Spatial-temporal variations of water quality in upstream and downstream of Three Gorges Dam

Abstract

In order to explore the spatio-temporal variations of water quality in the upstream and downstream (99.9 km upstream and 63.0 km downstream of the dam, total length 162.9 km) of the Three Gorges Dam, principal component analysis and variance analysis were used to analyze the vicinity dam water quality in 2016. Principal component analysis showed that the main factors dominating the water quality changes in the study area were hydrological factors, such as discharge (Q), temperature (T), water level (Z) and water quality factors, such as water temperature (WT), dissolved oxygen (DO), suspended substance (SS), permanganate index (CODMn), sulfate (SO42-), fluoride (F-), total hardness (T-Hard), nitrate nitrogen (NO3--N), total nitrogen (TN) and selenium (Se). The principal component scores of each sampling point and two-way ANOVA showed that the time variation of water quality factors in the study area were mainly different in season and operation period of different reservoirs, and the water quality changed in reservoir drawdown period (February to May) was mainly dominated by T-Hard, F-, SO42- and electronic conductivity (EC), while the main leading factors were Q, SS, CODMn, NO3--N, TN and Se in flood season (July to August), and the water quality in the end of flood season (September) was dominated by T and WT, and caused changes in the physical and chemical properties such as DO. In addition, chloride (Cl-) was the leading factor in the period of high water level (December). At the present stage, changes in DO, organic pollutants (CODMn), inorganic salts (SO42-, F-), nutrient salts (NO3--N and TN), Se and mineralization degree of water (T-hard) dominate the changes in regional water quality and make the greatest effect to the water quality among all parameters. The analysis of variance showed that the physical and chemical characteristics (DO, pH and SS), nutrient composition (NH3-N and NO3--N), inorganic salts (EC and Cl-), petroleum organic pollutants and fecal coliform (FC) of rivers were varied significant in the upstream and downstream of the dam. The seasonal influence factors such as temperature, water temperature, rainfall, sediment concentration and reservoir operation mode were the main factors affecting the time difference of water quality near the dam. The spatial difference were mainly affected by the pollution of urban and the change of hydrological and hydrodynamical conditions by Three Gorges Reservoir operation. Therefore, controlling the exogenous pollution caused by human activities in the study area and implementing a reasonable reservoir operation mode for the seasonal variation characteristics of types of pollutants are the key to water quality improvement in the near dam section.