Research Articles

Hydrochemical characteristics and controlling factors of natural water in the border areas of the Qinghai-Tibet Plateau

  • TIAN Yuan 1, 2 ,
  • YU Chengqun , 1, * ,
  • ZHA Xinjie 2, 3 ,
  • GAO Xing 3 ,
  • DAI Erfu 1
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  • 1.Lhasa National Ecological Research Station, Key Laboratory of Ecosystem Network Observation and Modelling, Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China
  • 2.University of Chinese Academy of Sciences, Beijing 100049, China
  • 3.State Key Laboratory of Resources and Environmental Information System, Institute of Geographic Sciences and Natural Resources Research, CAS, Beijing 100101, China
Yu Chengqun (1965−), Professor and Ph.D. supervisor, specialized in Tibet agriculture and regional development. E-mail:

Tian Yuan (1991−), PhD, specialized in Tibet water environment and health. E-mail: tiany.13s@igsnrr.ac.cn

Received date: 2019-01-30

  Accepted date: 2019-03-20

  Online published: 2019-12-05

Supported by

Key R&D and Transformation Program of Tibet(No.XZ201901NB08)

Major Science and Technology Project of Tibet(No.XZ201901NA03)

Major Science and Technology Project of Tibet(No.XZ201801NA02)

Copyright

Copyright reserved © 2019. Office of Journal of Geographical Sciences All articles published represent the opinions of the authors, and do not reflect the official policy of the Chinese Medical Association or the Editorial Board, unless this is clearly specified.

Abstract

The special geography and human environment of the Qinghai-Tibet Plateau has created the unique hydrochemical characteristics of the region’s natural water, which has been preserved in a largely natural state. However, as the intensity of anthropogenic activities in the region has continued to increase, the water environment and hydrochemical characteristics of the Qinghai-Tibet Plateau have altered. In this study, water samples from the western, southern, and northeastern border areas of the Qinghai-Tibet Plateau, where human activities are ongoing, were collected, analyzed, and measured. The regional differences and factors controlling them were also investigated. The key results were obtained as follows. (1) Differences in the physical properties and hydrochemical characteristics, and their controlling factors, occurred in the different boundary areas of the Qinghai-Tibet Plateau. These differences were mainly the consequence of the geographical environment and geological conditions. (2) The water quality was good and suitable for drinking, with most samples meeting GB (Chinese national) and WHO (World Health Organization) drinking water standards. (3) The chemical properties of water were mainly controlled by the weathering of carbonates and the dissolution of evaporative rocks, with the former the most influential. (4) The biological quality indicators of natural water in the border areas were far superior to GB and WHO drinking water standards.

Cite this article

TIAN Yuan , YU Chengqun , ZHA Xinjie , GAO Xing , DAI Erfu . Hydrochemical characteristics and controlling factors of natural water in the border areas of the Qinghai-Tibet Plateau[J]. Journal of Geographical Sciences, 2019 , 29(11) : 1876 -1894 . DOI: 10.1007/s11442-019-1994-y

