Quantifying nitrate and oxygen reduction rates in the hyporheic zone using ²²²Rn to upscale biogeochemical turnover in rivers

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, NO^- ₃, CO₂, DOC, and carbon quality (EEMs, SUVA) data to estimate Monod and first-order reaction rates. Monod parameters µ_max and k_sat for NO^- ₃ 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 NO^- ₃ mass loss over a 32 km reach of the RM. The aims were to (1) to estimate hyporheic efficiency using Damköhler numbers (Da), and (2) calculate NO^- ₃ mass loss in the HZ over the reach. The Da analysis suggests that the hyporheic zone is inefficient for NO^- ₃ processing, however, this is somewhat misleading as the largest NO^- ₃ 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 NO^- ₃ h⁻¹ over the reach, which was 20% of the NO^- ₃ flux from the catchment.