Estimating internal dissolved methane loading in rivers using a mass balance approach

The dynamics of dissolved methane concentration in rivers are influenced by losses through atmospheric emission and microbial oxidation, and by gains from internal (<i>e.g</i>., riverbed sediment) and external (<i>e.g</i>., groundwater) sources. Reducing riverine methane emissions, a key strategy for mitigating global warming, requires decreasing both internal and external loadings. To develop effective mitigation measures, it is essential to quantify these loadings separately. In this study, we estimated the internal methane loading in a river using a mass balance approach. We focused on river reaches without tributary inflow or significant discharge changes, assuming negligible external methane loading. Sampling was conducted at upstream and downstream sites of two short reaches (2.2 and 4.4 km) in the Kokai River, Japan, during 2022-23. Dissolved methane concentrations ranged from 237 ± 19 to 1,271 ± 6 nmol L^-1, with changes from upstream to downstream ([CH₄]_downstream-[CH₄]_upstream) varying from -113 to 363 nmol L^-1 . Methane oxidation rates and diffusive emission fluxes to the atmosphere were -1.2 ± 0.8~66 ± 19 µmol m^-2 h^-1 and 32 ± 10~199 ± 149 µmol m^-2 h^-1, respectively. The net flux of dissolved methane from the riverbed to the water varied from -33 to 160 µmol m^-2 h^-1. Compared to conventional methods, including benthic chambers and peeper sampling with model simulation, this approach is simple and facilitates methane flux measurements across multiple sites and diverse environmental gradients. By integrating the estimates from river reaches, the proposed method is applicable to large-scale assessments of internal methane loading in river systems.