Migration,Transformation And Simulation Prediction Of Nitrogen,Iron And Manganese During Artificial Recharge Of Groundwater In Rongcheng Funnel Area Of Xiong’an New Area

Artificial groundwater recharge is an important way to solve the problem of groundwater shortage,improve water environment and alleviate environmental geological problems caused by the over-exploitation of groundwater.Water-quality safety is a key issue of artificial groundwater recharge.Due to the differences in the hydrochemical composition,environmental conditions and organic matter composition conditions between the recharge source and groundwater,a series of complex hydrogeochemical processes will occur in the nitrogen,iron and manganese components in the groundwater during the recharge of river channels and wells,resulting in changes in groundwater quality.At present,the migration and transformation process of nitrogen,iron and manganese in groundwater caused by river and well irrigation and the relationship between the composition of organic matter and the migration and transformation of nitrogen,iron and manganese is not completely clear.This research takes Rongcheng funnel area where the starting area of Xiong’an New Area is located as the research area,and applies technologies such as indoor simulation,field simulation experiments and numerical simulation.On the basis of analyzing the artificial groundwater recharge conditions,to identify the migration and transformation process of nitrogen,iron and manganese components in groundwater during river recharge and well irrigation,and the correlation between organic matter composition and nitrogen,iron and manganese components,and predict the evolution trend of nitrogen,iron and manganese concentrations in groundwater under different artificial recharge schemes.The research results have important theoretical and practical significance for enriching the research content of groundwater nitrogen,iron and manganese evolution law and organic matter composition under artificial recharge conditions,and ensuring the water quality safety of groundwater artificial recharge in Xiong’an New Area.The following main conclusions and achievements have been obtained through this research:（1）The artificial recharge conditions in the study area are relatively good.The development of the river ecological water replenishment project in the study area（water diversion from Central Line Project of South-to-North Water Diversion-Nanjuma River-Baigou River）enables the Nanjuma River and Baigou River to provide stable water supply;The riverbeds of Nanjuma River and Baigouyin River have strong infiltration capacity;The aquifer has a large water storage space and strong water conductivity.The storage capacity of the aquifer is as high as 90.6million m³,and the coefficient of transmissivity is between 120 and 1000 m²/d.Along the direction of the river ecological water replenishment route in the study area,the content of nitrogen,iron and manganese components in the river water gradually increases.In the process of infiltration of river water to recharge groundwater,it will bring the risk of nitrogen,iron and manganese pollution to the groundwater.（2）The geochemical activities of IEF-N,SOEF-N,EXC-Fe/Mn and Fe Mn Ox-Fe/Mn in riverbed sediments,vadose zone and aquifer media are relatively strong.It is easily released into groundwater during the recharge process.The organic matter components in riverbed sediments,vadose zone and aquifer media are mainly microbial-derived humus components,terrestrial humus components and protein-like components.（3）In the process of river bed recharge,NH₄⁺in groundwater mainly occurs nitrification and adsorption,and NO₃^-mainly occurs denitrification and leaching.Fe²⁺in groundwater mainly occurs the reductive dissolution of Fe Mn Ox-Fe,and Mn²⁺occurs adsorption,oxidative precipitation and and the reductive dissolution of Fe Mn Ox-Mn.The protein-like components in organic matter are mainly involved in nitrogen denitrification,and the humus-like components are mainly involved in the reductive dissolution of Fe Mn Ox-Fe/Mn.（4）Physical mixing mainly occurs in the process of recharging groundwater by well irrigation.In addition,0.124 mmol/L and 0.013 mmol/L NH₄⁺undergo nitrification and adsorption,0.174 mmol/L and 0.291 mmol/L NO₃^-undergo denitrification and leaching,and 0.303 mmol/L hematite and 0.022 mmol/L pyrolusite undergo the reductive dissolution,etc.The protein-like component is the organic matter that mainly participates in the aerobic respiration and denitrification in the process of recharge,and the humic-like component has the ability to significantly enhance the reduction and dissolution of iron and manganese minerals.（5）Based on the future water diversion/diversion planning,different precipitation guarantee rates,pressure mining policy and field well irrigation experiments in the study area,different riverbed recharge schemes and well irrigation recharge schemes were designed.Under the above artificial recharge scheme,the groundwater level at the center of the funnel can be raised by 10～20 m by 2035.Except for the local concentration of nitrogen,iron,and manganese in groundwater increased due to the mixing of rivers,the leaching of nitrates in the vadose zone medium,and the reduction and dissolution of iron and manganese minerals,the regional groundwater quality generally improved.