Tracing controlling factors of riverine chemistry in a headwater tributary of the Yangtze River, China, inferred from geochemical and stable isotopic signatures

Inputs and transport of acid mine drainage-derived heavy metals in karst areas of Southwestern China

Heavy metal pollution caused by acid mine drainage (AMD) is a global environmental concern. The processes of migration and transformation of heavy metals carried by AMD are more complicated in karst areas where carbonate rocks are widely distributed. Water, suspended particulate matter (SPM), and sediments are the crucial media in which heavy metals migrate and it is important to elucidate the geochemical behavior of AMD heavy metals in these environments. This study tracked AMD heavy metals from release to migration and transformation in a natural river system in a karst mining area. AMD directly impacted the hydrochemical composition of the karst water environment, but the carbonate rock naturally neutralized the acidity of the AMD. AMD heavy metal concentrations decreased gradually after the tributaries from the mining area entered the main river, with the metals tending to accumulate in SPM and sediments. The forms in which heavy metals were present were influenced by pH and their relative concentrations. Raman spectroscopy and transmission electron microscopy of sediments from the mining area suggested that the presence of an iron phase plays an important role in the fate of AMD-derived heavy metals. It is, therefore, necessary to elucidate the mechanisms of iron phase precipitation from sediments in order to control AMD-derived heavy metals in karst mining areas. This study improves our understanding of the geochemical behavior of heavy metals in karst environments and provides direction for the prevention and control of AMD in affected areas.

Control mechanisms of water chemistry based on long-term analyses of the Yangtze River

Long-term series data can provide a glimpse of the influence of natural and anthropogenic factors on water chemistry. However, few studies have been conducted to analyze the driving forces of the chemistry of large rivers based on long-term data. This study aimed to analyze the variations and driving mechanisms of riverine chemistry from 1999 to 2019. We compiled published data on major ions in the Yangtze River, one of the three largest rivers in the world. The results showed that Na+ and Cl- concentrations decreased with increasing discharge. Significant differences in riverine chemistry were found between the upper and middle-lower reaches. Major ion concentrations in the upper reaches were mainly controlled by evaporites, especially Na+ and Cl- ions. In contrast, major ion concentrations in the middle-lower reaches were mainly affected by silicate and carbonate weathering. Furthermore, human activities were the drivers of some major ions, notably SO42- ions associated with coal emissions. The increased major ions and total dissolved solids in the Yangtze River in the last 20 years were ascribed to the continuous acidification of the river and the construction of the Three Gorges Dam. Attention should be given to the impact of anthropogenic activities on the water quality of the Yangtze River.

Reservoir NO3- pollution and chemical weathering: by dual isotopes of δ15N-NO3-, δ18O-NO3- and geochemical constraints

Reservoir dams alter the nutrient composition and biogeochemical cycle. Thus, dual isotopes of δ¹⁸O-NO₃^- and δ¹⁵N-NO^-₃ and geochemical signatures were employed to study the NO₃^- pollution and chemical weathering in the Three Gorges Reservoir (TGR), China. This study found that the TGR dam alters the δ¹⁵N-NO₃^- composition and is enriched in the recharge period. Values of δ¹⁵N-NO₃^- varied from 4.5 to 12.9‰ with an average of 9.8‰ in the recharge period, while discharge period δ¹⁵N-NO₃^- ranged from 3.2 to 12.5‰, with an average of 9.3‰. δ¹⁸O-NO₃^- varies (1.2-11.3‰) with an average of 6.5‰ and (2.4-12.4‰) with an average of 7.5‰, in the recharge and discharge periods, respectively. Stable isotopic values sharply decreased from upstream to downstream, indicating the damming effects. δ¹⁸O-NO₃^- and δ¹⁵N NO₃^- confirm that sewage effluents, nitrification of soil organic material, and NH₄⁺ fertilizers were the primary sources of NO₃^- in the reservoir. Carbonate weathering mainly provides ions to the reservoir. HCO₃^- + SO₄^2- and Ca²⁺ + Mg²⁺ represent 90% of major ions in the TGR. Downstream sampling sites showed low solute concentration during the recharge period, indicating the dam effect on solute concentration. Ca-Mg-Cl^-, Ca-HCO₃^- and Ca-Cl^- were the main water types in the TGR. The average percentage of solutes contribution revealed the carbonate weathering, evaporites dissolution, silicate weathering, and atmospheric input were 51.9%, 41%, 7.8%, and 1.7% for the recharge period. In contrast, the discharge period contributed 66.4%, 29.2%, 10%, and 4.3%, respectively. TGR water is moderately suitable for irrigation, and hardness is high in drinking water. This study provides new insight into the dual isotopic approach and geochemical signatures to interpret the NO₃^- cycle and chemical weathering process under dam effects in the TGR. However, this isotopic application has some limitations in source identification, isotope fractionation, and transformation mechanisms of nitrate. Thus, further studies need to be done on these topics for a better undestanding.

