Island and provides almost all of the drinking water needs at the present time. To overcome environmental problems associated with extensive groundwater exploitation, information about the hydrological cycle of freshwater bodies is needed for resource evaluation. For this purpose, stable oxygen and hydrogen isotope and chloride data are reported for samples from various streams, ponds, wells, springs throughout the island and adjacent portions of the sea. A comparison of isotopic data of groundwaters with rainfall taken over a two-year period (Lee et al., in preparation) indicates that precipitation during the rainy season, from June through September, which contributes approximately 65% of the annual rainfall to the island, contributes about 67% of groundwater recharge. This implies that the isotopic composition of groundwaters nearly equals the mean weighted annual isotopic composition at the island and that no seasonal bias to recharge exists. This also implies that recharge occurs throughout the year and that evapotranspiration effects may be minimal, contrary to estimates based on water budget models (Hahn et al., 1997). This situation contrasts with other environments where relatively thick soils overlay bedrock. In these environments, evapotranspiration can effectively prevent recharge to groundwater during much of the growing season, permitting only winter rainfall to effectively contribute to groundwater (Wenner et al., 1991; Winograd et al., 1998). A selection of springs from different altitude shows a distinct altitude effect of 20.15‰ for d 18O and 21.14‰ for d D per 100 m. Some samples from springs, streams and ponds show a marked departure from the summer local meteoric water line on a d D– d 18O diagram. From the isotopic and chloride data, it is clear that springs lying very close to sea level show a clear mixing relationship between groundwater and seawater. In contrast, several ponds and streams, based on their chloride concentrations, were almost
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certainly affected by evaporation and/or contamination by waste water. Acknowledgements This work was supported by the Korea Science and Engineering Foundation grant (1997 Post-Doc program). We thank J.R. Gat and J.R. Lawrence for their review comments and C.H. Kim for his help in analyses of chloride contents. References Ahn, J.S., Kim, S.J., Kim, J.W., 1992. Environmental isotope studies on seawater intrusion into the southeastern coastal aquifer on Cheju Island. In: Isotope Techniques in Water Resources Development, IAEA Symposium 319, March 1991, Vienna, pp. 740–747. Bartarya, S.K., Bhattacharya, S.K., Ramesh, R., Somayajulu, B.L.K., 1995. d 18O and d D systematics in the surficial waters of the Gaula river catchment area, Kumaun Himalaya, India. J. Hydrol. 167, 369–379. Burns, D.A., McDonnell, J.J., 1998. Effects of a beaver pond on runoff processes: comparison of two headwater catchments. J. Hydrol. 205, 248–264. Clark, I.D., Fritz, P., 1997. Environmental Isotopes in Hydrogeology Lewis, New York, 328pp. Coleman, M.L., Shepherd, T.J., Durham, J.J., Rouse, J.E., Moore, G.R., 1982. Reduction of water with zinc for hydrogen isotope analysis. Anal. Chem. 54, 993–995. Craig, H., 1961. Isotopic variations in meteoric waters. Science 133, 1702–1703. Davis, G.H., Lee, C.K., Bradley, E., Payne, B.R., 1970. Geohydrologic interpretations of a volcanic island from environmental isotopes. Water Resour. Res. 6, 99–109. Epstein, S., Mayeda, K., 1953. Variation of 18O content of waters from natural sources. Geochim. Cosmochim. Acta 4, 213–224.
