Abstract

Shigaze city is situated in the southwestern Tibetan Plateau and is the second largest city in the Tibet Autonomous Region. Groundwater is the major source of domestic and drinking water for urban inhabitants. In this study, the major ion chemistry and a water quality assessment of groundwater were studied using geochemical methods and fuzzy comprehensive assessment. Groundwater was classified as slightly alkaline soft and hard freshwater, and the influence of anthropogenic activities on groundwater was relatively weak. The dominant cations and anions were Ca2+ and Mg2+ and HCO3 and SO42−, respectively. Overall, the mean concentrations of major ions in groundwater increase gradually over time, except for NO3; however, the mean value of pH decreases over time. Most groundwater samples belong to the type of HCO3-Ca, and the groundwater has a trend of evolution from HCO3-Ca to the mixed type. Rock weathering was the main hydrogeochemical process controlling groundwater hydrochemistry, and the dissolution of carbonate and silicate minerals were the primary contributors to the formation of the major ion chemistry of groundwater. Major ions of groundwater in the urban area of Shigaze are below the standard limits, and the groundwater is excellent for drinking according to the fuzzy comprehensive assessment.

INTRODUCTION

Groundwater is closely related to human society, such as domestic, industrial and agricultural activities (Tiwari et al. 2017). China's development is heavily dependent on groundwater, and more than 70% of cities in China utilize groundwater for drinking and domestic purposes (Liu et al. 2018). As an ideal drinking water source, groundwater has the advantages of good water quality, wide distribution and convenient use (Zhou et al. 2016). However, industrialization, urbanization, economic and population growth and many other human activities have had significant impacts on the groundwater environment, especially in developing countries (Ma et al. 2014; Villa-Achupallas et al. 2018). Water quality deterioration has become a serious problem worldwide (Wu et al. 2018). The water chemistry characteristics of groundwater are affected by various factors, such as precipitation, climate, hydrogeology, geology, hydrodynamic conditions and anthropogenic inputs, and the main ion chemistry of groundwater is the result of long-term interactions between groundwater and the surrounding environment (Xing et al. 2018). Understanding the chemistry of major ions in groundwater helps reveal the hydrogeochemical process and the quality evaluation of groundwater (Li et al. 2016). Numerous studies have been presented regarding the major ion chemical characteristics of groundwater in the world, and these studies have indicated the processes controlling hydro-chemical and water quality (Maharana et al. 2015; Xiao et al. 2015a; Li et al. 2016). Fuzzy comprehensive assessment (FCA) methods are widely used in groundwater quality evaluation (Zhang et al. 2012, 2017; Zhou et al. 2013). The results of fuzzy comprehensive evaluation are consistent with the basic characteristics of water quality, which not only highlight the influence of the main over-standard components in groundwater but also comprehensively consider the influence of other non-primary over-standard components.

The Qinghai-Tibet Plateau is one of the cleanest areas in the world and has long been the focus of international academic attention (Liu et al. 2018, 2019). Shigaze is one of the most important cities of the Tibet Autonomous Region (TAR). Groundwater is the major source of water supply and essentially supports the sustainable and healthy development of the social economy. With the rapid and sustainable development of the social economy, the impact of anthropogenic activities on groundwater is bound to increase. Therefore, it is of great significance to study the major ion chemistry and water quality assessment of groundwater in the urban area of Shigaze. However, research on groundwater hydrochemistry and water quality in Shigaze is notably limited. Therefore, the objectives of this manuscript are to (1) identify the hydro-chemical characteristics of groundwater and its controlling factors and (2) determine the spatial and temporal variations of groundwater quality in the urban area of Shigaze. This paper will be useful for understanding the hydro-geochemical evolution of groundwater and will benefit the protection of groundwater environments.

STUDY AREA

Shigaze (Figure 1), which is the second largest city in TAR, is located in the northern foot of the Himalaya Mountains and the alluvial plain at the intersection of the Yarlung Zangbo River and the Nianchu River. The Yarlung Zangbo River flows from west to east from the north of the city. The Nianchu River flows through the city from south to north and then flows into the Yarlung Zangbo River. The urban area of Shigaze is distributed along the Nianchu River, with a long north-south distance, a short east-west distance, and an average altitude of 3,850 m. Influenced by the alluviation of the Nianchu River, the basic topography of Shigaze shows the topographic characteristics of being high in the south and low in the north. The surrounding bedrock mountains are seriously affected by wind erosion and weathering, and the mountain shape is steep. The climate type of Shigaze is a plateau temperate semi-arid continental monsoon climate. The climate has the characteristics of low temperature, indistinct seasons, large temperature differences between day and night, distinct dry and rainy seasons, less precipitation, strong evaporation, strong solar radiation and long periods of sunshine.

