3.1 Characteristics of groundwater hydrochemical parameters
As shown in Fig. 2, in the Cele and Fukang areas of South and North Xinjiang, the order of the milligram equivalent concentrations of the major cations in groundwater and surface water from high to low is Na+ > Ca2+ > Mg2+ > K+, and the order of the milligram equivalent concentrations of the major anions from high to low is SO42− > Cl− > HCO3−. In the Fukang region of North Xinjiang, the ion content of groundwater is generally higher than that of surface water, and in the Cele region of South Xinjiang, the difference between the ion content of surface water and groundwater is smaller. During the period 2006–2016, groundwater ion concentrations in the Northern Territory fluctuated, with a general downward trend and insignificant changes in surface water ion content; the concentrations of cations Ca2+ and K+ fluctuate and change more significantly in South Xinjiang, while anions Cl− and HCO3− have smaller variations and Mg2+, Na+ and SO42− are more stable in surface water and groundwater. In groundwater, the concentrations of Mg2+, Na+, Cl−, SO42− and TDS were significantly higher in the North than in the South; in surface water, the concentrations of Ca2+, Mg2+, K+, Cl− and SO42− were higher in the South than in the North.
3.2 Correlation analysis
Correlation analysis is effective in analysing the chemical characteristics of water and can more clearly characterise the consistency and variability of groundwater sources (Yang et al., 2020b). As can be seen from Fig. 3, in the Cele region of South Xinjiang, Cl− and SO42− correlated significantly with each cation except Ca2+, and had the highest correlation with Na+, reaching 0.62 and 0.7, respectively, and HCO3− had the highest correlation with Mg2+, indicating a similar origin; in the Fukang region of North Xinjiang, the correlations of Cl− and SO42− with other ions were similar to those of South Xinjiang, and were also highest with Na+, reaching 0.96 and 0.95, respectively, with significant correlations higher than those of South Xinjiang, and HCO3− had the highest correlation with Na+, indicating a similar origin. The correlation between TDS and each ion can be a good cause of natural water bodies. There is a good correlation between TDS and each ion content in the north and south of the border, with the highest correlation with SO42−, Cl− and Na+, indicating that the mineralisation of natural water bodies is controlled by Na+ and SO42−.
3.3 Types of water chemistry
Piper trilinear plots were used to analyse the hydrochemical characteristics of groundwater samples in the study area, i.e. the ratio of anions to cations per millilitre equivalent (%), to determine the relative contribution of different rock weathering to groundwater ion concentrations. When mainly affected by carbonate weathering, the anions and cations are mainly near the HCO3− and Ca2+ ends, respectively, while when mainly affected by evaporite salt weathering, the anions and cations are mainly near the SO42−~Cl− and K+~Na+ ends, respectively. As can be seen from Fig. 4, the cations in the Cele region of South Xinjiang and the Fukang region of North Xinjiang are mainly close to the Ca2+ end element and the anions are mainly close to the HCO3− end element, indicating the influence of carbonate weathering.
The diamond-shaped area of the Piper diagram is divided into nine zones indicating the nine types of water chemistry, or the Ca2+-HCO3−, Ca2+-SO42−, Na+-Cl−, Na+-HCO3− and mixed types (no pair of anions or cations greater than 50%) depending on whether the dominant anion or cation exceeds 50%, corresponding to zones 5, 6, 7, 8 and 9 in the diamond-shaped area of the Piper diagram, respectively. The ions in the Cele region of South Xinjiang are mainly concentrated in regions 7 and 9, indicating that the water chemistry type in this region is mainly Na+ -Cl− and mixed; the ions in the Fukang region of North Xinjiang are mainly concentrated in regions 5, 7 and 9, indicating that the water chemistry type in this region is mainly Ca2+-HCO3− and Na+-Cl− and mixed. Compared to the northern and southern regions, the northern region has more complex water chemistry types than the southern region, which is also closely related to the differences between the climatic environments of the two regions.
3.4 Gibbs plot based water chemistry analysis
The semi-logarithmic coordinate solution of the Gibbs plot is an important method for analysing the evolution of the water composition of groundwater in a study area (Ma et al., 2021). The water chemistry of natural water bodies has a variety of causes and is closely related to the various hydrogeochemical processes they undergo. Gibbs, through his study of natural water bodies around the world, summarised them into three genesis factors: atmospheric precipitation, lithology and evaporation-crystallisation (Akakuru et al., 2022).As can be seen from Fig. 5, the natural water bodies in the Cele region of South Xinjiang are mainly influenced by the control of rock weathering and a small part of them lie outside the Gibbs diagram, indicating that they are influenced by anthropogenic fac-tors; in contrast, the natural water bodies in the Fukang region of Northern Xinjiang are clearly controlled by factors, with surface water mainly controlled by rock weathering and groundwater mainly by the controlling effect of evaporative crystallisation.
