Compositions of the major ions, variations in their sources, and a risk assessment of the Qingshuijiang River Basin in Southwest China: a 10-year comparison of hydrochemical measurements

Irrigation water quality assessment

Irrigation water quality was evaluated by calculating the sodium adsorption ratio (SAR), soluble sodium percentage (Na%), and residual sodium carbonate (RSC). Higher SAR values indicate stronger adsorption of sodium ions by the soil in the basin, making it more difficult for the vegetation roots to absorb water (Wang et al., 2022b). The higher the Na% value, the worse the permeability of the soil, affecting the growth of vegetation. Different irrigation water salinity and alkalinity levels also affect soil quality attributes, thereby changing farmland yield. Therefore, SAR, Na%, and RSC were calculated using the concentration (meq/L) of ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, HCO₃^–, and CO₃^2–) in river water to comprehensively evaluate the saline-alkali hazard of irrigation water (Asare-Donkor, Ofosu & Adimado, 2018). The main calculation formulas are as follows:

(1) $$\mathrm{S}\mathrm{A}\mathrm{R}=\sqrt{2}\times \mathrm{N}{\mathrm{a}}^{+}/(\mathrm{C}{\mathrm{a}}^{2+}+{\mathrm{M}\mathrm{g}}^{2+}{)}^{1/2}$$

(2) $$\mathrm{N}\mathrm{a}\mathrm{\%}=\mathrm{N}{\mathrm{a}}^{+}/(\mathrm{C}{\mathrm{a}}^{2+}+{\mathrm{M}\mathrm{g}}^{2+}+{\mathrm{N}\mathrm{a}}^{+}+{\mathrm{K}}^{+})\times 100\mathrm{\%}$$

(3) $$\text{RSC}=({{\text{HCO}}_3}^{-}-{\text{CO}}_3^{2-})-({\text{Ca}}^{2+}+{\text{Mg}}^{2+})$$

Health risk assessment

Water pollutants in river water enter the human body mainly through both drinking water and skin contact, and unsafe drinking water intake has been shown to be the main way to threaten human health (Adimalla & Li, 2019; Adimalla, Qian & Nandan, 2020). Long-term exposure to high concentrations of nitrate and fluoride may increase the risk of disease and other negative health effects (Xia et al., 2021b). Recent research has shown that NO₃^– and F^–, as non-carcinogenic pollutants, are often used to assess the non-carcinogenic health risks of river water to the population (Han & Xu, 2022; Wang et al., 2017; Zeng, Han & Yang, 2020), though the hazard quotient (HQ) is the most commonly used metric for evaluating non-carcinogenic health risks. Its calculation method follows in Eqs. (4) and (5):

(4) $$\mathrm{A}\mathrm{D}{\mathrm{D}}_{\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=\mathrm{C}\times \mathrm{I}\mathrm{R}\times \mathrm{E}\mathrm{F}\times \mathrm{E}\mathrm{D}/(\mathrm{B}\mathrm{W}\times \mathrm{A}\mathrm{T})$$where the ADD_ingestion is the ingestion intake of daily doses, C is the concentrations of ions (mg/L), IR is the rate of daily ingestion (0.6 L/day for children, 1 L/day for adults), EF is the exposure frequency (365 days/year for both adults and children), ED is the exposure duration (25 years for adults and 12 for children), BW is the body weight (16 kg for children and 56 kg for adults), and AT is the average time (4,380 days for children and 10,950 days for adults; (Qasemi et al., 2019; Wu & Sun, 2016)).

(5) $$\mathrm{H}\mathrm{Q}={\mathrm{A}\mathrm{D}\mathrm{D}}_{\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}/\mathrm{R}f{\mathrm{D}}_{\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}$$where DfR_ingestion is the reference dose of different ions (0.04 and 1.60 ppm/day for F⁻ and NO₃⁻, respectively; (Li et al., 2016)). When HQ < 1, the human health risk caused by pollutants is permissible; when HQ > 1, non-carcinogenic effects should be considered.

Anthropogenic inputs

When analyzing sources of major ions in water chemistry, high Cl⁻ concentration in river water can generally be used as an important indicator of human input of domestic sewage (mainly concentrated in urban areas; (Tang, Jin & Liang, 2021)). Recent studies have shown that river nitrates from agricultural synthetic fertilizers have higher NO₃⁻ concentrations and higher NO₃⁻/Cl⁻ ratios, while domestic sewage has lower NO₃⁻/Cl⁻ ratios and higher Cl⁻ concentrations due to higher organic matter content (Ge et al., 2021; Liu et al., 2021c; Yue et al., 2020). To better explore the changes in the main ion sources in the Qingshuijiang River Basin in the past decade, the main ions were analyzed in the four sample periods to identify the sources of the ions. As shown in Fig. 4A, the Cl⁻ concentration in all water samples was between 0.02–1.00 mmol/L, the NO₃⁻/Cl⁻ ratio was between 0.01–10.13, and the coefficient of variation was 86.24% and 160.39%, respectively. This showed that some areas in the basin were affected by urban activities and agricultural activities, with large regional differences in the scope of the effects. Smaller flows were much more vulnerable to pollution than larger flows (Im et al., 2020). The comparison of data over the past decade showed that the average content of Cl⁻ in the Qingshuijiang River Basin doubled from 0.10 to 0.20 mol/L, while the average ratio of NO₃⁻/Cl⁻ decreased from 0.74 to 0.49. These results indicate that with the increases seen in population, urbanization, and construction in the Qingshuijiang River Basin, the main source of ions in river water shifted from agricultural input (fertilizer) to municipal wastewater input (Zheng et al., 2022). For both urban wastewater and agricultural fertilizer inputs, the impact was greater during the dry season than during the wet season.

