Sources and flux of trace elements in river water collected from the Lake Qinghai catchment, NE Tibetan Plateau

Applied Geochemistry 25 (2010) 1536–1546

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Sources and flux of trace elements in river water collected from the Lake Qinghai catchment, NE Tibetan Plateau Zhangdong Jin a,b,⇑, Chen-Feng You c, Tsai-Luen Yu c, Bo-Shian Wang c a

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China School of Human Settlement and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China c Earth Dynamic System Research Center, National Cheng Kung University, Tainan 70101, Taiwan b

a r t i c l e

i n f o

Article history: Received 30 November 2009 Accepted 2 August 2010 Available online 9 August 2010 Editorial handling by R. Fuge

a b s t r a c t River waters play a significant role in supplying naturally- and anthropogenically-derived materials to Lake Qinghai, northeastern Tibetan Plateau. To define the sources and controlling processes for river water chemistry within the Lake Qinghai catchment, high precision ICP-MS trace element concentrations were measured in water samples collected from the Buha River weekly in 2007, and from other major rivers in the post-monsoon (late October 2006) and monsoon (late July 2007) seasons. The distributions of trace elements vary in time and space with distinct seasonal patterns. The primary flux in the Buha River is higher TDS and dissolved Al, B, Cr, Li, Mo, Rb, Sr and U during springtime than those during other seasons and is attributed to the inputs derived from both rock weathering and atmospheric processes. Among these elements, the fluxes of dissolved Cr, B and Rb are strongly influenced by eolian dust input. The fluxes of dissolved Li, Mo, Sr and U are also influenced by weathering processes, reflecting the sensitivity of chemical weathering to monsoon conditions. The anthropogenic sources appear to be the dominant contribution to potentially harmful metals (Ni, Cu, Co, Zn and Pb), with high fluxes at onset of the main discharge pulses due, at least partially, to a runoff washout effect. For other major rivers, except for Ba, concentrations of trace elements are higher in the monsoon than in the post-monsoon season. A total of 38.5 ± 3.1 tons of potentially harmful elements are transported into the lake annually, despite human activities within the catchment being limited. Nearly all river water samples contain dissolved trace elements below the World Health Organization guidelines for drinking water, with the exception of As and B in the Daotang River water samples collected in late July probably mobilized from underlying lacustrine sediments. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The spatial and temporal variability of trace element cycles is the coordination of chemical, mechanical, and biological processes (Drever, 1997; Tribovillard et al., 2006). Since many trace elements are transported in rivers as either soluble or adsorbed phases, characterization of water chemistry draining terrain is crucial to precisely identify the various contributions to the waters of the different sources. On the one hand, trace elements hold great potential for understanding geochemical processes because of their individual sources or characteristic behaviors. Assessment of trace element behavior in various drainage systems encompassing a diverse range of lithology, climate and relief is helpful to improve knowledge of the processes which govern trace element distribution and biogeochemical cycles in aqueous systems and to better ⇑ Corresponding author at: State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China. Tel.: +86 29 88329660; fax: +86 29 88320456. E-mail address: [email protected] (Z. Jin). 0883-2927/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2010.08.004

constrain chemical budgets into the lakes/oceans. On the other hand, anthropogenically derived trace elements in waters (Nriagu, 1996) may exceed the standards established by various national governments for the protection of aquatic life and for drinking water (WHO, 2004), some toxic or harmful ones having significant effects on biological vitality and toxicity. Therefore, understanding sources, fluxes and variation of trace elements is very important in understanding environmental processes. Lake Qinghai (3194 m a.s.l., 4260 km2, Fig. 1), the largest lake on the Tibetan Plateau, is an outstanding, world-class site for high resolution of paleo-records available in sediments and is attracting increasing attention. Previous substantial researches on paleoenvironmental changes recorded in sediments of Lake Qinghai are reviewed in detail by Colman et al. (2007) and Henderson and Holmes (2009). To better understand the significant implications of the sediments, a complete picture of the material sources and their contributions is needed (Jin et al., 2009a, 2010b). In a hydrologically closed basin such as Lake Qinghai, rivers play a pivotal role in transferring dissolved components to the lake. It has been observed that the major rivers within the catchment have

Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546

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Fig. 1. Map showing the main substrate lithology and prominent structures of the Lake Qinghai catchment (modified from Zhang et al. (2009)). Also shown are the locations of river water and loess samples, and the Buha River Hydrological Station.

pronounced temporal and spatial variability in dissolved major elements, primarily in response to the lithologies they drain (Jin et al., 2009b). Meanwhile, an elemental input–output model of Lake Qinghai has shown that atmospheric input is important in budgets of the lake with an estimated 65% of total annual input being from dust deposition (Jin et al., 2009a). During recent decades, anthropogenic activities have introduced significant amounts of potentially harmful metals into the aqueous environment via runoff and/or direct atmospheric deposition. Potentially harmful metals on eolian surfaces can be transported long distances (Nriagu, 1989) and are susceptible to dissolution in water after deposition (Grousset et al., 1995). Therefore, this atmospheric input is a potential pollution source in Lake Qinghai (Jin et al., 2010a). However, knowledge about sources, fluxes and seasonal variation of trace elements, in particular those of atmospheric input, into Lake Qinghai is sparse. The selected study area is ideally suited to assess the different factors that control trace element transportation by rivers, because of (i) its seasonally alternating climatic conditions with springs having significant winds from the NW (Fig. 2a), mild summers which are sensitive to monsoons with easterly sources (Fig. 2b) and dry winters, (ii) the absence of any mineral exploitation, dams or reservoirs within the catchment, and (iii) its sparse population, minimizing the effects caused by local human activities. Through a detailed geochemical investigation of river waters from the Lake Qinghai catchment, this study addresses (i) variations of dissolved trace elements in river waters on a seasonal basis, (ii) potential

sources and processes controlling their seasonal variations, and (iii) annual flux of anthropogenic potentially harmful elements to the lake.

2. Sampling and analytical methods Lake Qinghai (36°320 –37°150 N, 99°360 –100°470 E) is located on the northeastern margin of the Tibetan Plateau (Fig. 1), with a catchment area of 29,660 km2. The geological and geographical setting and the modern climate are reviewed in detail by Colman et al. (2007) and Jin et al. (2010b). The six largest rivers ranked by discharge within the Lake Qinghai catchment are the Buha, Shaliu, Hargai, Quanji, Daotang and Heima Rivers. The rivers have, as yet, not been regulated by dams or other hydro-technical works and are strongly influenced by the hydrological cycle associated with the monsoons. These rivers represent more than 87% of the water, dissolved load and sediment discharge to the lake, more than half being from the Buha River (Jin et al., 2009b). To better assess the importance of seasonal fluxes of trace elements from rivers into the lake, 53 water samples from the Buha River were collected weekly at the Buha River Hydrological Station in 2007. Six snow and seven rain water samples were collected during 2007 and 2008 at the station. The sampling methods for the waters, including snow, after melting at room temperature, and rainwater, were those given in Jin et al. (2009b). Twelve water samples from the lower reaches of the six major rivers (two each)

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Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546

