Kinetics of Nsub2/subO production and reduction in a nitrate-contaminated aquifer inferred from laboratory incubation experiments

摘要

Knowledge of the kinetics of N₂O production and reduction ingroundwater is essential for the assessment of potential indirect emissionsof the greenhouse gas. In the present study, we investigated this kineticsusing a laboratory approach. The results were compared to field measurementsin order to examine their transferability to the in situ conditions. The study sitewas the unconfined, predominantly sandy Fuhrberger Feld aquifer in northernGermany. A special characteristic of the aquifer is the occurrence of thevertically separated process zones of heterotrophic denitrification in thenear-surface groundwater and of autotrophic denitrification in depths beyond2–3 m below the groundwater table, respectively. The kinetics of N₂Oproduction and reduction in both process zones was studied during long-termanaerobic laboratory incubations of aquifer slurries using the 15Ntracer technique. We measured N₂O, N₂, NO₃-,NO₂-, and SO₄2- concentrations as well as parameters ofthe aquifer material that were related to the relevant electron donors, i.e.organic carbon and pyrite. The laboratory incubations showed a lowdenitrification activity of heterotrophic denitrification with initial ratesbetween 0.2 and 13 μg N kg?1 d?1. The process was carbonlimited due to the poor availability of its electron donor. In theautotrophic denitrification zone, initial denitrification rates wereconsiderably higher, ranging between 30 and 148 μg N kg?1 d?1,and NO₃- as well as N₂O were completely removedwithin 60 to 198 days. N₂O accumulated during heterotrophic andautotrophic denitrification, but maximum concentrations were substantiallyhigher during the autotrophic process. The results revealed a satisfactorytransferability of the laboratory incubations to the field scale forautotrophic denitrification, whereas the heterotrophic process lessreflected the field conditions due to considerably lower N₂Oaccumulation during laboratory incubation. Finally, we applied aconventional model using first-order-kinetics to determine the reaction rateconstants k₁ for N₂O production and k₂ for N₂Oreduction, respectively. The goodness of fit to the experimental data waspartly limited, indicating that a more sophisticated approach is essentialto describe the investigated reaction kinetics satisfactorily.