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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.

Quantifying and upscaling chemical turnover in the hyporheic zone (HZ) is difficult due to limited reaction rate data, unknown carbon quality, and few methods for upscaling local measurements to river networks. Here we develop a method for quantifying reaction kinetics in situ in the HZ and upscaling biogeochemical turnover to catchment scales. Radon-222 was used to quantify water residence times in the HZ of the Roter Main River (RM), Germany. Residence times were then combined with O2, NO3−, CO2, DOC, and carbon quality (EEMs, SUVA) data to estimate Monod and first-order reaction rates. Monod parameters µmax and ksat for NO3− reduction were 11 µmol l−1 h−1 and 52 µmol l−1, respectively, while the first-order rate was 0.04 h−1. Carbon quality was highly bioavailable in the HZ and is unlikely to be limiting. Reaction kinetics was incorporated into the FINIFLUX model to upscale NO3− mass loss over a 32 km reach of the RM. The aims were to (1) to estimate hyporheic efficiency using Damkohler numbers (Da), and (2) calculate NO3− mass loss in the HZ over the reach. The Da analysis suggests that the hyporheic zone is inefficient for NO3− processing, however, this is somewhat misleading as the largest NO3− mass loss occurs at the shortest residence times where Da ≪ 1. This is due to the largest water flux occurring in the uppermost part of the sediment profile. Nitrate processing in the HZ accounted for 24 kg NO3− h−1 over the reach, which was 20% of the NO3− flux from the catchment.

Recent ground-based and space borne observations suggest the presence of significant amounts of iodine monoxide in the boundary layer of Antarctica, which are expected to have an impact on the ozone budget and might contribute to the formation of new airborne particles. So far, the source of these iodine radicals has been unknown. This paper presents long-term measurements of iodine monoxide at the German Antarctic research station Neumayer, which indicate that high IO concentrations in the order of 50 ppb are present in the snow interstitial air. The measurements have been performed using multi-axis differential optical absorption spectroscopy (MAX-DOAS). Using a coupled atmosphere – snowpack radiative transfer model, the comparison of the signals observed from scattered skylight and from light reflected by the snowpack yields several ppb of iodine monoxide in the upper layers of the sunlit snowpack throughout the year. Snow pit samples from Neumayer Station contain up to 700 ng/l of total iodine, representing a sufficient reservoir for these extraordinarily high IO concentrations.