Gat, J.R., 1981. Groundwater. In: Gat, J.R., Gonfiantini, G. (Eds.) Stable Isotope Hydrology: Deuterium and Oxygen-18 in the Water Cycle, IAEA Technical Report Series, 210. IAEA, Vienna, pp. 223–240. Hahn, J., Lee, Y., Kim, N., Hahn, C., Lee, S., 1997. The groundwater resources and sustainable yield of Cheju volcanic island, Korea. Environ. Geol. 33, 43–53. IAEA, 1992. Statistical treatment of data on environmental isotopes in precipitation, IAEA Technical Report Series, 331. IAEA, Vienna. Lee, K.S., Wenner, D.B., Choi, M.S., Climatic controls on the stable isotopic composition of precipitation in Northeast Asia, in preparation. Leontiadis, I.L., Vergis, S., Christodoulou, Th., 1996. Isotope hydrology study of areas in Eastern Macedonia and Thrace, Northern Greece. J. Hydrol. 182, 1–17. Public Health and Environmental Research Institute, 1992. Analysis of groundwater of Cheju Island, Cheju City, Korea, open file (in Korean). Richter, B.C., Kreitler, C.W., 1993. Geochemical Techniques for Identifying Sources of Ground-water Salinization. CRC Press, Boca Raton, FL. 258pp. Shivanna, K., Navada, S.V., Nair, A.R., Rao, S.M., 1993. Isotopic and geochemical evidence of past seawater salinity in Midnapore groundwaters. In: Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere, IAEA Symposium 329, April 1993, Vienna, pp. 199–211. Wenner, D.B., Ketcham, P.D., Dowd, J.F., 1991. Stable isotopic composition of waters in a small Piedmont watershed. In: Taylor Jr., H.P., O’Neil, J.R., Kaplan, I.R. (Eds.), Stable Isotope Geochemistry: a Tribute to Samuel Epstein, The Geochemical Society, pp. 195–203. Special Publication No. 3. Winograd, I.J., Riggs, A.C., Coplen, T.B., 1998. The relative contributions of summer and cool-season precipitation to groundwater recharge, Spring Mountains, NV, USA. Hydrogeol. J. 6, 77–93. Yurtsever, Y., Gat, J.R., 1981. Atmospheric waters. In: Gat, J.R., Gonfiantini, G. (Eds.), Stable Isotope Hydrology: Deuterium and Oxygen-18 in the Water Cycle, IAEA Technical Report Series, 210. IAEA, Vienna, pp. 103–142.
Using H- and O-isotopic data for estimating the relative contributions of rainy and dry season precipitation to groundwater: example from Cheju Island, Korea Kwang-Sik Lee a,*, D.B. Wenner b, Insung Lee c a
Korea Basic Science Institute, Isotope Research Group, Eoeun-dong 52, Yusung-ku, Taejeon 305-333, South Korea b Department of Geology, University of Georgia, Athens, GA 30602, USA c Department of Geological Science, Seoul National University, Seoul 151-742, South Korea Received 8 December 1998; received in revised form 26 May 1999; accepted 30 June 1999
Abstract A comparison of deuterium excess or d-values of precipitation and groundwater at Cheju Island, Korea, indicates that, unlike in many temperate climates, precipitation during the whole year contributes to groundwater recharge. This in turn suggests that evapotranspiration effects are minimal, consistent with the fact that the island contains highly permeable volcanic rocks overlain by thin soils. This hypothesis is contrary to current water budget models that ascribe a significant role to evapotranspiration processes. Three coastal springs, and several streams and ponds throughout the island have enriched d 18O and d D values that plot off the local meteoric water line and have relatively high Cl concentrations. This relationship is suggestive of seawater mixing for the coastal springs and evaporation for the streams and ponds. This database thus provides a useful means for evaluation of groundwater resources of the island. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Deuterium excess; Precipitation; Groundwater; Cheju Island; Korea
1. Introduction This investigation is an outgrowth of another study (Lee et al., in preparation) of the oxygen and hydrogen isotopic composition of precipitation at Cheju Island, Korea. This study revealed that summer d , 110‰ and winter precipitation d . 115‰ had distinct deuterium excess values, a pattern that could potentially provide a means for evaluating the relative importance of summer and winter recharge to groundwater. It is with this background that a study was initiated of selected surface and groundwaters. * Corresponding author. Fax: 182-42-865-3419. E-mail address: [email protected] (K.-S. Lee)
Cheju Island, located at the southern tip of the Korean Peninsula (Fig. 1(A)), outcrops over an aerial extent of approximately 1825 km 2, with a length of 74 km and a width of 32 km. It is predominantly composed of highly permeable volcanic rocks with thin soils. The island is gently sloping along its periphery and rises in the center to a height of 1950 m (Mt. Halla) (see Fig. 1(B)). Most habitation and development exist along the outer edge of the island. At the present time, groundwater supplies almost all of the drinking water needs of the island. Extensive groundwater development, initiated in the early 1970s, has produced a number of environmental problems whose severity has increased over time.