1 Introduction

Water is the most active factor in the geographical environment. It interconnects and interacts with the atmosphere, biosphere, pedosphere, and lithosphere in a dynamic process of water circulation. Water quantity and quality constantly changes during this process. Moreover, the water environment surrounding habituated areas can directly or indirectly influence human life and development (Zheng et al., 2007). The Qinghai-Tibet Plateau (QT) is termed the “Headwater of Asia,” as many rivers in China and Southeast Asia begin at the plateau. The area plays an important role in the protection and construction of national ecological security barriers, as freshwater has also become the most important strategic resource following energy resources (Sun et al., 2012). Because of environmental, transportation, and social historical development constraints, among others, the development level of water resources on the QT is lower than the average level in China. The QT is currently among the regions that are less affected by human activities and less polluted on earth. Its water environment still maintains a relatively complete native state (Tian et al., 2016). However, the special alpine environment makes the plateau ecological environment sensitive and extremely fragile. With the progress of society and the development of the economy, the instability of the ecological environmental system on the QT has increased along with the pressure on resources and the environment (Zhang et al., 2015).
At present, except for the uninhabited areas of the Qiangtang Plateau in the northern part of the QT, the degree of anthropogenic activities is increasing. Particularly, with the construction of the Xinjiang-Tibet Highway (on the western QT) and the Qinghai-Tibet Highway (on the northeastern QT), as well as the opening of the Zhangmu Port and the Jilong Port (on the southern QT), the QT is more frequently connected with the regions of southern Xinjiang in China, northwestern and southern China, and Nepal. Moreover, the boundary areas will gradually become an important channel for China to open to South Asia, an important pillar of the “the Belt and Road” strategy. This will further affect the environment of the plateau and its surrounding areas, changing the environment and hydrochemistry of the local natural water, which in turn will affect the production and life of local farmers and herdsmen. In recent years, research regarding the water chemistry on the QT has focused on the major rivers and lakes and their catchments, including Yarlung Zangbo (Sarin and Krishnaswami, 1984; Wang, 2016), Senge Zangbo (Li et al., 2012; Wang et al., 2012), Three River Resource areas (Deng, 1988; Wu et al., 2008; Cao, 2013; Tan et al., 2016), Qinghai Lake (Hou et al., 2009; Jin et al., 2009; Xiao et al., 2012), Yamzhog Yumco (Sun et al., 2012; Zhang et al., 2012; Sun et al., 2013; Zhe et al., 2017), Pumayum Co (Ju et al., 2010; Zhu et al., 2010), Nam Co (Gao et al., 2008; Guo et al., 2012; Wang et al., 2013), Mapam Yumco (Yao et al., 2015). Some scholars have also studied water chemistry in some areas of the QT (Zhang and Gustafsson, 1995; Guo and Wang, 2012; Tian et al., 2015), including microorganisms (Nie, 2011; Zhang et al., 2013; Zhao et al., 2017), heavy metals and trace elements (Cao et al., 2000; Grange et al., 2001; Li et al., 2006; Sheng et al., 2012; Tian et al., 2016).
However, the existing research lacks a systematic study of the hydrochemistry and the genesis of natural water in various boundary areas where human activities occur on the QT. The author collected water samples in the western, southern and northeastern boundary areas of the QT (Figure 1 and Table 1) and measured the physical properties on site during August, October and November 2017. During September and December of 2017, the author analyzed and tested the elemental contents of the water at the Physical and Chemical Analysis Center of the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (IGSNRR, CAS). Based on the collection and analysis of water samples from the three boundary areas of the QT, the hydrochemistry and causes of natural waters in different boundary areas of the QT were studied, and a comparison of regional differences between different boundary areas was completed.
Figure 1 Geographical location of water sampling points in Qinghai-Tibet Plateau (QT): (a) water sampling points at western border area of QT; (b) water sampling points at southern border area of QT; (c) water sampling points at northeastern border area of QT
Table 1 Sampling points of water samples in the border areas of the Qinghai-Tibet Plateau
No. Type Town, County Latitude (°) Longitude (°) Elevation (m) No. Type Town,
County
Latitude (°) Longitude (°) Elevation (m)
1 Well (14 m) Zhaxigang, Gaer 32.3722 79.7976 4262 45 Stream Yalai, Nyalam 28.4159 86.1307 4426
2 Stream Zhaxigang, Gaer 32.6942 79.4597 4271 46 Stream Borong, Nyalam 28.7547 85.5842 4643
3 Well (14 m) Rituzong, Ritu 33.3922 79.7046 4264 47 Stream Gyirong, Gyirong 28.3950 85.3294 2815
4 Well (15 m) Risong, Ritu 33.3682 79.6971 4281 48 Stream Gyirong, Gyirong 28.3952 85.3525 2731
5 Stream Risong, Ritu 33.1262 79.8355 4359 49 Stream Gyirong, Gyirong 28.4812 85.2251 3171
6 Stream Kunsha, Gaer 32.1069 80.0823 4293 50 Stream Gyirong, Gyirong 28.5006 85.2215 3261
7 Well (10 m) Tuolin, Zhada 31.4778 79.8051 3712 51 Stream Gyirong, Gyirong 28.5346 85.2195 3409
8 Well (8 m) Tuolin, Zhada 31.4757 79.6732 3621 52 Stream Gyirong, Gyirong 28.5418 85.2285 3510
9 Stream Daba, Zhada 31.5879 79.9703 4582 53 Stream Zongga, Gyirong 28.5635 85.2462 3632
10 Stream Daba, Zhada 31.5314 79.9843 4493 54 Stream Zongga, Gyirong 28.6056 85.2599 3760
11 Stream Daba, Zhada 31.4484 80.1229 4693 55 Stream Zongga, Gyirong 28.6301 85.2691 3736
12 Stream Menshi, Gaer 31.1789 80.7601 4450 56 Stream Zongga, Gyirong 28.6533 85.2781 3785
13 Stream Menshi, Gaer 31.1426 80.9014 4636 57 Stream Zongga, Gyirong 28.7397 85.2965 3955
14 Well (10 m) Pulan, Pulan 30.2945 81.1759 3905 58 Stream Zongga, Gyirong 28.8557 85.2968 4145
15 Well (5 m) Kejia, Pulan 30.1927 81.2707 3734 59 Stream Zheba, Gyirong 29.0409 85.4394 4733
16 Stream Kejia, Pulan 30.2633 81.1870 3891 60 Stream Zheba, Gyirong 29.2074 85.3636 4527
17 Stream Namumani, Pulan 30.5157 81.2018 4521 61 Stream Zhangmu, Nyalam 27.9887 85.9828 2277
18 Stream Huoer, Pulan 30.6872 81.8323 4752 62 Stream Zhangmu, Nyalam 27.9903 85.9829 2263
19 Stream Longzi, Longzi 28.4081 92.4628 3881 63 Stream Yalai, Nyalam 28.3828 86.1070 4373
20 Stream Ridang, Longzi 28.6125 92.2153 4998 64 Stream Nierudui, Kangma 28.4865 89.9269 4569
21 Stream Zhaxiraodeng, Milin 29.2319 94.0680 2993 65 Stream Nierudui, Kangma 28.4894 89.9265 4584
22 Stream Wolong, Milin 29.1447 93.6939 3032 66 Stream Nierumai, Kangma 28.6419 89.8728 4389
23 Stream Nanyi, Milin 29.1849 94.1883 2938 67 Stream Zhangxiong, Kangma 28.6183 89.3708 4489
24 Stream Nanyi, Milin 29.1279 94.2226 3009 68 Stream Samada, Kangma 28.3495 89.5377 4447
25 Stream Nanyi, Milin 29.0417 94.2321 3147 69 Stream Samada, Kangma 28.2517 89.6455 4573
No. Type Town, County Latitude (°) Longitude (°) Elevation (m) No. Type Town,
County
Latitude (°) Longitude (°) Elevation (m)
26 Stream Danniang, Milin 29.4613 94.7849 2945 70 Stream Gala, Kangma 28.2388 89.1817 4474
27 Stream Pai, Milin 29.5174 94.8811 2931 71 Stream Kangma, Kangma 28.5587 89.6807 4298
28 Stream Pai, Milin 29.6977 94.8978 2821 72 Stream Labuleng, Xiahe 35.1936 102.5151 2942
29 Stream Jiayu, Longzi 28.2941 92.7492 3380 73 Stream Sangke, Xiahe 35.0651 102.4624 3207
30 Stream Liemai, Longzi 28.4247 92.5742 3851 74 Stream Sangke, Xiahe 35.0126 102.5232 3399
31 Stream Ridang, Longzi 28.3930 92.1056 4253 75 Stream Sangke, Xiahe 34.9377 102.6594 3249
32 Stream Ridang, Longzi 28.4883 92.2845 4119 76 Stream Maai,
Luqu
34.5901 102.4854 3112
33 Stream Xuesha, Longzi 28.6346 92.5459 4245 77 Stream Gahai, Luqu 34.4331 102.2976 3432
34 Stream Xuesha, Longzi 28.6337 92.5489 4175 78 Stream Gahai, Luqu 34.2006 102.4419 3491
35 Stream Xuesha, Longzi 28.6038 92.5566 3905 79 Stream Benzilan, Luqu 34.1269 102.6116 3358
36 Stream Rerong, Longzi 28.4856 92.1489 4064 80 Stream Langmu, Luqu 34.0949 102.6318 3393
37 Well (8 m) Menbu, Nyalam 28.7857 86.2255 4452 81 Stream Hongxing, Nuoergai 34.0959 102.7545 3161
38 Stream Menbu, Nyalam 28.5812 86.1501 4857 82 Stream Zhagana, Diebu 34.2394 103.1792 2974
39 Stream Yalai, Nyalam 28.4565 86.1650 4560 83 Stream Zhagana, Diebu 34.2374 103.2025 2978
40 Stream Nyalam, Nyalam 28.1598 85.9805 3771 84 Stream Zhagana, Diebu 34.2370 103.1979 2939
41 Stream Nyalam, Nyalam 28.1634 85.9768 3788 85 Stream Dianga, Diebu 34.0558 103.2373 2355
42 Stream Yalai, Nyalam 28.2928 86.0248 4104 86 Stream Wangzang, Diebu 33.9520 103.6059 2007
43 Stream Yalai, Nyalam 28.3273 86.0474 4259 87 Stream Huayuan, Diebu 33.9885 103.9207 1733
44 Stream Yalai, Nyalam 28.3828 86.1070 4373 88 Stream Sigou, Minxian 34.2426 103.9113 2948

