Hydrochemistry and isotope geochemistry of Song ...

Geosciences Journal Vol. 11, No. 2, p. 157

− 164, June 2007

Hydrochemistry and isotope geochemistry of Song Stream, a headwater tributary of the South Han River, South Korea Jong-Sik Ryu

Kwang-Sik Lee* Ho-Wan Chang

Division of Isotope Geoscience, Korea Basic Science Institute, Daejeon 305-333, Korea School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea Division of Isotope Geoscience, Korea Basic Science Institute, Daejeon 305-333, Korea School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea

ABSTRACT: To investigate the geochemical and isotopic characteristics of a headwater tributary of the South Han River, we analyzed major elements, stable isotopes of oxygen, hydrogen, carbon, and sulfur, and strontium (Sr) isotopes of stream and groundwater samples collected from the Song Stream watershed in summer 2003. The stream water samples of the study area were divided into three water types, among which dissolved ion concentrations differed considerably. Our results strongly indicate that the chemical composition of Song Stream is controlled by silicate and carbonate weathering, as well as anthropogenic contamination, and variations in major dissolved ions and Sr isotopic ratios are mainly correlated to lithological variations in the watershed. The dissolved loads of the main channel of Song Stream are largely controlled by carbonate dissolution. Thus, the water chemistry of the main channel is probably dominated by the chemical weathering of carbonates, even where carbonates comprise only a minor proportion of the bedrock geology. The Sr isotopes and Mg/Ca molar ratios indicate that a dolomite end-member may exist in the study area, which would be compatible with the cationic characteristics of Song Stream. All groundwater samples from the study area, except for one, had significantly high nitrate concentrations (0.75– 2.42 mmol/L) that exceeded the drinking-water standard and possibly resulted from both sewage and agricultural inputs.

various industrial and agricultural activities and supplies freshwater to more than 20 million inhabitants of the central Korean Peninsula. Despite the river’s socio-economic value, only limited information is presently available on its chemical and isotopic compositions and anthropogenic contamination levels (Yu et al., 1994; Seo and Kim, 1996; Lee and Lee, 1999; Chae et al., 2004; Lee et al., 2007). The chemical and isotopic compositions of a pristine headwater tributary of the Han River watershed can serve as baseline information for understanding the geochemical evolution of the Han River system and for assessing anthropogenic inputs. However, no previous studies have reported the chemical and isotopic compositions of a pristine headwater tributary of the Han River system. We focused mainly on the fluvial geochemistry and its controlling factors of Song Stream, a headwater tributary of the South Han River. Our results provide baseline information on the chemical and isotopic characteristics of the uppermost headwaters of the South Han River and can contribute to future watershed-scale investigations of the main Key words: Song Stream, carbonates, silicates, chemical weathering, channel of the Han River. anthropogenic contamination

1. INTRODUCTION Surface water chemistry provides important information on the sources of dissolved loads, chemical weathering, and chemical and isotopic characteristics of a drainage basin, including its crust material (Drever, 1988; Négrel et al., 1993; Han and Liu, 2004). Over several decades, numerous studies have examined surface waters and the source rocks affecting their chemical compositions (e.g., Potter, 1978; Karim and Veizer, 2000; Quade et al., 2003). These studies have shown that chemical weathering of rocks in a drainage basin, mainly by carbonate dissolution, largely controls the water chemistry of that basin. The Han River is the largest river in South Korea in terms of both water discharge and total river length. It supports *Corresponding author: [email protected]

2. MATERIALS AND METHODS 2.1. Study Area Song Stream, which originates from Mt. Odae, is one of the uppermost tributaries of the South Han River. With a drainage area of 352 km , the 81.4 km long stream flows through Pyeongchang-Gun and Jeongseon-Gun before joining the Joyang River at Aurazi. Doam Dam, forming a 144.9 km reservoir, is the only dam on the stream (Fig. 1). The study area has a temperate climate with four distinct seasons. Monthly average temperatures vary from –2.5°C in January to 25.4°C in August. The 30-year (1971–2000) average annual precipitation was 1307 mm, with approximately two-thirds of annual precipitation occurring during the rainy season from June to September (Korea Meteorological Administration, website: http//www.kma.go.kr). In the geology of the Song Stream watershed, the upper 2