Fig. 2. Air mass HYSPLIT backward trajectory (500, 1000, and 2000 m above the ground level), indicating the origin of typical air masses observed at Lake Qinghai in (a) spring (1 May 2007) and (b) summer (29 July 2007), with a time interval of 6 h between two marks on the trajectory tracks, reflecting different velocity. The main moisture sources arriving at Lake Qinghai mainly came from the NW during the springtime and from East China during the summer, respectively.

were also included in this study (Jin et al., 2009b). These samples were collected in the post-monsoon season (late October 2006) under low-flow conditions and in the monsoon season (late July 2007) under high-flow conditions. The extent of the sample coverage is illustrated in Fig. 1. Six samples of fresh loess were collected from extensive sites within the catchment (Fig. 1), for XRF analyses. Trace element analyses of waters were conducted in the Isotope Geochemistry Laboratory at the National Cheng Kung University (Taiwan), using a high resolution sector field inductively coupled plasma mass spectrometer (Element II, Thermo Fisher Scientific). The instrument is equipped with a quartz torch and CD-1 Guard Electrode and three selectable resolutions, low, medium and high with m/Dm = 300, 4000 and 10,000, respectively. Sample introduction was performed using a PFA-50 nebulizer and a quartz dual spray chamber. Operational parameters (i.e., torch position and Ar gas flow rate) were optimized daily to obtain the best stability and maximum sensitivity before sample analysis. The analysis was conducted with sensitivity higher than 106 counts/s for 10 ng/mL In under medium resolution. The accuracy and the precision of the analytical methods were monitored using SLRS-4 as a standard and using NIST1640 and NIST1643e as working standards to double check. Analysis of standard materials gave values of 97– 102% of the certified values for all of the trace elements. The detection limits for trace elements, determined to be three times the standard deviation of a filter blank, were Mn 0.09, Fe 2.0, Co 0.06, Cu 1.0, Zn 1.0, Mo 0.2, Ni 0.5, and Cd 0.01 nmol/L, respectively. More than 95% of the concentrations measured in this study were at least one order of magnitude higher than the detection limits. The concentrations of Co, Cr, Cu, Ni, Pb, and Zn in bulk loess were measured by an Axios advanced wavelength dispersive Xray fluorescence (WD-XRF; PANalytical, Ea Almelo, The Netherlands) at the Institute of Earth Environment, Chinese Academy of Sciences. Samples were air-dried, then ground and homogenized in an agate mortar. Five gram of each sample was compacted into a disc with 32 cm diameter under 30 tf and then scanned using XRF. Calibration was done by compared with international reference samples (15 soil samples (GSS 1–7 and 9–16) and 11 stream sediment samples (GSD 1–11)), the recovery rates of each element for reference samples being Co 100.6%, Cr 100.8%, Cu 97.3%, Ni

102.0%, Pb 108.4%, and Zn 104.1%. Analytical uncertainties, as verified by parallel analysis of two international standards (GSS-8, GSD-12), were better than 2%. 3. Results and discussions The concentrations of dissolved major elements in waters from the Lake Qinghai rivers are reported elsewhere (Jin et al., 2009a,b) with relevant data summarized below. The waters are alkaline, with pH values ranging from 7.94 to 8.53 for the Buha River, and from 8.41 to 9.41 in July and from 8.41 to 9.28 in October for other rivers. The total dissolved solids (TDS) and major elements varied significantly among samples, depending on the season and dominant lithology in the tributaries. The TDS was generally higher at low flow, dominated by carbonate weathering. The highest value was observed in the Daotang River, Na–Ca–HCO3 waters, draining mainly previously deposited lacustrine sediments. The lowest value was observed in the Quanji River draining mainly sandstone and granitic rocks (Fig. 1). Low dissolved Si (averaging 93 lmol/ L) reflects a low intensity of silicate weathering. 3.1. Dissolved trace elements Concentrations of trace elements in Buha water samples collected weekly in 2007 are reported in Table 1, and those from other sites in Table 2. It should be noted that some of the dissolved Cd and Co concentrations listed in Tables 1 and 2 are close to the instrumental detection limits. Principal component analysis (PCA) was carried out for trace element concentrations in the Buha River waters to explore element associations and their origins (Gupta and Subramanian, 1998). Four principal components (PCs) were extracted from 18 trace elements and TDS. The PC1 has high loadings of TDS, Al, B, Ba, Cr, Mo, Rb, Sr, and U (hereafter referred to as PC1-elements), PC2 has Co, Cu, Ni, Pb, and Zn (PC2-elements) and PC3 has As and Cd (Table 3). Lithium, Ba, and Cd have negative loadings while Li has relatively high loadings for both PC1 and PC2. An exception is Mn, which is an exclusive element in PC4. The four PCs together explain about 80% (PC1 36.3%, PC2 26.8%, PC3 8.8%, and PC4 6.8%)

Table 1 Dissolved concentrations of trace elements in the Buha River water samples collected weekly at the Buha River Hydrological Station in 2007. Sampling date

TDS (mg/L)

Al (lg/L)

As (lg/L)

B (lg/L)

Ba (lg/L)

Cd (lg/L)

Co (lg/L)

Cr (lg/L)

Cu (lg/L)

Fe (lg/L)

Li (lg/L)

Mn (lg/L)

Mo (lg/L)

Ni (lg/L)

Pb (lg/L)

Rb (lg/L)

Sr (lg/L)

U (lg/L)

Zn (lg/L)

BH07-01 BH07-02 BH07-03 BH07-04 BH07-05 BH07-06 BH07-07 BH07-08 BH07-09 BH07-10 BH07-11 BH07-12 BH07-13 BH07-14 BH07-15 BH07-16 BH07-17 BH07-18 BH07-19 BH07-20 BH07-21 BH07-22 BH07-23 BH07-24 BH07-25 BH07-26 BH07-27 BH07-28 BH07-29 BH07-30 BH07-31 BH07-32 BH07-33 BH07-34 BH07-35 BH07-36 BH07-37 BH07-38 BH07-39 BH07-40 BH07-41 BH07-42 BH07-43 BH07-44 BH07-45 BH07-46 BH07-47 BH07-48 BH07-49 BH07-50 BH07-51 BH07-52 BH07-53

06-12-31 07-01-07 07-01-14 07-01-21 07-01-28 07-02-04 07-02-11 07-02-18 07-02-25 07-03-04 07-03-11 07-03-18 07-03-25 07-04-01 07-04-08 07-04-15 07-04-22 07-04-29 07-05-06 07-05-13 07-05-20 07-05-27 07-06-03 07-06-10 07-06-17 07-06-24 07-07-01 07-07-08 07-07-15 07-07-22 07-07-29 07-08-05 07-08-12 07-08-19 07-08-26 07-09-02 07-09-09 07-09-16 07-09-23 07-09-30 07-10-07 07-10-14 07-10-21 07-10-28 07-11-04 07-11-11 07-11-18 07-11-25 07-12-02 07-12-09 07-12-16 07-12-23 07-12-30