2 Materials and methods

2.1 Study area

The QT has a land area of 2.57 × 106 km2 (26.0033°-39.7806°N, 73.3144°-104.7831°E) along the southwestern border of China, with an average elevation higher than 4000 m above sea level (Zhang et al., 2002). The northeastern border areas of the QT are in the transitional zone between the QT and the Loess Plateau. The terrain is higher in elevation to the northwest (meadow steppe) and lower to the southeast (the Min-Die mountain area). The southern border areas are in the Himalayas and the geomorphological type is mainly deep-cutting alpine gorge and plateau strath lake basin. The western border areas are on the western Qiangtang Plateau, where the Kunlun Mountains, Gangdese Mountains, and Himalayas are distributed to the north, central, and south, respectively. Here, the geomorphological type is mainly plateau lake basin, river valley, terrestrial forest, and alpine gully. Since the Proterozoic, the strata of the system have been well developed, and the sedimentary rock types are diverse atop the QT, particularly the well-exposed Mesozoic-Cenozoic marine strata. Igneous rocks and metamorphic rocks are widely distributed, various rock types are exposed, and the rock types are complex (Lu et al., 2016). Rainfall and runoff are abundant to the east and south, and the supply of surface water and groundwater is sufficient; however, the climate to the west and north is arid, comprising a global cold and arid core. The average annual precipitation of each boundary sampling county is approximately 620 mm (northeastern boundary), 570 mm (southern boundary), and 130 mm (western boundary), respectively (NBS, 2017). In 2016, the county gross domestic product (GDP) of prefecture-level cities in the border areas of the QT was 1.707 billion yuan (along the northeastern border; Gannan Tibetan Autonomous Prefecture), 1.12 billion yuan (along the southern border; Shigatse City, Shannan City, and Nyingchi City), and 604 million yuan (along the eastern border, Ali (Ngari) Prefecture), respectively (NBS, 2017).

