Study On The Variation Of DOM Characteristics And The In Situ Reduction Effect By Mixing And Oxygenation In Jinpen Reservoir

Natural organic matter(NOM)exists widely in aquatic environment.Its concentration,properties and characteristics reflect the water quality of aquatic environment.Jinpan reservoir(JPR)is the main source water reservoir of Xi’an city.The concentration of organic matter directly affects the water supply safety of Xi’an city.In this study,the two-year in-situ monitoring of the JPR was carried out.The UV-Vis spectrophotometry and three-dimensional fluorescence excitation emission matrices was used to characterize the composition and source of dissolved organic matter(DOM)in the JPR.The reduction of organic pollution caused by the operation of the water lifting aerators(WLAs)provides a theoretical basis for the in-situ reduction of organic pollution in the water source reservoir.The main research contents include: 1)the seasonal variation of the organic pollution load and DOM characteristics of the JPR;2)the concentration,composition characteristics and optical derivative properties of the DOM in upstream during runoff event which produced by different rainfall conditions;3)the reduction effect of mixing and oxygenation of WLAs on DOM.(1)The DOM in the JPR has strong endogenous characteristics in dry season,it has smaller molecular weight,lower degree of humification,and lower hydrophobicity and aromaticity.The DOM of the JPR in rainy season has strong terrestrial characteristics and higher molecular weight.The degree of humification is high,and the hydrophobicity and aromaticity are high.Without a large scale of algae bloom,the input of stormwater runoff is the main reason for the input of organic pollution in the JPR.The organic pollutants inputed by the stormwater runoff are reduced under the action of the mixing and oxygenation by WLAs.(2)The rainfall season can significantly affect the characteristics of runoff DOM in the JPR.From winter to autumn of the second year,the aromatics and hydrophobicity of the DOM in upstream when runoff occurs gradually increase.The organic matter in the upstream of runoff in summer mainly consisted of terrestrial macromolecular humus.The fluorescence of humic-like substance was much higher than that of protein-like substance,which showed strong humus characteristics,BIX and β:α were low.The characteristics of the DOM of upstream in autumn runoff were similar to that in summer,but the terrestrial characteristics were weaker than that in summer.In winter and spring,the DOM in the upstream of runoff has strong endogenous characteristics,high proportion of protein-like,low molecular weight,weak humus,strong autogenic and high proportion of fresh DOM.(3)There is a significant positive correlation between DOC of runoff upstream and rainfall in the JPR,and there are significant differences in organic matter characteristics under different rainfall conditions.The DOM of runoff generated by light and moderate rain has strong endogenous characteristics,low degree of humification,relatively high proportion of fresh DOM,low molecular weight,and relatively high proportion of protein-like substance.Humic-like substance is the main component of runoff input caused by heavy rain and rainstorm.The DOM has strong terrestrial source characteristics,high degree of humification,relatively low proportion of fresh DOM,weak endogenous characteristics and high molecular weight.(4)There is no significant correlation between rainfall interval and organic pollution load,but it can significantly affect the characteristics of DOM inputed by runoff.In the same season,compared the two runoff events with similar rainfall and different rainfall intervals.Under the condition of long rainfall intervals,the protein-like component accounted for a relatively high proportion,the endogenous characteristics of DOM were strong,the degree of humification was low,the proportion of fresh organic matter was relatively high,and the molecular weight was relatively low.The DOM inputed by continuous rainfall with short rainfall intervals was dominated by terrestrial macromolecular humus.DOM had strong terrestrial source characteristics,high degree of humus,relatively low proportion of fresh organic matter,weak endogenous characteristics,and high molecular weight.(5)The mixing and oxygenation of WLAs can induce the earlier occurrence of complete mixing,provides microbes with long-term of high DO and relatively high temperature conditions which is beneficial to the respiration and mineralization of DOM.However,with the decline of water temperature,the microbial activity largely decreases,despite of the DO is relatively high.During the whole periods of induced mixing,the DOC decreases from 3.18 ± 0.51 to 2.13 ± 0.64 mg/L with a reduction of 39.18%,the CODMn declines from 3.76 ± 0.22 to 2.73 ± 0.18 mg/L,was reduced by 20.47%.When microbial metabolism activity(indicated by AWCD)was high(AWCD=1.57 ± 0.39),the microbial metabolites(C5)accounted for ～22%,and the protein-like components(C4)accounted for ～12%.As the AWCD declined to 1.22 ± 0.12,the proportion of C5 gradually decreased to 10%,while that of C4 increased to 19%.The results of optical properties showed that,with the decrease of the microbial metabolism activity,the micromolecular DOM with endogenous characteristics was gradually accumulated in the water column.