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

Indirect nitrous oxide (NO) emissions from rivers are currently derived using poorly constrained default IPCC emission factors (EF) which yield unreliable flux estimates. Here, we demonstrate how hydrogeological conditions can be used to develop more refined regional-scale EF estimates required for compiling accurate national greenhouse gas inventories. Focusing on three UK river catchments with contrasting bedrock and superficial geologies, NO and nitrate (NO) concentrations were analyzed in 651 river water samples collected from 2011 to 2013. Unconfined Cretaceous Chalk bedrock regions yielded the highest median NO-N concentration (3.0 μg L), EF (0.00036), and NO-N flux (10.8 kg ha a). Conversely, regions of bedrock confined by glacial deposits yielded significantly lower median NO-N concentration (0.8 μg L), EF (0.00016), and NO-N flux (2.6 kg ha a), regardless of bedrock type. Bedrock permeability is an important control in regions where groundwater is unconfined, with a high NO yield from high permeability chalk contrasting with significantly lower median NO-N concentration (0.7 μg L), EF (0.00020), and NO-N flux (2.0 kg ha a) on lower permeability unconfined Jurassic mudstone. The evidence presented here demonstrates EF can be differentiated by hydrogeological conditions and thus provide a valuable proxy for generating improved regional-scale NO emission estimates.