0022-1694/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0022-169 4(99)00099-2
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Fig. 1. Map of Northeast Asia showing the location of (A) Cheju Island and (B) the topography and sampling locations.
These problems include decline of groundwater levels, salinization of groundwater in coastal areas and degradation of groundwater quality (Hahn et al., 1997). Numerous wells on the island have been contaminated by seawater, sewage, animal wastes and chemical fertilizers. The present study focuses on measuring the D/H and 18O/ 16O compositions, along with the chloride concentration from selected samples, of various surface and groundwaters acquired from streams, ponds, springs and wells throughout the island. In this study, we have attempted to quantify the relative contribution of rainy and dry season precipitation to groundwater recharge and the extent of evaporation of surface water by comparing the isotopic composition of groundwater with precipitation data. Further, it was of interest to measure the altitude effect of precipitation from the isotopic composition of springs collected at different elevations. Finally, we were able to evaluate the cause of salinization process observed in some surface and groundwaters by using a combination of isotopic and chloride data.
To date, a limited number of groundwater samples have been analyzed for H- and O-isotopic compositions from Cheju Island. Davis et al. (1970) reported oxygen and hydrogen isotopic and tritium data and concluded that altitude effect of precipitation is not preserved in groundwaters. Based on both stable isotopic data and Cl/HCO3 ratios, Ahn et al. (1992) suggested that groundwater in the eastern part of the island is significantly contaminated by seawater, but not in the western part of the island. We address both of these conclusions in this study.
2. Hydrogeology and climate Cheju Island was formed by Cenozoic volcanism and is composed mainly of basalts, andesites and trachytes. The volcanic rocks are generally good aquifers because they are vesicular and have numerous joints, fractures and fissures. The best aquifers include pyroclastic deposits and clinkers interbedded between lava flows (Hahn et al., 1997). Sedimentary units, the
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Fig. 2. Monthly average (A) temperature and (B) precipitation amount during the 74-year period (1923–1996) at Cheju Island.
Seogipo and Sehwari formations, are confined to subsurface portions of the island. These sedimentary formations consist of reworked volcanic detritus and volcanic ashes deposited on the old sea bottom and serve as confining units because of their low permeability (Davis et al., 1970; Hahn et al., 1997). Despite the heavy rainfall in summer, there is little sustained streamflow on the island. Torrential runoff occurs during and right after large rain events, but this water immediately infiltrates into the ground due to high permeability of the volcanic rocks. Small streams fed by springs in the mid- and high-altitude areas flow locally and then disappear rapidly into the ground. Along the southern coast, a few major streams fed by coastal springs flow directly into the sea (Davis et al., 1970; Hahn et al., 1997). According to Hahn et al. (1997), the groundwater bodies on the island can be divided into three groups: high-level, basal and parabasal. The high-level groundwaters are perched and occur in the mid- and high-altitude areas. The basal groundwater is distributed widely along the coast. The parabasal groundwater is freshwater whose lower boundary is not in direct contact with seawater but occurs on imperme-