331.5 315.7 326.0 325.3 317.9 312.7 293.2 312.7 444.8 438.5 445.6 428.2 306.9 428.7 410.9 387.6 404.2 413.1 406.8 387.4 355.9 325.5 343.7 281.2 251.6 258.8 254.9 245.0 263.6 289.8 290.4 303.6 294.5 334.2 330.8 355.2 336.1 335.8 330.0 327.9 347.7 328.1 331.9 350.4 334.8 338.0 328.5 332.9 309.6 317.9 324.7 318.3 313.9

9.06 9.28 9.04 7.84 7.64 11.16 9.85 9.04 20.48 12.44 22.66 17.18 18.03 14.98 17.61 20.82 17.78 14.96 17.60 15.96 18.76 20.98 18.49 23.42 21.95 22.80 24.81 21.98 22.80 18.58 20.28 20.50 22.45 25.20 30.52 19.43 29.03 23.22 25.02 25.57 17.01 21.95 16.05 22.05 24.39 29.26 20.35 19.92 23.41 24.12 28.43 17.47 18.24

1.15 1.41 1.11 1.21 1.15 1.14 1.22 1.25 2.31 2.23 2.07 1.99 1.13 2.02 1.85 1.77 1.76 1.67 2.20 1.79 1.84 1.84 1.80 1.89 1.86 1.93 1.89 1.90 1.86 1.83 1.85 1.58 1.71 1.75 1.67 1.47 1.64 1.47 1.79 1.69 1.54 1.55 1.70 1.53 1.44 1.38 1.48 1.47 1.38 1.36 1.25 1.31 1.16

129.8 124.6 134.7 137.6 139.1 136.6 129.4 126.7 357.1 358.6 407.8 406.7 132.7 343.3 325.5 292.7 338.2 322.2 295.2 273.3 283.9 239.7 255.6 138.5 134.1 136.1 113.8 127.5 113.0 146.9 186.1 184.5 179.6 165.9 163.8 150.2 154.7 162.0 148.0 145.2 147.7 146.7 146.8 140.6 146.2 146.0 136.8 134.4 133.7 139.6 130.4 131.8 128.9

86.2 85.6 85.2 87.3 88.2 86.8 85.4 85.5 53.4 54.0 52.4 52.3 82.9 53.6 54.4 56.3 55.2 56.3 55.8 57.8 56.7 56.7 58.4 62.9 66.4 64.1 60.3 61.5 56.5 62.6 68.5 75.0 74.9 72.6 73.6 78.0 72.3 71.3 72.4 71.6 77.6 72.2 77.0 83.9 51.0 52.8 88.3 86.2 83.7 84.5 84.5 83.9 82.2

0.04 0.20 0.04 0.12 0.03 0.06 0.01 0.02 0.04 0.04 0.02 0.02 0.03 0.03 0.02 0.02 0.02 0.01 0.02 0.01 0.02 0.02 0.03 0.03 0.03 0.04 0.03 0.01 0.02 0.03 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.02 0.01 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01

0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.04 0.04 0.05 0.05 0.04 0.04 0.04 0.04 0.05 0.06 0.07 0.05 0.04 0.04 0.08 0.07 0.07 0.07 0.06 0.04 0.05 0.05 0.04 0.04 0.03 0.07 0.08 0.03 0.03 0.03 0.03 0.02 0.03 0.04 0.04 0.03 0.02 0.02 0.03 0.03 0.02 0.02 0.02 0.02 0.02

0.36 0.38 0.38 0.36 0.35 0.38 0.37 0.35 1.73 1.56 1.75 1.86 0.37 1.52 1.40 1.14 1.16 0.99 1.21 0.64 1.10 0.86 1.38 0.30 0.28 0.29 0.29 0.28 0.25 0.26 0.30 0.33 0.33 0.32 0.32 0.30 0.36 0.34 0.31 0.31 0.28 0.30 0.30 0.33 0.34 0.29 0.33 0.36 0.39 0.38 0.35 0.35 0.34

0.42 2.53 1.08 1.10 2.80 3.16 1.12 1.06 0.65 0.37 0.74 0.44 1.55 5.31 4.31 4.30 6.03 8.21 11.20 12.69 5.99 7.40 21.51 16.65 14.53 15.81 19.53 3.42 2.75 1.47 0.84 0.74 0.60 13.95 14.24 0.61 1.47 1.56 1.06 1.08 0.63 1.93 0.69 0.66 2.58 0.34 0.40 0.41 0.32 0.28 0.31 0.24 0.21

10.79 4.87 7.65 15.06 5.69 10.21 4.72 7.71 14.97 38.20 40.64 11.90 5.35 12.78 7.66 9.82 8.96 8.36 17.74 25.75 12.57 20.11 27.91 12.37 9.57 10.14 14.64 9.00 11.62 5.66 11.16 4.87 5.59 9.16 8.35 6.34 4.36 26.59 5.21 6.40 9.18 15.20 7.27 7.33 28.89 21.58 11.23 9.84 9.56 10.46 18.28 4.06 4.79

22.92 24.06 21.79 22.78 22.21 22.05 22.46 22.48 27.54 28.13 25.99 26.38 21.02 24.25 24.62 23.59 25.08 24.85 23.96 23.88 23.04 21.78 21.31 16.40 22.92 24.06 15.95 15.30 13.32 16.49 12.68 23.53 27.10 27.35 27.08 24.59 25.20 24.35 25.17 24.08 25.06 24.19 24.70 23.59 24.58 25.48 24.28 24.58 23.31 22.70 22.88 22.99 22.60

2.89 2.51 1.76 3.01 3.04 2.97 2.55 2.55 1.22 1.66 1.52 1.13 3.96 2.25 2.73 1.64 3.01 3.69 4.55 5.37 1.90 2.24 8.02 3.77 3.21 4.43 4.34 0.82 0.64 4.76 3.90 3.15 3.07 3.97 3.91 3.02 0.41 0.55 0.51 0.46 2.97 0.72 2.98 3.65 4.75 2.62 3.79 3.33 3.78 3.39 3.45 2.93 3.65

0.83 0.89 0.84 0.85 0.85 0.84 0.84 0.83 1.29 1.28 1.35 1.34 0.80 1.20 1.18 1.12 1.20 1.19 1.17 1.15 1.16 1.08 1.15 1.01 0.97 0.99 0.79 0.89 0.68 0.89 1.01 1.05 1.04 1.00 1.01 0.93 0.96 0.92 0.95 0.90 0.94 0.91 0.92 0.98 1.15 1.14 0.98 0.98 0.94 0.94 0.84 0.84 0.84

0.22 0.57 0.35 0.31 0.49 0.47 0.30 0.27 0.27 0.19 0.27 0.23 0.29 0.31 0.36 0.48 0.36 0.38 0.44 0.52 0.55 0.63 0.82 1.03 1.20 1.00 1.22 0.70 0.69 0.37 0.32 0.34 0.27 1.13 1.19 0.35 0.40 0.34 0.39 0.43 0.31 0.45 0.28 0.48 0.25 0.19 0.24 0.23 0.18 0.16 0.20 0.16 0.18

