pH, hardness, and TDS.
In general, surface waters in Longzi County are neutral to slightly alkaline because pH values range between 6.61–8.07 (mean = 7.39). The maximum pH value of 8.07 was obtained from a location near the tributary of the Longzi Xiongqu River in Liemai, while the minimum value of 6.61 was produced by a mountain spring sample from the Yumai. The measured pH values shown in Table 1 produce a range similar to the 6.87–8.14 reported by Tian, et al.6 for waters in the southeast border of Tibet. In the present study, pH values generally decreased from upstream to near neutral downstream, and values for samples from townships near the central area of the Longzi Xiongqu River including Liemai (7.50), Jiayu (7.55), and Zhunba (7.59) are higher than those upstream, such as Rerong (7.39), Ridang (7.27), and Longzi (7.38). Samples collected from townships affected by the KBD in Longzi County, such as Yumai and Zhari, exhibit relatively low pH values ranging between 6.61–7.91, with a mean value of 7.27 (Table. 1). Low water pH is usually associated with a reducing environment, in which humic acids and other organic substances accumulate; such an environment is conducive for the incidence of the KBD51.
Table 1
Summary of pH, TH, and TDS data for surface water samples from different townships in the Longzi County
Township
|
Sample size
|
pH
|
|
TH (mg L–1)
|
|
TDS (mg L–1)
|
Mean
|
Range
|
SDa
|
|
Mean
|
Range
|
SDa
|
|
Mean
|
Range
|
SDa
|
Longzi
|
3
|
7.38
|
6.63~7.78
|
0.57
|
|
210.38
|
164.3~260.2
|
48.09
|
|
216.94
|
170.0~267.0
|
48.58
|
Ridang
|
11
|
7.27
|
6.73~7.66
|
0.28
|
|
141.03
|
35.2~321.5
|
76.43
|
|
172.54
|
46.0~473.0
|
133.77
|
Liemai
|
6
|
7.50
|
7.06~8.07
|
0.35
|
|
438.42
|
340.6~510.8
|
72.17
|
|
372.06
|
305.0~447.0
|
57.35
|
Rerong
|
4
|
7.39
|
7.01~7.72
|
0.36
|
|
333.50
|
195.8~528.3
|
141.73
|
|
314.43
|
175.7~454.0
|
114.35
|
San’anqulin
|
8
|
7.30
|
6.93~7.77
|
0.25
|
|
213.06
|
92.5~293.5
|
59.86
|
|
217.88
|
96.0~277.0
|
58.23
|
Zhunba
|
7
|
7.59
|
7.44~7.75
|
0.12
|
|
225.25
|
96.2~365.3
|
99.12
|
|
234.86
|
115.0~337.0
|
83.69
|
Xuesa
|
9
|
7.55
|
6.96~8.04
|
0.37
|
|
308.36
|
65.1~489.9
|
141.25
|
|
280.59
|
80.0~387.7
|
110.09
|
Zhari
|
19
|
7.32
|
6.81~7.91
|
0.26
|
|
54.03
|
15.6~86.9
|
22.69
|
|
64.21
|
18.0~103.0
|
24.77
|
Yumai
|
17
|
7.22
|
6.61~7.83
|
0.34
|
|
60.49
|
9.0~281.4
|
63.05
|
|
71.29
|
14.0~266.0
|
59.29
|
Jiayu
|
12
|
7.55
|
7.25~8.03
|
0.20
|
|
315.67
|
181.5~484.3
|
110.98
|
|
265.34
|
184.0~364.0
|
69.46
|
Douyu
|
8
|
7.75
|
7.44~7.96
|
0.20
|
|
228.56
|
143.8~346
|
56.43
|
|
260.75
|
172.0~371.0
|
58.00
|
Non-KBD areas
|
68
|
7.49
|
6.88~8.07
|
0.30
|
|
262.06
|
35.2~528.3
|
123.29
|
|
253.25
|
46.0~473.0
|
100.39
|
KBD areas
|
36
|
7.27
|
6.61~7.91
|
0.30
|
|
57.08
|
9.0~281.4
|
45.74
|
|
67.56
|
14.0~266.0
|
44.00
|
Whole county
|
104
|
7.41
|
6.61~8.07
|
0.32
|
|
191.10
|
9.0~528.3
|
142.13
|
|
188.97
|
14.0~473.0
|
122.86
|
Note:a Standard deviation (SD). |
The TH and TDS are important indicators of the quality of drinking water. Based on the TH values, water in the area can be divided into soft (<150 mg L–1), slightly hard (150–300 mg L–1), hard (300–450 mg L–1) and extremely hard water (> 450 mg L–1). The TH values for surface waters from the nine non-KBD townships range between 35.2–528.3 mg L–1, while mean values vary from 141.0 to 438.4 mg L–1, and these indicate that the waters are soft to hard (Table 1). However, surface waters in both towns affected by the KBD are soft, with TH values ranging between 9.0–281.4 mg L–1, and mean values varying from 54.0 to 60.5 mg L–1. As presented in Table 1, the TDS values for surface waters in the nine non-KBD townships vary between 46.0–473.0 mg L–1 (mean = 253.3 mg L–1), while those for the two KBD townships range between 14.0–266.0 mg L–1 (67.6 mg L–1).
