Biogeochemical Reactions Of Nitrogen And Its Numerical Simulation During River Bank Infiltration

Groundwater pumping along the river bank not only enhances water supply by stimulating water infiltration into the aquifer from the parent river,but also improves water quality via attenuation or degradation of the pollutants.In fact,the biogeochemical processes is usually very complex during bank filtration due to significant physical and chemical gradients between the river water and groundwater.With the growing eutrophication and nitrogen pollution of river water,groundwater pumping along river bank is facing the risk of nitrogen pollution.However,the mechanism of variation of key environment variables,including hydrodynamics,redox conditions and mass flux resulting from groundwater pumping on the nitrogen biogeochemistry during bank filtration is still unknown.Funded by NSFC,this thesis chooses the Karadian Water Source along the Second Songhua River,NE China as a case,to investigate the biogeochemical processes of nitrogen during bank filtration affected by groundwater pumping through in-situ monitoring,environmental isotopes,molecular biology,laboratory experiments and geochemical modeling.Based on analysis of the hydrogeological conditions and redox zonation,the biogeochemical processes of nitrogen and its microbial response is identified.This study is of great scientific and practical significance for the explanation of the causes of NH₄⁺enrichment in groundwater in the bank well fields,the optimization of exploitation scheme,and the emergency response to sudden nitrogen pollution incidents.The main findings and conclusions of this research are summarized as follows:(1)Based on long time monitoring of water level,water chemistry and isotopes,two infiltration paths(shallow and deep,respectively)of the river water controlled by heterogeneity of lithology and permeability of the water-bearing medium is identified.According to the hydraulic conditions,the infiltration flow paths can be divided into three zones,i.e.the zone strongly affected by river water infiltration,the zone moderately affected by river water infiltration and the zone affected by groundwater pumping.Within 200 m from the river bank,a sequential redox zonation is formed,i.e.the aerobic respiration,denitrification,Mn(IV)reduction,Fe(III)reduction and SO₄^2-reduction zone,respectively,and the spatiotemporal distribution of each zone changes along with the hydraulic conditions between river and groundwater.(2)Through the analysis of hydrochemistry,nitrogen and carbon isotopes,it is found that the main biogeochemical processes controlling nitrogen migration and transformation in the process of river water infiltration are adsorption,denitrification,denitrification,dissimilatory nitrate reduction to ammonium(DNRA)and mineralization of organic nitrogen.It is found for the first time that DNRA is active within 1.5?6 m from the river bank,and is the main reason for the enrichment of NH₄⁺in groundwater in the zone strongly affected by river water infiltration.Denitrification mainly occurs in the zone within 10?20 m from the river bank,and is the principal reason for the attenuation of NO₃^-in the zone strongly affected by river water infiltration.Studies from molecular biology show that the community structure and diversity of nitrogen functional bacteria for nitrification,denitrification,DNRA and organic nitrogen mineralization have a certain response to the above-mentioned nitrogen biogeochemical reactions,and the functional bacteria show strong co-evolution with river and groundwater hydrochemistry.(3)The results from laboratory experiments show that the adsorption of NH₄⁺on water bearing medium follows the Langmuir adsorption model,the denitrification of NO₃^-follows the first-order reaction kinetics,and the DNRA and organic nitrogen mineralization follow the zero order reaction kinetics.The environmental adaptability of NO₃^-denitrification is stronger than that of DNRA,and DNRA is more competitive to NO₃^-under high temperature and carbon load.In the process of river water infiltration,microbial activity,hydraulic gradient,organic carbon and nutrient content could significantly change the rate of heterotrophic reduction.On one hand,these factors can affect the inhibition of the inhibition term on NO₃^-reduction by changing the spatial and temporal distribution of redox zonation;on the other hand,they can change the degree of denitrification and DNRA by changing the carbon load.(4)The results of reactive nitrogen transport simulation in typical infiltration flow path show that the contribution of DNRA to NO₃^-reduction in groundwater during wet season and dry season are 25?30%and 5?10%,respectively,and the contribution to NH₄⁺enrichment in groundwater in the zone strongly affected by river water infiltration amounts to 40%and 15%,respectively,while the contribution of organic nitrogen mineralization to NH₄⁺enrichment can reach 75%and 85%in wet season and dry season,respectively.