Hydrogeological Controls on Regional-Scale Indirect Nitrous Oxide Emission Factors for Rivers

Indirect nitrous oxide (N₂O) emissions from rivers are currently derived using poorly constrained default IPCC emission factors (EF_5r) which yield unreliable flux estimates. Here, we demonstrate how hydrogeological conditions can be used to develop more refined regional-scale EF_5r estimates required for compiling accurate national greenhouse gas inventories. Focusing on three UK river catchments with contrasting bedrock and superficial geologies, N₂O and nitrate (NO₃^-) concentrations were analyzed in 651 river water samples collected from 2011 to 2013. Unconfined Cretaceous Chalk bedrock regions yielded the highest median N₂O-N concentration (3.0 μg L^-1), EF_5r (0.00036), and N₂O-N flux (10.8 kg ha^-1 a^-1). Conversely, regions of bedrock confined by glacial deposits yielded significantly lower median N₂O-N concentration (0.8 μg L^-1), EF_5r (0.00016), and N₂O-N flux (2.6 kg ha^-1 a^-1), regardless of bedrock type. Bedrock permeability is an important control in regions where groundwater is unconfined, with a high N₂O yield from high permeability chalk contrasting with significantly lower median N₂O-N concentration (0.7 μg L^-1), EF_5r (0.00020), and N₂O-N flux (2.0 kg ha^-1 a^-1) on lower permeability unconfined Jurassic mudstone. The evidence presented here demonstrates EF_5r can be differentiated by hydrogeological conditions and thus provide a valuable proxy for generating improved regional-scale N₂O emission estimates.