Denitrification Within Riparian Floodplains Influenced by Inundation Time and Carbon Availability

Best management practices (BMPs) are commonly used for water quality and/or flow control purposes within watersheds. Typically, small BMPs such as bioretention basins do not provide enough residence time for efficient nitrate (NO3) removal and other systems such as wet ponds have variable metal/nitrogen removal due to the complexity of treatment processes. This study investigated if a floodplain regularly receiving discharge from a wet pond could further decontaminate stormwater containing nitrogen and metals. In soil columns, flooding conditions were established to mimic the discharge of wet pond water to floodplain soil. Denitrification was measured by Membrane-Introduction Mass Spectrometry (MIMS) to measure dinitrogen (N2) gas evolution over time. Additionally, porewater was analyzed for nitrogen speciation and common metals of concern in stormwater following flooding. Results showed that denitrification or increased N2 production was heavily influenced by inundation time and carbon availability, with denitrification rates increasing by 9-fold or higher in day two and three. Furthermore, carbon availability played a larger role during longer inundation periods. Within our system, dissolved oxygen (DO) concentrations were relatively high which suggests that complete anoxic conditions are not required. Additionally, this study saw metal concentrations increase regardless of carbon addition or inundation time. However, overall concentrations did not exceed water quality standard levels. This result emphasizes the possibility of using floodplains as a point of stormwater discharge to enhance nitrogen removal within the watershed and the role of residence time and carbon within these systems.