Endogenous Pollution Characteristics And In-situ Control Of Sediments In Subtropical Source Reservoirs

The subtropical region has four distinct seasons.Affected by temperature and light intensity,the reservoirs in this region will form obvious thermal stratification in summer,which has a serious impact on the water quality of the reservoir.The Xikeng Reservoir is located in the south subtropical zone.As the most important drinking water reservoir in the northwest of Shenzhen,the safety of water supply has attracted much attention.In this paper,the Xikeng Reservoir was taken as the research object,through the combination of in-situ continuous monitoring and stratified sampling and laboratory simulation experiments,the water quality evolution law of the Xikeng Reservoir and the characteristics of sediment endogenous pollution were studied,and the in-situ control effect of water-lifting aeration system operation on water pollution in reservoir was analyzed,in order to provide scientific basis and theoretical support for the improvement of reservoir water quality.The main research contents include:1)Changes of thermal stratification and water quality evolution characteristics of Xikeng Reservoir under natural conditions.2)Sediment oxygen consumption,sediment-water interface pollutant exchange flux in Xikeng Reservoir and their effects on reservoir water quality.3)The effect of artificial forced mixing and oxygenation of water-lifting aeration on the control of internal source pollution of the reservoir and the improvement of water quality.The main research results and conclusions are as follows:（1）In-situ monitoring showed that the thermal stratification of the water body in the Xikeng Reservoir was a typical subtropical single-cycle mixed mode,with the stratification period from February to October every year,and the mixing period from November to January of the following year.Air temperature was the main factor affecting the formation and disappearance of thermal stratification in water bodies,and rainfall and water transfer would also weaken the stability of thermal stratification in water bodies to a certain extent.Dissolved oxygen,p H and chlorophyll-a in the stratification period showed seasonal distribution characteristics of high and low levels.The bottom water body anaerobic induced the release of endogenous nitrogen and phosphorus in the sediment,so that the NH₃ and TP concentrations in the bottom water body were significantly higher than those in the middle and upper water bodies during the stratification period.（2）Laboratory experiment results showed that the oxygen consumption rate of sediment was 1.03g/（m²·d）which was about 1.45 times that of the water body,and the oxygen consumption of sediments was the main factor leading to the anaerobic state of the bottom of Xikeng Reservoir.The surface sediments were seriously polluted by carbon,nitrogen,and phosphorus with obvious differences in their spatial distribution.Generally speaking,the old reservoir area was larger than the new reservoir area.And the sediments of the Xikeng Reservoir had potential risks of internal release.（3）Studies on the exchange fluxes of sediment pollutants showed that the nitrogen and phosphorus exchange fluxes of the sediments obtained from the static release experiment and the interstitial water concentration diffusion model were both positive,NH₃ average exchange flux range was 7.36～37.92 mg/（m²·d）,PO₄^3--P average exchange flux range was 0.03～4.20 mg/（m²·d）.Indicated that the sediments of the Xikeng Reservoir are the"source"of the reservoir’s endogenous nitrogen and phosphorus.The release of nitrogen and phosphorus in sediments was an important reason why the concentrations of ammonia nitrogen and total phosphorus in the bottom water body were significantly higher than those in the surface and middle water bodies.（4）The operation results of the water-lifting aeration system showed that the mixing and oxygenating effect of the water-lifting aeration system was remarkable.After the system ran for a week,the thermal stratification of the water body disappeared,and the complete mixing time of the water body was about 3 months earlier than the same period in 2020.And the dissolved oxygen in the whole water body was maintained above 6 mg/L.The water-lifting aeration system also had a good control effect on typical pollutants in the reservoir.During the operation of the system,the total phosphorus reduction effect was obvious,the reduction rate was 46%-58%.The ammonia in the bottom water of the control area was about 0.203mg/L less than that before the system operation.The analysis of the water quality indicators of the downstream water plant showed that the operation of the system reduced the manganese concentration in the water plant’s influent from0.026 mg/L to 0.002 mg/L,which fundamentally solved the problem of excessive manganese in the water plant.The operation of the water-lifting aeration system improved dissolved oxygen in the middle and lower water bodies,eliminated the anaerobic environment of the bottom water body,suppressed the release of endogenous pollutants in the sediment,effectively reduced the nitrogen,phosphorus and manganese concentrations in the bottom water body,and reduced the water quality of the reservoir.continuous improvement.