Research On The Mechanism And Numerical Simulation Of Nitrate Natural Attenuation In The River Bank Filtration Zone In Seasonal Frozen Soil Region Of Northeast China

Riverside groundwater extraction is a critical component of developing and utilizing water resources.It can stimulate river water recharge to boost groundwater recharge resources while also effectively filtering poisonous and hazardous elements in river water and improving groundwater quality.With the eutrophication of river water and the increasing severity of nitrogen pollution in recent years,groundwater in areas with tight hydraulic connections to surface water has confronted the possible risk of nitrogen pollution caused by groundwater exploitation along the river.Among these,nitrate attenuation associated with river water infiltration is one of the most effective techniques to reduce nitrogen pollution concentrations.However,the nitrate attenuation process in river water infiltration zones caused by riverbank mining,as well as the factors that influence it,require further investigation.The surface water temperature in the northeast seasonal frozen soil area varies substantially with the seasons,and surface water infiltration will surely impact the groundwater temperature in the infiltration zone,affecting the nitrate attenuation capacity.The response mechanism of the nitrate attenuation process in the river water infiltration zone to seasonal changes in groundwater temperature produced by surface water infiltration is still unknown at this time.Based on this,this paper uses the riverside water source area of Kaladian,the second Songhua River in Northeast China,as a typical research area,and integrates the techniques and methods of in-situ monitoring,environmental isotope tracing,temperature tracing,geological multivariate statistical methods,laboratory experiments and numerical simulation,and so on,to analyze the hydrodynamics,heat migration,and so on.This research describes the process,influencing factors,and influencing mechanism of nitrate attenuation in the process of river water infiltration in Northeast China’s seasonal frozen soil region,with an emphasis on the effect of temperature variation on nitrate denitrification.Finally,a linked numerical simulation model of water and heat transfer,as well as nitrate reactive transport,is developed,which quantitatively shows the nitrate attenuation process in the river water penetration zone.The following are the primary results and understandings derived from this paper’s research:（1）The groundwater hydrodynamics can be divided into the coastal river strong influence zone（shore distance 0～5m）,the coastal river medium influence zone（shore distance 5～30m）,and the far shore river weak influence zone（shore distance30～420m）based on in-situ dynamic monitoring results of water level and temperature of surface water and groundwater in the river infiltration zone（from November 2020to November 2021）.（30～420m from the shore）.Temperature changes in river water and groundwater in the research area are constant,with sinusoidal swings producing seasonal dynamic characteristics of a low temperature area in winter,a warming period in spring and summer,and a cooling period in summer.（2）It is recognized that there are obvious oxidation-reduction zones in the infiltration flow path of water along the Yangtze River,which are aerobic respiration zone,NO₃^-N reduction zone,manganese（Ⅳ）reduction zone,and iron（Ⅲ）reduction zone,based on in-situ dynamic monitoring results of hydrochemistry and microorganism of river water and groundwater in the infiltration zone.Seasonal differences in the distribution range of oxidation-reduction zones are as follows:in fall（November 2020）,the range of NO₃^-reduction zones is 0～5m from the coast,in winter（February 2021）,0～14m from the shore,and in spring（May 2021）,0～8m from the shore.（3）The main processes controlling nitrate attenuation in the river infiltration zone are denitrification and convection mixing,according to the stable isotope tracer technique of hydrogen,oxygen,and nitrogen.Denitrification clearly contributes more to nitrate attenuation than convection mixing in the 0～5m range from the beach.Hydraulic gradient,nitrate infiltration flow,dissolved organic nitrogen and carbon content,and groundwater temperature all have an impact on nitrate denitrification.Within 2m of the beach,the proportional influence of temperature on nitrate attenuation is as high as 85.1%,which is substantially influenced by river water temperature.（4）The static experimental data suggest that the nitrate denitrification rate at19℃is approximately 2.4 times that at 10℃.The dynamic soil column experiment established the exponential positive correlation formula between the nitrate denitrification decay kinetic constant and temperature.The affinity of denitrifying enzyme for the nitrate of the reaction substrate was found to be decreasing temperature gradient（30℃→10℃）>zero temperature gradient（10℃）>increasing temperature gradient condition（0℃→10℃）,resulting in decreasing temperature gradient（30℃→10℃）,zero temperature gradient（10℃）,and increasing temperature gradient condition（0℃→10℃）.（5）Two-dimensional coupled simulation results of water-heat-reactive solute transport reveal that denitrification contributes significantly more to nitrate attenuation in the process of river water infiltration than convective mixing.Denitrification contribution rates in winter are 84.9%,94.6%,and 97.3%,respectively,and nitrate attenuation ability in the process of river water penetration is particularly susceptible to temperature change.