Influencing Mechanism Of Hydrogeochemical Spatial Variation On Groundwater Microbial Community Characteristics

There exist microbial communities in groundwater,Microorganisms participate in material circulation,energy conversion and information transfer during hydrogeochemical evolution.And the growth and reproduction of microorganisms are influenced and restricted by various factors of groundwater environment.Spatial changes of hydrogeochemical conditions will certainly affect the microbial community.The structure of hydrogeological units changes in space,correspondingly the hydrogeochemical conditions change accordingly.How macroscopic hydrogeochemical spatial variation influences microscopic microbial community characteristics is of great significance and challenge for the study of hydrogeochemistry.This paper relies on the Chinese geological survey project"Taihang Piedmont Loose aquifer multi-source pollution water quality survey"（DD20160309）and the Ministry of Environmental Protection nonprofit industry research special"preparatory study of methodology and key technology of groundwater environmental quality benchmark,standard formulation"（201409029）.Taking the alluvial-diluvial fan of Hutuo River（ADFHR）as the research area,based on the data and extensive historical materials from projects above mentioned,this study depicted the rule of groundwater hydrochemistry space change in the study area and analyzed the influence of hydrogeological conditions,including the formation lithology,lithology and thickness of vadose zone and groundwater velocity,and man-made pollution,on water chemical spatial variation.The characteristics of microbial community were revealed under the space changes of hydrogeochemistry.The quantitative method of construction of microbial community characteristics was established,and the influence of hydrogeochemical factors on microbial community characteristics was quantitatively analyzed.And the influencing mechanism of hydrogeochemical spatial variation on groundwater microbial community characteristics was definite.Specific research conclusions are as follows:1.The rule of groundwater hydrochemistry spatial variation（1）Along the flow direction of regional unconfined water in ADFHR,the concentrations of TDS,HCO₃^-,SO₄^2-and NO₃^-decreased as a whole,and those of Fe and Mn increased.in the middle of the alluvial-diluvial fan of the Hutuo River（fan-middle）,the concentration of TDS,SO₄^2-,NO₃^-,Fe and Mn was 0.8,0.5,0.5,3.3 and 3.3 times respectively as much as that on the top of the fan（fan-top）.（2）The main hydrochemical types were HCO₃-Ca,HCO₃·SO₄-Ca,HCO₃·SO₄-Ca·Mg in fan-top,and HCO₃-Ca·Mg,HCO₃·SO₄-Ca·Mg,HCO₃·Cl-Ca·Mg in fan-middle.2.The factors and their effects of groundwater chemical spatial variation（1）The groundwater chemistry was mainly characterized by dissolved salts,iron and manganese,and redox environment,which was primarily effected by six common factors:mineral dissolution in stratum,groundwater velocity,cation exchange and adsorption,man-made pollution,lithology and thickness of vadose zone in the study area.（2）The quantitative analysis of the influence factors of the general characteristics of the groundwater chemistry showed that water-rock interaction（mineral dissolution（standard coefficient 0.551）and cation adsorption alternations（0.603））occurring in strata was the most important source of water chemistry in regional unconfined groundwater,followed by anthropogenic pollution（0.337）,and the lithology of vadose zone（-0.108）played an important role in attenuation of surface pollution.The effect of groundwater velocity（0.064）on the spatial variation of groundwater hydrochemistry was relatively low.3.The characteristics of microbial community under the space changes of hydrogeochemistry（1）In fan-top,the concentration of TDS and Fe was 644 and 0.13 mg/L respectively in groundwater.Aquifer was mainly composed of coarser sandy gravel.The average thickness of the vadose zone was 36 m,and the groundwater velocity was about 0.24 represented by the value ofγCl^-/γCa²⁺.The average richness（ACE）of microbial OTU in groundwater was 1980.Species of bacterial genera was 1127,and the indicative species were Brevundimonas,Nocardioides and Sphingomonas.（2）In fan-middle,the concentration of TDS and Fe was 478 and 0.46 mg/L respectively in groundwater.Aquifer was mainly composed of coarse sandy gravel and medium-coarse sand.The average thickness of the vadose zone was 45 m,and the groundwater velocity was about 0.34 represented by the value ofγCl^-/γCa²⁺.The average richness（ACE）of microbial OTU in groundwater was 1551.Species of bacterial genera was 1275,and the indicative species were norank＿f＿Gallionellaceae、Pseudorhodobacter and Sulfuricurvum.4.The influencing mechanism of hydrogeochemical spatial variation on groundwater microbial community characteristics（1）The results by statistical framework based onβNTI/RC_Bray null model showed that the microbial community characteristics in the study area were mainly driven by stochastic process（93.82%）,among of which dispersal limitation was the main random factor.There existed differences of geographical distance in microbial community characteristics.Heterogeneous selection（6.18%）was the main deterministic process,that was,changes of environmental factors resulted in significant differences in microbial community characteristics.