0.04 0.09 0.07 0.12 0.21 0.21 0.07 0.05 0.12 0.06 0.24 0.03 0.06 0.07 0.10 0.04 0.11 0.04 0.16 0.02 0.04 0.08 0.25 0.18 0.15 0.29 1.07 0.19 0.09 0.04 0.02 0.45 0.01 0.07 0.07 0.04 0.01 0.14 0.01 0.13 0.01 0.04 0.02 0.02 0.03 0.04 0.02 0.03 0.07 0.04 0.04 0.03 0.02

0.51 1.01 0.58 0.46 0.55 0.52 0.45 0.44 1.87 1.58 1.76 1.70 0.40 1.42 1.44 1.41 1.40 1.39 1.29 1.34 1.38 1.26 1.16 1.24 0.96 0.99 0.84 0.78 0.67 0.76 0.95 0.87 0.84 1.22 1.23 0.64 0.76 0.76 0.71 0.75 0.66 0.72 0.67 0.57 0.44 0.45 0.48 0.47 0.46 0.44 0.43 0.40 0.39

451.3 451.5 437.9 444.8 447.8 445.2 448.3 448.5 540.0 541.5 522.5 526.7 435.0 503.9 493.6 476.1 506.4 497.1 489.6 476.9 460.6 435.7 452.9 381.3 374.2 371.1 356.5 373.2 349.5 382.2 406.0 435.4 435.6 460.0 459.6 481.6 463.6 460.4 468.1 455.1 491.6 468.7 484.1 489.7 491.6 483.1 479.4 466.3 445.3 446.7 446.5 442.5 440.2

2.31 2.22 2.32 2.36 2.39 2.34 2.40 2.40 3.72 3.71 3.76 3.75 2.38 3.31 3.13 2.94 3.28 3.18 3.17 2.98 2.84 2.55 2.76 1.85 1.81 1.84 1.59 1.79 1.55 1.96 2.15 2.28 2.25 2.52 2.50 2.63 2.53 2.51 2.61 2.51 2.69 2.55 2.65 2.74 3.22 3.11 2.75 2.68 2.50 2.49 2.37 2.38 2.35

2.62 46.66 3.40 2.70 3.88 4.22 3.49 3.06 7.89 4.49 6.02 3.81 6.13 37.01 14.48 15.76 21.87 28.19 49.99 43.58 19.71 24.25 92.19 170.61 176.32 160.38 88.03 4.44 3.69 1.81 3.47 2.56 1.97 71.06 74.26 5.51 4.64 5.10 4.22 3.66 5.28 6.04 2.50 2.30 22.77 130.04 6.61 8.87 55.65 67.99 2.97 3.85 1.95

Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546

Sample

1539

10

–

–

15

–

Table 3 Loading results of principal component analyses (PCA) for TDS and trace elements in waters collected weekly from the Buha River. PC

1

2

3

4

% of variance TDS As Al B Ba

37.25 0.819 0.025 0.650 0.956

27.36 0.368 0.323 0.544 0.084 0.511

8.36 0.228 0.839 0.134 0.129 0.134

6.81 0.133 0.076 0.318 0.089 0.257

0.016

0.809 0.093 0.228 0.046 0.021 0.098

0.015

0.092 0.051 0.019 0.157 0.126 0.004 0.003 0.043

0.837 0.126 0.029 0.114 0.190 0.226 0.171 0.285

Cd

0.658 0.104

Co Cr Cu Fe Li

0.313 0.912 0.022 0.540 0.566

400

70

20

Mn Mo Ni Pb Rb Sr U Zn

0.109 0.939 0.218 0.156 0.828 0.774 0.907 0.109

0.863 0.088 0.918 0.183 0.619 0.367 0.139 0.879 0.587 0.421 0.497 0.309 0.768

0.067 0.093 0.270 0.154 0.236

c

d

b

10 – – WHO guidelinesd

The numbers are the locations of water samples labeled in Fig. 1. Total dissolved solids. Samples collected at the middle reaches of Buha River. World Health Organization guidelines (WHO, 2004).

500

700

3

–

50

2000

–

–

High loadings in each PC are bolded, and higher loadings but negative are bolded and underlined.

a

47.65 4.61 2.84 2.35 1.15 0.79 2.46 4.13 10.75 16.65 0.94 0.97 06-10-17 07-07-20 QH03-2 QH07-2 6

Daotang

6.49 13.80

338.95 558.59

66.7 28.3

0.05 0.15

0.74 1.18

3.06 2.07

3.07 5.42

34.88 45.91

58.93 82.25

70.07 9.01

1.01 1.73

0.67 1.54

1024.6 824.0

1.55 2.97 1.96 1.71 0.47 0.62 1.06 1.13 11.68 14.47 0.32 0.28 06-10-18 07-07-22 QH03-31 QH07-27 5

Quanji

1.05 3.33

50.30 57.22

67.9 71.5

0.02 0.09

0.25 0.25

0.51 0.54

1.44 1.78

7.45 6.71

5.45 6.52

2.29 2.75

0.31 0.50

0.19 0.81

246.8 262.5

1.27 2.75 2.12 1.67 1.12 1.11 0.99 0.97 10.66 16.35 0.36 0.31 06-10-19 07-07-23 QH03-38 QH07-37 4

Hargai

1.66 3.70

106.20 105.37

107.3 108.3

0.02 0.09

0.25 0.26

0.61 0.63

1.33 1.44

5.40 6.24

10.66 12.68

3.04 4.03

0.31 0.41

0.17 0.84

213.7 242.1

1.37 2.89 2.07 1.63 0.37 0.49 0.96 0.98 7.46 17.57 0.33 0.27 06-10-19 07-07-23 QH03-32 QH07-28 3

Shaliu

0.95 4.82

52.14 56.51

89.7 92.5

0.02 0.10

0.25 0.25

0.54 0.56

1.39 1.63

5.73 5.63

5.71 6.22

2.24 2.79

0.33 0.38

0.28 0.97

272.0 286.8

38.63 47.52 4.48 2.84 297.2 434.6 0.91 1.20 2.05 3.13 7.90 8.95 0.40 0.46 06-10-17 07-07-20 QH03-15 QH07-14 2

Heima

2.36 3.98

99.79 69.89

89.2 197.6

0.02 0.08

0.27 0.27

1.98 27.59

1.90 1.80

4.61 69.85

9.30 7.06

2.64 2.92

0.41 0.74

0.37 0.54

2.84 1.86 437.5 353.3 2.88 1.37 1.51 1.24 1.73 3.67 10.52 22.86 0.37 0.29 06-10-18 07-07-21 QH03-21c QH07-19c 1

Buha

3.51 4.62

334.69 185.74

89.4 76.1

0.02 0.11

0.25 0.26

0.61 0.62

1.43 1.78

6.19 9.22

52.87 29.60

0.36 0.53

0.25 1.03

U (lg/L) Sr (lg/L) Rb (lg/L) Pb (lg/L) Ni (lg/L) Mo (lg/L) Mn (lg/L) Li (lg/L) Fe (lg/L) Cu (lg/L) Cr (lg/L) Co (lg/L) Cd (lg/L) Ba (lg/L) B (lg/L) As (lg/L) Al (lg/L) TDSb (g/L) River Sampling date Sample No.a

Table 2 Dissolved concentrations of trace elements in river waters from the Lake Qinghai catchment and WHO guidelines for drinking water.

