Natural and anthropogenic factors affecting geochemistry of the Jialing and Yangtze Rivers in urban Chongqing, SW China

Owing to the complex relationship between natural factors and anthropogenic activities, rivers passing through urban areas present special challenges to geochemists attempting to understand geochemical conditions and fluxes. Research here on a river impacted by urban development, influenced by both natural and anthropogenic factors, helps to understand the mechanisms of the processes that cause changes in water quality. In this study, we collected water samples weekly at three sampling sites from the Jialing and Yangtze Rivers in the urban area of Chongqing. To explore the evolutionary processes and factors influencing surface water quality, we used stoichiometry and principle component analysis (PCA). Results show that the samples belonged to the HCO3·SO4Ca hydrochemical facies. The hydrochemical characteristics of the two rivers were mainly controlled by rock weathering. Carbonate mineral weathering exhibited the largest contribution. Sulfuric acid was involved in carbonate mineral weathering reactions, but gypsum dissolution had little effect on surface water chemistry. PCA revealed that rock weathering, soil erosion, domestic sewage, and agricultural activities contribute to the dissolved chemical load in the Jialing and Yangtze Rivers. The chemical characteristics of the Yangtze River mainstream exhibited changes due to recharge from the Jialing River. Compared with historical data (1959–1990), we found that river water in our study area was more acidic because of coal emissions from industrial activities in Sichuan and Guizhou Provinces. As a result of urbanization and the increase in domestic sewage, the average Cl−/Na+ molar ratio at the sampling sites is approximately 50% greater than historical background data. Furthermore, NO3− was sensitive to agricultural activities, indicating that agricultural activities have an effect on the Jialing and Yangtze Rivers in the urban Chongqing.