Spatial and temporal variability of dissolved nitrous oxide in near-surface groundwater and bubble-mediated mass transfer to the unsaturated zone

摘要

Understanding spatial and temporal variability of dissolved nitrous oxide (N₂O) is essential to process understanding of N₂O emissions from near-surface groundwater to the unsaturated zone and to the atmosphere. We propose a conceptual model of bubble-mediated mass transfer within the exchange zone defined by the range of groundwater fluctuations. Based on this model, we discuss our experimental data collected over a period of 2 years from of a small-scale test site (6.5 m × 2.5 m × 5 m, 20 observation wells), where we measured the dissolved gases N₂O and O₂ at five different depths ( 0.1, 0.5, 0.8, 1.5, and 2.5 m below groundwater level).We show by visualization of the spatially interpolated data and by descriptive statistics that the N₂O concentration of near-surface groundwater exhibits a significant anticorrelation to O₂ concentration, a spatial coefficient of variation up to 260%, and a spatial-correlation range at the meter-scale. The temporal variation of the spatially averaged data is correlated to the temporal variation of the averaged groundwater level. The implications of high spatial and temporal variability on gradient-based flux models like the steady state–flat interface model usually used in literature are discussed. Our main conclusion is that both the steady-state assumption and the flat-interface model are far from being realistic, because of (1) the highly transient behavior of the exchange zone and (2) the oversimplification of the gas–water interface that underestimates mass transfer by order of magnitudes compared to bubble-mediated mass transfer.