1.74 3.48

Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546

Zn (lg/L)

1540

of the total variance in the data matrix and the controlling factors of each PC are explained below. Large differences of trace element concentrations exist between seasons and among the rivers (Table 2). The concentrations of Al, As, Ba, Cd, Cr, Cu, Fe, Li, Mn, Mo, Ni, Pb, Sr and Zn in the monsoon season are higher than those in the post-monsoon. The Daotang River has higher concentrations for most trace elements during both seasons; the Heima River has abnormally high Cr and Fe in July, possibly resulting from increased traffic during the tourism season. More than six PCs were extracted from trace elements of water samples from the other six sites, possibly resulting from various sources and lithologies among the river tributaries (Jin et al., 2009b). The concentrations of trace elements measured in rainwater and snow samples have ranges of more than an order of magnitude when all individual samples are considered (Table 4). The variability may be related to individual episodes and/or high evaporation rates, or air sources as shown by air mass back-trajectory (Fig. 2). However, most samples have relatively constant concentrations, within a factor of 2–3 of the average values. Aluminum, B, Ba, Sr, Zn, Cu, Cr, Pb and Fe in rainwater have relatively high concentrations which may be due, at least partially, to the influence of anthropogenic sources. Similarly high values for Cu, Pb and Zn have also been found in coastal regions between the southern Yellow Sea and the East China Sea (Liu et al., 2005). 3.2. PC1-elements: dissolutions of catchment rocks and atmospheric dust Most of the PC1-elements are lithophile, mainly deriving from weathering of catchment rocks and atmospheric dust. Fig. 3 shows seasonal variations of PC1-elements and TDS in the Buha River waters, along with daily water discharge, air temperature, and suspended sediment discharge throughout the river water sampling period. The PC1-elements can be further divided into two groups on the basis of seasonal variations. The first group includes Mo, Sr, U, and Li and their concentrations vary along with TDS; while the second group (Cr, B, and Rb) has a different pattern. The large difference in patterns of the PC1-elements between the two groups is marked by dust contribution and its mediated secondary carbonate precipitation.

83.1 0.22 83.0 1.83 0.71 0.89 0.80 12.57 2.15 11.57 9.58 Note: The abnormal values underlined were excluded in the calculation of the average values.

0.30 0.10 26.6 38.33 2.34 34.5 98.80 Average value

1541

3.2.1. Catchment weathering sources Seasonal variations in TDS, U, Sr and Mo show minima during the onset of the monsoon season and maxima in springtime (Fig. 3), with a similar pattern. The increasing concentrations of TDS, Mo, Sr and U in the Buha River during the monsoon season potentially indicate weathering of marine-deposited strata (late Paleozoic marine limestone) which release carbonate-derived and/or redox sensitive elements. Dissolved concentrations of U and Sr are well correlated (r2 = 0.961 for spring samples and r2 = 0.973 for other seasons, respectively, Fig. 4a), both behaving similarly to the major cations and related to the degree of weathering (Jin et al., 2009b), in agreement with their carbonate-complexes. The sensitivity of bedrock weathering to monsoons has been demonstrated (e.g., Lasaga et al., 1994; Tipper et al., 2006). Remarkable seasonal air temperature and precipitation variations in the studied area make catchment weathering more sensitive to climatic conditions. Water discharge in the Buha River varies by two orders of magnitude between the monsoon and other seasons. In the case of 2007, average river discharge was less than 2 m3/s in the dry season while in the monsoon season it was larger than 200 m3/s (Fig. 3), and about 85% of water flowed between June and October. In addition, average daily air temperature varied distinctly from 25 °C to +13 °C. These conditions lead to the underlying marine sedimentary rocks being subjected to varying physical erosion and chemical weathering, resulting in changing suspended sediment loads (Fig. 3c) and concentrations of mobile elements with seasons (Fig. 4a). At the onset of the monsoon season, decreased concentrations of TDS and most PC1-elements can be attributed to a dilution effect resulting from increased discharge by a factor of up to 30 (Fig. 3). As the monsoon intensifies, the concentrations of TDS, Mo, Sr and U increase coherently along with increased temperature and discharge, reflecting enhanced carbonate weathering under monsoon conditions. After the monsoon season, the concentrations of most PC1-elements stay at a high level and then decline gradually to a steady state along with decreasing discharge, in part related to the concentrating effect during the low water level period.

1.09

10.8 24.3 39.1 5.8 2.8 3.8 0.13 0.04 0.06 0.40 0.56 0.20 43.9 23.0 34.8 63.7 118.7 107.5 0.32 0.31 3.30 5.56 1.87 0.77 0.23 0.40 0.33 0.24 0.25 0.64 0.84 0.31 1.06 1.24 0.96 0.73 0.59 0.64 0.73 0.96 1.21 0.91 4.06 1.58 5.77 24.25 35.76 1.81 0.38 0.52 1.30 8.42 2.84 1.00 7.31 14.76 11.36 28.78 17.74 4.70 17.87 2.60 2.59 3.40 3.87 2.48 0.26 0.23 0.28 0.47 0.38 0.24 0.05 0.03 0.04 0.02 0.03 0.08 07-03-16 07-10-11 07-10-13 08-01-27 08-02-09 08-04-21 Snow QH07-S1 QH07-S2 QH07-S3 3-5-01 3-5-02 3-5-03

39.40 53.89 73.50 122.99 122.86 108.69

44.5 29.6 45.2 46.7 27.5 39.4

0.69 0.78 0.97 2.12 2.65 2.35

6.44 27.59 22.79 36.69 26.88 32.64

11.7 12.3 23.0 10.6 24.4 20.3

0.26 08-10-16 5-F-06

28.6

26.7

3.05

8.32

3.1

0.14

0.48 0.56 1.41 1.53 0.68 0.79

460.3 90.2 0.07 19.8 4.15 1.26 0.69 0.70 3.58 1.58 7.59 2.29

2.57 6.25 3.94 61.09 1.23 1.23 2.16 0.89 0.31 0.26 0.24 0.33 08-06-22 08-06-27 08-07-03 08-07-15 5-F-02 5-F-03 5-F-04 5-F-05

59.0 158.1 114.5 43.1

34.4 45.6 35.1 24.5

3.43 3.27 4.53 2.56

20.15 83.75 41.20 18.95

20.9 83.1 20.9 15.6

0.20 0.12 0.12 0.15

1.47

13.7 81.4 50.4

0.10 0.44 0.22 0.05 45.9 221.8 93.3 45.3 0.69 1.03 1.79 0.45 1.64 0.92 1.09 1.14 0.85 0.90 0.97 0.70 0.66 0.68 0.72 0.61 10.81 2.08 1.84 55.03 0.91 2.86 2.37 0.69

1893.2 14.4 0.19

0.38 154.4

107.5 0.85

2.70 0.73

0.41 1.04

1.28 1.28

0.77 14.95

1.94 3.45

1.61 7.17

112.41 6.80 9.36 8.99 14.30 6.09

9.45 0.52

1.18

0.37

0.25

0.21

0.09 40.4

59.3 49.26

123.60 3.15

0.88 19.6

29.7 157.7

86.3 07-10-10

08-05-25

Rain QH07-R1

5-F-01

Co (lg/L) Cd (lg/L) Ba (lg/L) B (lg/L) As (lg/L) Al (lg/L) TDS (mg/L) Sampling date Water type Sample

Table 4 Dissolved concentrations of trace elements in rain and snow collected at the Buha River Hydrological Station.

