Groundwater N₂O emission factors of nitrate-contaminated aquifers as derived from denitrification progress and N₂O accumulation

Abstract. We investigated the dynamics of denitrification and nitrous oxide (N₂O) accumulation in 4 nitrate (NO⁻₃) contaminated denitrifying sand and gravel aquifers of northern Germany (Fuhrberg, Sulingen, Thülsfelde and Göttingen) to quantify their potential N₂O emission and to evaluate existing concepts of N₂O emission factors. Excess N₂ – N₂ produced by denitrification – was determined by using the argon (Ar) concentration in groundwater as a natural inert tracer, assuming that this noble gas functions as a stable component and does not change during denitrification. Furthermore, initial NO⁻₃ concentrations (NO⁻₃ that enters the groundwater) were derived from excess N₂ and actual NO⁻₃ concentrations in groundwater in order to determine potential indirect N₂O emissions as a function of the N input. Median concentrations of N₂O and excess N₂ ranged from 3 to 89 μg N L⁻¹ and from 3 to 10 mg N L⁻¹, respectively. Reaction progress (RP) of denitrification was determined as the ratio between products (N₂O-N + excess N₂) and starting material (initial NO⁻₃ concentration) of the process, characterizing the different stages of denitrification. N₂O concentrations were lowest at RP close to 0 and RP close to 1 but relatively high at a RP between 0.2 and 0.6. For the first time, we report groundwater N₂O emission factors consisting of the ratio between N₂O-N and initial NO⁻₃-N concentrations (EF1). In addition, we determined a groundwater emission factor (EF2) using a previous concept consisting of the ratio between N₂O-N and actual NO⁻₃-N concentrations. Depending on RP, EF(1) resulted in smaller values compared to EF(2), demonstrating (i) the relevance of NO⁻₃ consumption and consequently (ii) the need to take initial NO⁻₃-N concentrations into account. In general, both evaluated emission factors were highly variable within and among the aquifers. The site medians ranged between 0.00043–0.00438 for EF(1) and 0.00092–0.01801 for EF(2), respectively. For the aquifers of Fuhrberg and Sulingen, we found EF(1) median values which are close to the 2006 IPCC default value of 0.0025. In contrast, we determined significant lower EF values for the aquifers of Thülsfelde and Göttingen. Summing the results up, our study supports the substantial downward revision of the IPCC default EF5-g from 0.015 (1997) to 0.0025 (2006).