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Jong-Sik Ryu, Kwang-Sik Lee, and Ho-Wan Chang

lyzed by ion chromatography (Dionex DX-500 IC) at the Geochemistry Laboratory, Seoul National University. The oxygen, hydrogen, carbon, and sulfur isotopic compositions of samples were determined using a VG Prism II stable isotope ratio mass spectrometer at the KBSI. Stable isotopic compositions are expressed in the usual δ notation relative to Vienna Standard Mean Ocean Water (VSMOW) values for oxygen and hydrogen isotopes, Vienna Pee Dee Belemnite (VPDB) for carbon isotopes, and Cañon Diablo Troilite (CDT) for sulfur isotopes, in which δ (‰) = (Rsample/ Rstandard – 1)×1000, where R represents O/ O, D/H, C/ C, or S/ S. The analytical reproducibility is ± 0.1‰ for δ O, ± 1‰ for δD, ± 0.2‰ for δ C, and ±0.2‰ for δ S. For Sr isotopic analysis, approximately 60 ml of each water sample was evaporated to dryness in ultraclean Teflon vessels and redissolved using distilled HCl. Strontium in the solution was then separated from other ions using cation exchange resin (BioRad AG 50W X8 200–400 mesh) in a quartz column. The total blank level was less than 0.1 ng, which was negligible for Sr. Sr isotopes were determined using a VG54-30 thermal ionization mass spectrometer at Geological map of the Song Stream watershed showing the the KBSI. The Sr/ Sr ratios were normalized to Sr/ Sr sampling locations. = 0.1194, and the mean Sr/ Sr ratio of the NBS987 standard (recommended value = 0.71025) during analysis was 0.710247 ± 0.000008 (2σ, n = 12). and middle reaches of the Song Stream catchment are dominated by Jurassic granites and sedimentary rocks (mostly 3. RESULTS AND DISCUSSION clastic sediments) and Paleozoic sedimentary rocks (sandstones, shales, sand shales, and siltstones; Fig. 1). The lower 3.1. Major Ions Geochemistry reaches are composed mainly of Paleozoic sedimentary rocks (limestones, coal-bearing sedimentary formations, dolomitic 3.1.1. General descriptions limestones, shales, and sand shales; data from the Korea The stream waters were mildly alkaline, with pH ranging Institute of Geoscience and Mineral Resources, website: from 7.05 to 8.93, averaging 8.06 (Table 1). The pH of the stream waters tended to increase toward downstream sites. http://geoinfo.kigam.re.kr). In contrast, the groundwater samples had weakly acidic to neutral pH ranging from 6.35 to 7.27, with an average of 6.84. 2.2. Sampling and Analytical Procedures The total dissolved cations (TZ =Na +K + 2Mg +2Ca ) To investigate the hydrochemical and multi-isotopic com- and total dissolved anions (TZ− = Cl− + NO − + 2SO − + position of the Song Stream watershed, stream and ground- HCO − ) were well balanced within ± 9% of the normalized water samples were collected at 19 sites in August 2003 inorganic charge balance (NICB), with the exception of (summer, high water; Fig. 1). The temperature, pH, and elec- G-4 and S-1, which had quite high concentrations of both trical conductivity (EC) of water samples were measured in Cl− and NO − (Table 1). Such a large unbalance in stoichithe field. Alkalinity was determined within 12 h after sam- ometry may be the result of anthropogenic contaminants. In this pling by the Gran method of titration using 0.1 N HCl. case, the NH ion can likely be considered well balanced. Samples for chemical and Sr isotopic analysis were passed Total dissolved solids (TDSs) in the groundwater ranged through 0.45-µm pre-cleaned membrane filters and kept from 162 to 273 mg/L, with an average 228 mg/L (Table 1). refrigerated at approximately 4°C before analysis. Samples The TDS of Song Stream ranged from 17.0 to 252 mg/L, for cation analysis were acidified in the field with ultrapure with an average of 120 mg/L. Two stream water samples HNO to pH < 2. Cations were analyzed by inductively cou- (S-6 and S-12) had quite low TDS, at 17.1 and 17.0 mg/L, pled plasma mass spectrometry (ICP-MS) and ICP atomic respectively. These stream waters were collected from very emission spectroscopy (ICP-AES) at the Korea Basic Sci- small headwater tributaries draining entirely sedimentary ence Institute (KBSI). Internal standards (In, Rh, and Tl) rocks. Such low TDSs are likely the result of a pristine were used to calibrate the data. The analytical uncertainties headwater draining silicates and a direct reflection of the ranged from 2 to 5% for major elements. Anions were ana- chemical composition of rainwater because of the short 18

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Fig. 1.

