Seasonal Variations of Hydraulic Exchange Between Surface Water and Groundwater in an Alluvial Plain Setting Using 222Rn

Understanding dynamic groundwater–surface water interactions in alluvial plains is critical for sustainable water resource management, yet seasonal variability and spatial heterogeneity of these exchanges remain imprecisely quantified. Here, we present an improved 222Rn mass balance model for evaluating the seasonal hydraulic exchange between groundwater and the Xintongyang Canal in the Taizhou alluvial plain over the course of a hydrologic year. To reduce the model uncertainty, the “background” 222Rn for non-groundwater sources was incorporated into the model to replace the influence of hyporheic exchange. The results indicate that the hydraulic exchange process of surface water and groundwater has significant spatiotemporal differences. Based on the calculations from the 222Rn mass balance model, the canal leakage flux follows the order of summer > autumn > winter > spring over the course of a hydrologic year. In contrast, the groundwater discharge flux follows the order of summer > spring > autumn > winter. During a hydrological year, summer demonstrated the most intense water exchange dynamics, with peak fluxes reaching 0.0455 m3/(s·m) for surface water leakage and 0.0013 m3/(s·m) for groundwater discharge, revealing pronounced spatial heterogeneity in dominant exchange processes. 222Rn activity in canal and groundwater varies significantly across different regions, with canal leakage being the dominant mode of hydraulic exchange within the study area. The change of the hydraulic exchange process was mainly affected by factors such as rainfall. In the process of promoting surface water leakage, precipitation will also strengthen the supplement of groundwater and contribute to the groundwater discharge in most of the canal sections. This study offers insight into the seasonal variations of groundwater and surface water interaction within an alluvial plain.