In this study, 24 surface water samples were collected from the main trunk/tributary of the Lijiang River during the wet season (April) and the dry season (December) in 2021. The total concentration of 11 heavy metal(loid)s (Al, Cu, Pb, Zn, Cr, Ni, Co, Cd, Mn, As, and Hg) was determined to investigate their physicochemical properties and spatial-temporal distribution characteristics. The heavy metal evaluation index (HEI) and the positive matrix factorization (PMF) model were employed to evaluate water quality and to reveal quantitatively identified pollution sources for further investigation to obtain a health risk assessment using the hazard index (HI) and carcinogenic risk (CR) of various pollution sources. The mean concentrations of heavy metal(loid)s in surface water in the wet and dry seasons were ranked as: Al > Mn > Zn > Ni > Cd > Cr > Cu > As >Hg = Pb > Co, with the mean concentration of Hg being higher than the national Class II surface water environmental quality standard (GB3838-2002). In terms of time scale, the concentration of most heavy metal(loid)s was higher in the wet season; most heavy metal(loid)s were distributed mainly in the midstream area. HEI index indicated that the main water quality status was “slightly affected” in the study area. Five potential sources of pollution were obtained from the PMF model, including industrial activities, traffic sources, agricultural activities, domestic waste emissions, and natural resources. The source-oriented risk assessment indicated that the largest contributions of HI and CR were agricultural sources in the Lijiang River. This study provides a “target” for the precise control of pollution sources, which has a broad impact on improving the fine management of the water environment in the basin.

Accurate estimate of carbonate weathering and the related carbon sink flux induced by anthropogenic H₂SO₄ is of great significance for improving understanding of the hydrogeochemical evolution and the global carbon cycle. Here, to quantitatively evaluate the influence of anthropogenic H₂SO₄ on different lithological carbonate weathering and the related carbon sink budget, karst spring water in the typical limestone and mixed limestone–dolomite catchments in Yaji and Beidiping affected by acid precipitation in southwest China were sampled monthly for the analysis of hydrochemical and δ¹³C_DIC characteristics. Results show for the period of sampling (August 2013 to December 2014) that the average contribution rates of atmospheric inputs and carbonate weathering to total dissolved cations are 2.24% and 97.8%, and 3.09% and 96.9% in Yaji and Beidiping, respectively. The δ¹³C_DIC values (−17.0% to −14.7‰) and the [Ca²⁺ + Mg²⁺]/[HCO₃⁻] (0.98 to 1.25) and [Ca²⁺ + Mg²⁺]/[HCO₃⁻ + SO₄²⁻] (approximately 1) equivalent ratios of samples prove that H₂CO₃ and H₂SO₄ simultaneously participate in carbonate weathering. The contribution rates of H₂SO₄ to [Ca²⁺ + Mg²⁺] and [HCO₃⁻] produced by carbonate weathering in Yaji and Beidiping are 0–30% and 0–18%, and 0–37% and 0–23%, with average values of 14% and 7%, and 19% and 11%, respectively, suggesting that the influence of H₂SO₄ on different lithological carbonate weathering is different. H₂SO₄ precipitation participating in carbonate weathering increases the weathering rate by 14–19%, whereas it decreases the flux of karst carbon sink by 7–11% in Southwest China. Therefore, anthropogenic acids have influenced the global carbon cycle and climate change by carbonate weathering due to the large karst areas in the world, and their influences on different lithological carbonate weathering should not be ignored in the regional and global carbon cycles in future studies.

In order to improve the understanding of the global carbon cycle and the stability of karst carbon sinks, it is necessary to better understand the source, distribution and transformation characteristics of organic matter (OM) in aquatic ecosystems. Here, stable isotope ratios (δ¹³C and δ¹⁵N), elemental analysis (C/N ratios), and lipid biomarkers were analyzed for dissolved organic matter (DOM) (0.7 μm) of water, and organic matter from sediment cores (SCOM) to identify the sources, distribution, and transformation of OM in a subtropical karst reservoir. The results showed that short-chain (C14–20) n-alkyl lipids were more abundant than long-chain (C21–34) n-alkyl lipids in both the DOM and SCOM samples, indicating that bacteria were the primary sources of these lipids, while terrestrial organic matter (OM) made only a minor contribution to the n-alkyl lipid pool, and aquatic plants (macrophytes) OM contributed major contribution to the n-alkyl lipid pool in POM. Microbial activity and lipid degradation were more pronounced in the DOM. Furthermore, terrigenous and macrophyte-derived lipids were found to be more abundant in POM than in DOM and SCOM, suggesting that they are relatively resistant to degradation compared with phytoplankton-derived OM.

The openness and sensitivity of karst water systems make the geochemical processes of groundwater in karst areas of great importance. In this study, the recharge processes have been identified based on physical-chemical indicators (T, DO, EC, HCO3−, SO42−, Cl-, Na+, Ca2+, and Mg2+), δ2H, δ18O, and δ13CDIC isotopes. The results are as follows: (1) Ions exchange is active in karst groundwater, particularly during the dry season. (2) Evaporation and precipitation are significant but seasonally varied processes governing the recharge of the Lihu underground river. Spring and summer show significant groundwater recharge driven by rainfall, with intense evaporation in summer and the isotopic and d-excess values in autumn reflecting the reduced rainfall and cooler weather. Despite the combined influence of surface water and rainfall in winter, low precipitation produces predominantly old groundwater, resulting in pronounced evaporative effects on groundwater isotopic composition. (3) δ13CDIC values are generally heavy (-10.74‰ to -4.34‰) in the underground water, indicating rapid surface water infiltration with short residence times and abbreviated karstification processes in summer versus longer residence times and prolonged karstification in winter. This study highlights chemical and isotope analysis to understand karst recharge and hydrodynamics in Southwest China and beyond.