冰封对乌梁素海水质污染特征的影响

Change in Water Quality of Ulansuhai Lake after Its Surface Freezing

【目的】明确乌梁素海冰封期的水质污染特征,揭示冰封对湖泊水质等级的影响。【方法】于2021年1月在乌梁素海布设10个采样点,采集冰层及冰下水样品。参照《地表水环境质量标准》(GB 3838—2002)和乌梁素海的污染特征,选取COD_(Mn)、NH_(3)-N、TP、TN、氟化物和铜6项水质指标作为目标污染物,采用单因子水质标识指数法、综合水质标识指数法和主成分分析法对乌梁素海冰封期与非冰封期的水质进行综合评价。【结果】各项目标污染物的单因子水质标识指数为:冰封期冰下水≥非冰封期≥冰封期冰融水;6项目标污染物的污染程度依次为:TN>TP>COD_(Mn)>NH_(3)-N>铜>氟化物,其中COD_(Mn)、NH_(3)-N、TN质量浓度都发生了水质等级的变化;各采样点在冰封期的综合水质标识指数结果全部劣于非冰封期,其中采样点I12由Ⅱ类恶化为Ⅲ类,其余采样点水质等级虽未发生变化,但其质量浓度距离相应的水质等级下限值更加接近;乌梁素海的整体水质受NH_(3)-N、TN和TP水质因子的影响较大,较高的氮、磷值将为乌梁素海水体富营养化带来潜在风险;各采样点的水质优劣顺序在空间上呈出水区>湖中区>进水区的变化。【结论】冬季结冰过程中,冰对目标污染物具有排斥效应,污染物由冰体向冰下水迁移致使冰下水中污染物质量浓度增大,加剧了乌梁素海的水质恶化,这为冰封期湖泊水环境管理带来严峻考验。

【Objective】Chemical quality of surface water bodies is affected by many factors but most of them are driven by oxygen.Bioavailable oxygen in water decreases after the water surface is frozen,and this paper investigates how this impacts water quality in Ulansuhai lake in Inner Mongolia.【Method】Ice and water samples were taken in January 2021 from 10 sites in the lake.Contents of COD_(Mn),nitrate,total phosphorus(TP),total nitrogen(TN),fluoride and copper in each sample were analyzed.They were compared to those measured before the lake was frozen.Change in these chemicals due to freezing was assessed using the single-factor and the comprehensive water quality identification index,as well as the principal component analysis.【Result】①Single factor water quality identification index analysis indicated that the content of all chemicals in fluid water was ranked in the order of water underneath ice≥water before freezing≥melted ice.The pollution degree of the six chemicals was ranked in the order of TN>TP>COD_(Mn)>NH_(3)-N>copper>fluoride.The chemicals that resulted in water quality classification change were COD_(Mn),NH_(3)-N and TN.②Comprehensive water quality identification index analysis showed that freezing worsened the water quality in all 10 sampling sites,though the deterioration varied with sampling sites.Nitrate,TN and TP had the greatest influence on water quality,as increasing nitrogen and phosphorus risked eutrophication.Spatially,water quality in the lake area was ranked in the order of outlet area>middle lake area>inlet area.【Conclusion】Surface freezing in Ulansuhai Lake increased chemical concentrations in the water beneath the ice,leading to water quality deterioration.To ameliorate water pollution,discharge of pollutants to the lake,especially for TN and TP in autumn and winter,should be reduced.