able or less permeable sub-surface sedimentary units. Springs on the island are classified as high-level and coastal (including sub-sea springs), depending upon their location. High-level springs originate from the high-level groundwater sources and the coastal springs from the basal groundwater. Climatically, the island is temperate and thus shows four distinct seasons with moderately hot summers and cold winters. Storm fronts reaching the island differ with the season. In winter, winds commonly originate from the north–northeast resulting in a wide range of temperatures typical of northern continental conditions. In summer, winds commonly come from south–southeast resulting in high temperatures and humidity. Temperatures recorded over a 74-year period from 1923 to 1996 show a typical cyclic variation between 5.0 and 26.28C with an annual mean temperature of about 158C (Fig. 2(A)). Rainfall is abundant; at Seongsan, on the east coast of the island, the 74-year average annual precipitation amount is 1467 mm. However, rainfall varies considerably from place to place, being the highest on the southern coast. High temperatures and humidity from the northern Pacific air mass usually results in a
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Table 1 Oxygen and hydrogen isotopic compositions, d-values and chloride concentrations of surface and groundwaters at Cheju Island Sample Sea CH3 CH8 CH10 CH12 CH20 CH27 CH33 Stream CH14 CH15 CH18 CH19 CH21 CH23 CH24 CH26 CH28 CH30 CH36 CH37 CH41 CH42 CH46 CH59 Pond CH31 CH35 CH61 Spring CH7 CH11 CH13 CH25 CH39 CH40 CH43 CH45 CH62 Well CH1 CH5 CH6 CH9 CH29 CH32 CH47 CH52 CH55 CH57 CH58 a
Cl 2 (ppm)
d 18O (‰)
d D (‰)
20.6 20.5 20.5 20.8 21.6 20.9 21.7
22 23 26 26 212 25 29
2.6 0.6 22.4 0.6 0.8 1.7 4.6
15 372 – 16 444 – 13 480 – 13 053
0.4 20.1 23.8 27.2 27.0 26.6 26.6 26.5 26.8 20.7 26.8 27.8 28.6 28.1 26.3 25.6
29 212 226 243 244 243 243 243 244 210 246 250 255 253 238 239
212.0 211.1 3.7 14.3 12.1 10.0 10.0 8.9 9.7 24.2 8.5 12.5 13.8 11.8 12.5 5.9
5 7 117 – – – – – – 38 – – – – – 3
24.7 23.1 1.2
233 227 27
4.7 22.1 216.1
78 22 39
24.1 22.6 22.8 26.8 28.5 28.8 29.1 28.3 26.8
224 217 219 241 255 256 261 252 244
9.1 4.1 4.0 13.2 12.9 14.3 11.5 15.0 10.9
9214 10 250 10 158 11 – 3 – – –
27.2 27.0 26.6 26.6 27.1 27.1 27.4 27.8 27.2 26.5 27.3
244 242 241 240 244 245 247 249 244 241 244
13.5 14.1 11.5 12.0 13.2 12.1 12.4 12.6 13.1 10.8 14.3
– – – – – – – – 8 – –
d dD28d 18 O:
d a (‰)
Altitude (m)
,0 ,0 ,0 120 1020 1431 1685 1080 153
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Fig. 3. Plot of d D versus d 18O in water samples in comparison to groundwaters reported by Davis et al. (1970). LMWL represents local meteoric water line from data of Lee et al. (in preparation).
monsoonal rainy season lasting from June to September. This period accounts for about 65% of the total annual rainfall.
3. Sampling and analytical methods Samples from streams, ponds, springs, wells and the sea adjacent to the island were collected for oxygen and hydrogen isotope measurements and chloride analysis in September 1996. These data are presented in Table 1. Sample locations are shown in Fig. 1(B). Water samples for oxygen isotopic analysis were prepared by H2O–CO2 equilibration (Epstein and Mayeda, 1953). About 2 ml of each water sample was equilibrated with tank CO2 gas at 258C. The CO2 gas was then extracted and cryogenically purified. For deuterium analysis, metallic zinc was used to produce hydrogen gas (Coleman et al., 1982). The oxygen and hydrogen isotopic compositions of the samples were determined using a VG Prism II stable isotope ratio mass spectrometer at the Korea Basic Science Institute (KBSI). The analytical reproducibility is ^ 0.1‰ for d 18O and ^ 1‰ for d D. All oxygen and hydrogen
isotopic analyses are reported in the usual d notation relative to the V-SMOW, in which d R=RV2SMOW 211000, where R represents either the 18O/ 16O or the D/H ratio of the sample and standard, respectively. Chloride concentrations were determined using an ion chromatograph at the Seoul National University.
