Groundwater nitrification and denitrification are not always strictly aerobic and anaerobic processes, respectively: an assessment of dual-nitrate isotopic and chemical evidence in a stratified alluvial aquifer

Nitrification and denitrification are traditionally assumed to occur under aerobic and anoxic conditions, respectively. However, new and interesting alternatives challenge the traditional assumption. Along this line, we provide dual-nitrate isotopic and chemical evidence for the occurrence of denitrification linked with heterotrophic nitrification in an upper oxic region and nitrification in an underlying low-oxygen (sub-oxic/anoxic) layer of a stratified and channelized alluvial aquifer. Particularly significant is the source of the oxidant required for nitrification within the deeper low-oxygen layer. Combined with the existence of steep geochemical gradients, the introduction of a favorable manganese oxide oxidant from the upper oxic layer into the underlying low oxygen region during diffusive mixing resulted in the higher concentrations of nitrate observed (due to the anoxic reoxidation of nitrite to nitrate) as well as the strong positive correlation of nitrite/nitrate with manganese (II) ion concentrations (in the deeper anoxic layer compared with the shallower oxic layer). The observations and findings presented herein have implications for not only reconciling the discrepancies in such unconventional pathways of nitrogen metabolism between groundwater ecosystems and river/stream/-soil/marine systems (where such processes have most commonly been reported) but also for devising effective nitrogen nutrient remediation and complex nitrogen-cycling modeling strategies.