Study On Physical And Chemical Propertiesand Phosphorus Forms Of Sediments Inbaiyangdian Dredging Area

Baiyangdian Lake is the largest grassy shallow water lake in the North China Plain.From 2019 to 2020,in order to solve the problem of endogenous pollution in the Baiyangdian Lake,the first pilot zone and the second expanded pilot zone carried out endogenous control measures,including ecological dredging.In this study,the sediment in Baiyangdian desilting area was taken as the main object of study,and the influence of desilting process on the basic physical and chemical properties of sediments and the occurrence form of phosphorus in sediments was explored through the measurement of the physical and chemical properties of sediments,so as to provide a scientific basis for evaluating the desilting effect of Baiyangdian Lake.After desilting,the sedimentation of particulate matter containing a large number of organisms and microorganisms will change the basic physical and chemical properties,phosphorus occurrence form and adsorption and release characteristics of surface sediments in the desilting area.Therefore,this thesis also studied the basic physical and chemical properties and phosphorus occurrence form of particulate matter,and discussed the possibility of its activation and regeneration of sediments in the desilting area.Compared with before desilting,desilting effectively reduced the TP content of water body.The TP content of water body in desilting area was 0.11～0.17 mg/L,which was still higher than the limit value of Class Ⅲ of surface water 0.05 mg/L,and the phosphorus pollution in water body was still very serious.There was no significant difference between the physical and chemical indexes of the water body after desilting and that of the area without desilting.In terms of longitudinal changes,water temperature,p H and DO showed a downward trend with the increase of depth,and the ability of light projection gradually weakened with the increase of depth,which was an important factor causing the stratification of water temperature.The aerobic degradation of OM substances in surface sediments formed a large amount of organic acids,which was an important reason for the low DO content and p H in bottom water.The sediment moisture content of Baiyangdian Lake after desilting is21.68～75.59%,which is significantly lower than that of the undesilted area.The desilting process reduces the content of free water in the sediment and inhibits the dissolution and release of nutrient salts.The sediment particles in the desilting area are mainly silt and fine sand,accounting for 20.06～58.33% and 33.94～67.51%,respectively.In the desilting process,the embankment and Feijoa were removed,the depth of the lake was increased,and the hydrodynamic condition of the lake was improved.As a result,the grain size of the surface sediment in the desilting area was enlarged.The results of scanning electron microscopy showed that OM and inorganic mineral particles were the main components of settling particles in the desilting area,and OM mainly contained mixed aggregates,algae and their fragments.In general,the desilting process effectively improved the hydrodynamic conditions of the lake,increased the density of the sediment,and effectively reduced the contents of TC,OM and TN in the sediment.However,the analysis of settling particulate matter found that the settling particulate matter carried a high content of OM and was loaded with a large number of microorganisms.When it settled to the bottom of the lake,It can significantly affect the basic physicochemical properties of surface sediments after desilting.The average TP content of Baiyangdian sediment is 1074.25 mg/kg,which is at the moderate pollution level.Ca-P with weak bioavailability was dominant in the sediment in the desilting area,and the proportion of Ca-P in the sediment was > Fe/Al-P > OP,which effectively reduced the content of bioavailable phosphorus in the sediment.The phosphate adsorption/analytical mass equilibrium concentration of sediment in the desilting area is 0.17-0.47 mg/L,which is higher than the water TP concentration of0.16mg/L,and there is a certain risk of release.Compared with non-desilting,desilting reduces the phosphorus release potential of sediments and increases the phosphorus adsorption capacity of sediments.Compared with the surface sediments in the desilting area,the proportion of phosphorus in the biological availability of the particulate matter is significantly increased,and its adsorption capacity for phosphorus is weakened.When particulate matter settled to the bottom of the lake,it would change the morphological characteristics of phosphorus in the surface sediments of the dredging area and reduce the adsorption capacity of phosphorus in the sediments.Correlation analysis showed that overlying water temperature,DO and OM content in sediment were important factors affecting the migration and transformation of phosphorus at the sediment-water interface.