Uranium isotopes as a tracer of sources of dissolved solutes in the Han River, South Korea

Chemical Geology 258 (2009) 354–361

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Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o

Uranium isotopes as a tracer of sources of dissolved solutes in the Han River, South Korea Jong-Sik Ryu a, Kwang-Sik Lee a,⁎, Ho-Wan Chang b, Chang-Sik Cheong a a b

Division of Earth and Environmental Science, Korea Basic Science Institute, 113 Gwahangno, Yusung-gu, Daejeon 305-333, Republic of Korea School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Republic of Korea

a r t i c l e

i n f o

Article history: Received 11 April 2008 Received in revised form 17 September 2008 Accepted 24 October 2008 Editor: B. Bourdon Keywords: 234

238

U– U fractionation Silicate weathering Carbonate dissolution Black shale Groundwater input Han River

a b s t r a c t The uranium (U) content and 234U/238U activity ratio were determined for water samples collected from Korea's Han River in spring, summer, and winter 2006 to provide data that might constrain the origin of U isotope fractionation in river water and the link between U isotope systematics in river waters and the lithological nature of the corresponding bedrock. The large difference in the major dissolved loads between the two major branches of the Han River, the North Han River (NHR) and South Han River (SHR), is reflected in the contrasting U content and 234U/238U activity ratio between the tributaries: low U content (0.08– 0.75 nM; average, 0.34 nM) and small 234U/238U activity ratio (1.03–1.22; average, 1.09) in the NHR; and high U content (0.65–1.98 nM; average, 1.44 nM) and large 234U/238U activity ratio (1.05–1.45; average, 1.24) in the SHR. The large spatial differences in U content and 234U/238U activity ratio are closely related to both lithological differences between the two tributaries and groundwater input. The low U content and small 234 U/238U activity ratio in the NHR arise mainly from a combination of surface and meteoric weathering of the dominant silicate rocks in this branch and congruent dissolution of already weathered (secular equilibrium) materials. In contrast, the high U content and large 234U/238U activity ratio in the SHR are ascribed to the dissolution of carbonates and black shales along with significant inputs of deep groundwater. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Numerous proposals have suggested the possibility of using the U–238U disequilibrium as a proxy in modeling chemical weathering of rocks (Bourdon et al., 2003 and references therein). Among others, Sarin et al. (1990), Plater et al. (1992), Pande et al. (1994), Riotte and Chabaux (1999), Chabaux et al. (2001), Riotte et al. (2003), Durand et al. (2005) and Vigier et al. (2005) have employed the uranium (U) disequilibrium in a manner similar to the strontium (Sr) isotope, to investigate the nature and origin of chemical weathering fluxes carried by rivers to the ocean. In particular, variations in measured 234 U/238U activity ratios of surface waters appear closely related to the weathering of bedrock. However, the studies of Riotte and Chabaux (1999) and Chabaux et al. (2001) have indicated that U fractionation in river waters is affected not only by surface weathering of rocks but also by water–rock interactions occurring in deeper environments. Uranium disequilibrium is well documented among the dissolved phases of surface waters and is thought to reflect preferential leaching of 234U relative to 238U during water–rock interactions (Osmond and Ivanovich, 1992). However, the nature and localization of the water– rock interaction that controls 234U–238U fractionation is open to debate (Osmond and Ivanovich, 1992; Riotte et al., 2003; Vigier et al., 2005). The Han River is the largest river system in South Korea, in terms of both water discharge and drainage area. It supports a variety of 234

⁎ Corresponding author. Tel.: +82 42 8653447; fax: +82 42 865 3963. E-mail address: [email protected] (K.-S. Lee). 0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2008.10.039

industrial and agricultural activities, and supplies freshwater to more than 20 million inhabitants of the central Korean Peninsula. The extent of geochemical information regarding the river's basin is relatively limited (Yu et al., 1994; Seo and Kim, 1996; Chae et al., 2004; Lee et al., 2007). Recently, we identified distinct differences in basin lithology, i.e., silicate versus carbonate, between the North Han River (NHR) and South Han River (SHR) branches, making this system a good natural laboratory in which to investigate weathering processes and the influence of basin geology on the chemistry of river water (Ryu et al., 2007, 2008). For the same reason, the Han River basin also appears to offer a suitable place in which to investigate the nature and localization of the water–rock interaction controlling 234U–238U fractionation in river waters. In this study, we also analyzed the major ion and Sr isotope compositions of dissolved loads in the Han River. Our objectives were mainly to discuss the origin of U isotope fractionation in river water by focusing on a new case study and to clarify the link between U isotope systematics in river waters and the lithological nature of the corresponding bedrock. 2. Study area 2.1. Geography and climate The Han River system consists of two major branches, the NHR and SHR, along with many minor tributaries (Fig. 1). The river drains an area of 26,018 km2, or 27% of South Korea. The average runoff is

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355

Fig. 1. Geological map of the Han River basin showing tectonic units and sampling locations (GM: Gyeonggi Massif, OFB: Ogcheon Fold Belt, OB: Ogcheon Basin, TB: Taebaeksan Basin, YM: Yongnam Massif, KB: Kyeongsang Basin).

