Pollution Characteristics And Environmental Risk Of Superficial Sediments From Streams In An Urban-rural Fringe

Headwater ditches have been identified as a vital part of river water system,nutrient stored or purified on stream or ditch beds.Residual nitrogen and phosphorus in the bottom sediment was considerable and can release back to the overlyingwater body through water-sediment exchange.Therefore,agricultural headwater stream is generally as source and sink of nitrogen and phosphorus.In the study,three headwater streams and ditches which were discharged from different types in Chaohu Lake basin was investigated.Seasonal water and sediment samples were collected from three headwater streams.The pollution characteristics of the overlying water and sediments were preliminarily determined,we analyzed the nitrogen and phosphorus factions,the spatial and temporal variations for the risk assessment on phosphorus release,resolved the nitrification and denitrification potential characteristic of superficial sediments.The main results were outlined as follows:?1?Significant differences in the water environment were found for each streams,Guanzhenhe Distributary was the most polluted stream with a mean content of 26.69 mg·L-1 for NH₄⁺,2.38 mg·L-1 for PO₄^3-.Obvious variations exsited in the aquatic water quality criteria for ammonia were found for each streams,the highest risk occurred in Guanzhenhe Distributary,the vast majority samples in Taochong Stream at high risk and few at security level,the risk was low in Modian Stream.As the comprehensive water quality identification index method,the result showed that only Modian Stream was equal to water function area goals.?2?Phosphorus and nitrogen pollution levels could be arranged in the order of Guanzhenhe Distributary>Taochong Stream>Modian Stream,Guanzhenhe Distributary was the most polluted stream with a mean content of 4512.23 mg·kg-1 for TN,which was 2.56 and 1.36 times higher than Modian Stream and Taochong Stream,respectively.Similarly,with a mean content of 1376.95 mg·kg-1 for TP,which was 2.94 and 1.91 times higher than others.Besides,three streams were all in the state of ecological toxicity and also in a serious pollution situationin in TN and organic nitrogen.?3?Significant differences and different sort orders in contents of phosphorus fractions were found for each of the three streams.Moreover,the highest values for NH₄⁺-N and NO₃^--N emerged in Guanzhenhe Distributary,with a mean content of 575.43 mg·kg-1 for NH₄⁺-N,which was 1.51 and 1.22 times higher than Modian Stream and Taochong Stream,and with a mean content of 72.31 mg·kg-1 for NO₃^--N,which was 3.39 and 1.54 times higher than others.?4?Highest values for PSI emerged in Modian Stream,with a mean value of 40.36?mg L?·?100g·?mol?-1,the lowest value occured in Guanzhenhe Distributary,with a mean value of 40.36?mg L?·?100g ·?mol?-1.Obvious seasonal variations existed in PSI for the three streams,namely,the maximum content emerged in autumn,fewer in winter,and the minimum in spring or summer.The same conclusions were drawn in phosphorus release risk according to the PSI,DPS and ERI,and the risk ranked as follows:Guanzhenhe Distributary>Taochong Stream>Modian Stream.?5?Maximum values emerged in Taochong Stream while the minimum in Guanzhenhe Distributary for PNR and ANR.And the arrangement in order for dentrification rates was Guanzhenhe Distributary>Taochong Stream>Modian Stream.According to the variable importance plot?Fvip?,ANR,PNR and rate of denitrification were almost significantly correlated with such physical and chemical factors as pH,OM,NH₄⁺-N,NO₃^--N,TN and TP in sediments for the three streams.