Cr (lg/L)

Cu (lg/L)

Fe (lg/L)

Li (lg/L)

Mn (lg/L)

Mo (lg/L)

Ni (lg/L)

Pb (lg/L)

Rb (lg/L)

Sr (lg/L)

U (lg/L)

Zn (lg/L)

Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546

3.2.2. Eolian dust sources In contrast, the concentrations of Cr, B and Rb show pulse-like distributions in springtime and decrease to a constant background during other seasons (Fig. 3c). Such elemental seasonal patterns cannot be solely attributed to a weathering contribution from the bedrock. Rock weathering would increase dissolved Cr in the system either when weathering is strong during the monsoon season or when discharge is low because of the long residence time of Cr in the water column (Sirinawin et al., 2000). However, the average concentration of Cr in the Buha waters for springtime is 1.31 lg/L (excluding the 25 March sample, see below), while for other seasons it is equable as 0.33 ± 0.06 lg/L (Table 1). It is also unlikely that human activities introduce Cr to the river in springtime alone. Whilst Cr was originally classified as a recycled element (Whitfield and Turner, 1987), a more recent investigation concludes that Cr shows characteristics of both a recycled and an accumulated element (Sirinawin et al., 2000) whose geochemical behavior is highly sensitive to the redox state during oxidative weathering (Oze et al., 2007). Since the most important source of Cr from the atmosphere is particles derived from crustal weathering (Buat-Menard and Chesselet, 1979), the major dust-fall events that typically occur during springtime would be expected to provide the most important source of dissolved Cr to the Buha River. Similar scenarios also occur for both B and Rb (Fig. 3c). Therefore, it is proposed that Cr, B and Rb in the Buha River are mainly delivered from dry atmospheric dust deposition during springtime when dust transportation increases dramatically.

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Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546 26

1.4

Monsoon season

Monsoon season 520

3.5 1.2

18

440

B (µg/L)

2.5

480

Sr (µg/L)

1.0

Li (µg/L)

3.0

400

1.6

300

1.2

200

0.8

22

Mo (µg/L)

U (µg/L)

2.0

500

560

Monsoon season

Rb (µg/L)

4.0

400 14

200

400

5.0

600

-5.0

500

-15.0

400

150

350

100

300

50

250

0

200 1

2

3

4

5

6

7

8

9

10

11

0

3

-25.0

300

12

1

2

3

4

5

6

7

8

9

10

11

2.5

0.0 2.0

2.0

1.6

1.5

1.2

1.0

0.8

0.5

0.4

0.0

0.0

12

Cr (µg/L)

700

320

B Rb Cr Sediment

Snow storm

15.0

Daily air temperature (ºC)

10

450

TDS (mg/L)

0.6

250

0.4

100

360

Li Sr Ba Air T

Daily suspended sediment (kg/m )

Snow storm

1.5

Daily water discharge (m3/s)

U Mo TDS Discharge

Snow storm

2.0

Ba (µg/L)

0.8

1

2

3

4

5

6

7

8

Month (2007)

Month (2007)

Month (2007)

(a)

(b)

(c)

9

10

11 12

Fig. 3. Weekly variations in concentrations of the PC1 trace elements and total dissolved solid (TDS) at the Buha River Hydrological Station during 2007. (a) TDS, U, Mo and (b) Sr, Li, Ba are characterized by weathering and/or secondary carbonate precipitation dominance and (c) Cr, B, and Rb by eolian dust sources during springtime. (a) Daily water discharge, (b) daily air temperature and (c) daily suspended sediment discharge are shown for correlating the seasons. The monsoon season is the period between the two dashed lines. The 25 March sample has decreased concentrations of all elements and TDS. The anomalous chemical signal of this sample reflects a rapid melting event after a large snow storm (see text).

4.0

2.0

y = 0.012 x - 2.944 r 2 = 0.961

3.5

1.6

Rb (µg/L)

U (µg/L)

3.0

2.5

2.0

Spring Summer Autumn Winter

y = 0.008 x - 1.280 r 2 = 0.973

1.5

1.2

River waters in springtime impacted by eolian dust input

0.8

0.4

25 March 1.0 320

360

400

440

480

520

560

0.0 0.0

0.4

0.8

1.2

Sr (µg/L)

Cr (µg/L)

(a)

(b)

1.6

2.0

Fig. 4. Plots showing concentrations of (a) Sr versus U and (b) Cr versus Rb in the waters collected seasonally from the Buha River.

Lake Qinghai adjoins the Qaidam Basin in the west and the arid desert in the north and west, both being the important potential sources of Asian dust (Prospero et al., 2002; Qiang et al., 2007). Field investigations (Darmenova et al., 2005; Uno et al., 2009) have indicated that dust events occur typically during springtime and episodically in other seasons. The entrained dust from the arid regions would be impeded partially by the surrounding high mountains (Shule and Datong Mt. with elevation >4000 m a.s.l., Fig. 1) when dust particles pass through the basin, and be deposited as loess within the catchment (Porter et al., 2001). Subsequently eolian dust is partially dissolved and associated elements are released to river water even when the temperature is relatively low.

The impact of eolian dust on dissolved metal concentrations in the river water column is dependent on the concentrations of the elements, the transport flux, and the solubility of mineral dust in water. Although there was no collection of eolian dust, fresh local loess with similar chemical compositions within the catchment is an appropriate candidate for dry atmospheric deposition to the waters. The average Cr concentration in local fresh loess is 70 ± 10 ppm, equal to that in central Loess Plateau (CLP) soils (64.8 ± 1.8 ppm, Yokoo et al., 2004), richer by about a factor of 2 than that of the average upper continental crust (35 ppm, Wedepohl, 1995). Similar enriched Cr concentrations (averaging 84.8 ppm) have also been reported in particles of <20 lm in the