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86

87

88

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+

+

+

2+

2+

2

3

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3

+

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4

Physico-chemical properties of water samples in the study area. Cl NO SO HCO NICB δ O Sr Si Mg Na K Ca Sam- T pH EC TDS (mmol/ ( µmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ (mmol/ ple (°C) (µS/cm) (mg/L) L) (%) (‰) L) L) L) L) L) L) L) L) L) Groundwater G-1 16.7 6.52 203 162 0.58 0.04 0.17 0.33 0.22 1.73 0.68 0.81 0.09 0.32 -7 -10.2 G-2 18.1 6.88 343 265 1.10 0.12 0.37 0.16 0.13 3.06 0.99 0.75 0.05 1.60 -7 -10.4 G-3 17.0 7.16 246 208 0.81 0.04 0.18 0.36 0.30 2.56 0.49 1.37 0.13 0.45 -9 -10.2 G-4 20.0 6.35 307 273 1.06 0.05 0.20 0.18 0.26 4.15 0.60 2.42 0.10 0.33 -28 -10.0 G-5 15.0 7.27 276 235 1.07 0.03 0.29 0.20 0.18 1.96 0.26 0.09 0.07 2.39 3 -10.2

Table 1.

3

Mean

17.4

Stream water S-1 17.8 S-2 16.9 S-3 16.8 S-4 18.0 S-5 21.5 S-6 13.0 S-7 22.1 S-8 14.8 S-9 17.0 S-10 12.5 S-11 17.4 S-12 19.8 S-13 -

Mean

17.3

6.84

275

0.24

0.25

0.22

2.69

0.60

1.08

0.09

1.02

7.26 155 176 0.44 0.05 7.05 137 98.4 0.31 0.03 7.94 125 104 0.36 0.04 8.07 129 108 0.40 0.05 8.53 123 92.2 0.34 0.05 7.19 21.6 17.1 0.03 0.01 8.93 122 95.0 0.36 0.05 8.34 378 200 0.67 0.03 8.69 283 252 1.28 0.03 7.94 110 99.1 0.49 0.01 8.71 217 192 0.99 0.02 7.35 26.3 17.0 0.03 0.01 8.84 134 107 0.42 0.05

0.11 0.09 0.09 0.10 0.08 0.02 0.09 1.42 0.39 0.07 0.27 0.02 0.14

0.25 0.19 0.19 0.19 0.20 0.05 0.18 0.05 0.05 0.05 0.05 0.04 0.17

0.21 0.24 0.22 0.21 0.14 0.08 0.12 0.12 0.07 0.10 0.08 0.08 0.11

1.29 1.69 1.24 1.31 0.99 0.08 1.02 0.80 1.12 0.38 0.78 0.08 1.04

2.04 0.25 0.27 0.28 0.34 0.09 0.37 0.10 0.07 0.10 0.06 0.33

0.37 0.45 0.39 0.44 0.24 0.03 0.25 0.02 0.03 0.04 0.04 0.23

0.05 0.02 0.05 0.05 0.05 0.02 0.04 0.04 0.02 0.05 0.02 0.06

0.52 0.36 0.43 0.42 0.45 0.02 0.48 1.90 3.17 0.94 2.32 0.02 0.63

8.06

0.22

0.13

0.14

0.91

0.36

0.21

0.04

0.90

120

0.92

0.47

0.03

- not measured NICB = Normalized Inorganic Charge Balance = (TZ - TZ−)/TZ × 100% calculated by PHREEQC (Parkhurst and Appelo, 1999) CSI = calcite saturation index