4. Results and discussion A completed oxygen and hydrogen isotopic study of precipitation was carried out at Seongsan on the east coast of Cheju Island (Lee et al., in preparation). During the study period (May 1995–May 1997), the isotopic composition of precipitation is quite variable, with d 18O ranging from 213.5 to 21.0‰ and d D from 2105 to 120‰ (see Fig. 3). No seasonal variation (temperature effect) was recognized in the oxygen and hydrogen isotopic data. However, the values of deuterium excess (d-values) for summer precipitation d , 110‰ were clearly distinct from winter precipitation d ., 115‰: This same pattern appears to exist in a number of locations
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throughout Northeast Asia (IAEA, 1992). Accordingly, two different local meteoric water regression lines (LMWLs) were drawn to describe the isotopic d D 7:94 ^ data for different seasons: 0:20d 18 O 1 8:84 ^ 1:47 for rainy season precipitation (June–September) and d D 6:92 ^ 0:42d 18 O 1 16:60 ^ 2:26 for dry season precipitation (October–May). The slope and intercept of the regression line for rainy season precipitation are virtually identical to the global meteoric water line (GMWL) defined by Craig (1961). Lee et al. (in preparation) explained such seasonal isotopic differences as reflecting different air masses affecting the island during different seasons: cold and dry continental Siberian air masses in winter and hot and humid maritime North Pacific air masses in summer. The stable oxygen and hydrogen isotopes of groundwater are generally considered to be transported conservatively in the absence of high temperature water–rock interaction and significant evaporation, whereas most of chemical constituents are potentially reactive and do not mix conservatively in the hydrological cycle. Only a few halogen series elements such as chloride and bromide are known to be chemically unreactive, that is, once in solution, they are not easily removed by processes other than precipitation at very late evaporation stages (Gat, 1981; Richter and Kreitler, 1993). Therefore, stable isotope and chloride data of groundwaters can provide important information about their origin, source, recharge area and salinization for groundwater resource evaluation. The results of the d D and d 18O analyses are plotted in Fig. 3. It can be seen from this figure that waters fall into two major groupings: (1) the majority of samples, obtained from springs, wells and some streams, that lie close to but slightly above the rainy season local meteoric water line (LMWL); and (2) samples that lie below the rainy season LMWL and are isotopically enriched. Each of these two groupings are discussed separately below. 4.1. Samples lying close to the rainy season LMWL All of the groundwater samples obtained from wells and springs plot close to the LMWL for rainy season precipitation. A regression line drawn through these data is: d D 7:45d 18 O18:75 n 17; r2 0:96;
which is sub-parallel to the rainy season LMWL with a slope of 7.94. If stream samples lying on or above the rainy season LMWL are included, the regression has a slightly different slope and intercept of: d D 6:95d 18 O14:54 n 28; r 2 0:93: Despite the limited number of samples examined and the one period of sampling, it is noteworthy that our data are very similar to those reported by Davis et al. (1970) as shown in Fig. 3. Their samples were collected during the spring and fall of 1966, with nine locations sampled quarterly in 1967–68. It can thus be concluded that the observations reported here are generally applicable over an extended time period beyond just our one period of sampling. The displacement of the isotopic composition of this group of samples above the rainy season LMWL can be determined from the d-values listed in Table 1. Among all of the samples examined in this study, the best assessment of the isotopic composition of groundwaters on the island can be obtained from the 17 well and spring samples shown in Fig. 3. The mean d-value among these 17 groundwater samples (springs and wells) is 112:8 ^ 1:2‰; which as Fig. 3 shows, is slightly displaced above the LMWL for rainy season precipitation (,110‰). For comparison, the weighted average dvalue for rainy season (June through September) precipitation is 19.2‰ and for dry season (October through May) precipitation is 120‰ (Lee et al., in preparation). Using this mean d-value, the relative contributions of rainy and dry season precipitation to the groundwater recharge can be calculated using a mass-balance equation: dGroundwater XdRainy
season 1 12XdDry season
where X and 12X are the fraction of rainy and dry season precipitation, respectively. Thus it appears, based on their d-values, that groundwaters are composed on average of approximately 67% rainy season precipitation and 33% dry season precipitation. It is noteworthy that this proportion is nearly the same on average as the proportion of the monsoonal summer and dry season precipitation observed at Seongsan over a 74-year period from 1923 to 1996. As Fig. 2(B) indicates, the bulk of rainfall at Seongsan, about 65%, occurs during the period June through September. If this same balance exists
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Fig. 4. d 18O and d D values of selected springs as a function of altitude.
