Impact of Mainstream Backwater on the Water Environment of the Tributaries of the Three Gorges Reservoir at Low Water Level

Abstract

To prevent the eutrophication of tributaries and guarantee water quality and safety in the Three Gorges Reservoir, research on the impact of mainstream backwater on tributary water environments is of great significance. The investigation and sampling of the Yangtze mainstream and its major tributaries in the reservoir region were performed from August 7 to August 12, 2016, through which the overall hydrochemical environment of the Three Gorges Reservoir has been revealed, and the impact of mainstream backwater on the hydrochemical characteristics of main tributaries has been determined during the low water level operation period. The results showed the following:① The electrical conductivity of the mainstream varied from 291 μS·cm-1 to 336 μS·cm-1, whereas that of the mainstream backwater unaffected zone of the tributary varied from 183.7 μS·cm-1 to 518 μS·cm-1. The electrical conductivity of the mainstream backwater affected zone of the tributary varied from 267 μS·cm-1 to 330 μS·cm-1, which was close to the mainstream variation range. ② The variation range of the δD and δ18 O values of the mainstream were -81.60‰--75.16‰ and -11.57‰--0.26‰, whereas that of the mainstream backwater unaffected zone of tributaries were -59.94‰--43.67‰ and -9.00‰--6.04‰; those of the mainstream backwater affected zone of tributary were -77.85‰--50.75‰ and -11.06‰--7.33‰, which showed the same pattern as those of electrical conductivity and mass concentration of main anions and cations. This means that the mainstream affected the waterbody composition of tributaries through backwater as well as the chemical characterization of tributary water. The extent of mainstream backwater influence on tributaries was negatively correlated to the distance between the tributary estuary and Three Gorges Dam as well as tributary discharge. The hydrochemical characteristics of the mainstream backwater unaffected zone of the tributary were related to the tributary catchment properties. Tributaries with denser populations and higher proportions of cultivated land have poorer water quality. Mainstream backwater can pollute tributaries of better water quality and optimize those with poor water quality.