Figure 1

Location of the study area and sampling locations.

Figure 1

Location of the study area and sampling locations.

The Nianchu River is the second tributary of the upper and middle reaches of the Yarlung Zangbo River, with a total length of 108.4 km and a basin area of 11,130 km2. The lower reaches of the basin are low hills and open valleys, which converge into the Yarlung Zangbo River north of Shigaze city. Groundwater in the urban area of Shigaze is mainly quaternary loose rock pore water. Groundwater exists in the pores of pebbles and gravel, and the groundwater depth is shallow. Both sides of the valley plain are bedrock mountainous areas with a modern riverbed in the middle, which is conducive to the collection and recharge of groundwater and the formation of a good storage space for groundwater. Atmospheric precipitation also controls the distribution and occurrence of groundwater. The infiltration recharge of atmospheric precipitation and the annual groundwater recharge of the Nianchu River are greater, while the groundwater recharge in the dry season is lower.

MATERIALS AND METHODS

Sampling and measurement

In this paper, groundwater samples were obtained from 10 wells (Figure 1) within the urban area of Shigaze city during March (dry season) and August (wet season) of 1996, 2002, 2009 and 2015. Before sampling, the clean plastic bottles used for sampling were cleaned 2–3 times at the sampling sites. After sampling, the water samples were refrigerated and sent to the laboratory as soon as possible for further water quality analysis.

Major cations such as Ca2+, Mg2+, Na+, and K+ were analysed by flame atomic absorption spectrophotometry (contrAA300, Jena, Germany), and total hardness (TH) and HCO3 were determined by a titrimetric method (Ministry of Health 2006). Major anions (SO42−, Cl and NO3) were measured using an ion chromatograph (833, Metrohm AG). Total dissolved solids (TDS) were determined by the oven drying method.

ANALYSIS METHODS

Traditional hydro-chemical methods

A Piper trilinear diagram is widely used in the classification and comparison of water types, and its advantages are that the method is not affected by human factors (Li et al. 2016; Zhang et al. 2018). The saturation index (SI) can identify the precipitation and dissolution state of related minerals in groundwater. In this paper, the SI values were calculated using PHREEQC software. 
formula
(1)
where IAP indicates the ion activity product and K is the mineral dissolution equilibrium constant. SI > 0, SI = 0 and SI < 0 represent the three states of the mineral in the supersaturated state, the equilibrium state and the unsaturated state, respectively.

Water-rock interactions, evaporation and atmospheric precipitation were three dominant natural mechanisms controlling water chemistry. Gibbs plots (Gibbs 1970) are widely used to identify the sources of water hydrochemistry compositions. The ionic ratios are a useful method for determining the types of hydro-geochemical processes controlling groundwater chemical composition (Colado et al. 2018; Zhang et al. 2018). In this study, the ratio graphs of ions were applied to determine the controlling factors of groundwater chemistry.

Chloro-alkaline indices (CAI) can be used to confirm the cation exchange process of groundwater systems (Schoeller 1967), and they are expressed as follows: 
formula
(2)
 
formula
(3)
where all values are expressed in meq/L. If CAI-1 > 0 and CAI-2 > 0, cation exchange processes have occurred, where the Na+ or K+ from the water has been replaced by Ca2+ and Mg2+ from aquifer material. If CAI-1 and CAI-2 < 0, then the inverse reactions have taken place.

Statistical analysis

Descriptive statistics are the basis of understanding the hydro-chemical characteristics. This study used descriptive statistics to analyse the general characteristics of major ions in groundwater in each year. In addition, the trend of ion change with time is determined by the least square method.

Principal component analysis (PCA) is a useful method to further understand the water chemistry data and identify the contribution of natural and human factors to the groundwater hydrochemistry components (Campo et al. 2014). PCA can reduce the dimension of the data matrix by converting the related variables into a smaller number of irrelevant parameters (Yang et al. 2016).

Water quality assessment

The groundwater in the study area is mainly used for irrigation, domestic and drinking purposes. In this study, the groundwater quality was assessed using an FCA according to the Standard for Groundwater Quality of China (SGQC) (SQGC 2017).

FCA is a popular method to assess the water quality of groundwater. The fuzzy membership function was computed using the following formulas (Zhang et al. 2012): 
formula
(4)
where is the fuzzy membership of factor i to j, j is the groundwater quality rank (1–5), is the determined value of parameter i, and is the standard value of the parameter.
The fuzzy membership matrix R is composed of a parameter and class and is expressed as 
formula
(5)
The normalized weight of each factor can be expressed as 
formula
(6)
where indicates the normalized weight of parameter i, and is the mean of the standard value of each class.
The fuzzy comprehensive evaluation is based on matrix D and is expressed as 
formula
(7)
where W is the weight of each water quality factor.