throughout the island as a whole, then it can be inferred that a substantial amount of summer precipitation on the island must recharge the groundwater system. If this were not the case, and evapotranspiration was an important process in controlling groundwater recharge, then one would expect that a greater proportion of winter precipitation would recharge groundwater, and therefore, the groundwater isotopic data would show a greater proportion of winter precipitation (a more enriched d-value). Evapotranspiration processes are most important in summer when plants are actively growing, and tend to be less important in winter when plants are dormant. Such a hypothesis, if true, is consistent with the fact that the island contains highly permeable volcanic rocks overlain by thin soils. Such an environment would permit rapid infiltration, even during times of the year when transpiration is active. This hypothesis is at odds with the study of Hahn et al. (1997) who estimate from a water budget model that approximately 37% of the water input onto the island is lost to evapotranspiration. This situation contrasts to environments where residual soils are thick and infiltration is relatively slow (Wenner et al., 1991; Clark and Fritz, 1997; Winograd
et al., 1998). For example, in the Piedmont of southeast US, thick soils (typically greater than 10 m) develop in situ over igneous and metamorphic bedrock (Wenner et al., 1991). This area is heavily vegetated, and transpiration is dominant in summer. Wenner et al. (1991) observed that only winter rainfall was able to recharge groundwater, since the effects of evapotranspiration from late spring to early fall prevented rainfall from percolating through the soil to any significant depth. Thus groundwaters in this area are dominated by winter rainfall because of the discriminating effects of transpiration during other parts of the year. It is apparent that samples, mainly from springs and small streams from the center of the island have the most depleted d D and d 18O values, reflecting the altitude effect. This effect, commonly observed in precipitation in many locations, can be used for identifying potential sources of recharge to groundwater (e.g. Leontiadis et al., 1996; Clark and Fritz, 1997). A quantitative value of this altitude effect can be determined from springs sampled from six locations at different elevations. Fig. 4 shows that a distinct correlation exists between the d 18O and d D values and elevation. This correlation defines an altitude
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Fig. 5. Plot of chloride content versus d 18O in water samples.
effect of 20.15‰ for d 18O r2 0:98 and 21.14‰ for d D r2 0:95 per 100 m, contrary to Davis et al.’s (1970) observations in which they did not see an altitude effect. The value of the altitude effect (20.15‰ for d 18O) is similar to that observed by Bartarya et al. (1995) from the Kumaun Himalaya region (20.14‰ for d 18 O),(but somewhat smaller than that observed by Leontiadis et al. (1996) for the Eastern Macedonia (20.44‰ for d 18O) and Northern Greece (20.21‰ for d 18O). This almost certainly reflects differences in local climate and topography. The reported overall range of values for the altitude effect in d 18O and d D for precipitation varies from 20.15 to 20.5‰ for d 18 O and from 21 to 24‰ for d D per 100 m rise in altitude (Yurtsever and Gat, 1981; Clark and Fritz, 1997). Our results fall in the lower part of this range. 4.2. Isotopically enriched samples lying below the rainy season LMWL These samples are from three freshwater springs lying adjacent to the sea and five streams and three ponds that, because of their setting, were potentially affected by evaporation. The three coastal springs,
which occur very close to sea level, are distinctly enriched in their stable isotopic compositions. These springs could be easily affected by the tidal effect of seawater because of their setting. Fig. 3 clearly shows a mixing relationship between groundwater and seawater. Such a mixing trend can be confirmed on the Cl– d 18O diagram shown in Fig. 5 (Shivanna et al., 1993). Here, two seawater samples (CH20 and CH33) show relatively depleted isotopic compositions compared to other seawaters (Fig. 3). This is presumably due to dilution by runoff of fresh waters from the island. This mixing is confirmed by the lower chloride contents of these samples (Table 1 and Fig. 5). Some of the pond and stream samples are also greatly enriched in their oxygen and hydrogen isotopic composition and deviate significantly from the rainy season LMWL (Fig. 3). They appear to have undergone varying degrees of evaporation and define an evaporation line with a slope of 4:81 ^ 0:34 n 8; r2 0:97; significantly less than that of local precipitation. When a water body loses water through evaporation, the slope of the d D– d 18O relation becomes smaller than eight, typically 3–6, because evaporation is a non-equilibrium process that enriches D less than 18O in the remaining water
