title: Role of oxbow lakes in controlling redox geochemistry of shallow groundwater under a heterogeneous fluvial sedimentary environment in an agricultural field: Coexistence of iron and sulfate reduction.
abstract: This study aimed to extend the knowledge of the vertical distribution of redox conditions of shallow groundwater in heterogeneous fluvial sediments near oxbow lakes. For this study, we revisited the study area of Kim et al. (2009) to examine the redox zoning in details. Three multi-level samplers were installed along a flow path near two oxbow lakes to obtain vertical profiles of the subsurface geology and hydrochemical and isotopic data (δ(18)O and δD of water, δ(15)N and δ(18)O of nitrate, and δ(34)S of sulfate) of groundwater. Geologic logging showed that characteristics of the heterogeneous subsurface geology are closely related to the pattern of vertical redox zoning. Hydrochemical data in conjunction with nitrogen and sulfur isotope data show that the redox status of groundwater near oxbow lakes is controlled by denitrification, iron reduction, and sulfate reduction. The oxidizing condition of groundwater occurs in the sand-dominant alluvium located in the up-gradient of oxbow lakes, whereas the reducing condition accompanying denitrification, iron reduction, and local sulfate reduction is developed in silt-rich alluvium in and the downgradient of oxbow lakes. The occurrence of sulfate reduction was newly found in this study. However, the vertical profiles of redox-sensitive parameters show that iron reduction and sulfate reduction occur concurrently near oxbow lakes, although the measured redox potentials suggest that thermodynamic conditions are controlled by the stability of Fe(2+)/Fe-oxides. Therefore, this study shows that the redox condition of groundwater in the iron-rich zone should be carefully interpreted. For this purpose, depth-specific sampling and careful examination of sulfur isotope data will be very useful for identifying the redox processes occurring in the zone with overlapping iron reduction and sulfate reduction in heterogeneous fluvial sediments.
authors.affiliation: Geologic Environment Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, South Korea; Department of Earth and Environmental Sciences and Green School, Korea University, Seoul 136-701, South Korea. Electronic address: choiby@kigam.re.kr.
authors.forename: Byoung-Young
authors.initials: BY
authors.lastname: Choi
authors.affiliation: Department of Earth and Environmental Sciences and Green School, Korea University, Seoul 136-701, South Korea.
authors.forename: Seong-Taek
authors.initials: ST
authors.lastname: Yun
authors.affiliation: Department of Earth and Environmental Sciences and Green School, Korea University, Seoul 136-701, South Korea.
authors.forename: Kyoung-Ho
authors.initials: KH
authors.lastname: Kim
mesh_terms: D058440:Denitrification; D004784:Environmental Monitoring / Q000379:methods*; D019015:Geologic Sediments / Q000737:chemistry*; D005844:Geology; D060587:Groundwater / Q000737:chemistry*; D007501:Iron / Q000032:analysis* / Q000737:chemistry; D060106:Lakes / Q000737:chemistry*; D009566:Nitrates / Q000032:analysis; D010084:Oxidation-Reduction; D013431:Sulfates / Q000032:analysis* / Q000737:chemistry; D013460:Sulfur Isotopes / Q000032:analysis; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry
chemical_list: D009566:Nitrates; D013431:Sulfates; D013460:Sulfur Isotopes; D014874:Water Pollutants, Chemical; D007501:Iron
keywords: Heterogeneous fluvial aquifer; Isotopes; Multi-level sampling; Redox geochemistry