Quantifying Groundwater–Surface Water Exchange on a River Island: A Comparative Analysis of Direct and Indirect Methods

Abstract

Abstract Groundwater–surface water exchange is a key hydrological process that governs river water quality and aquatic ecosystems. However, the quantification of such exchange remains uncertain, especially within river island aquifers where exchange processes are highly heterogeneous. In this study, direct seepage meter measurements and indirect Darcy‐based estimates of exchange fluxes were compared under consistent spatiotemporal conditions within the aquifer of Jungsa Island, a river island in the lower West Nakdong River, South Korea. At the downstream low‐permeability site (SP‐3), Darcy‐based exchange fluxes ranged from −2.14 × 10 −5 to 1.15 × 10 −5  m/hr. Owing to these small magnitudes, net intrusion and discharge were not distinguishable. However, seepage meter measurements consistently indicated intrusion of surface water into the island aquifer, with fluxes ranging from −8.46 × 10 −4 to −1.94 × 10 −4  m/hr. At the upstream high‐permeability site (SP‐6), seepage meter measurements captured both transient groundwater discharge during short measurement periods and sustained surface water intrusion during longer measurements. Darcy‐based estimates at this site consistently indicated intrusion, indicating that both methods showed the same flow direction during longer measurements. Consequently, Darcy‐based approaches may underestimate exchange fluxes under low‐permeability conditions. These findings suggest that short‐term monitoring is favorable for capturing immediate, transient flow dynamics, whereas extended continuous measurements help buffer short‐term fluctuations to provide a more representative estimate of daily integrated exchange. Ultimately, by establishing a methodological framework that accounts for localized riverbed heterogeneity, this study provides practical guidelines for optimizing field investigation strategies in complex riparian environments.