2.2 Field measurement

The location at each collection sample recorded the coordinate by using a handheld GPS (Global Position System) device (equipment model: 530HCx, produced by GARMIN). The pH, Ec (electrical conductivity) and T (temperature) were in-situ tested by using a pH tester (Limit of detection (LOD): 0.01, equipment model: SX-620, produced by Sanxing, Shanghai) and Ec tester (LOD: 0.1 μS/cm, equipment model: SX-650, produced by Sanxing, Shanghai) respectively. The TDS (total dissolved solids) were automatically converted by the instrument through Ec. Collection and preservation of all water samples are followed the Standard Examination Methods for Drinking Water (GB/T 5750-2006) (MH, 2007).

2.3 Laboratory analysis method

The bicarbonate (CO32-) and carbonate (HCO3-) were analyzed using an acid-base titration according to the method of Ministry of Health (MH) of the P.R.C. (MEP, 2002; MH, 2007). Anions of chlorine (Cl-), sulfate (SO42-), nitrate (NO3-), fluorine (F-), phosphate (PO43-) and nitrite (NO2-) were analyzed using IC (Ion Chromatography, LOD: 0.001 mg/L, equipment model: ICS-900, produced by Thermo Fisher Scientific) at the IGSNRR, following the U.S. Environmental Protection Agency (EPA) method 300.0 (EPA, 1993). Major cations of calcium (Ca2+), potassium (K+), magnesium (Mg2+), sodium (Na+) and silicon (Si) were determined using ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry, LOD: 0.001 mg/L, equipment model: Optima 5300 DV, produced by PerkinElmer). Quality assurance were controlled by using certified external standard solutions and retested sample measurements (retest 1 sample after run every 10 samples) during the analysis to ensure and verify the stability of the results. Certified external standard of cations solutions for Ca2+, K+, Mg2+, Na+, and Si were prepared from Multi-element ICP-MS calibration standards (No.Lot# 15-76JB and Cat# N9300233). Certified external standard of anions solutions were prepared from GB of F- (GBW080549), Cl- (GBW080268), SO42- (GBW080266), PO43- (GBW080435), NO3- (GBW080264) and NO2- (GBW080223), respectively.
The relative error of total anion and cation (equivalent ratio) in 88 samples ranged from 0.01% to 4.67%, less than 5.00% (Figure 2). Therefore, it can be said that the total anion and cation in the measured water are basically balanced (Shen et al., 1993), i.e., our data are accurate and reliable.
Figure 2 Total cation (mEq/L) and total anion (mEq/L) relative error of water samples on the Qinghai-Tibet Plateau

3 Results and discussion

3.1 Elements concentrations

The water samples of QT border areas have suitable hydrochemical characteristics (Table 2). The pH value ranged from 6.52 to 8.88 (mean value: 7.75), and the TDS ranged from 6.11 mg/L to 583 mg/L (mean value: 180 mg/L). The TH (hardness) of the QT border area water samples was calculated, ranging between 3.69 mmol/L and 512 mmol/L (mean value: 168 mmol/L).
Table 2 Parameters and elements concentration statistical summary of the water samples of the Qinghai-Tibet Plateau border areas
Parameters pH TDS TH Ca2+ K+ Mg2+ Na+ HCO3- Cl- SO42- NO3- Si
Unit mg/L mmol/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L
All Min 6.52 6.11 3.69 1.10 0.00 0.19 0.19 2.95 0.00 1.29 0.00 2.08
Max 8.88 583 512 149 8.43 71.0 32.8 305 21.7 356 32.2 20.7
Mean 7.75 180 168 44.7 1.03 13.5 6.06 127 2.33 59.2 2.60 8.77
Median 7.75 171 143 42.3 0.87 9.02 3.75 109 0.68 28.7 1.15 7.64
SD 0.50 130 127 30.9 1.06 14.6 6.30 95.8 3.98 80.8 4.48 4.10
Skewness -0.04 1.13 0.88 0.77 3.89 1.75 1.66 0.37 2.94 2.03 4.33 0.85
West Min 6.99 32.9 27.6 9.38 0.00 0.99 0.19 16.4 0.00 3.48 0.00 4.85
Max 8.88 391 394 69.3 8.43 52.9 32.8 302 21.7 199 32.2 20.7
Mean 8.38 142 122 31.0 1.44 10.6 10.8 106 5.62 46.9 3.41 10.2
Median 7.74 171 146 42.4 0.86 9.09 3.54 108 0.67 29.6 1.17 7.50
SD 0.44 79.1 82.8 17.4 1.77 11.1 8.37 70.1 6.79 45.0 7.42 4.96
Skewness -1.97 1.45 1.76 0.54 3.37 2.95 0.74 0.92 1.08 2.13 3.23 0.75
South Min 6.52 6.11 3.69 1.10 0.00 0.19 0.38 2.95 0.31 1.29 0.40 2.08
Max 8.07 583 512 149 2.72 58.7 23.7 305 9.84 356 19.2 18.3
Mean 7.57 187 163 44.5 0.77 12.5 4.32 98.6 0.96 74.4 1.74 8.46
Median 7.66 131 128 38.4 0.64 5.30 3.02 74.3 0.57 38.3 1.04 6.95
SD 0.36 156 146 36.0 0.68 15.2 4.68 90.6 1.68 95.7 2.71 4.04
Skewness -1.11 0.92 0.90 0.85 0.76 1.44 1.92 0.93 4.47 1.51 5.30 0.68
Northeast Min 6.87 119 148 40.6 0.34 4.44 0.99 162 0.97 3.35 0.92 3.51
Max 8.14 330 462 81.9 2.87 71.0 20.6 298 12.4 139 16.3 14.0
Mean 7.66 198 231 59.9 1.40 19.5 6.50 238 3.11 24.5 4.38 8.22
Median 7.77 198 224 58.1 1.00 17.1 4.34 250 2.54 15.7 2.09 8.04
SD 0.40 47.7 66.8 12.0 0.76 14.4 5.48 37.2 2.58 31.2 3.96 2.64
Skewness -0.68 0.91 2.27 0.14 0.47 2.54 1.55 -0.28 2.70 2.90 1.59 0.34