19.4 km3, and the total river length is 5417 km (data from Water Management Information System (WAMIS), http://www.wamis.go. kr). The river source is sited at an altitude of more than 1300 m above sea level in Mt. Taeback and traverses the mid-western parts of the Korean Peninsula before flowing into the Yellow Sea. The NHR and SHR join at the Paldang Dam to form the main channel of the Han River. The dam provides the first reservoir located in the main river channel and serves as a reference point for the sampling program used in this study (Fig. 1). The latitude and geography of Korea provide four distinct seasons. Wind and precipitation are largely affected by the surrounding Pacific Ocean in the south and the Eurasian landmass in the north. The mean annual rainfall is 1274 mm, of which about 70% is concentrated over the summer months between June and September. The mean monthly temperature varies from below freezing in winter to over 25 °C in summer. 2.2. Geology The southern Korean Peninsula consists of two Precambrian massifs that flank a series of mobile belts and sedimentary basins. Of these, the Han River basin transects the Gyeonggi massif and Ogcheon Fold Belt (Fig. 1). The basement rocks of Precambrian Gyeonggi massif consist of the Late Archean to Early Proterozoic high-grade gneisses and schists that

have suffered amphibolite- to granulite-facies metamorphism and have been intruded by Mesozoic plutons (Lee, 1987). The Ogcheon Fold Belt is divided into two zones based largely on lithology and metamorphic grade: the Ogcheon Basin in the southwest and the Taebaeksan Basin in the northeast. The Ogcheon Basin contains non-fossiliferous, low- to medium-grade metasedimentary and metavolcanic rocks of Precambrian to Paleozoic age (Lee et al., 1998 and references therein). The metasedimentary rocks consist predominantly of black shales containing coal seams (Lee, 1987). The Taebaeksan Basin comprises mainly the Choson Supergroup (Cambrian–Ordovician) and Pyongan Supergroup (Carboniferous– Triassic). The Paleozoic sediments of the Choson Supergroup are of shallow marine origin and consist predominantly of carbonates with lesser amounts of sandstone and shale, whereas the Pyongan Supergroup comprises thick clastic successions of marginal marine to nonmarine environments and contains economically important coal measures (Cheong, 1969). The NHR basin is occupied in the main by Precambrian gneisses and Mesozoic granites. In contrast, Ordovician limestones and Permo-Carboniferous coal-bearing clastic sedimentary rocks, including U-bearing black slates, are widely distributed in the upper reaches of the SHR basin. Locally, limestones have been metasomatized by intrusion of Mesozoic granites with numerous polymetallic skarn deposits developed along contact boundaries (Fig. 1).

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3. Sampling and analytical methods River water samples were collected from two stations in the main channel of the Han River, six stations in the NHR, and seven stations in the SHR during three sampling programs carried out in April 2006 (spring, rising-water stage), August 2006 (summer, high-water stage), and November 2006 (winter, low-water stage). All sampling sites were selected to avoid anthropogenic contamination and direct influences from small tributaries (Fig. 1). At each site, a depth-integrated sample was taken from the center of the river. The temperature and pH of water samples were measured on site. The alkalinity of the water samples was determined within 12 h of sampling by Gran titration using 0.1 N HCl. For comparison, shallow and deep groundwaters (0– 10 and 50–300 m in depth, respectively) were sampled from boreholes in May and October 2007. Two spring water samples (NG1 and NG-2) were also collected in the NHR basin, and one groundwater (SG-2) sample was taken from an abandoned coalmine in the SHR basin. Samples for chemical and isotopic analyses were passed through 0.2-μm, pre-cleaned membrane filters and kept refrigerated at approximately 4°C before analysis. Samples for cation, trace element and isotope analyses were acidified in the field with ultrapure HNO3 to a pH of b2. Cations and trace elements were analyzed by ICP-MS (X-7, Thermo Elemental) and ICP-AES (Optima 4300DU; Perkin-Elmer) at the Korea Basic Science Institute (KBSI). Anions were analyzed by ion chromatography (Dionex DX-500 IC) at the Korea Institute of Geoscience and Mineral Resources.

