The Reactive Transport Processes And Fate Of Groundwater Nitrogen Driven By Water Dynamics In The Central Yangtze River Basin

Inorganic nitrogen is the most ubiquitous pollutant in groundwater.The occurrence of excessive nitrogen in groundwater is threatening human and aquatic ecosystem health across many places worldwide.Moreover,the discharge of groundwater containing high concentrations of nitrogen to surface water will also lead to eutrophication of rivers and lakes.Therefore,it is of great significance for the management of surface water and groundwater quality to identify the source,reactive transport processes and fate of nitrogen in aquifer system.Variations in hydrodynamic conditions caused by fluctuations in surface water and groundwater levels will not only change the migration pathways of nitrogen components,but also affect different geochemical processes,and ultimately change the distribution of nitrogen compounds in groundwater.Identifying the controlling effect of hydrodynamic processes and geochemical processes on reactive transport processes of groundwater nitrogen is the focus of recent research.The Jianghan Plain,located in the central Yangtze River Basin,is a major agricultural region of China.Intensive human activities lead to the wide occurrence of increased concentrations of ammonium and nitrate in groundwater,however,the origin,migration and enrichment mechanisms are still controversial.This study is focused on the key scientific issue of“nitrogen reactive transport processes and release mechanisms driven by water dynamics”.The Jianghan Plain was selected as the study area.In present study,the source and primary controlling processes of nitrogen in the multi-layer aquifer system in was preliminarily identified by a comprehensive study that includes the collection of geological and hydrogeological data,analysis of water level,hydrochemical and stable isotopic data,combining the evaluation of the dissolved organic matter and the machine learning methods.By applying the high-resolution mass spectrometry method to obtain the molecular composition of dissolved organic matter in sediment,associating with the adsorption of ammonium on sediments,the form and abundance of nitrogen conserved in sediments has been revealed.The control effect of water dynamic on the reaction and migration process of nitrogen in groundwater was revealed by long-term monitoring of water level and water chemistry.The main processes governing the concentrations of ammonium and nitrate in groundwater under different surface water-groundwater interaction modes were subsequently investigated by indoor soil column experiments.Finally,the storage and release mechanism of nitrogen in groundwater controlled by hydrodynamic and geochemical processes in large river basins was recognized by constructing a reactive transport model of nitrogen in groundwater.The main achievements are as follows:1.High concentrations of nitrogen components occur in both the shallow and deep groundwater of the Jianghan Plain.Nitrogen in the shallow groundwater mainly comes from human activities,while it in the deep groundwater released from the decomposition of nitrogen-containing organic matter in sediment.Nitrogen-containing organic matter and exchangeable NH₄-N are the primary forms of nitrogen conserved in sediments.The main forms of inorganic nitrogen in both surface water and groundwater in the area are NH₄-N and NO₃-N.Vertically,high concentrations of NO₃-N are mainly distributed in shallower depth（0～10m）,and the concentration dramatically declines with depth;NH₄-N concentration usually peaks at20-30m below the surface,and then decreases with depth;in the confined aquifer（depth>100m）,the NH₄-N concentration increased significantly.The stable isotopic compositions of NO₃^-and NH₄⁺in shallow groundwater are distributed in theδ¹⁵N characteristic value range of nitrogen from animal manure and domestic sewage.Meanwhile,the high concentrations of NH₄-N have a strong correlation with the increased concentrations of K⁺and Cl^-,indicating the input of agricultural activities.The weak hydraulic connection between the shallow and deep aquifers limits the migration of nitrogen from surface sources to the deep aquifers.Thus,the high concentration of NH₄-N in the deep groundwater mainly comes from the decomposition and release of dissolved nitrogen-containing organic matter（DON）in sediment.Although total organic carbon（TOC）content in shallow sediments is high（～23930mg/kg）,bioavailable dissolved organic carbon（DOC）content is relatively limited（～670 mg/kg）,dissolved organic matter（DOM）The proportion of nitrogen-containing