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(Craig, 1961; Clark and Fritz, 1997; Burns and McDonnell, 1998). A spring sample (CH40) from near the center of the island has a low chloride concentration (3 ppm). This indicates that the chloride content of rainwater is generally low and the enhanced chloride contents observed in some streams, ponds and coastal springs is due to the secondary processes including evaporation, contamination by animal waste and mixing with seawater, as noted previously. A groundwater (CH55, 8 ppm) and a coastal spring (CH25, 11 ppm) are slightly enriched in chloride relative to the highlevel spring (CH40). If the enrichment of chloride in groundwater and surface waters resulted from only the evaporation, the chloride content of samples should be correlated with the amount of change in their oxygen isotopic composition (reflecting the degree of evaporation). On the Cl– d 18O plot (Fig. 5), the ponds and some streams show a relatively scattered pattern. It seems likely that such a pattern results from the combination of at least two processes: evaporation and contamination by waste water. With the present data, it is difficult to distinguish between these two options. Although the present chloride data for groundwaters are not enough to clearly identify the cause of salinization, we can evaluate these possibilities by using other parameters. Ahn et al. (1992) reported that the oxygen and hydrogen isotopic composition of groundwaters in the eastern part of the island are generally more enriched than those in the western part. They attributed this to contamination by seawater based on their Cl/HCO3 ratios. The present isotopic and chloride data of coastal springs sampled very close to sea level in the eastern part of the island in part support their conclusion that groundwater in the eastern part was contaminated by seawater. However, Hahn et al. (1997) report from data obtained from the Public Health and Environmental Research Institute (1992) that the chloride content is high even in the western coastal areas and its content decreases soon after rainfall. They argue that the main source of chloride is seawater. The conclusion of Ahn et al. (1992) based on oxygen and hydrogen isotopic data is not compatible with that of Hahn et al. (1997) based on the chloride data. With the present data, it is difficult to verify if groundwater in the eastern part of the island is more contaminated by seawater than that in
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the western part. Clearly, further study on salinization process of groundwaters at the island as well as surface waters is required.
5. Summary Groundwater is the most important freshwater resource at Cheju Island and provides almost all of the drinking water needs at the present time. To overcome environmental problems associated with extensive groundwater exploitation, information about the hydrological cycle of freshwater bodies is needed for resource evaluation. For this purpose, stable oxygen and hydrogen isotope and chloride data are reported for samples from various streams, ponds, wells, springs throughout the island and adjacent portions of the sea. A comparison of isotopic data of groundwaters with rainfall taken over a two-year period (Lee et al., in preparation) indicates that precipitation during the rainy season, from June through September, which contributes approximately 65% of the annual rainfall to the island, contributes about 67% of groundwater recharge. This implies that the isotopic composition of groundwaters nearly equals the mean weighted annual isotopic composition at the island and that no seasonal bias to recharge exists. This also implies that recharge occurs throughout the year and that evapotranspiration effects may be minimal, contrary to estimates based on water budget models (Hahn et al., 1997). This situation contrasts with other