For Sr isotopic analysis, approximately 60 ml of each water sample was evaporated to dryness in ultraclean Teflon vessels and redissolved in distilled 2.5 N HCl. Strontium in the solution was then separated from other ions by cation exchange resin (AG 50 W X8, 200–400 mesh; Bio-Rad) in a quartz column. The total Sr blank level was less than 0.1 ng. Strontium isotopic compositions were determined using MC-ICPMS (Axiom, VG Elemental) at the KBSI. The 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194, and the mean 87Sr/86Sr ratio of the NBS 987 standard (recommended value = 0.71025) obtained for reproducibility during analysis was 0.710247 ± 0.000008 (2σ, n = 24). For U isotopic analysis of the dissolved fractions, U was separated from about 8 L of filtered river water using iron hydroxide coprecipitation (Chen et al., 1986). All of the separations were performed in a clean room (CLASS 1000) with laminar flow benches. The acids and water used were double distilled with a quartz still or Teflon twobottle method. Uranium was then separated using Eichrom TRU-Spec resin (Luo et al., 1997). The chemical yield was better than 95% for U, and the Th/U ratio was less than 0.1 in the U cut. The total U blank was less than 40 pg. The U isotopes were analyzed by multi-collector inductively coupled plasma-mass spectrometry (MC-ICPMS) at the KBSI. The 234 U/238U ratio was measured using multi-static modes (Pietruszka et al., 2002) and was corrected for instrumental mass fractionation using external bracket methods (standard-sample-standard) with certified reference material (CRM) 129A as the standard. The reproducibility and reliability of the U isotopic analyses were tested by regular measurements of the CRM 129A standard, which is assumed to

Table 1 The chemical compositions of the dissolved load in rivers and groundwaters of the Han River basin Sample number

Date

pH

yyyy-mm River water Main channel of the Han River M-1 M-2 North Han River NS-3 NP-2 NP-3 N-5 N-6 N-6-1 South Han River S-2 S-3 S-3-1 S-5 S-5-1 S-7 S-10 Groundwater North Han River NG-1⁎ NG-1⁎ NG-2⁎ NG-2⁎ NG-3 NG-3 NG-4 NG-5 NG-6 NG-7 South Han River SG-1 SG-2⁎⁎ SG-3 SG-4 SG-5 SG-6