RESULTS AND DISCUSSION

Hydro-chemical characteristics

The statistical analysis results for groundwater in 1996, 2002, 2009 and 2015 in the Shigaze urban area are shown in Table 1. The mean pH values were 7.46, 7.70, 7.77 and 7.69 in the dry seasons of 1996, 2002, 2009 and 2015, respectively. With respect to the wet season, the mean pH values were 7.88, 7.79, 7.40 and 7.41 in each year, respectively. This result indicates that the groundwater of the study area is slightly alkaline. Overall, the average concentration of major ions in the groundwater increased gradually over time, except for NO3. The mean pH values decreased over time (Figure 2). In addition, the content of K+ in 1996 was higher than that in other years, and it has been increasing gradually since 2002.

Table 1

Chemical compositions of groundwater samples

TimeParametersNa+K+Ca2+Mg2+HCO3SO42−ClpHTDSNO3TH
1996D Max 26.22 7.64 101.73 32.60 344.50 74.49 41.23 8.00 686.89 56.70 383.72 
Min 5.14 0.75 37.57 0.94 137.68 7.12 0.01 7.10 231.16 0.54 97.68 
Mean 14.71 2.54 64.26 16.76 223.88 38.76 12.58 7.46 425.87 19.73 229.41 
SD 6.70 2.77 22.83 10.94 75.91 24.62 16.62 0.30 150.10 22.08 95.19 
1996 W Max 28.37 11.28 96.99 36.63 352.68 97.16 35.08 8.02 739.74 59.23 392.92 
Min 5.10 0.42 41.25 2.40 142.31 7.38 1.79 7.60 232.56 0.32 112.88 
Mean 16.97 3.73 70.77 20.65 248.27 50.07 21.07 7.88 442.41 25.40 261.68 
SD 8.65 4.71 20.22 13.14 84.27 33.58 14.68 0.14 158.94 26.96 101.60 
2002D Max 46.50 1.50 98.00 40.19 462.90 134.13 42.26 8.10 834.53 67.11 410.09 
Min 6.64 0.31 39.01 3.20 138.02 10.96 0.40 7.70 251.69 0.00 110.58 
Mean 20.72 0.67 66.65 18.56 245.25 56.89 15.18 7.86 464.66 17.94 242.79 
SD 13.03 0.37 20.99 13.47 106.65 37.99 14.55 0.15 194.63 22.50 107.86 
2002 W Max 36.59 0.89 97.61 32.48 335.50 71.87 56.18 8.22 677.57 56.07 377.39 
Min 7.96 0.22 48.33 1.68 138.02 11.31 1.04 7.40 267.15 1.38 127.59 
Mean 17.99 0.47 66.20 16.66 228.80 37.41 22.81 7.79 429.65 13.73 233.88 
SD 10.47 0.22 20.68 10.77 75.34 22.44 25.27 0.25 142.44 18.89 95.95 
2009D Max 48.73 1.95 129.03 45.97 525.28 97.60 61.44 8.08 946.66 67.20 511.35 
Min 11.10 0.60 41.13 5.87 141.62 18.36 5.65 7.40 249.64 0.49 126.86 
Mean 21.17 1.05 77.59 25.83 271.22 54.90 29.04 7.77 528.19 26.74 300.03 
SD 12.72 0.46 29.91 13.02 120.83 23.19 22.71 0.22 209.19 25.76 128.26 
2009 W Max 57.98 2.69 131.46 31.11 489.11 84.53 66.45 7.80 918.99 80.00 456.27 
Min 7.08 0.87 31.26 4.38 108.01 19.21 4.85 6.90 236.27 4.00 128.05 
Mean 22.27 1.24 77.76 17.29 237.99 53.69 34.34 7.40 493.62 28.28 265.29 
SD 16.12 0.56 33.82 9.41 129.08 20.64 24.22 0.28 227.91 27.52 116.35 
2015D Max 42.97 2.23 116.66 42.45 437.50 117.83 61.84 7.80 861.61 60.78 465.98 
Min 5.99 0.76 40.00 10.11 163.85 33.95 3.84 7.49 292.16 2.00 141.48 
Mean 17.26 1.17 76.66 26.38 246.79 77.09 27.43 7.69 514.29 18.75 299.97 
