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 (N_2O) is essential to process understanding of N_2O 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 N_2O and O_2 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 N_2O concentration of near-surface groundwater exhibits a significant anticorrelation to O_2 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.