T

Na

K

Mg

Ca

Cl

NO3

SO4

PO4

Alk

Si

TDS

Sr

°C

μM

μM

μM

μM

μM

μM

μM

μM

μEq

μM

mg/l

μM

87

Sr/86Sr

2006-04 2006-11

8.70 7.18

12.5 14.7

320 492

39.7 65.6

143 214

433 636

267 353

113 157

136 169

n.d n.d

1015 1372

– 101

114 163

0.955 1.34

0.72246 0.72261

2006-04 2006-04 2006-04 2006-04 2006-04 2006-04

7.05 6.82 7.22 8.70 8.47 7.80

10.7 13.6 7.9 11.9 13.1 17.0

136 193 128 197 212 155

16.7 35.7 23.6 28.7 31.9 29.4

56.0 118 53.7 79.1 73.4 68.5

173 313 177 263 211 366

117 141 61.3 121 174 153

136 98 84 99 114 132

58.9 77.3 49.3 63.3 70.7 77.3

n.d n.d n.d n.d n.d n.d

247 809 400 609 500 556

91.9 74.2 94.1 62.4 31.5 97.9

42.5 87.5 49.2 69.4 61.7 73.7

0.502 0.657 0.486 0.601 0.607 0.730

0.72372 0.72525 0.72490 0.72889 0.72701 0.72250

2006-04 2006-04 2006-04 2006-04 2006-04 2006-04 2006-04

7.62 8.31 8.04 7.68 8.11 7.72 8.73

8.2 10.0 11.3 11.2 17.8 9.8 14.0

135 118 156 131 575 221 385

18.9 22.3 24.4 22.1 62.6 35.9 43.8

103 192 138 171 91.7 195 195

417 711 409 596 365 621 596

139 133 174 131 465 181 308

161 170 169 157 113 133 134

76.6 184 76.7 156 87.2 140 202

n.d n.d n.d n.d n.d n.d n.d

805 1498 921 1203 924 1400 1410

88.3 78.0 91.5 74.5 91.8 30.7 –

89.8 155 100 129 119 143 156

0.859 1.65 0.599 1.31 1.14 1.22 1.20

0.71661 0.71473 0.71958 0.71629 0.71773 0.71665 0.71722

2007-05 2007-10 2007-05 2007-10 2007-05 2007-10 2007-10 2007-10 2007-10 2007-10

7.13 – 6.33 – 6.61 – 6.40 6.28 6.04 6.56

13.4 – 14.7 – 14.9 – 16.7 16.9 15.6 16.3

164 152 325 350 72.6 57.3 453 199 250 365

17.7 15.2 11.0 11.3 11.9 10.4 23.9 46.5 14.6 14.0

62.3 57.6 43.8 47.5 102 104 384 97.2 205 72.9

346 317 230 233 451 449 626 290 314 619

43.0 62.6 204 251 62.2 64.7 345 105 125 94.7

20.9 26.2 40.9 54.4 63.1 58.4 561 177 252 36.3

63.5 57.8 39.0 39.2 69.3 67.8 116 56.5 85.4 191

n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d

792 809 518 483 917 903 902 435 735 1226

264 285 466 506 192 208 383 236 282 293

77.1 76.9 63.3 64.5 91.4 89.8 159 67.1 97.0 134

0.783 0.665 0.864 0.934 0.811 0.730 2.01 0.708 0.682 4.92

0.76116 0.75863 0.71730 0.71615 0.72589 0.72416 0.74285 0.75082 0.75602 0.77670

2007-05 2007-05 2007-05 2007-11 2007-11 2007-11

7.30 7.14 7.31 – – –

14.1 12.7 12.7 – – –

165 51.0 149 406 653 582

17.4 21.4 30.1 55.2 64.6 66.5

907 780 729 1378 1390 1479

143 35.9 74.6 – – –

247 4.92 114 – – –

125 840 254 – – –

n.d n.d n.d – – –

1979 1362 1215 – – –

201 126 92.2 435 413 400

202 216 146 – – –

2.98 1.81 1.00 2.08 2.34 2.25

0.71151 0.73265 0.72414 0.71197 0.71214 0.71176

–: Not measured, n.d: not detected, ⁎: spring, ⁎⁎: abandoned coal mine drainage.

342 730 178 1500 1633 1825

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be in secular equilibrium. The mean 234U/238U activity ratio of the CRM 129A standard obtained for reproducibility during analysis was 1.000 ± 0.003 (2σ, n = 33).

Table 2 Uranium concentrations and 234U/238U activity ratios of the dissolved load in river and groundwaters of the Han River basin Sample no.

4. Results In a previous study, we presented preliminary results of geochemical and isotopic investigations of the Han River basin (Ryu et al., 2007). Subsequently, a companion paper reported the rates of chemical weathering and associated consumption of CO2 in the river basin (Ryu et al., 2008). In those two papers, we noted that the NHR drains silicate rocks and exhibits lower concentrations of total dissolved solids (TDS), major ions, and Sr, but higher Si concentration and a higher 87Sr/86Sr ratio. In contrast, the SHR drains karst terrains and displays opposite trends. These results clearly indicated that the major and trace element concentrations in the Han River water reflect the lithological differences in the respective catchments. 4.1. Major elements and Sr isotopes The concentrations of major ions and Sr isotopes of the dissolved loads in river waters and groundwaters of the Han River basin are shown in Table 1. The river water is neutral to mildly alkaline, with pH values ranging from 6.82 to 8.73 (average, 7.88). In contrast, the groundwater ranges from mildly acidic to neutral, with pH values ranging from 6.04 to 7.31 (average 6.71). The TDS in the SHR (average = 127 mg/L) is about twice that in the NHR (average = 64 mg/L). Similarly, the TDS in groundwater of the SHR (average 188 mg/L) is about twice that of the NHR (average 92 mg/L).

357

River water Main channel of the Han River M-1 M-1 M-1 North Han River NS-3 NS-3 NS-3 NP-2 NP-2 NP-2 NP-3 NP-3 NP-3 N-5 N-5 N-5 N-6 N-6 N-6 N-6-1 N-6-1 N-6-1 South Han River S-2 S-2 S-2 S-3 S-3 S-3 S-3-1 S-3-1 S-3-1 S-5 S-5 S-5 S-5-1 S-5-1 S-5-1 S-7 S-7 S-7 S-10 S-10 S-10 Groundwater North Han River NG-1⁎ NG-1⁎ NG-2⁎ NG-2⁎ NG-3 NG-3 NG-4 NG-5 NG-6 NG-7 South Han River SG-1 SG-2⁎⁎ SG-3 SG-4 SG-5 SG-6

(234U/238U)

Date

U

yyyy-mm

nM

2006-04 2006-08 2006-11

1.35 1.36 1.03

1.175 ± 0.003 1.140 ± 0.001 1.144 ± 0.003

2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11

0.285 0.542 0.320 0.351 0.175 0.168 0.084 0.302 0.309 0.747 0.414 0.423 0.307 0.336 0.284 0.248 0.406 0.432

1.217 ± 0.024 1.111 ± 0.001 1.150 ± 0.003 1.102 ± 0.003 1.077 ± 0.003 1.087 ± 0.004 1.081 ± 0.004 1.072 ± 0.003 1.087 ± 0.002 1.070 ± 0.002 1.117 ± 0.002 1.082 ± 0.003 1.054 ± 0.004 – 1.084 ± 0.003 1.039 ± 0.002 1.051 ± 0.003 1.032 ± 0.004

2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11 2006-04 2006-08 2006-11

0.713 1.41 1.23 1.56 1.52 1.58 0.646 1.57 1.98 1.05 1.74 1.78 0.981 1.46 1.06 1.60 1.42 1.54 1.92 1.73 1.68

