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The major ion chemistry in the Taihu watershed has dramatically changed due to human disturbances; however, little is known about the similarities and differences in the responses of the inflow rivers and Taihu lake to the disturbances. Using historical (1950s-1970s) and recent (2018-2021) water chemistry data of inflow rivers and the lake, as well as socioeconomic and land use data, we explored the drivers for the major ion chemistry change and different responses of the inflow rivers and the receiving lake. The results indicated that, compared with 1950s-1970s, all the major ions and TDS in rivers and Taihu lake significantly increased (by 91% for Mg2+ and by 395% for Cl- in rivers; by 68% for HCO3- and 134% for Na+ in the lake); however, their increases in major ion composition presented a clear difference, i.e., although current dominant cation remained Ca in inflow rivers, the second dominant cation has shifted from Mg2+ (1950s-1970s) to Na+ (2018-2021) for rivers, while for the lake, the second dominant cation has become frequently Na+ (2018-2021), followed by Ca2+, indicating a clear salinization tendency. Furthermore, the change of some indicative ratio indices of inflow rivers and the lake in the past decades presented an apparent difference, i.e., the river systems had a higher increase rate in Ca2+/Mg2+ and SO42-/Cl- than the lake, while the lake had a higher increase in (Ca2+ + Mg2+)/HCO3-, TH/TA, and Cl-/Na+ than the river systems. Analyses indicated that increased human disturbances were the major driver for the similar increase in the TDS and major ions for both river systems and the lake, while the different algal biomass in the rivers and lake, the land use change, and declined hydrological connectivity in this watershed played important roles in the different alterations of the water chemistry indices. Comparison of major ion correlation change between the running and stagnant waters indicated a clear "lacunification" trend of inflow rivers in terms of water chemistry characteristics in this dense river-network region. Our work revealed the cause and effect of the fundamental water chemistry change in a rapid development region and will provide scientific basis for the integrated management and recovery in the watershed.

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To investigate the impact of megacities on the chemistry of surface waters, monthly sampling and monitoring were conducted in the Chengdu section of the Minjiang and Tuojiang River basin, corresponding to the upper reaches of the Yangtze River since the spring of 2019, including the influent and effluent water samples from 57 sewage treatment plants in Chengdu. All the samples were analyzed for major ions and other water chemistry parameters, and compared with the historical data of the Minjiang and Tuojiang River. The results showed that the Chengdu surface water still presented a natural chemistry with medium-low total dissolved solids(TDS), and calcium bicarbonate chemistry type, which is the natural consequence of the weathering of carbonate rocks in the basin effected by the weathering of silicates and evaporites. The natural water chemistry of the surface waters in Chengdu presented monthly variation, i.e.,the concentration of major ions and TDS was higher in the dry season compared to the wet season, reflecting the variations of point source. Spatially, the concentration of major ions and TDS downstream of the city was higher than those in the upper reaches, and the concentration in the tributary was higher than that in the mainstream, which may reflect urban influence. Further analyses, such as simulation calculations, indicated that urban activities were the major driving factor for the chemistry change in the surface waters in Chengdu, which is evidenced by the significant contribution of the sewage discharge to the elevated Cl- and Na+ and the ratio of hardness/alkalinity>1 from anthropogenic acid gas emissions. A comparison with the water chemistry of the Minjiang and Tuojiang River in the 1960s indicated that, the current Cl-/Na+ ratio has significantly increased, which has been evidenced by a salinization trend. As a megacity nearest to the source of the Yangtze River, the impact of Chengdu on the natural water chemistry of the Yangtze River system and its environmental effects deserves more attention.

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Long-distance inter-basin water transfer solves the problem of unbalanced water resources in different regions. However, it also changes the natural water chemistry characteristics as well as the bioavailability of heavy metals in the receiving water. In this study, taking the South-to-North Water Transfer Project in China as an example, the basic physicochemical characteristics of the source water (in the Danjiangkou (DJK) Reservoir) and receiving water (in the Beitang (BT) Reservoir) were studied. The BLM (biotic ligand model) was used to study the effect of long-distance inter-basin water transfer on the bioavailability of Cu in receiving waters. The results showed that the TOC (total organic carbon) and TDS (total dissolved solids) in the BT Reservoir water were 10 times and 4.6 times greater than those of the DJK Reservoir water, respectively. The ions in the BT Reservoir were mainly (K++Na+)-(SO42-+CI-), while the ions in the DJK Reservoir were mainly (Ca2++Mg2+)-HCO3-. The results from the BLM showed that the main species of Cu in the water was total organic Cu (Torg Cu), which accounted for 98.69% and 99.77% of the Cu in the DJK Reservoir and BT Reservoir, respectively. The LC50 of Cu for Daphnia magna was 1203.40 ± 57.70 µg/l in the BT Reservoir and only 101.93 ± 7.60 µg/l in the DJK Reservoir. The criteria maximum concentration value of the BT Reservoir was 13.75 times that of the DJK Reservoir, while the criteria continuous concentration value of the BT Reservoir was 13.76 times that of the DJK Reservoir. These results showed that the heavy metals content in water bodies should not be used as the only consideration for water ecological security in the inter-basin water transfer process, and that differences in water quality criteria values caused by differing water environmental qualities in the river basins must be taken into consideration.

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To analyze the characteristics of shallow groundwater pollution in farmland ecosystem in south China and geochemical behavior of major pollutants in the environment, taking the typical farmland in Taihu River Basin as the object, combined with conventional analysis method of the water quality, fluorescence spectrum technology and multivariate statistical analysis method, the characteristics of the water chemistry in shadow groundwater and the distribution of the dissolved organic matter and the heavy metals were studied. The reasons for all the phenomena were researched as well. The result showed that, water body in the study area showed the reducing atmosphere, the main chemical types of shallow groundwater were HCO3-Ca·Na, and compared to the contents of NH3--N and NH2--N, the concentration of NH4+-N was higher. Dissolved organic matter of the shallow groundwater was composed of humic acid, tryptophan and tyrosine. And they were mainly derived from terrestrial and biological source. The distribution was affected by the flow direction of the surface water supplies and groundwater. In the detection of nine kinds of heavy metal elements, the average concentrations of Fe and Cr were higher than the national groundwater environmental quality in class Ⅲ standard. And the contents of Fe, Cr, Mn, Zn elements at some points exceeded the standard. The results showed that fluorescent organic matter characteristics could better reflect the status of shallow groundwater pollution, in which the kind of protein-like substances was closely related to the ammonia nitrogen. The heavy metal elements Cu and Ni were associated with the dissolved organic matter and they were mainly complexed in small molecular fluorescent organic matter.