| Hyporheic flow zone is the hub of material and energy exchange between surface water and groundwater,and it has saturated aquifer with physical,chemical and biological gradients.There is a strong biogeochemical process in the subsurface flow zone,which plays an important role in improving the quality of water environment and maintaining the stability of water ecosystem.In view of the problems such as the change characteristics of riverbed sediments and the structure and function of microbial ecosystem in the subsurface flow zone,further research is needed.In this study,the Hunhe sluice in Shenyang City section of Hunhe River is taken as the research area to study the biogeochemical behavior of nitrogen.It not only has important theoretical significance for the nitrogen cycle at the junction of groundwater and surface water,but also has complete practical significance for regulating the water supply safety of rivers by combining sluices and dams.The following are the research contents and achievements :(1)Analysis of hydrogeological conditions in the study area.The natural factors such as topography and geomorphology of monitoring section and sampling well,meteorology and hydrology of Hunhe hydrological station in different years were analyzed.And carry out underground well water sample collection and testing.As the vertical distance between aquifer and ground increases,the particle size of medium increases,the content of gravel increases,the specific surface area decreases,and the total organic carbon content decreases.The water level difference between river water and groundwater is large due to the discharge of sluices and dams in dry season.The water level of sluices and dams in wet season is higher than that in dry season due to the increase of atmospheric precipitation.The groundwater temperature in the observation well is consistent with the change of Hunhe River water.The farther away from the river bank,the groundwater temperature in the well is increasing.The calculation of hydraulic gradient i reflects that the groundwater level decreases with the distance from the river bank.With the increase of distance from the river bank,the mixing ratio of river water and groundwater decreases.The closer the river is,the greater the hydraulic slope is.The farther away from the river,the smaller the hydraulic gradient.(2)Spatial and temporal distribution characteristics of nitrogen in subsurface flow zone.The effects of spatial and temporal variation of DO,Eh,SO42-,p H and conventional ions in groundwater on the spatial and temporal distribution of TN were determined.The data of three nitrogen in groundwater from 1990 s,00 s and 2021 were collected,and the three nitrogen in Hunhe River from 1985 to 1996,2006 and 2007 were analyzed.The total flow of river water infiltration increased,and the concentration of ammonia nitrogen was higher than that in the flood season of September.Under the influence of river water concentration,the farther away from the river bank,the concentration decreased,and the nitrification rate in dry season was faster than that in wet season.the farther away from the river bank,the lower the concentration of nitrite nitrogen;the concentration of nitrate nitrogen was relatively low in the environment with low p H and high redox potential in the wet season.In acidic and coastal aerobic environment,the intensity of denitrification and alienation reduction is relatively weak,and the concentration of nitrate nitrogen remains stable.The farther away from the river bank,the concentration of nitrate nitrogen in dry season was lower than that in wet season.It can be determined that the dominant range of nitrogen nitrification in the subsurface flow zone is about 150 m – 2890 m perpendicular to the river bank between wells 1 – 3,and the linear distance between the two wells is 2740 m.The main range of denitrification in the subsurface flow zone is about 2890 m – 4240 m perpendicular to the river bank,and the linear distance between the two wells is 1350 m.(3)Laboratory experiments on biogeochemistry of nitrogen in subsurface flow zones.The adsorption and desorption kinetics and thermodynamic process of different media in the subsurface flow zone were studied.The adsorption capacity of ammonia nitrogen by sub-clay was stronger than that of sandy soil,and the desorption capacity was also higher than that of sandy soil.Due to the decrease of particle size and the increase of specific surface area,the adsorption capacity increased inversely.The lower the temperature is,the stronger the adsorption effect of subsurface flow media on ammonia nitrogen is.With the increase of temperature,the adsorption of ammonia nitrogen weakened and the desorption enhanced.On the contrary,with the decrease of river water temperature in dry season,the desorption effect of subsurface flow media on ammonia nitrogen is gradually enhanced,and the adsorption.The fitting results of pseudo-second-order adsorption kinetics models were more accurate.The quasi-first-order kinetic equation of the desorption process can better fit the subclay,and the quasi-second-order can better reflect the desorption kinetic characteristics of subclay for ammonia nitrogen.Langmiur adsorption model fitted better.(4)Study on microbial distribution characteristics and community diversity in groundwater of subsurface flow zone.Using 16 Sr RNA technology,PCR amplification and high-throughput sequencing,the characteristics and distribution of microbial species in groundwater of four wells in the phreatic zone were analyzed during the dry season of sluice and dam discharge.The abundance of nitrifying bacteria was higher in W1 well in the nearshore zone with high TN concentration and the nearest distance from the river line.With the interaction between river water and groundwater,the abundance of denitrifying bacteria in groundwater in Well W2 is higher.Microbial treatment of ammonia nitrogen,nitrite nitrogen capacity increased.Nitrification occurred in well W1 in the coastal zone with the infiltration of river water,and the content of Proteobacteria in well W2 was the highest,and the denitrification reaction was strong.Nitrification and denitrification are gradually carried out simultaneously when groundwater is infiltrated into the W study area to form an anaerobic environment,which is also affected by dissolved oxygen and hydrodynamic conditions.(5)Study on the prevention and control scheme of three nitrogen pollution in the phreatic zone.The three nitrogen pollution in the subsurface flow zone was proposed from the surface water prevention and groundwater treatment of Hunhe River.The invention idea of ecological floating island with multi-functional aeration device was proposed at Hunhe sewage outlet of sewage treatment plant,and the numerical model and analytical solution equation of ammonia nitrogen migration in groundwater were established to predict the biogeochemical behavior of nitrogen in subsurface flow zone under different conditions. |
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