Analysis Of The Seasonal Sudden Rise And Spatial-temporal Distribution Pattern Of Permanganate Index In A Certain River In The Yangtze River Delta

A river is the"mother River"of a city in the Yangtze River Delta.It runs through the whole City from west to east and is the main river of the city.In recent years,with the rapid development of the social economy and urbanization in the City,the water quality of the the River was seriously polluted.Since the 18th National Congress of the Communist Party of China,the government of the has intensified efforts to improve the the River,and the water quality of the river has improved year by year,but there are still some water quality control indicators that exceed the seasonal standards.For example,during the period from April to June 2021,the phenomenon of a sudden rise of permanganate index COD_Mnoccurred in the headwaters and all sections of the the River,which exceeded the class Il I standard of the national surface water environmental quality standard（GB3838-2002）for dozens of consecutive days,which has affected the image of the as a‘national civilized city’.Aiming at the phenomenon of the seasonal sudden increase of permanganate index COD_Mnin the river,this study aims to determine the cause of the sudden increase of permanganate index COD_Mnin the River.Through conventional water quality detection,three-dimensional fluorescence spectroscopy（EEM）,gel chromatography（HPSEC）and gas chromatography-mass spectrometry（GC-MS）,and other detection methods,the characteristics,composition,and source of related pollutants in the whole river of the River in the City were investigated.To obtain the cause of seasonal overproof of COD_Mnand the main source of pollutants in the the River national control section,and provide targeted suggestions for the treatment of the the River.The main conclusions of this study are as follows:（1）The water quality of the river changed greatly during the first continuous heavy rainfall at the turn of spring and summer.The river water quality index continued to rise from the lowest state before the rainfall and reached a peak with the rainfall,and continued to decline after the rainfall:COD_Mnincreased from 5.20mg/L to 6.40mg/L and then gradually decreased to 4.38mg/L,while NH₄⁺-N increased from 0.175mg/L to 1.225mg/L and then gradually decreased to 0.400mg/L.The overall COD_Crof the channel increased from18.25mg/L to 20.25mg/L and then gradually decreased to 11.00mg/L.（2）The results of three-dimensional fluorescence spectrum analysis showed that the organic matter in the water body was mainly composed of humic acid,and the proportion of dissolved organic matter in the water body increased from 72.81%to 76.93%with the rainfall course,and decreased to 75.39%10 days after the rainfall.The above results show that,from winter to early spring,the low water temperature of the river inhibited the microbial activity of the sediment,and there was no continuous mass precipitation to stir the sediment.Therefore,the pollutants imported into the river for a long time were deposited in the sediment and did not release to the overlying water of the river before rainfall.However,during the continuous heavy rainfall,the average temperature of the river water reached 27℃.Under the agitation of sediment by appropriate temperature and rainfall,the process of hydrolysis and acidification of sediment is promoted.Under a series of synergistic effects of endogenous and exogenous sources,a large number of pollutants are released and discharged into overlying water of the river channel in a short time,resulting in a sudden increase in the concentration of pollutants in the river channel and the rise of permanganate index.After the end of rainfall,the surface runoff generated by rainfall carried the particulate matter rich in organic impurities to the water body and deposited in the river sediment,providing nutrients for the hydrolysis and acidification of the sediment in the future.（3）The sources of dissolved organic matter in the river were influenced by both terrigenous and endogenous sources.The main source of pollutants was terrigenous input,and the main release mode was endogenous release.The fluorescence index analysis results showed that the FI index of overlying water in the river ranged from 1.38 to 1.86,with an average value of 1.49.HIX index ranges from 2.02 to 6.68,with an average value of 3.79.The BIX index ranges from 0.73 to 0.94,with an average of 0.80.（4）The molecular weight distribution of dissolved organic matter in the channel was determined by gel chromatography.The results showed that the organic matter in the channel water body was mainly small molecular organic matter with molecular weight<1000Da,and the water quality in each section of the channel had a main peak between 275-750Da and a secondary peak between 750-1000Da.The results showed that the dissolved organic matter in the river channel was mainly medium and small molecular weight organic matter,and almost did not contain macromolecular organic matter.The distribution law of organic matter was consistent,and the cross section of the middle and lower reaches of the river channel was consistent with that of the upper reaches of the river and agricultural non-point sources.（5）Gas chromatography-mass spectrometry（GC-MS）was used to determine the water body of the river.Dibutyl dicarboxylate（plasticizer）,dichlorobenzene（pesticide）,tetrachloroethylene and other industrial pollution sources were detected in all sections,but they were all lower than the detection limits.The test results showed that the water quality of the whole river was consistent with that of the source,and there was no obvious industrial point source pollution.（6）The pollution contribution rate of the river channel was calculated by three-position fluorescence difference spectrum analysis and principal component analysis.During rainfall,the sediment pollution contribution rate of the river channel was the highest,reaching 24.74%.Before and after rainfall,the contribution rate of pollution from agricultural non-point sources was the highest,reaching 28.18%.The above results show that,in the short term,the overall pollution of the river channel is dominated by the release of hydrolyzed acidifying substances from the sediment under the influence of rainfall.During rainfall,the water temperature,the increase of the concentration difference between the sediment and the overlying water,the rise of discharge and water level all promote the release of pollutants deposited in the sediment for a long time.At the same time,rainfall changes the hydraulic conditions of the river channel.The disturbance of sediment caused by heavy flow at the bottom of the channel increases the release rate of pollutants.In the long run,the import of agricultural non-point sources is the biggest factor that leads to the increase of permanganate index COD_Mnin the river channel,and the main source of river sediment is that heavy rainfall carries a large number of agricultural non-point sources into the water body of the river channel and accumulates at the bottom of the river channel.After long-term and continuous accumulation of pollutants from agricultural non-point sources in river sediment,once the suitable conditions brought by continuous rainfall are met,a large number of pollutants will be released into the overlying water of the river channel,and the concentration of pollutants in the river channel will rise,leading to the rise of COD_Mn.In summary,the main causes of the seasonal surge of COD_Mnin this river channel are as follows:pollutants that flow into the river channel and deposit in the sediment for a long time,are affected by continuous rainfall in spring and summer,and are released in large quantities under the synergistic effect of internal and external sources.