Study On CO₂-Water-Rock Interaction And Microbial Community Characteristics On The Precipitation-Groundwater Transformation Process

Precipitation is the most important source of groundwater recharge.The hydrochemical evolution of groundwater recharge by precipitation infiltration is a difficult and hot issue in the field of hydrogeology.In this paper,the CO2-water-rock interaction and changes of microbial community characteristics in the process of groundwater recharge by precipitation infiltration through vadose zone soil were studied,through field experiments were conducted to observe the changes of hydrochemistry and microbial community characteristics in the process of precipitation infiltration.The laboratory simulation study of CO2-water-rock interaction and microbial community characteristics changes reveals the mechanism of CO2-water-rock interaction under normal temperature and pressure,and discusses the interaction between CO2,water-rock interaction and microorganisms in the process of precipitation-groundwater transformation.In this paper,the following aspects are obtained:(1)Soil CO2 is an important participant of water-rock interaction in the process of groundwater recharge by precipitation infiltration,which has an important impact on the evolution of water chemical composition in the process of precipitation-groundwater transformation,and is closely related to the changes of soil microbial community characteristics.In the field,the content of water chemical components such as fCO2(free CO2),HCO3-,TDS,Ca2+,Mg2+ in the water samples after precipitation infiltration increased significantly compared with the precipitation water samples;and the content of the main water chemical components in the water samples taken from different test plots showed T2(peanut)>T5(pine)>T4(chestnut)>T3(peach)>T1(bare land),which was consistent with the change of soil CO2 concentration in each test plot;it shows that the increase of soil CO2 concentration promotes the formation of the initial water chemical composition of groundwater.In addition,with the increase of soil CO2 concentration,the microbial abundance increased and diversity decreased;the relative abundance of Actinobacteria increased,the relative abundance of Acidobacteria decreased,and the relative abundance of Nitrospirae spirochetes decreased in the secondary dominant.(2)The participation of CO2 accelerates the dissolution of minerals in the water-rock interaction,promotes the formation of the initial water chemical components of the groundwater during the precipitation-groundwater conversion process,and affects the distribution of microbial species in the water and soil.In the laboratory experiment of CO2-water-rock interaction,with the increase of CO2 partial pressure,the pH of aqueous solution decreases,the proportion of calcite and hornblende decreases,which promotes the dissolution of calcite,feldspar and other minerals,and increases the content of fCO2,HCO3-,TDS,Ca2+,Mg2+,K+,Na+in aqueous solution.The results of SEM-EDS analysis of soil samples also confirmed this phenomenon,with the increase of CO2 partial pressure,the surface corrosion of soil samples became more obvious,and the contents of Ca,Mg and other elements in soil samples decreased.The increase of reaction time can also increase the dissolution of minerals and increase the content of related chemical components in aqueous solution.With the increase of CO2 partial pressure,the relative abundance of Firmicutes and Acidobacteria increased,while the relative abundance of Proteobacteria,Actinobacteria decreased;the relative abundance of Acidobacteria decreased and the relative abundance of Cyanobacteria increased.(3)In the single mineral dissolution,CO2 participated in the dissolution process of the single mineral,changed the original dissolution reaction mode of the mineral,and increased the dissolution rate of the mineral,and CO2 partial pressure has a good quantitative relationship with the content of mineral characteristic ions in the aqueous solution.In the experiment,CO2 dissolve in water to form H2CO3 and then dissociates to produce H+,which provides suitable hydrochemical conditions for mineral dissolution.With the increase of CO2 partial pressure,the pH value decreases,the mineral dissolution increases,and the characteristic ion content of each mineral increases obviously.The changes of the microbial community characteristics during each mineral dissolution were different.With the increase of CO2 partial pressure,the relative abundance of Proteobacteria in calcite dissolution increased,Actinobacteria in K-fledspar dissolution decreased,while that relative abundance of Dependentiae decerease in albite dissolution test.(4)During the precipitation-groundwater transformation process,the interactions between CO2,water-rock interaction and microorganisms are mutual.The participation of CO2 can promote the dissolution of minerals in the water-rock interaction and affect the changes in the characteristics of microbial communities.The difference in mineral composition in the water-rock interaction and the difference in water chemical composition produced by mineral dissolution can also affect the distribution of microbial species.At the same time,the distribution and development of microbial species can also affect the dissolution of CO2 and minerals,and the evolution of water chemical composition in the process of precipitation-groundwater transformation.In the field and laboratory experiments,the development of Proteobacteria and Firmicutes can increase the dissolution of CO2,and promote the dissolution of calcite,hornblende and feldspar in water-rock interaction,resulting in the increase of Ca2+,k+,Na+,Mg2+ in water.However,the development of Nitrospirae,Chloroflexi and Bacteroides will inhibit the dissolution of CO2 and some minerals,and affect the process of CO2-water-rock interaction.This study is the main content of the research on the water chemistry evolution in the precipitation-groundwater tranformation process,which can effectively fill the data gaps in the experimental study of the hydrochemical evolution in the precipitation-groundwater transformation process and provide a theoretical basis for the research on the formation mechanism of groundwater chemical types.In addition,through the discussion of the relationship between microorganisms and CO2-water-rock interaction,it provides data support for the research on the mechanism of microorganism-driven CO2-water-rock reaction,which has important theoretical significance and practical application value.