1.319 ± 0.004 1.273 ± 0.007 1.327 ± 0.003 1.392 ± 0.010 1.426 ± 0.002 1.447 ± 0.002 1.248 ± 0.003 1.146 ± 0.002 1.197 ± 0.003 1.400 ± 0.004 1.296 ± 0.002 1.353 ± 0.003 1.047 ± 0.003 1.048 ± 0.001 1.059 ± 0.003 1.250 ± 0.002 1.161 ± 0.002 1.203 ± 0.002 1.209 ± 0.005 1.149 ± 0.001 1.156 ± 0.004

2007-05 2007-10 2007-05 2007-10 2007-05 2007-10 2007-10 2007-10 2007-10 2007-10

3.32 2.72 2.94 1.48 1.07 0.723 5.03 0.134 0.147 14.5

0.934 ± 0.007 1.009 ± 0.002 0.988 ± 0.008 1.008 ± 0.006 1.146 ± 0.025 1.312 ± 0.006 1.278 ± 0.002 1.079 ± 0.009 1.022 ± 0.028 1.154 ± 0.002

2007-05 2007-05 2007-05 2007-11 2007-11 2007-11

3.48 0.689 1.53 64.3 151 108

1.877 ± 0.024 1.644 ± 0.056 1.156 ± 0.017 3.728 ± 0.004 3.984 ± 0.008 1.620 ± 0.002

–: not measured, ⁎: spring, ⁎⁎: abandoned coal mine drainage.

Fig. 2. Plots of (a) Mg/Na and (b) U/Na versus Ca/Na, showing a mixing trend between silicate and carbonate end-members. Data for world rivers are from Gaillardet et al. (1999) and Chabaux et al. (2003).

In a plot of Mg/Na versus Ca/Na (Fig. 2a), the values for the total river water samples collected in both our present and previous studies (Ryu et al., 2007, 2008) fall between those for silicate and carbonate

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end-members. The NHR values cluster toward the silicate endmember, whereas the SHR values occur close to the carbonate endmember. A similar trend is exhibited in a plot of U/Na versus Ca/Na (Fig. 2b). This is also seen with the Sr isotope ratios, consistent with the data published by Ryu et al. (2007, 2008). In addition, similar to river water samples, the most radiogenic Sr isotope composition is observed in groundwaters from the silicate-rich NHR basin, whereas groundwater samples from the carbonate-rich SHR basin have lower Sr isotope values (Table 1). Deep groundwater samples in the SHR basin have a much higher Sr concentration and lower 87Sr/86Sr ratio than those in the NHR basin. The results are consistent with the predominance of silicate weathering in the NHR and carbonate weathering in the SHR.

single tributary of the SHR that entirely drains a silicate catchment, and its 234U/238U activity ratio of 1.05 is comparable to those of the NHR (Table 2). In comparison with other world rivers, the average 234 U/238U activity ratio (1.28) of the SHR draining a carbonate basin is similar to that of the Yellow (1.32), Mackenzie (1.40), and Mississippi (1.31) rivers, which also drain carbonate basins (Chabaux et al., 2001 and references therein; Vigier et al., 2001). In contrast, the low 234U/ 238 U activity ratio of 1.09 for the NHR is similar to that of rivers in the Ganges–Brahmaputra river system (1.03), which drain High Himalayan Crystalline (HHC) and Lesser Himalaya (LH) units among the main structural units of the Himalayas (Chabaux et al., 2001).

4.2. Uranium isotopes

5.1. Seasonal variation

The U concentrations and 234U/238U activity ratios of the dissolved loads in the Han River basin are listed in Table 2. Although there is seasonal and spatial variation in U concentrations in the Han River, the average U concentration in the SHR (1.44 ± 0.36 nM) is similar to that in the Yangtze River (1.90 nM) and the Mississippi River (1.30 nM), but much lower than that in the Ganges (16.70 nM) and the Indus (20. 8 nM) rivers, which drain the Himalayas (Palmer and Edmond, 1993; Chabaux et al., 2001). The average U concentration in the NHR (0.34 ± 0.14 nM) is similar to that in other rivers that drain zones such as the Amazon (0.14 nM) and Orinoco (0.10 nM) rivers, which drain the Andes (Palmer and Edmond, 1993). The NHR, with a silicate-dominated basin, is characterized by small 234 U/238U activity ratios between 1.032 and 1.217, with an average of 1.09. In contrast, the SHR, which drains carbonates, has large 234U/238U activity ratios that range from 1.156 to 1.447 (average: 1.28), with the exception of one sample (S-5-1). This sample was collected from a

In general, the dissolved U concentration in river water is lower during high-water stages than during low-water stages (Sarin et al., 1990). Given the very low U concentration in rainwater, rainfall should cause a decrease in the U concentration of river water (Riotte and Chabaux, 1999). However, little or no decrease occurred in the Han River basin (Fig. 3). The substantial variability in the U content and 234U/238U activity ratio can be interpreted in terms of rainfall, runoff, dam effects, tributary water mixing, and groundwater flux. During spring (a time of increased rainfall), a much larger variance is observed in the U content and 234U/238U activity ratio of the NHR, whereas that of the SHR appears to be equally variable in all seasons (Fig. 3). The greater variability in the NHR seems to occur because increasing discharge during spring releases a major portion of the 234U from superficial soil horizons (Riotte and Chabaux, 1999). In contrast, the equal variability in the SHR during all seasons could be ascribed to the fact that a

Fig. 3. Plots of (a) U content and (b)

234

5. Discussion

U/238U activity ratio versus runoff, showing a weak positive correlation in the NHR for summer samples.