Note: SD means standard deviation

Significant spatial variations in the major cations and anions indicate the impact of different lithologies and anthropogenic activities at a watershed scale (Li and Zhang, 2008). The statistical data (min, max, mean, median, SD, skewness) for the major elemental composition of the water samples in different QT border areas are listed in Table 2. The water samples major cations relative concentration order is K+ < Mg2+ < Na+ < Ca2+ (in the western QT border area) and K+ < Na+ < Mg2+ < Ca2+ (in the southern and northeastern QT border areas), while order of the anions in water samples is NO3- < Cl- < SO42- < HCO3- (in the western QT border area) and NO3- < Cl- < SO42- < HCO3- (in the southern and northeastern QT border areas) (Table 2). The lowest TDS and TH of the water samples on the QT was found in the western border area, with a mean value of 142.01 mg/L and 122.55 mg/L respectively.
Natural water in different QT border areas is mainly fresh-soft water and fresh-hard water. No water samples in the QT border areas with high TDS (> 1000 mg/L) and TH (> 1000 mg/L) were classified as brackish-hard water (Figure 3). Most water samples from western border of the QT were fresh-soft water, but water samples from the northeastern border of the QT were fresh-hard water (Figure 3).
Figure 3 Water quality of water samples in different border areas of the Qinghai-Tibet Plateau
In Table 3, TDS, Ca2+, Mg2+, Na+, and Cl- in all of the water samples of QT border areas were less than the drinking water standards of both GB and WHO. The pH value of all the water samples meets the WHO drink water standard and all the water samples of southern and northeastern QT border areas meet both standards. However, only 50% of the water samples from the western border of the QT meet the GB standard (Table 3), while 7.5% and 5.9% of the water samples in the southern and northeastern QT border areas exceed the GB TH standard, respectively. Only one water sample in the QT southern border area exceeds the WHO drinking water standard (Table 3). Approximately 11% of the water samples in the southern QT border areas exceed the SO42- value of the two drinking water standards. The NO3- value of all the water samples meets the GB and WHO drinking water standards. Therefore, the majority of the natural water in the border areas of the QT is suitable for drinking.
Table 3 Distribution of water samples exceeding the drinking water standards in border areas of the Qinghai-Tibet Plateau
Parameters GB WHO Number of exceeding the drinking water standards
(MH 2007) (WHO 2008) West South Northeast
pH 6.5-8.5 6.5-9.5 9 (50%) 0 0
TDS 1000 1000 0 0 0
TH 450 500 0 4 (7.5%) 1 (5.9%)
Ca2+ - 300 0 0 0
Mg2+ - 300 0 0 0
Na+ 200 200 0 0 0
Cl- 250 250 0 0 0
SO42- 250 250 0 6 (11%) 0
NO3- 44 50 0 0 0