Hydrochemical characteristics.
The concentrations of eight major ions including four cations (Na+, Mg2+, K+, and Ca2+) and four anions (Cl−, CO32−, HCO3–, and SO42−) determine the hydrochemical characteristics of natural waters. The representation of these ions using a Piper diagram provides insights into physical and chemical processes controlling water chemistry5. Therefore, this methodology was employed in the present study for evaluation and classification of the surface waters.
As depicted in Fig. 1, HCO3– and SO42– are the principal anions (approximately 57% and 42%, respectively) in surface waters of the non-KBD endemic areas in the west of the Longzi County, while the alkaline earth metal ions Ca2+ and Mg2+ are the primary cations (70% and 22%, respectively). Therefore, Ca–Mg–SO4–HCO3 and Ca–Mg–HCO3–SO4 are main types in water of these areas. In contrast, in the KBD endemic areas, HCO3– (approximately 82%) is the predominant anion, while SO42– (approximately 17%) is significantly lower. The alkaline earth metal ions Ca2+ (75%) and Mg2+ (15%) account for approximately 90% of cations, while the alkali metal ions K+ and Na+ are significantly low (< 10%). Therefore, the surface water of KBD endemic areas are dominated by low TDS waters with Ca–HCO3 and Ca–Mg–HCO3 as the principal hydrochemical species. Xiao52 reported negative correlations between the incidence of the KBD and Na+ and Mg2+ deficiencies in drinking water. In addition, Hu43 indicated that the consumption of low-salinity water with low Mg2+, Cl−, SO42−, and HCO3– is a major cause of KBD. Further, based on the analysis of major ions in waters in the Rangtang County, Wang and Xie51 suggested that most drinking water are acidic waters with low ion contents, especially for SO42−, in KBD endemic areas. Thus, based on previous studies, chemical characteristics of waters in the townships of Yuma and Zhari are conducive for KBD occurrence.
The measured total concentration of cations (Fig. 2a) and anions (Fig. 2b) are plotted as contours based on spatial interpolation. Obviously, both the cations and the anions exhibit lowest concentrations in the two townships in the east. Therefore, the spatial distribution of ions displays a relationship with the prevalence of the KBD in the Longzi County, which is consistent with previous studies in the Tibet Plateau23,38,48 and elsewhere in China40,43,52. These significant differences in the distribution of ions in the surface waters are attributed to many factors including the regional geology, local climate, and topography. As highlighted by the TDS and TH data, surface water TDS in the Longzi County is likely controlled by the Ca/Mg, carbonates/sulfates encountered. The interaction of rocks with flowing water causes dissolution of Ca/Mg carbonates and sulfates. The KBD endemic areas in Longzi County are on the south slope of the East Himalayas, which is characterized by a humid climate, lush vegetation, and waters with high organic contents. This regional setting can significantly limit water–rock interactions, thereby reducing the redox potential of the water system. Under such conditions, weathering is considerably decreased and this promotes low pH, salinity, and hardness, and these results are consistent with previous studies40,51. Other factors that affect the spatial distribution of ions in surface waters in the Longzi County, such as the geology, geography, and anthropogenic activities are examined subsequently.