SD 11.59 0.47 27.84 8.79 88.18 26.42 23.54 0.11 180.31 17.49 98.87 
2015 W Max 63.55 2.87 149.99 45.58 493.60 168.74 79.65 7.68 1,053.07 60.00 562.09 
Min 10.55 0.78 40.00 14.15 154.88 78.37 4.19 7.20 343.13 2.00 170.58 
Mean 29.22 1.34 86.49 29.02 254.60 125.45 33.22 7.46 603.91 19.44 335.38 
SD 16.83 0.65 33.64 9.79 105.44 30.86 26.79 0.14 216.38 22.72 115.99 
TimeParametersNa+K+Ca2+Mg2+HCO3SO42−ClpHTDSNO3TH
1996D Max 26.22 7.64 101.73 32.60 344.50 74.49 41.23 8.00 686.89 56.70 383.72 
Min 5.14 0.75 37.57 0.94 137.68 7.12 0.01 7.10 231.16 0.54 97.68 
Mean 14.71 2.54 64.26 16.76 223.88 38.76 12.58 7.46 425.87 19.73 229.41 
SD 6.70 2.77 22.83 10.94 75.91 24.62 16.62 0.30 150.10 22.08 95.19 
1996 W Max 28.37 11.28 96.99 36.63 352.68 97.16 35.08 8.02 739.74 59.23 392.92 
Min 5.10 0.42 41.25 2.40 142.31 7.38 1.79 7.60 232.56 0.32 112.88 
Mean 16.97 3.73 70.77 20.65 248.27 50.07 21.07 7.88 442.41 25.40 261.68 
SD 8.65 4.71 20.22 13.14 84.27 33.58 14.68 0.14 158.94 26.96 101.60 
2002D Max 46.50 1.50 98.00 40.19 462.90 134.13 42.26 8.10 834.53 67.11 410.09 
Min 6.64 0.31 39.01 3.20 138.02 10.96 0.40 7.70 251.69 0.00 110.58 
Mean 20.72 0.67 66.65 18.56 245.25 56.89 15.18 7.86 464.66 17.94 242.79 
SD 13.03 0.37 20.99 13.47 106.65 37.99 14.55 0.15 194.63 22.50 107.86 
2002 W Max 36.59 0.89 97.61 32.48 335.50 71.87 56.18 8.22 677.57 56.07 377.39 
Min 7.96 0.22 48.33 1.68 138.02 11.31 1.04 7.40 267.15 1.38 127.59 
Mean 17.99 0.47 66.20 16.66 228.80 37.41 22.81 7.79 429.65 13.73 233.88 
SD 10.47 0.22 20.68 10.77 75.34 22.44 25.27 0.25 142.44 18.89 95.95 
2009D Max 48.73 1.95 129.03 45.97 525.28 97.60 61.44 8.08 946.66 67.20 511.35 
Min 11.10 0.60 41.13 5.87 141.62 18.36 5.65 7.40 249.64 0.49 126.86 
Mean 21.17 1.05 77.59 25.83 271.22 54.90 29.04 7.77 528.19 26.74 300.03 
SD 12.72 0.46 29.91 13.02 120.83 23.19 22.71 0.22 209.19 25.76 128.26 
2009 W Max 57.98 2.69 131.46 31.11 489.11 84.53 66.45 7.80 918.99 80.00 456.27 
Min 7.08 0.87 31.26 4.38 108.01 19.21 4.85 6.90 236.27 4.00 128.05 
Mean 22.27 1.24 77.76 17.29 237.99 53.69 34.34 7.40 493.62 28.28 265.29 
SD 16.12 0.56 33.82 9.41 129.08 20.64 24.22 0.28 227.91 27.52 116.35 
2015D Max 42.97 2.23 116.66 42.45 437.50 117.83 61.84 7.80 861.61 60.78 465.98 
Min 5.99 0.76 40.00 10.11 163.85 33.95 3.84 7.49 292.16 2.00 141.48 
Mean 17.26 1.17 76.66 26.38 246.79 77.09 27.43 7.69 514.29 18.75 299.97 
SD 11.59 0.47 27.84 8.79 88.18 26.42 23.54 0.11 180.31 17.49 98.87 
2015 W Max 63.55 2.87 149.99 45.58 493.60 168.74 79.65 7.68 1,053.07 60.00 562.09 
Min 10.55 0.78 40.00 14.15 154.88 78.37 4.19 7.20 343.13 2.00 170.58 
Mean 29.22 1.34 86.49 29.02 254.60 125.45 33.22 7.46 603.91 19.44 335.38 
SD 16.83 0.65 33.64 9.79 105.44 30.86 26.79 0.14 216.38 22.72 115.99 
Figure 2

Bar diagram of the mean value of ions in groundwater.

