RECHARGE-DISCHARGE RELATIONSHIPS ON FLOODPLAINS INFLUENCE BIOGEOCHEMICAL PROCESSING OF FLOODWATER NITROGEN AND EXPORT BACK TO RIVERS

Floodplains influence the water quality of their adjacent channels by transforming, retaining, or removing particulate and dissolved constituents during high flow. Removal of nitrogen from floodwater, for example, lessens the load of nitrogen delivered to downstream estuaries where water quality has been degraded by decades of increasing riverine nitrogen inputs. We measured N-fluxes in 2 floodplain sloughs during 2 flood events along the Tangipahoa River in southeastern Louisiana. While one slough recharged the groundwater system, the other received groundwater discharge from the surrounding floodplain system. The two sloughs also differed in their nitrogen mass balances during the floods. Substantial nitrogen was exported back to the river in the slough that received groundwater discharge, while neither ammonia nor nitrate was exported back to the river from the slough where recharge occurred. We interpreted these different behaviors to be a consequence of oxygen replenishment to the sediment interface in the recharging slough that stimulated nitrification (and coupled nitrification/denitrification) in porewater just beneath the interface. Our interpretation was strengthened by isotopic tracer experiments in the sloughs that quantified denitrification rates and suggested that nitrification only occurred in the slough with recharge. We conclude that differing degrees of nitrogen removal in recharging and discharging sloughs is not explained by hydrology alone (i.e. greater N export from the discharging slough was not explained by the groundwater flux, which was volumetrically too small to be significant to the nitrogen mass balance). Rather, it was the effect that recharge had on delivering a reactant (oxygen) to the top few centimeters of sediment in the recharging slough that was important to nitrogen removal, through its effect in stimulating the coupled nitrification/denitrification reaction just beneath the floodplain sediment-water interface.