3.5 Gaillardet end metaplot analysis
Weathering of evaporite, silicate and carbonate rocks provides a material source for river solutes. Gaillardet studied water chemistry data from 60 large rivers around the world, developed a computational model based on normalised molar ratios of sodium ions, and mapped evaporite, silicate, and carbonate ends to identify possible sources of river solutes (Gaillardet et al., 1997).
The ratio relationships of Mg2+/Na+, Ca2+/Na+ and HCO3−/Na+ in water bodies are commonly used to study the interactions between water bodies and various rock masses (Buccianti et al., 2014).As shown in Fig. 6, in the Fukang area of North Xinjiang, flowing surface water is mainly distributed between the carbonate and silicate end elements, closer to the carbonate end elements, indicating that flowing surface water is mainly influenced by the weathering of the carbonate rocks; the stationary surface water is closer to the silicate end element and weathering of the silicate rocks has a greater impact on the stationary surface water; desert groundwater and arable groundwater are mainly distributed between evaporitic saline and silicate rock end elements, indicating that weathering of the silicate and evaporite rocks is the main controlling factor for groundwater components. In the Cele region of South Xinjiang, surface water and groundwater are mainly close to the silicate and carbonate end elements, indicating that the chemical components of surface water and groundwater are mainly controlled by the joint weathering of silicate and carbonate rocks, with the weathering of silicate and saline rocks having a greater influence on the chemical components of water than carbonate rocks.
3.6 Analysis based on ion ratios and ion sources
The ratio of Na++K+ to Cl− is equal to 1, indicating that the chemical composition of the water body comes mainly from the dissolution of evaporite (Pant et al., 2018). Figure 7 shows that the ion samples from the Fukang region of North Xinjiang all lie to the upper left of the 1:1 contour, indicating that Na+ and K+ are in excess compared to Cl−, while the ion samples from the Cele region of South Xinjiang are mainly distributed on both sides of the 1:1 contour, with some samples all lying on the 1:1 contour, indicating that their chemical composition is related to evaporation, with some samples particularly close to the 1:1 contour, indicating that dissolution of evaporite has a chemical composition has a greater influence.
In terms of the ratio of HCO3−+SO42− to Ca2++Mg2+, if a sample lies on the 1:1 equivalence, this suggests that Ca2+ and Mg2+ originate from a combination of silicate rock, evaporite weathering dissolution and carbonate rock dissolution (Singh et al., 2012). Samples located to the lower right of the contour are likely to contain Ca2+ and Mg2+ derived from dissolved carbonate rocks (Mahaqi et al., 2018), while samples located to the upper left of the contour are more likely to contain Ca2+ and Mg2+ derived from silicate weathering (Saleem et al., 2015).Surface and groundwater samples from Fukang, North Xinjiang, lie mainly to the upper left of the contours without excessive deviation, suggesting that Ca2+ and Mg2+ originate from dissolution and evaporation from silicate rocks. However, some samples from southern Cele deviated to the lower right of the 1:1 contour, suggesting that surface water groundwater dissolved some of the carbonate rocks. The distribution of surface water and groundwater sample sites indicates that dissolution of silicate rocks and evaporites are the main contributors of ions in water, while dissolution of carbonate rocks is not significant.
3.7 Irrigation water suitability assessment
The use of substandard water for irrigation poses a potential threat to human health and can also lead to a reduction in soil quality and crop yields (Jongman and Korsten, 2018). Many fac-tors contribute to water quality deterioration, and for natural water bodies with few organic pollutants, salt or alkali is the main factor affecting their quality as irrigation water (Akhtar et al., 2021).
Oases in the Northwest Arid Zone have been active in agriculture and pastoralism since ancient times, and modern agriculture is largely dependent on local surface and groundwater irrigation. As shown in the USSL diagram (Fig. 8), surface and groundwater samples from the Cele region of South Xinjiang are located in the S1C3 and S2C3 areas of the USSL diagram, indicating that they basically meet the requirements for irrigation water, and a small number of samples are located in S1C4 and S2C4, which do not meet the requirements for direct irrigation. In contrast, surface water in the Fukang area of North Xinjiang is located in the S1C1 and S2C2 zones of the USSL map, indicating that it meets the criteria for irrigation water, while ground-water samples are not shown in the map due to high EC and SAR, indicating that they are not suitable for direct irrigation water use.