Possible sources of ions.
In the Longzi County, surface water recharge is associated mainly with precipitation and melting of ice. The Gibbs boomerang envelope7 is a weight ratio simple plot of the TDS versus Na+/(Na++Ca2+) and the TDS versus Cl–/(Cl–+HCO3–), which highlights the relative significance of evaporation, weathering, and precipitation on hydrochemical characteristics. According to the Gibbs boomerang envelope, rock weathering is the principal control on major hydrochemical components of surface waters in the Longzi County (Fig. 3). This observation is consistent with that from a previous study of rivers in the Qinghai–Tibet Plateau6. However, in the KBD endemic areas (Yumai and Zhari townships), the composition of surface waters is dominantly controlled by precipitation, while evaporation prevails in other townships (Fig. 3).
Trace elements.
Although trace elements in the natural environment are present in low concentrations in humans because these cannot be synthesized directly, elements such as I, Cu, Mn, and Zn are essential for health. Among trace elements, ETEs and PTEs, such as Mn, Fe, Co, Cu, and Zn have been associated with the KBD in many research30,41,42,44–46. In particular, the Se-deficiency theory has been advanced in several studies50–52. However, the relationship between the Se content and the incidence of the KBD suggests that although Se deficiency is a likely environmental factor, it does not represent the principal cause of the disease.
Table 2
Summarized data for trace elements in surface waters from the non-KBD and KBD endemic areas in Longzi County.
Parameters
|
Non-KBD
|
|
|
|
KBD
|
|
|
|
K–W
|
Mina
|
Maxa
|
Meana
|
Mediana
|
SD
|
CVb
|
K–S
|
|
Mina
|
Maxa
|
Meana
|
Mediana
|
SD
|
CVb
|
K–S
|
|
p value
|
Al
|
0.37
|
20.37
|
5.34
|
3.17
|
5.09
|
13.88
|
0.00
|
|
0.93
|
22.22
|
6.19
|
4.13
|
5.58
|
6.00
|
0.01
|
|
0.21
|
Si
|
2.35
|
18.54
|
8.23
|
7.18
|
3.84
|
1.63
|
0.00
|
|
1.72
|
17.16
|
6.91
|
6.19
|
3.29
|
1.92
|
0.00
|
|
0.09
|
V
|
0.01
|
1.17
|
0.34
|
0.25
|
0.28
|
28.34
|
0.00
|
|
0.11
|
0.76
|
0.36
|
0.35
|
0.15
|
1.33
|
0.20
|
|
0.02
|
Mn
|
0.00
|
0.28
|
0.04
|
0.02
|
0.05
|
25.24
|
0.00
|
|
0.00
|
0.06
|
0.02
|
0.02
|
0.02
|
16.42
|
0.10
|
|
0.06
|
Fe
|
0.58
|
271.70
|
105.33
|
93.37
|
69.81
|
120.79
|
0.20
|
|
2.86
|
71.99
|
19.71
|
17.31
|
13.10
|
4.58
|
0.16
|
|
0.00
|
Co
|
0.02
|
0.18
|
0.06
|
0.06
|
0.03
|
1.80
|
0.12
|
|
0.01
|
0.44
|
0.04
|
0.02
|
0.08
|
15.94
|
0.00
|
|
0.00
|
Ni
|
0.02
|
2.47
|
1.15
|
1.06
|
0.52
|
25.76
|
0.20
|
|
0.11
|
0.84
|
0.32
|
0.29
|
0.17
|
1.58
|
0.01
|
|
0.00
|
Cu
|
0.00
|
0.64
|
0.16
|
0.10
|
0.17
|
165.49
|
0.00
|
|
0.02
|
0.44
|
0.15
|
0.14
|
0.10
|
5.43
|
0.00
|
|
0.37
|
Zn
|
0.03
|
5.49
|
1.03
|
0.55
|
1.18
|
34.84
|
0.00
|
|
0.01
|
3.57
|
0.47
|
0.18
|
0.89
|
177.33
|
0.00
|
|
0.00
|
As
|
0.01
|
1.55
|
0.29
|
0.18
|
0.33
|
41.57
|
0.01
|
|
0.17
|
1.49
|
0.58
|
0.49
|
0.32
|
1.95
|
0.02
|
|
0.00
|
Se
|
0.07
|
2.21
|
0.46
|
0.25
|
0.49
|
6.81
|
0.00
|
|
0.04
|
0.33
|
0.14
|
0.12
|
0.08
|
2.08
|
0.05
|
|
0.00
|
Mo
|
0.13
|
1.03
|
0.47
|
0.41
|
0.25
|
1.96
|
0.01
|
|
0.02
|
0.55
|
0.25
|
0.20