Stable isotopic characteristics of precipitation related to the environmental controlling factors in Ningbo, East China

Hydrogen and oxygen stable isotopes (δ²H and δ¹⁸O) in precipitation were analysed from June 2018 to May 2020 in Ningbo and were influenced by the subtropical monsoon climate in East China. The δ²H and δ¹⁸O values of precipitation in Ningbo varied from -90.0 to 6.0‰ and from -13.5 to -1.6‰, respectively. The local meteoric water line (LMWL) in Ningbo was obtained as δ²H = 9.27 δ¹⁸O + 35.95 and had a larger slope and intercept compared to the global meteoric water line (GMWL) because its water vapour sources were oceans. The more negative δ¹⁸O values and lower deuterium excess (D-excess) of precipitation in summer were due to water vapour sources from the East China Sea, the South Sea and the Western Pacific, which are controlled by the southeasterly monsoon. In contrast, the less negative δ¹⁸O and higher D-excess of precipitation in winter were influenced by water vapour sources from the North Asian continent and North China transported by the northwesterly monsoon. The precipitation amount effect was significant in Ningbo, especially in summer. The inverse temperature effect was appeared in Ningbo, except winter. These two effects may be caused mainly by the monsoon climate rather than by the secondary evaporation effect.

Major Elements in the Upstream of Three Gorges Reservoir: An Investigation of Chemical Weathering and Water Quality during Flood Events

Rivers transport terrestrial matter into the ocean, constituting a fundamental channel between inland and oceanic ecosystem and affect global climate change. To reveal chemical weathering processes and environmental health risks during flood periods, water samples were collected in the upper reaches of Three Gorges Reservoir (TGR) in 2020. HCO3− and Ca2+ were the most abundant anions and cations of the river water, respectively. The range of HCO3− concentration was between 1.81 and 3.02 mmol/L, while the mean content of Ca2+ was 1.03 mmol/L. The results of the Piper diagram and element ratios revealed that the river solutes were mainly contributed by carbonate weathering and gypsum-rich evaporite dissolution. A mass balance model indicated that the contribution order of sources to cations in the main channel (Yibin-Luzhou) was evaporites > carbonates > atmospheric input > silicates. The order in the Chongqing—Three Gorges Dam was carbonates > atmospheric input > evaporites > silicates. These results showed a lithologic control on hydrochemical characteristics. Most sampling sites were suitable for agricultural irrigation according to the water quality assessment. However, indexes sodium adsorption ratio (SAR) and soluble sodium percentage (Na%) were higher than 1.0 in Yibin-Luzhou and 30% in Yibin–Chongqing, respectively, suggesting a potential sodium hazard. In addition, except Tuojiang River and Shennong River, the risk of sodium hazard in tributaries was relatively low. High Na+ concentration in irrigation water can damage soil structure and function and ultimately affect agricultural production. Water quality in the upstream of a Piper diagram should attract enough attention.