Coastal stormwater wet pond sediment nitrogen dynamics

Wet ponds are a common type of stormwater control measure (SCM) in coastal areas of the southeastern US, but their internal nitrogen dynamics have not been extensively studied. Using flow-through intact sediment core incubations, net sediment N₂ fluxes before and after a nitrate addition from five wet ponds spanning a range of ages (3.25-10years old) were quantified through membrane inlet mass spectrometry during early summer. Multiple locations within a single wet pond (6.16years old) were also sampled during ambient conditions in late summer to determine the combined effects of depth, vegetation, and flow path position on net N₂ fluxes at the sediment-water interface. All pond sediments had considerable rates of net nitrogen fixation during ambient conditions, and net N₂ fluxes during nitrate-enriched conditions were significantly correlated with pond age. Following a nitrate addition to simulate storm conditions, younger pond sediments shifted towards net denitrification, but older ponds exhibited even higher rates of net nitrogen fixation. The pond forebay had significantly higher rates of net nitrogen fixation compared to the main basin, and rates throughout the pond were an order of magnitude higher than the early summer experiment. These results identify less than optimal nitrogen processing in this common SCM, however, data presented here suggest that water column mixing and pond sediment excavation could improve the capacity of wet ponds to enhance water quality by permanently removing nitrogen.