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Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546

icant correlation with A1. A good correlation is observed between Cu and Ni (Fig. 5), possibly indicating their similar origin. Seasonal variations in PC2-elements in the waters (Fig. 6) are believed to be closely tied to (1) dry/wet atmospheric fallout and/or (2) hydrological conditions. These potentially harmful metals might be introduced into the system via dry/wet atmospheric fallout, originating from mineral industries in western China where recently there has been increasing exploitation of ore deposits containing potentially harmful metals (Zhong, 2002). High concentrations of PC2-elements (such as Zn, Pb and Cu, Table 4) hosted in some local rainwater indicate that rainwater is a significant contribution to potentially harmful metals. Based on air mass HYSPLIT backward trajectory, this atmospheric supply may be stemming from pollutant sources from the eastern cities beyond the lake, because most rainwater is carried by the monsoon wind from the east (Fig. 2b). A significant input of Pb has previously been reported for Lake Qinghai (Jin et al., 2010a); this work showed that the increased Pb in recent sediments was from a predominantly anthropogenic atmospheric source. On the other hand, another important process may be a runoff washout effect. The concentrations of PC2elements show an initial increase at the onset of the main discharge pulses and then a decrease to a low level. A runoff washout effect would result in the initial increase in the concentrations of PC2-elements encountered at onset of the main pulses. Significant quantities of potentially harmful metals would initially settle on the soil surface mainly via atmospheric fallout. The runoff washout process causes the removal and gathering of these metals from the soil surface to waters. This washout effect would be more apparent in the early pulse during high flow, as shown in Fig. 6a. Concentrations of both PC1- and PC2-elements in water samples for other five major rivers show a wide range (Table 2). Concentrations of most PC2-elements are higher in the samples collected in July than in late October, indicating that the catchment receives significant anthropogenic inputs possibly resulting from increased tourism and/or discharge under monsoon conditions. Much higher concentrations of Pb, Zn and Cr in the Heima and Daotang Rivers also demonstrate the influence of increased tourism, because most tourists gather at southern part of the lake where the Qing-Zang highway passes through. Both PC3- and PC4-elements do not show systematic relationships with water discharge and major ions. Although both PC3-elements (As and Cd) may be attributed to anthropogenic inputs, their sources would be different. During winter, Cd is high but As is low (Fig. 7). Cadmium is from coal combustion, for heating purpose, 2.0

1.6

Ni (µg/L)

top soils from the western Tibetan Plateau which is regarded as one of the most significant dust source regions of central Asia (Li et al., 2009). The Cr concentration in local rain and snow is 1.09 ± 0.49 lg/L (Table 4), 4–5 times higher than total Cr concentrations reported globally (Kieber et al., 2002). Therefore, eolian dust deposited in the Buha River is characterized by enrichment of Cr, a potential Cr source to the waters. It has been reported that 4–17% of atmospherically transported Cr is released to solution in seawater (Chester and Murphy, 1988). Assuming 10% solubility, an input flux of 330 ± 48 g/m2/a dust to the Buha River is calculated from the dissolved Cr flux during springtime. This approximation of the dust flux is quite rough, because the solubility of eolian dust in the Buha River water is potentially different from that of seawater. However, this value is equivalent to a previous maximum estimate of dust flux (300 ± 45 g/m2/a), further indicating that the argument on Cr input from eolian dust is robust. The previous flux was estimated from a model of input–output budgets of major elements for Lake Qinghai (Jin et al., 2009a). High TDS and the first group of the PC1-elements in the river waters during springtime could also result in part from eolian dust dissolution. This leads to the separation of the springtime samples from other seasons in Fig. 4a and b. Among the PC1-elements, both Ba and Li have relatively high but negative loadings (except for Li in PC1) for both PC1 and PC2. In contrast to the other PC1-elements, the concentrations of Ba during springtime drop to minimum values for the year (Fig. 3b). The highest value in Li appears in the maximum of the monsoon, indicating its sensitivity to weathering, while the concentrations of Li during springtime are relatively low. Decreased Ba and to a lesser extent Li during springtime is attributed to calcite precipitation. The waters in the Buha River are supersaturated with respect to calcite (Zhang et al., 2009). Precipitation of carbonates is common under such conditions (Szramekw and Walter, 2004). Both Ba and, to some extent, Li during springtime might be removed from river waters by calcite precipitation. Increased carbonate-rich dust fallout during springtime may facilitate more calcite precipitation. Atmospheric deposition of carbonate-rich dust is common surrounding the Tibetan Plateau (Wake et al., 1994) and is characterized by a high loading during the spring (Zhang et al., 1997). The exact mechanism of calcite precipitation could merit further study. An exception in the spring water samples is the one collected on 25 March. The sample has anomalously low concentrations of the PC1-elements (except Al and Ba) and As relative to samples collected before and after (Fig. 3). This anomalous spike in the chemical signal can be ascribed to a rapid melting event after a large-scale snow storm, resulting from an increased temperature event. A large-scale severe snow storm occurred at Qinghai and neighbouring provinces (including Lake Qinghai) between 14 and 17 March 2007 which caused 10–16 cm of snow to accumulate. After the storm, daily average air temperature increased rapidly from 17 °C to +2 °C (Fig. 3). During this event, daily water discharge in the Buha River downstream showed a concomitant rapid increase by about a factor of 2, from 1.40 to 2.62 m3/s. The decreased values of TDS and most PC1-elements can be attributed to a dilution effect resulting from increased discharge during this event.

y = 0.348 x - 0.123 r2 = 0.952

1.2

0.8

y = 0.043 x + 0.266 r2 = 0.731

3.3. Anthropogenic contribution All PC2-elements (Co, Cu, Ni, Pb and Zn) are potentially harmful metals. High concentrations of the potentially harmful metals are believed to be associated with anthropogenic origin, partially via atmospheric deposition. Generally, the concentrations of PC2-elements in the Buha River waters are low during most seasons. Their different patterns from the PC1-elements suggest that compositions of the rock/dust weathering probably play only a minor role in PC2 variations. This argument is also supported by their insignif-

0.4

Buha River Other rivers 0.0

0

5

10

15

20

25

Cu (µg/L) Fig. 5. Plot showing concentrations of Cu versus Ni in the Buha River waters and other major rivers within the catchment.

1544

Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546

and is commonly found in northern China during winter (Zhang et al., 2003), reflecting high dissolved Cd during winter and early springtime. High As input during springtime seems to relate to eolian dust, but the gradual increase after the monsoon season may be related to water redox condition or biological degradation (Mucci et al., 2000). Nonetheless, the sources and controlling factors for the non-seasonal variation of Mn (PC4-element, Fig. 7) remain uncertain. 3.4. Water quality and potentially harmful element fluxes Based on the World Health Organization guidelines for drinking water (WHO, 2004), dissolved PC2 potentially harmful metals and 1.6

25.0

Monsoon season 20.0

Cu Ni

15.0

0.8 10.0

Ni (µg/L)

Cu (µg/L)

1.2

0.4 5.0

0.0

0.0 Pb Zn

200

other regulated elements such as As, B and Cd in river waters within the Lake Qinghai catchment were always below the limits established for drinking water. However, the water sample from the Daotang River collected in late July had dissolved As and B above the limits (Table 2). The word ‘‘Daotang” means ‘‘flow backwards” in Chinese, when the lake has shrunk. Therefore high B and As (and Li, Sr) in the Daotang River waters might be recharged from the previously deposited lacustrine sediments that it drains. The annual riverine fluxes of potentially harmful elements for the major rivers within the Lake Qinghai catchment are listed in Table 5. The fluxes of the Daotang River are not included because it flows completely into Lake Erhai, a small lake isolated from the main lake. In the flux calculations, the average concentration for all water samples was multiplied by the annual water discharge for the respective river. For comparison, the flux of the Buha River was also calculated from the sum of the weekly flux for each element. There is about 25% (12–37%) difference of the flux between the two calculations for the Buha River, further demonstrating that the flux estimation using data for limited samples has inherent uncertainties associated with seasonal variations in water discharge and/or atmospheric input. Despite this, considering that the water samples of the other major rivers were collected during typical monsoon and post-monsoon seasons, the calculated fluxes are robust for evaluating the inputs of the major rivers to the lake. Of the potentially harmful elements, Zn has highest fluxes for all rivers, followed by Cu and As. This ranking is the same as the average values for rain and snow (Table 4), again indicating the significant role of atmospheric inputs. Enrichments of these potentially