a

b

c

3

0.05

151

228

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+

a

-118 -7 -5 -1 -1 -4 -5 2 9 1 6 4

18

δD

C (‰) (‰) log pCO CSI δ

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2

S (‰) δ

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Sr/ Sr

87

86

-70 -72 -72 -18.8 -74 -75 -

-3.5 -3.3 -4.1 -3.4 -3.5

-2.2 -1.0 -1.4 -2.1 -0.5

-

0.716392 0.724476 0.714077 0.719273 0.713804

-10.2

-73

-18.8

-3.6

-1.4

-

0.717604

-10.1 -10.4 -10.2 -10.0 -10.0 -10.5 -10.1 -10.6 -10.5 -10.6 -10.7 -10.5 -10.1

-70 -74 -72 -70 -70 -73 -70 -76 -75 -76 -76 -75 -70

-10.4 -10.5 -13.9 -18.9 -8.9 -2.7 -9.7 -12.8 -9.1 -8.4

-4.1 -4.1 -4.9 -5.0 -5.5 -4.7 -5.9 -4.7 -4.9 -4.5 -5.1 -4.0 -5.7

-1.4 - 0.715413 -1.9 - 0.717706 -0.9 - 0.713231 -0.7 4.5 0.713857 -0.3 3.0 0.715775 -3.4 5.6 0.715311 0.2 4.2 0.715850 0.3 -0.1 0.716221 1.1 6.1 0.710302 -0.5 3.8 0.713072 0.9 5.5 0.711557 -2.2 - 0.712753 0.3 2.1 0.715663

-10.3

-73

-10.5

-4.9

-0.7

3.9

0.714362

H yd ro ch em is try an d is ot op e ge oc he m is try of So ng St re am ,a he ad w at er tri bu ta ry of th e So ut h H an R iv er ,S ou th K or ea 15 9

160


Piper diagram showing the three different water types. Arrows indicate the direction of the evolution of water chemistry. Fig. 2.

mean residence time of surface waters. The average TDS of Song Stream (120 mg/L) is much lower than that of the Ganges (187 mg/L; Dalai et al., 2002) and Indus (164 mg/L; Karim and Veizer, 2000) draining the Himalayas, the Mackenzie draining the Rockies (160 mg/L; Millot et al., 2003), and the Upper Huang He (Yellow River; 274 mg/L; Wu et al., 2005). The water samples from the Song Stream watershed could be divided into three distinct types with two end-members: (Ca+Mg)-rich and (Na+K)-rich waters (Fig. 2). These endmembers are distinct in anion chemistry. Type I waters were collected from small headwater tributaries draining granites and clastic sediments and were characterized by higher concentrations of Na, Cl, and NO ions compared to other dissolved ions. Type II waters belong to the Ca–Mg–HCO type and were collected from a tributary draining carbonates in the lower reaches of the watershed. Type III waters are a mixture of these two types. Major cation concentrations were generally in the order of Ca > Mg > Na > K (on a molar basis). The Ca concentration occupied over 50% of the total cation concentration, with two exceptions, i.e., S-6 and S-12, which were collected from pristine headwaters.

for atmospheric inputs consists of quantifying and subtracting the portion of chemical elements carried into rivers by rainwater (Stallard and Edmond, 1981). Yu and Park’s (2004) chemical analysis of rainwater in Chuncheon City, located near our study area, provides an estimate of the proportion of atmosphere-derived constituents in Song Stream. Given that no salt-bearing rocks or evaporites have been reported from the Han River basin (Chough et al., 2000), we can assume that the Cl in sample S-12, which was the lowest Cl concentration we found, came entirely from rainwater (Table 1). We then calculated the proportion of atmosphere-derived elements in the stream water using the element/Cl ratios in rainwater. The calculation suggested that 3–18% of total dissolved cations in Song Stream, but about 75% of total dissolved cations in two samples (S-6 and S-12) draining pristine headwater tributaries, originated from rainwater. In contrast, 6–19% of anions in Song Stream were estimated to have derived mainly from the atmosphere. The nitrate and chloride concentrations in stream and groundwater were much higher than those of precipitation reported by Yu and Park (2004), indicating anthropogenic contamination through sewage and/or agricultural inputs (Table 1). To discriminate the different anthropogenic components in the study area, we plotted Cl/Na versus NO /Na (Fig. 3; Roy et al., 1999). Most of the stream water samples deviated from the precipitation data, indicating that they were affected by other (i.e., anthropogenic) sources. Stream water sample S-1, which was collected near a small town, particularly deviated from the major grouping of stream water and had the highest Cl concentration among the stream water samples. This sample was likely affected by 3

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3.1.2. Atmospheric inputs and anthropogenic components

A number of sources such as atmospheric Cl and SO , natural Ca, and anthropogenic NO , contribute to the chemistry of river waters (Roy et al., 1999). Before discriminating the weathering sources and different rock types in a Plot of Cl/Na vs. NO /Na. The end-member composition river system, it is important to evaluate the atmospheric and for agricultural inputs is from Roy et al. (1999), and rainwater data anthropogenic contributions (Meybeck, 1983). Correction are from Yu and Park (2004). 4

3

Fig. 3.