Note: GB means Chinese State Standard

3.2 Hydrochemical characteristics

The relative amounts of major anions and cations in water samples of the QT border area are shown by using a Piper diagram; the hydrochemical characteristics of water was determined by the percentage of major anions and cations (Piper, 1944).
The QT border water samples are dominated by Ca2+-Mg2+-HCO3- (n=19), Ca2+-Mg2+-HCO3-- SO42- (n=13), Ca2+-Mg2+-SO42--HCO3- (n=13), Ca2+-HCO3- (n=13) and Ca2+-HCO3-- SO42- (n=10), with an average pH of 7.75 (Table 2). In different water samples, some observations are as follows: (1) Most water samples of the western border are weakly alkaline (average pH is 8.38) (Table 2); Ca2+-Mg2+-HCO3--SO42- (n=4) and Ca2+-Mg2+- SO42--HCO3- (n=2) are the most common water types. (2) Most water samples of the southern border are slightly alkaline (average pH is 7.57) (Table 2); Ca2+-Mg2+- SO42-- HCO3- (n=11), Ca2+-Mg2+-HCO3-- SO42- (n=9), Ca2+-HCO3--SO42- (n=9), Ca2+- Mg2+- HCO3- (n=7), and Ca2+-HCO3- (n=7) are the most common water types of these water samples. (3) Most water samples of the northeastern border are slightly alkaline (average pH is 7.66) (Table 2); Ca2+-Mg2+-HCO3- (n=11) and Ca2+-HCO3- (n=4) are the most common water types (Figure 4).
Figure 4 Piper diagrams for water samples from different border areas of the Qinghai-Tibet Plateau

3.3 Preliminary discussion of ion sources

Gibbs boomerang envelope (Gibbs, 1970) built a simple model by using the TDS values versus the Na+/(Na++Ca2+) (weight ratio) and the TDS values versus Cl-/(Cl-+HCO3-) (weight ratio) measuring the relative significance of the three types of natural factors (evaporation, weathering, and precipitation) that control surface water chemistry.
The Gibbs model plot shows that the rock weathering controls the main hydrochemical composition in the QT border areas (Figure 5), basically consistent with the previous study of rivers on the QT (Zhu et al., 2010; Yao et al., 2015; Zhe et al., 2017).
Figure 5 Gibbs boomerang envelope model plot

3.4 Mechanisms controlling hydrochemistry

A total of 11.6% of global river solutes originate from silicates, 17.2% from evaporites (although evaporites only account for approximately 1.25% of the surface rock distribution area), and approximately 50% from carbonates (Meybeck, 1987). The major anions and cations in water which have been weathered from stratum and dissolved in water can determine the rock type, e.g., Ca2+ and Mg2+ may originate from carbonates, evaporites, or silicates; Na+ and K+ from both evaporites and silicates; Cl- and SO42- mainly from the dissolution of evaporites; and HCO3- mostly from weathered carbonates (Li and Zhang, 2008).
The milliequivalent (mEq) ratio of Cl-+SO42- to HCO3- in the QT water samples from the western and southern border areas is approximately equal to 1 (Figure 6a). The mEq ratios of Ca2++Mg2+ to HCO3- (Figure 6b) and Ca2++Mg2+ to SO42- (Figure 6c) are greater than 1. The mEq ratio of Ca2++Mg2+ to HCO3-+SO42- is approximately equal to 1 (Figure 6d). This indicates that the ions in these water samples are mainly controlled by the carbonate weathering and the dissolution of evaporites. This further suggests the weathering of carbonate mineral (e.g., calcite and dolomite) and dissolution of sulfate mineral (e.g., gypsum) could be crucial reactions resulting in the Ca2+ and Mg2+ in water (Dalai et al., 2002). Meanwhile, the mEq ratios of Ca2+/SO42- to Mg2+/SO42- (Figure 6e) in most of the water samples are slightly greater than 1, meaning the content of Ca2+ is higher than that of Mg2+ and the sulfate dissolution contribution is less than that of carbonates weathering (Li and Zhang, 2008). In the water samples from the northeastern border area of the QT, the mEq ratio of Cl-+SO42- to HCO3- (Figure 6a) is much less than 1, the mEq of Ca2++Mg2+ is nearly equal in proportion to HCO3- (Figure 6b), and the mEq ratio of Ca2++Mg2+ to SO42- (Figure 6c) is much greater than 1, indicating that the Ca2+ and Mg2+ in the QT border areas water samples is closely related to HCO3-. The ions (their types and concentrations) are mainly controlled by carbonate weathering; and the Ca2+ and Mg2+ may originate from the weathering of carbonate minerals (e.g., calcite and dolomite).
Figure 6 Scatter diagrams of (a) Cl-+SO42- and HCO3-, (b) Ca2++Mg2+ and HCO3-, (c) Ca2++Mg2+ and SO42-, (d) Ca2++Mg2+ and HCO3-+SO42-, (e) Ca2 +/SO42- and Mg2+/SO42-, (f) Na+ and HCO3-, (g) Na+ and SO42-, (h) Na+ and Cl-, (i) Na++K+ and Cl-, (j) Ca2++Mg2+ and Na++K+ for the water samples in different border area of the Qinghai-Tibet Plateau
The mEq ratio of Na+ to HCO3- (Figure 6f) in most of the water samples is less than 1, while the mEq ratio of Na+ to SO42- (Figure 6g) in the western and northeastern border area water samples is approximately equal to 1, suggesting that the dissolution of sulfate minerals (e.g., mirabilite) may be among the sources of Na+ in the water samples of western and northeastern areas (Zhe et al., 2017). Meanwhile, the mEq ratio of Na+ to SO42- (Figure 6g) in the southern area water samples is less than 1. Therefore, the Na+ in the southern border area water samples may originate from silicates (Zhe et al., 2017). Generally, the ratio of Na++K+ to Cl- is equal to 1 when the dissolution of evaporites plays a major role in hydrochemical compositions (Gibbs, 1970). Both the mEq ratios of Na+/Cl- (Figure 6h) and (Na++K+)/Cl- (Figure 6i) are greater than 1, indicating that halite in evaporites (e.g., NaCl and KCl) are the main source of Cl- in the water samples.
The ratio of (Ca2++Mg2+)/(Na++K+) in the water can be used as an indicator to distinguish the relative intensity of different rock weathering. Silicate weathering releases more Na++K+ than Ca2++Mg2+ (Sarin et al., 1989). Relatively high (Ca2++Mg2+)/(Na++K+) (Figure 6j) and SO42-/Na+ (Figure 6g) ratios indicate that the rocks are rich in Ca2+, Mg2+ and SO42- (e.g., gypsum and dolomite) on the water sample hydrochemistry. The world’s rivers average ratio of (Ca2++Mg2+)/(Na++K+) is 2.2 (Meybeck and Helmer 1989). Carbonate weathering may cause the high ratio of (Ca2++Mg2+)/(Na++K+) in a river/stream, such as the Lake Qinghai catchment in China which ranges from 5.5-20.3 (Hou et al., 2009), the carbonate bedrock section of Ganges-Brahmaputra River in India which ranges from 5.2-11.5 (Sarin and Krishnaswami, 1984), the Yangtze River in China which is 5.1 (Chen et al., 2002), the Indus River in India which is 6 (Ahmad et al., 1998) and the Mackenzie River in Canada which is 6.9 (Reeder et al., 1972). Conversely, the low ratio of (Ca2++Mg2+)/(Na++K+) in a river/ stream means it is probably controlled by the dissolution of evaporites, such as rivers around the Taklimakan Desert which are approximately 0.9 (Zhu and Yang, 2007), the Orinoco River in South America which is 1.5 (Nemeth et al., 1982) and the Buha-Heima River of the Lake Qinghai catchment which ranges from 1.36-6.81 (Hou et al., 2009). The QT border area water samples average ratio of (Ca2++Mg2+)/(Na++K+) decreases in the following order: northeastern (23.8) > southern (18.2) > western (9.8). This means that the process of carbonate weathering in the water has intensifies from the western to eastern QT.

