南四湖及入出湖河流水环境质量变化趋势分析

南四湖作为南水北调东线工程重要的输水通道和调蓄湖泊，其水质状况一直受到人们的广泛关注. 根据南四湖流域主要入出湖河流14个监控断面及湖内5个监测站点的数据，从年际、年内、不同时间段和不同监测站点等进行了南四湖及河流水质变化趋势的综合分析. 结果表明，2007与2000年相比，南四湖入出湖河流监控断面的ρ（COD_Cr）总体下降率达到84.2%，ρ（NH₃-N）总体下降率达到79.7%. 南四湖及河流水质总体上出现了大幅度的改善，但距离稳定达到调水水质Ⅲ类标准的要求还有较大差距. 受水文情势、底泥释放和污染源以及各项污染控制措施等因素的影响，南四湖及入出湖河流监控断面水质改善存在一定的波动现象，水体富营养化形势严峻. 提出了科学合理地计算与分配南四湖养殖承载力和流域水环境承载力，重视非点源的控制与治理，大力推进“退耕还湿、退渔还湖”策略，是当务之急. 

Analysis of Water Environmental Quality Variation Trends in Nansi Lake and Its Joined Rivers

Nansi Lake is an important water transport channel and storage lake of the East Route Project of the South to North Water Transfer. Its water quality has been drawing wide public attention. This research investigates 14 monitoring sections in the main rivers flowing in and out the lake and 5 monitoring stations in the lake. The monitoring data is used to analyze the water quality variation trends of the rivers and the lake, including annual variation, intra-annual variation, different periods of time and different monitoring sites. The results show that the reduction ratio of COD_Cr and NH₃-N are 84.2% and 79.7% in 2007 compared with 2000 at the monitoring sections. The water quality in the Nansi Lake and its joined rivers has been greatly improved as a whole, but there are still large gaps compared with the target of consistently satisfying the water quality III standards. The water quality of the rivers and the lake has some fluctuation and is currently under eutrophication status, because of the effects of manifold factors, for example hydrology, sediment release, pollution source and the pollution control measures used in the basin. According to the current highlighted environmental problems, this paper proposes several appropriate countermeasures, such as calculating the distribution of the lake fishery capacity and the pollutant load capacity, paying attention to the control and management of non-point pollution sources, and promoting the strategy of restoring farmland to wetland and restoring fisheries to lakes. All of the above are urgent affairs.