Major ion geochemistry and nutrient behaviour in the mixing zone of the Changjiang (Yangtze) River and its tributaries in the Three Gorges Reservoir

摘要

Inorganic ions and nutrients were measured at different depths of the Xiangxi and Daninghe Rivers to explore the mixing processes of representative bays in the Three Gorges Reservoir (TGR). HCO_3~- and Ca~(2+) are the dominant ions. Carbonate weathering is the most important mechanism controlling the ion water chemistry; however, important differences exist between the main channel and its tributaries. Major ion levels in the TGR bays depend on hydrological mixing. Results show that the major ions of Ca~(2+), Mg~(2+), Na~+, K~+, Sr~(2+), SO_4~(2-) and Cl~- show chemically conservative behaviour during transit through the bays of the TGR. This means the ions can be used as tracers in the same way that salinity is used in estuaries to explore behaviour of other non-conservative elements and to indicate specific source waters. In contrast, nutrients are not conserved in the mixing zone. The mixing of the main channel and tributaries and biological utilization in backwater reaches were the key factor controlling nutrient distributions in Xiangxi and Daninghe Bays.