High concentrations of SO₄²⁻ in river water are generally derived from the input of mining and industrial production (Liu & Han, 2020a), while Ca²⁺ ions are usually derived from rock weathering and are not affected by human factors. Therefore, NO₃⁻ (representing agricultural and domestic input) and SO₄²⁻ (representing industrial and mining input) were compared with Ca²⁺ to determine the main anthropogenic contributor to solute in the Qingshuijiang River. As shown in Fig. 4, in addition to agricultural activities and urban sewage discharge, industrial activities and mining also impact the solute in the Qingshuijiang River. In the upstream area (Chonganjiang River Basin), which is affected by acid mine drainage, the sulfate in the river water (range 0.50–4.87 mmol/L, average: 1.57 mmol/L) was more than three times the average value (0.49 mmol/L) of the basin (Li et al., 2024). The average ratios of SO₄²⁻/Ca²⁺ and NO₃⁻/Ca²⁺ in the basin were also calculated for the study years. The ratio of SO₄²⁻/Ca²⁺ increased slightly over the 10-year period, while the ratio of NO₃⁻/ Ca²⁺ decreased slightly, indicating that some areas in the basin are affected by industrial activities.

Rock weathering input

Rock weathering includes both carbonate rock weathering and silicate rock weathering, which are the main sources of Ca²⁺, Mg²⁺, Na⁺, K⁺, and HCO₃⁻ in river water (Herath et al., 2022; Tsering et al., 2019). Ca²⁺/Na⁺ and HCO₃⁻/Na⁺ ratios are effective indices for tracing the source of ion weathering (Liu & Han, 2020b; Zheng et al., 2023). The ratios of Ca²⁺/Na⁺, Mg²⁺/Na⁺, and HCO₃⁻/Na⁺ in the Qingshuijiang River Basin were dispersed between silicate and carbonate end members (Figs. 5A and 5B), indicating that both carbonate and silicate rock weathering were involved. This reflects the fact that the upstream area is a carbonate rock area and the downstream area is a silicate rock area.

In general, the Ca²⁺ + Mg²⁺/(Na⁺ + K⁺) equivalent ratio can be used as an index to distinguish the relative intensity of different types of rock weathering (Gupta, Nayek & Chakraborty, 2016; Setia et al., 2021). The Ca²⁺ + Mg²⁺/(Na⁺ + K⁺) ratio in the Qingshuijiang River Basin ranged from 0.71 to 35.29, with an average of 5.51, which was higher than the world average (2.2) and the Indian average (2.5); (Setia et al., 2021), indicating that the chemical composition of the river was more controlled by the lithology of the carbonate rocks in the basin than by the lithology of the silicate rocks. Therefore, a further analysis of the weathering process of carbonate rocks was carried out (Figs. 6A–6C). Most of samples from the Qingshuijiang River Basin were distributed in the 2(Ca²⁺ + Mg²⁺)/HCO₃⁻ concentration ratio (Fig. 6A), indicating that in the process of carbonate rock weathering, Ca²⁺ and Mg²⁺ ions in the river water were more abundant than HCO₃⁻, and there were other exogenous acids (sulfuric acid or nitric acid) to balance the river water ions (Gong et al., 2024; Li et al., 2023). Compared with 2013/2014, more exogenous acids were needed in 2023.

In order to further explore the effect of sulfuric acid or nitric acid on weathering, 2(Ca²⁺ + Mg²⁺) and (HCO₃⁻ + 2SO₄²⁻) and 2(Ca²⁺ + Mg²⁺) and (HCO₃⁻ + 2SO₄²⁻ + NO₃⁻) were analyzed. As shown in Figs. 6B and 6C, most of the sampling points in the Qingshuijiang River were distributed on the 1:1 contour line on the relationship diagram of 2(Ca²⁺ + Mg²⁺) and (HCO₃⁻ + 2SO₄²⁻), while on the relationship diagram of 2(Ca²⁺ + Mg²⁺) and (HCO₃⁻ + 2SO₄²⁻ + NO₃⁻), some sampling points in the Qingshuijiang River fell farther away from the 1:1 contour line, indicating that sulfuric acid was involved in ion balance in the river rather than nitric acid. This result indicates that sulfuric acid might be involved in the weathering of carbonate rocks in the basin (Li et al., 2008; Ma et al., 2023; Tang & Han, 2021). The results of the PCA and CA analyses showed that Ca²⁺ and SO₄²⁻ were in the same component, and the correlation between them was high, which also provides evidence that sulfuric acid is involved in carbonate weathering. The 2SO₄²⁻ content involved in the ionic equilibrium of the river water increased from 0.39 to 0.58 mol/L over the 10-year period, suggesting that sulfuric acid played a more significant role in weathering in the clearwater river basin in 2023 than it did a decade prior.

