Source and Distribution of Dissolved Metal Ions in the Backwater Area of Pengxi River in Three Gorges Reservoir

Abstract

This study uses the Gaoyang Lake section of the Pengxi River, the largest tributary on the northern bank of the Three Gorges Reservoir (TGR), as an example for exploring the distributions and dynamics of Ca, Zn, Fe, Cr, Pb, Cu, and Hg ions in the tributaries of TGR where the water level fluctuates due to dam regulation. Samples were taken 21 times, once every 17.3 days, at four sampling sites in Gaoyang Lake, which is in a perennial backwater zone of the Pengxi River, during one year from June 5, 2013 to May 29, 2014. At each sampling site, water samples were taken from the surface layer (0-0.5 m), middle layer, and bottom layer (0.5 m above the bed mud). During winter when the water was not stratified, the middle layer samples were taken at 1/2 depth, and when water was stratified in other seasons, the middle layer samples were taken from the thermal layer. Inductively coupled plasma atomic emission spectrometry (ICP-AES) and cold-vapor atomic absorption methods were adopted to determine the concentrations of the metals. Excel and SPSS were used for data analysis and Matlab for building 3-D prisms displaying concentration distributions of Hg ions in the high water level period (175 m, November-April in the ensuing year), sluicing period (May-middle June), low water level in the flooding season (145 m, June-August), and the storage period (September-November). The results provided the following observations ① Concentrations of Cr, Pb, Cu, Zn, and Hg ions were lower than those in Class Ⅲ of the water environment quality standard (GB 3838-2002). ② Cr, Pb, and Cu had high peak values during the storage and sluicing period, and the lowest values during the high water level period. Cr, Pb, and Cu were derived from the main stream of Yangtze, while Fe and Zn were from the Pengxi River locally. The concentration of Hg ions was affected by both the main stream and endogenous sources. As the water column stratified, metal ions did not mix among the stratified layers in Gaoyang Lake. ③ The conductivity was significantly lower during the high water level period than during other water level periods. The main material that affects the conductivity of Gaoyang Lake could be nonmetallic ions.