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

Abstract

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.

Modeling Riverine N2O Sources, Fates, and Emission Factors in a Typical River Network of Eastern China

“…The considerable N 2 O contribution from groundwater was closely related to the high BFI (16−92%) and high N 2 O concentrations in groundwater. 9 These inferences were supported by the significant correlation (r = 0.56, p < 0.01) between riverine and groundwater N 2 O concentrations (Supporting Information, Part III, Table S3, 4). The higher BFI and lower groundwater N 2 O attenuation rate (K 1 ) led to higher groundwater contribution percentages in Trib res and Trib agr than in mainstream (Table 1).…”

“…The higher BFI and lower groundwater N 2 O attenuation rate (K 1 ) led to higher groundwater contribution percentages in Trib res and Trib agr than in mainstream (Table 1). 9 The highest groundwater contribution percentage occurred in Trib for having lower surface runoff and decreased in-stream biogeochemical processes because of lower DIN concentrations (Supporting Information, Part III, Table S2). This resulted in low N 2 O contributions from surface runoff (10%) and in-stream production (4%).…”

“…The Intergovernmental Panel on Climate Change (IPCC) proposed an indirect N 2 O emission factor for rivers (EF 5r ), which is defined either as the ratio of the annual N 2 O emission rate and the fraction (Frac LEACH ) of total fertilizer N inputs lost to waterbodies or simply as the ratio of dissolved N 2 O−N to dissolved inorganic nitrogen (DIN = NH 4 + NO 3 + NO 2 ). 6,9,10 The recommended default EF 5r value was revised from 0.0075 (kg N 2 O−N per kg NO 3 − −N) to 0.0025 in 2006 and 0.0026 in 2019. 11,12 Nevertheless, a uniform IPCC EF 5r value exhibits appreciable limitations in regional estimations, leading to proposals for site-specific EF 5r values (0.003−41%) based on ambient conditions.…”

“…4 Therefore, ignoring off-channel N 2 O inputs in developing emission factors and models introduces substantial uncertainty for riverine N 2 O emission estimates. 9,23 Quantitative information on riverine N 2 O sources and sinks is necessary for assessing mitigation and management options. 31 A recent study developed a modeling framework that included the entire N cascade from cropland to rivers for estimating N 2 O emissions from soils, riparian zones, and rivers.…”

Dynamics of greenhouse gases in groundwater: hydrogeological and hydrogeochemical controls

“…It is important to constrain and better understand the scope of uncertainties related to the upscaling procedures. That is why the studies devoted to the distribution and dynamics of GHGs in groundwater should consider the variability in hydrogeology, hydrogeochemistry and land use across the explored area (Choi et al, 2010;Cooper et al, 2017).…”