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significant part of the dissolved U transported by the SHR comes from groundwater in contact with U-bearing bedrocks regardless of season. This pattern in the SHR conflicts with the result of Snow and Spalding (1994), who found that only dissolved U associated with low flow (groundwater sources) tended toward higher U concentrations and activity ratios. The weak positive correlation between U concentration and runoff for the NHR in summer suggests that the U concentration is related to mechanical weathering (Fig. 3; Kronfeld et al., 2004). Since six dams in the NHR have a great influence on physicochemical weathering in the Han River (Ryu et al., 2008), increased physical weathering and the resulting long duration in dams could increase the number of alpharecoil fractures, allowing for an increased rate of 234U removal (e.g., Kronfeld et al., 2004; Robinson et al., 2004; Grzymko et al., 2007). Additional study of the 234U/238U activity ratio of the suspended loads is needed to investigate this hypothesis. As previous studies have suggested (Sarin et al., 1990; Chabaux et al., 2001; Grzymko et al., 2007), differences in the U concentrations of tributaries (Samples N-61, S-3-1, and S-5-1) and in the 234U/238U activity ratios due to lithological differences in drainage areas seem to control the observed seasonal variation (Fig. 4). 5.2. Spatial variation The NHR and SHR show large differences in the 234U/238U activity ratios of the dissolved loads (Figs. 3 and 4). In the SHR, the values far exceed unity, but in the NHR, the values are only marginally above unity. In particular, the dissolved U content and 234U/238U activity ratio become higher when water flows over carbonate terrain (Fig. 4), showing a trend similar to that already observed for major ions and Sr isotopes (Ryu et al., 2007). This variation seems to be linked to lithological changes and is well correlated with the alkalinity (not

Fig. 4. Plot of variation in the U content and

234

359

shown). The higher U content in the SHR might be closely related to the high alkalinity because the solubility of U is greatly increased by the soluble uranyl carbonate complex (Mangini et al., 1979). Uranyl carbonate complex is the principal species in solution at intermediate Ehs, with neutral to alkaline pHs in the presence of carbonate, and sorption processes could have a greater control of dissolved U fluxes in rivers than mineral precipitation or dissolution (Langmuir, 1978 and references therein). Our results are quite different from those reported by Sarin et al. (1990). They reported that small 234U/238U disequilibrium resulted from the dominant weathering of carbonate rocks. In contrast, high 234 U/238U disequilibrium resulted from the dominant weathering of silicate rocks, which is generally supposed to produce significant preferential leaching of 234U into the weathering fluids (Sarin et al., 1990). The U content of the Paleozoic limestones in the SHR basin ranges from11.6 to 13.9 μg/g, with an average of 12.6 μg/g (n = 5; Kwon and Park, 1993), whereas the U content of the granites and gneisses in the NHR basin varies between 0.6 and 11.5 μg/g, with an average of 3.6 μg/g (n = 14; Park et al., 1993). Even though the range of U concentrations for the carbonates and silicates somewhat overlaps, high U concentrations in the SHR could be partly ascribed to the high U content of carbonates. In addition, the black shales that are widely distributed in the Ogcheon Fold Belt (OFB) of the SHR are high (up to 308 μg/g) in U content (Lee et al., 1986; Kim, 1989; Chon and Jung, 1991). The weathering of these U-bearing black shales and slates can readily account for the increase in U concentrations in the SHR, similar to that observed in the Himalayan Rivers (Chabaux et al., 2001).and the Platte River (Snow and Spalding, 1994). If an anthropogenic contribution such as phosphate fertilizer input had been significant, we would expect to see a clear relationship between phosphate and U concentrations (Conceição and Bonotto,

U/238U activity ratio in Han River water along the flow path of the river, showing seasonal and spatial variation.

