Degradation Rate And Environmental Influencing Factors Of Typical Pollutants In Urban Streams

In the process of urbanization,urban rivers often show the decline of water environment quality and degradation of environmental ecological functions.From the perspective of urban river water environmental protection,it is necessary to carry out quantitative analysis and evaluation on the degradation potential of major pollutants in water bodies and their environmental impact factors.In this study,three small rivers in urban area of Hefei City were selected as the research object.The natural leaf litter of fast-growing poplar was selected as the decomposition material.The field experiment method was used to estimate the degradation rate of organic matter,identify the main environmental influencing factors,and evaluate the health status of rivers according to the Gessner model.Based on the in-site experiment,the comprehensive degradation rate and biodegradation rate of nutrients and COD were estimated,and the main environmental influencing factors were identified.The nitrification and denitrification rates and their variation characteristics of sediments in Shiwuli River and Guanzhen River were analyzed,and the main environmental factors affecting nitrification and denitrification were analyzed.The results showed that:（1）Nitrogen and phosphorus pollution were serious in Shiwuli River and Guanzhen River,and the water quality of banqiao river was better.The results of water quality evaluation based on the five-element correlation number method showed that the water environment status of the three rivers is ranked in the order of Banqiao Stream>Shiwuli River>Guanzhen Stream.The mean values of TN content in the sediments from the Shiwuli River and Guanzhen River were 1743.75 and 1812.07 mg·kg^-1,respectively,and the TP content was 759.66 and 559.47 mg·kg^-1,respectively.（2）The TN pollution index of the sediments in Shiwuli River and Guanzhen Stream are both greater than 2,which were classified as heavy pollution.According to the TP pollution evaluation index,the two rivers were at moderate and light pollution levels respectively.Based on the evaluation results of the organic nitrogen mass score,both rivers were at level IV.Based on the organic index evaluation results,the upper reaches of the two rivers were at level III,and the downstream were at level IV.（3）The degradation rate k of organic matter in leaf litter of Banqiao Stream with good water quality was relatively small,while that in the severely polluted Shiwuli River and Guanzhen Stream was relatively high,the mean values were 0.0193,0.0418and 0.0416 d^-1,respectively.Regardless of coarse mesh bag or fine mesh bag,the degradation rate of leaf litter in each river showed extremely significant differences between the two experiments,and the degradation rate of organic matter in leaf litter between three rivers also showed extremely significant differences.Compared with sediment indicators,the effect of water quality factors on the degradation rate of organic matter in leaf litter was more obvious.（4）The range of comprehensive degradation rates of NH₄⁺-N,TN,TP and COD in the three urban rivers of Hefei City were 0.0427-0.2109,0.0264-0.1196,0.0940-0.3344 and 0.0258-0.1714 d^-1,respectively.The range of biodegradation rates of NH₄⁺-N,TN,TP and COD were 0.0108-0.1124,0.0109-0.0690,0.0196-0.1915 and0.0054-0.0955 d^-1,respectively.After hydraulic correction,the degradation rate of pollutants was higher than before.Temperature showed significant importance to the degradation rate of nutrients and COD in three rivers.（5）The average nitrification rate of sediments in Shiwuli River and Guanzhen Stream were 0.758 and 0.937 mg·（kg·d）^-1,respectively,and the average nitrification activity were 0.758%and 0.934%,respectively.The average denitrification rates of the two rivers were 16.15 and 18.45 mg·（kg·d）^-1,respectively,and the denitrification activity were 51.43%and 58.86%,respectively.The rates and activities of nitrification and denitrification in summer were higher than those in autumn and winter.The content of NH₄⁺-N and NO₃^--N in sediments and the water temperature showed significant importance for the nitrification and denitrification rate and activity of the two rivers.