Hydrogeochemical Simulation Of Iron And Manganese Migration Under River Infiltration Condition

Recharge of reclaimed water to groundwater can effectively alleviate a series of environmental and geological problems caused by over-exploitation of groundwater.However,the recharge might change the environmental conditions of the aquifer and result in release of certain ions from the solid media,which can threaten the quality of groundwater.In this study,at the reclaimed water section of the Chaobai River,a profile which is perpendicular to the river course was selected.This profile consists of three groundwater monitoring wells at different distances(i.e.,50 m,100 m and 200 m,respectively)from the river.Based on the surface water and groundwater monitoring data from 2007 to 2015,the main components that affect the quality of groundwater and the possible hydrogeochemical processes have been identified.The TOUGHREACT module of Petrasim software was used to establish a two-dimensional reactive solute migration model in the study profile to evaluate the influences of various hydrogeochemical processes on the migration and transformation of Fe and Mn during the reclaimed water recharge process.The main conclusions are as follows:(1)Under the infiltration condition of the recharge water from the river,the change trends of groundwater level of monitoring wells at different distances from the river channel are consistent,but the changes of groundwater hydrochemical types and major ions are significantly different.During the monitoring period,the overall hydrochemical type of the recharge water did not change significantly,and mainly was SO₄·Cl-Na type.The hydrochemical types of the groundwater in the study area changed from HCO₃-Ca type to HCO₃-Na type.The farther away from the river,the longer it took to change to HCO₃-Na type.(2)Monitoring data show that ammonia nitrogen,manganese,barium,arsenic and iron in groundwater have a great impact on groundwater quality.The results of sodium adsorption ratio method,correlation analysis and mineral saturation index analysis indicate that the mixing dominated the conventional components in groundwater.Meanwhile,the ion exchange between K⁺and Ca²⁺and the desulfurization occurred during the recharge process.The dissolution-precipitation of siderite and rhodochrosite affected the migration of Fe²⁺and Mn²⁺in aquifer.(3)In the model identification and verification,the fitting results of groundwater level and Cl^-concentration are both good.Based on this,the concentration changes of conventional anion and cation components,Fe²⁺and Mn²⁺in groundwater with time were simulated.The modeling results show that Fe²⁺migration is mainly inhibited by the precipitation of siderite and goethite,and is also influenced the its adsorption to sediments;the process promoting Fe²⁺migration is cation exchange;the reduction of hematite has no obvious effect on Fe²⁺migration.The process inhibiting Mn²⁺migration is the precipitation of rhodochrosite;the processes promoting Mn²⁺migration are mainly cation exchange and pyrolusite reduction,and desorption of Mn from sediments also enhances its migration.