Figure 2

Bar diagram of the mean value of ions in groundwater.

As shown in Table 1, HCO3 and SO42− are the predominant anions in the groundwater. The mean concentrations of SO42− were 38.76, 56.89, 54.98 and 77.09 mg/L in the dry season of each year and 50.07, 37.41, 53.69 and 125.45 mg/L in the wet season (Table 1), respectively, which were lower than the national standards (250 mg/L). The average concentrations of Cl were 12.5, 15.18, 29.04 and 27.43 mg/L in the dry season and 21.07, 22.81, 34.34 and 33.27 mg/L in the wet season of each year, respectively, which were far below the permissible limit of the national standards (250 mg/L). Affected by human activities, NO3 has been one of the major groundwater pollutants worldwide (Liu et al. 2019). The concentrations of NO3 varied from 0.54–56.70, 0–67.11, 0.49–67.20 and 2–60.78 mg/L in the dry season of each year, and with respect to the wet season, the concentrations of NO3 ranged from 0.32 to 59.32, 1.38 to 56.07, 4.00 to 80.00 and 2.00 to 60.00 mg/L, respectively. The groundwater of the study area has a relatively low concentration of NO3, indicating that the influence of anthropogenic activities on groundwater in the urban area of Shigaze was relatively weak.

Ca2+ and Mg2+ were the dominant cations in the groundwater. The mean concentrations of Ca2+ were 64.26, 66.65, 77.59 and 76.66 mg/L in the dry season and 70.77, 66.20, 86.49 and 86.49 mg/L in the wet season of each year, respectively. The average concentrations of Na+ were 14.71, 20.72, 21.17 and 17.26 mg/L in the dry season and 16.97, 17.99, 22.27, and 29.22 mg/L in the wet season, respectively, which were also far below the national standard limit of 250 mg/L.

TDS and TH are two important indices used to assess water quality (Li et al. 2014). As shown in Table 2, the concentrations of TDS ranged from 231.16–686.89, 251.69–834.53, 249.64–946.66 and 292.16–861.61 mg/L in the dry season of each year and from 232.56–739.74, 267.15–677.57, 236.27–918.99 and 343.13–1,053.07 mg/L in the wet season, respectively. Sample Z02 had the highest TDS (1,053.07 mg/L) content in the wet season of 2015. The TH ranged from 141.51–466.11 mg/L, 97.68–383.72, 110.58–410.09, 126.86–511.35, and 141.48–465.98 mg/L in the dry season of each year and from 170.58–562.09, 127.59–377.39, 128.05–456.27 and 112.88–392.92 in the wet season, respectively.

Table 2

Rotating factor loading matrix and the variance contribution rate

ParameterComponent
PC1PC2
HCO3 0.917 0.097 
K+ 0.353 0.250 
Mg2+ 0.924 0.074 
Ca2+ 0.736 0.592 
SO42− 0.803 0.196 
Na+ 0.799 0.406 
Cl 0.429 0.782 
NO3 −0.12 0.955 
Eigenvalue 3.828 20,156 
Variance (%) 47.852 26.948 
Cumulative of variance (%) 47.852 74.800 
ParameterComponent
PC1PC2
HCO3 0.917 0.097 
K+ 0.353 0.250 
Mg2+ 0.924 0.074 
Ca2+ 0.736 0.592 
SO42− 0.803 0.196 
Na+ 0.799 0.406 
Cl 0.429 0.782 
NO3 −0.12 0.955 
Eigenvalue 3.828 20,156 
Variance (%) 47.852 26.948 
Cumulative of variance (%) 47.852 74.800 

As shown in Figure 3, most of the water samples belong to soft-fresh water and hard-fresh water, and only one sample (Z02) is hard-brackish water, indicating that the groundwater in the urban area of Shigaze is suitable for drinking based on the TDS and TH. In addition, the major ions (Ca2+, K+, Na+, SO42− and Cl) were below the national standard limits (Table 1), indicating that the groundwater in the urban area of Shigaze is good for drinking.

Figure 3

Groundwater quality in the study area based on TDS and TH.

Figure 3

Groundwater quality in the study area based on TDS and TH.