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Hydrochemistry and isotope geochemistry of Song Stream, a headwater tributary of the South Han River, South Korea

161

tern almost similar to that of rainwater and because no evaporite has been reported in the Han River basin (Chough et al., 2000). This result indicates that silicate weathering is predominant in the (Na+K)-rich waters, but carbonate weathering is significant in the (Ca+Mg)-rich waters. Based on a Gibbs-type (1970) TDS vs. Na/(Na+Ca) diagram that provided information on the three major mechanisms controlling surface water chemistry (atmospheric input, weathering, and evaporation; not shown), stream waters in the study area could be divided into two major groups: one in the rock-dominance field and the other in the precipitation-dominance field. All surface water samples, with two exceptions in the precipitation-dominance field, were plotted within the rock-dominance field. This indicates that the major mechanism controlling the surface water chemistry of the study area is chemical weathering of rocks, with a minor contribution from atmospheric input.

3.2. Isotope Geochemistry 3.2.1. Oxygen and hydrogen isotopes

The oxygen and hydrogen isotopic compositions of Song Stream water ranged from –10.7 to –10.0‰ and from –76 to –70‰, respectively (Table 1). All water samples were plotted on or close to the global meteoric water line of Craig (1961), indicating meteoric origin (Fig. 5). However, one groundwater sample (G-4) slightly deviated from the major grouping. This sample, which had the highest nitrate concentration among the water samples (2.42 mmol/L), was likely affected by irrigation water that had undergone evaporation before infiltration.

3.2.2. Carbon isotopes

The relationships among measured surface water variMajor ion chemistry of stream water and groundwater of the Song Stream watershed showing a mixing trend between sili- ables often provide useful information regarding the concate and carbonate end-members. End-member compositions of carbonates and silicates are from Gaillardet et al. (1999). Fig. 4.

point contamination such as sewage. Interestingly, one sample (G-4) was collected in a highland farming site near the agriculture end-member described by Roy et al. (1999).

3.1.3. Water-rock interaction

To examine the effect of lithological weathering of the Song Stream watershed, we plotted two mixing diagrams using Na-normalized molar cation ratios (Fig. 4; Gaillardet et al., 1999). Water samples from Song Stream were plotted between silicate and carbonate end-members. Two water samples (S-6 and S-12) were plotted near the evaporite endmember. However, these samples are probably not related to evaporite dissolution; it is more likely that their chemical compositions directly reflect that of rainwater because their Plot of δD vs. δ O values for the water samples. GMWL TDS are extremely low (17.0–17.1 mg/L), showing a pat- represents the global meteoric water line of Craig (1961). Fig. 5.

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Plot of (a) pH vs. HCO − , (b) δ C vs. HCO − , (c) δ C vs. log pCO , and (d) δ C vs. the calcite saturation index (CSI) for water samples from the study area. The δ C values are positively correlated with HCO − and CSI. 13

Fig. 6.

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3

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2

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3

trolling factors of carbon isotopes and water chemistry (Lee et al., 2007). There was a weak positive correlation between pH and HCO − and between δ C and HCO− in the water samples (Fig. 6a, b). This suggests that carbonate dissolution is a dominant source of dissolved inorganic carbon in the stream water and may thus be the major factor controlling the carbon isotope chemistry of surface water in the study area. The partial pressure of CO (pCO ) and the calcite saturation index (CSI) were computed using the PHREEQC program (Parkhust and Appelo, 1999). No meaningful relation was found between δ C and pCO (Fig. 6c), whereas the relationship between δ C and CSI showed a positive trend (Fig. 6d) and a meaningful correlation. This means that δ C values are closely related to carbonate dissolution in the study area. The (Na+K)-rich type sample (S-6) had the lowest CSI and d C values, whereas (Ca+Mg)rich type samples (S-8) had the highest CSI and δ C val13

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DIC

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DIC

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DIC

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DIC

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DIC

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DIC

ues (Fig. 6d). This indicates that water–carbonate interaction and/or the input of carbonate-saturated groundwater increases the δ C values of stream water in the study area (Lee et al., 2007). 13