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2003). However, phosphate was not detected in the Han River water, even that flowing through highly fertilized zones. 5.3. Role of groundwater input Contrary to the general theory of Sarin et al. (1990), several recent studies (e.g., Snow and Spalding, 1994; Riotte and Chabaux, 1999; Chabaux et al., 2001; Riotte et al., 2003; Grzymko et al., 2007) of localto large-scale watersheds around the world have found that high 234U/ 238 U activity ratios in stream or river water result from inputs of groundwater from deep sources involving water–rock interactions. In this situation, the inputs from bedrock and deep horizons of the weathering profiles, as well as from superficial soil horizons, have significant effects on the 234U/238U activity ratios of river water. Generally, an enhanced U activity ratio in groundwater can arise from the α-recoil process, which would require either very large U concentrations at the water–rock interface or very long residence times, or it can arise from a steady-state chemical etch/leach process within relatively short timescales (Dickson and Davidson, 1985; Bonotto and Andrews, 1993, 2000; Luo et al., 2000; Durand et al., 2005). To investigate possible impacts on the 234U/238U activity ratio in the Han River system, we collected samples from springs, drainage from an abandoned coalmine, and groundwater from the Han River basin (Table 2). The 234U/238U activity ratios were much higher in the groundwater samples collected from the SHR basin than in those from the NHR basin. Interestingly, the two spring-water samples (NG-1 and NG-2) had 234U/238U activity ratios below unity due to the migration front. That is, the previously stabilized accumulation of the absorbed U becomes mobilized, and this U experiences recoil-related loss of 234U and exhibits a low 234U/238U activity ratio in spring-water (Cowart and Osmond, 1980). Spring-water and groundwater samples from the NHR basin exhibit seasonal variation, with high U concentrations and low 234 U/238U activity ratios in the rising-water stage, but opposite trends in the falling-water stage (Table 2). Generally, the preferential leaching of radiogenic 234U relative to 238 U from silicates has been well documented (Sarin et al., 1990 and references therein). In contrast, the dissolution of aquifer rocks tends to decrease the 234U/238U ratio in groundwater because this ratio in rocks is close to unity. Precipitation lowers the U concentration in groundwater, but has little effect on 234U/238U ratios. Therefore, the relatively low U concentrations and 234U/238U ratios of groundwaters in the NHR basin suggest that dissolution and precipitation exert an important control on the U isotopes in the groundwater (Luo et al., 2000). Even though the flux of groundwater and spring-water could affect the U concentrations and 234U/238U activity ratios in the NHR, the effect would not be intense because the silicate basement of the NHR basin has low water storage capability and low fluid infiltration (Lee et al., 2000). Consequently, the relatively near-equilibrium values of the 234U/238U activity ratio in the NHR water are thought to result from either a combination of the incongruent silicate weathering and the total congruent dissolution of weathered materials from which most of the mobile 234U has already been lost (Sarin et al., 1990) or surface and meteoric weathering of the bedrock (Riotte et al., 2003). In contrast, the 234U/238U activity ratios were significantly higher in the groundwater samples of the SHR basin than in the groundwater samples of the NHR basin. In addition, the 234U/238U activity ratios in deep groundwater (SG-4 and SG-5) collected from the SHR basin were much higher than those of shallow groundwater (SG-1, SG-3, and SG-6) and abandoned coalmine drainage (SG-2). This result, together with the observation that the dissolved U in the SHR originates predominantly from U-rich lithologies, such as black shales, emphasizes that the higher U activity ratios in the SHR could be ascribed to the α-recoil process because the estimated pH and Eh of the black shales suggest a euxenic marine to organic-rich saline water environment in the OFB during U deposition (Lee et al., 1986). Furthermore, this is

Fig. 5. Plots of (a) 87Sr/86Sr versus 234U/238U activity ratio and (b) U/Na versus 234U/238U activity ratio in the Han River water, showing an evolution trend of the river water.

probably because groundwater generally flows much more slowly and because U may encounter reducing conditions and precipitate in deep aquifers (Kronfeld et al., 2004). Even at shallow depths, oxygenated groundwater α-recoil inputs have been shown to increase the 234U/ 238 U activity ratio directly as a function of residence time (Rogojin et al., 1998) or of a steady-state chemical etch/leach process (Bonotto and Andrews, 1993, 2000). Given that these signatures also exist in the groundwater collected from the SHR basin, groundwater flux could have a great impact on the SHR water because the thick carbonate/ sedimentary rock sequence, which has good reservoir capacity, could easily contribute groundwater to stream flow. A plot of the 234U/238U activity ratio versus the 87Sr/86Sr ratio (Fig. 5a) shows the evolution trends of the U and Sr isotopic compositions in the Han River. There are two end-members: one with a higher 234U/238U activity ratio and lower 87Sr/86Sr ratio and the other with a lower 234U/238U activity ratio and higher 87Sr/86Sr ratio. Two deep groundwater samples (SG-4 and SG-5) collected from karst terrain in the SHR basin have much higher 234U/238U activity ratios and lower 87Sr/86Sr ratios than do river water. Notably, sample SG-2, which was collected from an abandoned coalmine, deviates markedly from the evolution line, possibly forming another minor evolution trend. This sample is characterized by higher 234U/238U activity and 87 Sr/86Sr ratios than any of the river water samples. Perhaps the deep groundwater is an extension of the pathway to which the SHR water is evolving, consistent with the weathering of U from carbonate host rock. Surface water in karst terrain is often closely connected to the associated groundwater systems of karst aquifers because of the numerous caves, conduits, and sinkholes (Palmer, 1990). Consequently, river water that drains karst terrain is expected to be strongly influenced by groundwater inputs. Such a similar evolution trend is also apparent in a plot of the 234U/238U activity ratio versus U/Na (Fig. 5b).