Comparison with other regions

The valley plain area of Lhasa city is another area with intensive human activity in the Qinghai-Tibet Plateau. Compared with this study, the chemical compositions of groundwater have also increased significantly over time (Liu et al. 2018). The average ion contents in groundwater in 2015 were higher than those in groundwater and spring water in the Yarlung Zangbo River valley plain, except for SO42− (Liu et al. 2019). The TDS was lower than the shallow groundwater in the permafrost region on the Qinghai-Tibet Plateau (Cheng & Jin 2013). The mean concentrations of chemical components in groundwater in this study in 2015 were higher than those in the river water of the Tarim River basin (Xiao et al. 2012), except for the TDS, while the values were lower than those in the groundwater of the Tarim River basin, except for HCO3 (Xiao et al. 2015b).

Water types

As shown in Figure 4, groundwater hydro-chemical data from 1996, 2002, 2009 and 2015 in the study area were located in the Piper diagram using Aq·QA software. In terms of cations, most of the water samples were located in zone A of Figure 4, indicating that the samples belong to the calcium type. With respect to anions, most groundwater samples were plotted in zone E, suggesting bicarbonate-type water. As shown in Figure 4, most samples are located in zone 5, and only a few samples are plotted in zone 9, which means that HCO3-Ca·is the dominant hydro-chemical factor. In addition, it was obvious that the hydro-chemical types of groundwater in the study area evolved from HCO3-Ca to the mixed type over time.

Figure 4

Piper trilinear diagram of groundwater samples.

Figure 4

Piper trilinear diagram of groundwater samples.

Saturation index

The SI values were calculated using PHREEQC software. The results of the calculated SI are illustrated in Table 2. The SI values of calcite ranged from −0.62 to 1.02, with a mean of 0.44. With respect to dolomite, the calculated SI values ranged from −1.89 to 1.96, with an average of 0.63, and for gypsum, the SI values varied from −2.68 to −1.25, with a mean of −1.91. Most of the SI values of dolomite and calcite were greater than zero, which indicated that the groundwater was saturated with regard to carbonate minerals. In addition, the SIs of anhydrite, gypsum and halite were negative, and the SI of halite was quite negative, which means that the dissolution of halite was weak.

Gibbs plots

As shown in Figure 5, all water samples are located in the rock weathering dominance zone, suggesting that rock weathering is the major hydrogeochemical process controlling groundwater chemistry in the study area.

Figure 5

Gibbs diagram for groundwater samples.

Figure 5

Gibbs diagram for groundwater samples.

Ratio graphs of ions

Theoretically, the dissolution of halite (NaCl → Na+ + Cl) releases the same amount of Na+ and Cl. However, Figure 6(a) shows that all the groundwater samples are plotted above or below the 1:1 line, which means that the dissolution of halite is not the main source of Na+ in the groundwater. In addition, the excess Na+ probably originates from the weathering of silicate and cation exchange (Li et al. 2016). If the dissolution of carbonates (calcite and dolomite) and sulphate (gypsum) are the dominant reactions in the groundwater system, the ratio of (Ca2+ + Mg2+) versus (HCO3 + SO42−) should be close to 1 (Marghade et al. 2015; Yang et al. 2016). Figure 6(b) shows that most groundwater samples are plotted above or below the 1:1 line and some fall on the 1:1 line, suggesting that the weathering of carbonate, silicate and sulphate rocks were the dominant hydrogeochemical processes in groundwater. Likewise, if Ca2+ and SO42− originate from the dissolution of gypsum, the water samples should be plotted along the 1:1 line. However, all the groundwater samples above the 1:1 line (Figure 6(c)), which indicates that the dissolution of gypsum is not the main source of SO42−, and Ca2+ is the result of other rock weathering, such as carbonate or silicate. As shown in Figure 6(d), most of the water samples are close to the 1:1 line and are located between the 1:1 line and the 1:2 line, suggesting that the dissolution of calcite and dolomite are the primary factors resulting in the sources of Ca2+ and HCO3.

Figure 6

Plots of (a) Na+ versus Cl, (b) (Ca2+ + Mg2+) versus (HCO3 + SO42−), (c) Ca2+ versus SO42−, (d) Ca2+ versus HCO3, (e) CAI-2 versus CAI-1 and (f) (Na+ + K+)-Cl versus (Ca2+ + Mg2+)-(HCO3 + SO42−).

Figure 6

Plots of (a) Na+ versus Cl, (b) (Ca2+ + Mg2+) versus (HCO3 + SO42−), (c) Ca2+ versus SO42−, (d) Ca2+ versus HCO3, (e) CAI-2 versus CAI-1 and (f) (Na+ + K+)-Cl versus (Ca2+ + Mg2+)-(HCO3 + SO42−).

Cation exchange

The indices of CAI-1 and CAI-2 of the groundwater samples ranged from −59.42 to 0.59 and from −0.23 to 0.19, respectively. As shown in Figure 6(e), the CAI of most groundwater samples in the dry and wet seasons were negative, indicating the cation exchange of Ca2+ and Mg2+ in the solution with Na+ and K+ in the aquifer materials.