1.2 0.25

2.40

Monsoon season 160 0.20

80 0.4

Cd (µg/L)

Pb (µg/L)

0.15 1.60 0.10 Cd As

40 0.05

1.20

0.0

0

0.00 Co Precipitation

0.06

10 0.04 5 0.02

200

150

9.0 7.5

Snow storm

0.08

Daily water discharge (m3/s)

15

Mn Discharge

250

Co (µg/L)

20

0.80

0.10

Snow storm

Daily precipitation (mm)

25

6.0 4.5

100

Mn (µg/L)

Zn (µg/L)

120

As (µg/L)

2.00

0.8

3.0 50

1.5

0 1

2

3

4

5

6

7

8

9

10

11 12

Month (2007)

0

0.0 1

2

3

4

5

6

7

8

9

10

11 12

Month (2007) Fig. 6. Weekly variations in concentrations of PC2 trace elements at the Buha River Hydrological Station during 2007. Copper, Ni and Zn are related with runoff washout, while Co and Pb seem to relate to the tourism season. Daily precipitation is shown for correlating the seasons. The monsoon season is the period between the two dashed lines. The 25 March sample which was affected by a rapid melting event after a large snow storm seems to have no influence on PC2-elements (see text).

Fig. 7. Weekly variations in concentrations of PC3 (As, Cd) and PC4 (Mn) trace elements at the Buha River Hydrological Station during 2007. Daily water discharge is shown for correlating the seasons. The monsoon season is the period between the two dashed lines. The 25 March sample which was affected by a rapid melting event after a large snow storm seems to have an influence on As.

1545

Z. Jin et al. / Applied Geochemistry 25 (2010) 1536–1546 Table 5 Annual fluxes of potentially harmful elements for major rivers to the main lake of Lake Qinghai.

a b c

River

Discharge (km3/yr)

Zn

Cu

As

Cr

Ni

Pb

Co

Cd

Buhaa Buhab

0.785

25.94 ± 1.22 22.87 ± 0.48

4.30 ± 0.19 3.31 ± 4.44

1.68 ± 0.04 1.28 ± 0.24

0.34 ± 0.01 0.46 ± 0.37

0.54 ± 0.02 0.35 ± 0.22

0.12 ± 0.01 0.08 ± 0.12

0.04 ± 0.00 0.03 ± 0.01

0.02 ± 0.00 0.02 ± 0.02

Shaliub

0.246

0.52 ± 0.19

0.37 ± 0.03

0.71 ± 0.48

0.14 ± 0.00

0.09 ± 0.01

0.15 ± 0.08

0.06 ± 0.00

0.01 ± 0.01

Hargaib

0.242

0.49 ± 0.18

0.34 ± 0.01

0.65 ± 0.25

0.15 ± 0.00

0.09 ± 0.01

0.12 ± 0.08

0.06 ± 0.00

0.01 ± 0.01

Quanjib

0.055

0.12 ± 0.04

0.09 ± 0.01

0.12 ± 0.06

0.03 ± 0.00

0.02 ± 0.01

0.03 ± 0.02

0.01 ± 0.00

0.00 ± 0.00

Heimab

0.011

0.47 ± 0.05

0.02 ± 0.00

0.03 ± 0.01

0.16 ± 0.14

0.01 ± 0.00

0.01 ± 0.00

0.00 ± 0.00

0.00 ± 0.00

Totalc

1.339

27.54 ± 1.67

5.11 ± 0.24

3.19 ± 0.84

0.81 ± 0.15

0.74 ± 0.04

0.43 ± 0.19

0.18 ± 0.00

0.05 ± 0.02

Flux (t/yr)

The fluxes were the sum of weekly discharge multiplying concentration of each element of each sample. The fluxes were the average of total discharge multiplying average concentration of each element of samples. The sum fluxes of Buha River plus the average fluxes of other four major rivers.

harmful elements with same ranking were also observed in the muscle of the naked carp (Gymnocypris przewalskii, a unique fish of economic importance and an endangered cyprinid in the lake) (Qin et al., 2008). In total, the five major rivers annually deliver 27.54 ± 1.67 tons of dissolved Zn, 5.11 ± 0.24 tons of Cu, and 3.19 ± 0.84 tons of As, and minor amounts of other potentially harmful metals into the main lake (Table 5). The Buha River as the largest river within the catchment contributes 94% Zn, 84% Cu, 73% Ni, and 52% As, while Pb and Co contributions by either the Shaliu or the Hargai Rivers are similar to, or even higher than, those of the Buha River (Table 5). The Shaliu and Hargai Rivers with similar catchment areas also have equivalent contributions to other potentially harmful elements, both being significant for their input to the lake, despite the uncertainties of the estimates. 4. Conclusions This study presents high precision ICP-MS trace element data for river waters collected seasonally from the Lake Qinghai catchment, northeastern Tibetan Plateau. The seasonal geochemical data for river waters firstly demonstrate trace element background and its control on river water chemistry in the semiarid area where human activities are limited. The variable concentrations of trace elements depend on the relative importance of different sources and/ or geochemical processes along with the seasons. Observed variations in the lithophile elements (B, Ba, Cr, Mo, Rb, Sr and U) are closely tied to catchment weathering and/or atmospheric dust associated with regional weather and show (1) higher dissolved concentrations during springtime when the eolian dust prevails, (2) gradual increase in concentrations in the monsoon season when catchment weathering is enhanced, and (3) low and constant concentrations in other seasons when both temperature and flow are low. Notably, the springtime variations of Cr, B and Rb are indicative of the local eolian dust activities, indicating that eolian dust makes a significant contribution to river water chemistry. The potentially harmful metals, including Co, Cu, Ni, Pb and Zn, have higher concentrations in the monsoon season due to pollutant sources from the east and/or a runoff washout when the discharge is high. Although the impact of human activities on the Lake Qinghai system is limited and the concentrations of most of potentially harmful metals in river waters are below WHO regulations for drinking waters, the fluxes (total 38.5 ± 3.1 tons/a) of these dissolved potentially harmful elements to the lake via the major rivers can not be ignored, in particular for Zn and Cu. Acknowledgements This work was financially supported by National Natural Science Foundation of China through Grants 40873082 and

40599423. The authors especially thank Mr. Yuewei Shi at the Buha River Hydrological Station and Drs. Zhang Fei and Wu Feng at the Institute of Earth Environment, Chinese Academy of Sciences for their assistance with sample collection and analyses.

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