|
0.18
|
11.39
|
0.14
|
|
0.00
|
I
|
0.01
|
0.42
|
0.10
|
0.07
|
0.09
|
17.31
|
0.00
|
|
0.02
|
0.16
|
0.05
|
0.05
|
0.03
|
1.50
|
0.12
|
|
0.00
|
Note: a Concentrations data are in μg L–1; b Coefficient of variation (CV).
|
Data for trace elements and physicochemical parameters including Al, Si, V, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Mo, and I of surface waters in the non-KBD and KBD endemic areas in Longzi county, Tibet, are presented in Table 2. These elements were analyzed to evaluate their impact on the incidence of the KBD in the Longzi County. According to the K–S results, Co, Fe, and Ni are normally distributed in the non-KBD endemic areas, while Fe, I, Mn, Mo, and V are normally distributed in townships affected by the KBD (>0.1). According to data in Table 2, surface waters in the townships affected by the KBD exhibit lower concentrations for most trace elements compared to the non-KBD endemic areas. The K–W results reveal significantly higher Co, Fe, I, Mo, Ni, Se, and Zn in the non-KBD endemic areas (p<0.01), while the concentrations of As are significantly lower (p<0.01).
According to the Pearson correlation matrix in Fig. 4, relationships between trace elements in surface waters in the two areas differ, which indicate different sources or chemical behaviors53. For instance, in the non-KBD areas, strong positive correlations (p<0.01) with coefficients varying between 0.359–0.667 for Al and Co, Mn, and Ni, and 0.349–0.546 for Co and Cu, Ni, and Se were obtained. However, in the KBD endemic townships, Al displays a significant correlation only with I (0.513), while Co is significantly correlated with Ni (0.471), Si (0.705), and Zn (0.662). These differences in correlations between trace elements in the KBD and non-KBD endemic areas are attributed to local conditions, which are examined subsequently.
PCA was conducted to explore possible sources of trace elements by extracting influential factors associated with the dataset54. Five and six PCs were obtained for trace elements in surface waters from the non-KBD and KBD endemic areas, respectively, and the component matrixes and explained variance are presented in Table S1. In the non-KBD endemic areas, PC1 explains 26.40% of the total variance, with high loadings for Ni, Mn, Al, Co, and Fe; PC2 involves As and Se, and it accounts for 14.5% of the total variance; PC3 explains 14.04% of the total variance, and this is associated primarily with Cu, I, V, and Z; PC4 represents 9.49% of the total variance, and this mainly attributed to Si. The five PCs with eigenvalues > 1 account for 73.23% of the total variance. In the KBD endemic areas, PC1 is dominated by Co, Si, and Zn, and this represents 21.21% of the total variance; PC2 is accounts for 19.20% of the total variance, and it dominated by Se, Mo, Fe, and Ni; PC3 and PC4 are mainly controlled corresponding by Al and I and by Cu and Mn, which represent 14.64% and 11.38% of the total variance, respectively. The six PCs with eigenvalues > 1 account for 82.65% of the total variance. The PCs associated with trace elements for both areas are shown in Fig. 5.