（2）The results of redundancy analysis and variance decomposition showed that the contribution rate of hydrogeochemistry to groundwater microbial community characteristics in the study area was 14%.And the independent contribution rate of（lithology of stratum（the content of HCO₃^-）and thickness of vadose zone）and the groundwater velocity were（5.9+1.0）%and（6.1+1.0）%respectively.DO and p H significantly affectedαdiversity of microorganisms.（3）It was found that the spatial variation of hydrogeochemistry mainly affected the nutrition and habitat conditions of microbial community characteristics.○1E_AFor microbial community nutrition,its most fundamental source was groundwater chemistry environment,and the composition of groundwater chemistry was mainly originated from leaching of mineral in stratum.The leaching depended mainly on stratum lithology,and also was influenced by groundwater velocity and redox environment.And man-made pollution was the second source of the composition of groundwater chemistry.It was mainly affected by thickness of vadose zone and groundwater velocity._A○2E_AFor habitat conditions of microbial community,it mainly depended on the formation lithology and groundwater velocity.The main innovations of this paper are as flows:（1）The main factors affecting microbial community characteristics were clarified in hydrogeochemical conditions.In this paper,it was found that lithology of stratum（the content of HCO₃^-）,groundwater velocity and thickness of vadose zone had significant effects on microbial community characteristics by redundancy analysis.Current domestic research believed that hydrochemical features,anthropogenic pollution and the thickness of vadose zone had important effects on the composition of microbial community.Compared to previous studies,it was found that formation lithology（the content of HCO₃^-）and groundwater velocity also significantly affected the characteristics of microbial community in this study.These findings contribute to a better understanding of the spatial distribution of microbial community characteristics in groundwater.（2）The hydrogeochemical factors influencing microbial community characteristics were quantitatively researched.By variance decomposition in the paper,significant factors were divided into two groups,one group was lithology of stratum（the content of HCO₃^-）and thickness of vadose zone,the other was groundwater velocity.And the independent contribution rate of each group was（5.9+1.0）%and（6.1+1.0）%respectively.Different from the qualitative description in the present study,the quantitative analysis is more conducive to the deeply understanding of the influence of hydrogeochemical spatial changes on microbial community characteristics.In this paper,the quantitative analysis method of factors of influencing groundwater microbial community characteristics was established.which has practical significance for the study of groundwater microbial community characteristics.（3）Influencing mechanism of hydrogeochemical spatial variation on groundwater microbial community characteristics was revealed.Traditional microbial hydrogeochemistry mainly studies the microbial-mediated hydrogeochemistry in natural and man-made pollution situations,but there are few studies on the influencing mechanism of hydrogeochemical spatial variation on microbial community characteristics.In this paper,the influencing mechanism of hydrogeochemical spatial changes on groundwater microbial community characteristics was explored.And it was found that the spatial variation of hydrogeochemistry mainly affected the nutrition and habitat conditions of microbial community characteristics._A○1E_AFor microbial community nutrition,its most fundamental source was the groundwater chemistry environment,and the compositions of groundwater chemistry were mainly originated from leaching of mineral in stratum.The leaching depended mainly on stratum lithology,and also was influenced by groundwater velocity and redox environment.And man-made pollution was the second source of compositions of groundwater chemistry.It was mainly affected by thickness of vadose zone and groundwater velocity.All the factors montioned above together affected the quality of groundwater environment and constituted the influence of microbial community nutrition._A○2E_AFor habitat conditions of microbial community,it mainly depended on the formation lithology and groundwater velocity.The formation lithology affected the intergranular pores.The flow rate of groundwater determined the srength of scouring microorganisms by groundwater runoff.Both of them together affected the habitat conditions of microbial communities.This study provides a new idea for the establishment of a microbial-based investigation and evaluation method for regional groundwater quality.