3.5 Anthropogenic input

The hydrochemical characteristics of water can be affected by human activities (Meybeck and Helmer, 1989; Katz et al., 2001). Because of the rapid increase in industrial and agricultural activities, this impact has recently become significant in whole China (Chen et al., 2002). Generally, non-point source pollutants (i.e. fertilization) and surficial geological function (i.e. weathering) are the two important sources of NO3- (Holloway et al., 1998). The presence of biogenic substances such as nitrogen, sulfur, and phosphorus compounds (e.g., NO2-, NO3-, SO42-, and PO43-) in natural water can reflect the impact of human activities on the chemical composition of water, to some extent (Roy et al., 1999).
The average nitrogen compounds in the order from the lowest to the highest are as follows (Table 4 and Figure 7): northeast > west > south. This order basically coincides with the socio-economic development in these areas. The average sulfur compounds have a general trend of south > east > west (Table 4). Phosphorus compounds in all the QT water samples were less than the limit of detection or were not detected (Table 4). One can see from Table 4 that all the biogenic substances in the water samples are very low and far lower than GB and WHO drinking water standards, indicating the QT is less affected by human activities.
Table 4 Values of anthropogenic input geochemical variables in water compared with GB and WHO drinking water standards
Parameter Border area of QT GB WHO
Western Southern Northeastern (MH 2007) (WHO 2008)
NO2- (mg/L) n.a. 0.02 0.08 - 3
NO3- (mg/L) 3.41 1.71 4.38 44 50
SO42- (mg/L) 46.91 74.45 24.53 250 250
PO43- (mg/L) n.a. n.a. n.a. - -

n.a. (not applicable) means the concentration of parameter is lower than the LOD or is not detected.

Figure 7 Box and whisker plot of water nitrate concentrations in different border areas of the Qinghai-Tibet Plateau

3.6 Spatial pattern of major ions

Principal component analysis (PCA) of hydrochemical characteristics parameters derived three significant factors (Table 5). The percentages of variance indicate three components accounting for approximately 85.34%, 75.40%, and 80.61% of the total variability water samples in the western, southern, and northeastern QT border areas, respectively. The scores of variables on the principal component vector are plotted in Figure 8.
Table 5 Results of the PCA vectors, eigenvalues and % of variance
No. West South Northeast
Eigenvalue % of variance Eigenvalue % of variance Eigenvalue % of variance
1 7.3178 52.2703 6.9330 49.5215 6.9762 49.8304
2 3.4631 24.7362 2.1551 15.3934 2.3914 17.0814
3 1.1666 8.3326 1.4682 10.4873 1.9171 13.6934
4 0.6698 4.7843 0.9467 6.7622 1.1243 8.0310
5 0.4954 3.5387 0.7184 5.1313 0.7262 5.1872
6 0.3340 2.3859 0.7059 5.0418 0.5176 3.6968
7 0.2622 1.8729 0.4028 2.8774 0.1330 0.9501
8 0.1449 1.0349 0.3045 2.1753 0.1023 0.7308
9 0.1013 0.7236 0.2039 1.4561 0.0677 0.4835
10 0.0368 0.2627 0.0923 0.6594 0.0241 0.1724
11 0.0076 0.0542 0.0662 0.4728 0.0196 0.1397
12 0.0005 0.0037 0.0019 0.0135 0.0005 0.0034
13 0.0000 0.0000 0.0011 0.0079 0.0000 0.0000
14 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

Principal vectors are shown in bold.

