Biogeochemical Process Of Fe And Mn During River Bank Infiltration Affected By Groundwater Exploiting

Groundwater exploiting along the river bank not only induces the river water infiltrating into the aquifer but also affects the biogeochemical reactions in the infiltration zone.Due to the differences in physical,there are obvious physical,chemical and biological gradients in the river bank infiltration zone,resulting in a strong and complex biogeochemical process.This biogeochemical reaction during the interaction of surface water and groundwater,is of critical benefit to sustain the safety of water quality and ecology.To date,there is little information about how the variation of hydraulic conditions resulting from the groundwater exploiting does control and affect the biogeochemical reations in the infiltration zone.This thesis chooses Huangjia groundwater source area along the Liao river in Liaohe alluvial fan as case study area,and selects iron and manganese as the target elements in order to interpret the biogeochemical behavior of Fe and Mn in the infiltration zone affected by groundwater pumping with multi-methods,such as the in-situ monitoring,environmental isotope tracer,molecular biology method and geochemical modeling.Based on the investigation and analysis of the hydrogeological conditions in the study area,the objectives of this thesis is to determine the features of hydraulic exchange of river-groundwater in the infiltration zone induced by groundwater exploiting,to explore the temporal and spatial zoning of redox zoning,and to interpret the biogeochemical processes of Fe and Mn in the infiltration zone and the main influencing factors;to evaluate contribution of major geochemical reactions to migration and transformation of iron and manganese in groundwater.This research will scientically benefit to enrich the biogeochemical reactions in the exchange of river water and groundwater and to pretect the safety of groundwater supply.Through this study,the following conclusions can be highlighted:(1)Based on the analysis of hydrogeological conditions of the river infiltration zone,there were two groundwater flow paths,the shallow water flow path(the depth of circulation<10m)and the deep water flow path(the depth of circulation 10-50m)identified according to the multiple geochemical tracing method.Compared with the shallow water flow path,the permeability of the deep water flow path was better,closely connected with the river water,and intensity recharged from the river water was relatively strong.Compared with winter,the river stage was higher in summer when the hydraulic gradient,the river infiltration rate and flux were lager.(2)The geochemical activity of EXC-Mn,EXC-Fe,FeMnOx-Fe in aqueous media is relatively strong.The content of EFC-Fe and EFC-Mn in aqueous medium along the shallow water flow were higher than deep.Compared with the deep aqueous media,the microbial species in the shallow aquifers were relatively more abundant and had a stronger diversity,which showed a similarity with the microbial community structure of the sediments.(3)The long-term dynamic monitoring data of river water and groundwater hydrochemistry showed that,during the process of river water infiltrating into groundwater,there is an obvious sequential REDOX zoning regularity.Horizontally,the extent of redox active reaction zone along the shallow water flow path was in the range of river banks to funnel center extending to 17.0 m,and the zonal range was narrow;along the deep water flow path,the reaction zone ranged from the river bank to funnel center extending to 200.0 m.Vertically,the scope of 10.0 m below the surface of shallow water flow path acted as the sequential redox active reaction zone.Furthermore,the microbial community structure had a certain co-evaluation characteristics on redox zoning in groundwater.(4)The results of indoor physical simulation experiments and numerical simulation showed:during the process of river infiltration,the processes of dissolution,cation exchange,colloid adsorption,reduction of Mn(IV)and Fe(?)minerals and the complexation of ligands which were controlled by the solubility of iron and manganese minerals have occurred.Among them,the microbial mediated sequential oxidation reduction was the main biogeochemical process of manganese and iron.during the process of river infiltration,The biogeochemical reaction degree of Fe and Mn in shllow flow path was higher than deep,and the reaction degree of Fe was higher than Mn,It was depending on the hydrodynamic and microbial community structure conditions and the potential geochemical activity of iron and manganese.(5)There was a symbiotic relationship with Arsenic and Fe/Mn in nature.In the study area,along Mn(IV)/Fe(III)reduction,arsenic adsorbed on the surface and in the lattice of the medium was released into groundwater,resulting in content of arsenic more than 30.0?g/L;Along sulfate reduction,HS-and Fe2+ will form into sulfides(such as FeS)which can adsorb or coprecipitate arsenic,resulting in decrease content of arsenic in groundwater.It was suggested that the wells in the study area should be laid in the sulfate reduction zone,to reduce the risk of secondary pollution of groundwater.