Research On Isotope Tracer In The Biogeochemical Processes During River Bank Infiltration

Due to the increasing pollution of surface water,groundwater resources have become the main source of water for daily water supply,industrial and agricultural production in northern China.In the development and utilization of groundwater,groundwater exploitation in riverside is an important way to promote the transformation of the river water into the groundwater,and it also affects the biogeochemical action of the river water infiltration process.The study of biogeochemical effects during the infiltration of river water is of great significance for maintaining groundwater quality and ecological security.Because of the multiplicity of solutions and uncertainties in water chemistry methods,based on the application of multivariate statistical analysis,this paper selects carbon and sulfur stable isotopes that actively participate in the redox reaction of groundwater,and is accurate and effective of reveal the biogeochemical effects of river water infiltration from the microscopic point of view by isotope tracing methods.Through this study,the following points were made:?1?In the shallow and deep groundwater flow paths,groundwater environmental indicators and main components are different on the spatial scale,and can be divided into different groundwater environment zones:in the shallow groundwater flow path,within 4.0 m from the river bank,4.0 m to 17.0 m,and17.0 m to 350.0 m,they are divided into weak oxidation zone,weak reduction zone,and reduction zone.In the deep groundwater flow path,they are within 17.0 m from the river bank,17.0 m to 200.0 m,and 200.0 m to 350.0 m away from the bank.?2?Applying factor analysis method,successfully extracted the main factors affecting the chemical properties of groundwater in the study area from many monitoring indicators,combined with the spatial variation of the corresponding components of each main factor,and explained their actual significance respectively.Prove that the main role of groundwater hydrochemical composition is the oxidation and decomposition of organic carbon,denitrification,reduction of iron and manganese minerals and SO₄^2-reduction.On this basis,by analyzing the different scores of the main factor at each observation point,it can be seen that there are differences in the spatial distribution of biogeochemical effects in the infiltration process of river water:in the weak oxidation zone,decomposition of organic carbon is the main biogeochemical process;in the weak reduction zone,reduction of iron and manganese minerals is the main biogeochemical process;in the reduction zone,SO₄^2-reduction is the main biogeochemical process.?3?The carbon isotope in the process of river infiltration showed that there are many sources of organic carbon which drives biogeochemical action.In the weak oxidation zone,the organic carbon involved in biogeochemical action mainly come from river water during the infiltration process;In the weak reduction zone and reduction zone,there are amounts of organic carbon come from the sediment organic carbon.?4?During the infiltration process of river water,the gradual decrease of SO₄^2-mass concentration in groundwater was accompanied by the gradual increase of?³⁴S,which further indicated that SO₄^2-reduction occurred in the reduction zone.Using Rayleigh fractionation model,the enrichment coefficient of the ³⁴S in the study region is-13.86‰,which is consistent with the results reported in related studies.It is proved that SO₄^2-is used as the electron acceptor for sulfate reduction in the study area.