Groundwater–surface water interactions in Osongji, Korea: hydrogeochemical evidence from hyporheic zone profiling

Understanding groundwater and surface water interactions is crucial for gaining insights into several physical and biochemical processes in the water cycle. This study evaluated hydrogeochemical changes by depth in the hyporheic zone of Osongji (Osong Pond), Jeonju-si, Korea, to determine groundwater contributions to surface water. Conducted from April to August 2022, diffusion samplers were installed at two pond locations (Areas 1 and 2) for groundwater quality analysis by depth. Soil samples were collected (~ 70 cm below the pond bottom), and a rainwater collector was placed in Area 1 to assess the effects of rainwater and soil on geochemistry. Results showed that redox-sensitive indicators (i.e., O₂, Mn, Fe, and SO₄²⁻) below the pond bottom aligned with theoretical oxidation-reduction profiles. Electrical conductivity (EC) and alkalinity exhibited similar trends in both areas. In particular, the high alkalinity and elevated Ca and Mg concentrations in Area 2 were consistent between water and soil samples, likely due to carbonate dissolution from limestone aquifer materials. Groundwater cation analysis showed that Fe, Mn, Ca, and Mg distributions matched soil analysis results. The findings of this study suggest that Osongji’s surface water is influenced by the geological characteristics of aquifer materials and prolonged groundwater flows than direct precipitation. This study provides insights into water quality changes linked to groundwater-surface water interactions and serves as a basis for similar hyporheic zone studies.