Irrigation and guideline-based water quality

The Qingshuijiang River Basin is the main water source for agriculture, industry, and local residents in the Qiandongnan and Qiannan Prefectures of Guizhou Province. The quality of the water in the river basin is closely related to the health of these residents. As summarized in Table 1, the pH values of 82.93% of the sampling points in the Qingshuijiang River Basin were in line with both Chinese and WHO drinking water quality guidelines (6.5–8.5), though the pH values of some sampling points affected by carbonate weathering exceeded 8.5 (Ge et al., 2021). Most of the F⁻, Cl⁻, NO₃⁻, and SO₄²⁻ levels in the river water samples were lower than the recommended limits, but F⁻, NO₃⁻, and SO₄²⁻ levels at a few of the sampling points exceeded the recommended values. The percentage of sampling points exceeding the recommended values of F⁻, NO₃⁻, and SO₄²⁻ were 4.27%, 0.61%, and 2.44%, respectively. These excessive amounts might be related to the phosphorus and fluoride chemical enterprises and coal mining enterprises in the basin (Tang et al., 2022; Van Stempvoort et al., 2023).

Common indicators for evaluating river irrigation water quality include Na%, SAR, and RSC. Na% and SAR indicators can reflect the Na hazard of soil aggregates affected by irrigation on agricultural land (Li, Wu & Qian, 2015). A United States Salinity Laboratory (USSL) diagram and Wilcox diagram were drawn using the EC, SAR, and Na% values of the river water (Figs. 7A and 7B; (Bishwakarma et al., 2022)). Most of the Qingshuijiang River water samples were scattered in the C1S1 and C2S1 regions of the USSL map and in the ‘excellent’ region of the Wilcox map. Only a few sampling points in the tributaries of the Qingshuijiang River were scattered in the C3S1 region of the USSL plot and in the ‘good’ region of the Wilcox plot. For residual sodium carbonate (RSC), the RSC value of water samples in the basin ranged from −2.55 to 2.56, with an average of 0.56. Overall, the results show that the Qingshuijiang River water is not a hazard to the soil when used for agricultural irrigation. However, it is worth noting that the sampling points have slightly shifted toward the direction of poor water quality over the past 10 years, which may be related to the increase in human activity seen in the basin in the past decade. Therefore, the continuous long-term monitoring of the basin is still important.

Health risk assessment

The Qingshuiiang River Basin is an important source of drinking water for residents near both the Qiandongnan Prefecture and the Qiannan Prefecture. The ingestion of excessive F⁻ and NO₃⁻ can cause typical non-carcinogenic hazards, while SO₄²⁻ does not cause health problems (Liu & Han, 2020a). Therefore, in this study, NO₃⁻ and F⁻ were included in the health risk assessment, and the HQ values of nitrate (HQ_N) and fluoride (HQ_F) of each sampling point were calculated according to the corresponding concentrations in the river water, and the potential risks to human health were evaluated. As shown in Fig. 8, the HQ values for children in the whole Qingshuijiang River Basin were HQ_F (average: 0.22) > HQ_N (average: 0.14), and those for adults were HQ_F (average: 0.09) > HQ_N (average: 0.06). These results indicated that the overall health risk of water quality in the mainstream was low, and the health risk of children was higher than that of adults. These results also showed that the potential effect of F⁻ on health was greater than that of NO₃⁻. However, the difference between the mainstream and the tributaries was large, and the difference between different tributaries was also large. In some upstream tributaries, the HQ value of some sampling points (Z3, Z5, Z6, and Z8 in Chonganjiang River) exceeded 1, indicating a large health risk. These high values may be related to the large-scale phosphorus chemical base in Fuquan City (Tang et al., 2022). The HQ_F and HQ_N values for children in the whole Qingshuijiang River Basin were higher in the dry season (average: 0.24 and 0.21, respectively) than in the wet season (average: 0.19 and 0.06, respectively). In addition, the HQ_F and HQ_N values for children were higher in 2013/2014 (average: 0.27 and 0.14, respectively) than in 2023 (average: 0.15 and 0.13, respectively), indicating that the health risks to children of fluoride and nitrogen in the Qingshuijiang River Basin have declined in the past decade, which may be related to the management of the basin, especially the treatment of key pollution sources. However, it is worth noting that in some areas (Z6) during the dry season, the health risks increased, emphasizing the need for continued environmental management.