Numerical Analysis Of The Influence Of Bioclogging On Nitrogen Transport And Transformation In The Hyporheic Zone With An Undulate River-bed

With the intensification of the human activities and the development of agriculture,the contamination of nitrogen in surface water has been more serious.In natural environment,hyporheic exchange is generally induced by various bed topographies.Surface waters bring dissolved solutes,nutrients and contaminants into streambed sediments where is rich in hyporheic microbes,then they are involved into a series of biogeochemical reactions.Meanwhile,hyporheic microbes make use of nutrients for metabolism and propagation,gather in colonies and grow in the forms of biofilms,which may fill sediment grains,reduce sediment permeability,change the flow patterns and further affect nitrogen dynamics.Study the effect of bioclogging on nitrogen transport and transformation is helpful for us to understand the nitrogen cycle within hyporheic zone and make the prevention and treatment of nitrogen pollution in river basins.It's difficult to capture the development of bioclogging by laboratory flume experiment and filed sample monitoring.Numerical simulation proved to be an useful method for demonstrating the spatial changes and distributions of biogeochemical gradient within hyporheic zone.In this research,we made use of ANSYS Fluent CFD to simulate turbulent flow and got the pressure distribution over the dunes.Then we established numerical models by simultaneously coupling water flow,biogeochemical reactions and microbial metabolism to simulate the hyporheic exchange driven by an undulate river bed and explored the effect of bioclogging by COMSOL Multiphysics.Further more,the effects of dissolved organic carbon content,stream velocity,the height of dune and stream-groundwater interactions on nitrogen transformation and biofilm-induced clogging were also studied.In this paper,the main results are as follows:1.The influence of bioclogging on nitrogen cycling in a hyporheic zoneThe clogging appears in superficial zone of dunes caused by microbial growth and the sediment porosity is reduced from 0.3 to 0.22.Biofilm-induced clogging makes hyporheic shape oxidation-reduction gradient,as the upper section of stoss side is in oxic environment as the less side and deeper section is in anoxic environment.Bioclogging limits the spatial distribution of NO₃^-,and NH₄⁺becomes the mainly nitrogen contamination in hyporheic zone.2.The influence of bioclogging on nitrate net sink/source function within hyporheic zone1)The nitrate reaction rates are greater than zero in clogging area,and it acts as nitrate source.Meanwhile,the deeper part of hyporheic zone acts as nitrate sink.The area of nitrate source is swelled while the area of nitrate sink is shrinked under the development of clogging.2)The hyporheic zone acts as nitrate source at first 10.4 days,the nitrate production rate is low and decreases with time.Then hyporheic zone acts as nitrate sink after that period,the nitrate removal rate increases first and then decreases.When the reaction reaches stable state,nitrate removal efficiency is 35.00%.3)The Damk?hler number of dissolved oxygen is much larger than 1 near the sediment-water interface,and it decreases by flow paths to less than 1.The space distribution of Dao₂ is shrunk with the development of clogging.The distributions of RNO3>0 and Dao2>>1 are in similar shapes,it means that Dao2 is a useful indicator for hyporheic nitrate source/sink function.3.The influence of environment factors on bioclogging and nitrogen cycle1)Based on the comparison of different dissolved organic carbon concentrations,we found that when organic carbon is limited,there is nearly no clogging appeared.Hyporheic zone serves as the hotspot for nitrification and the reaction of denitrification doesn't occur.When organic carbon is excess,the scope of clogging is shrunk but the degree is enhanced.Denitrification prevails in hyporheic zone while nitrification is restrained,which results in ammonium contamination in streambed.2)Biofilm-induced clogging is influenced by stream velocity and topographic condition.The more fluctuated river bed surface and the larger surface water flow would increase pressure gradient along bed-dunes,promote microbial growth and propagation,swell the clogging scope,and enhance the biogeochemical reaction rates.The faster reaction rate and larger clogging scope are beneficial for nitrate and ammonium removal.3)Bioclogging degree and reaction rates are influenced by stream and groundwater interactions.Biogeochemical reaction rates,total microbial biomass and nitrate removal efficiency are increasing with downwelling flux and are decreasing with upwelling flux monotonously.The clogging degrees are enhanced under gaining conditions while the clogging degrees are damped under losing conditions.