Experimental Research On The Influence Of Surface Water-Groundwater Interaction On The Denitrification Process

Due to a large number of human activities such as burning of fossil fuels,industrial activities,and the application of agricultural fertilizers,the global emissions of reactive nitrogen have increased from 15 Tg to 210 Tg.These excessive nitrogen emissions have caused many environmental problems,such as water eutrophication,ocean acidification and the greenhouse effect,etc.The denitrification process mediated by microorganisms has been widely concerned and studied,because it is the main way to remove nitrogen from the environment.As an effective"water filter"in the natural environment,the hyporheic zone is a hotspot for biogeochemical processes,which distributes a large number of denitrifying bacteria.Under the period water level fluctuation,surface water and groundwater interaction occur in the hyporheic zone.This process brings not only O₂,organic matter and other substances into the hyporheic zone,but also pollutants like NO₃^-.The denitrifying bacteria in the hyporheic zone capture the NO₃^-in the water stream and convert it into the low bioavailable N₂.Under the coupling effect of microorganisms,geochemistry and solute transport,the hyporheic zone has become a hotspot for the denitrification process in the environment.Aiming at both denitrification and the hyporheic zone,sandboxes are used to simulate the denitrification process in the hyporheic zone at two dimensions.The change law of the water chemical composition of the in-situ pore water in the sandbox is used to characterize the dynamic characteristics and spatial distribution characteristics of denitrification,and also the process of water flow and solute transport in the hyporheic zone.Then,the influence mechanism of interaction intensity of surface water and groundwater on the denitrification process is studied.Besides,parallel sets of sandboxes are used to investigate the dynamic changes of water chemical composition,microbial community,and the key functional genes under different time and space conditions.As a result,the processes of microorganisms,geochemistry and solute transport in the hyporheic zone are coupled.The dynamic response law and response mechanism of the microbial community,as well as denitrification in the hyporheic zone,are deeply explored with changing the water chemical composition.The experimental results indicate that the denitrification rate and products quickly respond to the change of the hydrochemical composition,physical and chemical properties of the sediment,the hydraulic retention time and the intensity of the interaction.With the increasing of NO₃^-and hydraulic retention time,the denitrification rate greatly increases,and the intermediate product NO₂^-appears to accumulate.As the water interaction progresses,clear functional divisions appear in the sandbox along the direction of water flow.at the inlet of SGW（synthetic groundwater）,the concentration of NO₃^-is extremely low.Due to the high concentration of NH₄⁺,the nitrification process of NH₄⁺mediated by microorganisms in this region becomes the main source of substrates for the denitrification process.Nitrification and denitrification in the environment proceed simultaneously and quickly.at the inlet of SRW（synthetic river water）,a higher concentration of NO₃^-is input.Denitrification is carried out normally by using the input of NO₃^-.NH₄⁺with high concentration mainly participates in the adsorption and desorption of the sediment,while the nitrification of NH₄⁺is negligible.According to the experimental results,the microbial community,the abundance of functional genes and the changes in water chemical composition in different regions in the sandbox need to be investigated deeply.It is found that under the strong and rapid interaction of surface water and groundwater,the physicochemical properties of sediment and pore water,the abundance of functional genes and the composition of microbial communities tend to be distributed more homogeneous in time and space.On the contrary,under the slow interaction of surface water and groundwater,the above-mentioned parameters tend to be distributed more and more heterogeneous in time and space.The results of this study illustrate the importance of mutual coupling and influence among microbial communities,geochemical processes,solute transport and water flow processes in the hyporheic zone.In the previous researches,the influence of the surface water and groundwater interaction on the structure and function of the microorganisms was often ignored.at the same time,the evolution of microorganisms may influence geochemical processes.Under such feedback,the model simulated result must be quite different from the real situation.In future research,it is important to carefully consider the interaction between microbial composition,geochemical process,solute transport and water flow process,and should not simplify the changes in microbial composition,geochemical process or solute transport caused by the interaction process.