Accumulated heavy metals in surface sediments are released into the aquatic environment, causing secondary contamination of the hydrosphere, and increasing the risks to human health. To evaluate the pollution characteristics of heavy metals in the sediments of the Chishui River Basin, in the present study, the concentrations of five heavy metals in surface sediments of the Chishui River Basin in China were investigated using the geo-accumulation index, pollution load index, and potential ecological risk indexes. These indexes evaluated the degree of contamination and the influence of human activities on heavy metal levels in the basin. Cu, Zn, Cd, Hg, and As were found at concentrations of 5.12–120.40, 36.01–219.31, 0.03–1.28, 0.01–1.18, and 1.56–11.59 mg kg^–1, respectively, with mean values of 37.43, 91.92, 0.25, 0.07, and 5.16 mg kg^–1, respectively, in the order Zn > Cu > As > Cd > Hg. The contamination indices revealed Hg as the principal pollutant based on the spatial distribution, while Pearson’s correlation coefficients suggested that Cu, Zn, and As originated from a similar source. Hg had a different source from the other metals, whereas Cd originated from a different source compared with that of Zn, As, and Hg. This paper showed a Hg and Cd contamination in the Chishui River Basin.

This study collected 31 water samples from the Tongzi River, Guizhou Province, Southwest China to conduct a risk assessment to understand the hydrochemical characteristics and major ion sources of irrigation and drinking water quality and their effects on human health. The results showed that ion abundance in the Tongzi River is Ca²⁺ (66%) > Mg²⁺ (24%) > K⁺+Na⁺ (10%) for cations and HCO₃⁻ (75%) > SO₄²⁻(21%) > Cl⁻ (4%) for anions. Additionally, the hydrochemical type of the water is Ca-Mg-HCO₃, controlled by carbonate weathering. Methods including ion ratios, principal component analysis (PCA), and correlation analysis (CA) were used to analyze the source of main ions in the river water. PC1, with the most significant variance (54.9%), decides the hydrochemical characteristics and is affected by the positive loadings of SO₄²⁻ (0.92), pH (0.85), Ca²⁺ (0.80), Cl⁻ (0.72), Na⁺(0.66), NO₃⁻ (0.65), and K⁺ (0.57). PC2 explains 19.2% of the total variance, with strong positive loadings of Na⁺ (0.75), K⁺ (0.63) and Cl⁻ (0.59). Mg²⁺ (0.84) and HCO₃⁻ (0.85) exhibits high loadings in PC3, explaining 9.3% of the variance. The results showed that intensive agricultural activities in the basin were the main source of nitrate NO₃⁻, whereas SO₄²⁻ was mainly derived from mining activities. The lower concentrations of Na⁺, K⁺, and Cl⁻ were from coal combustion, domestic wastewater discharge, and agricultural fertilizer applications. The study area was mainly affected by carbonate rock weathering; natural processes (mainly the weathering of carbonate rocks) were still the main origin of Ca²⁺, Mg²⁺, and HCO₃⁻. Moreover, the United States Salinity Laboratory (USSL) diagram and the Wilcox diagram showed that 100% of the samples fell in the C2S1 zones, and the water quality had good suitability for irrigation. The health risk assessment (HRA) results showed that HQNO₃⁻ was much larger than HQ_NH4+ and indicated that nitrate pollution dominated non-health hazards. About 6.5% of the samples in the tributaries represented an unacceptable risk for infants and children, and the HQ value for infants and children was always higher than that for adults. Additionally, the non-carcinogenic health risk of riverine ions for infants and children in the Tongzi River was very noteworthy, especially in the tributaries.

Rivers in karst areas face increased risks from persistent growth in human activity that leads to changes in water chemistry and threatens the water environment. In this study, principal component analysis (PCA), ion ratio measurements, and other methods were used to study the water chemistry of the Qingshuijiang River Basin over the past 10 years. The results showed that the main ions in the river were Ca²⁺ and HCO₃^–, with a cation order of Ca²⁺ (mean: 0.93 mmol/L) > Mg²⁺ (mean: 0.51 mmol/L) > Na⁺ (mean: 0.30 mmol/L) > K⁺ (mean: 0.06 mmol/L) and HCO₃^– (mean: 2.00 mmol/L) > SO₄^2– (mean: 0.49 mmol/L) > Cl^– (mean: 0.15 mmol/L) > NO₃^– (mean: 0.096 mmol/L) > F^– (mean : 0.012 mmol/L). In the past 10 years, the concentration of major ions in the river water in the basin has increased significantly. The weathering input of rock (mainly upstream carbonate) was the main source of Mg²⁺, Ca²⁺, and HCO₃⁻, though sulfuric acid was also involved in this process. While K⁺ and Na⁺ were affected by the combination of human activity and the weathering input of silicate rock in the middle and lower reaches of the river, human activity was the main source of SO₄²⁻, NO₃⁻, and F⁻ ions. Irrigation water quality and health risks were evaluated by calculating the sodium adsorption ratio (SAR), soluble sodium percentage (Na%), residual sodium carbonate (RSC), and hazard quotient (HQ). The findings indicated that the river water was generally safe for irrigation and drinking, and the health risks were gradually reduced over time. However, long-term monitoring of the river basin is still essential, especially for the risk of excessive F⁻ in a few tributaries in the basin.

Reservoirs in karst regions exhibit significant carbon sink potential; however, how different reservoir types influence carbon sequestration remains poorly understood. In this study, dual carbon isotopes (δ¹³C–Δ¹⁴C) were applied to trace dissolved organic carbon (DOC) sources in an open reservoir (Aha Reservoir, AHR) and a closed reservoir (Guanshan Lake, GSL) in southwestern China, and to evaluate their carbon sequestration potential. DOC concentrations in GSL were significantly higher than those in AHR (4.14 ± 0.28 mg/L > 3.37 ± 0.30 mg/L) (p < 0.01), along with lower δ¹³C values (−30.34 ± 0.51‰ < −28.18 ± 0.31‰) and more enriched Δ¹⁴C values (−6.94 ± 11.07‰ > −93.74 ± 6.76‰). The δ¹³C–Δ¹⁴C tracing revealed that plants were the primary DOC source for AHR (61 ± 2%), whereas algae dominated DOC sources in GSL (70 ± 2%). Inflow rivers and water retention time (WRT) likely drive differences in DOC sources and concentrations between the two reservoirs. The absence of inflow rivers and the longer WRT in GSL created favorable conditions for algal growth, resulting in substantially higher chlorophyll a (Chl.a) concentrations (103.00 ± 29.87 μg/L > 13.10 ± 3.29 μg/L) and enhanced production of autochthonous DOC through a stronger biological carbon pump (BCP) effect. These conditions further facilitate the formation and accumulation of recalcitrant DOC (RDOC), ultimately increasing DOC concentrations in GSL. Our findings highlight that closed karst reservoirs may represent important yet underappreciated carbon sinks and should receive greater attention in future carbon-sink assessments.