environments where relatively thick soils overlay bedrock. In these environments, evapotranspiration can effectively prevent recharge to groundwater during much of the growing season, permitting only winter rainfall to effectively contribute to groundwater (Wenner et al., 1991; Winograd et al., 1998). A selection of springs from different altitude shows a distinct altitude effect of 20.15‰ for d 18O and 21.14‰ for d D per 100 m. Some samples from springs, streams and ponds show a marked departure from the summer local meteoric water line on a d D– d 18O diagram. From the isotopic and chloride data, it is clear that springs lying very close to sea level show a clear mixing relationship between groundwater and seawater. In contrast, several ponds and streams, based on their chloride concentrations, were almost
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certainly affected by evaporation and/or contamination by waste water. Acknowledgements This work was supported by the Korea Science and Engineering Foundation grant (1997 Post-Doc program). We thank J.R. Gat and J.R. Lawrence for their review comments and C.H. Kim for his help in analyses of chloride contents. References Ahn, J.S., Kim, S.J., Kim, J.W., 1992. Environmental isotope studies on seawater intrusion into the southeastern coastal aquifer on Cheju Island. In: Isotope Techniques in Water Resources Development, IAEA Symposium 319, March 1991, Vienna, pp. 740–747. Bartarya, S.K., Bhattacharya, S.K., Ramesh, R., Somayajulu, B.L.K., 1995. d 18O and d D systematics in the surficial waters of the Gaula river catchment area, Kumaun Himalaya, India. J. Hydrol. 167, 369–379. Burns, D.A., McDonnell, J.J., 1998. Effects of a beaver pond on runoff processes: comparison of two headwater catchments. J. Hydrol. 205, 248–264. Clark, I.D., Fritz, P., 1997. Environmental Isotopes in Hydrogeology Lewis, New York, 328pp. Coleman, M.L., Shepherd, T.J., Durham, J.J., Rouse, J.E., Moore, G.R., 1982. Reduction of water with zinc for hydrogen isotope analysis. Anal. Chem. 54, 993–995. Craig, H., 1961. Isotopic variations in meteoric waters. Science 133, 1702–1703. Davis, G.H., Lee, C.K., Bradley, E., Payne, B.R., 1970. Geohydrologic interpretations of a volcanic island from environmental isotopes. Water Resour. Res. 6, 99–109. Epstein, S., Mayeda, K., 1953. Variation of 18O content of waters from natural sources. Geochim. Cosmochim. Acta 4, 213–224.
Gat, J.R., 1981. Groundwater. In: Gat, J.R., Gonfiantini, G. (Eds.) Stable Isotope Hydrology: Deuterium and Oxygen-18 in the Water Cycle, IAEA Technical Report Series, 210. IAEA, Vienna, pp. 223–240. Hahn, J., Lee, Y., Kim, N., Hahn, C., Lee, S., 1997. The groundwater resources and sustainable yield of Cheju volcanic island, Korea. Environ. Geol. 33, 43–53. IAEA, 1992. Statistical treatment of data on environmental isotopes in precipitation, IAEA Technical Report Series, 331. IAEA, Vienna. Lee, K.S., Wenner, D.B., Choi, M.S., Climatic controls on the stable isotopic composition of precipitation in Northeast Asia, in preparation. Leontiadis, I.L., Vergis, S., Christodoulou, Th., 1996. Isotope hydrology study of areas in Eastern Macedonia and Thrace, Northern Greece. J. Hydrol. 182, 1–17. Public Health and Environmental Research Institute, 1992. Analysis of groundwater of Cheju Island, Cheju City, Korea, open file (in Korean). Richter, B.C., Kreitler, C.W., 1993. Geochemical Techniques for Identifying Sources of Ground-water Salinization. CRC Press, Boca Raton, FL. 258pp. Shivanna, K., Navada, S.V., Nair, A.R., Rao, S.M., 1993. Isotopic and geochemical evidence of past seawater salinity in Midnapore groundwaters. In: Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere, IAEA Symposium 329, April 1993, Vienna, pp. 199–211. Wenner, D.B., Ketcham, P.D., Dowd, J.F., 1991. Stable isotopic composition of waters in a small Piedmont watershed. In: Taylor Jr., H.P., O’Neil, J.R., Kaplan, I.R. (Eds.), Stable Isotope Geochemistry: a Tribute to Samuel Epstein, The Geochemical Society, pp. 195–203. Special Publication No. 3. Winograd, I.J., Riggs, A.C., Coplen, T.B., 1998. The relative contributions of summer and cool-season precipitation to groundwater recharge, Spring Mountains, NV, USA. Hydrogeol. J. 6, 77–93. Yurtsever, Y., Gat, J.R., 1981. Atmospheric waters. In: Gat, J.R., Gonfiantini, G. (Eds.), Stable Isotope Hydrology: Deuterium and Oxygen-18 in the Water Cycle, IAEA Technical Report Series, 210. IAEA, Vienna, pp. 103–142.