Effects of carbon load on nitrate reduction during riverbank filtration: Field monitoring and batch experiment.

Riverbank filtration (RBF) is a well-established technique worldwide, and is critical for the maintenance of groundwater quality and production of clean drinking water. Evaluation of the decay of exogenous nitrate (NO) in river water and the enrichment of ammonium (NH) in groundwater during RBF is important; these two processes are mainly influenced by denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) controlled by the groundwater carbon load. In this study, the effects of carbon load (organic carbon [OC]: NO) on the competing nitrate reduction (DNRA and DNF) were assessed during RBF using field monitoring and a laboratory batch experiment. Results show the groundwater OC: NO ratio did not directly affect the reaction rate of DNRA and DNF, however, it could control the competitive partitioning between the two. In the near-shore zone, the groundwater OC: NO ratio shows significant seasonal variations along the filtration path owing to the changing conditions of redox, OC-rich, and NO-limited. A greater proportion of NO would be available for DNRA in the wet season with higher OC: NO ratio (> 10), resulting in a significantly NH-N enrichment rate (from 1.43 × 10 to 9.54 × 10 mmol L d) in the near-shore zone where the zone of Mn (IV) oxide reduction. However, the activity of DNRA was suppressed with lower OC: NO ratio (< 10) in the dry season, resulting in a stable NH-N enrichment rate (from 3.12 × 10 to 1.30 × 10 mmol L d). Benefiting from seasonal variation of OC-rich and NO-limited conditions, DNRA bacteria outcompeted denitrifiers, which eventually led to seasonal differences in NO reduction in the near-shore zone. Overall, under the effect of DNRA induced by continuous high carbon load in RBF systems, nitrogen input is not permanently removed but rather retained in groundwater during RBF.