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6. Conclusions We focused mainly on factors controlling the 234U–238U fractionation in the Han River system. The variation in the U concentrations and 234 U–238U activity ratios appears to be related to variation in the supply of groundwater and surface water. During spring (a time of increased rainfall), the much greater variability of dissolved U in the NHR is ascribed to the preferential leaching of 234U from the superficial soil horizon by increasing discharge, whereas the equal variability in the SHR during all seasons is attributable to the effect of groundwater in contact with U-bearing bedrocks. The increased mechanical weathering by high runoff seems to account for the variability in dissolved U in summer, although additional studies are needed to test this hypothesis. Low U content and 234U/238U activity ratio in the NHR are attributed to a combination of meteoric and surface weathering of silicates and congruent dissolution of already weathered materials with a minor flux of groundwater. In contrast, the SHR is characterized by higher U content and 234U/238U activity ratio. This result is opposite to what might be expected from congruent carbonate dissolution alone. Therefore, the higher U concentration in the SHR originates predominantly from U-rich lithologies, such as black shales, and the higher 234U–238U activity ratios in the SHR strongly point to a significant contribution from the α-recoil process or the steady-state chemical etch/leach process of deep groundwater in contact with uraniferous rocks. Acknowledgements We would like to thank Y.J. Jeong for valuable discussion concerning U isotope analysis, Y. Park and W.J. Shin for assistance in the field, and H.S. Shin, M.S. Yi and D.C. Koh for help with sample analysis. Our work was supported by a KBSI grant (N28052) to K.S. Lee and by a grant (code 3-2-3) from the Sustainable Water Resources Research Center of 21st Century Frontier Research. References Bonotto, D.M., Andrews, J.N., 1993. The mechanism of 234U/238U activity ratio enhancement in karstic limestone groundwater. Chem. Geol. 103, 193–206. Bonotto, D.M., Andrews, J.N., 2000. The transfer of uranium isotopes 234U and 238U to the waters interacting with carbonates from Mendip Hills area (England). Appl. Radiat. Isotopes 52, 965–983. Bourdon, B., Henderson, G.M., Lundstrom, C.C., Turner, S.P., 2003. Uranium-series geochemistry. Geochemical Society, Mineralogical Society of America, Washington. Chabaux, F., Riotte, J., Clauer, N., France-Lanord, Ch., 2001. Isotopic tracing of dissolved U fluxes of the Himalyan rivers: implications for present and past U budgets of the Ganges–Brahmaputra system. Geochim. Cosmochim. Acta 65, 3201–3217. Chabaux, F., Riotte, J., Dequincey, O., 2003. U–Th–Ra fractionation during weathering and river transport. In: Bourdon, B., Henderson, G.M., Lundstrom, C.C., Turner, S.P. (Eds.), Uranium-series Geochemistry. Geochemical Society, Mineralogical Society of America, Washington, pp. 533–576. Chae, K.T., Yun, S.T., Kim, K.H., Lee, P.K., Choi, B.Y., 2004. Atmospheric versus lithogenic contribution to the composition of first- and second-order stream waters in Seoul and its vicinity. Environ. Int. 30, 73–85. Chen, J.H., Lawrence, E.R., Wasserburg, G.J., 1986. 238U, 234U and 232Th in seawater. Earth Planet. Sci. Lett. 80, 241–251. Cheong, C.H., 1969. Stratigraphy and paleontology of the Samchang coalfield, Gangweondo, Korea. J. Geol. Soc. Korea 5, 13–56 (in Korean with English abstract). Chon, H.T., Jung, M.C., 1991. Dispersion of toxic elements in the area covered with uranium-bearing black shales in Korea. J. Korean Inst. Mining Geol. 24, 245–260 (in Korean with English abstract). Conceição, F.T., Bonotto, D.M., 2003. Use of U-isotope disequilibrium to evaluate the weathering rate and fertilizer-derived uranium in São Paulo state, Brazil. Environ. Geol. 44, 408–418. Cowart, J.B., Osmond, J.K., 1980. Uranium isotopes in groundwaters as a prospecting technique. U.S. Dept Energy Report GJBX 119. Dickson, B.L., Davidson, M.R., 1985. Interpretation of 234U/238U activity ratios in groundwaters. Chem. Geol. 58, 83–88. Durand, S., Chabuax, F., Rihs, S., Duringer, P., Elsass, P., 2005. U isotope ratios tracers of groundwater inputs into surface waters: example of the Upper Rhine hydrosystem. Chem. Geol. 220, 1–19. Gaillardet, J., Dupré, B., Louvat, P., Allègre, C.J., 1999. Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers. Chem. Geol. 159, 3–30.

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