molecules（CHON）is also low（～17%）.The content of exchangeable NH₄-N in sediments can reach 240mg/kg.Thus,the content of bioavailable dissolved nitrogen-containing organic matter is low.2.According to the column experiments,different hydrochemical compositions would result in variable reactive transport processes of groundwater nitrogen.Nitrification-denitrification would be promoted during the infiltration of surface water while amount of NH₄-N adsorbed in sediments increases during the groundwater discharge.Particularly,denitrification and adsorption are favored under frequent exchange of surface water and groundwater.Column experiments have been conducted by changing the concentration of Dissolved oxygen（DO）and major ions,respectively.The concentrations of NH₄-N and NO₃-N in groundwater are primarily controlled by nitrification,denitrification and cation exchange reactions.The hydrochemical compositions change under different patterns of surface water-groundwater interactions,resulting in different geochemical and biogeochemical processes.Infiltration of surface water with high concentration of DO would promote nitrification and following denitrification processes.The NH₄-N adsorbed in sediment is limited.Groundwater discharge with high concentration of NH₄-N and major cations(i.e.,Ca²⁺and Mg²⁺)and low concentration of DO would result in the inhabitation of nitrification-denitrification,however,the adsorbed NH₄-N in sediment increases.When surface water and groundwater exchange frequently,the denitrification and adsorption of NH₄⁺are both promoted.3.The temporal and spatial distribution of NH₄-N and NO₃-N in shallow aquifer system is controlled by interaction of surface water and groundwater.Induced by variations of hydrochemical characterizations,the nitrification-denitrification and adsorption-desorption processes would change,which eventually influence the fate of groundwater nitrogen.A large proportion of terrestrial NH₄-N was rapidly adsorbed by upper clay sediments.Nitrification is promoted under oxic condition,leading to the enrichment of NO₃-N in the shallower groundwater.DO is consumed along the groundwater flow path,after entering the anaerobic environment,denitrification and dissimilatory nitrate reduction to ammonium（DNRA）are promoted.Since the rate of DNRA is lower than denitrification,indicating denitrification is the primary mechanism of NO₃^-reduction.High concentrations of NH₄-N in deep groundwater are associated with the decomposition process of DON in sediment.The seasonal fluctuations of water levels in the Shahu monitoring area changes the interaction of surface water and groundwater.The recharge of surface water to groundwater increases the concentration of DO in groundwater and enhances the nitrification-denitrification reaction;while the discharge of groundwater to the surface water with high concentrations of NH₄⁺exchanges the adsorbed Ca²⁺and Mg²⁺out of sediments,resulting in an enrichment of NH₄-N in sediment.In addition,the groundwater level in the study area has dropped significantly over the past decade,resulting in an increase in the infiltration intensity of surface water,which promotes the desorption of NH₄⁺,synchronously the accumulating DO in groundwater enhanced the transformation of NH₄-N to NO₃-N.4.The regional distribution of nitrogen concentration is controlled by both groundwater dynamic and geochemical/biogeochemical processes.The depletion of NO₃-N and enrichment of NH₄-N in the groundwater along the flow path in the large river basin has been observed.The surface water-groundwater interaction pattern in large river basins varies along the flow direction,the surface water recharges the groundwater in the recharge zones,while the groundwater discharges to the surface water in the discharge zones with seasonal fluctuation.In recharge zones,DO enriched surface water enters the aquifer and promotes the nitrification reaction,oxidizing NH₄-N to NO₃-N,so the NO₃-N is enriched,while the NH₄-N concentration in groundwater drops and NH₄-N adsorbed in the sediment decreases.With the DO concentration decreasing along the flow path,nitrification gradually weakens,and denitrification increases.Consequently,the NH₄-N gradually accumulated,and NO₃-N consumed.Entering the discharge zones,NH₄-N concentration reaches the highest,while NO₃-N is relatively lowest.The intensified surface water-groundwater exchanges will change the concentration of NH₄-N and cations in the aquifer to reinforce the adsorption of NH₄-N on the sediment surface.Therefore,the content of exchangeable NH₄-N in the sediments in the discharge area is much higher than that in the recharge area.