Biogeochemical Process Of Arsenic In Groundwater And Its Simulation Affected By Groundwater Exploitation In Riverside

There is often frequent and intense interaction between the river and the groundwater,affected by groundwater exploitation in riverside,resulting in changes in the hydrodynamic conditions,acid-base conditions,and redox conditions during river water infiltration.Due to the significant differences in river water and groundwater environmental characteristics,complex biogeochemical reactions take place while the river water infiltrates the groundwater,triggering the release of arsenic from the aqueous medium into the groundwater.At present,the biogeochemical processes during river water infiltration affected by groundwater exploitation in riverside and their contribution to arsenic in groundwater are not fully understood.Based on this,this paper takes the Huangjia water source in Shenyang,the typical water source of groundwater in northern China,as the study area,identifying the geological and hydrogeological conditions,the application of insitu monitoring technology,molecular biology technology,indoor experiment and insitu experimental techniques,and hydrogeochemical simulation techniques have identified the temporal and spatial distribution characteristics of arsenic in groundwater and the biogeochemical reaction mechanism,establish the hydrogeochemical reaction model for arsenic in groundwater,identify and differentiate the biogeochemical process of arsenic in groundwater and its contribution to arsenic,which providing a scientific basis for monitoring and protection of groundwater,safe operation of water sources and ensuring the water safety of local residents.Through this study,we achieve the following major understandings:1.Along the groundwater flow path,the groundwater environment in the study area is partitioned from the river to the cone center of groundwater level.There are differences in the spatial scale of groundwater environment in different groundwater flow paths.There is a good correlation between arsenic content in groundwater and environmental zoning.In the weakly oxidizing environment,oxidizing river water causes oxidative dissolution of arsenic-containing minerals,which results in the release of arsenic in the medium into the groundwater.In the weakly reduced environmental,reductive dissolution of iron and manganese minerals occurs,resulting in arsenic adsorbed on the surface and lattice of ferromanganese minerals release into groundwater.The reduction of sulfate in the reduced environment is relatively strong,the resulting sulphide and low-price iron and manganese ions form precipitation with a certain amount of arsenic is absorbed,while arsenic is precipitated as secondary sulfide,which lead to lower arsenic content in groundwater.2.In the process of river water infiltration,the fluctuation of groundwater level can cause obvious changes in the water chemical composition,redox environment and microbial activity intensity of the groundwater system.The dynamic change of arsenic content in groundwater in the study area is closely related to the water level.The arsenic content in groundwater increases with the rise of groundwater level,and the response of the shallow to water level change is the most obvious.With the increase of depth,the response of arsenic content in groundwater to the change of water level is decreased,and the seasonal variation is decreased.3.In addition to the high content of residual arsenic,the arsenic in riverbed sediments and aqueous media is mainly present in the iron-manganese oxides bound state.In the different forms of arsenic in the study area,exchangeable arsenic,exchangeable iron coexisting with arsenic,and iron-manganese oxide bound iron have relatively strong geochemical activity.In the process of river water infiltration,the arsenic enters the groundwater from the solid phase through the adsorption and desorption of exchangeable arsenic and exchangeable iron in the medium and the reductive dissolution of iron and manganese oxide bound iron.4.Groundwater microbial community structure,abundance and diversity are closely related to arsenic content in groundwater along the groundwater flow paths.There is a clear relationship between microbial diversity and arsenic content,and the microbial diversity of groundwater with relatively low arsenic concentration is greater.The abundance and diversity of microbial species in medium show a strong spatial heterogeneity,and the microorganism abundance and diversity of shallow are relatively high.5.The indoor experimental studies show that the adsorption and desorption of arsenic in the aqueous medium are nonlinear,and pH conditions,temperature,solid-liquid ratio,ionic strength,phosphate,carbonate,organic matter,etc.have great influence on the adsorption and desorption of arsenic in groundwater.Under the same conditions,As?V?is more easily adsorbed by aqueous media than As?III?.In the reduced environment,reductive dissolution of arsenic-containing iron minerals mediated by microorganisms occurr,and the arsenic on the mineral surface or in the crystal lattice is continuously released.The content of arsenic and iron in the solution continuously increases and there is a significant positive correlation between the two.In the oxidizing environment,oxidative dissolution of arsenic-containing iron minerals mediated by microorganisms occurr,causing the release of arsenic.As the reaction proceeds,Fe²⁺that is produced by the dissolution of arsenic-containing iron minerals is oxidized under the action of oxygen,and then hydrolyzed to form Fe?III?minerals.Fe²⁺ions are adsorbed on the surface of the formed minerals,which results in a decrease in Fe²⁺content.6.The insitu experimental studies show that the hydrodynamic conditions have a significant effect on the dynamic characteristics of pore water chemical indicators during river water infiltration.When the hydrodynamic conditions are strong,the microbial metabolism speeds up,the reaction time is short,and the distance of the water flow is relatively long.When the hydrodynamic conditions are weak,the microbial growth and metabolism process is complete,the biological activity is strong,and the reaction rate is relatively high,then the minerals have more reductive dissolution and more arsenic in the medium is released into groundwater.The higher DOC content can provide more energy for the growth and metabolism of microorganisms,making the reaction rate faster,which is beneficial to the reductive dissolution of iron and manganese mineral,leading to an increase of arsenic in groundwater.7.The numerical simulation studies show that during river water infiltration,the contribution rate of various biogeochemical processes of arsenic in groundwater from high to low is as follows:iron minerals reductive dissolution,adsorption and desorption,and mineral oxidative dissolution,the contribution rates are 55%⁷0%,20%²7%and 10%¹8%,respectively.Among them,the reductive dissolution of iron minerals is the main biogeochemical process of arsenic in groundwater during river water infiltration affected by groundwater exploitation in riverside.The change of hydrodynamic conditions has a significant effect on the biogeochemical process of arsenic in groundwater,and has the most significant effect on the reductive dissolution of iron minerals,but has little effect on adsorption and desorption,and mineral oxidative dissolution processes.