三峡库区长江干流入出库水质评价及其变化趋势

水质问题关系着三峡工程的安全运行与库区社会经济的可持续发展.选取重庆朱沱断面和湖北南津关断面分别代表三峡库区长江干流的入出库断面,在对DO、COD_(Mn)和NH_3-N的年均、季均值统计基础上,运用基于灰色关联系数矩阵的TOPSIS法评价入出库水质,运用R/S法查明入出库水质的未来变化趋势.结果表明:入出库水质在2004—2014年间均介于Ⅰ、Ⅱ类间(《地表水环境质量标准》(GB 3838—2002)),其中,2004—2007年、2012—2014年均为Ⅰ类,2008年均为Ⅱ类,2009—2010年均在Ⅰ、Ⅱ类间变动,且均距Ⅰ类较近;入出库断面每年4个季度水质的相对贴近度均在Ⅰ、Ⅱ类间波动,但更偏近于Ⅰ类,这一趋势在多年平均情况下最为显著,2005—2007、2009、2010和2013年整体上两断面水质在4个季度中存在着相反的变化趋势;年均入库水质变化明显呈现良好趋势,而出库水质变化趋势基本不变.入出库季均水质变化呈现明显的阶段性差异,入出库过去的水质变好趋势会延续到未来相同的时间序列里,但这种水质变好趋势在将来的变化中并不稳定,具有一定的随机性.本研究结果有助于提高人们对三峡工程阶段性的蓄水对长江干流水质产生明显影响的认识,丰富人们对近年实施的环库生态治理工程成效的理解.

Water quality assessment and its changing trends in the reservoir inflow and outflow along the Yangtze River mainstream in the Three Gorge Reservoir Area

Water quality is essential to the safe operation of the Three Gorges Project and the sustainable socioeconomic development in the Three Gorges Reservoir Area (TGRA). By setting Zhutuo section in Chongqing as the inflow section and Nanjinguan section in Yichang, Hubei province as the outflow section of the TGRA along the Yangtze River mainstream, the TOPSIS (Technique for order preference by similarity to an ideal solution) model based on gray correlation coefficient matrix was applied to assess inflow and outflow water quality based on the annual and seasonal average statistics of three water quality indexes, i.e. DO, COD_Mn and NH₃-N.The R/S analysis method was used to analyze future changing trends in water quality. The results showed that water quality in the inflow and outflow sections was between classes I and II from 2004-2014. It was class I from 2004-2007 and 2012-2014, class II in 2008, and fluctuated between classes I and II from 2009-2011. The relative closeness of water quality in two sections fluctuated between classes I and II and closer to class I during all seasons, which was more evident in the annual average. The inflow water quality improved significantly while the outflow changed very little. At the same time, the seasonal variation of water quality evidently presented temporal-spatial difference. The favorable changing trend in water quality will continue, however, this trend is somewhat random and unstable. Results of this study will promote the awareness of the public on the influence of stage impoundment of the Three Gorges Reservoir on water quality of the Yangtze River, and also enrich the understanding of the effect of ecological protection over the TGRA.