DIC

3.2.3. Sulfur isotopes

Dissolved SO − in water originates from atmospheric input, dissolution of sedimentary sulfates, oxidation of sulfide minerals or organic materials, and anthropogenic inputs such as sewage (Thode, 1991; Grasby et al., 1997). Various geochemical tools have been used as tracers to identify the sources of SO − in water. Of these, the sulfur isotopic signature of SO − is most diagnostic (Grasby et al., 1997). On a plot of δ S values vs. SO − concentrations (Fig. 7), Song Stream water was plotted within or near the range for precipitation found by Yu and Park (2004). Based on sulfur isotopes and factor analysis results, they argued that atmospheric input could be the major contributor of sulfates 2

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2

SO4

4

Hydrochemistry and isotope geochemistry of Song Stream, a headwater tributary of the South Han River, South Korea

Plot of Sr/ Sr vs. Mg/Ca showing the three end-members: limestone, dolomite, and silicate.

Fig. 8.

Plot of δ S values vs. SO − concentrations of the water samples. The dissolved SO − in the stream water is apparently mostly related to the deposition of atmospheric sulfates. Fig. 7.

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4. CONCLUSIONS

We focused on the hydrochemistry and isotope geochemin surface waters of the North Han River. Our results also istry of Song Stream, one of the headwater tributaries of the suggest that the dissolved sulfates in Song Stream originate South Han River. We analyzed dissolved major ions, stable isotopes of oxygen, hydrogen, carbon, and sulfur, and stronmainly from atmospheric deposition. tium isotopes of stream and groundwater collected from the Song Stream watershed in summer 2003. The variations in 3.2.4. Strontium isotopes Generally, the Sr isotopic compositions of river water are major ions and Sr isotopic ratios are mainly correlated with similar to those of the parent rocks of the drainage basin the lithology of the watershed. The dissolved loads of Song because Sr isotopes are not fractionated during weathering Stream originate mainly from carbonate dissolution and silprocesses (Faure, 1986). Hence, Sr isotopes have been used icate weathering, with minor contributions from rainwater. as a powerful tool to identify the sources of dissolved loads The oxygen and hydrogen isotopic compositions of Song in river water (Négrel and Dupré, 1995; Zhang et al., 1995; Stream indicate meteoric origin, and δ S data suggest Gaillardet et al., 1997; Viers et al., 2000; Quade et al., 2003). that dissolved sulfates in Song Stream are mostly derived The Sr concentration of Song Stream ranged from 0.08 to from atmospheric deposition. Carbonate dissolution appears 1.69 µmol/L, with an average of 0.91 µmol/L (Table 1), which to play an important role in carbon isotopic values of disis very similar to the global average of 0.89 µmol/L (Palmer solved inorganic carbon (DIC) in the study area. Combined, and Edmond, 1989). The Sr/ Sr values of Song Stream the Sr isotopes and Mg/Ca ratios also suggest that three ranged from 0.710302 to 0.717706, with an average of 0.714362 end-members (silicate, limestone, and dolomite) control the (Table 1). Interestingly, samples S-6 and S-12 had Sr con- water chemistry of Song Stream. centrations and Sr/ Sr ratios very similar to those of rainwater in the study area (unpublished data). This result agrees ACKNOWLEDGEMENTS: This work was supported by the KBSI with the finding that the samples were mainly derived from grant (N27052) to K.S. Lee and by a grant (code 3-2-3) from the Sustainable Water Resources Research Center of 21st Century Frontier atmospheric input, as described before. Program. We thank the BK 21 Program, School of Earth & The chemical composition of Song Stream water was Research Environmental Sciences, Seoul National University. We also thank controlled by three end-members: silicate, limestone, and Prof. S.T. Yun and an anonymous reviewer for constructive comments dolomite (Fig. 8). Most samples were plotted to a limestone– and Prof. C.-E. Baag for editorial assistance. silicate mixing line, whereas only a few samples were plotted to a limestone–dolomite mixing line. Combined, the Sr REFERENCES isotopes and Mg/Ca ratios suggest that the main factor conK.T., Yun, S.T., Kim, K.H., Lee, P.K. and Choi, B.Y., 2004, trolling Song Stream water chemistry is the dissolution of Chae,Atmospheric versus lithogenic contribution to the composition of both limestone and silicate, with a minor contribution of dolofirst- and second-order stream waters in Seoul and its vicinity. mite dissolution. It is apparent that carbonate weathering preEnvironmental International, 30, 73−85. dominates over silicate weathering in the study area. Chough, S.K., Kwon, S.-T., Ree, J.-H. and Choi, D.K., 2000, Tec34

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