Temporal And Spatial Distribution Characteristics And Source Analysis Of COD In Daihai

In recent years,as an important ecological barrier,the function of Daihai Lake in northern China has been weakening continually,the lake area has been shrinking rapidly,the salinization degree has been increasing constantly,the water quality indexes such as COD_Crand COD_Mnhave been raising consistently,and the aquatic ecosystems has been damaged seriously.Based on the historical data of water quality indexes in Daihai from 1983 to 2019 and the actual water indexes from 2019 to 2020,this paper focused on the spatial-temporal distribution change of COD,an important index of Daihai water quality exceeding the standard.Based on UV-Vis,Fourier transform infrared spectroscopy,three-dimensional fluorescence spectroscopy and other characterization technologies,the sources of DOM in the water environment of the lake were identified to comprehensively grasp the spatiotemporal variation rules,sources and characteristics of organic matter in Daihai and its watershed.Then,the spatial and temporal distribution pattern of organic pollution in natural lakes in arid and semi-arid regions was revealed,and the characteristics of organic pollution in natural lakes were evaluated scientifically.The main research results are as following:（1）The water quality indicators of Daihai Lake show a certain temporal and spatial differences.The concentration of COD_Crvaries from 86.78 to 426.66 mg/L,and the overall pattern is summer>autumn>winter>spring;the concentration of COD_Mnvaries from 3.45 to 17.66 mg/L.Overall,there is a change pattern of summer>spring>winter>autumn.The lake area presents pollution characteristics in which the coasts of the southern and northern waters are higher than the center of the lake,and the coasts of the eastern and western waters are lower than the center of the lake.（2）For the 7 rivers that inflowing into the lake,COD_Cris at the inferior V level of the surface water environmental quality standards as a whole,and COD_Mnis at the level ofⅣ.Among them,the poorest water quality is the Muhua River and Damiaoxigou,and the concentrations of COD_Cr,COD_Mnand TP are all inferior V level,the TN concentration in Sandaohe is 1.76 mg/L,which is in Category III water,and the rest are inferior to Category V level.The average DOC concentration is 16.66mg/L,and the highest value is 24.56 mg/L in Muhua River,the lowest value in Tianchenghe is 11.52 mg/L.（3）There are a lot of protein substances,fatty compounds,aromatic unsaturated substances,and polysaccharide substances in the Daihai Lake.The functional groups mainly contained in DOM are C=O,C—H,N—H,C—O.The western waters have more organic and complex components,and receive a large amount of urban non-point source pollution sewage.The southern waters have complex organic components,stable structures,and more difficult-to-degrade substances,and accept urban residents’domestic sewage and tertiary industry pollutions and nd the tail water of the sewage treatment plant.The types of pollution components in the northern waters are relatively single,but the chemical bonds are stable,most of which are difficult-to-degrade organics.The organic components of the eastern waters have a stable structure and are easy to accumulate in water and difficult to degrade.（4）The Daihai Lake area mainly has C1（tyrosine-like）,C2（tryptophan-like）,C3（like fulvic acid）,C4（soluble microbial metabolites）,and C5（like h umic acid）,among which the content of refractory fulvic acid humic acid and humic acid is high,and the content of humus-like substances is higher in spr ing,and the water is greatly affected by external sources;the content of trypt ophan-like and soluble microbial metabolites is higher in summer,and the wat er is affected by both external and internal sources at the same time;the cont ent of tryptophan of protein-like,humus-like fulvic acid and soluble microbial by-products is higher in autumn,and the high value is mainly distributed in th e northern of Daihai Lake,which is more affected by human factors;the cont ent of fulvic acid and humus is higher in winter,and the high value is close to the human gathering places.