Transient storage zones (TSZs) are located at the interface of rivers and their abutting aquifers and play a vital role in hydrological and biogeochemical functioning of rivers. The natural radioactive tracer 222Rn is a particularly well-suited tracer for studying TSZ water exchange and water age. Although 222Rn measurement techniques have developed rapidly recently, there has been little progress in modelling 222Rn activities in TSZs. Here we use field measurements with the numerical model HydroGeoSphere (HGS) to model 222Rn emanation, decay and transport during steady state (riffle-pool sequence) and transient (bank storage) conditions. Comparing the HGS mean water ages with the conventional 222Rn apparent ages during steady state showed a systemic underestimation of water age with increasing dispersion and especially where large concentration gradients exist within the subsurface. Large underestimation of apparent water age was observed at the advective front during bank storage where regional groundwater mixes with newly infiltrated surface water. The explicit modelling of radiogenic tracers such as 222Rn offers a physical interpretation of this data as well as a useful way to test simplified apparent age models.