Figure 8 PCA results for variables: (a) component 1 and component 2 of water samples in the western border area, (b) component 1 and component 3 of water samples in the western border area; (c) component 1 and component 2 of water samples in the southern border area, and (d) component 1 and component 3 of water samples in the southern border area; (e) component 1 and component 2 of water samples in the northeastern border area, and (f) component 1 and component 3 of water samples in the northeastern border area of the Qinghai-Tibet Plateau
PCA (Table 5, Figures 8a and 8b) of the western QT border area water samples shows strong relationships in the first component between Ec, TDS, TH, Ca2+, Mg2+, K+, SO42-, and HCO3-, which indicate the carbonate weathering and evaporite dissolution (Gibbs, 1970; Chen et al., 2002). The relationships between Na+, Cl-, NO3-, and SiO2 in the second component reflect the silicate weathering, evaporite dissolution, and some anthropogenic input (Gibbs, 1970; Cruz and Amaral, 2004). Because of the minimal amount of precipitation and human activities, silicate contributes more to the western QT (Yao et al., 1996; Chang et al., 2012). The F- shows the dominant nature in the third component (Figure 8b), which represents the geological and chemical characteristics of the rocks and soils (Meenakshi and Maheshwari, 2006).
Figures 8c and 8d show strong relationships of the southern QT border area water samples among EC, TDS, TH, Ca2+, Mg2+, Na+, SO42-, and HCO3- in the first component, indicating carbonate weathering and evaporite dissolutions; K+, Cl- and NO3- in the second component, reflecting evaporite dissolutions and some anthropogenic input; and SiO2 and F- in the third component, showing silicate weathering and geological characteristic influence. Most of the parameters have some relationships in the first component, while only the Ca2+ correlation is greater in the second vector; Cl- and NO3- are higher in the third vector. The correlations, predict common effects of multiple factors in the water samples of the northeastern border area of the QT (Figures 8e and 8f).

4 Conclusions

In summary, through the collection, experimental measurement, and analysis of natural water samples from the three boundary areas of the QT where human activities exist, we found that water quality in the western, southern, and northeastern border areas of the QT was generally good, and most samples meet the drinking water standards. In addition, some indicators of individual water samples were even superior to the standards for drinking mineral water in China.
The QT water samples have the pH values ranging from 6.52 to 8.88, with an average of 7.75, being weakly alkaline. Furthermore, there were differences among the different boundary areas as follows: western areas (8.38), southern areas (7.57), and northeastern areas (7.66). The average TDS value in the QT water samples was 171 mg/L and the TDS were generally small. The QT water samples have the TH ranging from 3.69 mg/L (very soft) to 512 mg/L (very hard), with the average value of 168 mg/L (indicating a lower hardness).
The main cations were Ca2+ and Mg2+, while HCO3- and SO42- were the main anions. The main types of hydrochemistry in the three border areas were Ca2+-Mg2+-HCO3-- SO42- and Ca2+-Mg2+-SO42--HCO3- to the west; Ca2+-Mg2+-SO42--HCO3-, Ca2+-Mg2+- HCO3-SO42-, Ca2+-HCO3-SO42-, Ca2+-HCO3-, and Ca2+-Mg2+-HCO3- to the south; and Ca2+-Mg2+-HCO3- and Ca2+-HCO3- to the northeast, respectively.
The genetic type of the QT border area water sample mainly was rock weathering, and the ions in the water were mainly controlled by the carbonate weathering and the evaporite dissolution; the weathering process of carbonate rocks was more intense. The main source of Cl- in water samples of the QT border areas was the halite of the evaporites. Dissolution of sulphate minerals in the water may be among the sources of Na+ in the western and northeastern border areas, while Na+ in the water samples in the southern border area may have originated from silicates.
The concentration of nitrogen compounds from water samples in each boundary area from high to low were in the northeast, west, and south; this was largely consistent with the local socio-economic development level. Meanwhile, the general trend in the average concentration of sulfur compounds from high to low was in the south, east, and west. The biological quality indicators of the natural water in the border areas was far superior to GB and WHO drinking water standards, indicating that these areas have been little affected by human activities.
The regional differences in the hydrochemistry of water samples from various boundary areas mainly are the result of the combined effects of geographical environment and geological conditions. In addition, human activities have had certain effects on the regional hydrochemistry.
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