The bivariate diagram of (Na+ + K+)-Cl versus (Ca2+ + Mg2+)-(HCO3 + SO42−) can be used to determine the cation exchange in groundwater. If cation exchange plays a dominant role in groundwater, water samples should be distributed along a line with a slope of −1 (Li et al. 2016). Figure 6(f) shows that most of the samples are above the y = −x line and some samples are located on the y = −x line, indicating that cation exchange exists, but it is not dominant in the groundwater system.

Principal component analysis

In this paper, 80 hydro-chemical data were taken each year and from each point individually, including information on the major ions (Ca2+, Mg2+, Na+, K+, HCO3, Cl, SO42− and NO3) of groundwater, and these data were used for PCA to interpret the principal factor controlling water chemistry. Data were standardized before PCA. In addition, the PCA method is based on eigenvalues >1 (Figure 7) and was also checked by the Kaiser-Meyer-Olkin measure (0.617). As shown in Table 2, the two most significant principal factors were obtained, accounting for 74.800% of the total variance, which suggests that the two principal components could reflect most of the groundwater chemistry.

Figure 7

Scree plot and factor loading for PC1 and PC2.

Figure 7

Scree plot and factor loading for PC1 and PC2.

PC1 explained 47.852% of the total variance and had a strong loading of HCO3, Mg2+, Ca2+, Na+ and SO42−, indicating that PC1 represents water-rock interactions, including weathering and dissolution of rocks and minerals such as carbonate, silicate and sulphate minerals. PC2 was responsible for 26.948% of the total variance and had a positive loading of Cl and NO3. The higher loading of NO3 could be attributed to anthropogenic activities such as domestic sewage, agriculture and industry. Therefore, PC2 mainly reflects the impacts of human activities on groundwater chemistry. However, the contents of Cl and NO3 in the study area were low; thus, the impact of human activities in the study area was relatively weak.

Groundwater quality assessment

Understanding the quality of groundwater is important because it determines whether groundwater is suitable for drinking purposes (Yang et al. 2016; Colado et al. 2018; Rabeiy 2018). Groundwater is the main drinking water source in the urban area of Shigaze. To determine the quality of the groundwater, FCA was used to evaluate the groundwater quality in the urban area of Shigaze. According to the SQGC (SQGC 2017), the TDS, TH, Na+, Cl, SO42− and NO3 were used to assess groundwater quality for drinking.

The results of the groundwater quality assessment according to FCA are shown in Table 3 and Figure 8. The classes of water quality ranged from I to III in the dry and wet seasons, indicating that the groundwater quality ranged from excellent to medium. Overall, the groundwater quality of the urban area of Shigaze is quite suitable as a source of drinking water.

Table 3

Groundwater quality assessment based on FCA

Sample no.1996
2002
2009
2015
DWDWDWDW
Z01 II 
Z02 II III III II III III III III 
Z03 II II II II II II II II 
Z04 II II II II II III II II 
Z05 II II 
Z06 
Z07 II 
Z08 II II II 
Z09 II II II II III II II II 
Z10 II II II 
Sample no.1996
2002
2009
2015
DWDWDWDW
Z01 II 
Z02 II III III II III III III III 
Z03 II II II II II II II II 
Z04 II II II II II III II II 
Z05 II II 
Z06 
Z07 II 
Z08 II II II 
Z09 II II II II III II II II 
Z10 II II II 
Figure 8

Results of groundwater quality for drinking purposes.

Figure 8

Results of groundwater quality for drinking purposes.

This study shows that the groundwater quality in the urban area of Shigaze city is excellent and is an ideal drinking water source. However, it is also obvious that the content of major ions in groundwater increases with time, which indicates that the development of cities and human activities have a certain impact on groundwater. In the process of continuous urban development, we should strengthen the protection of the groundwater environment, control potential pollution sources, and strengthen dynamic groundwater monitoring to achieve sustainable utilization of groundwater resources.

CONCLUSIONS

  • (1)

    The concentrations of major anions and cations were relatively low in the groundwater, suggesting that the influence of anthropogenic activities on groundwater is relatively weak. Groundwater in the study area is classified as slightly alkaline freshwater and hard water. The major cations are Ca2+ and Mg2+, and the major anions are HCO3 and SO42−.

