Submarine groundwater discharge (SGD, m-to-km scale) and cm-scale porewater exchange (PEX) are important benthic water flow processes that regulate nutrient and inorganic carbon dynamics in coastal seas. Partitioning SGD and PEX is useful for accurately quantifying their water and solute contributions. This is particularly important in deltaic systems underlain by well-developed, yet understudied, paleochannels. Here we employ an optimized multi-radium mass balance model to distinguish SGD from PEX and quantify the dissolved inorganic carbon (DIC), total alkalinity (TA), and CO₂ fluxes in the Yellow River deltaic sea. Benthic water exchange hotspots were identified in the southern estuary, where the combined SGD and PEX fluxes were 3-6 times greater than those in the northern region. The extensive paleo-river channel network in the south could likely facilitate SGD. More spatial heterogeneity was observed during the wet season, with total SGD and PEX fluxes approximately twice those in the dry season. PEX dominated the total benthic water flux, exceeding SGD by a factor of 2-4 across regions and seasons. Both SGD and PEX contributed substantially to inorganic carbon export, especially during the wet season. Lateral exports of SGD- and PEX-driven HCO₃^- and CO₃^2- exceeded vertical sedimentary carbon burial by 2-4 times on average level. Considering the potential offset from CO₂ and CaCO₃ precipitation, SGD/PEX still represents an important carbon sink contributor in the Yellow River delta. We propose that benthic exchange processes should be considered in future assessments of coastal carbon sinks.