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 (EF5r) which yield unreliable flux estimates. Here, we demonstrate
how hydrogeological conditions can be used to develop more refined
regional-scale EF5r 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), EF5r (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), EF5r (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), EF5r (0.00020), and N2O–N flux (2.0
kg ha–1 a–1) on lower permeability
unconfined Jurassic mudstone. The evidence presented here demonstrates
EF5r can be differentiated by hydrogeological conditions
and thus provide a valuable proxy for generating improved regional-scale
N2O emission estimates.