Reaction chain modeling of denitrification reactions during a push-pull test

摘要

Field quantitative estimation of reaction kinetics is required to enhance our understanding of biogeochemical reactions in aquifers. We extended the analytical solution developed by Haggerty et al. (1998) to model an entire 1st order reaction chain and estimate the kinetic parameters for each reaction step of the denitrification process. We then assessed the ability of this reaction chain to model biogeochemical reactions by comparing it with experimental results from a push-pull test in a fractured crystalline aquifer (Ploemeur, French Brittany). Nitrates were used as the reactive tracer, since denitrification involves the sequential reduction of nitrates to nitrogen gas through a chain reaction (NO+3~-→NO_2~-→NO→N_2O→N_2) under anaerobic conditions. The kinetics of nitrate consumption and by-product formation (NO_2~-, N_2O) during autotrophic denitrification were quantified by using a reactive tracer (NO_3~-) and a non-reactive tracer (Br~-). The formation of reaction by-products (NO_2~-, N_2O, N_2) has not been previously considered using a reaction chain approach. Comparison of Br~- and NOT breakthrough curves showed that 10% of the injected NO_3~- molar mass was transformed during the 12 h experiment (2% into NO_2~-, 1 % into N_2O and the rest into N_2 and NO). Similar results, but with slower kinetics, were obtained from laboratory experiments in reactors. The good agreement between the model and the field data shows that the complete denitrification process can be efficiently modeled as a sequence of first order reactions. The 1st order kinetics coefficients obtained through modeling were as follows: k_1 = 0.023 h~(-1),k_2=0.59 h~(-1),k_3=16 h~(-1),and k4=5.5 h~(-1).Anext step will be to assess the variability of field reactivity using the methodology developed for modeling push-pull tracer tests.