Explicit Modeling of Radon-222 in HydroGeoSphere During Steady State and Dynamic Transient Storage

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