Sediment Characteristics Of Overbank Sediments In The Nanjing-Zhenjiang Reaches Of The Yangtze River And Their Environmental Implication

Stable overbank contains much information of flood events and environmental pollution of river basin.Extensive overbanks distribute along the Nanjing-Zhenjiang Reach in the lower reaches of the Yangtze River.In this thesis,NB1 Core,NB2 Core(outcrop section)and GB88 Core were sampled in Nanjing reach of the Yangtze River and ZR99 Core,ZH51 Core were sampled in Zhenjiang reach of the Yangtze River.Based on field and laboratory observations,together with the results of 137Cs dating,grain size analyses,magnetic susceptibility analyses and geochemical elements analyses,the sediment characteristics of modern overbank sediments and their environmental significance were analysed systematically,the flood events recorded in the modern overbank sediments were discussed,the sedimentary environment quality of the modern overbank sediments were evaluated,and the history of heavy metal pollution recorded in the sediments was reconstructed.The main results were as following1)The main colors of the modern overbank sediments were gray and brown.The grain size of the modern overbank sediment was fine,which were dominated by clayey silt,silt,sandy silt and a small amount of sand.In the outcrop of overbank,muddy layers with dozens of centimeters thick were developed,between the muddy layers were coarser sand layers with a flew centimeters The muddy layers contained a series of sandy silt coupled with thin silty layers with a few millimeters thick,which were dominated by horizontal beddings,and developed wave beddings locally.On the surface of the overbank,mud cracks were developed2)The grain size characteristics of the modern overbank sediment indicated that the hydrodynamics on the overbank was weak and the cores were mainly composed of fine sediments.The hydrodynamics on the overbanks where NB1 Core and ZR99 Core were sampled was stronger relatively.In some layers of NB1 Core,the sand contents were high,corresponding to flood events.The sand contents increased upwardly in ZR99 Core,indicating increasing hydrodynamics resulted by the riverbed evolution.Influenced by plant evolution,the sorting of the ZH51 Core had two obvious stages.The river channel gradient,river channel regimes,the evolution of the braided river and the vegetation conditions on the overbank were the main factors influencing the grain size characteristics of the modern overbank sediment in the Nanjing-Zhenjiang reach of the Yangtze River.3)The magnetic susceptibility of the overbank cores in the Nanjing-Zhenjiang reach of the Yangtze River was high,while the frequency magnetic susceptibility was low.There were almost no correlations between magnetic susceptibility and grain size,while there were correlations between frequency magnetic susceptibility and grain size.In NB1 Core,the correlations between magnetic susceptibility and trace element content were not obvious,but there were obvious correlations between frequency magnetic susceptibility and trace element content.The sediment source,organic matter/plant residue content,and the chemical environment were the main factors influencing the magnetic susceptibility of the overbank sediment.4)In the modern overbank sediment,the element contents were influenced by geological background,chemical weathering,the sedimentary hydrodynamic and human activities.Apart from CaO,all the elements were closely related with the grain size.The vertical phased changes of trace metal in NB1 Core indicated the contaminated history influenced by human activities.5)Sand content in modern overbank sediment was an indicator of flood event.The major element content was an indicator of flood event as well:the larger flood resulted in less Al2O3,Fe2O3,K2O,MgO,MnO,P5O2,TiO2 and more SiO2,Na2O.In NB1 Core,the changes of grain size and major element content indicated the massive flood in 1954 and 1983 clearly.6)Sediment quality guidelines in main stream sediment of the Yangtze River,sediment quality guidelines of FDEP,enrichment factor,geoaccumulation index,and potential ecological harm index method were used to evaluate the environment quality of NB1 Core.The results showed that:though the trace elements were enriched in the overbank sediment(the Pb and Cu were the most enriched trace metal),the environment quality of the overbank sediment was not bad."Diluting environment" formed by large amount water and sediment from the upper Yangtze river was the main reason resulting the low heavy metal content in the overbank sediment.Based on results of potential ecological harm index method,the history of heavy metal pollution archived by overbank was reconstructed and the pollution history could be divided into 6 phases:The first phase was 1954a,the sediments deposited were not contaminated by heavy metals and had no ecological harm;the second phase was from 1954a to early 1980s,the degrees of heavy metal pollutant and ecological harm increased gradually with the processes of agricultural,industrialization and urbanization.The third phase was 1983a,massive flood brought large flow,which diluted the heavy metal concentration,leading obvious decline of the degrees ofheavy metal pollutant and ecological harn.The fourth phase was middle 1980s,the degrees of heavy metal pollutant and ecological harm increased quickly.The fifth phase was from 1990s to 2003a,influenced by fast processes of agricultural,industrialization and urbanization,the degrees ofheavy metal pollutant and ecological harn were worst,but because of high frequency massive flood,the degrees ofheavy metal pollutant and ecological harm didn't increase obviously.The last phase was from 2003a to 2012a,influenced by strengthened environment supervision and the construction of Three Gorge Dam,the degrees ofheavy metal pollutant and ecological harm decreased.During the study,coarser and finer laminated layers with sandy silt or clayey silt were found to be the dominated characteristics of the modern overbank sediment in the lower reaches of the Yangtze River.Thin sand layers or coarse layers in some part of the overbank sediment core were corresponding to flood events.Based on 137Cs dating,grain size analyses,and geochemical elements analyses of the modern overbank sediment,the sedimentary characteristics of the overbank sediment were analysed,the flood events archived in the modern overbank sediment since 1950s were reconstructed,the sediment environmental quality was evaluated and the polluted history of the overbank was reconstructed.In the lower reaches of the Yangtze River,researches on the flood events archived in modern overbank can provide references to the researches of overbank,terrace and paleoflood.