In this study, the concentration of eight dissolved heavy metals (Ti, Cr, Mn, Fe, Ni, Mo, Sb, and Ba) in 42 water samples from the Jiulongjiang River, southeast China, were determined by inductively coupled plasma mass spectrometry (ICP-MS). Multivariate statistical methods, including correlation analysis (CA) and factor and principal component analysis (FA/PCA), were analyzed to identify the sources of the elements. Water quality index (WQI) and health risk assessment, including hazard quotient (HQ) and hazard index (HI), were used to evaluate water quality and the impacts on human health. Our results were compared with the drinking water guidelines reported by China, the World Health Organization (WHO), and the United States Environmental Protection Agency (US EPA), revealing that Ti, Mn, and Sb were not within approved limits at some sites and might be the main pollutants in the drainage basin. Based on the spatial distributions, Ti, Mn, Fe, Ni, and Mo showed good similarity, indicating that they might come from similar sources along the river. The CA results also showed that Ti, Mn, Fe, Ni, and Mo had a high correlation coefficient. The FA/PCA results identified three principal components (PC) that accounted for 79.46% of the total variance. PC 1 suggested that a mixed lithogenic and urban land source contributed to Ti, Mn, Fe, Ni, and Mo; PC 2 showed that Cr, Ni, and Mo were influenced by the discharge of industrial effluents; Sb had a strong loading on PC 3, which was controlled by mining activities. The results of the WQI indicated that the water in the Jiulongjiang River was basically categorized as excellent water, but the water quality levels in site W5 and N4 were poorer due to urban land use. Hazard quotient and HI values showed that Sb was a potential threat to human health, indicating that preventive actions should be considered in regard to mining activities in the upper reaches of Beixi stream.

Forty-eight suspended particulate matter (SPM) samples were collected from the Mun River, northeast Thailand and its junction with the Mekong River, to investigate the relationship between the distribution of rare earth elements (REE) in SPM and the soils in the watershed. The total REE contents (∑REE) in SPM in the Mun River ranged from 78.5 to 377.8 mg/kg with the average of 189.3 mg/kg, which was lower than ∑REE of 222.3 mg/kg at the Mekong River (one sample at junction). The Post Archean Australia Shale (PAAS)-normalized ratios of light REE (LREE), middle REE (MREE) and heavy REE (HREE) were averaged to 1.0, 1.3 and 1.0, which showed a clear enrichment in MREE. In short, along the Mun River, the REE contents in SPM were decreasing, and the PAAS-normalized patterns of REE showed gradually flat. The REE content in SPM and soils are highest in the upper catchment, indicating that soil/bedrock is the most important source of REE in SPM. Additionally, the positive Eu anomaly was enhanced by the higher Ca content in SPM (R = 0.45), which may be caused by more feldspars or carbonates with Ca and Eu substituting Ca. The results present the REE behaviors of SPM in the Mun River and relationship between REE in SPM and soil/bedrock, the findings may support the other studies in catchment weathering.

Dissolved inorganic carbon isotope composition (δ¹³C_DIC), together with major ion concentrations were measured in the Mun River and its tributaries in March 2018 to constrain the origins and cycling of dissolved inorganic carbon. In the surface water samples, the DIC content ranged from 185 to 5897 μmol/L (average of 1376 μmol/L), and the δ¹³C_DIC of surface water ranged from −19.6‰ to −2.7‰. In spite of the high variability in DIC concentrations and partial pressure of carbon dioxide (pCO₂), the δ¹³C_DIC values of the groundwater were relatively consistent, with a mean value of −16.9 ± 1.4‰ (n = 9). Spatial changes occurred in the direction and magnitude of CO₂ flux through water-air interface (F_CO2). In the dry season, fluxes varied from −6 to 1826 mmol/(m²·d) with an average of 240 mmol/(m²·d). In addition to the dominant control on hydrochemistry and dissolved inorganic carbon isotope composition by the rock weathering, the impacts from anthropogenic activities were also observed in the Mun River, especially higher DIC concentration of waste water from urban activities. These human disturbances may affect the accurate estimate contributions of carbon dioxide from tropical rivers to the atmospheric carbon budgets.

River water samples between August 2017 (wet season) and March 2018 (dry season) from the Mun River Basin in northeast Thailand were collected and their dissolved heavy metals concentrations (Al, Mn, Fe, Cu, Zn, and Ba) were measured. Compared with international drinking water guidelines, Mn was the dominant pollutant in the dry season. The correlation analysis (CA) suggested that similar sources were shown in each element pair of Al-Fe, Mn-Zn, and Fe-Ba in both seasons. The principal component analysis (PCA) results showed that the dominant source of dissolved heavy metals was sedimentary inputs or colloid destabilization in the wet season, while anthropogenic inputs were the main sources in the dry season, such as agricultural runoff, industrial effluents, and domestic discharge. On the basis of water quality index (WQI), water at most sites in the wet and dry seasons can be categorized as excellent water, except for a few sites with substandard values. The river water posed no significant health risks according to the health risk assessment, but Mn, Fe, and Ba needed to be paid more attention due to the relatively high values. Al, Fe, and Ba were the main dissolved heavy metals flowing into the Mekong River, and Cu contributed to the background value in the Mekong river.