  • (2)

    Overall, the hydro-chemical types of groundwater in the urban area of Shigaze are the HCO3-Ca type, and the groundwater has a trend of evolution from HCO3-Ca to the mixed type. The Gibbs diagram shows that the groundwater hydrochemistry was controlled by the weathering of rocks, and the ratio graphs of ions show that the dissolution of carbonate and silicate were the major hydrogeochemical processes affecting groundwater hydrochemistry.

  • (3)

    Major cations and anions of groundwater in the urban area of Shigaze are below the standard limits, indicating good water quality. Based on TDS, TH and an FCA, the groundwater in the study area is excellent for domestic use and drinking.

ACKNOWLEDGEMENTS

This research was supported by the Tibet Institute of Geological Environment Monitoring and the Center for Hydrogeology and Environmental Geology, China Geological Survey (12120114059601, DD20160298). The authors are thankful to the staff of the Tibet Institute of Geological Environment Monitoring for their efforts in the monitoring of groundwater quality.

REFERENCES

REFERENCES
Campo
M. A. M. D.
Esteller
M. V.
Expósito
J. L.
Hirata
R.
2014
Impacts of urbanization on groundwater hydrodynamics and hydrochemistry of the Toluca Valley aquifer (Mexico)
.
Environ. Monit. Assess.
186
(
5
),
2979
2999
.
Gibbs
R. J.
1970
Mechanisms controlling world water chemistry
.
Science
170
(
3985
),
870
.
Liu
J. T.
Gao
Z. J.
Wang
M.
Li
Y. Z.
Ma
Y. Y.
Shi
M. J.
Zhang
H. Y.
2018
Study on the dynamic characteristics of groundwater in the valley plain of Lhasa City
.
Environ. Earth Sci.
77
(
18
),
646
.
Liu
J. T.
Gao
Z. J.
Wang
M.
Li
Y. Z.
Shi
M. J.
Zhang
H. Y.
Ma
Y. Y.
2019
Hydrochemical characteristics and possible controls in the groundwater of the Yarlung Zangbo River Valley, China
.
Environ. Earth Sci
.
76
(
78
).
Maharana
C.
Gautam
S. K.
Singh
A. K.
Tripathi
J. K.
2015
Major ion chemistry of the Son River, India: weathering processes, dissolved fluxes and water quality assessment
.
J. Earth Syst. Sci.
124
(
6
),
1293
1309
.
Ministry of Health
2006
Standard Examination Methods for Drinking Water-organoleptic and Physical Parameters, 2006. GB5750.4-2006
.
Ministry of Health of the People's Republic of China
,
Beijing, China
.
Schoeller
H.
1967
Qualitative evaluation of groundwater resources
. In:
Methods and Techniques of Groundwater Investigation and Development
.
Water Resource Series 33
,
UNESCO
,
Paris, France
, pp.
54
83
Standard for groundwater quality of China
2017
Ministry of Natural Resources of the People's Republic of China
.
Villa-Achupallas
M.
Rosado
D.
Aguilar
S.
Galindo-Riaño
M. D.
2018
Water quality in the tropical Andes hotspot: the Yacuambi river (southeastern Ecuador)
.
Sci. Total Environ.
633
,
50
.
Wu
Z.
Wang
X.
Chen
Y.
Cai
Y.
Deng
J.
2018
Assessing river water quality using water quality index in Lake Taihu Basin, China
.
Sci. Total Environ.
612
,
914
922
.
Xing
L. T.
Huang
L. X.
Hou
X. Y.
Yang
L. Z.
Chi
G. Y.
Xu
J. X.
Zhu
H. H.
2018
Groundwater Hydrochemical Zoning in Inland Plains and its Genetic Mechanisms
.
Water
10
(
6
),
752
.
Zhang
B.
Song
X. F.
Zhang
Y. H.
Han
D. M.
Tang
C. Y.
Yu
Y. L.
Ma
Y.
2012
Hydrochemical characteristics and water quality assessment of surface water and groundwater in Songnen plain, Northeast China
.
Water Res.
46
(
8
),
2737
2748
.
Zhang
Q.
Wang
S. L.
Yousaf
M.
Wang
S. X.
Nan
Z. R.
Ma
J. M.
Wang
D. P.
Zang
F.
2017
Hydrochemical characteristics and water quality assessment of surface water in the northeast Tibetan plateau of China
.
Water Sci. Technol. Water Supply
18
(
5
),
1757
1768
.
Zhang
T.
Cai
W. T.
Li
Y. Z.
Geng
T. T.
Zhang
Z. Y.
Lv
Y. G.
Zhao
M.
Liu
J. W.
2018
Ion chemistry of groundwater and the possible controls within Lhasa River Basin, SW Tibetan Plateau
.
Arabian J. Geosci.
11
,
510
.