Dissolved aluminum and silica in the Changjiang (Yangtze River): Impact of weathering in subcontinental scale

Water samples were collected from the river mouth upstream over a distance of 3500-4000 km in the Changjiang (Yangtze River) and its 15 tributaries in April-May 1997, and at Nantong close to the river mouth in 1996-1998. Dissolved Al and Si were determined and compared with data of water flow, total suspended matter, and alkalinity. Data obtained show that dissolved Al in the Changjiang illustrates a rather uniform distribution from upstream toward river mouth, and a broad increasing of Al with higher Si to alkalinity ratio in tributaries has been found. The plots of Al:Si ratio against specific runoff and sediment yields reveal different weathering mechanisms involved in tributaries. At Nantong, concentration of Al is considerably stable relative to important seasonal variations of water discharge and sediment load. Acidification of bulk riverine samples releases an amount of Al that can be one order of magnitude higher than the natural concentrations, indicating potential remobilization by acid deposition and/or waste drainage. Compared with other world rivers, concentration of Al in the Changjiang is low and similar to those from primitive systems.