Evaluation of flood-driven bank infiltration effects on hyporheic zone groundwater age distributions

The hyporheic zone is the shallow subsurface beneath and adjacent to streams characterized by bidirectional groundwater-surface water exchange. This exchange can intensify during high-flow events, driving stream water into streambanks where it may persist as bank storage for weeks to months. However, groundwater ages in shallow near-stream wells (often used as proxies for hyporheic exchange) are frequently interpreted with exponential mixing assumptions that may not be valid under prolonged bank storage. As a result, bank-infiltrated water can be misidentified as young groundwater discharge, leading to biased estimates of stream-aquifer exchange and erroneous source attribution. In this study, we quantify how flood-driven bank infiltration perturbs near-stream groundwater age distributions in Pyeongjeong Stream (Chungcheongnam-do, South Korea) by integrating hydraulic head, electric conductivity (EC), and tritium data. Event-scale hydraulic gradients show that flood onset rapidly enhances stream-to-aquifer flow as stream stage rises faster than groundwater head. Consistent with this mechanism, EC-stage hysteresis indicates contrasting recovery behavior: stream EC decreases during precipitation events and quickly returns to pre-event levels, whereas groundwater EC remains suppressed much longer, implying persistent bank-stored water and delayed flushing. Tritium concentrations further support sustained contributions of modern recharge and/or infiltrated stream water. We then evaluate transit time distributions (TTDs) using conceptual mixing models that explicitly represent episodic bank infiltration and extended storage. The resulting TTDs deviate from a single exponential form, exhibiting a composite structure that combines a short-transit event component with a broader, older background associated with bank storage. These results highlight the need to account for flood-driven bank infiltration to interpret near-stream groundwater ages and to constrain the water sources and timescales governing hyporheic zone exchange and associated solute transport.Keywords: Hyporheic exchange, Bank infiltration, Flood event, Transit time distributionAcknowledgement: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (RS-2025-00552981).