近18年长江干流水质和污染物通量变化趋势分析

长江是我国第一大河，2000年以来长江流域水环境形势发生了巨大变化，长江水质现状及其变化和原因备受关注.采用水质、水量、污染物通量、污染负荷等多要素综合分析方法，研究了近18年长江干流水质和污染物通量的时空分布、变化趋势及原因.结果表明:①宜宾以下长江干流总磷浓度高于金沙江；从源区至入海口，长江干流氨氮浓度总体呈沿程上升趋势.②2011—2013年是长江干流水质重要转折期.2003—2010年，长江下游江段氨氮浓度总体呈明显上升趋势，2013—2018年大幅下降，下降约65%；2012—2018年，长江干流大部分江段总磷浓度呈明显下降趋势，其中上游下降最大，为45%~60%；2003—2018年，长江干流高锰酸盐指数、重金属和石油类污染均明显减轻.③2000年以来，长江水量未有明显增大或减小趋势，但输沙量大幅下降.总磷年通量与年径流量密切相关，年内丰水期总磷通量较高.2001—2006年宜昌断面、汉口37码头断面氨氮年通量大幅下降；2013—2018年，大通断面氨氮年通量呈明显下降趋势.④2018年，大通断面总磷、氨氮年通量分别约为9.37×104和21.47×104 t.总磷汇入量中游强于下游，氨氮汇入量下游强于中游.上游向下游磷的输送由21世纪初以颗粒态为主转变为2017—2018年以溶解态为主.⑤长江下游江段氨氮浓度和大通断面氨氮年通量的显著下降，以及长江整体石油类超标率大幅下降均主要归因于水污染防治；长江干流大部分江段总磷的明显下降主要归因于随泥沙汇入水体磷的减少，以及长江流域水污染防治.研究显示，近18年来长江干流污染物浓度、时空特征、输送形态发生了巨大变化. 

Analysis on Alteration of Water Quality and Pollutant Fluxes in the Yangtze Mainstem during Recently 18 Years

The Yangtze River is the largest river in China. Since 2000, great changes have taken place in the water environment of the Yangtze River Basin. The changes in the water quality of the Yangtze River and its causes have attracted widespread concern. Therefore, the status and trends of water quality and pollutant flux in the mainstem of the Yangtze River in recently 18 years were analyzed, and results indicate: (1) The concentration of total phosphorus (TP) in the reach downstream from Yibin was higher than that in the Jinsha River, and the ammonia nitrogen concentration showed an upward trend from the source area to the estuary. (2) Ammonia nitrogen concentration in the lower reach increased significantly during 2003-2013, then decreased by 65% during 2013-2018. Since 2013, the concentration of TP in the mainstem has declined significantly, especially in the upper mainstem with the reduction of 45%-60%. From 2003 to 2018, Permanganate Index, the concentration of heavy metals and petroleum in the mainstem were greatly reduced. (3) Since 2000, there has been no significant increase or decrease in the water volume of the Yangtze River, but the sediment transported has decreased greatly. The annual flux of total phosphorus is closely related to the annual runoff, and the flux in wet season is larger than that in other seasons. From 2001 to 2006, the annual ammonia nitrogen fluxes in Yichang section and No.37 Hankou Port section decreased significantly, and from 2013 to 2018, the annual ammonia nitrogen flux at Datong section decreased significantly. (4) The TP flux and ammonia nitrogen flux in Datong section were 9.37×10⁴ and 21.47×10⁴ t, respectively, in 2018. Phosphorus inflow in the middle reach was stronger than that in the lower reach, and the ammonia nitrogen inflow in the lower reach was stronger than that in the middle reach. The main transport form of phosphorus from upstream to downstream has changed from the granular state at the beginning of this century to the dissolved state at present. (5) Significant decrease of ammonia nitrogen concentration in the lower reach and declination of standard-exceeding rate of petroleum in the mainstem are mainly due to water pollution control; the significant decrease of TP, permanganate index in the mainstem are mainly due to the reduction of sediment entering the water and water pollution control. The results show that the concentration, temporal and spatial characteristics and transport patterns of pollutants in the mainstem of the Yangtze River have changed dramatically in the past 18 years.